Low-temperature synthesis of boron carbide powder
|School||Dalian University of Technology|
|Course||Chemical Engineering and Technology|
|Keywords||Synthesis Boron carbide Polyvinyl alcohol Precursor Sol - gel method|
Boron carbide because of its high hardness, high modulus, low density, high chemical stability and neutron absorption cross section and thermoelectric properties and other characteristics, it has been widely in many fields of applications, such as for the production of bulletproof armor, wear corrosion-resistant parts, functional ceramics, reactor control rods and the thermoelectric components. Industrial preparation B4C, often used as raw materials in boric acid and carbon carbon thermal reduction smelting. This method not only high energy consumption, manufacturing B4C powder coarse granularity, milling is easy to introduce impurities, so that lower purity. Therefore, the research and development of a low-cost, low-power method of preparation of boron carbide powder is necessary. Sol - gel method is a molecular level from the design and control of materials, particle size uniformity and preparation method, high-purity, fine and uniform nanomaterials. If they can find a suitable source of boron, carbon form a gel method has been applied in a uniform mixing of raw materials at the molecular level, the reaction temperature is low, the product leavening and other characteristics of ultrafine B 4 C powder great benefit. The study is in this context the development of a low-temperature process for synthesis of boron carbide powder. First of polyvinyl alcohol and boric acid as raw materials by sol - gel synthesis of polyvinyl borate gel precursor for preparing boron carbide. To understand the various factors on the polymerization reaction, investigated the degree of polymerization of polyvinyl alcohol and polyvinyl alcohol / acid molar ratio on the polymerization reaction. Polymerization product by the infrared spectroscopy, elemental analysis and chemical analysis test that: the greater the degree of polymerization of polyvinyl alcohol, the more difficult to carry out the polymerization reaction; with alcohol / acid molar ratio increases, the utilization increased acid However, polymerization yields some fluctuations occur. According to the experimental results to determine the appropriate reaction molar ratio of 4:1. Precursor polymer gel obtained in air temperature lysis 2h, investigated cleavage product composition and temperature on the composition of pyrolysis. By IR, XRD, TG-DTG and elemental analysis means that: after cleavage product was a mixture of boron oxide, and carbon; pyrolysis temperature changes, boron oxide and carbon product also changes the ratio, the appropriate pyrolysis temperature of 600 ℃. After lysis of the product in a tubular furnace under nitrogen atmosphere, temperature reducing boron carbide. The effects of different reduction temperature, reduction time impact on the product. The results showed that: the higher the reduction temperature, the reduction time is longer, more favorable formation of boron carbide, but also a corresponding increase in the content of impurities, particle agglomerate, particle size increases. Suitable reduction temperature of 1400 ℃, the reduction time of 3h. In the synthesis of the appropriate conditions, the availability of the boron carbide particle size of the powder of about 6μm. To investigate the mechanism of high-temperature reduction reaction for the preparation of boron oxide and carbon reduction process of carbonization whetstone a thermodynamic theory. Theoretical calculations show that the reaction at 1550 ℃ the following will not occur spontaneously. However, the experimental results show that boron has appeared at 1300 ℃, this is because in a nitrogen stream atmosphere, the reaction equilibrium is changed, reducing the partial pressure of CO, B4C can generate a temperature below 1550 ℃. The same temperature, β-SiC and 9Al2O3 · 2B2O3 of ΔGθ less than B4C's ΔGθ, instructions easier to produce β-SiC and 9Al2O3 · 2B2O3. You can use an appropriate increase in the reaction temperature or the use of graphite utensils measures to reduce these two impurities.