Solid-electrolyte-based amperometric sensors for measuring concentrations of CO2 in air are being developed for use in detection of fires, environmental monitoring, and other applications where liquid-based electrochemical cells are problematic. These sensors are small (sizes of the order of a millimeter), are robust, are amenable to batch fabrication at relatively low cost, and exhibit short response times (seconds) and wide detection ranges.
A sensor of this type at a previous stage of development included a solid electrolyte of Na3Zr2Si2PO12 deposited mainly between interdigitated Pt electrodes on an alumina substrate, all over-coated with an auxiliary solid electrolyte of (Na2CO3:BaCO3 in a molar ratio of 1:1.7). It was necessary to heat this device to a temperature as high as 600 °C to obtain the desired sensitivity and rapid response. Heating sensors increases the power consumption of the sensor system and complicates the use of the sensor in some applications. Thus, decreasing a sensor’s power consumption while maintaining its performance is a technical goal of ongoing development.
A sensor of this type at the present state of development (see Figure 1) has the same basic structure, except that it includes an additional outer layer of nanocrystalline SnO2, which is an n-type (electron-donor-type) semiconductor that provides additional electrons for reduction reaction at the working electrode to detect CO2. [This use of SnO2 as a CO2-sensor material should not be confused with the use of SnO2 in a related development described in “CO2 Sensors Based on Nano crystalline SnO2 Doped With CuO” (LEW-18247-1), NASA Tech Briefs, Vol 32, No. 10 (October 2008), page 44. The SnO2 layer makes it possible to obtain the desired sensor responses at a lower temperature (355 °C), thereby making it possible to operate the sensor at lower power. Figure 2 shows the comparison in response between a sensor with and without the armor layer of nanocrystalline SnO2. Concentrations of CO2 from 0.5 to 4% in air were also detected at 375 °C.
A sensor of this type can be fabricated in the following sequence:
- The platinum interdigitated electrodes, typically having width and spacing of 30 μm, are formed on the alumina substrate by use of standard techniques of sputter deposition, photolithography, and liftoff.
- In a second process involving the use of standard techniques of sputter deposition, photolithography, and liftoff, the Na3Zr2Si2PO12 solid electrolyte is deposited mainly between (and touching) the platinum interdigitated electrodes.
- The workpiece is heated to a temperature of 850 °C for 2 hours.
- The Na2CO3:BaCO3 auxiliary solid electrolyte is deposited on the electrodes and the Na3Zr2Si2PO12 solid electrolyte by sputtering through a shadow mask.
- The workpiece is heated to 686 °C for 10 minutes, then to 710 °C for 20 minutes.
- The layer of nanocrystalline SnO2 is deposited on the Na2CO3:BaCO3 layer by a sol-gel process.
- The workpiece is heated to 500 °C for 2 hours.
The workpiece is then ready for use as an amperometric CO2 sensor.
Research will continue to optimize CO2 sensor performance, while decreasing the operating temperature and power consumption. The objective of future work is to decrease the power consumption to enable, for example, long-term battery operation of CO2 sensor systems.
This work was done by Gary W. Hunter and Jennifer C. Xu of Glenn Research Center.
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