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.