May 9, 2024

Properties of Silicon Nitride Ceramics

There are two types of silicon nitride (SI3N4), both of which belong to the hexagonal crystal system. Both are three-dimensional spatial networks composed of tetrahedrons sharing vertices. There is only a difference in symmetry (α phase is relatively symmetrical), but there is no distinction between high and low temperature phases. The crystalline phase is the basic composition of special ceramics. The main crystalline phase of each special ceramic material determines the performance of the material. For example, the main crystalline phase in silicon nitride ceramics is Si:N, because the bonding force between nitrogen and silicon atoms in the silicon nitride lattice is very strong. Strong, stable at high temperature, can still maintain high strength before decomposition, and the thermal expansion coefficient of silicon nitride is very small. Therefore, silicon nitride has high hardness, excellent wear resistance, corrosion resistance and thermal stability. an important high temperature structural material.

Silicon nitride ceramics sintered at high temperature are composed of polycrystals aggregated by many crystallites. It is inevitable that the atoms on the grain boundaries cannot be arranged in an orderly manner, which has a transitional nature and a relatively loose structure, so the grain boundaries are atoms. (Ion) is an important channel for rapid diffusion and is the most vulnerable part of ceramics in an acid-base environment.

Ceramic materials are a combination of polycrystals. The junctions between grains of the same composition with different crystallographic directions are called grain boundaries, and the junctions between grains of favorable components are called phase boundaries, which are important microstructures that must be paid attention to. The research object studies the characteristics of the material structure at the grain boundary, which is called the grain boundary structure.

It is a reasonable way to improve the fracture energy and impact toughness of raw materials. It is also beneficial to have appropriate micro-cracks in porcelain components to resist thermal shock damage.

Silicon nitride is a covalent compound, so atoms are bonded to each other with strong covalent bonds, so it has high hardness and melting point. Silicon nitride ceramics, as atomic crystal silicon nitride, have high bond energy and can produce metal oxide protective films in the gas, so it has excellent organic chemical reliability, and its shocking heat resistance and high performance. Toughness, high toughness properties.

Silicon nitride has excellent chemical properties and is resistant to corrosion by all inorganic acids except hydrofluoric acid and sodium hydroxide solution below 25%.

Silicon nitride special ceramic materials also have a glass phase. Due to the formation of a liquid phase during the sintering process, it can reduce the sintering temperature, prevent polycrystalline transformation, inhibit grain growth and promote grain bonding.

Silicon nitride ceramic molten salt corrosion, generally called molten inorganic compound as molten salt, is an ionic melt composed of cations and anions, and has many characteristics that aqueous solutions do not have. Molten salt is an ionic melt with a wide temperature range, large heat capacity and stable chemical properties. It can be widely used in molten salt electrolysis for gold production. However, high temperature molten salt is highly corrosive, and even very stable ceramic materials are easy to use. corroded. In the process of preparing metal aluminum by molten salt electrolysis, ceramic materials, generally silicon nitride or silicon carbide materials, are often used as the inner wall of the reaction vessel, so the corrosion of cryolite molten salt to ceramics has also attracted attention.

Silicon nitride ceramics are chemically stable and resistant to corrosion. They do not react with other inorganic acids except hydrofluoric acid. They do not react with oxygen in a dry atmosphere of 800 °C. When the temperature exceeds 800 °C, a silicon oxide film begins to form on the surface. The silicon oxide film gradually becomes stable as the temperature rises, and a dense silicon oxide film can be formed with oxygen at about 1000°C, which can be kept stable until 1400°C.

Silicon nitride and water have almost no effect; slow hydrolysis reaction in concentrated strong alkali aqueous solution converts into ammonia salt and silicon dioxide; soluble in hydrochloric acid, and dilute alkali failure concentrated strong acid aqueous solution can slowly erode silicon nitride, melting Strong acids can rapidly change silicon nitride to aluminosilicates and ammonia.

There are many types of silicon nitride ceramic products, and their applications are becoming more and more extensive.