CRYSTALLIZATION OF IRON(III) ZIRCONIA CO-GELS

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 degrees 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 degrees C in air, but this range rapidly decreases at higher temperatures; alpha-Fe2O3 forms in samples of overall composition Zr0.95Fe0.05O1.975 heated to 1200 degrees C in air.