IE60287B1 - Barium titanate coforms - Google Patents

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. [CA1300870C]

Description

This invention relates to barium titanate based dielectric compositions and, more particularly, relates to stoichiometric, dispersible, subinicron barium titanate or coforms, with very narrow particle size distributions» The high dielectric constant and strength or barium titanate make it an especially desirable material from which capacitors, condensers, and other electronic components can be fabricated. Especially attractive is the tact 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 ABOj type. This crystal structure consists of a regular array of corner-sharing oxygen octahedra with smaller titanium (IV) cations occupying the central octahedral II site and barium (11, 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 both 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 TiO,-octahedra which, because of their high polarizability, essentially determine the dielectric properties of the structure. The high polarizability 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 point temperature of about 130°C. Below 130°C, the Ti(SV) ions occupy off-center positions. This transition to the oif-1eenfcer position results in a change in crystal structure from cubic to tetragonal between temperatures of 5°C and 130°C, to orthorhombic between -90°C and 5°C and finally to rhombohedral at temperatures less than -SO°C Needless to say8 the dielectric constant and strength also decreases relative to these temperature and crystal structure changes. * / The dielectric constant of barium titanate ceramic has a strong temperature dependence and exhibits a pronounced? maximum dielectric constant at or around the Curie point. in view of the temperature dependence o£ the dielectric constant and its relatively low value at room temperature, pure QaTiO^ is rarely used in the production of commercial dielectric compositions.

Hence, In practice? additives are employed to upgrade the dielectric properties of barium titanate. For example, it is known in the art that the Curie temperature can be shifted to lower temperatures and broadened by effecting a partial 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. Alternatively, the Curie temperature can be Increased by .a partial substitution of lead (ΙΪ) for barium. Additionally, the substitution of smell 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 !3. Jaffe, "Piezoelectric Ceramics, Academic Press,, SLY. 1971, can cause profound changes in the nature of the dielectric properties.

In cornmecical practice, barium titanate based dielectric powders are produced either by blending the required pure titanates, zirconates, stannates and dopants or by directly λ producing the desired dielectric powder by © high temperature solid state reaction of en intimate mixture of the appropriate ' stoichiometric amounts of the oxides e»r oxide precursors {e.g., carbonates, hydroxides or nitrates} of barium, calcium, titanium. etc. The pure titanates, zirconates, stannates, etc. are also, typically, produced by a high temperature solid phase reaction process. In such calcination processes the required reactants are wet milled to accomplish the formation of an intimate mixture™ The resulting slurry is dried and calcined et elevated temperatures, ranging from about 700 to 12OO°C, to attain the desired solid state reactions. Thereafter, the calcine is remilled to produce a dispersible powder for use in making green bodies.

Although the barium titanate based dielectric formulations produced by Solid pSsase reactions are acceptable for many electrical applications, they do suffer from several disadvantages. Firstly, the milling step serves as a source of contaminants which can adversely affect electrical properties.

Compositional inhomogenleties on a microscale can lead to the formation of undesirable phases, such as barium orthotitanate» which can give rise tc moisture sensitive properties. Moreover, during calcination substantial particle growth and interparticle sintering can occur. As a consequence, the milled product consists of irregularly shaped fractured aggregates which have a wide particle size distribution ranging from about 0.2 up to 10 microns.

Published studies have shown that green bodies formed iron such aggregated powders with 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 sintering temperatures rather then 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. 2n 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 detects 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 undispeirsed 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 signs £icantly increases the overall manufacturing 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 include the thermal decomposition of barium titanyl oxalate and barium titanyl citrate and the high temperature oxidation of atomized solutions of either barium and titanium alcoholates dissolved in alcohol 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 BaTiO^ and Other Ceramic Powders by Coprecspitat ion of Citrates in an Alcohol", Ceramic Bulletin, 49, No. 11, 1970, pages 990-993, that BaTiO^ based compositions or coforms can be prepared by a coprecipitation process. In this process aqueous solutions of Ti(IV), Zr(XV) and/or Sn(iV) citrates and formates of Ba{ΣΪ), Mg(II), Ca(II), Sr(IS) and/or Pb(I1) are sprayed into alcohol to effect coprecipitation. The precipitates are decomposed by calcination in a stream of air diluted with at 00-@00oC to give globular and rod shaped particles having an average size of 3 to .10 microns.

Barium titanate based coforms have been prepared by precipitation and subsequent calcination of mixed alkali metal * and/or Fb(II) titanyl and/or sircony1 oxalates as disclosed by Gallagher' et , al. in an article entitled 65 Proper at ion of SemiConducting Titanates by Chemical Methods63, Amer. Ceramics Soc. .. 45, Mo.. 3, 1953 pages 359-305, These workers demonstrated that BeTiO^ based compositions in which Ba is replaced by Sr or Pb in the range of 0 to 50 mole percent or in which Ti(IV) is replaced by £r(lV) in the range of 3 to 20 mole percent may be produced.

Faxon et al. discloses in U.S. Patent No. 3,637,531 that SaTlO^ based coforms can be synthesised by heating a solution of a titanium chelate or a titanium 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 synthesise 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 fragments with wide size disfcribution®.

The prior art has also attempted to circumvent the disadvantages of conventionally prepared BaTiO^ powders by synthesizing a mixed alkaline earth titanate-zirconate composition through a molten salt reaction. Such a process is disclosed in U.S. Patent No. 4,293,534 to.Arendt. In the practice of this process titania or zirconia or mixtures thereof and barium oxide, strontium oxide*or mixtures thereof ace mixed with alkali metal Q hydroxides and heated to temperatures sufficient to melt the i 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 Sa^Sr n«.xjTi Zr p_V)O3. The products are characterized as chemically homogeneous, relatively monodisperse, submicron crystallites. This 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. Dalduzzi and Steinemann in British Patent No,. 715,762 heated aqueous slurries of hydrated TiO2 with stoichiometric amounts of alkaline earth hydroxide to temperatures between 200 and 400°C to form mixed alkaline earth titanates. Although it was slated that products of any desired size up to about 100//in could be produced, it is doubtful that, other than in the case of Sr-containing coforms, products wish the morphological characteristics of this invention could be obtained. This contention is based on the fact that whereas UafOIDj *s soluble in aqueous media Ca(OH)_, and Mg(OH)o, especially in the presence of I3a(01!) 2*’ afs relatively insoluble. Accordingly, in the case of Caconfaining coforms it has been found that under the experimental conditions of Qaldussi and Steinemann that MaTiO, is first formed and then Ca(0H)9 reacts with the balance of the unreacted titania to form CaTiO^ during fhe heating process to 200 to 400°C.

Matsushita et al. in European patent application No. 34306925.1 (Dublication number 0 141 551 Al) demonstrated that dilute slurries of hydrous titania can be reacted with Ba(0H)2 and/or Sr(OH)? by heating to temperatures up to 110°C to produce either UaTiOj or Sr-containing coforms. The morphological characteristics of these coiorms 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 BaTiOj Powder, by Abe et al. discloses a hydrothermal process for synthesising a barium titanate based coform with the formula BaTl jSn,tO3. in this process a 0.6M TA ,_J5jSnJrOo slurry, prepared by neutralizing an aqueous solution of SnOCl2 and TiCl^, is mixed with 0.9M 3,;ίΟΗ)2 and subjected to a hydrothermal treatment at 200°C 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 BaTiOj product producer] by the •n same process had a surface area of 11 m /g, a particle size of 0.1 j/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(IX), since, like Ba(OH)9, Sr(OJI)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 9a.

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 titanate based coform is represented by the general formula ba r i um Sa (ϊ-χ-χ’ -X ) PbxCax "Sr X "’’Γ; jjL-y-y’-y6’) ^ny^Ey " •’^y,,©3 where x, x‘ and x and ys y' and y each have independent values greater than zero and less than 0.3 and the sum of x+x'+x is less than 0.4 and the sum of y+y'+y is less than 0.4.

Ba (1-X ' , Cax sTi (l-y-y ’ -y8’ ) Sny2ry y °3 In another important embodiment of the present invention, the barium titanate corona is represented by the general formula 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 (1-x) pbxT" (3-v-v*-yB) Sry2rv' "sfy °3 f’ therein lead is substituted for the divalent barium. In each of the latter embodiments,, the Independent values for the mole fractions x, xB„ x and y, ya, yw are consistent with those already cited for the more complex coform having the general formula a® (l-x-x9-x8,1)PbxC£ixBSffx*'ri (l-y-y ’-y, SnySiL'y »H£yM°3 Notwithstanding the chemical composition of the cotorm, 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 sLoichiometric such that the divalent to tetravalent mole ratio of the varyingly composed coforms is 1.000 r 0.015 regardless of the n urn be?, and mole percent of any divalent and tetravalent cation substitions. Nonsfcoichiometric compositions, where the divalent to tetravalent cation mole ratio of the varyingly composed coforms is in the range of 0.9 fco 1.1» can also be produced. The mean, primary particle size of the barium titanate based coforms ia In the range of 0.05 fco 0.4 siorons» 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 distribution curve of the coform particles has a quartile 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 dielectric 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 & wide variety of compositions of such BaTiO^ based coforms having primary pas tide sizes which can be controlled in the size range of 0.05 up to about 0.4.^/m.

Xt is another object of the present invention to provide a wide variety of coforms which are synthesizable by a single general hydrothermal process. xnvcntion coform It is another object of the present a stoichiometric barium titanate based substantially free of mill media.

It is another object of the present a coform of barium titanate containing a variet shifts and/or broadens the CsArie point to the regions and reduces the temperature dependence formed. to provide which is invention to provide y of additives which desired temperature of the dielectric so It is another object of the present invention to provide dispersible BaTiO^ based dielectric composi t. ions 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 costs idee: ably 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. 1 is a transmission electron micrograph nt 50,000x magnification of a stoichiometric, dispersible, submicron complex coform according to the present invention having the general formula BiS0.056?b0.097Ca0.074Ti 0.830Zr0.099Sn0.071°3; and Fig. 2 is a transmission electron micrograph at 50,000x magnification of pure barium titanate powder which exhibits a morphology substantially similar to the morphology of the complex coform of Fig. 1. overall prefer r general Ba , ·, (1-xwherein c.ati ons At the outset, aspects,, with a ed embodiment of type the invention more detailed the present i is described in its broadest description following. The nvention is a coform of the [«.,,η^χ^,δΓ^Τΐ p_y-y’-y)SnySys^iy"03 x, Xs and x*5 represent the mole fractions of the divalent x and x’ have independent values which are greater than zero and less than 0.3 and3 more oreferably greater than zero and less than 0.2 while x can have values ranging from 0 to 0.3 and the sum of x + x' + x can have values ranging from 0 to 0.4 and more preferably from 0 to 0.3s vs y' and y represent the mole fractions of the tetravalent cations and have independent values ranging from 0 to 0.3 and3 more preferably., from 0 to 0.25 and the sum c£ y ·? y" + y have values ranging from 0 to 0»4 and, more preferably, from 0 to 0.3.

When the suras of (x + χ· 4- χ) and (y + y9 + ·/) both equal zero the coforra simply constitutes barium titanate powder.

When x » x ryK y =y" E 0 and x* is greater than 0, the resulting product is a barium titanate based coform where x mole fractions of Sa(iX) in BaTiO-g have been replaced by Cajll, to give a product with the nominal formula Baf„ „,.Ca„.TiO·,. Conversely,, when I jil - x ) x .4 than zerOt the coform has X9 --- X- = y =y’ the composition Since -y" = 0 and x is greater Dn(l-x,PbxTi03« 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 o£ 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 a divalent metal ion and, optionally, tetravalent metal ion substitutions. The divalent barium ion,optionally partially replaced by strontium, can be partially replaced by either lead, calcium, or mixtures thereof.

-The tetravalent titanium ion of the coform 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 calcium titanate as well as more complex coforms including barium lead stannate titanate and barium lead strontium stannate zirconate titanate. 0£ course, the selection of the divalent and/or tetravalent cation replacement anti the mole percent of the substitution is dependent upon whether the Curie temperature is desired to be raised or lowered as well as by whether the Curse 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 coforms 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 4 0,015, avers when both the divalent and tetravalent ions have been replaced by one or more other ions.

The narrow particle size distribution and submicron size of the barium titanate based dielectric compositions make the coforms of the present invention particularly attractive for further 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 silverpalladium alloy can be increased as the sintering temperature is reduced. In addition, since these BaTiO-j based diolectt ic compositions are all dispersible and have few aggregates exceeding a size of 1 micron, they can be employed in the formation of dielectric films of reduced thickness. Hence, the spherical, unaggregated, submicron and narrowly distributed barium titanate dielectric coform powder of the present invention should be particularly wall 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 inhomogenities are an advantage. in these instances, the availability of products with varying primary particle site can be utilized to produce a dispersion os 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 nn’croinhomogeneities. 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 titanate 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 atc advantageous.

The preferred approach for producing the barium titanate based coforms is to heat slurries containing the hydrous tetravalent oxides with selected divalent oxides or hydroxides. After formation of the divalent titanates, the slurry still contains substantial quantities of hydrous TiO^ and/or hydrous SnC^, ZrO9 or The slurry temperature and concentration are then adjusted and a stoichiometric excess of ba (Oil) 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 complex coforms of the present invention having the formula B®f^x-x'-xl,PbxCaxlSrxllTil~y-y,-y,,ZrySiy Hfy03 are similar to those of BaTiO3 produced by a similar process. This becomes readily apparent from the transmission electron micrograph of the complex coform ^ao.856pbO.G97Cs0.07^*I’i0.8307'‘ 0.099SnG»071°3 in i?ig„ 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 0.10 microns with a narrow size distribution. A comparison ox 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 tctravalent 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 he reduced to the specified range by minor variations in the synthesis conditions without 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 was used to determine product primary particle size and primary particle size distribution. 500 to 1000 particles wore sized in a plurality οί 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 equivalent 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 bo 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 OR value of 1 and, for our testing purposes, products with QR values ranging from 1.0 to about 1.5 were classified as having narrow size distributions, those with OR values ranging from 1.5 to about 2.0 were classified as having fairly narrow distributions while those with values substanfially greater than 2.0 were classified as having broad size distributions, The «suartile ratio of the barium titanate coforms indicating that the primary particles have a narrow size distr ibution.

Surface areas were calculated from the coiorm’s primary particles and were found to be consistent with the surface areas determined by nitrogen adsorption, indicating that the primary particles are essentially nonporoun. ϊη cases where the surface area substantially exceeded the TEM surface area, it was found that the difference could be readily accounted Cor by the presence oi unreacted high surface area hydrous oxides.

Since th® coiorms oi 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 where D is particle diameter (microns)/S> is density (g/cc) and S is surface area (m /g) , 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 oi 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, KJ.K.) and the Micromer i tics Sedigraph (Norcross, Georgia) were employed to determine cumulative mass percent distributions in terms of Stokes diameters from which the median Stokes diameters and the QK values were calculated.

In determining particle size by sedimentation, the powders were dispersed by a 15 to 30 minutes sonification in either water containing 0.08g/L sodium tripolyphosphate at pH 10 or in isopropanal containing 0.08 or 0.12 weight percent Smphos PS-2!A (Witco Organics Division, 520 Madison Ave., Sew York).

Since particle size determined by image analysis and by sedimentation depend on different principles, an exact correspondence iiirti size by these 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 o£ some bonding (e.g., necking) between the primary particles to give cemented aggregates which cannot be readily broken down during the soni(ication process and because of less than optimum dispersion stability which leads to some £loculation. 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 o£ 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 TEMs were present as single particles. When substantial necking was observed the product was classified as aggregated. Xn each o.£ these tests, 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 dissolution. The precision of the analyses was about *1%. The mole ratio oi divalent cations to tetravalent cations of the coforms, regardless o£ 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 coiorms of stoichiometr ic.

The unique properties coforms are further illustrated examples. the present invention are of the barium titanate based by the following non-limiting Reagent grade chemicals or their equivalents were used throughout the Examples. The reagent grade employed contained 1 mole percent Sr. Experiments have shown that Sr (II) is more readily incorporated than Ha(IX) in the cofoini. for this reason all coforms described here contain Sr(11). This cation represents about I mole percent of the total divalent cation content of the coform. For simplicity, the Sr(11) mole fraction has been included in the Ba(II) mole fraction. ha(0il)2 and/or Sr (Oil) 2 solutions, maintained at 70-100°C, were filtered prior to use to remove any carbonates present. CaCOj was calcined at 800°C to give CaO. The latter compound when contacted with water gives Ca(Oh)2» PbtOHJj was prepared by neutralizing a solution with aqueous NH-. The washed hvdroxsde wet cake was used in subsequent experiments. hydrous oxides of TiO^,, SnO^ and ZrOj prepared by neutralizing aqueous solutions of their respective chlorides with aqueous MHj at ambient temperatures. The products were filtered off and washed until chloride-free (as determined by AgEsSO^) filtrates were obtained. The surface areas of the hydrous oxides, determined after drying at 110 G, were about 300, 290 and 150 m*/g for TiC^e SnO-j and ZrO^, respectively. In addition coprec ipi tales of hydrous TiO^ and ZrO9 or hydrous TiO, and SnOo 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 all wetted parts of the autoclave were coated with polytetraflljo roe thy lens and every effort was made u0 allude C02 from all parts of the system. Ba(0H)2 or BafOH)2 and Sr(0H)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, into 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 treatment of 0.54 L of a slurry containing 0.20 moles of hydrous TiO2 and 0.04 moles of Ca(01l)2 to 200°C. The slurry was cooled and 0.45 L of 0.41H BaiOiOj was added to the slurry at 120°C„ The resulting slurry temperature was raised to 150°C 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.

Ba S/k Ca Ca; Cation- Mole Ratio in Solids Sr Tr 2,52 0.445 0.127: 0.042: 0.019: 1.00 Divalent/ Tetravalent Cation Mole 8 0.908 N Area 12.0 Primary Part Icis Size (TEM) 0.15 micron Size Distribution Narrow Example 2 Λ lead containing coform was prepared by hydrothermal treatment of 0.54 L of a slurry containing 0.2 moles of hydrous T'Oj and 0.04 moles PbO. 0.45 L of Ba(OI!)2 was added to the slurry „ *3 e J\O« cF. . was held at 150°C for 50 minutes and then temperature for complete conversion of the the perovski te structures . The slurry was sampled and characterized. The results obtained are as follows: Filtrate 3/Ρ Ba Pb Ba Cation Mole Hat io in Sol ids_ ; Ph: Sr: Tl Divalent/ Tetravalent Cat ion Mole Ratio .6 2.74 0.810: 0.173: 0.024s 1.000 1.007 Primary Particle Size (TEM) 0.07 micron Size Di sfcr i but ion Marrow Example_3 Complex coforms are formed in which the Ba{11) and Ti(IV) in nal'iO-j are partially replaced by one or more divalent and tetravalent cations. A preheated BatOH)^ solution was introduced into slurries heated to 150°C or 120°C containing the tetravalent hydrous oxides and presynthesized perovskites of Pb(11) and/or Ca(ll). 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: Cation Mole Ratio in Solids Ba : Pb : Ca : Ti : Zr : Sn Divalent/ Tetravalent Cation Arses Mole Ratio iti /g Sample 1 Sample 2 Sample 3 0.908:0.090:0.000:0.904:0.096:0.000 0.001:0.000:0.123:0.881:0.119:0.000 0.856:0.097:0.074:0.830:0.099:0.071 0.990 8.0 1.0C1 12.2 1.028 9.0 Primary Particle Si 7.C Size (TEM) D i s t r i but ion Sample 1 0.14 micron Very 5 Harrow Sample 2 0.2 microns Mar row Sample 3 0.2 microns Mar EQW Image Analyst s Sedimentat ion size (microns) QH size (microns) ±>K Sample 1 0.12 1.33 0.2« 2.2 Sample 2 0.19 1.31 0.2« 1.6 Sample 3 0.18 <1 a ffl «· «I 0.2« 1»5 The quantative data for samples 2 and 3 corresponds well with the estimated particle size, size distribution and dispersibility data drawn from the transmission electron micrographs. Sample 1, however, as assessed by the QR value is only moderately dispersible Nevertheless, the sedimentation 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 ί 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.

Claims (5)

1. A barium titanate based coform comprising substantially spherical particles having the formula Ba (l-x')^ a x' Tl (l-y-y'-y)^ n y Zi y' H, y B 3 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 1 is greater than zero and 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 in the range from 1.0 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 2.2.

2. The coform of barium titanate of claim 1 wherein the mole ratio of (Ba + Ca)/(Ti +Sn + lr + Hi) 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 (i-x) Pb x Ti (l-y-y'-y) Sn y Zf y ,Hl y°3 wherein y, y' and v have independent values ranging from zero to 0.3, the sum of y + v 1 + y is less than 0.4, and x is greater than zero and 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 in the range from 1.0 to 1.5. - 22 (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 2.2. 5. The coform of barium titanium of claim 4 wherein the mole ratio of (Ba + Pb)/(Ti + Sn + Zr + Hi) is in the range between 0.9 and 1.1.

5. The coform of barium titanate of claim 4 wherein the mole ratio of (Ba + Pb)/(Ti 4- $n * Zr * Hf) is 1.000 + 0.015.

7. A barium titanate based coform comprising substantially spherical particles having the formula Ba (l-x-x , -x'') l>li x ( ' a x' Sr x Tl (l-y-y , -y“) Sn y Z, 'y ,Hf y 0 3 wherein x, x' and x, y, y* and y each have independent 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* + v 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 in the range from 1.0 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 2.2.

8. The coform of barium titanate of claim 7 wherein the mole ratio of (Ba + Ca * Pb + Sr)/(Ti + Sn ·< Zr 4- Hf) is within the range between 0.9 and 1.1.

9. The coform of barium titanate of claim 7 wherein the mole ratio of (Ba * Ca -s-pb 4-Sr)/(Ti * Sr +Zr *Hf) is 1.000 + 0.015.

10. A barium titanate based coform as claimed in claim 1 and substantially as herein described with reference to any of the Examples.