Research On The Forming Processes And Properties In Selective Laser Melting Of N-type CoSb_2.85Te_0.15 Thermoelectric Materials

At present,the industrialization of human society is developing by leaps and bounds,and the demand for energy is increasing day by day.However,there are two problems to be solved in the utilization of energy.On the one hand,the main kinds of energy used by human beings are fossil fuels,including coal,oil and natural gas,which are limited in reserves and non-renewable,hence the tendency that those fuels are increasingly depleted.Meanwhile,the use of fossil fuels has brought serious environmental problems.On the other hand,most of the energy is utilized through thermal power generation,which will generate a lot of waste heat,resulting in inefficient energy utilization.Therefore,to find new renewable energy sources and to recycle the waste heat generated in traditional energy utilization are the most urgent things to do.According to the statistics,the proportion of waste heat in the range of temperature from 200?to 500??the medium temperature?is incredibly large,so,it is of great significance to recover the waste heat in this range.Thermoelectric?TE?technology can build up a direct relationship between heat and electric energy,which is a good choice to solve above problems.However,the fabrication of medium-temperature thermoelectric devices is with some disadvantages,such as too complex technology and high energy consumption.At the same time,the medium and high temperature thermoelectric materials and electrode materials are difficult to combine perfectly.Based on the above problems,n-type CoSb_2.85Te_0.15 thermoelectric material of the best thermoelectric properties in the medium temperature region,was chosen as the research object.Self-propagating High-temperature Synthesis?SHS?was used to prepare CoSb_2.85Te_0.15 powder in large quantities.Combined with Selective Laser Melting?SLM?technique,CoSb_2.85Te_0.15 bulk thermoelectric material was directly fabricated on electrode material.The SLM technology makes it reality the thermoelectric material forming and the one-step preparation of the material and electrode.Also,the SLM technology has the advantages of simple process,short preparation period and high forming precision.In this paper,the forming process,chemical composition,microstructure and thermoelectric properties of n-type CoSb_2.85Te_0.15 thermoelectric materials are studied.The main research contents and conclusions are as follows:N-type CoSb_2.85Te_0.15 materials were prepared by SHS technology,and SLM powder was obtained by ball milling.Single track and single layer of CoSb_2.85Te_0.15were prepared by SLM.The effects of different process parameters on the qualities of single track and single layer forming of CoSb_2.85Te_0.15 were systematically studied.With the planetary ball milling,the particles with smaller average diameter could be obtained.However,the particle size is so small that it is easy to get agglomerated.The particle size of the powder obtained by horizontal ball milling is in a smaller range,and the average particle size is 4.338?m.The mismatch between laser power and scanning speed will lead to structure defect of single track and single layer such as balling,porosity and warping deformation.The thicker powder coating will lead to the thin thickness of single layer,so it needs more superposition to get a certain thickness of block and to increase the workload.However,if the thickness of powder coating is too large,it will be difficult for laser to penetrate the powder bed.As a result,the powder bed cannot be completely melted.Finally,we chose the thickness of powder bed of 40?m.The larger hatch space will result in the single track not being connected,and the excessive scanning spacing will result in the distortion of the single tracks.Finally,we chose the hatch space of 60?m.On this condition,the laser energy density E_V corresponding to the optimal forming process is in the range of58¹30 J/mm³.The changes of chemical composition and microstructure of n-type CoSb_2.85Te_0.15 materials were systematically studied in the process of single-track and single-layer forming by SLM.Through the calculation of Langmuir formula,it is known that the gasification rate of Sb is higher than that of Co.At the same time,we have collected and analyzed the volatilized matter in SLM process,and analyzed the element composition of SLM forming layer.It is concluded that Sb elements do selectively volatilize in the SLM process of CoSb_2.85Te_0.15 materials.Meanwhile,because the SLM process is a non-equilibrium process of rapid heating and cooling,the heterogeneous phases,such as CoSb₂,are produced in the cooling process of CoSb_2.85Te_0.15 materials.The appearance of these heterogeneous phases will seriously affect the thermoelectric properties of materials.Therefore,it is necessary to eliminate the impurity phase by vacuum annealing in the later stage.The multi-layer annealing of SLM shows that the transition temperature of CoSb₃ phase in the sample is about400?.Therefore,the optimum annealing temperature is 450?.The results of FESEM and XRD with different annealing time at 450?show that the heterogeneous phases disappears completely when the annealing time is 120 minutes.In order to make the sample annealed more absolutely,the annealing time we chose is240 min.Making use of the characteristics of CoSb_2.85Te_0.15 material is easy to separate in the non-equilibrium process of SLM,and the more serious the phase separation is,the lower the absolute value of Seebeck coefficient is.By using PSM to measure the distribution of Seebeck coefficient in different positions of single tracks,we can observe the phase separation in different regions.ANSYS finite element simulation software and PSM were used to simulate the temperature field distribution in the melting pool of n-type CoSb_2.85Te_0.15 material.According to the parameters of the optimal surface forming process,bulk samples of CoSb_2.85Te_0.15 with the thickness of 1.7 mm were fabricated by individually fitting 80 layers together on the Ti electrode.The surface of the bulk material was flat and there was no obvious macroscopic defect.After annealing at450?for 240 min,the sample was found to be a better pure phase.The average Seebeck coefficient of the annealed sample at room temperature was increased from-26.1?V/K to-107.3?V/K,and the sample was more well-distributed.After annealing,the Seebeck coefficient of the annealed sample can reach-175?V/K at high temperature.The Seebeck coefficients of bulk samples prepared by SHS-PAS are close to those of bulk samples.Because of the influence of micro-defects in the samples on the transport of electrons and phonons,the electric conductivity and thermal conductivity of the SLM-AN samples are lower than those of the SHS-PAS samples,and the ZT value of the final bulk samples reaches 0.56 at 550?.The contact resistivity between the CoSb_2.85Te_0.15 bulk material and the electrode material is 37.13??cm².