Preparing Monodisperse Metal Powders in Micrometer and Submicrometer Sizes by the Polyol Process

Abstract

One of the newer tendencies in materials science has been to tailor-make classical products (long associated with old applications) with controlled properties for special uses, especially in high technology. Preparing dispersed systems in which all particles have nearly uniform size (monodisperse solids) is a typical example. This goal can be achieved in some cases through cleverly controlled particle growth from a liquid medium. Examples of such preparations include gold colloids prepared by Zsigmondy and later by Turkevich et al., sulfur sols obtained by LaMer, metal oxides and hydrous oxides prepared by Matijević et al., silica, etc. These dispersions have been used either to check theories of colloid science, or to a lesser extent, for industrial purposes. In the case of fine metal particles, a uniform size distribution associated with a low degree of agglomeration, and sometimes the spherical shape, appear as particularly convenient characteristics for certain applications. The production of conductive inks or pastes for electronic materials and for the preparation of conductive paints are particularly good examples.In so-called thick film technology, conductive inks and pastes are screen printed on a ceramic substrate in order to form, after firing, a conductive film with a thickness less than 10 μm. This technique is, for instance, used to form the network in hybrid integrated circuits or the internal electrodes of multilayer ceramic capacitors.Metallic powders in thick film compositions are usually precious metals (Au, Ag, Pt, Pd), their mixtures, or alloys. Cheaper metals such as copper or nickel are tested and may be potential substitutes for precious metals in different specific applications. Powders for thick film composition are mainly obtained through chemical precipitation from aqueous or organic solutions, which yield high purity powders. Modification of precipitation parameters (such as the nature and the concentration of the starting metallic compound and of the reducing agent, reaction temperature, viscosity of the medium) and the addition of additives and surfactants, can often be used to control particle size and agglomeration.Over the past few years, we have developed a new process for preparing finely divided metal powders of easily reducible metals (such as precious metals and copper) or less reducible metals (such as cobalt, nickel, cadmium, or lead) by precipitation in liquid polyols. This reaction will be used as an example in order to discuss the mechanism of formation of uniform micrometer and submicrometer size metal particles by precipitation reactions.