Exploring electrical conductivity within mesoscopic phases of semiconducting poly(3,4-ethylenedioxythiophene):poly(4-styrene-sulfonate) films by broadband dielectric spectroscopy

Poly(3,4-ethylenedioxythiophene): poly(4-styrene sulfonate) (PEDOT:PSS), an optically transparent organic semi-conductor, constitutes a suspension of conducting PEDOT:PSS grains, shelled by an insulating layer of PSS. While a percolation network enhances dc conductivity, structural and electrical inhomogeneity hinters electric charge flow giving rise to capacitance effects. In the present work, two distinct relaxation mechanisms are detected by Broadband Dielectric Spectroscopy. Double polarization mechanisms are predicted by bi-phase suspension dielectric theory. Within the frame of interfacial polarization, we propose a methodology to have an insight into the local conductivity of the interior of mesoscopic conducting phase. © 2013 AIP Publishing LLC

Low frequency dielectric relaxation phenomena in conducting polypyrrole and conducting polypyrrole-zeolite composites

The dielectric properties of polypyrrole-zeolite composites up to 50 % w/w zeolite were analyzed. Investigations show that in the formalism of complex permittivity, the dispersive region of the conductivity revealed the presence of a dielectric mechanism. It was found that the direct current (dc) conductivity was as increasing function of the percentage w/w content in zeolite. The results show that the long-range electric charge transport and relaxation corresponded to short-range forward and backward motions exhibited different processes

Hopping charge transport mechanisms in conducting polypyrrole: Studying the thermal degradation of the dielectric relaxation

Isotherms of the imaginary part of the permittivity from 10(-2) to 2x10(6) Hz from liquid nitrogen to room temperature for fresh and thermally aged specimens of conducting polypyrrole reveal a dielectric loss peak, which is affected by the reduction of conducting grains with aging. Charge trapping at the interfaces separating the conductive islands seems invalid. Thermal aging indicates that macroscopic conductivity and short range one have different aging evolution. The first (dc conductivity) is dominated by the tunneling of the carriers between neighboring grains through the intermediate insulating barriers, though the second (ac conductivity) is due to a backward-forward movement of the carriers and is controlled by the intrachain transport of them and their hopping between the chains. (C) 2005 American Institute of Physics

Thermal degradation of the dielectric relaxation of 10-90% (w/w) zeolite-conducting polypyrrole composites

The effect of thermal aging of 10-90 wt% zeolite-conducting polypyrrole composite on its dielectric properties is studied in the frequency range 10 -2 to 2 × 106 Hz from room temperature to liquid nitrogen temperature. A dielectric relaxation mechanism, which appears in the fresh samples, is influenced by the thermal annealing. The frequency f max where a maximum of a dielectric loss peak is located decays exponentially with the aging time and the intensity of the loss peaks shows a maximum at intermediate aging time. A modified Williams-Landel-Ferry law describes the temperature variation of fmax in all specimens. Increasing activation energy values on increasing the aging duration are obtained. The temperature dependence of fmax and the activation energy (regarded as the height of a potential barrier) are different from those characterizing the macroscopic conductivity, which is described by the charging energy limited tunneling model. The intensity of the dielectric mechanism in thermally treated samples deviates from the linear decrease with inverse temperature occurring in fresh polypyrrole. Although the thermal degradation of the logarithm of the dc conductivity decays proportional with the root of the aging time, the equivalent conductivity obtained from the dielectric data decays exponentially with aging duration. Time constants are obtained in both cases. The model of Barton-Nakajima-Namikawa (BNN) can hardly interconnect the dc conductivity with the relaxation process in fresh sample. The divergence augments with the aging time. The thermal aging law and the inadequacy of the BNN model probably indicates that the dc process is probably irrelevant to the relaxation process. © 2005 Elsevier B.V. All rights reserved

Low frequency a.c. conductivity of fresh and thermally aged polypyrrole-polyaniline conductive blends

The low frequency (10-2 to 106Hz) a.c. conductivity of fresh and thermally aged polypyrrole-polyaniline blends of various compositions was studied at room temperature. The cross-over frequency that separates the d.c. regime from the dispersive region is a function of the composition and is influenced by the thermal aging for the polyaniline-rich blends. The frequency-dispersive conductivity is determined by a sublinear law. The fractional exponent is close to unity for the fresh samples. The thermal aging results in values of the fractional exponent, which depend on the composition. This picture reflects what has been observed previously in d.c. vs. temperature studies: thermal aging reduces the size of conducting grains in polyaniline, while in polypyrrole, results in a modification of the backbone matrix. It seems that a.c. measurements at room temperature may well serve as a parallel way to the time consuming d.c. conductivity vs. temperature technique, to detect thermal degradation of the transport properties in conducting polymers. © 2003 Elsevier B.V. All rights reserved

An insight into the localization of charge carriers in conducting polyaniline by analyzing thermally stimulated depolarization signals

A dielectric relaxation mechanism, which is attributed to the localized motion of trapped polarons, has been recorded in conducting polyaniline by employing the thermally stimulated depolarization current technique. The signal was analyzed within the frame of the normal distribution in the activation energy value. The experimental dielectric relaxation results were manipulated in order to evaluate the length of the jump distance that the trapped polarons transfer along and the concentration of trap centers. The concentration of trapped carriers is calculated from two different viewpoints: the pair approximation that assumes phonon-assisted tunneling through the barrier separating two adjacent sites and the pinning model that considers the trapped polaron oscillating around its pinning point. Both models provide compatible results. © 2003 Elsevier Science Ltd. All rights reserved

ART: sub-logarithmic decentralized range query processing with probabilistic guarantees

We focus on range query processing on large-scale, typically distributed infrastructures, such as clouds of thousands of nodes of shared-datacenters, of p2p distributed overlays, etc. In such distributed environments, efficient range query processing is the key for managing the distributed data sets per se, and for monitoring the infrastructure’s resources. We wish to develop an architecture that can support range queries in such large-scale decentralized environments and can scale in terms of the number of nodes as well as in terms of the data items stored. Of course, in the last few years there have been a number of solutions (mostly from researchers in the p2p domain) for designing such large-scale systems. However, these are inadequate for our purposes, since at the envisaged scales the classic logarithmic complexity (for point queries) is still too expensive while for range queries it is even more disappointing. In this paper we go one step further and achieve a sub-logarithmic complexity. We contribute the ART (Autonomous Range Tree) structure, which outperforms the most popular decentralized structures, including Chord (and some of its successors), BATON (and its successor) and Skip-Graphs. We contribute theoretical analysis, backed up by detailed experimental results, showing that the communication cost of query and update operations is O(log2blogN) hops, where the base b is a double-exponentially power of two and N is the total number of nodes. Moreover, ART is a fully dynamic and fault-tolerant structure, which supports the join/leave node operations in O(loglogN) expected w.h.p. number of hops. Our experimental performance studies include a detailed performance comparison which showcases the improved performance, scalability, and robustness of ART