Thermoelectric power factor of YBCO polycrystalline samples

A study of thermoelectric power factor, PF=S²/ρ, of polycrystalline YBa₂Cu₃O_7-δ (YBCO) samples synthesized from oxides by solid state reaction have been carried out. The Seebeck coefficient, S(T) and electrical resistivity, ρ(T) were measured in the temperature range between 77 K and 300 K. The oxygen present in the samples was modified by means of an annealing process at 600°C for different periods of time. The Seebeck coefficient is positive in whole measured temperature range and shows a linear behavior with the temperature, its magnitude increases from 30 μV/K for the samples annealed during one hour to 1000 μV/K for the samples annealed through 50 hours. The behavior of electrical resistivity changes from metallic to semiconducting as the annealing time is increasing. The magnitude of ρ(T) increases from 10^-3 Ω-cm to 10^-1 Ω-cm as the annealing time increases. The thermoelectric power factor reaches values around 18 μW/K²cm cm in the samples annealed for 30 hours. The behavior of the transport properties suggests that the carrier density and the scattering mechanisms can be modified by decreasing the oxygen level, which open the possibility of use these kind of materials as active thermoelements.