A collision model for fume formation in metal oxidation
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IN the preceding paper,
the in situ observation of copper oxidation revealed a Cu20 fume residue with some small Cu particles at the Cu20/gas interface. The particular geometry of the hot-stage environmental scanning electron microscope (HSESEM) resulted in a small oxidized spot (about 2 mm diameter) on an unoxidized copper plane of about 1 cm diameter. This fume formation in the absence of an inert gas in the HSESEM or in a conventional furnace with pure 02 of reduced pressure (as reported) is believed to initiate from bimolecular collisions of copper atoms and oxygen molecules in the gas phase. In this paper, a collision model is advanced for explaining the mechanism of such oxide fume formation. The formation of oxide fume between metal vapor and dilute oxygen in an inert gas has been previously observed and explained.~-4 The thermodynamically stable bulk oxide in the Cu-O system at T > 800 ~ in the HSESEM is Cu20. An evaporation contribution from molecules such as Cu2(g) from copper metal can be neglected. 5'6 The vapor pressures for Cu:O, CuO, and Cu species over Cu20(s) or Cu(s) have been calculated using the dissociation energies of Cu20(v) and CuO(v) and the free energy functions derived from references 6 and 7. These values for 927 ~ and Po2 = 3 x 10-4 atm are listed in Table I. According to Table I, the predominant vapor species prior to any collision are Cu(v) from solid copper and O2(g) in the gas beam. The equivalent Po2 of about 3 x 10 -4 atm in the HSESEM corresponds to the transitional flow regime, i . e . , the calculated mean free path of 0.026 cm for oxygen molecules is somewhat smaller than the dimensions (about 1 mm) of the collision system. The evaporation of copper follows a cosine law directional distribution, i . e . , evaporation of atoms in random directions. If fume formation arises from the collision of copper atoms and oxygen molecules in the gas phase, the first step of the reaction must be:
since large numbers of collisions by three or more bodies are a statistical impossibility. The existence of the binary dioxygen vapor complexes has been demonstrated. 8:9 Lever et al. 1o have confirmed the existence of a CuO2 molecule. Darling et al. ,8 when condensing copper vapor at low temperatures in argon containing 10 pet oxygen, proved the existence of a Cu(O2)2 molecule. Furthermore, the reaction:
Cu(v) + O2(g) ~ CuO2*(v)
where ncu and no2 are the densities (no./cm 3) of copper atoms and oxygen molecules, Vcu is the average relative velocity of copper atoms to oxygen molecules, and o- is the effective cross-section for collision defined as:
[11
GUY M. RAYNAUD, formerly Graduate Student, Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210, is now Research Scientist, IREQ, Varennes, Quebec J0L 2P0, Canada. ROBERT A. RAPP is Professor, Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210. Manuscript submitted May 25, 1983. METALLURGICALTRANSACTIONSA
Cu + O 2 ~ C u O
+ O
[2]
is highly endothermic, t~ On the contrary
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