Thermal expansion behavior of bulk Bi-based (2223) superconductors

The thermal expansion coefficient of Bi-based 2223 and 2212 ceramics has been measured using dilatometric apparatus. Different typologies of bulk superconducting samples have been tested, in particular (2223) phase fully dense hot-pressed samples both parallel and perpendicular to the direction of applied pressure, pressureless sintered samples, and finally hot-pressed (2212) phase specimen, in order to gather information on the contribution of secondary phase to thermal expansion behavior. From the observed α (thermal expansion coefficient) values, accounting for the sample orientation degree, it was possible to distinguish the contributions along the basic crystallographic directions and to determine the following for (2223) phase: α_a = 12.2 × 10^-6 °C^-1, and α_c = 20.8 × 10^-6 °C^-1 for temperatures 200-600 °C. Theoretical a values were then calculated for the different samples and successfully compared with the measured ones. As regards (2212) phase, thermal expansion coefficient was evaluated for the oriented sample, and a value for random one extrapolated; the results are very similar to that of (2223), but lower with respect to previously reported data in air atmosphere.