10,452 research outputs found
Two-dimensional simulations of temperature and current-density distribution in electromigrated structures
We report on the application of a feedback-controlled electromigration
technique for the formation of nanometer-sized gaps in mesoscopic gold wires
and rings. The effect of current density and temperature, linked via Joule
heating, on the resulting gap size is investigated. Experimentally, a good
thermal coupling to the substrate turned out to be crucial to reach electrode
spacings below 10 nm and to avoid overall melting of the nanostructures. This
finding is supported by numerical calculations of the current-density and
temperature profiles for structure layouts subjected to electromigration. The
numerical method can be used for optimizing the layout so as to predetermine
the location where electromigation leads to the formation of a gap.Comment: 9 pages, 6 figure
Funnel control for boundary control systems
We study a nonlinear, non-autonomous feedback controller applied to boundary
control systems. Our aim is to track a given reference signal with prescribed
performance. Existence and uniqueness of solutions to the resulting closed-loop
system is proved by using nonlinear operator theory. We apply our results to
both hyperbolic and parabolic equations.Comment: 26 pages, thoroughly revised version. The system class has been
generalized considerably. Added general example class of parabolic problem
Analysis of Granular Flow in a Pebble-Bed Nuclear Reactor
Pebble-bed nuclear reactor technology, which is currently being revived
around the world, raises fundamental questions about dense granular flow in
silos. A typical reactor core is composed of graphite fuel pebbles, which drain
very slowly in a continuous refueling process. Pebble flow is poorly understood
and not easily accessible to experiments, and yet it has a major impact on
reactor physics. To address this problem, we perform full-scale,
discrete-element simulations in realistic geometries, with up to 440,000
frictional, viscoelastic 6cm-diameter spheres draining in a cylindrical vessel
of diameter 3.5m and height 10m with bottom funnels angled at 30 degrees or 60
degrees. We also simulate a bidisperse core with a dynamic central column of
smaller graphite moderator pebbles and show that little mixing occurs down to a
1:2 diameter ratio. We analyze the mean velocity, diffusion and mixing, local
ordering and porosity (from Voronoi volumes), the residence-time distribution,
and the effects of wall friction and discuss implications for reactor design
and the basic physics of granular flow.Comment: 18 pages, 21 figure
Stability of Big Surface Bubbles: Impact of Evaporation and Bubbles Size
Surface bubbles have attracted much interest in the past decades. In this
article, we propose to explore the lifetime and thinning dynamics of
centimetric surface bubbles. We study the impact of the bubbles size as well as
that of the atmospheric humidity through a careful control and systematic
variation of the relative humidity in the measuring chamber. We first adress
the question of the drainage under saturated water vapor conditions and show
that a model including both capillary and gravity driven drainage provides the
best prediction for this process. Additionally, unprecedented statistics on the
bubbles lifetimes confirm experimentally that this parameter is set by
evaporation to leading order. We make use of a model based on the overall
thinning dynamics of the thin film and assume a rupture thickness of the order
10-100 nm to obtain a good representation of these data. For experiments
conducted far from saturation, the convective evaporation of the bath is shown
to dominate the overall mass loss in the cap film due to evaporation
Thermophotovoltaic space power system, phase 3
Work performed on a research and development program to establish the feasibility of a solar thermophotovoltaic space power generation concept was summarized. The program was multiphased. The earlier work is summarized and the work on the current phase is detailed as it pertains to and extends the earlier work. Much of the experimental hardware and materials development was performed on the internal program. Experimental measurements and data evaluation were performed on the contracted effort. The objectives of the most recent phase were: to examine the thermal control design in order to optimize it for lightweight and low cost; to examine the concentrator optics in an attempt to relieve pointing accuracy requirements to + or - 2 degrees about the optical axis; and to use the results of the thermal and optical studies to synthesize a solar thermophotovoltaic (STPV) module design that is optimized for space application
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Design of a Solid Freeform Fabrication Diamond Reactor
Solid Freeform Fabrication (SFF) has progressed from the visualization aided
stage of computer aided designs (CAD) to rapid prototyping of structural parts.
Among the promising techniques for producing structural prototypes is the technology
ofchemical vapor deposition (CVD) ofpolycrystalline diamond. This paper
discusses the thermodynamic and kinetic theories that suggest that structural
diamond may be rapidly deposited at rates approaching 1 mmJhr from the vapor
phase at metastable thermodynamic conditions. The design of a reactor that will
produce structural diamond prototypes is discussed. This reactor combines downstream
microwave plasma enhanced chemical vapor deposition (DMWPECVD)
with a scanned CO2 laser that locally heats the substrate to diamond deposition
temperatures. The input:Fases are H2, 02' CH4, and Ar. The operating pressure range
of the reactor is 1 x 10- to 7 x 102 Torr. The reactor is designed for in situ determination
of deposit thickness while deposition occurs as well as having the capacity
of fitting on an existing resonance enhanced multiphoton ionization time of flight
mass spectroscopy (REMPITOFMS) apparatus that will allow for plasma diagnostics
immediately above the heated substrate. Plasma diagnostics will be
employed to determine the active metastable species that results in diamond
deposition so that optimization can be made ofthe operating parameters to maximize
diamond selectivity and deposition rate.Mechanical Engineerin
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