Densification mechanisms of UO2 consolidated by spark plasma sintering

Despite the growing interest in the spark plasma sintering (SPS) of uranium dioxide, its sintering mechanisms have yet to be studied in great detail. Herein we propose a direct method to calculate the apparent activation energy for densification, Q(act), and the stress exponent, n, for SPS of nearly stoichiometric UO2. A set of experiments performed at different heating rates (CHR) and different pressures levels allowed us to calculate Q(act) and n, respectively, though we were limited to a theoretical density between 50% to 75 %. The master sintering curve was employed as a complementary method to compare Q(act). The average values were Q(act) =96 kJ/mol (CHR), Q(act) = 100 kJ/mol (MSC) and n = 1.4. We have therefore proposed grain boundary diffusion coupled with grain boundary sliding as the densification mechanism. The activation energy in SPS tends to be lower compared with that in other processes like conventional sintering (250-450 kJ/mol), creep (350-550 kJ/mol) and hot pressing (222 kJ/mol and 480 kJ/mol). This decrease could be due to the effect of the electric field combined with the higher heating rates, typical of SPS.

Automated summary

A direct method for calculating the apparent activation energy and stress exponent in the sintering of UO2 was proposed, with grain boundary diffusion coupled with grain boundary sliding as the densification mechanism. The activation energy in SPS was found to be lower compared to other processes, possibly due to the effect of the electric field combined with higher heating rates.