Novel materials for thermoelectric applications

Research project P 13778 Novel materials for thermoelectric applications E. BAUER and P. ROGL 11.10.1999 Thermoelectric devices are ideally suited to applications were simplicity and reliability, as well as the absence of moving parts and silent operation outweigh their relatively high cost and low efficiency: spot cooling of electronics, air conditioning, water cooler, cooling of superconducting electronics and infrared detectors or home refrigerators. The application of thermoelectric devices for power generation is expected to be useful for: remote wheather stations, remote navigational systems, submarines, conversion of waste heat (steel industry, operation of catalytic converter in cars, chemical industry, ...) into electric power. Although unoptimized, the figures of merit for some of the newly observed and/or investigated skutterudite phases (EP1-xM4X12) are comparable to the best values previously attained. Furthermore, in order to maximise the figure of merit, the peculiarities of the crystal structure with its defect nature offer the possibility by substitution of the constituents via subtle changes in the electron density at the Fermi-level and via changes in the phonon spectrum to in- or de-crease electric conductivity, thermal conductivity and the Seebeck coefficient. Hitherto, there are only few attempts known, to prepare compounds of those homologous elements, which are prone to valence instabilities with a large volume excited state such as europium or ytterbium, for which a skutterudite structure might become stable. Furthermore, in a recent paper the proposers of the present research project have shown without doubt, that the theory of Dannebrook may not be generally applicable. With respect to the above-mentioned currently existing controversy in the theoretical understanding of the physico-chemical behaviour of the skutterudite compounds the scope of the proposed research is therefore to investigate the physical properties of these new and technologically interesting materials in detail with emphasis towards the discovery and development of a new generation of thermoelectric materials. The dimension of the proposed project results from the scientific complementarity of the two laboratories involved (Institut für Experimentalphysik, Vienna University of Technology and Institute of Physical Chemistry of the University Vienna) as well as through an efficient use of complementary equipment and know-how. The following investigations are planned: * Search for new ternary compounds in the systems Yb1-x.M4X12 where T stands for a transition metal from the iron and platinum group and X is one of the elements As, Sb or Bi. * Lebeau method for the preparation of flux grown single crystals from the above mentioned systems. * Structural characterisation of the obtained samples by X-ray techniques; structure analyses of compounds by Rietveld powder refinement or from single crystal data. * Extent of solid solutions, characterisation of crystal structure, and control of physical parameters by atom substitution in higher order solid solutions: Yb1-xM4X12, 0 <= x <= l,Yb1-x(M1-yM`y)4X12, 0 <= y <=1 and Yb1-x(M1-yM`y)4(X1-zX`z)12, 0 <= z <=1. * Mechano-synthesis of solid solutions, inaccessible by conventional preparation methods. This will particularly refer to substitution by bismuth which is known to form a huge solubility gap for instance in the Fe-Bi binary, reducing ternary compound formation via melting and high temperature sintering processes. * Evaluation of physical properties in a wide parameter range, particularly of transport phenomena (electrical resistivity, thermal conductivity and thermopower) as well as magnetic and thermodynamic quantities. * Optimisation of physical properties within the above mentioned solid solutions with respect to maximum achievable figure of merit for thermal/electrical energy conversion. * Further optimisation of the figure of merit with respect to parameteres of materials processing, i.e., density of compaction by controlled applied pressure, selection of proper sintering temperature, addition of innert fillers. Direct scientific contacts between the proposers and the research institutes in Villigen, Switzerland (P. Fischer), CNRS Paris-Vitry (C. Godart), CEA Saclay, France (P. Bonville) and Austria (E. Bauer, P. Rogl) have been successfully intensified during recent years and have led to fruitful co-operations evidenced by a series of publications. Although a selection of sophisticated experiments can be performed in Vienna, specific knowledge can additionally be acquired from various centers of excellence. For the research described above it would be highly benefitial to gain direct access to the neutron spallation facility at PSI - Villigen, X-ray absorption spectroscopy at LURE - Orsay and to Yb-Mößbauer spectroscopy in CEA - Saclay, as these facilities do not yet exist in Austria. Such experiments abroad will help to clarify certain questions on: (a) magnetic state and spin order; vacancy and atom distribution in the crystal lattice; possible amorphous state, crystal field "citations; (b) character and degree of Yb valence; (c) magnetic state of the Yb ions.