18 research outputs found
Measurement of the microwave effective permittivity in tensile-strained polyvinylidene difluoride trifluoroethylene filled with graphene
We report an interesting effect in the form of a rise (up to 13%) in the permittivity of graphene (GE) filled polyvinylidene difluoride trifluoroethylene (P(VDF-TrFE)) subjected to a small uniaxial deformation (up to 7% in the principal direction). Our findings differ from GE-PVDF homopolymer samples that show a decrease of permittivity upon elongation. We argue that the VDF content which controls the spontaneous polarization has a profound effect on the charge storage through the addition of interface density by the GE phase. (C) 2014 AIP Publishing LLC
Microwave and mechanical properties of quartz/graphene-based polymer nanocomposites
We report microwave spectroscopy studies of graphene-based polymer-matrix composite materials subject to uniaxial elongation. The samples were prepared via shear mixing under the same thermal processing conditions of amorphous styrene butadiene rubber (SBR) with quartz grains on the order of micrometers in size and/or graphene sheets with thickness 10-20 nm and average lateral size 200 mu m. An important result is the observation of a significant increase (up to 25%) in the effective microwave permittivity of hybridized nanocomposites comprising both quartz and graphene compared to the nanocomposites with quartz only. We suggest that the coating of quartz grains by graphene sheets is the most likely origin of this synergetic effect. In all cases, we also observe that the permittivity spectrum is unaffected by strain up to 8%. By examining the mechanical response, it is shown that the elasticity network of SBR polymer chains is significantly affected in the rubbery state by filling SBR with graphene and quartz particles. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793411
Le champ critique de claquage de films d'oxyde de polyphénylène réalisés par voie électrochimique
The electrical breakdown of thin films of polyphenylene oxide electrochemically deposited on stainless steel plane substrates has been studied. First it was examined the dependence of the medium surrounding the dielectric and the electrodes (nature, hydrostatic pressure) on the breakdown voltage and on its statistical distribution. Between sphere and plane electrodes, it appears that even for pressurified gases, breakdown of the film is caused by the gas breakdown. We have analysed the discharges occurring at atmosphere pressure in the test cell. Breakdown of the film occurred when the electric field due to the charge deposited on its surface was about 230 V/m. We have also studied self-healing capacitors with PPO as a dielectric, and determined the life-time of this material.On a étudié le claquage électrique de films minces d'oxyde de polyphénylène de quelques microns d'épaisseur déposés par voie électrochimique sur un plan en acier inoxydable. L'étude a d'abord été faite en rampe de tension continue dans la géométrie d'électrodes sphèreplan, en fonction du milieu ambiant liquide ou gazeux. L'influence de la pression sur la rigidité diélectrique du matériau, les distributions statistiques de Weibull et les cratères formés au moment du claquage, dans les différents milieux et dans les deux polarités de l'électrode sphérique montrent que le claquage du matériau est causé par des décharges qui se produisent dans le milieu environnant. A partir de l'analyse quantitative de ces décharges, on propose comme critère de caractérisation de la rupture d'un matériau sounmis aux décharges, le champ créé au moment de la rupture par les charges déposées à sa surface. On a réalisé ensuite des échantillons plans autocicatrisables par dépôt de couches minces d'aluminium (quelques milliers d'Å d'épaisseur) sur le film de PPO. On étudie dans cette configuration la durée de vie du matériau
Claquage electrique de films d'oxyde de polyphenylene realises par voie electrochimique
SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc
Latitudinal profile of the ionospheric disturbance dynamo magnetic signature: comparison with the DP2 magnetic disturbance
During magnetic storms, the auroral electrojets intensification affects the
thermospheric circulation on a global scale. This process which leads to
electric field and current disturbance at middle and low latitudes, on the
quiet day after the end of a storm, has been attributed to the ionospheric
disturbance dynamo (Ddyn). The magnetic field disturbance observed as a
result of this process is the reduction of the H component amplitude in the
equatorial region which constitutes the main characteristic of the
ionospheric disturbance dynamo process, associated with a westward electric
current flow. The latitudinal profile of the Ddyn disturbance dynamo magnetic
signature exhibits an eastward current at mid latitudes and a westward one at
low latitudes with a substantial amplification at the magnetic equator. Such
current flow reveals an "anti-Sq" system established between the mid
latitudes and the equatorial region and opposes the normal Sq current vortex.
However, the localization of the eastward current and consequently the
position and the extent of the "anti-Sq" current vortex changes from one
storm to another. Indeed, for a strong magnetic storm, the eastward current
is well established at mid latitudes about 45° N and for a weak magnetic
storm, the eastward current is established toward the high latitudes (about
60° N), near the Joule heating region, resulting in a large
"anti-Sq"
current cell. The latitudinal profile of the Ddyn disturbance as well as the
magnetic disturbance DP2 generated by the mechanism of prompt penetration of
the magnetospheric convection electric field in general, show a weak
disturbance at the low latitudes with a substantial amplification at the
magnetic equator. Due to the intensity of the storm, the magnitude of the DP2
appears higher than the Ddyn over the American and Asian sector contrary to
the African sector
Statistical study of the night-time F-layer dynamics at the magnetic equator in West Africa during the solar minimum period 1995–1997
In this paper, we report on the night-time equatorial F-layer height
behaviour at Korhogo (9.2° N, 5° W; 2.4° S
dip lat), Ivory Coast, in the West African sector during the solar minimum
period 1995–1997. The data were collected from quarter-hourly ionograms of
an Ionospheric Prediction Service (IPS) 42-type vertical sounder. The main focus of this work was to study
the seasonal changes in the F-layer height and to clarify the equinox
transition process recently evidenced at Korhogo during 1995, the year of
declining solar flux activity. The F-layer height was found to vary strongly
with time, with up to three main phases. The night-to-night variability of
these morphological phases was then analysed. The early post-sunset slow
rise, commonly associated with rapid chemical recombination processes in the
bottom part of the F layer, remained featureless and was observed regardless of the date. By contrast, the following event, either presented like the
post-sunset height peak associated with the evening <b><i>E</i></b> Ă— <b><i>B</b></i> drift, or was delayed
to the midnight sector, thus involving another mechanism. The statistical
analysis of the occurrence of these events throughout the solar minimum
period 1995–1997 revealed two main F-layer height patterns, each
characteristic of a specific season. The one with the post-sunset height
peak was associated with the northern winter period, whereas the other, with the
midnight height peak, characterized the northern summer period. The
transition process from one pattern to the other took place during the
equinox periods and was found to last only a few weeks. We discuss these
results in the light of earlier works
Correction to 'Simulation of electric field and current during the 11 June 1993 disturbance dynamo event: Comparison with the observations'
International audienceNot Availabl