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Pure and doped MgB 2 thick films were fabricated on Ni substrates by applying a coating mixture of powders of elemental magnesium and boron with varying amounts of elemental Cu and nano-SiC powders, followed by a high pressure press, and sintering at 840 or 900 °C for just a few minutes, and then quenching in liquid nitrogen. For films sintered at 900 °C, critical current densities J c were achieved as high as 1.4 × 10 6 A/cm 2 at 20 K and 2.3 × 10 5 A/cm 2 at 20 K and 2 T for the pure and SiC added films. Films doped with 5 wt.% of Cu powders were observed to have better adherence to the Ni substrate without degradation in T c , and J c was found to be slightly decreased, but still remains as high as 7 × 10 5 A/cm 2 at 20 K in zero field. It was observed that J c and irreversibility field increase with an increasing sintering temperature up to 900 °C. Furthermore, nano-SiC addition has significantly improved the irreversibility field compared to undoped MgB 2 films.

The flux dynamics of a (Tl,Pb)(Sr,Ba) 2 Ca 2 Cu 3 O y single crystal have been investigated by means of magnetic relaxation measurements, with the applied field parallel to the c -axis. A power law dependence of the activation energy on the current density for fields less than 2 T over the temperature range 15–45 K has been observed, consistent with three-dimensional large bundle vortex creep. For intermediate fields (2–3 T), the flux dynamics are dominated by plastic vortex creep, while for fields higher than 3 T over the temperature range 16–46 K, the vortex glass scaling approach implied that a two-dimensional vortex structure is unlikely for fields less than 5 T.

Flux motion and its influence on the properties of the new discovered superconductor MgB 2 was studied by electric transport resistance as well as by ac susceptibility measurements. Different bias current I was applied perpendicular and parallel to applied dc field B d up to 7 T. The upper critical field B c2 and irreversibility field B irr were determined as functions of temperature T and bias current I . It is found that the larger the bias current I the smaller the B irr , and B c2 is observed to be affected by flux motion. The B irr ( T ) and B c2 ( T ) data determined by ac susceptibility measurement confirm the transport result. These results suggest that the influence of flux motion should be taken into account if resistively determined parameters for MgB 2 are considered. The present study also implies that the flux dynamics of MgB 2 is more like as that of high temperature superconductors rather than of low temperature superconductors.

The tube filling/forming powder-in-tube (PIT) technique was employed at Hyper Tech Research for the continuous processing of MgB 2 /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 ( J c ) of 2–3×10 4 A/cm 2 at 4 K/4 T, 4×10 4 A/cm 2 at 4 K/2 T, and at 4 K in self field an extrapolated J c of at least 2×10 5 A/cm 2 . Batch-processed PIT MgB 2 /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 J c is extracted, and found to be comparable to the extrapolated transport value.

Vibrating sample magnetization (VSM) measurements have been made on round Fe-clad monocore MgB 2 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.

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 powderin-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 DH 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. 2002 Published by Elsevier Science B.V.

We report the preparation of Mg$_{1-x}$B$_{2}$ (0$\le$x$\le$0.5) compounds with the nominal compositions. Single phase MgB$_{2}$ was obtained for x=0 sample. For 0$

Fe-clad MgB2 long tapes have been fabricated using a powder-in-tube technique. An Mg + 2B mixture was used as the central conductor core and reacted in-situ to form MgB2. The tapes were sintered in pure Ar at 800 ^(o) C for 1 h at ambient pressure. SEM shows a highly dense core with a large grain size of 100 micron. The Fe clad tape shows a sharp transition with transition width of 0.2 K and Tc0 at 37.5 K. We have achieved the highest transport critical current reported so far at 1.6 times 10^(4) A/cm^2 for both 29.5 K in 1 Tesla and 33 K in null field. R-T and critical current were also measured for fields perpendicular and parallel to the tape plane. The iron cladding shielded on the core from the applied external field, with the shielding being less effective for the field in the tape plane. Fe cladding may be advantageous for some applications as it could reduce the effects of both the self-field and external fields., 14 pages, 5 figures, submitted to Physica C on May 7, 2001

We study the flux pinning of Bi2Sr2CaCu2O8+δ (Bi2212) single crystals doped with up to 2.2% of Fe on the Cu positions using measurements of magnetization hysteresis loop. We found that the critical current density increased with the doping for doping less than 0.5 at.%. This proves that Fe point defects contribute to the flux pinning. However, high doping levels deteriorate the Jc. The effect of Fe doping on the peak effect was also studied. The peak position shifts to high temperature, but the peak field decreased with increasing doping levels.

The peak effect in heavily Pb-doped Bi 2 Sr 2 CaCu 2 O 8+ δ single crystals was studied by measuring the magnetisation hysteresis ( M – H ) loop. In contrast to the weak peak effect in pure single crystals, the Pb-doped crystals showed a very strong peak effect in a very wide temperature range, from 20 K up to T c . The peak field, H peak , was as high as 0.4 T. The effect of annealing on the peak effect was also studied. Oxygen overdoping enhances the peak effect more than the underdoping. A comparison study of the peak effect was done for pure, Pb-doped Bi2212 crystals and Y123 samples. A novel method was used to characterise the peak effect. Results showed that the peak effect in the Pb-doped Bi2212 is even stronger than in Y123.

Atomic force microscopy (AFM), X-ray diffraction, polarized microscopy and scanning electron microscopy (SEM) were used to characterise Bi 2 Sr 2 CaCu 2 O y (Bi-2212) single crystals grown at the surface of KCl. The crystals growth were mainly controlled by a spiral growth mechanism. Morphologies of the growth spirals varied with growth conditions. Single, double and even multi-spirals with square and polygonized shapes were observed. (110) growth twins were found to co-exist with square shape single or multi-spirals and lead to the square shape growth spirals. The widths and heights of growth steps were determined to be 0.5–6 μm and 3–5 unit cells by AFM.