Ultra-high temperature synthesis of high-performance Zintl thermoelectrics

Technologies that enable the efficient use of energy could have an enormous impact on the most pressing issues of today: global warming and the reliance on ever-dwindling supplies of fossil fuels.

The proposed research addresses this topical challenge through the investigation of the next generation of thermoelectric materials that harvest waste heat and transform it into useful electricity. In particular, the research is focused on thermoelectric materials that can operate at high temperatures, which is essential as the Carnot efficiency (the thermodynamic maximum) increases with temperature difference.

The scientific challenge is to optimise three competing material parameters; the Seebeck voltage; the electrical and thermal conductivity, and to do this in a material with good temperature stability.

The novelty of the proposed research derives from the use of ultra-high temperature synthesis to achieve temperature stability, and the synergistic exploitation of Zintl chemistry and interfaces in nanocomposites to obtain large thermoelectric figures of merit.

Zintl phases are key high-performance thermoelectric materials because the simultaneous presence of ionic and covalent regions enables a more independent optimisation of the thermoelectric parameters compared to electronically homogeneous materials. Two classes of promising Zintl-type phases have been identified, and the performance of outstanding bulk materials will be further enhanced through the use of interfaces in nanocomposites.

This ambitious and transformative research programme will contribute towards the development of high-performance thermoelectric materials operating at temperatures most suitable to power generation, enabling 20-30% energy conversion efficiencies. The research will also lead to an increased understanding of the relation between composition, structure and thermoelectric properties.

The large scale implementation of power generation from waste heat depends primarily on the discovery of thermally stable high-performance thermoelectric materials, which the proposed research intends to address. The most promising application areas are in energy recovery from hot exhaust gases in automobiles, and exploitation of solar infrared radiation not collected using photovoltaic cells.

About 60% of the energy stored in petrol is lost as heat in combustion engines. Most large car manufacturers, including general motors and BMW, are looking to integrate thermoelectric devices into the exhaust system (temperatures of the order of 600 degrees Celsius) and use the electricity to power the on-board electronics and air-conditioning. This makes the alternator redundant, which reduces roll friction, and results in substantial reductions in fossil fuel consumption.

About 40% of the solar spectrum is in the infrared domain and is not used by conventional solar cells. The goal is to use photovoltaic and thermoelectric cells in tandem to utilize the whole solar spectrum. In this scenario the solar light would be concentrated using optical lenses to temperatures above 500 degrees Celsius to achieve sufficiently large temperature gradients.

Achieving maximum impact relies on effective communication of the proposed research to both society and potential industrial partners.

A sustainable world energy supply is of great importance for the United Kingdom. The contribution this research makes will be communicated through a number of public engagement activities, including writing a general interest article and presentations aimed at the tax-paying public.

The research has a clear potential economic impact if it results in the discovery of high-performance materials. UK industry could play a role in the bulk manufacture of these materials, in fabrication of the devices, or in the design and implementation of bespoke thermoelectric modules using high-efficiency materials. A strong effort will be made to engage with possible industrial partners. This will not just focus on chemical companies that manufacture materials but also on engineering companies that may wish to exploit thermoelectric modules in their products. The initial contact will be aimed at gauging interest and exploring possible collaborations. In case of the discovery of commercially interesting materials the intellectual property will be protected through Heriot-Watt University. The established contacts will then be used to explore possible scale-up and fabrication of devices, which requires a significant amount of engineering expertise.