Zinc oxide varistor, a method of preparing the same, and a cryst

Details Zinc oxide varistor, a method of preparing the same, and a cryst Zinc oxide varistor, a method of preparing the same, and a cryst

1991-06-26

1994-03-15

Lateef, Marvin M.

Electrical resistors

Resistance value responsive to a condition

Current and/or voltage

H01C 710

Patent

active

052949087

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention particularly relates to a zinc oxide varistor used in the field of an electric power system, a method of preparing the same, and a crystallized glass composition used for coating an oxide ceramic employed for a thermistor or a varistor.


BACKGROUND ART

A zinc oxide varistor comprising ZnO as a main component and several kinds of metallic oxides including Bi.sub.2 O.sub.3, CoO, Sb.sub.2 O.sub.3, Cr.sub.2 O.sub.3, and MnO.sub.2 as other components has a high resistance to surge voltage and excellent non-linearity with respect to voltage. Therefore, it has been generally known that the zinc oxide varistor is widely used as an element for a gapless arrestor in place of conventional silicon carbide varistors in recent years.
For example, Japanese Laid-open Patent Publication No. 62-101002, etc., disclose conventional methods of preparing a zinc oxide varistor. The aforesaid prior art reference discloses as follows: first, to ZnO as a main component are added metallic oxides such as Bi.sub.2 O.sub.3, Sb.sub.2 O.sub.3, Cr.sub.2 O.sub.3, CoO, and MnO.sub.2 each in an amount of 0.01 to 6.0 mol % to prepare a mixed powder. Then, the mixed powder thus obtained is blended and granulated. The resulting granules are molded by application of pressure in a cylindrical form, after which the molded body is baked in an electric furnace at 1200.degree. C. for 6 hours. Next, to the sides of the sintered body thus obtained are applied glass paste consisting of 80 percent by weight of PbO type frit glass containing 60 percent by weight of PbO, 20 percent by weight of feldspar, and an organic binder by means of a screen printing machine in a ratio of 5 to 500 mg/cm.sup.2, followed by baking treatment. Next, both end faces of the element thus obtained are subjected to surface polishing and then an aluminum metallikon electrode is formed thereon, thereby obtaining a zinc oxide varistor.
However, since a zinc oxide varistor prepared by the aforesaid conventional method employed screen printing, a high resistive side layer was formed with a uniform thickness. This led to an advantage in that discharge withstand current rating properties did not largely vary among varistors thus prepared, whereas since the high resistive side layer was made of composite glass consisting of PbO type frit glass and feldspar, the varistor also had disadvantages as follows: the discharge withstand current rating properties were poor, and the non-linearity with respect to voltage lowered during baking treatment of glass, thereby degrading the life characteristics under voltage.


DISCLOSURE OF INVENTION

The present invention overcomes the above conventional deficiencies. The objectives of the present invention are to provide a zinc oxide varistor with high reliability and a method of preparing the same. Another objective of the present invention is to provide a crystallized glass composition suited for coating an oxide ceramic employed for a varistor or a thermistor.
In the present invention, for the purpose of achieving the aforesaid objectives, to the sides of a sintered body comprising ZnO as a main component is applied crystallized glass comprising PbO as a main component such as PbO-ZnO-B.sub.2 O.sub.3 -SiO.sub.2, MoO.sub.3, WoO.sub.3, NiO, Fe.sub.2 O.sub.3, or TiO.sub.2 type crystallized glass, followed by baking treatment, to form a high resistive side layer consisting of PbO type crystallized glass on the sintered body, thereby completing a zinc oxide varistor.
Furthermore, the present invention proposes a crystallized glass composition for coating an oxide ceramic comprising PbO as a main component, and other components such as ZnO, B.sub.2 O.sub.3, SiO.sub.2, MoO.sub.3, WO.sub.3, NiO, Fe.sub.2 O.sub.3, and TiO.sub.2.
Since crystallized glass comprising PbO as a main component according to the present invention has high strength of the coating film due to the addition of SiO.sub.2, MoO.sub.3, WO.sub.3, NiO, Fe.sub.2 O.sub.3, TiO.sub.2, etc., and excellent adhesion to a sintered body, it has excellent disch

REFERENCES:
patent: 3959543 (1976-05-01), Ellis
patent: 4319215 (1982-03-01), Yamazaki et al.
patent: 4400683 (1983-08-01), Eda et al.
patent: 4420737 (1983-12-01), Miyoshi et al.
patent: 4559167 (1985-12-01), Julke et al.

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