Patent Publication Number: US-2010122916-A1

Title: Sensor with electrodes of a same material

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a sensor, and more particularly, to a sensor with electrodes of a same material. 
     BACKGROUND 
     The composition of exhaust produced by the combustion of hydrocarbon fuels is a complex mixture of oxide gases (NO x , SO x , CO 2 , CO, H 2 O), unburned hydrocarbons, and oxygen. Measurement of the concentration of these individual exhaust gas constituents in real time can assist in improved combustion efficiency and lower emissions of polluting gases. Prior art discloses a variety of sensors configured to measure a concentration of different exhaust gas constituents. In general, these sensors include Nernstian (also called equilibrium sensors) and non-Nernstian sensors (also called nonequilibrium sensors). 
     In Nernstian sensors, a reference electrode and a sensing electrode are exposed to different environments. These different environments may be environments containing gases that have different concentrations of a chemical species to be measured (different gases). When the two electrodes are exposed to different environments, an electric voltage is generated between the electrodes. This electric voltage is used as an indicator of the concentration of the chemical species. In these sensors, the measured electric voltage follows the Nernst equation. In Nemstian sensors, both the reference electrode and the sensing electrode may be made of a same or of different materials and the electric voltage between them is generated by the difference in electrochemical activity between the two electrodes due to the different environment that each electrode is exposed to. 
     In Non-Nernstian sensors, a reference electrode and a sensing electrode, made of different materials, are both exposed to same or different environments, and an electric voltage (indicative of the concentration of the electrochemical species) is measured between the two electrodes. In these sensors, the measured electric potential across the two electrodes do not follow the Nernst equation. In Non-Nemstian sensors, the electric voltage is generated due to the differences in electrochemical activity between the same gas and the different electrode materials. 
     Non-Nemstian sensors are used for the detection and measurement of various oxidizable (CO, NO, etc.) and reducible gases (O 2 , NO 2 , etc.). Typical non-Nemstian sensors include an ionically conductive electrolyte, such as yttria stabilized-zirconia (YSZ), a reference electrode, and a sensing electrode. The two electrodes are typically made of different materials which may include various metals, such as platinum (pt), and various perovskite-type metal oxides. Differences in the reduction/oxidation reactions occurring at the gas/electrode/electrolyte interface at the two electrodes may induce a potential difference between the two electrodes. These reduction/oxidation reactions (redox reactions) at the gas/electrode/electrolyte interface (triple phase boundary) are generally referred to herein as electrochemical activity. Some problems with non-Nernstian sensors known in the art include low sensitivity due to signal drift and the difficulty of maintaining a pristine reference voltage. 
     Hasei et al., U.S. Pat. No. 6,274,016, issued Aug. 14, 2001 (the &#39;016 patent), discloses a NO x  sensor having high sensitivity to NO x . The sensor of the &#39;016 patent includes a reference and a sensing electrode formed on a zirconia solid electrolyte substrate. The sensitivity of the sensor of the &#39;016 patent is increased by fabricating the reference electrode out of platinum and making the sensing electrode by laminating a layer of rhodium on a layer of platinum and dispersing zirconia in the laminated electrode. While the sensitivity of sensor of the &#39;016 patent may be enhanced by the particular choice of the electrode materials, the sensor may have some of the other drawbacks discussed above. The disclosed sensor assembly is directed at overcoming shortcomings as discussed above and/or other shortcomings in existing technology. 
     SUMMARY 
     In one aspect, a sensor for monitoring concentration of a constituent in a gas is disclosed. The sensor may include an ionically conductive layer and a sensing electrode coupled to the ionically conductive layer. The sensing electrode may be exposed to a gas. The sensor may also include a reference electrode that is exposed to the gas and made of substantially a same material as the sensing electrode. 
     In another aspect, a method of fabricating a sensor is disclosed. The method may include creating a sensing electrode on an ionically conducting substrate and creating a reference electrode on the ionically conducting substrate. The sensing electrode and the reference electrode may be made of a same material and have different microstructures. The method may also include positioning the sensing electrode and the reference electrode such that both the reference electrode and the sensing electrode are exposed to a same gas during operation of the sensor. 
     In yet another aspect, a method of measuring a constituent of a gas using a sensor is disclosed. The method may include directing the gas over a sensing electrode coupled to an ionically conducting substrate. The method may also include directing the gas over a reference electrode coupled to the ionically conducting substrate. The sensing electrode and the reference electrode may be made of a same material and have different microstructures. The method may further include measuring an electric voltage across the sensing electrode and the reference electrode. The electric voltage may be indicative of a concentration of the constituent. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of an exemplary sensor of the current disclosure; 
         FIG. 2  is an cross-sectional illustration of an exemplary sensor assembly of the current disclosure; 
         FIG. 3A  is an illustration of an exemplary integrated sensor of the sensor assembly of  FIG. 2 ; 
         FIG. 3B  is a schematic illustration of a heating component and two sensing components included in the integrated sensor of  FIG. 2 ; 
         FIG. 4  is a flow chart illustrating an exemplary method of fabrication of a sensing component of the sensor assembly of  FIG. 2 ; 
         FIG. 5A  is an scanning electron microscope (SEM) image of a sensing electrode of the sensor assembly of  FIG. 2 ; and 
         FIG. 5B  is a scanning electron microscope image of a reference electrode of the sensor assembly of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an embodiment of a sensor  20 A of the current disclosure. Sensor  20 A may be a Non-Nernstian sensor. Sensor  20 A may include a substrate  38 A made of an ionically conductive material, and a reference electrode  40 A and a sensing electrode  50 A coupled to substrate  38 A. Any ionically conductive materials known in the art, may be used as substrate  38 A. Although reference electrode  40 A and sensing electrode  50 A are illustrated in  FIG. 1  as being on opposite sides of substrate  38 A, it is contemplated that, in some embodiments, both reference and sensing electrode  40 A,  50 A may be on same side of substrate  38 A. Both reference electrode  40 A and sensing electrode  50 A may be made of substantially the same material. The term substantially the same material is used to account for the possibility that, although reference electrode  40 A and sensing electrode  50 A may be fabricated using the same material, in practice, impurities, contaminants, and trace elements of materials may cause some measurable differences in the materials of reference electrode  40 A and sensing electrode  50 B. 
     Reference electrode  40 A and sensing electrode  50 A, may however, have different microstructures. For example, the porosities and/or the pore size of the electrode material of the reference electrode  40 A and sensing electrode  50 B may be different. During operation, sensor  20 A may be exposed to a gas having a chemical species as a constituent. The concentration of this chemical species may be measured by sensor  20 A. The differences in electrochemical activity between the gas and the two electrodes, due to the differences in microstructure between the electrodes may generate an electric voltage between the two electrodes. This electric voltage may be indicative of the concentration of the chemical species in the gas. Although not shown in  FIG. 1 , sensor  20 A may also include circuits that may be configured to measure the electric voltage between the reference and sensing electrodes  40 A,  50 A, and support structures that may be configured to enable sensor  20 A to be applied to a specific application. In the description that follows, an embodiment of a sensor of the current disclosure that is used in an engine application will be described. 
       FIG. 2  is an illustration of a sensor assembly  100  that may be configured to measure constituents of exhaust gases of an engine. In such an application, sensor assembly  100  may be positioned in an exhaust duct that transports exhaust gases from the engine. Sensor assembly  100  may include multiple components enclosed in a housing  10 , that cooperate to allow one or more constituents of the exhaust gas to be measured. These components may include an integrated sensor  20 . Sensing component  20  may extend within housing  10  along a longitudinal axis  98 . A grommet  12  and a flow head  14  may enclose integrated sensor  20  within housing  10 . Housing  10  may also include components such as connectors and crimp rings (generally referred to herein as sealing members  16   a ,  16   b ,  16   c ) that constrain integrated sensor  20  snugly within housing  10 . A measurement chamber  18  may also be enclosed within housing  10  along side integrated sensor  20 . Flow head  14  may include inlet openings and passages (not shown) that direct exhaust gases flowing in the exhaust duct to the measurement chamber  18 . These exhaust gases may pass though one or more catalysts (not shown) positioned in the flow path as they flow to the measurement chamber  18 . The catalyzed exhaust gases may flow through the measurement chamber and exit housing  10  through an outlet opening (not shown) in flow head  14 . Integrated sensor  20  may include one or more sensing regions  28  positioned in measurement chamber  18 . Integrated sensor  20  may also include a heating component  22  configured to heat sensing regions  28  and the one or more catalyst positioned in flow head  14 . The sensing regions  28  may measure the concentration of one or more exhaust gas constituents as they pass through measurement chamber  18 . Terminals  8  that extend into housing  10  through grommet  12  may transfer this measured concentration to a control system of the engine. 
       FIG. 3A  illustrates integrated sensor  20  of sensor assembly  100 . Integrated sensor  20  may be of a multilayer ceramic construction, and may include the one or more sensing regions  28 . Although, in general, sensing regions  28  may include any number of sensing regions positioned anywhere on integrated sensor  20 , in this discussion integrated sensor  20  is depicted as including two sensing regions positioned on one side thereof. These two sensing regions  28  may each be configured to measure a separate constituent of the exhaust gases. In some embodiments, one of these two sensing regions  28  may be an oxygen sensor  26  that is configured to measure a concentration of oxygen in the exhaust gases, and the second sensing region  28  may be a NO, sensor  24  that is configured to measure a concentration of NO x  in the exhaust gases. As indicated before, sensor assembly  100  may include additional or different sensing regions than those described herein. Heating component  22  may include one or more heating elements (not shown) embedded in integrated sensor  20 . In some embodiments, separate heating elements may be embedded below each sensing region to heat each sensing region independently. The heating component may also include electrical connections that electrically couple the heating elements, NO x  sensor  24 , and the oxygen sensor  26  to electrical contacts  32  of integrated sensor  20 . Terminals  8  may electrically couple these contacts  32  to the control system of the engine. 
       FIG. 3B  illustrates a schematic view of the sensing and heating components that make up sensor assembly  100 . In addition to heating component  22  being of multi-layer ceramic construction, oxygen sensor  26  and NO x  sensor  24  may also be of multi-layer ceramic construction. In the embodiment of  FIG. 3B , oxygen sensor  26  may be an Nemstian sensor while NO x  sensor  24  may be a non-Nemstian sensor. Heating component  22 , NO x  sensor  24  and oxygen sensor  26  may be fabricated separately and may be bonded together after fabrication. Heating component  22  may include cavities  24   a  and  26   a  that may be sized to fit NO x  sensor  24  and oxygen sensor  26  therein. The separately fabricated NO x  sensor  24  and oxygen sensor  26  may be positioned and bonded in the respective cavities  24   a  and  26   a  of heating component  22 . Heating component  22  and oxygen sensor  26  may be of any type known in the art, and may be fabricated by any known fabrication technique. Since the construction and fabrication of heating component  22  and oxygen sensor  26  are well known in the art, they will not be discussed herein. The construction and method of fabrication of NO x  sensor  24  is described in the following paragraphs. 
     NO x  sensor  24  may include multiple layers of ceramic sheets that are sandwiched together and sintered to form NO x  sensor  24 .  FIG. 4  illustrates a flow chart for fabricating NO x  sensor  24 . In the description that follows, reference will be made to both  FIGS. 3B and 4 . The multiple layers of NO x  sensor  24  may include a first layer  34 , second layer  36 , and a third layer  38 . As is well known in the art, the design of NO x  sensor  24  may include an open reference chamber. As will be described in more detail below, the individual layers of the NO x  sensor  24  may include openings configured to form these reference chambers when they are laminated together. 
     First layer  34 , second layer  36 , and third layer  38  may be formed from a powder (or paste) of an ionically conductive material. As with substrate  38 A of sensor  20 A (illustrated in  FIG. 1 ), any ionically conductive material known in the art may be used to fabricate first layer  34 , second layer  36 , and third layer  38  (step  110 ). In one exemplary embodiment, yttria stabilized zirconia (YSZ) may be used as the ionically conductive material. YSZ powder material may be mixed with binders, solvents, and/or plasticizers and tape cast and dried to form relatively flexible layers of YSZ. This relatively flexible form of the ceramic material is known in the art as green layers. Some of these green YSZ layers may include openings configured to form the reference chamber when the individual layers are laminated together. 
     The openings of the different layers may be formed on the green sheets by any technique known in the art, such as laser cutting (step  120 ). These openings may include opening  36   a  on second layer, and openings  38   b  and  38   c  on third layer. Holes, called via holes (not shown), may also be drilled through some or all of the layers in this step. When first layer  34 , second layer  36 , and third layer  38  are stacked together, opening  36   a  along with first layer  34  and third layer  38  may define the reference chamber, with openings  38   b  and  38   c  providing access to exhaust gases from measurement chamber  18  (see  FIG. 2 ) into the reference chamber. The via holes may then be filled with an electrically conductive material (step  130 ) to conduct electrical signals between the different layers. 
     Reference electrode  40 , and lead wires  40 ′, that electrically interconnect reference electrode  40  to a mating electrical connection  50   b  on heating component  22 , may then be formed on one side of the green third layer  38  (step  140 ). Reference electrode  40  and the lead wires may be patterned on third layer  38  by any method, such as screen printing, known in the art. First layer  34 , second layer  36 , and third layer  38  may then be stacked together and laminated to assemble NO x  sensor  24  (step  150 ). When the layers are stacked together, reference electrode  40  may be positioned in the reference chamber formed by openings  36   a ,  38   b , and  38   c . Lamination may be carried out under heat and pressure. The temperature and pressure used during lamination may depend upon the design of NO x  sensor  24  and the specific material used as the ionically conductive material. In some embodiments, lamination may be carried out by stacking first layer  34 , second layer  36 , and third layer  38 , and subjecting the stack to a pressure between about 1,500-10,000 psi and a temperature between about 25-100° C. 
     The shape of openings  36   a ,  38   b , and  38   c  may be such that an unsupported span of third layer  38  above the reference chamber is minimized. Minimizing the unsupported span of the third layer  38  may improve the structural integrity of the reference chamber, and help preserve the shape of the reference chamber during lamination and other subsequent operations. In one embodiment, projections  36   b  and  36   c  (see  FIG. 3B ) may be provided on second layer  36  to support third layer  38  above the reference chamber. Although rectangular projections  36   b ,  36   c  that project into opening  36   a  from opposite side walls of second layer  36  are depicted in  FIG. 3B , it should be emphasized that these projections may have other shapes, sizes, and orientations. 
     In some embodiments, multiple NO x  sensors  24  may be included in the same stack of layers. In these embodiments, individual NO x  sensors  24  may be singulated from the stack after lamination (step  160 ). Any processes known in the art, such as laser cutting, sawing, punching, etc., may be used for singulation. The singulated NO x  sensors  24  may then be sintered to drive the organic components off the green ceramic and densify the ceramic material (step  170 ). Sintering may be carried out by exposing the laminated NO x  sensors  24  to a high temperature for a prolonged time. Sintering may form a NO x  sensor  24  of unitary structure with reference electrode  40  and the electrical connections to the reference electrode  40 , embedded therein. The time-temperature profile employed during sintering may depend upon the application. As an illustrative example, if a YSZ based ionically conductive material is used to fabricate NO x  sensor  24 , sintering may include heating the stacked and laminated layers (first layer  34 , second layer  36 , and third layer  38 ) together for a temperature greater than about 1000° C. for over 2 hours. In some embodiments, the sintering may include heating the laminated layers to a temperature greater than about 1300° C. for about 2 hours or more. 
     Sensing electrode  50 , along with lead wires  50 ′ that electrically couple the sensing electrode  50  to the mating electrical connection  50   b  on heating component  22 , may then be formed on the sintered NO x  sensor  24  (step  180 ). Any known method, such as screen printing, may be used to form the sensing electrode  50 . The NO x  sensor  24  may then be heated (“fired”) to adhere the sensing electrode material to the ceramic material of NO x  sensor  24 . As is known in the art, the firing conditions may depend upon the application. In some embodiments, firing may include heating the NO x  sensor  24  to a temperature between about 800-1400° C. for about 15 minutes to about 2 hours. 
     In NO x  sensor  24 , both reference electrode  40  and sensing electrode  50  may be made of substantially the same material but may have different microstructures. For example, the porosities and/or the pore size of the electrode material of the reference electrode  40  and sensing electrode  50  may be different. These different microstructures may be created by any known technique. For instance, the sintering conditions and firing conditions may be controlled to obtain a desired microstructure of reference electrode  40  and sensing electrode  50 , respectively. In some embodiments, the maximum temperature that one of the electrodes (reference electrode  40  or sensing electrode  50 ) is exposed to during the manufacturing process may be at least 50° C. lower than the maximum temperature that the other electrode is exposed to during the manufacturing process. This difference in temperature may assist in forming reference electrode  40  and sensing electrode  50  having different microstructures. 
     J  FIGS. 5A and 5B  show scanning electron microscope (SEM) images of sensing electrode  50  and reference electrode  40 , respectively, having different microstructures (including porosity and pore size). In this disclosure, porosity is generally defined as the percentage area occupied by pores  45  in a unit area of the material. The difference in microstructure may produce a difference in the length of the triple phase boundary (gas-electrode-electrolyte interface) at the reference electrode  40  and the sensing electrode  50 . The difference in length of the triple phase boundary may cause a difference in electrochemical activity at the two electrodes (reference electrode  40  and sensing electrode  50 ). This difference in electrochemical activity at the two electrodes may generate an electric voltage, which is indicative of the concentration of a chemical species in the gas, across these two electrodes. 
     In general, any metal or metal oxide (such as platinum (Pt) and perovskite-type oxides) may be used as the electrode material. The reference electrode  40  and sensing electrode  50  may also have any microstructure as long as the microstructure of the two electrodes are different. In some embodiments, reference electrode  40  and sensing electrode  50  may have different porosities and/or pore sizes. In some embodiments, the porosity and/or pore size of the reference electrode  40  may be greater than the porosity and/or pore size of the sensing electrode  50 , while in other embodiments, the porosity and/or pore size of the sensing electrode  50  may be greater than the porosity and/or pore size of the reference electrode  40 . In some embodiments, the ratio of the porosities of the two electrodes may be greater than or equal to about 1.3. 
     Industrial Applicability 
     The presently disclosed sensor may be utilized to measure the concentration of a chemical species in a gas. In one embodiment, the sensor may be used to measure the concentration of one or more chemical species in an exhaust flow of an engine, while maintaining a high degree of accuracy. Heating and sensing components, that make up the sensor, may be separately fabricated and bonded together to form the sensor. The sensing components may include both Nemstian sensor and non-Nernstian sensors. The non-Nernstian sensors may include a reference electrode and a sensing electrode made substantially from the same material, but having different microstructures. The difference in microstructure of the two electrodes may cause a difference in electrochemical activity at the two electrodes, thereby generating a voltage across the two electrodes. 
     Fabricating the two electrodes of the same material having different microstructures may improve accuracy and reliability of the sensor by reducing signal drift and high oxygen sensitivity. In operation, both sensing and reference electrodes are exposed to the same oxygen partial pressure. The electric potential caused by different oxygen partial pressures at the two electrodes may thereby be minimized. In other words, the change in the oxygen concentration at the two electrodes may have little or no influence on the output signal. By controlling the microstructure of the reference electrode and the sensing electrode, the rate of electrochemical reaction at the two electrodes may be controlled, thereby reducing signal drift. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed sensor. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed sensor. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.