Note: Descriptions are shown in the official language in which they were submitted.
~59~i3~
HYDROTHERMAL METHOD FOR PRODUCING STABILIZED
ZIRCONIA AND_THE PRODUCT THEREOF
FIELD OF THE INVENTION
This invention relates to a hydrothermal method
for the production of fully and partially stabilized
zirconia with controlled particle size by hydrothermal
processing. The process further includes stabilization
of zirconia with more than one stabilizer. Powders
with special stabilization properties may he produced.
BACKGROUND OF THE INVENTION
In the last several years there has been an
increasing interest in zirconia ceramics. This
interest emanates largely from the discovery by
Garvie, et al that partially stabilized zirconia
(PSZ) ceramics could be fabricated with high strength
and high fracture toughness. Garvie named the material
"ceramic steel". Current applications include extru-
sion and drawing dies, cyclone heads, and tool bits.
A large number of wear applications are envisaged.
The largest potential application is in automotive
engines where partially stabilized zirconia is being
tested as a material for piston crowns, valve guides,
and valve seatings. In the advanced diesel engines,
currently under development, partially stabilized
zirconia is the leading candidate material for the
cylinder liner and piston cap.
The term stabilized zirconia as used herein
refers to both fully and partially stabilized zirconia
materials. Fully and partially stabilized zirconia
can be fabricated with a number of dopants or stabi-
lizers. Examples of these are yttria, calcium oxide,
or magnesium. The quality of the powder and distribu-
tion of dopant in the powder play an important role
ln determining the microstructure of the ceramic
after sintering. Since the effectiveness of the
~25~
martensitic transformation responsible for stabilized
zirconia's strength and fracture toughness depends
upon its microstructural features, control of the
physical and chemical properties are essential.
The following factors affect the martensitic transfor-
mation in PSZ:
(1) Grain size and grain size distribution
(2) The transformation temperature
(3) Type and concentration of stabilizing
agent
(4) Density of the sintered ceramic
(5) Purity and the presence of grain boundary
phases
(6) Microstructural homogeneity
(7) Twin spacing
(8) Phase type and concentration.
Many of these factors can be related to the character-
istics of the starting powder. For example, if
the particle size of the starting powder is too
large the strain energy in the grain will be sufficient
to convert tetragonal grains spontaneously to the
stable monoclinic form. A similar effect will occur
if the ceramic does not have a high density due
to lack of constraint by the surrounding matrix.
This may also occur near porous areas in high density
materials. Such effects are due to the presence
of agglomerates in the starting powder.
The purity of the powder may affect the sintering
characteristics of the powder as well as retention
of the tetragonal phase through the formation of
grain boundary phases which aid in sintering, but
lower the high temperature properties of the ceramic
- as well as reduce the concentration of the stabili~er
in the grains comprising the bulk of the ceramic.
Use of a high quality powder is thus an essential
prerequisite for producing optimum PSZ ceramics.
~2~34
The objectives of microstructure control in
any ZrO2-containing ceramic are (a) to obtain as
high a volume fraction o~ the tetragonal particles
as possible, and (b) to optimize the particle size
and size distribution. Large particles trans~or~
spontaneously and do not contribute to toughening
while very small particles will require very high
stresses for trans~ormation. It is desirable to
have a narrow particle-size distribution about the
optimum size. This can be achieved by the present
invention.
There are five main methods of producing partially
stabilized zirconia powder: (1) powder mixing,
(2) coprecipitation and decomposition, (3) vapor
phase decomposition, (4) sol-gel processing, and
(5) hydrothermal processing. The preferred method
of preparing stabilized zirconia powder is the one
that gives the best combination of cost and performance
in terms of the cost of producing the powder and
the powder's technical features. Mixing commercially
available powders is an inexpensive method to prepare
the powder, ~ut may result in ceramics with poorer
properties because solid state mixing does not al~ays
result in homogeneous distribution of the dopant
throughout the powder.
A popular method for preparing stabilized zirconia
powder is coprecipitation and decomposition in which
salts of both the zirconia and the stabilizer are
: first precipitated from solution. This mixture
of salts is then calcined to form the oxide. Pine
reactive powders can be prepared by this method,
but the calcination step requires a high temperature
step to produce the oxide and may create agglomerates
in the powder. Furthermore, the grinding operation
required to break down the agglomerates can contaminate
the powder. The vapor phase decomposition pxocess
is a thermal (or plasma assisted) chemical vapor
i25~6~3A
decomposition reaction in which chlorides or metal-
lorganic compounds of zirconia and the stabilizer
are used as the starting materials. Powders made
by this technique are generally extremely fine and
difficult to handle because of their low bulk density.
The process is also relatively expensive. In the
sol-gel process alkoxides are polymerized and subse-
quently heat treated to form the oxide. Ultrafine
powder is generally produced, but with appropriate
engineering a wide variety of particle sizes can
be prepared; however, unless excess water is avoided,
the powder may contain chemically bound water which
prevents the particles from being fully dense.
The raw materials for this process, alkoxides of
zirconia and yttria, are also relatively expensive.
The method has the advantage that partially stabilized
powder (not a mixture of zirconia and the oxide
of the stabilizing agent) is produced directly.
The hydrothermal method of preparing zi~conia
powder in the present invention ofers the best
possibility of producing a high quality partially
or fully stabilized zirconia powder at attractive
production costs.
The objectives of the invention are to produce
a high quality stabilized or partially stabilized
zirconia powder from zirconyl nitrate, from zirconyl
oxychloride, from zircon sands and other appropriate
feedstocks, with controlled particle sizes that
can be used to fabricate ceramics with superior
properties, to produce a lower cost powder, and
to produce a reactive free flowing powder exhibiting
a high degree of crystallinity and crystalline perfec
tion, a high degree of homogeneity and containing
little or no bound water.
BRIEF DESCRIPTION OF THE IMVENTIO~
Unlike many other powders that are mixtures,
the process of the invention produce5 powders in
~S !3~i:34
which th~ individual crystallites themselves are
partially or fully stabilized. Furthermore the
process (l) enables the particle size to be controlled
within narrow limits, (2) produces a very active,
homogeneous powder, (3) gives rise to an agglomerate
fr~e pow~er or one containing only weak agglomerates,
(4) produces a powder containing little or no chemically
bound water, and (5) produces a powder with a high
degree of crystallinity and crystalline perfection.
These factors, together with the cost advantages
of the hydrothermal process, and the potential use
for using low cosk starting materials (such as zircon
sand or zirconyl oxychloride), are indicative of
the invention's advantages.
The invention uses two basic approaches or
a combination of these approaches to control particle
size and achieve other desired objectives. The
first is a colloidal method using zirconyl nitrate,
zirconyl oxychloride or feed materials from zircon
sands. In the first method the aqueous feed material
is first titrated with a base such as NaOH, ammonium
hydroxide or tetraethylammonium hydroxide to the
appropriate pH of greater than about 9Ø Single
; or multiply stabilized zirconia with stabilizers
chosen from MgO, CaO, and Y2O3 or other stabilizers
with similar characteristics may be used. The material
is then hydrothermally treated at appropriate temperatures
and pressures and for a time adapted to ~ive the
stabilized powdered zirconia of appropriate particle
size. Complexing agents may or may not be present.
The powders produced by this method can range in
size from 0.1 to l~m with smaller and larger sizes
present according to process conditions.
The second approach uses the same feed solutions
as above but must involve complexing agents and
careful feedstock preparation. The feedstock is
carefully titrated to a pX of about 9 at a slow
S963fl
rate adapted to produce a homogeneous solution that
results in a homogeneous powder by hydrothermal
precipitation. Again the material is hydrothermally
treated at appropriate temperature, pressure, and
for a time adapted to give the stabilized powdered
zirconia. A typical complexing agent is ethylene-
diaminetetraacetic acid (EDTA). The resulting powder
is in the particle size range from 0.5 - 3.0~m.
A combination of the above two methods may
be used to produce a powdered product containing
more than one major particle size.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a photomicrograph of the powder
produced by the colloidal method with EDTA complexing
agent composed of ZrO2 partially stabilized with
3.38% of MgO and 3.39% of Y2O3. (Ex. 24 Table III)
Figure 2 is a photomicrograph of the powder
produced by the colloidal method with EDTA complexing
agent composed of ZrO2 partially stabilized with
3.29% of Y2O3. (Ex. 20 Table III)
Figure ~ is a photomicrograph of the powder
produced by the colloidal method oE the invention
composed of ZrO2 partially stabilized with 2.75
of CaO. ~Ex. 18 Table III)
Figure 4 is a photomicrograph of the powder
produced by the homogenous solution method of the
invention composed of ZrO2 partially stabilized
with 2.01~ of CaO. (Ex. 16 Table II)
Figure 5 shows photomicrographs of a sintered
sample obtained by use of powders from Example 20,
Table III. View 5A shows a fracture surface and
- View 5B a thermally etched surface of the sintered
sample.
~:5963~L
DETAILED DESCRIPTION OF THE INVENTION AND PREFEF~RED
EMBOD ~MENT
The invention is characterized by two methods
of producing stabilized or partially stabilized
zirconia powders with controlled particle size by
hydrothermal treatment. A further embodiment of
the invention is characterized by a combination
of the first two methods to produce powders containing
more than one major particle size. The invention
also allows use of low cost readily available starting
material, controlled doping level and incorporation
of dual constituents such as MgO, CaO; Y2O3, MgO;
or Y2O3, CaO in the structure for production of
stabilized zirconia. Triply stabilized zirconia
containing Y2O3, MgO and CaO may also be produced.
Zircon sand may be used as a source of zirconium
ion to prepare powders via either the colloidal
or homogeneous solution processes. Zircon sand
i5 mixed with an alkali salt such as sodium hydroxide,
sodium carbonate, etc. The mixture is heated to
between 500C and 1000C to form water soluble alkali
silicates. The alkali silicates are dissolved by
a water leach. The resulting, undissolved material
hereafter called the zircon frit is used to recover
zirconium ions via the selective leaching of the
same in acidic conditions. The undissolved zircon
frit is recovered from the supernatant containing
the dissolved alkali silicates. The recovered zircon
frit is placed in suitable acid solutions such as
hydrochloric acid, nitric acid, sulfuric acid, etc.
and hydrothermally treated to selectively leach
the zirconium ion values from the zircon frit feedstock.
Typical temperatures to hydrothermally leach the
zirconium ions range from 120C to 175C. Typical
acid concentrations range from 75 to 200 grams per
liter. The undissolved sediment composed primarily
of silica is removed from the zirconium rich superna-
tant. The supernatant is then used as the source
~259~3~
of zirconium ions in the hydrothermal preparation
of zirconia powders. Baddeleyite may also be used.
The zirconium rich supernatant is used to prepare
feedstocks by titration with base solutions after
addition of appropriate complexing agent and stabiliz-
ing agent(s) for either the colloidal or homogeneous
solution approaches to produce the zirconia powders
as further discussed below.
Preparation of stock solution. The preparation
of stock solution is typical for all these examples
using zirconyl nitrates. As alternatives zirconium
sulphates, zirconium chlorides, zirconium hydroxides,
zirconium carbonates, zirconium acetates, or zirconium
oxides could be used as a feed material following
similar preparation procedures. All concentrations
are in moles per liter.
EXAMPLES 1-2
Stock solution was prepared by mixing 1 M ZrO2-
~NO3)2 and 1 M Ca(NO3)2 ~ 4H20 to produce a feedstockcontaining 0.176 M ZrO(NO3)2 with 0.088 M Ca(NO3)2
4H20. No EDTA was added. This resulted in an
initial molar ratio of zirconia to calcia of 2:1.
The initial pH was 1Ø The sample was placed in
a teflon lined container which was placed in an
autoclave and treated at 193C for two hours. A
white precipitate with a primary particle size of
1-3~m that appeared heavily agglomerated was obtained.
Analysis of the white precipitate confirmed that
acid conditions as present here prevented the Ca++
ions from precipitating as the stabilizing agent
in the zirconia. Accordingly higher pH values were
tested.
EXAMPLES 3-6
.
This stock solution was prepared as Examples
1-2 but 0.176M EDTA was also added.
~L2S9~3~1L
Six samples were prepared ~rom these ~eedstocks
using various ranges of p~ from 1.0-8.23 as shown
in Table I. Samples 1 and 2 were treated at low
acidic pH. They illustrate that the stabilizing
agent, in this case CaO, does not form a solid solution
with zirconia at this low pH. This was confirmed
by analysis of the precipitate of Sample 1. The
level of CaO was within the limits of error of the
analysis and was therefore essentially ~ero.
Samples 3-6 were treated at a slightly alkaline
pH and using EDTA. Only Sample 4 obtained a precipi-
tate. This precipitate consists of very fine particles
(<O.l~m) that appear slightly agglomerated. Particle
size analysis using a deionized water sol~ent resulted
in an average particle size of 0.05~m. Since Sample
3 (ph 8.23) did not produce a powder, this example
shows that the presence of stabilizing agen~s and
still higher pH levels for this temperature and
time are necessary to produce undoped zirconia.
Table II lists further examples o~ the homogeneous
method of the invention. Here dually stabilized
zirconia is produced. Feedstock material was prepared
as follows:
EXAMPLES 7-12
A stock solution was prepared by diluting a
lM ZrO(N03)2 with distilled water to produce a 0.338
M solution. 0.338 M H4EDTA was added to the above
while stirring. 1.207 M NH40H was added to raise
the pH to approximately 9Ø Appropriate quantities
of calcium nitrate, magnesium nitrate and yttrium
nitrate were added to give a dual concentration
of 0.0845 moles/liter as shown in Table II.
EXAMPLES 14-15
A stock solution was prepared as a~ove but
appropriate NaOH was added to bring the pH to approxi-
mately 11Ø
~;25~634
EXAMPLES 16-17
-
These were prepared as in Examples 14-15 above
except only a single stabilizing agent was added
at a concentration of 0.158 and 0.164 moles per
liter respectively. The zirconium oxynitride concen-
tration also varied as shown in Table III.
Results for these examples show that it is
apparent that between 1-3 hours at a pH above 9.0
are needed to produce a precipitate in that Example
7, 9, and 11 at 1 hour had no precipitate while
Examples 8, 10, and 12 had a characteristic white
precipitate. Example 13 illustrates that higher
pH levels reduce the time needed to obtain a product.
The homogeneous solution method is based on
the thermal decomposition of complexing agents present
in the feedstock. This results in the precipitation
of stabilized zirconia particles. This is accomplished
by the chemical breakdown of zirconium ion - complexing
agent couples at hydrothermal temperatures. Complexing
agents are added to zirconium containing feedstocks
initially at low pH prior to titration with base
to the feedstock pH required to produce PSZ powders.
The complexing agent is used to prevent the formation
of metastable zirconium ~hydrous) oxide precipitates
which normally form as the pH is adjusted above
pH 3. This requires the slow addition of base to
the feedstock during titration accompanied by rapid
stirring or mixing. If the base is added too fast,
the zirconium ion - complex is not formed fast enough
3~ to prevent precipitation of the zirconium (hydrous)
oxide.
The mode of action of the complexing agent
in control of the particle size during precipitation
operates by one or a combination of three mechanisms.
35 Complexing agents may control the rate o~ crystal
growth to produce larger particles by three mechanisms:
their presence decreases the effective concentration
~LZ59~i3~
of ions available for nucleation and growth; complex
ions may adsorb at growing crystal surfaces inhibiting
the delivery of growth ions to the crystal lattice;
and the decoupling of ligand from complex usually
requires a finite time. Thus, the latter limits
ionic concentrations available for nucleation and
growth at any given time.
Selection of a specific complexing agent is
dependent on: (1) the equilibrium constants dictating
the stability of the cation - complex agent bond~;
(2) whether side reactions are possible resulting
in undesirable products; (3) removal of complex
agents from the final product; and (4) the thermal
decomposition temperature of the complexing agent
in solution.
For example, EDTA forms reasonably stable,
water soluble complexes with zirconium ions. When
pH of the zirconium ion feedstocks is adjusted to
higher pH, the formation of the soluble zirconium
- EDTA complex ion prevents formation of the relatively
insoluble zirconium (hydrous) oxide. This results
in a homogeneous feedstock solution at pH 9 prior
to hydrothermal treatment. The thermally initiated
chemical decomposition of the EDTA is used to release
; 2S zirconium ions to the solution for precipitation.
It is known that EDTA decomposes at about 215C
; to species with less affinity for metal ions. Thus,
the chemical decomposition o~ the complexing agent
or the decrease in affinity of the complexing agent
for the zirconium ion under hydrothermal conditions
may be used to release precipitating ions to the
solution. This technique may be used to control
the size of particles based on the feedstock pH
and zirconium and complexing agent concentrations.
Additional complexing agents to EDTA include:
carboxylic acids or salts thereof such as tartaric
acid, citric acid, oxalic acid, etc; inorganic ions
~;~59~3~
such as the salts or acids of nitrates, chlorides,
sulfates, hydroxides, etc.; polyelectrolytic inorganic
species such as sodium hexametaphosphate; aminocarbox-
ylic acids or salts thereof in addition to EDTA
such as HEDT~ (hydroxyethylethylenediaminetriacetic
acid~, DTPA (diethylenetriaminepentaacetic acid),
etc.; polyelectrolytic acids and salts thereo~ such
as ammonium polymethylmethacrylate; organic phosphonic
acids such as aminotri(methylenephosphonic acid);
and organic amino compounds such as TEA (triethylamine)
and TEOA (triethanolamine~.
A general way to describe the homogeneous version
of the invention would be as a process for the produc-
tion of stabilized zirconia powders by hydrothermal
treatment that comprises: Providing an aqueous
feedstock selected from the group consisting of
zirconium nitrates, zirconium sulphates, zirconium
chlorides, zirconium hydroxides, zirconium carbonates,
zirconium acetates, zirconium oxides and zirconium
ion containing solutions derived from the hydrothermal
leach of zircon sand or baddeleyite with mineral
acids; adding and mixing a complexing agent with
the feedstock; mixing with-the feedstock and complexing
agent at least one and mixtures thereof of materials
selected from the group consisting of nitrates,
sulphates, chlorides, hydroxides, carbonates, acetates,
and oxides of yttrium, magnesium and calcium; titrating
the resulting mixture to a pH > 9 with a base in
a manner adapted to prevent precipitation of a hydrous
oxide; treating the titrated mixture at hydrothermal
conditions adapted to produce a stabilized zirconia
precipitate; and separating the stabilized zirconia
precipitate from the mixture.
The precipitate is then washed and the suspension
may be used directly in slip, centripetal t or pressure
casting. It may be directly spray dried or granulated
as well as cold extruded. The precipitate may also
~25963~
- 13 - 2649~-105
be separated and dried to form a powder subsequent to washing.
Washing may be done with an organic acid such as acetic acid and
with deionized water. Separation may be by centrifugation,
sedimentation or filtration.
The titrating step may use a base selected from the
group consisting of NH4OH, NaOH, or tetraethylammonium hydroxide
and the hydrothermal treating conditions are for at least 5
minutes at about 350C, for at least 15 minutes at about 300C,
for at least 45 minutes at about 250C, and at least l hour at
about 190C. The autogenous pressure produced in the autoclave
may be used.
The invention includes a stabilized zirconia precipitate
or powder produced with the aforementioned homogeneous process.
This precipitate or powder may be singly, dually or triply stabil~
ized with oxides selected from the group consisting o~ calcium
oxide, magnesium oxide, and yttrium oxide.
Specific conditions o~ the homogeneous method
illustrated in Table II include: Providing an aqueous feedstock
of zirconium nitrate; adding and mixing EDTA with the feedstock;
mixing with the feedstock and EDTA at least one and mixtures
thereof of materials selected from the group consisting of yttrium
nitrate, magnesium nitrate and calcium nitrate; titrating the
resulting mixture to a pH > 9 with NH40M in a manner adapted to
prevent precipitation of a hydrous oxide; treating the titrated
mixture at hydrothermal conditions of > 190C for l hour at
elevated autogenous pressures, which are adapted to produce a
1~:59~
- 14 - 26494-105
stabilized zirconi.a precipitate, and separating the stabilized
zirconia precipitate from the mixture.
Another way, using sodium hydroxide, is by providing an
aqueous feedstock of zirconium nitrate; adding and mixing EDTA
with the feedstock; mixing with the feedstock and EDTA at least
one and mixtures thereof of materials selected from the group
consisting of yttrium nitrate, magnesium nitrate and calcium
nitrate; titrating the resulting mixture to a pE ~ 11 with NaOH in
a manner adapted -to prevent pr~cipitation of a hydrous oxide;
treating the titrated mixture at hydrothermal conditions of ~
190C for > 1 hour at autogenous pressures, which are adapted to
produce a stabilized zirconia precipitate, and separating the
precipitate from the mixture.
EXAMPLES 18-20, 26-28
These examples use chemical solutions prepared in the
manner of previous examples but with concentrations of zirconium
oxynitrate as illustrated in Table III. EDTA is added to Examples
18-20 to give single stabilization with calcia, magnesia, or
yttria.
EXAMPLES 21-25
These examples use chemical solutions prepared in the
manner of previous examples but with concentrations of zirconium
oxychloride as illustrated in Table III. EDTA is not used and
dual stabiliæation is obtained for Examples 22, 24, 25. Examples
21 and 23 were performed to confirm results for unstabilized
and singly stabilized powders of the prior art.
X
25963~
- 14a - 26494-105
Table III lists results obtained for Examples 18 to 28
for hydrothermal treatment using process conditions that produce
colloidal suspensions of the zirconium (hydrous) ox;de plus
stabilizing agent(s). The feedstock materials for these examples
are prepared by the colloidal process technique. An important
aspect of this technique is the proper mixing of feedstock
materials at low pH. After mixing the solution is brought to
proper pH (preferably 9.0 or above). If this procedure is not
done correctly proper control of particle size will not result.
~2~634
While EDTA is something present it is not needed
at all of the conditions shown; however a hybrid
powder of more than one major size distribution
would al~ays require EDTA or other complexing agent.
Figure 1 illustrates the powder obtained by the
process of Example 24 of Table III.
The approach incorporated in the colloidal
feedstock process is to obtain a stable dispersion
of precursor hydrous oxide particles. The precursor
hydrous oxide particles composed of the zirconium
ions and stabilizing agent(s) ion(s) in atomic mixture
within each particle are obtained by the rapid titra-
tion of a zirconium salt solution containing the
stabilizing agent(s) using a suitable concentrated
base solution such as ammonium hydroxide. Suitable
zirconium salt solutions may be obtained from the
dissolution of salts of zirconium oxychloride, oxyni-
trates, oxysulfates, etc. or from the zirconium
ion containing solution resulting from the recovery
of zirconium ions from the leaching of zircon frit
or from baddeleyite. The stable dispersion of precursor
hydrous oxide particles resulting from the titration
described above is hydrothermally treated under
the appropriate conditions to obtain 0.1 to 1 micron
particles of the anhydrous oxide containing the
stabilizing agent(s) in solid solution with the
zirconium oxide. The use of complexing agents is
optional in the colloidal feedstock approach. However,
the complexing agents are preferred and may be used
to control the degree of dispersion in the precursor
feedstock.
Complexing agents are added to produce well
dispersed feedstocks via surface charge effects.
Complexing agents are used as such in both the homoge-
neous solutions and colloidal solutions. In thehomogeneous solutions the complexing agents are
used as a way to maintain a soluble complex species.
~259639~
16
In the colloidal solutions the complexing agent
is used for its surface adsorbing properties. In
both solutions the complexing ions prevent or minimize
the ~ormation of agglomerations and are used to
control particle size. It is expected that particle
size will not be controlled when the complexing
agent is not present in sufficient quantities. In
some reactions as with zirconium o~ychlorides or
zirconium chlorides, the complexing agent, the chloride
ion, is already present but does not control particle
size fully on its own. Additional complexing agents
must be added to control the particle size of the
precipitate appropriately. Other workers using
a colloidal approach to produce singly stabilized
zirconias but without the use of additional complexing
agents were not able to obtain the particle si~es
of the present invention.
A general way to describe the colloidal version
of the invention would be as a process for the produc-
tion of stabilized zirconia powders by hydrothermaltreatment that provides an aqueous feedstock selected
from the group consisting of zirconium nitrates,
zirconium sulphates, zirconium chlorides, zirconium
hydroxides, zirconium carbonates, zirconium acetates,
zirconium oxides and zirconium ion containing solutions
derived from the hydrothermal leach of zircon sand
or baddeleyite with mineral acids; adding and mixing
a complexing agent with the feedstock; mixing with
the feedstock and complexing agent at least one
and mixtures thereof of materials selected from
the group consisting of nitrates, sulphates, chlorides,
hydroxides, carbonates, acetates, and oxides of
yttrium, magnesium and calcium; titrating the resulting
mixture to a pH ~ 9 with a base in a manner adapted
to precipitate hydrous oxide; treating the titrated
mixture at hydrothermal conditions adapted to produce
~25~63~L
a stabilized zirconia precipitate; and sepaxating
the stabilized zirconia precipitate from the mixture.
The precipitate i5 then washed and the suspension
may be used directly in slip, centripetal, or pressure
casting. It may be directly spray dried or granulated
as well as cold extruded. The precipitate may also
be separated and dried to form a powder subsequent
to washing. Washing may be done with an organic
acid such as acetic acid and with deionized water.
Separation may be by centrifugation, sedimentation
or filtration.
The titrating step may use a base selected
from the group consisting of NH40H, NaOH, or tetraeth-
ylammonium hydroxide and the hydrothermal treating
conditions are for at least 5 minutes at about 350C,
for at least 15 minutes at about 300C, for at least
45 minutes at about 250C, and at least 1 hour at
about 190C. Autogenous pressure produced in the
autocla~e may be used.
The invention includes a stabilized zirconia
precipitate or powder produced with the aforementioned
colloidal process. This precipitate or powder may
be singly, dually, or triply stabilized with oxides
selected from the group consisting of calcium oxide,
magnesium oxide, and yttrium oxide.
Specific conditions of the colloidal method
illustrated in Table III include providing an aqueous
feedstock selected from the group consisting of
zirconium nitrate; adding and mixing EDTA with the
3n feedstock; mixing with the ~eedstock and EDTA at
least one and mixtures thereof of materials selected
from the group consisting of yttrium nitrate, magnesium
nitrate and calclum hydroxide; titrating the resulting
mixture to a pH > 9 with NaOH or NH40H in a manner
adapted to precipitate a hydrous oxide; treating
the titrated mixture at hydrothermal conditions
of 190C for > 1 hour at autogenous pressure, which
~Z59~i3~
18
are adapted to produce a stabilized zirconia precipitate;
and separating the stabilized zirconia precipitate
from the mixture.
More than one major particle size distribution,
in this case two, may be obtained by a combination
of the homogeneous and colloidal approaches in two
ways. First, by preparing a homogenous solution,
performing the titration step in the homogenous
method, and adding a controlled amount of colloidal
hydrous oxide particles. Second, by controlled
addition of a base.
The first way of producing a stabilized zirconia
powder having two major particle size distributions
is by providing an aqueous feedstock selected from
the group consisting of zirconium nitrates, zirconium
sulphates, zirconium chlorides, zirconium hydroxides,
zirconium carbonates, zirconium acetates, zirconium
oxides and zirconium ion containing solutions derived
from the hydrothermal leach of zircon sand or bad-
deleyite with mineral acids; adding and mixing a
- complexing agent with the feedstock; mixing with
the feedstoc~ and complexing agent at least one
and mixtures thereof of materials selected from
the group consisting of nitrates, sulphates, chlorides r
hydroxides, carbonates, acetates, and oxides of
yttrium, magnesium and calcium; titrating the resulting
mixture to a pH > 9 with a base in a manner adapted
to prevent precipitation of a hydrous oxide; adding
colloidal hydrous oxide particles to the mixture;
treating the mixture at hydrothermal conditions
adapted to produce a stabilized zirconia precipitate;
and separating the stabilized zirconia precipitate
from the mixture.
Treatment of the precipitate is the same as
that discussed earlier. The powder product of the
process may be singly, dually, or triply stabilized.
~Z59~i3~
19
The second way of producing a stabilized zirconia
powder having two major particle size distributions
is by providing an aqueous feedstock selected from
the group consisting of zirconium nitrates~ zirconium
sulphates, zirconium chlorides, zirconium hydroxides,
zirconium carbonates, zirconium acetates, zirconium
oxides and zirconium ion containing solutions derived
from the hydrothermal leach o~ zircon sand or bad-
deleyite with mineral acids; adding and mixing a
complexing agent with the feedstock; mixing with
the feedstock and complexing agent at least one
and mixtures thereof of materials selected from
the group consisting of nitrates, sulphates, chlorides,
hydroxides, carbonates, acetates, and oxides of
yttrium, magnesium and calcium; titrating the resulting
mixture to a pH > 9 with a base in a manner adapted
to precipitate only a portion of the precipitatable
hydrous oxide; treating the titrated mixture at
hydrothermal conditions adapted to produce a stabilized
zirconia precipitate; and separating the stabilized
zirconia precipitate from the mixture.
Again further treatment of the precipitate
is the same as that discussed earlier. This powder
product may also be singly, dually, or triply stabilized.
The advantages of the dually stabilized zirconia
was discussed above. These advantages are also
inherent in triply stabilized zirconias which may
be produced by the aforementioned methods. It has
further been discovered by the inventors that dually
and triply stabilized zirconias may be produced
by a colloidal method where no additional complexing
agent is added. This latter method has the disadvantage
that particle size is not as controlled and is expected
to be smaller than that obtained with the additional
complexing agent.
~Z~963~L
This colloidal method without the addition
of complexing agent was used to obtain dually stabilized
zirconia; however, it is expected that triply stabilized
are also possible. This dually or triply stabilized
zirconia is produced hy providing an aqueous feedstock
selected from the group consisting of zirconium
nitrates, zirconium sulphates, zirconium chlorides,
zirconium hydroxides, zirconium oxides and zirconium
ion containing solutions derived from the hydrothermal
leach of zircon sand or baddeleyite with mineral
acids; mixing with the feedstock at least two and
mixtures thereof of materials selected from the
group consisting of nitrates, sulphates, chlorides,
hydroxides, carbonates, acetates, and oxides of
yttrium, magnesium and calcium; titrating the resulting
mixture to a pH > 9 with a base in a manner adapted
to precipitate a hydrous oxide; treating the titrated
mixture at hydrothermal conditions adapted to produce
a stabilized zirconia precipitate; and separating
the stabilized zirconia precipitate from the mixture.
The dually stabilized product may contain zirconia
stabilized by CaO, MgO or Y203, CaO or Y203, MgO.
The triply stabilized zirconia product would contain
Y20, CaO and MgO.
Specific conditions shown in Table III for
this special case.of producing dually and triply
s~abilized zirconias includes providing an aqueous
feedstock of zirconium oxynitrate or zirconium oxychloride;
mixing with the feedstock at least two of materials
selected from the group consisting of yttrium nitrate,
magnesium nitrate and calcium hydroxide; titrating
the resulting mixture to a pH > 9 with NaOH or NH40H
in a manner adapted to precipitate a hydrous o~ide;
treating the titrated mixture at hydrothermal conditions
35 of 190C for > 1 hour at autogenous pressure, which
are adapted to produce a stabilized zirconia precipitate;
and separating the precipitate from the mixture.
~59634
Overall the results indicate it is possible
to tailor properties of the stabilized zirconia
powders. It had been determined that compositional
control is most easily accomplished for the yttria
partially stabilized zirconias and dual partially
stabilized zirconia powders with yttria as one of
the stabilizing agents. A summary of the chemical
compositions of selected powders is shown in Table
IV. In all cases evaluated, a uniform distribution
of the stabilizing agents has been indicated by
energy dispersive x-ray analysis However, the
ability to prepare a stabilized powder is strongly
dependent on the feedstock pH. If the feedstock
pH is less than 9.0, the powders may contain very
little of the stabilizing agent and the powder is
monoclinic. In special cases no precipitation will
take place if the feedstock pH is not within a certain
pH range. When stabilizing agents are present in
the precipitated powders, the phase present in all
powders synthesized to date is cubic.
It is possible to produce stabilized zixconia
powders with a wide range of different particle
sizes. The shape of particles in all powders produced
to date have been equiaxial. The median particle
25 sizes range from 0.05 microns to 3.60 microns.
Typically the median particle size is between 0.2
and 0.5 microns. Specific surface areas of selected
powders have ranged from 30 m2 per g to 150 m2 per
gram.
Particle size analyses and crystallite domain
size determined by XRD line broadening indicate
that powders produced from h~drous oxide feedstocks
are microcrystalline. Average equivalent spherical
diameters calculated from the specific surface areas
compared to the median particle sizes determined
by particle size analysis indicates that the particles
shown in Figures 1 through 3 are microcrystalline.
~259~i3~
This finding is supported by the XRD patterns for
these types of particles. The particles shown in
the photomicrograph in Figure 4 produced from a
homogeneous solution feedstock are single crystals.
Partially stabilized powders composed of micro-
crystalline particles compact to reasonably high
bulk densities. The compaction behavior for three
microcrystalline powders were tested. Bulk densities
up to 3.20 g per cm3 were achieved at 340 MPa (50
ksi) uniaxial pressures. This corresponds to approxi-
mately 55 percent of theoretical density. The yttria
partially stabilized zirconia used had a median
particle size of approximately 0.4 microns. This
powder compacted to about 50 percent of theoretical
density. This may be due to some agglomerates present
in the powder greater than 1 micron. The agglomerates
levels may be reduced by shorter reaction hydrothermal
times.
Preliminary sintering studies have been performed,
primarily on the ~ttria partially stabilized zirconia
powders because of the lower temperatures required
to consolidate them. Results indicate that hydrother-
mally derived partially stabilized zirconia powders
are highly reactive if the particle size is in the
range 0.2 to 1.0 micron. Sintering conditions for
the powders have not yet been optimized (the optimum
sintering temperature appears to be between 1350
and 1450C), but the microstructure of a sintered
3.3 mole percent YPSZ shown in Figure 5 indicates
that a high density (> 95 percent theoretical) was
achieved by sintering at 1350C.
It is believed that the advantages of the dually
stabilized zirconia are at least twofold. First,
lower levels of the higher cost yttria can be used
to obtain similar physical properties since CaO
or MgO are also complexed therewith. Second, the
dual stabilization will allow lower sintering tempera-
tures since lower amounts of calcia or magnesia
~:5~63~
are employed. The physical features o~ the product
powder, their particle size, narrow particle size
distribution, and low agglomeration indicate that
they should sinter at a relatively low temperature,
produce a sintered ceramic with minimal variation
in properties and be an easy powder to handle.
This latter feature will result in cost savings
since granulation to produce a free flowing powder
will not be necessary. The ability to produce the
powder by a chemical reaction in the liquid phase
will enable a high purity powder to be produced.
The process enables the stabilizer to be dist~ibuted
in each particle homogeneously on an atomic scale.
This unique feature of the process means tha~ the
high temperature heat treatment needed to form partially
stabilized zirconia particles from mixed oxides
is avoided as is the possibility of grain growth
resulting from this heat treatment. It also ensures
that variations in properties due to nonuniform
distribution of the stabilizer in the ceramic are
avoided. The ability to add one or more stabilizers
to the basic process means that several zirconia
powder products can be made by small modifications
to the main process.
An advantage of the combined method mentioned
above is that it would be possible to produce powders
with two particle size distributions. This would
allow better initial packing of the powders prior
to sintering and improve the quality of the final
product.
Other variations on the basic method of both
the colloidal and homogeneous approaches include
seeding, where the hydrous oxides are separated
from the aqueous medium and redispersed in a fresh
aqueous medium having desired conditions; conducting
the hydrothermal treatment under an inert, reducing
or oxidizing atmosphere; conducting the hydrothermal
~25~63~
24
treatment under an overpressure of an added gas;
conducting the hydrothermal treatment at temperatures
ranging from 175C to 350C and the corresponding
autogenous steam pressure or the autogenous steam
pressure plus the added pressure of an inert, reducing,
or oxidizing atmosphere for ~ period of time greater
than 5 minutes; using an organic acid to wash the
precipitate, acetic acid may be the organic acid;
and conducting the process as a continuous or batch
type process.
While the forms of the invention herein disc~osed
constitute presently preferred embodiments, many
others axe possible. It is not intended herein
to mention all of the possible equivalent forms
or ramifications of the invention. It is to be
understood that the terms used herein are merely
descriptive rather than limiting, and that various
changes may be made without departing from the spirit
or scope of t,he invention.
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~25~i3~
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