Abstract:
A compact and high sensitive gas detecting sensor for detecting the partial pressure of oxygen gas in the exhaust gases of an internal combustion engine is disclosed. The sensor is provided with a sensing element made of the ceramic material having an electrical characteristic which varies in response to variations in the partial pressure of oxygen gas, such as CoO, a base member made of ceramic material having a characteristic of generating heat when an electric current is supplied thereto, such as silicon carbide. The sensing element is integrally fixed to the base member through an insulating layer made of ceramic material. To the base member, electrodes are connected for supplying an electric current thereto so that the base member is maintained at a predetermined temperature regardless of the temperature change of the exhaust gases.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a gas detecting sensor for detecting the partial pressure of oxygen gas contained within exhaust gases discharged from an internal combustion engine and measuring air/fuel ratio of the combustion mixture to be supplied into the internal combustion engine. 
     Recently, &#34;lean burn system&#34; that is, the system of operating an internal combustion engine with an air/fuel ratio larger than the stoichiometric value thereof, is proposed and employed in practice in order to reduce harmful components contained within the exhaust gases and lower the fuel consumption. 
     The above described &#34;lean burn system&#34; requires a detecting means for accurately detecting air/fuel ratio in a range of lean mixtures. 
     U.S. Pat. Nos. 3,933,028 and 4,012,709 show examples of such a detecting sensor as described above. 
     These detecting sensors are provided with a sensing element made of cobalt monoxide (CoO) or an alloy of cobalt monoxide (CoO) and magnesium oxide (MgO). These detecting sensors are further provided with a heating means for heating and maintaining the sensing element at a predetermined temperature, for example 900° C. in order to prevent cobalt monoxide from changing into tricobalt tetroxide (Co 3  O 4 ) and to compensate output fluctuation caused by the temperature change. 
     These conventional detecting sensors have problems as follows. 
     Since the heating coil is used for heating the sensing element, the structure of the sensor becomes complex and it is troublesome to mount the sensor to the exhaust pipe. 
     Since the sensing element is formed into a cylindrical or disc-shaped block and the heating coil is arranged around the housing which supports the sensing element, the device becomes large. 
     Since the sensing element is thick, the sensing element does not exhibit excellent responsive characteristic to the change of the partial pressure of oxygen gas. 
     Since the sensing element which is supported within the housing, is not directly contacted with the heating coil which is arranged around the housing, high heating efficiency is not obtained so that it takes a while for heating the sensing element to a predetermined temperature and that the consumption of electric power becomes large. 
     Accordingly, one object of the present invention is to provide a compact gas detecting sensor having simple construction of which sensing element exhibits excellent responsive characteristic. 
     Another object of the present invention is to provide an improved gas detecting sensor which can be produced with high working efficiency. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent from the follwing description of embodiments with reference to the accompanying drawings wherein: 
     FIG. 1 is a sectional veiw of a gas detecting device wherein a gas detecting sensor of the present invention is accomodated; 
     FIG. 2 is a perspective view of the sensor of a first embodiment; 
     FIG. 3 to FIG. 6 are views showing the process of producing the sensor of a first embodiment; 
     FIG. 7 to FIG. 9 are views showing the arrangement of electrodes and lead wires in sensors of a second to a fourth embodiments of the present invention, respectively; 
     FIG. 10 to FIG. 13 are views showing the steps for producing a sensor of a fifth embodiment; 
     FIG. 14 is a plane view of a sensor of a sixth embodiment; and 
     FIG. 15 is an end view of the sensor of the sixth embodiment. 
    
    
     SUMMARY OF THE INVENTION 
     The gas detecting sensor of the present invention is provided with a base member which also operates as a heating means, electrodes formed on the base member and a sensing element formed on the base member so as to be contacted with the electrodes. The base member is made of ceramic materialwhich generates heat when an electric current is supplied thereto. 
     According to the present invention, since the base member on which the sensing element is formed, also operates as the heating means, the structure of the sensor can be made simple and the size thereof can be made small. 
     Since the sensing element is formed like a film, the sensing element exhibits excellent responsive characteristic. 
     And since the sensing element is formed on the base member which also operates as a heating means, heating efficiency can be made high. 
     Furthermore, according to the present invention, a gas detecting sensor canbe easily produced by printing the materials for the electrodes and the sensing element on the surface of the ceramic base member and sintering together. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the present invention will be explained in detail in accordance with several embodiments with reference to the accompanying drawings. 
     FIG. 1 and FIG. 2 show a first embodiment of the present invention. 
     In the first embodiment, the gas detecting sensor S comprises a plate shaped base body 2, one pair of electrodes 3c which are continuously formed along both side edges of the upper surface of the base body 2 and on both side surfaces of the top end portion thereof for supplying electric current to the base body 2, a film shaped sensing element 1 whichis formed on the top end portion of the surface of the base body 2 so as tobe electrically insulated therefrom, another pair of electrodes 3e which are formed on the surface of the base body 2 so as to be electrically insulated therefrom, one end of which is connected to the sensing element 1, respectively, lead wires 3a and 3d which are connected to the electrodes 3c and 3e, respectively, and an insulating layer covering the electrodes 3e. 
     The base body 2 is made of ceramic material which generates heat when an electric current is supplied thereto, such as silicon carbide, lanthanchromite, silicon nitride and titanium carbide. 
     The sensing element 1 is made of ceramic material of which electric resistance changes in accordance with the change of the partial pressure of oxygen gas, such as cobalt monoxide, titanium oxide, nickel monoxide, zirconium oxide, tin oxide and zinc oxide. 
     The electrodes 3c and 3e are made of platinum, platinumrhodium alloy or thelike. 
     The insulating layer is made of alumina, spinel, mullite or the like. 
     The base body 2 is accomodated within a protecting cover member 9 made of heat resistant metal and provided with holes 9a for introducing exhaust gases therein, and a pipe member 10 connected to the cover member 9. In the connecting portion of the protecting cover member 9 and the pipe member 10, a flange member 11 for fixing the cover member 9 and the pipe member 10 to an exhaust pipe (not shown) is mounted. 
     The sensor S is supported by a retaining member 6 made of a sintered body such as alumina within the pipe member 10. And to the base end portion of the base body 2, the lead wires 3a and 3d and the sub lead wires 5 which are connected to the lead wires 3a and 3d are fixed within the pipe member10 by means of inorganic binding agent 7. 
     To the pipe member 10, pipe members 13 and 14 are connected in order. Within the pipe members 13 and 14, an insulating pipe 8 made of alumina orthe like, a bush 12 made of fluorine-contained rubber or the like and a heat resistant rubber member 15 made of silicon rubber or the like are accomodated and the sub-lead wires 5 are inserted therethrough and extendsoutside the pipe member 14 with being covered by a covered member 16. 
     Hereinafter, the structure of the detecting sensor S of the present invention will be explained together with the producing steps thereof withreference to FIGS. 3 to 6. 
     As shown in FIGS. 3A, 3B and 3C, on the upper surface of the sintered ceramic base plate 2, insulating layers 4a made of alumina paste or the like are printed along both side edges thereof so as to extend in the longitudinal direction thereof and dried. Then, one portion 3c 1  of each electrode 3c is printed on each insulating layer 4a. And on each sidesurface of the top end portion of the base plate 2, the other portion 3c 2  of each electrode 3c is printed so as to continue from one portion 3c 1  of each electrode 3c. 
     Next, as shown in FIGS. 4A and 4B, on the whole surface of the base plate 2, on which one electrode portion 3c 2  are formed, an insulating layer4b made of alumina paste or the like is printed except for the base end portion (righthand portion in FIG. 4A) of each of one electrode portion 3c 1 . 
     After the insulating layer 4b is dried, a pair of electrodes 3e are printedin the central portion of the surface of the insulating layer 4b. Then, theobtained base plate on which the insulating layers and the electrodes are printed is sintered at 1500° to 1600° C. for about 5 hours. 
     Then, as shown in FIGS. 5A and 5B, the vase end portion of each of the electrodes 3c 1  and 3e is coated with nickel and lead wires 3a and 3d are copper-soldered thereto, respectively. And the surface of the base plate 2 except for the top end portion thereof is covered with electric insulating material such as the paste of glass matter so as to completely cover the electrodes 3c 1  and 3e to form a protecting layer 4c. 
     Next, as shown in FIGS. 6A and 6B, on the surface of the top end portion ofthe base plate 2 on which the protecting layer 4c is not formed, paste likecobalt monoxide (CoO) is printed so as to cover the end of each of the electrodes 3e and sintered at 800° C. for about 1 hour to form a sensing element 1. 
     In FIGS. 3B, 4B, 5B and 6B, each of the electrodes 3c 1 , 3c 2 , and 3e and the insulating layers 4a, 4b and 4c is drawn thicker than the actual thickness thereof in order to clarify the structure thereof. 
     In the detecting sensor produced by the above described producing steps, the base plate 2 which supports the sensing element 1 also operates as theheating means. When the electric current is supplied to the base plate 2 through the lead wires 3a and the electrodes 3c 1 , the temperature of the top end portion of the base plate 2 to which the sensing element 1 is fixed, rapidly rises so that large electric consumption is not required for heating the sensing element 1. 
     Since the sensing element 1 is formed into a thin film, the exhaust gases diffuse within the sensing element 1 for a very short time so that excellent responsive characteristic can be obtained. 
     Furthermore, since the electrodes 3c 1  and 3e which are fixed to the upper surface of the base plate 2 are completely covered with the protecting layer 4c and the sensing element 1, short circuit which would occur between the electrodes 3c and 3e when electric-conductive substance such as carbon contained within the exhaust gases is accumulated therebetween, can be prevented. 
     The electrodes for supplying an electric current to the base body can be modified into various forms due to the resistance value of the base body, electric power, heating temperature, etc. 
     In the second embodiment shown in FIG. 7, electrodes 3c are formed in nearly whole width of each of the top and base ends of the base plate 2. One of the electrodes 3c is connected to one of lead wires 3a through a longitudinally extending portion 3c 1  thereof which is formed along one of the side edges of the base plate 2 while the other electrode 3c is connected to the other lead wire 3a. In the gas detecting sensor of the second embodiment, the whole of the base plate 2 is heated. 
     In the third embodiment shown in FIG. 8, the base plate 2 is formed into a U-shape and lead wires 3a are connected to two base ends of the base plate2. The gas detecting sensor of the third embodiment provided with the U-shaped base plate 2 does not require such insulating layers 4a as shown in FIG. 3. 
     In The fourth embodiment shown in FIG. 9, on the whole of the side surfacesof the base plate 2, electrodes 3c are formed and a lead wire 3a is connected to each end of the electrodes 3c. 
     FIGS. 10 to 13 show a fifth embodiment of the present invention. The structure of the gas detecting sensor of the fifth embodiment will be explained together with the producing method thereof. 
     A pair of semi-columnar base bodies 2a and 2b are made of ceramic material which generates heat by supplying electric current thereto. 
     As shown in FIG. 10, on the flat surface and one semicircular end surface of the base body 2a, insulating layers 4d and 4e are printed. 
     And as shown in FIG. 11, on the flat surface of the base body 2b, an insulating layer 4f is printed except for rectangular portions 21 whcih are formed in both sides of one end portion thereof. And also on one semicircular end surface of the base body 2b, an insulating layer 4h is printed. 
     Next, as shown in FIG. 12, on the flat surface of the base body 2b, electrodes 3f are printed along the side edges thereof. One end of each electrode 3f is located in each rectangular portion 21 and is printed on the base body 2 directly. And on the flat surface of the base body 2b, a pair of electrodes 3h are printed between the electrodes 3f, so as to extend on the side surface of the base body 2b on which the insulating layer 4h is formed. Then, the obtained base body 2b is sintered at 1500° C. to 1600° C. for about 5 hours. 
     Next, as shown in FIG. 13, on the flat surface of the base body 2b, the flat surface of the base body 2a is laid and the base bodies 2a and 2b areheated under pressure to form a columnar structure 2. At this time, the electrodes 3f and 3h are connected to lead wires (not shown), respectively. 
     At last, on one end surface of the structure 2 on which the electrodes 3h are formed, paste of cobalt monoxide is printed and sintered at about 800° C. for about 1 hour to form a sensing element 1. 
     FIGS. 14 and 15 show a sixth embodiment of the present invention. To the top end of a cylindrical supporting member 20, a disc-shaped base body 2 made of ceramic material which generates heat when electric current is supplied thereto, is adhered. On the surface of the base body 2, an insulating layer 4k is formed and on the surface of the insulating layer 4k, a sensing element 1 is formed. Within the base body 2, a pair of electrodes 3c which are connected to lead wires 3a, respectively are embedded. And also within the base body 2 and the insulating layer 4k, a pair of electrodes 3e are embedded so that one end thereof extends to the sensing element 1. The other end of each of the electrodes 3e is connectedto each of lead wires 3d, and the electrodes 3e is covered with an insulating layer 4n within the base body 2. 
     As described above, according to the present invention, the base member on which the sensing element is fomed, also operates as the heating means so that the structure of the sensor can be made simple and the size thereof can be made small. 
     Since the sensing element is formed on the base member which also operates as a heating means, like a film, the sensing element exhibits excellent responsive characteristic and high heating efficiency is obtained. 
     Furthermore, according to the present invention, a gas detecting sensor canbe easily produced by printing the materials for the electrodes and the sensing element on the surface of the ceramic base member and sintering together. 
     Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as setforth herein.