Silicon nitride ceramics are widely used in aerospace, machinery industry, electronic power, armor, chemical industry and other fields because of their excellent performance in strength, wear resistance and corrosion resistance. Moreover, because of their excellent dielectric properties, silicon nitride ceramics are often studied as a new type of transmission material for hypersonic aircraft. Compared with traditional structural materials, silicon nitride ceramics still have excellent physical properties at high temperatures, and its excellent properties have become a research hotspot of many scholars. Preparation and Research Development of Silicon Nitride Ceramic Materials
Up to now, silicon nitride ceramics have been documented only for more than a hundred years. Since Deville et al. successfully synthesized silicon nitride for the first time in 1859, the preparation and properties of silicon nitride ceramics have become the focus of many research groups. The properties of solid silicon nitride ceramics with stoichiometric ratios of silicon and nitrogen closest to 3:4 were measured and evaluated in detail in 1910 and 1926 respectively.
Silicon nitride and silicon carbide are structural ceramics with similar chemical properties and preparation methods. Literature reports on their applications first appeared in the 1950s. Silicon nitride was initially used as a bond between silicon carbide and oxide for refractories, and then used for crucibles, thermocouples, rocket nozzles and so on. In the 1960s, with the increasing demand for new materials with excellent high temperature performance (especially thermal shock performance), silicon nitride ceramics, as a potential application material for high temperature structural engines, has been developing rapidly. Since the research and development of silicon nitride ceramic gas turbine project in 1970s, the development of silicon nitride ceramic materials has been mainly in the ceramic and electronic industries, and has been expanding to other application fields. Its research direction includes five basic branches: basic physical properties of silicon nitride, amorphous film silicon nitride, powder preparation, porous nitriding.
Basic physical properties of silicon nitride ceramic materials
In 1994, Popper published a paper on the application of silicon nitride ceramic materials, summarizing all the published literatures of silicon nitride ceramic materials from 1967 to 1992, including theoretical research and measurement evaluation of the basic properties of silicon nitride ceramic materials by different scholars. According to this literature, Krstic et al. listed the basic physical properties of silicon nitride ceramic materials, which can be seen in table 1-1. Silicon nitride ceramic materials have obvious advantages over other ceramic materials: the material has good thermal shock resistance so that it shows excellent high temperature resistance and chemical inertia to molten metal; Silicon nitride is a high hardness material, so it has high wear resistance and high fracture toughness in ceramic materials. Thin Film Silicon Nitride
Because of its excellent physical properties and chemical inertia, silicon nitride thin films are often used as passivation, insulation and dielectric materials in semiconductor, microelectronics and microelectromechanical systems. At present, the preparation methods of silicon nitride films can be divided into two categories: chemical vapor deposition (CVD) and physical vapor deposition (PVD).
LPCVD method and PECVD method are often used to prepare silicon nitride thin films. LPCVD method improved from APCVD technology can efficiently deposit uniform and low hydrogen concentration silicon nitride thin films at low pressure (10-10³Pa). Its disadvantage is higher deposition temperature: PECVD method with lower temperature requirement is flexible and sensitive to reactive gases. Silicon nitride thin films prepared by this method have high hydrogen concentration, although they have great advantages in terms of requirements and deposition rate. Silicon nitride thin films have wide application prospects, and the improvement of its preparation technology has become the main direction of research on silicon nitride thin films.
Preparation of Silicon Nitride Powder
The preparation process of silicon nitride powder determines the intrinsic properties and impurities (microcrystalline size, alpha phase content, oxygen content, etc.) of the raw materials, thus affecting the densification behavior of silicon nitride ceramic materials in sintering process. With the research of scholars, it has been found that impurities have a significant effect on the high temperature mechanical properties of nitrogen-impregnated ceramics.
The requirement for purity of silicon nitride powder for sintering is increasing. There are three common preparation methods of commercial silicon nitride powder, which are silicon nitriding, carbothermal reduction of silicon dioxide in nitrogen atmosphere and high temperature decomposition of silicon diimide.
Porous Silicon Nitride Ceramic Materials
Porous silicon nitride is a new type of ceramic material, which combines the advantages of traditional silicon nitride ceramics and porous ceramics. It has excellent physical properties and uniform permeability. It can be used as carrier materials for wave transmission, filtration, catalyst and Bioceramics in aerospace, environment, chemical industry, biology and other fields. The preparation methods of porous silicon nitride ceramics can be divided into two categories according to whether pore-forming agents are available or not: pore-forming agent method and sintering method.
Xu et al. have successfully prepared porous silicon nitride ceramics with high strength and more than 50% porosity by pore-forming agent method. The commonly used pore-forming agents are starch, organic whiskers, porous polymers and organic solvents used in freeze-drying process. The main sintering methods include pressureless sintering, tape casting, reaction sintering, gel casting, sintering, forging and carbothermal reduction. Carbothermal reduction has become a research hotspot in the fabrication of porous silicon nitride ceramics due to its high efficiency and high porosity. Dense Silicon Nitride Ceramic Materials
Two conditions must be satisfied when sintering ceramics: 1) the atomic diffusivity must be high; 2) the free energy of the newly formed grain boundary phase should be low enough to make the total energy of the system decrease. As a covalent compound, the self-diffusion coefficient of silicon nitride is low, and the activation energy of self-diffusion is very high. Moreover, the grain boundary energy of pure silicon nitride is high, even if sintered at high temperature for a long time, its densification is difficult. Experiments show that pure silicon nitride powder is easy to form pore during normal sintering process (hot pressing sintering, temperature: 1750 ~1900℃). Sintering aids are often needed to reduce the boundary energy of silicon nitride and increase its diffusivity, so as to improve the densification degree of materials. The sintering aids used in the early preparation of dense silicon nitride ceramics are mostly metal nitrides and metal oxides, such as Mg₃N₂.
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