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Highlights: [•] We notice that when MgB2 is doped so as to increase its Birr and Bc2 , the low-field Jc is decreased. [•] We propose that this phenomenon is intrinsic to superconductors with a large variability in Bc2 . [•] Using the surface pinning function, we have shown that increases in Bc2 will decrease low-B Jc . [•] For an application where low-B Jc needs to be maximized, doping will be deleterious. [•] Rather, increases in connectivity should be explored. [Copyright &y& Elsevier]

Abstract: The critical current densities (Jc ) and stabilities of Tube Type Nb3Sn conductors have been measured. The strands had superconducting subelement counts ranging from 192 to 744, and flat-to-flat filament sizes (for 0.7mm OD wire) of from 35μm down to 15μm. These Tube Type conductors had a very simple structure: prior to heat treatment the filaments consist of a Sn core surrounded by a thin Cu tube, itself surrounded by a Nb or Nb alloy tube. Eight different strand types were investigated using various techniques including SEM, residual resistance ratio (RRR), transport Jc , and stability measurement. Most strands were studied at 0.7mm OD, with one representative at 0.42mm. The transport measurements were made at 4.2K in fields up to 14T. Numerous heat treatment schedules were investigated, with reaction temperatures ranging from 615°C to 650°C, and times ranging from 36–500h. The highest Jc s were seen for the lowest reaction temperatures, with 12 T transport Jc values as high as 2450 A/mm2 observed. The RRRs were lower for longer time and higher temperature reactions and ranged from 4 to 180. Strand stability was a strong function of the effective filament diameter, deff, and RRR. The most stable strands showed stability currents, Js, of 8700A/mm2 and 15,300A/mm2 for 0.7mm OD and 0.42mm OD conductors, respectively. [Copyright &y& Elsevier]

Abstract: In a continuation of a series of reports on the fabrication and properties of several demonstration coils and solenoids wound with MgB2 strand this paper describes the winding, testing, and load-line comparisons for a racetrack coil (R-4) and two solenoids (S-1 and S-2). Racetrack R-4 was wound with 19-filament strand, S-1 and S-2 with 7-filament strand. The filaments were clad in Nb-lined Cu and the restack bundle was enclosed in a monel sheath; some Cu stabilization was included. Transport J c measurements were performed initially in liquid He, and then as a function of temperature up to 30K. Racetrack R-4 achieved 240A in self-field at 7.7K corresponding to a J c of 240kA/cm2, leading to a strand J e of 43kA/cm2, and a winding J e,w of 22kA/cm2. At 4K, S-1 (310m of strand) achieved an I c of 138A and a bore field of 3.4T; S-2 (742m of strand) an I c of 85A and a bore field of 3.9T. The magnetic field profiles for all coils were calculated using a finite element approach in conjunction with the Biot–Savart equation. The windings were treated as a series of small segments, each tangential to the winding, numerical errors were estimated. These calculations were used to generate load-line comparisons for the coils. [Copyright &y& Elsevier]

Abstract: Improvements in B c2, B irr, and high field transport J cs are crucial for many MgB2 applications. As transport J c is inextricably linked with microstructure, a knowledge of the detailed influence of two of the most practical variables – Mg/B ratios and SiC additions – on microstructure, B c2/B irr, and high field J c, is of interest. In this work, the influence of Mg/B ratios on microstructure, B irr, and transport J c was investigated in MgB2 strands with and without SiC doping. The binary compositions chosen were Mg x B2 where x =0.85, 0.90, 1.0, 1.10, and 1.15 in monofilamentary strands. In general, high Mg molar percentages were seen to increase transport J c, especially at higher fields. At 8 T, for example, heat treatments at 700°C for 40min yielded J cs at 4.2K which ranged from 5.8×103 A/cm2 for x =0.85 to 2.73×104 A/cm2 at x =1.15. Microstructures evolved from powdery (B-rich) to web-like (Mg-rich). In a second set of samples, Mg-rich stoichiometries were investigated with and without SiC. High field transport J cs increased, and the microstructure appeared to densify. Both excess Mg and SiC doping significantly increased high field J c. The highest transport result was seen for SiC dopant (5mol%) added to 15% excess Mg strands, which yielded a transport J c at 4.2K and 8T of 5×104 A/cm2. Variable temperature analysis was performed on the 15% excess Mg strands with and without SiC in fields of 0–15T. At temperatures below 20K, excess Mg+SiC doping performed best, while above 20K, excess Mg without SiC had better transport properties. [Copyright &y& Elsevier]

Magnetization, AC loss, χDC and deff were measured for several designs of rod-in-tube based internal-Sn Nb3Sn type superconducting multifilamentary strands. Two kinds of subelement geometries were used in strand construction. The first had the standard annular Nb/Cu ring surrounding a Sn source; the second was similar but included an internal split intended to reduce magnetization and loss. Strands with 18 and 36 subelements were measured, at strand diameters of 0.5–0.8 mm. Optical, SEM, and EDS measurements were performed on these samples; average radii are reported and physical barrier integrity is found to be good. The magnetizations of these structures were analyzed in terms of a deff parameter, in this case calculated for annular structures. Analytical and numerical results of these calculations are presented. It was found that in general annular structures should be expected to have deff values somewhat larger than the subelement diameter; the value of this enhancement is reported. Also, the effect of subelement splitting on deff and magnetization was calculated. The results of these calculations are compared to the experimentally measured results. Reductions in deff due to subelement splitting are compared to direct, low-field susceptibility measurements. Magnetization values are seen to be nearly uniformly lower in the split subelement strands, and this leads in some but not all cases to significantly lower deff values. Possible reasons for these discrepancies are discussed. [Copyright &y& Elsevier]

Various MgB2/sheath composite strands have been tested using transport and magnetic measurement techniques. The strands had bimetallic sheaths of (1) Fe/monel, (2) Fe/Cu and (3) Cu/Cu. In strands (1) and (2) the Fe played the role of diffusion barrier. Transport current densities of more than 5 × 105 A/cm2 at 4.2 K in self-field and 2.7 × 104 A/cm2 at 5 T were obtained on short lengths of Fe/monel strand. In coils, 6 × 104 A/cm2 in 1 T at 4.2 K for Cu/Cu sheath strands and 2.2 × 105 for Fe/monel sheath strands have been measured. The strands had diameters of 0.8–1.1 mm and superconducting fractions ranging from 27% to 40% of the total wire cross-section. They were heat-treated (HT) at temperatures of from 675 to 900 °C under various heat-treatment schedules. The best results were obtained at 675 °C for Cu/Cu and 700 °C for Fe/monel strands. [Copyright &y& Elsevier]

Vibrating sample magnetization (VSM) measurements have been made on round Fe-clad monocore MgB2 wire fabricated by a powder-in-Fe-tube technique. Magnetization, AC loss, and transport measurements are reported. The M–H and loss measurements were made with a field sweep amplitude of 17 kOe over a temperature range of 4–40 K. In a round monocore wire the superconducting core was completely shielded against an external field of ±2 kOe; at higher fields a fixed ΔH was observed (partial shielding). It was found that the signal picked up by the VSM was itself also partially shielded, i.e. reduced in magnitude during its ‘return passage’ through the sheath, leading to a low field dip in the magnetization. FEM calculations of this latter effect are presented. The fact that the field which reaches the inside of the conductor is less than the external field leads to a real loss suppression, even at moderate fields. The low field dip caused by a suppression of the signal emanating from the superconducting core leads to an apparent loss suppression, i.e., the magnetically measured loss is less than the real (e.g., calorimetric) loss. The difference between real and apparent loss is estimated by FEM calculation, and the field sweep amplitude dependence of these two components was computed neglecting the effects of partial flux penetration. [Copyright &y& Elsevier]

Vibrating sample magnetization (VSM) measurements have been made on Fe-clad monocore and multifilamentary MgB2 wire fabricated by the powder-in-Fe-tube technique. The measurements were made with a field sweep amplitude of 17 kOe over a temperature range of 4–40 K. Although the benefits of magnetic shielding to both high-Tc and low-Tc superconductors (SC) have been listed, and in the former case demonstrated, its applicability to the intermediate-Tc material MgB2 is new. Since Fe or low-alloy steels seem necessary primary cladding (sheath) materials for powder-in-tube MgB2 both from mechanical- and chemical-compatibility standpoints, the resulting strands automatically become ideal candidates upon which to explore and exploit the principles of magnetic shielding. The effectiveness of the shield depends on its thickness, relative permeability (and hence strength of the applied field) and shape-dependent demagnetization. In the round monocore wire the SC core was completely shielded against an external field of ±2 kOe; at higher fields a fixed ΔH was observed (partial shielding). Finally, in considering AC loss in the partially shielded regime, allowance must be made for the fact that the signal picked up by the VSM is itself also partially shielded, i.e. reduced in magnitude during its “return passage” through the sheath. FEM calculations of this latter effect are presented. [Copyright &y& Elsevier]

Discusses the fabrication of a multifilamentary high temperature superconducting bismuth (Bi):2212/silver strand using the powder-in-tube process and heat treatment (HT) in oxygen by a modified standard procedure. Tests for determining possibility of strand poisoning by the core during HT; Calorimetrically measured alternating current loss results for the cables.

Conventional magnetization measurements of proximity effect (PE) coupling in superconducting (S) multifilamentary (MF) strands have been made, and a previously developed technique has been used to quantify the coupling strength in terms of a PE coupling supercurrent, J/sub cn/. This quantity has been determined for a number of MF strand types processed under various conditions. All are NbTi/Cu MF composites in some of which the NbTi filaments were surrounded by Nb "barriers", while in the others they were not. The magnetic field dependence of J/sub cn/ was also explored. The measured differences in J/sub cn/ are attributed to differences in the properties of the filament/matrix (normal metal, N) interface treated as a S1-S2-N trilayer. These results, together with those of an earlier series of experiments confirm that PE coupling between the filament and the matrix, and hence the normal-state coherence length, /spl xi//sub N/, is enhanced by the presence of a Nb (S2) barrier; and furthermore that the field-dependent coupling strength, gauged in terms of J/sub cn/ or /spl xi//sub N/, depends on the thickness of that barrier. It is demonstrated that magnetic PE-coupling studies not only provide a technique for extracting /spl xi//sub N/, a quantity central to artificial pinning center (APC) design, but also may help in the investigation of S-N boundary conditions in the clean limit. [ABSTRACT FROM PUBLISHER]

Non-stoichiometric compounds may develop a composition gradient when they are formed by reactive diffusional processes. This paper reports an interesting phenomenon that in compounds with low bulk diffusivities, which rely mainly on grain boundary diffusion for their growth, the final bulk compositions may be far from equilibrium, with a very low bulk diffusivity leading to fixed (persistent) compositions – one such example is Nb 3 Sn, a superconductor. We investigated the microchemistry at the reactive interface using atom probe tomography to clarify the diffusion reaction mechanism for this low-bulk-diffusivity case and thus propose a theory for what determines the compound composition, using Nb 3 Sn as an example for concreteness. Using certain approximations, we derive an explicit analytical equation that illustrates what factors determine its composition profile. We compare our model with the known facts of Nb 3 Sn and see good agreement. In particular, this model predicts that internal oxidation may lead to higher Sn contents than conventional, non-oxidized Nb 3 Sn. Our measurements show that this is indeed true, and that the internally-oxidized Nb 3 Sn also has higher upper critical fields, achieving up to 28.2 T at 4.2 K. We discuss the general applicability of this model to non-stoichiometric compounds with low bulk diffusivity, and propose it as a tool to help in the design and processing of such materials for compositional control. Image 1 • Compounds with low bulk diffusivity have fixed compositions far from equilibrium. • We use atom probe tomography to clarify the diffusion reaction mechanism for them. • We derive an explicit analytical equation to show what determines the composition. • The model agrees well with facts for Nb 3 Sn superconductor, with predictive power. • Internally-oxidized Nb 3 Sn has high Sn content and superior high-field performance. [ABSTRACT FROM AUTHOR]