Abstract
The effect of iron concentration and conditioning pH on the crystallisation of amorphous zirconia gels has been investigated by differential thermal analysis/thermogravimetry (DTA/TG) and X-ray methods. Evidence for the existence of a co-gel as opposed to a mixture of two gels is given. Increasing iron content inhibits the crystallisation so that reaction occurs at higher temperatures in the range 480–720 °C. The pH of gel formation is primarily responsible for determining the crystal structure after transformation: acidic pHs produce the tetragonal Zr1–XFeXO2–X/2 polymorph and alkaline pHs the cubic polymorph, regardless of the iron concentration. Activation energies for the two processes are 173 kJ mol–1 for the crystallisation to the cubic form and between 194 and 270 kJ mol–1 for the tetragonal form. The role of oxygen vacancies and hydrogen ions in determining the course of the crystallisation is discussed. The crystalline phase has a wide range of composition, from ZrO2 to Zr0.45Fe0.55O1.725 at 720 °C in air, but this range rapidly decreases at higher temperatures; α-Fe2O3 forms in samples of overall composition Zr0.95Fe0.05O1.975 heated to 1200 °C in air.
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