Fundamental Properties, Synthesis, and Performance of Boron Carbide
Release date:
2025-10-24 12:53
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Boron carbide is a general term for compounds of carbon (C) and boron (B). Depending on the synthesis conditions, two distinct compounds—B4C and B6C—are formed; when referring to boron carbide in general, it usually means B4C. I. Fundamental Properties of Boron Carbide B4C belongs to the trigonal crystal system, with 12 boron atoms and 3 carbon atoms in each unit cell. The carbon atoms are arranged along the body diagonals of the cubic unit cell, making them highly mobile and capable of being substituted by boron atoms, thereby forming substitutional solid solutions. Additionally, carbon may leave the lattice, resulting in high‑boron compounds containing defects. B4C has a molecular weight of 52.25, containing 21.74% carbon and 78.26% boron. It typically appears as a dark gray to black powder, with a density of 2.519 g/cm³, a Mohs hardness of 9.36, and a microhardness of approximately 50 GPa—second only to diamond and cubic boron nitride. Consequently, B4C powder exhibits exceptionally high abrasive performance, achieving 60–70% of the grinding efficiency of diamond and surpassing silicon carbide by about 50%, while offering 1–2 times the grinding power of alumina. The melting point of B4C is 2450°C (with decomposition occurring at this temperature). Its coefficient of thermal expansion between 0 and 1000°C is 4.5 × 10⁻⁶ °C⁻¹. At 100°C, its thermal conductivity is 121.4 W/m·K, decreasing to 62.79 W/m·K at 700°C. B4C is primarily used as an abrasive material and in abrasive tools. Hot‑pressed B4C products serve as wear‑resistant and heat‑resistant components. In the refractory materials industry, B4C is mainly employed as an additive—for example, added to carbon‑bonded refractories to act as an antioxidant, or incorporated into amorphous refractories to enhance green strength and corrosion resistance. II. Composition and Typical Properties of Boron Carbide The most common industrial method for synthesizing B4C powder involves reducing boron trioxide with excess carbon: 2B₂O₃ + 7C → B₄C + 6CO↑ This reaction can be carried out in either a resistance furnace or an electric arc furnace. In a resistance furnace, boron trioxide (B₂O₃) is heated together with carbon (C) under conditions below B4C’s decomposition temperature, yielding B4C with minimal free carbon (sometimes containing 1–2% free boron). This approach produces higher‑quality material. By contrast, in an electric arc furnace, due to the extremely high temperatures involved, B4C decomposes around 2200°C into a carbon‑rich phase and elemental boron. The elevated temperature also causes some boron to evaporate, leading to a product with a significantly higher content of free carbon (20–30%). Consequently, the quality of B4C produced in an arc furnace is somewhat inferior. When synthesizing B4C in an electric arc furnace, typical raw materials include boric acid (containing >92% B₂O₃), artificial graphite (with >95% fixed carbon), and petroleum coke (with >85% fixed carbon). Based on stoichiometric calculations, the amount of boric acid is set approximately 2% above the theoretical requirement, while artificial graphite and petroleum coke each account for 50% of the total carbon input, with an additional 3–4% over the theoretical amount. These three components are thoroughly mixed in a ball mill, then loaded into the electric arc furnace and subjected to reduction and carburization at temperatures ranging from 1700 to 2300°C, ultimately producing B4C. Finally, the resulting melt is refined through processes such as screening, washing, crushing, fine grinding, acid leaching, and sedimentation‑based classification to obtain B4C powders of various particle sizes.
Boron carbide is a general term for compounds of carbon (C) and boron (B). Depending on the synthesis conditions, two distinct compounds—B4C and B6C—are formed; when referring to boron carbide in general, B4C is typically meant.
I. Fundamental Properties of Boron Carbide
B4C belongs to the trigonal crystal system, with 12 boron atoms and 3 carbon atoms in the unit cell. In the unit cell, the carbon atoms are arranged along the body diagonals of the tetrahedral framework; the carbon atoms are highly mobile and can be substituted by boron atoms, forming a substitutional solid solution, and may also leave the lattice to form high‑boron compounds containing defects.
B4C has a molecular weight of 52.25, with 21.74% carbon and 78.26% boron. It typically appears in shades ranging from grayish‑dark to black, with a density of 2.519 g/cm³, a Mohs hardness of 9.36, and a microhardness of approximately 50 GPa—surpassed only by diamond and cubic boron nitride. Consequently, B4C powder exhibits exceptionally high abrasive performance; its grinding efficiency can reach 60%–70% of that of diamond, exceeds that of SiC by 50%, and is 1–2 times greater than that of corundum.
B4C has a melting point of 2450°C (with decomposition). Its coefficient of thermal expansion between 1000°C is 4.5 × 10⁻⁶ °C⁻¹. Its thermal conductivity is 121.4 W/m·K at 100°C and 62.79 W/m·K at 700°C. B4C is primarily used as an abrasive and in grinding tools; hot‑pressed B4C products serve as wear‑resistant and heat‑resistant components. In the refractory materials industry, B4C is mainly employed as an additive—e.g., incorporated into carbon‑bonded refractories to act as an antioxidant, or added to amorphous refractories to enhance green strength and corrosion resistance.
II. Composition and Typical Applications of Boron Carbide
The industrially common method for preparing B4C powder is to reduce boron trioxide with excess carbon:
2B₂O₃ + 7C → B₄ + 6CO↑
The synthesis reaction can be carried out in either a resistance furnace or an arc furnace. In the resistance furnace, boron trioxide (B₂O₃) is heated with carbon (C) and the mixture at a temperature below the decomposition point of B₄C, yielding B₄C with only a small amount of free carbon (sometimes containing 1%–2% free boron). This is a relatively favorable synthesis method. In contrast, when the reaction is performed in an arc furnace, the arc temperature is sufficiently high; B₄C decomposes at around 2200°C into a carbon‑rich phase and boron, and the boron further volatilizes at elevated temperatures, resulting in a product that contains a substantial fraction of free carbon (20%–30%). Consequently, the quality of the B₄C obtained is somewhat inferior.
When synthesizing B4C in an electric arc furnace, boric acid (with a purity exceeding 92%), synthetic graphite (with fixed carbon content greater than 95%), and petroleum coke (with fixed carbon content greater than 85%) are typically used as raw materials. The theoretical stoichiometric amounts are calculated based on the reaction equation; the amount of boric acid added is approximately 2% higher than the theoretical requirement, while synthetic graphite and petroleum coke each account for 50% of the total carbon input, with their quantities further increased by 3–4% above the theoretical values. The prepared mixture of these three materials is then blended in a ball mill and fed into the electric arc furnace, where reduction and carburization are carried out at temperatures between 1700 and 2300°C to produce B4C. Finally, the resulting melt‑solidified mass is sorted, washed, crushed, finely ground, acid‑leached, and subjected to sedimentation and classification processes to obtain B4C powders of various particle sizes.
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