Energy demand is growing and our society faces a challenge to find sustainable sources with minimal environmental impact. Existing technologies such as solar, wind and geothermal have been deployed and effort to improve their physical and cost effectiveness is ongoing. Another source of renewable energy available which has not been harvested to its full potential so far is "waste" heat. It arises from a variety of sources, from household boiler to large scale power plant, and a striking example is the conventional combustion engine in which 60 % of the energy produced is lost in the form of heat. The possibility to design a semiconductor device made of p-n junctions which when exposed to a temperature gradient will output electrical power is an attractive solution for the automotive industry to improve fuel efficiency, lower the carbon foot print and end-user costs. This device, called a thermoelectric generator has been successfully used for aero-spatial application or in its converse form as Peltier cooler, contributes to all component of the energy trilemma. The major barrier for a widespread dissemination of this technology as energy harvester is the high raw material costs and a lack of material for high temperature operation. This research will investigate new classes of inorganic oxide composed of earth abundant elements presenting electrical and thermal properties suitable for integration in a high temperature thermoelectric generator. Efficient thermoelectric materials possess high electrical conductivity and low thermal conductivity which, in a standard semiconductor picture, are antagonistic properties. Focusing on the high temperature spectrum, oxides materials will display the chemical stability required for the device to function reliably. Since the majority of these materials are electrically insulating, the concept is based on identifying structure patterns that have hidden electronic lattice which could act as conducting channel. Similar concept has been successfully applied on layered oxides where only competitive p-type thermoelectric materials where produced. The project aims to explore the possibility to use the strong correlation between electronic, thermal and magnetic lattice to circumvent the limitations encountered in this class of materials and expand our understanding of this complex compounds. A specific objective of the project is to prepare poly- and single crystalline layered oxides derived from the trirutile structure, measure the high temperature conductivity and thermopower and optimise the thermoelectric property using chemical doping to obtain both p and n type compounds. The layered structures of the proposed compounds are conducive to exotic magnetic properties and more complex phenomena such as Nernst-Ettinghausen effect and spin Seebeck effect will be investigated.

The Seebeck effect is a thermoelectric effect whereby a temperature gradient across a material is converted to a voltage, which can be exploited for power generation. The growing concern over fossil fuels and carbon emissions has led to detailed reviews of all aspects of energy generation and routes to reduce consumption. Thermoelectric (TE) technology, utilising the direct conversion of waste heat into electric power, has emerged as a serious contender, particular for automotive and engine related applications. Thermoelectric power modules employ multiple pairs of n-type and p-type TE materials. Traditional metallic TE materials (such as Bi2Te3 and PbTe), available for 50 years, are not well suited to high temperature applications since they are prone to vaporization, surface oxidation, and decomposition. In addition many are toxic. Si-Ge alloys are also well established, with good TE performance at temperatures up to 1200K but the cost per watt can be up to 10x that of conventional materials. In the last decade oxide thermoelectrics have emerged as promising TE candidates, particularly perovskites (such as n-type CaMnO3) and layered cobaltites (e.g. p-type Ca3Co4O9) because of their flexible structure, high temperature stability and encouraging ZT values, but they are not yet commercially viable. Thus this investigation is concerned with understanding and improving the thermoelectric properties of oxide materials based on CaMnO3 and ZnO. Furthermore, not only do they represent very promising n-type materials in their own right but by using them as model materials with different and well-characterised structures we aim to use them to identify quantitatively how different factors control thermoelectric properties. The conversion efficiency of thermoelectric materials is characterised by the figure of merit ZT (where T is temperature); ZT should be as high as possible. To maximise the Z value requires a high Seebeck coefficient (S), coupled with small thermal conductivity and high electrical conductivity. In principle electrical conductivity can be adjusted by changes in cation/anion composition. The greater challenge is to concurrently reduce thermal conductivity. However in oxide ceramics the lattice conductivity dominates thermal transport since phonons are the main carriers of heat. This affords the basis for a range of strategies for reducing heat conduction; essentially microstructural engineering at the nanoscale to increase phonon scattering. The nanostructuring approaches will be: (i) introduction of foreign ions into the lattice, (ii) development of superlattice structures, (iii) nanocompositing by introducing texture or nm size features (iv) development of controlled porosity of different size and architecture, all providing additional scattering centres. Independently, TE enhancement can also be achieved by substitution of dopants to adjust the electrical conductivity. By systematically investigating the effect of nanostructuring in CaMnO3 and ZnO ceramics, plus the development of self-assembly nanostructures we will be able to define the relative importance of the factors and understand the mechanisms controlling thermal and electron transport in thermoelectric oxides. A key feature of the work is that we will adopt an integrated approach, combining advanced experimental and modelling techniques to investigate the effect of nanostructured features on the properties of important thermoelectric oxide. The modelling studies will both guide the experimentalists and provide quantitative insight into the controlling mechanisms and processes occurring at the atom level to the grain level, while the experiments will provide a rigorous test of the calculation of the different thermoelectric properties. We will assess the mechanical performance of optimised n-type and p-type materials, and then construct thermoelectric modules which will be evaluated in automobile test environments.

Electronic thermoelectrics use semiconductors to convert waste heat into electricity. This is an established energy generation technology, for example, used by NASA to power the Mars Rovers. However, it is not very suitable for low-grade waste heat recovery due to poor power generation from small temperature differences. Ionic conductors generate much larger thermal voltages and are better suited to near room temperature operation. However, they cannot be used in the same mode of operation as this would require a continuous flow of ions. The innovative solution proposed is to couple the ionic conductor with an energy storage system that converts the ionic potential to an electronic one. This proposal will investigate novel ionic thermoelectric power generation devices consisting of an ionic conductor sandwiched between two energy stores, including supercapacitors and insertion materials. This is a novel approach targeted at recovery of low-grade waste heat, increasing the sustainability of industrial processes and reducing carbon emissions.

Application of the Spin-Seebeck-Effect (SSE), only demonstrated in 2008/10, potentially allows new types of large area single layer thermoelectric (TE) devices for heat-energy exchange (waste heat energy recovery or micro Peltier cooling) under near ambient temperature applications. Present SSE research demonstrations (based on synthetic garnets coated with Pt) are unsuited and unsustainable for real-World application. We proposed two new ways to attain more sustainable, lower cost SSE devices: (i) Use of a facilitating organic interface between the SSE and metal layers to facilitate spin transfer out of the SSE layer; (ii) replacement of the Pt metal layer by more sustainable metals.

The UK is committed to achieving Net Zero by 2050. Waste heat is a huge cause of energy losses in domestic and industrial settings. Large scale thermoelectric recovery of waste heat into electricity can lead to significant reductions in CO2 emissions. In addition, there is a need to power the internet of things (IoT), which dictates the deployment of billions of interconnected sensor devices. Here thermoelectrics can provide free electricity by scavenging waste heat, eliminating the need for batteries or grid connectivity. However, despite the many advantages of the use of thermoelectricity in energy generation and scavenging, commercially it is still an inefficient and expensive technology which relies on scarce materials, mainly Tellurium compounds. New, abundant materials with ease of processing, which can enable large scale production in order to become competitive sources of electricity are needed. Amongst the many new materials investigated lately to increase performance and replace the prominent Bi2Te3 and PbTe for use in thermoelectric generators, half-Heuslers are leading contenders for mass production and commercialisation. They are stable, mechanically robust and are composed of abundant, inexpensive elements. However, a substantial improvement in their power output (i.e. improving W/£), which would largely exceed the power output of current thermoelectric devices is also needed. To radically improve the power output from thermoelectric materials, new approaches are required, beyond reducing the heat transport through them, which has been the key paradigm in the field. We propose an alternative, challenging and disruptive approach based on insights from advanced modelling of charge transport in half-Heusler materials. This shows that the power output, even of already studied materials, can be increased by 2-10-fold by improved materials growth, control of defect chemistry, doping and bandstructure engineering. This will reduce the £/W cost by up to an order of magnitude as the overall material compositions remain similar. This work is a paradigm shift in thermoelectric materials research away from the mainstream focus on nanostructuring and thermal conductivity reduction, to materials with huge electronic responses to a temperature difference. Success of this research will enable the application of Heusler alloys in large-scale waste-heat recovery (kW range energy harvesting) and/ or powering the internet of things (mW-W range of energy scavenging). The project team brings together leading UK expertise in Heusler materials synthesis and thermoelectric materials modelling and will work closely with Industrial and Academic partners to ensure success and translation into working technologies. The resulting developments in synthetic and computational methodologies will be highly relevant to other electronic and opto-electronic materials fields as well.