Microstructural, mechanical, and electrical characteristics of alumina-reinforced ytterbia-stabilized cubic zirconia-bas
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Al2O3-dispersed Yb2O3-stabilized cubic-ZrO2 (YbSZ) composites are fabricated by pressureless sintering of composite powders to obtain fine and homogeneous microstructures by the solution chemistry route. Al2O3 particles are deposited on ZrO2 powders by the precipitation of aluminum nitrate followed by calcination in air. The sinterability of the composites was affected by the calcination temperature. Microstructures of the sintered bodies are dependent on the Al2O3 content. For the 5 vol% Al2O3-dispersed composite, fine Al2O3 particles were mainly located inside the grains of zirconia, whereas relatively large Al2O3 particles almost dispersed at the grain boundaries when the Al2O3 content was increased. The grain growth of YbSZ was suppressed by the Al2O3 addition, and the refinement of the matrix grain improved the fracture strength of YbSZ. The YbSZ and YbSZ/Al2O3 composites exhibited almost similar ionic conductivity at high temperatures of around 1000 °C.
I. INTRODUCTION
Cubic zirconia (c-ZrO2), which is stabilized by the doping of certain oxides, is a fast oxide ionic conductor at high temperatures. Because of its high stability in both reducing and oxidizing environments, c-ZrO2 has been used as an electrolyte material for solid oxide fuel cells (SOFC) and oxygen sensors.1–3 Despite these high expectations, its mechanical properties are poor. The working temperature of SOFC system is usually high, around 900–1000 °C because the oxide ionic conductivity of c-ZrO2 at low temperatures is insufficient to achieve the appropriate efficiency. It should be noted, however, that fuel cells made up of ceramics may suffer severe thermal fatigue and/or stresses as well as creep deformation during operation, which may reduce the lifetime of the fuel cell. Therefore, it is necessary to improve the mechanical properties of c-ZrO2 to guarantee the lifetime and the reliability of the SOFC system. The properties of such ceramics in general depend on the microstructure to be used as a SOFC system. Niihara4 has shown that the dispersion of nanometer-sized particles into matrix grains and/or at the grain boundaries to make ceramicsbased nanocomposites is an effective technique for improving the mechanical properties of ceramics at both a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0194 J. Mater. Res., Vol. 19, No. 5, May 2004
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room and high temperatures. Recently, several kinds of ceramic nanocomposites have been studied, e.g., ceramic/ceramic, 5–9 ceramic/metal, 10–14 and ceramic/ intermetallic15,16 composite systems. One of the benefits of nanocomposite materials is shown in the Al2O3/SiC system by Ohji et al.5 in which nanometer-sized SiC in Al2O3 greatly affects the enhancement of creep resistance due to the anchoring effect of SiC nanoparticles by inhibiting the grain boundary sliding at high temperatures. These previous findings imply that nanodispersion in c-ZrO2 contributes to the improvement of mechanical p
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