U.S. patent application number 09/917150 was filed with the patent office on 2002-07-18 for drift compensation for gas component sensors.
Invention is credited to Essalik, Abdeltif, Rosen, Howard.
Application Number | 20020092779 09/917150 |
Document ID | / |
Family ID | 23980050 |
Filed Date | 2002-07-18 |
United States Patent
Application |
20020092779 |
Kind Code |
A1 |
Essalik, Abdeltif ; et
al. |
July 18, 2002 |
Drift compensation for gas component sensors
Abstract
The present invention is a gas component sensor comprising novel
electrolyte compositions. The electrolyte compositions in bulk,
sintered or thin film embodiments are capable of forming with
different-metal sensing and reference electrodes a highly stable
gas oxide sensors. The novel electrolyte composition changes
electrochemical reactions at the sensing and reference electrodes
and the overall reaction of the electrodes and electrolyte. The
novel electrolyte compositions have: (1) excellent chemical
stability and thermal compatibility as to the electrodes and a
preferred ceramic substrate, (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. An
improvement is disclosed where a correction of calculated gas
concentration is made by comparison of measured voltages of two gas
sensors.
Inventors: |
Essalik, Abdeltif;
(St-Romuald (PQ), CA) ; Rosen, Howard; (Montreal,
CA) |
Correspondence
Address: |
David T. Bracken
The Law Office of David T. Bracken
4839 Bond Avenue
Orange
CA
92869
US
|
Family ID: |
23980050 |
Appl. No.: |
09/917150 |
Filed: |
July 26, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09917150 |
Jul 26, 2001 |
|
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|
09498211 |
Feb 4, 2000 |
|
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6270651 |
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Current U.S.
Class: |
205/781 ;
204/426; 205/784.5 |
Current CPC
Class: |
G01N 27/4073
20130101 |
Class at
Publication: |
205/781 ;
205/784.5; 204/426 |
International
Class: |
G01N 027/407 |
Claims
we claim:
1. A drift correction method comprising gas sensor assemblies
selected from the types of a carbon dioxide gas sensor assembly, an
oxides of nitrogen gas sensor assembly, and an oxides of sulfur gas
sensor assembly, comprising: (a) each carbon dioxide gas sensor
assembly comprises: (i) a first electrolyte comprising one or more
alkali metal carbonates, one or more alkaline earth metal
carbonates, and one or more first reference electrode metal
halogens, wherein the reference electrode metal of the first
reference electrode metal halogens is chosen from the group
consisting of ruthenium, rhodium, palladium, rhenium, osmium,
iridium, platinum and gold and the halogens of the first reference
metal halogens are chlorine, bromine or iodine; (ii) the first
electrolyte in electrochemical connection between a first sensing
electrode and a first reference electrode, such that the first
reference electrode consists of a first reference electrode metal
and the sensing electrode consists of any other reference electrode
metal; and (iii) the first electrolyte and first electrodes are
supported on a first substrate; (b) each oxides of nitrogen gas
sensor assembly comprises: (i) a second electrolyte comprising one
or more alkali metal nitrates, one or more alkaline earth metal
nitrates, and one or more second reference electrode metal
halogens, wherein the reference electrode metal of the second
reference electrode metal halogens is chosen from the group
consisting of ruthenium, rhodium, palladium, rhenium, osmium,
iridium, platinum and gold and the halogens of the second reference
metal halogens are chlorine, bromine or iodine; (ii) the second
electrolyte in electrochemical connection between a second sensing
electrode and a second reference electrode, such that the second
reference electrode consists of a second reference electrode metal
and the sensing electrode consists of any other reference electrode
metal; and (iii) the second electrolyte and second electrodes are
supported on a second substrate; (c) each oxides of sulfur gas
sensor assembly comprises: (i) a third electrolyte comprising one
or more alkali metal sulfate, one or more alkaline earth metal
sulfates, and one or more third reference electrode metal halogens,
wherein the reference electrode metal of the third reference
electrode metal halogens is chosen from the group consisting of
ruthenium, rhodium, palladium, rhenium, osmium, iridium, platinum
and gold and the halogens of the third reference metal halogens are
chlorine, bromine or iodine; (ii) the third electrolyte in
electrochemical connection between a third sensing electrode and a
third reference electrode, such that the third reference electrode
consists of a third reference electrode metal and the sensing
electrode consists of any other reference electrode metal; and
(iii) the third electrolyte and third electrodes are supported on a
third substrate; (d) a first gas sensor assembly of a first type
exposed to a gas to be sensed; (e) a second gas sensor assembly of
the first type sealed against the gas to be sensed; and (f) having
the first and second gas sensor assemblies electrically connected
to means for separately measuring a voltage across them and for
comparing those measured voltages, exposing the first gas sensor
assembly to a gas to be sensed, operating the first and second gas
sensor assemblies, and measuring and comparing the voltages of the
first and second gas sensor assemblies.
2. The method of claim 1 wherein the first and second gas sensors
assemblies are carbon dioxide gas sensor assemblies and the gas to
be sensed comprises carbon dioxide.
3. The method of claim 1 wherein the first and second gas sensors
assemblies are oxides of nitrogen gas sensor assemblies and the gas
to be sensed comprises oxides of nitrogen.
4. The method of claim 1 wherein the first and second gas sensors
assemblies are oxides of sulfur gas sensor assemblies and the gas
to be sensed comprises oxides of sulfur.
5. The method of claim 1 wherein a difference in the voltages
across the first and second gas sensors assemblies is used to
calculate a correction to a calculated concentration of a component
in the gas to be sensed based on the voltage across the first gas
sensor assembly.
6. The method of claim 1 wherein a difference in the voltages
across the first and second gas sensors assemblies is used to
calculate a correction to a calculated concentration of a component
in the gas to be sensed based on the voltage across the first gas
sensor assembly.
7. The sensor of claim 6 wherein a the corrected and calculated
concentration of a component of the gas to be sensed is displayed,
stored or used in the operation of a control system.
8. An array of two or more gas sensor assemblies selected from the
types of a carbon dioxide gas sensor assembly, an oxides of
nitrogen gas sensor assembly, or an oxides of sulfur gas sensor
assembly, comprising: (a) each carbon dioxide gas sensor assembly
comprises: (i) a first electrolyte comprising one or more alkali
metal carbonates, one or more alkaline earth metal carbonates, and
one or more first reference electrode metal halogens, wherein the
reference electrode metal of the first reference electrode metal
halogens is chosen from the group consisting of ruthenium, rhodium,
palladium, rhenium, osmium, iridium, platinum and gold and the
halogens of the first reference metal halogens are chlorine,
bromine or iodine; (ii) the first electrolyte in electrochemical
connection between a first sensing electrode and a first reference
electrode, such that the first reference electrode consists of a
first reference electrode metal and the sensing electrode consists
of any other reference electrode metal; and (iii) the first
electrolyte and first electrodes are supported on a first
substrate; (b) each oxides of nitrogen gas sensor assembly
comprises: (i) a second electrolyte comprising one or more alkali
metal nitrates, one or more alkaline earth metal nitrates, and one
or more second reference electrode metal halogens, wherein the
reference electrode metal of the second reference electrode metal
halogens is chosen from the group consisting of ruthenium, rhodium,
palladium, rhenium, osmium, iridium, platinum and gold and the
halogens of the second reference metal halogens are chlorine,
bromine or iodine; (ii) the second electrolyte in electrochemical
connection between a second sensing electrode and a second
reference electrode, such that the second reference electrode
consists of a second reference electrode metal and the sensing
electrode consists of any other reference electrode metal; and
(iii) the second electrolyte and second electrodes are supported on
a second substrate; (c) each oxides of sulfur gas sensor assembly
comprises: (i) a third electrolyte comprising one or more alkali
metal sulfate, one or more alkaline earth metal sulfates, and one
or more third reference electrode metal halogens, wherein the
reference electrode metal of the third reference electrode metal
halogens is chosen from the group consisting of ruthenium, rhodium,
palladium, rhenium, osmium, iridium, platinum and gold and the
halogens of the third reference metal halogens are chlorine,
bromine or iodine; (ii) the third electrolyte in electrochemical
connection between a third sensing electrode and a third reference
electrode, such that the third reference electrode consists of a
third reference electrode metal and the sensing electrode consists
of any other reference electrode metal; and (iii) the third
electrolyte and third electrodes are supported on a third
substrate; (d) a first gas sensor assembly of a one type is adapted
to be exposed to a first gas to be sensed; and (e) a second gas
sensor assembly of one other type is adapted to be exposed to the
first gas to be sensed.
9. The array of claim 8 wherein a third gas sensor assembly of a
last type is adapted to be exposed to the first gas to be
sensed.
10. The array of claim 8 wherein a selectively permeable membrane
seals at least one of the gas sensor assemblies against the first
gas to be sensed such that at least one component of the first gas
to be sensed is substantially excluded from the first gas to be
sensed reaching the electrolyte of the gas sensor assembly.
11. The array of claim 8 wherein the first gas sensor assembly is a
carbon dioxide gas sensor assembly and the second gas sensor
assembly is an oxides of nitrogen gas sensor assembly.
12. The array of claim 8 wherein the first gas sensor assembly is a
carbon dioxide gas sensor assembly and the second gas sensor
assembly is an oxides of sulfur gas sensor assembly.
13. The array of claim 8 wherein the first gas sensor assembly is a
oxides of nitrogen gas sensor assembly and the second gas sensor
assembly is an oxides of sulfur gas sensor assembly.
Description
[0001] This application is a division and continuation in part of
U.S. patent application Ser. No. 09/498211 filed Feb. 4, 2000.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to correction of calculated
gas component concentration measured by voltage across gas
component sensors .
[0003] Prior art electrolytic 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:
[0004] sensing electrode:
1/2 O2+2e.sup.-+2 Na+Na2O
CO2+Na2ONa2CO3
[0005] reference electrode:
AgAg.sup.++1e.sup.-
[0006] where Na.sup.+ and Ag.sup.+ are the mobile ions and Na2O and
Na2CO3 are in solid state. The cell EMF can be written according to
the Nernst equation as:
EMF=K-[(2.3.sup.RT Log a.sub.Ag+)/F]+[(2.3.sup.RT Log P.sub.O2
)/4F]+[(2.3.sup.RTLog P.sub.CO2)/2F]
[0007] 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.sub.O2 and silver-ion activity a.sub.Ag+,
the EMF depends only on the CO2 partial pressure.
[0008] 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.degree. C.) has been required, although the
Essalik et al sensor displayed superior operational response at
about 250.degree. C.
[0009] 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
[0010] The present invention is an electrolyte composition. The
electrolyte composition in bulk, sintered 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 changing the electrochemical reactions at the sensing and
reference electrodes and the overall reaction of the electrodes and
electrolyte.
[0011] The novel electrolyte has: (1) excellent chemical stability
and thermal compatibility as to the electrodes and a preferred
ceramic substrate, (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.
[0012] 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 electrical connection with
sensing and reference electrodes, whereby the sensing and reference
electrodes are of different metals.
[0013] The invention electrolyte has been additionally found to
have gas concentration reactive sensitivity to NOx, SOx, H2S,
chloride ions, flouride ions and bromide ions. Although insensitive
and non-reactive with carbon monoxide, gas concentration carbon
monoxide may be indirectly determined by its calculation from the
gas concentrations of carbon dioxide and/or nitrogen oxides and
sulfur oxides, as the gas species and concentration ranges of a
sensed gas are typically known for a specific application of the
invention sensor.
[0014] The invention electrolyte is effective in bulk or thin layer
for chemical specie detection, although the thin layer embodiment
has a somewhat lower power requirement and improved response
time.
[0015] In one embodiment of the invention sensor, a hydrophobic
layer filter substantially excludes water from the sensed gas
reaching the sensor. Such 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 or other undesirable condition. Increased carbon dioxide,
oxides of sulfur and nitrogen and carbon monoxide in air typically
indicate high temperatures and combustion products.
[0016] 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 equilibrium
reaction at the electrolyte/reference metal interface of the
following reaction:
[Reference electrode metal][Reference electrode
metal].sup.Z++ze.sup.-
[0017] The above benefits were not realized or taught in the prior
art with respect to the novel electrolyte composition of the
invention. The invention electrolyte comprises electrochemically
effective amounts of one or more alkali metal carbonates, one or
more alkaline earth metal carbonates, one or more reference
electrode metal halogens and optionally an amount of one or more
alkali halogens. Although not specifically described herein, the
invention electrolyte may comprise substantially electrochemically
neutral components while still achieving the objects of the
invention. Thus, a description of the invention electrolyte will
include a composition with such substantially neutral components so
long as the invention benefits are obtained with an
electrochemically effective amount of the electrolyte applied
across the electrodes of the invention sensor.
[0018] The invention electrolyte also comprises a method for
conversion from a carbon dioxide sensing electrode to an electrode
capable of sensing oxides of nitrogen or sulfur or hydrogen
sulfide. At operating temperature, the invention electrode is
exposed to substantial amounts of oxides of nitrogen or sulfur or
hydrogen sulfide, resulting in absorption and reaction of a portion
of that component with the invention CO2 electrolyte. The resulting
electrolyte (such as including sodium or barium nitrate or sulfate)
thereby becomes sensitized to that component in addition to some
sensitivity to carbon dioxide. Calculation of the concentrations of
the sensed components is possible from a previously determined
potential range of component concentrations for carbon dioxide and
the other component, or a filter may be arranged such that
substantially all carbon dioxide is excluded from the sensed
gas.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a top view of the interdigitated reference and
sensing electrodes and RTD's on a top side of a thin support layer,
with the outline of the invention electrolyte shown in broken
lines.
[0020] FIG. 2 is a bottom view of a bottom side of the thin support
layer, having adhered thereon a resistive heating element.
[0021] FIG. 3 is a graph of the EMF response of the invention thin
film electrolyte to CO2 in a sensed gas.
[0022] FIG. 4 is a graph demonstrating stability of the invention
CO2 sensor.
[0023] FIG. 5 is a graph demonstrating the response of the
invention CO2 sensor to CO2 levels in a gas.
[0024] FIG. 6 is a graph of the invention CO2 sensor response to
non-CO2 components of air as to the response of the sensor shown in
FIG. 5.
[0025] FIG. 7 is an infrared spectra of the bulk invention
electrolyte.
[0026] FIG. 8 are the infrared spectra of the bulk and thin film
invention electrolyte maintained at 130.degree. C. and 400.degree.
C. for 14 days, demonstrating component integrity and continued
stability of the invention electrolyte over time.
[0027] FIG. 9 is a side cutaway view of one embodiment of the
electrode.
[0028] FIG. 10 is a top perspective view of an electrolyte
electrode selectively sealed against an outside atmosphere only as
to some gas components.
[0029] FIG. 11 is a side cutaway view of the device of FIG. 11.
[0030] FIG. 12 is a side cutaway view of an alternate embodiment of
the device of FIG. 11.
[0031] FIG. 13 is a side cutaway view of one type of composite
membrane.
[0032] FIG. 13A is a side cutaway view of another type of composite
membrane.
[0033] FIG. 14 is a microphotograph of a carbon fiber cloth that
can be used as a support in the composite membrane.
DETAILED DESCRIPTION OF THE INVENTION
[0034] The invention electrolyte comprises the following specific
ranges of electrochemically effective components relative to each
other in weight percent:
[0035] alkali metal carbonate(s): from about 20% to about 99%;
[0036] alkaline earth metal carbonate(s): from about 20% to about
99%;
[0037] reference electrode metal halogen(s): from an
electrochemically effective amount, but most preferably from about
0.005% to 10%; and
[0038] alkali halogen(s) (optional): about 0.5% to 1.0%.
[0039] Exemplary alkali metal carbonate are NaCO3, Li2CO3, K2CO3,
and Rb2CO3. Exemplary alkaline earth metal carbonates are BaCO3,
CaCO3, and SrCO3. Exemplary reference electrode metal halogens
comprise any inorganic salt of the reference electrode metal and
halogen ions or oxy-anions as one group or the group of Cl, Br or
l. Exemplary alkali halogens are NaX, LiX, KX, and RbX where X is
chosen from the group of Cl, Br or I. Sensing and reference
electrode metals are preferably chosen from the group Ru, Rh, Pd,
Re, Os, Ir, Pt, or Au. It does not matter which two of the
preferred metals are chosen from among this group for the sensing
and reference electrode so long as the two metals are different.
Optimizing price and ease in fabrication will result in a choice
from the above group of sensing and reference electrodes.
[0040] For a thin film embodiment of the present invention, it has
been found that a ceramic substrate of a-alumina is a preferred
material, although many other appropriate supports are appropriate
depending on adhesion and heat transfer characteristics of the
substrate. A preferred thickness of the ceramic substrate is from
about 50 um to less than about 1 millimeter. It is preferred to
prepare the surfaces for thin film deposition by cleaning in baths
of warm acidic solution (Citronox), warm acetone solution, warm
isopropanol solution, and warm deionized water dried in nitrogen
gas at about 140.degree. C.
[0041] FIGS. 1 and 2 show a thin film invention sensor assembly 100
having ceramic substrate with a top side 101, thickness 102 and
bottom side 107. Sensing electrode 104 is interdigitated with
reference electrode 105 between RTD's 103, all of which are applied
as thin films to top side 101 via sequential e-beam evaporation and
patterned by lift-off. When using Au or Pt for electrodes or
otherwise depositing such on the substrate, it is preferable to
first deposit a layer of chromium to improve adherence of the thin
layer metals. It is preferred that electrode layers have the
following thicknesses: Ag/>15000 angstroms; Pt--Cr/>1000
angstroms; Au--Cr/>5000 angstroms.
[0042] The RTD's are preferably Pt, as well as the thin film heater
108 having wire connections 109. It is apparent to the skilled
person that the embodiment of FIGS. 1 and 2 comprise sites for wire
lead attachment for connection to electrodes 104 and 105, RTD's 103
and heater 108. The outline 106 of FIGS. 1 and 2 shows the
preferred extent of deposited invention electrolyte on assembly
100, thereby covering a substantial portion of the interdigits of
electrodes 104 and 105.
[0043] After deposition of the electrodes 104 and 105, electrolyte
106 is deposited in the following manner. A sputter target disk of
about 2 inches diameter and 5 millimeter thickness is prepared from
the bulk powder form of the invention electrolyte. The disk is
prepared similarly to the process of Essalik et al by hydraulic
(6,000 psi) and cold isostatic press (40,000 psi). The target was
mounted in the RF magnetron sputtering guns in a UHV chamber for
low pressure chemical vapor deposition to the thin film. The target
was hygroscopic and therefore was maintained in vacuum protected by
load lock. The sputtering chamber of the RF magnetron was
back-filled with pure argon to a working pressure. Sputtering of
the target to the substrate was done at a power level of about 85 W
at about 1.1 angstroms/s, producing a thin film of about 20,000
angstroms. The outline of the deposited electrolyte is maintained
only such that it is substantially electrochemically in touch with
opposing edges of the interdigits of electrodes 104 and 105.
[0044] The following list are actual examples sputtering target
disk compositions for four embodiments of the invention electrolyte
incorporated into a CO sensor assembly 100 and having CO2 sensing
capabilities according to the performance characteristics described
herein:
[0045] 1. BaCO3, 17.7615 g.; Na2CO3, 5.2995; AgCl, 0.3583 g.; NaCl,
0.0055 g. or 0.0 g.
[0046] 2. NaCO3, 17.7615 g.; Ba2CO3, 5.2995; Agl, 0.5869 g.; Nal,
0.0055 g. or 0.0 g.
[0047] 3. Li2CO3, 17.7615 g.; Ba2CO3, 5.2995; AgCl, 0.3583 g.;
NaCl, 0.0055 g. or 0.0 g.
[0048] 4. Li2CO3, 17.7615 g.; Ba2CO3, 5.2995; Agl, 0.3583 g.; Nal,
0.0055 g. or 0.0 g.
[0049] FIGS. 3-8 show performance characteristics of the sensor
assembly 100 wherein heater 108 causes the electrolyte 106 to reach
about 250.degree. C. and a higher concentration CO2 containing gas
is introduced to and withdrawn from the electrolyte 106 surface.
For a single, exemplary sensed gas, FIG. 3 shows that full response
time to the higher concentration CO2 containing gas requires only
about 20 seconds (from about 65 seconds to 85 seconds) to reach
accurate measurement and about 65 seconds to recover once the
higher concentration CO2 gas was replaced with the previous gas. In
general, the thin film embodiments of assembly 100 comprise sensors
for CO2, NO2 and SO2. The results for CO2 sensor assembly 100 at
250.degree. C. were about 45.+-.3 mV/decade sensitivity, 10-30
seconds for response time and around 60 seconds recovery time. The
results for NO2 sensor assembly 100 at 250.degree. C. were about
48.+-.3 mV/decade sensitivity, 2-10 seconds for response time and
around 10 seconds recovery time. The results for SO2 sensor
assembly 100 at 250.degree. C. were about 50.+-.3 mV/decade
sensitivity, 1-2 seconds for response time and around 5 seconds
recovery time. Exemplary electrolyte compositions for NO2 and SO2
sensor assemblies are respectively: (1) BaNO3, AgCl, and NaCl and
(2) BaSO4, Na2SO4, AgCl, V2O5, and NaCl. Sputtering target and thin
film deposition techniques for NO2 and SO2 sensor assemblies are as
above.
[0050] FIG. 4 is a graph demonstrating stability of the invention
CO2 sensor. It easily seen that sensor response is substantially
flat after an initial period of decline.
[0051] FIG. 5 is a graph demonstrating the response of the
invention CO2 sensor to CO2 levels in a gas. The lower CO2
concentration sensitivity at about 2000 ppm and below is especially
useful for devices incorporating the CO2 sensor into a display,
recording or control system where the gas will come into contact
with breathing air of animal life.
[0052] FIG. 6 is a graph of the invention CO2 sensor response to
non-CO2 components of air as to the response of the sensor shown in
FIG. 5.
[0053] FIG. 7 is an infrared spectra of the bulk invention
electrolyte.
[0054] FIG. 8 are the infrared spectra of the bulk and thin film
invention electrolyte maintained at 130.degree. C. and 400.degree.
C. for 14 days, demonstrating component integrity and continued
stability of the invention electrolyte over time. Component
locations on the spectrum are shown in the Figure. Traces A and C
show, respectively, bulk powder composition after 14 days of
exposure to heating in air at 400.degree. C. and 130.degree. C.
Traces B and D show, respectively, thin film composition after 14
days of exposure to heating in air at 130.degree. C. and
400.degree. C. It will be readily appreciated that the powder
electrolyte experiences a greater percentage loss of
electrochemically effective components in the increase in
temperature from 130.degree. C. to 400.degree. C. than the thin
film electrolyte. The thin film embodiment of the invention
electrolyte remains effective in composition through long periods
of exposure to much higher temperatures than required for effective
operation.
[0055] The specific example of the above thin film CO2 sensor
operates effectively with a current of about greater than about
5-10 mA. The low power usage translates into low ambient heat
transfer, thereby permitting use in compact, temperature sensitive
devices. It is, however, an alternate embodiment of the sensor
assembly to do without the RTD's and thin film heater where ambient
conditions of a sensed gas are within the temperature range
required by the invention sensor assembly.
[0056] Embodiment for Composite Membrane for Protection of a Gas
Sensor
[0057] The present invention of this embodiment is one of the many
forms of gas components sensors in the prior art or to be developed
later that are protected from some gas components by a dual layer
membrane or a membrane that is effectively made to exhibit
properties of such dual layer membranes. Specifically, it is known
that electrolyte based gas sensors may impaired by gas components
such as water, oxides of nitrogen or sulfur, or other such
components. The impairment components typically react to
permanently damage the gas sensor abilities or may compete for
sensing surface area or sites on the gas sensor. In either case, a
gas sensor can require that one or more gas components be excluded
for short term or long term sensing stability.
[0058] The following discloses prior art gas sensors. Essalik et al
(Study of a new solid electrolyte thin film micropotentiometric
carbon dioxide gas sensor, J. New Mat. Electrochemical Systems 1,
67-70 (1998) discloses a thin film CO2 sensor with a composition
similar to the invention electrolyte without ions of the electrode
metal or different electrode metals.
[0059] U.S. Pat. No. 4,388,155 discloses a shielded side sensor
illustrating the operation problems of exposing both electrodes to
the sensed gas.
[0060] EP Application 91113350.2 discloses a sensor of the NASICON
type requiring specific ratios of alkali metal carbonates to
alkaline metal carbonates.
[0061] U.S. Pat. No. 5,759,366 discloses a solid electrolyte
ceramic with alkali ion conductivity, and two electrodes of
conductor material inert with respect to the electrolyte.
[0062] U.S. Pat. No. 4,715,944 discloses an allegedly stable
operation CO2 sensor having a gas shielding layer with respect to
the electrolyte thereby protected.
[0063] U.S. Pat. No. 5,910,239 discloses titanium dioxide or tin
dioxide used in alkali/alkaline earth metal based electrolyte.
[0064] However, gas sensors operate on the principle of gas
exchange, i.e., that a volume of gas, typically small, is sensed
and is replaced with another volume that is in turn sensed. The
volumes exchanged must be representative of the larger volume whose
composition is of concern. The requirement of gas exchange has
resulted in development of some selective membranes for gas
sensors. Selectivity by membrane necessarily slows response over
the condition where no membrane is used. Thus, the benefits of
selectivity for protection of a gas sensor must be balanced against
the capability of the membrane to exchange volumes of gas with the
external environment in time for a sensor response to be of
use.
[0065] The art of membranes for protection of gas sensors has been
expanded with the present invention. The present invention is a
composite membrane comprising a support layer arranged so that it
is bonded to an exclusion layer, the dual layer forming a barrier
against an external environment having one or more undesirable gas
components. The composite membrane has adequate porosity for
required gas volume exchange and is preferably near the gas sensor
without an intervening layer. The exclusion layer is applied to the
side of the support layer that would be exposed to the external
environment.
[0066] In a specific embodiment, a general class of composite
membranes is disclosed as those dual layer membranes used in fuel
cell technology for gas diffusion electrodes, with or without
electrocatalyst loading, a commercial embodiment of which is
currently sold by E-Tek Inc., a well known provider of fuel cell
components to the industry. That commercial product has a porous
support layer overlain with Teflon.RTM. (PTFE) that has been
pressed onto or into the surface of the support layer. The sole
uses of this composite membrane in the prior art has been for a
liquid to gas interface. The commercial product is disclosed in
combination with a system for fuel cells at their Internet web site
at www.etekinc.com/about.html where they describe their product as
an electrode or catalyst (support layer) that can be adhered
directly to a polymer electrolyte membrane. The constructions are
known to E-Tek, Inc. as membrane electrode assemblies used in fuel
cells for power generation. This art of composite membranes has
been developed for electrical power generation as a liquid to gas
interface since 1930. Although the technology has advanced
dramatically, the uses in the prior remain as a liquid to gas
interface with unidirectional flow of gas components. No prior art
reference suggests their use as an interface for gas to gas where
gas exchange back and forth across the membrane occurs. The present
invention uses the composite membrane in just such a manner to
exclude from a gas sensor a component in the external environment
gas.
[0067] The support layer of the E-Tek, Inc. composite membrane has
a porosity and other characteristics that are designed as a liquid
water to gas interface. The exclusion layer of the E-Tek, Inc.
composite membrane is shown to be Nafion.RTM. or a polymer of the
customer's choosing. In preferred embodiment, the exclusion layer
is Teflon.RTM. to form a hydrophobic layer on the support layer.
The composite layer thereby forms a water exclusion dual layer
membrane that surprisingly permits sufficient diffusion back and
forth across the membrane so that a gas sensor located fairly close
to the membrane is protected but is capable of providing a timely
response as to the concentration of one or more components in the
external environment. Clearly, if one or more gas components are
excluded from the gas sensed by a protected sensor, the
concentration of the sensed gas component in the protected gas
volume is not precisely that of the external environment. If it is
likely that this will introduce substantial error in the
concentration desired to be known by measurement by the gas sensor,
a second sensor for the excluded components may sense them in the
external environment and the result directed to a logic means such
a computer with the result of the concentration of the desired gas
component so that a correction may be made before transmittal to a
user of the information.
[0068] The E-Tek, Inc. product is a flat and relatively thin
support layer having pressed into or onto one side a polymer as an
exclusion layer. U.S. Pat. Nos. 5798668, 6130175, and 6156461
disclose fluorinated polymers for both the porous support layer and
a hydrophobic exclusion layer.
[0069] U.S. Pat. No. 5,126,216 and 5,298,343 are directed to a
catalyst whose activity as a catalyst is of no particular use to
the present invention, although the solid material produced from
production of the invention catalysts forms a material useful as
support material for the present invention. The prior art discloses
some dual layer membranes. U.S. Pat. No. 6,045,697 and the parent
patents to it disclose a mineral oxide support with a polymer
exclusion layer of PS, PVA, PFV, and others. U.S. Pat. No.
6,048,383 tells of a dual layer membrane with a support layer made
of carbon fiber in a thermoset resin or glass fiber in an epoxy and
an exclusion layer made of a fluorinated polymer for
hydrophobicity.
[0070] FIGS. 10 and 11/12 are respectively perspective and cut away
side views of a specific embodiment of the invention system 200. A
base plate 203 has sealingly mounted to it cover 202 which has in
its top surface a hole 204 that allows external environment gas to
pass to the cavity formed between plate 203 and cover 202. Within
that cavity and on the plate 203 is a representative gas sensor
206, such as the above disclosed electrolyte gas sensors or other
gas sensors that require gas contact with the sensor. Sensor 206 is
open to the external environment except for composite membrane 201
sealed at its periphery 205 to the underside of the top part of
cover 202, where the sealing is gas tight so that substantially all
gas exchange between the external gas environment and the gas
sensor passes back and forth through the composite membrane 201. In
a preferred embodiment, the exclusion layer of membrane 201 is
adjacent to the underside of the top part of cover 202 so that gas
tight sealing between cover 202 and membrane 201 is more easily
accomplished and so that the support layer is sealed against the
external environment that may have components damaging to the
support layer materials. The preferred embodiment of FIGS. 10 and
11 teaches the skilled person by its disclosure that a composite
membrane is sealed in a manner that does not permit external
environment gas to reach the sensing surface of a gas sensor
without first passing through the composite membrane. This means
that the composite membrane 201 may contact or be sealed directly
to the sensor 206 as in FIG. 12. Fabrication may permit sputtering
or other such fabrication of a support layer of a membrane 201
directly on a top surface of a sensor 206 with an exclusion layer
applied later to the top surface of the support layer.
[0071] Alternately, support layers may be applied as the above
described polymers or mixtures thereof and the exclusion layer
applied thereafter.
[0072] In a most dual layer composite membrane, FIG. 13 shows a
diagram of a commercial product of E-Tek, Inc. The product A-6
solid polymer electrolyte electrode, single sided version has
support layer 209 as a plain weave carbon cloth of 3.4 oz/yd2 (116
g/m2). The support thickness is about 0.36 mm. FIG. 14 is a
microphotograph of an exemplary carbon cloth Vulcan XC-72 (Cabot
Industries Corp.) used in the support layer of the composite
membrane of composite membrane 207. A finished catalyzed electrode
ranges from 0.45 mm to 0.50 mm in thickness depending on the
catalyst loading 210. Gas-side wet-proofing is by means of a
hydrophobic fluorocarbon/carbon layer 208 on one side of cloth
only. In the commercial product of FIG. 13, E-Tek, Inc. uses
Nafion.RTM. as the hydrophobic fluorocarbon for layer 208, which is
pressed at high pressure into the support layer 209 to obtain the
hydrophobic fluorocarbon/carbon composite. The water exclusive
effect of the inventive system may also be provided by other
polymer in layer 208, as 30 disclosed in the research of Jochen
Kerres et al in the article "DEVELOPMENT OF MEMBRANES FOR
ELECTROLYSIS AND MEMBRANE FUEL CELLS" (Institut fur Chemische
Verfahrenstechnik, Universitt Stuttgart , Dec. 24, 1996,
Collaborative Research Center SFB 270/Project A7, published at
http://www.uni-stuttgart- .de/sfb270/A7_E.htm). As is well
demonstrated in the prior art, no one heretofore has shown
inclination to attempt to use the composite membranes of this
embodiment of the invention system in a gas to gas interface for
water exclusion. In the composite membrane of FIG. 13, a model is
disclosed for preparation of other component exclusive layers,
whereby layer 208 is a polymer/carbon composition formed from
pressing or combining by other method on to support layer 209 with
high porosity and adequate support such as is found in carbon fiber
cloth or carbon paper. As for any physicochemical phenomena, the
determining step is the slowest step and in this embodiment the
slowest step is the diffusion of the gas through the hydrophobic
part of the membrane which is equipped with the smallest porosity.
It is most preferred where the support layer is at least somewhat
hydrophilic that the support layer be sealed from the outside
environment by the hydrophobic layer and other structure or
materials.
[0073] FIG. 13A shows an alternate form 211 of a composite
membrane. Operation of electrolyte gas sensor typically requires
heating to substantially over the vaporization temperature of
water. Hydrophilic layers 208 are impressed or adhered to both
sides of a dessicant layer 212, although in one form of this
embodiment a hydrophilic layer 207 is optionally eliminated from
the side of the membrane facing the electrode. Dessicant layer 212
comprises either a dessicant material such as a zeolite or porous
silica as the support for the hydrophobic layers 207 or a material
such as carbon cloth or other porous material impressed or
impregnated with water absorbent but material that results in a
porous support layer as in the device of FIG. 13. The composite
membrane form 211 of FIG. 13 takes advantage of the heating of the
gas sensor to dry the dessicant layer 212. During operation of a
heated electrolyte electrode, water molecules are typically not
absorbed on the electrolyte in such quantity to seriously affect
electrode performance. However, in a non-operation state when the
temperature of the electrode is reduced below the vaporization
temperature of water, water molecules can absorb into the
electrolyte of the electrode at an unacceptable rate and cause long
term reduction in gas sensing accuracy. The present composite
membrane uses dessicant in layer 212 to capture water molecules. In
non-heated states, gas will pass from an atmospheric side of the
membrane and through a layer 207 to layer 212 where water will be
absorbed. The substantially dry gas will pass directly to the gas
sensor or through yet another hydrophilic layer 207 to enhance
water molecule retention in the dessicant layer 212. During heated
operation, the water molecules in layer 212 are driven off and
layer 212 is again ready to capture water molecules in non-heated
states. Alternately, heated operation of the gas sensor may be
initiated periodically not to specifically to sense a gas
concentration but instead to drive off water molecules from the
dessicant layer. A dessicant layer may also be used with reduced
temperate of the atmospheric gas to cause condensation of water on
the dessicant layer and drain means are provided to draw off the
condensed water, whereafter heated operation will drive the water
off. The dessicant layer thus becomes an effective barrier to water
molecule intrusion into an electrolyte gas sensor.
[0074] Embodiment for Drift Compensation with Dual Gas Sensors
[0075] The above invention gas sensors for carbon dioxide exhibit
effective lives much in excess of the prior art electrolytic
sensors. However, operation of the sensors in some environments
indicates that continuous operation at effective temperatures leads
to a drift in accurate measurement of carbon dioxide in the
measured gas volumes passing across the exposed surface of the
electrolyte.
[0076] FIG. 15 is a top view of a drift compensating embodiment of
the present invention. An invention electrolyte gas sensor 100 as
in FIG. 1 is shown in FIG. 15 closely associated with a
substantially identically functioning electrolyte gas sensor 100'.
The aspect numbers of sensor 100' identify substantially the same
structure and function of the aspects of sensor 100, although the
prime (') designation with an aspect number indicates the aspect's
association with sensor 100'. The support 111 preferably brings
sensors 100 and 100' into such close supporting association with
effectively about the same heat transfer from a heating element or
means that the electrolytes of sensors 100 and 100' experience
about the same temperature during operation. Sensor 100' comprises
a modification of a gas tight overlay 110 that seals the
electrolyte of sensor 100' against the atmosphere. The sealing of
the sensor 100' preferably takes place in a typical ambient air
atmosphere, although the sealing step may take place in a sensed
component-rich or sensed component-absent gas atmosphere.
[0077] It has been found that integrated operation of the sensor
100' with one or more sensors 100 at about the same temperature and
in effective connection with logic means recording and comparing
the potentials across the sensors 100 and 100' results in a
substantially constant potential across sensor 100' comparable to
reduced potential drift of sensor 100 over a very long period of
time. The logic means is programmed to record over a number of
short periods the potential across sensor 100, whereby a sequence
of relatively constant recorded potentials indicates a period of
substantially constant sensed gas concentration of a sensed
component. Where the potential of sensor 100 has declined from the
start of the period of constant sensed gas concentration of a
sensed component to the end of that period, the rate of that
decline is compared with the rate of a decline or change, if any,
in the potentials across sensor 100' for the same period of time.
The logic means uses the overall of instantaneous rate of change of
sensor 100 mathematically compared with the overall of
instantaneous rate of change of sensor 100' to generate a
correction factor which is applied to the current potential from
sensor 100 which is then mathematically transformed to a sensed
component concentration for use or display in display means such as
in a display screen or printed on media from a printer. For
example, if no change in potentials occurs across sensor 100' in a
30 hour period but a 10% decline has occurred in the potentials of
sensor 100 in the same period where inputs to the logic means
indicates a substantially constant sensed component concentration
for the period, then a correction factor of 1.10 (110%) in a simple
ratio could be applied to the currently sensed potential across
sensor 100 before application of the logic means of mathematical
conversion of the potential to sensed gas component
concentration.
[0078] The above design disclosures present the skilled person with
considerable and wide ranges from which to choose appropriate
obvious modifications for the above examples. However, the objects
of the present invention will still be obtained by the skilled
person applying such design disclosures in an appropriate
manner.
* * * * *
References