EMC IN APT · Towards the analysis of energy conversion materials at the atomic scale

Towards the analysis of energy conversion materials at the atomic scale

This project aimed to characterise the structure and chemistry of novel thermoelectric materials at the nanometre scale to reveal the structure-property relationship in this class of energy conversion materials, especially using Atom Probe Tomography coupled with Electron Microscopy. Thermoelectrics are likely to play a major role in the development of new energy sources, undoubtedly one of the major challenges of the 21st century. Thermoelectric materials can be used either to harvest energy directly from sunlight, or to capture waste heat, and convert it into electricity. The project was focused on bulk materials, which are more likely to have a deep impact on the energy production. As an IEF the project involved a large part of training and acquisition of knowledge available at the host institution. During the first year of the project, between May 2009 and May 2010, an active collaboration was set up between Dr Baptiste Gault and a world-leader in the fabrication of thermoelectric materials, Prof. G.J. Snyder from the California Institute of Technology in Pasadena, California. They provided two types of samples, BaGaGe alloys and Zinc antimonides, the latter is the materials showing the highest performance of all bulk thermoelectrics. The first steps of the proposed time frame were followed, and Dr Gault was trained on the high-end scanning electron microscope (SEM) combined with a focused ion beam at the University of Oxford, and followed a course on transmission electron microscopy that included some practicals. Preliminary atom probe tomography and SEM experiments on ZnSb were performed, leading to the publication of a letter in Scripta Materialia and presentations at three international conference (2 posters, 1 oral presentation). In parallel, Dr Gault went on with his work on the investigation of performance of atom probe tomography and development of data treatment methodologies to improve them or facilitate data interpretation, leading to the publication of 16 articles in peer-review journals. Despite the early termination of the contract, he has acquired knowledge and skills in SEM/FIB, TEM and finite-element method simulations, as expected during the first year of the project. The work on simulation has enabled a better quantification of the spatial performance of atom probe tomography, which was highly necessary, and to explain some of the most detrimental artefacts intrinsic to the technique. The results obtained on the zinc antimonide have enabled significant advance in the understanding of the structure-properties relationship in this class of materials. Indeed, it was shown that the multiple grain boundaries within the solid and compacted material were wetted by a layer of different chemistry and potentially the location of a very high segregation of antimony. These original and unprecedented observations enable to propose a plausible explanation about the actual high performance of such materials, as the change in chemistry at the grain boundary is likely to lower the thermal conductivity of the material, while the electrical conductivity should remain mostly unaffected. This clearly open pathways for design of materials with enhanced performance via reduction of the grain size and potential doping of the material by elements known to segregate at grain boundaries in these alloys.

Data: CORDIS, © European Union