What is boron carbide?
Boron carbide is a black metallic glossy crystal, commonly known as black diamond, which is a kind of powder product made from boric acid as the main raw material, adding carbonaceous materials such as petroleum coke, smelting and crushing at high temperature. It belongs to superhard materials as well as diamond and cubic boron nitride.
Boron carbide is insoluble in water and organic solvents, has strong chemical stability, is resistant to acid and alkali corrosion, and does not react with almost all acid and alkali solutions. Boron carbide also has the following characteristics: large thermal neutron capture cross section, strong neutron absorption capacity, so it is called neutron absorber, semiconductor, etc.
Boron carbide Properties
Boron carbide (B4C) is grayish black and is a very hard man-made material. Its Mohs hardness is 9.3 and its microhardness is 5500~6700kg/mm2, second only to diamond and cubic boron nitride. The crystal structure of boron carbide is hexagonal crystal. The density is 2.52g/cm3. The melting point is 2450 ℃, and it decomposes and volatilizes rapidly when the temperature is higher than 2800 ℃. Its linear expansion coefficient is 4.5 × 10-6 / ℃ and its thermal conductivity is 121.42 W (m ·K) and 62.80 W / (m ·K), respectively. Its resistivity is 0.44 (20 ℃) Ω cm and 0.02 (500 ℃) Ω cm. Boron carbide is resistant to acid and alkali corrosion and is not wetted with most molten metals and has high chemical stability. Boron carbide can resist the oxidation of air at 1000 ℃, but it is easy to be oxidized above 900C in oxidizing atmosphere. Boron carbide powder has a very high grinding capacity, which is 50% higher than that of silicon carbide and 1 / 2 times higher than that of corundum. Boron carbide powder is an excellent grinding material and wear-resistant material.
What is boron carbide used for?
Boron carbide can be used as abrasive and raw material for manufacturing abrasive tools. It is suitable for grinding, polishing, drilling and other processing of all kinds of cemented carbide tools, moulds, parts, components and gemstones. Boron carbide can be made into grinding paste and polishing paste with appropriate amount of oil or water as lubricant. Boron carbide can also be used as raw material for manufacturing metal boride, boron alloy, boron steel and so on. In addition, in special needs, it can be used to manufacture boron carbide hot-pressed products as wear-resistant and high-temperature resistant components, such as nozzles, sealing rings, gyroscopes, petrochemical parts, as well as lightweight and high-strength components in military engineering and control rods of atomic reactors.
Synthesis of Boron Carbide
The industrial method of synthesizing boron carbide is to reduce boric anhydride with excess carbon. The principle is:
The synthetic reaction can be carried out in resistance furnace or electric arc furnace. A better method is to heat the mixture of boron anhydride and carbon to prepare boron carbide in a resistance furnace at a decomposition temperature lower than that of boron carbide. The boron carbide synthesized by this method contains little free carbon and sometimes some free boron. When melting in the electric arc furnace, because the temperature of the arc is quite high, and boron carbide begins to decompose into carbon-rich phase and boron at 2200 ℃, boron will volatilize from the reaction space at high temperature, so that the final reaction product contains a lot of free carbon. The quality is poor.
The production process of arc fusion to boron carbide is shown in figure 1. Using boric acid (mass fraction > 92%), artificial graphite (mass fraction of fixed carbon > 95%) and stone coke (mass fraction of fixed carbon > 85%), the reaction formula is synthesized according to the reaction formula calculation:
Among them, 50% of artificial graphite and 50% of petroleum coke. Considering factors such as high temperature volatilization and oxidation of arc melting, the addition of boric acid is 2% higher than the calculated value, and the addition of artificial graphite and petroleum coke is 3% and 4% higher than the calculated value. After the three kinds of raw materials were uniformly mixed in the ball mill, the reduction and carbonization reaction was carried out in a single-phase double-electrode arc furnace or a three-phase arc furnace. The arc melting temperature was controlled at 1700-2300 ℃. In the smelting process, it is best to feed in batches to cover the arc to achieve closed arc smelting. After the melting, the melt is cooled on carbon black or carbon electrodes, and then the bulk is smashed into 50mm-sized pieces with a hammer. Then, according to the shape of the cross section of the fragments, the unqualified blocks of overburning and raw burning are picked out. According to experience, the cross section of the raw burning block is white or gray; the cross section of the overburned block is like graphite flake or porous melting body; all those with dense cross section similar to steel or metallurgical coke streak section or sand grain section are qualified blocks, the mass fraction of boron carbide is 94%-97%, and the mass fraction of free carbon is less than 1.5%. The qualified block is then soaked in hot water and steamed, and then washed with water to remove boric acid, boric anhydride and carbon substances adhered to it. Wash and dry the chunks and then crush them through the 2.362mm sieve. Further rough grinding and fine grinding can be carried out in a rotary ball mill or a vibrating ball mill, with shorter rough grinding time and longer fine grinding time. The iron admixture in the ball milling process can be washed and removed by hot sulfuric acid at 80 ℃. The amount of sulfuric acid is 30% of the material. The removal of iron by pickling is carried out in the pickling tank. First add the material, then add acid, then add 60 'water, stir with leaf pulp, and heat it with steam to improve the reaction speed. After pickling and soaking for 12 hours, dilute with water, aerate and stir. After holding for another 12 hours, remove the acid and rinse to neutral with water. Finally, the purified materials are sorted into different particle sizes with settling semicolons and serial washing semicolons. The settling semicolon is to divide the fine abrasive into the fine powder of W35~W40, and the series washing semicolon is to divide the coarse abrasive into the coarse particles of W4012000.The sedimentation semicolon is the fine abrasive powder which is divided into fine abrasive powder and water washing semicolon. If the coarse particles are further classified, 120 particles can be divided into 100, 80, 70, 60 and more coarse particles by screening method.
Boron carbide Price
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