U.S. patent application number 13/938372 was filed with the patent office on 2013-11-07 for palladium thiolate bonding of carbon nanotubes.
The applicant listed for this patent is Baratunde A. Cola, Timothy S. Fisher, Stephen L. Hodson, Giridhar U. Kulkarni, Bhuvana Thiruvelu. Invention is credited to Baratunde A. Cola, Timothy S. Fisher, Stephen L. Hodson, Giridhar U. Kulkarni, Bhuvana Thiruvelu.
Application Number | 20130295288 13/938372 |
Document ID | / |
Family ID | 43497545 |
Filed Date | 2013-11-07 |
United States Patent
Application |
20130295288 |
Kind Code |
A1 |
Fisher; Timothy S. ; et
al. |
November 7, 2013 |
PALLADIUM THIOLATE BONDING OF CARBON NANOTUBES
Abstract
Carbon nanotube (CNT) arrays are attractive thermal interface
materials with high compliance and conductance that can remain
effective over a wide temperature range. Disclosed herein are CNT
interface structures in which free CNT ends are bonded using
palladium hexadecanethiolate Pd(SC.sub.16H.sub.35).sub.2 to an
opposing substrate (one-sided interface) or opposing CNT array
(two-sided interface) to enhance contact conductance while
maintaining a compliant joint. The palladium weld is mechanically
stable at high temperatures. A transient photoacoustic (PA) method
is used to measure the thermal resistance of the palladium bonded
CNT interfaces. The interfaces were bonded at moderate pressures
and then tested at 34 kPa using the PA technique. At an interface
temperature of approximately 250.degree. C., one-sided and
two-sided palladium bonded interfaces achieved thermal resistances
near 10 mm.sup.2 K/W and 5 mm.sup.2 K/W, respectively.
Inventors: |
Fisher; Timothy S.; (West
Lafayette, IN) ; Hodson; Stephen L.; (West Lafayette,
IN) ; Thiruvelu; Bhuvana; (Chennal, IN) ;
Kulkarni; Giridhar U.; (Bangalore, IN) ; Cola;
Baratunde A.; (Atlanta, GA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fisher; Timothy S.
Hodson; Stephen L.
Thiruvelu; Bhuvana
Kulkarni; Giridhar U.
Cola; Baratunde A. |
West Lafayette
West Lafayette
Chennal
Bangalore
Atlanta |
IN
IN
GA |
US
US
IN
IN
US |
|
|
Family ID: |
43497545 |
Appl. No.: |
13/938372 |
Filed: |
July 10, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12719759 |
Mar 8, 2010 |
8541058 |
|
|
13938372 |
|
|
|
|
61158187 |
Mar 6, 2009 |
|
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61234270 |
Aug 15, 2009 |
|
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Current U.S.
Class: |
427/337 ;
427/226 |
Current CPC
Class: |
B82Y 40/00 20130101;
B05D 3/108 20130101; B05D 3/0254 20130101; B82Y 10/00 20130101;
B82Y 30/00 20130101; C09K 5/14 20130101 |
Class at
Publication: |
427/337 ;
427/226 |
International
Class: |
B05D 3/02 20060101
B05D003/02; B05D 3/10 20060101 B05D003/10 |
Claims
1.-38. (canceled)
39. A method for fabricating a thermal interface, comprising:
providing a growth substrate; growing carbon nanotubes from the
growth substrate, each nanotube being anchored at one end to the
growth substrate; aligning the nanotubes in a direction generally
perpendicular to the growth substrate; increasing the number of
defect sites in the CNTs by growing carbon nanotubes by microwave
plasma enhanced chemical vapor deposition; and altering the density
of states of an energy carrier in the nanotubes; wherein said
increasing occurs prior to said altering.
40. The method of claim 39 wherein the energy carriers are
electrons or phonons.
41. The method of claim 39 wherein said altering is at the
interface of the nanotube and the substrate.
42. The method of claim 39 wherein said altering is by exposing the
carbon nanotubes to an electron-donating material.
43. The method of claim 42 wherein the electron-donating material
includes one of tetracyanoquinodimethane,
tetramethyltetrathiafulvalene, tetramethylselenafulvalenes, or
dimethylanthracene.
44. The method of claim 39 wherein said altering is by exposing the
carbon nanotubes to an electron-withdrawing material.
45. The method of claim 44 wherein the electron-withdrawing
material includes one of tricyanomethane, tetracyanoethylene,
tetracyanoquinodimethanide, nitrobenzene,
1,3,6,8-pyrenetetrasulfonic acid tetra sodium salt hydrate, or
9,10-dibromoanthracene.
46. The method of claim 39 wherein said altering is by exposing the
carbon nanotubes to one of phenylamine, o-toluidine,
2,4,6-trimethylaniline, anisidine, or 3-trifluoromethylaniline.
47. The method of claim 39 wherein said altering is by exposing the
carbon nanotubes to one of tetrathiafulvalene,
tetracyanoquinodimethane, tetramethyltetrathiafulvalene,
tetramethylselenafulvalenes, or dimethylanthracene.
48. The method of claim 39 wherein said altering is by exposing the
carbon nanotubes to one of tetracyanoethylene, tricyanomethane,
tetracyanoethylene, tetracyanoquinodimethanide, nitrobenzene,
1,3,6,8-pyrenetetrasulfonic acid tetra sodium salt hydrate, or
9,10-dibromoanthracene.
49. The method of claim 39 wherein said altering is by exposing the
carbon nanotubes to an aromatic amine.
50. The method of claim 39 wherein said altering is by exposing the
carbon nanotubes to an organosulfur compound.
51. The method of claim 39 wherein the substrate includes a layer
of at least one of Ti or Cr to promote adhesion of the carbon
nanotubes.
52. The method of claim 39 wherein the substrate includes material
chosen from the group of aluminum, platinum, gold, nickel, iron,
tin, lead, silver, titanium, indium, or copper.
53. The method of claim 39 wherein said growing is by microwave
plasma chemical vapor deposition.
54. The method of claim 39 wherein said growing is a density of
nanotubes greater than about 10.sup.7 nanotubes per square
millimeter.
55. The method of claim 39 wherein said growing is a density of
nanotubes less than about 10.sup.9 nanotubes per square
millimeter.
56. The method of claim 39 wherein said carbon nanotubes are
multi-walled.
57. The method of claim 39 further comprising: providing a contact
substrate; contacting carbon nanotubes to the contact substrate;
and increasing the density of states of the energy carrier by
creation of metallic-like bonds between the nanotubes and the
contact substrate.
58. The method of claim 57, wherein the contact substrate includes
a metal.
59. The method of claim 57, wherein the contact substrate includes
silver.
60. The method of claim 57, wherein the metallic-like bonds are
created by thermal treatment of a metal ethiolate.
61. The method of claim 57, wherein the metallic-like bonds are
created from metal nanoparticles.
62. The method of claim 57, wherein the metallic-like bonds are
created from palladium nanoparticles.
63. A method for fabricating a thermal interface, comprising:
providing a first growth substrate and a second growth substrate;
growing a first plurality of carbon nanotubes from a first surface
of the first growth substrate, each nanotube of the first plurality
being anchored at one end to the first surface; growing a second
plurality of carbon nanotubes from a second surface of the second
growth substrate, each nanotube of the second plurality being
anchored at one end to the second surface; placing the first
plurality of nanotubes in contact with the second plurality of
nanotubes with the first surface being opposite of the second
surface; increasing the number of defect sites in the CNTs by
growing carbon nanotubes by microwave plasma enhanced chemical
vapor deposition; and altering the density of states of an energy
carrier in the contacting nanotubes; wherein said increasing occurs
prior to said altering.
64. The method of claim 63 which further comprises pressing the
first surface against the second surface before said altering.
65. The method of claim 63 wherein altering the density of states
includes increasing the density of states.
66. The method of claim 63 wherein increasing the density of states
includes creation of metallic-like bonds between free ends of the
first plurality of nanotubes and the second plurality of nanotubes.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of priority to U.S.
Provisional Patent Application Ser. No. 61/158,187, filed Mar. 6,
2009, entitled MOLECULAR DOPING OF CARBON NANOTUBE THERMAL
INTERFACE MATERIALS, and U.S. Provisional Patent Application Ser.
No. 61/234,270, filed Aug. 15, 2009, entitled PALLADIUM THIOLATE
BONDING OF CARBON NANOTUBES, both of which are incorporated herein
by reference.
FIELD OF THE INVENTION
[0002] Various embodiments of the present invention pertain to
apparatus and methods for reducing the thermal or electrical
interface resistance between two devices that are in contact and,
more particularly, to improvements by using an interface including
carbon nanoparticles that have been altered with either
electron-donating molecules or electron-withdrawing molecules.
BACKGROUND OF THE INVENTION
[0003] As the size of electronic devices scales down and power
densities increase, the demand for innovative cooling solutions
becomes more imperative. Thermal interface materials (TIMs) such as
thermal greases and gels with highly conductive particle additives
are commonly used in microprocessor cooling solutions where
operating temperatures are near 100.degree. C. However, recent
reliability tests on polymeric TIMs using thermogravitic analysis
revealed a dramatic increase in thermal interface resistance as
operating temperatures and exposure times increased. Because of
their high thermal conductivity, mechanical compliance, and
stability over a wide temperature range, carbon nanotubes have been
extensively studied as conductive elements. Several recent reports
have shown that dense, vertically aligned CNT arrays are viable
alternatives to current state-of-the-art TIMs. However, when
contact sizes between a nanotube and an opposing surface become
comparable to the mean free path of the dominant energy carriers,
nanoscale constriction resistance becomes important. For CNT TIMs
similar to those in this study, the resistive component at the CNT
`free tip` and opposing metal substrate has been shown to cause the
largest constriction of heat flow in comparison to the bulk CNT and
growth substrate resistances [9]. Reduction of this `free tip`
constrictive resistance using novel CNT TIM composite structures is
shown in several embodiments of the inventions disclosed
herein.
[0004] Recent thermal resistance values for CNT based TIMs have
been measured to be between 1-20 mm.sup.2 K/W. The thermal
resistance values include both bonded and non-bonded interfaces,
and measurements were obtained using different characterization
techniques (1D reference bar, thermoreflectance, photoacoustic, and
3-omega). Weak bonding at heterogeneous interfaces, differences in
phonon dispersion and density of states, and wave constriction
effects are factors that could hinder further reduction in thermal
contact resistance. Adverse phonon constriction can be moderated by
increasing the interfacial contact area. In an effort to increase
the interfacial contact area, developments in bonded and
semi-bonded CNT TIMs have rendered thermal interface resistances as
low as 1.3 mm.sup.2 K/W and 2 mm.sup.2 K/W, respectively. CNTs
exhibit ballistic conduction of electrons in the outermost tubes
and ohmic current-voltage characteristics with certain metals. When
this effect is coupled with a strong metallic-like bond at the
CNT/metal substrate interface, phonon constriction could be
circumvented by using electrons as a secondary energy carrier. A
possible way to achieve electron transmission is through a strong
CNT/metal substrate bond and sufficiently high electron DOS at the
interface.
[0005] Silicon carbide (SiC), with a band gap near 3.3 eV, is
attractive for high-temperature power electronic applications such
as high-voltage switching for more efficient power distribution and
electric vehicles, powerful microwave electronics for radar and
cellular communications, and fuel efficient jet aircraft and
automobile engines. At high temperatures, phonon scattering with
charge carriers increases while the charge carrier mobility, which
dictates electrical conductivity, is adversely affected. In order
to maintain sufficient electrical conductivities for reliable
operation in such applications, innovative heat dissipation methods
that can withstand high temperature environments are necessary.
[0006] With regards to the performance of a Si/CNT interface with a
commercial phase change material (PCM) applied to a CNT array,
there is a decrease in thermal resistance of approximately 10
mm.sup.2 K/W between Si/PCM/Cu interfaces and Si/CNT/PCM/Cu
interfaces, which achieved a low value of approximately 5 mm.sup.2
K/W at 350 kPa. Also, there can be a 50% reduction in thermal
interface resistance by wicking paraffin wax into CNT arrays grown
on both sides of Cu foil. It is possible that such improvement is
the result of an increase in contact area and reduction in
constriction resistance at the `free tip` interface. However, PCMs
and paraffin wax suffer similar disadvantages as polymeric TIMs at
high temperatures. In contrast, thermal resistances near 10
mm.sup.2 K/W in a dry SiC/CNT/Ag interface can be achieved, with
the possibility of a weak dependence of thermal interface
resistance on temperatures up to 250.degree. C., indicating that
the TIM was suitable for high temperature applications.
[0007] What is needed are thermal interface materials and
construction methods that have lowered thermal interface resistance
and improved long term characteristics. Various embodiments of the
present invention do this in novel and unobvious ways.
SUMMARY OF THE INVENTION
[0008] Various embodiments of the present invention pertain to
altering the charge density of carbon nanotubes to add to the
effectiveness of CNT thermal interface materials.
[0009] While heat conduction in carbon nanotubes is commonly
dominated by phonon transport, some embodiments of the present
invention pertain to methods for opening channels for electron
transport to provide an alternative transport path that is
particularly useful at interfaces, where the longer phonon
wavelength and/or lower electron densities can dramatically impede
transport.
[0010] In some embodiments, CNT TIMs are enhanced with Pd
nanoparticles using a method for CNT synthesis and a process for
bonding interfaces using Pd hexadecanethiolate. Structures enhanced
with Pd nanoparticles exhibited improved thermal performance and
thermal interface resistances that are comparable to previously
reported values in the literature and that can outperform some
state-of-the-art TIMs used in industry.
[0011] Various embodiments of the present invention pertain to
thermal interface materials (TIMs) that have enhanced thermal
stability across a wide temperature range. Structures made
according to these various embodiments are suitable for a variety
of applications, particularly high temperature electronics.
[0012] One embodiment of the present invention pertains to a method
including providing a growth substrate and growing carbon nanotubes
from the growth substrate. Still further embodiments include
applying a solution containing a metal organic compound to the
nanotube array and thermally decomposing the solution around the
nanotubes and substrate.
[0013] Another embodiment of the present invention pertains to a
method including growing carbon nanotubes from the growth
substrate, each nanotube being anchored at one end to the growth
substrate, and altering the density of states of energy carriers in
the nanotubes.
[0014] Yet another embodiment of the present invention pertains to
a method for fabricating a thermal interface, including providing a
first growth substrate and a second substrate. Yet other
embodiments include growing a first plurality of carbon nanotubes
from a first surface of the first growth substrate, each nanotube
of the first plurality being anchored at one end to the first
surface. Still other embodiments include placing a second plurality
of carbon nanotubes on a second surface of the second growth
substrate. Still further embodiments include placing the first
plurality of nanotubes in contact with the second plurality of
nanotubes and applying a solution containing a metal organic
compound to the contacting nanotubes, and thermally decomposing the
solution.
[0015] Another embodiment of the present invention pertains to a
method for fabricating a thermal interface. Other embodiments
include providing a first growth substrate and a second growth
substrate. Yet other embodiments include growing a first plurality
of carbon nanotubes from a first surface of the first growth
substrate, and growing a second plurality of carbon nanotubes from
a second surface of the second growth substrate. Yet other
embodiments further include placing the first plurality of
nanotubes in contact with the second plurality of nanotubes with
the first surface being opposite of the second surface and altering
the density of states of an energy carrier in the contacting
nanotubes.
[0016] It will be appreciated that the various apparatus and
methods described in this summary section, as well as elsewhere in
this application, can be expressed as a large number of different
combinations and subcombinations. All such useful, novel, and
inventive combinations and subcombinations are contemplated herein,
it being recognized that the explicit expression of each of these
combinations is excessive and unnecessary.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic representation of a
Pd(SC.sub.16H.sub.35).sub.2 structure.
[0018] FIG. 2 are photographic representations showing CNT arrays
synthesized on Si substrate: (a) FESEM cross-section image
illustrating array height and (b) FESEM image illustrating CNT
diameter.
[0019] FIG. 3 is a photographic representation showing a
post-thermolysis FESEM image of CNT array on Si substrate according
to one embodiment of the present invention.
[0020] FIG. 4 are schematic representations of cross-sections of
various TIM structures according to various embodiments of the
present invention (a) Si/CNT/Ag and (b) Si/CNT/CNT/Cu.
[0021] FIG. 5 is a schematic representation of a photoacoustic
experimental setup.
[0022] FIG. 6 are graphical representations showing bulk thermal
interface resistance as a function of temperature: (a) Si/CNT/Ag
with and without Pd nanoparticles and (b) Si/CNT/CNT/Cu with and
without Pd nanoparticles.
[0023] FIG. 7 is a schematic representation of a CNT TIM enhanced
with an electron-donating or withdrawing species according to
another embodiment of the present invention.
[0024] FIG. 8 is an illustration according to another embodiment of
the present invention of enhanced thermal conduction by
electrons.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025] For the purposes of promoting an understanding of the
principles of the invention, reference will now be made to the
embodiments illustrated in the drawings and specific language will
be used to describe the same. It will nevertheless be understood
that no limitation of the scope of the invention is thereby
intended, such alterations and further modifications in the
illustrated device, and such further applications of the principles
of the invention as illustrated therein being contemplated as would
normally occur to one skilled in the art to which the invention
relates. At least one embodiment of the present invention will be
described and shown, and this application may show and/or describe
other embodiments of the present invention. It is understood that
any reference to "the invention" is a reference to an embodiment of
a family of inventions, with no single embodiment including an
apparatus, process, or composition that must be included in all
embodiments, unless otherwise stated.
[0026] The use of an N-series prefix for an element number (NXX.XX)
refers to an element that is the same as the non-prefixed element
(XX.XX), except as shown and described thereafter. As an example,
an element 1020.1 would be the same as element 20.1, except for
those different features of element 1020.1 shown and described.
Further, common elements and common features of related elements
are drawn in the same manner in different figures, and/or use the
same symbology in different figures. As such, it is not necessary
to describe the features of 1020.1 and 20.1 that are the same,
since these common features are apparent to a person of ordinary
skill in the related field of technology. Although various specific
quantities (spatial dimensions, temperatures, pressures, times,
force, resistance, current, voltage, concentrations, wavelengths,
frequencies, heat transfer coefficients, dimensionless parameters,
etc.) may be stated herein, such specific quantities are presented
as examples only. Further, with discussion pertaining to a specific
composition of matter, that description is by example only, and
does not limit the applicability of other species of that
composition, nor does it limit the applicability of other
compositions unrelated to the cited composition.
[0027] This application incorporates by reference U.S. patent
application Ser. No. 11/873,952, titled ELECTROTHERMAL INTERFACE
MATERIAL ENHANCER to inventors Fisher and Cola.
[0028] Some embodiments of the present invention utilize CNT TIMs
enhanced with palladium (Pd) nanoparticles to achieve low thermal
interface resistances suitable for electronics in a wide
temperature range. Several possible enhancements of CNTs with Pd
nanoparticles are disclosed herein. One enhancement is an increase
in contact area between the CNT `free tips` and an opposing metal
substrate that is formed from the Pd weld. This increase in contact
area mitigates the phonon bottleneck at the CNT/metal substrate
interface. Another concerns increasing electron density of states
(DOS) near the Fermi level at the CNT/metal substrate interface
that is a result of charge transfer between CNTs and Pd
nanoparticles. In some embodiments, it is possible that electrons
are used as a secondary energy carrier at the interface. One- and
two-sided interfaces, comprised of CNT arrays grown on Si
substrates, are bonded to opposing metal substrates using a new
method that utilizes the behavior of Pd hexadecanethiolate upon
thermolysis. Using a transient PA technique, bulk and component
thermal interface resistances of the Pdbonded CNT interfaces were
resolved.
[0029] In one embodiment, CNT TIMs enhanced with Pd nanoparticles
were fabricated using a new process for bonding interfaces using Pd
hexadecanethiolate. A transient photoacoustic technique was used to
resolve bulk and component thermal interface resistances.
Structures enhanced with Pd nanoparticles exhibited improved
thermal performance and thermal interface resistances that are
comparable to previously reported values in the literature and that
outperform most state-of-the-art TIMs used in industry. It is
possible that the majority of improved performance can be
attributed to the Pd weld that reduced phonon reflection at the
interface by increasing the contact area between the CNT
`free-tips` and an opposing metal substrate. In addition, some
embodiments contemplate utilizing electrons as a secondary energy
carrier at the interface because of an increase in electron density
of states at the CNT/Ag interface. In some embodiments, it is
possible that there is a dependence that electron transmission has
on wave vector conservation and disorder. With thermal stability
across a wide temperature range, these structures are suitable for
a variety of applications, particularly high-temperature
electronics.
[0030] Functionalizing CNTs with metal nanoparticles (Pt, Au, Pd,
Ag, Au) has been an area of growing interest for a diverse set of
applications. For example, a biosensor involving Au/Pd
nanocubeaugmented SWCNTs showed significant increases in glucose
sensing capabilities. The increased performance was attributed to a
highly sensitive surface area, low resistance pathway at the
nanocube-SWCNT interface, and selective enzyme adhesion, activity,
and electron transfer at the enzyme, Au/Pd nanocube interfaces.
Metal nanoparticles can adhere to CNTs through covalent or van der
Waals interactions, which can lead to charge transfer.
Single-walled CNTs interacting with Au and Pt nanoparticles can
exhibit an increase in the ratio of metallic to semiconducting
tubes. Charge density analysis showed a decrease in electron
density in the valance band of Au and an increase in the outer
orbitals of C, indicating direct charge transfer. There can also be
changes in the Raman G-band peak intensity for pristine and silver
nanoparticle-decorated metallic SWCNTs, indicating that the
nanoparticles alter the electronic transitions of the tubes. With
its high work function and strong adhesion to CNTs, Pd can be a
metal that electronically couples well to CNTs. Additionally,
efficient carrier injection from Pd monolayers to graphene can be
accomplished because of the band structure that results from the
hybridization between the d orbital of Pd and orbital of
graphene.
[0031] Metal alkanethiolates can serve as sources of metal clusters
upon thermolysis and yield either metal or metal sulfide
nanoparticles. While some metal alkanethiolates are insoluble in
most organic solvents, Pd alkanethiolates have been reported to be
soluble in these solvents and also exhibit repeated self-assembly.
The soluble nature of Pd alkanethiolates in such solvents makes
them attractive for forming smooth, thin films on substrates. Some
embodiments of the present invention use Pd hexadecanethiolate
(FIG. 1) to coat the CNT sidewalls with Pd nanoparticles.
[0032] Pd hexadecanethiolate has been patterned using electron beam
lithography and subsequent formation of Pd nanoparticles on
thermolysis was demonstrated. Energy-dispersive spectral (EDS)
values before and after thermolysis were 21:71:8 and 90:9.6:0.4 for
(Pd:C:S), respectively. Most notably, electrical measurements
yielded resistivity values of Pd nanoparticles that were similar to
that of bulk Pd. Although what is shown and described is the use of
a particular metal alkanethiolate, the present invention is not so
limited, and pertains to the use of compounds including other
metals, including iron, gold, and silver as examples. Further,
other embodiments include the use of butane thiolates, including
various metal butane thiolates, including palladium butane
thiolate.
[0033] In one embodiment of the present invention, an apparatus is
constructed as follows: an electron beam evaporative system was
used to deposit a tri-layer metal catalyst stack consisting of 30
nm Ti, 10 nm Al, and 3 nm Fe on polished intrinsic Si substrates.
For a two-sided interface, the tri-layer catalyst was deposited on
both a Si substrate and 25 .mu.m thick Cu foil purchased from Alfa
Aesar (Puratronic.RTM., 99.999% metals basis). Vertically oriented
CNT arrays of moderately high density were then synthesized in a
SEKI AX5200S microwave plasma chemical vapor deposition (MPCVD)
system. The growth chamber was evacuated to 1 Torr and purged with
N.sub.2 for 5 min. The samples were heated in N.sub.2 (30 sccm) to
a growth temperature of 900.degree. C. The N.sub.2 valve was then
closed and 50 sccm of H.sub.2 was introduced to maintain a pressure
of 10 Torr in the growth chamber. After the chamber pressure
stabilized, a 200 W plasma was ignited and 10 sccm of CH4 was
introduced to commence 10 minutes of CNT synthesis.
[0034] The samples were imaged using a Hitachi field-emission
scanning electron microscope (FESEM). FIG. 2 contains images of the
vertically oriented CNT arrays 40 synthesized on a substrate 30
(such as silicon). CNT arrays 40 grown on Cu foil are similar. The
array densities were approximately 10.sup.8-10.sup.9 CNTs/mm.sup.2.
The average CNT diameter for each array was approximately 30 nm
while the array heights were approximately 15-25 .mu.m.
[0035] For preparation of Pd hexadecanethiolate, an equimolar
solution of Pd(OAc).sub.2 (Sigma Aldrich) in toluene was added to
hexadecanethiol and stirred vigorously. Following the reaction, the
solution became viscous and the initial yellow color deepened to an
orange-yellow color. The hexadecanethiolate was washed with
methanol and acetonitrile to remove excess thiol and finally
dissolved in toluene to obtain a 200 mM solution. Using a
micropipette, approximately 16 .mu.L of Pd hexadecanethiolate was
added to the CNT array. The CNT array 40 was then heated for 5
minutes at 130.degree. C. to evaporate the toluene. Finally, the
sample was formed by sandwiching the structure under a pressure of
273 kPa and commencing thermolysis by baking at 250.degree. C. for
2 hours in air. In yet other embodiments, the CNT array has applied
to it solution including a metal acetate, thiol and toluene
[0036] Although a particular process for decomposing has been shown
and described, the present invention is not limited to these
parameters, which are by way of example only. Further examples
include, for those embodiments using a hexadecane thiolate, a range
of temperatures from about 230 C to about 300 C. In those
embodiments using a butane thiolate, another non-limiting range of
temperatures is from about 160 C to about 300 C. Further, although
thermolysis has been shown and described as being performed at a
pressure of 273 kPa, it is understood that in other embodiments any
range of pressure can be used, include 0 kPa applied pressure.
Further, although thermolysis was accomplished in air, it is also
understood that other embodiments are not so limited, and
contemplate atmospheres of nitrogen or high vacuum.
[0037] FIG. 3 contains an FESEM image of the CNT array 40 after
thermolysis at 250.degree. C. The Pd nanoparticles that decorate
the CNT walls range from approximately 1 to 10 nm. It is possible
that Pd nanoparticles 50 preferentially attach to defect sites in
the CNT sidewalls. Therefore, in some embodiments of the present
invention it is contemplated that additional defect sites may be
created in the CNTs prior to their exposure to Pd. The control
samples (no Pd hexadecanethiolate) were prepared under the same
heating and loading conditions as above.
[0038] Various embodiments of the present invention contemplate
thermal interfaces comprising a single layer of carbon nanotubes
attached to a substrate and having free tips in contact with a
second, opposing substrate. Yet other embodiments of the present
invention contemplate two substrates, each having carbon nanotubes
attached, in which the free tips of both sets of carbon nanotubes
are in contact with each other.
[0039] FIG. 4(a) shows a thermal interface comprising a substrate
30 (such as silicon) from which a forest of nanotubes 40 has been
grown. The nanotubes 40 are attached at one end to substrate 30.
The other end of the CNTs 40 is free, such that the length of the
nanotubes substantially extends away from substrate 30. A second
substrate 60 (such as silver, copper, gold, or any suitable
heat-conducting substrate) is placed opposite of the growth
substrate 30, such that the array of carbon nanotubes 40 is located
between the two substrates. The metal nanoparticles 50 (such as
palladium nanoparticles) are subsequently introduced into the
assembly 20 preferably by any of the methods described herein. As
is noted herein, the nanoparticles can both decrease the thermal
resistance from the free tip of the CNT 40 to the opposing
substrate 60 as a "weld," but also the nanoparticles 40 can bond
with more than one carbon nanotube 40, possibly through defect
sites.
[0040] In some embodiments, substrate 30 can be utilized as part of
a component with which it is desired to exchange heat. In some
embodiments, assembly 20 can be a separate component that is
applied to a thermal interface of a device such as an electronic
component.
[0041] FIG. 4(b) shows an assembly 120 in which carbon nanotubes
have been grown both from a first substrate 130, and also from a
second substrate 160. Each array of carbon nanotubes (array 140
grown from substrate 130; and array 170 grown from substrate 160)
are attached at one end to their respective substrate, with the
other, free tip end of the CNTs being located away from the
substrate. As shown in FIG. 4(b), assembly 120 is fabricated by
placing the free tips of array 140 in contact with the free tips of
array 170. After the two arrays of carbon nanotubes are placed into
contact as shown in FIG. 4(b), metal nanoparticles can be attached,
preferably by any of the methods shown herein. However, it is
understood that yet other embodiments can have metal nanoparticles
placed on these nanotubes by other means.
[0042] A transient photoacoustic (PA) technique was used to
characterize thermal interface resistances. FIG. 4 contains
cross-sectional sketches for each multilayer sample type tested,
and FIG. 5 shows the experimental setup. For a multilayer
structure, the PA technique can resolve both bulk and component
resistances in which the bulk resistance in FIG. 4a is defined
as
R.sub.bulk=R.sub.Si-CNT+R.sub.CNT+R.sub.CNT-Ag (1)
where RCNT is the resistance of the CNT array and RSi-CNT and
RCNT-Ag are the contact resistances at the Si-CNT and CNT-Ag
interfaces, respectively. Briefly, in a given PA measurement the
sample surface is surrounded by a sealed acoustic cell that is
pressurized with He gas at 34 kPa. The sample is then heated over a
range of frequencies by a 350 mW, modulated laser source. The
thermal response of the multilayer sample induces a transient
temperature field in the gas that is related to cell pressure. A
microphone housed in the chamber wall measures the phase shift of
the temperature-induced pressure response in the acoustic chamber.
Using the acoustic signal in conjunction with a thermal model
developed that is based on a set of one-dimensional heat conduction
equations, thermal interface resistances are determined using a
least-squares fitting method.
[0043] The PA technique was used to resolve bulk thermal interface
resistances of one- and two-sided TIMs with configurations of
Si/CNT/Ag and Si/CNT/CNT/Cu. The latter samples had CNT arrays
grown on both the Si and Cu substrates, and the resulting interface
formed a two-sided, Velcro.TM.-like structure (see FIG. 4b). In
addition, component resistances were resolved on a separate
Si/CNT/Ag sample to elucidate possible mechanisms for enhanced
performance. The samples used for measuring component resistances
were not identical to those used to measure overall resistance.
Specifically, a lower Pd thiolate concentration and bonding
pressure were employed.
[0044] In order to ensure proper operation of the pressure-field
microphone used in the PA setup, the maximum temperature tested was
250.degree. C., and the chamber pressure was limited to 34 kPa.
Bulk resistance measurements for the Si/CNT/Ag and i/CNT/CNT/Cu
samples were taken in a temperature range of 27.degree. C. to
250.degree. C. while the component resistance measurement on the
second Si/CNT/Ag sample was performed at 27.degree. C. FIG. 6 shows
bulk thermal resistance values as a function of temperature for the
Si/CNT/Ag and Si/CNT/CNT/Cu samples. The resolved component
resistances for the second Si/CNT/Ag are tabulated in Table 1.
TABLE-US-00001 TABLE 1 Component thermal resistances for Si/CNT/Ag
structure with and without Pd nanoparticles. R.sub.Si-CNT R.sub.CNT
R.sub.CNT-Ag Sample (mm.sup.2K/W) (mm.sup.2K/W) (mm.sup.2K/W)
Si/CNT/Ag 2 .+-. 1 <1 40 .+-. 4 Si/CNT/Ag + Pd <1 <1 15
.+-. 1
[0045] Within the temperature range, the Si/CNT/Ag and
Si/CNT/CNT/Cu structures decorated with Pd nanoparticles
outperformed the structures without Pd nanoparticles where the
average thermal resistance value for the Pd nanoparticle-enhanced
structures was 11 mm.sup.2 K/W and 5 mm.sup.2 K/W, respectively.
Averaging thermal resistances across the temperature range yielded
reductions of thermal resistance across the interface of
approximately 50% in both cases. In addition, all structures
exhibited only small variations in performance across the
temperature range, indicating thermal stability and applicability
to high-temperature devices.
[0046] The summary results in Table 1 indicate that reductions in
bulk thermal resistance between decorated and undecorated TIMs
occurred at the Si-CNT and CNT-Ag interfaces, with the latter
having the largest reduction. These results indicate that the
dominant thermal resistance was at the CNT `free tip` interface as
opposed to the growth substrate interface where the CNTs are well
adhered. This reduction at the CNT-Ag interface can be attributed
to two mechanisms. First, upon thermolysis, a strong bond between
at the CNT/Ag was created such that greater contact area was
achieved. It is possible that the bond results in the majority of
improvement to the reduced phonon reflection at the CNT/Ag
interface. In addition, the increase in contact area may reduce
phonon reflection at the boundary consisting of nano-sized contacts
and provided enhanced pathways for heat conduction.
[0047] Thermal treatment of Pd hexadecanethiolate at 230 C in air
can produce metallic Pd nanowires with a specific electrical
resistivity near 0.300 .mu..OMEGA.m. Thermal treatment of
structures in this study could have produced a metallic-like bond
between CNT free ends and Ag foil via Pd nanoparticles in which a
higher electron DOS near the Fermi level at the CNT/Ag interface
was established.
[0048] Two types of contacts can exist at a CNT/metal interface:
side- and end-contacted. Although the general orientation of the,
CNT arrays in FIG. 2a is vertical, it is possible that the majority
of the contacts have side-contacted geometries upon compression
into an interface. For non-bonded, side-contacted geometries, the
contact quality depends on tunneling of electrons across an energy
barrier created by van der Waals interaction at the metal/CNT
interface and since the physical separation between the metal and
CNT is comparable to the carbon/metal bond length, tunneling
depends on the chemical composition and configuration of electronic
states at the surface. If the Ag makes uniform contact to graphene
and then transmits an electron across the CNT/Ag interface, then
in-plane wave vector conservation is enforced and for good
coupling, the metal Fermi wave vector (k.sub.f,Ag=1.2.sup.-1)
should be comparable to that of graphene
(k.sub.f,graph=4.pi./3a.sub.o=1.70.sup.-1). Under weak coupling
assumption (i.e., van der Waals interaction), calculated
transmission probabilities at a uniform metal/graphene contact have
been shown to exhibit a monotonic increase with contact length
depending on CNT chirality.
[0049] For larger diameter tubes, such as the CNTs in some
embodiments of the present invention, wavevector conservation
becomes a consideration. However, such conservation principles can
be relaxed when disorder (defects and impurities) are present.
Plasma-enhanced chemical vapor deposition (PECVD) grown CNTs in
previous work have exhibited relatively high defects at the
sidewalls due to plasma etching. Thus, the additional disorder from
sidewall defects caused by PECVD synthesis and Pd impurities at the
CNT/Ag interface could relax wave vector conservation constraints.
In this case, additional scattering from defects and Pd impurities
could increase the transmission probability across the CNT/Ag
interface, mediated by the presence of the Pd nanoparticles. It is
possible that similar effects are operative for the two sided TIM
configuration (FIG. 4b), with most of the improvement localized at
the CNT/CNT and CNT/Cu interfaces.
[0050] Referring to FIGS. 7 and 8, another embodiment of the
present invention pertains to a method to reduce the thermal and/or
electrical interface resistance between two connecting devices such
as an electronic component and a heat sink or another electrical
component. The process includes synthesizing CNT arrays on various
substrates such as Si or Cu and subsequently applying electron
donating or accepting molecular species to the arrays. By altering
the charge density in the nanotubes via exposure to
electron-donating molecules such as aniline (C6H7N) and
tetrathiafulvalene (C6H4S4, TTF) as well as electron-withdrawing
molecules such as tetracyanoethylene (C6N4, TONE), the electronic
contribution to thermal conductivity at an interface can be
enhanced. This approach combined with mechanical conformability of
CNT arrays creates a material that is effective in conducting heat
(electricity) and providing low interface resistance (thermal or
electrical).
[0051] Yet other embodiments of the present invention pertain to an
apparatus to be inserted between a processor chip or an integrated
circuit (IC) device and a heat sink to allow the chips or IC
devices to operate at lower temperatures. Yet other embodiments
pertain to an apparatus to be inserted between an electrical device
and a connecting electrical device to allow electricity to pass
between the devices with minimal resistance.
[0052] An interface with a CNT array directly synthesized on one
side of the interface has been measured to have a thermal
resistance less than 8 mm.sup.2K/W, which is better than the
resistance of the state of the art commercial thermal interface
materials. An interface with a CNT array directly synthesized on
both sides of the interface has been measured to have a thermal
resistance of 4 mm.sup.2K/W, which is similar to the resistance of
a soldered joint. Park et al. studied the effects of a CNT layer on
electrical resistance between Cu substrates. For an apparent
contact area of 0.31 mm2, they reported an 80% reduction in
electrical resistance from a bare Cu/Cu interface to Cu/MWCNT/Cu
interface.
[0053] When contact sizes between a nanotube and another material
(e.g. CNT-Cu interface) are relatively small there is exists a
major impediment to heat flow at the true heterogenous material
interface in which ballistic phonon effects can substantially
decrease conductance. These effects include impedance mismatch and
wave constriction at nanoscale contacts. Some embodiments of the
present invention use an approach in which this phonon `bottleneck"
is circumvented by altering the charge density in the nanotubes via
exposure to electron-donating and withdrawing molecular species.
The process includes increasing the electronic contribution to
thermal conductivity at a CNT-substrate interface by chemically
opening electron channels for the transport of heat.
[0054] The thermal interface resistance of a CNT TIM with this
process is measured using a photoacoustic technique. At moderate
pressures, a 40% reduction in bulk thermal interface resistance
from the minimum to maximum aniline concentrations was observed.
The use of another electron-donating molecule, TTF, is expected to
provide much improved performance because of the increased
alteration of the charge density. These results indicate that the
enhanced thermal conduction by electrons, with their smaller
wavelengths, opens a new route for heat transfer and improves
thermal conductance, particularly between the free CNT tips and
opposing substrate. FIG. 7 shows a CNT TIM in application which has
undergone the process (not to scale). FIG. 8 illustrates the
enhanced thermal conduction by electrons in comparison to
phonons.
[0055] The CNT arrays can be synthesized using any CNT synthesis
technique that allows for dense, vertically oriented CNT arrays to
be directly grown on both sides or one side of substrates while
having strong mechanical adhesion between the CNTs and the surfaces
(e.g. microwave plasma chemical vapor deposition (MPCVD) and a Ti
underlayer on the substrates surfaces to promote adhesion).
Addition of the molecular species (aniline, TTF, TONE) can be
accomplished by dip coating the array in the chemical solution and
drying in vacuum. TTF and TONE can to be mixed with toluene prior
to dip coating the array and drying in vacuum.
[0056] In addition to the use of aniline (or phenylamine), the
present invention contemplates the use of other aromatic amines,
including as examples o-toluidine; 2,4,6-trimethylaniline;
anisidine; or 3-trifluoromethylaniline. Further, although the use
of TTF has been shown and described, it is understood that other
embodiments of the present invention contemplate the use of other
organosulfur compounds, including as examples
tetracyanoquinodimethane; tetramethyltetrathiafuvalene;
tetramethylselenafulvalenes; or dimethylanthracene. In addition,
although the use of TONE has been shown and described, although it
is recognized that other embodiments of the present invention
contemplate the use of other organic compounds such as
tricyanomethane; tetracyanoethylene; tetracyanoquinodimethanide;
nitrobenzene; etra sodium; 1,3,6,8-Pyrenetetrasulfonic acid; or
9,10-dibromoanthracene.
[0057] Further, some embodiments of the present invention pertain
to the use of electron-donating materials, some non-limiting
examples of which include tetracyanoquinodimethane;
tetramethyltetrathiafulvalene; tetramethylselenafulvalenes; or
dimethylanthracene. The use of an electron-withdrawing material has
been disclosed, and it is recognized that non-limiting list of
examples includes tricyanomethane; tetracyanoethylene;
tetracyanoquinodimethanide; nitrobenzene; etra sodium;
1,3,6,8-pyrenetetrasulfonic acid; or 9,10-dibromoanthracene.
[0058] FIG. 7 shows an assembly 220 similar to that of assembly 20
of FIG. 4(a). However, instead of attaching metal nanoparticles to
the carbon nanotubes 240, assembly 220 contemplates processing the
array of nanotubes 240 with either an electron-donating species or
an electron-withdrawing species. As discussed with regards to FIG.
4(a), the nanotubes 240 are grown from a substrate 230, and the
free tips of the nanotubes 240 are placed in contact with another
substrate 260. The process described herein for changing the charge
density of the carbon nanotubes is applied to this assembly and
provides molecules 250 of the donating or withdrawing species
attached to CNTs 250 at various locations along their length. The
charge density of the attached carbon nanotube is changed, and
therefore its heat conducting ability is enhanced. Yet other
embodiments of the present invention contemplate an assembly
related to FIG. 7, but also similar to that of FIG. 4(b). In these
embodiments there are two substrates from which CNTs have been
grown. The free tips of the two forests of CNTs are brought into
contact, and subsequently placed in solution with an
electron-donating species or an electron-withdrawing species.
[0059] While the inventions have been illustrated and described in
detail in the drawings and foregoing description, the same is to be
considered as illustrative and not restrictive in character, it
being understood that only the preferred embodiment has been shown
and described and that all changes and modifications that come
within the spirit of the invention are desired to be protected.
* * * * *