A. Bailey

Josephson behaviour for high critical current density YBCO+Ag thick films on YSZ substrates

Abstract High critical current density thick films of yttrium barium copper oxide plus 10 wt.% silver have been fabricated on yttria stablised zirconia substrates. The films are approximately 10 μm thick, superconducting with T c ( R = 0) in the range 90.5 to 91.0 K and are found to have j c s close to 3000 A cm -2 at 77 K in zero applied magnetic field. The thermal cycle for the production of these films requires the use of temperatures in excess of the peritectic and therefore involves melt processing. This has a number of advantages, including film texturing, which is significantly improved with the addition of Ag, and the formation of an inert BaZrO 3 barrier layer which has fine CuO needles dispersed throughout. This layer prevents atomic diffusion and film poisoning. At 77 K, the j c values decreased markedly for applied magnetic fields increasing from 0–20 Oe, and then decreased slowly for increasing fields, being almost constant for field values ∥#62; 150 Oe. The temperature variation, near T c , of the critical current i c was best described by i c £ (1 − T / T c ) 1.52 , the Ginzburg-Landau 3 2 power law characteristic, and indicates that the granular films may be modelled as an array of Josephson-coupled grains which have a short coherence length. Variable thickness bridges patterned into the thick films showed very clear DC and AC characteristics that were very similar to those expected from a single Josephson junction.

High current capacity textured thick films of YBCO on YSZ obtained by melt processing

Abstract By processing thick films of YBCO on stabilized zirconia at temperatures in excess of the peritectic value critical current densities in excess of 103 A cm−2 have been obtained at 77 K in film thicknesses of ≈ 40 μm. The microstructures of these films include extensive, crystalline sheets which appear to have been produced from a liquid phase system. These sheets are strongly textured with the c-axes perpendicular to the substrate. With increasing processing temperature, severe microcracking appears, resulting in a major limitation on the current carrying capacity. Variable thickness microbridges cut into the films exhibit many features characteristic of Josephson junctions. The addition of silver to these thick films during processing greatly improves their surface morphology, increases the degree of texture obtained and results in critical current densities in excess of 1300 A cm−2 at 77 K.

Effects of helium absorption on the superconducting mechanism of YBa2Cu3Oy

Abstract During the course of a series of measurements of the electrical conductivity and Hall effect in the YBa2Cu3Oy superconductors, it was observed that the presence of an helium atmosphere during the measurements led to reversible changes in the electrical properties including increases in the critical temperature. The results of a series of measurements to investigate this effect are described and possible mechanisms considered. It is suggested that helium enters the lattice and leads to different internal pressure effects in different parts of the unit cell.

High critical density thick films of YBCO on YSZ

Abstract High critical current density thick films of yttrium barium copper oxide have been fabricated on yttrium stabilized zirconia substrates. The films are approximately 40 μm thick, superconducting with Tc(R=0) ≈91.5 K and are found to have jcs in the range 750 to 1200 A cm-2 at 77 K in zero applied magnetic fields. This jc range was found to correspond to a 5°C variation in the highest processing temperature. At 77 K, jc decreased rapidly for applied magnetic fields, 0–20 Oe, but was almost constant for field values 100 Oe.

Surface texturing of bulk YBa2Cu3O7−δ samples

Abstract A process technique is described which results in significant surface texturing of bulk YBa 2 Cu 3 O 7-δ samples. Using standard calcining and sintering cycles, surface textured material with grain size ⩾ 50 μm has been produced. The T c of the material is 93 K and J c (transport), for predominantly textured material, is in excess of 1.5×10 3 A cm -2 in zero applied field at 77 K.

YBa2Cu3O7−σ thick films on alumina substrates

To attempt to solve the important technological problem of developing techniques for the growth of thick film YBa2Cu3O7−σ layers on polycrystalline Al2O3 substrates, we investigated a number of different barrier layer systems. In general these do not provide high current capacity structures; however, by the use of silver doping and sapphire substrates, critical current densities of 200 A cm−2 were obtained. While this is more than an order of magnitude less than comparable films on yttrium-stabilized zirconia, it represents a significant step towards the fabrication of films on the technologically more attractive substrate material.

High critical current density thick films of (YBaCuO): Ag obtained by melt processing

Abstract Following the development and optimization of a processing route to fabricate high current capacity thick films of YBaCuO on yttrium stabilized zirconia, attempts were made to improve the film properties further. The addition of silver powder to the printing ink before processing has been found to produce films of high surface smoothness and of thickness less than 10 μm, with critical current densities at 77 K of almost 3000 A cm −2 in zero applied magnetic field.

A comparative study of the YBa2Cu3Ox and Y2Ba3Cu5Oy superconductors: Structural, electrical and mechanical properties

Abstract Measurements of the Seebeck coefficients of the two commonly studied superconducting compositions YBa 2 Cu 3 O x and Y 2 Ba 3 Cu 5 O y show that the value for the former compound is always significantly greater than that for the latter. Structural studies show a slight change in the lattice parameters of the orthorhombic structure which is common to both. Damping loss and elastic modulus observations in the compound show a significant change near the 240 K as well as at the superconducting transition.

Low frequency (5 MHz) impedance measurements of thick YBCO films

Abstract Two-point low frequency impedance measurements of microstrip transmission lines, fabricated from thick YBCO ( + 10 wt, 9% Ag) films on YSZ and sapphire substrates, have provided intrinsic properties for these granular thick films. The variation of the resistance with temperature for all the films studied, with and without an external DC magnetic field, followed that found by the conventional four-point DC technique. However, the temperature variation of the reactance for these granular thick films showed four important features: (1) for temperatures below T c ( R =0) the reactance was purely inductive even when superconductivity had been destroyed by the application of an external magnetic field, but the tail of the resistive transition was still present; (2) high quality, melt textured, thick films had a small double peak (or broad single peak) structure in the inductance for temperatures above T c which corresponded to changes in the tail of the resistivity transition and these peaks are interpreted in terms of magnetic field penetration into the inter- and intra- granular regions; (3) poor quality films and those intentionally doped with Y 2 BaCuO 5 had a constant inductive value extending from low temperatures to a temperature corresponding to the end of the resistive tail; (4) as the temperature is increased beyond the onset of superconductivity the inductive reactance decreased rapidly, for all films studied, to a negative value (i.e. a capacitive reactance) and then decreased approximately linearly for further increases in temperature. The value of the capacitive reactance appeared to depend on the amount of (211) YBCO present in the film. Analysis of the film conducting path in terms of parallel conductance and susceptance components gives rise to a temperature dependence very similar to the susceptibility curves obtained from bulk YBCO material and their observed structure may be interpreted in terms of a granular model.

Enhancement of Tc in YBa2Cu3O7−δ superconductors by gas absorption

Abstract Carefully controlled absorption of gas into the yttrium barium cuprate superconductors at low temperatures leads to significant and reproducible increases in the critical temperature. In the limit a value of 141K has been achieved with nitrogen. The effect has now also been observed with helium, argon and oxygen and we believe that the mechanism responsible for the increases may be associated with a catalytic absorption of gas into the open structure of the lattice.