Evolution of the diamagnetic magnetization, Mdia, critical current density and irreversibility field, Hirr, of MgB2 was investigated by changing the water-exposure time. The evolution was found to follow a universal exponential law of AN(tR) = AN(0) exp(-tR/tN), where AN represented Mdia, JC and Hirr for N = 1, 2 and 3, respectively, and tN was the corresponding time constant. When Mdia showed the most rapid decay with exposure time, Hirr exhibited the slowest degradation, with a difference between the decay-time constants of more than 2 orders of magnitude. In the case of Ti-doped MgB2, the exponential decay law could be still applied, but with a much longer decay-time constant. Microstructure analyses revealed that the grain boundaries of MgB2 were corroded by water when the exposing time was long enough. However, Ti doping significantly enhances the resistance to the corrosion of water.

Dynamical Characteristics of the Degradation of Superconducting Properties in Undoped and Ti-Doped MgB2 by Exposing to Water. C.H.Cheng, Y.Zhao, Y.Feng, X.T.Zhu, N.Koshizuka, M.Murakami: Superconductor Science and Technology, 2003, 16, 125-9