Characterization and densification of lanthana-zirconia powders prepared by high temperature hydrolysis

Abstract

Zirconia-lanthana powders containing 4.5, 7, 10, 15 and 20 mol % La₂O₃ were prepared by hydrolysis. The hydrolysis process was carried out in a laboratory stainless steel autoclave for their equivalent hydroxides for 2 h at 200‡ C. The powders were investigated using X-ray diffraction, infrared spectrometry, and transmission electron microscopy techniques. No other phases except the cubic phase zirconia of fluorite-type structure were detected. The prepared materials were examined for their thermal stability and phase constitution, by X-ray and infrared analyses, on heating up to 1400‡ C. The cubic phase zirconia remains stable up to 1000‡ C at which it starts to decompose yielding monoclinic zirconia and lanthanum zirconate. At 1200‡ C, the cubic phase nearly disappears in the sample containing 4.5 mol % La₂O₃. Increasing La₂O₃ content up to 20 mol % retards its destabilization, reduces the yielded monoclinic phase, and in the same time increases the formed lanthanum zirconate phase. At 1500‡ C only monoclinic ZrO₂ and La₂Zr₂O₇ are present. The La₂Zr₂O₇/monoclinic ZrO₂ ratio increases with increasing La₂O₃ content. Pressed specimens of the prepared materials were fired for 2 h at 800 to 1400‡ C. The sintering activity of the prepared powders resulted in a 92% theoretical density body for the 4.5 mol % ZrO₂ material. The densification properties in relation to changes in the phase constitution are discussed.