Electrochemical interfacial phenomena under microgravity: Part I. Anodic dissolution of copper in drop shaft

Abstract

As part of a series of physical modeling studies on the electrochemical etching of a printed wiring board, copper foil was anodically dissolved in a small cavity. Coupling of the ionic mass transfer rate with the undercutting phenomena must be precisely understood in order to finely control high aspect ratio anisotropic etching. Interferometry has been applied to the study of the ionic mass transfer rate accompanying electrochemical reactions, and it may help in the visualization of the concentration profile of the metal ion within a narrow cavity. As an idealistic case, the electrochemical dissolution of copper in a purely diffusional field, especially the Cu2+ ion transfer rate itself, is now the focus of attention. The copper anode was galvanostatically dissolved in an aqueous solution (0.1M CuSO4-1M H2SO4) contained in a quasi-two-dimensional electrolytic cell for 8 seconds during free fall in a drop shaft. A disk type of anode, 1 mm in diameter and 100 µm in thickness, and a flat shaped ring cathode 20 mm in inner diameter were sandwiched by two sheets of slide glass. The electrolyte layer was 200 µm in effective thickness. The interference fringe pattern accompanying the copper dissolution was measured in situ with a common path-type microscopic laser interferometer. Terrestrial experiments were also conducted with a horizontally installed cell under the same electrolytic conditions. No natural convection would be expected in the present electrolytic cell design with such a shallow and horizontal electrolyte layer. The distance between the anode surface and the location at which the outer interference fringe appeared was measured by the duration time. It increased linearly with the square root of time under microgravity, but started to deviate from this linearity a few seconds after starting the electrolysis in the terrestrial experiment. A significant difference in the growing distance between the environments had not been a priori anticipated for such a quasi-two-dimensional cell. The local current density distribution along the smaller electrode surface may be influenced by induction of a kind of natural convection.