arXiv subjects

A. N. Papathanassiou

Publications and source records attributed to A. N. Papathanassiou.

14 recordsLinked to original sources

arXiv2021

Thermal and electrical characterization of poly(vinyl)alcohol)/poly(vinylidene fluoride) blends reinforced with nano-grapheneplatelets

Nano-graphene /polymer composites can functionas pressure induced electro-switches, at concentrations around their conductivity percolation threshold. Close to the critical point, the pressure dependence of the electron tunneling through the polymer barrier separating nanon-graphenes results from thecompetition among shorteningof the tunneling length and the increase of the polymer's polarizability. Such switching behaviorwas recentlyobserved inpolyvinyl alcohol (PVA) loaded withnano-graphene platelets (NGPs). In this work, PVA is blended withh alpha-poly(vinylidene fluoride) (PVdF)and NGPs. Coaxial mechanical stress and electric field render the nano-composite piezoelectric. We investigate the influence of heterogeneity, thermal properties, phase transitions and kinetic processes occurring in the polymer matrix on the macroscopicelectrical conductivity and interfacial polarization in casted specimens. Furthermore, the effect of electro-activity of PVdF grains on the electric and thermal properties are comparatively studied. Broadband Dielectricspectroscopy is employed to resolve and inspect electron transport and trapping with respect to thermal transitions and kineticprocessestraced via Differential Scanning Calorimetry. The harmonic electric field applied during a BDS sweep induces volume modifications of the electro-active PVdF grains, while, electro-activity of PVdF grains can disturb the internal electric field that free (or bound) electric. The dc conductivity and dielectric relaxation was found to exhibit weakdependencies.

cond-mat.mtrl-sci↗

arXiv2019

Piezoelectric polyvinylidene fluoride-based epoxy composites produced by combined uniaxial compression and poling

We present a simple, efficient method, which combines uniaxial compression and subsequent poling, to produce piezoelectric polyvinylidene fluoride-based epoxy composites. The values of the piezoelectric factors obtained are slightly higher than those of neat piezoelectric polyvinylidene fluoride. The composites respond rapidly and reach a saturation voltage output, to the application of mechanical stimulus quickly. The composites are promising for the creation of bulk piezoelectric devices, different from the usual stretched films, exploiting the physic-chemical a of the epoxy matrix. The piezoelectric factor d31 versus the mechanical stimulus for the specimens studied, scale according to a double logarithmic representation.

physics.app-ph↗

arXiv2017

Dynamics of electric charge transport and determination of the percolation insulator-to-metal transition in polyvinyl-pyrrolidone / nano-graphene platelet composites

Polyvinyl-pyrrolidone (PVP) loaded with different fractions of dispersed nano-graphene platelets (NGP) were studied by Broadband Dielectric Spectroscopy in the frequency range from 1 mHz to 1 MHz. Complex permittivity and dynamic ac conductivity as a function of frequency, temperature and composition were explored. The concentration-dependent insulator-to-conductor transition was traced through dependence of the dc conductivity and the onset of the dispersive ac ac conductivity. The temperature evolution of the dielectric spectra, below and above the fractional threshold exhibits different dynamics and signs the critical percolation threshold. Percolation is dictated by quantum penetration of the effective potential barrier set by the polymer matrix operating in parallel with conduction along physical contact of NGP, in accordance with predictions for systems consisting of a semi-conducting matrix and dispersed conducting inclusions.

cond-mat.mes-hall↗

arXiv2013

Exploring electrical conductivity within mesoscopic phases of semi-conducting PEDOT:PSS films by Broadband Dielectric Spectroscopy

Poly(3,4-ethylenedioxythiophene) : poly(styrene sulfonic acid (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 inhomogenity hinters electric charge flow giving rise to capacitance effects. In the present work, two distinct relaxation mechanisms are detected by Broadband Dielectric Spectroscopy (BDS). Double polarization mechamisms 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.

cond-mat.mtrl-sci↗

arXiv2011

Dielectric properties of Granodiorite partially saturated with water and its correlation to the detection of seismic electric signals

Transient electric signals emitted prior to earthquake occurrence are recorded at certain sites in the Earth's crust termed sensitive. These field observations enforce the laboratory investigation of the dielectric response of rocks forming these localities. The dielectric relaxation of granodiorite rock coming from such a sensitive locality (Keratea, Greece) reveals, through complex impedance spectroscopy, that the activation volume for relaxation of this rock is negative which so far has been reported only rarely. This result, however, supports a theoretical model on the pre-seismic electric signals and is likely to be correlated with the sensitivity of the site and hence with the selectivity.

physics.geo-ph↗

arXiv2011

Long aging time thermal degradation of the ac conductivity and complex permittivity of conducting polypyrrole

The modification of the ac conductivity and the complex permittivity of conducting polypyrrole was monitored throughout a two years thermal aging at 343K. Reduction of the cross-over frequency is correlated with the degradation of dc-conductivity, while the ac conductivity region corresponding to the so-called 'universal' dielectric relaxation remains practically invariant during the first year of ageing, which implies a collective co-operativity among multiple degradation processes that yield a practically time-independent effective disordered environment. A broad dielectric loss peak recorded in fresh specimens splits into two distinct relaxations for intermediate stages of the annealing process. The ageing-time evolution of the dc component and the relaxations are qualitatively analysed and time constants are determined.

cond-mat.mtrl-sci↗

arXiv2010

Pressure dependence of the dielectric loss in semi-conducting polypyrrole aged at room temperature

The effect of physical aging of semi-conducting polypyrrole at ambient temperature for two years duration on the dielectric loss at various pressures is investigated. Changes of the dielectric loss spectra and the modification of the values of the activation volume for relaxation are interpreted through the division of chain clusters into smaller components and the reduction of the size of the conductive grains.

cond-mat.mtrl-sci↗

arXiv2010

Self-diffusivity as a function of density and temperature in crystalline solids and compensating rules for self-diffusion parameters in Carbon - Subgroup crystals

The self-diffusion coefficient of crystalline solids as a function of density and temperature may derive from thermodynamics concepts and an earlier elastic thermodynamic point defect model [P. Varotsos and K. Alexopoulos, Phys. Rev. B 15, 4111 (1977); Phys. Rev. B 18, 2683 (1978)]. Compensation laws ruling self-diffusion parameters in carbon-subgroup crystals obtained from theoretical calculations are predicted, as well.

cond-mat.stat-mech↗

arXiv2010

Negative activation volume for dielectric relaxation in hydrated rocks

Negative defect activation volumes are extremely rare in solids. Here, we report for the first time that this holds in a couple of hydrated rocks for dielectric relaxation by exploring the complex impedance spectra at various pressures and temperatures. The present findings mean that the relaxation time of the relevant relaxation mechanisms decreases upon increasing pressure, thus it may become too short at higher pressure and hence lead to the emission of transient electric signals before fracture. This may constitute the long-standing laboratory confirmation for the explanation of the generation of electric signals prior to an earthquake, as recently pointed out by Uyeda et al [Tectonophysics 470 (2009) 205-213].

cond-mat.mtrl-sci↗

arXiv2009

Density scaling of the diffusion coefficient at various pressures in viscous liquids

Fundamental thermodynamics and an earlier elastic solid-state point defect model [P. Varotsos and K. Alexopoulos, Phys. Rev B 15, 4111 (1977); 18, 2683 (1978)] are employed to formulate an analytical second-order polynomial function describing the density scaling of the diffusion coefficient in viscous liquids. The function parameters are merely determined by the scaling exponent, which is directly connected with the Gruneisen constant. Density scaling diffusion coefficient isotherms obtained at different pressures collapse on a unique master curve, in agreement with recent computer simulation results of Lennard-Jones viscous liquids, [D. Coslovich and C.M. Roland, J. Phys. Chem. B 112, 1329 (2008)].

cond-mat.soft↗

arXiv2009

Effect of composition on the dielectric relaxation of zeolite-conducting polyaniline blends

The complex permittivity of conducting polyaniline and zeolite - polyaniline blends was measured in the frequency range 0.01 to 2000000 Hz from room temperature to liquid nitrogen temperature. A loss peak is detected for 25, 35 and 50 wt % zeolite blends. Its position in the frequency domain, activation energy and intensity is a function of composition. The experimental results are interpreted through the competing multiple role of zeolite: as being less conducting than polyaniline, it impedes the electric charge flow by dividing large conducting polyaniline regions into smaller pieces, subsequently provides short-range pathways and, moreover, enriches the blend in interfaces separating zeolite from polyaniline.

cond-mat.dis-nn↗

arXiv2008

Transformation of polarons to bipolarons in disordered matter

The polaron to bipolaron transition in disordered media under the influence of a broadband ac electric field is suggested: ac conductivity vs. frequency measurements in disordered media with inhomogeneous disorder induce spatio-temporal fluctuations of the density of polarons yielding polaron to bipolaron transformation. The external field results in the transition, alternatively to an increasing doping level. The assertion is confirmed by performing successive complex impedance measurements in disordered media. A systematic increase of the real part of the ac conductivity in the frequency domain, followed by mutual reduction of the magnetic inductivity of conducting polypyrrole, are explained.

cond-mat.dis-nn↗

arXiv2008

Universal frequency-dependent ac conductivity of conducting polymer networks

A model based on the aspect of the distribution of the length of conduction paths accessible for electric charge flow reproduces the universal power-law dispersive ac conductivity observed in polymer networks and, generally, in disordered matter. Power exponents larger than unity observed in some cases are physically acceptable within this model. A saturation high frequency region is also predicted, in agreement with experimental results. There does not exist a universal fractional power law (and is useless searching for a unique common critical exponent), but a qualitative universal behavior of the ac conductivity in disordered media.

cond-mat.dis-nn↗

arXiv2008

Migration volume for polaron dielectric relaxation in disordered materials

A theoretical study of the influence of pressure on the dielectric relaxation related with polaron tunneling and phonon assisted hopping in disordered solids is developed. The sign and absolute value of the migration volume, which is obtained by employing the present formulation, evidence about the nature of the relaxation. As a paradigm, positive and negative values of migration volume are evaluated by analyzing recently published dielectric loss measurements under pressure in semiconducting polypyrrole. A straightforward relation between the value of the migration volume and the nature of short-range polaron flow and the size of polaron distortion is revealed.

cond-mat.dis-nn↗