Development and testing of a solid-state CO2 gas sensor for use in reduced-pressure environments

A planetry instrument called the thermal and evolved analyzer (T/EGA) is currently being developed, consisting of a combined differential scanning calorimeter (DSC) and evolved gas analyzer (EGA) to subject regolith samples to a temperature ramp and monitor gas-evolution events. This paper reports the development and testing of a miniature carbon dioxide (CO2) solid-state electrochemical gas sensor (SSEGS) for use in the EGA portion of the instrument. A miniature potentiometric CO2 sensor has been developed that is configured in a planar geometry. The sensor uses solid-state reference and measuring electrodes co-located one one side of a beta alumina electrolyte substrate, and a thick-film platinum heater on the opposite side. Experiments are performed under reduced-pressure conditions to characterize the sensor response in terms of heater power, sensor temperature, total pressure, and gas-composition parameters. The sensor response to carbon dioxide (CO2) is found to be a complex function of these variables, and to act through two separate and independent mechanisms to influence the sensor output. The sensor oxygen sensitivity is also measured in the presence of CO2. A sensor temperature of 530°C is found to result in an oxygen-response slope coefficient of zero. Operation of the sensor at this temperature results in selective carbon dioxide response, and an output that is independent of oxygen partial pressure.