  The effect of water exposure on MgB2 is studied by submerging an 800 nm thick
MgB2 film into deionized water at room temperature for 1 hour, 4 hours, 10
hours, and 15 hours, and by analyzing the resulting material using scanning
electron microscopy and resistance vs. temperature measurements. It is clearly
observed that Tconset of these films (obtained by an ex-situ reaction of a
e-beam evaporated boron layer) remains unchanged throughout this process,
indicating that at least a portion of the sample retains its original bulk-like
properties. The data is consistent with an interpretation in which a portion of
the exposed film - likely the region closest to the substrate - becomes
superconducting only at about 25 K. It is possible that this low-Tc region
already exists in the as-prepared film, and we observe that its Tc coincides
with that of MgB2 films obtained by annealing precursor films prepared by
pulsed laser deposition. Therefore the data presented here not only illustrate
the degradation of MgB2 in water but also shed light onto the differences and
similarities between films obtained via different routes.
