Cryogenic bathysphere for rapid variable-temperature characterization of high-T c superconductors

A bathysphere consisting of an inverted Dewar flask for submersible operation in cryogenic fluids is used to measure the resistance of superconductors, including high-Tc superconducting copper oxides, as a function of temperature from 4 to 300 K. We describe the cryostat incorporating the bathysphere and present data on NbTi (44% Ti) and YBa2Cu3O7−δ with respective superconducting transitions temperatures of 9.5 and 91.5 K. There are several advantages of the bathysphere method. The cryostat is of simple, compact design easily adapted to high-field applications where magnet bore size is a limiting factor. The sample and thermometer are thermolyzed in the dry vapor trapped at the top of the bathysphere. Temperature can be varied rapidly from 300 to 4 K at a rate of 1 K min with less than a 0.1 K thermal lag between the sample and thermometer.