Ultrafine metal particles

Abstract

In this paper we present a novel and versatile technique for the production of ultrafine metal particles by evaporation from a temperature-regulated oven containing a reduced atmosphere of an inert gas. An extensive investigation of particles of oxidized AI, with diameters of 3 to 6 nm, has been performed. We have also studied ultrafine particles of Mg, Zn, and Sn produced in the same manner. A supplementing investigation has been carried out for particles of Cr, Fe, Co, Ni, Cu, and Ga, as well as larger Al particles, produced by "conventional" inert-gas evaporation from a resistive filament. Diameter as a function of evaporation rate, inert-gas pressure, and the kind of inert gas are reported. Crystalline particles smaller than 20 nm look almost spherical in the electron microscope, while larger ones often display pronounced crystal habit. Size distributions have been investigated in detail, and consistently the logarithm of the particle diameter has a Gaussian distribution to a high precision for the smallest sizes, whereas larger particles deviate from such a simple behavior. A statistical growth model, based on the Central Limit Theorem, has been formulated for liquid-like coalescence of particles; this theory accounts satisfactorily for all our data, as well as for most size distributions published in the literature. Applications of the model to colloids, discontinuous films, and supported catalysts are discussed. By comparing size distributions for particles produced by a variety of techniques we found a number of empirical rules for the width of the distributions, as defined by a (geometric) standard deviation (J". For crystalline inert-gasevaporated particles we obtained consistently 1.36 ~ (J" ~ 1.60; for coalescing islands in discontinuous films we found 1.22 ~ (J" ~ 1.34; and similar rules are applicable to colloids, supported catalysts, and to ultrafine droplets.

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