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The Expansion upon Cooling of Thick Cu2O Films Grown on Copper Substrates

  • T. F. Swank (a1) and K. R. Lawless (a1)

Abstract

During the course of catalytic experiments on bulk copper single crystals, several crystals were intentionally oxidized to form thick (6000 Å) Cu2O films on the copper substrates. These oxidized crystals were investigated by means of a high temperature chamber installed on a General Electric XRD-5 X-ray diffractometer. It was discovered subsequently that the lattice spacings of the Cu2O decreased upon heating and increased upon cooling. Bulk single crystals and polycrystals of copper were oxidized at 3 Torr of air for several hours. All of the oxidized crystals were examined with copper and chromium radiation and both showed similar results. Typical of the results was an oxidized (110) copper disc which showed a net contraction upon heating of 1% for the (110) Cu2O planes. This slightly oriented Cu2O film was distinguished because -it contracted on heating to 440°C from room temperature, then expanded from 440°-540°C, and then expanded again when cooled from 540°C to room temperature. CuO also was detected in the diffraction pattern and the CuO and copper spacings were behaving normally with the temperature changes. A polycrystal of Cu2O was'examined and that, of course, also acted normally as its temperature was varied. Borie and co-workers have reported and explained very nicely similar anomalous behavior for thinner (500 Å) (110) oriented Cu2O films grown on (110) copper substrates. They showed that the epitaxial forces would cause an oxide film grown at high temperatures to contract parallel to the metal interface and expand normal to the interface as the copper cools and contracts. The oxide would expand normal to the surface in. order to keep its unit cell volume constant.

It is felt that epitaxial forces are not causing the anomalous behavior in the present work mainly because the 6000 Å of Cu2O is too thick for epitaxy to exert a meaningful force. The oxide film on the (110) copper was slightly (110) oriented but all of the Cu2O reflections behaved similarly. An additional reason to discount epitaxy is that this Cu2O film expanded upon heating from 440°-540°C. For these thick oxide films epitaxial forces do not seem to be the controlling factor; therefore, a point defect mechanism must be the cause. Changing oxidation and diffusion rates with temperature would produce various vacancy concentrations in the oxide layer and cause the spacings to vary.

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References

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1. Borie, B., “A Diffraction Measurement of the Structure of Cu2O Films Grown on Copper,” Acta Cryst. 13: 542, 1960.
2. Borie, B., Sparks, C. J., and Cathcart, J. V., “Epitaxially Induced Strains in Cu2O Films on Copper Single Crystals,” Acta Met. 10: 691, 1962.
3. Wagner, J. B. and Gwathmey, A. T., “The Formation of Powder and its Dependence on Crystal Face During Catalytic Reactions of Hydrogen and Oxygen on a Single Crystal of Copper,” J. Am. Chem. Soc. 76: 390, 1954.
4. Intrater, J., “High Temperature, High Vacuum Diffractometer Attachment,” Rev. Set. Imtr, 32: 982, 1961.
5. Tylecote, R. F., “The Adherence of Oxide Scales on Copper,” J. Inst. Metal. 78: 301, 1950.
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8. Swalin, R. A., Thermodynamics of Solids, John Wiley and Sons, Inc., New York, 1962.

The Expansion upon Cooling of Thick Cu2O Films Grown on Copper Substrates

  • T. F. Swank (a1) and K. R. Lawless (a1)

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