Experiments concerning the connective nature of superconductivity in YBa2Cu3O7

In order to understand why the inter-and intra-granular current densities of polycrystalline superconducting oxypnictides differ by three orders of magnitude, we have conducted combined magneto-optical and microstructural examinations of representative randomly oriented polycrystalline Nd and Sm single-layer oxypnictides. Magneto optical images show that the highest J c values are observed within single grains oriented with their c axes perpendicular to the observation plane, implying that the intragranular current is anisotropic. The much lower intergranular J c is at least partially due to many extrinsic factors, because cracks and a ubiquitous wetting As-Fe phase are found at many grain boundaries. However, some grain boundaries are structurally clean under high resolution transmission electron microscopy examination. Because the whole-sample global J c (

Section: Resultsmentioning

confidence: 99%

Combined microstructural and magneto-optical study of current flow in polycrystalline forms of Nd and Sm Fe-oxypnictides

Kametani

Polyanskii

Yamamoto

et al. 2008

“…However, it was very early established that grain boundaries were not intrinsic obstacles to current flow in MgB 2 [39]. Of greater interest is the comparison to the least anisotropic of the cuprates, those with the RE-123 structure, randomly oriented polycrystalline examples of which have global J c values of only ~100 A/cm 2 at 4 K [15,16]. Thus it may be that any intrinsic weak-link effect at oxypnictide grain boundaries is less serious than in the cuprates.…”

Section: Discussionmentioning

confidence: 99%

“…Based on the rather high upper critical field B c2 (0) values of 63-65 T observed by Hunte et al [6] on the same sample and the nanoscale coherence length ξ (ξ ab (0) = 5 nm, ξ c (0) = 1.2 nm), we would expect strong vortex pinning even from naturally occurring atomic-scale defects. By analogy to randomly oriented polycrystalline cuprates which also show small hysteretic current loops and large intragrain current densities of 10 5 -10 6 A/cm 2 at 4 K [15,16], we proposed [12] that electromagnetic granularity was likely to be characteristic of polycrystalline oxypnictides. Evidence for granular behavior were also presented in two subsequent studies of a Sm oxypnictide (Senatore et al [13]) and a Nd oxypnictide (Prozorov et al [14]), the latter of which also presented magneto optical images of confined current flow within intragranular regions.…”

Section: Introductionmentioning

confidence: 99%

Evidence for two distinct scales of current flow in polycrystalline Sm and Nd iron oxypnictides

Yamamoto¹,

Polyanskii²,

Jiang³

et al. 2008

127

177

Early studies have found quasi-reversible magnetization curves in polycrystalline bulk rare-earth iron oxypnictides that suggest either wide-spread obstacles to intergranular current or very weak vortex pinning. In the present study of polycrystalline samarium and neodymium rare-earth iron oxypnictide samples made by high pressure synthesis, the hysteretic magnetization is significantly enhanced. Magneto optical imaging and study of the field dependence of the remanent magnetization as a function of particle size both show that global currents over the whole sample do exist but that the intergranular and intragranular current densities have distinctively different temperature dependences and differ in magnitude by about 1000. Assuming that the highest current density loops are restricted to circulation only within grains leads to values of ~5×10 6 A/cm 2 at 5 K and self field, while whole-sample current densities, though two orders of magnitude lower are 1000-10000 A/cm 2 , some two orders of magnitude higher than in random polycrystalline cuprates. We cannot yet be certain whether this large difference in global and intragrain current density is intrinsic to the oxypnictides or due to extrinsic barriers to current flow, because the samples contain significant second phase, some of which wets the grain boundaries and produces evidences of SNS proximity effect in the whole sample critical current. 2 Introduction:The recent discovery of superconductivity in the LaFeAsO 1-x F x compound [1] has stimulated a rapid exploration of superconductivity in the rare earth iron oxypnictides [2][3][4][5][6][7][8][9][10][11][12][13][14]. It has now been established that the iron oxypnictides can be superconducting when doped to x ~0.05-0.2 and that they can have transition temperature T c above 40 K when La is replaced by Ce [5] and above 50 K by Pr, Nd, Sm and Gd [7][8][9][10][11]. In a recent paper [12] we addressed the issue of electromagnetic granularity in polycrystalline La iron oxypnictides, finding an asymmetric M(H) loop that indicated an irreversible moment due to hysteretic bulk currents that was almost as small as the reversible magnetization of the superconducting state. In that case we were not able to distinguish definitively between a state where the intragrain pinning was very weak, leading to very low intragrain current densities or to the state where currents were largely confined to the intragrain regions and might have been rather high. Based on the rather high upper critical field B c2 (0) values of 63-65 T observed by Hunte et al.

“…As mentioned above, the first Sm-(or La-)1111 and Sr-122 superconducting wires [13][14][15] had magnetic global J c values of ~4000 A/cm 2 at 5 K, some ten times higher than in random polycrystalline cuprates, which are typically 100 A/cm 2 due to large intrinsic weak-linked effects [35,42]. Thus it may suggest that the weak-linked effect at pnictide grain boundaries is less serious than in the cuprates.…”

Section: The Use Of Silver As the Sheath Materialsmentioning

confidence: 99%

Progress in wire fabrication of iron-based superconductors

Ma¹

2012

220

192

Abstract:Iron-based superconductors, with T c values up to 55 K, are of great interest for applications, due to their lower anisotropies and ultrahigh upper critical fields. In the past 4 years, great progress has been made in the fabrication of iron-based superconducting wires and tapes using the powder-in-tube (PIT) processing method, including main three types of 122, 11, and 1111 iron-based parent compounds. In this article, an overview of the current state of development of iron-based superconducting wires and tapes is presented. We focus on the fabrication techniques used for 122 pnictide wires and tapes, with an emphasis on their meeting the critical current requirements for making high-performance conductors, such as a combination of using Ag sheath, addition element and optimized heat treatment to realize high J c , ex situ process employed to reduce non-superconducting phases and to obtain a high relative density, and a texture control to improve grain connectivity. Of particular interest is that so far transport J c values above 10 4 A/cm 2 at 4.2 K and 10 T are obtained in 122 type tapes, suggesting that they are prospective candidates for high-field applications. Finally, a perspective and future development of PIT pnictide wires are also given.*