The Research On The Immicibility And Its Effect On Crystallization Of Lithium Aluminosilicate Glass Ceramics

Lithium aluminosilicate (Li2O-Al2O3-SiO2, LAS) glass-ceramics have been extensively applied in microelectronics, aeronautics, national defense and civil field mainly due to their beneficial properties, e.g., low thermal expansion coefficient, good thermal and chemical stability, high mechanical strength and unique optical properties. In LAS system glass, the process of phase transition has not been sufficiently investigated, which included the process of nucleation and controllable crystallization.In the present dissertation, the immiscibility, nucleation and crystallization of LAS glass ceramics was investigated by means of HRTEM, Raman, FESEM, SEM, XRD, DTA. Firstly, the phase transition of Li2O-Al2O3-SiO2-TiO2 (LAST) system glass at nucleation stage was investigated. Secondly, the effect of phase separation on microstructure and mechanism of crystallization in LAS system glass ceramics was also investigated. Finally, the relationship between phase separation and properties of glass ceramics was studied.The results showed that the Ti-rich phase and Si-rich phase were formed in the LAST initial glass after annealed. The size of titanium-rich was 50nm. With increasing heat treatment temperature at nucleation stage, the amount of anatase and AlxTi1-xO2 precipitated in the glass increased and the density of glass increased. In the low nucleation temperature (660℃), the AlxTi1-xO2 was not easy to form. Prolonging treated time promoted the formation of AlxTi1-xO2 while restrained the formation of anatase. However, when increasing heat treatment time in the high nucleation temperature (720℃), the AlxTi1-xO2 decomposed to anatase, which greatly promoted the precipitation of anatase. In addition, (3-quartz solid solution was formed under the condition.The phase separation was not formed in the commercial composition of LAS glass ceramics after annealed. However, the droplet microphase separation was formed with appropriate heat treatment around Tg for initial glass. The droplet phase was the titanium-rich phase and the size was 5nm. The phase separation influenced the crystallization indirectly by its effect on nucleation. The phase separation promoted the precipitation of anatase and ZrTiO4 and increased the nucleation density. The phase separation didn't change the main phase and the main phase was still theβ-spodumene solid solution. The phase separation decreased the crystallization activation energy (E) and changed the mechanism of crystallization of glass ceramics, which promoted the evolution of the nanostructure of glass-ceramics. The properties were correlated with crystallization and morphology of glass ceramics. The phase separation increased the density of LAS glass ceramics from 2.5291 to 2.5387g/cm-3, the flexural strength from 75.24 to 93.64MP and the microhardness from 558.93 to 606.86Kg/mm2 and decreased the CET from 13.960×10-7K-1to 10.950×10-7K-1(20-300℃).