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@INPROCEEDINGS{Mohamadi:827174,
      author       = {Mohamadi, Maryam Beig and Tavabi, Amir Hossein and
                      Dunin-Borkowski, Rafal and Pöhle, Georg and Pacheco,
                      Vicente},
      title        = {{T}he influence of {Y}b and {B}i doping on the
                      thermoelectric properties of {M}g2{S}i0.4{S}n0.6 studied
                      using transmission electron microscopy},
      address      = {Weinheim, Germany},
      publisher    = {Wiley-VCH Verlag GmbH $\&$ Co. KGaA},
      reportid     = {FZJ-2017-01372},
      pages        = {1046 - 1047},
      year         = {2016},
      comment      = {European Microscopy Congress 2016: Proceedings / Mohamadi,
                      Maryam Beig ;ISBN: 9783527808465},
      booktitle     = {European Microscopy Congress 2016:
                       Proceedings / Mohamadi, Maryam Beig
                       ;ISBN: 9783527808465},
      abstract     = {Current research in thermoelectric materials is focused on
                      increasing the figure of merit ZT=(S2σ/κ)T (where S is the
                      Seebeck coefficient and σ is the electrical conductivity)
                      by maximizing the power factor PF (S2σ) and/or minimizing
                      the thermal conductivity (κ). Attempts to maximize the PF
                      include the development of new materials and optimization of
                      existing materials by doping and nano-structuring. A
                      reduction in thermal conductivity can be achieved by
                      alloying, by producing disordered or complex unit cells or
                      by nanostructuring. Here, we investigate a Bi-doped and a
                      Bi- and Yb- doped Mg2Si0.4Sn0.6 alloy. We discuss the
                      influence of composition, crystal structure and
                      microstructure on the thermoelectric performance of the
                      materials, in order to assess new opportunities for
                      enhancing the performance of bulk nano-structured composite
                      materials.Samples were produced by powder metallurgical
                      processes, starting from a stoichiometric mixture of a
                      melt-spun Mg or Mg-Yb pre-alloy and Si, Sn and Bi powders.
                      After performing high energy milling to mix the components
                      homogeneously under a protective Ar atmosphere, the material
                      was simultaneously compacted and synthesized during a FAST
                      process.Pure Mg2Si0.4Sn0.6 is an n-type semiconductor with a
                      low value of σ. S is negative between room temperature and
                      600 °C. σ increases approximately linearly with Bi
                      concentration. An optimized doping content leads to a value
                      for σ of 140000 - 180000 S/m and a value for S of - 150
                      µV/K at room temperature. Strong doping results in
                      degeneracy of the semiconductor. Therefore, σ decreases
                      with temperature, while S increases. The temperature
                      dependence of κ shows two “branches”. In samples that
                      have an optimized Bi doping concentration, κ decreases from
                      room temperature to approximately 400 °C due to a dominant
                      phonon-phonon scattering mechanism, with a minimum of 2
                      W/mK. At higher temperatures, thermal excitation of charge
                      carriers across the band gap increases κ. Bi-Yb-doped
                      Mg2Si0.4Sn0.6 shows a larger ZT than the Yb-free sample.We
                      prepared specimens for high-resolution transmission electron
                      microscopy (HRTEM) using an FEI Helios Nanolab 400s focused
                      ion beam (FIB) dual-beam system. HRTEM images were acquired
                      at 300 kV using an FEI Titan 80-300 TEM equipped with a
                      spherical aberration (Cs) corrector on the objective lens.
                      High-angle annular dark-field (HAADF) scanning TEM (STEM)
                      images and elemental maps were acquired at 200 kV on an FEI
                      Titan G2 80-200 TEM equipped with a Cs corrector on the
                      condenser lens system.An inspection of the microstructures
                      of the materials by TEM reveals a homogeneous Mg2Si0.4Sn0.6
                      matrix and a similar grain size distribution in both
                      samples. The average grain sizes are in the range 1 - 3 μm,
                      which shows that an improvement in the thermoelectric
                      properties of the Bi- and Yb- doped alloy cannot be
                      attributed to grain size. High spatial resolution
                      energy-dispersive X-ray spectroscopy (EDXS) shows that the
                      elemental distribution inside the grains differs from that
                      at the grain boundaries. Our results show that Yb does not
                      form a solid solution with Mg2Si0.4Sn0.6, but instead forms
                      distinct grains by reacting with Bi and Sn. The formation of
                      Bi-rich precipitates in Bi- and Yb- doped Mg2Si0.4Sn0.6
                      reduces the Bi content in the otherwise homogeneously doped
                      matrix. Some oxygen enrichment in the region of the grain
                      boundaries, associated with the formation of MgO and SiOx,
                      was observed in both samples. Sn and Si nanoscale
                      precipitates were detected in the Bi-doped sample.},
      month         = {Aug},
      date          = {2016-08-28},
      organization  = {16th European Microscopy Congress (EMC
                       2016), Lyon (France), 28 Aug 2016 - 2
                       Sep 2016},
      cin          = {PGI-5 / ER-C-1},
      cid          = {I:(DE-Juel1)PGI-5-20110106 / I:(DE-Juel1)ER-C-1-20170209},
      pnm          = {143 - Controlling Configuration-Based Phenomena (POF3-143)},
      pid          = {G:(DE-HGF)POF3-143},
      typ          = {PUB:(DE-HGF)8 / PUB:(DE-HGF)7},
      doi          = {10.1002/9783527808465.EMC2016.6207},
      url          = {https://juser.fz-juelich.de/record/827174},
}