Production of spherical ceramic powders

A sol-gel method is employed to produce ceramic oxide spheres from precursors thereto. An alkanol of the sol precursor(s) is introduced to an immiscible hydrophilic liquid phase to cause spheroidizing of the sol. Moisture, which initially may be present in the immiscible liquid or may subsequently be added, is employed to effect hydrolysis of the precursor to oxide form. The ceramic oxide spheres are recovered from the liquid phase and calcined to their final form.

FIELD OF THE INVENTION 
The present invention is directed to the production of ceramic spheres 
using a sol-gel method. More particularly the present invention is 
directed towards the production of high performance ceramic powder spheres 
by way of sol-gel synthetic methods using polymerized alkoxides or similar 
ceramic precursors. 
BACKGROUND OF THE INVENTION 
Chemical methods of ceramic powder synthesis have been increasing in 
prominence in recent years because of their potential to produce 
homogeneous powders with controlled particle size and morphology. The 
majority of work using polymerized alkoxides has been done not for 
polycrystalline ceramics or powders but in silica or high silica glasses 
for optical fibre applications. 
The principle behind sol-gel synthetic methods using polymerized alkoxides 
is basically a decomposition-recomposition reaction. Metal alkoxides 
(metal-organic precursors) are partially hydrolyzed or decomposed, then 
are recombined by polymerization or condensation to form a 
metal-oxygen-metal bond. An example of this technology is found in the use 
of aluminum isopropoxide to form alumina. Aluminum alkoxide is added to 
water with stirring, and hydrolysis begins. Typically a sol is stabilized 
by acid addition and heating, the solution then is gelled and fired to 
yield a solid alumina body. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a novel method of producing 
ceramic spheres suitable for calcination to ceramic powders is provided, 
which comprises contacting an anhydrous alkanol sol of at least one 
ceramic oxide precursor with liquid with which said sol is immiscible to 
form spheres of said sol in said immiscible liquid. 
In accordance with an aspect of the present invention a novel method of 
producing ceramic spheres is provided wherein an anhydrous alkanol-based 
hydrophobic sol containing at least one ceramic oxide precursor is 
contacted with a liquid with which the sol is immiscible to form spheres 
of the sol in the immiscible fluid. The ceramic oxide precursor then is 
hydrolyzed to form ceramic oxide spheres resulting in an amorphous powder 
with spherical morphology. The ceramic spheres so produced are recovered 
from the liquid, by filtration or other suitable method, and calcined to 
form a final ceramic powder. 
The spheroidizing and hydrolyzing steps preferably are effected 
simultaneously employing an immiscible hydrophilic spheroidizing medium. 
Alternatively, the spheroidizing and hydrolyzing steps may be effected 
sequentially. 
The present invention permits the preparation of spherical particles of any 
size ranging from sub-micron size to large spherical particles (&gt;1 mm) out 
of structural ceramics and the application of this technology to the 
synthesis of hollow spheres from electronic or functional ceramic 
materials. The latter materials may be used as piezoelectric materials for 
transducer and/or sonar applications.

GENERAL DESCRIPTION OF INVENTION 
In the present invention, ceramic oxide spheres are formed from an 
anhydrous alkanol-based hydrophobic sol containing at least one ceramic 
oxide precursor and often a mixture of such precursors. For example, for 
producing transformation toughened alumina, a mixture of 
aluminum-tri-sec-butoxide and zirconium n-butoxide may be employed. The 
ceramic oxide precursor may be any suitable precursor which, when 
moistened, produces the ceramic oxide. Alkoxides are the preferred 
precursors. 
Two main considerations were focussed upon in developing the preferred 
embodiments of the present invention: 
(1) the sol is stable for a prolonged period (up to many months); and 
(2) the powder preparation method should use properties of the solvents and 
liquids employed to promote the production of spherical particles. 
The first consideration was addressed by making the sol anhydrous and 
hydrophobic thereby limiting the hydrolysis of the ceramic precursor upon 
storage. 
The second consideration was addressed by using the property of immiscible 
liquids, i.e. emulsions. In such systems, energy is minimized by 
minimizing the contact area between immiscible liquids. A spherical 
particle or droplet is the morphology at which the surface area to volume 
ratio is at a minimum, thus minimizing the contact area between the two 
liquids and consequently the energy of the system. 
The properties of ceramic oxide particles in accordance with a preferred 
embodiment of the invention is shown in the schematic illustration of FIG. 
1 and the process flow sheet of FIG. 2. 
Generally, in the sol preparation stage, the sol is prepared by dispersing 
particles of the ceramic oxide precursor of colloidal particle size in the 
hydrophobic solvent. The solvent comprises at least one alkanol, for 
example, isobutanol, generally along with an organic hydrocarbon solvent, 
such as toluene, to render the solvent highly hydrophobic. The sol is 
maintained anhydrous until spheroidizing is effected. 
An important aspect of the present invention is that the moisture content 
in the sols is maintained low until ceramic oxide particles are to be 
formed from the sol, to minimize hydrolysis of the ceramic oxide 
precursors. To achieve this result, a dehydrating molecular sieve or other 
suitable desiccant may be added to maintain a low moisture content in the 
sol, particularly where long term storage is desirable. 
The concentration of ceramic precursor in the sol may vary widely. 
Successful sphere formation may be achieved at an alkoxide concentration 
from about 1 to about 80 wt. %. The spheres tend to increase in size and 
decrease in number as the concentration of precursor increases. 
The concentration of alkanol present in the sol also may vary widely. 
Successful sphere formation may be achieved at a concentration of about 20 
to about 99 wt. %. The spheres tend to decrease in size and increase in 
number as the concentration of alkanol increases. 
The hydrophobic solvent component of the sol also may vary widely in 
concentration. Spheres may be formed at concentrations from about 12 to 
about 40 wt. %. The spheres tend to increase in size and decrease in 
number as the concentration of hydrophobic solvent increases. 
In the powder preparation step, the sol is reacted with a liquid in which 
the sol is immiscible and which is hydrophilic. To effect such reaction, 
the sol is most conveniently introduced in droplet form, to a continuous 
phase of the immiscible hydrophilic liquid, as illustrated in FIG. 1, 
whereby the immiscible liquid provides a spheroidizing effect on the sol 
droplet. The metal alkoxide in the sol droplets is partially hydrolyzed by 
water present in the hydrophilic liquid and the hydrolysis product then 
recombines by polymerization or condensation to form a metal-oxygen-metal 
bond in ceramic oxide particles which result. 
The spheroidizing hydrophilic liquid is a liquid immiscible with the liquid 
phase of the sol and contains water. Sphere formation has been found to be 
independent of water concentration in the immiscible liquid. The rate of 
hydrolysis and the physical stability of the spheres, however, tend to 
depend on water concentration. Generally, the aqueous component may 
comprise less than 1/2 to about 5 wt. % of immiscible liquid. 
The combination of the hydrophobic nature of the sol, the immiscibility of 
the liquids and the presence of water in the hydrophilic liquid ensures 
that fine powder particles of ceramic are formed. 
In a modification to the above described process, the spheroidizing and 
hydrolysis may be effected sequentially rather than simultaneously. In 
this modification, the sol also may be introduced initially into dry 
immiscible liquid, which effects the spheroidizing and yields unreacted 
spherical droplets of sol. These droplets then are contacted with water, 
such as by introducing water to the immiscible liquid, ceramic precursor 
in the spheres of sol hydrolyses and the droplet/liquid interface 
rigidizes. This modification has the advantage of decreasing the volumes 
of non-alkoxide components required to prepare the powders. 
Common mixed oxide ceramics used for structural applications include 
alumina, Transformation Toughened Alumina (TTA), Partially Stabilized 
Zirconia (PSZ), mullite, cordierite and spinel. Any of these materials may 
be formed from suitable alkoxide precursors prepared in accordance with 
the process of the present invention. 
It is appreciated that the present invention has a number of useful 
applications, including its suitability for space (zero gravity) 
production of high performance ceramic ball bearings. The stability of the 
anhydrous sol prior to reaction with a suitable polymerizing agent is 
advantageous in these applications. 
DESCRIPTION OF PREFERRED EMBODIMENT 
In one preferred embodiment, Transformation Toughened Alumina (TTA) was 
synthesized, details of such synthesis appearing in the Examples below. 
TTA is alumina dosed with zirconia as a toughening agent. There are two 
main steps involved in making the precursors for TTA, the first being 
synthesis of a stable sol containing the precursors required (sol 
preparation stage), and the second being preparation of a powder through 
reaction of this sol with the liquid (powder preparation stage). TTA is 
then prepared by heat treatment of the powder formed. 
TTA, like other common mixed oxide ceramics for use in structural 
applications, uses ceramic precursor alkoxides which are highly reactive 
and to-date have resulted in sols with high moisture sensitivity. As such, 
the reactions have been difficult to control. The anhydrous sols of the 
present invention overcome this problem and permit the development of a 
reliable reproducible method for the production of ceramic oxide 
structural spheres of TTA. The anhydrous sols of the present invention not 
only enhance the reliability and reproducibility of the sol-gel 
methodology employed herein but are also stable for long periods of time 
(on the order of months). 
In a preferred embodiment for the preparation of TTA, the sol components 
are aluminum-tri-sec-butoxide, zirconium-n-butoxide, isobutanol and 
toluene. The amount of each alkoxide is selected to yield an Al.sub.2 
O.sub.3 :ZrO.sub.2 ratio of about 9:1. This corresponds with the mid-range 
of ratios commonly used in TTA synthesis via conventional dry mixing 
routes. The isobutanol concentration was selected to be the lowest which 
yielded intact spherical particles, namely about 45 wt. % isobutanol in 
the sol. The toluene concentration, also was the lowest which yielded 
mainly intact spherical particles, namely about 17 wt. %. It is desirable 
to have the sol as concentrated as possible to minimize solvent 
evaporation and particle shrinkage. 
In this preferred embodiment for the preparation of TTA, the dry 
alcohol/toluene-based sol was added to an immiscible wet solvent, namely 
SPHEROL.TM.. 
SPHEROL is a trade mark for a proprietary mixture containing mainly 
acetonitrile, along with toluene isopropanol, ethanol and water. In this 
preferred embodiment, it was noted that the addition of toluene to the sol 
to make it strongly hydrophobic is an important aspect of this invention. 
The strongly hydrophobic nature of the toluene combined with keeping the 
sol dry allowed for a highly reactive and successful formation of ceramic 
particles during the powder preparation stage. As shown in the SEM 
photographs in FIGS. 3 and 4, micron and sub-micron sized spheres were 
produced. The ratio of sol to spheroidizing liquid is best kept to a 
minimum to maintain total volumes, cost and solvent recovery systems at a 
minimum. For the formation of TTA, a volume ratio of 10:1 (SPHEROL to sol) 
is preferred. 
Toluene is extremely non-polar and is an important component in making 
toughened alumina spheres. Toluene makes the sol less polar and enhances 
sphere formation in the TTA system. The usefulness of toluene to enhance 
sphere formation and/or size depends on the ceramic precursors used. 
Other ceramics, in addition to Transformation Toughened Alumina (TTA), have 
potential application as structural ceramics. These include mullite 
(3Al.sub.2 O.sub.3.2SiO.sub.2), cordierite (xMgO.yAl.sub.2 
O.sub.3.zSiO.sub.2) and spinel (MgO.Al.sub.2 O.sub.3). In addition, hollow 
electronic ceramic spheres have potential for use as magnetic devices. 
Barium titanate (BaTiO.sub.3) is an example of an electronic ceramic. 
EXAMPLES 
Example 1 
This Example illustrates the use of aluminum-tri-sec-butoxide and 
tetraethylorthosilicate to form mullite ceramic particles. 
Aluminum-tri-sec-butoxide was diluted with isopropanol to form a slightly 
hazy mixture. Tetraethylorthosilicate (TEOS) was diluted with dry ethanol. 
A precipitate formed immediately upon addition of the 
aluminum-tri-sec-butoxide mixture to the stirred TEOS mixture. After 1/2 
hour of stirring, the precipitate was stored overnight, refluxed at 
.about.86.degree. C. under a nitrogen atmosphere for about 6 hours, cooled 
and stored in a stoppered flask. The resulting sol (sol number 25) was 
still slightly hazy. 
The powder was prepared by injecting droplets of the sol into stirred, wet 
SPHEROL (150 mL). Stirring was continued for 5 minutes. The resulting 
powder was filtered, washed with acetone and dried. 
Optical microscopy examination of the powder revealed several spheres of 
various sizes (16 to 54 .mu.m in diameter). SEM examination revealed large 
spheres and lumps of powder at low magnifications. High magnification 
showed the powder lumps consist of tightly-packed sub-micron spheres. SEM 
photographs are shown in FIG. 5. 
Example 2 
This Example illustrates the preparation of ceramic powders of cordierite 
(xMgO.yAl.sub.2 O.sub.3.zSiO.sub.2) and spinel (MgO.Al.sub.2 O.sub.3) 
Anhydrous sols containing cordierite and spinel precursors were provided 
by UES of Dayton, Ohio. The powders were prepared by injecting sol into 
stirred, wet SPHEROL. Upon completion of sol addition, the mixture was 
stirred for 5 minutes. The resulting powder was filtered, washed with 
acetone and dried. 
The dried powders were examined using optical microscopy and SEM. The 
microscopy observations are given in Table 1, below. SEM photographs of 
the products are shown in FIGS. 6 and 7. 
TABLE 1 
______________________________________ 
PREATION OF CORDIERITE AND SPINEL 
CERAMIC POWDERS 
Powder Particle Size/Morphology 
Number Sol Used Observations on SEM 
______________________________________ 
23-xSAM-5 (Cordierite 
Low magnification (.about.1000 X): 
from UES) some spheres, fines 
High magnifications (.about.5000 X 
and .about.25000 X): fines consist 
of very small spheres; some 
have irregular shape. 
24-50AM-5 (Spinel Low magnification (.about.1000 X): 
from UES) few large spheres; fines on 
large spheres. 
High magnifications (.about.5000 X 
and .about.25000 X): fines consist 
of small spheres; many 
spheres have irregular 
shape. 
______________________________________ 
The powders produced were examined using energy dispersive x-ray analysis 
(EDXA) to determine the atomic percentages of their elemental components. 
In order to form the desired crystal structures upon calcination, the 
amorphous powders must contain the required ratios of metallic elements. 
EDXA results and elemental requirements are given in following Tables 2 
and 3. 
TABLE 2 
______________________________________ 
EDXA RESULTS AND ELEMENTAL REQUIREMENT 
FOR POWDER NUMBER: 23-xSAM-5, CORDIERITE 
Elements Required* 
Elements Found 
Trial (atomic percent) 
(atomic percent) 
Number Si Al Mg Si Al Mg Cu Cl 
______________________________________ 
1 45.5 36.4 18.2 42.20 
49.12 
8.32 0.35 
2 41.42 
49.04 
8.44 1.10 
3 41.95 
48.50 
7.90 0.68 0.97 
______________________________________ 
*for most favoured composition 
TABLE 3 
______________________________________ 
EDXA RESULTS AND ELEMENTAL REQUIREMENTS 
FOR POWDER NUMBER: 24-50AM-5, SPINEL 
Elements 
Trial Required Elements Found 
Num- (atomic percent) 
(atomic percent) 
ber Al Mg Al Mg Cu Si Cl Ba 
______________________________________ 
1 66.67 33.33 84.09 
15.21 
0.70 
2 81.18 
14.30 2.19 2.00 0.32 
3 83.21 
13.09 2.70 1.00 
4 77.39 
15.82 
6.79 
______________________________________ 
Example 3 
Transformation toughened alumina (TTA) was prepared using zirconia as a 
toughening agent. The sol had the following composition: 
______________________________________ 
isobutanol 34.8 wt % 
zirconium n-butoxide 3.8 wt % 
aluminum-tri-sec-butoxide 
39.7 wt % 
toluene 21.7 wt % 
______________________________________ 
The amount of each alkoxide was selected to yield a mole ratio AL.sub.2 
O.sub.3 :ZrO.sub.2 of 9:1. The sol was kept dry by continuous contact with 
a Type 4A Molecular Sieve. 
Anhydrous sol was introduced into SPHEROL. The sol: SPHEROL ratio was 1:10. 
Spheres of different sizes were yielded including many sub-micron spheres 
as seen in the SEM photograph of FIG. 8. 
SUMMARY OF DISCLOSURE 
In summary of this disclosure, the present invention provides a novel 
method of producing ceramic spheres employing a sol-gel method, in which 
an anhydrous hydrophobic sol of ceramic oxide precursors is introduced to 
an immiscible hydrophilic liquid to form spheres, which then are calcined 
to the final ceramic sphere. Although preferred embodiments of the 
invention have been described herein in detail, it will be understood by 
those skilled in the art that variations may be made thereto without 
departing from the spirit of the invention or the scope of the appended 
claims.