Ti and Zr doped MgB2 bulk superconductors

We report on a thorough characterisation of MgB2 bulk superconductors doped with 10 at% Ti (Tc = 36.54 K) or 10 at% Zr (Tc = 37.10 K). The upper critical field was determined resistively. It extrapolates to 28 T at 0 K in both cases and is, therefore, considerably larger than for pure MgB2 (∼20 T at 0 K). The irreversibility lines were also determined resistively (offset of the resistive transition). The irreversibility fields are much higher than in pure MgB2 and reach, e.g., 15 T at 7.5 K in MgB2 (10 at% Ti). The critical current densities were determined from ac measurements employing the flux profile technique. They are enhanced by a factor of 2 and more at 20 K compared to pure MgB2. Finally, we report on results obtained after neutron irradiation of the samples, which show further significant improvements of their transport properties, especially at low temperatures and high magnetic fields