Compositions – Electrically conductive or emissive compositions – Metal compound containing
Patent
1998-02-20
1999-11-09
Kopec, Mark
Compositions
Electrically conductive or emissive compositions
Metal compound containing
501126, 264621, 264671, H01B 108, C04B 3514, B28B 100
Patent
active
059807881
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a lateral high-resistance additive for forming a lateral high-resistance layer of a zinc oxide varistor mainly used in the field of electric power, a zinc oxide varistor using the same, and a process for producing the zinc oxide varistor.
BACKGROUND ART
A conventional process for producing a zinc oxide varistor is disclosed, for example, in Japanese Laid-open Patent No. 61-259502, and its procedure is as follows.
First, ZnO is a principal ingredient, and small amounts of metal oxides such as Bi.sub.2 O.sub.3, Co.sub.2 O.sub.3, MnO, Cr.sub.2 O.sub.3, Sb.sub.2 O.sub.3, NiO, and Al.sub.2 O.sub.3 are added as auxiliary ingredients. Mixing them sufficiently together with water, binder, and dispersant, slurry is prepared, which is dried and granulated by a spray dryer, and the obtained power is formed in a disk of 55 mm in diameter and 30 mm in thickness. After baking at 500.degree. C. in order to remove organic matter, it is calcined at 1020.degree. C., and a calcined material is obtained. A prepared slurry for forming a high-resistance layer is applied on this calcined material by means of a spray gun.
This slurry for forming a high-resistance layer is prepared by reacting Fe.sub.2 O.sub.3, ZnO and Sb.sub.2 O.sub.3 to produce ZnFe.sub.2 O.sub.4 and Zn.sub.7 Sb.sub.2 O.sub.12, weighing powder of ZnFe.sub.2 O.sub.4 and Zn.sub.7 Sb.sub.2 O.sub.12 so that the ratio of Fe and Sb may be 2:1, adding purified water so that the ratio by weight to this powder may be 1:1, and adding binder such as polyvinyl alcohol for increasing the strength of the coat film by about 0.1 wt. %.
Consequently, the calcined material on which the slurry for forming a high-resistance layer is applied is baked in air at 1200.degree. C. to obtain sinter, and both ends of the sinter is polished to form an Al sprayed electrode, thereby obtaining a zinc oxide varistor having a lateral high-resistance layer.
In this conventional method, as the slurry for forming a high-resistance layer, ZnFe.sub.2 O.sub.4 and Zn.sub.7 Sb.sub.2 O.sub.12 preliminarily synthesized at high temperature are used, and when a lateral high-resistance layer is formed by using them, the reactivity of the calcined material with ZnFe.sub.2 O.sub.4 and Zn.sub.7 Sb.sub.2 O.sub.12 is not sufficient, and the contact between the sinter and the lateral high-resistance layer is poor, and the lateral high-resistance layer is likely to be peeled off during discharge current withstand test, and hence the discharge current withstand capacity characteristic is low.
DISCLOSURE OF THE INVENTION
It is hence an object of the invention to present a zinc oxide varistor excellent in reliability including discharge current withstand capacity characteristic.
To achieve the object, the invention forms a lateral high-resistance additive for zinc oxide varistor by using a metal oxide comprising 1-40 molar % (in terms of Fe.sub.2 O.sub.3) iron, 0-20 molar % (in terms of Bi.sub.2 O.sub.3) bismuth, and the balance consisting of SiO.sub.2.
This lateral high-resistance additive is applied and baked on a lateral face of a molded or calcined material containing zinc oxide as principal ingredient and at least antimony as auxiliary ingredient to form a high-resistance layer on the lateral face of the zinc oxide varistor, and therefore the iron, bismuth and silicon in the lateral high-resistance additive react very well with the ingredients in the molded or calcined material to produce a high-resistance layer containing Zn.sub.2 SiO.sub.4 as principal ingredient and at least Zn.sub.7 Sb.sub.2 O.sub.12 dissolving Fe as auxiliary ingredient. This high-resistance layer is homogeneous and excellent in contact with the sinter, and hence discharge current withstand capacity characteristic and other properties can be enhanced substantially.
Moreover, since this lateral high-resistance additive is also extremely excellent in reactivity with the molded material, it can be directly applied on the molded material, and the conventional calcining process of molded mat
REFERENCES:
patent: 4111852 (1978-09-01), Ho et al.
patent: 4500642 (1985-02-01), Reiji et al.
patent: 4933659 (1990-06-01), Imai et al.
patent: 5770113 (1998-06-01), Iga et al.
Kanaya Osamu
Katsumata Masaaki
Kopec Mark
Matsushita Electric - Industrial Co., Ltd.
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