Boron Carbide

Boron carbide (B4C) is one of the hardest man-made materials utilized in lots of fields. It is the third hardest chemical compound, ranking after diamond and cubic boron nitride (formula CBN). Boron carbide is a dark black crystal, so it is also called a “black diamond”. This strong material does not exist in nature, and could only be obtained from boron and carbon in a furnace. Boron carbide can be shaped by pressing at temperatures exceeding 2000 Thanks to its hardness and other unique properties, boron carbide has significant applications in a variety of industries. Boron carbide, with a combination of special properties, is now manufactured in large quantities. It was discovered in the mid of the 19th century as a by-product of reactions under high temperatures. Yet this compound was not known to the world until the 1930s. Boron carbide is produced in a heat-resistance furnace. Only 15 percent of raw material can be converted into B4C in this process. Thereafter, the valuable boron carbide has become an important material for bulletproof armors, nuclear control rods, abrasives, etc.

Carbothermal reduction
The carbothermal reduction method usually uses boric acid or boric anhydride as raw material and carbon as a reducing agent to carry out high-temperature reduction reaction in an electric arc furnace. At present, this method is the primary method of industrial production of boron carbide, which has the advantages of simple response and low cost.

Self-propagating thermal reduction
The self-propagating thermal reduction method uses carbon black (or coke) and boric acid (or boric anhydride) as raw materials, active metal substance (usually mg) as reducing agent or flux, and the heat generated by self-propagating combustion reaction of metal substance are used to synthesize boron carbide. The reaction equation is as follows: 6mg + C + 2b2o3 = 6mgo + B4C .This method has the advantages of low initial reaction temperature (1000-1200 ℃), energy saving, fast reaction, and simple equipment. The synthesized B4C powder has high purity and excellent particle size (0.1 - 4.0 μ m) and generally does not need to be crushed.

Mechanochemistry
The mechanochemical method uses boron oxide powder, magnesium powder and graphite powder as raw materials, using the rotation or vibration of the ball mill to make the harder ball milling medium impact, grind and stir the raw materials vigorously, and induce the chemical reaction at a temperature slightly higher than the room temperature to prepare boron carbide powder. The preparation temperature of this method is low, so it is a promising preparation method.

Direct synthesis
The direct boron carbide production method is to prepare by thoroughly mixing the carbon powder and boron powder and reacting in a vacuum or inert atmosphere of 1700 - 2100 ℃. The purity of boron carbide prepared by direct synthesis is high, and the B / C ratio in the reaction is easy to control. Still, the preparation process of boron carbide used for synthesis is relatively complex and high cost. Therefore, this method has some limitations.

Sol-gel method
Sol-gel method (Sol-gel) refers to the technique of solidifying inorganic or metal alkoxides through the solution, sol, and gel, and then heat-treated to obtain stable compounds. The superiority of this method is that the mixture of raw materials is more uniform, the reaction temperature is low, the product is bulky, and the particle size of B4C powder is small.Sinha et al. Mixed the boric acid and citric acid under the conditions of pH=2-3 and 84-122. Transparent and stable gold gel can be formed. When heated to 700 degrees in a vacuum furnace, the porous soft borate citric acid precursor can be obtained.