Gels and Gel-Derived Glasses in the Na 2 O-B 2 O 3 -SiO 2 System
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GELS AND GEL-DERIVED GLASSES IN THE Na20-B 2 0 3 -Si0
SHYAMA P. MUKHERJEE Battelle's Columbus Laboratories,
321
2
505 King Avenue,
SYSTEM
Columbus,
Ohio
43201
ABSTRACT The containerless melting of high-purity multicomponent homogeneous gels and gel-monoliths offers a unique approach to making ultrapure multicomponent optical glasses in the Procedures for prereduced gravity environment of space. paring and characterizing gels and gel-derived glasses in Preparation is the Na 20-B 0 -SiO 2 system are described. based on tLe polymerization reactions of alkoxysilane with trimethyl borate or boric acid and a suitable sodium compound. The chemistry of the gelling process is discussed in terms of process parameters and the gel compositions. The physicochemical nature of gels prepared by three different procedures were found to be significantly different. Infrared absorption spectra indicate finite differences in The meltthe molecular structures of the different gels. ing of the gel powders and the transformation of porous gel-monoliths to transparent "glass" without melting are described.
INTRODUCTION The possibility of containerless melting of glass in the reduced gravity environment of space will open a unique approach to preparing ultrapure glass Consequently, the optical or melts free from contamination from containers, and electrical properties of the melt which are strongly influenced by trace However, because of the absence of impurities can be of superior quality. gravity-induced convection currents, homogenizing glass during melting of Homoconventional raw materials will be difficult in the space environment. geneous, glass-like, gels and gel-monoliths prepared by the polymerization reaction of alkoxysilane with other metal alkoxides and/or with suitable metal 1 1 salts[ -7 can be used as the batch materials for the containerless melting This approach could of glasses in the reduced gravity environment in space. lead to a process for the preparation of ultrapure optical glasses. In the present work, three procedures for the preparation of gels in the The physicomulticomponent Na 2 0-B 2 0 3 -SiO 2 system are investigated. chemical nature of gels and gel-derived glasses has been characterized. Attempts are made to understand the physicochemical properties of the gels in The transformation of relation to the chemistry of the gelling process. different gels and gel-monoliths to glasses by melting and by densifying monolithic gels are investigated. EXPERIMENTAL WORK AND RESULTS Compositions The starting batch composition of the glasses investigated are given in Table I. The glass transition temperatures of the same compositions prepared by the melting conventional batches are 520 C and 600 C for Composition 1 and
322 Composition 2, respectively. Composition 1 having high Na 2 0 content lies away from the metastable liquid-liquid immiscibility zone. Composition 2 lies inside the metastable liquid-liquid immiscibility zone and was chosen for investigating the liquid-liquid phase-separation behavior 8 of gel-
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