Note: Descriptions are shown in the official language in which they were submitted.
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INORGAMIC COMl?OSITE STRUCTURES
TECHNI CAL FI~:LD
Inorganic products, such as refractory hodies, and method
of producing such products by freezing aqueous slurries
of the inorganic particles of which the inorganic products
are composed.
BACKGROUND ART
Freezing a slurry of particulate inorganic materials
to form refractory products has been disclosed in U.S.
Patents Nos. 3,177,161; 3,512,571; 3,816,572 and 3,885l005O
The production of refractory structures in accordance with
the prior art resulted in products which often contain
large voids which adversely affect the strength of the
ceramic product, which in turn adversely affects their
properties such as thermal conductivity and thermal shock
resistance.
These deficiencies in the resulting ceramic products are
believed to be caused by nucleation at one of the cooling
sur~aces due to contact with an ice crystal or with the
mold surface that has a nucleation temperature higher
than that of the slurry itself as the dispersion is
cooled to its freezing point. This nucleation is believed
to cause large ice crystals to grow out from these points
and eventually entrap the last remaining liquid, resulting
in rupture, cracking, weak, and non-uniform ceramic
structures.
Many attempts have been made to overcome this problem,
such as thorough waxing of the molds, the use of different
mold materials, thorough cleaning of mold surfaces, even
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with acids, bu-t the problems still pers:isted.
DISCLO_URE OF THE INVENTION
This invention rela-tes to a process which substantially
eliminates the problems discussed above by fxeezing an
inorganic particulate slurry or suspension containing
a freeze sensitive ceramic colloidal sol which comprises
regulating or ad~usting the relative zeta po-tentials of
the particulate particles and the sol particles -to cause
the sol particles to precipitate onto the surface of the
particulate particles during the mixing of the particles.
The invention may include the addition of lithium ions to
the freezing media with or without supercooling. The in-
organic particulate slurry preferably contains magnesia or
zirconia particles. The invention may be used to make com-
posite structures in the nature of laminates where at least
one inorganic structure, such as a zirconia plate, is adhered
to a different inorganic structure, such as an alumina plate.
The various inorganic particles that can be used accordingto this inven-tion include, without limitation, aluminas
such as mullite and tabular alumina, silicas such as
fused silica, magnesia, chromite, spinels such as chromite
spinel, kyanite, carbomul, zirconia, mica, carbon, graphite,
molydisulfide, uranium oxide~ thoria, titania, clays, etc.
The invention, however, is broadly applicable to suspen-
sions of inorganic particles in general inciuding othermetal compounds. Mixtures may be used if desired.
The inorganic particle size is not critical. Best results
seem to have been obtained to date when the majority
of the particles, or 40-50% of them, are below about
200 mesh. Small particle or grain size seems -to be
particularly advantageous when using zirconia. Much
_ 3 ~ 3
smaller particle sizes can also be used even in a colloidal
size. The ultimate particle size and size distribution will
depend to some e~tent on the end use of the structures and
the properties desired therein.
The inorganic products produced according to this inven-
tion are porous and the si7e of the grains or particles
employed in the slurries will to a large extent determine
the degree of porosity. The products of the invention
have a wide ~ariety of uses depending to some extent on
the type of particle being employed in the process. For
example, if ceramic particles are employed, the products
can be used in the same manner as ceramic and refractories
are used such as fire brick, linings for furnaces in the
steel, glass and other industries. They can also be used
as filters, carriers for catalysts, thermal shock resis-
tant dinnerware, grinding wheels, etc. When particles such
as graphite and molydisulfide are employed their uses can
include many of the above but adding thereto the lubrica-
tion properties of these ~aterials. Generally the prod-
ucts are useful in any area where porosity is desired or
in areas where porosity is not desired but is not detri-
mental. r.
The freeze-sensitive colloidal ceramic sols useful accord-
ing to this invention are well known and include colloidal
ceramic sols, such as disclosed in the Smith-Johannsen
U.S. Patent 3,177,161 and U.S. Patents 3,512,571 to
Phelps, 3,816,572 to Roelofs and 3,885,005 to Downing et
al. A freeze-sensitive sol is one which, when frozen,
will break down and no longer exist as a sol or colloidal
suspension when thawed. Both cationic and anionic silica
sols can be used with the anionic preferred at least with
alumina and zirconia refractories. Ammonia stabilized
silica sols, such as Dupont's AM LUDOX*, may be advantageous
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where elimination of sodium is desired. Other freeze
sensitive colloidal ceramic s015, such as zirconia and
magnesia sols, can also be used. Silica s015 have been
used because they are readily available on the market.
Rlthough not necessarily preferable due to insufficient
experimental data to date, most present experiments mainly
utilize a freeze-sensitive sodium stabilized colloidal
silica sol having about 30% colloidal silica supplied by
Nalco Chemical Company.
The total amount of sol stabilizer such as sodium, ammonium
and/or ;ithium should be sufficient to stabilize the sol
but not be so high as to render the sol non-freeze sen-
sitive or to lower the strength of the fired silica or
other ceramic contained in the sol when fused or fired or
to lower the strength of the fused or fired product to an
unacceptable level. ~his can readily be determined by
routine experimentation by one skilled in the art. For
example a mole ratio of silica to lithi.a of about 85 in a
lithium stabilized silica sol works quite well but when
the ratio is lower~d to about 48 the sol appears to lose
some freeze sensitivity resulting in weaker bonds. The
optimum amounts have not as yet been determined.
Generally, the sodium stabilized silica sol are quite
adequate to practice the invention disclosed herein. With
some inorganic particles, namely zirconia and magnesia~
some adjustments can be made to improve the results.
These adjustments are desirable to improve pot life
~nd to preserve the distribution of particle sizes during
the filling operation so that an optim~ degree of uniform
packing can be obta,ned.
When zirconia, for example is mixed with a negatively charged
sodium stabilized silica sol (DIlPont Ludox HS-40*) it is well
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we-tted and the particles quickly segrega-te. It is be-
lieved that this segregation occurs because the sol
particles and -the zirconia particles are so charged to
prevent or minimize particle association. The zirconia
particles generally have a charge of about -20 m.v. (zeta
poten-tial) in deionized water while the above Ludox
particles are even more highly charged. To overcome
particle segregation problem, the zeta potential of the
particles comprising the various mixes is altered.
One way of al-tering the zeta potential is to reduce the pH
of the above Ludox from about lO to about 8 by adding
dilute HCl which lowers the zeta potential of the silica
sol particles and rendering them less stable. Under these
conditions, the silica particles begin to precipitate onto
the ceramic (zirconia) particles creating a degree of
association between all of the particles of the mix and
segregation of coarse and fine ceramic particles such as
zirconia is greatly inhibited. The acid can also be added
to the inorganic particles or -to a mixture of the sol and
particles. The amount of acid is tha-t which is sufficient
to prevent settling or segrega-tion of the particles. In
practice, it is advantageous to add the acid directly to
the sol. With zirconia, the amount of acid found practi-
cal to accomplish the result is about 0.6 percent by
weight (based on the total weight of zirconia) of a 37
percent HCl solution. If such a problem is encountered
with other inorganic particles the zeta potential can be
measured and appropriate adjustment with acid or alkali to
alter the zeta potential can be made in such a manner as
to insure particle association.
Zirconia, and especially magnesia, also appear to react
with the sodium stabilizer (and also ammonia somewhat) to
cause limited pot life. In fact, the reaction with
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magnesia is so rapid that mixing itself becomes difficult.
The use of a lithium stabilized silica sol was found to
eliminate this reaction to the extent that magnesia
dispension could be readily mixed and cast without concern
of short pot life. The use of lithium stabilized sols
also overcomes the particle segregation problem referred
to above with respect to zirconia. Thus it may not be
necessary to adjust the zeta potentials of the particles
if a sol having the requisite zeta potential can initially
be used.
When the lithium stabilized silica sol was used with
magnesia and zirconia another new and very significant
property was observed. These sols inhibited ice crystal
growth even in the absences of supercooling. In fact,
when nucleation was deliberately initiated, in the case of
magnesia dispersion containing lithium ions, from the
surface with an ice crystal, no macro or large crystal
growth was detectable for more than two millimeters from
the initiation site. Thus the use of lithium stabilized
ceramic sols not only solved the pot life problems and
particle segregation problems of zirconia and magnesia but
has been found extremely advantageous for producing small
uniform ice crystals during the freezing step with regard
to all inorganic dispersions. The use of a lithium
stabilized ceramic sol in combination with supercooling
has been found most advantageous.
When using lithium in the inorganic particle slurries
containing a freeze sensitive ceramic sol, it is of course
most practicable to employ a lithium stabilized ceramic sol
available on the market. ~ silica sol having a silica to
lithia ratio of 85 worked quite well, however this sol,
DuPont's Lithium Polysilicate 85*, is not being marketed
today. One lithium stabilized silica sol which is avail- <
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able today contains a silica to lithia ratio of about 48
(DuPont's Lithium Polysilicate 48*). This amount o~ lithia
however minimizes the freeze sensitivity of the sol and
when used alone produced fired products having weaker
bonds. This commercial lithium stabilized sol can be used
however by using it in admixture with a sodium or prefer-
ably an ammonia stabilized sol. A 50-50 mixture has
worked well but the optimum has not as yet been determinedO
It is the presence of the lithium ion which produces the
surprising ice crystal growth inhibition rather than the
absence of sodium or ammonia. Thus lithium ions can be
added to the slurries by the addition of ionizable lithium
compounds such as lithium chloride, lithium hydroxide,
lithium sulfate, lithium succinate and so orth. It is
preferred to add the lithium ions to the ceramic sol. The
amount of lithium ions added to an inorganic particle
slurry should be sufficient to inhibit ice crystal growth
to the desired degree but insufficient to adversely affect
the freeze sensitivity of the sol. This can be determined
by routine experimentation with respect to any particular
system being frozen. Only a very small amount of lithium
ion is necessary to inhibit ice crystal growth. Higher
amounts may however be required to increase the pot life
when magnesia is used as can be observed in Examples
below.
To accomplish the supercooling and substantial instan-
taneous freezing, it is not a simple matter of inserting a
mold filled with the slurries into a cold freezing media
even at -40 or -60F. This invention includes a p~ocess
of insuring supercooling of the ceramic slurries by
treating the mold with a hydrophobic liquid such as
xylene, mineral spirits, or perchloroethylene to cover
at least the entire working surface of the mold, and
inserting the slurries or suspensions into the mold while
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it is still wet. This can be accomplished ~y simply
dipping the mold in the hydrophobic liquid. The mold
can then be closed and the s]urry frozerl. It is also
advantageous to cover the aqueous slurry or suspension in
the mold with a thin layer of the hydrophobic liquid. The
mold itself is preferably of light weight and of low mass
relative to the freezing media and the ceramic or partic-
ulate slurry being frozen. The mold and freezing ~edia
should also have a high thermal conducti~ity. Although
the freezing temperature can be varied, it should be
sufficiently low to insure supercooling and a rapid
freeze. Temperature of about -45 to -50F can be used.
When lithium is used temperatures of about -10F can
advantageously be used.
When producing large articles, supercooling may only occur
to a certain depth from the mold surface toward the center
of the slurry because all of the heat within the centar
cannot be removed before freezing of a portion of the
slurry closer to the mold. When large articles are to be
made it is thus advantageous to cool the entire slurry to
near the freezing point before inserting it inko the
freezing media to insure complete supercooling. The
presence of lithium ions in large slurries is alsc advan-
tageous.
The supercooling can also be carried out without the use
of mold such as by extruding cylinders, sheets or films of
the aqueous slurry or suspension on a belt treated with
the hydrophobic liquid and then into the hydrophobic
freezing media. The transport through the freezing media
can be between a pair of belts and the process can be
continuous. The terms "mold" as used herein is intended
to include any structure for supporting and/or encompassing
the slurries or suspensions.
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Supercooling of the slurry to a temperature where it
spontaneously nucleates results in a structure that is
uniform throughout. ~t the time of nucleation not all
the water freezes because the heat of fusion raises the
temperature back to the freeæing point. However, as
cooling proceeds further, ice crystal growth is completed
from all of these nucleation sites at substantially the
same time. The structure that develops is therefore
much more uniform and fine grained regardless of the
thickness of the structure to be produced or frozen.
Random tests made on some of the freezing steps set
forth herein indicate that the temperature of supercooling
is about 4 degrees below the freezing temperatures of
the aqueous slurry.
Various freezing media can be used to freeze the slurry
structures such as those described in the above-mentioned
patents. A hydrophobic freezing media such as Freon* or
perchloroethylene is advantageously used to prevent
penetration of the freezing media into the aqueous slur-
ries to prevent the growth of large or variable sizedice crystals, and to insure supercooling.
The various inorganic materials or ceramics useful according
to this invention have different and known firing or
sintering tempera~ures in conventional refractory pro-
cesses. For example, alumina is generally fired at atemperature of 1400C or slightly above, and zirconia at
about 1700C, in conventional refractory processes. As
a general rule, freeze-cast ceramics are most advantage-
ously fired at about 50~ to 65~ of their melting temper-
ature. Thus, when firing freeze-cast ceramics, alumina
is advantageously fired at about 1250C while zirconia
is advantageously fired at about 1400C.
Other inorganic structures can be sintered at their known
or determined sintering temperatures. The temperature
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used and time of heating should be sufficient tc bond the
particles together into a strong integral structure but
insufficient to significantly reduce or adversely affect
the desired porosity or uniformity of the product.
Examples of such temperatures are given, for example, in
U.S. Patent 3,177,161.
The molds or patterns are usually made of lightwei~ht steel
or aluminum if more thermal conductivity i5 desired. The
wetting of a mold with a hydrophobic liquid to aid in
supercooling can also act as a release agent.
The slurries should be as free from entrapped air as
practical. En~rapped air can be a~oided to some extent
by the manner by which the slurries are first mixed, and
any entrapped air can be removed in various known manners,
such as using long periods of holding time, vibration, or
vacuum treatment techniques.
After freeziny, the frozen slurry structures are removed
from the mold, thawed and dried. Although various
manners of thawing and drying can be employed, the thawing
and drying can be accelerated by the use of heat. The use
of a conventional drying oven has been found satisfactory
for this purpose.
~fter selection of the specific particles to be formed
into a structure, they can be mixed in the conventional
manner having due re~ard to particle size, and the freeze-
sensitive colloidal ceramic added to each of the dried
ceramic materials selected. ~he freeze-sensitive col-
loidal ceramics are contained in water and the solid
colloidal content may range from 15% to 50~ solids. Thus,
the addition of the freeze-sensitive colloids to the dried
ceramic material usually automatically adds the neces-
sary water for handling. For example, a mix commonly
S~
used in slip casting containing up to about 10~ water,
having a consistency somewhat like pancake batter can
be poured into a mould or injected by simple mean.s. This
can readily be accomplished by maintaining the proper
consistency of the ceramic slurries either by using
high solid content freeze-sensitive colloidal ceramic
sol or by removing water prior to freezing. One manner
of accomplishing this mixing is to dry mix the ceramic
- grain in a ribbon blender and-add the freeze-sensitive
ceramic sol together with its liquid component, slowing
the ribbon blender and continuing until thorough mixing
is obtained. The particulate suspensions or slurries,
should have a particle content sufficient to insure
particle to particle contact during the ~reezing step
as described in U.S. patent 3,177,161. If the particles
are dispersed too thinly, no structure will be formed
when the ice melts. The amount of water is desirably
held to a minimum practical amount for economic reasons.
The amount of the freeze-sensitive ceramic sol can be
as reported in the above-noted U.~. patents. The most
suitable percentage appears to be about 15% by weight
of the colloidal ceramic sol (30% solids) based on the
weight of the dried inorganic particles.
BEST MODE EOR CARRYING OUT THE INVENTION
_ _ _ _
Example 1
Alumina Mix (19 kilograms~ - crucible
Tabular Alumina Alcoa T61
- 2~ + 48 Mesh 55
- 100 Mesh 25
- 325 Mesh 20~
Sol 30~ Colloidal Silica (NALCO)*
14.3% by weight pH about 10
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Example 2
Zirconia Mix (367.5 grams) - plate
Monoclinic Zirconia
- 100 " 50%
- 325 " 50~
Sol 30% Colloidal Silica (NALCO)
12.5% by weight
Modified by 0.6% HCL. to pH 7.5
Example 3
Zircon (380.5 grams) - plate
- 80 Mesh 70
- 325 Mesh 30~
Sol 30% Silica (REMASIL SP-30), pH about 10
11.7% by weight
15 Exam~le 4
Mullite Remasil ~60 (1180 grams) - plate
- 20 Mesh 20~
- 70 Mesh 20%
- 200 Mesh 40
- 325 Mesh 20~
Sol 30~ Silica (REMASIL SP-30), pH about 10
14.6% by weight
The above mixes are thoroughly blended and each mix has
the con~istency of thick pancake batter and can ~e
poured and placed in a mold with the aid of a spatula.
The molds are thoroughly wet with perchloroethylene.
~ach mix is placed in the mold while it is still wet with
the perchlorethylene to the desired depth, about two
inches. The top layers of each mix is then covered with
s~
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a layer of perchloroethylene freezing liquid, the molds
closed and molds inserted entirely in perchloroethylene
freezing liquid at a temperature of -48F. The liquid
slurries are supercooled and then frozen. The frozen
structures are then removed from the mold while frozen,
thawed and dried in a radiant heated oven at a temperature
of about 120F. No significant large voids were detected
and the products after freezing were extremely uniform.
After drying the structures are each fired in a conven-
tional kiln at a temperature of 1250C for about 4 hours,
after which the moldings were allowed to slowly cool to
ambient temperature. The moldings were then subjected to
heat and thermal shock by applications of a torch (about
6000F~ directly to the moldings at ambient temperature D
The moldings remained substantially u;naffected after theapplication of heat with no visible or physical defects
except for surface melting where thè torch was applied.
Repeated tests such as those described above gave con-
sistent and repeatable results.
Example 5
60 parts by weight of magnesia (50%-14 mesh and 50~-48
mesh) were mixed with 12 parts by weight of a lithium
stabilized 30% aqueous silica sol having a silica-lithium
ratio of 85, formerly marketed by DuPont under the designa-
tion Litnium Polysilicate 85. The mixture was supercooledand frozen in the same manner as set forth in the above
examples. No initial reaction was noted and the pot life
of the mix was very good.
Example 6
60 parts by weight of alumina (50%-28 + 48 mesh and
20%-325 mesh) were mixed~together with 2.4 parts by weight
of a lithium stabilized 30% aqueous silica sol having a
5~
silica-lithium ratio of ~8, marketed by DuPont under -the
designation Lithium Polysilicate 48, and 9.6 parts by
weight of an ammonia stabilized 30% aqueous silica sol
marketed by DuPont under the designation Ludox AS-~0 and
the mixture supercooled and frozen in the same manner as
set forth in the above examples.
Example 7
Example 6 was repeated using 12 parts by weight of the
ammonia stabilized sol and 0.025 parts by weight of a
saturated solution of lithium hydroxide substituted for
the lithium stabilized sol.
The ice formed during the freezing in examples 5, 6 and 7
was much finer than the ice formed in examples 1-~. The
unfired products were more uniform and were stronger than
those produced in examples 1-4.
The thawed products can also be broken up into particles
and then fired for various uses such as fillers for resins
or plastics. The particles have a unique shape which is
advantageous for use as fillers. A clay slurry, for
example, was frozen, thawed, and dried according to this
invention, broken up or ground into small particles; their
shape was particularly suitable for filler particles. The
jagged nature of these particles makes them particularly
suited to mixing with small amounts of powdered thermo-
plastic or thermosetting resins to form, under heat and
pressure, ceramic-like products such as roofing tiles.
Composite inorganic structures such as ceramics, can be
considered as being in the nature of laminates where one
inorganic structure, such as a zirconia plate, is adhered
to a different structure, such as an alumina plate. Prior
~ 15 ~ ?t~
attempts to produce such composite ceramics by simul-
taneous firing of the ceramics in a single mold or by
firing the ceramics separately and cementing them together
have not been success~ul to applicant's knowledge. The
interfacial bonds hetween the different ceramics obtained
in these manners have not been adequate to withstand the
high temperatures and thermal shock to which they are
frequently subjected. This interfacial bond failure is
mainly due -to the differences in the thermal coef.ficients
of expansion of the different refractories or ceramics.
A further problem involved in producing composite ceramics
is the difference in the firing temperatures required
of the different ceramic materials.
The composite inorganic articles produced in accordance
with this invention, particularly ceramics, may be com-
posed of different inorganic materials adhered or lamin-
ated together and having an interfacial bond of extra-
ordinary strength sufficient to withstand extremely high
temperatures of thermal shock, such as experienced in the
metals inductry. Composite ceramics produced according
to this invention have been subjected to temperatures as
high as 6000F without any noticeable effect on the inter-
facial bonds.
This invention also includes a process for producing
composite ceramics in a single mold by inserting the
different ceramic materials into the mold in layered
fashion and in contact with each other, freezing the
layered ceramics while in contact with each other, thawing
th~ frozen composite, and then firing the thawed composite.
There are a number of important and surprising results
obtained when composite ceramic articles are made ac-
cording to this invention. The bond between the two
different ceramics is at least as strong as the bond
between the particles of the individual ceramics employed.
.,
- 16 -
The composite frozen structure can be fired at a single
temperature despite the fact the ceramics individually
are known to require different firing temperatures.
The differences in the thermal coefficients or the mass
thermal coefficients of expansion of the different
ceramics does not cause disruption or weakening of the
interfacial freeze bonding during the firing operation
and the fired bond is not significantly affected during
high temperature use or by subjecting the composite ceramic
to extreme thermal shock. It is further surprising that
experiments to date indicate that a very wide range of
different ceramics can be employed to make the composite
ceramics of this invention having wide differences in
their firing temperatures and thermal coefficients of
expansion.
The thermal coefficient of expansion refers to the ex-
pansion of the individual particles or crystals of the
refractory. When these particles are bonded or sintered
together in a mass and heat is applied, it is the
individual particles that expand causing the dimensions
of the entire mass to change due to the cumulative effect
of the expansion of the individual particles. This
phenomenon is referred to herein as the mass thermal co
efficient of expansionO
When refractories are prepared by the conventional pro-
cess, they act as a single body and when the body is sub-
jected to heat the entire body expands due to the cumu-
lative effect of -the individual particles or crystals
making up the bod~. In the composites of this invention,
the body acts as individual particles and although the
individual particles or crystals expand when the body is
heated.
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- 17 -
Although many different inorganic materials can be used
to form composites, some of the more commercially
important composites are ceramic or refractory composites
such as zirconia-alumina, zircon-alumina, magnesia-alumina,
silica-alumina, zircon-silica, zirconia-magnesia and
zircon-magnesia composites. The zirconia-alumina com-
posi-te, for example, is particularly valuable to the steel
industry. Zirconia has a low thermal conductivity when
compared to alumina, and is very corrosive. and errosive
resistant to molten steel. Zirconia, however, is not very
strong when compared to alumina (especially in its soft
form) and is also much more expensive than al~mina. Thus,
a zirconia-alumina composite brick or slab having about
one-quarter inch of zirconia adhered to one side of two
to three inches of alumina gives a product of high strength
and at a significantly reduced cost while retaining all
of the advantages of zirconia in its most desirable soft
form. The advantages of other combinations of ceramics
will depend somewhat upon their intended use and variations
can be produced to meet the requirements of any particular
end use. ~lumina is presently preferred as a basic ceramic
to which other ceramics are bonded because it is inex-
pensive, strong, eas~ to work with, and has a convenient
firing temperature.
Various shapes can be produced; for example, alumina can
be faced on one or more sides with zirconia or magnesia,
the internal surface of alumina cylinders can be lined
~ith zirconia, alumina nozzles and ladles can be lined
with zirconia and so forth.
In firing the composite ceramics, it has been found to
be advantageous to fire at a temperature for the ceramic
having the lower firing temperature. For example, when
firing a composite composed of alumina and zirconia, it
has been found advantageous to fire it at about 1250C.
- 18
Such a firing temperature, however, is below that normally
used for zirconia and also below that which is used for
firing the individually freeze-cast zirconias, namely
1~00C. When firing such a composite at 1250C, the
zirconia would normally be considered as somewhat under-
fired. Soft zirconia is even more corrosive and errosive
resistant to molten uranium than hard zirconia fired at
its normal temperature; and the fact that the zirconia is
bound or adhered to the alumina renders the composite
as a whole very strong, thus permitting one to take
maximum advantage of the properties of zirconia while
maintaining excellent strength. If desired, the zirconia
portion of such a substance could be locally fired to
increase its hardness.
The formation of the composite structures can take place
- in a single mold. The different ceramics are formed into
slurries containing the free~e-sensiti~e colloidal ceramic
sols. One ceramic slurry can then be placed at the
bottom of the mold and the different ceramic slurry placed
on top thereof, the composite fro~en, thawed, and subse-
quently fired. To prevent any substantial mixing of the
different ceramics, the slurries can be formed in a
viscous state or other techniques can be used to prevent
significant mixing, such as inserting a thin separating
slip or shield bet~een the ceramic slurries and pouring
the other slurry on top or on the other side of the slip
to temporarily effect a physical separation of the dif-
ferent slurries and then removing the separating slip
just prior to the freezing of the slurries.
Example 8
Laminate Zirconia-Alumina (11" melting crucible)
Alumina ~ix (15 kilograms)
Tabular Alumina Alcoa* T61
- 28 + 48 Mesh 55%
*Trade Mark
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- 100 Mesh 25%
- 325 Mesh 20%
Sol 30% solids Colloidal Silica (NALCO)
pH about 10
14.3% by weight
Zireonia Mix (7 kilograms)
Monoclinie 7.irconia
- 100 Zireonia 50%
- 325 Zixeonia 50%
Sol 30~ solids Colloidal Siliea (NALCO)
12.4% by weight
Modified by 0.6% HCL. to pH 7.5
Exam~le 9
Laminated Zireon-Alumina (plate)
Alumina Mix (1060 grams)
Tabular Alumina Aleoa T61
- 28 ~ 48 Mesh 55%
- 100 Mesh 25%
- 325 Mesh 20~
Sol 30% solids Colloidal Siliea (NALCO)
pH about 10
14.3% by weight
Zircon (600 grams)
- 80 Mesh 70%
- 325 Mesh 30%
Sol 30% solids Siliea (NALCO)
pH about 10
11.7% by weight
Example 10
Laminated Alumina ~ Mullite (plate)
Tabular Alumina Aleoa T61 (300 grams)
- 14 Mesh 40~
- 48 Mesh 44%
- 325 Mesh 16%
Sol 30% Silica (NALCO), pH about 10
14.7% by weight
.
- 20 _
Mullite-Remasil ~60 (89 grams)
- 20 Mesh 30%
- 70 Mesh 30~
200 Mesh 30%
- 325 Mesh 10~
Sol 30% Silica INALCO), pH about 10
14.6% by weight
The above mixes were thoroughly blended and each mix
had the consistency of thick pancake batter and could be
poured and placed in a mold with the aid of a spatula.
The molds were thoroughly waxed and highly polished.
The zirconia mix was first placed in each mold to the
desired depth, about 1/4 inch in these examples, and the
alumina, zircon, and mullite in each of the above examples
placed in the same molds on top of the zirconia to a
depth of about two inches. The top layers of alumina,
zircon and mullite are each then covered with a layer of
hydrophobic freezing liquid, the molds closed and molds
inserted entirely in perchloroethylene freezing liquid
at a temperature of -48F.
The frozen composite bodies were then removed from the
mold while frozen, thawed and dried in a radiant heated
oven at a temperature of about 120F. After drying the
composite bodies were each fired in a conventional kiln
at a temperature of 1250C for about 4 hours, after which
the moldings were allowed to slowly cool to ambient
temperature. The moldings were then subjected to heat
and thermal shock by applications of a 3000F torch
directly to the moldings at ambient temperature. The ~old-
ings remained substantially unaffected after the appli-
cation of heat with no visible or physical effect on the
interfaced bonds. The zirconia-alumina composite was also
hit with a gas torch at about 6000F with no appar-ent
damage to the interfacial bond although it melted the
alumina.
- 21 -
Examp1e 11
Example 8 was repeated using a mold previously immersed
in perchloroethylene and the mold filled while still
wet. Supercooling occurred quite readily when the com-
posite ceramics were inserted into the freezing liquid
and a fine uniform grained composite structure obtained.
Example 12
Example 11 was repeated substituting for the NALCO, a
50-50 mixture of the sodium stabilized NALCO silica sol
and a 30 percent solids lithium stabllized silica sol
having a silicalithia ratio of about 48.
Exam~le 13
Example 8 was repeated in which about 0.6 percent by
weight a 3.7 percent HCl solution base on the weight of
the zirconia was added to the zirconia grains and about
15.6 percent by weight of the NALCO silica sol (30%
solids) having a pH of about 10 was used. The amount
of acid used is about the same amount that would be re
quired to bring the pH of the sodium stabilized silica
sol to about 7-8.
Example 14
60 parts by weight o~ magnesia 150%-14 mesh and 50%-48
mesh) were mixed with 12 parts by weight of a lithium
stabilized 30~ aqueous silica sol having a silica-lithium
ratio of 85, formerly marketed by DuPont under the desig-
nation Lithiu~ Polysilicate 85. The mixture was super-
cooled and frozen in the same manner as set forth in the
above examples. No initial reaction was noted and the
pot life of the mix was very good.
