Classification of special graphite

Special graphite covers a wide range. Electric carbon products industry, natural graphite product industry and metallurgical carbon products industry have different understandings and classification methods. As for the customary classification of metallurgical carbon products industry, special graphite mainly refers to high-strength , High-density, high-purity graphite products (referred to as three- high graphite). Three- high graphite can be divided into three types in terms of material structure: coarse particle, fine particle and ultra-fine particle ; In terms of molding method, there are mainly three types: mold forming, extrusion forming and isostatic pressing. In addition, vibration molding also can be used to produce special graphite.

High-quality specialty graphite - isotropic graphite is produced using an isostatic pressing process. According to their main applications, they could be divided into: special graphite for EDM; special graphite for making casting molds and dies; special graphite for continuous casting of steel or copper and aluminum; high-purity graphite for Czochralski monocrystalline silicon furnace or for smelting precious metals and high-purity materials; graphite for synthetic diamond; special graphite for rocket and missile technology.

In a broad sense, special graphite also includes pyrolytic carbon (pyrolytic graphite), carbon materials for bioengineering, glassy carbon, porous carbon and porous graphite, graphite interlayer compounds (such as flexible graphite, fluorinated graphite), graphite for lasers, etc. , The mechanical and electronic industry also uses many special carbon and graphite materials, such as carbon brushes, carbon-graphite bearings, feeder sliders for electric locomotives, and graphite molds for drawing optical fibers, which will not be described in this article.

Main varieties of special graphite

Graphite for EDM

EDM is a new type of machining process in the machinery manufacturing industry. EDM can process many metals with high hardness, and can process parts with complex shapes and high precision requirements. It can be used as a tool electrode for anode instead of copper material.

Graphite materials used as tool electrodes for EDM must meet the following conditions:

1. The structure is dense and uniform, and there should be no coarse particles and large pores;

2. It has high mechanical strength and good processing performance, and can process complex shapes or acute angles and flakes;

3. Graphite tool electrodes have a certain loss in the process of EDM, and this loss should be as low as possible;

4. The discharge characteristics are stable and the processing speed is fast.

Therefore, graphite for EDM generally adopts fine-grained graphite or ultra-fine-grained graphite, and its physical properties are preferably isotropic. Therefore, at present, the isotropic graphite with fine particle structure is more used in China's EDM graphite material market.

Die graphite

The foundry industry in the machinery industry uses a large number of graphite materials as molds for pressure casting, centrifugal casting, and hot extrusion of superhard alloys. Graphite molds can be used for large train wheels and small precision parts. Graphite molds can be reused many times, and the castings after demolding have a high finish, and some can be used without further processing. The graphite material used for casting molds should be graphite with dense texture, low thermal expansion coefficient and good oxidation resistance.For the casting of big dimension parts coarse particle graphite could be used. But for the casting of precision parts fine-grained graphite must be used.

Among the dies graphite, the metal continuous casting graphite has the most stringent quality requirements, requiring high thermal conductivity, good thermal stability and thermal shock resistance, good lubricity, no infiltration with molten metal, no reaction with cast metal, and easy processing into dimensionally accurate molds.

Graphite for Synthetic Diamond

Diamonds are an important cutting and grinding material in industry, but naturally mined diamonds are rare and expensive. Graphite and diamond both belong to carbon elements, but the crystal forms are different. Graphite can be converted into diamond crystal form under high temperature and high pressure. As early as 1954, Sweden and the United States successfully synthesized synthetic diamonds. China also synthesized synthetic diamonds in the 1960s. In the 1970s, special graphite materials for synthetic synthetic diamonds were produced, and a certain production scale was formed.

High purity graphite

High-purity graphite generally refers to graphite with a carbon content of more than 99.99%. It can be divided into three types from the structure: coarse particle graphite, fine particle graphite and ultra-fine particle graphite. High-purity graphite is widely used in Czochralski monocrystalline 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 Czochralski method. Other methods include magnetic field Czochralski method, area method and double crucible crystal pulling method. The graphite parts are consumables in the Czochralski monocrystalline silicon furnace. High-purity graphite are processed into the heating system in this area.

Another important use of high-purity graphite is to process into various types of crucibles for the production of precious metals, rare metals or high-purity metals and non-metallic materials. The graphite electrode for spectroscopic analysis is also a kind of high-purity graphite, which can be used for spectrochemical analysis of all elements except carbon, and the graphite electrode for spectroscopic analysis is formed by extrusion. The content of impurity elements in the finished product should not be greater than 6*10-5. Spectrally pure carbon powder or spectroscopically pure graphite powder is required to prepare standard samples and chemically capture impurities in spectral analysis. The impurity content of the graphite couldn’t exceed 6*10-5; In some cases the carbon content needs to reach 99.9995%, and the total ash content is less than 5*10-6. The molding methods of high-purity graphite include extrusion forming, mold forming and isostatic pressing.

Graphite Materials for Nuclear Energy

Graphite is one of the deceleration materials and reflective materials used in nuclear reactors. The early reactors were all graphite reactors. Graphite used as a structural material for nuclear reactors is much stricter and more expensive than graphite electrodes in the selection of raw materials, 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 for fast neutrons, dimensional stability under irradiation, and very little impurity content.

Nuclear graphite must have a higher bulk density, 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 deceleration effect, so the bulk density It is one of the main indicators of nuclear graphite. The bulk 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 should be reduced to a certain level.

Isotropic graphite

Although the international definition of isotropic graphite needs to be further clarified, it is generally to measure some physical performance indicators in the diameter direction and length direction of the product and calculate the ratio. Some are expressed by the ratio of thermal expansion coefficients, but the simplest is expressed by the ratio of resistivity. The ratio in the range of 1.0-1.1 is called an isotropic product, and if it exceeds 1.1, it is called an anisotropic product. In addition to general petroleum coke, the production of isotropic graphite also uses modified pitch coke, natural pitch coke, oxidized petroleum coke, uncalcined raw petroleum coke, natural graphite, etc.