Graphite is a crystalline mineral of carbonaceous element. Its crystalline framework is a hexagonal layered structure. Graphite is soft, black-gray, greasy, and has excellent properties such as high temperature resistance, electrical conductivity, and thermal conductivity. Therefore, it has a good application prospect. Especially in recent years, the rapid development of photovoltaic industry has promoted the application of graphite materials more widely. This paper mainly analyzes the production methods of high-purity graphite and its practical application in the production of photovoltaic industry, and discusses the future development of high-purity graphite materials in combination with the production characteristics of photovoltaic industry.

      Graphite materials can be divided into natural graphite and artificial graphite, and the application of natural graphite is limited due to the large number of powder forms, so the development of high-purity graphite has become more and more important; the application of high-purity graphite in photovoltaic production has increased in recent years. Due to the rapid development of the photovoltaic industry, it has rapidly heated up. The process of photovoltaic industry from basic raw material silicon ore to final photovoltaic system application is shown in Figure 1.

      With the rapid development of the solar photovoltaic industry, the demand for its incoming and outgoing raw material monocrystalline silicon cells and polycrystalline silicon cells is increasing. In recent years, large-scale production of monocrystalline silicon processing plants and polycrystalline silicon processing plants have sprung up. Therefore, the production of monocrystalline silicon The raw and auxiliary materials required for polysilicon and polycrystalline silicon are also increasing rapidly, and high-purity graphite is an important one of these raw and auxiliary materials. The development of high-purity graphite also promotes the production of high-quality monocrystalline silicon and polycrystalline silicon. Graphite plays a pivotal role in the photovoltaic industry.

Properties of graphite

      Because of its special structure, graphite has the following special properties:

① High temperature resistance, the melting point of graphite is 3850+-50℃, and the boiling point is 4250℃. Even if it is burned by an ultra-high temperature arc, the weight loss is very small, and the thermal expansion coefficient is also small; the strength of graphite increases with the increase of temperature. At 200 °C, the strength of graphite is doubled;

② Excellent electrical and thermal conductivity, the electrical conductivity of graphite is one hundred times higher than that of ordinary non-metals, and the thermal conductivity exceeds that of steel, iron, lead and other metal materials, and its thermal conductivity decreases with the increase of temperature, even at higher temperatures down, it becomes an insulator; the smaller the coefficient, the

③ Lubricity, the lubricating performance of graphite depends on the size of graphite flakes, the larger the flakes, the better the friction and lubrication performance of graphite;

④ Chemical stability, graphite has good chemical stability at room temperature, and is resistant to acid, alkali and organic solvent corrosion;

⑤ Plasticity, graphite has good toughness and can be rolled into very thin sheets;

⑥ Thermal shock resistance, when graphite is used at room temperature, it can withstand severe changes in temperature without damage. When the temperature changes suddenly, the volume of graphite changes little, and fission will not occur.

      It can be seen from the above excellent characteristics of graphite that graphite as a raw material for industrial production has many unique advantages. Therefore, graphite materials have a wide range of applications in industrial production, especially in the rapid development of the photovoltaic industry today. more widespread application.

Production method of high-purity graphite

      The premise of graphite deep processing industry is purification. Graphite purification is a complex physical and chemical process. The purification methods mainly include flotation method, alkaline acid method, hydrofluoric acid method, chlorination roasting method and high temperature method. The equipment requirements, product carbon content, advantages and disadvantages of each purification method are shown in Table 4.

2.1 Flotation method

      Flotation is a common and important beneficiation method. Graphite has good natural floatability. Basically, all graphite can be purified by flotation. In order to protect graphite flakes, graphite flotation mostly adopts multi-stage process. The graphite flotation collector generally uses kerosene, and the dosage is 100-200g/t, and the foaming agent generally uses terpineol oil or butyl ether oil, and the dosage is 50-250g/t.

      Because graphite has good natural floatability, flotation method can increase the grade of graphite to 80% to 90%, or even up to about 95%. The greater advantage of this method is the one with the lowest energy and reagent consumption and lower cost of all purification schemes. However, the compounds of silicate minerals and potassium, calcium, sodium, magnesium, aluminum and other elements mixed in the graphite flakes in a very fine state cannot be dissociated by the grinding method, and it is not conducive to the protection of large graphite flakes. . Therefore, flotation method is only the primary means of graphite purification. In order to obtain high-carbon graphite with a carbon content of more than 99%, other methods must be used for purification.

2.2 Alkaline acid method

      The alkali-acid method includes two reaction processes: alkali melting process and acid leaching process. The alkali melting process is a chemical reaction between the alkali in the molten state and the acidic impurities in the graphite under high temperature conditions, especially the impurities containing silicon, to generate soluble salts, and then washing to remove the impurities, so that the purity of the graphite can be improved. The basic principle of the acid leaching process is to use acid to react with metal oxide impurities, which do not react with alkali during the alkali melting process. The metal oxides are converted into soluble salts, and then washed to separate them from graphite, and the combination of alkali melting and acid leaching has a good effect on graphite purification.

      The alkaline-acid method is the most widely used method in the industrial production of graphite purification in my country. It has the characteristics of less one-time investment, high product quality, strong adaptability, etc., as well as the advantages of simple equipment and strong versatility. The disadvantage is that it requires high temperature calcination, large energy consumption, long process flow, serious equipment corrosion, large loss of graphite and serious waste water pollution.

2.3 Hydrofluoric acid method

      Hydrofluoric acid is a strong acid that can react with almost any impurities in graphite, and graphite has good acid resistance, especially hydrofluoric acid, which determines that graphite can be purified with hydrofluoric acid. The main process of the hydrofluoric acid method is to mix graphite and hydrofluoric acid. The hydrofluoric acid reacts with impurities for a period of time to produce soluble substances or volatiles, which are washed to remove impurities, dehydrated and dried to obtain purified graphite.

      The hydrofluoric acid method to purify graphite has the advantages of simple process flow, high product quality, relatively low cost, and little impact on the performance of graphite products. However, hydrofluoric acid is highly toxic, and safety protection measures must be taken during use. The generated wastewater must be treated before it can be discharged, otherwise it will cause serious pollution to the environment.

2.4 Chlorination roasting method

      The chlorination roasting method is to mix graphite and a certain reducing agent together, and roast at high temperature under a specific equipment and atmosphere. The valuable metal in the material is converted into metal chloride in the gas phase or condensed phase, and is separated from the remaining components to make the graphite Purification process.

The advantages of the chlorination roasting method lie in energy saving, high purification efficiency and high recovery rate, but there are also problems such as toxic chlorine, severe corrosiveness and serious environmental pollution. The purity of graphite produced in the process is limited, and the process stability is not good, which affects the application of the chlorination method in actual production, and needs to be further improved and improved.

2.5 High temperature purification method

      The melting point of graphite is 3850℃±50℃, which is one of the substances with a higher melting point in nature, much higher than the boiling point of impurity silicate. Using the difference of their melting and boiling points, the graphite is placed in a graphitized graphite crucible, and in a certain atmosphere, heated to 2700 ℃ with specific equipment, the impurities can be vaporized and escaped from the graphite to achieve the effect of purification. . This technology can purify graphite to more than 99.99%.

      The high-temperature method purifies graphite, the product quality is high, and the carbon content can reach more than 99.995%. This is a greater feature of the high-temperature method, but at the same time, it consumes a lot of energy and requires extremely high equipment. It needs special design and investment. Graphite raw materials also have certain requirements.

Application of High Purity Graphite

      With the rapid development of the photovoltaic industry, the amount of high-purity graphite, an important raw and auxiliary material in production, has risen sharply. The domestic production technology of high-purity graphite products is still in its infancy, and most materials are imported and then processed.

      Many raw and auxiliary materials in the photovoltaic industry need to use high-purity graphite with stable performance and few interference factors as processing raw materials; high-purity graphite is used in the entire photovoltaic industry from silicon smelting to polysilicon production, to polysilicon ingots and Czochralski monocrystalline and more processes.

3.1 Application of graphite in metal silicon smelting

      It can be seen from Figure 1 that the production of solar photovoltaic cells starts from the smelting of metallic silicon. A key equipment used in the smelting of metallic silicon is the metallic silicon smelting furnace; An important part, the current is input into the furnace through the electrode to generate an arc, which is an important link in chemical silicon smelting.

The requirements for electrode materials in terms of production characteristics are:

①Good conductivity and low resistivity to reduce power loss;

② The melting point should be high, the thermal expansion coefficient should be small, and it is not easy to deform;

③ Sufficient mechanical strength at high temperature and low impurity content.

The electrode made of high-purity graphite has low ash content, good electrical conductivity, heat resistance and corrosion resistance, and is a better choice for chemical silicon smelting. Therefore, high-purity graphite has broad application prospects in metal silicon smelting.

3.2 Application of graphite in polysilicon production

At present, more than 85% of solar-grade polysilicon products on the market are produced by the modified Siemens process. In the production process of the modified Siemens process, high-purity graphite is widely used in multiple processes due to its excellent characteristics; for example, the reduction of polysilicon vapor deposition A large number of graphite consumables are used in the furnace, and high-purity graphite materials are also used in the thermal hydrogenation process of the by-product silicon tetrachloride.

3.2.1 High-purity graphite material in vapor deposition reduction furnace

The production of polysilicon by vapor deposition method is carried out in a closed reduction furnace, and the mixed gas of trichlorosilane and hydrogen in a specific ratio is passed into the reduction furnace, and the mixed gas is deposited on the high temperature silicon core carrier to obtain polysilicon products; A special material is required between the silicon core of the deposition carrier and the electrode that transmits current, which can not decompose at high temperature and does not participate in the reaction of silicon. High-purity graphite can just meet this requirement; graphite parts made of high-purity graphite are today An irreplaceable material for fixing silicon core and transmitting current, the development of high-purity graphite is beneficial to improve the inherent quality of polysilicon products.

Under the existing technical conditions, the application of high-purity graphite products in polysilicon production is irreplaceable, so if the pollution of graphite parts to polysilicon is to be minimized, the quality of graphite parts must be started. First, select high-purity or ultra-high-purity graphite with low impurity content.

Then, using more advanced processing technology, such as improving the processing technology, the graphite has less ash and higher strength; in addition, with the help of today's advanced surface treatment technology, graphite parts can be coated, such as silicon coating, carbonization Silicon etc.

It is also reported that high-purity graphite material composite plates are added in the polysilicon reduction furnace. These graphite material composite plates are distributed between the inner wall of the reduction furnace and each pair of electrodes, which can well maintain the thermal field distribution in the reduction furnace and improve the thermal energy. The utilization rate, from the perspective of energy saving, can reduce the production cost of polysilicon.

3.2.2 High-purity graphite material in thermal hydrogenation furnace

The thermal hydrogenation technology uses silicon tetrachloride and hydrogen as raw materials, and is heated by a graphite heating element at 120-1250 ℃ to carry out a reduction reaction to generate trichlorosilane. The silicon tetrachloride hydrogenation furnace is the key equipment in the thermal hydrogenation process. During the thermal hydrogenation reaction in the hydrogenation furnace, a certain amount of silicon powder will be produced. When the silicon powder is deposited on the graphite heating element, the deposited silicon powder will The distance between the graphite heating elements will become smaller, and there is a potential difference between the graphite heating elements, which will lead to discharge between the graphite heating elements, which will damage the equipment and reduce the service life of the equipment.

At present, some studies have pointed out that in order to reduce heat loss and improve thermal efficiency in the thermal hydrogenation furnace, the method of heat exchange equipment prepared by ink material is adopted to exchange heat between the high temperature reaction gas discharged from the hydrogenation furnace and the gas entering the hydrogenation furnace; and The heat loss is reduced by means of built-in heat radiation screen and rigid high-density high-purity graphite. Therefore, high-purity graphite materials are more and more widely used in thermal hydrogenation furnaces.

3.3 Application of graphite in polysilicon ingot furnace

In a polysilicon ingot furnace, graphite material is required for several components. In particular, the heating material and heat insulating material used in the heater are important supporting materials at present. Therefore, the development of high-purity graphite material promotes the further development of the production process of polysilicon ingots.

3.3.1 Heating materials used in heaters

In the design of the polycrystalline silicon ingot furnace, in order to melt the silicon material, a suitable heating method must be adopted. In terms of heating effect, both induction heating and radiation heating can achieve the desired temperature. Radiation heating is generally used, which can control the heat transfer in the crystallization process, and is easy to form a vertical temperature gradient inside the accretion.

Therefore, the heating capacity of the heater must exceed 1650 °C, and the material of the heater cannot react with the silicon material, so it will not cause pollution to the silicon material, and can be used for a long time in vacuum and inert atmosphere. There are metal tungsten, molybdenum and non-metallic graphite for the heaters that can be selected according to the conditions of use. Because tungsten and molybdenum are expensive and difficult to process, graphite has a wide range of sources and can be processed into various shapes.

In addition, graphite has the characteristics of small thermal inertia, rapid heating, high temperature resistance, good thermal shock resistance, large radiation area, high heating efficiency, and stable basic performance, so high-purity graphite material can be used as a good heater material. widely used.

3.3.2 Insulation materials used in heaters

For the ingot casting process, in order to improve the production efficiency, the heating rate of the equipment is required to be as fast as possible; due to the vacuum process, the outgassing amount of the materials in the furnace should be as small as possible, and the vacuum exhaust time should be shortened; The formation of the gradient also requires the improvement of the thermal insulation layer, and the mass of the thermal insulation layer should be as light as possible to reduce the inertia during lifting and affect the control accuracy.

In summary, the selection requirements for thermal insulation materials are: high temperature resistance, low density, low thermal conductivity, less heat storage, good thermal insulation effect, less outgassing, light weight, and small expansion coefficient. Among many refractory thermal insulation materials, High-purity graphite is the most ideal.

Graphite material plays an irreplaceable role in the production process of polysilicon ingot, and its excellent characteristics provide a guarantee for maintaining a high temperature field and excellent thermal insulation effect in the ingot furnace. Therefore, the future development of graphite material can further promote polysilicon casting. The optimization and transformation of the ingot furnace also contributed to the reduction of the production cost of polysilicon ingots.

3.4 The use of graphite in Czochralski monocrystalline silicon

The thermal system of the Czochralski single crystal furnace refers to the whole system for melting the silicon material and maintaining the single crystal growth at a certain temperature. There are more than 30 kinds of heat shielding plates, seed crystal holders, bases for rotating disasters, various circular plates, and heat reflecting plates.

Among these high-purity graphite products, the heater is the most important part in the thermal system, it is a direct heating body, and the temperature is higher than 1600 ℃; the graphite electrode connected to the heater should not only support the heater smoothly, but also need to pass through it. The heater is heated, so the graphite electrode needs to be thick and durable. The contact surface between it and the metal electrode and the heater needs to be smooth and stable, so as to ensure good contact and no ignition when energized. This requires special treatment of the graphite electrode. ; The high-purity graphite citrus used to hold the silicon material must have a certain strength to bear the weight of the silicon material.

With the increasing application of graphite materials in Czochralski single crystals