EFFECT OF HEAT TREATMENT ON THE PHYSICOCHEMICAL PROPERTIES OF ULTRAFINE POWDER IN THE ZrO₂–Y₂O₃–CeO₂–Al₂O₃–СоО SYSTEM

Abstract

Variations in the phase composition, specific surface area, and morphology of structural components in the ultrafine powder of composition (wt.%) 70 (90 ZrO₂ (3 Y₂O₃, 2 CeO₂)–10 Al₂O₃)–30 CoAl₂O₄ (70ZA30CoA) produced by hydrothermal synthesis combined with mechanical mixing during heat treatment up to 1300 °C were studied. The study employed X-ray diffraction, scanning and transmission electron microscopy, petrography, and BET. The formation of CoAl₂O₄ in the heat reatment of the 70ZA30CoA powder was accompanied by a reversible phase transformation: T-ZrO₂ → M-ZrO₂ → T-ZrO₂. The M-ZrO₂ content increased from 15% to 46% in the temperature range 850–1000 °С and decreased to 13% after heating up to 1150 °С. The process was accompanied by slight coarsening of the primary T-ZrO₂ particles, while the size of the primary M-ZrO₂ particles remained practically unchanged. This phase transformation is due to a decrease in the free energy of the thermodynamically nonequilibrium system represented by the ultrafine 70ZA30CoA powder. The phase composition influences the color of the 70ZA30CoA powder, changing in the following sequence: gray → gray blue → dark cyan → bright blue. The morphological analysis of the structural components showed that the formation of CoAl₂O₄ and the reversible T-ZrO₂ → M-ZrO₂ phase transformation were accompanied by a change in shape, loosening, and subsequent sintering of the agglomerates. The appearance of chain-like agglomerates of various shapes and sizes is indicative of the high activity of the 70ZA30CoA powder to sintering at 1300 ° C. The decrease in the specific surface area from 46 to 1 m²/g and its dependence on the heat treatment temperature were determined by the development of three structural transformation processes: formation of CoAl₂O₄, phase transformation of the ZrO₂-base solid solution, and sintering of the 70ZA30CoA powder. The established regularities are of fundamental importance for the microstructural design of ZrO₂ composites such as ZrO₂–Y₂O₃–CeO₂–Al₂O₃–CoO materials of blue and other colors for various applied purposes.