engleski

Thermal behaviour of the system ZrO2-Fe2O3

Ceramics based on the system ZrO2-Fe2O3 have found important applications as catalysts for hydrocarbon isomerisation, hydrogenation of carbon monoxide, Fischer-Tropsch synthesis and ammonia synthesis. The incorporation of Fe3+ cations in ZrO2 lattice could stabilize high temperature cubic ZrO2 polymorphs, but because of its smaller ionic radius in comparison with the Zr4+ cation, this stabilization was not efficient. In our previous investigation [S. Popović, B. Gržeta, G. Štefanić, S. Musić and Cako-Nagy, J. Alloys Comp., 241 (1996) 10.], we examined the existence of solid solutions in the thermodynamically stable system m-ZrO2-a-Fe2O3 formed after calcination at 1100 oC. It was found that in the whole concentration range there existed two types of solid solutions of the form Zr1-xFe2xO2+x, structurally very closely related to m-ZrO2 or a-Fe2O3. The terminal solid solubility limits at RT were estimated to be (2.0 ą 0.3) mol% of a-Fe2O3 in m-ZrO2 and (1.0 ą 0.3) mol% of m-ZrO2 in a-Fe2O3. In the present work we investigate thermal behavior of the amorphous precursors of ZrO2-Fe2O3 system at ZrO2-rich side of the concentration range. Thermal behavior of the samples, precipitated from aqueous solutions containing different molar fraction of ZrO(NO3)2ˇ2H2O and Fe(NO3)3ˇ9H2O salts, were investigated using in situ high-temperature X-ray diffraction and differential scanning calorimetry. It was found that the heating treatment caused crystallization of examined samples. The crystallization temperature increased linearly with the increase of the molar fraction of Fe2O3 up to 0.3. The first crystallization product of these samples contained only one crystal phase, structurally closely related to tetragonal or cubic ZrO2. Unit cell of the crystallization product decreased with the increase of iron content. The increase of temperature caused segregation of the initial crystal phase into two phases structurally closely related to tetragonal ZrO2 and a-Fe2O3. The first crystallization product of the sample with Fe2O3 molar fraction 0.5 contained one crystal phase structurally closely related to a-Fe2O3. In further heating the content of this phase increased and the additional crystal phase, structurally closely related to tetragonal or cubic ZrO2, appeared on account of the amorphous remnant.

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