The Fundamental Properties, Synthesis, and Performance of Boron Carbide

Boron carbide is a general term for compounds of carbon (C) and boron (B), forming two distinct compounds—B4C and B6C—depending on the reaction conditions. When people refer to "boron carbide" without further specification, they typically mean B4C.

I. Fundamental Properties of Boron Carbide

B4C belongs to the trigonal crystal system, with 12 boron atoms and 3 carbon atoms in its unit cell. Within the cell, the carbon atoms are arranged along the body diagonals, leaving the C atoms in a highly mobile state—capable of being replaced by boron atoms, thus forming a substitutional solid solution. Alternatively, these C atoms may even detach from the lattice altogether, leading to the formation of high-boron compounds with structural defects.

B4C has a molecular weight of 52.25, with carbon accounting for 21.74% and boron making up 78.26%. It typically appears in shades ranging from grayish-black to black. Its density is 2.519 g/cm³, and it exhibits a Mohs hardness of 9.36 and a microhardness of around 50 GPa—ranking just below diamond and cubic boron nitride. As a result, B4C powder possesses exceptionally high abrasive capabilities, with its grinding efficiency reaching 60%–70% of that of diamond, surpassing SiC by 50% and demonstrating 1 to 2 times the abrasiveness of corundum.

B4C has a melting point of 2450°C (decomposition). Its coefficient of thermal expansion ranges from 4.5 × 10⁻⁶/°C between 1000°C. At 100°C, its thermal conductivity is 121.4 W/m·K, while at 700°C, it drops to 62.79 W/m·K. Primarily used as an abrasive material, hot-pressed B4C products are also employed in manufacturing wear-resistant and heat-resistant components. In the refractory industry, B4C serves mainly as an additive—for instance, when incorporated into carbon-bonded refractories, it acts as an antioxidant; when added to unshaped materials, it enhances the green body's strength and resistance to corrosion.

II. Composition and Typical Applications of Boron Carbide

A commonly used industrial method for producing B4C powder involves reducing boron oxide with excess carbon:

2B₂O₃ + 7C → B₄ + 6CO↑

The reaction can be carried out in either a resistance furnace or an arc furnace. When conducted in a resistance furnace, heating boron trioxide (B₂O₃) together with carbon (C) at temperatures below the decomposition point of B₄C yields B₄C containing minimal free carbon—though occasionally it may still include 1%–2% free boron. This method is considered a relatively effective approach for synthesizing B₄C. In contrast, when the reaction is performed in an arc furnace, the high arc temperatures cause B₄C to decompose into a carbon-rich phase and elemental boron around 2200°C. Moreover, the boron tends to evaporate at these elevated temperatures, leading to a final product with a significantly higher content of free carbon—often ranging from 20% to 30%. As a result, the quality of B₄C produced via this method is slightly inferior.

When producing B4C using an electric arc furnace, the typical materials selected are boric acid (with a boron content exceeding 92%), synthetic graphite (containing more than 95% fixed carbon), and petroleum coke (with fixed carbon levels above 85%). The theoretical amounts of these materials are calculated based on the chemical reaction. Specifically, about 2% more boric acid is added than the calculated amount, while synthetic graphite and petroleum coke each account for roughly 50% of the total carbon input—though in practice, it’s advisable to add an additional 3%–4% beyond the theoretical requirement. Once the three materials are properly mixed in a ball mill, they are fed into the electric arc furnace, where temperatures range from 1700°C to 2300°C. Under these conditions, the raw materials undergo reduction and carburization processes, ultimately yielding pure B4C. Finally, the resulting solid product is carefully sorted, followed by a series of processing steps: washing, crushing, grinding to fine particles, acid treatment, and sedimentation-based size classification. These steps ensure that B4C of various desired particle sizes is produced with high purity and quality.