Oxide thermoelectrics: The challenges, progress, and outlook
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Ryoji Funahashi Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, Ikeda, Osaka 563-8577, Japan; and CREST, Japan Science and Technology Agency, Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan (Received 17 January 2011; accepted 28 March 2011)
Most state-of-the-art thermoelectric (TE) materials contain heavy elements Bi, Pb, Sb, or Te and exhibit maximum figure of merit, ZT~1–2. On the other hand, oxides were believed to make poor TEs because of the low carrier mobility and high lattice thermal conductivity. That is why the discoveries of good p-type TE properties in layered cobaltites NaxCoO2, Ca4Co3O9, and Bi2Sr2Co2O9, and promising n-type TE properties in CaMnO3- and SrTiO3-based perovskites and doped ZnO, broke new ground in thermoelectrics study. The past two decades have witnessed more than an order of magnitude enhancement in ZT of oxides. In this article, we briefly review the challenges, progress, and outlook of oxide TE materials in their different forms (bulk, epitaxial film, superlattice, and nanocomposites), with a greater focus on the nanostructuring approach and the late development of the oxide-based TE module.
I. INTRODUCTION
While electricity remains the most convenient form of energy in the foreseeable future, heat has been an abundant but low quality source of energy as more than half of all the energy generated by mankind is lost as waste heat. The simplest technology applicable for direct heat–electricity conversion is thermoelectricity.1 Heat emanating from various sources (e.g., solar, geothermal, and exhaust from automobiles or other industrial processes) can be directly converted into clean electricity by a thermoelectric (TE) device, which is an all-solid assembly without moving parts or greenhouse gas emissions, compact, responsive, and feasible for miniaturization. The TE device can also work reversely as a heat pump for refrigeration (heat management). The conversion efficiency of a TE device is determined by the Carnot efficiency and the figure of merit, ZT, of the TE material.2 Assuming a negligible contact resistance and optimized load condition, the maximum conversion efficiency of TE power generation, g, is expressed as pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ ZTm 1 Thot Tcold g5 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; ð1Þ Thot 1 þ ZTm þ ðTTcold Þ hot where the Carnot efficiency is given as the ratio of the temperature difference between the hot-end temperature and the cold-end temperature (Thot Tcold), to Thot, and a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.108
Tm is the mean temperature. ZT is the material parameter of prime importance in thermoelectrics research: ZT 5
a2 T PF a2 T 5 5 qj j qðjl þ je Þ
;
ð2Þ
where a is the thermopower (or Seebeck coefficient), q the electrical resistivity, j the thermal conductivity (jl the lattice thermal conductivity and je the carrier thermal conductivity), PF the power factor, and T the temperature in Kelvin. Ideally, a TE material simultaneously possesses low q and l
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