Silicon nitride ceramics are regarded as structural ceramic materials with the best comprehensive performance because they not only have a high melting point, high hardness, and wear resistance, but also a high bending strength and good thermal conductivity. In crucial fields such as national defense, the military industry, and electronic information, silicon nitride ceramics hold an irreplaceable position.
Powders of High Performance Silicon Nitride Ceramics
Powder must not only be highly pure but also meet the indicators of low oxygen, ultrafine, and high alpha phase in order to prepare ceramic substrates and high-performance bearing balls with high bending strength and high thermal conductivity. The defects (lattice oxygen, pores), impurities, and grain boundary sizes in the substrate and ceramic balls will be directly determined by these indicators, which will affect thermal conductivity and flexural strength.
When the particle size of the powder is 0.4-1.5um, the specific surface area is 4.0-9.0m2/g, and according to the mass percentage, when the alpha phase content is >95%, the O content is 0.2%-0.95%, the C content is 0.2%, and the F content is 0.003%, Cl content is 0.01%, and other metal impurities such as Ca+Fe+Al 0.05%, it has high sintering activity. For commercial applications, 90W/(m.K) of thermal conductivity and 600MPa of flexural strength are the minimum requirements.
Preparation Methods of Silicon Nitride Ceramic Powder
Typically, Si3N4 powders produced by various methods have distinct specific surface areas, which also have an effect on the final properties of Si3N4 ceramics.
Silicon Powder Nitriding Method
This technique relies on the reaction of silicon powder with nitrogen-containing gases like nitrogen and ammonia at high temperatures to produce silicon nitride. This method, which is the most popular and developed for producing silicon nitride powder in large quantities, has the benefits of good product stability and low cost. This process is used by well-known powder producers around the world, including VESTA in Sweden and ALZ in Germany, to mass-produce silicon nitride powder.
Carbothermal Reduction
High-purity carbon powder(coke or charcoal) and quartz powder with a specific purity are used as the method's primary raw materials. The raw materials are evenly mixed before being placed in the reaction furnace and heated to 1400°C for reaction with nitrogen or ammonia gas. Carbon undergoes a reduction reaction to produce elemental silicon, which is then transformed into silicon nitride by reacting with nitrogen or ammonia.
The method of carbothermal reduction has the advantage of producing micropowder with small particle size, high purity, and significant amounts of alpha phase. Comparing the reaction process to direct nitriding, the reaction is easier, quicker, and more effective. The drawback of this method is that silicon dioxide is difficult to completely reduce and nitride, and the residual silicon dioxide has a significant impact on the performance of ceramics at high temperatures.
Liquid Phase Reaction Method
The liquid phase reaction method is also known as the silicon imine chemical decomposition method. In order to create high-purity imino silicon and amino silicon, which are then thermally decomposed at a specific high temperature to produce amorphous silicon nitride, silicon tetrachloride is reacted with a specific amount of anhydrous ammonia in zero-degree dry ethane. Further heat treatment transforms amorphous silicon nitride into stable alpha-phase silicon nitride.
The primary advantages of this process are the rapid production rate, intense chemical reaction, and ability to produce high-purity silicon nitride powder. However, this method is difficult to prepare, the technical threshold is high, and the raw materials must be very pure. The issue is that stable solid imino silicon (Si(NH)2) is challenging to obtain. The first and only manufacturer to use this technique to mass produce silicon nitride powder products with excellent performance and consistent quality is Japan UBE Company. The liquid phase reaction method is used by the Japan UBE company to create silicon nitride powder with fine grain, high purity, high alpha phase content, and good sintering activity. It is an essential choice for precision ceramic bearing balls with high strength and hardness, as well as high-end structural parts.
High Temperature Self-Propagating Synthesis
In recent years, a novel technique for producing high-temperature inorganic compound materials has emerged: self-propagating high-temperature synthesis. The approach is to rely on external energy to ignite metal silicon powder. Due to the exothermic nature of the reaction between silicon and nitrogen, as soon as the reactant is lit, it will automatically spread to the unreacted region until the reaction is finished. Fast reaction times, high powder purity, and low costs are all advantages of this method, but the preparation process is difficult to control and the silicon nitride powder has low alpha phase content.