N.J. Long

Structure and Electrical Properties of Ni-substituted Lanthanum Gallate Perovskites

Crystal structure, electrical conductivity and thermal expansion of materials in the system La0.9Sr0.1Ga1–yNiyO3 (LSGN) with 0 < y ≤ 0.5 have been studied as a function of Ni content, temperature and Po2. The materials have an orthorhombic structure at low dopant content and a hexagonal structure for higher Ni content. The σ(Po2, T) results show increasing electronic conductivity at high Po2 with increasing Ni content. At low Po2 the conductivity fits a model for predominant ionic conductivity. AC impedance spectroscopy on an electron blocking cell of the form M/LSG/LSGN/LSG/M was used to isolate the ionic conductivity in the y=0.1 and 0.2 materials. The materials were found to have appreciable ionic conductivity in air with a similar magnitude and activation energy to the electrolyte materials. An analysis of the low frequency impedance of the blocking cell provided values for D at 800°C of the order of 9 × 10−5 cm2/s. An evaluation of the so called chemical capacitance enabled determination of the electron density and mobility. The calculated electronic mobility of 3.8 × 10−3 cm2/Vs and activation energy of 0.14 eV for the y = 0.2 material are in excellent agreement with expectations of an electronic transport model involving electron hopping within the Ni impurity band. The thermal expansion coefficients of the Ni doped materials were determined as a function of temperature and dopant level. The presence of mixed conductivity suggests that this material may be useful as an electrode for a lanthanum gallate based fuel cell or other electrochemical device.

Performance of La0.9Sr0.1Ga0.5Ni0.5O3 as a Cathode for a Lanthanum Gallate Fuel Cell

The cathodic overpotential of a La0.9Sr0.1Ga0.5Ni0.5O3 (LSGN) electrode on a La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) electrolyte was studied using the current interruption technique. The electrode performance was found to be clearly superior to La0.85Sr0.15MnO3 and comparable to La0.6Sr0.4CoO3 in the temperature range of 700–850°C. The exchange current density is a maximum in air and is smaller in either 100% O2 or 2% O2 suggesting that the occupation of surface adsorption sites is important. The overpotential-current density curves suggest that charge transfer is the limiting process for high current densities. An energy dispersive spectrometry line scan did not show any significant interdiffusion of Ni ions across the electrode/electrolyte boundary.

Modeling of Vortex Paths in HTS

A random walk model of the vortex paths in high temperature superconductors is investigated for different relative densities of c-axis, ab-plane and point pinning defects. The model suggests an origin for some of the more unusual features seen in the field angle dependence of the critical current. It also predicts greater noise in critical currents for fields parallel to the plane of the dominant pinning defects and anomalous behavior of n-values. The random walk model also suggests an alternative form for the expected field angle dependence of the upper critical fields in disordered layered superconductors.

Formation of epitaxial YBCO thin films by ex-situ processing of a polymerized complex

High T/sub c/ and J/sub c/ YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films have been formed by ex-situ processing of a spin coated organic sol-gel precursor based on a polymerized complex. To form the precursor we have reacted metal cations with a chelating organic acid, then formed polyesterified complexes through reaction with a polyhydroxy alcohol. This method permits a simple, cheap and reliable route to the fabrication of YBCO films. The value of J/sub c/ achieved for the film deposited onto a LaAlO/sub 3/ [100] substrate at 77 K under a zero field is routinely as high as 0.25 MA/cm/sup 2/ with T/sub c,zero/ of 90.6 K.

Relating Critical Currents to Defect Populations in Superconductors

Analyzing critical currents from an information theoretic or statistical point of view allows one to identify distinct populations of microstates contributing to the critical current under particular conditions of temperature and applied field. We show how this knowledge can be correlated with the known microstructure of a sample to identify how different physical populations of pinning centers are contributing to these statistical populations of microstates. We will then show that by tracking the variation of critical current with temperature, field, and field angle we can construct a picture of the relative contributions of different defect populations under different conditions. We particularly focus our analysis on YBCO thin film coated conductors with potential commercial application.

Low-Temperature Pinning Behavior of MOD YBCO Coated Conductors

We have investigated the temperature and field dependence of the critical current for YBCO coated conductors fabricated by metal-organic deposition. Samples with Dy2O3 or BaZrO3 nanoparticle pinning centers were compared with undoped and Dy-substituted YBCO. While emphasis in development is often limited by practical constraints to measurements at 77 K and self-field, the majority of applications will require operation at much lower temperatures and moderate-to-high magnetic fields. We compare Ic(B,T) for temperatures in the range 20-77 K and magnetic fields up to 7 T. Pinning enhancements observed at 77 K and attributed to nanoparticle additions are less conspicuous at 20 K.

Structure property relationships in a nanoparticle-free SmBCO coated conductor

We examine the temperature, field and field angle dependence of the critical current of a SmBa2Cu3Oy coated conductor produced by reactive co-evaporation. A transmission electron microscopy based microstructural analysis shows the film contains extended c-axis defects, stacking faults, and two different species of inclined defects. By applying a maximum entropy decomposition of the field angle dependent critical current I-c(theta) datasets we are able to identify the individual contributions of these defects to the critical current even though they do not produce distinct peaks but rather an anisotropy in I-c(theta). We are able to confirm the structure property relationships by determining the matching fields where each of the individual defect contributions are a maximum and showing that these are consistent with the observed microstructure. For a critical current component having a maximum magnitude at an intermediate temperature we propose a model of thermally activated depinning to explain the behaviour.

Formation of Nanoparticles in Zr and Dy Doped YBCO MOD Superconducting Films

We study the formation mechanism of nanoparticles in thin films of the superconductor YBa2Cu3O7-δ (YBCO). We form the films by metal-organic deposition (MOD) on buffered, textured metal substrates. Through the addition of Dy or Zr salts to the precursor solution we precipitate (Y,Dy)2O3 and BaZrO3 nanoparticles, uniformly distributed through the film thickness. By quenching samples during the film growth, we show the nanoparticles form in the precursor layer before YBCO growth. The size of the nanoparticles was quantitatively analysed by TEM. We found that Zr doping produces smaller nanoparticles than Dy doping.