Characteristics and application range of graphite materials
Carbon materials are inorganic non-metallic materials mainly composed of carbon. Carbon materials are mainly composed of non graphite carbon materials, while graphite materials are mainly composed of graphitic carbon materials. Graphite can be divided into natural graphite and artificial graphite.
Graphite material is mainly composed of polycrystalline graphite. Graphite is a kind of layered crystal with hexagonal plane grid between carbon atoms. Although graphite belongs to inorganic nonmetal material, it is called semi metal because of its good thermoelectric conductivity. Graphite has higher thermoelectric conductivity than some metals, and has much lower coefficient of thermal expansion than metal and high chemical stability, which makes it has important value in engineering application 。 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 kind of high temperature resistant material. At high temperature, graphite will not melt, and the gasification temperature is very high. Only 335018 begins to sublimate into gas under normal pressure. The strength of general materials decreases gradually at high temperature, while the strength of graphite increases with the increase of temperature when the temperature is less than 2 500'c. the strength of graphite is twice as high as that at room temperature above 2000 ℃. Graphite also has excellent thermal shock resistance, so graphite has its unique advantages as a high temperature material.
Due to its high strength at high temperature, conductive heat transfer, thermal shock resistance, corrosion resistance and good lubricity, graphite has become an indispensable structural material, high temperature material, conductive material, anti-wear material and functional material in the development of national economy. At present, graphite materials have been widely used in metallurgy, chemical industry, electronics, electrical appliances, machinery, nuclear energy, aerospace industry and other departments, can be used as electrodes, electrolytic anodes, casting molds and high temperature bearings; as neutron deceleration materials and surface coating of nuclear fuel in nuclear reactors; in aerospace field, graphite materials It can be used in satellite antenna, shell of space shuttle and throat liner of rocket generator nozzle.
The physical and chemical changes of graphite materials at high temperature and its application characteristics at high temperature are as follows
Graphite is a kind of corrosion resistant material because of its stable chemical properties. However, under certain conditions, carbon can also interact with other substances. The main reactions are: oxidation with oxidizing atmosphere or strong oxidizing acid at high temperature; melting into metal and forming carbide at high temperature; forming graphite intercalation compound.
At room temperature, there is no chemical reaction between carbon and various gases. At about 350 ℃, the oxidation reaction of amorphous carbon is obvious, and the oxidation reaction of graphite begins at about 450 ℃. The higher the degree of graphitization, the more complete the crystal structure of graphite, the greater the reaction activation energy and the better oxidation resistance. When the temperature reaches the same oxidation rate within 800 ~ C, the graphite material is about 50 ~ 100 ℃ higher than that of carbon material. In the same material, the binder carbon has the tendency of preferential oxidation, so the oxidation reaction proceeds to a certain extent The aggregate particles will fall off. At a lower temperature, if the air supply is sufficient, the reaction between carbon and graphite is mainly as follows: C + O 2-C O 2 at a higher temperature, the reaction between carbon and graphite begins again: C + 1 / Zo 2-C O red hot carbon and graphite materials start to react with water vapor at about 700 ℃: C + H The red hot carbon and graphite materials of 2O-C O + H 2C + 2 h 2O-C 0 2 + 2 h 2 and C o. The oxidation reaction of C + c o 2-2 C 0 carbon and gas should be gas-solid reaction. The rate of oxidation reaction is related to the size of reaction area, the porosity of material and the gas pressure. The reaction rate depends not only on the chemical reaction rate on the surface, but also on the diffusion of gas molecules into the material Off. If the porosity of the material is high, especially when the open porosity is large, the gas molecules are easy to diffuse into the material, the surface area involved in the reaction is large, and the oxidation rate is fast. When the service temperature is low, the oxidation reaction rate is not high, and the gas molecules have enough time to diffuse into the material The stomatal structure and reactivity were related. When the temperature is higher than 800 ~ C, the chemical reaction rate is fast, but the diffusion of gas molecules into the pores of the material is slowed down due to the thermal movement. The oxidation reaction only takes place on the surface. The oxidation rate is controlled by the surface air flow velocity and has little relationship with the material types. The impurities in graphite materials catalyze the oxidation reaction, so the oxidation of high purity graphite is superior to that of ordinary graphite. Differences. (b) carbides