Thermoelectric Power Generation: Giant Thermopower 2DEG in SrTiO3
[Nature Materials 2007/ Materials Today 2007/ Appl. Phys. Lett. 2007/ Appl. Phys. Express 2008/ phys. stat. sol. (b) 2008/ Inorg. Chem. 2008/ phys. stat. sol. (RRL) 2008/ Phys. Rev. B 2010]
Thermoelectric energy conversion technology attracts great attention to convert the waste heat into electricity. For the practical thermoelectric application, figure of merit ZT > 1 (ZT = S2・sigma・T・kappa-1, where Z, T, S, sigma, kappa are a figure of merit, the absolute temperature, Seebeck coefficient, electrical conductivity and thermal conductivity, respectively). Although heavy metal based materials such as Bi2Te3 and PbTe exhibit ZT ~ 1, they are not attractive, particularly operating at high temperatures (T~1,000K), because they are mostly toxic, and decomposition, vaporization or melting of the constituents can easily occur at high temperatures. This review provides a new class of thermoelectric material: SrTiO3 having two-dimensional electron gas (2DEG). The high-density (~1021 cm-3) 2DEG confined within the thickness of a SrTiO3 unit cell layer (a = 0.3905 nm), which is realized at SrTiO3/Nb-doped SrTiO3 superlattices or TiO2/SrTiO3heterointerfaces. The 2DEG demonstrates that enhanced |S| by a factor of ~5 compared to that for bulk. And the optimized ZT for the 2DEG reaches 2.4, which is twice of conventional thermoelectric materials.