Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Reexamination Certificate
2001-08-21
2002-12-10
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
C338S020000
Reexamination Certificate
active
06492895
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a voltage non-linear resistor, a method for manufacturing the same, and a varistor using the same.
2. Description of the Related Art
Recent trends toward smaller circuits and higher reference frequencies have demanded size reductions of electronic components capable of withstanding higher frequencies. A varistor, which functions as a surge absorber, is one such electronic component.
Conventionally, SiC-based varistors and ZnO-based varistors are known in the art as nonlinear resistors.
Although the conventional ZnO varistors have a voltage nonlinearity coefficient a of several tens, the apparent relative dielectric constant thereof is 200 or more and the electrostatic capacitance must be kept low when using the ZnO varistors.
The conventional SiC varistors, on the other hand, have a low apparent relative dielectric constant. However, the voltage nonlinearity coefficient a thereof is low compared to other types of varistors and is approximately 7 to 8 at most.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a voltage non-linear resistor for making a varistor having a low apparent relative dielectric constant and a voltage nonlinearity coefficient a at the same level as that of ZnO varistors. A method for making the powder and a varistor using the same are also provided.
To this end, the present invention provides a voltage non-linear resistor including semiconductive SiC particles doped with an impurity. The semiconductive SiC particles have an oxide layer on the surface thereof. The oxide layer has the thickness in the range of about 5 to 100 nm, and aluminum is diffused into the oxide layer.
The present invention also provides a varistor including: a body made of the above-described voltage non-linear resistor; and electrodes provided on the body.
The present invention further provides a method for manufacturing the voltage non-linear resistor. The method includes a steps of forming an oxide layer on the surface of the semiconductive SiC particles; adding one of elemental Al and an Al compound in the semiconductive SiC particles to prepare a mixture, and performing a heat treatment to the mixture in a reducing atmosphere or a neutral atmosphere to diffuse Al into the oxide layer and to form a potential barrier in the oxide layer.
Preferably, the rate of change in weight of the semiconductive SiC particles DM with respect to a specific surface area S (m
2
/g) of the semiconductive SiC particles satisfies the relationship:
0.01
×S
2+0.37
×S≦DM
≦7.34
×S
wherein DM (%)={(M2−M1)/M1}×100, M1 represents the weight of the semiconductive SiC particles before the formation of the oxide layer, and M2 represents the weight of the semiconductive SiC particles after the formation of the oxide layer.
Preferably, the thickness of the oxide layer formed on the surface of each of the semiconductive SiC particles is in the range of about 5 to 100 nm.
The step of forming the oxide layer may include performing a heat treatment to the semiconductive SiC particles in an oxidizing atmosphere.
The step of forming the oxide layer on the surface of the SiC particle may include performing oxidation in air at a temperature in the range of about 1,000 to 1300° C. Preferably, the step of diffusing Al into the oxide layer is performed at a temperature in the range of about 1,000 to 1,400° C.
The voltage non-linear resistor manufactured according to the method of the present invention exhibits low apparent relative dielectric constant and has a voltage nonlinearity coefficient a at the same level as the ZnO-based varistors. Thus, the voltage non-linear resistor of the present invention is suitable for the varistor material.
Moreover, the respective conditions for the step of forming an oxide layer on the surface of a SiC particle and for the step of diffusing Al into the oxide layer can be controlled separately. Thus, the stability of the characteristics can be improved.
REFERENCES:
patent: 2150167 (1939-03-01), Hutchins et al.
patent: 2916167 (1959-12-01), Van der Beck, Jr.
patent: 4147572 (1978-02-01), Vaadajiv et al.
patent: 4086559 (1978-04-01), Knippenberg et al.
patent: 5972801 (1999-10-01), Lipkin et al.
patent: 6157290 (2000-12-01), Glatz-Reichenbach et al.
Kamoshida Yukihiro
Nakamura Kazutaka
Dickstein , Shapiro, Morin & Oshinsky, LLP
Easthom Karl D.
Murata Manufacturing Co. Ltd.
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