Effect of Al2O3 content on amorphous phase-separation and self-limited crystallization of phosphosilcate glasses

Amorphous phase-separation and crystallization behavior of phosphosilicate glasses with different Al2O3 contents were systematically investigated. We demonstrate that nanophase-separation spontaneously occurs during cooling of all the glass melts. The two separated phases are found to be the phosphate-rich droplet phase with high ion movability and the rigid silicate-rich matrix. With increasing Al2O3 content, nucleation rate decreases whereas growth rate is enhanced in terms of nucleation-growth type phase-separation. In addition, for the compositions with 0 and 2 mol% Al2O3 , sodium phosphate nanocrystals simultaneously form in the droplets in the cooling process, this is further confirmed from the micro-morphologies the samples heat-treated at 1.2 Tg for various durations. In this case, the growth of sodium phosphate is self-limited by the surrounding silicate-rich phase. Our results show that the addition of Al2O3 tends to inhibit nucleation of sodium phosphate rather than crystal growth. This work indicates that phase-separation can be used as a route to the preparation of amorphous/nanocrystal composites by adjusting Al2O3 content in phosphosilcate glass system.