Abstract:
A planar device includes a heating circuit that is disposed between ceramic layers in a planar device and co-fired with the ceramic. The heating circuit material and geometry are controlled so as to provide a targeted resistance characteristic as a function of temperature that allows interchangeability in an engine management system that was designed for a heater circuit based on a material system that cannot be co-fired with the planar device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 61/351,348, filed Jun. 4, 2010, the contents of which are hereby incorporated by reference. This application additionally claims priority to U.S. Provisional Application Ser. No. 61/351,396, filed Jun. 4, 2010, the contents of which are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to a heating circuit in a co-fired planar oxygen sensor that can be used to replace an oxygen sensor in existing applications without requiring vehicle recalibration. 
       BACKGROUND OF THE INVENTION 
       [0003]    Current planar oxygen sensors use ceramic tapes and appropriate metallizations to form a sensor structure. Tapes (ceramic substrates) include insulating materials (alumina or zirconia) and an electrolyte (zirconia) in addition to metallizations to form functional Nernst cell exhaust sensors. Such planar devices are formed as multi-layer co-fired ceramic circuits, where all components are assembled in “green” (unfired) state, laminated to form a contiguous structure, and co-fired at temperatures appropriate for densification of ceramic body and formation of a monolithic structure after sintering. Due to the high sintering temperatures required for the ceramic substrates (1400°-1600° C.), metallization is limited to PGM (Platinum Group Metals) materials. Exhaust Oxygen sensors typically use platinum for the heater circuit in the co-fired device. Platinum has a TCR (Temperature Coefficient of Resistance) of around 3850 ppm/K. This gives Pt heater circuits a very specific electrical signature. 
         [0004]    Previous oxygen sensor technologies utilized conical (thimble) elements with a separate heating element comprised of a tungsten alloy co-fired with an alumina ceramic. Zirconia and tungsten cannot be co-fired. The high-temperature oxidation characteristics of tungsten dictate that a reducing atmosphere is required for sintering. However, zirconia requires an oxidizing atmosphere to prevent reduction of the oxide to its metallic form and so tungsten is not a good choice for co-firing with zirconia. 
         [0005]    As conical exhaust sensors go out of regular production, a market has emerged for replacement sensors. EMS (Engine Management System) diagnostics and heater controls for conical sensors in many applications depend on the TCR of the heater circuit for proper function, as the system controls are based on heater resistance at operating temperature. For a consistent sensor performance, many applications measure the heater current during a cold start after 8 hours or more soaking time. From the heater current measure, the resistance of the heater can be calculated since the current and voltage supply is known (Ohm&#39;s Law). Based on the cold heater resistance, the heater duty cycle is controlled for a high or low heater resistance to maintain a desire element tip temperature. The TCR of the tungsten alloy used in heaters for conical exhaust sensors is much lower than the TCR of the platinum typically used in planar oxygen sensors, making it difficult for a co-fired planar sensor to match the electrical characteristics of a conical sensor sufficiently closely to enable direct replacement without reprogramming the EMS. 
         [0006]    Accordingly, a need exists in the sensor art for a co-fired planar exhaust oxygen sensor that can replicate the characteristics of a conical exhaust oxygen sensor. 
       SUMMARY 
       [0007]    Disclosed herein is a heater circuit for a co-fired planar exhaust sensor that matches the characteristics of a conical exhaust sensor. The heater circuit alloy and the heater circuit geometry are both controlled to achieve a target effective base resistance and effective TCR. The heater circuit is sufficiently matched to the base resistance and TCR of a tungsten alloy heater used with a conical sensor such that an EMS that is calibrated to the characteristics of the tungsten alloy heater can operate with the co-fired planar exhaust sensor with no recalibration. 
         [0008]    The above described and other features are exemplified by the following figures and detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Refer now to the Figures, wherein like elements are numbered alike. 
           [0010]      FIG. 1  is a plan view of a heater circuit in a planar device. 
           [0011]      FIG. 2  is a plot showing the temperature profile over a heater circuit. 
           [0012]      FIG. 3  is an equivalent electrical circuit for a heater circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    At the outset of the description, it should be noted that the terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another, and the terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., includes the degree of error associated with measurement of the particular quantity). It is noted that the terms “bottom” and “top” are used herein, unless otherwise noted, merely for convenience of description, and are not limited to any one position or spatial orientation. Furthermore, all ranges disclosed herein are inclusive and combinable (e.g., ranges of “up to about 25 weight percent (wt. %), with about 5 wt. % to about 20 wt. % desired, and about 10 wt. % to about 15 wt. % more desired,” are inclusive of the endpoints and all intermediate values of the ranges, e.g., “about 5 wt. % to about 25 wt. %, about 5 wt. % to about 15 wt. %”, etc.). Finally, unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. The suffix “(s)” as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term (e.g., the metal(s) includes one or more metals). 
         [0014]    An exemplary heater  10  as used in a planar oxygen-sensing element is shown in  FIG. 1 . The sensor comprises an electrically conductive material disposed on a substrate  14  in a heater circuit  12 . The heater circuit  12  shown in  FIG. 1  includes a first contact pad  16  connected to one end of a first lead  22 . The other end of the lead  22  connects to one end of a serpentine pattern  20 . The other end of the serpentine pattern  20  connects to a first end of a second lead  24 . The other end of the second lead  24  connects to a second contact pad  18 . It will be appreciated that the first contact pad  16 , first lead  22 , serpentine pattern  20 , second lead  24 , and second contact pad  18  are not required to be distinct elements, but rather may refer to segments of a single continuous element. It will also be appreciated that reference to first or second ends of a segment refers to a location where an electrical connection is made and is not limited to a location that is spatially opposite another location on the segment. 
         [0015]    The heater circuit  12  is operated by connecting a source of electric power to contact pads  16  and  18 . It will be appreciated that the electrically conductive material has an associated electrical resistance, which is a function of the resistivity of the material and of the geometry of the heater circuit. As electrical current passes through the heater circuit  12 , power is dissipated in the heater circuit through resistive heating according to the relationship P=I 2 R, where P is the power, I is the current, and R is the resistance. This power dissipation becomes thermal energy, raising the temperature of the heater and any other elements that are in thermal communication with the heater. The heater circuit  12  is designed such that a desired temperature distribution is obtained. In an exemplary embodiment, serpentine pattern  20  is located close to the electrochemical cell. The exemplary heater circuit  12  is designed so that the maximum heating is achieved in the vicinity of serpentine pattern  20 . In such a way, the heater can be used to heat the electrochemical cell in an exhaust oxygen sensor to a temperature required by the electrochemical cell to produce a usable output voltage. 
         [0016]    It will be appreciated that heat will be produced as a result of current flow at each incremental segment of heater circuit  12 , and that the sum of the heat contributions of each incremental segment will contribute to an overall temperature profile over the entire substrate area.  FIG. 2  illustrates an exemplary temperature profile at the end of substrate  14  where the serpentine pattern  20  of heater circuit  12  is located, indicating temperatures obtained by passing a particular level of current through the heater circuit  12  shown in  FIG. 1  at a particular ambient temperature. In  FIG. 2 , points lying along the line marked  510  indicate the locations on substrate  14  where the temperature is 510° C. Similarly, line  520  on  FIG. 2  indicates points having a temperature of 520° C., line  530  on  FIG. 2  indicates points having a temperature of 530° C., line  540  indicates points that are at a temperature of 540° C., line  550  indicates points at 550° C., lines  570   a  and  570   b  indicate points that are at 570° C., lines  580   a  and  580   b  indicate points that are at 580° C., and lines  590   a  and  590   b  indicate points that are at 590° C. The actual thermal profile for a heater circuit depends on many factors, including the ambient temperature, the material used to form the heater circuit, the voltage level applied to the heater circuit, and the geometry of the conductor pattern that defines the heater circuit. 
         [0017]    In addition to having an associated resistivity, the electrically conductive material has an associated temperature coefficient of resistivity (TCR). The TCR of a material is commonly referred to as alpha (a), and the resistance of a resistive element at a temperature T can be described as R(T)=R 0 (1+α(T−T 0 )), where T is the temperature at which the resistance R(T) is measured and R 0  is the resistance of the resistive element at a reference temperature T 0 . Metals typically have a positive TCR, meaning that the resistance increases with increasing temperature. To provide a heater circuit that can be used to replace a tungsten alloy heater used in earlier generation conical oxygen sensor without requiring engine management system recalibration, a palladium-rhodium alloy was found to provide a compatible TCR. More particularly, to achieve the targeted characteristics in an exemplary embodiment, an alloy comprising about 95% palladium and 5% rhodium was found to be suitable. 
         [0018]      FIG. 3  shows a simplified electrical schematic equivalent circuit for the heater circuit in  FIG. 1 . In  FIG. 3 , heater circuit  12  is modeled as having seven resistive segments RA, RB, RC, RD, RE, RF, and RG connected electrically in series between the first contact pad  16  and the second contact pad  18 . It is to be noted that the choice of seven segments is merely for convenience, and is in no way to be construed as limiting. 
         [0019]    The total resistance indicated between contact pads  16  and  18  is the sum of the individual resistances. For the example depicted in  FIG. 3 , 
         [0000]      Rtotal= RA+RB+RC+RD+RE+RF+RG    
         [0020]    It must be noted that each resistive segment that comprises the total resistance has an associated TCR, and is operating at its own associated temperature as depicted in  FIG. 2 . Assuming the TCR has the same value a for each resistive segment, the resistance of each segment can be determined as: 
         [0000]        RA=RA   0 (1+α( T   A - T   0 ))
 
         [0000]        RB=RB   0 (1+α( T   B - T   0 ))
 
         [0000]        RC=RC   0 (1+α( T   C - T   0 ))
 
         [0000]        RD=RD   0 (1+α( T   D - T   0 ))
 
         [0000]        RE=RE   0 (1+α( T   E - T   0 ))
 
         [0000]        RF=RF   0 (1+α( T   F - T   0 ))
 
         [0000]        RG=RG   0 (1+α( T   G - T   0 ))
 
         [0000]    where:
       RA 0  is the resistance of RA at a temperature T 0 , and T A  is the temperature of RA;   RB 0  is the resistance of RB at a temperature T 0 , and T B  is the temperature of RB;   RC 0  is the resistance of RC at a temperature T 0 , and T C  is the temperature of RC;   RD 0  is the resistance of RD at a temperature T 0 , and T D  is the temperature of RD;   RE 0  is the resistance of RE at a temperature T 0 , and T E  is the temperature of RE;   RF 0  is the resistance of RF at a temperature T 0 , and T F  is the temperature of RF;   RG 0  is the resistance of RG at a temperature T 0 , and T G  is the temperature of RG.       
 
         [0028]    If the temperature at the locations of each of the resistive segments are known (for example by knowing a temperature profile as exemplified in  FIG. 2 ), the total resistance can be determined by performing the calculations indicated in the foregoing equations. It will be appreciated that actual analysis may involve fewer or more resistive segments than the seven segments shown in the illustrative example of  FIG. 3 . 
         [0029]    It is known to one skilled in the art that the electrical resistance of a resistive element is given by the relationship R=ρL/A, where the bulk resistivity ρ is a material property of the resistive material, L is the length of the resistive element in the direction of current flow, and A is the cross sectional area of the resistive element perpendicular to the direction of current flow. To achieve a heater circuit characteristic that allows replacement of a tungsten alloy heater circuit, it will be appreciated that the resistance of each segment that forms a heater circuit can be adjusted so that the sum of the resistances, each of which is at its own distinct temperature, produces a targeted total resistance when measured between contact pad  16  and contact pad  18 . The resistance of a resistive segment can be changed by changing its length and/or by changing its cross sectional area. Changing the cross sectional area can be achieved by changing the thickness and/or the width of the resistive segment. In the exemplary embodiment shown in  FIG. 1 , it can be seen that the width of lead segment  22  and lead segment  24  are each tapered from a narrow width near the serpentine segment  20  to a wider width near the contact pads  16 ,  18  to achieve a desired heater circuit characteristic. 
         [0030]    It will also be appreciated that the temperature of a given resistive segment may be influenced by the resistance of the segment, as the electrical power that is converted to heat is related to the resistance by the relationship P=I 2 R, where P is the power in watts, I is the current in amperes, and R is the resistance in ohms. It is to be noted that the resistance depends on the temperature (because of TCR effects), the temperature depends on the power dissipation (because of the conversion of electrical energy to thermal energy), and the power dissipation depends on the resistance (because of the relationship between power, current, and resistance). Accordingly, an iterative process may be required to produce a heater circuit having a desired total resistance when measured between the contact pads  16 ,  18  at a given level of heater drive voltage or current. 
         [0031]    An engine management system may be programmed to perform diagnosis of the proper condition of a heater circuit. Diagnosis may include providing a predetermined voltage to the heater circuit and measuring the current flowing through the heater circuit to determine the resistance of the heater circuit. It will be appreciated that the resistance of the heater circuit is not a constant value, but is dependent on the temperature of the resistive material that is included in the heater circuit. An engine management system may be calibrated based on characteristics of a particular heater circuit, where the characteristics include a particular heater circuit material and a particular heater circuit geometry. The engine management system may provide a predetermined voltage to a heater circuit and provide indication of a heater circuit fault if the current flow resulting from the application of the predetermined voltage does not fall within predetermined limits. The present invention provides a heater circuit that can be used as a drop-in replacement in an engine management system without necessitating recalibration of the engine management system diagnostic characteristics by matching the electrical characteristics of a particular heater circuit (e.g. a tungsten rod heater in a conical oxygen sensor) by controlling the composition (e.g. palladium rhodium alloy) and geometry (e.g. cross sectional area as a function of location on the substrate) of a heater circuit in a planar sensor. 
         [0032]    While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation of material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as including the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.