Abstract:
Ceramic exhaust gas sensors are disclosed that offer enhanced dimensional stability during curing, with reduced occurrence of deformations like bending or warping, and can be used in a variety of exhaust gas component sensing applications. The sensors of the invention utilize appropriate selection and orientation of the various layers of green ceramic tape that make up the sensor structure to provide enhanced dimensional stability.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to sensors used to detect various constituents (e.g., oxygen, ammonia, hydrogen, nitrogen oxides, carbon monoxide, hydrocarbons, etc.) in combustion exhaust, such as the exhaust from internal combustion engines. Such sensors often include a flat or planar sensing element having multiple ceramic layers that provide for fluid flow on and through the sensor element and on which are disposed various components such as sensing electrodes, ground or reference electrodes, resistors, heater elements, and the like used to detect constituents of interest. Planar ceramic sensor elements are often manufactured by forming a multilayer element of layers of uncured ceramic material known as ceramic tape using known ceramic tape casting methods. Alternative methods may also be used, such as die pressing, roll compaction, stenciling, screen printing and the like. Electrodes and similar components may be disposed onto any of the various layers of ceramic tape before additional layers are placed over the top. Metal to form the electrodes can be applied onto a ceramic layer by known techniques, such as sputtering, vapor deposition, screen printing, or stenciling. 
         [0002]    Ceramic layers in exhaust gas sensing elements may be used as solid electrolytes, across which gas ions (e.g., oxygen ions) can move as part of the detection mechanism. Although various materials may function as a solid electrolyte, zirconia (e.g., yttria stabilized zirconia, calcia stabilized zirconia, magnesia stabilized zirconia) is most often used due to its compatibility with extreme environments. Ceramic layers in exhaust gas sensing elements may also be used as dielectric materials to separate various components (i.e., an insulating layer), protect portions of the sensor (i.e., a protective layer), and/or to enhance the structural integrity of the sensing element. Although various materials may function as a dielectric material, alumina (e.g., alpha alumina) is often used due to its compatibility with extreme environments. 
         [0003]    After the sensing element, including the various electrodes and other components, is formed from uncured or ‘green’ ceramic layers, the element is cured or hardened by a sintering process in which it is heated to temperatures of 1375° C. to 1575° C. for periods of 1 to 3 hours. During this sintering process, the sensing element may be subject to undesirable physical deformation of the element, which can, in extreme cases, render it unusable. It would therefore be desirable to provide ceramic sensing elements for exhaust gas sensors that could be manufactured using known techniques, but which do not suffer from undesirable physical deformation during sintering. 
       SUMMARY OF THE INVENTION 
       [0004]    Therefore, according to the present invention, there is provided a flat ceramic exhaust gas sensor comprising a ceramic layer structure including one or more solid electrolyte layers, one or more insulating layers, and two or more electrodes, wherein the ceramic layer structure has:
       (a) a flat central layer structure having a top side and a bottom side, said central layer structure selected from the group consisting of
           (1) a single alumina insulating layer;   (2) two alumina insulating layers of equal thickness;   (3) a single zirconia layer; and   (4) two zirconia layers of equal thickness;   
           (b) on each of the sides of the central layer structure, in order:
           (1) optionally, a first layer structure having a first predetermined thickness, selected from the group consisting of:
               (i) one or more zirconia layers if the central layer structure is one or two zirconia layers;   (ii) one or more alumina insulating layers if the central layer structure is one or two alumina insulating layers;   
               (2) a second layer structure having a second predetermined thickness, selected from the group consisting of:
               (i) one or more zirconia layers if the central layer structure is one or two alumina insulating layers, and   (ii) one or more alumina insulating layers if the central layer structure is one or two zirconia layers;   
               (3) optionally, a third layer structure having a third predetermined thickness, selected from the group consisting of:
               (i) one or more alumina insulating layers if the second layer structure (b)(1) is one or more zirconia layers; and   (ii) one or more zirconia layers if the second layer structure (b)(1) is one or more alumina insulating layers;   
               (3) if the third layer structure is present and is one or more zirconia layers, then optionally, a fourth layer structure having a fourth predetermined thickness, comprising one or more alumina insulating layers; and   (4) an alumina protective layer having a fifth predetermined thickness.   
               
 
         [0022]    Ceramic exhaust gas sensors according to the present invention offer enhanced dimensional stability during curing, with reduced occurrence of deformations like bending or warping, and can be used in a variety of exhaust gas component sensing applications. 
         [0023]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0025]      FIG. 1  is an exploded perspective view of a sensing element according to the invention that can be used for sensing ammonia and/or NO x . 
           [0026]      FIG. 2  is an exploded perspective view of a sensing element according to the invention that can be used for sensing ammonia and/or NO x . 
           [0027]      FIG. 3  is an exploded perspective view of a sensing element according to the invention that can be used for sensing oxygen. 
           [0028]      FIG. 4  is an exploded perspective view of a sensing element according to the invention that can be used for sensing oxygen. 
           [0029]      FIG. 5  is an exploded perspective view of a sensing element according to the invention that can be used for sensing oxygen. 
           [0030]      FIG. 6  is an exploded perspective view of a ceramic layer structure that can be utilized in exhaust gas sensor elements according to the invention. 
           [0031]      FIG. 7  is an exploded perspective view of a ceramic layer structure that can be utilized in exhaust gas sensor elements according to the invention. 
           [0032]      FIG. 8  is an exploded perspective view of a ceramic layer structure that can be utilized in exhaust gas sensor elements according to the invention. 
           [0033]      FIG. 9  is an exploded perspective view of a ceramic layer structure that can be utilized in exhaust gas sensor elements according to the invention. 
           [0034]      FIG. 10  is an exploded perspective view of a ceramic layer structure that can be utilized in exhaust gas sensor elements according to the invention. 
           [0035]      FIG. 11  is an exploded perspective view of a ceramic layer structure that can be utilized in exhaust gas sensor elements according to the invention. 
           [0036]      FIG. 12  is an exploded perspective view of a ceramic layer structure that can be utilized in exhaust gas sensor elements according to the invention. 
           [0037]      FIG. 13  is an exploded perspective view of a ceramic layer structure that can be utilized in exhaust gas sensor elements according to the invention. 
           [0038]      FIG. 14  is an exploded perspective view of a prior art sensing element used as in a comparative example. 
           [0039]      FIG. 15  is a set of photographic edge views of layered element structures showing a comparison of deformation observed during curing for elements according to  FIG. 2  versus prior art elements according to  FIG. 14 . 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    Referring now to the Figures, where the invention will be described with reference to specific embodiments, without limiting same. 
         [0041]    It should be noted that the terms “first,” “second,” and the like herein do not denote any order 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. 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.). 
         [0042]    Turning now to  FIG. 1 , an exploded perspective view is shown of an exemplary embodiment of a sensing element structure according to the invention that can be used for sensing ammonia and/or NO x . 
         [0043]    Alumina protective layer L 9  is configured with a heater element  1  thereon. The heater  1  can be any heater capable of maintaining the sensor end of the ammonia sensor element at a sufficient temperature to enable the sensing of ammonia. The heater  1  can comprise platinum, palladium, tungsten, molybdenum, and the like, or alloys or combinations comprising at least one of the foregoing, or any other heater compatible with the environment. The heater  1  can be printed (e.g., thick film printed) onto the alumina layer a sufficient thickness to attain the desired resistance and heating capability. The heater thickness can be, for example, about 10 micrometers to about 50 micrometers, or so. 
         [0044]    Alumina layer L 8  is shown configured with EM shield  2 , although the shield  2  may be disposed anywhere between the heater  1  and the other components that could be subject to EM interference from the heater  1 . The shield  2  can comprise, for example, a closed layer, a line pattern (connected parallel lines, serpentine, and/or the like), and/or the like. The shield  2  can comprise any material capable of enhancing the electrical isolation of the heater from the temperature sensor. Possible shield materials include precious metal (such as platinum (Pt), palladium (Pd), gold (Au) and the like, as well as alloys and combinations comprising at least one of the foregoing materials. Zirconia layer L 7  is disposed over alumina layer L 8 , and alumina insulating layer L 6  is disposed over zirconia layer L 7 . 
         [0045]    Zirconia layer L 5 , which serves as the central layer structure in this embodiment of the invention, is configured with an impedance electrode  3  that functions as a resistance temperature detector to measure temperature on the sensing end of the sensor element. Potential materials for the temperature electrode  3  can be any material having a sufficient temperature coefficient of resistance to enable temperature determinations, and have a sufficient melting point to withstand the co-firing temperature (e.g., of about 1,400 degree. C. or so). Some possible materials include those employed for the heater  1 . The temperature sensor can comprise a serpentine portion with a line width of less than or equal to about 0.15 mm. Alumina insulating layer L 4  is disposed over zirconia layer L 5 . 
         [0046]    The exhaust component sensing section of the element comprises the ammonia sensing electrode  6  and backing electrode  7 , along with NOx sensing electrode  5  disposed on alumina insulating layer in ionic communication with zirconia solid electrolyte layer L 3 . On the opposite side of solid electrolyte layer L 3  from the sensing electrodes is reference electrode  4 . The element also has gas flow channels between layers L 3  and L 4  for reference gas (which in this case is same as the exhaust gas being sensed), and also gas flow channels on each side of layer L 5  for enhancing the responsiveness of the temperature sensor. Electrically conductive pads  8  are disposed on the outside of protective layer L 1  to be in electrical contact with the sensing electrodes  5  and  6 , the reference electrode  4 , and one of the impedance electrodes  3  through vias (not shown) in the layers. Electrically conductive pads  9  are disposed on the outside of protective layer L 9  to be in electrical contact with the heater element  1  and the other of the impedance electrodes  3  through vias (not shown) in the layers. Alumina protective layer L 1  is shown as not extending over the sensing electrodes  5  and  6 ; however, layer L 1  may also include a porous section that can extend over the sensing electrodes. Each of the ceramic layers L 1 -L 9  in  FIG. 1  has an identical cured thickness of 172 μm. 
         [0047]      FIG. 2  represents an exploded perspective view of an alternative exemplary embodiment of a sensing element structure according to the invention that can be used for sensing ammonia and/or NO x . The electrode structure and function for this element is the same as for  FIG. 1 . In the structure of  FIG. 2 , the layer structure is different than that of  FIG. 1 , with the central structure having two alumina layers L 14  and L 15 , the first layer structure not present, the second layer structure having a single zirconia layer L 13 , L 16  on each side of the central layer structure, the third layer structure having a single alumina layer L 12 , L 17  disposed on each layer of the second layer structure, and the fourth layer structure has a single alumina insulating layer L 11 , L 18  disposed on each layer of the third layer structure. Layers L 11  and L 18  each represents an alumina protective layer. Each of the layers L 11  through L 18  has a cured thickness of 172 μm. Layer L 13  functions as a solid electrolyte layer by selectively allowing oxygen ions to pass through it during operation. Other components disposed on or between the ceramic layers of the sensing element are as described for  FIG. 1 . 
         [0048]    Each of  FIGS. 3-13  represents exploded perspective views of exemplary alternative embodiments of ceramic layer structures for exhaust gas sensing elements of the invention. Unlike  FIGS. 1-3 , components necessary for sensing exhaust gas components (e.g., sensing electrodes, reference electrodes, impedance electrodes, heater elements, and the like) are not shown in these Figures, as one skilled in the art would readily be able to configure the layer arrangements shown in  FIGS. 3-13  with such components using design and manufacturing techniques well-known in the art. Accordingly,  FIGS. 3-13  show only the ceramic layer structures of such alternative exemplary embodiments. 
         [0049]    Ceramic layer structures according to the present invention such as those shown in  FIGS. 3 ,  4 , and  5  may be adapted for use in sensing oxygen in combustion exhaust. The principles by which such a sensor operates, along with materials and methods for its manufacture, are described in detail in U.S. Pat. Nos. 5,384,030, 6,555,159, 6,572,747, and 7,244,316, the disclosures of which are incorporated herein in their entirety. Turning now to  FIG. 3 , in this exemplary embodiment, the central layer structure has two alumina insulating layers L 33  and L 34 , the first layer structure is not present, the second layer structure has a single zirconia layer L 32 , L 35  on each side of the central layer structure, and the third and fourth layer structures are not present. Layers L 31  and L 36  each represents an alumina protective layer. Each of layers L 31  through L 36  has an identical thickness of 172 micrometers. Layer L 32  functions as a solid electrolyte layer by selectively allowing oxygen ions to pass through it during operation. In one exemplary embodiment when used as an oxygen sensor, the element of  FIG. 3  would have a sensing electrode on top of layer L 32 , a reference electrode between layers L 32  and L 33 , and a heater element between layers L 33  and L 34 . 
         [0050]      FIG. 4  is configured similarly to the embodiment shown in  FIG. 3 , except that the central layer structure has a single alumina insulating layer L 44  and the first layer structure is present, having a single alumina insulating layer L 43 , L 45  disposed on each side of the central layer structure. The rest of the element is similar to that shown in  FIG. 3 , with a second layer structure having a single zirconia layer L 42 , L 46  disposed on each layer of the first layer structure, third and fourth layer structures not present, and alumina protective layers L 41  and L 47  disposed on each layer of the second layer structure. Each of layers L 41  through L 46  has an identical thickness of 172 micrometers. In one exemplary embodiment when used as an oxygen sensor, the element of  FIG. 4  would have a sensing electrode on top of layer L 42 , a reference electrode between layers L 42  and L 43 , a heater element between layers L 44  and L 45 , and optionally an EM shield between layers L 43  and L 44 . 
         [0051]      FIG. 5  is configured similarly to the embodiment shown in  FIG. 3 , except that the second layer structure has two alumina zirconia layers L 52 , L 53 , L 56 , L 57  disposed on each side of the central layer structure. The rest of the element is similar to that shown in  FIG. 3 , with a central layer structure having two alumina insulating layers L 54 , L 55 , first, third and fourth layer structures not present, and alumina protective layers L 51  and L 58  disposed on each layer of the second layer structure. Each of layers L 51  through L 58  has an identical thickness of 172 micrometers. In one exemplary embodiment when used as a wide-range oxygen sensor, the element of  FIG. 5  would have an outer sensing electrode on top of layer L 52 , an inner sensing electrode and an outer reference electrode separated by a chamber between layers L 52  and L 53 , a reference electrode between layers L 53  and L 54 , and a heater element between layers L 54  and L 55 . 
         [0052]    Turning now to  FIG. 6 , there is shown an exploded perspective view of an exemplary embodiment of a layer structure according to the invention having a central layer structure with a single alumina insulating layer L 63 , a second layer structure having a single zirconia layer L 62 , L 64  disposed on each side of the central layer structure, first, third and fourth layer structures not present, and alumina protective layers L 61  and L 65  disposed on each layer of the second layer structure. Each of the layers L 61 -L 65  has an identical cured layer thickness of 172 micrometers. 
         [0053]    Turning now to  FIG. 7 , there is shown an exploded perspective view of an exemplary embodiment of a layer structure according to the invention having a central layer structure with a single zirconia layer L 63 , a second layer structure having a single alumina insulating layer L 72 , L 74  disposed on each side of the central layer structure, first, third and fourth layer structures not present, and alumina protective layers L 71  and L 75  disposed on each layer of the second layer structure. Each of the layers L 71 -L 75  has an identical cured layer thickness of 172 micrometers 
         [0054]    Turning now to  FIG. 8 , there is shown an exploded perspective view of an exemplary embodiment of a layer structure according to the invention having a central layer structure with two zirconia insulating layer L 83 , L 84 , a second layer structure having a single alumina insulating layer L 82 , L 85  disposed on each side of the central layer structure, first, third and fourth layer structures not present, and alumina protective layers L 81  and L 86  disposed on each layer of the second layer structure. Each of the layers L 81 -L 86  has an identical cured layer thickness of 172 micrometers 
         [0055]    Turning now to  FIG. 9 , there is shown an exploded perspective view of an exemplary embodiment of a layer structure according to the invention having a central layer structure with a single alumina insulating layer L 94 , a second layer structure having a single zirconia layer L 93 , L 95  disposed on each side of the central layer structure, a third layer structure having a single alumina insulating layer L 92 , L 96  disposed on each layer of the second layer structure, third and fourth layer structures not present, and alumina protective layers L 91  and L 97  disposed on each layer of the third layer structure. Each of the layers L 91 -L 97  has an identical cured layer thickness of 172 micrometers 
         [0056]    Turning now to  FIG. 10 , there is shown an exploded perspective view of an exemplary embodiment of a layer structure according to the invention having a central layer structure with a single zirconia layer L 104 , a second layer structure having two alumina insulating layers L 102 , L 103 , L 105 , L 106  disposed on each side of the central layer structure, first, third, and fourth layer structures not present, and alumina protective layers L 101  and L 107  disposed on each layer of the second layer structure. Each of the layers L 101 -L 107  has an identical cured layer thickness of 172 micrometers 
         [0057]    Turning now to  FIG. 11 , there is shown an exploded perspective view of an exemplary embodiment of a layer structure according to the invention having a central layer structure with two zirconia layers L 114  and L 115 , a second layer structure having a single alumina insulating layer L 113 , L 116  disposed on each side of the central layer structure, a third layer structure having a single zirconia layer L 112 , L 117  disposed on each layer of the second layer structure, a fourth layer structure having a single alumina insulating layer L 111 , L 118  disposed on each layer of the third layer structure, the first layer structure not present, and alumina protective layers L 110  and L 119  disposed on each layer of the fourth layer structure. Each of the layers L 110 -L 119  has an identical cured layer thickness of 172 micrometers. In an alternate exemplary embodiment, layers L 111  and L 118  each has a cured thickness of 86 μm while layers L 110 , L 119 , and L 112  through LL 117  each has a cured thickness of 172 μm. 
         [0058]    Turning now to  FIG. 12 , there is shown an exploded perspective view of an exemplary embodiment of a layer structure according to the invention having a central layer structure with two alumina insulating layers L 124  and L 125 , a second layer structure having two zirconia layers L 122 , L 123 , L 126 , L 127  disposed on each side of the central layer structure, a third layer structure having a single alumina insulating layer L 121 , L 128  disposed on each layer of the second layer structure, first and fourth layer structures not present, and alumina protective layers L 120  and L 129  disposed on each layer of the third layer structure. Each of the layers L 120 -L 129  has an identical cured layer thickness of 172 micrometers. 
         [0059]    Turning now to  FIG. 13 , there is shown an exploded perspective view of an exemplary embodiment of a layer structure according to the invention having a central layer structure with two zirconia layers L 134  and L 135 , a second layer structure having two alumina insulating layers L 132 , L 133 , L 136 , L 137  disposed on each side of the central layer structure, a third layer structure having a single zirconia layer L 131 , L 138  disposed on each layer of the second layer structure, first and fourth layer structures not present, and alumina protective layers L 130  and L 139  disposed on each layer of the third layer structure. Each of the layers L 130 -L 139  has an identical cured layer thickness of 172 micrometers. 
         [0060]    One of the features of the present invention is that the layers in each of the layer structures are described as having a predetermined thickness. In an exemplary non-limiting embodiment of the invention, Since the layers of each layer structure are symmetrically disposed on each side of the element, this ensures that the thickness of certain layer structure&#39;s layer that is disposed on one side of the element will have substantially the same thickness as that layer structure&#39;s corresponding layer disposed on the opposite of the element. Also, the characterization of the layer structure as being symmetrically disposed on each side of the element also ensures that a zirconia layer on one side of the element will be matched with a zirconia layer on the opposite side of the element, and likewise for the alumina layers. In an exemplary non-limiting embodiment of the invention, each layer will have substantially the same composition as matching layer on the opposite side of the element. In another exemplary non-limiting embodiment of the invention, each layer will have the identical composition as matching layer on the opposite side of the element, and more particularly will be from the same production ceramic green tape production batch. The thicknesses of the individual layers within a layer structure may vary as long as the thickness of each layer is symmetrically matched by the thickness a corresponding layer on the opposite side of the element, and of course the thickness of individual layers may vary from one layer structure to another layer structure. Representative layer thicknesses of 172 micrometers (6.8 mils) and 102 micrometers (4 mils) have been described above in  FIGS. 1-13 ; however, it is understood that varying cured ceramic layer thicknesses may be employed as is known in the art, for example from 25 micrometers to 500 micrometers in one exemplary embodiment and from 50 micrometers to 200 micrometers in another exemplary embodiment. 
         [0061]    The advantages of the invention are readily apparent when the sensor element is made by bulk ceramic technology where layers green ceramic sheets or tapes of ceramic material are laid together along with electrodes, fugitive materials, and other components deposited on the ceramic sheets or tapes by known methods, e.g., ink deposition methods (screen printing), vapor deposition, etc. The sandwiched layers of green ceramic sheets or tapes are then sintered at temperatures of about 1400° C. to about 1500° C. to fire the element. 
         [0062]    The zirconia layers are capable of permitting the electrochemical transfer of oxygen ions, although each zirconia layer used in elements of the invention is not necessarily used as a solid electrolyte for that purpose. The zirconia layers described herein may be optionally stabilized with calcium, barium, yttrium, magnesium, aluminum, lanthanum, cesium, gadolinium, and the like as is known in the art. 
         [0063]    After completion of the manufacture of the sensor element, the sintered sensor element may be disposed in a housing or package to form the completed sensor. Such a sensor may comprise the sintered sensor element, an upper housing shell, a lower housing shell, and a shield for the sensing element. The shield has opening(s) to enable fluid communication between the sensing end of the sensor element and the gas to be sensed. To provide structural integrity to the sensor element  38 , insulators (e.g., ceramic, talc, mesh (metal or other), and/or the like) may be disposed between the sensor element and the shell. The terminal end of the sensor within the upper shell in electrical commutation with a terminal interface such that cables can be disposed in electrical communication with the sensor via the contact pads. During operation, the sensor is disposed in an area where a gas is to be sensed (e.g., within an exhaust conduit of a vehicle). When a gas passes down the conduit, the gas enters the sensor through shield openings and contacts the sensor element. The output signal(s) of the sensor are transmitted through the contact pads through electric cables to a signal processor and/or microprocessor controller that is in operable communication with a vehicle. Based upon the output of the sensor, vehicle operating parameters may be adjusted. 
       EXAMPLES 
       [0064]    Sensor elements with the structures shown in  FIGS. 14 and 2  were prepared from tape and ink raw materials. For each design, the appropriate number and thickness of tape cast alumina and zirconia tapes were blanked into sheets sized for producing seven elements in an array pattern. Via holes and electrode holes were punched into the sheet layers. The conductive circuits were applied to the sheets by screen printing platinum inks onto them. Fugitive carbon inks were printed for forming the chamber and channel features. For each design, the sheets were stacked in the correct order and orientation on a metal plate, sealed in an evacuated plastic bag, and laminated together in an isostatic laminator. Individual green ceramic elements were cut from the laminated tiles using a hot-knife. The organic binder and fugitive carbon material were burned away during a controlled temperature ramp up to a 120 minute hold at a sintering temperature of 1435° C. in a high temperature kiln.  FIG. 15  shows an edge view photograph of the resulting two types of sintered elements with the element  152  according to the invention ( FIG. 2 ) on the right and the comparison element  151  ( FIG. 14 ) on the left. The comparison element  151  was warped and could not be assembled for sensor testing. The element  152  according to the invention was flat within acceptable tolerances. 
         [0065]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing.