262 research outputs found
Top and side gated epitaxial graphene field effect transistors
Three types of first generation epitaxial graphene field effect transistors
(FET) are presented and their relative merits are discussed. Graphene is
epitaxially grown on both the carbon and silicon faces of hexagonal silicon
carbide and patterned with electron beam lithography. The channels have a Hall
bar geometry to facilitate magnetoresistance measurements. FETs patterned on
the Si-face exhibit off-to-on channel resistance ratios that exceed 30. C-face
FETs have lower off-to-on resistance ratios, but their mobilities (up to 5000
cm2/Vs) are much larger than that for Si-face transistors. Initial
investigations into all-graphene side gate FET structures are promising
Influence of different design parameters on a coplanar capacitive sensor performance
Coplanar capacitive sensors are employed in Non-destructive Testing (NDT) methods to measure the difference in dielectric properties of the materials. The most important design parameters for a coplanar capacitive sensor include the shape, size, and separation distance of the electrodes which affect the sensor performance. In addition, the impact of the shielding plate and guard electrode should be considered. In the framework of this paper, numerical simulations and physical experiments are studied for two shapes of electrodes, triangular and rectangular, by examining different sizes and different separation distances between electrodes to assess and analyze the important features of the coplanar capacitive electrodes, such as the penetration and strength of the electric field as a function of sensor geometrical properties. Therefore, a detailed analysis of numerical simulation using Finite Element Modelling (FEM) is provided to study these geometric parameters. In addition, the influence of the different frequencies, lift-off, and the presence or absence of a metal shielding plate and guard electrode on the output result is analyzed. Finally, sensors were manufactured and several experiments were carried out under different configurations. Comparison of the numerical simulation results and physical experiments illustrate that they are in good qualitative agreement
Atomic Friction Investigations on Ordered Superstructures
We review recent friction measurements on ordered superstructures performed by atomic force microscopy. In particular, we consider ultrathin KBr films on NaCl(001) and Cu(001) surfaces, single and bilayer graphene on SiC(0001), and the herringbone reconstruction of Au(111). Atomically resolved friction images of these systems show periodic features spanning across several unit cells. Although the physical mechanisms responsible for the formation of these superstructures are quite different, the experimental results can be interpreted within the same phenomenological framework. A comparison between experiments and modeling shows that, in the cases of KBr films on NaCl(001) and of graphene films, the tip-surface interaction is well described by a potential with the periodicity of the substrate which is modulated or, respectively, superimposed with a potential with the symmetry of the superstructur
Adhesion measurements in MOS2 dry lubricated contacts to inform predictive tribological numerical models : comparison between laboratory-tested samples and ball bearings from the niriss mechanism
International audiencePredicting the tribological behaviour of dry lubricants remains difficult because it greatly depends on their mechanical and physicochemical environment. While it is difficult to analytically model dry lubrication, Discrete Element Method (DEM)-based modelling has been able to provide valuable insight into the tribological behaviour of dry lubricated contacts. The present study aims to experimentally define interactions between the discrete elements used for simulating different materials in contact, in order to accurately model and predict the tribological behaviour of dry lubricants. Those interactions are here defined by using the work of adhesion (W) between engineering materials: AISI440C, pristine MoS2 coating, as well as the related transfer film. A method was developed and applied on regular laboratory tribological test samples and ball bearings from the Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument of the James Webb Space Telescope. Measured W values were consistent between all worn surfaces. The first DEM modelling results exhibit behaviours similar to those observed experimentally including surface plasticization and transfer
Self-lubricating polymer composites : using numerical trbology to hightlight their design criterion
International audienceAfter the cessation of RT/Duroid 5813, manufacturing tests were performed by CNES and ESA/ESTL in order to find an alternative material. Although PGM-HT was selected as the best candidate, limitations about its tribological capabilities to replace RT/Duroid 5813 were later pointed out. Today, the predictability of the tribological behaviour of those materials is not fully overcome. The motivation to this work is to complement studies of self-lubricating materials by coupling experimental analyses with numerical modelling, in order to predict their tribological behaviour. A Discrete Element Method is chosen to construct the numerical material, because it allows to represent wear and the third body generation at the scale of the ball/retainer contact. An underlying role of the adhesion between components in controlling the tribological properties of the transfer film has been observed
Self-lubricating composite bearings: Effect of fibre length on its tribological properties by DEM modelling
International audienceSelf-lubricating polymer-based composites are used in space and in aircraft mechanisms as materials for solid lubricated systems. Such composites mostly consist of a polymeric matrix and fillers of two kinds: hard fillers (fibres made of glass, or of minerals) and solid lubricating particles (made of MoS 2). Their advantages are that they provide their own lubrication, and they can be used in both very high and very low temperatures (from −40 up to ~200 F). Precision ball bearings with these composites are manufactured since the 60's in these bearings the retainer material itself provides the lubrication. From the experimental analyses implemented (X-ray tomography, SEM observations, and experiences in a tribometer); it is possible to observe that the geometry of the fillers has a strong influence on the third body rheology. Nevertheless, the confined nature of the contact does not allow in-situ observation. To overcome this difficulty a combined numerical/experimental approach is carried out. To be able to reproduce the evolution of third-body particles within the contact, Discrete Element Methods (DEM) is used. Such an approach allows to represent wear: by the construction of an equivalent continuous medium resulting from the incorporation of interaction laws between the discrete particles. The motivation to this work is the understanding of the impact of filler geometry o tribological behaviour of these materials. More specifically, the goal is to study the influence of the fibre length in the tribological behaviour of self-lubricating composites by Discrete Element Methods (DEM)
Size-selective nanoparticle growth on few-layer graphene films
We observe that gold atoms deposited by physical vapor deposition onto few
layer graphenes condense upon annealing to form nanoparticles with an average
diameter that is determined by the graphene film thickness. The data are well
described by a theoretical model in which the electrostatic interactions
arising from charge transfer between the graphene and the gold particle limit
the size of the growing nanoparticles. The model predicts a nanoparticle size
distribution characterized by a mean diameter D that follows a scaling law D
proportional to m^(1/3), where m is the number of carbon layers in the few
layer graphene film.Comment: 15 pages, 4 figure
Mobility variations in mono- and multi-layer graphene films
The electric properties of mono- and multi-layer graphene films were
systematically studied with the layer number determined by their optical
contrast. The current modulation increased monotonically with a decrease in the
layer number due to the reduction of the interlayer scattering. Carrier
mobility in the monolayer was significantly greater than that in the multilayer
due to linear dispersion relation. On the other hand, in the monolayer, carrier
transport was extremely sensitive to charged impurity density due to the
reduction in screening effect, which causes larger mobility variation.
Reduction of the charged impurity density is thus key for high mobility.Comment: 4 page
Surface energy engineering of graphene
Contact angle goniometry is conducted for epitaxial graphene on SiC. Although
only a single layer of epitaxial graphene exists on SiC, the contact angle
drastically changes from 69{\deg} on SiC substrates to 92{\deg} with graphene.
It is found that there is no thickness dependence of the contact angle from the
measurements of single, bi, and multi layer graphene and highly ordered
pyrolytic graphite (HOPG). After graphene is treated with oxygen plasma, the
level of damage is investigated by Raman spectroscopy and correlation between
the level of disorder and wettability is reported. By using low power oxygen
plasma treatment, the wettability of graphene is improved without additional
damage, which can solve the adhesion issues involved in the fabrication of
graphene devices
Ultrahard carbon film from epitaxial two-layer graphene
Atomically thin graphene exhibits fascinating mechanical properties, although
its hardness and transverse stiffness are inferior to those of diamond. To
date, there hasn't been any practical demonstration of the transformation of
multi-layer graphene into diamond-like ultra-hard structures. Here we show that
at room temperature and after nano-indentation, two-layer graphene on SiC(0001)
exhibits a transverse stiffness and hardness comparable to diamond, resisting
to perforation with a diamond indenter, and showing a reversible drop in
electrical conductivity upon indentation. Density functional theory
calculations suggest that upon compression, the two-layer graphene film
transforms into a diamond-like film, producing both elastic deformations and
sp2-to-sp3 chemical changes. Experiments and calculations show that this
reversible phase change is not observed for a single buffer layer on SiC or
graphene films thicker than 3 to 5 layers. Indeed, calculations show that
whereas in two-layer graphene layer-stacking configuration controls the
conformation of the diamond-like film, in a multilayer film it hinders the
phase transformation.Comment: Published online on Nature Nanotechnology on December 18, 201
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