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M.T. Smolle, R. Bachofner, H. Mahr, G. Schmid, C. Eisenmenger-Sittner:
"Determination of the thickness uniformity on coated hollow glass microspheres";
Vortrag: 15th Joint Vacuum Conference (JVC15), Hotel Kaiserwasser, Wien; 15.06.2014 - 20.06.2014.

Hollow glass microspheres can be used for the safe storage of hydrogen [1] and other gases. The stored gas can be released if the pressurized microspheres (diameter approx. 50µm, wall strength approx. 1 µm) are subjected to elevated temperatures. Heat may be supplied by external heating or by exothermic chemical reactions. In the latter case a catalyst is vital to trigger the reaction. The catalytic material can be applied to the surface of the microspheres by a special magnetron sputtering process [2], which allows for the deposition of relatively uniform coatings on granulates with diameters ranging from 20 µm to 500 µm. Nonetheless, a quantitative assessment of the coating thickness on such granulates is not trivial.
In this work a method to determine the film thickness of transparent coatings on a transparent granulate is used to determine the thickness and the thickness distribution of thin metallic coatings (< 100 nm) on the hollow glass microspheres. The method can directly be applied to the catalytic coating on the glass microspheres (Pt with a thickness of around 10 nm, which renders the film transparent). Nonetheless also thickness distributions for other coating materials (Cu, Al) were determined. An image acquisition system was implemented into an optical microscope. By using a high power LED array the coated microspheres can be easily penetrated by light in the visible range. The emission spectrum of the LED source as well as the sensitivity of the CCD chip of the digital camera was quantitatively analyzed to allow for a reliable thickness determination.
Using a custom designed image processing software full maps of the thickness of the metallic coatings on the microspheres as well as thickness histograms cold is determined. The results of this work yield valuable input to the optimization of the coating equipment in respect to maximum thickness uniformity.
The financial support of the Austrian Science Fund (FWF) under Grant Nr. P-22718 and TRP-6 is gratefully acknowledged.
References
[1] M. Keding, G. H. S. Schmid, M. Tajmar, Proc. Hydrogen & Fuel Cells Conf. 2009, Vancouver, Canada
[2] G. H. S. Schmid, C. Eisenmenger-Sittner, Surface & Coatings Technology 236 (2013) 353.