JP3340643B2 - Composite PTC material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a composite PTC suitably used for a current limiting element for suppressing a fault current, and the like.
It is about materials.

[0002]

2. Description of the Related Art PTC (positive temperature coeff)
Since the material has a property that the electric resistance rapidly increases at a specific temperature, the material is used as a current limiting element for suppressing a fault current in a breaker. For example, a composite material containing a polymer as a base material and a conductive substance as an additive has PTC characteristics.

That is, a composite material in which a crystalline polymer such as polyethylene as an insulator and conductive particles such as carbon are mixed at a specific mixing ratio and a conductive path is formed in a polymer matrix to impart conductivity. When the temperature of the crystalline polymer reaches the melting temperature due to the temperature rise, only the crystalline polymer, whose thermal expansion is much larger than the conductive particles, expands abruptly. Are separated and the conductive path is cut, so that the electric resistance sharply increases. However, when an organic material such as a polymer is used as a base material,
Since heat resistance is low and operation cannot be maintained at high temperatures, quartz,
A composite material in which conductive particles are mixed with a silica-based substance having a high coefficient of thermal expansion such as cristobalite has been proposed.

[0004]

However, the above-mentioned silica-based composite material has a high room temperature resistivity and a large conduction loss, and furthermore, the conductive particles are oxidized during firing or use of the material, resulting in a low room temperature resistivity. However, there is a problem that the size becomes larger. The present invention has been made in view of such problems of the prior art, and has as its object to have heat resistance, low power loss, and conductivity during material firing or use. An object of the present invention is to provide a composite PTC material in which oxidation of particles is prevented.

[0005]

According to the present invention, there is provided a composite PTC material comprising cristobalite as a base material, and the base material and a conductive filler. In comparison, a reduced component having a smaller standard energy of formation of an oxide in an oxidation reaction,
0.1 to 10 of the total weight of the base material and the conductive filler
A composite PTC material containing 0.1 wt% and having a room temperature resistivity of 10 ^-1 Ωcm or less.

In the composite PTC material of the present invention, the reducing component is preferably at least one of a transition metal element, metallic silicon, and carbon, and preferably contains 0.1 to 5% of the reducing component. preferable. Further, in the composite PTC material of the present invention, the room temperature resistivity of the single sintered body of the conductive filler is preferably 10 ⁻³ Ωcm or less, and the particle size of the conductive filler is 2 to 50 Ωcm.
μm, and the relative density after firing of the composite PTC material is preferably 90% or more.

In the composite PTC material of the present invention, the conductive filler is preferably at least one of a metal simple substance, a metal silicide, a metal carbide, and a metal boride, and the conductive filler is made of MoSi ₂ , WSi ₂ , Mo, and W. More preferably, at least one of them is used. Further, in the composite PTC material of the present invention, it is preferable that the addition rate of the conductive filler is 10 to 35 vol%.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention mainly comprises a cristobalite, which is a high thermal expansion material, and a conductive filler, and further has, as a third component, a reducing component having a smaller standard energy of formation of an oxide in an oxidation reaction than the conductive filler. Is from 0.1 to the total weight of the base material and the conductive filler
A composite PTC material (hereinafter, referred to as a PTC material) containing 10 wt% and having a room temperature resistivity of 10 ^-1 Ωcm or less. According to the present invention, it is possible to provide a PTC material which has heat resistance, has a small power loss, and prevents oxidation of conductive particles during firing or use of the material. The PTC material is required to have a large jump rate as a characteristic, that is, a large difference between the resistivity at the time of device operation and the initial resistivity. However, the PTC material of the present invention has a jump rate of three digits or more. It is possible to secure.

In the present invention, cristobalite (quartzite) is used as a base material of the PTC material. Cristobalite is made of SiO ₂ along with quartz and tridymite.
A type of mineral polymorph, a high thermal expansion material that has the property of rapidly expanding as the crystal structure changes from alpha (tetragonal) to beta (cubic) at around 230 ° C. Therefore, although cristobalite itself is an insulator, for a material obtained by mixing a conductive material at a predetermined ratio to impart conductivity, the formed conductive path is cut off due to the thermal expansion of cristobalite accompanying a rise in temperature. , P
Can exhibit TC properties.

Cristobalite has a melting point of 173.
Since the temperature is as high as 0 ° C., the heat resistance is higher than that of a polymer which is an organic material, and there is no damage due to melting or the like even when exposed to a high temperature for a long time. Therefore, it is suitable as a base material of a PTC material. Cristobalite is obtained by calcining quartz at a high temperature, but can be obtained by calcining at a lower temperature in the presence of an alkali metal or alkaline earth metal that stabilizes cristobalite. In the present invention, quartz may be used as a raw material, and may be used after being converted to cristobalite during a step such as a firing step after molding.

[0011] The conductive filler is an additive for imparting conductivity to cristobalite, which is an insulator. In the present invention, a simple metal, a metal silicide, a metal carbide,
It is preferable to use a metal boride, but it is more preferable to use metal particles such as molybdenum and tungsten, which are high melting points, and a metal silicide such as molybdenum silicide and tungsten silicide. In the present invention, the conduction loss is suppressed by defining the room temperature resistivity of the plain sintered body of the conductive filler to be 10 ⁻³ Ωcm or less and reducing the room temperature resistivity of the PTC material itself to 10 ⁻¹ Ωcm or less. are doing.
Therefore, carbon having a room temperature resistivity of 10 ⁻³ Ωcm or more and low conductivity has a large conduction loss, and is not suitable as the conductive filler of the present invention.

In the present invention, the particle size of the conductive filler is:
Preferably it is 2 μm or more. Usually, to increase the jump rate, the amount of filler, which is a conductor, should be reduced with respect to the amount of cristobalite, which is an insulator, but this directly leads to an increase in room temperature resistivity and, consequently, an increase in conduction loss. I do. Therefore, in the present invention, the particle diameter of the conductive filler is adjusted to 2 μm or more, and the conductive filler is brought into contact with the base material particles by making the most of the surface area of the conductive filler. By doing so, it is possible to reduce only the useless conductive filler that does not contribute to the improvement of the conductivity, and it is possible to improve the jump rate while preventing an increase in the room temperature resistivity.

Further, in the present invention, the particle size of the conductive filler is preferably 50 μm or less. If the filler particle size is 50 μm or more, it becomes difficult to uniformly disperse the filler in the base material. If the added amount of the filler is small, the conductive path is not formed and the room temperature resistivity rises. If it is large, the conductive path cannot be cut even if the temperature rises, and no resistance jump occurs. Base material particles 0.1
In the range of 10 to 10 μm and the filler particles in the range of 2 to 50 μm, the filler addition amount is preferably in the range of 10 to 35 vol%.

In the present invention, the sintered PT
Preferably, the relative density of the C material is densified to 90% or more, and more preferably to 95% or more. If the relative density is 90% or less, even if a resistance jump occurs, it does not return to the initial resistivity after the temperature is lowered. The relative density of the sintered body is affected not only by the particle size of the raw material but also decreases when the firing temperature is low.

Further, in the present invention, in addition to the above-described base material and conductive filler, as a third component, a reducing component having a smaller standard energy of formation of an oxide in an oxidation reaction than the conductive filler (ie, a conductive component) (A component more easily oxidized than the filler). By adding the reducing component to the material, the reducing component is oxidized in preference to the conductive filler, so that it is possible to suppress the oxidation of the conductive filler and, consequently, the increase in the room temperature resistivity.

The reducing component used in the present invention is not particularly limited as long as the standard energy of formation of the oxide in the oxidation reaction is smaller than that of the conductive filler used (that is, it is more easily oxidized than the conductive filler). However, since the oxidation product is preferably a substance that does not contribute to an increase in the room temperature resistivity of the material, the oxidation product (CO
₂ ) It is preferable that the metal to be released into the outside of the material as gas becomes carbon, and that the metal to be oxidized (Si) be metallic silicon which does not have a low room temperature resistivity and whose oxidation product does not adversely affect the properties of the material. .

In the present invention, a reducing component that is 0.1 to 10% by weight of the total weight of the base material and the conductive filler is added. This is because the addition of 0.1 wt% or less does not have a sufficient effect of preventing the filler from being oxidized.
This is because, when the above addition is performed, for example, when a conductive reducing component such as carbon is used, the conductive path increases, so that the resistance jump is reduced or the resistance jump is not exhibited. Therefore, when a conductive reducing component such as carbon is used, it is more preferable to add 0.1 to 5 wt% which has little effect on the resistance jump.

Hereinafter, an example of the method for producing a PTC material of the present invention will be described. The method for producing a PTC material of the present invention comprises, for example, three steps as shown in FIG. 1, and raw materials are prepared as follows. When cristobalite is used as a base material, the quartz powder is calcined at a high temperature, or the quartz is calcined in the presence of an alkali metal or an alkaline earth metal to form cristobalite, which is then pulverized by a wet pot mill. A powder having a particle size of 1 μm or less is prepared. When quartz is used as the base material, the average particle size is 0.5 to 2 by pulverizing with a wet pot mill.
Prepare a μm powder.

As the conductive filler material, a metal simple substance,
After pulverizing a metal silicide, a metal carbide, or a metal boride, the powder is classified to prepare a powder having a desired particle size. The reducing component is adjusted by adding carbon powder or pulverizing metal powder by a wet pot mill. Note that an organic substance may be added instead of adding a reducing component. This is because an organic substance consumes oxygen by a decomposition reaction at the time of firing, and the same effect as the addition of carbon can be expected even if it remains in the form of carbon after firing. For example, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), a paraffin-based or cellulose-based material that can be used as a binder can be preferably used.

The first step is a mixing step of mixing a base material, a conductive filler material, and a reducing component.
Measure the conductive filler raw material and the reducing component at a predetermined ratio,
The mixture is obtained by mixing with a wet or dry ball mill. When quartz is used as a base material,
Since it is necessary to convert to cristobalite during the process, an alkali metal or an alkaline earth metal may be added as a stabilizing material for cristobalite at the time of mixing.

The second step is a molding step of molding a mixture, and the mixture obtained in the first step is press-molded to obtain a molded body. When firing under normal pressure, the molded body may be further subjected to isotropic pressure molding to obtain a molded body.

The third step is a sintering step of sintering the molded body. The molded body obtained in the second step is subjected to a predetermined load in an inert gas stream such as nitrogen gas at a high temperature. Hot pressing is performed to obtain a sintered body. The green body isotropically pressed is subjected to normal-pressure sintering in a stream of an inert gas such as argon at a high temperature to obtain a sintered body.

[0023]

EXAMPLES Hereinafter, the present invention will be described with reference to specific examples, but the present invention is not limited to these examples. Example 1 Cristobalite powder having an average particle size of 0.8 μm and molybdenum silicide powder having an average particle size of 35 μm were mixed with 7
For a mixture of 5:25 ratio, 1 w of its total weight
t% of carbon was mixed by a wet ball mill.

The mixture is press-molded under a load of 200 Kg / cm ² , and the obtained molded body is subjected to hot pressing under a load of 200 Kg / cm ² at 1450 ° C. for 3 hours in a nitrogen stream to obtain a sintered body. I got The obtained sintered body was processed into a column having a size of 5 × 5 × 30 mm. Then, the resistivity ρ ₀ at room temperature and the resistivity ρ ₃₀₀ at 300 ° C. were measured by a DC four-terminal method, and the resistance jump rate (ρ ₃₀₀ / Ρ ₀ ) was calculated. In addition, the room temperature resistivity ρ ₀ of the material is 10 at 300 ° C.
The resistivity ρ ₂ after holding for a time was measured, and a high-temperature holding evaluation (ρ ₂ / ρ ₀ ) was calculated. Table 1 shows the results.

(Example 2) A mixture of cristobalite powder having an average particle diameter of 0.8 μm and molybdenum silicide powder having an average particle diameter of 35 μm in a ratio of 75:25 was subjected to wet ball milling with 5 wt% of carbon based on the total weight thereof. By mixing. The mixture was press-molded at a load of 200 Kg / cm ² , and the obtained molded body was subjected to hot pressing under a load of 200 Kg / cm ² at 1450 ° C. for 3 hours in a nitrogen stream to obtain a sintered body. . The same evaluation as in Example 1 was performed on the obtained sintered body. Table 1 shows the results.
Shown in

Example 3 Powdered quartz having an average particle size of 1.2 μm and powdered metal molybdenum having an average particle size of 3.1 μm were mixed with 7
For a mixture of 5:25 ratio, 1 w of its total weight
t% of metallic silicon was added, and mixed by a dry ball mill in the presence of 1 mol% of sodium hydrogen carbonate with respect to the quartz powder. The mixture was press-molded under a load of 200 kg / cm ² and then isostatically pressed under a load of 7 t / cm ^2.
A normal-pressure baking was performed at 0 ° C. for 3 hours to obtain a sintered body.
The same evaluation as in Example 1 was performed on the obtained sintered body.
Table 1 shows the results.

Example 4 Powdered quartz having an average particle size of 1.2 μm and powdered metal molybdenum having an average particle size of 3.1 μm were mixed with 7
For a mixture of 5:25 ratio, 5 w of its total weight
t% of metallic silicon was added, and mixed by a dry ball mill in the presence of 1 mol% of sodium hydrogen carbonate with respect to the quartz powder. The mixture was press-molded under a load of 200 kg / cm ² and then isostatically pressed under a load of 7 t / cm ^2.
A normal-pressure baking was performed at 0 ° C. for 3 hours to obtain a sintered body.
The same evaluation as in Example 1 was performed on the obtained sintered body.
Table 1 shows the results.

Example 5 Powdered quartz having an average particle size of 1.2 μm and powdered metal molybdenum having an average particle size of 3.1 μm were mixed with 7
For a mixture of 5:25 ratio, 10% of the total weight
wt% of metallic silicon was added and mixed by a dry ball mill in the presence of 1 mol% of sodium hydrogen carbonate with respect to the quartz powder. Load the above mixture with a load of 200 kg / cm ²
Press molding and then isostatic pressing under a load of 7 t / cm ^2.
Atmospheric pressure sintering maintained at 00 ° C. for 3 hours was performed to obtain a sintered body. The same evaluation as in Example 1 was performed on the obtained sintered body. Table 1 shows the results.

Comparative Example 1 A mixture of cristobalite powder having an average particle diameter of 0.8 μm and molybdenum silicide powder having an average particle diameter of 35 μm in a ratio of 75:25 was mixed by a wet ball mill without adding a reducing component. The mixture was press-molded at a load of 200 kg / cm ² , and the obtained molded body was further subjected to a load of 200 kg / c in a nitrogen stream.
Hot pressing was performed at 1450 ° C. for 3 hours at m ² to obtain a sintered body. The same evaluation as in Example 1 was performed on the obtained sintered body. Table 1 shows the results.

(Comparative Example 2) A mixture of cristobalite powder having an average particle size of 0.8 μm and molybdenum silicide powder having an average particle size of 35 μm in a ratio of 75:25 was wet ball milled with 20 wt% of carbon based on the total weight. By mixing. The mixture was press-molded at a load of 200 Kg / cm ² , and the obtained molded body was subjected to hot pressing under a load of 200 Kg / cm ² at 1450 ° C. for 3 hours in a nitrogen stream to obtain a sintered body. . The same evaluation as in Example 1 was performed on the obtained sintered body. Table 1 shows the results.

Comparative Example 3 Powdered quartz having an average particle size of 1.2 μm and powdered metal molybdenum having an average particle size of 3.1 μm were mixed with 7
The mixture having a ratio of 5:25 was mixed by a dry ball mill in the presence of 1 mol% of sodium hydrogen carbonate with respect to quartz powder without adding a reducing component. The mixture was press-molded under a load of 200 Kg / cm ² , and then subjected to isotropic pressure molding under a load of 7 t / cm ² , and subjected to normal-pressure sintering at 1600 ° C. for 3 hours in an argon stream. Thus, a sintered body was obtained. The same evaluation as in Example 1 was performed on the obtained sintered body. Table 1 shows the results.

[0032]

[Table 1]

As shown in Examples 1 to 5, when 1 to 10% of the reducing component is added, low room temperature resistivity and high jump rate can be obtained regardless of the starting materials, the mixing method, and the firing method. Also, the result of the high-temperature holding evaluation is as good as 10% or less.
On the other hand, as shown in Comparative Examples 1 and 3, when a reducing component was not added, although a resistance jump was exhibited, the high temperature retention evaluation was as high as 10 to 20%, and the room temperature resistivity accompanying the oxidation of the filler was high. Is observed. Further, as shown in Comparative Example 2, when 10% or more of the conductive reducing component (carbon) is added, the room temperature resistivity decreases as the number of conductive paths increases, but the transition temperature of cristobalite (the expansion temperature) ), The conductive path cannot be cut and no resistance jump is exhibited.

[0034]

As described above, the composite PTC material of the present invention has heat resistance, low power loss, and prevents oxidation of conductive particles during firing or use of the material. It should be noted that the jump rate, which is a characteristic of the PTC material, can ensure a jump rate of three digits or more.

[Brief description of the drawings]

FIG. 1 is a process chart showing an example of a production method of the present invention.

Claims (9)

(57) [Claims] 1. A composite PTC material comprising cristobalite as a base material and a base material and a conductive filler, and further comprising, as a third component, an oxide standard in an oxidation reaction as compared with the conductive filler. The reducing component having a small generation energy is used as 0.1% of the total weight of the base material and the conductive filler.
At room temperature with a resistivity of 10 ^-1 Ωcm
A composite PTC material characterized by the following. 2. The composite PTC material according to claim 1, wherein the reducing component is at least one of a transition metal element, metallic silicon, and carbon. 3. The composite PTC material according to claim 1, which contains 0.1 to 5% of a reducing component. 4. The composite PTC material according to claim 1, wherein the room temperature resistivity of the plain sintered body of the conductive filler is 10 ⁻³ Ωcm or less. 5. The composite PTC according to claim 1, wherein the conductive filler has a particle size of 2 to 50 μm.
material. 6. The composite PT according to claim 1, wherein a relative density after firing is 90% or more.
C material. 7. The composite P according to claim 1, wherein the conductive filler is at least one of a simple metal, a metal silicide, a metal carbide, and a metal boride.
TC material. 8. The conductive filler is made of MoSi ₂ , WSi ₂ , M
The composite PTC material according to claim 7, which is at least one of o and W. 9. An addition rate of the conductive filler is 10 to 35 vo.
The composite PTC material according to any one of claims 1 to 8, which is 1%.