Transitions in the growth mode of branched silver electrodeposits under isothermal and non-isothermal ionic mass transfer kinetics

Abstract

Sequential growth patterns of silver electrodeposits are obtained in a rectangular cell consisting of vertical parallel plate electrodes of a relatively small height, under both isothermal and non-isothermal conditions. These patterns show first a tree-like open branching (regime I), then a dense branching (regime II), and finally a dendritic branching (regime III). An enhancement of transitions I/II and II/III occurs when a unidirectional stationary thermal gradient is established between the cathode and the anode. The dynamic scaling exponents ( α and β) and the geometric dimension ( D F) of the different patterns are evaluated. Regime I ( α≅1, β≅1) can be related to the formation of silver islands. Regime II ( α≅1/2, β≅1/3 and D F≅2) occurs when the average thickness of the deposit is about one-half the length of the hydrodynamic profile. The dynamic scaling exponents resulting from regime II approach the values predicted by the Kardar, Parisi and Zhang equation in the asymptotic limit. This fact is attributed to the weight of the non-linear term in the equation, which is related to the change in the fluid velocity components parallel and perpendicular to the electrode surface. Regime III ( β≅1 and D F≅1.7) involves a change in the growth mechanism with a local exchange of stability at the growing front that results from a balance between surface diffusion and surface relaxation processes. The presence of a stationary thermal field (Δ T) results in a change of the transition parameters that depends on both the magnitude and sign of (Δ T).