Graphite raw material specifications

High-purity graphite generally refers to graphite with a carbon content of more than 99.99%. Its organizational structure can be divided into three categories: coarse-grained structure, fine-grained structure, and ultra-fine-grained structure. High-purity graphite is widely used in Czochralski single-crystal silicon furnaces. The basic material of integrated circuits is mainly silicon single-crystal chips. At present, the growth process of silicon single crystal mainly adopts the Czochralski (CZ) method. Other methods include the magnetic field Czochralski method (MCZ), zone (FZ) method, and double crucible crystal pulling method. Czochralski single-crystal silicon used in the global electronics industry accounts for about 80% of the total monocrystalline silicon consumption. Graphite parts in Czochralski single-crystal silicon furnaces are consumables. High-purity graphite materials are used to process the heating system of Czochralski single-crystal silicon furnaces.

Another important use of high-purity graphite is to be processed into various types of crucibles for the production of rare metals or high-purity metals and non-metallic materials. Graphite electrode for spectral analysis is also a kind of high-purity graphite, which can be used for spectrochemical analysis of all elements except carbon. Graphite electrode for spectral analysis is formed by the extrusion method. The impurity element content of the finished product should not be greater than 6*10-5. When preparing standard samples for spectral analysis and capturing impurities using chemical methods, spectrally pure carbon powder or spectrally pure graphite powder must be used. The impurity content requirements of these two high-purity materials are 6*10-5; in some applications, the carbon content needs to reach 99.9995%, and the total ash content is less than 5*10-6. There are three molding methods for high-purity graphite: extrusion molding, compression molding, and isostatic pressing.

Graphite is one of the deceleration materials and reflective materials used in building nuclear reactors. Early reactors were all graphite piles. Graphite for nuclear reactors used as structural materials is much stricter and more expensive than graphite electrodes in terms of raw material selection, process control, and finished product inspection. Graphite for nuclear reactors must have the following properties: small absorption of slow neutrons, good high-temperature strength, high thermal shock resistance, good deceleration performance of fast neutrons, stable size under irradiation, and minimal impurity content. The content of elements such as boron and cadmium that are easy to absorb neutrons must be strictly controlled.

Nuclear graphite must have a higher volume density M because the deceleration effect of graphite on fast neutrons is achieved by the collision of fast neutrons with carbon atoms. The more carbon atoms in a unit volume, the better the effect. Therefore, volume density is one of the main indicators of nuclear graphite. Volume density is also directly related to the porosity and permeability of graphite. In order to avoid the loss of nuclear fuel and heat carrier, the porosity and permeability must be reduced to a certain level. The mechanical strength of graphite is also related to its volume density. Generally, the volume density of core graphite used in graphite stacks is 1.7-1.75g/cm³.

Nuclear reactors used for power generation are currently mainly pressurized water reactors that do not require graphite, but experts predict that high-temperature gas-cooled reactors have great development prospects. High-temperature gas-cooled reactors require a large amount of advanced graphite materials. 9 In high-temperature gas-cooled reactors, since helium is used as the coolant, carbon and ceramic materials are used as fuel coating materials, and graphite or carbonaceous materials are used as deceleration materials and furnace core structural materials, high-temperature gases close to -5 can be exported out of the reactor for use as energy.

Although the international definition of isotropic graphite needs to be further clarified, certain physical performance indicators in the diameter and length directions of the product are generally measured, and their ratios are calculated. Some are expressed by the ratio of thermal expansion coefficients, and the simpler ones are expressed by the ratio of resistivity. The anisotropy ratio is in the range of 1.0~1.1, which is called isotropic products, and when it exceeds 1.1, it is called anisotropic products. In addition to general petroleum coke, isotropic graphite also uses modified pitch coke, natural pitch coke, oxidized petroleum coke, uncalcined raw petroleum coke, natural graphite, etc.