Crystal structure of Silicon Nitride Si3N4 has two kinds of crystal structure: α-Si3N4 is granular crystal and β-Si3N4 is needle crystal. Both of them are three-dimensional networks composed of common vertices of [SN4] tetrahedron and belong to hexagonal system. The difference between them lies in the order of [SiN4] tetrahedral layers. The β phase is formed by the overlap of hexagonal ring layers composed of six almost completely symmetrical [SN4] tetrahedrons in the c-axis direction, while the α phase is formed by two layers of tangible transformation and different non-hexagonal ring layers. α phase can dissolve oxygen in the range of crystal structure, and the internal strain of α phase is larger than that of β phase, so the free energy is higher than that of β phase. From the point of view of thermodynamics, the β phase is more stable at higher temperature. The α phase has low symmetry and is easy to form. At the temperature of about 1500 ℃, the α phase undergoes a reconstruction transformation and is transformed into β phase. This transformation is irreversible, and the existence of some process conditions and quality is more beneficial to the transformation from α phase to β phase. α-Si3N4 is formed when the temperature is lower than 1350 ℃, and β-Si3N4 can be directly prepared at a temperature higher than 1500 ℃.
Silicon Nitrideproperties The molecular formula of silicon nitride is Si3N4, of which Si accounts for 60.06% and dint N accounts for 39.94%. There is a strong covalent bond between Si and N (of which the ion bond accounts for only 30%), so the Si3N4 has high hardness (Morse hardness 9), high melting point and stable structure.
The Si-N in silicon nitride crystal is mainly covalent bond, and the bonding strength is high, so it has a large elastic modulus (4.7 × 105kg/cm2). The coefficient of thermal expansion is low, but the thermal conductivity is large, so this material is not easy to produce thermal stress, so it has good thermal shock resistance and good thermal shock resistance. It has toughness, high mechanical strength at high temperature and small deformation at high temperature. (the high temperature deformation of silicon nitride ceramic with 2.5g/cm3 density is 0.5% at 1200 × 1000h ℃ and 23 × 7kg/cm2 load). Not eroded by many metals, due to the formation of a silicon dioxide layer, good oxidation resistance and good electrical insulation.
Silicon nitride has no melting point and sublimates and decomposes at 1900 ℃ under atmospheric pressure. The specific heat is 711.8J/ kg ℃. The microhardness of α phase and β phase is 10~16GPa and 24.5~32.65GPa respectively. Because it is a strong covalent bond compound, no liquid phase will be formed below its decomposition temperature (about 1900 ℃), so silicon nitride materials can be sintered with the help of oxide additives. The oxide materials that promote sintering are mainly Y2O3, Al2O3, etc., and the high addition amount can reach 20%. The reaction principle is that the SiO2 oxide film formed on the surface of silicon nitride particles reacts with the added oxide to form liquid phase and permeate at the grain boundary to ensure high diffusion ability during material migration.
Chemical Stability of Silicon Nitride Si3N4 is a thermodynamically stable compound. Silicon nitride ceramics can be used up to 1400 ℃ in oxidation atmosphere and 1850 ℃ in neutral or reducing atmosphere. In fact, the oxidation reaction of Si3N4 occurs above 800C:
Si3N4+3O2=3SiO2+N2.
The weight of the sample increased and a dense silica protective layer was gradually formed on the surface, which prevented the further oxidation of Si3N4. The weight gain was not obvious until the temperature was above 1600 ℃. But in the humid atmosphere, Si3N4 is very easy to oxidize, and the surface begins to oxidize at 200 ℃, which is about twice as fast as that in dry air. The oxidation activation energy of Si3N4 powder in water vapor is obviously lower than that in oxygen and air. The reason is that water vapor can react with Si3N4 through amorphous SiO2 films:
Si3N4+6H2O=3SiO2+NH3.
Silicon nitride is stable to most metal solutions, free from corrosion and infiltration, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag and so on. However, for Cu solution, it is not eroded only in vacuum or inert atmosphere; Mg can react weakly with Si3N4; silicon solution can wet Si3N4 and erode slightly; transition element solution can strongly wet Si3N4 and form silicide with Si to quickly decompose silicon nitride and escape N2 at the same time. Si3N4 is stable to alloy solutions such as brass, hard aluminum and nickel silver, and has good corrosion resistance to cast iron and medium carbon steel, but it is not resistant to Ni-Cr alloy and stainless steel.
Except for molten NaOH and HF, silicon nitride has good chemical corrosion resistance. But most of the molten alkali and salt can interact with Si3N4 to decompose it.
Silicon Nitride used in Refractories. 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. Due to the strong covalent bond and low diffusion coefficient at high temperature, the manufacture of Si3N4 ceramics must rely on high temperature, high pressure and sintering agent, and the cost is too high, otherwise it is difficult to produce high quality silicon nitride materials. These limitations of production costs and equipment are difficult to accept in the metallurgical industry, so the research in the field of refractories starts relatively late, and the research is not in-depth. Many theories come from ceramics, but there is not much innovation. In the past, silicon nitride generally existed in the form of bonding phase in refractories. Through nitriding and firing of metal Si, aggregates such as corundum or silicon carbide were combined with fine powder, so as to achieve the purpose of combining refractory materials. Ceramic shed plate is silicon carbide aggregate and part of fine powder. Silicon nitride is formed by nitriding metal Si to form silicon nitride. Silicon carbide is combined to form silicon nitride bonded silicon carbide material, which is used in 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 plate, the high temperature performance of the material is very good, which solves the problem of bulging failure caused by the oxidation of silicon carbide when the clay bonded silicon carbide shed plate is used.
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