Mechanisms of crystal structure organisation in magnesium aluminosilicate glass: HREM and analytical study

摘要

The results of a high resolution transmission electron microscopy study (HRTEM) and energy dispersive x-ray analysis (EDAX) of the crystal structural organisations formed during the early stages of ceramisation within a model glass from the cordierite area of the SiO2-Al2O3-MgO(ZnO) system, activated by TiO2 and/or ZrO2 as nucleating agents, are presented. It is shown that the roles of TiO2 and ZrO2 in the crystallisation of glass are different leading to glass-ceramics with different phase compositions and textures. With regard to the SiO2-Al2O3-MgO(ZnO)-(12 wt%)TiO2 glass, the HRTEM study proved that the first crystal forms appear at 725°C. They are (Mg,Al)Ti2O5, titanate grains 3-5 nm in diameter, high temperature quartz structure solid solutions with Mg, Al, Ti admixtures and SiO2-TiO2 solid solution crystals. The SiO2-Al2O3-MgO(ZnO)-ZrO2 (5 wt%) glass starts to crystallise at 900°C, which is above the softening temperature, and baddelyite (ZrO2) and zircon (ZrSiO4) crystals are formed. Furthermore, large micron size cordierite crystals crystallise around them. When both oxides [TiO2(7 wt%) and ZrO2 (5 wt%)] are introduced the crystallisation starts at the lower temperature of 750°C (equal to Tg). Large TiO2 and ZrO2 solid solution dendrites and high quartz solid solution crystallise with cordierite crystals between them. ZrO2 unlike TiO2 hampers the formation of high quartz and promotes cordierite crystallisation; this is a result of the different crystallochemical properties of ZrO2 and TiO2. Medium range order domains existing in the parent glass can be the precursors of crystal embryo formation, and they determine their composition and structure of the crystals formed (i.e. structural inheritance).