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Electrothermal interface material enhancer

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Electrothermal interface material enhancer


Vertically oriented carbon nanotubes (CNT) arrays have been simultaneously synthesized at relatively low growth temperatures (i.e., <700° C.) on both sides of aluminum foil via plasma enhanced chemical vapor deposition. The resulting CNT arrays were highly dense, and the average CNT diameter in the arrays was approximately 10 nm, A CNT TIM that consist of CNT arrays directly and simultaneously synthesized on both sides of aluminum foil has been fabricated. The TIM is insertable and allows temperature sensitive and/or rough substrates to be interfaced by highly conductive and conformable CNT arrays. The use of metallic foil is economical and may prove favorable in manufacturing due to its wide use.

Browse recent Purdue Research Foundation patents - West Lafayette, IN, US
Inventors: Baratunde A. Cola, Timothy S. Fisher
USPTO Applicaton #: #20120276327 - Class: 428119 (USPTO) - 11/01/12 - Class 428 
Stock Material Or Miscellaneous Articles > Structurally Defined Web Or Sheet (e.g., Overall Dimension, Etc.) >Including Sheet Or Component Perpendicular To Plane Of Web Or Sheet

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The Patent Description & Claims data below is from USPTO Patent Application 20120276327, Electrothermal interface material enhancer.

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CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 60/829,753, filed Oct. 17, 2006, entitled ELECTROTHERMAL INTERFACE MATERIAL ENHANCER, incorporated herein by reference.

GOVERNMENT RIGHTS

The United States government may have rights to certain aspects of this invention as a result of funding from Air Force Research Lab Grant No. FA8750-04-D-2409.

FIELD OF THE INVENTION

This invention pertains to flexible structures having nanostructures attached to a surface, and in particular to deformable thermal and electrical interface materials using multiwalled carbon nanotubes.

BACKGROUND OF THE INVENTION

Electrical contacts are vital elements in many engineering systems and applications at the macro, micro, and nano scales. Reliability and functionality of electrical contacts can often be a limiting design factor. A major portion of electrical contact resistance comes from the lack of ideal mating between surfaces. Primary causes of this problem involve the mechanical properties of the surfaces and surface roughness. When two surfaces are brought together, the actual contact area may be much smaller than the apparent contact area. The contact between two surfaces can actually be thought of as the contact of several discrete points in parallel, referred to as solid spots or α-spots. Thus, only the α-spots act as conductive areas and can be a small percentage of the total area.

Since their discovery, carbon nanotubes (CNTs) have been studied intensively throughout many communities in science and engineering. Several researchers have reported on the mechanical, electrical, and thermal properties of individual single-wall carbon nanotubes (SWNTs). The electrical properties of SWNTs are affected by the chirality of the SWNTs to the degree that the SWNTs can exhibit metallic or semiconducting electrical conductivity. The electrical transport properties of a single SWNT are a well studied subject. It has been shown that for ballistic transport and perfect contacts, a SWNT has a theoretical resistance of 6.45 KΩ, which is half of the quantum resistance h/2e2. In MWCNTs, each layer within the MWCNT can have either a metallic or semi-conducting band structure depending on its diameter and chirality. Due to this variation among layers, the net electrical behavior of a MWCNT is typically metallic and a wide range of resistance values, e.g., from 478Ω to 29 KΩ, have been reported.

The use of an individual MWCNT may not be low enough to reduce contact resistance at an interface significantly. However, by using an array of MWCNTs as an interfacial layer, it is expected that numerous individual contact spots and contact area enlargement can create current flow paths through each contact, thus reducing overall resistance. An additional advantage to using CNTs is that they can tolerate high current densities. Therefore a MWCNT layer can be a potential solution to the reliability and functionality issues faced at electrical interfaces.

Various embodiments of the present invention present novel and nonobvious apparatus and methods for improved structural, electrical, and thermal interfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a photoacoustic (PA) test apparatus.

FIG. 1B is a schematic representation of a nanoparticle assembly according to one embodiment of the present invention.

FIG. 1C is a schematic representation of a nanoparticle assembly according to another embodiment of the present invention.

FIG. 2 is a comparison of contact resistance between a bare Cu—Cu Interface and a Cu-MWCNT-Cu Interface.

FIG. 3 depicts a classification of the Contact Surface.

FIG. 4a is a typical contact configuration of a bare Cu—Cu contact.

FIG. 4b shows a contact resistance reduction by parallel contacts created by MWCNTs according to one embodiment of the present invention.

FIG. 5 shows SEM images according to one embodiment of the present invention of a CNT array synthesized on a Si substrate on a silicon substrate. (a) A 30°-tilted plane, top view of the vertically oriented and dense CNT array. The array height is estimated to be 15 μm. The CNT array has a part across the top of the image that helps illustrate the uniformity of growth. (b) An image with higher magnification showing individual CNTs. CNT diameters range from 15-60 nm.

FIG. 6 shows SEM images according to one embodiment of the present invention of a CNT array synthesized on a Cu sheet according to one embodiment of the present invention. (a) Cross-section view of the vertically oriented and dense CNT array. The array height is estimated to be approximately 20 μm; the inset shows the CNT array grown on a 1 cm tall Cu bar. (b) An image with higher magnification showing individual CNTs. The CNT diameters range from 15-60 nm.

FIG. 7 is a schematic representation of a system for preparing apparatus according to one embodiment of the present invention.

FIG. 8 is a schematic representation of different analytical models of the inventive sample assemblies during PA measurement. (a) The CNT array is not considered a layer in the PA model, but rather as a contributor to the interface resistance between the Si wafer and the Ag foil, RSi—Ag. (b) The CNT array is considered a layer in the PA model; therefore, the component resistances, RSi—CNT and RCNT-Ag, and the thermal diffusivity of the CNT array can be estimated. (c) The CNT arrays are not considered as layers in the PA model, but rather as contributors to the interface resistance between the Si wafer and the Cu sheet, RSi—Cu. (d) The CNT arrays are considered as layers in the PA model; therefore, the component resistances, RSi-CNT, RCNT-CNT, and RCNT-Cu, and the thermal diffusivity of each CNT array can be estimated.

FIG. 9 show phase shift as a function of modulation frequency for CNT interfaces under 0.241 MPa of pressure. (a) Lumped one-sided interface fitting results. The mean-square deviation is 0.5° in phase shift. (b) Resolved one-sided interface fitting results. The mean-square deviation is 0.5° in phase shift. (c) Lumped two-sided interface fitting results. The mean-square deviation is 0.9° in phase shift. (d) Resolved two-sided interface fitting results. The mean-square deviation is 0.3° in phase shift. The two-sided fitting data is typical of measurements at each pressure.

FIG. 10 shows thermal resistance as a function of pressure for a two-sided CNT interface (RSi-CNT-CNT-Cu) measured with the PA method and the 1-D reference bar method according to one embodiment of the present invention.

FIG. 11 is a schematic representation of an apparatus according to one embodiment of the present invention.

FIG. 12 is a schematic representation of an apparatus according to another embodiment of the present invention.

FIG. 13 is a schematic representation of an apparatus and method according to another embodiment of the present invention.

FIG. 14 is a schematic representation of an apparatus and method according to another embodiment of the present invention.

FIG. 15 is a schematic representation of an apparatus and method according to another embodiment of the present invention.

FIG. 16 shows CNT arrays synthesized on both sides of a 10 μm thick CU foil according to another embodiment of the present invention. The density is ˜108 CNTs/mm2. Both CNT arrays are approximately 50 μm in height and the average CNT diameter is approximately 20 nm.

FIG. 17 Thermal resistances of bare foil interfaces, Rfoil and CNT/foil interfaces, RCNT/foil, as a function of contact pressure.

FIG. 18 Thermal circuit for the CNT/foil interface. The local resistances sum to give RCNT/foil.

FIG. 19 Thermal resistance between the two free surfaces of the samples. For the bare foil, the resistance is the same as Rfoil. For the CNT/Foil the resistance is the sum of the two free CNT tip interface resistances.

FIG. 20 CNT arrays synthesized on both sides of aluminum foil according to another embodiment of the present invention. The insert is a higher magnification SEM image that illustrates the CNT diameters in the array.

FIG. 21 Resistive network for the aluminum foil/CNT interface.

FIG. 22 is an exploded schematic representation of an apparatus according to another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

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.

The present invention pertains to nanoparticles that are deposited on at least one side of a flexible, easily deformable substrate. The substrate with attached nanoparticles can then be placed in contact with the interface of a device. The easily deformable substrate permits the substrate and nanoparticles to closely conform to irregularities on the surface of the object. By virtue of this intimate contact of the nanoparticles with the object, an interface is formed with improved properties due to the presence of the nanoparticles and an apparatus prepared according to various embodiments of the present invention include improvements in one, some, or all of the following properties: increased thermal conductance, reduced electrical resistance, absorption of electromagnetic radiation, increased efficiency in converting electromagnetic radiation to heat, and mechanical support. This list of properties provided by the nanoparticles is by way of example only, and is not an exhaustive list.

In one embodiment of the present invention, a plurality of thermally conductive nanoparticles are grown or otherwise adhered to a thin, readily deformable substrate, such as a flexible sheet of any solid material, including a foil of metal, and further including foil of noble metal. The side of the substrate or foil with nanoparticles is placed in contact with a heat source, such as a package containing an integrated circuit. Because of their small size and the easy deformation of the foil, the nanoparticles readily occupy many surface irregularities of the package. Thus, the heat transmitted through the wall of the package is more effectively spread into the foil. The heat transfer to the foil can be removed by convection or by phase change if a phase change material is placed in contact with the nanoparticles, or if another object is placed in contact with the foil, through conduction.

In another embodiment of the present invention, nanoparticles are placed on both sides of a substrate that is plastically deformable with small amounts of pressure.

In one embodiment, this member is placed inbetween a source of heat and a sink for heat, such as between an integrated circuit package and a finned heat exchanger. Since the member plastically deforms under light pressure, it readily adapts to irregularities on the adjacent surfaces of the integrated circuit package and finned heat exchanger. Further, the nanoparticles will fill in some surface voids and small irregularities and any adjacent surface. Therefore, heat is more effectively transferred out of the heat source and more effectively transferred into the heat sink.

In some embodiments, the nanoparticles are multiwalled carbon nanotubes (MWCNTs). Although an individual MWCNT has an electrical resistance measured in thousands of ohms, by arranging a high density of MWCNTs on surface of the member, the overall resistance is greatly reduced, since the MWCNTs act as resistances in parallel.

In yet other embodiments of the present invention, the MWCNTs are exposed to an electromagnetic field that preferentially aligns the MWCNTs during deposition and formation. In one embodiment, the MWCNTs are arranged such that the central axes of the tubes are substantially perpendicular to the surface to which they are attached. However, the present invention is not so limited and contemplates other directions of alignment.

In yet another embodiment, a plurality of nanoparticles is deposited on a thin, metallic, easily deformable substrate and used as a shield from electromagnetic interference (EMI). This member can be placed at the mating interface between electrical components or housings. As one example, an electrically conductive metallic foil having a plurality of vertically aligned MWCNTs on opposing sides is placed between a lid of an electronics housing and the base of the electronics housing. This foil easily conforms to irregularities in the adjoining surfaces, and both: (1) enhances the housing\'s blockage of external and internal EMI; and (2) reduces the electrical resistance between the lid and the base.

In yet another embodiment, the ability of MWCNTs to convert electromagnetic energy to heat is utilized to provide localized heating of a component subjected to an electromagnetic field. As one example, a member populated with MWCNTs can be placed at an interface where two thermosetting plastic materials come into contact. When the assembly of the plastic materials and nanopopulated member is subjected to a microwave field, the MWCNTs cause the thermoset joint to heat and fuse into a structural joint.

In one embodiment of the invention there is a product to be used to reduce the thermal (electrical) interface resistance between two connecting devices such as an electronic component and a heat sink (another electrical component). The invention includes a metal foil with dense carbon nanotube (CNT) arrays directly synthesized on the surface of both sides. Under moderate applied pressure, the metal foil deforms to the shape of the interface and the CNTs act to produce a plurality of thermally (electrically) conductive surface to surface contact spots which in effect increases the real contact area in the interface and reduces the resistance of the interface to heat conduction and electrical flow. The invention can also be used with existing commercial, wax-based phase change materials (PCM) to enhance the stability of the PCM in the interface and to produce increased thermal conduction through the PCM.



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stats Patent Info
Application #
US 20120276327 A1
Publish Date
11/01/2012
Document #
13466259
File Date
05/08/2012
USPTO Class
428119
Other USPTO Classes
165185, 428143, 1562722, 427331, 427569, 977773, 977742, 977890
International Class
/
Drawings
23



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