Abstract:
A gas sensor for sensing NO x  having electrochemical cells wherein dielectric material surrounds electrolytes except where electrodes are attached. Thereby, the exhaust gas is effectively prevented from contacting the electrolytes of the sensor&#39;s electrochemical cells. With the use of this technique, signal cross talk is minimized while enhancing NOx sensing sensitivity. Further, the total number electrodes needed are reduced which allows for more complex sensors structures.

Description:
TECHNICAL FIELD 
     The present disclosure relates to exhaust gas sensors. More particularly, the present disclosure relates to an exhaust gas sensor with enhanced nitrous oxides sensing capabilities. 
     BACKGROUND 
     Exhaust sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, exhaust sensors have been used for many years in automotive vehicles to sense the presence of exhaust gases. In automotive applications, the direct relationship between various exhaust gas concentrations and the air-to-fuel ratios of the fuel mixture supplied to the engine allows the sensor to provide concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions. 
     Particularly with nitrogen oxides (NO x ), there are several different ways to detect NO x  in exhaust gas. These methods are thermal, optical, electronic resistive, and electrochemical. U.S. Pat. No. 5,486,336 to Betta et al., U.S. Pat. No. 4,822,564 to Howard, U.S. Pat. No. 5,800,783 to Nanaumi et al., and U.S. Pat. No. 4,927,517 to Mizutani et al. demonstrate each of these methods of detecting NO x , respectively. Among the conventional NO x  detection methods, the electrochemical method has proven to be particularly effective because the sensor materials are compatible with the high temperature environment created by the exhaust gas. With the electrochemical method, there are two basic principles involved in NO x  sensing: the Nernst principle and the polarographic principle. 
     With the Nernst principle, chemical energy is converted into electromotive force (emf). A gas sensor based upon this principle typically consists of an ionically conductive solid electrolyte material, a porous electrode on the sensor&#39;s exterior exposed to the exhaust gases with a porous protective overcoat, and a porous electrode on the sensor&#39;s interior surface exposed to the partial pressure of a known gas. Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of a particular gas, such as oxygen for example, that is present in an automobile engine&#39;s exhaust. This is particularly relevant as NO x  sensors catalytically reduce NO x  to nitrogen gas and oxygen, wherein the liberated oxygen is then measured. When opposite surfaces of the 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:        E   =       (       -   RT       4      F       )                   ln                   (       P     O   2     ref       P     O   2         )               where:                 E   =     electromotive                 force                   (   emf   )                   R   =     universal                 gas                 constant                 F   =     Faraday                 constant                 T   =     absolute                 temperature                 of                 the                 gas                   P     O   2     ref     =     oxygen                 partial                 pressure                 of                 the                 reference                 gas                   P     O   2       =     oxygen                 partial                 pressure                 of                 the                 exhaust                 gas                                  
     With the polarographic principle, the sensors utilize electrolysis; that is, by measuring the current required to decompose a gas, such as NO x , the concentration of that gas can be determined. Generally, this type of sensor is composed of a pair of current pumping electrodes where both are in contact with an oxide conductive solid electrolyte and one electrode is in contact with a gas diffusion limiting medium. The gas diffusion limiting means in conjunction with the pump electrode creates a limiting current which is linearly proportional to the measured gas concentration in the sample. 
     For example, one known type of exhaust sensor includes a flat plate sensor formed of various layers of ceramic and electrolyte materials laminated and sintered together with electrical circuit and sensor traces placed between the layers in a known manner. Within the sensor, a flat plate sensing element is employed. This sensing element can be both difficult and expensive to package within the body of the exhaust sensor since it generally has one dimension that is very thin and is usually made of brittle materials. Consequently, great care and time consuming effort must be taken to prevent the flat plate sensing element from being damaged by exhaust, heat, impact, vibration, the environment, etc. This is particularly problematic since most materials conventionally used as sensing element supports, for example, glass and ceramics, cannot withstand much bending. After the sensor is formed, exhaust gas can be sensed. 
     Particular to NO x  sensors, treatment of the exhaust gas is employed prior to being analyzed utilizing the Nernst and/or polarographic principles. Typically, this is achieved using catalyst and/or by maintaining the other gasses at constant levels within an enclosed or semi-enclosed environment. Once the exhaust is treated, the gas encounters the sensor&#39;s electrochemical cells. 
     A typical prior art NO x  sensor will have two electrochemical cells. The first cell has an exhaust gas diffusion limiting means, two oxygen pumping electrodes, and two oxygen sensing electrodes separated by an oxide conducting solid electrolyte. The second cell has a gas diffusion limiting means that connects to the first cell, two pumping electrodes, two sensing electrodes, and an oxide conducting solid electrolyte between the electrodes. The first cell has one pumping electrode exposed to ambient exhaust gas and the other pumping electrode exposed to the inside of the first cell. As to the first cell&#39;s sensing electrodes, one is exposed to a reference gas while the other is located within an interior portion of the first cell. The pumping electrodes of the second cell have one electrode exposed to exhaust gas and the other electrode exposed to the interior of the second cell. As with the first cell, the second cell has one sensing electrode exposed to a gas and the other exposed to the interior of the second cell. In use, the electrodes located inside the first cell have substantially no effect on the NO x  concentration so that only the oxygen concentration is modulated and not the NO x  concentration. The electrodes inside the second cell have an effect on the NO x  concentration via using a catalyst. Thereby, NO x  sensing can be achieved with either the Nernst and/or the polarographic principles. Generally, a heater is provided to maintain a constant operating temperature within the sensor. 
     As such, existing electrochemical NO x  sensors employ multiple electrochemical cells that share a common oxide conducting solid electrolyte. These cells have a frequent tendency to electrically cross-communicate and interfere with each other. Accordingly, there remains a need in the art for a NO x  sensor having minimal cross-communication and interference between sensor electrochemical cells. 
     SUMMARY 
     The deficiencies of the above-discussed prior art are overcome or alleviated by the gas sensor and method of making the same. One embodiment of the gas sensor comprises: a first electrochemical cell having a first electrolyte disposed between and in ionic communication with first and second electrodes; a second electrochemical cell having a second electrolyte disposed between and in ionic communication with third and fourth electrodes wherein said first and second electrochemical cells are ionically isolated from each other; and a third electrochemical cell having a fifth electrode disposed on the same side of the second electrolyte as the third electrode. The fifth electrode and third electrode are arranged to be disposed in a spaced relation. Additionally, the first and second electrolytes are each disposed in a separate layers of dielectric material. 
     In another embodiment, the gas sensor, comprises: a first electrochemical cell having a first electrolyte disposed between and in ionic communication with first and second electrodes; a second electrochemical cell having a second electrolyte disposed between and in ionic communication with third and fourth electrodes wherein said first and second electrochemical cells are ionically isolated from each other; a third electrochemical cell having a fifth electrode disposed on the same side of the second electrolyte as the third electrode, wherein the third and fifth electrodes are disposed in a spaced relation; and a fourth electrochemical cell disposed on a side of the second electrochemical cell opposite the first electrochemical cell, the fourth electrochemical cell having a third electrolyte disposed between and in ionic communication with sixth and seventh electrodes, wherein the fourth electrochemical cell is disposed in a dielectric layer, and wherein the first, second, and fourth electrochemical cells substantially ionically isolated from one another. The first and second electrolytes are each disposed in a separate layers of dielectric material. 
     Alternatively to avoid electrical cross-communication and interference between electrochemical cells using a shared electrolyte, a dielectric insulation layer is inserted between the two electrochemical cells. Or to avoid electrical cross-communication and interference between electrochemical cells, the cells arrangement involve both of the two schemes mentioned above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings wherein like elements are numbered alike in the several Figures. 
     FIG. 1 is a perspective layout view of a dielectric material and electrolyte prior to the electrolyte&#39;s placement within dielectric material. 
     FIG. 2 is a perspective layout view of a dielectric material and electrolyte prior to the electrolyte layer being joined with a dielectric layer. 
     FIG. 3 is an exploded view of a portion of a NO x  sensor employing the placement of electrolyte within dielectric. 
     FIG. 4 is an exploded view of a portion of a NO x  sensor employing a dielectric layer under the electrolyte layer. 
     FIG. 5 is a perspective view of an example arrangement of a NO x  sensor wherein two of the electrochemical cells with an embedded layout. 
     FIG. 6 is a perspective view of an example arrangement of a NO x  sensor wherein two of the electrochemical cells with a layered layout. 
     FIG. 7 is a perspective view of an example arrangement of a NOx sensor that is similar to FIG. 5 except an additional electrolyte is employed with all the cells in an embedded layout. 
     FIG. 8 is a perspective view of an example arrangement of a NOx sensor that is similar to FIG. 7 except an additional electrolyte is employed a layered layout. 
     FIG. 9 is a perspective view of an example arrangement of a NO x  sensor that is similar to FIG. 7 with an embedded layout for all of the cells. 
     FIG. 10 is a perspective view of an example arrangement of a NO x  sensor that is similar to FIG. 9 with a layered layout for all of the cells. 
     FIG. 11 is a perspective view of an example arrangement of a NO x  sensor that is similar to FIG. 9 except that mixed structure are used layered structures mixed with an embedded structure. 
     FIG. 12 is a perspective view of an example arrangement of a NO x  sensor that is similar to FIG. 10 except that mixed structure used embedded structures mixed with layered structure. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     A NO x  sensor is similar to other gas sensors, particularly oxygen sensor, in that both Nernst and current-pumping type electrochemical cells can be utilized. As stated above, sensors typically contain multiple electrochemical cells that share an oxide conducting solid electrolyte. To avoid electrical cross-communication and interference between electrochemical cells, an electrolyte preferably is embedded in dielectric material. By embedding the electrolyte in dielectric material and between electrodes, the electrolytes of the two cells avoids direct contact with each other. Alternatively, to avoid electrical cross-communication and interference between electrochemical cells using a shared electrolyte, the sensor can have a dielectric insulation layer inserted between the two electrolyte layers of the two electrochemical cells. In another alternative, the sensor can have the cells arranged in a way to involve both of the two schemes just mentioned above. 
     Typically in use with a NO x  sensor, at least one electrochemical cell is provided as an oxygen pumping cell. The cell is positioned beneath a coating layer for poison protection from exhaust gas. The electrode facing the coating layer is typically comprises of a precious metal, such as palladium (Pd), rhodium (Ru), platinum (Pt), and the like, as well as combinations and alloys comprising at least one of the forgoing. The electrode facing an interior portion of the sensor is also typically comprised of a metal such as gold (Au) alloyed with palladium (Pd), rhodium (Ru), platinum (Pt), osmium (Os), ruthenium (Ru), iridium (Ir), zirconium (Zr), yttrium (Y), cerium (Ce), calcium (Ca), aluminum (Al), and the like, as well as other similar alloys, oxides, which have less electrochemical pumping effect on NOx, and combinations comprising at least one of the foregoing metals. By applying a current, oxygen ions can be conducted out of the interior portion of the cell. Thereby, the relative concentration of NO x  is much greater than the oxygen concentration. 
     Used in conjunction with the oxygen pumping cell is an exhaust gas oxygen sensing cell(s) which use the Nernst principle to create an emf across an electrolyte. Typically, the electrodes comprise a precious metal, with platinum preferred for the reference electrode and platinum or platinum alloy (such as PT/Rh alloy) preferred for the sensing electrode facing the interior portion of the sensor. The reference electrode faces a reference gas while the sensing electrode faces the gas within the sensor being sampled. This difference in oxygen concentration will generate an emf that can be analyzed and compared to determine the oxygen concentration within the gas being analyzed. Depending on this determination, a process control device can operate the above mentioned oxygen pumping cell to remove additional oxygen from the sensing gas so that the NO x  concentration can be easily determined. 
     The NO x  is then determined by the NO x  cell which has one electrode facing the sensing gas that has the ability to reduce NO x  to nitrogen and oxygen. This is attained by using rhodium, rhodium alloy (such as a rhodium/platinum alloy), or other NO x  catalyst in the electrode facing the sensing gas. By reducing the NO x , the oxygen can then either be sensed using emf comparison or determined using oxygen pumping by analyzing the amount of current required to conduct the oxygen created out of the sensor. 
     Referring now to FIG. 1, an electrolyte  10  is shown prior to being embedded in a dielectric material  20 . While electrolyte  10  is depicted as a disc shaped object, any shape that will lead to a functioning sensor structure is possible, for example a circular, elliptical, rectangular, multi-sided, or the like. Referring now to FIG. 2, an electrolyte  10  layer is shown with a dielectric material  20  layer. Electrolyte  10  and dielectric material  20  are typically manufactured as tapes (sheets of rolled material). In one method of manufacturing, tapes of electrolyte  10  and dielectric material  20  are punched so that a punched disc of electrolyte  10  will fit within a corresponding punched opening in dielectric  20 . 
     Advantageously, the materials for electrolyte  10  and dielectric  20  are selected to have similar shrinkage and thermal coefficient factors, preferably within about 5%, during manufacturing firing stage (the process of maturing ceramic products by the application of heat) and similar thermal coefficients after firing. This can be achieved with the use of doping or other additives added to the starting materials. To form tapes of the electrolyte  10  and dielectric material  20 , any known method of manufacturing can be used such as roll compaction, tape casting, slip casting, or calendaring, for example. Possible electrolyte materials include any material conventionally employed as sensor electrolytes, including, but not limited to, zirconia which may optionally be stabilized with calcium, barium, yttrium, aluminum, lanthanum, cesium, gadolinium, and the like, as well as alloys, oxides, and combinations comprising at least one of the foregoing. For example, the electrolyte can be alumina and yttrium stabilized zirconia. Typically, the electrolyte has a thickness of up to about 500 microns, with a thickness of approximately 100 microns to about 250 microns preferred. Possible dielectric materials include alumina or another dielectric material capable of inhibiting electrical communication and providing physical protection. The dielectric materials can be up to about 500 microns thick, with a thickness of about 100 to about 250 microns preferred. 
     The electrolytes  10  can be solid or porous. 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 sensor is utilized. Typically, 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. Commonly assigned U.S. Pat. No. 5,762,737 to Bloink et al., which is hereby incorporated in its entirety by reference, further describes porous electrolytes that may be useful in the instant application. 
     Referring to FIG. 3, an exploded view showing a structure wherein electrolytes  10  and  12  are fitted within corresponding openings in dielectric materials  20  and  22 , respectively. The subassembly of electrolyte  10  and dielectric material  20  is positioned adjacent to and in agreement with the subassembly of electrolyte  12  and dielectric  22 . Disposed on opposite sides of electrolyte  10 , are electrode  34  and electrode  30 , to form a first electrochemical cell  11 . Disposed on opposite sides of electrolyte  12 , are electrode  32  and electrode  36 , to form a second electrochemical cell  13 . Electrodes  30  and  32  are positioned between electrolytes  10  and  12 , with an electrode gap  40 , if desired, disposed there between. 
     To form the electrode gap  40  (an open gas space), during sensor production, a fugitive material (e.g., a carbon based material) is positioned between electrodes  30  and  32 . Upon formation, the fugitive material will burn off leaving electrode gap  40  between electrodes  30  and  32 . When the fugitive material is burned off, the first electrochemical cell  11  having electrode  30  will be separate and ionically isolated from the second electrochemical cell having electrode  32 . 
     In operation, the electrode gap  40  is in fluid communication with the gas to be sensed either via a channel or through the first electrochemical cell  11 . The first electrochemical cell  11  is used as an oxygen pumping cell to pump oxygen out from the electrode gap. The second electrochemical cell  13  can then be used as a NO x  sensing or oxygen cell to determine the concentration of NO x  or oxygen. Alternatively, a design can be created wherein electrodes  30  and  32  are shared by the top and bottom electrochemical cells and still maintain an ionic isolation (not depicted). To achieve this, electrodes  30  and  32  can be joined together in another part of the sensor, or electrode gap  40  can be eliminated whereby electrodes  30  and  32  can be combined into one electrode. If electrodes  30  and  32  are combined, the electrode ink (electrode coating) preferably should not contain any oxide electrolytic materials. 
     Referring to FIG. 4, an exploded view showing a structure wherein electrolytes  10  and  12  are separated by a dielectric material layer  20 . Disposed on opposite sides of electrolyte  10 , are electrode  34  and electrode  30 , is forming a first electrochemical cell  11 . Disposed on opposite sides of electrolyte  12 , are electrode  32  and electrode  36 , forming a second electrochemical cell  13 . Electrodes  30  and  32  are positioned between electrolytes  10  and  12  wherein electrode gap  40  and  42  are disposed in between and separated by the dielectric layer  20 . To form the electrode gap  40  and  42  (an open gas space), during sensor production, a fugitive material (e.g., a carbon based material) is positioned between electrodes  30  and  32 . Upon formation, the fugitive material will burn off leaving electrode gap  40  and  42  between electrodes  30 ,  32  and the dielectric layer  20 . There is a hole or aperture  50  on dielectric layer  20  to allow fluid communication between the electrode  30  and  32 . 
     FIGS. 5-10, various embodiments of NO x  sensors employing structures based on those explained above in FIGS. 1-4. 
     Referring to FIG. 5 comprising three electrochemical cells with two of these cells sharing the same electrolyte, a first electrochemical cell  111  comprises an electrolyte  110  and electrodes  130  and  132 . Electrodes  130  and  132  are oxygen pumping electrodes that are in electrical communication with electrolyte  110 . For example, electrode  130  can comprise platinum (Pt) and electrode  132  can comprise a gold (Au)/platinum alloy. Further, gas diffusion control is provided by a coating layer, the electrolyte (porous electrolyte or aperture(s) through the electrolyte), or by an aperture inserted between electrolytes  110  and  112 . In other words, the gas to be sensed can enter the sensor and contact electrode  132  by traveling through electrolyte  110  or by passing through an aperture or passageway (not shown) disposed from electrode  132  to the exterior of the sensor. 
     The remaining electrochemical cells are an oxygen sensing cell and a NO x  sensing cell. The second electrochemical cell  113 , used for oxygen sensing, comprises electrolyte  112  and electrodes  134  and  138 . The third electrochemical cell  115  used for NO x  sensing, comprises sensing electrode  136 , electrolyte  112 , and the reference electrode  138 . Reference electrode  138  is maintained in fluid communication with a reference gas source, such as oxygen or air; e.g., reference gas can be provided by oxygen pumping and an oxygen chamber, and/or with the use of an air channel connected to ambient air. 
     When gas, e.g., exhaust gas, enters the sensor and contacts electrode  132 , the first electrochemical cell  111  acts as an oxygen pumping cell, which, upon application of current, removes the oxygen between electrodes  132  and  134 . Using reference electrode  138  and sensing electrode  134  and the first electrochemical cell  111 , the oxygen concentration in the area of electrodes  132 ,  134 , and  136  is controlled at a constant value so that NO x  concentration can be determined. 
     One method of determining the NO x  concentration is by measuring the NO x  emf in proximity with NO x  sensing electrode  136 , which is attributable to the decomposition of NO x  in addition to residual oxygen content, and by measuring the oxygen emf in proximity with oxygen sensing electrode  134 . Another method of determining the NO x  concentration is by pumping the oxygen, which is attributable to the decomposition of NO x  in addition to residual oxygen content, through electrode  136  to electrode  138 . By measuring this pumping current and comparing to a current developed between electrodes  134  and  138 , the concentration of NO x  can be determined. Alternatively, gap  154 , which can be formed from fugitive material which is burn off during firing, can be partially eliminated and electrodes  132  and  134  can be joined together to allow electronic conductance between the electrodes  132  and  134 . In use, an electronic control circuit operates the sensor in determining the amount of current to be applied. 
     In FIG. 6, there is a dielectric layer  120  inserted between the electrolyte layers  110  and  112 . Disposed in dielectric layer  120  is preferably a hole or an aperture  150  which allows gas communicated between electrodes  132 ,  134 , and  136 . 
     Referring to FIG. 7, which is similar to FIG. 5 with an additional electrochemical cell  117 . This fourth electrochemical cell  117  comprises electrolyte  114  and electrodes  140  and  142  and is situated between the first and second electrochemical cells  111 ,  113 . Electrolyte  114  can be a porous electrolyte, or a solid electrolyte comprising a hole, optionally has an aperture or the like,  160  and/or porosity control material to control the amount of NO x  and any other residual gas entering contact with electrodes  134 ,  138 , and  142 . Electrodes  140  and  142  can be used as either pumping or sensing electrodes for improved removal of oxygen and improved control of the oxygen pumping cells. With this additional electrochemical cell, additional oxygen reduction can be achieved in relation to the NO x  concentration. Also, due to the use of the embedded electrolytes  110 ,  112 ,  114  and/or layered electrolytes with dielectric material gaps  154 ,  156 , cross-talk between the electrolytes is reduced verses conventional systems. Actually, the crosstalk is essentially eliminated. 
     In FIG. 8, the sensor has layered structure (as opposed to FIG.  7 &#39;s embedded structure) with dielectric insulation layers  120  and  122  inserted between the electrolyte layers of  110 ,  112 , and  114 . Apertures or holes of  150 ,  164  and  160  are created on dielectric layers  120 ,  122  and electrolyte layer  114  so that fluid gas communication can be achieved between electrodes  132 ,  140 ,  142 ,  134 , and  136 . As with FIG. 9, electrolyte  114  can comprise a hole or aperture  160  to allow passage of exhaust gas to electrode  142 . Note, if electrolyte  114  is porous, aperture  160  is not needed. 
     Another setup is shown in FIG.  9 . This setup is similar to FIG. 7 except that the forth (middle) cell  117  recesses to one side (is askew) so that reference electrode  144  can be disposed on the third cell  117 , adjacent but electrically separated from electrode  142 , as opposed to the second cell  119 , allowing for cell isolation. The reference electrode  144  can be oxygen pumping or can have a reference (e.g., vent, reference gas storage chamber and/or material, and the like, as well as combinations comprising at least one of the forgoing references) vent connected to ambient air (not shown). With this sensor, electrode  148  is exposed to exhaust through an aperture pores, or the like, that is opened between electrode  148  and a dielectric layer (not shown) disposed on a side of the electrode  148  opposite electrolyte  112 . In this arrangement, during operation, both sides of the sensor have direct access to the exhaust gas, e.g., electrodes  130  and  148  , with the air reference being electrode  144 . In such an arrangement, electrolytes  110  can be solid or porous, with a solid electrolyte  112  and  114  preferred. For example, FIG. 10 illustrates a layered structure version of FIG. 9 with dielectric layers  120  and  122  inserted between electrolyte layers  110 ,  114  and  112 . 
     With the arrangement of FIGS. 9 and 10, several different modes of sensor operation are possible. Depending upon which mode of operation is selected, different electrodes can be shared. For example, sensing electrode  140  and reference electrode  144  can be arranged to control the pump electrodes  130  and  132  of the first electrochemical cell  111 . Thereby, the oxygen pumping activity can be maintained at a constant level in the area around electrodes  132 ,  140 ,  142 , and  134 . Electrodes  134  and  148  can be then utilized as NO x  pumping or sensing electrodes for the determination of the NO x  concentration. 
     Referring to FIGS. 11 and 12, a mixed structure views of the sensor depicted in FIGS. 9 and 10 are shown. As displayed in FIG. 11, the two electrolyte layers  110 ′ and  112 ′ are separated by a dielectric insulation layer which also has an electrolyte disk  114 ′ inserted. In FIG. 12 the electrolyte  114 ″ is a layer structure while electrolyte  110 ″ and  112 ″ are embedded structure. 
     In FIGS. 1-12, we did not show heaters, poison resistive coating layers, air reference channels, exhaust gas diffusion limiting means, channels to the ambient exhaust gas. These items can be easily incorporated into the sensor layouts shown in FIGS. 1-12, as the dielectric layers are used in either embedded or layered structures. 
     The NO x  sensor arrangement avoids the cross talk and interference associated with having multiple cells sharing the same leaders that connect to the electronic controller and sensing signal reading electronics. As stated, this is achieved with the placement of the electrolyte within a corresponding opening within a dielectric strip, or with dielectric layer inserted between the electrolyte layers. The dielectric materials, such as alumina, have a high electrical resistivity and dielectric brake down voltage, which provide ionic and electronic isolation between the electrochemical cells (basically, the dielectric layer, used at 800° C. and under a 30 second duration of a 10 volt pulse applied to a 1.0 square centimeter (cm 2 ) electrode area, will generate a leakage current of about 2.8 nano-ampere (nA) to about 4.2 nA.) With these sensor arrangements, improved NO x  sensing and sensor operation is obtained. 
     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. 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.