Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For oxygen or oxygen containing compound
Reexamination Certificate
2000-02-04
2001-08-07
Warden, Sr., Robert J. (Department: 1744)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For oxygen or oxygen containing compound
C205S785000, C204S424000, C252S062200
Reexamination Certificate
active
06270651
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to gas component sensors.
Prior art electrochemical sensors for carbon dioxide have disclosed the following useful relationship. As disclosed in “Study of a new solid electrolyte thin film based micropotentiometric carbon dioxide gas sensor” (A. Essalik et al., J. New Mat. Electrochem. Systems 1, p.67-70 (1998)) electrode reactions giving the EMF of such a sensor are as follows:
sensing electrode:
½O
2
+2e
−+2
Na
+
<=>Na
2
O
CO
2
+, Na
2
O<=>, Na
2
CO
3
reference electrode:
Ag<=>Ag
+
+1e
−
where Na
+
and Ag
+
are the mobile ions and Na
2
O and Na
2
CO
3
are in solid state. The cell EMF can be written according to the Nernst equation as:
EMF=K−[(2.3RT Log a
Ag+
)/F]−[(2.3RT LogPo
2
)/4F]−[(2.3RTLogPco
2
)/2F]
where K is a constant, F and R are the Faraday and gas constants respectively and T is the temperature. According to this equation, at constant P
o2
and silver-ion activity a
Ag
+
, the EMF depends only on the CO
2
partial pressure.
Also disclosed therein is an inherent restriction on the usefulness of that prior art electrode. “However, for practical use, stability of the sensors should be improved.” (Essalik et al, p. 70) and the article explained that the sensor lasted only a few hours at operating temperature. This limitation is a common problem of prior art electrolyte based carbon dioxide sensors. Typically, in other prior art carbon dioxide sensors, high temperature operation (400-500° C.) has been required, although the Essalik et al sensor displayed superior operational response at about 250° C.
There is a need for a carbon dioxide sensor after the Essalik et al device for which stable operation is maintained over a long period of time, sufficiently long for application to control or sensing systems wherein low power, low temperature carbon dioxide sensing may used to advantage.
SUMMARY OF THE INVENTION
The present invention is an electrolyte composition. The electrolyte composition in bulk, thick or thin film embodiments are capable of forming with different-metal sensing and reference electrodes a highly stable carbon dioxide sensor. The sensor of Essalik et al. is improved with changed electrolyte composition thereby improving the chemical, mechanical and physical stability of the sensor and at the same time preserving its operational characteristics. The novel electrolyte has: (1) excellent chemical stability and thermal compatibility as to the electrodes and to the preferred ceramic substrates, (2) excellent chemical stability with the environment as to the reference and sensing electrodes, which need not be sealed against the atmosphere to be sensed, (3) effective adherence to the substrate and electrode metals. The novel electrolyte has solved the stability problems of the Essalik et al. sensor.
The invention electrolyte comprises a relatively small amount of reference electrode metal halide and optionally an alkaline. The invention sensor comprises an electrochemically effective amount of the electrolyte in electrochemical connection with sensing and reference electrodes, whereby the sensing and reference electrodes are of different metals.
The invention electrolyte has been additionally found to be reactive with the following species at high concentration: NO
x
, SO
x
, chlorine and its ion, fluorine and its ion and bromine and its ion. Although insensitive and non-reactive with carbon monoxide, carbon monoxide gas concentration may be determined from the equilibrium between CO and CO
2
when a catalyst is used to enhance the CO
2
formation.
In one embodiment of the invention sensor, a hydrophobic layer filter substantially excludes water from the sensed gas reaching the sensor. Other filters are effective in selecting out or permitting in some chemical species such as water carbon monoxide, and oxides of nitrogen and sulfur. Where such filters are effectively used, separate invention sensors may be used as an array to determine a profile of multiple gas components in a sensed gas, thereby providing a process determination to recording, control and/or display means. For example, such a profile for a gas composition as air after combustion with oxidizeable components and compared with a prior air composition will indicate the presence of a fire both with and without flame and smoke or other undesirable condition. Increased carbon dioxide, oxides of sulfur and nitrogen and carbon monoxide in air typically indicate high temperatures and combustion products.
In a further development of the concept of the preceding paragraph, several selectively permeable membranes are currently available and in the prior art that selectively permit passage of gas components while excluding others, as the above example of water exclusion illustrates. An important improvement of the present invention will be to provide one or more of either prior art and/or invention sensors for detection of carbon dioxide, carbon monoxide, and oxides of nitrogen and sulfur with a gas to be sensed after passage through one of such selectively permeable membranes. The present invention provides that a profile of gases sensed at one or more sensors may be indicative of a fire or other alarm or hazard condition may be passed through a selectively permeable membrane to exclude from a sensed gas one or more or the gas components, most preferably components that could interfere with the particular sensor used to sense a gas component for such a profile. The selectively permeable membrane, for example, may exclude oxides of nitrogen to the sensor array or single sensor where the sensor array or single sensor comprises a sensor for oxides of sulfur. An example of a commercially available source of such selectively permeable membranes is from the disclosure of U.S. Pat. Nos. 5,073,176 and 5,922,104, which are incorporated herein, wherein are disclosed several classes of selectively permeable membranes appropriate for use in the present embodiment.
The composite membrane used in this invention as a moisture barrier can be made by mixing any chemical stable solid with any chemical stable hydrophobic polymer. A preferred moisture barrier membrane comprises polyfluorocarbon polymers. These polyfluorocarbon polymers include at least the following: polytetrafluoroethylene, polyvinylidine fluoride, polychlorotrifluoroethylene, polyvinyl fluoride, and perfluoro(alkyl vinyl ethers).
In a preferred embodiment of the invention sensor, thin layer sensing and reference electrodes are in effective connection through the invention electrolyte, the assembly adhered to a top side of a thin ceramic layer also comprising a resistive temperature detector (RTD), whereby a heating layer is adhered to a bottom side of the thin ceramic layer. As described above, a change in the gas concentration of the sensed component changes the EMF across the electrodes, thereby permitting direct or indirect calculation of the concentration of the sensed component. The invention sensor in this embodiment is inexpensive, easy to fabricate, compact, operates at low temperature, and uses very low power. The small size and low power use of the invention sensor enable the skilled person to now use a stable carbon dioxide sensor in low power and temperature sensitive assemblies. The invention sensor has a broad operating range as to carbon dioxide, typically up to and exceeding 10,000 ppm. Such a range of operation allows use of the invention sensor in devices for sensing, recording or controlling air quality, fire detection, chemical, biochemical and biological (including medical) processes, agricultural processes and the like. The present sensor senses a sensed component independent of the flow rate of the component across the sensor. The present sensor eliminates gas tight sealing of at least one electrodes against a sensed or reference gas. The present sensor now permits stable operation through a substantially equilib
Currie John F.
Essalik Abetif
Olsen Kaj K.
Warden, Sr. Robert J.
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