engleski

The influence of temperature treatment on the phase development in ZrO2-Fe2O3 and HfO2-Fe2O3 systems

In continuation of our research on the materials with improved catalytic efficiency we investigated the systems ZrO2-Fe2O3 and HfO2-Fe2O3 having Fe2O3 content up to 50 mol %. Amorphous precursors of ZrO2-Fe2O3 and HfO2-Fe2O3 systems were co-precipitated from aqueous solutions of the corresponding salts. Detailed studies of the crystallization behavior of the precursors were performed by differential thermal analysis and thermogravimetric analysis. Crystallization temperature of both systems increased with an increase of the Fe2O3 content indicating that in both cases the amorphous precursor was a single co-gel. The influence of temperature treatment (500, 600, 800 and 1100 °C) on the phase development in the ZrO2-Fe2O3 and HfO2-Fe2O3 systems was studied by X-ray powder diffraction (XRD) and, in some cases, by laser Raman spectroscopy. XRD patterns were taken by step-scanning and determination of unit-cell parameters of the appropriate phases was performed by means of the powder-pattern-fitting methods. It was found that incorporation of Fe3+ ions partially stabilizes high temperature polymorphs of zirconia and hafnia during the calcination and cooling at standard air pressure. Incorporation of Fe3+ ions into ZrO2 and HfO2 lattices decreases with the increase of temperature, which was followed by the transformation of metastable polymorphs to the thermodynamically stable monoclinic polymorphs of zirconia and hafnia. The terminal solid solubility limits of Fe2O3 in ZrO2, estimated at RT, was ~30 mole % for calcination at 600 °C, ~9 mole % for calcination at 800 °C and  2.0 mole % for calcination at 1100 °C. The terminal solid solubility limits of Fe2O3 in HfO2, estimated at RT after the same calcination procedures, showed to be very similar, only ~1 mole % smaller. In both systems the unit cell volume of the crystallization products decreased with the increase of iron content, but in the system ZrO2-Fe2O3 the rate of decrease was bigger. The status of the Fe3+ ions in the HfO2- and ZrO2-type lattice and the process of their segregation into the  -Fe2O3 phase were examined by Mössbauer spectroscopy.

ZrO2-Fe2O3; HfO2-Fe2O3; XRD; DTA

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