Introduction of Boron carbide ceramics



Boron carbide ceramics is a class of ceramics which the main chemical ingredient is boron carbide. The chemical formula of boron carbide is B4C, it belongs to hexagonal diamond crystal, there are 12 boron atoms in the unit cell, lattice parameter co = 1.212nm, ao = 0. 56nm. Crystal structure exists atom which can accommodate up to 0.18nm diameter, so it can remain the lithium or helium atoms within the crystal structure. Synthesis of boron carbide powder is mainly used carbon thermal reduction method, except the direct reduction with boron anhydride, it can also obtained in the presence of carbon (C), boron anhydride is reduced by Mg, the reaction is: 2B2O3 + 6Mg + C → B4C + 6MgO, the reaction temperature is 1000~1200 ℃. This reaction is highly exothermic, the final product requires H2SO4 or HCl acid pickling, then be washed with hot water to obtain more pure and fine grain size (0.1~5μm), no C boron carbide powder. Boron carbide ceramic is main hot pressing, hot isostatic pressing and non-pressure sintering is also used. Hot press sintering temperature is 2000~2100 ℃, generally Mg, Al, Cr, Si, Ti or a metal such as Al2O3, MgO, etc. or an oxide glass is added to use as sintering aid. The melting point of boron carbide is 2450 ℃, the theoretical density is 2.519g/cm3, elastic modulus is 360~460GPa, the thermal expansion coefficient is 5.73 × 10-6/K (300~1970K), the thermal conductivity at 200 ℃ is 24W/( m • K), hardness is 4950kg/mm2, just below the diamond and cubic boron nitride, it has a large thermal neutron capture cross-section. It is used to process precious stones, ceramics, molds, tools and bearings. It is also used as nozzles, bulletproof materials and nuclear reactor neutron absorber.