Effects of Gadolinium Doping on Electrical Properties of U02 Grain Boundaries

Abstract

Complex impedance measurements were made on Gd-doped U02 to study Gd doping effects on electrical properties of the matrix and grain boundary. The Gd contents ranged from 0.5 to 10 wt%; temperature from 295 to 1.273 K; and frequency from 5 to 4 X 10 7 Hz.The matrix conductivity increased almost linearly with Gd content. indicating that the number density of electron holes also increased almost linearly with increasing Gd. The grain boundary capacitance of the Gd-doped uo. was larger than that of U02 by about 3 orders of magnitude. The grain boundary conductivity of the Gd-doped U02 was also larger than that of UO •. but decreased rapidly with increasing Gd content, between 0.5 to 10 wtq6. The migration energy of electron holes across a grain boundary was larger than that in the matrix and seemed to increase with Gd content.It was presumed from these results that Gd ions segregated to the grain boundaries to form a potential barrier for the migration of electron holes. The barrier thickness was estimated to be a few nanometers from the grain boundary capacitance. The energy barrier estimated from the migration energy of electron holes seemed to increase with Gd content.

Defect segregation facilitates oxygen transport at fluorite UO 2 grain boundaries

“…Our results show that the M 3+ cations present at the boundary strongly attract oxygen vacancies (figures 6-8), pulling them to the boundary where they will remain trapped and facilitate high levels of oxygen transport along the boundary and thereby increase the rate of corrosion. Kubo et al have seen a similar effect in UO 2 whereby at Gd 3+ concentrations at or exceeding 10 wt%, a potential barrier for the migration of electron holes formed [50]. This suggests that this behaviour (i.e.…”

“…Xe can undergo further chain decay leading to other stable fission products, e.g. La/Gd 3+ has been shown to segregate to the grain boundaries, resulting in an increase in oxygen conductivity [50]. Recognizing the potential significance of this, in this work we investigate the combination of chemical impurities (fission products and fabrication impurities) within extended structural defects (grain boundaries) and the effect that this combination has on the oxygen transport properties of UO 2 .…”