M. Suenaga

Microstructure and structural defects in MgB2 superconductor

Abstract We report a detailed study of the microstructure and defects in sintered polycrystalline magnesium diboride (MgB2). Both transmission electron microscopy and X-ray data reveal that MgO is the major second-phase in our bulk samples. Although MgB2 and MgO have different crystal symmetries, being P6/mmm and Fm-3m, respectively, their stacking sequence of Mg and B (or O) and lattice spacings in certain crystallographic orientations are very similar. The size of MgO varies from 10–500 nm, and its mismatch with the MgB2 matrix can be a source for dislocations. Dislocations in MgB2 often have a Burgers vector of 〈1 0 0〉 . 1/3 〈1 −1 0〉 and 1/3 〈2 1 0〉 partial dislocations and their associated stacking faults were also observed. Since both dislocations and stacking faults are located in the ( 0 0 1 ) basal plane, flux pinning anisotropy is expected. Diffuse scattering analysis suggests that the correlation length along the c-axis for defect-free basal planes is about 50 nm. ( 0 0 1 ) twist grain boundaries (GBs), formed by rotations along the c-axis, are major grain boundaries in MgB2 as a result of the out-of-plane weak bonding between Mg and B atoms. An excess of Mg was observed in some grain boundaries. High-resolution nano-probe electron-energy loss spectroscopy reveals that there is a difference in near edge structure of the boron K-edge acquired from GBs and grain interiors. The change at the edge threshold may be suggestive of variation of the hole concentration that would significantly alter boundary superconductivity.

Superconductivity in H/sub x/YBa/sub 2/Cu/sub 3/O/sub 7/

The preparation of H/sub x/YBa/sub 2/Cu/sub 3/O/sub 7/ by the direct reaction with hydrogen gas is described. The expansion of the original orthorhombic unit cell, as determined by x-ray diffraction, indicates that the system forms a hydrogen solid solution up to x--0.2; at higher H contents a hydrogen-rich phase is precipitated. The solid-solution phase exhibits a superconducting transition at --94 K, which was usually slightly higher than the original oxide. The H-rich phase(s), are not superconducting at 4 K or above. The maximum H content so far achieved corresponds to x = 5.9. The H-rich phases are stable at room temperature, at elevated temperatures (>473 K) they will decompose slowly under vacuum to form water and various solid products. They also slowly react upon exposure to room air.

The calorimetric measurement of losses in HTS tapes due to ac magnetic fields and transport currents

Abstract We report a simple technique for the measurement of energy losses in HTS tape carrying ac currents in ac magnetic fields at liquid nitrogen temperatures. This method is based on measuring the temperature rise of a thermally insulated tape and in the present form is able to measure losses down to 0.01 W m −1 . We present data obtained by this method confirming previously published results for the losses at 100 Hz due to in phase ac currents and magnetic fields as measured electrically. Using this calorimetric method we are able to measure the effect of phase differences between current and field.

Superconducting critical temperatures, critical magnetic fields, lattice parameters, and chemical compositions of ‘‘bulk’’ pure and alloyed Nb3Sn produced by the bronze process

Superconducting critical temperatures Tc and magnetic fields Hc2, lattice parameters a0, and chemical compositions were measured for ‘‘bulk’’ layers (∼6 μm or greater) of ‘‘pure’’ and alloyed Nb3Sn which were made by the bronze process. The values of Tc, a0, and the composition of pure Nb3Sn layers were ∼18 K, 0.52900±0.00005 nm, and 25±0.5 at. % Sn, respectively, independent of heat‐treatment temperature (between 650–780 °C) and of the bronze composition, as long as the thickness of the layers was greater than ∼6 μm. Small additions of Ti (∼1 at. %) or Ta (∼3 at. %) slightly increased the value of Tc (by ∼0.2–0.4 K) above that for pure Nb3Sn. However, additions of larger amounts of these elements or addition of other transition elements (V, Zr, and Mo) significantly decreased Tc. Also, small additions of these elements significantly increased Hc2. Specifically, the largest values of Hc2 (∼27 T at 4.2 K) were obtained for Nb3Sn layers containing ∼1.5 and ∼4 at. % of Ti and Ta, respectively, compared with ...

Grain boundary in textured YBa 2 Cu 3 O 7−δ superconductor

The misorientations of over 200 pairs of adjacent grains separated by grain boundaries in textured YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} were measured using a transmission electron microscopy technique. The results indicate that there exist discrete preferred rotation angles and rotation axes. The existence of low-energy boundaries is inferred. The results are analyzed based on the Constrained Coincidence Site Lattice (CCSL) and O2-lattice theories and imply the applicability of such theories for the case of large-angle grain boundaries in a complex crystal structure such as YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. The results of analysis also show that some boundaries are likely to be reduced in oxygen near the boundary to satisfy the constraint of the coincidence site lattice.

A model of flux pinning by grain boundaries in type‐II superconductors

A relationship between pinning force Fp and grain size D in type‐II superconductors derived on the basis of interaction of fluxoids with dislocations forming the grain‐boundary walls is reported. While confirming approximately the experimentally observed Fp∝1/D relationship, the theory predicts a peak in Fp at a0/D≃0.2, where a0 is the lattice constant of the flux line lattice.

Effects of stresses, induced by thermal contraction of a bronze matrix, on the superconducting properties of Nb3Sn wires

The superconducting critical temperature Tc and high‐magnetic‐field critical‐current densities Jc have been measured on bronze‐processed Nb3Sn single‐core wires prior to and after removing the bronze cladding. Both Tc and Jc, at high magnetic fields, are increased after removal of the cladding. Tc measurements on a series of wires with various core‐to‐matrix ratios R show that the depression in Tc increases as R decreases. The results are interpreted in terms of a stress imposed on the Nb3Sn layer by the thermal contraction of the outer bronze cladding. Results point to the possibility of a martensitic phase transformation occurring in the bronze‐clad Nb3Sn wires.

Chemical compositions at and near the grain boundaries in bronze‐processed superconducting Nb3Sn

The chemical composition of bronze‐processed Nb3Sn at grain boundaries and its variation as a function of distance from the grain boundary were determined using Auger electron spectroscopy. The composition at the grain boundary was 44.1, 30.6, and 25.4 at. % for Nb, Sn, and Cu, respectively, and approached to the bulk value (∼75 at. % Nb and ∼25 at. % Sn) at ∼20 A into the grain, as determined by a depth‐profiling method.

Synchrotron x‐ray scattering measurements of bulk structural properties in superconducting (Bi,Pb)2Sr2Ca2Cu3O10–Ag tapes

The structural properties of superconducting (Bi,Pb)2Sr2Ca2Cu3O10–Ag (2223) tapes have been measured using synchrotron x‐ray scattering techniques. The x‐ray photon energy was tuned just below the silver K absorption edge so the penetration depth was large, which allowed the measurements to be performed in a transmission geometry without removing the silver cladding. Analysis of the peaks in 2θ scans indicates that residual (Bi,Pb)2Sr2CaCu2O8 (2212) superconductor starting material is present in all samples studied. The amount of 2212 varied widely among the tapes, and was not homogeneous along the length of each individual tape. Residual 2212 content increased near the ends of most samples, suggesting that 2223 phase development is sensitive to whether the superconducting material is encased in silver or not. The bulk c‐axis alignment was measured in ∼100 mono‐ and multifilament samples, and correlations between c‐axis alignment and current carrying capacity at 77 K were found. Multifilament samples gene...

Nucleation and growth of YBa 2 Cu 3 O x on SrTiO 3 and CeO 2 by a BaF 2 postdeposition reaction process

The nucleation and growth of the c -axis-aligned Yba 2 Cu 3 O x on SrTiO 3 and CeO 2 , from precursor films, were studied by examining quenched and fully processed specimens using transmission electron microscopy techniques. The precursor films, a stoichiometric mixture of fine-grained Y, Cu, and BaF 2 , were deposited using physical vapor deposition methods. An Y-Ba oxy-fluoride formed from the precursor contributed to the nucleation of Yba 2 Cu 3 O x , while a liquid layer between the unreacted precursor and the Yba 2 Cu 3 O x layer played an important role in the growth of Yba 2 Cu 3 O x . However, the process of nucleation of Yba 2 Cu 3 O x on SrTiO 3 and CeO 2 were significantly different.