Optimisation of Heusler Fe2VAl-based thermoelectric compounds through innovative metallurgical processing

van der Rest, Camille [UCL]

It has become a widely accepted fact that conventional resources will not indefinitely satisfy the rising global demand in energy, and very few people still deny the negative impact of fossil fuels on the environment. In light of these facts, energy harvesting has become an increasingly important issue, notably in industrial applications. Industries, such as steel, glass or cement manufacturing, reject large quantities of waste heat at temperatures below 673-773K and are therefore interested in heat harvesting solutions. Thermoelectricity could help in converting part of this waste heat into usable electricity. However, the materials currently used in commercial thermoelectric generators (Bi2Te3, SiGe, etc.) do not fit with the requirements of large-scale applications: they are expensive and based on toxic and/or scarce elements. The present project thus aims at assessing the potentialities of Fe2VAl-based Heusler compounds for thermoelectric generators, with interesting conversion properties and by manufacturing processes suitable with industrial applications. In the present work, liquid-phase processing but also powder metallurgy allowed to manufacture relatively pure and dense Fe2VAl-based compounds with the right composition and the right shape for use in thermoelectric generators (with additional hot-rolling if a plate-shape is needed). Innovative processes were also considered to increase the flexibility in the composition of the Fe2VAl-based compounds and to obtain directly « near net shape » materials: reactive sintering for cylinder-shaped materials and roll-bonding or friction melt bonding (followed by diffusion heat-treatments) for plate-shaped materials. Furthermore, in order to optimise the composition of Fe2VAl-based compounds to enhance their thermoelectric properties, the Al- and Fe off-stoichiometry strategies were adequately combined. The Seebeck coefficient of 104 μV·K−1 reached in the present thesis with some undoped samples is an outstanding result. The significant influence of the Fe/V ratio on the Seebeck coefficient, hence the non-universality of the representation based on the concentration of valence electrons (often used in the literature), has also been highlighted. Several characterisation techniques were combined to identify the crystal structures of Fe2VAl-based compounds and assess their order-disorder transformations. Concerning the optimum microstructure, L21 ordering through heat-treatments at 1273K was confirmed as being favourable for the thermoelectric properties. Finally, the implementation of Fe2VAl-based compounds in conventional thermoelectric conversion modules was performed by Joule bonding with Cu sheets and the resulting prototype gave interesting power densities. However, such a configuration requires many assembling steps not really scalable and affordable. A planar geometry, developed during the present work, seems thus to be a promising alternative solution for large-scale thermoelectric generators. An experimental proof-of-concept was tested on a single p-n couple and manufacturing processes have been suggested for future work.