Sintering and crystallization of sol-gel derived lithium aluminosilicate (LAS) glass-ceramic powders

The present study focused on the fundamental understanding of crystallization and sintering of sol gel derived lithium aluminosilicate (LAS) glass ceramic powders and how they are related to gel microstructure.; The gel powders were obtained by drying an alkoxide gel, calcining at 640{dollar}spcirc{dollar}C and milling to a particle size less than 1 um. The as-dried powder was extremely porous with a specific surface area of {dollar}sim{dollar}350 m{dollar}sp2{dollar}/g and a particle density of {dollar}sim{dollar}67% relative to melt glass. After calcining at 640{dollar}spcirc{dollar}C, the gel microstructures were not affected significantly but the particle density increased to {dollar}sim{dollar}95% relative to melt glass and the specific surface decreased to {dollar}sim{dollar}186 m{dollar}sp2{dollar}/g. At 750{dollar}spcirc{dollar}C, the particle density approached that of melt glass and the porous gel particles became solid glass particles.; A metastable {dollar}beta{dollar}-quartz solid solution was formed first and transformed to a stable {dollar}beta{dollar}-spodumene solid solution at higher temperature. A time-temperature-transformation curve for the crystallization of {dollar}beta{dollar}-quartz solid solution was not C-shaped and indicated that a large number of nuclei were already present in the calcined powder. This was supported by the negligible effect of particle size, nucleation treatment and its temperature on the crystallization temperature. From the independence of crystallization temperature of the existence of nucleating agents, it was proposed that a large number of surface nuclei were formed at low temperature in the gel powder.; The calcined gel powder sintered well. Sintering for 10 minutes at 900{dollar}spcirc{dollar}C resulted in well sintered glass ceramic of {dollar}sim{dollar}97% theoretical density but some large irregular-shaped pores which were due to processing defects and further interrupted by crystallization, were present. By decreasing the glass viscosity with partial replacement of SiO{dollar}sb2{dollar} by P{dollar}sb2{dollar}O{dollar}sb5{dollar}, a translucent sintered glass ceramic was achieved.; In the melt glass, nucleation from the solid glass particle surface was dominant whereas surface nucleation from the micropore walls occurred throughout the individual gel particles. Consequently, the glass ceramic microstructure from the sol gel powder was similar to that of the nucleated bulk crystallized glass ceramic. The lower viscosity and the unique crystallization behavior of the sol gel powder resulted in rapid elimination of the particle surfaces (pores). This is in sharp contrast to the melt glass powder which sintered poorly. This difference in sinterability is attributed to the interruption of glass powder sintering by crystallization of particle surfaces, a process that is not present in the sol gel particles.