Dynamical characteristics of the degradation of superconducting properties in undoped and Ti-doped MgB₂by exposing to water

Evolutions of diamagnetic magnetization ( M dia ), critical current density ( J c ) and irreversibility field ( H irr ) for MgB 2 have been investigated by changing the water-exposure time. The evolutions are found to follow a universal exponential law of A N ( t R ) = A N (0) exp(− t R / t N ), where A N represents M dia , J c and H irr for N = 1, 2 and 3, respectively, and t N is the corresponding time constant. When M dia shows the most rapid decay with exposure time, H irr exhibits the slowest degradation, with a difference between the decay-time constants of more than two orders of magnitude. For the case of Ti-doped MgB 2 , the exponential decay law can be still applied, but with a much longer decay-time constant. Microstructure analyses reveal that the grain boundaries of MgB 2 are corroded by water when the exposing time is long enough. However, Ti doping significantly enhances the resistance to the corrosion of water.