Conversion of uniform colloidal Cu 2 O spheres to copper in polyols
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Colloidal copper particles with a high degree of crystallinity were obtained by heating solutions of copper(II) acetate in ethylene glycol (EG) and tetraethylene glycol (TEG). The formation mechanism of copper particles involved first the formation of sparingly soluble nano-sized copper(I) oxide, which aggregated into uniform spheres. On prolonged heating the Cu2O particles were reduced to nano-sized copper crystallites followed by their sintering to colloidal metal. The conversion of the copper(I) oxide to metallic copper proceeds uniformly within the body of each particle.
I. INTRODUCTION 1,2
Colloidal copper has found uses as a catalyst, antimicrobial agent,3 or in powder metallurgy,4 but its most notable applications are in the electronic industry.5 Its excellent electrical conductivity and low cost make it the material of choice for “constructing” highly conductive layers needed in electronic devices, especially for rapidly growing high-frequency telecommunications. Such conductive structures are obtained by “thick-film” technology, in which the metallic particles, dispersed in highviscosity liquid vehicles (pastes), are first deposited onto various substrates by screen-printing. The resulting deposits of closely packed metallic particles are subsequently converted into solid, conductive layers, following the removal of the organic matter and sintering in inert atmosphere at appropriate temperatures.6–8 While the uniformity in particle size and lack of agglomeration are necessary to obtain well-packed deposits, the internal structure of the copper particles plays a critical role during the sintering and, therefore, the formation of highly conductive layers. Consequently, in the preparation of copper powders for such uses it is essential to produce crystalline uniform particles with few internal grain boundaries. This aim requires the control of all stages in their formation, including the size, shape, and structure of intermediate compounds and to elucidate the mechanism through which they are converted into the final products. While different techniques (atomization, chemical vapor deposition, spray pyrolysis) have been successfully used to produce highly dispersed copper particles, the a)
On leave from the National Chemistry Institute, Ljubljana, Slovenia. b) Address all correspondence to this author. J. Mater. Res., Vol. 18, No. 4, Apr 2003
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reduction of copper salts in homogeneous solutions remains the most versatile preparation method. By proper selection of the precursor metallic species, the reaction medium, the protective colloid, and the reducing agent, nonagglomerated copper particles ranging in size from a few nanometers to several micrometers have been obtained.9–13 In most cases the described processes involve several stages. For example, monodispersed spheres of copper basic carbonate were first prepared by decomposition of urea in copper salt solutions. In a separate second process these particles were then reduced to copper using hydroxy
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