Preparing Monodisperse Metal Powders in Micrometer and Submicrometer Sizes by the Polyol Process
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N/DECEMBER1989
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 /um. This technique is, for instance, used to form the network in hybrid integrated circuits or the internai électrodes of multilayer ceramic capacitors.11"16 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 différent spécifie applications. Powders for thick film composition are mainly obtained through chemical précipitation from aqueous or organic solutions,11"13 which yield high purity powders. Modification of précipitation parameters (such as the nature and the concentration of the starting metallic compound and of the reducing agent, reaction température, viscosity of the médium) and the addition of additives and surfactants, can often be used to control particle size and agglomération. Over the past few years, we hâve developed a new process for preparing finely divided métal powders of easily
reducible metals (such as precious metals and copper) or less reducible metals (such as cobalt, nickel, cadmium, or lead) by précipitation in liquid polyols.17 This reaction will be used as an example in order to discuss the mechanism of formation of uniform micrometer and submicrometer size métal particles by précipitation reactions.
Description of the Polyol Process Expérimental Conditions In the polyol process a powdered inorganic metallic compound is suspended in a liquid polyol (ethyleneglycol, diethyleneglycol, or mixture of both hâve been mainly used). The suspension is stirred and heated to a given température, which can reach boiling point of the polyol for less reducible metals, e.g., cobalt or nickel. The starting compound may be rather soluble in the polyol (e.g., silver nitrate or cobalt, nickel and copper acétate hydrate) or it may be only slightly soluble (e.g., cobalt and nickel hydroxides, or copper oxides, CuO, and Cu 2 0). A complète réduction of thèse compounds can be achieved within a few hours, the métal being recovered as a finely divided powder. In some cases the water and the volatile products evolved during the reaction must be distilled off, while the polyol is refluxed in order to shorten the reaction time. General Mechanism of the Reaction The gênerai mechanism of the reaction involved in this process has been investigated mainly for the réduction into métal of Co(OH)2 and Ni(OH)2 in ethyleneglycol. Itappears, fromdetailed results published elsewhere,18"20 that the reaction proceeds via dissolution rather than solid phase transformation. In this reaction the polyol at first acts as a solvent for the starting compound due to its rather high dielectric constant. Subsequently, ethyleneglycol reduces the cobalt(II) and nickel(II) species in the liquid phase, in which nucleation and growth of the metallic phase occur. Thus, ethy
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