Synthesis and Thermoelectric Properties of Y-doped Ca 3 Co 4 O 9 Ceramics
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Synthesis and thermoelectric properties of Y-doped Ca3Co4O9 ceramics J. E. Rodríguez1 and L. C. Moreno2 1 Department of Physics, Thermoelectric Materials group, Universidad Nacional de Colombia 2 Department of Chemistry, Universidad Nacional de Colombia ABSTRACT Polycrystalline ceramics with a nominal composition of Ca3-xYxCo4O9+δ (0≤x≤0.10) were grown by using the citrate-complex method. Thermoelectric properties were studied using Seebeck coefficient S(T) and electrical resistivity ρ(T) measurements. These transport properties were studied in the temperature range between 100 and 290 K. For low doping levels in Y substituted samples (x≤0.06), the magnitude of S(T) and ρ(T) decreases with yttrium content. The temperature behavior of S(T) and ρ(T) was interpreted in terms of the small-polaron hopping mechanism. From S(T) and ρ(T) data, it was possible to calculate the thermoelectric power factor PF, which reaches a maximum value close to 23 µW/K2-cm. These values become these compounds promissory thermoelectric compounds for use in low temperature thermoelectric applications. INTRODUCTION Thermoelectric effects offer an alternative pathway for direct energy conversion. The best thermoelectric materials available near room temperature are bismuth telluride and its alloys. However, these materials are not stable at high temperatures. On the contrary, at low temperature the best thermoelectric materials are monocrystalline Bi-Sb alloys, which are n-type semiconductors. However, their use in practical thermoelectric devices is limited by both their poor mechanical properties and because no suitable p-type material has been found with compatible properties. The research for efficient thermoelectric materials often involves no conventional semiconductors [1-3]. In this sense, oxide compounds such as LaCoO3, La1-xSrxCoO3, Bi2Ca2Co2Ox, NaCo2O4 and Ca3Co2O6 are promising candidates as thermoelectric materials, because of their transport properties and their physical-chemical stability [4-7]. Ca3Co4O9+δ (CCoO) compounds are members of the oxide-family, which consists of alternating layers of conducting CoO2 and insulating rock-salt Ca2CoO3+δ. For this reason, these kinds of compounds are considered to be natural superlattices, which are potential candidates for thermoelectric applications because of their high electrical conductivity and Seebeck coefficient values, low cost, and high thermal stability [8-11]. Increasing the efficiency of thermoelectric materials is the biggest challenge of thermoelectric materials research. The energy conversion performance of a thermoelectric device is evaluated using the dimensionless figure of merit ZT, which is defined as follows [1,2]:
ZT =
S 2T
(1)
ρκ
where S is the Seebeck coefficient, ρ the electrical resistivity, κ the total thermal conductivity and T the absolute temperature. ZT determines the fraction of the Carnot efficiency that can be obtained by a thermoelectric device. The quantity S 2 / ρ is called the power factor (PF) and is the key to attaining high the
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