Abstract:
An exhaust sensor includes a first sheet of ceramic that is perforated with a vent orifice and, a second sheet of ceramic that is laminated to the first sheet. A palladium circuit trace is positioned between the first sheet and the second sheet of ceramic and a fugitive ink is printed on one of the sheets that is in communication with the vent orifice and the palladium. The fugitive ink volatilizes during a firing process and created a void space that is occupied by a palladium oxide that forms at temperatures around  625 - 900 C.

Description:
[0001]    The present disclosure relates to eliminating structural stress due to palladium oxide within a ceramic laminate structure. 
       BACKGROUND OF THE INVENTION 
       [0002]    Planar exhaust sensor elements are manufactured using multiple layers of alumina, porous, and zirconia tapes. Metallic features are printed on the various layers using platinum conductive ink. The multiple layers of printed tapes are laminated together, and this laminated composite is fired at high temperatures to yield a fully dense multi-layer ceramic element. The metalized features can be designed either on the surface of, or embedded within, the element. For embedded features, small holes through the tape called “vias” are filled with the conductive ink in order to carry the electrical circuit through the layers of insulating tape. All of the metalized features, both surface and embedded, typically come together in a pad of metal at the surface where an electrical connection to the outside circuit is made by way of soldering, brazing, pressure contacts, etc. This pad is known as the contact pad. 
         [0003]    Referring now to  FIG. 1 , an exploded view is shown of an exhaust sensor  10  in accordance with the prior art just described Exhaust sensor  10  may implement a temperature sensor, a heater, a lambda sensor, an oxygen sensor, or the like. Exhaust sensor  10  is formed from layers of green ceramics and metal inks that are layered and then fired together to form a unitary laminar structure. 
         [0004]    A sensor layer  12  may be formed from zirconia. Sensor layer  12  may be sandwiched between a sensor protective layer  14  and a stack of support/insulating layers  16   a - 16   c , which are collectively referred to as support/insulating layers  16 . Sensor protective layer  14  and support/insulating layers  16  can be formed from alumina. A sacrificial material  20 , such as carbon ink, can be applied to support/insulating layers  16  at the interface with sensor layer  12 . Sacrificial material  20  volatilizes during firing and leaves a void air reference channel that is in communication with one side of sensor layer  12 . A porous protection layer  15  allows exhaust gas to reach one side of sensor layer  12  while protecting sensor layer  12  from moisture and/or particulates. 
         [0005]    Exhaust sensor  10  also includes a heater that heats sensor layer  12 . A separator layer  18  may be formed from alumina and positioned between support/insulating layers  16  and a heater protective layer  22 . The heater is positioned between separator layer  18  and heater protective layer  22 . The heater is formed from a platinum heater serpentine  24 . Platinum may also be employed to form pairs of pads  26   a - 26   b  and  28   a - 28   b.  Pads  26   a - 26   b  and  28   a - 28   b  provide a power connection to heater serpentine  24  via associated heater leads  30  and  32 . 
         [0006]    Platinum may also be employed to form pads  40   a - 40   b  and vias  42   a - 42   c , which provide a connection to a signal voltage that is developed across sensor layer  12 . Pads  40   a - 40   b  and  42   a - 42   c  communicate with electrode lead  44  and electrode lead  46 , respectively. Electrode leads  44  and  46  are also formed of platinum. 
         [0007]    The electrical features of exhaust sensor  10 , such as heater serpentine  24 , heater leads  30  and  32 , pads  26 ,  28 , electrode leads  44  and  46 , and pads  40  and  42 , are formed of platinum conductive ink. The zirconia, alumina, and porous layers are initially green, e.g. unfired, ceramic tapes. The electrical features are printed on the various layers. The multiple layers of tapes, at least some of which are printed, are then laminated together and fired at high temperatures to yield a fully dense multi-layer ceramic element. The metallized features, e.g. heater serpentine  24 , electrode leads  44  and  46 , and the various pads, can be designed either on the surface of, or embedded within, exhaust sensor  10 . For embedded metallized features, vias are punched through the green ceramic. The vias are filled with the conductive ink and fired to make electrical connections with a circuit. Exhaust sensor  10  also includes contact pads that are formed on an exterior of exhaust sensor  10 . The contact pads provide an external connection to the metallized features regardless of whether they are internal or external of exhaust sensor  10 . 
         [0008]    Referring now to  FIGS. 2 and 3 , cross sections are shown of a portion of exhaust sensor  10 . Heater lead  32  is laminated between separator layer  18  and heater protective layer  22 . A lamination region  40  indicates where separator layer  18  and heater protective layer  22  are laminated together. Pad  28   b  provides an attachment point for an electrical lead  42 . Electrical lead  42  can be resistance welded, brazed, soldered, or otherwise attached to via  28 . A via  44  includes a plated or inked conductor  44  that electrically connects pad  28   b  and heater lead  32 .  FIG. 3  omits pad  28   b  and via  44  for the sake of clarity. 
         [0009]    As shown in  FIG. 4 , there are some drawbacks to this design. Firing the green laminated ceramics releases gases from organic binders. These gases expand and can cause defects such as the delamination of region  40  or bubble formation in the ceramic material. Both are causes of scrap and in-service failures. These problems can be minimized by adding a venting feature, e.g. one or more open vias  44 , to allow the gasses to escape without causing cracks and/or bubbles during firing. 
         [0010]    However, the venting feature introduces another failure mode in service. The open venting feature can permit contaminants, such as dissolved ionic compounds in water, to enter the element under some service conditions. This weakens the ceramic materials and may cause spalling, cracking, and other failures. 
         [0011]    Another drawback is the high cost of the platinum metallization. Platinum is one of the few metals that have a melting/vaporization point high enough to withstand the firing temperature of the ceramic materials. It also has catalytic properties that are well suited to sensor functions. For metal features not related to those sensor functions, however, efforts have been made to replace platinum with palladium, palladium alloy, or related metals and alloys thereof, all of which are collectively referred to herein as “palladium.” 
         [0012]    Palladium is less expensive than platinum and will stand up to the temperatures needed to co-fire the ceramic materials reasonably well. Unlike platinum, however, palladium forms an oxide. Palladium oxide reversibly forms at approximately the same temperature where oxygen sensors operate, beginning at around 625 deg C. and decomposing around 850-900 deg C. There is a considerable volume increase, such as around 38%, associated with the formation of the related metal oxidation. 
         [0013]    The volume difference between oxidized and unoxidized palladium causes some risk when palladium is used for embedded metallized features. During firing and potentially during service, the oxygen sensor will cycle through temperatures that will cause formation and decomposition of the oxide. This oxidation/decomposition process occurs during the firing process and may also occur during operation in service if the element is not hermetically sealed. The volume change on oxidation is likely to cause cracking and/or delamination. 
       SUMMARY OF THE INVENTION 
       [0014]    An exhaust sensor includes a first sheet of ceramic that is perforated with a vent orifice and a second sheet of ceramic that is laminated to the first sheet. A palladium circuit trace is positioned between the first sheet and the second sheet of ceramic and a fugitive ink is printed on one of the sheets that is in communication with the vent orifice and the palladium. The fugitive ink volatilizes during a firing process and creates a void space. This allows expansion space for the palladium oxide that forms as a result of oxidation. 
         [0015]    A method of making an exhaust sensor includes perforating an orifice vent opening through a first ceramic sheet and printing a palladium circuit trace on a second ceramic sheet. Thereafter, printing a fugitive ink to register with the palladium circuit trace and laminating the first ceramic sheet with the second ceramic sheet such that the palladium circuit trace and fugitive ink are sandwiched between the first ceramic sheet and the second ceramic sheet and registered with the orifice vent opening. After volatilizing the fugitive ink leaves a void space between the first and second sheets. This void space allows expansion space for the palladium oxide, thus preventing crack formation. 
         [0016]    An exhaust sensor includes a first sheet of ceramic that is perforated with a plurality of vent orifices and a second sheet of ceramic that is laminated to the first sheet. Palladium circuit traces are positioned between the first sheet and the second sheet of ceramic and a fugitive ink is provided that is in communication with the vent orifices and the palladium circuit traces. A palladium oxide is disposed within a void space created by the fugitive ink volatilizing after a firing process. 
         [0017]    Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0019]      FIG. 1  is an exploded view of an exhaust sensor in accordance with the prior art; 
           [0020]      FIGS. 2-4  are cross-sections of different details of exhaust sensors in accordance with the prior art; 
           [0021]      FIG. 5  is an exploded view of an exhaust sensor in accordance with the present invention; 
           [0022]      FIG. 6  is a cross section showing a detail of the present invention before firing; 
           [0023]      FIG. 7  is a cross section showing the detail of  FIG. 6  after firing; 
           [0024]      FIG. 8  is a cross section showing another detail of the present invention before firing; 
           [0025]      FIG. 9  is a cross section showing the detail of  FIG. 8  after firing; 
           [0026]      FIG. 10  is a cross section of yet another detail of the present invention; 
           [0027]      FIG. 11  is a flow diagram of a process that forms the present invention; and 
           [0028]      FIG. 12  is a flow diagram of another process that forms the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    Referring now to  FIGS. 5 through 12 , where the invention will be described with reference to specific embodiments, without limiting same,  FIG. 5  is an exploded view of an exhaust sensor  100 . In some aspects exhaust sensor  100  is similar to exhaust sensor  10  of the prior art, however exhaust sensor  100  employs an expansion zone and venting feature as will be further described herein. The expansion zone and venting feature enable palladium to be employed instead of platinum for forming circuit features such as heaters, circuit traces and pads, sense elements, and the like. 
         [0030]    Exhaust sensor  100  is formed from layers of ceramics and metal inks that are fired together to form a unitary laminar structure. A sensor layer  102 , such as zirconia, can be sandwiched between a sensor protective layer  104  and a structure of one or more support/insulating layers  106   a - 106   d,  which are collectively referred to as support/insulating layer  106 . Sensor protective layer  104  and support/insulating layers  106  can be formed from alumina or the like. A sacrificial (“fugitive”) material  108 , such as carbon ink, is applied to support/insulating layers  106  at the interface with sensor layer  102 . Sacrificial material  108  volatilizes during firing and leaves a void air reference channel that is in communication with one side of sensor layer  102 . A porous protection layer or diffusion restriction layer  110 , applied to sensor layer  102 , allows exhaust gas to reach one side of sensor layer  102  while protecting sensor layer  102  from moisture and/or contamination with particulates. 
         [0031]    Exhaust sensor  100  also includes a heater  114  that heats sensor layer  102 . The heater  114  is positioned between support/insulating layers  106  and a heater protective layer  112 . The heater  114  is formed from palladium, and as shown, is serpentine in shape. Palladium is also employed to form pairs of pads  116   a  and  116   b  and associated conductive vias  117   a,    117   b  and  117   c.  In a like manner, palladium is employed to form corresponding pairs of pads  118   a  and  118   b  associated conductive vias  119   a,    119   b  and  119   c.  Pads  116   a  and  118   a  provide a power connection to heater  114  through a heater lead  120  and  122 , respectively. Heater leads  120  and  122  are also formed with palladium. 
         [0032]    A fugitive ink  124  is printed over heater  114  and heater leads  120  and  122 . Fugitive ink  124  may be organic in nature and may use carbon latex spheres, flour, carbon black powder, or other materials that volatilize during firing and therefore leave a void in the ceramic lamination. The volatilized ink escapes through one or more venting features that are described below. The resulting void provides space for palladium oxide to form without stressing the adjoining layers of exhaust sensor  100 . It will be appreciated that while fugitive ink  124  is shown as a trace for palladium heater  114  and heater leads  120  and  122 , its use is not limited to those applications. Fugitive ink  124  may be employed between any of the layers in exhaust sensor  100  to provide space for palladium oxide to form without generating pressure that could separate adjacent laminated layers. 
         [0033]    Pads  130   a - 130   b  and  132   a - 132   b  are disposed on opposite sides of protective layer  104 , while pads  132   b  and  132   c  are adjacent. As shown in  FIG. 5 , pads  130   a - 130   b  and  132   a - 132   b  are interconnected with vias  131   a,    131   b  and  131   c  and vias  133   a,    133   b  and  133   c,  respectively. Pad  132   c  is connected to vias  135   a,    135   b  and  135   c  on sensor layer  102 . Pads  132   a - 132   c,  in combination with vias  133   a - 133   c  and vias  135   a - 135   c  communicate with a lead  136 . In a like manner, pads  130   a  and  130   b,  in combination with  131   a - 131   c,  communicates with a lead  134 . The construction described and shown provides a connection to a signal voltage that develops across sensor layer  102 . 
         [0034]    Referring now to  FIG. 6 , a cross-section is shown of a portion of an exhaust sensor  200 .  FIG. 6  shows exhaust sensor  200  in a green or unfired state. Exhaust sensor  200  includes a palladium circuit trace  206 . Palladium circuit trace  206  is formed or printed with palladium ink. A first ceramic tape layer  202  is laminated to a second ceramic tape layer  204 . The first ceramic layer  202  and second ceramic layer  204  is formed from alumina. It will be appreciated that other suitable materials known to one in the art may be substituted. Palladium circuit trace  206  is positioned on ceramic tape layer  204  and will ultimately be between first ceramic layer  202  and second ceramic layer  204 . Fugitive ink  124  is printed over palladium circuit trace  206  prior to ceramic tape layers  202  and  204  being laminated together. Fugitive ink  124  therefore occupies space between palladium circuit trace  206  and first ceramic layer  202  when ceramic layers  202  and  204  are fired. 
         [0035]    Referring now to  FIG. 7 , a cross section is shown of the exhaust sensor  200  of  FIG. 6  after firing. Fugitive ink  124  has volatilized, leaving a void or expansion space  208 . Expansion space  208  provides room for palladium circuit trace  206  to form palladium oxidation  210  without cracking or delaminating first ceramic layer  202  and second ceramic layer  204 . 
         [0036]    Referring now to  FIG. 8 , a second embodiment is shown of exhaust sensor  200  in which an orifice venting feature  212  has been added. Orifice venting feature  212  vents the gasses generated by fugitive ink  124  when exhaust sensor  200  is volatilized during firing. Venting feature  212  can simply be the open orifice prior to firing, or venting feature  212  can contain fugitive ink  124 , as shown in  FIG. 8 , to assure that orifice venting feature  212  stays open during lamination. Referring now to  FIG. 9 , a cross section is shown of the exhaust sensor  200  of  FIG. 8 , after firing. Fugitive ink  124  has volatilized and escaped through venting feature  212 , leaving behind expansion space  208  and palladium oxidation  210 . 
         [0037]    The embodiment shown in  FIG. 10  is a cross section of the exhaust sensor  200  first seen in  FIG. 6 , with the addition of a contact pad  214 . Contact pad  214  is printed after exhaust sensor  200  is fired by filling orifice venting feature  212  with palladium ink and forming contact pad  214  on the exterior surface of ceramic tape layer  202 . The resulting sensor  200  has contact orifice pad  214  providing an electrical contact through venting feature  212  to palladium circuit trace  206 . Contact pad  214  and the palladium in orifice  212  also seals orifice venting feature  212  to prevent moisture, debris, and the like from entering expansion space  208 . 
         [0038]    The process flow diagram of  FIG. 11  depicts a first method  300  of manufacturing the exhaust sensor  200 . Method  300  begins by forming venting features  212  with a punching process  302  through a first sheet of green ceramic that will ultimately be singulated. The punched sheet of green ceramic after singulation, will become a plurality of first layers  202  of a plurality of exhaust sensors  200 . 
         [0039]    As shown at step  304 , a palladium circuit trace  206  is printed on a second sheet of green ceramic, such as second layer  204 . The fugitive ink  124  is also printed over circuit trace  206  in step  304 . The method then proceeds to step  306 , where assembly occurs by registering the venting features  212  of the first sheet with its associated palladium circuit traces  206  on the second sheet and the fugitive ink  124  on the second sheet. Thereafter, first and second sheets are laminated together in step  308 . The lamination step  308  can be performed with heat and pressure. The laminated sheets form first layer  202  and second layer  204 , which are best shown in  FIG. 6 . 
         [0040]    Individual exhaust sensors  200  are formed in step  310  by cutting or singulating the first and second sheets of laminated first layer  202  and second layer  204 . This results in a plurality of individual exhaust sensor  200  that still require further processing. As identified at block  312  laminated layers  202  and  204  are then fired. Firing burns out a laminating binder and sinters the ceramic layers. An inspection step, as identified at block  314  inspects exhaust sensor  200  for defects such as plugged venting features  212 , delaminated first layer  202  and second layer  204 , and the like. The porous protection layer or diffusion restriction layer  110 , identified as step  318 , is added during the printing steps  304 , if desired. Porous protection layer  110  is primarily employed when exhaust sensor  200  implements an oxygen sensor. 
         [0041]    Contact pads  214  are applied and the filling of orifice venting features  212  are carried out in step  320 . A heat treating step shown as step  322 , secures contact pads  214  into orifice venting features  212  and provides a secure connection to palladium circuit trace  206 . The method of manufacture of exhaust sensor  200  finishes at block  324  where the finished exhaust sensors  200  is tested to verify that each one is operative. 
         [0042]    The process of  FIG. 12  depicts a second method  400  of manufacturing exhaust sensor  200 . Second method  400  differs from method  300  in that method  400  singulates the sensors after they are fired instead of before they are fired. Depending on available equipment, this may allow second method  400  to be more efficient by printing contact pads  214 , vias, or orifice venting features  212  while a plurality of exhaust sensors  200  are still together in a single laminate. Exhaust sensors  200  are then singulated after being fired. 
         [0043]    Method  400  begins at block  402  by punching orifice venting features  212  through a sheet of green ceramic. The sheet of green ceramic will, upon singulation, provide a plurality of first layers  202  for a plurality of sensors  200 . At step  404  the palladium circuit trace  206  and fugitive ink are printed on a second sheet of green ceramic. As with the first sheet, the second sheet will become, upon singulation, a plurality of second layers  204 . As identified in block  406 , first and second green ceramic sheets are aligned so that orifice venting features  212  are registered with associated palladium circuit traces  206 . The aligned green sheets may also be hot pinned to maintain the registration. Thereafter, as shown in block  408 , the green layers are laminated together; thereby forming a plurality of first layer  202  and second layer  204  in a single lamination. 
         [0044]    Unlike the method  300  described in relation to  FIG. 11 , method  400  then proceeds to, if necessary, removing offal from the laminated first and second layers  212 , as shown in step  410 . The entirety of laminated first and second layers  202 ,  204 , still in a single lamination are then fired, as seen in step  412 . The firing burns out any binder that was employed and sinters the ceramic sheets. Method  400  then proceeds to block  414  and prints contact pads  214  and orifice venting features  212 , also shown in  FIG. 5  as  117   a - 117   c,    119   a - 119   c  vias  131   a - 131   c,    133   a - 133   c  and  135   a - 135   c.  Method  400  then proceeds to block  416 , where heat treating of contact pads  214  fixes them on the exterior surface of the laminate, thereby sealing venting features  212 . Unlike method  300 , singulation of the fired laminate into individual exhaust sensors  200  occurs after heat treating, as shown in step  418 . Inspection of exhaust sensors  200  for defects such as cracking, lamination separation, and the like occurs as step  420 . The porous protection layer or diffusion restriction layer 110  is applied to exhaust sensors  200 , if needed at step  422 . Porous protection layer  110  is primarily employed when exhaust sensor  200  implements an oxygen sensor. Method  400  then proceeds to block  424  and heat treating again is performed, this time on the completed exhaust sensors  200  prior to final testing in step  426 . 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 description.