Silicon carbide powder, as a synthesis raw material, directly affects the growth quality and electrical properties of silicon carbide single crystals. In recent years, preparing high-purity silicon carbide powder has gradually become a research hotspot in the field of silicon carbide single crystal growth.
The PVT method for growing SiC single crystals has high purity requirements for SiC powder, usually requiring 5N-grade purity (purity ≥99.999%). However, during the preparation of SiC powder, many steps-such as the processing of carbon powder, silicon powder, and graphite crucibles, incomplete reaction of raw materials, and post-processing of the powder-can easily introduce various impurities, such as C, Si, SiO2, Fe, Al, N, and B, including metallic and non-metallic impurities. The presence of these impurities not only affects the physical and chemical properties of SiC powder, but also affects the crystal quality and electrical properties of subsequently grown SiC single crystals. Therefore, to ensure the purity of the synthesized SiC powder, purification treatment is required.
01
Removal of Carbon from Silicon Carbide Powder
Oxidation Method
The oxidation method involves heating the synthesized SiC powder to 700–900°C for high-temperature calcination, using the reaction between carbon and oxygen in the air to generate CO and CO2, so that free C in the SiC powder is separated from the SiC powder in gaseous form, thereby achieving carbon removal. For micron-sized silicon carbide powder with relatively small particle size, because the specific surface area of the powder is relatively high, silicon carbide is more likely to oxidize during high-temperature carbon removal, leading to the introduction of new silica impurities. To solve this problem, one cannot simply rely on extending the oxidation time or increasing the oxidation temperature to achieve complete carbon removal.
Oxidation-based carbon removal is a relatively simple, economical, pollution-free, and efficient carbon removal method. It is widely used for removing free carbon from SiC powder, but the carbon removal temperature and time must be controlled to avoid problems such as severe oxidation caused by excessively high temperatures.
Flotation Method
Flotation utilizes the different degrees to which the surfaces of different substances are wetted by a liquid medium, and separates and floats minerals according to differences in contact angle between particles. Impurity carbon powder and SiC powder in the powder have different forms of adhesion. One form is physical adhesion to SiC particles through surface forces. This form of adhesion makes the interaction between carbon powder and SiC powder relatively weak. At the same time, SiC powder is hydrophilic, while carbon is hydrophobic. Therefore, through the different hydrophilicity of SiC and carbon, flotation can be used to remove carbon.
Flotation-based carbon removal has a long working cycle. Most flotation agents used are chemical reagents, which cause certain adverse effects on the environment. Because the dispersion medium is relatively toxic, it may also pose potential hazards to operators. In addition, because silicon carbide powder has a small particle size, silicon carbide powder may be entrained in the free carbon, resulting in loss of raw material.
Air Classification Method
The basic principle of air classification is based on the difference in suspension velocity between the material and impurities. That is, under the action of airflow, solid particles with higher density have higher settling velocity and travel a shorter distance, while those with lower density have lower settling velocity and travel a longer distance.
Air classification can perform large-scale impurity removal from silicon carbide powder. At the same time, the equipment is simple to operate, the cost is relatively lower than other methods, and there are no environmental issues. However, the carbon removal efficiency is not high. To obtain high-purity silicon carbide powder, further processing is still required.
In summary, carbon removal from SiC powder should not rely on a single method alone. Multiple processes can be combined: first, crush the powder to fully expose encapsulated carbon; then use deionized water for simple flotation to preliminarily remove free carbon; after drying, perform heating/oxidation treatment to effectively remove residual carbon impurities. The synergistic effect of multiple methods helps significantly improve impurity removal efficiency and increase the purity of SiC powder.
02
Removal of Other Impurities from Silicon Carbide Powder
The processing of carbon powder, silicon powder, and graphite crucibles, as well as the crushing of SiC powder, inevitably introduces some metal impurities, such as Al, Fe, and Mn. These impurities exist in SiC powder in the form of oxides or metal ions and have a significant impact on the electrical properties of SiC. In particular, for powder used for SiC crystal growth, its impurities directly affect crystal growth quality. Therefore, removal of metal impurities is particularly important.
Magnetic Separation Method
Magnetic separation uses the magnetism of metals and applies magnetic force to the powder so that metal impurities move and separate from the SiC powder. It is mainly used to remove iron-containing impurities and magnetic minerals. Electromagnetic iron removal has high efficiency and a high degree of automation and meets the needs of industrial production. However, for impurities with low iron content or weak magnetism, the effect of electromagnetic impurity removal is not ideal.
High-Temperature Method
The high-temperature method mainly takes advantage of the fact that SiC is difficult to volatilize at high temperatures. Through high-temperature calcination, low-melting-point metal or non-metal impurities in SiC powder are discharged in vaporized form, thereby purifying the SiC powder. The high-temperature method is a common impurity removal method currently studied for synthesizing high-purity SiC powder, but during impurity removal, the temperature must be controlled to avoid loss of SiC powder as much as possible.
Acid Washing Method
Acid washing uses acidic solvents to react with metal and oxide impurities in SiC powder to form soluble salts, which are then washed with water and dried to achieve impurity removal. For example, elemental iron, iron oxides, SiO2, etc. in SiC powder can be removed by reaction with hydrochloric acid, sulfuric acid, sodium hydroxide, or hydrofluoric acid.
Acid washing has high removal efficiency for metal impurities, but after acid washing, the SiC powder must be rinsed with water until neutral. This step consumes a large amount of water, and the various acids generated during impurity removal cause environmental problems.
Chlorination Roasting Method
In chlorination roasting for impurity removal, a specific gas is introduced to react with impurities in silicon carbide powder to form chlorides and other impurities with lower melting and boiling points, which are then volatilized under high-temperature and low-pressure conditions to achieve purification. Chlorination roasting has advantages such as low energy consumption and high impurity removal efficiency, and can be used for deep impurity removal of already-purified, relatively high-purity powder. However, because the presence of chlorides is highly damaging to purification equipment, the process is not stable enough, and environmental pollution is relatively serious, this method is still difficult to apply on a large scale at present.