Boron carbide is hard black metallic luster crystal, commonly known as black diamond. It is a kind of powder product made of boric acid and carbon materials by high temperature smelting and crushing in electric furnace. The theoretical density is 2.52 g / cm3, the melting point is 2450 ℃, and the microhardness is 4950 kgf / mm2. Its hardness is second only to diamond and CBN, and belongs to superhard material with diamond and CBN. It has been widely used in many fields, such as high temperature stability, light acid resistance, high alkali resistance and so on. Boron carbide is insoluble in water and organic solvents. It has strong chemical stability and is resistant to acid and alkali corrosion. It hardly reacts with all acid and alkali solutions. Boron carbide also has the following characteristics: it has large thermal neutron capture cross section, strong neutron absorption capacity, so it is called neutron absorber; it has semiconductor properties, etc.

History of boron carbide: it was discovered in the 19th century as a by-product of the study of metal borides and was not scientifically studied until the 1830s. Its Mohr hardness is 9.3, which is the third known strong hardness material after boron nitride and diamond.

Application and characteristics:

(1) in the field of national defense industry:

Making bulletproof materials, such as bulletproof board in bulletproof vest, ceramic bulletproof tile in cockpit of military aircraft pilot and ceramic bulletproof board of modern armored personnel carriers and tanks. It can be used as the nozzle of gun in military industry. At present, the Al2O3 based ballistic ceramics have been used in "502 project" and "212 project", but when the Al2O3 based ceramic composite armor is used in the side of the combat vehicle body, its weight reduction effect is not obvious, and the weight of the same thickness high-performance boron carbide ceramic composite armor is reduced by 15% ~ 20% compared with the Al2O3 based ballistic ceramics, and the ballistic performance is further improved. Therefore, the key equipment engineering ceramic composite armor development project puts forward an urgent demand for high-performance, low-cost boron carbide bulletproof ceramics. Therefore, the development and application of high-performance and low-cost boron carbide bulletproof ceramic materials can greatly improve the performance of relevant military defense equipment, which has significant military and economic benefits. The application directions of boron carbide bulletproof ceramic materials are: key equipment engineering, light armored vehicles such as future main battle tanks, infantry combat vehicles, airdrop vehicles, and armor protection of web plates of armed helicopters and superstructure of ships and boats.

(2) in the field of nuclear industry:

Boron carbide can absorb a large number of neutrons without forming any radioisotopes, so it is an ideal neutron absorber in nuclear power plants, and the neutron absorber mainly controls the rate of nuclear radiation. Boron carbide is mainly made into controllable rod in nuclear reactor field, but sometimes it is made into powder because of increasing surface area.

When Chernobyl nuclear accident happened in 1986, the former Soviet Union dropped nearly 2000 tons of boron carbide and sand, and the nuclear reactor eventually stopped the chain reaction in the reactor. In the nuclear energy industry, it can be used to manufacture ultra-high pressure gun nozzle. Boron carbide, very hard and wear-resistant, can not react with acid and alkali, high / low temperature resistance, high pressure resistance, density ≥ 2.46g/cm3; Microhardness ≥ 3500kgf / mm2, bending strength ≥ 400MPa, melting point 2450 ℃. Boron carbide is the second most important material for nuclear reactor, which is used to make control rod, regulating rod, accident rod, safety rod and shielding rod, to make boron carbide tile, plate or neutron absorber (made of high B10 content powder) for radiation protection, or to make shielding layer of nuclear reactor with cement Functional components. Characteristics: boron carbide can absorb a large number of neutrons without forming any radioisotopes, so it is an ideal neutron absorber in nuclear power plant, and it is also a core component of nuclear reactor. The main purpose of making it into a neutron absorber is to increase the surface area of the reactor and to control the rate. Boron carbide is an important neutron absorbing material because of its high neutron absorption cross section, wide absorption energy spectrum, low price, abundant raw materials and no strong secondary radiation of λ - ray after absorbing neutrons.

(3) in the field of refractories:

Boron carbide is used as antioxidant additive in low carbon magnesia carbon brick and castable. It is used in the key parts of high temperature and erosion resistance in iron and steel industry. Such as ladle, taphole (nozzle), slide plate, stopper, etc. With the need of energy saving and low carbon steel smelting in iron and steel industry, the research and development of low carbon magnesia carbon brick with excellent performance (carbon content is generally less than 8%) has been paid more and more attention by the industry at home and abroad. At present, the performance of low-carbon magnesia carbon brick is generally improved by improving the combined carbon structure, optimizing the matrix structure of magnesia carbon brick and adding high-efficiency antioxidants. The graphitized carbon black composite powder composed of industrial grade B4C and part of graphitized carbon black is used as carbon source and antioxidants for low-carbon magnesia carbon brick, and good results are achieved. The low carbon magnesia carbon brick with B4C has good conventional physical properties, oxidation resistance and thermal shock stability.

Characteristics: boron carbide plays an anti-oxidation role in carbon containing refractories, which can densify the products, prevent the oxidation of carbon in carbon containing refractories, and react at 1000 ℃ ~ 1250 ℃, forming (9Al2O3 · 2b2o3) columnar crystals distributed in the matrix and gap of refractories, so as to reduce the porosity, improve the medium temperature strength, and generate crystals with volume expansion, healing, volume shrinkage and reduction Few cracks.

(4) for other engineering ceramic materials:

Boron carbide is used to make nozzles for sandblasting machine, nozzles for high pressure water cutting machine, sealing rings, ceramic tools and moulds, etc. Characteristics: boron carbide nozzle has the characteristics of wear resistance and high hardness, which will gradually replace the known hard alloy (tungsten steel) and silicon carbide, silicon nitride, alumina, zirconia and other materials of sandblasting nozzle. In addition, the application of boron carbide in the field of composite ceramics: boron carbide is a compound with strong covalent bond, and the plasticity of boron carbide is very poor, the resistance of grain boundary is very large, so it is difficult to obtain dense sintered body. In addition to some special occasions, such as microcrystalline boron carbide gas bearing material, boron carbide block used as neutron absorbing material in nuclear reactor, sintering additives are usually added In order to improve the sintering behavior of boron carbide, more cheap and practical boron carbide products are obtained. It is worth noting that B4C SiC composite ceramics can keep the excellent physical and mechanical properties of boron carbide ceramics while reducing the sintering conditions. B4C SiC ceramic is considered to be a kind of high temperature corrosion-resistant and wear-resistant material with wide application prospect. It has been applied in industrial nozzle, pump sealing, hot extrusion die and other fields.

In recent decades, due to the rapid development of science and technology, especially the rapid development of electronic technology, space technology, computer technology, there is an urgent need for materials with special properties. Boron carbide has become an important member of high-grade ceramic family because of its many excellent properties. At present, many problems about the preparation of boron carbide powder and the sintering of boron carbide ceramic materials have been solved. In the future material field, boron carbide will occupy an important position with its excellent performance.

(5) In general industry:

Boron carbide is used to make high-grade wear-resistant electrode to enhance the wear-resistant strength of welding surface; it is used as grinding and polishing material, abrasive for water cutting and correction material for diamond abrasives; it is used for high-precision polishing and grinding in jewelry industry. Characteristics: boron carbide is a solid harder than silicon carbide or tungsten carbide. Long ago, it was used as a coarse abrasive material. Because of its high melting point, it is not easy to cast into industrial products, but through high-temperature melting powder, it can be processed into a simple shape, used for grinding, grinding, drilling and polishing of cemented carbide, gem, etc.

(6) Electrical properties and applications of boron carbide:

Boron carbide graphite thermocouple consists of graphite tube, boron carbide rod and boron nitride bushing between them. In inert gas and vacuum, the temperature is up to 2200 ℃. The linear relationship between potential difference and temperature is good between 600 ℃ and 2200 ℃.

(7) Application of boron carbide as chemical raw material:

Activated by halogen, boron carbide powder can be used as boronizing agent for steel and other alloys to Boronize on the surface of steel to form a thin layer of iron boride, which can enhance the strength and wear resistance of materials. Boron carbide can also be used as a nonmetal additive for some metal based friction materials. When boride powder is prepared by reduction combination method, boron carbide can be used as boron source to produce TiB2, ZrB2, CrB2 and other powders, which is called "boron carbide method" for powder preparation.

(8) Application of boron carbide in sapphire chip (LED):

In recent years, the state's strong support and promotion for the LED industry has made the LED industry develop better, which has brought great business opportunities to the enterprises producing and processing sapphire crystal. Due to the high strength and hardness of sapphire crystal (Mohs hardness 9), it brings great difficulties to the processing enterprises. From the point of view of materials and grinding academia, the materials for processing and grinding sapphire crystal are synthetic diamond, boron carbide and silicon dioxide. Because the hardness of synthetic diamond is too high (MOH's hardness is 10), the surface of sapphire wafer will be scratched when grinding, which affects the transmittance of the wafer and is expensive. However, the hardness of silicon dioxide is not enough (MOH's hardness is 7) and the grinding force is poor. Therefore, boron carbide abrasive (Morse hardness 9.3) is an ideal material for processing and grinding sapphire crystal. Boron carbide abrasives have excellent performance in the double-sided grinding of sapphire wafer and the back thinning and polishing of sapphire based LED epitaxial wafer. Some national key universities also have major research on boron carbide in grinding sapphire crystal. In short, with the rapid development of LED industry, boron carbide will also rise rapidly.