739 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
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
A Streamwise Constant Model of Turbulence in Plane Couette Flow
Streamwise and quasi-streamwise elongated structures have been shown to play
a significant role in turbulent shear flows. We model the mean behavior of
fully turbulent plane Couette flow using a streamwise constant projection of
the Navier Stokes equations. This results in a two-dimensional, three velocity
component () model. We first use a steady state version of the model to
demonstrate that its nonlinear coupling provides the mathematical mechanism
that shapes the turbulent velocity profile. Simulations of the model
under small amplitude Gaussian forcing of the cross-stream components are
compared to DNS data. The results indicate that a streamwise constant
projection of the Navier Stokes equations captures salient features of fully
turbulent plane Couette flow at low Reynolds numbers. A system theoretic
approach is used to demonstrate the presence of large input-output
amplification through the forced model. It is this amplification
coupled with the appropriate nonlinearity that enables the model to
generate turbulent behaviour under the small amplitude forcing employed in this
study.Comment: Journal of Fluid Mechanics 2010, in pres
Bridging tools to better understand environmental performances and raw materials supply of traction batteries in the future EU fleet
Sustainable and smart mobility and associated energy systems are key to decarbonise the EU and develop a clean, resource efficient, circular and carbon-neutral future. To achieve the 2030 and 2050 targets, technological and societal changes are needed. This transition will inevitably change the composition of the future EU fleet, with an increasing share of electric vehicles (xEVs). To assess the potential contribution of lithium-ion traction batteries (LIBs) in decreasing the environmental burdens of EU mobility, several aspects should be included. Even though environmental assessments of batteries along their life-cycle have been already conducted using life-cycle assessment, a single tool does not likely provide a complete overview of such a complex system. Complementary information is provided by material flow analysis and criticality assessment, with emphasis on supply risk. Bridging complementary aspects can better support decision-making, especially when different strategies are simultaneously tackled. The results point out that the future life-cycle GWP of traction LIBs will likely improve, mainly due to more environmental-friendly energy mix and improved recycling. Even though second-use will postpone available materials for recycling, both these end-of-life strategies allow keeping the values of materials in the circular economy, with recycling also contributing to mitigate the supply risk of Lithium and Nickel
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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