ﻻ يوجد ملخص باللغة العربية
A strong effect of sample size on magnetic Jc(H) was observed for bulk MgB2 when Jc is obtained directly from the critical state model. Thus obtained zero-field Jc (Jc0) decreases strongly with the sample size, attaining a constant value for the samples larger than a few millimetres. On the other hand, the irreversibility field (Hirr) defined at Jc = 100 A/cm2 increases with the sample size. The decrease of Jc0 is described in terms of voids in the bulk MgB2 samples and superconducting screening around the cells of superconducting material between these voids (35 micro-m), because of concentration of the current in the narrow bridges connecting the cells. For samples larger than a few millimetres, the value of magnetic Jc is in agreement with the transport Jc and it is restricted by the voids. The critical state model is not suitable for obtaining Jc for small bulk MgB2. The increase of Hirr with the sample size is an artefact of defining Hirr by the value of Jc at which an additional superconducting screening on 1mm scale dominates Dm.
Systematic ac susceptibility measurements have been performed on a MgB$_2$ bulk sample. We demonstrate that the flux creep activation energy is a nonlinear function of the current density $U(j)propto j^{-0.2}$, indicating a nonlogarithmic relaxation
Measurements of the critical current density (Jc) by magnetization and the upper critical field (Hc2) by magnetoresistance have been performed for hafnium-doped MgB2. There has been a remarkable enhancement of Jc as compared to that by ion irradiatio
MoSi2 doped MgB2 tapes with different doping levels were prepared through the in-situ powder-in-tube method using Fe as the sheath material. Effect of MoSi2 doping on the MgB2/Fe tapes was investigated. It is found that the highest JC value was achie
Polycrystalline MgB2-xCx samples with x=0.05, 0.1, 0.2, 0.3, 0.4 nano-particle carbon powder were prepared using an in-situ reaction method under well controlled conditions to limit the extent of C substitution. The phases, lattice parameters, micros
We report on significant flux pinning enhancement in MgB2/Fe tapes that has been easily obtained by a simple and cheap route using acetone as both an efficient ball-milling medium and liquid additive through the in situ method. Results showed that th