Reliable, Scalable and Affordable Thermoelectrics: Spin Seebeck Based Devices for Energy Harvesting

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As part of the Energy Efficiency Directive, the UK has committed to a 20% increase in energy efficiency, a reduction of greenhouse gas emissions by at least 20% and an increased share of renewable energy sources (compared to 1990 levels) by 2020. To address these challenges a stable and diverse range of energy sources will need to be developed and, unsurprisingly, this has been the focus of an intense international research effort. The associated research challenges can be loosely categorised into renewable sources (solar, wind, tidal), sustainable sources (e.g. carbon capture, fusion), and micro generation (e.g. energy harvesting from thermal, light, sound, or vibrational sources). One example of such sources is the harvesting of waste heat with thermoelectric generators (TEGs), a technology that has the advantage of reliability (no moving parts), but is limited by high costs (use of critical elements such as Te) and low efficiencies (<10% for a 200K temperature difference). Given the abundant sources of waste heat in everyday life (boilers, engines, computers, district heat networks), development of low-cost TEGs that could easily be applied to various surfaces could present a significant vector for change. For example, harvesting just 5% of the energy lost as waste heat by car engines in the UK would save the equivalent of 1 hundred thousand equivalent tonnes of oil per year (or ~1% of the UK's total energy usage in 2014).Conventional TEGs are typically based on the Seebeck effect: a physical process that results in the generation of an electric current when a temperature difference exists between two ends of a material. One of the bottlenecks for improvement of the efficiency of these devices is the co-dependence of two key material properties: the thermal and electric conductivity. Whilst some progress has been made to circumvent this by nano-engineering, there is still some way to go before widespread commercialisation becomes viable. This could, however, be overcome with TEGs based on the spin Seebeck effect, where an additional degree of freedom - the spin of the electrons - results in a device architecture that scales with surface area (unlike conventional thermoelectrics), enables separation of the thermal and electric conductivities that drive the efficiency of the device and boasts active materials that could be sourced from abundant sources (such as iron or copper, rather than bismuth telluride). The aim of this Fellowship is to investigate the spin Seebeck effect with regards to its application as a TEG. There are 5 key challenges that will be addressed: (1) precise determination of the efficiency of such spin Seebeck based TEGs; (2) discovery of new materials (from abundant sources); (3) development of prototype TEGs; (4) identifying the controlling factors with regards to the efficiency of the overall device; and(5) understanding the underlying physics of this effect. For example, harnessing the maximum spin polarised current generated by the spin Seebeck effect typically requires the use of expensive platinum contacts. For such technology to become economically viable would therefore require discovery of cheaper alternatives, such as the doped metals that will be investigated. In addition, precise characterisation of the spin Seebeck effect is limited by instrumentation that typically only monitors the temperature difference (rather than heat flow), hence instrumentation will be developed to monitor both these parameters so that the power conversion can be determined. There is also, as of yet, no comprehensive coefficient that can be used to compare different material systems (such as the Seebeck coefficient for conventional thermoelectrics), nor a rigorously tested figure of merit. Once this has been established, a comprehensive comparison of different materials and engineering of the overall device can be made.