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Register a new userAtmospheric conditions and their effect on ball-milled magnesium diboride
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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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We have studied the effect of damage induced by 2-MeV alpha particles on the critical temperature, Tc, and resistivity of MgB2 thin films. This technique allows defects to be controllably introduced into MgB2 in small successive steps. Tc decreases linearly as the intragrain resistivity at 40 K increases, while the intergrain connectivity is not changed. Tc is ultimately reduced to less than 7 K and we see no evidence for a saturation of Tc at about 20 K, contrary to the predictions of the Tc of MgB2 in the dirty limit of interband scattering.
We present results on all-MgB2 tunnel junctions, where the tunnel barrier is deposited MgO or native-oxide of base electrode. For the junctions with MgO, the hysteretic I-V curve resembles a conventional underdamped Josephson junction characteristic with critical current-resistance product nearly independent of the junction area. The dependence of the critical current with temperature up to 20 K agrees with the [Ambegaokar and Baratoff, Phys. Rev. Lett. 10, 486 (1963)] expression. For the junctions with native-oxide, conductance at low bias exhibits subgap features while at high bias reveals thick barriers. As a result no supercurrent was observed in the latter, despite the presence of superconducting-gaps to over 30 K.
We report on the microwave (mw) properties of coaxial cavities built by using bulk MgB2 superconductor prepared by reactive liquid Mg infiltration technology. We have assembled a homogeneous cavity, by using an outer MgB2 cylinder and an inner MgB2 rod, and a hybrid cavity by using an outer copper cylinder and the same MgB2 rod as inner conductor. By the analysis of the resonance curves, in the different resonant modes, we have determined the microwave surface resistance, Rs, of the MgB2 materials as a function of the temperature and the frequency, in the absence of DC magnetic fields. At T = 4.2 K and f = 2.5 GHz, by a mw pulsed technique, we have determined the quality factor of the homogeneous cavity as a function of the input power up to a maximum level of about 40 dBm (corresponding to a maximum peak magnetic field of about 100 Oe). Contrary to what occurs in many films, Rs of the MgB2 material used does not exhibit visible variations up to an input power level of about 10 dBm and varies less than a factor of 2 on further increasing the input power of 30 dB.
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.
High energy milling of MgB2 pre-reacted powder renders the material largely amorphous through extreme mechanical deformation and is suitable for mechanically alloying MgB2 with dopants including carbon. Bulk samples of milled C and MgB2 powders subjected to hot isostatic pressing and Mg vapor annealing have achieved critical fields in excess of 32T and critical current density approaching 10^6 A/cm^2.
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