M.J. Goringe

Transmission electron microscopy and high-resolution transmission electron microscopy studies of the early stages in the degradation of yba2cu3o7-δ superconductor in water vapour

Abstract The initial stages of water vapour attack of the high-temperature superconductor YBa2Cu3O7-δ have been studied by transmission electron microscopy (TEM). The planar defects both of thickness 0·2 nm and of thickness 0·4 nm reported previously have been observed. The defects are sometimes associated with preexisting sites of strain in the material, but such strain is certainly not required for their nucleation, since often they occur in initially perfect material. A model for the thinner defect has been developed, which fits well with the TEM observations, as well as with bond valence calculations and previous work on the degradation of YBa2Cu3O7-δ by water. The implications of the model for that degradation process are discussed.

The fabrication of Bi-based superconductor tape by electrophoretic deposition and melt-texturing techniques

A processing technique has been developed for the fabrication of high-current Bi-2212 superconductor tapes. The technique comprises three steps: (i) treating the particulate ceramic superconductor with trichloroacetic acid in 2-butanol to generate a negative charge on the particle surface; (ii) electrophoretic deposition of the particulate ceramic onto an Ag-based substrate and (iii) melt-texturing of the deposit to form a superconductor coating on the substrate. It has been found that the 2212 superconductor can only be melt textured on an Ag substrate in a narrow temperature window around 885 degrees C. Slow cooling from the molten state not only allows the development of superconductor phase texture, but also suffers from the growth of Ca-Cu-O oxides. These oxides can be transformed into 2212 phase by annealing at 850 degrees C. However, the 2212 grains formed from the Ca-Cu-O precipitates are invariably oriented at right angles to the 2212 phase formed in the course of solidification. The presence of such large-angle grain boundaries is detrimental to the superconductor properties. An optimal processing procedure has been proposed and some important processing parameters discussed. A pilot plant has been set up and semi-continuous tape of Bi-2212 superconductor on Ag has been produced. Critical currents as high as 155 A have been measured over a length of 1 m of multilayer coil at 4.2 K. The performance corresponds to a critical current density of about 70000 A cm-2 in the superconductor layer in a self-field up to about 64 mT. The uniformity of Jc measured on adjacent 10 cm lengths in the multilayer coil is much better than that in a single layer coil, about 30% variation in the former compared with 80% in the latter. The results show that electrophoretic tapes have a promising future for integration into small magnet windings.

Fabrication and properties of TBCCO films via air-atomising spray deposition of thallium-free precursors

Abstract The high-field properties of polycrystalline superconducting TlBaCaCuO films fabricated by the incorporation of thallium vapour into air-atomised BaCaCuO precursors are described. Thick films with T c values in the range 106–111 K have been prepared on polycrystalline yttria-stabilised zirconia substrates. The surface morphology, crystal structure and composition of the films are related to their high-field transport and magnetisation properties. Typical 10 mm × 9 mm films show J c values > 1×10 4 A/cm 2 at 77 K (0 T). The best film has a J c =1.3×10 4 A/cm 2 ( I c =3.6 A) at 77 K (0 T). Films prepared on 26 mm×9 mm substrates show typical large-area J c values > 0.5×10 4 A/cm 2 (77 K, 0 T). A square planar specimen of dimensions 4.3 mm ×4.3 mm exhibited magnetisation J c values=1.2×10 5 A/cm 2 at 4.2 K (0.1 T), 9.3×10 4 A/cm 2 at 10 K (0.1 T), 3.3×10 4 A/ cm 2 at 4 K (8 T), and 1.6×10 4 A/cm 2 at 10 K (8 T).

The critical current of the Chevrel phase superconductor lead-molybdenum-sulphur with gadolinium

Abstract The critical current density J c has been measured in a series of Chevrel phase Pb 1− x Gd x Mo 6 S 8 ( x = 0, 0.1, 0.2 and 0.3) bulk samples made using a hot isostatic pressing process. Adding Gd markedly increases the critical temperature, and the upper critical field found using resistivity, susceptibility and heat capacity measurements. The grain boundaries are found to be clean when observed using TEM. However, increasing the level of Gd, causes J c and the irreversibility field B irr to decrease dramatically. In particular, J c becomes more field dependent as more Gd is added. The pinning force density for all samples can be described either by a Kramer relation F p = J c × B ∝ b 1 2 (1 − b) 2 or by an exponential relation J c = α ( T )exp[− B / β ( T )], over a wide field and temperature range. This indicates that one mechanism controls J c in all the samples. This study shows that adding Gadolinium improves the intrinsic properties ( T c , B c2 ( T )) and yet simultaneously degrades the extrinsic properties ( J c , B irr ). It is suggested that the Gadolinium acts as an oxygen getter which improves the intrinsic properties of the grains but causes local compositional variations at the grain boundaries which reduces J c .

Orientation and thermal cycling effects on the critical currents of high Tc superconducting composites

Abstract Transport critical currents of short lengths of high T c superconducting composite tapes have been measured as a function of magnetic field applied parallel to and perpendicular to the tape surface. Anisotropy factors have been calculated from the results on a number of samples, differing only in their final annealing temperatures, and show great similarity for samples whose critical current densities vary widely. We explain these results in terms of the different microstructures of the samples. We discuss n values and present some data on Ag magnetoresistance. Mechanisms leading to a degradation in J c as a result of thermal cycling are described along with possible solutions.

Fabrication of TBCCO films via sequential electrodeposition and co-electrodeposition of thallium-free precursors

Tl-Ba-Ca-Cu-O (TBCCO) superconducting films have been prepared via the incorporation of Tl vapour into co-electrodeposited and sequentially electrodeposited Ba-Ca-Cu-O (BCCO) precursors on ceramic substrates and silver foils. The BCCO films were electrodeposited at a constant potential of -3 to -5 V (versus Hg/Hg2SO4, (SO42-)=1 M) from a single electrolyte or multiple electrolytes. The results suggest that the BCCO precursors deposited by the novel hybrid sequential electrodeposition provide a better stoichiometry control than those deposited by co-electrodeposition due to the great difference (>0.2 V) in standard reduction potentials among the cation species (Ba2+/-2.906 V, Ca2+/-2.866 V, Cu2+/0.337 V versus SHE). Superconducting films with reliable properties have been prepared on polycrystalline zirconia plates and rods. These films contain multiple superconducting phases with a plate-like morphology and have transition temperatures in the range 95-105 K. The best film on MgO (100) has a thickness of 2*103 A cm-2 at 77 K. The best film on silver foil shows a promising Jc>103 A cm-2 at 77 K.

Structure and current transport mechanisms in Bi(2223)/Ag tapes

Abstract The effects of 2223 phase content, grains morphology, texture and core density in Ag sheathed tapes were studied and related to the transport currents. Four Bi(2223)/Ag tapes differing in thermomechanical treatment have been analyzed structurally (XRD, EDX, SEM and TEM), mechanically (microhardness) and electrically (transport I c measurement in self field, I c in external magnetic field up to 1.2 T, and I c field angular dependence, all at 77 K). The obtained structural results do not always give a clear relation to transport current because of a difference between the volume (layer) of structural analysis and the real current path.

Prototype coils wound from high-T/sub c/ superconducting composites

The construction and performance of three prototype pancake coils wound from high-T/sub c/ superconducting composite tapes fabricated using the techniques of Powder-In-Tube (PIT) and Electrophoretic Deposition (EPD) are described. Numerous potential taps were incorporated in the coils to enable determination of conductor homogeneity. Results are presented of measurements of transport critical currents of complete coils and sections of coils. The homogeneity of these sectional critical currents is shown to be much improved when the composite tape is multilayer rather than single layer. In addition data are presented on critical current anisotropy with respect to applied field direction, at 4.2 K and 77 K, for a short sample of the tape used in the construction of one of the coils. >

Raman-active phonon softening in a TlBaCaCuO superconducting thin film

Abstract We report on a Raman scattering study of a large-area spray pyrolysed TlBaCaCuO superconducting thin film ( T c = 111 K). We have analysed the temperature dependence of the lineshape of the apical and bridging oxygen vibration (A 1 g ) modes taken from an intimate mixture of TBCCO-1223 and -2223 crystallites and have found evidence for anomalous phonon self-energy effects at or near the critical transition temperature. Possible mechanisms that could account for this behaviour are discussed.

HREM studies of defect structure in YBa1.85La0.15Cu4O8

Abstract The defect structure in YBa 1.85 La 0.15 Cu 4 O 8 has been studied by high-resolution electron microscopy. It is found that the introduction of La has resulted in a high density of single and triple CuO chain type planar defects, which may act as flux pinning centers to enhance the current density J c . The present results suggest that the La-substitution combined with a small excess of CuO could be an important technique for J c enhancement in the 1-2-3 phase, and may also provide a means of stabilising bulk 1-2-5 phase.