Carbon Dioxide Gas Sensors and Method of Manufacturing and Using Same

Abstract

A gas sensor includes a substrate and a pair of interdigitated metal electrodes selected from the group consisting of Pt, Pd, Au, Ir, Ag, Ru, Rh, In, and Os. The electrodes each include an upper surface. A first solid electrolyte resides between the interdigitated electrodes and partially engages the upper surfaces of the electrodes. The first solid electrolyte is selected from the group consisting of NASICON, LISICON, KSICON, and β″-Alumina (beta prime-prime alumina in which when prepared as an electrolyte is complexed with a mobile ion selected from the group consisting of Na+, K+, Li+, Ag+, H+, Pb2+, Sr2+ or Ba2+). A second electrolyte partially engages the upper surfaces of the electrodes and engages the first solid electrolyte in at least one point. The second electrolyte is selected from the group of compounds consisting of Na+, K+, Li+, Ag+, H+, Pb2+, Sr2+ or Ba2+ ions or combinations thereof.

Description

ORIGIN OF THE INVENTION[0001]The invention described herein was made by employees and by employees of a contractor of the United States Government, and may be manufactured and used by the government for government purposes without the payment of any royalties therein and therefor.FIELD OF THE INVENTION[0002]The invention is in the field of carbon dioxide gas sensors.BACKGROUND OF THE INVENTION[0003]The detection of CO2 is essential for a range of applications including reduction of false fire alarms, environmental monitoring, and engine emission monitoring. For example, traditional smoke detectors monitoring particles can have false fire alarm rates as high as 1 in 200 in aircraft applications. Alternatively, monitoring the change of CO and CO2 concentrations and their ratio (CO / CO2) can be used to detect the chemical signature of a fire. Electrochemical CO2 sensors which use super ion conductors (such as Na Super tonic Conductor or NASICON) as the solid electrolyte, and auxiliary e...

AI Technical Summary

Benefits of technology

[0011]A miniaturized amperometric electrochemical (solid electrolyte) carbon dioxide (CO2) sensor using a novel and robust sensor design has been developed and demonstrated. Semiconductor microfabrication techniques were used in the sensor fabrication and the sensor is fabricated for robust operation in a range of environments. The sensing area of the sensor is approximately 1.0 mm×1.1 mm. The sensor is operated by applying voltage across the electrodes and measuring the resultant current flow at temperatures from 450 to 600° C. Given that air ambient CO2 concentrations are ˜0.03%, this shows a sensitivity range from below ambient to nearly two orders of magnitude above ambient. Sensor current output versus ln [CO2 concentration] (natural logarithm of the carbon dioxide concentration) shows a linear relationship from 0.02% to 1% CO2. This linear relationship allows for easy sensor calibration. Linear responses were achieved for CO2 concentrations from 1% to 4% and to the logarithm of the CO2 concentrations from 0.02% to 1%. These sensing measurement results, but not the method of sensor fabrication, were disclosed in the April 2004 American Ceramic Society presentation and at the Fire Prevention Conference in Lisbon November 2004. This CO2 sensor has the advantage of being simple to batch fabricate, small in size, low in power consumption, easy to use, and fast response time.

Problems solved by technology

Current bulk or thick film solid electrolyte carbon dioxide sensors have the disadvantages of being large in size, high in power consumption, difficult in batch fabrication, and high in cost.

This structure is a schematic and not ideally achievable for a number of reasons.

Statistically, given manufacturing tolerances the structure depicted in FIG. 1 is very difficult to achieve.

Any misalignment of the photolithographic mask will result in photoresist trapped between NASICON and electrode finger and therefore result in a failed sensor.

Simply put, the structure of FIG. 1 is very difficult to manufacture exactly as shown.

Errors in manufacturing probably will result in a failed structure such as that depicted in FIG. 5D.

The power consumption of these sensors is very high and batch fabrication is very difficult.

Porous electrodes are typical: Electrodes formed by the thick film technique are not sufficiently porous.

Using a non-porous electrode can lead to the formation of sodium carbonate Na2CO3 which hinders the working electrode.

The formation and dissociation of sodium carbonate Na2CO3 at the electrodes results in slower response time.

Humidity, liquid chemical processing, and / or physical vibration tends to erode or loosen the electrolyte underneath the electrodes.

This structure limited the application of standard microprocessing techniques one might employ such as photolithography.

These properties limited the miniaturization of the sensor using this structure, because the electrodes could only be deposited by a shadow mask, which usually produces electrodes with less integrity when the feature is very small.

Employing these techniques can fundamentally change and improve the sensors produced; a significant technical challenge is to apply these techniques for some material systems such as those used for CO2 sensor production.

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