Mechanochemical synthesis of zirconia nanoparticles: Formation mechanism and phase transformation

Studies on the synthesis and formation mechanism of zirconia nanopowder prepared by mechanochemical technique have been carried out by means of X-ray powder diffraction (XRPD), differential thermal and thermogravimetric analysis (DSC-TG), Fourier transform infrared spectroscopy (FT-IR), and transmission electron microscopy (TEM). A dry powder mixture of anhydrous ZrCl4 and CaO was milled in stoichiometric ratio to produce ZrO2. The milling resulted in the formation of crystalline CaO and partially amorphous ZrCl4 while there was no sign of chemical reactions during milling. Heating the as-milled powder resulted in the formation of amorphous zirconium hydroxide (ZrOH)4) and calcium chloride (CaCl2(2H2O)). According to our results, ZrCl4 hydrolyses during the heating stage while CaO adsorbs the produced HCl forming CaClOH and CaCl2 subsequently. Heat treatment of Zr(OH)4 resulted in the production of zirconia (ZrO2). Based on Rietveld refinement it has been shown that an average critical crystallite size around 46 nm exists above which tetragonal zirconia (t-ZrO2) transforms to monoclinic zirconia (m-ZrO2).

► The formation mechanism of zirconia from a mixture of ZrCl4 and CaO has been determined. ► ZrO2 synthesis by mechanochemical treatment follows a hydrolysis reaction and not a solid-state displacement reaction. ► CaO oxide does not act as an oxygen donor but its role is as a consumer of HCl produced by the hydrolization process. ► Heating the as-milled powder resulted in the formation of amorphous Zr(OH)4 and calcium chloride. ► An average critical crystallite size around 46 nm exists above which t-ZrO2 transforms to m-ZrO2.