1,172 research outputs found
"Wet-to-Dry" Conformational Transition of Polymer Layers Grafted to Nanoparticles in Nanocomposite
The present communication reports the first direct measurement of the
conformation of a polymer corona grafted around silica nano-particles dispersed
inside a nanocomposite, a matrix of the same polymer. This measurement
constitutes an experimental breakthrough based on a refined combination of
chemical synthesis, which permits to match the contribution of the neutron
silica signal inside the composite, and the use of complementary scattering
methods SANS and SAXS to extract the grafted polymer layer form factor from the
inter-particles silica structure factor. The modelization of the signal of the
grafted polymer on nanoparticles inside the matrix and the direct comparison
with the form factor of the same particles in solution show a clear-cut change
of the polymer conformation from bulk to the nanocomposite: a transition from a
stretched and swollen form in solution to a Gaussian conformation in the matrix
followed with a compression of a factor two of the grafted corona. In the
probed range, increasing the interactions between the grafted particles (by
increasing the particle volume fraction) or between the grafted and the free
matrix chains (decreasing the grafted-free chain length ratio) does not
influence the amplitude of the grafted brush compression. This is the first
direct observation of the wet-to-dry conformational transition theoretically
expected to minimize the free energy of swelling of grafted chains in
interaction with free matrix chains, illustrating the competition between the
mixing entropy of grafted and free chains, and the elastic deformation of the
grafted chains. In addition to the experimental validation of the theoretical
prediction, this result constitutes a new insight for the nderstanding of the
general problem of dispersion of nanoparticles inside a polymer matrix for the
design of new nanocomposites materials
A Scintillating Fiber Tracker With SiPM Readout
We present a prototype for the first tracking detector consisting of 250
micron thin scintillating fibers and silicon photomultiplier (SiPM) arrays. The
detector has a modular design, each module consists of a mechanical support
structure of 10mm Rohacell foam between two 100 micron thin carbon fiber skins.
Five layers of scintillating fibers are glued to both top and bottom of the
support structure. SiPM arrays with a channel pitch of 250 micron are placed in
front of the fibers. We show the results of the first module prototype using
multiclad fibers of types Bicron BCF-20 and Kuraray SCSF-81M that were read out
by novel 32-channel SiPM arrays from FBK-irst/INFN Perugia as well as
32-channel SiPM arrays produced by Hamamatsu. A spatial resolution of 88 micron
+/- 6 micron at an average yield of 10 detected photons per minimal ionizig
particle has been achieved.Comment: 5 pages, 7 figures, submitted as proceedings to the 11th Topical
Seminar on Innovative Particle and Radiation Detectors (IPRD08
Silicon photomultiplier arrays - a novel photon detector for a high resolution tracker produced at FBK-irst, Italy
A silicon photomultiplier (SiPM) array has been developed at FBK-irst having
32 channels and a dimension of 8.0 x 1.1 mm^2. Each 250 um wide channel is
subdivided into 5 x 22 rectangularly arranged pixels. These sensors are
developed to read out a modular high resolution scintillating fiber tracker.
Key properties like breakdown voltage, gain and photon detection efficiency
(PDE) are found to be homogeneous over all 32 channels of an SiPM array. This
could make scintillating fiber trackers with SiPM array readout a promising
alternative to available tracker technologies, if noise properties and the PDE
are improved
Impact of Demand-Response on the Efficiency and Prices in Real-Time Electricity Markets
International audienceWe study the effect of Demand-Response (DR) in dynamic real-time electricity markets. We use a two-stage market model that takes into account the dynamical aspects of gen-eration, demand, and DR. We study the real-time market prices in two scenarios: in the former, consumers anticipate or delay their flexible loads in reaction to market prices; in the latter, the flexible loads are controlled by an independent aggregator. For both scenarios, we show that, when users are price-takers, any competitive equilibrium is efficient: the players' selfish responses to prices coincide with a socially optimal policy. Moreover, the price process is the same in all scenarios. For the numerical evaluation of the properties of the equilibrium, we develop a solution technique based on the Alternating Direction Method of Multipliers (ADMM) and trajectorial forecasts. The forecasts are computed us-ing wind generation data from the UK. We challenge the assumption that all players have full information. If the as-sumption is verified, then, as expected, the social welfare increases with the amount of DR available, since DR relaxes the ramping constraints of generation. However, if the day-ahead market cannot observe how elastic loads are affected by DR, a large quantity of DR can be detrimental and leads to a decrease in the welfare. Furthermore, the DR operator has an incentive to under-dimension the quantity of avail-able DR. Finally, we compare DR with an actual energy storage system. We find that storage has a faster response-time and thus performs better when only a limited amount is installed. However, storage suffers from charge-discharge in-efficiency: with DR, prices do concentrate on marginal cost (for storage, they do not) and provide a better welfare
Gel transitions in colloidal suspensions
The idealized mode coupling theory (MCT) is applied to colloidal systems
interacting via short-range attractive interactions of Yukawa form. At low
temperatures MCT predicts a slowing down of the local dynamics and ergodicity
breaking transitions. The nonergodicity transitions share many features with
the colloidal gel transition, and are proposed to be the source of gelation in
colloidal systems. Previous calculations of the phase diagram are complemented
with additional data for shorter ranges of the attractive interaction, showing
that the path of the nonergodicity transition line is then unimpeded by the
gas-liquid critical curve at low temperatures. Particular attention is given to
the critical nonergodicity parameters, motivated by recent experimental
measurements. An asymptotic model is developed, valid for dilute systems of
spheres interacting via strong short-range attractions, and is shown to capture
all aspects of the low temperature MCT nonergodicity transitions.Comment: 12 pages, LaTeX, 5 eps figures, uses ioplppt.sty, to appear in J.
Phys.: Condens. Matte
Application of a magnetic suspension balance to the oxidation study of the zirconium based alloys under high pressurewater vapour
International audienceThe fuel claddings in the Pressurised Water Reactor are corroded in water at high temperature and high pressure. The technical device ableto follow continuously the corrosion rate in conditions close to this medium does not yet exist. That is the reason why a high pressure thermogravimetric installation based on magnetic suspension has been designed to study in situ the oxidation kinetics of the zirconium based alloys under water vapour until 50 bars of pressure at 415°C. The accuracy of measurements is about 5•10–5 g under 2 bars, and 10–4 g under 50 bars. The reproducibility of measurements was verified and the deviation regarding post test weighing at room temperature is around 5•10–5 g what is clearly satisfying. Finally, the results presented in this work allow validating the high pressure thermogravimetric measurements obtained with this magnetic suspension device
Critical phenomena in colloid-polymer mixtures: interfacial tension, order parameter, susceptibility and coexistence diameter
The critical behavior of a model colloid-polymer mixture, the so-called AO
model, is studied using computer simulations and finite size scaling
techniques. Investigated are the interfacial tension, the order parameter, the
susceptibility and the coexistence diameter. Our results clearly show that the
interfacial tension vanishes at the critical point with exponent 2\nu ~ 1.26.
This is in good agreement with the 3D Ising exponent. Also calculated are
critical amplitude ratios, which are shown to be compatible with the
corresponding 3D Ising values. We additionally identify a number of subtleties
that are encountered when finite size scaling is applied to the AO model. In
particular, we find that the finite size extrapolation of the interfacial
tension is most consistent when logarithmic size dependences are ignored. This
finding is in agreement with the work of Berg et al.[Phys. Rev. B, V47 P497
(1993)]Comment: 13 pages, 16 figure
Lithium Phthalocyanine: A Probe for Electron Paramagnetic Resonance Oximetry in Viable Biological Systems.
Lithium phthalocyanine (LiPc) is a prototype of another generation of synthetic, metallic-organic, paramagnetic crystallites that appear very useful for in vitro and in vivo electron paramagnetic resonance oximetry. The peak-to-peak line width of the electron paramagnetic resonance spectrum of LiPc is a linear function of the partial pressure of oxygen (pO2); this linear relation is independent of the medium surrounding the LiPc. It has an extremely exchange-narrowed spectrum (peak-to-peak line width = 14 mG in the absence of O2). Physicochemically LiPc is very stable; its response to pO2 does not change with conditions and environments (e.g., pH, temperature, redox conditions) likely to occur in viable biological systems. These characteristics provide the sensitivity, accuracy, and range to measure physiologically and pathologically pertinent O2 tensions (0.1-50 mmHg; 1 mmHg = 133 Pa). The application of LiPc in biological systems is demonstrated in measurements of pO2 in vivo in the heart, brain, and kidney of rats
Worm Epidemics in Wireless Adhoc Networks
A dramatic increase in the number of computing devices with wireless
communication capability has resulted in the emergence of a new class of
computer worms which specifically target such devices. The most striking
feature of these worms is that they do not require Internet connectivity for
their propagation but can spread directly from device to device using a
short-range radio communication technology, such as WiFi or Bluetooth. In this
paper, we develop a new model for epidemic spreading of these worms and
investigate their spreading in wireless ad hoc networks via extensive Monte
Carlo simulations. Our studies show that the threshold behaviour and dynamics
of worm epidemics in these networks are greatly affected by a combination of
spatial and temporal correlations which characterize these networks, and are
significantly different from the previously studied epidemics in the Internet
Evolutionary game of coalition building under external pressure
We study the fragmentation-coagulation (or merging and splitting)
evolutionary control model as introduced recently by one of the authors, where
small players can form coalitions to resist to the pressure exerted by the
principal. It is a Markov chain in continuous time and the players have a
common reward to optimize. We study the behavior as grows and show that the
problem converges to a (one player) deterministic optimization problem in
continuous time, in the infinite dimensional state space
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