Abstract:
A mixed potential NO x  sensor apparatus for measuring the total NO x  concentration in a gas stream is disclosed. The NO x  sensing apparatus comprises a multilayer ceramic structure with electrodes for sensing both oxygen and NO x  gas concentrations and includes screen-printed metalized patterns that function to heat the ceramic sensing element to the proper temperature for optimum performance. This design may provide advantages over the existing technology by miniaturizing the sensing element to provide potentially faster sensor light off times and thereby reduce undesired exhaust gas emissions. By incorporating the heating source within the ceramic sensing structure, the time to reach the temperature of operation is shortened, and thermal gradients and stresses are minimized. These improvements may provide increased sensor performance, reliability, and lifetime.

Description:
GOVERNMENT RIGHTS 
   This invention was made in part with government support under Grant Numbers 68-D-02-076 and 68-D-03-061 awarded by the United States Environmental Protection Agency. The Government has certain rights in the invention. 

   RELATED APPLICATIONS 
   This application is related to and claims the benefit of U.S. patent application Ser. No. 11/137,693, of Balakrishnan Nair, Jesse Nachlas, and Michael Middlemas filed on May 25, 2005, and entitled “NO x  Sensor Method and Device” and U.S. Provisional Patent No. 60/574,622 of Balakrishnan Nair, Jesse Nachlas, and Michael Middlemas filed on May 26, 2004, and entitled “NO x  Gas Sensor Method and Device.” Each of these applications is incorporated herein by reference in their entirety. 
   FIELD OF THE INVENTION 
   The present invention relates in general to the measurement of NO x  gases in exhaust streams generated from the combustion of hydrocarbons, and more particularly, to the measurement of NO x  gases in exhaust gas streams produced by the combustion of gasoline and/or diesel fuels. 
   BACKGROUND OF THE INVENTION 
   The composition of exhaust gases produced by the combustion of hydrocarbon fuels is a complex mixture of oxide gases (NO x , SO x , CO 2 , CO 2 , H 2 O), unburnt hydrocarbon gases, and oxygen. Measurement of the concentration of these individual constituents of exhaust gases in real time can result in improved combustion efficiency and lower emissions of polluting gases. In some cases, the concentration of one gas may influence or control the concentration of a second gas. In these situations, it may be required to know the concentration of the first gas in order to measure the concentration of a second, or even third, gas accurately. Various devices have been proposed to operate as exhaust gas sensors that have the capability of measuring the gas concentration of two or more gases in an exhaust stream. 
   One NO x  sensor known in the art is configured as a flat plate multilayer ceramic package designed to include two or more chambers. The first chamber has electrodes attached to an oxygen ion-conducting electrolyte membrane to form an oxygen pump for removing oxygen from a flow of gas entering the sensor. The first chamber also catalyzes the decomposition of NO 2  to NO and one-half O 2 . The oxygen pump in the first chamber also removes the oxygen formed by this process. Thus, in theory, the only oxygen-containing gas that enters the second chamber is NO. The second chamber includes a NO decomposing element that removes the oxygen from the NO using a second oxygen pump. The electrical current produced by the transport of oxygen from the decomposition of NO in the second chamber is correlated to the concentration of NO. 
   A number of concerns affect the commercial application of this known NO x  sensor. For example, when the NO x  concentration to be detected is low, residual oxygen can cause significant interference. In addition to the above, the signal current produced by the sensor is very small, thus making it susceptible to interference from the electronic noise commonly found in an automobile. Also, the flow of exhaust gas monitored by such sensors typically has pulsations in its flow rate caused at least in part by engine cylinder firings. This impairs the ability of the oxygen pump to effectively remove all of the free oxygen and may result in measurement error. This device may also contain a small diffusion aperture used to limit the passage of gas into the measurement chambers. This structure has been demonstrated to be prone to clogging during use. 
   Another known NO x  sensor utilizes a similar flat plate multilayer ceramic package design. There are a few significant differences in the operation principle for this sensor; namely, the sensor is a mixed potential type rather than amperometric, and the first chamber is used to convert NO to NO 2  and vice versa. It is well established that in mixed potential NO x  sensors, the voltage signals generated from the gas species NO and NO 2  are of opposite sign. As a result, it is difficult to distinguish a meaningful voltage signal when both gases are present since cancellation may occur. 
   Some sensor designs have attempted to address this problem by utilizing a flat plate multilayer package design with two separate chambers built into the sensor. Attempts have also been made to convert all of the NO x  gas species into a single species with the use of an electrochemical oxygen pump that pumps oxygen into the first chamber to attempt to convert all of the gas to NO 2 . Other efforts conversely attempt to remove oxygen from the chamber and reduce all of the NO 2  to NO. This “conditioned” gas then passes into the second chamber where the NO x  concentration is measured by the voltage signal generated from a mixed potential type sensor. 
   There are a number of limitations to this approach that have hampered the commercialization of this configuration. One significant concern is the reproducibility of the conversion system to completely convert all the NO x  gases into a single species under varying gas concentration conditions. In addition, the oxygen pump conversion cell tends to degrade with time, further contributing to the issue of reproducibility. Because the effects of these concerns are magnified in the low concentration range, this measurement approach is not well suited for detecting low concentrations of NO x  gases. 
   Additional drawbacks common to both of the sensor mechanisms discussed above stem from the fundamental design of the flat plate ceramic multilayer system. Response times tend to be slow because of the complexity of the device requiring gas to first enter through a diffusion port, be conditioned in a first chamber, and then to diffuse into a second chamber. Achieving rapid gas exchange that can keep up with the dynamic environment of the engine exhaust is difficult in these configurations. Also, the corrosive nature of the gas itself and the fact that it bears fine particulates may result in the clogging of the diffusion controlling port, or at the very least, changes in the gas flow dynamics with time. Finally, pulsations in gas flow rates due to cylinder firings and the electrical noise typical of automobiles make it difficult to control and monitor the low voltage and current circuits associated with these devices. 
   Thus, it would be an improvement in the art to provide alternative configurations for NO x  sensing elements usable in a NO x  sensor system designed to address these and other considerations. Such a device is provided herein. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is directed to a method and design for constructing the NO x  sensing element of a NO x  sensor system previously described in patent application Ser. No. 11/137,693, filed May 25, 2005, and incorporated by reference herein. The NO x  sensing element comprises a multilayer ceramic structure with electrodes for sensing both oxygen and NO x  gas concentrations and has included within the structure screen-printed metalized patterns that heat the ceramic sensing element to the proper temperature for optimum performance. This design provides advantages over the existing technology by miniaturizing the sensing element, which results in faster sensor light off times, thereby reducing undesired exhaust gas emissions. By incorporating the heating source within the ceramic sensing structure, the time to reach the temperature of operation is shortened and the thermal gradients and stresses are minimized, thus resulting in improved sensor performance, reliability and lifetime. 
   Other advantages and aspects of the present invention will become apparent upon reading the following description of the drawings and detailed description of the invention. These and other features and advantages of the present invention will become more fully apparent from the following figures, description, and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In order that the manner in which the above-recited and other features and advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
       FIG. 1A  is a schematic view of an embodiment of a planar multilayer ceramic sensing assembly of the present invention; 
       FIG. 1B  illustrates each of the individual layers of the planar sensing assembly of the present invention, with the outermost layer being designated A, the next inward being designated B, the next C, the following D, the next E, and the lowest layer being designated F; 
       FIG. 2  illustrates the individual segments of green ceramic tape used to create the layers of the planar sensing assembly of the present invention, the appropriate segments showing electrode and heater patterns used in the device; 
       FIG. 3  shows a pair of assembled multilayer NO x  sensors of the invention comprised of the layers illustrated in  FIG. 2  having been stacked, laminated, and cut to their final shape in preparation for sintering; 
       FIG. 4A  is an isolated top view of a sintered multilayer NO x  sensor according to the invention; 
       FIG. 4B  is an isolated bottom view of a sintered multilayer NO x  sensor according to the present invention; 
       FIG. 5  is a plan view of another embodiment of the multilayer NO x  sensors of the present invention having a tubular form that incorporates two heaters, an oxygen sensor, and a NO x  sensor along with a shared air reference electrode; 
       FIG. 6  illustrates the patterns used for screen printing heaters on unsintered zirconia tape for use in constructing the tubular sensor body; 
       FIG. 7  is a perspective view of a sintered zirconia tubular NO x  sensor constructed from the tape illustrated in  FIG. 6 ; 
       FIG. 8  illustrates a test setup for characterizing the performance of the heater of the tubular NO x  sensor of  FIG. 7 ; and 
       FIG. 9  illustrates the individual layers of another embodiment of the multilayer planar sensing assembly of the present invention, with an optional first layer being designated A, the next inward being designated B, the next C, the next D, and the final E, the final layer being shown twice, E showing its inward face and E′ showing its outer face. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The presently preferred embodiments of the present invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the multilayer ceramic NO x  gas sensor device of the present invention, as represented in  FIGS. 1A through 9 , is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention. 
   One embodiment of the present invention is a method for fabricating a multilayer ceramic structure to be used as a NO x  sensing element. A complete NO x  sensing apparatus was described in U.S. patent application Ser. No. 11/137,693, filed May 25, 2005, which is incorporated by reference herein in its entirety. The apparatus disclosed in that Application includes a sensor element. One of the features of the referenced NO x  sensor apparatus is its ability to create two distinct temperature zones. One of these temperature zones is associated with the gas conditioning catalyst and oxygen sensor. A second of these temperature zones is associated with the mixed potential NO x  sensing element. The present invention provides a novel sensor element for use in such sensing apparatus. 
   The sensor elements of the present invention may improve overall system performance by miniaturizing the ceramic sensing element and including multiple features within the miniaturized ceramic element. The ceramic sensor elements of the present invention may include a single sensing electrochemical cell, such as a NO x  gas sensor, or may include at least two sensing electrochemical cells, such as oxygen and NO x  gas sensors. The sensor elements of the invention additionally include at least one, and often two metalized patterns that function as “heater elements” to heat the entire ceramic structure when a voltage and current are applied to contact points of the metalized patterns. 
   By incorporating these heater elements into the ceramic structure of the sensor element, the heat transfer rate to the sensing electrodes is increased. This provides more rapid light off times for the sensor components of the sensor element. In addition to the above, thermal stresses due to rapid changes in temperature are minimized by optimization of the heater design pattern and the construction of the multilayer ceramic package. These features may result in improved lifetime performance and reliability of the sensor apparatus. 
   Several examples are provided below which discuss the construction, use, and testing of specific embodiments of the present invention. These embodiments are exemplary in nature and should not be construed to limit the scope of the invention in any way. 
   EXAMPLE 1 
   Referring first to  FIG. 1A , the basic features of the multilayer gas sensor element  10  are illustrated. More specifically, the gas sensor element  10  is shown in a schematic view such that features of the individual layers  30 ,  40 ,  50 ,  60 ,  70 ,  80  used to make up the sensor body  12  are shown to overlap as they would in the completed sensor element  10 . This view illustrates the relationship between features of the sensor element  10 . 
   In the sensor element  10 , the oxygen sensor  32  is positioned spacially near the heater element  52 , but on an outer face of the element  10 . A reference electrode  34  is positioned on an inner face of the oxygen sensor layer  30  in a substantially similar position. As a result, when viewed as in  FIG. 1A , the oxygen sensor  32  and reference electrode  34  overlap. Similarly, the NO x  sensor  82  is positioned spacially near the heater element  54  on an outer surface of the element  10 . A reference electrode  84  is positioned on an inner face of the NO x  sensor layer  80  in a substantially similar position. As a result, when viewed as in  FIG. 1A , the NO x  sensor  82  and the reference electrode  84  overlap. In some embodiments of the sensor elements of the invention, a gas sensor such as a NO x  sensor that is insensitive to oxygen may be used. In such cases, the oxygen electrode may be omitted. Other sensors such as hydrocarbon sensors and/or CO sensors may be substituted in the place of the sensors described herein. 
   The heater  52  is configured to heat the oxygen sensor  32  to a temperature of from about 500° C. to about 900° C. and more preferably from about 650° C. to about 750° C. to create a first temperature zone  51 . In some specific embodiments of the invention, the heater  52  heats the first temperature zone  51  encompassing the sensor  32  to a temperature of about 700° C. The heater  54  is configured to heat the NO x  sensor to a temperature of from about 400° C. to about 600° C., and more preferably from about 450° C. to about 550° C. to create a second temperature zone  53 . In some specific embodiments, the heater  54  heats the second temperature zone  53  encompassing the sensor  82  to a temperature of about 500° C. It should be noted that when installed in a sensing apparatus such as that disclosed in U.S. patent application Ser. No.: 11/137,693, these heating elements  52 ,  54  may additionally provide heat to the catalyst, thus further improving the function of the apparatus as a whole. 
     FIG. 1B  provides a top view of each individual layer  30 ,  40 ,  50 ,  60 ,  70 , and  80  of the sensor element  10  of the invention. Each of the layers  30 ,  40 ,  50 ,  60 ,  70 , and  80  are initially produced from a green ceramic tape made using zirconia powder mixed with binders, solvents and plasticizers into a slurry that was suitable for tape casting. A variety of ion-conductive ceramic materials are known in the art and would be suitable for constructing conductive portions of the sensor body  12  of the sensor element  10  of the present invention, as would be understood by one of ordinary skill in the art. In some embodiments it may be advantageous to add a non-conductive or insulating region to the device. A variety of insulative ceramic materials are also known in the art and could be used for constructing the sensor body  12  of the sensor element  10  of the present invention, as would be understood by one of ordinary skill in the art. Following production of the zirconia slurry, the slurry was tape cast and dried prior to further manufacturing steps used in producing the final sensor element. Segments of the dried tape were cut to approximate shape using techniques common in the art. 
   As illustrated in  FIG. 1B , an oxygen sensor layer  30  is provided for placement of an oxygen sensor electrode (not shown) and a reference electrode  34 . The oxygen sensor electrode  32  is generally composed of platinum, but is not printed onto the oxygen sensor layer  30  until after the multilayer sensor  10  of  FIG. 1A  has been assembled and sintered (discussed in detail below). Although the oxygen sensor  32  may be printed onto the layer  30  prior to sintering in some circumstances, sintering of the sensor  32  may reduce its porosity, and hence, its sensitivity and effectiveness. 
   A first channel layer  40  is next provided, as illustrated in  FIG. 1B . This layer  40  is cut to include a channel  42  extending into the sensor  10  to allow entry of the reference gas, which is typically air. The length and geometry of the channel  42  may be varied widely within the scope of the invention. The second channel layer  70  is also illustrated in  FIG. 1B , the layer  70  including a channel  72  extending into the sensor  10 . Channels  42 ,  72  allow air to enter the sensor  10  to reach reference electrodes  34  and  84  placed on interior surfaces of oxygen sensor layer  30  and NO x  sensor layer  80 , respectively. As with the channel  42  provided in the first channel layer  40 , the channel  72  of the second channel layer  70  may be varied in size and geometry within the scope of the invention. 
     FIG. 1B  further illustrates the heater layer  50  adapted to include heating elements  52 ,  54  that produce first and second temperature zones  51 ,  53 . These heaters  52 ,  54  may be constructed to be independently-controlled, having distinct power sources; or to be controlled by the same power source and rendered capable of producing first and second temperature zones  51 ,  53  by varying the resistance of the individual heater  52 ,  54 . Resistance may be varied in many ways, as understood by one of ordinary skill in the art, including increasing the length of the heater  52 ,  54 . The heaters  52 ,  54  are positioned to be near the oxygen and NO x  sensors  32 ,  72 , on opposing sides of sensor body  12  making up the sensor  10  when it has been assembled. The electrodes provided for the heaters  52 ,  54  are screen printed and dried in an oven at 80° C. for 2 hours prior to assembly of the sensor  10 . The individual layers  30 ,  40 ,  50 ,  60 ,  70 , and  80  are shown overlaid with the patterns used to facilitate the screen-printing process (in the case of layers  30 ,  50 , and  80 ) used to deposit the electrodes on each of the layers in  FIG. 2 , and to facilitate cutting of channels  42 ,  72  in layers  40  and  70 . 
   After screen-printing the electrodes, the green ceramic layers  30 ,  40 ,  50 ,  60 ,  70 , and  80  may be laminated together using a technique such as solvent bonding, heat lamination, or another technique known to one of ordinary skill in the art. In methods using heat lamination, the individual layers are pressed together using a lamination press. After lamination of the layers  30 ,  40 ,  50 ,  60 ,  70 , and  80 , the sensor elements  10  are cut to final shape using techniques known to those of ordinary skill in the art, and are then ready to be sintered. Two laminated and cut multilayer ceramic sensor packages  10  prepared for sintering are shown in  FIG. 3 . 
   The green laminated ceramic tape sensor package  10  was then sintered for two (2) hours at 1475° C. to produce the sensor element shown in  FIGS. 4A and 4B . Following sintering, the ceramic sensor element structure  10  was coated with a platinum electrode for the oxygen sensor  32  on the side corresponding to the oxygen sensor layer  30  as schematically illustrated in  FIGS. 1A and 1B . The opposing side of the ceramic structure  10  corresponding with original NO x  sensor layer  80  was also coated with a composite electrode of WO 3 /ZrO 2  to make up the NO x  sensor  82 . The NO x  sensor electrode  82  is preferably placed on the sensor element  10  after sintering to prevent high-temperature chemical reaction with the zirconia in the green tape. After placement of the electrodes, the sensor element  10  was fired at a high temperature in the range of from about 800° C. to about 1000° C., and in some instances from about 850° C. to 950° C. to promote good adhesion of the oxygen sensor  32  and the NO x  sensor  82  to the exterior of the sensor body  12 . 
   In some embodiments of the sensor  10  of the present invention, the sensors  32 ,  82  may be mixed potential sensors constructed using a semi-conductive oxide material. In some specific embodiments, the semi-conductive oxide material may include at least one of the following: WO 3 , Cr 2 O 3 , Mn 2 O 3 , Fe 2 O 3 , TiO 2 , and CO 3 O 4 . In others, a multi-component oxide material may be used. The multi-component oxide material may be, for example, a spinel or perovskite. In some specific embodiments, the multi-component oxide material may be at least one of the following: NiCr 2 O 4 , ZnFe 2 O 4 , CrMn 2 O 4 , LaSrMnO 3 , LaSrCrO 3 , and LaSrFeO 3 . 
   One of ordinary skill in the art would understand that the number and configuration of the layers  30 ,  40 ,  50 ,  60 ,  70 , and  80  used to construct the gas sensor element  10  could be widely varied within the scope of the invention. Specifically, sensors  32 ,  82  or heaters  52 ,  54  could be placed in a variety of locations, including on opposing surfaces of single layers, to reduce the number of layers used to create the sensor body  12 . Further, channels  42 ,  72  could be embossed or partially etched from a layer instead of being cut completely through. Other variations, including variations of electrode material, shape, and in some instances, placement could be made within the scope of the invention by one of ordinary skill in the art. 
   EXAMPLE 2 
   While there are many advantages to the planar multilayer sensor element  10  characterized in Example 1 above, it may also be advantageous to utilize similar processing techniques to produce a multilayer sensor element  110  in the form of a tubular sensor body  112 , as illustrated in  FIG. 5 .  FIG. 5  shows a conceptual schematic of a multilayer tubular sensor element  110  which, like the sensor element  10  of  FIGS. 1A-4B , incorporates two different heating zones  151 ,  153 , along with both an oxygen sensing electrode  132  and a NO x  sensing electrode  182 . Both sensors  132 ,  182  share a common air reference electrode  134 . It should be noted that the first and second heating zones  151 ,  153  illustrated in  FIG. 5  are not in practice discrete zones, but are temperature regions with no concrete border separated instead by a continuum of intermediate temperatures. 
   To fabricate the tubular sensor element  110  illustrated in  FIG. 5 , the first step was to produce a ceramic tubular multilayer structure that contained two separate heaters  152 ,  154  to produce two different temperature zones  151 ,  153  associated with the electrodes  132 ,  182 . To produce the ceramic structure, zirconia powder was mixed with binders, solvents and plasticizers into a slurry that was suitable for tape casting. The slurry was tape cast and dried to produce a green ceramic tape  114  with a thickness of approximately 0.015″.  FIG. 6  shows the green tape  114  having been cut to length and screen-printed with a platinum ink to form heater elements  152 ,  154 . These heater elements  152 ,  154  are provided with distinct patterns  156 A,  156 B to produce two different temperature zones. The patterns  156 A,  156 B shown in  FIG. 6  are exemplary only, and may be widely varied within the scope of the invention. Specifically, the size and length of the heater elements  152 ,  154  may be widely varied to provide differentially heated zones. In one example, the heater element  152  adapted to produce temperature zone  151  for the oxygen sensor  132  is longer and more tortuous to provide increased heat. 
   As briefly mentioned above,  FIG. 6  provides a picture of the green zirconia tape  114  that has been screen printed with platinum ink to produce the heaters  152 ,  154 . After the platinum ink has properly dried, the green tape  114  is wrapped onto a tubular mandrel using terpineol to bond the wrapped layers of the tubular sensor body  112  together as they are wrapped around the mandrel. Once the tape  114  has been completely wrapped around the mandrel and dried it is then fired to 1475° C. for a 2-hour hold.  FIG. 7  illustrates the sensor element  110  in the form of a sintered zirconia tube sensor body  112  showing the platinum heating pattern  156 A on the inside surface of the sensor body  112 . The sintered ceramic sensor element  110  was then ready for testing the performance of the heater elements  152 ,  154 . 
   The performance of the heater elements  152 ,  154  of the sensor element  110  was tested by first attaching lead wires to the contact points of the heaters  152 ,  154 , and then attaching a DC power supply to each of the two heaters  152 ,  154 . The heater elements  152 ,  154  performed as desired, producing 500° C. and 700° C. temperature zones. The heater elements  152 ,  154  were tested for over 500 hours.  FIG. 8  illustrates the heaters  152 ,  154  being tested for heating rate and temperature profile. The heater patterns  156 A,  156 B used on heaters  152 ,  154 , respectively, as shown in this example successfully produced the two different temperature zones  151 ,  153  required for the catalyst/oxygen sensor  132  and the NO x    182  sensor of the sensor element  110 . 
   Another embodiment of the multilayer sensors of the present invention is illustrated schematically in  FIG. 9 .  FIG. 9  illustrates the individual layers of another embodiment of the multilayer planar sensing assembly  210  of the present invention arrayed as in  FIG. 1B . This embodiment may be assembled similarly to that described with reference to  FIGS. 1A-4  discussed in greater detail above. The sensor  210  may first include an optional first layer  230 . This layer  230  may include via holes  232  to allow access to the heaters  252 ,  254  of the heater layer  240 . The heater layer  240  may be spaced from the channel layer  260  by an intermediate layer  250 . The channel layer  260  may include a channel  262  to allow entry of air being channeled to the air reference electrode  272  found on an interior surface  274  of the sensor layer  270  illustrated in E. The oxygen-sensing and NO x  sensing electrodes  274 ,  276 , respectively, are placed as instructed above with reference to the embodiment of  FIGS. 1A-4  on an exterior surface of the sensing layer  270  shown in E′. 
   While specific embodiments of the present invention have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying claims.