Effect of Sn Substitution on the Thermoelectric Properties of Nanostructured Bulk Bi 2-x Sb x Te 3 Alloy.
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Effect of Sn Substitution on the Thermoelectric Properties of Nanostructured Bulk Bi2-xSbxTe3 Alloy. Sumithra.Santhanam1, Nathan J.Takas1, Westly M.Nolting1,3, Pierre F.P.Poudeu1,2 and Kevin L.Stokes1,3 * 1
Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana, 70148.
2
Department of Chemistry, University of New Orleans, New Orleans, Louisiana, 70148.
3
Department of Physics, University of New Orleans, New Orleans, Louisiana 70148.
ABSTRACT The carrier concentration and electronic transport properties in Bi2-xSbxTe3 alloy can be tuned by varying the Bi to Sb ratio, for high thermoelectric figure of merit. The concentration of intrinsic antisite defects in these alloys is also known to change with Bi to Sb ratio. Here we report the thermoelectric figure of merit of Sn doped Bi0.5Sb1.5Te3 alloy. Different atomic percentages of Sn was substituted at Bi/Sb site in Bi0.5Sb1.5Te3 alloy, synthesized by planetary ball milling. The electrical conductivity decreases with increasing Sn doping but for higher Sn content the electrical conductivity increases compared to undoped alloy. The Seebeck coefficient changes in accordance to electrical conductivity, resulting in small decrease in power factor for highest Sn doping. The lattice thermal conductivity shows a systematic decrease, with increasing Sn concentration resulting in a significant thermal conductivity reduction. Hence an increase in thermoelectric figure of merit could be achieved for the highest Sn (3at%) doping in Bi0.5Sb1.5Te3 alloy as compared to undoped alloy. INTRODUCTION Bismuth telluride (Bi2Te3) and its solid solution alloy Bi2-xSbxTe3 (p-type), Bi2Te3-xSex (n-type) are the commercial state of the art materials used for thermoelectric devices near room temperature. The dimensionless thermoelectric figure of merit is defined as ZT= (S2σ)T/K, where S is seebeck coefficient, σ is electrical conductivity and K is thermal conductivity which has electronic and lattice contributions (ie K=Kelectronic+Klattice). It is well known that ZT can be enhanced in mixed crystals (Bi2-xSbxTe3) by reducing the lattice thermal conductivity arising due to mass difference scattering. The ZT in most of these materials was restricted to 1, but it was shown by Poudel et.al. [1] that nanostructuring can further decrease the lattice thermal conducitivity (Klattice) by phonon scattering at grain boundaries for enhancement in ZT. The electronic transport parameters S and σ are sensitive to Bi/Sb ratio in Bi2-xSbxTe3 alloy, and so the composition needs to be optimized for the highest ZT. Hence a careful control of Bi/Sb ratio is essential to obtain highest ZT in the mixed crystal Bi2-xSbxTe3 series. The presence of antisite defects also changes the electronic transport properties in Bi2Te3 and Bi2-xSbxTe3 mixed crystal series. In Bi2-xSbxTe3 mixed crystals, the BiTe and SbTe (Bi/Sb can occupy the Te position and vice versa) are the most probable antisite defects and these defects
are electrically active which can change the carrier concentration
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