Preparation and application of three typical nitride ceramics

Nitride ceramic materials have been widely used in aerospace, mechanical, electronic and national defense fields due to their high hardness, high melting point, high thermal conductivity, good chemical stability and excellent electromagnetic optical properties. The preparation methods of the nitride ceramics include direct nitriding, solid phase sintering, CVD, precursor impregnation-cracking, and self-propagating high-temperature synthesis. Among them, direct nitriding method, solid phase sintering method and self-propagating high-temperature synthesis method are mainly used for preparing nitride powder, which requires high temperature; CVD method and precursor impregnation-cracking method are mainly used for preparing nitride coating and compounding. Materials require pre-design and synthesis of precursor molecules. The equipment is complex, difficult to synthesize, expensive, and requires a long period of time to produce a product with satisfactory performance.

Take three typical nitride ceramic materials as examples to see their preparation and application.

01 silicon nitride ceramic

Silicon nitride (Si3N4) ceramic is a kind of engineering ceramic which develops faster in non-oxide ceramics. The [SiN4] tetrahedral structural unit is formed by covalent bonding between silicon and nitrogen, which makes the ceramic have high strength, high hardness and excellent Anti-oxidation and corrosion resistance. These excellent properties are largely determined by the properties of the raw material powder, so the preparation of silicon nitride powder having a high purity, a high alpha phase content, and a narrow particle size distribution is essential. The synthesis methods of silicon nitride powder mainly include direct nitridation method, carbothermal reduction method, silicon imide decomposition method, plasma method, etc., wherein the direct nitridation method is simple in process, non-toxic by-products and products in the production process. Good repeatability is the most commonly used method in industrial production.

Silicon nitride has two crystal forms, namely fine-grained α-Si3N4 and needle-shaped β-Si3N4. The α-Si3N4 fine particles in the green body can be converted into needle-shaped β-Si3N4 at the sintering temperature, which acts as a self-toughening effect. Therefore, the silicon nitride ceramic has higher strength and toughness than the silicon carbide ceramic, and is more suitable. Preparation of ceramic tools, bearings and other ceramic products requiring high strength and high toughness. Silicon nitride ceramic tools are usually hot pressed to reduce the firing temperature, suppress grain growth, and increase the degree of densification. Silicon nitride ceramic bearing materials not only require high strength and toughness, but also have high requirements for grain size and densification. For this reason, the two-sintering process is often used, that is, the gas is first sintered to a relative density of 90% or more, so that the surface pores are substantially closed, and then subjected to hot isostatic pressing to have a relative density of 99.8% or more.

02 boron nitride ceramics

Boron nitride (BN) has two typical crystal structures, namely hexagonal boron nitride and cubic boron nitride. High-purity boron nitride powder is the premise for the preparation of high-performance boron nitride ceramics. The main preparation methods include precursor method, hydrothermal method, chemical vapor phase synthesis method and high temperature self-propagation synthesis method. However, these synthetic methods have some shortcomings. For example, the boron nitride powder prepared by the precursor method has a low density, and the hydrothermal method has a low yield. The chemical vapor phase synthesis method requires precise control of the process parameters and the recovery of by-products is difficult. Wait. Therefore, the industrial synthesis of low-cost high-purity boron nitride powder is still a technical bottleneck for the wider application of boron nitride ceramics.

At present, hexagonal boron nitride ceramics are mainly used as high-temperature insulating materials, lubricating materials (such as mold release agents for hot-pressing molds), and crucibles for melting metals. Cubic boron nitride ceramics are mainly used for making cutting tools, grinding tools and grinding or Polished material.

03 aluminum nitride ceramics

Aluminum nitride (AlN) ceramic is a ceramic material with high thermal conductivity (thermal conductivity exceeding 200W·m-1·K-1) but insulated. It has high strength, high hardness and high temperature resistance, so it is used for high power. Thermal substrates for integrated circuits and electronic components have unique advantages. In order to obtain high thermal conductivity aluminum nitride ceramics, firstly, high purity and low oxygen content aluminum nitride powder should be synthesized; secondly, the amount of sintering aid should be reduced as much as possible to reduce the grain boundary phase of heat conduction resistance; The sintering of the aluminum nitride is complete, the grain size is uniform, and the structure is dense. Therefore, the use of high purity, stable performance, fine particle size and narrow particle size distribution of aluminum nitride powder and a small amount of sintering aid (nano-sized cerium oxide), combined with hot pressing sintering process is ideal for the preparation of high thermal conductivity aluminum nitride ceramics Technical solutions.

At present, methods for synthesizing aluminum nitride powder include aluminum powder direct nitriding method, carbothermal reduction method, gas phase synthesis method, and sol-gel method. Among them, the direct nitriding method of aluminum powder has the advantages of abundant raw materials and simple process, and has been applied to industrial production at present, but since the aluminum powder nitriding is a strong exothermic reaction, the process is difficult to control, and it is difficult to synthesize high-purity aluminum nitride powder. body. The carbothermal reduction method can synthesize aluminum nitride powder with high purity and stable performance, but at the same time, it has high reaction temperature, long synthesis time, and needs secondary carbon removal. The gas phase synthesis method can synthesize high-purity, uniform particle size and fine aluminum nitride powder in a short time, and has controllability and continuous operation, but the production efficiency is not high, and the raw materials used are high-priced organic aluminum. Therefore, this method is mostly used for experimental research.