A. Kezionis

Electrical conductivity dispersion in Co-doped NASICON samples

Impedance spectra of Co-doped NASICON polycrystalline samples (x = 2.0) were measured in the frequency range 0.5 MHz-1.2 GHz and analyzed in terms of the conductivity dispersion. The dispersion of bulk electrical conductivity obeys the power law frequency dependence σ(ω) = σ 0 + Kω n . It is observed from room temperature up to 450 K, i.e. up to the temperature of phase transformation of NASICON. The activation energy of the conductivity of grain interiors and the activation energy of the frequency, at which the dispersion sets on, have the same value of 0.313 eV. The activation energy value of the frequency dependent part of conductivity (0.077 eV) constitutes the proper fraction of the activation energy of the dc conductivity. Similar dispersion is found at lower frequencies for the grain-boundary conductivity. It extends in temperature up to 550 K, well over the temperature of phase transformation of NASICON. The phase transformation, taking place in the crystal grains of the material, does not influence the electrical behavior of grain boundaries. The activation energies for the grain boundary conductivity and for the grain-boundary onset frequency also have equal values (0.440 eV).

Broadband Method for the Determination of Small Sample's Electrical and Dielectric Properties at High Temperatures

A model for a measurement circuit consisting of a telescopic coaxial transmission line with the sample placed in the gap of the central conductor has been developed and a measurement method, based on this model, was tested. To determine the samples electrical properties, the method requires the measurement of the two-port scattering matrices of the short-circuited line, the line with one calibration sample, and the line with the sample to be measured inserted. The model takes into account the thermal elongation of the measurement line and can be used for measuring electrical properties of materials at high temperatures in the megahertz and gigahertz ranges. This model also reduces the influence of parasitic reflections in the measuring coaxial line on the measurement results and takes into account a complicated distribution of the electromagnetic field in the measuring capacitor without solving a complicated electrodynamics problem.

Electrical and acoustical relaxation in fast ionic conductors

Abstract The ultrasonic velocity, attenuation coefficient, electric conductivity and dielectric permittivity have been measured in wide temperature and frequency ranges in various crystalline, polycrystalline and glassy solid electrolytes. The high relaxational ultrasonic attenuation maxima and corresponding velocity dispersion have been observed. Such behaviour is supposed to be determined by acoustoionic interactions of two types: (i) the piezoelectric in the piezoelectric superionic; (ii) interaction caused by acoustic wave modulation of chemical potential felt by mobile ions in the centrosymmetric materials. In polycrystalline and glassy superionics the non-Debye behaviour for ultrasonic relaxation has been established. In such materials the characteristic frequency dependencies of ionic conductivity and dielectric permittivity were obtained. A simple theoretical model describing both frequency and temperature dependencies of electrical and acoustical properties of solid electrolytes is proposed. This model is based on continuous distribution of activation energies in a limited interval and describes the experimental data fairly well.

Electric properties of NH4Sn2F5 polycrystals in the frequency range from 20 to 3.2 · 1010 Hz

Abstract The ionic conductivity ( σ ac ) and the real part of complex dielectric permittivity (e′) of F − conductor NH 4 Sn 2 F 5 have been investigated between 300 and 380 K in the frequency range from 20 to 3.2 · 10 10 Hz. The crystals NH 4 Sn 2 F 5 at the temperatures T 1 = 343 K and T 2 = 357 K undergo two superionic phase transitions, accompanied by the anomalies of σ ac and e′. At room temperature and electric field frequency v = 10 3 Hz, σ ac = 5 · 10 − 2 S/m and its activation energy ΔE 1 = 0.47 eV; at temperature T ≥ 357, K σ ac ( T ) changes with activation energy ΔE 2 = 0.34 eV. In the temperature range 343 ≤ T ≤ 357 K, σ ac ( T ) dependence does not obey Arrhenius law. In this frequency region two relaxation dispersions of σ ac and e′ attributed to the ion transport in the grain and grain boundaries of our polycrystals have been found.

Synthesis and characterization of Li1/3Ce2/3PO4 and LiCe2/3PO4 ceramics

Li1/3Ce2/3PO4 and LiCe2/3PO4 compounds were synthesized by a solid state reaction and studied by x-ray diffraction and thermogravimetric analysis. At room temperature the investigated compounds exhibit monoclinic symmetry (space group P 21/n) with four formula units in the lattice. The compounds are stable up to 1200 K in air and show no weight loss. The ceramic samples were fabricated with varying sintering times. The surfaces of the ceramics were studied by scanning electron microscopy. The increase of sintering time of the ceramics leads to an increase of the grain size of the materials. The results of an x-ray photoelectron spectroscopy study have shown that the external electric field changes the Li amount on the surfaces of the ceramic samples. The electric properties of the samples were investigated by complex impedance spectroscopy in the frequency range from 50 to 1.2 × 109 Hz in the temperature range from 300 to 650 K. Varying the sintering time of the ceramics affects the values of the total conductivity, activation energy, dielectric permittivity and dielectric losses of the ceramics.