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What is boron carbide - Let's understand boron carbide


Boron carbide crystal structure  


The crystal cell of B ₄ C. The green sphere and icosahedron are composed of boron atoms, while the black sphere is composed of carbon atoms .

Fragments of boron carbide crystal structure


Fragments of boron carbide crystal structure.

Boron carbide has a complex crystal structure, which is a characteristic of borides centered on the pentahedron. The B12 octahedron forms a rhombic lattice unit around the C-B-C chain at the center of the unit cell (space group: R3m, lattice constant: a=0.56 nm and c=1.212 nm), and all carbon atoms bridge adjacent three octahedra. The B12 octahedron and bridging carbon form a network plane parallel to the c-plane, which is stacked along the c-axis to form a layered structure. The two basic structural units of a lattice are the B12 octahedron and the B6 octahedron. Due to the small size of the B6 octahedron, it cannot be combined. On the contrary, they bind to the adjacent B12 octahedra, which weakens the bonding of the c-plane. 

The chemical formula of "ideal" boron carbide is sometimes written as B12C3, and the carbon deficiency of boron carbide is defined by the combination of B12C3 and B12C units, due to the B12 structural unit. Some studies have shown that one or more carbon atoms can be embedded into the boron octahedron, resulting in formulas like B11CCBC=B4C at the stoichiometric carbon heavy end, but formulas like B12 (CBB)=B14C at the boron rich end. Therefore, "boron carbide" is a series of compounds with different compositions, rather than a single compound. B2 (CBC)=B6.5C is a common intermediate that approximates the commonly found elemental ratio. According to quantum mechanics calculations, the symmetry of the crystal composed of B4C and the non-metallic electrical properties of B13C2 are determined by the disordered configuration of boron and carbon atoms at different positions in the crystal. 

The physical properties of boron carbide

The density of boron carbide is close to 2.52 g/cm ³.

The melting point of boron carbide is 2445 ° C.

The hardness range of boron carbide is 2900-3580Kg/mm2 (Knoop 100g).

The fracture toughness of boron carbide is 2.9-3.7 MPam-1/2.

The Young's modulus of boron carbide is 450-470 GPa

The conductivity of boron carbide at 25 ° C is 140 S.

The thermal conductivity at 25 ° C is 30-42 W/m.K

The thermal neutron capture of boron carbide is 600 bars.

Chemical properties of boron carbide

Boron carbide is a tough substance with high toughness (Mohs hardness of approximately 9.5 to 9.75), a large neutron absorption cross-section (i.e. strong neutron shielding performance), and resistance to ionizing radiation and most chemical substances. The Vickers hardness (38 GPa), elastic modulus (460 GPa), and fracture toughness (3.5 MPam-1/2) are similar to those of diamonds (1150 GPa and 5.3 MPam-1/2).

Boron carbide was identified as the third hardest material in 2015, second only to diamonds and cubic boranes, and is therefore praised as a "black diamond".

Boron carbide is a semiconductor whose electronic properties are dominated by hopping transport. The band gap is determined by both composition and degree of order. The band gap measurement is 2.09 eV, and the photoluminescence spectrum is complicated by several intermediate band gap states. 

The reaction of boron carbide

The oxidation of boron carbide powder begins at temperatures as low as 250 ° C in the presence of water vapor and 450 ° C in the absence of water vapor. Water vapor removes B ₂ O ₂ oxides at a faster rate than oxidation, at temperatures below 550 Å -600 Å C. B ₂ O ₂ inhibited H ₂ O oxidation on the B ₄ C surface, but did not inhibit air oxidation. There is a linear relationship between the rate and the partial pressure of water. The activation energy of the water B ₄ C reaction is 11 kcal/mol, while the activation energy of the air B ₄ C reaction is 45 kcal/mol. The oxidation rate of dry air is slower than that of water vapor before reaching 700 (for 235 mm water pressure). 

The History of Boron Carbide

Boron carbide was discovered as a byproduct of metal boride reactions in the 19th century, but its chemical composition is unknown. It was not until the 1930s that its chemical composition was determined to be B4C. The precise 4:1 stoichiometric ratio of this material is still controversial, as in nature, this formula still slightly lacks carbon, and X-ray crystallography reveals its highly complex structure, with a mixture of C-B-C chains and B12 octahedra.

These characteristics do not match the exact B4C empirical formula. The chemical formula of "ideal" boron carbide is sometimes written as B12C3, and the carbon deficiency of boron carbide is defined by the combination of B12C3 and B12CBC units, due to the B12 structural unit.