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Reactive hot pressing of boehmite Bradbeer, Ross Stanley

Abstract

The compaction behaviour of boehmite has been studied under isothermal conditions, with special emphasis devoted to hot pressing in the temperature range 300 to 600°C. The present work indicates that it is possible to produce a hard dense compact under certain conditions. However, the behaviour of the material during reactive hot pressing appears to be more complicated than can be explained by simple sintering or kinetic theories. To aid in understanding the mechanisms of compaction during a phase transformation, the behaviour of the system during reactive hot pressing was studied from a purely phenomenological point of view, a viscoelastic model. By using viscoelastic theory it is possible to relate ideal elastic and energy-absorbing or damping viscous parameters to the behaviour of boehmite during R.H.P. While the apparent compact density varied as a complex function of temperature, it was found that the overall compaction behaviour of boehmite could be adequately described by a second order linear differential equation, which in turn could be related to a combination of elastic (displacement sensitive) and viscous (strain rate sensitive) components. The viscous nature of the powder during R.H.P. reached a maximum value just before the boehmite to gamma transition (380 to 443°C), suggesting that strong particle interaction was occurring. It is anticipated that R.H.P. from 380 to 443°C will lead to the most favourable particle rearrangement for producing a hard gamma phase at 500°C. In producing the "hard phase" it was important to maintain a critical water concentration. Approximately 4% retained water was necessary for forming a hard dense compact. On the other hand in the presence of an excessive water vapor pressure, the unreacted boehmite powder appeared to transform directly to alpha alumina, resulting in a friable compact. Thus the need for maintaining the correct vapor pressure during R.H.P. is essential. Production of the "hard phase" material at 500°C appears promising as an intermediate step in producing strong translucent bodies upon subsequent sintering at 1000°C.

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