Crystallization sequence and microstructure evolution of Synroc samples crystallized from CaZrTi 2O 7 melts

Abstract

Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM) studies have been conducted on samples crystallized from melts with a composition of zirconolite {(Ca 0.9Gd 0.1)Zr(Ti 1.9Al 0.1) 2O 7}. The formation of a whole suite of Synroc phases (zirconia, ZrTiO 4, zirconolite, perovskite and rutile) has been observed. In the CaZrTi 2O 7 system, the formation of these phases follows the crystallization sequence of Ti-bearing zirconia→ ZrTiO 4 phase → Zr-rich zirconolite → Zr-poor zirconolite → rutile/ perovskite . This sequence is induced by a fractional crystallization process, in which Zr-rich phases tend to crystallize first, resulting in continuous depletion of Zr in melt. Consistent with this melt compositional evolution, the Zr content in the zirconolite decreases from the area next to the ZrTiO 4 phase to areas next to rutile or perovskite. High-resolution TEM images show that there are no glassy phases at the grain boundary between zirconolite and perovskite. The fractional crystallization-induced textural heterogeneity may have a significant impact on the incorporation of radionuclides into crystalline phases and the resistance of radionuclides to leaching processes. Exsolution lamellae and multiple twinning resulting from the phase transition from tetragonal zirconia to monoclinic zirconia may decrease durability of the Synroc. Fast cooling of the melt may produce more zirconolite phase and relatively uniform texture. In general, however, a Synroc prepared by melting is less uniform in texture than that prepared by a sol–gel method.