Size Resolved Kinetics of Nickel Nanoparticle Oxidation by Ion-Mobility Classification

Recent advancement on field of so called “nanoenergetic” materials are focused on either enhancing or tuning reactivity. On one level this issues reduces to a length-scale argument, whereby smaller fuel/oxidizer combinations result in smaller diffusion lengths and therefore higher reactivity. On another level, this discussion leads to choices of other materials beyond aluminum as the primary thermite based fuel to Ti, or Ni, or even metal based composites such as Ni/Al. While the oxidation of Ni metal in form of bulk sample or thin films has been studied for over a century, there are only a few studies on the oxidation of nickel nanoparticles and most of the studies were carried out by using conventional dynamic thermal techniques such as thermogravimetry. It is well know that those methods are greatly influenced by heat and mass transfer effects such that the results are biased by experimental artifacts . In this work we employ aerosol based techniques to study the oxidation kinetics of Ni nanoparticles. The basic idea of the experiment approach is to prepare Ni particles of characterized size (i.e. Monodisperse), and monitor changes during oxidation in free-flight (i.e. no substrate). The study consists of two experiments, a Tandem Differential Mobility Analyzer (DMA) system is used to measure the size change after oxidation, while the mass change is tracked by a DMAAPM (Aerosol Particle Mass Analyzer) system. This technique allow a direct measure of mass and volume change of individual particles and thus enables us to explore the intrinsic reactivity of nanoparticles with minimizing the sampling error introduced by mass and heat transfer. The mass and size changes of nanoparticles are studied from room temperature to 1100C. The average density obtained show the nickel nanoparticle oxidation process can be correlated to the formation of both NiO and Ni2O3 and a phase change region where both the oxidation of nickel and thermal decomposition of Ni2O3 to NiO occur simultaneously. The reaction kinetics parameters (activation energy and effective diffusion coefficient) were then extracted from the experiment data as a function of particle size.