Flash sintering of stoichiometric and hyper-stoichiometric urania

Abstract

Flash sintering (FS), a novel fabrication technique belonging to the family of field assisted sintering (FAS) techniques, has been utilized in this study to fabricate uranium dioxide (UO2) pellets. Stoichiometric (UO2.00) and hyper-stoichiometric (UO2.16) pellets were flash sintered at 600 °C within a few (2–3) minutes. This is in sharp contrast to conventional sintering where temperatures hundreds of degrees higher are necessary and the sintering time extends to hours. Relating this in terms of the homologous temperature ratio (TH) for both conditions shows that in the case of flash sintering at 600 °C, TH = 0.3 versus TH = 0.6 for conventional sintering at 1600 °C. The highest density achieved for a UO2.00 pellet was 81% theoretical density (TD) when flash sintered at 600 °C for 185 s at a field of 188 V/cm and a current density of 442 mA/mm2. For the UO2.16 pellet, the highest achieved density was 91% TD when flash sintered at 600 °C for 123 s at a field of 188 V/cm and a current density of 632 mA/mm2. X-ray diffraction (XRD) characterization of the sintered pellets showed the final sintered material to be single cubic fluorite phase. Scanning electron microscopy (SEM) of longitudinal sections revealed non-uniform microstructures with regions of high density where the grain size ranged from 1 to 15 μm. Comparisons between conventionally and flash sintered pellets that achieved equivalent shrinkage strains were also conducted. In all cases, the flash sintered pellets achieved similar densification to the conventionally sintered pellets at much lower furnace temperatures and shorter times.