Ekaterina Axelrod

New nano-composite metal-polymer materials: dielectric behaviour

Abstract The design, structure and dielectric properties of a new generation of metal-containing polymer systems produced by cryochemical solid-state synthesis were investigated. The synthesis consists of low-temperature co-condensation of metal and monomer vapours followed by low-temperature solid-state polymerisation of the co-condensate. By this method, one obtains nano-heterogeneous composites with high content of stable metal nanoparticles of various sizes trapped inside a polymer matrix. Synthesized materials show specific dielectric responses, which are discussed in relation to material structure and electron transfer between nanoparticles

Dielectric relaxation of porous glasses

Abstract The dielectric properties of porous glasses, obtained from sodium borosilicate glass, were investigated in the frequency range 20 Hz to 1 MHz and temperature range −100°C to +300°C for the purpose of inferring the geometric properties of porous materials. The features of the dielectric properties due to the geometrical disorder were analysed by using models describing the non-Debye slow decay dynamics. The dielectric response is affected by the geometrical micro- and mesostructural properties of the porous matrix and the properties of the material filling the pores. It provides information on the hindered dynamics of water molecules, located within the pores and affected by the surfaces. An analysis of the dielectric parameters enables us to describe the porosity of the materials.

Optimization of tunable laser glasses with the aid of dielectric relaxation and adsorption measurements

Abstract The recently developed tunable dye lasers in the visible are obtained by incorporation of stable laser dyes into glasses prepared by the sol–gel method. In order that the lasers be photostable and more efficient it is essential that the dye molecules penetrate into the glass matrices in a unimolecular form and be protected from the surroundings. In this respect, the size of the pores and their distribution in the glass are of major importance. In this paper the influence of the various catalysts on the porosity of the glasses is presented. The pore sizes, their surface area, pore volume, pore size distribution and glass effective density are presented, and the solid state tunable lasers prepared under the optimal conditions are demonstrated. The results obtained by the conventional adsorption methods (water, nitrogen adsorption) and mercury porosimetry are compared with dielectric measurements and the conclusion is that the latter provide information about charge movement in glasses with the smallest pores. The water vapor from the atmosphere is adsorbed within the smallest pores in a monolayer resulting in the formation of dipoles, that can be detected by dielectric measurements.

Dielectric relaxation and porosity determination of porous silicon

Abstract Results of the dielectric spectroscopy study of porous silicon (PS) samples in the frequency range, 20 Hz–1 MHz, and in the temperature range, 173–493 K, are presented. Three relaxation processes that dominate at low, moderate and high temperatures respectively were observed. The low temperature dispersion follows the Cole–Cole type with two activation energies of 0.28–0.4 and 0.2–0.3 eV for samples with thickness 20 and 30 μm, respectively. At moderate temperatures the time domain dielectric response function demonstrates stretch exponential behaviour associated with the percolation of charge carries near threshold. At temperatures above 400 K, we found a strong contribution of dc-conductivity, with approximately the same activation energy of 0.47±0.01 eV for both samples. An additional Havriliak–Negami relaxation process with typical relaxation times of the order of 10−3 s is observed in this temperature interval. The dielectric response is found to be very sensitive to the geometrical micro- and meso-structural features of the PS. The dynamical aspects of the processes are discussed.

Fast-sol–gel derived silsequioxane glasses embodying glycerol moieties: dielectric properties and morphology

Abstract The fast-sol–gel synthetic route was utilized to prepare glassy matrices of varying porosity. Glycerol doping of these matrices was carried out at various stages of the sol–gel reaction. Dielectric spectroscopy data were collected from all these matrices, and interpreted in terms of various relaxation processes from which the internal morphology of the matrices could be deduced. Glycerol dopant added to the sol–gel-derived glass during preparation is adsorbed and partly chemically bound at the ends of the polymer-like chains of the matrix, acting as a cross-linking agent. It does not form bulk moieties at concentrations up to 2%. Cluster glycerol moieties are formed inside the glass at a concentration of 4%.

Broadband dielectric spectroscopy of oxidized porous silicon

Dielectric measurements accompanied by infrared absorption and photoluminescence (PL) spectroscopy were used to investigate the electrical and optical properties of oxidized porous silicon (PS). As opposed to non-oxidized PS, only high temperature relaxation processes could be resolved for oxidized PS. Two relaxation processes have been observed. The first process is related to dc-conductivity that dominates at high temperatures and low frequencies. After subtraction of dc-conductivity we could analyse a second high-temperature relaxation process that is related to interface polarization induced by charge carriers trapped at the host matrix–pore interfaces. We found that, while the main effect of the oxidation on the PL appears to be a size reduction in the silicon nanocrystals that gives rise to a blue shift of the PL spectrum, its main contribution to the dielectric properties turns out to be blocking of transport channels in the host tissue and activation of hopping conductivity between silicon nanocrystals.