‘Strong’ grain boundaries in Ag-BSCCO(2223) tapes

Journal of Applied Physics

Soltanian

Pan

et al. 2004

Field dependence of irreversible magnetic moment ∆m was obtained from magnetic hysteresis loops for a number of bulk MgB 2 superconductors. The field dependence of ∆m exhibits inflections at two characteristic fields, H t and H i (H t < H i ). These two fields increase linearly with the logarithm of the sample size, each extrapolating to zero at a different characteristic sample size. Magneto-optical, optical and scanning electron microscopic examinations show that these extrapolated sample sizes match the sizes of the main microscopic features of the samples. The inflection at H t and the sample size dependence of ∆m and H t are associated with voids scattered through the samples, which are observed for all bulk MgB 2 samples. The voids encircle cells of MgB 2 material of tens of micrometers in size. The cells are connected by narrow bridges. The superconducting currents screening the whole of the sample have to cross these bridges and they are concentrated in them. This promotes additional superconducting screening around the cells, where the current density would otherwise be smaller than the critical current density J c . The field dependence of the currents pertinent to each of the screenings follows the stretched exponential law. However, these two screening currents decrease with H with a different rate, and H t is a field of the transition from the dominance of one of the currents in ∆m to the other. The screening that gives predominant contribution to ∆m for H < H t is confined to inside the cells, circulating on ~10 µm-scale. For H t < H < H i , the dominant screening currents circulate the whole of the sample, percolating through the bridges. If J c is calculated from the critical state model assuming only the screening around the whole of the sample, erroneous value of J c and its functional dependence on the field are obtained. This also leads to an artifact of the sample size-dependent J c . However, because these two stretched exponential contributions decrease with H with a different rate, they can be separated for most of the samples. The magnetically obtained J c for H t < H < H i is defined by the screening currents percolating around the whole of the sample, and it is in good agreement with the transport J c . There is also a third tier of superconducting screening on a micrometer scale, inside the cells. It is associated with clusters of superconducting crystals that make up the cells. However, its contribution to ∆m is negligible for H < H i , which are the fields of technological interest.

Section: A Model With Two Superconducting Screening Lengthsmentioning

confidence: 99%

Superconducting screening on different length scales in high-quality bulk MgB2 superconductor

Journal of Applied Physics

Soltanian

Pan

et al. 2004

“…This resembles the inter-and intra-grain currents in high-temperature superconductors, with a significant difference that the bridges between the cells in high quality MgB 2 are not Josephson junctions. The sample size dependence of magnetically obtained J c can be explained by a model devised for high-temperature superconductors 11 . Approximating the shape of the sample and cells by round cylinders, the irreversible magnetic moment obtained from the hysteresis loop is 11 :…”

mentioning

confidence: 99%

Effect of sample size on magnetic Jc for MgB2 superconductor

Applied Physics Letters

Soltanian

Wang

et al. 2004

A strong effect of sample size on magnetic J c (H) was observed for bulk MgB 2 when J c is obtained directly from the critical state model. Thus obtained zero-field J c (J c0 ) 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 (H irr ) defined at J c = 100 A/cm 2 increases with the sample size. The decrease of J c0 is described in terms of voids in the bulk

“…There are many types of links between the grains, with different mechanical and electrical properties [5][6][7][8][9][10][11][12][13][14]. Mechanical damage can affect the weak links versus the strong links to a different extent, which can be revealed in the change of the field dependence of I c [15].…”

Section: Resultsmentioning

confidence: 99%

Effects of mechanical deformation on critical current of Bi-2223/Ag tapes

Supercond. Sci. Technol.

Guo

Bhasale

et al. 1998

The influence of mechanical deformation on critical current has been studied, in order to assess the effect of sample handling on the current carrying capabilities of Bi-2223/Ag tapes. Transport critical current was measured at 77 K using a criterion. The effect of progressive bending of the tapes on grain connectivity and flux pinning were investigated by measuring against H. It was observed that bending can affect weak and strong links to a different degree. Damaged samples have a low , but often have a large critical current which is carried through strongly linked paths, compared to carried through weak links. The influence of tensile deformation on was also studied. was found not to degrade until the deformation of the tape approached its elastic limit.