Polymers for stabilization of colloidal cobalt particles

Abstract

A process for preparing metiallic cobalt particles of uniform size in the 10–1000 A. range is described. Dicobalt octacarbonyl is thermally decomposed in solutions of dispersant polymers to form stable colloids of discrete particles which are separated by polymer coatings. Variatin of polymer composition, molecular weight, and solvent used results in a variation of particle size and colloid stability. Preparation of single‐domain ferromagnetic cobalt particles with good magnetic properties depends on a very delicate balance between dispersant polymer, solvent, and the growing metal particle. A number of addition and condensation polymers can act as dispersants. The most successful are linear addition polymers of high molecular weight having relatively nonpolar backbones. Ideally, groups with a polarity greater than the backbone are attached at intervals of at least 100 but not more than 200 backbone atoms. The need for more polar groups and their spacing becomes les critical with increasing polymer molecular weight. The polymers appear to stabilize the metal particles by adsorption to form a thick fiml which separates the particles sufficiently to keep van der Walls energy of attraction below thermal energy levels. The solvent must solubilize the polymer coating sufficiently to allow the particle to grow. It should be less polar than the most polar group in the polymer and be chemically inert.