Determination of different oxygen transport mechanisms and measurement of the oxygen ion conductivity of Y1Ba2Cu3O7−x

By undertaking AC electrochemical impedance experiments on yttria stabilised zirconia electrolytes with polished Y₁Ba₂Cu₃O_7−x electrodes, the activation energy for oxygen ion transport within the bulk of Y₁Ba₂Cu₃O_7−x, in air, over the temperature range 823 K–1043 K, was determined to be 1.50 ± 0.05 eV. At 1000 K the oxygen ionic conductivity was calculated to be around one order of magnitude lower than that in yttria stabilised zirconia. Typical calculated values were σ=5×10⁻⁵ (ω cm)⁻¹ and 6×10⁻³ (ω cm)⁻¹ at the respective temperatures 823 K and 1043 K. By employing a similar cell but with Y₁Ba₂Cu₃O_7−x paste electrodes, oxygen transfer between the Y₁Ba₂Cu₃O_7−x and the electrolyte was found to occur via a surface diffusional processes. Over the temperature range 873 K–1098 K, in air, the activation energy for in-diffusion at the surface was found to be 1.4±0.1 eV and that for out-diffusion at the surface to be 1.76±0.05 eV.     

Magnetoresistance in a constricted domain wall

We show that a thin Gd layer inserted between two thicker layers of permalloy contains an in-plane domain wall whose width can be controlled by varying the thickness of the Gd layer. The magnetoresistance of this structure has been measured with the current perpendicular to the plane, thus eliminating spurious contributions which have complicated previous measurements. This is the first measurement to show unambiguously that the domain wall contributes an additional resistance whose magnitude is in good agreement with theory.     

The sputtering of high Tc A15 Nb3Si and V3Ge

We report some initial results on the preparation of A15 Nb₃Si and V₃Ge using a getter sputtering technique. Under sufficiently clean conditions we observe an increase in the superconducting transition temperature. DC onsets in excess of 14 and 11 K have been observed for Nb₃Si and V₃Ge respectively. In each case a positive identification of the A15 phase has been made.     

Flux vortices and transport currents in type II superconductors

This article is concerned with the mechanisms by which type II super-conductors can carry currents. The equilibrium properties of the vortex lattice are described and the generalized driving force in gradients of temperature and field is derived using irreversible thermodynamics. This leads to expressions for thermal cross effects which can include pinning forces.

The field distributions which occur in a range of situations are derived and a number of useful solutions of the critical state given. In particular, the distribution in a longitudinal field is obtained, and the conditions under which force-free configurations can break down by the cutting of vortices discussed.

The effects of lattice rigidity on the summation of pinning forces is considered and it is shown that a summation based on statistical arguments uses the same approximations and leads to the same results as a dissipation argument. Theoretical expressions are derived for the vortex pinning interaction to a number of different metallurgical defects. The theoretical models are compared critically with experimental measurements of pinning forces and other related phenomena, such as flux creep, low amplitude vortex oscillations and vortex lattice defect effects. Finally, the implications for technological materials are assessed.