Bottom-up Fabrication of Metal/Metal Nanocomposites from Nanoparticles of Immiscible Metals

Abstract

The combination of immiscible metals has traditionally escaped preparation as such metal’s largely different surface energies lead to nonwetting and separation of the two metals during synthesis. The simultaneous preparation of two metals as nanoparticles in a gas phase process can result in the formation of random agglomerates if rapidly cooled. Compaction and subsequent sintering then allows combining otherwise immiscible metals in a bottom-up approach to form metal/metal nanocomposites. In this work, bismuth and cobalt were chosen as model materials which cannot be alloyed by traditional metallurgy due to their large difference in physical properties such as hardness and melting point. Combining bismuth with cobalt (continuous phase) at the nanometer scale resulted in a metal/metal nanocomposite. This class of materials is formally an extension of the current oxide/metal nanocomposites which we conceptually demonstrated through the combination of two distinctly different properties of the composite’s base metals: The bismuth/cobalt nanocomposite displayed a low friction value of around 0.2 (a property of soft bismuth) while maintaining a high hardness (a property of nanocrystalline cobalt). These previously difficult to access properties are attractive for the development of lead-free bearings in energy efficient engines.