LU86859A1 - COMPOSITIONS OR COFORMATIONS BASED ON BARIUM TITANATE - Google Patents

Description

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Compositions or coformations based on barium titanate.

The present invention relates to dielectric compositions based on barium titanate 5 and, more particularly, it relates to stoichiometric and dispersible compositions or co-formations of barium titanate with a granularity of 1 less than a micron in particles having very narrow granulometries. .

The high dielectric constant and high disruptive resistance of barium titanate make it particularly desirable as a material from which capacitors, condensers and other electronic components can be made.

Of particular interest is the fact that the electrical properties of barium titanate can be controlled over a wide range through the formation of a solid solution (mixed crystals) and doping.

The very simple cubic structure of perovskite possessed by barium titanate is the crystalline form for high temperatures for many mixed oxides of the ABO ^ type. This crystal structure consists of a regular network 25 of octahedra of oxygen sharing the corners with smaller titanium cations (iv) occupying the seat ί 2 B central octahedral and barium cations (ll) filling the interstices between octahedra in 1 the largest seats A 12-coordinated. This struc-; Particularly important is its crystalline structure, since it allows a plethora of multiple cation substitutions on both seats A and B, so that many more complex ferroelectric compounds can easily be obtained.

The relatively simple reticular structure. 10 of barium titanate is characterized by the octahedra of TiOg which, due to their high polarizability, essentially determine the dielectric properties of the structure. The high polarizability is due to the fact that the small Ti ions (IV) have relatively more space inside the octahedra of oxygen. However, this cubic unit cell is stable only above the Curie point temperature of about 130 ° C. Below 130 ° C, the Ti (IV) ions occupy eccentric positions. This transition to the eccentric position gives rise to a change in the crystal structure from the cubic structure to the tetragonal structure at temperatures between 5 ° C and 130 ° C, to the orthorhombic structure between -90 ° C and 5 ° C and, finally, the rhombohedral structure at temperatures below -90 ° C. It goes without saying that the dielectric constant and the disruptive resistance also decrease with respect to these changes in temperature and crystal structure.

The dielectric constant of a ceramic barium titanate material is largely dependent on temperature and reaches a pronounced maximum at or around the Curie point.

Given the extent to which the dielectric constant is dependent on temperature and its relatively low value at room temperature, the

Pure BaTiOg is rarely used in the preparation of industrial dielectric compositions. Therefore, in practice, additives are used to enhance the dielectric properties of barium titanate. For example, in the art, it is known that the Curie temperature can be shifted to lower and enlarged values by partially substituting strontium and / or calcium for barium and zirconium and / or tin for titanium, thereby giving materials with a maximum dielectric constant of 10,000 to 15,000 at room temperature.

Alternatively, the Curie temperature can be raised by partially substituting lead (II) for barium.

Furthermore, by substituting small quantities of other metal ions of an appropriate size but having valences different from those of barium and titanium (as summarized by B. Jaffee, WR Cook, Jr. and H. Jaffe in 20 "Piezoelectric Ceramics", Academie Press, NY 1971), profound changes in the nature of dielectric properties can be brought about.

According to industrial practice, dielectric powders based on barium titanate are formed: by mixing the doping agents, stannates, zirconates and pure titanates required or by directly forming the desired dielectric powder by subjecting an intimate mixture of stoichiometric amounts suitable oxides or oxide precursors (e.g. carbonates, hydroxides or nitrates) of barium, calcium, titanium, etc., for a reaction in the solid state at high temperature.

Pure titanates, zirconates, stannates, etc. are also specifically formed by a high temperature solid phase reaction process.

4

In these calcination processes, the required reagents are wet milled to ensure the formation of an intimate mixture. The resulting porridge is dried! and calcined at elevated temperatures between about 700 and 1,200 ° C, to provide the desired reactions in the solid state. Then, the calcined product is again ground to form a dispersible powder intended to be used for the formation of raw materials. Although the barium titanate dielectric formulations obtained by solid phase reactions are acceptable for many electrical applications, they nevertheless have several drawbacks.

Firstly, the grinding step constitutes a source of impurities which can exert a harmful influence on the electrical properties. Defects in homogeneity in the compositions on a microscopic scale can lead to the formation of untimely phases such as barium orthotitanate, which can give rise to moisture-sensitive properties. In addition, during calcination, there can be significant growth of the particles and sintering between them. As a result, the milled product consists of fractured aggregates of irregularly shaped particles which may have a large particle size ranging from about 0.2 to 10 microns. Published studies have shown that raw bodies formed from these agglomerated powders whose aggregates have large granulo-metries, required high sintering temperatures and gave rise to the formation of sintered bodies whose grains had large particle sizes. However, in the manufacture of complex dielectric bodies such as monolithic multi-layer capacitors, an important economic advantage lies in the adoption of tem-. lower sintering peratures, rather than higher sintering temperatures, since, as the sintering temperature is reduced, the percentage of lower cost silver involved in the silver-palladium electrode 1 may increase.

As is known in the art, the capacity of a dielectric layer is inversely proportional to its thickness. In common multi-layer capacitors, the thickness of the dielectric layer is of the order of 25 microns. Although very desirable, this value cannot be significantly reduced because, as the layer thickness decreases, the number of defects occurring in the dielectric film, for example, pitting, increases. have a detrimental influence on the efficiency of the capacitor. A major source of such defects is the presence of undispersed aggregates having granularities comparable to the thickness of the film. During sintering, due to the presence of these aggregates , non-uniform shrinkage occurs and pitting occurs. Therefore, the use of dielectric formulations based on barium titanate, obtained by solid state reactions, substantially increases the overall cost of manufacture of monolithic multi-layer capacitors.

In view of the limitations of the product obtained by conventional solid state reaction methods according to the prior art, several other methods have been developed for forming barium titanate. These methods include the thermal decomposition of barium titanyl oxalate and barium titanyl citrate, as well as the oxidation, at elevated temperatures, of atomized solutions of barium alcoholates and dissolved titanium, 'in alcohol or barium and titanium lactates! dissolved in water. In addition, barium titanate 5 was formed from molten salts by hydrolysis of barium and titanium alkoxides dissolved in; alcohol and by the reaction of barium hydroxide with titanium oxide, both hydrothermally and in aqueous media. Since the morphologies of the products obtained by some of these methods approximate those desired here, in the prior art, attempts have been made to prepare compositions based on barium titanate by the same methods as those adopted for preparing pure barium titanate. For example, in an article entitled "Preparation of BaTiO„ and Other Ceramic Powders by Coprécipitation of Citrates in an Alcohol ", Ceramic Bulletin, 49, N ° 11, 1970, pages 990-993, BJ Mulder reveals that one can prepare 20 co-formations or compositions based on BaTi0o by a coprecipitation process. In this process, aqueous solutions of citrates of Ti (IV), Zr (IV) and / or Sn (IV) are sprayed, as well as formates of Ba (II), Mg (II), Ca ( II), Sr (II) and / or Pb (II) in alcohol, to effect the co-precipitation. The precipitates are decomposed by calcination in a stream of air diluted with Ng at 700-800 ° C to obtain globular and rod-shaped particles having an average granularity of 3 to 10 microns.

Co-formations based on barium titanate were prepared by precipitation and subsequent calcination of mixed titanyl and / or zirconyl oxalates of alkali metals and / or Pb (II) as described by Gallagher et al. in an article titled / 7}

"Preparation of Semi-Conducting Titanates by Chemical Methods", J. Amer. Ceramics Soc., 46, N ° 8, 1963, pages 359-365. These researchers demonstrated that it was possible to prepare compositions based on BaTiOQ

O

5 in which Ba is replaced by Sr or Pb in; the range of 0 to 50 mol% or in which

Ti (IV) is replaced by Zr (IV) in the range of 0 to 20 mol%.

In U.S. Patent No. 3,637,531, Faxon et al. reveal that BaTiOQ-based coformations can be synthesized by heating a solution of a titanium chelate or titanium alkoxide, an alkaline earth metal salt and a lanthanide salt to form a 15 semi-solid mass. This mass is then calcined to obtain the desired co-formation of titanate. However, in each of the aforementioned references of the prior art, calcination is adopted to synthesize the particles of the 20 co-formations based on barium titanate. For the reasons already given, this operation at high temperature gives agglomerated products which, after spraying, give fragments of smaller aggregates having large particle sizes.

In the prior art, an attempt has also been made to avoid the drawbacks of BaTiOq powders prepared in the usual manner, by synthesizing a mixed composition of alkaline earth metal titanate-zirconate by means of a reaction of a molten salt. This process is described in U.S. Patent No. 4,293,534 to Arendt. During the implementation of this process, titanium oxide or zirconia or their mixtures are mixed, as well as barium oxide, strontium oxide 35 or their mixtures with hydroxides of 8 i alkali metals and heated to temperatures sufficient to melt the hydroxide solvent. The reagents are; "dissolve in the molten solvent and they precipitate 'in the form of a metal titanate or zirconate, - 5 alkaline earth or also in the form of a solid solution corresponding to the general formula Ba Sr, .. -> Ti ZrM λ0ο.

x y. i — xj y K1 — y) o

The products obtained are characterized in that they are in the form of chemically crystalline: homogeneous, relatively monodispersed and of a granularity less than one micron. However, this method * is limited by the fact that it only makes it possible to obtain co-formulations containing Sr and / or Zr.

Hydrothermal processes have also been described in which co-formations are formed.

In British Patent No. 715,762 Balduzzi and Steinemann heated aqueous slurries of hydrated TiO 2 with stoichiometric amounts of an alkaline earth metal hydroxide at temperatures between 200 and 400 ° C to form mixed titanates of alkaline earth metals.

Although it has been stated that products of any desired granularity of up to about 100 µm can be formed, it is doubtful that, except in the case of co-formulations containing Sr, products can be obtained having the morphological characteristics of the present invention.

This design is based on the fact that, while Ba (0H) 2 is soluble in aqueous media,

Ca (OH) 2 and Mg (OH) 2, in particular, in the presence of 30 Ba (OH) 2, are relatively insoluble. Consequently, in the case of coformations containing Ca, it has been found that, under the experimental conditions of Balduzzi and Steinemann, BaTiO3 was formed first, Ca (OH) 2 then reacting with the rest of l unreacted titanium oxide for i 9 <<to form CaTiOg during the heating process at a temperature of 200 to 400 ° C.

: 7 'In European patent application No. 34306926.1, Matsushita et al. have demonstrated that dilute oxide slurries can be reacted * 5

of titanium hydrated with Ba (0H) 2 and / or Sr (0H) 2 in. heating at temperatures up to 110 ° C

to obtain co-training containing BaTiO „or

• O

Sr. The morphological characteristics of these 10 co-formations appear to be comparable to those of the present invention. However, this method is again limited only to the formation of co-formations containing Sr.

In a publication of "Sakai Chemical 15 Industry Company", entitled "Easily Sinterable

BaTiOg Powder ", by Abe et al., Discloses a hydrothermal process for the synthesis of a co-formation based on barium titanate and corresponding to the formula BaTi £ 1_xjSnx03. In this process, a mixture of Ti (^ _x) Snx ° 2 prepared by neutralizing an aqueous solution of SnOCl2 and TiCl4, with 0.9M Ba (0H) 2 and the mixture obtained is subjected to a hydrothermal treatment at 200 ° C. for at least 5 hours. do not explicitly point this out, Abe et al suggest that the slurry is heated to temperature, although no description is given of the morphology of the co-formations, the product of BaTiO 4 obtained by the same process has a specific surface of 11 m2 / g, a granularity of 0.1 μm and, moreover, it seems to be dispersible.

Probably, the co-formations containing Sn have comparable morphologies and, therefore, they can be assimilated to those of the present invention. However, Abe et al. imply limitations in that they only reveal that one can i 10; synthesize Sn (IV) into a co-formation of barium titanate. Probably, by analogy, they suggest the use of other tetravalent cations such as Zr (IV) and possibly the use of Sr (ll). - 5 bivalent because, just like Ba (0H) 2, Sr (0H) 2 is very soluble in aqueous media. However, the method of Abe et al. cannot be adopted for the substitution of divalent Curie point shift agents such as Pb and Ca to bivalent Ba.

Therefore, in the prior art, there is no co-formation of stoichiometric barium titanate, dispersible, spherical and of a granularity less than one micron with narrow particle sizes, including calcium substitutions 15 and / or lead or multiple bivalent and tetravalent cations.

- The present invention encompasses a wide variety of dispersible co-formations of barium titanate, which are practically spherical, stoichiometric and of a granularity less than one micron with narrow particle sizes. According to a most important characteristic, the dielectric compositions based on barium titanate according to the present invention include coformations in which the bivalent lead and / or calcium are partially substituted for the bivalent barium, as well as coformations in which the barium bivalent is partially replaced by lead, calcium and strontium and in which also tetravalent titanium is partially replaced by tin, zirconium and hafnium.

According to an important embodiment of the present invention, the co-formation based on barium titanate is represented by the general formula: 4 11 i

Ba (1-χ.χ. -X ") PbXGax. Srx-.T1 (1-yy. -Y ·.) SnyZry. HfyM ° 3; T in which x, x 'and x" represent the molar fractions i of bivalent cations and have values; * 5 independent lying between 0 and 0.3, the sum of x + x '+ x "having a value lying between 0 and 0.4, while y, y' and y" represent the molar fractions of the tetravalent cations and have independent values between 0 and 0.3, 10 the sum of y + y '+ y "having a value between 0 and 0.4.

In another important embodiment of the present invention, the co-formation of barium titanate is represented by the general formula Bah vMCa .Tih. , nSn Zr, Hf ..0 "U-x ') x' Uy-y'-y") yy 'y "3 in which the calcium is partially substituted for the bivalent barium cation and, in yet another important embodiment of the invention, the dielectric composition of barium titanate is represented by the general formula

Ba (l-x) PbxTi (l-y-y'-y ") SnyZry, Hfy" ° 3 danS lac * uelle lead is substituted for bivalent barium. In each of the latter embodiments, the independent values for the molar fractions 25 x, x ', x "and y, y', y" are compatible with those already mentioned for the more complex co-formation corresponding to the general formula

Ba (l-x-x'-x ") PbxCaxSrx · Ti (1-y-y'-y") SnyZry "Hfy" ° 3 ·

Despite the chemical composition of the co-formation, each of the barium titanate co-forms of the present invention has the same exceptional chemical and physical properties. Dielectric formulations based on barium titanate are stoichiometric such that the bivalent / tetravalent molar ratio 12 of co-formations of various compositions is 1.000 + 0.015 whatever the number and the; * molar percentage of one or the other substitution: by bivalent and tetravalent cations. Non-stoichiometric compositions can also be formed in which the molar ratio of bivalent cations to tetravalent cations of co-formations of various compositions is in the range of 0.9 to 1.1. the average granularity of | 10 primary particles from · co-training. barium titanate is in the range of 0.05 to 0.4 micron. In addition, the average particle size, determined by image analysis, is comparable to the average particle size determined by sedimentation, thus demonstrating that the co-formations are dispersible. The granulometric curve of the co-formation particles has a quartile ratio less than or equal to 1.5, thus establishing that the co-formulations based on barium titanate have a narrow particle size. An additional important feature is that any dispersible dielectric compositions based on barium titanate and having a granularity of less than one micron according to the present invention can be prepared by a single general hydrothermal process.

Consequently, a main object of the present invention is to provide a dispersible co-formation of barium titanate with a granularity of less than one micron into particles of narrow particle size.

Another object of the present invention is to provide a wide variety of BaTiO 3 co-formation compositions in primary particles of granularity which can be controlled in the meantime ranging from 0.05 to about 0 , 4 pm.

Another object of the present invention is to provide a wide variety of coformations which can be synthesized by a single general hydrothermal process.

; Another object of the present invention is to provide a stoichiometric co-formation based on barium titanate which is practically free from grinding media.

Another object of the present invention is to provide a co-formation of barium titanate containing various additives which shift and / or widen the Curie point to the desired temperature zones, while reducing the extent to which the dielectric thus formed depends of the temperature.

Another object of the present invention is to provide dispersible dielectric compositions based on BaTiO 2 which can be used to form dielectric layers of reduced thickness, which are practically free from defects.

Yet another object of the present invention is to provide a barium titanate-based dielectric formulation which undergoes uniform sintering at high density at temperatures well below normal temperatures.

These details and advantages of the invention, as well as others, will be described with reference to the appended drawings in which: FIG. 1 is an electron micrograph of transmission, at an enlargement of 50,000 ×, of a complex co-formation, stoichiometric, dispersible and of a granularity less than one micron according to the invention, this co-formation corresponding to the general formula: 35 Ba0.856Pb0.097Ca0.074Tl0.830Zr0.099Sn0.071 ° 3 5 and / 14 Figure 2 is a micrograph transmission electronics, at a magnification of 50,000x, of a pure barium titanate powder having a morphology practically similar to that of the co-formation; ^ 5 complex of figure l.

To begin with, the invention is described in its broadest general aspects, then giving a more detailed description. The preferred embodiment of the present invention is a co-training of the general type:

Ba (1-x-x'-x ") PbxCax'Srx" Ti (1-y-y'-y ") SnyZry'Hf y" ° 3 where x, x 'and x "represent the molar fractions of bivalent cations and have independent values between 0 and 0.3, better still, between 0 and 15 0.2, the sum of x + x '+ x "can have values between 0 and 0.4, better still , between 0 and 0.3, while y, y 'and y "represent the molar fractions of the tetravalent cations and have independent values between 0 and 0.3, better still, between 0 and 0.25, the sum of y + y '+ y "having values between 0 and 0.4, better still, between 0 and 0.3.

When the sums of (x + x '+ x ") and (y + y' + y") are both zero, the co-formation consists simply of barium titanate powder. When x = x "= y = y '= y" = o and when x' is greater than 0, the product obtained is a co-formation based on barium titanate in which the molar fractions x 'of Ba (ll) in 30 BaTiOg have been replaced by Ca (II) to give a product corresponding to the nominal formula Ba, Λ,> Ca, TiOQ. On the other hand, when x '= x "= y = y' = y" = 0 and when x is greater than zero, the co-formation has the composition BaM sPb TiO ".

35 i 15 {

Since the values of x, x ', x ",' y" y 'and y "can each lie within a wide range (within the limits mentioned), many combinations of co-formations having compositions can be prepared lying in a wide; interval. However, whatever the composition formed, each of the co-formations based on barium titanate is exceptionally characterized by its high purity, its fine granularity less than one micron and its particles of narrow particle size.

Preferably, the fine dispersible powder with a granularity of less than one micron according to the present invention consists of a co-formation of barium titanate comprising both a substitution by tetravalent metal ions and a substitution by bivalent metal ions between 0 and 30 mol%. The bivalent barium ion can be partially replaced by lead, calcium, strontium or mixtures thereof.

On the other hand, the tetravalent titanium ion can be partially replaced by tin, zirconium, hafnium or their mixtures. Consequently, the barium titanate dielectric compositions according to the present invention include simple co-formations of barium lead-titanate or barium-strontium titanate, as well as more complex coformations including barium-lead stannate-titanate and barium-lead-strontium stannate-zirconate-30 titanate. Of course, the choice of replacement with bivalent and / or tetravalent cations, as well as the molar percentage of the substitution, depends on whether the Curie temperature should be raised or lowered, as well as whether '' the Curie peak should be widened or offset 16. Whatever the wide variety of barium titanate compositions formed, the co-formulations of barium titanate according to the present invention are, however, always exceptionally identified by the above-mentioned morphological and chemical characteristics. Consequently, both the simple and the complex co-formations of barium titanate are made up of practically spherical dispersible particles, the primary particles of which have a granularity ranging from 0.05 to 0.4 micron with d Narrow particle sizes and a bivalent / tetravalent molar ratio of 1,000 + 0.015, even when the bivalent ions and the tetravalent ions have been replaced by one or more other ions.

Owing to the narrow particle size distribution and the less than one micron granularity of the dielectric compositions based on barium titanate, the co-formations of the present invention are particularly advantageous for other applications in the manufacture of complex dielectric bodies. Previous studies have established that raw bodies formed from non-agglomerated powders having narrow particle sizes undergo sintering at reduced temperatures and give sintered bodies in grains of narrow particle size. The economic advantage of using a dielectric formulation having a lower sintering temperature is obvious, since the lower cost percentage of silver in the silver / palladium alloy can be increased as the sintering temperature is reduced. Furthermore, since these BaTiOg-based dielectric compositions are all dispersible and contain few aggregates with a granularity greater than 1 micron, they can be used when forming dielectric films of a reduced thickness. Consequently, the dielectric co-formation powder of barium titanate, spherical, non-agglomerated, of one; . 5 granularity less than one micron and a narrow particle size according to 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 of the primary particle composition is relatively low. However, in some cases, homogeneity in the composition is an advantage. In this case, use may be made of the availability of products comprising primary particles of varying granularity to form a dispersion of two or more powders having different compositions comprising either comparable numbers of primary particles or substantially different numbers. primary particles. Such dispersions give raw bodies and hence sintered bodies with microscopic homogeneity defects at controlled degrees. In such applications, the lack of homogeneity of the composition may be inherent in the co-formation of barium titanate chosen or rather, it may result from a small amount of a co-formation of barium titanate of a selected composition, which is added to a barium titanate dispersion in order to obtain the desired homogeneity of composition. Since, according to the present invention, it is possible to form coformations deficient in bivalent barium and / or in tetra-valent titanium, the compositions based on barium titanate Λ

J

18 of the present invention are also well suited for applications in which homogeneity in the compositions is advantageous.

[5 The preferred method for forming co-formations based on barium titanate is to heat slurries containing tetravalent oxides hydrated with selected bivalent oxides or hydroxides. After formation of the bivalent titanates, the slurry still contains large quantities of hydrated TiO 4 and / or SnO 4, ZrO 2 or hydrated HfO 2. Next, the temperature and the concentration of the slurry are adjusted and then, under isothermal conditions, a stoichiometric excess of Ba (OH) 2 solution is added. In order to ensure the complete conversion of the tetravalent oxides to their corresponding oxy-anions , the slurry is preferably subjected to a final heat treatment at a higher temperature.

The granularity and the granulometry of the primary particles according to the present invention are obtained, whether the co-formation of barium titanate is simply BaTiO 4 or whether it is rather the more complex co-formation corresponding to the formula Bal-xx '-x "PbxCax 'Srx "Tii-y_y' -y" ZrySny, Hfy "03 '

This characteristic becomes easily evident if one observes the transmission micrograph of the complex co-formation

Ba0,856Pb0,097Ca0,074Tl0,830Zr0,099Sn0,071 ° 3 in FIG. 1 appended which shows the presence of primary particles mainly simple and practically spherical, although some firmly linked doublets and triplets are also present. The granularity of the primary particles of this co-formation is 0.18 micron with a narrow particle size.

; * 19 "! • If we compare the complex co-formation based on barium titanate of figure 1 with the micro-. = electronic graph of transmission of pure barium titanate in FIG. 2, it can be seen that the morphologies of the compositions based on barium titanate are very similar. It should be noted that, in the two micrographs, the particles are practically spherical, non-agglomerated and of a uniform granularity less than one micron. It may also be noted that in this product the molar ratio of; cations bivalent to tetravalent cations, i '1.027, is somewhat greater than the value 1,000 + 0.015 specified for stoichiometric products.

This ratio can easily be reduced to the specified range by making slight variations in the synthesis conditions without influencing the morphology.

In order to evaluate the physical and chemical properties of the co-formulations based on barium titanate according to the present invention, various laboratory tests were carried out. Image analysis was adopted to determine the granularity and particle size of the primary particles of the products. 500 to 1,000 particles were calibrated in several TEM fields (= transverse electromagnetic) in order to determine the equivalent spherical diameters of the primary particles. Two or more particles in contact were visually disaggregated and the granularities of the individual primary particles were measured. The equivalent spherical diameters were used to calculate the cumulative percentage particle size of the mass as a function of the granularity of the primary particles. The average particle size (by weight) was considered to be the particle size of the primary particles in the sample. The ratio Γ 20 quartile (QR), defined by the upper quartile diameter (by weight) divided by the lower quartile diameter, was considered as a measure of the width of the grain size. Monodispersed products T. 5 have a QR value of 1 and, for the purposes of the tests carried out: according to the invention, products having QR values between 1.0 and approximately 1.5 have been classified among the products having d narrow grain sizes, those with QR values between 1.5 and about 2.0 were classified among the products with fairly narrow grain sizes, while those with QR values significantly above 2.0 were classified among products with large particle sizes. The quartile ratio of the barium titanate co-forms of the present invention has been determined to be between 1.0 and 1.5, indicating that the primary particles have a narrow particle size.

Specific areas were calculated from the primary particles of the co-formations and found to be compatible with the specific areas determined by. nitrogen adsorption, which indicates that the primary particles are essentially non-porous. When the specific surface determined by nitrogen adsorption has significantly exceeded the specific surface determined in TEM fields, it has been found that the difference can be easily explained by the presence of hydrated oxides with high reactive surface. .

Since the co-formations of the present invention have a narrow particle size, the average granularity of the primary particles was readily determined by calibrating 20 to 30 part-35 cells. We found that we could use the; '21.

f i. >.

1 t 'relation D = 6 // * S where D represents the diameter, of the particles (microns), ^ indicates the, density (g / cm3) ^ L and S, the specific surface determined by adsorption:' nitrogen ( m2 / g) to obtain a good measure of the granularity of the primary particles of the co-formations.

te.

According to this formula, it has been found that co-formations based on barium titanate contain primary particles with a granularity in the range of 0.05 to 0.4 micron, regardless of the composition of the co-formation submissive! for testing.

The dispersibility of the co-formation products was determined by comparing the granularities and particle sizes of the primary particles, determined by image analyzes, with the comparable values determined by sedimentation methods. The sedimentation process; gives the Stokes diameter of the particles which roughly corresponds to the equivalent spherical diameter.

20 Two sedimentation methods were adopted, namely the "Joyce Loebl Disc Centrifuge" method ("Vickers Instruments, Ltd.", London, United Kingdom) and the "Micromeritics Sedigraph" method (Norcross, Georgia), to determine the cumulative percentage of mass granulometry in terms of Stokes diameters from which the average Stokes diameters and QR values were calculated.

When determining the granularity of the particles by sedimentation, the 30 powders were dispersed by ultrasonic treatment for 15 to 30 minutes, either in water containing 0.08 g / l of sodium tripolyphosphate at a pH of 10, either in isopropanol containing 0.08 or 0.12% by weight of "Emphos PS-21A" ("Witco Organics Division, 35 520 Madison Ave., New York).

f • '. 22 *: 1 '·, I *' ·· _ /

Since the granularities of parti-; · 'Cules, determined by image analysis and by sedimentation, depend on different principles, we': did not always obtain an exact correspondence * 5 in the granularities determined by these two methods, i Moreover, as we 1'a already indicated, during the image analysis, visually disaggregating particles which touch. In the sedimentation process, bound or flocculated particles behave like 10 individual entities. These entities result from the existence of a certain bond (for example, throttling) between the primary particles to give cemented aggregates which cannot be easily disintegrated during the ultrasonic treatment process, as well as the dispersion stability below the optimum value, which leads to some flocculation. Therefore, as was to be expected, the QR values determined by sedimentation were somewhat. greater than 20 those found by image analysis.

In the barium titanate co-formations of the present invention, the granularity of the primary particles, determined by image analysis, was in reasonable agreement with the granularity of the primary particles, determined by sedimentation. The determined average granularity of the particles varied by a factor not exceeding two, thus demonstrating that the co-formations are dispersible.

Two additional measurements were taken to determine dispersibility. In the first method, the mass fraction of the product having a Stokes diameter greater than 1 micron was used as a measure of the amount of difficult to disperse aggregates. In the second method, r. : t. 23 ..

i ': V' ··. * <. · -; ··. · · ·. . ,,;. f il, ·>: '.' ·· ·. ’F, i r! '„. ; ; . i: "a product was classified as a dispersible product t · if all the primary particles, determined in> i. the TEM fields were present in the form of individual particles. When a significant constriction was observed, the product was classified as agglomerated product. In each of these tests, the co-formations based on barium titanate were again classified as dispersible co-formations. The composition and stoichiometry of the 10 co-formation products were determined by elemental analysis using an inductively coupled plasma spectroscopy after dissolution of the samples. The precision of the analyzes was approximately + 1%. Whatever the number or the molar weight percent of the substitutions by bivalent cations and tetravalent cations, the molar ratio of bivalent cations to tetravalent cations of coformations was 1,000 + 0.015.

"This report indicates that the barium titanate co-formations of the present invention are stoichiometric.

The exceptional properties of co-formations based on barium titanate are further illustrated by the nonlimiting examples below.

Throughout the examples, suitable chemicals were used as reactants or their equivalents. The Ba (0H) 2.8H20 used and suitable as reagent contained 1 mol% of Sr.

Experiments have shown that Sr (II) is more readily incorporated into co-training than Ba (II).

For this reason, all the co-formations described ~ here contain Sr (II). This cation represents approximately 1 ° / o molar of the total content of bivalent cations in the co-formation. For reasons of simplicity, the molar fraction of Sr (II) has been included in /: ·:. 24 ...

· ·· · t. · '·. ·. [y · '' "’! i! at ; molar fraction of Ba (II). Before their use,: solutions of Ba (OH) 2 and / or of Sr (OH) 2, kept at 70-100 ° C, were filtered, in order to remove any * - carbonates possibly present. F 5 CaCOg was calcined at 800 ° C to obtain CaO. When it is brought into contact with water, the latter compound gives

Ca (OH) 2 <Pb (OH) 2 was prepared by neutralizing a 1 solution of Pb (NO3) 2 with aqueous NH3. In subsequent experiments, the wet cake of washed hydroxides was used.

Hydrated oxides of TiO 4 were prepared,

SnOg and ZrO ^ by neutralizing aqueous solutions of their respective chlorides with aqueous NH3 at ambient temperatures. The products were separated by filtration and washed until chlorine-free filtrates were obtained (determined by AgN03). The specific surfaces of the oxides - hydrates, determined after drying at 110 ° C, were? approximately 380, 290 and 150 m2 / g for TiOg, Sn02 and 20 ZrOg Γβ8Ρθ ° ΐΐνβιηβη ^ · In addition, co-precipitates of hydrated Ti02 and Zr02 or hydrated TiO2 or SnOg were prepared by neutralizing aqueous solutions of chlorides of Ti (IV) and Sn (IV) or of Ti (IV) and Zr (IV).

All experiments were carried out in a two liter autoclave. In order to avoid product contamination, all wet parts of the autoclave were coated with Teflon and every effort was made to exclude 30 CO 2 from all parts of the system. Solutions of Ba (0H) 2 or Ba (0H) 2 and Sr (0H) 2 were introduced into the autoclave either by means of a high pressure pump or by rapidly discharging a solution of or hydroxides (contained in a heated bomb) in the autoclave using nitrogen under high pressure.

. r, '. . · 25 ί ·.

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The contents of the autoclave were stirred at 1,500 rpm for the duration of the synthesis process.

Example 1 'Z A co-formation was prepared containing; f 5 calcium by hydrothermal treatment of 0.64 1 of a boiled v containing 0.20 mole of hydrated TiO 2 and 0.04. mole of Ca (OH) 2, at 200 ° C. The slurry was cooled and, to 120 ° C, 0.46 1 of Ba (0H) 2 0.41M was added thereto.

The temperature of the slurry obtained was brought to; 10 150 ° C and held there for 60 minutes. The sample was filtered and the concentrations of divalent cations existing in the filtrates were determined. The filter cake was dried and its specific surface area, nominal stoichiometry and morphological characteristics were determined.

Filtrate Molar ratio of g / 1 cations in solids Surface ratio. _ ______ specific molar 20 Ba Ca Ca: Ba: Sr: Ti cations that bivalent / ended tetravalent by adsorption of N2 25 m2 / g 2.62 0.446 0.127: 0.842: 0.019: 1.00 0.988 12.0

Granularity of Primary particle size distribution 30 (TEM) _ _ 0.15 micron narrow = Example 2

A lead-containing co-formation was prepared by hydrothermal treatment of 0.64 1 of a slurry containing 0.2 mole of hydrated TiOg and 0.04 26 ...

: '··' - '' Γ Ifeïf: ·

mole of PbO. At 150 ° C, to this slurry was added ’0.46 1 of Ba (0H) 2. The porridge was kept at 150 ° C.

; ^ for 60 minutes, then brought to a high temperature to ensure complete conversion of the T 5 tetravalent oxides to perovskite structures.

! Porridge samples were taken and.

characterized them. The results obtained are as follows:

Filtrate molar ratio of surface ratios 10 g / 1 cations in specific molar solids; cations m2 / g bivalent / tetravalent 15 Ba_ Pb_ Ba; Pb: Sr: Ti_ 10.6 2.74 0.810: 0.173: 0.024: 1,000 1.007 11.5

Granularity of Granulometry ~ primary particles (TEM) _ _ 20 0.07 micron narrow

Example 3

Complex coformations are formed in which Ba (II) and Ti (IV) of BaTiOg are partially replaced by one or more bivalent and tetravalent cations. A preheated solution of Ba (OH) 2 was introduced into slurries heated to 150 ° C or 120 ° C, containing the tetravalent hydrated oxides, as well as pre-synthesized perbskites of Pb (II) and / or Ca (II). After having kept them at temperature for about 20 to 30 minutes, the slurries were brought to a final temperature in order to ensure the conversion of the tetra valent hydrated oxides into stoichiometric perovskites. The ‘porridge obtained was characterized in accordance with the following 35 results:>,: '' 27 -. -; · V '·'. : i CD · Η · ': 1? O "H O C \ J '00 CÖ -H Ö0 CH O \ - CO CM O)

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Quantitative results for the traders; · 1 tillons2 and 3 correspond well to the results ·; estimates, recorded in transmission electron micrographs, concerning the dispersibility, * 5 the particle size and granularity of the particles, i 'However, as indicated by the QR value, sample 1 is only moderately dispersible. Nevertheless, the: sedimentation results indicate that less than 5% by weight of the material is present in the form of aggregates having a granularity greater than 1; micron.

Consequently, from the above examples and description, it can be seen that the co-formulations of barium titanate which is the subject of the present invention includes dielectric compositions containing calcium and / or lead or substitutes. multiples for cations of bivalent barium and / or tetravalent titanium, these compositions being characterized in an exceptional manner in that they are in spherical particles, in that their primary particles have a granularity lying in the range ranging from 0.05 to 0.4 micron, in that they have a bivalent to tetravalent cations molar ratio of 1.000 +: 0.015, as well as particles of narrow particle size. None of the dielectric compositions of the prior art based on barium titanate comprising dir calcium ;, lead or the complex forms described in this specification, have these exceptional morphological and chemical characteristics.

It is understood that the description which precedes, is given simply by way of illustration without in any way limiting the invention and that various modifications can be made without departing from the spirit of the invention as it stands. is claimed below.

Claims (17)

1. Co-formation based on barium titanate comprising practically spherical particles corresponding to the formula 5 Ba (lx ") Cax'Ti (i-y_y._yM) SnyZry.Hfy..03 in which y" y 'and y "have independent values between 0 and 0.3, the sum of y + y '+ y "being less than 0.4, while x' is greater than 0 and **; less than 0.4. 10 .2. Co-formation of barium titanate according to claim 1, characterized in that the average granularity of its primary particles is in the range from 0.05 to 0.4 microns. 3. Co-formation of barium titanate according to claim 1, characterized in that the granularities of the primary particles, determined by image analysis and by sedimentation, agree to a factor of 2. 4. Co-formation of barium titanate according to claim 1, characterized in that its particles have a narrow particle size and in that the particle size curve of its primary particles 0 has a quartile ratio less than or equal to 1.5. 5. Co-formation of barium titanate according to claim 1, characterized in that the ratio! '* *. - i -. , I,:. ‘30 '3' 'V' 't' * i”. i. *. i molar of (Ba + 'Ca) / (Ti + Sn + Zr + Hf) is of j'. 1,000. + 0.015. 6. Co-formation of barium titanate according to claim 1, characterized in that the * 5 molar ratio of (Ba + Ca) / (Ti + Sn + Zr + Hf) is located! in the range of 0.9 to 1.1. 7. Co-formation based on barium titanate | 'comprising practically spherical particles corresponding to the formula: 10 Ba (ix) PbxTi (iyy> -yn) SnyZry.HfyM03 in which iy, y' and y "have independent values lying between 0 and 0.3, the sum of y + y '+ y "being less than 0.4, while x is greater than 0 and less than 0.4. 8. Co-formation of barium titanate according to claim 7, characterized in that the average granularity of its primary particles is "in the range from 0.05 to 0.4 micron. 9. Co-formation of barium titanate according to claim 7, characterized in that the granularities of its primary particles, determined by image analysis and by sedimentation, agree to a factor of two. 10. Co-formation of barium titanate according to claim 7, characterized in that its particles have a narrow particle size and in that the particle size curve of its primary particles has a quartile ratio less than or equal to 1.5. 11. Co-formation of barium titanate according to claim 7, characterized in that the molar ratio of (Ba + Pb) / (Ti + Sn + Zr + Hf) is ^ 1,000 + 0.015. 12. Co-formation of barium titanate according to claim 7, characterized in that the molar ratio of (Ba + Pb) / (Ti + Sn + Zr + Hf) is: ·. 31 i '* y * **' -, 1 '. * * »· '*. . In the range between 0.9 and 1.11. 13. Co-formation based on • barium titanate comprising practically spherical particles corresponding to the formula ’'! £ 5 Ba (lx-x'-x ") PbxCax, Srx" Tl (ly-y'-y ") SnyZr'y'Hfy" ° 3] in which x, x ', x ", y, y' and y "each have -: independent values greater than - 0 and less than 0.3, the sum of x + x '+ x" being less than 0.4, while the sum of y + y' + y " is 1 10 less than 0.4. 14. Co-formation of barium titanate according to claim 13, characterized in that the average granularity of its primary particles is in the range from 0.05 to 0.4 15 microns. 15. Co-formation of barium titanate according to claim 13, characterized in that the ~ granularities of its primary particles, determined by image analysis and by sedimentation, correspond to a factor of two. 16. Co-formation of barium titanate according to claim 13, characterized in that its particles have a narrow particle size and in that the particle size curve of its primary particles has a quartile ratio less than or equal to 1.5. 17. Co-formation of barium titanate according to claim 13, characterized in that the molar ratio of (Ba + Ca + Pb + Sr) / (Ti + Sn + Zr + Hf) is 1,000 + 0.015. 18. Co-formation of barium titanate according to claim 13, characterized in that the molar ratio of (Ba + Ca + Pb + Sr) / (Ti + Sn + Zr + Hf) is in the range between 0.9 and 1.1. y