New insights into the formation mechanism of zircon in a ZrO2–SiO2 nanocrystalline glass-ceramic: A TEM study

We have previously observed that doping of Ca ions was beneficial to the formation of zircon (ZrSiO 4 ). It is well known that synthetic ZrSiO 4 is typically formed via a solid-state reaction between ZrO 2 and SiO 2 , in which the interfaces between the reactant and resultant play an important role. However, the interfaces are lacking detailed microstructural observation. This follow-up study aims at exploring the formation mechanism of ZrSiO 4 by inspecting the interfaces at the nano and atomic scales with TEM techniques. Results demonstrated that ZrSiO 4 was formed in the Ca-doped sample after sintering at 1200 °C, whereas, no ZrSiO 4 was formed in the undoped sample even after sintering at 1230 °C. The Ca-doped sample consisted of a continuous ZrSiO 4 matrix with dispersed ZrO 2 nanocrystallites . Doping of Ca ions promoted the formation of ZrSiO 4 by causing lattice distortion and oxygen vacancies in ZrO 2 lattices. Thin amorphous grain boundary complexions were found between ZrO 2 nanocrystallites and between ZrO 2 and ZrSiO 4 crystallites . These amorphous complexions acted as reaction sites and an intermediate metastable state for the solid-state reaction. A detailed formation mechanism of ZrSiO 4 at the nanometer scale and atomic scale has been proposed.