Application of Neutron Diffraction NDE to High-Temperature Superconducting Composites

Since the discovery of superconducting materials with relatively high transition temperatures (Tc), there has been a considerable effort both to understand the reason for the high Tc and to improve the mechanical properties, (the latter has been a limiting factor for practical applications). The YBa2Cu3O7-δ (YBCO) compounds have received considerable attention because of their high Tc and high upper critical magnetic field [l–3]. Additions of silver have recently been shown to improve the mechanical properties (toughness and strength) of these compounds [4]. Furthermore, the addition of the Ag can improve the conductive path between grains of superconducting YBCO and possibly help reduce the “weak-link” effect [5]. Note that whereas the addition of a low-volume fraction of silver does not adversely affect the superconductivity the introduction of transition metals to YBCO can have a detrimental effect on the superconducting properties. Also, the addition of silver has a minimal affect on the stress free lattice spacing. During fabrication of YBCO/Ag composites, differential thermal expansion upon cooling can lead to potentially troublesome residual stresses. Since the Ag contracts more than the YBCO, good bonding between the ceramic and silver could lead [6] to tensile stresses in the silver and compressive stresses in the YBCO for relatively small percentages of Ag. These residual stresses could lead to premature failure of the composite, debonding of the YBCO and Ag, and/or microcracking, which will affect the flow of superconducting current and the life expectancy of components made from this material. An understanding of the nature and magnitude of these stresses will help improve the design of these composites

Repetition Enhancement for Frequency-Modulated but Not Unmodulated Sounds: A Human MEG Study

BACKGROUND: Decoding of frequency-modulated (FM) sounds is essential for phoneme identification. This study investigates selectivity to FM direction in the human auditory system. METHODOLOGY/PRINCIPAL FINDINGS: Magnetoencephalography was recorded in 10 adults during a two-tone adaptation paradigm with a 200-ms interstimulus-interval. Stimuli were pairs of either same or different frequency modulation direction. To control that FM repetition effects cannot be accounted for by their on- and offset properties, we additionally assessed responses to pairs of unmodulated tones with either same or different frequency composition. For the FM sweeps, N1m event-related magnetic field components were found at 103 and 130 ms after onset of the first (S1) and second stimulus (S2), respectively. This was followed by a sustained component starting at about 200 ms after S2. The sustained response was significantly stronger for stimulation with the same compared to different FM direction. This effect was not observed for the non-modulated control stimuli. CONCLUSIONS/SIGNIFICANCE: Low-level processing of FM sounds was characterized by repetition enhancement to stimulus pairs with same versus different FM directions. This effect was FM-specific; it did not occur for unmodulated tones. The present findings may reflect specific interactions between frequency separation and temporal distance in the processing of consecutive FM sweeps

Complex Processes from Dynamical Architectures with Time-Scale Hierarchy

The idea that complex motor, perceptual, and cognitive behaviors are composed of smaller units, which are somehow brought into a meaningful relation, permeates the biological and life sciences. However, no principled framework defining the constituent elementary processes has been developed to this date. Consequently, functional configurations (or architectures) relating elementary processes and external influences are mostly piecemeal formulations suitable to particular instances only. Here, we develop a general dynamical framework for distinct functional architectures characterized by the time-scale separation of their constituents and evaluate their efficiency. Thereto, we build on the (phase) flow of a system, which prescribes the temporal evolution of its state variables. The phase flow topology allows for the unambiguous classification of qualitatively distinct processes, which we consider to represent the functional units or modes within the dynamical architecture. Using the example of a composite movement we illustrate how different architectures can be characterized by their degree of time scale separation between the internal elements of the architecture (i.e. the functional modes) and external interventions. We reveal a tradeoff of the interactions between internal and external influences, which offers a theoretical justification for the efficient composition of complex processes out of non-trivial elementary processes or functional modes

Time Scale Hierarchies in the Functional Organization of Complex Behaviors

Traditional approaches to cognitive modelling generally portray cognitive events in terms of ‘discrete’ states (point attractor dynamics) rather than in terms of processes, thereby neglecting the time structure of cognition. In contrast, more recent approaches explicitly address this temporal dimension, but typically provide no entry points into cognitive categorization of events and experiences. With the aim to incorporate both these aspects, we propose a framework for functional architectures. Our approach is grounded in the notion that arbitrary complex (human) behaviour is decomposable into functional modes (elementary units), which we conceptualize as low-dimensional dynamical objects (structured flows on manifolds). The ensemble of modes at an agent’s disposal constitutes his/her functional repertoire. The modes may be subjected to additional dynamics (termed operational signals), in particular, instantaneous inputs, and a mechanism that sequentially selects a mode so that it temporarily dominates the functional dynamics. The inputs and selection mechanisms act on faster and slower time scales then that inherent to the modes, respectively. The dynamics across the three time scales are coupled via feedback, rendering the entire architecture autonomous. We illustrate the functional architecture in the context of serial behaviour, namely cursive handwriting. Subsequently, we investigate the possibility of recovering the contributions of functional modes and operational signals from the output, which appears to be possible only when examining the output phase flow (i.e., not from trajectories in phase space or time)