Thermistor Composition and Thick Film Thermistor

1. Field of the Invention

This invention relates to an element suitable for carrying out the temperature sensing by the changes of resistivity with changes of temperature (said element is referred to as "thermistor" hereinafter) and more particularly it relates to a thermistor composition capable of providing a thermistor in which a resistivity in the range of 1.OMEGA.-1M.OMEGA. can be obtained, dispersion of resistivity is within .+-.15%, voltage-dependence of resistivity, namely, varistor constant is 1.00-1.05, current noise is 2% or less and resistivity aging characteristic is 2% or less, and furthermore, this invention relates to a thick film thermistor prepared using said composition.

2. Description of the Prior Art

Heretofore, thick film thermistors have been produced, in the same manner as in the production of general thick film elements, by preparing a thermistor paste from a mixture of a complex oxide powder of traditional metals such as nickel, manganese, cobalt and the like which has thermistor characteristic, and shows change of resistivity with changes of temperature (said powder will be referred to as "thermistor characteristic powder" hereinafter), a glass frit and an organic vehicle. Said thick film thermistors, however, have various defects, namely, (1) low resistivity of less than 1 k.OMEGA. cannot be obtained, (2) resistivity greatly changes with changes of thickness of thermistor film and dispersion of resistivity is very large, i.e., .+-.30%, (3) voltage-dependence of resistivity is high, i.e., varistor constant is about 1.10, (4) current noise is high, i.e., about 10%, (5) resistivity aging characteristic is more than 5%, etc. Thus, thick film thermistor excellent in aging characteristic and high in accuracy cannot be obtained unless all of these defects have been removed.

As the result of researches on various materials, the said defects (1)-(4) have been eliminated by the addition of noble metal powder, especially, silver and palladium as the third component, but it is still necessary to search for the materials capable of improving the resistivity aging characteristic as mentioned in the above (5).

Accordingly, the object of this invention is to provide a thermistor composition suitable for obtaining a thick film thermistor which has no defects as enumerated above and which is excellent in aging characteristic and high in accuracy and furthermore to provide the thick film thermistor obtained therefrom.

As the results of the inventor's various researches on thermistor composition, it has been found that said object of this invention can be achieved by the addition of a mixture of ruthenium oxide powder and silver powder or ruthenium oxide powder and gold powder as the third component to thermistor characteristic powder and glass frit (bismuth-containing lead borosilicate glass frit). That is, this invention is characterized by a thermistor composition comprising a thermistor characteristic powder, a glass frit (bismuth-containing lead borosilicate glass frit) and a mixture of ruthenium oxide powder and silver powder or ruthenium oxide powder and gold powder wherein the thermistor characteristic powder is 20-70% by weight, the glass frit is 70-20% by weight and the ruthenium oxide powder and the noble metal powder are 10-50% by weight and the amount of the ruthenium oxide powder is 15-95% by weight of the sum of the ruthenium oxide powder and the noble metal powder. The thick film thermistor produced from said composition has a resistivity of 1.OMEGA.-1M.OMEGA., a varistor constant of 1.00-1.05 and a current noise of 2% or less and dispersion of the resistivity being within .+-.15% and resistivity aging characteristic being 2% or less. Thus, said thick film thermistor has an excellent aging characteristic and a high accuracy.

Next, the materials used in this invention will be explained below.

The metal oxide powder having the thermistor characteristic is preferably complex oxide of traditional metals such as nickel, cobalt, manganese and the like. The glass frit is preferably bismuth-containing lead borosilicate glass which desirably has the composition shown in Table 1. The electrically conductive powder is a mixture of ruthenium oxide powder and noble metal powder, specifically a mixture of ruthenium oxide powder and silver powder or a mixture of ruthenium oxide powder and gold powder. The amount of the powder having thermistor characteristic is limited to 20-70% by weight of the total amount of powders because when the amount of the thermistor characteristic powder is less than 20% by weight, resistivity does not change with change of temperature and thermistor constant is less than 500 K to lose utility value as a thermistor and when the amount of the thermistor characteristic powder is more than 70% by weight the contents of glass frit, ruthenium oxide, silver and the like are below those required for obtaining the desired characteristics. The amount of the electrically conductive powder comprising ruthenium oxide and silver or gold is limited to 10-50% by weight because when it is more than 50% by weight metallic conduction takes place to show nearly no change of resistivity with change of temperature and when less than 10% by weight the effect of adding the electrically conductive powder is not exhibited, dispersion of resistivity is greater than .+-.15%, varistor constant which indicates voltage-dependence of resistivity is more than 1.05 and current noise is higher than 2%.

Furthermore, the amount of the glass frit is limited to 20-70% by weight because when it is more than 70% by weight a resistivity of lower than 1 M.OMEGA. cannot be obtained and the varistor constant is more than 1.05 to lose utility value as a thermistor and when less than 20% by weight adhesive strength between the powders and between substrate and the powders decreases to result in problems in production.

The amount of the ruthenium oxide powder is limited to 95-15% by weight of the electrically conductive powder because when it is more than 95% by weight or less than 15% by weight the voltage-dependence of resistivity, namely, varistor constant is more than 1.05.

FIG. 1 is a triangular diagram illustrating the range of mixing ratio of the thermistor characteristic powder, the glass frit and the mixture of ruthenium oxide powder and noble metal powder according to this invention.

FIG. 2 is a cross sectional view of the thick film thermistor.

The following examples illustrate this invention.

A thermistor characteristic powder prepared by solid phase reaction of MnO.sub.2, CoO and NiO in a molar ratio of 2:1:3, a glass frit having the composition of Table 1, ruthenium oxide powder and silver powder were weighed in the various ratios as shown in Table 2 and in the total amount of 10 g and they were mixed by a mixing and grinding machine for one hour. Then, 4cc of .alpha.-terpineol solution containing 10% of ethyl cellulose as a vehicle was added to each of the mixtures and each mixture was kneaded for additional one hour to obtain thermistor pastes.

A silver palladium electroconductive paste was printed by screen printing method on alumina substrate 1 shown in FIG. 2 and this was fired at 850.degree. C. for 10 minutes to obtain lower electrode 2. Said thermistor paste was printed on said electrode 2 and this was dried at 130.degree. C. for 10 minutes to obtain thermistor layer 3. Then, a silver palladium electroconductive paste was printed thereon so that this overlapped the lower electrode through the thermistor layer 3. This was fired at 800.degree. C. for 10 minutes to form upper electrode 4. Thus, a thick film thermistor of sandwich structure was produced. The overlap of the lower and upper electrodes, namely, effective surface area was 9 mm.sup.2 and thickness of the thermistor layer after firing was 40 .mu.m. Characteristics of thus obtained thick film thermistor are shown in Table 2.

In Table 2, the current noise .delta. is obtained from the following formula: ##EQU1## wherein .DELTA.i is fluctuation of current in Amp. when 1 VD.C was applied to a thermistor having a resistivity R.sub.o (.OMEGA.) at no load.

The resistivity aging characteristic is changing rate when the thermistor is left to stand at 150.degree. C. for 1000 hours.

In the case of sample No. 3, 6, 7, 8, 11, 12, 13, 15, 16 and 17, in other words, when the mixing ratio of the thermistor characteristic powder, the glass frit and the mixture of silver powder and ruthenium oxide powder was in the area of the quadrilateral ABCD in the triangular diagram of FIG. 1 and when the amount of ruthenium oxide powder was in the range of 15-95% by weight of the sum of silver powder and ruthenium oxide powder, resistivity of the thick film thermistors was 1.OMEGA.-1M.OMEGA., dispersion of the resistivity was within .+-.15%, varistor constant was 1.00-1.05, current noise was 2% or less and resistivity aging characteristic was 2% or less.

Sample No. 1 had smaller amount of the thermistor characteristic powder and thermistor constant of this sample was small. Moreover, this sample had higher glass content and so the resistivity of this sample was high and the varistor constant and current noise were also great. Sample No. 2 was low in content of the thermistor characteristic powder and thermistor constant of this sample was small. Sample No. 4 was low in contents of RuO.sub.2 and Ag and varistor constant and current noise of this sample was great and the resistivity aging characteristic was also great. Sample No. 5 was low in content of the thermistor characteristic powder and thermistor constant of this sample was small. Sample No. 9 was low in contents of RuO.sub.2 and Ag and varistor constant and current noise were great and moreover the resistivity aging characteristic was also great. Sample No. 10 was low in content of the thermistor characteristic powder and high in contents of RuO.sub.2 and Ag and thermistor constant of this sample was zero. Sample No. 14 was high in contents of RuO.sub.2 and Ag and thermistor constant of this sample was small. Sample No. 18 was low in contents of RuO.sub.2 and Ag and varistor constant and current noise of this sample were great and resistivity aging characteristic was great. Sample No. 19 and 20 were low in content of glass and adhesive strength between powders was small and so there was problem in production.

Table 3 shows compositions and various characteristics of thick film thermistors produced in the same manner as in Example 1 except that thermistor characteristic powder of MnO.sub.2, CoO and NiO in a molar ratio of 2:3:1 was used. In the case of sample No. 1, 3, 4, 6, 8, 9 and 10, namely, when the mixing ratio of the thermistor characteristic powder, the glass frit and the mixture of silver powder and ruthenium oxide powder and the mixing ratio of the silver powder and the ruthenium oxide powder were in the same ranges as refered to in Example 1, the same effects as in Example 1 were obtained.

Sample No. 2 was low in content of the thermistor characteristic powder and thermistor constant of this sample was small. Sample No. 5 was low in contents of RuO.sub.2 and Ag and varistor constant and current noise of this sample were great. Resistivity aging characteristic of this sample was great. Sample No. 7 was high in contents of RuO.sub.2 and Ag and thermistor constant of this sample was small.

Gold powder was added to the thermistor characteristic powder, the glass frit and the ruthenium oxide powder used in Example 1 and thick film thermistors were produced therefrom in the same manner as in Example 1. Relations between the compositions and the various characteristics of thus obtained thermistors are shown in Table 4.

Even when the silver powder in the compositions of Example 1 was replaced with gold powder, in the case of sample No. 1, 3, 4, 6, 8, 9 and 10, namely, when the mixing ratio of the thermistor characteristic powder, the glass frit and the mixture of gold powder and ruthenium oxide powder was in the area of the quadrilateral ABCD in the triangular diagram of FIG. 1 and when the amount of ruthenium oxide powder was in the range of 15-95% by weight of the sum of the gold powder and the ruthenium oxide powder, resistivity of the thick film thermistors was 1.OMEGA.-1M.OMEGA., dispersion of the resisitivity was within .+-.15%, varistor constant was 1.00-1.05, current noise was 2% or less and resistivity aging characteristic was 2% or less as in Example 1.

Sample No. 2 was low in content of the thermistor characteristic powder and thermistor constant of this sample was small. Sample No. 5 was low in contents of RuO.sub.2 and Au and varistor constant and current noise were great. Moreover, resistivity aging characteristic was great. Sample No. 7 was high in contents of RuO.sub.2 and Au and thermistor constant of this sample was small.

As mentioned above, this invention can provide a thick film thermistor excellent in aging characteristic and high in accuracy which is characterized in that (1) a low resistivity of less than 1 K.OMEGA. can be obtained and thermistor element has a wide range of resistivity of about 1.OMEGA.-1M.OMEGA., (2) dispersion of resistivity is within .+-.15%, (3) voltage-dependence of resisitivity is small and varistor constant is within the range of 1.00-1.05, (4) current noise is controlled to 2% or less and (5) resistivity aging characteristic is 2% or less.