U.S. patent application number 09/855466 was filed with the patent office on 2003-02-06 for discontinuous carbon fiber reinforced metal matrix composite.
Invention is credited to Cornie, James A., Cornie, Stephen S., Ryals, Mark A..
Application Number | 20030024611 09/855466 |
Document ID | / |
Family ID | 25321326 |
Filed Date | 2003-02-06 |
United States Patent
Application |
20030024611 |
Kind Code |
A1 |
Cornie, James A. ; et
al. |
February 6, 2003 |
Discontinuous carbon fiber reinforced metal matrix composite
Abstract
Disclosed are methods and materials for preparing metal matrix
composite (MMC) components that have low weight, good thermal
conductivity and a controllable in-plane coefficient of thermal
expansion. One embodiment of the invention features a metal matrix
composite that includes a metal alloy and random in-plane
discontinuous fibers. In some embodiments, the metal alloy includes
aluminum, copper or magnesium. In certain embodiments, the metal
matrix composite includes additives that enable solution hardening.
In other embodiments, the metal matrix composite includes additives
that enable precipitation hardening. Another embodiment of the
invention features a method of manufacturing a metal matrix
composite. The method includes contacting random in-plane
discontinuous fibers with a binder, and pressurizing the random
in-plane discontinuous fibers and the binder to form a bound
preform. The preform is pressurized to a pressure greater than the
molten metal capillary breakthrough pressure of the bound preform.
Subsequently, the bound preform is placed in a mold, infiltrated
with a molten infiltrant, and the molten infiltrant is cooled to
form the metal matrix composite.
Inventors: |
Cornie, James A.;
(Cambridge, MA) ; Ryals, Mark A.; (Newton, MA)
; Cornie, Stephen S.; (Watertown, MA) |
Correspondence
Address: |
TESTA, HURWITZ & THIBEAULT, LLP
HIGH STREET TOWER
125 HIGH STREET
BOSTON
MA
02110
US
|
Family ID: |
25321326 |
Appl. No.: |
09/855466 |
Filed: |
May 15, 2001 |
Current U.S.
Class: |
148/420 ;
148/432; 148/437; 164/113; 164/97; 257/E23.112 |
Current CPC
Class: |
C22C 47/025 20130101;
C22C 49/00 20130101; B22F 2998/00 20130101; C22C 9/00 20130101;
C22C 47/08 20130101; H01L 2924/0002 20130101; H01L 2924/0002
20130101; B22F 2998/00 20130101; C22C 47/06 20130101; H01L 23/3733
20130101; C22C 47/025 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
148/420 ;
148/432; 148/437; 164/97; 164/113 |
International
Class: |
C22C 023/00; C22C
009/00; C22C 021/00 |
Goverment Interests
[0001] This invention was made with government support under Grant
No. N00167-99-C-0072. The government has certain rights in the
invention.
Claims
What is claimed is:
1. A metal matrix composite comprising: a metal alloy; and random
in-plane discontinuous fibers, wherein the random in-plane
discontinuous fibers comprise carbon.
2. The metal matrix composite of claim 1 wherein the metal alloy
comprises a major component selected from the group consisting of
aluminum, copper, and magnesium.
3. The metal matrix composite of claim 1 wherein a majority of the
random in-plane discontinuous fibers have a length less than
approximately 750 .mu.m.
4. The metal matrix composite of claim 1 wherein the random
in-plane discontinuous fibers comprise graphite.
5. The metal matrix composite of claim 1 wherein the random
in-plane discontinuous fibers are milled.
6. The metal matrix composite of claim 1 wherein the metal matrix
composite has a volume fraction of the random in-plane
discontinuous fibers in a range of approximately 0.15 to
approximately 0.6.
7. The metal matrix composite of claim 1 wherein a minority of the
random in-plane discontinuous fibers are oriented out of plane by
an angle greater than 10.
8. The metal matrix composite of claim 7 wherein less than 20% of
the random in-plane discontinuous fibers are oriented out of plane
by an angle greater than 10.degree..
9. The metal matrix composite of claim 1 wherein an in-plane
coefficient of thermal expansion is in a range of approximately 3
ppm/.degree. K to approximately 12 ppm/.degree. K.
10. The metal matrix composite of claim 1 wherein an in-plane
coefficient of thermal expansion is greater than the coefficient of
thermal expansion of silicon.
11. The metal matrix composite of claim 2 wherein the major
component of the metal alloy is aluminum, and the metal alloy
further comprises more than approximately 4 wt % silicon.
12. The metal matrix composite of claim 11 wherein the silicon
composition is approximately the eutectic composition.
13. The metal matrix composite of claim 1 wherein the random
in-plane discontinuous fibers are uniformly distributed within the
metal matrix composite.
14. The metal matrix composite of claim 11 wherein the metal alloy
further comprises a minor component that enables precipitation
hardening.
15. The metal matrix composite of claim 14 wherein the minor
component is less than approximately 2 wt % magnesium.
16. The metal matrix composite of claim 2 wherein the major
component of the metal alloy is copper, and the metal alloy further
comprises less than approximately 5 wt % chromium.
17. The metal matrix composite of claim 16 wherein the metal alloy
further comprises a minor component that enables solution
hardening.
18. The metal matrix composite of claim 17 wherein the minor
component is at less than approximately 2 wt % zirconium.
19. The metal matrix composite of claim 18 further comprising a
nickel plating.
20. An article of manufacture comprising the metal matrix composite
of claim 1.
21. A metal matrix composite comprising: a metal alloy; and random
in-plane discontinuous fibers, wherein the random in-plane
discontinuous fibers comprise carbon and are uniformly distributed
within the metal matrix composite, and wherein the metal matrix
composite has a volume fraction of the random in-plane
discontinuous fibers in a range of approximately 0.15 to
approximately 0.6.
22. A metal matrix composite comprising: a metal alloy consisting
essentially of aluminum, silicon and magnesium, wherein the silicon
is approximately 5 wt % to approximately 20 wt % of the metal
alloy, and the magnesium is approximately 0.1 wt % to approximately
2 wt % of the metal alloy; and random in-plane discontinuous
graphite fibers uniformly distributed within the metal matrix
composite.
23. A metal matrix composite comprising: a metal alloy consisting
essentially of copper, chromium and zirconium, wherein the chromium
is approximately 0.3 wt % to approximately 2 wt % of the metal
alloy, and the zirconium is approximately 0.1 wt % to approximately
1 wt % of the metal alloy; and random in-plane discontinuous
graphite fibers uniformly distributed in the metal matrix
composite.
24. A method of manufacturing a metal matrix composite, the method
comprising the steps of: contacting random in-plane discontinuous
fibers with a binder; pressurizing the random in-plane
discontinuous fibers and the binder to form a bound preform,
wherein the random in-plane discontinuous fibers and the binder are
pressurized to a pressure greater than the capillary breakthrough
pressure of the bound preform; placing the bound preform in a mold;
infiltrating the bound preform with a molten infiltrant under a
pressure at least equal to the capillary breakthrough pressure; and
cooling the molten infiltrant to form the metal matrix
composite.
25. The method of claim 24 further comprising the steps of: placing
a second bound preform adjacent to the bound preform in the mold
prior to the step of infiltrating; contacting a surface of the
bound preform with a surface of the second bound preform; and
removing the binder prior to the step of infiltrating to merge the
surface of the bound preform with the surface of the second bound
preform.
26. The method of claim 24 further comprising the steps of: heating
the bound preform in the mold; evacuating the bound preform in the
mold to create a reduced pressure within the bound preform; and
transporting a charge of the molten infiltrant into the mold while
maintaining the reduced pressure within the preform.
27. The method of claim 24 further comprising the step of: forming
a preform of random in-plane discontinuous fibers, wherein the step
of forming the preform comprises agitating discontinuous fibers to
promote a random in-plane orientation.
28. The method of claim 24 wherein the binder comprises a
particulate, and the method further comprises the steps of:
liquefying the binder; and solidifying the binder to form the bound
preform.
29. The method of claim 24 wherein the random in-plane
discontinuous fibers comprise carbon.
Description
FIELD OF THE INVENTION
[0002] The present invention relates to methods of forming metal
matrix composites for thermal or structural applications, and the
resulting compositions. More specifically, the invention relates to
methods of infiltration casting to form metal matrix composites
with controlled thermal expansion and mechanical properties, and
the resulting compositions.
BACKGROUND OF THE INVENTION
[0003] Technological developments from cellular phones to imaging
satellites push current semiconductor device capabilities to their
performance limits. In particular, modern devices often must
dissipate a great amount of heat. Integrated circuit devices
typically require integration with heat sinks due to the
potentially deleterious effects of heat generated by the device. A
semiconductor die, typically a portion of a silicon wafer, can be
directly attached to a heat sink. More commonly, the die is encased
in a ceramic package that protects the die and provides electrical
connections.
[0004] Common ceramic package materials include aluminum oxide,
aluminum nitride and beryllium oxide. The coefficient of thermal
expansion of the semiconductor die and the ceramic package are
purposely matched to avoid thermal cycle induced mechanical stress
failures. Thermal cycling arises during power up and power down
cycles in combination with resistive heating due to current flow in
the device.
[0005] Heat sinks are commonly fabricated from metals, for example
copper, molybdenum, tungsten and aluminum. A metal heat sink is
often plated with nickel prior to attachment to a ceramic package
at an elevated temperature, for example, via brazing.
Alternatively, silver-filled adhesives, or other conductive metal
powder-filled adhesives, can be used for bonding.
[0006] Choosing a metal or other material for a heat sink often
involves a trade-off between desirable and undesirable properties.
For example, aluminum and copper have high thermal conductivity,
but coefficients of thermal expansion several times greater than
that of a ceramic package or semiconductor die. During power
cycling of the integrated circuit, resistive heating causes the
temperature of the integrated circuit, and the attached heat sink,
to fluctuate. Consequently, such metals apply mechanical stress to
the heat sink bonding material during power cycling. The
differential expansion of the heat sink relative to the ceramic
package or semiconductor die can cause failure of the bond material
or cracking of the package or die.
[0007] In contrast, some metals, such as tungsten and molybdenum,
have relatively small coefficients of thermal expansion. Although
such metals can permit a reliable bond, they have lower thermal
conductivity than aluminum or copper substrates and they are
difficult to electroplate. Further, tungsten and molybdenum are
undesirable for applications that require minimal weight.
[0008] Composites of copper and tungsten, or of copper and
molybdenum, can partially mitigate these deficiencies. These
composites can be made by powder metallurgical methods, such as
infiltrating copper into a sintered body of tungsten or molybdenum,
or sintering a mixed powder of the two metals. It is difficult,
however, to obtain an elongated plate by rolling a sintered ingot
of tungsten or molybdenum. Alternatively, layers of metal can be
joined by cladding or lamination of sheets. Cladded and laminated
products require precise machining, which is difficult and
increases costs.
[0009] As an alternative to an all metal heat sink, some heat sinks
combine a sintered ceramic with a metal matrix. The fabrication
process involves the formation of a ceramic preform, for example,
by sintering silicon carbide powder. The ceramic preform
microstructure typically has a predetermined void volume fraction
that is subsequently filled with molten metal, typically aluminum.
An aluminum ceramic heat sink can employ copper-based inserts to
improve its thermal conductivity. Such heat sinks, however, can be
difficult to machine and are usually limited in their ability to
match coefficients of thermal expansion with integrated
circuits.
[0010] As another alternative, a metal matrix composite can include
an inorganic fiber material. Infiltration of fibers has its own
difficulties, for example, problems with fiber wetting and
non-uniform fiber distribution. In addition, molten metal
infiltration of fibers under pressure can displace the fibers due
to the fiber breakthrough pressure threshold. Further, it is often
difficult to control fiber volume fraction, and thus to obtain a
desired property of the composite. These factors have limited use
of metal matrix fiber composites as heat sinks.
[0011] As the semiconductor industry continues to implement ever
increasing semiconductor die sizes and transistor densities to
permit enhanced integrated circuit complexity, the heat generated
by state-of-the-art integrated circuits also increases. Thus, the
challenge of coping with resistive heating is expected to become an
ever more central concern in integrated circuit design.
[0012] Beyond the electronics industry, precision motion and
control components and other mechanical hardware must be light
weight, stiff, and damp unwanted vibrations. In many instances,
conventional materials, such as aluminum and copper, are unable to
meet the performance demands of many emerging technologies.
SUMMARY OF THE INVENTION
[0013] It has been discovered that a metal matrix composite ("MMC")
that includes random in-plane discontinuous carbon fibers and a
method of forming an MMC from a pressure-formed preform can solve
many problems of prior art heat sinks. The invention can overcome
numerous problems, such as: fiber collapse during molten metal
infiltration; coefficient of thermal expansion ("CTE") mismatch;
heat sink weight; limits on range of CTE values; difficulty in
obtaining high fiber density; limits in control of fiber
orientation; machinability of a heat sink; and/or heat sink cost.
The invention addresses these problems through the use of one or
all of the following: preforms prepared with pressures greater than
the breakthrough pressure used during metal infiltration; in-plane
oriented fibers; short fibers; and carbon fibers.
[0014] Use of random in-plane discontinuous fibers permits a high
fiber volume fraction in the MMC ("in-plane" as used herein is
understood as the X-Y plane, for example, the plane parallel to the
bonded surface of a heat sink). Further, by using in-plane oriented
fibers, substantially all of the fibers can contribute to the
control of the CTE in the X-Y plane. Though Z-direction CTE is not
controlled by in-plane fibers, such control is generally
unnecessary for heat sink applications because the integrated
circuit or other object is attached to an X-Y oriented surface of
the heat sink.
[0015] Use of these in-plane oriented fibers permits selection of a
CTE over a wide range of values. A desired volume fraction of
in-plane oriented fibers is selected to obtain a desired CTE. By
orienting substantially all fibers in the X-Y plane, a very high
fiber volume fraction can be obtained. This permits selection of
volume fraction over a wide range and a corresponding ability to
select a wide range of CTE values.
[0016] Carbon fibers, in particular graphite fibers, have excellent
mechanical and thermal properties for use in heat sinks of the
invention. In combination with an aluminum or other light metal,
such heat sinks typically are easily machined, have excellent heat
conductivity, and are lightweight. An aluminum and graphite fiber
MMC of the invention thus realizes the advantages of
aluminum--lightweight, easy machinability and good heat
conduction--in combination with the advantages of graphite
fibers--high Young's Modulus, small to negative CTE, high tensile
strength, high thermal conductivity and strong damping
properties.
[0017] The invention also solves the problem of non-uniform fiber
distribution within the MMC. A preform that includes fibers and a
binder can be prepared via application of a pressure in a preform
mold that is greater than the molten alloy breakthrough pressure
for the preform. The binder maintains the compressed configuration
of the fibers in the preform while the preform is removed from the
preform mold and placed in a metal infiltration mold. The metal
infiltration mold can maintain the compressed fiber configuration
upon removal of the binder, if the infiltration mold is sized and
shaped to conform to the preform. Because the fiber configuration
remains in its compressed state, it is substantially undisturbed
during infiltration of molten metal at the molten metal
breakthrough pressure.
[0018] In a broad aspect, the invention features a metal matrix
composite that includes a metal alloy and random in-plane
discontinuous fibers. The random in-plane discontinuous fibers may
be carbon, and preferably are graphite. The fibers typically are
milled, and preferably are ball milled. In preferred embodiments,
the metal alloy includes aluminum, copper or magnesium.
[0019] In one embodiment, the metal matrix composite has a volume
fraction of random in-plane discontinuous fibers in a range of
approximately 0.15 to approximately 0.6. In another embodiment, a
minority of the random in-plane discontinuous fibers are oriented
out of plane by an angle greater than 10.degree.. In a preferred
embodiment, the random in-plane discontinuous fibers are uniformly
distributed within the metal matrix composite.
[0020] In certain embodiments, the metal matrix composite may
include a component that enables solution hardening. In other
embodiments, the metal matrix composite may include a component
that enables precipitation hardening. In one preferred embodiment,
the metal matrix composite includes aluminum, silicon and
magnesium. In another preferred embodiment, the metal matrix
composite includes copper, chromium and zirconium.
[0021] In another aspect, the invention provides a method of
manufacturing a metal matrix composite. The method includes
contacting random in-plane discontinuous fibers with a binder, and
pressurizing the random in-plane discontinuous fibers and the
binder to form a bound preform. In the later step, the random
in-plane discontinuous fibers and the binder are pressurized to a
pressure greater than the capillary breakthrough pressure of the
bound preform. Subsequently, the bound preform typically is placed
in a mold, heated under a vacuum to remove the binder, then heated
to above the metal liquidus and infiltrated with a molten
infiltrant. The molten infiltrant is then cooled to form the metal
matrix composite.
[0022] In another embodiment, the method includes placement of a
second bound preform adjacent to the bound preform in the mold
prior to infiltration with the molten infiltrant. A surface of the
bound preform contacts a surface of the second bound preform, and
removal of the binder prior to infiltration causes the contacted
surfaces of the two preforms to merge, creating one continuous,
metal matrix composite.
[0023] The binder may be removed (called debindering) prior to
infiltration with a molten infiltrant, e.g., via evaporation.
Alternatively, the binder may partially or completely remain in the
preform during infiltration. For example, a volatile component of a
binder may be removed prior to infiltration, leaving a residue in
the preform.
[0024] Reference to the figures herein is intended to provide a
better understanding of the methods and apparatus of the invention
but are not intended to limit the scope of the invention to the
specifically depicted embodiments. The drawings are not necessarily
to scale, emphasis instead being placed upon illustrating the
principles of the invention. Like reference characters in the
respective figures typically indicate corresponding parts.
[0025] It should be understood that the order of the steps of the
methods of the invention is immaterial so long as the invention
remains operable, i.e., e.g., a preform is made prior to
infiltration of the preform. Moreover, two or more steps may be
conducted simultaneously.
[0026] The foregoing, and other features and advantages of the
invention, as well as the invention itself, will be more fully
understood from the description, drawings, and claims which
follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a graph that shows the variations of CTE with
in-plane volume fraction for a theoretical model and for
experimental embodiments of aluminum and copper matrix composites
of the invention.
[0028] FIGS. 2a and 2b are scanning electron micrographs of an
experimental embodiment of a metal matrix composite having an
aluminum matrix and graphite fibers. FIG. 2a shows a cross-section
through the X-Y plane. FIG. 2b shows a cross-section through the
X-Z plane.
[0029] FIGS. 3a-3e illustrate formation of a preform according to
an embodiment of the invention.
[0030] FIG. 3a is a cross-sectional illustration of dispensing
fibers and binder into a preform mold base portion.
[0031] FIG. 3b shows the fibers and binder residing in the preform
mold base portion.
[0032] FIG. 3c shows the fibers and binder under compression in the
preform mold.
[0033] FIG. 3d shows binding of the fibers by the binder.
[0034] FIG. 3e shows a completed, bound preform after removal from
the preform mold.
[0035] FIG. 4 illustrates an embodiment of a stacked preform having
three individual preforms layered with two graphite foils between
the preforms.
[0036] FIGS. 5a and 5b illustrate an embodiment of forming a larger
preform from a combination of smaller preforms. FIG. 3a shows a
stack of preforms without any intermediate layers. FIG. 3b shows
the larger preform after merging of the interfaces of the smaller
preforms.
[0037] FIG. 6 illustrates an embodiment of the stacked preform of
FIG. 4 in a metal infiltration mold.
[0038] FIG. 7 illustrates an embodiment of a horizontally oriented
preform in a metal infiltration mold.
DETAILED DESCRIPTION OF THE INVENTION
[0039] A metal matrix composite ("MMC") that includes random
in-plane discontinuous carbon fibers and a method of forming an MMC
from a pressure-formed preform can solve many problems of prior art
heat sinks. The composite and method can alleviate such problems
as: fiber collapse during molten metal infiltration; coefficient of
thermal expansion ("CTE") mismatch; heat sink weight; limits on
range of CTE values; difficulty in obtaining high fiber density;
limits in control of fiber orientation; machinability of a heat
sink; and/or heat sink cost. The following describes various
embodiments of the invention that may include, for example,
preforms prepared with pressures greater than the breakthrough
pressure used during metal infiltration, random in-plane oriented
fibers, short or discontinuous fibers, and carbon fibers.
[0040] Definitions
[0041] As used herein, "molten metal infiltration" is understood to
mean any casting process with or without an externally applied
pressure to facilitate infiltration of a mold vessel cavity that
contains a preform. Examples of pressure infiltration casting
include, but are not limited to, pressure infiltration casting such
as the Advanced Pressure Infiltration Casting (APIC.TM.) process as
described in U.S. Pat. Nos. 5,322,109; 5,553,658; and 5,983,973;
high throughput pressure infiltration casting as described in U.S.
Pat. No. 6,148,899; squeeze casting; and die-casting.
[0042] As used herein, "metal" is understood to mean a metal or
metal alloy. Examples of common metals or metal alloys are, among
others, aluminum, aluminum alloys, bronze, beryllium, beryllium
alloys, chromium, chromium alloys, cobalt, cobalt alloys, copper,
copper alloys, gold, iron, iron alloys, steel, magnesium, magnesium
alloys, nickel, nickel alloys, lead, lead alloys, copper, tin, tin
alloys such as tin-bismuth and tin-lead, zinc, zinc alloys,
superalloys such as International Nickel 100 (IN-100) or
International Nickel 718 (IN-718), and combinations thereof.
[0043] As used herein, "molten infiltrant", "liquid infiltrant,"
"molten metal," or "liquid metal" is understood to mean a
respective material which is at least at or above approximately its
liquidus temperature.
[0044] As used herein, "fugitive" is understood to mean
substantially removable, i.e., removable to a great extent.
[0045] As used herein, "preform" is understood to mean a fibrous,
non-metallic material such as, e.g., an oxide, a boride, a nitride,
a carbide or a form of carbon which is to be infiltrated with an
infiltrant. Infiltration of a preform by a molten metal followed by
solidification produces a metal matrix composite (MMC).
[0046] As used herein, "bound preform" is understood to mean a
preform in which the fibers are held in a more or less fixed
physical relationship due to the action of a binder material.
[0047] As used herein, "preform mold vessel" and "preform mold" are
understood to mean any container capable of holding or applying
pressure to preform materials during formation of a preform.
[0048] As used herein, "metal infiltration mold vessel" and "metal
infiltration mold" are understood to mean any container capable of
holding a preform, and confining the preform and molten metal
during metal infiltration of the preform.
[0049] As used herein, "in-plane" is understood to mean the X-Y
plane or the plane normal to the Z direction in an X-Y-Z coordinate
system. It is also understood to mean the plane that is parallel to
the bonded surface of a heat sink. This is commonly referred to as
the "base" plane in the electronics industry.
[0050] Overview of Materials in an MMC Component
[0051] Some embodiments of MMC components of the invention are well
suited as heat sinks for use with a variety of integrated circuit
semiconductor and ceramic packaging materials. These components
have relatively low density, high thermal conductivity and a
coefficient of thermal expansion ("CTE") that can be controlled
over a wide range to match a companion integrated circuit material.
Properties of common semiconductor and packaging materials are
illustrated in Table I. Table I also shows the preferred heat sink
CTE ranges for a good match with each of the listed materials.
1 TABLE I Semiconductor CTE Preferred Density or Ceramic
(ppm/.degree. K) Heat Sink CTE (g/cc) Si 4.2 4.5-5.0 2.3 GaAs 6.5
7.0-8.0 5.3 AlN 4.5 5.0-6.0 3.26 Al.sub.2O.sub.3 6.5 7.0-8.0 3.6
BeO 7.6 8.0-9.0 2.9
[0052] In one embodiment, an MMC includes random in-plane
discontinuous fibers. Use of discontinuous fibers, particularly
fibers less than approximately 1 mm in length, permits good control
of the volume fraction of the fibers in the finished MMC. Further,
for in-plane oriented fibers, substantially all of the fibers
contribute to control of CTE in the X-Y plane.
[0053] An MMC of the invention may include fibers of a narrow or
wide range of fiber lengths. In some embodiments, an MMC includes
fibers as short as approximately 30 .mu.m. In some embodiments, an
MMC includes chopped fibers as long as approximately 12 mm.
[0054] Some applications do not require control of Z-direction CTE,
for example, heat sinks. For these applications, MMC components can
be fabricated with a desired X-Y plane CTE over a very wide range
by selection of a corresponding fiber volume fraction. Hence, the
invention provides heat sinks that can be CTE-matched to a wide
variety of integrated circuit materials. Fibers of various
materials may be used. For example, the fibers may include silicon
carbide. In preferred embodiments, MMC components include carbon
fibers, preferably graphite fibers, in a light metal matrix. Carbon
fibers may, e.g., be prepared from a pitch or pan precursor.
Preferred embodiments employ pitch precursor fibers due to a
superior elastic modulus and thermal conductivity relative to pan
precursor fibers.
[0055] Graphite fibers have excellent mechanical and thermal
properties for use in heat sinks of the invention. In combination
with a light metal, e.g., aluminum, magnesium, or copper, or their
alloys, such heat sinks are easily machined, have excellent heat
conductivity and are very lightweight. An aluminum and graphite
fiber MMC of the invention thus realizes the advantages of
aluminum--lightweight, easy machinability and good heat
conduction--in combination with the advantages of graphite
fibers--high Young's Modulus, small to negative CTE, high tensile
strength, high thermal conductivity and strong damping
properties.
[0056] In addition to high thermal conductivity, graphite fibers
have the unusual feature of a high modulus of elasticity combined
with a negative coefficient of thermal expansion. Thus, these high
modulus, negative CTE fibers can be embedded within a matrix metal
alloy to restrain the matrix from expanding to its full extent
during heating. The fibers further prevent excessive contraction
during cooling from a processing temperature, and contribute to
in-plane thermal conductivity. By combining graphite fibers with a
metal such as Al and by controlling the volume fraction of fibers
and the orientation, one can design a material with a specified
CTE.
[0057] In some embodiments, a heat sink of the invention can be
soldered or brazed to an integrated circuit package to improve heat
transfer. The heat sink has a CTE that is preferably chosen to be
slightly greater in value than the package so that the package is
under compression at room or operating temperature. Package
cracking or failure of the heat sink/package bond is less likely
under this condition.
[0058] The CTE of the heat sink is also preferably chosen in view
of the temperature range to be used during attachment of the heat
sink to a ceramic package. For example, a eutectic copper-silicon
braze alloy requires a temperature of 780.degree. C. during
attachment of a nickel plated copper alloy matrix heat sink to a
metallized ceramic package. A gold-germanium braze alloy requires a
temperature of 380.degree. C. during attachment of an aluminum
matrix heat sink to a ceramic package.
[0059] The discontinuous fibers can be inorganic. Preferably, the
discontinuous fibers are carbon-based. The fibers are more
preferably graphite. The fibers can be chopped. In some embodiments
the fibers are less than 25 mm in length. In some embodiments, the
fibers are preferably less than 1 mm in length, more preferably
less than 0.75 mm. In some embodiments, the fibers are milled. In
preferred embodiments, the fibers are ball milled. In a preferred
embodiment, the fibers graphite and have an average length of
approximately 0.2 mm (200 .mu.m) and a diameter of approximately 10
.mu.m.
[0060] Fiber Orientation and Fiber Volume Fraction
[0061] Theoretical considerations can assist in the selection of a
fiber volume fraction that will provide a desired in-plane CTE. For
example, a uniaxial laminate-based theory (see, e.g., R. S.
Schapery, "Thermal Expansion Coefficients of Composite Materials
Based on Energy Principles," J. Composite Materials, Vol. 2, pages
380-404, (1968)) approximates the CTE of a composite material as: 1
11 = E f f v f + E m m ( 1 - V f ) E f v f + E ( 1 - v f ) Equation
1
[0062] where,
[0063] .alpha..sub.11 is the CTE of the composite in the orthogonal
direction (parallel to the 0.degree. fibers in an uniaxial
laminate, units of ppm/.degree. K),
[0064] .alpha..sub.f is the axial CTE of the fiber (units of
ppm/.degree. K),
[0065] .alpha..sub.m is the CTE of the matrix (e.g., 24
ppm/.degree. K for Al and 16 ppm/.degree. K for Cu)
[0066] .sub.v.sub.f is the volume fraction of fibers oriented
parallel to the 0.degree. axis,
[0067] (1-v.sub.f) is the matrix volume fraction,
[0068] E.sub.f is the Young's modulus of elasticity for graphite
fiber (units of GPa), and
[0069] E.sub.m is the Young's modulus of elasticity for the Matrix
(e.g., 69 GPa for Al and 110 GPa for Cu).
[0070] One can model orthogonal in-plane CTE properties, i.e., as
obtained with two sets of orthogonally oriented in-plane fibers, by
adding a second laminate of fibers oriented at 90.degree. in the
above model. Such a model should be considered a lower bound
approximation for orthogonally oriented fiber reinforced
metals.
[0071] One can approach the random in-plane condition by extension
of the theoretical model to a multi-laminate composite. This will
give an approximate CTE for randomly oriented in-plane fibers where
the X and Y components of in-plane fibers are equal.
[0072] For most electronic thermal management applications when a
matching CTE is important, it is desirable to maintain orthogonal
CTE values, i.e., the CTE in the 0.degree. or X direction should
equal the CTE in the 90.degree. or Y direction. When the in-plane
CTE is, for example, 8 ppm/.degree. K in all in-plane directions,
one can simply say that the in-plane CTE is 8 ppm/.degree. K.
[0073] Balanced 0.degree.-90.degree. composites can be achieved by
use of orthogonally oriented in-plane fibers. Fibers in an MMC can
be orthogonally oriented by weaving or by stacking alternating
plies of uniaxially wrapped fibers to form a laminate of
orthogonally oriented layers.
[0074] In preferred embodiments of an MMC, substantially uniform
in-plane orthogonal properties are obtained by use of in-plane
randomly oriented discontinuous fibers. Preferably, very few of the
fibers are oriented out of plane, i.e., in the Z axis direction. By
controlling the volume fraction of a preform, whether it includes
woven, wrapped or random-in-plane discontinuous fibers, one can
obtain a graphite reinforced MMC having a selected value of
CTE.
[0075] Theoretical curves are plotted in FIG. 1 for an MMC that
includes an aluminum matrix or a copper matrix, with fiber
properties corresponding to those of P-120 graphite fibers, a fiber
in the ThermalGraph.RTM. family of products available from BP Amoco
(Alpharetta, Ga.). These fibers have an average length and width of
approximately 200 .mu.m and 10 .mu.m, respectively, E.sub.f=827
GPa, .alpha..sub.f=1.45 ppm/.degree. K, and density=2.17 g/cc.
Other useful fibers in the ThermalGraph.RTM. family include those
sold under the developmental names DKA X and DKD X. Similar fibers
are available from other suppliers, such as Conoco Carbon Fibers
(Houston, Tex.).
[0076] Theoretical curves can be used to assist control of the CTE
of a reinforced metal alloy by selecting an appropriate fiber
volume fraction. For example, referring to FIG. 1, a CTE in a range
of 4 to 12 ppm/.degree. K would require a corresponding fiber
volume fraction between approximately 0.40 and 0.18. Similarly, a
P-120 reinforced copper alloy MMC would require selection of fiber
volume fraction between 0.4 and 0.14 to obtain CTE values in the
same range.
[0077] More generally, MMC components can be prepared with a wide
range of CTE values. For example, a CTE can be zero or negative in
value, or can be 12 ppm/.degree. K or greater in value. The ability
to densely pack fibers permits fiber volume fractions to be chosen,
preferably, from within a range of approximately 0.15 to 0.60. For
some applications, fiber volume fraction may lie outside this
range.
[0078] For aluminum-based MMCs, a wide range of useful properties
may be obtained with a fiber volume fraction in a range at least as
broad as approximately 0.15 to approximately 0.55. For example, a
fiber volume fraction of 0.30 provides a CTE of approximately 8.0
ppm/.degree. K, while a fiber volume fraction of 0.40 provides a
CTE of 4.0 ppm/.degree. K.
[0079] Deviation of a portion of the fibers in an MMC from in-plane
orientation reduces the observed CTE from that predicted by theory.
Deviations from in-plane randomness in experimental samples cause
the in-plane CTE of a reinforced metal alloy to be greater than
that predicted theoretically. Accordingly, empirical calibration
curves can be constructed that are based on experimental data.
Consequently, the CTE versus volume fraction curves are more
accurate for manufacturing purposes than curves obtainable from
theoretical relationships such as given in Equation 1.
[0080] The graph in FIG. 1 shows two CTE volume fraction curves for
aluminum matrix and copper matrix MMC samples prepared with P-120
graphite fiber preforms. The aluminum MMC curve was obtained from
CTE measurements obtained during cooling of samples from
400.degree. C. to 50.degree. C. (800.degree. C. to 50.degree. C.
for the copper matrix samples). As shown, P-120 reinforced aluminum
samples were prepared having fiber volume fractions in a range of
0.2 to 0.4. The CTE of the resulting samples varied between 13 and
4 ppm/.degree. K.
[0081] The degree of deviation from theory is at least in part due
to a population of fibers with some out of plane orientation. To
the extent that a fiber is oriented out of plane, the elastic
restraint of the fiber on the in-plane CTE of the matrix is
reduced.
[0082] The microstructure of a sample aluminum alloy MMC with 0.30
volume fraction of P-120 fibers is shown in the scanning electron
micrographs of FIGS. 2a and 2b. The sample was prepared by blending
743 grams of DKDX fibers and 119 grams of Carbowax.RTM.
polyethelyne glycol 8000 from Union Carbide Chemical and Plastics
Co. (Danbury, Conn.) as a binder.
[0083] The blend was loaded into a 190.5 mm.times.190.5 mm press
die mold and pressed to a thickness of 31.7 mm. Subsequent to
heating to a temperature in a range of 80.degree.-100.degree. C. to
liquefy the binder, cooling to 10.degree. C. solidified the binder
to produce a stable, bound preform. The bound preform was
infiltrated with aluminum alloy containing 12.5% silicon and 0.4%
magnesium. To obtain these micrographs, polished sections were
taken from the sample MMC along the X-Y plane and along the X-Z
plane.
[0084] The X-Y section of FIG. 2a shows a substantial number of
fibers laying parallel to the X-Y plane. In the X-Z section of FIG.
2b, only a few fibers lie with any component of their orientation
parallel to the Z direction. Quantitative analysis of several
micrographs showed that over 84% of the fibers laid within
10.degree. of the X-Y plane, and that over 96% of the fibers laid
within 300 of the X-Y plane.
[0085] In one embodiment of an MMC of the invention, less than half
of the fibers are oriented out of the X-Y plane by more than
10.degree.. In a more preferred embodiment, less than 25% of the
fibers are oriented out of plane by more than 10.degree.. In a more
preferred embodiment, less than 20% of the fibers are oriented out
of plane by more than 10.degree.. In a further preferred
embodiment, less than 15% of the fibers are oriented out of plane
by more than 10.degree..
[0086] Preparation of a Preform
[0087] Properties of an MMC are affected by both the volume
fraction and the orientation of the fibers. For liquid metal
infiltrated composites, preferred embodiments use a fibrous preform
that does not "swim" or become disturbed during the inrush of
molten metal. These embodiments include a preform that is stable
and that does not lose its shape or fiber distribution during the
infiltration process.
[0088] In some embodiments, a stable preform is obtained by densely
packing a preform mold. After removing the packed and bound preform
from the preform mold, the preform is typically placed into a metal
infiltration mold, in preferred embodiments, a steel can.
Preferably, the preform completely fills the metal infiltration
mold or the mold cavity of the metal infiltration mold.
[0089] When the preform is heated, the binder may begin to release
the fibers. The fibers can then relax and press against the metal
infiltration mold. Some binders evaporate during heating. After the
binder is removed, e.g., with the assistance of an applied vacuum,
and the preform has reached a molten metal infiltration
temperature, molten metal infiltration can take place.
[0090] In one embodiment, woven fibers in the form of a fabric
cloth are cut and loaded directly into a metal infiltration mold
for subsequent pressure infiltration. Since a fabric has a discrete
thickness, controlling the thickness of an MMC component formed
from fabric is difficult. For volume fractions above the natural
woven volume fraction of a fabric, the fabric is compressed and
clamped into a mold, increasing tooling costs. Moreover, it is
typically difficult to pack woven fabrics to a fiber volume
fraction greater than approximately 0.45. Conversely, loading molds
with fabric to a fiber volume fraction less than 0.40 can lead to
non-uniform distribution of the fiber plies. Even when preform
cloth plies have been well compressed into a mold, non-uniform ply
loading can result in warping after removal from the mold.
[0091] An alternative embodiment uses a continuous fiber preform.
Such a preform may be fabricated by drum winding continuous fibers,
and fixing the wound fibers onto a transfer sheet by applying a
fugitive binder. Such plies can be stacked with orthogonal
orientations, or more mixed orientations, for example, including
plies oriented at 45.degree. to other plies. These embodiments can
include fiber volume fractions of approximately 0.55 to 0.6 or
more.
[0092] Another alternative preform material includes a paper-like
product produced from chopped discontinuous fibers. The material
includes a fugitive binder to provide stability and facilitate
handling. In a preferred embodiment, the fibers in the "paper" are
randomly orientated in the X-Y plane. This material can be
compressed to a desired fiber volume fraction and further
stabilized with additional binder.
[0093] In another embodiment, a preform of the invention provides
substantially uniform fiber distribution after molten metal
infiltration. The preform is typically prepared by compressing
fibers and a binder in a preform mold. In a preferred embodiment,
the fibers are discontinuous. The fibers and binder usually are
mixed, then compressed at a pressure that is greater than the
molten alloy breakthrough pressure for the finished preform. Prior
to metal infiltration, the binder maintains the compressed
configuration of the fibers in the preform so the preform can be
removed from the preform mold and placed in a metal infiltration
mold. In certain applications, a bound preform may be stored for
some time prior to metal infiltration. Preferably, the bound
preforms are stored below room temperature.
[0094] The binder is often removed from the preform while the
preform resides in the metal infiltration mold. Under the
constraints of the metal infiltration mold, the preform can
maintain its compressed fiber configuration. Molten metal is then
infiltrated into the preform. Because the fiber configuration
remains in its compressed state, it is substantially undisturbed
during infiltration of molten metal at the molten metal
breakthrough pressure, and proper in-plane orientation of the
fibers is maintained.
[0095] In some embodiments, the in-plane distribution of fibers in
the preform mold is enhanced prior to compression and fixation with
a binder. For example, the preform mold can be agitated, such as by
vibration, prior to compression. Vibration may also break up
clumps, or "hair balls", of fiber. Further, the compression of the
fibers and the binder also serves to enhance the in-plane
orientation of the fibers, as well as increase the volume fraction
of fibers in the preform.
[0096] Some embodiments utilize thin sheets of paper-like or
felt-like material formed from random in-plane oriented fibers. In
a preferred embodiment, a sheet is produced with a volume fraction
in a range of approximately 0.05 to approximately 0.20. The sheets
may be weighed to select a proper amount for a desired preform. The
sheets may then be placed in a preform mold and compressed to
obtain a final desired fiber volume fraction in the preform.
[0097] Various binder materials can be employed to form and
maintain a preform. A binder material is generally required to
maintain fibers in a desired orientation and state of compression.
For example, water may be a binder which is set via freezing. Solid
forms of polyethylene glycol ("PEG") may be a binder as well as
acrylic. Solid binder materials usually are heated to approximately
or above their melting point, then cooled to solidify. One suitable
PEG material is Carbowax.RTM. polyethylene glycol 8000 from Union
Carbide Chemical and Plastics Co. (Danbury, Conn.), which is
liquefied at approximately 80.degree.-100.degree. C.
[0098] Some binders are fully removed prior to molten metal
infiltration. Other binder materials partially or fully remain
during and after molten metal infiltration. For example, a
phenolic-based binder may have a volatile component removed prior
to molten metal infiltration. For example, the volatile component
leaves in vapor form, leaving behind a carbon-based residue. In
some embodiments, non-binder organic materials may also escape from
the preform during evaporation of some or all of a binder
material.
[0099] FIGS. 3a through 3e illustrate in cross-section one
embodiment of the formation of a preform 10. Referring to FIG. 3a,
a material dispenser 22 randomly dispenses discontinuous fibers 11
and a binder 12 into a preform mold base portion 20. The binder 12
can be dispensed as discontinuous particles as shown, or can be
mixed with the fibers by other means. For example, the fibers can
be coated with binder subsequently or prior to distribution of the
fibers into the preform mold. One or more dispensers may be
employed. In other embodiments, fibers and binder are dispensed
from different dispensers.
[0100] Referring to FIG. 3b, to promote a random, in-plane
distribution of the fibers, the preform mold base portion 20 often
is agitated. For example, the base portion 20 can be vibrated after
filling with fibers 11 and binder 12. Preferably, agitation is
applied continuously during the dispensing of the fibers and binder
into the preform mold.
[0101] Referring to FIG. 3c, a preform mold cap portion 21 is
placed in contact with the fibers 11 and binder 12. Pressure is
applied via the preform mold cap portion 21 to the mixture of
fibers 11 and binder 12 to compress the mixture at a pressure
greater than the breakthrough pressure required to infiltrate the
formed preform with the appropriate molten metal. The binder 12
serves to fix the configuration of fibers obtained in this
compressed state by adhering neighboring fibers 11 to one another
to produce the preform 10 as shown in FIG. 3d. FIG. 3e shows the
preform 10 after it is removed from the preform mold base portion
20.
[0102] In a preferred embodiment, in a tumble mill to uniformly mix
the fibers and binder. The mixture then may be redispersed in a
rotary brush mill to untangle the fiber clusters. A measured
portion of the mixture is selected by weighing for production of an
MMC of a desired size and fiber volume fraction.
[0103] The weighed portion of fiber and PEG binder is placed in a
preform mold base portion. After leveling the mixture by vibration,
the preform mold is closed and the preform is compressed to a
predetermined volume to obtain the desired volume fraction of
fibers in the complete preform.
[0104] When forming a preform from a mixture of PEG powder and
fibers having an average length of approximately 200 .mu.m,
pressure of approximately 450 psi is required to obtain a fiber
volume fraction of 30%, while a pressure of approximately 1,100 psi
is required to obtain a volume fraction of 40%. These pressures
exceed typical liquid metal capillary breakthrough pressures of the
resulting preform.
[0105] Heating to approximately 85.degree. C. melts the PEG.
Subsequent cooling allows the PEG to solidify and bind the fibers
to one another. The preform can then be stored under refrigeration
for later use.
[0106] Metal Matrix Materials
[0107] Various metals and metal alloys can be used in the invention
depending on the particular application. Aluminum, copper and
magnesium are preferred. With applications involving aluminum or an
aluminum alloy, it is desirable to add silicon to the metal to
reduce the reactivity of the metal with graphite, which undesirably
forms aluminum carbide. For example, an MMC formed from 6061
aluminum alloy with 0.45 volume fraction of graphite fibers and had
approximately 4.0% carbide formation after pressure infiltration
casting. Fabrication of an MMC from an aluminum alloy having
approximately 7.0% by weight silicon, reduced carbide formation to
approximately 0.5%. Using 12.5% silicon in the aluminum alloy,
further reduced the carbide formation to approximately 0.3%.
[0108] Thus, in embodiments which include aluminum, silicon is
preferably added. In preferred embodiments, the alloy includes at
least approximately 7% by weight silicon, and more preferably
approximately 12.5% by weight silicon. The eutectic composition for
an aluminum-silicon alloy has 12.5% silicon. In addition to
reducing the activity of carbon in aluminum, addition of silicon
reduces the melting point of the alloy. This, in turn, further
decreases the kinetics of carbide formation, provided that metal
infiltration temperatures also are reduced.
[0109] The matrix alloy should also be able to withstand
micro-scale deformation that can occur during thermal cycling. In
some applications, such as integrated circuit heat sinks, an MMC
experiences large temperature cycles during use. Micro-deformation
during thermal cycling can cause thermal ratcheting, i.e., a change
in dimension of the MMC after each thermal cycle. An accumulation
of dimensional changes can lead to damage, for example, of an
electronic assembly attached to an MMC heat sink. It is thus
desirable to limit deformation over the use temperature ranges of
an MMC, such as from approximately -30.degree. C. to approximately
150.degree. C.
[0110] To reduce thermal cycling deformation, the metal matrix
alloy can be hardened. In certain embodiments, magnesium is added
to aluminum. In combination with silicon, the magnesium provides an
age hardenable aluminum alloy due to formation of precipitates
during cooling from, for example, a brazing temperature. The
precipitation hardened matrix suffers from less plastic deformation
during thermal cycling.
[0111] One embodiment of an aluminum-based MMC of the invention
includes 2.0% by weight or more magnesium in the alloy. A preferred
embodiment includes magnesium in a range of approximately 0.1% to
approximately 1.0%. More preferred embodiments include magnesium in
a range of approximately 0.2% to 0.5%, or approximately 0.3% to
0.4%. It should be understood that the aluminum alloy can contain
minor impurities, for example, iron, manganese and titanium.
[0112] Other embodiments of the invention use a copper alloy and
graphite fibers. Because the bond strength between copper and
graphite is extremely low, chromium is often included in the alloy.
The chromium reacts with carbon in the fibers to form chromium
carbide at the fiber-metal interface, which aids the bonding
between the alloy and the fibers.
[0113] One embodiment includes approximately 5.0% by weight or more
chromium in the alloy. A preferred embodiment includes chromium in
a range of approximately 0.3% to 5.0%. A more preferred embodiment
includes chromium in a range of approximately 0.3% to 2.0%. Other
more preferred embodiments include chromium in a range of
approximately 0.5% to 1.5%, or approximately 0.7% to 1.0%.
[0114] Another embodiment includes a copper alloy having improved
yield strength through addition of zirconium. Zirconium promotes
solid solution hardening and reduces thermal ratcheting. A
preferred embodiment includes zirconium in a range of approximately
0.1% to 2.0% by weight. A more preferred embodiment includes
zirconium in a range of approximately 0.1% to 1.0%. Other more
preferred embodiments include zirconium in a range of approximately
0.1% to 0.5%, or approximately 0.12% to 0.3%. Alloy additions, such
as those described above, have a minimal effect on the thermal
conductivity of an alloy.
[0115] Infiltration
[0116] Using methods known in the prior art, it is typically
difficult to control MMC volume fraction by stacking woven cloth
preforms into a mold. Since graphite fibers are not easily wetted
by some alloys, such as aluminum, magnesium and copper, pressure
infiltration is used to overcome the wettability difficulty.
However, a pressure infiltrated metal exerts a compressive pressure
on a preform prior to capillary breakthrough and subsequent
infiltration. In some cases, the infiltrating metal compresses the
preform and causes veining and gross displacement of the preform to
the mold wall upon breakthrough into less dense regions. Obtaining
a particular desired volume fraction of fibers is also difficult
when using stacked wrapped lamina to make a preform.
[0117] As described above, methods of the invention overcome these
difficulties by providing a preform which has been compressed at a
pressure above that experienced during infiltration casting. For
example, by pressing preforms to a pressure greater than the
capillary breakthrough pressure, activating a binder to constrain
the preform, loading the fiber and binder into a fixed volume mold,
removing the binder by evacuation and heating just prior to
pressure infiltration casting, a metal matrix composite can be
manufactured free of breakthrough defects and at a controlled
volume fraction reinforcement.
[0118] Preforms of the invention can be infiltrated individually or
collectively. FIG. 4 illustrates an embodiment of multiple preforms
prepared for infiltration. The preforms 10 are layered with
separator sheets 15, for example, graphite foil sheets, to permit
production of more than one MMC component during one infiltration
cycle. The separator sheets may also be, e.g., slices of graphite,
graphite coated steel sheets, or colloidal graphite coated sheets.
The separator sheets ease separation of the MMC components after
cooling of the metal.
[0119] In another embodiment, as illustrated in FIG. 5a, one or
more preforms 10 are placed adjacent to each other without
separator sheets. Hence, surfaces of the preforms 10 are in direct
contact. After packing the layered preforms 10 into a metal
infiltration mold vessel, the binder is removed, for example, by
heating. As illustrated in FIG. 5b, upon release of the binder, the
contacted surfaces of the preforms 10 can merge with one another
and permit formation of an effectively larger preform and
ultimately larger MMC component.
[0120] A preform or stack of preforms can be infiltrated with a
molten metal by any method known to one skilled in the art. In one
embodiment, as illustrated in FIG. 6, a stack of preforms 10
layered with separator sheets 15 is placed in a metal infiltration
vessel 30. A filter 33 is placed on top of the stack to prevent
premature infiltration of the preform, especially if the preform is
evacuated prior to introduction of the metal. Note, however, that
alternative arrangements of preforms in mold vessels are possible
which may not required a filter, for example, use of gated top
plates or caps.
[0121] In another embodiment, illustrated in FIG. 7, a preform 10
is horizontally positioned in a metal infiltration vessel 30. A cap
33 with gates 39, for admission of molten metal, is placed on the
preform 10. The cap may be held in place by means known in the art
which includes welding. In embodiments where a high volume fraction
of fibers is desired, the preform(s) typically need to be isolated
in a confined space so that upon removal of the binder, the fibers
maintain their position, orientation and compactness.
[0122] One can employ a mold release agent in the vessel. For
aluminum alloy and magnesium alloy, the mold release agent
preferably is one or more layers of colloidal carbon, e.g.,
colloidal graphite or boron nitride, which is dispersed in a
suitable volatile vehicle. However, other ceramic slurry coatings
may be used. For copper alloy, a slurry of zirconium oxide in a
slightly acidic vehicle sold under the trade name Zircwash.TM. may
be used. Other parting compounds may be used as mold release agents
or washes such as boron nitride or graphite foil.
[0123] In one embodiment, a preform is tightly loaded into a molten
metal infiltration vessel. The preform is heated to remove binder
via evaporation. Upon removal of the binder, compressive stresses
stored in the preform cause the preform to relax against the walls
of the vessel. Since the preform is constrained by the walls of the
vessel, the vessel walls now maintain a compressive stress on the
preform that is greater than the breakthrough pressure of the
molten metal.
[0124] In a preferred embodiment, the process described in U.S.
Pat. No. 6,148,899 is used to infiltrate molten metal into a
preform. Briefly, liquid metal is transferred by vacuum siphon into
the metal infiltration mold vessel which is under reduced pressure.
The mold vessel is placed in an autoclave and pressurized to
approximately 60 atm using nitrogen gas, forcing the molten metal
into the preform.
[0125] The mold vessel then is contacted with tin-bismuth at the
eutectic composition. Heat from the vessel causes the tin-bismuth
to melt. This heat transfer process increases the solidification
rate of the molten metal in the preform and assists directional
solidification to help eliminate shrinkage porosity.
[0126] After cooling, the MMC component is removed from the vessel.
The MMC component can than receive other processing, for example,
machining into a final desired shape for use as a heat sink, or
plating in preparation for some types of brazing.
[0127] The invention may be embodied in other specific forms
without departing from the spirit or essential characteristics
thereof. The foregoing embodiments are therefore to be considered
in all respects illustrative rather than limiting on the invention
described herein. Scope of the invention is thus indicated by the
appended claims rather than by the foregoing description, and all
changes which come within the meaning and range of equivalency of
the claims are intended to be embraced therein.
[0128] Each of the patent documents and scientific publications
disclosed hereinabove is incorporated by reference herein.
* * * * *