New Thermoelectric Arsenides and Antimonides for High Temperature Applications
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New Thermoelectric Arsenides and Antimonides for High Temperature Applications Hong Xu1, Navid Soheilnia1, Huqin Zhang2, Paola N. Alboni2, Terry M. Tritt2, and Holger Kleinke1 1 University of Waterloo, Waterloo, N2L 3G1, Canada 2 Clemson University, Clemson, SC, 29634-0978 ABSTRACT Three different materials crystallizing in the cubic Ir3Ge7 type are under investigation in our group, namely Mo3(Sb,Te)7, Nb3(Sb,Te)7, and Re3(E,As)7 (with E = Si, Ge, Sn). Our electronic structure calculations reveal a band gap to occur at 55 valence-electrons in all three cases, namely Mo3Sb5Te2, Nb3Sb2Te5, and Re3EAs6. Cubic holes exist in these structures that may be filled with small cations such as 3d transition metal atoms. Ni0.06Mo3Sb5.4Te1.6 is a degenerate p-type semiconductor that reaches ZT = 0.96 at 750°C, while Re3Ge0.6As6.4 is a degenerate n- type semiconductor with slightly lower ZT values. Preliminary results indicate that the Re3(Sn,As)7 system may be the most promising of the rhenium arsenides. INTRODUCTION The thermoelectric (TE) energy conversion is commercially used both for power generation utilizing the Seebeck effect and for cooling applications utilizing the Peltier effect. One particularly exciting opportunity is the conversion of waste heat into useful electricity, e.g. stemming from combustion in car engines, which is normally lost to the environment [1]. Advanced thermoelectrics are narrow band gap semiconductors comprising heavy elements. Ideally, these exhibit a large Seebeck coefficient, S, high electrical conductivity, σ, and low thermal conductivity, κ. TE materials are ranked by their thermoelectric figure-of-merit, ZT, defined as ZT = TS2σ/κ [2-5]. The strong renewed interest in thermoelectrics led to the development of several new materials with high ZT values at various temperatures, including the low temperature TE CsBi4Te6 [6] and several high temperature TEs, e.g. the filled skutterudites [7], Yb14MnSb11 [8], and AgPbmSbTe2+m [9]. In the late 90s, we commenced to investigate the properties of a new material, Mo3Sb5+δTe2–δ [10], a ternary substitution variant of the metallic antimonide Mo3Sb7 (Ir3Ge7 type) [11, 12]. As subsequently characterized in the Jet Propulsion Laboratories, Mo3Sb5.4Te1.6 is one of the leading high temperature p-type thermoelectrics with its ZT of 0.8 at 1050 K [13]. This value is comparable to Yb14MnSb11 [8] and the best filled (also antimony based) skutterudites. After proving that small cations such as for example Mg, Ni, Cu can be intercalated into the cubic voids of Mo3Sb5+δTe2–δ [14], our low temperature investigations showed that adding Ni occurs with increases in all three key properties, i.e. Seebeck coefficient, electrical conductivity, and thermal conductivity, leading to an increase of ZT of 30% at 300 K [15]. The increase in ZT originates primarily from the significant increase in electrical conductivity with the addition of Ni. Our most recent investigations led to the identification of another isostructural high temperature TE, namely the n-type s
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