Large scale production of coated fragile granulates

Materials prepared as granulates, powders, fibers or other particles with small geometrical extensions play manifold roles in science, engineering and technology. Examples are basic studies of molecular adsorption, the employment of colloidal matter in catalysis or powder compaction in hot pressing and sintering processes. Also the possibility to include granulates or fibers into matrices as fillers or reinforcements is an important issue in the manufacturing of composite materials. Often the small particles are fragile due to their size, their composition or their geometry which makes their manipulation and modification an involved task. The surface of the particles is of significant importance. It may have to be optimized in respect to surface energy and wettability to allow for good mixing and blending properties with solvents or carrier materials. Catalysts make use of the high surface to volume ratio of granulates and colloidal matter. Therefore surface modification often is decisive for activating several diverse classes of physio-chemical processes. Physical Vapor Deposition (PVD) methods allow for the deposition of an extremely wide range of materials as single layers or complex multilayer systems. Unfortunately the line of sight characteristic of PVD processes makes it hard to coat particles if uniform exposure to the vapor beam is not guaranteed. It is the aim of this project to design and optimize a container for fragile granular materials which shall be coated by means of PVD techniques. Metallic, oxidic and nitridic coatings in the thickness range from 1 - 100 nm shall be deposited onto large quantities of hollow glass microspheres with diameters of 5 - 70 m and a wall thickness of 1 m. The core feature is that the container design guarantees optimal exposure of the particles to the vapor beam while ensuring that the fragile objects are not destroyed by the movement necessary for intermixing. The goal of the project is to coat 1 litre of granulate within one deposition run. The coated glass microspheres may have applications in gas storage, catalytic processes and anti- counterfeiting techniques. Common to all these topics is that there have to be sufficient amounts of coated microspheres to produce e. g. gas storage containers of sufficient size or to implement infrared reflective microspheres into mass products for anti counterfeiting purposes. Within the project this shall be established by the proposed high throughput PVD process.

Materials prepared as granulates, powders, fibres or other particles with small geometrical extensions play manifold roles in science, engineering and technology. Examples are basic studies of molecular adsorption, the employment of colloidal matter in catalysis or powder compaction in hot pressing and sintering processes. Often the small particles are fragile due to their size, their composition or their geometry which makes their manipulation and modification an involved task. The surface of the particles may have to be optimized in respect to surface energy and wettability to allow for good mixing and blending properties with solvents or carrier materials. Catalysts make use of the high surface to volume ratio of granulates and colloidal matter. Therefore surface modification is decisive for activating several diverse classes of physio-chemical processes. Physical Vapor Deposition (PVD) methods allow for the deposition of a wide range of materials as single layers or complex multilayer systems. Unfortunately the line of sight characteristic of PVD processes makes it hard to coat particles if uniform exposure to the vapor beam is not guaranteed. Within this project to a container for fragile granular materials which shall be coated by means of PVD techniques was designed and optimized. Metallic, oxidic and nitridic coatings in the thickness range from 1 100 nm can be deposited onto large quantities of hollow glass microspheres and other particles with diameters of 10 100 m. The hollow glass microspheres with a wall thickness of 1 m are extremely fragile, but can be coated without any noticeable destruction. The core feature of the container design guarantees optimal exposure of the particles to the vapor beam while ensuring that fragile objects are not destroyed by the movement necessary for intermixing. The goal of the project, which was to coat 1 litre of granulate within one deposition run, could easily be achieved by the present set-up and even was exceeded in special cases. In synopsis, within the project a high throughput PVD process could be established to coat significant amounts of arbitrarily shaped granular materials with diameters ranging from 10 to 100 m. Apart from handling granulate volumes of around, and sometimes even more than, one litre, also several methods could be developed to characterize the coating thickness and thickness distribution on single grains and on large ensembles of particles. These comprise methods based on gravimetry, optical methods, and methods based on the determination of the electrical conductivity of coated granulates. Therefore not only a robust deposition process could be developed, but also the non-trivial task of coating characterization on small and often irregular bodies could successfully be addressed.