Following a suggestion by Kingery and Francois, the theory of grain growth in porous compacts controlled by the migration of pores with the boundaries is developed for all possible transport processes. Contrary to the assumption of these authors, it is shown that the cubic rate law reported for both UO2 and Al2O3 compacts, if it is due to pore migration, must be interpreted as being controlled by vapor transport through the pore with the pressure maintained at 2γ/r, where γ is the surface tension and r the pore radius. Available data for UO2 and Al2O3 are analyzed according to this model and found to give satisfactory agreement, with the calculated activation energies being reasonably close to the heats of vaporization for the two oxides. An interesting prediction of the model is that the compact should in fact show a decrease in density during grain growth. This behavior has been observed in the case of UO2 but apparently has not yet been reported for Al2O3.
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