Oxidation kinetics of nickel particles

Abstract: Nanoparticles have an immense surface to volume ratio. This is a key factor to the new properties of some materials made up of such particles. The oxide layer formed on all materials will have a large influence on the behaviour of these materials. The layer isadvantageous in some cases but causes problems in other. To predict the behaviour and to find techniques to prevent unwanted oxidation, the mechanisms by which the oxide film is formed, need to be studied.The focus of this thesis is on kinetics measurements on nickel particles with sizes between 15 nm and 150 µm. The oxidation mechanisms valid for bulk nickel was first tested on the large particles. New information on sintering could be extracted hereand brought on to the evaluation of experiments on the smaller particles.The kinetics was measured with a thermogravimetric technique where the weight change with time during exposure to oxygen at different temperatures was recorded. The surface structure and sintering behaviour of the particles was studied with scanning electron microscopy. An Infrared Spectroscopic technique was used todetermine the oxidation kinetics of nickel nanoparticles in absorber coatings for thermal solar collectors. The experiments were performed without the alumina matrix normally embedding the particles. A master plot technique was used to find the activation energy of the process. The assumptions of nickel vacancy limited diffusion in grain boundaries with initial grain growth, valid for bulk nickel, is found to be applicable also for particles. The sintering process however gives a deviation from bulk behaviour. Indications of a non-linear growth, as predicted for very thin oxide films, is found for the smallest particles. From the results of previous experiments on the solar absorber particles in the alumina matrix it is concluded that the matrix is the only reason for the improved oxidation resistance compared to that of bulk nickel.