On the Role of the Electrical Field in Spark Plasma Sintering of UO2+x

Abstract

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.

Bulk Li mobility enhancement in Spark Plasma Sintered Li(7−3x)AlxLa3Zr2O12 garnet

“…In the two pristine samples S1 and S2, LaAlO 3 impurity was found in XRD patterns and confirmed by 27 Al MAS NMR (see below). Moreover we suspected the existence of a second impurity Li 1.82 ZrO 3…”

“…For 6 Li, additional data were collected at a higher magnetic field of 20 T (850WB) to obtain a better resolution at 10 kHz spinning frequency. 27 Al (I = 5/2) MAS NMR spectra were acquired using a single short pulse excitation (1 ms, tip angle of ~15°) to ensure quantitativeness (i.e., working in the non-selective linear regime at short pulse length for quadrupolar nuclei). For 6 Li (I = 1), due to the very long nuclear spinlattice relaxation time, a pre-saturation period followed by a recovery delay (200 s) was systematically employed before a single pulse excitation.…”

“…However, a clear narrowing in the peak widths was observed in SPS samples spectra which indicates an increase of Li mobility. 27 Al MAS NMR spectra are displayed in Fig. 7.…”

“…Importantly, concentrations in the garnet phase were deduced after removal of the LaAlO 3 parasitic phase contribution, estimated from 27 Al MAS NMR as explained in section 3.3. With SPS treatment, Al content has increased in the two pellets as suggested by XRD analysis.…”

“…Site occupancies in the prepared LLAZO were identified using 27 Al and 6 Li MAS NMR [1,[14][15][16]. NMR relaxation time measurements allowed getting Li dynamics parameters, namely the Li mean jump rate and activation energy [11,12].…”