Effect of Approach Speed and Electrode Geometry on Electrostatic Discharges Off Floating Dielectrics

With the damaging effects of electrostatic discharges (ESDs), it is important to study them within various parameter regimes relevant to real-world scenarios. One such scenario studied here is a floating dielectric with no nearby ground plane with a grounded object approaching at a high rate of speed. One may encounter this scenario when moving objects by hand. Discharge current and radiated fields are captured for discharges drawn from polytetrafluoroethylene (PTFE) and poly methyl methacrylate (PMMA) with approach speeds ranging from 20 to 100 mm/s. Previous studies have shown that at lower speeds in metal-to-metal or metal-to-ground backed dielectric geometries, the peak current and discharge distance decrease with increasing speed. However, in the speed range studied here, an increase in distance and current are observed for increasing speed. Namely, the highest speeds coincide with the highest peak currents and discharge distances of approximately 800 mA and up to 24 mm. With no grounded backing, as opposed to setups in other ESD studies, the electric field between the dielectric and an approaching electrode has a more uniform distribution with highest fields at the electrode, which is elucidated to be the driving factor in the differences revealed in this study. Two spheres of differing diameters and a conical electrode are used to draw discharges off the dielectric surfaces. Captured radiated fields via B-dot sensor, plasma imagery via intesified charge-coupled device (ICCD), and mapping of surface potential reveal that a majority of the energy lost during a discharge goes to gas losses and radiated fields.

I. Introduction

Electrostatic discharge (ESD) and surface potentials have been and are studied extensively due to the inherent danger to delicate electronic systems. ESD generally starts with charging of a dielectric or other surface commonly through triboelectric charging, whereby two similar or dissimilar materials are rubbed against each other exchanging charges [1]. As an example, removing a dielectric object from foam padding may generate several kilovolts on the surface of the dielectric, which then discharges to nearby conductive objects. While the underlying physics behind the triboelectric effect is not fully understood [2], the relative magnitude of this effect may be characterized in a triboelectric series, which ranks a material’s ability or propensity to exchange and accumulate charges and the relative polarity—i.e., positive/negative—of the charging [3]. Polytetrafluoroethylene (PTFE)—a commonly used material in scientific and chemical processes—is often observed to exhibit a strongly negative charging characteristic; however, other common materials, such as polyamides, polyethylenes, and rubbers, can also be highly charged via tribology. The maximum surface potential, and consequently charge density, is primarily governed by the breakdown strength of the surrounding gas, approximately 30 kV/cm for atmospheric air, which corresponds to a maximum surface charge density of approximately 2–3 nC/cm². Discharges from these charged surfaces exhibit short rise times with pulsed currents, and as such, it is important to evaluate both the charging and discharge characteristics to determine safe operating conventions and/or storage conditions. Of interest for this study is the case where a dielectric is in a “floating” configuration, whereby no ground exists close enough to influence the electric field between charge and discharge electrodes.