The crystal structure of silicon nitride
Si3N4 has two crystal structures: α-Si3N4 is a granular crystal, and β-Si3N4 is a needle-shaped crystal. Both are three-dimensional spatial networks composed of [SN4] tetrahedra sharing vertices, and both belong to the hexagonal crystal system. Their difference lies in the arrangement order of the [SiN4] tetrahedral layers. The β phase is composed of almost completely symmetrical six [SN4] tetrahedrons composed of hexagonal ring layers superimposed in the c-axis direction; while the α phase is composed of two deformed and different non-hexagonal ring layers superimposed. The α phase can dissolve oxygen in the crystal structure, and its internal strain is larger than that of the β phase, so the free energy is higher than that of the β phase. From a thermodynamic point of view, the beta phase is more stable at higher temperatures. The α phase has low symmetry and is easy to form. At about 1500°C, the α phase undergoes a reconstructive 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 transformation of α phase to β phase. α-Si3N4 is formed when it is lower than 1350℃, and β-Si3N4 can be directly prepared at a temperature higher than 1500℃.
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%. There is a strong covalent bond between Si and N (in which the ionic bond only accounts for 30%), so Si3N4 has high hardness (Mohs hardness 9), high melting point and stable structure.
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 up to 1850°C in a neutral or reducing atmosphere.
Silicon nitride is stable to most metal solutions, not corroded or wetted, such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag, etc. But for Cu solution, it is not corroded only in vacuum or inert atmosphere; Mg can weakly react with Si3N4; silicon solution can wet Si3N4 and erode it slightly; transition element solution can strongly wet Si3N4 and form silicide with Si And quickly decompose silicon nitride, while escaping N2. Si3N4 is very stable to alloy solutions such as brass, hard aluminum, nickel-silver, etc., and has good corrosion resistance to cast iron, medium carbon steel, etc., but is not resistant to corrosion of nickel-chromium alloys and stainless steel.
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