|Publication number||US3911386 A|
|Publication date||Oct 7, 1975|
|Filing date||Jan 15, 1975|
|Priority date||Jan 15, 1975|
|Publication number||US 3911386 A, US 3911386A, US-A-3911386, US3911386 A, US3911386A|
|Inventors||Beaudoin Gordon L, Foote Lawrence R, Hough Jerome F, Merchant Stanley R|
|Original Assignee||Ford Motor Co|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (3), Referenced by (22), Classifications (8)|
|External Links: USPTO, USPTO Assignment, Espacenet|
United States Patent [191 Beaudoin et al.
[4 1 Oct. 7, 1975 1 EXHAUST GAS AIR FUEL RATIO SENSOR  Assignee: Ford Motor Company, Dearborn,
221 Filed: Jan. 15, 1975 21 Appl. No.: 541,365
 U.S. Cl. 338/34; 23/254 E; 73/27 R  Int. Cl. H01C 13/00  Field of Search 338/34; 23/254 E, 255 E,
23/288 F, 288 EC; 340/237 R, 237 P; 200/6103; 252/477 R Hardtl 338/34 Taguchi 338/34 Primary ExaminerC. L. Albritton Attorney, Agent, or FirmRobert A. Benziger; Keith L. Zerschling [5 7] ABSTRACT An exhaust gas air/fuel ratio sensor is formed of a plurality of wafers of ceramic material. A pair of resistance sensing leads are situated within a titania exhaust gas sensor wafer. The operating temperature of the sensor wafer is maintained at a desired level by forming a pair of heater wafers and situating the heater wafers on either side of the titania wafer. Each heater wafer is comprised of a length of platinum heater wire embedded within an alumina ceramic wa- 4 Claims, 2 Drawing Figures References Cited fer.
UNITED STATES PATENTS 2,806,991 9/1957 White 338/34 X POWZIQ JZ/PPZ) Z6 /2 xii/V301? [([CTKO lV/C j 0 MfH/VS EXHAUST GAS AIR FUEL RATIO SENSOR CROSS-REFERENCE TO RELATED APPLICATIONS The present invention is an improvement to the inventions disclosed and claimed in copending, commonly assigned patent applications Ser. Nos. 391,424 and 393,698 filed in the names of H. L. Stadler et al. Co-pending commonly assigned patent application Ser. No. 375,993 filed in the names of Kushida et al. entitled Circuit for Converting a Temperature Dependent Input Signal to a Temperature Independent Output Signal is a related patent application.
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to the field of exhaust gas sensors and more particularly to that portion of the above-noted field which is concerned with the analysis of the exhaust gases of an internal combustion engine to generate electrical signals indicative of the air/fuel ratio of the combustion mixture. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with devices for sensing exhaust chemistry, which devices rely upon the resistance change of titania ceramic oxide material in response to variations in the partial pressure of oxygen of its immediate gaseous environment. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with provision of means for maintaining the temperature of the ceramic sensor material at a predetermined controlled level without contaminating or otherwise influencing the measured titania resistance. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with the provision of a device for insertion within the exhaust system of an internal combustion engine for performing selective chemical analysis thereof, which device may give reliable performance over extended periods of time. More particularly still, the present invention is directed to that portion of the abovenoted field which is concerned with devices having the capability of providing long duration generation of signals indicative of an exhaust gas chemistry characteristic which signals may be used to provide feed-back control of an internal combustion engine.
2. Description of the Prior Art It is known that certain ceramic materials exhibit a predictable, variable electrical behavior in the pres ence of variations in the partial pressure of oxygen in the gaseous environment of the material. One example of this electrical phenomenon is the galvanic voltage developed by a zirconia ceramic body used to separate gaseous mixtures having differing partial pressures of oxygen. Another example is the variation in resistance of a titania ceramic material maintained at elevated temperature, set forth in the above-noted copending, commonly assigned patent applications Ser. Nos. 39l,424 and 393,698.
It has been proposed that the ability to generate an electrical signal indicative of a variation in the partial pressure of oxygen may be utilized, with suitable support electronics, to provide a feed back control system for an internal combustion engine. Zirconia and titania exhaust gas sensors, situated within an internal combustion engine exhaust system, have demonstrated large scale electrical signal transitions for variations in the air/fuel ratio of the combustion mixture being provided to the engine as that mixture ratio is changed from a rich mixture to a lean mixture through a stoichiometric mixture ratio. Any combustible air/fuel mixture having a quantity of fuel in excess of the quantity of fuel necessary for complete combustion of the available quantity of air is considered to be a rich mixture. Conversely, any combustible air/fuel mixture having a quantity of air in excess of the quantity of air necessary for complete combustion of the available quantity of fuel is considered to be a lean mixture. A stoichiometric mixture has quantities of fuel and of air sufficient for complete combustion.
In working with these devices, it has become apparent that each device has peculiarities which render the device less than ideal. In the case of zirconia, exposure of one surface of the zirconia sensor to atmospheric conditions is necessary in order to provide a reference partial pressure of oxygen. In the case of the titania, the measured resistance is a function of temperature as well as of the partial pressure of oxygen. Titania sensors therefore require additional techniques for maintaining the ceramic titania sensor at a predetermined temperature level.
According to the above-noted co-pending commonly assigned patent application Ser. No. 391,424 the titania wafer may be maintained at a selected temperature, for example, in the range of 600C to 900C, by fabricating the sensor from three wafers of ceramic material with a heater element contained between the first and second wafers and the resistance sensing leads contained between the second and third wafers. One problem with this approach has resulted in the fact that a temperature gradient is established within the device which gradient passes through the resistance sensing zone. This gradient causes the maintenance of the desired temperature, which should not vary more than 2 or 3 centigrade, at the resistance sensing zone to be diffi cult. One way to achieve closely controlled temperatures at the resistance sensing zone would be to use a thermocouple, positioned in the resistance sensing zone, to control the application of heater energy. This approach is not desirable since thermocouples are relatively expensive devices and they add to the complexity of the resulting sensor device. It is also an object of the present invention to provide a means for maintaining a titania exhaust gas sensor at a selected temperature without the establishment of any signficiant temperature gradients and which may be used reliably with external electronic heater control means to maintain the selected temperature without the use of thermocouples.
As described in the above-noted co-pending commonly assigned patent application Ser. No. 393,698, it is believed that one important feature of a resistive titania exhaust gas sensor used in a feedback control system for an internal combustion engine resides in rapid response time achieved through maintaining a high degree of porosity. In order to avoid the undesired temperature gradient heating previously mentioned, a pair of heaters could be situated on either side of the titania sensing wafer. Such an approach would require that the rapid gas transfer through the device be maintained. It is therefore a further and specific object of the present invention to provide a ceramic material for encasement of a pair of heater conductors which material will be resistive to microfracturing. It is also a further and specific object of this invention to provide such a material which may be provided with a high degree of porosity consonant with maintaining a good response time for an included titania exhaust gas sensor wafer. A further problem which can be anticipated to occur in any sensor arrangement which provides for a mixture of ceramic material having interface zones is to provide such materials which will not result in contamination of the active sensor element by migration of the materials.
across the interface zone.
The existence of the temperature gradient may be alleviated by using a pair of heater wires operated in unison and placed on opposite sides of the resistance sensing zone. This approach presents a further problem in that the application of resistance wire heating of the titania from wires embedded within the titania may produce microfractures. On the one hand, these microfractures may extend into the resistance may extend into the resistance sensing zone causing a measurable resistance change such as may stimulate false regulation. On the other hand, these microfractures may expose the embedded heater wire to the gaseous atmosphere surrounding the sensor body. This may cause the heater wires to burn out. In those instances where the heater wires are coupled to a temperature controller as described in co-pending commonly assigned patent application Ser. No. 484,896, filed July 1, 1974 in the name of Lawrence R. Foote and titled Electrical Control System for an Exhaust Gas Sensor, false temperature signals may result even if burn out does not occur. This will cause the sensor output signal to drift from the output signal which should be generated under the existing conditions. It is therefore an object of this invention to provide a ceramic element, for inclusion with the exhaust system of an internal combustion engine, to include a titania exhaust gas chemistry responsive element, which need not include temperature sensing means in the resistance sensing zone. It is a further object of the present invention to provide such a device which is resistant to microfracture induced resistance changes in the sensing zone. It is therefore a further and specific object of the present invention to provide an exhaust gas air/fuel ratio sensor having an active section comprised of a variably resistive titania ceramic material which includes a pair of resistance sensing leads and having a pair of heater wires embedded within alumina ceramic material disposed on opposite sides of the active section to maintain the active section at an operating temperature without requiring thermocouple control techniques and which will not result in any ceramic contamination or degradation of the active titania section of the sensor and which will not provide any substantial masking of the active titania section to changes in the partial pressure of oxygen in the environment of the sensor.
SUMMARY OF THE PRESENT INVENTION The present invention provides a multi-layered ceramic exhaust gas sensor body having an active variably resistive titania section sandwiched between a pair of heater sections. The active titania section is comprised of a pair of resistance sensing leads disposed within a wafer or disc-like body of titania ceramic material. Each of the heater sections is comprised of a length of heater wire embedded within a ceramic wafer comprised essentially of alumina. The two heater sections and the active titania section are formed from green ceramic material and are co-fired into a unitary ceramic body. The heater sections are. connected electrically in series and may be communicated to, for example, an electronic heater control circuitresponsive to the heater resistance for maintainingthe current flow through the heater wires at a level whichwill provide for maintenance of the heater sections and the active titania section sandwiched therebetween at a selected temperature level.
Each of the three sections of the exhaust gas sensor may be comprised of a pair of wafers or discs of the selected ceramic material, in a green or un-fired state. The necessary electrical leads and heater wires may be placed between the wafers while the ceramics are in a green state and the wafers may be stacked together prior to final firing.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates an electrical schematic block diagram for an exhaust gas sensor and its associated control.
FIG. .2 illustrate an exhaust gas sensor fabricated in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing wherein like numbers designate like structure throughout the various views thereof, FIG. 1 illustrates an exhaust gas sensor body 10 according to the present invention having an associated sensor electronic means 12. Exhaust gas sensor body 10 is illustrated as having resistance heater means 14 and a pair of resistance sensing leads l6, l8 embedded therein. Electrical heater means 14 are energized from a power supply 20 through leads 22, 24. Resistance sensing leads l6, 18 are connected to the sensor electronic means 12 which also receives energy from the power supply 20 over conductors 26, 28. This structure is generally as described in the aforementioned copending commonly assigned patent application Ser. No. 391,424 and further description is not believed to be necessary to an understanding of the instant invention.
Power supply 20 will provide electrical energy through the electrical heater conductor 14 to apply electric heating energy to the exhaust gas sensor 10. Heater conductor 14 may be, for example, a noble metal conductor such as platinum. similarly, resistance sensing leads l6, 18 would be formed of a noble metal since such material readily withstand the temperature of ceramic firing. Power supply 20 may be, for example, the electronic heater control of co-pending commonly assigned patent application Ser. No. 484,896. Conversely, it could also be a simple battery with a thermocouple control arrangement.
When resistance sensing probes l6, 18 are mutually embedded in spaced-apart relation within a titania sensor section, the electrical resistance of the titania between sensing leads 16, 18 will be a function of a) the operating temperature and b) the partial pressure of oxygen in the environment in which sensor body 10 is immersed. With power supply 20 operated to maintain the temperature generated by electrical heating means 14 at a constant value, variations in the electrical resistance measured by sensing probes l6, 18 will be solely a function of the partial pressure of oxygen in the environment in which sensor is located. The resistance sensing zone in the body of material of the discs 38 of wafer 36 which lies between resistance sensing leads l6, l8 and is here denoted by numeral 42. Sensor electronic means 12 will receive an electrical energy over conductors 26, 28 and will measure the amount of resistance sensed by sense leads l6, 18 to generate an output signal on conductor 30 which signal may be made readily indicative of the air/fuel ratio of the combustible mixture generating the exhaust gas environment of sensor 10. The electrical character of the signal appearing at output 30 will be a function of the type of control desired in the feedback system in which the present invention may be utilized and the sensor electronic means may be readily adapted to generate the proper kind of electrical signal.
Referring. now to FIG. 2, the exhaust gas sensor body 10 according to the present invention is illustrated in an exploded view. Exhaust gas sensor body 10 is made up of a sandwich of heater wafer means 32, 34 which are arranged to be on opposite sides of a central active titania exhaust gas sensing section 36 is illustrated as being comprised of a pair of discs 38 of titania ceramic material generally as described in the above-noted copending commonly assigned patent applications. A pair of resistance sensing leads I6, 18 are maintained in spaced-apart relation between discs 38. The heater wafer sections 32, 34 are comprised of a pair of discs 40 of ceramic material, the preferred composition of which is described hereinbelow. A heater conductor 14 is situated between each of the discs 40 of each of the heating wafer means 32, 34 and are shown to be connected electrically in series.
In fabricating the exhaust gas sensor 10 of the instant invention, the active titania wafer 36 may be fabricated generally in accordance with the teachings of the above-noted co-pending commonly assigned patent ap plications Ser. Nos. 391,424 and 393,698. A pair of resistance sensing probe leads 16, 18 would be sandwiched between a pair of green ceramic discs 38 formed from a titania material as noted above and the discs 38 would be uniaxially compressed to provide for an initial bonding therebetween. According to the present invention, discs 40 are formed of alumina ceramic material. Each of the heater wafer sections 32, 34 would be fabricated by sandwiching a heater conductor 14 between a pair of discs 40 which would be compressed uniaxially to provide for initial bonding. The active wafer 36 would then be sandwiched between a pair of heater wafer means 32, 34 with further uniaxial pressure applied to provide for initial bonding between the green ceramic material forming each of the heater wafer sections 32, 34 and the green ceramic material forming the active wafer 36. Thereafter, the composite would be fired to produce a finished ceramic body. The heater wafers 32, 34 are formed advantageously of alumina ceramic for several reasons. The coefficient of thermal expansion of the alumina ceramic more closely matches the coefficient of thermal expansion of the platinum noble metal conductors used as the electrical heating means 14. The alumina is sufficiently highly porous at thicknesses consistent with internal heater tem perature control to permit rapid gas transfer from the surrounding atmosphere to the titania sensor wafer 36. The alumina also forms a strong ceramic material which aids in adding to the strength of the sensor body 10. In addition, the alumina heater wafers 32, 34, when cofired with the titania sensor wafer 36 will form a strong ceramic bond with the titania at the interface of a ceramic disc 40 with a ceramic disc 38.
Each of the discs 38, 40 illustrated in the FIG. 2 embodiment may be for example one quarter of an inch in diameter and 0.025 in (twenty five thousandths of an inch) thick prior to final firing. Resistance sensing leads 16, 18 may be fabricated from platinum conductive wire and positioned in parallel approximately an eighth of an inch apart on the surface of one of the wafers 38. Similarly, the heater wire 14 may be fabricated from platinum resistance wire with a substantial length thereof deposited on the surfaces of the wafers 40 to provide a sufficient length to generate the necessary level of heating.
In the accomplishment of the objectives of the present invention, the wafers 40 are fabricated from an ultra fine alumina powder having a mean particle size of less than 1.0 microns. Use of the fine particle size alumina powder permits co-firing of the sensor assembly in order to provide for a strong ceramic bond formation between the active sensor wafer and each of the heater wafer sections 32, 34. The alumina ceramic material is comprised essentially of alumina having a chemical composition A1 0 and a purity of about 95 percent. It may include minor quantities of various metal oxides. In fabricating wafers 40, a mixture of approximately ceramic powder and 20% plastic ma terial such as polyvinyl butyral and dibutyl phthalate may be used. This material is deposited on a sheet and is controlled to have a thickness of approximately 0.025 in. The wafers are thereafter cut from the sheet while in a plastic state and the sensor 10 is fabricated by first forming the heater wafers 32, 34 and then plac ing an active sensor wafer 36 therebetween. The composite is thereafter uniaxially compressed to establish initial bonds between the discs 38 and their contacting discs 40. The composite of green ceramic discs is thereafter fired to form a finished ceramic. The final firing may follow the time temperature schedule set forth in the above-mentioned patent applications Ser. Nos. 391,424 and 393,698. The ceramist will recognize that other time-temperature relationships could be followed to arrive at a finished sensor body 10. The resulting heater wafer portions 32, 34 of the sensor body 10 have a typical pore size (diameter) of 0.15 microns with about 65% of the pore volume being comprised of pores having a size of from about 0. 10 microns to about 0.20 microns.
It will be seen that the present invention readily accomplishes its stated objectives. By using alumina ceramic material to provide the ceramic portion of each of the heater wafers 32, 34, a strong ceramic material is provided to protect the heater elements with the ceramic material being fully compatible with the titania material while resisting any tendency to produce ceramic material migration or contamination. By use of the fine particle size a relatively porous ceramic body is produced so that, when considering the overall thickness being in the neighborhood of fifty thousandths of an inch, the transport time for gas to penetrate through the heater wafers 32, 34 of the sensor body 10 to arrive at the active sensing wafer 36 is sufficiently low that good response time for the sensor body 10 can be obtained. Furthermore, the alumina and titania are sufficiently compatible so that strong ceramic .bonds are formed between the mating interface surfaces of the heater wafers 32, 34 and the active wafer 36. By providing two heater wafers disposed on opposite sides of the active section, the generation of undesirable temperature gradients through the acitve section such as each of said heater wafers comprising electrical resistance heater means embedded within an alumina ceramic wafer.
2. The sensor of claim 1 wherein each of said heater wafers is comprised of alumina ceramic forming powders having a purity of at least 3. The sensor of claim 2 wherein said alumina ceramic forming powders have a mean particle size of less than about 1.0 microns.
4. The sensor of claim 1 wherein each of said heater wafers comprises a pair of discs of green alumina ceramic material having the electrical resistance heater means sandwiched therebetween with the discs matured by cofiring with the titania wafer.
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|U.S. Classification||338/34, 338/28, 422/94, 73/31.5, 338/23|