Étude Raman de l'effet de taille nanométrique des cristallites sur le diagramme de phase P-T de la zircone

The phase transformations of 7, 13, 18nm nanocrystalline zirconia prepared by pyrosol technique and 10-15nm zirconia prepared by oxidation of zirconium alloys have been monitored by Raman spectrometry. It is shown that for these zirconia, the tetragonal form can be obtained either under its metastable state at ambient conditions or from the monoclinic form by application of pressure below 4 GPa and with temperature increment smaller than 300 K. The nanometric monoclinic-tetragonal equilibrium line is strongly down-shifted in temperature compared to literature data measured on usual polycrystalline zirconia. In addition, the monoclinic-orthorhombic equilibrium line is slightly shifted to higher pressure (i.e. 7 GPa). These results confirm that stresses can play a primordial role in the formation of tetragonal form in the oxidation process of zirconium alloys.