Figure of merit zT mapping of p-Mg2Si1−xSnx with respect to carrier concentration.

Polycrystalline higher manganese silicides (HMS) with Ge additions were prepared by induction melting followed by hot-pressing. The phase structures and microstructure of the pellets were investigated, and their thermoelectric properties were measured from room temperature to 650°C. It was found that the solubility of Ge in HMS was limited to around 1.6%, beyond which an extra phase of Si y Ge 1Ày appeared. The electrical conductivity was continuously enhanced by Ge additions, while the Seebeck coefficient was slightly decreased. The thermal conductivity showed first a decreasing then an increasing relationship with increasing Ge additions. The HMS cells, mainly along the c-axis, were remarkably enlarged by the substitution of Ge, which probably resulted in the enhancement of phonon scattering due to an increased number of defects, reducing the phonon thermal conductivity. The dimensionless figure of merit of the optimized HMS polycrystals was improved by more than 30% compared with the pure HMS material.

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Stoichiometric CrSi2 was prepared by arc melting and compacted by uniaxial hot pressing for property measurements. The crystal structure of CrSi2 was investigated using the powder x-ray diffraction method. From the Rietveld refinement, the lattice parameters were found to be a=4.42757 (7) and c=6.36804 (11)Å, respectively. The thermal expansion measurement revealed an anisotropic expansion in the temperature range from room temperature 800K with αa=14.58×10−6∕K, αc=7.51×10−6∕K, and αV=12.05×10−6∕K. The volumetric thermal expansion coefficient shows an anomalous decrease in the temperature range of 450–600K. The measured electrical resistivity ρ and thermoelectric power S have similar trends with a maxima around 550K. Thermal conductivity measurements show a monotonic decrease with increasing temperature from a room temperature value of 10Wm−1K−1. The ZT values increase with temperature and have a maximum value of 0.18 in the temperature range studied. An analysis of the electronic band structure is provided.

The electronic and thermal transport properties of Mg 2 Sn crystals prepared by radio-frequency (RF) induction melting, were investigated in the temperature range 10À700 K to study the effect of adding excess silver to the melt on the microstructure and thermoelectric properties. As the Agcontent increases the amount of silver incorporated at doping sites tends to saturate so that the carrier concentration increases asymptotically to 6.3 Â 10 19 cm À3 . The excess amount of silver reacts with magnesium to form a fine MgAg þ Mg 2 Sn eutectic structure. The temperature dependence of the carrier mobility varies from $T À1.5 for the heavily doped samples in the extrinsic range to $T À2.8 for the undoped sample in the intrinsic range. From electrical conductivity and Hall measurements on the undoped samples we find the energy gap to be 0.36À0.38 eV. Doped samples show broad Seebeck peaks of 150À210 mV K À1 at T ¼ 350À550 K. The thermal conductivity decreases sharply as the addition of Ag is increased, and reaches a minimum value of k ¼ 3.4 W m À1 K À1 at 450 À 500 K for the doped sample containing a uniformly distributed eutectic phase, leading to a maximum figure of merit ZT max ¼ 0.30.