Preparation and application of boron carbide powder

Boron carbide is a kind of black crystal with metallic luster, also known as black diamond, belongs to inorganic nonmetallic material.  At present, everyone is familiar with boron carbide material may be due to the application of bullet-proof armor, because it has the lowest density in ceramic materials, has the advantages of high elastic modulus and high hardness, can achieve a good use of micro fragmentation to absorb projectile energy effect, while minimizing the load.  But boron carbide has many other unique properties that could make it useful in abrasives, refractories, the nuclear industry, aerospace and other fields.

Properties of boron carbide  

In terms of physical properties, the hardness of boron carbide is only after that of diamond and cubic boron nitride, and it can still maintain high strength at high temperature, so it can be used as an ideal high-temperature wear-resistant material.  Boron carbide has a very low density (theoretical density is only 2.52g/cm3), which is lighter than ordinary ceramic materials and can be used in aerospace fields.  Boron carbide has strong neutron absorption capacity, good thermal stability and melting point of 2450℃, so it is also widely used in the nuclear industry. Meanwhile, its neutron absorption capacity can be further improved by adding ELEMENT B.  Boron carbide materials with specific morphology and structure also have special photoelectric properties.  In addition, boron carbide has the advantages of high melting point, high elastic modulus, low expansion coefficient and good oxygen absorption capacity, which make it become a potential application material in many fields such as metallurgy, chemical industry, machinery, aerospace and military industry.  For example, wear resistant parts, bulletproof armor, reactor control rods and thermoelectric elements.

In terms of chemical properties, boron carbide does not react with acid, alkali and most inorganic compounds at room temperature, and hardly reacts with oxygen and halogen gases at room temperature, so its chemical properties are stable.  In addition, boron carbide powder, as a boriding agent of steel, is activated by halogen, which penetrates boron into the surface of steel to form iron boride film, thus enhancing the strength and wear resistance of the material, and its chemical properties are excellent.  

We all know that the properties of materials determine the use, so boron carbide powder in which applications have outstanding performance?

Application of cubic boron carbide  

1. Boron carbide is used as a polishing abrasive  

The application of boron carbide as an abrasive is mainly used in the grinding and polishing of sapphire. Among the superhard materials, the hardness of boron carbide is superior to alumina and silicon carbide, second only to diamond and cubic boron nitride.  Sapphire is the most ideal substrate materials such as semiconductor GaN/Al2O3 light-emitting diode (LED), large-scale integrated circuit SOI and SOS, and superconducting nanostructured films. The surface smoothness of sapphire is required to be super smooth without damage.  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 material and abrasive field, the best materials for processing and grinding sapphire crystal are artificial diamond, boron carbide, silicon carbide and silicon dioxide.  The hardness of artificial diamond is too high (Mohs hardness 10), which will scratch the surface of sapphire crystal and affect the transmittance of the crystal, and the price is expensive.  Silicon carbide cutting usually roughness RA is high, flatness is poor;  And silica hardness is not enough (Mohs hardness 7), grinding force is poor in grinding engineering time, labor.  Therefore, boron carbide abrasive (Mohs hardness 9.3) is the most ideal material for sapphire crystal processing and grinding, and has excellent performance in sapphire wafer double-side grinding and sapphire based LED epitaxial wafer back thinning polishing.

It is worth mentioning that, because boron carbide in 600℃ above, the surface will be oxidized into B2O3 film, so that it has a certain softening, so in the abrasive application is not suitable for high temperature dry grinding, only suitable for polishing fluid grinding.  However, this property can prevent the further oxidation of B4C, so it has a unique advantage in the application of refractory materials.  

2. Application in refractory materials  

Boron carbide has the characteristics of oxidation resistance and high temperature resistance, and is generally used as advanced shaped and unshaped refractory material widely used in various fields of metallurgy, such as iron and steel stoves, kiln tools and so on.  

With the energy saving and consumption reduction of iron and steel industry and the need of smelting low carbon steel and ultra-low carbon steel, the research and development of low carbon magnesium carbon brick with excellent performance (carbon content generally <8%) is increasingly concerned by the industry at home and abroad.  Currently, usually by improving the carbon structure, optimize the structure of magnesia carbon brick matrix, increase effective antioxidants and other measures to improve the use performance of low-carbon magnesia carbon brick, which adopts industrial-grade consisting of boron carbide and part of the graphitized carbon black composite powders, graphitized carbon black as carbon source and antioxidant used in low-carbon magnesia carbon brick, good results have been achieved.  

Due to boron carbide at high temperature will occur certain softening, so it can be attached to the surface of other material particles, even the densification of the product, the surface of the B2O3 oxide film can also form a certain protection, play an antioxidant role.  At the same time, because the reaction generated columnar crystals distributed in the matrix and gap of refractory materials, thus reducing porosity, improving the strength of medium temperature, and the volume expansion of the crystal, can heal volume shrinkage, reduce cracks.

3. Bulletproof material used to strengthen national defense  

Due to its high hardness, high strength, small specific gravity and high elastic resistance grade, boron carbide is especially in line with the trend of lightening anti-projectile material, and is the best anti-projectile material for aircraft, vehicles, armor and human body protection.  At present, the General Armaments Department of China has put forward the research of low-priced boron carbide bullet-resistant armor, aiming at promoting the use of boron carbide bullet-resistant armor in China's national defense industry.

4. Application in the nuclear industry  

Boron carbide is recognized as the best neutron absorber in the nuclear industry due to its high neutron absorption cross section and wide neutron absorption spectrum.  The thermal cross section of boron-10 isotope is as high as 347×10-24cm2, only second to gadolinium, samarium, cadmium and a few other elements, so it is an efficient thermal neutron absorber.  In addition, boron carbide is rich in resources, corrosion resistance, good thermal stability, does not produce radioactive isotopes, low secondary ray energy, so boron carbide is widely used as a control material and shielding material in nuclear reactors.  

In the nuclear industry, for example, the high temperature gas cooled reactor using boron absorption sphere shutdown system as the second shutdown system, the accident cases, when the first shutdown system fails, the second shutdown system USES a large amount of boron carbide balls free fall into the core reflector methods of channel, make the reactor shut, and achieve cold shutdown, the absorption is containing boron carbide graphite ball.  The main function of boron carbide core in high temperature gas cooled reactor is to control reactor power and safety. Boron carbide neutron absorbing material permeated in carbon brick can reduce the neutron irradiation of reactor pressure shell.  

At present, boride materials used in nuclear reactors mainly include the following materials: boron carbide (control rod, shield rod), boric acid (moderator, coolant), boron steel (control rod and storage material of nuclear fuel and nuclear waste), europium boride (combustible toxic material of reactor core), etc.