Monolithic varistor

Details Monolithic varistor Monolithic varistor

1999-04-07

2001-02-06

Donovan, Lincoln (Department: 2832)

Electrical resistors

Resistance value responsive to a condition

Current and/or voltage

C338S020000, C338S224000, C029S610100

Reexamination Certificate

active

06184770

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a monolithic varistor, more particularly, to a monolithic varistor which comprises ZnO as a primary component and has a varistor voltage of 100 V or more. The present invention also relates to a ceramic for producing the varistor and to a method for producing the varistor. Throughout the specification, “varistor voltage” refers to voltage across the varistor measured at a current of 1 mA.
2. Background Art
In recent years, development of a chip-type element and employment of higher frequencies have progressed along with the trend of miniaturization of electronic devices and higher-speed circuit operation. In addition, such an element is required to have a reduced size, especially in terms of height, in order to increase the packaging density of a circuit. A non-linear resistor, i.e., varistor serving as a noise-absorbing element, is not an exception; a chip-type varistor which is formed of a ceramic predominantly comprising zinc oxide or strontium titanate has brought on the market. In contrast, a single-layer varistor having lead terminals or a varistor in which a single varistor layer is “molded-in” a resin or glass has been used as a varistor having a high varistor voltage such as a varistor for alternating current.
However, the conventionally employed single-layer varistor has a drawback that when the maximum peak current is desired to be increased, the electrode area must also be enlarged, thus failing to attain miniaturization of the varistor; whereas miniaturization of the varistor is possible only at the cost of maximum peak current. Thus, miniaturization of a varistor having a varistor voltage of 100 V or more has seen no progress. To cope with the dilemma, a monolithic ceramic varistor comprising a layered ceramic body in which a plurality of internal electrodes are formed is desirable. In this case, however, the varistor voltage per unit thickness thereof must be increased. To this end, the grain size of the ceramic must be reduced without lowering the maximum peak current per unit area.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a monolithic varistor which is small and inexpensive, and which has excellent varistor characteristics.
Another object of the present invention is to provide a ceramic for producing the varistor.
Another object of the present invention is to provide a varistor which predominantly comprises ZnO and has a high varistor voltage of 1000-2500 V/mm.
Still another object of the present invention is to provide a method for producing the varistor.
Accordingly, in a first aspect of the present invention, there is provided a monolithic varistor which includes a layered ceramic body having a plurality of internal electrodes within the product and which is monolithically sintered, wherein the layered ceramic body comprises ZnO as a primary component, and, based on 100 mol % ZnO, an Al component in an amount of about 100-350 ppm calculated as Al
2
O
3
, a Bi component in an amount of about 1.0-3.0 mol % calculated as Bi
2
O
3
, a Co component in an amount of about 0.1-1.5 mol % calculated as Co
2
O
3
, an Mn component in an amount of about 0.1-1.0 mol % calculated as MnO, at least one of an Sb component and an Sn component in an amount of about 0.1-2.0 mol % calculated as SbO
3/2
or SnO, a Y component in an amount of 0-about 3.0 mol % calculated as Y
2
O
3
, an Si component in an amount of about 0.1-1.0 mol % calculated as SiO
2
, and a B component in an amount of about 0.1-2.0 mol % calculated as B
2
O
3
; and which has an average grain size of about 0.9-3.0 &mgr;m at least in a characteristic portion which exhibits the varistor characteristic and is sandwiched by internal electrodes.
In a second aspect of the present invention, there is provided a monolithic varistor which includes a layered ceramic body having a plurality of internal electrodes within the product and which is monolithically sintered, wherein the layered ceramic body comprises ZnO as a primary component, and, based on 100 mol % ZnO, an Al component in an amount of about 100-350 ppm calculated as Al
2
O
3
, a Bi component in an amount of about 1.0-3.0 mol % calculated as Bi
2
O
3
, a Co component in an amount of about 0.1-1.5 mol % calculated as Co
2
O
3
, an Mn component in an amount of about 0.1-1.0 mol % calculated as MnO, at least one of an Sb component and an Sn component in an amount of about 0.1-2.0 mol % calculated as SbO
3/2
or SnO, a Y component in an amount of 0-about 3.0 mol % calculated as Y
2
O
3
, an Si component in an amount of about 0.1-1.0 mol % calculated as SiO
2
, and a B component in an amount of about 0.1-2.0 mol % calculated as B
2
O
3
; and which has a varistor voltage per unit thickness of about 1000-2500 V/mm when an electric current of 1 mA is applied.
In a third aspect of the present invention, there is provided a ceramic for a varistor which comprises ZnO as a primary component, and, based on 100 mol % of ZnO, an Al component in an amount of about 100-350 ppm calculated as Al
2
O
3
, a Bi component in an amount of about 1.0-3.0 mol % calculated as Bi
2
O
3
, a Co component in an amount of about 0.1-1.5 mol % calculated as Co
2
O
3
, an Mn component in an amount of about 0.1-1.0 mol % calculated as MnO, at least one of an Sb component and an Sn component in an amount of about 0.1-2.0 mol % calculated as SbO
3/2
or SnO, a Y component in an amount of 0-about 3.0 mol % calculated as Y
2
O
3
, an Si component in an amount of about 0.1-1.0 mol % calculated as SiO
2
, and a B component in an amount of about 0.1-2.0 mol % calculated as B
2
O
3
.
In a fourth aspect of the present invention, there is provided a varistor which has a ceramic layer containing ZnO as a primary component and a plurality of internal electrodes in the ceramic layer, and which has a varistor voltage per unit thickness of 1000-2500 V/mm when an electric current of 1 mA is applied.
In a fifth aspect of the present invention, there is provided a method for producing a varistor which comprises the following steps:
mixing starting raw materials including ZnO, and components of Al, Bi, Co, Mn, Y, Si, B, and at least one of Sb and Sn;
calcining the resultant mixture;
forming ceramic green sheets containing the calcined product;
forming an internal electrode on each of the ceramic green sheets;
laminating the green sheets;
sintering the layered product; and
providing on outer surfaces of the sintered product outer metallized portions which are connected to the internal electrodes.
Preferably, the starting raw materials in the method have the same composition as described in the first aspect of the invention.
The calcining temperature, the calcining time, the sintering temperature the sintering time, and the composition of the internal electrodes and the outer metallized portions are selected appropriately.
Preferably, the sintering step further includes a step for decomposing organic substances at about 600° C. for removal thereof.


REFERENCES:
patent: 5231370 (1993-07-01), Arnold, Jr. et al.
patent: 5234641 (1993-08-01), Rutt
patent: 5269972 (1993-12-01), Arnold, Jr. et al.
patent: 5569495 (1996-10-01), Evans et al.
patent: 5973588 (1999-10-01), Cowman et al.

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