Preparation Of Boron Carbide Using Biochar As Carbon Source And Study On The Properties Of Boron Carbide

Boron carbide is an important ultra-high temperature structural ceramic material widely used in military protection materials,abrasives,and reactor control rods due to its low density,high hardness,high chemical stability,and neutron absorption performance.The prerequisite for obtaining high-performance boron carbide ceramic materials is to synthesize boron carbide powders with high purity,small particle size,and good particle size uniformity.The main technology for industrial preparation of boron carbide is the carbothermal reduction method.In this method,boric acid and sodium borate are mixed with coke and graphite,and then entered into a closed reactor for reduction reaction at high temperature（about 2000°C）to produce boron carbide.The reaction product is processed by crushing,sieving,and other treatments to obtain boron carbide powder.Although the carbothermal reduction process is suitable for the mass production of boron carbide,the use of toxic chemicals as carbon sources has adverse effects on the ecological environment.In addition,the single morphology and uniformity limit its technical application.To overcome these limitations,the article describes the preparation of boron carbide powders using four different methods,namely direct carbothermal reduction method,low-temperature heat treatment carbothermal reduction method,hydrothermal treatment carbothermal reduction method,and polymer precursor pyrolysis/sol-gel treatment carbothermal reduction method,using corn cob as the carbon source and boric acid as the boron source.The study investigates the thermal decomposition mechanism of corn cob,the ratio of boric acid to corn cob,the morphology of corn cob,the precursor process before pyrolysis,the carbothermal reduction temperature,and the carbothermal reduction time,and analyzes the formation mechanism of boron carbide prepared from corncob.The main research contents are as follows:（1）Utilizing the direct carbon thermal reduction method,boron acid and corncob were mechanically ground and directly fed into the atmosphere furnace for a high temperature reaction at 1600°C or above.By studying the impact of raw material ratio,synthesis temperature,and synthesis time on the boron carbide powder,it was discovered that the optimal process path in this method was achieved with a boron acid to corncob ratio of 1:15,a pyrolysis temperature of 1800°C,and a pyrolysis time of 4 h.This resulted in irregular block-shaped boron carbide powder with a particle size range of 40-60μm and a purity of 78.32%.As the synthesis temperature increased,the particle size showed an obvious trend of enlargement.The volatile gases contained in the maize cob caused the synthesized boron carbide to be oxidized in the high-temperature area.Mechanism analysis revealed that the decomposition of maize cob and boron acid at high temperatures produced solid-phase derived carbon and gas-phase oxidized boron,which stimulated a reduction reaction to form granular boron carbide powder.（2）Utilizing the low-temperature heat treatment carbon thermal reduction method,after mixing boron acid and corncob,they were first subjected to a low-temperature pyrolysis treatment at 350-550°C and then carbon thermal reduction at 1350°C or higher.It was found that the optimal process path was achieved with a boron acid to corncob ratio of 1:7.After grinding,the mixture underwent pyrolysis treatment at 500°C for 4 h.Following this,the produced carbide composite material was ground once again and then subject to heat treatment for reduction at 1500°C for 4 h to yield pure boron carbide powder.The layer-like structure of the carbonized corncob affected the morphological properties of the final boron carbide product,which had a particle size range of approximately 10μm and a morphology that is both flaky and granular.The purity of the boron carbide reached 85.67%.Compared to the direct carbon thermal reduction reaction,low-temperature heat treatment effectively reduced the temperature required for the carbon thermal reduction reaction,and the intermediate products that underwent grinding helped to reduce the particle size distribution of the boron carbide.Mechanism analysis revealed that the synthesis reaction of B₄C in a carbon tube furnace was primarily a liquid-solid reaction,and during this process,B₂O₃ and carbon generated B₄C while B₂O₃ evaporated.Factors related to the reaction driving force,reaction velocity,and B₂O₃ evaporation associated with the reaction temperature and B₂O₃ content in the reactants all affect the residual carbon in the B₄C powder.（3）The hydrothermal treatment carbon thermal reduction method involves hydrothermal carbonization of corncob and boron acid in a hydrothermal reaction kettle,followed by low-temperature pyrolysis at 450-550°C and high-temperature carbon thermal reduction.Using a boron acid to maize cob ratio of 1:7,after hydrothermal reaction at 180°C for 12 h and low-temperature pyrolysis at 500°C for 3 h,the carbonized material underwent high-temperature carbon thermal reduction at 1500°C for 3 h to yield boron carbide powder with particle sizes ranging from 2-10μm.The morphology of the powder is primarily irregular block-shaped,but also features some flake-like and hexagonal particle-shaped structures.The purity of the product reached 93.21%.Hydrothermal treatment allowed for the boron acid to enter the interlayers of the maize cob,effectively improving the reaction efficiency,shortening the preparation time,reducing the minimum synthesis temperature of boron carbide,and lowering the particle size of the resulting boron carbide.（4）The glycerol assisted boric acid corncob carbothermal reduction method introduces glycerol as an additive in the raw material of the low-temperature heat treatment carbothermal reduction method.The effect of glycerol content,carbon thermal reduction temperature and time,and precursor boron-carbon ratio on the preparation system of boron carbide powder is explored.Using a boron acid to maize cob ratio of 1:6 and a glycerol content of 10%,the low-temperature pyrolysis was conducted at 550°C for 3 h,followed by carbon thermal reduction at 1400°C for2.5 h to yield pure boron carbide powder.The presence of glycerol builds a bridge between the maize cob and boron acid,increasing the reaction rate between them.Glycerol acts as a template,making the carbon pore size more uniform and dense,resulting in a significant reduction in the particle size of the resulting boron carbide powder,with particle sizes concentrating around1-5μm.