Effect of sintering temperature on crystal growth and evolution of CaZrTi2O7–CaNdTiNbO7 ceramics: Minor actinides immobilization host materials

Regulation of microstructure and processing cost of waste form are important for the design and application of high-level radioactive waste immobilization. In this study, CaZrTi₂O₇ (zirconolite) – CaNdTiNbO₇ (pyrochlore) ceramics were synthesized via conventional sintering method at 1000–1500 °C to investigate the effect of sintering temperature on the microstructure, solidification, and crystal evolution. The powder x-ray diffraction (XRD) and backscattered scanning electron microscopy–energy dispersive spectroscopy (BSEM–EDS) results indicated gradual formation of zirconolite and pyrochlore ceramics, following with their densification when increasing sintering temperature. It was found that sintering temperature should be higher than 1300 °C to form zirconolite-pyrochlore solid solutions. Densified pyrochlore was obtained at 1500 °C. In addition, transformation from relatively low-symmetry lattice to higher-symmetry lattice was found with an increase in sintering temperature. Furthermore, higher temperature can enhance the crystal growth and densification in the ceramic composite. The finding in this study could provide important suggestions to design and process zirconolite-pyrochlore waste forms for industrial application.