With the rapid development of the photovoltaic industry, the use of high-purity graphite, which is an important raw material in production, has risen sharply. The domestic production technology of high-purity graphite products is still in its infancy, and most of the materials are imported and then processed.
From the jump growth of foreign exchange volume of high-purity graphite for photovoltaic solar energy, it can be seen that the high-purity graphite has increased greatly with the rapid development of the photovoltaic industry in recent years, which has a great relationship with the wide application of high-purity graphite in the photovoltaic industry, 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 whole photovoltaic industry from silicon smelting to polysilicon production, to polycrystalline silicon ingots and straight pull single crystals and other processes.
1 Application of high-purity graphite in metal silicon smelting
Solar photovoltaic cell production begins with metal silicon smelting. One of the key equipment used in metal silicon smelting is a metal silicon smelting furnace. In a metal silicon smelting furnace, the electrode is the heart and is an important part of the conductive system. The current is passed the electrode is input into the furnace to generate an arc, which is an important part of chemical silicon smelting. The requirements for electrode materials from the production characteristics are: 1 good conductivity, low resistivity to reduce power loss; 2 high melting point, small thermal expansion coefficient, not easy to deform; 3 high mechanical strength at low temperature, low impurity content .
The electrode made of high-purity graphite has low ash content, good electrical conductivity, heat resistance and corrosion resistance, and is the best choice for chemical silicon smelting. Therefore, high-purity graphite has broad application prospects in metal silicon smelting.
2 Application of high-purity graphite in polysilicon production
At present, more than 85% of the solar grade polysilicon products on the market are produced by the modified Siemens process. In the process of improving the Siemens process, high-purity graphite is widely used in many processes with its excellent characteristics; For example, a large number of graphite parts are used in the reduction furnace of polycrystalline silicon vapor deposition, and high purity graphite materials are also used in the thermal hydrogenation process of treating by-product silicon tetrachloride.
High-purity graphite material in vapor deposition reduction furnace
The polycrystalline silicon produced by the vapor deposition method is carried out in a closed reduction furnace, and a specific mixture of trichlorosilane and hydrogen is introduced into the reduction furnace, and the mixed gas is deposited on the high temperature silicon core carrier to obtain a polycrystalline silicon product; A special material is needed between the electrode for depositing the deposited carrier and the electrode for carrying current. It can not decompose at high temperature and does not participate in the reaction of silicon. High-purity graphite can meet this requirement. Graphite parts prepared from high purity graphite is now an irreplaceable material for fixed silicon core and transmission current, and the development of high-purity graphite is beneficial to improve the intrinsic quality of polysilicon products.
Under the existing technical conditions, the application of high-purity graphite products in the production of polycrystalline silicon is irreplaceable. Therefore, if the contamination of polycrystalline silicon by graphite parts is to be minimized, it is necessary to start from the quality of the graphite parts. First, high-purity or ultra-high-purity graphite with a low impurity content is selected. Then, using more advanced processing techniques, such as improved processing, the graphite has less ash and higher strength. In addition, with today’s advanced surface treatment technology, graphite parts can be coated, such as silicon and silicon carbide.
It is also reported that a high-purity graphite material composite plate is added in the polysilicon reduction furnace, and the graphite material composite plates are distributed between the inner wall of the reduction furnace and the pair of electrodes, It can keep the heat field distribution in the reduction furnace well, improve the utilization rate of heat energy, and reduce the production cost of polysilicon from the point of view of energy saving.
High-purity graphite material in hot hydrogenation furnace
The thermal hydrogenation technology uses silicon tetrachloride and hydrogen as raw materials, and is heated by a graphite heating element at 1200 to 1250 °C to carry out a reduction reaction to form trichlorosilane. The silicon tetrachloride hydrogenation furnace is the key equipment in the thermal hydrogenation process. In the thermal hydrogenation reaction occurring in the hydrogenation furnace, a certain amount of silicon powder is generated. When the silicon powder is deposited on the graphite heating element, the deposition of silica fume will make the distance between graphite heaters smaller, and there is a potential difference between the graphite heating elements, which causes discharge between the graphite heating elements, causes damage to the equipment and reduces the service life of the equipment.
At present, it is pointed out that in the thermal hydrogenation furnace, in order to reduce heat loss and improve thermal efficiency, a heat exchange device with a built-in graphite material is adopted to exchange heat between the high-temperature reaction gas discharged from the hydrogenation furnace and the gas entering the hydrogenation furnace; The use of built-in thermal radiation screen and rigid high-density high-purity graphite to reduce heat loss.
Therefore, the application of high purity graphite materials in hot hydrogenation furnaces is more and more extensive.
3 Application of high purity graphite in polycrystalline silicon ingot furnace
In polycrystalline silicon ingot furnaces, multiple components require graphite material. In particular, the heating materials and heat insulating materials used in the heaters are currently important supporting materials, so the development of high-purity graphite materials has promoted the further development of the polycrystalline silicon ingot production process.
Heating material used in the heater
In the design of a polycrystalline silicon ingot furnace, in order to melt the silicon material, a suitable heating method must be employed. In terms of heating effect, both induction heating and radiant heating can reach the desired temperature. Radiation heating is generally used, which can precisely control the heat transfer of the crystallization process, and it is easy to form a vertical temperature gradient inside the disaster. Therefore, the heating capacity of the heater must exceed 1650 °C, and the material of the heater cannot react with the silicon material, it does not pollute the silicon material, and can be used for a long time in a vacuum and an inert atmosphere. Heaters that meet the conditions of use are available in metallic tungsten, aluminum and non-metallic graphite.
Since tungsten and aluminum are expensive and difficult to process, graphite is widely available 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 properties. Therefore, high-purity graphite material can be obtained as a good heater material to widely used.
Insulation material used in the heater
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 amount of gas released from the furnace should be as small as possible, and the time of vacuum evacuation should be shortened; The formation of the gradient also requires the precise improvement of the insulation layer. The quality of the 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 the heat insulating material are: high temperature resistance, low density, small heat conduction, low heat storage, good heat insulation effect, low gas release, light weight, and small expansion coefficient. Among many refractory insulation materials, It is most ideal for high purity graphite.
Graphite material has an irreplaceable role in the production process of polycrystalline silicon ingots. 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 materials can further promote polysilicon casting. The optimization of the ingot furnace also contributes to the decline in the production cost of polycrystalline silicon ingots.
4 Application of graphite in Czochralski 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. In the Czochralski silicon thermal field, there are about 30 kinds of graphite parts, such as vortex, heater, electrode, insulation shielding plates, seed crystal holders, bases for rotation and disaster reduction, various circular plates, and heat reflection plates.
Among these high-purity graphite products, the heater is the most important component in the thermal system, and it is a direct heating element with a temperature of up to 160 °C. The graphite electrode connected to the heater not only needs to support the heater smoothly but also needs to pass it. The heater is heated, so the graphite electrode is required to be heavy and durable, and its contact surface with the metal electrode and the heater needs to be smooth and stable, ensuring good contact, and does not ignite when energized, which requires special treatment of the graphite electrode; The high-purity graphite used to hold the silicon material must have a certain strength to withstand the weight of the silicon material.
With the increasing use of graphite materials in Czochralski single crystals, the development of graphite materials can improve the Czochralski single crystal process, which is beneficial to further reduce the production cost of Czochralski single crystals and improve the production of Czochralski single crystals.
4 Development proposals for graphite materials in the photovoltaic industry
With the continuous development of the solar photovoltaic industry, the demand for raw and auxiliary materials in the production process of solar photovoltaic materials is also increasing rapidly. Therefore, the development direction of graphite materials for the photovoltaic industry should be studied according to the unique characteristics of the photovoltaic industry, and its future development direction should be From the following aspects:
(l) The graphite materials required by the solar photovoltaic industry are characterized by ultrafine and high purity. Therefore, ultrafine and high purity can be studied as two core problems of graphite deep processing. Different pulverization process have certain purification processes for graphite. The effect of stripping and purifying is beneficial to protect the large scales of graphite, but the corrosion-resistant grinding medium loss directly affects the effect of acid purification. The pulverization and purification process should be designed as the whole process to avoid interference from human factors and produce high-purity ultrafine graphite powder.
(2) At present, the graphite material in the photovoltaic industry can withstand high temperature, but the service life is not long enough, so that there are more consumables in the production; therefore, it is necessary to further develop a new type of graphite material, so that it can maintain the high temperature resistance and resistance of the existing high-purity graphite. Oxidation and other characteristics can also enhance mechanical strength and extend service life. This can reduce the amount of graphite consumables in solar photovoltaic production and greatly reduce the cost of solar photovoltaic production.
(3) Conducting research on the application of high-purity graphite composite materials, materials such as carbon-carbon materials, high-purity graphene and isostatic graphite are being studied more and more, and some performance improvements can be better for high-purity graphite materials. Applied to the photovoltaic industry, these improved materials based on high-purity graphite materials are the mainstream of future development.
Author: Shi Hewu，Tang Chuanbin
