CA2431018A1 - Combined oxygen and nox sensor - Google Patents
Combined oxygen and nox sensor Download PDFInfo
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- CA2431018A1 CA2431018A1 CA002431018A CA2431018A CA2431018A1 CA 2431018 A1 CA2431018 A1 CA 2431018A1 CA 002431018 A CA002431018 A CA 002431018A CA 2431018 A CA2431018 A CA 2431018A CA 2431018 A1 CA2431018 A1 CA 2431018A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/403—Cells and electrode assemblies
- G01N27/406—Cells and probes with solid electrolytes
- G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
- G01N27/4071—Cells and probes with solid electrolytes for investigating or analysing gases using sensor elements of laminated structure
Abstract
A combined oxygen and NOx sensor is provided. Generally, the combined sensor employs a sensor body (220) that includes two different types of electrodes -oxygen-porous electrode layers (16a, 16c) and dissociative oxygen-porous electrode layers (16b, 16d). In accordance with one embodiment of the present invention, the sensor comprises a sensor body, an oxygen content electrical signal output, and a NOx content electrical signal output. The sensor body is disposed in the gas and comprises a plurality of oxygen-porous electrode layers and a plurality of dissociative oxygen-porous electrode layers. The dissociative oxygen-porous electrode layers comprise a material selected to dissociation of NOx into nitrogen and oxygen.
Description
COMBINED OX'Y~GEN ~-1,ND NOx SENSOR
CROSS REFERENCE TO RELATED APPLrCATIONS
"his application claims the benefit of U.S_ provisional Application Serial No.
601254,081, filed December 7, 2000. This application is also a continuation-in-part o~'CJ.S.
Patent application Serial No. 09/662,773, filed September 15, 2000, which claims the beneft of U.S. Provisional Application Serial No. 601155,817, fled September 23, 1999.
'$ACI~GROUND OF THE fNV~NT~O1V
The pz~eserat invention, relates to a device for sensing the partial pressure of oxygen in a gas, and more particularly to an active nnultilayer sensor utilizing an oxygen ion conducting matexial. The present invention also relates to a combined sensor for measuring oxygen content and NOx content in a gas. ~tOx is utiaized herein to represent nitzic oxide, zaitrogen dioxide, nitrogen trioxide, etc_ It is vtridely recognized that one of the most important diagrxostics for monitoring the efficiency of any combustion process is the measuzemeut of the oxygen partzaJ
pressure in ara exhaust gas. Thus, oxygen sensoxs have loug been used to measure the oxygen content of exhaust gases from such divezse combustion processes as internal combustion engines in motor vehicles and coal, natural gas, or oil burning power generations facilities.
2O The most widely lrnown and used oxygen sensors are based on partially sfiabilized zirconia (PSZ) as the ion conductor. Such sensors function by monitoring the electxarnotive force (EMF) developed across an ion conductor which is exposed to different partial pressures of oxygen. Oxygen tends to move from a gas containing a high concerxtration of oxygen to vne of lower concentration. Tftwo gases are separated from each other by an electroded oxygen ion conductor, the oxygerx molecules will dissociate on one surface of tlae couductar and absorb electrons to loran oxygen ions. These ions then diffuse through the ionic conductor, leaving the entry surface with a def cieracy of electrons (OZ + 4e = 20'2). One the exit or lorw oxygen concenixation side of the conductor, oxygen iozzs leaving the conductor must give up electrons to form molecular oxygen, thus leaving the exit surface with an excess of electrons. This creates the $M11 between the two surfaces of floe ion eonduetor_ One problem with the use of partially stabilized zirconia sensors is that they must be operated at temperatures in the range of about 800 C to reduce intez-rtal resistance to a point where a current carp be measured. Further, the raw material costs of stabili2ed zireonia is zelatively high, and the ~xlelting point of zizconia is quite high (2700 C) so that formation of sensors is expensive.
Lawless, in U.S. Patent No. 4,462,89 x, describes a passive oxygen sensor using eeram ion conducting materials based ors nickel niobates and Bismuth oxides. The oxygen sensor includes a plurality of layers of tlxe ceramic material and a porous metallic conductor axrangec form a body having alterxzating ceramic and metallic layers, with first alternate ones of the metallic layers being exposed along one side of the body and second alternate ones of the metallic layers being exposed along an opposite side of the body. The first and second alterna ones of the metallic layers are exposed to separate gases, one of the gases being a reference ga in order to cxeate a voltage output signal across electrodes connected to alternate raaetallic laye:
The voltage output signal is indicative ofthe relative oxygen partial pressures of the separate gases. Thus, the passive oxygen sensor cannot provide an oxygen partial pressure indication unless the first arid second metallic layers present in the body are exposed, respectively, to a sample gas and a separate reference gas having a known oxygen partial pressure, i.e., each side of the sensoz body r~nust be exposed to a separate gas.
More recently, arnperomeiric sensors have been introduced which also use partially stabilized zircoz~ia but which do not require a reference gas to operate. Such a sensor 80 is illustrated in Fig, 1 aad comprises a cavity 100 iz~ communication with the unknown gas ihrou~
a diffusion hole 120. The base of the cavity 100 is a PSZ electrolyte 1~.0 which is connected through electrodes 160, 160' to a voltage source x 70. The application of a voltage causes oxyg tv be pumped from the cavity through diffusion into the suzround~g gas as shown by the arrov ~f the cavity is sealed atop the base, and if the top of the cavity has the small diffusion hole '12C
then a point is rEached on increasing the voltage where no more oxygen can be pumped out of the cavity than is entering through the diffusion hole. The current drawn at this point is called the amperometric current. The larger the oxygen partial pressure in the surrounding gas, the larger will be the amperometric current. Thus, a measurement of the amperometric current gelds the oxygen parl5al pressure. Again, however, this sensor suffers .from some of the same drawbacks in that materials and fa'6rication costs are relatively high. An extremely small diffusion hole is required, about Spm, and requires precise machining because the size is cz~itical to the operation of the sensor. Additionally, the manufacture o~ the sensor of Fig. 1 requires five silk screen operations and four burnout steps. Finally, these sensor. s lose their sensitivity above about 80% oxygen and the diffusion bole is prone to plugging.
Accordingly, there renr~a~ins a need int the art for art amperometric oxygen sensor which is relatively inexpensive to manufacture and provides enhanced oxygen sensitivity. There is also a need in the art for a sensor which is capable of providing an independent indication of NOx content in a gas.
BRTIJF SUMMAT~Y OF THE INVENTION' These needs are met by the present invention wherein a combined oxygen and NOx 1 S sensor is provided. Generally, the combined sensor employs a sensor body that iuncludes two different types of electrodes - oxygen-porous electrode layers and dissociative oxygen-porous electrode layers.
In accordance with one embodiment of the present invention, a combined sensor for measuring oxygen content and NOx content in a gas is provided. The sensor comprises a sensor body, an oxygen content electrical sigual output, and a N'Ox content elecixical signal output. The sensor body is disposed in the gas and comprises (t) a plurality of oxygen porous electrode layers, (ii) a plurality of dissoczative oxygen porous electrode layers, wherein the dissociative oxygen porous electrode layers comprise a material selected to catalyze dissociation of~l'Ox into nitrogen arid oxygen, and (iii) a plurality of oxygen ion conductive ceramic layers interposed between respective ones of the oxygen-porous electrode layers and respective ones of the dissociative oxygen-porous electrode layers. The oxygen content electrical signal output is coupled to the plurality o~oxygen-porous electrode layers. Similarly, the NOx content electrical signal output is coupled to the plurality of dissociative oxygen porous electrode layers. The NOx content electrical signal output is electrically isolated from the oxygen content electrical signal output.
In accvzdance with another embodiment of the present invention, a combined sensor for measuring oxygen content and NOx content in a gas is provided where the dissociative oxygen-porous electrode layers comprise sufficient Rh to catalyze dissociation of NOX
ixtto nitrogen and oxygen.
Tn accordance with yet another embodiment of tlxe present invention, a cozobined sensor for measuring oxygen content and NOx content in a gas is provided. The sensor comprises a partial enclosure defining a gas passage, a sensoz body, and a diffusion baxz~ier_ The diffusion barrier defines a diffusion-limited portion of the gas passage and the sensor body is disposed in the diffusion-limited portion ofthe gas passage.
In accordance with yet another embodiment of the present invention, a sensor body is provided comprising a plurality of oxygen porous electrode layers, a plurality of dissociative oxygen-porous electrode layers, and a plurality of oxygen ion conductive cerazxuic layers. The dissociative oxygen-porous electrode layers comprise a material selected to catalyze dissociation of NOx into nitrogen and oxygen. The plurality o~ oxygen ion conductive ceramic layers are 1 ~ intezposed betweEn respective ones of the oxygen-porous electrode layers and zespective ones of the dissociative oxygen-porous electrode layers.
Accordingly, it is an object of the present invention to provide an improved oxygen and NOx sensing device. Other objects of the pzesent invention will be apparent art light of the description of the invention embodied hezein.
BRIEF DESCRIPTxON OF ~E SEVERAh V'rE'hfS OF'1~ DRA'WIN'GS
The following detailed desc~.ption of the preferred embodixx~ents of the present invention can be best understood when read ix~ conjunction with the following drawings, where like stricture is indicated with like reference numerals and in which:
Fig. 1 is a schematic representation of a pz~or art oxygen sensor;
Fig. 2 is a schemai~ic representation of an oxygen sensor in accordance with the present invention;
Figs. 3-5 are illustrations of an alternative heating circuit arrangement according to the IO present invention;
Figs. C~~, and 6B are ilhtsirations of a packaging scheme according to one embodiment of.
the present invention;
Fig. 7 is an illustration of a sensor body for use in a combined sensor for measuring oxygen content and'~1'Ox content in a gas; and 15 Figs. 8.A.-8C illustzate a combined sensor for measuring oxygen content and NC7X content m a gas.
DETAILEA DESCI~fTTOZV
The present invention is described herein with initial reference to an amperometric oxygen sensor and with subsequent reference to a combined oxygen and I~Ox sensor that utilizes an oxygen sensor and additional structure similar to the basic oxygen sensor structure.
~eror~netric Ox yen Sensor A, seherrxatie representation of an amperometric oxygen sensor constructed according to the present invention is shown an Fig. 2. As seen in Fig. 2, oxygen sensor 10 includes a sensor body I2 having alternating layers of an oxygen ion conducting xnatcrial 14 and an oxygen-porous electrically conductive material 16a, 16b, I 6c, 16d. A Erst set of oxygen porous conductive layers 16a and 16b have end portions that are exposed along a first edge 18 of the seztsor body 12. For the purpose of describing and defining the present invention, an oxygen ion conductor is any material capable of achieving electrical conductivity due to displacement of oxygen ions within its crystal lattice.
Electrical connections are made to the conductive layers I6a and 16b by firing electrically conductive oxygen-porous terminations.22 onto the ends of the conductive layers 1 Gay 16b to form a plurality of cathode layers. A second set of oxygen porous conductive layers 16c and 16d have end portions that are exposed along a second edge 20 of the sensor body 12.
The conductive layers 16c and I6d are electrically connected to one another by an electrically conductxvc oxygen-porous termination 24, to forxxi a plurality of anode layers. Silver or oxygen-porous platinum are suitable nnaterials for use as the electrically conductive oxygen porous termit~ations 22, 24. The tezxninations 22, 24 are used to electrically connect the ceramic layers in pan~allel to reduce the electrical resistance of the sensor and allow increased amperometri,e current.
Each of the conductive layers 16a 16d includes two major surfaces. For example, cortduetive layer 16a includes major surfaces 2 and 4_ Each oxygen ion coz~duetor layer 14 is disposed between major surfaces of opposing conductive layers. Further, both major surfaces of each conductive layer are unexposed, i.e., enclosed by the sensor body 12. It is contemplated by the present invention that any number of oxygen-porous conductive layers and ion conductor layers may be used to construct the sensor body 12. The number of layers shown in Fig. 2 is merely pzesented for illustrative pwcposes.
A voltage source 26 is electrically connected to the t~rminatxvr~s 22 and 24.
Tn this manner, a fiucst pole 2Ga of the voltage source 26 is electrically eonuected to tlae cathode layers forr~aed by conductive layers I 6a and 1 Gb and a second pole 26b of the voltage source 26 is ' electzically connected to the anode layers formed by conductive layers 16e and I6d. An amperoanetric current meter 28 is connected betwveen the voltage source 2G and the termination 2A. A voltage meter 30 is connected across the voltage source 2G.
The oxygen-porous electrically conductive material fornvng conductive layers 16a-d I O preferably comprises oxygen-porous platinum, although any suitable electxically conductive material which is porous to oxygen and catalyzes oxygen molecules to ions at the cathode layers and catalyzes ions to oxygen molecules at the anode layers nn.ay be used.
Platinum electrodes can be made porous to oxygen by well-known methods. For example, the use of coarse Pt panicles in elecitodzng ink results in porous electrodes. Otk~er additions to the electrvding ink, such as zireonia particles, further increase the porosity. A
platinum electrode having 530% of its volume occupied by pores is oz~e preferred example. As another example, 85 parts, by weight, of a coarse Pt powder available as platinum gorwder number 5432\0101 froxxj Uemetron, GMBH, Hanau, Germany, xnay be combined with 1S parts, by weight, o~ a d.00 mesh zirconia powder in a suitable silk screening slurry.
:Cn one ezobodxxn~ent o~ the present invention, the width of the sensor body 12; i.e., the dimEnsion of the sensor body ~rom the first edge I 8~ to the second edge 20, is about 0.20" (0.5 can), the short ends of the conductive layers lGa,16b, 16c, 16d terminate about 0.030" (0.075 em) from respective side edges, leaving a 0.14" (0.3b cm) conductive layer overlap. The lez~gtb of the sensor body 12 is about 0.18" (0.46 cm). The thi,cltc~ess of the sensor body 12 is defined by the number and thickness of the oxygen ion conductor layers 19., the conductive layers 1 fia, lGb, 16c, 16d, and any layers dedicated to a heating circuit (desczibed below). In one embodiment ofthe present invention, eleven oxygen ion conductor layers 14 are positioned between alternate ones of twelve conductive layers 16a,16b, 16e, 16d. The oxygen ion conductor layers 14 may comprise 0.0030" (0.076 mm) thick yttria-stabilized zireonia layers.
The conductive layers comprise 0.0001" (0.0025 mm) thick porous platinum. The result is an oxygen sensor that is relatively compact in size and relatively inexpensive to produce.
A number of eerarnie oxygen ion conductor materials may be used in accordance witty the present Invention. Indeed, the present invention's advantages of simplicity ofconstruction and reduced electrical resistance due to sensor geometry are applicable to any of a wide vaxiety of ceramic materials used. Preferably, the oxygen ion conductor of tlae present invention is a ceramic eXectrolyte and more specifically, comprises yttria-stabilized zirconia (ZrOz stabilized with Y203) but may also comprise stabilized bismuth oxide, stabilized ceria, etc. The zirconia ceramic may be stabilized with materials other than Yz43-Fine gzain sized powders of ZrQz:'Y2C~ can be sintered to high density at 1150-1300 C, making it possible to manufacture multi-layer seffsor bodies from 'this oxygen ion conductor.
Because of the convenient sintering temperatures of the ceramic materials of the present invention, the ceramics can be "tape cast" into a monolithic body. As is well lrnown in the ceramic art, tape casting is a process for making a multilayered body (for example, a cerarrxic capacitor) wherein appropriate metal electrodes arE interdispersed between the ceramic layers. A, tape casting technique may be employed such as that described in U.S. Patent No. 4,462,891, incorporated herein by reference. The cet~amic layers are quite thin, having a thickness of from about 25-100 ~.xr~. Further, this tape casting method requires only a single sills screening operation and a single burnout step.
Higher porosity levels in the conductive layers are more suitable for sensing very low levels of oxygen in a gas, e.8., as low as 1 ppm oxygen partial pressure.
Conversely, lower porosity levels in the conductive layers are more suitable far sensing applications over a broad range of oxygen paatial pressure up to a maximum of l Os ppm. According to one eznbodino.ent of the present invention, the amperometrac oxygen sensoz 10 is pxoduced by sintering the entire sensoz body 12, i.e., the oxygen ion canductvr layers 14, the conductive layers 16a, I 6b,16c, 16d, and any layers dedicated to the heating circuit 12, at a sintering temperature selected to yield a predetermined oxygen porosity in the conductive layers 16a, 16b,16c, 16d. Sintering at relatively high temperatures for relatively large amounts of time decreases the porosity in the electrode layers because the density of the sensor body increases. Conversely, sintering at relatively low temperatures fez' relatively short amounts of time does not lead to equally significant decreases in porosity in the electrode layers because the density of the sensor body does not izacrease as much as is the case for higher temperature arid longer duration sintering.
Accordixtgly, a~~ amperametric oxygen sensor according to the present invention may be produced by providing an unsintered sensor. body, selecting a target porosity for the oxygen-porous electrode layers, and selecting a corresponding sintering temperature for the sensor body, 'fhe sintering temperature is selected to correspond to the target porosity anal may be determined ~khrough experimentation. Tlae sensor body is sintered at the selected sintering temperature to yield a siu~tered sensor body including oxygen porous electrode layers having a target porosity.
For example, where the conductive layers are sintered at about 1200°C, for a duration of about 2 hours, the sintered sensor body is suitable for oxygen sensing in gases having art o~tygen~ content ranging from a value typically found in air to values as low as 1 ppm or lower. Xf the sensor 'body is sintered at a higher temperature, e.g.,1275°C, for the same duration, a less porous layer is formed and the sintered sensor body is more suitable for oxygen sensing of gases having higlaer oxygen concentrations, e.g_, up to I00% oxygez~_ There may be some increase in resistance in the oxygerx porous electrode layers over time as a result of sintering of platinum particles in the electrodes at the operating temperature of the sensor. The long term stability of sensors according to the present invention may be improved it some irxstanees by stabilizing the oxygen porous electrode layers against sintering. If should be appreciated by those practicing the present invention that a variety of methods are available for stabilizing platinum electrodes agai~ast sintering.
Tn operation, the oxygen sensor I O is immersed in a gas whose oxygen partial pressure is to be determined_ If there is not already oxygezz present in the porous conductive layers 16a-d, oxygen froru the gas passes through the porous terminations 22 and 24 and enters the porous electrodes 16a-d through diffusion. A. voltage from voltage source 26 is applied across the terxninations 22 and 24. The resulting voltage difference between the conductive layers 16a and 16b, also referred to herein as the cathode layers, and the conductive layers 16c and 16d, also referred to herein as the anode layers, will cause oxygen to be pumped through the Layers of oxygen ion conducting material 14. Since the porous electrode layers lda d catalyze oxygen molecules to ions at the cathode layers 16a, 16b and catalyze ia~ns to oxygen zx~olecules at the anode layers I6e, 16d, oxygen enters at the cathode layers 16a, 16b, is pumped througia the layers of ion conductor material 14, grad exits through the anode layers 16c,16d. The resulting electrical current is measured by the amperor~aetric meter 28 and is indicative of the oxygen partial pressure of the gas.
Sensors based on stabilized zircouia tend to have operating temperatures above 700°C_ The applied voltage is monitored by the voltage meter 30_ It has been found that applied do voltages at and above 0.2 volts often lead to instabilities in the sensor and that an applied voltage of 0.05 volts has been found to yield unstable current signals at Iarge oxygen partial pressures.
~1n applied voltage of 0.1 volts is the preferred bias voltage. The voltage source rnay be a do voltage source or oat ac voltage source operating at about 3 Hz. The preferred ac frequency is less than 50Hz sine, as the ac .frequency increases, the sensor response to oxygen decreases_ Because the oxygen sensor of the present invention operates at an elevated temperature, it is preferable to provide a heater and thermometer fox the sensor body.
Resistive heating electrodes 35 are provided in the mamaer ihustrated in Figs.
CROSS REFERENCE TO RELATED APPLrCATIONS
"his application claims the benefit of U.S_ provisional Application Serial No.
601254,081, filed December 7, 2000. This application is also a continuation-in-part o~'CJ.S.
Patent application Serial No. 09/662,773, filed September 15, 2000, which claims the beneft of U.S. Provisional Application Serial No. 601155,817, fled September 23, 1999.
'$ACI~GROUND OF THE fNV~NT~O1V
The pz~eserat invention, relates to a device for sensing the partial pressure of oxygen in a gas, and more particularly to an active nnultilayer sensor utilizing an oxygen ion conducting matexial. The present invention also relates to a combined sensor for measuring oxygen content and NOx content in a gas. ~tOx is utiaized herein to represent nitzic oxide, zaitrogen dioxide, nitrogen trioxide, etc_ It is vtridely recognized that one of the most important diagrxostics for monitoring the efficiency of any combustion process is the measuzemeut of the oxygen partzaJ
pressure in ara exhaust gas. Thus, oxygen sensoxs have loug been used to measure the oxygen content of exhaust gases from such divezse combustion processes as internal combustion engines in motor vehicles and coal, natural gas, or oil burning power generations facilities.
2O The most widely lrnown and used oxygen sensors are based on partially sfiabilized zirconia (PSZ) as the ion conductor. Such sensors function by monitoring the electxarnotive force (EMF) developed across an ion conductor which is exposed to different partial pressures of oxygen. Oxygen tends to move from a gas containing a high concerxtration of oxygen to vne of lower concentration. Tftwo gases are separated from each other by an electroded oxygen ion conductor, the oxygerx molecules will dissociate on one surface of tlae couductar and absorb electrons to loran oxygen ions. These ions then diffuse through the ionic conductor, leaving the entry surface with a def cieracy of electrons (OZ + 4e = 20'2). One the exit or lorw oxygen concenixation side of the conductor, oxygen iozzs leaving the conductor must give up electrons to form molecular oxygen, thus leaving the exit surface with an excess of electrons. This creates the $M11 between the two surfaces of floe ion eonduetor_ One problem with the use of partially stabilized zirconia sensors is that they must be operated at temperatures in the range of about 800 C to reduce intez-rtal resistance to a point where a current carp be measured. Further, the raw material costs of stabili2ed zireonia is zelatively high, and the ~xlelting point of zizconia is quite high (2700 C) so that formation of sensors is expensive.
Lawless, in U.S. Patent No. 4,462,89 x, describes a passive oxygen sensor using eeram ion conducting materials based ors nickel niobates and Bismuth oxides. The oxygen sensor includes a plurality of layers of tlxe ceramic material and a porous metallic conductor axrangec form a body having alterxzating ceramic and metallic layers, with first alternate ones of the metallic layers being exposed along one side of the body and second alternate ones of the metallic layers being exposed along an opposite side of the body. The first and second alterna ones of the metallic layers are exposed to separate gases, one of the gases being a reference ga in order to cxeate a voltage output signal across electrodes connected to alternate raaetallic laye:
The voltage output signal is indicative ofthe relative oxygen partial pressures of the separate gases. Thus, the passive oxygen sensor cannot provide an oxygen partial pressure indication unless the first arid second metallic layers present in the body are exposed, respectively, to a sample gas and a separate reference gas having a known oxygen partial pressure, i.e., each side of the sensoz body r~nust be exposed to a separate gas.
More recently, arnperomeiric sensors have been introduced which also use partially stabilized zircoz~ia but which do not require a reference gas to operate. Such a sensor 80 is illustrated in Fig, 1 aad comprises a cavity 100 iz~ communication with the unknown gas ihrou~
a diffusion hole 120. The base of the cavity 100 is a PSZ electrolyte 1~.0 which is connected through electrodes 160, 160' to a voltage source x 70. The application of a voltage causes oxyg tv be pumped from the cavity through diffusion into the suzround~g gas as shown by the arrov ~f the cavity is sealed atop the base, and if the top of the cavity has the small diffusion hole '12C
then a point is rEached on increasing the voltage where no more oxygen can be pumped out of the cavity than is entering through the diffusion hole. The current drawn at this point is called the amperometric current. The larger the oxygen partial pressure in the surrounding gas, the larger will be the amperometric current. Thus, a measurement of the amperometric current gelds the oxygen parl5al pressure. Again, however, this sensor suffers .from some of the same drawbacks in that materials and fa'6rication costs are relatively high. An extremely small diffusion hole is required, about Spm, and requires precise machining because the size is cz~itical to the operation of the sensor. Additionally, the manufacture o~ the sensor of Fig. 1 requires five silk screen operations and four burnout steps. Finally, these sensor. s lose their sensitivity above about 80% oxygen and the diffusion bole is prone to plugging.
Accordingly, there renr~a~ins a need int the art for art amperometric oxygen sensor which is relatively inexpensive to manufacture and provides enhanced oxygen sensitivity. There is also a need in the art for a sensor which is capable of providing an independent indication of NOx content in a gas.
BRTIJF SUMMAT~Y OF THE INVENTION' These needs are met by the present invention wherein a combined oxygen and NOx 1 S sensor is provided. Generally, the combined sensor employs a sensor body that iuncludes two different types of electrodes - oxygen-porous electrode layers and dissociative oxygen-porous electrode layers.
In accordance with one embodiment of the present invention, a combined sensor for measuring oxygen content and NOx content in a gas is provided. The sensor comprises a sensor body, an oxygen content electrical sigual output, and a N'Ox content elecixical signal output. The sensor body is disposed in the gas and comprises (t) a plurality of oxygen porous electrode layers, (ii) a plurality of dissoczative oxygen porous electrode layers, wherein the dissociative oxygen porous electrode layers comprise a material selected to catalyze dissociation of~l'Ox into nitrogen arid oxygen, and (iii) a plurality of oxygen ion conductive ceramic layers interposed between respective ones of the oxygen-porous electrode layers and respective ones of the dissociative oxygen-porous electrode layers. The oxygen content electrical signal output is coupled to the plurality o~oxygen-porous electrode layers. Similarly, the NOx content electrical signal output is coupled to the plurality of dissociative oxygen porous electrode layers. The NOx content electrical signal output is electrically isolated from the oxygen content electrical signal output.
In accvzdance with another embodiment of the present invention, a combined sensor for measuring oxygen content and NOx content in a gas is provided where the dissociative oxygen-porous electrode layers comprise sufficient Rh to catalyze dissociation of NOX
ixtto nitrogen and oxygen.
Tn accordance with yet another embodiment of tlxe present invention, a cozobined sensor for measuring oxygen content and NOx content in a gas is provided. The sensor comprises a partial enclosure defining a gas passage, a sensoz body, and a diffusion baxz~ier_ The diffusion barrier defines a diffusion-limited portion of the gas passage and the sensor body is disposed in the diffusion-limited portion ofthe gas passage.
In accordance with yet another embodiment of the present invention, a sensor body is provided comprising a plurality of oxygen porous electrode layers, a plurality of dissociative oxygen-porous electrode layers, and a plurality of oxygen ion conductive cerazxuic layers. The dissociative oxygen-porous electrode layers comprise a material selected to catalyze dissociation of NOx into nitrogen and oxygen. The plurality o~ oxygen ion conductive ceramic layers are 1 ~ intezposed betweEn respective ones of the oxygen-porous electrode layers and zespective ones of the dissociative oxygen-porous electrode layers.
Accordingly, it is an object of the present invention to provide an improved oxygen and NOx sensing device. Other objects of the pzesent invention will be apparent art light of the description of the invention embodied hezein.
BRIEF DESCRIPTxON OF ~E SEVERAh V'rE'hfS OF'1~ DRA'WIN'GS
The following detailed desc~.ption of the preferred embodixx~ents of the present invention can be best understood when read ix~ conjunction with the following drawings, where like stricture is indicated with like reference numerals and in which:
Fig. 1 is a schematic representation of a pz~or art oxygen sensor;
Fig. 2 is a schemai~ic representation of an oxygen sensor in accordance with the present invention;
Figs. 3-5 are illustrations of an alternative heating circuit arrangement according to the IO present invention;
Figs. C~~, and 6B are ilhtsirations of a packaging scheme according to one embodiment of.
the present invention;
Fig. 7 is an illustration of a sensor body for use in a combined sensor for measuring oxygen content and'~1'Ox content in a gas; and 15 Figs. 8.A.-8C illustzate a combined sensor for measuring oxygen content and NC7X content m a gas.
DETAILEA DESCI~fTTOZV
The present invention is described herein with initial reference to an amperometric oxygen sensor and with subsequent reference to a combined oxygen and I~Ox sensor that utilizes an oxygen sensor and additional structure similar to the basic oxygen sensor structure.
~eror~netric Ox yen Sensor A, seherrxatie representation of an amperometric oxygen sensor constructed according to the present invention is shown an Fig. 2. As seen in Fig. 2, oxygen sensor 10 includes a sensor body I2 having alternating layers of an oxygen ion conducting xnatcrial 14 and an oxygen-porous electrically conductive material 16a, 16b, I 6c, 16d. A Erst set of oxygen porous conductive layers 16a and 16b have end portions that are exposed along a first edge 18 of the seztsor body 12. For the purpose of describing and defining the present invention, an oxygen ion conductor is any material capable of achieving electrical conductivity due to displacement of oxygen ions within its crystal lattice.
Electrical connections are made to the conductive layers I6a and 16b by firing electrically conductive oxygen-porous terminations.22 onto the ends of the conductive layers 1 Gay 16b to form a plurality of cathode layers. A second set of oxygen porous conductive layers 16c and 16d have end portions that are exposed along a second edge 20 of the sensor body 12.
The conductive layers 16c and I6d are electrically connected to one another by an electrically conductxvc oxygen-porous termination 24, to forxxi a plurality of anode layers. Silver or oxygen-porous platinum are suitable nnaterials for use as the electrically conductive oxygen porous termit~ations 22, 24. The tezxninations 22, 24 are used to electrically connect the ceramic layers in pan~allel to reduce the electrical resistance of the sensor and allow increased amperometri,e current.
Each of the conductive layers 16a 16d includes two major surfaces. For example, cortduetive layer 16a includes major surfaces 2 and 4_ Each oxygen ion coz~duetor layer 14 is disposed between major surfaces of opposing conductive layers. Further, both major surfaces of each conductive layer are unexposed, i.e., enclosed by the sensor body 12. It is contemplated by the present invention that any number of oxygen-porous conductive layers and ion conductor layers may be used to construct the sensor body 12. The number of layers shown in Fig. 2 is merely pzesented for illustrative pwcposes.
A voltage source 26 is electrically connected to the t~rminatxvr~s 22 and 24.
Tn this manner, a fiucst pole 2Ga of the voltage source 26 is electrically eonuected to tlae cathode layers forr~aed by conductive layers I 6a and 1 Gb and a second pole 26b of the voltage source 26 is ' electzically connected to the anode layers formed by conductive layers 16e and I6d. An amperoanetric current meter 28 is connected betwveen the voltage source 2G and the termination 2A. A voltage meter 30 is connected across the voltage source 2G.
The oxygen-porous electrically conductive material fornvng conductive layers 16a-d I O preferably comprises oxygen-porous platinum, although any suitable electxically conductive material which is porous to oxygen and catalyzes oxygen molecules to ions at the cathode layers and catalyzes ions to oxygen molecules at the anode layers nn.ay be used.
Platinum electrodes can be made porous to oxygen by well-known methods. For example, the use of coarse Pt panicles in elecitodzng ink results in porous electrodes. Otk~er additions to the electrvding ink, such as zireonia particles, further increase the porosity. A
platinum electrode having 530% of its volume occupied by pores is oz~e preferred example. As another example, 85 parts, by weight, of a coarse Pt powder available as platinum gorwder number 5432\0101 froxxj Uemetron, GMBH, Hanau, Germany, xnay be combined with 1S parts, by weight, o~ a d.00 mesh zirconia powder in a suitable silk screening slurry.
:Cn one ezobodxxn~ent o~ the present invention, the width of the sensor body 12; i.e., the dimEnsion of the sensor body ~rom the first edge I 8~ to the second edge 20, is about 0.20" (0.5 can), the short ends of the conductive layers lGa,16b, 16c, 16d terminate about 0.030" (0.075 em) from respective side edges, leaving a 0.14" (0.3b cm) conductive layer overlap. The lez~gtb of the sensor body 12 is about 0.18" (0.46 cm). The thi,cltc~ess of the sensor body 12 is defined by the number and thickness of the oxygen ion conductor layers 19., the conductive layers 1 fia, lGb, 16c, 16d, and any layers dedicated to a heating circuit (desczibed below). In one embodiment ofthe present invention, eleven oxygen ion conductor layers 14 are positioned between alternate ones of twelve conductive layers 16a,16b, 16e, 16d. The oxygen ion conductor layers 14 may comprise 0.0030" (0.076 mm) thick yttria-stabilized zireonia layers.
The conductive layers comprise 0.0001" (0.0025 mm) thick porous platinum. The result is an oxygen sensor that is relatively compact in size and relatively inexpensive to produce.
A number of eerarnie oxygen ion conductor materials may be used in accordance witty the present Invention. Indeed, the present invention's advantages of simplicity ofconstruction and reduced electrical resistance due to sensor geometry are applicable to any of a wide vaxiety of ceramic materials used. Preferably, the oxygen ion conductor of tlae present invention is a ceramic eXectrolyte and more specifically, comprises yttria-stabilized zirconia (ZrOz stabilized with Y203) but may also comprise stabilized bismuth oxide, stabilized ceria, etc. The zirconia ceramic may be stabilized with materials other than Yz43-Fine gzain sized powders of ZrQz:'Y2C~ can be sintered to high density at 1150-1300 C, making it possible to manufacture multi-layer seffsor bodies from 'this oxygen ion conductor.
Because of the convenient sintering temperatures of the ceramic materials of the present invention, the ceramics can be "tape cast" into a monolithic body. As is well lrnown in the ceramic art, tape casting is a process for making a multilayered body (for example, a cerarrxic capacitor) wherein appropriate metal electrodes arE interdispersed between the ceramic layers. A, tape casting technique may be employed such as that described in U.S. Patent No. 4,462,891, incorporated herein by reference. The cet~amic layers are quite thin, having a thickness of from about 25-100 ~.xr~. Further, this tape casting method requires only a single sills screening operation and a single burnout step.
Higher porosity levels in the conductive layers are more suitable for sensing very low levels of oxygen in a gas, e.8., as low as 1 ppm oxygen partial pressure.
Conversely, lower porosity levels in the conductive layers are more suitable far sensing applications over a broad range of oxygen paatial pressure up to a maximum of l Os ppm. According to one eznbodino.ent of the present invention, the amperometrac oxygen sensoz 10 is pxoduced by sintering the entire sensoz body 12, i.e., the oxygen ion canductvr layers 14, the conductive layers 16a, I 6b,16c, 16d, and any layers dedicated to the heating circuit 12, at a sintering temperature selected to yield a predetermined oxygen porosity in the conductive layers 16a, 16b,16c, 16d. Sintering at relatively high temperatures for relatively large amounts of time decreases the porosity in the electrode layers because the density of the sensor body increases. Conversely, sintering at relatively low temperatures fez' relatively short amounts of time does not lead to equally significant decreases in porosity in the electrode layers because the density of the sensor body does not izacrease as much as is the case for higher temperature arid longer duration sintering.
Accordixtgly, a~~ amperametric oxygen sensor according to the present invention may be produced by providing an unsintered sensor. body, selecting a target porosity for the oxygen-porous electrode layers, and selecting a corresponding sintering temperature for the sensor body, 'fhe sintering temperature is selected to correspond to the target porosity anal may be determined ~khrough experimentation. Tlae sensor body is sintered at the selected sintering temperature to yield a siu~tered sensor body including oxygen porous electrode layers having a target porosity.
For example, where the conductive layers are sintered at about 1200°C, for a duration of about 2 hours, the sintered sensor body is suitable for oxygen sensing in gases having art o~tygen~ content ranging from a value typically found in air to values as low as 1 ppm or lower. Xf the sensor 'body is sintered at a higher temperature, e.g.,1275°C, for the same duration, a less porous layer is formed and the sintered sensor body is more suitable for oxygen sensing of gases having higlaer oxygen concentrations, e.g_, up to I00% oxygez~_ There may be some increase in resistance in the oxygerx porous electrode layers over time as a result of sintering of platinum particles in the electrodes at the operating temperature of the sensor. The long term stability of sensors according to the present invention may be improved it some irxstanees by stabilizing the oxygen porous electrode layers against sintering. If should be appreciated by those practicing the present invention that a variety of methods are available for stabilizing platinum electrodes agai~ast sintering.
Tn operation, the oxygen sensor I O is immersed in a gas whose oxygen partial pressure is to be determined_ If there is not already oxygezz present in the porous conductive layers 16a-d, oxygen froru the gas passes through the porous terminations 22 and 24 and enters the porous electrodes 16a-d through diffusion. A. voltage from voltage source 26 is applied across the terxninations 22 and 24. The resulting voltage difference between the conductive layers 16a and 16b, also referred to herein as the cathode layers, and the conductive layers 16c and 16d, also referred to herein as the anode layers, will cause oxygen to be pumped through the Layers of oxygen ion conducting material 14. Since the porous electrode layers lda d catalyze oxygen molecules to ions at the cathode layers 16a, 16b and catalyze ia~ns to oxygen zx~olecules at the anode layers I6e, 16d, oxygen enters at the cathode layers 16a, 16b, is pumped througia the layers of ion conductor material 14, grad exits through the anode layers 16c,16d. The resulting electrical current is measured by the amperor~aetric meter 28 and is indicative of the oxygen partial pressure of the gas.
Sensors based on stabilized zircouia tend to have operating temperatures above 700°C_ The applied voltage is monitored by the voltage meter 30_ It has been found that applied do voltages at and above 0.2 volts often lead to instabilities in the sensor and that an applied voltage of 0.05 volts has been found to yield unstable current signals at Iarge oxygen partial pressures.
~1n applied voltage of 0.1 volts is the preferred bias voltage. The voltage source rnay be a do voltage source or oat ac voltage source operating at about 3 Hz. The preferred ac frequency is less than 50Hz sine, as the ac .frequency increases, the sensor response to oxygen decreases_ Because the oxygen sensor of the present invention operates at an elevated temperature, it is preferable to provide a heater and thermometer fox the sensor body.
Resistive heating electrodes 35 are provided in the mamaer ihustrated in Figs.
2~5. As is illustrated in Figs. 2-5, cover plate heating electrodes 35 in the form of platinum tracks are embedded in the ion conductor zoaterial 14 of the sensor body 12, more specifically in the top and bottom cover plates 32. Referring specifically to Figs. 3-5, tlae sensor body 12 is provided with a top heater track 2 and a bottom heater track 4. 'fhe rear face S of the sensor body 12 is provided wiih a conductive tez~xaxnation arranged to couple conductively the top heater track 2 to the bottom heater track 4. In addition, the front face 7 of the sensor body 12 is provided with a pair of conductive tenninations 6 coupled conduetively to respective ones of the top heater track 2 and the bottom heater track 4. Tin this manner, a complete circuit is formed by coupling a heating voltage source (incorporated in heating circuit controller 50) grad terminals 8 to respectiwe~ ones of the conductive tezminations 6.
The measured resistance in the embedded platinum heater track 35 typically varies from about 2.3 to about 6.5 ohms between 2S° C and 800° C, respectively. The measured beater power required to maintain the sensor body 12 ranges up to about 2 watts at 800° C, a pz~eferred sensor operating temperature_ A heating voltage is applied across the heating circuit by connecting a heating voltage source across the heating electrodes 35. ~'he resistivity of the heating circuit generates heat when a voltage is applied_ The resistance of the heating electrodes 35 varies as a function of temperature. This temperature/resistance relation provides a means for to measuring the tennperaturc of the sensor body 12. l.'referably, the heating electrodes 35 are coupled to a beating circuit controller 50 programmed to control the resistance of the heating electrodes 35 by applying a constant current to the beating electrodes 35 and controlling the voltage applied thereto.
S As is illustrated in Figs. 2-S, top and bottom dielectric cover plates 32 preferably comprise a 0_02" (0.05 em) thick dielectric material added above and below the uppermost and lowermost electroded layers of the sensor body 12 for electrical insulation and structural integrity. The sensor body 12 may be incorporated into a four pin package, two connections fox the heating circuit, a cathode connection, and an anode connection, surrounded by thermal insula~on, and enclosed by a Teflon parhiculate filter.
Co~ad~uctive Au or 1't leads may be coupled to the various sensor electrodes by attaching the leads to the exposed electrode portions on the sensor body 12 wifih an Au or Pt paste.
Aleernatavely, sensor packaging can be simplified by er~nbedding the conductive leads in the sensor body 12. Specificahy, small holes (~0.6 mm) may be drilled in the sensor body 12 prior to sintering and Pt or Au vvizes may be inserted, with a suitable conductive paste, into the holes.
.~ preferxed heating contxol scheme involves applying tl~e constant current to the heater electrodes 35 in square-wave pulses and using the voltage signal to control the pulse width of the current pulses (pulse-width modulation)_ Undex feedback conixol the pulse width is modulated td maintain the voltage constant, thereby maintaining the resistance of the heating electrodes 35 constant, as desired. Stated differently, modulating the pulse width of the current controls the heating power applied to the heating electrodes 35 to maintain the se~asor temperature constant.
The voltage can easily be read using a x b bit A/l~ converter to an accuracy of ~ 0.0015°fo.
Conventional Eurrent control schemes allow maintenance of a constant current within about O.OX°.lo. Therefore, the temperature of the integrated sensor body can be controlled within acceptable ranges.
A preferred microprocessor-based heating circuit cor~traller 50 consists of a temperature's control section and a sensor-output section. The latter section would supply a constant voltage to the heating electrodes 3S and read the amperometric current in the heating electrodes 35. The c~urent signal may be converted to a readout of the oxygen partial pressure and may be converted to an output suitable for controlling a combustion process_ The sensor I O xnay be calibrated and used by first identifying the resistance of the heating electrodes 35 in the desired operating temperature range_ This resistance value, e.g. 9-10 ohms at 600 C, is known and typically is well defined within a given temperature range.
Corresponding current and voltage parameters, e.g., 0.47 A and 4.1 volts, arc prograaxuned into the heating circuit controller 50~ and the controller 50 is programmed to maintain these values.
The actual operating tempezature of any individual sensor is held constant within the sensor's operating range.
As an illustrative example, where 1 mil = 0.001 inches = 0.0254 mm, a preferred sensor body is 166 znil x 124 mil x 53 rxail (4.22 mm x 3.15~mm x 1.35 mm) and weighs 144 mg, rn the embodiment of tlae present invention where cover plate heating electrodes 35 are exnployed, the total electrode overlap area per layer is pzeferably about 12_7 znu2 and the total area to thickness ratio of the oxygen sensor body 12 is about 199 em. The exposed edge of each eleci~ode is 50 rail (1.27 mna) wide, and each electrode extends 1 S3 mil (3.89 mm) into the body. 1'he resistive heating electrodes are preferably porous Pt tracks approximately x 66 mil (4.22 mrx~) in length and 22 mil (0.559 zrnn) in width, whereby a heater current of 223 rxaA is typical for a control temperature of about 600° C.
Referring now to Figs. 6A and 6E, a packaging scberrae according to orte embodiment of the present invention is illustrated. In the illustrated embodiment, the sensor body 12 is enclosed in a stainless steel tube 60. The thicla~,ess of the tube 60 is preferably selected to be machinablE
for threads for mounting the package into a bulkhead or exhaust flue. The sensor bod~r 12 is stabilized and thermally insulated within the tube 60 by means of suitable gas permeable thermal.
insulation 62 (e.g., N'extel 312 thezznal insulation). A back end 64 of the tube 60 is sealed with a ceramic 66_ Electrical connections 68 to the sensor body 12 ate potted in the ceramic 66 and routed through the insulation b2. Preferably, the electrical connections cozxiprise 20 gauge copper leads coupled to the few sensor leads. A front ez~d 65 of the tube 60 is provided with a stainless steel screen 69 to permit gas to reach the sensoz body 12.
VPlzile cextaiz~ representative embodiments and details have been shown far purposes of illustrating the invention, it will be apparent to those skilled in the alt that various changes in the methods and apparatus disclosed herein may be made without departing from the scope of the invention, which is de .fined in the appended claims. For example, although the sensor 10 of the present invention is well suited for measuring excess oxygen partial pressure because the o~cygen-porous terminatzons 22, 24 present a catalysis area for the combustion of CO and other combustibles, it is noted that the present invention may be arranged for measuring actual oxygen partial pressurE rather than excess oxygen partial pressure. Specil5eally, the cathode electrodes 16a, 16b exposed on the first edge i8 ofthe sensor body 12 are very thin and present a very small catalysis area for the combustion of CO and other combustibles.
Accordingly, by omitting the oxygen-porous tcrminations 22, 24, the sensor 10 of tl~e present invention may be arranged for measuring actual oxygen paaiial pressure rather than excess oxygen partial pressure.
huxther, it is contemplated by tlae present invention that a pair of sensors could be packaged to yield both actual and excess oxygen measurerner~is simply by providing the o~cygen-porous terminaiions 22, 24 on one sensor body only. Finally, it is noted that an alteznate method of measuring actual and excess oxygen using two sensors would be to maintain one sensor below the ignition te~.nperature of CO (600-650°C) and the second sensor above this temperature, also in a single package.
Combined Oxy~and NOx Sensor Referring now to Figs. 7 and 8A-8C, a combined sensor 200 for measuring oxygen content and 1\10X content in a gas is described. 'fhe sensor 200 comprises a partial enclosure 210, a sensor body 220 disposed in the partial enclosure 210, a diffusion barrier 230, and axr oxygen sensor 240. As will be described in further detail below, the sensor body 220 is conFgured to provide an indication of the N'Ox content of tl~e gas and the oxygen sensor 240 is configured to provide an indication of the oxygen content of the gas. The sensor 200 includes many components identical or similar in structure to those described in detail above with reference to Fig. 2. Like reference numerals are utilized in Figs. 2 and 7 corresponding to the life elements and reference is made to the discussion of Fig_ 2 for a description of these elements_ The partial enclosure 210 defines a gas passage 2I2 and as referred to herein as "partial's because it encloses a defined space but also defines the gas passage 212, an inlet portion 2I4, and an outlet portion 216. ~'he partial enclosure 210 typically comprises au o~tygen-ion conductive ceramic tube. It is noted that, although the enclosure is illustrated with a rectangula.e cross-section, an enclosure with a circuaar cross seci~on is likely to be more efFective and easier to mariufacture_ The diffusion barrier 230 extends across the gas passage 212 and defines a diffusion-limited portion 2I8 of the gas passage 2I2 between the inlet portion 214 and the outlet portion 216. The enclosure 210, the diffusion barzaer 230, and the sensor body 220 are configured such that the diffusion-limited portion 218 of the gas passage 212 comprises a hermetically sealed Zone including a diffusion inlet defined by the diffusion barrier 230 and a sensor outlet defined by the sensor body 220. An oxygen pumping portion 250, described in detail below, is also provided in the hermetically sealed zone.
The diffusion barrier 230 is porous to oxygen and NOx and may comprise, for example, a substantially uniform zirconia partition. Typically, the diffusion barrier is configured to pass an amount of gas that varies as a function of oxygen partial pressure of gas within an inlet portion of the gas passage. It is contemplated that the diffusion barrier may define a variety of con~~gurativns including, for example, a perforated plate, a plate including a single restricted x5 aperture, etc.
The sensor body 220 extends across the outlet porkion 216 of the gas passage 212 and is disposed in the diffusion-limited portion 218 of the gas passage 212. The sensor body 220 differs from the sEnsor body 12 illustrated in Fig. 2 in that selected ones of the oxygen porous conductive layers are formed from a material that catalyzes the dissociation of NOx into N2 and 02. In this manner, dissociated 02 may be measured as an amperometric current sad the amperometric current may be related to NOx contex~t_ 'fhe conductive layexs that do not catalyze the dissociation of NOx into N2 and 02, i.e., the non-dissociative electrode layers, are utilized to pxovide an indication of oxygen content, as will be described in further detail herein.
Specifically, the sensor body 220 comprises a plurality of oxygen-porous electrode layers ZS 16a, 16c and a pluo:ality of dissociative oxygen-porous electrode layers 16b, 16d. As is described above with reference to the oxygen sensor of Fig. 2, the oxygen-porous electrode layers 16a, 16c catalyze cause oxygen to be pumped through the layers of oxygen ion conducting rx~ate~al 14 by catalysing oxygen molecules to ions at the cathode layers and catalyzing ions to oxygen molecules at the anode layers. The resulting electrical current is measured by the arnperometric meter 28 and is indicative of the oxygen partial pressure o~the gas_ The dissociative oxygen-porous eIecixode layers 16b, 16d pump oxygen through this process as well but additionally pump oxygen dissociated from NOx in the gas by catalyzing the dissociation of NOx into N2 and 02 at the cathode layers. As a result, the resulting electrical current at the dissoeiative oxygen-porous electrode layers 16b, 16d provides an indication of NOx present in the gas.
As is the case with the embodiment ofFig_ 2, a plurality of oxygen ion conductive cerarr~ic layers are interposed between respective ones of the oxygen=porous electrode layers 16a, 16c and respective ones of the dissociative oxygen-porous electrode layers 16b,16d. As will be appreciated by those practicing the present invention, an oxygen content electrical signal output is provided in the form of electrical leads coupled to the plurality of oxygen porous electrode layers 16a,16c. Similarly, a NOx content electrical signal output is provided in the fozxu of electrical leads coupled to the plurality of dissoeiative oxygen-porous electrode layers 16b, 16d.
In this manner, the o~tygen-porous electrode layers 16a, 16e are coupled to an electrical signal output indicative of an oxygen content of gas within the diffusion-lizxaited portion 218 of the gas passage 212 and the dissociative oxygen-porous electrode layers 16b, x6d are coupled to an electrical signal output indicative of an NOX content of gas within the diffusion-limited portion 218 of the gas passage 212.
The NOx content electrical signal output is electrically isolated frorr~ the oxygen content electrical signal output to ensure proper device performance. To further enhance device performance, flee power source 30 and the electrode layers 16a, 16b, 16c,16d are arranged such that the oxygen-porous electrode layer 16a and the dissociative oxygen porous electrode layer 16b define the sole adjacent pair of different-type electrode layers and have matching polarity.
The electrode layers 16a, 16b are also at substantially equivalent electrical potential (e_g_, 0.1VDC). In this manner, pumping of oxygen between the oxygen-porous electrode layer 16a and the dissociative oxygen-porous electrode layer 1Gb is .inhibited. In contrast, the sensor arrangement illustrated in Fig 2 nncludes electrode layers of alternating polarity.
At elevated temperatures, e.g., above about 600°C, Rh catalyzes the dissociation of NOx into NZ and Oz. Accordingly, the dissociative oxygen-porous electrode layers ldb, 16d may comprise Rh_ The non-dissoeiative electrode layers 16a, 16c may comprise oxygen porous platinum, as described above, and may additionally include Au in an amount sufficient to discourage catalysis of the dissociation of NOx. As is noted above with reference to the oxygen is sensor of Fig. 2, a heater or heating electrode is preferably configured to elevate the operating temperature of the combined sensor well above room temperature, typically an the vicinity of an operating temperature of about 800°C. The sensor is temperature independent in this range. The heater may, for example, be providEd in the form of a heating electrode formed about the enclosure 210_ The paz-tial enclosure 2I0 also defines an oxygen pumping portion 250 that is configured to maintain a favorable NOx to oxygen ratio in the diffusion limited portion 218 of the gas passage 212. Depending upon the operation. constraints of the equipment used with the present invention, accurate measurement of l~Ox content may be problematic if the amount of oxygen in the diffusion limited portion relative to the amount of NOx is too high. The oxygen pumping portion 250 comprises an oxygen-porous cathode electrode 252, an oxygen-porous anode electrode 254, and axz oxygen-ion conductive ceramic material 256. The oxygen-porous cathode electrode 252 is positioned over an interior surface of the partial enclosure 210 within the diffusion-limited portion 218 of the gas passage 212. The oxygen porous anode electrode Z54 is 1 S positioned over azi exterior surface of the partial enclosure 210 outside of the diffusion-limited portion 218 of the gas passage 212. The oxygen-ion conductive ceramic material 2SG is typically formed by the body of the enclosure 210 and, as such, is interposed between the cathode electrode 252 and the anode electrode 254. 'f he oxygen-porous anode electrode 254 may eonoprise platinum and the oxygen-porous cathode electrode 252 may also comprise platinum with an amount of gold additive su:~cient to discourage dissociation of NOx.
Preferably, the NOx to oxygen ratio in the diffusion lix~aited portion 218 is below about 5 parts oxygen to 1 part NOx but x~c.~ay be higher if the equipment used to measure amperometric current and control the voltages at the electrodes is optimized to accowlt for higher oxygen levels. Accurate measurement of NO~c content is problematic if the amount of oxygen in the diffiusion limited portion relative to the amount o~NOx is too high. For example, there is a logarithmically linear relafiionship between amperometric current and oxygen partial pressure below about 1000 ppm but accurate measurement is problexx~atic above this level. A feedback loop xnay be coupled between the sensor body 220 and the oxygen pumpxz~g portion 250. The feedback loop may be configured to control the oxygen pumping porhion 250 in response to the amount of oxygen sensed by the sensor body 220. Specifically, using the oxygen measurement from the sensor body 220, the rate of pumping oxygen out of the diffusion limited portion 218 can be continuously adjusted so that no more oxygen is pumped out ofthe tube interior than is needed to provide an accurate measurement of t'he NOx content (e.g., tv keep the ratio ofNOx-released oxygen to background oxygen at, say, 1:5). The feedback loop may also be configured to switch the pumping funetioxx on and offin response to the amount of sensed oxygen. Tn this mannez~, operation of the oxygen pumping portion 250 rxxay be operated to minimize power eonsurnption of the combined sensor 200.
The oxygen sensor 240 is positioned in the inlet portion 214 of the gas passage 212 and provides a signal indicative of the oxygen partial pressure of the gas ixx the inlet portion 214.
'Thus, the combined sensor 200 is 'configured to provide independent indicatioxas of oxygen partial pressure and NOX content.
Turning now to the manner in which the NOX content is determined, NOx present in the gas v~rithin tile diffusion limited portion 218 dissociates on the dissociative oxygen porous electrode layers 16b, 16d and the released oxygen creates an ampexometric current at the NOx content electrical signal output. Oxygen in the surrou~oding gas also contributes to the NOx content electrical signal output, increasing the amperometric current because tlae dissociative electrodes lbb, 16d pump the oxygen in the gas and the oxygen dissociated $rom the NOx present in the gas. This "background" oxygen axad the inczeased amperometric current can be accounted for using the oxygen content electrical signal output from the electrodes 16a,16c ZO because the corresponding amperometric current at the non-dissociative olect~rodes 1,6a,16c provides an izxdependent xr~easure of the background oxygen.
As is nol~ed above, to accurately measure tk~e NOx content, it is also z~eecssaacy to reduce the background oxygen in the diffusion limited portion to a level commensurate with the NOx released oxygen (e.g., to a ratio of about 5:1 (oxygen to NO~.
As is noted above, the sensor body 220 has two separated sets of porous electrodes, one of which catalyzes the dissociation of NOx to nitrogen and o~cygen. For convenience of illustration, Fig_ 7 merely illustrates a pair of electa-ode layers in eack~
set. However, it is contemplated that a large number of electrode layers could be provided in each set. Preferably, an equal number of electrode layers are provided in each set. However, it is contemplated by the present izzvention that more electrode layers could be providEd in one set, relative to the other, as long as the difference in nurxaber is accounted for in the subsequent NOx content calculation.
The sensor 200 may be mounted directly iz~ an exhaust or sample gas. There is no need for a reference gas supply. Particulate filters or ether types of. filters may be provided to present damage to the sensor and extend sensor life.
The sensor 200 is preferably manufactured in a manner similar to that discussed above with reFerence to the oxygen sensor of Fig. 2. Although a variety ofznanu~acturir~g techniques are available, multi-layering manufaeturiz~g processes have the flexibility of producing layers electroded with PtlAu and separate layers electroded with Rh in tb,e same sensor body. Sensor leads are preferably embedded in the sensor body by drillixrg small holes (~0.5 xnm) in the sensor body 220 in the green state. The sensor body 220 is thezt sintered and Pt wires are feed in the holes with a Pt paste. The stiffness of the ft wires has the advantage ofprovidiz~g mechanical support. Leads for the oxygen sensor 240 are similarly errabedded.
The length of the actual coxxibined sensor nnay be about one inch (2.5 cm) and the major outside diameter may be about %x 111C11 (1.25 cm). The enclosure 210 may comprise a zircania tube made by slip casting. The tube i.s ypically milled in the green state to pmvide passageways for electrical leads and is subsequently sintered. The PtJAu and Pt electrodes 252, 254 are then fired on the intezior and exterior of the tube, respectively. Finally, using a commercial glass for sealing zirconia parts together, the sensor body 220, the diffusion battier 230, anal the oxygen sensor 240 are sealed in the zirconia tube in a single firing_ The first two components are sealed hermetically. .A. Pt lead for the internal Pt/Au electrode passes through the wall of the tube and is also sealed hermetically. A slot in the zirconia tube at the large open end provides the passagerWay fez two oxyge~a sensoz leads, and opposing slots in the snrxall closed end provide passageways for four du.a.1-sensor leads. The sensor body or dual sensor 220 arid its four leads are hermetically sealed in the xirconia tube rwith a commerciahy available glass.
Generally, the operation of the combined sensor 200 is as follows: The device Xs heated to, and maintained at, the operafiing temperature (e.g., 800 °C), axed the oxygen sensor 240 measures the oxygen partial pressure of the exhaust or sample gas. The gas diffuses through the diffusion bawier 230 into the interior diffusion limited portion 218 of the enclosure or tube 210.
A voltage applied across tlae cathode 252 and the anode 254 causes the oxygen in the interior to be pumped to a sufficiently low Ievel. The sensor body 220 measures this low oxygen level with the non-dissociative layers 16a, 16e. The dissociative electrode layers 16b, 16d measure both the low oxygen level and the oxygen released from the NOx dissociation. ~laese amperometrie currents from both sets of electrodes are then used to determine the NOx content.
The 2irconia diffusion barrier 230 diffusion-limits the amount of exhaust gas entering the interior of the tube and thereby ensures that a low level of oxygen can be reached in the interior by the pumping process (i.e., without this plug the interior would be constantly flooded with the exhaust gas). The NOx diffuses through this plug as molecular NOx.
Tlae heater (not shown in Fig. 2) has a temperature-dependent resistance and thereby provides a aneans for measuring and controlling the operating temperature. ,A.
tradeoff is involved with the operating temperature, however: On the one hand, the higher the temperature, the more power is consmned by the lieater in maintaining this temperature. On the other hand, the temperatuze slZVUld be high enough to reduce the resistance of the zirconia tube to a low value to avoid consuming large amounts of powex in pumping the oxygen out of the tube interior.
For tlae purposes of describing and defining the present invention it is noted that the term "substantially" is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The terra "substantially" is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.
.'l3aving described tl~e invention in~ detail and by reference to preferred erxxbodiments thereof, it will be apparent that modifZCations and variations are possible without departing from the scope ofthe invention defined iu~ the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred oz-particularly advantageous, it is contemplated that the present invezttian is not necessarily limited to these preferred aspects of the invez~tion_ What is claimed is:
The measured resistance in the embedded platinum heater track 35 typically varies from about 2.3 to about 6.5 ohms between 2S° C and 800° C, respectively. The measured beater power required to maintain the sensor body 12 ranges up to about 2 watts at 800° C, a pz~eferred sensor operating temperature_ A heating voltage is applied across the heating circuit by connecting a heating voltage source across the heating electrodes 35. ~'he resistivity of the heating circuit generates heat when a voltage is applied_ The resistance of the heating electrodes 35 varies as a function of temperature. This temperature/resistance relation provides a means for to measuring the tennperaturc of the sensor body 12. l.'referably, the heating electrodes 35 are coupled to a beating circuit controller 50 programmed to control the resistance of the heating electrodes 35 by applying a constant current to the beating electrodes 35 and controlling the voltage applied thereto.
S As is illustrated in Figs. 2-S, top and bottom dielectric cover plates 32 preferably comprise a 0_02" (0.05 em) thick dielectric material added above and below the uppermost and lowermost electroded layers of the sensor body 12 for electrical insulation and structural integrity. The sensor body 12 may be incorporated into a four pin package, two connections fox the heating circuit, a cathode connection, and an anode connection, surrounded by thermal insula~on, and enclosed by a Teflon parhiculate filter.
Co~ad~uctive Au or 1't leads may be coupled to the various sensor electrodes by attaching the leads to the exposed electrode portions on the sensor body 12 wifih an Au or Pt paste.
Aleernatavely, sensor packaging can be simplified by er~nbedding the conductive leads in the sensor body 12. Specificahy, small holes (~0.6 mm) may be drilled in the sensor body 12 prior to sintering and Pt or Au vvizes may be inserted, with a suitable conductive paste, into the holes.
.~ preferxed heating contxol scheme involves applying tl~e constant current to the heater electrodes 35 in square-wave pulses and using the voltage signal to control the pulse width of the current pulses (pulse-width modulation)_ Undex feedback conixol the pulse width is modulated td maintain the voltage constant, thereby maintaining the resistance of the heating electrodes 35 constant, as desired. Stated differently, modulating the pulse width of the current controls the heating power applied to the heating electrodes 35 to maintain the se~asor temperature constant.
The voltage can easily be read using a x b bit A/l~ converter to an accuracy of ~ 0.0015°fo.
Conventional Eurrent control schemes allow maintenance of a constant current within about O.OX°.lo. Therefore, the temperature of the integrated sensor body can be controlled within acceptable ranges.
A preferred microprocessor-based heating circuit cor~traller 50 consists of a temperature's control section and a sensor-output section. The latter section would supply a constant voltage to the heating electrodes 3S and read the amperometric current in the heating electrodes 35. The c~urent signal may be converted to a readout of the oxygen partial pressure and may be converted to an output suitable for controlling a combustion process_ The sensor I O xnay be calibrated and used by first identifying the resistance of the heating electrodes 35 in the desired operating temperature range_ This resistance value, e.g. 9-10 ohms at 600 C, is known and typically is well defined within a given temperature range.
Corresponding current and voltage parameters, e.g., 0.47 A and 4.1 volts, arc prograaxuned into the heating circuit controller 50~ and the controller 50 is programmed to maintain these values.
The actual operating tempezature of any individual sensor is held constant within the sensor's operating range.
As an illustrative example, where 1 mil = 0.001 inches = 0.0254 mm, a preferred sensor body is 166 znil x 124 mil x 53 rxail (4.22 mm x 3.15~mm x 1.35 mm) and weighs 144 mg, rn the embodiment of tlae present invention where cover plate heating electrodes 35 are exnployed, the total electrode overlap area per layer is pzeferably about 12_7 znu2 and the total area to thickness ratio of the oxygen sensor body 12 is about 199 em. The exposed edge of each eleci~ode is 50 rail (1.27 mna) wide, and each electrode extends 1 S3 mil (3.89 mm) into the body. 1'he resistive heating electrodes are preferably porous Pt tracks approximately x 66 mil (4.22 mrx~) in length and 22 mil (0.559 zrnn) in width, whereby a heater current of 223 rxaA is typical for a control temperature of about 600° C.
Referring now to Figs. 6A and 6E, a packaging scberrae according to orte embodiment of the present invention is illustrated. In the illustrated embodiment, the sensor body 12 is enclosed in a stainless steel tube 60. The thicla~,ess of the tube 60 is preferably selected to be machinablE
for threads for mounting the package into a bulkhead or exhaust flue. The sensor bod~r 12 is stabilized and thermally insulated within the tube 60 by means of suitable gas permeable thermal.
insulation 62 (e.g., N'extel 312 thezznal insulation). A back end 64 of the tube 60 is sealed with a ceramic 66_ Electrical connections 68 to the sensor body 12 ate potted in the ceramic 66 and routed through the insulation b2. Preferably, the electrical connections cozxiprise 20 gauge copper leads coupled to the few sensor leads. A front ez~d 65 of the tube 60 is provided with a stainless steel screen 69 to permit gas to reach the sensoz body 12.
VPlzile cextaiz~ representative embodiments and details have been shown far purposes of illustrating the invention, it will be apparent to those skilled in the alt that various changes in the methods and apparatus disclosed herein may be made without departing from the scope of the invention, which is de .fined in the appended claims. For example, although the sensor 10 of the present invention is well suited for measuring excess oxygen partial pressure because the o~cygen-porous terminatzons 22, 24 present a catalysis area for the combustion of CO and other combustibles, it is noted that the present invention may be arranged for measuring actual oxygen partial pressurE rather than excess oxygen partial pressure. Specil5eally, the cathode electrodes 16a, 16b exposed on the first edge i8 ofthe sensor body 12 are very thin and present a very small catalysis area for the combustion of CO and other combustibles.
Accordingly, by omitting the oxygen-porous tcrminations 22, 24, the sensor 10 of tl~e present invention may be arranged for measuring actual oxygen paaiial pressure rather than excess oxygen partial pressure.
huxther, it is contemplated by tlae present invention that a pair of sensors could be packaged to yield both actual and excess oxygen measurerner~is simply by providing the o~cygen-porous terminaiions 22, 24 on one sensor body only. Finally, it is noted that an alteznate method of measuring actual and excess oxygen using two sensors would be to maintain one sensor below the ignition te~.nperature of CO (600-650°C) and the second sensor above this temperature, also in a single package.
Combined Oxy~and NOx Sensor Referring now to Figs. 7 and 8A-8C, a combined sensor 200 for measuring oxygen content and 1\10X content in a gas is described. 'fhe sensor 200 comprises a partial enclosure 210, a sensor body 220 disposed in the partial enclosure 210, a diffusion barrier 230, and axr oxygen sensor 240. As will be described in further detail below, the sensor body 220 is conFgured to provide an indication of the N'Ox content of tl~e gas and the oxygen sensor 240 is configured to provide an indication of the oxygen content of the gas. The sensor 200 includes many components identical or similar in structure to those described in detail above with reference to Fig. 2. Like reference numerals are utilized in Figs. 2 and 7 corresponding to the life elements and reference is made to the discussion of Fig_ 2 for a description of these elements_ The partial enclosure 210 defines a gas passage 2I2 and as referred to herein as "partial's because it encloses a defined space but also defines the gas passage 212, an inlet portion 2I4, and an outlet portion 216. ~'he partial enclosure 210 typically comprises au o~tygen-ion conductive ceramic tube. It is noted that, although the enclosure is illustrated with a rectangula.e cross-section, an enclosure with a circuaar cross seci~on is likely to be more efFective and easier to mariufacture_ The diffusion barrier 230 extends across the gas passage 212 and defines a diffusion-limited portion 2I8 of the gas passage 2I2 between the inlet portion 214 and the outlet portion 216. The enclosure 210, the diffusion barzaer 230, and the sensor body 220 are configured such that the diffusion-limited portion 218 of the gas passage 212 comprises a hermetically sealed Zone including a diffusion inlet defined by the diffusion barrier 230 and a sensor outlet defined by the sensor body 220. An oxygen pumping portion 250, described in detail below, is also provided in the hermetically sealed zone.
The diffusion barrier 230 is porous to oxygen and NOx and may comprise, for example, a substantially uniform zirconia partition. Typically, the diffusion barrier is configured to pass an amount of gas that varies as a function of oxygen partial pressure of gas within an inlet portion of the gas passage. It is contemplated that the diffusion barrier may define a variety of con~~gurativns including, for example, a perforated plate, a plate including a single restricted x5 aperture, etc.
The sensor body 220 extends across the outlet porkion 216 of the gas passage 212 and is disposed in the diffusion-limited portion 218 of the gas passage 212. The sensor body 220 differs from the sEnsor body 12 illustrated in Fig. 2 in that selected ones of the oxygen porous conductive layers are formed from a material that catalyzes the dissociation of NOx into N2 and 02. In this manner, dissociated 02 may be measured as an amperometric current sad the amperometric current may be related to NOx contex~t_ 'fhe conductive layexs that do not catalyze the dissociation of NOx into N2 and 02, i.e., the non-dissociative electrode layers, are utilized to pxovide an indication of oxygen content, as will be described in further detail herein.
Specifically, the sensor body 220 comprises a plurality of oxygen-porous electrode layers ZS 16a, 16c and a pluo:ality of dissociative oxygen-porous electrode layers 16b, 16d. As is described above with reference to the oxygen sensor of Fig. 2, the oxygen-porous electrode layers 16a, 16c catalyze cause oxygen to be pumped through the layers of oxygen ion conducting rx~ate~al 14 by catalysing oxygen molecules to ions at the cathode layers and catalyzing ions to oxygen molecules at the anode layers. The resulting electrical current is measured by the arnperometric meter 28 and is indicative of the oxygen partial pressure o~the gas_ The dissociative oxygen-porous eIecixode layers 16b, 16d pump oxygen through this process as well but additionally pump oxygen dissociated from NOx in the gas by catalyzing the dissociation of NOx into N2 and 02 at the cathode layers. As a result, the resulting electrical current at the dissoeiative oxygen-porous electrode layers 16b, 16d provides an indication of NOx present in the gas.
As is the case with the embodiment ofFig_ 2, a plurality of oxygen ion conductive cerarr~ic layers are interposed between respective ones of the oxygen=porous electrode layers 16a, 16c and respective ones of the dissociative oxygen-porous electrode layers 16b,16d. As will be appreciated by those practicing the present invention, an oxygen content electrical signal output is provided in the form of electrical leads coupled to the plurality of oxygen porous electrode layers 16a,16c. Similarly, a NOx content electrical signal output is provided in the fozxu of electrical leads coupled to the plurality of dissoeiative oxygen-porous electrode layers 16b, 16d.
In this manner, the o~tygen-porous electrode layers 16a, 16e are coupled to an electrical signal output indicative of an oxygen content of gas within the diffusion-lizxaited portion 218 of the gas passage 212 and the dissociative oxygen-porous electrode layers 16b, x6d are coupled to an electrical signal output indicative of an NOX content of gas within the diffusion-limited portion 218 of the gas passage 212.
The NOx content electrical signal output is electrically isolated frorr~ the oxygen content electrical signal output to ensure proper device performance. To further enhance device performance, flee power source 30 and the electrode layers 16a, 16b, 16c,16d are arranged such that the oxygen-porous electrode layer 16a and the dissociative oxygen porous electrode layer 16b define the sole adjacent pair of different-type electrode layers and have matching polarity.
The electrode layers 16a, 16b are also at substantially equivalent electrical potential (e_g_, 0.1VDC). In this manner, pumping of oxygen between the oxygen-porous electrode layer 16a and the dissociative oxygen-porous electrode layer 1Gb is .inhibited. In contrast, the sensor arrangement illustrated in Fig 2 nncludes electrode layers of alternating polarity.
At elevated temperatures, e.g., above about 600°C, Rh catalyzes the dissociation of NOx into NZ and Oz. Accordingly, the dissociative oxygen-porous electrode layers ldb, 16d may comprise Rh_ The non-dissoeiative electrode layers 16a, 16c may comprise oxygen porous platinum, as described above, and may additionally include Au in an amount sufficient to discourage catalysis of the dissociation of NOx. As is noted above with reference to the oxygen is sensor of Fig. 2, a heater or heating electrode is preferably configured to elevate the operating temperature of the combined sensor well above room temperature, typically an the vicinity of an operating temperature of about 800°C. The sensor is temperature independent in this range. The heater may, for example, be providEd in the form of a heating electrode formed about the enclosure 210_ The paz-tial enclosure 2I0 also defines an oxygen pumping portion 250 that is configured to maintain a favorable NOx to oxygen ratio in the diffusion limited portion 218 of the gas passage 212. Depending upon the operation. constraints of the equipment used with the present invention, accurate measurement of l~Ox content may be problematic if the amount of oxygen in the diffusion limited portion relative to the amount of NOx is too high. The oxygen pumping portion 250 comprises an oxygen-porous cathode electrode 252, an oxygen-porous anode electrode 254, and axz oxygen-ion conductive ceramic material 256. The oxygen-porous cathode electrode 252 is positioned over an interior surface of the partial enclosure 210 within the diffusion-limited portion 218 of the gas passage 212. The oxygen porous anode electrode Z54 is 1 S positioned over azi exterior surface of the partial enclosure 210 outside of the diffusion-limited portion 218 of the gas passage 212. The oxygen-ion conductive ceramic material 2SG is typically formed by the body of the enclosure 210 and, as such, is interposed between the cathode electrode 252 and the anode electrode 254. 'f he oxygen-porous anode electrode 254 may eonoprise platinum and the oxygen-porous cathode electrode 252 may also comprise platinum with an amount of gold additive su:~cient to discourage dissociation of NOx.
Preferably, the NOx to oxygen ratio in the diffusion lix~aited portion 218 is below about 5 parts oxygen to 1 part NOx but x~c.~ay be higher if the equipment used to measure amperometric current and control the voltages at the electrodes is optimized to accowlt for higher oxygen levels. Accurate measurement of NO~c content is problematic if the amount of oxygen in the diffiusion limited portion relative to the amount o~NOx is too high. For example, there is a logarithmically linear relafiionship between amperometric current and oxygen partial pressure below about 1000 ppm but accurate measurement is problexx~atic above this level. A feedback loop xnay be coupled between the sensor body 220 and the oxygen pumpxz~g portion 250. The feedback loop may be configured to control the oxygen pumping porhion 250 in response to the amount of oxygen sensed by the sensor body 220. Specifically, using the oxygen measurement from the sensor body 220, the rate of pumping oxygen out of the diffusion limited portion 218 can be continuously adjusted so that no more oxygen is pumped out ofthe tube interior than is needed to provide an accurate measurement of t'he NOx content (e.g., tv keep the ratio ofNOx-released oxygen to background oxygen at, say, 1:5). The feedback loop may also be configured to switch the pumping funetioxx on and offin response to the amount of sensed oxygen. Tn this mannez~, operation of the oxygen pumping portion 250 rxxay be operated to minimize power eonsurnption of the combined sensor 200.
The oxygen sensor 240 is positioned in the inlet portion 214 of the gas passage 212 and provides a signal indicative of the oxygen partial pressure of the gas ixx the inlet portion 214.
'Thus, the combined sensor 200 is 'configured to provide independent indicatioxas of oxygen partial pressure and NOX content.
Turning now to the manner in which the NOX content is determined, NOx present in the gas v~rithin tile diffusion limited portion 218 dissociates on the dissociative oxygen porous electrode layers 16b, 16d and the released oxygen creates an ampexometric current at the NOx content electrical signal output. Oxygen in the surrou~oding gas also contributes to the NOx content electrical signal output, increasing the amperometric current because tlae dissociative electrodes lbb, 16d pump the oxygen in the gas and the oxygen dissociated $rom the NOx present in the gas. This "background" oxygen axad the inczeased amperometric current can be accounted for using the oxygen content electrical signal output from the electrodes 16a,16c ZO because the corresponding amperometric current at the non-dissociative olect~rodes 1,6a,16c provides an izxdependent xr~easure of the background oxygen.
As is nol~ed above, to accurately measure tk~e NOx content, it is also z~eecssaacy to reduce the background oxygen in the diffusion limited portion to a level commensurate with the NOx released oxygen (e.g., to a ratio of about 5:1 (oxygen to NO~.
As is noted above, the sensor body 220 has two separated sets of porous electrodes, one of which catalyzes the dissociation of NOx to nitrogen and o~cygen. For convenience of illustration, Fig_ 7 merely illustrates a pair of electa-ode layers in eack~
set. However, it is contemplated that a large number of electrode layers could be provided in each set. Preferably, an equal number of electrode layers are provided in each set. However, it is contemplated by the present izzvention that more electrode layers could be providEd in one set, relative to the other, as long as the difference in nurxaber is accounted for in the subsequent NOx content calculation.
The sensor 200 may be mounted directly iz~ an exhaust or sample gas. There is no need for a reference gas supply. Particulate filters or ether types of. filters may be provided to present damage to the sensor and extend sensor life.
The sensor 200 is preferably manufactured in a manner similar to that discussed above with reFerence to the oxygen sensor of Fig. 2. Although a variety ofznanu~acturir~g techniques are available, multi-layering manufaeturiz~g processes have the flexibility of producing layers electroded with PtlAu and separate layers electroded with Rh in tb,e same sensor body. Sensor leads are preferably embedded in the sensor body by drillixrg small holes (~0.5 xnm) in the sensor body 220 in the green state. The sensor body 220 is thezt sintered and Pt wires are feed in the holes with a Pt paste. The stiffness of the ft wires has the advantage ofprovidiz~g mechanical support. Leads for the oxygen sensor 240 are similarly errabedded.
The length of the actual coxxibined sensor nnay be about one inch (2.5 cm) and the major outside diameter may be about %x 111C11 (1.25 cm). The enclosure 210 may comprise a zircania tube made by slip casting. The tube i.s ypically milled in the green state to pmvide passageways for electrical leads and is subsequently sintered. The PtJAu and Pt electrodes 252, 254 are then fired on the intezior and exterior of the tube, respectively. Finally, using a commercial glass for sealing zirconia parts together, the sensor body 220, the diffusion battier 230, anal the oxygen sensor 240 are sealed in the zirconia tube in a single firing_ The first two components are sealed hermetically. .A. Pt lead for the internal Pt/Au electrode passes through the wall of the tube and is also sealed hermetically. A slot in the zirconia tube at the large open end provides the passagerWay fez two oxyge~a sensoz leads, and opposing slots in the snrxall closed end provide passageways for four du.a.1-sensor leads. The sensor body or dual sensor 220 arid its four leads are hermetically sealed in the xirconia tube rwith a commerciahy available glass.
Generally, the operation of the combined sensor 200 is as follows: The device Xs heated to, and maintained at, the operafiing temperature (e.g., 800 °C), axed the oxygen sensor 240 measures the oxygen partial pressure of the exhaust or sample gas. The gas diffuses through the diffusion bawier 230 into the interior diffusion limited portion 218 of the enclosure or tube 210.
A voltage applied across tlae cathode 252 and the anode 254 causes the oxygen in the interior to be pumped to a sufficiently low Ievel. The sensor body 220 measures this low oxygen level with the non-dissociative layers 16a, 16e. The dissociative electrode layers 16b, 16d measure both the low oxygen level and the oxygen released from the NOx dissociation. ~laese amperometrie currents from both sets of electrodes are then used to determine the NOx content.
The 2irconia diffusion barrier 230 diffusion-limits the amount of exhaust gas entering the interior of the tube and thereby ensures that a low level of oxygen can be reached in the interior by the pumping process (i.e., without this plug the interior would be constantly flooded with the exhaust gas). The NOx diffuses through this plug as molecular NOx.
Tlae heater (not shown in Fig. 2) has a temperature-dependent resistance and thereby provides a aneans for measuring and controlling the operating temperature. ,A.
tradeoff is involved with the operating temperature, however: On the one hand, the higher the temperature, the more power is consmned by the lieater in maintaining this temperature. On the other hand, the temperatuze slZVUld be high enough to reduce the resistance of the zirconia tube to a low value to avoid consuming large amounts of powex in pumping the oxygen out of the tube interior.
For tlae purposes of describing and defining the present invention it is noted that the term "substantially" is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The terra "substantially" is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.
.'l3aving described tl~e invention in~ detail and by reference to preferred erxxbodiments thereof, it will be apparent that modifZCations and variations are possible without departing from the scope ofthe invention defined iu~ the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred oz-particularly advantageous, it is contemplated that the present invezttian is not necessarily limited to these preferred aspects of the invez~tion_ What is claimed is:
Claims (30)
1. A combined sensor for measuring oxygen content and NO X content in a gas, said sensor comprising:
a sensor body disposed in said gas, wherein said sensor body comprises a plurality of oxygen-porous electrode layers, a plurality of dissociative oxygen-porous electrode layers, wherein said dissociative oxygen-porous electrode layers comprise a material selected to catalyze dissociation of NO X into nitrogen and oxygen, and a plurality of oxygen ion conductive ceramic layers interposed between respective ones of said oxygen-porous electrode layers and respective ones of said dissociative oxygen-porous electrode layers;
an oxygen content electrical signal output coupled to said plurality of oxygen porous electrode layers; and a NO X content electrical signal output coupled to said plurality of dissociative oxygen-porous electrode layers, wherein said NO X content electrical signal output is electrically isolated from said oxygen content electrical signal output.
a sensor body disposed in said gas, wherein said sensor body comprises a plurality of oxygen-porous electrode layers, a plurality of dissociative oxygen-porous electrode layers, wherein said dissociative oxygen-porous electrode layers comprise a material selected to catalyze dissociation of NO X into nitrogen and oxygen, and a plurality of oxygen ion conductive ceramic layers interposed between respective ones of said oxygen-porous electrode layers and respective ones of said dissociative oxygen-porous electrode layers;
an oxygen content electrical signal output coupled to said plurality of oxygen porous electrode layers; and a NO X content electrical signal output coupled to said plurality of dissociative oxygen-porous electrode layers, wherein said NO X content electrical signal output is electrically isolated from said oxygen content electrical signal output.
2. A combined sensor for measuring oxygen content and NO X content in a gas, said sensor comprising:
a sensor body disposed in said gas, wherein said sensor body comprises a plurality of oxygen-porous electrode layers, a plurality of dissociative oxygen porous electrode layers, wherein said dissociative oxygen-porous electrode layers comprise sufficient Rh to catalyze dissociation of NO X into nitrogen and oxygen, and a plurality of oxygen ion conductive ceramic layers interposed between respective ones of said oxygen porous electrode layers and respective ones of said dissociative oxygen-porous electrode layers;
an oxygen content electrical sigma output coupled to said plurality of oxygen porous electrode layers; and a NO x content electrical signal output coupled to said plurality of dissociative oxygen-porous electrode layers, wherein sand NO x content electrical signal output is electrically isolated from said oxygen content electrical signal output.
a sensor body disposed in said gas, wherein said sensor body comprises a plurality of oxygen-porous electrode layers, a plurality of dissociative oxygen porous electrode layers, wherein said dissociative oxygen-porous electrode layers comprise sufficient Rh to catalyze dissociation of NO X into nitrogen and oxygen, and a plurality of oxygen ion conductive ceramic layers interposed between respective ones of said oxygen porous electrode layers and respective ones of said dissociative oxygen-porous electrode layers;
an oxygen content electrical sigma output coupled to said plurality of oxygen porous electrode layers; and a NO x content electrical signal output coupled to said plurality of dissociative oxygen-porous electrode layers, wherein sand NO x content electrical signal output is electrically isolated from said oxygen content electrical signal output.
3. A combined sensor for measuring oxygen content and NO x content in a gas, said sensor comprising:
a partial enclosure defining a gas passage;
a sensor body disposed in said partial enclosure,wherein said sensor body comprises a plurality of oxygen porous electrode layers, a plurality of dissociative oxygen porous electrode layers, and a plurality of oxygen ion conductive ceramic layers interposed between respective ones of said oxygen-porous electrode layers and respective ones of said dissociative oxygen-porous electrode layers; and a diffusion barrier defining a diffusion limited portion of said gas passage, wherein said sensor body is disposed in said diffusion-limited portion of said gas passage.
a partial enclosure defining a gas passage;
a sensor body disposed in said partial enclosure,wherein said sensor body comprises a plurality of oxygen porous electrode layers, a plurality of dissociative oxygen porous electrode layers, and a plurality of oxygen ion conductive ceramic layers interposed between respective ones of said oxygen-porous electrode layers and respective ones of said dissociative oxygen-porous electrode layers; and a diffusion barrier defining a diffusion limited portion of said gas passage, wherein said sensor body is disposed in said diffusion-limited portion of said gas passage.
4. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 3 wherein said gas passage defined by said partial enclosure defines an inlet portion and an outlet portion and wherein said sensor body extends across said outlet portion of said gas passage.
5. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 3 wherein at least a portion of said partial enclosure defines an oxygen pumping portion configured to maintain a favorable NO X to oxygen ratio.
6. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 5 wherein said predetermined NO x to oxygen ratio is below about 1:5.
7. A combined sensor for measuring oxygen content and NO X content in a gas as claimed in claim 3 wherein at least a portion of said partial enclosure defines an oxygen pumping portion, said combined sensor further comprises a feedback loop coupled between said sensor body and said oxygen pumping portion, and said feedback loop is configured to control said oxygen pumping portion as a function of an amount of oxygen sensed by said sensor body.
8. A combined sensor for measuring oxygen content and NO X content in a gas as claimed an claim 7 wherein said feedback loop is configured to decrease a pump rate of said oxygen, pumping portion as said amount of sensed oxygen decreases.
9. A combined sensor for measuring oxygen content and NO X content in a gas as claimed in claim 3 wherein at least a portion of said partial enclosure defines an oxygen pumping portion comprising:
an oxygen-porous cathode electrode positioned over an interior surface of said partial enclosure within said diffusion-limited portion of said gas passage;
an oxygen-porous cathode electrode positioned oven an exterior- surface of said partial enclosure outside of said diffusion-limited portion of said gas passage; and an oxygen-ion conductive ceramic material interposed between said cathode electrode and said anode electrode.
an oxygen-porous cathode electrode positioned over an interior surface of said partial enclosure within said diffusion-limited portion of said gas passage;
an oxygen-porous cathode electrode positioned oven an exterior- surface of said partial enclosure outside of said diffusion-limited portion of said gas passage; and an oxygen-ion conductive ceramic material interposed between said cathode electrode and said anode electrode.
10. A combined sensor for measuring oxygen content and NO X content in a gas as claimed in claim 3 wherein said oxygen-parous anode electrode comprises platinum and said oxygen-porous cathode electrode comprises platinum and gold.
11. A combined sensor for measuring oxygen content and NO X content in a gas as claimed in claim 3 wherein said plurality of oxygen-porous electrode layers comprise a material selected to inhibit dissociation of NO X into nitrogen and oxygen.
12, A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 11 wherein said plurality of oxygen-porous electrode layers comprise Pt and Au.
13. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 3 wherein said plurality of plurality of dissociative oxygen-porous electrode layers comprise a material selected to catalyze dissociation of NO x into nitrogen and oxygen.
14. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 13 wherein said dissociative oxygen-porous electrode layer material is selected to catalyze dissociation of NO x into N2 and O2.
15. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 14 wherein said plurality of dissociative oxygen-porous electrode layers comprise Rh.
16. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 3 wherein said combined sensor further comprises a power source, said power source is configured such that an oxygen-porous electrode layer and a dissociative oxygen porous electrode layer define respective ones of an adjacent pair of electrode layers having matching polarity and substantially equivalent electrical potential such that pumping of oxygen between said oxygen porous electrode layer and a dissociative oxygen-porous electrode layer as inhibited.
17. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 3 wherein said oxygen-porous electrode layers are electrically isolated from said dissociative oxygen-porous electrode layers.
18. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 3 wherein said plurality of oxygen porous electrode layers are coupled to an electrical signal output that is independent of an electrical signal output to which said dissociative oxygen-porous electrode layers are coupled.
19. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 18 wherein said oxygen porous electrode layers are coupled to an electrical signal output indicative of an oxygen content of gas within said diffusion-limited portion of said gas passage and said dissociative oxygen-porous electrode layers are coupled to an electrical signal output indicative of an NO x content of gas within said diffusion-limited portion of said gas passage.
20. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 3 wherein said partial enclosure comprises an oxygen-ion conductive ceramic tube and said diffusion barrier extends across an inside diameter of said tube defining a barrier between said diffusion-limited portion of, said gas passage and an inlet portion of said gas passage.
21. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 3 wherein said diffusion-limited portion of said gas passage comprises a hermetically sealed zone including a diffusion inlet defined by said diffusion barrier and a sensor outlet defined by said sensor body.
22. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 21 wherein said hermetically sealed zone further comprises an oxygen pumping portion.
23. A combined sensor for measuring oxygen content aid NO x content in a gas as claimed in claim 3 wherein said diffusion barrier defines a barrier between said diffusion-limited portion of said gas passage and an inlet portion of said gas passage and wherein said combined sensor includes an oxygen sensor positioned in said inlet portion of said gas passage.
24. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 3 wherein said diffusion barrier comprises a zirconia partition.
25. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 3 wherein said diffusion barrier extends across said gas passage.
26. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 25 wherein said diffusion barrier comprises a substantially uniform partition.
27. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 3 wherein said diffusion barrier is configured to pass an amount of gas that varies as a function of oxygen partial pressure of gas within an inlet portion of said gas passage.
28. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 3 further comprising a heater configured to elevate an operating temperature of said combined sensor to about 800°C.
29. A combined sensor for measuring oxygen content and NO x content in a gas as claimed in claim 28 wherein said partial enclosure comprises a zirconia tube and said heater is formed about said zirconia enclosure.
30. A sensor body comprising:
a plurality of oxygen-porous electrode layers;
a plurality of dissociative oxygen porous electrode layers, wherein said dissociative oxygen-porous electrode layers comprise a material selected to catalyze dissociation of NO x into nitrogen and oxygen; and a plurality of oxygen ion conductive ceramic layers interposed between respective ones of said oxygen-porous electrode layers and respective ones of said dissociative oxygen porous electrode layers.
a plurality of oxygen-porous electrode layers;
a plurality of dissociative oxygen porous electrode layers, wherein said dissociative oxygen-porous electrode layers comprise a material selected to catalyze dissociation of NO x into nitrogen and oxygen; and a plurality of oxygen ion conductive ceramic layers interposed between respective ones of said oxygen-porous electrode layers and respective ones of said dissociative oxygen porous electrode layers.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US25408100P | 2000-12-07 | 2000-12-07 | |
| US60/254,081 | 2000-12-07 | ||
| PCT/US2001/047465 WO2003008957A1 (en) | 2000-12-07 | 2001-12-07 | COMBINED OXYGEN AND NOx SENSOR |
Publications (1)
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|---|---|
| CA2431018A1 true CA2431018A1 (en) | 2003-01-30 |
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|---|---|---|---|
| CA002431018A Abandoned CA2431018A1 (en) | 2000-12-07 | 2001-12-07 | Combined oxygen and nox sensor |
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| EP (1) | EP1350090A4 (en) |
| JP (2) | JP4116543B2 (en) |
| CA (1) | CA2431018A1 (en) |
| RU (1) | RU2269121C2 (en) |
| WO (1) | WO2003008957A1 (en) |
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| US9381382B2 (en) | 2002-06-04 | 2016-07-05 | The Procter & Gamble Company | Composition comprising a particulate zinc material, a pyrithione or a polyvalent metal salt of a pyrithione and a gel network |
| US8852414B2 (en) | 2009-04-15 | 2014-10-07 | Emd Millipore Corporation | Converter for use with sensing devices |
| RU2548374C2 (en) * | 2012-12-06 | 2015-04-20 | Федеральное государственное унитарное предприятие "Государственный научный центр Российской Федерации - Физико-энергетический институт имени А.И. Лейпунского" | Solid electrolyte detector of oxygen concentration in gas media |
| DE102013222195A1 (en) * | 2013-10-31 | 2015-04-30 | Siemens Aktiengesellschaft | Gas sensor for the detection of nitrogen oxides and operating method for such a gas sensor |
| JP6825992B2 (en) * | 2017-05-31 | 2021-02-03 | 株式会社東芝 | Oxygen measurement method in the reactor containment vessel |
| DE102017218327B4 (en) | 2017-10-13 | 2019-10-24 | Continental Automotive Gmbh | Method for operating an internal combustion engine with three-way catalytic converter and lambda control |
| CN115166000A (en) * | 2022-06-21 | 2022-10-11 | 湖北天瑞电子股份有限公司 | Sensor chip for fuel inerting oxygen measurement and preparation method thereof |
Family Cites Families (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2288873A (en) * | 1939-12-30 | 1942-07-07 | Standard Oil Co | Polymerization of olefins |
| US4462891A (en) * | 1983-02-07 | 1984-07-31 | Lawless William N | Oxygen sensor and high temperature fuel cells |
| US5250169A (en) * | 1991-06-07 | 1993-10-05 | Ford Motor Company | Apparatus for sensing hydrocarbons and carbon monoxide |
| SE513477C2 (en) * | 1993-11-08 | 2000-09-18 | Volvo Ab | Sensor for detecting nitric oxide compounds |
| JP2885336B2 (en) * | 1994-04-21 | 1999-04-19 | 日本碍子株式会社 | Method and apparatus for measuring NOx concentration in gas to be measured |
| US5672811A (en) * | 1994-04-21 | 1997-09-30 | Ngk Insulators, Ltd. | Method of measuring a gas component and sensing device for measuring the gas component |
| JP3537628B2 (en) * | 1996-05-16 | 2004-06-14 | 日本碍子株式会社 | Method for measuring nitrogen oxides |
| DE19846487C5 (en) * | 1998-10-09 | 2004-12-30 | Basf Ag | Measuring probe for the detection of the instantaneous concentrations of several gas components of a gas |
| EP1174712A4 (en) * | 1999-03-23 | 2002-06-12 | Hitachi Ltd | Gas components measuring device |
-
2001
- 2001-12-07 EP EP01271049A patent/EP1350090A4/en not_active Ceased
- 2001-12-07 JP JP2003514250A patent/JP4116543B2/en not_active Expired - Fee Related
- 2001-12-07 WO PCT/US2001/047465 patent/WO2003008957A1/en active Application Filing
- 2001-12-07 CA CA002431018A patent/CA2431018A1/en not_active Abandoned
- 2001-12-07 RU RU2003120058/28A patent/RU2269121C2/en not_active IP Right Cessation
-
2008
- 2008-02-29 JP JP2008049626A patent/JP2008203265A/en not_active Ceased
Also Published As
| Publication number | Publication date |
|---|---|
| JP4116543B2 (en) | 2008-07-09 |
| RU2269121C2 (en) | 2006-01-27 |
| WO2003008957A1 (en) | 2003-01-30 |
| EP1350090A4 (en) | 2006-06-07 |
| RU2003120058A (en) | 2005-02-10 |
| JP2004536307A (en) | 2004-12-02 |
| EP1350090A1 (en) | 2003-10-08 |
| JP2008203265A (en) | 2008-09-04 |
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| EEER | Examination request | ||
| FZDE | Discontinued |