Impedance spectroscopy study of ionic diffusion in polycrystalline ZrO 2 :Y 2 O 3 solid solution
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Impedance spectroscopy study of ionic diffusion in polycrystalline ZrO2:Y2O3 solid solution Fábio C. Fonseca, Eliana N. S. Muccillo and R. Muccillo Multidisciplinary Center for Development of Ceramic Materials CCTM-Energy and Nuclear Research Institute CP 11049, Pinheiros, S. Paulo, SP, Brazil, 05422-970 [email protected] ABSTRACT The ZrO2:Y2O3 solid solution formation has been followed by impedance spectroscopy and X-ray analysis. The experimental sequence, after mixing 8 mol% Y2O3 to ZrO2, was: attrition milling the mixture, drying, weighing, cold pressing, thermally treating at several different temperatures and times, performing the X-ray diffraction measurements at room temperature, applying metallic electrodes, and performing the impedance spectroscopy measurements in the 300°C-600°C temperature range. A good correlation was found between the decrease of the yttria main diffraction line and the increase of the stabilized zirconia main diffraction line, showing that solid solution is attained at the expenses of yttria, as expected. The impedance spectroscopy data Z(ω, T, t) show that the bulk response follows a t1/2 law, an evidence of yttrium diffusion to zirconia. Moreover, a relationship is found between the bulk resistivity and the elimination of ion blockers for increasing sintering times. The result allowed for the determination of the activation energy for the diffusion of the slowest diffusing species (Zr4+) in ZrO2:Y2O3. INTRODUCTION Zirconia-based solid electrolytes are used as electrochemical transducers in a series of devices like oxygen sensors, oxygen pumps and fuel cells [1]. Oxygen sensors find major applications in the steel making process and in the automotive industry [2]. Oxygen pumps are used to provide known oxygen partial pressures in closed systems [3,4]. Fuel cells can produce environmental clean electrical energy during conversion of oxygen and hydrogen to water [5]. All these applications rely on the fact that zirconia-based solid electrolytes are oxide ion conducting materials in a wide range of temperature and partial pressure of oxygen. The defects responsible for the ionic conduction are oxygen-ion vacancies produced in zirconia matrices by partial substitution of aliovalent (2+ or 3+) ions for zirconium ions. In particular, the substitution of Y3+ for Zr4+stabilizes the zirconium oxide in the cubic phase with the highest value of electrical conductivity [6] in comparison to that of the tetragonal and monoclinic polymorphs of zirconia. ZrO2:8 mol% Y2O3 is the most important zirconia-based solid electrolyte for its electrical performance and the stabilization of zirconia is the main phenomenon behind its electrical properties. Mass transport in ceramics is important for their fabrication, functional properties and long term stability. Mass transport controls the rates of sintering, solid state reactions and grain growth [8]. Mass transport occurs via diffusion of chemical species. It is a fundamental process occurring in solid materials during processing - when atoms or ions exchange
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