The Hudson Bay Lithospheric Experiment (HuBLE) : Insights into Precambrian Plate Tectonics and the Development of Mantle Keels

The UK component of HuBLE was supported by Natural Environment Research Council (NERC) grant NE/F007337/1, with financial and logistical support from the Geological Survey of Canada, Canada–Nunavut Geoscience Office, SEIS-UK (the seismic node of NERC), and First Nations communities of Nunavut. J. Beauchesne and J. Kendall provided invaluable assistance in the field. Discussions with M. St-Onge, T. Skulski, D. Corrigan and M. Sanborne-Barrie were helpful for interpretation of the data. D. Eaton and F. A. Darbyshire acknowledge the Natural Sciences and Engineering Research Council. Four stations on the Belcher Islands and northern Quebec were installed by the University of Western Ontario and funded through a grant to D. Eaton (UWO Academic Development Fund). I. Bastow is funded by the Leverhulme Trust. This is Natural Resources Canada Contribution 20130084 to its Geomapping for Energy and Minerals Program. This work has received funding from the European Research Council under the European Unions Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement no. 240473 ‘CoMITAC’.Peer reviewedPublisher PD

Variability and origin of seismic anisotropy across eastern Canada: evidence from shear-wave splitting measurements

Measurements of seismic anisotropy in continental regions are frequently interpreted with respect to past tectonic processes, preserved in the lithosphere as “fossil” fabrics. Models of the present-day sublithospheric flow (often using absolute plate motion as a proxy) are also used to explain the observations. Discriminating between these different sources of seismic anisotropy is particularly challenging beneath shields, whose thick (≥200 km) lithospheric roots may record a protracted history of deformation and strongly influence underlying mantle flow. Eastern Canada, where the geological record spans ∼3 Ga of Earth history, is an ideal region to address this issue. We use shear wave splitting measurements of core phases such as SKS to define upper mantle anisotropy using the orientation of the fast-polarization direction ϕ and delay time δt between fast and slow shear wave arrivals. Comparison with structural trends in surface geology and aeromagnetic data helps to determine the contribution of fossil lithospheric fabrics to the anisotropy. We also assess the influence of sublithospheric mantle flow via flow directions derived from global geodynamic models. Fast-polarization orientations are generally ENE-WSW to ESE-WNW across the region, but significant lateral variability in splitting parameters on a ≤100 km scale implies a lithospheric contribution to the results. Correlations with structural geologic and magnetic trends are not ubiquitous, however, nor are correlations with geodynamically predicted mantle flow directions. We therefore consider that the splitting parameters likely record a combination of the present-day mantle flow and older lithospheric fabrics. Consideration of both sources of anisotropy is critical in shield regions when interpreting splitting observations

Development of lead-free thin-film dielectrics for capacitor applications

This PhD project aims to develop lead-free thin-film dielectric materials for fixed value, voltage tunable and high-k zipping variable capacitors using growth techniques that can be scaled for silicon batch fabrication. The thesis specifically details the growth and characterisation of barium zirconate titanate (BZT) and bismuth zinc niobate (BZN) dielectric thin films. Fixed value and tunable capacitors have been realised through the use of low and high permittivity dielectric thin film materials in both the amorphous and crystalline states. Planar devices fabricated using BZT and BZN thin-film dielectrics were grown by sol-gel and RF magnetron sputtering, respectively. The effects of different bottom electrodes were also investigated. Capacitors in metal-insulator-metal (MIM) structure have been fabricated to characterise the dielectric films at low frequency (to 300 kHz). Finding alternative higher permittivity dielectrics to SiO2 for various capacitor and isolation layer applications can be a challenge. Trials were conducted that looked at using amorphous BZT and nano-crystalline/crystalline BZN as a low-k dielectric insulator. Dielectric constants of ~50 were typical for BZN, but much lower permittivity was achieved for amorphous BZT, between 2 and 15 when deposited on Cr/Au bottom electrode. Breakdown strength of amorphous BZT was extremely high (2.0 MV/cm) and far superior to that of BZN (0.35 MV/cm). The dielectric strength of BZN was increased to 0.56 MV/cm when BZN was grown on a BZT seed layer due to a change in the microstructure of the BZN film from granular to columnar. The development of a suitable dielectric BZN for use with polymer substrate was also investigated and MIM capacitors fabricated by sputter deposition. Preliminary results for nano-crystalline BZN thin film growth on gold coated liquid crystal polymer (LCP) substrates appeared encouraging with dielectric constant of 27 and loss 0.005. Crystallised BZT thin films can be used to good effect as lead-free dielectric material in tunable devices. For BZT in the ferroelectric phase, excellent tunabilities of 80% were realised when deposited on platinised silicon. This wasfound not to be the case for BZT in the paraelectric phase where tunabilities tended to be ~60% at best. The dielectric properties of thin-film MIM capacitors can be enhanced by the use of lower resistivity bottom electrode such as gold. Previous research has failed to demonstrate growth and crystallisation of BZT on gold electrode due to the fact that it is technically difficult using the sol-gel method. Films tend to crack after annealing. I have found that films can be stabilised, and the tunability of BZT thin film in the paraelectric phase can be increased significantly, by growing BZT on gold bottom electrode using a BZN buffer layer 25nm thick. A peak tunability of 83% was achieved while maintaining a low dielectric loss of ~0.01. Novel BZT multilayer structures incorporating both ferroelectric and paraelectric compositions were grown on platinised silicon resulting in a tunability of 82% at a bias field 600 kV/cm. Based on the success of growing good quality BZN thin films on gold bottom electrode, it was decided to use BZN thin film as one of the high-k dielectrics in the zipping varactor, a miniature MEMS tunable device. Trials were performed that looked at depositing BZN on thick (800nm) gold coated silicon and glass. This was successful on small sample pieces but failed when scaled up to full device wafer size (100 mm diameter) due to Cr/Au diffusion into the dielectric layer. To overcome this, a 300nm thick BZN film was sputter deposited on Ti/Pt and Ti/Au/Pt coated 100 mm glass device wafers and processed to form the dielectric layer and bottom electrode of the capacitor. As part of the process the BZN layer required patterning. Wet etching of the small features was inappropriate due to undercutting of the structure; dry etching was therefore necessary. Prior to this work there had been nothing reported on the dry etching of BZN, only wet etched using a 1:10 HF-deionised H2O solution. In this work, thin-film BZN was reactively ion etched in Ar/CHF3 plasma at a rate of 6nm per minute with excellent selectivity over platinum of 10:1. Fabrication of the curved top electrode, final assembly and device testing were undertaken by a group at Imperial College London who were collaborators on a work programme entitled, “Integrated Functional Materials for System-in-Package Applications”.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

How does flow in a pipe become turbulent?

The transition to turbulence in pipe flow does not follow the scenario familiar from Rayleigh-Benard or Taylor-Couette flow since the laminar profile is stable against infinitesimal perturbations for all Reynolds numbers. Moreover, even when the flow speed is high enough and the perturbation sufficiently strong such that turbulent flow is established, it can return to the laminar state without any indication of the imminent decay. In this parameter range, the lifetimes of perturbations show a sensitive dependence on initial conditions and an exponential distribution. The turbulence seems to be supported by three-dimensional travelling waves which appear transiently in the flow field. The boundary between laminar and turbulent dynamics is formed by the stable manifold of an invariant chaotic state. We will also discuss the relation between observations in short, periodically continued domains, and the dynamics in fully extended puffs.Comment: for the proceedings of statphys 2

An experimental model to measure the ability of headphones with active noise control to reduce patient's exposure to noise in an intensive care unit.

BACKGROUND: Defining the association between excessive noise in intensive care units, sleep disturbance and morbidity, including delirium, is confounded by the difficulty of implementing successful strategies to reduce patient's exposure to noise. Active noise control devices may prove to be useful adjuncts but there is currently little to quantify their ability to reduce noise in this complex environment. METHODS: Sound meters were embedded in the auditory meatus of three polystyrene model heads with no headphones (control), with headphones alone and with headphones using active noise control and placed in patient bays in a cardiac ICU. Ten days of recording sound levels at a frequency of 1 Hz were performed, and the noise levels in each group were compared using repeated measures MANOVA and subsequent pairwise testing. RESULTS: Multivariate testing demonstrated that there is a significant difference in the mean noise exposure levels between the three groups (p < 0.001). Subsequent pairwise testing between the three groups shows that the reduction in noise is greatest with headphones and active noise control. The mean reduction in noise exposure between the control and this group over 24 h is 6.8 (0.66) dB. The use of active noise control was also associated with a reduction in the exposure to high-intensity sound events over the course of the day. CONCLUSIONS: The use of active noise cancellation, as delivered by noise-cancelling headphones, is associated with a significant reduction in noise exposure in our model of noise exposure in a cardiac ICU. This is the first study to look at the potential effectiveness of active noise control in adult patients in an intensive care environment and shows that active noise control is a candidate technology to reduce noise exposure levels the patients experience during stays on intensive care

Travelling waves in pipe flow

A family of three-dimensional travelling waves for flow through a pipe of circular cross section is identified. The travelling waves are dominated by pairs of downstream vortices and streaks. They originate in saddle-node bifurcations at Reynolds numbers as low as 1250. All states are immediately unstable. Their dynamical significance is that they provide a skeleton for the formation of a chaotic saddle that can explain the intermittent transition to turbulence and the sensitive dependence on initial conditions in this shear flow.Comment: 4 pages, 5 figure

Fractal Stability Border in Plane Couette Flow

We study the dynamics of localised perturbations in plane Couette flow with periodic lateral boundary conditions. For small Reynolds number and small amplitude of the initial state the perturbation decays on a viscous time scale $t \propto Re$. For Reynolds number larger than about 200, chaotic transients appear with life times longer than the viscous one. Depending on the type of the perturbation isolated initial conditions with infinite life time appear for Reynolds numbers larger than about 270--320. In this third regime, the life time as a function of Reynolds number and amplitude is fractal. These results suggest that in the transition region the turbulent dynamics is characterised by a chaotic repeller rather than an attractor.Comment: 4 pages, Latex, 4 eps-figures, submitted to Phys. Rev. Le