Mullite-cordierite composite ceramic and method for preparation

Method for producing a polycrystalline body comprised of from about 50% by weight to about 95% by weight of mullite and from about 5% by weight to about 50% by weight of cordierite, said body having a porosity of less than about 10% by volume.

The present invention relates to the production of a polycrystalline body, 
i.e. a composite ceramic, comprised of a mullite phase and a cordierite 
phase. In a preferred embodiment, the present invention relates to the 
production of a polycrystalline material comprised of mullite and 
cordierite useful as a substrate for silicon with a thermal expansion 
coefficient identical or close to that of silicon. 
To minimize stress at the silicon-substrate interface of an IC device 
during operation, it is desired that the substrate material have a thermal 
expansion coefficient which matches that of silicon as closely as 
possible. Such a match is expected to be especially critical in projected 
high power semiconductor devices where heating and cooling cycles during 
operation of the device are expected to subject the silicon-substrate 
interface to more severe thermal stresses than are encountered in present 
devices. Present technology utilizes alpha alumina as the substrate 
material. However, the thermal expansion mismatch between alumina and 
silicon is judged to be too great for this material to be satisfactory for 
future applications. In addition, high speed integrated circuits require a 
substrate material with a low dielectric constant. For projected high 
speed devices, alumina is judged to have too high of a dielectric 
constant. A replacement substrate material for alumina is then needed 
whose thermal expansion coefficient provides a closer match to that of 
silicon and whose dielectric constant is lower than that of alumina. 
Mullite composite substrate materials have been investigated in the past. 
Leipold and Sibold, J. Amer. Ceram. Soc., 65, C147 (1982), report making a 
two phase mullite based ceramic with a thermal expansion coefficient close 
to that of silicon by preparing a silica-rich mullite composition. After 
firing, this resulted in a body consisting of mullite and a silica-rich 
glass which contained approximately 50% glass. Fiori and Vincenzini, 
Collogque Internationale sur les Nouvelles Orientations des Compososants 
Possifs, p. 203, Paris, Mar. 29-Apr. 1, 1982, also report making similar 
mullite-silica glass compositions for use as a substrate material with a 
thermal expansion to match that of silicon. 
Mullite is a crystalline aluminum silicate phase which has the approximate 
chemical formula, 3 Al.sub.2 O.sub.3.2SiO.sub.2. It is a potential 
substrate material as its thermal expansion coefficient is a reasonable 
match for that of silicon (3.3.times.10.sup.-6 /.degree.K. at 300.degree. 
K. for mullite vs. 2.5.times.10.sup.-6 /.degree.K. at 300.degree. K. for 
silicon). However, as discussed above, a closer match than this is desired 
for substrates for projected high power devices. Therefore, according to 
one aspect of the present invention, the thermal expansion coefficient of 
mullite is lowered to match that of silicon through the addition of a 
second phase having a low thermal expansion coefficient. Specifically, in 
accordance with the copresent invention, cordierite (a magnesium aluminum 
silicate) is added. Cordierite has a thermal expansion coefficient of 
0.5.times.10.sup.-6 /.degree.K. at 300.degree. K.

In accordance with the present invention, a mullite-cordierite ceramic can 
be fabricated to match the thermal expansion coefficient of silicon. An 
advantage of the present mullite-cordierite ceramic over the mullite-glass 
ceramics is that the present ceramic has a potentially higher thermal 
conductivity because of the higher thermal conductivity of the crystalline 
ceramics over the glass. 
Briefly stated, the present process for producing a polycrystalline body 
comprised of from about 50% by weight to about 95% by weight mullite and 
from about 5% by weight to about 50% by weight cordierite comprises 
providing a mixture comprised of Al.sub.2 O.sub.3, MgO and SiO.sub.2 and a 
nucleating agent, said Al.sub.2 O.sub.3, MgO and SiO.sub.2 being used in 
amounts required to produce said polycrystalline body, shaping said 
mixture into a compact, sintering said compact at a temperature ranging 
from about 1290.degree. C. to about 1550.degree. C. at which sufficient 
liquid phase is generated to liquid phase sinter the compact and produce a 
densified body having a porosity of less than about 10% by volume of said 
body, said sintering temperature having no significant deleterious effect 
on said body, said sintered body containing said mullite phase and a 
glassy phase of said cordierite, nucleation-annealing said sintered body 
at a temperature ranging from about 600.degree. C. to about 800.degree. C. 
to nucleate said glassy cordierite phase, crystallization-annealing the 
resulting nucleated body at a temperature ranging from about 1200.degree. 
C. to below the temperature at which liquid forms in said body to produce 
said polycrystalline body, said nucleating agent being an agent for 
nucleating said glassy cordierite phase, said nucleating agent being used 
in an amount sufficient to nucleate said glassy cordierite phase to 
produce said polycrystalline body, said sintering, said 
nucleation-annealing and said crystallization-annealing being carried out 
in an atmosphere or vacuum which has no significant deleterious effect on 
said compact or body. 
In the present invention, the composition of the mullite phase is one which 
can exist in thermal equilibrium with a cordierite phase. Likewise, the 
composition of the cordierite phase is one which can exist in thermal 
equilibrium with the mullite phase. The present polycrystalline body, 
therefore, is comprised of a mullite phase composed of from about 71.8 
weight % to about 73.3 weight % Al.sub.2 O.sub.3 balance SiO.sub.2, and a 
cordierite phase comprised of a magnesium aluminum silicate with the 
formula of or about 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, i.e. about 13.7 
weight % MgO, about 34.9 weight % Al.sub.2 O.sub.3 and about 51.4 weight % 
SiO.sub.2. 
In the present invention, the phase composition of the present 
polycrystalline body depends largely on its final application. In one 
embodiment of the present invention, the phase composition of the present 
polycrystalline body is tailored to match the thermal expansion 
coefficient of silicon at 300.degree. K. 
In carrying out the present process, a uniform or at least a substantially 
uniform particulate mixture or dispersion of the ingredients or 
components, i.e. Al.sub.2 O.sub.3, MgO, SiO.sub.2 and nucleating agent, is 
formed. The particular amount of Al.sub.2 O.sub.3, MgO and SiO.sub.2 used 
in forming the mixture is determined by the particular phase composition 
desired in the polycrystalline body. In the present process, there is no 
significant loss of the reactants, i.e. Al.sub.2 O.sub.3, MgO and 
SiO.sub.2, forming the mullite and cordierite phases. 
The present nucleating agent is one which nucleates the glassy cordierite 
phase in the present process and has no significant deleterious effect on 
the body. Representative of such a nucleating agent is titanium dioxide, 
titanium isopropoxide, lithium fluoride, magnesium fluoride and mixtures 
thereof. Titanium isopropoxide thermally decomposes below the present 
sintering temperature to titanium dioxide and by-product gas which 
vaporizes away. 
The particular amount of nucleating agent used is determinable empirically 
and depends largely on the amount of glassy cordierite phase present. The 
nucleating agent must be present in at least an amount which nucleates the 
glassy cordierite phase sufficiently to allow its crystallization to 
produce the present polycrystalline body. Generally, in the present 
process, the nucleating agent is used in an amount ranging from about 5% 
by weight to about 10% by weight, preferably from about 6% by weight to 
about 8% by weight, and more preferably about 7.5% by weight, of the total 
weight of the glassy cordierite phase in the body. An amount of nucleating 
agent less than about 5% by weight may not be enough to be operable 
whereas an amount in excess of about 10% by weight may effect the 
properties of the final product, i.e. particularly its thermal and 
electrical properties. Specifically, the nucleating agent should not be 
used in an amount which has a significantly deleterious effect on the 
present polycrystalline body. 
The components of the mixture can be of commercial or technical grade. 
Specifically, they should not contain any impurities which would have a 
significantly deleterious effect on the properties of the resulting 
polycrystalline body. The larger the amount of impurities in the starting 
materials, the greater is the amount of glassy phase in the final product. 
The present mixture of components or ingredients can be formed by a number 
of conventional techniques such as, for example, ball milling or jet 
milling. Representative of the mixing techniques is ball milling, 
preferably with balls of a material such as .alpha.-Al.sub.2 O.sub.3 which 
has low wear and which has no significant detrimental effect on the 
properties desired in the final product. Preferably, milling is carried 
out in a liquid mixing medium which is inert to the ingredients. Typical 
liquid mixing mediums include hydrocarbons such as benzene and chlorinated 
hydrocarbons. The resulting wet milled material can be dried by a number 
of conventional techniques to remove the liquid medium. Preferably, it is 
dried in an oven maintained just below the boiling point of the liquid 
mixing medium. 
The particulate components of the present mixture should be of a size which 
allows the present reaction, i.e. formation of mullite and cordierite 
phases, to take place. Preferably, the present starting components or 
mixture thereof have an average particle size which is submicron. 
A number of techniques can be used to shape the particulate mixture into a 
compact. For example, the mixture can be extruded, injection molded, 
die-pressed, isostatically pressed, slip cast or tape cast to product the 
compact of desired shape. Any lubricants, binders or similar material used 
in shaping the mixture should have no significant deteriorating effect on 
the compact or the resulting polycrystalline body. Such materials are of 
the type which evaporate away on heating at temperatures below the present 
sintering temperature, and preferably below 200.degree. C., leaving no 
significant residue or any effective contaminants. 
The shaped compact can be in any desired form. For example, it can be 
simple, hollow and/or complex in shape. Preferably, for use as a 
substrate, it is in the form of a tape. 
The compact is sintered at a temperature at which sufficient liquid phase 
is generated to carry out the present liquid phase sintering and such 
sintering temperature can range from about 1290.degree. C. to about 
1550.degree. C. The sintering temperature should have no significantly 
deleterious effect on the compact or body, i.e., the sintering temperature 
should not generate so much liquid phase as to slump the compact or body. 
Specifically, the particular sintering temperature used depends largely on 
the amount of liquid phase generated at such temperature for the 
particular composition of the compact. For the present composition, where 
the nucleating agent has no effect on sintering temperature, the sintering 
temperature is one at which the glassy cordierite is molten, and in such 
instance it ranges from higher than about 1465.degree. C. to about 
1550.degree. C. Temperatures higher than about 1550.degree. C. are not 
operable because they generate so much liquid phase as to slump the 
compact or body. With increasing temperatures, increasing amounts of 
mullite dissolve in the molten cordierite generating more liquid at the 
higher sintering temperatures. 
Generally, however, the nucleating agent lowers the present sintering 
temperature. For example, when titanium dioxide or titanium isopropoxide 
are used as nucleating agents in the present invention, the present 
sintering temperature ranges from about 1450.degree. C. to about 
1490.degree. C., and temperatures higher than about 1490.degree. C. 
generate so much liquid as to slump the compact or body. On the other 
hand, when lithium fluoride is the nucleating agent, the present sintering 
temperature ranges from about 1290.degree. C. to about 1340.degree. C., 
and temperatures higher than about 1340.degree. C. generate so much liquid 
as to slump the compact or body. When magnesium fluoride is the nucleating 
agent, the present sintering temperature ranges from about 1390.degree. C. 
to about 1490.degree. C., and temperatures higher than about 1490.degree. 
C. generate so much liquid as to slump the compact or body. Mixtures of 
nucleating agents can be used to attain a desired sintering temperature. 
During sintering, the liquid formed is comprised primarily of molten 
cordierite with a minor amount of dissolved mullite and nucleating agent. 
The present sintering is carried out to produce a sintered body having a 
porosity of less than about 10% by volume, preferably less than about 5% 
by volume, and more preferably less than about 2% by volume, of the total 
volume of the sintered body. Sintering time is determinable empirically. 
Generally, a sintering time of about two to ten hours is satisfactory. 
The sintered body, which is comprised of crystalline mullite phase and a 
glassy cordierite phase, is nucleation-annealed to nucleate the glassy 
cordierite phase sufficiently to allow its crystallization to produce the 
present polycrystalline body. Specifically, the sintered body is annealed 
at a temperature ranging from about 600.degree. C. to about 800.degree. 
C., preferably from about 650.degree. C. to about 750.degree. C., and most 
preferably, it is annealed at about 700.degree. C. At a temperature below 
about 600.degree. C., the rate of nucleation is too slow to be useful 
whereas a temperature higher than about 800.degree. C. is not operable in 
the present process. 
Nucleation-annealing time is determinable empirically by standard 
techniques such as, for example, by observations of the degree to which 
the final product is crystallized. Generally, a nucleation-annealing time 
period of about two hours is satisfactory at about 700.degree. C. 
The nucleation-annealed sintered body is then crystallization-annealed to 
crystallize the nucleated glassy cordierite phase to produce the present 
polycrystalline body. Specifically, the nucleation-annealed sintered body 
is crystallization-annealed at a temperature ranging from about 
1200.degree. C. to a temperature at which the body remains totally solid. 
More specifically, the present crystallization-annealing is carried out at 
a temperature ranging from about 1200.degree. C. up to a temperature at 
which any liquid forms in the body, i.e. from about 1200.degree. C. to a 
temperature below the liquid-forming temperature, and such maximum 
crystallization-annealing temperature depends on the particular 
composition of the body being annealed. For example, when the nucleating 
agent has no effect on the temperature at which liquid forms in the 
present system, the crystallization-annealing temperature ranges from 
about 1200.degree. C. to a temperature below the melting point of pure 
cordierite, i.e. below about 1460.degree. C. However, when the nucleating 
agent does have an effect on the temperature at which liquid forms in the 
present system, the crystallization-annealing temperature ranges from 
about 1200.degree. C. to that temperature below the liquid forming 
temperature of that particular composition. For example, when TiO.sub.2 is 
used as the nucleating agent in the present invention, the maximum 
crystallization-annealing temperature would be below about 1450.degree. C. 
When lithium fluoride is used as the nucleating agent in the present 
invention, the maximum crystallization-annealing temperature would be 
below about 1290.degree. C. When magnesium fluoride is the nucleating 
agent, the maximum crystallization-annealing temperature would be below 
about 1390.degree. C. At a temperature lower than about 1200.degree. C. 
the rate of such crystallization is too slow to be useful. 
Crystallization-annealing time is determinable empirically by standard 
techniques such as, for example, by observations of the degree to which 
the final product is crystallized. Generally, a crystallization-annealing 
time of about two hours is satisfactory. 
The present sintering, nucleation-annealing and crystallization-annealing 
is carried out in an atmosphere or a vacuum which has no significantly 
deleterious effect on the compact or body. Representative of a useful 
atmosphere is air, hydrogen, wet hydrogen, nitrogen, argon and mixtures 
thereof. When a sintering atmosphere is used, it is preferably at ambient 
pressure since there is no advantage in using a pressure higher than 
ambient. Preferably, to produce a more highly dense sintered body, 
sintering is carried out in a vacuum below about 200 microns of Hg. 
In one preferred embodiment, the present sintering, nucleation-annealing 
and crystallization-annealing are carried out in a single run using the 
same atmosphere or vacuum throughout the run. 
The present polycrystalline body is comprised of mullite and cordierite 
phases, and more specifically, it has a phase composition comprised of 
polycrystalline mullite in an amount ranging from about 50% by weight to 
about 95% by weight of the total weight of the body and polycrystalline 
cordierite in an amount ranging from about 5% by weight to about 50% by 
weight of the total weight of the body. 
The present polycrystalline body has a porosity of less than about 10% by 
volume, preferably less than about 5% by volume, and more preferably less 
than about 2% by volume of the total volume of said body. 
In a preferred embodiment, the present polycrystalline body has a phase 
composition comprised of polycrystalline mullite ranging from about 60% by 
weight to about 70% by weight of the total weight of the body, and 
polycrystalline cordierite ranging from about 30% by weight to about 40% 
by weight of the total weight of the body, and has a thermal expansion 
coefficient within about 10% of that of silicon at 300.degree. K. 
In a more preferred embodiment, the present polycrystalline body has a 
phase composition comprised of about 64% by weight of polycrystalline 
mullite and about 36% by weight of polycrystalline cordierite, and has a 
thermal expansion coefficient within about 5% of that of silicon at 
300.degree. K. 
The present polycrystalline body can contain a glassy phase in an amount of 
less than about 5% by volume, preferably less than about 2% by volume, and 
more preferably less than about 1% by volume, of the total volume of said 
body. Even more preferably, the present polycrystalline body contains only 
a detectable amount of glassy phase. Therefore, glassy phase in the 
present polycrystalline body can range from a detectable amount up to 
about 5% by volume of the total volume of the body. The amount of glassy 
phase present in the present polycrystalline body depends largely on the 
impurities in the starting materials. 
The present polycrystalline body may or may not contain a minor amount of a 
phase comprised of the nucleating agent, and the presence of such phase 
depends largely on the particular nucleating agent used, i.e. the extent 
to which the nucleating agent vaporizes away, if at all, during the 
present process. Specifically, the present polycrystalline body contains a 
phase of nucleating agent ranging from none to about 5% by weight of the 
total weight of the body. In one embodiment, the present polycrystalline 
body contains a phase of nucleating agent ranging from about 0.5% by 
weight to about 5% by weight of the body. In another embodiment, the 
present polycrystalline body contains a phase of nucleating agent ranging 
from about 0.6% by weight to about 4% by weight of the body. In yet 
another embodiment, the present polycrystalline body contains a phase of 
nucleating agent of from about 0.75% by weight to about 3.25% by weight of 
the body. For example, in the present process the fluoride nucleating 
agents vaporize significantly and are not detectable as a phase in the 
present polycrystalline body using standard techniques such as, for 
example, X-ray diffraction analysis or by optically examining the 
microstructure. On the other hand, a nucleating agent such as titanium 
dioxide does not appear to vaporize away to any significant extent in the 
present process and does leave a TiO.sub.2 phase in the final product 
which corresponds, or substantially corresponds, to the amount of 
TiO.sub.2 present before sintering. The TiO.sub.2 phase is barely 
detectable in X-ray diffraction patterns and can range from about 0.5% by 
weight to about 5% by weight of the total weight of the final product, 
i.e. of the present polycrystalline body. 
In the present polycrystalline body, the phases are distributed uniformly, 
substantially uniformly or at least significantly uniformly. Also, the 
present polycrystalline body has a uniform, substantially uniform or at 
least a significantly uniform microstructure. 
The cordierite phase in the present polycrystalline body can be 
discontinuous or continuous. Specifically, in the present polycrystalline 
body, when the cordierite phase ranges from about 5% by weight to about 
10% by weight of the body, it is discontinuous. As the cordierite phase 
increases in amount from 10% by weight of the body, it exhibits some 
continuity, and at about 15% by weight of the body, it is likely to be 
continuous and interconnecting. The cordierite phase ranging in amount 
from in excess of 15% by weight to about 50% by weight of the body is 
continuous and interconnecting and envelops small islands of the mullite 
phase. 
The present polycrystalline body has a number of uses, but because of its 
relatively low thermal expansion coefficient and dielectric constant, it 
is particularly useful as a substrate material, especially as a supporting 
substrate for semiconductors in information processing equipment such as 
computers. Specifically, the present polycrystalline body has a thermal 
expansion coefficient of less than 3.3.times.10.sup.-6 /.degree.K. at 
300.degree. K., and its thermal expansion coefficient decreases as its 
content of cordierite increases. This is illustrated by the calculated 
graph of FIG. 2. The present polycrystalline body containing about 5% by 
weight cordierite would have a thermal expansion coefficient of about 
3.2.times.10.sup.-6 /.degree.K. at 300.degree. K., and the present body 
containing about 50% by weight cordierite would have a thermal expansion 
coefficient of about 2.times.10.sup.-6 /.degree.K. at 300.degree. K. 
The present polycrystalline body has a dielectric constant ranging from 
higher than 5 to lower than 7 at 300.degree. K. The dielectric constant is 
a function of the amount of cordierite present. The more cordierite 
present, the lower is the dielectric constant of the present body. 
The present polycrystalline body can be in any desired form. For example, 
it can be simple, hollow and/or complex in shape. Preferably, for use as a 
substrate, it is in the form of a tape. 
The invention is further illustrated by the following examples wherein the 
procedure was as follows unless otherwise stated: 
All firing of the compact or body was carried out in a molybdenum 
disilicide resistance furnace. 
At the end of each run, the power was switched off and the body was 
furnace-cooled to room temperature. 
The porosity of the body was measured by optical microscopy using standard 
techniques. 
Phase composition of the final product was determined by standard 
metallographic techniques such as optical microscopy and/or X-ray 
diffraction analysis. 
Thermal expansion coefficient was measured at 300.degree. K. in an quartz 
dilatometer. 
EXAMPLE 1 
A starting powder corresponding to a composition of 64% by weight mullite 
and 36% by weight cordierite was prepared from high-purity (greater than 
99% pure) oxide powders of aluminum oxide, silicon oxide and magnesium 
oxide. TiO.sub.2 was used as a nucleating agent. Specifically, 57.0% by 
weight of Al.sub.2 O.sub.3 with an average particle size of about 
0.15.mu., 35.6% by weight of SiO.sub.2 with an average particle size of 
about 1.0.mu., 4.8% by weight of MgO with an average particle size of 
about 0.5.mu. and 2.6% by weight of TiO.sub.2 of 99% purity with an 
average particle size of about 0.1.mu. were ball-milled using alumina 
media in acetone with a polyacrylic acid added as a dispersant. After 
milling for 3 hours, the solvent was removed by drying in air at about 
40.degree. C. The resulting powder mixture was substantially uniform and 
had an average particle size of about 0.5.mu.. 
The powder mixture was pressed into pellets at ambient temperature under a 
pressure of about 20,000 psi. Each pellet was in the form of a disc of 
substantially the same size of about 1/2 inch in diameter and about 1/8 
inch in thickness. 
One disc was sintered at 1490.degree. C. for two hours. The temperature was 
then lowered to 700.degree. C. where it was held for 2 hours to nucleate 
the glassy cordierite phase. The temperature was then raised to 
1300.degree. C. where it was held for 2 hours to crystallize the 
cordierite phase. The disc was then furnace cooled to ambient temperature. 
All firing, i.e. sintering, nucleation- and crystallization-annealing was 
carried out in air at ambient pressure, and the fired disc was furnace 
cooled in the same atmosphere. 
A polished cross-section of the resulting fired disc, i.e. the present 
polycrystalline body, is shown in FIG. 1. FIG. 1 shows that the disc is 
comprised of mullite (lighter phase), cordierite (darker colored phase) 
and a minor amount of TiO.sub.2 phase (tiny circles). 
FIG. 1 illustrates that the present body of this composition has a 
continuous interconnecting phase of cordierite enveloping islands of the 
mullite phase. 
The fired disc had a porosity of about 5.3% by volume of the body. It was 
comprised of about 62.3% by weight of crystalline mullite, about 35.1% by 
weight of crystalline cordierite and about 2.6% by weight of crystalline 
TiO.sub.2 phase. The mullite had a composition of about 71.8% by weight 
Al.sub.2 O.sub.3 balance SiO.sub.2 and the cordierite had a composition of 
about 13.7 weight % MgO, about 34.9 weight % Al.sub.2 O.sub.3 and about 
51.4 weight % SiO.sub.2. 
The disc had a thermal expansion coefficient at 300.degree. K. of 
2.5.+-.0.1.times.10.sup.-6 /.degree.K. which matches that of silicon at 
300.degree. K. 
This disc would be useful as a supporting substrate for a silicon chip for 
use in ceramic packaging for use in computers. 
This example is illustrated in Table I. 
Examples 2-10 of Table I were carried out in substantially the same manner 
as Example 1 except as noted in Table I and except as noted herein. 
Specifically, in Example 4 lithium fluoride was used instead of titanium 
dioxide; in Example 5, magnesium fluoride was used instead of titanium 
dioxide; and in Example 8, a mixture of titanium dioxide and lithium 
fluoride was used instead of titanium dioxide alone. The lithium fluoride 
and magnesium fluoride had an average particle size of about 1.0 micron 
and were greater than 99% pure. 
In Example 6, a slip composition was prepared to cast a tape. The slip was 
comprised of 57 grams Al.sub.2 O.sub.3, 35.6 grams SiO.sub.2, 4.8 grams 
MgO, 2.6 grams TiO.sub.2, 5.0 grams polyvinylbutanol, 4.0 cc polyethylene 
glycol, 5.0 cc glycerine trioleate, 1.0 cc dioctylphthalate and 60 cc 
toluene. The polyvinyl butanol functioned as a binder, i.e. it was 
substantially a solid at room temperature, but soluble in the toluene. The 
polyethylene glycol, glycerine trioleate and dioctylphthalate were 
plasticizers. 
The slip was cast at ambient temperature on the surface of a glass 
substrate where it was allowed to dry to remove the solvent. The resulting 
dried green tape was of substantially uniform thickness of about 0.019" 
and it was about 1 foot square. It was lifted from the glass and fired as 
shown in Table I. Its organic content vaporized away before sintering 
temperature was reached. The final fired tape, i.e. the present 
polycrystalline body, had a thickness of 0.009". 
In Example 7, the compact was first fired at 1480.degree. C. for 15 minutes 
in a vacuum of about 10 microns Hg, the firing atmosphere was then changed 
to air at ambient pressure, the temperature was then raised to 
1490.degree. C. and the firing schedule was then as shown in Table I. 
In Example 8, the compact was prefired in air at ambient pressure at a 
temperature of 1300.degree. C. for 5 hours, the temperature was then 
raised to 1490.degree. C. and the firing schedule was then as shown in 
Table I. 
TABLE I 
__________________________________________________________________________ 
Crystal- 
Nucleation 
lization 
Starting Composition Sintering 
Annealing 
Annealing 
Al.sub.2 O.sub.3 
SiO.sub.3 
MgO TiO.sub.2 
LiF MgF.sub.2 
Method of 
Temp 
Time 
Temp 
Time 
Temp 
Time 
Example 
wt % 
wt % 
wt % 
wt % 
wt % 
wt % 
Forming .degree.C. 
hr. 
.degree.C. 
hr. 
__________________________________________________________________________ 
.degree.C. 
hr. 
1 57.0 
35.6 
4.8 2.6 -- -- Pressing 20 ksi 
1490 
2 700 2 1300 
2 
2 57.0 
35.6 
4.8 2.6 -- -- Pressing 20 ksi 
1490 
2 700 2 1200 
2 
3 57.0 
35.6 
4.8 2.6 -- -- Pressing 20 ksi 
1490 
2 700 2 1300 
2 
4 57.0 
35.6 
4.8 -- 2.6 -- Pressing 20 ksi 
1290 
2 700 2 1200 
2 
5 57.0 
35.6 
4.8 -- -- 2.6 Pressing 20 ksi 
1390 
2 700 2 1200 
2 
6 57.0 
35.6 
4.8 2.6 -- -- Tape Cast 
1490 
2 700 2 1300 
2 
7 57.0 
35.6 
4.8 2.6 -- -- Pressing 20 ksi 
1490* 
2 700 2 1300 
2 
8 56.8 
35.5 
4.8 2.6 0.3 -- Pressing 20 ksi 
1490** 
2 700 2 1300 
2 
9 57.0 
35.6 
4.8 2.6 -- -- Pressing 20 ksi 
1490 
2 -- -- 1300 
2 
10 58.5 
36.6 
4.9 -- -- -- Pressing 20 ksi 
1490 
2 700 2 1300 
2 
__________________________________________________________________________ 
Final Fired Body 
Phase Composition Thermal Expansion 
Dielectric 
Firing Mullite 
Cordierite 
Other Porosity 
Coefficient 
Constant 
Example 
Atmosphere 
wt % 
wt % wt % vol % 
@ 300.degree. K. 
@ 25.degree. C. 
__________________________________________________________________________ 
1 air .about.62.3 
.about.35.1 
.about.2.6(TiO.sub.2) 
5.3 2.5 .+-. 0.1 .times. 10.sup.-6 
/.degree.K. 
2 air " " " 6.3 
3 H.sub.2 
" " " 8.7 
4 air .about.64 
.about.36 
-- 
5 air " " -- 
6 air .about.62.3 
.about.35.1 
.about.2.6(TiO.sub.2) 
8.0 6.2 
7 air " " " 1.3 
8 air " " " 2.3 
9 air .about.64 
-- .about.36(Glass) 
10 air .about.64 
-- .about.36(Glass) 
__________________________________________________________________________ 
*run included a 15 min. prefire in vacuum at 1480.degree. C. 
**run included a 5 hr. prefire at 1300.degree. C. in air 
Examples 1-8 of Table I illustrate the present invention. All of the final 
fired bodies of Examples 1-8, i.e. the present polycrystalline bodies, 
would be useful as a substrate for a semiconductor such as a silicon chip. 
Based on other work, the porosity of the polycrystalline bodies of 
Examples 4 and 5 would have a porosity of less than about 10% by volume of 
the body. 
Example 9 illustrates the importance of the present nucleation-annealing. 
Specifically, in Example 9, the sintered body was not nucleation-annealed 
and the final fired body did not contain the present crystalline 
cordierite phase. 
In Example 10, no nucleating agent was used, and the present 
polycrystalline body was not produced. 
EXAMPLE 11 
Two pellets, i.e. green discs, were prepared according to the present 
invention according to the disclosure of Example 1. Each disc had the same 
composition as disclosed for Example 1. 
Tungsten ink in the form of an X was brushed on one surface on one of the 
discs. A sandwich of the two discs with the deposited tungsten ink between 
them was formed and sintered at 1490.degree. C. for two hours. The 
temperature was then lowered to 700.degree. C. where it was held for 2 
hours to nucleate the glassy cordierite phase. The temperature was then 
raised to 1300.degree. C. where it was held for 2 hours to crystallize the 
cordierite phase. The resulting fired body was then furnace cooled to 
ambient temperature. All firing, i.e. sintering, nucleation- and 
crystallization-annealing was carried out in wet hydrogen having a dew 
point of 25.degree. C. at ambient pressure, and the fired body was cooled 
in the same atmosphere. 
The resulting product, i.e. body, had a density greater than 96% of 
theoretical, i.e. its porosity was less than 5% by volume of the total 
volume of the body. 
The low oxygen partial pressure, which was necessary to prevent the 
oxidation of the tungsten, resulted in some volatization of SiO from the 
surface of the ceramic. However, this resulted only in the slight 
depletion of cordierite only on the surface as indicated by X-ray 
diffraction and was not considered significant. 
The resulting fired ceramic product had an electrical resistivity on the 
order of 10.sup.13 ohms at 25.degree. C. The tungsten interconnects 
sintered to sufficient density to provide a continuous electrical pathway 
through the assembly. The high resistivity of the ceramic in conjunction 
with the electrical continuity of the tungsten illustrate the usefulness 
of the present polycrystalline body in ceramic packaging of 
semiconductors. 
The fired polycrystalline body was comprised of about 62.3% by weight of 
mullite, about 35.1% by weight of cordierite and about 2.6% by weight of 
TiO.sub.2 phase. 
EXAMPLE 12 
A starting powder corresponding to 64 weight % mullite and 36 weight % 
cordierite was prepared. 
In this example, there was used, in the proper ratio to produce such 
starting powder, a colloidal aluminum monohydroxide (CATA SB), 
colloidal silica (Cab-O-Sil), and a colloidal magnesium hydroxide which 
was prepared by precipitating a solution of magnesium nitrate with a 
concentrated ammonia solution. These materials were dispersed in a nitric 
acid solution with a pH of about 3 and subsequently gelled by the addition 
of concentrated ammonia. The gel was then frozen and allowed to thaw. This 
step aided in the subsequent filtration of the gel. During filtering of 
the thawed gel, the filtercake was washed with acetone to insure the 
complete removal of all water prior to drying. The filtercake was then 
allowed to dry at room temperature. After drying, the resultant powder was 
calcined at 500.degree. C. in air at ambient pressure for about 10 hours 
to decompose the hydroxides. Following calcination, the powder, which had 
an average particle size on the order of 200 Angstroms, was pressed into 
pellets at 100,000 psi at ambient temperature. Each pellet was in the form 
of a disc of substantially the same size of about 1/2 inch in diameter and 
about 1/8 inch in thickness. 
One disc was sintered at 1490.degree. C. for two hours. The temperature was 
then lowered to 700.degree. C. where it was held for 2 hours to nucleate 
the glassy cordierite phase. The temperature was then raised to 
1300.degree. C. where it was held for 2 hours to crystallize the 
cordierite phase. The disc was then furnace cooled to ambient temperature. 
All firing, i.e. sintering, nucleation- and crystallization-annealing was 
carried out in air at ambient pressure, and the fired disc was furnace 
cooled in the same atmosphere. 
The fired disc, i.e. the present polycrystalline body, had a porosity of 
about 5.3% by volume of the body. It was comprised of about 62.3% by 
weight of crystalline mullite, about 35.1% by weight of crystalline 
cordierite and about 2.6% by weight of crystalline TiO.sub.2 phase. This 
fired disc appeared to be the same as that produced in Example 1. 
This disc would be useful as a supporting substrate for a silicon chip for 
use in ceramic packaging for use in computers. 
EXAMPLE 13 
In this example, the process and procedure were the same as disclosed in 
Example 12 except that the calcined powder was pressed into a pellet at 
50,000 psi. 
The resulting fired disc, i.e. the present polycrystalline body, did not 
differ in any significant manner from that produced in Example 1.