Crystallisation of iron(III)-zirconia co-gels

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 Zr_1–XFeXO_2–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 Zr_0.45Fe_0.55O_1.725 at 720 °C in air, but this range rapidly decreases at higher temperatures; α-Fe₂O₃ forms in samples of overall composition Zr_0.95Fe_0.05O_1.975 heated to 1200 °C in air.