Research On Liquid Mass Transfer Mechanism Of Electrode Process During Electrolyte Vacuum Boiling Electrodeposition

To solve the two problems of low electrodeposition rate and easy existence ofdefects, a new technology carried out in abnormal environment, namely electrolytevacuum boiling electrodeposition, was developed. It has been proved by experimentsthat with no additives and a considerably high deposition rate of up to seventy timesthan that under conventional conditions, nickel deposits with almost no pinhole andnodule defects can be obtained even at a high cathode current density of75A/dm2usingthis technique. However, many theory problems lying behind the experiment resultsurgently need to be explored and studied. The first and most important one was theliquid mass transfer mechanism of electrode process. This dissertation was supported byNational Natural Science Foundation (51175152) and Henan Province UniversityScience and Technology Innovation Talent Support Plan (2012HASTIT012), and theliquid mass transfer mechanism of electrode process during electrolyte vacuum boilingelectrodeposition was taken as the research topic, the liquid mass transfer characteristicsas well as the electrodeposition behavior characteristics dominated was analyzed andinvestigated simultaneously. Furthermore, the technological effect based onelectrodeposition behavior characteristics was evaluated in order to verify and reflectthe mass transfer mechanism. The concrete contents were as follows:(1) The electrolyte vacuum boiling electrodeposition process was developed, andits technological superiority was summarized. Meanwhile, the necessity of investigatingliquid mass transfer mechanism was pointed out.(2) The evolution law of bubble dynamic behavior and flow pattern as environmentfactors and operation conditions were observed from flow field angle relying on thedeveloped electrolyte vacuum boiling electrodeposition apparatus firstly. The resultsshowed that, as the vacuum degree decreased, the cathode surface temperature raised,the cathode current density increased and the cathode surface roughening, the bubblegrowth cycle was no fundamental changed, but the generation frequency evidentlyaccelerated, meanwhile, the average departure diameter varied changed. Moreover, theflow pattern evolved from unobvious bubbly flow to weak bubbly flow, bubbly mixingflow, and then to mixing flow as the vacuum degree reduced, however it changed slightly with the increasing of cathode surface temperature and cathode current density.Afterwards, the disturbance mass transfer mechanism of gas bubbles along with theflow field distribution law induced was discussed from two aspects of physical modeland numerical simulation.(3) The indexes, such as cathode polarization characteristics, cathode limit currentdensity, electrodeposition rate and cathode current efficiency, dominated under differentmass transfer characteristics conditions that can characterize the electrodepositionbehavior characteristics were systematically studied from the electrochemical behaviorperspective. The results showed that as the flow pattern evolved from unobvious bubblyflow to weak bubbly flow, bubbly mixing flow, and then to mixing flow, as well as theliquid layer physical state transformed from weak natural convection state to owethermal nucleate boiling state and then to surface boiling state, the cathodic polarizationincreased, the electrodeposition rate accelerated and the cathode current efficiencyraised. When the flow pattern in the layer nearby cathode surface presented mixing flow,as the mass transfer effect gradually improved, the cathodic polarization decreased, theelectrodeposition rate increased, but the cathode current efficiency varied little. In theboiling environment, with the disturbance effect initiated by bubble growth cyclebecame more and more strong, the cathodic polarization obviously increased, theelectrodeposition rate significantly improved, however the cathode current efficiencyincreased first and then decreased.(4) In order to verify and reflect the liquid mass transfer mechanism, thetechnological characteristic measurement indexes, e.g. morphology, microstructure andproperties, were evaluated from the process test aspect. The results showed that depositsand microstructures featuring almost no defects as well as excellent morphology qualitycan be obtained in the boiling mass transfer environment. Also, the electrodepositionbehavior characteristics have a significant effect on the texture and the properties. Asthe cathode overpotential raised along with the deposition rate accelerated, the grainsrefined and the microstructure varied, the microhardness progressively increased as awhole, but the corrosion resistance gradually weakened.