Crossover between Re-Nucleation and Dendritic Growth in Electrodeposition without Supporting Electrolyte

This work deals with the formation of dendritic structures by electrodeposition of Cu2+ and Ag+ without supporting electrolyte in Hele-Shaw cells. The transition between the two main patterns, ramified branches and dendrites, is specifically addressed at the scale of branch microstructure using careful SEM observations. Ramified branches, composed only of grain assemblies, are obtained at low current densities because of a re-nucleation process induced by space charge dynamics (Fleury, Nature, 1997). For current densities higher than a given threshold, ramified branches are also formed by re-nucleation but another growth mode, the dendritic growth, is also observed while, at the macro-scale, the pattern remains fractal and isotropic. This shows that 1) pattern transition originates from a morphological transition at microstructure scale and 2) the re-nucleation process enables a freedom in local growth direction allowing the pattern to be fractal at the macro-scale. The onset of the dendritic growth mode, from shape instability of the grains, is considered with Mullins & Sekerka model. This latter disagrees with the observations by predicting that the grains are always unstable. It is proposed that the space charge plays a key role by controlling the shape stability and thus the transition between the two growth modes