Preparation of high purity doped oxides for fundamental studies

Abstract

Yttria-stabilized zirconia (YSZ) is an important ceramic and is critical in electrochemical applications. Small amounts of transition metal dopants are shown to influence mechanical and electronic properties of YSZ. In this work, a novel method was used to synthesize 0.01, 0.1, 0.5, and 1.0 mol % NiO, Co3O4, and Fe2O3-doped YSZ from all acetate precursors. The synthesized materials were calcined at temperatures between 600 degrees C and 1000 degrees C to produce powders that were then pressed into compacts and sintered at 1500 degrees C. The powders and compacts were evaluated with x-ray diffraction, electron microscopy, and SQUID magnetometry. The particle size was observed to increase with increasing calcination temperature and a correlation between the particle size and formation of NiO was noted. In particular, NiO is absent as a second phase in the finer particle size material (lower calcination temperatures) but present in coarser material (calcination temperature at 1000 degrees C), even when the concentration is well below the known solid solubility limit of NiO in YSZ. The results suggest Ni2+ ions dissolve readily in nanoscale YSZ, but form NiO once the grains are larger than about 150 nm. When all materials are sintered at 1500 degrees C, NiO dissolves fully into YSZ. Thus, the dissolution of NiO in YSZ occurs in two stages: at temperatures up to 1000 degrees C, Ni2+ ions dissolve only in the grain boundaries of YSZ; at 1500 degrees C, Ni2+ ions dissolve in the bulk YSZ. The results of initial experiments for Co3O4-10YSZ and Fe2O3-10YSZ systems are reported and show an increased complexity compared with NiO-YSZ, likely because the dopant solubilities are not well-known and the effect of multivalent dopant ions existing simultaneously in YSZ is not well-characterized. This work shows that magnetometry is extremely useful for characterizing the dopant concentration in all stages of processing.