| Boron carbide has various prominent properties, such as high temperature stability, high hardness, high cross section for neutron capture, and excellent high temperature thermoelectric properties. Due to these intriguing properties, boron carbide can be applied to many fields, including used as an abrasive wear resistant material, ceramic armor, and a neutron moderator in nuclear reactors. In industry, B4C is generally produced by a carbothermal reduction route which is a high temperature process. The boron carbide production by the carbothermal reaction of boric acid or boron oxide with carbon at high temperatures requires higher energy consumption and it is difficult to afford B4C micro-nano powder. Therefore, an insight into the synthesis process of boron carbide at low temperatures has attracted some researchers'interests. The sol-gel method is a process of preparing materials by which the uniform and particle size of materials can be designed and controlled in molecular level and high purity, ultrafine and uniform nano-materials can be obtained. If suitable boron and carbon sources can be obtained to afford B-C containing gel, the mixture of raw materials at the molecular level can lead to low reaction temperature and is helpful for the preparation of B4C superfine powder. Hence, to synthesize boron carbide in a low-temperature polymer pyrolysis route is an additional economic incentive.In the present work, a boron-containing polymeric precursor was synthesized by the reaction of boric acid with polyvinyl alcohol in a sol-gel method. To get an insight into the polymeric reaction, the effects of the polymerization degree of polyvinyl alcohol and polyvinyl alcohol/boric acid molar ratio on the reaction were investigated. The polymeric precursor was characterized by IR, elemental analysis and chemical analysis. The results indicated that the greater polymerization of polyvinyl alcohol will result in poor reaction. With increasing polyvinyl alcohol/boric acid molar ratio, the utilization of boric acid was increased and while the polymerization yield was fluctuation. The precursor was pyrolyzed in air at moderate temperatures for 2h and the effect of pyrolyzed temperature on the composition of pyrolyzed product was investigated. The pyrolyzed product was characterized by IR, XRD, TG-DTG and elemental analysis. The results showed that the pyrolyzed product consists of B2O3 and C. With the pyrolyzed temperature varying, the B2O3/C molar ratio of pyrolyzed product also changed. The suitable pyrolyzed temperature was 600℃.The pyrolyzed product was placed in a tube furnace under N2 flow to afford boron carbide at high temperature. The effect of reduction temperature and time on product was investigated. The results indicated that with increasing reduction temperature and time, more boron carbide can be formed and particle aggregation took place to lead to particle growing large. The suitable reduction temperature and time are 1400℃and 3h. Boron carbide powder with 6μm of particle size was obtained under suitable conditions.In order to probe in the mechanism of the high temperature reduction reaction, the thermodynamics analysis of the reaction between B2O3 and C was made. The theory analysis indicated that the reaction between B2O3 and C can not occur below 1550℃. However, boron carbide occurred at 1300℃in this work due to decreasing CO partial pressure in N2 flow. TheΔGθvalues of B-SiC and 9Al2O3·2B2O3 is lower than that of B4C at same temperature, indicating that the formation of B-SiC and 9Al2O3·2B2O3 is easier than that of B4C. Therefore, the measurements of increasing reduction temperature and using graphite crucible should be made.
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