Phase-transition toughening of high-Tc superconducting ceramics

The effect of the superconducting transition (SCT) on the plane-strain fracture toughness K1c of YBCO and BSCCO is studied by the method of three-point bend loading at room temperature and at 77 K. Toughening is observed and related to variation of the Gibbs free energy through the SCT, and hence to the condensation energy of a Cooper-pair gas. This is explained by the additional energy needed to excite superconducting electrons to normal ones in crack surfaces owing to magnetic field penetration. The superconducting gap parameters of YBCO and BSCCO are estimated and the results are reasonable.