Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~?~ 5 57 78
pHIGH DENSITY LEUCITE AND/OR POLLUCITE
BASED CERAMICS FROM ZEOLITE"
FIELD OF THE INVENTION
s The present invention relates to a novel method of preparation of leucite
based and/or pollucite based ceramic articles having less than 5% porosity and
being substantially free of cracks.
BACKGROUND OF THE INVENTION
Ceramic articles have many uses including catalyst supports, dental
io porcelain, heat exchangers, turbine blades, substrates for integrated
circuits, etc.
The particular ceramic which is used in a given application depends on the
prop-
erties required for the given application. For example, leucite ceramics can
be
used as dental porcelains, coatings for metals and metal/ceramic seals. A
review
of the importance of potassium aluminosilicate compositions in dental ceramics
is
i5 given in C. Hahn and K. Teuchert in Ber. Dt. Keram. Ges., 57, (1980) Nos. 9-
10,
208-215. One drawback to the use of leucite in dental applications is that it
is
fragile and hard to repair. For this reason, dental restorations usually
require a
metal framework. Accordingly, there is a need for a leucite ceramic with
higher
strength. There is also a need for a process which can form a leucite ceramic
at
2 0 lower temperatures so that the processes of high temperature glass melting
fol-
lowed by fritting and milling are eliminated.
U.S. Patent No. 4,798,536 teaches the addition of potassium salts to vari-
ous feldspars to produce a porcelain having a greater amount of a leucite
phase
and increased strength. A partially crystallized leucite glass ceramic has
been
2 5 produced by the present invention with strengths greater than those
reported in
the '536 reference, by taking a potassium exchanged zeolite Y powder and
heating it at a temperature of about 1050°C to give an amorphous
powder. This
amorphous powder is then formed into a desired shape and sintered at a
temperature of about 1150-1400oC to give a leucite ceramic article. Thus,
glass
~ o melting and preparation of frits are unnecessary.
Although the prior art describes the preparation of ceramics from zeolites,
there is no report of a process to make a dense leucite ceramic article. For
ex-
ample, D.W. Breck in ZEOLITE MOLECULAR SIEVES, John Wiley & Sons, New
York (1974), pp. 493-496 states that Mg-X can be heated to form cordierite.
The
2
~~aas
disclosed process involves heating the Mg-X zeolite at 1500oC to form a glass
and then heating the glass above 1000oC to form cordierite. Thus, two steps
are
required to form cordierite.
Another reference which teaches the preparation of a cordierite based ce
ramic article is U.S. Patent No. 4,814,303 to Chowdry et al. Chowdry discloses
producing a monolithic anorthite, anorthite-cordierite or cordierite based
ceramic
article by heating the Ca, Ca/Mg and Mg forms of zeolites X, Y and A at a tem
perature of about 900°C to about 1350°C. Example 33 of Chowdry
discloses
preparing a potassium exchanged zeolite X followed by sintering at
1000°C,
io thereby forming predominantly KAISi206 which supposedly showed the X-ray
diffraction pattern of leucite (JCPDS File No. 15-47).
Finally, European Patent Publication Number 298,701 (to Taga et al.) de-
scribes the preparation of a ceramic article having an anorthite phase from a
cal-
cium zeolite. The process involves a calcination to form an amorphous product
which can then be shaped into an article and sintered at temperatures of about
850-950oC.
The process of the present invention differs considerably from this prior art.
First, the instant process is a two-step process whereas Chowdry discloses a
one-step process. As the examples herein show, a two step process is critical
for
2 o producing usable ceramic articles. Second, the type of zeolites used and
sintering conditions used in the instant process are completely different from
that
in the Taga reference.
The process of this invention can also be used to produce ceramic articles
whose principal crystalline phase is pollucite. Pollucite ceramic articles can
be
2 s used in applications where there is a need for low thermal shock and high
refrac
toriness since pollucite has a coefficient of thermal expansion of less than 2
x 10-6
oC-1 over the temperature range 50-700oC, and has a melting point of greater
than 1900oC. This type of ceramic article can be produced by using a cesium
exchanged zeolite instead of a potassium exchanged zeolite and sintering at a
3 o temperature of about 1250°C.
Another drawback to leucite in certain applications is it has a large coeffi-
cient of thermal expansion. Leucite goes through a phase change (from tetra-
gonal to cubic) at a temperature between 400 and 600°C which results in
a unit
cell volume increase of about 5%. Even at temperatures below this structural
3 5 transition, leucite and its glass ceramics show relatively large thermal
expansion
coefficients. The prior art describes that thermal expansion in leucite glass
3
~~ ~8
ceramics can be varied over a somewhat narrow range by changing the ratio of
leucite crystals to residual glass in the sintered ceramic. This method of
thermal
expansion variation is described in U.S. Patent No. 4,604,366, which teaches
that
thermal expansion can be adjusted over a range of 10 x 10'6 to 19 x 10~ by
s blending two different glass frits with two different pulverized glass
ceramic pow-
ders in varying ratios.
A process has now been discovered by which the coefficient of thermal
expansion of the leucite can be varied from 2 x 10'6 to 27 x 10-6oC-1 in the
50 to
700oC temperature range.
io The coefficient can be varied by introducing a pollucite phase into the
leucite ceramic. Pollucite is a relatively low thermal expansion cesium-silica-
alu-
mina ceramic which has the cubic high-leucite structure at room temperature
and
forms a continuous series of solid solutions with leucite over the full
subsolidus
temperature range. As the cesium level in the leucite ceramic is increased the
i5 thermal expansion coefficient decreases to a point that the
leucite/pollucite as-
sumes the high leucite cubic structure at room temperature, after which time
the
coefficient of expansion continues to decrease with increased cesium content.
The leucite/pollucite ceramic article can be made by exchanging a zeolite
such as zeolite Y with both potassium and cesium and then following the
process
2 o described above. By varying the amounts of potassium and cesium content in
the starting zeolite and processing as described above, one can obtain any
desired leucite/pollucite solid solution. The use of a potassium and cesium
exchanged zeolite as the starting material provides a uniform distribution of
these
cations in the starting zeolite which in turn results in a homogeneous
distribution
25 of these cations in the ceramic article. By varying the amounts of cesium
and
potassium in the starting zeolite, the thermal expansion coefficient of the
ceramic
article can be adjusted to whatever value is desired between the coefficients
given
above. Thus, the instant process greatly simplifies the control of the
coefficient of
thermal expansion over that found in the prior art and allows a wider range of
the
3 o thermal expansion coefficient to be attained.
SUMMARY OF THE INVENTION
This invention relates to a process for preparing a ceramic article whose
principal crystalline phase is tetragonal leucite, a process for preparing a
ceramic
article whose principal crystalline phase is pollucite, a process for
preparing a
4 y55~~'$
ceramic article whose principal crystalline phase is rubidium
le~e~,~2'~fi'6C~~s for
preparing a ceramic article whose principal crystalline phase is a
leucite/pollucite
solid solution and to a ceramic article comprising a leucite/pollucite solid
solution.
Accordingly, one embodiment of the invention is a process producing a substan-
tially crack free ceramic article having less than 5°~ porosity and
whose principal
crystalline phase is tetragonal leucite comprising calcining a powder of a
potas-
sium exchanged zeolite, the zeolite having a Si02/AI2~3 ratio of 3.5 to 7.5,
at a
temperature of 900 to 1100°C for a time effective to collapse the
zeolite
framework and provide an amorphous powder, forming the amorphous powder
io into a shaped article and sintering the shaped article at a temperature of
at 1150
to 1400°C, for a time of 0.5 to 12 hours, thereby forming said ceramic
article.
Another embodiment of the invention is a process for producing a sub-
stantially crack free ceramic article having less than 5% porosity and whose
prin
cipal crystalline phase is a leucite/pollucite solid solution, comprising
calcining a
i5 powder of a potassium and cesium co-exchanged zeolite or a powder of a
potas
sium only exchanged zeolite and a cesium only exchanged zeolite at a tempera-
ture of 900 to 1100°C for a time effective to collapse the zeolite
framework and
provide an amorphous powder, the zeolite having a Si02/AI203 of 3.5 to 7.5,
has
a potassium content of greater than zero but less than 100% of the ion
exchange
2 o capacity of the zeolite, a cesium content of greater than zero but less
than 100%
of the ion exchange capacity of the zeolite and the sum of the potassium and
cesium content is at least 50% of the total ion exchange capacity of the
zeolite;
forming the amorphous powder into a shaped article and sintering the shaped
article at a temperature of 1150° to 1400°C, for a time of 0.5
to 12 hours, thereby
2 5 forming said ceramic article.
Yet another embodiment of the invention is a process for producing a sub-
stantially crack free ceramic article having less than 5% porosity and whose
prin-
cipal crystalline phase is pollucite comprising calcining a powder of a cesium
exchanged zeolite having a Si02/AI203 ratio of 3.5 to 7.5 at a temperature of
900
3 o to 1100°C for a time effective to collapse the zeolite framework
and provide an
amorphous powder, forming the amorphous powder into a shaped article and
sintering the shaped article at a temperature of 1150 to 1400°C, for a
time of 0.5
to 12 hours, thereby forming said ceramic article.
A further embodiment of the invention is a substantially crack free ceramic
s s article having less than 5% porosity, having as its principal crystalline
phase a
y055778
leucite/pollucite solid solution having an empirical formula expressed in
terms of
the metal oxides:
xK20:yCs20:zSi02:A1203
where x varies from 0.01 to 0.99, y varies from 0.99 to 0.01 and z varies from
3.5
5 to 7.5 except that when z is 7.5, y is greater than 0.19, the ceramic
article
characterized in that it has a coefficient of thermal expansion of 2 x 10-6 to
27 x 10-6oC-1 over the range 50° to 700oC.
DETAILED DESCRIPTION OF THE INVENTI N
One necessary component of the process of this invention is a zeolite.
io Zeolites are well known microporous three-dimensional framework structures.
In
general the crystalline zeolites are formed from corner sharing A102 and Si02
tetrahedra and are characterized as having pore openings of uniform
dimensions,
having a significant ion-exchange capacity and being capable of reversibly
desorbing an adsorbed phase which is dispersed throughout the internal pores
or
i5 voids of the crystal without displacing any atoms which make up the
permanent
crystal structure.
Zeolites can be represented on an anhydrous basis, by the formula
M2/nO:A1203:XSi02
where M is a canon having the valence n and X is generally equal to or greater
2 o than 2. In naturally occurring zeolites, M can be Li, Na, Ca, K, Mg and
Ba. The M
rations are loosely bound to the structure and frequently can be completely or
partially replaced with other rations by conventional ion exchange techniques.
The zeolites which can be used in this invention include any zeolite which
can be synthesized with a Si02/AI203 ratio between 3.5 and 7.5. It is also nec
25 essary that the ration present in the zeolite be exchangeable with
potassium,
cesium, rubidium or a mixture of potassium and cesium. Illustrative of the
zeolites
which have these properties are zeolite Y, zeolite L, zeolite LZ-210, zeolite
B, zeo
lite omega, zeolite LZ-202, and zeolite W. Zeolite LZ-210 is a zeolite Y whose
sili
con content has been increased by treatment with aqueous ammonium fluorosili
s o rate ((NH4)2SiFg). The preparation and characterization of this zeolite is
described in U.S. Patent No. 4,503,023 , zeo-
6
life t1-202 is an omega-type zeolite prepared without a templating agent,
whose
preparation is disclosed in U.S. Patent No. 4,840,779.
Of these zeolites, zeolite Y, L, B, W, and ~ga are preferred.
In the description which follows, zeolite Y will be used to exemplify the pro
s cess. However, this is not to be construed as limiting the invention in any
way to
zeolite Y.
Zeolite Y is a synthetic zeolite having the formula Na20:A1203:xSi02 where
x ranges from 3 to 6. The synthesis of zeolite Y is described in U.S. Patent
No.
3,130,007 . The synthesis essentially entails
to forming a mixture of sodium aluminate, sodium silicate, colloidal silica
and sodium
hydroxide heating this mixture at a temperature of 20° to 175°C
under
autogenous pressure for a time sufficient to ensure complete crystallization,
usually about 16 to 40 hours and isolating the product.
Two techniques are generally used to remove the sodium cation or other
15 cation and replace it with potassium, cesium, rubidium or a mixture of
potassium
and cesium. One technique is a multiple ion exchange with the potassium cation
while the other technique involves pre-exchanging the zeolite with a cation
such
as NH4+ followed by ion exchange with the potassium ion.
Ion exchange is conveniently carried out by contacting the zeolite with an
2 o aqueous solution of the metal ion to be exchanged. For example, a dilute
(about
1 molar) aqueous solution of potassium chloride or potassium nitrate is
prepared
and the pH of the solution adjusted to about 8.5 using potassium hydroxide.
The
volume of solution which is prepared is that amount which provides from about
5
to about 10 times the amount of potassium ion needed to fully ion exchange the
2 s sodium or other unwanted alkali metal in the zeolite.
The contacting of the potassium salt solution with the zeolite can conve-
niently be carried out in a batch process. Accordingly, the solution is mixed
with
the zeolite powder and the mixture is refluxed for about 2 hours. Next the
mixture
is filtered, thereby isolating the zeolite powder. This procedure is repeated
with a
3 o fresh batch of solution until the potassium content is at least 50% and
preferably
at least 90% of the ion exchange capacity of the zeolite. The ion exchange
capacity for a zeolite in units of moles/g is defined as the moles/g of
aluminum in
the framework when a monovalent cation is being exchanged into the zeolite.
Alternatively, the potassium exchange can be carried out using a continuous
pro-
s 5 cess employing methods well known in the art such as placing the zeolite
in a
column and flowing the potassium solution through the column or using a basket
7
'~' ~ ~~ ~$
centrifuge. A continuous process has the advantage of allowing a more
efficient
utilization of the potassium solution.
The potassium exchanged zeolite Y is now calcined, i.e., heated in air, at a
temperature of 900 to 1100oC and preferably at 1000 to 1075oC for a time of
0.5
to 2 hours. This calcination collapses the zeolite framework and produces an
amorphous powder which, when formed into a ceramic article (a green or
unsintered article) has a higher density than if the uncalcined zeolite were
used.
The effect of this calcination step is that cracks and warping in the finished
ceramic article are minimized or eliminated, i.e., the finished article is
substantially
Z o crack and warp free.
During the calcination agglomeration of the zeolite may occur. It is pre-
ferred that the calcined or amorphous powder be sieved and only the powder
which goes through a 60 mesh U.S. Standard Sieve (250 micron opening) be
used to prepare the ceramic powder. Of course the powder can be milled using
conventional milling means such as ball milling, attrition milling, impact
milling, etc.
in order to reduce the particle size to 60 mesh or less. A powder with smaller
par-
ticles will produce a ceramic article with fewer cracks and allow for more
facile
processing.
The amorphous powder is now formed into a desired shape by means well
2 o known in the art. A typical method of forming a shaped article involves
placing
the zeolite powder into a metal die and then pressing the powder at pressures
of
500 to 50,000 psi (3,440 to about 344,000 kPa).
It is also desirable to add a binder to the powder as an aid in forming the
shaped article. The binder may be selected from those well known in the art
such
2 5 as polyvinyl alcohol, and polyethylene glycol. If a binder is added, the
amount
which is to be added is up to about 15 weight percent of the powder.
Having formed the potassium exchanged zeolite Y into a desired shape
(green article), the green article is now sintered at a temperature of
1150°C to
about 1400°C and preferably at a temperature of 1200°C to
1300°C for a time of
3 0 2 to 6 hours. The resultant ceramic article obtained after sintering has
been found
to have as its principal crystalline phase tetragonal leucite. By principal is
meant
that at least 90% of the crystalline phase of the article is leucite. The
ceramic
article which is obtained is substantially crack free and has less than 5%
porosity.
By substantially crack-free is meant that no cracks are visible to the naked
eye.
35 Porosity can be measured by conventional techniques such as microstructure
analysis by Scanning Electron Microscopy or Transmission Electron Microscopy.
8
~~5~~'8
A ceramic article containing pollucite as its principal crystalline phase can
be prepared in a analogous way to that described for a leucite ceramic
article.
Thus a zeolite is exchanged using a cesium salt, e.g., cesium nitrate
following the
procedure outlined above for potassium exchange. The amount of cesium to be
exchanged should be at least 50% and preferably at least 90% of the ion
exchange capacity of the zeolite. The cesium exchanged zeolite is processed in
the same manner as the potassium exchanged zeolite powder described above
to produce a ceramic article with its principal crystalline phase being
pollucite.
In an analogous manner a zeolite can be exchanged with rubidium instead
to of potassium or cesium. Rubidium exchange is carried out in the same manner
as potassium or cesium exchange except that a rubidium chloride or rubidium
nitrate solution is used. Next, the rubidium exchanged zeolite is processed in
the
same way as described for the potassium exchanged zeolite to produce a
ceramic article having as its principal crystalline phase a rubidium leucite
phase.
i5 This invention also relates to a process for preparing a ceramic article
whose principal crystalline phase in a leucite/pollucite solid solution. By
varying
the amount of pollucite in the article, one can vary the coefficient of
thermal
expansion over a range from 2 x 10'6 to 27 x 10'6oC'1 in the temperature range
of 50 to 700oC. In preparing a ceramic article composed of a leucite/pollucite
2 o solid solution a zeolite, such as zeolite Y, is first exchanged to obtain
the
potassium form as described above and then exchanged with a cesium salt such
as cesium chloride, cesium hydroxide or cesium nitrate. When both potassium
and cesium are present in the zeolite, i.e. co-exchanged, the potassium
content
is greater than zero but less than 100% of the ion exchange capacity of the
zeolite
2 s and the cesium content is greater than zero but less than 100% of the ion
exchange capacity of the zeolite and the sum of the potassium and cesium
content is at least 50% and preferably at least 90% of the ion exchange
capacity
of the zeolite. As the amount of cesium in the zeolite increases, the
coefficient of
thermal expansion decreases. Therefore, by varying the concentration of potas-
3 o sium and cesium one obtains a process for controlling the thermal
expansion
coefficient of a leucite/pollucite solid solution containing ceramic article.
Once the zeolite containing both potassium and cesium is obtained, it is
processed as described above to obtain a ceramic article having as its
principal
phase a leucite/pollucite solid solution. Instead of using one zeolite that
has been
3 s exchanged with both potassium and cesium, one can use two zeolite powders
(either the same structure type or different structure type), one exchanged
with
only potassium and one exchanged with only cesium and blending the two zeolite
powders to achieve the desired ratio of potassium and cesium which leads to
the
desired ratio of leucite and pollucite. The amounts of potassium and cesium
pre-
sent are the same as in the co-exchanged case. Although both methods can be
s used, they do not necessarily give the same results. Thus, it is preferred
that one
zeolite powder that contains both potassium and cesium be used.
The leucite/pollucite ceramic article can be described in terms of the metal
oxides by the empirical formula
xK20:yCs20:zSi02:A1203
1 o where x varies from 0.01 to 0.99, y varies from 0.99 to 0.01 and z varies
from 3.5
to 7.5, except that when z is 7.5, y is greater than 0.19. The ceramic article
is
characterized in that it has a coefficient of thermal expansion of 2 x 10-6 to
27 x 10-6oC-~ over the range 50o to 700oC, has less than 5% porosity and is
extremely refractory, i.e., has a melting point greater than 1450oC. Finally,
the
15 principal crystalline phase of the ceramic article is a leucite/pollucite
solid
solution. The leucite/pollucite ceramic articles of this invention have
several uses
including dental porcelains, metal/ceramic seals where the coefficient of
thermal
expansion can be graded in the transition zone between the metal and ceramic.
EXAMPLE 1
2 o This example shows the preparation of potassium exchanged zeolite Y
from NaY zeolite. In a container 223.7 grams of KCI were dissolved in 3 liters
of
distilled water and the pH of the solution was adjusted to 8.5 by adding a
small
amount of KOH. To this solution there were added 150 g. of NaY zeolite, pre-
pared according to the procedure in U.S. Patent 3,130,007, whose chemical anal-
2 s ysis was: 19.52 wt.% AI203, 41.45 wt.% Si02, 12.82 wt.% Na20 and 26.21
wt.%
LOI. The chemical formula expressed as ratio of oxides on an anhydrous basis
was determined to be: 1.08 Na20 : 1.00 AI203 : 3.61 Si02. The resulting slurry
was heated to reflux while stirring for two hours.
The zeolite powder was isolated by filtration, after which the powder was
s o reexchanged three more times, each time with equal amounts of freshly
prepared
KCI solution (adjusted to pH 8.5 as above), followed by another filtration.
Finally
the powder was washed with 9 liters of distilled water. The resulting powder
was
~~~ ~~ ~~
dried at room temperature. Elemental analysis showed the presence of: 20.2
wt.% AI2O3, 41.0 wt.% Si02, 0.188 wt.% Na20, 17.0 wt.°~ K20 and 22.2
wt.%
LOI. The chemical formula expressed as the ratio of the oxides on an anhydrous
basis was determined to be: 0.02 Na20 : 0.91 K20 :1.0 AI2O3 : 3.4 Si02.
5 EXAMPLE 2
A 53.3 Ib. sample of LZ-Y62 (ammonium exchanged Y zeolite with nomi-
nally 2.7 wt.% residual Na20 and Si02/AI203 about 5) was slurried in a
solution
of 360 Ib of H20 and 40 Ib. of NH4CI. The mixture was refluxed for 1 hour,
then
filtered in a filter press, after which the powder was left in the filter
press for the
io remainder of the ion exchanges. A new solution of 40 Ib. of NH4CI in 360 Ib
of
H20 was prepared and heated to reflux in a kettle which was fitted with piping
to
the filter press. The hot solution was circulated through the filter press
containing
the zeolite powder for two hours, while recycling through the heated kettle in
order to keep the solution as close to reflux temperature as possible. Three
more
i5 exchanges were carried out by the above circulation procedure, each time
with
equal amounts of freshly prepared NH4CI solution. Finally, the zeolite powder,
while still in the filter press, was washed with about 75 gallons of H20. The
resulting wet powder was removed from the filter press and dried overnight at
100oC. Elemental analysis showed the presence of: 17.8 wt.% AI203, 51.7 wt.%
2o Si02, 8.7 wt.% (NH4)20, 0.31 wt.% Na20, and 29.7 wt.% LOI. The chemical
formula expressed as the ratio of the oxides on an anhydrous basis was
determined to be:
0.03 Na20 : 1.0 AI203 : 4.9 Si02 : 0.96 (NH4)20.
EXAMPLE 3
25 A 500 g. portion of ammonium exchanged zeolite Y prepared in example 2,
was exchanged as follows. In a container 1011.1 g. of KN03 was dissolved in 10
liters of H20, and the pH was adjusted to about 9 with a small amount of KOH.
The zeolite powder was slurried in the solution and then the mixture was
heated,
with stirring, to reflux for 2 hours. The zeolite powder was isolated by
filtration,
3 o after which the powder was reexchanged three more times, each time with
equal
amounts of freshly prepared KN03 solution (adjusted to pH 9 as above).
11
T
Finally, the powder was washed with 15 liters of distillE:d water and dried in
air at room temperature. Elemental analysis showed the folllowing composition:
16.4 wt.% AI2O3, 48.0 wt.% Si02, 14.5 wt.% K20, and 21.0 ~wt.% LOI, which can
be expressed as the following ratio of anhydrous oxides: 0.96 K20 : 1.0 AI203
5.0 Si02.
EXAMPLE 4A
This example shows the preparation of ceramic pellets using potassium
exchanged zeolite Y made as in Example 3. Two pellets were formed by placing
about 1 gram portions of potassium exchanged zeolite Y into 0.5 inch (1.27 cm)
1 o diameter steel dies and pressing at 10,000 psi (68950 kPa). 'The two
pellets were
heated at 6°C/minute to 1050°C and held at 1050°C for 4
hours. The densities
of the fired pellets, which were white and chalky and clearly not sintered,
were
1.55 and 1.55 g/cc. One of the pellets was ground into a fine powder and
analyzed by X-ray diffraction which indicated that the pellet was amorphous.
EXAMPLE 4B
This example shows the preparation of ceramic pellets using potassium
exchanged zeolite Y made as in Example 3. Two pellets were formed by placing
about 1 gram portions of the potassium exchanged zeolite Y into 0.5 inch (1.27
cm) diameter pellet dies and pressing at 10,000 psi (68950 kPa). The two
pellets
2 o were heated at 6°C/minute to 1150°C and held at
1150°C for 4 hours. The
densities of the sintered pellets, which were glassy and a light gray color,
were
2.31 and 2.32 g/cc. One of the pellets was ground into a fine powder. X-ray
diffraction analysis of the powder indicated that the ceramic was amorphous.
EXAMPLE 4C
Two more pellets were made as in Example 4A above using the same
potassium exchanged zeolite powder. The pellets were heated at
6°C/minute to
1150°C and held at 1150°C for 12 hours. The sintered densities
of the two pel-
lets, which were similar in appearance to the pellets in Example 4A above,
were
2.32 and 2.29 g/cc. X-ray analysis of one of the pellets after grinding
revealed the
3 o presence of tetragonal leucite {JCPDS File No. 15-47).
12
~5~'~'8
EXAMPLE 4D
A pellet was formed by placing approx. 25 grams of potassium exchanged
zeolite Y, made as in example 3, into a 2.25 inch (57 mm) cliameter steel die
and
pressing at 3000 psi (20685 kPa). The green pellet was 5'7.15 mm in diameter.
The pellet was heated at 10°C per minute to 1050°C, then at
4°C per minute to
1250°C, and held at 1250°C for 4 hours. The resultinc,~ sintered
pellet was
severely cracked. A measurement of the diameter from a small uncracked area
was 39.3 mm, indicating a 31% shrinkage in the pellet diameter.
EXAMPLE 4E
to A small rectangular pellet of a potassium exchanged zeolite Y prepared as
in Example 3 and measuring 0.26" (6.6 mm) in length was loaded into a
horizontal
recording dilatometer, with the longest dimension used as the measured axis of
shrinkage. The pellet was heated at 6°C per minute to 1 ~~00°C.
The sintered
pellet had a final length of 0.19" (4.8 mm), representing a 27% linear
shrinkage.
The pellet was ground into a fine powder and analyzE:d by X-ray powder
diffraction which showed the presence of tetragonal leucite, as in Example 4B
above.
Examples 4A to 4E show that preparing ceramic articles in one step gives
2 o very unsatisfactory results. Leucite begins to form only after heating at
1150°C
for 12 hours. Additionally, the green articles (pellets) shrink considerably
upon
sintering (at least 27% shrinkage).
EXAMPLE 5A
2 s About 5 grams of potassium exchanged zeolite Y made as in Example 3,
was heated as a loose powder to 1050°C for one hour. Six pellets were
made by
pressing the precalcined powder in a 0.5 inch (12.7 mm) stE:el dies at 10,000
psi
(68950 kPa). The heating rate used for the following experiments was
4°C per
minute. Three pairs of pellets were heated for 4 hours at 1150°C,
1250°c, and
3 0 1350°C respectively. The average densities of the sintered pellets
for the three
processing temperatures were 2.31, 2.35, and 2.39 g/cc respectively. One
pellet
13
from each pair was ground into a fine powder and analyzed by x-ray
diffraction.
The x- ray patterns of the three powders revealed the following crystalline
phases,
as referenced to the respective sintering temperatures: '1150°C -
tetragonal
leucite; 1250oC - tetragonal leucite; 1350oC - tetragonal leucite. The leucite
glass
s ceramic processed at 1250oC showed the highest degree of crystallinity
EXAMPLE 5B
Approximately 100 g. of potassium exchanged zeolite Y prepared as in
example 3, was heated as a loose powder at lOoC per minute to 1050oC and
held at 1050oC for 1 hour. About 45 grams of the calcined powder was loaded
Zo into a circular steel, 57.15 mm, die and pressed into a pellE~t at 3000 psi
(20685
kPa). Similarly about 9 grams of the powder were loaded into a steel 82.55 by
9.5
mrn die and pressed at 4,000 psi (27580 kPa). The pellets were then heated in
a
furnace at lOoC per minute to 1050oC then at 4oC per minute to 1250oC, then
held at 1250°C for 4 hours. This heating schedule was identical to the
one used
15 in example 4D above. The resulting parts showed minimal warping and were
crack- free. The circular pellet had a diameter of 44.95 mm <~nd a density of
2.37
g/cc, while the rectangular bar had a length of 66 mm and a density of 2.26
g/cc.
The linear shrinkages, resulting during sintering, for these ceramic parts
derived from precalcined powders were 20-21%, which are significantly less
than
2 o parts made from uncalcined powder, which typically show 27-33% shrinkage.
The
degree of shrinkage in parts made from uncalcined powders essentially
precludes
the consistent production of crack- free, unwarped ceramics, while the use of
precalcined powders allows for facile production of strong defect-free parts.
EXAMPLE 5C
25 The rectangular bar made in Example 5B above was c:ut to a length of 2.0
in (50.8 mm) using a diamond grit cutoff wheel. The shori:er piece which was
obtained was ground into a fine powder and submitted for x-ray analysis. The x-
ray revealed the presence of tetragonal leucite as the only crystalline phase.
The 2.0 in piece was loaded into a recording dilatorneter. The bar was
s o heated at approximately 4°C per minute to 800°C. The
calculated average coef
ficient of thermal expansion over the 50°-700°C range, corrected
with a standard
AI203 reference, was 26.7 x 10-6oC-1. The tetragonal to cubic (low to high
14
5~~~
leucite) transformation was centered at about 410°C in the dilatometer
trace.
EXAMPLE 6A
A small rectangular pellet of potassium exchanged zeolite Y, made in
Example 1, was prepared by pressing the powder in a rectangular die at 5000
psi
(34475 kPa). The long dimension of the pellet was 0.272 in (6.9 mm). The
pellet
was loaded into a recording dilatometer with its long dimension parallel to
the
measuring axis, then heated at 6°C per minute to 1350°C. The
sintered pellet
had a final length of 0.181 in (4.6 mm), indicating a linear shrinkage of 33%.
The
pellet was ground into a fine powder and analyzed by x-ray diffraction, which
to confirmed that the only crystalline component was tetragonal leucite.
EXAMPLE 6B
About 15 grams of potassium exchanged zeolite Y, made in example 1,
was heated as a loose powder to 1000°C for 1 hour. An 82.55 mm
rectangular
bar was made from the calcined powder in a steel die, as in example 5A. The
bar
was heated at 10°C per minute to 1000°C, then 4°C per
minute to 1250°C, then
held at 1250°C for 4 hours. The resulting bar was cut with a diamond
cutoff
wheel to a length of 2.0 in (50.8 mm). The measured density of the bar was
2.36
g/cc. The short piece which was cut off was ground into a fine powder and
analyzed by x-ray diffraction. The x-ray pattern revealed the presence of
tetra
2 o gonal leucite.
The bar was loaded into an automatic recording dilatometer and was
heated at about 4°C per minute to 900°C.
The calculated average coefficient of thermal expansion over the 50-
700°C
range, corrected with a standard AI203 reference, was 28.0 x 10-6°C-1.
The
tetragonal to cubic {low to high leucite) transformation was not well defined
in this
ceramic composition, but was indicated by subtle slope changes in the dilato-
meter trace between 500 and 650°C.
EXAMPLE 7
This example shows the preparation of a cesium and potassium
3 o exchanged zeolite. A 100 gram portion of a potassium exchanged zeolite Y,
pre-
pared as in Example 3, was exchanged with cesium as follows. In a container
15
331.35 g. of cesium nitrate was dissolved in 1.7 liters of water, then the pH
was
adjusted to 8 with a small amount of CsC03. The zeolite powder was slurried in
the solution and the mixture was heated with stirring to reflux for two hours.
The
powder was isolated by filtration, after which the powder was reexchanged two
more times as above, each time with equal amounts of freshly prepared pH
adjusted CsN03 solutions. The final powder was isolated by filtration, washed
with 15 liters of deionized water, and dried in air at room temperature.
Elemental
analysis revealed the presence of: 14.1 wt.% AI203, 41.4 wt.% Si02, 3.01 wt.%
K20, 27.2 wt.% Cs20, and 15.8 wt.% LOI, which can be expressed in anhydrous
io oxide ratios as 0.70 Cs20 : 0.23 K20 : 1.0 AI203 : 4.95 Si02.
EXAMPLE 8A
About 5 grams of the cesium and potassium exchanged zeolite Y, made in
Example 7 was heated as a loose powder to 1050oC for one hour. Six pellets
were made by pressing the precalcined powder in 0.5 in (12.7 mm) steel dies at
i5 10,000 psi (68950 kPa). The heating rate used for the following experiments
was
4°C per minute. Three pairs of pellets were heated for 4 hours at
1150°C,
1250oc, and 1350oC respectively. The average densities of the sintered pellets
for the three processing temperatures were 2.71, 2.76, and 2.77 g/cc
respectively. One pellet from each pair was ground into a fine powder and
2 o analyzed by x-ray diffraction. The x- ray patterns of the three powders
revealed
the following crystalline phases, as referenced to the respective sintering
temperatures: 1150oC - amorphous, 1250oC - pollucite (cubic leucite), 1350oC -
pollucite (cubic leucite).
EXAMPLE 8B
2 5 About 15 grams of cesium, potassium exchanged zeolite Y, made in
Example 7, was heated as a loose powder to 1050°C for 1 hour. The
powder
was passed through a standard 60 mesh screen (aperture of 0.21 mm) to remove
large agglomerates, then a 82.55 x 9.5 mm rectangular bar was made from the
calcined, meshed powder in a steel die. The bar was heated at 10°C per
minute
3 o to 1050°C, then 4°C per minute to 1250°C, then held
at 1250°C for 4 hours. The
resulting bar, which was crack free, was cut with a diamond cutoff wheel to a
length of 2.0 in (50.8 mm). The measured density of the bar was 2.73 g/cc. The
16
short piece which was cut off was ground into a fine powder and analyzed by x-
ray diffraction. The x-ray pattern revealed the presence of cubic leucite.
The bar was loaded into an automatic recording dilatometer and was
heated at about 4°C per minute to 875°C.
The calculated average coefficient of thermal expansion over the 50-
700°C
range, corrected with a standard AI203 reference, was 4.47 x 10-6 /°C-
1. No
structural transition was apparent in the dilatometer trace.
EXAMPLE 9
A 50 gram portion of potassium exchanged zeolite Y, prepared as in
to example 3 was exchanged as follows. In a container 7.14 g. of cesium
chloride
was dissolved in 212.5 ml. of water, then the pH was adjusted to 7.5 with a
small
amount of CsCOg. The zeolite powder was slurried in the solution and the mix-
ture was heated with stirring to reflux for two hours. The powder was isolated
by
filtration and washed chloride free with deionized water and dried in air at
room
temperature. Elemental analysis revealed the presence of: 15.18 wt.% AI203,
10.9
wt.% K20, and 8.9 wt.% Cs20, indication that the cation ratio within the
exchanged zeolite was 78% K and 22% Cs.
EXAMPLE 10
About 10 grams of the potassium and cesium exchanged zeolite Y, pre-
2 o pared in example 9, were heated as a loose powder at 10°C per
minute to
1050°C for 1 hour. A 82.55 x 9.5 mm rectangular bar was made from the
cal-
cined powder in a steel die. The bar was heated at 10°C per minute to
1050°C,
then 4°C per minute to 1250°C, then held at 1250°C for 4
hours. The resulting
bar, which was crack-free, was cut with a diamond cutoff wheel to a length of
2.0
2 s in (50.8 mm). The measured density of the bar was 2.49 g/cc. The short
piece
which was cut off was ground into a fine powder and analyzed by x-ray
diffraction.
The x-ray pattern revealed the presence of high tetragonal leucite.
The bar was loaded into an automatic recording dilatometer and was
heated at about 4°C per minute to 775°C.
3 o The calculated average coefficient of thermal expansion over the 50-
700°C
range, corrected with a standard AI203 reference, was 14.1 x 10-6 /°C-
1. No
structural transition was apparent in the dilatometer trace.
17
EXAMPLE 11
A potassium zeolite L identified as product number 3069 and whose
analysis in anhydrous oxide ratios was: 1.1 K20 : 1.0 AI203 : 6.4 Si02. About
5
grams of this sample was heated as a loose powder to 1050°C for one
hour. Six
s pellets were made by pressing the precalcined powder in a 0.5 (12.7 mm) inch
steel dies at 10,000 psi (68950 kPa). The heating rate used for the following
experiments was 4°C per minute. Three pairs of pellets were heated for
4 hours
at 1150oC, 1250oC, and 1350oC respectively. The densities of the sintered
pellets were difficult to measure due to significant viscous flow during the
so sintering. One pellet from each pair was ground into a fine powder and
analyzed
by x-ray diffraction. The x-ray patterns of the three powders revealed the
following crystalline phases, as referenced to the respective sintering
temperatures: 1150oC - tetragonal leucite, 1250oC - tetragonal leucite, 1350oC
-
tetragonal leucite.
