Abstract
This work reports an investigation on mass transfer by ultrasound agitation during electrodeposition on electrodes separated by a narrow inter-electrode gap. Polarisation experiments were performed to identify the mass transfer limiting current. The limiting current density was used to calculate mass transfer boundary layer thicknesses which were used to develop mass transfer correlations. Experiments were carried out using a cell with parallel copper discs which were positioned at gaps of 1, 0.5 and 0.15cm. The distance between the ultrasonic probe and electrodes was varied between 3 and 1.5cm. The polarisation data showed clear limiting current plateaux when the distance between the electrodes was larger, however significant distortions were observed when the gap was 0.15cm. It was found that lower ultrasound powers of 9–18W/cm2 provided more effective agitation at narrower electrode gaps than powers exceeding 18W/cm2. Sherwood correlations showed that in this system, developing turbulence occurs for larger inter-electrode spacing, whereas for narrow electrode gaps fully turbulent correlations were obtained. A 2-D current distribution model showed that potential distortions that were observed in the polarisation data were caused by the close placement of the metallic US probe to the two parallel electrodes.
Highlights
It is well known that a diffusion layer forms at an electrode surface; if the reaction is carried out for short periods of time when diffusion is the dominant transport mechanism, classical equations developed by Cottrell (1903) and Sand (1901) may be used
If the reaction results in a movement of ions in the bulk leading to natural convection flow formation, laminar or turbulent correlations for free convection, such as Eq (1-a) (Wagner, 1949) and (1-b) (Fouad and Ibl, 1960) respectively, may be used
Limiting current plateaux are observed as expected during copper deposition, the data for lower probe–electrode distances and narrow electrode gaps exhibit smaller plateaux regions, resulting in a difficulty in measurement of the limiting current density
Summary
It is well known that a diffusion layer forms at an electrode surface; if the reaction is carried out for short periods of time when diffusion is the dominant transport mechanism, classical equations developed by Cottrell (1903) and Sand (1901) may be used. For the case of turbulent flow between two parallel plates the general Sherwood number correlation is a Re. For the case of turbulent flow between two parallel plates the general Sherwood number correlation is a Re While these equations have been used extensively (Wragg, 1971; Wragg and Ross, 1967), they are valid only when the two electrodes are sufficiently far from each other, and no interaction of boundary-layer takes place
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