silicon carbide structure and chemical properties

 Silicon carbide structure

Silicon carbide is a typical covalently bonded stable compound.Theoretically, silicon carbide is made of SiC tetrahedra, but the difference is only parallel or antiparallel bonding. 75 variants of SiC are available, such as α-SiC, β-SiC, 3C-SiC, 4H-SiC, 15R-SiC, etc. All these structures can be divided into square crystal system, hexagonal crystal system and rhombic crystal system, among which α-SiC and β-SiC are the most common. β-SiC can be transformed into α-SiC at 2100-2400 ℃, and β-SiC can be produced from a simple mixture of silicon and carbon at a temperature of about 1450 ℃. The SiC microstructure can be analyzed and quantified in multiple forms using transmission electron microscopy and X-ray diffraction detection techniques. In order to distinguish between the different structures, nomenclature is required. This nomenclature is incompatible with the phase law convention and mineralogical nomenclature, but is popular because it is convenient

 Silicon carbide Chemical properties

Silicon carbide itself is easy to oxidize, but it oxidizes to form a silicon dioxide film, and the oxidation process is gradually hindered. In air, the oxidation of silicon carbide begins at 800 ℃, but very slowly; as the temperature increases, the oxidation rate accelerates sharply. The rate of oxidation of silicon carbide is 1. 6 times faster in oxygen than in air; the rate of oxidation slows down with time. If the amount of oxidation is plotted over time, a typical parabolic graph is obtained. This reflects the hindering effect of the silica protective layer on the oxidation rate of silicon carbide ceramic. When oxidation, if there are substances that can remove or break the silicon dioxide film, the silicon carbide will be easily further oxidized. For example: iron, manganese and other metals have several compounds, silicon carbide can be oxidized by their oxides , and can generate low melting point compounds with silicon dioxide, can erode silicon carbide ceramic. For example, FeO in 1 300 ℃, MnO in 1 360 ℃ can erode silicon carbide; and CaO, MgO in 1 000 ℃ can erode silicon carbide ceramic.