The high dielectric constant and strength of barium titanate make it an especially desirable material from which capacitors, condensers, and other electronic components can be fabricated. Especially attractive is the fact that barium titanate's electrical properties can be controlled within a wide range by means of mixed crystal formation and doping.
The very simple cubic perovskite structure exhibited by barium titanate is the high temperature crystal form for many mixed oxides of the ABO.sub.3 type. This crystal structure consists of a regular array of corner-sharing oxygen octahedra with smaller titanium(IV) cations occupying the central octahedral B site and barium(II) cations filling the interstices between octahedra in the larger 12-coordinated A-sites. This crystal structure is of particular significance since it is amenable to a plethora of multiple cation substitutions at 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.sub.6 -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.degree. C. Below 130.degree. C., the Ti(IV) ions occupy off-center positions. This transition to the off-center position results in a change in crystal structure from cubic to tetragonal between temperatures of 5.degree. and 130.degree., to orthorhombic between -90.degree. C. and 5.degree. C. and finally to rhombohedral at temperatures less than -90.degree. C. Needless 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 of the temperature dependence of the dielectric constant and its relatively low value at room temperature, pure BaTiO.sub.3 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(II) for barium. Additionally, the substitution of small amounts of other metallic ions of suitable size but with valencies which are different to those of barium and titanium, as summarized in B. Jaffee, W. R. Cook, Jr. and H. Jaffe, "Piezoelectric Ceramics", Acedemic Press, N.Y. 1971, can cause profound changes in the nature of the dielectric properties.
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 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.degree. to 1200.degree. 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 phase 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 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 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 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 sintering temperatures rather than higher sintering temperatures, since the percentage of lower cost silver in the silver-palladium electrode can be increased as the sintering temperature is reduced.
As is known in the art, the capacitance of a dielectric layer is inversely proportional to its thickness. In current multilayer capacitors, the dielectric layer thickness is of the order of 25 microns. Although very desirable, this value cannot be substantially reduced because as layer thickness is decreased the number of defects in the dielectric film, such as pin holes, increases. The defects adversely affect the performance of the capacitor. One major source of such defects is the presence of undispersed aggregates having sizes comparable with the film thickness. During sintering, because of the presence of such aggregates, non-uniform shrinkage occurs and pin holes are formed. Hence, utilization of barium titanate based dielectric formulations formed by solid state reactions significantly increases the overall 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.sub.3 and Other Ceramic Powders by Coprecipitation of Citrates in an Alcohol", Ceramic Bulletin, 49, No. 11, 1970, pages 990-993, that BaTiO.sub.3 based compositions or coforms can be prepared by a coprecipitation process. In this process aqueous solutions of Ti(IV), Zr(IV) and/or Sn(IV) citrates and formates of Ba(II), Mg(II), Ca(II), Sr(II) and/or Pb(II) are sprayed into alcohol to effect coprecipitation. The precipitates are decomposed by calcination in a stream of air diluted with N.sub.2 at 700.degree.-800.degree. C. 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 Pb(II) titanyl and/or zirconyl oxalates as disclosed by Gallagher et al. in an article entitled "Preparation of Semi-Conducting Titanates by Chemical Methods", J. Amer. Ceramic Soc., 46, No. 8, 1963 pages 359-365. These workers demonstrated that BaTiO.sub.3 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 Zr(IV) in the range of 0 to 20 mole percent may be produced.
Faxon et al. discloses in U.S. Pat. No. 3,637,531 that BaTiO.sub.3 based coforms can be synthesized 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 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.sub.3 powders by synthesizing a mixed alkaline earth titanate-zirconate composition through a molten salt reaction. Such a process is disclosed in U.S. Pat. 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 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, zirconate or a solid solution having the general formula Ba.sub.x Sr.sub.(1-x) Ti.sub.y Zr.sub.(1-y) O.sub.3. 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. Balduzzi and Steinemann in British Pat. No. 715,762 heated aqueous slurries of hydrated TiO.sub.2 with stoichiometric amounts of alkaline earth hydroxide to temperatures between 200.degree. and 400.degree. C. to form mixed alkaline earth titanates. Although it was stated that products of any desired size up to about 100 microns could be produced, it is doubtful that, other than in the case of Sr-containing coforms, products with the morphological characteristics of this invention could be obtained. This contention is based on the fact that whereas Ba(OH).sub.2 is soluble in aqueous media Ca(OH).sub.2, and Mg(OH).sub.2, especially in the presence of Ba(OH).sub.2, are relatively insoluble. Accordingly, in the case of Ca-containing coforms it has been found that under the experimental conditions of Balduzzi and Steinemann that BaTiO.sub.3 is first formed and then Ca(OH).sub.2 reacts with the balance of the unreacted titania to form CaTiO.sub.3 during the heating process to 200.degree. to 400.degree. C.
Matsushita et al. in European patent publication No. 0141551 demonstrated that dilute slurries of hydrous titania can be reacted with Ba(OH).sub.2 and/or Sr(OH).sub.2 by heating to temperatures up to 110.degree. C. to produce either BaTiO.sub.3 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 BaTiO.sub.3 Powder", by Abe et al. discloses a hydrothermal process for synthesizing a barium titanate based coform with the formula BaTi.sub.(1-x) Sn.sub.x O.sub.3. In this process a 0.6M Ti.sub.(1-x) Sn.sub.x O.sub.2 slurry, prepared by neutralizing an aqueous solution of SnOCl.sub.2 and TiCl.sub.4, is mixed with 0.9M Ba(OH).sub.2 and subjected to a hydrothermal treatment at 200.degree. 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 BaTiO.sub.3 product produced by the same process had a surface area of 11 m.sup.2 /g, a particle size of 0.1 microns and appeared to be dispersible. Presumably the Sn-containing coforms have comparable morphologies and are thus comparable with those of this invention. However, Abe et al. is limited in that it teaches only that Sn(IV) can be synthesized into a barium titanate coform. Perhaps, by analogy, it does suggest the use of other tetravalent cations such as Zr(IV) and possibly the use of divalent Sr(II), since, like Ba(OH).sub.2, Sr(OH).sub.2 is quite soluble in aqueous media.
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 except when these reagents are present at impurity levels. For example, Abe et al. found that their hydrothermally derived BaTiO.sub.3 product contained 0.01 weight % Ca and, since not mentioned, probably less than 0.01 weight % Pb. In practice, it can be expected that the amounts of such impurities present in precipitated BaTiO.sub.3 and BaTiO.sub.3 based dielectric compositions will vary with the source of the reactants employed. From an examination of the purities of a number of typical commercially available reactants or reactant precursors, such as hydrous titania, TiCl.sub.4, TiOSO.sub.4, ZrO(NO.sub.3).sub.2, Ba(OH).sub.2, Ba(NO.sub.3).sub.2, etc., it is concluded that the levels of either Pb(II) or Ca(II) to be found in prior art precipitated BaTiO.sub.3 and BaTiO.sub.3 based composition will be, typically, much less than 0.1 weight %. In other words, the mole fraction of Ca(II) or Pb(II) present in such products will be less than 0.006. The effects of such minor levels of Ca(II) and Pb(II) on the dielectric properties, such as the Curie temperature, of these BaTiO.sub.3 based compositions is small. In general, the mole fractions of these additives which are present in practical dielectric compositions have values which exceed 0.005 and, more preferably, exceed 0.01.
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