Abstract:
Disclosed herein is a critical temperature sensitive resistor material which comprises 60 to 90% by weight of VO 2  and 40 to 10% by weight of RuO 2 . This material exhibits hysteresis of resistance that decreases remarkably over a temperature range in which the resistance varies greatly, and is hence used for measuring the temperature maintaining a high precision.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a critical temperature sensitive resistor material of which the electric resistance changes greatly at a given temperature, and particularly relates to a thermally sensitive low hysteresis resistor material. 
     Thermally sensitive resistor materials include a critical temperature sensitive resistor material of which the electric resistance changes greatly at a given temperature. That is, a single crystal of VO 2  or a sintered body of VO 2  exhibits a resistance that changes by a factor of several thousand to several tens of thousands at a temperature of about 68° C. It is said that the phenomenon of the great resistance change is attributed to the fact that VO 2  undergoes a change in crystalline structure at a temperature of about 68° C. between the monoclinic system (below 68° C.) and the tetragonal system (above 68° C.), and the fact that the electric conduction mechanism changes from electric conduction mechanism of semiconductor type of a high resistance into electri conduction mechanism of metallic type of a low resistance. Such a change in crystalline structure, i.e., a change which involves migration of atoms cause hysteresis in which change of resistance is delayed during the transient temperature conditions, and resistance characteristics are different between when the temperature rises from a low temperature to a high temperature and vice versa. At a certain temperature of about 68° C., therefore, the resistor material exhibits two resistances that differ greatly. Usually, the temperature width of hysteresis is 2° C. Therefore, the precision for detecting the temperature or for controlling the temperature is ±1° C., which is not adequate for high-precision measurement. 
     Japanese Patent Publication No. 8547/1971 is quoted to show the state of the art. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a critical temperature sensitive resistor material which is capable of measuring temperature while maintaining a precision higher than that of the aforementioned conventional art. 
     The above object is accomplished by reducing the hysteresis of electric resistance, that is attained by mixing VO 2  and RuO 2  together followed by the heat-treatment in order to impart distortion to the crystal structure of VO 2  by RuO 2 . 
     That is, if distortion is imparted to the crystal structure in advance, the hysteresis of resistance decreases remarkably within a temperature range in which the electric conduction mechanism is changed by phase transformation, i.e., in which the resistance changes greatly. Though the change of resistance is not so abrupt and the resistance ratio decreases to some extent over the temperature range in which the resistance changes greatly, the atoms start to migrate gradually from the distorted crystal structure over the temperature range in which the resistance changes greatly, and the resistance changes gradually. Further, great change in resistance takes place over a broad temperature range. However, the measuring error falls within ±1° C. 
     Here, the sintered body of VO 2  and RuO 2  should preferably be composed of 60 to 90% by weight of VO 2  and 40 to 10% by weight of RuO 2 . The sintered product is obtained by effecting the sintering in a non-oxidizing atmosphere of, for example, nitrogen or helium gas at a temperature of 950° C. to 1100° C. for 1.5 hours to 3 hours. The VO 2  and RuO 2  should preferably have an average particle size of 1 μm to 4 μm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a graph showing resistance vs. temperature characteristics of a critical temperature sensitive device using the material of the present invention and of device using materials of different compositions. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be described below in further detail by way of an embodiment. 
     VO 2  powder having an average particle size of 2 μm and the RuO 2  powder having an average particle size of 2 μm were mixed at ratios of examples Nos. 1 to 14 of Table 1. In Table 1, the amount of VO 2  gradually decreases from 100% by weight to 50% by weight starting from example No. 1 toward example No. 14. Examples Nos. 6 to 12 lie within the composition range of the present invention in which the amount of VO 2  is from 90% by weight to 60% by weight. In examples Nos. 1 to 5, the amount of VO 2  is larger than the amounts of the above range. In examples Nos. 13 and 14, on the other hand, the amount of VO 2  is smaller than the amounts of the above range. The mixtures were compressed to prepare compact pellets of a square column shape each having sides of 2 mm, 2 mm and 1 mm. The pellets were heat-treated at a temperature of as high as 1000° C. for two hours. The heat-treatment must be effected in a nonoxidizing atmosphere in order to suppress substantially the oxidation of VO 2 . For this purpose, therefore, the heat-treatment was carried out in an nitrogen atmosphere containing 30 ppm of oxygen. 
     A silver paste was applied to both surfaces of the thus obtained sintered bodies to form electrodes, and their characteristics were measured. Table 1 shows resistances at 50.00° C., resistances at 100.00° C., ratios of resistances at 50.00° C. to resistances at 100.00° C., and maximum resistance ratios of hysteresis. Further, FIG. 1 shows temperature vs. resistance characteristics of representative samples. A curve A of FIG. 1 represents the data of example No. 10 (70% by weight of VO 2 ) which is encompassed by the embodiment of the invention, a curve B represents the data of a sample which contains RuO 2  in an amount smaller than 10% by weight, and a curve C represents the data of a sample which contains RuO 2  in an amount greater than 40% by weight. 
     To obtain a resistance change which is greater than a conventional thermistor element, it is desired that the ratio of resistance at 50.00° C. to resistance at 100.00° C. is greater than 10. It is further desired that a maximum resistance ratio of hysteresis is less than 1.05 such that the difference thereof is smaller than 5%. Moreover, the resistor material should desirably have a resistance of less than 100 kiloohms at 50.00° C. so that it can be employed for practical circuits. 
     As will be apparent from Table 1, characteristics that satisfy all of the above-mentioned requirements are obtained when the content of RuO 2  ranges from 10% by weight to 40% by weight. Little effect of RuO 2  is exhibited when its amount is less than 10% by weight. That is, although a great change in resistance is exhibited at around 68° C., the hysteresis is so great that the resistance ratio exceeds 1.05 (curve B of FIG. 1). Further, as the amount of RuO 2  exceeds 40% by weight, the resistance change required for the thermally sensitive element becomes small; i.e., ratio of resistance at 50.00° C. to resistance at 100.00° C. becomes smaller than 10. In this case, the change of resistance relative to the temperature decreases as indicated by the curve C of FIG. 1. The curve A of FIG. 1 represents data (temperature vs. resistance characteristics of example No. 10) of a preferred embodiment that lies within the composition range of the present invention mentioned earlier. It will be understood that satisfactory resistance change is exhibited with very little hysteresis. Accordingly, the resistor material of the invention makes it possible to measure with high precision, and can be fully adapted to forming a thermally sensitive element for measuring very small changes in temperature. 
     
                                           TABLE 1__________________________________________________________________________Composition of thermallysensitive materials           Resistance at                  Resistance at  Maximum resistance   VO.sub.2    RuO.sub.2           50.00° C.                  100.00° C.                         Resistance ratio                                 ratio ofNo.   (wt %)    (wt %) R.sub.1 (ohms)                  R.sub.2 (ohms)                         R.sub.1 /R.sub.2                                 hysteresis__________________________________________________________________________1  100    0     35.5.sup.K                  7.20   4,660   2,1352  99     1     22.3.sup.K                  5.62   3,967   1,1203  95     5     16.5.sup.K                  5.53   2,984   8934  92     8     10.8.sup.K                  5.47   1,974   3565  91     9     9.86.sup.K                  5.36   1,840   1.106  90    10     4.35.sup.K                  4.67   931     1.057  89    11     1.06.sup.K                  4.16   255     1.048  85    15     638    3.95   162     1.039  80    20     365    3.16   116     1.0210 70    30     177    2.81   63.0    1.0111 65    35     86.4   2.45   35.3    1.0112 60    40     26.5   2.03   13.1    1.0113 55    45     10.8   1.79   6.03    1.0014 50    50     5.01   1.58   3.16    1.00__________________________________________________________________________