Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Of electrical article or electrical component
Reexamination Certificate
1995-06-06
2001-02-06
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Of electrical article or electrical component
C264S104000, C264S157000, C264S160000
Reexamination Certificate
active
06183685
ABSTRACT:
RELATED APPLICATIONS
This application is related to applications Ser. No. 07/543,528, filed Jun. 26, 1990, entitled “Varistor Ink Formulations”, now abandoned; Ser. No. 07/543,921, filed Jun. 26, 19990, entitled Varistor Structures, now U.S. Pat. No. 5,115,221; Ser. No. 07/543,516, filed Jun. 26, 1990, entitled “Varistor Powder Compositions”, now U.S. Pat. No. 5,235,310. The teachings of these applications are incorporated herein by reference.
BACKGROUND OF THE INVENTION
Zinc oxide varistors are ceramic semiconductor devices based on zinc oxide. They have highly non-linear current/voltage characteristics, similar to back-to-back Zener diodes, but with much greater current and energy handling capabilities. Varistors are produced by a ceramic sintering process which gives rise to a structure consisting of conductive zinc oxide grains surrounded by electrically insulating barriers. These barriers are attributed to trap states at grain boundaries induced by additive elements such as bismuth, cobalt, praseodymium, manganese and so forth.
Fabrication of zinc oxide varistors has traditionally followed standard ceramic techniques. The zinc oxide and other constituents are mixed, by milling in a ball mill, and are then spray dried, for example. The mixed powder is dried and pressed to the desired shape, typically tablets or pellets. The resulting tablets or pellets are sintered at high temperature, typically 1,000 to 1,400° C. The sintered devices are then provided with electrodes, typically using a fired silver contact. The behavior of the device is not affected by the configuration of the electrodes or their basis composition. Leads are then attached by solder and the finished device may be encapsulated in a polymeric material to meet specified mounting and performance requirements.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of and apparatus for manufacturing a multilayer varistor.
A multilayer varistor suitably comprises a plurality of layers of ceramic material and a plurality of layers of electrode material. The layers are interleaved with each ceramic material layer sandwiched between two electrode material layers, at least a portion of at least one of the layers of electrode material extending to a first surface portion of the varistor, and at least a portion of at least one other of the layers of electrode material extending to a second surface portion of the varistor. A first body of conductive material is adhered at least to the first surface portion for electrical communication with the portion of the at least one electrode material layer. The portion of the at least one electrode material layer being spaced from all other surface portions of the varistor by ceramic material. A second body of conductive material is adhered to at least the second surface portion for electrical communication with the portion of the at least one other electrode material layer. The portion of said at least one other electrode material layer is spaced from all other surface portions of the varistor by ceramic material. The bodies of conductive material define terminals of the varistor. Each of the ceramic material layers sandwiched individually between two electrode material layers have a thickness dimension less than 30.0 microns.
In an alternative embodiment, a multilayer varistor may comprise a plurality of layers of ceramic material and a plurality of layers of electrode material. The layers are interleaved with each ceramic material layer sandwiched between two electrode material layers. At least a portion of at least one of the layers of electrode material extends to a first surface portion of the varistor, and at least a portion of at least one other of the layers of electrode material extends to a second surface portion of the varistor. A first body of conductive material is adhered at least to the first surface portion, for electrical communication with the portion of the at least one electrode material layer. The portion of the at least one electrode material layer is spaced from all other surface portions of the varistor by ceramic material. A second body of conductive material is adhered to at least the second surface portion for electrical communication with the portion of the at least one other electrode material layer. The portion of the at least one other electrode material layer is spaced from all other surface portions of the varistor by ceramic material. The bodies of conductive material define terminals of the varistor. Each ceramic layer is individually sandwiched between two electrode material layers, and each is formed by deposition of a powder suspension and subsequent heat treatment to provide a dense continuum of ceramic material of low porosity. Each ceramic layer may be formed by a multiple depositions of powder suspension aggregated by the heat treatment to provide a dense continuum of low porosity ceramic material.
Each layer of ceramic material separating two layer of electrode material is of substantially the same thickness as every other layer of ceramic material separating two layers of electrode material, and the thickness is substantially uniform over the entire area of the separating layer of ceramic material. Each layer of electrode material may be of substantially the same thickness as every other layer of electrode material, with the thickness being substantially uniform over the entire area of the layer of electrode material.
At least one of the layers of electrode material may be separated from an external surface portion of the varistor by a layer of ceramic material of greater thickness than the thickness of any of the layers of ceramic material separating two layers of electrode material. Alternatively or in addition, at least one of the layers of electrode material may be separated from an external surface portion of the varistor by a layer of ceramic material of a different composition from that of the separating layer of ceramic material.
At least one of the plurality of layers of electrode material may be defined by a single region of electrode material. Alternatively, at least one of the plurality of layers of electrode material may be defined by a plurality of individual regions of electrode material.
In a preferred embodiment and construction, a multilayer varistor of the invention is of generally rectangular block form configuration, and the layers of electrode material are substantially planar, and extend substantially parallel to those side faces of the varistor which are of maximum planar dimensions. The end faces of the rectangular block-form varistor define the first and second surface portions.
In an alternative embodiment, a multilayer varistor may be of generally cylindrical configuration, with the layers of electrode material being substantially planar and extending transverse to the axis of the generally cylindrically configured varistor. The first surface portion and the second surface portions are defined by curved surface portions of the varistor. One of the first and second surface portions may be an external, convexly-curved surface portion of the annular varistor, and the other of the first and second surface portions may be an internal, concavely-curved surface portion of a central aperture passing through the annular member.
In any configuration of multilayer varistor, of the various embodiments of the invention at least one layer of electrode material, and the at least one other layer of electrode material, together define said plurality of electrode layers. Thus, a multilayer varistor may also consist of three layers of ceramic material and two layers of electrode material, with one of the ceramic layers being sandwiched between the two electrode material layers. A first layer of the layers of electrode material extends to a first external surface portion of the varistor, and the other of the layers of electrode material extends to a second external surface of the varistor. A first body of conductive material is adhered at least to the first external surface portion for electrical
Cowman Stephen P.
Nicker Derek A.
Oliver Anthony L.
Ratcliffe Alan J.
Shreeve John M.
Chapman and Cutler
Fiorilla Christopher A.
LittleFuse Inc.
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