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G. Schmid, C. Eisenmenger-Sittner, J. Hell, M. Horkel, M. Keding, H. Mahr:
"Optimization of a container design for depositing uniform metal coatings on glass microspheres by magnetron sputtering";
Surface & Coatings Technology, 205 (2010), 205; S. 1929 - 1936.

Coating granular substrates by PVD methods like magnetron sputtering is a very challenging process. Although many of such substrates may also be coated by other means like the sol gel method, there are coating materials (e. g. refractory metals) for which PVD processes are the method of choice. One of these substrates is hollow glass microspheres with 2-80 μm diameter which can be used for hydrogen storage if a proper catalytic film is applied. To achieve a uniform film by magnetron sputtering on all the spheres a special apparatus was used which basically consists of rotating vessels positioned beneath the target. The arising problems of agglutination of the powdery substrate were solved by designing a special coating vessel, where the spheres are contained during deposition.
For testing the system first copper was used as a target material, which was then replaced by platinum since the glass microspheres are used for a catalytic application. The film thickness on the spheres was determined by optical absorption and matches well with the thickness calculated for the special vessel geometry. Additionally it is shown that the glass microspheres can be coated with a uniform layer by magnetron sputtering whereas coatings produced by a chemical deposition process are not continuous.

http://dx.doi.org/10.1016/j.surfcoat.2010.08.076