Periodic current reversal

Current density can be increased without impairing the quaUty of the copper by polishing the cathode surface by brief periodic current reversals (PCR). Reversed current electrolysis, first developed for electroplating, was tested in 1952 for copper refining. Although good results were obtained, no suitable electrical equipment for current reversal was available. The thyristor-controUed siUcon rectifier, introduced in the 1960s, provided a means for... 

MeOH/CH2Cl2, UCIO4, Pt, periodic current reversal in divided cell... 

The present procedure offers an alternative electrochemical setup to accomplish the Kolbe electrolysis of half esters to that reported earlier for the preparation of dimethyl octadecanedioate.17 In the present case the apparatus offers general versatility and electrode coating is prevented by an additive (pyridine). In the earlier case periodic current reversal was necessary. 

When the feed solutions to ED stacks are clean and are relatively free of sparingly soluble materials, ED stacks can be operated for years with little concern about membrane fouling. However, most feedwaters have constituents that can cause problems for ED stacks. Such problems are usually handled by reducing current densities, periodic current reversal (the EDR process), or pretreatment of the feedwater. 

It is a rather old experience in electroplating practice that periodic changes of current lead to improvement in the quality of the deposit. Such changes are of three types periodic current reversal, sinusoidal ac superimposed on direct cathodic current, and pulsating current. A detailed review of the subject is available.The effect is well demonstrated in Figure 23, showing the yield of dendrites during cathodic deposition of zinc as a function of the ratio of the superimposed ac density to the basic direct current density. ... 

Perhaps the most significant recent development in copper refining has been periodic current reversal which permits an increase (of < 15%) in production rates via two effects (1) the anodic current density at which passivations occur may be increased and (2) selective removal of Cu high spots (e.g. nodules) during the reverse current gives smoother cathode deposits. 

Optimum conditions of periodic current reversal permit cathode current densities of up to 36 mAcm The preferred ratio of forward urrent time re-verse-current time is 20 or 30 to 1, with identical forward and reverse currents, for a cycle time of 0.5-3 min. 

The use of programmed current waveforms (e.g. periodic current reversal) in order to produce thin, dense, pore<free coatings, particuJarty for demanding electronic applications (section 8.1.2). 

The majority of practical power supplies are (nominally) square wave in form, the technique being applied in a largely empirical fashion. It is clear, however, that periodic current reversal may generally affect charge transfer, mass transport and surface electrocrystallization processes. 

Stringers that tend to form during deposition of Al on the cathode may cause short circuits within a short time. Technical devices such as scrapers have been developed for stripping the aluminium from the cathode [296, 310] see also [421]. In order to minimize short circuits by particulate metal collecting on diaphragms, periodic current reversal is proposed [494]. The process was also suggested to serve In simultaneously refining commercially pure aluminium [291, 294, 301, 314, 350]. 

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