[en] This thesis is divided into two sections, each dealing with the response of a superconducting proximity system to an applied magnetic field. In Section I, the maximum supercurrent that can be carried in an applied perpendicular magnetic field by a superconducting normal-metal superconducting (SNS) junction consisting of a square normal-metal layer sandwiched between two crossed perpendicular superconducting strips is investigated theoretically. For weak applied fields, the critical current is suppressed reversibly as induced Meissner screening currents flow into the SNS sandwich and generate a spatially varying magnetic field largely parallel to the junction, thereby altering the local phase difference. For stronger applied fields, the critical current is changed irreversibly as vortices enter and become pinned in the junction. In Section II, Eilenberger's quasi-classical formulation of superconductivity is used to derive an equation for the upper critical field of a multilayered SN system in a perpendicular magnetic field. The alternating layers are coupled via the proximity effect and are in the dirty limit

[en] Ni-Ti superconducting wires with multifilamentar configuration were produced, using the grouping technique. Some basic concepts on superconductivity and its main critical parameters are presented. The criteria for stabilizing superconductors in terms of the geometry are studied. The main critical parameters, H_c, J_c, T_c in function of the composition and the metallurgical structure of Ni-Ti alloy are analysed. The development of Ni-Ti superconducting wires is described. (M.C.K.)

[en] Critical parameters were calculated using the one-dimensional multigroup transport theory. Calculations have been performed for water mixture of uranium metal and uranium oxides and plutonium nitrates to determine the dimensions of simple critical geometries (sphere and cylinder). The results of the calculations were plotted showing critical parameters (volume, radius or critical mass). The critical values obtained in Handbuch zur Kritikalitat were used to compare with critical parameters. A sensitivity study for the influences of mesh space size, multigroup structure and order of the S sub(n) approximation on the critical radius was carried out. The GAMTEC-II code was used to generate multigroup cross sections data. Critical radius were calculated using the one-dimensional multigroup transport code DTF-IV. (Author)

[en] In this thesis, the authors studied physical properties of various high-T_c superconductors by means of the ac susceptibility technique. They present the physics of ac susceptibility from first principles without relying on any specific models. From this, they can explain some fundamental but interesting features of experimental results such as symmetry properties of the ac susceptibility and its harmonics with respect to dc magnetic field. By measuring ac hysteresis they were able to obtain lower critical fields which were compared with some theoretical predictions. They found that the results were consistent with the BCS theory with modified gap parameter. By measuring ac hysteresis of granular samples at ultra low magnetic fields, they revealed the Meissner state and obtained Josephson lower critical fields, which was also found to be in consistence with the BCS theory. They studied the critical state models in detail and obtained some useful equations which were used in analyzing magnetic field dependence of ac susceptibility. By comparing the equations with experimental results, they were able to perform quantitative analysis of some important physical parameters such as critical current density. From numerical calculations of ac susceptibility with the parameter values obtained by this procedure, they confirmed good agreement of theory with experimental data. They incorporated the theory of flux creep into the equation governing the steady state flux motion, from which they were able to explain the shift of ac susceptibility with operating frequency toward high temperatures. Through this study, quantitative results on the activation energy were obtained

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[en] 36 different NbTi superconductor wires with 42, 49 and 54 weight % Ti and different thermo-mechanical treatments have been investigated. Critical currents up to the critical magnetic field Bsub(c2) have been measured, the maximum pinning force Pv max derived and the curves Bmax=B(Pv max) drawn. With 49 and 54 % Ti there is additional pinning by precipitates. For high fields, >8T the positive effect of higher pinning forces is outweighted by lower critical fields Bsub(c2) and the critical currents are smaller than at 42 %. In the region B >= 0, 8 Bsub(c2) there is good agreement with R Schmecker's (1977) model of plastic deformation of the flux line lattice. The line pinning hypothesis was also confirmed for this material and this field range. (G.Q.)

[en] Liquid ³He under zero external magnetic field undergoes a second-order transition to a superfluid state at a pressure dependent temperature, T/sub c/, of about 2 mK. When a magnetic field is applied to liquid ³he, the transition splits into two second-order transitions occurring at T/sub c1/ (>T/sub c/) and T/sub c2/ (< T/sub c/). These two transition temperatures, separated by approximately 5 μK per kilogauss applied field, delineate the A₁ phase. The superfluid component of the A₁ phase is made up of pairs of ³He atoms whose nuclear magnetic moments are directed parallel to the external magnetic field. The second order transition at T/sub c1/ is a transition in which relative spin-gauge symmetry is broken. This unique broken symmetry results in the appearance of a hydrodynamic mode involving transport of mass, entropy, and spin angular momentum. One of the consequences of the existence of this mode is that superfluid flow can be generated by applying a magnetic field gradient across a superleak connecting two vessels of ³He-A₁. This is called the magnetic fountain effect. The magnetic fountain effect was used to determine the orientation of the magnetic moments in the external field. Measurements were made of the width (in temperature) of the A₁ phase at two pressures. Critical flow was driven magnetically and the critical velocity was measured through the A₁ phase at two pressures. Critical flow was driven magnetically and the critical velocity was measured through the A₁ phase to within 0.5 μK of T/sub c1/

[en] In high magnetic fields, the critical current density is strongly dependent on the upper critical field, which is determined primarily by the stoichiometry of the Nb₃Al. The critical temperature (T/sub c/), like the upper critical field, is considered to be a measure of the ''intrinsic'' quality of the superconductor, indicating the stoichiometry, order, and strain. If the A15 phase is stoichiometric and well ordered, a high T/sub c/ (and high H/sub C₂/) is expected, regardless of the volume fraction of superconductor. On the other hand, if sigma phase is present with the A15, the resultant composition gradient across the sigma-A15 interface(s) requires that some of the A15 be off-stoichiometric, and therefore that the T/sub c/ (and H/sub C₂/) be low. Thus the extent of the A15 (Nb₃Al) reaction and the quality of the A15 formed are interdependent. This work focuses on the factors that control the extent of Nb₃Al formation in Nb/Al powder wires. The morphology and content of the reacted and unreacted wires are studied in optical, SEM, and TEM micrographs. Critical current density data and its dependence on processing are explained in terms of the unreacted microstructure and its effect on the extent of Nb₃Al formation. As a method of improving the critical current density, a new variation of the conventional powder process for wire manufacturing is developed and tested

[en] In this thesis, the magnetic characterization of some Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀, Bi_1.84Pb_0.34Sr_1.91Ca_2.03Cu_2.06O₁₀, Tl₂Ba₂Ca₂Cu₃O₁₀ and Bi-2212/Ag high temperature superconductors were given by ac magnetic susceptibility and electrical resistivity measuring techniques. Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀ and Bi₁.₈₄Pb_0.34Sr_1.91Ca_2.0₃Cu_2.06O₁₀ high temperature superconductors were prepared by liquid ammonium nitrate method, while conventional solid state reaction route was applied to make Tl₂Ba₂Ca₂Cu₃O₁₀ superconductor. On the other hand, Powder-In Tube (PIT) technique was used to make multi-filamentary Bi-2212/Ag tapes. Structural characterizations were carried out by X-ray powder diffraction (XRD) patterns and scanning electron microscopy (SEM). Unit cell parameters of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀ and Tl₂Ba₂Ca₂Cu₃O₁₀ high temperature superconductors which have tetragonal crystal structure were calculated as a=b=5.3538 AA and c=37.1137 AA, a=b=3.8520 AA and c=35.5970 AA respectively. The results are consistent with the literature. Some information with regards to the grain size and phase content were given by the SEM studies. Fundamental and high order ac susceptibilities, χ=χ_n'''+iχ_n''⁽n=1, 2, 3, 5 and 7) of the samples were measured for the temperature range 15 K-140 K, ac field range 8 A/rn-1600 A/m and the frequency range 11 Hz-2110 Hz. All the susceptibilities exhibit field, frequency and temperature dependences. The observed dependences were analyzed using Bean model. For temperature scaling a function of the form Hp=Hα(l-t)''β was found empirically. The best fitting parameters for the Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀ superconductor were found as Hα 3.3x104 A/m and β=2.05. Odd-order harmonic susceptibilities were calculated from the Bean model to make comparison with the experiments. In addition, ac losses were also calculated from the Bean model to compare with the experimentally measured values. At low temperatures and fields, the Bean model is observed to account for the ac losses. However, some deviations from the model were seen for high temperature and high field values. Finally, the ac field was applied to parallel (PAR) and perpendicular (PER) to the multi-filamentary tapes of Bi-2212/Ag. The fundamental harmonic susceptibilities exhibit field and frequency dependences. The results were dependent on the orientation of the specimen which was placed parallel or perpendicular to the applied field