Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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4250-1/2 88-374 MIS/CJL 1990 05 07 D4
TITLE OF INVENTION
PRODUCTION OF SPHERICAL ~ERAMIC POWDE~S
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
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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 (> 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.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 is a schematic illustration of a
spheroidizing/polymerization process for forming
ceramic spheres in accordance with one embodiment of
the invention;
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Figure 2 is a schematic flow sheet for a process
for forming ceramic spheres in accordance with one
embodiment of the invention;
Figure 3 is a scanning electron photomicrograph
(SEM) of a first sample of ceramic spheres produced
using the process of Figures 1 and 2;
Figure 4 is a scanning electron photomicrograph
(SEM) of a second sample of ceramic spheres produced
using the process of Figures 1 and 2;
Figure 5 is a scanning electron photomicrograph
(SEM) of a third sample of ceramic spheres using the
process of Figures 1 and 2;
Figure 6 is a scanning electron photomicrograph
(SEM) of a fourth sample of ceramic spheres using the
process of Figures 1 and 2;
Figure 7 is a scanning electron photomicrograph
(SEM) of a fifth sample of ceramic spheres using the
process of Figures 1 and 2.
Figure 8 is a scanning electron photomicrograph
(SEM) of a sixth sample of ceramic spheres using the
process of Figures 1 and 2.
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:
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(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
5employed 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.
10The 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
15to 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
20is shown in the schematic illustration of Figure 1 and
the process flow sheet of Figure 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
25solvent. 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.
30An 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 ceramlc oxide
precursors. To achieve this result, a dehydrating
35molecular sieve or other suitable desiccant may be added
2~235
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 ~9
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 Figure 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
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water concentration. Generally, the aqueous component
may comprise less than ~ 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 al~mina, 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
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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
A12O3:ZrO2 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.
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In this preferred embodiment for the preparation of
TTA, the dry alcohol/toluene-based sol was added to an
immiscible wet solvent, namely SPHEROL
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 Figures 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 (SP~EROL 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
(3A1203 2SiO2), cordierite (xMgO yA12o3 zsio2) and
spinel (Mg A1203). In addition, hollow electronic
ceramic spheres have potential for use as magnetic
devices. Barium titanate (BaTiO3) is an example of an
electronic ceramic.
2Q1 ~2~~a
~PhES
Example l
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 ~ hour of stirring, the precipitate
was stored overnight, refluxed at ~86C 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~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 Figuxe 5.
Example 2
This Example illustrates the preparation of ceramic
powders of cordierite (xMgO yAl2O3 zSiO2) and spinel
(MgO Al2O3). 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.
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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 Figures 6 and 7.
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11
TABLE 1
PREPARATION OF CORDIERITE AND SPINEL CERAMIC POWDERS
Powder Particle Size/Morphology
Number Sol Used Observations on SEM
23-xSAM-5 tCordierite Low magnification (~lOOOX):
from UES) some spheres, fines
High magnifications (~5000X
and ~25000X): fines consist
of very small spheres; some
have irregular shape.
24-50AM-5 (Spinel Low magnification (~lOOOX~:
from UES) few large spheres; fines on
large spheres.
High magnifications (~5000X
and ~25000X): fines consist
of small spheres; many
spheres have irregular
shape.
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12
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 REOUIREMENT
FOR POWDER NUMBER: 23-xSAM-5. CORDIERITE
TrialElements Required* Elements Found
Number(atomic percent) (atomic percent~
Si Al Mq Si Al Mq Cu Cl
1~5.5 36.4 18.2 42.20 49.12 8.320.35
2 41.42 49.04 8.441.10
3 41.95 48.50 7.900.68 0.97
*for most favoured composition
TABLE 3
EDXA RESULTS AND ELEMEi~AL REOUIREMENTS
FOR POWDER NUMBER: 24-50AM-5, SPINEL
Trial Elements Required Elements Found
Number(atomic percentl (atomic percent)
Al Ma Al Mq Cu Si Cl Ba
1 66.67 33~33 84.09 15.21 0.70
2 81.18 14.30 2.192.00 0.32
3 83.21 13.09 2.701.00
4 77.39 15.82 6O79
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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 AL2O3:ZrO2 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 Figure 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 immisaible 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.
