BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a thick film thermistor composition. 
     2. Description of the Prior Art 
     Heretofore, thick film thermistors have been formed according to the ordinary thick film technique comprising steps of screen printing a thermistor paste comprising powders having a thermistor characteristic, glass powder, and an organic vehicle on an insulating substrate, firing, etc., and their structures can be classified into two main groups: thick film resistor type structure (which will be hereinafter referred to as a sheet type) and thick film condenser type structure (which will be hereinafter referred to as a sandwich type). Thermistor materials having a high stability now employed in the bead-form thermistor or disc-form thermistor, or the like have a high specific resistance, for example, 500 Ω-cm or higher, and the glass itself has a very high specific resistance. Thus, when a thick film thermistor is prepared from these materials, the structure is always of sandwich type which naturally provides a low resistance. The sandwich type thick film resistor is thus applied to the ordinary electric circuit. 
     However, the sheet type thermistor has more advantages such as a low cost, a high reliability, etc. in the process for producing thick film thermistors and the structure than the sandwich type thermistor, because of less processing steps, wide interelectrode distance, etc. That is, the sheet type thermistor is industrially more advantageous than the sandwich type thermistor, if the thermistor film itself can be made to have a low resistance. To this end, two methods can be expected: (i) an electroconductive powder is added to a thick film thermistor composition, and (ii) a material having a low resistance is used as the thermistor powder itself. However, when the electroconductive powder is added to the composition according to said method (i) until the resistance of the sheet type thermistor becomes less than 10 kΩ, the thermistor constant is decreased to less than one-half of the thermistor constant of the thermistor powder itself. Thus, it is very difficult to prepare a sheet type thermistor having such a characteristic that a thermistor constant is more than 2,000 K, while the thermistor has the practical resistance. According to said method (ii), a sheet type thermistor having the desired characteristics can be prepared, using thermistor powders having a specific resistance of less than 100 Ω-cm and containing Cu. However, the thermistor material containing Cu has a problem in stability, and a large change in resistance, and thus cannot be used as a heat-sensitive element of high precision. 
     Compound metal oxides of pyrochlore (compound oxides of Cd, Bi, Nb, and Ru) are known as a thermistor material containing an oxide of Ru as a noble metal, but require firing at 1,200° C. for 16 hours (Japanese Laid-Open Patent Application Specification No. 118295/75). 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a thick film composition being freed from said drawbacks of the prior art, and having such advantages that (i) the firing can be completed within a few hours, and the resulting thick film thermistor has (ii) a high thermistor constant, (iii) a low resistance, (iv) a small change in resistance with time, and (v) no development of cracks when baked onto an alumina substrate. 
     As a result of various studies to accomplish said object of the present invention, the present inventors have found that a composition obtained by mixing metal oxide powders of at least two of Mn, Co and Ni, and an oxide powder of Ru as a noble metal, firing the resulting mixture, thereby obtaining a compound oxide thermistor of spinel structure, pulverizing the resulting thermistor, and mixing and kneading the resulting thermistor powder with a glass powder and an oxide powder of Ru for adjusting a resistance is effective. 
     A mixing proportion of the metal oxide powders of at least two of Mn, Co and Ni is preferably within an area surrounded by lines A-B-C-D-E in a triangular diagram shown in FIG. 1 in the accompanying drawings, and points A, B, C, D and E in the triangular diagram have the following compositions: 
     
         ______________________________________Mn (% by atom)         Ni (% by atom)                       Co (% by atom)______________________________________A    80            0            20B    10            0            90C    10           50            40D    50           50             0E    80           20             0______________________________________ 
    
     An amount of the oxide of Ru in the powdery mixture of said metal oxide powders and the oxide power of Ru as the noble metal in said composition is preferably 0.5 to 50% by atom on the basis of total of metals in the powdery mixture. When thermistor powders are prepared from powdery mixtures comprising the metal oxide powders and the oxide powder of Ru as the noble metal outside said range, thick film thermistor compositions are prepared from the resulting thermistor powders, and thick film thermistors are prepared therefrom, the resulting thermistors have a coefficient of heat expansion of more than 120×10 -7  K -1 , and cracks develop on the thermistor films. Thus, such thermistors cannot be practically used. 
     An amount of the oxide powder of Ru for adjusting the resistance in the thick film thermistor composition prepared by aiding the oxide powder of Ru for adjusting the resistance and the glass powder to powders of thermistor of composite oxides of spinel structure obtained by firing the powery mixture of said metal oxide powders and the oxide power of Ru is preferably 1-12% by weight on the basis of the total weight of the thick film thermistor composition. 
     An amount of the glass powder is 20-60% by weight on the basis of the total weight of the thick film thermistor composition. If the amount of the glass powder exceeds 60% by weight, the resistance of the thick film thermistor is so elevated that the thermistor becomes less practical. If the amount of the glass powder is less than 20% by weight, the adhesiveness between the thermistor powders and the oxide powder of Ru baked onto the alumina substrate, or the adhesiveness of these powders to the alumina substrate are so weak that a good film cannot be obtained. When the amount of the oxide powder of Ru for adjusting the resistance is more than 12% by weight, the thermistor constant becomes less than 500K, and the thermistor is less practical. When it is less than 1% by weight on the other hand, the resistance unpreferably becomes dependent upon voltage. 
     The present invention is valid, even if the thermistor contains oxides of Al and Fe. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing a mixing proportion of oxide powders of Mn, Ni and Co in % by atom, and 
     FIG. 2 is a cross-sectional view of a sheet type thermistor. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will be described in detail, referring to Examples. 
     EXAMPLE 1 
     MnO 2  powder, Co 3  O 4  powder and RuO 2  powder were weighed out in a ratio by mole of 1:2:1, and milled and mixed together in an agate mortar for 4 hours. The resulting powdery mixture was placed in an alumina crucible and fired at 900° C. for 2 hours to proceed with solid phase reaction to some extent. Then, the fired mixture was again milled and pulverized in an agate mortar for 4 hours. The resulting powders were fired at 1,250° C. for 2 hours to complete the solid phase reaction, and a thermistor of compound oxides of spinel structure was obtained thereby. The resulting thermistor was pulverized to powders in a ball mill, and the resulting powders were mixed with glass powder having the composition shown in Table 1 and RuO 2  powder for adjusting the resistance in proportions shown in Table 2, Nos. 2-9. 
     10 g each of the resulting powdery mixtures were weighed out, and each powdery mixture was mixed in an agitating grinder for one hour, then admixed with an organic binder (an α-terpineol solution containing ethyl cellulose), and further kneaded for one hour, whereby a thermistor paste was obtained. 
     A silver-palladium electroconductive paste was screen printed on an alumina substrate 1 shown in FIG. 2, and fired at 850° C. for 10 minutes to form electrodes 2 with an electrode width of 3.5 mm at an electrode distance of 0.5 mm. Then, said thermistor paste was printed thereon, fired at 800° C. to form a thermistor layer 3 with a thermistor width of 3.0 mm and a thermistor thickness of 40 μm, and a sheet type thermistor was obtained thereby. 
     The thermistor itself had a specific resistance of 5 Ω-cm and a thermistor constant of 2,450K. Resistance, thermistor constant, and change in resistance when left standing at 150° C. for 2,000 hours of the thus formed sheet type thermistors are shown in Table 2, Nos. 2-9. 
     As is evident from Table 2, all of Nos. 2-8 had lower resistances than No. 1 containing no RuO 2 , and had thermistor constants substantially equal to that of No. 1 containing no RuO 2 . That is, sheet type thermistor elements having a low resistance and a high thermistor constant could be obtained. Their stability was also good. 
     On the other hand, No. 1 containing no RuO 2  in Table 2 had a higher resistance than those containing RuO 2 , and No. 9 containing 14% by weight of RuO 2  had a small thermistor constant, and the thick film thermistor No. 1 containing no RuO 2  had a dependency of resistance upon voltage, and thus they had a problem in practice. 
     
                       TABLE 1______________________________________SiO.sub.2  PbO    B.sub.2 O.sub.3                  Al.sub.2 O.sub.3                        Bi.sub.2 O.sub.3                                CaO  BaO______________________________________24     25     20       4     6       6    15______________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________              Sheet type thermistor characteristicsMixing proportion of powders    Stability,(wt %)                    Thermistor                           change in                                Voltage   Thermistor    Glass         RuO.sub.2              Resistance                     constant                           resistance                                depend-No.   powder    powder         powder              (Ω, 25° C.)                     B(K)  (%)  ency__________________________________________________________________________1  60    40   0    1.6 × 10.sup.4                     2320  +1.6 poor2  59    40   1    1.3 × 10.sup.4                     2320  +1.6 good3  58    40   2    7.3 × 10.sup.3                     2330  +1.6 good4  56    40   4    5.2 × 10.sup.3                     2320  +1.4 good5  54    40   6    3.4 × 10.sup.3                     2310  +1.5 good6  52    40   8    1.3 × 10.sup.3                     2300  +1.3 good7  50    40   10   9.8 × 10.sup.2                     2300  +1.2 good8  48    40   12   2.3 × 10.sup.2                     2000  +1.3 good9  46    40   14     1.4 × 10.sup.-1                      340  +1.0 good__________________________________________________________________________ 
    
     EXAMPLE 2 
     MnO 2  powder, NiO powder, Fe 2  O 3  powder and RuO 2  powder were weighed out in a ratio by mole of 3:2:0.5:0.5, and subjected to solid phase reaction in the same manner as in Example 1 to obtain a thermistor of compound metal oxide of spinel structure. The resulting thermistor was pulverized to powders in the same manner as in Example 1. The resulting powders were mixed with the glass powder having the composition given in Table 1 and RuO 2  powder for adjusting the resistance in proportions given in Table 3, Nos. 2-9 and 11-18. 10 g each of the resulting mixtures was prepared into a thermistor paste in the same manner as in Example 1, and a sheet type thermistor was prepared therefrom. Resistance, thermistor constant and change in resistance when left standing at a high temperature of the sheet type thermistors are given in Table 3, Nos. 2-9 and 11-18. The thermistor material itself had a specific resistance of 42 Ω-cm and a thermistor constant of 3,000K. 
     As is evident from Table 3, Nos. 2-9, and 11-18 had a lower resistance than No. 1 and No. 10 containing no RuO 2  in Table 3, when the content of glass powder was constant and had a thermistor constant substantially equal to that of No. 1 containing no RuO 2 . That is, sheet type thermistor elements having a low resistance and a high thermistor constant could be obtained. Their stability was also good. 
     On the other hand, No. 1 and No. 10 containing no RuO 2  in Table 3 had a higher resistance than those containing RuO 2 , and No. 9 and No. 18 containing 14% by weight of RuO 2  in Table 3 had a small thermistor constant, the thick film thermistors No. 1 and No. 10 containing no RuO 2  had a dependency of resistance upon voltage, and thus they had a problem in practice. 
     
                                           TABLE 3__________________________________________________________________________              Sheet type thermistor characteristicsMixing proportion of powders    Stability,(wt %)                    Thermistor                           change in                                Voltage   Thermistor    Glass         RuO.sub.2              Resistance                     constant                           resistance                                depend-No.   powder    powder         powder              (Ω, 25° C.)                     B(K)  (%)  ency__________________________________________________________________________ 1 65    35   0    1.3 × 10.sup.5                     2850  +1.2 poor 2 64    35   1    9.7 × 10.sup.4                     2850  +1.1 good 3 63    35   2    9.3 × 10.sup.4                     2840  +1.0 good 4 61    35   4    6.6 × 10.sup.4                     2830  +1.3 good 5 59    35   6    4.3 × 10.sup.4                     2840  +1.2 good 6 57    35   8    9.7 × 10.sup.3                     2830  +1.0 good 7 55    35   10   7.8 × 10.sup.3                     2820  +0.9 good 8 53    35   12   1.8 × 10.sup.3                     2060  +1.3 good 9 51    35   14     2.3 × 10.sup.-1                      440  +1.1 good10 80    20   0    6.5 × 10.sup.4                     2850  +1.2 poor11 79    20   1    9.3 × 10.sup.3                     2850  +1.1 good12 78    20   2    4.7 × 10.sup.3                     2840  +1.0 good13 76    20   4    3.3 × 10.sup.3                     2830  +1.3 good14 74    20   6    2.6 × 10.sup.3                     2840  +1.2 good15 72    20   8    4.5 × 10.sup.2                     2830  +1.0 good16 70    20   10   3.7 × 10.sup.2                     2820  +0.9 good17 68    20   12   1.1 × 10.sup.2                     2060  +1.3 good18 66    20   14     1.4 × 10.sup.-1                      440  +1.1 good__________________________________________________________________________ 
    
     EXAMPLE 3 
     MnO 2  powder, NiO powder, Fe 2  O 3  powder, Al 2  O 3  powder and RuO 2  powder were weighed out in a ratio by mole of 3:3:0.3:0.4:1 and subjected to solid phase reaction in the same manner as in Example 1 to obtain a thermistor of compound oxides of spinel structure. The thermistor was pulverized to powders in the same manner as in Example 1. The resulting powders were mixed with glass powder having the composition given in Table 1 and RuO 2  powder for adjusting the resistance in proportions given in Table 4, Nos. 2-9 and 11-18. 10 g each of the resulting mixtures was weighed out and formed into a sheet type thermistor in the same manner as in Example 1. The thermistor material itself had a specific resistance of 10 Ω-cm and a thermistor constant of 2,640K. Resistance, thermistor constant, and change in resistance when left standing at a high temperature of the sheet type thermistors are given in Table 4, Nos. 2- 9 and 11-18. 
     As is evident from Table 4, Nos. 2-9 and Nos. 11-18 had a lower resistance than No. 1 and No. 10 containing no RuO 2 , when the content of glass powder was constant and had a thermistor constant substantially equal to that of No. 1 and No. 10 containing no RuO 2 . That is, sheet type thermistor elements having a low resistance and a high thermistor constant could be obtained. Their stability was also good. 
     On the other hand, No. 1 and No. 10 containing no RuO 2  in Table 4 had a higher resistance than those containing RuO 2 , and No. 9 and No. 18 containing 14% by weight of RuO 2  in Table 4 had a small thermistor constant, and the thick film thermistors No. 1 and No. 10 containing no RuO 2  had a dependency of resistance upon voltage, and thus they had a problem in practice. 
     
                                           TABLE 4__________________________________________________________________________              Sheet type thermistor characteristicsMixing proportion of powders    Stability,(wt %)                    Thermistor                           change in                                Voltage   Thermistor    Glass         RuO.sub.2              Resistance                     constant                           resistance                                depend-No.   powder    powder         powder              (Ω, 25° C.)                     B(K)  (%)  ency__________________________________________________________________________ 1 65    35   0    3.4 × 10.sup.4                     2530  +2.0 poor 2 64    35   1    2.7 × 10.sup.4                     2530  +2.0 good 3 63    35   2    1.5 × 10.sup.4                     2520  +2.0 good 4 61    35   4    1.1 × 10.sup.4                     2530  +1.5 good 5 59    35   6    7.1 × 10.sup.3                     2520  +1.6 good 6 57    35   8    2.7 × 10.sup.3                     2510  +1.5 good 7 55    35   10   2.1 × 10.sup.3                     2320  +1.7 good 8 53    35   12   4.8 × 10.sup.2                     2010  +1.8 good 9 51    35   14     1.4 × 10.sup.-1                      380  +1.5 good10 40    60   0    9.8 × 10.sup.4                     2530  +2.0 poor11 39    60   1    6.4 × 10.sup.4                     2530  +2.0 good12 38    60   2    4.3 × 10.sup.4                     2520  +2.0 good13 36    60   4    2.7 × 10.sup.4                     2530  +1.5 good14 34    60   6    1.6 × 10.sup.4                     2520  +1.6 good15 32    60   8    6.8 × 10.sup.3                     2510  +1.5 good16 30    60   10   6.0 × 10.sup.3                     2320  +1.7 good17 28    60   12   2.3 × 10.sup.3                     2010  +1.8 good18 26    60   14   1.4 × 10.sup.                      380  +1.5 good__________________________________________________________________________ 
    
     EXAMPLE 4 
     Mn 3  O 4  powder, Co 3  O 4  powder, NiO powder, and RuO 2  powder were weighed out in a ratio by mole of 2:1:1.5:1, and subjected to a solid phase reaction in the same manner as in Example 1 to obtain a thermistor of compound oxide of spinel structure. The resulting thermistor was pulverized to powders in the same manner as in Example 1. The resulting powders were mixed with glass powder having the composition given in Table 1 and RuO 2  powder for adjusting the resistance in proportions given in Table 5, Nos. 2-9. 
     10 g each of the resulting mixtures was weighed out, and prepared into a sheet type thermistor in the same manner as in Example 1. The thermistor material itself had a specific resistance of 10 Ω-cm and a thermistor constant of 2,640K. Resistance, thermistor constant and change in resistance when left standing at a high temperature of the sheet type thermistors are shown in Table 5, Nos. 2-9. 
     As is evident from Table 5, Nos. 2-9 have a lower resistance than No. 1 containing no RuO 2 , and had a thermistor constant substantially equal to that of No. 1 containing no RuO 2 . That is, sheet type thermistor elements having a low resistance and a high thermistor constant can be obtained. Their stability was also good. 
     On the other hand, No. 1 containing no RuO 2  in Table 5 had a higher resistance than those containing RuO 2 , and No. 9 containing 14% by weight of RuO 2  in Table 5 had a small thermistor constant, and No. 1 containing no RuO 2  had a dependency of resistance upon voltage and thus they had a problem in practice. 
     
                                           TABLE 5__________________________________________________________________________              Sheet type thermistor characteristicsMixing proportion of powders    Stability,(wt %)                    Thermistor                           change in                                Voltage   Thermistor    Glass         RuO.sub.2              Resistance                     constant                           resistance                                depend-No.   powder    powder         powder              (Ω, 25° C.)                     B(K)  (%)  ency__________________________________________________________________________1  60    40   0    6.3 × 10.sup.4                     2860  +0.7 poor2  59    40   1    5.1 × 10.sup.4                     2850  +0.6 good3  58    40   2    2.8 × 10.sup.4                     2840  +0.7 good4  56    40   4    2.0 × 10.sup.4                     2830  +0.4 good5  54    40   6    1.3 × 10.sup.4                     2830  +0.5 good6  52    40   8    4.1 × 10.sup.3                     2800  +0.4 good7  50    40   10   3.6 × 10.sup.3                     2800  +0.6 good8  48    40   12   9.6 × 10.sup.2                     2790  +0.5 good9  46    40   14   2.6 × 10.sup.                      480  +0.5 good__________________________________________________________________________
A thick film thermistor composition is prepared by mixing metal oxide powders of at least two of Mn, Co and Ni, and oxide powder of Ru as a noble metal, firing the resulting mixture, thereby obtaining a compound oxide thermistor of spinel structure, pulverizing the resulting compound oxide thermistor, and mixing and kneading the resulting thermistor powder with glass powder and oxide powder of Ru for adjusting a resistance.