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Method for preparation of boron carbide

wallpapers News 2021-05-14
Boron carbide has excellent properties and is widely used. The hardness of boron carbide is second only to diamond and cubic boron nitride. It has the advantages of high melting point, low density, high strength, large neutron absorption cross section, excellent thermoelectric properties and good mechanical stability, etc. It has been widely used in aerospace, national defense, nuclear energy and wear resistance technology. At present, the carbothermal reduction method is the main method of industrial production of boron carbide, in addition, the preparation of boron carbide methods and self-spreading thermal reduction method, mechanization method, direct synthesis method, sol gel method, etc.
2.1 Carbonothermic reduction method
Carbothermal reduction method usually uses boric acid or boron anhydride as raw material, carbon as reducing agent, in an electric arc furnace for high temperature reduction reaction. At present, this method is the main method for industrial production of boron carbide, which has the advantages of simple reaction, easy operation and low cost.
Using boric acid and carbon black as raw materials, Yu Guoqiang et al. calcined boron carbide powder with high purity at 1700~1850℃ for 0.5~1.0h. The mass fraction of carbon content was 20.7%, which was close to the theoretical value. However, the disadvantage of this method is that it needs to be carried out at a higher temperature and consumes a lot of energy. The prepared boron carbide is easy to caking and needs to be pulverized. The inclusion of unreacted carbon in the product needs to be removed by subsequent treatment.
2.2 Self-propagating thermal reduction method
The self-propagating thermal reduction method uses carbon black (or coke) and boric acid (or boric anhydride) as raw materials, with active metal elements (usually Mg) as reducing agent or flux, and the heat generated by the self-propagating combustion reaction of metal elements to synthesize boron carbide. The reaction equation is as follows: 6Mg+C+2B2O3=6MgO+B4C (1)
This method has the advantages of lower initial reaction temperature (1000~1200℃), energy saving, faster reaction and simple equipment. The synthesized B4C powder has higher purity and finer particle size (0.1~4.0μm), and generally does not need to be crushed.
With carbon black, B2O3 and magnesium as raw materials, Alkan et al. prepared B4C powder by self-propagating thermal reduction method.
With Na2B4O7, Mg and C as raw materials, Jiang et al. prepared B4C powder with a particle size of 0.6μm by self-propagating thermal reduction. However, the MgO produced by the reaction must be removed by additional process, and it is extremely difficult to remove it completely.
2.3 Mechanized learning method
The mechanization method is based on boron oxide powder, magnesium powder and graphite powder as raw materials, using the rotation or vibration of the ball mill to make the hard ball mill medium to impact, grind and stir the raw materials strongly, and induce the chemical reaction to produce boron carbide powder at a temperature slightly higher than room temperature. The preparation temperature of this method is low and it is a promising method.
Deng et al. prepared B4C powder with the mass ratio of B2O3: C: Mg of 10:1:11 by mechanization method, and the particle size of the powder obtained was 100~200 nm. Yaghoubi et al. believed that the optimal mass ratio of Mg: C was between 9:2 and 10:1. However, MgO, the by-product of this method, is difficult to be completely removed, and generally requires a long time of ball milling.
2.4 Direct synthesis method
The direct synthesis method is to prepare boron carbide under the condition of vacuum or inert atmosphere at 1700~2100 ℃ after the carbon powder and boron powder are fully mixed. The boron carbide prepared by direct synthesis method has high purity, and the B/C ratio in the reaction is easy to control, but the preparation process of the elemental boron used for synthesis is relatively complex and the cost is high. Therefore, this method has some limitations.
2.5 Sol-gel method
Sol-gel method refers to the method of solidifying inorganic substances or metal alcohols through solution, Sol and gel, and then obtaining solid compounds through heat treatment. The advantages of this method are more uniform mixing of raw materials, low reaction temperature, leavening of products and smaller particle size of B4C powder prepared.
Sinha et al. experimented with the mixed solution of boric acid and citric acid at pH 2~3 and temperature 84~122 ℃, forming a transparent and stable golden gel. When heated to 700 ℃ in a vacuum furnace, porous and soft blocked boric acid-citric acid gel precursor could be obtained. The precursor was held at 1000~1450℃ in vacuum for 2h to obtain B4C powder with a particle size of about 2.25 μm.
Hadian et al studied the effects of reaction time, temperature and different ratio of raw materials on B4C in the boric-citric acid gel reaction system. When the initial mass ratio of boric acid to citric acid was controlled to be 2.2:1 and the reaction time was 3.5h at 1500℃, the content of free carbon in the product was 2.38%. However, the production efficiency of this method is low and it is difficult to be applied on a large scale.

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