Mechanical Effect on Oxygen Mobility in Yttria Stabilized Zirconia
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Mechanical Effect on Oxygen Mobility in Yttria Stabilized Zirconia Wakako Araki1, and Tadaharu Adachi2 1 Mechanical Sciences and Engineering, Tokyo Institute of Technology, 2-12-1-I6-5 O-okayama, Meguro-ku, Tokyo, 152-8552, Japan 2 Mechanical Sciences and Engineering, Tokyo Institute of Technology, 2-12-1-I6-1 O-okayama, Meguro-ku, Tokyo, 152-8552, Japan ABSTRACT The mechanical effect on the oxygen ion mobility in zirconia stabilized with 8 mol% yttria was investigated in this study. A dynamic mechanical thermal analysis showed that the dynamic modulus decreased gradually with temperature while the mechanical loss had two peaks due to different relaxation mechanisms. From the comparison of activation energies between the ionic conductivity and the mechanical relaxation, the dominant factor for oxygen mobility was determined to be the migration of oxygen vacancies in the simple complexes. The result also illustrated the strong relationship between the modulus and the conductivity. An impedance analysis under mechanical tensile-loading conditions showed that the mechanical load improved the ionic conductivity by 6 % at maximum although the improvement was a temporary effect. INTRODUCTION Oxygen ionic conductive materials are promising materials as electrolytes for solid oxide fuel cells (SOFCs) and oxygen sensors and have been thoroughly investigated in recent years [1,2]. Zirconia and ceria doped with rare earths have been found to show an ionic conductivity due to oxygen vacancies. Especially, yittria-stabilized zirconia (YSZ) has been widely used as the oxygen ionic conductive material because of its high ionic conductivity as well as its excellent mechanical properties. Much research have been done on the mechanical properties and behaviors of YSZ, such as its elastic modulus, bending strength [2], fracture toughness, superplasticity3, and also stressinduced phase transformation [4]. The modulus and strength were experimentally shown to decrease with increasing temperature [5]. More recently, the mechanical loss, i.e. internal friction, of YSZ has been investigated and examined under various temperatures and frequencies [6-9]. It has been discussed that the mechanical loss can be attributed to migration of oxygen vacancies. Thus, the ionic conductivity could be strongly related to the mechanical viscoelastic relaxation behavior. In addition to the relationship between the mechanical viscoelastic behavior and the conductivity, the effect of mechanical stress and strain on the conductivity is very interesting. Dielectric materials such as lead zirconate titanate are widely known to show the piezoelectricity due to intrinsic polarization caused by the mechanical stress. The electron mobility in the semiconductors such as doped silicon is known to be improved by mechanical strain, which cause the band structures. Few studies [10.11], however, have reported on the relationship between the mechanical stress or strain and the ionic conductivity. It has been found out that the plastic compressive deformation
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