Michael Matlosz

Competitive Adsorption Effects in the Electrodeposition of Iron‐Nickel Alloys

Two-step reaction mechanisms involving adsorbed monovalent intermediate ions for the electrodeposition of iron and nickel as single metals can be combined to form a predictive model for the codeposition of iron-nickel alloys. Inhibition of the more noble nickel in the presence of iron is caused by preferential surface coverage of the adsorbed iron intermediate resulting from a difference between the two elements in Tafel constant for the electrosorption step. The role of hydrolyzed cations and surface pH is investigated and methods for evaluating the influence of pH are explored. The analysis shows that changes in surface pH with potential are not necessary for iron-rich (anomalous) deposits, but that variations in pH from one electrolyte to another may influence deposit composition. The tendency toward iron-rich deposits with increasing overpotential exists in all systems, however, and can be prevented only by decreasing the iron concentration of the bath. An extension of the analysis to account for transport limitations in baths with low iron concentration is developed and calculations with the model are presented to illustrate the effects of current density and electrolyte convection under conditions similar to those investigated experimentally in the literature.

Blocking Inhibitors in Cathodic Leveling I. Theoretical Analysis

A model for predicting leveling during electrodeposition in the presence of an inhibiting additive is presented. Based on a diffusion-adsorption mechanism, the model assumes that the additive is consumed at the cathode by electroreduction. Using the approximation of a flat, stagnant diffusion layer, leveling during metal electrodeposition into triangular and semicircular grooves is simulated. The variation of the leveling agent concentration along the groove profile is determined by solving a concentration field problem with a boundary element method, and the advancement of the groove profile is simulated with a flexible moving-boundary algorithm. Leveling performance depends on three dimensionless groups characterizing leveling-agent reduction, metal-ion reduction, and the geometrical ratio of the diffusion layer thickness to the groove depth.

Experimental investigation of a porous carbon electrode for the removal of mercury from contaminated brine

A flow(through porous electrode, made of reticulated vitreous carbon (RVC), has been designed to remove mercury from contaminated brine solutions. Experiments with a bench-scale reactor show that the mercury concentration of contaminated brine solutions can be reduced by as much as a factor of five thousand during a single pass through the electrode. The process is mass-transfer limited, and the results of the experiments are used to develop a general correlation for the dependence of the mass-transfer coefficient on the flow rate of electrolyte through RVC. In addition, the effect of counterelectrode placement on the cell resistance is examined, and the experimental data are compared to predictions from a mathematical model of the system. The model agrees favorably with the experimental results, and the benefits of upstream counterelectrode placement, indicated by the model, are verified.

Shape Changes during Through‐Mask Electrochemical Micromachining of Thin Metal Films

Shape change simulations of the electrochemical etching of lines and holes into thin metal films sandwiched between a photoresist mask and an insulating support are presented. For the moving‐boundary simulations, which use a boundary‐element method, it is assumed that the primary current distribution is applicable. The Appendix explains how formulating the current distribution problem in terms of a stream function instead of an electric potential can improve the efficiency of the numerical procedure. Two aspect ratios of photoresist thickness to cavity width are considered. In one case, large aspect ratios are assumed, and various metal film thicknesses are investigated. At long times, but before the insulating support is first exposed to the electrolyte, the shapes of the profiles can be fit to an ellipse. The simulations are continued following exposure of the support, and these results are summarized by a design curve that gives the cavity width necessary to etch a groove of a desired width or a hole of a desired radius into a metal of a given thickness. The second case that is investigated was studied experimentally by Rosset et al. [17]. The aspect ratio of this geometry is 0.08, and the thickness of the metal is large compared to the cavity width. The argeement between theory and experiment is discussed in terms of the likelihood of achieving in practical situations the modeling assumptions.

Complexation Chemistry in Copper Plating from Citrate Baths

An experimental and theoretical study of the influence of solution chemistry on the electrodeposition of copper from complexing citrate baths is proposed and discussed. The behavior of the system is described in terms of the relative distribution of various copper-citrate complexes, combined with a model mechanism for electrodeposition kinetics involving an adsorbed blocking intermediate. Studies of partial-current polarization curves for copper deposition over a wide range of solution pH and free citrate concentration substantiate the mechanism and offer convincing evidence for the significant role of solution chemistry in the electroreduction process. In addition to the copper system, the mechanism proposed offers a framework that may be useful for the study of other metals and alloys electrodeposited from complexing baths containing citrate or citrate-like molecules.

Nonuniform Current Distribution and Thickness during Electrodeposition onto Resistive Substrates

The interaction of electrode and electrolyte resistances during plating onto resistive substrates is investigated. Following presentation of an asymptotic treatment for the estimation of current distributions on highly resistive substrates, more general cases are examined by numerical calculation to illustrate the influence of counterelectrode position, Tafel kinetics, and mass-transfer limitations. The calculations are compared with approximate models presented in the literature or developed in this study in order to determine under which conditions they are applicable

Normalized and Average Current Distributions on Unevenly Spaced Patterns

Spatial variations in electrochemical etch rate are predicted for an unevenly spaced pattern of lines. Two models, based on primary current distribution calculations, are presented. One model is used to estimate the normalized current distribution on an individual line of the pattern. The other mode, based on boundary integral equations, is used for an estimation of the distribution from line to line of the average current densities

A Multisectioned Porous Electrode for Synthesis of D‐Arabinose

A multisectioned flow-through porous electrode, consisting of alternating conducting and insulating sections, was designed, built, and tested for the electrosynthesis of D-arabinose in aqueous solution. The laboratory prototype, consisting of ten independent porous graphite slices each of 5 mm thickness, can be adapted by external electrical connection to allow both traditional and sectioned operations to be run in a single device. The experimental study in this work focuses on use of the multisectioned arrangement to improve current-distribution uniformity in the porous-electrode reactor. A model electrosynthesis system, consisting of oxidation of sodium gluconate to D-arabinose in competition with the parallel reaction of oxygen evolution, has been chosen for investigation. Measurements in the sectioned electrode show that the performance, expressed in terms of current efficiency for gluconate oxidation, increases significantly with an increase in the number of sections. A comparison of predicted theoretical performance with experimental measurements on the ten-section prototype has been used to extrapolate the performance of the sectioned electrode to configurations with larger numbers of sections. Extrapolation indicates that optimal performance in the 5 cm electrode can be achieved with fewer than 100 sections, a design that is feasible to construct with modern microfabrication technology

Complexation Chemistry in Nickel and Copper-Nickel Alloy Plating from Citrate Baths

Following a previous study of the influence of solution chemistry on the electrodeposition kinetics for copper deposition from complexing citrate baths, the present work examines the corresponding influence for nickel andcopper-nickel alloys. The distributions of various metal-citrate chelates, computed as a function of bath composition and pH, are combined with model mechanisms for electrodeposition kinetics involving adsorbed blocking intermediates. The methodology permits not only interpretation of the partial-current curves measured in nickel-citrate baths, but also of the partial-current curves determined for copper and for nickel in codeposition citrate baths. For the case of codeposition, the copper behavior is similar to that observed without nickel, whereas the nickel deposition is catalyzed by the copper. When correctly accounting for solution chemistry, a simple model is sufficient to describe the essential features of the single nickel and the codeposition kinetics.

Segmented thin-gap flow cells for process intensification in electrosynthesis

Design calculations are presented for a single-pass high-conversion electrochemical reactor suitable for process intensification in electroorganic synthesis. The key feature of the design is the use of a segmented working electrode, combined with a small anode—cathode gap. Each working electrode segment is operated at an optimal local current density, defined with respect to the local diffusion—limited current density of the reacting species. Two reactor configurations are considered:(i) an adiabatic reactor, and (ii) an isothermal reactor with integrated heat exchange. Calculated results for the devices in a classical electroorganic synthesis system, the methoxylation of 4-methoxy-toluene, are presented and the general features and performance characteristics of the cell are compared with those of a more conventional capillary-gap cell, currently used industrially. For an electrode gap of 0.1 mm, the average current density attainable in the novel design is of the order of 2700 A m−2 in the adiabatic reactor and of the order of 7100 A m−2 in the isothermal reactor, respectively, 5 and 14 times higher than the current densities applied in the current industrial process. In addition to process intensification, other advantages of the proposed technology are the absence of reactant recycle, short residence times and plug flow of the reagents, all of which contribute to improved process selectivity.