Electrochemical and speciation studies in the electroplating of aluminum from non-aqueous media.

Abstract

This thesis describes the mechanism of the electrodeposition and dissolution of aluminum (Al) from a non-aqueous, aprotic bath. In order to electrodeposit Al, however, media which are aprotic are required, but additionally Al complexes that are reactive. One system for such a purpose, based upon mixtures of AlCl$\sb3$ and LiAlH$\sb4$ dissolved in etheric solvents, was examined as a starting point for further exploration. A limitation in the use of these baths is the necessity to accommodate the reactive H$\sp-$ species. Baths based on tetrahydrofuran (thf) had not been properly characterized. Electrochemical results derived from cyclic-voltammetry, steady-state polarization and a.c, impedance applied in experiments on hydride-baths were carefully examined in order to establish the mechanism of Al electrodeposition and dissolution. It was established that the mechanism was dependent upon the composition of the hydride-bath, for which, in fact, the symmetry factor, $\beta$, in the electrode-kinetics varied between the compositional extremes. Interestingly, from neither of the two compounds composing the baths could Al be deposited when examined separately in thf, but baths of nearly all proportions of the two combined gave good quality Al electrodeposits at high rates. The success of the latter baths has been found to lie in the ligand mixing, catalyzed by H$\sp{-}$, which gives a distribution of aluminate complexes of varying ligation in solution. The speciation of the plating bath, which was found to contain up to nine different aluminate complexes, was examined by $\sp{27}$Al-NMR over a wide range of bath compositions. Additionally, a comparative examination of relative chemical shifts, linewidths and concentration dependences was conducted in an effort to characterize the reactivity of the individual chloro-hydrido-aluminates that were found to be responsible for the composition-dependent behaviours observed for these baths. The Al deposit from the hydride-bath was generally of very good quality. The morphology of deposits from the hydride-bath was characterized by electron microscopy and the mechanism of initial phase formation examined in terms of nucleation and growth theories and was also found to depend upon the composition of the plating bath. (Abstract shortened by UMI.)