Enhancement of B and B in bulk MgB₂ superconductors with SnO₂ Additions

Three (MgB₂)_1- (SnO₂) samples with x ranging from 0 to 5 wt% were prepared by the in situ route to study the effect of tin dioxide additions on the superconducting properties of MgB₂ bulk materials. All of the reacted samples were slightly Mg deficient although the starting Mg:B precursor powder ratio was 1:2. A heat treatment (HT) temperature of 700 °C with a dwell time of 30 min was used. XRD results showed evidence of peak shifts for MgB₂ phases with SnO₂ addition. The magnitude of the a-axis lattice constant change (0.361 ± 0.075 %) calculated for the 3 wt% doped samples is comparable in magnitude to that seen previously for the C-doped MgB₂ bulks which exhibited enhanced B . The upper critical fields (B ) and the irreversibility fields (B ) were measured resistively in fields up to 14 T at 5 K to T . The best B value at 20 K (15.2 T based on extrapolation) was seen for sample IS3 (x = 3 wt%), and was comparable to the best B values (≈ 15 T at 20 K) seen for C-doped MgB₂ bulks. IS3 had a corresponding B = 10.8 T (20 K). The superconducting transition temperature (T ) appeared to increase slightly with doping, although within the range of error bars (37.4 K to 37.6 K for 1.6 T B increase at 20 K), in contrast to C doping which is accompanied by a significant decrease in T (39 K to 36 K for 3.8 % C doped MgB₂ bulk). We attribute the observed increase in both B and B for SnO₂-additions to lattice strain caused by the introduction of precipitates within the grains.