879 research outputs found
A local field emission study of partially aligned carbon-nanotubes by AFM probe
We report on the application of Atomic Force Microscopy (AFM) for studying
the Field Emission (FE) properties of a dense array of long and vertically
quasi-aligned multi-walled carbon nanotubes grown by catalytic Chemical Vapor
Deposition on a silicon substrate. The use of nanometric probes enables local
field emission measurements allowing investigation of effects non detectable
with a conventional parallel plate setup, where the emission current is
averaged on a large sample area. The micrometric inter-electrode distance let
achieve high electric fields with a modest voltage source. Those features
allowed us to characterize field emission for macroscopic electric fields up to
250 V/m and attain current densities larger than 10 A/cm. FE
behaviour is analyzed in the framework of the Fowler-Nordheim theory. A field
enhancement factor 40-50 and a turn-on field 15 V/m at an inter-electrode distance of 1 m are estimated.
Current saturation observed at high voltages in the I-V characteristics is
explained in terms of a series resistance of the order of M. Additional
effects as electrical conditioning, CNT degradation, response to laser
irradiation and time stability are investigated and discussed
Probing the superconducting condensate on a nanometer scale
Superconductivity is a rare example of a quantum system in which the
wavefunction has a macroscopic quantum effect, due to the unique condensate of
electron pairs. The amplitude of the wavefunction is directly related to the
pair density, but both amplitude and phase enter the Josephson current : the
coherent tunneling of pairs between superconductors. Very sensitive devices
exploit the superconducting state, however properties of the {\it condensate}
on the {\it local scale} are largely unknown, for instance, in unconventional
high-T cuprate, multiple gap, and gapless superconductors.
The technique of choice would be Josephson STS, based on Scanning Tunneling
Spectroscopy (STS), where the condensate is {\it directly} probed by measuring
the local Josephson current (JC) between a superconducting tip and sample.
However, Josephson STS is an experimental challenge since it requires stable
superconducting tips, and tunneling conditions close to atomic contact. We
demonstrate how these difficulties can be overcome and present the first
spatial mapping of the JC on the nanometer scale. The case of an MgB film,
subject to a normal magnetic field, is considered.Comment: 7 pages, 6 figure
“In conspiracies we trust”: interpersonal/institutional trust and beliefs in conspiracy theories during the COVID-19 pandemic
Highly optimized transitions to turbulence
We study the Navier-Stokes equations in three dimensional plane Couette flow geometry subject to stream-wise constant initial conditions and perturbations. The resulting two dimensional/three component (2D/3C) model has no bifurcations and is globally (non-linearly) stable for all Reynolds numbers R, yet has a total transient energy amplification that scales like R/sup 3/. These transients also have the particular dynamic flow structures known to play a central role in wall bounded shear flow transition and turbulence. This suggests a highly optimized tolerance (HOT) model of shear flow turbulence, where streamlining eliminates generic bifurcation cascade transitions that occur in bluff body flows, resulting in a flow which is stable to arbitrary changes in Reynolds number but highly fragile in amplifying arbitrarily small perturbations. This result indicates that transition and turbulence in special streamlined geometries is not a problem of linear or nonlinear instability, but rather a problem of robustness
Vacuum freeze-drying effect on bioactive compounds of eggplant (Solanum Melongena L.).
Vacuum Freeze Drying (VFD) is a low temperature drying technique that may be used for food preservation. The aim of this work is to evaluate how VFD operating conditions affect eggplants bioactive compounds loss after drying. Samples were freeze-dried under different pressure and temperature conditions, and had their ascorbic acid, total polyphenol and antioxidant capacity percent loss measured after processing. Under the tested conditions, lower temperatures resulted in lower antioxidant capacity in the final product, while lower chamber pressures resulted in lower total polyphenol content
Local Tunneling Study of Three-Dimensional Order Parameter in the -band of Al-doped MgB Single Crystals
We have performed local tunneling spectroscopy on high quality
MgAlB single crystals by means of Variable Temperature Scanning
Tunneling Spectroscopy (STS) in magnetic field up to 3 Tesla. Single gap
conductance spectra due to c-axis tunneling were extensively measured, probing
different amplitudes of the three-dimensional as a function of Al
content. Temperature and magnetic field dependences of the conductance spectra
were studied in S-I-N configuration: the effect of the doping resulted in a
monotonous reduction of the locally measured down to 24K for x=0.2. On
the other hand, we have found that the gap amplitude shows a maximum value
meV for x=0.1, while the ratio increases
monotonously with doping. The locally measured upper critical field was found
to be strongly related to the gap amplitude, showing the maximum value
for x=0.1 substituted samples. For this Al concentration the
data revealed some spatial inhomogeneity in the distribution of on
nanometer scale.Comment: 4 pages, 3 figure
Diffuse interface modeling of eggplants vacuum freeze-drying process
Vacuum freeze-drying (VFD) can be used for preserving food with small effects on nutritional qualities. VFD modelling is useful to select off-line the best operating conditions to avoid product overheating and reduce the drying time, thus saving energy. A 3D diffuse interface model was developed to simulate in-silico eggplant VFD. By comparing the experimental drying time and product temperature, heat transfer coefficient and vapor diffusivity were estimated under different operating conditions
Perturbation Energy Production in Pipe Flow over a Range of Reynolds Numbers using Resolvent Analysis
The response of pipe flow to physically realistic, temporally and spatially continuous(periodic) forcing is investigated by decomposing the resolvent into orthogonal forcing and response pairs ranked according to their contribution to the resolvent 2-norm. Modelling the non-linear terms normally neglected by linearisation as unstructured forcing permits qualitative extrapolation of the resolvent norm results beyond infinitesimally small perturbations to the turbulent case. The concepts arising have a close relationship to input output transfer function analysis methods known in the control systems literature. The body forcings that yield highest disturbance energy gain are identified and ranked by the decomposition and a worst-case bound put on the energy gain integrated across the pipe cross-section. Analysis of the spectral variation of the corresponding response modes reveals interesting comparisons with recent observations of the behavior of the streamwise velocity in high Reynolds number (turbulent) pipe flow, including the importance of very long scales of the order of ten pipe radii, in the extraction of turbulent energy from the mean flow by the action of turbulent shear stress against the velocity gradient
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