engleski

Factors influencing the stability of low temperature tetragonal ZrO2

Various factors that influence the appearance of a tetragonal (t-) ZrO2 polymorph at room temperature have been extensively investigated. Several proposed models emphasize the role of anionic impurities (SO42-, OH-), crystallite size (surface energy), structural similarities between the starting material and t-ZrO2, lattice strains, water vapor, lattice defects (oxygen vacancies), etc. Our investigations, focused on the stability of low temperature t-ZrO2, showed that, regardless of the structural differences in the starting zirconium materials, their thermal decomposition products crystallized into a metastable t-ZrO2. The t-ZrO2 --> m-ZrO2 transformation occurred during the cooling or further calcination in the presence of air at atmospheric pressure. On the other hand, if these processes are performed in vacuum, the metastable phase is preserved. These observations indicate that a metastable t-ZrO2 appears at room temperature as a result of stabilization caused by introduction of oxygen vacancies, similarly as in the solid solutions with aliovalent cations. A decrease in the specific surface area of ZrO2 grains or the presence of the substances that enter into strong surface interactions with ZrO2 (SO42-, Cr2O3) prevents the diffusion of oxygen from the atmosphere into the ZrO2 lattice and due to this fact the metastable t-ZrO2 is stabilized. On the other hand, lattice strain and grain size of metastable t-ZrO2 could not be clearly related to its stability.

T-zro2. M-zro2. Hydrous zirconia. Oxygen vacancies. Lattice strain. Xrd. Dsc.

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