JPH10208908A - Current limiting element and method of manufacturing the same - Google Patents

Abstract

(57) 【Abstract】 PROBLEM TO BE SOLVED: To join electrodes to both ends of a columnar resistor made of vanadium oxide-based ceramics via a brazing material, and apply an antioxidant film of low melting glass on the resistor surface. In the current limiting element, the defect of the glass coat is eliminated,
Provided are a current limiting element having excellent long-term reliability and a method for manufacturing the same. An electrode (3) and a V ₂ O ₃ resistor (1) are made larger in diameter than that of a V ₂ O ₃ resistor (1).
A fillet is formed on the outer periphery of the joint portion with the filler to form a fillet, thereby improving poor wetting at the interface, and preventing voids and cracks in the glass coat. The same effect can be obtained by using the tapered electrode 6 having a taper on the bonding surface side of the outer periphery of the electrode. Brazing material as electrode, V ₂ O ₃ resistor 1
May be formed on the end face.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current limiting element using a (PTC) resistor having a positive temperature coefficient of resistance, and more particularly to a current limiting element excellent in long-term reliability and a method of manufacturing the same.

[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 a load is short-circuited has been increased accordingly. In response to such technical trends, use of a current limiting element using a PTC resistor mainly composed of vanadium oxide (hereinafter referred to as V ₂ O ₃ ) -based ceramics as an overcurrent protection element for large current and large power. Is expected.

[0003] V ₂ O ₃ -based ceramics are
It has the property of transitioning from a metal to an insulator between 0 ° C, and its specific resistance is as small as 10 ^-3 Ω · cm around room temperature.
It gradually increases from 100 to 150 ° C., rapidly increases by about two digits near 150 ° C. (this rapidly increasing temperature is referred to as a transition temperature), peaks at 150 to 200 ° C.,
It has the property of decreasing at higher temperatures.

The temperature of a V ₂ O ₃ resistor having such properties increases due to Joule heat when an overcurrent flows,
By utilizing the increase in the resistance value, the overcurrent can be limited by the increase in the resistance. And it is used as a current limiting element which joined the electrodes. However, vanadium trioxide (V ₂ O ₃ ) having PTC characteristics is an unstable phase in the atmosphere, so that oxidation proceeds easily even with a slight heat generation due to energization in a normal state where a normal current flows. For example, it is transformed into a peroxide of V ₂ O ₃ such as VO ₂ , and the PTC characteristic is deteriorated. Since the deterioration of the PTC characteristic significantly lowers the function as an overcurrent protection element, V ₂
It is important to prevent O ₃ oxidation.

First, the group of the inventors applied a paste of a low melting point glass powder such as zinc glass as an antioxidant film on the surface of the non-electrode portion of the current limiting element, and heat-treated in a neutral atmosphere such as nitrogen. The glass was melted and solidified, and a glass coat was found to be suitable and proposed. [Patent Application 8
No.-128889 (filed on May 24, 1996)]

[0006]

However, during the manufacture of current limiting devices, it has been found that some of the defects are caused by incomplete glass coatings for preventing oxidation.
3A to 3C are cross-sectional views illustrating an example of the mechanism in order of a manufacturing process. Conventional V ₂ O ₃
Although the diameter of the resistor 1 and the diameter of the electrode 3 were the same, in this case, the diameter of the V ₂ O ₃ resistor 1 and the displacement of the brazing foil 2 joining the electrode 3 [FIG. )], The molten solder 2a flows out to the side surface of the electrode 3, and a partial wetting failure may occur at the bonding interface [FIG. For this reason, pores 8 and cracks 9 are generated in the recessed portions of the glass coat 4 due to the shrinkage of the glass accompanying the solidification during the heat treatment of the above-mentioned glass paste [FIG. In the current limiting element having such a defect, the oxidation of V ₂ O ₃ proceeds during long-term use, and the characteristics are degraded.

An object of the present invention is to solve the above-mentioned problems, and an object of the present invention is to provide a current limiting element which eliminates defects in a glass coat covering a V ₂ O ₃ resistor and has excellent long-term reliability, and a method of manufacturing the same. To provide.

[0008]

SUMMARY OF THE INVENTION The present invention for the above problems solved, cylindrical V ₂ O consisting of V ₂ O ₃ based ceramics
₍₃ ) In a current limiting element in which electrodes are vacuum brazed to both ends of a resistor through a brazing material and an antioxidant film of low-melting glass is applied to the surface of the V ₂ O ₃ resistor, the diameter of the electrode is V ₂ O
_{3 It} shall be larger than the diameter of the resistor.

In the case of a current limiting element using such an electrode, a brazing with a base angle of 90 ° can be formed on the outer periphery of the bonding interface, and poor wetting of the bonding interface due to the flow out to the electrode side can be eliminated. In addition, since the surface of the accumulated solder becomes tapered (fillet) and becomes a continuous surface, cracks in the glass coat can be prevented. At least a part of the outer peripheral surface of the electrode may be provided with a small taper on the outer surface.

The same effect as described above can be obtained even if a taper is provided on the bonding surface side of the outer periphery of the electrode. As a method of manufacturing the current limiting element of the present invention, brazing is performed using a brazing foil having a diameter larger than the diameter of the V ₂ O ₃ resistor and a thickness of 5 to 500 μm. By doing so, displacement of the brazing material can be prevented. When the thickness is 5 μm or less, the bonding becomes insufficient.
If it is 500 μm or more, the brazing material flows out.

Also, a brazing material may be sputtered to at least one of the V ₂ O ₃ resistor and the end face of the electrode to a thickness of 1 to 10 μm, and brazing may be performed with the brazing material. The same effect as described above is obtained, and handling is further facilitated. If the thickness is less than 1 μm, the bonding becomes insufficient and 10 μm
If it is more than m, time is required for sputtering, and it is not suitable for mass production.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings. [Embodiment 1] FIG. 1C is a sectional view of a current limiting element according to a first embodiment of the present invention, and FIGS. 1A and 1B are sectional views in the middle of the manufacturing process.

In FIG. 1A, 1 is V_TwoO_FiveOr
V_TwoO_ThreeAnd Cr_TwoO_Three, Fe_TwoO _ThreeRaw material, wet type
Mix and crush with a ball mill for 12 hours to form a dry powder
And then fired in a hydrogen stream at 1500 ° C. for 1 hour._Two
O_ThreeA resistor having a composition of, for example, (V_0.9965Cr
_0.0035)_TwoO_Three+ Fe5wt%
Is φ12 × 20 mm.

Reference numeral 2 denotes an Ag—Cu eutectic brazing foil having a size of φ12 × 50 μm. Reference numeral 3 denotes a Mo electrode of φ15 × 5 mm. A Cu electrode may be used. V ₂ O ₃ resistor 1, Ag
A Cu eutectic brazing foil 2 and a Mo electrode 3 were stacked in the longitudinal direction [FIG. 1 (a)], and baked in a vacuum at 750 ° C. for 5 minutes to attach electrodes [FIG. 1 (b)]. There was no poor wetting at the joining interface, and a fine fillet was formed over the entire circumference by the molten and solidified solder 2a.

Thereafter, a low-melting glass paste containing ZnO as a main component was applied and baked in nitrogen at 700 ° C. for 10 minutes to form a glass coat 4 (FIG. 1C). In the glass coat 4, voids, cracks and the like were not observed not only visually but also microscopically. As a result, the defect rate due to the defect of the glass coat was reduced to about 1/10 or less of the related art. This is because the diameter of the Mo electrode 3 was made larger than that of the V ₂ O ₃ resistor 1 and a solder pool was formed to eliminate poor wetting and solder flow at the bonding interface.

[Embodiment 2] FIG. 2C is a sectional view of a current limiting element according to a second embodiment of the present invention, and FIGS. 2A and 2B are sectional views in the course of the manufacturing process. It is. FIG.
5 (a), a 5 μm thick Ag—Cu eutectic brazing layer 7 containing 1% Ti was formed on the bonding surface of the V ₂ O ₃ resistor by sputtering.
m formed V ₂ O ₃ resistor.

Reference numeral 6 denotes a tapered electrode having a diameter of 12.5 mm on the bonding side and 15 mm on the electrode surface side.
It is better that the diameter of the tapered electrode 6 on the joining side is slightly larger than that of the V ₂ O ₃ resistor 1. The angle of the taper is not so steep with respect to the joining surface so that the melted solder does not flow out, and is preferably 45 degrees or less. 5,
6 in the longitudinal direction [FIG. 2 (a)]
It was baked at 0 ° C. for 5 minutes, and electrodes were attached [FIG. There was no poor wetting at the joint interface, and a fillet was formed cleanly over the entire circumference. And because the amount of brazing material is limited,
Extrusion of the brazing material from the joint surface is reduced, and a good surface shape is obtained.

Thereafter, a low-melting glass paste was applied and baked in nitrogen at 700 ° C. for 10 minutes to form a glass coat 4 (FIG. 3C). Also in this example, the glass coat 4
No voids or cracks were generated. The Ag-Cu eutectic brazing layer 7 may be deposited on the Mo electrode side to be joined or on both sides by a sputtering method.

In this example, the V ₂ O ₃ resistor on which the Ag-Cu eutectic brazing layer was formed and the tapered electrode were used at the same time. However, both are not required, and the tapered electrode, the brazing foil, Alternatively, a combination of a V ₂ O ₃ resistor on which an Ag—Cu eutectic brazing layer is formed and an electrode without a taper may be used.

[0020]

As described above, according to the present invention, V
By making the diameter of the electrode larger than the diameter of the ₂ O ₃ resistor and providing a taper on the bonding surface side of the outer periphery of the electrode, poor wetting at the interface of the bonding portion is improved, and the occurrence of defects in the glass coat as an antioxidant film is generated. Therefore, a current limiting element having excellent long-term reliability can be obtained.

If the brazing material is previously sputtered on one or both of the V ₂ O ₃ resistor and the electrode, the handling is easy and the amount of the brazing material is limited. The protrusion of the brazing material is reduced, and a good joint shape is obtained.

[Brief description of the drawings]

1 (a) and 1 (b) are cross-sectional views of a current limiting element according to a first embodiment of the present invention during a manufacturing process, and FIG.

2 (a) and 2 (b) are cross-sectional views of a current limiting element according to Embodiment 2 of the present invention during a manufacturing process, and FIG. 2 (c) is a cross-sectional view after the process is completed.

3 (a) and 3 (b) are cross-sectional views in the middle of a conventional process of manufacturing a current limiting element, and FIG. 3 (c) is a cross-sectional view after the process is completed.

[Explanation of symbols]

1 V ₂ O ₃ resistor 2 brazing foils 2a melt, V ₂ O ₃ resistor 6 tapered electrode 7 with would have solidified 3 electrode 4 glass coat 5 Ag-Cu eutectic brazing layer Ag-Cu eutectic brazing layer 8 Hole 9 crack

Claims (4)

[Claims] 1. A current limiting device comprising a columnar vanadium oxide resistor made of vanadium oxide-based ceramics, electrodes joined to both ends by brazing, and an antioxidant film of low-melting glass adhered to the surface of the resistor. A current-limiting element, wherein the diameter of the electrode is larger than the diameter of the vanadium oxide resistor. 2. The current limiting element according to claim 1, wherein at least a part of the outer peripheral surface of the electrode is provided with a small taper on the bonding surface side. 3. The current limiting element according to claim 1, wherein the brazing is performed using a brazing foil having a diameter larger than that of the vanadium oxide resistor and a thickness of 5 to 500 μm. Method. 4. A brazing layer is sputtered to at least one of the end faces of the vanadium oxide resistor and the electrode to be joined to a thickness of 1 to 10 μm, and brazing is performed with the brazing material. 3. The method for manufacturing a current limiting element according to claim 1.