  This work focuses on the microstructure of metallic deposits formed by
galvanostatic electrodeposition inside a Hele-Shaw cell without both supporting
electrolyte and flow. For a low applied current density j, the deposit grows
under the form of ramified branches. As shown by Fleury (Nature, 390,, 1997),
these branches are composed of small metallic crystals. This microstructure is
built up by a re-nucleation process induced by the dynamics of a space charge
region (non-electrically neutral solution) ahead of the growth front. When
increasing j the crystal size decreases whereas the nucleation frequency
increases. These latter tendencies are reversed for high j when, as
experimentally observed, dendrites are formed instead of ramified branches.
There must be a transition between the nucleation/growth regime (ramified
branches) and the pure growth regime (dendrites). This transition is examined
experimentally by carefully observing the branch microstructure by SEM. For
copper and silver branches, when j is lower than a critical current density j_c
(concentration-dependent), the branches are composed only of non-dendritic
crystals. Whereas, when j>j_c, dendritic crystals are observed and they become
the main kind of crystals constituting the branches for higher j. These
observations show that the morphological transition on the pattern scale,
between ramified branches and dendrites, originates from a morphological
transition on the scale of the crystals constituting the branches. This latter
is considered theoretically by analyzing the shape stability of the growing
crystals. The Mullins & Sekerka model (shape stability of a spherical particle
growing by diffusion) disagrees with these observations by predicting that the
crystals are always unstable. It is proposed that the space charge layer,
surrounding the growing crystals, induces a stabilizing effect.
