Compositions – Barrier layer device compositions – Groups ii and v elements containing binary compund; e.g.,...
Reexamination Certificate
2001-04-25
2003-09-30
Gupta, Yogendra N. (Department: 1751)
Compositions
Barrier layer device compositions
Groups ii and v elements containing binary compund; e.g.,...
C252S06230Q, C252S06230Q, C252S518100, C252S519500, C338S021000, C361S127000, C423S622000
Reexamination Certificate
active
06627100
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a current/voltage non-linear resistor having main component of zinc oxide (ZnO), applied in an overvoltage protection device such as an arrester or a surge absorber, and in particular, relates to a current/voltage non-linear resistor capable of improving a resistance distribution in the current/voltage non-linear resistor and a component composition of an auxiliary component included in the main component. The present invention also relates to a sintered body for the current/voltage non-linear resistor of the character mentioned above.
In general, overvoltage protection devices such as arresters or surge absorbers are employed in power systems or circuits of electronic equipments to protect the power system or electronic equipments by removing the overvoltage state that is superimposed on the normal voltage. As overvoltage protection devices, current/voltage non-linear resistors are frequently used. The current/voltage non-linear resistors have a characteristic that practically shows an insulating characteristic at an ordinary voltage, but shows low resistance when the overvoltage is applied.
A current/voltage non-linear resistor may be manufactured by procedures described in Japanese Patent Publication No. HEI 4-25681, for example. First of all, a raw material is prepared by adding Bi
2
O
3
, Co
2
O
3
, MnO, Sb
2
O
3
and NiO as auxiliary component to zinc oxide (ZnO) as main component. This raw material is then thoroughly mixed with water and binder and then granulated by using a spray drier etc, and a sintered body is obtained through molding and sintering processes. Thereafter, an insulating layer is formed on the side surfaces of the sintered body by applying an insulating substance to prevent surface flashover to the side surfaces of the sintered body, followed by a thermal (heat) treatment. After the formation of the insulating layer, the current/voltage non-linear resistor is manufactured by polishing both end surfaces of the sintered body and then attaching electrodes thereto.
However, in recent years, with increased demand for power, increased sub-station capacity and installation of sub-stations underground, a reduction in the size of sub-station equipment has been required.
Although the current/voltage non-linear resistor whose main component is zinc oxide is employed in the arrester on account of its excellent non-linear resistance characteristic, this non-linear resistance characteristic only offers the protection level of the arrester and it is hence necessary to further improve such characteristic.
For example, Japanese Patent Publication No. HEI 4-25681 discloses an attempt to improve the non-linear resistance characteristic and life characteristic by restricting the contents of auxiliary components such as Bi
2
O
3
, Co
2
O
3
, MnO, Sb
2
O
3
and NiO added to the ZnO as main component.
Furthermore, Japanese Patent Publication No. HEI 2-23008 discloses an attempt to improve life characteristic by restricting the contents of the auxiliary component such as Bi
2
O
3
, Co
2
O
3
, MnO, Sb
2
O
3
and NiO and restricting the crystal phases of the Bi
2
O
3
contained in the sintered body having the main component of ZnO.
Furthermore, Japanese Patent Laid-open Publication No. HEI 8-264305 discloses an attempt to improve the energy endurance by making the resistance in a peripheral region lower than the resistance in a central region in a sintered body.
However, the characteristics that are required for the conventional current/voltage non-linear resistors are currently becoming increasingly strict, and it becomes difficult to satisfy the characteristics required with the prior arts described above.
Specifically, it becomes also difficult to achieve sufficient equipment reliability and stability of the power supply since a sufficient life characteristic is not obtainable because the normal voltage that is applied to the current/voltage non-linear resistor may be deteriorated.
Furthermore, it is difficult to achieve miniaturization of the arrester since the number of sheets of current/voltage non-linear resistor laminated in the lightning arrester cannot be reduced since the resistance per sheet of the current/voltage non-linear resistor is insufficient.
It is also difficult to minimize transformers and switches for the reason that, although it is required to improve the energy endurance, i.e. the surge, that can be absorbed without damage by the current/voltage non-linear resistor, if the number of sheets of the current/voltage non-linear resistor be reduced, the surge energy endurance obtained would be insufficient.
SUMMARY OF THE INVENTION
In view of these problems, an object of the present invention is to provide a voltage/current non-linear resistor in which an excellent current/voltage non-linear resistor resistance characteristic is obtained and which has an excellent life characteristic and energy endurance characteristic.
Another object of the present invention is to also provide a sintered body for the current/voltage non-linear resistor of the characters mentioned above.
In order to achieve these and other objects, the present inventors of the subject application made repeated studies of various types of the component composition of current/voltage non-linear resistors and the resistance distribution, as a result of which the inventors have perfected the present invention.
That is, according to the present invention, there is provided in one aspect a current/voltage non-linear resistor comprising a sintered body having a main component of ZnO, an electrode applied to a surface of the sintered body and an insulation material also applied to the surface of the sintered body, the main component containing, as auxiliary components, Bi, Co, Mn, Sb, Ni and Al, the contents of the auxiliary components being respectively expressed as Bi
2
O
3
, Co
2
O
3
, MnO, Sb
2
O
3
, NiO and Al
3+
, of Bi
2
O
3
: 0.3 to 2 mol %, Co
2
O
3
: 0.3 to 1.5 mol %, MnO: 0.4 to 6 mol %, Sb
2
O
3
: 0.8 to 7 mol %, NiO: 0.5 to 5 mol % and Al
3+
: 0.001 to 0.02 mol %; Bi
2
O
3
crystalline phase in the sintered body including an &agr;-Bi
2
O
3
phase representing at least 80% of the total Bi
2
O
3
phase.
The reason why the component composition range and crystalline phase are restricted in this way according to the present invention of the above aspect is that if these ranges are departed from, the non-linear resistance characteristic is adversely affected.
The Bi
2
O
3
that is added as the auxiliary component is a component, existing at the grain boundaries of the ZnO produces a non-linear resistance characteristic. The Co
2
O
3
and NiO are component which, dissolved in a solid solution in the ZnO grains, are effective for improving the non-linear resistance characteristic. Sb
2
O
3
is a component which controls grain growth of the ZnO grains during the sintering process by forming spinel grains and has the action of improving uniformity, conferring the benefit of improving the non-linear resistance characteristic. MnO is a component that is effective for improving the non-linear resistance characteristic by dissolving in the solid solution in the ZnO grains and spinel grains. Al
3+
is a component that is effective for improving the non-linear resistance characteristic by dissolving in the solid solution in the ZnO grains, thus lowering the electrical resistance of the ZnO grains.
Furthermore, by restricting the amount of &agr;-Bi
2
O
3
phase in the orthorhombic system to at least 80% of the total bismuth phase, the insulation resistance of the Bi
2
O
3
crystalline phase in the sintered body is raised and the non-linear resistance characteristic can be improved.
In another aspect of the present invention, there is also provided a current/voltage non-linear resistor comprising a sintered body having a main component of ZnO, an electrode applied to a surface of the sintered body and an insulation material also applied to a surface of the sintered body, the main component containing, as auxiliary components, Bi
Ando Hideyasu
Higashibata Koji
Imai Toshiya
Ito Yoshiyasu
Narita Hiroyoshi
Gupta Yogendra N.
Vijayakumar Kallambella
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