Study On Preparing Low Thermal Expansion Glass-ceramic Of Li₂O-Al₂O₃-SiO₂ System By Sintering Method

Li2O-Al₂O₃-SiO₂ system glass-ceramics has been prepared by sintering method, using chemically pure Li₂CO₃, Al₂O₃, SiO₂ as main raw materials, adding TiO₂, ZrO₂, P₂O₅ as nucleating agents. The interrelationship and interaction of glass composition, heat treatment schedule, microstructure and performance was investigated by orthogonal test and DTA, XRD, TEM, SEM analysis. Reasonable composition of glass ceramics and heat treatment schedule have been confirmed, and low expansion glass-ceramics with relatively high mechanical properties and reasonable crystal phase composition has been prepared. On the basis of above-mentioned results, the effects of nucleating agents such as TiO₂, ZrO₂, P₂O₅ on the structure and performance has been further studied.The results show that:there is only granular crystal when the molar ratio of SiO₂ /Al₂O₃ is 5∶1, and with the decrease of the molar ratio of SiO₂ /Al₂O₃, fibrous crystal precipitates appear. The thermal expansion coefficient of glass ceramics increases with the decrease of the molar ratio of SiO₂/Al₂O₃ while the flexural strength increases first and then decreases on the condition of a bit high sintering temperature and crystallization. The effects of sintering temperature on performance has been investigated. It shows that the main crystal phase isβ-quartz solid solution and the thermal expansion coefficient and the density of the products is low when the sintering temperature is 1000℃. With the increase of temperature,β-quartz solid solution of quartz changes intoβ-spodumene and the expansion coefficient increases. The transition temperature increases gradually with the decrease of the molar ratio of SiO₂/Al₂O₃. When the sintering temperature increases to 1200℃, the strength is greater than 70MPa.The effects of heat treatment of LAS glass ceramics have also been researched. The appropriate heat treatment as follow: the nucleation temperature and the nucleation time are 780℃and 1h, the crystallization temperature and the crystallization time are 860℃and 1.5h respectively. The thermal expansion coefficient of as-prepared glass ceramics is 0.67×10-6/℃and the strength reaches 80.48MPa when the sintering temperature increases to 1200℃. The crystallization process of LAS system glass ceramics is a process controlled by phase separation promoting crystallization, and the whole process controlled by the theory of nucleation - growth. The quantity of crystal nucleus is few when the temperature is 760℃. While on the contrary the same set up. It is only at the best temperature that the crystal nucleus is small and in a large quantity.The species and dosage of nucleating agent only affect the morphology, it can not change the main crystal phase. The glass ceramics with good crystallinity can be still obtained without any nucleating agent. P₂O₅ is beneficial to the precipitation fibrousβ-spodumene solid solution, while TiO₂, ZrO₂ are favorable to the precipitation of granularβ-spodumene solid solution, and with the increase of the total amount of TiO₂ and ZrO₂. The thermal expansion coefficient and the strength are 0.61×10-6/℃and 85.71Mpa respectively, when the molar ratio of TiO₂:ZrO₂:P₂O₅ is 1:1:3.