Upper critical field of cellular magnesium diboride

A cellular superconducting material consisting of thin (1–20 μm) MgB_2−x layers and magnesium granules of about 100 μm has been produced. The critical temperature T _c of this superconductor decreases with the thickness of the MgB_2−x layers. In unalloyed magnesium diboride, the curvature of the temperature dependence of the upper critical field H _c2(T) changes gradually from downward to pronounced upward as the temperature T _c decreases from 38 to 36 K.