25,639 research outputs found
Magnetic bearings for free-piston Stirling engines
The feasibility and efficacy of applying magnetic bearings to free-piston Stirling-cycle power conversion machinery currently being developed for long-term space missions are assessed. The study was performed for a 50-kWe Reference Stirling Space Power Converter (RSSPC) which currently uses hydrostatic gas bearings to support the reciprocating displacer and power piston assemblies. Active magnetic bearings of the attractive electromagnetic type are feasible for the RSSPC power piston. Magnetic support of the displacer assembly would require unacceptable changes to the design of the current RSSPC. However, magnetic suspension of both displacer and power piston is feasible for a relative-displacer version of the RSSPC. Magnetic suspension of the RSSPC power piston can potentially increase overall efficiency by 0.5 to 1 percent (0.1 to 0.3 efficiency points). Magnetic bearings will also overcome several operational concerns associated with hydrostatic gas bearing systems. These advantages, however, are accompanied by a 5 percent increase in specific mass of the RSSPC
Two Modes of Solid State Nucleation - Ferrites, Martensites and Isothermal Transformation Curves
When a crystalline solid such as iron is cooled across a structural
transition, its final microstructure depends sensitively on the cooling rate.
For instance, an adiabatic cooling across the transition results in an
equilibrium `ferrite', while a rapid cooling gives rise to a metastable twinned
`martensite'. There exists no theoretical framework to understand the dynamics
and conditions under which both these microstructures obtain. Existing theories
of martensite dynamics describe this transformation in terms of elastic strain,
without any explanation for the occurence of the ferrite. Here we provide
evidence for the crucial role played by non-elastic variables, {\it viz.},
dynamically generated interfacial defects. A molecular dynamics (MD) simulation
of a model 2-dimensional (2d) solid-state transformation reveals two distinct
modes of nucleation depending on the temperature of quench. At high
temperatures, defects generated at the nucleation front relax quickly giving
rise to an isotropically growing `ferrite'. At low temperatures, the defects
relax extremely slowly, forcing a coordinated motion of atoms along specific
directions. This results in a twinned critical nucleus which grows rapidly at
speeds comparable to that of sound. Based on our MD results, we propose a
solid-state nucleation theory involving the elastic strain and non-elastic
defects, which successfully describes the transformation to both a ferrite and
a martensite. Our work provides useful insights on how to formulate a general
dynamics of solid state transformations.Comment: 3 pages, 4 B/W + 2 color figure
Thickness dependent magnetotransport in ultra-thin manganite films
To understand the near-interface magnetism in manganites, uniform, ultra-thin
films of La_{0.67}Sr_{0.33}MnO_3 were grown epitaxially on single crystal (001)
LaAlO_3 and (110) NdGaO_3 substrates. The temperature and magnetic field
dependent film resistance is used to probe the film's structural and magnetic
properties. A surface and/or interface related dead-layer is inferred from the
thickness dependent resistance and magnetoresistance. The total thickness of
the dead layer is estimated to be for films on NdGaO_3 and for films on LaAlO_3.Comment: 11 pages, 4 figure
Lattice Green's function for crystals containing a planar interface
Flexible boundary condition methods couple an isolated defect to a
harmonically responding medium through the bulk lattice Green's function; in
the case of an interface, interfacial lattice Green's functions. We present a
method to compute the lattice Green's function for a planar interface with
arbitrary atomic interactions suited for the study of line defect/interface
interactions. The interface is coupled to two different semi-infinite bulk
regions, and the Green's function for interface-interface, bulk-interface and
bulk-bulk interactions are computed individually. The elastic bicrystal Green's
function and the bulk lattice Green's function give the interaction between
bulk regions. We make use of partial Fourier transforms to treat in-plane
periodicity. Direct inversion of the force constant matrix in the partial
Fourier space provides the interface terms. The general method makes no
assumptions about the atomic interactions or crystal orientations. We simulate
a screw dislocation interacting with a twin boundary in Ti using
flexible boundary conditions and compare with traditional fixed boundary
conditions results. Flexible boundary conditions give the correct core
structure with significantly less atoms required to relax by energy
minimization. This highlights the applicability of flexible boundary conditions
methods to modeling defect/interface interactions by \textit{ab initio}
methods
Entanglement enhanced atomic gyroscope
The advent of increasingly precise gyroscopes has played a key role in the
technological development of navigation systems. Ring-laser and fibre-optic
gyroscopes, for example, are widely used in modern inertial guidance systems
and rely on the interference of unentangled photons to measure mechanical
rotation. The sensitivity of these devices scales with the number of particles
used as . Here we demonstrate how, by using sources of entangled
particles, it is possible to do better and even achieve the ultimate limit
allowed by quantum mechanics where the precision scales as 1/N. We propose a
gyroscope scheme that uses ultra-cold atoms trapped in an optical ring
potential.Comment: 19 pages, 2 figure
A theoretical review of the operation of vibratory stress relief with particular reference to the stabilization of large-scale fabrications
Vibratory stress relief (VSR) is widely used on large welded fabrications to stabilize the structures so that they do not distort during further machining or during operational duty. The level of applied stress achieved during VSR on such structures is only 5–10 per cent of the yield stress. It is, therefore, not obvious how these applied loads come to modify the level of residual stress. It is suggested here that the reason for the success of VSR applied to large fabrications lies (a) in the origin of the residual stresses and (b) in the partial relief of these residual stresses by the initiation of the transformation of retained austenite particles (in the size range from 1 to 25 µm) by the movement of dislocations into positions that are favourable for the nucleation of martensite embryos. The shear deformation associated with the transformation of retained austenite into martensite will reduce the residual stress field to the point where the stability of the structure may be assured
Unsupervised decoding of long-term, naturalistic human neural recordings with automated video and audio annotations
Fully automated decoding of human activities and intentions from direct
neural recordings is a tantalizing challenge in brain-computer interfacing.
Most ongoing efforts have focused on training decoders on specific, stereotyped
tasks in laboratory settings. Implementing brain-computer interfaces (BCIs) in
natural settings requires adaptive strategies and scalable algorithms that
require minimal supervision. Here we propose an unsupervised approach to
decoding neural states from human brain recordings acquired in a naturalistic
context. We demonstrate our approach on continuous long-term
electrocorticographic (ECoG) data recorded over many days from the brain
surface of subjects in a hospital room, with simultaneous audio and video
recordings. We first discovered clusters in high-dimensional ECoG recordings
and then annotated coherent clusters using speech and movement labels extracted
automatically from audio and video recordings. To our knowledge, this
represents the first time techniques from computer vision and speech processing
have been used for natural ECoG decoding. Our results show that our
unsupervised approach can discover distinct behaviors from ECoG data, including
moving, speaking and resting. We verify the accuracy of our approach by
comparing to manual annotations. Projecting the discovered cluster centers back
onto the brain, this technique opens the door to automated functional brain
mapping in natural settings
Comprehensive assessment of a hotspot with persistent bancroftian filariasis in coastal Sri Lanka
Metallization of Fluid Hydrogen
The electrical resistivity of liquid hydrogen has been measured at the high
dynamic pressures, densities and temperatures that can be achieved with a
reverberating shock wave. The resulting data are most naturally interpreted in
terms of a continuous transition from a semiconducting to a metallic, largely
diatomic fluid, the latter at 140 GPa, (ninefold compression) and 3000 K. While
the fluid at these conditions resembles common liquid metals by the scale of
its resistivity of 500 micro-ohm-cm, it differs by retaining a strong pairing
character, and the precise mechanism by which a metallic state might be
attained is still a matter of debate. Some evident possibilities include (i)
physics of a largely one-body character, such as a band-overlap transition,
(ii) physics of a strong-coupling or many-body character,such as a Mott-Hubbard
transition, and (iii) processes in which structural changes are paramount.Comment: 12 pages, RevTeX format. Figures available on request; send mail to:
[email protected] To appear: Philosophical Transaction of the Royal
Society
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