Being a sought-after choice in the high-end industry, the strength of high-purity graphite cannot be underestimated. With its excellent self-lubrication properties, high strength, good electrical conductivity, resistance to acid and alkali, corrosion resistance, high temperature resistance, small expansion coefficient, excellent chemical stability, high sealing performance, and uniform and dense structure, it firmly establishes itself in industries such as electrical, metallurgy, national defense and aerospace, energy, and chemical engineering. Moreover, it holds a core position in emerging fields such as the third-generation semiconductors, nuclear industry, and photovoltaics, and can be regarded as a "key piece" in technological development.
Two birth paths: the natural and the artificial "purification path"
High-purity graphite can be classified into natural high-purity graphite and artificial high-purity graphite based on the raw materials and preparation methods. High-purity natural graphite is widely used in various fields due to its excellent electrical conductivity and heat transfer properties, such as in refractory materials, anti-rust coatings, carbon electrodes, generators, lubricants, and protective materials in the nuclear industry. On the other hand, artificial high-purity graphite is applied in pre-baked anodes, special graphite, and the nuclear industry due to its uniform morphology and particle size distribution.
The production of high-purity graphite from natural graphite often involves first using the flotation method to initially purify the graphite ore, then using the alkali-acid method or hydrogen fluoride method to further purify the flotation concentrate, and finally obtaining high-purity graphite products through high-temperature methods or high-temperature chlorination methods. However, artificial high-purity graphite is generally obtained by initially processing the raw materials and then placing them in a specially designed high-temperature reaction furnace for graphitization and purification. The main process is to use the extremely high temperature of the furnace to graphitize petroleum coke and asphalt coke, and introduce halogen gases to react with impurities, causing them to vaporize and volatilize, thereby obtaining high-purity graphite.
Whether it is the production of high-purity natural graphite or high-purity artificial graphite, strict requirements are imposed on the quality of raw materials. To achieve the requirements of high-purity graphite from natural graphite, only the high-temperature method can meet the production needs. However, the high-temperature method has high energy consumption and high requirements for equipment, so it also has certain requirements for the quality of raw materials. Usually, it uses graphite products with a purity of 99% obtained through other purification methods for purification, ultimately reaching the requirement of 99.99% high-purity graphite. For artificial high-purity graphite, high-quality petroleum coke and asphalt coke are often selected for production. The ash content of petroleum coke should be less than 0.5%, and usually only low-sulfur petroleum coke is used for production. Therefore, the sulfur content of petroleum coke is below 0.5%.
"Shining Bright in the High-End Field": The Application of High-Purity Graphite
The industries using high-purity graphite as raw material mainly focus on high-tech fields such as semiconductor materials, photovoltaics, graphene, and nuclear graphite.
Third-generation semiconductor materials
In the semiconductor industry, graphite products are widely used in high-temperature heat treatment equipment such as crystal growth furnaces, carbonization furnaces, and graphitization furnaces. The high-purity graphite products used in the third-generation semiconductor silicon carbide include graphite crucibles for crystal growth, graphite heaters, and graphite powder, etc. The purity of these products plays a decisive role in the quality of silicon carbide crystals.
At present, most enterprises use the physical vapor transport method when producing silicon carbide single crystals. In this method, the silicon carbide powder is placed at the bottom of the graphite crucible, and the silicon carbide seed crystal is placed at the top of the graphite crucible. Then, the graphite crucible is heated until the silicon carbide reaches its sublimation temperature. Subsequently, the silicon carbide powder gradually decomposes into gaseous substances such as Si vapor, Si2C, and SiC2. These substances will gradually sublimate to the top of the crucible under the axial temperature gradient and condense on the surface of the SiC seed crystal at the top of the crucible, thereby obtaining silicon carbide single crystals.
Nuclear graphite
Nuclear-grade high-purity graphite is a widely used material in nuclear power engineering. Nuclear-grade graphite materials are applied in the core of
high-temperature gas-cooled reactors, such as tubes, fuel elements, and support components.
Nuclear-grade graphite must be high-purity graphite. High-purity graphite mainly consists of aggregates, binders and impregnating agents. The aggregates mainly consist of needle-shaped petroleum coke and asphalt coke. Nuclear-grade high-purity graphite has relatively strict requirements for controlling trace elements. Under an airtight condition, it undergoes long-term high-temperature calcination for purification, removing impurities and reducing the content of trace elements such as halogens and low-melting-point metals. The carbonization molding of nuclear-grade high-purity graphite adopts the isostatic pressing process. The material is placed in an elastic body mold, sealed tightly, and then the air is removed to maintain a vacuum environment. The mold is placed in the isostatic hydraulic cylinder, and the isotropic liquid exerts the same pressure on the mold, transmitting the pressure to the carbon material, causing the raw material to become dense.
Photovoltaic field
In the photovoltaic industry, especially as the electrode material for solar cells, graphite has extensive applications. Its excellent conductivity and chemical stability make it an ideal material for producing the electrodes of solar cells. Moreover, the pull-type silicon single crystal growth furnace is the main equipment for preparing silicon single crystal materials. The heating system of the single crystal furnace mainly includes high-purity graphite heaters, quartz crucibles, graphite crucibles, and insulation materials, etc. By applying direct current electricity to the graphite heater, a thermal source is provided to melt the polycrystalline silicon raw material. The power of the heater can be adjusted through the voltage or current of the power supply. The heat released by the heater is insulated by the insulation system, which can form a thermal field environment suitable for the growth of silicon single crystals.
Graphene
Graphene is currently the thinnest nanomaterial in the world, featuring high chemical stability and thermal stability, as well as excellent mechanical and electrical properties. It has broad application prospects. The main raw material for preparing graphene is high-purity graphite.
Others
In the more challenging aerospace field, high-purity graphite still performs exceptionally well. It is used in the nozzle throat liners of rocket engine tails, the neutron moderator of atomic reactors, the heat insulation and heat-resistant materials of missiles, and even the radio connection and conductive structural materials on artificial satellites all feature the presence of high-purity graphite.
