Better than milk on breakfast cereals: New precision coating method for industrial granular material
As anyone who eats their cereal with milk in the morning knows: coating large volumes of granular material homogeneously is no mean feat. In a recent paper published in EPJ D, an Austrian team has developed a new method, based on physical vapour deposition, to upscale the quantity of coating without affecting the quality and homogeneity of the film. In this study, Andreas Eder from Vienna University of Technology and colleagues also developed a model capable of predicting the film thickness. This represents a major step forward for industrial materials, as previous approaches relied on optical measurement after the coating had been deposited. Because this coating system is capable of implementing a plasma close to the granular substrate, it opens the door to new surface treatment and modification possibilities.
Coating granular materials is a science in itself. A coating method based on plasma vapour deposition, called magnetron sputtering, previously only covered up to 20ml of granular material. Now, the authors have up-scaled the hardware and optimised the geometry for coating up to one litre of granular material, irrespective of particle shape or size. To do so, they use a special coating vessel designed to prevent the coating material clumping together. They also developed a semi-empiric model to predict the film thickness, which takes into account key factors such as the surface area exposed to the vapour beam or trickling behaviour, which have been approximated in the model. They found that their results were consistent with traditional thickness- measurement methods.
Applications are expected for the many granular materials used in industry, including, for example, a novel hydrogen-storage system, which stores hydrogen in hollow glass spheres. Hydrogen stored in microbeads can be released by applying heat to the spheres. The new method helps meet the challenge of applying heat to the beads, thanks to a chemical reaction triggered by a catalyst, which is applied to the sphere's surface.
A. Eder, G.H.S. Schmid, H. Mahr, and C. Eisenmenger-Sittner (2016), Aspects of Thin Film Deposition on Granulates by Physical Vapor Deposition, European Physical Journal D 70:207, DOI: 10.1140/epjd/e2016-70435-7