A. N. Papathanassiou

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 exists not a “universal fractional power law” (and it is useless searching for a unique common critical exponent) but a qualitative universal behavior of the ac conductivity in disordered media.

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

Isotherms of the imaginary part of the permittivity from 10−2to2×106Hz 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.

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 are studied in the frequency range from 10−2to2×106Hz from room temperature to liquid nitrogen temperature. The complex permittivity formalism reveals a temperature dependent relaxation in all samples except for the 25%w∕w zeolite composite. The frequency fmax where a maximum of a loss peak is located varies with temperature by the Williams-Lander-Ferry law. The values of the activation energy of the relaxation process (which are of the order of polaronic dc conductivity) have the tendency to reach a minimum in the 25%w∕w composition, which is a loss-free composite. The 50%w∕w zeolite behaves as a dielectric where ionic relaxation dominates. The temperature variation of the strength of the dielectric mechanism follows a Curie law, apart from 50%w∕w zeolite where the dielectric strength is practically constant. The frequencies, where loss peaks are maximum, as well as dc conductivity follow qualitatively the same temperature l...

Correlation of the scaling exponent γ of the diffusivity-density function in viscous liquids with their elastic properties

Fundamental thermodynamical concepts and a solid-state point defect elastic model are used to formulate a diffusivity-density scaling function for viscous liquids. It is proved in a straightforward manner that the scaling exponent gamma describing the density scaling of the diffusivity is related with the pressure derivative of the isothermal bulk modulus.

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); Phys. Rev. B 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 Grüneisen 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. 112, 1329 (2008)].

Electrical conductivity and TSDC study of the thermal aging in conductive polypyrrole/polyaniline blends

The thermal aging of conducting polypyrrole/polyaniline (PPy/PA) blends heated at 70°C for up to about 600 hours was studied by d.c. conductivity measurements and by thermally stimulated depolarization current (TSDC) spectroscopy in the temperature range from 80 to 300 K. The composition of the samples varied from pure polypyrrole to pure polyaniline with the PA content increasing in steps of 10%. Although the σ = σ(T) curves seem smooth, the corresponding Δσ/ΔT = f(T) curves exhibit systematically ascattering of points in the temperature ranges from 100 to 140 K and from 225 to 320 K approximately for all the samples before and after the heat treatment. TSDC measurements with the MISIM (metal-insulator-sample-insulator-metal) configuration, show a low temperature peak at 100–120 K and a high temperature peak at 280–300 K. The first disappears when the polypyrrole content exceeds 70% and for all the samples after heat treatment, although the high-temperature peak persists. The explanation given to this correspondence between d.c. conductivity and TSDC signals is based on the destruction of conformons and the mobility change of the polymer chains with rising temperature.

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 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.

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.

Investigation of the dielectric relaxation and the transport properties of porous silicates containing humidity

The dielectric relaxation responses of pelite, which is a porous silicate sediment containing a low content of inherent humidity, were identified and characterized by employing the experimental scheme of the thermally stimulated depolarization current (TSDC) spectroscopy. Comparative experiments were performed on dry samples. The elementary responses that compose the dielectric spectrum were recorded by applying certain sampling techniques. The dielectric relaxation spectrum consists of two low-temperature mechanisms, which are related to different modes of relaxation of water molecules. A third one is probably produced by permanent dipoles consisting of point defects in the calcium participant. Three relaxation mechanisms were sampled within the intermediate temperature region and were strongly affected by the outgassing of the pore network. They correspond to polarization processes occurring in the multi-layer shell of humidity over the surface the solid aggregates. At higher temperatures, two mechanisms were traced: the first is related to the homogeneous polarization of the specimen as charge carriers migrate within conducting territories until they are trapped at internal boundaries and the latter is described as a long-distance conduction mechanism which is enhanced by the presence of humidity. The activation energy profiles of the above-mentioned relaxation mechanisms were obtained from the analysis of the experimental signals of the thermal sampling and the partial heating schemes.

Separation of electric charge flow mechanisms in conducting polymer networks under hydrostatic pressure

To distinguish between different electric charge flow mechanisms in conducting polymer networks, the authors performed ac conductivity and complex permittivity measurements in conducting polypyrrole at various hydrostatic pressure values. Pressure influences capacitive coupling of chains or conducting grains. The measurement of ac conductivity and complex permittivity in the frequency domain at different hydrostatic pressures seems to be a tool for resolving different conductivity modes in conjugated polymer networks and understanding electric signal propagation in random networks.