Time evolution of superconducting properties of MgB₂ exposed to water

The degradation of the superconducting properties, such as the diamagnetic magnetization (M_dia), the critical current density (J_c), and the irreversibility field (H_irr) for MgB₂ exposed to water has been investigated. The time evolution of the degradation obeys an exponential law with a different decay-time constant for each of these properties. M_dia shows the most rapid decay with exposure time, whereas H_irr exhibits the slowest degradation. By doping with Ti, the degradation of these superconducting properties was significantly suppressed, although the decay time still obeyed an exponential law. The experimental results have been reasonably explained in terms of a grain-boundary-degradation model. Our study suggests that the degradation of M_dia is related to the degradation of the relatively-high-angle grain boundaries near the sample's surface, whereas the degradation of J_c is associated with the degradation of the relatively lower-angle grain boundaries. The degradation of H_irr is probably due to the reduction of the surface pinning force.