KJ-Ceramics 31
Program & Abstracts

TE-I03A study on thermoelectric properties of perovskite-type oxides
Hiroshi Nakatsugawa1*, Masaki Kubota1 , Takanori Yamamoto1, Yosuke Watanabe1
Yokohama National University,79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan

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Keywords: thermoelectric, perovskite-type oxides, 3d transition metal, Heikes formula
Thermoelectric energy conversion technology attracts renewed interest as a new energy resource.
Thermoelectric materials can directly convert waste heat from automobiles, facilities, and power
plants into electrical energy. Since electrons in solids carry electricity together with thermal entropy,
there is a finite coupling between the electrical and heat currents, which is known as thethermoelectric phenomena. The potentiality of the thermoelectric materials is indicated in terms of thedimensionless figure of merit, i.e., ZT=S2σT/κ, where S, σ, κ, and T are the Seebeck coefficient, theelectric conductivity, the thermal conductivity, and the absolute temperature, respectively. The
thermoelectric energy conversion generates energy from the temperature gradient and the heat flow.
The thermoelectric energy conversion efficiency η is defined by the ratio between the output electrical
power P with the heat flux Q, i.e., η=P/Q. By optimizing the efficiency with respect to a loadresistance, the maximum efficiency ηmax is obtained as a monotonically increasing function of ZT.
Obviously, a higher temperature with a larger temperature difference and to be a larger ZT gives alarger ηmax. In this respect, oxides are quite attractive because they are thermally and chemically stable
for long time use at high temperatures in air for thermoelectric conversion. Recently, oxides havebeen studied extensively as a possible candidate for thermoelectric materials at high-temperatures
since the discovery of large Seebeck coefficient (S=100μV/K) and low electric resistivity(ρ=0.2mΩcm) at room temperature in the layered cobalt oxide NaxCoO2.[1] The misfit-layered cobalt
oxide Ca3Co4O9 has also been studied as a candidate for p-type thermoelectric materials and typicallyexhibits S=130μV/K, ρ=15mΩcm, and κ=1.0W/mK at room temperature.[2] For instance, fabricationand power generation of oxide thermoelectric modules consisting of p-type Ca2.7Bi0.3Co4O9 and n-type
La0.9Bi0.1NiO3 bulks have been reported.[3] In face, power charging a portable phone has beensuccessful using this oxide thermoelectric module, however, the maximum efficiency ηmax was as low
as 1.4%.[4] To enhance ηmax, Urata et al.[5] have built thermoelectric modules consisting of p-type
Ca2.7Bi0.3Co4O9 and n-type CaMn0.98Mo0.02O3 bulks and calculated ηmax to be 2.0% from ZT values.
However, the n-type legs were damaged by thermal stress between the alumina substrate. [5] To
overcome the lack of oxide thermoelectric materials, we have paid attention to perovskite-type oxides
which form both n-type and p-type conductors, since many kinds of metal elements can fit into theperovskite structure with the general formula ABO3. Perovskite-type oxides such as titanate,manganate, ferrate, and cobaltate phases reveal a large Seebeck coefficient, S, which is one of theessential prerequisites for potential thermoelectric materials. The large S values are caused by a strong
interplay between charge, orbital states, electron spin states, and crystal structure. Koshibae et al.[6]proposed that the high-temperature limit of S values in 3d transition metal oxides is given by Heikes
formula from a localized picture. When both a degeneracy of spin and orbital degrees of freedom of
3d transition metal and the Heikes formula give large values, the large S at room temperature can beexpected. Therefore, control of spin and orbital states of 3d transition metal ions would improve theZT values of thermoelectric oxides. In this paper, the thermoelectric properties S, ρ, and κ, of theperovskite-type oxides, Pr1-xSrxMnO3, Pr1-xCaxMnO3, and La1-xSrxFeO3 will be investigated in order toclarify the possibility of the thermoelectric modules consisting of perovskite-type oxides.
References:[1] I.Terasaki, Y.Sasago, and K.Uchinokura, Phys. Rev. B56, R12685 (1997).
[2] Y.Miyazaki, K.Kudo, M.Akoshima, Y.Ono, Y.Koike, and T.Kajitani, Jpn, J.Appl.Phys. 39, L531 (2000).
[3] R.Funahashi, S.Urata, K.Mizuno, T.Kouuchi, and M.Mikami, Appl.Phys.Lett. 85, 1036 (2004).
[4] R.Funahashi, M.Mikami, T.Mihara, S.Urata, and N.Ando, J.Appl.Phys. 99, 066117 (2006).
[5] S.Urata, R.Funahashi, T.Mihara, A.Kosuga, S.Sodeoka, and T.Tanaka, Int.J.Appl.Ceram.Technol. 4, 535(2007).
[6] W.Koshibae, K.Tsutsui, and S.Maekawa, Phys.Rev. B62, 6869 (2000).

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