HfC and TaC are typical ultra-high temperature ceramics (UHTCs), which are important candidate materials for thermal protection components in high-speed aircraft. However, their oxidation resistance at medium and low temperatures is relatively poor. Due to the similar crystal structures of HfC and TaC, and the close atomic radii of Hf and Ta (r_Hf = 1.585 Å, r_Ta=1.457 Å, Δr < 0.15), theoretically, they can form an infinitely miscible substitutional solid solution (Hf_xTa_1-x)C (0 < x < 1). Among them, Hf_0.2Ta_0.8C has a melting point as high as 4 300 K, which is recognized as the highest melting point carbide. Compared with single-component HfC and TaC, (Hf_xTa_1-x)C exhibits higher initial oxidation temperature and lower oxidation rate, indicating significantly improved oxidation resistance. In this work, a series of processable liquid single-source precursors (SSPs) were successfully prepared by introducing HfCl₄ and TaCl₅ into allylhydridopolycarbosilane (AHPCS). Through the polymer-derived ceramic (PDC) method, SiC/(Hf_xTa_1-x)C/C nanocomposites with core-shell structured SiC@C and (Hf_xTa_1-x)C@C nanoparticles were obtained, and the evolution of phase composition and microstructure during the ceramic transformation process was investigated. Combining PDC and spark plasma sintering (SPS) technology, nearly dense SiC/(Hf_xTa_1-x)C/C bulk materials were prepared, and the mechanical properties of the resulting bulk ceramic were investigated.

Chemically stoichiometric amounts of HfCl₄ and TaCl₅ were weighed and reacted with AHPCS in CHCl₃ solvent at 60 ℃ to prepare a series of SSPs. The molecular structure of the synthesized SSPs was characterized using Fourier-transform infrared spectroscopy (FT-IR, Nicolet iS10, Thermo Fisher Scientific). Thermal behavior analysis of the SSPs was conducted using a thermogravimetric analyzer (TGA, STA 449 F5 Jupiter, NETZSCH). The SSPs were heat-treated at temperatures ranging from 900 ℃ to 1 600 ℃ to obtain SiC/(Hf_xTa_1-x)C/C nanocomposite powders. The phase composition of the ceramic powders was analyzed using X-ray diffraction (XRD, SmartLab-SE, Rigaku Corporation) with Cu Kα1 radiation (λ=1.540 6 Å). The microstructure and phase composition of the samples were analyzed using transmission electron microscopy (TEM, Talos F200S, Thermo Fisher Scientific) and selected area electron diffraction (SAED). The powders were sintered using a spark plasma sintering (SPS, CXSPS-40A, Chenxin Corporation) at 50 MPa and 2 200 ℃ to prepare SiC/(Hf_0.2Ta_0.8)C/C bulk materials, named as LAH2T8-bulk. The surface morphology of LAH2T8-bulk was observed using scanning electron microscopy (SEM, CLARA, TESCAN). The hardness and elastic modulus of the bulk materials were measured using a nano-indenter (iNano, Nanomechanics, Inc.) with a Berkovich tip under a maximum load of 20 mN. The volume density and porosity of LAH2T8-bulk were measured using the water immersion method.

During the synthesis of liquid SSPs, the main reactions are dehydrochlorination between metal chlorides and AHPCS. The crosslinking of liquid SSPs is achieved through further consumption of Si-H via dehydrochlorination reactions. The ceramic yield of the obtained SSPs at 900 ℃ exceeds 90%. By combining the PDC method with SPS, nearly dense SiC/(Hf_0.2Ta_0.8)C/C ceramic bulk with an open porosity of 4.87% were successfully prepared. In the crystalline phases, the mass fractions of β-SiC and (Hf_0.2Ta_0.8)C are 60.7% and 39.3%, respectively, with average grain sizes of both phases being less than 90 nm. The measured nanoindentation hardness is (7.72±1.36) GPa, and the elastic modulus is (76.71±7.89) GPa.

By altering the feed ratio of HfCl₄/TaCl₅ during the synthesis process of the SSPs and designing their molecular structure, the composition of the transition metal carbide solid solution products in the final SiC/(Hf_xTa_1-x)C/C nanocomposites can be effectively controlled. After heat treatment at 1 600 ℃, core-shell structured (Hf_0.2Ta_0.8)C@C and SiC@C nanoparticles were in-situ formed in the ceramic matrix. The combination of the PDC approach with SPS technology results in the bulk SiC/(Hf_0.2Ta_0.8)C/C ceramic nanocomposites with good mechanical properties.

Uniform distribution in molecular level of ceramics could be achieved by polymer-derived method. Comparing to other methods, fewer crystal seeds existed in microstructure of polymer-derived ceramics, which made it more difficult to sinter. Up to now, few reports focused on the direct sintering of the polymer-derived ceramics. For silicon nitride sintering, the introduction of metal oxide additives caused defects in the lattice of silicon nitride because of oxygen atoms, which could affect the properties of the material. This work focused on the sintering behavior of polymer-derived ceramics and the issue of performance degradation caused by introducing oxygen elements in sintering additives. The metal elements were connected to the precursor molecule and formed the metal nitride sintering additives during pyrolysis process. The method could help to achieve the in-situ non-oxygen addition and uniformly mixing of sintering additives, and the followed low-temperature sintering could achieve the high-purity silicon nitride ceramics.

4 mL of propylamine was added into a solution of trimethylsilyl chlorosilane monomer (14 g), yttrium chloride (0.14 g) and magnesium chloride (0.085 g) in pyridine (20 mL) at room temperature in N₂ atmosphere. The mixture was warmed up to 120 ℃ and stirred for 2 d. The mixture was then heated to 400 ℃ for 4 h in N₂ atmosphere and further heated to 1 000 ℃ with the rate of 10 ℃/min, finally the mixture were holding at 1 000 ℃ for 4 h and then cooled to room temperature naturally. A white silicon nitride powder SiN-1 was obtained. Silicon nitride powders SiN-2 and SiN-3 were prepared by the same method, in which 0.055 g magnesium chloride was added in SiN-2 and no magnesium chloride was added in sample SiN-3. Silicon nitride ceramics was prepared through spark plasma sintering of silicon nitride powders in 1 450 ℃, 45 MPa for 30 min. Before characterization of the silicon nitride ceramics, surface of it was pre-polished.

Based on trimethylsilyl-substituted polysilazane precursors, the Y and Mg elements were successfully connected to N with the reaction of M-Cl to N-H and coordination. Through pyrolosis process, three kinds of ceramic powders SiN-1, SiN-2 and SiN-3 were obtained. The main component of these powders was silicon nitride, and content of C was below 0.1%, that of O was below 1.6%. As the content of metal decreased, there is a slight decrease in oxygen contents, being due to the high reactivity of metal nitrides. When treated at 1 000 ℃, a large amount of amorphous silicon nitrides with small amount of magnesium nitrides via the dope of magnesium. Small amount of YSi₃N₅ with Si₃N₄ via the dope of yttrium. When ceramics were heated to 1 500 ℃, amorphous silicon nitride began to crystallize, and sample SiN-3 only contained α-Si₃N₄. As the amount of magnesium increased, the content of β-Si₃N₄ crystal gradually increased. These results demonstrate that magnesium nitrides could promoted the transformation of amorphous Si₃N₄ to β-Si₃N₄ crystalline, while yttrium nitrides activated the lattice of Si₃N₄ crystal.

Silicon nitride ceramics were prepared by spark plasma sintering when three types of silicon nitride ceramic powders acted as main raw materials, but only SiN-1 powders with the highest magnesium content could obtain pure β-Si₃N₄. The frame of the obtained ceramics was mainly composed of stacked rod-shaped β-Si₃N₄ crystals. These results prove that the pores in porous ceramics were mainly through-pores, and few closed-pores. The porous ceramics showed a hardness of 29.1 GPa, an elastic modulus of 229.1 GPa, which attributed to the speical porous structure. At 25 ℃, the ceramics possessed a thermal conductivity of 20.451 (W/m·K), a specific heat capacity of 0.355 J/(g·K), a thermal diffusion coefficient of 18.169 mm²/s, and an overall thermal expansion coefficient less than 4×10^-6/K in the range of room temperature to 1 200 ℃. Due to the presence of pores, a significant decrease in thermal conductivity and thermal diffusion coefficient can be obtained, while the coefficient of thermal expansion remained relatively low.

Metal element Y and Mg were successfully connected to trimethylsilyl-substituted polysilazane precursors and contents could be adjusted as needed. Metal nitride sintering additives were in-situ generated during pyrolysis process and polymer-derived high-purity silicon nitride ceramics with sintering additives were prepared. This method could help to achieve both in-situ non- oxygen addition and molecular level uniform mixing of sintering additives in ceramic powders. Low-temperature sintering could help to achieve the silicon nitride ceramic powders which resulting in the obtained silicon nitride ceramics with good mechanical and thermal properties.

With the rapid development of high-technology fields, advanced aerogel acts as the thermal insulation materials under high-temperature environment attracted extensive attention. Polymer derived ceramic (PDC) aerogels were considered to be a promising thermal protection materials due to its low density, high porosity, excellent thermal insulation performance and high-temperature stability. Silicon oxycarbide (SiOC) ceramic aerogels, one kind of typical representative PDC materials, were mainly formed by partial substitution of C atoms for O atoms in SiO₂ tetrahedra. Compared the traditional silica aerogels, silicon oxycarbide (SiOC) aerogels exhibited better thermal and chemical stability as well as higher mechanical strength. Previously research reported that SiOC aerogels were mainly prepared by sol-gel method and PDC route, exhibited a higher density, which limited its applications and development in lightweight thermal insulation filed. Moreover, the obtained SiOC aerogels exhibited a necklace-like structure, poor high-temperature stability and mechanical strength due to the weak contact between nanoparticles. And most researches on SiOC aerogels focused on its high-temperature pyrolysis behavior. In this paper, a lightweight and high-strength SiOC aerogel were prepared using polyhydromethylsiloxane (PHMS) as precursor and tetramethyltetravinylcycletetrasiloxane (D₄Vi) as crosslinker through solvothermal, freeze casting and pyrolysis. The influence of D₄Vi content on the microstructure and thermal stability of SiOC aerogels were discussed. The evolution of phase composition, mechanical strength and thermal insulation performance was further explored via high-temperature heat treatment and oxidation treatment.

In a typical process, 1.5 g D₄Vi and 1.0 g PHMS were added into 52.4 g cyclohexane solvent and mixed uniformly by magnetic stirring for 30 min. Then, 1% Pt catalyst was added into the mixture for another 15 min stirring to obtain the transparent polysiloxane (PSO) solution. The PSO solution was then poured into an 80 ml Toflon-lined autoclave and underwent hydrothermal treatment at 150 ℃ for 20 h. Subsequently, the sample was freeze-dried at -50 ℃ for 48 h to obtain PSO aerogels. Finally, the PSO aerogels were pyrolyzed at 1 000 ℃ for 2 h with a heating rate of 5 ℃/min in Ar atmosphere. The pyrolyzed SiOC aerogels were further treated at 1 400 ℃ in Ar and 800 ℃ in air to explore their high-temperature stability and anti-oxidation.

The bulk density of SiOC aerogels was calculated by the measurement of the mass and dimension of the samples. The pore size distribution of SiOC aerogels was characterized using mercury intrusion porosimetry. The chemical structure and phase composition of SiOC aerogels was explored by Fourier transform infrared spectrometer and X-ray diffractometer. The microstructure of SiOC aerogels was characterized by scanning electron microscopy. The thermal conductivity of SiOC aerogels was measured by the hot-disk method using a thermal constant analyzer with the sample's dimension of Φ15 mm ×20 mm. The compressive strength of SiOC aerogels was investigated by an electronic universal testing machine using the samples with a dimension of Φ15 mm×20 mm.

lightweight SiOC aerogels were prepared by solution-based freeze-drying method under the mass ratio of D₄Vi: PHMS of 1.5 and 2. Both kinds of obtained aerogels exhibited low density of 0.12-0.13 g·cm^-3, much lower than that of previously reported SiOC aerogels and porous SiOC ceramics (0.23-0.62 g·cm^-3). With the increase in DVB content, the ceramic yield of SiOC aerogels decreased from 75.40% to 71.68% as the decreased crosslinking degree of PSO precursor, further leading to the transition from relatively disordered spider-web structure to ordered honeycomb-like structure. The stable porous structure endowed SiOC aerogels with low thermal conductivity of 0.061 9-0.065 5 W·m^-1·K^-1 and high compressive strength of 0.60-0.96 MPa.

Moreover, the SiOC aerogels possessed excellent high-temperature stability, when heat treatment at 1 400 ℃ in Ar, density of it was still ranged between 0.12 and 0.13 g·cm^-3 with original honeycomb-like structure. Meanwhile, the SiOC aerogels remained low thermal conductivity of 0.066 7-0.070 3W·m^-1·K^-1 and high compressive strength of 0.56-0.95 MPa. When oxidation at 800 ℃ in air, the SiOC aerogels still maintained porous structure and a density of 0.12-0.13 g·cm^-3 with a low mass loss of about 1.2%. The related thermal conductivity showed a minor variation of 0.060 7-0.064 3 W·m^-1·K^-1 and the compressive strength lightly decreased to 0.58-0.91 MPa, indicating a good anti-oxidation performance.

In this work, SiOC aerogels with lightweight, high-strength and thermal performance were prepared using PHMS as precursor and D₄Vi as crosslinker through solvothermal, freeze casting and pyrolysis. Porous honeycomb-like structure endowed SiOC aerogels with low density (0.13 g·cm^-3) and thermal conductivity (0.065 5 W·m^-1·K^-1) as well as high compressive strength (0.96 MPa). Meanwhile, SiOC aerogels exhibited excellent high-temperature stability and oxidation resistance. The corresponding bulk density, phase composition, thermal conductivity and compressive strength still remained and without any structural collapse even heat treatment at 1 400 ℃ in Ar, which could be an ideal candidate for lightweight, high strength, and high-temperature stability materials.

SiC ceramics, known for their lightweight, high strength, high temperature resistance and oxidation resistance, find extensive applications in aerospace, energy and transportation industries. The polymer-derived ceramics (PDC) method is an important method for the preparation of SiC ceramics, employing special polymers as raw materials and converting them into inorganic ceramics via high-temperature pyrolysis. This method offers advantages such as controllable elemental composition, ease of molding, and lower processing temperatures. As one of the most important raw materials in the PDC method, hyperbranched liquid polycarbosilanes, synthesized via the Grignard coupling reaction and the subsequent reduction reaction, are prized for their low viscosity, high ceramic yield, and ease of processing and molding. Therefore, it has been widely used in the fabrication of ceramic matrix composites, coatings and membrane materials. A large number of articles have been published on the synthesis and applications of polycarbosilanes. The molecular structure of SiC polymer precursors not only influences their curing and pyrolysis processes, including weight loss and gas release behavior, but also plays an important role in determining the elemental composition (e.g., C/Si ratio), crystallization behavior and oxidation resistance. There are many studies conducted to investigate the structure of precursors affecting their curing and pyrolysis properties. However, it is very necessary to investigate the relationship between molecular structures of different types of liquid polycarbosilanes with varying C/Si ratios and their curing, pyrolysis and crystallization behaviors. In this work, the curing and pyrolysis behaviors as well as the crystallization behavior of two major types of liquid hyperbranched polycarbosilanes vinylhydridopolycarbosilane (VHPCS) and allylmethylhydridopolycarbosilane (AMHPCS)) with different carbon-to-silicon ratios and molecular structures were comprehensively characterized by a variety of testing methods. The relationship between the molecular structure and the ceramization process, as well as the properties of the ceramic products, was established using five types of liquid polycarbosilanes with varying C/Si ratios and molecular structures.

0.3% of DCP was added to the VHPCS precursor, while 0.5% of DCP was added to the AMHPCS precursor, the mixtures were then stirred at room temperature until the DCP fully dissolved. The compounds were then poured into tetrafluoroethylene coated stainless steel molds and defoamed in vacuum before being transferred to the blast oven for curing. The curing procedure of VHPCS/AMHPCS precursor was as follows: the temperature was increased from room temperature to 120 ℃/160 ℃ at a rate of 3 ℃/min, held for 1 h, then increased from 120/160 ℃ to 170/200 ℃ at a rate of 2 ℃/min, held for 2 h. Finally, the samples were cooled to room temperature at a rate of 3 ℃/min to obtain transparent cured cylindrical. The cured samples were pyrolyzed in an Argon atmosphere with a ramping rate of 3 ℃/min to 1 100/1 500 ℃ and held at the target temperature for 2 h.

The molecular structure of the VHPCS and AMHPCS precursors was characterized by ¹H-NMR and FT-IR spectroscopy. The curing temperatures of the two types of precursors were analyzed by DSC tests. The cross-linking reactions temperatures was increased with the higher methyl, vinyl and allyl content. When DCP was added to the precursor, the peak of curing temperature reduced by about 80 ℃. By comparing the DSC curves of the precursor A1-A3, it can be discovered that the contents of Si—H, Si—Me, and Si—allyl groups collectively determine the curing temperature of the five precursors. Due to the different molecular structure, significant differences were observed in the TG curves of VHPCS and AMHPCS, as represented by precursor V1 and precursor A3. The reasons for these differences were explored through TG-DTG and TG-MS curves. During the pyrolysis process, the two precursors released different amounts of gas at different temperature ranges. The crystallization behavior and antioxidant properties of the PDCs were also investigated. It was observed that the VHPCS precursors are easier to form SiC crystal compared to the AMHPCS precursors. This phenomenon was evidenced by the larger grain sizes observed in VHPCS-derived ceramics under identical pyrolysis temperatures. Additionally, an increase in carbon content in the pyrolysis products of the precursors was found to inhibit the growth of β-SiC. TG analysis tests conducted in air atmosphere revealed that designing the molecular structure of the single-source SiC precursor to adjust the carbon-to-silicon ratio of the pyrolysis products can improve the antioxidation properties of polymer-derived ceramics.

The relationship between the molecular structure and the ceramization process as well as the properties of ceramic products was established by analyzing the molecular structure characteristics, curing and pyrolysis, crystallization as well as high-temperature oxidation behavior of the pyrolyzed VHPCS and AMHPCS precursors. The results show that the type and amount of the side-chain functional groups could affect the curing temperature and ceramic yield of the precursors, and the addition of appropriate amount of DCP could reduce the curing temperature by about 80 ℃. The ceramic precursor has low viscosity and high ceramic yield, but the reduction of active groups can lead to higher curing temperature and lower ceramic yield. For precursor A3 with 40% Si—Me groups, it reached its curing peak temperature at 188 ℃ and exhibited a ceramic yield of 75%, while precursor A1 with 10% Si—Me groups reached its curing peak temperature at 160 ℃ and a ceramic yield of 84.4% when pyrolyzed at 1 000 ℃. Side groups affect the pyrolysis gas release behavior and different precursors lead to different C/Si ratios in the ceramic products. For precursor V1, the main cause of weight loss is the release of H₂, whereas for precursor A3, the main factor leading to weight loss is the release of small molecules of methane or olefins. Carbon-rich compositions inhibit the crystalline growth of SiC, and the single-source precursor with a C/Si ratio of 1.7 in the pyrolytic ceramic product has good high-temperature oxidation resistance.

The SiC_xO_y phase in SiC fibers will decompose at high temperature, which results in the loose structure, declining mechanical performance, and lowing the high temperature resistance of the fibers. Introduction of some elements can play the role of sintering assistant at high temperature. Nearly stoichiometric polycrystalline SiC fibers can improve the oxidation resistance of fibers at high temperature. Al and Y are commonly used as sintering additives for SiC ceramics. Al and Y can form yttrium aluminum garnet (YAG) liquid phase at high temperature. The YAG liquid phase is conducive to reduce the sintering temperature and improve the densification of SiC ceramics. If the SiC fibers can form YAG phase at high temperature, the performance of SiC fibers can be improved. In this work, the both Y and Al are introduced for the fabrication of high performance SiC fibers.

A certain amount of polydimethylsilane (PDMS) was placed in a flask, raised to 470 ℃ and hold for 42 h in N₂, then cooled to room temperature, filtered to obtain yellowish viscous polysilacarbosilane (PSCS). 0.4 g yttrium acetylacetone (Y(AcAc)₃) and 0.6 g aluminum acetylacetone (Al(AcAc)₃) were uniformly mixed with 20 g PSCS in a flask, polymerized in N₂, and then dissolved, filtered, and distilled under vacuum to obtain polyyttrio-aluminocarbosilane (PYACS) precursor.

A certain amount of PYACS precursor was added to a spinning cylinder, heated to 300 ℃ in N₂, for defoaming, then reduced to 240 ℃. Using a pressurized N₂ (flow rate of 0.1 L/min), the melt was extruded through a single hole spinneret and the fiber was stretched and wound on a spool by a drafting device at a winding speed of 350 r/min. Curing of the fibers was carried out by exposing fibers in iodine with mass ratio of 3∶5, covered with glass and placed in oven. The system was heated up to 80 ℃ for 3 h, then cooling to room temperature.

PYACS fibers were pyrolysied in N₂ and converted into ceramic fibers at different temperature schdule, such as 20 ℃ to 400 ℃ with a heating rate of 150 ℃/h, 400 ℃ to 800 ℃ with a heating rate of 100 ℃/h, 800 ℃ to 1 000 ℃ with heating rate of 150 ℃/h. Si-C-O-(Y/Al) fibers are fired at 1 000 ℃ for 1 h. The Si-C-O-(Y/Al) fibers were heated to 1 400, 1 600 and 1 900 ℃ in argon according to certain procedures.

Due to the higher molecular weight of polycarbosilane (LPCS), it tends to crosslink during polymerization, which is notably affecting the spinning properties and fiber characteristics. To solve this problem, PYACS fibers were synthesized by addition of Y(AcAc)₃ and Al(AcAc)₃ into liquid PSCS, providing uniform molecular weight with excellent spinnability. The Y and Al elements are effectively introduced into the PYACS precursor as the form of Si—O—Y and Si—O—Al. PYACS fibers were obtained successfully by melt-spinning and iodine vapor curing. The cross-linking of fibers can be achieved with the iodine vapor curing, where iodine plays the critical role of accelerating the cleavage of Si—H, Si—Si, and C—H bonds, and recombination to form cross-linked networks of Si—O—Si. The amorphous Si—C—O—(Y/Al) fibers were obtained by pyrolysising at 1 000 ℃, and the ceramic yield was 73.96%. The effects of different heat treatment temperatures (1 400, 1 600, 1 900 ℃) on the morphology and structure of the Si—C—O—(Y/Al) fibers were investigated. After heat treatment at 1 400 ℃, the fibers remained amorphous, with smooth surface and dense cross section. After heat treatment at 1 600 ℃, the fibers has obvious β-SiC diffraction peak. And the fibers surface and the cross sections are loose porous structure, the diameter becomes smaller. The degradation reaction occurs in the fibers may be the main reason for the formation of porous degraded fibers, SiC grains are formed in Si—C—O—(Y/Al) fibers, and the release of CO gas. After heat treatment at 1 900 ℃, the crystalline degree of SiC fibers doped with Al and Y is higher, the grain size increases, and the fibers diameter decreases further. Al and Y in Si—C—O—(Y/Al) fibers exist as Al—O—Y at 1 900 ℃. The doping of Al and Y plays a role of sintering additives at high temperature.

The main conclusions of this paper are summarized as following. The Y and Al elements are effectively introduced into the PYACS precursor as the form of Si—O—Y and Si—O—Al. The crosslinking of fibers can be achieved with the iodine vapor curing. The amorphous Si—C—O—(Y/Al) fibers were obtained by pyrolysising at 1 000 ℃, and the ceramic yield was 73.96%. The effects of different heat treatment temperatures (1 400, 1 600, 1 900 ℃) on the morphology and structure of the fibers were investigated. After heat treatment at 1 400 ℃, the fibers remained amorphous, with smooth surface and dense cross section. After heat treatment at 1 600 ℃, the fibers has obvious β-SiC diffraction peak. The surface and cross section of the fibers are loose and porous structures. After heat treatment at 1 900 ℃, the crystalline degree of SiC fibers is higher, the grain size of SiC is increased, the diameter of the fibers is further reduced, and the structure is gradually densified. Al and Y in Si—C—O—(Y/Al) fibers exist as Al—O—Y at 1 900 ℃. This study demonstrates that the presence of Al and Y can reduce the formation of SiC crystal particles and nano-pores on the fiber surface at high temperature. The doping of Al and Y plays a role in the sintering densification of SiC fibers. This work has practical significance for the fabrication of SiC fibers containing heterogeneous element.

SiC ceramic matrix composites have excellent mechanical properties and high-temperature resistance, and hence attracts increasing research attentions in aerospace and other fields. Precursor infiltration and pyrolysis (PIP) is one of the most common methods for the preparation of SiC ceramic matrix composites. The synthesis of organic precursors is the basis of the PIP process. Liquid polycarbosilane (LPCS) is an ideal precursor for the preparation of SiC ceramic matrix composites by PIP process. LPCS can be prepared using the Grignard coupling reaction, while this method suffers from the problem of long synthesis route and high cost. The Wurtz reaction, as another method for synthesizing LPCS, has a shorter route and less cost. However, it is difficult to directly synthesize LPCS containing active groups because of the severe reaction conditions. Ultrasound can significantly promote the effective contact between sodium and chlorosilanes and reduce the reaction conditions in the Wurtz reaction, and some ether additives can improve the activity of the reaction groups. Therefore, the introduction of ultrasound and ether additives is expected to realize the efficient synthesis of LPCS containing active groups. In this work, an active LPCS was prepared directly from chlorosilane monomer containing active groups by using ultrasonic and ether additives in Wurtz reaction. The basic structure of the product, ceramic yield and pyrolytic properties of the transformed ceramics were characterized.

CH₂Br₂, MeHSiCl₂ and ViMeSiCl₂ were used as raw materials, and mixed with the molar ratio of 2∶1∶1. Under the conditions of no ultrasonic or ether auxiliary agent, ether auxiliary agent only and ultrasonic together with ether auxiliary agent, the reaction was carried out at 40 ℃ for 4 h, and the excess Si—Cl was reacted by Me₃SiCl. The corresponding products were obtained by filtration and distillation. Diethylene glycol dimethyl ether (DGDME) or tetrahydrofuran (THF) was selected as ether auxiliary agent, and THF is easier to remove due to its lower boiling point, thus providing an advantage. On this basis, the molar ratio was adjusted to 3∶1∶1 and 4∶1∶1 to synthesize the products.

According to the ratio of actual yield to theoretical yield, the yield of each product was calculated. The basic structures and functional groups of the synthesized products under different conditions of ultrasonic and ether were characterized by Fourier transfer infrared spectroscopy (FT-IR) and nuclear magnetic resonance hydrogen spectroscopy (¹H-NMR). For the synthesized products with THF as ether auxiliary agent and the molar ratios of raw materials as 2∶1∶1, 3∶1∶1 and 4∶1∶1, the functional groups and structures were characterized and compared with FT-IR, and the existence of Si—Si bonds was determined and compared with UV-Vis spectroscopy. The mass change curves of the pyrolysis products at 900 ℃ were analyzed by thermogravimetric analyzer (TG). The crystal structures of the pyrolysis products prepared at 1 000, 1 200 ℃ and 1 400 ℃ were analyzed by X-ray diffractometer (XRD).

For the precursors synthesized under different conditions of ultrasonic and ether, it was found that liquid products could not be obtained by Wurtz reaction at 40 ℃ without ultrasonic and ether. On the contrary, when ultrasonic is introduced, the reaction can take place to obtain the product, and when ultrasound and ether auxiliary agent are used simultaneously, the yield can be further improved. FT-IR and ¹H-NMR spectra show that Si—H, CH=CH₂ exist in all products, and Si—C—Si chain is formed, which proves that LPCS containing active groups can be successfully synthesized by this method. According to IR spectra, Si—CH₂—Si in the product is the most when the molar ratio of raw materials is 3∶1∶1, which indicates that proper increase of CH₂Br₂ content may promote polymerization, which may be due to CH₂Br₂ affecting the concentration of reactive free radicals in the reaction. The TG curves show that the mass loss of each precursor occurs at room temperature to 250 ℃ and 500-600 ℃, respectively, due to the volatilization of small molecular oligomers and the thermal crosslinking process dominated by active groups. When the molar ratio of raw materials is 3∶1∶1, the highest ceramic yield is obtained up to 48.8%. UV-Vis analysis shows that Si-Si bond exists in LPCS. XRD results shows that there is no obvious diffraction peak in the pyrolysis product curve at 1 000 ℃. With the temperature increasing to 1 200 ℃, diffraction peaks of the pyrolysis products appeared at 35.7°, 59. 9° and 71.7°, corresponding to the crystal structure of β-SiC. Further, at 1 400 ℃, the characteristic diffraction peaks of β-SiC were enhanced. All above indicate that LPCS can be partially transformed into β-SiC crystal after 1 200 ℃.

Through the introduction of ultrasonic and ether auxiliary agent, LPCS containing reactive groups can be rapidly synthesized using the Wurtz reaction at a temperature of 40 ℃ with a yield of more than 90%. The synthesized LPCS takes Si—C and Si—Si bonds as the main chain structure, and is rich in Si—H, CH=CH₂ active groups. When the molar ratio of CH₂Br₂, MeHSiCl₂, ViMeSiCl₂ is 3∶1∶1, the ceramic yield of the product reaches the highest value up to 48.8%. This study demonstrates that Wurtz reaction assisted by ultrasonic and ether auxiliary agent is simple in process, high efficient and low cost. It is an effective method to prepare liquid SiC ceramic precursors containing active groups.

In recent years, with the rapid development of the flexible displays and wearable electronic devices, higher requirements have been placed on the performance of electronic devices, such as lightweight and flexible. Polymer films such as transparent polyimide (CPI) and polyethylene terephthalate (PET) have gained widespread attention in the field of flexible optoelectronic materials owing to their high transparency, light weight, good bendability and high mechanical strength. However, due to the characteristics of their molecular structure, polymer films generally have the problems of low surface hardness and poor abrasion resistance. Therefore, enhancing the surface hardness and abrasion resistance of polymer films without compromising their characteristics such as transparency, light weight and flexibility become an urgent problem to be solved. Among the various enhancement strategies that have been developed, applying optically transparent coatings with abrasion and bending resistance to the surface of polymer films is one of the most effective and promising approaches. Hard flexible coatings are mainly achieved by organic-inorganic hybridization to achieve both hardness and flexibility. Organic polysilazanes can retain their organic groups while forming the Si—O—Si network structure by hydrolysis condensation reaction, which can realize organic-inorganic hybridization and providing a basis for constructing hard flexible coatings. In this work, an organic-inorganic hybrid hard flexible coating was prepared at 80 ℃ 90%RH by hybridizing organic polysilazane OPZ181 with triethoxysilane (KH550). The effect of KH550 content on the conversion process of OPZ181 was investigated, and the structure and properties of the hard flexible coatings were systematically characterized.

Using n-butyl ether as solvent, OPZ181 and KH550 were mixed homogeneously at room temperature to obtain a colorless, transparent and clarified OPZ181/KH550 polymer precursor solution. OPZ181/KH550 hybrid coatingswere prepared by liquid-phase coating methods, including spin-coating, scraping and drenching. The chemical structure and compositions of the coatings were characterized by using FT-IR, ²⁹Si NMR and XPS. The micro-morphology of the surface and cross-section of the coatings was observed by SEM. The optical transmittance of the samples was tested using a UV-Vis-NIR spectrophotometer. The mechanical properties of the coatings were characterized using a pencil hardness tester and nano-indenter. The bending test was carried out using a homemade experimental setup. The abrasion resistance of the coatings was tested on the reciprocating abrasion meter using 0000^# type steel wool.

The experimental results and analysis of FT-IR test show that when the ratio of OPZ181 to KH550 in the hybrid hard flexible coating is more than 2∶1, the coatings conversion efficiency to Si—O structure under hygrothermal conditions is increased, and the converted coating structure is dominated by Si—O bonds. The XPS results show that KH550 can effectively promote the conversion of OPZ181 to the Si—O structure at higher content, and —CH₃ can be retained in the coating after the conversion, which is consistent with the results of the FT-IR. Further deconvolution of Si 2p data shows that the coating is dominated by the CH₃SiO₃(T) structure with a high degree of crosslinking by the Si—O—Si structure. The ²⁹Si NMR results similarly show that OPZ181 transforms to the Si—O—Si network structure under hygrothermal conditions, and —CH₃ remains in the coating after the transformation. In addition, as the content of KH550 in OPZ181 increases, the degree of conversion and cross-linking of the Si—O—Si structure increases, and CH₃SiO₃ is the main structure of the hybrid coating. The pencil hardness as well as the nanoindentation hardness and modulus of the coatings increased with the increase of KH550 content, which is attributed to the fact that the coatings are dominated by the CH₃SiO₃ structure with a denser Si—O—Si network at the higher content of KH550. The coating has a pencil hardness up to 6 H, a nanoindentation hardness up to 0.7 GPa, and a modulus up to 5.7 GPa, indicating that the coating has good mechanical properties. Calculated from the load-displacement curves, the H/E of the coating is >0.1 and We >60%, indicating that the coating has good hard flexible properties. In the visible light range, the transmittance of the coated PET film is almost unchanged relative to that of the blank PET film, which can be kept above 90%, indicating that the coating has good optical transparency. From the SEM image, it can be observed that the surface and cross-section of the coating are flat and smooth, and there are no obvious defects such as holes and cracks produced during the transformation process. From the microscopic photographs and SEM surface morphology of the coatings after bending test 10,000 times with a bending radius (R) of 1.5 mm and 1 mm, respectively. The surface is kept flat and smooth, and there is no damage, which indicates that the coating is flexible and has excellent bending resistance. Blank PET film shows visible scratches on its surface after 20 abrasions cycles with 0000^# steel wool. In contrast, the surface of the coated PET film remained flat and smooth after 200 abrasion cycles without any scratches. This shows that the OPZ181/KH550 hybrid coatings have excellent abrasion resistance.

In this work, an organic-inorganic hybrid hard flexible coating was prepared at 80 ℃ 90% RH by hybridizing organic polysilazane OPZ181 with triethoxysilane (KH550). KH550 promotes the hydrolysis condensation reaction of OPZ181 under hygrothermal conditions and improves its degree of conversion. The hybrid hard flexible coatings are dominated by Si—O—Si network structure and contain organic groups such as—CH₃. The degree of cross-linking of the Si—O—Si network in the coatings increases with the increase of KH550 content, resulting in the mechanical properties of the coatings enhanced. The coating has a pencil hardness up to 6 H, nanoindentation hardness up to 0.7 GPa, and a modulus up to 5.7 GPa. The H/E of the coating is >0.1 and W_e>60%, indicating that the coating has good hard flexible properties. The surface of the coating is flat and smooth, and has excellent optical transparency. The coating has excellent bending resistance over 10 000 times under the bending radius of 1.5 mm and 1 mm. The coating can effectively improve the abrasion resistance of PET film. The above experimental results show that organic polysilazane is one of the potential materials for the preparation of organic-inorganic hybrid hard flexible coatings, which is expected to be applied to the protection of flexible devices such as flexible displays and wearable electronic devices.

PDCs as sensing material is one of the most promising materials for high-temperature extreme environment applications. This paper summarized the development of PDCs sensors and elaborated the corresponding sensing mechanism. According to the connection type, the properties of existed wired/wirelesses PDCs sensors was compared, results exhibited that the PDCs sensors possess excellent sensitivity, repeatability, stability and fast response, exhibiting competitive performance in high temperature. Meanwhile, PDCs sensor can be prepared on complex surfaces based on the distinctive liquid precursor. Temperature, pressure and strain signals of PDCs sensor can be monitored in-situ and long-distance.

However, the development of PDCs sensors is still in infancy stage, and it still exists many obstacles to be overcome. Firstly, it is necessary to eliminate the sensor transmission signal interference through the structural design, and improve the anti-interference ability; Secondly, it is still a challenge to realize the application of PDCs sensors at higher temperature and more complex environments; Finally, the repeatability of wireless PDCs sensors needs to be enhanced by attenuating the interference of wireless transmission signals so that the longer distance transmission can be achieved. In a summary, to realize the practical application of PDCs sensors, some parameters need to be optimized such as structural components, material properties, and production costs.

The preparing process of ceramics and composites via PDCs becomes more and more sophisticated. The oxygen-containing system requires a carbon thermal reduction process to obtain ultra-high temperature ceramics, resulting in a lower yield of ultra-high temperature ceramics with more pores and cracks, which leads to excessive PIP times, long densification time, and poor mechanical properties. Therefore, the development of oxygen free ultra-high temperature ceramic precursors is one of the important direction for the future development. Moreover, compared to single-phase ultra-high temperature ceramics, diversified and multiphase ultra-high temperature ceramics exhibite superior performance. Therefore, the development of precursors that could achieve a diversified ultra-high temperature multiphase ceramics was also an important direction to develop in the future. Polymer precursors can also be used in new application fields such as preparing ultra-high temperature thermal protective coatings, ultra-high temperature binders, and ultra-high temperature ceramic fibers. Polymer precursors would have extremely wide applications in the future.

Although significant progress has been achieved in the design and preparation of PDC-NCs for the heterogeneous catalysts, the prevenient research of this field is still superficial and unsystematic, and the "structure-property relationship" of porous PDC-NCs is also unclear. Based on the above-mentioned deficiencies, the challenges faced to PDC-NCs in the field of catalysis preparation have been analyzed in this review and a series of prospects have been proposed as follow: 1) Precursors played an important role in the preparation of PDC-NCs, in order to design and synthesize single-source precursors accurately, the reaction mechanism of chemical modification of polymeric precursors with metallic compounds needed to be studied intensively; 2) Preparing multi-doped PDC-NCs and exploring the synergistic effect of multiple elements in catalytic reactions; 3) In order to prepare catalysts with controllably and optimized catalytic performance, the catalytic mechanism of PDC-NCs need to be investigated thoroughly via the combination of experimental research with theoretical models to establish a scientific connection between "chemical composition, microstructure, catalytic properties" of PDC-NCs; 4) Further explore should focused on the catalytic properties and application of PDC-NCs, especially in the clean energy fields such as water-splitting and fuel cells; 5) Based on the excellent physical and chemical stabilities, high-temperature resistance of PDC-NCs, the catalytic application can be further explored in extreme environments where oxide-based or carbon-based supports did not use because of the lower structural and chemical instabilities; 6) Due to the complex molecular structure of precursors and pyrolysis process, the research on simulation of the microstructure of PDC-NCs is difficult, new technologies such as machine learning should be used to accelerate the development of new PDCs-based catalysts.

In addition, silicon carbide crystal with nanostructure exhibits great potential in photocatalytic water-splitting and photocatalytic CO₂ reduction due to its unique characteristic of semi-conductivity. However, because of the intrinsic defects such as low quantum efficiency, severe carrier recombination, and complex nanostructures, the development and application of SiC in photocatalysis fields have been seriously restricted. SiC nanomaterials with various low dimensional structures, like nanospheres, nanowires, and nanosheets can be prepared easily via PDC approach, the surface decoration and heterojunction construction can also be conveniently achieved. The semi-conductivity of PDCs and their applications in photocatalysis deserved further study, which can expand the application scope of PDCs and explore synthesis strategies of new materials.

Compared with oxide ceramics for wave transparent, nitride ceramics with better high-temperature resistance are the most promising high-temperature wave transparent materials to meet the needs of high Mach number aircraft. Polymer-derived method presenting the advantages of low processing temperature and plasticity, is the promising method for preparing nitride ceramics for the wave transparent at high temperature. This review mainly summarized the research progress on the precursor of nitride wave transmitting ceramics in recent years, involving the synthesis of precursors for ceramics such as silicon nitride, boron nitride, silicon boron nitrogen, and silicon nitrogen oxide, as well as the preparation of target ceramics through thermal decomposition conversion. It also looks forward to the research directions in the future.

Boron nitride fiber is widely used in the field of high-temperature radiation resistance due to its excellent heat resistance and wave transmission properties. the mechanical properties could be improved via the optimizing the composition structure of the polymeric precursor, regulating the heat treatment process and fiber microstructure, which are also the focus and future directions of research on boron nitride fiber prepared by PDCs. Although the mechanical properties of boron nitride fibers prepared currently have been improved to some extent, great challenges still remain in the scientific research and practical applications. The composition structure of the boron nitride precursor plays an important role in the properties of boron nitride fibers. Currently, widely used boron nitride precursors have a relatively high carbon content and low ceramic yield, making the fibers prone to defects and a significant decrease in strength. An ideal boron nitride precursor should have good spinnability and high atomic utilization. Therefore, the optimization and control of composition structure of precursor is a problem that must be solved to prepare high-performance boron nitride fibers in the future. Due to the lack of electron structure of B atoms, boron nitride precursors are generally sensitive to water and oxygen, easily combustible when encountering water, and prone to absorption and deterioration in air, requiring high storage conditions and production processes, which have an adverse impact on the properties of BN fibers. Therefore, regulating the electronic structure of BN precursors to give them good chemical stability and anti-water oxygen properties is an urgent need for large-scale industrial production. The microstructure regulation of boron nitride fibers is mainly a qualitative analysis of crystal orientation at present. By utilizing deep learning of artificial intelligence, clarifying the correspondence between fiber crystallinity, grain size, orientation arrangement, and mechanical properties by combining theoretical calculation and experimental data can provide important theoretical guidance for structural design and improvement of mechanical properties. The strength of boron nitride fibers prepared currently still needs to be improved, especially compared with the high tensile strength of structurally similar carbon fibers. Therefore, how to further effectively improve the tensile strength was a key focus for the future development of boron nitride fibers. In summary, although progress has been made in improving the mechanical properties of boron nitride fibers, it still existed many challenges to be addressed regarding their strength, precursor composition, chemical stability, and microstructure-property relationships. Further optimization in these aspects through multi-disciplinary approaches including precursor design, process control, and machine learning is needed to unlock the full potential of boron nitride fibers for high-temperature structural applications.

Photocatalysis is an effective technology to solve environmental pollution. Ferroelectric ceramics, as a new type of catalyst, have been received widespread attention mainly attributed to the ability to reduce the recombination rate of photo^- induced carriers and to improve catalytic performance by building a built-in electric field. In order to avoid the decrease of remanent polarization in ceramic bulks during the traditional poling technology and simplify the poling process, corona poling technology has been used for the poling of powders. Several researches indicated that corona poling could significantly improve the catalytic performance, while the relationship among the particle size, phase structure, and piezo-photocatalytic performance was still not so clear. Therefore, BaTiO₃ (BT) nanoparticles with different particle sizes were prepared via sol-gel method and then polarized these powders for 1 h under a voltage of 6.56 kV. The phase structures and the lattice distortion before and after poling were carefully explored. In addition, the effect of corona poling on the piezo-photocatalytic performance of BT powders was investigated, Rhodamine B (RhB) was utilized as the target pollutant.

Barium acetate (C₄H₆O₄Ba) and tetrabutyl titanate (C₁₆H₃₆O₄Ti) were used as raw materials, both were dissolved in acetic acid and ethanol, respectively. Subsequently, these two solutions were mixed and then stirred for 3 h, after that, the solution was left to stand for 16 h, and then dried in an oven at 70 ℃ for 24 h. The mixtures were calcined at 800 ℃ for 2, 5 h and 12 h, respectively, in order to obtain BT powders with different particle sizes (the corresponding BT powders were respectively referred to as 2, 5 h and 12 h).

A SmartLab type X-ray diffractometer (XRD) was used to analyze the crystal structure of the obtained powders. The obtained powders were measured using an Invia Reflex model laser micro Raman spectrometer (Raman). In addition, the Helios G4 UC type scanning electron microscopy (SEM) was used to observe the surface morphology of the obtained powders. A Mapada UV-6300 UV-Vis spectrophotometer (UV-Vis) was used to characterize the transmittance of the obtained powders, thereby evaluating the degradation rate of powders. A home-made corona poling set-up at 6.56 kV for 1 h was used to polarize the BT powders. The unpoled/poled BT powders were mixed in RhB and stirred for 30 min to complete the dark adsorption. A Xe lamp (300 W) was used to simulate the sunlight irradiation, and an ultrasonic machine (30 kHz, 100 W) was used to assist in completing the piezo-photocatalytic test.

Particle sizes of the 2, 5 h and 12 h BT powders were 55, 110 nm, and 130 nm, respectively. XRD patterns showed that the cubic and tetragonal phases coexisted for all kinds of samples, and Raman spectroscopy confirmed the presence of tetragonal phases. The c/a values of tetragonal phase (c_T/a_T) in these BT powders, evaluated by Rietveld refinement analysis, were 1.007 85 (2 h-unpoled), 1.008 00 (5 h-unpoled), and 1.009 10 (12 h-unpoled), respectively, these were closely related to the particle size. The larger size corresponded to the higher c_T/a_T, illustrating that the ratio of the tetragonal phase would be increased in 12 h BT nanoparticles. After corona poling process, the lattice distortion further strengthened and the c_T/a_T values were 1.008 89 (2 h-poled), 1.008 44 (5 h-poled), and 1.009 32 (12 h-poled), respectively. The difference of c_T/a_T between unpoled and poled BT powders (Δ(c_T/a_T)) was significantly decreased with the increase of particle size, demonstrating the higher poling effectiveness in nanoparticles with smaller size.

Moreover, effect of poling on the piezo-photocatalytic performance of samples was evaluated via observing the degradation of RhB. It was found that the piezocatalysis existed a "bottleneck period", being related to the pressure pulse generated by cavitation bubbles and the free charges generated by particles through surface friction. The reaction rate constants k for unpoled BT powders were 0.003 28 min^-1 (2 h-unpoled), 0.004 39 min^-1 (5 h-unpoled), and 0.003 82 min^-1 (12 h-unpoled), respectively. After poling, the k values were 0.016 20 min^-1 (2 h-poled), 0.009 69 min^-1 (5 h-poled), and 0.006 20 min^-1 (12 h-poled), 4.94, 2.21 and 1.62 times higher than before poling, respectively, mainly attributed to the enhancement of the built-in electric field caused by corona poling, which could increase the driving force for the separation of photo-induced carriers and prolong the lifetime of carriers. Among them, the 2 h-poled powder exhibited the highest rate constant and a degradation efficiency of 96.81%, as that particle size of 2 h-poled samples was smaller than that of 5 h powders and 12 h powders, resulted in a high degree of lattice distortion and enhanced the tetragonality. Moreover, the small particle size of samples was considered to be favorable for the sufficient contact with RhB. Therefore, highest reaction rate constant and degradation efficiency were achieved for the 2 h-poled sample.

BT powders with different particle sizes were prepared and poled by corona poling technology. Effect of the poling process on the degree of lattice distortion, phase structure, and piezo-photocatalytic performance for degrading RhB of BT powders was systematically investigated. Results indicated that the corona poling could deepen the crystal lattice distortion and increase the tetragonality of BT nanoparticles. In addition, poling process could enhance the built-in electric field, reduce the recombination rate of photo-induced carriers, and significantly improve catalytic performance. The rate constants k of the poled powders were 4.94 times (2 h-poled), 2.21 times (5 h-poled), and 1.62 times (12 h-poled) higher than those of unpoled BT powders. Among all samples without and with poling process, the 2 h-poled BT powder achieved the highest degradation efficiency of 96.81% within 180 min, indicated the optimal piezo-photocatalytic performance. This work could provide a simple and rational method to design ferroelectric materials for achieving the high catalytic performance.

Micro-displacement actuators are widely used in micromachining, precision measurement, aerospace and biomedical. The performance of micro-displacement actuators can be evaluated in terms of displacement, hysteresis, linearity and temperature stability. For a long time, researchers have been hoping to obtain a ferroelectric material with high field-induced strain, low hysteresis and good temperature stability for a variety of different applications. Pb(Mg_1/3Nb_2/3)O₃ (PMN) is a typical perovskite type relaxor ferroelectric, and PMN-PbTiO₃ (PT) solid solution possesses a morphotropic phase boundary (MPB) region from PT mole fractions of 0.27 to 0.35. The field-induced strain and hysteresis of PMN-PT could meet the requirements of high-performance micro-displacement actuators. However, the temperature stability of field-induced strain still needs to be improved before practical application. In this work, effect of Sm³⁺ doping on the temperature stability of field-induced strain in PMN-0.32PT ceramics was investigated.

Experimental samples of nominal composition Pb_1-1.5xSm_x[(Mg_1/3Nb_2/3)_0.68Ti_0.32]O₃ (xSm-PMN-0.32PT, x=0, 0.020, 0.030, 0.035, 0.040, 0.045, 0.050) were prepared via a two-step columbite precursor method. The sintered disks were sintered at 750 ℃ for 15 min, poled in silicone oil bath under 1 kV/mm for 10 min at room temperature. The electrochemical properties of sintered samples were measured 24 h late after poling. The crystal structure of sintered samples was determined by PANalytical X'Pert3 X-ray diffractometer (XRD) at room temperature. The dielectric, piezoelectric, ferroelectric and resonant properties of sintered samples were measured by TH2816A LCR meter, ZJ-3 quasi-static d₃₃ meter, Radiant Precision Multiferroic Materials Analyzer and ZX80A impedance analyzer, respectively. The field-induced strain of sintered samples was measured using LVDT linear variable differential transformer probe controlled by MDS micro-displacement meter and Keithley 2 410 source meter. The temperature environment was provided by GZ-ESPEC MC710P environmental test chamber.

All samples exhibited typical perovskite phase, and the chemical compositions of them were similar as that of MPB, a mixture of rhombohedral R, monoclinic M_C, monoclinic M_B and tetragonal T phases. The MPB shifted towards to the PMN-rich direction with the increase of x value. The dielectric temperature spectra showed that the dispersion coefficient (γ) increased slightly with the increase of x, 1.374 ≤γ ≤1.67. As the frequency increased, values of ε_r-m shifted to a higher temperature and the frequency dispersion increased significantly with the increased x value. d₃₃ values decreased from 929 pC/N at x=0.020 to 546 pC/N at x = 0.050. These results indicate that all samples are relaxor ferroelectrics. The substitution of Sm³⁺ for Pb²⁺ broke the long-range ordering of lattice, resulted in the existence of numerous nanodomains below depolarization temperature (T_d), and the spontaneous polarization vector was more easily switchable with external electric field. The ferroelectric properties showed that both the remanent polarization (P_r) and coercive field (E_C) decreased monotonically with increased x value. In addition, the maximum value of field-induced strain (S_m) was very small at low temperature, but increasesd rapidly nears -10 ℃. The temperature stability of S_m for x=0.040 sample was much better than other samples when temperature ranged between -10 ℃ to +60 ℃. It is known that the ferroelectric domains in relaxor ferroelectric would be gradually into the small domains with increased temperature, which were easily oriented for external electric field. The field-induced strain was the largest near T_d, but decreased sharply when the temperature was higher than T_d. For xSm-PMN-0.32PT ceramics, value of T_d shifted to lower temperature due to substitution of Sm³⁺ for Pb²⁺, and T_d also showed a broad distribution due to the increased inhomogeneity of chemical composition in the microregion. As the temperature approached the T_d of certain microregions, the ferroelectric domains will be gradually cleaved into the small domains, and thus the field-induced strain will show good stability over a wide temperature range. As the field-induced strain decayed remarkedly below -10 ℃, it may be related to the diffuse phase transition of PMN.

PMN-PT was the preferred material for micro-displacement actuators due to its high field-induced strain and low hysteresis. The substitution of Sm³⁺ for Pb²⁺ broke the long-range ordering of lattice, increased the inhomogeneity of chemical composition in the microregion, resulted in a strong dielectric frequency dispersion, and the appearance of numerous nanodomains and polar nanoregions made samples exhibit large field-induced strains and high temperature stability. The field-induced strain was much smaller below -15 ℃ and increased rapidly near -10 ℃ under the influence of diffuse phase transition of PMN. A field-induced strain of 4.2×10^-4 and hysteresis of less than 13.1% in x=0.040 sample were achieved with excellent temperature stability that the strain fluctuation was smaller than 3.8% in the temperature between -10 ℃ and +60 ℃.

Microwave dielectric ceramics are widely used in microwave frequency (i.e., 300 MHz-300 GHz) circuits as communication electronic components such as resonators, filters, and antennas. The dielectric permittivity (ε_r), quality factor (Q×f) and the temperature coefficient of resonant frequency (τ_f) are the main performance index to measure the quality of microwave dielectric ceramics. However, there is a constraint relationship between the performance parameters. In general, the giant ε_r may cause a large dielectric loss and a low Q×f, and the volatility of ε_r originates an enormous τ_f value. Therefore, materials with superior intrinsic microwave dielectric properties are rare. The existing methods of controlling microwave dielectric properties mainly include ion substitution, composite control, laminated control and non-stoichiometric ratio control. These methods often require sacrificing one parameter to satisfy other parameters. High-entropy strategy is a novel regulation method used in inorganic materials. High-entropy ceramics refer to a ceramic system with a configuration entropy (S_c) of 1.5R. The high-entropy effect, sluggish diffusion effect, lattice distortion effect and synergy in components (i.e., cocktail effect) are four special effects of high-entropy ceramics. The high-entropy strategy shows a great potential in dielectric ceramics due to these unique effects. In this paper, high-entropy strategy was used to design rare-earth vanadate ceramics. The impact of high-entropy structure on the microwave dielectric properties was investigated.

High-entropy ceramics of ReVO₄ were synthesized by a solid-phase method with high-purity rare earth oxides (Re₂O₃ with a purity of > 99.9%) and V₂O₅. The ceramics with different chemical formulas (La_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)VO₄, (La_0.2Ce_0.2Nd_0.2Sm_0.2 Gd_0.2)VO₄, and (La_0.2Ce_0.2Nd_0.2Sm_0.2Y_0.2)VO₄, were denoted as Eu-5RVO, Gd-5RVO, and Y-5RVO, respectively. The raw materials were weighed according to a stoichiometric ratio and mixed in a ball mill at a rotational speed of 300 r/min for 6 h. The slurry after milling was dried in an oven at 120 ℃ for 24 h. After grinding, the powders were pre-fired in a Muffle furnace at 800 ℃ for 6 h. Then the prepared powders were further ground for 6 h, dried, and added with a polyvinyl alcohol solution (PVA, 5%) as a binder. The granulated powders were sieved by a 80/120 mesh screen and then pressed into cylindrical bulks with a thickness of 5.5-6.5 mm and diameter of 10 mm at 10 MPa. The samples were heated at 550 ℃ for 6 h to eliminate the binder and sintered at 1 400-1 500 ℃ for 6 h with an increment of 20 ℃ to obtain the dense ceramics.

The crystal structure of the ceramics was analyzed by X-ray diffraction (XRD, PANalytical Co., the Netherlands). The apparent density was determined by Archimedes' drainage method. The microstructure was evaluated by a model S-4800 scanning electron microscope (SEM, Hitachi High-Technologies Co., Japan). The dielectric properties at different microwave frequencies (i.e., 1-20 GHz) were measured by a model Keysight E5071C vector network analyzer (Keysight Technologies Co., USA) in TE₀₁₁ mode based on the Hakki-Coleman method. The temperature coefficient of resonant frequency (τ_f) was calculated via recording the frequency drifting at 25-85 ℃ (i.e., f₂₅ and f₈₅).

The high-entropy ceramics Eu-5RVO, Gd-5RVO, and Y-5RVO exhibit a tetragonal zircon structure with a space group I4₁/amd. The SEM images of the fractured cross-section of high-entropy ceramics ReVO₄ show uniform grains and clear grain boundaries, along with the spatial distribution of elements based on EDS analysis. The ceramics all exhibit a dense microstructure, as well as a promising microwave dielectric performance. The dielectric permittivity is analyzed via the Clausius-Mossotti formula, which is positively correlated to ion polarization. The quality factor is primarily determined by the packing fraction. The bond valence theory is used to reflect the degree of lattice distortion, having a strong relationship with the temperature coefficient of resonance frequency. The results show that the high-entropy strategy can effectively regulate the microwave dielectric properties.

A series of high-entropy ceramics of ReVO₄ were prepared by a solid-state reaction method. All the ceramics were a tetragonal zircon structure with a space group I4₁/amd. The phase structure, microscopic morphology, and vibration characteristics of high-entropy ceramics ReVO₄ were analyzed, and the microwave dielectric properties were analyzed by the Clausius-Mossotti formula, packing fraction, and bond valence theory. The results showed that ceramic Y-5RVO exhibited an optimal microwave dielectric properties (i.e., ε_r of 10.69, Q×f of 11 704 GHz, and τ_f of -21.70×10^-6/℃). It was indicated that the high-entropy ceramics of ReVO₄ could be used as competitive microwave dielectric materials.

High-entropy zirconate ceramics (HEZCs) have been studied extensively in recent years for the potential applications in thermal barrier coatings and high-level nuclear waste immobilization. While these HEZCs have the issue of poor toughness, which impedes their applications. The toughness of HEZCs can be improved by decreasing its grain size. While preparation of ultrafine-grained or nanocrystalline HEZCs is a challenge because the rapid grain growth inevitably occurred at high temperature densification process (typically above 1 500 ℃).

In this work, the challenge of preparing dense ultrafine-grained high-entropy ceramics through conventional pressureless sintering process was addressed via a simple two-step sintering method. Ultrafine-grained (Ce_0.2Nd_0.2Sm_0.2Gd_0.2Y_0.2)₂Zr₂O₇ high-entropy zirconate with 99.0% theoretical density and 162 nm grain size was fabricated. Compared to the conventional method, two-step sintering provided the high-entropy zirconate with finer grain size and better microstructural uniformity, and excellent comprehensive mechanical properties including high hardness of 12.5 GPa and high fracture toughness of 2.4 MPa·m^1/2. This work could help to understand the sintering kinetics of HEZCs, and also supplied a guidance to prepare the ultrafine-grained or nanocrystalline HEZCs by pressureless sintering method.

The starting high-entropy (Ce_0.2Nd_0.2Sm_0.2Gd_0.2Y_0.2)₂Zr₂O₇ ceramic powders were prepared via a polyacrylamide gel method. Analytical grade rare earth nitrate hexahydrates RE(NO₃)₃·6H₂O (RE=Ce, Nd, Sm, Gd, Y) with a purity of 99.9%, zirconium oxychloride (ZrOCl₂·8H₂O, 99.0% purity), acrylamide (AM, C₃H₅NO, 99.0% purity), N-N'-methylene-bis-acrylamide (MBAM, C₇H₁₀N₂O₂, 99.0% purity) and ammonium persulfate ((NH₄)₂S₂O₈, 99.0% purity) were used as starting materials, which were purchased from Macklin Chemical (Shanghai, China). First, RE(NO₃)₃·6H₂O and ZrOCl₂·8H₂O were mixed in distilled water according to the stoichiometric ratio of (Ce_0.2Nd_0.2Sm_0.2Gd_0.2Y_0.2)₂Zr₂O₇. After a vigorous stirring for 30 min, a clear solution was obtained and the concentration of Zr⁴⁺ was 0.02 mol/L. Then the AM, MBAM and (NH₄)₂S₂O₈ with a mole ratio of 24/2/1 were added in the above solution and dissolved completely by vigorous stirring. The mole ratio of AM/Zr⁴⁺ (abbreviated as A/Zr) in mixed solution was 80/1. After that, the mixed solution was heated in a water bath at 80 ℃ until it converted into wet gel. Subsequently, the wet gel was dried in an oven at 80 ℃ for 72 h to obtain the xerogel. Finally, (Ce_0.2Nd_0.2Sm_0.2Gd_0.2Y_0.2)₂Zr₂O₇ powders were prepared after a calcination of the xerogel at 1 000 ℃ for 2 h in air.

These (Ce_0.2Nd_0.2Sm_0.2Gd_0.2Y_0.2)₂Zr₂O₇ powders were uniaxially pressed into green pellets with a diameter of 8 mm and a thickness of (2±0.1) mm under 300 MPa, then it was sintered in air through conventional sintering and two-step sintering. For the conventional sintering, the green pellets were heated to 1 200-1 500 ℃ and held for 2 h directly; For the two-step sintering, the green pellets were firstly heated to a higher temperature of T₁ and held for time of t₁, then immediately cooled down to a lower temperature of T₂ and held for time of t₂. After that, the samples were naturally cooled down to room temperature in the furnace. It should be noted that in the heating process, the heating rate was 10 ℃/min before 1 000 ℃. After the furnace temperature reached to 1 000 ℃, the heating rate was down to 5 ℃/min.

The crystalline phases of samples were detected by X-ray diffractometer (XRD, Bruker D8 Davinci with Cu Kα radiation, Germany). The microstructure and element distribution of HEZC powders were characterized by transmission electron microscopy (TEM, JEOL-2100 F, Japan) equipped with energy dispersive X-ray spectroscopy (EDS). The morphology of sintered sample was observed by field emission scanning electron microscopy (SEM, Hitachi S-4800, Japan). The average grain size of sintered sample was determined by Nano Measurer software. At least 200 grains were randomly selected in measurement. The density of sintered sample was measured by Archimedes method. The relative density was calculated according to the theoretical density derived from the Rietveld refinement. The Vickers hardness and fracture toughness were evaluated by a micro-Vickers hardness tester (HV-1000A, Laizhou Huayin, China) under a load of 9.8 N with dwelling time of 15 s. The nanoindentation tests were carried on a Nano Indenter G200 (U9820A, Agilent Technologies), using a constant strain rate of 0.05/s. The Young's modulus was calculated according to the Oliver-Pharr method from the loading-unloading curve.

All diffraction peaks of the starting powders (Ce_0.2Nd_0.2Sm_0.2Gd_0.2Y_0.2)₂Zr₂O₇ prepared via polyacrylamide gel method matched well to that of the defective fluorite phase, indicating that the starting powder was single-phased HEZC with defective fluorite structure. The TEM image of HEZC powders presented that the average particle size was about 25 nm. The EDS mapping of HEZC revealed the homogeneous distribution of elements without segregation.

Samples sintered at 1 200 ℃ and 1 300 ℃ exhibited a porous structure; When sintered at 1 400 ℃, it exhibited a relatively dense structure, and some closed pores were still remained in the sample; When sintered at 1 500 ℃, a dense and pore-free microstructure can be observed. It should be noted that the dense HEZC bulks with relative density of 99.4% can be prepared when sintered at 1 500 ℃ for 2 h, implying a high sintering activity. The grain size increased slowly as the sintered temperatures increased from 1 200 ℃ to 1 400 ℃, whereas grain size increased rapidly beyond 1 400 ℃. Therefore, the rapid grain growth could be avoided if sintering temperatures below 1 400 ℃ in the conventional sintering. However, the densification process could not be accomplished under such sintering conditions. The pore pinning effect was of great significance for preparing the ultrafine-grained or nanocrystalline HEZCs via pressureless sintering method. And based on the XRD analysis, it can be concluded that the structure of (Ce_0.2Nd_0.2Sm_0.2Gd_0.2Y_0.2)₂Zr₂O₇ evolved from the defective fluorite toward to the pyrochlore at temperatures above 1 400 ℃.

With the understanding of the sintering behavior of HEZC, two-step sintering (TSS) could be provided to achieve the high densification and fine grain size by suppressing the rapid grain growth in the final sintering stage. the ultrafine-grained HEZC with an average grain size of 162 nm was prepared by TSS4, the sample showed that the compact microstructure with homogeneous ultrafine grains was obtained. In addition to retarding the rapid grain growth and obtaining the finer grain size, two-step sintering also could help to achieve a uniform microstructure with a narrower grain-size distribution compared to the conventional sintering. Two-step sintered sample (TSS4) has a lower σ (0.31, the ratio of the standard deviation Σ of grain-size distribution to the average grain size G_avg) than that of the conventionally sintered sample (CS4, σ=0.84). The Vickers hardness and fracture toughness of the HEZC sample prepared by CS4 were 11.0 GPa and 2.0 MPa·m^1/2, respectively. Meanwhile, these values of sample prepared by TSS4 were 12.5 GPa and 2.4 MPa·m^1/2, increased by 13.6% and 20.0%, respectively. The hardness and fracture toughness of samples prepared by two-step sintering were much better than those of conventional sintering, owing to the ultra-fined and homogeneous microstructure.

Pressureless two-step sintering process is successfully used to prepare the dense and ultrafine-grained high-entropy (Ce_0.2Nd_0.2Sm_0.2Gd_0.2Y_0.2)₂Zr₂O₇ ceramics with a relative density of 99.0% and an average grain size of 162 nm, as the phase transformation from defective fluorite to pyrochlore occurred over 1 400 ℃. In addition, this process could achieve the higher grain-size uniformity and finer grain size, as compared to that of the conventional sintering. Owing to the ultrafine grain size and high microstructural uniformity, the high-entropy (Ce_0.2Nd_0.2Sm_0.2Gd_0.2Y_0.2)₂Zr₂O₇ ceramics possessed the excellent mechanical properties with higher Vickers hardness of 12.5 GPa and fracture toughness of 2.4 MPa·m^1/2. It was believed that pressureless two-step sintering method could be used to prepare other high-entropy ceramics with fine grain size and high quality.

Compared with the preparation process of blade turbine using precision investment casting (PIC), additive manufacturing technology, especially binder jetting additive manufacturing (BJAM), possess many advantages such as the decreased cost, simplified process, easily preparation of complex porous materials, and diversified selectivity of binder types, therefore, it has been widely used to fabricate the core parts. Al₂O₃ ceramic owns better oxidation and corrosion resistance, highly serviced temperature and high hardness, is widely used as the core materials. However, the solid sintering method is hard to meet the mechanical requirement of Al₂O₃ ceramic cores. Therefore, a solid-liquid sintering additives, such as CuO and TiO₂, was added to assist the sintering of Al₂O₃ ceramic. In this work, influence of sintering additives contents ranged from 0 to 10% (mass fraction) and layer thickness ranged from 60-100 μm on the porosity, microstructure, and mechanical properties of Al₂O₃ ceramic cores were investigated. the results could construct a theoretical foundation for the rapid and low-cost printing of high-performance ceramic cores, and further expand the application scope of Al₂O₃ ceramics.

Irregular Al₂O₃ powders were used as raw material, irregular CuO and ball-like TiO₂ powders were used as liquid-solid phase sintering additives. Based on the mass fraction of 4∶1 corresponding to the eutectic temperature of CuO and TiO₂, 0-10% (mass fraction) of CuO-TiO₂ mixture was respectively weighted and added into Al₂O₃ powders in the polystyrene jar with ethanol as medium, and then these powders with Al₂O₃ balls (weight ratio of ball to powder was 5∶1) were ball milled at 180 r/min for 2 h. After milling, the mixed powder was dried at 90 ℃ for 12 h and then sieved through 800-mesh screen (about 18 μm) to gain the eventual printed powders. The complex parts were printed by a binder jetting 3D printer, the corresponding parameters were the layer thickness of 60-100 μm, inkjet amount of 60%-80% and inkjet number of 2 Ps. After printing, the powders were cured at 120 ℃ for 4 h, and then the samples were firstly sintered to 600 ℃ for 1 h with a heating rate of 5 ℃/min, then further sintered to 1 500 ℃ for 2 h with a heating rate of 5 ℃/min. Finally, the samples were cooled to ambient temperature with naturally cooling.

The thermogravimetric curve of binder was measured by simultaneous thermal analyzer. The relative density and porosity of samples were measured via Archimedes' methods. The microstructure and element distribution of powders, sintered parts and fracture surface were observed via a JSM-7 600F SEM and XRD-6 100 X-ray diffractometer. The three-dimensional surface roughness of sample was measured by laser scanning confocal microscope. The bending and compressive strengths were determined using a universal testing machine.

Al₂O₃ ceramic cores with an optimal porosity of 43.76% ± 0.72%, bending strength of (29.4 ± 1.57) MPa, and compressive strength of (61.82±1.73) MPa were obtained when Al₂O₃ powders with 5.0% CuO-TiO₂, 60 μm layer thickness and sintering at 1 500 ℃ for 2 h. The effects of CuO-TiO₂ amounts and layer thicknesses on the properties of Al₂O₃ ceramic cores were investigated. The relative density of green ceramics gradually increases with the contents of CuO-TiO₂ increased from 0% to 10% under a low layer thickness.

When sintered at 1 500 ℃ for 2 h, the porosity of ceramic with different content of CuO-TiO₂ mixture equaled to 64.94% ±1.2%, 42.36% ±1.44%, 43.76% ±0.72%, 42.21% ±0.35%, and 41.85% ±1.82%, respectively, when the layer thickness was 60 μm, which can be attributed to that the sintering process could decline the surface energy of particles and rise the particles contact, and the generation of liquid phases such as CuAlO₂ and CuAl₂O₄ by the chemical reaction between CuO/TiO₂ and Al₂O₃ can fill the pores. For pure Al₂O₃ ceramic, the dimensional shrinkage of X-Z axis was less than 4% when layer thickness ranged between 60 μm and 100 μm, but that of Al₂O₃ ceramics with 2.5%-10% CuO-TiO₂ addition could reach about 17%. The surface roughness of Al₂O₃ ceramics firstly decreased and then increased with the increase of CuO-TiO₂ amounts, and roughness of X-Y surface was better than that of X-Z surface due to the press and smoothing from powder roller and the filling effect of liquid phases. The bending and compressive strength of the sintered Al₂O₃ ceramic firstly increased and then declined with the increase of CuO-TiO₂ amounts, the maximum values were (29.4±1.57) MPa and (61.82±1.73) MPa, repetitively when CuO-TiO₂ content equaled to 5.0%.

The BJAM Al₂O₃ ceramic core with a suitable porosity and mechanical properties was obtained by regulating the amounts of CuO-TiO₂ and layer thicknesses. Results showed that ceramics with lower layer thickness could obtain relatively better performance. The relative density of sintered samples increased by 11.72%, 65.66%, 58.82%, 51.24% and 54.81% with the increase of CuO-TiO₂ amounts compared with the green bodies. And the sintered porosity gradually decreased from 64.94 ± 1.20% to 41.85±1.82%, which was attributed to the generation of CuAlO₂ and CuAl₂O₄ phases. Additionally, the roughness values of X-Y surface were better than that of X-Z surface of all samples, but the roughness firstly decreased and then increased with the increase of CuO-TiO₂ amounts, the optimal values of (6.00 ± 0.63) μm and (7.00±0.78) μm were obtained at 5.0% Al₂O₃ ceramic. The bending and compressive strength values similarly first increased and then declined with the CuO-TiO₂ amounts, and the maximum values of (29.40±1.57) MPa and (61.82±1.73) MPa were obtained by the 5.0% Al₂O₃ ceramic, which were mainly affected by the distribution of pores in the microstructure and the generation amounts of the second phases.

Silicon nitride (Si₃N₄) ceramics are considered to be one of the most promising ceramic substrate materials due to their good insulation, low coefficient of thermal expansion and high mechanical properties. However, the thermal conductivity of Si₃N₄ ceramics prepared by conventional methods is low (< 50 W·m^-1·K^-1), which greatly affects the reliability of chips and even causes the failure of electronic devices. New methods such as gas pressure sintering (GPS), and sintered reaction-bonded silicon nitride (SRBSN) have been employed to fabricate Si₃N₄ ceramics with high thermal conductivity and high bending strength. Nevertheless, the sintering temperature of both GPS and SRBSN is high and the holding time is too long, which lead to the high cost and limit its applications. Spark plasma sintering (SPS) can produce ceramics with high density in a relatively short time, while the short soaking time is not suitable to improve the thermal conductivity. High-temperature heat treatment can promote the growth of Si₃N₄ grains, and thus improves the thermal conductivity of Si₃N₄ ceramics. Therefore, it is expected that Si₃N₄ ceramics with high bending strength and high thermal conductivity can be prepared at a lower temperature and a shorter time by the combination of SPS and high-temperature heat treatment processes.

Si₃N₄ ceramics were prepared by SPS at 1 400, 1 500, 1 600 ℃ and 1 700 ℃ for 5 min using α-Si₃N₄, Y₂O₃ and MgSiN₂ as raw materials. The obtained samples with the best overall properties were then selected and put into a gas pressure furnace to be heated at different temperatures (1 600, 1 700 ℃, and 1 800 ℃) for 3 h. Density and open porosity of the sintered samples were tested by Archimedes method. The phase composition of samples was analyzed by X-ray diffractometer (XRD). Samples were polished and then etched in molten NaOH at 500 ℃ for 20 s to observe the microstructures using a field emission scanning electron microscope (FESEM). The diameters of at least 200 grains in the SEM images were measured by Nano measure software and statistically analyzed to obtain the average grain size. The bending strength was examined via three-point bending test using an electronic universal testing machine. The thermal diffusion coefficient of Si₃N₄ ceramics was determined by a laser thermal conductivity analyzer.

As the sintering temperature increased from 1400 ℃ to 1 700 ℃, the relative density of samples increased rapidly from 80.43% to 97.81% at first and then slowly to 99.09%. With the increase of sintering temperature, the viscosity of liquid phase produced by the reaction of sintering additives gradually reduced, and the rate of material transport improved during sintering process, which led to a higher densification. When sintering temperature reached to 1 600 ℃, the content of β-Si₃N₄ in sample reached 100%, indicating that the phase transition from α-Si₃N₄ to β-S_i3N₄ has been completed. The average grain size of samples gradually grew from 0.31 μm to 1.35 μm with the increase of sintering temperature from 1 400 ℃ to 1 700 ℃, and the thermal conductivity of the samples rapidly increased from 9.88 W·m^-1·K^-1 to 61.80 W·m^-1·K^-1, being attributed to the phase transition of α→β-S_i3N₄. Besides, the process of phase transition could purify the crystal lattice, while the increase of densification and grain growth reduced the scattering of phonons by defects such as pores, grain boundaries, and so on. Therefore, the thermal conductivity of the Si₃N₄ ceramics was improved.

After the heat treatment of sample sintered by SPS at 1 600 ℃, the relative density reduced in a certain degree compared with that of the untreated samples. The average grain size of β-Si₃N₄ in samples before heat treatment was 1.16 μm, and grows to 1.33 μm after heat treatment at 1 600 ℃. Rising the heat treatment temperature to 1 700 ℃ and 1 800 ℃, the average grain size of the samples grew to 1.35 μm and 1.51 μm, respectively. when heat treatment temperatures increased from 1 600 ℃ to 1 800 ℃, the thermal conductivity of the samples rosed from 50.87 W·m^-1·K^-1 to 57.36 W·m^-1·K^-1, 0.45% and 13.27% higher than that of the un-treated samples. Bending strength of samples decreased with the increase of heat treatment temperature. When heat treated at 1 800 ℃, the bending strength of sample reduced to 743 MPa. which still meet the requirement of bending strength >700 MPa for Si₃N₄ ceramic substrates.

The relative density, thermal conductivity, and β-Si₃N₄ content of the Si₃N₄ ceramics increases with the increase of SPS temperature. When SPS temperature was 1 600 ℃, the bulk sample was composed of β-Si₃N₄ with a high relative density of 98.78% and a thermal conductivity of 50.64 W·m^-1·K^-1. After heat treated at the same temperature, the thermal conductivity of the samples was improved, while the bending strength displayed the opposite trend. The thermal conductivity of samples heat-treated at 1 800 ℃ achieved 13.27% higher than that of the untreated sample, while the bending strength still exceed 700 MPa. Therefore, the combination of SPS and high-temperature heat treatment is a promising process to produce Si₃N₄ ceramics with high thermal conductivity and bending strength.

Nanostructured amorphous materials represent a novel class of materials characterized by nanostructured amorphous particles and amorphous interfaces between particles. Due to the nanometer scales, amorphous disorder, controllable atomic and electronic structure, this kind of material exhibits numerous unique properties distinct from conventional materials. Recently, scholars discovered that composites composed of nanostructured amorphous materials as the matrix with precipitated nanocrystals exhibited excellent mechanical properties. However, densification of both nanostructured amorphous materials and nanostructured amorphous composites was primarily achieved through ultra-high-pressure forming. During ultra-high-pressure forming, the plastic deformation of nanostructured amorphous powder under high pressure enabled amorphous particles to contact each other, extrude pores, and diffused through interfaces to form new interfaces. However, these studies required high-pressure forming equipment, and the associated costs constrained further applications.

In contrast to the ultra-high-pressure molding mentioned earlier, Rosenflanz et al. employed viscous sintering to consolidate amorphous powder within the supercooled liquid region. This method utilized amorphous powder with low viscosity above the glass transition temperature (T_g) and its viscous flowed under external pressure to fill the pores between particles, thereby achieving densification. This approach only required 34 MPa pressure, effectively circumventing the unrealistically high pressures previously required for amorphous material preparation. However, crystallization during viscous sintering may impede the viscous flow of amorphous powder, necessitating a sufficiently wide kinetic window between the glass transition temperature (T_g) and crystallization temperature (Tx) to achieve densification. To address this issue, Rosenflanz et al. again employed an alumina-rare earth oxide system (Al₂O₃-RE₂O₃) to modulate the polyhedral structure of Al-O, regulated the glass formation ability of the system, substantially widen the kinetic window of the oxide system, and fabricated a range of dense amorphous ceramics. Nonetheless, viscous sintering process has never been reported in the realm of nanostructured amorphous materials.

Amorphous powder of 57.3% Al₂O₃-19.7 %La₂O₃-23 %ZrO₂(ALZ) was synthesized via sol-gel method and subsequently calcined at 800 ℃ for 1 h. The ALZ amorphous powder was then densified using the SPS system under vacuum conditions. Uniaxial compression was applied using graphite punches at the top and bottom, with a loading pressure of 50 MPa. The heating rate from room temperature to sintered temperatures (Ts) was set at 100 ℃/min. The sintered temperatures ranged between 875 ℃ and 1 000 ℃, with a holding time of 5 min. The obtained sample was cylindrical with a diameter of 15 mm and a thickness of 1 mm.

The powders calcined at 800 ℃ for 1 h was amorphous with an average particle size of 11.6 nm, The glass transition temperature (T_g) and crystallization onset temperature (Tx) of the obtained powders are 844 ℃ and 914 ℃, respectively. Numerous pores can be observed in samples sintered at 875 ℃, indicating an incomplete densification at this temperature, whereas no pores were detected in samples sintered at 900 ℃. The relative densities of samples sintered at 875, 900, 927 ℃ and 1 000 ℃ were 87.6%, 98.3%, 96.6% and 91.5%, respectively. The main diffraction peaks in XRD pattern for the sample sintered at 875 ℃ corresponds to La_0.28Zr_0.72O_1.8 with a grain size of 14.2 nm, when sintered at 900 ℃, diffraction peaks of samples corresponded to La₂ (Zr₂O₇) with a grain size of 9.9 nm, which suggested that high degrees of crystallization of La_0.28Zr_0.72O_1.86 sintered at 875 ℃ impeded the viscous flow of the amorphous powder, resulting in a formation of numerous pores. However, the relatively low crystallinity of La₂ (Zr₂O₇) phase sintered at 900 ℃ facilitated the viscous flow of the amorphous powder, and densification was achieved only under a pressure of 50 MPa. Thus, the phase transition formed La_0.28Zr_0.72O_1.86, which owned a higher crystallinity, to La₂(Zr₂O₇) with lower crystallinity was advantageous for the densification of the amorphous powder via viscous flow. A further increase of crystallinity of samples sintered at 927 ℃ and 1 000 ℃ was not conducive to the viscous flow of the amorphous powder, resulted in the formation of numerous pores. STEM-BF images of sample sintered at 900 ℃ exhibited noticeable light-dark contrast. Upon further magnification, distinct lattice fringes can be observed at the interfaces of dark regions, while no lattice fringes were observed in the bright regions. The precipitated crystals were composed of La(AlO₃), La_0.28Zr_0.72O_1.86, and La₂(Zr₂O₇) phases, being consistent with that of XRD results. Both EDS line scanning and mapping revealed that the bright areas were composed of Al-enriched materials, whereas the interfaces between dark areas were Zr and La-enriched materials. Furthermore, the HRTEM image revealed that the nanocrystalline phase precipitates from the amorphous phase matrix.

Nanostructured amorphous ALZ materials with a relative density of 98.3% can be prepared through spark plasma sintering at 900 ℃. Nanocrystalline phases precipitated from the amorphous matrix were observed via STEM-BF and HRTEM, revealing the presence of La(AlO₃), La_0.28Zr_0.72O_1.86, and La₂(Zr₂O₇) phases. The average hardness and Young's modulus of samples were 12.0 GPa and 159.9 GPa, respectively. Meanwhile, samples sintered at 875, 927 ℃ and 1 000 ℃ existed notable pores, being originated from the changes in crystallinity of ALZ under different temperatures. The significant crystallization of La_0.28Zr_0.72O_1.86 sintered at 875 ℃ impeded the viscous flow of amorphous powder. Concurrently, the phase transition from La_0.28Zr_0.72O_1.86 to La₂(Zr₂O₇) with lower crystallinity occurred at 875-900 ℃, which facilitated the viscous flow of amorphous powder. However, a further increase in crystallinity at 927 ℃ and 1 000 ℃ was not conducive to the viscous flow of amorphous powder.

During the sintering process, porous ceramics undergo significant shrinkage with volume changes even half. Such volumetric alterations may lead to materials cracking or deformation, severely affecting the yield of the prepared products. Researchers have proposed several methods such as adjusting the sintering temperature, utilizing reactions and expansion phases during the sintering process to address these issues. The most commonly method was to utilize the expansion characteristics of the materials themselves to solve the shrinkage problem. However, this method requires complex raw materials and lacks universality. Recently, our group proposed an in-situ hollow sphere method with the addition of spherical metal powders, successfully preparing porous ceramics without sintering shrinkage. The main theoretical support for this process is the Kirkendall effect that occurs during two-phase diffusion. When metal spherical particles are oxidized, the difference in diffusion rates forms a hollow structure, effectively offsetting the shrinkage caused by sintering. In this work, hollow zinc oxide particles were successfully prepared using the direct oxidation of metallic zinc powders and applied in the preparation of zinc oxide porous ceramics. During the oxidation process of metallic zinc powder, its addition amount, particle size, and their effects on the properties and microstructure of porous ceramics were investigated. The relationship between the pore size of porous ceramics and the particle size of metallic zinc particles was analyzed. This study could broad the application scenarios of this process and hollow zinc oxide particles.

Firstly, zinc oxide powder and metallic zinc powder with different particle size were thoroughly ground and mixed in a grinding bowl according to a certain ratio (the volume percentage of metallic zinc powder to zinc oxide powder). Subsequently, a 5% PVA aqueous solution (50 g/L) was added to the total mass of powder as a binder, then stired further until the raw powders and binder were uniformly mixed. Secondly, the mixture was poured into molds for dry pressing at a pressure of 5 MPa for 2 min. Finally, the samples were demolded and subjected to oxidation and sintering with a rate of 1 ℃·min^-1 and a holding time of 2 h. The linear shrinkage were calculated by measuring the changes in diameter of samples before and after sintering. The volume density and apparent porosity of samples were determined using the Archimedes' displacement method, and relative density calculated as the measured volume density divided by the theoretical density of zinc oxide (5.6 g·cm^-3). Weight changes, as well as endothermic and exothermic effects during heating were measured using a thermal analyzer. The morphology of the samples was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray diffraction (XRD) was employed to analyze the phase composition of the samples. The three-point bending strength of the samples was tested using an electronic universal testing machine, with a span of 30 mm, a crosshead speed of 0.5 mm·min^-1, and sample dimensions of 3 mm × 4 mm, ensuring the length exceeded the span.

From TG-DTA analysis, it can be observed that the oxidation process of metallic zinc powder can be divided into three stages: a low-speed oxidation stage before 400 ℃, a high-speed oxidation stage from 400 ℃ to 900 ℃, and a complete oxidation stage after 900 ℃. However, when varied the particle size of metallic zinc powders, XRD results revealed that some unoxidized metallic zinc remained in the samples containing 1μm metallic zinc powder, being due to the relationship between the particle size of metallic zinc and zinc oxide; Regarding the shrinkage rate, when amount of added metallic zinc powder increased and the particle size enlarged, the reduction in the linear shrinkage of the porous ceramic during sintering process became notably apparent, suggesting that the shrinkage rate of samples were mainly dependent on the size of the metallic particles. When size of particle was 10 μm and addition amount equaled to 50%, the linear shrinkage rate of the sample was only -0.36%, with a porosity of 25%, achieving the near net-size preparation of the sample. As for the microscopic structure, it can be observed that distinct pore structures existed inside the material, and the pores size of was closely related to the particle size of the added metallic zinc powder. In TEM analysis, it can be clearly seen that particle size of metallic zinc affected the temperature at which metal zinc begins to hollow, and the radius of Kirkendall pores formed inside the material after the oxidation of metallic zinc was 0.54 to 0.99 times the initial radius of metallic zinc.

The in-situ hollow sphere method allowed for the preparation of near-net-size zinc oxide porous ceramics, offering a simple and efficient process without the addition for any organic pore-forming agents. When temperatures exceeded it's melting point, metallic zinc powder undergone vigorous oxidation, typically concluding around 900 ℃. The linear shrinkage of the sintered zinc oxide porous ceramics was closely related to the amount and particle size of added metallic zinc; higher amounts and larger particle sizes of added metallic zinc powders resulted in a lower linear shrinkage rate, potentially leading to negative shrinkage (expansion). After computational analysis, it was determined that the pores formed in the porous ceramics were smaller than the particle size of metallic zinc powder, consistent with the Kirkendall effect. The pore diameter was approximately 0.54 to 0.99 times the initial spherical metallic zinc particle size.

Ceramic foams is a kind of inorganic non-metallic material with a three-dimensional porous network structure after calcination at high temperatures. The mechanical properties are one of the important factor to evaluate the application of ceramic foams, which affected by the porosity, pore size, pore distribution and skeleton structure, etc. High porosity would reduce the bearing capacity of ceramic foams and limit its application fields. Therefore, this research is mainly focused on how to balance porosity and strength of ceramic foams, as compressive strength could be improved effectively by reducing the pore size, narrowing the pore size distribution or forming a closed pore structure.

When coal gangue is used as raw materials to prepare ceramic foams, it still exists some problems, such as the low utilization, high energy consumption and unclear foaming principle. The key to producing a ceramic foam is to form a melt matrix with appropriate viscosity and to generate a certain amount of foaming gas in the matrix. Due to the complex composition of coal gangue, it is difficult to control the viscosity of the melt matrix at high temperatures, resulting in the pore structure distribution uneven and reducing the mechanical properties of ceramic foams.

In this work, coal gangue was used as the main raw materials, and diatomite powder, talc powder, a small amount of silicon carbide (SiC) foaming agent were added to prepare the ceramic foams with high compressive strength. The effects of raw materials composition, SiC content (mass fraction) and particle size of feed mixture on the pore structure, apparent density and compressive strength and other properties of ceramic foams were investigated.

The coal gangue was crushed and sieved to less than 74 μm. Based on the SiO₂-Al₂O₃-MgO ternary system, the coal gangue, diatomite powder and talc powder were mixed at different mass ratios, and then SiC powders were added as the foaming agent, the mixture and agate balls in a mass ratio of 1∶1 were milled with a speed of 300 r/min. After ball milling, the powders were placed in a corundum boat and heated at 1 190 ℃ for 25 min, then cooled to room temperature in furnace. The chemical compositions of the raw materials were determined by X-ray fluorescence spectrometer (XRF). The crystalline structure of raw materials and ceramic foams was determined by X-ray diffraction (XRD). The thermogravimetric analyzer (TG-DSC) was used to analyze the mass change and thermal analysis of raw materials. The particle size of mixed materials was analyzed via a laser particle size analyzer. The compressive strength of ceramic foams was measured via a pressure test machine. The pore size distribution of the sample was measured by Nano Measurer 1.2, and the water absorption and porosity of ceramic foams were measured by Archimedean method.

When amount of coal gangue reached 60% and 70%, the main phases of ceramic foams were cordierite and quartz, and when content of coal gangue exceeded 75%, sekaninaite phase appeared. When content of coal gangue reached 85%, the average pore size of the ceramic foams decreased from 2.01 mm to 0.80 mm, and the distribution of pore size became uneven as the increase of coal gangue could reduce the SiO₂/Al₂O₃ mass ratio, which would lead to an increase of melting temperature of the matrix, and thus the generated silicate molten material at high temperature decreased and the viscosity increased. As a result, the pore structure with uneven distribution would decrease the bearing capacity. Compressive strength reached the maximum value of 2.2 MPa at the coal gangue content of 75%. When content of SiC foaming agent increased, more small pores were generated in the matrix, these pores could connect to form large pores during sintered process, resulting in a gradual increase in pore size. When content of SiC increased from 0.4% to 1.4%, the compressive strength of ceramic foams decreased from 2.9 MPa to 0.8 MPa, the reduction of particle size of the feed materials could be benefit to the agglomeration and sintering behavior of SiO₂ at low temperature. When the average particle size decreased from 21.1 μm to 5.4 μm, the apparent density and porosity of ceramic foams changed slightly, while the compressive strength gradually increased from 2.6 MPa to 6.1 MPa, being due that the small particle size of SiO₂ in feed mixtures could improve the densification of the pore wall and finally improve the bearing capacity of the ceramic foams.

The main conclusions of this work can be summarized as follows: With the increase of coal gangue content in the feed materials, SiO₂/Al₂O₃ mass ratio decreased from 2.66 to 2.05, the pore structure of the sample deteriorated, and the compressive strength first increased and then decreased. When the content of SiC increased, more gases were released during calcination process, and thus increasing the porosity. When it exceeded 0.8%, the excess SiC should increase the open porosity and thus reduces the compressive strength of ceramic foams. Reducing the particle size of the feed mixture could improve the reaction activity of powders during the high temperature foaming process, the small pores and uniform pore structure could obviously improve the compressive strength of ceramic foams. When mass ratio of coal gangue: diatomaceous earth powder: talc was 7∶2∶1, content of SiC was 0.6%, the average particle size of the mixed materials was 5.4 μm, the apparent density, porosity and compressive strength of the prepared samples were 0.45 g/cm³, 80.6%, and 6.1 MPa, respectively, expected to be applied in the field of heat preservation wall.

Porous ceramics are-widely used in a variety of environmental fields such as solid-liquid separation and hot gas filtration due to their high mechanical strength, excellent chemical resistance, and high thermal stability. Conventional porous ceramics are usually made of particle-packing of inorganic powders, which are usually characterized by a low porosity, a high self-weight, a low permeability and a high filtration resistance. It is thus necessary to develop novel porous ceramics with a high porosity, a high mechanical strength and a low packing density. Mullite whisker-based porous ceramics have attracted recent attention in the treatment of various wastewaters. Compared with porous ceramics prepared by conventional particle-packing, mullite whisker-based porous ceramics are constructed by whisker interlocking, resulting in fibrous structures with a high porosity, a high pore connectivity and a low packing density. However, these mullite porous ceramics are seldom used in industry due to the high cost of the high-purity starting materials. Fly ash is a major by-product of coal combustion. The elemental analysis shows that fly ash is mainly composed of unburned carbon, metal oxides (i.e., Si, Al, and Fe) and other inorganic materials. Fly ash can be used to prepare mullite porous ceramics due to the presence the high content of aluminum silicates. However, the effect of holding time on the growth of mullite whiskers and the permeance-selectivity of the fibrous structures is still unclear. In order to elucidate the growth mechanism of mullite whiskers based on gas-solid reaction, the effects of sintering temperature and holding time on the microstructure, phase composition and filtration properties of the porous ceramics were investigated.

Fly ash powder provided by Huaneng Thermal Power Plant in Zibo, China, and Al(OH)₃ purchased from Sinopharm Chemical Reagent Co. Ltd., China, were used as major materials. AlF₃·3H₂O purchased from Sinopharm Chemical Reagent Co. Ltd., China, was employed as a catalyst for mullite whisker growth. In order to prepare porous ceramics with a high fracture strength, sodium carboxymethyl cellulose (CMC) from Sinopharm Chemical Reagent Co., China was chosen as a binder. Fly ash, Al(OH)₃, AlF₃·3H₂O and CMC were mixed and ground in a ball mill, and then the slurry was dried with a rotary evaporator and sieved through a stainless steel sieve to obtain a homogeneous mixture. After dry pressing, the green body was sintered in a muffle furnace. The effect of holding time on the formation and growth of mullite whiskers was elucidated at different holding time (i.e., 2, 4 h and 6 h). The microstructure of the porous ceramics was determined by scanning electron microscopy (SEM). The phase composition of the porous ceramics was analyzed by X-ray diffraction (XRD). The selectivity of the porous ceramics was investigated by oil-in-water emulsion separation experiments.

To clarify the growing mechanism of mullite whiskers based on solid-vapor reaction, the influences of sintering temperature and holding time on the microstructure, phase composition and filtration performance of the porous ceramics are investigated. The gas AlF₃·3H₂O acts as a catalyst for the formation and growth of mullite whiskers. The thermogravimetric analysis curves indicate that the mullite whisker growth follows a gas-solid reaction mechanism, where AlF₃·3H₂O acts as a catalyst for the formation and growth of mullite whisker. The mass loss of the samples sintered at different temperatures is similar while the shrinkage increases from 2.0% to 4.2% as the holding time increases. A liquid phase formed during the sintering process enhances the particles rearrangement, the mass diffusion and migration, and the densification of the specimens. The XRD patterns indicate that a high sintering temperature and a long holding time were favorable for the formation of the mullite phase. The whisker aspect ratio is related to the sintering temperature and the concentration of the atmosphere provided by the additives. The analysis of the length-to-diameter ratio of the mullite whiskers reveals that the increase in holding time favors the growth of mullite whiskers. Based on the analysis of the pore structure of the porous ceramics, the sintering temperature is a key factor affecting the porosity, water absorption and pore size of the porous ceramics, while the holding time has a slight effect. According to the results of flux tests on the specimens sintered at 1 200 ℃ for 6 h, the permeation flux decreases from 210.61 L/(m²·h) to 192.35 L/(m²·h). When the trans-membrane pressure increases from 0.1 bar to 0.4 bar, the oil rejection of the porous ceramic reaches 90%.

The sintering temperature affected shrinkage of the porous ceramics. For the specimen sintered at 1 100 ℃, only a slight shrinkage of approximately 0.2% occurred during sintering. For the specimen sintered at 1 200 ℃, the shrinkage increased from 2.0% to 4.2% due to the formation of silica-rich liquid phase. Also, the high sintering temperature favored the formation of mullite phase. The growth of mullite whiskers increased both in porosity and pore size of the whisker-constructed porous ceramics. With the growth of mullite whiskers, the porosity of the porous ceramics increased from 56.4% to 57.6% for the specimen sintered at 1 200 ℃, and the average pore size increased from 0.75 to 1.24 μm. The emulsion test indicated that the permeate flux decreased from 210.61 to 192.35 L/(m²·h) for the specimen sintered at 1 200 ℃. Oil rejection of the porous ceramics decreased from appropriately 90% to 87% as TPM increased from 0.1 to 0.4 bar.

The aimed environmental barrier coatings (EBCs) need to meet the characteristics of low oxygen permeability, match thermal expansion coefficient, and keep phase stability under service conditions, which usually need to be designed as a multi-layer structures to avoid premature failure of gas turbine engines during operation. Yb₂Si₂O₇ (YbDS) is considered the most promising candidate due to its excellent resistance to water vapor corrosion and damage tolerance, a similar coefficient of thermal expansion (4.1×10^-6/K) to that of the matrix material (SiC, 4.7×10^-6/K), and does not generate high thermal cycling stress in EBCs systems. However, when YbDS was exposed to an environment containing with high-temperature water vapor, corrosion transformation may occur, which would reduce the durability of EBCs. To analyze the corrosion process, stress characteristics, and crack propagation laws of YbDS under the coupling effect of water-oxygen corrosion and thermal cycling, the compositional and structural changes of YbDS under high-temperature water-oxygen action were determined through experiments, also the influence of water-oxygen corrosion on the stress evolution of the coating was explored using finite element software Abaqus. The extended Finite Element Method (XFEM) was used to simulate the propagation path of cracks in YbDS, and the accelerating effect of water-oxygen corrosion on crack propagation was discussed finally.

The Si powder was used for bond coat deposition, and Yb₂Si₂O₇ powder was used for top coat deposition. Powders used in this experiment were designed with a particle size distribution suitable for thermal spray applications. Both the bond coat and top coat were deposited onto the φ25.4 mm×3 mm SiC substrate using the Praxair 3710 air plasma spraying system. Thermal cycling tests of the EBC system was conducted in an aluminum oxide tube furnace at 1 325 ℃, The ratio of water vapor to oxygen was controlled to 9∶1 by a gas-liquid mixing device, samples were took out and cooled to room temperature when insulation for 5 h. Repeated this steps until the coating failed. The cross-sectional and surface morphology changes of the coatings were observed via a scanning electron microscope for both secondary electron mode (SE2) and backscattered electron mode (BSE). Energy dispersive spectroscopy was used for elemental trace analysis, and X-ray diffraction was used to analyze the phase composition of the coating surface at different stages of water oxygen corrosion. The fracture toughness of the coating was calculated via the indentation methods. The changes in phase, structure, and mechanical properties collected from the experiment were compiled into the USDFLD and UEXPAN subroutines in Abaqus, and then substituted them into the established two-dimensional finite element model for calculation. By prefabricating XFEM cracks in the model, the influence of the cracks on the coating was analyzed.

The XRD results showed that the Yb₂SiO₅ increased from 21.6% in the initial spray state to 58.1% and 95.1%, and the phase content of Yb₂Si₂O₇ decreased from 73.8% in the spray state to 33.6% and 1.5% when the sample surface suffered 20 h and 40 h of water oxygen corrosion. After 60 h corrosion, phase composition of Yb₂Si₂O₇ was not detected, and the content of Yb₂SiO₅ increased slightly to 91.5%, and some Yb₂O₃ was observed results show that the Yb₂Si₂O₇ would preferentially decompose into Yb₂SiO₅ under high temperature water vapor environment, and the Yb₂SiO₅ has better high temperature stability than that of Yb₂Si₂O₇.

The finite element results showed that the corrosion zone of YbDS changed from compressive stress to tensile stress under the joint action of corrosion and thermal cycle. After cooling, the stress increased to 631.4 MPa, resulting in an early corrosion EBC cracking and rapid water vapor penetration. The volume shrinkage caused by the volatilization of the components in corrosion area made a great influence on the stress of the corrosion layer, and the residual tensile stress caused by it was about 490 MPa, while the effect of thermal mismatch in cycle stage was relatively small, only about 130 MPa. It should be noted that the new pores in high temperature corrosion stage would change the stress field in the coating, and the more the corrosion depth, the more the new pores effect on the stress field. Through the simulation of crack growth process, it was found that the volume shrinkage in high temperature corrosion stage exhibited a more important impact on the surface crack growth when the corrosion aera was shallow. The thermal mismatch caused by water oxygen corrosion gradually became the main driving force for the crack propagation when corrosion depth reached 147 μm. The above two alternate actions led to the surface cracking.

The composition of environmental barrier coating would decompose to Yb₂SiO₅ corroded under high temperature water-oxygen environment. The volatilization of Si—O caused the increase of shrinkage stress in surface coating, and thermal mismatch between Yb₂SiO₅ and the matrix material also increased, the residual stress accumulated as the cyclic thermal load continued. The alternating action of these two factors caused the surface layer to crack. At the same time, the pores generated during the high-temperature corrosion stage could also resulted in the internal stress field of the coating, and its impact increases with the increase of corrosion degree.

The modern gas turbines are constantly pursuing the development trend of high heat insulation and long service life, making the operating temperature of turbine blade continuously rising. Thermal barrier coatings (TBCs) have high temperature resistance and high heat insulation to protect metal alloy substrates. The ceramic topcoat layer is typically composed of yttria-stabilized zirconia (YSZ) material. However, the spallation failure of YSZ is easy to occur during high-temperature service. Gd₂Zr₂O₇(GZO) is one of the candidate materials for the new generation of thermal barrier coatings, due to its low thermal conductivity and phase stability at high temperatures. The porous structure of ceramic coatings inevitably undergoes sintering and densification during high-temperature service, the hardness and elastic modulus of coating were decreased.

Adjusting the spraying distance influence both the temperature and velocity of particles, thereby influencing the microstructure of the porous coating. In this work, three kinds of Gd₂Zr₂O₇ (GZO) coatings deposited at different spraying distances were prepared by atmospheric plasma spraying (APS). The as-deposited coatings were thermal exposure to 1 300 ℃ for different hours. The phase structure, microstructure and mechanical properties of the obtained coatings were characterized. The relationships of pore structure and mechanical properties of GZO coatings were revealed during high temperature thermal exposure.

Atmospheric plasma spraying equipment was used to deposit GZO coatings with a thickness of about 300 μm at different spraying distances. The GZO coatings were placed into a Muffle furnace to carry the thermal exposure tests at 1 300 ℃ for 10, 20, 50, 100 h and 200 h, respectively. The microstructure of the as-prepared GZO coating was observed by SEM. The porosity of the coating was statistically obtained using Image-J software. The phase structure was analyzed by D8-Advance X-ray diffractometer (XRD) using Cu Kα radiation with an operating voltage and current of 40 kV and 40 mA, respectively. Vickers hardness tester (American Micromet-5104 microscopy) was used to test the hardness on the polished section of the coating, with a load of 300 g for 30 s. The elastic modulus of the coating was measured by Knoop indentation method, and parameters were similar with that of Vickers hardness.

Results show that the as-deposited GZO coatings exhibited a typical layered structure. After thermal exposure, the inter-splat cracks, intra-splat cracks and porosity of GZO coatings decreased, while the elastic modulus and hardness increased.

With the increase of thermal exposure time, the apparent porosity of GZO coatings decreased sharply in the early stage (0-20 h) then decreased slowly in the middle stage (20-100 h), and remained stable in the late stage (100-200 h). In the early stage, the crack tip and narrow cracks were preferentially healed by multi-point connection, which hindered the quickly decrease of apparent porosity in the followed two stages, and after 200 h thermal exposure, the apparent porosity of GZO coatings were maintained at 7%-9%. In addition, with the increase of thermal exposure time, varieties of hardness and elastic modulus of GZO coating also presented a multi-stage change rule, similar to that of apparent porosity. The apparent porosity of GZO coatings was increased with the spraying distance, while the hardness and elastic modulus were decreased. The phase structure of GZO coating remained the same before and after thermal exposure. Namely, the mechanical behavior of GZO coatings under high temperature thermal exposure process was mainly affected by its microstructure.

When thermal exposure at 1300 ℃, compositions of GZO coating were defective fluorite (F phase), in consistent with that of the as-sprayed coatings. When thermal exposure for different time, values of apparent porosity, elastic modulus and hardness of coatings all present multi-stage changes, varieties of these parameters mainly occurred in the early stage of thermal exposure (0-20 h). After thermal exposure at 1 300 ℃ for 200 h, the porosity of coatings was remained at about 8%. The hardness and elastic modulus of coating prepared at a spray distance of 120 mm were about (7104.5±658.2) MPa, and (40.1±1.5) GPa.

Magnesia, acted as a refractory material, is widely used in industrial metallurgical furnaces owing to its high refractoriness and excellent corrosion resistance to basic slag and metal melt. Generally, the fabrication of high-quality magnesia aggregates with high purity and high bulk densities (>3.40 g/cm³) plays a critical role in producing MgO-based refractories that show satisfying mechanical properties and corrosion resistance. In China, magnesite is mainly used as a raw material to produce sintered magnesia at high-temperature. while, as the generally coarse-grained of magnesite, it is difficult to produce high density magnesia even sintered at high temperatures, being due to the poor sintering property. As reported that the introduction of oxides additives such as TiO₂, Al₂O₃ ZrO₂, etc. could effectively promote the sintering of magnesia. However, the additives also resulted in a low melting phase formation, which significantly reduced the high-temperature performance of magnesia. In fact, besides magnesite, China also has abundant magnesium resources in salt lakes. Compared to magnesite, magnesia derived from salt lake owns the advantage of higher purity and without CO₂ emissions during the production process. In this work, magnesium hydroxide produced from salt lake was used as raw material to prepare the high-purity dense magnesia via a two-step method. Effects of calcination temperature and grinding time on the morphology and sintering activity of MgO were studied, microstructure and densification behavior of magnesia sintering at high-temperature were investigated also.

Magnesium hydroxide powder [Mg(OH)₂] was used as raw material. Firstly, it was lightly-burned to produce MgO when calcined at 650, 750, 850 ℃ and 950 ℃ with a holding time of 1 h and a heating rate of 5 ℃/min, repetitively. Secondly, the obtained powder was finely ground using mechanical ball milling for 0.5, 1.0, 3.0, 5.0, 12.0 ℃ and 24.0 h, respectively, here, zirconia balls acted as the media and anhydrous ethanol as solvent. Thirdly, the finely ground and dried powders were pressed into a green body with a size of Φ20 mm × 20 mm under a pressure of 100 MPa. Finally, the green body was sintered at 1 600 ℃ for 3 h to prepare the magnesia samples.

Mg(OH)₂ would decompose into MgO agglomerates with crystallite and nano-pores when calcined at 650-850 ℃. The crystal size of MgO continuously increased with the increase of calcination temperature, and rapidly increased to 320 nm when temperature reached to 950 ℃. Correspondingly, the specific surface area of calcined MgO decreased with the increase of the temperature. In addition, a small amount of undecomposed Mg(OH)₂ still existed in lightly-burned MgO even treated at 750 ℃. When calcined at 850 ℃, the Mg(OH)₂ decomposed completely and the obtained MgO crystallite possessed high activity and best sintering properties. With the increase of grinding time, the number of MgO agglomerates decreased continuously and well disappeared when the grinding time reached to 12 h, and the average size of MgO agglomerates decreased to 1.72 μm. However, by further increasing the grinding time to 24 h, the average size of MgO agglomerates only decreased to 1.70 μm. The destruction of MgO aggregates could reduce the number of large pores and improve the contact between agglomerates in the green specimens, both could enhance the densification of the sintered specimens.

When the calcination temperature increased from 650 ℃ to 850 ℃, the sintering activity of crystallite in MgO agglomerations reduced slightly due to the increase of average size of MgO crystallite from 138 nm to 180 nm, while the density of sintered magnesia exhibited an increasing trend as content of residual Mg(OH)₂ in MgO agglomerations reduced. When calcined at 950 ℃, the sintering activity of the crystallite reduced dramatically and average size sharply increased to 320 nm, which obviously decreased the sintering activity and reduced the grain size and density of the specimens. The MgO agglomerations can be effectively crushed via the ball milling process, whose average size sharply reduced from 55.90 to 2.69 μm when ball milling for 0.5 h, and then this value decreased slowly to 1.72 μm when ball milling increasing to 12 h. However, a further increase of ball milling time would not produce the average size. The crush of MgO agglomerations could reduce the large pores in green compacts and enhanced the bonding between MgO crystallites, therefore enhancing the grain growth and densification of the specimens. When calcination temperature and milling time were 850 ℃ and 12 h, respectively, a dense magnesia with an average grain size of 30.4 μm and bulk density of 3.42 g/cm³ could be prepared when sintered at 1 600 ℃ for 3 h.

Chalcogenide glasses are primarily composed of elements from group VI of the periodic table, such as sulfur, selenium, and tellurium, and may include other non-oxide elements like germanium and arsenic. These glasses exhibit properties such as high refractive index (2-3.5), low phonon energy (<350 cm^-1), and tunable performance. As the most popular chalcogenide glasses, As-Se and As-S glasses are widely used in the fields of mid- and far-infrared imaging, supercontinuum generation, infrared laser power transmission, etc. However, the soluble impurity absorption and insoluble particle impurities inside the glass lead to a lot of trouble. In particular, particle-induced scattering is one of the key factors for the decrease of the glass transmittance. Such particles may be sourced from various conditions, such as the original presence of raw materials and introduction during glass preparation. So, it is urgent to find the real source and remove these impurities. For example, Nguyen et al. prepared As₂Se₃ fiber with the lowest loss below 1 dB/m, but found that there are a large number of submicron particles-Al₂SiO₃-inside the fiber, which enhances the scattering loss of the glass. To address this challenge, this work investigated the optical properties of glass by adding deoxidizers and filter sheets to the dynamic distillation process.

The glass was prepared using three-stage quartz ampoules. Filters with apertures of 250 μm, 160 μm, 100 μm and 10 μm were added to the multi-stage dynamic distillation process for removing particulate impurities from the glass. Metal Al as a deoxidizer was loaded in the ampoule during the distillation process for removing oxide impurities such as H₂O, O—H, etc. After the glass was ground and polished, the particles inside the glass samples were observed using a Dino Capture infrared camera and analyzed in the context of scattering theory to determine how the particles affect the transmittance of the glass; Then, transmittance maps were plotted to analyze the relationship between particles and transmittance of the glass prepared with different filter apertures.

The glass prepared using the same filter aperture was ground into thin slices of different thicknesses ranging from 2-20 mm to research the relationship between glass thickness, transmittance and cut-off wavelength. At the same time, As₂Se₃ glass with the highest purity in this study was loaded on the newly constructed CO₂ laser experimental platform for testing, and the relationship between the content of particles inside the glass and the CO₂ laser damage threshold of the glass was investigated.

High-purity, low-scattering As₂Se₃ glasses were prepared by adding filters as well as deoxidizers to the multistage dynamic distillation process. The transmittance of the glass samples obtained with a 250 μm pore size filter was only 43 % at 10.6 μm, while the transmittance obtained with a 160 μm pore size filter was increased to 57 %. When the pore size of the filter was further reduced to 100 μm, the transmittance reached to 59 %, and finally, the transmittance reached the highest level of 62 % at a pore size of 10 μm.

Additionally, the content of oxygen impurities (O—H, H₂O, Se—O) and heterogeneous impurities inside the glass was significantly reduced after purification. The difference in transmittance between samples with different thicknesses (from 2 to 20 mm) was not more than 2 %, suggesting that the glass has a high degree of homogeneity and reduced internal particle scattering.

Excluding the influence of external factors, the resistance of glass to laser damage is related to the glass component and the concentration of inclusions, under the premise of determining the glass component, it is necessary to improve the laser threshold by reducing the internal particles of the glass. The absorption of heat by particulate impurities within the glass affects its laser threshold, so the increase of the glass purity also increases the CO₂ laser damage threshold of the glass, specifically to 32.83 kW cm^-2 (Spot diameter of 0.025 cm).

The results of this study showed that the optical transmittance of the glass after filtration with a 10 μm pore size screen was close to the theoretical transmittance - 62.22 % - without taking into account any scattering losses. Meanwhile, the content of oxygen impurities (O—H, H₂O, Se—O) and heterogeneous impurities inside the glass has been significantly reduced. The difference in transmittance between samples with different thicknesses from 2 to 20 mm is not more than 2% with the reduced scattering inside the glass, while the preparation of ultra-low-scattering As₂Se₃ glass requires filters with smaller filter apertures. In summary, this study provides a new technique to further reduce the internal scattering of sulfur-based glass and to prepare ultra-low-loss sulfur-based optical fibers.

Platinum group metals (PGM), including ruthenium (Ru), rhodium (Rh), and palladium (Pd) are enriched in high-level liquid waste (HLLW) during the reprocessing of spent nuclear fuels. Vitrification is currently the only practical technology to immobilize HLLW. However, the high density and low solubility of PGM in glass cause them to settle down during vitrification process, resulting in the increased viscosity and conductivity of the glass melt at the bottom of the melter. This would potentially lead to concerned issues such as blockages in the discharge port and damage of electrodes. Therefore, understanding the deposition of PGM during vitrification is quite crucial in the management of HLLW. However, few research focused on the deposition process and the spatial distribution of PGM particles in glass melts. Thus, this work took Ru as the representative of PGM and investigated spatial distribution of Ru precipitates in glass melts.

A simplified simulated waste glass with the composition of 50.00%SiO₂-18.89%B₂O₃-2.79%Li₂O- 5.00%CaO-13.30%Na₂O-6.68%Al₂O₃-3.34%ZrO₂ (mass fraction) was formulated. Batches were thoroughly mixed and placed in corundum crucibles, and then melted at 1200 ℃ for 1 h in a muffle furnace. The molten glass was quenched in air and then crushed to obtain the glass frits with the size smaller than 70 μm. Subsequently, glass frits and additional 1.00%RuO₂ (mass fraction) were then fully mixed. 3 g of mixtures was placed in a corundum crucible with a lid to prevent Ru volatilization, and remelted at 1200 ℃ for 0.5 h, 1 h, 2 h and 4 h, respectively, and then annealed at 500 ℃ for 2 h to release the thermal stress.

XRD was used to analyze the precipitated crystalline phases of the Ru-containing glasses. SEM-EDS was utilized to examine the morphology of Ru precipitates in glass samples and determine the distribution density of Ru precipitates particles in agglomerates. The glass samples were then cut into blocks with the size of 7 mm ×7 mm ×6 mm, and XRM was applied to detect the content of Ru precipitates, as well as changes in the size and quantity of agglomerates.

XRD analysis suggested that RuO₂ and Ru precipitated from the glasses in different melting durations. SEM showed both RuO₂ and Ru formed the agglomerates, but RuO₂ agglomerates exhibited higher content and the larger sizes compared to Ru agglomerates. Through calculating the distribution density of RuO₂ particles in the agglomerates, it was determined that RuO₂ particles in the agglomerates coarsened and the RuO₂ agglomerates became more compact as the melting time increased.

Results from SEM and XRM indicate that RuO₂ particles aggregated into a larger size (~20 μm) in the short-time stage (t <1 h), and the content of precipitated RuO₂ and Ru increased rapidly and reached to its added level. In the middle-time stage (1 h< t <2 h), the coalescence of the agglomerates occurred and aggregated to a much larger size (>130 μm). Consequently, the number density of agglomerates decreased, whereas the volume fraction of agglomerates remained relatively stable. In the long-time stage (2 h< t <4 h), the agglomerates turn to the denser one. This led to the increase of particles distribution density and the decrease of agglomerates size. At this stage, the number density of agglomerates remained even.

The three-dimensional images reconstructed from XRM were segmented into 12 layers from top to bottom. The analysis focused on the content of Ru precipitates in each layer of glasses melted for different durations. The samples were categorized into three regions: the surface layer (0~1.5 mm), middle layer (1.5~5 mm), and bottom layer (5~6 mm). The content of precipitated particles in the bottom layer were notably larger compared to those in the middle and surface layers, when the melting time was the same. Furthermore, the content of precipitated particles in the bottom layer increased with prolonged melting time, and approximately 50% of the precipitates settled down in the crucible bottom after 4 h melting. The rate of precipitates deposition decreased as the melting time increased.

Ruthenium primarily precipitates as the form of RuO₂ particles in the simulated waste glass. As the melting time is prolonged, the Ru precipitates undergo various processes, including formation of agglomerates, coalescence between agglomerates, and densification of agglomerates. The content of precipitates in the bottom layer was larger than those in the middle and surface layers. The volume fraction of precipitates in the bottom layer increases with extended melting time, and the rate of deposition of precipitates decreases as melting time increases.

Sustainable H₂ generation from water splitting by solar-driven photocatalysis has been proposed as a promising alternative to increasingly reduced fossil fuels. However, such techniques still face great challenges in dynamics and thermodynamics. Biomass as a hole sacrificial agent is conducive to the generation of H₂. Local surface plasmon resonance (LSPR) can effectively use solar energy and convert it into high-energy "hot carriers", and realize high efficiency solar energy to chemical energy conversion. Copper alloy inherits the LSPR effect of copper particles. Palygorskite (Pal) is a natural hydrated magnesium silicate mineral with large surface area and certain acid sites. In this work, Pal-loaded plasma alloy nanocomposite was prepared for photocatalytic reforming cellulose for synergistic hydrogen production. The phosphate group-modified Pal provides a large number of acidic sites for cellulose hydrolysis and glucose. At the same time, the LSPR effect of CuNi alloy increases the reaction temperature, realizes the photothermal cooperative catalysis, and further enhances the catalyst activity.

2 g Pal was evenly dispersed in 100 ml phosphate solution, then the mixture was stirred in an 80℃ water bath for 12 h. Thereafter, they were washed with deionized water and dried at 80℃ for 16 h to obtain H-Pal. 0.1 mol of CuCl₂·2H₂O and 0.1 mol Ni(NO₃)₂·6H₂O were dissolved in 30 ml of sodium hydroxide solution, and various-Pal was added with 0.5 g ethylenediamine by stirring. About 1 mol of sodium phosphates was added to the solution, and the mixture was transferred to the reaction kettle, reacted at 180℃ for 90 min, and then naturally cooled to room temperature. After washing, CuNi/H-Pal composite was obtained after vacuum drying for 6 h.

The photocatalytic reforming cellulose was produced under simulated sunlight. The conversion of cellulose reached a maximum of 82% for 30% CuNi/H-Pal. In the end product of photoreforming cellulose, the content of formic acid dominated, reaching the highest 36.8 μ mol · g^-1 · h^-1, followed by lactic acid 13.8 μmol·g^-1·h^-1. After the conversion of cellulose into glucose, the C—C bond in glucose finally breaks to generate formate. Compared with Cu/H-Pal and Ni/H-Pal, the CuNi/H-Pal hydrogen production efficiency is higher, because the synergistic action of bimetals can effectively promote charge separation, while retaining the LSPR effect of copper particles, thus achieving the photothermal cooperative catalysis, and the highest hydrogen evolution rate is 1.68 mmol·g^-1·h^-1.

In the aqueous solution, cellulose is first adsorbed on the surface of Pal rod crystals, and under light irradiation, photogenerated electrons reduce H⁺ to H₂ in water at the reduction end of CuNi/H-Pal. Due to the plasmon resonance effect of CuNi, CuNi/H-Pal has a significant photothermal effect, and the activity of acidic sites on the catalyst is enhanced under the synergistic effect of photothermal interaction, which promotes the cleavage of cellulose β-1,4-glycosidic bond and hydrolysis to glucose. Subsequently, the resulting glucose is converted to C5 sugar and formic acid by hole oxidation, and C5 sugar is converted to C4 sugar and formic acid through the Ruff degradation mechanism, and all intermediates are further gradually converted to formic acid through the C1-C2 α-splitting process. At the same time, cellulose conversion products contain small amounts of lactic acid. Cu on the surface of the catalyst acts as a small amount of Lewis acid active site during the catalytic reaction, promoting the conversion of glucose to lactate under the joint action of holes.

In summary, a new plasma-metal-alloy supported by clay mineral nanocomposite material was prepared. Different amounts of CuNi alloys were loaded on the surface of phosphorylated Pal, which not only retained the plasmon resonance effect of Cu element, but also improved its stability and inhibited its oxidation, and significantly improved the charge transfer efficiency due to the Schottky junction formed by the metal alloy and modified Pal. The hydrogen production rate with 30% CuNi/H-Pal catalyst could reach 1.68 mmol g^-1·h^-1 under simulated sunlight irradiation. At the same time, cellulose was converted into high value-added formic acid and lactic acid, and the yields reached 36.8 μmol·g^-1·h^-1 and 13.8 μmol·g^-1·h^-1, respectively. This work provides a new strategy for hydrogen production and the production of valuable biochemicals through biomass photocatalytic reforming.

With the continuous promotion of the "carbon neutrality" and "carbon peak" strategies, reducing carbon emissions from cement production is urgent. Replacing cement with a combination of one or multiple mineral materials can not only improve the physical properties of cement products, but also reduce the use of cement clinker, thereby reducing carbon emissions in cement production and saving resources. However, due to the complex properties and multiple interrelated properties of cement matrix composites, the optimization design of these materials in cement matrix composites faces challenges. Therefore, it is necessary to find more efficient and accurate methods for optimizing the design of cement matrix composites.

Response surface experiments were conducted by using Shangfeng cement clinker that meets the Chinese national standard GB 21372-2008, standard sand that meets the European standard EN 196-1∶2005, multiple mineral materials (grade II fly ash, S95 mineral powder, limestone powder, and tuff powder), and external gypsum (natural gypsum, desulfurization gypsum). The response surface experiment was conducted using Design Expert 13.0 software for Box Behnken response surface design. The response surface design consists of four factors and three levels. Four factors were selected: fly ash content (A), limestone powder content(B), tuff powder content(C), and slag content(D). The response values were cement 3 d compressive strength, 28 d compressive strength, and the cumulative hydration heat release of cement within 3 d, and 29 experimental points were designed.

Firstly, use Monte Carlo sampling method to analyze the reliability of each response quantity. Select process factors as noise, randomly select 8 000 samples points to analyze the contribution of each random input variable to the response, and confirm its distribution characteristics including mean value, standard deviation, and Sigma level. Next, Further construct optimization objectives that meet the minimum 3 d compressive strength value (Y₁), maximum 28 d compressive strength value (Y₂), and minimum the cumulative hydration heat release value of cement within 3 d (Y₃), as well as the maximum content of multiple mineral materials (Y₄). The deterministic constraint range for Y₁ is 17.0-27.0 MPa, and the deterministic constraint range for Y₂ is 42.5-52.5 MPa. Y₃ is set to be no higher than the mean value of reliability analysis and not lower than 170.0 J/g; Y₄ is set to not less than 30% of the total material quantity. And the reliability of each performance response and design variable should be not less than 6-Sigma. A multi-objective optimization design for multi mineral materials, based on the mathematical model obtained from response surface experiments and the multi-objective optimization design of multivariate mineral materials, the non-dominated sorting genetic algorithm (NSGA-II) is used for multi-objective optimization. When optimizing, set the initial population size of NSGA-II to 200, genetic algebra to 20, cross distribution index to 10, and mutation operator to 20.

Based on the response surface experimental matrix, the experimental results were obtained. After removing the insignificant items (p-value>0.05) from the response surface experimental results, the mathematical model was obtained as follows:

Y₁=12.148+0.748A+0.690B-0.392C+2.089D-0.078A×D-0.066B²-0.065D²

Y₂=49.595-1.179A+0.893B+0.654C+0.381D+0.042A×C+0.073B×C-0.048C×D+0.048A²-0.155B²-0.094C²

Y₃=368.801-12.309A+0.369B-12.273C-7.958D-0.192A×B+0.258A×C+0.757A×D+0.569C×D

A 6-Sigma reliability analysis was conducted on the response surface mathematical model, and the sigma level values for Y₁ and Y₂ were 1.144 and 1.497, respectively, with reliability values of 74.76% and 86.55%, both lower than 90%. However, the sigma level value for Y₃ was 3.953, with reliability values of 93.83%. After multi-objective optimization of the mathematical model, the obtained Pareto solutions were sorted to get the optimal process parameters: A of 2.72%, B of 7.14%, C of 9.95%, and D of 19.93%. The optimized Y₁ of cement is 23.43 MPa, and the Y₂ is 46.82 MPa, the Y₃ is 214.36 J/g, and the total content of cement multi mineral materials is 39.74%. Five repeated experiments were conducted to verify the optimal process parameters, and the experimental values were close to the optimized values. The maximum relative error is 3.71%, which is within an acceptable error range (>90% confidence level).

According to the mathematical model obtained from the response surface, it is found that there is a good correlation between the multi mineral material system and the mechanical properties of cement. There is an excellent linear correlation between the multi mineral material system and the hydration heat of cement. Based on mathematical models, a multi-objective optimization method based on NSGA-II can be used to solve the optimal formula. In addition, by using the 6sigma robust multi-objective optimization method, the reliable multi-objective optimization results can be obtained. When the content of fly ash is 2.72%, the content of limestone powder is 7.14%, the content of tuff powder is 9.95%, and the content of slag is 19.93%, the three-dimensional compressive strength value of cement is 23.43 MPa, the 28 d compressive strength value is 46.82 MPa, and the Y₃ is 214.36 J/g. This study validated the robustness and feasibility of composite design methods (including response surface design and multi-objective optimization based on NSGA-II) in multi-component mineral systems.

In recent years, mordenite molecular sieves with various properties have been successfully synthesized by template method. Although the template method has received a lot of attention, the addition of the templating agents especially organic templating agents, would increase the cost of raw materials, and could be harmful to environment. Therefore, the method without organic template is more economical and environmentally friendly. Recently, the synthesis and applications of mordenite modified by hetero-atom have getting more and more attentions, and the performance of modified mordenite molecular sieve shows better. The common methods for modifying mordenite molecular sieve include ion exchange method and impregnation method. Some researchers have synthesized monolithic iron-containing mordenite (Fe-MOR) by template-free method, which has excellent CO₂/N₂ separation performance and high carbon dioxide capture performance. In addition, Fe-MOR zeolite can catalytically decompose N₂O into N₂ and O₂, which can effectively reduce the N₂O content in the atmosphere. In this work, Fe-MOR zeolite was synthesized by hydrothermal method without organic template. A novel rod-cluster-shaped Fe-MOR zeolite was controllably synthesized by fine adjustment of pH and crystallization time, and its CO₂ adsorption properties were investigated.

Deionized water, tetraethyl orthosilicate, concentrated hydrochloric acid, ferric nitrate, aluminum sulfate and sodium hydroxide were used. A mixture with n(Fe) : n(SiO₂) : n(Na₂O) : n(H₂O) : n(Al₂O₃) : n(HCl)=0.002 5∶1.000 0∶x∶30.000 0∶0.066 5∶y (n is molar ratio, x=0.300 0~0.580 0, y=0.100 0~0.150 0) was obtained, then the mixture was transferred into a polytetrafluoroethylene lined stainless steel autoclave and crystallized at 180°C for 7 to 14 days. When the reaction finished, a white powder sample was obtained after post-treatment.

The crystalline phase and crystallinity of the samples were analyzed by PANalytical X'Pert Powder (XRD). S-3400N scanning electron microscope (SEM) was used to observe the morphology of the samples. The elemental composition of the samples was analyzed by S8-tiger X-ray fluorescence spectrometer (XRF). The static CO₂ adsorption performance of the samples was measured by ASAP-2460 physical adsorption analyzer. A fixed-bed apparatus and a Hiden Analytical HPR-20 on-line mass spectrometer were used to determine the dynamic CO₂ adsorption performance of the samples.

The crystalline phase of the product is effected by the amount of sodium hydroxide or the concentration of hydrochloric acid used in the initial gel mixture. The pH value of the initial gel mixture is crucial for the synthesis of pure phase Fe-MOR. The XRD results show that the peak intensity of MOR decreases gradually with the decrease of Na₂O molar ratio from 0.4 to 0.36, indicating the crystallinity of samples related tod the basicity in a certain range. When the molar ratio of Na₂O/SiO₂ is 0.4 or 0.375, the pure phase MOR with rod-shaped cluster was obtained, in which the single rod is approximately hexagonal and the morphology is uniform. When the molar ratio of Na₂O/SiO₂ is in the range of 0.3-0.58, the analcite will arise if the alkalinity of the system is too high, and the amorphous phase will appear if the alkalinity is too low. This means that moderate alkalinity is conducive to the interaction of silicon, aluminum and other components, promoting the nucleation and growth of the crystallization, contributing to the synthesis of pure mordenite. Moreover, when the molar ratio of Na₂O/SiO₂ is 0.4, the pure phase of Fe-MOR can be controllably synthesized by adjusting the molar ratio of concentrated hydrochloric acid.

The pure phase Fe-MOR molecular sieves can also be synthesized by shortening the crystallization time under the constant pH. The axial size of the synthesized samples also decreases with the decrease of the crystallization time. The results show that the longer the crystallization time is, the better the axial growth of the crystals, but when the crystallization time is less than 7 days, the hetero-crystalline phase of analcite appears.

The static adsorption capacity of CO₂ on Fe-MOR samples was similar because the adsorption mainly depends on the atmosphere pressure. The capacity of dynamic adsorption of CO₂ is due to the difference of the suitable site of electric field in the 8-membered ring channel of MOR, which determines the adsorption capacity in the dynamic CO₂ penetration under low partial pressure of CO₂. The microstructure of Fe-MOR molecular sieves synthesized under different conditions are different, which results in the difference of CO₂ dynamic adsorption capacity of several samples.

The results show that the pH of the gel mixture and crystallization time affected the synthetic products. When the amount of hydrochloric acid is constant, the high alkalinity results in analcite crystals while the low alkalinity results in amorphous phase. When the amount of sodium hydroxide is high, increasing the amount of hydrochloric acid will result in the pure Fe-MOR molecular sieves. The axial size of the crystal will decrease with the shortening of the synthesis time under appropriate pH value. When the crystallization time was less than 7 days, the hetero-crystalline phase of analcite appeared, which was attributed to the short synthesis time leading to incomplete crystallization. Rod cluster Fe-MOR molecular sieves were synthesized by hydrothermal method without organic template and showed good static and dynamic adsorption capacity for CO₂, and the static adsorption capacity was up to 3.57 mmol/g.

Supercapacitors (SCs) are known as one of the most promising energy storage devices due to their high-power density and excellent cycle stability. The performance of SCs is closely related to the electrochemical performance of electrode materials. The widely-used electrodes are carbon-based materials and their derivatives. Carbon materials contribute energy storage based on double layer capacitance associated with ion adsorption and desorption, achieving excellent power density and cycle stability. However, they lack in high capacitance because of limitation of surface area. Instead, Fe-based Prussian blue (PB) is a promising pseudocapacitive material with open frame structure and high theoretical capacitance, but the poor electrical conductivity limits its rate performance and cycle stability. Combining PB with carbon materials is considered as an effective solution to develop advanced electrode composites. Herein, nanoporous carbon (NPC) derived from lignite has been prepared with high specific surface area and good electrical conductivity. Then, Fe-Prussian blue was successfully loaded on NPC via co-precipitation method to obtain Fe-Prussian blue@nanoporous carbon composite (PB@NPC). The fabricated PB@NPC composite possesses high specific surface area, high specific capacitance and excellent cycle stability.

The lignite powder was obtained by crushing the lignite with 100 mesh and sifting, and then mixed in 4 mol/L HNO₃ and stirred for 8-10 h. After washing and drying, the collection was dispersed in 5 mol/L NaOH for 8-10 h with stirring, following by washing with deionized water and drying to obtained pre-treated lignite. Then, the pre-treated lignite was mixed with KOH at a mass ratio of 1∶3 and heat treated at 800 ℃ in Ar atmosphere for 1 h, and then washed and dried to obtain nanoporous carbon (NPC).

0.32　g FeCl₃·6H₂O was dissolved in 100 mL deionized water. 0.5 g NPC and 0.37 g K₄Fe(CN)₆·3H₂O were mixed with 100 mL deionized water and ultrasonicated to obtain a black suspension. Then, ferric chloride solution was added into the suspension under stirring for 4 h and aging for 2 h. The precipitate formed was collected by centrifugation and then dried at 80 ℃ for 12 h. The as-prepared sample was labeled as PB@NPC. The preparation method of PB was similar to that of PB@NPC but without NPC addition in the preparation process.

The phase structure, valence state, bonding mode, specific surface area and pore size distribution of NPC, PB and PB@NPC were investigated. The mechanism of improved properties of PB@NPC was discussed. The electrochemical performance of both PB and PB@NPC symmetrical capacitor were conducted.

From XRD and Raman tests, the characteristic peaks of PB and NPC can be observed, which proves the successful synthesis of PB@NPC. The XPS spectra show that C-N bond binding energy in PB@NPC is significantly shifted compared with PB. It is concluded that the shift of binding energy of C and N in the composite is related to the change of electronegativity, which is the result of the interface binding effects between PB and NPC. The specific surface area of PB@NPC reached to 2122 m²/g with a large number of micropores and mesopores by N₂ adsorption/desorption analysis.

The electrochemical performance was tested in a three-electrode system. The CV curves of PB@NPC have clear redox peaks, revealing typical pseudocapacitive behavior. The curve near the high potential and low potential is approximately rectangular, also indicating the characteristics of double layer capacitance. The specific capacitance of PB@NPC is 309.35 F/g at a current density of 0.2 A/g and even maintains 213.09 F/g at 1.0 A/g, which is much higher than PB (112.20 F/g), showing remarkable rate performance of PB@NPC. The EIS results demonstrate that the internal resistance R_s and diffusion resistance R_ct of PB@NPC are both lower than that of PB, showing better electrical conductivity and ions diffusion rate. It is proposed that the porous structure of NPC provides more ion transportation channels for PB and accelerates ion transfer. At a sweep speed of 5 mV/s, PB@NPC yields a specific capacitance of 364.14 F/g after 500 cycles. The capacitance retention rate is 75%, which is higher than that of PB (65%).

Compared with PB capacitors, PB@NPC capacitors have higher capacitance under different current density and can still return to the initial capacitance after high current density, reflecting the good structural stability. As shown in Ragone plot, PB@NPC capacitor has much better performance than PB capacitor, providing 22.4 Wh/kg energy density at the power density of 800 W/kg. Moreover, the capacitance retention of the capacitor after 1000 cycles is 91%, much higher than that of PB capacitor (34%). It is proposed that the porous structure of NPC in the composite can delay the lattice distortion of PB and promote the penetration and diffusion of electrolyte.

The PB@NPC composite was prepared via co-precipitation method. It possesses high specific surface area and excellent electrical conductivity. The fabricated porous structure provides abundant ion transport paths to promote the penetration and diffusion of electrolyte. PB@NPC demonstrates high capacitance due to synergistic effect of double-layer capacitance and pseudo-capacitance. The specific capacitance of PB@NPC electrode is 483.45 F/g at a scan rate of 5 mV/s. The specific capacitance is 309.35 F/g at a current density of 0.2 A/g. The symmetrical capacitor assembled with PB@NPC electrodes exhibits capacitance retention of 91% after 1 000 cycles, revealing excellent cycle stability. PB@NPC is a new type of supercapacitor electrode material with excellent performance and a potential electrode material to be commercialized.

Rare earth gypsum is an industrial by-product generated from the treatment of wastewater produced by the rare earth metallurgical process, and its recycling is a strategic measure for the current green metallurgy and cleaner production. In addition, the filler reinforcing agent for natural rubber in polymers is mainly carbon black or other materials such as calcium carbonate, which will cause a large amount of carbon emissions in the production process. Calcium sulfate whisker (CSW) is a new reinforcing agent in rubber composites, while its surface is hydrophilic and has poor compatibility with organic matrix. This work aims at the above problem by using inorganic trisodium phosphate-organic stearic acid composite modification, thus changing the surface polarity of CSW and improving the compatibility with organic matrix.

20 g/L CSW was dispersed in 100 mL of trisodium phosphate solution (0.005, 0.010, 0.015, 0.020, 0.025 mol/L), then ultrasonicated for 2 min, and stirred at room temperature to make the solution mixed homogeneously. The solution was filtered and washed to obtain inorganic modified CSW (1-CSW). Subsequently, 1-CSW was dispersed in 100 mL of ethanol, and stearic acid (0, 1%, 6%, 11%, 16%, 21% of CSW addition) was added to the solution, and the slurry was heated (60, 70, 80, 90 ℃, and 100 ℃) in an oil bath for a certain time (10, 15, 20, 25 min, and 30 min) under magnetic stirring. The final product was cooled to room temperature, then filtered and washed with anhydrous ethanol for five times, and dried at 100 ℃ for 1 h to obtain the composite modified CSW (2-CSW).

The optimal modification conditions for inorganic-organic surface modification of CSW were determined by one-way experiments: CSW addition as 20 g/L, optimal amount of stearic acid as 6% of CSW, modification time as 10-15 min, modification temperature as 80 ℃, and concentration of trisodium phosphate as 0.02 mol/L. XRD patterns indicated that the modified products were mainly monoclinic hemihydrate (HH) calcium sulfate with the presence of a small amount of monoclinic dihydrate (DH) calcium sulfate, It can be seen that the surface modifier contributed to the transformation of CSW from dihydrate to hemihydrate calcium sulfate. TEM analysis was performed that the surface modifier existed in amorphous form on the surface of the whisker. A transparent coating layer on the surface of 2-CSW can be clearly observed. The enlarged TEM image shows that the average thickness of the hydrophobic layer on the surface of 2-CSW is about 121.26 nm. Infrared spectroscopy analysis shows that inorganic trisodium phosphate and organic stearic acid were successfully modified on the surface of CSW. Thermal stability analysis shows that the first mass loss of CSW is perfectly agreement with the theoretical water content in CSW. The second weight loss is due to the decomposition of stearic acid. XPS analysis of the surface elements and contents proves that trisodium phosphate and stearic acid are successfully encapsulated on the surface of 2-CSW.

The surface modification of CSW was successfully realized through a series of one-factor experiments to change the surface property of this whisker from hydrophilic to hydrophobic. The optimal modification conditions were as follows: concentration of CSW: 20 g/L, stearic acid modifier: 6% of CSW, trisodium phosphate: 0.02 mol/L; modification time: 15 min at 80 ℃. The morphology of CSW remained the original form after the modification, but the crystal structure of the whisker was transformed from DH to HH, and the surface property was changed from hydrophilic to hydrophobic. The contact angle of 2-CSW reached a maximum of 104.23 °, and the thickness of its hydrophobic layer was 121.26 nm. The modification mechanism is that the surface of CSW firstly generates CaHPO₃ and Ca(OH)₂, then undergoes a bonding reaction with stearic acid, generating a nanoscale hydrophobic layer with a C₁₇H₃₅COO-Ca -structured nanoscale. The layer protects CSW from denaturation by the external environment, maintains the unique properties, and confers compatibility with organic substrates.

Previous researches clearly showed the large potential of high-entropy strategy on the adjustment of multiple properties in FEs materials, However, the high configuration entropy only implied the substitution of a high ratio of multiple exotic atoms, and the characteristic of these atoms as well as their influence on the structure of the final compositions were not revealed clearly, even though a large number of high-entropy FEs did not exhibit outstanding performance. Therefore, designing high-entropy FE materials did not involve directly adopting the high-entropy strategy used in traditional structural materials. Instead, it entailed ensuring chemical disorder while utilizing highly adjustable chemical compositions to design functional unit disorder. To achieve functional materials with excellent performance, it is necessary to simultaneously satisfy chemical disorder, structural disorder, and functional unit disorder, it is also the core principle behind designing new high-entropy functional materials. Polarization configuration design is the core for achieving excellent physical properties in FE materials, many special polarization configurations have been reported to show unique electrical properties in many studies. The high-entropy strategy shows strong operability on polarization configuration design because of the unique properties and structure of high-entropy FEs. Furthermore, the high-entropy strategy would also benefit the grain refinement, mechanical property enhancement, and entropy stabilization effects. To elucidate the regulatory mechanism of chemical disorder on polarization disorder, researchers utilized neutron/synchrotron total scattering techniques combined with reverse Monte Carlo methods to achieve atomic-level three-dimensional structure reconstruction. They quantitatively analyzed the intrinsic mechanisms of sub-nanometer ion clusters and the formation of strong polarization fluctuation polarization configurations, it served as a bridge for understanding the relationship between chemical disorder and structural functional unit disorder. Some new technologies, such as machine learning and phase field simulation, could be used for assisting the prediction of high-performance materials among endless high-entropy compositions, and high throughput experiment method can also greatly improve efficiency for exploring new compositions with novel polarization configurations. In the future, further research on high-entropy FEs should focus on the design of more novel polarization configurations to attain better performance of this kinds of materials, thereby to facilitate the application of high-entropy FEs in a wide field.