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L. Armelao, C. Eisenmenger-Sittner, S. Gross, C. Sada, U. Schubert, E. Tondello:
"Zirconium and hafnium oxoclusters as molecular building block for highly dispersed ZrO₂ or HfO₂ nanoparticles in silica matrix";
Poster: Euro Nano Forum 2003, Triest/Italien; 09.12.2003 - 12.12.2003.

Oxide nanoparticles covalentely embedded in dielectric matrices represent a class of nanocomposite systems endowed with outstanding optical, mechanical, electronic and thermal properties. To ensure the desired performances of the final material, a uniform distribution of the doping clusters inside the host matrix and the absence of grain agglomeration are necessary requirements. To this aim, a novel synthetic route to synthesize ZrO2 or HfO2 doped-SiO2 films was developed. It is based on the copolymerisation of a organically modified oxozirconium (Zr4O2(OMc)12) or oxohafnium cluster (Hf4O2(OMc)12 with OMc = methacrylate) with (methacryloxymethyl)triethoxysilane (MAMTES). The crystalline clusters, which are precursor for the corresponding metal oxides (MO2) were prepared via the sol-gel route by reaction of zirconium or hafnium butoxide with methacrylic acid. This bidentate ligand lowers the reactivity of the alkoxide at the same time allowing the introduction of the methacrylate functional groups. The copolymerization of the cluster with the methacrylate-functionalized siloxane allows the anchoring of the cluster to the forming silica network. This route represents a valuable strategy to yield a very homogeneous dispersion of the MO2 (M= Zr, Hf) precursors inside the silica matrix. The polymerization kinetics of MAMTES as well as the hydrolysis and condensation of the ethoxy groups were investigated through IR, 1H and 13C NMR. To the optimised solution, Irgacure 184 (1% w/w with respect to the silane) was added as photoinitator. The solution was then cast into films by dip-coating and UV cured (5?, 100 W) to promote the copolymerisation of the methacrylate groups of the cluster with those of the silane. Transparent and homogeneous films about 200 nm thick were obtained after annealing at 800°C in air. The surface and in-depth composition of the thin films were investigated through IR, X-Ray Photoelectron Spectroscopy (XPS) and Secondary Ionisation Mass Spectrometry (SIMS). IR spectra show the formation of Si-O-Si groups already in the not calcinated samples and a strong decrease of the intensity of the C=C band (evaluated as C=C/C=O). The SIMS depth profiles evidence a very homogenous distribution of both the zirconium or hafnium oxides within the silica films. In particular, the oxides are well distributed within the whole thickness of the films and a sharp interface could be detected. The surface morphology of the coatings was investigated through Atomic Force Microscopy (AFM), which showed smooth, homogeneous and crack-free samples. The adhesion of the films on the substrate was evaluated by pull-off tests which showed a adhesiveness of about 650 N/cm2. This value can be viewed as a lower bound of the adhesiveness since in most cases the bonding tape (SCOTCH-WELDTM brand bonding film 588) used to contact the test-stud to the coating failed. A direct bonding of the test stud to a glass substrate yielded similar values, so the adhesion of the coating to the substrate can be considered as excellent.