Note: Descriptions are shown in the official language in which they were submitted.
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1 PROCESS FOR FORMING MULLITE
Description
Technical Field
This invention relates to a method of forming mullite, more par-
ticularly to a method of forming a ceramic material which includes high
purity mullite in any desired composition.
An object of this invention is to provide a method of forming high
purity dense mullite from A1203 and glass.
Another object of this invention is to provide a method of forming
10 a dense sintered ceramic material which includes high purity mullite as
a component thereof.
Yet another object of this invention is to provide a method for
forming a dense sintered ceramic material wherein A1203 and SiO2 are
reacted to form high purity mullite in any desired composition of the
, material.
~ Another object of this invention is to provide a method for forming
< ceramic material which includes mullite by reacting A1203 and SiO2
~ during a sintering step wherein the reaction is initiated by a rela-
`~ tively small amount of mullite included in the original particulate
2a material.
Another object of this invention is to provide a method of forming
a ceramic composition where the mullite portion can be varied from
approximately 10% to 100%.
The invention provides a method of making a dense ceramic material
which includes high purity mullite in any desired percentage. A par-
ticular mixture of at least 3% mullite, an amount of A1203 which is
between the stoichiometric amount ot the excess necessary to form the
desired mullite composition percentage less the initially added mullite
and glass in an amount to provide suFficient SiO2 to combine with the
30 A12a3 to forrn the desired mullite composition percentage is formed. The
mixture is tllen sintered at a temperature in the range of 1300C to
1800C to form mullite under the influence of the initially added mullite.
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1 Background Art
Mullite (3A1203.2SiO2) has long been known in the ceramic and
refractory industries. Mullite is one of the most stable compounds
in the A1203-SiO2 system. Consequently it occurs as a main consti-
tuent in a large number of ceramic products which are fabricated
from aluminosilicate materials. Considerable amounts of mullite are
used to produce refractory bodies designed to withstand high temp-
eratures. Its relatively low thermal coefficient of expansion makes
such refractories more resistant to thermal stresses in contrast to
similar bodies prepared from aluminum oxide materials.
Mullite possesses a dielectric constant of approximately 5 to 6
and therefore presents a very attractive electrical characteristic
as integrated circuit technology continues advancing to higher speed
circuit devices. Moreover, mullite's low thermal coefficient of ex-
pansion offers an excellent match to large silicon integrated circuit
chîps or glasses which may be placed on substrates. Although mullite
has been mentioned for use as an electronic substrate for integrated
circuit devices, high grade dense substrates are not known to exist.
Commercially available mullite always contains significant amounts
of impurities such as silica, iron oxide, and titania. These impurities
influence the physical, electrical, and chemical properties of the
mullite, which in turn affect the ceramic compositions in which mul-
lite may be embodied. Mullite can be produced, as described in U.S.
Patent 3,857,9?3; U.S. Patent 3,922,333; U.S. Patent 3,615,778; and
U.S. Patent 3,336,108. In order to combine mullite into a ceramic
mixture it must be reduced to a fine particle size. Since mullite is
a very hard material this process is quite expensive, and more im-
portantly, additional impurities are introduced into the mullite by
the grinding process.
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1 Attempts to manufacture high purity mullite from aluminum
oxide and silica are very expensive and time consuming because
the alumina and silica must diffuse together through the mullite
which forms at the interfaee between the two. This diffusion is
so slow that repeated heat treatment with grinding between each
heat treatment may be required. The temperatures must be at least
1500C and usually higher. Therefore a recognized need for a
practical commercial process for producing high purity mullite has
not been met by the prior art techniques. It is particularly de-
sirable that the mullite can be fornmed in various concentrations in
ceramic compositions whereby the physical, electrical, and chemical
properties of the ceramic composition can be tailored.
Disclosure of the Invention
In this process a particulate mixture is formed which includes
alpha A1203 and SiO2 added as an alumino-silicate glass. Also in-
cluded is a relatively minor amount, at least 3%, of mullite. When
the mixture is sintered the initial mullite acts as a "seed" or a
nucleating agent, which promotes the combining of A1203 and SiO2 to
form mullite. Any desired amount of mullite can be formed, up to
substantially 100%. In order to form 100% mullite ceramic material
the stoichiometric amounts of A1203 and SiO2 (added as a glass) are
combined with the initial mullite seed. During the sintering the
components are formed into mullite. If less than 100% mullite is
desired, the ratio of total alumina content to silica content must
be in excess of the stoichiometric ratio. After reaction, this will
leave the desired mullite plus alumina phase structure. The glass
phase will have been depleted. The glass acts as a flux during
sintering which hastens the mullite formation reaction. Other cera-
mic materials can be combined in the particular mixture to provide
desirable physical, electrical, or chemical properties in the sintered
ceramic material.
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1 The particulate mixture is preferably combined with an organic
binder, solvent, and plasticizer, to form a slurry which can be
doctor bladed and dried to form ceramic green sheet, as described in
U.S. Patent 2,966,119. The green sheet is then punched, and metal-
lurgy is screened on, a plurality of sheets is laminated, and the
resultant unit sintered to form a multi-layer ceramic substrate.
The desirability of combining various amounts oF mullite into
ceramic materials used as semiconductor package substrate is apparent
when one considers the coefficient of expansion of mullite, silicon,
and other ceramic materials. The coefficient of expansion of pure
mullite from room temperature to 300C is 40 x 10 7~oc. The coef-
ficient of expansion of A1203 is 75 x 10-7/C, and typical high
alumina ceramics is on the order of 70 x 10 7~oc. The coefficient
of expansion of molybdenum, used as a refractory conductive metal-
lurgy material on MLC substrates, is 55 x 10 7Oc which is between
high alumina ceramic materials and mullite. A match of coefficients
of expansion of molybdenum and a ceramic substrate material which
includes the proper amount of mullite can thus be achieved, which
would reduce cracking, distortion and internal stress of the sub-
strate which uses molybdenum metallurgy. Also, a pure mullite sub-
strate would provide a relatively good coefficient of expansion match
to silicon which has a coefficient of expansion of 32 x 10 7~oc.
This substrate could be important when a silicon device is solder bond-
ed to the substrate.
The dielectric constant of mullite is 5-6 as compared to high
alumina cerarnic with a dielectric constant of on the order of 9 to
9 1l2. The use of mullite in a ceramic would reduce the dielectric
constant which is important in reducing inductance in high speed
switching aplplications.
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1 With this process it is possible to achieve a dense ceramic
with a very low or essentially 0 percent glassy phase. Chemical
attack of a ceramic material occurs normally in the glassy phase.
Thus, the process could be used to produce a ceramic body with
improved chemical inertness.
In the forming of the particulate material, the A1203 is pre-
ferably alpha alumina of 99+ percent purity which is commercially
available. The mullite used as seed can be any mullite material
including commercially available mullite of 90+ percent purity.
The impurities present in the seed mullite when dispersed through
the body would not significantly effect the electrical, chemical,
and physical properties of the resultant ceramic material. Any
suitable type of glass having a high percentage of SiO2 can be used
in the practice of the invention. As a practical application the
mullite seed is added to an existing glass which includes both SiO2
and A1203. A typical glass useful in practicing the invention con-
tains 52% SiO2, 31% A1203, lOYo MgO and 7% CaO by weight. When mul-
lite is added to this glass and the mixture sintered, the alumina and
silica in the glass react at a ratio of 72 to 28 to form additional
mullite.
The types of glasses that are suitable for use in this process
are the aluminosilicate glasses with the glass viscosity at the firing
temperature and the desired amount of silica being the governing fac-
tors. Too fluid a glass will deform the body while too high a vis-
cosity will make the reaction proceed too slowly. The glass composi-
tion should have between 80 and 40% silica content. The particle size
of the particulate mixture of the invention is preferably on the order
of -325 mesh. However, the particle size can be varied if desired.
The ultimate fineness of the particles possible is governed by the
difficulty o1 manufacturing the desired shapes. The maximum coarse-
ness of the powder used is governed by the length of time that it is
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1 required to complete the reaction. The more coarse the powder the
greater the time the body must be sintered. The method of the in-
; vention is particularly applicable to multilayer ceramic technology
where the particulate mixture is combined with a binder, a solvent,
and preferably a plasticizer, the resultant slurry doctor bladed
and dried to form green ceramic sheet. However, other fabrication
methods can be used to form a desired shape such as dry pressing,
extruding, ramming, jiggering, and casting. The sintering atmosphere
used in the practice of the method of the invention can include
either wet hydrogen, air, forming gas, nitrogen, argon, or any othersuitable atmosphere. In general, the nature of the atmosphere does
not effect the mullite formation. However, various types of atmos-
pheres may be desirable in order to avoid adverse effects on the metal-
lurgy associated with the ceramic substrate. The sintering tempera-
ture used in the method of the invention is generally in the range
of 1200C to 1800C, preferably in the range of 1350C to 1600C and
more preferably in the range of 1450C to 1550C. The required
sintering time will vary depending on the nature of the particulate
mixture and the coarseness of the particle. In general, the sinter-
ing time will range from 1/2 to 24 hours, more typically in therange of 1 to 6 hours. The particle size of the components of the
particulate mixture can be any suitable size, preferably in the
range from 100 nanometers to 8 mesh and, more preferably in the order
of -325 mesh. The mullite component in the particulate mixture is
present in an amount to provide sufficient nucleation. The preferred
amount is from 3 to 90% by weight, more preferably 5 to 30%. The
amount of SiO2 in the mixture can be any amount up to the stoichio-
metric amount necessary to result in a final mixture of up to 100%
mullite. ~lowever, any lesser amount can be used.
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1 The following examples are included to illustrate typical
illustrations of the practice of the method of the invention and
should not be construed to limit the scope of the invention.
Example 1
A mixture of 20 parts by weight of mullite, 11 parts by weight
of glass, and 69 parts by weight of alumina were placed in a ball
mill. An organic mixture consisting of 21 parts by weight of
methanol, 63 parts by weight of methylisobutylketone, 3 parts by
weight of dipropylene glycol dibenzoate, and 12 parts by weight
of polyvinylbutyral were added such that the weight ratio of ceramics
to organics was 1.82. The ceramic was milled so that all particles
; passed through a 325 mesh screen. The slurry was de-aired and cast
under a doctor blade onto a moving organic film to form a ceramic
green sheet. The sheet was then dried and sized. Ten green sheets
were laminated together in a cavity die at a temperature of 95C
and a pressure of 3000 psi.
The samples were then placed in a batch kiln and heated to 1560C
in a wet hydrogen atmosphere. The samples were held at temperature
for three hours. The fired samples were dense with no visible poro-
sity.
After cooling, the samples were ground into a minus 200 meshpowder. This powder was then analyzed using an x-ray powder dif-
fractometer with Cu Ka radiation. The results obtained were compared
to the ASTM x-ray data. The x-ray trace showed a complete absence
of the very broad peak indicative of a silicate glass phase. The only
crystal phases detected were alumina and mullite.
This example proved that when the alumina to glass ratio in -the
original ra~ material mixture was such that there was an excess of
alumina, the in situ mullite synthesis raction occurred until the
SiQ2 Was esslentially depleted. The amount of alumina in excess of
the stoichiometric ratio did not react and remained as crystalline
alpha alumina.
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1 Example 2
A mixture of 74.5 parts mullite, 14.5 parts alumina, and 11
parts glass powder was processed as described in Example 1. After
sintering for 3 hours at 1560C a portion of the sample was then
ground into minus 200 mesh powder, ~and the powder was analyzed using
an x-ray diffractometer, as per the procedure in Example 1.
The results of this investigation showed that no crystalline
alpha alumina phase remained. The alumina to glass ratio in this
composition was tailored to give the stoichiometric ratio that would
react fully to form mullite. The results do show that the reaction
occurred as predicted, with no crystalline alumina remaining.
Example 3
An additional sample was prepared as described in Example 1,
except that the ceramic raw material composition was 30 parts by
weight mullite, 11 parts by weight of glass powder, and 59 parts by
weight alumina. Following the casting, drying, laminating, and sin-
tering steps described in Example 1, a portion of the sample was
mounted in molding material, and polished using standard metalla-
graphjc techniques. The sample was then analyzed using an optical
microscope, on which was mounted a recording micrometer stage. The
stage was used to quantitiatively measure by line intercept method
the amount of the phases present. Extensive analysis showed that the
above composition had a ratio of phase contents of 55.9% by volume,
mullite, and 44.1% alpha alumina. Only residual (less than 2% by
volume) glass phase was observed in this composition. Phase equili-
brium diagrams for the A1203-SiO2 system predict that the expected
phases are mullite and alumina, in a ratio of 55 to 45, by volume.
Therefore, this example proves that by adding mullite to a composi-
tion containing alumina and an aluminosilicate glass, the mullite
nucleation step is bypassed and the growth of the mullite phase occurs
by the reaction between the alumina and the glass phase to produce
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1 additional mullite. The amount of porosity and pullout observed
was measured and found to be discontinuous and less than 7.6% by
volume. Hence, the technique allowed for the production of dense
; mullite samples.
Example 4
Mullite formation as a function of the starting amounts of mul-
lite was measured by forming as in Example 1, bodies having various
initial compositions and firing them for 3 hours at 1560C. The
final percentages of mullite were calculated from x-ray diffraction
patterns.
Starting Compositions Fired Mullite
by weight Percentage
by weight
Mullite % Glass % Alpha
11 84 7
11 79 13
1l 74 22
11 69 27
11 64 35
11 59 45
The results showed that the amount of mullite formed varied
from 30% to 50% as compared to the starting amounts and that the
ease of mullite formation increased with increased mullite addi-
tions which provided additional nucleation sites. The glass-alumina
reaction to mullite did not go to completion in cases where less
than 3Q% mullite and 11% glass was present due to heterogeneity in
the ceramic body limiting the reaction.
Example 5
The following raw material compositions were prepared as de-
scribed in Example 1:
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l A. 3 parts mullite, 2l.8 parts glass powder, -/5.2 parts alumina,
all by weight;
B. 5 parts mullite, 20.7 parts glass powder 74.3 parts alumina,
all by weight.
Samples A and B, were sintered in air for 3 hours at l3l9C
and l328C respectively.
Portions of the samples were mounted on carbon-coated electron
microscope grids. Using the defocussed electron diffraction pattern
technique, the amount of glass phase could be measured. For both
compositions, 200 particles were examined to determine if the parti-
cles ~ere crystalline (either mullite or alumina) or amorphous, i.e.,
glass particles. The results are as follows:
Sample Glass Particles per 200 Particles
A
B O
With no glass depletion a count of approximately 40 should have
been observed.
The compositions of samples A and B should have yielded excess
alumina. The glass phase should be depleted. As seen above in the
results, this was indeed the case. The reaction to form mullite went
to completion at l320C.
Example 6
An alpha alumina-glass body having a composition of 89 parts by
weight alpha alumina and ll parts by weight glass was manufactured
as in Example 1 and sintered at l560C for 3 hours. The sample was
ground and passed through a 200 mesh screen. An x-ray diffraction
pattern of the powder was made using copper Ka radiation. Only alpha
alumina was visible in the x-ray pattern with a complete absence of
mullite.
The glass phace was also looked for using the transmission electron
microscopy technique as described in Example 5. Eleven particles of
glass were found indicating the presence of the glass phase.
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