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تعداد صفحات این فایل: ۲۴ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:Mechanism of Codeposition of Silicon Carbide with Electrolytic Cobalt~~en~~

Abstract

The mechanism of codeposition of silicon carbide with electrolytic cobalt was studied. A theoretical model of codeposition is developed here which provides a more complete insight to the mechanism of electrolytic codeposition than Guglielmi’s model. The model gives a more general expression relating weight percent of embedded particles to current density for different suspension particle concentration for the Co-SiC codeposition system which cannot be described by Guglielmi’s model.

Interest in electrodeposited composite coatings has increased rapidly 16 due to their expected new engineering applications. ~ Such coatings are produced by codeposition of fine inert particles in a metal matrix from electrolytic or electroless baths. For example, a cobalt coating in conjunction with silicon carbide particles was developed and has been proven highly successful.

Although the technique has been developed considerably from the practical point of view, the theoretical details on the codeposition mechanism are not well understood. Three mechanisms, namely, mechanical inclusion, electrophoresis, and adsorption of inert particles onto the cathode, have been proposed to explain the observed codeposition. The relative importance of these three processes cannot be deduced from published experimental work as some results are contradictory. Guglielmi 7 has proposed a mathematical model for the electrodeposition of inert particles which is based on two successive adsorption steps. In the first step a loose adsorption which has an essentially physical character results in a high degree of coverage of cathode by particles which are in equilibrium with the particles in suspension solution. The particles are still surrounded by adsorbed ions and solvent molecules. In the second step this screen is broken through so that a strong electrochemical adsorption of the particles on the cathode takes place, namely, strong adsorption. The inert particles are now permanently bound to the cathode and consequently are embedded in the deposit. The formula so deduced is

[۱]

Where Cp is the concentration of suspended particles, a is the volume fraction of particles in the deposit, F, W,, and d are the Faraday constant, atomic weight, and density of the electrodeposited metal, respectively. The parameters i0 and A are related to the metal deposition and are the constants in the Tafel equation. Similarly, the parameters v0 and B are related to the inert particle deposition. K is derived from the Langmuir adsorption isotherm and depends essentially on the intensity of the interaction between particles and cathode. The validity of this model has been verified for different codeposition systems such as SiC and TiO2 with nickel from sulfamate baths, 7 a-A1203 with copper from acid copper sulfate baths, 8 A1203 with nickel from Watts bath, 9 and TiQ with copper from acidic copper sulfate baths. ~~ However, some codeposition systems question the generality of the model, such as ~-Al~O3 with copper from acid copper sulfate baths ~ and A12Q with gold from cyanide baths. ~ Guglielmi’s model cannot describe the weight percent of the particles in the deposit and the current density relationship shown in Fig. 1. A more general model for the codeposition system must be developed.

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