Isostatic graphite is a new type of graphite materials which developed in the 1940s and has a series of excellent properties. Isostatic graphite has good heat resistance, in an inert atmosphere, its mechanical strength increased with the increase of temperature and reached the highest value at around 2500 °C, compared with ordinary graphite, Isostatic graphite has fine structure, and uniform good performance; low coefficient of thermal expansion, excellent thermal shock resistance; isotropy, chemical resistance, good thermal and electrical conductivity; excellent mechanical processing properties. Because of this series of excellent performance that isostatic graphite has been widely used in chemical, semiconductor, electrical, metallurgy, mechanical, nuclear and aerospace fields. Moreover, the application field is still expanding with the development of science and technology.
1. Main uses of isostatic graphite
1.1 Graphite for Solar Cells and Semiconductor Wafers
In the solar and semiconductor industries, isostatic graphite is used in large quantities to produce single-crystal direct-heating furnace hot-field graphite parts, polycrystalline silicon casting furnace heaters, compound semiconductor manufacturing heaters, and crucibles. In recent years, the development of solar photovoltaic power generation has been rapid. The monocrystalline silicon and polysilicon production in the photovoltaic industry has a great demand for graphite. At present, monocrystalline and polycrystalline silicon products have grown toward large-scale and high-end development, and the isostatic graphite also has higher requirements, greater specifications, higher strength, and higher purity.
1.2 Nuclear graphite
Isostatic graphite has moderate mechanical properties, excellent mechanical properties at high temperature, high thermal conductivity, and low coefficient of linear expansion. In high-temperature gas-cooled reactors, it is mainly used as a reflector, a moderator, and an active zone structural material, and constitutes a nuclear fuel assembly together with nuclear fuel. At a temperature of 400-1200° C., high-energy γ-rays and fast neutron radiation are liable to cause radiation damage for a period of several years, thereby changing the structure and properties of graphite. Therefore, the degree of graphitization of the material is required to be high. It should has good isotropic property, uniform composition and low elastic modulus.
1.3 EDM graphite
Graphite has no melting point, is a good conductor of electricity, has good thermal shock resistance and is an excellent electrode material for EDM machining. Ordinary graphite materials, which are low-density anisotropic graphite with coarse grain structure, can not meet the demand of EDM. However, the isostatic graphite electrode structure is uniform, dense, and has high processing accuracy, which can meet the requirements in this respect.
1.4 Continuous casting mould graphite and molded graphite
It is mainly used for continuous casting moulds and superhard materials for high temperature and high pressure mould materials. Due to its fine particle structure, high mechanical strength, and uniform heat conduction, isostatic graphite makes the surfaces of continuous casting and molded products smooth, with high internal quality and long service life. It is the best material for crystallizers. Moreover, for large sintered materials,the mould wall should be as thin as possible, and it is necessary to use a fine-strength, isotropic graphite with high strength.
1.5 Other uses
In the use of carbon brushes, mechanical seals, contact plates, etc., where precision is required, high lubricity and high electrical conductivity are extremely important. Ordinary graphite materials need to be impregnated with resins and metals in order to increase strength and airtightness, but their use is limited in terms of corrosion resistance and high temperature resistance. Isostatic graphite has a low coefficient of friction and good thermal conductivity. It is often used as a sliding friction material for bearings, mechanical seal rings, and piston rings. In addition, isostatic graphite is also used to make diamond tools, thermal field components (heaters, insulation tubes, etc.) of fiber drawing machines, thermal field components (heaters, load frames, etc.) of vacuum heat treatment furnaces, and precision graphite heat exchanger.
2. Development of isostatic graphite
Due to the excellent performance of high-density isotropic carbon materials, wide application, and high added value, all developed countries have invested more manpower and resources to develop the material. According to relevant data in Japan, the annual production value of carbon graphite materials doubles every five years due to the development of isostatic graphite materials.
The United States is the first country in the world to develop and produce isostatic graphite, and products were available in the late 1950s.
3. Isostatic graphite, molded graphite and their production proces
As shown in Fig. 1, the conventional production process for isostatic graphite is similar to the molded graphite.
Fig. 1 Flow chart of isostatic graphite production process
To prepare isostatic graphite, it is generally necessary to use a structurally isotropic carbonaceous feedstock and grind it to a specific particle size. In order to avoid the directional alignment of the powder during the pressing process, cold isostatic pressing technology needs to be applied. In order to ensure the balance of temperature inside and outside the billet during the roasting process, the temperature must be very slow. In order to achieve the desired density, it is generally necessary to perform a plurality of immersion-roasting cycles. Finally, the graphitization cycle of the sample is also much longer than that of ordinary graphite materials.
In addition to the above conventional preparation methods, another method is a self-sintering method. The self-sintering method is a method of producing isostatic graphite without using a binder, which is a self-sintering powder. The production method of Wuyu Chemical Industry Co., Ltd. of Japan is to carry out special treatment of asphalt in two stages to obtain raw materials with special structure, and then directly press, roast and graphitize without using binder to obtain high-performance isostatic graphite. Because there are few related publications, this article does not describe this method.
3.1 Raw materials
The raw materials for the production of isostatic graphite include aggregates, binders and small amounts of additives. Petroleum coke and pitch coke are the most common isostatic graphite aggregates. In addition, natural graphite, anthracite, and carbon black are also commonly used as aggregates. Under normal circumstances, in order to reduce the shrinkage of the sample during roasting and graphitization, petroleum coke and pitch coke need to be calcined at 1200~1400°C to remove water and volatile matter before use. However, in order to improve the mechanical properties and structural compactness of products, there is also the production of isostatic graphite directly from raw coke. For example, Tokai Carbon Co., Ltd. of Japan uses oxygen in its publicly patented isotropic graphite manufacturing method. Raw coke below 4% is used as aggregate. Coking is characterized by the presence of volatiles, self-sintering, and simultaneous expansion and contraction with the binder coke. Coal tar pitch is the most commonly used binder, and phenolic resin and other lipid materials are also often used as binders. For example, Tsinghua University, in its publicly patented “isotropic graphite product and its preparation method”, classifies asphalt, phenolic resin, furfural resin, and epoxy resin as lipid backup materials. The additives in isostatic graphite are mainly boron and its compounds, which are used to promote the sintering of carbon materials, but also introduce boron impurities that can be removed during the purification process.
The performance of isostatic graphite is greatly influenced by the raw materials, and the selection of raw materials is the key to the production of the desired final product. The material properties and uniformity must be strictly tested before feeding.
3.2 Grinding
Grinding, including one-time grinding and two-time grinding. Primary grinding refers to the crushing of aggregates in raw materials. It is generally believed that the smaller the aggregate size, the better the density, strength and isotropy of the final product obtained. The particle size of isostatically pressed graphite is usually required to be less than 20μm. At present, the finest isostatic graphite has a particle diameter of 1μm. For example, the diameter of EDM-AF5 isostatic graphite used in EDM in the United States reaches 1μm. To grind aggregate coke into such a fine powder, an ultrafine crusher is needed. Grinding powders with an average particle size of 10-20 μm requires the use of a vertical roller mill, whereas milling of powders with an average particle size of less than 10 μm requires the use of an air-jet mill. Secondary grinding refers to the crushing of the cooled paste after kneading, and the particle size can be in the range of tens of micrometers to several hundreds of micrometers, and can be accomplished by using a vertical roller mill or a ball mill. The paste was broken and sieved to become pressed powder.
3.3 Kneading
The milled aggregate powder, binder, and additives are put into a heated kneader in proportion and kneaded thoroughly to uniformly attach a layer of asphalt to the surface of the aggregate particles. The kneading process is relatively simple, and it is necessary to control the kneading temperature and time. The kneading temperature is determined according to the binder used, generally not exceeding 150° C. The kneading time is determined according to the ratio of the aggregate and the binder, and is generally not less than 1 h. After the kneading is completed, the paste is taken out and allowed for a secondary grinding after cooling.
3.4 isostatic compaction
Isostatic compaction is a key process for ensuring the isotropy of isostatic graphite materials. The basic principle of isostatic pressing is Pascal’s law, that is, in a closed container filled with liquid, the pressure applied at any point in the fluid must be transmitted to any part of the container at the same value. In the isostatic pressing process, pressure is transmitted to the rubber mold through a liquid medium such as water, and the pressure in each direction is equal. In this way, the powder is not oriented in the filling direction in the mould but is compressed in an irregular arrangement. Thus, although graphite is anisotropic in its crystallographic properties, isostatically-pressed graphite is, on the whole, isotropic.
The formation of isostatic graphite is roughly divided into three steps: feed, boosting, and depressurization. The raw material powder is filled into a rubber mold and the compaction is made dense by high-frequency electromagnetic vibration. After the material is loaded, the mould is manually shaped and then sealed. At this time, the powder in the mold also contains a lot of air, which will affect the molding performance and density of the product. Therefore, the sealed mold needs to be evacuated to remove the air between the powder particles. In the production of certain spherical products, the powder should be pre-compacted into spheres by compression molding and then put into correspondingly sized isostatically pressed moulds, such as Chengdu Carbon Co., Ltd. in its public patent 《A kind ofnuclear graphite material combination and pretreatment method》, the moulding process of the first press moulding and then the isostatic pressing is adopted. After the filling is completed, the mold is transferred into a high pressure vessel for pressing. The pressurization process needs to be carried out step by step. For example, the pressure is raised by 5 MPa for a period of time to allow the residual gas in the mold to be partially discharged. At this time, since the powder is compressed and the volume shrinks, the pressure in the high-pressure vessel slightly decreases. The pressure is then raised again to about 20 MPa. After the partial gas is discharged, the volume of the powder is once again shrunk, and then the pressure is increased to the required working pressure, generally 100 to 200 MPa, and maintained at the selected high pressure for a certain period of time (20 to 60 minutes). ). The depressurization process also needs to be carried out slowly because a small amount of air must remain in the powder, and the volume decreases rapidly with the compression of the powder. If the pressure suddenly decreases, these compressed gases will expand rapidly, causing the body to crack.
3.5 Roasting
Roasting is the process of removing the volatiles in the green body and coking the binder. The maximum temperature generally does not exceed 1250°C. During the roasting process, a complex chemical reaction occurs between the aggregate and the binder, and the binder decomposes to release a large amount of volatiles while conducting a polycondensation reaction. During the low-temperature preheating phase, the green body expands due to the heat, and in the subsequent temperature increase process, the volume shrinks due to the polycondensation reaction. The larger the volume of the green body, the more difficult it is to release the volatiles. At the same time, the surface and the interior of the green body are more prone to temperature difference and non-uniform shrinkage, which may cause cracks in the green body. Because of the compact structure of isostatic graphite, the roasting process requires particularly slow, and the furnace temperature is very uniform, especially in the temperature phase of asphalt volatile emissions, the heating process should be carried out carefully, the heating rate can not exceed 1 °C / h, The temperature difference in the furnace is less than 20°C. This process generally takes more than 1 month. For example, Tianjin Jinmei Carbon Material Science and Technology Development Co., Ltd. has described in its public patent “An Isotropic Graphite Preparation Method” that the maximum temperature for one-time calcination is about 1200° C. and the average temperature increase rate is 3° C./h. Where 350 ~ 400 °C, heating rate: ≤ 1 °C / h; 400 ~ 500 °C, heating rate: ≤ 0.7 °C / h; 500 ~ 600 °C, heating rate: ≤ 1 °C / h.
3.6 Impregnation
During roasting, binder volatiles are discharged and fine pores are left in the product, and almost all open pores. The presence of these pores can impair the bulk density, mechanical strength, electrical conductivity, thermal conductivity, and chemical resistance of the product. In production, the porosity is mainly reduced by the asphalt impregnation method, ie, the coal pitch is impregnated into the interior of the product through the open pores, and then the secondary asphalt is used to coke the asphalt and fill the pores.
General process of impregnation: first preheat the product in impregnated cans with good tightness. The preheating temperature depends on the type of impregnated asphalt selected, usually around 100°C; then the impregnation tank is evacuated and the vacuum degree is controlled. At about -0.06MPa, the product is degassed; then the molten coal bitumen is poured into the impregnation tank until the asphalt is immersed in the product, and the temperature of the impregnation tank is raised, generally not exceeding 300°C; finally pressurize the tank to promote the entry of the asphalt. Inside the product, the pressure generally does not exceed 3 MPa.
Isostatic graphite undergoes several cycles of impregnation-roasting cycles, but not more than three times generally, the effect of the impregnation step on the improvement of the product performance is very limited if more than three times. In Tsinghua University’s public patent, “An isotropic graphite product and its preparation method,” three impregnation-roasting cycles are used. In the immersed sample, in the second baking, the phenomenon that the asphalt is heated and exuded easily occurs, and the impregnation effect is affected. Studies have shown that the use of hot isostatic pressing technology for secondary baking products, while heating, while applying a pressure of about 5MPa, you can effectively avoid the phenomenon of asphalt seepage. The disadvantage of this method is that the equipment is expensive and the productivity is extremely low.
3.7 Graphitization
The calcined product is heated to about 3000 DEG C. The lattice of carbon atoms is arranged in an orderly manner, and the process of converting from carbon to graphite is called graphitization. The graphitization methods include Acheson method, internal heat series method, high frequency induction method, and the like. The usual Acheson method takes about 1 to 1.5 months from the time the product is installed to the furnace. Each furnace can handle several tons to several tens of tons of baked goods. The thermal efficiency of the internal heat series connection method is high, about 1.5 times that of the Acheson method, and it has gradually replaced the Acheson method. After graphitization, the bulk density, electrical conductivity, thermal conductivity and corrosion resistance of the product are greatly improved, and the mechanical processing performance is also improved. However, graphitization reduces the flexural strength of the product.
3.8 Inspection
After graphitization, it is also necessary to check the density, hardness, strength, resistivity, and ash of the product to determine whether it meets the specifications. Table 1 shows the performance indicators of isostatic graphite of XRD Graphite.
1. The figures above are typical values, and to be considered minimum or maximum.
2. Unit conversion: μΩ•m=μΩ•cm×0.01 MPa=kgf/cm2×0.098 GPa=kgf/mm2×0.0098 W/(m•K）=kcal/h•m℃×1.16
3. There are other product sizes in addition to those described above. Contact XRD Graphite for more details.
Table 1 Various Performance Indicators of Isostatic Graphite in XRD Graphite.
3.9 Purification
When isostatic graphite is used in the fields of semiconductors, monocrystalline silicon, and atomic energy, it requires high purity and must be removed by chemical methods before it can be used in these fields. The usual practice for removing impurities from the graphite is to put the graphitized product in a halogen gas and heat it to about 2000°C. The impurities are then halogenated to a low-boiling halide and volatilized. Almost all impurity elements in graphitized products can be removed by halogenation with chlorine gas. But with the exception of boron, it can only be removed by fluorination. The halogen gases used for purification are chlorine, fluorine, or halogenated hydrocarbons that decompose at high temperatures to produce these gases, for example, carbon tetrachloride (CCl4), dichlorodifluoromethane (CCl2F2). For example, U.S. Graf Technology International Holdings Co., Ltd. uses the high-temperature halogen gas to remove impurities in its public patent “Low CTE High Isotropic Graphite”: the graphitized sample is between 2200 and 2600°C. The halogen gas is purified to remove impurities such as boron to obtain high-purity, high-isotropic graphite.
