Effect of alumina on densification of binary borosilicate glass composite
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The effect of alumina on densification of the binary borosilicate glass composite, containing low-softening borosilicate glass (BSG) and high-softening high silica (HSG) glass, has been investigated. It is found that with a small amount of alumina, 2-10 vol. %, present as a dopant in the binary glass mixture of BSG and HSG, both densification and densification rate are significantly reduced, but the activation energy of densification at a given densification is dramatically increased. However, no significant change in densification behavior with increasing alumina content from 2 to 10 vol. % is observed. These results are attributed to a chemical reaction taking place at the interface of alumina/BSG, forming a reaction layer adjacent to alumina. Since the composition of the reaction layer is known to be rich in aluminum and alkali ions and poor in silicon, the alkali ions content in BSG is continuously decreased during sintering. Accordingly, the resultant loss of alkali ions from BSG causes a rise in viscosity of BSG, thus slowing down the densification kinetics and increasing the activation energy of densification.
I. INTRODUCTION In the first paper of this series,1 a crystalline phase of cristobalite was found to precipitate continuously out of the initial amorphous binary glass mixture, containing low-softening borosilicate glass (BSG) and high-softening high silica (HSG) glass, at temperatures ranging from 800 to 1200 °C. The precipitation of cristobalite exhibits a characteristic incubation period that decreases with increasing temperature, and originates in HSG rather than in BSG. However, it is the transport of alkali ions (e.g., K, Na, and Li) from the BSG to the HSG that controls the crystal growth kinetics and acts as the ratecontrolling step. In the second paper of this series,2 the densification kinetics and mechanism of the above binary glass composite have been studied. It is found that the densification is nearly complete prior to the onset of cristobalite formation, indicating that the crystallization does not play a significant role in densification. Furthermore, the densification is found to be achieved primarily through a combination of rearrangement and solutionprecipitation kinetics, similar to those observed in the reactive liquid-phase sintering.3-4 From activation analysis, the rate-controlling kinetics during densification is identified to be viscous flow of pure BSG when the sintered density is below a critical value and when above viscous flow of silica-rich (reacted) BSG. In addition, the critical value of densification increases with increasing
^Present address: Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, Republic of China.
1990
http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 8, Aug 1994
Downloaded: 24 Mar 2015
BSG content. These results are attributed to a dissolution process taking place at the interface of HSG/BSG sintering. Since HSG (95 wt. % silica) has a higher silica content than that of BSG (75 wt. % SiO 2 )
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