High Temperature Performance of Graphite Materials

High Temperature Performance of Graphite Materials

The graphite material is mainly composed of polycrystalline graphite. Graphite is a layered crystal with a hexagonal planar grid between carbon atoms. Although graphite is an inorganic non-metallic material, it is called a semi-metal because of its good thermal and electrical conductivity. Graphite has higher thermal and electrical conductivity than some metals, and has a much lower thermal expansion coefficient than metals. chemical stability, which makes it of great value in engineering applications. Graphite is chemically inert in non-oxidizing medium and has good corrosion resistance. Except for strong acid and strong oxidizing medium, graphite is not corroded by other acid and alkali salts, and does not react with any organic compounds.

Graphite material is also a high temperature resistant material. Graphite will not melt at high temperature, and the gasification temperature is very high, but 335018 begins to sublime into gas under normal pressure. Generally, the strength of the material decreases gradually at high temperature, while graphite is within 2500°C, and its strength increases with the increase of temperature. Above 2000°C, its strength is doubled compared with the normal temperature strength. The graphite material also has excellent heat resistance. Therefore, graphite material has its unique advantages as a high temperature material.

Because graphite material has the advantages of high temperature strength, electrical conductivity and heat transfer, thermal shock resistance, corrosion resistance and good lubricity, it has become an indispensable structural material, high temperature material, conductive material, anti-wear material and functional material. At present, graphite materials have been widely used in metallurgy, chemical industry, electronics, electrical appliances, machinery, nuclear energy and aerospace industries, etc., and can be used as electrodes, electrolytic anodes, casting molds and high-temperature bearings; in nuclear reactors as neutron deceleration materials and the surface of nuclear fuel Coating; in the aerospace field, graphite materials can be used in components such as satellite antennas, space shuttle casings, and rocket generator nozzle throat linings.

The physical and chemical changes of graphite materials at high temperature and the characteristics of high temperature use:

The chemical properties of graphite material are stable, so it is a corrosion-resistant material. However, under certain conditions, carbon will also interact with other substances, and its main reactions are: oxidation in oxidizing atmosphere or strong oxidizing acid at high temperature; melting in metal at high temperature and forming carbide; forming graphite interlayer compound.

At room temperature, carbon and various gases do not react chemically at around 350 °C, amorphous carbon has obvious oxidation reaction, and graphite also begins to undergo oxidation reaction at around 450 °C. The higher the degree of graphitization, the more complete the crystal structure of graphite, the larger the reaction activation energy, and the better the oxidation resistance. Within 800~C, the temperature of reaching the same oxidation rate, the graphite material is about 50~100℃ higher than the carbon material. In the material, the binder carbon has a tendency to be preferentially oxidized, so when the oxidation reaction progresses to a certain extent, the aggregate particles will fall off. At a lower temperature, if the air supply is sufficient, the carbon and graphite materials mainly undergo the following reactions: C+O2—CO2 At higher temperatures, the carbon and graphite materials begin to react as follows: C+1/ZO2— CO red hot carbon and graphite materials and water vapor start to react at about 700 ℃: C+H2O—CO+H2C+2H2O—CO2+2H2 red hot carbon and graphite materials and CO. The oxidation reaction can only be carried out at a higher temperature: the reaction between C+CO2-2C0 carbon and gas should be a gas-solid reaction, and the oxidation reaction speed is related to the size of the reaction area at that time, the porosity of the material and the gas pressure and other factors. The reaction speed depends not only on the chemical reaction speed of the surface, but also on the diffusion of gas molecules into the material. If the porosity of the material is high, especially when the open porosity is large, the gas molecules can easily diffuse into the material, the surface area involved in the reaction is large, and the oxidation speed is fast. When the operating temperature is low, the oxidation reaction speed is not high, and the gas molecules have enough Time diffuses into the material, and the oxidation reaction rate is related to the pore structure and reactivity of the material. When the temperature is higher than 800~C, the chemical reaction rate is fast, and the diffusion of gas molecules into the pores of the material is slowed down due to thermal motion. The oxidation reaction only occurs on the surface, and the oxidation rate is dominated by the surface airflow velocity.

(b) Generation of Carbides At high temperature, carbon melts into metals such as Fe, A1, Mo, Cr, Ni, Ti and non-metals such as B and Si to form carbides.

(c) Formation of graphitic interlayer compounds The carbon atoms of graphite are firmly connected together by covalent bonds within their layers, and are bound by weaker van der Waals forces between layers. Therefore, by inserting various molecules, atoms, and ions between the layers of graphite without destroying its two-dimensional lattice, only increasing the interlayer spacing, a graphite-specific compound called a graphite interlayer compound can be made to make a graphite layer. Intermediate materials usually use natural flake graphite as raw material. Among the graphite intercalation compounds, flexible graphite has been widely used in industry. Flexible graphite not only has self-lubrication and high temperature resistance, but also has flexibility, flexibility and compression resilience. It can be used as a thermal 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, binders such as boric acid, thermosetting resin, and inorganic colloid are added to flexible graphite. It can be seen that the carbon material integrates heat resistance and electrical conductivity in a non-oxidizing medium, but in an oxidizing environment where the temperature is higher than 627K, the oxidation reaction begins, and as the temperature increases, the The oxidation rate is accelerated, and the structure is therefore corroded, damaged, and affects its use at high temperatures. Therefore, the anti-oxidation protection of graphite materials is receiving extensive attention.