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

Abstract

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.

Investigation of Coatings for Langmuir Probes: Effect of Surface Oxidation on Photoemission Characteristics

“…After recleaning, the photoemission flux of Ir, Rh, and Cu returned to the level before oxidation, while Au and TiN did not fully return to the same level. These results are consistent with previous results of the removal of the oxidation layers on these materials after recleaning (Samaniego et al, ). Figure shows that the photoemission remains low once Nb becomes oxidized, indicating its oxidation layer is difficult to remove.…”

“…Figure shows the setup of the vacuum chamber. The oxidation process followed the same procedure as our previous experiment (Samaniego et al, ). Oxygen gas (O 2 ) was filled in the chamber with 20% of oxygen atoms (O) created due to photodissociation using an ultraviolet lamp (wavelength: 172 nm, full width at half maximum ±10 nm; photon flux to the probe surface: 10 16 photons·cm −2 ·s −1 ).…”

“…In this work we supplement the work by Samaniego et al () by providing the effect of oxidation on the photoemission of various materials, including DAG213 (a resin‐based graphite coating), AquaDAG (a simple graphite coating), Au, TiN, Ir, Rh, and copper (Cu). Additionally, because of its use on the Parker Solar Probe mission (Bale et al, ), we tested the photoemission response of niobium (Nb).…”

“…It has been observed that the oxidized layer of most materials will reduce the surface conductivity or photoemission yield, causing a change in the probe's current‐voltage ( I ‐ V ) curves and subsequent derived plasma parameters (Ergun et al, ) or errors in electric field measurements (Mozer, ). Recently, various surface coatings have been tested to characterize the oxidation effect on the probe's I ‐ V curve measurements of ambient plasma electrons and ions (Samaniego et al, ). The tested coating materials include new materials iridium (Ir) and rhenium (Rh) against currently used materials DAG (graphite coatings), gold (Au), and titanium nitride (TiN).…”

“…The tested coating materials include new materials iridium (Ir) and rhenium (Rh) against currently used materials DAG (graphite coatings), gold (Au), and titanium nitride (TiN). It has been shown that Ir outperformed all the other materials with almost unchanged I ‐ V curves (and therefore the derived plasma characteristics) after the oxidation process, which is attributed to the high conductivity of its oxidized form (Samaniego et al, ).…”