Abstract:
A method of producing a flat solid electrolyte layer of a flat film type oxygen sensor is disclosed. The method comprises in steps, (a) preparing first and second electrolyte pastes each containing stabilizer, the content of stabilizer in the first electrolyte paste being smaller than that in the second electrolyte paste, (b) applying the first electrolyte paste onto an electrode layer and then applying the second electrolyte paste onto the outer face of the first electrolyte paste to form a layered paste heap on the electrode layer, and (c) firing the layered paste heap to form a solid electrolyte layer on the electrode layer. With this production method, the stabilizer is uniformly and homogeneously distributed into the body of the fired electrolyte layer.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to an oxygen sensor for measuring the oxygen concentration in a fluid, the sensor being of the type having a layer of an oxygen ion conductive solid electrolyte. More specifically, the present invention is concerned with a method of forming a so-called flat film type oxygen sensor which is suitable for measuring the oxygen concentration in an engine exhaust gas. 
     BACKGROUND OF THE INVENTION 
     It is known that flat film type oxygen sensors which use a flat solid electrolyte layer show better performance in EMF (electromotive force) characteristic and responsiveness as compared with tubular type oxygen sensors which use a tubular solid electrolyte. However, some of such oxygen sensors fail to exhibit their maximum performance because of inhomogeneous distribution of a stabilizer in the body of the solid electrolyte, originating from the inherent production method of the sensors. This drawback encountered in the conventional oxygen sensors will become apparent hereinafter. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a method of producing a flat film type oxygen sensor including an oxygen ion conductive solid electrolyte layer. The method comprises in steps (a) preparing first and second electrolyte pastes each containing stabilizer, the content of stabilizer in the first electrolyte paste being smaller than that in the second electrolyte paste, (b) applying the first electrolyte paste onto an electrode layer and then applying the second electrolyte paste onto the outer face of the first electrolyte paste to form a layered paste heap on the electrode layer, and (c) firing the layered paste heap to form a solid electrolyte layer on the electrode layer. 
     It is an object of the present invention to provide a method of producing a flat film type oxygen sensor including a flat solid electrolyte layer of which stabilizer is uniformly and homogeneously distributed in the body of the electrolyte layer. 
     It is another object of the present invention to provide a method which assures production of a flat solid electrolyte layer without formation of cracks therein. 
     Other objects and advantages of the present invention will become clear from the following description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section view of a conventionally used flat film type oxygen sensor; 
     FIG. 2 is a graph depicting the electrical conductivity of Y 2  O 3  -ZrO 2  system solid electrolyte with respect to Y 2  O 3  content in the electrolyte; 
     FIG. 3 is a cross section view of a flat film type oxygen sensor which is produced through a method according the present invention; 
     FIG. 4 is a plan view of the sensor of FIG. 3; 
     FIG. 5 is a graph showing the relationship between the distance from the outer surface of the solid electrolyte layer and Y 2  O 3  content in the portion at the distance from the outer surface, on six samples; 
     FIG. 6 is a graph showing the specific resistances of the solid electrolyte layers, of the six samples, containing different amounts of Y 2  O 3  ; and 
     FIG. 7 is a graph showing the output characteristics of the oxygen sensors with respect to the condition of the exhaust gas to which the sensors are exposed. 
    
    
     DESCRIPTION OF PRIOR ART 
     Prior to describing in detail the invention, an outlined explanation of conventional flat film type oxygen sensor will be made with reference to FIG. 1 in order to clarify the invention. 
     Referring to FIG. 1 of the drawings, there is shown a conventional flat film type sensor which is designated by numeral 10. The sensor 10 generally comprises a partition layer 12 of ceramics designed to serve as a structurally basic member of the sensor 10, a first or reference electrode layer 14 deposited on the partition layer 12, a layer 16 of an oxygen ion conductive solid electrolyte deposited on the first electrode layer 14, a second or measurement electrode layer 18 deposited on the solid electrolyte layer 16, and a protective layer 20 covering both the second electrode layer 18 and side surfaces of the solid electrolyte layer 16 entirely and intimately, as shown. Usually, such sensor as mentioned above is produced by taking the following production process. 
     To produce the sensor 10, a platinum paste is applied or painted onto a surface of a sintered alumina sheet 12, and then the heaped paste on the sheet 12 is air-dried and then fired to form the first or reference electrode layer 14 on the sheet 12. An electrolyte paste composed of a certain amount of powdered electrolyte material containing a certain amount of stabilizer, and a certain amount of organic vehicle is applied or painted onto the fired electrode layer 14, and then the heaped electrolyte paste is air-dried and then fired to form the solid electrolyte layer 16. Then, the same platinum paste as producing the first electrode layer 14 is applied or painted onto the fired electrolyte layer 16, and the heaped platinum layer on the layer 16 is air-dried and then fired to form the second or measurement electrode layer 18. Finally, a protective layer material such as powdered calcium zirconate is deposited on the entire front surface of the multi-layered article by plasma spraying technique to form the porous protective layer 20. 
     Hitherto, the application of the electrolyte paste onto the reference electrode layer 14 has been made by a screen-printing method in which for obtaining the desired thickness of the electrolyte paste heap, several coats are repeatedly applied to the first electrode layer 14. However, in this production method, it has been revealed that emission of the stabilizer which inevitably occurs at the firing process of the electrolyte paste causes inhomogeneous distribution of components of the electrolyte in the direction of the thickness of the same, with the result that the oxygen ion conductivity of the solid electrolyte 16 thus produced lowers considerably thereby to decrease the output characteristic of the oxygen sensor. 
     Therefore, elimination of this drawback is an essential object of the present invention. 
     DESCRIPTION OF THE INVENTION 
     As will become clear as the description proceeds, the feature of the present invention resides in that for deposition of the electrolyte paste on the first or reference electrode layer, several kinds of electrolyte paste which contain different amounts of stabilizer are painted onto one over another so that upon completion of firing of the multi-layered paste heap, even or uniform distribution of the stabilizer is obtained in the fired solid electrolyte. 
     For putting the concept of the invention into practical use, several experiments were carried out. 
     EXPERIMENT 
     For studying the amount of lost stabilizer of the produced electrolyte layer with respect to the firing condition of the electrolyte paste, several test samples were prepared. Each sample was produced by applying via &#34;screen-printing method&#34; an electrolyte paste composed of a certain amount of powdered Y 2  O 3  -ZrO 2  and a certain amount of organic vehicle onto a suitable sheet member, air-drying and firing the paste on the sheet member. Then, the samples thus produced were subjected to several evaluation tests. The results of these tests are shown in Tables I, II and III. 
     
                       TABLE I______________________________________            Y.sub.2 O.sub.3 reductionFiring Temperature            rate at the outer surface(°C.)     of the solid electrolyte (%)______________________________________1300             311400             361500             441600             56______________________________________ 
    
     
                       TABLE II______________________________________Firing time for which             Y.sub.2 O.sub.3 reduction ratethe electrolyte paste             at the outer surfacewas fired (hours) of the solid electrolyte (%)______________________________________0                 312                 364                 406                 458                 50______________________________________ 
    
     
                       TABLE III______________________________________Distance from the outersurface of the solid              Y.sub.2 O.sub.3 reduction rateelectrolyte layer (μm)              (%)______________________________________ 0                 36 5                 1810                 815                 420                 0______________________________________ 
    
     Table I depicts the reduction rate of the stabilizer (Y 2  O 3 ) at the surface of the solid electrolyte layer with respect to the temperature at which the electrolyte paste was fired for two hours. As will be seen from this table, the Y 2  O 3  reduction rate becomes greater as the firing temperature increases. Table II shows the relationship between the reduction rate of the stabilizer (Y 2  O 3 ) at the surface of the fired solid electrolyte layer and the firing time for which the electrolyte paste was fired at a temperature of 1400° C. As will be known from this table, the emission degree of the stabilizer (Y 2  O 3 ) in the electrolyte layer becomes greater as the firing time increases. Further, it will be noted that considerable degree of emission of the stabilizer occurs at the initial stage of the firing process. Table III shows the relationship between the distance from the outer surface of the solid electrolyte layer and Y 2  O 3  reduction rate at the position, on a sample which was produced by firing the electrolyte paste at a temperature of 1400° C. for two hours. From this table, it will be noted that greatest emission of the stabilizer occurs at the outer surface of the solid electrolyte layer. 
     From the above, it will be known that the emission of the stabilizer (Y 2  O 3 ) takes place inevitably at the firing process. Experiments have further revealed that such undesired emission phenomenon occurs also in a case using CaO or MgO as the stabilizer. 
     The present invention is provided by taking the above mentioned facts into consideration. As will become apparent hereinafter, the electrolyte layer of the oxygen sensor of the present invention is produced by preparing several kinds of electrolyte paste which have different amounts of stabilizer, and applying these pastes one over another in such a manner that a layer of the layered electrolyte paste thus heaped which may exhibit greater stabilizer emission at the firing process is formed by a paste which contains a larger amount of stabilizer, the heaped electrolyte paste thus prepared being air-dried and then fired. With this production method, even or uniform distribution of the stabilizer is achieved throughout the body of the solid electrolyte produced, so that sufficient oxygen ion conductivity is given to the electrolyte from low temperature to high temperature. 
     FIG. 2 is a graph showing the relationship between Y 2  O 3  content in a solid electrolyte layer and the electrical conductivity of the electrolyte layer at a temperature of about 800° C., the electrolyte layer being constructed of yttria stabilized zirconia (Y 2  O 3  -ZrO 2 ). As will be seen from this graph, favorable electrical conductivity is achieved within a range from about 2 wt.% to about 23 wt.%. It has been revealed that in using CaO or MgO as the stabilizer, substantially the same result as of using Y 2  O 3  is given. 
     From this graph, it will be noted that the stabilizer content should be determined within the range from about 2 wt.% to about 23 wt.%. If the stabilizer content is less than 2 wt.%, the electrical conductivity of the solid electrolyte is greatly decreased as is known from the graph, and if the stabilizer content is greater than 23 wt.%, there occurs severe problem in which the electrolyte layer may be cracked at the firing process. 
     The present invention will be further illustrated by the following example. 
     EXAMPLE 
     Four kinds of electrolyte paste were prepared for making the solid electrolyte layer according to the present invention, these pastes being designated by references No. 1, No. 2, No. 3 and No. 4 in Table-IV. As shown in the Table, each of these pastes was composed of certain amount of ZrO 2  stabilized by certain amount of Y 2  O 3 , and organic vehicle (mixture of ethylcellulose and terpineol) which are kneaded together. 
     
                       TABLE IV______________________________________                         content ofPowdered materials of         the powderedthe electrolyte     organic   materialsY.sub.2 O.sub.3 (wt. %)        ZrO.sub.2 (wt. %)                   vehicle   (wt. %)______________________________________No. 1 12         88         ethyl-  50                       cellulose +                       terpineolNo. 2 14         86         ethyl-  50                       cellulose +                       terpineolNo. 3 16         84         ethyl-  50                       cellulose +                       terpineolNo. 4 18         82         ethyl-  50                       cellulose +                       terpineol______________________________________ 
    
     Six samples of oxygen sensors A, B, C, D and F each having a construction as illustrated in FIGS. 3 and 4 were produced. Each sensor 22 was produced by taking the following production process. 
     A platinum paste composed of 70 wt.% powdered platinum and 30 wt.% organic vehicle (such a lacquer thinner) was applied or painted via the screen-printing method onto a sintered alumina sheet 24. The paste on the sheet 24 was air-dried and fired to form a first or reference electrode layer 26. Then, selected three of the pastes described on Table IV were successively applied or painted by means of the screen-printing method onto the electrode layer 26 one over another in such an order as indicated by Table-V to form a triple-layered paste heap 28 (28a, 28b and 28c) on the electrode layer 26. The paste heap 28 was air-dried and fired under the conditions indicated by Table-V to form a solid electrolyte layer 28. Then, the same platinum paste as producing the reference electrode layer 26 was applied or painted by the screen-printing method onto the solid electrolyte layer 28. The paste heap on the layer 28 was air-dried and then fired to form a second or measurement electrode layer 30. 
     
                       TABLE V______________________________________Selected electrolyte pastesFirst   Second    ThirdSam- coating coating   coating                        Firingples (28a)   (28b)     (28c) condition                                 Note______________________________________A    No.4    No.4      No.4B    No.3    No.3      No.3           conventionalC    No.1    No.1      No.1           methodD    No.1    No.1      No.3  1400° C. ×                                 methodE    No.1    No.1      No.2  2 hours  according toF    No.1    No.2      No.4           the invention______________________________________ 
    
     EXAMINATION I 
     The oxygen sensors A, B, C, D, E and F were subjected to a stabilizer distribution test in which the relationship between the distance from the outer surface of the solid electrolyte and Y 2  O 3  content in the portion at the distance from the outer surface was measured on each sensor. The results are shown in the graph of FIG. 5. 
     As is understood from the graph, in the sensors A, B and C which were produced by conventional method, there is considerable difference in Y 2  O 3  distribution in the direction of the thickness of the electrolyte layer, while in the sensors D, E and F which were produced by the method of the invention, the difference in Y 2  O 3  distribution is smaller than that in the conventional sensors. Now, it should be noted that the solid electrolyte layers of the sensors D, E and F were each produced such that the paste which forms the outermost race 28c of the solid electrolyte layer contains greater amount of Y 2  O 3  than the other two races 28a and 28b, by considering that at firing process of the paste, greater emission of the stabilizer occurs at the outer surface of the solid electrolyte layer. It has been revealed that the sensors D, E and F exhibite excellent stability in generating electromotive force (EMF). 
     EXAMINATION II 
     Specific resistances of the solid electrolyte layers of the sensors were measured. The result is shown in the graph of FIG. 6. As is understood from this graph, in the electrolyte layers of the sensors D, E and F produced by the method of the invention, the specific resistances were considerably low as compared with those of the sensors A, B and C produced by the conventional method. Further, the specific resistance in each sensor D, E or F with respect to measured points was not so greatly dispersed as that of the sensor A, B and C. The desired phenomenon given to the electrolyte layers of the sensors D, E and F brings about decrease in internal resistance loss of these sensors, so that stable generation of electromotive force (EMF) is expected. 
     EXAMINATION III 
     Output characteristics of the oxygen sensors A, B, C, D, E and F were examined. The test was such made that the sensors A to F were disposed in a conduit through which two kinds of exhaust gas were alternately passed, one of which was a gas richer than stoichiometric and the other of which was a gas leaner than stoichiometric. The result is shown by the graph of FIG. 7. As is seen from this graph, the oxygen sensors A, B and C produced by the conventional method showed an undesired result wherein the electromotive force thereof was considerably low as compared with that of the sensors D, E and F produced by the method of the present invention, and the electromotive force thereof dropped rapidly to zero level even when the richer exhaust gas which has sufficient amount of oxygen to cause generation of electromotive force (EMF) of the sensors was present. It was found that the rapid drop in electromotive force of the sensors was originated from formation of cracks in the solid electrolyte layers of the sensors A, B and C. While, in the oxygen sensors D, E and F produced by the method of the invention, such problems were not recognized.