Fused silica crucibles are widely used in the production of polysilicon ingots because of their good thermal stability, low coefficient of thermal expansion and thermal conductivity, and no chemical reaction with silicon at high temperatures. In addition, quartz ceramics also have good dielectric strength, which is a good electrical insulator and light wave reflector. Due to the above-mentioned excellent properties, fused silica ceramics have been rapidly promoted and applied since their inception, and their application fields involve metallurgy, chemical industry, electrical engineering, aerospace and other industrial fields.
1. Introduction to fused silica crucibles.
As a clean energy source, solar cells have received widespread attention around the world. As shown in Figure 1-1 below, polysilicon is the material with the highest industrialization ratio due to its high conversion efficiency and yield, and relatively low raw material requirements.
Fused silica crucible is a kind of fused silica ceramic, widely used in the polysilicon ingot technology of solar photovoltaic industry, because the current polysilicon ingot is basically square, so the fused silica crucible is often called quartz square crucible In the process of polysilicon ingot, the use environment of the fused silica crucible is very harsh, to withstand the high temperature of 1550 °C and work continuously for more than 50h at one time, so the purity of the raw materials and various properties used in the quartz crucible are higher, so the production technology of the crucible is difficult. It is a key consumable in the production process of multi-product silicon. At present, the mainstream products in the market are G5 series and G6 series, with dimensions of 883mmx883mmx420mm, 480 mm, and 1040mmx1040mmx500mm respectively. With the rapid development of the photovoltaic industry, G6 series crucibles have gradually become the mainstream demand for containers for multi-product silicon ingots. At present, the development trend of quartz pots is large size and thin wall, so higher process requirements are put forward for the production of crucibles.
According to a recent survey by the China Electronic Materials Industry Association, there are currently six major manufacturers of quartz crucibles in China, and their production capacity is shown in Table 1-2.
Quartz ceramic crucible is a key consumable component in the process of solar polysilicon ingot, although the current high polysilicon raw materials and the shortage of polysilicon make the industry seem to ignore other problems, but quartz ceramic crucible is the auxiliary material in the production process of polysilicon in addition to the cost of the largest consumption, but also subject to others, is still an important problem faced by enterprises. In addition, the number of imported crucibles in China still occupies a large proportion, so the realization of the localization of crucibles, thereby greatly reducing the production cost of multi-grade silicon ingots is still the goal that many enterprises continue to strive for.
Second, the preparation process of quartz ceramic crucible.
There are four molding methods for quartz ceramic crucibles: grouting molding, injection molding, filter pressing molding and injection coagulation molding.
1.Grouting molding
Slip casting is to use the water absorption characteristics of porous molds (mostly gypsum molds) to inject the configured slurry into the mold, and the water is absorbed to form a green blank with a certain strength. The advantage of this molding method is that the equipment is simple, the cost is low, and it is convenient to prepare products with large size and complex shape. Generally speaking, the grouting type is mainly divided into single-sided grouting and double-sided grouting suction.
In this molding process, in order to obtain high-quality green body, the main requirements for slurry are: (1) good fluidity, that is, slurry viscosity should be small, can ensure that slurry is easy to flow to all parts of the model in the grouting process; (2) Good stability to ensure the consistency of each component of the slurry, as well as the uniformity of the green body after molding; (3) a certain thixotropy, that is, after a certain period of time, the viscosity change of the slurry should not be too large, which is convenient for transportation and storage; (4) High solid content, ensure the strength of the green body after molding and reduce the shrinkage rate of the green body after drying; (5) Good filtration, to ensure that the water medium in the slurry can be absorbed by the gypsum mold; (6) does not contain bubbles, so the slurry should generally be degassed before grouting .
2.Injection molding
Injection molding is the process of melting plastic from an injection machine into a mold under the action of a plunger or screw thrust, and then cooling to obtain a product. The principle is: the solid plastic is melted according to a certain melting point, injected into the mold at a certain speed through the pressure of the injection machine, and the mold is cooled by the water channel to solidify the plastic and obtain the same product as the designed mold cavity. The advantage of this molding method is that the parts with uniform structure, accurate size and complex shape can be formed; The disadvantage is that the mold design process adopted in the molding process is complicated, and it is difficult to exclude organic additives in the product in the drying process.
3.Filter press molding
Filter press molding (PSC) is a molding process developed under pressure on the basis of grouting molding. Its principle is to inject the slurry into the porous mold, and the liquid medium in the slurry is filtered out by applying pressure, and the body of a certain shape is obtained.
Although from the principle point of view, filter press molding is similar to pressure grouting, that is, the use of external pressure to make the ceramic slurry remove part of the liquid medium and form, so in a broad sense, filter press molding can be regarded as pressure grouting molding. But there is this significant difference between the two: first, the filter press molding adopts a very thin porous filter layer, so the requirements for slurry medium content are not as strict as pressure grouting, and the rheological properties and particle gradation of slurry can be adjusted in a wide range to facilitate the molding of high-performance products; Second, the adjustable pressure range during filter press molding is wider; Thirdly, for the parts with complex shapes, in order to obtain a uniform green body structure, parameters such as model structure and permeability coefficient of different parts can be adjusted in the process of filter press molding, so that different parts have different curing rates.
4.Injection molding
Gel Casting is a new molding process invented by the United States at the end of the 20th century and applied to ceramic preparation. The principle is that the organic polymer and the crosslinker are configured into an aqueous or non-aqueous solution of the precursor, and a certain amount of ceramic powder is dispersed in the precursor to obtain a suspension with a certain solid content; Then add a certain amount of initiator and catalyst, and after full mixing, pour the slurry into a mold of a certain shape; Through the polymerization reaction of organic monomers, the whole system forms a three-dimensional network structure, and the ceramic powder dispersed in it is solidified in situ to form a certain shape of the green body. The so-called in-situ curing refers to the force between the particles or the change of the carrier properties within the system to make the suspended body become solid, and the position of the particles in the system is fixed. In this process, there is almost no volume shrinkage in the whole system, and there is no quantitative change in the number of carriers in the system.
According to the different media used, injection coagulation molding can be divided into non-water-based injection molding and water-based injection coagulation. The non-aqueous injection molding process is suitable for systems where ceramic particles can react with water during molding. The water-based injection molding process is suitable for the molding of quartz ceramics and most other ceramics, and is a molding process with good development prospects [18]. For the water-based injection molding process, the most widely used organic polymer monomer is acrylamide, which can be adsorbed on the surface of the particles, and the monomer polymerization reaction occurs after the addition of crosslinkers, initiators and catalysts, so that the particles are solidified in situ to form a dense green billet.
Source**: Tianjin University; Zhang Xiaoyan, Du Haiyan.
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