YBCO samples as a possible thermoelectric material

Abstract

We have performed a study of thermoelectric properties of polycrystalline Y Ba2Cu3O7–δ (YBCO) samples syntherized from oxides by solid state reaction. The Seebeck coefficient, (S), electrical resistivity (ϱ) and thermal conductivity (κ) of these Y-based high temperature superconductors were measured in the temperature range between 77 K and 300 K. The samples were submitted to different thermal processes at 600 °C in vacuum in order to change both the oxygen content (7–δ) and its ordering. The Seebeck coefficient was measured by differential method in which the sample is heated periodically with a frequency of 50 mHz. It is positive in whole measurement range and show a weak linear behavior with the temperature and its magnitude change from 3 × 10–5 V/K for the samples annealed during one hour to 1 × 10–3V/K for the samples annealed through 50 hours. The electrical resistivity was measured by standard four-probe method and its behavior changes from metallic to semiconductor as the annealing time is increasing. The magnitude of ϱ(T) increases when the annealing time increases from 6.9 × 10–3 Ω cm to 9.2 × 10–1 Ω cm for the samples annealed 50 hours. The thermal conductivity data was obtained by means of longitudinal heat-flow method in which the sample is heated with a frequency of 30 mHz and κ(T) was calculated from Fourier law. The accuracy of the κ(T) data was 0.1 W/mK. In all samples κ(T) is less than 4 W/mK and exhibit a weak linear behavior with the temperature, its magnitude decreases with the annealing time. The performance of thermoelectric material is determined by the dimensionless merit figure ZT, which is defined in terms of Seebeck coefficient, electrical resistivity and thermal conductivity as: ZT = S2 T/ϱκ. ZT(T) exhibit an enhancement up to reach values close to 0.6 which are comparable to those of conventional thermoelectric semiconductors. This behavior suggests that the charge carrier density and scattering mechanisms can be modify by decreasing the oxygen level present in YBCO samples, which opens the possibility to use these kind of materials as active thermo-elements below room temperature. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)