Improved thermoelectric design through multi-lengthscale structural analysis

Session

Stream 1: EMAG - Functional Materials

Authors

Dr DA MacLaren (1)

Affiliations

1. University of Glasgow

Keywords

thermoelectrics, Heusler alloys, nanocharacterisation, electron spectrocopy, atom probe tomography, correlative microscopy

Abstract text

Thermoelectric generators (TEGs) convert heat into electrical power and their robust, reliable power generation underpins their deployment in the Mars rovers. A more down-to-Earth application is to scavenge waste thermal energy from energy-intensive industries including glass, steel and cement manufacture. These currently account for ~15% of global CO₂ emissions, so improving their energy efficiency could make a significant contribution to efforts to reduce climate change.  

Half-Heusler alloys (hHAs) are a family of intermetallic compounds that have become leading contenders for mass production and commercialisation of TEGs: they are stable, mechanically robust and employ abundant, inexpensive elements. However, they are also compromised by relatively low thermoelectric performance. Almost all research has focused on optimising hHAs through the twin strategies of (i) electronic doping via atomic substitutions and (ii) enhanced phonon scattering by introducing structures on the nanometre length-scale. Nevertheless, the predicted performance is usually degraded by electron and phonon scattering arising from structural features with micrometre, if not millimetre dimensions. Here, I outline a comprehensive suite of microscopies in order to develop a multi-lengthscale appreciation of the links between materials structure and ultimate TEG performance.

I will focus on structural characterisation of the most promising TiNiSn and related Heusler alloys. Elemental mapping by energy dispersive x-ray analysis [1] revealed remarkably inhomogeneous structuring, phase-segregation effects and grain-by-grain, micron-scale variations [2]. Dopants will be shown to improve the structural homogeneity and Cu is found to be an effective electronic dopant. It has surprisingly low solubility in the bulk material, with excess being extruded to grain boundaries. On the atomic scale, a combination of scanning transmission electron microscopy, electron energy loss spectroscopy and atom probe tomography was used to identify two main types of grain boundary. In the first type, Cu forms ‘wetting layers’ that facilitate coherent grain boundary formation [2]. In the second type, Cu acts as a surfactant, drawing impurities (notably oxides) out of the bulk [3,4]. Both grain boundary complexions will influence TEG performance. 

I will then turn to thin film studies, using pulsed laser deposition to deposit epitaxial thin TiNiSn films that act as model systems for studies of phase segregation [5,6]. The ideal deposition conditions will be discussed; absolute quantification of the stoichiometry using electron energy loss spectroscopy will be outlined [7]; and annealing experiments in-situ to the electron microscope will be shown to reveal novel aspects of phase segregation and spontaneous nanostructuring. A unique combination of the Quantum Detectors MediPix direct electron counting detector and NanoMegas DigiStar precession electron diffraction system was used for rapid, high-fidelity acquisition of pseudo-kinematical electron diffraction patterns with high dynamic range. The resulting structural assessment was then correlated with EELS measurements in a form of correlative spectrum imaging. We derive a non-linear relationship between lattice parameters and composition, and find a preference for doped hHAs to segregate into ‘full’ and ‘half’ Heusler alloy phases.

My main theme is that no one microscopy technique is sufficient for full structural characterisation of bulk materials; but that with multiple techniques spanning multiple length-scales, much progress can be made towards viable devices.

ACKNOWLEDGEMENTS

This work was conducted through collaboration with Dr. RWH Webster, Dr. JE Halpin (University of Glasgow), Dr Paul Bagot (University of Oxford) and Dr. J-WG Bos (Heriot-Watt University). It was funded principally by the EPSRC through grants EPSRC grants EP/N017218/1 and EP/P001483/1.

References

[1] J. Mat. Chem. C 7 (2019) 6539, DA Ferluccio, JE Halpin, KL MacIntosh, RJ Quinn, E Don, RI Smith, DA MacLaren and J-WG Bos.

[2] APL Materials 7 (2019) 013206, RWH Webster, JE Halpin, SR Popuri, J-WG. Bos, and DA MacLaren.

[3] H. He, J.E. Halpin, S.R. Popuri, L. Daly, J.-W.G. Bos, M.P. Moody, D.A. MacLaren & P.A.J. Bagot, in press.

[4] Ultramicroscopy 202 (2019) 121, JE Halpin, RWH Webster, H Gardner, MP Moody, PAJ Bagot and DA MacLaren.

[5] ACS Applied Materials and Interfaces 10 (2017) 4786, SA Barczak, JE Halpin, J Buckman, R Decourt, M Pollet, RI Smith, DA MacLaren and J-WG Bos.

[6] Appl. Surf. Sci. 512 (2020) 145649, RWH Webster, MT Scott, SR Popuri, JWG Bos and DA MacLaren.

[7] Ultramicroscopy 217 (2020) 113069, RWH Webster, AJ Craven, B Schaffer, S McFadzean, I MacLaren, DA MacLaren.