Abstract:
A method of manufacturing an exhaust gas sensor includes providing a sensor element having an open end, a closed end, and an inner surface defining a chamber between the open and closed ends. The method further includes inserting a nozzle into the chamber, supplying an electrode material to the nozzle, and without substantially any relative movement between the nozzle and the sensor element, atomizing the electrode material to form a mist of electrode material that substantially surrounds the tip of the nozzle and deposits onto the inner surface of the sensor element to form an inner electrode.

Description:
BACKGROUND  
       [0001]     The present invention relates to exhaust gas sensors.  
         [0002]     Exhaust gas sensors are well known in the automotive industry for sensing the oxygen, carbon monoxide, or hydrocarbon content of the exhaust stream generated by internal combustion engines. Stoichiometric or “Nernst”-type oxygen sensors (a widely-used type of exhaust gas sensor) measure the difference between the partial pressure of oxygen found in the exhaust gas and oxygen found in the atmosphere. By determining the amount of oxygen in the exhaust gas, the oxygen sensor enables the engine control unit to adjust the air/fuel mixture and achieve optimal engine performance. Other types of exhaust gas sensors that operate based on different principles are also known and widely used in the automotive industry.  
       SUMMARY  
       [0003]     There are a number of conventional methods for applying electrode material to an inner surface of a generally cup-shaped sensor element. Many of these prior art methods use more of the expensive electrode material than is actually needed to create the inner electrode of the sensor element. The use of an excessive amount of electrode material adds to the cost of manufacturing the exhaust gas sensors. Additionally, some of the prior art application methods require both relative translation and rotation between the sensor element and the device that applies the electrode material. These methods are complex and require machinery and parts capable of achieving the required translational and rotational movements. It can also be difficult to control the thickness, size, and position of the electrode material using these techniques.  
         [0004]     The invention provides an improved method of manufacturing exhaust gas sensors, and more specifically an improved method for applying electrode material to an inner surface of a sensor element to form at least part of the inner or reference electrode. With the method of the invention, the application of excessive amounts of electrode material is greatly reduced or eliminated, and the need for relative rotational and/or translational movement between the sensor element and the device applying the electrode material is eliminated. The method of the present invention results in a substantially uniform and well-controlled layer of electrode material on the inner surface of the sensor element.  
         [0005]     In one embodiment, the invention provides a method of manufacturing an exhaust gas sensor. The method includes providing a sensor element having an open end, a closed end, and an inner surface defining a chamber between the open and closed ends. The method further includes atomizing an electrode material using an ultrasonic spraying device to deposit a layer of electrode material onto the inner surface of the sensor element.  
         [0006]     In another embodiment, the invention provides a method of manufacturing an exhaust gas sensor. The method includes providing a sensor element having an open end, a closed end, and an inner surface defining a chamber between the open and closed ends, inserting a nozzle into the chamber, supplying an electrode material to the nozzle, and without substantially any relative rotation between the nozzle and the sensor element, atomizing the electrode material to deposit a layer of electrode material onto the inner surface of the sensor element substantially 360 degrees around the nozzle.  
         [0007]     In yet another embodiment, the invention provides a method of manufacturing an exhaust gas sensor. The method includes providing a sensor element having an open end, a closed end, and an inner surface defining a chamber between the open and closed ends, inserting a nozzle into the chamber, supplying an electrode material to the nozzle, and without substantially any relative movement between the nozzle and the sensor element, atomizing the electrode material to form a mist of electrode material that substantially surrounds the tip of the nozzle and deposits onto the inner surface of the sensor element to form an inner electrode.  
         [0008]     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a section view of an exhaust gas sensor embodying the invention.  
         [0010]      FIG. 2  is an enlarged section view of a sensor element of the exhaust gas sensor of  FIG. 1 .  
         [0011]      FIG. 3  is a plan view of an ultrasonic spraying device and support fixture used in applying electrode material to the sensor element of  FIG. 2 .  
         [0012]      FIG. 4  is an enlarged section view taken along line  4 - 4  of  FIG. 3 .  
         [0013]      FIG. 5  is section view of the sensor element illustrating the application of a conductive lead to the inner surface of the sensor element. 
     
    
       [0014]     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.  
       DETAILED DESCRIPTION  
       [0015]      FIG. 1  illustrates an exhaust gas sensor  10  according to the invention. The sensor  10  is shown mounted to an exhaust conduit  12  of an automobile or other vehicle powered by an internal combustion engine. The illustrated sensor  10  is a case-grounded, unheated, single wire sensor, however, those skilled in the art will understand that the sensor  10  could be modified to be a heated, multiple-wire sensor. Except for the method of applying the inner electrode and the resulting applied inner electrode described in detail below, the general construction of the illustrated sensor  10  is described in detail in U.S. Patent Application Publication No. 2004/0074284 published Apr. 22, 2004, and the entire contents of that application are incorporated by reference herein. It is also to be understood that the invention is applicable to other exhaust gas sensor designs that include a cup-shaped or “thimble” type sensor element, as described in detail below. The invention can also be adapted to other applications in which a substantially uniform and well-controlled layer of material is applied to an inner surface of a generally tubular substrate.  
         [0016]     The sensor  10  includes a housing  14 , a sleeve  18  coupled to the housing  14 , and a lead wire  22  exiting the sleeve  18  through a grommet  26 . An insulating bushing  30  is housed within the sleeve  18 , and includes a bore that houses and electrically isolates a contact pin  34 .  
         [0017]     The sensor  10  also includes a ceramic, cup-shaped or thimble-shaped sensor element  38  of the type commonly known and made from materials such as stabilized ZrO 2 , CaO— and/or Y 2 O 3 — stabilized ZrO 2 , Al 2 O 3 , Mg-spinel, and forsterite. The sensor element  38  is retained in the housing  14  and, as shown in  FIGS. 2 and 5 , includes an open end  42 , a closed end  46 , an outer surface  50 , and an inner surface  54 . The inner surface  54  defines a chamber  58  extending between the open end  42  and the closed end  46 .  
         [0018]     As best seen in  FIGS. 1 and 2 , an outer or exhaust electrode  62  of conductive and catalytically active electrode material, such as platinum or other similar conductive and catalytically active materials (e.g., Pd and Rh), is positioned on the outer surface  50 . A lead portion  66  of the exhaust electrode  62  extends along the outer surface  50  toward the open end  42  of the sensor element  38  to be in electrical contact with a bore  70  of the housing  14 , thereby grounding the exhaust electrode  62  through the housing  14 . The exhaust electrode  62  communicates with the exhaust gas stream (depicted by the arrows  74  in  FIG. 1 ), as is understood by those skilled in the art.  
         [0019]     An inner or reference electrode  78  of conductive and catalytically active electrode material, such as platinum or other similar conductive and catalytically active materials (e.g., Pd and Rh), is positioned on the inner surface  54  of the sensor element  38  within the chamber  58 . A lead portion  82  of the reference electrode  78  extends along the inner surface  54  toward the open end  42  of the sensor element  38  and out of the chamber  58  along an end surface  86  (see  FIGS. 2 and 5 ) defining the open end  42  of the sensor element  38 . The lead portion  82  is configured to be in electrical contact with the contact pin  34  housed in the sensor bushing  30 . The reference electrode  78  communicates with reference air inside the chamber  58 , as is also understood by those skilled in the art.  
         [0020]     The sensor  10  also includes a tube  90  that substantially surrounds and protects the end of the sensor element  38  extending into the exhaust gas stream  74 . The illustrated tube  90  is made of stainless steel or other heat-resistant metal alloys and is secured to the housing  14 . The tube  90  allows exhaust gas to enter therein for communication with the sensor element  38 , yet protects the sensor element  38  from debris particles contained within the exhaust gas stream  74 .  
         [0021]     The method of applying the reference electrode  78  to the inner surface  54  of the sensor element  38  will now be described with reference to  FIGS. 3-5 .  FIG. 3  illustrates an ultrasonic spraying device  94  that is used to apply at least a portion of the reference electrode  78  to the inner surface  54  of the sensor element  38 . The illustrated ultrasonic spraying device  94  includes a frame or stand  98  that supports a movable carriage  102 . The illustrated carriage  102  can translate both vertically (as indicated by the arrows  106  in  FIG. 3 ) and horizontally (into and out of the page in  FIG. 3 ). An ultrasonic nozzle assembly  110  is mounted on the carriage  102  and includes a nozzle or tip  112 . An input device  114  enables the user to control movement of the carriage  102  and operation of the nozzle assembly  110 .  
         [0022]     The nozzle assembly  110  is electrically connected to a broadband ultrasonic generator  118 . While any suitable ultrasonic nozzle assembly and ultrasonic generators can be used, the illustrated nozzle assembly  110  and broadband ultrasonic generator  118  are available from Sono-Tek Corporation of Milton, N.Y. as a Model Number 8600-6015 nozzle assembly with a MicroSpray nozzle, and a Part Number 06-05108 broadband (20-120 kHz) ultrasonic generator. The electrode material is stored in paste or slurry form in a storage reservoir  122  and is provided to the nozzle assembly  110  via conduit  126 . In the illustrated embodiment, the slurry or paste contains ceramic and metal particles having a diameter of less than about sixty microns suspended in liquid medium (water or solvent based with auxiliary additives, e.g., dispersing agents, binder systems, and the like). In the illustrated embodiment, the solid ceramic and metal particles constitute between about thirty percent and about seventy percent of the total weight of the slurry or paste, and the liquid components constitute the remaining percentage. The slurry or paste used in the illustrated embodiment has a viscosity of between about fifty to about one thousand mPas.  
         [0023]     Also shown in  FIGS. 3 and 4  is a fixture  130  for supporting one or more sensor elements  38 . Those skilled in the art will understand that any suitable fixture can be used to support and retain the sensor elements  38 . To apply the electrode material to the inner surface  54  of the sensor element  38 , the nozzle  112  is inserted into the chamber  58  of the sensor element  38  as shown in  FIG. 4 . As the electrode material is provided to the nozzle  112 , the broadband ultrasonic generator  118  energizes the nozzle assembly  110  such that the electrode material exiting the nozzle  112  is atomized into a fine mist (as represented by the reference numeral  126  in  FIG. 4 ) that is deposited on the inner surface  54  of the sensor element  38  substantially 360 degrees around the nozzle  112 . In other words, the mist  126  travels outwardly from the nozzle  112  in all directions to substantially coat the entire inner surface  54  of the closed end  46  of the sensor element  38  without requiring substantially any relative movement (e.g., rotational or translational) between the nozzle  112  and the sensor element  38  during the application of the atomized electrode material  126 . The atomized mist of electrode material  126  provides a substantially uniform and well-controlled layer of electrode material on the inner surface  54 .  
         [0024]     Using the ultrasonic spraying device  94  to apply at least a portion of the inner electrode  78  substantially reduces the excess usage of expensive electrode material deposited in the chamber  58  of the sensor element  38 . In a test of twenty-five sample runs, the average electrode material paste usage using the ultrasonic spraying device  94  was 23.7 mg with an average standard deviation of 1.02. Twenty-five sample runs were also conducted for prior art “fill &amp; extract” and “drip &amp; blow” processes. Using a prior art “fill &amp; extract” process, an average of 36.0 mg of paste was used per run with a standard deviation of 8.0. Using a prior art “drip &amp; blow” process, an average of 60.0 mg of paste was used per run with a standard deviation of 10.0.  
         [0025]     The use of the ultrasonic spraying device  94  for the method of the present invention greatly reduces the amount of expensive electrode material needed to apply the portion of the inner electrode  78  adjacent the closed end  46  of the sensor element  38 . In addition, the inner electrode  78  can be accurately sized and positioned, and the thickness of electrode material can be accurately controlled. Furthermore, the deposited atomized mist  126  results in good electrode homogeneity, and the ultrasonic vibration also maintains a well-dispersed suspension of the paste or slurry prior to application. There is also no air pressure required for application of the inner electrode  78 . This eliminates problems occurring in prior art processes associated with excessive over-spray due to the use of air pressure.  
         [0026]     In addition, the method of the present invention eliminates the need for substantially any relative movement (e.g., rotational or translational) between the sensor element  38  and the nozzle  112  during application of the electrode material  126  because the atomized mist of electrode material  126  spreads outwardly, 360 degrees around the nozzle  112 . This is in contrast to prior art methods that apply the inner electrode in a ring form by brushing or spraying paste from a nozzle that is rotating and/or translating relative to the sensor element. In twenty-five sample runs conducted using a prior art “ring electrode” process, an average of 35.4 mg of paste was used per run with a standard deviation of 2.90. Therefore, the method of the present invention uses less electrode material than prior art “ring electrode” processes, and also eliminates the need for any mechanically-complex relative rotation and/or translation between the nozzle  112  and the sensor element  38  during application of the electrode material.  
         [0027]     It is to be understood that devices utilizing technology other than ultrasound technology, and that can create an atomized mist of electrode material  126  capable of being deposited to form the inner electrode  78  in the manner discussed above, can also be substituted for the ultrasonic spraying device  94 . This includes technology currently in existence as well as technology yet to be developed. For example, a spraying device utilizing air pressure to create the atomized mist of electrode material  126  could be used to form the inner electrode  78  without requiring relative movement, or at least without requiring relative rotation, between the air pressure spraying nozzle and the sensor element  38  during application. In another example, a mechanical vibration nozzle could be used to create the atomized mist of electrode material  126  without requiring relative rotation between the mechanical vibration nozzle and the sensor element  38  during application.  
         [0028]     Once the portion of the inner electrode  78  is applied using the ultrasonic spraying device  94 , the nozzle  112  is removed from the chamber  58 . Next, the lead portion  82  of the inner electrode  78  is formed by dripping some of the electrode material down the inner surface  54  of the sensor element  38  as illustrated in  FIG. 5 . This procedure can occur while the sensor element  38  remains in the fixture  130 , or as shown in  FIG. 5 , can occur after the sensor element  38  has been removed from the fixture  130 . As discussed above, the lead portion  82  provides an electrical connection with the remaining portion of the inner electrode  78  that was applied by the ultrasonic spraying device  94 .  
         [0029]     Once the inner electrode  78  and lead portion  82  have been applied, the outer electrode  62  and lead portion  66  can be applied to the outer surface  50  using any suitable technique. Next, the sensor element  38  is sintered at between about 500 degrees C. and about 1,500 degrees C. to bond the electrode material to the ceramic substrate of the sensor element, thereby forming cermet-type inner and outer electrodes,  78  and  62 , respectively. The resulting electrodes  78 ,  62  have large amounts of three-phase boundaries and are therefore highly active and resistant to contamination. The metal-to-ceramic oxide weight ratio in the sintered cermet electrodes  78 ,  62  can range from about 10:1 to about 3:2. The layer thickness of the cermet electrodes  78 ,  62  can range from about two to about thirty microns. After sintering, the sensor element  38  is ready for installation into the exhaust gas sensor 10.  
         [0030]     Various features and advantages of the invention are set forth in the following claims.