  Here, we report a study on the radiation resistance enhancement of Gd2Zr2O7
nanograin ceramics, in which amorphization, cell volume expansion and
multi-stage helium (He) bubble formation are investigated and discussed.
Gd2Zr2O7 ceramics with a series of grain sizes (55-221 nm) were synthesized and
irradiated by 190 keV He ion beam up to a fluence of 5x10^17 ions/cm2. Both the
degree of post irradiation cell volume expansion and the amorphization fraction
appear to be size dependent. As the average grain size evolves from 55 to 221
nm, the degree of post irradiation cell volume expansion increases from 0.56 to
1.02 %, and the amorphization fraction increases from 6.8 to 11.1 %.
Additionally, the threshold He concentrations (at. %) of bubbles at different
formation stages and locations, including (1) bubbles at grain boundary, (2)
bubble-chains and (3) ribbon-like bubbles within the grain, are all found to be
much higher in the nanograin ceramic (55 nm) compared with that of the
submicron sample (221 nm). We conclude that grain boundary plays a critical
role in minimizing the structural defects, and inhibiting the multi-stage He
bubble formation process.
