Note: Descriptions are shown in the official language in which they were submitted.
~484~:~
Cas~ 3555
THERMAL 8HOCR AND C~EEP RE~IS~AN~ POROU~ MULLITE AR~IC~E~
PREPARED FROM TOPAZ AND PROCESS FOR MAN~FACTURE
BA~RGROUND OF THE INVENTION
This invention relates to ceramic articles based on a three-
dimensional interconnected mullite whisker crystal network and is
specifically directed to the use of topaz crystals as a reactant
in an ln situ synthesis of mullite whiskers to densify and
strengthen mullite whisker articles produced without conventional
sintering. In particular, the invention relates to the reaction
ln situ of interconnected topaz crystals and silicon dioxide or
topaz crystals, silicon dioxide and hydrated aluminum fluoride in
shaped green bodies, especially extruded honeycombs.
Shaped ceramic articles which exhibit a high degree of
thermal shock and creep resistance have a number of commercially
impor~ant applications, for example, as diesel particulate traps,
hot gas filters, molten metal filters, substrates for exhaust
catalysts, catalytic substrates for ozone conversion, catalytic
substrates for precious metal combustors (Catcom) used to power
gas turbines and in metal matrix composites. Several major
problems, however, hinder such uses of ceramics. First, ceramics
.
~ o ~
Case 3555
are susceptible to damage, such as cracking, caused by thermal
shock and/or creep. Another problem results from the fact that
ceramic structures frequently are difficult to machine or ~o
join, making it difficult to manufacture ceramic articles having
complex shapes. This is especially true of thin-walled
structures such as honeycombs.
Mullite is widely used in numerous ceramic applications and
is a crystalline aluminum silicate of the approximate empirical
composition 3Al203.2SiO2. Mullite is characterized by a
distinctive diffraction pattern, but mullite occurs in distinctly
different crystalline shapes, the most common being prismatic
cigar-shaped crystals, sometimes referred to as "acicular" form.
This form of mullite occurs as "clumps" and may be obtained, for
example, by firing clays. Mullite can also be synthesized as
smooth elongated single crystals (whiskers). Whiskers have a
significantly higher aspect ratio than the prismatic crystals in
conventional mullite materials. Mullite can also be synthesized
as highly elongate fibers. Generally, all forms of mullite have
many of the known valuable properties of alumina, such as a high
melting point and, in addition, exhibit other valuable physical
and chemical properties. However, when mullite is formed as
whiskers, the unusual strength associated with single crystals is
7/8/91 2
.
.
,
~ aRe 3555
obtained. Topaz ( A~Sio4F2) is usually obtained as stocky, bar-
like crystals which have a significantly lower aspect ratio than
mullite whiskers. Topaz is characterized by a unique x-ray
diffraction pattern and can be converted to various forms of
mullite by reactions with silicon dioxide.
The art is replete with suggestions to use various whiskers,
including mullite whiskers, to reinforce ceramics. Generally, in
such use the whiskers are employed as discrete, nonagglomerated
crystals which are formed into composites by conventional
sintering technology. This inherently limits the content of
mullite in the ceramic articles so produced and introduces
potentially fluxing materials. Thus mullite articles based on
composites including addition of discrete single whiskers cannot
be used at temperatures as high as those that pure mullite can
survive, and production of such composites necessitates handling
fibrous material.
Various methods have been suggested in the prior art for the
production of mullite in whisker form. Generally, these involve
solid-solid reactions at high temperatures with evolution of
gaseous by-product. Formation of mullite whiskers from the
reaction of anhydrous aluminum trifluoride (or aluminum
7/8/91
~ o l~
- Case 3555
trifluoride and alumina) and silicon dioxide with a topaz
intermediate is described in u.s. 4,910,172, 4,911,902, and
4,984,766, all to Talmy et al. The reactants pass through a
topaz crystalline phase before mullite crystals are formed.
According to the teachings of the paten~s, the solid reactants
must be anhydrous and an anhydrous silicon tetrafluoride
atmosphere must be present to form the mullite whiskers. In U~S.
4,984,766 a porous shaped preform containing the reactants and an
organic binder is converted to a highly porous felt, exemplified
in examples as small discs, the green bodies going through a
topaz intermediate stage without isolation of the topaz
intermediate. The initial green bodies are highly porous as are
the felts.
Our copending application, USSN 07/386,186, is directed to
an ln situ chemical route for making porous mullite ceramic parts
in near net shape from preformed precursors. The invention
permits the formation of highly complex shapes such as thin-
walled honeycombs, and it entails heating coherent green bodies
containing a mixture of finely powdered hydrated aluminum
fluoride and silicon dioxide in a molar ratio of approximately
12:13, along with a fugitive binder, while sweeping the volatile
reaction products including silicon tetrafluoride and water from
7/8/91
~8~
Case 355
the bodies. The reactants form mullite according to the
following equation:
12AlF3.xH20 ~ 13SiOz = 2 (3A1203.2SiO2) + 9SiF4 ~ xH20 (1~
X is suitably about 3 and can be as high as 9. At temperatures
of about 600C - 800C a topaz intermediate (A12SiO4F2) is formed
and above 890C this reaction results in the formation of mullite
whiskers. The topaz intermediate is not recovered in this
process.
Mullite whiskers products obtained by reaction (1) are
highly porous, typically about 80% porous, and thus they are
relatively weak. However, many of the important potential
commercial applications for high mullite whisker content shaped
articles require higher strength while still possessing desirable
micro and macro-structures.
7/8/91
Ca~e 3555
~UMNA~Y OF THE INYENTION
It has now been found that useful mullite whisker articles
can be produced by preparing interconnected topaz crystals from
near stoichiometric mixtures of hydrated aluminum fluoride and
silicon dioxide in the form of coherent green bodies by an in
situ process and then using the interconnected topaz thus
recovered (or interconnected topaz crystals from another source)
as a reactant with additional silica or silica and aluminum
fluoride in the form a shaped bodies to form densified mullite
whisker articles by ln situ synthesis.
In one embodiment of the invention, topaz crystals prepared
from a near stoichiometric mixture of hydrated aluminum fluoride
and silicon dioxide are mixed with and formed into coherent green
bodies, such as a spaghetti-like extrudate, along with a fugitive
binder, and the bodies are heated to temperatures in the range of
about 600 - 800C to form bodies in essentially the same size and
shape as the green bodies, but composed of interconnected bar-
like topaz crystals. During the reaction, e~olved volatiles,
including water and silicon tetrafluoride, are swept from the
solids. Since the topaz crystals are not whiskers, they are
only weakly interconnected~ Thus the topaz bodies are not
7/8/~1 6
2 ~
Ca~e 3555
relatively frangible and can be ~round, for example, to a size
finer than 1~0 mesh, or the extrudate can be mixed in a ball mill
with other ingredients which will result in the breakdown of the
extrudates into small granules of in~erconnected topaz crystals.
In practice of one preferred embodiment of the invention,
the topaz so formed is then made into another shaped green body,
preferably a honeycomb, along with additional hydrated aluminum
fluoride and silicon dioxide, these materials being used in an
amount, relative to the topaz, such that a portion of the silicon
dioxide reacts with the topaz to form mullite whiskers, and
another portion of the dioxide reacts with the aluminum fluoride,
to form additional mullite whiskers. This green body is then
fired while sweeping volatiles by means of air or nitrogen to
form an interconnected network of mullite whiskers and produce an
article of greater density and strength than is obtained by
Reaction (1). In an especially preferred embodiment, the fired
body derived from these two sources of mullite whiskers is
further densified by being infiltrated with an alumina-silica sol
precursor to mullite. This composite is fired to convert the sol
to additional mullite which will not be, however, in whisker
form.
7/8/91
4 ~ ~
Case 3555
In another preferred embodiment of the invention, mullite
whisker articles of greater strength and density are prepared by
mixing agglomerates of topaz crystals, prepared as described
above, with a quantity of silicon dioxide stoichiometric to form
mullite along with a fugitive binder. The mixture is formed into
green bodies which are f ired at a time and temperature suf f icient
to form mullite, e.g. 890C or above, whereby all of the mul,ite
whiskers are formed by the reaction of the interconnected crystal
topaz precursor with the added silicon dioxide. This leads to
less porous (denser) articles than are obtained by Equation (1),
as well as articles that are denser than those obtained by
reaction of agglomerates of topaz crystals with appreciable
amounts of added aluminum fluoride and silica dioxide.
A preferred use of honeycomb whisker products is as the
substrate for a precious metal catalyst for use with Catcom
applications. In such applications substrate material of thermal
shock resistance is required because it must survive thermal
stresses induced by significant thermal gradients created as a
result, for example, of emergency shutdowns of the gas turbine.
The shutdowns are anticipated to occur numerous times during the
lifetime of the generator. A simplified way of expressing
7/8/91 8
2 ~ 4 ~ ~
Case 35S5
thermal stress in a solid cylindrical body can be given by the
following relation:
~ = E~(Ts-TC)/(2(l-v))
where ~ is thermal stress, E is the elastic modulus, ~ is thermal
expansion coefficient, v is Poisson's ratio, Ts and Tc are the
surface and center temperatures of the cylinder, respectively.
7/8/91
Cas~ 3555
DETAILED DE8~RIPTION OF TH~ ~NVENTION
In putting the instant invention into practice, the topaz
reactant is preferably prepared by mixing aluminum fluoride
hydrate, e.g., AlF3.3H2O, with amorphous or crystalline silicon
dioxide, preferably using proportions substantially
stoichiometric to form topaz without free silicon dioxide, e.g.,
from about 54 to 62 parts by weight aluminum trifluoride
(anhydrous basis) to 42 parts by weight silicon dioxide
corresponding to about 1 (one) mole of AlF3 per mole SiO2. The
aluminum fluoride and silicon dioxide are in particulate form.
The mixture is thoroughly mixed with a temporary binder material
such as methyl cellulose, the binder either being added dry
followed by addition of a liquid vehicle, preferably water, or
binder is added as an aqueous solution or dispersion. Sufficient
liquid vehicle is added to provide a mix of extrudable
consistency. Prior to extrusion, the mixture is thoroughly mixed
and extruded to form green bodies in a form amenable to drying
and firing. Sufficient binder is used to achieve a green
extrudate of adequate strength to facilitate handling. Excellent
results have been obtained by extruding the mixture into thin,
spaghetti-like strands which can be of any appropriate diameter,
e.g., 0.8 mms. to 6.4 mms. These strands can be chopped into
7/8/91
2 ~
Case 3555
pellets before drying and firing, or the strands can be dried and
fired. Extrudates of interconnected topaz crystals are then
crushed into finer particles when milled.
It is preferable to dry the green bodies before firing in
order to strengthen the green bodies by the gelation of a binder
such as methyl cellulose so that the pieces can be handled with
more ease during the consequent firing step. Firing is
preferably carried out by a furnace which has capabilities of
operating at temperatures as high as 1500C with capabilities of
driving out the ~luorine containing gases evolved during the
topaz and mullite forming reactions. Removing of fluorine
containing gases is accomplished by peristaltic pump(s) which are
placed outside the furnace but are connected via refractory
tubing to the exit gate of the furnace. The pumps, when
operational, suck the gases from inside the furnace outside and
send gases to a scrubbing unit to convert the products to
relatively non-hazardous solid products such as NaF and SiO2.
Peristaltic pumps are needed to protect the furnace furniture and
heating elements by removing the fluorine containing gases,
particularly HF, from the reaction chamber by minimizing the
residence time of these gases inside the furnace. As a result of
7/8/91 11
4 ~ ~
Ca~ 3555
this pumping action, silicon tetrafluoride is continuously
removed from the reaction chamber, and also faster than it
normally would.
In the practice of the present invention, the reactants to
produce mullite including interconnected topaz crystals
preferably formed as described above, are mixed thoroughly with
silicon dioxide or silicon dioxide and hydrated aluminum fluoride
along with a suitable binder, such as methyl cellulose and
water. Other suitable binders include, e.g., alginate,
polyethylene oxides, resins, starches, guar gum and waxes.
Choice of suitable binders for making the topaz intermediate as
well as the mullite whiskers article are discussed in U.S. Patent
4,551,295, the teachings of which are incorporated herein by
reference. Following mixing with a binder the reactants are
formed into a desired shape using extrusion, injection molding,
low pressure injection molding, pressing, tape casting or any
other suitable ceramic processing technique.
Aluminum oxide may also be added to the reactive ingredient
mix comprising aluminum fluoride hydrate, silicon dioxide and
topaz as fine particles and at a weight ratio of 1 to 10% based
on the total dry weight of the reactive ingredients, i.e.,
7/8/91 12
' '
:
Ca~e 3555
aluminum fluoride hydrate plus silicon dioxide, topaz and
alumina. Alpha alumina is suitable, although other forms can be
used. The main reason for aluminum oxide addition is to have it
react with unreacted silicon dioxide at elevated temperatures to
form mullite particulates. A portion of the silicon dioxide may
remain unreacted because a portion of aluminum fluoride may
volatilize before it reacts with silicon dioxide. In cases where
stoichiometric amounts of aluminum fluoride and silicon dioxide
are added to the reactive ingredient mix, this would result in
excess silicon dioxide, which may not be desired in the final
product. Excess aluminum oxide will then react with this silica
to form mullite particulates. When aluminum oxide is added to
the reactive ingredient mix, some portion of it would react with
this excess silica while the remainder may stay unreacted. Thus
the final fired honeycomb product may contain unreacted aluminum
oxide.
In carrying out the invention, formed pieces of the powdered
reactants and binder are fired to a desired temperature (900C
and above) in a flowing stream of air or nitrogen to sweep
volatiles including but not limited to silicon tetrafluoride and
water, and, while the original shape is retained, the precursor
mix is transformed to mullite whiskers. There is no matrix, and
7/8/91 13
Caso 3555
the product is in the form of very porous shaped articles
consisting essentially of interconnecting and branched mullite
whiskers.
Inherent in this method of manufacturing mullite is the
possibility that the fired articles may contain trace amounts of
fluoride ions. since even very low concentrations of fluoride
ion can poison many catalysts, it is important that the formed
mullite articles be essentially fluoride-free, if they are to be
used as support for metallic catalysts. One aspect of the
present invention involves preparation of mullite articles by the
described route, and obtaining such articles in a fluoride-free
condition. Practical methods of fluoride ion removal include the
use of steam or superheated steam. For example, a honeycomb
useful as the support for metallic catalyst can, after it is
fired, be immersed in a super-heated (900C) concentrated
sulfuric acid bath and/or have steam passed therethrough to
remove fluoride ion. Alternatively, a hydrogen purge is an
efficient remover of fluoride ion. The result is an essentially
fluoride-ion-free mullite honeycomb support.
7/8/91
14
:.
- ~ :
2 i~
- casa 3s55
The method of making porous mullite articles described here
is especially attractive because by this route shaped articles
with very high thermal shock resistance can be produced at
comparatively low cost. The articles are lightweight and have
very high creep resistance. According to the invention, articles
with complex or thin shapes can be produced with relative ease.
Examples are honeycomb shapes, corrugated sheets, reticulated
(comb-shaped) pieces, foams, donuts or any other desired
functional shape chosen to serve as a filter, catalytic
substrate, particulate trap or other functional purpose. These
articles can be made into a near net-shape and/or machined
extensively without cracking. Catalytic substrate walls with
high roughness can be produced, and high specific strength
(strength/density) mullite articles attained without any residual
glassy phase present.
Refractory catalyst supports such as those described in U.S.
Patent 3,565,830 (incorporated herein by reference) constitute an
advantageous utilization of the present invention. Mullite
supports prepared according to the present invention may be
readily coated with catalytically active oxide, such as alumina,
and then impregnated with a platinum group metal for use with
catalysts such as those described in the reference patent.
7/8/91
Cas~ 3555
Refractory catalyst supports are also required for process
directed to catalytically supported thermal combustion (Catcom)
described for example in U.S. Patents 3,928,961 and 4,B93,465
(incorporated herein by reference). Mullite catalyst supports
made according to the present invention are exceptionally well
adapted for use in catalytic combustion processes such as those
described in these patents.
Two articles made by the precursor mix according to the
present invention can be joined to each other via thermal
treating. Preparing a good joint between two similar materials
has advantages. Certain catalytic applications require large
size of honeycomb pieces. A diesel particulate filter can be
about 12 inches in diameter or more. A large piece of honeycomb
shaped article is required for catalytic combustion applications.
Extrusion of such large pieces is very difficult. Since the
joint between two articles made from this material has good
quality, then large pieces can be assembled by joining smaller
extruded parts. For example, a cylindrical honeycomb material
can be assembled by fusing four pieces of quadrant cylinders. In
this method, several small pieces of green particulate aluminum
fluoride, silicon dioxide and binder admixture are joined either
by placing in integral contact or pressing smaller unfired pieces
7/8/91
16
~ Ca3e 3555
together before firing, so that upon firing large articles of
relatively more complex shapes are formed. See Examples 4 and 16.
Articles made with a topaz reactant, e.g. Example 16, to form a
joint are preferred because of the increased density.
Pure mullite ceramics of the present invention have a very
high melting point (>1880C). Production of articles composed of
whiskers is an especially straightforward and clean process,
since there is no need to disperse mullite or other whiskers, the
whiskers being formed during heating of the shaped article made
of the precursor mix, thus avoiding health and safety problems
associated with respirable whiskers.
Clay can also be used in small amounts to improve the
extrudability of a honeycomb. Clay is more plastic than either
silica or aluminum fluoride or interconnected topaz. The
increased plasticity of the extrudate mix which accompanies clay
addition enhances the ease of extrusion. Such clay can be added
to the dry mix or entrapped in the binder solution. Suitable
clays include, for example, bentonite, attapulgite, palygorskite,
montmorillonites, pyrophyllite and kaolin.
7/8/91
17
~ ~ L~
Ca3~ 3S55
A further aspect of the invention involves improving the
strength of mullite articles by grinding the reactants prior to
conversion to topaz and prior to mixing the topaz with other
reactants to form mullite whiskers. Grinding of reactants to
particle sizes of less than about 45 microns can improve the
compression strength of the mullite articles. Such grinding or
particle size reduction can be conducted by a variety of methods,
ball milling being one of the more convenient well known methods.
Ball mixing will also serve to reduce a spaghetti-like extrudate
of interconnected topaz crystals to small granules, e.g. finer
than 45 microns, of interconnected topaz.
Finally, as is appreciated by those skilled in the art,
whisker processing has the potential for creating serious health
hazards. In the manufacturing route of the present invention,
whiskers are formed in-situ from powders at elevated temperatures
and remained interconnected. Thus, there is no need to handle
loose whiskers during the manufacture route which is the subject
of the instant invention.
7/8/91 18
case 3555
Following is a detailed explanation of factors to be
considered in selecting ingredients used in practice of the
invention, especially as such factors affect the micro-structure
and macro-structure of mullite whisker products of the invention.
The mullite whisker articles of our copending application
(07/386,186) are made of mechanically interconnected mullite
single crystal whiskers and have low elastic modulus. Low
elastic modulus is mainly due to the very open nature of the
product. An article made by reacting aluminum fluoride
trihydrate and silica in accordance with Equation(1) is 80% or
more porous. In a
typical reaction:
12(AlF3.3HzO)+13SiO2 = 2(3Al203.2SiO2)~9SiF4(g)+36H20(g)
1656 gr 780 gr 852 gr
A total of 2436 grams of solids yield 852 grams of solid mullite.
This number suggests that 65% of the starting solids react to
form gaseous products. However, additional porosity is imparted
by evolution of volatile byproducts derived from agents used in
binding these reactants to a new net shaped article. Thus the
7/8/91
~ ~L~ 3 ~ ~ ~
C~se 3555
reactants are mixed with a binder, preferably an organic binder,
such as methyl cellulose, in a weight ratio preferably between
2 - 8 weight percent. A liquid binder/plasticizer, preferably
water, is added to the mixture at a weight ratio between 10 -
30%, and typically about 20% to facilitate forming. The mixture
is then homogenized to a dough by using one of several available
mixing techniques, such as pug milling followed by extruding the
mix through a die orifice to form honeycomb shapes. The
honeycomb is then dried and fired in a furnace to complete the
reaction to mullite.
The extruded green body obtained by reaction (1) is never
100% dense. There will be some porosity in the body, depending
on the quality of dispersion, particle size distribution,
extrusion pressure, die shape and some of the extruded
parameters. Porosity of the green body will vary between 1 -
15%, which is significantly less than that of green discs of U.S.
4,984,766. If one assumes 5% porosity in the green honeycomb
(i.e. the actual density is 5% less than the theoretical density)
and assumes water and binder contents of 20% and 5% of the
reactant solids, and the density of the fired piece is expected
to be 23% of the theoretical density of mullite. As an example,
1656 grams of aluminum fluoride trihydrate, 780 grams of silicon
7/8/91
case 3~55
dioxide, 487 grams of water and 122 grams of binder are mixed and
formed to a honeycomb, which is then fired to obtain 852 grams of
mullite. The product is only 28% of the original weight.
Combined with the 5% inherent porosity, the mullite whisker
honeycomb is about 23~ dense or about 77% porous. This
calculation suggests that fired honeycomb pieces of our copending
patent application are 80% porous. Measurements confirm this
estimate.
A very important factor involved in the preparation of
mullite whisker honeycomb is that the fired honeycomb must retain
the shape of the green one despite the fact that the green body
loses a significant portion of its original weight. If the
microstructure of mullite obtained from the reaction were such
that the mullite grains were dense and equiaxed, i.e., more or
less isotropic in three dimensions, then there could not be
strong connectivity between each grain. With lack of strong 3-
dimensional connectivity, the original honeycomb shape could not
be retained. The product would turn into loose powder form upon
firing, and collapse from the honeycomb shape. A rigid honeycomb
shape is obtained upon firing because of the unique micro-
structure as shown in Figures 2 - 4. The mullite crystals are
formed in the shape of whiskers. Each whisker is dense, single
7/8/91 21
~ ~ L~
Case 3,55
crystalline which is branched as shown in Figure 4, with the
whiskers being mechanically connected to another making a 3-
dimensional rigid body. Whiskers are anisotropic. Their aspect
ratio is high, generally loO or more. A three-dimensional body
made of interconnected whiskers can have 80% or more void space
and still be rigid. This characteristic of the mullite made from
the hydrated aluminum fluoride route is unique. Any dense, solid
mullite powder, single or polycrystalline, but equiaxed with very
little or no anisotropy, cannot produce a 3-dimensional rigid
body with 80% void space.
The highly open nature of the microstructure leads to
desirable low elastic modulus. Porosity, in general, decreases
elasticity of a ceramic material. There is no well understood
relation between the elastic modulus and porosity, but several
models have been developed and exist in the literature. One
model is described by:
E = Eoe~,~bP or E = EOexp(-bP)
where Eo is elastic modulus of the 100% dense body, P is
porosity, b is an empirical constant. The relation suggests that
E is decreased significantly with porosity. Low E values are
desirable in a Catcom substrate. Lower E means lower thermal
7/8/91 22
caQs 3555
shock susceptibility. A material composed of dense solid single
crystals, which can be formed in 80% or more void space due to
highly anisotropic nature of its individual grains, and where
these grains are interconnected to give rigidity to the body is a
good candidate for Catcom application due to its exceptionally
low elastic modulus.
However, strength of the substrate is another key parameter.
stronger substrates have more chance to survive service during
use. The mullite whisker material obtained from reaction (1) is
weak. Our copending application teaches the substrate obtained
by that reaction can be strengthened by one of several routes or
by their combination. However, none of these routes provides the
multiple benefits obtained by growing mullite crystals from
agglomerates of interconnected topaz.
In accordance with the instant invention, bar-like topaz
crystals, preferably interconnected topaz crystals, are added to
the mix. The interconnected topaz is preferably formed at a
temperature between 600 - 800C via the following reaction:
7/8/91 23
Case 3555
2(AlF3.3H2o) ~ 2sio2 = Al2siO4F2 + SiF4(g) + 6H20(g~
276 grams 120 184
Topaz crystals formed by this reaction are stocky or bar-like and
interconnected. At higher temperatures, topaz reacts with silica
to form interconnected mullite whiskers via reaction.
6Al2SiO4F2 + Sio2 - 2(3Al203.2SiO2) + 3 SiF4(g) (2)
1104 grams 60 85Z
Re~ction 2 suggests that loss of weight is about 27%. This
number is significantly lower than that obtained from Reaction 1.
Thus if a substrate is made via Reaction 2, the interconnected
mullite whisker product is much denser as shown by comparison of
in the micrographs in Figures 9 and 10. The strength of the
mullite substrate obtained from topaz is therefore higher than
that obtained from aluminum fluoride trihydrate and silica.
As noted above, increasing density increases elastic modulus
which then increases thermal shock susceptibility. Thus, there
is a trade-off between density and strength. Rewriting the
7/8/91
24
~8~
case 3555
equation above for the relationship of factors involving thermal
stress in solid body, it can be calculated that optimi~ation of
the product for thermal shock susceptibility is achieved by
maximizing the a/E ratio.
The densest mullite whisker product can be obtained by
Reaction (2) and is hereinafter denoted T-100, meaning that the
product is composed of interconnected whiskers produced by using
interconnected topaz and silica only. The product obtained by
Reaction (1) is denoted by T-O, since no topaz was added to the
original reaction mix. ~ T-25 product means appropriate amounts
of topaz and silica dictated by ~eaction (2) are added to a
mixture of aluminum fluoride trihydrate and silica dictated by
Reaction (1) such that, upon firing, 25% of the interconnected
mullite whiskers are obtained by the reaction of added topaz
powder with silica (Reaction 2) while the rest of the whiskers
originate from the reaction of hydrated aluminum fluoride with
silica (Reaction 1).
Theoretical porosity contents for mullite whisker materials
were calculated using several assumptions namely:
a) The extruded green honeycomb is 95% dense, i.e. it
retains 5% porosity.
7/8/91
4 l
Case 3555
b) The extruded green honeycomb has 5% organic binder by
weight.
c) The extruded green honeycomb has a total 20% water and 5%
liquid plasticizer by weight of solid reactants.
Based on these assumptions, theoretical porosity contents are:
MATERIAL POROSITY. %
T-0 77
T-25 73
T-50 67
T-75 59
T-100 46
These calculated porosity values clearly indicate that
mullite whisker honeycombs become denser as their topaz content
increases. This is due to the fact that more fluoride is lost in
AlF3 3H2O than in Al2SiO4Fz and also that there are 3 moles of
water in a hydrous aluminum fluoride molecule whereas there is no
water in anhydrous topaz. At high temperatures water in fluoride
will evaporate, causing significant material loss from the
reactive ingredient mix. Also fluoride will be lost at even
higher temperatures, causing additional loss from the solid
7/8/91
26
h ~ l 8 ~
caBe 35~5
phase. Thus, as the aluminum fluoride trihydrate content of the
starting mixture decreases ( iOe. T-number increases) there will
be less loss of matter and the final product becomes denser.
Within the scope of the invention are products prepared from
mixes in which topaz (T) content is defined by the equation T -
X, in which X is greater than 0. T-25 through T-100 articles
(about 50% to 75% porosity) are preferred and T-50 through T-lOo
(about 45% to 70% porosity) are especially preferred.
Addition of topaz to the reactive mix is not the only way to
increase density of the honeycomb. However topaz is unique
because the densified body is still in the form of a body
composed substantially completely of interconnected whiskers.
Thus, another approach to densify and strengthen an
interconnected mullite whisker substrate is to increase its
weight after firing. This technique can be used to further
densify mullite whisker products obtained in accordance with this
invention from a topaz reactant. Densification can be
accomplished by different techniques. ~ne is infiltration. In
this case, fired mullite whisker honeycombs are infiltrated with
a slurry or the like by dipping. The assembly is then dried and
subsequently calcined. The composition of the slurry can be any
7/8/91 27
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Ca~e 3555
form of aluminosilicate, or aluminum or silicon based compound,
including clays. Preferred is the use of a mixture of alumina
silica sols in proportion stoichiometric to form mullite. The
slurry is then dried and calcined at a suitable sintering
temperature, time and atmospheric condition. The calcined
substrate is denser and presumably stronger than the uncoated
substrate. A colloidal solution can also be used instead of a
slurry. Chemical vapor deposition, chemical vapor infiltration,
sputter deposition or any other coating technology may also be
suitable for densification. Another densification route is to
blend the reactive ingredients with densification aids or fillers
and then extrude this mixture, dry and calcine the honeycomb.
These additives can be alumina, silica, zirconia or mullite
powders, mullite whiskers or any other silicon, aluminum or
zirconium based compounds including clays. Illustrative examples
demonstrate producing mullite whisker products made with
additives such as clays, alumina, mullite or silica sols as part
of the reactant mixture and using mullite precursor sol as an
infiltration vehicle on the fired honeycomb that can also be
applied to whisker products having a topaz precursor.
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Ca3e 35S5
However, densification increases elastic modulus which is
not desired because higher elastic modulus increases thermal
shock susceptibility. Thus, a trade-off between strength and
thermal shock susceptibility must be made to produce honeycombs
which have both adequate strength and thermal shock resistance.
Another key physical requirement for a substrate is high
temperature strength and durability. Catcom substrate is
operated at high temperatures such as 1250C and above in steady-
state conditions and for long durations. It is also subjected to
steady stress levels. The pressure of the flowing gas can be as
high as 10 atmospheres. The pressure acts as stress on the face
of the substrate. Thus, the conditions of creep, i.e. time
dependent deformation of a material under stress at elevated
temperatures, exist in Catcom application. The choice of
material should address this issue. The material should possess
sufficient strength at elevated temperature but must also be
creep resistant.
Example 6 demonstrates that only a 10% drop in strength was
observed at 1300C for a T-0 article. The drop was limited to
25% of the room temperature strength when a sample of T-0 was
tested at 1400C. It is clear from this example that this
7/8/91
29
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ca~e 3555
mullite whisker material is capable of withstanding high
temperatures in short term and this is expected of articles
ranging from T-0 through T-100. This is expected because the
melting point of mullite is around 1800C and all of these
articles can be prepared so that they are composed entirely of
mullite. Most of the commercial mullite products have limited use
at _emperatures in excess of 1300C. This is because of the
viscous deformation of the glassy phase which is common to see
along the grain boundaries of mullite and is present because of
sintering techniques generally used in preparing mullite
ceramics. This glassy grain boundary phase becomes less viscous
at elevated temperatures when operated under a steady stress.
The viscous relaxation which takes place microscopically creates
a damaged zone at the high stress concentration regions causing
grain boundary sliding, void growth, etc. These time dependent
deformation phenomena called viscous creep, weakens the material
and eventually leads it to failure even when it is operated at a
low stress level. The TEM microscopy done on the mullite
whiskers showed that each individual whisker is single
crystalline and the region where two whiskers connect i5 free of
glassy phases (Figures 4 & 5). Thus, the mullite whisker
material is substantially more creep resistant compared to most
of the conventional mullite products.
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case 3~ss
Another form of long term failure is corrosion related. At
high temperatures the atomic mobility of sp~cies, or dif~sivity,
increases causing corrosion or stress corrosion. These phenomena
can limit the use of a refractory material which has otherwise
good properties such as high strength, thermal shock and creep
resistance. Washcoat degradation due to diffusion of species
from or to the substrate, or segregation of cationic or anionic
species present in the substrate composition towards grain
boundaries are among the possible causes of degradation. Mullite
material has an advantage here. Mullite is a mixed oxide
material with very high degree of covalent bonding for an oxide.
Diffusivity of A1, Si and O are low. The mullite whisker
material is pure and prepared with no cationic additives. Thus,
diffusivity of Group I or II cations is not a major problem.
Mullite whisker material is resistant to corrosion related long
term failures.
Gas turbines vary in size and capacity. Some systems
require a substrate as wide as 22" in diameter. This size is
very difficult to extrude. One way of producing large substrates
is to make smaller ones and join them. Smaller mullite whisker
substrates can be extruded and joined to form larger pieces.
Mullite whisker material can be joined relatively easily.
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case 3555
Example 4 demonstrates that mullite whisker articles can be
joined to form larger pieces and this can be achieved using
topaz-derivad products. See Example 16. Topaz derived ln situ
joints are preferred because the mullite thus derived will be
denser and the joint stronger. Whiskers from both of the pieces
to-be-joined grow and mechanically join to each other producing a
clean joint boundary.
As the temperature difference between the surface and the
center of the substrate becomes larger, the radial thermal stress
induced on the sample increases. Substrate breaks when the
induced stress reaches the strength of the material.
The temperature differential, thus thermal stress, depends
on the size of the catalytic unit. Obviously, units with larger
diameters would experience larger temperature differential and
thermal stress. The severity of thermal shock varies from one
gas turbine manufacturer to another. However, regardless of the
type and size of the power generator, the Catcom substrate should
be designed to survive the emergency shutdown trips.
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c~se 3~55
The equation for thermal stress given above shows that
variables such as E, v and ~ are the key physical parameters in
determining the induced thermal stress at a given T. A candidate
material should possess high strength, low elastic modulus and
thermal expansion coefficient so that the magnitude of thermal
stress will be low and smaller than the strength of the material
to avoid failure during emergency shutdown.
In addition to the microstructure factors affecting design
of substrates for applications such as Catcom, there are a number
of important design requirements for the substrate which carries
the catalyst in Catcom. These are macrostructure factors. The
support should be manufactured in large sizes such as 22" in
diameter; it should have uniform cross section; it should possess
high open frontal area such as 70% or more.
It is essential that momentum profile (mass x velocity) of
the incoming gas across the face of th0 turbine should be uniform
for proper functioning of the system. Uniform velocity profile
is necessary to maintain constant momentum. Honeycomb structures
are used to smooth velocity pro~iles of flow streams in many
applications including gas turbines. Thus, a Catcom substrate
made into a honeycomb structure has an advantage over other
7/8/91
Case 3S55
shapes in improving the incoming gas stream and maintaining its
flow uniformity. Uniformity in the Gell walls of the honeycomb
is also very critical. Gas will flow with higher velocity
through a larger cell. The variation of velocity from one cell
to another should be avoided. Honeycombs can be manufactured by
a number of techniques. Extrusion process produces uniform cell
sizes across the face of the substrate and is the preferred
method of manufacturing. Any other method used to make Catcom
substrate must produce uniform cell sizes in order to be
compatible with extrusion. Uniformity in the cell size is
essential not only for maintaining a uniform velocity profile but
also to maintain constant temperature across the outlet face of
the catalyst. It is essential for operating conditions that the
gas phase should have a uniform temperature profile when it
reaches the turbine. The quality of combustion is affected by
cell size. The boundary layer, defined as the stagnant film of
gas between the washcoat and the gas phase, is thicker in the
larger cell. The honeycomb walls are the heat sources to the
system. Heat transfer from the honeycomb walls to the gas phase
becomes harder with the increased thickness of the boundary
layer. Same is true for mass transfer. Thus, reaction is slower
in the more open cells. If nonuniformity exists in the
honeycomb, unequal reaction rates will occur as a re~ult of
7/8/91
34
Caqe 3S55
nonuniformity in the cell size. The combustion reactions are
exothermic and generate heat. More heat will be generated in the
smaller cells, increasing the temperature of the substrate walls
and the gas phase which is not desired for turbine operating
conditions.
A uniform cross section other than that of a honeycomb can
be obtained by extrusion or by some other method. An example is
a cross section with a circular hole pattern with each circle
having the same radius. This pattern will also allow uniform gas
velocity and temperature profile. A very impoxtant advantage of
honeycomb profile over any other design is that the former will
produce less back pressure to the incoming gas stream. Obtaining
minimum back pressure is very critical in Catcom application.
Higher back pressure will cause higher compression force which
results in fuel penalty for the system. Minimum pressure drop
increases the efficiency of the system. Energy is provided to
turbine by the expansion of the gas phase and is characterized by
a PV term. An increase in pressure drop results in a decrease in
the amount of energy provided to the turbine.
Finally, in honeycombs, the inner surface of each wall is
available for catalysis.
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case 3555
Ideally, cell sizes should be infinitely small with wall
thicknesses infinitely thin. However, in practice, when walls
are made thinner, extrusion becomes more difficult and also
strength of the substrate diminishes. Therefore, cell size and
wall thickness in a honeycomb to be used for catcom application
should be optimized amongst the engineering constraints.
An extruded profile with uniform cross section is the
preferred structure as Catcom substrate since extrusion produces
a pattern with uniform cross section which is essential in
producing and maintaining uniform gas velocity and temperature
profile. Honeycomb shape, in particular, is preferred because
this geometry minimizes back pressure while it provides high
available surface area for catalysis.
Mullite whisker honeycomb substrates for Catcom can be used
with various precious metal catalysts, including palladium oxide,
such as described in U.S. 4,893,463. A mixture of a refractory
inorganic binder and a catalytically effective amount of a binary
oxide of the formula Pr4PdO7 is recommended. See U.S.
07/684,631, filed April 12, 1991, the teachings of which are
lncorporated herein by cross-reference.
7/8/91 36
~3`~1
case 3555
BRIEF DE8CRIPTION OF T~E DRAWING8
The invention can be further understood by reference to the
drawings in which:
Fig. 1 is an X-ray diffraction pattern of a T-O specimen.
Fig. 2 is a scanning electron micrograph of the surface of a
T-O specimen.
Figs. 3 and 5 are micrographs of T-0 material.
Fig. 4 is a micrograph depicting a joint formed between two
pieces of mullite whisker material.
Fig. 6 is a plot of compression strength of a mullite
whisker article vs. carbon content of reactants.
Fig. 7 is a plot of compression strength vs. temperature for
mullite whisker articles.
Fig. 8 is a plot of relative stiffness vs. temperature for
mullite whisker articles.
7/8/91
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: '
2 ~
Ca~e 3555
Fig. 9 is a micrograph of the structure of a honeycomb
produced from the topaz (T-100) route.
Fig. lo is a micrograph of the structure of a honeycomb
produced directly from aluminum fluoride and silica.
Figure 11 is an SEM photomicrograph at 20,000 magnification
showing the microstructure of microspheres of kaolin clay
fired to mullite and then leached with caustic to remove
free silica formed during the reaction.
Figure 12 i5 an SEM micrograph of a honeycomb wall of a
mullite whisker material with T-50 composition.
To illustrate more completely the invention, the following
examples are given. These examples are for purposes of
illustration only and are not to be construed as limitations of
the invention.
7/8/91
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2 ~ f~
Cas~ 3555
EXAMPLB 1
This example illustrates the production of a T-o honeycomb.
Two-hundred and fifty four grams of aluminum fluoride (Aldrich
ALF3. 2.8 H2O) was added to 123 grams of -325 mesh amorphous
silica (Thermal American, Montville, NJ~ and ground and mixed in
a ball mill for 24 hours. The mix was then placed in a pug mill
where 120 ml of 7% Methocel solution was slowly added to the
batch during mixing. The mixing continued until an extrudable
plastic mass was obtained. The charge was then fed to the
extrusion chamber and extruded through a simple honeycomb die,
and shapes of approximately 1 1/2" diameter and 2" length were
formed. The cell density of the pieces were approximately 11
cells/square inch.
A number of such extruded pieces were placed in an oven at
75C so that the Methocel binder gelled. Then the pieces were
placed uncovered in a nitrogen flowing tube furnace and heated at
a rate of 10C/min until 350C was reached. The pieces were
soaked at this temperature for one hour and heated at a rate of
10C/min until 1000C was reached. The samples were then furnace
cooled, with room temperature being obtained over the course of
several hours. The appearance of these samples (referred to
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' :
Case 3555
herein as l(a)) was porous, strong enough to handle without
breaking, cohesive, integral and dark grey in color.
One sample was reheated to loooDc at a rate of 8C/min,
followed by heating it to 1300C at a rate of 5C/min. The
honeycomb piece was held at this temperature for 6 hours.
Finally, the piece was cooled from this temperature to room
temperature at a rate of 25C/min. This honeycomb (referred to
herein as l(b)) appeared white, extremely porous, cohesive,
integral, strong enough to handle without breaking and very
lightweight. The initial and final weights of the honeycomb were
26.13 and 8.42 grams, respectively.
Once piece prepared as in example l(b) was thermal shocked
in a gas fired furnace from 1000C to 600C at a rate of 25C/sec.
Visual examination showed no evidence of cracking. The same
piece was subsequently shocked from 1100, 1200, 1300 and 1400C
to 600C at the rates of 23.8, 25, 25.9, 26.7C/sec,
respectively. After each shock, the piece was visually examined
and no cracking was observed. Two three-point flexural beams
were cut from this piece. The strength of each beam was measured
using Instron equipment (Model 4202) at a crosshead speed of
0.0075 in/min. The strength of the porous honeycomb specimens
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2 ~
Case 3555
with two cells in width and one cell thickness was measured as
111 psi. The apparent density of the honeycomb wall was measured
as 0.48 gr/cc.
The X-ray diffraction (XRD) pattern of the honeycomb
material is shown in Figure 1. No evidence of glassy phase was
observed. The material as is evident from this pattern is very
crystalline mullite. Chemical analysis done using EDX analysis
showed the whiskers have a mullite composition (40.0% Al, 11.4%
Si and 48.6% O).
The scanning electron micrograph (SEM) of the fracture
surface reveals the microstructure of this material as shown in
Figure 2. The material is basically an open 3-dimensional
whisker structure, with interconnected whiskers of sizes ranging
from 0.05 to 10 microns or larger in diameter. Individual
whiskers appear to be very crystalline with very smooth surfaces
(Figure 3). The transmission electron micrograph of the whiskers
showed that they are branched and interconnected with very clean
boundaries, exhibiting very little or no glassy phase at the site
of interconnection (Figure 4).
7/8/91 41
Case 3555
The honeycomb pieces were machinable. Four holes were
drilled using an electric drill with a 3/32 drill bit. The holes
were 1/4" apart from each other. There was no apparent cracking
around the holes.
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f') ~
Ca~o 3555
EXAMPLE 2
Discs of green material composed of hydrated aluminum
fluoride, silica, Methocel and water were dry pressed into forms
13 millimeter in diameter with heights varying between 5 to 9
millimeters. The AlF3/SiO2 ratio was kept at 2/0.968. The discs
were fired in flowing nitrogen at 1400C. Compression strength
of the discs was evaluated using a screw driven Instron equipment
(Model 4202) at a crosshead speed of 0.0075 in/min. The average
strength was obtained as 310+56 psi. Elevated temperature tests
showed compression strengths at 1200C, 1300C and 1400C of
306+49, 243+31 and 220~+37 psi, respectively.
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case 3555
EXA~PLB ~
Aluminum fluoride hydrate and silica powders each with top
sizes of 45 microns were ground separately in anhydrous ethanol
in an Eiger mill using zirconia ball media. Handling of aluminum
fluoride in a water-free environment at this point is important
in order to maintain a flowable powder. The powders were then
ground to less than 10 microns and oven dried and in a molar
ratio of 12:13 and mixed in ethanol in the above described Eiger
mill. No ball media was used. The stoichiometric mixture was
then oven dried. Methocel in the form of 0.2% solution was
manually added to the mix. Discs were formed, dried in an oven
then fired in flowing nitrogen at 1400C. Room temperature
compression strength of these discs was 667+41 psi. The
described pregrinding of the reactants, which resulted in initial
powders finer than 45 microns, thus improved the strength of the
porous mullite discs.
Backpressure measurements on these discs were performed by
flowing nitrogen through them. Back pressures were measured
using a differential pressure gauge placed in parallel to the
sample disc. Flow rate of the incoming gas was carefully
monitored. Backpressure increased as the flow rate increased.
7/8/91
44
Case 3555
As expected, thicker samples showed higher back pressures. The
permeability coefficient of the filter material was calculated
using Darcey's equation. Its value was determined as 3.86 + 1.12
(xlO-13) mZ
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Ca~e 3555
EXAMP~E 4
Aluminum fluoride hydrate and silica powder were ground and
discs were prepared as described in Example 3. Two discs were
placed on top of each other and fired in flowing air at 1400C.
Figure 5 shows the interface between the two discs after firing.
The joint zone is approximately 20 microns. The darker color
background is epoxy used in sample preparation for microscopy.
Whiskers grew from each disc and joined the two discs. The joint
appears unifo1~ and no glassy phase is apparent at the boundary.
7/8/91
46
., ' ~
Ca30 3555
EXAMPLE 5
An alumina fluoride hydrate, silica, Methocel mixture was
prepared as described in Example 3, extruded through an 11
cell/square inch die to form a honeycomb and fired in flowing
nitrogen 1400C.
A mullite precursor sol mixture was then prepared using 15
grams of a 26% silica and 4% alumina sol mixture (NALCO ISJ-612)
and 93.3 grams of a 10~ alumina sol (NALC0-ISJ-614). The sol
mixture was stirred for four days using a magnetic stirrer. The
honeycomb piece was then dipped in the sol mixture twice, dried
at 120C and dipped twice again followed by drying at 120C. The
honeycomb was then fired to 1400C and weighed upon cooling. A
weight gain of 10% was noted. Repeated examples showed weight
gains of up to 20% or more, demonstrating that densification of
fired honeycombs could be easily achieved.
7/8/91
47
cas~ 3555
EXAMPLE 6
Two sample geometries of mullite whisker material were
prepared. These were simple honeycombs (1.5 inch dia., with 11
cells/s~uare inch) and discs (13 mm dia.). The apparent
densities were varied to observe the effect of porosity on key
parameters such as strength and backpressure. These samples were
prepared by adding graphite into the mix, the graphite burning
off at a temperature range of 600 - 800C, resulting in increased
porosity.
1) Strenath
Several discs (13 mm dia.) were prepared by dry pressing.
Samples were heat treated in nitrogen at 1300C for 12 hours.
The average compressive strength of the fired discs was 310 psi.
The specific strength (strength/density) was 16610 inches.
Samples with greater porosity, prepared using graphite, had less
strength as illustrates in (Figure 6).
ii) ~urability
Strength as a function of testing temperature is shown in
Figure 7. Room temperature strength was maintained at 1200C. A
10% drop was observed at 1300C. At 1400C strength was 220 psi,
7/8/91
48
ca~ 3555
only 25% less than obtained at room temperature. The material
was still quite usable at this temperature. Figure 8 show
relative stiffness of the discs as a function of testing
temperature. These values were obtained by measuring the 510pe
of the elastic portion of the load vs. time curves.
iii) Thermal Shock Resistance
One honeycomb piece, fired at 1300C, was thermally shocked
in a gas fired furnace from 1000C to 600C at an approximate
rate of 25C/sec. Visual examination showed no evidence of
cracking. The same piece was subsequently shocked from 1100,
1200, 1300 and 1400C at the same rate to 600C. After each
shock, the piece was visually examined and not cracking was
observed. A second honeycomb was shocked from 1300C for 5
cycles at a rate of 25C/sec and ultimately failed. The XRD
analysis showed that this piece had cristobalite in it which is
expected to be detrimental because of high temperature phase
transformations. More severe thermal shock experiments were done
with other honeycomb samples. One sample survived water
quenching from 100 and 1300C at an approximate rate of
280C/sec. Minor spalling was observed along the circumference
of the cylinder.
7/8/91
49
2~d~
~ Ca~e 3555
~XAMPL~ 7
A commercial calcined kaolin clay (Satintone #5, a fine
particle size calcined kaolin, marketed by Engelhard corporation~
was mixed with powdered AlF3 hydrate (5.52 grams AlF3, 1.80 grams
SiO2, 2.22 grams clay and 0.48 gram Methocel), pressed into a
pellet and fired in nitrogen to 1300C. XRD results showed that
the final product was principally mullite, alpha-alumina and a
minor amount of cristobalite. Since the presence of cristobalite
may impair thermal shock resistance, it may be advantageous to
compensate for the excess silica in the fired product by adding
alumina powder or aluminum fluoride hydrate to the initial powder
mix and thereby obtain 100% mullite composition.
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C ~e 3555
EXAMP~ 8
Thirteen grams of a commercial fine particle size hydrous
kaolin ~Engelhard ASP-172) was added to 465 ml water in a beaker.
The beaker ~as then heated to 90C followed by transferring of
the contents to a blender in which the mixture was stirred at low
speed. Dry Methocel powder, 65 gram, was added to prepare a 14%
methocel solution and the mixture chilled and refrigerated.
Aluminum fluoride hydrate and silica powders, 297.5 and
143.9 grams, respectively, were mixed in a ball mill jar and
blended overnight, the dry fluoride/silica mixture pug milled,
145.2 grams of the above described Methocel solution being added
during milling. After 30 minutes of pugging, the mixture was
extruded to a honeycomb shape of approximately 50 cells per
square inch using a piston extruder. This mixture was softer and
far more easily extruded than those extruded without clay
addition to the Methocel mixture. Clay addition reduced the
amount of water needed to extrude a cohesive, integral piece.
7/8/91
51
L~
Ca~8 3555
EXANPLE 9
This example describes a method to produce mullite whisker
honeycom~ using topaz as the only aluminum source~ T-100) in.
A quantity of 348.5 grams of aluminum fluoride trihydrate
was mixed in a pug mill with 151.5 grams of amorphous anhydrous,
fused amorphous silicon dioxide, from Thermal American,
Montville, N.J. (-325 Mesh). To this mixture, 21.6 grams ~f dry
MethocelX methyl cellulose was added. The mix was pugged for 15
minutes. A total of 130 mls. deionized water was added to the
mix. Pugging continued for 30 more minutes. The paste was then
extruded through a multiple die with 1/16" openings using a
piston extruder to spaghetti shape. The uncovered extrudates
were then fired in flowing air at 750C for 12 hours. Weight
loss upon firing was 52%. The XRD analysis showed that the sole
crystalline phase in the fired product was topaz. A total of 441
grams of topaz was prepared as described above were mixed with 24
grams of silicon dioxide (the same as used above) in a ball mill
with liquid medium being ethyl alcohol. After being ball milled
24 hours, the mixture was filtered and dried. The dried mixture
was mixed with 23 grams of Methocel 20-213 binder and mixed in a
pug mill. A total of 140 mls. of water was added to produce an
7/8/91
52
'.
Ca~e 3555
extrudable paste. The mix was then extruded through a 50 cpsi
1.5" die to a honeycomb shape using a piston extruder; wall
thickness was about 0.9 mm. The honeycomb pieces were then fired
in flowing air using a heating schedule of 10C/min to 350C;
hold 7 hours at 350C; 10 C/min to 1000C; 8C/min into 1300C:
5C/min to 1400C hold 90 minutes. The furnace was then cooled
25C/min to 1400C to produce mullite. The final extrudate was
stronger than that obtained by extruding aluminum fluoride and
silica.
Figures 9 and 10 show the microstructures of this topaz
produced honeycomb and one obtained by firing aluminum fluoride
trihydrate with silica alone, respectively. The denser
interconnected whisker network present in the material produced
by the topaz route of this example is apparent from a comparison
of these figures.
The pieces had large cracks and therefore porosity was not
measured; however 35% weight loss was observed in a similar run
(see Example 11). Surface area of the product (BET) was
lm2/gram. Aspect ratio of the whiskers was greater than 100 : 1.
Whiskers are 100% dense. See whisker SEM micrographs.
7/8/91
C~3~ 3555
Similar tests were carried out with both nitrogen and air
firing to 1300 - 1500C as the peak temperature. Results did not
show a difference in the products. For further purposes of
comparison, Figure 11 shows the microstructure of a mullite
article prepared by firing microspheres kaolin to mullite and
leaching the free silica. The particles on the surface of the
microspheres are cigar shaped with an aspect ratio of 3 :1 with
about 0.1 microdiameter.
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Case 355
EXAMPLE 10
This example shows that topaz induced mullite whisker
honeycomb can be produced by extrusion using a glycol plasticizer
as an extrusion aid.
A topaz/silica mixture was prepared as described in Example
9. Polyethylene glycol was added to the dry mixture in amount
less than 1% by wt. A total of 5% of dry Methocel binder was
added. A total of 110 ml. water was added to produce extrudable
paste. The mix was then extruded and fired as described in
Example 9. The appearance and properties of the honeycombs were
same as described in Example 9. The weight lost on firing was
35%.
7/8/91
Ca3~ 3555
E$A~P~s ll
This example shows that topaz can be added to aluminum
fluoride trihydrate and sil con mixture, extruded to honeycomb
shape and fired to produce an interconnected mullite whiskers
honeycomb. In this example, 30% of the mullite whiskers were
generated from reaction of topaz and silica, whereas the rest was
formed in situ from reaction of aluminum fluoride trihydrate and
silica.
A dry mix was made by blending 249 grams of aluminum
fluoride trihydrate, 120 grams of silicon oxide and 71 grams of
topaz prepared as described in Example 9. Methocel binder, 26
grams, was added to the dry mix. A total of 107 mls. of water
was added and pugged for 40 minutes. The mix was then extruded
to a honeycomb shape through a 50 cpsi die via a piston extruder.
The extrudates were then fired at 1400C in air. Weight loss of
55% was observed upon firing. The resulting extrudates were all
composed of interconnected mullite whiskers, of which 30% were
generated by the reaction of topaz and silica. The rest of the
whiskers were formed by the reaction of aluminum fluoride and
silica. The honeycomb was denser and stronger than the one
described in Example 1 which is T-0 (no topaz added to initial
7/8/91
56
Case 3555
dry mix). The product of Example 11 is designated as T-30 since
30% of the whiskers were generated from the barlike topaz added
to the initial mix. Examples 9 and 10 describe preparation of T-
100 product (i.e. no AlF3. 3H2O was used in the initial mix).
Only topaz and silica were used to produce honeycombs.
It was found that strength and density were higher when
using the higher topaz content in the reactant mix, but porosity
was lower.
7/8/91
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Ca~e 3555
EXAMPLE 12
A four-component dry powder mix was blended in a pug mill.
The mix was composed of 249 grams of aluminum fluoride
trihydrate, 120 grams of fused silica, 71 ~rams of topaz prepared
as described in Example 9, and 22 grams of dry Methocel powder.
Polyethylene glycol (Dow Chemical Polyglycol E-400) was added, in
amount of 0.5 wt.% of dry materials, to the mixture. A water
volume of 91 mls. was added to the mix and pugged for 45 minutes.
A second batch was made following the same procedure to double
the size of the paste. Both batches were fed to a twin screw
extruder and honeycomb pieces with 64 cpsi and 2" x 2" were
extruded. Some of the extrudates were air dried while others
were dried in a conventional kitchen type microwave oven at
medium heat. Microwave dried pieces appeared to be smoother on
the outside surfaces with fewer cracks. Some pieces were heated
in the microwave in the presence of water vapor which was
generated by water present in a one liter beaker located inside
the microwave cavity. These pieces had the least number of flaws
on the outside. The pieces were then fired at 1400C to have the
same composition as described in Example 11.
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ca~e 3555
EXAMPLE 13
This example describes the preparation of a T-50 honeycomb
using a twin-screw extruder where mixing and extruding are both
done using the same piece of equipment. Aluminum fluoride was
slightly more than the stoichiometric amount required to ma~e T-
50 product to compensate for the volatile fluorine loss at
elevated temperatures before the onset of the reaction of
aluminum fluoride with silica.
A total of 24.4 lbs. of aluminum fluoride trihydrate was
blended with 15.7 lbs. of topaz prepared a~ in Example 9 in a
sigma mixer. This mix was fed dry via a loss-in-weight type
feeder to a co-rotating twin screw extruder at a rate of 73.8
lbs./hr. A mixture of silica (same as Example 1) and dry
Methocel, at a ratio of 100:15, was fed at a rate of 26.2
lbs./hr. simultaneously. A water/glycol mixture (2.7% glycol)
was also independently fed to the extruder. The powders and
liquid were mixed and conveyed through the chilled barrels of the
extruder and through a 2" x 2" 64 cpsi honeycomb die. The
extrudates were then sliced to various lengths and dried either
in air or in microwave oven in the presence of water vapor as
described in Example 9. The extrudates did not show cracking on
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C~e 3555
the outside surfaces and fired to form interconnected mullite
whiskers, of which 50% was ger.erated by ths reaction of topaz and
silica.
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Ca~e 3555
EXA~PLE 14
-
This example shows that colloidal silica can be used to
replace amorphous fused silica powder to produce shapes which
produces stronger product.
A quantity of 9.6 grams of topaz prepared as described in
Example 9 and 0.4 grams of dry Methocel powder was mixed with
1.92 grams of hydrous colloidal silica (NALCO-1050) containing
50% silica by weight, the balance being water. Water was added
gradually so that the paste had 21% water. The mix was then
pressed to 1.25" diameter discs at a pressure of 5000 psi at
80C. The discs were then fired and cut to make bend bars with
approximately 6.8 x 3.3 mms. cross section. The bars were tested
for strength by three point bending tests and their strength
varied between 7000-9000 psi. Strength of the material made
using colloidal silica was higher by 2000-3000 psi than that made
by amorphous powder silicau
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~XAMP~E 15
A total of 450 grams of aluminum fluoride trihydrate,
silica/Methocel mixture and -100 mesh topaz prepared as in
Example 9, was blended in the same ratio used in Example 13 in a
sigma blade pug mill. To this mixture water/glycol mixture was
added as a 2.7% glycol solution and the mixture was pugged until
a paste suitable for extrusion was obtained. The paste was then
pressed through a 50 cpsi die using a ram press to obtain
honeycomb shaped profiles of 1.5" diameter. The pieces were then
dried and fired in air at a temperature of 1400C for full
conversion of reactants to mullite. One piece was then coated
with a standard commercial precious metal containing autocatalyst
washcoat based on alumina. The piece was then dried and calcined
to observe adhesion of catalyst washcoat to the substrate. The
washcoat adhered to the substrate uniformly after calcining. The
piece was then tested for catalytic activity for C7H16 oxidation,
SO2 to S03, CO to CO~, and NO to NO2 using a diagnostic reactor.
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case 3555
The results of the conversion are listed below:
Speciesconversion Rate
C~H16 oxidation 37%
SO2 to SO3 23%
Co to CO2 62%
NO to NO2 3%
These results demonstrate that mullite whisker honeycombs
can be coated with commercial precious metal based washcoats and
the catalyst deposited on the substrate is active in oxidizing
hydrocarbons, O, NO and SO2. Also, the catalyst deposited on
mullite whisker material is active and oxidizes hydrocarbons, CO,
NO and SO2.
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Case 35s5
EXAMPLE 16
This example describes a method to join two pieces of fired
honeycombs to form larger pieces using topaz to form a strong,
dense joint.
55.2 grams of topaz, produced in a whisker furnace in
spaghetti form, as described in Example 9, was mixed with 3.0
grams of fused silica (Thermo American, -325 Mesh) in a 200 cc
capacity ball mill filled halfway with alumina balls. Water was
added to cover the balls and the mixture was milled for 48 hours
to a very fine powder suspension. The slurry was then recovered
and placed in a closed jar. The jar was not disturbed for seven
days. It was observed that the contents in the jar were gelled.
Two mullite honeycomb pieces with lO0 cpsi and T-50
composition were sliced parallel to the honeycomb wall direction.
The topaz/silica gel was then spread on both faces of the
honeycomb walls and two pieces were pressed under l kg load
overnight. The joined dry piece was then fired in the whisker
furnace in air to 1400C for 12 hours. The product appeared to
have a solid joint upon firing.
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Case 3555
Various changes and modifications can be made in the process
and processes of this present invention without departing from
the scope and spirit thereof. The various embodiments disclosed
herein are for the purpose of further illustrating the invention
but are not intended to limit it.
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