Chemical properties of carbon graphite materials:
Carbon graphite materials are chemically stable and therefore a corrosion resistant material. However, under certain conditions, carbon also interacts with other substances. The main reactions are: oxidation at high temperatures with oxidizing gases or in strong oxidizing acids; melting at high temperatures to form metals and forming carbides, Interlayer compound.
A. At normal temperature, carbon does not react with various gases. At about 350 ° C, the amorphous carbon has obvious oxidation reaction, and the graphite begins to oxidize at about 450 ° C. The higher the degree of graphitization, the more complete the crystal structure of graphite, the greater the activation energy and the better the oxidation resistance. At 800 ° C, reaching the same oxidation rate, the graphite material is about 50-100 ° C higher than the carbon material. In the same material, the binder carbon tends to be preferentially oxidized, so that when the oxidation reaction proceeds to a certain extent, the aggregate particles may fall off.
The reaction between carbon and gas belongs to gas-solid reaction. When the reaction rate is within 800 °C, the reaction rate is related to the porosity of the material, and the porosity is large. Especially when there are many open pores, the oxidation rate is fast. When the temperature is higher than 800 ° C, the oxidation reaction only occurs on the surface, and the oxidation rate is governed by the surface gas velocity, which is less related to the material type.
The impurities contained in the carbon graphite material catalyze the oxidation reaction, so the oxidation of high-purity graphite and ordinary graphite is significantly different.
B, the formation of carbide:
At high temperatures, carbon is dissolved in metals such as Fe, Al, Mo, Cr, Ni, V, U, Th, Zr, Ti, and non-metals such as B and Si to form carbides. Carbides formed from carbon and IV, V, and VI elements have good chemical stability, high hardness, and generally have electrical conductivity, and some also exhibit superconductivity. Some carbide solids such as 4TaC+1ZrC or 4TaC+1HfC have a melting point of 4200K, which is the highest melting point known. Carbon and alkali metals, alkaline earth metals, Al and rare earth elements form salt-based carbides, which are generally insulators, most of which are less stable in chemical properties and decompose in water and dilute acids.
