Table of contents

The effect of discreteness of the Josephson medium on the electrodynamics of high- superconducting ceramics has been studied both theoretically and experimentally. It is shown that two scenarios are possible for the penetration of magnetic field into grained superconductors. The first is in line with the conventional pattern for a continuous type-II superconductor, considered within the framework of the flux-line-cutting concept sensitive to the anisotropy induced by a constant field. The second, corresponding to a Josephson penetration depth much less than the characteristic grain size, is described by an isotropic local current-voltage characteristic. For these two models, expressions are presented describing the harmonic spectrum of a response to an oscillating field. Experiments with a linearly polarized oscillating field orientated at various angles to a constant field and also with a rotating oscillating field superimposed on a constant field have shown that the isotropic model is preferable for describing high- superconducting ceramics.

We prepared superconducting films at lower temperatures by a pulsed laser deposition method. The growth temperature usually decreased with decreasing oxygen pressure in the deposition chamber. However, the superconducting critical temperature of the films deposited at a low oxygen pressure was drastically degraded because of insufficient oxidation. In this work, we employed a mixed gas consisting of oxygen and ozone as the oxidation gas. As a result, increased to 86 K, when films were deposited in a low-pressure oxygen atmosphere (5 mTorr) mixed with ozone (5 mol%) and at a low laser fluence of . In addition to superconducting properties, the surface morphology could be improved by using low-pressure oxygen gas mixed with ozone and by optimizing the laser fluence.

A laser deposition arrangement for the preparation of smooth and films is presented. An excimer laser beam is split by a knife-edge prism into two beams, which are focused onto two rotating targets oriented perpendicular to each other. The two laser-induced molecular streams collide and form a new stream containing mainly light molecules and almost no droplets. The molecules from this stream deposit on a substrate that is placed in the shadow of the original molecular streams. The use of copper-poor targets prevented the occurrence of precipitates.

Careful examination of the system has allowed the synthesis of three materials with different compositions but constant . Their superconducting properties have been correlated with structural and electronic changes deduced from neutron powder diffraction data. This has shown that, while remains constant, the hole density within the superconducting sheets increases with Ca doping. The relevance and significance of this are discussed with respect to a proposed increase in the optimum hole density with Sr content in systems based on .

The electron backscatter diffraction technique has been used to study the depth distribution of the structural strain in a-axis oriented films made by a self-template method. It has been found that the structural strain was mainly formed in the upper layer near the interface between the template and upper layer. In contrast, there is little strain in the template layer. The strain can be relaxed when the thickness of the upper layer exceeds a certain value. The results agree with the observation of a dramatic improvement in the transition temperature and metallic behaviour when the upper layer was thicker than about 200 nm. Such a depth distribution of strain has been discussed in correlation with the degradation of in a-axis films. The microstructures of these films have been investigated by transmission electron microscopy.

Superconductor/ferroelectric (SF) and normal conducting metal/ferroelectric (NF) film structures were grown using DC and RF magnetron sputtering of ceramic and targets. Low-energy ion beam etching was used for patterning an SF planar capacitor with an electrode gap of about . Ferroelectric films displayed a rather smooth temperature dependence of with a maximal value of about 850 at K . Planar ferroelectric capacitors were intended for application in microwave tunable devices, so structures without hysteresis in voltage-capacitance characteristics were demanded. The SF and NF structures demonstrated a high enough coefficient of capacitance tuning (more than 1.5) and lower hysteresis for structures with YBCO electrodes.

The reaction leading to the formation of the Pb free phase from calcined precursor powders has been studied by means of scanning electron microscopy, x-ray diffraction and differential thermal analysis. The observation of the transformations occurring at a fixed location on the surface of a pellet has shown that the formation of the phase crystallites occurs simultaneously with a gradual decomposition of the platelets initially present in the mixture. The present investigations are discussed in relation with the formation of the Pb substituted phase. It is shown that there is a close relationship between the formation mechanisms of the Pb free and the Pb substituted phases, which both can be described as a nucleation and growth process.

The difference in mechanical deformation of the cladding material and ceramic superconductor during cold working of Ag-sheathed Bi-based superconducting composite tapes has been analysed. From this, a optimal rolling procedure has been developed in order to improve the uniformity of the tapes by gradually increasing the reduction per pass during rolling. Using this technique, a uniform stress was achieved within the tapes which improved both the transverse and longitudinal filament interfacial homogeneity and reduced the sausaging effect. An improved and more uniform was observed in long-length tapes fabricated using this procedure than in those fabricated using a non-optimized route.

A small coil (35 mm bore) was made by winding a superconducting Ag/Bi-2223 multifilamentary tape. The coil, composed of eight pancakes, showed reduced performance in the pancakes placed at the sides with respect to central ones. This effect was attributed to the radial field at the coil ends. In order to explain this behaviour, a general method for predicting the critical current of a coil, starting from basic information on short samples, was developed. The method was used to understand the possible advantages from a reduction of radial field in the coil through a suitable iron yoke for flux return. The measured performance (in terms of critical current) of the coil with iron improved by 10%. The central field at 77 K was 1100 Gauss and 6000 Gauss at 4.2 K.

Substitution of Pr at the Ca site in , with , has been reported. Pr is shown to substitute in the (BiPb)-2223 system, albeit with a simultaneous formation of Pr-doped (BiPb)-2212 phase in the samples. The x-ray analysis indicates that the latter phase grows continuously with increasing Pr concentration up to x = 0.4. For only Pr-doped (BiPb)-2212 phase is observed in the samples. The superconductor transition temperatures measured resistively shift to lower temperatures, with increasing Pr, at a rate of 5 K/(% Pr). The normal-state resistivity value and its temperature dependence are found to depend on the amount of Pr doping. We have further analysed the resistivity data of the pure and 5% (x = 0.1) doped samples for the fluctuation conductivity. Both samples exhibit 2D behaviour for the superconducting order parameter in the critical regime, within the Aslamazov-Larkin formulation.

We investigated planar gradiometers on the basis of galvanically coupled high- DC SQUIDs for application in non-destructive testing (NDT). The stability of the sensor of better than 1% permits NDT investigations in unshielded environments. The layout of the gradiometer sensors and a corresponding determination of their effective areas and baselines will be discussed as well as the properties of the Josephson junctions in the DC SQUID based on step-edge and bicrystal grain boundaries. We show measurements with this type of sensor applied in a testing system for NDT to determine spatial and field gradient resolution as well as the influence of sensor position and experimental environment such as the dewar material on the performance of the sensor and the whole testing system. As an example first investigations of the permanent magnetization of hardened valves will be presented where we look for a correlation between hardening and magnetic field distribution.

Fatigue behaviour at room temperature and its influence on superconducting current and upper critical magnetic field at 4.2 k were studied using multifilamentary composite wire with a copper ratio of 1.55 and an overall diameter of 0.812 mm in which 241 filaments are embedded. The filament itself was not fractured by the fatigue when tested separately. In the composite, the fatigue cracks nucleated in the copper, which grew and caused the fracture of the filaments. When the maximum stress in the fatigue test was low, one crack among many grew preferentially; when the maximum stress was intermediate, many cracks could grow at different cross-sections before the overall fracture of the composite; when the maximum stress was high, the composite was fractured before large fatigue crack growth. Because of such a maximum stress dependence of the growth of the fatigue damage, the critical current measured using the segments of the fatigue-fractured specimens for the intermediate maximum stress range was lower than that for the low and high maximum stress ranges. The mode I fracture toughness of the present composite wire, estimated from the size of the strength-determining fatigue crack in combination with the maximum stress, was around .