The physical and chemical changes that occur in graphite materials at high temperatures

The physical and chemical changes that occur in graphite materials at high temperatures and their high-temperature usage characteristics:

Graphite materials have stable chemical properties, making them a corrosion-resistant material. But under certain conditions, carbon can also interact with other substances, and its main reactions include: oxidation occurs at high temperatures with oxidizing atmospheres or strong oxidizing acids; Melt in metals at high temperatures and generate carbides; Generate graphite interlayer compounds.

At room temperature, carbon does not undergo chemical reactions with various gases at around 350 ℃. Amorphous carbon undergoes visible oxidation reactions, and graphite also begins to undergo oxidation reactions at around 450 ℃. The higher the degree of graphitization, the more complete the crystal structure of graphite. Its reaction activation energy is high, and its antioxidant performance is good. Within 800~C, when reaching the same oxidation rate temperature, graphite material is about 50~100 ℃ higher than carbon material. In the same material, binder carbon has a tendency to preferentially oxidize. Therefore, when the oxidation reaction reaches a certain degree, the aggregate particles will fall off. At lower temperatures, if air supply is sufficient, carbon and graphite materials mainly undergo the following reactions: C+O2 CO2 At higher temperatures, carbon and graphite materials begin to undergo the following reactions again: C+1/ZO 2-CO The red hot carbon and graphite materials start to react with water vapor at around 700 ℃: C+H 2O CO+H 2C+2 H 2O C 0 2+2 H 2 The red hot carbon and graphite materials react with CO. The oxidation reaction can only be carried out at higher temperatures: the reaction between C+COO2-2C0 carbon and gas should belong to gas-solid reaction, and the oxidation reaction rate is related to factors such as the reaction area, material porosity, and gas pressure at that time. The reaction rate depends not only on the surface chemical reaction rate, but also on the diffusion of gas molecules into the material. If the porosity of the material is high, especially when there are many open pores, gas molecules are easily diffused into the interior of the material, and the surface area participating in the reaction is large, resulting in a fast oxidation rate. When the operating temperature is low, the oxidation reaction rate is not high, and gas molecules have enough time to diffuse into the interior of the material. At this time, the oxidation reaction rate is related to the pore structure and reaction activity of the material. When the temperature is above 800~C, the chemical reaction rate is fast, while the diffusion of gas molecules into the material pores slows down due to thermal motion. The oxidation reaction only occurs on the surface, and the oxidation rate is less dependent on the surface airflow speed and the type of material. The impurities contained in graphite materials play a catalytic role in the oxidation reaction, so the oxidation performance of high-purity graphite is significantly different from that of ordinary graphite. Differences. (b) The formation of carbides occurs at high temperatures when carbon melts into metals such as F e, A 1, M o, C r, N i, T i, and non-metals such as B and Si to form carbides. (c) The generation of graphite interlayer compounds: The carbon atoms of graphite are firmly connected together through covalent bonds within its layer, while they are bound by weaker van der Waals forces between the layers. Therefore, by inserting various molecules, atoms, and ions into the interlayer of graphite without damaging its two-dimensional lattice and only increasing the interlayer spacing, a graphite specific compound called graphite interlayer compound can be produced. The production of graphite interlayer compounds usually uses natural flake graphite as the raw material. Flexible graphite is widely used in industrial applications in graphite interlayer compounds. Flexible graphite not only has self-lubricating and high-temperature resistance, but also flexibility, flexibility, and compression resilience. It can be used as an insulation material for refining furnaces and high-temperature furnaces, and is widely used as a sealing material. In order to improve the oxidation resistance of flexible graphite, adhesives such as boric acid, thermosetting resin, and inorganic colloids are added to flexible graphite. It can be seen that carbon materials integrate heat resistance and conductivity in non oxidizing media. However, in environments where oxidation is dominant, oxidation reactions begin to occur at temperatures above 627K. As the temperature increases, the oxidation rate accelerates, and the structure is corroded, damaged, and its use at high temperatures is affected. So, the issue of antioxidant protection of graphite materials is receiving widespread attention