Electrical properties of zeolitic catalysts

The zeolites belong to the category of high surface area ceramics. Their exceptional catalytic properties and performance as solid electrolytes are linked with the composition pore geometry, the distribution of T-atoms, and the nature and mobility of the adsorbed phases. Parallel conductivity and dielectric relaxation studies, carried out mostly during the last three decades of growing scientific interest on zeolites, are utilised in the investigation of the necessary conditions for optimal catalytic activity and conductive behaviour. Their ionic conductivity and a number of dielectric relaxation phenomena, are strongly affected by the presence of various molecular and ionic species. The thermodynamic quantities related to the above processes show a marked dependence on the kind (size and charge), density (absolute number and relative partition in coordination site groups) and mobility of the current carriers since these parameters determine the strength of the electrostatic interactions. Moreover, the conduction mechanisms show dependence upon the three-dimensional matrix (channels' size and interconnection), which influences the cation jumping rate, as well as miscellaneous processes related to the conditions of material preparation and measurement (structure-directing templates, thermal treatments, vapour adsorption at controlled pressures with molecules of variable dimensions and polarisability, irradiation, clustering processes, etc). © 1998 Scitec Publications

A TSD current dielectric investigation of the effects of xanthene and perylene dye incorporation in PMMA

The potentials of developing tunable lasers with the use of photostable dyes, incorporated at solid-state hosts of high optical quality, have attracted considerable scientific interest. A critical factor determining the optical response of the dye is related to the extend to which its intrinsic physical and chemical properties are maintained in the environment provided. The thermally stimulated depolarization (TSD) current technique has been employed in order to investigate possible interactions between the typical thermoplastic carrier PMMA and dispersed xanthene and perylene derivatives. The dielectric β-relaxation of PMMA, related to the (re)orientation of the polar ester carbonyl side groups, exhibits a broad distribution in the relaxation times that remains unaffected by the presence of the dye molecules. The result is supportive of the model considering the chromophores being encapsulated in the polymer without strong (chemical) bonding, in agreement with the observed laser behavior. Further aspects of the dielectric response are discussed in terms of the specific sample preparation procedures and structural relaxation phenomena occurring in PMMA

Dielectric response of natural zeolite (stilbite) single crystals

The dielectric properties of single crystals of the natural zeolite stilbite were studied employing two complementary methods; the Thermally Stimulated Depolarization Currents (TSDC) technique applied over the temperature range 77–300 K and the Dielectric Relaxation Spectroscopy (DRS) method with frequency measurements over the range 5 mHz—I GHz. A distinct contribution to the dielectric relaxation response of the mineral arises from the activated hopping of Na+ counterions between vacant interstitial sites, and is described by both TSDC and dielectric measurements. © 1995, Taylor & Francis Group, LLC. All rights reserved

Multiphased mixed crystals: Interconnection of the conductivity activation energy with bulk properties

Using the miscible alkali halides, viz. NαBr and KCl, multiphased binary and ternary mixed crystals have recently been grown. Such crystals are important because of their potential applications, for example, for the construction of laser optical windows. The temperature dependence of their conductivity and the corresponding activation energy E have also been recently reported for various compositions. Here, we show that, upon combining experimental data alone, an interesting correlation emerges, which indicates that the energy E varies linearly with BΩ, where B stands for the measured bulk modulus of the multiphased mixed crystals and Ω denotes the mean volume per atom. This conforms with an early model that interconnects the Gibbs energy for the formation and migration of defects in solids with bulk properties. © 2008 Elsevier B.V. All rights reserved

The diverse effect of antiplasticizer in the molecular dynamics of an organic dye-doped polymer observed at different motional lengthscales

Complementary thermal analysis techniques were used to study blending-induced perturbations in polymer dynamics pertaining to different motional lengthscales. The antiplasticizing role of common neutral and apolar fluorescent perylimides on the cooperative relaxation dynamics of poly(methyl methacrylate) (PMMA) chain segments is evidenced by a clear rise of both the glass transition (Tg) and liquid-liquid transition (TLL) temperatures. Simultaneously, the dielectric strength, Δεβ, of the signal ascribed to restricted rotational movements of lateral groups, shows a substantial reduction. Most aspects of the observed behavior bear analogies with recent experimental observations in nanoconfined PMMAs (e.g., PMMA with homogeneously dispersed SiO2 nanospheres, in-situ polymerized in silica nanopores or in the form of supported ultrathin films), suggesting that a common mechanism is operational. In our mixtures, confinement effects, such as a modification in the macroconformation of the polymer, and changes in the orientation and packing of the polymer random coil, provide a plausible explanation of the observed changes regardless of the motional lengthscale concerned. Consonant with this scenario are reports of advanced optical properties for perylimide + PMMA blends, ascribed to the high rigidity of the matrix together with weak intercomponent interactions. © 2009 Elsevier Ltd. All rights reserved