Electrocrystallization of nickel

Abyaneh, Morteza Youssef (1980) Electrocrystallization of nickel. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

A critical review of the models for electrocrystallization reactions is presented.The statistical treatment of overlap of growth centres, originally discussed by Avrami, is studied in depth. The extent to which the Avrami theorem takes care of the ingestion of sites (or the related areas) is recognised. A modified version of the Evans' statistical treatment of overlap is proposed. General current-time transients for two- and three dimensional nucleation and two- and three-dimensional growth of centres are derived. A number of other models such as 'death' and 'rebirth' of three-dimensional centres and simultaneous two- and three-dimensional nucleation and crystal growth are developed and are shown to be operative in the electrocrystallization of nickel. It is shown that highly detailed models for the growth mechanism can be examined using computer programmes based on analysis by non-linear regression. The pre-pulse technique for separating the growth rate constant from that of nucleation is shown to be redundant and produce incorrect values for the crystal growth rates.A simple analysis of the growth transients to obtain the kinetic parameters is described. Electrocrystallization of nickel is shown to be initiated via the formation of a monolayer of the deposit followed by either further monolayers (very low overpotentials) and/or the nucleation and three-dimensional growth of centres (intermediate and high overpotentials). The outward growth of these three-dimensional centres into the solution is shown to be of limited duration ('death' of crystal growth); synchronised 'rebirth' of secondary centres takes place after a short -induction period leading to a possible mechanism for the formation of microcrystalline deposits. The formation of the primary growth centres is shown under some conditions to be synchronised with the two dimensional centres of the second monolayer; an indication that the kinetics of thick deposits are governed to some extent by the very initial stages of deposition. Simple, first order behaviour is demonstrated for the concentration dependence of the4-kinetics of the electrocrystallization of nickel; sulphate and chloride ions respectively inhibited and accelerated the process. It is also found necessary to propose complex formation between nickel and borate ions in solution. Dislocations are shown to be absent in the initial stages of growth or, if present, they do not participate in the growth process.

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