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What is SiC?
Silicon carbide is a highly hard compound, second only to diamond (hardness 15) and boron carbide (hardness 14). Used as a refractory material, its consumption is only surpassed by alumina and magnesia, making it a crucial substance for heat resistance. The production of silicon carbide products (silicon carbide powder) consists of two processes: bulk material production and crushing of bulk materials. More than half of the silicon carbide powder used worldwide is produced in China.
The crystal structure and powder of silicon carbide
Silicon carbide (SiC) primarily exists in two crystallographic forms: one is the low-temperature stable cubic crystal system known as B-SiC, and the other is the high-temperature stable hexagonal crystal system known as α-SiC. In general, the unit cell of silicon carbide crystal consists of four silicon (Si) atoms surrounding each carbon (C) atom and vice versa, meaning four carbon atoms also encircle one silicon atom. These atoms are bonded through atomic orbitals to form the basic unit structure, resulting in SiC4 and CS tetrahedral structures. As shown in the diagram ((a) for β-SiC crystal structure; (b) for α-SiC crystal structure).
The main crystal structure of β-SiC is cubic, also known as the 3C structure, where silicon and carbon form a face-centered cubic lattice. However, the thermal stability of 3C-SiC is relatively poor, and below 1800°C, some 3C-SiC structures may transform into the 2H-SiC structure. As the temperature rises to approximately 1800°C, the crystal structure transitions from the 3C cubic structure to the 4H and 6H structures of silicon carbide. With further temperature increase beyond 2000°C, the crystal structure may shift from 6H to 4H. On the other hand, α-SiC exists in nearly a hundred polymorphs, with the 6H crystal structure being the most common in industrial applications.
Types of silicon carbide powder
Silicon carbide is mainly divided into black silicon carbide and green silicon carbide. Pure silicon carbide is colorless and transparent. It turns black or green due to impurities (such as Al and N) being dissolved in it; the more impurities, the darker the color. Black silicon carbide has lower hardness compared to green silicon carbide and is used for grinding softer materials, such as cast iron and non-metallic materials. Green silicon carbide is suitable for grinding hard alloys, optical glass, and similar materials.
Both black and green silicon carbide in China are produced using silica, with the addition of salt to remove Al in the case of green silicon carbide. In addition to China, black silicon carbide in European countries and others is produced using silica sand, and its chemical composition is close to that of green silicon carbide.
Production of Silicon Carbide Powder
(1) Solid-Phase Method:
The solid-phase method involves using solid-state materials to prepare silicon carbide powder. This includes the carbothermal reduction method and the self-propagating high-temperature synthesis method (SHS). In the carbothermal reduction method, commonly used raw materials are quartz sand and coke. Under high-temperature conditions (usually above 1900°C), direct reduction is carried out through methods such as electric arc furnaces.
(2) Liquid-Phase Method:
The liquid-phase method involves preparing oxide micro-powders through aqueous solutions. It usually starts with preparing silicon dioxide and aluminum oxide. This method is widely used in modern applications. The advantage of the liquid-phase method is that it is easy to control the composition of materials, can synthesize composite oxide powders, and is convenient for adding trace components, achieving good mixing uniformity. However, strict control of operating conditions is necessary to maintain the chemical homogeneity of the generated powder at the ionic level.
(3) Gas-Phase Method:
The gas-phase method mainly includes two types: the evaporation-condensation method (PVD) and the chemical vapor deposition method (CVD).
In the evaporation-condensation method, raw materials are heated to a high temperature to vaporize them and then rapidly cooled under large temperature gradients to condense them into particulate materials. Common heating methods include electric arcs or plasma flows.
The chemical vapor deposition method utilizes the vapor of volatile metal compounds through a chemical reaction to synthesize the desired substance.
Properties and Applications of Silicon Carbide Powder
Silicon carbide powder materials can be divided into single crystal SiC and polycrystalline SiC, with polycrystalline SiC mainly referring to SiC ceramics.
Silicon carbide possesses several excellent properties, including low density, high strength, high hardness, high thermal conductivity, wear resistance, and outstanding chemical stability. Therefore, it is an important structural ceramic material widely used in fields such as chemical industry, aerospace, and defense.
In industrial applications, SiC ceramics are commonly used for structural components such as sealing materials, bearings, and beams. In thermal power plants, SiC is primarily applied to thermal components like nozzles and heat exchange tubes. In the field of ballistic protection, SiC ceramics are used for manufacturing bulletproof components in armored vehicles and tanks. In the aerospace industry, SiC mirrors find extensive use in remote sensing and observation systems. Additionally, silicon carbide materials play a crucial role in applications related to nuclear energy and electronics.
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