Effects of nucleating agents on crystallization and microstructure of fluorophlogopite mica-containing glass-ceramics

The alteration of crystallization behavior, microstructure, and thermal properties of fluorophlogopite mica-containing glass-ceramics by nucleating agent is systematically studied. TiO2, TiO2 + ZrO2, and ZrO2 have been doped as the nucleating agents in the SiO2-MgO-Al2O3-B2O3-K2O-MgF2 (BMAPS) glass system and prepared by the melt-quench technique. The glass without nucleating agent is also prepared to ascertain the influence of nucleating agent. Addition of nucleating agents effectively increases the softening as well as glass transition temperatures. From the DSC study, it is found that the fluorophlogopite mica crystallization exotherm exhibited in the temperature range 800-850 degrees C and the activation energy varies in the range 167-182 kJ/mol. The opaque mica glass-ceramics are derived from these BMAPS glasses by a controlled heat treatment process and heat treatment at 1050 degrees C is found to be optimum. The mica crystals were identified as fluorophlogopite for all the four BMAPS glasses by X-ray powder diffraction (XRD) and subsequently confirmed by FTIR spectroscopy. Excellent matching with fluorophlogopite crystal was obtained in Zirconia-containing glass-ceramic as perceived from the XRD and FTIR studies. The microstructure of interlocked card-like mica flake crystals is found to form as seen from scanning electron microscopy, and such microstructure is obtained when ZrO2 has been used as nucleating agent. Glass-ceramic without nucleating agent possesses Vickers hardness value 4.58 Gpa and it is increased with addition of the nucleating agent (5.67-6.56 GPa), ZrO2-containing glass-ceramic possess lower hardness (5.67 GPa) and better machinability. Therefore, ZrO2 is the most efficient nucleating agent to generate fluorophlogopite mica in these glass-ceramics useable for SOFC applications