FR2652675A1 - Composition for thermistors having a negative temperature coefficient and a very low electrical resistivity - Google Patents

Abstract

The invention relates to a composition for thermistors having a negative temperature coefficient, of the type comprising manganese, nickel and copper, corresponding to the following formula:

Description

 The present invention relates to thermistor compositions with a negative temperature coefficient, more particularly negative temperature coefficient thermistor compositions having a very low electrical resistivity, this resistivity being stable over time.

 Among the thermistor compositions with a negative temperature coefficient based on semiconducting oxides, known as NTC thermistors, currently available on the market, some contain at least two types of cations, those of manganese and nickel, these compositions being obtained using manganese oxide, most often in the form Mu3O4, and nickel oxide, most often in the form of NiO.

However, thermistors of this type have a resistivity of between 1000 and 100,000 cm.

 It is known, to lower the resistivity, to add to the Mn-Ni system above, the copper element. 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 25 Ω · cm is obtained for a copper atomic concentration of approximately 10%, the resistivity decreasing when the copper content is increased. However, the resistivity of this composition is not stable over time. Thus, for a system comprising as atomic percentage 78% of Mn, 12% of Ni and 10% of Cu, a variation is obtained after 500 hours. of resistance t. R / R> 15% and for a system comprising in atomic percentage 63% of Mn, 22% of Ni and 15% of Cu, there is obtained after 500 hours, a variation of resistance R / R of about 8%, the above tests having been performed at 1250 C, the thermistors not being energized.

 In general, the thermistors of very low resistivity have the least good stabilities.

 In order to limit this resistance drift, it has been proposed in PR 2 598 021 to add barium or calcium or strontium ions to the Mn-Ni-Cu system. This solution is effective but has the disadvantage of increasing the electrical resistivity of the material and causing a chemical decarbonation reaction of barium salts, calcium or strontium used in sintering ceramics.

 According to the present invention, it has been discovered that it is possible, by means of a judicious choice of manganese, nickel and copper ion concentrations, to obtain thermistors with very low electrical resistivity, which are stable as a function of time without resorting to doping. above, thus avoiding the disadvantages mentioned above.

The subject of the invention is therefore a composition for thermistors with a negative temperature coefficient, of the type comprising manganese, nickel and copper, characterized in that it corresponds to the following formulation
Mn Ni Cu O
3-xy xy 4
with 0.9 <x <1.8
0.45 <y <0.9
x + y <2.25.

 The invention also relates to a thermistor developed from this composition.

 The present invention will be better understood and other advantages will become apparent from the description which follows, given in a non-limiting manner.

According to one characteristic of the present invention, the manganese ions are supplied in the form of manganese oxide, the nickel ions in the form of nickel oxide, the copper ions under the copper oxide. Preferably, the manganese oxide will be in the form of MnO 2, Mn 2 O 3, Mn 3 O 4 or a mixture of these oxides, nickel oxide in the form of
NiO, copper oxide in the form CuO, Cu2O. However, other types of oxides can be envisaged.

 The CTN thermistors having the compositions given above can be made using a powder technique known per se which consists in mixing the oxide powders in a mill mixer, drying them, sieving and squeezing them in the form of disc in known manner, then sintering them at a temperature preferably between 11800C and 12500 C, the sintering being preferably performed by a step of at least one hour.

According to another embodiment, the thermistors
CTN having the above compositions can be obtained chemically, that is by heating a mixed oxalate of manganese, nickel, copper or a hydrate of said oxalate, in an atmosphere consisting of a mixture of an inert gas and oxygen having an oxygen partial pressure reduced to 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 at a temperature of between 4200.degree. C. and 8000.degree. C. for a time sufficient to obtain particles having the desired surface area dimensions, and then the particulate ceramic compositions are converted according to the usual methods.

 Table 1, placed at the end of the description, includes a number of compositions according to the present invention and numbered from 1 to 5. From these compositions, thermistors were developed on which tests were carried out. The compositions were prepared chemically.

 In this case of chemical synthesis, the powders are prepared in the following manner: oxalates of manganese, nickel and copper are mixed in the proportions corresponding to the formulas indicated in Table 1, Examples 1 to 5. Oxalate The resulting mixture is thermally decomposed at 7000 C. The powder thus obtained is ground by liquid (water) in the presence of grinding elements of zirconia for two hours.

A solution containing a binder of polyvinyl alcohol is added to the slip for 1/4 hour. The composition thus obtained was dried in an oven at 120 ° C. and 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 tons / cm.
The discs thus obtained are sintered at a temperature of 11800C for two hours. They are then covered with silver electrodes in known manner and heat treated at 5500C.

Table 1 indicates, for each case, the resistivity
in A. cm, B the thermal sensitivity index in kelvin, the variation of resistivity AR / R in
To study the aging of the different samples, and thus deduce the variation of resistivity R / R with time, they were placed in an enclosure under a temperature of 1250C plus or minus 20C, for 1000 hours and without application of voltage . The resistivity of the parts is measured under a current of 5 mA.

 From Table 1, it can be seen that the variation in the resistivity of the materials is always less than or equal to 3% for extremely low electrical resistivity values (of the order of 1 Ω cm). These results are very interesting, not only from the point of view of the electrical characteristics, but also from the point of view of the raw materials involved which remain limited to the essential elements manganese, nickel, copper.

Satisfactory results are obtained for manganese, nickel and copper concentrations corresponding to the following inequalities
0.9 <x <1.8
0.45 <y <0.9
x + y <2.25.

<Tb>

N <SEP> x <SEP> y <SEP>><SEP><SEP> B
<tb><SEP> 1 <SEP> 0.99 <SEP> 0.66 <SEP> 0.5 <SEP> 1526 <SEP> 2
<tb><SEP> 2 <SEP> 1.02 <SEP> 0.66 <SEP> 0.75 <SE> 1381 <SEP> 3
<tb><SEP> 3 <SEP> 1.23 <SEP> 0.69 <SEP> 1.2 <SEP> 1000 <SEP> 2
<tb><SEP> 4 <SEP> 1.53 <SEP> 0.66 <SEP> 3 <SEP> 1100 <SEP> 2
<tb><SEP> 5 <SEP> 1.23 <SEP> 0.51 <SEP> 1 <SEP> 1400 <SEP> 3
<Tb>
TABLE 1

Claims (2)

 1 - Composition for thermistors with a negative temperature coefficient, of the type comprising manganese, nickel and copper, characterized in that it corresponds to the following formulation  Mn 3-x-y Nix Cuy 4  with 0.9 <x <1.8  0.45 <y <0.9  x + y <2.25.  2 - Thermistor with a negative temperature coefficient, characterized in that it is produced from a composition according to claim 1.