Research progress on application of SICI monocrystalline silicon carbon/carbon thermal field materials

Solar photovoltaic power generation has become the most promising new energy industry in the world. Compared with polycrystalline silicon and amorphous silicon solar cells, monocrystalline silicon, as a photovoltaic power generation material, has high photoelectric conversion efficiency and outstanding commercial advantages, and has become the mainstream of solar photovoltaic power generation. Czochralski (Czochralski,CZ) is one of the main methods for the preparation of monocrystalline silicon, and the composition of the Czochralski single crystal furnace includes furnace system, vacuum system, gas system, thermal field system and electrical control system. Among them, the thermal field system is one of the most important conditions for the growth of monocrystalline silicon, and the quality of monocrystalline silicon is directly affected by the temperature gradient distribution of the thermal field.

The thermal field components are mainly composed of carbon materials (graphite materials and carbon/carbon composite materials), and are divided into supporting parts, functional parts, heating elements, protection parts, thermal insulation materials, etc. according to their functions, as shown in Figure 1. With the increasing size of monocrystalline silicon, the size requirements for thermal field components are also increasing, and carbon/carbon composites have become the first choice for thermal field materials for monocrystalline silicon due to their dimensional stability and excellent mechanical properties.

In the process of Czochralski monocrystalline silicon, the melting of silicon material will produce silicon vapor and molten silicon splash, resulting in silicidation erosion of carbon/carbon thermal field materials, and the mechanical properties and service life of carbon/carbon thermal field materials are seriously affected. Therefore, how to reduce the silicidation erosion of carbon/carbon thermal field materials and improve their service life has become one of the common concerns of monocrystalline silicon manufacturers and carbon/carbon thermal field material manufacturers. Silicon carbide coating has become the first choice for surface coating protection of carbon/carbon thermal field materials due to its excellent thermal shock resistance and wear resistance.

In this paper, the main preparation methods and advantages and disadvantages of silicon carbide coatings are introduced from the carbon/carbon thermal field materials used in the production of monocrystalline silicon. On this basis, the application and research progress of silicon carbide coatings in carbon/carbon thermal field materials are reviewed according to the characteristics of carbon/carbon thermal field materials, and some suggestions and development directions for the surface coating protection of carbon/carbon thermal field materials are put forward.

1 Preparation Technology of Silicon Carbide Coating

1.1 embedding method

The embedding method is commonly used in the preparation of silicon carbide inner coating in the C/C-SiC composite system, which first wraps the carbon/carbon composite material with mixed powder, and then heat treats it at a certain temperature, and a series of complex physical and chemical reactions occur between the mixed powder and the surface of the sample to form the coating. The advantage is that the process is simple, only a single process can be prepared compact, crack-free matrix composite materials; from pre-forming to the final product, the size change is small; for any fiber-reinforced structure is applicable; between the coating and the substrate can form a certain composition gradient, and the substrate is better combined. However, there are also shortcomings, such as chemical reactions at high temperatures are prone to damage the fibers, and the mechanical properties of the carbon/carbon matrix are reduced; the uniformity of the coating is difficult to control, and the coating is uneven due to factors such as gravity.

1.2 slurry coating method

The slurry coating method is to mix the coating material and the binder into a mixture, evenly brush on the surface of the substrate, and then sinter the coated specimen at high temperature in an inert atmosphere after drying to obtain the required coating. The advantage is that the process is simple and easy to operate, and the thickness of the coating is easy to control; the disadvantage is that there is a poor bonding strength between the coating and the substrate, and the thermal shock resistance of the coating is poor, and the coating uniformity is low.

1.3 chemical vapor reaction method

The chemical vapor reaction method is a process method in which the solid silicon material is evaporated into silicon vapor at a certain temperature, and then the silicon vapor diffuses into the interior and surface of the substrate, and reacts with the carbon in the substrate to generate silicon carbide. The advantages include uniform atmosphere in the furnace, consistent reaction speed and deposition thickness throughout the coated material, etc. The process is simple and easy to operate, and the coating thickness can be controlled by changing parameters such as silicon vapor pressure and deposition time. The disadvantage is that the sample is greatly affected by the placement position in the furnace, and the silicon vapor pressure in the furnace can not reach the theoretical uniformity, resulting in uneven coating thickness.

1.4 chemical vapor deposition

Chemical vapor deposition is a process in which hydrocarbons are used as the gas source and high purity N2/Ar as the carrier gas, and the mixed gas is introduced into the chemical vapor reaction furnace. Hydrocarbons are decomposed, synthesized, diffused, adsorbed and resolved at a certain temperature and pressure, and solid films are formed on the surface of carbon/carbon composite matrix. The advantages are that the density and purity of the coating are controllable; it is also applicable to workpieces with more complex shapes; by adjusting the deposition parameters, the crystal structure and surface morphology of the product can be controlled. The disadvantage is that the deposition rate is too low, the process is complicated, the production cost is high, and there may be coating defects, such as cracks, network defects and surface defects.

To sum up, the embedding method is limited to its process characteristics and is suitable for the development and production of laboratory and small-size materials. The coating method is not suitable for the production of batch products due to its poor process consistency. CVR method can meet the batch production of large-size products, but has high requirements on equipment and process. CVD method is an ideal preparation method for silicon carbide coating, but it is difficult to control the process, the cost is higher than the CVR method.

Application and Research Progress of Silicon Carbide Coating in Carbon/Carbon Thermal Field Materials

Application and Research Progress of 2.1 in Crucible Preparation

The carbon/carbon crucible is mainly used as a bearing container for the silicon material in the single crystal thermal field, and is in contact with the quartz crucible, as shown in Figure 2. The working temperature of carbon/carbon crucible is about 1450 ℃, which is eroded by solid silicon (silicon dioxide) and silicon vapor, and finally the crucible fails due to thinning or circumferential cracks.

Zhang Yonghui et al. prepared a composite coating carbon/carbon composite crucible by chemical vapor infiltration process and in-situ reaction. The composite coating consists of silicon carbide coating (100~300 μm), silicon coating (10~20 μm) and silicon nitride coating (50~100 μm), which can effectively inhibit the erosion of silicon-containing vapor on the inner profile of carbon/carbon composite crucible. In the process of production and use, the single furnace loss of the composite coating carbon/carbon composite crucible is 0.04mm, and the service life can reach 180 furnaces.

Wang Jinduo and others used chemical reaction method to control the mass ratio of silicon dioxide and metal silicon as raw materials in a high-temperature sintering furnace. Under certain temperature conditions and the protection of carrier gas, a uniform silicon carbide coating was generated on the surface of the carbon/carbon composite crucible. The results show that after high temperature treatment, not only the purity and strength of the silicon carbide coating are improved, but also the wear resistance of the carbon/carbon composite surface is greatly improved, and the corrosion of the crucible surface by SiO vapor and volatile oxygen atoms in the single crystal silicon furnace is prevented, and the service life is increased by 20% compared with the crucible without silicon carbide coating.

Application and Research Progress of 2.2 in Diversion Tube

The guide tube is located above the crucible (as shown in Figure 1), in the process of pulling crystal, its temperature field up and down inside and outside the difference is large, especially the bottom outer surface from the molten silicon material closest, the highest temperature, the most serious corrosion by silicon vapor.

Liao Jicao and others invented a simple process and good oxidation resistance oxidation resistance coating for the diversion tube and its preparation method. First, a layer of silicon carbide whisker is grown in situ on the diversion tube substrate, and then a dense silicon carbide outer layer is prepared, thus forming a SiCw transition layer between the substrate and the dense silicon carbide surface layer. As shown in Figure 3, its thermal expansion coefficient is between the substrate and silicon carbide, which can effectively reduce the thermal stress caused by mismatch of thermal expansion coefficient.

The analysis shows that with the increase of SiCw content, the size and number of cracks in the coating decrease, and the weight loss rate of the coating sample is only 0.87~8.87 after oxidation in air at 1100 ℃ for 10h, the oxidation resistance and thermal shock resistance of the silicon carbide coating are greatly improved, and the whole preparation process is completed continuously by chemical vapor deposition, the preparation of silicon carbide coating is greatly simplified, the overall performance of the entire diversion tube is enhanced.

Jiang Jianchun and others proposed a method of substrate strengthening and surface coating of graphite diversion cylinder for Czochralski monocrystalline silicon. The obtained silicon carbide slurry was evenly coated on the surface of graphite diversion cylinder by brush coating method or spraying method, with a coating thickness of 30 ~ 50μm, and then placed in a high-temperature furnace for in-situ reaction. The reaction temperature was 1850~2300 ℃ and kept for 2~6h, thus SiC outer layer was prepared, it can be used in a 24 in(60.96cm) single crystal growth furnace at a temperature of 1500 ℃ and 1500h. It is found that the surface of the graphite deflector is free of cracking and powder falling.

Application and Research Progress of 2.3 in Insulation Cylinder

As one of the key components of monocrystalline silicon thermal field system, the main function of the insulation tube is to reduce heat loss and control the temperature gradient of the thermal field environment. As a support for the insulation layer on the inner wall of the single crystal furnace, the product slagging and cracking due to silicon vapor corrosion will eventually lead to product failure.
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