Thermoelectric (TE) materials, which could directly convert temperature difference into electricity, are attracting increasing attentions among energy harvesting technologies. Generally, the performance of TE materials is evaluated in terms of a dimensionless figure of merit, ZT=S2σTκ−1, where Z is the figure of merit, T is the absolute temperature, S is the thermopower (≡Seebeck coefficient), σ is the electrical conductivity and κ is the sum of the electronic (κele) and lattice thermal conductivities (κlat) of a TE material. In addition to reducing κlat, enhancing S2σ, which is regarded as power factor (PF) is also a promising strategy.
Two-dimensional electron system (2DES)–carrier electrons are confined within a narrow layer (the thickness < de Broglie wavelength, λD)–is known as one of efficient strategies to achieve an enhanced PF because it could promise an enhanced S without reducing σ. [1,2] Since the degree of S-enhancement strongly depends on the two-dimensionality of 2DES, a conducting material with longer λD would be efficient to enhance PF if the carrier electrons are confined within a defined thickness layer.
Recently, we found that with increasing x in SrTi1−xNbxO3, carrier effective mass (m*) exerts a reducing tendency from 1.1me to 0.7me, when x increases across x = 0.3 point. So 2DES of 1 u.c. layer thick SrTi1−xNbxO3 (x > 0.3) is hypothesized to exhibit greatly enhanced S due to its longer λD and correspondingly stronger two-dimensionality.
Here we report the TE properties of oxide 2DESs, [N unit cells SrTi1−xNbxO3|11 unit cells SrTiO3]10 superlattices (1 ≤ N ≤ 12, x=0.2−0.9), in which the λD of x > 0.3 is ~5.2 nm while that of x ≤ 0.3 is ~4.1 nm. The S-enhancement factor (Sobsd./Sbulk) of the 2DES for x=0.8 was ~1000%, while that for x=0.2 and 0.3 were 400−500%, clearly indicating that two-dimensionality can be enhanced by using a conducting material with longer λD. As a result of precise control of N and x, PF of the superlattice (N=1, x=0.6) exceeded ~5 mW m−1 K−2, which is double of the optimized bulk SrTi1−xNbxO3 (PF~2.5 mW m−1 K−2). The present results might be fruitful to design efficient TE materials with 2DES.
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