Precursor transformation of silicon carbide ceramic matrix composites is mainly used for the preparation of high-temperature, oxidation resistance, good wear resistance, low thermal expansion rate, good electrical and thermal conductivity, high hardness and corrosion resistance, etc. High-performance ceramic materials and fiber-reinforced ceramic matrix composites are now widely used in high-end technology and defense military fields such as lightweight remote-sensing imaging optical system lightweight support structures, aerospace engine hot-end components, and reusable aerospace Carrier thermal protection materials, hypersonic transport propulsion systems, etc. In the civil sector, precursor structural transformation materials also gradually reflect its enormous economic value and irreplaceable role, such as aircraft, high-speed rail, automotive and other modern transport system brake discs, high-temperature combustion Steam turbine hot end parts, High temperature gas waste heat recovery, Industrial dust filtration, Corrosion-resistant regenerated catalyst carrier, Large-scale high-temperature system heating parts, Metallurgical high-temperature furnace carbon sheaths, etc. With the advancement of civil-military fusion work, precursors transform high-performance structures Ceramic material preparation technology will become more green and cost-effective, and its products will be more With extensive penetration into all aspects of the economic and social system, the impetus for economic development will increase.
The preparation of precursors for the CMCs matrix is crucial. From the process flow, the ideal precursors should have both a 'three low' (low viscosity, low temperature cross-linking, low shrinkage), 'two no' (no impurities, no foaming ), "High" (high ceramic yield) characteristics. At present, the CFMs have the fastest development of Cf/SiC and SiCf/SiC systems. The domestic use of solid polycarbosilane (PCS) as the precursor of SiC exists. The disadvantages are: need to be dissolved in organic solvents, reduce impregnation efficiency and bring pollution; PCS itself cannot be cross-linked, foam after pyrolysis; PCS ceramics yield is not high, about 55%; the price is more expensive, 3000~4000 yuan / kg. At the same time, the best service temperature of SiC-based composites is around 1600°C. For the service temperature of 1000-1300°C, the materials can meet the application requirements, but it will result in redundant performance. The cost of production limits the scope of application and the expansion of production, such as high-temperature structures, heat-resistant components, high-temperature chemical pumps, valve applications, etc. It is urgent to find cost-effective alternatives. Through the LC3 program, the United States first launched the CMCs low-cost Research, determined that SiOC is suitable for applications up to 1300°C System, and developed precursor product is suitable for PIP process.
The Nuclear Energy Materials Engineering Laboratory of the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences undertook the nuclear energy lead task of the Chinese Academy of Sciences during the “Twelfth Five-Year Plan” period and conducted in-depth research on high-performance silicon carbide precursor materials. In the research work, it was found that a large number of small-molecule liquid by-products were generated during the conventional synthesis process. Recently, under the guidance of the research ideas of green manufacturing and cost reduction, the team conducted research on the reuse of the above liquid byproducts. In this study, The laboratory scientific research personnel analyzed the composition of the liquid by-products and determined that it was a liquid low-molecular-weight PCS with a weight average molecular weight between 200 and 800 and a main chain of Si-C structure. Further, the introduction of 'C=C 'Liquid-active precursors (LC-PCS) that can be converted into SiOC ceramics were prepared. LC-PCS has the following characteristics: 1 Room temperature viscosity of about 30 mPa·s; 2 Fully cross-linked below 400°C; The rate is greater than 70%. Finally, LC-PCS and PCS were used as precursors to prepare CMCs by PIP process. The results show that the required 'dip-cracking' cycle time is reduced from 14 to 10 for obtaining dense samples. It can be seen that the new synthesis route reduces the cost of CMCs from both the precursor itself and the composite material preparation process, and prepares cost-effective CMCs that can be serviced at 1000-1300°C. The reuse of liquid by-products also realizes the green of PCS. The synthesis reflects the environmental benefits. The research results paved the way for military and nuclear high-end materials to enter the civilian field.
Ningbo Institute of Materials has patented the green and low-cost technology (201810433805.3) and actively promoted the docking of downstream products. The above work has been supported by the National Natural Science Foundation of China (91426304) and the Chinese Academy of Sciences Strategic Priority Technology Project (XDA03010305). .
Figure 1 (a) LC-PCS; (b) Crosslinked cured product; (c) Lysate at 1200°C; (d) TG curve for PCS and LC-PCS
Figure 2 (a) LC-PCS and PCS solution impregnation cycles - weight gain curves; (b) LC-PCS prepared as precursors for 2D Cf/SiOC
