Percolation Theory (percolation + theory)

Distribution by Scientific Domains


Selected Abstracts


How good are the Electrodes we use in PEFC?

FUEL CELLS, Issue 3 2004
M. Eikerling
Abstract Basically, companies and laboratories implement production methods for their electrodes on the basis of experience, technical capabilities and commercial preferences. But how does one know whether they have ended up with the best possible electrode for the components used? What should be the (i) optimal thickness of the catalyst layer? (ii) relative amounts of electronically conducting component (catalyst, with support , if used), electrolyte and pores? (iii) "particle size distributions" in these mesophases? We may be pleased with our MEAs, but could we make them better? The details of excellently working MEA structures are typically not a subject of open discussion, also hardly anyone in the fuel cell business would like to admit that their electrodes could have been made much better. Therefore, we only rarely find (far from systematic) experimental reports on this most important issue. The message of this paper is to illustrate how strongly the MEA morphology could affect the performance and to pave the way for the development of the theory. Full analysis should address the performance at different current densities, which is possible and is partially shown in this paper, but vital trends can be demonstrated on the linear polarization resistance, the signature of electrode performance. The latter is expressed through the minimum number of key parameters characterizing the processes taking place in the MEA. Model expressions of the percolation theory can then be used to approximate the dependence on these parameters. The effects revealed are dramatic. Of course, the corresponding curves will not be reproduced literally in experiments, since these illustrations use crude expressions inspired by the theory of percolation on a regular lattice, whereas the actual mesoscopic architecture of MEA is much more complicated. However, they give us a flavour of reserves that might be released by smart MEA design. [source]


Dielectric studies of conductive carbon black reinforced microcellular ethylene,propylene,diene monomer vulcanizates

JOURNAL OF APPLIED POLYMER SCIENCE, Issue 1 2007
S. P. Mahapatra
Abstract The alternating-current and electrical conductivity of conductive, carbon black reinforced, microcellular ethylene,propylene,diene monomer vulcanizates was measured in the frequency range of 100 Hz to 1 MHz. The effects of variations in the filler and blowing-agent loadings on the dielectric constant and percolation behavior were studied. The phenomenon of percolation was examined on the basis of measured changes in the electrical conductivity and morphology of composites with different concentrations of the filler. Scanning electron microphotographs showed the agglomeration of the filler above these concentrations and the formation of a continuous network structure. The experimental results were not in agreement with the predictions of the statistical percolation theory; this deviation was explained in light of the formation of an interphase or mesostructure in the composites. The variation of the dielectric constant with the filler and blowing-agent loadings was explained on the basis of polarization of the filler in the polymer matrix. Additionally, the use of dielectric mixture laws in describing the dielectric constants of both solid and microcellular composites was investigated. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007 [source]


Conductivity and Permittivity of Nickel-Nanoparticle-Containing Ceramic Materials in the Vicinity of Percolation Threshold

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2006
Umar Abdurakhmanov
Conductivity and static permittivity of ceramic materials containing nanoparticles of Ni were measured in the vicinity of percolation threshold. It is found that, below this threshold, the experimentally obtained dependences of conductivity and static permittivity on the fractional Ni content in these materials are different from those calculated in the frame of the percolation theory. The origin of this discrepancy is discussed in terms of the network hierarchy model proposed recently by Balberg et al. for composite materials. [source]


Optimization of a Composite Working Electrode for a New Family of Electrochemical Cell for NO Decomposition

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2003
Kazuyuki Matsuda
The electrochemical properties of a composite (NiO)x,(yttria-stabilized zirconia (YSZ))1,x working electrode for a new type of electrochemical cell for NO decomposition in the presence of excess oxygen are investigated. It is shown that the dependence of the NO conversion on the value of the current passed through the electrochemical cell with a nanoporous (NiO)x,(YSZ)1,x working electrode is linear and that the value of current efficiency depends on the NO and O2 gas concentrations only (,= [NO] /([NO] + 2[O2]). The optimum NiO addition (35% by volume) to the YSZ resulted in a decrease of the cell operating voltage and, as a result, in a decrease in the electrical power required for NO decomposition. The observed high performance of the composite working electrode at this composition is consistent with the effective medium percolation theory, which predicts the ambipolar transport behavior of the composite mixed ionic,electronic (YSZ,NiO) conductors as a function of the volume fraction of each of the randomly distributed constituent phases. [source]


Evidence of Nearest-Neighbor Ordering in Wet-Processed Zirconia,Nickel Composites

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2001
Carlos Pecharromán
Monolithic zirconia,nickel (ZrO2/Ni) cermets have been prepared by a wet-processing method with nickel volume concentrations of 16%,40%. Microstructural analysis performed on scanning electron microscopy images has revealed evidence of a partial ordering of metallic particles inside the ceramic matrix. This ordering does not appear in mullite/molybdenum cermets. Complex impedance measurements have shown that the percolation threshold of ZrO2/Ni cermets appears at a filling factor (fc) of 0.34, exceeding the theoretical value (fc= 0.16), as a consequence of its microstructural order. Electrical measurements display the expected increase of capacity near the percolation threshold. These results open the possibility to design new devices with the appealing electric, magnetic, and mechanical properties that are predicted by the percolation theory. [source]


A fluorescence study on critical exponents during sol-gel phase transition in complex monomeric systems

MACROMOLECULAR SYMPOSIA, Issue 1 2003
Demet Kaya
Abstract Methyl methacrylate (MMA), ethyl methacrylate (EMA) and various combinations of MMA with EMA were used during FCC experiments. Pyrene (Py) was introduced as a fluorescence probe and fluorescence lifetimes from its decay traces were measured during sol-gel phase transitions. The fast transient fluorescence (FTRF) technique was used to study the critical exponents during sol-gel phase transition in free-radical crosslinking copolymerization (FCC). The results were interpreted in the view of percolation theory. The critical exponents of gel fraction, , and weight average degree of polymerization, , were measured near the point of gel effect and found to be around 0.37 ± 0.015 and 1.69 ± 0.05 in all systems studied respectively. [source]


Using Nano-Cast Model Porous Media and Integrated Gas Sorption to Improve Fundamental Understanding and Data Interpretation in Mercury Porosimetry

PARTICLE & PARTICLE SYSTEMS CHARACTERIZATION, Issue 1 2006
Sean P. Rigby
Abstract The mechanisms of entrapment, and the nanoscopic spatial distribution, of the residual mercury within nano-cast and amorphous porous media (pore sizes ~1,100 nm) following high-pressure penetration have been studied. It has been shown that, even at the nano-scale, one of the same two principle mechanisms that have been observed previously in mercury porosimetry experiments on macroscopic glass pore models also occur within a given amorphous, nanoporous solid. Using percolation theory to interpret novel, integrated gas sorption experiments, entrapment was shown to arise, either because of the presence of sufficiently narrow pore necks interspersed between larger voids, or due to non-random, longer-range structural heterogeneity. The threshold "snap-off" ratio parameter for the entrapment process has also been directly measured but found to be considerably smaller than seen previously for macroporous materials. The techniques employed here enable information not previously available for nanoporous systems to be determined, and therefore to be incorporated into simulations of mercury porosimetry on those materials. [source]


New nanocomposite materials made of an insulating matrix and conducting fillers: Processing and properties

POLYMER COMPOSITES, Issue 2 2000
L. Flandin
The work described in this paper deals with the preparation and the characterization of an homogeneous composite material composed of electrically conductive fillers dispersed in a thermoplastic insulating matrix. These fillers were chosen to have either a spherical shape or a high aspect ratio. Processing of these conductive polypyrrole particles, which were obtained either through a classical polymerization of pyrrole in the presence of stabilizer, or polymerization onto the surface of cellulose monocrystals is detailed. The relationship of filler content in the composite to electrical and mechanical properties was investigated. In order to characterize the connectivity of the fillers in the matrix, electrical measurements were performed and these results have been compared with the predictions of the statistical percolation theory. Further analysis was undertaken by considering the influence of the network of rigid particles on the viscoelastic properties of the composites. These results were compared with two models; first, with a mechanical percolation approach which was based on strong interactions between fillers, and second, with a mean field model, which in no way accounted for interactions bewteen fillers. In recent developments, it has been shown that these materials used as coating on various substrate could be good sensors, allowing to determine some specific features of the substrate deformation. [source]


Percolation phenomena in carbon black,filled polymeric concrete

POLYMER ENGINEERING & SCIENCE, Issue 9 2000
L. Rejón
Percolation in carbon black-filled polymeric concrete, is discussed based on the measured changes in electrical conductivity and morphology of the composite at different concentrations of carbon black. The percolation threshold ranged between 6 and 7 wt% (based on resin weight) of carbon black. Above this concentration, the filler particles formed agglomerates in contact with each other, suggesting that the conduction process is nearly ohmic in nature. A power law predicted by percolation theory described the behavior of the conductivity as a function of carbon black content. Microscopic analysis showed the presence of a continuous structure formed by the polyester resin and carbon black, in which silica particles were embedded. [source]


Routing complexity of faulty networks

RANDOM STRUCTURES AND ALGORITHMS, Issue 1 2008
Omer Angel
Abstract One of the fundamental problems in distributed computing is how to efficiently perform routing in a faulty network in which each link fails with some probability. This article investigates how big the failure probability can be, before the capability to efficiently find a path in the network is lost. Our main results show tight upper and lower bounds for the failure probability, which permits routing both for the hypercube and for the d -dimensional mesh. We use tools from percolation theory to show that in the d -dimensional mesh, once a giant component appears,efficient routing is possible. A different behavior is observed when the hypercube is considered. In the hypercube there is a range of failure probabilities in which short paths exist with high probability, yet finding them must involve querying essentially the entire network. Thus the routing complexity of the hypercube shows an asymptotic phase transition. The critical probability with respect to routing complexity lies in a different location than that of the critical probability with respect to connectivity. Finally we show that an oracle access to links (as opposed to local routing) may reduce significantly the complexity of the routing problem. We demonstrate this fact by providing tight upper and lower bounds for the complexity of routing in the random graph Gn,p. © 2007 Wiley Periodicals, Inc. Random Struct. Alg., 2008 [source]


Relevance of percolation theory to power-law behavior of dynamic processes including transport in disordered media

COMPLEXITY, Issue 2 2009
Allen Hunt
First page of article [source]


A new conceptual model for forest fires based on percolation theory

COMPLEXITY, Issue 3 2008
Allen Hunt
First page of article [source]