Fabrication and Characterization of Nanostructured Bulk Skutterudites

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Fabrication and Characterization of Nanostructured Bulk Skutterudites Mohsen Y. Tafti 1, Mohsin Saleemi1, Alexandre Jacquot2, Martin Jägle2, Mamoun Muhammed1, Muhammet S. Toprak1 1 Department of Materials and Nano Physics, KTH - Royal Institute of Technology, Stockholm, Sweden 2 Fraunhofer-Institute for Physical Measurements IPM, Freiburg, Germany ABSTRACT Latest nanotechnology concepts applied in thermoelectric (TE) research have opened many new avenues to improve the ZT value. Low dimensional structures can improve the ZT value as compared to bulk materials by substantial reduction in the lattice thermal conductivity, κL. However, the materials were not feasible for the industrial scale production of macroscopic devices because of complicated and costly manufacturing processes involved. Bulk nanostructured (NS) TEs are normally fabricated using a bulk process rather than a nanofabrication process, which has the important advantage of producing in large quantities and in a form that is compatible with commercially available TE devices. We developed fabrication strategies for bulk nanostructured skutterudite materials based on FexCo1-xSb3. The process is based on precipitation of a precursor material with the desired metal atom composition, which is then exposed to thermochemical processing of calcination followed by reduction. The resultant material thus formed maintains nanostructured particles which are then compacted using Spark Plasma Sintering (SPS) by utilizing previously optimized process parameters. Microstructure, crystallinity, phase composition, thermal stability and temperature dependent transport property evaluation has been performed for compacted NS FexCo1-xSb3. Evaluation results are presented in detail, suggesting the feasibility of devised strategy for bulk quantities of doped TE nanopowder fabrication. INTRODUCTION Thermoelectric (TE) materials have recently found a lot of interest and investment. They can directly interconvert between heat and electricity and have no moving parts, with almost close to zero noise [1-8]. In order to have good TE material, the material’s thermal conductivity should behave similar to a glass and on the other hand have the electronic properties of a crystal [21]. This concept is called Phonon Glass Electron Crystal (PGEC). Skutterudites based on CoSb3 are among the materials which may behave similarly to the PGEC concept. [9] The skutterudite structure is based on MX3 where M is a transition metal (Fe, Co, Ni, etc.) and the X can be P, As, or Sb [1,4-6,8]. Skutterudite unit cell is represented by the face centered cubic arrangement of M atoms forming 8 sub-cubic voids/cages. Four membered rings of X atoms fill six of these eight cages. The empty two voids can allow introduction of heavy elements forming so called filledskutterudites. Several strategies have been introduced to enhance the electronic properties as well as reducing the thermal conductivity of skutterudites including doping, filling, and nanoengineering. Doping can dramatically enhance the electrical con