102 research outputs found
Percolative conductivity in alkaline earth silicate melts and glasses
Ion conducting glasses and melts show a threshold
behaviour in dc conductivity near , with conductivities increasing
linearly at . We show that the behaviour can be traced to a rigid
() elastic phase transition near . In the
floppy phase, conductivity enhancement is traced to increased mobility or
diffusion of carriers as the modified network elastically softens.Comment: 15 pages, 5 figures. Europhysics Letters (2003), in pres
Electrode Polarization Effects in Broadband Dielectric Spectroscopy
In the present work, we provide broadband dielectric spectra showing strong
electrode polarization effects for various materials, belonging to very
different material classes. This includes both ionic and electronic conductors
as, e.g., salt solutions, ionic liquids, human blood, and
colossal-dielectric-constant materials. These data are intended to provide a
broad data base enabling a critical test of the validity of phenomenological
and microscopic models for electrode polarization. In the present work, the
results are analyzed using a simple phenomenological equivalent-circuit
description, involving a distributed parallel RC circuit element for the
modeling of the weakly conducting regions close to the electrodes. Excellent
fits of the experimental data are achieved in this way, demonstrating the
universal applicability of this approach. In the investigated ionically
conducting materials, we find the universal appearance of a second dispersion
region due to electrode polarization, which is only revealed if measuring down
to sufficiently low frequencies. This indicates the presence of a second
charge-transport process in ionic conductors with blocking electrodes.Comment: 9 pages, 6 figures, experimental data are provided in electronic form
(see "Data Conservancy"
Rings and rigidity transitions in network glasses
Three elastic phases of covalent networks, (I) floppy, (II) isostatically
rigid and (III) stressed-rigid have now been identified in glasses at specific
degrees of cross-linking (or chemical composition) both in theory and
experiments. Here we use size-increasing cluster combinatorics and constraint
counting algorithms to study analytically possible consequences of
self-organization. In the presence of small rings that can be locally I, II or
III, we obtain two transitions instead of the previously reported single
percolative transition at the mean coordination number , one from a
floppy to an isostatic rigid phase, and a second one from an isostatic to a
stressed rigid phase. The width of the intermediate phase and the
order of the phase transitions depend on the nature of medium range order
(relative ring fractions). We compare the results to the Group IV
chalcogenides, such as Ge-Se and Si-Se, for which evidence of an intermediate
phase has been obtained, and for which estimates of ring fractions can be made
from structures of high T crystalline phases.Comment: 29 pages, revtex, 7 eps figure
Vibrational analysis of d-PCL(530)/siloxane based hybrids doped with two lithium salts
Published online: 22 May 2013The present study has been focused on environmentally friendly sol-gel derived electrolytes based on a di-urethane cross-linked d-PCL(530)/siloxane network (where d represents di, PCL identifies the poly(ε–caprolactone) biopolymer and 530 is the average molecular weight in g.mol-1) doped with a wide range of concentration of lithium perchlorate (LiClO4) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Fourier Transform Infrared and Raman (FT-IR and FT-Raman, respectively) spectroscopies have been applied to evaluate the extent of ionic association. Characteristic bands of the PCL(530) segments, of the urethane cross-links and of the anions have been examined to gain insight into the cation/biopolymer, cation/anion and cation/cross-link interactions. In both electrolyte systems “free” ions and contact ions have been identified. The addition of salt modifies the hydrogen-bonded array of the host matrix, causing the destruction/formation of the urethane/urethane aggregates.Fundação para a Ciência e a Tecnologia (FCT
Elaboration of photonic crystal heterostructures by the Langmuir–Blodgett method
This paper describes the elaboration of photonic crystal heterostructures with a perfectly defined architecture. The transfer of a Langmuir film of functionalized silica particles onto a solid substrate gave us the ability to build three-dimensional (3D) colloidal crystals. The influence of the withdrawing speed of the substrate on the crystalline quality of the 3D photonic materials has been studied. Photonic crystal heterostructures have been fabricated by the successive depositions of layers of silica spheres with different diameters. The composite materials have been characterized by scanning electron microscopy (SEM) and near infrared (NIR) spectroscopy. Our results establish that the optical properties of these multilayer systems can be tailored by adjusting either the thickness or the stacking order of each component crysta
Electrochemical codeposition of multilamellar vesicles in an inorganic matrix
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