Abstract
We have previously observed that doping of Ca ions was beneficial to the formation of zircon (ZrSiO4). It is well known that synthetic ZrSiO4 is typically formed via a solid-state reaction between ZrO2 and SiO2, 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 ZrSiO4 by inspecting the interfaces at the nano and atomic scales with TEM techniques. Results demonstrated that ZrSiO4 was formed in the Ca-doped sample after sintering at 1200 °C, whereas, no ZrSiO4 was formed in the undoped sample even after sintering at 1230 °C. The Ca-doped sample consisted of a continuous ZrSiO4 matrix with dispersed ZrO2 nanocrystallites. Doping of Ca ions promoted the formation of ZrSiO4 by causing lattice distortion and oxygen vacancies in ZrO2 lattices. Thin amorphous grain boundary complexions were found between ZrO2 nanocrystallites and between ZrO2 and ZrSiO4 crystallites. These amorphous complexions acted as reaction sites and an intermediate metastable state for the solid-state reaction. A detailed formation mechanism of ZrSiO4 at the nanometer scale and atomic scale has been proposed.
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