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Densification and microstructural evolution of TiO2 films prepared by sol-gel processing

: Bockmeyer, M.; Löbmann, P.


Chemistry of Materials 18 (2006), Nr.18, S.4478-4485
ISSN: 0897-4756
ISSN: 1520-5002
Fraunhofer ISC ()

Soluble precursor powders for TiO2-coating solutions were prepared and dissolved in solvent mixtures containing ethanol and 1,5-pentanediol in different ratios. Materials isolated by rotational evaporation of the sols (sol powders) and scraping of films deposited by dip-coating (film powders) were annealed at different temperatures and thoroughly characterized in order to investigate the influence of solvent composition and drying conditions on the film densification. Organic groups present in the precursor are partially exchanged during solvent evaporation. The chemical composition of these materials has a significant influence on the respective densification behavior and properties such as surface area and porosity. This observation can be correlated to the evolution of the crystalline polymorphs of TiO2, anatase, and rutile. The large area-to-volume ratio and the presence of air moisture during film drying cause significant chemical and microstructural differences between film powders and sol powders prepared by rotational evaporation. The rapid solvent evaporation in films leads to a fast densification and the inclusion of organic groups, which cause the transient presence of CO2 entrapped in closed pores upon sintering. The conclusions drawn from the characterization of the different powders are the basis to understanding and avoiding the formation of defects on a microscopic level.