Preparation of fine copper powders from organic media by reaction with hydrogen under pressure: Part II. The kinetics of
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I.
INTRODUCTION
THE production of particulate inorganic materials with controlled physical and chemical properties is central to the efforts of a large sector of the international metallurgical Research and Development community.tZl An example of particulate materials requiring close control of their three physicochemical properties are the very fine metallic powders (noble metals and copper) used by the electronics industry for thick film applications. In the present study, a novel method of preparing very fine copper powders, possessing the desired properties (particle size, size distribution, particle morphology, and oxygen content) for thick film applications, was investigated. The method described in part I involves the production of copper powders with uniform spheroidal and discrete particles by the reaction of copper-loaded Kelex 100-decanolVersatic 10-kerosene solutions with H2 gas in autoclaves. In particular, the process involved a limited (only 3 pct yield) nucleation event (heating of the loaded organic solvent at 488 K for 1 hour under Nz) to produce nuclei and then heterogeneous deposition of the bulk of the metal on the surface of the nuclei by H 2 reduction at 573 K and 1.7 MPa, H> A full account of the experimental results was given in part I of this two-part series. In the present article (part lI), the kinetic results reported in part [ in the form of percent yield vs time are analyzed with the aid of the Avrami-Erofeev kinetic model, c2~ the parameters of which are correlated to the particle morphology of the produced powders. In addition, the mechanism of nucleation and growth is elaborated and the cooperative action of Versatic 10 and decanol in bringing steric stabilization to the powder digression is discussed. R. SARRAF-MAMOORY, formerly Doctoral Student, Department of Metallurgical Engineering, McGill University, is Assistant Professor, Faculty of Engineering, Tarbiot Modarres University, Tehran, lran. G.P. DEMOPOULOS and R.A.L. DREW, Professors, are with the Department of Metallurgical Engineering, McGi[[ University. Montreal. PQ, Canada H3A 2A7. Manuscript submitted August 2. 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
II.
HOMOGENEOUS NUCLEATION
Homogeneous nucleation is believed to occur during the thermal treatment of the Cu(Kelex)2 loaded solvent under N=. Copper nuclei form, presumably, via intramolecular redox coupling of Cu(II)L2 complex,~31 which leads to the generation of the intermediate Cu(I)L species (step I). Cu(1)L further undergoes disproportionation and produces metallic particles (step 2). Step l: lntramolecular redox coupling 2Cu(II)L 2 --> 2Cu(I)L + L-L L-L a dimer
[1]
Step 2: Disproportionation of Cu(I)L 2Cu(I)L ~ Cu(II)L2 + Cu
[2]
It is the latter reaction which is responsible for the precipitation of the very fine monodispersed spherical particles (Figure 2(a) and Figure 3 (curve n) in part I). These particles act as nuclei in the present process. Provided that the yield is low, the population of nuclei is rather small, and thus, discrete particles are
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