13,961 research outputs found
Non-contact method for measurement of the microwave conductivity of graphene
We report a non-contact method for conductivity and sheet resistance
measurements of graphene samples using a high Q microwave dielectric resonator
perturbation technique, with the aim of fast and accurate measurement of
microwave conductivity and sheet resistance of monolayer and few layers
graphene samples. The dynamic range of the microwave conductivity measurements
makes this technique sensitive to a wide variety of imperfections and
impurities and can provide a rapid non-contacting characterisation method.
Typically the graphene samples are supported on a low-loss dielectric
substrate, such as quartz, sapphire or SiC. This substrate is suspended in the
near-field region of a small high Q sapphire puck microwave resonator. The
presence of the graphene perturbs both centre frequency and Q value of the
microwave resonator. The measured data may be interpreted in terms of the real
and imaginary components of the permittivity, and by calculation, the
conductivity and sheet resistance of the graphene. The method has great
sensitivity and dynamic range. Results are reported for graphene samples grown
by three different methods: reduced graphene oxide (GO), chemical vapour
deposition (CVD) and graphene grown epitaxially on SiC. The latter method
produces much higher conductivity values than the others.Comment: 8 pages, 2 figures and 2 table
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Fluid drag-reducing effect and mechanism of superhydrophobic
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.In this paper, drag-reducing property and mechanism of superhydrophobic surface are investigated.
Superhydrophobic surfaces with micro-nano textures were fabricated and tested using SEM and contact
angle measurement. Experiments on a channel and a flat plate with superhydrophobic surface were
conducted separately. For the channel flow, the drag was acquired by measuring the pressure loss. A 54%
drag reduction was found both in laminar and turbulent flow over Re range from 500 to 5000. For flow over
a plate, PIV measurement was used to obtain the velocity distribution at Reδ=12000. There was a 19%
reduction on the total stress in the whole boundary layer. Suppressions of the turbulence intensities and the
Reynolds shear stress were found, which may cause the drag reduction
The electrorheology of suspensions consisting of Na-Fluorohectorite synthetic clay particles in silicon oil
Under application of an electric field greater than a triggering electric
field kV/mm, suspensions obtained by dispersing particles of the
synthetic clay fluoro-hectorite in a silicon oil, aggregate into chain- and/or
column-like structures parallel to the applied electric field. This
micro-structuring results in a transition in the suspensions' rheological
behavior, from a Newtonian-like behavior to a shear-thinning rheology with a
significant yield stress. This behavior is studied as a function of particle
volume fraction and strength of the applied electric field, . The steady
shear flow curves are observed to scale onto a master curve with respect to
, in a manner similar to what was recently found for suspensions of laponite
clay [42]. In the case of Na-fluorohectorite, the corresponding dynamic yield
stress is demonstrated to scale with respect to as a power law with an
exponent , while the static yield stress inferred from
constant shear stress tests exhibits a similar behavior with . The suspensions are also studied in the framework of thixotropic fluids:
the bifurcation in the rheology behavior when letting the system flow and
evolve under a constant applied shear stress is characterized, and a
bifurcation yield stress, estimated as the applied shear stress at which
viscosity bifurcation occurs, is measured to scale as with to 0.6. All measured yield stresses increase with the particle
fraction of the suspension. For the static yield stress, a scaling law
, with , is found. The results are found to be
reasonably consistent with each other. Their similarities with-, and
discrepancies to- results obtained on laponite-oil suspensions are discussed
Learning State Representations via Retracing in Reinforcement Learning
We propose learning via retracing, a novel self-supervised approach for learning the state representation (and the associated dynamics model) for reinforcement learning tasks. In addition to the predictive (reconstruction) supervision in the forward direction, we propose to include “retraced” transitions for representation/model learning, by enforcing the cycle-consistency constraint between the original and retraced states, hence improve upon the sample efficiency of learning. Moreover, learning via retracing explicitly propagates information about future transitions backward for inferring previous states, thus facilitates stronger representation learning for the downstream reinforcement learning tasks. We introduce Cycle-Consistency World Model (CCWM), a concrete model-based instantiation of learning via retracing. Additionally we propose a novel adaptive “truncation” mechanism for counteracting the negative impacts brought by “irreversible” transitions such that learning via retracing can be maximally effective. Through extensive empirical studies on visual-based continuous control benchmarks, we demonstrate that CCWM achieves state-of-the-art performance in terms of sample efficiency and asymptotic performance, whilst exhibiting behaviours that are indicative of stronger representation learning
The distribution of silicate strength in Spitzer spectra of AGNs and ULIRGs
A sample of 196 AGNs and ULIRGs observed by the Infrared Spectrograph (IRS)
on Spitzer is analyzed to study the distribution of the strength of the 9.7
micron silicate feature. Average spectra are derived for quasars, Seyfert 1 and
Seyfert 2 AGNs, and ULIRGs. We find that quasars are characterized by silicate
features in emission and Seyfert 1s equally by emission or weak absorption.
Seyfert 2s are dominated by weak silicate absorption, and ULIRGs are
characterized by strong silicate absorption (mean apparent optical depth about
1.5). Luminosity distributions show that luminosities at rest frame 5.5 micron
are similar for the most luminous quasars and ULIRGs and are almost 10^5 times
more luminous than the least luminous AGN in the sample. The distributions of
spectral characteristics and luminosities are compared to those of optically
faint infrared sources at z~2 being discovered by the IRS, which are also
characterized by strong silicate absorption. It is found that local ULIRGs are
a similar population, although they have lower luminosities and somewhat
stronger absorption compared to the high redshift sources.Comment: Accepted for publication on ApJ
Determination of the electronic structure of bilayer graphene from infrared spectroscopy results
We present an experimental study of the infrared conductivity, transmission,
and reflection of a gated bilayer graphene and their theoretical analysis
within the Slonczewski-Weiss-McClure (SWMc) model. The infrared response is
shown to be governed by the interplay of the interband and the intraband
transitions among the four bands of the bilayer. The position of the main
conductivity peak at the charge neutrality point is determined by the
interlayer tunneling frequency. The shift of this peak as a function of the
gate voltage gives information about less known parameters of the SWMc model,
in particular, those responsible for the electron-hole and sublattice
asymmetries. These parameter values are shown to be consistent with recent
electronic structure calculations for the bilayer graphene and the SWMc
parameters commonly used for the bulk graphite.Comment: (v2) 11 pages, 7 figures; Important typo fixes and bibliography
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