Abstract:
A gas sensor assembly is described, comprised of conductive and non-conductive plastic sections, and having a receptacle formed therein. A gas-sensing agent is disposed in the receptacle, and electrodes are positioned adjacent to the receptacle in fluid contact with the gas sensing agent. Each electrode is also in contact with a conductive plastic section of the sensor assembly. The conductive plastic sections are physically and electrically isolated from each other by a non-conductive plastic section.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is directed to a gas sensor used to detect the presence of gases, such as ethanol.  
       BACKGROUND OF THE INVENTION  
       [0002]     Many commercially available gas sensors are of the amperometric type having two or more electrodes in which a catalytically active metal is fixed to a porous substrate. The porous substrate may operate as a gas permeable membrane and structural support for the electrode. The electrodes are located on the inside surface of the membrane where they make contact with an electrolyte such as sulfuric acid. External circuitry maintains one of the electrodes, the working electrode, at a selected electrical potential with respect to one of the other electrodes during operation.  
         [0003]     When the gas of interest diffuses through the porous membrane to reach the working electrode, the diffused gas is oxidized or reduced at the interface of the working electrode and the electrolyte. That reaction generates an electrical current that is proportional to the concentration of the gas. In some cases, the gas of interest reacts with another chemical which, in turn, is oxidized or reduced at the electrode. In some cases, sensors are of a galvanic design wherein a metal such as lead is oxidized to provide the opposite current to that occurring at the working electrode.  
         [0004]     In amperometric ethanol sensors in the prior art, a large platinum black electrode has been needed to form a stable signal, making the sensor bulky and difficult to miniaturize. In addition, the need for a large amount of noble metal for the electrode made the sensor expensive.  
         [0005]     Also in the prior art, the sensors were connected to the external circuit through wires. For example, a platinum contact wire was connected to the catalytically active electrode and passed through the sensor body to an external contact. Since most sensors contain a corrosive, liquid electrolyte, a difficulty with sensors has been providing secure electrical contact with the electrodes while maintaining an electrolyte-tight seal at the location where the conductor passes through the sensor body. In the prior art, seals around conductors have included Teflon gaskets. In other methods, the seal has been made of thermoplastic material or epoxy resin.  
         [0006]     U.S. Pat. No. 5,744,697 to Martell, et al. discloses a gas sensor of the type described above. The Martell, et al. gas sensor has a plastic housing comprised of a plurality of conductive housing portions integrally formed with a plurality of non-conductive housing portions. The housing has a receptacle disposed therein, and a gas-sensing agent is provided in the receptacle. A support sheet that has a plurality of electrodes formed thereon is disposed above the receptacle wherein electrical contact is made with electrodes to the conductive plastic portion, and a wick disposed in the receptacle causes the gas-sensing agent to maintain electrolytically conductive contact with the electrodes formed on the support sheet inside the sensor assembly.  
         [0007]     The present invention uses electrodes that are in electrical contact with conductive plastic portions, as in the Martell, et al. patent, but in a design that uses simpler electrode and sensor housing construction. This results in a smaller sensor that is less expensive to manufacture, easier to interface to external circuitry, and less prone to leaking.  
       SUMMARY OF THE INVENTION  
       [0008]     The invention comprises a gas sensor assembly having a housing with a receptacle formed therein. The housing has non-conductive housing portions and conductive housing portions, each of which is physically and electrically isolated from the other, with the conductive housing portions being comprised of a conductive plastic material. A gas-sensing agent is disposed in the receptacle, and a plurality of electrodes are disposed in conductive contact with the gas-sensing agent and the conductive housing portions.  
         [0009]     In one embodiment of the invention, the gas sensor assembly comprises a bottom conductive layer, a middle non-conductive layer, and a top conductive layer. The receptacle is formed by an opening through the middle and top layers, with the bottom layer being solid with no opening. The bottom electrode, also known as the counter electrode, is in physical contact with the bottom conductive layer, with its active surface facing up towards the receptacle opening. The top electrode, also known as the working electrode, is in physical contact with the top conductive layer, with its active surface facing down towards the receptacle opening. A gas-sensing agent is disposed in the receptacle. Conductive wires are in physical contact with each conductive layer (and not in direct contact with the electrode surfaces) in order to facilitate connection to an external circuit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed the same will be better understood from the following description taken in conjunction with the accompanying drawings in which:  
         [0011]      FIG. 1  is a schematic illustration of a cross-sectional view of an exemplary sensor assembly of the present invention;  
         [0012]      FIG. 2  is a schematic illustration of an exemplary sensor assembly of the present invention;  
         [0013]      FIG. 3  is a schematic illustration of a cross-sectional view of an exemplary sensor core of the present invention depicting the bottom electrode heat welding process;  
         [0014]      FIG. 4  is a schematic illustration of a cross-sectional view of an exemplary sensor core of the present invention depicting the top electrode heat welding process;  
         [0015]      FIG. 5  is a schematic illustration of an exemplary sensor core of the present invention; and  
         [0016]      FIG. 6  is a schematic illustration of an exemplary apparatus to fabricate electrodes. 
     
    
       [0017]     The embodiments set forth in the drawings are illustrative in nature and not intended to be limiting of the invention defined by the claims. Moreover, individual features of the drawings and the invention will be more fully apparent and understood in view of the detailed description.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Reference will now be made in detail to various embodiments of the invention, examples of which are illustrated in the accompanying drawings, wherein like numerals indicate similar elements through the views.  
         [0019]     One exemplary embodiment of the gas sensor  1  in accordance with the invention is illustrated in  FIG. 1  in a cross-sectional view. The sensor core  12  comprises the bottom portion  2 , which could be a solid rectangular piece of conductive plastic. A rectangular electrode  3 , also known as the counter electrode, is sized to fit within the cylindrical receptacle  7 . Electrode  3  is in physical and electrical contact with the bottom portion  2 , with its active surface facing upwards towards the cylindrical receptacle  7 . The middle portion  4  of the sensor is in direct contact with the bottom portion  2  and is comprised of non-conductive plastic with a round center opening that forms the lower side walls of the receptacle  7 . The middle portion  4  also serves to electrically insulate the lower portion  2  from the upper portion  5 . The upper portion  5  of the sensor is in direct contact with the middle portion  4  and is comprised of conductive plastic with a round center opening that forms the upper side walls of the receptacle  7 . A rectangular electrode  6 , also known as the working electrode, is in physical and electrical contact with the upper portion  5 , with its active surface facing downwards towards the receptacle  7 . The receptacle  7  is filled with wick material  8  and with a gas sensing agent (not shown). The wick material  8  maintains electrode  3  in fluid contact with electrode  6 . Wire  13  is in electrical contact with bottom portion  2  only and wire  14  is in electrical contact with upper portion  5  only. Wires  13  and  14  bring the electrical signal of sensor  1  to an external circuit (not shown). A cover  9  comprised of non-conductive plastic is fitted tightly over sensor core  12 . Cover  9  forms a cavity  10  above the electrode  6 . Holes  11  in the cover  9  allow the gas to be sensed to pass into and out of the cavity  10  and across the surface of electrode  6 .  
         [0020]     Typically, a sensor of this type would require a seal between each electrode ( 3  and  6 ) and its associated conductive portion ( 2  and  5 , respectively). In this invention, the counter electrode  3  is fully contained within the body of the gas sensor, so there is no potential for there to be a leak of the gas-sensing agent from electrode  3  to the outside of the sensor body. This results in a two-electrode gas sensor with only one electrode sealing area that has the potential for leaks, thus increasing the reliability of the sensor.  
         [0021]     Core  12  may be formed of non-conductive plastic by injection molding machinery using a two step process with two molds.  FIG. 2  illustrates the middle portion  34  that is formed in the first mold in the first step of the process. Middle portion  34  is rectangular in shape and has a circular opening  35  that serves as part of the receptacle  7 . Middle portion  34  also has an opening  36  to allow for subsequent insertion of wire  14 . In the second step of the process, wire  13  is first inserted into the second mold, middle portion  34  is next inserted, and wire  14  is finally inserted by passing it through opening  36 . Upper portion  5  and lower portion  2  of core  12  are then molded with conductive plastic around portion  34  and wires  13  and  14 . As a result, wire  13  will be in electrical contact with electrode  3  through lower portion  2 , and wire  14  will be in electrical contact with electrode  6  through upper portion  5 .  
         [0022]      FIG. 3  is a cross sectional view of the sensor core  12  and is used to illustrate the first step that may be used in the fabrication of the sensor  1 . The projections  16  are part of the conductive plastic portion  18  and form the rectangular cavity  17 . The rectangular electrode  19  is placed within the cavity  17  with its active surface facing upwards toward cylindrical receptacle  20 . A heated metallic cylinder  21  as is used in heat welding equipment is lowered into receptacle  20  and makes contact only with extensions  16 . Extensions  16  are melted down and flow over top of the active surface of electrode  19 , thus causing the surface of electrode  19  to be in electrical contact with portion  18 .  
         [0023]      FIG. 4  is a cross sectional view of the sensor core  12  and is used to illustrate the next step that may be used in the fabrication of the sensor  1 . Wick material  22  is placed within cylindrical receptacle  23 . Edge  25  is circular in shape and forms the upper boundary of receptacle  23 . Electrode  24  is rectangular in shape and is placed onto edge  25  of upper portion  26  with its active surface facing in towards the receptacle  23 . A heated metallic cylinder  27  as is used in heat welding equipment is lowered onto electrode  24  and melts the edge  25  into the electrode  24 , creating electrical contact and a liquid-tight seal between the electrode  24  and the upper portion  26 .  
         [0024]     Referring to  FIG. 5 , the hole  29  forms a passageway from the outside surface of the core  28  to the receptacle  30 . A gas sensing agent (not shown) is dispensed into receptacle  30  through the hole  29 . The hole  29  may be sealed by applying heat to the outside surface of core  28  at the hole opening, melting plastic over the hole  29  causing a liquid-tight seal. The cover  9  may then be fitted over the core  28  to complete the sensor assembly.  
         [0025]     One exemplary way to fabricate the electrodes  3  and  6  is by using the apparatus  37  illustrated in  FIG. 6 , shown in cross-sectional view. The vacuum chamber  38  is connected to a valve and a vacuum source (neither shown) by way of the passageway  39 . The rectangular plate  40  is connected to the vacuum chamber  38  with a hinge  41  along one edge. A foam gasket  48  is attached to the bottom edge of the plate  40 . The clamp  42  is attached to the vacuum chamber  38  and when in the closed position holds the plate  40  securely to the vacuum chamber  38 . Plate  40  and clamp  42  are shown in the closed position, and plate  43  and clamp  44  are shown in the open position. The electrode support sheet  45  is placed between the foam gasket  48  and the top surface of the vacuum chamber  38 , and is held in place by the plate  40  and the clamp  42 . The electrode support sheet  45  may be fabricated from porous Teflon (PTFE), which may be part number PM23JSF commercially available from MuPore, that is approximately 1.75 inches square and 0.010 inches thick. The electrode support sheet  45  may alternatively be made of any other porous non-reactive thermoplastic, such as polypropylene, polyethylene, or a combination of the two. The top area of the vacuum chamber  38  that is immediately underneath the frame  40  contains holes that allow for the passage of liquids from the area  47  above the electrode support sheet into the interior  46  of the vacuum chamber  38 .  
         [0026]     Another embodiment of the present invention comprises the a method to form the liquid electrode mixture. The method comprises: combining from about 60 to about 240 milligrams (preferably about 120 milligrams) of platinum black catalyst (e.g., fuel cell grade platinum black, part number S3002 commercially available from Englehard) with from about 900 milligrams to about 1100 milligrams (preferably about 1000 milligrams) of water and from about 300 microliters to about 400 microliters (preferably about 350 microliters) of 1-propanol (high purity grade). In one exemplary embodiment, the water comprises Type II 18 megaohm deionized water. The water, platinum black and 1-propanol are mixed together with a blender or mixer on high speed for about 3 to about 15 minutes (preferably about 5 minutes) or until evenly mixed.  
         [0027]     About 200 to about 250 microliters (preferably 225 microliters) of 60% PTFE by weight (commercially available as PTFE 30 from DuPont) is mixed with about 8 to about 12 grams (preferably about 10 grams) of water. About 100 to about 150 microliters (preferably about 125 microliters) of the resulting solution is added to the water, platinum black and 1-propanol mixture described above. The combined mixture and solution is then mixed for about 2 to about 5 (preferably 3) more minutes.  
         [0028]     A vacuum is applied to the vacuum chamber  38  and the electrode support sheet  45  is pre-wet with 1-propanol. The vacuum is maintained until the electrode support sheet  45  is clear. The vacuum is then stopped for 1 to 5 minutes. The liquid electrode mixture is then applied with a pipette to the electrode support sheet  45 . A vacuum is then applied to the vacuum chamber  38  for approximately one minute, sucking the liquid electrode mixture through the electrode support sheet  45 , leaving the catalyst material on top.  
         [0029]     The electrode support sheet  45  is removed from the apparatus  37 . It may then be cut into individual electrodes using a steel rule die, or by other means, with electrode  3  being 0.250 inches square, and electrode  6  being 0.500 inches square.  
         [0030]     The foregoing description of the various embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or limit the invention to the precise form disclosed. Many alternatives, modifications, and variations will be apparent to those skilled in the art of the above teaching. Accordingly, this invention is intended to embrace all alternatives, modifications, and variations that have been discussed herein, and others that fall within the spirit and broad scope of the claims.