Anisotropic thermal conductivity of YBa2Cu3O7- delta thin films

Abstract

Room-temperature thermal conductivity values were determined for sputtered c-axis oriented YBa2Cu3O7- delta thin films. These values were measured through the plane of the films (Kperpendicular to ) using a thermal comparator technique, and calculated in the plane of the films (K/sub ///) using the Wiedemann-Franz-Lorenz conversion of electrical sheet conductivity values. The films show a thermal conductivity anisotropy, with values of Kperpendicular to approximately=,0.26 W m-1 K-1 and K/sub /// approximately=2.4-4.5 W m-1 K-1. The thermal conductivities are depressed in both directions from expected values based on room-temperature measurements of single-crystal thermal conductivities. While some anisotropy is expected due to the crystallographic orientation of the films, the observed anisotropy is greater than that typically seen in single crystals of YBa2Cu3O7- delta . Possible explanations are presented based on thin film microstructure and phonon and charge carrier scattering mechanisms, but due to the short room-temperature mean free path of this material, phonon scattering mechanisms such as the size effect or scatter from stacking faults may be discounted as significant sources of thermal conductivity reduction. Instead, the formation of a second phase during film deposition is the most likely source of thermal resistance in the films.