Isostatic Graphite
Introduction of Isostatic Graphite
Isostatic graphite is a new type of graphite material developed in the 1960s. With series of excellent properties, isostatic graphite gets more attention in many fields. Under inert atmosphere, isostatic graphite’s mechanical strength will not weaker with the temperature rising, but will become stronger reaching the strongest value at about 2500℃. So its heat resistance is very good. Compared with ordinary graphite, more advantages it owns, such as the fine and compact structure, good uniformity, low thermal expansion coefficient, excellent thermal shock resistance,strong chemical resistance, good thermal and electrical conductivity and excellent mechanical processing performance.
The Process of Isostatic Graphite
Different from ordinary extrusion molding and compression molding, isostatic graphite is formed by cold isostatic pressing technology. The raw material of pressed powder is filled into a rubber mold, and the pressed powder is compacted through high-frequency electromagnetic vibration. After sealing, vacuum is performed to exhaust the air between the powder particles and put into a high-pressure container filled with liquid medium such as water or oil, then press into cylindrical or rectangular shape. According to Pascal’s principle, pressure is applied to the rubber mold through a liquid medium such as water, and the pressure in all directions is equal. In this way, the compressed powder particles are not oriented in the filling direction in the mold, but are compressed in an irregular arrangement. Therefore, although graphite is anisotropic in crystallographic properties, on the whole, isostatic graphite is isotropic.
Isotropic Graphite Application
● Solar Cells and Semicoductor Wafers
In the solar energy and semiconductor industries, a large amount of isostatic graphite is used to produce graphite parts for the thermal field of single crystal Czochralski furnaces, heaters for polysilicon melting and casting furnaces, heaters for compound semiconductor manufacturing, crucibles and other parts. In recent years, solar photovoltaic power generation has developed rapidly, and the production of monocrystalline silicon and polycrystalline silicon in the photovoltaic industry has a huge demand for graphite. At present, monocrystalline and polycrystalline silicon products are developing towards large-scale and high-end products, and there are higher requirements for isostatic graphite, namely: larger specifications, higher strength, and higher purity.
● Nuclear Graphite
Isostatic graphite has medium mechanical properties, excellent high-temperature mechanical properties, high thermal conductivity and low linear expansion coefficient. In high temperature gas cooled reactor, it is mainly used as reflector, moderator and active zone structural material to form nuclear fuel assembly together with nuclear fuel. At the temperature of 400 ~ 1200 ℃, it is subject to high energy γ The radiation of X-ray and fast neutron for several years, which is easy to cause radiation damage and change the structure and properties of graphite. Therefore, the material is required to have high graphitization, good isotropy, uniform composition and low elastic modulus. At present, China can only produce a small amount of nuclear graphite for high temperature gas cooled reactor, which mainly depends on import.
● EDM
Graphite has no melting point. It is a good conductor of electricity and has good thermal shock resistance. It is an excellent electrode material for EDM. Ordinary graphite material, which is low-density anisotropic graphite with coarse particle structure, can not meet the demand of EDM, while isostatic pressing graphite electrode has uniform structure, dense and high machining precision, which can meet these requirements.
