E. Rysiakiewicz-Pasek

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.

Calorimetric Investigations of Phase Transitions in KNO3 Embedded Into Porous Glasses

Results of the Differential Scanning Calorimetry (DSC) studies of porous glasses filled by KNO3 ferroelectric in wide temperature range are presented. Anomalous heat capacity of the potassium nitrate confined in nanoscale porous glasses with two different radius of pore 23 and 160nm show that the phase I-III transition temperature was slightly affected by particle size, whereas the phase III-II transition temperature significantly decreased. During the heating, the phase II-I transition temperature increased up to 12.5°C comparing to the bulk material. The lowering of the potassium nitrate particle size significantly reduced the phase I–liquid transition temperature.

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.

Size effects in KDP-porous glass ferroelectric nanocomposites

An influence of the size effect on the phase transition temperature in potassium dihydrogen phosphate (KDP)-porous glass nanocomposites was studied in this work. Results of the dielectric measurements in a wide temperature range for KDP crystals embedded in porous glasses are presented. The phase transition temperature shift toward higher temperatures was observed for KDP embedded in porous glasses with the mean value of the pore size of 312 and 160 nm, whereas for smaller pore sizes (71 and 23 nm) the phase transition temperature decreased. The phase transition temperature dependence of the mean values of the pores dimensions of KDP-porous glass nanocomposites is the experimental evidence showing the non-monotonical character of the size effect on the phase transition temperature in ferroelectrics particles. It was found that the dielectric permittivity of KDP embedded into the porous glasses with the mean values of pore sizes of 160 nm and 312 nm practically did not depend on frequency, whereas for KDP embedded into the pores with dimensions of 71 nm and 23 nm a dispersion of the dielectric permittivity is observed in both para- and ferro-electric phases.

Application of porous glasses for humidity control

Abstract We have fabricated and investigated humidity sensors on the basis of silica porous glasses obtained by leaching of initial sodium-borosilicate glasses. Correlation between the performance of humidity sensors and size distribution of voids was revealed and investigated. We observed reproducible changes of electrical resistance three-four orders of magnitude for changes of humidity in the range from 20 to 98%. It was shown that fabricated humidity sensors can operate at temperatures as low as −20°C.

Water Absorption and Mechanical Properties of Silica Porous Glasses

We have estimated the mechanical properties of silica porous glasses (SPG) using special two-layered structures consisting of a SPG layer on a solid glass substrate. These structures turned curved after keeping them in moist atmosphere. The linear dimensions of dry free-standing SPG wafers with the same pore size distribution increased significantly (up to 1.5%) after water vapour soaking. Bending angles of two-layered structures and the values of linear expansions were used to estimate SPG elastic constants and the mechanical stresses arising in porous glasses as the result of water absorption.

Phase transitions in nanostructured potassium nitrate

Dielectric properties and temperature evolution of the crystal structure of nanocomposites on the basis of porous glasses and KNO3 embedded into the pores have been studied on heating and cooling. It is shown that the stability of the ferroelectric phase depends on nanoparticle sizes and temperature prehistory of sample preparation and measurement procedure. The temperature interval, where the ferroelectric phase exists, increases on decreasing of the nanoparticle size. In the composite of KNO3 and porous glasses with the average pore diameters of 7 nm, the ferroelectric phase becomes stable down to 100 K after the first heating–cooling circle.

Dielectric properties of the P(VDF60/Tr40) copolymer in the porous glass matrix

The dielectric properties and electrical conductivity of the composite material, which was prepared by incorporating the P(VDF60/Tr40) copolymer into the porous glass matrix (the average pore diameter is approximately equal to 320 nm), and the bulk sample of the P(VDF60/Tr40) copolymer have been investigated in the temperature range 290–440 K. It is revealed that the incorporated material is characterized by an increase in the melting temperature and a considerable decrease in the temperature at which the ferroelectric phase formed in polymer inclusions becomes unstable. It is shown that charge transfer in the composite material occurs predominantly through channels filled with the polymer.

Neutron and X-ray Diffraction Studies of KNO3 Embedded into Porous Glasses

Temperature evolution of crystal structure of KNO3 embedded into porous glasses with average pore diameters 7 nm, 46 nm and 320 nm have been studied by neutron and X-rays diffraction. For 320 nm samples it is shown that KNO3 phase transition temperatures and their sequence corresponds to the bulk. For 46 nm sample the temperature interval of the ferroelectric phase stability enlarges significantly and there is a temperature range where all three phases coexist. It has been demonstrated that for 7 nm samples ferroelectric phase becomes stable down to 100°K and remains stable after consequent heating-cooling cycles.

Temperature dependences of the order parameter for sodium nitrite embedded into porous glasses and opals

The temperature dependences of the order parameter η(T) for sodium nitrite NaNO2 embedded in porous glasses with average pore diameters of 320 and 20 nm, as well as in artificial opals, have been investigated. It has been demonstrated that the dependence η(T) for sodium nitrite in the porous glass almost coincides with that for the bulk material, whereas this dependence for NaNO2 in opals differs substantially from that observed in the bulk material and from those previously determined for sodium nitrite in porous glasses with average pore diameters of 3 and 7 nm. It has been revealed that the dependence of the order parameter for sodium nitrite in opals exhibits a temperature hysteresis (approximately equal to 8 K). The temperature dependence η(T) has been described using a simple model, which takes into account the nanopore diameter distribution existing in artificial opals.