JPH0669001A - Positive temperature coefficient characteristic thermistor - Google Patents

Abstract

PURPOSE:To facilitate the control of positive temperature coefficient(PTC) characteristics furthermore enhancing the thermal resistance by a method wherein electric insulating particles comprising ion crystals and conductive particles previously mixed with one another are compression-molded. CONSTITUTION:Sodium chloride as electric insulating particles put in a resin- made vessel is crushed up using a ball mill and then sieved out to produce specific sodium chloride particles 2. Besides, as for conductive particles, carbon particles 3 are prepared. Next, the sodium chloride particles 2 are wet-mixed with the carbon particles 3 and then a catalyst is removed to produce the mixed powder. The mixed powder filled up in a molding mold is compression-molded to produce a molded product 1. At this time, the conductive particles 3 for controlling the temperature characteristics are blended at higher ratio than conventional one. Besides, the thermal resistance of the electric insulating particles comprising ion crystals is higher than that of conventional organic high molecules. Through these procedures, the control of PTC characteristics can be facilitated furthermore enabling the thermal resistance to be enhanced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive temperature coefficient thermistor having heat resistance.

[0002]

2. Description of the Related Art Positive temperature coefficient thermistors [hereinafter referred to as PTC (Posi
A positive resistance-temperature coefficient is a positive resistance-temperature coefficient, that is, an element whose resistance value increases with increasing temperature.

Such a positive temperature characteristic, PTC
As the characteristics, for example, as shown in FIG. 5, there are A characteristics in which the resistance value gradually increases and B characteristics in which the resistance value sharply increases when the temperature exceeds a predetermined temperature. The A characteristic is widely used mainly for temperature compensation, while the B characteristic is widely used mainly as a switching element.

Conventionally, as the above PTC thermistor, ceramics containing barium titanate (BaTiO ₃ ) as a main component has been known. The above ceramics are manufactured by adding a small amount (0.5 mol% or less) of semiconducting agent particles, for example, oxide particles of a rare earth element, to barium titanate particles which are an insulator, followed by molding and firing.

As described above, in the above-mentioned ceramics, since the amount of the semiconducting agent particles added is very small, the PTC may be caused by a weighing error or the like.
There is a problem in that it is difficult to control PTC characteristics because the characteristics are likely to change greatly, and it is difficult to produce a material having a low room temperature resistance value because a small amount of semiconducting agent particles is added to the insulator.

Therefore, in order to avoid the above problems,
An organic polymer such as a polyester resin in which a conductive substance such as carbon is dispersed and mixed has been proposed as a polymer PTC thermistor (JP-A-62-232902). Such a polymer PTC thermistor can add conductive particles such as carbon over a wide range, so that the room temperature resistance value can be made low. In addition, since a large amount of conductive material can be mixed in, the weighing error can be reduced and the PTC characteristics can be controlled. It has the property of being easy to do.

[0007]

However, the above-mentioned conventional polymer PTC thermistor has a problem that it cannot be used at high temperature because the organic polymer has poor heat resistance.

Therefore, an object of the present invention is to provide a PTC thermistor whose PTC characteristics are easily controlled and which has excellent heat resistance.

[0009]

In order to solve the above-mentioned problems, the PTC thermistor of the present invention has been earnestly studied on various materials, and as a result, sodium chloride particles and carbon having heat resistance higher than that of organic polymers have been obtained. It was discovered that a molded product consisting of the particles of the conductive substance described above exhibits PTC characteristics,
The present invention has been completed.

That is, the PTC according to claim 1 of the present invention.
The thermistor is characterized in that it is formed by mixing electrically insulating particles made of ionic crystals and conductive particles and then pressure-molding the mixture.

The PTC thermistor according to claim 2 of the present invention is the positive temperature coefficient thermistor according to claim 1, wherein
The electrically insulating particles are sodium chloride particles, and the conductive particles are carbon particles.

The material used for the above-mentioned electrically insulating particles is electrically insulating and has a coefficient of thermal expansion (3 to 5 ×).
It is not particularly limited as long as 10 ^-5 / deg) is large, but sodium chloride (NaCl), potassium chloride (KCl), potassium bromide (KBr), calcium bromide (CaBr ₂ ), potassium iodide (K)
Ionic crystals such as I) and sodium bromide (NaBr) can be used, and sodium chloride is particularly preferable because it is easy to handle and easily available.

The material of the above-mentioned conductive particles is not particularly limited as long as it has conductivity and heat resistance and has a coefficient of thermal expansion (<1 × 10 ⁻⁵ / deg) smaller than that of the above-mentioned electrically insulating particles. However, amorphous carbon is preferable because it is relatively easily available.

[0014]

According to the above-mentioned structure of the present invention, the electrically conductive particles can be interposed between the electrically insulating particles, and the electrically conductive particles can be adjacent to each other for electrical conduction.
On the other hand, when the electrically insulating particles expand with increasing temperature, the coefficient of thermal expansion of the electrically conductive particles is smaller than that of the electrically insulating particles. The thickness of the conductive particles can be reduced, or the absent section of the conductive particles can be increased in a part between the electrically insulating particles, so that the resistance value increases with the temperature rise.
It becomes possible to have characteristics.

Moreover, since the melting point of the electrically insulating particles made of ionic crystals is higher than the melting point of the conventional organic polymer,
The heat resistance can be increased, and more conductive particles that control the temperature characteristics can be blended than the semiconducting agent particles in the conventional ceramics PTC thermistor, so that poor control of the PTC characteristics due to weighing error or the like can be improved.

Further, according to the second aspect of the present invention, since the sodium chloride particles are used as the electrically insulating particles and the carbon particles are used as the conductive particles, they can be easily obtained and a positive temperature coefficient thermistor can be obtained quickly. be able to.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following will describe one embodiment of the present invention as Embodiment 1 with reference to FIGS. Example 1 The positive temperature coefficient thermistor of the present invention (hereinafter referred to as PTC
(Thermistor), the manufacturing method will be explained first. In the preparation of the raw material, sodium chloride (NaCl) as electric insulating particles (manufactured by Wako Pure Chemical Industries, purity 99.5%)
(Melting point, 800.4 ° C) is put into a resin container together with zirconia balls as one raw material, crushed by a ball mill for 1 week, then classified by a sieve, and sodium chloride having an average particle size of 50 μm (particle size range 38 to 75 μm) The particles were obtained. In addition,
The coefficient of thermal expansion of sodium chloride is 4.4 × 10 ^-5 / deg.

As the other raw material, as conductive particles, carbon particles shown in FIG. 2 (made by Nippon Carbon Co., Ltd., carbon microbeads PC type, average particle size 5 μm, particle size range 4 to 7) were used.
μm, specific surface area ≦ 2 m ² / g, bulk specific gravity 0.80-0.90, true specific gravity 1.
40 to 1.57, and the shape is substantially spherical). The carbon particles are made of amorphous carbon and have a coefficient of thermal expansion of 0.54 × 10 ⁻⁵ / deg (40 ° C.).

Next, the volume fractions (%) of the carbon particles to the sodium chloride particles were weighed so as to be 8.5%, 10% and 30%, respectively, and subsequently, the sodium chloride particles weighed as described above were weighed. And carbon particles are put into 2-propanol as a solvent at the time of wet mixing and uniformly mixed by a wet method to obtain a mixture, and then the mixture is put into a rotary evaporator, and the solvent is distilled while stirring and mixing. It was removed to obtain a mixed powder.

The mixed powder thus obtained has a diameter of
After being filled in a 10 mm round-shaped molding machine, pressure molding was carried out in a uniaxial direction for 90 seconds to obtain columnar molded articles having different amounts of carbon particles mixed with sodium chloride particles. In addition,
The molding pressure was 2 ton / cm ² , and the average relative density of each obtained molded product was 95%.

Next, although not shown, silver paste (manufactured by DuPont), which is not shown, is applied to both axial end surfaces of each of the above-obtained molded products, and then heated by an infrared lamp to remove the solvent in the silver paste. After removal, electrodes were respectively formed on both end faces to obtain respective specimens. Then, for each of the above samples, when a silver wire (0.3 mm) was connected as a lead wire to each of these electrodes and electricity was applied,
It turned on and showed conductivity.

As shown in FIG. 1 (a), the molded article 1 has carbon particles 3 between sodium chloride particles 2.
It was assumed that carbon particles 2 that are adjacent to each other and that are adjacent to each other are present.

Next, the volume fraction (%) of the carbon particles 3 with respect to the sodium chloride particles 2 is 8.5%, 10%, 30
% -Resistivity-temperature characteristics of each molded product 1 were measured over time, and the results are also shown in FIG. The two-terminal method was used for the measurement, with a constant voltage of 0.1 V and a temperature rising / falling rate of 7 ° C /
I went at min.

When the temperature of the sample was raised to 500 ° C. or higher, oxidization of the carbon particles 3 was observed by TG (thermogravimetric) measurement of the carbon particles 3. Therefore, when heating to 400 ° C. or higher, nitrogen (N ₂ ) The resistivity-temperature characteristic was measured in a gas atmosphere. The measuring instruments used are as follows.

Ammeter: manufactured by ADVANTEST, DISITAL ELECTROMET
ER TR8652 DC constant voltage power supply: HIOKI, 7005 PROGRAMMABLE DC S
TANDARD Controller: CHINO, KP series digital program controller Electric furnace: Ishizuka Electric Co., openable electric furnace Recorder: NEC Saneisha, HYBRID RECORDER
RD3218 As is clear from FIG. 3, each of the constituents of the above-mentioned Example 1 exhibits conductivity at room temperature, and the resistance value increases as the temperature increases.
It was found to have TC (Positive Temperature Coefficient) characteristics. In particular, a steep PTC characteristic was exhibited at a carbon content of 8.5%.

As shown in FIG. 1B, the coefficient of thermal expansion of the sodium chloride particles 2 with respect to the carbon particles 3 is large, and each sodium chloride particle 2 expands as the temperature rises, and each sodium chloride particle 2 expands. It was assumed that the carbon particles 3 that were adjacent to each other around the circumference of 2 were separated from each other, and therefore the carbon particles 3 that were conducting were in a non-conductive state.

Another embodiment of the present invention will be described below as a second embodiment with reference to FIGS. 1, 4 and 5. [Example 2] A method for producing the PTC thermistor of the present invention will be described first. As one raw material, sodium chloride (NaCl, manufactured by Wako Pure Chemical Industries, Ltd., purity 99.5%) (melting point, 80
0.4 ° C.) was put into a resin container together with zirconia balls, crushed by a ball mill for 1 week, and then classified by a sieve to obtain sodium chloride particles having an average particle size of 250 μm (particle size range 150 to 350 μm). The carbon particles shown in Example 1 were used as the other raw material.

Next, the sodium chloride particles and the carbon particles were weighed so that the volume fraction (%) of the carbon particles to the sodium chloride particles was 4%, 5% and 10%, respectively.

The following operations are carried out in the same manner as in Example 1,
Molded articles having different carbon particles with respect to sodium chloride particles were obtained. In order to show the particle structure of the molded product, an example of a fracture surface of the molded product 1 in which the volume fraction (%) of carbon particles to sodium chloride particles is 10% is shown in FIG.

From this, as shown in FIG. 1 (a), it can be seen that substantially spherical carbon particles 3 are continuous between the sodium chloride particles 2 and adjacent carbon particles 3 are present. Resistivity-temperature characteristics of the molded products 1 having different volume fractions (%) of carbon particles were measured in the same manner as in Example 1, and the results are shown in FIG.

As is apparent from FIG. 5, it was found that each structure of the above-mentioned Example 2 exhibits the conductivity at room temperature and has the PTC characteristic in which the resistance value increases as the temperature rises. In particular, when the carbon content was 5%, a 6-digit increase in resistance value was observed as the temperature increased, and a sharp PTC characteristic was exhibited.

Next, as still another embodiment of the present invention, as a third embodiment, molded articles having different average particle diameter ratios of sodium chloride particles and carbon particles and having various carbon compound volume fractions are implemented. The results of measuring the resistivity-temperature characteristics prepared in the same manner as in Example 1 will be described with reference to FIGS. 1, 6 and 7. Unless otherwise specified, the raw materials used and the manufacturing method are the same as in Example 1 above.

[Third Embodiment] a. Sodium chloride particles (average particle size 25 μm, particle size range 15
~ 38 μm), carbon particles (PC type, average particle size 50 μm,
Particle size range 45-60 μm), volume fraction of carbon particles (10
%, 20%, 25%, 30%, 40%), molding pressure 2 ton / cm ² , press time 90 seconds.

B. Sodium chloride particles (average particle size 50μ
m, particle size range 38 to 75 μm), carbon particles (PC system, average particle size 50 μm, particle size range 45 to 60 μm), volume fraction of carbon particles (5%, 10%, 20%, 23%, 25% , 30%, 40
%, 50%), molding pressure 2 ton / cm ² , press time 90 seconds.

C. Sodium chloride particles (average particle size 50μ
m, particle size range 38 to 75 μm), carbon particles (PC
System, average particle size 20μm, particle size range 16-26μm), volume fraction of carbon particles (5%, 10%, 15%, 17%, 20%, 25
%, 40%), molding pressure 2 ton / cm ² , press time 90 seconds.

In this way, sodium chloride particles and carbon particles having different average particle diameters from those in Examples 1 and 2 were used, and respective constitutions having different carbon volume fractions were prepared. When the resistivity-temperature characteristics of each of the above-described structures were examined in the same manner as in Example 1, a PTC characteristic in which the resistance value increased with temperature increase was shown, although not shown. However, it has been found that its use as a PTC thermistor is limited because of its large resistance value at room temperature.

In order to show the particle structure of each of the above-mentioned constitutions, as an example, the above-mentioned sodium chloride particles (average particle size 50
micron), carbon particles (PC system, average particle size 50 μm), and a fracture surface of a molded product having a volume fraction of carbon particles of 10% are observed with a scanning electron microscope and shown in FIG. Sodium particles (average particle size 50 μm), carbon particles (P
C type, average particle size 50 μm), volume fraction of carbon particles 40%
The fracture surface of the molded product of No. 1 was observed with a scanning electron microscope and is shown in FIG.

From this, as shown in FIG. 1 (a), in the obtained molded article 1, the substantially spherical carbon particles 3 are continuous around the sodium chloride particles 2, and the adjacent carbon particles are adjacent to each other. It can be seen that particles 3 are provided.

Next, still another embodiment of the present invention will be described as a fourth embodiment with reference to FIGS. [Example 4] Instead of the PC-based carbon particles in Example 2, MC-based carbon particles (average particle size 5
μm, particle size range 4 to 7 μm, specific surface area ≦ 10 m ² / g, bulk specific gravity
0.60 to 0.80, true specific gravity 1.37 to 1.39, shape is substantially spherical, and the volume fraction of the above carbon particles (1%, 1.3%,
1.5%, 2%, 3%, 5%, 7.5%, 10%), and other manufacturing methods were the same as in Example 1 to obtain molded articles 1 as shown in FIG.

The above MC-based carbon particles have carbon black fine particles coated on the surface of the carbon particles to provide irregularities on the surface, and have a specific surface area (m ² / g)
Is ≦ 10, which is 5 to 10 times larger than the PC system.

Resistivity-temperature characteristics of the respective structures having different volume fractions of carbon particles as described above were examined in the same manner as in the first embodiment. Although not shown, in each of the above structures, the steepness associated with the temperature rise is shown. Although no significant increase in resistance was observed, it showed a nearly constant increase in resistance over a wide temperature range from room temperature to 500 ° C. As a result, each of the above structures can be suitably used for temperature compensation in a wide temperature range, especially in a high temperature range.

Next, still another embodiment of the present invention will be described as a fifth embodiment with reference to FIG. [Example 5] Instead of the carbon volume fractions used in Example 1, the carbon volume fraction was 15%, and the molding pressure (ton / cm ² ) was 0.5, 1, 1.5 and ² , respectively. ,
Others were the same as in Example 1 above, and molded articles 1 were prepared as shown in FIG.

Resistivity-temperature characteristics of each of the above-described molded products 1 were examined in the same manner as in Example 1 above.
Each of the above-mentioned molded products 1 exhibited PTC characteristics. Moreover, in the molded product 1 molded at a molding pressure of 2 ton / cm ^2, a gradual increase in the resistance value was observed as the temperature increased, while
In the molded product 1 molded at a molding pressure of 0.5 ton / cm ² , a 7-digit steep increase in the resistance value was observed as the temperature increased.
From this, it was shown that the above-mentioned respective configurations can adjust the PTC characteristics by controlling the molding pressure.

As described above, in the constitution of each of the above-described embodiments, the sodium chloride particles 2 and the carbon particles 3 are mixed and pressure-molded, so that as shown in FIG. It is possible to interpose the conductive carbon particles 3 with each other so that the carbon particles 3 ... Can be adjacent to each other to provide conductivity.

On the other hand, in each of the above structures, as the temperature rises, the sodium chloride particles 2 expand and the absent section of the carbon particles 3 between the sodium chloride particles 2 increases as shown in FIG. 1 (b). Or the thickness of the carbon particles 3 between the sodium chloride particles 2 decreases, the resistivity increases, and the resistivity increases as the temperature rises.
It becomes possible to provide TC characteristics.

At this time, the coefficient of thermal expansion of the carbon particles 3 is
As described above, since the coefficient of thermal expansion of the sodium chloride particles 2 is about 1/8, the expansion of the sodium chloride particles 2 is not hindered, and the carbon particles 3 are soft and easily elastic. As described above, the absent section of the carbon particles 3 between the sodium chloride particles 2 can be increased, and the thickness of the carbon particles 3 between the sodium chloride particles 2 can be decreased.

In addition, in each of the above-mentioned constitutions, since the melting point of sodium chloride is higher than the melting point or softening point of the conventional organic polymer, heat resistance can be enhanced, and further, the semiconducting agent in the conventional ceramics PTC thermistor. Unlike the minute amount (0.5 mol% or less) of compounding amount such as particles, PT
Since the carbon particles 3 for controlling the C characteristics can be blended in a large amount of 4 to 30 (V / V)%, poor control of the PTC characteristics due to weighing error or the like can be improved.

Further, in each of the above-mentioned constitutions, the use range of the blending amount of the carbon particles 3 is wider than that of the ceramics PTC thermistor, a wide range of resistance values can be realized, and the room temperature resistance value can be set low. As compared with the conventional ceramics PTC thermistor, it is possible to expand the applicable range of the power supply voltage and the like.

On the other hand, since each of the above-mentioned constitutions may be mixed and pressure-molded, the firing step in the conventional ceramics PTC thermistor can be omitted and the manufacturing time can be shortened. In addition, in the above configuration, since the electric insulation may change due to moisture absorption, it is preferable to apply a waterproof treatment such as a waterproof coat at the time of use.

In each of the above examples, 2-propanol was used as the solvent for wet mixing.
It is not particularly limited to the above as long as it has a high boiling point so as to have low volatility at the temperature at the time of mixing and a high viscosity so as to facilitate mixing, and other alcohols such as 1
-Propanol or butanol can be used.

[0051]

As described above, the positive temperature coefficient thermistor according to the first aspect of the present invention is constructed by mixing the electrically insulating particles made of ionic crystals and the electrically conductive particles and then press-molding the mixture. .

Therefore, the above structure can have a PTC characteristic in which the resistance value rises as the temperature rises.
In addition, since the conductive particles for controlling the temperature characteristics can be added in a larger amount than the small amount of the semiconductor agent particles in the conventional ceramics PTC thermistor, it is possible to improve the control failure of the temperature characteristics due to a weighing error or the like. Has the effect.

In addition, the above-mentioned structure has the effect that the heat resistance can be increased more than that of the conventional organic polymer by using the electrically insulating particles made of ionic crystals, and therefore the range of application can be expanded.

A positive temperature coefficient thermistor according to a second aspect of the present invention is the positive temperature coefficient thermistor according to the first aspect, wherein the electrically insulating particles are sodium chloride particles and the conductive particles are carbon particles. .

Therefore, in the above structure, in addition to the effect of the invention described in claim 1, since sodium chloride particles are used as the electrically insulating particles and carbon particles are used as the conductive particles, they can be easily obtained. Thus, it is possible to obtain a positive temperature coefficient thermistor quickly.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing a particle structure of a positive temperature coefficient thermistor of the present invention, in which (a) is a schematic cross-sectional view at room temperature and (b) is a schematic cross-sectional view of a main part at a temperature rise. .

FIG. 2 is a drawing-substituting photograph showing the particle structure of carbon particles used in the PTC thermistor in Example 1 of the present invention, using a scanning electron microscope.

FIG. 3 is a graph showing the resistivity-temperature characteristics of each positive temperature coefficient thermistor in the first embodiment of the present invention.

FIG. 4 is a drawing-substituting photograph showing the particle structure of the positive temperature coefficient thermistor in Example 2 of the present invention using a scanning electron microscope.

FIG. 5 is a graph showing the resistivity-temperature characteristics of each positive temperature coefficient thermistor in the second embodiment of the present invention.

FIG. 6 is a drawing-substituting photograph showing the particle structure of the positive temperature coefficient thermistor in Example 3 of the present invention, using a scanning electron microscope.

FIG. 7 is a drawing-substituting photograph showing the particle structure of another positive temperature coefficient thermistor in Example 3 of the present invention using a scanning electron microscope.

FIG. 8 is a drawing-substitute photograph showing a particle structure of carbon particles used in Example 4 of the present invention, using a scanning electron microscope.

FIG. 9 is a graph showing an example of resistivity-temperature characteristics of a conventional positive temperature coefficient thermistor.

[Explanation of symbols]

 2 Electrically insulating particles (sodium chloride particles) 3 Conductive particles (carbon particles)

Claims (2)

[Claims] 1. A positive temperature coefficient thermistor characterized by being formed by mixing electrically insulating particles made of ionic crystals and conductive particles and then pressure-molding the mixture. 2. The positive temperature coefficient thermistor according to claim 1, wherein the electrically insulating particles are sodium chloride particles and the conductive particles are carbon particles.