  The temperature dependence of the upper critical field, Hc2, for both field
directions in partially substituted polycrystalline MgB2 was determined.
Whereas the suppression of Tc is similar for aluminum and carbon substituted
samples, Hc2 is affected by the substitution in profoundly different ways. In
the case of Al substitution changes can tentatively be described by intrinsic
effects (shift of the Fermi level). In the C substituted samples, Hc2 is
increased drastically, and extrinsic effects (disorder) have to play a major
role. The strong contrast between the two substitutions is discussed, taking
into account three relevant scattering rates.
