Abstract:
A NO x  sensor and a method of manufacturing a NO x  sensor array. The NO x  sensor includes a base substrate, a plurality of potentiometric sensors, and a plurality of connectors. The plurality of potentiometric sensors are coupled to the base substrate. Each potentiometric sensor generates a potential difference in response to the presence of NO X  in a gas specimen. The plurality of connectors are coupled to the plurality of potentiometric sensors. The plurality of connectors connect the plurality of potentiometric sensors to combine the potential differences of the plurality of potentiometric sensors to produce a combined potential difference indicative of a level of NO x  within an ambient gas specimen. Use of a filter and appropriate temperature control of filter and sensor minimizes interference from contaminants.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/890,342, filed on Feb. 16, 2007, which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    Nitrogen oxides (NO x ) contribute to ground level ozone formation and acid deposition in the form of acidic particles, fog, and rain. Ground level ozone is a key ingredient of urban smog that causes many respiratory problems. Acid deposition, on the other hand, causes acidification of lakes and streams, damage of forest soils, and decay of building materials and paints. The major source of NO x  is from the combustion of fossil fuels in power plants, vehicles, and airplanes. High temperature NO x  sensors to optimize combustion and minimize emissions are mandated for many industries, including automotive engine control and development. Various solid-state NO x  sensing devices and materials are being examined for operation at elevated temperatures. Among these, electrochemical devices using oxygen-ion-conducting yttria-stabilized-zirconia (YSZ) are appropriate for applications at temperatures higher than 500° C. The basis for NO x  measurement in these devices is based on the difference of NO x  electrochemistry between the two electrodes on the sensor, which results in an EMF (electromotive force) response. In the presence of oxygen, the chemical reactions on the surface of metal-oxide electrodes and electrolytes compete with electrochemical reactions. In some instances, the catalytic property of the electrode material influences the sensing performance. The development of solid oxide NO x  sensors is an interdisciplinary study of solid-state electrochemistry and heterogeneous catalysis. 
         [0003]    Electrochemical devices usually exhibit response to many different gases, thus minimizing selectivity. For example, on potentiometric solid-oxide sensors, any gas that can react with the oxygen ions in YSZ should generate a signal. In the case of NO x  detection, the two main components of nitrogen oxides in combustion environments are NO and NO 2 . NO 2  tends to be reduced and NO tends to be oxidized, resulting in the generation of opposite signals. Many NO x  sensors focus on NO since it is the major component of NO x  at high temperatures. However, in lean-burn conditions, NO 2  is also present in significant, or measurable, amounts. The interference from other reactive species in the combustion environment, including CO, hydrocarbons, as well as NH 3  also may influence performance. Oxygen and water can also act as interfering species. In typical engine exhausts with NO x  reduction/storage device, the NO x  concentration is 1 to 10 ppm, in the presence of 20% CO 2 , 10% water, 3%O2, 10 ppm NH 3 , 1000 ppm hydrocarbons, and 2000 ppm CO. 
         [0004]    In order to eliminate the interference from reducing and oxidizing gases, many applications use catalytic filters. In at least on conventional implementation, a platinum-loaded zeolite Y (PtY) filter has good performance on equilibrating NO x  and oxidizing CO in the presence of oxygen. Zeolite Y may be selected as the support because the Pt nanoclusters stabilized on the high surface area microporous zeolite cages exhibit excellent catalytic properties. The equilibrated NO x  after the PtY filter may be measured by a YSZ-based sensor with a metal oxide electrode. The zeolite filter is kept at a different temperature from the sensor to produce a signal. 
         [0005]    Potentiometric sensors provide a promising approach for NO x  detection in harsh environments, but typically suffer from interferences with other gases. Potentiometric sensors use two electrodes, and both chemical and electrochemical reactivity at each electrode influence sensor performance. 
       SUMMARY 
       [0006]    In some embodiments, a Pt electrode is covered with Pt containing zeolite Y (PtY) and WO 3  as two electrode materials. The electrode may be affected by temperature programmed desorption of NO from NO x /O 2 -exposed PtY and WO 3 . In addition, the ability of PtY and WO 3  to equilibrate a mixture of NO and O 2  may vary over a temperature range of about 200-600° C. Significant reactivity differences may be manifest between PtY and WO 3 , with the latter being largely inactive toward NO x  equilibration. With gases passing through a PtY filter, it may be possible to remove interferences from 2000 ppm CO, 800 ppm propane, 10 ppm NH 3 , as well as minimize effects of 1˜13% O 2 , CO 2 , and H 2 O. By maintaining a temperature difference between the filter (typically at 400° C.) and the sensor at about 600° C., total NO x  concentration (NO+NO 2 ) measurements may be performed. By connecting three sensors in series, in some embodiments, the sensitivity of the sensor system is improved relative to conventional, single-sensor systems. 
         [0007]    An embodiment of a NO x  sensor is described. The NO x  sensor includes a base substrate, a plurality of potentiometric sensors, and a plurality of connectors. The plurality of potentiometric sensors are coupled to the base substrate. Each potentiometric sensor generates a potential difference in response to the presence of NO x  in a gas specimen. The plurality of connectors are coupled to the plurality of potentiometric sensors. The plurality of connectors connect the plurality of potentiometric sensors to combine the potential differences of the plurality of potentiometric sensors to produce a combined potential difference indicative of a level of NO x  within an ambient gas specimen. 
         [0008]    In some embodiments, the plurality of connectors are connected to the plurality of potentiometric sensors to connect the plurality of potentiometric sensors in series. In the series configuration, the combined potential difference is a sum of the potential differences of each of the potentiometric sensors. In some embodiments, each of the potentiometric sensors includes a sensing electrode and a reference electrode. In some embodiments, the NO x  sensor also includes a first electrode lead and a second electrical lead. The first electrical lead is coupled to the sensing electrode of a first potentiometric sensor within the series of potentiometric sensors. The second electrical lead is coupled to the reference electrode of a last potentiometric sensor within the series of attention nitric sensors. In this configuration, the combined potential difference is measurable at the first and second electrical leads. 
         [0009]    Additionally, the first and last potentiometric sensors may be connected together directly, or connected via one or more additional, intermediate potentiometric sensors. In one embodiment, the NO x  sensor includes a third potentiometric sensor coupled between the first and last potentiometric sensors within the series of potentiometric sensors. In this configuration, the sensing electrode of the first potentiometric sensor is connected to the reference electrode of the third potentiometric sensor, and the sensing electrode at the third potentiometric sensor is connected to the reference electrode of the last potentiometric sensor. 
         [0010]    In some embodiments, the sensing electrode is tungsten oxide (WO 3 ). In some embodiments, the reference electrode is platinum (Pt). In some embodiments, the reference electrode is platinum coated with platinum zeolite (PtY). In some embodiments, each of the potentiometric sensors includes an electrolyte substrate. An exemplary electrolyte substrate is an oxygen-ion conducting ceramic. In some embodiments, the electrolyte substrate is yttria-stabilized zirconia (YSZ). In some embodiments, the connectors are platinum. Other embodiments of the NO x  sensor are also described. 
         [0011]    A method for manufacturing a NO x  sensor array is also described. In one embodiment, the method includes disposing a plurality of electrolyte substrate on a base substrate, disposing a sensor electrode on each electrolyte substrate, and disposing a reference electrode on each electrolyte substrate. The method also includes connecting, in a series configuration, each of the sensing electrodes, other than a first sensing electrode, to corresponding reference electrodes, other than a last reference electrode, on adjacent electrolyte substrates. In other words, the electrodes are connected together in a chain, except for the first and last electrodes, which are used for electrical leads to connect to a controller, or other device. 
         [0012]    In some embodiments, the method also includes using a metallic paste disposed electrolyte substrates on a substrate. In some embodiments, the method also includes connecting the electrolyte substrates in series with a plurality of platinum wires. In some embodiments, the method also includes painting the sensing electrode onto the electrolyte substrate. The sensing electrode may be platinum. In some embodiments, the method also includes painting the reference electrode onto the electrolyte substrate. The reference electrode may be tungsten oxide (WO 3 ). Other embodiments of the method are also described. 
         [0013]    A sensing system to measure the NO x  in a gas specimen is also described. In one embodiment, the system includes a sensor array, a filter, and a plurality of temperature-control devices. The sensor array includes a plurality of NO x  sensors coupled in series. The sensor array detects a nitrogen oxide compound in the gas specimen. The filter removes a contaminant compound from the gas specimen. The temperature-control devices maintain the gas specimen at a substantially consistent temperature at the sensor array. 
         [0014]    In some embodiments, the NO x  sensors are implemented with substantially similar materials and structures. In some embodiments, each of the NO x  sensors includes a sensing electrode and a reference electrode. The sensing electrodes and the reference electrodes of the adjacent NO x  sensors, respectively, are coupled together to produce a combined potential difference indicative of a sum of potential differences of the plurality of NO x  sensors. In some embodiments, the sensor array also includes a plurality of connectors coupled to the plurality of NO x  sensors. The connectors connect the plurality of NO x  sensors in series. In some embodiments, the filter removes the contaminant compound from the gas specimen prior to introduction of the gas specimen at the sensor array. In some embodiments of sensor array is calibrated to detect parts per million (ppm) and sub-ppm quantities in the gas specimen. In some embodiments, the sensor array also generates an electrical potential difference in response to detection of the nitrogen oxide compound. In some embodiments, the temperature-control devices also maintaining a temperature difference between the sensor array and the filter. Other embodiments of the system are also described. 
         [0015]    Other aspects and advantages of embodiments of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which are illustrated by way of example of the various principles and embodiments of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  illustrates a schematic block diagram of one embodiment of a NO X  sensor testing setup. 
           [0017]      FIG. 2A  illustrates a schematic diagram of one embodiment of a NO x  sensor. 
           [0018]      FIG. 2B  illustrates a schematic diagram of one embodiment of a sensor array with multiple NO x  sensors. 
           [0019]      FIG. 3  illustrates a temperature programmed desorption (TPD) graph depicting nitric oxide (NO) peak absorption in PtY with 5% oxygen (O 2 ) at room temperature. 
           [0020]      FIG. 4  illustrates a graph of NO x  equilibration as a function of temperature. 
           [0021]      FIG. 5  illustrates a TPD graph depicting NO peak absorption in tungsten oxide (WO 3 ) with 5% O 2  at room temperature. 
           [0022]      FIG. 6  illustrates a graph of electromotive force (EMF) vs. log NO x  at 600° C. 
           [0023]      FIG. 7  illustrates a graph of EMF as a function of filter (PtY) temperature. 
           [0024]      FIG. 8  illustrates response curves and an EMF-log graph for a single and 3-sensor array. 
           [0025]      FIG. 9-11  illustrates a graph depicting response transients of multiple conditions in a sensor test setup. 
           [0026]      FIG. 12A  illustrates response curves of NO 2  in different oxygen levels. 
           [0027]      FIG. 12B  illustrates a graph of an EMF-Oxygen level with and without filtration at varying concentrations. 
           [0028]      FIG. 13  illustrates a graph of EMF vs. water level in 10% O 2  with a PtY filter. 
           [0029]      FIG. 14  illustrates the stability of an NO sensor signal vs. time. 
           [0030]      FIG. 15  illustrates a schematic flow chart diagram of depicting one embodiment of a method of making a sensor array with multiple sensors. 
       
    
    
       [0031]    Throughout the description, similar reference numbers may be used to identify similar elements. 
       DETAILED DESCRIPTION 
       [0032]    In the following description, specific details of various embodiments are provided. However, some embodiments may be practiced with less than all of these specific details. In other instances, certain methods, procedures, components, structures, and/or functions are described in no more detail than to enable the various embodiments of the invention, for the sake of brevity and clarity. 
         [0033]    At least some embodiments use PtY electrodes. In particular, the high chemical reactivity of PtY may be exploited in the sensor design by using PtY/Pt as a reference electrode. Because of the poor chemical reactivity of NO x  on WO 3 , WO 3  may be used as the sensing electrode with the assumption that NO x  species will reach the WO x /YSZ triple-point boundaries chemically unmodified and produce a more sensitive electrochemical response. A combination of the PtY filter with sensors effectively minimizes interferences from 2000 ppm CO, 1000 ppm propane, and 10 ppm NH 3 . Other gases including 30% CO 2 , 5˜10% H 2 O, 1˜13% O 2  also do not cause significant interference. In some embodiments, the signal magnitude can be enhanced by connecting the sensors in series. 
         [0034]    Preparation and characterization of sensor materials. A Pt-loaded zeolite Y powder is prepared from Na-exchanged zeolite Y (Si/Al=2.5, Union Carbide, LZY-52) by ion-exchange. 1.0 g of NaY powder is dried at 100° C. for 4 hours followed by mixing with 2.5 mM [Pt(NH 3 ) 4 ]C12 (Alfa Aesar) solution. The mixture is stirred overnight at room temperature for ion-exchange. After washing and centrifuging with distilled water several times, the Pt-exchanged powder is dried at 70° C. for 3 hours and then calcined at 300° C. for 2 hours. The heating rate of calcination is set to 0.2° C./min to increase the Pt dispersion by preventing the autoreduction of ammonia ligand. The calcined zeolite is exposed to 5% H 2  to reduce Pt 2 +  in the zeolite framework to metallic Pt. WO 3  is used from a commercial powder (99.8%, Alfa Asaer) without any further treatment. 
         [0035]    A FEI XL30 FEG ESEM may used to investigate the microstructure of PtY and WO 3 . A Rigaku Geigerflex X-Ray Powder Diffractometer may be applied to examine the crystal structure of PtY and WO 3 . The dispersion of Pt clusters may be inspected by a FEI Tecnai TF-20 transmission electron microscope with the HAADF detector. The Pt loading may be determined with an inductively coupled plasma-optical emission spectroscopy (ICP-OES). The BET surface area may be measured by a Micrometrics ASAP 2020 analyzer. 
         [0036]    Catalytic NO x  conversion measurements.  FIG. 1  illustrates a schematic block diagram of one embodiment of a NO x  sensor testing setup  150 . Catalytic activity for NO x  conversion may be measured by a chemiluminescent NO x  analyzer  156  (Eco-Physics CLD 70S) as shown in  FIG. 1 . In one embodiment, a 100 mg sample may be placed on a quartz wool support inside a U-shape quartz tube  164  with 4 mm in diameter. The quartz tube  164  may be heated by a vertical tube furnace  152 . 600 ppm NO 2 , 3% O 2  (balance N 2 ) may be delivered through the quartz tube  164  at a total flow rate of 200 cc/min and the NO x  product after reacting with PtY may be examined by the chemiluminescent analyzer  156 . As one example, the gas hourly space velocity (GHSV) may be around 90,000 h −1 . The NO x  analyzer  156  may be calibrated daily with 600 ppm or 30 ppm NO primary standards (Praxair) depending on the concentration range of interest. Before evaluating the WO 3  and PtY sample, a blank experiment may be carried out with only quartz wool support to examine the NO x  conversion from quartz wool and quartz tube  164 . Because the purpose of this experiment is to demonstrate the difference in the catalytic activity between PtY and WO 3 , as well as examining the NO x  conversion when PtY was used as a filter  160 , the gas flow rate and the mass of sample in the filter  160  should be the same of catalytic and sensor testing. 
         [0037]    Temperature programmed desorption measurement. Temperature programmed desorption (TPD) may be performed to study the co-adsorption of NO and oxygen on PtY and WO 3 . A 300 mg sample may be placed on a quartz wool support inside a U-shape quartz tube  164  (4 mm in diameter). Before gas adsorption, the sample is heated to 650° C. in 10% oxygen for 30 min and cooled down to room temperature in He. 2500 ppm NO and 5% oxygen are passed through the sample tube for 20 min at a flow rate of 60 cc/min for gas adsorption. The sample may be purged with 30 cc/min He for 10 min to remove NO x  and O 2 . The sample temperature then may be increased from room temperature to 600° C. at the rate of 10° C./min. The desorbed species are then analyzed by a gas chromatography-mass spectrometer  156  (Shimadzu QP-5050). In one embodiment, the fragments monitored by the mass spectrometer  156  may be m/z=18(H 2 O), 28(N 2  or CO), 30(NO), 32(O 2 ), 44(N 2 O or CO 2 ), and 46(NO 2 ). For both PtY and WO 3 , only m/z=30 and 46 have notable desorption features. Exemplary data for NO (m/z=30) is shown in  FIG. 3  and  FIG. 5 , since the peak for NO 2  (m/z=46) exhibits similar features. 
         [0038]    Sensor fabrication. Electrochemical sensors for use in studying the electrodes may be based on YSZ electrolytes  172  with two electrodes  178  and  180 , as shown in  FIG. 2   a . The YSZ substrate  172  is prepared from YSZ green sheets (3 mole % tetragonal YSZ, NexTech Materials). The 10 mm by 5 mm YSZ green sheets are sintered in air at 1450° C. for 2 hours to form dense bodies. Two Pt lead wires  174  and  176  (99.95%, 0.13 mm in diameter, Fischer Scientific) are attached to YSZ  172  with a small amount of commercial Pt ink (Englehard, A4731). In some embodiments, the end attaching to YSZ is shaped into a disc  180  of 2 mm diameter in order to increase the mechanical stability, although other shapes may be used in other embodiments. The Pt ink is cured at 1200° C. for two hours to secure bonding between the Pt wire  176  and YSZ  172 . WO 3  powder is mixed with α-terpineol to form a paste, which is then painted on top of the Pt lead wire  174  and YSZ  172 . The WO 3  layer  178  is spread over as much YSZ  172  as possible. After sintering at 700° C. in air for 2 hours, the WO 3  layer  178  is about 200 μm thick. PtY is also mixed with α-terpineol and painted on the top of another Pt lead wire  176  to form the reference electrode  180 . The PtY layer is around 100 μm thick after calcination in air at 600° C. for two hours. 
         [0039]    The sensor array of  FIG. 2B  is fabricated by connecting three sensors in series. Other embodiments may use two sensors connected in series. Other embodiments may use more than three sensors coupled in series. Other embodiments may use one or more sensors coupled in parallel with one or more sensors coupled in series. As shown in  FIG. 2B , YSZ  172  without WO 3    178  and PtY  180  are attached first on an alumina substrate  182  with gold paste (Heraeus C5789) followed by calcination at 800° C. for 2 hours. Pt wires  184  are used as the interconnect between sensors. The PtY  180  and WO 3    178  are then applied on the sensors following the method described above. 
         [0040]    Gas sensing measurements. Gas sensing experiments may be performed within a quartz tube placed inside a tube furnace  154  (Lindberg Blue, TF55035A). The quartz tube is wrapped with a grounded aluminum foil to screen against electric noise. A computer-controlled gas delivery system with calibrated mass flow controllers (MFC) is used to introduce the test gas stream. Four certified N 2 -balanced NO x  cylinders (30 ppm NO, 30 ppm NO 2 , 2000 ppm NO, and 2000 ppm NO 2 ) are used as NO x  sources. A pure CO 2  cylinder and certified N 2 -balanced 300 ppm NH 3 , 2000 ppm CO, and 2000 ppm propane are also connected to the gas delivery system. Certified cylinders may be obtained, for example, from Praxair. 
         [0041]    The sensor tests are carried out by mixing dry or humidified air with NO x , balancing N 2 , and CO/CO 2 /NH 3 /propane at a total flow rate of 200 cc/min. A pair of Pt wires is used to connect the sensor to the external leads. As schematically shown in  FIG. 1 , the gas mixture from MFCs could be introduced into the tube furnace either through or bypassing  158  the PtY filter  160 . The filter  160  is in a U-shape quartz tube  164  with 100 mg PtY placed on quartz wool, similar as what may be used in the catalytic NO x  conversion measurements. A chemiluminescent NO x  analyzer  156  (Eco-Physics CLD 70S) is connected to the outlet of the vertical tube furnace  152  for NO x  monitoring. The open circuit potential of the sensors is recorded, for example, by a Hewlett-Packard 34970A data acquisition system with 10 GΩ internal impedance. The sensor devices may be conditioned in a 600° C. furnace in air for 15 hours prior to performing sensor tests. 
         [0042]    Sensor Design.  FIG. 2A  illustrates a schematic diagram of one embodiment of a NO x  sensor  162 . The basic sensor design involves the use of two electrodes  178  and  180  on an oxygen-ion conducting ceramic  172 , tetragonal yttria-stabilized zirconia (YSZ), across which the potential is measured in the presence of NO x  species.  FIG. 2  shows two exemplary types of sensor devices.  FIG. 2   a  is the basic sensor  162  with two electrodes  180  and  178 , including a reference electrode  180  and a sensing electrode  178 . As one example, PtY/Pt may be used as the reference electrode  180 , and WO 3  may be used as the sensing electrode  178 .  FIG. 2   b  depicts a sensor array  170  with three sensors  162  connected in series. In some embodiments, gases pass through a PtY filter  160  placed ahead of the sensor  162  or sensor array  170 . 
         [0043]    Electrode Materials. Several physical and chemical characteristics of the PtY and WO 3  may be examined. Transmission electron microscopy of PtY shown in the graph  190  of  FIG. 3  suggests that Pt clusters are highly dispersed and no clusters larger than 10 nm are found on the exterior of zeolite crystals. The Pt loading determined by elemental analysis (ICP-OES) is 4.36% wt. The surface area of PtY obtained by BET method is 443 m 2 /g. A sample of PtY is exposed under ambient conditions to NO or NO 2  in 5% O 2 . Thermal desorption of NO (peak at m/z=30) may be monitored and data for an exemplary sample  194  exposed to NO is shown in  FIG. 3  (similar results may be obtained with samples exposed to NO 2 ). Four desorption peaks at 100, 128, 270 and 335° C. are observed. The ability of PtY for equilibrating a mixture of 600 ppm NO 2  in 3% O 2  is carried out using the chemiluminescence NO x  analyzer as the detector (refer to the graph  200  of  FIG. 4 ). The equilibration of NO x  passing through PtY is complete at temperatures higher than 400° C. 
         [0044]    The SEM images  214  in  FIG. 5  shows that an average grain size of WO 3  is about 300 nm, and the XRD of WO 3  indicates a monoclinic structure (although the data not shown). The surface area given by the BET method is 2.76 m 2 /g for WO 3  heated at 700° C. After the co-adsorption of NO(NO 2 ) and O 2  on WO 3  at room temperature, NO evolution (m/z=30) may be monitored as a function of temperature and the resulting data for NO is shown in the graph  210  of  FIG. 5  (NO 2  gives similar results). There is no noticeable desorption of NO. The ability of WO 3  to equilibrate a mixture of 600 ppm NO 2  in 3% O 2  may be studied, and as shown in  FIG. 4 , there is no significant equilibration of NO x  even at 600° C. 
         [0045]    Sensor Characteristics. Sensors may be manufactured, or produced, according to  FIG. 2   a  with WO 3  sensing electrodes  178  and PtY/Pt reference electrodes  180 . The change in potential upon exposure to 1-800 ppm NO 2  and NO measured in 3% O 2  may be monitored and the data is shown in the graph  220  of  FIG. 6 . The potential displays a logarithmic relation to NO x  concentration. The slope of the NO 2  calibration curve is in the opposite direction as compared to NO. 
         [0046]    With the gases passing through the PtY filter  160 , NO 2  and NO with the same concentration generates almost the same signal on the sensor, as shown in  FIG. 6 . The calibration curves have better linearity than those without the PtY filter  160 . The temperature of the PtY filter  160  may be changed from 250 to 600° C. and the potential may be measured upon exposure of the sensor  162  to 10 ppm NO in 3% O 2 . As shown in the graph  230  of  FIG. 7 , with a larger temperature difference between the filter  160  and the sensor  162 , the signal from the sensor  162  is also enhanced. 
         [0047]    Performance of a single sensor  162  (refer to  FIG. 2   a ) and three sensors  162  linked in series (refer to the sensor array  170  of  FIG. 2   b ) may be compared. The calibration curves  242  and  244  in  FIG. 8  show that the signal magnitude is roughly triple for the 3-linked sensor array  170  as compared to a single sensor  162 . The increased signal indicates that the EMF is additive by connecting the sensors in series. 
         [0048]    Interferences. For cross interference studies, CO, CO 2 , NH 3 , propane, O 2 , and H 2 O may be introduced along with NO with and without the gases passing through the PtY filter. During the interference studies, the PtY filter  160  may be maintained at 400° C., and the sensor  162  may be maintained at 600° C. The data is shown for concentrations of NO between 1-13 ppm. Similar results may be obtained for NO 2  if the gases are passed through the PtY filter  162 . The relative error is defined as the change in potential with 10 ppm NO in 3% O 2  by itself and in the presence of the interfering gas. Table 1 summarizes exemplary results with the interfering gases. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Relative changes due to the presence of CO 2 , CO, propane, NH 3 , oxygen, 
               
               
                 and water on NO signal with and without a 400° C. PtY filter. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 CO 2   
                 CO 
                 Propane 
                 NH 3   
               
             
          
           
               
                   
                 20% 
                 30% 
                 1500 ppm 
                 2000 ppm 
                 500 ppm 
                 800 ppm 
                 10 ppm 
                 50 ppm 
               
               
                   
               
               
                 Without Filter 
                 — 
                 — 
                 −199% 
                 −252% 
                 −821% 
                 −1040% 
                 −299% 
                 −1135% 
               
               
                 With Filter 
                 0.9% 
                 0.2% 
                   0.2% 
                  −0.3% 
                  −0.5% 
                   0.1% 
                   2% 
                   28% 
               
               
                   
               
             
          
           
               
                   
                 O 2 * 
                 H 2 O** 
               
             
          
           
               
                   
                 4% 
                 7% 
                 10% 
                 5% 
                 10% 
                 15% 
               
               
                   
               
               
                 Without Filter 
                 73.5% 
                 39.2% 
                 16.7% 
                 — 
                 — 
                 — 
               
               
                 With Filter 
                 13.2% 
                   11% 
                  6.4% 
                 14.3% 
                 14.7% 
                 19.2% 
               
               
                   
               
               
                 *Compared with 13.5% oxygen 
               
               
                 **Compared with the dry condition 
               
             
          
         
       
     
         [0049]    CO 2  and CO interference. The graph  250  of  FIG. 9(   a ) shows exemplary results for NO and NO+CO 2  with gases passing through the PtY filter  160 . In particular, four scenarios are shown, with the first and third trace due to NO (1-13 ppm) and the second and fourth traces with NO and 20% and 30% CO 2 , respectively. Less than 1% error may be observed for 10 ppm NO in the presence of 20 to 30% CO 2 . In the case of CO, substantial interference is noted as shown in  FIG. 9(   c ), with the NO signal completely overwhelmed. When the gas stream is passed through the PtY filter  160 , less than 1% difference in signal is observed for 10 ppm NO in the presence of 1500 and 2000 ppm CO, and this data is shown in  FIG. 9   b.    
         [0050]    NH 3  interference.  FIG. 10   b  shows that NH 3  produces a strong response, with magnitude of the potential &gt;−100 mV with 50 ppm NH 3  and completely overwhelms the NO signal. An irreversible drop in the NO x  signal is also observed after passing 70 ppm NH 3  for one hour over the sensor  162  (refer to  FIG. 14 ), indicating that NH 3  modifies the electrode-electrolyte interface. Upon passing 50 ppm NH 3  and NO through the PtY filter  160 , 28% error is still observed for 10 ppm NO (refer to  FIG. 10   a ). However, with 10 ppm NH 3 , which is the expected level of NH 3  slip from controlled NO reduction/storage devices (e.g., in automobiles), the signal change is ˜2% as long as these gases pass through the PtY filter  160  (refer to the second data set in the graph  260  of  FIG. 10   a ). 
         [0051]    Propane interference. The graph  270  of  FIG. 11   c  shows that 800 ppm propane generates a signal of about −100 mV, significantly higher than NO. Note that the NO only signal is in the opposite direction than what is expected for pure NO (e.g., the first and third sets of data in  FIG. 9   c ), and appears only if propane is tested on the sensor  162  just prior to NO. After switching the gases through a PtY filter  160  at 400° C., 800 ppm propane does not cause significant interference (e.g., &lt;1% for 10 ppm NO; refer to the fourth data set in  FIG. 11   b ). However, the temperature of the PtY filter  160  may influence the ability to reduce or minimize propane interference.  FIG. 11  a shows substantial propane interference (e.g., &gt;100% for 10 ppm NO with 500/800 ppm propane; refer to the second and fourth data sets in  FIG. 11   b ) if the PtY filter  160  is maintained at 300° C. 
         [0052]    Oxygen interference. The graph  280  of  FIG. 12   a  compares the potential changes with concentration of NO 2  (10 ppm) in varying O 2  concentrations with and without the PtY filter. Plots  1 ,  2 ,  3  and  7 ,  8 ,  9  in the graph  290  of  FIG. 12   b  demonstrate the effect of oxygen on NO 2  and NO signals without the PtY filter. Plots  4 ,  5 ,  6  are for passing NO through the PtY filter. For 60 ppm NO 2 , a change of oxygen from 1 to 10% results in 90% relative change in signal. The interference decreases when the PtY filter is applied, as is evident from the graph  280  of  FIG. 12   a  and the graph  300  of  13 . The error is 4% for 60 ppm NO (refer to Plot  6 ) between oxygen levels of 1% and 10%. 
         [0053]    Water interference. In order to examine the effect of water, air may be bubbled through a water bottle and then mixed with other gases. The temperature of the water bottle may be adjusted from 40° C. to 70° C., and the water concentration in the test chamber is calculated from the saturated vapor pressure.  FIG. 13  indicates that there is a 14.3% increase in signal for 10 ppm NO when the water level is switched from totally dry to 5% relative humidity. Nevertheless, only 4.5% change is observed from 5% to 15% water. NO x  equilibration measurements on the PtY filter  160  do not show any change in catalytic activity after being in the stream of 15% water at 400° C. for 2 hours. 
         [0054]    Stability. The signal change over a one-week time period is shown in the graph  310  of  FIG. 14  for 1-13 ppm NO passing through a PtY filter  160  at 400° C. Tests with 70 ppm NH 3 , 2000 ppm CO, and 800 ppm propane may be performed over this period. Overall, the worst-case error is 7.5% for 10 ppm NO. After a 7-day test, 70 ppm NH 3  is passed over the sensor for one hour without the PtY filter  160 . A 20% decrease in signal may be observed for 10 ppm NO when the sensor is tested again with the PtY filter  160 , and is a permanent effect. 
         [0055]    Choice of Electrodes. One embodiment of the sensor structure is shown in  FIG. 2 , as described above. In some embodiments, no NO x  electrochemistry occurs at the reference electrode  180 , and the sensing electrode  178  is primarily responsible for the sensor signal. In the presence of NO x  and O 2 , the steady-state potential rises when the two redox reactions shown below occur simultaneously on the same electrode: 
         [0000]      O 2 +4 e   −           2O 2−   (1) 
         [0000]      2NO+2O 2−           2NO 2 +4 e   −   (2) 
         [0000]    where O 2−  represents an oxygen ion on YSZ. The measured potential may be referred as a non-Nernstian or mixed-potential because of the deviation from a typical Nernstian relation. Mixed-potential arises when a nonequilibrium state exists, involving two or more electrochemical reactions, and is the steady-state potential where the partial currents for each reaction (i cathodic +i anodic ) is equal to zero. 
         [0056]    When NO x  molecules adsorb on the sensor surface, they can either participate in the charge-transfer reaction (2), and in turn change the open circuit potential, or react with the adsorbed surface oxygen promoting the following reaction: 
         [0000]      2NO+O 2           2NO 2   (3) 
         [0057]    Reactions (2) and (3) compete with each other. On electrode surfaces where reaction (3) is predominant, NO x  is brought to thermodynamic equilibrium before the gas reaches the triple-point boundary. Since the NO/NO 2  is already in equilibrium, there is no driving force for the electrochemical reaction. Thus, there is a lack of an electrochemical signal. Such a material is appropriate for the reference electrode  180 . In contrast, the sensing electrode  178  for non-Nernstian sensors may have low catalytic activity for reaction (3). This relation is analogous to the anode reaction on solid oxide fuel cells. Non-electrochemical surface reactions could consume the fuel and result in lower open-circuit potential. 
         [0058]    Tungsten oxide has low catalytic activity toward NO x  equilibrium, according to NO x  conversion measurements and TPD data shown in  FIG. 5 . Additionally, fully oxidized WO 3  has weak interaction with NO x . Typically, nitrosyl species do not form upon NO adsorption on oxides with cations in their highest oxidation state, examples being V 5+ , W 6+ , and Mo 6+ , and is consistent with the TPD data in  FIG. 5  for WO 3 . On the other hand, W 4+  and W 5+  can form stable nitrosyl species with NO x . The absence of NO x  desorption features on WO 3  also implies that the substoichiometric component is not present in the WO 3 . Thus, WO 3  is a good choice for the sensing electrode  178  for potentiometric NO x  sensing at high temperatures. Other embodiments may use other materials for the sensing electrode  178 . 
         [0059]    The reference electrode  180  with Pt-loaded zeolite Y promotes reaction (3), and it is likely that NO and NO 2  will reach equilibrium upon passing through the PtY before reaching the triple phase boundary. TPD studies in  FIG. 3  show that the coadsorption of NO and O 2  on PtY forms a variety of adsorbed species. Studies on Pt catalysts indicate that the two low temperature desorption peaks (&lt;200° C.) can be attributed as arising from the support, while the peaks around 300° C. are characteristic of the interaction between Pt and NO x  species. No desorption is noted above 400° C. and so PtY is a good candidate for reference electrode  180 . Other embodiments may use other materials for the reference electrode  180 . 
         [0060]    Total NO x  sensing. As is clear from plots (b) and (c) in  FIG. 6 , both NO and NO 2  gives rise to the same calibration curve, essentially functioning as a total NO x  sensor. When NO or NO 2  passes through the PtY filter  160  in the presence of oxygen, an equilibrium mixture of NO and NO 2  is formed. The NO/NO 2  ratio depends only on the filter temperature when the oxygen level is fixed, as shown in  FIG. 4   d , (e.g., in 3% oxygen, NO 2  is 37.7% of total NO x  at 400° C. and only 5.3% at 600° C.). Thus, a NO/NO 2  equilibrated mixture emerging from the PtY filter at 400° C., upon contact with a sensor  162  at 600° C., generates a “NO 2 -like” signal as the new equilibrium is attained at 600° C. (NO 2  converting to NO) and is the basis for the total NO x  sensing, since the filter  160  equilibrates any mixture of NO/NO 2 . If the filter  160  is at higher temperatures than the sensor  162 , “NO-like” signal is generated. 
         [0061]    Interference from Oxidizing Gases. CO, NH 3 , and hydrocarbons can react with lattice oxygen ions in YSZ via the following reactions and generate a mixed-potential response, as indicated by the data in  FIGS. 9-11 . 
         [0000]      2CO+2O 2−           2CO 2 +4 e   −   (4) 
         [0000]      H x C y +( x/ 2+2 y )O 2−           ( x/ 2)H 2 O+ y CO 2 +( x+ 4 y ) e   −   (5) 
         [0000]      2NH 3 +3O 2−           N 2 +3H 2 O+6 e   −   (6) 
         [0062]    Reaction (6) is only one of the possible pathways for the reaction of NH 3 . The standard oxidation potential of CO, hydrocarbon, and NH 3  is significantly higher than NO, implying that a small amount of CO, NH 3 , or hydrocarbons can totally overwhelm the signal from NO x , as noted in  FIGS. 9   c ,  10   b , and  11   c.    
         [0063]    Supported platinum catalysts are known for promoting CO, NH 3 , and hydrocarbon oxidation, following the reaction pathways (7)-(9). Again, N 2  formation in reaction (9) is only one of the possible products from NH 3  oxidation. 
         [0000]      2CO+O 2           2CO 2   (7) 
         [0000]      H x C y +( x/ 4 +y )O 2           ( x/ 2)H 2 O+ y CO 2   (8) 
         [0000]      4NH 3 +3O 2           2N 2 +6H 2 O  (9) 
         [0064]    In order to minimize interference from CO/propane/NH 3  on the NO x  signal, in some embodiments, PtY is used to drive reactions (7)-(9) to completion. Also, the reactions between NO x  and propane/NH 3 , often referred as selective catalytic reduction (SCR), may be negligible compared with reactions (8)-(9), since the reaction of NO influence the NO x  sensor signal. 
         [0065]    For NH 3 , there may be several oxidization paths. Pt-based catalysts can oxidize NH 3  directly to NO x  and N 2 O with the product ratio depending on the property of catalysts. If NO x  is produced from NH 3  oxidation, it will increase the signal, and is probably the reason for increase in signal of 28% with 50 ppm NH 3 , as shown in  FIG. 10   a . However, the fact that the signal with 50 ppm NH 3  is not much stronger indicates that N 2  and N 2 O may also be forming. In some embodiments, N 2  is not formed at 400° C., suggesting that N 2 O may be the primary product when passing through the PtY filter  160 . This also implies that N 2 O does not generate pronounced signal on WO 3  electrodes at 600° C. Less than 2% error may be observed from 10 ppm NH 3 , and would be suitable for most practical applications. 
         [0066]    For propane, the temperature window of selective reduction with supported Pt catalysts is about 200-300° C. At 400° C., propane should be oxidized by O 2  instead of NO x . As can be seen in  FIG. 11   b , the PtY filter at 400° C. reduces the error due to 800 ppm propane to less than 1%. However, with the PtY filter  160  at 300° C., the signal observed in  FIG. 11   a  implies that propane might not be fully oxidized, or NO x  is reduced to N 2 /N 2 O by SCR reactions. For the Pt-based catalysts at 230° C. under stoichiometric condition, NO even as low as 1 ppm can effectively prohibit propane oxidation. 
         [0067]    Another advantage of the reaction of NH 3  on the PtY filter is that it provides protection against electrode microstructure degradation and change of surface stoichiometry caused by reactive NH 3  gas. As can be seen in  FIG. 14 , exposing the sensor under 70 ppm NH 3  for one hour without the PtY filter  160  may cause permanent decrease in NO signal. Thus, with a PtY filter at 400° C., CO/propane/NH 3  can be converted to less reactive CO 2 , H 2 O and N 2 O. In addition, signal drift is minimal. 
         [0068]    Oxygen Interference. Among all gases in the combustion environment, O 2  interference is probably the most difficult to overcome because oxygen is involved in both the ionic conduction process and catalytic NO x  conversion. The concentration of oxygen is typically more than 100 times higher than NO x , and the fluctuation is also large. In many designs of NO x  sensors, oxygen is pumped out by an additional pair of electrodes to reach a low level before the gas mixture reaches the sensing electrode  162 . An additional oxygen sensor may be applied to correct the error from oxygen fluctuations. 
         [0069]    For the filter/sensor, the fluctuation of oxygen from 1 to 13.5% shows errors of 4-14% on the NO x  signal when the PtY filter  160  is applied (refer to  FIG. 12   b ). The use of the PtY filter  160  reduces the oxygen interference since NO/NO 2  equilibration at 400° C. reduces or minimizes the further effect of O 2  reequilibrating the reaction. For example, 100 ppm NO 2  without a filter in 1% O 2  equilibrates at the 600° C. electrode to 3 ppm NO 2 /97 ppm NO and at 20% O 2  to 13 ppm NO 2 /87 ppm NO, thereby producing a lower signal at the higher O 2  concentrations (refer to  FIG. 12   a ). If the 100 ppm NO first goes through a 400° C. filter, then at 1% O 2 , 26 ppm NO 2 /74 ppm NO reacts at the 600° C. electrode and, in 20% O 2 , the gas mixture is 61 ppm NO 2 /39 ppm NO, indicating that the signals are smaller and more comparable, consistent with  FIG. 12 . This also suggests that the O 2  interference increases as the filter temperature is lowered. 
         [0070]    Strategies to Increase Sensitivity. There are two strategies to increase sensitivity in the present filter/sensor design. The first is to increase the temperature difference between the sensor  162  and the filter  160 , as shown in  FIG. 7 . The result can be understood by simple free energy calculations based on reaction (3) and outlined in equation (1). If the filter  160  temperature is at 400° C. and sensor  162  is at 600° C., then the reaction quotient Q in the equation is equal to the equilibrium constant at 400° C. The change in free energy on the surface of a 600° C. sensor  162  can be written as: 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     G 
                   
                   = 
                     
                    
                   
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         G 
                         600 
                         ° 
                       
                     
                     + 
                     
                       RT 
                        
                       
                           
                       
                        
                       ln 
                        
                       
                           
                       
                        
                       Q 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         G 
                         600 
                         ° 
                       
                     
                     + 
                     
                       RT 
                        
                       
                           
                       
                        
                       ln 
                        
                       
                           
                       
                        
                       
                         K 
                         400 
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     
                       
                         - 
                         RT 
                       
                        
                       
                           
                       
                        
                       ln 
                        
                       
                           
                       
                        
                       
                         K 
                         600 
                       
                     
                     + 
                     
                       RT 
                        
                       
                           
                       
                        
                       ln 
                        
                       
                           
                       
                        
                       
                         K 
                         400 
                       
                     
                   
                 
               
             
           
         
       
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     G 
                   
                   = 
                     
                    
                   
                     RT 
                      
                     
                         
                     
                      
                     
                       ln 
                        
                       
                         ( 
                         
                           
                             K 
                             400 
                           
                           
                             K 
                             600 
                           
                         
                         ) 
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     34.68 
                      
                     
                         
                     
                      
                     kJ 
                      
                     
                       / 
                     
                      
                     mole 
                   
                 
               
             
           
         
       
     
         [0071]    The positive value shows that the signal on the sensor  162  is being produced due to the reduction of NO 2  on the sensing electrode  178  (reverse of reaction 3). This value rises as the filter temperature is lowered, leading to a greater driving force for the reaction. 
         [0072]    A second method to increase sensitivity is by connecting sensors in series, as exemplified with the three-sensor array shown in  FIG. 8 . In principle, many potentiometric sensors can be connected in series (analogous to a battery) to increase sensitivity, as long as the internal impedance of the electrometer is high enough to handle the sensor impedance. For low NO x  detection (e.g., &lt;ppm), a single sensor should generate a measurable response for NO x  (e.g., ten times the instrument noise level of background). Thus, the proper selection of sensing and reference electrode is useful for developing linked sensors in an array. 
         [0073]    Catalytic activity measurements and temperature programmed desorption indicate that WO 3  is almost inactive toward NO x  equilibration and no chemisorbed NO x  species are released from the WO 3  surface. On the contrary, PtY has much higher activity toward NO x  equilibration. The dissimilar catalytic activity of PtY and WO 3  may be exploited to fabricate compact solid-state potentiometric sensors using PtY/Pt as the reference electrode and WO 3  as the sensing electrode. The use of a PtY filter makes it possible to measure total NO x . Additionally, interferences from CO, propane, NH 3 , H 2 O and CO 2  may be reduced or minimized. The PtY filter  160  also provides protection against irreversible changes at the electrode-electrolyte interface from reactions with NH 3 . By connecting multiple (e.g., three) sensors in series, the sensitivity is improved by a corresponding factor (e.g., three) and allows for sub-ppm total NO x  detection. 
         [0074]      FIG. 15  illustrates a schematic flow chart diagram of depicting one embodiment of a method  320  of making a sensor array with multiple sensors. Although the method  320  is described in conjunction with the sensor of  FIG. 2  and the sensor array of  FIG. 3 , other embodiments of the method  320  may be implemented in conjunction with other sensors and sensor arrays. 
         [0075]    In the depicted embodiment, a plurality of electrolyte substrates are disposed  322  on a basic substrate. A sensing electrode is disposed  324  on each electrolyte substrates. A reference electrode is also disposed  326  on each electrolyte substrate. He sensing electrodes are then connected to  328  to the reference electrodes on the adjacent electrolyte substrates so that the sensors are connected together in series within a sensor array. The illustrative method  320  then ends. 
         [0076]    Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that the described feature, operation, structure, or characteristic may be implemented in at least one embodiment. Thus, the phrases “in one embodiment,” “in an embodiment,” and similar phrases throughout this specification may, but do not necessarily, refer to the same embodiment. 
         [0077]    Furthermore, the described features, operations, structures, or characteristics of the described embodiments may be combined in any suitable manner. Hence, the numerous details provided here, such as examples of electrode configurations, housing configurations, substrate configurations, channel configurations, catalyst configurations, and so forth, provide an understanding of several embodiments of the invention. However, some embodiments may be practiced without one or more of the specific details, or with other features operations, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in at least some of the figures for the sake of brevity and clarity. 
         [0078]    Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. The scope of the invention is to be defined by the claims appended hereto and their equivalents.