Silicon carbide ceramics have excellent comprehensive properties such as high temperature resistance, wear resistance, corrosion resistance, radiation resistance, oxidation resistance, low thermal expansion rate and high thermal conductivity. They are important in key fields such as aerospace, nuclear power, high-speed locomotives, and weaponry. Application value. SiC ceramics are difficult to shape due to their high thermal stability and strength.
At present, the international preparation of ceramic materials mainly uses traditional powder molding methods, including micro powder preparation, molding (calendering, extrusion, dry pressing, isostatic pressing, casting, injection, etc.), sintering (hot pressing sintering, reaction sintering, Atmospheric pressure sintering, atmospheric pressure sintering, hot isostatic pressing, discharge plasma sintering, etc., processing, etc. In the last 30 years, new preparation methods for ceramic materials have emerged in an endless stream, with breakthroughs in all aspects, but there are still limitations. Properties, high preparation temperature (although the addition of sintering aid can reduce the sintering temperature, but the sintering aid will affect the performance of the ceramic), it is difficult to obtain uniform chemical composition and microstructure, difficult to finish and difficult to solve the high brittleness of ceramic materials. problem.
Advanced ceramic preparation technology must make breakthroughs in raw material preparation, molding, sintering, etc. Since the preparation of SiC ceramic fiber by polycarbosilane by Yajima et al in 1975, the precursor-transformed ceramic technology has entered people's field of vision. According to BCC Research In 2017, the global ceramic precursor market was US$437.6 million (of which SiC ceramic precursors accounted for 40.4% of the market share), and is expected to reach US$712.4 million by 2022, with an average annual growth rate of 10.2%. The so-called precursor conversion ceramics is the first to pass. The chemical synthesis method produces a polymer which can be converted into a ceramic material by pyrolysis at high temperature, and after molding, obtains a ceramic material by high temperature conversion. It has many advantages: Molecular designability: chemical composition of the precursor can be designed by molecular design Design and optimization with the structure to achieve the control of ceramic composition, structure and performance; Good processability: Ceramic precursors are organic polymers, inheriting the advantages of good polymer processing, such as soluble impregnation, spinnable , can be molded, foamable, 3D printing, etc., so it can be used to prepare low-dimensional materials that are difficult to obtain in traditional powder sintering processes. Complex configuration, such as ceramic fiber, ceramic film, complex three-dimensional member, etc.; low temperature ceramization, no need to introduce sintering aid; can prepare ternary and multi-valent covalent bond compound ceramics; can obtain fiber toughened ceramic materials, thus solving High brittleness of ceramic materials.
Precursor-transformed ceramic technology can flexibly control and improve the chemical structure, phase composition, atomic distribution and microstructure of ceramic materials. It has the advantage that traditional ceramic preparation technology can't match. The key to the preparation of ceramic materials by precursor conversion method is The ability to prepare a suitable precursor directly determines whether a ceramic material with excellent properties can be successfully prepared. The SiC ceramic precursors that have been successfully developed and applied are mainly solid polycarbosilane (PCS). But PCS is the pioneer of SiC ceramics. There are still some shortcomings, such as C/Si in PCS, the pyrolysis product is rich in carbon, which ultimately affects the performance of SiC ceramics; PCS ceramics have lower yield; it is solid at room temperature and is used to form ceramic matrix in composite materials. At the time of the impregnation process, a solvent such as xylene or tetrahydrofuran is required, and the solvent needs to be evaporated before the cracking, resulting in a long preparation cycle and a cumbersome process.
Recently, the Nuclear Energy Materials Engineering Laboratory of the Institute of Materials Technology and Engineering of the Chinese Academy of Sciences has studied to prepare a fluidity (complex viscosity 0.01~0.2Pa·S), long storage time (> 6 months), low oxygen content. (~ 0.1 wt%), high ceramic yield (~79wt% ceramic yield at 1600°C), liquid hyperbranched polycarbon with C/Si of ~1.1 in ceramic products and less than 3% change in mass after static oxidation at 1500°C Silane (LHBPCS). The quality of the sample has been confirmed by many application units. In addition, the research team has also conducted in-depth research on the LHBPCS curing cross-linking mechanism, which can achieve its photo-curing and low-temperature thermosetting molding, and the gelation time is only a few minutes. , and the structure is dense and free of cells.
Relevant research results were published in J. Eur. Ceram. Soc., Adv. Appl. Ceram., J. Am. Ceram. Soc. and other related journals. Relevant research has received major research projects of the National Natural Science Foundation of China, key deployment projects of the Chinese Academy of Sciences, etc. Funding.
Figure 1. Preparation of LHBPCS and cross-linking curing and compact morphology after sintering
Figure 2. Change in cross-linking rate of prepared LHBPCS at different thermal initiator (TBPB) levels
