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تعداد صفحات این فایل: ۴۷ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:Analysis of the Electrolytic Codeposition of Non-Brownian Particles with Metals~~en~~

Abstract

A model for the electrolytic codeposition of spherical particles with metals on a rotating disk electrode is presented, based on a trajectory analysis of the particle deposition, including convective mass transport, geometrical interception, and migration under specific forces, coupled to a surface immobilization reaction. A number of relevant forces were included and their effects determined. Theoretical predictions of this model are compared with experimental results for the codeposition of spherical polystyrene particles with copper during electrolysis from an acid copper sulfate solution. The influence of fluid flow velocities, particle concentration, and current density on the rate of particle deposition is illustrated. Experiments done on a rotating disk electrode allow the adhesion forces to be determined from the distribution of particles on the surface. It is shown that codeposition is governed by colloidal interactions that can, in first order, be approximated by the Derjaguin-Landau-Verwey-Overbeek interactions plus an additional short range repulsion that was associated with the hydration force.

۱ Introduction

The entrapment of ceramic, polymer, and metal powders suspended in an electroplating bath during metal electrodeposition is a process called electrolytic codeposition or composite plating. The simultaneous deposition of particles and metal brings about interesting changes in physical and mechanical properties of the coatings (1). Recently, it has been found that the electrolytic codeposition of liquid-containing microcapsules enables the production of self-lubricating coatings through the inclusion of oil (2). The codeposition of polymer particles provides an excellent aohesion of paint or plastic top coatings. The synthesis of these new coatings has spurred considerable industrial interest. Despite the growing number of applications, the underlying physics of the codeposition process have eluded systematic analysis. One of the first attempts to describe the mechanism of electrolytic codeposition is the pioneering work of Guglielmi (3). But the state of modeling is still far from the ultimate goal, being the prediction of the rate of particle codeposition.

The deposition of particles in other industrial processes, such as deep-bed filtration, flotation, and fouling, has led to a growing body of experimental and theoretical investigations on the mechanism of particle deposition (4-6). However, the link to the field of electrolytic codeposition has never been made, nor has it been shown that the trajectory description can be useful in the context of electrolytic codeposition. The most pronounced difficulties in the transfer of knowledge between the area of filtration and electrolytic codeposition are obvious; filtration deals with dilute solutions, rarely more concentrated than 10 -2 molar, with a low particle concentration. Furthermore, in codeposition the collector is not a dielectric but a metal electrode. In fact, the electrolytic codeposition of particles proceeds under nonequilibrium conditions and is accompanied by a flow of ions across the electrode, the progressive development of a surface texture and a roughening of the deposit during electrodeposition.

Two factors that have further plagued the theoretical progress and have led to considerable confusion lie in the nature of codeposition experiments carried out so far. Most codeposition experiments deal with mineral particles, irregularly shaped, with jagged edges; the result of grinding or milling, having irreproducible charge distributions and an unknown surface chemistry. The incomplete and sketchy information on such systems may explain the contradictory data reported on electrolytic codeposition. The irregular shape and properties also made the theoretical modeling more complex as most theories about particle-wall interactions assume spherically shaped, isotropic particles on a flat surface. Furthermore, most of the early work on electrolytic codeposition was carried out on stationary plane electrodes in cells, where the particles were kept in suspension by gas bubbling or vibrating perforated bottom plates. Therefore, due to the ill-defined and poorly reproducible hydrodynamical plating conditions, comparison of literature data is difficult. This is especially true since the electrolytic codeposition process is strongly affected by the hydrodynamical conditions prevailing in the vicinity of the cathode.

Two basic questions await explanation in analyzing the codeposition of particles from aqueous plating baths. Upon arrival at the cathode, a particle must, somehow, stick to the surface, otherwise it will not contribute to the electrolytic codeposition. How does a particle become captured from the plating bath and what are the forces responsible for this On the other hand, not all particles reaching the electrode surface will necessarily be trapped. Consequently, some of the particles that come into contact with the electrode detach again. What is the force required to release the deposited particulate material

It should be pointed out that the specific surface interactions during codeposition should be viewed as dynamic rather than static. Since codeposition of particles takes place on a renewable (growing) surface, time is an important variable. The continuous buildup of metal influences the geometry of the electrode, especially the area of the electrode in closest contact with the particle, thus changing the initial particle-electrode interaction with time.

The aim of this paper is to present a theoretical framework in which electrolytic codeposition experiments of non-Brownian particles can be analyzed. For this purpose, the trajectory analysis is used as a starting point.

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