CA1128610A - Ceramic element sensor - Google Patents
Ceramic element sensorInfo
- Publication number
- CA1128610A CA1128610A CA343,134A CA343134A CA1128610A CA 1128610 A CA1128610 A CA 1128610A CA 343134 A CA343134 A CA 343134A CA 1128610 A CA1128610 A CA 1128610A
- Authority
- CA
- Canada
- Prior art keywords
- sensor
- ceramic element
- charge transfer
- titania
- oxygen
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
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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
- G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
- G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
- G01N27/12—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid
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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
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T436/00—Chemistry: analytical and immunological testing
- Y10T436/20—Oxygen containing
- Y10T436/207497—Molecular oxygen
- Y10T436/208339—Fuel/air mixture or exhaust gas analysis
Abstract
ABSTRACT OF THE INVENTION
An improved sensor having a ceramic element that undergoes a change in an electrical characteristic in response to a change in the partial pressure of oxygen in a mixture of gases to which the ceramic element is exposed. Sensors of this type are used to detect the air-fuel ratio of mixtures supplied to internal combustion engines. Prior art titania and zirconia sensors are characterized by little change in their respective electrical characteristics at operating temperatures below about 350°C. A charge transfer material, platinum, has been applied to the ceramic element to facilitate or make possible the electron transfers required for sensor operation. Loss of the platinum charge transfer material by vaporization, as the result of operation at normal elevated temperatures, has been discovered to be the cause of a loss of sensor response at low sensor operating temperatures. The improved sensor has a charge transfer material comprised of an alloy of platinum and rhodium.
This alloy, which preferably is 90% platinum and 10%
rhodium, reduces the minimum temperature of operation for the sensor and substantially eliminates the afore-mentioned vaporization loss of the charge transfer material.
An improved sensor having a ceramic element that undergoes a change in an electrical characteristic in response to a change in the partial pressure of oxygen in a mixture of gases to which the ceramic element is exposed. Sensors of this type are used to detect the air-fuel ratio of mixtures supplied to internal combustion engines. Prior art titania and zirconia sensors are characterized by little change in their respective electrical characteristics at operating temperatures below about 350°C. A charge transfer material, platinum, has been applied to the ceramic element to facilitate or make possible the electron transfers required for sensor operation. Loss of the platinum charge transfer material by vaporization, as the result of operation at normal elevated temperatures, has been discovered to be the cause of a loss of sensor response at low sensor operating temperatures. The improved sensor has a charge transfer material comprised of an alloy of platinum and rhodium.
This alloy, which preferably is 90% platinum and 10%
rhodium, reduces the minimum temperature of operation for the sensor and substantially eliminates the afore-mentioned vaporization loss of the charge transfer material.
Description
IMPROVED CERAMIC ELEMENT SENSOR
This inven~ion relates ~o an Lmproved sensor of the type having a ceramic element tha~ undergoes a change in an electrical characteristic in response to a change in the partial pressure of oxygen in a mixture of gases to which the ceramic element is exposed. The ceramic element or the sensor may be either titania or zirconia under the current state of development, but other electxically responsive ceramics are known and may be used in the future.
The preferred titaria ceramic element is porous to provide a large sura~e area for effecting the transfer of oxygen from the titania to the gases to which the ceramic element is exposed and vice versa. The titania ceramic element has a porous or discontinuous coating of a precious metal charge transfer material. This material in ~he past has been platinum applied ~o the titania ceramic element by immersion in a solution con~aining platinum~
Sensors of the type having a zirconia ceramic element also utilize a porous pla~inum charge transfer material, but the zirconia ceramic is very dense and the platinum is applied to the zirconia surfaces by vapor deposition. The surface platinum to be exposed to engine exhaust gases is ; usually covered with a porous re~ractory material to aid in bonding and for the protection of the platinum~
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Sensors of the type di~cussed above are particularly suited for use in detecting excursions, above and below stoichiometry, of the air-fueI ratio of the mixture of air and fuel supplied to an internal comhustion engine. In accomplishing this detection, the sensor is positioned in the path of the exhaust gases emanating from the engine.
As the mixture supplied to the engine changes from rich to lean, the exhaust gases change from a composition including very little oxygen to a composition containing an excess of oxygen. As the exhaust gases change from lean to rich, the reverse changes in composition occur.
The sensors have an electrical characteristic that under-goes a step-function change as a xesult of the mixture excursions across the stoichiometric air-fuel ratio.
The titania ceramic material undergoes a change in its resistance as a function of the oxygen concentration gradient between the titania and the exhaust gases. The zirconia ceramic element undergoes a change in ~he EMF
produced across its platinum change transfer electrodes as a function of the oxygen concentration differential on opposite sides of the zirconia material. ~ith the zirconia sensor, a reference gas, usually air, is applied to one side of the zirconia and the exhaust gas composition is allowed to contact ~he other side of the zirconia. The use of a reerence gas is unnecessary in connection with titania sensors, and the entire titania ceramic element is immersed in the exhaust gases.
The present invention is particularly directed to a titania sensor, but has possible application to zirconia sensors and others if problems peculiar to these sensors are eliminated or become less extreme as the art progresses.
The specific problem solved by the present invention is the loss of response to air-fuel mixture variations that occurs with the prior art titania sensor as a result of its use over a reIatively short period of time. This loss of response occurs in the lower portion of the noxmal . .: ,, ........... . ..... .. , , , . . - ~
~ c~ 3 operating temperature range, which extends from about 300C
to ab~u~ 900~C~ The failure of the sensor to operate at low temperatures due to loss of its low temperature response is a ~ery serious problem because it means that t~e feedbac~
s fuel control system associated wit~ the sensor for con-trolling the mixture ratios supplied to an internal combustion engine cannot be operated un~il the exhaust gases have heated t~e sensor sufficiently to main~ain its temperature above that at which it is able to respond to LO air-fuel ratio variations. These may increase undesirable engine exhaust emissions and reduce fuel economy during engine warm-up conditions.
The inventors have discovered that the porous or discontinuous platinum coating on the porous titania cQramic element forms an oxide, PtO2, that vaporizes and that thereby i5 remo~ed from the titania element when it i exp~sed to the higher tempera~ures within the aforementioned normal range of operàting temperatures. These temperatures occur during normal sensor usage when the s~nscr is sub-jected to exhaust gases produced by lean mixtures havingexcass oxygen content. If the sensor temperatuxe is at about 850C during this time, the platinum oxide forms, vaporizes, and is lost. While this may not adversely affect the operation of th sensor when being used under high temperature conditions, it very ad~ers ly affects th~
sens~r with respect to its low-temperature operability and response time. T~is can cause ~he forementioned deleterious effects on the fuel control system.
It has been found that the vaporiæation loss of the 3~ platinum charge transfer material applied to the titania ceramlc element of an exhaust gas sensor can be sub-stantially eliminated. Also, the minimum operating temperature of the prior art titania sensorO even with its platinum charge transer material intact, can be ~ub-stantially reduced so that the sensor can be used earlier .
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;`' :, in the operation of a cold internal combustion engine to achieve closed-loop feedback air-fuel ratio control.
The present invention, therefore, provides an improved sensor of the type having a ceramic element that undergoes a change in an electrical characteristic of the ceramic element in response to a change in the partial pressure of oxygen in a mixture of gases to which the ceramic element is exposed, the ceramic element having electrodes connected thereto in spaced relationship and extending from the ceramic element to permit the resistance between them to be sensed, the ceramic element being porous to permit oxygen in the gases to which the ceramic element is exposed to migrate into and out of its interior regions, and the ceramic element having a discontinuous or porous charge transfer lS material deposited thereon.
The improvement of the invention comprises a rnajority of the charge transfer material by weight is Pt and Rh is included in an amount sufficient to allow the charge transfer material to be retained on the ceramic element when the ceramic element is exposed to gaseous mixtures produced by lean-mixture combustion at ceramic element temperatures in excess of about 850C.
The eleimination of the prior art problem and the additional advantages of the invention are obtained without degradation in the response time of the sensor. The sensor improvement is brought about by the addition of the rhodium, whic~ forms an oxide (RhO2) at elevated temperatures that is considerably more stable than is the oxide of platinum and increases the life of the charge transfer material.
The greater the stability of the oxide of the charge transfer material, the longer one would expec-t the sensor response time to be due to the lower electrical conductivity of the metal oxide and the interference of the oxide with oxygen transfer from and to the titania crystal structure.
Pure platinum would be expected to facilitate rapid transfer .
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of oxygen from the ceramic element to the exhaust gases and vice versa and, therefore, platinum in its pure form would be expected to be the optimum metal for a titania sensor charge transfer material. Unfortunately, the inventors have ~ound that the high vapor pressure of platinum and its oxide is responsible for its loss from the titania element during operation of the snesor above about 850 C.
Of course, if the titania sensor is not to be operated at temperatures and conditions at which platinum oxide is formed, then pure platinum or some other electrically conductive material mày be the "optimum" charge transfer material for such application. The charge transfer material is an elec-trical conductor; it is not regarded as a catalyst because it does not promote chemical reaction of exhaust gas con-stituents in its activity related to variation of the ceramicelectrical characteristic.
With a ~resh sensor having a charge transfer material of pure platinum, the prior art sens~r response time initially is very fast, ~ut as a result of vaporization of the platinum or its oxida at elevated sensor operating tempera~uras, the response time at low operating tempera-tures degrades substantially. The addition of rhodium, however, in accordance with this invention, to the platinum has been found to prevent the rapid vaporization of the charge transfer material and allow the sensor to respond to variations in the oxygen content of gases to which the sensor is exposed at considerably lower operating temperatures than is characteristic of the prior art device.
The invention may be better understood by reference to the detailed description which follows and to the accompanying drawings, wherein :
Figure 1 is an eleva~ional view of a ti~ania exhaus~
gas oxygen sensor suitable or installation in ~he i~take manifold of an external combustion engine;
Figure 2 is a sectional end view, taken along the line II-II in Figure 1, of the sensor of Figure 1 a~d is shown in enlarged scale;
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Figure 3 is a sectional view,taken along the line III-III in Figure 2, showing the internal structure of the sensor of Figures 1 and 2 also on an enlarged scale;
Figure 4 is a circuit di gram illus~rating the S manner in which the titania oxygen sensing element and the thermis~or shown in Figures 1 through 3 are electrically connected with circuitry deslgned to receive the sensor output voltage;
Figures ; and 6 are photomicrographs o~ the titania oxygen sensor and ~he thermistor illustrated in Figures 1 ~hrough 4;
Figure 7 is a graph of both the oxygen sens~r element and thermistor element resistance as a runction of tempera-ture over the normal operating range frsm about 30~C to about 900C;
Figure 8 is a graph of sensor output voltage as a percent of the input (reference) voltage versus equivalent air-fuel ratio;
Figure 9 is a graph illustrating the voltage response f the tita~ia sensor as a function o~ time with ar. air-fueI ratio that varies by about 0.1 ra~ios above and below stoichiometry;
Figure 10 is a graph of the response time of unaged titania sensors ~ersus the sensor tip temp~rature for both ~t and Pt/Rh charge ~ransfer materials;
Figure 11 is a gra~h of ~he response t me o~ an aged sensor versus the sensor tip temperat~re for both Pt and Pt/Rh charge transfer materials;
Figure 12 is a graph of titania s~nsor response time as a function of the percent composition of the charge transfer ma~erial on the titania oxygen sensor element;
and Figures 13 through 18 are graphs that illustrate the response time a~d ~oltage characteristics of the ~mproved titania sensors at different operating temperatures during and after durability testins to 850 hours on a~ engine dynamometer.
With p rticular reference now to Figures 1 through 3, wherein like numerals refer to ~ike parts in the several . . . .
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`' ' ~ ; , '' ' ~' ' ' ' , ~ .7_ views, there is shown a complet~ titania. exhaust gas sensor as5embly generally designate~ by t~e numeral 10. The sensor lO includes a steel. housing or body 12, which may be substantially identical to a typical spar~ pLug body, having a ~hreaded portion 14 for engagement. with a suitably threaded aperture provided within the exhaust system of an internal combustion engine (not shown). In most cases, . the sensor lO would be installed in an a~erture at a location in the exhaus~ manifold near the flange that would connec~ to an exhaust pipe. ~ ceramic insulator 1 extends through the body 12 and has a tapered portion 26 projecting outwardly trom the ~ody 12 into the volume de~ined by the boundaries of a perforated shield 18.
There are three longitudinal passages 20, 22 and 24 extending from the projecting end 26 of the c~ramic ; insulator to its opposite end 28. Wires 30, 32 and 34 æ e located i~ the respectiveIy corresponding passages 20, 22 and 24 and are o a.heat resistan~ character, preferably being made from an al.Ioy such as 80% ~ic~el 20% chromium wire.. These electrically conducti~e wires are welded to precious-metal wire leads 40, 42 and 44, which are ~mbedded in disc-shaped ceramic elements 46 and ~8.
Element 46 is a ceramic titania 2 sensor responsive to the partial pressure of oxygen in the gaseous medium to which this ~lement is axposed~ Sen~or el~,ment 46 may be fabricated in accordance with the teachings of U.S. Patents 3,886,785 issued June 3, 1975 and 3,332,246 issued January 13, lg76, both in the names of Stadler et al and assigned to Ford Motor Company. The teachings of the present inventlon must, however, also be considered in the fabrication of the oxygen sensing element 46. The present invention teaches the application to the porous oxygen sensor of a platinum alloy (atomic or fine particle mixture) charge transfer material for reasons which are hereinafter made clear.
The element 48 is a thermistor. The termistor may be made from titania ceramic material of greater density, , ' ',' - ' ~ ' .
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near its theoretical density, than the density of the porous titania oxygen sensor 46. Alternatively, the termistor 48 may be constructed in accordance with the teachings of U.S. Patent No. 4,162,431 in the names of LogothetiS, Laud and Park, entitled "Rare Earth - Yttrium, transition metal oxide Thermistors", and assigned to E~ord Motor Company. The thermistor 48 is intended to provide temperature compensation in accordance with the circuitry illustrated in Figure 4 and is intended to be substantially nonresponsive to variations in the partial pressure of oxygen in the gaseous medium to which it is exposed.
The sensor of ~igures 1 t~rough 3 is intended to be used in con~unction with electronic circuitry for closed-loop feedback contxol of the amount of fuel supplied to an internal combustion engine. T~e sensor indicates whether tne exhaust gases contain a substantial amount of 8C and CO or whether instead there is a substantial amount of oxygen, thereby, indicating whether or no~ the air-fuel ratio of the mixture supplied to ~he engine was rich or lean with respect to the stoichiometric value of a~out 14.7 parts of air to each part of fuel by weight. This air-uel ratio typically is expressed as a normalized air-fuel ratio lambda, whereln the actual ratio is divided by the stoichiometric value and the stoichiometric ratio therefore is represented as 1.0 in accordance with well known practice.
The exhaust gas sensor 10 has terminals S0, 52 and 54 designed for connec~ion`to external circuitry as speci~ied above to enable it to be used in a feedbac~ fuel control system~ With particular reference now to Figure 4, there is shown a circuIt that schematically represents the manner in wh~ch the sensor 10 is utilized in association ~ith such extexnal ci~cuitxy~ A DC source of requlated re~erence volt~ge 60 has its positi~e terminal connected to terminal 5a o~ the sensor o~ygen . .
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responsive elemen~ 46. The lead wires 40, 42 and 44 from the sensor 46 and thermistor 48 are welded or o~herwise joined, respectively, to lead wires 30, 32 and 34 to interconnect the two ceramic elements 46 and 4~ as shown.
The thermistor element 48 is connected ~hrough a response-shaping resistor 62 to ground potential at 64. The output voltage of the sensor 10 is taken between the sensor terminal 54 and ground potential. This signal is applied across the input impedance or load resistance RL (about two megohms) of the engine control electronic circuitry.
The input voltage to the circuit of Figure 4 is obtained from the source reference 60 and is applied across the voltage divider comprising the seriçs-connected variable resistances of oxygen sensor 46 and thermistor 48 is series with the response-shaping resistor 62. The output voltage is taken across the load resistance ~ ~
The resistance values of both the oxygen sensor 46 and the thermistor 48 vary as a function of temperature and in the same directiont that is, the resistance of these elements decreases with increasing temperature. As a result, the voltage di~iding effect provides an output voltage across the load resistance RL ~hat is independent o~ temperature. The oxygen sensor 46, however, has a resistance which varies not only with temperature but also with the partial pressure of oxygen in the gaseous medium to which the sensor is exposed. An increase in ~he resistance of the oxygen sen~or 46 causes the output voltage across the load RL to decrease, and a reduction in the resistance of the oxygen sensor causes a corresponding increase in the output voltage across ~he resistance RL.
Otherwise stated, an increase in oxygen conten~ in the gaseous medium surrounding the oxygen sensing device 46 causes its resistance to increase in a manner hereinafter described and thereby causes a reduction in the voltage across the load resistance ~. A decrease in the oxygen content o~ the gaseous medium causes ~he resistance o~ ~he oxygen sensor 46 to decrea~e in a corresponding manner .
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and this c~uses an incxe~se ln the voltage across the load resistance RL~
Figure 5 ~s a photo~icrograph o~ the t~tan~a oxyyen senso~ 46 ~t~ a ma~nification of 700 times size. Figure 6 ~s a p~otomicro~raph of a titania thermistor 48 with a similar magnif~cation of 700 times s~ze~ From the ~igure 5 titania sensor photom~cro~raph, it may be seen quite clearly that the oxygen sensor structure is ver~ porous~
Also~ its grain size is very small as compared to the much larger grain size o~ the titania thermistor~ which is much more dense and which lacks the porosity of the titania oxygen sensor.
Titanium d~oxide (titania~ is a material that occurs naturally in mixture with other minerals~ The titania is obtained by precipitation from a solution of minerals that include titania~ When thus obtained by precipitation, the titania has an anatase cr~stal structure. When the titania material in thi~ crystal structure is formed into an exhaust gas oxygen sensor~ it is first thermally treated in a manner that allows the crystal structure to change from anatase to rutile. An increase in the temperature of the rutile material above room temperature induces oxygen vacancies into ~he crystal structure. This results in ionization of the titanium atoms interstitiall~ located in the crystal structure. The concentration of the inter-stitial titanium ions and oxygen vacancies increase as temperature rises~ and ~hese variations in concentration are of considerable significance in the use of titania as a sensor material~
Figure 7 illustrates the manner in which the resistance of the oxygen sensing element 46 and the thermistor element 48 va~ a~ a ~unction of temperature Curve 70 represents the ~esist~nce o~ the ox~en sensor when ~t is located ~n the ex~ust ~a~ e~anatln~ from an internal comDustion engine supplxed ~ith a lean air~fuelmixture~ t~at ~s~ a m~xture that has a qu~ntity of oxy~en greater than that requ~red for sto~c~iometric combustion.
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The curve 72 represen~s the resistance of that sensor when located in the exhaust gases emanating from an engine supplied with a rich mixture. Curve 74 illustrates the resistance of the thermistor 48 as a function of temperature. The curve is of alternating character indicating the small variation of the thermistor resistance that occurs as the air-fuel ratio supplied to the engine oscillates back and forth about stoichiometry. From curve 74, it is quite evident that there is but very minor variation in the resistance of the thermistor 48 as a function of the oxygen content in the gaseous medium surrounding the sensor. This is much in contrast to the curves 70 and 72 representing, respectively, the lean and rich resistance values over the normal operating range of exhaust gas sensor 10. of course, the actual resistance values for the oxygen sensor element 46 would vary back and forth between the curves 70 and 72 as the air-fuel ratio supplied to the engine was varied about stoichiometry. At ~he left side of the graph of Figure 7, it m~y be seen that the curves 70 and 72 come together at low temperatures.
This indicates that titania is not responsive to the surrounding oxygen concentration at low temperatures.
A very significant feature of the present invention is that the portion of the curves 70 and 72 at which the sensor becomes responsive to oxygen concentration occurs at a lower temperature than with the prior art device.
This feature, together with elimination of the degradation in low temperature response experienced in the prior art device, are very substantial benefits.
The fact that rutile titania, as previously described, has deficiencies where atoms of oxygen are missing is responsible for much of the resistance variation indi-cated in Figure 7.
If it is assumed that a titania sensor, such as sensor element 46, is located in an environment in which the oxygen concentxation is constant and only the tempera-ture varies, then the number of vacancies in the titania ,~
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structure may change due to thermal energy~ Ho~ever~ the tit~nium atom in those titarlium oxide molecules h~ing ~ut one oxygen ~tom~ ~ave onl~ t~o of the~r four v~lence electrons covalentl~ bonded with ox~gen. As the temperature of the titania increa$es~ the thexmal energy supplied to the molecules in the s~ructure increases and the oxygen ~acancies therein have greater mobility~ As the oxygen deficiency and concentrat~on of Ti interstitials increases, more electrons ~ecome a~ailable for the conduction processr and the resistivity of the material decreases~ The conductivity of the titania increases or, othexwise stated, its resistance decreases as a function of temperature, as is indicated in ~igure 7 for both the thermistor and oxygen sensor element.
If it is now assumed that a sensor element 46 of titania is positioned in an environment of varying oxygen partial pressure and that it is at a temperature within the titania operating range~ for exampl~, 600C, then the number of vacancies in titania increases or decreases as a function of oxygen partial pressure.
If a titania oxygen sensor 46 is positioned in the exhaust stream of an internal combustion engine and if the air-fuel mixture supplied to such engine continually varies between lean and rich with rexpect to stoichiometry, the partial pressure of oxygen to which the sensor is exposed varies cyclically. When the mixture is lean, there is an excess of oxygen in the exhaust gas and few oxidizable carbon compounds. The titania element has a relatively high resistance~ on the order of about 0-5 megohms. This is because oxy~en from the exhaust gases will have been adsorbed on the suxface of the titania element The adsorbed oxygen ato~s on the titania surface annihilate oxy~en Vacanc~es ~nd interstitial titanium ions and mi~xate into the titan~ cx~stal stxuctu~e, In ~n oxygen d~f~cient ox~de~ ~oth oxygen Vacancies and `' ' ,:
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~ 13 interstitial ions may be in~olved in an equilibriu~
reaction w~th oxygen In the surrounding environment~ In this equilibrium reactionr the partial pressure of oxygen in the en~ironment deter~ànes whether the interstitial ions or the oxygen vacancies play the predominant role in the oxygen trans~er process~ In both cases, there is an acquisition o~ electrons followed by an annihilation of a ~acancy and an interstitial ion. The electrons at low sensor operating temperatures are provided by the charge transer material, which is an electrical conductor having a ~pool" of available electrons~ At higher temperatures, thermal energy is suf~icient to provide electrons required at the titania sur~ace or the process of vacancy annihilation.
The lower the number of vacancies in the titania crystal structure, the higher is its electrical resistance.
On the other hand, the moxe vacancies that are created in the crystal structure, the lower is the titania resistance.
When the exhaust gases change from lean-to-rich (L-R), a percentage of the oxygen atoms in the titania structure are removed to create additional vacancies. The oxygen leaves the titania crystal structure probably as a negatively charged ion. As a result, there is a positively charged vacancy left behind. At the titania surface, either the oxygen ion reacts with an oxidizable carbon compound in the exhaust gas or two oxygen atoms or ions unite to form an oxygen molecule.
When the exhaust gases change to a composition corresponding to a lean mixture, the concentration of oxidizable carbon compounds is drastically reduced and an excess o oxy~en appears in the exhaust gas~ The oxygen concentration gradient xeVerse~, and oxygen atoms are adsorbed on t~e titania ~u~f~ce and ~ acanc~es therein as ~as previousl~ mentioned~
Figure 8 illustra~es the manner in which ~he outpu~
voltage of the sensor lO, connected in the circuit of Figure 4, varies as a function of air-fuel ratio where this ratio changes from rich (below 14.7) to lean (above 14.7). When the mix~ure is rich, the sensor element 46 has a low resistance and the sensor output voltage is almost lO0 percent, the percentage figure being the ratio of the actual output voltage to the input reference volt~ge multiplied by 100 percent. It may be seen that, with the temperature compensation provided by the thermistor 48, there is very little variation in the sensor output voltage as a function of variation in temperature between 350C and 350C. Under rich conditions, the removal o~ oxygen from the titania structure to create new vacancies provides additional electrons from the titanium atoms that may be used for the purpose of conduction. This explains the greatly increased conduc-tivity of titania when exposed to exhaust gases produced by the combustion of rich mixtures. The opposite effect explains the very high resistance and low conductivity of the titania sensor element when exposed to exhaust gases produced by lean mixtures.
In Canadian Patent Application Serial No. 310,561 filed September 1, 1978, entitled "Catalytic Material Impregnated Porous Variably Resistive Exhaust Gas Sensor and Method of Impregnation", there is described the use of a "catalys~" o~ platinum on the surface of the titania sensor element 46. The platinum is dispersed throughout the porous titania element for the purpose of enhancing 30 its response characteristics at low temperatures.
The use of the term "catalyst" to describe the metallic platinum deposited on the titania element is believed to be a misnomer, in that little or no catalytic action is believed to take place. The metal on the titania 35 functions as a charge transfer material to promote, particularly at the lower portion of the sensor operating ;
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range, the acquisition and removal of oxygen from the titania crystal structure as a result of air-fuel changes from rich-to-lean and lean-to-rich, respectively. The metal is very thin and may be located a~ or near the grain boundaries of the titania and is preferably dis-continuous to facilitate the acquisition and removal of oxygen from the titania.
Figure 9 illustrates the actual output-voltage response of a titania exhaust gas sensor exposed to exhaust gases produced by ~oth rich and lean mixtures where these mixtures cycled between rich and lean once each second. The sansor response tLme is defined as the tLme required for the sensor to traverse from 33% to 66~
of the re~erence voltage when the air~fuel ratio changes from lean to rich and the tLme required for the sensor to traverse from 66% to 33~ of the reference voltage when the air-fuel ratio changes from rich to lean. The sensor response depicted in Figure 9 is taken from an output-voltage trace of a titania sensor. The trace was
This inven~ion relates ~o an Lmproved sensor of the type having a ceramic element tha~ undergoes a change in an electrical characteristic in response to a change in the partial pressure of oxygen in a mixture of gases to which the ceramic element is exposed. The ceramic element or the sensor may be either titania or zirconia under the current state of development, but other electxically responsive ceramics are known and may be used in the future.
The preferred titaria ceramic element is porous to provide a large sura~e area for effecting the transfer of oxygen from the titania to the gases to which the ceramic element is exposed and vice versa. The titania ceramic element has a porous or discontinuous coating of a precious metal charge transfer material. This material in ~he past has been platinum applied ~o the titania ceramic element by immersion in a solution con~aining platinum~
Sensors of the type having a zirconia ceramic element also utilize a porous pla~inum charge transfer material, but the zirconia ceramic is very dense and the platinum is applied to the zirconia surfaces by vapor deposition. The surface platinum to be exposed to engine exhaust gases is ; usually covered with a porous re~ractory material to aid in bonding and for the protection of the platinum~
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Sensors of the type di~cussed above are particularly suited for use in detecting excursions, above and below stoichiometry, of the air-fueI ratio of the mixture of air and fuel supplied to an internal comhustion engine. In accomplishing this detection, the sensor is positioned in the path of the exhaust gases emanating from the engine.
As the mixture supplied to the engine changes from rich to lean, the exhaust gases change from a composition including very little oxygen to a composition containing an excess of oxygen. As the exhaust gases change from lean to rich, the reverse changes in composition occur.
The sensors have an electrical characteristic that under-goes a step-function change as a xesult of the mixture excursions across the stoichiometric air-fuel ratio.
The titania ceramic material undergoes a change in its resistance as a function of the oxygen concentration gradient between the titania and the exhaust gases. The zirconia ceramic element undergoes a change in ~he EMF
produced across its platinum change transfer electrodes as a function of the oxygen concentration differential on opposite sides of the zirconia material. ~ith the zirconia sensor, a reference gas, usually air, is applied to one side of the zirconia and the exhaust gas composition is allowed to contact ~he other side of the zirconia. The use of a reerence gas is unnecessary in connection with titania sensors, and the entire titania ceramic element is immersed in the exhaust gases.
The present invention is particularly directed to a titania sensor, but has possible application to zirconia sensors and others if problems peculiar to these sensors are eliminated or become less extreme as the art progresses.
The specific problem solved by the present invention is the loss of response to air-fuel mixture variations that occurs with the prior art titania sensor as a result of its use over a reIatively short period of time. This loss of response occurs in the lower portion of the noxmal . .: ,, ........... . ..... .. , , , . . - ~
~ c~ 3 operating temperature range, which extends from about 300C
to ab~u~ 900~C~ The failure of the sensor to operate at low temperatures due to loss of its low temperature response is a ~ery serious problem because it means that t~e feedbac~
s fuel control system associated wit~ the sensor for con-trolling the mixture ratios supplied to an internal combustion engine cannot be operated un~il the exhaust gases have heated t~e sensor sufficiently to main~ain its temperature above that at which it is able to respond to LO air-fuel ratio variations. These may increase undesirable engine exhaust emissions and reduce fuel economy during engine warm-up conditions.
The inventors have discovered that the porous or discontinuous platinum coating on the porous titania cQramic element forms an oxide, PtO2, that vaporizes and that thereby i5 remo~ed from the titania element when it i exp~sed to the higher tempera~ures within the aforementioned normal range of operàting temperatures. These temperatures occur during normal sensor usage when the s~nscr is sub-jected to exhaust gases produced by lean mixtures havingexcass oxygen content. If the sensor temperatuxe is at about 850C during this time, the platinum oxide forms, vaporizes, and is lost. While this may not adversely affect the operation of th sensor when being used under high temperature conditions, it very ad~ers ly affects th~
sens~r with respect to its low-temperature operability and response time. T~is can cause ~he forementioned deleterious effects on the fuel control system.
It has been found that the vaporiæation loss of the 3~ platinum charge transfer material applied to the titania ceramlc element of an exhaust gas sensor can be sub-stantially eliminated. Also, the minimum operating temperature of the prior art titania sensorO even with its platinum charge transer material intact, can be ~ub-stantially reduced so that the sensor can be used earlier .
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;`' :, in the operation of a cold internal combustion engine to achieve closed-loop feedback air-fuel ratio control.
The present invention, therefore, provides an improved sensor of the type having a ceramic element that undergoes a change in an electrical characteristic of the ceramic element in response to a change in the partial pressure of oxygen in a mixture of gases to which the ceramic element is exposed, the ceramic element having electrodes connected thereto in spaced relationship and extending from the ceramic element to permit the resistance between them to be sensed, the ceramic element being porous to permit oxygen in the gases to which the ceramic element is exposed to migrate into and out of its interior regions, and the ceramic element having a discontinuous or porous charge transfer lS material deposited thereon.
The improvement of the invention comprises a rnajority of the charge transfer material by weight is Pt and Rh is included in an amount sufficient to allow the charge transfer material to be retained on the ceramic element when the ceramic element is exposed to gaseous mixtures produced by lean-mixture combustion at ceramic element temperatures in excess of about 850C.
The eleimination of the prior art problem and the additional advantages of the invention are obtained without degradation in the response time of the sensor. The sensor improvement is brought about by the addition of the rhodium, whic~ forms an oxide (RhO2) at elevated temperatures that is considerably more stable than is the oxide of platinum and increases the life of the charge transfer material.
The greater the stability of the oxide of the charge transfer material, the longer one would expec-t the sensor response time to be due to the lower electrical conductivity of the metal oxide and the interference of the oxide with oxygen transfer from and to the titania crystal structure.
Pure platinum would be expected to facilitate rapid transfer .
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of oxygen from the ceramic element to the exhaust gases and vice versa and, therefore, platinum in its pure form would be expected to be the optimum metal for a titania sensor charge transfer material. Unfortunately, the inventors have ~ound that the high vapor pressure of platinum and its oxide is responsible for its loss from the titania element during operation of the snesor above about 850 C.
Of course, if the titania sensor is not to be operated at temperatures and conditions at which platinum oxide is formed, then pure platinum or some other electrically conductive material mày be the "optimum" charge transfer material for such application. The charge transfer material is an elec-trical conductor; it is not regarded as a catalyst because it does not promote chemical reaction of exhaust gas con-stituents in its activity related to variation of the ceramicelectrical characteristic.
With a ~resh sensor having a charge transfer material of pure platinum, the prior art sens~r response time initially is very fast, ~ut as a result of vaporization of the platinum or its oxida at elevated sensor operating tempera~uras, the response time at low operating tempera-tures degrades substantially. The addition of rhodium, however, in accordance with this invention, to the platinum has been found to prevent the rapid vaporization of the charge transfer material and allow the sensor to respond to variations in the oxygen content of gases to which the sensor is exposed at considerably lower operating temperatures than is characteristic of the prior art device.
The invention may be better understood by reference to the detailed description which follows and to the accompanying drawings, wherein :
Figure 1 is an eleva~ional view of a ti~ania exhaus~
gas oxygen sensor suitable or installation in ~he i~take manifold of an external combustion engine;
Figure 2 is a sectional end view, taken along the line II-II in Figure 1, of the sensor of Figure 1 a~d is shown in enlarged scale;
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Figure 3 is a sectional view,taken along the line III-III in Figure 2, showing the internal structure of the sensor of Figures 1 and 2 also on an enlarged scale;
Figure 4 is a circuit di gram illus~rating the S manner in which the titania oxygen sensing element and the thermis~or shown in Figures 1 through 3 are electrically connected with circuitry deslgned to receive the sensor output voltage;
Figures ; and 6 are photomicrographs o~ the titania oxygen sensor and ~he thermistor illustrated in Figures 1 ~hrough 4;
Figure 7 is a graph of both the oxygen sens~r element and thermistor element resistance as a runction of tempera-ture over the normal operating range frsm about 30~C to about 900C;
Figure 8 is a graph of sensor output voltage as a percent of the input (reference) voltage versus equivalent air-fuel ratio;
Figure 9 is a graph illustrating the voltage response f the tita~ia sensor as a function o~ time with ar. air-fueI ratio that varies by about 0.1 ra~ios above and below stoichiometry;
Figure 10 is a graph of the response time of unaged titania sensors ~ersus the sensor tip temp~rature for both ~t and Pt/Rh charge ~ransfer materials;
Figure 11 is a gra~h of ~he response t me o~ an aged sensor versus the sensor tip temperat~re for both Pt and Pt/Rh charge transfer materials;
Figure 12 is a graph of titania s~nsor response time as a function of the percent composition of the charge transfer ma~erial on the titania oxygen sensor element;
and Figures 13 through 18 are graphs that illustrate the response time a~d ~oltage characteristics of the ~mproved titania sensors at different operating temperatures during and after durability testins to 850 hours on a~ engine dynamometer.
With p rticular reference now to Figures 1 through 3, wherein like numerals refer to ~ike parts in the several . . . .
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`' ' ~ ; , '' ' ~' ' ' ' , ~ .7_ views, there is shown a complet~ titania. exhaust gas sensor as5embly generally designate~ by t~e numeral 10. The sensor lO includes a steel. housing or body 12, which may be substantially identical to a typical spar~ pLug body, having a ~hreaded portion 14 for engagement. with a suitably threaded aperture provided within the exhaust system of an internal combustion engine (not shown). In most cases, . the sensor lO would be installed in an a~erture at a location in the exhaus~ manifold near the flange that would connec~ to an exhaust pipe. ~ ceramic insulator 1 extends through the body 12 and has a tapered portion 26 projecting outwardly trom the ~ody 12 into the volume de~ined by the boundaries of a perforated shield 18.
There are three longitudinal passages 20, 22 and 24 extending from the projecting end 26 of the c~ramic ; insulator to its opposite end 28. Wires 30, 32 and 34 æ e located i~ the respectiveIy corresponding passages 20, 22 and 24 and are o a.heat resistan~ character, preferably being made from an al.Ioy such as 80% ~ic~el 20% chromium wire.. These electrically conducti~e wires are welded to precious-metal wire leads 40, 42 and 44, which are ~mbedded in disc-shaped ceramic elements 46 and ~8.
Element 46 is a ceramic titania 2 sensor responsive to the partial pressure of oxygen in the gaseous medium to which this ~lement is axposed~ Sen~or el~,ment 46 may be fabricated in accordance with the teachings of U.S. Patents 3,886,785 issued June 3, 1975 and 3,332,246 issued January 13, lg76, both in the names of Stadler et al and assigned to Ford Motor Company. The teachings of the present inventlon must, however, also be considered in the fabrication of the oxygen sensing element 46. The present invention teaches the application to the porous oxygen sensor of a platinum alloy (atomic or fine particle mixture) charge transfer material for reasons which are hereinafter made clear.
The element 48 is a thermistor. The termistor may be made from titania ceramic material of greater density, , ' ',' - ' ~ ' .
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near its theoretical density, than the density of the porous titania oxygen sensor 46. Alternatively, the termistor 48 may be constructed in accordance with the teachings of U.S. Patent No. 4,162,431 in the names of LogothetiS, Laud and Park, entitled "Rare Earth - Yttrium, transition metal oxide Thermistors", and assigned to E~ord Motor Company. The thermistor 48 is intended to provide temperature compensation in accordance with the circuitry illustrated in Figure 4 and is intended to be substantially nonresponsive to variations in the partial pressure of oxygen in the gaseous medium to which it is exposed.
The sensor of ~igures 1 t~rough 3 is intended to be used in con~unction with electronic circuitry for closed-loop feedback contxol of the amount of fuel supplied to an internal combustion engine. T~e sensor indicates whether tne exhaust gases contain a substantial amount of 8C and CO or whether instead there is a substantial amount of oxygen, thereby, indicating whether or no~ the air-fuel ratio of the mixture supplied to ~he engine was rich or lean with respect to the stoichiometric value of a~out 14.7 parts of air to each part of fuel by weight. This air-uel ratio typically is expressed as a normalized air-fuel ratio lambda, whereln the actual ratio is divided by the stoichiometric value and the stoichiometric ratio therefore is represented as 1.0 in accordance with well known practice.
The exhaust gas sensor 10 has terminals S0, 52 and 54 designed for connec~ion`to external circuitry as speci~ied above to enable it to be used in a feedbac~ fuel control system~ With particular reference now to Figure 4, there is shown a circuIt that schematically represents the manner in wh~ch the sensor 10 is utilized in association ~ith such extexnal ci~cuitxy~ A DC source of requlated re~erence volt~ge 60 has its positi~e terminal connected to terminal 5a o~ the sensor o~ygen . .
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responsive elemen~ 46. The lead wires 40, 42 and 44 from the sensor 46 and thermistor 48 are welded or o~herwise joined, respectively, to lead wires 30, 32 and 34 to interconnect the two ceramic elements 46 and 4~ as shown.
The thermistor element 48 is connected ~hrough a response-shaping resistor 62 to ground potential at 64. The output voltage of the sensor 10 is taken between the sensor terminal 54 and ground potential. This signal is applied across the input impedance or load resistance RL (about two megohms) of the engine control electronic circuitry.
The input voltage to the circuit of Figure 4 is obtained from the source reference 60 and is applied across the voltage divider comprising the seriçs-connected variable resistances of oxygen sensor 46 and thermistor 48 is series with the response-shaping resistor 62. The output voltage is taken across the load resistance ~ ~
The resistance values of both the oxygen sensor 46 and the thermistor 48 vary as a function of temperature and in the same directiont that is, the resistance of these elements decreases with increasing temperature. As a result, the voltage di~iding effect provides an output voltage across the load resistance RL ~hat is independent o~ temperature. The oxygen sensor 46, however, has a resistance which varies not only with temperature but also with the partial pressure of oxygen in the gaseous medium to which the sensor is exposed. An increase in ~he resistance of the oxygen sen~or 46 causes the output voltage across the load RL to decrease, and a reduction in the resistance of the oxygen sensor causes a corresponding increase in the output voltage across ~he resistance RL.
Otherwise stated, an increase in oxygen conten~ in the gaseous medium surrounding the oxygen sensing device 46 causes its resistance to increase in a manner hereinafter described and thereby causes a reduction in the voltage across the load resistance ~. A decrease in the oxygen content o~ the gaseous medium causes ~he resistance o~ ~he oxygen sensor 46 to decrea~e in a corresponding manner .
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and this c~uses an incxe~se ln the voltage across the load resistance RL~
Figure 5 ~s a photo~icrograph o~ the t~tan~a oxyyen senso~ 46 ~t~ a ma~nification of 700 times size. Figure 6 ~s a p~otomicro~raph of a titania thermistor 48 with a similar magnif~cation of 700 times s~ze~ From the ~igure 5 titania sensor photom~cro~raph, it may be seen quite clearly that the oxygen sensor structure is ver~ porous~
Also~ its grain size is very small as compared to the much larger grain size o~ the titania thermistor~ which is much more dense and which lacks the porosity of the titania oxygen sensor.
Titanium d~oxide (titania~ is a material that occurs naturally in mixture with other minerals~ The titania is obtained by precipitation from a solution of minerals that include titania~ When thus obtained by precipitation, the titania has an anatase cr~stal structure. When the titania material in thi~ crystal structure is formed into an exhaust gas oxygen sensor~ it is first thermally treated in a manner that allows the crystal structure to change from anatase to rutile. An increase in the temperature of the rutile material above room temperature induces oxygen vacancies into ~he crystal structure. This results in ionization of the titanium atoms interstitiall~ located in the crystal structure. The concentration of the inter-stitial titanium ions and oxygen vacancies increase as temperature rises~ and ~hese variations in concentration are of considerable significance in the use of titania as a sensor material~
Figure 7 illustrates the manner in which the resistance of the oxygen sensing element 46 and the thermistor element 48 va~ a~ a ~unction of temperature Curve 70 represents the ~esist~nce o~ the ox~en sensor when ~t is located ~n the ex~ust ~a~ e~anatln~ from an internal comDustion engine supplxed ~ith a lean air~fuelmixture~ t~at ~s~ a m~xture that has a qu~ntity of oxy~en greater than that requ~red for sto~c~iometric combustion.
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The curve 72 represen~s the resistance of that sensor when located in the exhaust gases emanating from an engine supplied with a rich mixture. Curve 74 illustrates the resistance of the thermistor 48 as a function of temperature. The curve is of alternating character indicating the small variation of the thermistor resistance that occurs as the air-fuel ratio supplied to the engine oscillates back and forth about stoichiometry. From curve 74, it is quite evident that there is but very minor variation in the resistance of the thermistor 48 as a function of the oxygen content in the gaseous medium surrounding the sensor. This is much in contrast to the curves 70 and 72 representing, respectively, the lean and rich resistance values over the normal operating range of exhaust gas sensor 10. of course, the actual resistance values for the oxygen sensor element 46 would vary back and forth between the curves 70 and 72 as the air-fuel ratio supplied to the engine was varied about stoichiometry. At ~he left side of the graph of Figure 7, it m~y be seen that the curves 70 and 72 come together at low temperatures.
This indicates that titania is not responsive to the surrounding oxygen concentration at low temperatures.
A very significant feature of the present invention is that the portion of the curves 70 and 72 at which the sensor becomes responsive to oxygen concentration occurs at a lower temperature than with the prior art device.
This feature, together with elimination of the degradation in low temperature response experienced in the prior art device, are very substantial benefits.
The fact that rutile titania, as previously described, has deficiencies where atoms of oxygen are missing is responsible for much of the resistance variation indi-cated in Figure 7.
If it is assumed that a titania sensor, such as sensor element 46, is located in an environment in which the oxygen concentxation is constant and only the tempera-ture varies, then the number of vacancies in the titania ,~
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structure may change due to thermal energy~ Ho~ever~ the tit~nium atom in those titarlium oxide molecules h~ing ~ut one oxygen ~tom~ ~ave onl~ t~o of the~r four v~lence electrons covalentl~ bonded with ox~gen. As the temperature of the titania increa$es~ the thexmal energy supplied to the molecules in the s~ructure increases and the oxygen ~acancies therein have greater mobility~ As the oxygen deficiency and concentrat~on of Ti interstitials increases, more electrons ~ecome a~ailable for the conduction processr and the resistivity of the material decreases~ The conductivity of the titania increases or, othexwise stated, its resistance decreases as a function of temperature, as is indicated in ~igure 7 for both the thermistor and oxygen sensor element.
If it is now assumed that a sensor element 46 of titania is positioned in an environment of varying oxygen partial pressure and that it is at a temperature within the titania operating range~ for exampl~, 600C, then the number of vacancies in titania increases or decreases as a function of oxygen partial pressure.
If a titania oxygen sensor 46 is positioned in the exhaust stream of an internal combustion engine and if the air-fuel mixture supplied to such engine continually varies between lean and rich with rexpect to stoichiometry, the partial pressure of oxygen to which the sensor is exposed varies cyclically. When the mixture is lean, there is an excess of oxygen in the exhaust gas and few oxidizable carbon compounds. The titania element has a relatively high resistance~ on the order of about 0-5 megohms. This is because oxy~en from the exhaust gases will have been adsorbed on the suxface of the titania element The adsorbed oxygen ato~s on the titania surface annihilate oxy~en Vacanc~es ~nd interstitial titanium ions and mi~xate into the titan~ cx~stal stxuctu~e, In ~n oxygen d~f~cient ox~de~ ~oth oxygen Vacancies and `' ' ,:
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~ 13 interstitial ions may be in~olved in an equilibriu~
reaction w~th oxygen In the surrounding environment~ In this equilibrium reactionr the partial pressure of oxygen in the en~ironment deter~ànes whether the interstitial ions or the oxygen vacancies play the predominant role in the oxygen trans~er process~ In both cases, there is an acquisition o~ electrons followed by an annihilation of a ~acancy and an interstitial ion. The electrons at low sensor operating temperatures are provided by the charge transer material, which is an electrical conductor having a ~pool" of available electrons~ At higher temperatures, thermal energy is suf~icient to provide electrons required at the titania sur~ace or the process of vacancy annihilation.
The lower the number of vacancies in the titania crystal structure, the higher is its electrical resistance.
On the other hand, the moxe vacancies that are created in the crystal structure, the lower is the titania resistance.
When the exhaust gases change from lean-to-rich (L-R), a percentage of the oxygen atoms in the titania structure are removed to create additional vacancies. The oxygen leaves the titania crystal structure probably as a negatively charged ion. As a result, there is a positively charged vacancy left behind. At the titania surface, either the oxygen ion reacts with an oxidizable carbon compound in the exhaust gas or two oxygen atoms or ions unite to form an oxygen molecule.
When the exhaust gases change to a composition corresponding to a lean mixture, the concentration of oxidizable carbon compounds is drastically reduced and an excess o oxy~en appears in the exhaust gas~ The oxygen concentration gradient xeVerse~, and oxygen atoms are adsorbed on t~e titania ~u~f~ce and ~ acanc~es therein as ~as previousl~ mentioned~
Figure 8 illustra~es the manner in which ~he outpu~
voltage of the sensor lO, connected in the circuit of Figure 4, varies as a function of air-fuel ratio where this ratio changes from rich (below 14.7) to lean (above 14.7). When the mix~ure is rich, the sensor element 46 has a low resistance and the sensor output voltage is almost lO0 percent, the percentage figure being the ratio of the actual output voltage to the input reference volt~ge multiplied by 100 percent. It may be seen that, with the temperature compensation provided by the thermistor 48, there is very little variation in the sensor output voltage as a function of variation in temperature between 350C and 350C. Under rich conditions, the removal o~ oxygen from the titania structure to create new vacancies provides additional electrons from the titanium atoms that may be used for the purpose of conduction. This explains the greatly increased conduc-tivity of titania when exposed to exhaust gases produced by the combustion of rich mixtures. The opposite effect explains the very high resistance and low conductivity of the titania sensor element when exposed to exhaust gases produced by lean mixtures.
In Canadian Patent Application Serial No. 310,561 filed September 1, 1978, entitled "Catalytic Material Impregnated Porous Variably Resistive Exhaust Gas Sensor and Method of Impregnation", there is described the use of a "catalys~" o~ platinum on the surface of the titania sensor element 46. The platinum is dispersed throughout the porous titania element for the purpose of enhancing 30 its response characteristics at low temperatures.
The use of the term "catalyst" to describe the metallic platinum deposited on the titania element is believed to be a misnomer, in that little or no catalytic action is believed to take place. The metal on the titania 35 functions as a charge transfer material to promote, particularly at the lower portion of the sensor operating ;
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range, the acquisition and removal of oxygen from the titania crystal structure as a result of air-fuel changes from rich-to-lean and lean-to-rich, respectively. The metal is very thin and may be located a~ or near the grain boundaries of the titania and is preferably dis-continuous to facilitate the acquisition and removal of oxygen from the titania.
Figure 9 illustrates the actual output-voltage response of a titania exhaust gas sensor exposed to exhaust gases produced by ~oth rich and lean mixtures where these mixtures cycled between rich and lean once each second. The sansor response tLme is defined as the tLme required for the sensor to traverse from 33% to 66~
of the re~erence voltage when the air~fuel ratio changes from lean to rich and the tLme required for the sensor to traverse from 66% to 33~ of the reference voltage when the air-fuel ratio changes from rich to lean. The sensor response depicted in Figure 9 is taken from an output-voltage trace of a titania sensor. The trace was
2~ obtained during a test conducted with a vapor-carburetor facility. During the test, air-fuel ratio was modulated in a step-function manner with + 1.5 air-fuel ratio variation about the stoichiometric value. Response time is ~ast, on the order of 10 to 20 milliseconds, and these very low values are difficult to measure.
With respect to sansors manufactured in accordance with the teachings o~ the aforementioned Canadian Patent Application Serial No. 310,561, the platinum charge transfer material applied to the porous titania oxygen 3a sensor element was found to substantially Improve the sensor response time as compared to a titania oxygen sensor having no charge transfer material, pax~icularly at low sensor operating temperatuxes. Unfortunately, it was found that the sensors having platinum as a charge 35 transfer material developed substantially leng~hened response times after the sensors had been in us~ for a , -16~
number of hours. Not only did the response time become very slow fox these sensors, but also the minimum temperature at which the sensor output response occurs increased substantially. This was very undesirable~ The sensor should respond at as low a temperature as possible so that feedback fuel control of the air-fuel ratio supplied to the internal combustion engine may be achieved at an early stage during its warm-up period.
The inventors have discovered that the change in response time of the sensor and the increased minimum operating temperature are the result of loss of the platinum charge transfer material that had been deposited on the titania sensor element. This loss is ~he result of the oxidation of the platinum at the higher sensor operating tempera~ures normally encountered~ Platinum forms platinum oxide PtO2 at elevated temperatures, with substantial amounts being formed at about 850C.
Unfortunately, platinum oxide vaporizes quite readily and leaves the porous titania oxygen sensing element 46. In a cooler portion of the exhaust conduit, the platinum oxide decomposes and i deposited as pure platinum. In fact, it has been found that some of the platinum is deposited on the nearby thermistor 48, which tends to be somewhat cooler than the oxygen sensor 46. This adversely affects the behavior of the thermistor hy increasing its response to variations in the partial pressure of oxygen in the exhaust gas. The thermistor is intended to respond only to temperature.
The inventors have found that if a small amount of rhodium is added to the platinum charge transfer material, not only can the vaporization of the charge transfer material be prevented, but the response time of the sensor can he made very fast, about equal to that of a fresh titania sensor with a platinum charge transfer material. Also, the minimum operating tempexature of the sensor can be substantially reduced below the minimum operating temperature of a titania sensor having pure ;
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platinum as the charge transfer ma~erial. An important chaxacteristic of the improvement is that a small amount of rhodium, a material which has a lower vaporization pressure than platinum and which has a lower ionization potential as well, provides the enhanced sensor behavior.
A solution of platinum and rhodium in 2 l/2% con-centration may be used to apply the platinum/rhodium charge transfer material to a porous titania sensor element 46. Solution containing nine parts by weight platinum to one part by weight rhodium (a 90/10 platinum-rhodium composition) is believed to be very satisfactory in achieving the results described herein. A solution containing platinum in the above ratio and in the amount of 2 l/2% by weight is formed by mixture of chlcroplatinic acid and rhodium chloride. The platinum and rhodium ions - in the solution are intimately mixed on an atomic scale.
A fresh titania sensor element is immersed in the solution.
The solution then is allowed to evaporate leaving crystals of platinum chloride and rhodium chloride on the sensor element. It is believed that these crystals are very fine and are so intimately mixed that, upon heating of the titania sensor element to about 900C, the salts decompose to leave platinum and rhodium atoms mixed with one another on an atomic scale. Thus, it is ~elieved that a true alloy of platinum and rhodium is formed by use of the above procedure. This platinum/rhodium alloy forms the charge transfer material for titania that produces the results hereinafter described.
With particular reference to Figur~ 10, there is 3Q shown a comparison of the response tLme of unaged (fresh) titania oxygen sensors as a function of titania element tip temperature in ~C. Temperatures are measured with a thermocouple positioned between, but not quite in contact with, the oxygen sensing element 46 and the thermistor 480 35 The data is plotted for rich-to-lean and lean-to~rich responses of some sensor elements having lO0~ platinum deposited by solution thereon as described ln the afore-.. . .. . .. . .. . . . . . . .
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mentioned Canadian Patent Application Serial No. 310,561 and other sensor elements having the platinum/rhodium alloy of the present invention deposited thereon as taught herein.
Each data point in this graph represents the average of the response times of twenty-three sensors. The R-L
response time of fresh sensors having a pure Pt charge transfer materail applied in a 5% solution of chloro-platinic acid (H2PtC16) may be seen to go from infinity to about 30 msec in the 300C temperature range. The L-R
response time for this group of sensors having Pt charge transfer material is essentially identical to the R-L
response, except where the respective curves diverge at the temperature region of about 330C~
Also represented in Figure 10 is a group of sensors having a charge transfer material of 90~ Pt and 10% Rh applied to the porous titania in a solution of H2PtC16 and RhC13.3H20 containing Pt and Rh in total amount of 2 1/2% by weight. Both the R-L and L-R response times for this group arP illustrated. These response times go from infinity to less than 50 msec over the temperature range from about 200 to 27SC.
As previously mentioned, all of the data for Figure 10 was obtained from fresh (unaged) titania sensors. It is evident that the use of the 90% Pt/10% Rh charge transfer material has substantially reduced the minimum operating temperature of the fresh titania sensors while also providing the same fast response time over the entire normal sensor operating range as the sensors with pure Pt 30 charge transfer material.
Figure 11 is similar to Figure 10, but shows results obtained for the twenty-three sensors after they had been exposed, for 250 hours, to the exhaust gases produced by the combustion of a natural gas/air mixture that varied 35 cyclically each second about the stoichiometric mixture level. The data establishes some degradation in response ' time for sensors having either the pure Pt or the 90~ Pt/
10~ Rh charge transfer materials. However, the substantial improvement obtained with the Pt/Rh charge transfer material on fresh sensors is retained after the 250 hours of aging. The minimum operating temperature is down to about 25CC for the Pt/Rh sensors and is about 400C for the Pt sensors. The time response over the entire normal operating temperature range is very satisfactory for the Pt/Rh sensors. The shift to a minimum operating tempera-lQ tuxe of 400C for the Pt sensor group is caused byvaporization loss o the Pt charge transfer material.
Figure 12 is a graph of percent Pt-Rh composition versus sensor response time~ The plotted data points for Pt-Rh alloys each are the average o data obtained from lS measurements made on six fresh sensors. Otherwise stated, there were six sensors having a 100% Rh charge transfer material, six sensors having a 50% Pt/50% Rh charge transfer material, six sensors having a 70~ Pt/30% Rh charge transfer material, and six sensors having a 90% Pt/
10% Rh charge transfer material. The data for sensors having 100% Pt charge transfer material was obtained from a group of fresh sensors.
In Figure 12, cur~es are provided for R-L response time at 350C, for L-R response time at 350C, for R-L
response time at 850C and for L-R response time at 850C.
The L-R response time at the lower operating temperature of 350C is independent of the percent Pt/Rh composition~
On the other hand, the R-L response times at both 350C
and the higher 850C temperature increase as the %Rh in 3Q the charge transfer material is increasedO In other words, it appears that Rh in the charge transfer material degrades the sensor performance by increasing the response time of the fresh sensors, particularly at 350C in the lower portion of the normal sensor operating temparature rangeO
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The efect of increasing Rh content in the Pt/Rh charge txansfer material is especially pronounced with respect to the L-R response time at 850C.
From the data plotted in Figure 12, it is clear that for Pt/Rh charge transfer materials in the range of compositions from 70% Pt/30~ Rh to 100~ Pt/0% Rh, there is little if any degradation in the response time of the sensors. On the other hand, the L-R response at 850C for compositional ranges between 70%Pt/30%Rh and 0%Pt~100%Rh is very much affected by the increasing rhodium content. The xeason for this is descxibed in the following paragraphO
At higher temperatures in a lean exhaust gas environment, the titania sensor having both Pt and Rh mixed together in the charge transfer material forms oxides of both Pt and Rh. The Pt oxide (Pt02) îs less stable than the Rh oxide (Rh02) a When the mixture chanyes from L-R so that there is substantially less oxygen in the gaseous exhaust mixture, then oxygen is removed from the titania sensor and from the charge transfer material.
The Pt oxide breaks down easily and its oxygen is removed quite rapidly, but the Rh oxide is more stable than the Pt oxide and it takes longer to decompose into oxygen and rhodium. The presence of the Rh oxide delays the removal of oxygen from the titania crystal structure and therefore delays a reduction in its resistanceO The more Rh there is present in the chaxge transfer material, the slower is the titania sensor respo~se, as is demonstrated by he L-R curve at 850C in Figure 120 However, as shown by Figure 12, for compositions of the charge transfer material having 30% or less Rh by weight, there is little if any degradation in response time, but yet this small amount of Rh is sufficient to prevent vapoxization of the oxidized charge transfer material. This is believed to be because Rh has a lower vapor pressure than does platinum and the platinum/rhodium alloy also has a lower -;
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vapor pressure than pure Pt. This prevents loss of the oxidized charge trans~er material at elevated normal operating temperatures, in contrast to the vaporization loss which occurs where the charge transfer material is pure Pt.
Figures 13 through 18 graphically depict, for specified sensor operating temperatures~ the durability of the sensors having Pt/Rh charge transfer material applied as taught hereinO The solid lines 70 in Figures la 13, 15 and 17 represent the maximum permissible response time for exhaust gas sen ors as currently specified by Ford Motor Company. In Figures 14, 16 and 18, the broken lines having the letters R and L depict, respectively, the rich minimum and lean maximum output voltage specified by Ford Motor Company for exhaust gas sensors in use over a period of hours.
The data ~or Figures 13 through 18 was obtained from titania exhaust gas sensors having Pt/Rh charge transfer materials applied in a 2~5% by weight solution wherein 2Q the ratio of Pt to Rh was nine parts Pt to one part Rh.
The titania exhaust gas sensors were used on a feedback i carburetor engine of 2.3 liter displacement. The values depicted are for 25 sensors tested for durability on a dynamometer. Figures 13, 15 and 17 show response times, both R-L and L-R for these sensors as a function of durability in hours. Figures 14, 16 and 18 show the output voltages of these sensors Ipercent of the input reference voltage) as a function of durability hours.
In Figures 13 and 14, it may be seen that the lean-to-rich and rich-to-lean response times for these sensors remain below the specification limit for 350C over the entire durability time of 850 hours where the sensor tip temperature was between 250 and 290C. Specific temperatures are indicated for all of the data points in Figure 13. Figure 14 shows that the rich and lean ~ :.... . . :: , ~, ~ , i ,' r~ . ' , output vol~ages were substantially above the minimum-for-rich and below the maximum-for-lean specification limits for sensor temperatures from 250~ to 290C.
Figures 15 and 16 show the durability of the sensors when operated at a sensor tip temperature of 350C. Again it may be seen that the response time for the sensors is below the specification limit fox 350C and that the output ~oltages for rich are above the rich voltage specification minimum and for lean are below the lean maximum voltage limit~
Figures 17 and 18 show that, for operation of the sensor at 850C, the response time is below the specifi-cation limit applicable thereto and the output voltages are above the minimum voltage limit for the rich condition and below the maximum voltage limit for the lean condition.
Zirconia exhaust gas sensors are known in the prior art. These sensors are ionically conductive, in contrast to the titania sensors which have a variable resistance electrical characteristic as has ~een described in detail herein The zirconia sensor, however, is a device that utilizes a charge transfer material in the production of an EMF having a magnitude proportional to the log of the ratio of the respective partial pressures of oxygen applied to opposite sides of the zirconia ceramic material. This EMF is produced due to a difference in the oxygen concentrations on opposite sides of this ceramic material. Where there is such a difference, the side of the material having the greater oxygen con-centration has oxygen molecules that are disassociated into oxygen atoms that pick up two electrons at the charge transfer material and ~hus becomes ions. The oxygen ions migrate through the zirconia cer~mic material to the opposite side thereof. On this opposite side, there is another charge transfer material to which the .
oxygen ion gives up the two eIectrons previously picked upO In so doing, oxygen molecules again æ e formed, but there is an excess of negative charges on the side of the zirconia sensor to which the oxygen ion has migrated S and then given up its charge. On ~he opposite side, the charge transfer material will have lost electrons and as a result an EMF or potential difference can be sensed.
The external sensing circuitry connecting the charge transfer materials on opposi~e sides of the zirconia ceramic providesa high impedance path for electron flow from one of ~he charge transfer materials ~o the othex.
The charge transfer~ material typically used with zirconia sensors is pure Pt. The zirconia ceramic is very dense and the Pt do~s not adhere well to ~his material. For this reason and for the protection of the Pt, the Pt is covered with a refractory material. The refractory material prevents vaporization or other 105s of the Pt.
If suitable adhesion of a Pt/Rh charge transfer material could be achieved on the zirconia sensor, then these charge transfer materials would be able to achieve the benefits of the invention described herein and the elimination of the refractory material on the charge transfer material may be possible.
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With respect to sansors manufactured in accordance with the teachings o~ the aforementioned Canadian Patent Application Serial No. 310,561, the platinum charge transfer material applied to the porous titania oxygen 3a sensor element was found to substantially Improve the sensor response time as compared to a titania oxygen sensor having no charge transfer material, pax~icularly at low sensor operating temperatuxes. Unfortunately, it was found that the sensors having platinum as a charge 35 transfer material developed substantially leng~hened response times after the sensors had been in us~ for a , -16~
number of hours. Not only did the response time become very slow fox these sensors, but also the minimum temperature at which the sensor output response occurs increased substantially. This was very undesirable~ The sensor should respond at as low a temperature as possible so that feedback fuel control of the air-fuel ratio supplied to the internal combustion engine may be achieved at an early stage during its warm-up period.
The inventors have discovered that the change in response time of the sensor and the increased minimum operating temperature are the result of loss of the platinum charge transfer material that had been deposited on the titania sensor element. This loss is ~he result of the oxidation of the platinum at the higher sensor operating tempera~ures normally encountered~ Platinum forms platinum oxide PtO2 at elevated temperatures, with substantial amounts being formed at about 850C.
Unfortunately, platinum oxide vaporizes quite readily and leaves the porous titania oxygen sensing element 46. In a cooler portion of the exhaust conduit, the platinum oxide decomposes and i deposited as pure platinum. In fact, it has been found that some of the platinum is deposited on the nearby thermistor 48, which tends to be somewhat cooler than the oxygen sensor 46. This adversely affects the behavior of the thermistor hy increasing its response to variations in the partial pressure of oxygen in the exhaust gas. The thermistor is intended to respond only to temperature.
The inventors have found that if a small amount of rhodium is added to the platinum charge transfer material, not only can the vaporization of the charge transfer material be prevented, but the response time of the sensor can he made very fast, about equal to that of a fresh titania sensor with a platinum charge transfer material. Also, the minimum operating tempexature of the sensor can be substantially reduced below the minimum operating temperature of a titania sensor having pure ;
.
.
- ' , .
.
platinum as the charge transfer ma~erial. An important chaxacteristic of the improvement is that a small amount of rhodium, a material which has a lower vaporization pressure than platinum and which has a lower ionization potential as well, provides the enhanced sensor behavior.
A solution of platinum and rhodium in 2 l/2% con-centration may be used to apply the platinum/rhodium charge transfer material to a porous titania sensor element 46. Solution containing nine parts by weight platinum to one part by weight rhodium (a 90/10 platinum-rhodium composition) is believed to be very satisfactory in achieving the results described herein. A solution containing platinum in the above ratio and in the amount of 2 l/2% by weight is formed by mixture of chlcroplatinic acid and rhodium chloride. The platinum and rhodium ions - in the solution are intimately mixed on an atomic scale.
A fresh titania sensor element is immersed in the solution.
The solution then is allowed to evaporate leaving crystals of platinum chloride and rhodium chloride on the sensor element. It is believed that these crystals are very fine and are so intimately mixed that, upon heating of the titania sensor element to about 900C, the salts decompose to leave platinum and rhodium atoms mixed with one another on an atomic scale. Thus, it is ~elieved that a true alloy of platinum and rhodium is formed by use of the above procedure. This platinum/rhodium alloy forms the charge transfer material for titania that produces the results hereinafter described.
With particular reference to Figur~ 10, there is 3Q shown a comparison of the response tLme of unaged (fresh) titania oxygen sensors as a function of titania element tip temperature in ~C. Temperatures are measured with a thermocouple positioned between, but not quite in contact with, the oxygen sensing element 46 and the thermistor 480 35 The data is plotted for rich-to-lean and lean-to~rich responses of some sensor elements having lO0~ platinum deposited by solution thereon as described ln the afore-.. . .. . .. . .. . . . . . . .
_ . _ _ _ . . _ .. _ . . .. _ .. . _ . . _ . . . .. . _ . . ..
. . .. . . . . . ..
. .
.1, ;, .~:
mentioned Canadian Patent Application Serial No. 310,561 and other sensor elements having the platinum/rhodium alloy of the present invention deposited thereon as taught herein.
Each data point in this graph represents the average of the response times of twenty-three sensors. The R-L
response time of fresh sensors having a pure Pt charge transfer materail applied in a 5% solution of chloro-platinic acid (H2PtC16) may be seen to go from infinity to about 30 msec in the 300C temperature range. The L-R
response time for this group of sensors having Pt charge transfer material is essentially identical to the R-L
response, except where the respective curves diverge at the temperature region of about 330C~
Also represented in Figure 10 is a group of sensors having a charge transfer material of 90~ Pt and 10% Rh applied to the porous titania in a solution of H2PtC16 and RhC13.3H20 containing Pt and Rh in total amount of 2 1/2% by weight. Both the R-L and L-R response times for this group arP illustrated. These response times go from infinity to less than 50 msec over the temperature range from about 200 to 27SC.
As previously mentioned, all of the data for Figure 10 was obtained from fresh (unaged) titania sensors. It is evident that the use of the 90% Pt/10% Rh charge transfer material has substantially reduced the minimum operating temperature of the fresh titania sensors while also providing the same fast response time over the entire normal sensor operating range as the sensors with pure Pt 30 charge transfer material.
Figure 11 is similar to Figure 10, but shows results obtained for the twenty-three sensors after they had been exposed, for 250 hours, to the exhaust gases produced by the combustion of a natural gas/air mixture that varied 35 cyclically each second about the stoichiometric mixture level. The data establishes some degradation in response ' time for sensors having either the pure Pt or the 90~ Pt/
10~ Rh charge transfer materials. However, the substantial improvement obtained with the Pt/Rh charge transfer material on fresh sensors is retained after the 250 hours of aging. The minimum operating temperature is down to about 25CC for the Pt/Rh sensors and is about 400C for the Pt sensors. The time response over the entire normal operating temperature range is very satisfactory for the Pt/Rh sensors. The shift to a minimum operating tempera-lQ tuxe of 400C for the Pt sensor group is caused byvaporization loss o the Pt charge transfer material.
Figure 12 is a graph of percent Pt-Rh composition versus sensor response time~ The plotted data points for Pt-Rh alloys each are the average o data obtained from lS measurements made on six fresh sensors. Otherwise stated, there were six sensors having a 100% Rh charge transfer material, six sensors having a 50% Pt/50% Rh charge transfer material, six sensors having a 70~ Pt/30% Rh charge transfer material, and six sensors having a 90% Pt/
10% Rh charge transfer material. The data for sensors having 100% Pt charge transfer material was obtained from a group of fresh sensors.
In Figure 12, cur~es are provided for R-L response time at 350C, for L-R response time at 350C, for R-L
response time at 850C and for L-R response time at 850C.
The L-R response time at the lower operating temperature of 350C is independent of the percent Pt/Rh composition~
On the other hand, the R-L response times at both 350C
and the higher 850C temperature increase as the %Rh in 3Q the charge transfer material is increasedO In other words, it appears that Rh in the charge transfer material degrades the sensor performance by increasing the response time of the fresh sensors, particularly at 350C in the lower portion of the normal sensor operating temparature rangeO
.
,";
: ;:. . ,: .
The efect of increasing Rh content in the Pt/Rh charge txansfer material is especially pronounced with respect to the L-R response time at 850C.
From the data plotted in Figure 12, it is clear that for Pt/Rh charge transfer materials in the range of compositions from 70% Pt/30~ Rh to 100~ Pt/0% Rh, there is little if any degradation in the response time of the sensors. On the other hand, the L-R response at 850C for compositional ranges between 70%Pt/30%Rh and 0%Pt~100%Rh is very much affected by the increasing rhodium content. The xeason for this is descxibed in the following paragraphO
At higher temperatures in a lean exhaust gas environment, the titania sensor having both Pt and Rh mixed together in the charge transfer material forms oxides of both Pt and Rh. The Pt oxide (Pt02) îs less stable than the Rh oxide (Rh02) a When the mixture chanyes from L-R so that there is substantially less oxygen in the gaseous exhaust mixture, then oxygen is removed from the titania sensor and from the charge transfer material.
The Pt oxide breaks down easily and its oxygen is removed quite rapidly, but the Rh oxide is more stable than the Pt oxide and it takes longer to decompose into oxygen and rhodium. The presence of the Rh oxide delays the removal of oxygen from the titania crystal structure and therefore delays a reduction in its resistanceO The more Rh there is present in the chaxge transfer material, the slower is the titania sensor respo~se, as is demonstrated by he L-R curve at 850C in Figure 120 However, as shown by Figure 12, for compositions of the charge transfer material having 30% or less Rh by weight, there is little if any degradation in response time, but yet this small amount of Rh is sufficient to prevent vapoxization of the oxidized charge transfer material. This is believed to be because Rh has a lower vapor pressure than does platinum and the platinum/rhodium alloy also has a lower -;
:
vapor pressure than pure Pt. This prevents loss of the oxidized charge trans~er material at elevated normal operating temperatures, in contrast to the vaporization loss which occurs where the charge transfer material is pure Pt.
Figures 13 through 18 graphically depict, for specified sensor operating temperatures~ the durability of the sensors having Pt/Rh charge transfer material applied as taught hereinO The solid lines 70 in Figures la 13, 15 and 17 represent the maximum permissible response time for exhaust gas sen ors as currently specified by Ford Motor Company. In Figures 14, 16 and 18, the broken lines having the letters R and L depict, respectively, the rich minimum and lean maximum output voltage specified by Ford Motor Company for exhaust gas sensors in use over a period of hours.
The data ~or Figures 13 through 18 was obtained from titania exhaust gas sensors having Pt/Rh charge transfer materials applied in a 2~5% by weight solution wherein 2Q the ratio of Pt to Rh was nine parts Pt to one part Rh.
The titania exhaust gas sensors were used on a feedback i carburetor engine of 2.3 liter displacement. The values depicted are for 25 sensors tested for durability on a dynamometer. Figures 13, 15 and 17 show response times, both R-L and L-R for these sensors as a function of durability in hours. Figures 14, 16 and 18 show the output voltages of these sensors Ipercent of the input reference voltage) as a function of durability hours.
In Figures 13 and 14, it may be seen that the lean-to-rich and rich-to-lean response times for these sensors remain below the specification limit for 350C over the entire durability time of 850 hours where the sensor tip temperature was between 250 and 290C. Specific temperatures are indicated for all of the data points in Figure 13. Figure 14 shows that the rich and lean ~ :.... . . :: , ~, ~ , i ,' r~ . ' , output vol~ages were substantially above the minimum-for-rich and below the maximum-for-lean specification limits for sensor temperatures from 250~ to 290C.
Figures 15 and 16 show the durability of the sensors when operated at a sensor tip temperature of 350C. Again it may be seen that the response time for the sensors is below the specification limit fox 350C and that the output ~oltages for rich are above the rich voltage specification minimum and for lean are below the lean maximum voltage limit~
Figures 17 and 18 show that, for operation of the sensor at 850C, the response time is below the specifi-cation limit applicable thereto and the output voltages are above the minimum voltage limit for the rich condition and below the maximum voltage limit for the lean condition.
Zirconia exhaust gas sensors are known in the prior art. These sensors are ionically conductive, in contrast to the titania sensors which have a variable resistance electrical characteristic as has ~een described in detail herein The zirconia sensor, however, is a device that utilizes a charge transfer material in the production of an EMF having a magnitude proportional to the log of the ratio of the respective partial pressures of oxygen applied to opposite sides of the zirconia ceramic material. This EMF is produced due to a difference in the oxygen concentrations on opposite sides of this ceramic material. Where there is such a difference, the side of the material having the greater oxygen con-centration has oxygen molecules that are disassociated into oxygen atoms that pick up two electrons at the charge transfer material and ~hus becomes ions. The oxygen ions migrate through the zirconia cer~mic material to the opposite side thereof. On this opposite side, there is another charge transfer material to which the .
oxygen ion gives up the two eIectrons previously picked upO In so doing, oxygen molecules again æ e formed, but there is an excess of negative charges on the side of the zirconia sensor to which the oxygen ion has migrated S and then given up its charge. On ~he opposite side, the charge transfer material will have lost electrons and as a result an EMF or potential difference can be sensed.
The external sensing circuitry connecting the charge transfer materials on opposi~e sides of the zirconia ceramic providesa high impedance path for electron flow from one of ~he charge transfer materials ~o the othex.
The charge transfer~ material typically used with zirconia sensors is pure Pt. The zirconia ceramic is very dense and the Pt do~s not adhere well to ~his material. For this reason and for the protection of the Pt, the Pt is covered with a refractory material. The refractory material prevents vaporization or other 105s of the Pt.
If suitable adhesion of a Pt/Rh charge transfer material could be achieved on the zirconia sensor, then these charge transfer materials would be able to achieve the benefits of the invention described herein and the elimination of the refractory material on the charge transfer material may be possible.
,. , . , - ::
,, , " ,. ; ~ :
.
Claims (5)
1. An improved sensor of the type having a ceramic element that undergoes a change in an electrical characteristic of the ceramic element in response to a change in the partial pressure of oxygen in a mixture of gases to which the ceramic element is exposed, the ceramic element having electrodes connected thereto in spaced relationship and extending from the ceramic element to permit the resistance between them to be sensed, the ceramic element being porous to permit oxygen in the gases to which the ceramic element is exposed to migrate into and out of its interior regions, and the ceramic element having a discontinuous or porous charge transfer material deposited thereon, wherein the improvement comprises:
a majority of the charge transfer material by weight is Pt and Rh is included in an amount sufficient to allow the charge transfer material to be retained on the ceramic element when the ceramic element is exposed to gaseous mixtures produced by lean-mixture combustion at ceramic element temperatures in excess of about 850°C.
a majority of the charge transfer material by weight is Pt and Rh is included in an amount sufficient to allow the charge transfer material to be retained on the ceramic element when the ceramic element is exposed to gaseous mixtures produced by lean-mixture combustion at ceramic element temperatures in excess of about 850°C.
2. An improved sensor according to Claim 1, wherein the charge transfer material consists essentially of Pt and Rh, the Rh being present in an amount not exceeding about 30% by weight of the total weight of Pt and Rh.
3. An improved sensor according to Claim 2, wherein the charge transfer material consists essentially of Pt and Rh in alloy ratio of about nine parts by weight Pt to one part by weight Rh.
4. An improved sensor according to Claim 3, wherein the titania ceramic element is porous and wherein the Pt and Rh are applied in a 2 1/2% by weight solution to said porous titania ceramic element, the ceramic element being immersed in the solution.
5. An improved sensor according to Claim 4, wherein said solution comprises a mixture of H2PtC16 and RhC13.3H2O.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US5,425 | 1979-01-22 | ||
| US06/005,425 US4225559A (en) | 1979-01-22 | 1979-01-22 | Ceramic element sensor |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1128610A true CA1128610A (en) | 1982-07-27 |
Family
ID=21715791
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA343,134A Expired CA1128610A (en) | 1979-01-22 | 1980-01-07 | Ceramic element sensor |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US4225559A (en) |
| EP (1) | EP0014091B1 (en) |
| JP (1) | JPS55136949A (en) |
| CA (1) | CA1128610A (en) |
| DE (1) | DE3062913D1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5683570A (en) * | 1993-06-04 | 1997-11-04 | Dalhousie University | Gas detection method |
Families Citing this family (14)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3118522A1 (en) * | 1981-05-09 | 1982-12-02 | Process-Electronic Analyse- und Regelgeräte GmbH, 7321 Wangen | Method of monitoring the serviceability of an oxygen probe |
| DE3217700A1 (en) * | 1981-05-15 | 1982-12-02 | Nitto Chemical Industry Co., Ltd., Tokyo | METHOD FOR IMPROVING THE ACTIVITY OF METAL OXIDE CATALYSTS CONTAINING TELLUR |
| JPS6130760A (en) * | 1984-07-24 | 1986-02-13 | Toyota Central Res & Dev Lab Inc | Oxygen sensor and manufacture thereof |
| JPS61101446A (en) * | 1984-10-22 | 1986-05-20 | 矢崎総業株式会社 | Oxygen ion electroconductor material |
| JPS61195338A (en) * | 1985-02-25 | 1986-08-29 | Ngk Spark Plug Co Ltd | Air fuel ratio sensor |
| JPS61223643A (en) * | 1985-03-29 | 1986-10-04 | Nohmi Bosai Kogyo Co Ltd | Gaseous hydrogen detecting element and its production |
| JPS61223642A (en) * | 1985-03-29 | 1986-10-04 | Nohmi Bosai Kogyo Co Ltd | Gaseous hydrogen detecting element and its production |
| US4911890A (en) * | 1986-01-29 | 1990-03-27 | The United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration | Device for quickly sensing the amount of O2 in a combustion product gas |
| US4698313A (en) * | 1986-02-07 | 1987-10-06 | Phillips Petroleum Company | Method and device for controlling a delayed coker system |
| US4857275A (en) * | 1986-03-19 | 1989-08-15 | Ngk Spark Plug Co., Ltd. | Thick-film gas-sensitive element |
| USRE33980E (en) * | 1986-03-19 | 1992-06-30 | Ngk Spark Plug Co., Ltd. | Thick-film gas-sensitive element |
| US4903648A (en) * | 1989-04-14 | 1990-02-27 | Outboard Marine Corporation | Engine with improved exhaust gas sensing |
| US5871375A (en) * | 1996-10-15 | 1999-02-16 | Itt Manufacturing Enterprises, Inc. | High temperature sensor assembly |
| US10273907B2 (en) * | 2014-12-30 | 2019-04-30 | Ford Global Technologies, Llc | Systems and methods for engine-off natural vacuum leak testing |
Family Cites Families (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2782103A (en) * | 1951-10-12 | 1957-02-19 | Phillips Petroleum Co | Analyzer for gaseous mixtures |
| BE771767A (en) * | 1970-08-26 | 1971-12-31 | Nat Res Dev | SEMICONDUCTOR DEVICES SENSITIVE TO GAS |
| GB1461891A (en) * | 1974-06-05 | 1977-01-19 | Nat Res Dev | Detection of carbon monoxide |
| US4058485A (en) * | 1974-12-26 | 1977-11-15 | Union Carbide Corporation | Porous metal-alumina composite |
| US4066413A (en) * | 1975-03-03 | 1978-01-03 | Nippon Soken, Inc. | Gas component detection apparatus |
| JPS5276991A (en) * | 1975-12-23 | 1977-06-28 | Nippon Soken | Detector for gas composition |
| IT1089129B (en) * | 1976-12-15 | 1985-06-18 | Uop Inc | ELECTROCHEMISTRY CELL HAVING A SOLID SURFACE ELECTROLYTE AND PROCEDURE TO PREPARE IT |
| CA1119084A (en) * | 1977-10-05 | 1982-03-02 | Stanley R. Merchant | Catalytic material impregnated, porous, variably resistive exhaust gas sensor and method of impregnation |
-
1979
- 1979-01-22 US US06/005,425 patent/US4225559A/en not_active Expired - Lifetime
-
1980
- 1980-01-07 CA CA343,134A patent/CA1128610A/en not_active Expired
- 1980-01-21 JP JP561780A patent/JPS55136949A/en active Pending
- 1980-01-22 DE DE8080300195T patent/DE3062913D1/en not_active Expired
- 1980-01-22 EP EP80300195A patent/EP0014091B1/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5683570A (en) * | 1993-06-04 | 1997-11-04 | Dalhousie University | Gas detection method |
Also Published As
| Publication number | Publication date |
|---|---|
| EP0014091A1 (en) | 1980-08-06 |
| EP0014091B1 (en) | 1983-05-04 |
| US4225559A (en) | 1980-09-30 |
| DE3062913D1 (en) | 1983-06-09 |
| JPS55136949A (en) | 1980-10-25 |
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