Copper is utilized in a variety of industrial applications (e.g. heat conductors, electronic components, protective coating), where it is frequently exposed to high temperatures [1]. However, contact with the atmosphere at these high temperatures causes uncontrolled oxidation of the metal. Previous research investigated the oxidation mechanism by oxidizing at specific temperatures and then analyzing the oxide layers with a scanning electron microscope [2].

This work presents a new method for the simultaneous investigation of the oxide layer formation by electrochemical impedance spectroscopy and cyclic voltammetry. The high-temperature electrochemical polarization method allows a dynamic adjustment of the oxygen partial pressure on the surface of the copper, leading to oxide layers with different compositions in respect to their thickness and defect density. The common combination of a potentiostat and a frequency response analyzer (FRA) in a three-electrode setup suffers from bandwidth limitations and artifacts [3]. Therefore, a novel method developed by Fafilek in 2009 [3] for a potential controlled electrochemical spectroscopy was adapted to our high-temperature oxidation setup. Thereby the two measuring units, potentiostat and FRA are electrically separated with regard to their grounding. Cyclic voltammograms are obtained as the result of the polarization of a sputtered copper ring versus a reference electrode, while the impedance spectra are recorded between the same copper ring (large contact area) and a copper sphere (small contact area) as shown in Figure 1. The FRA has nearly no internal resistance which ensures that both samples are adopting the same potential during the potential sweep resulting in an equal oxide formation on them. As the copper ring shows a significantly larger area than the copper sphere, the current received by the potential sweep is assumed to stem from the copper ring. Conversely, it is considered that the impedance spectra originate from the copper sphere, since the impedance of the copper ring can be neglected due to its large size. The data obtained from the potential sweep, such as the oxide structure and film thickness, are combined with the electrical properties acquired by impedance spectroscopy. This allows accurate characterization of the oxide layer that forms during electrochemically controlled oxidation.

[1] Davis, J.R. ed., 2001. Copper and copper alloys. ASM international.
[2] Zhu, Y., Mimura, K. and Isshiki, M., 2002. Oxidation mechanism of copper at 623-1073 K. Materials transactions, 43(9), pp.2173-2176.
[3] Fafilek, G., 2009. A novel experimental method for potential controlled electrochemical impedance spectroscopy. Monatshefte für Chemie-Chemical Monthly, 140(9), pp.1121-1127.