The influence of
Y
b and
B
i doping on the thermoelectric properties of
Mg2Si0.4Sn0.6
studied using transmission electron microscopy
Authors: Mohamadi Maryam Beig , Tavabi Amir Hossein , Dunin‐Borkowski Rafal Edward , Pöhle Georg , Pacheco Vicente 
Published: December 2016

Current research in thermoelectric materials is focused on increasing the figure of merit ZT=(S
2
σ/κ)T (where S is the Seebeck coefficient and σ is the electrical conductivity) by maximizing the power factor PF (S
2
σ) 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 Mg
2
Si
0.4
Sn
0.6
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 Mg
2
Si
0.4
Sn
0.6
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 Mg
2
Si
0.4
Sn
0.6
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 Mg
2
Si
0.4
Sn
0.6
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 Mg
2
Si
0.4
Sn
0.6
, but instead forms distinct grains by reacting with Bi and Sn. The formation of Bi‐rich precipitates in Bi‐ and Yb‐ doped Mg
2
Si
0.4
Sn
0.6
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 SiO
x
, was observed in both samples. Sn and Si nanoscale precipitates were detected in the Bi‐doped sample.

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