The Fabrication And Properties Of Bi₂Te_2.7Se_0.3 Thermoelectric Materials Based On Selective Laser Melting

Bismuth Telluride,which has been studied since 1954,it is a widely used thermoelectric material for commercial applications at present.The traditional thermoelectric device will undergo multiple cutting processes during the manufacturing process,which will cause great waste of raw materials.The mechanical properties of the thermoelectric materials are not high,and the deformation of the green body may occur during the cutting process,reducing the mechanical properties of the device.Therefore,it is of great significance to find a new preparation method to improve the utilization rate of raw materials and avoid the destruction of the mechanical properties of the samples.Based on the above problems,this paper takes n-type Bi₂Te_2.7Se_0.3 as the research object,and according to the nominal ratio,the n-type Bi₂Te_2.7Se_0.3 raw material is rapidly prepared by SHS,and the powder is obtained after ball milling,selective laser melting was used to prepare thermoelectric bulk with better performance,based on this,the printing work of thermoelectric leg was studied.The selective laser melting technology is a kind of“additive manufacturing”technology,which can directly form the raw material powder to avoid the cutting process,thereby improving the utilization efficiency of raw materials and shortening the preparation cycle of the device.The key to this research is to explore a more suitable laser process window to print large bulk without obvious defects,and study the phase composition,microstructure,chemical composition,thermoelectric properties,etc.during laser printing.The main research content and results are shown as follow.The n-type Bi₂Te_2.7Se_0.3 sample was prepared by Self-propagating high-temperature synthesis.The powder was obtained by horizontal ball milling.The effects of laser processing parameters such as laser power,scanning rate,scanning pitch and powder thickness on the quality of single track and single layer forming were studied by using the selective laser melting technology.For the single track study,when the laser line energy density is between 0.025 and 0.125 J/mm,the energy is moderate and the powder is fully melted,forming a continuous,smooth single track.For the single layer study,it is found that the sample forming quality is better when the laser body energy density is 33.3⁴6.67 J/mm³ under the premise of the fixed scanning distance d=50?m and the thickness of the powder h=30?m.During the bulk sample printing experiment,it was found that the sample showed two quality deteriorations.When the printing thickness exceeded about 0.1 mm,the forming quality deteriorated.The possible reason is that the printed sample has a strong orientation,resulting in anisotropy of thermal conductivity and a large difference from the substrate,thereby affecting the melting and the solidification process of the powder.Therefore,in the subsequent printing process,we adopted the remelting method to eliminate the deformation deterioration caused by the change of the substrate thermal conductivity.When the printing thickness exceeded about 1 mm,the sample forming quality deteriorated again.Because the thickness of the sample was already large enough,and the influence of the substrate on the temperature field near the molten pool during the forming process was gradually reduced,and it was mainly affected by the printed sample.Taking into account the thermal conductivity of the printed sample,we further improved the process and increased the bulk energy density of the laser,resulting in a sample with a radius of 20 mm and a thickness of approximately 15 mm.The thermoelectric properties of the samples were characterized.The conductivity of the samples was higher,the Seebeck coefficient was lower,the thermal conductivity was higher,and the final ZT value was lower.The vertical sample obtained a maximum ZT value of 0.32 at 523 K,while the horizontal sample obtained a maximum ZT value of 0.13 at 523 K which is much lower than the ZT value of the sample obtained by the conventional preparation method.The main reason is that the second phase of BiTe is produced,resulting in a decrease in the Seebeck coefficient of the sample,an increase in thermal conductivity and electrical conductivity.The relationship between the initial composition and the components after laser treatment was studied.According to the nominal composition of Bi₂Te_3.0Se_0.4 to take raw materials,after the laser printed bulk was annealed,the maximum ZT value of 0.76was obtained at 423 K in the vertical direction.On this basis,the printing work of thermoelectric leg was studied.It was found that bismuth telluride can form a flat surface with no obvious defects on Al and Sn substrates.On the Al substrate,due to the high thermal conductivity of Al,the sample is prone to form some microcracks.When multi-layer sample printing was performed on the Sn substrate,it was found that the sample was warped,mainly due to the infiltration of Sn into the sample to form the second phase of SnTe.