A Model of Mass-Transport Limited Copper Electrodeposition from Cyanide Electrolyte

摘要

A model of copper discharge from cyanide electrolyte on a planar electrode is developed to provide a quantitative framework for investigating the deposition mechanism. The model describes mass transport by diffusion and migration of all complexes formed by cuprous and cyanide (CuCN_(aq), Cu(CN)_2~-, Cu(CN)_3~(2-), and Cu(CN)_4~(3-)) through a boundary layer near the cathode surface, and allows investigation of the effects that varying solution composition (pH and stoichiometric concentrations of CuCN, NaCN, Na_2CO_3) has on current density, electrode potential, and the concentrations of species at the electrode surface. The focus in this paper is on mass-transfer limited deposition conditions. Dimensionless current density goes through a local maximum as CuCN concentration is increased from 0.2 to 0.6 M in 0.6 M NaCN, 0.1 M Na_2CO_3 at pH = 11.2, and the model is used to explain this unanticipated phenomena. As NaCN concentration is increased from 0.2 to 1.0 M in 0.2 M CuCN, 0.1 M Na_2CO_3 at pH = 11.2, the distribution of the coppercontaining species shifts towards higher-order cyanide complexes that diffuse at a slower rate, and current density decreases. The major effect of increasing the Na_2CO_3 concentration from 0.01 to 10 M and the NaOH concentration from 0.001 to 10 M is to reduce the electric field, thereby lowering the current density. In all electrolytes examined, the distribution of the copper containing species shifts towards higher-order cuprous cyanide complexes in the vicinity of the electrode surface compared to that in the bulk solution. This work is part of a larger program which aims to find non-cyanide bath formulations which yield current distributions of similar uniformity to that obtained in cyanide plating baths. (AESF Project #91)