Fundamental properties, synthesis and performance of boron carbide

Boron carbide is a general term for carbon (C) and boron (B) compounds, and two compounds B4C and B6C are generated according to different cooperative conditions. Generally speaking, boron carbide is generally referred to as B4C.

Fundamental properties of 1. boron carbide

B4C is a trigonal crystal system. There are 12 B atoms and 3 C atoms in the unit cell. The connecting three-dimensional diagonal composed of C atoms in the unit cell is equipped. C is in an active state and can be replaced by B atoms to form a replacement solid solution. It may be separated from the crystal lattice to form a high boron compound with defects.

B4C has 52.25 molecular weight, C21.74% and B78.26%, generally dark to black, density 2.519g/cm3, Mohs hardness 9.36, microhardness 50GPa, second only to diamond and cubic boron nitride. Therefore, B4C powder has very high grinding ability, and its grinding power can reach 60%-70% of diamond, higher than SiC50%, and 1-2 times of corundum grinding ability.

B4C melting point 2450 degrees C (differentiation). The coefficient of expansion between 1000°C is 4.5 x 10-6°C -1. The thermal conductivity is 121.4 W/m · k at 100 ℃ and 62.79 W/m · k at 700 ℃. B4C is primarily used as an abrasive tool, and hot-pressed B4C products can be used as wear-resistant and heat-resistant components. In the refractory industry, B4C is primarily used as an additive, such as adding to carbon-bonded refractory materials for antioxidant effect, adding to amorphous materials to improve the strength and corrosion resistance of the blank.

Composition and Typical Function of 2. Boron Carbide

The industrially common method of composing B4C powder is to recover the boric anhydride with an excess of carbon:

2b2o3 7C → b4 6co ^

The composition reaction can be carried out in a resistance furnace or an electric arc furnace. When the composition is carried out in a resistance furnace, B4C containing little free C (sometimes containing 1%-2% free boron) can be obtained by heating boron anhydride B2O3 and carbon C and the mixture at a differentiation temperature lower than B4C. It is a better composition method. When the composition is carried out in an electric arc furnace, the arc temperature is appropriately high, however, B4C is differentiated into carbon-rich phase and boron at about 2200 ℃, and boron will transpiration at high temperature to form reaction products containing a large amount of free C(20%-30%), so the quality of B4C obtained is slightly poor.

When B4C is composed of electric arc furnace, boric acid (content greater than 92%), artificial graphite (fixed carbon greater than 95%) and petroleum coke (fixed carbon greater than 85%) are generally selected as materials. According to the theoretical dosage of reaction calculation, the participation amount of boric acid is about 2% higher than the theoretical dosage, artificial graphite and petroleum coke each account for 50% of the total carbon participation amount, and then 3%-4% higher than the theoretical dosage, the prepared three materials are mixed in a ball mill, and B4C can be obtained by participating in an electric arc furnace for recovery and carbonization at 1700-2300 ℃. Finally, B4C of various particle sizes can be obtained by sorting and washing, crushing, grinding, pickling, sedimentation classification and other processes.