Effect of gallium oxide in preventing cristobalite formation in binary borosilicate glass composite
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Tapan K. Gupta Alcoa, Alcoa Center, Pennsylvania 15069 (Received 30 September 1992; accepted 23 April 1993)
When an appropriate mixture of low-softening borosilicate (BSG) and high-softening high silica (HSG) glasses is sintered at temperatures ranging from 800 to 1000 °C, a crystalline phase, identified as cristobalite by XRD, is known to precipitate out of the initial amorphous binary mixture of glasses as the sintering continues. The precipitation of cristobalite is found to originate in HSG and is controlled by the transport of alkali ions (e.g., K, Na, and Li) from BSG to HSG. 1 In this paper we report that when a small amount of gallium oxide is present as a dopant in the above binary mixture of BSG and HSG, the cristobalite formation is completely prevented at the sintering temperatures investigated. The above result is attributed to a strong affinity between Ga +3 from gallium oxide particle and alkali ions from BSG, which diverts the diffusion of alkali ions from HSG to gallium oxide, thus forming a K + and Ga +3 -rich reaction layer adjacent to gallium oxide particles far too rapidly compared with that of cristobalite formation.
I. INTRODUCTION 1
In our previous investigation, the devitrification kinetics of a binary glass composite, containing lowsoftening borosilicate (BSG) and high-softening high silica (HSG) glasses, has been studied. XRD results show that the pure glasses do not crystallize under sintering conditions studied, but when mixed in appropriate proportions the cristobalite gradually precipitates out of the initial amorphous binary mixture as the sintering continues at temperatures ranging from 800 to 1000 °C. The precipitation of cristobalite in the initially amorphous glass mixture is discouraging because its large thermal expansion coefficient and volume change associated with its martensitic transformation from the cubic to the tetragonal phases at about 150-200 °C3 dramatically reduces the thermal shock resistance and mechanical strength of the glass composite. Therefore, it is imperative to prevent the cristobalite from forming if any practical applications are envisioned for this binary glass composite. In the second paper of this series,2 we devised and successfully demonstrated a strategy to prevent the cristobalite formation in the above binary glass mixture by altering its crystal growth kinetics. The strategy lies in diverting the transport of alkali ions from the desired target to a more attractive but harmless target and to do so in a time frame shorter than the characteristic incubation period for cristobalite formation. In other words, the new target must provide a greater affinity for, and a faster reaction kinetics with, alkali ions than the old target. By J. Mater. Res., Vol. 8, No. 9, Sep 1993
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adding a small amount of alumina, 1-10 vol. %, into the binary glass mixture, the cristobalite formation is completely inhibited at the same temperature range. The above result is attributed to the strong coupling between
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