GB2190076A - Barium titanate coforms - Google Patents

Barium titanate coforms Download PDF

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GB2190076A
GB2190076A GB08710244A GB8710244A GB2190076A GB 2190076 A GB2190076 A GB 2190076A GB 08710244 A GB08710244 A GB 08710244A GB 8710244 A GB8710244 A GB 8710244A GB 2190076 A GB2190076 A GB 2190076A
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barium titanate
coform
coforms
mole ratio
particle size
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Jameel Menashi
Robert C Reid
Laurence P Wagner
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Cabot Corp
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Description

GB 2 190 076 A 1
SPECIFICATION
Barium titanate coforms Field of art 5
Th is i nventi o n rel ates to ba ri u m tita nate based diel ectric corn positions a nd, mo re particu 1 a rly, rel ates to stoich iometric, d ispersib le, su bm icro n ba ri u m tita nate or cofo rms, with very n a rrow pa rticle size distributions.
Background of the art 10
The high dielectric constant and strength of barium titanate make it an especially desirable material from which capacitors, condensers, a nd other electronic components can be fabricated. Especially attractive is the fact that bariu m tita n ate's el ectrical properties ca n be contro 11 ed with in a wide ra n ge by mea ns of mixed crystal formation and doping.
The very simple cubic perovskite structure exhibited by barium titanate is the high temperature crystal 15 form for many mixed oxides of the AB03 type. This crystal structure consists of a regular array of corner-sharing oxygen octahedra with smallertitanium (]V) cations occupying the central octahedral B 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 complexferrolectric compounds can be easily produced. 20 Barium titanate's relatively simple lattice structure is characterized bythe Ti06-Octahedra which, because of their high polarizability, essentially determine the dielectric properties of the structure. The high polarizability is due to the factthatthe small Ti(IV) ions have relatively more space within the oxygen octahedra. This cubic unit cell, however, is stable only above the Curie point tem peratu re of about 13WC.
Below 13WC, the Ti(IV) ions occupy off-center positions. This transition to the off center position results in a 25 change in crystal structure from cubic to tetragonal between tem pe ratu res of YC and 13WC, to orthorhombic between -90'C and 50C and finally to rhombohedral attemperatures less than -90'C. Needless to say,the dielectric constant and strength also decrease 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 thetemperature 30 dependence of the dielectric constant and its relatively low value at room temperature, pure BaTiO3 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 be effecting a partial substitution by strontium and/or calcium for barium and by zirconium and/ortin fortitanium, thereby resulting in materials 35 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 (11) for barium. Additionally, the substitution of small amounts of other metallic ions of suitable size but with valenices 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 dielectric properties. 40 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 intimate mixture of the appropriate stoichiometric amounts of the oxides or 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 45 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 at elevated temperatures, ranging from about 700 to 12000C, to attain the desired solid state reactions. Thereafter,the calcine is remilled to produce a dispersible powderfor use in making green bodies.
Although the barium titanate based dielectric formulations produced by solid phase reactions are 50 acceptable for many electrical applications, they do sufferfrom 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 to moisture sensitive properties. Moreover, during calcination substantial particle growth and interparticle sintering can occur. As a consequence, the milled product consists of 55 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 f rom 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 monolilthic multilayer capacitors, there is a substantial economic advantage to employing lower sintering 60 temperatures ratherthan higher sintering temperatures, since the percentages 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 layerthickness is of the order of 25 microns. Although very desirable, this value cannot be substantially reduced because as layerthickness is decreased the number of 65 2 GB 2 190 076 A 2 defects in the dielectric film, such as pinholes, increases. The defects adversely affectthe performance of the capacitor. One major source of such defects is the presence of undispersed aggregates having sizes comparable with the film thickess. During sintering, because of the presence of such aggregates, non-uniform shrinkage occurs and pinholes are formed. Hence, utilization of barium titanate based dielectric formulations formed by solid state reactions sign if icantly increases the overall manufacturing cost of 5 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 thethermal 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 10 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 15 discloses in a article entitled "Preparation of BaTiO3 and Other Ceramic Powders by Coprecipitation of Citrates in an AlcohoV, Ceramic Bulletin, 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(W) andlor SnOV) citrates and formates of Ba(li), Mg(h), Caffl), Sr(I1) and/or PWII) are sprayed into alcohol to effect coprecipitation. The precipitates are decomposed by calcination in a stream of air diluted with N2 at 20 700-800OCto 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 andlor Pb(11) titanyl andlorzirconyl oxalater as disclosed by Gailagher et al. in an article entitled "Preparation of Semiconducting Titanates by Chemical Methods", J. Amer. Ceramics Soc., 46, No. 8,1963 pages 359-365. These workers demonstrated that BaTi03 based compositions in which Ba is replaced by Sr or 25 Pb in the range of 0 to 50 mole percent or in which 11OV) is replaced by Zr(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 BaTi03 based coforms can be synthesized by heating a solution of a titanium chelate or a titanium alkoxide, an alkaline earth salt and a lanthanide saitto form a semisolid mass. The mass is then calcined to produce the desired titanate coform. 30 In each of the prior art references cited above, however, calcination is ernpioyed 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 f ragments with wide size distributions.
The prior art has also attempted to circumventthe disadvantages of conventionally prepared BaTi03 35 powders by synthesizing a mixed alkaline earth titanate-Arconate composition through a molten salt reaction. Such a process is disclosed in U.S. Patent No. 4,293,534to Arendt. 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 sufficient to meItthe hydroxide solvent. The reactants dissolve in the molten solvent and precipitate as an alkaline earth titanate, zirconate or a solid solution having 40 the general formula Ba,,Sr(i-,,)Ti,Zr(,-,)03. The products are characterized as chemically homogeneous, relatively monodisperse, submicron crystallites. This method is limited, however, in that it can only produce Sr andior Zr containing coforms.
Hydrothermal process have also been described in which coforms are produced. Baiduzzi and Steinemann in British Patent No. 715,762 heated aqueous slurries of hydrated Ti02 with stoichiometric amounts of 45 alkaline earth hydroxide to temperatures between 200 and 40WC to form mixed alkaline earth titanates.
Although itwas stated that products of any desired size up to about 1 OO[Lm could be produced, it is doubtful that, otherthan in the case of Sr-containing coforms, products with the morphological characteristics of this invention could be obtained. This contention is based on thefactthat whereas Ba(OW2 is soluble in aqueous media Ca(OH6 and Mg(OH)2, especially in the presence of Ba(OH)2, are relatively insoluble. Accordingly, in 50 the case of Ca-containing coforms it has been found that underthe experimental conditions of Baiduzzi and Steinemann that BaTi03 is first formed and then Ca(01-1)2 reacts with the balance of the unreacted titania to form CaTi03 during the heating processto 200 to 4000C.
Matsushita et al. in European patent application No. 34306926.1 demonstrated that dilute slurries of hydrous titania can be reacted with Ba(OW2 and/or Sr(O11)2 by heating to temperatures up to 11 O'Cto 55 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", byAbe eta]. discloses a hydrothermal process for synthesizing a barium titanate based coform with theformula 60 BaTia,)Sn,,03. In this process a 0.6M Tig,)Snx02 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 200'Cfor atleast five hours. Although not explicity delineated,Abe et al. implythe slurrywas heated to temperature. Although no description of the coform morphologywas indicated, the BaTi03 product produced bythe same process had a surface area of 11 M21g, a particle size of 0.1 1Lm and appeared to be dispersible. Presurnablythe 65 3 GB 2 190 076 A 3 Sn-containing coforms havecomparable morphologies and arethus comparable with those of this invention. However,Abe et al. is limited in that itteaches oniythatSn(IV) can be synthesized into a barium titanate coform. Perhaps, by analogy, it does suggest the useof other tetravalent cations such asZr(IV) and possiblythe use of divalentSr(li), since, like Ba(OH)2, Sr(O1-1)2 isquite soluble in aqueous media. However,the process of Abe et al. cannot be used for substitution of divalentCurie point shifters such as Pb and Caforthe 5 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.
10 Summary ofthe invention
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 a nd/orcalciu m for the divalent barium as wel I as 15 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 ofthe present invention, the barium titanate based coform is represented by the general formula Ba(,-,,-,,'-,,")Pb.,Ca,,'Sr."Ti(l-y-y'- y")SnyZry'Hfy"03 where x,x'and x" represent the mole fractions ofthe divalent cations and have independent values ranging from 0 to 0.3 and the sum x + x'+ x" 20 has a value ranging from 0 to O.4,whiley,y'andy" 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 0.4.
In another important embodiment ofthe present invention, the barium titanate coform is represented by the general formula Ba(l.x')Ca,,'Ti(i-y-y'-y")SnyZry'Hfy"03wherein calcium is partially substituted for the divalent barium cation and in another important embodiment ofthe invention the barium titanate dielectric 25 composition is represented by the general formula Ba(i-x)Pb>,Ti(l-y-y'y")SnyZry'Hfy"03, wherein lead is substituted for the divalent barium. In each ofthe latter embodiments, the independent values for the mole fractions x,x',x" and y,y',y" are consistent with those already cited for the more complex coform having the general formula Ba(i-x-x'-x")PbxCax'Srx'Ti((,-y-y'-y")SnyZry'Hfy"03.
Notwithstanding the chemical composition ofthe coform, each ofthe barium titanate based coforms ofthe 30 present invention possessthe same unique chemical and physical properties. The barium titanate based dielectric formulations are stoichiometric such thatthe divalentto tetravalent mole ratio ofthevaryingly composed coforms is 1.000.t 0.015 regardless ofthe number and mole percentofany divalantand tetravalent cation substitions. Non-stoichiometric compositions, wherethe divalentto tetravalent cation mole ratio ofthe varying ly composed coforms is in the range ofO.9to 1.1, can also be produced. The mean 35 primary particle size ofthe barium titanate based coforms is in the range ofO.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 thatthe coforms are dispersible. The size distribution curve ofthe coform particles has a quartile ratio lessthan or equal to 1.5which establishes that the barium titanate based coforms have a narrow particle size distribution. Additionally significant isthe factthat any ofthe dispersible, 40 submicron barium titanate based dielectric compositions ofthe present invention can be produced bya single, general hydrothermal process.
Accordingly, it is a primary object ofthe present invention to provide a dispersible, submicron barium titanate coform with a narrow particle size distribution.
It is another object ofthe present invention to provide a wide variety ofcompositions ofsuch BaTiO3 based 45 coforms having primary particle sizes which can be controlled in the size range ofO.05 up to aboutOAKm.
It is another object ofthe present invention to provide a wide variety ofcoformswhich are synthesizable by a single general hydrothermal process.
It is another object ofthe present invention to provide a stoichiometric barium titanate based coform which is substantially free of mill media. 50 It is another object ofthe present invention to provide a coform of barium titanate containing a varietyof additiveswhich shifts and/or broadensthe Curie pointto the desired temperature regions and reducesthe temperature dependence ofthe dielectric soformed.
It is another object ofthe present invention to provide dispersible BaTi03 based dielectric compositions which can be used to give dielectric layers of reduced thickness which are substantially defectfree. 55 It is still a furtherobject ofthe present invention to provide a barium titanate based dielectricformulation which uniformly sinters to a high density at considerably less than conventional temperatures.
BriOdescription ofthe drawings
These and other details and advantages ofthe invention will be described in connection withthe 60 accompanying drawings in which:
Figure 1 is a transmission electron micrograph at 50,000x magnification ofa stoichiometric, dispersible, submicron complex coform according to the present invention having the general formula BaO.856Pbo.097CaO.074TiO.830Zro.oggSnO.07103; and Figure2 is a transmission electron micrograph at50,000x magnification ofpure barium titanate powder 65 4 GB 2 190 076 A 4 which exhibits a morphology substantially similarto the morphologyof the complexcoform of Figure 1.
Description of thepreferred embodiment
Attheoutsetthe invention is described in its broadest overall aspects, with a more detailed description following. The preferred embodimentofthe present invention is a coform of the generaltype 5 Ba(i-x-x'-x")PbxCax'Srx"Ti(,,-,'-y")Sn,Zry'Hfy"03 wherein x,x'andx" representthe molefractions of thedivalent cationsand have independent values ranging from Oto 0.3 and, more preferably,from Oto 0.2 andthesurrix + x'- x" can havevalues ranging from Oto 0.4 and more preferablyfrom Oto 0.3,y,y'andy" representthe molefractions of the tetrava lent cations and have independent values ranging from Oto 0.3 and, more preferably,from Oto 0.25 andthesum of y + V'+ y" havevalues rangingfrom Oto 0.4and, morepreferably, 10 from 0 to 0.3.
Whenthesums of (x + x'+ xl and (y + y'+ V") both equal zerothecoform simply constitutes barium titanate powder.When x = x" =y= V'=y"= 0 and x'is greaterthan 0,the resulting product is a barium titanate based coform where x'mole fractions of 13a00 in BaTiO3 have been replaced byCa(I1) to givea productwiththe nominal formula Ba(l,')CaxM03. Conversely,when x'= x" = y =V'=V" = 0 and x isgreater 15 than zero,thecoform hasthe composition Ba(1,)PbxTi03.
Sincethevalues of x,x',x",y,y', andy" can each adoptawide range of values (within thecited 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. 20 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 divalanet barium ion can be partially replaced by either lead, calcium, strontium or mixtures thereof.
Conversely, the tetravalent titanium ion can be partially replaced bytin, zirconium, hafnium or mixtures thereof. Hence, the barium titanate based dielectric compositions of the present invention include simple 25 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 selection of the divalanet andlortetravalent cation replacement and the mole percent of the substitution is dependent upon whetherthe Curie temperature is desired to be raised or lowered as well as by whetherthe Curie peak is desired to be broadened orshifted. Regardless of which of the wide variety of barium titanate based 30 compositions isformed, 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 asthe 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 divalanentto tetravalent mole ratio of 1.000 0.015, even when both the divalent and tetravalent ions have 35 been replaced by one or more other ions.
The narrow particle size distribution and submicron size of the barium titanate based dielectric compositions makethe coforms of the present invention particularly attractive for further application inthe production of complex dielectric bodies. Priorstudies have established that green bodies formedfrom unaggregated powderswith narrowsize distributions will sinter at reduced temperatures and give sintered 40 bodieswith a narrowgrain size distribution. The economic advantage of employing a dielectricformulation with a lowersintering temperature is obvious sincethe percentage of lower cost silver in the silver-palladium alloy can be increasted asthe sintering temperature is reduced. In addition, since these BaTi03 based dielectric compositions are all dispersible and havefew aggregates exceeding a size of 1 micron,they can be employed in theformation of dielectricfilms of reduced thickness. Hence, the spherical, unaggregated, 45 submicron and narrowly distributed barium titanate dielectric coform powder of the present invention should be particularly well suitedfor use in complex dielectric applications requiring sintesing.
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 size can be utilizedto 50 produce a dispersion of two or more powders with differing compositions having either comparable numbers of primary particles orsubstantially different numbers of primary particles. Such dispersions give green bodies, and hence sintered bodies,with controlled degrees of microinhomogenities. In such applications, the compositional inhomogenity may be inherent in the barium titanate coform selected or, instead, may resuitfrom a small amount of a barium titanate coform with a selected composition being 55 added to a barium titanate dispersion in orderto achievethe desired compositional inhomogenity. Since eitherdivalent barium and/ortetravalenttitanium dificient coforms can beformed according tothis invention the barium titanate based compositons of the present invention are also well suited for applications where compositional inhomogenities are advantageous.
The preferred approach for producing the barium titanate based coforms is to heat slurries containing the 60 hydrous tetravalent oxides with selected divalent oxides or hydroxides. Afterformation of the divalent titanates,the slurry still contains substantial quantities of hydrous Ti02 and/or hydrous Sn02, Zr02 or Hf02.
The slurry temperature and concentration are then adjusted and a stoichiometric excess of Ba(01-1)2 solution is then added under isothermal conditions. In orderto ensure the complete conversion of thetetravalent oxides to their corresponding oxyanions, the slurry is preferably taken to a final, higher heattreatment. 65 GB 2 190 076 A 5 The primary particle size and size distribution of the present invention are achieved whetherthe barium titanatecoform is simply BaTi03 or instead is the more complex coform having the formula Bal-,x',"PbxCax'Srx"Tii-y-y'-y"ZrySny'Hfy"03. This becomes readily apparent from the transmission electron micrograph of the complex coform Bao.856Pbo.o97CaO.074TiO.830Zro.()99SnO. 07103 in Figure 1 which shows the presence of predominantly single, substantially spherical primary particles, although a few firmly bound 5 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 Figure 1 with the transmission electron micrograph of pure barium titanate in Fig ure2 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 thatthe divalent 10 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 variations in the synthesis conditions without affecting morphology.
In order to evaluate the physical and chemical properties of the barium titanate based coformsaccording to the present invention, a variety of laboratory tests were performed. Image analysis was used to determine 15 product primary particle size and primary particle size distribution. 500 to 1000 particles 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 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 be the 20 primary particle size of the sample. The quartile ratio, QIR, defined as the upper quartile diameter (byweight) divided by the lower quartile diameter, was taken as the measure of the width of the distribution.
Monodisperse products have a QRva I ue of land, for our testing purposes, products with QRvalues ranging from 1.0 to about 1.5 were classified as having narrow size distributions, those with QRvalues ranging from 1.5 to about 2.0 were classified as having fairly narrow distributions while those with values substantially 25 greater than 2.0 were classified as having broad size 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 primary particles have a narrow size distribution.
Surface areas were calculated from the coform's primary particles and were found to be consistentwith the surface areas determined by nitrogen adsorption, indicating the primary particles are essentially nonporous. 30 In cases where the N2 surface area substantially exceeded the TEM surface area, it was found thatthe difference could be readily accounted for by the presence of unreacted high surface area hydrous oxides.
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. ltwas found thatthe relationship D=6/pS, where D is particle diameter (microns) p is density (g/cc) and S is N2 surface area (m 2/g), could be used to obtain a good 35 measure of the coformprimary particle size. According to thisformula itwasfound thatthe bariumtitanate based coforms have a primary particle size in the range between 0.05 and 0.4 microns, regardless of which coform composition wastested.
Product dispersibility of the coformswas assessed by comparing the primary particle sizes and size distributions determined by image analyseswith the comparable values determined bysedimentation 40 procedures. The sedimentation process gives the particle Stokes diameterwhich, roughly, correspondsto 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 employedto determine cumulative mass percent distributions in terms of Stokes diameters from which the median Stokes diameters and the OR valueswere calculated. 45 In determining particle size by sedimentation, the powders were dispersed by a 15 to 30 minutes sonification in eitherwater containing 0.08g/Lsodium tripolyphosphate at pH 10 or in isopropanal containing 0.08 or 0.12 weight percent Emphos PS-21A (Witco Organics Division, 520 Madison Ave., NewYork).
Since particle size determined by image analysis and by sedimentation depend on different principles, an exact correspondence in size bythese two methodswas not always obtained. Moreover, as already noted, in 50 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 particlesto give cemented aggregateswhich cannot be readily broken down during the sonification process and because of less than optimum dispersion stabilitywhich leadsto some floculation. Thus,the OR values determined by sedimentation, as expected, were somewhat largerthan 55 thosefound by image analysis.
In the barium titanate based coforms of the present invention, the primary particle size determined by image analysiswas in reasonable agreementwith the primary particle size determined by sedimentation.
The median particle size determined varied by no morethan a factor of two. This demonstrates thatthe coforms are dispersible. 60 Two additional measureswere used to assess dispersibility. In thefirstmethod, the massfraction of the product having a Stokes diameter greaterthan one micron was used as a measure of the amountof hard-to-disperse aggregates. In the second method, a productwas classified as being dispersible if the bulk of the primary particles in theTEM'swere present as single particles. When substantial necking was observed the product was classified as aggregated. In each of these tests, the barium titanate based coforms 65 6 GB 2 190 076 A 6 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 :t 1 %. The mole ratio of divalent cations to tetravalent cations of the coforms, regardless of the numberor mole weight percent of the divalent and tetravalent cation substitutions, was 1.000 0.015. This ratio 5 indicates thatthe barium titanate coforms of the present invention are stoichiometric.
The unique properties of the barium titanate based coforms are further illustrated bythefollowing non-limiting examples.
Reagent grade chemicals ortheir equivalentswere used throughoutthe Examples. The reagentgrade Ba(OH)2.8H20 employed contained 1 mole percent Sr. Experiments have shown that Sffil) is more readily 10 incorporated than Ba(I1) in the coform. Forthis reason all coforms described here contain SrOl). This cation represents about 1 mole percentof thetotal divalentcation content of the coform. For simplicity, the Sr(I1) molefraction has been included in the Ba(I1) mole fraction. Ba(01-1)2 andlor Sr(O1-1)2 solutions, maintained at 70-1000C, werefiltered priorto useto remove any carbonates present. CaC03was calcined at 800'Cto give CaO.The lattercompound when contacted with water gives Ca(OH)2. Pb(O11)2was prepared by neutralizing a 15 Pb(NW2 solution with aqueous NH3. Thewashed hydroxide wet cakewas used in subsequent experiments.
Hydrous oxides of Ti02, Sn02 and Zr02were prepared by neutralizing aqueous solutions of their respective chlorides with aqueous NH3 at ambienttemperatures. The products were filtered off and washed until chioride-free (as determined byAgNO3) filtrateswere obtained. The surface areas of the hydrousoxides, determined afterdrying at 11 O'C,were about380,290 and 150 M2/g forTi02, Sn02 and Zr02, respectively. In 20 addition coprecipitates of hydrousTi02 and Zr02 or hydrousTi02 and Sn02were prepared by neutralizing aqueous solutions of the chlorides of Ti(IV) and Sn(IV) orTi(IV) and Zr(IV).
All experimentswere performed in a 2 literAutoclave. To prevent product contamination all wetted parts of the autoclavewere coated with Teflon and every effortwas madeto exclude C02from all parts of thesystem.
Ba(01-1)2or Ba(Offi2 and Sr(O1-1)2 solutions were introduced into the autoclave either by means of a high 25 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 autoclavewere stirred at 1500 RPM throughoutthe synthesis process.
Example 1 30
A calcium containing coform was prepared by hydrothermal treatment of 0. 64 L of a slurry containing 0.20 moles of hydrous Ti02 and 0.04 moles of Ca(01-1)2to 20WC. The slurry was cooled and 0.46 L of 0.41 M Ba(01-1)2 was added to the slurry at 1200C. The resulting slurry tem peratu re was raised to 1500C 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 35 determined.
Divalentl Filtrate Cation Mole Ratio Tetravalent g1L in Solids Cation NArea 40 Ba Ca Ca: Ba: Sr. Ti. Mole Ratio M21g 2.62 0.446 0.127: 0.842: 0.019: 1.00 0.988 12.0 PrimaryParticle Size 45 Size (TEM) Distribution 0.15 micron Narrow Example 2 50
A leading containing coform was prepared by hydrothermal treatment of 0. 64 L of a slurry containing 0.2 moles of hydrousTi02 and 0.04 moles PbO. 0.46 L of BaWl-1)2 was added to the slurry at 1 500C. The slurrywas held at 150'Cfor60 minutes and then raised to an elevated temperature for complete conversion of the tetravalent oxidesto the perovskite structures. The slurrywas sampled and characterized. The results obtained are asfollows: 55 Divalentl Filtrate CationMoleRatio Tetravalent 91L in Solids Cation Area Ba Pb Ba: pb: Sr. Ti. Mole Ratio m21g 60 10.6 2.74 0.810: 0.173: 0.024: 1.000 1.007 11.5 Primary Particle Size Size (TEM) Distribution 0.07 micron Narrow 65 7 GB 2 190 076 A 7 Example 3
Complex coforms are formed in which the Ba(I1) andTi(IV)in BaTi03 are partially replaced byoneormore divalent and tetravalent cations. A preheated Ba(C1-1)2 solution was introduced into slurries heated to 150'Cor 1200C containing the tetravalent hydrous oxides and presynthesized perovskitesof Pb(11) and/or Ca(li). After holding attemperaturefor about 20 to 30 minutes, the slurries were raised to a final temperature to ensure 5 that the tetravalent hydrous oxides converted to stoichiometric perovskites. The resulting si u rry was characterized with the following results:
Divalent Tetravalent 10 Cation Mole Ratio in Solids Cation Area Ba: Pb: Ca: Ti Zr Sn Mole Ratio M21g Sample 1 0.908 W.090 W.000 W.904 W.096 W.000 0.998 8.0 Sample2 0.881:0.000 W.123 W.881 W.119 W.000 1.004 12.2 15 Sample3 0.856 W.097 W.074 W.830 W.099 W.071 1.028 9.8 PrimaryParticle Size Size (TEM) Distribution 20 Samplel 0.14micron Very narrow Sample2 0.2 microns Narrow Sample3 0.2 microns Narrow ImageAnalysis Sedimentation 25 size (microns) OR size (microns) OR Sample 1 0.12 1.33 0.24 2.2 Sample2 0.19 1.31 0.24 1.6 Sample3 0.18 1.25 0.24 1.5 30 The quantative data forsamples 2 and 3 correspondswell with the estimated particlesize, size distribution and dispersibility data drawn from thetransmission electron micrographs. Sample 1, however, asassessed bythe QRvalue is only moderately dispersible. Nevertheless, the sedimentation data indicatesthat lessthan 5weight percentofthe material is present as aggregates having a size greaterthan 1 micron. 35 Itcantherefore be seen from the preceding examples and disclosure, that the coforms ofbariumtitanate encompassed bythe present invention includethose dielectric compositions containing calcium and/orlead or multiple replacements for either or both ofthe divalent barium and tetravalenttitanium cationswhich are uniquely characterized in thattheyare spherical, have a primary particle size inthe rangefrom 0.05to 0.4 microns, a divalent to tetravalent mole ratio of 1.000 0.015, and a narrow particle size distribution. No prior 40 art barium titanate based dielectric compositions which include calcium, lead orthe complexforms disclosed herein possessthese unique morphological and chemical characteristics.

Claims (13)

  1. 45 1. A barium titanate based coform comprising substantially spherical particles having the formula Ba(,->,')Cax'Ti(l-y-,'-y")SnyZr,'Hf,"03, wherein y, y', and y" have independent values ranging from zero to 0.3, the sum ofy + y' + y" is less than 0.4, and x'is greaterthan zero and less than 0.4.
  2. 2. The coform of barium titanate ofelaim 1 wherein the mole ratio of (Ba + Ca)/(Ti + Sn + Zr + 1-1f) is inthe range between 0.9 and 1.1 50
  3. 3. The coform of barium titanate ofclaim 2 wherein the mole ratio of (Ba + Ca)/(Ti + Sn + Zr + Hf) is 1.000 0.015.
  4. 4. A barium titanate based coform comprising substantially spherical particles having the formula Ba(,,)PbxTi(i-y-y'-y")SnyZry'Hfy"03, wherein y, y'and y" have independent values ranging from zero to 0.3, the sum ofy + y'+ y" is less than 0.4, and xis greaterthan zero and less than 0.4. 55
  5. 5. The coform of barium titanate ofciaim 7 wherein the mole ratio of (Ba + Pb)/(Ti + Sn + Zr + Hf) is inthe range between 0.9 and 1.1.
  6. 6. The coform of barium titanate ofclaim 5 wherein the mole ratio of (Ba + Pb)/(Ti + Sn + Zr + 1-1f) is 1.000 0.015.
  7. 7. A barium titanate based coform comprising substantially spherical particles having the formula 60 Ba(i-x-x'-x")PbxCax'Srx"Ti(l-y-y'-y")SnyZry'Hfy"03, wherein x, x'and x", y, y'and y" each have independent values greaterthan zero and lessthan 0.3Ahe sum ofx + x'+ x" is lessthan 0.4 and the sum ofy + y'+ y" is less 8 GB 2 190 076 A 8 than 0.4.
  8. 8. Thecoform of barium titanate of claim 7whereinthe mole ratio of (Ba + Ca + Pb + Sr)l(Ti + Sn + Zr+ Hf) is within the range between 0.
  9. 9 and 1.1 9. Thecoform of bariumtitanateof claim8whereinthe mole ratio of (Ba + Ca + Pb + Sr)/M + Sn + Zr+ Hf) iswithin the range between 1.000 0.015. 5
  10. 10. The coform of barium titanate of anyone of claims 11- 9 wherein the mean primary particle size of the coform is in the range of 0.05 and 0.4 microns.
  11. 11. The coform of barium titanate of anyone of claims 1 - 9 wherein the primary particle sizes determined by image analysis and by sedimentation agree within a factor of two.
  12. 12. The coform of barium titanate of anyone of claims 1 - 9 wherein the coform has a narrow particle size 10 distribution and the primary particle size distribution curve for the coform has a quartile ratio less than or equal to 1.5.
  13. 13. A barium titanate based coform substantially as herein described with reference to the Examples.
    Printed for Her Majesty's Stationery Office by Croydon Printing Company (U K) Ltd,9187, D89916B5. Published by The Patent Office, 25 Southampton Buildings, London WC2AlAY, from which copies maybe obtained.
GB8710244A 1986-05-05 1987-04-30 Barium titanate coforms Expired - Fee Related GB2190076B (en)

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FR2619370A1 (en) * 1987-08-12 1989-02-17 Cabot Corp COMPOSITIONS BASED ON BARIUM TITANATE DOPE
GB2193713B (en) * 1986-07-14 1990-12-05 Cabot Corp Method of producing perovskite-type compounds.
WO2009136320A1 (en) * 2008-05-08 2009-11-12 Nxp B.V. Tunable capacitor

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DE3745158C2 (en) * 1986-07-14 2003-10-02 Cabot Corp Divalent cation perovskite-type cpds. with controlled particle size
DE3723082C2 (en) * 1986-07-14 2003-09-25 Cabot Corp Process for the production of submicron perovskites
JPH02263528A (en) * 1989-04-05 1990-10-26 Ishikawajima Harima Heavy Ind Co Ltd Transmission shaft fixing device for transfer feed press
US5405554A (en) * 1990-02-08 1995-04-11 Cytec Technology Corp. High performance dewatering aids
DE19635406B4 (en) * 1996-08-31 2005-09-01 Philips Intellectual Property & Standards Gmbh Capacitor and multilayer capacitor with a tungsten-containing BCZT ceramic dielectric
CN108558391B (en) * 2018-06-26 2020-10-09 桂林电子科技大学 Lead-free piezoelectric ceramic with giant piezoelectric response and preparation method thereof
CN111410530B (en) * 2020-05-12 2021-07-20 武汉理工大学 Anti-reduction BaTiO3Base medium ceramic and preparation method thereof

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FR2619370A1 (en) * 1987-08-12 1989-02-17 Cabot Corp COMPOSITIONS BASED ON BARIUM TITANATE DOPE
WO2009136320A1 (en) * 2008-05-08 2009-11-12 Nxp B.V. Tunable capacitor
US8767373B2 (en) 2008-05-08 2014-07-01 Nxp, B.V. Tunable capacitor

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Effective date: 20040430