Doctor of Philosophy (PhD)
Electrodeposited NiCoFe ternary alloys are of interest for their magnetic and thermophysical properties. The reaction rates and resulting composition and current efficiency are often determined empirically, due to a lack of understanding of the electrodeposition mechanism and coupled mass transport. Therefore, the effects of electrolyte concentration, bulk pH and solution agitation are studied over a wide range of applied current densities to investigate the interrelated behavior of the partial reaction rates. The Fe rate during alloy deposition is independent of Co2+ and Fe2+ bulk concentrations, but is enhanced compared with its single metal rate. Both catalytic and inhibiting effects are observed for Co deposition. Such behavior has not been observed before, and is considered unique to the ternary alloy group. The Co rate increases with the Co2+ bulk concentration but is inhibited with the Fe2+ bulk concentration. The Ni rate is also inhibited with an increase of the Fe2+ bulk concentration, but appears unaffected by changes in the Co2+ bulk concentration. To date, there is no model of ternary alloy electrodeposition outside of this work, despite the material¡¯s importance in the microelectronics area. Two numerical models are developed here for the NiCoFe ternary deposition, one assuming metal hydroxides are the main reacting species referred to as a hydroxide model while the other does not specify the form of the reacting metal species, referred to as a non-hydroxide model. Both models assume metal depositions occur in a two-step manner and mixed metal intermediate species are formed and adsorb on the electrode surface. The effect of the electrolyte concentration is simulated successfully by both models through the preferential surface adsorption by the Fe species, which is responsible for not only the enhanced Fe rate, but also the inhibited Co and Ni rates. Chemical equilibria of metal sulfates, bisulfate, metal hydroxides and boric acid are included in the hydroxide model, which permit a more realistic hydrogen ion diffusion coefficient to be used in the simulation.
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Zhuang, Yun, "Mechanism of the NiCoFe ternary alloy deposition" (2002). LSU Doctoral Dissertations. 1463.
E. J. Podlaha