IONIC TRANSPORT IN CALCIUM-EXCHANGED BETA'-ALUMINA AND DIFFUSION STUDIES ON PROTONIC CONDUCTORS

Single crystals of calcium beta''-alumina were prepared from as-grown sodium beta''-alumina monocrystals by ion-exchange in molten calcium salts at 773 K. Ionic conductivities of both fully- and partially-exchanged beta'' crystals were measured by means of ac technique over the temperature range of 523 to 723 K and a frequency range of 10 Hz to 700 KHz, using ion-blocking electrodes. Ionic conductivity of fully-exchanged beta''-alumina was found to be 3.9 x 10('-3) (ohm-cm)('-1) at 573 K with an activation energy of 0.57 eV for Ca('2+) ion mobility. Ionic conductivity of partially-exchanged beta''-alumina was also discussed. A "picture frame" model was invoked to explain the conduction behavior of the partially-exchanged crystals in which a non-equilibrium distribution of Ca('2+) ions existed due to the ion-exchange method used.;A pulsed-field gradient nuclear magnetic resonance (NMR) spectrometer was constructed for the direct measurements of self-diffusion coefficient in solids. Ammonium-hydronium beta''-alumina single crystals were prepared from as-grown sodium beta''-alumina by means of ion-exchange in molten ammonium nitrate at 473 K. NMR relaxation times T(,1) and T(,2) were measured at a frequency of 33.0 MHz as a function of crystal orientation and temperature. Direct measurements of proton diffusivities were carried out as a function of temperature on hydrogen uranyl phosphate tetrahydrate powder samples and on ammonium-hydronium beta''-alumina single crystals.;A nonlinear least-squares curve fitting technique has been described for the analysis of a complex impedance (admittance) spectrum in which distortions of the various shapes constituting the complex plane plot occur because of overlapping time constants. With this technique, estimates of the parameter values of an equivalent circuit are first obtained by a geometrical-fitting procedure and these are then employed as the initial parametric values in a complex least-squares method which simultaneously fits the real and the imaginary parts of the impedance (admittance) to the equivalent circuit, thereby yielding best-fit values of the circuit parameters as well as their confidence intervals. The method has been illustrated by analyzing: (1) artificial, computer-generated impedance (admittance) data constructed for simple RC circuits; (2) measured admittance response of the fast sodium ion conductor NASICON at 486 K.