CA1096051A - Reconstituted metal oxide varistor - Google Patents

Reconstituted metal oxide varistor

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Publication number
CA1096051A
CA1096051A CA297,606A CA297606A CA1096051A CA 1096051 A CA1096051 A CA 1096051A CA 297606 A CA297606 A CA 297606A CA 1096051 A CA1096051 A CA 1096051A
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CA
Canada
Prior art keywords
varistor
metal oxide
metal
composite
reconstituted
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
CA297,606A
Other languages
French (fr)
Inventor
Roland T. Girard
James F. Burgess
Francois D. Martzloff
Constantine A. Neugebauer
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
General Electric Co
Original Assignee
General Electric Co
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by General Electric Co filed Critical General Electric Co
Priority to CA297,606A priority Critical patent/CA1096051A/en
Application granted granted Critical
Publication of CA1096051A publication Critical patent/CA1096051A/en
Expired legal-status Critical Current

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Abstract

ABSTRACT OF THE DISCLOSURE
Reconstituted metal oxide varistors are formed by hot pressing powdered metal oxide varistor ceramic with plastic resin. Metal electrodes can be pressed directly into the ceramic-plastic composite to provide improved contact characteristics.

Description

S~ :

This invention relates to metal oxide varistors. More specifically, this invention relates to varis~ors which com-prise a composite of finely ground metal oxide varistor ceramic in a plastic resin matrix.
There are a few known materials which exhibit non-linear resistance characteristics and which require resort to the following equation to relate current and voltage quantatively.

,, I = (V/C) where V is a voltage between two points separated by a body of the material under consideration, I is the current flowing between the two points, C is a constant, and dis an exponent greater than 1. Materials such as silicon carbide exhibit non-linear or exponential resistance characteristics and have been utilized in commercial silicon carbide varistors, however, ` such non-metallic varistors generally exhibit an ~ exponent of not more than six. ;
Recently, a family of polycrystalline metal oxide 20 varistor materials have been produced which exhibit an exponent in excess of ten. These new varistor materials comprise a sintered body of zinc oxide crystal grains, ` including additionally an intergranular phase of other metal ;` oxides and/or halides, for example: beryllium oxide, bismuth ~`
oxide, bismuth fluoride, or cobalt fluoride, and are described, ~ ~

~ RD-7115 for example, in U.S. Pat 3,682,841 dated Aug/8/1972 to Matsuoka et al and U.S. Pat.3,687,871 dated August 29,1972 to Masuyama et al .
The non-linear resistance relationship of metal oxide varistors is such that the resistance is very high (up to at least 10,000 megohms) at current levels in the micro~
ampere range, and progresses in a non-linear manner to an extremely low value (tenths of an ohm) at high current levels.
The non-linear resistance characteristics result in a voltage versus current characteristic wherein the voltage is effectively limited, the voltage limiting or clamping action ; being more enhanced at the higher values of thec~exponent.
Thus, the voltage versus current characteristics of metal oxide varistor material is similar to that of the Zener diode with the added characteristic of being symmetrically bidirectional. The "breakdown voltage" of a metal oxide varistor device is determined by the particular composition of the material and by the distance between the electrodes on the varistor body.
` 20 Metal oxide varistors of the prior art are fabricated ' by pressing and sintering a mixture of metal oxide powder at temperatures in the region of 1300C to form a generally hard, brittle ceramic body. Circuit components of metal `~ oxide varistor ceramics are generally formed by pressing and :
sintering disks of the material, applying the electrodes, for example, by painting or screening conductive ma-terials ` on the surface of the disks, affixing wire leads, and encapsulating the finished component in a suitable dielectric.
It has been suggested that metal oxide varistor ceramics be pressed or machined into complex shapes and bonded to . .

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metal terminals and contacts to form specialized circuit components, as for example in U.S. Patents 3,742,420 issued June 26, 1973 to Harnden and 3,693,053 issued September 19, 1972 to Anderson. The manufacture of metal oxide varistors in shapes other than flat disks requires dimensional control, however, which is difficult to attain in a sintering process (due to shrinkage and deformation) and the temperatures encountered in the sintering processes are generally incompatible with common, low cost electrical metals. Machining of sintered parts generally involves grinding brittle materials and is not an economically attractive process for large scale mass production.
Metal oxide varistor components have been formed in the prior art by screening a paste of ground metal oxide varistor ceramic and glass frit on a dielectric substrate and firing to produce a thick film device; as described for example in U.S. Patent No. 3,725,836 issued April 3, 1973 t:o Wada et al.
Varistors are formed by hot pressing a mixture of ground metal oxide varistor ceramic material and plastic resin powder to form a solid composite body. Temperatures utilized in the hot pressing process are much less than those utilized for sintering the ceramic and are generally compatible with low cost metals and contact materials. Complex shapes may be formed with good dimensional stability.
Electrical contacts are most suitably formed on ;
these ho~ pressed reconstituted varistors by pressing flat aluminum or copper disks or other shapes into the ceramic-plastic material. Insulating films of plastic with high contact resistance, which characterize painted electrical contacts on such devices, are thereby eliminated ~ RD-7115 It is, therefore, an ob~ect of this invention to provide low cost methods for producing complex shapes from metal~oxide varistor materials.
Another object of this invent~on i8 to provide metal oxide varistors which incorporiate int~gral metal component~;

Br~ef Description of the Drawin s The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects a~d advantages thereof, may be understood by reference to the following detailed description taken in connection with the appended drawings in which:
FIG. 1 is a reconstituted metal oxide varistor of the present invention;
FIG. ~ ls a plot of the breahdown voltage as a function of ~he plastic content in reconstit:uted metal oxide varistor bodies;
FIG. 3 is a plot of the breakdown voltage versus the ram pressure utilized to form reconstituted metal oxide varistors;
FIG. 4 is a plot of the a exponent as a function of the ramp pressure used to fonm reconstituted metal oxide varistors;
FIG. 5 is a tracing of a microphotograph of a pressed metal contact on a reconstituted metal oxide viaristor; and FIG. 6 is a plot of voltage gradient versus current density for reconstituted metal oxide varistors whlch include a variety of metal contact types.

`

~ RD-7115 Description of the Preferred Embod ents FIG. 1 is a reconstituted metal oxide varistor of the present invention. A mi~ture of metal oxide varistor powder and a thermoplastic resin powder is hot pressed, in a method more particular described ln the following examples, to form a solid plug of a reconstituted met~l oxid~ varistor-plastic matrix 9. The plug 9 is, for ease of description, illustrated as a simple square or cylindrical form but it may, of course9 assume any complex shape whlch is suitable for hot pressing by any of the methods which ~
are well known to the plastic fabricating arts. ~ ;
At least two electrical contacts 10 and 11 are applied to the surface o plu~ 9, typically on opposing faces and most suitably by hot pressing copper or al~minum disks into the surface of the plug. Alterna~ely, screenlng, printing, metal evaporation, or any other o~ the contact-forming techniques which are well known to the varistor arts may be utilized. Wire leads 12 and 13 may, if desired, be attached to the contacts 10 and 11 to provide interconnection with other clrcuit elements.
` Alternately, two or more metal electrodes may be '~ imbedded directly in the body o the plug to form any of the two terminals of the multi-terminal varistor " configurations which are known in ~he art.

Example of a Method for Forming a Reconstituted Pellets of metal oxide varistor materials were formed by sintering a mixture of approximately 97 mol percent zinc oxideg 1/2 mol percent bismuth oxide and antimony oxide, .

:' ' :
, , tin oxide, cobalt oxide, manganese oxide, barium carbona~e, and boric acid at approximately 1350C in the well-known manner of the prior art. The pellets were crushed in a steel die and separated lnto ~he following particle ranges Particle Siæe Average Particle 5creen Mesh _(micron) Size (micron~
.
-10 +20 2000-841 1420 -20 +35 841-500 6~0 -35 +100 500-149 325 10-200 ~325 74-44 59 The metal oxide varistor particles were mixed with LexanR polycarbonate powder, manufactured by the General Electric Company, Schenectady, New York, and placed in a steel die. The die cavity was a cylinder with an area of approximately 1 cm2. The die plunger had a 0.5 mill~meter 1at on one side to act as a riser for excess plastic durlng pressing. The die set was placed on a hot press, ~-without pressure, and givena 10 minute preheat to 220C.
Pressure was then applied to the sample for 5 minutes.
The hot die set was then removed from the press and cooled.
A~ter removal from the die~ the plastic disks were approximately 1 millimeter thick. The faces of the pressed dlsks were then coated with silver paint contacts and air dried.
Ideally, adjacent varlstor ceramic particles in the composite would be in intimate contact and the amount of plastic binder should be no more than that required to fill the empty spaces between the metal oxide varlstor particles~ The - : -Q~

proportion o~ plastic can be determined experimentally by gradually lncreasing the proportion of plastic and me~sur-ing ~he thickness of a var~stor plug produced under constant die pressure. The volume increases only slowly at first9 then, it increases more rapid~y with plastic content. For metal oxide varistor particles of approximately 500 microns, this occurs at approximately 50 percent of volume.
The clamping voltage of a reconstituted metal oxide varistor produced from metal oxide varistor powder in the 500 micron to 841 micron range is illustrated in FIG. 2 as a function of the volume percentage of polycarbonate resin.
As the resin content increases, the breakdown voltage i~creases in a substantially linear fashion. This is expected because of the formation of increased plastic barriers between the ceramic particles, adding addition~l IR drop.
The effect of molding pressure on reconstituted metal oxide varistor-plastic resin plugs is illustrated in FIGS. 3 and 4. FIGo 3 illustrates the relation between the breakdown voltage and molding pressure. Moldlng pressure has little effect on the breakdo~l voltage indicating that lt does not affect the plastic barriers between the particlesO In the measurements of FIGS. 3 and 4, the pressure was applied only during the molding process, when the plastic was llquid, `~ 25 and not during the hardening or electrical measurements.
One must distinguish between ~wo particle sizes in \powders produced from ground metal oxide varistor ceramics.
; ~irst, there is a zinc oxide grain size in a ceramic, generally of the order of about 10 microns~

.

- -6~ ~ ~

Secondly, there is a particle size of the metal oxide var~'stor powder itself, which may be larger or smaller than the grain slze. If the particle size is smaller than thc zinc oxide grain size, it would be expected to af?t substantially as a pure zinc oxide p~xticle without an intergranular barrier layer, Thus, no varistor action ~s to be expected. In a reconstituted metal oxide ~aristor utilizing this slze particle, most of the voltage drop is taken up ~n the binder material be~een the particles.
If the particle size is greater than the grain size, the intPrgranular barrier layer can be expected to remain intact. Thus, in a reconstituted composite,as the particle sizes increase, more of the voltage drop is across the tunneling barriers and less across the binder between the particles.
If the particle size is greater than the grain size, one must consider the particle size in relation to the distance between the electrodes in the reconstituted composite. Thus, if the particle size is smaller than the electrode spacing some of the voltage drop will be taken up in the binder between the particles. On the other hand, if ~he particle size is equal or larger than the electrode spacing there will be only intraparticle voltage drops. The upper curves of FIG. 3 illustrate reconstituted varistors wherein the particles of metal oxide varistor ceramic are larger than the zinc oxide grain siz~
smaller than the interelectrode spacing. The bottom curve of FIG. 3 illustrates a reconstituted varistor whereln the ceramic particles are of the same order of size as the interelectrode spacing.

.' ' '~

~ ' ' ' ' '. . . , ::': :' , ' ~ ~ 6~ 5 ~ RD-7115 FIG. 4 illustrates the dependence of the a exponent of reconstituted varistors as a function o~ molding pressure.
The ~ exponent for a smaller particle size i8 lower because of the presence of more plastic barriers between the particles. The effect of molding pressure on alpha exponents is not, however, understood.

Method for Forming Electrodes on Reconstituted Metal Oxide Varistor-Plastic Devlces When using pressed plastic plugs of reconstituted metal oxide varistor material, one problem is-to form good electrical contact with the faces of the device. It has been found that it is possible to press metal disks directly into both surfaces of a reconstituted metal oxide varistor-plastic plug to form contacts.

Example of a Method for ForminR-contacts A 0.01 millimeter metal disk is placed on the bottom of the die and the plastic-metal oxide varistor powder mi~ture is added. Mold~release compound is ~prayed on the plunger and acts as a temporary adhesive for a second 0.01 millimeter metal disk, which is placed on the plunger. The sample is then hot pressed i~ the manner described above. The resulting plug shows good electrode adhesion and electrical contact. FIG. 5 is a tracing of a microphotograph of an aluminum disk electrode on a reconstituted metal oxlde varistor. It may be seen that the ceramlc particles in the composite actually penetrate the aluminum electrode at the metal-plastic interface and thus eliminate any thin Plastic fi~m which might otherwise form on the plug surface.
_g_ : -' ' '" .. ' `' .,, . "

~ ~ 6~ ~ ~ RD-7115 :' FIG. 6 illustrates the electrical characteristics of recon~tltuted metal oxide varistor-polycarbonate devices produced from ceramic particle~ in the 841 micron-2000 micron rang~ with conventional painted over silver paste contacts, pressed aluminum contacts, and pressed copper contacts.
In addltion to the low co~t and ease o processing, the pressed metal contacts have a number of additional advant~ges, By plac~ng thick metal electrodes on the d~vice, heat sinking is improved at operating power levels. Thus, a device with pressed metal electrodes can be soldered directly into circuits. It is also possible to produce thinner devices with lower clamping voltage because the metal disk contacts are less sensitive t:o shorting than painted on lS paste electrodes.
Reconstituted metal oxide varistors may, alternately, be ~ormed in accordance wlth the present invention by pressing varistor powders in a matrix of thermosetting plastic resin, for example, epoxy resin. It is, in all cases,however, necessary to press the powder-plastic mixture during the forming process, to assure intimate contact between at least a fraction of ad~acent varistor particles.
Reconstituted metal oxide varistors of the present invention may be formed in more complex shapes and at lower cost than conventional sintered ceramic disk varistors.
The process temperatures are compatible with conventional electrical metals and allow the production of complex devices, incorporating metal components, in large quantity. Pressed electrodes of the present inventlon provide better electrical contact and improved heat sinking over the painted electrodes of the prior art.
" -10-.' ~

` ` .1L~GC3~i3L

While the invention has b~en described herein in : accordance with certain preferred embodiments thereof, many modlficat~ons and changes will be apparent ~o those skilled in the art. Accordingly, it is intended by the appended claims to cover all such modiflcations and changes as fall within the true spirit and scope of the invention.

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Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. A reconstituted metal oxide varistor com-prising a composite of metal oxide varistor ceramic particles in a plastic resin matrix.
2. The varistor of claim 1, wherein adjacent ceramic particles in said composite are in intimate physical contact.
3. The varistor of claim 1, wherein said composite comprises a quantity of thermoplastic resin which is at least sufficient to fill voids between said ceramic particles.
4. The varistor of claim 1, wherein the metal oxide varistor ceramic comprises a sintered mixture of zinc oxide, bismuth oxide, and other metal oxides.
5. The varistor of claim 1, wherein the resin comprises polycarbonate plastic.
6. The varistor of claim 1, further com-prising at least one metal electrode in contact with said composite.
7. The varistor of claim 6, wherein the electrodes comprise metal sheets in contact with at least one surface of said composite.
8. The varistor of claim 7, wherein said metal sheets comprise copper.
9. The varistor of claim 7, wherein said metal sheets comprise aluminum.
10. The varistor of claim 6, wherein adjacent ceramic particles penetrate the metal electrodes.
11. The varistor of claim 6, wherein the electrodes comprise conductive metal paste applied to at least one surface of said composite.
12. The varistor of claim 6, comprising two electrodes on opposite faces of said composite.
CA297,606A 1978-02-23 1978-02-23 Reconstituted metal oxide varistor Expired CA1096051A (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
CA297,606A CA1096051A (en) 1978-02-23 1978-02-23 Reconstituted metal oxide varistor

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
CA297,606A CA1096051A (en) 1978-02-23 1978-02-23 Reconstituted metal oxide varistor

Publications (1)

Publication Number Publication Date
CA1096051A true CA1096051A (en) 1981-02-17

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2858073A3 (en) * 2013-10-03 2015-06-03 Kabushiki Kaisha Toshiba Composite resin and electronic device

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2858073A3 (en) * 2013-10-03 2015-06-03 Kabushiki Kaisha Toshiba Composite resin and electronic device
US9419192B2 (en) 2013-10-03 2016-08-16 Kabushiki Kaisha Toshiba Composite resin and electronic device

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