CA1300870C - Barium titanate coforms - Google Patents

Abstract

ABSTRACT

Barium titanate based dielectric compositions having the general formula Ba(1-x-x'-x'')PbxCax'Srx''Ti(1-y-y'-y'')SnyZry,Hfy',O3, wherein x, x', x'' and y, y', y'' represent mole fractions of the divalent and tetravalent cations and have independent values greater than zero and less than 0.3, such that the sums (x + x' + x'') and (y +
y' + y'') do not exceed 0.4. Regardless of the specific composition selected, the coforms of the invention have a mean primary particle size in the range of 0.05 to 0.4 microns with a very narrow particle size distribution. The products are dispersible so that the mean particle size determined by image analysis and by sedimentation are comparable.
The mole ratio of the divalent to tetravalent cations of the coforms is 1.000 ? 0.015 notwithstanding the number nor mole percent of the divalent and tetravalent cation substitutions.

Description

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This inven-tion relates to barium titanate based dielectric compositions and, more particularly, relates to stoichiome-tric, dispersible, submicron barium ti-tanate or coforms, with very narrow particle size distributions.
The high dielectric constant and strength of barium titanate make it an especially desirable material from which capacitors, condensers, and other electronic components can be fabricated. Especially attractive is the fact that barium titanate's electrical properties can be controlled within a wide range by means of mixed crystal formation and doping.
The very simple cubic perovskite structure exhibited by barium titanate is the high temperature crystal form for many mixed oxides of the ABO3 type.
This crystal structure consists of a regular array of corner-sharing oxygen octahedra with smaller titanium (IV) cations occupying the central octahèdral B site and barium (II) cations filling the interstices between octahedra in the larger 12-coordinated A-sites. This crystal structure is of particular significance since it is amenable to a plethora of multiple cation substitutions at bo-th the A and B
sites so that many more complex ferrolectric compounds can be easily produced.
Barium titanate's relatively simple lattice structure is characterized by the TiO6-octahedra which, because of their high polarizability, essentially determine the dielectric properties of the structure. The high polarizabiIity is due to the fact that the small Ti(IV) ions have relatively more space within the oxygen octahedra. This cubic unit cell, however, is stable only above the Curie poin-t temperature of about 130C. Below 130C, the Ti(IV) ions occupy off-center positions. This transition to - ' ~

' the off-center position results in a change in crystal structure from cubic to tetragonal between temperatures of 5C and 130C, to orthorhombic between -90C and 5C and finally to rhombohedral at temperatures less than -90C. Needless to say, the dielectric constant and strength also decreases relative to these temperature and crystal s-tructure changes.
The dielectric constant of barium titanate cerarnic has a strong temperature dependence and exhibits a pronounced, maximum dielectric constant at or around the Curie point. In view of the temperature dependence of the dielectric constant and its relatively low value at room temperature, pure BaTiO3 is rarely used in the production of commercial dielect:ric compositions. E~ence, in practice, additives are employed to upgrade the dielectric properties of barium -titanate. For example, i-t is known in -the art that the Curie temperature can be shi~-ted to lower temperatures and broadened by effecting a par-tial substitution by strontium and/or calcium for barium and by zirconium and/or tin for titanium, thereby resulting in materials with a maximum dielectric constant of 10,000 to 15,000 at room temperature. Alternative1y, the Curie temperature can be increased by a partial substitution of lead (II) for barium. Additlonally, the substitution of small amounts of other metallic ions of suitable size but with valencies which are different to those of barium and titanium, as summarized in B. Jaffee, W. R. Cook, Jr. and H. Jaffe, "Piezoelectric Ceramics", Academic Press, N.Y. 1971, can cause profound changes in the nature of the ~ielectr1c ~roperties.

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In commercial practice, barium titanate based dielectric powders are produced either by blending the required pure titanates, zirconates, stannates and dopants or by directly producing the desired dielectric powder by a high temperature solid state reaction of an in-timate mixture of the appropriate stoichiometric amounts of the oxides or oxide precurors (e.g., carbonates, hydroxides or nitrates) of barium, calcium, titanium, etc. The pure titanates, zirconates, stannates, ete. are also, typically, produced by a high temperature solid phase reaction process. In such caleination proeesses the required reactants are wet milled to accomplish the formation of an intimate mixture. The resulting slurry is dried and caleined at elevated temperatures, ranging from about 700 to 1200C, to attain the desired solid state reaetions. Thereafter, the ealeine is remilled to produee a dispersible powder for use in making green bodies.
Although the barium titanate based dieleetxic formulations produced by solid phase reaetions are aeeeptable for many eleetrieal applieations, they do suffer from several disadvantages. Firstly, the milling step serves as a souree of eontaminants whieh ean adversely affect electrical properties. Compositional inhomogeneities on a mieroseale can lead to the formation of undesirable phases, sueh as barium orthotitanate, which ean give rise to moisture sensitive properties.
Moreover, during ealeination substantial partiele growth and interparticle sintering can occur. As a consequence, the milled produet eonsists of irregularly shaped fraetured aggregates which have a wide particle size distribution ranging from about 0.2 up to 10 micronsO Published studies have shown that green bodies formed from sueh aggregated powders with ~.. ,;,;.,,. - - . ~

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broad aggregate size distributions require elevated sintering temperatures and give sintered bodies with broad grain size distributions. In the production of complex dielectric bodies, however, such as monolithic multilayer capacitors, there is a substantial economic advantage to employing lower sin-tering temperatures rather than higher sintering temperatures, since the percentage of lower cost silver in the silver-palladium electrode can be increased as the sintering temperature is reduced.
As is known in the art, the capacitance of a dielectric layer is inversely proportional to its thickness. In current multilayer capacitors, the dielectric layer thickness is of the order of 25 microns. Although very desirable, this value cannot be substantially reduced because as layer thickness is decreased the number of defects in the dielectric film, such as pin holes, increases. The defects adversely affect the performance of the capacitor.
One major source of such defects is the presence of undispersed aggregates having sizes comparable with the film thickness. During sintering, because of the presence of such aggregates, non-uniform shrinkage occurs and pin holes are formed. Hence, utilization of barium titanate based dielectric formulations formed by solid state reactions significantly increases the overall manufactur.ing cost of monolithic multilayer capacitors.
In view of the limitations of the product rendered by conventional solid state reaction : processes~, the prior art has developed several other methods for producing barium titanate. These methods nclude the thermal decomposition of barium titanyl oxalate and barium titanyl citrate and the high : 35 temperature oxidation of atomized solutions of either barium and titanium alcoholates dissolved in alcohol .

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or barium and titanium lactates dissolved in water.
In addition, barium titanate has been produced from molten salts, by hydrolysis of barium and titanium alkoxides dissolved in alcohol and by the reaction of barium hydroxide with titania both hydrothermally and in aqueous media. Because the product morphologies derived from some of these processes approach those desired here, the prior art has attempted to produce barium titanate based compositions with the same methods used to produce pure barium titanate. For example, B. J. Mulder discloses in an article entitled "Preparation of BaTiO3 and Other Ceramic Powders by Coprecipitation of Citrates in an Alcohol", Ceramic Bulltein, 49, No. 11, 1970, pages 990-993, that Batio3 based compositions or coforms can be prepared by a coprecipitation process. In this process aqueous solutions of TI(IV), Zr(IV) and~or Sn(IV) citrates and formates of Ba(II), Mg(II), CatII), Sr(II) and/or Pb(II) are sprayed into alcohol to effect coprecipitation. The precipitates are decomposed by calcination in a stream of air diluted with N2 at 700-800C to give globular and rod shaped particles having an average size of 3 to 10 microns.
Barium titanate based coforms have been prepared by precipltation and subsequent calcination of mixed alkali metal and/or Pb(II) titanyl and/or zirconyl oxalates as disclosed by Gallagher et al. in an article entitled "Preparation of Semi-Conducting Titanates by Chemical Methods", J. Amer. Ceramics 30 Soc., 46, No. 8, 1963 pages 359-365. These workers demonstrated that BaTiO3 based compositions in which Ba is replaced by Sr or Pb in the range oE 0 to 50 mole percent or in which TitIV) is replaced by Zr(IV) in the range of 0 to 20 mole percent may be produced.

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Faxon et al. discloses in U.S. Patent No.
3,637,531 that BaTiO3 based coforms can be synthesized by heating a solution of a titanium chelate or a ti-tanium alkoxide, an alkaline earth salt and a lanthanide salt to form a semisolid mass. The mass is then calcined to produce the desired titanate coform.
In each of the prior art references cited above, however, calcination is employed to synthesize the particles of the barium titanate based coforms.
For reasons already noted this elevated temperature operation produces aggregated products which after comminution give smaller aggregate ~ragments with wide size distributions.
The prior art has also attempted to circumvent the disadvantages of conventionally prepared BaTiO3 powders by synthesizing a mixed alkaline earth titanate-zirconate composition through a molten sal-t reaction. Such a process is disclosed in U.S. Patent No. 4,293,534 to ~rendt. In the practice of this process -titania or zirconia or mixtures thereof and barium oxide, strontium oxide or mixtures thereof are mixed with alkali metal hydroxides and heated to temperatures suffi.cient to melt the hydroxide solvent. The reactants dissolve in the molten solvent and precipitate as an alkaline earth titanate, zirconate or a solid solution having the general formula BaxSr(l x)TiyZr(l y)O3. The products are characterized as chemically homogeneous, relatively monodisperse,. submicron crystallites. This - 30 method is limited, however, in that it can only produce Sr and/or Zr containing coforms.
:Hydrothermal: processes have also been described in which coforms are produced. Balduzzi and : Steinemann in British Patent No. 715,7~2 heated aqueous slurries of hydrated Tio2 with stoichiometric amounts of alkaline earth hydroxide to temperatures , ., .~, ~. . :

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between 200 and 400C to form mixed alkaline earth titanates. Although it was stated that products of any desired size up to about 100 ~lm could be produced, it is doubtful that, other than in the case of Sr-containing coforms, products with the morphological characteristics of this invention co~ld be obtained.
This contention is based on the fact that whereas Ba(OH)2 is soluble in aqueous media Ca(OH)2, and Mg(OH)2, especially in the presence of Ba(OH)2, are relatively insoluble. Accordingly, in the case of Ca-containing coforms it has been found that under the experimental conditions of Balduzzi and Steinemann that BaTiO3 is first formed and then Ca(OH)2 reacts with the balance of the unreacted titania to form CaTiO3 during the heating process to 200 to 400C.
Matsushita et al. in European patent publication No.
0141551 published May 15, 1985 demonstrated that dilute slurries of hydrous titania can be reacted with Ba(OH)2 and/or Sr(OH)2 by heating to temperatures up to 110C to produce either BaTiO3 or Sr-containing coforms. The morphological characteristics of these coforms appear to be comparable with those of this invention. The method, however, is again limited to producing only Sr-containing coforms.
A publication of the Sakai Chemical Industry Company entitled "Easily Sinterable BaTiO3 Powder" by Abe et al. discloses a hydrothermal process for synthesizing a barium titanate based coform with the formula BaTi(l-x)snxo3 In this process a 0.6M
Ti(l_x)SnxO2 slurry, prepared by neutralizing an aqueous solution of SnOC12 and TiC14, is mixed with 0.9M Ba(OH)2 and subjected to a hydrothermal treatment at 200C for at least five hours. Although not explicitly delineated, Abe et al. imply the slurry was heated to temperature. Although no description of the coform morphology was indicated, the BaTiO3 product .,, .. ~ , .

produced by the same process had a surface area of 11 m2/g, a particle size of 0.1 ~m and appeared to be dispersible. Presumably the Sn-containing coforms have comparable morphologies and are thus comparable with those of this invention. However, Abe et al. is limited in that it teaches only that Sn(IV) can be synthesized into a barium titanate coform. Perhaps, by analogy, it does suggest the use of other tetravalent cations such as Zr(IV) and possibly the use of divalent Sr(II), since, like Ba(OH)2, Sr(OH)2 is quite soluble in aqueous media. However, the process of Abe et al. cannot be used for substitution of divalent Curie point shifters such as Pb and Ca for the divalent Ba.
Hence, there is absent in the prior art any coforms of barium titanate which include calcium and/or lead or multiple divalent and tetravalent cation substitutions which are stoichiometric, dispersible, spherical, and submicron with narrow particle size distributions.
The present invention includes a wide variety of dispersible coforms of barium titanate which are substantially spherical, stoichiometric, and submicron with narrow particle size distributions.
Most importantly, the barium titanate based dielectric compositions according to the present invention include those coforms having a partial substitution by divalent lead and/or calcium for the divalent barium as well as coforms in which the divalent barium is partially replaced by lead, calcium and strontium and the tetravalent titanium is partially replaced by tin, zirconium and hafnium.
In one important embodiment of the present invention, the barium titanate~ based coform is represented by the general formula (l-x-xl-xll)pbxcax~srxllTi(l-y-yl-yll)snyzrylHf ,,O3 ..". ~, '"

)8~'0 where x, x' and x'' represent the mole fractions of the divalent cations and have independent values ranging from 0 -to 0.3 and the sum x + x' + x'' has a value ranging from 0 to 0.4, while y, y' and y'' represent the mole fractions of the tetravalent cations and have independent values ranging from 0 to 0.3 and the sum of y + y' + y'' has a value ranging from 0 to 0.~.
In another important embodiment of the present invention, the barium titanate coform is represented by the general formula Ba(l_xl)caxlTi(l-y-yl y,,)SnyZry,HF ,,O3 wherein calcium is partially substituted for the divalent barium cation and in another important embodiment of the invention the barium titanate dielectric composition is represented by the general formula Ba Pb Ti , " Sn Zr ,Hf ,,O , wherein lead tl-x) x (l-y-y -y ) y y y 3 is substituted for the divalent barium. In each of the latter embodiments, the independent values for the mole fractions x, x', x'' and y, y', y'' and consistent with those already cited for the more complex coEorm having the general formula (l-x-xl-x~l)pbxcaxlsrxlli(l-y-y~-yll)snyzr ,Hf ,,O3.
Notwithstanding the chemical composition of the coform, each of the barium titanate based coforms of the present invention possess the same unique chemical and physical properties. The barium titanate based dielectric formulations are stoichiometric such that the divalent to tetravalent mole ratio of the vafyingly composed coforms is 1.000 + 0.015 regardless of the number and mole percent of any divalent and tetravalent cation substitutions. Non-stoichiometric compositions, where the divalent to tetravalent cation : mole ratio of the varyingly composed coforms is in the range of 0.9 to 1.1, can also be produced~ The mean : primary particle size of the barium titanate based g 3~

coforms is in the range of 0.05 to 0.4 microns.
Moreover, the mean particle size determined by image analysis is comparable to the mean particle size determined by sedimentation demonstrating that the coforms are dispersible. The size dis-tribution curve of the coform particles has a ~uartile ratio less than or equal -to 1.5 which establishes that the barium titanate based coforms have a narrow particle size distribution. Additionally significant is the fact that any of the dispersible, submicron barium titanate based dielec-tric compositions of the present invention can be produced by a single, general hydrothermal process.
Accordingly, it is a primary object of the present invention to provide a dispersible, submicron barium titanate coform with a narrow particle size distribution.
It is another object of the present invention to provide a wide variety of compositions of such BaTiO3 based coforms having primary particle sizes which can be controlled in the size range of 0.05 up to about 0.4 ~m.
It is another object of the present invention to provide a wide variety of coforms which are syn-thesizable by a single general hydrothermal process.
It is another object of the present invention to provide a stoichiometric barium titanate based coform which is substantially free of mill media.
It is another object of the present invention to provide a coform of barium titanate containing a variety of additives which shifts and/or broadens the Curie point to the desired temperature regions and reduces the temperature dependence of the dielectric compositions so formed.

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~30~70 It is another object of the present invention to provide dispersible BaTiO3 based dielectric compositions which can be used to give dielectric layers of reduced thickness which are substantially defect free.
It is still a further object of the present invention to provide a barium titanate based dielectric formulation which uniformly sinters to a high density at considerably less than conventional temperatures.
These and other details and advantages of the invention will be described in connection with the accompanying drawings in which:
Fig. l is a transmission electron micrograph 15 at 50,000x magnification of a stoichiometric, dispersible, submicron complex coform according to the present invention having the general formula 0.856 0.097Ca0.074Ti0.830zr0 099Sn0 0713; and Fig. 2 is a transmission electron micrograph 20 at 50,000x magnification of pure barium titanate powder which exhibits a morphology substantially similar to the morphology of the complex coform of Fig.l.
At the outset, the invention is described in its broadest overall aspects, with a more detailed description following. The preferred embodiment of the present invention is a coform of the general type (l-x-xl-x~l)pbxcaxlsrxllTl~l , Sn Zr ~f O
wherein x, x' and x'' represent the mole fractions of the divalent cations and have independent values ranging from 0 to 0.3 and, more preferably, from 0 to 0.2 and the sum x + x' + x'' can have values ranging from 0 to 0.4 and more preferably from 0 to 0.3, y, y' and y'' represent the mole fractions of the tetra-valent cations and have independent values ranging .

13~0870 from 0 to 0.3 and, more preferably, from 0 to 0.25 andthe sum of y + y' + y'' have values ranging from 0 to 0.~ and, more preferably, from 0 to 0.3.
When the sums of (x + x' -t x'') and (y + y' + y'') both equal zero the coform simply constitutes barium titanate powder. When x = x'' = y = y' = y'' =
0 and x' is greater -than 0, the resulting product is a barium titanate based coform where x' mole fractions of Ba(II) in BaTiO3 have been replaced by Ca(II) to give a product with the nominal formula Ba(l x,)Cax,TiO3. Conversely, when x' = x'' = y = y' = y'' = 0 and x is greater than zero, the coform has the composition Ba(l X)PbxTiO3.
Since the values of x, x', x'', y, y', and y'' can each adopt a wide range of values (within the cited limits), many combinations of coforms with a large range of compositions can be prepared.
Regardless of which composition is formed, however, each of the barium titanate based coforms is uniquely characterized by its high purity, fine submicron size and narrow particle size distribution.
Preferably, the fine, dispersible submicron powder of the present invention consists of a barium titanate coform having both a tetravalent and a divalent metal ion substitution of between zero and 30 mole percent. The divalent barium ion can be partially replaced by either lead, calcium, strontium or mixtures thereof. Conversely, the -tetravalent titanium ion can be partially replaced by tin, zirconium, hafnium or mixtures thereof. Hence, the barium titanate based dielectric compositions of the present invention include simple coforms of barium lead titanate or barium strontium titanate as well as more complex coforms including barium lead stannate titanate and barium lead strontium stannate zirconate ; titanate. Of course, the sel~ection of the divalent ~ 12 -~L~00870 and/or tetravalent cation replacement and the mole percent of the substitution is dependent upon whether the Curie temperature is desired to be raised or lowered as well as by whethex the Curie peak is desired to be broadened or shifted. Regardless of which of the wide variety of barium titanate based compositions is formed, however, the barium titanate coEorms according to the present invention are still uniquely identified by the aforementioned morphological and chemical characteristics. Hence, both the simple as well as the complex coforms of barium titanate consist of substantially spherical, dispersible particles having a primary particle size in the range of 0.05 and 0.4 microns with narrow size distributions and a divalent to tetravalent mole ratio of 1.000 + 0.015, even when both the divalent and tetravalent ions have been replaced by one or more other ions.
The narrow particle size distribution and submicron slze of the barium -titanate based dielectric compositions make the coforms of the present invention particularly attractive for fur-ther application in the production of complex dielectric bodies. Prior studies have established that green bodies formed from unaggregated powders with narrow size distributions will sinter at reduced temperatures and give sintered bodies with a narrow grain size distribution. The economic advantage of employing a dielectric formulation with a lower sintering temperature is obvious since the percentage of lower cost silver in the silver-palladium alloy can be increased as the ~sintering temperature is reduced. In addition, since these BaTiO3 based dielectric compositions are all dlspersible and have few aggregates exceeding a size of l micron, they can be employed in the formation of dielectric films of reduced thickness. Hence, the ~30(:~870 spherical, unaggregated, submicron and narrowlydistribu-ted barium ti-tanate dielectric coform powder of the present invention should be particularly well suited for use in complex dielectric applications requiring sintering.
In most dielectric applications, the preferred products are those in which the variability in primary particle composition is relatively small.
In some circumstances, however, compositional inhomogeneities are an advantage. In these instances, the availability of products with varying primary particle size can be utilized to produce a dispersion of two or more powders with differing compositions having either comparable numbers of primary particles or substantially different numbers of primary particles. Such dispersions give green bodies, and hence sintered bodies, with controlled degrees of microinhomogeneities. In such applications, the compositional inhomogeneity may be inherent in the barium titanate coform selected or, instead, may result from a small amount of a barium titanate coform with a selected composition being added to a barium titana-te dispersion in order to achieve the desired compositional inhomogeneity. Since either divalent barium and/or tetravalent titanium deficient coforms can be formed according to this invention, the barium titanate based compositions of the present invention are also well suited for applications where compositional inhomogeneities are advantageous.
The preferred approach for producing the barium titanate based coforms is to heat slurries con-taining the hydrous tetravalent oxides wi~h ~selected divalent oxides or hydroxides. After formation of the divalent titanates, the slurry still contains substantial quantitites of hydrous TiO2 and/or hydrous SnO2, ZrO2 or HfO2. The slurry : ,.

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temperature and concentration are then adjusted and a stoichiometric excess of Ba(OH)2 solution is then added under isothermal conditions. In order to ensure the complete conversion of the tetravalent oxides to their corresponding oxyanions, the slurry is preferably taken -to a final, higher heat treatment.
The primary particle size and size distribution of the present invention are achieved whether the barium titanate coform is simply ~aTiO3 or instead is the more complex coform having the formula l-x-xl-xllPbxcaxlsrxllTil-y-yl-~llzrysnylHf ,,o3.
This becomes readily apparent from the transmission electron micrograph of the complex coform 0.856 0.097Cao.074Tio 830Zr0 099Sn0 0713 in Fig 1 which shows the presence of predominantly single, substantially spherical primary particles, although a few firmly bound doublets and triplets are also present. The primary particle size of this coform is 0.18 microns with a narrow size distribution. A
comparison of the complex barium titanate based coform of Fig. 1 with the transmission electron micrograph of pure barium titanate in Fig. 2 indicates that the morphologies of the barium titanate based compositions are very similar. Note that in both micrographs the particles are substantially spherical,unaggregated, submicron and uniformly sized. It may also be noted that the divalent to tetravalent cation mole ratio in this product, 1.027, is somewhat larger than the value 1.000 + 0.015 specified for stoichiometric products.
This ratio can easily be reduced to the specified range by minor varia-tions in the synthesis conditions wlthout affecting morphology.
In order to evaluate the physical and chemical properties of the barium titanate based coforms according to the present invention, a variety of laboratory tests were performed. Image analysis ,.. ~. ~. : .

~3ao~70 was used to determine product primary particle size and primary particle size distribution. 500 to 1000 par-ticles were sized in a plurality of TEM fields in order to determine the equivalent spherical diameters of the primary particles. Two or more touching particles were visually disaggregated and the sizes of the individual primary particles were measured. The e~uivalent spherical diameters were used to compute the cumulative mass percent distribution as a function of primary particle size. The median particle size, by weight, was taken to be the primary particle size of the sample. The quartile ratio, QR, defined as the upper quartile diameter (by weight) divided by the lower quartile diameter, was taken as the measure of the width of the distribution. Monodisperse products have a QR value of 1 and, for our testing purposes, products with QR values ranging from l.0 to about 1.5 were classified as having narrow size distributions, those with QR values ranging from 1.5 to about 2.0 were classiied as having fairly narrow distributions, while those with values substantially greater than 2.0 were classified as having broad size dis-tributions. The quartile ratio of the barium titanate coforms of the present invention was determined to be between 1.0 to 1.5, indicating that the primary particles have a narrow size distribution.
Surface areas were calculated from the coform's primary particles and were found to be consistent with the surface areas determined by nitrogen adsorption, indicating that the prirnary par-ticles are essentially nonporous. In cases where the N2 surface area substantially exceeded the TEM
surface area, it was found that the difference could be readily accounted for by the presence of unreacted . .
high surface area hydrous o~ides.

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Since the coforms of the present invention have a narrow size distribution, average primary particle size was readily determined by sizing 20 to 30 particles. It was found that the relationship D=6 S, where D is particle diameter (microns)p is density (g/cc) and S is N2 surface area (m2tg), could be used to obtain a good measure of the coform primary particle size. According to this formula it was found that the barium titanate based coforms have a primary particle size in the range between 0.05 and 0.4 microns, regardless of which coform composition was tested.
Product dispersibility of the coforms was assessed by comparing the primary particle sizes and size distributions determined by image analyses with the comparable values determined by sedimentation procedures. The sedimentation process gives the particle Stokes diameter which, roughly, corresponds to the equivalent spherical diameter. Two sedimentation methods, the Joyce Loebl Disc Centrifuge (Vickers Instruments, Ltd., London, U.K.) and the Micromeritics Sedigraph (Norcross, Georgia) were employed to determine cumulative mass percent distributions in terms of Stokes diameters from which the median Stokes diameters and the QR values were calculated.
In determining particle size by sedimentation, the powders were dispersed by a 15 to 30~ minutes sonification in either water containing 30 0.~08g/L sodium tripolyphosphate at pH 10 or in sopropanal contalnlng 0.08 ~ or 0~12 weight percent Emphos PS~21A (Witco Organics~ Division, 520 Madison Ave., New York)~
Since particle siz~e ~de~ermined by image ~analysis and by sedimentation depend on different pr~inciples, ~an exact correspondence in size by these :

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two methods was not always obtained. Moreover, as already noted, in image analysis touching particles are visually disaggregated. In the sedimentation process bound or flocculated particles act as single entities. These entities arise because of the existence of some bonding (e.g., necking) between the primary particles to give cemented aggregates which cannot be readily broken down during the sonification process and because of less than optimum dispersion stability which leads to some flocculation. Thus, the QR values determined by sedimentation, as expected, were somewhat larger than those found by image analysis.
In the barium titanate based coforms of the present invention, the primary particle size determined by image analysis was in reasonable agreement with the primary particle size determined by sedimentation. The median particle size determined varied by no more than a factor of two. This demonstrates that the coforms are dispersible.
Two additional measures were used to assess dispersibility. In the first method, the mass fraction of the product having a Stokes diameter greater than one micron was used as a measure of the amount of hard-to-disperse aggregates. In the second method, a product was classified as being dispersible if the bulk of the primary particles in the TEM's were present as single particles. When substantial necking was observed the product was classlfled as aggregated.
In each of these testsj the barium titanate based ; coforms were again classified as dispersible.
Product composition and stoichiometry of the :: :
coforms was determined by elemental analysis using inductively coupled plasma spectroscopy after sample ~ 35 dissolution. The precision of the analyses was about ; ~ ~1%. The mole ratio of divalent cations to :

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-tetravalent cations of the coforms, regardless of the number or mole weight percent of the divalent and tetravalent cation substitutions, was 1.000 + 0.015.
This ratio indicates that the barium titanate coforms of the present invention are stoichiometric.
The unique properties of the barium titanate based coforms are further illustrated by the following non-limiting examples.
Reagent grade chemicals or their equivalents were used throughout the Examples. The reagent grade Ba(OH)2 8H2o employed contained l mole percent Sr.
Experiments have shown that Sr(II) is more readily incorporated than Ba(II) in the coform. For this reason all coforms described here contain Sr(II).
This cation represents about l mole percent of the total divalent cation content of the coform. For simplicity, the Sr(II) mole fraction has been included in the Ba(II) mole fraction. Ba(OH)2 and/or Sr(OH)2 solutions, maintained at 70-100C, were filtered prior to use to remove any carbonates present. CaCO3 was calcined at 800C to give CaO. The latter compound when contacted with water gives Ca(OH)2. Pb(OH)2 was prepared by neutralizing a Pb(NO3)2 solution with aqueous NH3. The washed hydroxide wet cake was used in subsequent experiments.
Hydrous oxides of TlO2, SnO2 and ZrO2 were prepared by neutralizing aqueous solutions of their respective chlorides with aqueous NH3 at ambient temperatures. The products were filtered off and washed until chloride-free (as determined by AgNO3) ~filtrates were obtained. The surface areas of the hyd~ous oxides, detexmined after drying at 110C, were about 380, 290 and 150 m /g for TiO2, SnO2 and Zr2, respecti,vely. In addition coprecipitates of hydrous ,....

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TiO2 and ZrO2 or hydrous TiO2 and SnO2 were prepared by neutralizing aqueous solutions of the chlorides of Ti(IV) and Sn(IV) or Ti(IV) and Zr~IV).
All experiments were performed in a 2 liter Autoclave. To prevent product contamination a]l wetted parts of the autoclave were coated with Teflon and every effort was made to exclude CO2 from all parts of the system. Ba(OH)2 or Ba(OH)2 and Sr(OH)2 solutions were introduced into the autoclave either by means of a high pressure pump or by rapidly discharging a solution of the hydroxide or hydroxides, contained in a heated bomb, in-to the autoclave by means of high pressure nitrogen. The contents of the autoclave were stirred at 1500 RPM throughout the synthesis process.
Example 1 A calcium containing coform was prepared by hydrothermal treatmen-t of 0.64 L of a slurry containing 0.20 moles of hydrous TiO2 and 0.04 moles 20 of Ca(OH)2 to 200C. The slurry was coo]ed and 0.46 L
of 0.41M Ba(OH)2 was added to the slurry at 120C.
The resulting slurry temperature was raised to 150C
and held there for 60 minutes. The sample was filtered and the divalent cation concentrations in the filtrates were determined. The filter cake was dried and its surface area, nominal stoichiometry and morphological characteristics was determined.

87~

Divalent/
Filtrate Cation Mole Ratio Tetravalent g/L in Solids Cation N Area ... __ 2 Ba Ca Ca: Ba: Sr: Ti Mole Ratio m /g 2.62 0.446 0.127 0.842 0.019 1.00 0.988 12.0 Primary Particle Size Size(TEM) Distributlon 0.15 micron Narrow x~ e 2 A lead containing coform was prepared by hydrothermal treatment of 0.64 L of a slurry containing 0.2 moles of hydrous TiO2 and 0.04 moles PbO. 0.46 L of Ba(OH)2 was added to the slurry at 150C. The slurry was held at 150C for 60 minutes and then raised to an elevated temperature for complete conversion o the tetravalent oxides to the perovskite structures. The slurry was sampled and characterized.
The results obtained are as follows:

Divalent/
Z0 Filtrate Cation Mole Ratio Tetravalent g/Lin Solids Cation Area .... _ - ,~
_ Pb Ba: Pb: Sr: Ti: Mole Ratio m /g_ : ~ 10.6 2.74 0.810:0.173;0.024:1.000 1.007 11.5 ~ .
: : Primary Particle Size ~ 25 Size (TEM) Distribution .. . .. _ _ , .. .
~0.07 micron Narrow ', ' ' ' '' ' ' ~3W~

Example 3 Complex coforms are formed in which the Ba(II) and Ti(IV) in BaTiO3 are partially replaced by one or more divalent and tetravalent cations. A
preheated Ba(OH)2 solution was introduced into slurries heated to 150C or 120C containing the tetravalent hydrous oxides and presynthesized perovskites of Pb(II) and/or Ca(II). After holding at temperature for about 20 to 30 minutes, the slurries were raised to a final temperature to ensure that the tetravalent hydrous oxides converted to stoichiometric perovskites. The resulting slurry was characterized with the following results:

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~3008 ~'0 The quantitative data for samples 2 and 3 corresponds well with the estimated particle size, size distribu-tion and dispersibility data drawn from the transmission electron micrographs. Sample 1, however, as assessed by the QR value is only moderately dispersible. Nevertheless, the sediment~tion data indicates that less than 5 weight percent of the material is present as aggregates having a size greater than 1 micron.
It can therefore be seen from the preceding examples and disclosure, that the coforms of barium titanate encompassed by the present invention include those dielectric compositions containing calcium and/or lead or multiple replacements for either or both of the divalent barium and tetravalent titanium cations which are uniquely characterized in that they are spherical, have a primary particle size in the range from 0.05 to 0.4 microns, a divalent -to tetravalent mole ratio of 1.000 + 0.015, and a narrow particle size distribution. No prior art barium titanate based dielectric compositions which include calcium, lead or the complex forms disclosed herein possess these unique morphological and chemical characteristics.
It is understood that the preceding description is given merely by way of illustration and not in limitation of the invention and that various ; ~ modifications may be made thereto without departing from the spirit of the invention as claimed.

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Claims (9)

1. A barium titanate based coform comprising substantially spherical particles having the formula:
Ba(1-x')Cax'Ti(1-y-y'-y'')SnyZry'Hfy''O3 wherein y, y', and y" have independent values ranging from zero to 0.3, the sum of y + y' + y" is less than 0.4, and x' is greater than zero and less than 0.3 and wherein (a) the median primary particle size, as determined by image analysis, is in the range of 0.05 to 0.4 microns, (b) the primary particle size distribution, as determined by image analysis, has a quartile ratio less than or equal to 1.5, (c) the median primary particle size, as determined by image analysis and by sedimentation, agree within a factor of two, and (d) the particle size distribution, as determined by sedimentation, has a quartile ratio less than or equal to 2Ø

2. The coform of barium titanate of claim 1 wherein the mole ratio of (Ba + Ca)/(Ti + Sn + Zr +
Hf) is in the range between 0.9 and 1.1.

3. The coform of barium titanate of claim 1 wherein the mole ratio of (Ba + Ca)/(Ti + Sn + Zr +
Hf) is 1.000 ? 0.015.

4. A barium titanate based coform comprising substantially spherical particles having the formula:
Ba(1-x)Pbx'Ti(1-y-y'-y'')SnyZry'Hfy''O3, wherein y, y', and y" have independent values ranging from zero to 0.3, the sum of y + y' + y" is less than 0.4, and x' is greater than zero and less than 0.3 and wherein (a) the median primary particle size, as determined by image analysis, is in the range of 0.05 to 0.4 microns, (b) the primary particle size distribution, as determined by image analysis, has a quartile ratio less than or equal to 1.5, (c) the median primary particle size, as determined by image analysis and by sedimentation, agree within a factor of two, and (d) the particle size distribution, as determined by sedimentation, has a quartile ratio less than or equal to 2Ø

5. The coform of barium titanate of claim 4 wherein the mole ratio of (Ba + Pb)/(Ti + Sn + Zr +
Hf) is in the range between 0.9 and 1.1.

6. The coform of barium titanate of claim 4 wherein the mole ratio of (Ba + Pb)/(Ti + Sn + Zr +
Hf) is 1.000 ? 0.015.

7. A barium titanate based coform comprising substantially spherical particles having the formula:
Ba(1-x-x'-x")PbxCax'Srx"Ti(1-y-y'-y")SnyZry'Hf"O3, wherein x, x' and x", y, y' and y" each have inde-pendent values greater than zero and less than 0.3, the sum of x + x" + x" is less than 0.4 and the sum of y + y' + y" is less than 0.4 and wherein (a) the median primary particle size, as determined by image analysis, is in the range of 0.05 to 0.4 microns, (b) the primary particle size distribution, as determined by image analysis, has a quartile ratio less than or equal to 1.5, (c) the median primary particle size, as determined by image analysis and by sedimentation, agree within a factor of two, and (d) the particle size distribution, as determined by sedimentation, has a quartile ratio less than or equal to 2Ø

8. The coform of barium titanate of claim 7 wherein the mole ratio of (Ba + Ca + Pb + Sr)/(Ti +
Sn + Zr + Hf) is in the range between 0.9 and 1.1.

9. The coform of barium titanate of claim 7 wherein the mole ratio of (Ba + Ca + Pb + Sr)/(Ti +
Sn + Zr + Hf) is 1.000 ? 0.015.