Nanoparticle-in-alloy-approach without alloy: Processing and thermoelectric properties of Si-WSi2
Roland Schmechel, Universität Duisburg-Essen, Duisburg, GermanyJulia Stoetzel, Institute of Technology for Nanostructures / University Duisburg-Essen and CENIDE, Duisburg, GermanyTom Schneider, Institute for Combustion and Gas Dynamics / University Duisburg-Essen and CENIDE, Duisburg, GermanyMathis M. Mueller, Institute for Applied Geo Sciences / Technical University Darmstadt, Darmstadt, GermanyHans-Joachim Kleebe, Institute for Applied Geo Sciences / Technical University Darmstadt, Darmstadt, GermanyHartmut Wiggers, Institute for Combustion and Gas Dynamics / University Duisburg-Essen and CENIDE, Duisburg, GermanyGabi 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.