EP0243256B1 - Compositions for thermistors having a negative temperature coefficient - Google Patents

Abstract

1. A composition of matter for negative temperature coefficient thermistors based on oxides, comprising at least four cations including manganese, nickel and copper, characterized in that the fourth cation is barium or strontium, the composition comprises 40 to 67.6 atomic percent manganese, 20 to 33 atomic percent nickel, 12 to 20 atomic percent copper and 0.4 to 7 atomic percent barium or strontium, and the composition comprises 100 atomic percent.

Description

The present invention relates to compositions for thermistors with negative temperature coefficient, more particularly compositions for thermistors with negative temperature coefficient having a low resistivity, this resistivity being stable over time. Such a composition comprising manganese, nickel and copper oxides is known from BE-A 475 405.

Among the compositions for thermistors with a negative temperature coefficient based on semiconductor oxides, called CTN thermistors, currently available on the market, some contain at least two types of cations, those of manganese and nickel, these compositions being obtained by using manganese oxide, most often in the form Mna04, and nickel oxide, most often in the form NiO. However, thermistors of this type have a resistivity between 1,000 a.cm and 100,000 α.cm.

To lower the resistivity, it is known to add the copper element to the Mn-Ni system above. This Cu element is most often added to the composition in the form of copper oxide CuO. In this case, a resistivity of less than 25Ω.cm is obtained for an atomic copper concentration of approximately 10%, the resistivity decreasing when the copper content is increased. However, as shown in the aging curves 2 and 3 in FIG. 1, the resistivity of this composition is not stable over time. Thus, for a system comprising in atomic percentage 78% of Mn, 12% of Ni and 10% of Cu, one obtains at the end of 500 hours, a variation of resistance _A R / R> 15% and for a system comprising in percentage atomic 63% of Mn, 22% of Ni and 15% of Cu, one obtains at the end of 500 hours, a variation of resistance _A R / R of approximately 8%, the above tests having been carried out at 125 ° C , the thermistors not being energized.

The present invention therefore aims to remedy this drawback by proposing new compositions for NTC thermistors of low resistivity, namely less than 25a.cm with a resistivity of high stability over time at a temperature of approximately 125 _° C.

Consequently, the subject of the present invention is a composition for NTC thermistors based on oxides, comprising at least four cations including manganese, nickel and copper, characterized in a first embodiment in that, the fourth cation being barium or strontium, the composition comprises 40 to 67.7% in atomic percentage of manganese, 20 to 33% in atomic percentage of nickel, 12 to 20% in atomic percentage of copper and 0.4 to 7% in atomic percentage of barium or strontium, the composition comprising 100% by atomic percentage and characterized in a second embodiment in that, the fourth cation being calcium, the composition comprises 45 to 67.2% in atomic percentage of manganese, 20 to 33% by atomic percentage of nickel, 12 to 20% by atomic percentage of copper and 0.8 to 2% by atomic percentage of calcium, the composition comprising 100% by atomic percentage.

Indeed, the applicant has noticed, following numerous tests, that the fact of adding barium or strontium ions to the known Mn-Ni-Cu system, although slightly increasing the resistivity of the thermistor, makes it possible to '' obtaining a variation _A R / R of the resistivity over time at 3% after 1000 hours, as can be clearly seen on the aging curve 1 of FIG. 1 which corresponds to an addition of barium. On the other hand, calcium also decreases the electrical resistivity while giving still satisfactory results as for the variation in time _A R / R of the resistivity.

The percentage of the element chosen from barium, calcium and strontium was chosen so as to obtain a resistivity of less than approximately 25a.cm with an AR / R variation ≦ 3% after approximately 1000 hours. In fact, it is the ratio of the percentage of the element to the percentage of copper which must be adjusted to obtain a given resistivity with a given AR / R variation because, as mentioned above, by increasing the percentage of the element chosen from barium or strontium, the resistivity of the thermistor is increased while increasing the percentage of copper, the resistivity of the thermistor is decreased. On the other hand, calcium decreases the electrical resistivity. However, the percentage of copper should be limited to 20% to avoid sintering problems. On the other hand, for a percentage of copper less than 12%, the desired percentage of aging cannot be obtained.

In addition, a lower percentage of barium, calcium or strontium than those indicated above does not allow the desired stability to be obtained.

The nickel level was chosen taking into account the fact that, in a manganese-nickel system, the resistivity decreases with an increase in the nickel rate passing through a minimum at a nickel rate of 22% in atomic percentage and that beyond this point, the resistivity increases again with the nickel content, as shown on the curve in FIG. 2.

In fact, the nickel content will be between 20 and 33%, the upper limit being determined by the manufacturing process used. In fact, in the case of a conventional powder mixing process, the nickel content must not exceed 24% if the desired resistivity is to be obtained. In the case of a chemical process, the atomic percentage of Ni can be 33% while giving good results.

According to another characteristic of the present invention, the manganese ions are brought in the form of manganese oxide, the nickel ions in the form of nickel oxide, the copper ions in the form of copper oxide and the barium ions in the form of barium salt or oxide. Preferably, the manganese oxide will be in the form Mn0 ₂ , Mn ₂ 0 ₃ , Mns0 ₄ or a mixture of these elements, the nickel oxide in the NiO form, copper oxide in the form CuO, Cu ₂ 0, the barium salt or oxide in the form BaS0 ₄ , BaNOa, BaCO ₃ , the salt or calcium oxide in the form CaS0 ₄ , CaNOa, CaCOa, strontium salt or oxide in the form SrS0 ₄ , SrNO ₃ , SrCO ₃ . However, other types of oxides or salts can be envisaged.

In fact, the CTN thermistors having the compositions given above are produced using a powder technique known per se which consists in mixing the oxide powders in a grinder mixer, in drying them, in sieving them and in pressing them in the form of a disc in a known manner, then subjecting them to sintering at a temperature preferably between 1180 _° C. and 1250 _° C., the sintering preferably being carried out in a plateau of at least one hour.

As will be explained in more detail below, the sintering temperature is important in the context of the present invention in order to obtain good stability of the resistivity over time. According to another embodiment, the CTN thermistors having the compositions given above are obtained chemically, that is to say by heating a mixed oxalate of manganese, nickel, copper and barium or strontium or calcium, or a hydrate said oxalate, in an atmosphere consisting of a mixture of an inert gas and oxygen having a reduced partial pressure of oxygen at the decomposition temperature of the oxalate, the temperature is maintained until complete decomposition of the oxalate, the product obtained is heated in an inert atmosphere to a temperature between 420 _° C and 800 ° C for a time sufficient to obtain particles having the dimensions of the specific surface desired, then the compositions are transformed ceramic particles according to the usual methods.

Other characteristics and advantages of the present invention will appear on reading the tables below giving the results of tests carried out on various compositions in accordance with the present invention.

Tests 1 to 10 of Table I and those of the other tables were carried out on thermistors prepared using the same process, namely a conventional process for the preparation of ceramics. Only the proportions between the various components or the sintering temperature have been modified as will be explained in each table. Tests 11 and 12 in Table 1 were carried out using the chemical method.

Thus, in the first case, each thermistor has been prepared in a conventional manner. Powders of Mna0 ₄ , NiO, CuO and BaCO ₃ or SrCOs or CaCO _s incorporated in predetermined proportions so that the atomic percentages of the ions Mn, Ni, Cu and Ba or Sr or Ca are in accordance with the present invention been mixed and ground by liquid (water) in a mill containing zirconia beads, for example in a "Turbula" mill. The grinding was carried out for approximately two hours. A solution containing a polyvinyl alcohol binder is added to the slip for 1/4 hour. The composition thus obtained was dried in an oven at 120 _° C, then sieved through a 315 micron sieve. The composition thus sieved was pressed in the form of discs using a hydraulic machine of known type under a pressure of between 3 and 4 tonnes / cm ^2. The discs thus obtained are sintered at a temperature between 1180 _° C and 1250 _° C , with a stop time of a total duration of one hour. The sintering temperature is chosen as a function of the copper content, so as to obtain a resistivity and an _A R / R ratio corresponding to those fixed.

In the case of the chemical route, the powders are prepared in the following manner: the manganese, nickel, copper and barium oxalates are mixed in the proportions corresponding to the formulas indicated in Table I, Examples 11 and 12. L 'mixed oxalate obtained is thermally decomposed at 700 _° C. The powder is shaped identically to the powder process previously described. The discs thus obtained are sintered at a temperature of 1180 _° C for two hours.

The discs are then covered with silver electrodes in a known manner and heat treated at 550 ° C.

To study the aging of the different parts, they are placed in an enclosure at 125 ° C ± 2 ° C without applying a voltage, the resistivity of the parts is measured under a current of 5mA.

Table 1 below gives the influence of the barium level on a composition containing 15% Cu and 22% Ni.

From Table I, it can be seen that doping the Mn-Ni-Cu system with barium increases the density by about 10%. However, the density hardly varies with the barium content since, from a barium content of 0.6% in atomic concentration, it remains constant.

On the other hand, as mentioned above, the resistivity increases slightly with the barium content for a constant copper content. In addition, the stability of ceramics containing barium is significantly improved since with a barium content of 0.4%, a relative change in resistance of 1.9% is obtained after 1000 hours.

Table II represents the influence of the percentage of copper when the percentage of barium is 1.6% and that of nickel 22%.

From Table II, it can be seen that for a barium content of 1.6% the copper content can drop to 12% to obtain good aging. Below this concentration, the aging is 6.2% at 500 hours. On the other hand, we note that an acceptable aging can also be obtained for a copper content of 3%, but in this case the resistivity is 288 α.cm with a density of the thermistor of 4.45.

Table III represents the influence of the sintering temperature on certain compositions of the type of those tested in Table II.

Tables IV and V are identical to Table I and represent respectively the influence of the strontium rate and the calcium rate on the electrical characteristics of the NTC thermistors developed from the Mn, Ni, Cu system. In these tables, the thermistors are obtained using powder technology.

From the tables, it can be seen that strontium gives results substantially equivalent to those of barium while calcium gives less good aging, but decreases electrical resistivity.

Claims (5)

1. A composition of matter for negative temperature coefficient thermistors based on oxides, comprising at least four cations including manganese, nickel and copper, characterized in that the fourth cation is barium or strontium, the composition comprises 40 to 67.6 atomic percent manganese, 20 to 33 atomic percent nickel, 12 to 20 atomic percent copper and 0.4 to 7 atomic percent barium or strontium, and the composition comprises 100 atomic percent.

2. A composition of matter for negative temperature coefficient thermistors based on oxides, comprising at least four cations including manganese, nickel and copper, characterized in that the fourth cation is calcium, and the composition comprises 45 to 67.2 atomic percent manganese, 20 to 33 atomic percent nickel, 12 to 20 atomic percent copper and 0.8 to 2 atomic percent calcium, and the composition comprises 100 atomic percent.

3. The composition as claimed in claim 1 or claim 2, characterized in that the manganese is present in the form of manganese oxide, the nickel is in the form of nickel oxide, the copper is in the form of copper oxide and the barium, strontium or calcium is in the form of a salt.

4. The composition as claimed in claim 3, characterized in that the manganese oxide is in the form of Mn0₂, Mn₂0₃, Mn₃0₄, the oxide of nickel is in the form of NiO, the copper oxide is in the form of CuO, Cu₂0, the barium salt is in the form of BaCO₃, BaS0₄, BaNO₃, the calcium salt is in the form of CaCOs, CaS0₄, CaN0₃ and the strontium salt is SrCO_s, SrS0₄ or SrNO_a.

5. A method of manufacturing negative temperature coefficient thermistors having a composition as claimed in any one of the preceding claims 1 through 4, which consists in mixing the powders for constituting the composition, drying them, screening them, pressing them into the form of a disk and subjecting the disk to fritting, characterized in that the fritting temperature is comprised between 1180°C and 1300°C.

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