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Nanoporous Ceria Films Prepared from Colloidal Suspension Vladimir Petrovsky, Brian Gorman, Harlan U. Anderson, and Tatiana Petrovsky EMARC, University Missouri-Rolla, MO 65401. ABSTRACT The influence of porosity and densification on optical properties of films on sapphire substrates that were prepared from water colloidal suspensions of small (~5nm) particles of ceria was investigated. The colloidal ceria films have initially very porous structure (porosity about 50%) and densification starts at about 600oC accompanied by grain growth. The concurrence of these two processes makes it difficult to interpret the results of the optical spectrophotometry, but the combination of transmittance and reflectance measurements provides enough data to separate these two influences and to calculate the porosity, particle size and energy band gap separately. XRD, SEM, ellipsometry and mechanical profilometry were used to confirm the results obtained from the specrophotometric measurements. All these methods gave results, which are in good agreement: the change in the porosity from 50% to 15% and the particle size increased from 5 to 65nm in the temperature region from 400 to 1000oC. An important result of the investigation is the fact that the main optical properties of the coating such as refractive index and band gap energy depend only on the porosity, but not on the grain size. The grain size influences the scattering properties of the coating, which allows the grain size to be estimated from optical measurements. INTRODUCTION It was shown in our previous publications that optical spectroscopy is an effective way to determine many of the parameters of nanocrystalline coatings [1,2]. For example optical spectroscopy provides information about the thickness, porosity and grain size of the coating. However, in order to extract such information, optical models of the structure must be used and it is important to confirm the correctness of the interpretation by other independent methods. It is the intent of this study to compare the optical results to those obtained by SEM, XRD and mechanical profilometry. Colloidal suspensions were used for the film preparation since colloidal suspensions are one of the promising precursors for the film preparation in the thickness region of 0.5 to 5µm. Porous structure of the films in combination with relatively low sintering temperature makes these precursors of interest for different applications including fuel cells, gas separation membranes, catalytic media’s, planarizing interlayers, etc [1]. Cerium oxide has high refractive index (n~2.4) and can be used for dielectric waveguides and other optical applications [2,3]. At the same time it is a well-known ionic conductor [4,5] and is of interest for different ionic devices. Different techniques have been used for the ceria film preparation [4,6,7,8] and it has been found that optical properties of the coatings depend strongly on the technique used and deposition conditions. In the case of the coatings used in this study, the porosity varied fro