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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: Ac loss measurements have been made on YBCO coated conductors in externally applied sinusoidal ac magnetic fields using the pick-up coil method. Various arrangements of YBCO coated conductor have been measured at frequencies of 50Hz, 75Hz, 100Hz, 150Hz, and 200Hz, with amplitudes ranging from 0 to 140mT. Three different sets of IBAD-based YBCO coated conductor were measured: (i) unfilamented conductors with Cu added either on one side only, or on both sides (as well as the edges), (ii) conductors with various numbers of filaments, (iii) 4mm wide conductors. All samples were 4cm long and untwisted. The total ac loss per cycle, per unit length of conductor, Q t , displayed in the format Q t /B m (in J/Tm) vs. B m where B m is the field sweep amplitude enabled separation of the eddy current (or coupling current) and hysteretic components of Q t , after which the results were analyzed and discussed. The intercept values were compared to the product of critical current and sample width. Similarly, calculated and measured values of the penetration field were compared. Additionally, the eddy current component of the loss was extracted and compared to that expected from the stabilizer and possible interfilament connections. The power loss in W/m was then compared for conductors of various width, stabilizer, and filamentation level and put in the context of applications. [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]

Various heat-treatment schedules have been applied to the optimization of Tc and Jc,m in bulk MgB2. Samples were prepared by first mixing in a ‘V’ shaped mixer and then planetary milling a stoichiometric mixture of Mg and B powders. These powders were then compacted and heat-treated at different schedules under 200 Torr of Ar. The heat-treating schedules investigated involved different time-temperatures, heating rates, and cooling rates. Magnetic critical current densities (at 20 K) of more than 2×105 A/cm2 in 10 kOe and 4×105 A/cm2 in 5 kOe were obtained. The influence of sodium silicate additions on the bulk properties has been studied. [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]

Factors affecting the measurement of critical current density (Jc) in high performance Nb3Sn strands are discussed. Sample holder design and sample mounting procedures are considered in the light of cryogenic heat flow considerations. Also discussed are: the steady state measurability of metastable strands at current densities (J) at and above their nominal Jc, and mechanisms governing their quench at sufficiently high values of J greater than Jc. [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]

The tube filling/forming powder-in-tube (PIT) technique was employed at Hyper Tech Research for the continuous processing of MgB2/Fe composite strand. Demonstration lengths of up to 70 m of 1.2 mm diameter precursor strand––an Fe-clad Mg + B elemental powder mixture––have so far been produced. Short samples of this strand, heat treated for 3–30 min/800–950 °C, have yielded transport critical current densities (Jc) of 2–3×104 A/cm2 at 4 K/4 T, 4×104 A/cm2 at 4 K/2 T, and at 4 K in self field an extrapolated Jc of at least 2×105 A/cm2. Batch-processed PIT MgB2/Fe strands and tapes were made at the University of Wollongong’s Institute for Superconducting and Electronic Materials. These were the focus of magnetization- and magnetic-shielding studies. Depending on the thickness of the Fe sheath, the superconducting core could be completely shielded at low fields (e.g. below 2 kOe). Taking this into account, the magnetic Jc is extracted, and found to be comparable to the extrapolated transport value. [Copyright &y& Elsevier]

The influences of strand twisting and bending (applied at room temperature) on the critical current densities, J c , and n -values of MgB 2 multifilamentary strands were evaluated at 4.2 K as function of applied field strength, B . Three types of MgB 2 strand were evaluated: (i) advanced internal magnesium infiltration (AIMI)-processed strands with 18 filaments (AIMI-18), (ii) powder-in-tube (PIT) strands processed using a continuous tube forming and filling (CTFF) technique with 36 filaments (PIT-36) and (iii) CTFF processed PIT strands with 54 filaments (PIT-54). Transport measurements of J c (B) and n -value at 4.2 K in fields of up to 10 T were made on: (i) PIT-54 after it was twisted (at room temperature) to twist pitch values, L p , of 10–100 mm. Transport measurements of J c (B) and n -value were performed at 4.2 K; (ii) PIT-36 and AIMI-18 after applying bending strains up to 0.6% at room temperature. PIT-54 twisted to pitches of 100 mm down to 10 mm exhibited no degradation in J c (B) and only small changes in n -value. Both the J c (B) and n -value of PIT-36 were seen to be tolerant to bending strain of up to 0.4%. On the other hand, AIMI-18 showed ±10% changes in J c (B) and significant scatter in n -value over the bending strain range of 0-0.6%. [ABSTRACT FROM AUTHOR]

Abstract: Cu sheathed polycrystalline Ba0.6K0.4Fe2As2 superconducting wire was prepared by a two-step powder-in-tube method. A pressure of 2GPa was applied to a short sample before heat treatment. Magnetization and transport measurements were performed to investigate the effects of cold high pressure densification on the microstructure and superconductivity. The cold pressed sample shows an improved self-field transport critical current density of 2.8×104 A/cm2 (I c =83A) at 4.2K, which is nearly as twice as the unpressed sample. However, both samples manifest pronounced weak-link behavior, suggesting the technique need to be further optimized. The comparison of properties between pressed and unpressed sample and related mechanism was discussed. [Copyright &y& Elsevier]

Abstract: The decomposition of malic acid (C4H6O5) in the presence of Mg and B was studied using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) which revealed that malic acid reacted with Mg but not B. Also, the addition of toluene (C7H8) to dissolve malic acid followed by subsequent drying resulted in no reaction with Mg, indicating that the malic acid had decomposed during the dissolution/drying stage. The total carbon contributed by toluene versus a toluene/5wt.% malic acid mixture was measured using a LECO CS600 carbon analyzer. The toluene sample contained ∼0.4wt.% C while the toluene/malic acid mixture had ∼1.5wt.% C, demonstrating that the toluene contributed a significant amount of carbon to the final product. Resistivity measurements on powder-in-tube MgB2 monofilamentary wires established that the toluene/malic acid doped sample had the highest Bc 2. However, the toluene-only sample had the highest transport Jc over most of the magnetic field range (0–9T), equaled only by that of toluene/malic-acid sample in fields above 9T. [Copyright &y& Elsevier]

AC losses in multi-filamentary tapes depend on various parameters. Among them, the overall tape width and thickness are expected to have an important influence. In order to study this geometrical effect, five Bi-2223/Ag tapes with different aspect ratios from 5 to 26 have been prepared. AC losses have been measured at 77 K when a perpendicular AC magnetic field is applied. It has been found that at any frequencies the magnetic loss per cycle increases as the aspect ratio increases. For AC magnetic loss, with increasing frequency from 3 to 9000 Hz the losses as a function of frequency show a maximum if the field amplitude is much less than the full penetration field or increase continuously if the field amplitude is larger. [Copyright &y& Elsevier]