The fundamental properties, synthesis, and performance of boron carbide.

Boron carbide is a general term for compounds of carbon (C) and boron (B). Depending on the cooperative conditions, two compounds B4C and B6C are generated, with B4C generally referred to as boron carbide.

1. Fundamental Properties of Boron Carbide

B4C belongs to the trigonal crystal system, with 12 B atoms and 3 C atoms in the unit cell. The C atoms in the unit cell form connecting spatial diagonal lines, and C is in an active state, which can be replaced by B atoms, forming a substitutional solid solution, and may leave the lattice to form high-boron compounds with defects.

The molecular weight of B4C is 52.25, containing 21.74% C and 78.26% B. It generally appears gray to black, with a density of 2.519 g/cm3, Mohs hardness of 9.36, and microhardness of about 50 GPa, second only to diamond and cubic boron nitride. Therefore, B4C powder has a very high grinding ability, with its grinding efficiency reaching 60%-70% of diamond, higher than SiC by 50%, and 1-2 times that of corundum.

The melting point of B4C is 2450℃ (decomposes). The expansion coefficient between 1000℃ is 4.5×10-6℃-1. The thermal conductivity at 100℃ is 121.4 W/m·k, and at 700℃ it is 62.79 W/m·k. B4C is mainly used as an abrasive material. Hot-pressed B4C products can be used as wear-resistant and heat-resistant components. In the refractory materials industry, B4C is mainly used as an additive, such as being added to carbon-bonded refractory materials to act as an antioxidant, and added to amorphous materials to improve the strength and corrosion resistance of the body.

2. Composition and Typical Functions of Boron Carbide

The commonly used method for producing B4C powder industrially is to reduce boron oxide with excess carbon:

2B2O3 + 7C → B4 + 6CO↑

The synthesis reaction can be carried out in a resistance furnace or an electric arc furnace. In the resistance furnace, when synthesizing, boron oxide B2O3 and carbon C are heated below the decomposition temperature of B4C, resulting in B4C containing very little free C (sometimes containing 1%-2% free boron), which is a better synthesis method. In the electric arc furnace, due to the high temperature of the arc, B4C decomposes into a carbon-rich phase and boron at around 2200℃, and boron will evaporate at high temperatures, resulting in the reaction product containing a large amount of free C (20%-30%), thus the quality of the obtained B4C is slightly inferior.

When synthesizing B4C in an 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. The amount of boric acid added is about 2% higher than the theoretical amount calculated based on the reaction formula, with artificial graphite and petroleum coke each accounting for 50% of the total carbon input, and then about 3%-4% higher than the theoretical amount. The three prepared materials are mixed in a ball mill and then fed into the electric arc furnace for reduction and carbonization at 1700-2300℃ to obtain B4C. Finally, the melt is selected and subjected to washing, crushing, grinding, acid washing, and sedimentation classification to obtain various particle sizes of B4C.