An innovative method to study the activity of copper electrorefining additives: The time-varying behavior of thiourea as copper electrorefining additive

Onderzoeksoutput: PhD Thesis

Samenvatting

Electrodeposition additives are key components to enhance the electrochemical deposition of metals, either in pure plating applications or during the electrorefining of metals. Many different additives, either organic or inorganic,are applied in industry, depending on the type of process. This work focuses on thiourea and chloride as common additives used for copper electrorefining.This electrochemical industrial process consumes an impure copper anode and produces a pure (99.995 %) copper cathode, while the anode impurities remain in the electrolyte. Additives assist the electroreduction process to produce smooth copper deposits, essential to avoid short circuits and the formation of unwanted cathode morphologies which can entrap impurities.
Thiourea is a crucial but complex additive in copper refining. It is knownto degenerate into formamidine disulfide (FDS) and both thiourea and FDScan undergo interactions with either copper ions in the solution or the cathode surface. Unfortunately, time dependent additive studies and fundamental knowledge of the effect of aged electrolyte on the cathodic part of the electrorefining process are scarce.
The approach applied in this work enables the investigation of the time de-pendent effect of additives, in this case thiourea and chlorides, assisting the electroplating of copper. Fundamental knowledge of the assisted plating process is extended to offer possibilities for applied research, subsequently, semi-industrial circumstances are used to describe the additive behaviour industrial relevant conditions. Operando Odd Random Phase Electrochemical Impedance Spectroscopy (ORP-EIS) is used to study the electrochemistry ofthe additive system. Hereby, a multisine input signal is added to the DC current to perform the ORP-EIS experiment simultaneously with the plating process. This measurement technique allows an in situ monitoring of the electrochemical system for hour-long electrode position processes. The ORP-EIS technique enables a quantification of the nonlinearities and non-stationarities during the EIS experiment. Additionally, the present non-stationarities are modeled and the nonlinearities are considered as nuisance factor to result ina best linear time-varying approximation. An equivalent electrical circuit isused as model to extract the relevant physical parameters from the impedancedata. The EIS study is complimented with an ex situ analysis of the thiourea concentration, through ion chromatography, as a function of plating time.Correlations could be made between electrochemical parameters and actual thiourea concentration.
Thiourea has a intriguing effect on the copper plating process. First of all, it affects the plating kinetics positively or negatively, depending on the plating system. Secondly, it affects the double layer even when no effect of thiourea is detected anymore on the electrode kinetics. This double layer effect might arise from the reaction products of thiourea degradation but this is not confirmed yet.
Originele taal-2English
Toekennende instantie
  • Vrije Universiteit Brussel
Begeleider(s)/adviseur
  • Hubin, Annick, Promotor
Datum van toekenning1 sep 2022
StatusPublished - 2022

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