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تسجيل مستخدم جديدCritical Current Density and Resistivity of MgB2 Films
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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.
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In this paper, we analyze the upper critical field of four MgB2 thin films, with different resistivity (between 5 to 50 mWcm) and critical temperature (between 29.5 to 38.8 K), measured up to 28 Tesla. In the perpendicular direction the critical fiel
ds vary from 13 to 24 T and we can estimate 42-57 T range in other direction. We observe linear temperature dependence even at low temperatures without saturation, in contrast to BCS theory. Considering the multiband nature of the superconductivity in MgB2, we conclude that two different scattering mechanisms influence separately resistivity and critical field. In this framework, resistivity values have been calculated from Hc2(T) curves and compared with the measured ones.
A relatively high critical temperature, Tc, approaching 40 K, places the recently-discovered superconductor magnesium diboride (MgB2) intermediate between the families of low- and copper-oxide-based high-temperature superconductors (HTS). Supercurren
t flow in MgB2 is unhindered by grain boundaries, unlike the HTS materials. Thus, long polycrystalline MgB2 conductors may be easier to fabricate, and so could fill a potentially important niche of applications in the 20 to 30 K temperature range. However, one disadvantage of MgB2 is that in bulk material the critical current density, Jc, appears to drop more rapidly with increasing magnetic field than it does in the HTS phases. The magnitude and field dependence of Jc are related to the presence of structural defects that can pin the quantised magnetic vortices that permeate the material, and prevent them from moving under the action of the Lorentz force. Vortex studies suggest that it is the paucity of suitable defects in MgB2 that causes the rapid decay of Jc with field. Here we show that modest levels of atomic disorder, induced by proton irradiation, enhance the pinning, and so increase Jc significantly at high fields. We anticipate that chemical doping or mechanical processing should be capable of generating similar levels of disorder, and so achieve technologically-attractive performance in MgB2 by economically-viable routes.
Ex-situ Powder-In-Tube MgB2 tapes prepared with ball-milled, undoped powders showed a strong enhancement of the irreversibility field H*, the upper critical field Hc2 and the critical current density Jc(H) together with the suppression of the anisotr
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