Direct Simulation Monte Carlo Analysis of Microscale Field Emission and Ionization of Atmospheric Air

Abstract

Ionic winds are formed when air ions are drawn through the atmosphere by applied electric and/or magnetic fields. The ions collide with neutral air molecules, exchanging momentum, causing the neutral molecules to move. Continued collisions and momentum exchanges generate a net flow called an ionic wind [1]. Ionic winds formed near flat plates can produce local boundary layer distortion in the presence of a bulk flow. This concept has been studied experimentally at the macroscale as a method for drag reduction [2] and has been suggested at the microscale for convective cooling enhancement [3]. Specifically, microfabricated ion wind engines can be integrated onto electronic chips to provide additional local cooling at "hot-spot" locations. In our previous work, continuum modeling of the ionic wind phenomena showed an approximately 50% increase in the local heat transfer coefficient at the location of the ion wind engine [3]. However, in that work, ionization physics were not modeled, rather assumptions for ion current and concentrations were used as a basis for modeling ion transport. At the microscale, ionization occurs when field-emitted electrons from closely spaced electrodes collide with neutral air molecules, stripping away electrons and forming molecular ions. Geometric enhancement of the electrodes using nanostructured materials enables low ionization voltages conducive to microelectronic devices. Understanding the microscale ionization process is necessary to accurately predict the ensuing ionic wind and cooling. Direct Simulation Monte Carlo (DSMC) is used in the present work to predict field emission between two planar electrodes and the consequent ionization of the interstitial air.