Views: 0 Author: Site Editor Publish Time: 2023-05-04 Origin: 中国粉体网
Solar polycrystalline silicon cell is the most widely used and the most mature product in solar cell technology. However, in the process of melting polysilicon ingot, the container containing polysilicon is very demanding. As a refractory material that can resist temperature change, fused quartz ceramic is regarded as the perfect container containing polysilicon ingot.
Fused quartz ceramics is made of quartz glass or fused quartz as raw material, the products prepared by grinding, forming, sintering and other processes are a new kind of high purity and high temperature resistant quartz materials, also known as fused quartz sintered products, quartz glass ceramics.
Compared with quartz glass, quartz ceramics can be regarded as a comprehensive upgrade of quartz glass. The excellent characteristics of fused quartz ceramics are mainly reflected in the following aspects:
(1) Excellent corrosion resistance, except 300 ℃ hot concentrated phosphoric acid and hydrofluoric acid can react with it, it almost does not react with other acids and bases.
(2) Good thermal shock resistance, samples at 25℃-1100 ℃ thermal shock more than 30 times will not produce microcracks or surface spalling.
(3) the coefficient of thermal expansion is low, the temperature is 0℃-800 ℃, the coefficient of thermal expansion is only 0.54X10-6/℃, therefore, when the fused quartz ceramics are used in this temperature section, the volume change is small, the thermal shock resistance is good, so that the fused quartz ceramics overcome the harsh use environment.
(4) fused quartz ceramics have low thermal conductivity, dielectric constant and dielectric loss, the thermal conductivity is 2.09W /mK, dielectric constant is 3.1-3.8, dielectric loss is 4X10-4.
(5) Good insulation, resistance at room temperature 1015Ω.
(6) The flexure strength of fused quartz ceramics increases with the increase of temperature. The flexure strength at 1000 ℃ is 30%-60% higher than that at room temperature. This is because the atom migration rate in fused quartz ceramics is increased at high temperature, which makes part of the crack tip dull, thus increasing the flexure strength.
In the 1960s, the fused quartz ceramics was first developed by the Georgia Institute of Technology in the United States. Its excellent characteristics make it quite popular in the developed countries, and it is rapidly promoted and put into use in the developed industrial cities of the United States, Japan, France and other countries. Our country also began to be put into production and use in 1972.
The world's leading countries in molten quartz processing technology are the United States, Germany, Japan and Russia, but all implement technology blockade on our country, domestic restricted by purification technology and cost, large-scale production of molten quartz SiO2 content has been unable to break through 99.99%, Faced with low added value of products, high energy consumption, environmental problems prominent, serious waste of resources and other problems.
At present, the preparation methods of fused quartz ceramics mainly include gel injection molding, grouting molding, isostatic pressing molding, hot die casting molding, among which, gel injection molding is one of the commonly used methods in China.
4.1 Grouting molding
Grouting molding is a production process in which materials are put into the solution of a few water-soluble organic substances, stirred evenly, and formed into slurry, and then injected into the porous mold (generally gypsum mold). The capillary force of the porous mold and the water absorption function of gypsum are used to absorb the moisture in the slurry and make the slurry forming.
The process of preparing fused quartz ceramics by grouting is simple, the cost is low, and it is suitable for preparing products with complex shapes. However, there are some problems such as long forming time and poor uniformity of the billet.
4.2 Injection Molding
Injection molding is to add ceramic powder and solvent to a certain nature and function of the organic carrier, heating mixed evenly, through the equipment to use a very fast speed into the mold to let the sample natural molding, and finally the organic matter in the body of the billet through a more complex chemical physical method to get rid of.
Injection molding can be large-scale production and automation, but the sample is easy to produce defects, the success rate of the product is low, and the organic binder in the injection molding process sticks to the mold, and it is particularly difficult to remove.
4.3 Condensate injection type
The gel system is introduced into the molten quartz slurry to form a three-dimensional network structure, so that the ceramic particles can be in-situ cured to form a blank body. In situ curing is to keep the particles in the original position of the suspension, relying on the force between the particles and the gel system, so that the slurry from liquid to solid, to obtain the body.
Injection molding is a kind of molding process similar to grouting molding and injection molding. Compared with the grouting molding, the products prepared by the injection molding are more uniform inside, higher precision and near net size molding. Compared with injection molding, injection molding has more advantages. Injection molding does not need expensive molds, only needs a small amount of organic matter, and can be used as the medium of water for automated production.
4.4 Hot die casting
Hot die casting is to use the rheological characteristics of organic matter (mostly paraffin) at high temperature to pour the slurry into the mold made of paraffin. Generally, heating makes the slurry in a molten state. After cooling, it is put together into the metal mold to solidify into a blank body.
Hot die casting molding operation is simple, forming time is short, forming products basically do not need post-processing, sintering products have good density. Based on the above advantages, hot die casting has been widely used.
But the hot die casting process is complex, large energy consumption, the cost is relatively high, and the amount of paraffin and surfactant (oleic acid, stearic acid) will also affect the fluidity of molten quartz and the strength of the billet, and then affect the forming process.
Fused quartz ceramics have strong corrosion resistance, high thermal shock resistance, low thermal conductivity and excellent performance, as structural materials and functional materials used in the glass industry, metallurgy industry, aerospace industry, polysilicon industry, electronics industry and many other fields.
5.1 Glass industry
In the glass industry, because of the good thermal stability of fused quartz ceramic products, it is used as fused quartz ceramic roller, air nozzle, ceramic inner pouring cup, ram brick in float glass, etc.
5.2 Metallurgical industry
In the metallurgical industry, because of its high strength, small thermal expansion, good thermal shock resistance and other excellent properties, molten quartz ceramics are used as molten quartz riser pipe, molten quartz nozzle, etc.
5.3 Aerospace industry
In the aerospace industry, fused quartz ceramic products are used as missile radomes and meet the requirements of missile shape and strength because of their low loss and low distortion during radar wave transmission.
5.4 Polysilicon industry
In the polysilicon industry, the fused quartz ceramic crucible is an indispensable component for the preparation of solar polysilicon ingot. Because the composition of the fused quartz ceramic crucible is similar to that of polysilicon, it has a small coefficient of thermal expansion, good thermal shock resistance and high volume density and bending strength. At the same time, the crucible is a disposable consumable in the process of preparing polysilicon ingot, and the fused quartz ceramic products can also save a lot of cost.
5.5 Electrical Engineering Industry
In the electrical industry, fused quartz ceramics because of its high breakdown voltage, low dielectric constant, low loss Angle tangent value, good thermal shock stability and other advantages, fused quartz ceramic products are widely used as gas discharge room insulation cover, diversion heater, gas laser discharge room components.
