Boron carbide, also regarded as black diamond, has a molecular formula of B4C and is usually a gray-black micro powder. It is one of the three hardest materials known (the other two are diamond and cubic boron nitride). Boron carbide is applied in tank armor, bulletproof vests and many industrial occasions. Its Mohs hardness is 9.3. The boron carbide can be obtained by reducing the diboron trioxide with carbon in an electric furnace. Boron carbide can absorb a large number of neutrons without forming any radioactive isotopes, so it is an ideal neutron absorber in nuclear power plants. The neutron absorber mainly controls the rate of nuclear fission. Boron carbide is mainly made into controllable rods in the field of nuclear reactors, but sometimes it is made into powder form due to the increase in the surface area because of its low density, high strength, good high-temperature stability, and good chemical stability.
Boron carbide‘s crystal structure
Boron carbide has a typical complex crystal structure based on icosahedral boride. This structure is stacked: the B12 icosahedron and bridging carbon form a network plane parallel to the difference C plane, and are stacked along the C axis. The crystal lattice has two basic structural units-B 12 icosahedra and B 6 octahedron. Since the B 6 octahedrons are small in size, they cannot be connected to each other. Instead, they are combined with the B 12 icosahedron in the adjacent layer, thereby reducing the bonding strength of the c-plane.
Because of the B 12 structural unit, the chemical formula of "ideal" boron carbide is usually not written as B 4 C, but B 12 C 3. The combination of B 12 is used to describe the C 3 and B 12 C 2 units. Some studies have shown that one or more carbon atoms can be added to the boron icosahedron, resulting in the stoichiometric heavy carbon end such as (B 11 C) CBC = B 4 C, but the formula is such as B 12 (CBB) = B14C Located at the boron-rich end. Therefore, "boron carbide" is not a single compound, but a series of compounds with different components. B12 (CBC) = B6.5C is an ordinary intermediate, which approximates the ratio of ordinary elements. Quantum mechanics calculations show that the configuration disorder between boron and carbon atoms in different positions in the crystal determines the material properties—especially the crystal symmetry of the B4C component and the non-metallic electrical properties of the B13C2 component.
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