FULLY AUTOMATED APPARATUS FOR THERMAL-DIFFUSIVITY MEASUREMENTS ON HTSC IN HIGH MAGNETIC-FIELD

This paper describes an apparatus to measure the thermal diffusivity of high temperature superconductors in an extended range of temperature (10-300 K) and magnetic field (up to 8 T). Heat losses are taken into account by adopting an ac technique. Effects introduced by power dissipated on the heater and thermal contacts between thermocouples and sample are considered. In fact the presence of a thermocouple relaxation time not negligible in respect to the sample relaxation time causes an error in the thermal diffusivity evaluation (up to 13% in a sample with poor thermal contact). We have estimated that, also at the higher temperatures, frequencies over 0.03 Hz keep the thermal losses low enough; in this case the poor thermal contact of the thermocouples can be a major problem. All of the experiment is computer controlled. The accuracy of the measurement, mainly determined by evaluation of the sample length, is 10%, while the sensitivity to the relative variations is much higher (0.1%).