Graphite properties and applications

    The performance of graphite is closely related to the impurities contained. Natural graphite has a large impurity content, so artificial graphite is generally used. Recently, pyrolytic graphite manufactured by chemical vapor deposition has appeared, and its anisotropy is particularly prominent. There are a large number of pores in graphite, so it can absorb gas, and the shape characteristics (opening, closed) and number of pores also affect the strength and electrical properties.

    Some physical properties of graphite are shown in the table. It can be seen from the table that the melting point of graphite is extremely high, and the thermal conductivity of graphite in the direction parallel to the layer is comparable to copper, but the coefficient of thermal expansion is smaller than that of tungsten. In addition, the strength of graphite increases as the temperature rises until 2400~2600°C, and then decreases sharply. The strength at 2500°C is approximately doubled at room temperature. Graphite is very brittle at room temperature, and its plasticity increases when the temperature rises. Graphite cannot be spot welded, so the graphite electrode can only be fixed by screwing or inserting the screw, shaft, etc. into the small hole on the electrode. Because graphite is loose and porous, it cannot be cleaned in any solvent. It can be heated in the air for a short time to a temperature higher than 1000℃ and roasted to remove impurities, dust and incompletely graphitized charcoal. Then it needs to be in a vacuum furnace. Degassing by medium heating (heating to 1400℃, 20~30min), this is because graphite often adsorbs a large amount of gas, such as H2, N2, CO, CO2 and O2. Graphite has no effect on general chemical solvents, but it can be dissolved in the following mixtures to form colloidal graphitic acid. HNO3(20cm3)+H2SO4(40cm3)+KClO3(0.02kg) can dissolve 0.001kg graphite. This mixture can be used for Clean the graphite layer deposited on the glass bulb. Because graphite has a strong reducibility, it is necessary to avoid contact with glass or ceramics at high temperatures, otherwise the alkali metals and other components in it will be reduced, causing it to be broken down under a strong electric field, and graphite and mercury have no effect.

    Graphite has a high melting point, good thermal conductivity, and small expansion coefficient. It is also a black body material with a large emissivity coefficient, so it is a good material for making anodes and grids in power tubes. Graphite rods can be used as heating elements in vacuum electric furnaces.

    When graphite is used as the grid, the grid power can be increased, the distance between electrodes is small, and the thermionic emission and secondary emission of the grid are low. At present, the pyrolytic graphite grid can be made by sandblasting, and the cross-sectional area of the grid wire can be as small as 0.01mm2. Graphite is also used as the anode of mercury arc rectifiers, gas-filled diodes, thyratrons, and graphite electrodes in vacuum electric furnaces.

    Graphite emulsion is also used in electric vacuum devices. It is prepared by dissolving graphite powder (2~10μm) in an aqueous solution and adding stabilizers such as ammonia. It is mainly used to coat the inner and outer surfaces of the glass bulb as a conductive layer, or Coated on the surface of the electrode to increase the emissivity.

(Huiputech)