First-principles electronic band structure investigations of monoclinic, tetragonal, and cubic ZrO 2 reveal the highly anisotropic nature of the conduction and valence band topologies in the monoclinic phase with electron and hole effective masses differing by over an order of magnitude in perpendicular directions. The planes of relatively high implied electron and hole mobilities intersect along a single crystallographic direction, making this the only direction readily available for exciton motion. Conversely, in the tetragonal and cubic phases, charge carrier effective masses are more isotropic and exciton motion is less restricted. These findings may explain recent experimental observations suggesting that exciton production via gamma irradiation in zirconia crystallites immersed in water is responsible for the accelerated dissociation of adsorbed water molecules on crystallite surfaces, and for the specificity of the effect to the tetragonal zirconia phase.
Thehigh-temperature-waterwmosionpetioman-ofhafniumisevaluated. Corrosion kinetic data are used to develop correlations that are a function of time and temperature. The evaluation is based on oorrosion tests conducted in out-of-pile autoclaves and in out-of-flux locations of the Advanced Test Reactor (ATR) at temperatures ranging from 288°C to 360"C. Similar to the corrosion behavior of unalloyed zirconium, the high-temperature-water corrosion response of hafnium exhibits three corrosion regimes: pretransition, posttransition, and spalling. In the pretransition regime, cubic corrosion kinetics are exhibited, whereas in the posttransition regime, linear corrosion kinetics are exhibited. Because of the scatter in the spalling regime data, it is not reasonable to use a best fit of the data to describe spalling regime mrrosion. Data also show that neutron irradiation does not alter the mrrosion performance of hafnium. Finally, the data illustrate that the corrosion rate of hafnium is significantly less than that of Zkaloy-2 and Zircaloy-4.
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