ABSTRACT
MgB2 superconducting wires made using a Mg infiltration method have reached a higher performance than either in-situ or ex-situ mixed powder based routes. Indeed, very high layer J c coupled with whole-strand J e (critical current per total strand cross section) exceeding 104 A cm-2 at 4.2 K, 10 T have been found for monocore MgB2 wires. However, previous multicore infiltration route wires have not reached their potential for J e due to partially reacted and non-uniform MgB2 layers. This study shows that 18-core MgB2 AIMI wires processed using a low temperature route can attain higher and more uniform J e values due to a more uniform MgB2 reaction layer. The formation of fully reacted, uniform MgB2 layers is attributed to the switch from a liquid-solid to a vapor-solid reaction route.
ABSTRACT
MgB2 superconducting wires and bulks with nano-La2O3 addition have been studied. A series of MgB2 superconducting bulk samples with nano-La2O3 addition levels of 0, 5, 7, 18wt% were prepared. AC resistivity data showed slight increases of Bc 2 and unchanged B irr for the bulk samples with doping levels lower than 7 wt% and decreased critical fields for the heavily doped (18 wt%) bulk. X-ray diffraction (XRD) showed the presence of LaB6 in the nano-La2O3 doped MgB2 bulk samples and decreased MgB2 grain size in nano-La2O3 doped bulks. Monocore powder-in-tube (PIT) MgB2 wires without and with 5 wt% nano-La2O3 addition (P-05) were prepared for transport property measurement. 2mol%C-doped Specialty Materials Inc. (SMI) boron powder was used for wire P-05 and previously prepared control wires (control wires were made without the addition of nano-La2O3 powder, W-00 and P2). Low field magnetic properties were obtained from magnetization loop (M-H), transport critical current density (J c ) was measured at 4.2 K for the nano-La2O3 doped PIT wire (P-05) and the control samples (P2 and W-00). The transport critical current density J c (B) of P-05 at 4.2 K and 8 T (6.0 ×104 A/cm2) was twice that of the control wire. The critical magnetic fields (Bc 2 and B irr ) of P-05 and the control sample P2 were compared. The critical fields of P-05 were slightly less than those of P2. Kramer-Dew-Hughes plots indicated a change from surface pinning to a mixture of volume pinning and surface pinning. It is shown that enhancement of P-05's transport properties is due to additional flux pinning by the fine-size rare-earth borides rather than enhanced Bc 2 or B irr .
ABSTRACT
Three (MgB2)1-x (SnO2) x 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 C2 . The upper critical fields (B C2 ) and the irreversibility fields (B irr ) were measured resistively in fields up to 14 T at 5 K to T c . The best B C2 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 C2 values (≈ 15 T at 20 K) seen for C-doped MgB2 bulks. IS3 had a corresponding B irr = 10.8 T (20 K). The superconducting transition temperature (T c ) 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 C2 increase at 20 K), in contrast to C doping which is accompanied by a significant decrease in T c (39 K to 36 K for 3.8 % C doped MgB2 bulk). We attribute the observed increase in both B C2 and B irr for SnO2-additions to lattice strain caused by the introduction of precipitates within the grains.
