Research On Effect Of Hydroxy Acid On Preparation Of Boron Carbide Powder By Solid Phase Precursor Method

Boron carbide has been widely used in various fields because of its excellent physical and chemical properties.At present,the industrial carbothermic reduction method for producing boron carbide powder has problems such as high temperature,low yield,and heavy environmental pollution.In contrast,the precursor derived method has a lot of advantages such as low energy consumption,simple process,easy availability of raw materials,and small product size.However,the traditional sol-gel method for synthesizing precursors has a long reaction time,complicated operation,and carbon residues in boron carbide.This paper proposes a method of boron carbide prepared by adding a certain amount of hydroxy acid to the raw materials,which using a solid-phase reaction to synthesize a precursor and continuously pyrolyzed process.Polyvinyl alcohol,boric acid,and hydroxy acid,while maintaining the solid phase,were used as raw materials.The high purity B₄C powder was synthesized through three steps:esterification reaction,pyrolysis reaction,and carbothermal reduction reaction.Effect of the addition of hydroxy acid to the preparation of boron carbide powder was investigated.Firstly,the product synthesized by three kinds of raw materials through the esterification method was used as a precursor compound for the preparation of B4C.The precursors were analyzed by FT-IR to determine the formation of new B-O-C bonds,proving the synthesis of the precursors.After analyzing and testing the effect of the addition amount of hydroxy acid and reaction time on the conversion rate of the precursor,the best reaction conditions were obtained as the addition amount of tartaric acid and citric acid were 15mol%and 10 mol%,respectively,and the reaction was carried out at 250?for 4 h.The ratio of polyvinyl alcohol and boric acid is 3:1 stoichiometricAfter the synthesized precursors were directly pyrolyzed,the effects of different amounts of hydroxy acid addition on the composition and structure of the pyrolyzed products were investigated.The pyrolyzed products are characterized by FT-IR,XRD,TGA,SEM and chemical analysis.The pyrolyzed product is a mixture of boron oxide and amorphous carbon.The amorphous carbon exhibits a continuous network structure.Addition of hydroxy acid can significantly reduce the mesh size of carbon,increase the uniformity,improve the dispersion of amorphous carbon and boron oxide,and change the ratio of boron oxide and carbon in the pyrolyzed product.The suitable pyrolyzed conditions for the precursors with tartaric acid addition amount of 0-20 mol%and citric acid addition amount of 0-15 mol%are pyrolyzed at 650°C,and for precursors with tartaric acid addition amount of 25 mol%and citric acid addition amount of 20-25 mol%are pyrolyzed at 700°C.B4C was prepared by carbothermal reduction cracking products,combined with XRD,SEM,EDS and LPSA analysis and detection methods to explore the effect of adding hydroxy acid on the synthesis behavior of boron carbide.The results indicate that the addition of hydroxy acid can reduce the reaction time and the critical forming temperature of B4C,improve the degree of crystallization and the purity of B4C.The complete crystal is rhombic with smooth surface.The B4C powder with 15 mol%tartaric acid by heating treatment at 1450? for 2.5 h can be prepared with a uniform particle size.The particle size and purity are about 8 um and 97.8%,respectively.