Transport properties of ceria–zirconia–yttria solid solutions {(CeO₂)_x(ZrO₂)_1−x}_1−y(YO_1.5)_y (x = 0–1, y = 0.2, 0.35)

Abstract

The electron and hole conductivity, oxygen transport property and thermal expansion behavior of {(CeO₂)_0.5(ZrO₂)_0.5}_0.65(YO_1.5)_0.35 solid solution were investigated and the effect of yttrium content was examined by comparing the data with those of {(CeO₂)_1−x(ZrO₂)_x}_0.8(YO_1.5)_0.2. The electron conductivity of {(CeO₂)_0.5(ZrO₂)_0.5}_0.65(YO_1.5)_0.35 measured by ion blocking method was one order lower than that of {(CeO₂)_0.5(ZrO₂)_0.5}_0.8(YO_1.5)_0.2. The increase of yttrium content suppressed the phase transition from cubic to pyrochlore structure in reducing atmosphere. The oxygen isotope diffusivity and surface exchange rate constant of {(CeO₂)_0.5(ZrO₂)_0.5}_0.65(YO_1.5)_0.35 are considered to be comparable to those of {(CeO₂)_0.5(ZrO₂)_0.5}_0.8(YO_1.5)_0.2. The chemical expansion due to the reduction of cerium ion was observed as well as for rare earth doped ceria. The compatibility of the material as an anode substrate in solid oxide fuel cells is discussed.

Introduction

Ceria–zirconia solid solutions have been investigated from viewpoints of interesting catalytic activity and mixed conductivity. Ozawa et al. reported the application of CeO₂–ZrO₂ solid solutions to the three-way catalysts for automobiles [1]. Moreover, the CeO₂–ZrO₂–YO_1.5 solid solution is investigated as a promising material for oxygen permeable membranes, which can be applied to electro-catalytic reactors or oxygen gas separators. The temperature and oxygen partial pressure dependence of total and ionic conductivity of {(CeO₂)_1−x(ZrO₂)_x}_0.9(Y₂O₃)_0.1 have been reported by several researchers [2], [3], [4].

We have investigated the transport properties of the system {(CeO₂)_x(ZrO₂)_1−x}_0.8(YO_1.5)_0.2, such as oxygen isotope diffusivity ($D_{\text{O}}^{*}$) and surface exchange rate (k) [5] or electron/hole conductivity (σ_e, σ_h) [6] because we think such transport properties in the oxides may affect the anode reactions of solid oxide fuel cells (SOFC) in which the fuel reacts with oxide ions come through the electrolyte. The electron conductivity (σ_e) of {(CeO₂)_x(ZrO₂)_1−x}_0.8(YO_1.5)_0.2 drastically increased with cerium content (x) and had a maximum around x = 0.5 [6]. Concerning the oxygen transport properties, the oxygen isotope diffusivity has a minimum at x = 0.5 [5], however, the oxygen surface exchange rate constant (k) has a maximum around x = 0.4, which is similar to the compositional dependence of electron conductivity.

However, in the electron conductivity measurement of {(CeO₂)_x(ZrO₂)_1−x}_0.8(YO_1.5)_0.2, it was difficult to obtain the precise data in lower oxygen partial pressures, the oxygen partial pressures lower than 10⁻⁸ Pa at T = 973–1273 K. The data exhibited some scattering and their reproducibility was poor. It indicates the phase transition from fluorite to pyrochlore structure. This phase transition has been reported for CeO₂–ZrO₂ solid solutions by several researchers including complicated metastable phases [7], [8], [9], [10], [11]. Ikryannikova et al., reported that 10 mole% YO_1.5 addition to CeO₂–ZrO₂ solid solution may stabilize the cubic phase, however, it does not suppress the formation of pyrochlore phase in reducing atmosphere at high temperatures [8].

In this paper, we investigated ceria–zirconia–yttria solid solution with higher yttria content, {(CeO₂)_0.5(ZrO₂)_0.5}_0.65(YO_1.5)_0.35, in order to suppress the phase transition. The hole and electron conductivity, and oxygen isotope diffusivity was measured and compared with the results of samples with lower yttrium content. Since we have been interested in this material as a candidate of anode support materials in SOFC, the thermal expansion behavior was also checked in air and in reducing atmospheres.