Synthesis Of B₄C Via Microwave And Mechanical Properties Of B₄C/Al Composites

Recently,microwave sintering technique has attracted increasing attention of domestic and foreign researchers by advantages its special heating process which allows high-efficiency,energy-saving and can cause some specificity by the high frequency electromagnetic field.At present,the production efficiency of boron carbide crystal is low and with some more impurities and large energy consumption in the traditional industy,which is not suitable for the sustainable development in national resource saving and environmental protection.Herein,if make use of the dielectric loss of carbon materials and combined it with microwave sintering technology,we could obtain a fast and efficient synthesis of boron carbide crystals and broaden the application of boron carbide composite material,which will lead benefits to boron carbide industry.Boron carbide samples with different morphology were successfully synthesized by microwave sintering through rapid crabon-thermal reduction?RCR?,with a 2.45 GHz microwave furnace?TE666?,During the synthsis period,cheap coal and boric acid were used as raw minerals and no catalysts or shielding gas atmosphere were employed.The effects of heating temperature,holding time and Na2CO3 additives content were investigated and the behaviors in microwave heating were analyzed.We successfully prepared B₄C/Al composites with excellent performance by vacuum hot pressing though powder metallurgy,and some experimental parameters such as sintering temperature,holding time,samples interface bonding behavior and fracture mode were investigated.The results show that the obtained boron carbide is effectively and efficiently synthesized by microwave sintering at low temperatures,and the special microwave heating mode can make boron carbide grow on the surface of the coal particles,which will form B4 C nanosheets with the side length less than 150 nm and thickness of 40 nm at 1600 ?.When the temperature is 1700 ?,we could obtain boron carbide spherical particles with the diameter of 50 nm,when increase the temperature to 1800 ?,the diameter of particles grow to 150 nm;extend the time of heat preservation can make the boron carbide nanosheets grow up and crosslink and the particles can form more whisker with many stacking faults;decomposition of Na2CO3 enhances the gas-solid reaction which could promote the interface reaction,and ionized CO2 gas will become spatial hot spots;with Na2CO3 content of 9 wt%,after sintered at 1800 ?,the obtained boron carbide crystals show hollow "bamboo" microstructure.By powder metallurgy hot-press sintering method,we can prepare B₄C/Al composite material with enhanced mechanical performance which is significantly improved than that of traditional aluminum alloy products.The optimum parameters for hot-pressing sintering process is as follows: under a press of 30 MPa,heating at 700 oC for 45 min.Wet ball mill mixing can allow a thin aluminum oxide layer generated on the surface of aluminum particles,which makes Al and B2O3 have a better interface compatibility;when the content of boron carbide is 10 wt% we can get composites with complete densification with the hardness of 2.53 GPa and bending strength of 350 MPa;when the content is 12.5 wt%,the bending strength reaches the highest of 425 MPa.The possible exitence of Al3BC?a new phase?in the composites allows soluting atoms paly a pinning role to increase the flexural strength of B₄C/Al composites;there are three main formed fracture,tearing aluminum matrix,boron carbide particles transgranular fracture and particles crack extension fracture after pulling.