Densification kinetics and modeling of glass-filled alumina composite
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Tapan K. Gupta Alcoa, Alcoa Center, Pennsylvania 15069 (Received 27 July 1992; accepted 12 November 1993)
Kinetics and mechanism of sintering in a model borosilicate glass (BSG) + alumina composite system have been investigated isothermally at 600 to 1000 °C. A faster and greater densification is observed with higher sintering temperature, greater BSG content, and larger alumina particle size. Using measurable densification kinetics, it is found that the activation energy of densification (Q) increases continuously with increasing BSG content, from predominant 90 vol. %. The rate-limiting step during densification is identified to be diffusion of alkali ions in BSG when BSG < 40 vol. % (Q = 110 kJ/mol), diffusion of both alkali and aluminum ions when BSG = 40-60 vol.% (Q = 110-170 kJ/mol), diffusion of aluminum ion in BSG when BSG = 60-80 vol. % (Q = 170 kJ/mol), and viscous flow of BSG when BSG > 90 vol. % (Q = 245 kJ/mol). These observations are attributed to a chemical reaction taking place at the interface of alumina/BSG, resulting in 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, when the BSG content in the mixture is low, the resultant loss of alkali ions from BSG causes a rise in viscosity of BSG, thus slowing down the densification kinetics and changing the densification mechanism. As the amount of BSG is increased to >90 vol. % at the expense of alumina, the reaction as described above becomes less dominant and the sintering proceeds by viscous flow of glass.
I. INTRODUCTION 1
In our previous investigation on the reaction kinetics between borosilicate glass (BSG) and sapphire, we noted that there existed a strong coupling between aluminum ion from alumina and alkali ions from BSG. To briefly review those results, we refer to Fig. 1, which was described elsewhere.1 We found that when BSG and sapphire were brought together as a diffusion couple and heated at temperatures from 800 to 950 °C, a reaction layer was immediately formed adjacent to the alumina surface without any sign of an incubation period. The composition of the reaction layer is shown to be rich in Al and alkali ions and poor in Si [Figs. l(b) and l(c)], and appears amorphous to XRD. Although the kinetics of this reaction was found to be controlled by the diffusion of Al ion from the sapphire,1 the strong coupling between Al +3 from sapphire and alkali ions from BSG was concluded to occur from a need to balance the electrical charge caused by the substitution of a tetravalent Si by a trivalent Al in the glass network former. To maintain electroneutrality at the local level, the monovalent alkali ions were therefore forced to always J. Mater. Res., Vol. 9, No. 3, Mar 1994
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segregate at the alumina interface as determined from elemental mapping of the microstructure, thus demonstrating a strong affin
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