Nanostructured materials for thermoelectric conversion of waste heat into electricity

Thermoelectricity is the direct conversion of heat into electricity, without any moving parts. The optimization of a thermoelectric material depends on decreasing thermal conductivity while maintaining high electrical conductivity and Seebeck coefficient. Two ways to achieve this are by nanostructuring and by doping. Nanostructuring leads to scattering of long wavelength heat propagation, while doping introduces impurity scattering centres which may scatter short wavelength heat propagation. We have in this project used a grinding technique where powder is cooled by liquid nitrogen while the grain size is mechanically reduced (cryomilling). We have successfully obtained particles with grain size below 10 nm, which is very promising for thermoelectric applications. The materials produced by the cryomilling technique were then compacted into pellets. This was done by heating the materials under high pressure in vacuum or an inert atmosphere (Hot pressing), using an in-house hot press constructed within this project. This method is easily scalable and is therefore well suited for large scale manufacturing. Furthermore, the project has investigated different doping routes for the binary ZnSb phase, using both first-principles calculations and experimental methods. The modelling indicated that we can expect p-type behaviour with Tl, In, Al, Cu and Pb on the Sb site. Indeed, Cu-doped ZnSb exhibited very high thermoelectric performance. In conclusion, the project has demonstrated that ZnSb is a very promising material for thermoelectric applications, given the environmentally friendly elements and the high thermoelectric potential with proper doping and nanoscaled material.

Waste heat is a huge and growing part of the global energy challenge. If only a small fraction of the domestic and industrial low-grade thermal waste could be utilized, it would give a tremendous contribution to the overall energy balance and fight carbon dioxide emissions. Thermoelectricity represents a very attractive and promising path towards efficient and safe exploitation of this resource. Obvious benefits of thermoelectric (TE) devices include their compact and highly scalable design, with no movin g parts and virtually no need for maintenance. The current project aims to bring a new generation of TE materials up to the stage of demonstrating their performance in a prototype device. This generation relies on nanostructured materials which exhibit pr onounced decrease in their thermal conductivity parallel to an optimized carrier concentration, thus reducing heat loss without sacrificing electronic performance. This will be achieved with a nanosized network of grains and atomic substitutions, pressed into pellets from very fine powder. We will use a newly purchased cryogenic mill to reduce the powder size into the nano range and at the same time incorporate substituting elements to align the carrier concentration for maximized performance; this gives the opportunity to synthesize compounds with optimized bulk pro¬perties without decomposition or sintering during milling. The resulting materials will be used to construct a TE prototype device based on non-toxic and abundant materials.