Silicon carbide and its application in refractory materials

Overview of Silicon Carbide

Silicon carbide is a typical compound with covalent bonding, which is rare in nature and belongs to artificial synthetic materials. Silicon carbide has many excellent properties, such as resistance to grinding, high temperature, corrosion, high thermal conductivity, high chemical stability, wide bandgap, and high electron mobility. Due to the extraordinary hardness of silicon carbide, it was initially used in various grinding tools, such as grinding wheels, sandcloths, sand, and various abrasives, and was widely used in material processing and grinding in the mechanical industry. Later, it was also used as a reducing agent and heating element in steel smelting. People have found that it also has a series of excellent properties such as high temperature thermal stability, high thermal conductivity, acid and alkali corrosion resistance, low coefficient of expansion, and good resistance to thermal shock.

Pure silicon carbide is a colorless and transparent crystal, while industrial silicon carbide has colorless, light yellow, light green, dark green, light blue, deep blue, and even black, with a decreasing degree of transparency. According to color, silicon carbide is generally divided into two categories: black silicon carbide (commonly referred to as C in China) and green silicon carbide (referred to as GC). SiC from colorless to dark green is classified as green silicon carbide, while those from dark blue to black are classified as black silicon carbide.

1.1 Development History of Silicon Carbide

Natural silicon carbide, also known as silicon carbide (also known as mullite), is rare. Silicon carbide used in industry is an artificially synthesized material, commonly known as diamond sand. In 1891, American scientist Acheson accidentally discovered a type of carbide during the diamond melting experiment. At that time, it was mistakenly believed to be a mixture of diamond, so it was named diamond sand. In the same year, Acheson studied the method of industrial smelting of silicon carbide, also known as the Acheson furnace, which has been used to this day. A resistance furnace with carbon material as the core body, electrically heats a mixture of quartz SiO2 and carbon to generate silicon carbide.

Since the production of silicon carbide using electric furnaces, people have first used its high hardness as artificial abrasives. It began to be used as a refractory material in 1893. The development of silicon carbide in China was later than that of developed countries such as Europe and America. It was successfully developed by Zhao Guanghe in 1949. In June 1951, the first industrial furnace for producing SiC was built at the First Grinding Wheel Factory, marking the end of China's history of not being able to produce SiC.

1.2 Structure and properties of silicon carbide

The molecular formula is tetrahedral, with silicon atoms located at the center and carbon atoms surrounding it. The molecular weight is 40.07, with a content of Si70.045% and C29.955%. A compound composed mainly of covalent bonds (88% covalent bonds), with the basic unit being a Si-C tetrahedron, with silicon atoms located at the center and carbon atoms surrounding it. All SiC structures are formed by stacking Si-C tetrahedra, with the only difference being parallel or antiparallel stacking, as shown in Figure 1.

Figure 1 Crystal Structure of Silicon Carbide Silicon carbide is a typical stable compound with covalent bonding, and there are 75 variants of SiC, such as α- SiC β- SiC, 3C SiC, 4H SiC, 15R SiC, etc., all of these structures can be divided into cubic, hexagonal, and rhombic crystal systems, among which α- SiC β- SiC is the most common. α- SiC is a high-temperature stable type, β- SiC is a low-temperature stable type. β- SiC can transform into α- SiC, β- SiC can be generated by the reaction of silicon and carbon at a temperature of approximately 1450 ℃. Transmission electron microscopy and X-ray diffraction detection techniques can be used to perform polytype analysis and quantitative determination of SiC microstructures. In order to distinguish different structures, corresponding naming methods are needed, and low-temperature cubic silicon carbide is commonly referred to as β- SiC, while the other hexagonal and rhombic crystal cell structures are all referred to as α- SiC.

Industrial silicon carbide sand usually contains impurities other than the main component SiC. These impurities mainly include:

(1) Free silicon. It partially dissolves in SiC crystals, while a portion forms an alloy with its impurities (such as iron, aluminum, calcium, etc.) and adheres to or is embedded in the crystal.

(2) Free silica. Usually present on the surface of crystals. Most of them are generated due to the oxidation of SiC in contact with oxygen or water vapor in the air during the cooling process of the resistance furnace. When there is an excess of siliceous raw materials in the ingredients, they will also evaporate and condense on the surface of silicon carbide crystals, and white fluffy SiO2 may also appear in the furnace.

(3) Carbon. Part of the carbon is wrapped in SiC crystals, while another part forms carbides with metal impurities. When there is an excess of carbon in the ingredients, obvious free carbon particles can be seen.

(4) Iron, aluminum, calcium, magnesium, etc. Due to the high temperature reduction atmosphere in the furnace, most of these impurities contained in the crystalline blocks are in the form of alloys or carbides. Impurities such as iron, magnesium, and calcium do not enter the lattice, but accumulate at the grain interface and pores. The main impurities entering the SiC lattice include nitrogen, aluminum, boron, etc., which have a significant impact on the conductivity of the crystal.

1.3 Production methods

Silicon carbide is mainly produced by resistance furnaces, and the structure of the Acheson smelting furnace is shown in Figure 2.

Figure 2 Acheson's initial silicon carbide smelting furnace

A resistance furnace is a brick bed made of refractory bricks, which contains a mixture of silica sand, coke, and table salt. Two carbon electrodes are deeply embedded in the bed. The dedicated graphite furnace core is configured between the telegrams, providing an initial conductive path, and the generator is connected to the electrodes. A large current passes through the furnace core, generating a large amount of heat. The mixture surrounding the furnace core is converted into silicon carbide according to the following general equation:

SiO2+3C=SiC+2C

1.4 Application Fields

Silicon carbide has four main application areas, namely: abrasives, refractory materials, functional ceramics, and metallurgical raw materials.

(1) As an abrasive, it can be used as a grinding tool, such as grinding wheels, oilstones, grinding heads, sand tiles, etc.

(2) As a metallurgical deoxidizer and high-temperature resistant material.

(3) High purity single crystals can be used to manufacture semiconductors and silicon carbide fibers.

Main use: Used for wire cutting of monocrystalline silicon, polycrystalline silicon, potassium arsenide, quartz crystals, etc. Engineering processing materials for the solar photovoltaic industry, semiconductor industry, and piezoelectric crystal industry.

Used in fields such as semiconductors, lightning rods, circuit components, high-temperature applications, ultraviolet detectors, structural materials, astronomy, disc brakes, clutches, diesel particulate filters, fine wire pyrometers, ceramic films, cutting tools, heating elements, nuclear fuels, jewelry, steel, protective equipment, catalyst supports, etc.

The current situation of China's silicon carbide industry

2.1 Development History

China's silicon carbide was successfully developed by Zhao Guanghe in June 1949. In January 1951, the first silicon carbide smelting furnace was built at the First Grinding Wheel Factory, ending the history of China's inability to produce silicon carbide. In August 1952, the First Grinding Wheel Factory successfully produced green silicon carbide. Subsequently, silicon carbide, cubic silicon carbide, cerium silicon carbide, and non abrasive silicon carbide were developed for lightning arresters. In 1969, the first and second grinding wheel factories built 4000kVA movable resistance furnaces. In 1980, the First Grinding Wheel Factory established an 8000kVA large resistance furnace. To this day, many silicon carbide smelters use 12500-40000 kVA resistance smelting for smelting.

2.2 Current situation

At its peak, there were over 200 smelting enterprises in China's silicon carbide industry, with an annual production capacity of over 2.2 million tons (including over 1.2 million tons of green silicon carbide blocks and about 1 million tons of black silicon carbide blocks). The power of most green silicon carbide smelting transformers ranges from 6300 to 12500kVA, while the maximum smelting transformer for black silicon carbide is 32000kVA. There are over 300 sand processing and micro powder production enterprises with an annual production capacity of over 2 million tons.

There are currently around 60 smelting enterprises with an annual production capacity of over 900000 tons (including 120000 tons of green silicon carbide and approximately 800000 tons of black silicon carbide). There are over 100 sand processing and micro powder production enterprises with an annual production capacity of over 1 million tons.

Silicon carbide smelting enterprises are mainly concentrated in Gansu, Ningxia, Xinjiang, Inner Mongolia, and Sichuan. The production enterprises of silicon carbide processing sand powder are mainly distributed in provinces such as Henan, Shandong, Jiangsu, and Heilongjiang.

China's silicon carbide smelting production process, technical equipment, and energy consumption per ton have reached the world's leading level. The quality level of black and green silicon carbide blocks is also world-class. The gap between China's silicon carbide and the world's advanced level is mainly concentrated in several aspects: first, low production efficiency. Large mechanical equipment is rarely used in the production process, and many processes rely on human labor to complete, resulting in lower per capita silicon carbide production; The second issue is poor product quality. The quality management of silicon carbide deep processing products is not precise enough, and the stability of product quality is insufficient; Thirdly, there is a lack of high-end varieties. There is a certain gap in performance indicators between certain high-end products and similar products in developed countries. With the development of environmental protection situation, the improvement from open smelting to closed smelting has been basically completed, and all carbon monoxide has been recovered.

2.3 Advantages and Existing Problems

There are 13 countries with a global silicon carbide production capacity of over 10000 tons, accounting for 98% of the total global production capacity. China's silicon carbide production capacity accounts for over 80% of the global total production capacity. China is undoubtedly the world's largest country in silicon carbide, but its silicon carbide products are mainly low-end, with high-end refractory materials, high-end silicon carbide abrasives, and silicon carbide raw materials for silicon carbide wafers mainly relying on imports. The silicon carbide industry in China belongs to a typical labor-intensive basic industry, which is large but not strong, and is located at the low end of the silicon carbide production chain.

The application of silicon carbide in refractory materials

3.1 Wear resistance

Silicon carbide has a hardness second only to diamond and strong wear resistance. It is an ideal material for wear-resistant pipes, impellers, pump chambers, cyclones, and ore hopper linings. Its wear resistance is 5-20 times longer than that of cast iron and rubber, and it is also one of the ideal materials for aviation runways. Applying silicon carbide powder to the inner wall of the impeller or cylinder body of a water turbine through a special process can improve its wear resistance and extend its service life by 1-2 times.

3.2 Thermal shock resistance

Due to the high thermal conductivity and low thermal expansion coefficient of silicon carbide, this silicon carbide refractory material has excellent heat resistance and impact resistance. The heat resistance and shock resistance of silicon carbide products are closely related to the type and properties of the bonding substrate. Test proof: Quickly heat the sample in a 1200 ℃ electric furnace for 20 minutes, then remove it and cool it in air to measure the change in elastic modulus. The elastic modulus of silicate bonded silicon carbide products shows a relatively gentle and gradual decreasing trend with the increase of cold and hot impact tests, while silicon nitride bonded silicon carbide products are different. Before 30 cold cycle tests, their elastic modulus changes very little with the increase of thermal impact tests, and can maintain a relatively constant value. However, after 31 thermal impact tests, the elastic modulus of the sample rapidly decreases, Sudden destruction. Silicon oxynitride bonded silicon carbide products are similar to silicate bonded silicon carbide products in that there is no sudden damage phenomenon, and the elastic modulus shows a gentle downward trend with the increase of the number of thermal shock tests. In practical applications, the expansion, cracking, and deformation of silicate bonded silicon carbide products can be observed after being subjected to thermal shock, making it easy to predict the material's service life.

3.3 High thermal conductivity

Due to the good thermal conductivity of silicon carbide itself, refractory materials with high silicon carbide content have higher thermal conductivity, with thermal conductivity mostly exceeding 14.4W/(m • K). Used for water-cooled walls of heat exchangers, saggers, coal gasifiers, and kiln furniture products for indirect heating. The thermal conductivity of the particle surface of silicon carbide products will gradually decrease during use. The properties of the bonding material have a certain influence on the thermal conductivity of silicon carbide products. The thermal conductivity of silicon oxynitride bonding and silicon nitride bonding silicon carbide is higher, while the thermal conductivity of silicon silicate bonding silicon carbide is lower.

3.4 Antioxidant properties

Silicon carbide has good antioxidant properties, with weak oxidation below 1300 ℃ and significant oxidation only occurring above 1300 ℃, forming a SiO2 glass protective film that can inhibit the oxidation of silicon carbide.

The oxidation resistance of silicon carbide refractory products also shows significant differences with the type of binder. The oxidation resistance of silicon nitride combined with silicon carbide products is relatively low, which can be explained by their microstructure characteristics. Because the base material of silicon nitride combined with silicon carbide products is interwoven in a fibrous shape, with high breathability, the protective effect on silicon carbide particles is relatively small; In silicate bonded and silicon oxynitride bonded silicon carbide products, continuous substrates are wrapped on the surface of silicon carbide particles, thus possessing strong antioxidant properties. The antioxidant properties of silicates combined with silicon carbide and silicon oxynitride combined with silicon carbide showed similar characteristics in the above tests, but the differences between them can be clearly displayed in long-term use.

3.5 Slag resistance

SiC is a compound with strong covalent bonding, which maintains high bonding strength at high temperatures. Therefore, SiC has good chemical stability and will not be corroded by most acidic or alkaline solutions. Silicon carbide has a larger wetting angle with molten metal and slag, and compared with oxide refractory materials, it has good corrosion resistance to various solids, liquids, and gases. Such as Al2O3-SiC-C based castables and products used in ironmaking systems, silicon-molybdenum bricks and castables containing silicon carbide for cement kilns, various acid-base reaction containers, and so on.

epilogue

Silicon carbide is a typical covalent bonding compound with many excellent properties, such as resistance to grinding, high temperature, corrosion, high thermal conductivity, high chemical stability, wide bandgap, and high electron mobility. Widely applied in industries such as abrasive tools, refractory materials, functional ceramics, metallurgical accessories, etc. With the development of social economy and technology, silicon carbide will also be applied in more fields. China is a major producer of silicon carbide in the world, but has long been at the low end of the silicon carbide industry chain. We need to achieve new breakthroughs in high-end silicon carbide products, truly achieving both big and strong, and promoting high-quality development of the silicon carbide industry.