5,134,540
`[11] Patent Number:
`United States Patent
`Rutt
`[45] Date of Patent:
`Jul. 28, 1992
`
`
`[19]
`
`US005134540A
`
`[54] VARISTOR OR CAPACITOR AND METHOD
`OF MAKING SAME
`
`3,794,707 2/1974 OPNeill et ale ccescccsesceeeeeee 264/56
`3,798,762
`3/1974 Harris et al.
`». 264/61 X
`4,153,491
`5/1979 Swiss et al.
`4,289,719
`9/1981 McIntosh et al. oss 264/61
`4,598,107
`7/1986 Herronetal. ......
`. 264/63 X
`4,761,711
`8/1988 Hiremath voccrccrccsccccncn 361/321
`
`scccscccssseesseenseee 264/63
`
`Truman Rutt, Myrtle Beach, S.C.
`Inventor:
`[75]
`:
`:
`.
`[73] Assignee: AVX Corporation, New York, N.Y.
`[21] Appl. No.: 758,623
`Primary Examiner—Donald A. Griffin
`Attorney, Agent, or Firm—Mark T. Basseches
`[22] Filed:
`Sep. 12, 1991
`[57]
`ABSTRACT
`Related U.S. Application Data
`A method of manufacturing an improved varistor or
`[62]
`Division of Ser. No. 191,123, May 6, 1988.
`capacitor, and the resultant improved varistor or capac-
`itor is described. In accordance with the method the
`[S21]
`Int. CLS oes H01G 4/10; H0O1G 7/00;
`ceramic layers of the varistor are formed by providing
`[2] US.C
`361/321,2/084,
`0S.
`CV. vecccccsesernescscsretseeneees
`;
`42;
`at least two strata separated by a boundary layer which
`264/61
`resists grain growth thereacross. The ceramic bodyis
`.
`.
`.
`[58] Field of Search............ 361/320, xeaveL+& sintered under temperature conditions sufficiently low
`that grain growth within the strata defining the ceramic
`[56]
`References Cited
`layers is restricted to the strata such that grain growth
`U.S. PATENT DOCUMENTS
`across the boundary material
`is minimized. By this
`161/162
`2778-762.
`1/1957 Fist
`method the ceramic layers have a predictable numberof
`3419-759 1Mtoe Hayakawaeeees 3aon
`grain boundaries between adjacent electrodes.
`
`ccceseeesssessenenee 361/321
`3,426,250 2/1969 Kahin ou... cs
`3,496,008
`2/1970 Haskinset al. woe 117/215
`4 Claims, 2 Drawing Sheets
`
`16
`
`000001
`
`Exhibit 1006
`Exhibit 1006
`PGR2017-00010
`PGR2017-00010
`AVX CORPORATION
`AVX CORPORATION
`
`.
`
`000001
`
`

`

`U.S. Patent
`
`July 28, 1992
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`Sheet 1 of 2
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`5,134,540
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`XXONANAXKKKKNATL KS
`16
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`PYOVOR 927
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`U.S. Patent
`
`July 28, 1992
`
`Sheet 2 of 2
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`5,134,540
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`1
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`5,134,540
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`VARISTOR OR CAPACITOR AND METHOD OF
`MAKING SAME
`
`This application is a division of application Ser. No.
`07/191,123 filed May 6, 1988.
`
`BACKGROUNDOF THE INVENTION
`
`1. Field of the Invention
`The present invention is directed to a varistor or
`capacitor, and more particularly to a monolithic ce-
`ramic device of the type described.
`2. General Discussion
`A ceramic varistor comprises essentially an inter-
`granular barrier layer capacitor including a monolithic
`ceramic body: having a multiplicity of electrode layers
`separated by ceramic layers. The odd numbered elec-
`trode layers, i.e. the first, third, fifth, etc., are electri-
`cally connected as are the even numbered electrode
`layers. The varistor is typically employed in shunting
`relation of an electronic circuit to be protected. It is the
`function of a varistor to provide a high resistance (and
`a degree of capacitance) when voltages impressed on
`the electronic circuit are maintained below a predeter-
`mined threshold voltage, and to provide a low resis-
`tance shunt when voltages exceed the threshold.
`Heretofore,it has been exceedingly difficult to manu-
`facture varistors having a predictable breakdown volt-
`age, and particularly varistors which are rendered con-
`ductive at low voltages, i.e. 15 volts or less. The practi-
`cal solution heretofore adopted by the industry has been
`to manufacture the varistor in a conventional manner,
`i.e. in the same manneras capacitors are conventionally
`manufactured utilizing known formulations
`com-
`pounded to function as varistors. Thereafter, the varis-
`tors produced are individually tested as to break down
`voltage and classified. It will be readily recognized that
`the individual testing or batch testing of varistors con-
`stitutes a complicating and costly step in the manufac-
`ture of varistors. A further.desirable characteristic of a
`varistor is that when the same becomes conductive as a
`result of exposure to voltages beyond a threshold volt-
`age, that the current carrying capacity ofthe varistor be
`as great as is possible. This characteristic is best realized
`where substantially the entirety of the ceramic compo-
`nents become simultaneously conductive thus provid-
`ing the greatest current path between the variouselec-
`trodes of opposite polarity. In conventional varistors,
`and even those varistors which have been classified to
`break down at a particular voltage, the break down
`does not occur uniformly, especially where the im-
`pressed voltage only slightly exceeds the threshold
`voltage. As a result, the ability of such varistors to
`function as an effective shunt is greatly reduced since
`conduction between opposed electrodes is focused at
`limited areas with remaining areas of ceramic continu-
`ing to be highly resistive.
`It has been experimentally determined that the break-
`down voltage of a varistor-ceramic formulation is a
`function of the number of grain boundaries of the ce-
`ramic grains intervening between adjacent electrode
`layers. The greater the number of boundaries between
`adjacent
`layers,
`the higher the break down voltage
`necessary to provide a conductive path. Conversely, in
`the event of a grain size such that grains of ceramic
`directly span the distance between adjacent electrodes,
`the device will exhibit break down or pass current at
`extremely low voltages. From the foregoing experimen-
`
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`tal findings, it will be evident that a highly undesirable
`condition results where the numberof grain boundaries
`between adjacent electrodes varies greatly across the
`expanse of the ceramic layers. In such case, the break
`down voltage will be a function of and will occur atthat
`area or those areas where there are concentrations of a
`limited number of grain boundaries. Where the break
`downis concentrated in a limited numberofareas, it
`will be readily recognized that the current carrying
`capacity is substantially lower than would be the caseif
`the break down occurred more or less uniformly
`throughoutthe entire area of the ceramic.
`Efforts have been made to provide a ceramic having
`uniform grain boundary concentrations across the
`thickness of the ceramic. These efforts have heretofore
`been relatively unsuccessful on a commercial scale.
`Such efforts have included close control of the ceramic
`particle size embodied in the “green” ceramic layers;
`processing the ceramic undercarefully controlled heat-
`ing conditions during the sintering procedure; modify-
`ing sintering times, etc. As noted, none of the above
`methods have provensatisfactory.
`A particularly acute problem arises whenit is desired
`to provide a varistor having a relatively low break
`downvoltage, i.e. in the order of 15 volts or less. The
`manufacture of such varistors to provide for break
`downat low threshold voltages and yet provide high
`current carrying capacity when the threshold voltageis
`exceeded has heretofore been very difficult.
`SUMMARYOF THE INVENTION
`
`The present invention may be summarized as directed
`to an improved method for manufacturing varistors and
`capacitors having predictable and readily repeatable
`break down or operating voltage characteristics. While
`it is considered that the principle utility of the instant
`inventionlies in the production ofvaristors, it would be
`understood from the ensuing description that capacitors
`of the intergranular barrier layer type having improved
`characteristics will likewise benefit from the disclosed
`technology.
`The invention is predicated in large measure on the
`discovery that ceramic grain growth is inhibited by the
`higher binder concentrations present at the upper sur-
`face of a green ceramic tape or stratum, such that when
`the tape or stratum is processed within controlled heat-
`ing parameters, ceramic grains will not grow across the
`high-binder concentration boundary. By thus construct-
`ing a green ceramic layer which is comprised of two or
`more strata each of which strata incorporates a high-
`binder concentration at its upper extremity, and by
`processing such multi-stratum layer in such mannerthat
`the grain size does not extend across the boundary be-
`tween adjacent strata,
`it is now possible to create a
`ceramic layer within the varistor wherein the numberof
`grain boundaries within the layer is a function of the
`numberofstrata, and is thus controllable in accordance
`with the number of strata of which the layer is com-
`prised.
`The layer which is comprised of a numberofstrata
`maybe fabricated by any of a numberofdifferent tech-
`niques. Specifically in addition to the tape stacking
`process briefly described above, a binder and ceramic
`formulation may be deposited as by screening or doc-
`toring on a surface and the thus formed stratum may be
`overcoated with an organic ink as by swabbing. After
`drying of the organic ink layer, a further thickness of
`resin bonded ceramic may be overcoated atop the or-
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`5,134,540
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`3
`ganic layer, dried, and the process repeated. In this
`instance, the thin layers of organic ink define the grain
`growth barriers which provide a predictable grain
`structure. Numerous other techniques for forming the
`layers comprising the various strata are known. These
`techniques includesilk-screening, dipping, doctor-blad-
`ing, spraying,etc.
`It is accordingly an object of the invention to provide
`a varistor or capacitor comprised of one or morelayers
`of ceramic, such layers being characterized by being
`formed ofessentially discreet strata, the said strata hav-
`ing a predictable grain growth characteristic whereby
`the grain or grains of a given stratum cannot project
`across the boundary between the next adjacent stratum.
`In this manner, there may be formed a ceramic layer
`wherein the number of grains taken in a depth-wise
`direction may be accurately controlled. A further ob-
`ject of the invention is the provision of a method of
`forming a varistor or capacitor of the type described
`and the resultant product.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Reference is now made to the accompanying draw-
`ings wherein.
`FIG.1. is a schematic sectional view through a ca-
`pacitor or varistor in accordance with the invention.
`FIG. 1A is a magnified section of the circled compo-
`nent portion of FIG. 1.
`FIG.1B is a view similar to FIG. 1A depicting the
`grain size distribution which is typical of prior art ca-
`pacitors and or varistors.
`FIG. 2 is reproduction through a ceramic section
`taken by a scanning electron microscope (SEM).
`FIG, 3 is a further reproduction of an SEM photo-
`graph through a ceramic section magnified 2400 times.
`DETAILED DESCRIPTION OF DRAWINGS
`
`Referring now to FIG. 1, there is disclosed a sche-
`matic representation of a varistor 10 formed of a mono-
`lithic block of ceramic having internal electrodes sepa-
`rated by ceramic material. More particularly, the ce-
`ramic block or monolith includes upper and lowerseal-
`ing or encapsulating layers 11, 12 respectively integrally
`formed with a series of ceramic layers, (three being
`shownin the illustrated embodiment) the ceramic layers
`being numbered 13, 14, and 15 respectively.
`The varistor includes a pair of electrodes 16, 16 of a
`first polarity which are joined by a termination area 17.
`Electrodes 18, 18 of opposite polarity are joined by
`termination 19. The electrodes 16 are separated from
`electrodes 18 by the intervening dielectric layers 13, 14
`and 15.
`Asthusfar described, the construction of the varistor
`or capacitor is entirely conventional,
`the invention
`hereofrelating to the nature of the dielectric layers 13,
`14, and 15.
`Asis schematically shown in FIG. 1B representing a
`typical prior art dielectric ceramic layer, the layer is
`comprised of a multiplicity of ceramic grains. The
`grains intervening between the electrode layers 16, 18
`are randomly distributed in size in such manner that
`larger grains such as grain 20 may span theentire dis-
`tance between the electrodes 16, 18 there thus being no
`inter-granular boundaries between the electrodes in the
`area occupied by grain 20. In a second area a pair of
`grains 21 may span the electrodes thus defining a single
`inter-granular boundary 21A in the area between elec-
`trodes 16 and 18 in registry with grains 21. In similar
`
`4
`three
`fashion, grains 22 are dimensioned such that
`grains span the distance between the electrodes result-
`ing in a structure in which there are two grain bound-
`aries intervening between the electrodes.
`A dielectric ceramic matrix in accordance with that
`illustrated in FIG. 1B is highly disadvantageoussince as
`previously noted, the break downresistanceofthe layer
`will vary as a function of the numberofintervening
`grain boundaries between the electrodes. Thus in the
`example of FIG. 1B a break downwill occurfirst in the
`area of grain 20 before a break down would occur be-
`tween grains 21, and in turn the area between grains 21
`will become conductive before the area between grains
`22, etc. It will thus be readily recognized that a varistor
`wherein the ceramic dielectric layer is of the consis-
`tency of the prior art type shown in FIG. 1B is undesir-
`able in that the threshold voltage for break down will
`first occur in the areas of no grain boundaries or limited
`grain boundaries. Accordingly, the desired conductive
`nature of the varistor will be limited to those electrical
`current paths which register with grains such as grains
`20, and perhaps 21 and accordingly, even when the
`varistor becomes conductive, the conductive carrying
`paths will be limited to the areas of fewest intervening
`grain boundaries.
`In FIGS. 1 and 1A there is disclosed an idealized
`grain structure for the dielectric layers wherein the
`number of grain boundaries intervening between elec-
`trodes 16 and 18 are essentially equal throughout the
`entire area of the dielectric material. For purposes of
`simplicity ofillustration, the dielectric components of
`FIGS.1 and 1A aredisclosed as providing a single grain
`boundary 23 betweenthe strata defined by grains 24 and
`grains 28. As will be morefully explained hereinafter, as
`a practical matter a varistor will be formed with a pre-
`dictable numberof strata and hence a predictable num-
`ber of grain boundaries,
`the number of boundaries
`sometimes being substantially greater than the single
`grain boundarystructure illustrated.
`By wayofillustration, FIG. 2 comprises a photo
`micrograph through a section of ceramic formed in
`accordancewith the invention.In the illustrated photo-
`graph, the layer 30 is comprised offive distinct strata
`30A, 30B, 30C, 30D, and 30E wherebythe layer 30
`exhibits four grain boundary areas 31A, 31B, 31C, and
`31D. In the photo micrograph of FIG.2 there are illus-
`trated void areas 32, 33 intervening between layer 30
`and adjacent layers 34 and 35. These intervening areas
`can be subsequently be filled with electroding material,
`typically molten lead, in a known mannerin accordance
`with one or more of the following U.S. Pat. Nos.
`3,965,522; 3,679,950. Alternatively,
`the areas 32, 33
`between layers 34 and 30 and 35 and 30 may,prior to
`conversion of the green ceramic to a sintered ceramic
`monolith be provided, as by screening orthe like, with
`an electrode forming ink in the mannerset forth, by way
`of example, in U.S. Pat. Nos. Re 26,421 of Jul. 2,-1968
`and 4,347,650 of Sep. 7, 1982.
`It is to be understood that the manner of forming a
`varistor or capacitor from the once formed green ce-
`ramic layers 30 is conventional and well known to the
`art, the present invention being directed to the concept
`of and manner of formation of layers 30 which will,
`following sintering, provide a predictable and regular
`numberof grain boundaries between intervening elec-
`trode layers.
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`5
`
`DETAILED DESCRIPTION OF
`MANUFACTURING METHODS
`
`5,134,540
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`6
`compressed strata with the psuedo ink formulation
`emerging at opposite sides of the individually cut struc-
`tures in the manner described.
`The cut preform structure was heated for a period of
`3 hrs at 370° C. to decompose and remove binder and
`dispersants from the ceramic formulation and from the
`pseudoink. A slow heating rate (12 hrs to reach temper-
`ature) was used to slowly decompose the binder with-
`out disruption of the structure. The resultant structures
`were then fired at 1400° C. for one hourto sinter the
`ceramic.
`FIG. 2 of the instant application shows the grain
`structure of the resultant ceramic monolith. Asis appar-
`ent from the enlargement, each layer 30 of the monolith
`was comprised of five strata each stratum being com-
`posed essentially of a single ceramic grain extending
`throughoutthe entire depth of the stratum,the grains of
`each stratum terminating at a boundary defined by the
`next adjacent stratum. Grain size is shown to be substan-
`tially constant,
`the structure of each layer including
`four grain boundaries between the void areas 32,33.
`This structure and varistors manufactured in accor-
`dance with the invention were heated between 1000° to
`1100° centigrade in the presence of Bi203 in order to
`effect deposits of such substance in the grain bound-
`aries, The purpose of such procedure is to facilitate
`observance of the grain structure by an SEM,thebis-
`muth also functioning to enhance the varistor proper-
`ties. The heating was done before end termination.
`EXAMPLEII
`
`A varistor ceramic slurry formulation was formed
`using 65 parts by weight of ceramic powder comprised
`essentially of zinc oxide containing one or more small
`additions chosen from thelist of Bi, Co, Mn, Ti, Cr, Si,
`Al, K. This mixture was milled in water with 35% by
`weight waterutilizing the dispersant and percentage of
`dispersant as set forth in Example 1. Milling of the mix-
`ture was effected for a period of 18 hours in the manner
`set forth in Example 1. Relative to the weight of ce-
`ramic, 18 parts by weight of Rohm and Haas HA-12 and
`2 parts by weight of Rohm and Haas HA-16 binders
`were added, the resultant slurry having a viscosity of
`about 20-30 cps. Films were formed of the resultant
`slurry.as previously described, the dried films having a
`thickness of approximately 0.72 mils. The films were
`employed to define strata in the manner previously
`described. In this instance, the varistors were formed by
`screening between adjacent stacked layers areas ofplat-
`inum electrode paste in accordance with a. procedure
`set forth by way of example in U.S. Pat. No. Re. 26,421.
`The ceramic layers defining the varistors were formed
`of three strata and four strata respectively. The stacked
`green varistors were then heated to remove binder and
`dispersants as in Example 1. They werethereafterfired
`at 1180 degrees centrigrade for four hours and treated
`as described in Example 1. In the specific constructions
`cited, the finished varistor body made with three strate
`had the following dimensions: 0.118 in. distance be-
`tween terminated ends; 0.059 in. width; 0.033 in. thick-
`ness. The Thickness of each of the layers 30 of the
`. preform was approximately 0.0019 in., the specific pre-
`form including 5 operative layers (15 total strata), and 3
`electrodes of each polarity. The finished varistor body
`made with four strata per layer had essentially the same
`dimensions with the thickness of each ofthe layers 30 of
`the preform being approximately 0.0021 in. thick. It also
`had 3 electrodes of each polarity.
`
`0a
`
`Central to the method oftheinstant invention is the
`concept of forming each of the respective green dielec-
`tric layers of the varistor of a series of strata each stra-
`tum of which is provided at its major surfaces with a
`boundary forming material which functions to inhibit
`grain growth of the ceramic across the boundary during
`the sintering. A further aspect of the method is carrying
`out a sintering procedure at a temperature sufficiently
`low, and for time period sufficiently short that the ce-
`ramic grains do not grow across the respective bound-
`aries between the strata. It is, for example, possible’
`—_ 5
`utilizing.a green ceramic structure in accordance with
`the invention to produce a monolithic ceramic structure _
`having the random grain growth diagrammatically il-
`lustrated in FIG. 1B by sintering under temperatures
`sufficiently high that the grains will eventually break
`down the resistance of the barriers and become ran-
`domly sized. Thus, to achieve the desired results of the
`inventionit is necessary, in addition to providing barrier
`effects at the major surfaces of the strata to process the
`green ceramic preform in a manner which will preserve
`the barrier effect. Since skilled workers in the art of
`ceramic capacitor fabrication are well aware of the
`interplay of time, temperature, and formulation factors
`in relation to grain size, the skilled worker familiarized
`with the instant disclosure will be readily able to derive
`fabricating parameters which will assure the effective
`functioning of the barriers.
`EXAMPLE1: VARISTOR STRUCTURE
`FABRICATION
`
`20
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`25
`
`A water slurry was made by milling 65% by weight
`zinc oxide with 35% by weight water. A small addition
`of dispersant (approximately 0.5% to 1.0% by weight of
`Rohm and Haas T-901) was added to reduce viscosity.
`The slurry was milled for approximately five hoursin a
`vibro-energy mill made by SWECO CORPORATION.
`The milling operation was effected using } inch zirconia
`milling media. Milling was continued for approximately
`five hours following which 20% by weight of a com-
`mercially available acrylic latex binder was added
`(Rohm and Haas HA-12) to form the mixture into a thin
`paint. The paint was applied to a stainless steel belt
`coated with a thin lethechin coating to facilitate release
`of the film after drying, the film being formedto a thick-
`ness of approximately one mil after drying. During the
`drying procedure a degree of segregation of the latex
`binder was observed, the binder being concentrated at
`the top drying surface of the film. The degree of con-
`centration of the binder was found to be a function of
`drying time,
`longer drying times favoring increased
`surface concentrations of binder.
`The dried film was cut into rectangular pieces and
`stacked in groups of five. Increments of groupsoffive
`were stacked to form a structure, the physical formation
`of the green ceramic preform being effected in accor-
`dance with the procedures outlined in the previously
`cited U.S. Pat. Nos. 3,965,552 and 3,679,950. The pre-
`form was then consolidated by bonding of the stacks
`under heat and pressure. Bonding was effected by sub-
`jecting the preform under pressure of between 1500 to
`2500 psi and temperature of 113 degrees centigrade.
`The individual unit structures were then cut from the
`stack. In accordance with such patents a so called
`pseudoink is applied between intervening layers of the
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`7
`A numberofvaristors fabricated in the manner de-
`fined were tested as to capacitance and percent dissipa-
`tion factor, using a 1 volt RMS, 1 Khztest signal. The
`units fabricated with three strata had an average capaci-
`tance of 3.94 nf, and 8.65% DF. The units fabricated
`with four strata had an average capacitance of 3.49 nf,
`and 7.30% DF.
`Breakdown voltage will be defined as the voltage
`impressed uponthe units for a current of 1 ma. The units
`utilizing three strata layers had an average breakdown
`voltages of 7.43 volts. The units with four strata layers
`had an average breakdown of8.28 volts.
`Asa control, varistors were fabricated from the for-
`mulation set forth in the instant example, thesole differ-
`ence in fabrication techniques being that each of the
`ceramic bodies of the control varistors was fabricated
`by employing as the ceramic layers 30 a single sheet of
`the processed ceramic having a thickness approximately
`equal to the combined thickness defined by layers 30
`which were comprised of the three strata. The dried
`tape thickness was approximately 2.2 mils. Control
`varistors were formed from the control ceramic sheets
`in exactly the same manneras described in respect of
`the varistors manufactured from the multi-stratum lay-
`ers,. The control varistors were tested as to capacitance
`and percent dissipation factor as describe. the average
`capacitance was 4.68 nf, and 20.1% DF. The average
`breakdown voltage was 3.74 volts.
`For a limited voltage-current range, the current (ID
`througha varistor is proportional to the applied voltage
`(V), raised to some poweralpha (a). That is, I~Va. A
`large value for alpha is desirable, since then the device
`conducts a large current with a very small rise in volt-
`age over breakdown,providing an effective shunt to
`protect other circuit elements. Over a limited current
`range, an effective alpha can be defined as
`
`aeff =
`
`Aln()
`Aln(¥y)
`
`°
`
`In the following tests current pulses of short duration
`were applied to the varistors and the voltage measured.
`The current pulses were short in duration to prevent
`appreciable heating of the units. Following are the ef-
`fective values of alpha for the three stratum, four stra-
`tum, constructions and the controlvaristors, calculated
`for different current ranges.
`
`VARISTOR
`Control
`
`EFFECTIVE
`CURRENT RANGE USED
`ALPHA
`TO CALCULATEA In (1D)
`11.7
`0.5 to 1.0 amp
`4.8
`0.5 to 10.0 amp
`3.2
`0.5 to 18.8 amp
`19.8
`0.5 to 1.0 amp
`8.6
`0.5 to 10.0 amp
`47
`0.5 to 18.8 amp
`Four Stratum
`34.0
`0.5 to 1.0 amp
`10.3
`0.5 to 10.0 amp
`
`0.5 to 18.8 5.8
`
`Three Stratum
`
`Thebetter properties of the stratum constructed units
`relative to the control units can be understood in terms
`of the microstructures. A reproduction through a ce-
`ramic section of the three stratum constructed varistor
`taken by a scanning electron microscope (SEM) may be
`compared with a ceramic section of the control varistor
`taken by SEM.
`The control varistor has some large grains and re-
`gions of few grain boundaries which allow 1 ma of
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`8
`current to flow at a low voltage. The stratum con-
`structed units have a more uniform grain structure
`whichresults in a higher effective alpha overall current
`rangestested.
`With the multi-stratum construction it was easy to
`change the breakdownvoltage by changing the number
`of strata and grains. Changing the breakdown voltage
`of the control construction would involve more diffi-
`cult, less directly definable, process changes to modify
`average grain size.
`
`EXAMPLEIII
`
`20
`
`A zinc oxide varistor formula in accordance with
`Example 2 was prepared as thin sheets approximately
`0.8 mils thick. The ceramic layers defining three and
`four stratum varistors were formed in the mannerprevi-
`ously described in Example 1. The varistor monoliths
`were processed to form varistors by injecting Jead into
`the various void areas remaining following votalization
`of the pseudo ink and were terminated in the mannerset
`forth in the above referenced U.S.patents.
`The resultant varistors were tested against a control
`following the testing proceduresset forth in accordance
`with Example II. Varistors in which the layers of ce-
`ramic were fabricated by multiple sub-layers or strata
`were again found to be markedly superior in each of the
`test particulars as compared to a control batch formed
`in identical manner by wherein the layers of green ce-
`ramic were defined by a thicker single film the total
`thickness of which approximately equaled the plurality
`of stacked films.
`Fractured samples of the ceramic bodies of the con-
`trol varistors and those manufactured in accordance
`with the process of the instant application were sub-
`jected to SEM inspection. In the examples fabricated in
`accordance with the disclosure of the instant applica-
`tion, grain growth within the layers was foundto corre-
`spond closely to the configurationsofthe strata defining
`40 the layer, and hence a predictable number of grain
`boundaries between electrode areas was found to exist,
`An inspection of the control sample evinced a wide
`variation of grain sizes and the numberof grain bound-
`aries between electrode adjacent areas.
`The four stratum construction had a predictably
`higher breakdown voltage than the three stratum con-
`struction, and again gave an easier method of control-
`ling breakdownvoltage than the control construction.
`EXAMPLEIV
`
`.
`
`50
`
`55
`
`60
`
`65
`
`In this example a zinc oxidevaristor formulain accor-
`dance with Example II was prepared as thin sheets
`approximately 0.70 mils thick. The films were prepared
`in accordance with Example II, with an exception in
`binderlevels. Only 15 parts by weight total of binders
`were added.
`Multi-stratum layers were constructed from the thin
`films in two ways. In the first method the as formed
`sheets were slacked and laminated.
`In the second
`method a very thin layer of Rohm and Haas HA-16
`binder was applied to one surface of each tape and dried
`before stacking and lamination. The binder was applied
`by a cotton swabusing 90 parts by weight water with 10
`parts by weight of the HA-16 binder. The binderitself
`contained approximately 46 weight percent of latex
`organic. The laminated strata of both methods were
`then fired, and thermally treated as in Example II.
`
`000007
`
`

`

`9
`SEManalysis showed that the swabbed on binder
`acted as an effective grain growth barrier. The as cast
`tape did not have sufficient binder segregation to limit
`grain growth to within the tape strata.
`EXAMPLE V
`
`In this example samples of the thin films prepared in
`Example IV were used. As discussed in Example IV,
`the as cast film did not have sufficient binder segrega-
`tion to limit grain growth to within the tape strata.
`Stratum were constructed from the thin films by stack-
`ing and laminating. Before stacking and laminating one
`surface of each film was sprayed using an airless paint
`sprayer with a mixture of 90 parts water with 10 parts
`Lanthanum Carbonate. The laminated multi-strata were
`then fired and thermally treated as in Example II.
`SEManalysis showed that the thin coating of Lantha-
`num Carbonate acted as an effective grain growth bar-
`rier.
`
`EXAMPLEVI
`
`In this example a ceramic slurry formulation was
`prepared in accordance with Example IJ. The slurry
`was not cast as previously described. Multi-stratum
`layers were constructed by spraying thin ceramic films
`with an airless paint sprayer. Each film was dried and
`then the top surface coated with an organic film as
`described in Example IV before spraying the next thin
`ceramic film. A glass plate was used as a substrate for
`supporting the multi-stratum layer during build up. The
`build up was removed,fired and treated as in Example
`I.
`
`SEM analysis showed that grain growth was essen-
`tially limited to within the multi-strata sprayed films.
`In addition to the tape stacking and spraying tech-
`niques foundto be effective in producing ceramic layers
`having predictable grain sizes and hence predictable
`numbersof grain boundaries, similar results are achiev-
`able using doctor techniques, screening techniques,
`roller spreading techniques, etc. While at present, the
`tape stacking technique is considered to be the “best
`mode” of forming the multi-stratum layers, the inven-
`tion is not to be construed as limited to any specific
`technique or techniques, since numerous meansof form-
`ing multi-stratum layers with intervening grain growth
`barriers are possible. Rather,
`the invention is to be
`broadly construed to encompass the generic concept of
`providing ceramic layers wherein the numberof grain
`boundaries within the layer are controlled withinstatis-
`tically predicatable limits through the use of barriers
`formed at the major surfaces of the strata forming the
`ceramic layer. The examples have used organic resins
`for bonding of ceramic in the ceramic stratum during
`buildup of the structure. This should not be construed as
`being essential. This is demonstrated by the structures
`being in a binderless state after the binder burnout oper-
`ation but before sintering. It is anticipated that even
`material deposition techniques such as sputtering, or
`CVDcould be used to produce stratum layers of high
`green density bonded by thin layers of low green den-
`sity or other barriers at the stratum interfaces, and
`which could then be fired to produce the desired grain
`structures. The method is of course dependant on pro-
`cessing the green ceramic bodies in such a mannerthat
`grain growth across the adjacent strata does not de-
`velop,
`it being understood that notwithstanding the
`barriers, processing at unduly high temperatures or for
`
`20
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`25
`
`35
`
`40
`
`45
`
`50
`
`55
`
`65
`
`5,134,540
`
`10
`overly extended periods will induce such trans-barrier
`growth.
`While the present in