Nanoanalytical electron microscopy of advanced thermoelectric materials

Thermoelectric materials convert waste heat into useful electric power.
Even inefficient thermoelectric power generation recovery can have a substantial impact on UK and global energy consumption because more than half of primary energy is ultimately wasted as heat. So far, thermoelectric generators (TEGs) have been restricted to niche applications, such as powering the Voyager space probes, where durable, reliable and low-maintenance power generation is essential. However, the market for thermoelectric energy harvesters is projected to approach $1bn within a decade.* Potential applications for TEGs include scavenging heat from car exhausts, producing combined heat and power units for use in remote, off-grid locations, and replacing batteries in wearable microelectronic devices. A major limitation has been to develop cheap, efficient TEGs that do not rely on toxic or scarce resources. For example, the most efficient thermoelectric material for automobile heat recovery is currently a compound of toxic lead and scarce tellurium.

In this project, we aim to develop a viable, non-toxic alternative to lead telluride TEGs, using 'Heusler alloys', which combine abundant elements such as titanium, nickel and tin. They also meet the majority of industrial requirements for thermoelectric power generation, having good thermal and mechanical stability, mechanical strength and ease of processing. However, a TEG's thermal conductivity is also critical and optimising the thermal conductivity of Heusler alloys has been problematic. We aim to capitalise on our recent advances in Heusler alloy synthesis and nanostructuring, which currently represents the only UK efforts in this fast-growing field.

The ultimate aim of this proposal is to develop new means of controlling the thermal conductivity of Heusler alloys in order to build a TEG prototype of comparable performance to existing lead telluride devices.
Our insight is that there are a variety of alloy phases and intentional defects that can be used to introduce structural texture on the nanoscale, thereby reducing the thermal conductivity. What is exciting is that many of these structures have not previously been studied. A critical aspect is the size and distribution of the texturing, which should be long enough to avoid reducing the material's electrical conductivity but short enough to impede the flow of heat. We will investigate the optimum length-scales for texturing by performing a systematic study of the impact of processing conditions on the HA nanoscale structure. We will use world-leading electron microscopy, neutron scattering facilities and theoretical modelling to probe the atomic-scale structure and dynamics of the new materials in order to optimise the synthesis parameters. We will then use this technical know-how in collaboration with our industrial partner European Thermodynamics Ltd. to build prototype TEG modules.

This collaborative project, involving three academic institutions, national facilities and a UK small business, has substantial potential for impact, with notable prospects for making a contribution to lowering the UK's carbon footprint. It also provides excellent opportunities for knowledge transfer to a vibrant new industry and for high-quality training.

* H. Zervos, "Thermoelectric Energy Harvesting 2014-2024: Devices, Applications, Opportunities," 2014