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The oxygen concentration and distribution in the microstructure of MgB2- and YBa2Cu3O7-δ-based materials affect the formation of nanostructural defects and thus influence the critical current density, upper critical magnetic field and irreversibility field. For MgB2 oxygen containing additions (Dy-O, Ti-O) in the form of nanograins occurred not to be very effective for an increase of critical current density, jc. Sn-O additions to MgB2 can provoke even a significant decrease of jc due to a chemical interaction of Sn with MgB2. The processing pressure and temperature influence the defect density of the relevant pinning centers and the regularity of their distribution in doped and undoped MgB2 as well as in MT-YBCO. [ABSTRACT FROM AUTHOR]

High field pinning centers in MgB2 doped with 2 at. % carbon under a low and a high hot isostatic pressures have been investigated by transport measurements. The field dependence of the transport critical current density was analyzed within the different pinning mechanisms: surface pinning, point pinning, and pinning due to spatial variation in the Ginzburg-Landau parameter (Δκ pinning). Research indicates that a pressure of 1 GPa allows similar pinning centers to Δκ pinning centers to be obtained. This pinning is very important, because it makes it possible to increase the critical current density in high magnetic fields at 20K and 25 K. Our results indicate that the δTc and dl pinning mechanisms, which are due to a spatial variation in the critical temperature (Tc) and the mean free path, l, respectively, create dislocations. The high density of dislocations with inhomogeneous distribution in the structure of the superconducting material creates the δl pinning mechanism. The low density of dislocations with inhomogeneous distribution creates the δTc pinning mechanism. Research indicates that the hot isostatic pressure process makes it possible to obtain a high dislocation density with a homogeneous distribution. This allows us to obtain the δTc pinning mechanism in MgB2 wires. In addition, a high pressure increases the crossover field from the single vortex to the small vortex bundle regime (Bsb) and improves the δTc pinning mechanism. Our research has proved that a high pressure significantly increases the crossover field from the small bundle to the thermal regime (Bth), with only a modest decrease in Tc of 1.5K, decreases the thermal fluctuations, increases the irreversibility magnetic field (Birr) and the upper critical field (Bc2) in the temperature range from 4.2K to 25K, and reduces Birr and Bc2 above 25K. [ABSTRACT FROM AUTHOR]

MgB2 precursor wires were prepared using powder in tube technique by Institute of High Pressure PAS in Warsaw. All samples were annealed under isostatic pressure generated by liquid Argon in the range from 0.3 GPa to 1 GPa. In this paper, we show the effects of different processing routes, namely, cold drawing (CD), cold rolling (CR), hot isostatic pressure (HIP) and doping on critical current density (Jc), pinning force (Fp), irreversible magnetic-field (Birr), critical temperature (Tc), n value, and dominant pinning mechanism in MgB2/Fe wires with ex situ MgB2 barrier. The results show that medium pressures (∼0.35 GPa) lead to high Jc in low and medium magnetic fields (0 T - 9 T). On the other hand, higher pressures (∼1 GPa) lead to enhanced Jc in high magnetic fields (above 9 T). Transport measurements show that CD, CR, and HIP have small effects on Birr and Tc, but CD, CR, HIP, and doping enhance Jc and Fp in in situ MgB2 wires with ex situ MgB2 barrier. Transport measurements on in situ undoped MgB2 wire with ex situ MgB2 barrier yield a Jc of about 100 A/mm2 at 4.2K in 6 T, at 10K in 4 T and at 20K in 2 T. The results also show that cold drawing causes increase of n value. [ABSTRACT FROM AUTHOR]

This article discusses the influence of MgB2 wire diameter on critical parameters, dominant pinning mechanism, and n value. Studies were conducted on multifilament MgB2 wires of the 0.84 mm and 0.64 mm diameter. The wires were annealed under 1 GPa isostatic pressure, between 650 and 730 °C. Studies show that the best results (Jc, Birr, n value, and point pinning mechanism) for a 0.64-mm wire were obtained with annealing temperature of 650 °C. However, the best results for wires with a 0.84-mm diameter were obtained with annealing temperature of 730 °C. This research was conducted for exploring the use of MgB2 wires in multi-section superconducting coils, e.g., main coils generating magnetic fields and compensation coils—increasing the homogeneity of the magnetic field. Our research indicates that the main coil made with larger diameter wires should be heated at higher temperature (730 °C). In contrast, compensating coils with smaller diameter wires should be heated at a lower temperature (650 °C). [ABSTRACT FROM AUTHOR]

This article presents the impact of MgB2 wire bending and diameter on transport critical current density and irreversible magnetic field of a resultant coil. Unreacted MgB2 wires 500 mm in length and 0.63 or 0.83 mm in diameter have been used in the fabrication of small diameter (14 mm) superconducting coils. The coils were subsequently annealed under isostatic pressure of 1 GPa for 15 min at 700 °C and 725 °C. Our results indicate that larger wire diameter, higher annealing temperature, and bending lead to slight reduction of critical current density and irreversible magnetic field in the coil. [ABSTRACT FROM AUTHOR]

Field, temperature, and strain dependences of the critical current for a SiC doped multifilamentary in situ MgB2 wire have been studied. Measurement results were compared with that of the undoped wire, and the origin of the difference in the critical current is discussed. The critical current can be calculated with the percolation model considering the effect of anisotropy. The temperature dependence of the fitting parameters, the upper critical field along the ab-plane, and c-axis is compared with the dirty-limit two-gap theory. To assess the validity of the fitting parameters, resistive transition has been measured especially to extract the upper critical field directly. It is shown that even the resistive broadening can be well explained by a simple parallel path model using the fitting parameters obtained from the critical current analysis. [ABSTRACT FROM AUTHOR]

MgB2/Fe wires with 10 at. % excess Mg produced by in situ powder-in-tube processing were compared to normal stoichiometric MgB2/Fe wires prepared by the same method. It was found that the critical current density (Jc) and the irreversibility field (Hirr) were significantly enhanced for MgB2/Fe wires with excess Mg. The transport Jc for 10 at. % Mg excess samples sintered at 800 °C, measured at a field of up to 14 T, increased by a factor of 2 compared to that for the normal MgB2 wires. The best Jc results for the 10 at. % Mg excess sample were obtained by heating the sample at 600 °C for 1 h; the Jc at a field of 8 T and at temperature of 10 K reached 3×104 A/cm2. The detailed analysis of the effect of excess Mg on the microstructures, the Jc, and the Hirr, of MgB2/Fe wires are presented in this paper. [ABSTRACT FROM AUTHOR]

MgB2 samples were prepared by using 96% boron (B) powder with strong crystalline phase that had been ball milled for various times. We observed samples that contained ball-milled 96% B in comparison with one made from as-supplied commercial 96% B, with the results showing a significant enhancement in the high field critical current density (Jc) due to small grain size and better reactivity. Specifically, many grain boundaries for MgB2 could be acting as strong flux pinning centers. Based on Rowell connectivity analysis, when the ball-milling time increased, the connectivity factor, described as the active cross-sectional area fraction (AF), was decreased. This implies that the intergrain connectivity became worse. These properties could lead to poor Jc in low field. However, the pinning force strength, Jc1/2×B1/4, of samples using ball-milled 96% B is larger than that of the reference sample using commercial 96% B powder. These results accompany enhanced irreversibility (Hirr) and upper critical fields (Hc2). [ABSTRACT FROM AUTHOR]

Field, temperature and strain dependence of the critical current for a SiC doped multifilamentary in situ Mg[B.sub.2] wire are analyzed by using percolation model. The validity of the fitting parameters is assessed by measuring the resistive transition to extract the upper critical field directly.

Mg[B.sub.2]/Fe wires with excess Mg produced by in situ powder-in-tube processing are compared to normal stoichiometric Mg[B.sub.2]/Fe wires prepared by the same method. The effect of excess Mg on the microstructures, the critical current density and the irreversibility field of Mg[B.sub.2]/Fe wires is analyzed.

The superconducting properties of Mg[B.sub.2] made from low-grade 96% B powder with a strong crystalline phase as a function pf ball-milling time are examined. The pinning force strength of the samples using ball-milled 96% B is larger than that of the reference sample using commercial 96% B powder and these have accompanied enhanced irreversibility and upper critical fields.

The composition of MgB2-based materials (wires, bulks, and thin films) with high critical current densities, jc, prepared at different pressure (0.1 MPa–2 GPa)—temperature (600–1050 °C) conditions was analyzed by the X-ray and JAMP−9500F Auger spectrometer (after removing the oxidized layers at the sample surfaces by Ar ion etching performed directly in the vacuum chamber of a microscope). Similar regularly distributed inhomogeneities connected with Mg, B, and admixture O content variation on the nanolevel were observed in all types of the MgB2-based materials. The correlations between the character of the material inhomogeneities and the attained superconducting characteristics are discussed. [ABSTRACT FROM AUTHOR]

This article reports the influence of isostatic pressure (from 0.1 MPa to 1.1 GPa), low annealing temperature of 570 ∘C, and annealing time for the formation of high-field pinning centers in 2% C-doped MgB2 wires. Measurements indicate that 1.1 GPa pressure significantly increases the density of high-field pinning centers below 20 K. However, lower pressure (0.6 GPa) slightly increases the density of high-field pinning centers. Increasing the annealing time from 120 to 210 min leads to a reduction of critical temperature (Tc), irreversibility field (Birr), critical current density (Jc), and upper critical field (Bc2), suggesting that a long annealing time leads to a reduction of high-field pinning center density and the number of connections between superconducting grains. The high pressures and low annealing temperature lead to a high critical current density of 1000 A/mm2 in 7.2 T and 100 A/mm2 in 12.5 T at 4.2 K in MgB2 wires. [ABSTRACT FROM AUTHOR]