SINTERING OF PASSIVATED ULTRAFINE NI POWDER IN H-2 AND HE

Usually gas evaporated metal particles have to be passivated in order to keep the main part of the particles in metallic form. Passivation o f 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 subs equent particle growth. Particle growth is observed even if the surfac e oxide has not disappeared completely. As expected, annealing in hydr ogen reduces the oxide shell which is then followed by sintering, and the particle growth is here considerable above 498 K. Electron diffrac tion shows no difference in the formed phases when the particles are p assivated (as-prepared) or passivated-reduced-passivated (annealed sam ples). The repassivation seems to be independent of the gas used in th e reduction process. High resolution electron microscopy reveals Ni2O3 H as an additional phase which is not observed only by electron diffra ction.