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Anthony N. Papathanassiou

Publications and source records attributed to Anthony N. Papathanassiou.

12 recordsLinked to original sources
arXiv2026

Tracking Coupled Granular Temperature and Entropy Dynamics in Granular Materials via Dielectric Spectroscopy

In glass-forming liquids, structural dynamics are governed by configurational entropy and temperature, with dielectric relaxation time scaling alongside structural relaxation time as described by the Adam-Gibbs (AG) model. Under Edwards's athermal statistical thermodynamics, a modified AG law similarly governs granular matter, provided that granular temperature and configurational entropy are appropriately defined. This study investigates whether variations in the structural relaxation of granular systems can be probed via thermally activated processes, specifically electric charge hopping and trapping. By progressively reducing the volume of graphite powder to vary its packing fraction, we estimated relative configurational entropy and granular temperature from volumetric data, while evaluating electrical conductivity and capacity via impedance spectroscopy. We demonstrate that the logarithm of the dielectric relaxation time, derived from complex impedance, scales with granular temperature and entropy across both loose and compact states. Consequently, changes in the complex impedance resulting from packing fraction variations are tuned by granule configuration, strictly adhering to an AG-like relationship for thermal systems. These findings establish dielectric spectroscopy as a viable, non-destructive tool for tracing configurational dynamics in granular matter, analogous to its established use in polymers and glass formers.

cond-mat.soft↗
arXiv2026

Theory of melting lines with a variable enthalpy of fusion

Conventional derivations of phase boundaries from the Clausius-Clapeyron (CC) relation often employ the constant latent heat approximation to maintain analytical functions of the sublimation and boiling curves. To address the complex thermodynamics of the solid-liquid transition, we develop a two-phase analytical model by modifying the CC equation to account for a variable enthalpy of fusion along the melting line (ML). Our methodology utilizes recent theoretical and experimental progress demonstrating that the isobaric heat capacity of crystalline solids near the melting point features a dominant anharmonic, volume-dependent component. Consequently, the latent heat is correlated to the specific volumes of the coexisting phases. Differentiation of this modified CC relation yields a second-order differential equation governing ML. By imposing appropriate e boundary conditions, physically acceptable approximate parabolic solutions are derived. The parameters of these analytic functions are defined exclusively by fundamental thermophysical properties, including the bulk moduli, thermal expansion coefficients, and specific volumes of the coexisting phases, as well as the isobaric heat capacity of the solid. Our derivation, rooted in solid-state anharmonicity, yields approximate parabolic scaling laws that corroborate with a recent universal model derived from the Phonon Theory of Liquids [K. Trachenko, Phys. Rev. E 109, 034122 (2024)], supporting the universal parabolic nature of melting curves from a completely distinct theoretical foundation.

cond-mat.stat-mech↗
arXiv2025

Compensation between the parameters of the Jonschers's Universal Dielectric Response Law in disordered materials

Experimental results for a huge number of different materials published during the past fifty years confirm the validity of the Jonscher's Universal Dielectric Response Law. Accordingly,the ac conductivity is a fractional power of frequency. AC conductivity spectra recorded at different temperatures evidence for a proportionality between the logarithm of the pre-exponential factor to the fractional exponent, as well. The dc conductivity, pre-exponential factor and fractional exponent of the ac conductivity are three state variables, which describe the electric and dielectric properties. These constitute a unique relation by merging the Jonscher's Dielectric Response Law and the Ghosh - Pan Scaling Rule, respectively. A partial differentiation chain theorem combined with the temperature dependencies of the dc conductivity, pre-exponential factor and fractional exponent of the ac response, establishes a compensation rule between the parameters of the Universal Dielectric Response Law. The compatibility of the present theorynwth published experimental results is discussed.

cond-mat.dis-nn↗
arXiv2025

Pressure dependence of the interfacial polarization and negative activation volume for dielectric relaxation in heterogeneous systems

Negative activation volumes for dielectric relaxation are rarely reported in solid state physics and are limited to atomic scale processes. Broadband Dielectric Spectroscopy on heterogeneous systems, such as polycrystalline calcite and magnesite, hosting water in their porous spce, detectedintense dielectric relaxation mechanisms related to the interfacial polarization. The characteristic relaxation frequency increased uponhydrostatic compression, indicating that the activation volumes for relaxation are negative. However, a theoretical interpretation for the negative sign of the activation volume is lacking up to date. Within the frame of effective medium approximation for mesoscopic heterogeneous two phase solid - fluid systems, we investigate how the synergyof the pressure depedencies of polarization and electric charge transport, respectively, dictate the pressure dependence of interfacial polarization . predicting the value of the effective activation volume. Our theoretical approach succeeds in predicting the negative sign and magnitude of the activation volume in water saturated polycrystalline materials.

physics.geo-ph↗
arXiv2025

Broadband Dielectric Spectroscopy on dry Poly(vinyl alcohol)/Poly(vinylidene fluoride) blends reinforced with Nano-Graphene Platelets at combined pressure and temperature

Poly(vinyl alcohol)/Poly(vinylidene fluoride) blends at mass ratio 3:1 with Nano - Graphene Platelet fillers constitute mechanically and thermally stable systems, which are used for developing piezoelectric devices. Blends host a fraction of water molecules absorbed by the poly(vinyl alcohol) phase. Dc conductivity and dielectric relaxation occur via fluctuation induced tunneling of electrons. Electric charge transport is affected by the glass transition of the polymer matrix and the rotational and translational dynamics of absorbed water molecules. In the present work, absorbed water was subtracted by annealing and pumping and, subsequently, dry blends were characterized by employing Broadband Dielectric Spectroscopy at combined temperature and pressure states. Conductivity and relaxation obey fluctuation induced tunneling temperature dependencies. The corresponding activation energies and activation volumes reveal the role of absorbed polar water molecules on the tunneling current for macroscopic or localized electron transport.

physics.chem-ph↗
arXiv2019

Pressure-induced electro-switching of polymer/nano-graphene composites

The pressure and temperature dependency of the electrical conductivity of poly(vinyl alcohol)/poly(vinyl pyrrolidone) (1/1, w/w) and poly(vinyl alcohol) and composites with dispersed nano-graphene platelets were studied. Above the critical platelet fraction for electric charge percolation, the composites function as pressure-induced electro-switches. The conductor to insulator transition is optimally intense and stable. The electrical conductivity drops by two orders of magnitude at a critical pressure around 750 bars. The transition is stable over tenths of degrees above room temperature. The reduction of the conductivity upon pressure results from the competition between the pressure dependencies of the polarizability of the polymer matrix and the inter-platelet separation, respectively. Both contributions control the fluctuation induced tunneling of electrons through the polymer barrier separating adjusting conductive platelets. The role of the local electric field at the polymer-platelet interfaces by assisting tunneling is suppressed by the decrease of the polarizability upon pressure.

cond-mat.mtrl-sci↗
arXiv2019

Metal to insulator transition in Conducting Polyaniline/Graphene Oxide composites

Broadband Dielectric Spectroscopy (BDS) measurements of Pοlyaniline/Graphene oxide composites were conducted for an as-prepared and a thermally annealed specimen, respectively, from 15K to room temperature. Electrical conductivity values of the annealed composite display a very modest rise denoting the important contributions of the GO component to achieving electrical stability of the polymer. Patterns of the dc conductivity as a function of temperature also reveal a metal to insulator transition around 75K. The transition is dominated by two key factors; temperature and annealing process. Metal-like and insulating features are subsequently detected, as well, and accordingly described to provide a qualitative inspection of the charge transfer mechanisms involved

cond-mat.mtrl-sci↗
arXiv2018

Evidence of local softening in glassy poly(vinyl alcohol)/poly(vinyl pyrrolidone) (1/1, w/w) nano-graphene platelets composites

Complex permittivity studies on glassy poly(vinyl alcohol)/poly(vinyl pyrrolidone) (1/1) at proper pressure-temperature condition, provide the activation volume and energy values for both dc conductivity and \b{eta}-relaxation. The temperature dependence of the activation volume, which signatures volume-fluctuations accompanying a dynamic process, maximizes at temperature near the glass transition temperature of neat polyvinyl alcohol. The phenomenon is interpreted by a local softening of polyvinyl alcohol domains, while the blend remains in its glassy state. The scenario is also supported by the dependence of the activation energy upon pressure. Dispersed nano-graphene platelets at volume fractions around the critical conductivity percolation threshold are comparatively studied, so as to determine whether local softening affects the formation of a conductivity percolation network.

cond-mat.mtrl-sci↗
arXiv2017

Electric properties of carbon nano-onion/polyaniline composites: a combined electric modulus and ac conductivity study

The complex electric modulus and the ac conductivity of carbon nanoonion/polyaniline composites were studied from 1 mHz to 1 MHz at isothermal conditions ranging from 15 K to room temperature. The temperature dependence of the electric modulus and the dc conductivity analyses indicate a couple of hopping mechanisms. The distinction between thermally activated processes and the determination of cross-over temperature were achieved by exploring the temperature dependence of the fractional exponent of the dispersive ac conductivity and the bifurcation of the scaled ac conductivity isotherms. The results are analyzed by combining the granular metal model(inter-grain charge tunneling of extended electron states located within mesoscopic highly conducting polyaniline grains) and a 3D Mott variable range hopping model (phonon assisted tunneling within the carbon nano-onions and clusters).

cond-mat.mtrl-sci↗
arXiv2010

A plausible interpretation of the density scaling of the diffusivity in viscous liquids

Fundamental thermodynamic concepts and an earlier elastic solid-state point defect model are employed to formulate an analytical second-order olynomial function describing the density scaling of the diffusion coefficient in viscous liquids. The scaling exponent is correlated, within the approximations made in the present approach, with the pressure derivative of the isothermal bulk modulus. Our findings are compared with computer simulation results.

cond-mat.soft↗
arXiv2009

Density scaling of the diffusivity in viscous liquids: Identification of the scaling exponent with the pressure derivative of the isothermal bulk modulus

A density scaled diffusivity function for viscous liquids derived earlier [Phys. Rev. E 79, 032501 (2009)] is revisited, based on an improved equation of state assuming that the isothermal bulk modulus increases linearly with pressure. Without making any assumption on the interconnection between the scaling exponent and the Gruneisen parameter, we prove that the scaling exponent is identical with the pressure derivative of the isothermal bulk modulus. We further discuss probable interconnection between the scaling exponent and the Gruneisen parameter.meter.

cond-mat.soft↗