Properties of metallic Na x Co 2 O 4 thermoelectric materials
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Properties of metallic NaxCo2O4 thermoelectric materials Xiaofeng Tang1, Terry M. Tritt2, Ed Abbott3, J. K. Kolis3 1. Dept. of Material Science & Engineering, Clemson University, Clemson, SC 2. Dept. of Physics and Astronomy, Clemson University, Clemson, SC 3. Dept. of Chemistry, Clemson University, Clemson, SC
Abstract: Since the quite favorable thermoelectric properties of transition-metal oxide NaCo2O4 were first reported by Terasaki in 1997, extensive research work has been conducted, including the efforts to improve TE properties through doping or new synthesis approaches. In addition, theoretical investigations about the enhanced thermopower coupled with the small resistivity values for the metallic NaxCo2O4 have been investigated. The advent of the large thermopower and low resistivity appears not able to be explained via conventional free-electron theory. In this paper, thermoelectric and magnetic properties, including resistivity, thermopower, thermal conductivity, magnetic susceptibility and moment of single crystals and polycrystalline NaxCo2O4 are reported. The effect of Na concentration on the transport properties will also be discussed.
1. Introduction Thermoelectric (TE) materials can convert heat into electricity through the Seebeck effect; also electricial energy can be converted into thermal energy via Peltier effect. Thermoelectric devices exhibit high stability and reliability, quietness (without moving parts) and small-scale localized cooling/heating. Despite these advantages, current thermoelectric materials also exhibit low efficiency, which makes the TE devices more appropriate for applications where the high reliability, or specific demands other than efficiency are the major concerns, such as power generation for NASA space probe and refrigeration for computer chip and IR detector cooling to enhance the device performance. The performance of thermoelectric materials, which is proportional to the device efficiency, can be evaluated by the dimensionless figure-of-merit (FOM) which is defined as ZT = α 2T / ρ (κ L + κ e ) , where α is the Seebeck coefficient, also called thermopower, ρ is the resistivity, κL and κe is lattice and electronic thermal conductivity, respectively. It is certainly a challenging problem to increase the efficiency of a device since ZT is the combination of transport properties of materials and the three transport parameters, α, ρ,
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and κ, which are strongly correlated, especially the resistivity and thermopower typically being strongly dependent on each other as a function of carrier concentration. At present, the state-ofart TE materials are still the traditional Bi2Te3, BiSb or their alloys for solid-state cooling application at low temperature and the semiconducting SiGe alloy for power generation use at high temperature. These materials exhibit a maximum ZT ≈ 1 at their respective usage temperature [1].
Because of its high thermal and chemical stability and non-toxicity, ceramic oxide materials are promising for possible power generation use.
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