简介概要

Rapid fabrication and thermoelectric performance of SnTe via non-equilibrium laser 3D printing

来源期刊：Rare Metals2018年第4期

论文作者：Tian-Le Chen Chuang Luo Yong-Gao Yan Ji-Hui Yang Qing-Jie Zhang Ctirad Uher Xin-Feng Tang

文章页码：300 - 307

摘要：Thermoelectric technologies based on Seebeck and Peltier effects,as energy techniques able to directly convert heat into electricity and vice versa, hold promise for addressing the global energy and environmental problems. The development of efficient and low-cost thermoelectric modules is the key to their large-scale commercial applications. In this paper, using a non-equilibrium laser3 D printing technique, we focus an attention on the fabrication of mid-temperature p-type SnTe thermoelectric materials. The influence of laser power, scanning speed and layer thickness on the macro-defects, chemical and phase composition, micro structure and thermoelectric performance was systematically investigated. First and foremost,the processing parameter window for printing a highquality layer is determined. This is followed by the finite element method used to simulate and verify the influence of the laser-induced molten pool temperature distribution on the final composition and microstructure. Finally, the high-performance SnTe layer with 10 mm × 10 mm in area is produced within seconds with room temperature Seebeck coefficient close to that of SnTe manufactured by the traditional methods. Consequently,this work lays a solid foundation for the future fabrication of thermoelectric modules using laser non-equilibrium printing techniques.

详情信息展示

稀有金属(英文版) 2018,37(04),300-307

Rapid fabrication and thermoelectric performance of SnTe via non-equilibrium laser 3D printing

Tian-Le Chen Chuang Luo Yong-Gao Yan Ji-Hui Yang Qing-Jie Zhang Ctirad Uher Xin-Feng Tang

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology

Department of Materials Science and Engineering,University of Washington

Department of Physics,University of Michigan

收稿日期：7 December 2017

基金：financially supported by the National Natural Science Foundation of China (Nos. 51401153 and 51772232);the Program of the Ministry of Education of China for Introducing Talents of Discipline to Universities of China (No. B07040);

Rapid fabrication and thermoelectric performance of SnTe via non-equilibrium laser 3D printing

Tian-Le Chen Chuang Luo Yong-Gao Yan Ji-Hui Yang Qing-Jie Zhang Ctirad Uher Xin-Feng Tang

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology

Department of Materials Science and Engineering,University of Washington

Department of Physics,University of Michigan

Abstract：

Thermoelectric technologies based on Seebeck and Peltier effects,as energy techniques able to directly convert heat into electricity and vice versa, hold promise for addressing the global energy and environmental problems. The development of efficient and low-cost thermoelectric modules is the key to their large-scale commercial applications. In this paper, using a non-equilibrium laser3 D printing technique, we focus an attention on the fabrication of mid-temperature p-type SnTe thermoelectric materials. The influence of laser power, scanning speed and layer thickness on the macro-defects, chemical and phase composition, micro structure and thermoelectric performance was systematically investigated. First and foremost,the processing parameter window for printing a highquality layer is determined. This is followed by the finite element method used to simulate and verify the influence of the laser-induced molten pool temperature distribution on the final composition and microstructure. Finally, the high-performance SnTe layer with 10 mm × 10 mm in area is produced within seconds with room temperature Seebeck coefficient close to that of SnTe manufactured by the traditional methods. Consequently,this work lays a solid foundation for the future fabrication of thermoelectric modules using laser non-equilibrium printing techniques.

Keyword：

Selective laser melting; Laser non-equilibrium heating; SnTe compound; Thermoelectric performance;

Author： Yong-Gao Yan e-mail:yanyonggao@whut.edu.cn;

Received： 7 December 2017

1 Introduction

Thermoelectric (TE) technology,based on Seebeck and Peltier effects,can realize direct conversion between heat and electricity [ 1, 2, 3] by purely solid-state means.The TE conversion process is considered environmental-friendly,noiseless,exceptionally reliable and has been widely used in thermoelectric power generation and refrigeration [ 4, 5, 6, 7] .However,most of the commercial TE devices are produced using traditional techniques,which entail the preparation of p-and n-type materials,followed by multiple steps including slicing,cleaning,plating,assembling,soldering and packaging.Such processes require specialized equipment,are time-consuming and involve considerable material's loss and high cost.The drawbacks of the traditional processing routes are magnified during the fabrication of highly marketable miniature TE devices [ 8, 9] .Therefore,it is of vital importance to develop novel and inexpensive manufacturing processes for fabrication of TE modules.

Unlike traditional subtractive manufacturing,selective laser melting (SLM) is a kind of additive manufacturing technologies [ 10, 11, 12, 13, 14] ,in which a thin layer of powder is melted using a laser beam and then rapidly solidified.In this way,3D objects of full density can be built by successive steps of powder deposition followed by laser-induced melting [ 15] .Owing to rapid heating and cooling rates during the non-equilibrium laser processing,the technique is able to produce materials with the desired fine nanostructure.Moreover,the raw materials are utilized more efficiently.With the above advantages,SLM has been widely employed in the manufacture of metallic parts for aerospace and automotive industries [ 12, 16, 17, 18] .If successfully applied in the parallel production of p-and n-type TE legs,as well as in the formation of electrodes and insulating substrates,the TE module fabrication could be carried out in just one integrated step.This would dramatically improve the efficiency,reduce the cost and truly revolutionize the TE manufacturing industry.

The key step in the SLM-based technique of TE module fabrication is the ability to produce TE legs with good control over their chemical and phase composition,microstructure and TE performance.However,TE materials are usually heavily doped semiconductors,which typically exhibit a much lower thermal conductivity,poor ductility and low thermal shock resistance compared to metals [ 19] .Consequently,large temperature gradients arising during laser heating of TE materials may lead to macro-and micro-defects [ 20] .Even more important,in order to maintain good TE performance,it is essential to exercise accurate control over the material's composition,phase and microstructure.Here,one must take into account different laser coupling characteristics of semiconductors and metals.

In this paper,we focus on SnTe,one of the mid-temperature TE materials with promising properties [ 21, 22, 23, 24] .Starting from the SnTe powder produced by an ultra-fast,low-cost,self-propagating high-temperature synthesis(SHS),recently developed by our group [ 25, 26] ,we systematically investigate the influence of SLM processing parameters,such as the laser power,scanning speed and the powder layer thickness,on the composition,phase,microstructure,macro-defects and thermoelectric performance of the resulting SnTe material.Meanwhile,the finite element analysis was used to simulate and verify the effect of the molten pool temperature distribution on the material composition and microstructure.Our results show that a high-performance SnTe material can be produced rapidly,which serves as a solid foundation for the future fabrication of thermoelectric materials and modules via the SLM process.Although specific material parameters may differ,the SLM-based processing should be applicable to other families of TE materials.

2 Experimental

Pure elemental powders of Sn(4N) and Te(5N) were weighted according to the stoichiometric ratio of SnTe and mixed in an agate mortar.The mixed powders were coldpressed into a pellet and sealed in a quartz tube under vacuum.The SHS process was ignited by flame heating the bottom of the tube.The reaction zone passed through the ingot in a few seconds,leaving behind a fully reacted single-phase SnTe ingot.The non-spherical SnTe powder with an average particle size (D₅₀) of 7μm was obtained after crushing the ingot by ball milling for a couple of hours.The SnTe powder was blended with alcohol to obtain a slurry [ 27, 28] .The optimal slurry was found to contain about 29 vol%alcohol.The slurry was spread smoothly and dried on a SnTe substrate prepared previously by spark plasma sintering (SPS).

As shown in Fig.1,a customized apparatus was used for SLM experiments,in which SnTe powder-toped substrates were placed inside a gas tight chamber and scanned by a CW YAG-fiber laser via a top viewport of the chamber.The laser's wavelength was 1064 nm with the beam focused to a diameter of 100μm.The fiber laser beam can raster on the powder with the aid of oscillating mirrors along x-and y-axis,which is controlled by a computer.The key SLM parameters include the laser power (P) and the scanning speed (v),which are combined to determine the laser linear energy density (E_L),defined [ 29] as E_L=P/v.In this work,the SLM experiments were conducted in a high-purity argon atmosphere to avoid oxidation.The powder layer thickness was set at 25 and 40μm for singletrack and layer formation processes,respectively.During the single-track formation process,the laser power was set at 5-50 W,the scanning speed at 100-300 mm·s^-1,and the track length was 8 mm.For the layer formation experiments,the laser power was set at 3-14 W,the scanning speed at 100-700 mm·s^-1,the hatch spacing at40μm,and the layer area was 3 mm×3 mm.In order to study the effect of laser processing on the chemical composition,phase purity and the Seebeck coefficient of SnTe,five-layer SLM specimens with area of 10 mm×10 mm were fabricated under different linear densities of E_L=0.100,0.050,0.025 and 0.014 J·mm^-1,while the powder layer thickness,hatch spacing and laser power were fixed at25,40μm and 10 W,respectively.

The surface morphology and microstructure of a single track and a layer were examined by field emission scanning electron microscopy (FESEM,SU-8020 Hitachi).The actual chemical composition of the SLM-processed specimens was determined by electron probe microanalysis(EPMA,JXA-8230 m JEOL).The phase identification was performed by X-ray diffractometer (XRD,PANalytical Empyrean) apparatus operating with a Cu Kαradiation at40 kV and 40 mA.The spatially resolved Seebeck coefficient on the layer surface was determined by a potentialSeebeck-microprobe instrument (PSM,Panco) with a spatial resolution of 20μm.

The molten pool temperature distribution during the laser-powder interaction was simulated with the commercially available ABAQUS finite element software package [ 30] .The mesh size was 40μm,and the processing and material parameters used are listed in Table 1 [ 31, 32, 33, 34, 35, 36, 37] .According to Foroozmehr et al. [ 38] ,modeling five tracks can provide an adequate information about the molten pool size and the temperature distribution.

Fig.1 Schematic diagram illustrating customized apparatus used for SLM experiments

3 Results and discussion

SLM starts from a single point,track and layer and build up to a 3D object.Properties of the final bulk product depend on the quality of every single track and layer;hence,it is necessary to study the formation process of a single track and layer [ 39] .Figure 2a-h displays typical SEM images of single track and layer.Similar to the case of a metal,SnTe specimens prepared by SLM also show defects,such as droplets and holes [ 40, 41] .Figure 1a shows a track formed with the distinct balling features,which usually happens when E_L is too small.In this case,the molten pool generally has a low temperature and high surface tension,and therefore,it does not spread on a substrate but rather tends to contract and forms round-shaped droplets instead,to reduce the surface energy [ 42] .With E_L increasing,the molten pool can spread more freely,but the droplets can still exist,as shown in Fig.2b.A well-formed track,characterized by its consistent width and smooth surface free of macro-irregularities,is shown in Fig.2c.However,a further increase in E_L leads to significant vaporization and holes start to form at the surface.Moreover,the

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Table 1 Parameters used in finite element simulation of molten pool temperature

Fig.2 SEM images (top view) of four typical laser-melted SnTe single track with layer thickness of 25μm,scanning speed of 150 mm·s^-1 and different laser powers of a P=5 W,b P=15 W,cP=30 W and d P=50 W;SEM images (top view) of four typical SnTe surfaces with layer thickness of 25μm,scanning speed of 400 mm·s^-1,hatch spacing of 40μm and laser powers of e P=3 W,f P=5 W,g P=10 W and hP=12 W

Marangoni effect [ 43] becomes significant and results in a molten pool instability and track irregularity,as observed in Fig.2d.The layer formation characteristics of SnTe are similar to those of the single track.Figure 2e-h shows SEM images of four typical formed layers,with features similar to those observed in the case of a single track.As shown in Fig.2g,the layer surface is smooth,a consequence of the tracks having the same width and overlapping in an orderly manner.Moreover,there are no obvious droplets,holes or other defects visible on the surface of this well-formed layer.

In order to avoid the defects depicted in Fig.2a-f and obtain single track and layer of the quality shown in Fig.2c,g,the SLM processing parameter window was explored for the single-track and layer formation processes with the powder layer thickness of 25 and 40μm.Figure 3 a,b shows the experimental results of the single-track formation process under different laser powers and scanning speeds.When the powder layer thickness was 40μm,a well-formed single track was obtained for E_L varying from 0.200 to 0.500 J·mm^-1.Once the laser linear power density E_L exceeds 0.500 J·mm^-1,the holes caused by vaporization appear and the single track becomes distorted.At the other extreme,as E_L decreases below 0.200 J·mm^-1,the droplets appear.As E_L fell below 0.050 J·mm^-1,the full balling process has taken place.As shown in Fig.3b,with the layer thickness decreasing to 25μm,the optimal E_L turns out to be between 0.150 and 0.450J·mm^-1,consistent with the fact that the required E_L to completely melt the powder decreases with the layer thickness decreasing.

Based on the single-track formation process,we explored parameters that affect the layer formation.Figure 3c,d shows the experimental results of layer formation when the layer thickness is 40 and 25μm,respectively.As Fig.3c indicates,there are two notable features regarding how the optimal parameters vary with the formation type and layer thickness.First,at a constant powder layer thickness,the optimal processing window for the layer formation greatly narrows down,compared to the track formation,and the critical laser energy density (E_L) shifts to values about 10 times smaller than those for the track formation.This phenomenon is due to the fact that the heat accumulates up and is preserved within the powder bed during the layer formation,instead of dissipating into the substrate as happens during the track formation.This is especially so on thermally resistive TE substrates where the minimal laser energy density to melt the powder is substantially decreased.Second,with the powder layer thickness decreasing from 40 to 25μm,the optimal processing window for the layer formation opens up,showing that a thin powder layer provides more opportunities to vary the SLM parameters.As shown in Fig.3d,the optimal parameter window for the layer formation at the powder layer thickness of 25μm opens up for E_L of0.015-0.030 J·mm^-1,which is considerably less than the E_L range of 0.150-0.450 J·mm^-1 required for the track formation.

Fig.3 Single-track formation process results for SnTe with layer thickness of a 40μm and b 25μm,respectively;layer formation process results for SnTe with powder layer thickness of c 40μm and d 25μm

Laser heating during SLM process will influence the chemical composition,phase and microstructure,which are all critical to the performance of TE modules.Figure 4shows the chemical composition,phase and micros true ture of several 5-layer-thick SnTe specimens prepared with E_Lof 0.014-0.100 J.mm^-1.Figure 4a indicates that,compared to the bulk specimen of SnTe produced via SPS,the SLM samples are more Te deficient in their stoichiometry.When E_L increased from 0.014 to 0.100 J·mm^-1,the atomic ratio of Te to Sn decreases from 0.904 to 0.840.The reason is that the vapor pressure of Te is much higher than that of Sn [ 44] .An increase in E_L leads to a higher temperature of the molten pool,and therefore,a higher evaporation rate of Te relative to Sn causes a decrease in the ratio of Te to Sn [ 45] .Among several five-layer specimens,the one obtained with E_L of 0.014 J·mm^-1 shows the least Te deficiency and a good formation quality.EDS mapping on this specimen,shown in Fig.4a,proves the homogenous distribution of Sn and Te.The inset in Fig.4b shows XRD pattern collected on the processed layer and confirms its phase purity.Figure 4b presents SEM image of a specimen,in which nanograins with size of 20-50 nm are observed.They form as a result of rapid heating and cooling rates during the SLM process via a mechanism similar to that found in bulk samples prepared by melt spinning-spark plasma sintering (MS-SPS) process [ 46] .The presence of the nanostructure will likely reduce thermal conductivity [ 47] and thus optimize TE properties of the SLM bulk material.

During the laser non-equilibrium fabrication of TE materials,the molten pool temperature distribution has a significant influence on the crystal nucleation,the growth progress,the composition and the resulting micros true ture.However,it is difficult to measure the temperature distribution experimentally.Hence,the finite element analysis was used to simulate the molten pool temperature distribution under different E_L,as shown in Fig.4c-f.Figure 4c indicates that when E_L is 0.100 J·mm^-1,the peak molten pool temperature can reach 2441 K,which is far beyond the boiling temperature of SnTe.Consequently,significant evaporation happens and holes on macroscale occur in the formed layer.The simulated molten pool has the length of700μm and the width of 240μm,the area much larger than the laser spot size.This suggests that the high temperature and the large size of the molten pool lead to evaporation of Te,as shown in Fig.4a.As E_L decreases from 0.100 to 0.014 J·mm^-1,the size of the molten pool gradually diminishes.As shown in Fig.4f,the peak molten pool temperature for E_L of 0.014 J·mm^-1 is just 1588 K,well below the boiling point of SnTe.The corresponding length and width of the molten pool decrease down to 350and 80μm,respectively.This explains why this sample is less deficient in Te,as Fig.4a suggests.Moreover,when E_L is 0.014 J·mm^-1,the molten pool cools down at a rate of 2.6×10⁶ K·s^-1,according to the simulation results.Such a rate of cooling is of the same order of magnitude as the literature data relevant to simulations of the SLMprocess in metallic materials [ 48] .Crystal nucleation and growth progress are impeded under such high cooling rates,giving rise to nanostructures,as shown in Fig.4b.

Fig.4 Atomic ratio of Te to Sn in SLM-prepared SnTe layers with E_L of 0.100,0.050,0.025 and 0.014 J·mm^-1 and insets being EDS distribution maps for Te and Sn elements in SLM-prepared SnTe layer with E_L of 0.014 J·mm^-1;b typical microstructure and inserted XRD pattern of SLM-prepared SnTe layer with E_L=0.014 J·mm^-1;temperature distribution in molten pool at various E_L of c 0.100 J·mm^-1,d 0.050 J·mm-1,e 0.025 J·mm^-1 and f 0.014 J·mm^-1 using finite element analysis method

Figure 5 shows Seebeck coefficient distribution in a thin layer of SnTe prepared by SLM with E_L of 0.014 J·s^-1-mm^-1,and the structure exhibits uniform chemical composition,phase purity and nanostructure features.For comparison,the distribution of Seebeck coefficients in a bulk sample of SnTe consolidated by SPS is also presented in Fig.5.The data suggest that the Seebeck coefficient and its homogeneity in the SLM-prepared SnTe layers are close to those of the SPS-consolidated bulk sample.Specifically,Seebeck coefficients of the SLM-prepared SnTe and the bulk sample sintered by SPS are 28.08 and 29.77μV·K^-1,respectively.The corresponding values of full width at half maximum (FWHM) of the Seebeck coefficient distribution function are 3.78 and 4.61μV·K^-1,respectively.Thus,while having comparable magnitudes of the Seebeck coefficient,the SLM-prepared material actually shows a better homogeneity than the SnTe specimen prepared by SPS.This attests to the viability of SLM in preparing materials with uniform Seebeck coefficient distributions,even though the process involves an abrupt temperature gradient and large heating and cooling rates.

4 Conclusion

In this paper,it is demonstrated the rapid non-equilibrium laser heating technique as a means of fabricating homogeneous SnTe thermoelectric materials.We systematically studied the influence of the processing parameters (laser power,scanning speed and powder layer thickness) on the quality of the track and layer formation of SnTe,and on the chemical composition,phase purity and micros true ture of the resulting structure.Making use of the finite element analysis,the experimental results are explained by modeling the temperature distribution of the molten pool.The results show that,when the powder layer thickness is set at25μm,the hatch spacing at 40μm and E_L at0.014 J mm^-1;one can obtain high-quality single-phase SnTe material with uniform composition and nanostructures in just a few seconds using the laser power of 10 W.Most important,the Seebeck coefficient measured on such SLM-fabricated SnTe is very close to that of the bulk SnTe material fabricated via SPS.Moreover,the homogeneity of the Seebeck coefficient distribution in the SLM-fabricated SnTe is even better than that in the SPS-prepared bulk counterpart.The work lays a solid foundation for the fabrication of thermoelectric modules using the laser nonequilibrium heating technique and is likely directly applicable to the fabrication of other thermoelectric materials.