In Situ observation of copper oxidation at high temperatures
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I.
INTRODUCTION
MANY investigators have studied the reaction of copper with oxygen at high temperatures, and most of the results are conveniently summarized in two reviews. 1,2 Only a few studies have been made under conditions guaranteeing the formation of only one oxide, although the temperature/ pressure regime for the thermodynamic stability of a single oxide is well known. 3 The growth of cuprous oxide occurs by outward diffusion of cations, and at temperatures above 850 ~ follows a parabolic rate law. 4'5'6 However, in two studies at 1000 ~ 7'8 it was concluded that reaction at the Cu20/O2 interface may compete with diffusion through the oxide layer in the rate control. However, later, Bridges and Fassell9 concluded that copper oxidation fits Wagner's theory. Deviation from the previously reported o~J7 I O 2 dependence for the parabolic rate constant has been found in the high temperature regime.~~ In addition, the oxidation of copper has been the subject of numerous investigations using electron diffraction and electron microscopy. 12'13 However, very few studies of the morphology of cuprous oxide scales and its evolution have been reported for correlation to TGA, defect structure studies, etc. Then for the temperature regime where cation volume diffusion is known to limit the kinetics, questions arise about the exact mechanism for the growth of the oxide lattice at the oxide/gas interface, the generation of grain boundary fissures, migration of oxide grain boundaries, and other unforeseen features. Consequently, in this study, the oxidation of copper has been investigated in situ in a hot-stage environmental scanning electron microscope (HSESEM), as well as by post-oxidation TEM characterization. This paper describes Cu20 formation in the high temperature (T -> 850 ~ regime. The succeeding paper describes in detail the mechanism for oxide fume formation. The study of Cu20 growth at lower temperatures will be reported separately.
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. WILLIAM A. T. CLARK, Associate Professor, and ROBERT A. RAPP, Professor, are with the Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210. Manuscript submitted May 25, 1983. METALLURGICALTRANSACTIONS A
II.
EXPERIMENTAL EQUIPMENT AND PROCEDURES
The HSESEM for high temperature in situ oxidation studies has been described in two previous papers.13'14 In short, the top surface of a 1 cm diameter, 3 cm tall cylinder is exposed to a beam of oxygen gas from a small stainless capillary tube to oxidize a spot about 1 mm diameter. The specimen is heated by radiation from a W filament, and is surrounded by three concentric radiation shields. A thcrmocouple contacts the specimen close to the oxidized spot. Some considerable improvements in the HSESEM have been made more recently: (a) the coiled W heating wire has been inserted inside the cylindrical specimen,
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