Fine particles of various metals (Mg, Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag, Cd, Sn, Au, Pb and Bi) were prepared by evaporation in argon gas at low pressure. The evaporation was carried out in an ordinary vacuum evaporation unit using a tungsten wire basket heater, after introducing the gas into the vacuum chamber. The average particle size was controlled by changing the pressure of the argon. Particle diameter varied from about one hundred Angstoms at 1 mm Hg to a few tenths of a micron at 30 mm Hg. It was proved by electron diffraction that the particles of all the metals (except Pb) were not seriously oxidized in the air. Electron micrographs showed well defined crystal habits for some metals, e.g. hexagonal plates for magnesium and cubes for chromium. Remarkable “necklace-like arrangements” were observed for particles of the ferromagnetic metals. Many electron micrographs and diffraction patterns are reproduced to show the size, shape, arrangement and state of oxidation of the particles.
Fine metal crystallites with diameters 200∼2000 Å made by evaporating a metal in an atmosphere of an inactive gas were studied by electron microscopy. It is found that the crystal habit common to Be, Cr, Mn and Fe, which are bcc just below the melting point, is rhombic dodecahedra truncated by {100}. Fcc metals show six different types of crystal habit. However,only one, viz. octahedra truncated by {100}, is a single crystal and all the others have twins. Various degrees of truncation were found both for rhombic dodecahedra and octahedra. Crystallites with certain degrees of truncation are possibly Wulff polyhedra.
The appearance of smoke was studied by a video tape recorder. A typical smoke consisted of inner zone, inner front, outer zone and outer front. Smoke particles collected at different parts were studied by electron microscopy. They were large and often showed clear-cut habits in the inner front, while they were small and uniform in the inner and outer zones. The particle size varied with the gas pressure, evaporation temperature and position of collecting particles. The particles grow remarkably by coalescence where no metal vapour exists. All the experimental results on the size can be explained with an empirical rule that particles become larger when the nucleation starts in vapour of higher density.
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