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Register a new userResistivity and Magneto-Optical Evidence for Variable and Incomplete Connectivity in Dense, High Critical Current Density C-alloyed Magnesium Diboride
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Carbon-doped magnesium diboride was fabricated from pre-reacted pure MgB2 by mechanical alloying. The sample set had excellent critical current densities Jc(8T,4.2K) ranging from 15-60 kA/cm2, depending on composition. Magneto-optical imaging detected regions up to 0.5 mm in size which were nearly 100% dense with Jc 2-6 times that of the matrix. Evaluation of resistivity curves using the Rowell method predicts that only 10-50% of the cross sectional area carries the normal state measurement current, suggesting that considerable increases in Jc in these ~80% dense MgB2 samples would be possible with complete grain and particle connectivity.
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Magnesium diboride (MgB2) powder was mechanically alloyed by high energy ball milling with C to a composition of Mg(B0.95C0.05)2 and then sintered at 1000 C in a hot isostatic press. Milling times varied from 1 minute to 3000 minutes. Full C incorporation required only 30-60 min of milling. Grain size of sintered samples decreased with increased milling time to less than 30 nm for 20-50 hrs of milling. Milling had a weak detrimental effect on connectivity. Strong irreversibility field (H*) increase (from 13.3 T to 17.2 T at 4.2 K) due to increased milling time was observed and correlated linearly with inverse grain size (1/d). As a result, high field Jc benefited greatly from lengthy powder milling. Jc(8 T, 4.2 K) peaked at > 80,000 A/cm2 with 1200 min of milling compared with only ~ 26,000 A/cm2 for 60 min of milling. This non-compositional performance increase is attributed to grain refinement of the unsintered powder by milling, and to the probable suppression of grain growth by milling-induced MgO nano-dispersions.
The high resistivity of many bulk and film samples of MgB2 is most readily explained by the suggestion that only a fraction of the cross-sectional area of the samples is effectively carrying current. Hence the supercurrent (Jc) in such samples will be limited by the same area factor, arising for example from porosity or from insulating oxides present at the grain boundaries. We suggest that a correlation should exist, Jc ~ 1/{Rho(300K) - Rho(50K)}, where Rho(300K) - Rho(50K) is the change in the apparent resistivity from 300 K to 50 K. We report measurements of Rho(T) and Jc for a number of films made by hybrid physical-chemical vapor deposition which demonstrate this correlation, although the reduced effective area argument alone is not sufficient. We suggest that this argument can also apply to many polycrystalline bulk and wire samples of MgB2.
The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. With twice the critical temperature of Nb_3Sn and four times that of Nb-Ti alloy, MgB_2 has the potential to reach much higher fields and current densities than either of these technological superconductors. A vital prerequisite, strongly linked current flow, has already been demonstrated even at this early stage. One possible drawback is the observation that the field at which superconductivity is destroyed is modest. Further, the field which limits the range of practical applications, the irreversibility field H*(T), is ~7 T at liquid helium temperature (4.2 K), significantly lower than ~10 T for Nb-Ti and ~20 T for Nb_3Sn. Here we show that MgB_2 thin films can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding H*(4.2 K) above 14 T. In addition, very high critical current densities at 4.2 K, 1 MA/cm_2 at 1 T and 10_5 A/cm_2 at 10 T, are possible. These data demonstrate that MgB_2 has credible potential for high-field superconducting applications.
We study the effect of 100 MeV Silicon and 200 MeV Gold ion irradiation on the inter and intra grain properties of superconducting thin films of Magnesium Diboride. Substantial decrease in inter-grain connectivity is observed, depending on irradiation dose and type of ions used. We establish that modification of sigma band scattering mechanism, and consequently the upper critical field and anisotropy, depends on the size and directional properties of the extrinsic defects. Post heavy ion irradiation, the upper critical field shows enhancement at a defect density that is five orders of magnitude less compared to neutron irradiation. The critical current density however is best improved through light ion irradiation.
Magnesium diboride bulk pellets were fabricated from pre-reacted MgB2 powder ball milled with different amounts of exposure to air. Evidence of increased electron scattering including increased resistivity, depressed Tc, and enhanced Hc2 of the milled and heat treated samples were observed as a result of increased contact with air. These and other data were consistent with alloying with carbon as a result of exposure to air. A less clear trend of decreased connectivity associated with air exposure was also observed. In making the case that exposure to air should be considered a doping process, these results may explain the wide varibability of undoped MgB2 properties extant in the literature.
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