Vertically aligned arrays of carbon nanotubes formed on multilayer substrates

Abstract

Multilayer substrates for the growth and/or support of CNT arrays are provided. These multilayer substrates both promote the growth of dense vertically aligned CNT arrays and provide excellent adhesion between the CNTs and metal surfaces. Carbon nanotube arrays formed using multilayer substrates, which exhibit high thermal conductivity and excellent durability, are also provided. These arrays can be used as thermal interface materials.

Claims

1. A method of improving the transfer of heat from a heat source to a heat sink, comprising placing or affixing in between the heat source and the heat sink an array of carbon nanotubes formed on a multilayer substrate, the substrate comprising an inert support; an adhesion layer between about 10 nm and about 150 nm in thickness present on one or more surfaces of the support; an interface layer between about 5 nm and about 50 nm in thickness present on the adhesion layer; and a catalytic layer between about 10 nm and about 1 nm in thickness located on the interface layer; wherein the adhesion layer consists essentially of iron; wherein the interface layer consists essentially of aluminum or aluminum oxide; wherein the adhesion layer and the catalytic layer have the same chemical composition; and wherein the interface layer is formed on a plurality of metal oxide nanoparticles or aggregates; the catalytic layer is formed of a plurality of nanoparticles or aggregates deposited on the metal oxide nanoparticles or aggregates; and a plurality of vertically aligned carbon nanotubes are attached to the catalytic nanoparticles or aggregates.

2. The method of claim 1, wherein the inert support is a metal selected from the group consisting of aluminum, platinum, gold, nickel, iron, tin, lead, silver, titanium, indium, copper, or combinations thereof.

3. The method of claim 1, wherein the inert support is a metal alloy.

4. The method of claim 3, wherein the alloy is copper-tungsten pseudoalloy, diamond in copper-silver alloy matrix, or combinations thereof.

5. The method of claim 1, wherein the support is selected from the group consisting of silicon carbide in an aluminum matrix, beryllium oxide in beryllium matrix, or combinations thereof.

6. The method of claim 1, wherein the adhesion layer is between about 10 nm and about 100 nm in thickness.

7. The method of claim 1, wherein the interface layer is between about 7 nm and about 30 nm in thickness.

8. The method of claim 1, wherein the catalytic layer is between about 5 nm and about 1 nm in thickness.

9. The method of claim 1, wherein the adhesion layer is about 30 nm in thickness, the interface layer is about 10 nm in thickness, and the catalytic layer is about 3 nm in thickness.

10. The method of claim 1, wherein the nanotubes of the array are present at a density of between about 110.sup.7 to 110.sup.11 nanotubes per mm.sup.2 on the inert support.

11. The method of claim 1, wherein the nanotubes of the array are present at a density between about 110.sup.8 and 110.sup.10 nanotubes per mm.sup.2 on the inert support.

12. The method of claim 1, wherein the nanotubes of the array are present at a density between about 110.sup.9 and 110.sup.10 nanotubes per mm.sup.2 on the inert support.

13. The method of claim 1, wherein at least 90% of the carbon nanotubes remain on the surface of the array after sonication in ethanol.

14. The method of claim 1, further comprising one or more polymers absorbed to the distal ends of the carbon nanotubes.

15. The method of claim 14, wherein the one or more polymers are selected from the group consisting of poly(3-hexylthipohene), polystyrene, polyvinyl alcohol, poly(methyl methacrylate), polydimethylsiloxane, and blends thereof.

16. The method of claim 1, further comprising one or more metal nanoparticles absorbed to the distal ends of the carbon nanotubes of the array.

17. The method of claim 16, wherein the one or more metal nanoparticles comprise a metal selected from the group consisting of palladium, gold, silver, titanium, iron, nickel, copper, and combinations thereof.

18. The method of claim 1, further comprising a flowable or phase change material in the space between carbon nanotubes.

19. The method of claim 18, wherein the flowable or phase change material is selected from the group consisting of paraffin waxes, polyethylene waxes, liquid metals, oils, organic-inorganic eutectics, inorganic-inorganic eutectics, and blends thereof.

20. The method of claim 18, wherein the array of carbon nanotubes is placed or affixed in between an integrated circuit package and a finned heat exchanger.

21. The method of claim 1, wherein the morphology of the carbon nanotubes of the array are modified by evaporating a liquid in which the array was immersed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross section of a multilayer substrate for the formation and/or support of carbon nanotube arrays.

(2) FIG. 2 is a cross section of a carbon nanotube array formed by chemical vapor deposition on a multilayer substrate. For clarity, only a single nanotube, catalytic nanoparticle or aggregate, and metal oxide nanoparticle or aggregate are illustrated.

(3) FIG. 3 is a diagram showing the transfer printing of long carbon nanotubes onto an array of short carbon nanotubes.

(4) FIG. 4 is a schematic showing the change in morphology when a CNT array is immersed in a liquid. The SEM image shows the aggregation of CNTs into discrete islands due to the capillary action of the solvent evaporation.

(5) FIG. 5 is a schematic showing CNT arrays immersed in flowable of phase change materials.

(6) FIG. 6 is a diagram showing the distal ends of the CNT arrays on both sides of the aluminum foil coated with P3HT. The polymer-coated sample is adhered to gold-coated silver and quartz surfaces.

(7) FIG. 7 is a graph showing the measured voltage (V) as a function of current (Amperes) at 180 C. for CNT on copper, CNT on aluminum, and Grafoil.

DETAILED DESCRIPTION OF THE INVENTION

(8) I. Definitions

(9) Thermal Interface Material (TIM), as used herein, refers to a material or combination of materials that provide high thermal conductance and mechanical compliance between a heat source and heat sink or spreader to effectively conduct heat away from a heat source.

(10) Carbon Nanotube Array or CNT array, as used herein, refers to a plurality of carbon nanotubes which are vertically aligned on a surface of a material. Carbon nanotubes are said to be vertically aligned when they are substantially perpendicular to the surface on which they are supported or attached. Nanotubes are said to be substantially perpendicular when they are oriented on average within 30, 25, 20, 15, 10, or 5 degrees of the surface normal.

(11) II. Multilayer Substrates

(12) Multilayer substrates for the formation of carbon nanotube arrays promote the growth of dense vertically aligned CNT arrays and provide excellent adhesion between the CNTs and metal surfaces. Multilayer substrates also promote high CNT growth rates on metal surfaces. Multilayer substrates contain three or more metallic thin films deposited on the surface of an inert, preferably metallic support.

(13) An exemplary multilayer substrate (100) is shown in FIG. 1. The multilayer substrate contains three layers (an adhesion layer, 104; an interface layer, 106; and a catalytic layer, 108) deposited on the surface of an inert support (102).

(14) A. Supports

(15) A variety of materials can serve as a support for multilayer substrates. Generally, the support is inert, meaning that the support does not chemically participate in the formation of nanotubes on the multilayer substrate.

(16) Generally, the support is formed at least in part from a metal, such as aluminum, cobalt, chromium, zinc, tantalum, platinum, gold, nickel, iron, tin, lead, silver, titanium, indium, copper, or combinations thereof and/or one or more metal oxides, such as oxides of the metals listed above. Other materials include ceramics and silicon or silicon compounds, such as silicon dioxide.

(17) In some instances, the support is a readily deformable and/or flexible sheet of solid material. In certain embodiments, the support is a metallic foil, such as aluminum foil or copper foil.

(18) The support may also be a surface of a device, such as a conventional heat sink or heat spreader used in heat exchange applications. Such heat sinks may be formed from a variety of materials including copper, aluminum, copper-tungsten pseudoalloy, AlSiC (silicon carbide in an aluminum matrix), Dymalloy (diamond in copper-silver alloy matrix), and E-Material (beryllium oxide in beryllium matrix).

(19) In some embodiments, the surface of the support may be treated to increase adhesion with the adhesion layer. Such treatment may include the use of plasma-assisted or chemical-based surface cleaning. Another treatment would include the deposition of a metal or metal oxide coating or particles on the support.

(20) Multilayer substrates can be formed on one or more surfaces of a suitable support. For example, in certain embodiments, the support is a metallic foil. In these instances, multilayer substrates can be formed on one or both sides of the metallic foil as required for a particular application.

(21) The support, and conditions under which the CNTs are formed, should be selected such that the support resists reacting with the catalyst, process gases, and/or residual gases through reactions, such as oxidation, silicidation, alloying, and/or carbide formation. For example, C, O, H, and N are the elements composing most CNT CVD process and contamination gases. Under certain conditions, the support can react to form oxides, carbides, and other byproducts which significantly reduce CNT growth which in turn leads to loss of electrical conduction in the support. Reaction conditions, such as temperature, can be selected in order to minimize adverse reactions of the support.

(22) B. Adhesion Layers

(23) Adhesion layers are formed of a material that improves the adhesion of the interface layer to the support.

(24) In preferred embodiments, the adhesion layer is of the same chemical composition as the catalytic layer. In these cases, the adhesion layer may be designed, in combination with the interface layer, to reduce migration of the catalytic layer into the interface layer during nanoparticle synthesis.

(25) In some embodiments, the adhesion layer is iron or an iron alloy. In other embodiments, the adhesion layer is nickel or a nickel alloy. The adhesion layer may also be any transition metal, or alloy of that metal, that can also serve as CNT catalyst.

(26) In embodiments where the multilayer substrate is employed as a substrate for the growth of carbon nanotubes, the adhesion layer must be thick enough to remain as a continuous film at the elevated temperatures utilized to form CNTs. In certain cases, the adhesion layer may have a thickness of between about 10 nm and about 150 nm, more preferably between about 10 nm and about 100 nm, more preferably between about 10 nm and about 75 nm, most preferably between about 15 nm and about 50 nm. In certain embodiments, the adhesion layer has a thickness of about 30 nm.

(27) The adhesion layer should provide good resistance to oxide and carbide formation during CNT synthesis at elevated temperatures. In certain cases, the energy of oxide formation for the adhesion layer may be greater than 4.5 eV, more preferably greater than 3.5 eV, most preferably greater than 2.75 eV. In certain cases, the energy of carbide formation for the adhesion layer may be greater than 2.5 eV, more preferably greater than 1.5 eV, most preferably greater than 0.5 eV.

(28) C. Interface Layers

(29) In certain embodiments, the interface layer is formed from a metal which is oxidized under conditions of nanotube synthesis or during exposure to air after nanotube synthesis to form a suitable metal oxide. Examples of suitable materials include aluminum, titanium, gold, copper, silver, and tantalum.

(30) Alternatively, the interface layer may be formed from a metal oxide, such as aluminum oxide, silicon oxide, or titanium dioxide.

(31) In preferred embodiments, the interface layer is thin enough to allow the catalytic layer and the adhesion layer to diffuse across its thickness. In embodiments wherein the catalytic layer and the adhesion layer have the same composition, this reduces migration of the catalyst into the interface layer, improving the lifetime of the catalyst during nanotube growth.

(32) In certain embodiments, the interface layer has a thickness of between about 5 nm and about 50 nm, more preferably between about 7 nm and about 30 nm, most preferably between about 7 nm and about 15 nm. In certain embodiments, the interface layer has a thickness of about 10 nm.

(33) D. Catalytic Layer

(34) The catalytic layer is typically a thin film formed from a transition metal that can catalyze the formation of carbon nanotubes via chemical vapor deposition. Preferably, the catalytic layer is formed of a material that is resistant to oxidation and/or carbide formation under the chemical vapor deposition conditions used to form CNT arrays.

(35) Examples of suitable materials that can be used to form the catalytic layer include, but are not limited to, iron, nickel, cobalt, rhodium, palladium, osmium, iridium, platinum, and combinations thereof. In particular embodiments, the catalytic layer contains only materials that catalyze CNT formation, such as one or more transition metals, including those listed above. In other embodiments, the catalytic layer materials that catalyze CNT formation do not contain one or more non-catalytic materials. In preferred embodiments, the catalytic layer is formed of iron.

(36) The catalytic layer is of appropriate thickness to aggregate into small catalytic particles under annealing conditions. The catalytic layer typically has a thickness of less than about 10 nm. In preferred embodiments, the catalytic layer has a thickness of between about 10 nm and about 1 nm, more preferably between about 5 nm and about 1 nm, more preferably between about 2 nm and about 5 nm. In certain embodiments, the catalytic layer has a thickness of about 3 nm.

(37) E. Methods of Making

(38) Multilayer substrates can be formed using a variety of well-developed techniques for the deposition of metallic thin films. Non-limiting examples of such techniques include evaporation, sputter deposition, and chemical vapor deposition. In some embodiments, the multilayers are formed by sputter deposition and/or chemical vapor deposition, which can be easier to scale up.

(39) Evaporation can be used to deposit thin films of a variety of metals. The source material to be deposited (e.g., a metal) is evaporated in a vacuum. The vacuum allows vapor particles to travel directly to the target object (support), where they condense back into a solid state, forming a thin film on the target object. Methods of forming thin films using evaporation are well known in the art. See, for example, S. A. Campbell, Science and Engineering of Microelectronic Fabrication, 2.sup.nd Edition, Oxford University Press, New York (2001). Evaporation typically requires a high vacuum; however, it is applicable to a variety of metals, and can deposit metal at rates of up to 50 nm/s. If desired, masks can be used to pattern the metallic thin films on the target object.

(40) Metallic and metal oxide thin films can also be formed by chemical vapor deposition (CVD). Gas precursors containing the source material to be deposited by CVD (e.g., a metal or metal oxide) are feed into closed chamber. The chamber can be at atmospheric pressure or at various grades of vacuum. The chamber walls can be hot or a heated stage can be used with cold chamber walls to increase the deposition rate on the target object (support). Methods of forming thin films using CVD are well known in the art. See, for example, S. A. Campbell, Science and Engineering of Microelectronic Fabrication, 2.sup.nd Edition, Oxford University Press, New York (2001). CVD deposition of metals, such as iron, aluminum, and titanium, has been demonstrated, so has CVD deposition of oxides such as aluminum oxide and silicon oxide. CVD deposition rates can be as low as 1 nm/cycle.

(41) In one embodiment, electron-beam evaporation is used to form the multilayer structure on the support. Each layer is deposited at a pressure less than 0.001 mTorr. The adhesion layer is deposited at an evaporation rate of 0.3 nm/s. The interface and catalytic layers are each deposited at an evaporation rate of 0.1 nm/s.

(42) III. CNT Arrays

(43) CNT arrays contain a plurality of carbon nanotubes which are vertically aligned on the surface of a material. In some embodiments, the CNTs are vertically aligned on the multilayer substrate described above.

(44) In other embodiments, the CNT arrays are grown on the multilayer substrates described above by chemical vapor deposition. In these instances, the process of CNT growth alters the morphology of the multilayer substrate. Specifically, upon heating or exposure to air after growth, the interface layer is converted to a metal oxide, and forms a layer of metal oxide nanoparticles or aggregates deposited on the adhesion layer. The catalytic layer similarly forms a series of catalytic nanoparticles or aggregates deposited on the metal oxide nanoparticles or aggregates. During CNT growth, CNTs form from the catalytic nanoparticles or aggregates.

(45) The metal oxide nanoparticles or aggregates typically contain metal oxide formed from a metal used to form the interface layer. For example, in embodiments where the interface layer is formed from aluminum, the metal oxide nanoparticles or aggregates are formed from aluminum oxide. In embodiments where the interface layer is formed from a metal oxide, the metal oxide nanoparticles or aggregates may be composed of the metal oxide used to form the parent interface layer. The metal oxide nanoparticles or aggregates may further contain one or more metals which diffuse into the metal oxide nanoparticles or aggregated from the catalytic layer, adhesion layer, or combinations thereof. The catalytic nanoparticles or aggregates may be composed of the metal used to form the parent catalytic layer.

(46) The structure of a CNT array grown on the multilayer substrates described above (200) is shown in FIG. 2. These CNT arrays contain CNTs (210) anchored to an inert support, preferably a metal surface, (202) via an adhesion layer (204), metal oxide nanoparticles or aggregates (206), and catalytic nanoparticles or aggregates (208).

(47) Generally, the nanotubes are present at a sufficient density such that the nanotubes are self-supporting and adopt a substantially perpendicular orientation to the surface of the multilayer substrate. In some embodiments, the nanotubes are oriented, on average, within 30, 25, 20, 15, 10, or 5 degrees of the surface normal of a line drawn perpendicular to the surface of the support. Preferably, the nanotubes are spaced at optimal distances from one another and are of uniform height to minimize thermal transfer losses, thereby maximizing their collective thermal diffusivity.

(48) In some embodiments, the nanotube density on the substrate surface ranges from about 110.sup.7 to 110.sup.11 nanotubes per mm.sup.2, more preferably from about 110.sup.8 to 110.sup.10 nanotubes per mm.sup.2, most preferably from about 110.sup.9 to 110.sup.10 nanotubes per mm.sup.2.

(49) The CNTs display strong adhesion to the multilayer substrate. In certain embodiments, the CNT array will remain substantially intact after being immersed in a solvent, such as ethanol, and sonicated for a period of at least five minutes. Substantially intact as used herein, means that more than 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 99.9% of the CNTs remained on the surface after sonication, and there was less than 1% change in the thermal resistance of the CNT-multilayer support interface after sonication. In some embodiments, the thermal resistance of the CNT-support interface ranges from 1 to 0.1 mm.sup.2K/W, more preferred from 0.5 to 0.1 mm.sup.2K/W, most preferred from 0.25 to 0.1 mm.sup.2K/W.

(50) The adhesion of CNT arrays to the substrate can also be measured using industry standard die shear testing. In this test the free ends of the CNTs are affixed to another substrate, which is pushed with controlled force parallel to the substrate until the CNTs are torn from their interface with the multilayer support. In some embodiments, the die shear strength of the CNT-multilayer support interface ranges from 0.2 to 3 MPa, more preferably from 0.5 to 3 MPa, most preferably 1 to 3 MPa.

(51) In certain embodiments, one or more polymers are applied to the CNT array. One or more polymers may be adsorbed to the distal ends of the CNTs to bond the distal ends of the CNTs to a surface, reduce thermal resistance between the CNT array and a surface, or combinations thereof. Polymers can be applied to CNT arrays using a variety of methods known in the art. For example, polymers can be dissolved in a suitable solvent, and sprayed or spin coated onto the distal end of the CNTs. A representation is shown in FIG. 3.

(52) Examples of suitable polymers include conjugated and aromatric polymers, such as poly(3-hexylthipohene) (P3HT), polystyrene, and blends thereof. Other examples of suitable polymers that are neither conjugated nor aromatic include polyvinyl alcohol (PVA), poly(methyl methacrylate) (PMMA), polydimethylsiloxane (PDMS), and blends thereof.

(53) In certain embodiments, one or more metal nanoparticles are applied to the CNT array. One or more metal nanoparticles may be adsorbed to the distal ends of the CNTs to bond the distal ends of the CNTs to a surface, reduce thermal resistance between the CNT array and a surface, or combinations thereof. Metal nanoparticles can be applied to CNT arrays using a variety of methods known in the art. Suitable metal nanoparticles include, but are not limited to, palladium, gold, silver, titanium, iron, nickel, copper, and combinations thereof. For example, a solution of metal thiolate such as palladium hexadecanethiolate can be sprayed or spin coated onto the distal ends of the CNTs, and the organics can be baked off to leave palladium nanoparticles. In another example, electron-beam or sputter deposition can be used to coat metal nanoparticles or connected film-like assemblies of nanoparticles onto the distal ends of the CNTs.

(54) In certain embodiments, one or more polymers are applied together with one or more metal nanoparticles to the CNT array. The polymers and metal nanoparticles are both adsorbed to the distal ends of the CNTs to bond the distal ends of the CNTs to a surface, reduce thermal resistance between the CNT array and a surface, or combinations thereof. The polymers and metal nanoparticles can be applied together using a variety of methods known in the art. For example, a solution of metal thiolate such as palladium hexadecanethiolate can be sprayed or spin coated onto the distal ends of the CNTs, and the organics can be baked off to leave palladium nanoparticles. Then, polymers can be dissolved in a suitable solvent, and sprayed or spin coated onto the distal ends of the CNTs that were coated in the previous step with metal nanoparticles.

(55) In certain embodiments, flowable or phase change materials are applied to the CNT array. Flowable or phase change materials may be added to the CNT array to displace the air between CNTs and improve contact between the distal ends of CNTs and a surface, and as a result reduce thermal resistance of the array and the contact between the array and a surface, or combinations thereof. Flowable or phase change materials can be applied to CNT arrays using a variety of methods known in the art. For example, flowable or phase change materials in their liquid state can be wicked into a CNT array by placing the array in partial or full contact with the liquid. A representation is shown in FIG. 4.

(56) Examples of suitable flowable or phase change materials include paraffin waxes, polyethylene waxes, hydrocarbon-based waxes in general, and blends thereof. Other examples of suitable flowable or phase change materials that are neither wax nor polymeric include liquid metals, oils, organic-inorganic and inorganic-inorganic eutectics, and blends thereof.

(57) In certain embodiments, a liquid is added to the CNT array and then evaporated to alter the morphology of the array. Capillary forces that result from liquid evaporation can draw CNTs together into patterns, which facilitate the addition of flowable or phase change materials to the array, and/or pull additional CNTs in contact with a surface, and as a result reduce thermal resistance of the contact between the array and a surface, or combinations thereof. Capillary-driven altering of CNT arrays can be accomplished using a variety of methods known in the art. For example, solvent can be applied to the CNT array and the array can be placed in an interface in the wet state and allowed to dry, activating the capillary forces that ultimately drive CNTs into contact with the surface. In another example, the CNT array soaked with solvent can be allowed to dry free from surface contact to form patterns in the array. A representation is shown in FIG. 5.

(58) Examples of suitable liquids that can be evaporated from CNT arrays to change their morphology include solvents such as toluene, isopropanol, and chloroform, and any other liquid that wets the CNT arrays sufficiently to penetrate their entire depth.

(59) A. Carbon Nanotubes

(60) The CNT arrays contain nanotubes which are continuous from the top of the array (i.e., the surface formed by the distal end of the carbon nanotubes when vertically aligned on the multilayer substrate) to bottom of the array (i.e., the surface of the multilayer substrate). The array may be formed from multi-wall carbon nanotubes (MWNTs), which generally refers to nanotubes having between approximately 4 and approximately 10 walls. The array may also be formed from few-wall nanotubes (FWNTs), which generally refers to nanotubes containing approximately 1-3 walls. FWNTs include single-wall carbon nanotubes (SWNTs), double-wall carbon nanotubes (DWNTS), and triple-wall carbon nanotubes (TWNTs). In certain embodiments, the nanotubes are MWNTs. In some embodiments, the diameter of MWNTs in the arrays ranges from 10 to 40 nm, more preferably 15 to 30 nm, most preferably about 20 nm. The length of MWNTs in the arrays can range from 1 to 5,000 micrometers, preferably 5 to 5000 micrometers, preferably 5 to 2500 micrometers, more preferably 5 to 2000 micrometers, more preferably 5 to 1000 micrometers.

(61) B. Methods of Forming CNT Arrays

(62) In preferred embodiments, the CNTs are grown on the multilayer substrate using chemical vapor deposition.

(63) Generally, CNT formation begins by annealing the multilayer substrate. A suitable carbon source gas is then introduced, and the temperature is increased to the growth temperature.

(64) The multilayer substrate is generally annealed for a short period of time, for example for approximately ten minutes. Typically, the multilayer substrate is annealed under flow of an inert gas, such as nitrogen or argon. In certain embodiments, the annealing temperature is between about 500 C. and about 650 C., more preferably between about 500 C. and about 600 C., most preferably between about 525 C. and about 575 C.

(65) In preferred embodiments, the CNTs are grown on the multilayer substrate at a growth temperature that is less than the melting temperature of aluminum (approximately 660 C.). In certain embodiments, the CNTs are grown on the multilayer substrate at a growth temperature of between about 600 C. and about 660 C., more preferably between about 610 C. and about 650 C., most preferably between about 620 C. and about 640 C. In certain embodiments, the CNTs are grown on the multilayer substrate at a growth temperature of about 630 C.

(66) Any suitable carbon source gas may be used. In some embodiments, the carbon source gas is acetylene. Other suitable carbon source gases include ethene, ethylene, methane, n-hexane, alcohols, xylenes, metal catalyst gases (e.g., carbonyl iron), and combinations thereof. In some embodiments, the source gas is a metal catalyst gas, which can be used with or without the catalyst layer.

(67) In other embodiments, arrays of vertically aligned CNTs are fabricated on another surface, and transferred, using methods known in the art, to the distal ends of CNTs on the multilayer substrate. For example, a CNT array that is 5 micrometers or shorter is grown on the multilayer substrate. Then a very tall CNT array, around 500 micrometers in length, is transferred distal-end-to-distal-end onto the short CNTs adhered to the multilayer substrate. The distal ends of the two CNT arrays are bonded by polymers, metal nanoparticles, or a combination of both by coating the distal ends with such before the transfer. This technique is referred to as transfer printing. In the case of metal nanoparticle bonding, the CNT arrays and multilayer substrate are heated to promote metal diffusion and to secure the bond. As an example, the heating is done at 300 C. in air for 30 min to and 1 hour; and the two CNT arrays are placed under 20 to 40 psi of pressure during heating.

(68) IV. Methods of Use

(69) The CNT arrays described herein can be used as thermal interface materials. The CNT arrays can be formed and/or deposited, as required for a particular application.

(70) For example, in one embodiment, the inert support for the multilayer substrate and CNT arrays is a piece of metal foil, such as aluminum foil. In these cases, CNTs are anchored to a surface of the metal foil via an adhesion layer, metal oxide nanoparticles or aggregates, and catalytic nanoparticles or aggregates. In some instances only one surface (i.e., side) of the metal foil contains an array of aligned CNTs anchored to the surface. In other cases, both surfaces (i.e., sides) of the metal foil contain an array of aligned CNTs anchored to the surface. If desired, one or more polymers, metal particles, or combinations thereof may be applied to the CNT array.

(71) These materials may be placed or affixed in between a heat source and a heat sink or heat spreader, such as between an integrated circuit package and a finned heat exchanger, to improve the transfer of heat from the heat source to the heat sink or spreader.

(72) CNT arrays of this type exhibit both high thermal conductance and mechanical durability. As a consequence, these arrays are well suited for applications where repeated cycling is required. For example, foils of this type can be employed as thermal interface materials during turn-in testing of electrical components, such as chips.

(73) In other embodiments, the inert support for the multilayer substrate and CNT arrays is a surface of a conventional metal heat sink or spreader. In these cases, CNTs are anchored to a surface of the heat sink or spreader via an adhesion layer, metal oxide nanoparticles or aggregates, and catalytic nanoparticles or aggregates. This functionalized heat sink or spreader may then be abutted or adhered to a heat source, such as an integrated circuit package.

(74) The CNT arrays described herein can be used as thermal interface materials in personal computers, server computers, memory modules, graphics chips, radar and radio-frequency (RF) devices, disc drives, displays, including light-emitting diode (LED) displays, lighting systems, automotive control units, power-electronics, solar cells, batteries, communications equipment, such as cellular phones, thermoelectric generators, and imaging equipment, including MRIs.

(75) The CNT arrays can also be used for applications other than heat transfer. Examples include, but are not limited to, microelectronics, through-wafer vertical interconnect assemblies, and electrodes for batteries and capacitors. Currently, copper and aluminum foil are used as the backing materials for the anode and cathode in lithium ion batteries. A slurry of activated carbon and the lithium materials is pasted onto the foils. The electrical contact between the paste and the foil is a point of parasitic resistance. In addition to reduced electrical output this resistance can impede heat rejection from the device. Well adhered vertical CNT arrays placed at this interface would improve performance electrically and thermally.

(76) The CNT foils could also be used for electromagnetic shielding. The CNTs act to effectively absorb electromagnetic irradiation as well as solar absorbing material, to enhance solar absorption in solar hot water heaters.

EXAMPLES

Example 1

Preparation of Carbon Nanotube (CNT) Arrays

(77) Aluminum foil was purchased at a thickness of 10 micrometers from Alfa Aesar. A piece of aluminum foil was placed in a square sample holder in a Denton Explorer electron-beam evaporator. The sample holder clamped the aluminum foil around its edges and a 55 inch square of the aluminum foil was exposed on the front- and backside of the sample holder, which could be flipped in-situ to deposit metal on both sides of the foil without breaking vacuum.

(78) One side at a time, an adhesion layer of iron was deposited to a thickness of 30 nm, then an interface layer of aluminum was deposited to a thickness of 10 nm, and finally a catalytic layer of iron was deposited to a thickness of 3 nm. The aluminum layer was allowed to cool for 10 minutes before depositing the catalytic iron film. The depositions all occurred at a chamber pressure of approximately 0.0008 mTorr. The iron adhesion layers were deposited at a rate of 0.1 nm/s; the aluminum interface layers were deposited at a rate of 0.1 nm/s; and the iron catalytic layer was deposited at a rate of 0.05 nm/s. The deposited multilayer substrates were allowed to cool for 15 min before venting the chamber and removing the aluminum foil.

(79) An Aixtron Black Magic CVD tool was used to grow CNTs on the multilayer substrates. The aluminum foil with multilayers on both sides was placed on a stage in the CVD tool. The sample was heated in a nitrogen atmosphere at 10 Torr to a temperature of 550 C., and then the sample was annealed at this temperature for 10 minutes in nitrogen at 10 Torr. Hydrogen was fed into the chamber at the end of the nitrogen annealing step and the sample was held at the annealing temperature for an additional 3 minutes in the nitrogen and hydrogen atmosphere. Acetylene was introduced to the chamber and nitrogen flow was stopped at the end of the 3 minutes, and then the sample was heated to 630 C. CNT growth commenced for 5 minutes at 630 C. and 10 Torr with 700 standard cubic centimeters per minute (sccm) of hydrogen and 100 sccm of acetylene as process gasses. Hydrogen and acetylene gas flow was stopped at the end of 5 minutes and the aluminum foil with CNTs adhered via multilayers was allowed to cool to 200 C. in a nitrogen flow.

(80) Dense vertical CNT arrays approximately 12 micrometers tall were produced on the side of the aluminum foil facing up, and dense vertical CNT arrays approximately 10 micrometers tall were produced on the side of the aluminum foil facing the sample stage. The densities of CNTs on both sides of the foil were determined by scanning electron microscopy (SEM) to be about 110.sup.9 nanotubes per mm.sup.2. The diameters of the CNTs on both sides of the foil were determined by SEM to be about 10 nm. The produced CNTs were MWNTs, which had 5 walls on average.

(81) The produced aluminum foil sample with CNTs adhered with multilayers on both sides was placed in a sonication bath of ethanol for 5 minutes. No CNTs were observed to release from the substrate during the sonication, which demonstrates the excellent adhesive and cohesive integrity of the joint. Upon removal from the ethanol, the CNTs in the array were patterned into discrete islands, demonstrating that solvent evaporation from the array can be an effective method to alter the morphology of the array.

(82) The distal ends of the CNT arrays on both sides of the aluminum foil were coated with P3HT by spray coating. The structure is shown in FIG. 6. The polymer-coated sample was pressed at 20 psi between gold-coated silver and quartz surfaces that were wet with chloroform. The interface was allowed to dry and the thermal resistance was measured using a photoacoutic technique. The thermal resistance was estimated to be approximately 7 mm.sup.2K/W, which is a 70% reduction in resistance compared to the sample structure tested without the polymer coating.

Example 2

Preparation of Carbice Carbon Nanotube (CNT) Arrays Using First Nano CVD System

(83) Thermal CNT Growth using First Nano CVD System

(84) Thermal CNT growth was performed in a First Nano Easy Tube Chemical Vapor Deposition (CVD) furnace at sub-atmospheric pressures (300-400 torr) with H.sub.2, C.sub.2H.sub.2 as the growth gases. CNT growth was performed on Al and Cu foils in this furnace using the Carbice Fe30/Al10/Fe3 nm catalyst (multi-layer substrate) and some variations to the catalyst as described in the following sections.

(85) CNT Growth on Al Foils Using Carbice Catalyst (Fe30/Al10/Fe3 nm)

(86) CNT growth was performed on 25 m thick Al foil in the First Nano CVD furnace using the following low pressure chemical vapor deposition (LPCVD) procedure at 630 C. The sample was placed in the CVD furnace and the temperature increased to 530 C. in Ar at 400 sccm. The sample was annealed in H.sub.2 at 350 sccm for 3 mins. C.sub.2H.sub.2 was then introduced into the chamber at 50 sccm. The temperature was increased to 630 C. with the sample in H.sub.2 at 350 sccm and C.sub.2H.sub.2 at 50 sccm. CNT growth commenced for 20 mins in H.sub.2 at 350 sccm and C.sub.2H.sub.2 at 50 sccm at 630 C. at 330 torr pressure.

(87) Results

(88) Under these growth conditions, a fully densified array of CNTs was produced. The CNTs were approximately 17 micrometers tall, vertically aligned and well adhered to the substrate. Dry contact thermal resistances for CNTs grown under these conditions, measured in a stepped 1D reference bar apparatus, was about 1.6 cm.sup.2-K/W. Sample variability in CNT heights for this growth time is dependent on the temperature distribution and flow conditions inside of the furnace as well as the quality of the deposited catalyst. Typical CNT heights for this growth time range from about 15-25 micrometers. Shorter CNTs (5-7 micrometers) were grown by reducing the growth times with the same catalyst and growth gases. Under this condition, CNTs with thermal resistances that were smaller by a factor of three or more were produced.

(89) CNT growth on Al and Cu Foils using Modified Carbice Catalyst (Fe10/Al100Fe30/Al10/Fe3 nm)

(90) The CNTs heights can be increased or decreased by changing the growth time in kind, however growth typically terminates at about 50-60 micrometers, at which point increasing growth time does not continue to increase CNT height. Because the growth termination mechanism is partially related to sub surface diffusion, a slightly modified five layer catalyst system was implemented to combat the diffusion process. Using a catalyst of Fe10/Al100/Fe30/Al10/Fe3 nm, CNTs of 75-100 micrometers in height were grown on a 50 micrometer Al substrate with 45 minute growth time. The other growth parameters (e.g. growth gases, temperature, etc.) remain the same as described above. This modified catalyst represents a double stacking of the Carbice catalyst with thickness modifications in the first two layers. The first Fe layer serves as an adhesion promoter for the rest of the catalyst stack, and the Al acts as a diffusion barrier. In addition to the increased relative diffusion distance associated with the modified Carbice catalyst, the additional interfaces also provided some resistance to interlayer diffusion.

(91) Catalyst poisoning due to Cu diffusion from Cu substrates is much more problematic than the analogous problem seen when growing on Al substrates. For this reason, the standard three-layer Carbice catalyst (Fe30/Al10/Fe3 nm) is often not sufficient for repeatable growth on Cu. The modified 5 layer Carbice catalyst (Fe10/Al100/Fe30/Al10/Fe3 nm) overcomes this problem while allowing repeatable growth on Cu up to very tall CNT heights.

(92) For example, using the Fe10/Al100/Fe30/Al10/Fe3 nm catalyst, 150 micrometers CNTs were grown on an oxygen-free high conductivity copper substrate with a 90 minute growth time at 650 C. The higher melting temperature of Cu allows for this slight increase in growth temperature. The growth gases and ramp rates remain the same as described above.

Example 3

Performance Study of Carbice TIM Compared to Grafoil

(93) The following experiment determines the impact of the Carbice product in a potential end use application compared to a competing product (Grafoil).

(94) The output voltage of a thermoelectric module was measured. Ten (10) baseline measurements using Grafoil were performed. Ten (10) measurements with Carbice CNT TIM both on copper and aluminum substrate were also performed. All measurements were performed at hot side temperature set points of 30, 60, 90, 120, and 180 C. All tests were performed on a Marlow Industries RC3-6 Bi.sub.2Te.sub.3 thermoelectric module.

(95) Results

(96) A comparative curve of CNT on copper, CNT on aluminum, and Grafoil is shown in FIG. 7. A summary of the results is shown in Table 1. Under these test conditions, Carbice TIM significantly improves system performance compared to conventional graphite-based TIMs. Specifically, output voltages increased 20% or more for Copper and Aluminum substrates compared to Grafoil.

(97) TABLE-US-00001 TABLE 1 Summary of comparative study. Improvement Interface Voltage (over baseline) Grafoil (baseline) 1.148 Carbice TIM Copper Substrate 1.383 20% Carbice TIM Aluminum Substrate 1.465 28%