High-quality powder is the foundation for producing high-performance silicon nitride ceramics, and low-cost powder preparation technology is key to the widespread application of silicon nitride ceramics. Since the 1930s, the quality of silicon nitride powder has been significantly improved through the design of reaction systems and the adoption of enhanced heat or mass transfer methods. The main preparation methods include direct nitridation of silicon powder, self-propagating high-temperature synthesis (SHS), carbothermal reduction, thermal decomposition, sol-gel, and chemical vapor deposition. Currently, the commercial production methods for silicon nitride powder on the market primarily include:
Direct Nitridation of Silicon Powder:
Direct nitridation of silicon powder is the earliest method used to produce silicon nitride powder. At present, this process is relatively mature in industrial production, enables large-scale manufacturing, and has relatively low production costs, making it the most widely adopted method by domestic and international enterprises. Major international powder manufacturers such as ALZ (Germany), H.C. Starck (Germany), VESTA (Sweden), and Denka (Japan) use this method for mass production of silicon nitride powder. However, the powder produced by this method tends to contain impurity elements such as Fe, Ca, and Al, along with relatively high β-phase and oxygen contents, which are detrimental to the preparation of high-performance silicon nitride ceramics.
Self-Propagating High-Temperature Synthesis (SHS):
Self-propagating high-temperature synthesis, also known as SHS, is an emerging preparation method in recent years. This method offers advantages such as high efficiency, energy savings, and environmental friendliness. However, due to the extremely fast reaction rate during powder preparation, the reaction process is difficult to control.
Carbothermal Reduction:
The carbothermal reduction method uses low-cost raw materials and produces powder with fine particle size, fast reaction rate, and high α-Si₃N₄ content, making it suitable for large-scale production. However, the silicon nitride powder prepared by this method often contains residual carbon or silicon carbide, resulting in low purity and affecting product quality and application. Carbothermal nitridation has been successfully commercialized.
Silicon Diimide Thermal Decomposition:
The liquid-phase reaction method (also known as silicon diimide chemical decomposition) produces silicon nitride powder with an extremely high α-phase content and excellent sintering activity. However, this method is difficult to implement, has high technical barriers, and requires high-purity raw materials. The challenge lies in obtaining stable solid-state silicon diimide (Si(NH)₂). Currently, this method has become the most important approach for commercial production of high-purity, high-quality silicon nitride powder. Japan's UBE Corporation was the first manufacturer to scale up this method to produce silicon nitride powder products with excellent performance and stable quality.