IE871157L - 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

60287 This invention relates to barium titanate based dielectric compositions andP more particularlyt relates to stoichiometric dispersible, stibinicron barium titanate or coforms, with very narrow particle size distributions.

The high dielectric constant and strength o£ 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 oE mined 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 type- This crystal structure consists of a regular array of corner-sharing oxygen octahedra with smaller titanium (IV) cations occupying the central octahedral H site and bar I. um (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 mult iple cation substitutions at both the A 2nd 0 sites so that many more complex ferrolectric compounds can be easily produced, Barium titanate's relatively simple lattice strucluie is characterised by the TiO^-octahedra which, because of their high polar inability? essentially determine the dielectric properties of the structure. The high polarisabi1ity is due to the fact th.it the small Ti(IV) ions have relatively more spacer within the oxygen octahedra.. This cubic unit eel 1 .• however, is stable only above the Curij point temperature of about 130°C. Eelow 130°C,r the Ti(sv) ions occupy o££-ccnter positions. This transition to the of£- -1- eenfcer position results in a Changs in crystal structure from cubic to tetragonal between temperatures of 5°C and I30°Ce to orthorhombic between -90°C and 5°C and finally to rhombohadral at temperatures less than ~90°C„ Heedless to say? 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 oz the temperature dependence o£ the dielectric constant and its relatively low value at room temperature, pure BaTiOj is rarely used In the production o£ commercial dielectric compositions. Hence, in practice, additives are esaployed 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 stsonfciism 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 IS,000 at room temperature. Alternatively, the Curie temperature can be increased by .3 partial substitution of lead (II) for barium. Additionally, the substitution of small amounts of other metallic ions of suitable si a© but with valencies which are different to those of barium and titanium, as summarized in B. Jaffee, W. 8. Cook, Jr. and H. Jaffe, "Piezoelectric Ceramics", Academic Press, S.y. 1971, can cause profound changes in the nature of the dielectric properties. . .

In commerical practice, barium titanate based dielectric powders are produced either by blending the required pure tltanatea, zirconates, stannates and dopants or by directly producing the desired dielectric powder by a high temperature solid state reaction of an intimate mixture of the appropriate stoichiometric amounts of the oxides ct oxide precursors (e,g,, carbonates* hydroxides or nitrates} ©f barium# calcium, titanium. etc0 The pare titanates* sirconafcesj. stannales* etc. ere also,, typically t, produced by a high temperature solid phase reaction process™ isi such calcination processes the required reactants are wet sallied to accomplish the formation of an intimate mixture™ The resulting slurry is dried and calcined at elevated temperatures^ ranging from about 700 to 1200°CL, to attain the desired solid state reactions® Thereafter * the calcine is remilled to produce a dispersible powder Cor use in making green bodies.

Although the barium titanate based dielectric formulations produced by solid phase reactions are acceptable Cor 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 inhomogenieties 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 interpar tide sintering ce^ occur™ As .3 consequence* the milled product consists of Irregularly shaped fractured aggregates which have a wide particle sise distribution! ranging from about 0.2 up to 10 microns™ Published studies have shown that green bodies formed from 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 sinter ing temperatures rather than higher sintering temperatu&es* since the percentage o£ lower cost silver in the-'silver-palladium electrode / can be increased as the sintering temperature is reduced. .

As is Known in the act* 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 4 substantially reduced because as layer thickness is decreased the number of defects in the dielectric film,, such as pin holes 0 increases* The defects adversely affect the performance of the capacitor,, One major source of such defects is the presence of undispensed aggregates having sizes comparable with the film thickness. During sintering,, because o£ the presence of such aggregates, non-uniform shrinkage occurs and pin holes are formed. Hencen utilization of barium titanate based dielectric formulations formed by solid state reactions signi£icantly increases the overall menu Cactus: 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 include the thermal decomposition of barium titanyl okelate and barium titanyl citrate and the high temperature oxidation of atomised solutions of either barium and titanium aicoholates dissolved in alcohol or barium and titanium lactates dissolved in fe?ster„ In addition,, barium titanate has been produced fjrom molten salts, by hydrolysis of barium and titanium aJkosides dissolved in alcohol and by the reaction of barium hydroxide with eltania both hydrothermally and in aquenuE 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, D. J. Mulder discloses in an article entitled "Preparation of BaTiOj and Other Ceramic Powders by Coprecipitat ion of Citrates in an Alcohol™, Ceramic bulletin, 49, No. 11, 1970, pages 950-59 3., that ESaTiO^ based compositions or coforms can be prepared by a coprecipitation process. In this process aqueous solutions of Ti(IV)e Z-; (IV J and/or Sn(IW) citrates and formates of Qa(XI), Mg (XI), Ca(XI) , St(XXJ and/or Pb(X X) are sprayed into alcohol to effect copcecipitation. The precipitates are decomposed by calcination in a stream o£ air diluted with at 5 "00-800°C to give globular and rod shaped partic3.es having an average sixe of 3 to .10 microns- Barium titandte based coforms have been prepared by precipitation and subsequent calcination of missed alkali metal and/or Pb(II) titanyl and/or sirconyl oxalates as disclosed by Gallagher' et , al. in en article entitled "Preparation of Serai-Conducting Titanates by Chemical i'iethods", J. Amer . Ceramics Soc„ 4 5, No„ 1553 pages 3S5-355, These workers demonstrated that BsTiO^ 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 Est {IV) in the range of 0 to 20 mole percent may be produced.

Faxon et al. discloses in U.S. Patent No. 3,637,531 that based coSorsas can be synthesized by heating a solution o£ 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 employee! 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 fragments with wide size distributions.

The prior art has also attempted to circumvent the disadvantages of conventionally prepared BaTiO^ powders by synthesizing a mixed alkaline earth titanate-sirconate composition through a molten salt reaction™ Such a process is disclosed in ' U.S. Patent No. 4 r 293,534 to.Arendt. In the practice of this process titania or zicconia or mixtures thereof and barium oxide,, strontium oxide -or mixtures thereof are mixed with alkali metal hydroxides and heated to temperatures sufficient to melt the hydroxide solvent. The reactants dissolve in the molten solvent and precipitate as an alkaline earth titanate, xirconate or a solid solution having the general formula 3axSrn_xjTi Zrp_vj03. The 6 products are characterized as chemically homogeneous, relatively monodisperse, submicron crystallites. This method is limited, however, in that it can only produce Sr and/or 'it containing coforms.

Ilydrothermal processes have also been described in which coforms are produced, Balduzzi and Steinemann in British Patent No,. 7151,7 6 2 heated aqueous slurries of hydrated TiOj with stoichiometric amounts of alkaline earth hydroxide to temperatures between 200 and 400°C to form mixed alkaline earth titanates. Although it was stated that products oz any desired size up to about 100/4/m could be produced,, it is doubtful that, other than in the case of Sir-containing coCorms9 products wish the morphological characteristics o£ this invention could be obtained. This contention is based on the fact that whereas IJa (01!) 2 soluble in aqueous media CalOH)^* and Mg(OH)0? especially in the presence of Be, (Oil) 2 «* are rel ativelv insoluble. Accordingly,,, an the case of Ca-coei ca ining coforms it has been found that under the experimental conditions of Balduzxi and Steineinann that BaTiO^ is first formed and then Ca(01i)7 reacts with the balance of the unreached titania to form CaTiO^ during the heating process to 200 to 400°C.

Matsushita et al. in European patent application No. 34306926.1 (Dublication number 0 141 551 Al) demonstrated that dilute slurries of hydrous titania can be reacted with Ba(0H)2 and/or Sr(0H)p by heating to temperatures up to 110°C to produce either UaTiO^ or Sr-containing coforms. The morphological characteristics of these coforms appear to be comparable with those of this invention. The method,- -however p is again limited to producing only Sr-containing coforms.

A publication of the Sakak Chemical Industry Company entitled "Easily Sinterable DaTiO^ Powder*1, by Abe et «L. discloses a hydrotherma1 process for synthesizing a barium titanate based coform with the formula HaTi ^ Sn^Oj. in this process a 0»SH Tin_ .Sn O, slurry, prepared by neutralising an aqueous 1 fflrt solution of SnOCl2 and TiCl^ , is xctl! with 0.9M 3,; JOH) 2 and 7 subjected to a hydrothermal treatment at 200°C tor at leant five hours. Although not explicitly delineatedP Abe et al. imply the 511) r r y was heated to temperature. Although no description of the coform morphology was indicated, the DaTiO^ product produced by the same process had a surface area of 11 m /g, a particle sixe of 0.1 Jim 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 Da (Oil),, Sr(OJl)2 is quite soluble in aqueous media* However, the process of Abe et al. cannot be used £ or substitution of divalent Curie point shifters such at, 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 end tetravalent cation substitutions which are stoichiometir ic, dispersible, spherical, and submicron with narrow particle sise distributions.

The present invention includes a wide variety of dispersible, coforms of barium titanate which are substantially spherical, stoichiometrics 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 winch the divalent barium is partially replaced by lead, calcium and strontium and the tetravalent titanium is partially replaced by tin, sicconium and hafnium.

In one important embodiment of the present invent Ion, the barium titanate based coform is represented by the general formula 8 Ba (l-x-x'-jt") pfcV,Cax •jScx"Ti (1-y-y -y^) SnySEy 11 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.

In another important embodiment of the present invention, the barium titanate coform is represented by the general formula p _x« jCb>; sTi j, _y_y i _v!B) SnyZty >" H£y.,03 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(l-x)PbxTi(I-v-V-y-)SVryI!fy-°3' wl,erein lead is substituted for the divalent barium. In each ot the latter embodiments,, the independent values for the mole fractions x, xB„ x" and y, y", yw are consistent with those already cited for the more complex coform having the general formula (1-X-X 8 -x") "Slf X (1-v-y * -yH ) SnyZcy «H^y«*°3 - Notwithstanding the chemical composition of the coform,, each of the barium titanate basset! coforms of the present invention possess the same unique chemical and physical properties., The barium titanate based dielectric formulations are sioichiometr sc such that the divalent to tetravalent mole ratio of the varyingly composed coforms is 1-000 + 0.015 regardless of the nuinbet and mole percent of any divalent and tetravalent cation substitions. Non- stoichiometr a c compos it ions „ where the divalent to tetravalent cation mole satio of the varyingly composed coforms is in the range of 0.9 to !. 1, con also be produced. The mean primary particle size of the barium titanate based coforms is in the range of 0.05 to 0.4 Bicirons,, Moreover* the mean particle size determined by image analysis is comparable to the me/«n particle size determined by sedimentation demonstrating that the coforms are dispersible. The 9 size distribution curve of the coform particles has a quae tile 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 ot the dispersible, subra icfcon 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 di spers ible e subaticron 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 BaTiO^ based r.oforms having primary pat tide sises which can be controlled in the size range of Q»05 up to about 0.4.^/m.

St is another object of the present invention to provide a wide variety of coforms which are synthesizable by a single general hydrothermal process.

St is another object of the present invention to provide & .stoichiometric barium titanate based coform which is substantially free o£ mill media.

It is another object ok the present invention to provide © 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 so formed.

It is another object of the present invention to provide dispersible BaTiO^ 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 et consider ably less than conventional temperatures. 10 These and other details an<1 advantages n£ the invention will be described in connection with the accompanying drawings in which: Fig. 1 is a transmission electron micrograph at 50f000x magnification of a stoichiometric, dispersible,, submicron complex coform according to the present invention having the general formula Bn0.056?bO»097CaO„07 4Ti0.830Zr0.09fJSn0.071°3; and Fig. 2 is a transmission election micrograph at 50,000k magnification o£ pure barium titanate powder which exhibits n morphology substantially similar to the morphology of the complex coform of Fly. 1. i 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 na (1-X-Xg -X") PbXCax,Srx,,Ti { 1-y-y «-y")SnyZry sS,£y«,03 wherein and xra represent the mol® fractions of the divalent cations, x and x' have independent values which are greater than zero and less than 0,3 and,, more preferably 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 tetravalant cations and have independent falues ranging from 0 to 0.3 and3 more preferably,, from 0 to 0.25 and the sum e£ y -f y° + y" have values ranging from 0 to 0»<• and,, more preferably,, from 0 to 0.3.

When the suras ot (x + x* * x") and (y + y9 + •/") both equal ssro the* cofocm simply constitutes barium titanate powder.

Wlten x » x" myB y" ey" 13 0 end x* is greater than 0« the resulting product is a barium titanate based coform where x* mole fractions of 3a(IX) In DaTiO-, have been replaced by Ca(X X) to give a product J} with the nominal formula 0ef, „,. Ca„»TflO-». Conversely, when I Jl — X I X .4 11 x9 = x" = y =y11 -~y" = 0 and x is greater than zero, the coform has the composition Da ^ 7 _K j Pb^TiO-j« Since the values of x, x% x*% yt, 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 sixe distr ibution.

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 ion5optionally partially replaced by strontium., can partially replaced by either lead5 calcium9 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 o£ 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 stagnate zirconate titanate. Of course,, the selection of the divalent and/or tetravalent cation replacement and the mole percent of the subst itut ion i e dependent upon whether the Curie temperature is desired to be raised or lowered as well as by whether 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 coforms according to the present Invention ace still uniquely identified by the aforement ioncd 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 an the range of 0.05 and 0.4 microns with narrow size distributions and a divalent to tetresvalent mole ratio of 1.000 4 12 0,015; even when both the divalent and tetravalent ions have been replaced by one our more other ions.

The narrow particle size distribution and submicron size of the barium titanate based dielectric compositions make the coforms o£ the present invention particularly attractive for further application in the production of complex dielectric bodies. Pcior 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 sise distribution. The economic advantage or" 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 an the sintering temperature is reduced, In additions since these UaTiO^ based dielectric compositions are all dispersible and have few aggregates exceeding a sise of 1 micron. they can be employed in the formation o£ dielectric films of reduced thickness. Hence3 the spherical, unaggregated, submicron and narrowly distiibuted barium titanate 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 inhomogeni t ies are an advantage. In these instancesu, 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. j n such applications, the compositional inhomogeneity may be inherent in the barium titanate coform selected or, insteadmay result from a small 13 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 Cor applications where compositional i nhomogenei ties are 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 o£ hydrous Ti02 and/or hydrous SnC^e ZrO, or HfO^. The slurry temperature and concentration are then adjusted and a stoichiometric excess of bafOll^ solution is then added under isothermal conditions. In order to ensuie the complete conversion of the tetravalent oxides to their corresponding oxvanions , 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 Ba&rx-x'-x"PbxCax1 SVTil-y-y'-y"ZrySr|y1 Hfy"°3 are similar to those of BaTi03 produced by a similar process. This becomes readily apparent from the transmission electron micrograph of the complex coform ■Ba0.856pfa0.Q97Co0.074Tl0.830Zr0.099Sn0.071O3 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 sise 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 o£ pure barium titanate in Fig. 2 indicates that the morphologies of the barium titanate based compositions are very similar. Note that in both anicEographs the particles are substantially spherical„ unaggregated, submicron and uniformly sis:ed. Xt may also be noted that the divalent to tetravalent cation mole ratio in this product, 1 „ 0 2 7, is somewhat larger than the value 1.000 + 0.015 specified for stoichiometric products. This ratio can easily be reduced to the specified range bv 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 sis sd in a plurality of TEM fields ir> order to determine the equivalent spherical diameters of the primary particles. Two or more touching particles were visually disaggregated and the sizes o£ the individual primary particles were measured. The equivalent spherical diameters were used to compute the cumulative mass percent . distir ibution as a function of primary particle s i. z c . The median particle size, by weight, was taken to be the primary particle sise of the sample. The quar t ile ratio, QR, defined as the upper quartile diameter {by weight) divided by the lower quartile diameter, was taken as the measure of the width o£ the distribution. Monodisperse products have a QR value of 1 and,, for our testing purposes,, products with QR values ranging from 1.0 to ©bout 1.5 were classified as having narrow size distributions, those with QR values ranging from 1.5 to about 2.0 were classified as having fairly narrow distributions while those with values substantially greater than 2.0 wore classified as having broad si so distributions. 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 pristfitry particles Biave & narrow size diatr ibution.

Surface areas were calculated from the coCorm's primary particles and were found to be consiotent with the surface areas 15 determined by nitrogen adsorpt ion, indicating that the primary particles are essentially nonporous, In cases where the Nj surface area substantially exceeded the TEM surface area? it was found that ' the difference could be readily accounted for by the presence ol unreached high surface area hydrous oxides.

Since the coforms of the present invention have a narrow size distribution, overage primary particle size was readily determined by sizing 20 to 30 particles. It was found that the relationship l)=6/fS, where D is particle diameter (microns) is 2 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 of which coform composition was tested.

Product dispesrs i hi 1 i ty of the coforms was assessed by comparing the primary particle sixes 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, O.K.) and the Microliter i tics Sedigranh (i'.orcross, Georgia) wese employed to determine cumulative mass percent distributions in terras of Stokes diameters from which the median Stokes diameters and the QR values were calculated.

In determining particle sixe by sedimentation, the powders were dispersed by a 15 to 30 minutes Bonification in either water containing 0.08g/L sodium tr ipolyphosphate at pSI 10 or in isopiropanal containing 0.08 or 0.12 weight percent Emphos PS-23A (Witco Organics Division, 520 Madison Ave., Sew York).

Since pair tide sise determined by Image analysis and by sedimentation depend on different principles, an exact correspondence funs sise by these two methods was not always obtained. Moreover, as already noted, in image analysts touching particles are visually disaggregated. In the sedimentation process bound or flocculated particles act as single entities. These entitles arise because o£ the existence o£ some bonding (e.g., neckieig) between the primary particles to give cemented aggregates which cannot be readily broken down during the sonifixation process ®nd because of less than optimum dispersion stability which lends to some Cloculation. 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 Mas in reasonable agreement with the primary particle size determined by sedimentation. The median particle sixe determined varied by no more than a factor of two. This demonstrates that the eoEorias are dispersible.

Two additional measures were used to assess dispersjibi 31 ty. In the first method, the saass fraction of the product having a Stokes diameter greater than one micron was used as & 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 TEH's were present as single particles. When substantial necking was observed the product was classified as aggregated. 2n each of 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 o£ divalent cations to tetravalent cations of the colorms, regardless of the number or mole weight percent o£ the divalent and tetravalent cation + substitutions,^ was 1.000 — 0.013, This sc&t£© indicates that the 17 barium titanate cotorms of the present invention are stoichiometr ic.

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 employed contained 1 mole percent Sr. Experiments have shown that Sir (II) is more readily incorporated than Ba(lX) in the coform. For this reason all coforms described here contain Sr(II). This cation represents about 1 mole percent o£ the total divalent cation content o£ the coform. For simplicity, the Sr(II) mole fraction has been included in the Ba(II) mole fraction. and/or Sr (Oil)solutions, maintained at 70-!00°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(013)2» Pb(0ll)2 w®s prepared by neutralizing a PbJHO^Jj solution with aqueous Nll^. The washed hydroxide wet cake was used in subsequent experiments.

Hydrous oxides of 7i0-,,, SnO^ and XrO? were prepared by neutralizing aqueous solutions of their respective chlorides with aqueous HII^ at ambient temperatures. The products were filtered off and washed until chloride-free (as determined by AgWO-j) filtrates were obtained. The surface areas of the hydrous oxides, o ^ determined after drying at 110 C, were about 300, 290 and 150 m*"/g for TiO^ e SnO^ and ZrC^ e respectively. I n addition coprec ipi tates of hydrous TiC>2 and 2rO, or hydrous TiO, and Sn07 were prepared by neutralising aqueous solutions ol the chlorides of T i (XV) and Sn(IV) or T i(IV) and Zr(IV).

All experiments were performed in a 2 1 Iter Autoclave. To prevent product contamination all vetted parts of the autoclave were coated with p.olytetrafluopQetbylene and every effort was made to allude C02 from all parts of the system. Ba(0H)2 or BaC0H)2 and Sr(OH) 18 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 ware stirred at 1500 RPM throughout the synthesis process.

Example 1 A calcium containing coform was prepared by hydrOthermal treatment o£ 0,64 L of a slurry containing 0,20 molos of hydrour. TiO^ and 0„04 moles of Ca(OII)2 to 200°C. The slurry wan cooled and 0.46 L of 0.41M Da(OH)2 was added to the slurry at 120°C„ The resulting slurry temperature was raised to I50°C and held there for SO 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 stoichiomctry and morphological characteristics was determined.

P * "3 <;*- ? e Cation- Mole Ratio in Solids Ba Ca Cas Ba: Sr ; T i Di valent/ Tetravalent Cat i on Mole Hatio N k r e a _niAZa_ 2 - 62 0,446 0„127 s 0.042s 0-019: 1.00 0.988 12.0 Primary Particle Size Siza (TEM) Dlstr ibut ion 0.15 microi'. Marrow Example 2 h lead containing coform was prepared by hydrothermal treatment of 0.64 iL. of as slurry containing 0.2 moles of hydrous TiOj and 0„04 moles PbO. 0.46 h of Qa(OH)^ was added to the slurry at 150°C» The slurry was held at 150°C for 60 minutes and then raised to an elevated temperature for complete conversion of the tetravalent ©Hides to the perovskite structures. The slurry war. sampled and characterised. The results obtained are as follows: 19 Di val ent/ Filtrate Cation Mole Hatio Tetravalent q/L in Solids _ Cation As;ea Ua Pb Ua: Pb: " Sr: Ti Mole Ratio nr/q 10.6 2.74 0.810: 0.173: 0.024s 1.000 1.007 Primary Particle Si?.e Size ITEM) Di str i but ion 0.07 macron Narrow Example 3 Complex coforms are formed in which the Ua{11) and Ti(IV) in naTiOj are partially replaced by one or more divalent and tetravalent cations. h preheated BatOH)^ solution wan introduced into slurries heated to 150°C or 120°C containing the tetravalent hydrous oxides and prcsynthesized perovskites of Pb(11) and/oi 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 characterised with the following results: Divalent/ Tetravalent Cation Mole Ratio in Solids Cation Ajsci Ba : Pb : Ca s Ti ; £r :Sn Hole Hatio m /g Sample 1 0.908:0.090:0.000:0.904:0.096:0.000 0.990 0.0 Sample 2 0.081:0.000:0. 1 23i0»801;0.119:0.000 1.0C4 12.2 Sample 3 0.856:0.097:0,074i0.830:0.099:0.07 I 1.028 9.0 Sample 1 Sample 2 Sample 3 Primary Par t icl< Size (TEM) 0.14 micron 0.2 microns 0.2 miccons S i 7. c Di str ibut ion Very Narrow Mar row Mar row Imaae Analvsis Sedimentation size (microns) QR aise (microns) J)R_ Sample 1 0.12 1.33 0.24 2.2 Sample 2 0.19 1.31 0.2 4 1.6 Sample 3 0.18 1.25 0.24 1.5 20 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.^ 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. - 21 ~

Claims (10)

1. CLAIMS 1. A barium titanate based coform comprising substantially spherical 5 particles having the formula Ba(l~x,)^ax,Tl(l-y-y'-y")^ny~!y,H,y"®3 wherein y, v' and y" have independent values ranging from zero to 0.3, the sum of y + y1 * y" is less than 0.4, and x1 is greater than zero and less than 0.4 and wherein 10 (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 sue 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 15 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. 20

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 4- Ca)/(Ti + Sn + Zr + Hf) is 1.000* 0.015. 25

4. A barium titanate based coform comprising substantially spherical particles having the formula Ba(l~x)P'\\T^(l-y-y,-y")^ny*ry,Hly"^3 wherein v, y' and y" have independent values ranging from zero to 0.3, the sum of y * v1 * 30 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 35 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 5 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 + Hf) is in the range between 0.9 and 1.1. 10 5.

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

7. A barium titanate based coform comprising substantially spherical particles having the formula 15 Ba(l-x-x,-x,,)PbxCax,Srx"Tl(l-y~y,-y,,)SnyZry,HV°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 + y1 + y" is less than 0.4 and wherein (a) the median primary particle size{ as determined fay image 20 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, 25 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 30 (Ba + Ca * Pb + Sr)/(Ti + Sn * Zr + 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 *Pb *Sr)/(Ti + Sr +Zr +Hf) is 1»000 + 0.015* 35 - 23 -

10. A barium titanate based coform as claimed in claim 1 and substantially as herein described with reference to any of the Examples. Dated this 11th day of Hay, 1987. 20 25 30 5 10 5, Dartmouth Road, DUBLIN 6.