Research On The Forming Processes And Properties In Selective Laser Melting Of N-type Bi₂Te_2.7Se_0.3 Thermoelectric Materials

Bi₂Te₃-based thermoelectric?TE?materials possess one of the best thermoelectricperformance near room temperature and have been extensively used in the commercial TE power generation and refrigeration.The fabrication route of commercial bulk thermoelectric modules based on Bi₂Te₃ includes growing zone melting?ZM?ingots followed by a series of processes,such as slicing,plating,cleaning and soldering.The process is generally time-consuming with considerable material loss and high cost,especially when manufacturing micro TE devices.Selective Laser Melting?SLM?,as one of the additive manufacturing?AM?technologies,utilize a high-energy laser beam to scan and melt a layer of raw powder in a specific region to form a densified sheet,and eventually achieve the manufacturing of 3D objects by repeating the process.The technology has a wide application prospect in the field of thermoelectric materials and modules.Indeed,there are very few reports in the literature employing the SLM technique in the preparation of thermoelectric materials or modules.In this research,the traditional n-type Bi₂Te_2.7Se_0.3 thermoelectric materials were prepared by SLM,and the forming process,chemical composition,microstructure and thermoelectric properties were studied systematically.The main conclusions of this research are as follows:The effects of different powder size,laser power,scanning speed,hatch space and layer thickness on the forming quality of n-type Bi₂Te_2.7Se_0.3 material were systematically studied.The particle size of the powder prepared by two kinds of milling methods is below 30?m,the milling speed is too high or the milling time is too long are both likely to cause powder agglomeration.Inappropriate laser power and scan rate matching can lead to defects such as balling,vaporization,and distortion.Hatch space should not be too large,otherwise it will influence the connection between tracks.The laser energy density E_V corresponding to the optimal forming process is in the range of10-33.3 J/mm³ with a hatch space of 0.05mm.The chemical composition,phase structure and microstructure of n-type Bi₂Te_2.7Se_0.3 thermoelectric materials prepared by SLM were studied.In the SLM process,the evaporation in the connection between tracks area will be more serious than other regions,leading to the inhomogeneous of chemical composition in the resulted materials,as the number of stacked layers increases,the loss of Te and Se elements are increasing.The loss of Te and Se significantly increased with the increasing energy density.Some vaporization of Se and Te during the SLM process is unavoidable even at a low laser energy density less than 33.3 J/mm³,but its effect seems minimal as the single phase Bi₂Te₃ structure is maintained.However,secondary phases of Bi₄Te₅ and BiTe gradually form as the laser energy density exceeds 33.3 J/mm³ and the loss of Te and Se becomes a serious problem.SLM samples show a typical layered structure,the grain growth direction is basically parallel to the construction direction,the grains arranged regularly at low energy density and disorderly at high energy density.The n-type Bi₂Te_2.7Se_0.3 bulk sample prepared by SLM shows a smoth surface and no warping deformation formed.However,the appearance of the crack and the loss of the Te and Se elements resulted in poor mechanical properties and thermoelectric performance.Preheating substrate can effectively reduce the temperature gradient during the solidification of melted materials,which is beneficial to reduce the residual stress and to inhibit the generation of cracks.Meanwhile,the grain size become larger and the pores can move to the surface due to the extended solidifying time.By modifying the content of Te and Se in starting raw materials,the Bi₂Te_2.7+xSe_0.3+y?x=0.3-0.7,y=0.16-0.24?samples prepared by SLM shows single phase,and materials with optimized chemical composition.The thermoelectric properties can be significantly improved by annealing.After annealing for 24 hours at 673 K,the Seebeck coefficient of all samples on the surface was homogenously distributed,with an average value of about-160?V/K.Eventually,by optimizing the process parameters and the starting material composition,the temperature dependent TE properties of the SLM-fabricated bulk sample result in the maximum ZT value of 0.84 at 400 K,which is comparable to that of the commercially available material.