Densification mechanisms of UO2 consolidated by spark plasma sintering

Abstract Despite the growing interest in the spark plasma sintering (SPS) of uranium dioxide, its sintering mechanisms have not been studied extensively so far. Here we propose a direct method to calculate the apparent activation energy for densification, Qact, and the stress exponent, n, for SPS of nearly stoichiometric UO2. A set of experiments performed at different heating rates (CHR) and at different pressures allows the calculation of Qact and n, respectively, but limited to the range of 50–75 % of theoretical density. The master sintering curve is employed as complementary method to compare Qact. The average values are Qact =96 kJ/mol (CHR), Qact = 100 kJ/mol (MSC) and n = 1.4. Grain boundary diffusion coupled with grain boundary sliding is the proposed densification mechanism. The activation energy in SPS results lower compared to the one of the 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 explained by the effect of the electric field combined with the higher heating rates, typical of SPS.