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Good thermoelectric materials are characterized by a large Seebeck coefficient S, high electrical conductivity σ and low thermal conductivity λ. A high electrical conductivity is necessary to minimize Joule heating, whilst a low thermal conductivity helps to retain heat at the junctions and maintain a large temperature gradient. These three properties are embodied in the so-called figure-of-merit
figure-of-merit

which in conjunction with the applied temperature T describes the efficiency of a thermoelectric material

carnot

ηC known as the Carnot efficiency marks the reachable upper limit.

Nanostructured SiGe thin films through MIC processing

SiGe

Silicon-germanium (SiGe) alloys are materials for thermoelectric devices especially suited for application in the range of 600-1000 °C. Not only is their use in radio-isotope thermoelectric generators for space craft missions proof to their reliability, but also lead to increased interest in this type of material for terrestrial applications such as waste heat recovery. Recent improvements in thermoelectric efficiency were achieved by nanostructuring through sintering of SiGe nanoparticles into nanocomposites with greatly reduced thermal conductivity due to increased phonon scattering at grain boundaries. Since the production and sintering of said nanoparticles is time and cost intensive no widespread industrial application could be achieved by now.

Our research aims for the production of nanograined SiGe via sputter deposition of alternating stacks of SiGe and Al(p-type) or repectively Sb(n-type). These dopants were chosen to utilize the effect of metal induced crystallization (MIC). MIC combined with the multilayer approach leads to nanograined, polycrystalline SiGe thin films. Since MIC also lowers crystallization temperature the use of flexible, low temperature substrates becomes possible.

Ca3Co4O9 and Nb:SrTiO3 multilayers by pulsed laser deposition (PLD):

LayerOxides

We investigate the structural and electrical properties of the misfit layered cobaltite Ca3Co4O9 and Nb-doped SrTiO3 on silicon. These materials promise high potential for application in thin film energy harvesting systems. These thin films are ideally suited for studying and developing reliable high temperature stable metal contacts to these materials.

PLDChamberWeb

uebergang-ysz-cacoo-70-15_web

(left) PLD plasma plume

(right) Cross-sectional high resolution transmission electron microscopy(HRTEM) image

Novel materials – Skutterudite CoSb3
cosb3_2

An example for a modern thermoelectric material is the skutterudite CoSb. CoSb3 is a semiconductor and has in general good thermoelectric properties. It belongs to the group of “Phonon glass electron crystals” (PGEC) with big voids in their atomic structure. These voids can be filled with loose bounded guest ions. Because of this weak binding the ions are able to oscillate ("rattle") very strong and phonons are effectively scattered. This results in a reduced thermal conductivity and much better thermoelectric properties. Possible filling ions are for example rare earths metals, alkali earth metals, Ytterbium and Thallium. Many interesting properties of filled CoSb3 depend strongly on the filling ion, e.g. ferromagnetism, superconductance. With this special properties filled CoSb3 is a promising candidate for future thermoelectric applications.