Low-Temperature Oxidation of Fine UO₂ Powders: A Process of Nanosized Domain Development

The nanostructure and phase evolution in low-temperature oxidized (40-250 °C), fine UO₂ powders (<200 nm) have been investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). The extent of oxidation was also measured via in situ thermogravimetric analysis. The oxidation of fine powders was found to proceed differently as compared to oxidation of coarse-grained UO₂. No discrete surface oxide layer was observed and no U₃O₈ was formed, despite the high degree of oxidation (up to O/U = 2.45). Instead, nanosized (5-15 nm) amorphous nuclei (interpreted as amorphous UO₃), unmodulated and modulated U₄O₉, and a continuous range of U₃O_7-z phases with varying tetragonal distortion (c/a > 1) were observed. Oxidation involves formation of higher uranium oxides in nanodomains near the grain surface which, initially, have a disordered defect structure ("disordered U₄O₉"). As oxidation progresses, domain growth increases and the long-period modulated structure of U₄O₉ develops ("ordered U₄O₉"). A similar mechanism is understood to happen also in U₃O_7-z.