Modified Sol−Gel Based Approaches for Synthesizing Borophosphosilicate Glasses and Glass-Ceramics

Abstract

Modified sol-gel based processes were used to synthesize borophosphosilicate glasses and glass-ceramics, utilizing boron oxide and phosphorus pentoxide as starting precursors. In these processes, the alkoxides of boron and phosphorus were synthesized in situ using oxide precursors, which were then subjected to controlled or rapid hydrolysis and condensation reactions to form either gels (modified oxide sol -gel process: MOSG) or precipitate powders (modified oxide sol-precipitation process: MOSP). The dried gels and precipitates obtained in both processes were heat-treated to 800 °C to crystallize the BPO4 phase. The crystallinity of the as-prepared and heat-treated powders was analyzed using X-ray diffraction, while Fourier transform infrared spectroscopy (FTIR) was employed to identify the molecular linkages of the various components present. Results of these studies show that B-O-B species form during heat treatment of the MOSG-derived gels because of the presence of unreacted B-OH groups, leading to microsegregation of these units in the gels. However, these units are not detected in the powders obtained using the MOSP process, thereby suggesting differences in the gel structures. These structural differences lead to different pathways for the formation of BPO4 in the two processes. Nevertheless, independent of the process, both samples, although structurally different, exhibit almost the same extent of crystallization of BPO 4 after heat treatment at 800 °C.