The electric field has a large effect on the stoichiometry and grain growth of UO2+x during Spark Plasma Sintering. UO2+x is gradually reduced to UO2.00 as a function of sintering temperature and time. A gradient in the oxidation state within the pellets is observed in intermediate conditions. The shape of the gradient depends unequivocally on the direction of the electrical field. The positive surface of the pellet shows a higher oxidation state compared to the negative one. An area with larger grain size is found close to the positive electrode, but not in contact with it. We interpret these findings with the redistribution of defects under an electric field, which affect the stoichiometry of UO2+x and thus the cation diffusivity. The results bear implications for understanding the electric field assisted sintering of UO2 and non-stoichiometric oxides in general.

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Spark plasma sintering (SPS) is a rapidly developing method for densification of powders into compacts. It belongs to the so-called “field assisted sintering techniques” that enable rapid sintering at much lower temperatures than the classical approaches of pressureless sintering of green pellets or hot isostatic pressing. In this paper, we report the successful integration of a SPS device into a hermetic glovebox for the handling of highly radioactive material containing radioisotopes of U, Th, Pu, Np, and Am. The glovebox implantation has been facilitated by the replacement of the hydraulic system to apply pressure with a compact electromechanical unit. The facility has been successfully tested using UO2 powder. Pellets with 97% of the theoretical density were obtained at 1000 °C for 5 min, significantly lower than the ∼1600 °C for 5-10 h used in conventional pellet sintering.

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Flash spark plasma sintering (FSPS) is a newly developed technique for consolidation of powder materials into dense compacts. This method belongs to the group of electrical field-assisted sintering (FAST) techniques. It combines the voltage/current characteristics of flash sintering (FS), 1,2 usually performed in pressure-less dog-bone configuration, with the sintering cell design typical of spark plasma sintering (SPS). 3 Recent investigations in FSPS 4-8 use mostly a modification of the classical SPS to achieve FS conditions. This is usually done through the insertion of a nonconductive sleeve (eg, BN 9) between the powder (or pre-compacted/partially pre-sintered pellet) and the graphite die, or alternatively in a die-free configuration. Commercial instruments enabling FSPS processing with higher voltage characteristics (>10 V), using independent external heating of the die or precise time application of the electric field, are under development or in early application stage. In the presented paper, we describe the pressure-assisted FS compaction of powder in an SPS configuration using such a newly developed instrument. We have chosen ceria as a sintering probe not only for its