Thermoelectric Materials
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Our research is focused on extended solids that can contribute to the sustainable use of energy resources. Most of our current work is directed at thermoelectric materials that are used to harvest waste heat and turn this into electricity. Converting waste heat into electricity promises to increase the efficiency of any heat generating process.

 
 

Schematic of a thermoelectric device consisting of a p- and n-type leg. The p-type leg is made up from the intermetallic material TiCoSb

 

Thermoelectric waste heat recovery uses p and n-type semiconductors and a temperature gradient to generate electricity. A schematic module consisting of a single p- and n-type "leg" is shown in the figure below. The efficiency of this process depends on the temperature difference between the hot and cold side (the larger the better) and the energy conversion efficiency of the semiconducting materials, which is where solid state chemistry can have a real impact. The aim therefore is to design, synthesise and characterise new better performing thermoelectric materials. However, this is not a trivial endeavour as the efficiency depends on three materials parameters that are related through the electronic structure. This means that they cannot be optimised independently, and this limits the overall performance. The relevant materials parameters are (1) the Seebeck coefficient (the voltage response to a temperature gradient, (2) the electrical conductivity and (3) the thermal conductivity, which has an electronic and lattice part. In fact, only the lattice part can be manipulated independently from the the other variables, and lattice vibrations are therefore a primary target in thermoelectric materials research. For reference, current thermoelectric materials are about 5% efficient. A good web resource is the Caltech Thermoelectrics website.

 

Most of our recent work has focussed on semiconducting Zintl-type intermetallic materials. The fact that these materials are semiconducting despite only containing metallic elements signals that there is strong covalent and not metallic bonding. In fact, these materials often contain ionic and covalent regions, and this offers good opportunities to reduce the lattice thermal conductivity without adversely modifying the conducting covalent regions. Materials containing transition metals often contain dispersive bands based on s-p hybridisation and localised states derived from the d-bands, which are the kind of features in the electronic band structure that are often linked to good thermoelectric performance.

We are also interested in transition metal oxides. These materials are quite different, whereas the intermetallic materials are good semiconductors with high mobilities (little charge carrier scattering), oxides are characterised by much lower mobilities but larger carrier concentrations. In general, transition metal oxides don't support the same kind of outright performance but they have advantages in terms of cost, stability at high temperatures, and there is experience with this kind of materials in the semiconductor industry.

   

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