Preparation Of Ultrafine B₄C Powders By Reaction-diluted SHS Method

Boron carbide B₄C is widely used in abrasives,high performance engineering ceramics,bulletproof materials,cutting tools,nuclear reactor control rod and shielding material and other fields due to its excellent physical and chemical properties such as the advantages of low density,high hardness,high elastic modulus,high wear resistance,high chemical stability,strong acid corrosion and so on..Boron carbide materials have attracted much attention in recent years,the preparation of boron carbide powders by the SHS method had been studied by experts both at home and abroad,however,the SHS requires high temperature,high energy consumption,high requirement for equipment,it is difficult to control for reaction,and the particle size and purity of the product,moreover,there was no batch preparation technology of boron carbide ultrafine powder at home and abroad,which can not be mass,large-scale and industrialized production,which seriously hinders the application research of ultrafine boron carbide powder in the scientific community.For the defects of the preparation of the SHS for boron carbide powders,in this paper ultrafine boron carbide powders were prepared by the diluted self-propagation high-temperature synthesis?SHS?method using H₃BO₃/Mg/C₆H₁₂O₆ as reaction system,and a synthetic device and process for the continuous preparation of ultrafine boron carbide powders were developed,the optimum technological parameters were determined,and the aim of optimizing the material and process was achieved.The innovative research results of this paper are as follows:1.A novel synthesis method,reactive dilution self propagating high temperature synthesis,was proposed,which included a set of opposed endothermic and exothermic reactions.Ultrafine boron carbide powders were prepared by the diluted self-propagation high-temperature synthesis?SHS?method using H₃BO₃/Mg/C₆H₁₂O₆as reaction system.The SHS process and product particle sizes were controlled by adjusting the ratio of exothermic reactions which was adjusted by changing the ratio of reaction raw materials.The optimal conditions for the preparation process were found at endothermic ratio of 0.14 and reaction initiation temperature of 800?.The crystal form,morphology,structure and particle size distribution of the products were characterized by X-ray diffraction,scanning electron microscopy,raman and fourier infrared spectroscopy,laser particle size analyzer and thermogravimetry differential scanning calorimetry.The result shows that boron carbide with the B₄C structure and the highest purity is prepared and the optimum particle size of product was around400 nm,when the reaction material C:Mg:B is 1:2.7:5.5,and the reaction initiation temperature is 800?.A large amount of water vapor and CO₂ gas released during the reaction will take away the heat,reduce the temperature of the combustion system,and suppress the sintering and structural agglomeration of grain,which is beneficial to the formation of fine and uniform grains.2.A continuous self propagating reaction synthesis device consisting of four parts,a feed device,a reaction furnace,a ash storage,a cooling device,and a control device was invented.the best parameters of the equipment are determined by comparing the structure,morphology and particle size of boron carbide powder with the boric acid/magnesium/glucose reaction system by self propagating synthesis device with different ratio of raw materials.The feed quantity is 1-5Kg,the pushing speed is 12Hz,the stirring rate is 22Hz,the reaction furnace temperature is about800??heating rate is 20 degrees/min,holding heat is long 24h?,and the ash barrel discharge rate is 500g/min.3.A continuous self propagating reaction synthesis process is disclosed,the B₄C ultrafine powders were rapidly and continuously prepared by dilution self propagating reaction method,using continuous self propagating reaction synthesis equipment as production equipment.The optimum technological parameters were determined by comparing the powders with different proportions.A feed rate is about 1-5Kg,the ratio of raw materials is C/Mg/B 1/2.7/5.5,pushing speed in 12Hz;reaction temperature is800?,the endothermic reaction rate is 0.14;in 83%the yield of boron carbide powder;boron carbide production rate is 5.2Kg/h;the purity is about 97%;the average grain size is about 400nm.This process contains only raw material mixing,reaction and pickling,filtration and drying,is a simple operation,it has the advantages of low energy consumption,high yield,low cost and good product quality,and is suitable for large-scale production of new process technology.