Correlation between microstructure and degradation in conductivity for cubic Y2O3-doped ZrO2
Introduction
Yttria-doped Zirconia (YSZ) has been studied as an electrolyte material in solid oxide fuel cells (SOFCs) for many years. Doping with 8 to 9 mol% Y2O3 yields the highest ionic conductivity in this system [1]. The full stabilization of the cubic high-temperature phase at room temperature is often assumed at these doping concentrations, e.g. [2]. To obtain a sufficiently high ionic conductivity, SOFCs are operated at temperatures around 1000 °C. However, it is a well-known phenomenon that the ionic conductivity degrades significantly within less than 1000 h at high temperatures [3]. Despite the serious obstacle that degradation imposes on the commercialization of SOFCs, experimental verifications of the effect of aging on the microstructure combined with electrical measurements are relatively sparse. Several possible causes for degradation in YSZ have been suggested which are discussed in a comprehensive study by Kondoh et al. ([3], [4], [5], [6] and references therein): phase transformation from the cubic phase into phases with lower symmetry and lower ionic conductivity, complex formation between oxygen vacancies and Y3+-ions due to Coulomb interaction, formation of long-range ordered phases, increase of the grain boundary (GB) resistance by the formation of a glassy phase. It has been also pointed out that increasing Y-segregation at grain boundaries leads to the charging of grain boundaries. Using mainly extended X-ray absorption fine structure (EXAFS) measurements Kondoh et al. [5] concluded that increasing short-range order (SRO) of oxygen vacancies around Zr4+-ions occurs which reduce the concentration of mobile oxygen vacancies. Applying transmission electron microscopy (TEM), Kondoh et al. [6] analyzed microstructural changes of YSZ with similar doping concentrations and aging treatments as in our study. They observe the disappearance of “superstructure” reflections after aging, which they attribute to the relaxation of periodic lattice distortions by trapping of oxygen vacancies by Zr4+-ions. In an earlier study, Ciacchi et al. [7] came to a different conclusion because they attributed the degradation of ionic conductivity in ZrO2 doped with 6 to 8 mol% Y2O3 to the precipitation of the tetragonal ZrO2 phase from the cubic ZrO2 matrix.
TEM allows the analysis of several of the causes that have been suggested to be responsible for the degradation of the ionic conductivity. Selected area electron diffraction (SAED) yields information about the phases present in the material. It enables in particular the detection of phases, which occur as very small particles, which will not show up in X-ray diffraction due to the strong broadening of the Bragg reflections. Electrostatic potentials are expected if the oxygen vacancy concentration deviates in the vicinity of grain boundaries from the bulk grain values due to GB charging which can be visualized by electron holography with high spatial resolution as shown by Ravikumar et al. [8] for GBs in SrTiO3. SRO effects can be detected as a contoured diffuse background in SAED patterns. Dai and Wang [9] have in particular studied characteristic intensity contours, which are expected for SRO of oxygen vacancies (i.e. oxygen vacancy clustering).
In the present work we compare the microstructure and conductivity of as-sintered and aged YSZ with doping concentrations of 8 mol% and 10 mol%. The samples were prepared from commercially available powders and by subsequent tape casting and sintering. Thus, the fabrication procedure basically fulfils industrial requirements for a mass production of electrolyte substrates. Using various TEM techniques including electron holography, most of the factors attributed to cause degradation of ionic conductivity could be analyzed and significant microstructural changes were observed after aging.
Section snippets
Experimental techniques
YSZ samples with three different doping concentrations (8.5 mol% Y2O3: 8YSZ, 10 mol% Y2O3: 10YSZ and 15.6 mol% Y2O3: 16YSZ) were studied. The single crystalline 16YSZ samples were purchased from Zirmat (North Billerica, USA). This material was included in the study as a structural reference material for electron microscopy which contains only the cubic ZrO2 phase. Its conductivity was not measured in the present study because it is not interesting for application in SOFCs due to its low ionic
Conductivity
Fig. 2 shows the degradation of the conductivity of 8YSZ- and 10YSZ-samples, which was analyzed over a period of 2500 h for 8YSZ and 950 h for 10YSZ. The 8YSZ sample shows a strong conductivity reduction of 38%, which does not saturate even after 2500 h. In contrast, the reduction of the conductivity of 10YSZ is significantly lower. After about 400 h the conductivity of 8YSZ was already decreased below the value of 10YSZ at the same annealing time.
Due to the strong reduction of the conductivity
Phases in Y2O3-doped ZrO2
Many studies have been devoted to the analysis of the different YSZ phases and it was not the main purpose of our work to contribute information to the stability of various YSZ phases. However, a careful characterization of the as-sintered samples was necessary to analyze the changes after aging.
The SAED diffraction analyses clearly show that even doping with 10 mol% Y2O3 is not sufficient to obtain fully stabilized c-ZrO2. A significant volume fraction of t-ZrO2 is observed. Dark-field TEM
Summary
We have carried out a combined study of conductivity and microstructural changes in polycrystalline 8YSZ and 10YSZ electrolyte substrates before and after thermal treatment at 950 °C up to 2500 h. Various TEM techniques including transmission electron holography and electrical measurements were applied. 8YSZ shows a steep decrease in ionic conductivity of more than 35% over 2500 h at 950 °C, whereas 10YSZ degrades significantly less and tends towards a constant value after 950 h of thermal
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