U.S. patent application number 12/055789 was filed with the patent office on 2010-01-07 for method for producing heterogeneous composites.
Invention is credited to Timothy D. Fornes, Nicolas D. Huffman.
Application Number | 20100001237 12/055789 |
Document ID | / |
Family ID | 39590270 |
Filed Date | 2010-01-07 |
United States Patent
Application |
20100001237 |
Kind Code |
A1 |
Fornes; Timothy D. ; et
al. |
January 7, 2010 |
METHOD FOR PRODUCING HETEROGENEOUS COMPOSITES
Abstract
A method for selecting materials and processing conditions to
prepare a heterogeneous structure in situ via the reaction of a
homogeneous mixture of a reactive organic compound and a filler,
which may then optionally be sintered. The method is employed to
provide a heterogeneous composite possessing exceptionally high
thermal and/or electrically conductivities for a given
concentration of conductive filler. The choice of materials as well
as processing conditions employed, as will be described below, have
a strong effect on the rate domain formation/heterogeneity of the
structure formed, the extent of filler particle-particle
interactions within filler-rich domains, and ultimately the thermal
and/or electrical conductivity. Proper choice of these conditions
can lead to composites having enhanced properties at a reduced bulk
filler concentration.
Inventors: |
Fornes; Timothy D.; (Apex,
NC) ; Huffman; Nicolas D.; (Raleigh, NC) |
Correspondence
Address: |
LORD CORPORATION;PATENT & LEGAL SERVICES
111 LORD DRIVE, P.O. Box 8012
CARY
NC
27512-8012
US
|
Family ID: |
39590270 |
Appl. No.: |
12/055789 |
Filed: |
March 26, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60896961 |
Mar 26, 2007 |
|
|
|
Current U.S.
Class: |
252/500 ;
524/773; 524/780; 524/781; 524/785 |
Current CPC
Class: |
H01L 23/49883 20130101;
H01L 2924/0002 20130101; B22F 2001/0066 20130101; B22F 1/0059
20130101; B22F 2998/00 20130101; B22F 2999/00 20130101; B22F 3/1025
20130101; H01L 2924/0002 20130101; B22F 2999/00 20130101; B22F
2207/01 20130101; H01L 2924/00 20130101; B22F 1/0055 20130101; B22F
2202/11 20130101; B22F 3/14 20130101; B22F 2998/00 20130101; B22F
3/1025 20130101 |
Class at
Publication: |
252/500 ;
524/780; 524/773; 524/781; 524/785 |
International
Class: |
C08K 3/08 20060101
C08K003/08; C08K 5/09 20060101 C08K005/09; H01B 1/20 20060101
H01B001/20 |
Claims
1. A method for producing a composite comprising: a) selecting a
reactive organic compound; b) selecting an inorganic filler
component; c) mixing the reactive organic compound and the
inorganic filler component, wherein at room temperature the organic
compound and the filler component mix to form a substantially
homogeneous structure having a bulk filler concentration; and, d)
reacting the organic compound to form a polymer; wherein the
polymer has a repulsive interaction with the inorganic filler
thereby creating, in situ, a heterogeneous structure comprising
filler rich domains.
2. The method of claim 1, wherein the concentration of the filler
is higher than that of a bulk filler concentration.
3. The method of claim 1, wherein the reactive organic compound
comprises at least one of monomers, oligomers, prepolymers, or
reactive polymers.
4. The method of claim 3, wherein the organic compound further
comprises a cure agent.
5. The method of claim 1, wherein the reaction of step d) is
advanced by heating the mixture.
6. The method of claim 1, wherein the reaction of step d) is
advanced by exposing the mixture to activating ultraviolet
radiation.
7. The method of claim 1, further comprising a second filler
component.
8. The method of claim 7, wherein the second filler component
resides substantially with the polymer after the organic compound
has been reacted.
9. The method of claim 1, wherein the inorganic filler component
comprises an inorganic filler coated with an organic coating.
10. The method of claim 9, wherein the organic coating on the
filler has an affinity for the reactive organic compound.
11. The method of claim 9, wherein the coating on the filler
comprises stearic acid.
12. The method of claim 9, wherein the coating on the filler is
present in a single layer as averaged over substantially all of the
filler.
13. The method of claim 9, wherein the organic coating on the
filler has a repulsive interaction with the new polymer formed from
the step of reacting the organic component.
14. The method of claim 9, wherein the coating on the filler
comprises a non-polar coating.
15. The method of claim 1, wherein the polymer formed during step
d) comprises polar moieties.
16. The method of claim 1, wherein the filler is thermally
conductive.
17. The method of claim 1, wherein the filler is electrically
conductive.
18. The method of claim 1, wherein the filler comprises solder.
19. The method of claim 1, wherein the filler comprises less than
75 percent by weight based on the total weight of the
composition.
20. The method of claim 1, wherein the filler comprises less than
50 percent by volume based on the total volume of the
composition.
21. The method of claim 1, wherein the filler comprises at metallic
filler of at least one of nickel, copper, silver, palladium,
platinum, gold, and alloys thereof.
22. The method of claim 1, wherein the filler comprises a cold
worked silver flake.
23. The method of claim 1, wherein the filler comprises a
sinterable filler.
24. The method of claim 23, further comprising the step of
sintering the filler particles together.
25. The method of claim 24, wherein the step of sintering the
filler particles together and the step of reacting the organic
compound are performed simultaneously.
26. The method of claim 23, wherein substantially all of the
sinterable filler particles that are in direct contact with one
another are sintered.
27. The method of claim 24, wherein the step of sintering is
performed at a temperature above approximately 100.degree. C.
28. The method of claim 24, wherein the step of sintering is
performed at a temperature of above approximately 150.degree.
C.
29. The method of claim 24, wherein the sintering step is enhanced
by an applied pressure on the composition.
30. The method of claim 24, wherein the degree of sintering is
regulated through selection of sintering temperature and
pressure.
31. The method of claim 1, wherein the mixture of the reactive
organic component and the filler is a solvent-free 100% solids
composition.
Description
CROSS REFERENCE
[0001] This application claims the benefit of, and incorporates by
reference, U.S. Provisional Patent Application No. 60/896,961 filed
on Mar. 26, 2007 with the United States Patent and Trademark
Office.
FIELD OF THE INVENTION
[0002] The present invention relates to a method for creating
heterogeneous polymer-filler composites in situ via a reaction of a
homogeneous mixture of a filler and a reactive organic compound.
The heterogeneous structure comprises highly filler-rich areas
whose concentration is greater than that of the bulk filler
concentration.
BACKGROUND OF THE INVENTION
[0003] Electrically conductive polymer composites often consist of
electrically insulating polymers filled with electrically
conductive fillers. Such fillers often consist of metal- or
carbon-based fillers often in the form of flake or fibers. In order
to make the composite conductive, the fillers are added to the
point a critical filler concentration is reached at which the
composite changes from electrically insulating to electrically
conducting. This concentration, termed the percolation threshold,
is associated with a continuous electrical pathway formed from the
touching or "percolation" of conductive filler particles. Beyond
this threshold, the electrical conductivity can be further improved
by addition of filler to the polymer matrix. The ultimate
conductivity beyond the threshold will depend on the type of filler
used and the maximum obtainable filler loading before tradeoffs in
other composite properties becomes unacceptable from an application
standpoint
[0004] Electrically conductive polymer composites are commonly used
for such applications requiring electromagnetic shielding,
electrostatic discharge, or high conductivity for device
interconnects or circuitry. The type of application will dictate
the ultimate conductivity needed which will dictate the type and
concentration of conductive fillers used in the composite material.
In many instances the level of filler required leads to undesirable
sacrifices in other important physical characteristics of the
composite, such as dispense viscosity, adhesion, impact strength,
among other things. In some instances, the cost of the filler is a
limiting factor, particular for such fillers as gold, silver, or
carbon fibers. It would be thus desirable to achieve high levels of
electrically conductivity with minimal loss in polymer
attributes.
[0005] In the area of thermally conductive applications, surface
mounting of electronic composites via interface adhesives is well
developed in automated package assembly systems. Such adhesives are
used in several approaches to provide lid attach, sink attach and
mainly thermal transfer from flip chip devices, as well as against
mechanical shock and vibration encountered in shipping and use. As
semiconductor devices operate at higher speeds and at lower line
widths, the thermal transfer properties of the adhesive are
critical to device operation. The thermal interface adhesive must
create an efficient thermal pathway between the die or lid and the
heat sink as the adhesive itself due to interface resistance and
bulk resistance is typically the most thermally resistant material
in the die-adhesive-lid-adhesive-sink or die-adhesive-sink
configuration. The thermal interface adhesive must also maintain
efficient thermal transfer properties through reliability testing
which simulates actual use conditions over the life of the device.
Moreover, a suitable adhesive must have certain fluid handling
characteristics, and exhibit specific adhesion, controlled
shrinkage, and low corrosivity in order to provide long term
defect-free service over the thermal operating range of the
electronic circuit package.
[0006] As with traditional electrical applications mentioned above,
interface adhesives having high bulk thermal conductivities are
often made by adding large levels of thermally conductive filler to
the reactive organic resin. In many instances, undesirable
increases in viscosity occur to the point handling (or dispensing)
becomes an issue which often limits the thermal conductivity that
can be achieved. To help overcome this issue, low molecular
species, such as non-reactive solvent, plasticizer or other liquid
viscosity reducing diluents are added to the formulation. However,
a downside to this approach, as seen in epoxy based formulations,
is these low molecular weight species cause shrinkage issues, void
formation, and delamination when the adhesive is cured.
[0007] Other approaches for obtaining high bulk thermal
conductivities have employed fillers that are known to sinter as
temperatures amenable to electrical devices processing
temperatures. For example, U.S. Pat. No. 7,083,850 entitled
"Electrically Conductive Thermal Interface" describes a porous,
flexible, resilient heat transfer material which comprises a
network of metal flakes. The material is made by forming a
conductive paste comprising a volatile organic solvent and
conductive metal flakes. The conductive paste is heated to a
temperature below the melting point of the metal flakes, thereby
evaporating the solvent and sintering the flakes only at their
edges. While highly thermally conductive, this material is quite
limited in its ability to adhere to surfaces and has an
intrinsically high modulus owing to pure filler remaining once the
solvent is removed. Moreover, most packaging processes prefer low
solver to solvent-less materials owing to the complexities and
environment concerns with removing solvent.
[0008] It would thus be desirable to sinter metal flakes within
interface material obtaining adhesive. Unfortunately, sintering in
not achieved at low to moderate fillers loadings. This limitation
is associated with the lack of direct filler particle contacts
required for filler to occur due to the matrix material that coats
them. It is only at very high volume percent filler that some
sintering occurs, but at such concentrations the unreacted adhesive
composition becomes extremely viscous and even solid-like and lacks
the desirable polymeric attributes such as good adhesion,
toughness, etc. It is this reason that existing approaches have
resorted to using considerable amounts of solvents to address the
viscosity issue which again has its downsides as mentioned
earlier.
[0009] To this end, it would be desirable to provide a solvent-free
(or low solvent) adhesive composition comprising a matrix polymer
and low to moderate levels of filler material which exhibits high
conductivity resulting from sintered filler, and also provides
adhesive properties while maintaining beneficial properties such as
dispensability, proper coefficient of thermal expansion, improved
toughness, shock and vibration resistance, environmental
protection, and the like.
[0010] It is further desirable to provide a homogeneous material in
the unreacted state comprising filler particles in a reactive
organic matrix whose properties and cure condition could be
controlled to generate a heterogeneous structure during curing and
whose final properties exhibit high levels of thermal and/or
electrical conductivity among other attributes.
SUMMARY OF THE INVENTION
[0011] This invention describes a method for selecting materials
and processing conditions to prepare a heterogeneous structure in
situ via the reaction of a homogeneous mixture of a reactive
organic compound and a filler, which may then optionally be
sintered. In a preferred embodiment of the present invention, the
method is employed to provide a heterogeneous composite possessing
exceptionally high thermal and/or electrically conductivities for a
given concentration of conductive filler. The choice of materials
as well as processing conditions employed, as will be described
below, have a strong effect on the rate domain
formation/heterogeneity of the structure formed, the extent of
filler particle-particle interactions within filler-rich domains,
and ultimately the thermal and/or electrical conductivity. Proper
choice of these conditions can lead to composites having enhanced
properties at a reduced bulk filler concentration.
[0012] One aspect of the present invention is lower electrical
percolation threshold concentrations and enhanced conductivities
beyond percolation can be achieved through the formation of a
heterogeneous composite from the reaction of a homogeneous mixture
of filler and organic compound. Through appropriate selection and
reaction of the organic compound, a heterogeneous structure can be
formed comprising filler rich domains that are greater in
concentration that of the bulk filler concentration. Such a
structure, as will be shown, ultimately allows for electrically
percolation to occur at significantly lower concentrations of
filler than what is required in final composites that comprise a
homogeneous dispersion of filler. Moreover, the structure when
formed from sinterable or metal fillers and cured under the
appropriate conditions is capable of forming a fused network of
filler thereby further improving electrical conductivity at
levels.
[0013] In a first aspect of the present invention, a method for
producing a composite is provided comprising a) selecting a
reactive organic compound, b) selecting an inorganic filler
component, c) mixing the reactive organic compound and the
inorganic filler component, wherein at room temperature the organic
compound and the filler component mix to form a substantially
homogeneous structure having a bulk filler concentration, and d)
reacting the organic compound to form a polymer, wherein the
polymer has a repulsive interaction with the inorganic filler
thereby creating, in situ, a heterogeneous structure comprising
filler rich domains.
[0014] In one embodiment of the present invention, the
concentration of the filler is higher than that of a bulk filler
concentration. In a further embodiment of the present invention,
the reactive organic compound comprises at least one of monomers,
oligomers, prepolymers, or reactive polymers. In a further
embodiment of the present invention, the organic compound further
comprises a cure agent. In another embodiment of the present
invention, the reaction of step d) is advanced by heating the
mixture. In an alternate embodiment of the present invention, the
reaction of step d) is advanced by exposing the mixture to
activating ultraviolet radiation.
[0015] In yet another embodiment of the present invention, the
composition further comprises a second filler component. In an
additional embodiment of the present invention, the second filler
component resides substantially with the polymer after the organic
compound has been reacted.
[0016] In a further embodiment of the present invention, the
inorganic filler component comprises an inorganic filler coated
with an organic coating. In another embodiment of the present
invention, the organic coating on the filler has an affinity for
the reactive organic compound. In a preferred embodiment of the
present invention, the coating on the filler comprises stearic
acid. In another embodiment of the present invention, the coating
on the filler is present in a single layer as averaged over
substantially all of the filler. In yet another embodiment of the
present invention, the coating on the filler has a repulsive
interaction with the new polymer formed from the step of reacting
the organic component. In a preferred embodiment of the present
invention, the coating on the filler comprises a non-polar coating
and the polymer formed during step d) comprises polar moieties.
[0017] In another embodiment of the present invention, the filler
is thermally conductive and/or electrically conductive. In an
additional embodiment of the present invention, the filler
comprises solder. In a further embodiment of the present invention,
the filler comprises less than 75 percent by weight based on the
total weight of the composition. In a still further embodiment of
the present invention, the filler comprises less than 50 percent by
volume based on the total volume of the composition. In one
embodiment of the present invention, the filler comprises at
metallic filler of at least one of nickel, copper, silver,
palladium, platinum, gold, and alloys thereof. In a further
embodiment of the present invention, the filler comprises a cold
worked silver flake.
[0018] In a further aspect of the present invention, the filler
comprises a sinterable filler. In one embodiment of the present
invention, the method further comprises the step of sintering the
filler particles together. In another embodiment of the present
invention, the step of sintering the filler particles together and
the step of reacting the organic compound are performed
simultaneously. In still another embodiment of the present
invention, substantially all of the sinterable filler particles
that are in direct contact with one another are sintered. In one
embodiment of the present invention, the step of sintering is
performed at a temperature above approximately 100.degree. C. In
another embodiment of the present invention, the step of sintering
is performed at a temperature of above approximately 150.degree. C.
In a still further embodiment of the present invention, the
sintering step is enhanced by an applied pressure on the
composition. In another embodiment of the present invention, the
degree of sintering is regulated through selection of sintering
temperature and pressure. In a still further embodiment of the
present invention, the mixture of the reactive organic component
and the filler is a solvent-free 100% solids composition.
[0019] In a further aspect of the present invention, a composite
material is provided comprising, an organic compound, and, an
inorganic filler, wherein the organic compound and inorganic filler
comprise a heterogeneous structure comprising filler-rich domains
wherein the filler concentration is higher than the bulk filler
concentration, and said filler rich domains were formed in situ
from a homogeneous mixture of the organic compound and the
inorganic filler.
[0020] In a still further aspect of the present invention, a method
for increasing the conductivity of a conductive composition is
provided comprising, selecting an inorganic filler which is at
least one of thermally conductive and electrically conductive,
selecting an organic material, selecting a desired filler amount,
mixing the conductive filler and organic material, wherein the
homogeneous mixture of filler and matrix material comprises a bulk
conductivity, and selecting cure conditions to create a
heterogeneous structure so as to provide a cured composition
comprising a conductivity that is higher than the bulk conductivity
of the mixture having a homogeneous structure.
[0021] In an additional aspect of the present invention, a
thermally conductive adhesive is provided comprising, a filler
coated with a non-polar coating, an organic component comprising
polar functionality, and wherein the thermal conductivity of the
cured adhesive is greater than 15 W/mK and the silver flake
concentration is less than 50% by volume. Unlike other methods for
producing filled polymeric systems, the methods of embodiments of
the present invention provide higher thermal and/or electrical
conductivity at significantly lower filler contents. This typically
leads to lower viscosities, better toughness, better adhesion, and
other enhanced physical properties all while achieving higher
conductivities that bulk filler loading would allow.
[0022] Additionally, the lower concentration of filler permits less
adsorbed moisture and air in the formulation brought in by the
filler allows for better wetting of a substrate, and enables for
application methods such as screen printing due to the newly
achieved low-viscosity 100% solids system, which provides the same
or better conductivity as prior art systems.
[0023] As will be realized by those of skill in the art, many
different embodiments of the methods for producing a heterogeneous
composite from the reaction of homogeneous organic compound-filler
mixture according to the present invention are possible. Additional
uses, objects, advantages, and novel features of the invention are
set forth in the detailed description that follows and will become
more apparent to those skilled in the art upon examination of the
following or by practice of the invention.
[0024] Thus, there has been outlined, rather broadly, the more
important features of the invention in order that the detailed
description that follows may be better understood and in order that
the present contribution to the art may be better appreciated.
There are, obviously, additional features of the invention that
will be described hereinafter and which will form the subject
matter of the claims appended hereto. In this respect, before
explaining several embodiments of the invention in detail, it is to
be understood that the invention is not limited in its application
to the details and construction and to the arrangement of the
components set forth in the following description or illustrated in
the drawings. The invention is capable of other embodiments and of
being practiced and carried out in various ways.
[0025] It is also to be understood that the phraseology and
terminology herein are for the purposes of description and should
not be regarded as limiting in any respect. Those skilled in the
art will appreciate the concepts upon which this disclosure is
based and that it may readily be utilized as the basis for
designating other structures, methods and systems for carrying out
the several purposes of this development. It is important that the
claims be regarded as including such equivalent constructions
insofar as they do not depart from the spirit and scope of the
present invention.
[0026] So that the manner in which the above-recited features,
advantages and objects of the invention, as well as others which
will become more apparent, are obtained and can be understood in
detail, a more particular description of the invention briefly
summarized above may be had by reference to the embodiment thereof
which is illustrated in the appended drawings, which drawings form
a part of the specification and wherein like characters of
reference designate like parts throughout the several views. It is
to be noted, however, that the appended drawings illustrate only
preferred and alternative embodiments of the invention and are,
therefore, not to be considered limiting of its scope, as the
invention may admit to additional equally effective
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 A scanning electron micrograph (SEM) photomicrograph
of a heterogeneous composite according to an embodiment of the
present invention. Note the sample because of its high electrical
conductivity required no gold sputter coating prior to SEM
imaging.
[0028] FIG. 2 A SEM photomicrograph of a homogeneous composite
according to conventional technologies. Note the sample because of
its limited electrical conductivity required gold sputter coating
prior to SEM imaging.
[0029] FIG. 3 A SEM photomicrograph of a homogeneous composite
according to conventional technologies. Note the sample because of
its limited electrical conductivity required gold sputter coating
prior to SEM imaging.
[0030] FIG. 4 A graph illustrating the effect of silver flake
concentration on the thermal conductivity of composites based on
homogeneous structure versus heterogeneous structure (derived in
situ).
[0031] FIG. 5 A graph illustrating the effect of silver flake
concentration on the electrical volume conductivity of composites
based on homogeneous structure versus heterogeneous structure
(derived in situ).
[0032] FIG. 6 A SEM photomicrograph of a heterogeneous composite in
an embodiment of the present invention containing 5 volume percent
silver flake. Note: this image was taken at one half the
magnification of FIGS. 1-3.
[0033] FIG. 7 A graph illustrating the effect of cure temperature
on the thermal conductivity of pure silver flake (squares--left
axis) and heterogeneous composites according to an embodiment of
the present invention (circles--right axis).
[0034] FIG. 8(a) A SEM photomicrograph of pure silver flake at
25.degree. C.
[0035] FIG. 8(b) A SEM photomicrograph of pure silver flake heated
at 200.degree. C.
[0036] FIG. 9 A graph illustrating the effect of cure temperature
on the volume electrical conductivity of heterogeneous composites
containing 33 volume percent silver flake according to an
embodiment of the present invention.
[0037] FIG. 10 A SEM photomicrograph showing a heterogeneous
structure generated during the Michael's addition reaction between
ethoxylated bisphenol A diacrylate and PAA cured at 200.degree. C.
(33 volume percent Ag flake filled) in an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] In a first embodiment of the present invention a method is
presented for producing a heterogeneous structure in situ via the
reaction of a homogeneous mixture of a filler and reactive organic
compound. The mechanism of structure formation is achieved through
the proper selection of component materials and adherence to
particular processing conditions. In one embodiment of the present
invention, the filler component comprises a conductive filler
(thermal, electrical or both) and the organic compound comprises a
monomer and optionally a curative agent. The formation of filler
rich domains during reaction of the organic material allows for
direct filler-to-filler particle contacts to be made. In the
presence of heat the particles may further sinter together.
Sintering eliminates the contact resistance between the previously
non-sintered filler particles thereby substantially improving the
thermal and/or electrically conductivity of the composite.
[0039] While not fully understood and not wishing to be bound by
this theory, it is believed that domain formation and sintering in
the composition are sensitive to the organic material's cure
temperature, the cure time, and the level of pressure applied
during the cure. In other words, domain formation and sintering are
kinetically driven processes. In a still a further embodiment, the
rate at which the sample is heated will affect the extent of domain
formation and sintering. The effects of cure time, temperature, and
pressure are outlined in the exemplary embodiments discussed
herein. In total, the processing conditions can be tailored to
achieve a conductive adhesive having the best combination of
properties at minimal filler loading, which often translates to
lower cost and opportunity to take advantage other properties that
are adversely affected by high filler loadings. In some cases, when
the adhesive is employed in an application that is not able to
withstand high sintering temperatures, higher pressures or
non-traditional sintering techniques may used to achieve
exceptionally high conductivities.
[0040] The filler component and reactive organic compounds are
chosen so as to create a homogeneous mixture when mixed. However,
during the cure, it is believed that the resulting polymer formed
from the organic compound then has a repulsive interaction with the
filler so as to form a heterogeneous compound having filler-rich
domains wherein the filler composition is significantly higher than
the bulk filler concentration. Thus, while the overall (bulk)
filler concentration of the compound does not change, the filler
particles and the organic component separate in situ into
respective regions of high concentration. This phenomenon can lead
to a network of interconnected filler particles formed in situ from
a mixture having very few, if any, initial filler-filler
contacts.
[0041] There are several approaches which may be employed to create
the repulsive interaction between the filler component and the
organic compound. However, in a preferred embodiment of the present
invention, this is achieved by coating a metallic filler particle
with a non-polar coating and mixing the coated filler in a reactive
organic compound comprising a relatively non-polar resin and a
polar curing agent. In an uncured state, the resin, curative, and
filler form a relatively homogeneous mixture in which the coated
filler and the resin are compatible with one another and form a
relatively homogeneous mixture. However, with the application of
heat the curing agent reacts with the resin forming a polymer
having polar moieties thereon, resulting in a repulsive interaction
between the non-polar coating on the filler and the polar moieties
on the polymer. This leads to the gross separation of the two
materials to create polymer-rich and filler-rich domains whose
respective concentrations are significantly higher than the bulk
concentrations of polymer and filler, respectively. Moreover,
extensive domain formation is capable of creating continuous
filler-rich domains with substantial particle to particle contact
between most of the filler particles.
[0042] Other types of interactions capable of creating repulsive
effects upon curing of the organic compound in the presence of the
filler, could consist of, but are not limited to, electrostatic
interactions, hydrogen bonding interaction, dipole-dipole
interactions, induced dipole interaction, hydrophobic-hydrophilic
interactions, van der Waals interactions, and metallic interactions
(as with an organometallic compound and metallic filler). Other
forms of repulsive interactions could arise from entropic related
effects such as molecular weight differences in the polymers formed
from the organic compound(s). Additionally, repulsive interactions
could arise as a result of an external stimulus such as electrical
field.
[0043] The domains formed upon curing of the organic compound in
the presence of the filler results in filler-rich domains having a
higher than bulk (average) filler concentrations and in organic
rich domains having lower than bulk (average) filler
concentrations. The areas of higher than average filler
concentration can form semi-continuous or continuous pathways of
conductive filler material extending throughout the body of the
cured composition. These pathways provide a low resistance route
through which electrons and/or thermal phonons can travel. In other
words, the pathways or channels allow for greatly enhanced thermal
or electrical conductivity. This conductive pathway may be further
enhanced by sintering the filler particles together.
[0044] Sintering, as it is understood in the art, is a surface
melting phenomenon in which particles are fused together at
temperatures below the material's bulk melting temperature. This
behavior is brought about by a tendency of the material to relax
into a lower energy state. As such, selection of filler type, size,
and shape can greatly affect the sinterability of the filler
particles. Certain particles, such as thin, wide, flat, plates are
often formed by shearing large particles via various milling
processes. This process imparts a large amount of internal stress
in addition to creating a large amount of surface area. When a
certain amount of heat is added to the particles, they will have
the tendency melt and fuse together thereby relieving the internal
strain and decreasing the overall surface energy of the particles.
For this reason, the preferred filler particles for use in the
present invention are those that comprise some degree of thermal or
electrical conductivity and sinter easily. In a still further
embodiment of the present invention, the preferred filler comprises
a metallic particle that has been subjected to cold working which
has imparted strain into the structure of the filler.
[0045] The sintering temperature will vary according to the
material chosen as the filler, as well as the geometry of the
filler particle. However, in a preferred embodiment of the present
invention, it is advantageous to balance the cure of the organic
compound and the sintering of the filler such that they occur
simultaneously. In this embodiment, the cure temperature and
profile is selected to coincide with the sintering temperature of
the filler, so as the organic compound becomes repulsive to the
filler and the filler particles are forced together, the individual
filler particles can sinter once particle to particle contact is
made. This is believed to be responsible for the continuous filler
structure seen throughout the fully cured composition. In a
preferred embodiment of the present invention, the cure temperature
is at least about 100.degree. C., more preferably about 150.degree.
C., and even more preferably above 150.degree. C. for a silver
flake filler.
[0046] In a preferred embodiment of the present invention, the
compositions are cured and optionally sintered via application of
heat to the composition. This is commonly accomplished in a cure
oven whereby hot air or radiated heat is used to increase the
temperature of the composition. In alternate embodiments of the
present invention, other methods of cure may be employed such as
induction curing in an electromagnetic field, microwave curing,
infrared curing, electron beam curing, and ultraviolet curing.
Additionally, the cure reaction may be self accelerated through the
use of an exothermic cure reaction. A non-thermal cure may be
desirable, for example, when the composition is coated on a
temperature sensitive substrate such as a plastic.
[0047] In one embodiment of the present invention the filler
comprises inorganic fillers. Available fillers include pure metals
such as aluminum, iron, cobalt, nickel, copper, zinc, palladium,
silver, cadmium, indium, tin, antimony, platinum, gold, titanium,
lead, and tungsten, metal oxides and ceramics such as aluminum
oxide, aluminum nitride, silicon nitride, boron nitride, silicon
carbide, zinc oxide. Carbon containing fillers could consist of
graphite, carbon black, carbon nanotubes, and carbon fibers.
Suitable fillers additionally comprise alloys and combinations of
the aforementioned fillers. Additional fillers include inorganic
oxide powders such as fused silica powder, alumina and titanium
oxides, and nitrates of aluminum, titanium, silicon, and tungsten.
The particulate materials include versions having particle
dimensions in the range of a few nanometers to tens of microns. In
a preferred embodiment of the present invention, the filler
comprises a material that is either thermally conductive,
electrically conductive or both. Although metals and metal alloys
are preferred for use in several embodiments of the present
invention, the filler may comprise a conductive sinterable
non-metallic material.
[0048] In an embodiment of the present invention, the filler
component must be able to interact with the organic compound to
impart a heterogeneous structure in the finished material. In a
preferred embodiment of the present invention as discussed above,
this is accomplished through the interaction of a polar organic
compound with a non-polar filler. For preferred filler materials,
such as metals, the filler is coated with a material comprising the
desired degree of polarity. In one preferred embodiment of the
present invention, the filler coating comprises a non-polar fatty
acid coating, such as stearic acid. In a still further embodiment
of the present invention, the filler coating comprises another
non-polar material, such as an alkane, paraffin, saturated or
unsaturated fatty acid, alkene, fatty esters, or waxy coating. In
additional embodiments of the present invention, non-polar coatings
comprise ogranotitanates with hydrophobic tails or silicon based
coatings such as silanes containing hydrophobic tails or functional
silicones.
[0049] In an alternate embodiment of the present invention, the
polarity of the filler/coating and polymer are reversed wherein the
filler/coating comprises a polar moiety and the organic compound
comprises a non-polar polymer. Similarly, in an embodiment of the
present invention, in which a repulsive effect other than polarity
is employed to drive the in situ domain formation, the active
properties of the filler and organic components may be
interchanged.
[0050] In a preferred embodiment of the present invention the
organic compound comprises an epoxy resin and a cure agent. In this
embodiment, the organic compound comprises from about 60 to about
100 volume percent of the total composition. In this embodiment,
the organic compound comprises approximately from 70 to 80 percent
by weight of a diglycidal ether of a bisphenol compound, such as
bisphenol F, and 20 to 30 percent by weight of a cure agent, such
as a polyamine anhydride adduct.
[0051] In additional embodiments of the present invention, suitable
organic compounds comprise polysiloxanes, phenolics, novolac
resins, polyacrylates, polyurethanes, polyimides, polyesters,
maleimide resins, cyanate esters, polyimides, polyureas,
cyanoacrylates, and combinations thereof. The cure chemistry would
be dependant on the polymer or resin utilized in the organic
compound. For example, a siloxane matrix can comprise an addition
reaction curable matrix, a condensation reaction curable matrix, a
peroxide reaction curable matrix, or a combination thereof.
Selection of the cure agent is dependant upon the selection of
filler component and processing conditions as outlined herein to
provide a heterogeneous structure formed in situ.
EXAMPLE 1
[0052] Silver flake coated with stearic acid was first added to a
reactive organic resin, namely diglycidal either of bisphenol F
(DGEBF), in a 100 gram Hauschild.RTM. mixing cup and thoroughly
mixed for a minimum of two cycles at 2200 rpm for 1 minute/cycle. A
second reactive organic compound, i.e. a curing agent, was then
added and mixed for a minimum of two cycles at 2200 rpm for 1
minute/cycle. The resulting material was cast between 19 mm thick,
Teflon coated aluminum plates separated with 1 mm glass slides.
Samples were cured with a convection oven using a programmed ramp
which consisted of heating the sample from room temperature to
160.degree. C. over the course of 40 minutes followed by an
isothermal hold for 1 hour.
[0053] Bulk thermal conductivity was measured via the Flash Method
(ASTM E1461). Test specimens were cut from the cured samples.
Samples were 12.7 mm in diameter and .about.1 mm in thickness. All
samples were spray-coated with a thin film of graphite to ensure
complete absorption of the incident radiation.
[0054] Volume electrical conductivity was measured as follows:
Uncured samples were made into strips .about.1 mm in thickness,
.about.40 mm in length, and .about.2 mm in width. Copper wire was
placed at the ends of the uncured composite prior to curing. The
ends of the wire were lightly sanded prior to insertion. The
samples were cured using the same heating profile described above.
The electrical resistance (or conductance) was measured using a
Keithley 580 Micro-ohmmeter. The volume conductivity was calculated
from the sample dimensions and the measured resistance.
[0055] The morphology of the cured composites was analyzed via
Scanning Electron Microscopy (SEM) using a FEI XL30 SEM set at an
operating voltage of 10 kV and a spot size of 3. Highly
electrically conductive samples required no gold sputter coating,
i.e. Example 1, whereas poorly conductive samples did, i.e. Control
1-A and 1-B. Gold was applied under an Argon atmosphere, a pressure
of .about.50-75 mTorr, and deposition time of 20-30 s using a
Denton Desk II sputter coater.
[0056] Table 1 lists three different composites ultimately
differing in structure and corresponding conductivity. Choice of
curative in combination with the other constituents (epoxy resin,
filler and filler coating) and curing conditions will dictate the
structure formed during curing and the resulting properties of the
composite. Example 1 in Table 1 and the corresponding SEM
photomicrograph shown in FIG. 1 illustrate that by using a
polyamine anhydride adduct (PAA) to cure DGEBF in the presence of
stearic acid coated silver flake, a heterogeneous composite can be
formed during cure and thereby result in exceptional thermal and
electrical conductivity.
[0057] In the uncured state, Example 1 consists of homogeneous
mixture of DGEBF, PAA, and stearic acid coated silver. During cure,
it is believed that the resin and curative react to form a polymer
that has a repulsive interaction between the newly-formed polymer
and the stearic acid coated silver flake. The extent of repulsion
is large enough that the polymer molecules would prefer to reside
with other polymer molecules rather than with the stearic acid
coated filler. Thus, the polymer diffuses to isolated domains
thereby creating a highly heterogeneous structure of polymer-rich
and silver-rich domains.
[0058] FIG. 1 is an SEM photomicrograph of the heterogeneous
morphology consisting of discrete polymer-rich domains (very light,
globular regions typically 5-20 microns in size) distributed
throughout a continuous silver-rich phase. (Note that the Example 1
sample in FIG. 1 was not coated with gold prior to SEM imaging.
Thus regions that are electrically insulating will be heavily
charged under the microscope and appear very bright and amorphous,
i.e. polymer rich domains. Regions that are highly conductive will
help dissipate the incident electron beam and appear less charged
and have finer detail, i.e. silver rich domain.) The silver rich
regions comprise silver flake particles with define edges and
shapes, unlike the polymer rich areas.
[0059] In contrast to Example 1, Control 1-A and Control 1-B
samples based on diethylentriamine (DETA) and an imidazole
curative, respectively, and consist of a cured composite that has a
homogeneous structure comprising an even distribution of silver
throughout the sample as seen in FIGS. 2 and 3. (Note; FIGS. 1-3
were all taken at the same magnification.) In this type of
homogeneous structure, the local concentration of silver is
comparable to that of the bulk concentration, which is not the case
in the heterogeneous composites (Example 1).
[0060] Ultimately, the differences in structure formed upon cure
lead to dramatic differences in thermal and electrical properties.
The heterogeneous structure of Example 1 results in approximately
22 times the thermal conductivity and .about.3-4 orders of
magnitude higher electrical conductivity than that of the
homogeneous structure of Control 1-A and 1-B samples.
TABLE-US-00001 TABLE 1 Effect of composite structure on the
morphology and properties of silver flake filled - diglycidal
either of bisphenol F (DGEBF) composites. Ingredient by wt % (vol
%) Example 1 Control 1-A Control 1-B DGEBF 13.4 18.2 17.8
PAA.sup.(a) 4.6 -- -- DETA.sup.(b) -- 2.2 -- Imidazole.sup.(c) --
-- 0.7 Silver flake.sup.(d) 82.0 79.6 81.5 (33 vol %) (30 vol %)
(33 vol %) Total 100 100 100 Cured Structure Heterogeneous
Homogeneous Homogeneous Thermal 22.3 0.92 1.2 Conductivity, k (W/m
K) Electrical Volume 3.8E+04 3.2E+00 4.6E+01 Conductivity, (S/cm)
.sup.(a)Epoxy curative - PAA--polyamine anhydride adduct formed
from the reaction of diethylene triamine (DETA) and pthalic
anhydride .sup.(b)Epoxy curative - DETA--diethylene triamine
.sup.(c)Epoxy curative - 1-(2-cyanoethyl)-2-ethyl-4-methylimidazole
(d)Stearic acid coated silver flake (surface area = 0.83 m.sup.2/g,
weight loss in air at 538.degree. C. = 0.35%)
EXAMPLE 2
[0061] The samples corresponding to data shown in Table 2 and FIGS.
4-6 were prepared according the description provided in Example 1
with the exceptions of select electrical volume conductivity
measurements. The resistance (or conductance) of each sample
dictated the choice of resistivity instrumentation. Samples having
resistances in excess of .about.10.sup.10 ohms were measured via
ASTM D-257 using a HP 4339B High Resistance Meter equipped with a
16008B resistance cell. Samples were in the form of circular disks
.about.1 mm in thickness and >60 mm in diameter. Samples having
resistances in the range of .about.10.sup.2-10.sup.10 ohms were
measured using a Keithley 610C Electrometer. Samples in this case
were in the form of well-defined strips. Uncured samples were cured
into strips 1 mm in thickness, .about.40 mm in length, and .about.2
mm in width. Copper wire was placed at the ends of the sample prior
to curing. The ends of the wire were lightly sanded prior to
insertion. The samples were cured using the same heating profile
described in Example 1. The volume conductivity was calculated from
the sample dimensions and the measured resistance. Samples having
electrical resistances below .about.10.sup.2 ohms were measured
according the procedure described in Example 1.
[0062] Table 2 and FIGS. 4 & 5 show the effect of stearic acid
coated silver flake concentration on the thermal and electrical
conductivity of cured composites possessing a heterogeneous
structure versus a homogeneous structure (structures previously
described in Example 1). The heterogeneous structure in this
example was formed from the reaction two organic components, namely
DGEBF (resin) and PAA (curative) in the presence of the stearic
acid coated silver flake. As seen in FIG. 4, the thermal
conductivity of the heterogeneous structure dramatically increases
with silver flake concentration. At about 33 volume percent silver
flake the thermal conductivity of the heterogeneous composite is
roughly 100 times that of the unfilled polymer.
[0063] In contrast, when the DETA curative is used a homogeneous
structure is formed and the thermal conductivity follows a much
more shallow response with respect to filler loading. At about 33
volume percent silver flake the thermal conductivity of the
homogeneous material is only roughly 4 times that of the unfilled
polymer.
[0064] Comparing the two systems at a fixed thermal conductivity of
1 W/mK, the heterogeneous composite requires only about 5 volume
percent silver flake whereas the homogeneous composite requires 7
times the concentration of silver, i.e. about 35 volume percent to
achieve the same thermal conductivity. As will be discussed in
Example 3, the exceptional thermal conductivity observed in the
heterogeneous composite is a result of the segregation of the
polymerized material from the silver particles which enables direct
particle-particle contacts and subsequent particle sintering.
[0065] In the case of electrical conductivity, the heterogeneous
structure enables electrical percolation, i.e. the point at which
the material abruptly changes from electrically insulating to
conducting, to be achieved at a fraction of the silver
concentration needed to do the same in the homogeneous system. This
observation is evident in the data shown in Table 2 and FIG. 5. The
percolation threshold is about 3 volume percent stearic acid coated
silver flake in the heterogeneous composite (based on DGEBF and
PAA) versus roughly 27 volume percent stearic acid coated silver
flake in the homogeneous composite (based on DGEBF and DETA). This
much lower threshold is a result of creation of continuous,
concentrated domains of silver upon curing. FIG. 6 shows such
morphological features observed at 5 volume percent Ag Flake, i.e.
just beyond the electrical percolation threshold.
[0066] In addition to electrical percolation, the ultimate
electrical conductivity is considerably higher at silver
concentrations beyond the percolation threshold. For example, the
concentration of coated silver flake needed to achieve a volume
conductivity of about 100 S/cm is roughly 4 volume percent for the
heterogeneous composite whereas over 40 volume percent silver flake
is required for the homogeneous system. As will be discussed in
Example 3, the exceptional thermal conductivity observed in the
heterogeneous composite is a result of the segregation of the
polymerized material from the silver particles which enables direct
particle-particle contacts and subsequent particle sintering.
TABLE-US-00002 TABLE 2 The effect of composite morphology on the
thermal and electrical conductivity as function of silver flake
concentration. Thermal Electrical Volume Vol % Silver Conductivity,
k Conductivity, Flake (W/m K) (S/cm) DETA Cured.sup.(a) 0 0.22
2.4E-15 5 0.31 8.4E-16 15 0.67 3.2E-14 20 0.69 1.4E-13 25 0.69
3.3E-14 30 0.92 3.2E+00 40 1.23 3.5E+01 PAA Cured.sup.(b) 0.0 0.23
9.1E-16 1.0 0.21 4.8E-16 2.5 0.32 5.2E-15 4.0 0.44 1.6E+02 5 1.07
5.0E+02 10 2.82 4.3E+03 15 7.94 1.1E+04 20 9.10 1.7E+04 33.1 22.3
3.8E+04 .sup.(a)DETA--diethylene triamine used to cure DEGBF resin.
.sup.(B)PAA--polyamine anhydride adduct used to cure DEGBF
resin.
EXAMPLE 3
[0067] DGEBF (resin), PAA (curative), and stearic acid coated
silver flake (filler) were mixed (uncured state) and characterized
(cured state) as outlined in Example 1. The samples were cured by
placing them in a preheated convection oven and curing them for 2
hours. Studies on as-received flake involved first pressing the
powder in to 1-3 mm thick, 12.5 mm diameter pellet using a KBr hand
press, set at a compressive force of approximately 0.5 Mg. The
pellets were heat treated under the same conditions at which the
composites were cured in the previous example.
[0068] Table 3 and FIG. 7 show how temperature dramatically affects
the thermal conductivity of both the pure Ag flake and composites
thereof. For both materials, the higher cure temperatures result in
higher conductivities. Interestingly, both sets of data possess the
same sigmoidal shape (see FIG. 7) with a small increase in
conductivity observed below 120.degree. C., followed by a steep
temperature increase in the vicinity of 160.degree. C., and then a
plateau effect at about 200.degree. C. and above. Similar to the
composite, a 14 fold increase in conductivity is observed between
the silver flake at room temperature versus the flake heat treated
at 200.degree. C. The dramatic increase both sets of samples is a
result of sintering of the silver flake. Sintering of the particles
eliminates the contact resistance between particles by creating a
continuous pathway through which thermal phonons (and electrons)
can travel.
[0069] FIG. 8 provides morphological evidence of the sintering of
silver in the pure silver flake and heterogeneous composite based
on the same flake. The unsintered silver flake as shown in FIG.
8(a) comprises plate-like particles with sharp, well-defined edges.
Heat treating the flakes to moderate temperatures, i.e. 200.degree.
C. in the case of FIG. 8(b), causes the flakes to sinter,
ultimately forming a stable interconnected structure. In the case
of the heterogeneous composite based on coated silver flake, DGEBF,
and PAA, sintering is enabled by the formation of the silver rich
domains. Extensive silver domain formation during curing causes
direct silver particle-to-particle contacts to occur. In the
presence of sufficient heat, these contacting particles will
sinter. This ultimately creates interconnected network of silver
particles thereby minimizing the resistance to heat transfer.
[0070] As with thermal conductivity, electrical conductivity of the
heterogeneous composite increases with cure temperature. The data
provided in Table 3 and displayed in FIG. 9 show approximately a
four fold improvement in electrical conductivity when curing the 33
volume percent coated silver/DGEBF/PAA sample at 200.degree. C. as
compared to 80.degree. C.
TABLE-US-00003 TABLE 3 Effect of cure temperature on the thermal
and/or electrical conductivity of pure Ag flake and Ag flake filled
DGEBF cured with PAA. Thermal Conductivity, k Electrical Volume
Sample Cure (W/m K) Conductivity, (S/cm) Temperature 100% Ag 33% Ag
Flake Filled 33% Ag Flake Filled (.degree. C.) Flake Composite
Composite 25 11.3 1.3.sup.(a) -- 80 -- 2.0 1.1E+04 120 37.9 8.6
2.1E+04 160 136.5 16.1 3.8E+04 200 154.0 24.8 4.4E+04 240 -- 26.0
.sup.(a)Uncured sample.
EXAMPLE 4
[0071] Steric acid coated silver flake was first added reactive
organic resin, namely DGEBF, in a 100 gram Hauschild.RTM. mixing
cup and thoroughly mixed for a minimum of two cycles at 2200 rpm
for 1 minute/cycle. A second reactive organic component, i.e. a
curing agent (PAA), was then added and mixed for a minimum of two
cycles at 2200 rpm for 1 minute/cycle. The resulting material was
cast between 19 mm thick, Teflon coated aluminum plates separated
with 1 mm glass slides as spacers. Samples were cured in a two
stage process: (1) samples were placed in convection oven and
heated room temp to 120.degree. C. (approximately 40.degree. C./min
ramp) and held at 120.degree. C. with a total heating time=2 hours
(2) post-cure samples were placed in a preheated compression mold
set at 200.degree. C. for 1 hr under various applied pressures. The
resulting thermal conductivities of the samples were measured via
the Flash Method.
TABLE-US-00004 TABLE 4 Effect of pressure on the thermal
conductivity of in situ derived, heterogeneous composites. Applied
Pressure Thermal Conductivity, k (psi) (W/m K) 0 23.4 500 29.8
[0072] It is well known in the metallurgical literature, that the
sintering of metal powders is facilitated by pressure. Thus, it was
interest to determine how pressure affects the thermal conductivity
of a heterogeneous composite comprised of sinterable fillers, like
that of silver. Table 4 shows that indeed exposing the composites
to temperature and pressure leads to higher conductivity.
EXAMPLE 5
[0073] Samples in Tables 5 and 6 were mixed, cured, and
characterized for thermal conductivity in an identical manner as
described in Example 1.
[0074] Selection of base filler chemistry is another parameter
influencing the ultimate conductivity observed in in situ derived
heterogeneous composites. Table 5 shows the three examples of
heterogeneous composites based on pure silver, silver coated
copper, and aluminum flake. Each of the fillers possesses a similar
stearic acid coating. Comparing the thermal conductivity of each
Example on an equal volume percent filler basis to that of the
homogeneous (conventional) composite containing silver flake gives
an indication of the extent of sample heterogeneity and level of
particle sintering.
[0075] Example 5-A exhibits over a three fold improvement in
thermal conductivity relative to the homogeneous system at a filler
concentration of 5 volume percent. Example 5-B, based on silver
coated copper, has a thermal conductive that is over two times that
of the homogeneous composites at filler loading 6 volume percent.
Lastly, Example 5-C based on aluminum flake, exhibits a thermal
conductivity about 79% higher than that of the homogeneous
composite. The lower conductivity of this heterogeneous, aluminum
filled composite relative to heterogeneous samples based on pure
silver is a reflection of the aluminum's inability to sinter.
Nevertheless, the aluminum-rich domains formed are still enough to
significantly increase thermal conductivity.
TABLE-US-00005 TABLE 5 Effect of filler type on the thermal
conductivity of heterogeneous composites Ingredient by Example
Example Example wt % (vol %) Control 5-A 5-B 5-C DGEBF 8.27x 50.2
49.1 51.1 PAA -- 17.1 16.7 17.4 DETA x -- -- -- Silver
Flake.sup.(a) 100-x 32.7 (5) -- -- Silver coated copper -- -- 34.2
(6) -- flake.sup.(b) Aluminum flake.sup.(c) -- -- -- 31.5 (16)
Total 100 100 100 100 Cured Structure Homo- Hetero- Hetero- Hetero-
geneous geneous geneous geneous Thermal Conductivity, See 1.07 0.83
1.07 k (W/m K) FIG. 4/Table Ratio of k to k.sub.control 1.0 3.2 2.3
1.79 at same vol % filler (See FIG. 4/Table 2).sup.(d)
.sup.(a)Stearic acid coating. .sup.(b)Silver amount = 20 wt %.
Fatty acid coating is present on the outer silver layer.
.sup.(c)Fatty acid coating. .sup.(d)Linear regression of the
homogeneous data in Table 2 and FIG. 4 results in k = 0.0241 *(vol
% Ag) + 0.211 with a goodness of fit R.sup.2 = 0.96. This trend
line provides a basis for comparison of thermal conductivity a
homogeneous system (control) versus that of heterogeneous systems
(Examples 5-A, B and C).
EXAMPLE 6
[0076] Sample in Tables 6 and 7 were mixed, cured, and
characterized in identical manner described in Example 1.
[0077] Table 6 shows the effect amount of filler coating on the
thermal conductivity of composites based on DGEBF (resin), PAA
(curative), and coated silver flake. The data is based on a common
silver flake, yet the amount of stearic acid coating on the flake
differs. As seen in Table 6, a low to medium coating level, i.e.
less than a few monolayers result in exceptional thermal
conductivity. However, an excessive coating amount leads to a
substantial reduction in thermal conductivity.
[0078] The chemistry of the filler coating will also affect the
level of heterogeneity formed during curing and the corresponding
composite's properties. Table 6 shows the influence of two
different silver coatings on the thermal conductivity of composites
based on DGEBF, PAA, and silver flake. The data is based on the
same base silver flake with equivalent amounts of filler coating.
The use of an ester acid coating on the silver results in lower
composite thermal conductivity than using a long chain unsaturated
fatty acid coating. However, the heterogeneous effect in the cured
composite is still evident.
TABLE-US-00006 TABLE 6 The effect of filler coating amount on the
thermal conductivity of heterogeneous composites containing fatty
acid coated silver flake filled (33 vol %). Coating Thermal Amount
Conductivity, k Example.sup.(a) (mg/m.sup.2).sup.(b) (W/m K) A-6
Low 25.6 B-6 Med 23.0 C-6 High 13.7 .sup.(a)Hetergeneous composites
were formed from the reaction DGEBF (resin) and PAA(curative) in
the presence of the silver flake. .sup.(b)The flakes differ only in
the amount of fatty acid coating.
TABLE-US-00007 TABLE 7 The effect of coating chemistry on the
thermal conductivity of silver flake filled (25 vol %) DGEBF
composites cured with PAA. Coating Type Filler surface Coating
Thermal on Silver area Amount Conductivity, k Example Flake
(m.sup.2/g) (mg/m.sup.2).sup.(a) (W/m K) A-7 Long chain 0.71 4.78
14.0 unsaturated fatty acid B-7 Ester acid 0.66 4.24 10.9
.sup.(a)Determined by the ratio of filler surface area to that of
the weight loss (mg organic/g silver) at 538.degree. C. in air.
EXAMPLE 7
[0079] Samples in Table 8 and FIG. 10 were mixed (uncured state)
and characterized as outlined in Example 1. Each uncured sample was
cast between 19 mm thick, Teflon coated aluminum plates separated
with 1 mm glass slides as spacers. The Control, Examples 7-A and
7-B were cured with a convection oven using a programmed ramp which
consisted of heating the sample from room temperature to
160.degree. C. over the course of 40 minutes followed by an
isothermal hold for 1 hour. Example 7-C was cured by heating the
sample in a convection oven from room temperature to 200.degree. C.
(heating rate of approximately 40.degree. C./min) and held at
200.degree. C. The total cure time was 3 hours.
[0080] The chemical structure of organic components is another
parameter affecting the extent heterogeneity in the structure
formed during cure and the properties of the resulting composite.
Table 8 provides examples of different types of resins (organic
components) that produce heterogeneous structures when cured with
PAA in the presence of stearic acid coated silver flake. A control
sample possessing a homogeneous morphology provides a basis for
reference. The thermal conductivity of heterogeneous examples
relative to that of the control give an indication of extent of
heterogeneity in composite structure upon curing. DGEBF when cured
with PAA in the presence of silver results in about a 22 fold
higher thermal conductivity relative to the homogeneous system.
Similarly, an epoxy novolac cured with PAA results in a 16 fold
difference in thermal conductivity relative to the homogeneous
containing identical level (volume basis) of silver flake. Lastly,
curing an ethoxylated bisphenol A diacrylate with PAA via a
Michael's addition reaction results in a composite having over 6
times higher conductivity than that of the control. The SEM
photomicrograph of Example 7-C shows that the heterogeneous
structured during cure. As with previously in Example 1, the
heterogeneous structure comprises discrete polymer rich domains
(very light, globular regions) distributed throughout a continuous
silver-rich phase.
TABLE-US-00008 TABLE 8 Heterogeneous structures from alternative
resins Ingredient by Example Example Example wt % (vol %) Control
7-A 7-B 7-C DGEBF 8.27x 13.4 -- -- Epoxy novalac -- -- 18.1 --
Ethoxylated -- -- -- 13.0 bishpenol A diacrylate PAA -- 4.6 6.9 4.8
DETA x -- -- -- Silver flake.sup.(a) 100-x 82.0 (33) 75.0 (25) 82.2
(33) Total 100 100 100 100 Cured Structure Homo- Hetero- Hetero-
Hetero- geneous geneous geneous geneous Thermal See 22.3 13.1 6.5
Conductivity, k FIG. 4 (W/m K) Ratio of k to k.sub.control 1.0
22.sup. 16.sup. 6.4 at same vol % Ag (See FIG. 4).sup.(b)
.sup.(a)Stearic acid coated silver flake (surface area = 0.83
m.sup.2/g, weight loss in air at 538.degree. C. = 0.35%)
.sup.(b)Linear regression of the homogeneous data in Table 2 and
FIG. 4 results in k = 0.0241 *(vol % Ag) + 0.211 with a goodness of
fit R.sup.2 = 0.96. This trend line provides a basis for comparison
of thermal conductivity a homogeneous system (control) versus that
of heterogeneous systems (Examples 7-A, B and C).
[0081] Although the present invention has been described with
reference to particular embodiments, it should be recognized that
these embodiments are merely illustrative of the principles of the
present invention. Those of ordinary skill in the art will
appreciate that the compositions, apparatus and methods of the
present invention may be constructed and implemented in other ways
and embodiments. Accordingly, the description herein should not be
read as limiting the present invention, as other embodiments also
fall within the scope of the present invention as defined by the
appended claims.
* * * * *