U.S. patent application number 11/163486 was filed with the patent office on 2006-04-27 for spray deposition apparatus and methods for metal matrix composites.
This patent application is currently assigned to METAL MATRIX CAST COMPOSITES, LLC. Invention is credited to James Allen Cornie, Stephen Shawn Cornie, Shiyu Zhang.
Application Number | 20060086434 11/163486 |
Document ID | / |
Family ID | 37400893 |
Filed Date | 2006-04-27 |
United States Patent
Application |
20060086434 |
Kind Code |
A1 |
Cornie; James Allen ; et
al. |
April 27, 2006 |
SPRAY DEPOSITION APPARATUS AND METHODS FOR METAL MATRIX
COMPOSITES
Abstract
A spray deposition apparatus comprises a source of aqueous fiber
slurry that includes a mixture of milled graphite fibers in
suspension. A slurry input of the spray deposition apparatus is
coupled to the source of aqueous fiber slurry. The slurry input
receives the mixture of aqueous fiber slurry. A gas pressure input
receives pressurized gas. A nozzle aspirates the mixture of aqueous
fiber slurry with the pressurized gas to produce a stream of fiber
cluster droplets.
Inventors: |
Cornie; James Allen;
(Cambridge, MA) ; Cornie; Stephen Shawn;
(Watertown, MA) ; Zhang; Shiyu; (Cambridge,
MA) |
Correspondence
Address: |
RAUSCHENBACH PATENT LAW GROUP, LLC
P.O. BOX 387
BEDFORD
MA
01730
US
|
Assignee: |
METAL MATRIX CAST COMPOSITES,
LLC
101 Clematis Avenue
Waltham
MA
|
Family ID: |
37400893 |
Appl. No.: |
11/163486 |
Filed: |
October 20, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60621274 |
Oct 22, 2004 |
|
|
|
Current U.S.
Class: |
148/420 ;
118/308; 118/325; 264/518; 427/421.1 |
Current CPC
Class: |
B05B 7/1436 20130101;
B22F 3/22 20130101; B05B 7/1409 20130101; C23C 26/00 20130101; C22C
47/06 20130101; C22C 47/06 20130101; B22F 2999/00 20130101; C22C
47/12 20130101; C22C 9/00 20130101; C22C 21/02 20130101; B22F
2999/00 20130101 |
Class at
Publication: |
148/420 ;
118/308; 118/325; 427/421.1; 264/518 |
International
Class: |
C22C 23/00 20060101
C22C023/00 |
Claims
1. A spray deposition apparatus comprising: a source of aqueous
fiber slurry that includes a mixture of milled graphite fibers in
suspension; a slurry input that is coupled to the source of aqueous
fiber slurry, the slurry input receiving the mixture of aqueous
fiber slurry; a gas pressure input that receives pressurized gas;
and a nozzle that aspirates the mixture of aqueous fiber slurry
with the pressurized gas to produce a stream of fiber cluster
droplets.
2. The spray deposition apparatus of claim 1 wherein the source of
aqueous fiber slurry comprises a continuously recirculating source
of aqueous fiber slurry.
3. The spray deposition apparatus of claim 1 wherein the source of
aqueous fiber slurry comprises a binder that is dissolved in the
aqueous fiber slurry.
4. The spray deposition apparatus of claim 1 wherein the gas
pressure input is positioned in the apparatus so as to promote
forming the stream of fiber cluster droplets with a uniform spray
pattern.
5. The spray deposition apparatus of claim 1 wherein the nozzle
comprises a plurality of nozzles.
6. The spray deposition apparatus of claim 1 further comprising a
reciprocal scanning mechanism that is attached to the nozzle, the
reciprocal scanning mechanism providing a reciprocal scanning
motion to the spray nozzle.
7. The spray deposition apparatus of claim 1 wherein the stream of
fiber cluster droplets produced by the nozzle is essentially a
random stream.
8. A system for forming a composite preform, the system comprising:
a source of aqueous fiber slurry that includes a mixture of milled
graphite fibers in suspension; a slurry input that is coupled to
the source of aqueous fiber slurry, the slurry input receiving the
mixture of aqueous fiber slurry; a gas pressure input that receives
pressurized gas; a nozzle that aspirates the mixture of aqueous
fiber slurry with the pressurized gas to produce a stream of fiber
cluster droplets; a translating fiber belt that translates a fiber
preform adjacent to the nozzle so that the stream of fiber cluster
droplets deposits on the preform mat; and a vacuum plenum that is
positioned under the preform mat, the vacuum plenum removing excess
carrier fluid from the fiber cluster droplets deposited on the
preform mat.
9. The system of claim 8 wherein the preform mat is translated at a
constant rate of speed.
10. The system of claim 8 wherein the vacuum plenum is positioned
directly under the nozzle.
11. The system of claim 8 wherein the vacuum plenum is positioned a
distance away from the nozzle.
12. A method of forming a fiber preform mat, the method comprising:
forming an aqueous fiber slurry comprising milled graphite fibers
in suspension; aspirating the aqueous fiber slurry to produce a
spray of fiber cluster droplets; translating a fiber preform mat in
a path of the fiber cluster droplets so that a randomized
distribution of fibers is deposited on the fiber preform mat; and
extracting excess carrier fluid from the fiber cluster droplets,
thereby setting the binder in the fiber preform mat.
13. The method of claim 12 wherein the aqueous fiber slurry further
comprises a binder.
14. The method of claim 12 wherein the translating the fiber
preform mat comprises translating the fiber preform mat at a
constant rate.
15. The method of claim 12 further comprising spraying binder onto
the fiber preform mat.
16. The method of claim 12 wherein the extracting excess the
carrier fluid comprises reducing a pressure proximate to the
preform mat.
17. The method of claim 12 wherein the extracting the excess
carrier fluid comprises increasing a temperature of the fiber
preform mat to remove moisture from the fiber preform mat.
18. The method of claim 12 wherein the extracting the excess
carrier fluid comprises dehumidifying the fiber preform mat to
remove moisture from the fiber preform mat.
19. The method of claim 12 wherein the extracting the excess
carrier fluid is performed simultaneously with the deposition of
the randomized distribution of fibers on the fiber preform mat.
20. The method of claim 12 wherein the extracting the excess
carrier fluid is performed at a predetermined time after the
deposition of the randomized distribution of fibers on the fiber
preform mat.
21. A method of forming a fiber preform mat, the method comprising:
forming an aqueous fiber slurry comprising milled graphite fibers
in suspension; aspirating the aqueous fiber slurry to produce a
spray of fiber cluster droplets; translating a fiber preform mat in
a path of the fiber cluster droplets so that a randomized
distribution of fibers is deposited on the fiber preform mat;
extracting excess carrier fluid from the fiber cluster droplets,
thereby setting the binder in the fiber preform mat; and
controlling at least one of a rate of extracting the excess carrier
fluid from the fiber cluster droplets and a time at which the
excess carrier fluid is extracted from the fiber cluster droplets
to adjust an average fiber angular inclination above a basal
plan.
22. The method of claim 21 wherein the controlling the at least one
of the rate of extracting the excess carrier fluid and the time at
which the excess carrier fluid is extracted adjusts the average
fiber angular inclination above the basal plan in the range of
about 10%-30%.
23. The method of claim 21 wherein the controlling the at least one
of the rate of extracting the excess carrier fluid and the time at
which the excess carrier fluid is extracted adjusts the average
fiber angular inclination above the basal plan in the range of
about 10%-30%.
24. The method of claim 21 wherein the controlling the at least one
of the rate of extracting the excess carrier fluid and the time at
which the excess carrier fluid is extracted adjusts the average
fiber angular inclination above the basal plan in the range of
about 20%-30%.
25. A method for manufacturing metal matrix composites, the method
comprising: forming an aqueous fiber slurry comprising milled
graphite fibers in suspension; aspirating the aqueous fiber slurry
to produce fiber cluster droplets; translating a fiber preform mat
in a path of the fiber cluster droplets so that a randomized
distribution of fibers is deposited on the fiber preform mat;
extracting excess carrier fluid from the fiber cluster droplets,
thereby setting the binder in the fiber preform mat; stripping
filter material from the fiber preform mat to create a fiber
preform; pressing the fiber preform to a desired volume fraction;
and pressure infiltrating a metal matrix to form a composite
preform.
26. The method of claim 25 wherein the extracting the excess
carrier fluid comprises reducing a pressure proximate to the
preform mat.
27. The method of claim 25 wherein the extracting the excess
carrier fluid comprises increasing a temperature of the fiber
preform mat to a temperature that removes moisture from the fiber
preform mat.
28. The method of claim 25 wherein the extracting the excess
carrier fluid comprises dehumidifying the fiber preform mat to
remove moisture from the fiber preform mat.
29. The method of claim 25 wherein the extracting the excess
carrier fluid is performed simultaneously with the deposition of
the randomized distribution of fibers on the fiber preform mat.
30. The method of claim 25 wherein the extracting the excess
carrier fluid is performed at a predetermined time after the
deposition of the randomized distribution of fibers on the fiber
preform mat.
31. The method of claim 25 wherein the pressing the fiber preform
further comprises controlling a temperature of the fiber
preform.
32. The method of claims 25 further comprising cutting and stacking
the fiber preform to form a fiber preform with the desired
dimensions prior to pressing.
33. The method of claim 25 wherein the pressure infiltrating the
metal matrix comprises pressure infiltrating at least one of Al,
Mg, and Cu alloy matrix.
34. The method of claim 25 wherein the aqueous fiber slurry further
comprises a binder.
35. The method of claim 25 further comprising spraying binder onto
the fiber preform mat.
Description
BACKGROUND OF THE INVENTION
[0001] The section headings used herein are for organizational
purposes only and should not be construed as limiting the subject
matter described in the present application.
[0002] Modern electronic devices and systems, such as cellular
phones, radar systems, high power RF and microwave devices and
systems, and imaging systems push current semiconductor device
capabilities to their performance limits. Increasing operating
speeds and computing power of modern microelectronic devices have
given rise to increases in complexity and functionality of the
semiconductor structures. Such devices often must dissipate large
amounts of heat during normal operation. Consequently, state-of-the
art integrated circuit devices often require heat sinks to maintain
acceptable operating temperatures. The semiconductor die can be
directly attached to a heat sink or can be encased in a ceramic
package that protects the die and provides electrical
connections.
[0003] Common ceramic packages include silicon carbide, aluminum
oxide, aluminum nitride, gallium nitride, gallium arsenide, and
beryllium oxide. The coefficient of thermal expansion (CTE) of the
semiconductor die and the ceramic package are usually matched as
closely as possible to avoid thermal cycling induced mechanical
stress failures. Thermal cycling arises during power up and power
down cycles in combination with resistive heating caused by current
flowing in the device.
[0004] The semiconductor industry is continually increasing
semiconductor die sizes and device densities in order to provide
more functions and higher performance to semiconductor devices. The
heat generated by these devices continues to increase. Thus, there
is a continuing need to develop heat sinks that can dissipate
increasing amounts of heat.
BRIEF DESCRIPTION OF THE DRAWING
[0005] The invention, in accordance with preferred and exemplary
embodiments, together with further advantages thereof, is more
particularly described in the following detailed description, taken
in conjunction with the accompanying drawings. The drawings are not
necessarily to scale, emphasis instead generally being placed upon
illustrating principles of the invention.
[0006] FIG. 1 is a schematic diagram of one embodiment of a spray
deposition apparatus that manufactures fiber preforms according to
the present invention.
[0007] FIG. 2 is a schematic diagram of another embodiment of a
spray deposition apparatus that manufactures fiber preforms
according to the present invention.
[0008] FIG. 3 illustrates a schematic diagram of randomly dispersed
fibers of a composite material formed according to the present
invention.
[0009] FIG. 4 presents experimental data for preforms manufactured
according to the first example with an infiltrated Al alloy matrix
and with fiber angles of inclination above the basal plane of the
original preform mat that are approximately 25 degrees with a
distribution of fiber angles of inclination above the basal plane
as shown in FIG. 7A.
[0010] FIG. 5 presents experimental data for preforms manufactured
according to the second example infiltrated with a Cu alloy matrix
and with fiber angles of inclination above the basal plane of the
original preform mat that are approximately 25 degrees with a
distribution of fiber angles of inclination above the basal plane
as shown in FIG. 7A.
[0011] FIG. 6 is a comparison of experimental data and data
calculated using Shapery's equation for CTE, and Rule of Mixtures
calculations of thermal conductivity for Al-graphite composites
manufactured according to various embodiments of the present
invention with the distribution of fiber angles of inclination
above the basal plane of the original preform mat shown in FIG.
7B.
[0012] FIG. 7A illustrates a plot of the distribution of fiber
angles of inclination above the basal plane for measurements of 70
individual fibers in the original preform mats used to manufacture
Al-- and Cu-graphite composites according to various embodiments of
the present invention and described in connection with FIGS. 4 and
5.
[0013] FIG. 7B illustrates a plot of the distribution of fiber
angles of inclination above the basal plane for measurements of 70
individual fibers in the original preform mats used to manufacture
Al-graphite composites according to various embodiments of the
present invention and described in connection with FIG. 6.
DETAILED DESCRIPTION
[0014] While the present teachings are described in conjunction
with various embodiments and examples, it is not intended that the
present teachings be limited to such embodiments. On the contrary,
the present teachings encompass various alternatives,
modifications, and equivalents, as will be appreciated by those of
skill in the art.
[0015] It should be understood that the individual steps of the
methods of the present invention may be performed in any order
and/or simultaneously as long as the invention remains operable.
Furthermore, it should be understood that the apparatus of the
present invention can include any number or all of the described
embodiments as long as the invention remains operable.
[0016] Known heat sinks are commonly fabricated from metals, such
as 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. Alternatively, silver-filled adhesives,
or other conductive metal powder-filled adhesives, are sometimes
used for bonding.
[0017] Choosing a metal or other material for a heat sink often
involves a trade-off between desirable and undesirable properties.
Some metals, such as aluminum and copper have high thermal
conductivity, but have CTEs that are several times greater than
that of the ceramic package or semiconductor die. During power
cycling of an electronic component, the temperature of the
component and the attached heat sink fluctuate significantly.
Consequently, such metals cause 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.
[0018] Other metals, such as tungsten and molybdenum, have
relatively small CTEs. Although such metals can permit a reliable
bond, they have lower thermal conductivity than aluminum or copper
substrates and they are difficult to electroplate. Furthermore,
tungsten and molybdenum are undesirable for applications that
require minimal weight.
[0019] Composites of copper and tungsten or of copper and
molybdenum have certain advantages over elemental materials. These
composites can be made by various methods of powder metallurgy,
such as, for example, 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,
metal layers can be joined by cladding or lamination. Cladded and
laminated products, however, require precise machining, which is
labor-intensive, error-prone, and expensive.
[0020] Some heat sinks combine a sintered ceramic with a metal
matrix. The fabrication process involves the formation of a ceramic
preform, which can be made by, for example, 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 manufacture and are
usually limited in their ability to have a matched CTE with an
electronic device.
[0021] Other heat sinks are metal matrix composite which include
infiltrated inorganic fiber material. Infiltration of fibers has
its own limitations, such as, 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. Furthermore, 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.
[0022] Metal matrix composite ("MMC") materials that include
discontinuous high-modulus graphite fibers that are randomly
arranged in-plane at desired volume fractions have significant
advantages over many known heat sinks. Such composites are
disclosed in U.S. patent application, Ser. No. 10/379,044, filed
Mar. 4, 2003, entitled "Discontinuous Carbon Fiber Reinforced Metal
Matrix Composite," which is assigned to the present assignee. The
entire application of U.S. patent application Ser. No. 10/379,044
is incorporated herein by reference.
[0023] MMC composites enable design of base plates with relatively
high thermal conductivity and with CTEs that match the CTEs of
common ceramic package materials. It is particularly desirable for
such CTE-matching material to have high thermal conductivity in
various directions. There currently is a significant need in the
electronic thermal management and packaging industry for both heat
spreading, which relies on high in-plane thermal conductivity, and
heat sinking, which relies on high "z" or through-plane thermal
conductivity. The term "in-plane" as used herein refers to the
plane parallel to a bonded surface of a heat sink. The term
"through-plane" as used herein refers to a direction that is
orthogonal to an in-plane surface.
[0024] One method of manufacturing a MMC with randomly distributed
graphite fibers is disclosed in U.S. patent application Ser. No.
10/379,044. This method includes mixing dry PEG binder material
with dry-milled graphite fibers having average length of about 300
microns. The mix is then poured into a mold, pressed, and heated to
liquefy the binder. The mix is then chilled to set the binder prior
to removal from the die.
[0025] The resulting preform is inserted into a pressure
infiltration casting mold vessel for metal infiltration and
solidification. This process is relatively simple and inexpensive.
The fiber distribution obtained with this process is relatively
non-uniform and may results in a standard deviation on order of 2
ppm at the volume fraction that resulted in a CTE of 7 ppm/K. Thus,
this method may not be suitable for applications requiring
particularly close CTE matching. In addition, non-uniform and
largely unpredictable fiber distribution may result in warping of
plates machined from the casting while processing through the
various machining steps or through soldering operations.
[0026] Another known method of manufacturing a MMC with randomly
distributed graphite fibers is disclosed in U.S. Pat. No.
5,437,921. This method includes dispersing milled fibers in aqueous
slurry, which is then poured into a filter vessel. The aqueous
slurry is formed into a filter cake under vacuum and then pressed
to a desired volume fraction. The filter cake is then dried and
pressed to the desired volume fraction. This process, however, is
prone to localized non-uniform distribution due to localized flow
alignment of milled fibers during pouring and vacuum filtration
steps. In addition, the packing density tends to vary with
thickness. Furthermore, there are significant variations of CTE.
For example, the standard deviation can be 1.25 ppm/K at the
average level of CTE at 7 ppm/K.
[0027] Yet another known method of manufacturing a MMC with
randomly distributed graphite fibers includes incorporating chopped
CKD graphite fibers with an average chop length of 25 mm into a
paper product. This method is used commercially from Technical
Fibre Products of Cumbria in the United Kingdom. The method
requires adding a co-polyester fiber, which serves as a binder. The
paper product is laid out into a die and then heated to soften the
binder fiber. The preform is then pressed to the desired volume
fraction. Each ply is rotated through a sequence of orientations to
produce a substantially planar isotropic preform. This process
results in a lower standard deviation that is about 0.9 ppm/K.
However, the process is expensive and the through-plane thermal
conductivity is relatively low. In addition, the polyester binder
is typically difficult to remove and has relatively a high char
yield during the outgasing and preheating operation.
[0028] Milled fiber preforms in many known methods of manufacturing
a MMC tend to develop preferred orientations when they experience
flow alignment. Flow alignment occurs when dry milled fibers are
poured into a mold where they exhibit aligned flow that cannot be
re-randomized. It is difficult to prevent localized slurry flow and
local areas of fiber alignment when vacuum-assisted filtration of
filter composites is performed.
[0029] The methods and apparatus of the present invention relate to
forming metal matrix composite materials having discontinuous high
modulus graphite fibers for thermal or structural applications. The
methods and apparatus of the present invention can form metal
matrix composite containing discontinuous high modulus graphite
fibers that are arranged in-plane with a majority of fibers
oriented substantially off the basal plane at the desired volume
fraction. This material can have predetermined thermal expansion
properties that are controllably matched to those of a wide range
of materials that are commonly used as electronic substrates. In
addition, this material can have relatively high, nearly isotropic
thermal conductivity properties.
[0030] FIG. 1 is a schematic diagram of one embodiment of a spray
deposition apparatus 100 that manufactures fiber preforms according
to the present invention. The spray deposition apparatus 100 is
used to manufacture preforms for metal matrix composites. The metal
matrix composites can be manufactured with different coefficients
of thermal expansion that are suitable for many different
applications. In addition, the metal matrix composites can be
manufactured to have substantially isotropic thermal
conductivity.
[0031] The spray system 100 includes a slurry input port 102 that
receives an aqueous fiber slurry that includes milled graphite
fibers in suspension from a slurry source 104. In one embodiment,
the slurry source 104 continuously recirculates the aqueous fiber
slurry. In some embodiments, a binder is dissolved in the aqueous
fiber slurry. A gas pressure input 106 receives pressurized gas,
such as pressurized air from a gas source 108. In one embodiment,
the gas pressure input 106 is disposed asymmetrically in the spray
system 100 in order to promote forming swirling and uniform fan
patterns in the spray system 100.
[0032] A spray nozzle 110 aspirates the aqueous fiber slurry by
mixing the pressurized gas with the aqueous fiber slurry to produce
a random stream of fiber cluster droplets 112 that is sprayed
through the spray nozzle 110. It is understood that the stream of
droplets 112 is not perfectly random. However, each droplet 112 in
the stream of droplets contains fiber clusters that have a
relatively high degree of randomness relative to the fiber clusters
in each of the other droplets 112 in the stream of droplets.
[0033] In some embodiments, the aqueous fiber slurry includes a
dissolved binder. In other embodiment, the spray deposited mat is
sprayed with binder using a second set of spray jets that is down
stream from the fiber slurry spray jets (not shown) is used to
spray a binder on the aqueous fiber slurry after the aqueous fiber
slurry is deposited. A separate nozzle can be used to spray the
binder. In yet other embodiments, the aqueous fiber slurry includes
the dissolved binder and the spray nozzle 110 or a separate spray
nozzle is used to spray additional binder on the deposited aqueous
fiber slurry.
[0034] In some embodiments, the spray system 100 includes a
plurality of spray nozzles. In one particular embodiment, the spray
system 100 includes four or five spray nozzles. In some
embodiments, the spray nozzle 110 or the plurality of spray nozzles
is coupled to a reciprocal scanning mechanism 114 that provides a
reciprocal or other motion to the spray nozzle 110. In embodiments
that include multiple spray nozzles, the reciprocal scanning
mechanism 114 can be coupled to any number of the plurality of
spray nozzles to simultaneously scan the spray nozzles. Also,
separate and independent reciprocal scanning mechanisms can be
coupled to any number of the plurality of spray nozzles to
independently scan these spray nozzles. In one embodiment, the
width of the spray is in the range of about 3 to 7 inches. In some
embodiments, the spray nozzle 110 translates in the Z direction to
alter the spay pattern.
[0035] A translating filter belt 116 is positioned adjacent to the
spray nozzle 110 in the direction of the stream of random fiber
cluster droplets 112. A fiber preform mat 118 is positioned on the
translating filter belt 116 so that the stream of random fiber
cluster droplets 112 is sprayed directly onto the fiber preform mat
118 in a spray area 120 as the fiber preform mat 118 is translated
by the translating filter belt 116. The fiber preform mat 118 is a
carrier sheet that receives the droplets of random fiber clusters
112. In some embodiments, the fiber preform mat 118 comprises a
polymeric non-woven material that can be reusable. The resulting
film deposited on the fiber preform mat 118 comprises random fiber
clusters. The translating filter belt 116 translates the fiber
preform mat 118 relative to the spray nozzle 110. In some
embodiments, the translating filter belt 116 translates the fiber
preform mat 118 at a constant rate of speed in the longitudinal
direction.
[0036] A vacuum plenum 122 is positioned under the translating
filter belt 116. In the embodiment shown in FIG. 1, the vacuum
plenum 122 is positioned directly under the spray nozzle 110. The
vacuum plenum 122 extracts excess carrier fluid, such as water,
from the fiber cluster droplets. In other embodiments, a binder is
sprayed onto the translating fiber preform mat 118 as the fiber
preform mat 118 passes over the vacuum plenum 122 so that excess
carrier fluid is rapidly removed from the fiber preform mat
118.
[0037] In some embodiments, the translating filter belt 116
carrying the fiber preform mat 118 passes through a drying chamber
after receiving the fiber cluster droplets. The drying chamber
heats and dehumidifies the fiber preform mats. The heating and
dehumidification can be accomplished in either a continuous or a
discontinuous process. The heat and dehumidification removes
moisture, which permanently sets the binder. The filter material is
then stripped away from the fiber preform mat 118 to create a fiber
preform.
[0038] FIG. 2 is a schematic diagram of another embodiment of a
spray deposition apparatus 200 that manufactures fiber preforms
according to the present invention. The spray deposition apparatus
200 is identical to the spray deposition apparatus 100 that was
described in connection with FIG. 1 except that the vacuum plenum
122 is positioned a distance away from the spray area 120.
[0039] Placing the vacuum plenum 122 a distance away from the spray
deposition area 120 allows the fiber cluster droplets 112 to remain
on the fiber preform mat 118 for a predetermined time before the
excess carrier fluid is removed. The rate of translating the fiber
preform mat 118 and the distance between the vacuum plenum 122 and
the spray deposition area 120 are important process parameters. For
example, the average fiber angular inclination above the basal plan
can be controlled by adjusting these process parameters.
[0040] One embodiment of the present invention features a method of
forming a fiber preform mat. The parameters of the fiber preform
mats, such as the angle of inclination above the basal plane, can
be changed by varying processing parameters. The method includes
forming aqueous fiber slurry comprising milled graphite fibers in
suspension. In some embodiments, a binder is dissolved in the
aqueous fiber slurry. For example, the carrier fluid can include an
easily removable binder material, such as, PEG 8000, dissolved
therein. PEG-8000 is commercially available from the Dow Chemical
Company under the trade name Carbowax Sentry. The aqueous fiber
slurry is aspirated to produce a spray of fiber cluster
droplets.
[0041] A fiber preform mat 118 is translated in the path of the
fiber cluster droplets so that a randomized distribution of fibers
is deposited on the fiber preform mat 118. In some embodiments, the
fiber preform mat 118 is translated at a constant rate. In some
embodiments, a binder is also sprayed onto the fiber preform mat
118.
[0042] The milled fibers in the fiber cluster droplets tend to
flatten upon impact with the preform mat 118 and during excess
carrier fluid removal. The alignment of the milled graphite fibers
is specific to the particular volume of each droplet of the aqueous
fiber slurry. Each of the droplets sprayed onto the fiber preform
mat 118 contains a few fibers and is completely independent of the
other droplets. Thus, there is a substantially randomized
distribution of fibers deposited onto the fiber preform mat
118.
[0043] Excess carrier fluid is extracted from the fiber cluster
droplets. In some embodiments, the excess carrier fluid is
extracted by reducing the pressure proximate to the preform mat
118. In some embodiments, the excess carrier fluid is extracted by
increasing a temperature of the fiber preform mat 118 to remove
moisture from the fiber preform mat 118. In some embodiments, the
excess carrier fluid is extracted by dehumidifying the fiber
preform mat 118 to remove moisture from the fiber preform mat. Any
combination of reducing the pressure proximate to the preform mat
118, increasing the temperature of the fiber preform mat 118, and
dehumidifying the fiber preform mat 118 can be used to extract the
excess carrier fluid.
[0044] In some embodiments, the excess carrier fluid is extracted
simultaneously with the deposition of the randomized distribution
of fibers on the fiber preform mat 118. In other embodiments, the
excess carrier fluid is extracted at a predetermined time after the
deposition of the randomized distribution of fibers on the fiber
preform mat 118 so that the fiber preform mat 118 is exposed to the
binder for at least the predetermined time. Once the excess carrier
fluid is extracted, and the mats completely dry, the binder is set.
The fiber preform mats can then be handled by a technician. The
resulting fiber preform mat 118 contains a filter material, which
is usually stripped off before additional processing.
[0045] One embodiment of the present invention features a method of
forming a fiber preform from the fiber preform mat 118. In this
embodiment, the filter material is stripped from the fiber preform
mat 118 to create a fiber preform. The resulting fiber preform is
cut into pieces having the desired dimensions, and these pieces are
then stacked into a heating/cooling die that is mounted into a
press. The stack is then pressed to the desired volume fraction to
obtain the fiber preform. In some embodiments, the temperature of
the stack is controlled during pressing. The term "volume fraction"
is defined herein as the volume fraction of carbon fiber assuming a
density of 2.15 g/cc.
[0046] Fibers initially randomized in three dimensions will flatten
during the pressing. In some embodiments, the stack is heated
during pressing by passing heated air through passages beneath the
top and bottom face of the die cavity. The die can be chilled after
pressing. For example, the die can be chilled using liquid nitrogen
that is piped-through passages beneath the top and bottom face of
the die cavity.
[0047] The resulting volume fraction of the fiber preform depends
upon the number of sheets and the desired final thickness of the
fiber preform. In one example, the fiber preform mat 118 was
sprayed with a binder containing graphite fiber slurry and dried to
result in a volume fraction of approximately 0.23. In order to
manufacture a 0.3 volume fraction fiber preform, which is known to
have a CTE of 7 ppm/K in Al, a number of mat layers need to be
stacked into a mold and pressed to approximately 0.23/0.3 times
their initial thickness.
[0048] Some physical properties of the fiber preform are determined
by the volume fraction. One skilled in the art will appreciate that
it is important to maintain a certain thickness uniformity in the
resulting fiber preform in order to achieve the desired volume
fraction uniformity throughout the entire preform and thus, the
desired uniformity in physical properties. In some embodiments, the
spray pattern of the nozzle 110 is adjusted in order to maintain
the desired uniformity of physical properties of the final
preform.
[0049] A metal matrix is pressure infiltrated into the pressed
fiber preform to form a composite preform. In some embodiments, at
least one of Al, Mg, and Cu alloy matrix material is pressure
infiltrated into the pressed fiber preform to form the composite
preform. In one embodiment, the pressure infiltration is performed
using the method described in U.S. Pat. No. 6,148,899, entitled
"Methods of High Throughput Pressure Infiltration Casting," which
is assigned to the assignee of the present application. The entire
description of U.S. Pat. No. 6,148,899 is incorporated herein by
reference.
[0050] Composite materials formed using the spray deposition
apparatus 100, 200 have a relatively low CTE for a given volume
fraction. The CTE of the resulting metal matrix composite can be
adjusted by varying parameters, such as the volume fraction of the
graphite fibers. The CTE of the resulting metal matrix composite
can be matched to various ceramic substrates. For example, when
pressurized to the desired volume fraction, the CTE of such
composite can be matched to various ceramic and semiconductor
materials, such as silicon, silicon carbide, aluminum oxide,
aluminum nitride, gallium nitride, gallium arsenide, and beryllium
oxide over a range of about 1 to 12 ppm/K.
[0051] Composite materials formed using the spray deposition
apparatus 1 00, 200 have discontinuous high modulus graphite fibers
arranged in-plane with a majority of fibers oriented substantially
off the XY plane. A portion of the fibers in the composite material
is oriented randomly off the XY plane. In some experiments using a
Cu alloy matrix, a 35% volume fraction was achieved for fiber mats
prior to pressing to the desired volume fraction manufactured
according to the present invention and the measured distribution in
the average angular orientation of the fibers in the Z direction
was in the range of about 20 to 30 degrees from the basal plane. In
some particular experiments, the average fiber orientation was
about 26 degrees from the basal plane.
[0052] In other experiments using an Al alloy matrix, a 30% volume
fraction was achieved. The measured distribution in the average
angular orientation of the fibers in the Z direction in fiber mats
manufactured according to the present invention prior to pressing
to the desired volume fraction was about 19 degrees. These lower
angular distributions were achieved by allowing the fiber cluster
droplets 112 to remain on the mat for a period of time prior to
removal of the carrier fluid. This allows the fibers to align more
nearly parallel to the basal plane.
[0053] In other experiments, the fiber cluster droplets 112 were
allowed to remain on the fiber preform mat 118 for a predetermined
time before the excess carrier fluid was removed as described in
connection with FIG. 2. In these experiments, an Al alloy matrix
with a 30% volume fraction was achieved and the measured
distribution in the average angular orientation of the fibers in
fiber mats manufactured according to the present invention prior to
pressing to the desired volume fraction was in the range of about
5-15 degrees.
[0054] The average fiber angular inclination above the basal plan
can be controlled by adjusting process parameters. For example, the
average fiber angular inclination above the basal plan can be
adjusted by controlling the rate of extracting excess carrier fluid
from the fiber cluster droplets. The average fiber angular
inclination above the basal plan can also be adjusted by
controlling the time between depositing the fiber cluster droplets
on the fiber preform mat 11 8 and the extraction of the excess
carrier fluid.
[0055] FIG. 3 illustrates a schematic diagram 300 of randomly
dispersed fibers of a composite material formed according to the
present invention. The fibers are oriented at various angles with
respect to the XY plane of the material. Metal matrix composite
materials manufactured according to the present invention exhibit a
horizontal projection of the fibers onto the XY plane that is
substantially random with no preferred orientation.
[0056] Through-plane thermal conductivity is improved by randomly
orienting a significant portion of fibers off the XY plane within a
certain range of fiber angles of inclination from the basal plan.
In many embodiments, the average fiber angles of inclination from
the basal plan for an orientation that is parallel to the XY plane
is in the range of about 5 to 45 degrees. In some specific
embodiments, the range is between about 15 and 40 degrees. In other
specific embodiments, the range is between about 20 and 30 degrees.
In yet other specific embodiments, the range is between about 24
and 28. Experiments have shown that composites formed with fibers
that are randomly dispersed with fiber angles of inclination from
the basal plan of about 26.6 degrees from the basal plane display
substantially isotropic thermal conductivity in x, y, and z
directions.
[0057] The methods and apparatus of the present invention can form
numerous types of preforms. There are an almost unlimited number of
parameters that can be used to form these preforms. Two examples of
fabricating preforms using the methods and apparatus of the present
invention are given below. These examples are given to illustrate
specific embodiments of the invention and should not be used to
limit the scope of the invention in any way. Specifically, these
examples should not be used to limit the invention to specific
process steps or process parameters.
[0058] In the first example, preforms were spray-deposited with
slurry containing 300-micron milled graphite fiber DKD and a PEG
8000 binder. An appropriate number of spray deposited performs are
loaded into a die. The die is then heated and pressed to a desired
thickness that produces the desired volume fraction, which is a 0.3
volume fraction in this example. The die is then clamped and
chilled using liquid nitrogen to set the binder. The preforms are
then removed and inserted into a confining mold cavity where they
are pressure infiltrated with an alloy containing 12.5% Si and 0.3%
Mg with the balance being A1 using the pressure infiltration
casting process described in US Pat. No. 6,148,899.
[0059] It has been previously shown that such composition
effectively prevent the formation of Al4C3 during the casting
process. See J. A. Cornie, et. al., "Pressure Infiltration
Processing of P-55 Graphite Fiber Reinforced Al Alloys, Proceedings
of the Symposium on Composites Processing, Microstructures and
Properties," 2nd International Ceramic Sciences and Technology
Congress of ACM, Orlando, Fla. Nov. 13, 1990 and Qoing Li and J.
Cornie, "Microstructure of the interface and interfiber regions in
P-55 reinforced Al alloys manufactured by pressure infiltration
casting, Paper D.2.," Controlled Interphase Structures (ICCI-3), H.
Ishida, ed., Elsevier Science Publications, 1990, both publications
are incorporated herein by reference. In addition to preventing
carbide formation, this alloy melts and solidifies over a narrow
temperature range because it is formed near eutectic in
composition. Therefore, the alloy is substantially immune from
gross macrosegregation during solidification of the casting.
[0060] Eight two-inch long by 1/4 inch by 1/4 inch specimens were
removed from the casting in orthogonal directions and tested for
thermal expansion. In addition, specimens were removed from the
panel to test the thermal conductivity both in-plane and
through-plane using flash diffusivity techniques. For comparison,
specimens were prepared using a dry mix method that is disclosed in
U.S. patent application Ser. No. 10/379,044, filed Mar. 4, 2003,
entitled "Discontinuous Carbon Fiber Reinforced Metal Matrix
Composite" and tested for thermal expansion and thermal
conductivity.
[0061] FIG. 4 presents experimental data for preforms manufactured
according to the first example with an infiltrated Al alloy matrix
and with fiber angles of inclination above the basal plane of the
original preform mat that are approximately 25 degrees with a
distribution of fiber angles of inclination above the basal plane
as shown in FIG. 7A. Preforms manufactured according to the first
example had high thermal conductivity in the through-plane (Z)
direction. In addition, the standard deviation of CTE measurement
for preforms manufactured according to the first example was very
low, less than one quarter of the standard deviation of the
measurement for preforms manufactured according the dry mix
process. The very low standard deviation of CTEs indicates that the
fibers were uniformly randomized and that a large population of
fibers has an appreciable component in the through-plane (Z)
direction.
[0062] In the second example, the conditions of the first
experiment were repeated using a nominal Cu-0.5% Cr matrix alloy.
The Cu matrix composite database is less complete, but sufficient
data exists to make the same type of comparisons. In this
experiment, preforms were spray-deposited with slurry containing
milled graphite fiber DKD and a PEG 8000 binder. An appropriate
number of spray deposited performs are loaded into a die. The die
is then heated and pressed to a desired thickness that produces the
desired volume fraction, which is a 0.3 volume fraction in this
example. The die is then clamped and chilled using liquid nitrogen
to set the binder. The preforms were inserted into a confining mold
cavity and pressure infiltrated with an alloy containing Cu-0.5% Cr
using the pressure infiltration casting process described in U.S.
Pat. No. 6,148,899. The fiber angle of inclination above the basal
plane for the preform material was about 25 degrees. Preforms
having materials with substantially lower angles of inclination can
be produced using the methods of the present invention.
[0063] For comparison, specimens were prepared using the Technical
Fibre Products available from Cumbria in the United Kingdom as
described herein. This method incorporates chopped CKD graphite
fibers with an average chop length of 25 mm into a paper product.
The chopped CKD fibers have a substantially random distribution
in-plane. The resulting preforms were tested for thermal expansion
and thermal conductivity and the data is presented in FIG. 4.
[0064] FIG. 5 presents experimental data for preforms manufactured
according to the second example infiltrated with a Cu alloy matrix
and with fiber angles of inclination above the basal plane of the
original preform mat that are approximately 25 degrees with a
distribution of fiber angles of inclination above the basal plane
as shown in FIG. 7A. Composites manufactured from such preforms
according to the second example also had high thermal conductivity
in the through-plane (Z) direction. The thermal conductivity in the
through-plane (Z) direction is nearly as high as the thermal
conductivity in the in-plane direction. In addition, preforms
manufactured according to the first example had significantly lower
standard deviation of CTEs than preforms manufactured using the
Technical Fibre Products process. The very low standard deviation
of CTEs indicates that the fibers were uniformly randomized and
that a large population of fibers has an appreciable component in
the through-plane (Z) direction. The fiber angle of inclination
above the basal plan for the preform material was about 25 degrees.
Preforms having materials with substantially lower angles of
inclination can be produced using the methods of the present
invention.
[0065] FIG. 6 is a comparison of experimental data and data
calculated using Shapery's equation for CTE, and Rule of Mixtures
calculations of thermal conductivity for Al-graphite composites
manufactured according to various embodiments of the present
invention with the distribution of fiber angles of inclination
above the basal plane shown in FIG. 7B. Experimental data is
presented for Al 413/graphite composites. The preforms were spray
deposited according to the present invention. Initially, the fiber
angle of inclination above the basal plane was about 19 degrees,
which is significantly lower than the approximately 25 degree angle
of inclination from the basal plane that was observed in the first
and second examples that are described herein. After pressing, the
average fiber angle of inclination above the basal plane was about
15 degrees for a volume fraction of 0.3.
[0066] Shapery's equation for thermal expansion was simplified to
assume continuous fibers. The Rule of Mixtures ("ROM") calculations
for thermal conductivity include the fiber angle of inclination
above the basal plane and changes of the angle of inclination as a
function of compression. However, the ROM calculations were
simplified to remove the effects of contact resistance at the
fiber-matrix interfaces and at the fiber end interfaces.
Consequently, the simplified ROM calculations over-estimate the
actual thermal conductivity. The ROM calculations for thermal
conductivity that include the fiber angle of inclination above the
basal plane and changes of the fiber angle of inclination above the
basal plan as a function of compression agree well with the
experimental data for preforms with the 19 degree angle of
inclination above the basal plane.
[0067] The CTE experimental data is considerably above the data
calculated using Shapery's equation. The relatively high
experimental CTE data may be caused by a large population of fibers
below the critical aspect ratio. Poor interface strength may also
contribute to the relatively high CTE experimental data. Other
experimental data for flat paper derived preforms and for flat
spray deposited material are in close agreement with data
calculated using Shapery's equation. The CTE experimental data also
indicates that the standard deviation for preforms manufactured
using the spray deposition methods of the present invention is
relatively low (about 0.5 standard deviation), which will lead to a
relatively low rejection rate.
[0068] The preforms were analyzed to determine why the transverse
thermal conductivity measurements where high relative to the
in-plane thermal conductivity measurements. Metallographic sections
of resin infiltrated preform mats were taken parallel to the basal
plane. These sections were polished and photographed in a scanning
electron microscope. The length and diameter of 70 fibers were
measured and the ratio of the minor-to-major axis of the ellipse
was calculated using arc-tangent calculations. Angular deviation
from the in plane section were then determined.
[0069] FIG. 7A illustrates a plot of the distribution of fiber
angles of inclination above the basal plane for measurements of 70
individual fibers in the original preform mats used to manufacture
Al- and Cu-graphite composites according to various embodiments of
the present invention and described in connection with FIGS. 4 and
5. The plot in FIG. 7A indicates that there is a wide distribution
of fiber angles of inclination above the basal plane. However, the
peak of the distribution is in the 20-30 degree range.
[0070] FIG. 7B illustrates a plot of the distribution of fiber
angles of inclination above the basal plane for measurements of 70
individual fibers in the original preform mats used to manufacture
Al-graphite composites according to various embodiments of the
present invention and described in connection with FIG. 6. The plot
in FIG. 7B also indicates that there is a wide distribution of
fiber angles of inclination above the basal plane. However, the
peak of the distribution is in the 10-20 degree range with an
average fiber angle of inclination from the basal plan of about 19
degrees.
EQUIVALENTS
[0071] While the present teachings are described in conjunction
with various embodiments and examples, it is not intended that the
present teachings be limited to such embodiments. On the contrary,
the present teachings encompass various alternatives, modifications
and equivalents, as will be appreciated by those of skill in the
art, may be made therein without departing from the spirit and
scope of the invention as defined by the appended claims.
* * * * *