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Thermoelectric (TE) elements can generate heat from a voltage gradient
(Peltier refrigerators) or a voltage from a temperature gradient (thermogenerators). The search for new thermoelectric materials has become an important issue since there exists numerous sources of waste-heat which could be converted to electrical energy by using non polluting thermoelectric devices. The efficiency of such a device is characterized by the figure of merit defined as
Z=S2/ρκ, where S is thermoelectric power, ρ the resistivity and κ the thermo-conductivity. Materials used in TE devices usually have ZT in the order of 1, where T is the absolute temperature. The highest thermoelectric performance materials are intermetallic compounds such as
Bi2Te3/Sb2Te3 alloys. The drawbacks of the conventional TE materials are that they have low melting, decomposition, and oxidation-temperatures, and they have content of harmful or rare elements, and notably deficient in the temperature range of waste heat (600-1000K).
In 1997, it was discovered that large thermoelectric power and low resistivity could coexist in the
Na0.5CoO2 [I. Terasaki et al., Phys. Rev. B 56, R12685 (1997)], which made this compound an attractive candidate for TE applications. Soon after, the related cobaltates which posses the similar properties have been synthesized and investigated. Those cobaltates all have layered crystal structures, and all share the hexagonal
CoO2 blocks as a common unit. NaxCoO2 has relatively simple structure because of the smaller size of the Na atoms. The other cobaltates have misfit-layered structure with rock salt type structures in between the
CoO2 layers. The charge-carrier transport in those cobaltates is thought to be restricted mainly to these
CoO2 planes, and they seem to be crucial in the high thermoelectric power properties.
|Schematic structure of NaxCoO2
with the CoO2 layer and the Na ion separating layers.
(Figure reproduced from K. Takada, et. al., Nature 422,
structure of [Ca2CoO3][CoO2]1.62
with the CoO2 layer (red) and the Ca2CoO3
three-layer rock salt structure. (Figure reproduced from C. Simon's
The origin of the coexistence of metallicity with a large thermoelectric power is still unclear. Two models have been proposed. The first one takes into account the spin degeneracy associated to the different cobalt valency and spin states [W. Koshibae et al., Phys. Rev. B 62, 6869 (2000)]. In the second one [D. J Singh, Phys. Rev. B 61, 13397 (2000)], a two-band model has been proposed: a band of light carriers responsible for metallicity coexists with a narrow band of heavy carriers in which the Fermi energy level lies. A peak in the Density of States near
EF would be responsible for the large thermoelectric power.
In our group, we are trying to make different polycrystalline and single crystal
cobaltates, and cobaltate thin films as well. Magnetic, electrical and thermal transport properties are studied using our Quantum Design
PPMS, MPMS SQUID Magnetometer and other home made systems. Our goal is to investigate the mechanism of the high thermoelectric power, to improve the thermoelectric performance and to make TE devices using these layered cobaltates for practical application.
Last Modified: January 31, 2008
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