CA1119250A - Process for producing an activated oxygen gas sensor element - Google Patents
Process for producing an activated oxygen gas sensor elementInfo
- Publication number
- CA1119250A CA1119250A CA000327619A CA327619A CA1119250A CA 1119250 A CA1119250 A CA 1119250A CA 000327619 A CA000327619 A CA 000327619A CA 327619 A CA327619 A CA 327619A CA 1119250 A CA1119250 A CA 1119250A
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- sensor element
- oxygen gas
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- gas sensor
- activated oxygen
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/403—Cells and electrode assemblies
- G01N27/406—Cells and probes with solid electrolytes
- G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
- G01N27/4075—Composition or fabrication of the electrodes and coatings thereon, e.g. catalysts
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
- H01M4/90—Selection of catalytic material
- H01M4/92—Metals of platinum group
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
- H01M2004/8678—Inert electrodes with catalytic activity, e.g. for fuel cells characterised by the polarity
- H01M2004/8689—Positive electrodes
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/10—Fuel cells with solid electrolytes
- H01M8/12—Fuel cells with solid electrolytes operating at high temperature, e.g. with stabilised ZrO2 electrolyte
- H01M2008/1293—Fuel cells with solid oxide electrolytes
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M2300/00—Electrolytes
- H01M2300/0017—Non-aqueous electrolytes
- H01M2300/0065—Solid electrolytes
- H01M2300/0068—Solid electrolytes inorganic
- H01M2300/0071—Oxides
- H01M2300/0074—Ion conductive at high temperature
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M2300/00—Electrolytes
- H01M2300/0017—Non-aqueous electrolytes
- H01M2300/0065—Solid electrolytes
- H01M2300/0082—Organic polymers
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
- Y02E60/50—Fuel cells
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- Chemical & Material Sciences (AREA)
- Electrochemistry (AREA)
- Life Sciences & Earth Sciences (AREA)
- Health & Medical Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Biochemistry (AREA)
- Physics & Mathematics (AREA)
- Analytical Chemistry (AREA)
- Molecular Biology (AREA)
- General Health & Medical Sciences (AREA)
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- Immunology (AREA)
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- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- General Chemical & Material Sciences (AREA)
- Measuring Oxygen Concentration In Cells (AREA)
Abstract
Abstract An activated oxygen gas sensor element having an increased voltage output under rich gas conditions, short switching response and reduced internal resistance is produced by chemically treating the inner conductive catalyst electrode of the sensor element with an inorganic acid or acid salt and current activating the outer conductive catalyst electrode by applying a direct current to the sensor element, with the outer electrode as an anode, while the outer electrode is at an elevated temperature and in the presence of a nonoxidizing atmosphere.
Description
11~92~0 REFERENCE TO RELATED APPLICATION
Reference is made herein to our earlier Canadian Patent Application Serial No. 319,639, filed January 15, 1979.
Background of the Invention As discussed in our earlier application, oxygen gas sensors containing solid electrolyte oxygen gas sensor elements are used to measure the oxygen content of an automotive exhaust gas for the purpose of regulating the eficiency of the engine through control of the air to fuel 1~ ratio. These generally thimble-shaped sensor elements having an inner conductive catalyst electrode on the inner surface of the thimble and an outer conductive catalyst electrode on the outer surface of the thimble are conductively connected to a monitoring and actuating system to adjust said air-fuel ratio.
In our earlier application, the use of a chemical treatment, wherein the inner electrode was contacted with an inorganic acid or acid salt, produced a chemical treatment of the inner electrode and resulted in an increased voltage output in the positive range for the sensor element and also reduced the internal resistance of .
q~
.~ ~
dm~ J
' lll~Z~iO
the solid electrolyte sensor element, both of which are beneficial to the operation of the sensor. Also, as discussed thereinJ
when the chemically activated sensor elements are also subiected to a current treatment, wherein the sensor element is subjected to a direct current, with the outer catalytic electrode as a cathode, and at an elevated temperature and in the presence of a reducing gas, the above properties are further enhanced and, in addition, the switching response time required for swi~ching from rich to lean gas composition readings is reduced.
In using the combined chemical and current activation treatment of our previous application, however, the need for the presence of a reducing gas at the outer electrode during current activation was p.resent, as well as the need for a recovery period during which the sensor element was maintained at the elevated temperature, in order to provide a stable condition within the solid electrolyte body.
We have now discovered that if current activation of the sensor element is combined with the chemical activation, where the current activation is carried out by applying a direct current to the sensor element, with the outer electrode as an anode, only a nonoxidizing gas need be present and, in addition, the need for a recovery period is removed, where short time periods of current application are used.
Summar of the Invention Y
An activated oxygen gas sensor element having an increased voltage output under rich gas conditions, shortened switching response time and reduced internal resistance, where the element comprises a solid electrolyte body, such as zirconium dioxide, having an inner conductive catalyst electrode on the inner surface and an outer conductive catalyst electrode on the outer surface, is produced by contacting the inner conductive catalyst electrode with an inorganic acid or acid salt and by applying a direct current to the sensor element, with the outer electrode as an anode, with the current application effected while the outer electrode is in the presence of a nonoxidizing atmosphere and at a temperature in excess of 450C, the current density thereof being at least 5 milliamperes per square centimeter of the planar surface of the outer conductive catalyst electrode.
DETAILED DESCRIPTION
The gas sensor elements that are subjected to the present prccess to improve the properties thereof are generally in the shape of a closed tubuIar me~ber, thimble-like, with the sensor body formed of a solid electrolyte material, such as stabilized zirconium dioxide. mis general shape of the electrolyte body is known in the æ t, as well as the solid electrolyte usable~ The thimble-like shape of such sensor element, having a shoulder at the open end thereof, is illustrated in U.S. 3,978,006 issued August 31, 1976, and other existing publications, which also describe various solid electrolyte materials useful in forming such sensor elements.
The preferred composition for forming the solid electrolyte body is a mixture of zirconium dioxide and stabilizing materials such as calcium oxide or yttrium oxide.
~ To the interior surface of the electrolyte body, an inner electrode of conductive catalyst material is applied, such as by the coating of the surface with a platinum paste with or without a glass frit or other high temperature-resistance bonding material. This paste coating generally covers the interior surface of the closed terminal end and exte~ds to the shoulder of the electrolyte body. m is oombination is then fired at a temperature of 600-1000C or higher, as is kncwn in the art, for a sufficient period of time to convert the platinum paste to an electrically conductive inner elec~rode.
- A glass frit or other bonding agent, when used, while providing excellent adherence of the catalytic electrode to the interior surface of the X
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.
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solid electrolyte body, has an effect of increasing the internal electrical resistance of the sensor, reducing the positive output voltage of the sensor when the external surface thereof is exposed to a rich atmosphere and also causing a negative voltage output when the external surface thereof is exposed to a lean at~osphere.
As described in our co-pending application, Serial No. 319,639, the conductive catalyst electrode on the interior surface of the solid electrolyte body is subjected to a chemical activation treatment to improve the voltage output and to reduce the internal resistance of the sensor element. The treatment of the inner conductive catalyst electrode is by contact of the surface thereof with a solution of an inorganic acid or an acid salt. Solutions of an inorganic acid, such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, hydrofluoric acid and chloroplatinic acid, are preferred while acid salts, such as ammonium chloride, hydroxylamine hydrochloride, ammonium chloroplatinate or the like, are also usable.
In treating the conductive catalyst electrode with an acidic or acid slat solution, the electrode may be contacted with the solution and the same held in contact for a period of time before removing the solution and rinsing, or the electrode in contact with the solution may be heated to evaporate solvent from the solution and then heated further to elevated temperatures in the range of up to 1200C.
In addition to the aforedescribed chemical activation of the sensor element inner electrode, the outer electrode is subjected to a current activation treatment.
Both conductive catal~st electrodes, as is known, may comprise platinum or a platinum family-metal catalyst, such as palladium, rhodium or mixtures thereof, with the preferred material being platinum.
In the current activation treatment step of the present invention, a direct current is applied to the sensor element, with the outer conductive ~ .
n~ -4-ws/ ~
.`,, 11192~0 catalyst electrode as an anode, in the presence of a nonoxidizing gas and at an elevated temperature. m is current ac-tivation is described in detail in application Serial No. 327,798 of one of the inventors hereof, Ching T. Young, entitled "Process for Producing a Solid Electrolyte Oxygen Gas Sensing Element,"
filed on even date herewith. As described in said co-pending application, filed on even date herewith, the outer surface of the solid electrolyte body, with the outer conductive catalytic electrode thereon, is subjected to a nonoxidizing atmosphere, and while the outer surface is at a temperature in excess of 450C, a direct current is applied to the sensing element, with the outer electrode as an anode, the current density thereof being at least 5 milliamperes per squ æ e centimeter of the planar surface of said outer conductive catalyst electrode.
The nonoxidizing atm.osphere, to which the outer electrode is subjected, during the current activation step may be a reducing, neutral or inert atmosphere, provided that the atmosphere is nonoxidizing. Carbon monoxide, hydrogen or richexhaust gas mixtures are examples of reducing atmospheres, while nitrogen is the preferred neutral gas, and argon is an example of an inert gas. Mixtures of a reducing gas and a neutral or inert gas may, of course, be used, and a small amount of water vapor may also be present in the gaseous mixture.
The temperature to which the outer surface is heated prior to application of the direct current is about 450C and may he as high as about 1100C depending upon the solid electrolyte used and the other process conditions. A preferred temperature range of 600-900C provides an economical and efficient temperature range for the current activation.
Application of the direct current is made, to the sensor element, with the outer conductive catalytic electrode at the elevated temperature and in the presence of a nonoxidizing gas, with the outer electrode as an anode ws/~
, .
~llsz~o and the inner conductive catalytic electrode as a cathode.
A direct current power source is thus connected to the conductive catalyst electrodes, with the outer electrode connected to the positive terminal and the inner electrode connected to the negative terminal of the power source.
The current charge usa~le in the current activation step is one which provides a current density of at least 5 milliamperes per square centimeter af the planar surface of the outer conductive catalyst electrode. The term "current density," as used herein, is determined by dividing the current(in milliamperes) by the planar surface area of the outer conductive catalyst electrode (cm2) on the outer surface of the solid electrolyte body, while the term "planar surface of the outer èlec~rode" is used to define the surface that would be present if the conductive catalyst electrode were a smooth coating without porosity.
lhe preferred range of current density is between 20-150 milliamperes per square centimeter of the outer conductive catalys~ electrode surface. Current densities below 5 milliamperes/cm2 are ineffective to give the beneficial results and, while much higher current densities can be used, higher current densities far above the preferred range can cause fracturing of the element through shock.
The application of the direct current, as above described, for a period of only about two seconds has been fQund to provide the desired properties, while a time of current application of six seconds to about ten minutes is preferred. The longer times of current application, however, may require a recovery period for stabilization of the solid electrolyte. Such a recovery period is effected by maintaining the outer surface af the sensor element in the presence of a nonoxidizing gas and at the elevated temperature for a period of time after the current is turned off.
The following examples further illustrate the present invention. In these examples, the testing of thimbles, as sensor elements, to determine their ~llgZ;~O
performance in terms of voltage output under rich and lean conditions, the switching response to gas variation and their internal resistance, was made by inserting the thimbles into protective housings with conductive leads connected to the inner and outer electrodes to form sensors. The tests were conducted at 350C and a~ 800C
with testing at 800C effected first.
The sensor performance tests were conducted by inserting the sensors into a cylindrical metal tube and exposing them to oxidizing and reducing gaseous atmospheres within the tube through use of a gas burner adjustable to produce such atmospheres. Sensors placed in the desired positions in the tube were heated to testing temperature and the voltage output measured USihg ~ volt meter. The output was also connected to an oscilloscope to measure the speed of response of the sensor when the burner flame is changed from rich to lean and from lean to rich. A routine test consisted of setting the fLame to rich condition, measuring the voltage output of the sensor, switching the flame suddenly to lean condition, triggering the oscilloscope sweep at the same time to record the rich to lean switch of the sensor, switching the flame suddenly back to rich condition, again triggering the oscillo~cope to record the sensor output change, and finally adjusting the flame to a lean condition and measuring the sensor output voltage. The switching time is defined as the time period required for the output voltage, as recorded on the oscilloscope, to sweep between 600 and 300 millivolts. When the sensor output voltage under rich gas condition is less than 600 millivolts, the switching response time is not determlnable (n/d) according to the criteria used for this switching response measurement. Rich voltage output measurements were then made with different known values of shunting resistance across the sensor terminals. These -35 measurements provided data for calculating the internal resistance of the sensors.
A series of gas sensor electrolyte body thimbles 92~0 was prepared, for use in the following examples, from ball-milled zirconia, yttria and alumina, in a ratio of 80%, 14% and 6% by weight respectively, b~ isostatically pressing the same in the desired thimble shape and firing at high temperature.
Example I
Three of the series of electrolyte body thimbles (AEN-1, AEN-2 and AEN-3) had an inner electrode applied to the inner surface thereof by coating the inner surface with a platinum suspension containing a glass frit for bonding purposes. The thimble with its inner electrode was then heated in an oxidizing atmosphere to burn off the organic constituents of the suspension and bond the platinum to the zirconia surface. The external platinum catalyst electrode was next applied to the outer surface of the thimble by known thermal vapor deposition. A porous ceramic coating was applied over the external catalyst layer for protection. The thimbles were then formed into sensors and tested as to voltage output, switching response and internal resistance, as hereinbefore described. The results of the tests are listed in Table I under the designation "No Treatment."
The thimbles were then subjected to chemical treatment by applying to the inner surface thereof an aqueous solution of one normal hydrochloric acid by filling the interior portion of the thimbles with the acid. The sensors were maintained at 50C for a thirty minute period, and the acid solution was then removed and the interior of the sensor element washed with distilled water and dried at 100C for at least one hour. These sensor elements were then again tested as to voltage autput, swltching response and internal resistance. The results of these tests are listed in Table I under the heading "After Chemical Treatment." After this testing was effected, the sensor elements were subjected to current activation as follows.
The sensor elements, as sensors ~n a protective housing and with conductive leads, were inserted into a manifold with the outer surface of the sensor element, having the outer conductive catalyst coating thereon,exposed to a flow of reducing gas, 0.5% carbon monoxide in nitrogen (with 0.01 mg/cm3 water vapor), at a flow rate of 710 cm3/min. The elements were preheated to about 700C during a ten~minute period. The inner conductive catalyst electrode was in contact with air, and the temperature of the sensor was taken at the bottom of the inner region of the sensor element. The sensors were then subjected to a direct current, as indicated, for a ten-minute period, the direct current charge applied with the outer electrode as an anode at a current density of about 167 milliamperes/cm2 of the outer electrode planar surface, with the ga9 flow continued.
The direct current was then stopped and the sensor element allowed a recovery pe~iod of ten minutes at said temperature and with the outer electrode in said gas flow.
These sensor elements were then again tested as to voltage output, switching response and internal resistance. The results of these tests are listed in Table I under the heading "After Chemical Treatment and Current Activation."
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11192;~0 The tests results shown in Table I indicate the effect of the chemical treatment upon the voltage output and internal resistance of the sensor element, and also the effect o the combination of the chemical treatment and current activation upon the element, with resultant high voltage output, low internal resistance and significantly shortened response time.
Example~
Three other thimbles of the series of electrolyte - 10 body thimbles (AP7-11, AP7-12 and AP7-13) had inner and outer electrodes applied thereto as such application was effected in Example I. These three sensor elements were then chemically treated by applying to the inner surface - thereof a 2N aqueous solution (2 gram eguivalent per liter of solution) of hydrochloric acid. The inner thimble portion was filled with the acid to cover the inner electrode, the sensor heated to 50C for 0.5 hr. and, after pouring out the acid, the inner portion was rinsed twice with methanol. These three elements were then tested as to voltage output, switching response and internal resistance.
The results of the tests are listed in Table II under the designation "Chem. Treated." These three sensor elements were then sub~ected to current activation by insertion into a manifold with the outer conductive catalyst coating exposed to a flow of nitrogen atmosphere (710 cm3/min.) while the elements were heated to 750C during a ten-minute period. At a temperature of 750C, and with the outer electrode subjected to the nitrogen atmosphere, the sensor elements had applied thereto a direct current, with the outer electrode as an anode, the current density of which is listed in Table II for a period of ten minutes. The direct current was then stopped and the sensor elements allowed a recovery period of ten minutes, with the outer conductive catalytic electrode in the flow of nitrogen~ and ;35 at the elevated temperature. These sensor elements were then again tested. The test results are listed in Table II
;under the heading "After Chemical Treatment and Current Activation." -11 L
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As illustrated by the test results listed in Table II, the use of current densities as low as 8 ma/cm Eor a ten -minute period at 750C are effective in shortening the response time of the sensor element following the present process, although the degree of improvement is not as great as when current densities of 20 or 100 ma/cm2 are used.
Example III
Four other of the series of electrolyte body thimbles (AP7-10, AP7-21, AP7-22 and AP7-23) had inner and outer electrodes applied thereto as such application was effected in Example I. These four sensor elements were then chemically treated following the procedure described in Example`II. These four sensor elements were then tested.
The results of the tests are listed in Table III under the designation "Chem. Treated." These four sens~r elements were then subjected to current activation according to the procedure described in Example II, except that the current density used for each was 100 milliamperes/cm ; the temperature used for AP7-23 was 600C; and the time of application of the direct current as well as the recovery time were varied for the four sensors, these values being listed in Table III. These four sensor elements were again tested as to voltage output, switching response and internal resistance. The results of these tests are listed in Table III under the heading "After Chemical Treatment and Current Activation."
~' .
Reference is made herein to our earlier Canadian Patent Application Serial No. 319,639, filed January 15, 1979.
Background of the Invention As discussed in our earlier application, oxygen gas sensors containing solid electrolyte oxygen gas sensor elements are used to measure the oxygen content of an automotive exhaust gas for the purpose of regulating the eficiency of the engine through control of the air to fuel 1~ ratio. These generally thimble-shaped sensor elements having an inner conductive catalyst electrode on the inner surface of the thimble and an outer conductive catalyst electrode on the outer surface of the thimble are conductively connected to a monitoring and actuating system to adjust said air-fuel ratio.
In our earlier application, the use of a chemical treatment, wherein the inner electrode was contacted with an inorganic acid or acid salt, produced a chemical treatment of the inner electrode and resulted in an increased voltage output in the positive range for the sensor element and also reduced the internal resistance of .
q~
.~ ~
dm~ J
' lll~Z~iO
the solid electrolyte sensor element, both of which are beneficial to the operation of the sensor. Also, as discussed thereinJ
when the chemically activated sensor elements are also subiected to a current treatment, wherein the sensor element is subjected to a direct current, with the outer catalytic electrode as a cathode, and at an elevated temperature and in the presence of a reducing gas, the above properties are further enhanced and, in addition, the switching response time required for swi~ching from rich to lean gas composition readings is reduced.
In using the combined chemical and current activation treatment of our previous application, however, the need for the presence of a reducing gas at the outer electrode during current activation was p.resent, as well as the need for a recovery period during which the sensor element was maintained at the elevated temperature, in order to provide a stable condition within the solid electrolyte body.
We have now discovered that if current activation of the sensor element is combined with the chemical activation, where the current activation is carried out by applying a direct current to the sensor element, with the outer electrode as an anode, only a nonoxidizing gas need be present and, in addition, the need for a recovery period is removed, where short time periods of current application are used.
Summar of the Invention Y
An activated oxygen gas sensor element having an increased voltage output under rich gas conditions, shortened switching response time and reduced internal resistance, where the element comprises a solid electrolyte body, such as zirconium dioxide, having an inner conductive catalyst electrode on the inner surface and an outer conductive catalyst electrode on the outer surface, is produced by contacting the inner conductive catalyst electrode with an inorganic acid or acid salt and by applying a direct current to the sensor element, with the outer electrode as an anode, with the current application effected while the outer electrode is in the presence of a nonoxidizing atmosphere and at a temperature in excess of 450C, the current density thereof being at least 5 milliamperes per square centimeter of the planar surface of the outer conductive catalyst electrode.
DETAILED DESCRIPTION
The gas sensor elements that are subjected to the present prccess to improve the properties thereof are generally in the shape of a closed tubuIar me~ber, thimble-like, with the sensor body formed of a solid electrolyte material, such as stabilized zirconium dioxide. mis general shape of the electrolyte body is known in the æ t, as well as the solid electrolyte usable~ The thimble-like shape of such sensor element, having a shoulder at the open end thereof, is illustrated in U.S. 3,978,006 issued August 31, 1976, and other existing publications, which also describe various solid electrolyte materials useful in forming such sensor elements.
The preferred composition for forming the solid electrolyte body is a mixture of zirconium dioxide and stabilizing materials such as calcium oxide or yttrium oxide.
~ To the interior surface of the electrolyte body, an inner electrode of conductive catalyst material is applied, such as by the coating of the surface with a platinum paste with or without a glass frit or other high temperature-resistance bonding material. This paste coating generally covers the interior surface of the closed terminal end and exte~ds to the shoulder of the electrolyte body. m is oombination is then fired at a temperature of 600-1000C or higher, as is kncwn in the art, for a sufficient period of time to convert the platinum paste to an electrically conductive inner elec~rode.
- A glass frit or other bonding agent, when used, while providing excellent adherence of the catalytic electrode to the interior surface of the X
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.
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solid electrolyte body, has an effect of increasing the internal electrical resistance of the sensor, reducing the positive output voltage of the sensor when the external surface thereof is exposed to a rich atmosphere and also causing a negative voltage output when the external surface thereof is exposed to a lean at~osphere.
As described in our co-pending application, Serial No. 319,639, the conductive catalyst electrode on the interior surface of the solid electrolyte body is subjected to a chemical activation treatment to improve the voltage output and to reduce the internal resistance of the sensor element. The treatment of the inner conductive catalyst electrode is by contact of the surface thereof with a solution of an inorganic acid or an acid salt. Solutions of an inorganic acid, such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, hydrofluoric acid and chloroplatinic acid, are preferred while acid salts, such as ammonium chloride, hydroxylamine hydrochloride, ammonium chloroplatinate or the like, are also usable.
In treating the conductive catalyst electrode with an acidic or acid slat solution, the electrode may be contacted with the solution and the same held in contact for a period of time before removing the solution and rinsing, or the electrode in contact with the solution may be heated to evaporate solvent from the solution and then heated further to elevated temperatures in the range of up to 1200C.
In addition to the aforedescribed chemical activation of the sensor element inner electrode, the outer electrode is subjected to a current activation treatment.
Both conductive catal~st electrodes, as is known, may comprise platinum or a platinum family-metal catalyst, such as palladium, rhodium or mixtures thereof, with the preferred material being platinum.
In the current activation treatment step of the present invention, a direct current is applied to the sensor element, with the outer conductive ~ .
n~ -4-ws/ ~
.`,, 11192~0 catalyst electrode as an anode, in the presence of a nonoxidizing gas and at an elevated temperature. m is current ac-tivation is described in detail in application Serial No. 327,798 of one of the inventors hereof, Ching T. Young, entitled "Process for Producing a Solid Electrolyte Oxygen Gas Sensing Element,"
filed on even date herewith. As described in said co-pending application, filed on even date herewith, the outer surface of the solid electrolyte body, with the outer conductive catalytic electrode thereon, is subjected to a nonoxidizing atmosphere, and while the outer surface is at a temperature in excess of 450C, a direct current is applied to the sensing element, with the outer electrode as an anode, the current density thereof being at least 5 milliamperes per squ æ e centimeter of the planar surface of said outer conductive catalyst electrode.
The nonoxidizing atm.osphere, to which the outer electrode is subjected, during the current activation step may be a reducing, neutral or inert atmosphere, provided that the atmosphere is nonoxidizing. Carbon monoxide, hydrogen or richexhaust gas mixtures are examples of reducing atmospheres, while nitrogen is the preferred neutral gas, and argon is an example of an inert gas. Mixtures of a reducing gas and a neutral or inert gas may, of course, be used, and a small amount of water vapor may also be present in the gaseous mixture.
The temperature to which the outer surface is heated prior to application of the direct current is about 450C and may he as high as about 1100C depending upon the solid electrolyte used and the other process conditions. A preferred temperature range of 600-900C provides an economical and efficient temperature range for the current activation.
Application of the direct current is made, to the sensor element, with the outer conductive catalytic electrode at the elevated temperature and in the presence of a nonoxidizing gas, with the outer electrode as an anode ws/~
, .
~llsz~o and the inner conductive catalytic electrode as a cathode.
A direct current power source is thus connected to the conductive catalyst electrodes, with the outer electrode connected to the positive terminal and the inner electrode connected to the negative terminal of the power source.
The current charge usa~le in the current activation step is one which provides a current density of at least 5 milliamperes per square centimeter af the planar surface of the outer conductive catalyst electrode. The term "current density," as used herein, is determined by dividing the current(in milliamperes) by the planar surface area of the outer conductive catalyst electrode (cm2) on the outer surface of the solid electrolyte body, while the term "planar surface of the outer èlec~rode" is used to define the surface that would be present if the conductive catalyst electrode were a smooth coating without porosity.
lhe preferred range of current density is between 20-150 milliamperes per square centimeter of the outer conductive catalys~ electrode surface. Current densities below 5 milliamperes/cm2 are ineffective to give the beneficial results and, while much higher current densities can be used, higher current densities far above the preferred range can cause fracturing of the element through shock.
The application of the direct current, as above described, for a period of only about two seconds has been fQund to provide the desired properties, while a time of current application of six seconds to about ten minutes is preferred. The longer times of current application, however, may require a recovery period for stabilization of the solid electrolyte. Such a recovery period is effected by maintaining the outer surface af the sensor element in the presence of a nonoxidizing gas and at the elevated temperature for a period of time after the current is turned off.
The following examples further illustrate the present invention. In these examples, the testing of thimbles, as sensor elements, to determine their ~llgZ;~O
performance in terms of voltage output under rich and lean conditions, the switching response to gas variation and their internal resistance, was made by inserting the thimbles into protective housings with conductive leads connected to the inner and outer electrodes to form sensors. The tests were conducted at 350C and a~ 800C
with testing at 800C effected first.
The sensor performance tests were conducted by inserting the sensors into a cylindrical metal tube and exposing them to oxidizing and reducing gaseous atmospheres within the tube through use of a gas burner adjustable to produce such atmospheres. Sensors placed in the desired positions in the tube were heated to testing temperature and the voltage output measured USihg ~ volt meter. The output was also connected to an oscilloscope to measure the speed of response of the sensor when the burner flame is changed from rich to lean and from lean to rich. A routine test consisted of setting the fLame to rich condition, measuring the voltage output of the sensor, switching the flame suddenly to lean condition, triggering the oscilloscope sweep at the same time to record the rich to lean switch of the sensor, switching the flame suddenly back to rich condition, again triggering the oscillo~cope to record the sensor output change, and finally adjusting the flame to a lean condition and measuring the sensor output voltage. The switching time is defined as the time period required for the output voltage, as recorded on the oscilloscope, to sweep between 600 and 300 millivolts. When the sensor output voltage under rich gas condition is less than 600 millivolts, the switching response time is not determlnable (n/d) according to the criteria used for this switching response measurement. Rich voltage output measurements were then made with different known values of shunting resistance across the sensor terminals. These -35 measurements provided data for calculating the internal resistance of the sensors.
A series of gas sensor electrolyte body thimbles 92~0 was prepared, for use in the following examples, from ball-milled zirconia, yttria and alumina, in a ratio of 80%, 14% and 6% by weight respectively, b~ isostatically pressing the same in the desired thimble shape and firing at high temperature.
Example I
Three of the series of electrolyte body thimbles (AEN-1, AEN-2 and AEN-3) had an inner electrode applied to the inner surface thereof by coating the inner surface with a platinum suspension containing a glass frit for bonding purposes. The thimble with its inner electrode was then heated in an oxidizing atmosphere to burn off the organic constituents of the suspension and bond the platinum to the zirconia surface. The external platinum catalyst electrode was next applied to the outer surface of the thimble by known thermal vapor deposition. A porous ceramic coating was applied over the external catalyst layer for protection. The thimbles were then formed into sensors and tested as to voltage output, switching response and internal resistance, as hereinbefore described. The results of the tests are listed in Table I under the designation "No Treatment."
The thimbles were then subjected to chemical treatment by applying to the inner surface thereof an aqueous solution of one normal hydrochloric acid by filling the interior portion of the thimbles with the acid. The sensors were maintained at 50C for a thirty minute period, and the acid solution was then removed and the interior of the sensor element washed with distilled water and dried at 100C for at least one hour. These sensor elements were then again tested as to voltage autput, swltching response and internal resistance. The results of these tests are listed in Table I under the heading "After Chemical Treatment." After this testing was effected, the sensor elements were subjected to current activation as follows.
The sensor elements, as sensors ~n a protective housing and with conductive leads, were inserted into a manifold with the outer surface of the sensor element, having the outer conductive catalyst coating thereon,exposed to a flow of reducing gas, 0.5% carbon monoxide in nitrogen (with 0.01 mg/cm3 water vapor), at a flow rate of 710 cm3/min. The elements were preheated to about 700C during a ten~minute period. The inner conductive catalyst electrode was in contact with air, and the temperature of the sensor was taken at the bottom of the inner region of the sensor element. The sensors were then subjected to a direct current, as indicated, for a ten-minute period, the direct current charge applied with the outer electrode as an anode at a current density of about 167 milliamperes/cm2 of the outer electrode planar surface, with the ga9 flow continued.
The direct current was then stopped and the sensor element allowed a recovery pe~iod of ten minutes at said temperature and with the outer electrode in said gas flow.
These sensor elements were then again tested as to voltage output, switching response and internal resistance. The results of these tests are listed in Table I under the heading "After Chemical Treatment and Current Activation."
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11192;~0 The tests results shown in Table I indicate the effect of the chemical treatment upon the voltage output and internal resistance of the sensor element, and also the effect o the combination of the chemical treatment and current activation upon the element, with resultant high voltage output, low internal resistance and significantly shortened response time.
Example~
Three other thimbles of the series of electrolyte - 10 body thimbles (AP7-11, AP7-12 and AP7-13) had inner and outer electrodes applied thereto as such application was effected in Example I. These three sensor elements were then chemically treated by applying to the inner surface - thereof a 2N aqueous solution (2 gram eguivalent per liter of solution) of hydrochloric acid. The inner thimble portion was filled with the acid to cover the inner electrode, the sensor heated to 50C for 0.5 hr. and, after pouring out the acid, the inner portion was rinsed twice with methanol. These three elements were then tested as to voltage output, switching response and internal resistance.
The results of the tests are listed in Table II under the designation "Chem. Treated." These three sensor elements were then sub~ected to current activation by insertion into a manifold with the outer conductive catalyst coating exposed to a flow of nitrogen atmosphere (710 cm3/min.) while the elements were heated to 750C during a ten-minute period. At a temperature of 750C, and with the outer electrode subjected to the nitrogen atmosphere, the sensor elements had applied thereto a direct current, with the outer electrode as an anode, the current density of which is listed in Table II for a period of ten minutes. The direct current was then stopped and the sensor elements allowed a recovery period of ten minutes, with the outer conductive catalytic electrode in the flow of nitrogen~ and ;35 at the elevated temperature. These sensor elements were then again tested. The test results are listed in Table II
;under the heading "After Chemical Treatment and Current Activation." -11 L
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As illustrated by the test results listed in Table II, the use of current densities as low as 8 ma/cm Eor a ten -minute period at 750C are effective in shortening the response time of the sensor element following the present process, although the degree of improvement is not as great as when current densities of 20 or 100 ma/cm2 are used.
Example III
Four other of the series of electrolyte body thimbles (AP7-10, AP7-21, AP7-22 and AP7-23) had inner and outer electrodes applied thereto as such application was effected in Example I. These four sensor elements were then chemically treated following the procedure described in Example`II. These four sensor elements were then tested.
The results of the tests are listed in Table III under the designation "Chem. Treated." These four sens~r elements were then subjected to current activation according to the procedure described in Example II, except that the current density used for each was 100 milliamperes/cm ; the temperature used for AP7-23 was 600C; and the time of application of the direct current as well as the recovery time were varied for the four sensors, these values being listed in Table III. These four sensor elements were again tested as to voltage output, switching response and internal resistance. The results of these tests are listed in Table III under the heading "After Chemical Treatment and Current Activation."
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Example IV
Seven additional thimbles of the series of electrolyte body thimbles, (AP7-15, AP7-14, AP7-16, AP7-17, 1~ AP7-19, AP7-18 and AP7-20) had inner and outer electrodes appli.ed as in Example I and were chemically treated following the procedure described in Example II. These seven sensor elements were then tested as to voltage output, switching respQnse and internal resistance. The results of the tests are listed in Table IV under the designation "Chem. Treated." The-seven sensor elements were then current activated according to the procedure described i.n Example II, except that the temperature for activation, the current density, and the time of passing of the direct current were varied for particular of the sensor elements as indicated in Table IV. The preheating time and recovery time were both ten minutes in each case. The seven sensor elements were then again tested. The results of these tests are l.isted in Table IV under the heading "After Chemical Treatment and Current Activation."
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The present process provides a combined chemical and current treatment of solid electrolyte sensor elements which results in sensor elements of improved properties of high voltage output under rich conditions, fast response time and low internal resistance, all such properties of which result in an efficient, economical and stable operation of oxygen gas sensors containing such elements.
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Example IV
Seven additional thimbles of the series of electrolyte body thimbles, (AP7-15, AP7-14, AP7-16, AP7-17, 1~ AP7-19, AP7-18 and AP7-20) had inner and outer electrodes appli.ed as in Example I and were chemically treated following the procedure described in Example II. These seven sensor elements were then tested as to voltage output, switching respQnse and internal resistance. The results of the tests are listed in Table IV under the designation "Chem. Treated." The-seven sensor elements were then current activated according to the procedure described i.n Example II, except that the temperature for activation, the current density, and the time of passing of the direct current were varied for particular of the sensor elements as indicated in Table IV. The preheating time and recovery time were both ten minutes in each case. The seven sensor elements were then again tested. The results of these tests are l.isted in Table IV under the heading "After Chemical Treatment and Current Activation."
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The results listed in Table IV illustrate the effect of various temperatures, current densities and times of current application on the sensor properties. While the use of 450C in the test listed did not produce acceptable sensors for operational purposes, it should be noted that these sensors had inner electrodes of fluxed platinum (containing a glass or other bo~ding material) and the use of such a temperature where no flux is used on the inner electrode would provide the shortening of the switching response time desired.
The present process provides a combined chemical and current treatment of solid electrolyte sensor elements which results in sensor elements of improved properties of high voltage output under rich conditions, fast response time and low internal resistance, all such properties of which result in an efficient, economical and stable operation of oxygen gas sensors containing such elements.
.
Claims (16)
1. A process for producing an activated oxygen gas sensor element having an increased voltage output under rich gas conditions, shortened switching response time and reduced internal resistance, wherein the sensor element comprises a solid electrolyte body having an inner conductive catalyst electrode on the inner surface and an outer conductive catalyst electrode on the outer surface thereof, comprising:
a) contacting said inner conductive catalyst electrode with an acidic reactant selected from the group consisting of inorganic acids and acid salts;
and b) applying a direct current to the sensor element, with said outer conductive catalyst electrode as an anode, while subjecting said outer conductive catalyst electrode to a nonoxidizing atmosphere at an elevated temperature in excess of 450°C, the current density thereof being at least 5 milliamperes per square centimeter of the planar surface of said outer conductive catalyst electrode.
a) contacting said inner conductive catalyst electrode with an acidic reactant selected from the group consisting of inorganic acids and acid salts;
and b) applying a direct current to the sensor element, with said outer conductive catalyst electrode as an anode, while subjecting said outer conductive catalyst electrode to a nonoxidizing atmosphere at an elevated temperature in excess of 450°C, the current density thereof being at least 5 milliamperes per square centimeter of the planar surface of said outer conductive catalyst electrode.
2. The process for producing an activated oxygen gas sensor element as defined in claim 1 wherein said nonoxidizing gas is selected from the group consisting of reducing gases, neutral gases, inert gases and mixtures thereof.
3. The process for producing an activated oxygen gas sensor element as defined in claim 2 wherein said nonoxidizing gas is a reducing gas.
4. The process for producing an activated oxygen gas sensor element as defined in claim 2 wherein said nonoxidizing gas is a neutral gas.
5. The process for producing an activated oxygen gas sensor element as defined in claim 4 wherein said neutral gas is nitrogen.
6. The process for producing an activated oxygen gas sensor element as defined in claim 1 wherein said current density is between 20-150 milliamperes per square centimeter of the planar surface of said outer conductive catalyst electrode.
7. The process for producing an activated oxygen gas sensor element as defined in claim 1 wherein said elevated temperature is between 600-900°C.
8. The process for producing an activated oxygen gas sensor element as defined in claim 1 wherein said direct current is applied for a period of time between six seconds and ten minutes.
9. The process for producing an activated oxygen gas sensor element as defined in claim 1 wherein, following application of said current, said sensor element is maintained at said temperature for a period of time.
10. The process for producing an activated oxygen gas sensor element as defined in claim 1 wherein said solid electrolyte body comprises zirconium dioxide.
11. The process for producing an activated oxygen gas sensor element as defined in claim 1 wherein said inner conductive catalyst electrode and said outer conductive catalyst electrode comprise platinum.
12. The process for producing an activated oxygen gas sensor element as defined in claim 11 wherein said inner conductive catalyst electrode is bonded to said solid electrolyte body by a glass frit.
13. The oxygen gas sensor element produced according to the process of claim 1.
14. A process for producing an activated oxygen gas sensor element having an increased voltage output under rich gas conditions, shortened switching response time and reduced internal resistance, wherein the sensor element comprises a zirconium dioxide body having an inner platinum electrode on the inner surface and an outer platinum electrode on the outer surface thereof, comprising:
a) contacting said inner platinum electrode with an acidic reactant selected from the group consisting of inorganic acids and acid salts; and b) applying a direct current to the sensor element, with said outer platinum electrode as an anode, while subjecting said outer platinum electrode to a nonoxidizing atmosphere at an elevated temperature between 600-900°C, the current density thereof being between 20-150 milliamperes per square centimeter of the planar surface of said outer platinum electrode.
a) contacting said inner platinum electrode with an acidic reactant selected from the group consisting of inorganic acids and acid salts; and b) applying a direct current to the sensor element, with said outer platinum electrode as an anode, while subjecting said outer platinum electrode to a nonoxidizing atmosphere at an elevated temperature between 600-900°C, the current density thereof being between 20-150 milliamperes per square centimeter of the planar surface of said outer platinum electrode.
15. The process for producing an activated oxygen gas sensor element as defined in claim 14 wherein said nonoxidizing atmosphere comprises nitrogen.
16. The oxygen gas sensor element produced according to the process of claim 14.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US942,103 | 1978-09-13 | ||
| US05/942,103 US4169777A (en) | 1978-03-13 | 1978-09-13 | Process for producing an activated oxygen gas sensor element |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1119250A true CA1119250A (en) | 1982-03-02 |
Family
ID=25477582
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000327619A Expired CA1119250A (en) | 1978-09-13 | 1979-05-15 | Process for producing an activated oxygen gas sensor element |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPS5539097A (en) |
| AU (1) | AU523659B2 (en) |
| CA (1) | CA1119250A (en) |
| DE (1) | DE2934637A1 (en) |
| FR (1) | FR2436387A1 (en) |
| GB (1) | GB2030303B (en) |
| IT (1) | IT1123562B (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4253934A (en) * | 1979-04-17 | 1981-03-03 | General Motors Corporation | Aging treatment for exhaust gas oxygen sensor |
| JP2002048758A (en) | 2000-07-31 | 2002-02-15 | Denso Corp | Gas sensor element and its manufacturing method |
Family Cites Families (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1135097A (en) * | 1965-07-14 | 1968-11-27 | Komyo Rikagaku Kogyo Kabushiki | Improvements relating to the measurement of combustible gas concentrations |
| FR2034276A2 (en) * | 1969-03-06 | 1970-12-11 | Comp Generale Electricite | Solid electrolyte fuel cell |
| DE2065217A1 (en) * | 1970-09-10 | 1972-12-14 | Taguchi N | Method of manufacturing a gas sensor. Eliminated from: 2044851 |
| US3978006A (en) * | 1972-02-10 | 1976-08-31 | Robert Bosch G.M.B.H. | Methods for producing oxygen-sensing element, particularly for use with internal combustion engine exhaust emission analysis |
| CA1096441A (en) * | 1976-12-15 | 1981-02-24 | Anthony V. Fraioli | Electrochemical cell having enhanced-surface non- conducting solid electrolyte and method of making same |
| JPS6011428B2 (en) * | 1977-01-31 | 1985-03-26 | 日本碍子株式会社 | Manufacturing method of oxygen concentration battery |
| US4080276A (en) * | 1977-04-25 | 1978-03-21 | Bendix Autolite Corporation | Gas sensor with protective coating and method of forming same |
| US4136000A (en) * | 1978-03-13 | 1979-01-23 | Bendix Autolite Corporation | Process for producing improved solid electrolyte oxygen gas sensors |
-
1979
- 1979-05-15 CA CA000327619A patent/CA1119250A/en not_active Expired
- 1979-07-31 GB GB7926566A patent/GB2030303B/en not_active Expired
- 1979-08-20 AU AU50101/79A patent/AU523659B2/en not_active Ceased
- 1979-08-28 DE DE19792934637 patent/DE2934637A1/en not_active Ceased
- 1979-09-07 IT IT25535/79A patent/IT1123562B/en active
- 1979-09-11 JP JP11577879A patent/JPS5539097A/en active Granted
- 1979-09-13 FR FR7922905A patent/FR2436387A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| IT7925535A0 (en) | 1979-09-07 |
| AU523659B2 (en) | 1982-08-05 |
| GB2030303B (en) | 1983-03-02 |
| FR2436387A1 (en) | 1980-04-11 |
| DE2934637A1 (en) | 1980-03-27 |
| JPS5539097A (en) | 1980-03-18 |
| IT1123562B (en) | 1986-04-30 |
| GB2030303A (en) | 1980-04-02 |
| FR2436387B1 (en) | 1982-09-17 |
| JPS6244613B2 (en) | 1987-09-21 |
| AU5010179A (en) | 1980-03-20 |
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| Date | Code | Title | Description |
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| MKEX | Expiry |