Comportamiento eléctrico de monocristales 4.7mol%Y-PSZ

La función respuesta escalón de Kohlrausch-Williams-Watts ha permitido incorporar la contribución dieléctrica a la res-puesta eléctrica de los monocristales Y2O3 - Zr02. El modelo de divergencia infrarroja permite interpretar microscópica-mente los resultados. Mediante el análisis del comportamiento dieléctrico se ha mostrado la existencia de dipolos perma-nentes en las muestras de 4.7mol%itria-parcialmente-estabilizada-circona.The Kohlrausch-Williams-Watts step response function allows us to include the dielectric contribution in the electric res-ponse of Y2O3 - Zr02 single crystals. The infrarred divergence response model allov^s us to interpret the results from a microscopic point of view. By the analysis of the complex dielectric behaviour it have been shown that there are permanent dipoles in 4.7mol%Yttria-Partially-Stabilized-Zirconia samples.Junta de Extremadura (C.E.J.) Fondo Social Europeo (PRI96060081 - E1A96-19)Comisión Interministerial de Ciencia y Tecnología MAT-94-0120-C03-0

Electric Response of ZrO2- 3 mol% Y2O3 nanocrystals

Se ha estudiado la evolución con los tratamientos térmicos de la microestructura y el comportamiento eléctrico de nanocristales ZrO2 – 3mol% Y2O3.La muestra, que inicialmente estaba compuesta por 77.8%wt de fase tetragonal y 22.3%wt de fase monoclínica, sufre una transformación de fase t’m a baja temperatura en la que el porcentaje de fase monoclínica alcanza el 96%wt. Esta transformación afecta significativamente a las propiedades eléctricas del material de forma que la contribución del borde de grano ve incrementanda su resistividad en los sucesivos ciclos térmicos, mientras que la contribución del grano no cambia significativamente Creemos que el aumento de resistividad del borde de grano está relacionado con la aparición de microfisuras que acomodan el mayor tamaño de los granos monoclínicos.The evolution of microstructure and electrical behaviour of nanocrystalline ZrO2 – 3mol% Y2O3 samples with respect to thermal cycles have been investigated. The sample, that initially consisted of 77.8%wt of tetragonal and 22.3%wt monoclinic phases, eliminates the tetragonal phase by means of a low temperature t → m phase transformation in which the content of monoclinic phase reaches 96%wt. This transformation affects significantly the electrical properties of the sample, increasing the grain boundary resistivity in successive thermal cycles, whereas the bulk contribution remains unchanged. The growth of the grain boundary resistivity is believed to be due to the presence of microcracks that accommodate the larger size of monoclinic grains

Uniones superplásticas de muestras Y-TZP: comportamiento eléctrico

La respuesta eléctrica de la unión superplástica de dos muestras de 3mol% Y-TZP ha sido comparada con la de una muestra sin unir. La muestra unida presenta una conductividad eléctrica mayor que la muestra sin unir. Este aumento de conductividad se atribuye al proceso de unión

Ionic conductivity of ZrO2-12 mol% Y2O3 single crystals

Fast ionic conductors are important to study because of their use in the construction of technologically useful devices such as electrochemical cells, oxygen monitors, and the high-temperature fuel cell. Oxygen-ion conductors form a major subgroup of these materials, and, in particular, stabilized zirconia is one of the more important solid electrolytes. However, the ionic conductivity of this material is still only rather poorly understood. The aim of the present work is to describe, by means of a method of local fits (LF’s) to Arrhenius’s law, the experimental values of the ionic conductivity of ZrO2–12 mol % Y2O3 single crystals in the temperature range from 200 °C to 1600 °C. This method yields two sets of data: the preexponential factor, ALFi, and the activation enthalpy, ΔHLFi. The lnALFi versus ΔS(T)/k plot [where ΔS(T) is the entropy change in the process] is a very good test of the accuracy of the LF method. The ΔHLFi values are fitted by a least-squares procedure to an empirical temperature-dependence function with four adjustable parameters. In order to interpret these results and to understand the physical meaning of the fitted parameters, a microscopic model is proposed that allows us to deduce a theoretical function of temperature for the activation enthalpy similar to the empirical function.Then, from this function, we determine the association (0.57 eV) and migration (0.73 eV) enthalpies for oxygen vacancies, and analyze the temperature variation of the free energy (ΔG) and entropy (ΔS), as well as the degree of dissociation of the vacancies in the conduction process for this material. A noteworthy result is that, for the range of temperature studied here, the extrinsic dissociated regime (where it is assumed that all oxygen vacancies are free) is never reached. Finally, taking into account the contribution of the jumps up to the second-next-nearest anionic neighbors, we obtain the value of 1.31×1013 Hz for the attempt frequency of the oxygen vacancies

Activation entropy and Gibbs free energy for conduction in yttria–stabilized-zirconia single crystals

The temperature dependence of the activation Gibbs free energy [ΔG(T)] and entropy [ΔS(T)] associated with electric conduction in 12 mol % yttria–stabilized-zirconia single crystals was analyzed. In order to determine the ΔG(T) and ΔS(T) functions exactly, it was necessary to impose temperature-behavior conditions on the activation Gibbs free energy to give ΔS(T→∞)=0. The temperature dependence of the entropy was similar to that of the activation enthalpy. Both reach stable values at low temperatures (extrinsic associated range). In this temperature range, ΔG(T) shows a linear behavior. At higher temperatures, ΔG(T) decreases asymptotically to a value approximately equal to the migration enthalpy of the oxygen vacancies. A value of 5.6×10−7 eV/K was calculated for the variation of entropy associated with the migration of the vacancies. Assuming the cation-vacancy-cation association to be a first-neighbor structure, a value of 4.85×10−4 eV/K was found for the association entropy of the vacancies. These results are in very good agreement with those obtained by applying the method used in alkaline halide studies

Impedance spectroscopy of 4.7 mol% Y-FSZ

El ajuste mediante el método Rietveld del espectro de difracción de rayos-X de un monocristal Zr02-4.7mol%Y203, muestra la existencia de una microestructura de dos fases t+t'. Los espectros de impedancia de este material determinan que su respuesta eléctrica esta controlada por los mismos procesos que en monocristales de circonia totalmente estabilizados con 9.5 mol% de itria. El análisis de la impedancia con un modelo circuital de una resistencia en paralelo con un condensador dependiente de la frecuencia, como una función tipo Fiavriliak-Negami, permite determinar las energías de activación de 0.94 eV y 1.10 eV para la conductividad iónica en las muestras de dos fases (f+f) y un a fase (c), respectivamente.Fitting X-ray diffraction patterns of Zr02-4.7mol%Y203 single crystal by Rietveld method, demostrate the existence of a two-phases t+f microstructure. The impedance spectra of this material shoves that its electric response is controlled by the same process that in a singlephase 9.5 mol% yttria-fully-stabilized-zirconia. Impedance analysis using an equivalent model based on a resistor in parallel with a frequency-dependent capacitance, type tlavriliak-Negami function, allows us to calculate the activation energies of 0.94 eV and 1.10 eV of the ionic conductivity in two-phases {t+t') and single-phase (c) systems, respectively