Classifications

• • 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
  • G01N27/416—Systems
  • G01N27/417—Systems using cells, i.e. more than one cell and probes with solid electrolytes
  • G01N27/419—Measuring voltages or currents with a combination of oxygen pumping cells and oxygen concentration cells
• • 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

Definitions

• This invention relates to gas sensors, and, more particularly, to oxygen sensors.
• Oxygen sensors are used in a variety of applications that require qualitative and quantitative analysis of gases.
• the direct relationship between the oxygen concentration in the exhaust gas and the air- to-fuel ratio of the fuel mixture supplied to the engine allows the oxygen sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.
• a conventional stoichiometric oxygen sensor typically comprises an ionically conductive solid electrolyte material, a porous electrode on the exterior surface of the electrolyte exposed to the exhaust gases with a porous protective overcoat, and an electrode on the interior surface of the sensor exposed to a known oxygen partial pressure.
• Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with platinum electrodes, which operate in potentiometric mode to detect the relative amounts of oxygen present in the exhaust of an automobile engine. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation: where:
• an oxygen sensor based on solid oxide electrolyte such as zirconia, measures the oxygen activity difference between an unknown gas and a known reference gas.
• the known gas is the atmosphere air while the unknown gas contains the oxygen with its equilibrium level to be determined.
• the sensor has a built in air channel which connects the reference electrode to the ambient air.
• the sensor requires expensive sensor package which usually has complex features in order to provide sufficient gas sealing between the reference air and the unknown gas.
• these gas sealed sensor packages have demonstrated insufficient durability in the field. This problem can be avoided by using in-situ electrochemical oxygen pumping. In this method, the air reference electrode chamber is replaced by a sealed reference electrode with oxygen electrochemically pumped in from the exhaust gas.
• One embodiment of the gas sensor comprises: a first electrode and a reference electrode with an electrolyte disposed therebetween, wherein the first electrode and reference electrode are in ionic communication and a reference gas channel in fluid communication with the reference electrode and an exterior of the sensor, wherein the reference gas chamber has a diffusion limiter.
• One embodiment of the method comprises: using a gas sensor comprising a first electrode and a reference electrode with an electrolyte disposed therebetween, wherein the first electrode and reference electrode are in ionic communication and a reference gas channel in fluid communication with the reference electrode and an exterior of the sensor; introducing an exhaust gas to the first electrode; applying a voltage to the reference electrode; ionizing oxygen at the first electrode; transferring the ionized oxygen across the electrolyte to the reference electrode; forming molecular oxygen at the reference electrode; ionizing the molecular oxygen on the reference electrode; transferring the ionized oxygen across the electrolyte to the first electrode to create a voltage; and measuring the voltage.
• Figure 1 is an expanded view of a gas sensor design.
• Figure 2 is a graphical representation of exhaust flux measured from a gas-diffusion-limiting air channel against the leakage rates of the sensor package, the plateau of the data represents the limiting value of the exhaust flux.
• Figure 3 is a graphical representation of limiting exhaust flux values against the width distribution (in percent (%)) of the air channels.
• Figure 4 is an angled top view of one air channel design that has gas limiting means as well as gas buffering (storage) space to overcome oxygen pump problem related to the heater cyclic operation (heater on-off operation).
• Figure 5 is an expanded view of alternative design of a gas sensor which has electrical isolation (ground isolation between the heater and the sensor emf) where the emf element and oxygen pump element are on the opposite side of the heater.
• Figure 6 is an expanded view of alternative design of a gas sensor which has electrical isolation (ground isolation between the heater and the sensor emf) where the emf sensor and oxygen pump sensor are on the same side of the heater.
• Figure 7 is an expanded view of an alternative gas sensor design with connection of the emf electrode leads and the heater leads outside of the sensor element level to further address the issue of isolation between emf electrode leads and the heater leads.
• the gas sensor comprises one or more electrochemical cells (i.e., an electrolyte disposed between two electrodes; an exhaust side electrode and a reference electrode), a porous protective layer disposed adjacent to the outer electrode, a reference gas chamber in fluid communication with the reference electrode, and a heater in thermal communication with the electrochemical cell.
• the reference gas chamber is additionally in fluid communication with the atmosphere around the gas sensor, i.e., the air and the exhaust gas.
• the reference chamber does not need to be hermetically sealed from the exhaust gas.
• the gas sensor avoids the requirement of hermetic sealing the reference gas chamber by pumping-in oxygen to the reference electrode and reference chamber thereby creating a pressure gradient through the chamber.
• one or more gas diffusion limiters may also be employed within the reference chamber.
• the Nernst oxygen sensor is shown with the following basic features (components): exhaust side electrode 1, reference electrode 2, heater leads 3 and 4, porous protection layer 5, solid electrolyte layer 6, insulated alumina layers 7 and 8, air channel 9, and heater 12.
• exhaust side electrode 1 the exhaust gas electrode 1
• Reference electrode 2 is separated from exhaust electrode 1 by a dense solid electrolyte layer 6 and is exposed to the open channel 9 which is open to the ambient atmosphere at the tail 13 of the sensor.
• the heater leads 3 and 4 are connected to the positive and negative poles of a heater power supply 14, respectively.
• the senor can have the positive heater lead 3 connected to the reference electrode lead 17 through resistor R, and the heater negative lead 4 connected to the exhaust side lead 19.
• resistor R there is a resistor R disposed between leads 3 and 17, thereby enabling the connections and the resistor to optionally be disposed inside the sensor ceramic body.
• the exhaust gas migrates to the electrode 1 through the protection coating layer 5 and generates an emf between exhaust electrode 1 and reference electrode 2.
• oxygen flux is pumped into electrode 2 by electric power supplied from heater lead 3 with a positive polarity maintained at the reference electrode 2.
• the oxygen generated at the reference electrode 2 will diffuse out of the reference gas channel 9 through the open end 13 into the ambient atmosphere (e.g., in to the exhaust gas or air).
• the diffusion rate out of the reference gas chamber 9/9' see
• Figures 1 and 4 is linearly proportional to the difference of oxygen pressure between reference electrode (which is equal to or larger than about 1 atmosphere) and the ambient atmosphere (am .) (which is equal to or smaller than about 0.21 atmosphere). Because there is no gas diffusion limiting process employed in this embodiment, the pressure of oxygen within the reference gas channel will never build up to a level to damage the ceramic sensor body. Meanwhile, the diffusion rate into the reference gas chamber by air and/or exhaust gas is linearly proportional to the fuel concentration difference between the reference electrode (where the fuel concentration is kept at zero by the oxygen generated from the pump current) and the ambient atmosphere (which is typically about 21% or less). Since the fuel concentration difference doesn't exceed 21%, the exhaust flux is limited and can be described by Equation I:
• F ex is the exhaust gas flux (i.e., the rate of exhaust gas migration through the channel)
• D is the diffusion constant of exhaust gas constant
• C is the ambient atm. exhaust gas concentration at the open end of the reference gas channel; A is the average cross-sectional area of the gas channel; and L is the length of the gas channel.
• the oxygen flux has to be larger than the limiting flux of the fuel.
• the amount of the oxygen flux is adjustable by the resistance value of R in Figure 1 and the heater voltage; wherein the amount of the limiting flux of the fuel is determined by the cross-sectional area and the length of the gas channel.
• the gas sensor components i.e., protective layer 5, electrodes 1, 2, 3, and 4 (and leads thereto), heater 12, dielectric layers 7 and 8, and heater supply 14.
• additional conventional components can be employed, including but not limited to additional protective coatings (e.g., spinel, alumina, magnesium aluminate, and the like, as well as combinations comprising at least one of the foregoing coatings), lead gettering layer(s), ground plane(s), support layer(s), additional electrochemical cell(s), and the like.
• Dielectric layers 7 and 8 and any support layers typically comprise alumina or a similar material that is capable of inhibiting electrical communication and providing physical protection.
• layers 7,8 as well as optional support layers are preferably capable of effectively protecting various portions of the gas sensor, provide structural integrity, separate various components, electrically isolate heater 12 from the sensor circuits, cover reference electrode 2, heater 12, and/or lead(s), provide physical protection against abrasion, and/or electrically isolate the components of the gas sensor from the packaging.
• the materials employed in the manufacture of gas sensor comprise substantially similar coefficients of thermal expansion, shrinkage characteristics, and chemical compatibility in order to minimize, if not eliminate, delamination and other processing problems.
• the dielectric, as well as support layers can each be up to about 200 microns thick, with a thickness of about 50 microns to about 200 microns preferred.
• These layers can be formed using ceramic tape casting methods or other methods such as plasma spray deposition techniques, screen printing, stenciling and others conventionally used in the art.
• Heater 12 Disposed between two of the layers, e.g., 7 and 8 is heater 12, with a ground plane (not shown) optionally disposed between two support layers (not shown.
• Heater 12 can be any conventional heater capable of maintaining sensor end (i.e., end opposite open end 13) at a sufficient temperature to facilitate the various electrochemical reactions therein.
• Leads 17 and 19 are disposed across various dielectric layers to electrically connect the external wiring of the gas sensor with electrodes 1, 2.
• Leads are typically formed on the same layer as the electrode to which they are in electrical communication and extend from the electrode to the terminal end of the gas sensor where they are in electrical communication with the corresponding via (not shown).
• Heater 12 also has leads 3 and 4 that are in electrical communication with vias.
• Electrolyte layer 6, 6', 6" which is preferably a solid electrolyte that can comprise the entire layer or a portion thereof (see Figures 1, and 5 - 7), can be any material that is capable of permitting the electrochemical transfer of oxygen ions while inhibiting the physical passage of exhaust gases, has an ionic/total conductivity ratio of approximately unity, and is compatible with the environment in which the gas sensor will be utilized (e.g., up to about 1,000°C). Also, this electrolyte can be formed via many conventional processes including, but not limited to, die pressing, roll compaction, stenciling and screen printing, tape casting techniques, and the like.
• Possible solid electrolyte materials can comprise any material conventionally employed as sensor electrolytes, including, but not limited to, zirconia which may optionally be stabilized with calcium, barium, yttrium, magnesium, aluminum, lanthanum, cesium, gadolinium, and the like, as well as combinations comprising at least one of the foregoing.
• the electrolyte can be alumina and yttrium stabilized zirconia.
• the solid electrolyte has a thickness of up to about 500 microns, with a thickness of approximately 25 microns to about 500 microns preferred, and a thickness of about 50 microns to about 200 microns especially preferred.
• porous electrolyte may also be employed.
• the porous electrolyte should be capable of permitting the physical migration of exhaust gas and the electrochemical movement of oxygen ions, and should be compatible with the environment in which the gas sensor is utilized.
• porous electrolyte has a porosity of up to about 20%, with a median pore size of up to about 0.5 microns, or, alternatively, comprises a solid electrolyte having one or more holes, slits, or apertures therein, so as to enable the physical passage of exhaust gases.
• porous electrolytes that may be useful in the instant application.
• Possible porous electrolytes include those listed above for the solid electrolyte.
• the electrolytes 6,6', 6" and protective material 5 can comprise entire layer or any portion thereof; e.g. they can form the layer, be attached to the layer (protective material/electrolyte abutting dielectric material), or disposed an opening in the layer (protective material/electrolyte can be an insert). The latter arrangement eliminates the use of excess electrolyte and protective material, and reduces the size of gas sensor by eliminating layers.
• any shape can be used for the electrolyte and protective material, with the size and geometry of the various inserts, and therefore the corresponding openings, being dependent upon the desired size and geometry of the adjacent electrodes. It is preferred that the openings, inserts, and electrodes have a substantially similar geometry.
• the various electrodes 1, 2, 10, and 11 disposed in contact with electrolyte 6,6', 6" (see Figures 1, and 5 - 7) and porous electrolyte can comprise any catalyst capable of ionizing oxygen, including, but not limited to, metals such as platinum, palladium, osmium, rhodium, iridium, gold, and ruthenium; metal oxides such as zirconia, yttria, ceria, calcia, alumina and the like; other materials, such as silicon, and the like; and mixtures and alloys comprising at least one of the foregoing catalysts.
• metals such as platinum, palladium, osmium, rhodium, iridium, gold, and ruthenium
• metal oxides such as zirconia, yttria, ceria, calcia, alumina and the like
• other materials such as silicon, and the like
• mixtures and alloys comprising at least one of the foregoing catalyst
• Electrodes 1, 2, 10, and 11 can be formed using conventional techniques such as sputtering, chemical vapor deposition, screen printing, and stenciling, among others, with screen printing the electrodes onto appropriate tapes being preferred due to simplicity, economy, and compatibility with the subsequent co-fired process.
• reference electrode 2 can be screen printed onto dielectric layer 7 or over the solid electrolyte 6
• exhaust electrode 1 can be screen printed over solid electrolyte 6 or on protective layer 5.
• Electrode leads and vias in the dielectric and/or electrolyte layers are typically formed simultaneously with electrodes.
• the reference gas channel 9,9' Disposed in fluid communication with the reference electrode 2 is the reference gas channel 9,9' formed by depositing a carbon base material, i.e., a fugitive material such as carbon black, between reference electrode 2 and layer 7 such that upon processing the carbon burns out, and leaves a void.
• a carbon base material i.e., a fugitive material such as carbon black
• the reference gas channel 9' comprises two diffusion paths 21 and 23, with two chambers 23 and 25.
• the length and cross-sectional area of the various compartments (21, 23, 25, and 27) of the reference gas channel 9' are determined based upon Equation I above and the exhaust flux.
• two chambers and two diffusion paths are employed to account for a cycling of the heater voltage.
• the multi-stage diffusion channel is employed.
• the voltage employed for pumping is not cyclical, a single chamber and channel can be employed.
• the channel and chamber can be the same or a different size, based upon Equation I and the voltages to be employed.
• the oxygen pump current has to be larger than the fuel limiting current in order to move oxygen across the electrolyte into the reference gas channel.
• the current is too large it can create additional polarization at electrolyte (ohmic drop) and at electrodes (electrode polarization) which will create error signal to the emf measurement, especially during the light off time of the sensor operation. Consequently, a pump current of about 30 milliamperes per square centimeter (mA/cm ) or less of the exhaust electrode area can be used, with about 20 mA/cm or less preferred, and about 10 mA/cm or less especially preferred. This requirement can be achieved for example by selecting the right resistance value of R (see Figure 1), depending on the heater voltage used.
• the resistor can be carbon or metal oxide type resistor or can be thick film type which can be screen printed on the sensor ceramic body. If the heater uses an alternating current (ac) power supply, a diode can optionally be added to the circuit (in series with R, see Figure 1). Due to the fuel limiting current, the reference gas channel should have a physical dimension to attain a limiting exhaust gas flux of about 30 milliamperes per square centimeter (mA/cm ) of the electrode area or less, with about 20 mA/cm 2 or less preferred, and about 10 mA/cm 2 or less especially preferred. Since the various parameters are interrelated, various amounts can be employed, depending upon the particular design of the sensor. The design should be based upon a combination of the Equation I above, and Equation II:
• the channel length (L) can be about 35 mm to about 65mm, a width (W) of about 0.50 mm or less, and a height (H) of about 0.05mm or less; with a length of about 35mm to about 50mm, a width of about 0.30mm or less, and a height of about 0.025mm or less preferred; and a length of about 40mm to about 48mm, a width of about 0.13mm or less, and a height of about 0.015mm or less especially preferred.
• the lower limit of the cross-sectional area is based upon the desired diffusion rates. Although, the lower limits of both the width and height are about 0.005mm, the practical lower limits are substantially higher due to available production equipment limitations. For example, with a nominal V h of about 13.5 volts, R of about 2 megaohms (M ⁇ ), I p of about 7 microampheres (TJA), the dimensions of the channel are about 43 mm (L) by 0.2 mm (W) by 0.02mm (H).
• the reference gas channel can be accomplished via mechanical cutting-in duck, screen-printing fugitive material (such as carbon which can be burned off at high temperature), porosity controlled coating layering, laser drilling holes, and the like.
• the channel can be open to the ambient exhaust gas directly at the tip (13 in Figure 1) of the gas sensor with a channel dimension of about 10 millimeters (mm) in length (L) by 0.007mm in width (W) by 0.01mm in height (H).
• the heater or any other power device may be the power source for oxygen pumping. If the heater is the source and is operating in a cyclic mode, there is a period of time when no oxygen pumping occurs. If such period of time lasts too long, oxygen can be depleted at the reference electrode. A buffer zone can add into the gas channel design to avoid such difficulties. (See Figures 4-6) In these Figures, the reference electrode sits adjacent a small chamber 25 and a large chamber 27 functions as the buffer zone with a short diffusion-limiting path 21 connecting to the small chamber and a long diffusion path 23 connecting to the tail of the gas sensor. Pumped-in oxygen will be stored in the buffered zone to sustain the reference electrode over the period of time when no oxygen is supplied.
• Such gas channel can be made by screen printing with fugitive materials such as graphite or carbon which can be burned off during the sintering step.
• Oxygen storage materials can also optionally be added to all or a portion of the reference gas channel (e.g., to the entire channel, to one or more diffusion paths, to one or more chambers, or to any combination of these channel portions) to increase the effectiveness of its function.
• Such materials include metals such as platinum, rhodium, palladium, ruthenium, iridium, osmium, and the like; metal oxides such as cerium oxide and bismuth oxide, and the like; as well as other conventional oxygen storage materials, and mixtures and alloys comprising at least one of the foregoing materials.
• electric isolation can be established between the heater and the emf element by employing an additional isolated pair of electrodes to do the oxygen pumping. Possible ways to do so are illustrated in Figures 5 and 6.
• Figure 5 shows a symmetrical design.
• the sensor has two almost identical sensor layouts similar to the one shown in Figure 1 but put on opposite sides of the ceramic heater.
• One of the sensors has an opening reference gas channel with its two leads 3 and 4 connected to the heaters.
• the other sensor has a gas channel through the heater ceramic to expose one of its electrodes (2) to the oxygen generated by the pump electrodes 10 and 11 and measures emf by electrodes 1 and 2. Since electrodes 1, 2 are electrically isolated from electrodes 10 and 11, this sensor design has the feature of signal ground isolation. Also because of electrodes 1 and 2 are separated from electrodes 10 and 11, the maximum oxygen pump current is not limited to 30 mA/cm 2 .
• the hole through the ceramic heater can be made by mechanical punching, drilling on the green tapes before the thermal lamination step.
• Figure 6 shows another possible design that can achieve the same signal isolation. This time the emf sensing electrodes 1 and 2 and the oxygen pump electrodes 10 and 11 are put on the same side of the heater. Electrical isolation layer (e.g., a dielectric layer) such as alumina is put in between the two solid electrolytes cladded between the electrodes. The two isolated electrolytes can be in the form of button or strip. The open reference gas channel connects both electrodes 2 and 4. The two pairs of electrodes can be side by side or one after the other.
• Electrical isolation layer e.g., a dielectric layer
• the two isolated electrolytes can be in the form of button or strip.
• the open reference gas channel connects both electrodes 2 and 4.
• the two pairs of electrodes can be side by side or one after the other.
• exhaust electrode 1 , electrolyte 6, and reference electrode 2 form both a pumping cell and a reference cell, which can operate, even simultaneously.
• Oxygen in the exhaust enters the pumping cell through protective layer 5 where a voltage applied across electrodes 1 and 2 cause oxygen on electrode 1 to be ionized and pumped to reference electrode 2. Then the oxygen is available to provide the reference gas to determine whether the exhaust gas at the exhaust electrode fuel rich (A/F is less than about 14.7 for a gas engine) or fuel lean (A/F is greater than about 14.7 for a gas engine).
• Alumina and yttria doped zirconia were mixed with binders, plasticizers, and solvents and roll-milled into slurry as is conventional. The slurry was then casted into tapes by doctor blade tape casting methods.
• Electrodes 1, 4 were platinum and reference gas channel 9 was carbon).
• the print of the carbon channel 9 had a basic physical dimension of 49 mm L by 0.86mm W by 0.015mm H; with the W varied at 13%, 20%, 25%, 50% and 100%) of the value shown here.
• the reference gas channel had openings at the tail end of the gas sensor.
• the tapes were thermally laminated, cut and co-fired at 1,500°C for several hours and later packaged for final testing. Because the sensors were new and hermetically sealed, various sizes of leak holes were made into the sensor packages so that exhaust gas could leak into the tail part of the sensors.
• the sensors were then operated in hot rich engine operation conditions (e.g., 5.7 liter (1), eight valve (V8) engine at 2,700 revolutions per minute (rpm) and 70 kilopascals (kPa) exhaust pipe back-pressure, 10% rich from stoichiometric point (i.e., an A/F of about 13.2) , and an exhaust temperature 850°C), the limiting exhaust gas flux measured in terms of electrical current was measured as a function of the leak rate (which was measured against air at room temperature with a vacuum pump).
• hot rich engine operation conditions e.g., 5.7 liter (1), eight valve (V8) engine at 2,700 revolutions per minute (rpm) and 70 kilopascals (kPa) exhaust pipe back-pressure, 10% rich from stoichiometric point (i.e., an A/F of about 13.2) , and an exhaust temperature 850°C
• the limiting exhaust gas flux measured in terms of electrical current was measured as a function of the leak rate (which was measured against air at room
• the reference gas channel doesn't break.
• the current reference gas channel can withstand oxygen being pumped in at a voltage up to and even exceeding about 1.5 V without cracking due to excessive internal oxygen pressure build-up. This proves the reference gas channel indeed has a one-way-gas-diffusion-limiting feature. It limits the exhaust flux but does not limit the flow of oxygen and build up the oxygen pressure at the reference electrode to the point of cracking.
• the fuel limiting flux was measured as a function of the width of the reference channel.
• the results are plotted in Figure 3, in which the limiting exhaust flux are plotted against the percentage change of the width of the reference channel.
• the limiting exhaust flux is linearly proportional to the linear dimension of the reference gas channel.
• a further advantage of the sensor is, when a co-fired embodiment is employed, the production is simplified due to the ability to use fugitive material in the formation of the reference gas channel.

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