Cold Spray Deposition of Thermoelectric Materials

Thermoelectric materials convert heat flux to electricity (or vice versa as Peltier coolers); however, their application to harvest waste heat is limited by challenges in fabrication and materials optimization. Here, cold-spray deposition is used as an additive manufacturing technique to fabricate p- and n-type Bi2Te3, on substrates ranging from quartz to aluminum. The sprayed material has a randomly oriented microstructure largely free from pores (> 99.5% dense), and deposition is achieved without substantial compositional changes. The Seebeck coefficient and thermal conductivity are largely preserved through the spray process, but the defects introduced during deposition significantly increase electrical resistivity. Defects can be removed, and compressive strain relaxed by a post-deposition anneal, which leads to Bi2Te3 blocks with a typical ZT of 0.3 at 100°C. Generators fabricated on sheets or pipes made of copper compare favorably with similar designs constructed using bulk Bi2Te3, displaying a wider operating temperature range. These results demonstrate the power and versatility of cold-spray additive manufacturing and provide a pathway toward fabrication of thermoelectric generators in complex geometries that are inaccessible to generators made by traditional approaches.

Alexander A. Baker, Richard Thuss, Nathan Woollett, Alyssa Maich, Elissaios Stavrou, Scott K. McCall, Harry B. Radousky, “Cold Spray Deposition of Thermoelectric Materials,” JOM 73, 2853-2859 (2020). DOI: 10.1007/s11837-020-04151-2 abstract

Schematic of the cold-spray process, showing powder feed and gas handling. Note that the powder is injected after the expansion in the throat of the gun. (b) and (c) Depositions of Bi2Te3 bars on quartz and copper, respectively; the height of the material is approximately 3 mm. (d) Uniform deposition was achieved over the full surface by rotating the pipe during the spray process for lines of p- and n-type Bi2Te3. Kapton tape prevented overspray from making direct thermal and electrical contact between the two electrodes.

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(a) COMSOL simulation of gas (and thus particle) velocity during deposition on a flat surface, demonstrating that the supersonic shock occurs outside the nozzle, resulting in high gas pressures at the deposition site, which aid particle build-up. (b–d) The buildup process on a quartz substrate [seen in the right of (b)], with micron-scale particles embedding in the substrate to form a base layer, to which subsequent particles interlock and are compacted by subsequent particle impacts. (e) XRD results for p-type billet and spray material (before and after annealing) and the calculated pattern for Bi0.5Sb1.5Te3.05Se0.15. There is no evidence of a phase change, but the texture is modified, as seen in in (f) and (g), where 2D diffraction patterns become uniform rings as a result of the random orientation imposed during deposition.

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