Electrical Properties of Half Heusler Coatings Depending of Spray Process Geoffrey DARUT, Axel PORTEBOIS, Ludovic VITU, Marie Pierre PLANCHE, Hanlin LIAO UBFC, ICB-PMDM-LERMPS UMR6303, 90010 Belfort, France corresponding authors, @: geoffrey.darut@utbm.fr Shantanu MISRA, Christophe CANDOLFI, Bertrand LENOIR Institut Jean Lamour, UMR 7198 CNRS – Université de Lorraine, 54011 Nancy, France Abstract Microstructure and physico-chemical properties of a thermally sprayed coating depend on the dynamics of the particles interacting with the spray jet. According to the process used (plasma, flame, etc.), porosity and phase composition could be different. This is especially the case for electrical properties. In this study, different spray processes (APS and HVOF) were investigated to spray Half Heusler powders type p and n. For that, the thermoelectric powder of Hf20Zr75Ti05CoSb80Sn20 (p-kind) and Hf60Zr40NiSn98Sb02 (n- kind) were selected due to their highly interesting electrical properties. The spray processes were evaluated through the measures of coating compositions and mechanical properties. A coating has only be get with plasma spray processes. The results revealed significant modifications in coating properties (atomic mass of elements, porosity) due to the operating parameters of plasma spray processes (F4MB or cascaded torch, plasma gas, etc.). Introduction The thermoelectric (TE) effect refers to phenomena by which either a temperature difference creates an electric potential or an electric potential creates a temperature difference [1,2]. Thermoelectric materials are used in thermoelectric systems for cooling or heating in sharp-range applications, and are being studied as a way to regenerate electricity from waste heat [3]. The efficiency of any TE material is evaluated by measuring the material’s dimensionless figure of merit, ZT, defined by the following equation ZT = S 2 σT/κ, with S the Seebeck coefficient, σ the electrical conductivity, κ the total thermal conductivity, and T the absolute temperature in Kelvin [4]. Since S, σ, and κ depend all together upon the material properties, the main improvement obtained for ZT value has been achieved through the reduction of thermal conductivity from nano-structuring techniques. The rapid progress in this direction focused the efforts on the development of experimental methods and on the understanding of the phonons transportation to decrease thermal conductivity. The increase in ZT is not only a straight-forward matter of material choice but also involves smart design of material interfaces. Global energy uncertainty with increasing energy demand triggers the search for high-efficiency energy conversion technologies. Thermoelectric devices can play a relevant role in energy collection and recovery. TE devices are ‘fuel -free’ solid-state devices with static parts and therefore are extremely reliable. In a non-thermal equilibrium environment, a non-negligible ratio of heat is lost meaninglessly. A solution consists in using TE devices to convert heat into electricity directly. Actually, this kind of elements are used in diverse applications, ranging from charging batteries to providing electricity for small devices in remote areas to powering deep- space vehicles [5]. The applications of TE device are still limited by their low conversion efficiency. According to the latest researches on TE materials, nanotechnology and advanced processes offer unprecedented opportunities in designing and fabricating complex material architectures with controlled microstructures [6]. The key factor to improve the performance of TE devices applications will steer developing the fine-structured TE materials. In this frame, it has been demonstrated that thermoelectric materials in sub-micro or nano-structure have the potential ability to improve the “figure of merit” (TE materials performance evaluation value) performance by 2 to 13 times over the value for bulk materials. To date, advanced low-dimensional nano or submicro-structured TE materials are not applicable for large-scale commercial applications because they are fabricated by atomic layer deposition processes such as molecular beam epitaxy. Using this technique, the manufacturing takes a rather long time, remains too expensive and leads to very strong restrictions in the amount of material to be produced. Moreover, they do not allow the design of flexible geometries [7] and the contacts with the heat source are always imperfect [8,9]. Thermal spraying is an additive manufacturing technique used in many industrial applications because of its high deposition rate (4kg/h), its high flexibility in terms of sprayable materials (metals, complex multi-component alloys, ceramics, ...), its thermal protection properties, the high possible coating thicknesses (from about 20 μm to several mm) and its low cost [10]. Although this technique offers new and unique ways of integration, it has so far received little attention from the TE community and deserves further investigation [11-12]. The main advantages of the thermoelectric generator manufactured by thermal spraying are: - to create an intimate contact with the heat source and thus limit the parasitic contact thermal resistances; - to provide thin or thick coatings of different materials and for multi functionalities (insulation or electrical conduction, energy conversion, …); - to enable complex architectures that are not possible with current commercial TEGs; Thermal Spray 2021: Proceedings from the International Thermal Spray Conference May 24–28, 2021 F. Azarmi, X. Chen, J. Cizek, C. Cojocaru, B. Jodoin, H. Koivuluoto, Y. Lau, R. Fernandez, O. Ozdemir, H. Salami Jazi, and F. Toma, editors DOI: 10.31399/asm.cp.itsc2021p0422 Copyright © 2021 ASM International® All rights reserved. www.asminternational.org 422 Downloaded from http://dl.asminternational.org/itsc/proceedings-pdf/ITSC 2021/83881/422/487921/itsc2021p0422.pdf by guest on 23 September 2021