Silane polymers: Characteristics, Development and Applications

1. Unique molecular structure, establishing an important position
Silazane polymers are a type of inorganic polymers with Si-N bonds as repeating units in the main chain. Their special chemical structure enables them to transform into SiCN, SiCNO or silica ceramics at high temperatures, which is of great significance in fields such as high-temperature resistant coatings.

2. With a century-long research and development journey, achievements have been constantly emerging
In 1921, A. Stock and others first prepared polysilazane by ammonolysis of chlorosilane with ammonia gas, initiating A research journey of nearly a century. Compared with polysiloxane whose main chain is the Si-O chain as the repeating unit, polysilazane is relatively active by itself, prone to react with water, polar compounds, oxygen, etc., inconvenient for storage and transportation, and the preparation method is not perfect. The product is complex and has a low molar mass. Its development and application were once relatively lagging behind. However, with continuous exploration, Clariant of Switzerland, Teon of Japan and Azelectronic Materials of the United Kingdom have launched all-hydrogen polysilazane. Kion in the United States has the "Ceraset" brand of polyurea silazane and polysilazane. Some products are also available from the American company DowCorning and the German company Bayer. The Institute of Chemistry of the Chinese Academy of Sciences in China has developed the PSN series of polysilazanes. These commercial products have strongly promoted the application research of polysilazanes, especially the research on them as ceramic precursors.

3. Four development stages, witnessing technological breakthroughs
Looking back on its development, it can be divided into four stages. In the 1920s, researchers attempted to synthesize silazane rings and oligomers and classify them. A. Stock made pioneering contributions, but the development was slow. After World War II, polysiloxane was successfully commercialized in the 1950s and 1960s, which stimulated the enthusiasm for polysilazane research. People attempted to prepare polysilazane by ring-opening polymerization and similar methods and study its properties, but the progress was limited. In 1976, S. Yajima et al. obtained SiC fibers (trade name Nicalon) by cracking polysilane, which prompted researchers to focus on polysilane and expect to prepare Si₃N₄ and Si-C-N fibers. At this time, the spinnability and curing cracking of polysilane became the research focus. The polymer precursor method has also become a new method for preparing ceramics (high-temperature cracking of specific polymers to produce ceramics). In the 1990s, the research group of R. Reidel introduced element B to produce Si-B-C-N ceramics with a temperature resistance of 2200℃, which drove the research on modified polysilazane. Magnetic Si-Fe-C-N ceramics, antibacterial Si-Ag-C-N ceramics, anti-crystallization Si-Zr-C-N ceramics, etc. have emerged one after another.

4. Diverse application fields demonstrate the advantages of materials
Polysilazane is mainly used as the precursors of Si₃N₄ or Si-C-N ceramics and has expanded its applications in multiple fields. In terms of coating, SiO₂ or SiCN transformed from polysilazane is corrosion-resistant and easily adheres to metal substrates, making it an excellent high-temperature resistant anti-corrosion coating material. There are already commercialized products used in components such as automotive exhaust pipes. In the preparation of porous ceramic materials, its diverse modification methods and excellent molding capabilities enable the use of various pore-forming approaches to produce porous SiCN ceramics, meeting the demands of filtration, catalysis and other fields. It also has applications in the preparation of ceramic MEMS components, composite materials, etc. In addition, polysilazane has been studied less as a resin material, but it can be used to enhance the oxidation resistance of carbon materials and increase the thermal stability temperature of carbon fibers.

5. The current domestic development situation is characterized by both opportunities and challenges
In China, polysiloxane accounts for over 90% of the silicone industry chain, and there has been an overcapacity in recent years. The development of polysilazane is different. The global market's demand for ceramic-based materials is growing, which has driven up the demand for polysilazane. It is expected that the compound annual growth rate will exceed 16.5% from 2022 to 2026. However, the preparation process of polysilazane is difficult. Foreign countries monopolize the market and prohibit its sale to China. Although there have been certain achievements in China, polysilazane resin coatings and the like are still monopolized by foreign countries, and most applications are concentrated in the field of ceramic precursors.

6. Future research hotspots, indicating development directions
In the future, exploring mature synthesis methods for high-molecular-weight polysilazane, enhancing research on heat resistance performance, and synthesizing products with specific structures will become research hotspots.