N.M. Strickland

Enhanced in-field critical currents of YBCO second-generation (2G) wire by Dy additions

The addition of dysprosium oxide nanoparticles is shown to improve the critical current in perpendicular magnetic fields for second-generation (2G) wire formed by metal–organic deposition (MOD). Typical enhancements in Jc are from 0.17 MA cm−2 to over 0.33 MA cm−2 at 77 K and Bperp = 1.5 T. TEM analysis shows that we are introducing (Y,Dy)2O3 nanoparticles with dimensions of 10–50 nm. A simple theoretical analysis shows that the maximum pinning effect for additions is expected at excess concentrations of approximately 70% DyO1.5, i.e. for YBa2Cu3O7−δ+0.7DyO1.5 if the added nanoparticles are randomly dispersed and a strong pinning model is valid. An interesting feature is that the critical current in parallel field is reduced in these samples. We present evidence that shows this may be due to reduced planar defects in the YBCO.

TEM observation of the microstructure of metal-organic deposited YBa2Cu3O7−δ with Dy additions

The microstructure of metal-organic deposited YBa2Cu3O7−δ with dysprosium (Dy) additions has been investigated by transmission electron microscopy (TEM). Dy additions which increase the density of normal-state nanoparticles in the YBCO have been demonstrated to enhance the critical current densities in moderate magnetic fields. The influence of nanoparticles, stacking faults and other planar defects on flux pinning is discussed. We observed a high density of nanoparticles in the size range of 10–50 nm, which may act as flux pinning centres to enhance the critical current density of the material. Stacking faults and planar defects are observed which may also be effective flux pinning centres in YBCO samples with and without Dy addition.

Flux Pinning by Barium Stannate Nanoparticles in MOD YBCO Coated Conductors

Tin-based nanoparticles, proposed to be barium stannate, have been formed in YBCO films fabricated by metal-organic deposition, through modification of the precursor solution. These randomly-oriented 30-50 nm particles are dispersed throughout the film thickness, providing an enhancement in the isotropic flux pinning relative to undoped YBCO. For a low level of nanoparticle addition of 2 vol%, a flux-pinning force enhancement of up to 32% is achieved. We report field and field-angle dependence of the transport critical current, transmission electron microscopy, and X-ray diffraction of our YBCO films on RABiTS substrates, and compare with similar barium zirconate additions.

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.

Enhanced Flux Pinning in MOD Second Generation HTS Wires by Silver- and Copper-Ion Irradiation

We have studied the effect of medium-energy ion irradiation on flux pinning in second-generation HTS wires. Irradiation with 74 MeV silver ions and 50 MeV copper ions have produced up to 60% enhancements in critical current. The field-angle dependence shows enhancements over a wide angle range, suggesting that the irradiation defects are point-like defects rather than continuous columns. Such isotropic enhancements are generally preferable over narrow-angle anisotropic effects for wire applications. Ion irradiation can then be used to create a model population of defects to study concentration dependences. We present transport, magnetization, and TEM results to probe the nature of the defects formed by this irradiation.

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.