Powder materials prepared by advanced granulation techniques

The focus of the Ceramic Materials and Powder Metallurgy group of VITO has been the development of free-flowing ceramic powders for the micro-FAST process. To this end, various granulation techniques were applied to transform commercial available powder materials such as MgO stabilized zirconia, alumina and silicon nitride into powders with spherical shape and a consistent particle size distribution suitable for the micro-FAST process. The main technology used to achieve this objective has been spray-drying. In this process, a suspension containing the ceramic powders and organic additives is sprayed into the chamber giving droplets that are dried in heated air. Due to the rapid evaporation of water during the flight spherical granules are formed. The focus has been to limit the amount of organics, while at the same time obtaining a stable and free-flowing powder which can be fed to the micro-FAST machine and that avoids dust formation. Besides spray-drying, alternative suspension based granulation techniques, such as vibrational droplet coagulation and aerodynamically assisted jetting, were explored to prepare free-flowing powders. These allowed for more control resulting in very narrow size distributions of the granulated powder as well as an increased bulk density. For the demonstration tests close to 1 kg of 4 wt% magnesium stabilized zirconia (MSZ) powder was prepared with a size distribution classified between 25 and 44 µm. The process parameters usedto prepare the final batch have been optimised during the process. Only a low amount of organic material (1 wt%) was used, in contrast to more conventional spray drying process (5-6 wt%). The remaining organic material was removed during a calcination step. Because the spray-drying process gives a broad granule size distribution only 40 – 50 % of the produces material can be used for the Micro-FAST process. The discarded material (both the larger granules as the fines) can easily be introduced in the process again after calcining. This ensures that almost no material is lost. A batch of material was prepared from discarded materials from two prepared batched. The granules from the re-used material had exactly the same properties regarding size distribution, sphericity, density and material composition as the material that is not re-used.