JPH1167501A - Current-limiting element and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly reliable current-limiting element by preventing the deterioration of the element, composed of a vanadium oxide-based ceramic by repetitive transformations. SOLUTION: The material of a current-limiting element is constituted in such a texture that giant particles 5 larger than crystal grains 3 constituting a matrix are scattered in a matrix by mixing core particles which are increased in size by sintering in matrix powder composed of a vanadium oxide ceramic represented by (V1-x Ax )2 O3 (where, A represents at least one kind of elements selected from among aluminum, chromium, scandium, and lanthanoids and 0.001<=x<=0.30).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
Vanadium oxide based [(V_1-XA_X)_TwoO _Three A is
Aluminum, chrome, scandium or lantanoy
At least one element selected from metal] PTC resistor
(Hereinafter V_TwoO_ThreeCurrent limiter using PTC resistor)
For current-limiting devices with excellent long-term reliability
Related.

[0002]

2. Description of the Related Art In recent years, the capacity of a low-voltage distribution system has been increased, and the overcurrent flowing when the load is short-circuited has been increased accordingly. In response to such technical trends, the use of a V ₂ O ₃ -based PTC resistor element containing a vanadium oxide-based ceramic as a main component is expected as an overcurrent protection element for large current and large power.

A V ₂ O ₃ -based PTC resistor has a temperature of 100 ° C. to 20 ° C.
It has the property of transition from metal to insulator at 0 ° C. (MI transition), and has a specific resistance of 10 ⁻³ Ω · c near room temperature.
m, but gradually increases from room temperature to 100 ° C. to 150 ° C., rapidly increases by about two orders of magnitude near 150 ° C. (this rapidly increasing temperature is referred to as a transition temperature).
It has the property of peaking at 200 ° C. and decreasing at higher temperatures.

[0004] V ₂ O ₃ -based PTC having such properties
By utilizing the fact that the temperature of the resistor rises due to Joule heat when the overcurrent flows and the resistance value increases, the overcurrent can be limited by the increase in the resistance. Therefore, an electrode is joined to this to use as a current limiting element.

[0005]

However, the conventional V
_{In the 2} O ₃ ceramics, a large PTC magnification is obtained in the initial characteristics after firing, but due to repetition of transition, etc., the PTC magnification is much lower than that in the initial stage, and the expected current limiting effect is obtained. There was a problem that it could not be done.

The PTC phenomenon of a V ₂ O ₃ -based PTC resistor is as follows.
This is because at the transition temperature, the c-axis of the crystal contracts by 0.6% and the a-axis expands by 1.0%, whereby the crystal structure changes and the band structure changes. Then, at the time of transition, the crystal grains are deformed along with the expansion and contraction of the axis. For this reason, when the deformation directions of adjacent crystal grains are different, stress is generated between the grains or within the grains due to the transition. In order to alleviate this, microcracks enter the crystal grain boundaries or in the grains, and this process gradually progresses while repeating the transition, and the microcracks propagate to the entire device.

FIGS. 5A to 5C are organization diagrams schematically showing this process. FIG. 5A is a structural diagram before sintering, and 1 is a V ₂ O ₃ matrix powder. FIG. 5B is a structural diagram after sintering, in which the grown matrix crystal grains 3 are seen. Microcracks 4 generated during the cooling process after sintering due to the anisotropy in the coefficient of thermal expansion of the crystal
Some are seen. FIG. 5C shows the structure of a current limiting element that has undergone repeated transition, and a large number of microcracks 4 that have progressed between the matrix crystal grains 3 are seen.

FIGS. 5D and 5E are enlarged views schematically showing the generation and growth mechanism of microcracks.
Due to the expansion and contraction of the crystal axis during the transition, matrix crystal grains 3
A stress is generated between or in the matrix crystal grains 3 (FIG. 5D). In order to alleviate this, the microcracks 4 enter the crystal grain boundaries and the crystal grains, and the microcracks 4 further progress by repeating the transition [FIG. 5 (e)].

FIG. 6 is a PTC characteristic diagram of the current limiting element after sintering and after the transition is repeated about 100 times. It can be seen that in the current-limiting element that has repeatedly transitioned, the PTC magnification is low and the transition temperature has shifted to a higher temperature side.
One of the factors of this deterioration is the above-mentioned deterioration mode accompanying the stress relaxation. That is, in order to exhibit PTC characteristics in V ₂ O ₃ ceramics, it is necessary that a certain amount of stress be applied to the crystal grains.
As shown in (e), the microcracks progress and propagate due to the repetition of the transition, the stress of the crystal particles of the V ₂ O ₃ -based ceramics is relaxed, and the PTC magnification decreases.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to prevent the propagation of microcracks due to the repetition of the transition, maintain the stress in the crystal, maintain the characteristics, and improve the long-term reliability. It is to provide an excellent current limiting element.

[0011]

[MEANS FOR SOLVING THE PROBLEMS]
Ming is a vanadium oxide ceramic (V_1-XA_X) _Two
O_Three [A is aluminum, chromium, scandium or
At least one element selected from lanthanoids,
0.001 ≦ x ≦ 0.30] and has a positive resistance temperature characteristic.
Current limiting element using a PTC resistor
Trix grains are larger than matrix grains
It has a structure in which giant grains are dispersed.

[0012] By forming crystal grains that have grown large in a part of the crystal grains after sintering, microcracks due to a change in stress at the time of transition stop at the crystal grains that have grown large.
Further progress and propagation of microcracks can be prevented. In particular, if the matrix crystal grains and the giant grains are made of PTC resistors having the same composition, one kind of raw material can be used.

It is assumed that the ratio of the giant particles is in the range of 10 to 50% by volume. If the ratio of the giant grains is less than 10%, the effect of inhibiting the progress of microcracks is insufficient, and if it exceeds 50%, the stress acting on one giant grain in the ceramic after sintering increases, and Cracks propagate and propagate to giant grains. It is also possible that the matrix crystal grains are vanadium oxide containing no element A and the giant grains are PTC resistors containing element A.

In this case, since the matrix crystal grains do not transition, there is no anisotropic expansion and contraction accompanying the transition. In that case, the ratio of the giant particles is 55 to 90 in volume ratio.
%. When the ratio of the giant grains is less than 55%, the initial PTC magnification is insufficient. If it exceeds 90%, the sinterability is poor and a dense structure cannot be obtained.

The average size of the giant grains is in the range of 100 to 400 μm. The average particle size of the huge particles is 100μ
If it is less than m, the effect of inhibiting the progress of microcracks is insufficient, and if it exceeds 500 µm, the stress acting on one giant grain in the ceramic after sintering increases, and the microcracks progress to giant grains. , Propagate.

As a method of manufacturing the current limiting element as described above, the core particles which have been once sintered and coarsened are mixed with the matrix powder and fired. When nuclei particles are mixed with the matrix powder and sintered, grain growth proceeds with the nuclei particles as nuclei due to the difference in surface energy, and extremely large crystal grains are obtained as compared with the case where no nucleus particles are added. The matrix powder also grows by sintering, but the larger the crystal grains, the smaller the grains. By such a method, a structure in which giant grains for preventing microcracks are dispersed between matrix crystal grains is obtained.

When the particle size of the crystal grains of the matrix powder is 0.
0.5 to 2 μm, and the particle size of the core crystal is 50 to 300 μm.
It is good to With such a composition, after sintering 0.5 to 1
Matrix grains of 0 μm and giant grains of 100 to 400 μm. When both the matrix powder and the core particles are vanadium oxide-based ceramics containing the element A, the ratio of the core particles is preferably in the range of 5 to 30% by weight.

In this case, the ratio (volume ratio) of the giant grains after sintering becomes an appropriate 10 to 50%. When the matrix powder is vanadium oxide containing no element A and the core particles are vanadium oxide-based ceramics containing element A, the ratio of the core particles is preferably in the range of 50 to 90%.

By doing so, the ratio (volume ratio) of the giant grains after sintering becomes 55 to 90%, which is suitable for maintaining PTC characteristics and for suppressing microcracks.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings. Embodiment 1 FIG. 1 is a schematic diagram showing a PTC resistor according to an embodiment of the present invention. 3 is has e.g. composition PTC characteristics _{(V 0.9965 Cr 0.0035) 2 0} 3 matrix grains, 5 is huge grains of the same composition. For example, the particle size of the matrix crystal grains 3 is 0.5 to 1
0 μm, and the particle size of the giant grains 5 is 100 to 400 μm. In this way, the structure is such that the giant grains 5 are appropriately dispersed in the uniform crystal grains 3. The giant grains 5 occupy 10 to 50% of the cross section. That is, 10 to 5 by volume ratio
That is, it occupies 0%.

FIG. 2 is a flowchart showing a manufacturing process of the current limiting element having the structure shown in FIG. Hereinafter, the manufacturing process of the PTC resistor of the present invention will be described with reference to FIG. (1) Vanadium pentoxide (V ₂ O ₅ ) or vanadium trioxide (V ₂ O ₃ ), chromium oxide (Cr ₂ O ₃ ), iron oxide (Fe ₂ O ₃ ) _{0. 9965} C
r _0.0035 ) ₂ O ₃ + Fe 5% by weight
The slurry obtained by adding water and mixing and crushing with a wet ball mill for 12 hours is freeze-dried to obtain a matrix powder. The particle size of the matrix powder is about 0.05 to 2 μm. (2) A part of the matrix powder was placed in a hydrogen stream at 170
After baking at 0 ° C. for 10 hours, the mixture is finely crushed using an agate mortar or the like to obtain core particles serving as nuclei of giant particles. The average particle size of the core particles is preferably set to 50 to 300 μm for the reason described later. (3) Next, the core particles of (2) are put into a matrix powder, sufficiently mixed by a mixer, and formed into a predetermined shape by a mold. At this time, the mixing ratio of the core particles is set to 5 to 30% by weight. (Because the matrix powder and the core particles are the same substance, the weight ratio is equal to the volume ratio.) If the number of the core particles is too large, the proportion of the particles having a large particle diameter in the sintered ceramic increases. This is because the stress applied to one particle increases, and microcracks propagate and propagate in the grown giant particles in order to alleviate the stress. In addition, there is also a problem that the mechanical strength of the V ₂ O ₃ ceramics decreases due to the increase in the size of the grains. FIG.
FIG. 2 is a schematic structure diagram before sintering observed by EM (scanning electron microscope), in which core particles 2 dispersed in a matrix powder 1 can be seen. (4) Firing at 1550 ° C. for 1 hour in a hydrogen stream. FIG.
(B) is a structure diagram after sintering, in which grown matrix crystal grains 3 and giant grains 5 are seen. Due to the anisotropy in the coefficient of thermal expansion of the crystal, some microcracks 4 generated in the cooling process after sintering are observed. When the core particles are mixed with the matrix powder and sintered, grain growth proceeds with the core particles as nuclei due to the difference in surface energy, and very large crystal grains can be obtained as compared with the case where no core particles are added. The matrix powder also grows by sintering, but does not become as large as the large crystal grains described above. The core particles added with 5 to 30% grow by sintering and occupy 10 to 50% (= volume ratio) of the cross section. (5) The Mo electrode is brazed to the fired PTC resistor via a Ag-Cu eutectic solder by a heat treatment at 750 ° C for 5 minutes in a vacuum to form a current limiting element.

FIG. 4 is a PTC characteristic diagram of the current limiting element having the structure shown in FIG. 1 according to the present invention. It can be seen that the deterioration of the PTC characteristics is suppressed even after repeating the transition 100 times. FIG. 3C is a structural diagram of the current limiting element after the transition is repeated. The micro-cracks 4 that have progressed between the matrix crystal grains 3 are large
It can be seen that it has been stopped at that point, preventing further transmission.

Table 1 shows the results of an experiment in which the particle size of the core particles was changed, and the suitability and unsuitability of the current limiting element as ○ and ×. For comparison, the results for the conventional PTC resistor without the addition of nuclear particles are also shown in the table.

[0024]

[Table 1]

Even if the average particle size of the core particles was changed in the range of 10 to 500 μm, the initial PTC magnification did not change much. However, the PTC magnification after repeated metastasis is
For those with an average particle size of 10 μm and 500 μm, PT
A significant decrease in C magnification was observed. Average particle size 30-300
It can be seen that in the range of μm, the change of the PTC characteristics is suppressed much lower than that of the conventional one, and the characteristic deterioration due to the repetition of the transition can be prevented.

Table 1 also shows the results of SEM observation of the tissue after the repetition of the metastasis. The average particle size of the core particles is 10
In the case of μm, since the difference in particle size from the matrix powder was small, the core particles did not grow so much by firing, and it was observed that microcracks propagated throughout as in the conventional current limiting device. Average particle size of core particles 30 to 300 μm
In the range, microcracks concentrated around the giant grains, and propagation was prevented.

When the average particle diameter of the core particles was 500 μm, it was observed that microcracks were advanced in the giant particles. This is considered to be because the number of core particles is small and the stress is concentrated on the giant grains on which the microcracks have grown, and the stress increases. This may be the reason why the reduction in PTC magnification is larger than that by the conventional manufacturing method. Therefore, the average particle size of the core particles is suitably in the range of 50 to 300 μm. Further, as the core particles, powder obtained by firing a slurry obtained by granulating the slurry once and pulverizing the slurry may be used.

Example 2 As a matrix powder, a V ₂ O ₃ powder having no PTC characteristics after sintering is used.
That is, V ₂ O was added without adding a sub-component causing PTC characteristics.
₅ or V ₂ O ₃ raw material and Fe ₂ O ₃ serving as a sintering agent are used. The core particles are V ₂ O ₃ -based ceramics to which a subcomponent causing PTC characteristics is also added, and are obtained by the same method as in Example 1. The particle size of the matrix powder is about 0.05 to 2 μm, and the particle size of the core particles is 50
３００300 μm.

The manufacturing process of the current limiting element is almost the same as that of the first embodiment. The matrix powder and the core particles are sufficiently mixed by a mixer, formed into a predetermined shape by a mold, and then placed in a hydrogen stream at 1550. Calcination was carried out at ℃ for 1 hour. However, the mixing ratio between the matrix powder and the core particles is such that the core particles are 50 to 90% (by weight) for the following reasons. However, also in this case, since the specific gravity of the matrix powder and the specific gravity of the core particles are substantially equal, the weight ratio and the volume ratio are substantially the same. Observation of the structure after sintering revealed that the giant grains occupied 55 to 90% (= volume ratio) of the cross section. In particular, when there were many core particles, the ratio of the giant grains did not change much because there was little room for grain growth. Microcracks were scarcely found in the matrix crystal grains. This is because the matrix grains derived from the matrix powder
This is because they have no characteristics and there is no anisotropic expansion and contraction accompanying the transition.

Table 2 shows the results of an experiment in which the mixing ratio of the core particles was changed.

[0031]

[Table 2]

The current limiting element in which the mixing ratio of the core particles is 30% has a low initial PTC magnification. This is because there are few core particles having PTC characteristics to which sub-components are added, and the ratio of matrix crystal grains derived from the matrix powder having no PTC characteristics is large. However, in this current limiting element, the reduction in the PTC magnification was slightly smaller. This is because, as described above, the matrix grains
Since it has no properties, there is no change in the crystal structure due to the transition, so that micro-cracks do not progress much, and a decrease in PTC properties due to stress relaxation can be suppressed.

When the mixing ratio of the core particles is 50 to 90%, the initial PTC magnification is large, and the decrease after repeated transition is small. Table 2 also shows the results of SEM observation of the tissue after repeated metastasis. It was confirmed that microcracks were concentrated around the giant grains in the range of the core particle mixing ratio of 30 to 90%.

When the mixing ratio of the core particles exceeds 90%,
Sinterability deteriorated and a dense structure could not be obtained.
PTC characteristics were not measured. From the above, it is understood that the mixing ratio of the core particles is desirably in the range of 50 to 90% by weight. In the above embodiment, the powder is fired once to grow the grains and then crushed as the core particle production step, but the powder may be fired after granulation.

For example, Japanese Patent Publication No. 56-11203 discloses a method of adding a core particle to a raw material powder and firing the voltage non-linear resistor made of zinc oxide.
However, in the invention, although the means of adding and sintering core particles is similar, the invention intends to promote crystal growth using the core particles as nuclei and to form a structure of giant crystal grains as a whole. FIG. 7 is a schematic organization diagram. That is,
The present invention, which is intended to disperse giant grains in a fine grain structure, has completely different substances, problems to be solved, and structures after firing.

[0036]

As described above, according to the present invention, the core particles are mixed with the matrix powder, sintered, and the large grains that have grown greatly are dispersed in a part of the crystal grains, thereby enabling the transition during the transition. , The propagation and propagation of microcracks due to the change in stress can be prevented. In such a current limiting element, the stress is not relaxed due to the progress and propagation of the microcrack even if the transition is repeated, so that the deterioration of the PTC characteristic is prevented, and a current limiting element excellent in long-term reliability can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic organization diagram of a current limiting element according to an embodiment of the present invention.

FIG. 2 is a flowchart showing a manufacturing process of the current limiting element of the present invention.

FIG. 3A is a schematic structural diagram of a current limiting element before firing,
(B) is a structural diagram after firing, and (c) is a structural diagram after repeated transition.

FIG. 4 is a PTC characteristic diagram of the current limiting element of the present invention.

5 (a) to 5 (c) are schematic structural diagrams of a current limiting element according to a conventional manufacturing method, and FIGS. 5 (d) and 5 (e) show generation of microcracks due to expansion and contraction of a crystal axis during repeated transition. Enlarged view showing progress

FIG. 6 is a PTC characteristic diagram of a conventional current limiting element.

FIG. 7 is a schematic organization diagram of a voltage nonlinear resistor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Matrix powder 2 Core particle 3 Matrix crystal grain 4 Micro crack 5 Huge grain

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kenji Kunihara 1-1-1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa Prefecture Inside Fuji Electric Co., Ltd.

Claims (10)

[Claims] _1. A vanadium oxide-based ceramic (V1 _- XA)
_X ) ₂ O ₃ [A is at least one element selected from the group consisting of aluminum, chromium, scandium and lanthanoids, and is composed of 0.001 ≦ x ≦ 0.30] and has a positive resistance temperature characteristic PTC resistor. The current limiting element used has a structure in which giant grains larger than the matrix crystal grains are dispersed in the matrix crystal grains. 2. The method according to claim 1, wherein the matrix crystal grains and the giant grains are made of PTC resistors having the same composition.
Current limiting element as described. 3. The current limiting element according to claim 2, wherein the ratio of the giant particles is in the range of 10 to 50% by volume. 4. The current limiting element according to claim 1, wherein the matrix crystal grains are vanadium oxide containing no A element, and the giant grains are PTC resistors containing the A element. 5. The current limiting element according to claim 4, wherein the proportion of the giant particles is in the range of 55 to 90% by volume. 6. The current limiting device according to claim 1, wherein the average size of the giant particles is in the range of 100 to 400 μm. 7. A vanadium oxide-based ceramic (V _1-X A)
_X ) ₂ O ₃ [A is at least one element selected from the group consisting of aluminum, chromium, scandium and lanthanoids, and is composed of 0.001 ≦ x ≦ 0.30] and has a positive resistance temperature characteristic PTC resistor. A method for manufacturing a current-limiting element, comprising: mixing core particles that have been once sintered and coarsened with a matrix powder, followed by firing. 8. The matrix powder having a particle size of 0.05 to 2
The method according to claim 7, wherein the core particles have a particle diameter of 50 to 300 m. 9. Both the matrix powder and the core particles are A
The method for producing a current limiting element according to claim 7 or 8, wherein the element is a vanadium oxide-based ceramic containing an element, and a ratio of core particles is in a range of 5 to 30% by weight. 10. The matrix powder is vanadium oxide containing no element A, the core particles are vanadium oxide-based ceramics containing element A, and the ratio of the core particles is 5% by weight.
9. The method for manufacturing a current limiting element according to claim 7, wherein the current limiting element is set in a range of 0 to 90%.