Enhanced electrical conductivity in Si 80 Ge 20 B 0.6 alloys with Er addition prepared by spark plasma sintering
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Si80Ge20B0.6 thermoelectric alloys with minute erbium (Er) addition were prepared by the spark plasma sintering technique. The samples with different amounts of Er additions were analyzed by x-ray diffraction, x-ray fluorescence, and x-ray photoelectron spectroscopy. The thermoelectric properties were measured from 400 to 900 K. Effects of the amount of Er addition on the thermoelectric properties of Si80Ge20B0.6 alloys were investigated. New findings indicate that the Er-added alloys have larger carrier concentrations than the pristine sample. The larger carrier concentration appears to make a significant contribution to the electrical conductivity. Seebeck coefficient decreases with the increase of carrier concentration, whereas the power factor increases with increasing electrical conductivity. It was generally believed that the scattering of phonons by carriers may result in the thermal conductivity reduction. The samples with Er addition exhibit better figure of merits than the pristine sample. The optimal ratio of Er addition is actually in the range of 0.085–0.125 at.%. I. INTRODUCTION
Thermoelectric materials have been getting more and more extensively used in many cooling and power generating applications, such as in microelectronic devices, waste heat harvesting, etc.1,2 Significant research has been focused on further improving the comprehensive properties of thermoelectric materials to obtain higher conversion efficiency. One of the adoptable ways to achieve this purpose is to fabricate p- or n-type semiconductors by doping.3–7 Particularly, in recent years, the influence of rare earth doping on thermoelectric materials has been extensively studied.8,9 For example, the power factor of the Dysubstituted NaxCo2O4 sample was found to be remarkably higher than that of the nonsubstituted sample due to the enhancement of electrical conductivity.10 The carrier concentration decreased with the increasing amount of Gd dopant in n-type Bi2Te3 alloys. The dopant, Gd, therefore, was considered as an acceptor.11 The dopants of rare earth metals, Dy and Er, could improve the thermoelectric properties of MyCoSb3 by enlarging the lattice parameters.12 Sales13 showed that the thermal conductivity at room temperature was lowered by the presence of the rare earth rattlers filled in skutterudite structures. Among the various kinds of thermoelectric materials, SiGe alloys, used in thermoelectric power generators for space flights,14 have been attracting much attention. SiGe alloys were considered as potential thermoelectric materia)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.127 J. Mater. Res., Vol. 26, No. 15, Aug 14, 2011
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als for their wide energy gaps and low thermal conductivities. The physical properties were found to be closely dependent on the compositions of the alloys.6 Hence, composition adjustment could be practiced in an attempt to manipulate the performances of SiGe alloys.15 Conventional dopants (B, P, GaP, AlP, etc)
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