Sintering of passivated ultrafine Ni powder in H 2 and He

Abstract

Usually gas evaporated metal particles have to be passivated in order to keep the main part of the particles in metallic form. Passivation of ultrafine Ni particles forms a shell of NiO which is homogeneous and has little variation in thickness. Annealing of passivated ultrafine Ni particles in helium leads to formation of metallic nickel with subsequent particle growth. Particle growth is observed even if the surface oxide has not disappeared completely. As expected, annealing in hydrogen reduces the oxide shell which is then followed by sintering, and the particle growth is here considerable above 498 K. Electron diffraction shows no difference in the formed phases when the particles are passivated (as-prepared) or passivated-reduced-passivated (annealed samples). The repassivation seems to be independent of the gas used in the reduction process. High resolution electron microscopy reveals Ni 2O 3H as an additional phase which is not observed only by electron diffraction.