Diffusivity and solubility of oxygen in solid copper using potentiostatic and potentiometric techniques
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I.
INTRODUCTION
THE solubility of oxygen in solid copper has been determined earlier by several investigators.l-s Using analytical techniques, Rhines and Mathewson ~ found that the solubility of oxygen varies from 0.028 to 0.062 at. pct in the temperature range 600 to 1000 ~ whereas using the same technique, Phillips and Skinner2 reported this value to vary from 0.006 to 0.03 at. pct in the temperature range 550 to 1050 ~ Later, Pastorek and Rapp, 3 who employed electrochemical techniques, reported that the solubility of oxygen increased from 0.0016 to 0.014 at. pct with the increase in temperature from 800 to 1000 ~ The values of oxygen solubility obtained recently by Horrigon, 5 using vacuum fusion analysis, are, however, even lower than those reported by Pastorek and Rapp. 3 Previous values for the diffusivity of oxygen in solid copper are mostly estimated from the solubility-diffusivity product (N09 cu) obtained by either measuring the width of an internal oxidation zone6-L~or by using electrochemical 3'11'~2 cells. Verfurth and Rapp ~3 have shown that the value of the ,,o,--oAr~ ncu product depends upon the alloy content of copper and that the internal oxidation of copper alloys does not follow the ideal internal oxidation kinetics. Ramana Rao et al. 11 and Ramanarayanan et al. ~2have determined the values of N 6 D cu electrochemically using steady state current and galvanostatic methods, respectivel~r The only investigation in which the diffusivity of oxygen has been determined independent of its solubility is that by Pastorek and Rapp. 3 They used potentiostatic and galvanostatic electrochemical methods to determine oxygen diffusivity values in the temperature range 800 to 1030 ~ There is considerable disagreement among the data reported in the literature. The present investigation was therefore undertaken to determine the solubility-diffusivity product and diffusivity of oxygen in solid copper by electrochemical techniques.
M. L. NARULA is Research Engineer with Research and Development Steel Authority of India, Ranchi, India. V. B. TARE, Professor on leave of absence from the Banaras Hindu University, India, is presently at the Center for Solid State Science, Arizona State University, Tempe, AZ 85287. W. L. WORRELL is Professor, Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA. Manuscript submitted September 8, 1982.
METALLURGICAL TRANSACTIONS B
II.
T H E O R E T I C A L CONSIDERATIONS
In the galvanic cell, (A) schematically, Cu, Cu20/ZrO2(CaO)/Cu + O(dissolved)/Cu, Cu20
(A) |
Q potentiostat and current/emf recorder
el'
the activity of oxygen at the copper/electrolyte interface can be reduced to a negligible value potentiostatically by imposing an external voltage. The concentration gradient which is so generated across the copper pellet gives rise to the diffusion of oxygen throughout the copper pellet toward copper/electrolyte interface. Since transport number of oxygen ions in the calcia-stabilized zirconia electrolyte is substantially unity, the e
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