Devitrification inhibitor in binary borosilicate glass composite
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When an appropriate mixture of a low-softening borosilicate glass (BSG) and a high-softening high silica glass (HSG) 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 alumina 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 Al + 3 from alumina and alkali ions from BSG, which diverts the diffusion of alkali ions from HSG to alumina, thus forming a K + and Al +3 -rich reaction layer adjacent to the alumina particles far too rapidly compared to that of cristobalite formation.
I. INTRODUCTION In our previous investigation,1 the devitrification kinetics of a binary glass composite, containing a lowsoftening borosilicate glass (BSG) and a high-softening high silica glass (HSG), has been studied. XRD results show that the pure glasses do not crystallize under sintering conditions investigated, 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 TCE and volume change associated with its martensitic transformation from the tetragonal to the cubic phase at about 150-200 °C2 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. This paper presents an approach to prevent the growth of cristobalite in the binary glass composite of BSG and HSG by using a dopant that can act as a crystal growth inhibitor. II. STRATEGY TO PREVENT GROWTH OF CRISTOBALITE In order to prevent the growth of cristobalite, it is first necessary to review the salient features of the crystal growth kinetics of the binary mixture of BSG and HSG as described elsewhere1 and then to devise a strategy to alter the growth kinetics. Earlier work1 indicates that the precipitation of cristobalite occurs at 356 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 2, Feb 1993 Downloaded: 14 Mar 2015
heterogeneous nucleation sites in the glass composite, exhibits a characteristic incubation period that varies with temperature, and originates in HSG rather than in BSG. However, it is the transport of alkali ions (e.g., K + , Na + , and Li + ) from BSG to HSG that controls the crystal growth kinetics and serves as the rate-controlling step. Given that these reaction kinetics
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