V. Likar-Smiljanić

Dielectric investigation of electrical conductivity in the copper salt of the 12-tungstophosphoric acid hydrate

Abstract The electrical and dielectric properties of the copper salt of 12-tungstophosphoric acid (WPA) hydrate have been investigated by broadband dielectric relaxation spectroscopy (DRS). This work is a continuation of our previous investigations on the influence of monovalent and bivalent cations on the dielectric properties of WPA hydrates salts. Measurements were made in the frequency range 10 mHz–1 MHz in order to explore the long-range dynamics, following recent microwave measurements aiming at the local dynamics properties. Experiments were done in the temperature interval 293–350 K. Data treatment included complex permittivity e *, complex electric modulus M * and complex impedance Z * formalism, Arrhenius diagram and Cole–Cole diagrams. The results are discussed in terms of conductivity relaxation and conductivity current relaxation. Rather long conductivity relaxation times measured seem to correspond to ion jumping processes. A phase transition was observed at 323 K.

Structure and proton conductivity in a magnesium salt of 12-tungstophosphoric acid

Abstract Some structural properties of a magnesium salt of 12-tungstophosphoric acid, MgHPW12O40·nH2O (MgHWPA-n), are reported. The influence of the Mg2+ ion on the structure, dynamics of the protonic species, and dielectric and conductive characteristics of the salt have been studied. The interaction of protonic species with Keggin anion was discussed. Preliminary results of the crystal structure indicate that the structure is composed of Keggin anions and Mg(OH2)6 aqua-complexes. Some comparisons are made with other alkaline-earth and alkaline salts of WPA. The results of thermal analysis (TGA and DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (IR), conductivity and permittivity measurements of MgHWPA-n are reported and discussed.

Dielectric relaxation and conductivity in barium salt of 12-tungstophosphoric acid hydrate

Abstract Barium salt of 12-tungstophosphoric acid (WPA) hydrate BaHPW12O40·7H2O (BaHWPA·7H2O) has been investigated both in the microwave (X-band) region and in the lower frequency (5 Hz–500 kHz) region. This work is a continuation of our previous investigations on the influence of monovalent and bivalent cations on the dielectric properties of WPA hydrate salts. The real and imaginary parts of the permittivity (e′, e″) as a function of frequency and temperature, were determined in both frequency regions. The observed relaxations were attributed to specific relaxation processes. In the lower frequency range, the relaxation time τ=1.0×10−6 s was assigned to barium ion jumps. In the microwave region, the dielectric relaxation time τ=1.7×10−11 s was attributed to polyatomic ions (H3O+) and/or H2O molecules reorientations. A phase transition was observed at 330 K. The low value of the activation energy for reorientation in the microwave frequency region is discussed. The d.c. conductivity and its temperature dependence were determined. Measurements were made in the temperature interval from 286 to 353 K. In the upper part of this temperature interval a PTCR effect was observed.

Dielectric investigation of magnesium salt of 12-tungstophosphoric acid hydrate

Abstract Magnesium salt of 12-tungstophosphoric acid (WPA) hydrate MgHPW 12 O 40 –14H 2 O has been investigated in the microwave (X-band) and in the lower frequency (5 Hz–500 kHz) region. The specimens were obtained by pressing powder. Real and imaginary parts of permittivity were obtained for the specimens in both frequency regions. In the microwave region dipolar relaxation was observed with the relaxation time τ =1.6×10 −11 s. This relaxation is attributed to the fast reorientation of H 2 O molecules. In the lower frequency region ionic jump relaxation was observed with the relaxation time τ =3.9×10 −7 s. Results on ac and dc conductivity are discussed.

Microwave X-band permittivity measurements on 12-tungstophosphoric acid hexahydrate

Abstract Complex permittivity measurements of 12-tungstophosphoric acid hexahydrate (WPA·6H 2 O) have been performed in the X-band region at room temperature. Permittivity data were obtained by measuring reflection ( S 11 ) and transmission ( S 21 ) parameters with a network analyzer and also by using a short-circuited-waveguide. Results on powder samples are compared with the results on compressed samples using the model of mixtures. From Cole–Cole plots, a dipole relaxation time τ =1.9×10 −11 s was determined and attributed to polyatomic ion reorientation.

Conductivity of Grains and Grain Boundaries in Polycrystalline Heteropoly Acid Salts

In our previous work we investigated the conductivity and dielectric relaxation phenomena in heteropoly acids and their salts. In this work, we have studied the conductivity of grains and grain boundaries in compressed powders of 12-tungstophosphoric acid (WPA) salts with univalent and bivalent ions. The method of impedance spectroscopy has been employed in the frequency range from 5 Hz to 500 kHz. We obtained grains and grain boundaries conductivities as well as corresponding activation energies. Grain conductivity in all investigated salts is always higher than the grain boundary conductivity.

Nicolson-Ross Analysis of Absorption Coefficients of Nickel and Nickel-Zinc Ferrite Powders Prepared Using a Novel Method

Nickel and nickel-zinc ferrites are ones of the most abundant members of the soft magnetic materials. In the last few years, the interest in new methods of their preparation increases, because it is well known that the physical and chemical properties of these materials strongly depend on the preparing conditions. Powder samples of soft ferrites of NiFe2O4 and NixZn1-xFe2O4 were synthesized by a classical sintering procedure, planetary mill synthesis, and by a chemical procedure from complex compounds with acetylacetonato-ligands. Average particle sizes were 1μm, 40 nm and 20x20x4 nm (slabs), respectively. All of the obtained materials were measured in the frequency range of 7 12 GHz for the real and the imaginary part of permittivity and permeability, and the absorption coefficients were calculated. A concurrent analysis of the obtained values showed the influence of the method of synthesis, as well as the particle size on the absorption coefficients in the applied frequency range.