Sintering of glass films on rigid substrates studied by optical techniques
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The densification and shear viscosity of borosilicate glass (BSG) + silica films on rigid substrates were studied. Optical measurement techniques were devised to determine the shrinkage profiles of the free and constrained films and in-plane stress in the constrained films. These films were prepared from slurries of the powder by tape casting. Sintering was carried out isothermally in a hot stage between 665°C and 775°C. The densification rates of both films were observed to be the same in the early stage of sintering but slowed down in the constrained film during the later stage resulting in a lower density. The activation energies for both free and constrained sintering were found to be 385 ± 10 kJ/mol. In-plane stress measurements in constrained films of the pure glass showed the stress to rapidly rise to a maximum level of 20 kPa during the initial stage of sintering and gradually decreased back to zero during the final stage. Densification rates can be given as a product of mobility and driving pressure. Activation energy determinations did not indicate a change in the densification mechanism such that a change in mobility can be ruled out as the reason for the reduced densification rate in constrained films. The stresses are substantially smaller than the estimated lower limit of the sintering pressure and had no observed effect on the densification of the constrained film during the early stages of sintering. However, the stresses could have prevented a few large pores from shrinking during the early stage of sintering leading to the lower density and larger pores observed in the constrained film after sintering. Shear viscosity determinations were also done using data obtained from the sintering of constrained films. The results showed that the density dependence of the shear viscosity is consistent with other work in sinter-forging experiments. However, the results also indicated that the shear viscosity is strongly dependent on sintering temperature. This can be attributed to the different microstructures that evolved when the films were sintered at different temperatures.
- Doctoral Dissertations