A thermodynamic analysis of the Cu-O system with an associated solution model
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INTRODUCTION
T H E metallurgical importance of the copper-oxygen system has stimulated a large amount of experimental work on the oxygen activity in dilute ~-26 and concentrated "aT Cu-O liquid solutions, the phase diagram, 27-37 and the Gibbs energy of formation for the copper oxides Cu20 9'"'25'27 and CuO.9'38'-~9 All of these properties are related by the laws of 9thermodynamics. Therefore, a simultaneous evaluation of all these properties correlated with each other will exploit the experimental work to the maximum. The relationship between the phase diagram and the thermodynamic properties is especially strong in the copper-oxygen system because of the closed miscibility gap in the liquid phase. Like most metal-oxygen or metal-sulfur systems, the liquid Cu-O phase shows an extreme deviation from an ideal solution. There is a very rapid increase in the oxygen activity in a narrow concentration range above 33 at. pet O. It will be shown that an associated solution model assuming molecular "CuzO" species in the liquid phase gives a quantitative description in the complete concentration and temperature range. Associated solution models have been successfully used to describe the liquid phase behavior in various semiconductor systems, 4~ transition metal-sulfur systems, 42-45 and in the Fe-Fe203 system. 46This is, of course, no proof for the physical existence of such associated species. Essentially, this model has been chosen as a mathematical tool to fit all thermodynamic data of the liquid phase and to enable future calculations in multicomponent systems with a Cu-O binary. The nature of these liquid phases is very complex and not yet understood from first principles. An alternative approach to the associated model has been made in the Fe-S system with a two-sublattice model 47using a slightly greater number of experimentally adjusted parameters. A comparison of the two approaches will not be attempted here. The choice of the associated solution model will be discussed later in this study, but it may also be justified simply by the fact that it gives excellent results even in the Cu-O system in which a higher accuracy is needed than in the Fe-S system. RAINER SCHMID is Privatdozent at the lnstitut fur Eisenhuttenkunde und Giessereiwesen, Technical University Clausthal, D-3392 ClausthalZellerfeld, Germany, currently Visiting Associate Professor, Department of Metallurgical and Mineral Engineering, University of Wisconsin-Madison, Madison, WI 53706. Manuscript submitted November 23, 1982. METALLURGICAL TRANSACTIONS B
The aim of this study is to give an extension of the associated solution model and to provide a set of thermodynamic data for the various phases in this system which is internally consistent and consistent with the phase diagram.
II.
ASSOCIATED SOLUTION MODEL FOR THE LIQUID PHASE
As mentioned in the previous section, the existence of an associated species "Cu20" will be assumed in the homogenous liquid phase to account for the rapid increase of the oxygen activity in this region. The formation of
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