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Contributed talk

Nanoparticle-in-alloy-approach without alloy: Processing and thermoelectric properties of Si-WSi₂

Roland Schmechel, Universität Duisburg-Essen, Duisburg, Germany
Julia Stoetzel, Institute of Technology for Nanostructures / University Duisburg-Essen and CENIDE, Duisburg, Germany
Tom Schneider, Institute for Combustion and Gas Dynamics / University Duisburg-Essen and CENIDE, Duisburg, Germany
Mathis M. Mueller, Institute for Applied Geo Sciences / Technical University Darmstadt, Darmstadt, Germany
Hans-Joachim Kleebe, Institute for Applied Geo Sciences / Technical University Darmstadt, Darmstadt, Germany
Hartmut Wiggers, Institute for Combustion and Gas Dynamics / University Duisburg-Essen and CENIDE, Duisburg, Germany
Gabi Schierning, Institute of Technology for Nanostructures / University Duisburg-Essen and CENIDE, Duisburg, Germany

The nanoparticle-in-alloy-approach is a relatively new concept to improve the thermoelectric properties of materials by different phonon scattering processes. Phonons with short wavelength scatter dominantly on defects with atomic dimensions as realized in alloys. In contrast, phonons with longer wavelength are more scattered on larger objects like nanoinclusions. In this work, the system nano-Si / nano-WSi₂ is used to realize different phonon scattering processes on different length scales by an inhomogeneous distribution of these precipitates.

In fact, the thermoelectric figure of merit could be improved by about 50% from zT=0.4 for n-type nano-Si to zT=0.6 for nano-Si/WSi₂ at 1250K. This improvement is mainly due to a reduced lattice thermal conductivity. The primary material is synthesized in the gas phase by a plasma reactor, utilizing the precursors SiH₄ as Si source, PH₃ as phosphorus source for n-doping of silicon and WF6 as tungsten source. This results in a nanopowder of Si:P and WSi₂. In a subsequent current activated, pressure assisted densification (CAPAD) process a nanostructured bulk material is obtained. Its microstructure is more complex, but there are no indications for W alloyed in a Si-matrix. All tungsten is found in nano-WSi₂ precipitates only. However, the distribution of the WSi₂ precipitates is not homogeneous, but splits in two well-separated domains or areas where the nano-WSi₂ precipitate concentration is enhanced or lowered respectively. The length-scale for these domains is in the micrometer range and due to the uniaxial pressure during densification highly anisotropic. It acts probably as scatter center for phonons with long wavelength.

The microstructure and thermoelectric transport properties have been investigated for different nominal W-concentrations, varying from 1 to 17at%. There is no trivial relation between the microstructure and the nominal W-content, probably due to several counteracting processes, like diffusion, grain growth and phase separation. But the sample with 6at% W exhibits the best thermoelectric properties. For 17at% W, there are already percolation phenomena between different WSi₂ grains, reducing the Seebeck effect and lowering the thermoelectric efficiency significantly.

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