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High performance ceramic neutron absorption material: Boron carbide

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Update time : 2019-08-28 10:29:41
Boron carbide is an extremely hard ceramic material used in tank armor, body armor and many industrial applications. It has a Mohs hardness of 9.3 and is the fifth hardest known material after diamond, cubic boron nitride, fullerene compounds and diamond monofilaments. It was discovered as a by-product of metal boride research in the 19th century and was not scientifically studied until the 1930s. Boron carbide can be obtained by reducing boron trioxide with carbon in an electric furnace.
 
Method for preparing boron carbide
According to different reaction principles, different raw materials and equipment, the preparation methods of boron carbide are divided into the following categories: high temperature self-propagation synthesis method, carbon tube furnace or electric arc furnace carbon thermal reduction method, chemical vapor deposition method, direct preparation method, Jet milling, sol-gel method, solvothermal reduction method, mechanochemical method, and the like.
 
Self-propagating high temperature synthesis
The self-propagating high-temperature synthesis method requires a low reaction temperature, and makes full use of the heat released by the compounding reaction to continue the reaction, usually using flux magnesium.

The advantages of the self-propagating high-temperature synthesis method are: fast reaction, low reaction temperature, energy saving, simple process, and high purity of the synthesized boron carbide powder. Disadvantages are: uneven reaction: particle size distribution is wide. Magnesium oxide remaining in the reactants is difficult to completely remove the gas pressure even by pickling, which has a significant influence on the particle size of the boron carbide.
 
Carbon tube furnace or electric arc furnace carbothermal reduction method
The carbothermal reduction method is currently the main method for industrial production of boron carbide. In the method, boric acid or boric anhydride is uniformly mixed with carbon powder, and then placed in a carbon tube furnace or an electric arc furnace, and a protective gas and carbon are used as a reducing agent to synthesize a boron carbide powder at a certain temperature.

The working principle of the electrode of graphite is different. The electric arc furnace can be divided into a vertical curing furnace and a horizontal processing furnace. The graphite electrode of the vertical processing furnace has a large arc depth, and the high temperature area of the high-temperature processing zone is large, and the furnace material is fully cured.
 
Chemical vapor deposition
According to different processes, the chemical vapor deposition method can be further divided into: thermal chemical vapor phase reaction method, hot wire method, plasma vapor deposition method, laser induced vapor deposition method, synchronous heating irradiation method and the like.

The chemical vapor deposition method has the advantages of low pollution, high purity boron carbide powder, and the like, but the production rate of boron carbide is low, and the use of dangerous gas phase raw materials in the preparation process has high requirements on equipment and high production cost. Therefore, chemical vapor deposition is not suitable for mass production of boron carbide.
 
Direct preparation
The direct preparation method is to mix boron powder and carbon powder, pass through a protective gas, and react at a high temperature to form boron carbide. Its reaction equation is: 4B+C=B4C. Fu Bo et al. directly synthesized boron carbide powder with boron and carbon. The boron carbide powder prepared by the method has high purity and easy to control the carbon to boron ratio, but the preparation cost is high.
 
Airflow pulverization
The air jet pulverization method is to perform strong pulverization of the coarse powder on the jet mill, including volume pulverization and surface pulverization. Generally, the coarse powder is pulverized three times to obtain a boron carbide powder having an average particle diameter of less than 1μm. Yin Bangyue et al. used this method to prepare ultrafine boron carbide powder with a particle size of less than 1μm. Shampa Mondal et al. first synthesized a polymer precursor with boric acid and polyvinyl alcohol, and then decomposed at 400-800°C to obtain boron carbide.
 
Sol-gel method
The sol-gel method is a method in which an inorganic substance or a metal alkoxide is solidified by a solution, a sol or a gel, and then a compound is synthesized by heat treatment. By studying different carbon sources, Sinha et al. found that boric acid and citric acid can form a stable and transparent golden yellow gel at pH=2~3 and temperature of 84~122°C. Then, the prepared gel precursor is placed in a graphite mold and kept at 1000 to 1450 ° C for two hours under vacuum to obtain a boron carbide fine powder having an average particle diameter of 2.25μm.

The sol-gel method has the advantages of low reaction temperature, uniform mixing of carbon and boron, less loss of boron source, and small particle size of synthesized boron carbide. However, since the boride of the boron source is difficult to form a gel with other substances, the method is difficult to apply widely.
 
Solvothermal reduction
The solvothermal reduction method is to prepare boron carbide by adding an alkali metal as a reducing agent in a liquid phase solvent at a relatively low temperature. Shi et al. used Na as a reducing agent, BBr3 and CCl4 as a reactant, and reacted in an autoclave at 400°C to prepare a boron carbide powder. Gu et al. used Li as a reducing agent, and used amorphous B powder and CCl4 as reactants to prepare ultrafine boron carbide powder in a high pressure mark at 600°C.

The method realizes low-temperature synthesis of boron carbide, and the obtained boron carbide powder is ultrafine; however, the preparation process uses an alkali metal as a reducing agent, and has high requirements on reaction equipment and conditions, and the purity of the obtained boron carbide is low.
 
Mechanochemical method
Mechanochemical method is a new method for preparing boron carbide. It uses boron oxide powder, magnesium powder and graphite powder as raw materials, and uses the rotation of the ball mill to make the hard ball milling medium strongly impact, grind and stir the raw materials, plastically deform the raw material particles, and significantly reduce the diffusion activation energy of the material. A diffusion/reaction couple can be formed at room temperature, and a chemical reaction can be induced at a temperature slightly higher than room temperature to prepare a boron carbide powder. Tang Huaguo et al. synthesized boron carbide powder at low temperature by controlling the atomic ratio of boron oxide, magnesium powder and graphite powder.