Silicon carbide is a magical material, whether it is low quality or high quality, has its use, and even in many areas or the preferred material.
For example, in terms of smelting, low-quality silicon carbide is an excellent deoxidizer; in the field of processing, it is the most widely used grinding material; in the field of high temperature, it is a refractory material with excellent performance; in ceramics, Silicon carbide is one of the two most widely used ceramic materials (the other is alumina); more importantly, as a representative of the third generation of semiconductors, silicon carbide has become the hottest "star material".
Different application fields, the quality requirements of silicon carbide are not the same, and their prices are also very different. Let's look at a set of data first. The following is the price survey of silicon carbide micropowder by China Powder Network this week (data up to the end of November).
(Medium Powder Information/Silicon Carbide Weekly)
The above is the price of silicon carbide for some abrasives and ceramics, and the most expensive is about 50000 yuan/ton. What is the price of silicon carbide used in the semiconductor field? It is understood that the price of silicon carbide micropowder used for single crystal growth is2000~12000 yuan/kg, note that its price unit isYuan per kilogram.
Why is silicon carbide for single crystal growth so expensive?
It is understood that the particle size of silicon carbide powder for single crystal growth is about 300~500 μm, and the particle size requirement is not very high,But its purity needs to reach 99.999-99.9999%!
At present, the production of silicon carbide micropowder is mostly made of bulk silicon carbide synthesized by carbothermal reduction method, which is produced by rough breaking, grinding and other processes. In the production process, due to incomplete reaction of raw materials and influence of processing equipment and external environment, the processed silicon carbide micropowder has many impurities such as free carbon, graphite, metal elements such as Fe, Al and oxides.The existing silicon carbide purification process has certain limitations and instability, it is difficult to purify it to the purity required for single crystal growth, which is bound to put forward more stringent requirements for its production process.. Especially when it comes to mass production, few enterprises can do it. At present, companies that can produce high-purity SiC powder in large quantities include China's Tianke Heda, France's Saint-Gobain, Japan Pacific, etc. The purity and price of SiC powder synthesized by different companies are different.
Synthesis Method of Ordinary SiC Powder
The synthesis methods of ordinary SiC powder are varied, and in general, it can be roughly divided into three methods.The first method is the solid phase methodAmong them, the representative carbon thermal reduction method, self-propagating high-temperature synthesis method and mechanical crushing method;The second method is the liquid phase methodAmong them, the representative methods are mainly sol-gel method and polymer thermal decomposition method;The third method is the gas phase methodincluding chemical vapor deposition (CVD), plasma and laser induced methods.
Silicon carbide powder synthesized by solid-phase method is more economical, has a wide range of raw materials and low price, and is easy to be industrialized. However, silicon carbide powder synthesized by this method has high impurity content and low quality. The high-temperature self-propagating method uses high temperature to give the reactant initial heat to start a chemical reaction, and then uses its own chemical reaction heat to make the unreacted substance continue to complete the chemical reaction. However, since the heat released by the chemical reaction of Si and C is small, other additives must be added to maintain the self-propagating reaction, so that impurity elements are inevitably introduced, and this method is likely to cause uneven reaction.
At present, the technology of synthesizing silicon carbide powder by liquid phase method has been more mature, and the silicon carbide powder synthesized by liquid phase method has high purity and is nano-scale micro-powder, but the process is more complex, and it is easy to produce harmful substances to human body.
The silicon carbide powder synthesized by gas phase method has high purity and small particle size, which is a common method for synthesizing high purity silicon carbide powder. However, this synthesis method is not suitable for batch production because of its high cost and low yield.
Synthesis of High Purity SiC Powder
Among the many SiC powder synthesis methods, the gas phase method can obtain high purity SiC powder by controlling the impurity content in the gas source; only the sol-gel method can synthesize SiC powder with purity to meet the needs of single crystal growth in the liquid phase method; the improved self-propagating high-temperature synthesis method in the solid phase method is currently the most widely used and most mature method for the preparation of SiC powder.
1. Preparation of high purity SiC powder by gas phase method
(1) Preparation of high purity SiC powder by CVD method
CVD method is to obtain ultrafine and high purity SiC powder by high temperature reaction of gas, in which SiH is generally selected as Si source4and SiCl4C source generally choose CH4、C2H2and CCL4etc, while (CH3)2SiCl2、 Yes (CH3)4Such gases can provide both Si source and C source at the same time, and the purity of these gases is above 99.9999%.
(2) Preparation of high purity SiC powder by plasma method
The plasma method is to pass the reaction gas into the plasma container excited by the radio frequency power source, and the gas reacts with each other under the collision of high-speed electrons, and finally the high-purity SiC powder is obtained. The plasma method uses the same gas source as the CVD method, and the gas purity is 99.9999% or more.
2. Preparation of high purity SiC powder by liquid phase method
At present, only the sol-gel method can synthesize high-purity SiC powder in the liquid phase method, and its preparation process is to dissolve the inorganic salt or alkoxide in the solvent (water or alcohol) to form a uniform solution, to obtain a uniform sol, after drying or dehydration into a gel, and then after heat treatment to obtain the required ultrafine powder. Silicon carbide powder synthesized by sol-gel method was first used to sinter silicon carbide ceramics, and with the continuous improvement of the process, the purity of the synthetic powder is also increasing. At present, SiC powders prepared by sol-gel method can be used for the growth of single crystals.
3. Preparation of high purity SiC powder by solid phase method
The self-propagating high-temperature synthesis method belongs to the solid-phase synthesis method, which is carried out spontaneously and continuously by adding an activator under the condition of an external heating source. However, the addition of activator will inevitably introduce other impurities. In order to ensure the purity of the product, the researchers chose to increase the reaction temperature and continue to heat to maintain the reaction. This method is called improved self-propagating high-temperature synthesis. The improved self-propagating high-temperature synthesis method has simple preparation process and high synthesis efficiency, and is widely used in the production of high-purity SiC powders in industry. In this method, a solid Si source and a C source are used as raw materials to continuously react at a high temperature of 1400 to 2000 ° C., and finally a high-purity SiC powder is obtained.
At present, in the improved self-propagating synthesis method, researchers can control most impurities such as B, Fe, Al, Cu, P, etc. at 1 × 10 by controlling the impurity content in the starting Si source and C source and purifying the synthesized SiC powder.-6The following. However, in order to prepare semi-insulating SiC single crystal substrate, the content of N element in SiC powder must be reduced as much as possible, and both Si powder and C powder are easy to absorb a large amount of N element in the air, resulting in high content of N element in the synthesized SiC powder, which can not meet the requirements of semi-insulating single crystal substrate. Therefore, the current research on the preparation of SiC powders by improved self-propagating synthesis is focused on how to reduce the content of N in SiC powders.
The high price of high purity silicon carbide for single crystal growth is mainly due to its high purity requirements and the extremely complex preparation process. Among them, the solid phase method is the main method of synthesizing high-purity SiC powder, and the liquid phase method such as sol-gel method can also prepare high-purity silicon carbide powder, but due to the high preparation cost, the synthesis process is complex and other reasons can not meet the needs of industrial production. At present, there are only a few studies on the preparation of high purity SiC powder by gas phase method, and the gas phase method can better control the impurity content in SiC powder, especially the content of N element. However, the powder synthesized by gas phase method is mostly nanoscale, and the production efficiency is low, which can not meet the industrial demand. Therefore, how to increase the particle size of high purity SiC powder prepared by gas phase method and improve its production efficiency will become a new research direction.
Reference Source:
[1] Luo Hao, etc. Research Progress of High Purity Silicon Carbide Powder for Silicon Carbide Single Crystal Growth
[2] Ma Kangfu et al. Research Progress in Synthesis Process of SiC Powder for Single Crystal Growth
[3] China Powder Network
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