Observation of Low Temperature Oxidation Process of Pure Copper by 0.6 μ Monochromatic Light

Abstract

The measurement of changes in the reflectivity of a pure copper to the 0.6 μ monochromatic light during the oxidation process in air and the microstructure changes of the specimen surfaces were examined in order to study the oxidation phenomenon of copper. By the heat treatment from room temperature with a constant rate of 1° to 3.6°C/min, the reflectivity of copper decreased with temperature and lowered by about 80% when heated up to 200°C. The reflectivity restored about 90% of the pre-heating value by heating up to 250°C, but it again decreased rapidly by about 60% by heating up to 300°C. In the isothermal heating over the temperature range of 150°∼250°C the changes in reflectivity were similar to those observed in the constant rate heat treatment namely, with the heating time the reflectivity at first decreased to a minimum value and then attained a maximum value, after which it again decreased rapidly. Although a distinct microstructure was revealed on the specimen surfaces by thermal etching at the initial stage of oxidation, the structure turned into an indistinct brown structure when the reflectivity reached a maximum, and then exfoliation of the oxide film occurred with further oxidation. The non-monotonous change of the reflectivity of copper in the oxidation process can be attributed to the interferrence of the reflected rays at the air-copper oxide interface and the oxide-copper interface, and when the mean thickness of the oxide film on the polycrystalline copper specimens reached abut 1,000 and 1,500 Å, the reflectivity to the 0.6 μ monochromatic light attained the minimum and maximum values. The activation energy of the oxidation of copper was estimated by this experiment to be about 12,000 cal/mol at the initial stage when the reflectivity changed non-monotonously and about 19,000 cal/mol at the intermediate stage when the reflectivity decreased rapidly.