Atomic oxygen (AOX) quantification in the upper atmosphere of the Earth is of fundamental interest for the understanding of weather and climate development. Solid electrolyte sensors are suitable instruments for this task since they qualify as payload on all common spaceflight systems due to their small size and low power consumption. The High Enthalpy Flow Diagnostics Group (HEFDiG) at the Institute of Space Systems (IRS) (University of Stuttgart, Germany) has been developing solid electrolyte sensors (FIPEX) for applications aboard sounding rockets in the past 12 years.FIPEX sensors employ YSZ (yttria stabilized zirconia) as solid electrolyte. Together with platinum or gold electrodes they form an electrochemical cell that is operated in an amperometric mode of operation. At elevated temperatures and with a voltage applied between the electrodes this setup forces a migration of oxygen anions through the electrolyte. The corresponding electrical current is measured in an outer circuit and is a function of the AOX flux onto the surface of the sensor.On sounding rockets, the measurement of atomic oxygen is particularly difficult because of the rocket’s high vertical speed yielding strong gradients in AOX number densities along the trajectory. Therefore, our research focuses on the measurement of sensor response times and their reduction. Initially, FIPEX sensors were manufactured with screen printed electrodes that are robust yet comparably thick. Surface diffusion of adsorbed oxygen atoms at the cathode was identified as the rate limiting step in this configuration. In order to reduce the diffusion length, the screen printed electrodes were replaced by porous thin-film electrodes leading to significantly improved response times.This contribution focuses on the analysis of the sensor kinetics governing the behavior of the new thin-film FIPEX sensors. We demonstrate how porosity of the initially dense anode is triggered by high polarization. From this, we provide evidence that an increased porosity at the anode is sufficient to improve the response time of a FIPEX sensor. This is different from the conclusions drawn from the analysis of sensors with thicker screen printed electrodes. An experimental configuration to determine sensor step responses to sufficiently fast changes in oxygen particle flux is presented. Above that, we discuss cyclic voltammetry observations to develop a deeper understanding of the underlying kinetic processes.In conclusion, the transient behavior of thin-film sensors is shown to be dictated by kinetic steps different from what was found for screen printed sensors. Impacts on the suitability and further application of FIPEX sensors in space are presented.Fig. 1: a) Functional layers of a FIPEX sensor. b) Photography of a FIPEX sensor with thin film electrodes. Figure 1
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