Production process and main uses of isostatically pressed graphite

About the Isostatic graphite

Isostatic graphite is a new type of graphite material developed in the 1960s, which has a series of excellent properties. For example, isostatic graphite has good heat resistance. In an inert atmosphere, its mechanical strength not only does not decrease as the temperature increases, but increases, reaching the highest value at around 2500 ° C; compared with ordinary graphite, the structure Fine and dense, and good uniformity; low thermal expansion coefficient, excellent thermal shock resistance; isotropic; strong chemical corrosion resistance, good thermal and electrical conductivity; excellent machining performance.

It is precisely because of this series of excellent properties that isostatic graphite is widely used in the fields of metallurgy, chemistry, electrical, aerospace and atomic energy industries, and, with the development of science and technology, the application field is still expanding.

Isostatic graphite requires structurally isotropic raw materials, which need to be ground into finer powders. Cold isostatic pressing technology is required. The firing cycle is very long. In order to achieve the target density, multiple impregnation and firing cycles are required. , and the period of graphitization is much longer than that of ordinary graphite.

The raw materials for producing isostatic graphite include aggregate and binder. Aggregate is usually made of petroleum coke and pitch coke, and ground pitch coke is also used. For example, the AXF series isostatic graphite of an American company POCO is produced by using ground pitch coke Gilsonite coke.

About the production process

The performance of isostatic graphite is greatly affected by raw materials, and the selection of raw materials is the key link to produce the desired final product. The characteristics and uniformity of raw materials must be strictly checked before feeding.

Different from ordinary extrusion molding and compression molding, isostatic graphite is formed by cold isostatic pressing technology. Fill the raw material compressed powder into the rubber mold, and make the compressed powder dense through high-frequency electromagnetic vibration, vacuumize after sealing, discharge the air between the powder particles, and put it into a high-pressure container filled with liquid media such as water or oil. Pressurize to 100-200 MPa and press into cylindrical or rectangular products.

According to Pascal's principle, the pressure is applied to the rubber mold through a liquid medium such as water, and the pressure in all directions is equal. In this way, the pressed powder particles are not oriented in the filling direction in the mold, but are compressed in an irregular arrangement. Therefore, although graphite is anisotropic in crystallographic properties, as a whole, isostatic graphite But it is isotropic.

In addition to cylinders and rectangles, the formed products also have shapes such as barrels and crucibles.

Isostatic pressing machine is mainly used in powder metallurgy industry. Due to the needs of high-end industries such as aerospace, nuclear industry, cemented carbide, and high-voltage electromagnetics, isostatic pressing technology has developed very quickly. It has the ability to manufacture cold isostatic presses with a working cylinder inner diameter of 3000mm, a height of 5000mm, and a maximum working pressure of 600MPa.

At present, the maximum size of the cold isostatic press used in the carbon industry to produce isostatic graphite is Φ2150mm×4700 mm, and the maximum working pressure is 180MPa.

Due to the fine structure of isostatic graphite, the roasting process is required to be particularly slow, and the temperature in the furnace must be very uniform, especially in the temperature stage when the pitch volatiles are rapidly discharged, the heating process must be carried out carefully, and the heating rate cannot exceed 1°C/h. The temperature difference in the furnace is required to be less than 20°C, and this process takes about 1 to 2 months.

In order to improve the bulk density, mechanical strength, electrical conductivity, thermal conductivity, and chemical reactivity of the product, it can be treated by pressure impregnation, that is, the coal tar pitch is impregnated into the product through the open pores. Typically, isostatically pressed graphite undergoes multiple impregnation-calcination cycles.

When the calcined product is heated to about 3000°C, the carbon atom lattice is arranged in an orderly manner, and the transformation from carbon to graphite is completed, which is called graphitization.

After graphitization, the bulk density, electrical conductivity, thermal conductivity and corrosion resistance of the product are greatly improved, and the machining performance is also improved. However, graphitization reduces the flexural strength of the article.

After graphitization, it is also necessary to check the density, hardness, strength, resistivity, ash content and other indicators of the product to judge whether the indicators meet the requirements.

About the main uses

When isostatic graphite is used in the fields of semiconductors, single crystal silicon, and atomic energy, the requirements for purity are very high, and impurities must be removed by chemical methods before they can be used in these fields.

The usual way to remove impurities in graphite is to put the graphitized product into a halogen gas and heat it to about 2000°C, and the impurities will be halogenated into low-boiling halides and volatilized and removed.

Almost all impurity elements in graphitized products can be removed by chlorine halogenation. The exception is boron, which can only be removed by fluorination.

This purification method makes full use of the unique characteristics of graphite that does not react with halogens at high temperatures and that graphite is porous.

In the thermal field of Czochralski single crystal silicon, isostatic graphite components include crucibles, heaters, electrodes, heat insulation shielding plates, seed crystal holders, bases for rotating crucibles, various discs, heat reflection plates, etc. 30 kinds. Among them, 80% of isostatic graphite is used to make crucibles and heaters.

In recent years, the diameter requirements for single crystal silicon rods have become larger and larger, and the production of 300mm wafers has become increasingly mainstream. Correspondingly, the diameter of the heating zone of the single crystal furnace is mostly 800mm. In order to protect the quartz crucible placed inside the furnace, the diameter of the graphite crucible reaches 860mm. The diameter of the heater is about 960~1000mm. The diameter of other parts can reach up to 1000mm. 1500mm.

Since 2003, people's awareness of protecting the earth's living environment has gradually increased, and people are increasingly favoring natural energy sources that do not emit carbon dioxide. Under this trend, the production of solar cells has increased rapidly.

In the manufacturing process of polycrystalline silicon wafers for solar cells, polycrystalline silicon fragments must first be melted and cast into polycrystalline silicon square ingots. The heater of the ingot furnace needs to be made of isostatically pressed graphite.

In recent years, the global climate has warmed. Carbon dioxide produced by the use of fossil fuels is thought to be a major contributor to the problem. In recent years, although the economic growth of developing countries has achieved world-renowned results, the problem of power shortage has deeply troubled these countries. Under such circumstances, people's attention has turned to atomic power generation, which has a energy flow density that is much higher than that of solar cells and wind power, and does not emit carbon dioxide and sulfur oxides.

Currently, most of the nuclear reactors in use around the world are light water reactors. The working principle of this type of reactor is to use the heat energy generated during nuclear fission to vaporize cold water into 300°C water vapor, which drives the turbine to generate electricity. However, due to the low temperature of the water reactor, the power generation efficiency of the light water reactor is not very high. In contrast, high-temperature gas-cooled reactors do not have such problems. It uses inert gas (helium) as the coolant. Not only can the reactor core outlet temperature reach nearly 1000°C, it has high power generation efficiency, but it is also suitable for producing hydrogen. It can be said that power supply and environmental protection are both balanced.

Graphite is suitable as the core material of this high-temperature gas-cooled reactor because graphite not only resists high temperatures, but also absorbs less neutrons and has good heat transfer properties.

Graphite is a neutron moderator and an excellent reflector. Its many excellent properties have established its position in the nuclear industry. Graphite can not only meet the needs of industrial mass production, but also has the characteristics of high mechanical strength and high temperature resistance required by structural materials. Therefore, graphite is suitable as a structural material for high-temperature gas-cooled reactors.

The very safe feature of high-temperature gas-cooled reactors has led people to propose the design concept of modular high-temperature gas-cooled reactors. The next generation of ultra-high temperature nuclear reactors (UHTR) are moving towards high power density and high temperature. These technological developments have put forward higher requirements for the characteristics of the new generation of graphite materials, such as higher radiation damage tolerance, product homogenization, high quality and low price, long-term supply, etc.

As nuclear fusion devices gradually become larger, in order to generate high-temperature plasma, graphite materials with good thermal conductivity and high mechanical strength are used as the first wall material facing the plasma, and have shown good discharge pulse effects. In addition, even if they are mixed into the plasma, due to their low atomic number, the radiation loss caused is small, so the high-temperature plasma can be maintained stable. However, the incidence of hydrogen isotopes will cause the graphite material to generate consumptive chemical splashing of CH4 gas and the radiation-enhanced sublimation loss phenomenon (radiation-enhanced sublimation means that the plasma particles are in an irradiation environment, even if the current temperature does not reach the normal thermal sublimation of graphite Temperature, graphite material will also sublimate and lose). Therefore, when using graphite material as the plasma surface material, attention must be paid to the usage conditions of graphite, especially the temperature.

The plasma facing material and divertor plate of the critical plasma device JT-60U being developed by the Japan Atomic Energy Research Institute use components made of graphite materials. Among them, the divertor plate at the plasma outlet uses a special C/C composite material with high thermal conductivity, high thermal impact resistance, and carbon fiber as raw material. The first wall with relatively low thermal load adopts various materials. Isotropic graphite material.

In short, the world's atomic energy industry is undergoing various developments and changes. In the field of high-temperature gas-cooled reactors, commercial high-temperature gas-cooled reactors in South Africa and China are advancing. In the field of nuclear fusion reactors, there are experimental reactors. While the International Thermonuclear Experimental Reactor (ITER) project is being carried out, the transformation of Japan's JT-60 device is also in progress.

Electrical discharge machining, which mainly uses graphite or copper as electrodes, is widely used in processing fields such as metal molds.

Pre-process requirements for shape processing of graphite for electrical discharge machining: ① low tool consumption; ② fast processing speed; ③ good roughness of the machined surface; ④ no sharp protrusions, etc.

Requirements for the electrical discharge machining process: ① Fast electrical discharge machining speed; ② Less consumption of electrode length; ③ Less loss of electrode angle; ④ Good roughness of the machined surface of the workpiece; ⑤ Less unevenness of the machined surface of the workpiece, etc.

Compared with copper electrodes, graphite electrodes for electrical discharge machining have the following advantages: ① Lighter than copper, easy to transport, only 1/5 of the weight of copper in the same shape; ② Easy to process; ③ Cutting processing is not easy to produce stress and thermal deformation; ④ The melting point is above 3000°C, the thermal expansion coefficient is small, and graphite electrodes are rarely deformed by the heat generated by electrical discharge machining.

However, graphite electrodes also have some disadvantages, such as ① easy to produce dust during cutting; ② easy loss, etc.

Manufacturers of graphite electrodes for electrical discharge machining produce different grades of products from low-priced rough machining to finishing.

Recently, graphite electrodes for ultra-fine particle electrical discharge machining have appeared on the market, which are different from the traditional concept. This kind of electrode aims to reduce graphite consumption. Its development idea is simply: less electrode consumption → less graphite particles falling off from the electrode during electrical discharge processing → fine particles → high bonding strength between particles → efficient and rational use of asphalt aggregate → Adjust manufacturing parameters to reduce defective rate and manufacturing cost.

As for whether graphite electrodes for ultrafine particle electric discharge machining can be marketed, it also depends on the production technology level of graphite electrode manufacturers.

Under the current circumstances, the cutting process is still somewhat helpless in the deep and detailed processing of metal molds. Because the shape and strength of existing cutting tools are difficult to meet the requirements of deep and detailed processing. Therefore, graphite electrodes for precision discharge, which are processed from isostatically pressed graphite, were developed for finishing in order to take full advantage of the many advantages of graphite electrodes.

Due to the advantages of simplification of the casting process, improvement of product qualification rate, and uniformity of product structure, it has become very common to use continuous casting and rolling to produce non-ferrous metal plates, tubes, rods, etc. At present, the production of large-scale pure copper, bronze, brass, and white copper mainly adopts continuous casting. Among them, the crystallizer, which plays a vital role in product quality, is made of isostatic graphite.

Because isostatically pressed graphite material has good properties in terms of heat conduction, thermal stability, self-lubrication, anti-wetting and chemical inertness, it has become an irreplaceable material for making crystallizers.

Isostatic graphite is also used to make diamond tools and cemented carbide sintering molds, thermal field components (heaters, insulation cylinders, etc.) of optical fiber drawing machines, thermal field components (heaters, bearing frames, etc.) And precision graphite heat exchangers, mechanical seal components, piston rings, bearings, rocket nozzles, etc.

In today's industrial production, graphite has become an indispensable key material, especially in the solar industry, LED industry, semiconductor industry and nuclear industry, the demand for graphite is increasing sharply, and the quality requirements are getting higher and higher.