Investigation of Coatings for Langmuir Probes in an Oxygen-Rich Space Environment

摘要

Use of Langmuir probes in the atmospheres of planets is complicated by oxidation of the probe surface when high-density oxygen atoms/molecules and/or ions are present. Oxidation of most materials creates an electrically resistive layer on the probe surface that reduces the current collected at a given bias voltage, changing the probe's current-voltage (I-V) curves and consequently the measured plasma parameters. TiN (Titanium Nitride), DAG (a graphite coating), or Gold are currently used Langmuir probe coatings, yet they all have issues when exposed to oxygen-rich environments. Iridium and Rhenium are selected as new coating candidates because of the high conductivity of their oxidized forms and high hardness. Here we present the oxidation effect on the measurements of probes made of current coating materials (DAG, Gold, and TiN) and new coating materials (Iridium and Rhenium) against controls (Copper and Nickel) in the laboratory. The oxidation process is performed by bombarding oxygen ions on the probe surface with energies of 1.5-10 eV. The probe's I-V curves taken in an argon plasma are compared before and after oxidation. Our results show that the TiN, Gold, and DAG probes show significant to small changes in their I-V curves, while Iridium outperforms all the other testing materials with almost unchanged I-V curves after the oxidation process. Additionally, this new coating can be applied for other plasma instruments in which surface oxidation may pose an issue. However, for the application of Iridium to electric field probes, future work must be carried out to determine photoemission characteristics.