The Electronic Properties of Thin Film YBa2Cu3O7 Low Angle Grain Boundaries

Critical currents in the latest biaxially textured ‘coated conductors’ are now limited by 2D networks of low angle grain boundaries (LAGBs) with misorientation θm = 1 - 10°. In order to understand and optimise current transfer in these materials it is essential to elucidate the electromagnetic behaviour of the LAGB. This work presents an investigation into the transport properties of [001]-tilt LAGBs formed by the thin film deposition of YBa2Cu3O7 onto bicrystalline substrates. Through the use of a precision two-axis goniometer, measurements of the V-I characteristic and critical current density were made as magnetic field was rotated in angles θ and φ relative to the LAGB defect. It is found that for fields applied parallel to the LAGB defect plane, dissipation is dominated by the viscous flux flow of vortices along the boundary. Clear evidence for this is found in the V-I characteristic, which displays an increased linearity indicative of the viscous regime. It is shown that the number of vortex rows involved in the flow process can discontinuously switch, leading to a V-I characteristic made up of straight segments of different gradient. For fields applied away from the LAGB defect plane a kinked vortex structure develops and the boundary critical current density, JcGB, is determined by the channelling of vortex segments still lying within the LAGB. The channelling regime is seen in angular measurements as a marked decrease in JcGB(θ,φ) as field becomes aligned to within critical angles φK or θK of the LAGB. The aligned vortices experience a reduction in dimensionality that is manifest in a reduced temperature dependence of JcGB(T). For fields applied at a sufficiently large angle from the defect plane the deleterious effects of the LAGB on current transport are mitigated considerably. In this regime a collinear vortex structure is regained and transport is controlled by the intragranular (IG) sections of the LAGB track; JcGB = δJcIG where δ ~ 0.8, a result that is independent of field, temperature and angle for a 4.9° LAGB. For rotation of the applied field within the LAGB defect plane, the presence of a pinning peak in JcGB for field aligned to the dislocation array is confirmed and modelled. The peak is found to be absent in IG track measurements and increases relative to the intrinsic peak with both increasing field and temperature. In addition, a pronounced angular hysteresis is presented, which is directly linked to a corresponding ‘static’ hysteresis in JcGB(B) with field. Magneto-optic measurements confirm that this effect is controlled by the flux density profile in the IG regions of the LAGB track. Finally, above a temperature, angle and sample dependent merging field, B*, the LAGB is found to be effectively transparent, as δ ~ 1. This is due to the irreversibility line, above which dissipation occurs across the whole LAGB track.EPSR

Investigating learning in construction organizations

Learning in construction has received scant attention within extant theories of generic organizational learning. One of the apparently distinct characteristics of construction organization is that its business mainly runs through projects. In contrast, the origin of the organizational learning concept mainly stems from routine-based organizations. The present study investigates how these theories are applied in the construction domain. To be more specific, it focuses on contracting organizations that engage with the UK performance enhancement initiative known as Constructing Excellence. The paper summarises the theoretical perspective on the current state of knowledge about this topic and the full methodology to be adopted. In overall terms, the methodology takes a multifaceted approach involving six major stages. The first phases of this process are now complete. It takes the form of a business audit relating to the type and size of projects currently being undertaken and how the project teams are managed. In themselves, the results contain new empirical data that has informed the direction of the rest study. Two general groups of construction companies were identified: general contractors and specialist/subcontractors. Each of these groups has a different tendency for how they manage their project teams. The former tends to reform for each new project, while the latter favours staying together. The initial premise is that each of these practices implies different learning mechanisms. Further study and analysis will depart from these initial findings

One of Us: Social Identity, Group Belonging and Leadership

The title of this paper suggests a paradox. Leaders are simultaneously separate from and the same as their followers. They have higher status, greater influence, and more power, and occupy a different role, but they are also members of and identify with the same group as their followers. George W. Bush, as president, is certainly quite separate from most Americans, but he identifies himself as an American, and he spends a great deal of time making sure all Americans know this. However, if we take a fairly common type of definition of leadership as “a process of social influence through which an individual enlists and mobilizes the aid of others in the attainment of a collective goal” (Chemers, 2001, 376), then we can see that Bush is only really a leader to those who will follow—those who share his definition of American and therefore those who share his identity, group membership, and collective goal

Fretting wear and fatigue in taper junctions of modular orthopaedic implants

Multi-component, or modular, implants have a number of advantages over monoblock implants, but also a number of disadvantages related to micromotion and fretting at the taper interface. Depending on the fretting regime, either fatigue or wear damage may occur, resulting in greatly reduced fatigue lives and the production of metallic wear debris. Current revision rates of hip implants with replaceable necks are double those with fixed necks. To improve the understanding of taper performance and identify factors that can reduce wear and fatigue damage, 3-D finite element modelling of a taper connection representing the neck-stem junction of a dual modular hip prosthesis was performed. This included evaluations of short- and long-term taper strength, wear simulations and fatigue life predictions. Wear simulations included material removal due to wear. Fatigue damage calculations were performed using the critical plane Smith-Watson-Topper and Fatemi-Socie parameters together with an isotropic, linear damage accumulation model. To facilitate fatigue calculations, a unique method of tracking a consistent set of material points was presented. Taper geometry, assembly force and the magnitude of the cyclic load were all found to affect taper performance. Increasing the assembly load reduced micromotion, but reductions in wear were offset by an increase in contact pressure. Increased loads resulted in significant increases in fatigue damage. Clinically relevant wear rates were predicted, suggesting that wear volumes produced by neck-stem tapers are similar to rates of head-neck and bearing surfaces of large head metal-on-metal total hips. Fatigue crack initiation sites were predicted to be within the taper junction, located at the edges of the wear patches in regions of partial slip. Due to the evolution of the contact and sub-surface stress/strains, the inclusion of material removal was found to be critical in the prediction of both crack initiation site and fatigue damage

Astrometry.net: Blind astrometric calibration of arbitrary astronomical images

We have built a reliable and robust system that takes as input an astronomical image, and returns as output the pointing, scale, and orientation of that image (the astrometric calibration or WCS information). The system requires no first guess, and works with the information in the image pixels alone; that is, the problem is a generalization of the "lost in space" problem in which nothing--not even the image scale--is known. After robust source detection is performed in the input image, asterisms (sets of four or five stars) are geometrically hashed and compared to pre-indexed hashes to generate hypotheses about the astrometric calibration. A hypothesis is only accepted as true if it passes a Bayesian decision theory test against a background hypothesis. With indices built from the USNO-B Catalog and designed for uniformity of coverage and redundancy, the success rate is 99.9% for contemporary near-ultraviolet and visual imaging survey data, with no false positives. The failure rate is consistent with the incompleteness of the USNO-B Catalog; augmentation with indices built from the 2MASS Catalog brings the completeness to 100% with no false positives. We are using this system to generate consistent and standards-compliant meta-data for digital and digitized imaging from plate repositories, automated observatories, individual scientific investigators, and hobbyists. This is the first step in a program of making it possible to trust calibration meta-data for astronomical data of arbitrary provenance.Comment: submitted to A