Analysis of the contact electrification in metal-metal, metal-dielectric and dielectric-dielectric contacts applying broadband ultra-sensitive charge detection

• Applicants: Professor Dr. Rolf Möller; Professor Dr. Hermann Nienhaus
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 518939313

Despite the fact that contact electrification and triboelectricity has been observed for more than 2000 years the underlying physics are still unclear. However, it is not only of academic interest, it is of major importance in applications. On the one hand discharges following contact electrification may cause severe explosions of flammable liquids or dust. On the other hand, there are many useful purposes, like exhaust gas cleaning etc. We have developed a novel experimental technique which enables us to study the charge transfer between a free-falling spherical particle and a surface with unprecedented precision. It relies on self-built charge amplifiers providing a resolution < 1 fC and < 1 µs. Experiments performed for the contact between metals reveal that the particles are discharged during the contact but regain within less than a microsecond unexpectedly high charge when the electric contact breaks. Depending on the impact velocity the charge is much higher than predicted by the generally accepted model by Harper and Lowell. An extended model was proposed which explains the findings based on the deformation of the two objects brought into contact. Preliminary experiments including a ring electrode in proximity to the bottom plate demonstrate an even enhanced charge resolution. In addition, the extended setup allows to detect shifts between the center-of-mass and the center-of-charge of the sphere. The goal of this project is to analyze the key processes of contact electrification between metals (e.g. Au, Cu, Pt) and between metals and dielectrics (e.g. Si, InP, Al2O3, ZrO2) as well as between two dielectric materials. The study will be focused on semiconductors and inorganic insulators to address the role of intrinsic electronic states for the charge transfer at well-defined surfaces and at the contact interface. The experiment will be redesigned in a customized UHV chamber which allows to study the contact electrification under vacuum as well as defined gaseous atmospheres. Using low-energy electron beams or ultraviolet light the surface potential of insulating dielectric particles prior to contact is adjusted. The capability of the new setup to measure how the charge evolves in subsequent contacts shall be used to analyze the charge transfer as a function of impact velocity and charge before the contacts. By varying the materials, the effect of the contact potential, the surface band bending, the polarizability of the materials and the existence of adlayers (e.g. H2O) on the transferred charge will be investigated. The novel implementation of the ring electrode will be used to determine any asymmetric charge distribution on an insulating sphere before and after contact. Impingement induced damage and wear are investigated by chemical analysis microscopy in the analysis core facility on campus. The results are expected to gain a more profound insight into the mechanisms of contact electrification between inorganic materials.

• DFG Programme: Research Grants