In this article, we report the results of a study that shows the existence of pinning centers inside grains and between grains in NbTi wires. We accurately show the ranges of magnetic fields in which the individual pinning centers operate. The pinning centers inside grains are activated in high magnetic fields above 6 T. We show the range of magnetic fields in which individual defects, dislocations, precipitates inside grains and substitutions in the crystal lattice can operate. We show the existence of a new kind of high field pinning center, which operates in high magnetic fields from 8 to ∼9.5 T. We indicate that dislocations create pinning centers in the range of magnetic fields from 6 to 8 T. In addition, our measurements suggest that the peak effect (increased critical current density (Jc) near the upper critical field (Bc2)) could be attributed to martensitic (needle-shaped) α′-Ti inclusions inside grains. These centers are very important because they work very effectively in magnetic fields above 9.5–10 T. We also show that the α-Ti precipitates (between grains) with a thickness similar to the coherence length create pinning centers which work very effectively in magnetic fields from 3 to 6 T. In magnetic fields below 3 T, they act very efficiently in grain boundaries. The measurements indicate that the pinning centers created by dislocations only can be tested by transport measurements. This indicates that dislocations do not increase the magnetic critical current density (Jcm). Cold drawing improves pinning centers at grain boundaries and increases the dislocation density, and cold-drawing pinning centers are responsible for the peak effect.

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Magnetization measurements prove that the magnetic properties of large-angle ͑ Ͼ 30°͒ bismuth bicrystals with a crystallite interface ͑CI͒ of twisting types essentially differ from well-known results on single-crystalline specimens. Two superconducting phases with T c ϳ 8.4 K and ϳ4.3 K were observed at the CI of bicrystals while ordinary rhombohedral Bi is not a superconductor. We conclude that these phases have to do with the central part and the adjacent layers of the CI of bicrystals.

In this study we show that dominant point pinning mechanisms in SiC doped MgB2 wires can be obtained by annealing in high isostatic pressure. The results indicate that the point pinning centers increase the critical current density in medium and high magnetic fields, but not at low magnetic fields. In addition, our study shows that dominant pinning mechanism changes from point to surface type with increase of magnetic fields. An MgB2 wire heat treated in a high pressure of 1.4 GPa shows a high critical current density of 100 A mm−2 in 13 T at 4.2 K. Scanning electron microscope studies show that high isostatic pressure increases the density of the MgB2 material, eliminates voids, allows for small Si precipitates and homogeneous distribution of Si precipitates. Transport measurements E - B and E - I show that the MgB2 wires manufactured by Hyper Tech Research did not heat up after transition into a normal state. This is important for applications in coils.

Unreacted MgB 2 wires fabricated from SiC-doped precursor material by Hyper Tech Research, Inc. have been used to make small-diameter (14 mm) superconducting coils. All coils made of 500 mm length wires were subjected to hot isostatic pressure (HIP) treatment. The critical current density (J c ) parameters of coils were compared to straight samples characteristics. Both types of samples have been measured in perpendicular magnetic field configuration for J c and pinning force density (F p ) evaluation. No significant J c difference between the long wires on coils and straight wires was found. These results suggest that the critical current (I c ) for coils can be determined for straight samples (25 mm). SEM analysis indicated that a small diameter of the coil does not influence the structure of in situ MgB 2 wire.