Electrodeposition of copper under microgravity conditions

Copper was electrodeposited from 0.9 M CuSO4 aqueous solution for 8 s during a free-fall experiment in a drop shaft. A horizontally oriented quasi two-dimensional electrolytic cell, in which a 200 mu m thick electrolyte layer was sandwiched between two sheets of slide glass, fell in the shaft. Electrolysis was conducted at constant current densities from 0.05 to 0.3 A cm(-2), and the potential difference between electrodes was simultaneously monitored. A common-path microscopic interferometer was installed in the capsule. The time variation of interference fringe pattern, corresponding to the concentration profile due to the ionic mass-transfer rate accompanied with copper electrodeposition, was measured in situ around the periphery of a 1 mm diam circular cathode. The morphology of copper electrodeposited under microgravity conditions was compared with that obtained in a ground level experiment. The development of diffusion layer thickness measured by the interferometer increased with the square root of time during the 8 s experiment. The transient diffusion model combined with the migration effect reasonably explained the development of diffusion layer thickness. Calculated surface concentrations corresponded with the measured time variation of potential difference between electrodes. Under the gravitational field, the radius of annual interference fringe pattern developed at a rate much faster than that under microgravity, beginning 1 or 3 s after initiation of electrodeposition. Natural convection was evident in the ground level experiment, even using such a shallow electrolyte layer. Under microgravity, lower index planes preferentially grew to form a smaller number of larger grains, even though electrodeposition was conducted at 0.05-0.3 A cm(-2) only for 8 s.