First-Principles Molecular-Dynamics Simulations of Electrochemical Machining in Ultrapure Water

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Electrochemical machining process in ultrapure water is indnced by chemical reaction between work surface and OH ions which are generated by dissociations of water molecules by catalytic reaction in ultrapure water using catalyst. This method is one of the most ultraclean and ultraprecision machining method which can avoid contaminations of work surfaces by electrolytes, cleaning processes after machining process and waste water. Any metals can be machined in practical velocity except Si and Al since the anodic oxidations occur on their surfaces. In this study, analysis of surface chemical reactions using first-principles molecular-dynamics simulations was performed in order to reveal the chemical reaction processes on the anode and cathode surfaces. The simulation were carried out on the basis of the Kohn-Sham local-density-functional formalism. A plane-wave basis set and a norm-conserving pseudopotential were used. The standard molecular-dynamics method was adopted for the optimization of the ionic system and the preconditioned conjugate-gradient (CG) method for the quenching procedure of the electronic degrees of freedom. We determined the optimized atomic configurations and electronic distributions for H, H_2O and OH chemisorbed Si(001) and Al(001) surfaces. It was confirmed that an H atom reacts with H_2O molecules on the hydrogen-terminated Si(001) surface or Al(001) surface to produce an OH molecule, and chemisorptions of H atoms and OH molecules to the surface atom breaks the back-bonds and the surface atom is etched with forming an SiH_2(OH)_2 or AlH_2OH molecule. From these simulation results, we deduced that the electrochemical machining of Si and Al cathode surface in ultrapure water is possible. Furthermore, machining experiments in which samples act as cathodes have been performed and it was confirmed that both Si and Al are possible to be machined.