The growth of epitaxial uranium oxide observed by micro-Raman spectroscopy
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0893-JJ05-07.1
The growth of epitaxial uranium oxide observed by micro-Raman spectroscopy Niña Caculitan1, Wigbert Siekhaus2 1 Summer student at the Seaborg Institute at the Dep. of Chemistry and Materials Sciences, 2 Dep. of Chemistry and Materials Sciences to whom inquiries should be addressed, Lawrence Livermore National Laboratory Livermore, CA 94550-9234 ABSTRACT Raman spectroscopy can be performed with micrometer resolution and can thus be used to determine the dependence of oxide thickness on the substrate’s grain structure or local impurity inclusions. The Raman signal amplitude emitted from an epitaxial uranium oxide layer as a function of oxide thickness has been modeled for light of 632.8 nm wavelength incident on the oxide and reflected from the uranium substrate using the optical properties determined by spectrophotometry. The model shows that the Raman signal increases with oxide thickness and saturates at about 150 nm thickness. The model was compared with the measured Raman signal amplitude of an epitaxial uranium oxide layer growing in air with a known time dependence of oxide growth.
INTRODUCTION Micro-Raman spectroscopy has been performed on a variety uranium oxide single crystals [1,2,3,4], and characteristic Raman lines have been identified for stoichiometric, sub- and super-stoichiometric oxides and even ion bombarded oxides. The stoichiometry and impurity composition of the oxide grown in a laboratory atmosphere on a uranium substrate is of interest, because uranium oxide protects to some extent the underlying uranium from corrosive attack by other gases. Of particular interest is the attack by hydrogen, which initiates preferentially at isolated sites [4]. The mechanism of local initiation is not completely understood. One hypothesis [5] suggests that hydrogen attack occurs at a spot where the protective oxide presents the lowest impedance to diffusion of hydrogen. That impedance to diffusion depends on the thickness of the oxide layer, but potentially also on the stoichiometry or the impurity content of the oxide layer. It is therefore of interest to analyze the surface oxide with a tool that can identify with µ-m spatial resolution both the thickness and the stoichiometry of the oxide where hydride initiation occurs. In this study we observe a growing uranium oxide layer as a function of time with micro-Raman spectroscopy to determine whether the oxide thickness can be determined from the time-dependent integral of the Raman signal over a wavelength range, and compare the results with a model that describes the expected signal strength. Changes in oxide moiety as a function of thickness are also observed.
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EXPERIMENTAL DETAILS A uranium sample with a total weight impurity content of approximately 150 ppm was mechanically polished, finishing with 1µm SiO2 particles and exposed to laboratory air, typically having 35% relative humidity. Raman spectra were collected as a function of time using a commercial micro-Raman spectrometer ( HORIBA Jovin Yvon, Edison, New Jersey, NJ 0
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