Electric Conduction, Adhesion Forces and Discharge Processes in a Particle-Particle-Contact - Highly Resistive Materials

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In case of electrical conduction across the particle-particle-contacts in highly resistive dust layers, the generation of electrostatic adhesion force is strongly coupled to the mechanisms of electrical transport in the solid. High field strengths lead to a significant increase of adhesive forces. For the electrical resistivity, a pronounced non-ohmic behaviour with strong time effects is observed. Aim of the project is to investigate the underlying mechanisms by experiments and simulations on the microscopic scale. Our previous investigations have shown that the mechanisms of charge transport in highly resistive particle layers correspond to those in electrete materials. During current transport, positive and negative charge carriers are injected into the material, and significant electrostatic space charges are formed. The time effects are explained by the mechanisms of charge injection and by the immobilization of charges in the particle volume. In the contact gaps and pore spaces, charge transport can occur by the mechanisms of thermionic field emission and gas discharge. Even though strong electrostatic adhesion forces are generated on the microscopic level, the macroscopic space charge excess can lead to a mechanical failure of highly resistive dust layers by electrostatic repulsion. In the continuation of the project, the successful measurements on single particle-particle contacts shall be improved and continued. Here, the current uptake and the contact force are measured (time-resolving) as a function of particle diameter, contact gap distance, particle material and applied voltage. The mechanism of charge transport across the contact gap shall be verified by observing the emission of light and gas ions. Within the simulation part of the project, a model incorporating the mechanisms of charge transport in the solid and in the contact gap is being developed. Via local potentials, field strengths, charge carrier densities and current densities, the conduction or discharge mechanisms in the gap and the contact force are to be obtained. Here as well, the appropriate modelling of time effects is an important task.The simulations will be verified using results from the single particle experiments. Finally, we will try to simulate chain-like particle arrangements as a model for macroscopic particle layers. Additionally, experiments on particle layers and on massive layers of (dielectric) materials will be made.