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The crystal structure and chemical properties of silicon nitride

wallpapers Industry 2020-10-15
1. The crystal structure of silicon nitride

Si3N4 has two crystal structures: α-Si3N4 is a fine crystal, β-Si3N4 is a needle crystal. Both of them are three-dimensional networks of [SN4] tetrahedrons sharing vertex angles, and both belong to the hexagonal system. Their difference lies in the arrangement order of the [SiN4] tetrahedral layer. The β phase comprises six [SN4] tetrahedrons almost entirely symmetrical and overlapped in the c-axis direction. In contrast, the α phase is made up of two deformed and different non-hexagonal rings. The α phase can dissolve oxygen in the crystal structure, and the internal strain of the system is larger than that of the β stage, so the free energy is higher than that of the β phase. From a thermodynamic point of view, the β step is more stable at higher temperatures. α has low relative symmetry and is easy to form. At a temperature of about 1500°C, the α phase undergoes reconstruction transformation and transforms into the β phase. This transformation is irreversible, and the existence of certain process conditions and qualities is more conducive to the change from the α phase to the β degree. Α-Si3N4 is formed below 1350°C, and β-Si3N4 can be directly produced at a temperature higher than 1500°C.

2. The basic properties of silicon nitride

The molecular formula of silicon nitride is Si3N4, of which Si accounts for 60.06% and N accounts for 39.94%. Si and N are bonded by a strong covalent bond (the ionic bond is only 30%), so Si3N4 has a high hardness (Mohs hardness 9), a high melting point, and a stable structure.
The covalent bond between Si-N in silicon nitride crystal is the main, and the bonding strength is high, so it has a large elastic modulus (4.7×105kg/cm2). The thermal expansion coefficient is low, and the thermal conductivity is large. This material is not comfortable to generate thermal stress, so it has good thermal shock resistance and good thermal shock resistance. It has toughness, high-temperature mechanical strength, and low-temperature deformation. (Silicon nitride ceramics with a density of 2.5g/cm3 will be deformed to 0.5% at high temperature at 1200×1000h°C and a load of 23×7kg/cm2) Strong corrosion resistance. Many metals do not corrupt it, and due to the formation of a silicon dioxide layer, it has good oxidation resistance and good electrical insulation performance.

Silicon nitride has no melting point, and it decomposes by sublimation at 1900°C under normal pressure with a specific heat of 711.8J/kg·°C. The microhardness of the α phase and β phase are 10~16GPa and 24.5~32.65GPa, respectively. Because it is a potent covalent bond compound, there will be no liquid phase formation below its decomposition temperature (about 1900°C), so the silicon nitride material can only be sintered with the help of oxide additives. The oxide materials that promote sintering mainly include Y2O3, Al2O3, etc. The higher the addition amount is up to 20%, the reaction principle is to use the SiO2 oxide film formed on the surface of the silicon nitride particles and the added oxide to create a liquid phase and penetrate the crystal to ensure the high diffusion ability of the material migration.

3. The chemical stability of silicon nitride

Si3N4 is a thermodynamically stable compound. Silicon nitride ceramics can be used up to 1400°C in an oxidizing atmosphere and can be used up to 1850°C in a neutral or reducing atmosphere. Si3N4 will undergo oxidation reaction above 800℃:

Si3N4+3O2=3SiO2+N2↑(1)

The sample's weight increased, and a dense silicon dioxide protective layer was gradually formed on the surface, which prevented the continued oxidation of Si3N4. It is not until 1600℃ or higher that the weight gain is more prominent. However, in a humid atmosphere, Si3N4 is incredibly easy to oxidize, and the surface starts to deteriorate at 200°C, and the speed is about twice as fast as in dry air. The oxidation activation energy of Si3N4 powder in water vapor is significantly lower than that in oxygen and air. The reason is that water vapor can react with Si3N4 through the amorphous SiO2 film:

Si3N4+6H2O=3SiO2+NH3↑(2)

Silicon nitride is stable to most metal solutions, not corrupted or infiltrated, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag, etc. But for Cu solution, it is not corrupted only in vacuum or inert atmosphere; Mg can react weakly with Si3N4; silicon solution can wet Si3N4 and slightly corrode; transition element solution can enormously wet Si3N4 and form silicide with Si While rapidly decomposing silicon nitride, N2 escapes at the same time. Si3N4 is very stable to alloy solutions such as brass, duralumin, nickel and silver. It has good corrosion resistance to cast iron and medium carbon steel, but it is not resistant to nickel-chromium alloys and stainless steel.

Except for molten NaOH and HF, silicon nitride is resistant to chemical corrosion. But most molten alkalis and salts can interact with Si3N4 to decompose it.
 
Trunnano is one of the world's largest producers of silicon nitride powder. If you have any questions or needs, please contact Dr. Leo by email: brad@ihpa.net.


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