Small CO₂ Sensors Operate at Lower Temperature

Lower operating temperature translates to lower power demand.

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:

1. 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.
2. 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.
3. The workpiece is heated to a temperature of 850 °C for 2 hours.
4. The Na₂CO₃:BaCO₃ auxiliary solid electrolyte is deposited on the electrodes and the Na₃Zr₂Si₂PO₁₂ solid electrolyte by sputtering through a shadow mask.
5. The workpiece is heated to 686 °C for 10 minutes, then to 710 °C for 20 minutes.
6. The layer of nanocrystalline SnO₂ is deposited on the Na₂CO₃:BaCO₃ layer by a sol-gel process.
7. 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.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18324-1