Properties of isostatically pressed graphite
Isotropic
There are three main molding methods for carbon graphite products, namely: hot extrusion molding, such as the production of graphite electrodes for steelmaking; molding (including vibration molding) and for aluminum carbon and electric carbon products; isostatic molding.
Although the forming method is different, the principle of forming is the same. The direction perpendicular to the pressure surface and the direction horizontal to the pressure surface have different properties, which is called “anisotropy”. This “anisotropy” is useful and indispensable for graphite electrodes for steel making and electric brushes for motors. They need this characteristic. However, there are many other applications that do not need “anisotropy” but “isotropy”. Thus, isostatic graphite emerged.
Isostatic forming changes the uni-directional (or bi-directional) pressure of the material to multi-directional (all-round) pressure, and the carbon particles are always in a disordered state (as shown in Figure 1, c). As a result, there is no or little difference in performance of the final product. It is called “isotropic”, and the performance ratio in each direction is no more than 1.1
2. Homogeneity of bulk density
In order to produce fine structured, dense and homogeneous carbon graphite material, powder pressing (not paste) is the only method. The only way to produce fine structured, homogeneous carbon graphite is by powder pressing (not paste). In the use of molding, either single-sided pressing or double-sided pressing, by the friction (between the carbon particles and between the product and the mold), the pressure transfer will gradually reduce, resulting in the bulk density of non-uniform. This difference increases with the height of the product. The specific situation is shown in Figure 2.
The overall density of this blank is not uniform, not only for the future process – baking brings hidden problems, will also cause the blank processing finished parts, bringing individual product performance differences, is very harmful. When using isostatic press molding, the product is uniform in all directions, the bulk density is more uniform, and is not limited by the height of the product.
Main applications of isostatic graphite
Geochemistry and Semiconductor Industry
In this piece of ultra trace analysis, isostatic graphite can be used to make corrosion resistant heating plate, compared with ordinary graphite, the structure is fine and dense, and good uniformity; the coefficient of thermal expansion is very low, with excellent thermal shock resistance; isotropic; strong chemical resistance, good thermal and electrical conductivity; excellent mechanical processing properties. Together with PFA coating, it is resistant to acid and alkali corrosion, high temperature, reduces the wear and tear of graphite and reagent bottles during use, the plate surface is smooth and flat, the entire heating area is stable and extremely uniform, and multiple samples can be processed simultaneously.
It is also suitable for all trace metal laboratories for sample disintegration or evaporation, and is installed for use in clean laboratories and class 100 clean rooms. For example, Figure 3.
2. Direct-drawn monocrystalline silicon hot field and Heater for polysilicon ingot casting furnace
There are about 30 types of isostatically pressed graphite components in the direct-drawn single-crystal silicon hot field, including crucibles, heaters, electrodes, thermal shields, seed crystal holders, bases for rotating crucibles, various round plates, and heat reflective plates. Among them, 80% of isostatically pressed graphite is used for manufacturing crucibles and heaters, etc.
In the manufacturing process of polysilicon wafers for solar cells, first of all, the polysilicon scraps are melted and cast into polysilicon square ingots. The heater of the ingot casting furnace is made of isostatic graphite.
3. Atomic Energy Industry
For nuclear fission reactors ( high temperature gas-cooled reactors), graphite is a moderator of neutrons and an excellent reflector.
Nuclear fusion reactor, graphite material with good thermal conductivity and high mechanical strength is used as wall material facing the plasma.
4. Electrode for discharge processing
Electrical discharge machining with mainly graphite or copper electrodes is widely used in metal molds and other machining fields.
5. Graphite crystallizer for continuous casting of non-ferrous metals
Due to the good performance of isostatic graphite material in heat conduction, thermal stability, self-lubrication, anti-infiltration and chemical inertness, it makes it an irreplaceable material for making crystallizers.
6. Other Uses
Isostatic graphite is also used to make sintering dies for diamond tools and cemented carbide, hot field components for fiber optic wire drawing machines (heater, holding cylinder, etc.), hot field components for vacuum heat treatment furnaces (heater, load frame, etc.), and precision graphite heat exchangers, mechanical seal components, piston rings, bearings, rocket nozzles, etc.