Oriented Dioxide Films on Uranium

Abstract

The growth habit of UO~ on uranium during the oxidation by water vapor has been analyzed with the aid of detailed x-ray diffraction work and pole figures. The dioxide grows with a (110) planar texture which bears no epitaxial relation to the underlying metal crystallites. Although the polycrystalline alpha uranium has a strong and anisotropic preferred orientation as a result of fabrication, the oxide forms without azimuthal directionality in the plane of contact. The lack of alignment in the plane of contact also was confirmed in an experiment with a single crystal of uranium.The texture of UO~ formed during annealing in vacuum also was found to be planar without significant directionality. In such cases, the (100) planes were parallel to the surface of the metal substrate, and large amounts of uranium monoxide always were present in such films. Subsequent oxidation of specimens covered with the (100) texture yielded the characteristic (110) dioxide texture.In incidental experimental work on the vapor deposition of UO.., the octahedral or (111) texture was observed on glass and tantalum substrates, and the cubic or (100) texture was developed on several ionic substrates.In a preliminary investigation, the rate law for the formation of uranium dioxide under conditions which produce such oriented films was found to be logarithmic.

Electron Diffraction Study of the Removal and Formation in Vacuo of Oxide Films on Uranium and Plutonium

“…This observation is consistent with the preferred orientation observed for uranium oxidized at high temperature by low-pressure water vapor (13). X-ray diffraction studies of uranium following its exposure to moist hydrogen or water vapor have led to the conclusion that the (220) UO~ planes form parallel to the metal surface with little or no relation to the orientation of the underlying metal.…”

“…The x-ray diffraction pattern of 1802 corresponded to s-uranium plus uranium monoxide plus a small amount of strongly oriented uranium dioxide [the (200) line was abnormally strong]. Orientation and pole figure studies reported elsewhere (13) indicate that this cubic orientation texture having the (100) planes of the dioxide lattice parallel to the metal surface is obtained on vacuum heat-treated samples.…”

The crystallographic structure of the air-grown oxide on depleted uranium metal

“…In particular the crystallographic orientation of the oxide formation is apparently not influenced by the orientation of the underlying metal grains upon which they form. Instead, the oxide grows with a preferred orientation towards the [110] direction relative to the exposed metal surface, in agreement with the x-ray diffraction observations of Waber et al [22]. This was more apparent on the high carbon sample with a preference to orient along the (110) plane 7 times more often than that of a random direction (compared to 3.6 for the low carbon analysis) ( Figures 15 and 12 respectively).…”

“…As expected, the annealing process enlarged the average grain size without changing the overall preferred orientation previously found from x-ray diffraction measurements by Waber et al [22].…”

“…However, an early X-ray diffraction (XRD) study by Waber et al [22] reported that uranium dioxide formed by the reaction of uranium with water vapour occurs with a distinctive growth habit. The (110) planes of the dioxide were reported to lie parallel to the surface of the uranium substrate.…”