Exploring A New Method Of Pulse Electric Current Ultra-fast Forming To Synthesize Bi_0.5Sb_1.5Te₃ Alloy

Due to the requirements of energy and environment strategies,thermoelectric materials and their preparation methods have become the hotspot in both the domestic and foreign field.At present,there are mainly two kinds of methods to obtain bulk thermoelectric materials:one is zone melting or direction solidification method,and the other is powder metallurgy method(traditional sintering method or SPS method).The advanced Spark Plasma Sintering method is considered to be the development direction of synthesizing high-quality bulk thermoelectric materials.However,the SPS sintering equipment is very expensive and is difficult for maintenance.Moreover,the heating,holding and cooling process during sintering period takes extra long time,and the grain growth still exists in sintering process.In that way,is it possible to develop other methods for preparing thermoelectric materials superior to the traditional sintering or SPS method?Based on a set of pulse electric current ultra-fast forming device independently developed and designed,we chose Bi_0.5Sb_1.5Te₃alloy as the research object,and creatively explored and verified the feasibility of preparing thermoelectric materials by pulse electric current ultra-fast forming method.By adjusting the technological parameters,such as sintering voltage,loading pressure and discharge times,the structural optimization and thermoelectric performance improvement are realized through this new sintering method.Following was the main achievements:(1)With the device and mold modification,trial and error of improving methods and procedures as well as a large number of repeated tests,it is proved that the pulse electric current ultra-fast forming method can realize thermoelectric material powder sintering in millisecond time,and the effectiveness and feasibility of this new method is verified.The internal section of the synthesized Bi_0.5Sb_1.5Te₃ alloy is contined and has no macroscopic hole.Therefore,the surface of the sintered sample is smooth with metallic luster,and the preferred orientation of the crystal can be avoided by this new method.(2)Under the condition of one discharge sintering,the affecting law of sintering voltage,pressure and other process parameters were explored.The results show that the resistivity of B_0.5Sb_1.5Te₃ sample decreases continuously when the sintering voltage raises from 65V to 70V and 75V,while the resistivity increases significantly when the voltage go up to 80V.As the voltage increases,the thermal conductivity generally decreases.The Seebeck coefficient does not change significantly and the increasing of pressure load is beneficial to promote the density of samples.(3)Two discharge sintering experiment results show that the sample 65V-two discharge has higher Seebeck coefficient,lower resistivity and lower thermal conductivity than the sample 65V-one discharge,and finally obtain the maximum ZT value of 0.7 at 393K,which is 11%higher than that of the sample 65V-one discharge.For two discharge sintering,the improvement effect of the thermoelectric performance of the sample 70V-two discharge is not as significant as that of the sample 65V-two discharge.From the perspective of microstructure analysis,the grain growth tendency of the sample 70V-two discharge is relatively large than that of the sample 65V-two discharge.(4)The sample 70V-three discharge has lower resistivity and higher Seebeck coefficient than the sample 60V-three discharge,and finally obtain a higher maximum ZT value of 1.0 at 400K,which is 19%higher than that of the sample 60V-three discharge.In short,this paper explored and developed a new method--pulse electric current ultra-fast forming method for fabricating bulk thermoelectric materials,which is not only efficient(millisecond time),convenient but also requires no expensive equipment.Through this new method,the sample can obtain lower thermal conductivity than the sample prepared by other sintering methods,which is conducive to improving the final thermoelectric performance.Of course,this method still needs to be further improved,especially for further exploration on how to avoid the generation of microcracks under the condition of one discharge sintering.