Research On Process And Performance Of SnTe Thermoelectric Materials Via Selective Laser Melting

With the rapid development of levels of productive forces in human society,environmental pollution and the crisis of energy shortage are increasingly severe,so it is extremely urgent to develop clean,efficient and reliable new energy conversion technology.Thermoelectric conversion technology is the new energy technology that uses Seebeck effects and Peltier effects of materials to realize direct conversion from heat into electricity and plays an important role in solving problems of environmental pollution and energy shortage mentioned above.Thermoelectric conversion technology can achieve functions of thermoelectric generation and thermoelectric refrigeration with advantages of zero rotatable part,easy maintenance,small sizes,lightness,noiselessness and zero release etc.It has important application in fields of solar power generation,automobile exhaust generation and cogeneration etc.and broad application prospects in fields of micro power and cooling.The application prospect of thermoelectric conversion technology attracts much attention of researchers.Though researches about high-performance thermoelectric material continuously appear in recent years,there are few researches on structure design and manufacturing processes of thermoelectric devices.However,optimization of structure design of thermoelectric devices and reduction of their manufacturing costs will also play a critical role in promotion of commercial mass application of thermoelectric devices.The manufacturing technology of traditional thermoelectric devices generally has problems of demanding equipment,time consuming,high waste of raw materials and high costs etc.,which are more prominent on micro thermoelectric devices with an extensive application prospect.Therefore,it has great significance to develop the new technology of thermoelectric devices.Selective laser melting technology is extremely potential technology of additive manufacturing that is widely used in respect of structural materials such as metal and characterized by a high level of automation,high utilization of raw materials,avoiding mechanical cutting and high accuracy of manufacturing etc.If it is applied to manufacturing of thermoelectric devices,it will have a revolutionary impact.The core of manufacturing of thermoelectric devices is thermoelectric materials.By taking SnTe thermoelectric material as an object of study,this research prepared SnTe thermoelectric material via selective laser melting and systematically studied shaping quality,phase,composition,microstructure of SnTe thermoelectric material under a laser action and the change rule of Seebeck coefficients.In addition,it utilized the finite element modelling method to simulate temperatures field of molten pools,where simulation results were consistent with experimental phenomenon.Research contents and results of this paper are as follows:Through self-propagation high-temperature synthesis reaction,SnTe powder was prepared massively.Self-developed experimental selective-laser-melting equipment,which is appropriate for wet-process powder,was used to carry experiments of SnTe's single line and face formation.Results of research showed that:if the laser energy density was excessively high or low,during formation of a single line of SnTe,there would have macro defects such as balling,instability,gasification and cracking,where optimization of laser parameters could restrain these defects;when the thickness of powder was 40?m,its laser energy density was0.2-0.5 J/mm and high-quality SnTe single lines could be obtained;when the thickness of powder was 25?m,its laser energy density was 0.15-0.45 J/mm and high-quality SnTe single lines could be obtained;during face formation,there were also defects such as balling,instability,gasification and cracking,where macroscopic cracks were usually perpendicular to the direction of laser scanning and microcosmic cracks showed no clear orientation;when the thickness of powder was40?m and its laser energy density was more than 0.2 J/mm,good-quality SnTe faces could be obtained,but excessively high laser energy resulted in face gasification;when the thickness of powder was 25?m and its laser energy density was 0.15-0.03J/mm,SnTe faces without an obvious macroscopic defect could be obtained;in experiments of multi-layer formation of SnTe,curving and deformation occurred because the accumulated thermal stress exceeded the yield limit of SnTe.On the basis of obtaining high-quality SnTe faces,tests and characterization of phase,composition,microstructure of SnTe faces prepared at laser energy densities of 0.1,0.05,0.025 and 0.014 J/mm as well as their Seebeck coefficients were carried out.Research results showed that:under the laser action,SnTe compounds could be decomposed and formed oxides of Sn and Te by residual oxygen within cavities,where the laser energy density reduced and The degree of decomposition of SnTe weakened;the boiling point of Te is low,Te lacked because of selective volatilization,leading to reduction of proportions of Te and Sn with the increase of the laser energy density;with the decrease of the laser energy density,microstructure of SnTe surfaces was transformed from the micron scale to the nanoscale;the higher the laser energy density was,the lower the Seebeck coefficients of SnTe faces would be and the more uneven their distribution would be.When the laser energy density was 0.014 J/mm,SnTe faces with macroscopically evenly distributed Seebeck coefficients approaching to those of traditional methods could be obtained.The finite element simulation software,ABAQUS,was applied to simulate temperatures field of molten pools for single-line and face formation of SnTe.The results showed:with the decrease of the laser energy density,molten pools extended along the direction of the scanning direction,areas of molten pools reduced and the temperature dropped;due to uneven distribution of laser energy and mobile laser heating,molten pools were distributed asymmetrically along the direction of the scanning direction;the higher the laser energy density was,the gasification zones of molten pools were;when the laser energy density was 0.1 J/mm,the temperature of laser spots was higher than that of gasification of SnTe;with the decrease of the laser energy density,heating rates and cooling rates of molten pools enhanced;when the laser energy density was 0.014 J/mm,the cooling rate was 2.5×10~6 K/s,resulting in increase of surface micro cracks.The simulation results were consistent with the experimental results with consistent laws of physics,indicating that finite element simulation is an intuitive and reliable method of research on the distribution of temperature fields in the process of selective laser melting.