Relationships between structural and electrical properties in mixed conductors duplex materials in the ZrO2-Y2O3-TiO2ternary system

M T Colomer,J Chaboy,S Díaz-Moreno,M Maczka,R Boada

doi:10.1103/physrevb.89.094101

Abstract

Duplex materials constituted by 0.50 mol titania-doped yttria-stabilized zirconia (YSZ) and 0.50 mol titania-doped yttria-tetragonal zirconia polycrystalline (YTZP) solid solutions in the ZrO${}_{2}$-Y${}_{2}$O${}_{3}$-TiO${}_{2}$ ternary system can be obtained using different processing strategies. In this study, different amounts of TiO${}_{2}$ dopant and different sintering times have been used for the preparation of the duplex materials: Doping YSZ with 10 mol% of TiO${}_{2}$ and sintering in air for 8 h (10Ti8h) and doping YSZ with 15 mol% of TiO${}_{2}$ and sintering in air for 2 h (15Ti2h) are both successful routes to obtain duplex materials. If we compare the field emission scanning electron microscopy-energy dispersive x-ray analyses of each cubic and each tetragonal solid solution of the two duplex materials, we conclude that the composition of each phase is different from each other, as was expected. The total ionic conductivity of both duplex samples is strongly reduced with respect to that of YSZ, and this reduction increases with the Ti content. In addition, the activation energy for ionic migration in 10Ti8h and 15Ti2h is lower than that for YSZ but higher than that for YTZP, as expected in terms of the relative amount of both YSZ and YTZP fractions in the materials. X-ray absorption spectroscopy (XAS) results, both x-ray absorption near-edge structure and extended x-ray absorption fine structure, show that Ti is sixfold coordinated in both the cubic and tetragonal phases of the duplex materials, departing from the eightfold coordination expected if a simple substitution at the Zr sites would take place. The XAS results also point out that the ability of Ti to trap oxygen vacancies in the cubic phase increases as Ti content does, in agreement with the electrical conductivity behavior, which cannot be accounted for in terms of a dilution effect.

Highlights

Mixed conductors should be able to replace the current state-of-the-art anodes for solid oxide fuel cells (SOFCs)
Duplex materials constituted by 0.50 mol titania-doped yttria-stabilized zirconia (YSZ) and 0.50 mol titaniadoped yttria-tetragonal zirconia polycrystalline (YTZP) solid solutions in the ZrO2-Y2O3-TiO2 ternary system can be obtained using different processing strategies
Mixed conductors based on yttria-stabilized zirconia (YSZ) or yttria-tetragonal zirconia polycrystalline (YTZP) are usually obtained if a significant amount of a mixed valence dopant cation is introduced in solid solution

Summary

Introduction

Mixed conductors should be able to replace the current state-of-the-art anodes for solid oxide fuel cells (SOFCs). Both ionic and electronic defects are present in nonnegligible concentrations in mixed conductors, and their mobility is so significant that the electrode reaction can proceed at any point on the electrode surface. YTZP material has shown to be a tougher material [3] These two combined characteristics make Ti-doped YSZ/Ti-doped YTZP duplex materials a very attractive option for application in electrochemical devices, such as matrices in a SOFC anode. The desired properties of these mixed ionic conductors depend on the concentration and local environment of mobile ionic species, i.e. the lattice-oxygen vacancies and vacancy complexes. In order to understand the physical properties of these materials such as diffusion and/or ionic conduction, knowledge of the local structure and short-range order becomes necessary [4]

Methods

Results

Conclusion