Rapid fabrication and thermoelectric performance of SnTe via non-equilibrium laser 3D printing
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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 laser 3D 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, microstructure and thermoelectric performance was systematically investigated. First and foremost, the processing parameter window for printing a high-quality 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.
KeywordsSelective laser melting Laser non-equilibrium heating SnTe compound Thermoelectric performance
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 . 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 . Consequently, large temperature gradients arising during laser heating of TE materials may lead to macro- and micro-defects . 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.
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 cold-pressed 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 7 μm 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).
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 Kα radiation at 40 kV and 40 mA. The spatially resolved Seebeck coefficient on the layer surface was determined by a potential-Seebeck-microprobe instrument (PSM, Panco) with a spatial resolution of 20 μm.
Parameters used in finite element simulation of molten pool temperature
Process and material parameters
Spot diameter, D/μm
Laser power, P/W
Scanning speed, V/(mm·s−1)
100, 200, 400, 700
Hatch spacing, H/μm
Track length, L/mm
Convective heat-transfer coefficient, hc/(W·m−2·K−1)
Melting temperature, Tm/K
Evaporation temperature, Tv/K
Powder thermal conductivity, κp/(W·m−1·K−1)
Liquid SnTe thermal conductivity, κl/(W·m−1·K−1)
Powder density, ρ/(kg·m−3)
Specific heat capacity, Cp/(J·kg−1·K−1)
Absorption rate of powder, A
3 Results and discussion
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 25 μm, 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 (EL) 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 25 μm, 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 25 μm opens up for EL of 0.015–0.030 J·mm−1, which is considerably less than the EL range of 0.150–0.450 J·mm−1 required for the track formation.
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 microstructure. 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 EL, as shown in Fig. 4c–f. Figure 4c indicates that when EL 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 of 700 μm and the width of 240 μm, 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 EL 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 EL 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 350 and 80 μm, respectively. This explains why this sample is less deficient in Te, as Fig. 4a suggests. Moreover, when EL 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 SLM process in metallic materials . Crystal nucleation and growth progress are impeded under such high cooling rates, giving rise to nanostructures, as shown in Fig. 4b.
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 microstructure 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 at 25 μm, the hatch spacing at 40 μm and EL at 0.014 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 non-equilibrium heating technique and is likely directly applicable to the fabrication of other thermoelectric materials.
This work was financially supported by the National Natural Science Foundation of China (Nos. 51401153 and 51772232) and the Program of the Ministry of Education of China for Introducing Talents of Discipline to Universities of China (No. B07040).