Method of precipitating metal titanate powders

A titanium acylate solution is produced by adding glacial acetic acid to tetra-isopropyl titanate. Distilled water is then added until the solution clears, and a high grade alkaline-earth metal carbonate, such as BaCO.sub.3, SrCO.sub.3 or CaCO.sub.3, or a combination thereof, is added and agitated until the earth-metal dissolves and the solution again becomes clear. An alkaline hydroxide, such as NaOH, is added until the pH of the solution reaches 13 or 10 above, and causes crystals of an earth-metal titanate to form in the solution. The solution is thereafter filtered to produce an alkaline-earth metal titanate filter cake which is then dried and washed to produce the desired powder. At no time during the process is it necessary to apply any external heat to the solution. The heat of reaction is sufficient.

BACKGROUND OF THE INVENTION 
This invention relates to a novel method of precipitating alkaline-earth 
metal titanates in powder form from clear, homogeneous solutions of the 
desired titanate constituents. More specifically, this novel method 
involves the direct precipitation of titanates from such solutions 
rapidly, and without the need for the application of any external heating 
source. 
Barium titanate is a material of great interest for electronic applications 
due to its ferroelectric behavior, i.e., a spontaneous alignment of 
electric dipoles within the material itself. Modifying the transition 
temperature at which this ferroelectric behavior occurs allows the 
production of components such as multilayer ceramic capacitors and 
thermistors with optimized electronic properties. Materials such as other 
alkaline-earth titanates and zirconates are widely known as being 
effective in performing such modifications. 
As the trend in electronic circuitry continues toward higher levels of 
sophistication, increased board densities, and increased volumetric 
efficiencies in components, the need to supply newer, improved materials 
for component production is emerging. Conventionally, barium titanate and 
other titanate materials are prepared by high temperature calcination of 
the appropriate precursor materials. Specifically, barium carbonate 
(BaCO.sub.3) and titanium dioxide (TiO.sub.2) are mixed in the desired 
stoichiometric amounts and calcined at 1000.degree.-1200.degree. C. to 
form barium titanate (BaTiO.sub.3). Materials produced in this manner are 
hindered in their applications for the smaller, more sophisticated devices 
for several reasons. Mixing and post-calcination pulverization processes 
introduce uncontrollable and undesirable levels of impurities. The high 
calcination temperatures yield a powder which is of large and non-uniform 
grain sizes. In addition, the calcining process is expensive and thermal 
treatment is difficult to control. Also, mixing and calcining of 
precursors in the solid form is not entirely homogeneous and variations in 
particle to particle stoichiometry certainly exist. 
Another popular method for commercial production of barium titanate ceramic 
powders is the thermal decomposition of barium titanyl oxalate 
tetrahydrate BaTiO(C.sub.2 O.sub.4)2.4H.sub.2 O to form very fine 
BaCO.sub.3 and TiO.sub.2 crystallites which subsequently react in the 
solid state to form BaTiO.sub.3. (Clabaugh, W. Stanley, Swiggard, Edward 
M., and Gilchrist Raleigh, "Preparation of Barium Titanyl Oxalate 
Tetrahydrate for Conversion to Barium Titanate of High Purity", Journal of 
Research of the National Bureau of Standards, Vol. 56, No. 5, May 1956, 
pp. 289-291). Although this method developed by Clabaugh et al. produces a 
titanate material of higher purity and reactivity than the mixed oxide 
calcination process, high pyrolization/calcination temperatures of 
750.degree.-1100.degree. C. are still required to form single phase 
BaTiO.sub.3. In addition, Ba.sup.++ ion solubility in the precipitate 
mother liquor and the solid-state reaction processes necessary to form 
BaTiO.sub.3 do not allow a resultant material which is completely 
homogeneous in stoichiometry. 
Various other methods such as those disclosed in U.S. Pats. Nos. 3,330,697 
and 4,534,956, as well as others such as sol-gel or alkoxide methods have 
also been successfully employed. (Ritter, J. J., Roth, R. S., and 
Blendell, J. E., "Alkoxide Precursor Synthesis and Characterization of 
Phases in the Barium-Titanium Oxide System", Journal of the American 
Ceramic Society, Vol. 69, No. 2, 1986. pp. 155-162; Mazdiyasni K. S., 
Dolloff, R. T., and Smith J. S. II, "Preparation of High-Purity Submicron 
Barium Titanate Powders", Journal of the American Ceramic Society, Vol. 
52, No. 10, 1969, pp. 523-526; and Wu, Edward, Chen, K. C., and Mackenzie 
J. D., "Ferroelectric Ceramics--The Sol-Gel Method Versus Conventional 
Processing", Materials research Society Symposia Proceedings, Vol. 32, 
Better Ceramics Through Chemistry, Copyright 1984 by Elsevier Science 
Publishing Co, Inc. pp. 169-174). Although these methods result in fine 
sub-micron powders of near uniform size, they are also hindered by such 
factors as the need for calcination treatments, exotic manufacturing 
schemes, low product yields, and/or the use of exotic precursor materials. 
The literature has also suggested techniques for the hydrolysis of titanium 
esters in the presence of alkalineearth metal ions at higher pH values as 
a route to BaTiO.sub.3 formation. (Flachen, Steward S., "An Aqueous 
Synthesis of Barium Titanate", Journal of The American Chemical Society, 
Vol. 77, 1955, p. 6194; and Kiss, Klara, Mager, Jules, Vukasovich, Mark 
S., and Lockhart, Robert J., "Ferroelectrics of Ultra-fine Particle Size: 
I, Synthesis of Titanate Powders of Ultra-fine Particle Size", Journal of 
The American Ceramic Society, Vol. 49, No. 6, 1966, pp. 291-295.). 
However, sophisticated laboratory apparatus and external heating sources 
were employed to develop critical conditions necessary to form the desired 
product. The degree of control that is necessary would most certainly 
limit the commercialization potential for these processes. 
The recently issued U.S. Pat. No. 4,520,004 discloses a process for 
manufacturing fine alkaline-earth metal titanates by combining a water 
soluble salt of Ba, Sr, or Ca with a hydrolized product of a titanium 
compound in an aqueous alkaline solution having a pH greater than 13. More 
specifically, this method begins with the preparation of an inorganic 
titanium compound such as TiO.sub.2.xH.sub.2 O by neutralizing TiCl.sub.4 
or Ti(SO.sub.4).sub.2 in an aqueous or alkaline solution, and then 
reacting the product with a water soluble salt of Ba, Sr, or Ca in an 
aqueous alkaline solution having a pH of 13 or more and a temperature of 
approximately 100.degree. C. The process produces a fine precipitate which 
can then be filtered from the solution. The disadvantage of this process 
is that, as a practical matter, it requires the application of an external 
heating source to maintain elevated temperature during the reaction of the 
hydrolysis product with the water soluble salt. The temperatures necessary 
for conversion are selected to be preferably above 60.degree. C., and are 
actually in the 100.degree. C. area to optimize the process. 
Contrary to the above-noted teachings, applicants have discovered that 
metal titanates, and in particular alkaline-earth metal titanates and 
combinations thereof, can be directly synthesized or precipitated from a 
complex titanium alkoxide immersed in an acetic acid solution without 
requiring the application of any external heating source during the 
precipitation process (i.e., relying solely on the heat of chemical 
reaction to facilitate BaTiO.sub.3 production) and without requiring the 
use of a water soluble alkaline earth metal salt. 
A primary object of this invention, therefore, is to provide an improved 
method of precipitating fine metal titanates of precise stoichiometric 
proportions directly from a mixture of a complex titanium alkoxide and an 
alkaline earth metal, without having the need to apply any external 
heating source to enact the precipitation process. 
Still another object of this invention is to provide an improved method of 
the type described which obviates the need for utilizing the more costly 
and less commercially-available water soluble alkaline earth metal salt as 
a precursor in the process. 
Other objects of the invention will be apparent herein to one skilled in 
the art from the specification and from the appended claims, particularly 
when read in conjunction with the accompanying drawings. 
SUMMARY OF THE INVENTION 
A titanium acylate solution is produced by adding glacial acetic acid to 
tetra-isopropyl titanate. Distilled water is added until the solution 
clears. A high grade alkaline earth metal carbonate, such as BaCO.sub.3, 
SrCO.sub.3 or CaCO.sub.3, or combinations thereof, is added and agitated 
until the alkaline-earth metal dissolves and the solution becomes clear. 
An alkaline hydroxide, such as NaOH, is added until the pH of the solution 
reaches 13 or above. This causes crystals of the alkaline-earth metal 
titanate to form in the solution, which is thereafter filtered to produce 
an alkaline-earth metal titanate filter cake or residue. This cake is then 
dried and washed to produce the desired powder. 
During this process it is not necessary to apply any external heat applied 
to the solution. The solution is exposed only to the heat of reaction 
generated by the additions to the solution.

PREFERRED EMBODIMENTS OF THE INVENTION 
As noted hereinafter, applicants' novel method permits the production of 
various metal titanates by the direct precipitation of the desired 
titanates provided the stoichiometric amounts of alkaline earth metal 
carbonates are employed with a tetra-isopropyl titanate, such as Ti 
(OC.sub.3 H.sub.7).sub.4. A high grade carbonate is used in the process to 
assure the desired stoichiometric amounts of alkaline earth metal (for 
instance, B.sub.a.sup.+2, Sr.sup.+2 and Ca.sup.+2) carbonates. Examples of 
the direct prcipitations of the alkaline earth metal titanates, and 
combinations thereof are as follows: 
EXAMPLE I 
High purity BaTiO.sub.3 of precise stoichiometry and of particularly fine 
particle size was produced by mixing a tetra-alkyl titanate with glacial 
acetic acid to form a titanium acylate solution to which a suitable source 
of an alkaline earth metal was slowly mixed, after which the pH of the 
mixture was adjusted to a value greater than 13. The excess mother liquor 
was filtered, and the filter cake was dried and washed free of sodium to 
produce stoichiometric BaTiO.sub.3 with a mole ratio of 1:1. 
More specifically, 78.58 g (0.27522 moles) of tetraisopropyl titanate were 
mixed, while agitating, with 157 g of glacial acetic acid to form a 
titanium acylate solution. Distilled water in the amount of 52 ml was 
added to the acylate solution and agitated until all of the hydrolized 
mixture became clear. A high grade of BaCO.sub.3 in the amount of 54.44 g 
(0.27522 moles) was then slowly added to the acylate solution and agitated 
until the solution once again became clear. Thereafter the Ba-Ti solution 
was slowly added to a NaOH solution (600 mL of distilled water plus 120 g 
of NaOH pellets), while agitating vigorously. This raises the pH of the 
combined solution to a value greater than 13.0. 
The precipitated slurry was then filtered, and the filter cake dried at 
approximately 115.degree. C. The resultant powder was then washed with 
distilled water in order to remove the sodium previously introduced by the 
sodium hydroxide solution. Wet chemical analysis of the filtered mother 
liquor indicated that it contained less than 2.0 ppm of soluble Ba, and 
less than 0.5 ppm soluble Ti. An X-ray diffraction analysis of the dried 
filtrate produced a pattern as shown in FIG. 1, and established the 
presence of a single-phase BaTiO.sub.3 having a Ba/Ti mole ratio of 
approximately 1.0 within plus or minus 0.003. Peak 51 of the pattern 
(FIGS. 1 and 2), which occurred at approximately 45.4 degrees 2 theta, 
indicated the cubic phase of the powder. Tests also confirmed that the dry 
filtrate measurement HAc for soluble barium was approximately 4.0%; HCl 
for insoluble residue was 0.2%; LOI at 300.degree. C. was approximately 
1.4%; surface area was 22M.sup.2 /gm.; and scanning electron microscope 
(SEM) examination, in the form of the photomicrograph shown in FIG. 4, 
indicated an average particle size of less than 0.2 .mu.m. (In FIG. 4 the 
bar denoted as 1U corresponds to a measurement of one micron.) 
A differential thermal analysis of the powder (see the FIG. 3 graph) 
indicated the occurrence of a slight exothermal reaction in the powder, 
when heated in the range of 200.degree.-400.degree. C., and an endothermic 
reaction and conversion of the powder to its tetragonal phase, when heated 
into the range of 900.degree.-1000.degree. C., or more specifically to 
approximately 950.degree. C. This change is evidenced also by the 
difference between the peak 51 of the pattern as shown in FIG. 2, and the 
similar, but double peaked pattern 54, 54-1 (FIG. 5) of the heat treated 
powder. 
From these results, it is clear that precisely controlled stoichiometric 
metal titanate powders can be directly precipitated from a liquid solution 
without the need for expensive reaction vessels and associated external 
heating sources. Moreover, through the use of a metal carbonate, rather 
than a water soluble alkaline earth metal salt, it is possible to reduce 
the overall cost of the method, as compared to prior methods, since such 
carbonates are relatively abundant and inexpensive, as compared to 
precursors employed in other such known processes. 
EXAMPLE II 
Powdered SrTiO.sub.3 of high purity was also processed from a combination 
titanium alkoxide and acetic acid solution in the same manner as described 
above. In this example the same quantities of tetra-isopropyl titanate, 
glacial acetic acid and distilled water were combined and agitated as in 
Example I. Instead of using barium carbonate, 40.78 g (0.27522 moles) of 
strontium carbonate (SrCO.sub.3) were slowly added to the titanium acylate 
solution, together with an additional 300 ml of distilled water, which was 
required in order to dissolve the strontium carbonate. Thereafter the 
Sr-Ti solution was slowly added to the NaOh solution (consisting of 120 g 
NaOH pellets) while agitating, thereby producing a precipitated slurry. 
The excess mother liquor was filtered. and the filter cake dried at 
115.degree. C. The powder was then washed with distilled water to remove 
the sodium. The soluble Sr and Ti contents of the mother liquor were 
substantially the same minute quantities as in the case of Ba and Ti of 
Example I (Sr=2ppm, Ti=0.5 ppm). X-ray diffraction of the dry powder 
indicated a strong SrTiO.sub.3 pattern. Moreover the mole ratio of Sr to 
Ti was approximately 1.002; soluble strontium in the filtrate (HAc) was 
approximately 1.8%; the insoluble residue (HCl) was 0.2%; and LOI was 
3.79%. 
EXAMPLE III 
Very fine grain CaTiO.sub.3 was processed by the same method as described 
above, where, instead of BaCO.sub.3 or SrCO.sub.3, calcium carbonate 
(CaCO.sub.3) was slowly added to the titanium acylate solution in the 
amount of 27.82 g (0.27522 moles); and in order to completely dissolve 
this carbonate, distilled water had to be added in a total amount of 250 
ml. The other quantities of the constituents were the same as in Example 
I. Test results indicated soluble Ca in the filtered mother liquor was 
less than 0.5 ppm; X-ray diffraction indicated a strong CaTiO.sub.3 
pattern; soluble calcium in the powder (HAc) was 1.0%; and LOI 4.0%. 
EXAMPLE IV 
A combined barium strontium titanate (Ba.sub.0.8 Sr.sub.0.2 TiO.sub.3 was 
also prepared by the above-noted process in which the constituents were in 
the amounts as in Example I, except that distilled water amounted to 352 
ml, barium carbonate amounted to 43.55 g, and strontium carbonate amounted 
to 8.16 g. The Ba-Sr-Ti solution was slowly added to the NaOH solution as 
in preceding examples, after which the precipitant was filtered, dried and 
washed as above. Test results indicated soluble barium in the filter 
liquor at 10 ppm, soluble Sr at 1.4 ppm and soluble Ti less than 0.5 ppm. 
X-ray diffraction indicated a strong Ba.sub.0.8 Sr.sub.0.2 TiO.sub.3 
pattern; soluble barium in the powder (HAc) was 0.8%; insoluble residue 
(HCl) measured 0.3%; and LOI 3.6%. 
Tests were also conducted to determine whether or not there was in fact a 
significant difference between applicants' method of precipitating out 
metal titanate powders and those suggested by the prior art. This was done 
by substituting for the precursors employed in applicants' novel method, 
precursors of the type, disclosed by the prior art. In one such test, for 
example, tetraisopropyl titanate such as employed in applicants' 
above-noted Examples, was replaced by TiCl.sub.4, such as suggested in the 
above-noted U.S. Pat. No. 4,520,004. The TiCl.sub.4, containing a quantity 
of titanium equivalent to that employed in Example I above, was mixed with 
the same amount of acetic acid, distilled water, barium carbonate and 
sodium hydroxide as in Example I, and was processed in the same manner as 
in Example I, except that an additional 250 mL of distilled water had to 
be added to dissolve the BaCO.sub.3. The pH was increased, with a NaOH 
solution, to a value greater than 13. A precipitate was produced, 
filtered, dried and washed. Test results indicated that soluble Ba was 
present in the filtered mother liquor in the amount of 3,645 ppm (a loss 
of approximately 2.8 g of barium), and no soluble Ti. This significant 
loss of Ba led to a low mole ratio of 0.735. 
Still another test was conducted to determine whether or not stoichiometric 
barium titanate could be produced substituting in the method taught by the 
above-noted U.S. Pat. No. 4,520,004, barium carbonate in place of 
BaCl.sub.2, and by applying no external heat whatsoever to the reaction 
process. The precursors for this method included TiCl.sub.4 (50% diluted) 
in an amount of 102.90 g (0.27522 moles), distilled water in the amount of 
52 ml with an additional 178 ml needed to dissolve the BaCO.sub.3. The pH 
was then increased, with a NaOH solution to a value greater than 13. The 
slurry was, as above, filtered, dried, and washed. Soluble Ba in the 
filtered mother liquor amounted to 6,222 ppm (a loss of approximately 12 g 
of Ba), and soluble Ti in the amount of 2.5 ppm. Again, the significant 
loss of barium led to a very low mole ratio of 0.550. 
It was found that some water soluble salts, such as BaCl.sub.2, may be 
substituted as a source of the barium ion in the applicant's process, but 
BaCO.sub.3 is the preferred precursor due to its availability and low 
cost. 
From the foregoing, it will be readily apparent that the applicants have 
developed an improved method of effecting precipitation of metal titanate 
powders, and in particular alkaline earth metal titanate powders. Unlike 
prior art methods, it is not necessary with applicants' method to utilize 
external or additional heat in order to maintain the reaction temperature 
well above the normal reaction temperature or room temperature. 
Furthermore unlike the process disclosed in, for example, U.S. Pat. No. 
4,520,004 it is not necessary for applicant to utilize an inorganic 
titanium compound as a precursor, not for that matter is it necessary to 
use a water soluble salt of an alkaline earth metal for effecting 
precipitation of a crystalline metal titanate in a solution. On the 
contrary, it is possible to use water insoluble carbonates for producing a 
Ba.sup.+2, Sr.sup.+2 or Ca.sup.+2 ion's for reaction with the titanium 
acylate solution. As a consequence, this novel process eliminates the need 
for using, as noted above, expensive reaction vessels and external heating 
sources. 
The extremely fine BaTiO.sub.3 pwoder produced by this process can be 
readily converted to its tetragonal crystal structure simply by heating to 
approximately 950.degree. C., and in the latter form exhibits desirable 
ferro electric properties--i.e., when subjected to an electric field 
spontaneous polarization of its electric dipoles occurs. And in this fine 
powder form the material can be readily sintered to its optimum density. 
Although the SrTiO.sub.3, CaTiO.sub.3 and 20 Ba.sub.0.8 Sr.sub.0.2 
TiO.sub.3 powders produced in accordance with Examples II, III and IV do 
not convert to tetragonal crystal form upon heating (i.e., remain cubic), 
they nevertheless can likewise be readily sintered to optimum density, and 
can be added to modify the transition temperature of Curie point of a 
BaTiO.sub.3 composition. 
While this invention has been described in detail in connection with only 
certain embodiments thereof, it will be apparent that it is capable of 
further modification, and that this application is intended to cover any 
such modifications as may fall within the scope of one skilled in the art 
of the appended claims.