HYDROGENATED AMORPHOUS SILICON THIN-FILM DEPOSITION BY DIRECT PHOTO-ENHANCED DECOMPOSITION OF SILANE USING AN INTERNAL HYDROGEN DISCHARGE LAMP.

Hydrogenated amorphous silicon (a-Si:H) thin films have been deposited from silane using a novel photo-enhanced decomposition technique. The system comprises a hydrogen discharge lamp contained within the reaction vessel; this unified approach allows high energy photon excitation of the silane molecules without absorption by window materials or the need for mercury sensitisation. The film growth rates (exceeding 4 Angstrom/s) and material properties obtained are comparable to those of films produced by plasma-enhanced CVD techniques. The reduction of energetic charged particles in the film growth region should enable the fabrication of cleaner semiconductor/insulator interfaces in thin-film transistors.

Density, sp3 content and internal layering of DLC films by X-ray reflectivity and electron energy loss spectroscopy

A variety of hydrogenated and non-hydrogenated amorphous carbon thin films have been characterized by means of grazing-incidence X-ray reflectivity (XRR) to give information about their density, thickness, surface roughness and layering. We used XRR to validate the density of ta-C, ta-C:H and a-C:H films derived from the valence plasmon in electron energy loss spectroscopy measurements, up to 3.26 and 2.39 g/cm3 for ta-C and ta-C:H, respectively. By comparing XRR and electron energy loss spectroscopy (EELS) data, we have been able for the first time to fit a common electron effective mass of m*/me = 0.87 for all amorphous carbons and diamond, validating the `quasi-free' electron approach to density from valence plasmon energy. While hydrogenated films are found to be substantially uniform in density across the film, ta-C films grown by the filtered cathodic vacuum arc (FCVA) show a multilayer structure. However, ta-C films grown with an S-bend filter show a high uniformity and only a slight dependence on the substrate bias of both sp3 and layering.

The influence of repeated loading, residual stresses and shakedown on the behaviour of tribological contacts

Most tribological pairs carry their service load not just once but for a very large number of repeated cycles. During the early stages of this life, protective residual stresses may be developed in the near surface layers which enable loads which are of sufficient magnitude to cause initial plastic deformation to be accommodated purely elastically in the longer term. This is an example of the phenomenon of 'shakedown' and when its effects are incorporated into the design and operation schedule of machine components this process can lead to significant increases in specific loading duties or improvements in material utilization. Although the underlying principles can be demonstrated by reference to relatively simple stress systems, when a moving Hertzian pressure distribution in considered, which is the form of loading applicable to many contact problems, the situation is more complex. In the absence of exact solutions, bounding theorems, adopted from the theory of plasticity, can be used to generate appropriate load or shakedown limits so that shakedown maps can be drawn which delineate the boundaries between potentially safe and unsafe operating conditions. When the operating point of the contact lies outside the shakedown limit there will be an increment of plastic strain with each application of the load - these can accumulate leading eventually to either component failure or the loss of material by wear. © 2005 Elsevier Ltd. All rights reserved.

The effects of pile flexibility on pile-loading in laterally spreading slopes

Piles passing through laterally spreading slopes can be subjected to considerable loads by the soil flowing past them. Many case histories have been documented of piles which suffered failure as a result of horizontal loads exerted by the flowing soil. This paper details the results of a series of dynamic centrifuge tests carried out at Cambridge University Engineering Department, to investigate the transfer of load from the spreading soil to the piles passing through it, with particular emphasis on the effective stress state of soil elements immediately upslope and downslope of the pile. This soil stress state can be calculated by virtue of instrumentation measuring both horizontal total stress and pore pressures at locations close to the upslope and downslope faces of the piles. By comparison of results obtained for both rigid and flexible piles, conclusions will be drawn as to the effects of pile flexibility on modifying the behavior of the soil-pile system.

Maintaining internal representations: The role of the human superior parietal lobe

In sensorimotor integration, sensory input and motor output signals are combined to provide an internal estimate of the state of both the world and one's own body. Although a single perceptual and motor snapshot can provide information about the current state, computational models show that the state can be optimally estimated by a recursive process in which an internal estimate is maintained and updated by the current sensory and motor signals. These models predict that an internal state estimate is maintained or stored in the brain. Here we report a patient with a lesion of the superior parietal lobe who shows both sensory and motor deficits consistent with an inability to maintain such an internal representation between updates. Our findings suggest that the superior parietal lobe is critical for sensorimotor integration, by maintaining an internal representation of the body's state.

Modelling of waterfront cantilever sheet pile walls subjected to earthquake loading

The seismic performance of waterfront cantilever sheet pile retaining walls is of continuing interest to geotechnical engineers as these structures suffer severe damage and even complete failure during earthquakes. This is often precipitated by liquefaction of the surrounding soil, either in the backfill or in front of the wall. This paper presents results from a series of small-scale plane strain models that were tested on a 1-g shaking table and recorded using a high-speed, high-resolution digital camera. The technique of Particle Image Velocimetry (PIV) was applied in order to allow the failure mechanisms to be visualised. It is shown that using PIV analyses it is possible to obtain failure mechanisms for a cantilever wall in liquefiable soil. These failure mechanisms are compared with those obtained for a cantilever wall in dry soil, previously carried out at a similar scale. It was observed that seismic liquefaction causes significant displacement in much larger zones of soil near the retaining wall compared to an equivalent dry case. The failure mechanism for a cantilever wall with liquefiable backfill, but with a remediated zone designed not to liquefy, is also presented and compared to the unremediated case.

Tsunami wave loading on coastal houses: A model approach

The Asian tsunami of 26 December 2004 killed over 220 000 people and devastated coastal structures, including many thousands of traditional brick-built homes. This paper presents the results of model tests that compare the impact of a tsunami wave on a typical coastal house with that on a new tsunami resistant design developed in the USA and now built in Sri Lanka Digital images recorded during the test reveal how the tsunami wave passed through the new house design without damaging it but severely damaged the typical coastal house. Pressure sensor results also provided further insight into tsunami wave loading, indicating that the established Japanese method significantly underestimates maximum impact load.

Mechanical constraint and loading on ferroelectric memory capacitors

The influence of mechanical constraint imposed by device geometry upon the switching response of a ferroelectric thin film memory capacitor is investigated. The memory capacitor was represented by two-dimensional ferroelectric islands of different aspect ratio, mechanically constrained by surrounding materials. Its ferroelectric non-linear behaviour was modeled by a crystal plasticity constitutive law and calculated using the finite element method. The switching response of the device, in terms of remnant charge storage, was determined as a function of geometry and constraint. The switching response under applied in-plane tensile stress and hydrostatic pressure was also studied experimentally. Our results showed that (1) the capacitor's aspect ratio could significantly affect the clamping behaviour and thus the remnant polarization, (2) it was possible to maximise the switching charge through the optimisation of the device geometry, and (3) it is possible to find a critical switching stress at zero electric field and a critical coercive field at zero residual stress. © 2009 Materials Research Society.

Study of the electric loading aspects of the BDFM using a lumped parameter thermal model

Details of a lumped parameter thermal model for studying thermal aspects of the frame size 180 nested loop rotor BDFM at the University of Cambridge are presented. Predictions of the model are verified against measured end winding and rotor bar temperatures that were measured with the machine excited from a DC source. The model is used to assess the thermal coupling between the stator windings and rotor heating. The thermal coupling between the stator windings is assessed by studying the difference of the steady state temperatures of the two stator end windings for different excitations. The rotor heating is assessed by studying the temperatures of regions of interest for different excitations.

Towards topology-free optimization - An application to turbine internal cooling geometries

The application of automated design optimization to real-world, complex geometry problems is a significant challenge - especially if the topology is not known a priori like in turbine internal cooling. The long term goal of our work is to focus on an end-to-end integration of the whole CFD Process, from solid model through meshing, solving and post-processing to enable this type of design optimization to become viable & practical. In recent papers we have reported the integration of a Level Set based geometry kernel with an octree-based cut- Cartesian mesh generator, RANS flow solver, post-processing & geometry editing all within a single piece of software - and all implemented in parallel with commodity PC clusters as the target. The cut-cells which characterize the approach are eliminated by exporting a body-conformal mesh guided by the underpinning Level Set. This paper extends this work still further with a simple scoping study showing how the basic functionality can be scripted & automated and then used as the basis for automated optimization of a generic gas turbine cooling geometry. Copyright © 2008 by W.N.Dawes.

Shock transmission and eigenvalue veering within a coupled system.

The operation of dynamical systems in harsh environments requires continuous monitoring. Internal sensors may be used to monitor the conditions in real time. A typical example is the sensor and electronic components used in space structures which, especially during launch, are subject to huge g force. The paper will present an experimental and theoretical study on a simplified model used to analyze the possible cause of high acceleration on the enclosed sensors and equipments due to impulsive loading. The model system consists of two beams coupled using compliant connections. An impulse hammer excites one beam, and vibrations are transmitted to the indirectly driven beam. A theoretical model is developed using a Rayleigh-Ritz approach and validated using experimental results in both the frequency and time domains. Monto Carlo simulation was done with random masses positioned on the indirectly driven beam to determine the worst-case conditions for maximum peak acceleration. Highest acceleration levels were found when mode matching in the two beams led to veering behavior in the coupled modes. The results suggest guidelines for the detailed design of internal components of a structure exposed to shock loading from its environment. [The authors thank Schlumberger Cambridge Research for financial support.].

Impedance control and internal model use during the initial stage of adaptation to novel dynamics in humans.

This study investigated the neuromuscular mechanisms underlying the initial stage of adaptation to novel dynamics. A destabilizing velocity-dependent force field (VF) was introduced for sets of three consecutive trials. Between sets a random number of 4-8 null field trials were interposed, where the VF was inactivated. This prevented subjects from learning the novel dynamics, making it possible to repeatedly recreate the initial adaptive response. We were able to investigate detailed changes in neural control between the first, second and third VF trials. We identified two feedforward control mechanisms, which were initiated on the second VF trial and resulted in a 50% reduction in the hand path error. Responses to disturbances encountered on the first VF trial were feedback in nature, i.e. reflexes and voluntary correction of errors. However, on the second VF trial, muscle activation patterns were modified in anticipation of the effects of the force field. Feedforward cocontraction of all muscles was used to increase the viscoelastic impedance of the arm. While stiffening the arm, subjects also exerted a lateral force to counteract the perturbing effect of the force field. These anticipatory actions indicate that the central nervous system responds rapidly to counteract hitherto unfamiliar disturbances by a combination of increased viscoelastic impedance and formation of a crude internal dynamics model.

How are internal models of unstable tasks formed?

The results of recent studies suggest that humans can form internal models that they use in a feedforward manner to compensate for both stable and unstable dynamics. To examine how internal models are formed, we performed adaptation experiments in novel dynamics, and measured the endpoint force, trajectory and EMG during learning. Analysis of reflex feedback and change of feedforward commands between consecutive trials suggested a unified model of motor learning, which can coherently unify the learning processes observed in stable and unstable dynamics and reproduce available data on motor learning. To our knowledge, this algorithm, based on the concurrent minimization of (reflex) feedback and muscle activation, is also the first nonlinear adaptive controller able to stabilize unstable dynamics.