Rigid vortices in MgB2

Magnetic relaxation of high-pressure synthesized MgB$_2$ bulks with different thickness is investigated. It is found that the superconducting dia-magnetic moment depends on time in a logarithmic way; the flux-creep activation energy decreases linearly with the current density (as expected by Kim-Anderson model); and the activation energy increases linearly with the thickness of sample when it is thinner than about 1 mm. These features suggest that the vortices in the MgB$_2$ are rather rigid, and the pinning and creep can be well described by Kim-Anderson model.Comment: Typo corrected & reference adde

Efficiency of electrochemical chloride removal from concrete at different environmental temperatures

Electrochemical chloride removal (ECR) is an effective and curative method to treat existed reinforced concrete structures about to suffer or already suffering from chloride attack, however, its application is still limited due to its side effect and efficiency, including the velocity and maximum capacity of chloride removal. This paper presents a temperature related numerical transport model to study the effect of temperature on efficiency of electrochemical chloride removal from concrete. Based on Fick’s law and Nernst-Planck equation with Gauss’ Law, temperature effect, chloride binding, multi-species coupling, electrochemical reactions were taken into account in this model. Temperature effect was considered on diffusion coefficient, chloride binding, ions migration capacity as well as electrolyte concentration. The model was validated by the comparison between the calculated results and experimental data. The results indicate that temperature dose have a considerable influence on electrochemical chloride removal and controlling temperature during treatment is a practical method to improve the electrochemical chloride removal when applied current density is not amplified

catena-Poly[[bis­(nitrato-κ2 O,O′)cobalt(II)]-μ-4,4′-bis­(pyrazol-1-ylmeth­yl)biphenyl-κ2 N 2:N 2′]

In the title compound, [Co(NO3)2(C20H18N4)]n, the CoII atom lies on a crystallographic twofold axis and the coordination geometry can be considered as a slightly distorted tetra­hedron defined by two O atoms from two nitrate groups and two N atoms from two ligand mol­ecules. A distorted octa­hedron may be assumed when two of the symmetry-related nitrate O atoms with Co—O distances of 2.3449 (19) Å are added to the coordination environment. Another twofold axis, passing through the middle of the biphenyl bonds, is observed in the crystal structure. A chain is built up by the ligands linking the CoII ions along [101]

Current-voltage characteristics of NdFeAsO0.85F0.15 and NdFeAsO0.85 superconductors

The vortex phase diagrams of NdFeAsO0.85F0.15 and NdFeAsO0.85 superconductors are determined from the analysis of resistivity and current-voltage (I-V) measurements in magnetic fields up to 9 T. A clear vortex glass to liquid transition is identified only in the oxygen deficient NdFeAsO0.85, in which I-V curves can be well scaled onto liquid and glass branches consistent with the vortex glass theory. With increasing magnetic field, the activation energy U0, deduced from the Arrhenius plots of resistivity based on the thermally activated flux-flow model (TAFF), decays more quickly for NdFeAsO0.85F0.15 than for NdFeAsO0.85. Moreover, the irreversibility field Hirr of NdFeAsO0.85 increases more rapidly than that of NdFeAsO0.85F0.15 with decreasing temperature. These observations evidence the strong vortex pinning effects, presumably caused by the enhanced defects and disorders in the oxygen deficient NdFeAsO0.85. It is inferred that the enhanced defects and disorder can be also responsible for the vortex glass to liquid transition in the NdFeAsO0.85.Comment: 19 pages, 5 figure