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C. Eisenmenger-Sittner, J. Hell, M. Keding, G. Schmid:
"Assessment of the requirements for PVD systems for coating granular materials";
Poster: 15th International Conference on Thin Films (ICTF 15), Kyoto; 08.11.2011 - 12.11.2011.

Introduction
The coating of granulates by PVD methods is an important issue if other processes like electrochemistry or sol gel coating reach their limits. The material choice for liquid based processes is limlited, e. g. if refractory metals or oxides have to be deposited. Also for the formation of complex multilayered structures multi-source PVD systems are an excellent option. Nonetheless, the line of sight nature of PVD processes makes the deposition of uniform films on a granulate difficult. Also the the need for a vacuum environment can have implications on the dynamics of the granulate. This work presents two case studies one being related to coating diamond granulates which are used for heat sink materials and the other concerning hollow glass microspheres which can be used as hydrogen storage.
Experimental Procedures
An insert for a vacuum chamber was designed which allows for the planetary motion of three cups which contain the granulate. They can be titled with arbitrary angles relative to the horizontal [1]. Above the insert multiple sputter sources can be mounted which allow for the deposition of multilayered or gradient cotings on the granular material. In the present study only metals were deposited. Coating runs were performed using either diamond granulate with average diameters from 200 - 400 µm or hollow glass microspheres with average diameters from 2 - 80 µm. The over-all aim was to optimize the coating uniformity on the single particles as well as in the particle ensemble as a whole. To evaluate the coating thicknesses on a statistically significant ensemble of particles optical methods were employed. Since the particles are transparent in both cases and the metallic coatings are thin enough to be transparent the film thickness can be estimated by the absorption of visible light which passes the coated particles.
Results and Discussion
Case study I: metallic coatings on diamond granulate: carbide forming metals (Mo, Ti, Nb) were deposited on diamond granulate. They are intended to match the phononic mechanism of thermal conductivity in diamond to the electronic one in copper within a Cu-diamond metal matrix composite. It was found that the dynamics of the diamond granulate was influenced by the pressure within the chamber. Once the chamber pressure was beneath the vapor pressure of water the trickling of the granulate within the rotating cups was nearly completely suppressed. Within a systematic study of the cup rotation speed, tilt angle and diverter blade geometry a set of parametes could be identified which allowed for sufficient granulate dynamics to uniformly coat the particles.
Case study II: catalytic coatings on hollow glass microspheres: hollow glass microspheres can be filled with hydrogen under pressure [2]. Hydrogen may be released by diffusion through the wall of the microsphere (thickness approx. 0.2 - 1 µm) triggered by an exothermic reaction. To decrease the activation barrier of the reaction the spheres can be coated with a catalytic material, e. g. Pt. A special cup geometry with smooth surfaces and a small diverter blade was designed to guarantee a good intermixing of the very fragile spheres without destroying them. Pt could be homogeneously deposited on the spheres and hydrogen could be generated by a chemical process using the coated spheres whereas there was no hydrogen generated with uncoated ones. In addition it was shown that the sputter coatings on the spheres were more homogenous than Pt coatings deposited by a liquid based electrochemical process.
Acknowledgement
This work was supported by the Austrian Science Fund (FWF), Grants P19379 and P22718.
References
[1] J. Hell, M. Horkel, E. Neubauer, C. Eisenmenger-Sittner, Vacuum 84 (2010) 453-457
[2] M. Keding, M. Tajmar, International Patent Application for a "Method and Installation for Storing and Releasing Hydrogen",
International Patent Number WO2008/019414A2, 2008.