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.

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.

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.

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.

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.

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.

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.

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.