Surface features and energy considerations related to the erosion processes of Cu and Ni electrodes in a spark discharge nanoparticle generator

In the present study, the erosion of polished nickel and copper electrode surfaces (with an average roughness of ca. 4.5 nm) exposed to a low number (1-3) of sparks in a spark discharge nanoparticle generator were investigated with the purpose of better understanding the utilization of the energy pumped into the electrodes as well as the processes leading to the generation of aerosol nanoparticles. It was shown that even a single oscillatory discharge creates complex morphological changes on the electrode surfaces. Three main erosion features were identified (referred to as craters, undulated areas and dendritic areas) and characterized by optical, confocal laser scanning and atomic force microscopy. Their potential formation mechanisms are also discussed. By estimating the total energy needed to melt the electrode material corresponding to the volume of the craters individually identified and associated with the first half-cycle of the discharge, it was shown that the electric energy pumped into the electrodes in the form of Joule heating is mostly consumed by the melting of the electrode material, which is followed by re-solidification. Hence, only a small fraction of the electrode material is actually evaporated into the ambient gas. This explains the very low value of the energy-efficiency in the commonly used Jones model. Our results indicate that only a part of the material that leaves the electrode surface is actually converted into an aerosol. The input to the aerosol formation is either vapor or molten material ejected from craters. Ejected molten material can lead to the formation of micrometer-sized aerosol particles (splashing particles). Some of the metal vapor is deposited on the surface in the vicinity of craters forming dendritic areas, whereas only a fraction of the metal vapor is carried away by the ambient gas and may form aerosol nanoparticles. This study clearly indicates that erosion processes in spark discharge nanoparticle generators are very complex and can not be reasonably described by simplified erosion models.