Abstract:
Disclosed herein is a gas sensor having a small amount of lead oxide incorporated into an inner electrode and an outer electrode, and a method for depositing the lead oxide. The lead oxide is applied in an amount sufficient to effectuate consistent performance during sensor break-in. Lead oxide is transferred to the electrodes of the sensor element during the fabrication process by exposing the sensor element to glass having a known lead content during a heating step. Lead oxide from the glass is vaporized and deposited on the electrodes in the form of lead oxide. The deposited lead oxide is incorporated into the electrodes of the sensor element. The lead oxide reduces performance irregularities thereby improving performance during the initial use of the gas sensor.

Description:
BACKGROUND  
         [0001]    This disclosure relates generally to exhaust gas sensors, and specifically to reduction of inconsistencies in break-in performance in exhaust oxygen sensors.  
           [0002]    Oxygen sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, oxygen sensors have been used for many years in automotive vehicles to sense the presence of oxygen in exhaust gases, such as when an exhaust gas content switches from rich to lean or lean to rich. In automotive applications, the direct relationship between 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.  
           [0003]    A conventional stoichiometric oxygen sensor typically consists of an ionically conductive solid electrolyte material, a porous platinum electrode which is exposed to the exhaust gases, and a porous electrode on the sensor&#39;s interior surface exposed to a known oxygen partial pressure. Sensors typically used in automotive applications use a yttria-stabilized, zirconia-based electrochemical galvanic cell operating in potentiometric mode to detect the relative amounts of oxygen present in an automobile engine&#39;s exhaust. 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 electrolyte, according to the Nernst equation:  
       E   =       (         RT             4      F           )          ln        (           P     O   2     ref               P     O   2             )                               
 
           [0004]    where:  
           [0005]    E=electromotive force  
           [0006]    R=universal gas constant  
           [0007]    F=Faraday constant  
           [0008]    T=absolute temperature of the gas  
           [0009]    p O     2     ref =oxygen partial pressure of the reference gas  
           [0010]    P O     2   =oxygen partial pressure of the exhaust gas  
           [0011]    Due to the large difference in oxygen partial pressures between fuel rich and fuel lean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel rich or fuel lean, without quantifying the actual air to fuel ratio of the exhaust mixture.  
           [0012]    When first put into use, exhaust oxygen sensors exhibit a “green” effect, which produces inconsistent performance during the initial use of the sensor. Engine calibration must typically account for the green effect, which makes calibration more difficult. After several hours of use, the green effect disappears, and more reliable sensor performance is seen.  
           [0013]    To reduce the green effect, conventional oxygen sensors incorporate various elements into the ink used to form the electrodes. Sodium, magnesium, and potassium, in particular, have been incorporated into ink prior to electrode formation in an attempt to ameliorate the green effect. This approach, however, can incorporate excessive amounts of the elements in the finished sensor element, which causes a degradation in the performance of the sensor.  
           [0014]    What is needed in the art is a gas sensor with a reduced green effect.  
         SUMMARY  
         [0015]    The above-described and other disadvantages of the prior art are overcome by the sensor element described herein. The exhaust gas sensor element comprises an electrolyte body having a first surface and a second surface. Disposed in intimate contact with the first surface is a first electrode, while a second electrode is disposed in intimate contact with the second surface. The second electrode comprises lead oxide in an amount of about 0.1 to about 8 mg/cm 2 .  
           [0016]    The method for making the gas sensor element comprises forming an electrolyte body and forming an electrode ink comprising a first catalyst. The electrode ink is applied to a first surface and a second surface of the electrolyte body. The body is sintered to form a catalyst layer. Lead oxide is applied to the catalyst layer in an amount of about 0.1 to about 8 mg/cm 2 . A second catalyst is also applied to said catalyst layer, and the layer is sintering to form a first electrode and a second electrode.  
           [0017]    The method for depositing lead oxide on a gas sensor element, comprises applying a lead oxide containing glass to a substrate. The gas sensor element is placed in a closed container with the substrate and the element is heated causing lead oxide to be liberated from the substrate in vapor form and adsorbed by the gas sensor element. The resulting sensor has a first electrode and a second electrode comprising lead oxide in an amount of about 0.1 to about 8 mg/cm 2 .  
           [0018]    Finally, the gas sensor comprises a middle shell, with a lower shell and an upper shell disposed in contact with the middle shell. The sensor element is disposed in contact with the middle shell, protruding into the lower shell and the upper shell. At least one electrical connector disposed in contact with a first electrode and a second electrode of the sensor element, such that electrical access is provided to the sensor element from an external circuit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The apparatus and method will now be described by way of example, with reference to the accompanying drawings, which are meant to be exemplary, not limiting.  
         [0020]    [0020]FIG. 1 is a partial cross-section of one embodiment of a gas sensor.  
         [0021]    [0021]FIG. 2 is a cross-section of the sensor element of FIG. 1.  
         [0022]    [0022]FIG. 3 is an enlarged cross section of the outer surface of the sensor element of FIG. 2.  
         [0023]    [0023]FIG. 4 is a plot showing the relative performance of gas sensors treated with lead oxide and gas sensors without lead oxide.  
         [0024]    [0024]FIG. 5 shows several dynamic plots of various sensors with and without lead oxide.  
         [0025]    [0025]FIG. 6 shows several dynamic plots of various sensors with and without lead oxide. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    A gas sensor and method of making the same is described herein, wherein lead oxide is deposited on the sensor element prior to final assembly into the gas sensor. The sensor element comprises an electrolyte body with an inner electrode disposed on the inner surface, and an outer electrode disposed on the outer surface, and a protective layer disposed over the outer electrode. Lead oxide is incorporated onto the electrolyte body underneath the electrodes through a vapor deposition process in order to improve initial performance of the gas sensor. The lead oxide incorporation process and resulting sensor element can be used in any gas sensor, with use in fast light-off, heated and unheated, gas sensors preferred. It is hereby understood that although the apparatus and method are described in relation to making an oxygen sensor, the sensor could be a nitrous oxide sensor, hydrogen sensor, hydrocarbon sensor, or the like.  
         [0027]    [0027]FIG. 1 shows a cross section of one embodiment of the automotive exhaust gas sensor generally at  100 . The gas sensor includes an upper shell  102  having a lower end  104  laser welded and/or crimped to a thicker, middle shell  106 . A louvered, tubular, lower shell  108  is provided, having an upper end  110  welded and/or crimped to the middle shell  106 . The middle shell  106  includes a lower annular shoulder  115 . A sensor element  117  is disposed through in the middle shell. The sensor element  117  can include a conical-shaped electrolyte body  116  having inner and outer electrodes  118 ,  120  formed thereon. The electrolyte body  116  has a lower, sloped, annular shoulder  122  which is sufficient to engage the sloped shoulder  115 , with a lower gasket  114  preferably positioned between shoulders  122  and  115 .  
         [0028]    The middle shell  106  can include an annular ring  134 , crimped over an upper slanted shoulder  133  of an insulator  135 . The insulator  135  is disposed between the ring  134  and an optional area of compacted talc powder  136 , which contacts an upper, sloped, annular shoulder  150  of the sensor element  117 . The crimped annular ring  134  applies a force through the insulator  135  and talc powder  136  to hold the sensor element  117  in place, applying pressure to the annular lower gasket  114 . A heating element  132  extends into a cavity  137  of the conical-shaped electrolyte body  116 .  
         [0029]    The sensor element  117  and the heating element  132  are electrically connected to external circuits through clips. An internal electrode clip  152  is preferably formed so as to fit tightly in the cavity  137  of the sensor element  117 . The internal electrode clip  152  which applies an outward spring force, to ensure positive electrical contact with the inner electrode  118 , comprises prongs  154  which are angled inward toward the heating element  132  in order to secure the heating element  132  centrally in place in the cavity  137 .  
         [0030]    An external electrode clip  156  can be formed so as to fit tightly around the exterior surface of the sensor element  117 . By applying an inward spring force on the sensor element  117 , the external electrode chip  156  ensures a positive electrical contact with the outer electrode  120 . The internal electrode clip  152  and the external electrode clip  156  are disposed in a insulating clip securing block  158 , which holds the internal electrode clip  152  and the external electrode clip  156  in secure relative position.  
         [0031]    A connector assembly  160  holds heater connection clips  162  securely in place. The connector assembly  160  also comprises connectors  164 , securely engaged with the internal and external electrode clips  152 ,  156 , to provide an electrical path from an outside circuit to the heater connection clips  162  and the internal and external electrode clips  152 ,  156 .  
         [0032]    Four separate wires  138 ,  140 ,  141  (one not shown) are provided through a polymeric seal  144  in the upper shell  102  to make connections to the heating element  136  and the inner electrode  118  and outer electrode  120  of the sensor element  117 . The polymer seal  144  is sufficient to provide a water tight oxygen reference chamber  166  within the upper shell  102 . An elastomeric wire boot  148  is disposed between the upper shell  102  and an outer shell  146 , which is crimped and/or welded onto the upper shell  102  to secure the boot  148  and form a seal.  
         [0033]    The upper shell  102  is securely fastened and sealed to the middle shell  106  and the polymeric seal  144  to form an inner air reference chamber  166 . The reference chamber  166  extends into the cavity  137  of the sensor element  117 , between the heating element  136  and the inner electrode  118 . The heating element  136  is held centrally in the cavity  137  by the inner electrode clip  152  and a complementary fit between the heating element tip  170  and the cavity terminus  172 .  
         [0034]    [0034]FIG. 2 is a cross-section of the sensor element  117 . The electrolyte body  116  has an inner surface  202  and an outer surface  200 . The sensor element  117  can be formed in any generally cylindrical shape, with a generally tapered shape from the cavity opening  174  to the cavity terminus  172  preferred. A protrusion  176  defines the upper shoulder  150  and the lower shoulder  122 . The cavity  137  itself can be defined in any generally cylindrical shape. Preferably, a cylindrical top portion  178  is joined to a smaller cylindrical bottom portion  180  with a tapered portion  182 . The larger cylindrical top portion  178  allows for the proper locating of the interior electrode clip  152 , while the smaller cylindrical bottom portion  180  allows for a minimal gap between the heating element  132  and the inner electrode  118 .  
         [0035]    [0035]FIG. 3 is a magnified cross-section of the outer surface of the sensor element  117  below the protrusion  176 . The outer electrode is disposed between the electrolyte body  116  and a protective layer  188  which can comprise a porous material  184  and a high surface area material  186 .  
         [0036]    The method of manufacture of the gas sensor and the preferred materials for use in the gas sensor will now be discussed.  
         [0037]    Fabrication of the sensor element  117  begins with mixing and preparation of the electrolyte body  116 . The electrolyte body  116  can be any material that is capable of permitting the electrochemical transfer of oxygen ions while inhibiting the physical passage of exhaust gases, that preferably has an ionic/total conductivity ratio of approximately unity, and that is compatible with the environment in which the sensor will be utilized. metal oxides such as zirconia, and the like, which may optionally be stabilized with calcium, barium, yttrium, magnesium, aluminum, lanthanum, cesium, gadolinium, and the like, and oxides thereof, as well as combinations comprising at least one of the foregoing electrolyte materials. For example, the electrolyte can be alumina and yttrium stabilized zirconia. Typically, 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. Other additives, such as wax, organic powders, and the like can be added to improve the performance characteristics of the sensor element  117 .  
         [0038]    The electrolyte body  116  can be formed by any conventional technique. For example, the desired electrolyte materials in the form of near submicron powders and granulated additives can be combined to form a mixture which is compacted in a mold at pressures sufficient to achieve the desired density. The applied pressure is typically greater than about 8 ksi (kilopounds per square inch), with greater than about 10 ksi preferred. The mold, which can be a conventional mold, such as a urethane mold, produces an oversized electrolyte blank in order to allow for shrinkage in later steps. The electrolyte blank is ground to the desired shape using conventional grinding techniques, such as employing an appropriately contoured grinding wheel. The ground electrolyte body is then optionally sintered at high temperatures to impart strength. Sintering is carried out for a time and at a temperature sufficient to appropriately strengthen the part, e.g. at about 1,000° C. to about 1,200° C. for up to about 2 hours or so, with about 1,050° C. to about 1,150° C. for about 1 to about 2 hours preferred.  
         [0039]    The inner and outer electrodes  118 ,  120 , which are disposed in contact with the inner surface  202  and outer surface  200  of the electrolyte body  116 , can comprise any catalyst capable of ionizing oxygen, including, but not limited to, metals such as platinum, palladium, gold, osmium, rhodium, iridium, ruthenium, zirconium, yttrium, cerium, calcium, aluminum, and the like, as well as alloys, oxides, and combinations comprising at least one of the foregoing metals. The catalyst is combined with a vehicle, such as an organic vehicle, to form an electrode ink. The application of the outer electrode  120  is accomplished by the application of the electrode ink to the outer surface  200  of the electrolyte body  116  using conventional techniques, such as spraying, painting, dipping, physisorbing, imbibing, pad printing, and the like, and allowing the vehicle to adsorb into the electrolyte body  116 . Formation of the inner electrode  118  comprises filling the cavity  137  with the electrode ink, removing the excess electrode ink, and allowing the vehicle of the remaining electrode ink to absorb into the electrolyte body  116 . After vehicle adsorption, precursor catalytic electrode layers are present on both the outer surface  200  and the inner surface  202  of the electrolyte body  116 .  
         [0040]    The electrolyte body  116 , with electrode layers  118 ,  120 , is then subjected to a high temperature sintering step to preferably fully densify the part. The second sintering is performed at about 1,300° C. to about 1,700° C., with a temperature of about 1,400° C. to about 1,600° C. preferred. Sintering is performed for a sufficient period of time to preferably fully densify the part, e.g., typically greater than about 1 hour.  
         [0041]    At this point, the outer and inner electrodes  118 ,  120  are partially formed. Next, lead oxide is added to the electrode. To precisely control the lead oxide deposition for the gas sensor described above, a vapor transfer technique is used. Lead oxide (PbO) can initially be impregnated into a substrate in an amount of at least about 40 weight percent (wt %) PbO, with about 40 wt % to about 80 wt % PbO preferred, and about 60 wt % to about 80 wt % PbO especially preferred.  
         [0042]    Any conventional ceramic or other material that can both withstand the required temperatures and serve as a substrate for the glass can be used as the substrate. For example, a conventional ceramic sagger, a crucible containing a quantity of glass, or a parts tray. A coating of ink, which comprises a lead borosilicate glass and a vehicle, such as an organic vehicle (e.g., terpineol, or the like), is applied to the substrate. In one embodiment, the ink has a composition of 60 wt % lead oxide (PbO), 20 wt % silicon dioxide (SiO 2 ), 10 wt % boron oxide (B 2 O 3 ), and 10 wt % other oxides. The ink can be applied to the substrate in any conventional fashion such as spraying, dipping, and the like, with brushing the ink onto the substrate preferred.  
         [0043]    The electrolyte body  116  with the partially formed electrodes can now be loaded onto the lead oxide coated substrate and heated in a closed environment to a temperature sufficient to vaporize the desired amount of lead oxide from the substrate. Temperatures of about 800° C. to about 1,200° C. can typically be used, with about 900° C. to about 1,100° C. preferred, and about 975° C. to about 1,050° C. especially preferred, for about 0.5 hours to about 5 hours, with about 1 hour to about 3 hours preferred. The vaporized lead oxide is adsorbed by the outer surface of the electrolyte body  116  with the partially formed electrodes, thereby incorporating lead oxide into the partially formed electrodes at a controlled rate to form a precursor. The final concentration of adsorbed lead oxide on the exposed surfaces of the electrodes is about 0.10 to about 8 mg/cm 2 , with about 1 to about 6 mg/cm 2  preferred, about 2 to about 4 mg/cm 2  more preferred, and about 2.2 to about 2.5 mg/cm 2  especially preferred. The lead coated precursor is then cooled to room temperature (i.e., by forced-air cooling).  
         [0044]    Next, the precursor can be coated on the outside surface with more catalyst. The catalyst can be applied with conventional techniques, with a sputtering process preferred. The catalyst is preferably applied to a thickness of about 1 to about 50 angstroms, with a thickness of about 3 to about 10 angstroms preferred. The precursor is then sintered for a third time at a temperature sufficient to securely adhere the catalytic coating. The third sintering is typically performed at about 500° C. to about 1,000° C., with a temperature of about 700° C. to about 900° C. preferred.  
         [0045]    The electrodes  118 ,  120  are now complete, and a protective porous material  184  can be applied to the exterior surface of the precursor. Any porous material that allows passage of exhaust gases while preventing passage of unwanted contaminants can be used, such as magnesium aluminate, aluminum oxide, and combinations comprising at least one of the foregoing, among others. The porous material  184  is typically applied to a thickness of about 50 to about 200 microns, with a thickness of about 90 to about 140 microns preferred.  
         [0046]    To further protect the sensor element  117 , a high surface area material which can trap poisons in the exhaust  186  can optionally be applied to the exterior surface of the sensor element, preferably to the portion of the sensor element  117  between the protrusion  176  and the terminus  172  on the exterior surface. The high surface area material  186  can be any material that has a porosity sufficient to allow the passage of exhaust gases, such as aluminum oxide, and other metal oxides. The high surface area material  186  preferably has a surface area of greater than about 150 meters squared per gram (m 2 /g), with a surface area of greater than about 200 (m 2 /g) preferred. The high surface area material  186  is then dried, and the sensor element  117  is sintered at a temperature sufficient to harden the high surface area material  186 , e.g., a temperature of about 400° C. to about 600° C., with a temperature of about 450° C. to about 550° C. preferred.  
         [0047]    Manufacture of the sensor element  117  can optionally be completed with a heat treatment in a pure nitrogen atmosphere for at a temperature sufficient to remove any unwanted oxide film from the catalyst material. For example for a period of about 0.5 to about 1 hour at a temperature of about 600° C. to about 1,000° C., with about 700° C. to about 900° C. preferred.  
         [0048]    The completed sensor element is then incorporated into the gas sensor through conventional means.  
         [0049]    [0049]FIG. 4 is a plot showing the performance of several exemplary sensors incorporating lead oxide in the defined amounts and sensors not incorporating lead oxide. In FIG. 4, lines  203 ,  204 ,  206 ,  208 , and  210  represent the performance of sensors lacking lead oxide in the electrodes  118 ,  120 . Line  212  is an aged sensor reference, and line  214  is a de-greened sensor. Lines  216 ,  218 ,  220 ,  222 , and  224  represent the performance of sensors incorporating lead oxide at a temperature of 600° C. Lines  226 ,  228 ,  230 , and  232  represent the performance of sensors incorporating lead oxide at a temperature of 700° C. Lines  234 ,  236 ,  238 ,  240 , and  242  represent the performance of sensors incorporating lead oxide at an operating temperature of 800° C.  
         [0050]    [0050]FIGS. 5 and 6 represent the voltage outputs against time for oxygen sensors. The elements were tested at temperatures of 600° C., 700° C., and 800° C. The sensing elements were tested for 24 hours prior to collection of sampling data. A sample of a de-greened oxygen sensor is also illustrated with about 8.72 to about 15 mg/cm 2  of lead thereon. Following testing, the amount of lead detectable for each treatment temperature was; at 600° C. lead was not detected, at 700° C. lead was detected at 0.59 mg/cm 2 , and at 800° C. lead was detected at 2.25 mg/cm 2 . The curves indicate the variability of the pitch of the curve when the sensor switches from rich to lean conditions. The vertical pitch of the curve indicates a quick response time while the more horizontal pitch demonstrates a long response time. The Figures illustrate that at temperatures of 800° C. the elements closely mimic the results for a de-greened part.  
         [0051]    The gas sensor described above incorporates lead oxide into the electrodes  118 ,  120 , thereby improving sensor performance during initial use of the sensor by reducing the time required to desorb carbon monoxide. While other oxygen sensors also have a light-off temperature of 370° C., they do not perform as well as this sensor. The sensor performs well at low temperatures (i.e., at startup) and helps to achieve greater control over the sensor performance. When there is a sufficient amount of lead, there is a drastic reduction in part to part variability when switching from fuel rich to fuel lean. Another advantage is that this sensor makes it easier to calibrate engines, as well as making parts more repeatable.  
         [0052]    While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention, including the use of the geometries taught herein in other conventional sensors. Accordingly, it is to be understood that the apparatus and method have been described by way of illustration only, and such illustrations and embodiments as have been disclosed herein are not to be construed as limiting to the claims.