To anticipate future long-duration mission needs for life support sensors, we explored the feasibility of using thin-film metal-oxide semiconductors. The objective of this task was to develop gas sensors for life support applications which would be suitable for long-duration missions. Metal oxides, such as ZnO, SnO2, and TiO2 have been shown to react with oxygen molecules. Oxygen lowers the metal oxide's electrical resistance. Critical to the performance is the application of the oxide in a thin film on an inert substrate: the thinner the film, the more readily the oxygen penetration and hence the more rapid and sensitive the sensor. Metal oxides are not limited to oxygen detection, rather, oxides offer detection and quantification applications to the complete range of gases of interest, not only for life support systems, but for propellants as well.
After a preliminary assessment of various metal oxides, we chose ZnO as the active sensory element for oxygen - the most critical near-term need for life support sensors. Our results to date are that we:
Verified that metal oxides respond linearly to oxygen concentration
Experimented with deposition techniques for a thin film of ZnO
Determined the appropriate operational parameters by which ZnO could be used to measure oxygen in concentrations from 10% to 30%
Selected a conceptual design suitable for proof of concept
Developed techniques to fabricate the prototype sensor compatible with integrated circuit and microelectronic packaging
Subsequent work will optimize the ZnO properties, refine prototype design and fabrication, and install and test the sensors in the Kennedy Space Center (KSC) Plant Growth Chamber.