Self-assembly phenomena of graphite encapsulated nickel nanoparticles in an external magnetic field

Graphite encapsulated metal (GEM) nanoparticle is a composite material with a metal core and graphite shell structure, most particle sizes ranging 10-100 nm. To create a highly condensed bulk from nanoparticles, say over 80% relative density, generally it may take very high temperature and pressure processes. In this study, however, we accidentally discovered a very simple magnetic collecting procedure, by which the Ni-GEM automatically arrange themselves into a relatively dense bulk material. Based on the measurements of Archimedes method and SEM observations, the relative density of the bulks may be over 80%. Though it is impossible to directly observe the rearranging and densification processes of GEM particles in a magnetic field, the following three factors may all play an important role: First is when the GEM immersed in methanol, the methanol molecules adsorb on the surface of nanoparticles and decrease the van der Waals forces between them. Second is when the GEM put into an external magnetic filed, the ferromagnetic nickel nanoparticles could slide, rotate, and move themselves into a more compact position. Finally is when the methanol evaporates, and GEM becomes even denser through capillarity forces.