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Silicon nitride is a kind of synthetic refractory material with good abrasion resistance, high-temperature resistance, and corrosion resistance. In the application of refractory materials, it mainly appears in the form of a bonding phase.
Crystal Structure of Silicon Nitride
Si3N4 has two crystal structures: α-Si3N4 is a granular crystal, and β-Si3N4 is a needle-like crystal. Both of them are three-dimensional spatial networks composed of [SN4] tetrahedrons sharing vertex angles, and both belong to a hexagonal crystal system. Their difference lies in the sequence of the [SiN4] tetrahedral layers. The β phase is formed by overlapping hexagonal ring layers of six [SN4] tetrahedrons in the c-axis direction. The α phase is formed by the overlap of two deformed and different non-hexagonal ring layers. The α phase can solubilize oxygen in the crystal structure range, and its internal strain is larger than that of the β phase, so its free energy is higher than that of the β phase. From a thermodynamic point of view, the β phase is more stable at higher temperatures. The α phase has low symmetry and is easy to form. At about 1500℃, the α phase is transformed into the β phase by reconstruction. This transformation is irreversible, and some technological conditions and properties are more favorable to the transformation of the α to β phase. α-Si3N4 is formed at lower than 1350℃ and β-Si3N4 can be prepared directly at higher than 1500℃.
Basic Properties of Silicon Nitride
The molecular formula of silicon nitride is Si3N4, in which Si accounts for 60.06% and N for 39.94%. Si3N4 has a high hardness (Mohs hardness 9), high melting point, and stable structure due to the strong covalent bond between Si and N (ionic bond only accounts for 30%).
In silicon nitride crystal, Si-N is mainly bonded by covalent bonds, and the bonding strength is high, so it has a large elastic modulus (4.7×10^5kg/cm2). The low coefficient of thermal expansion and the large coefficient of thermal conductivity make this material not easy to produce thermal stress, so it has good thermal shock resistance and good thermal shock resistance. Toughness, high mechanical strength at high temperature, small deformation at high temperature. Good resistance to erosion. Not corroded by many metals, due to the formation of silicon dioxide layer, good oxidation resistance, and good electrical insulation performance.
Silicon nitride has no melting point and is sublimated at 1900℃ at atmospheric pressure with a specific heat of 711.8j /kg·℃. The microhardness of α and β phases are 10-16GPa and 24.5-32.65GPa, respectively. Because it is a strong covalently bonded compound, there will be no liquid phase formation below its decomposition temperature (about 1900℃), so the silicon nitride material can be sintered with the help of oxide additives. The oxide materials promoting sintering mainly include Y2O3, Al2O3, and so on, and the addition of a high amount can reach 20%. The reaction principle is to generate a liquid phase through the interaction of SiO2 oxide film formed on the surface of silicon nitride particles and the added oxide and permeate at the grain boundary to ensure the high diffusion ability of material migration.
Chemical Stability of Silicon Nitride
Si3N4 is a thermodynamically stable compound. Silicon nitride ceramics can be used up to 1400℃ in an oxidizing atmosphere and up to 1850℃ in a neutral or reducing atmosphere.
Silicon nitride is stable to most metal solutions and is not corroded or infiltrated, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag, etc. But for Cu solution, is not eroded only in a vacuum or inert atmosphere; Mg can react weakly with Si3N4. Si3N4 can be wetted and slightly eroded by silicon solution. The transition element solution can wet Si3N4 strongly and form a silicide with Si to decompose Si3N4 rapidly and escape N2 at the same time. Si3N4 is very stable to alloy solutions such as brass, duralumin, nickel-silver, etc., and has good corrosion resistance to cast iron, medium carbon steel, etc., but it is not resistant to corrosion of nickel-chromium alloy and stainless steel.
Silicon nitride is resistant to chemical corrosion except for molten NaOH and HF. But most molten bases and salts can interact with Si3N4 to decompose it.
Application of Silicon Nitride in Refractory Materials
Silicon nitride ceramics are called promising high-temperature structural materials because of their excellent high-temperature properties such as high-temperature strength, good wear resistance, and corrosion resistance. The strong covalent bond and low diffusion coefficient at high temperature lead to the high temperature and high pressure and sintering agent for the manufacture of Si3N4 ceramics, the cost is too high, otherwise, it is difficult to produce high-quality silicon nitride materials.
In the past, silicon nitride generally exists in the form of a bonding phase in refractory materials. Through the nitriding firing of metal Si, aggregates such as corundum or silicon carbide are combined with fine powder, to achieve the purpose of bonding refractory materials. Ceramic shelf plate is silicon carbide aggregate and part of the fine powder, with metal Si nitriding to form silicon nitride as the bonding phase, silicon carbide combined to form silicon nitride bonded silicon carbide material, used in the blast furnace body and other parts so that the performance of the material has been greatly improved. Compared with the clay bonded silicon carbide shed board, the high-temperature performance of the material is very good, which solves the problem of swelling and destruction of the clay bonded silicon carbide shed board caused by the oxidation of silicon carbide.
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