Carbon nanostructure-based devices for cooling and methods of making and using thereof

Abstract

Carbon nanostructure-based devices for cooling different heat sources, as well as methods of making and using thereof are described herein. Such cooling devices can be used for commercial applications, such as data center cooling.

Description

[0001] CARBON NANOSTRUCTURE-BASED DEVICES FOR COOLING AND METHODS OF MAKING AND USING THEREOF

[0002] CROSS-REFERENCE TO RELATED APPLICATION

[0003] The present application claims priority to U.S. Provisional Application No. 63 / 593,773, filed October 27, 2023, the contents of which is hereby incorporated by reference in its entirety.

[0004] FIELD OF THE INVENTION

[0005] This invention is in the field of devices for cooling having improved performance properties, as well as methods of making and using thereof.

[0006] BACKGROUND OF THE INVENTION

[0007] Various types of devices and materials are used for cooling high heat generating electronics and components thereof. Such devices and materials can be used, for example, with cooling fluids. A key challenge, however, in such devices and materials is their limits in providing high cooling performance.

[0008] For instance, in two phase cooling (boiling and condensation) the performance may be limited by surface properties or structure, in particular the ability of the surface to nucleate bubbles, as well as the ability to remove those bubble from the surface prior to coalescence with subsequent bubbles generated in the vicinity. In single phase cooling, a boundary layer forms near surfaces, and mixing within this boundary layer, interruption of its growth, as well as increasing surface area within the layer all serve to improve heat transfer between the fluid and the surface.

[0009] Thus, there is a need for providing enhanced and improved cooling of different heat sources.

[0010] Therefore, it is an object of the invention to provide devices demonstrating enhanced and improved cooling of different heat sources.

[0011] It is also an object of the invention to provide methods of making and using such devices demonstrating enhanced and improved cooling of different heat sources.

[0012] SUMMARY OF THE INVENTION

[0013] Carbon nanostructure-based devices including carbon nanostructure composites therein can provide for improved cooling of different heat source(s), such as chips, dies, and other heat generating sources. In some instances, the carbon nanostructure-based devices for cooling are formed from single-layered or multi-tiered carbon nanostructure composites. In a non-limiting instance, a carbon nanostructure-based device for cooling can include: a single-layered carbon nanostructure composite, which is planar and includes a top surface and a bottom surface; where at least the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0014] In yet another non-limiting instance, a carbon nanostructure-based device for cooling can include: a multi-tiered carbon nanostructure composite, which is planar and includes a top surface and a bottom surface; wherein at least the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0015] In some other instances, single- layered or multi-tiered carbon nanostructure composites can combined with a heat spreading substrate, as described below. Typically, the heat spreader is a planar substrate. See Figures 2A and 2B. In some instances, the heat spreading substrate may include structures, such as channels, fins, pins, and / or hydrofoils. In one non-limiting instance, a carbon nanostructure-based device for cooling including a heat spreader therein can include: a heat spreading substrate with a top surface and a bottom surface; a first single-layered carbon nanostructure composite on the top surface; and a second single-layered carbon nanostructure composite on the bottom surface; where at least the first single- layered carbon nanostructure composite on the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the second single-layered carbon nanostructure composite on the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0016] In a second non-limiting instance, a carbon nanostructure-based device for cooling including a heat spreader therein can include: a heat spreading substrate with a top surface and a bottom surface; a first multi-tiered carbon nanostructure composite on the top surface; and a second multi-tiered carbon nanostructure composite on the bottom surface; where at least the first multi-tiered carbon nanostructure composite on the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the second single-layered carbon nanostructure composite on the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0017] In certain cases, the heat spreading substrate is a planar substrate which can be made of copper, aluminum, steel, AlSiC, AlSilOMg, graphite, and combinations thereof.

[0018] Typically, at least the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer are in contact with a cooling fluid. In some cases, both the top and surfaces include nano- and / or micro- scale surface structures and both surfaces may be contacted with one or more cooling fluids.

[0019] In certain instances, it may be desirable to modify the surface energies of at least a portion of the carbon nanostructures present in the single-layered or multi-tiered carbon nanostructure composites described.

[0020] The single-layered or multi-tiered carbon nanostructure composites of the heat spreaders include at least nano-scale surface structures due to the presence of carbon nanostructures (i.e., carbon nanotubes) of the array(s) present in the composites. In certain instances, it may also be desirable to form pattem(s) on one or more surfaces of at least a portion of the carbon nanostructures present in the single-layered or multi-tiered carbon nanostructure composites described. The carbon nanostructure -based device for cooling described above can be formed according to various methods. In one non-limiting example, a method of making a carbon nanostructure-based device for cooling includes the steps of:

[0021] (a) forming or providing a single-layered carbon nanostructure composite, which is planar and includes a top surface and a bottom surface, to form the device; where at least the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0022] In another non-limiting example, a method of making a carbon nanostructure-based device for cooling includes the steps of:

[0023] (a’) forming or providing a multi-tiered carbon nanostructure composite, which is planar and includes a top surface and a bottom surface, to form the device; where at least the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the bottom surface is a thermal interface for attachment for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0024] In yet another non-limiting example, a method of making a carbon nanostructure-based device for cooling includes the steps of:

[0025] (a’ ’ ) attaching a first single-layered carbon nanostructure composite onto a top surface of a heat generating substrate;

[0026] (b”) attaching a second single-layered carbon nanostructure composite onto a bottom surface of the heat generating substrate to form the device; where at least the first single-layered carbon nanostructure composite on the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the second single-layered carbon nanostructure composite on the bottom is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0027] In still another non-limiting example, a method of making a carbon nanostructure-based device for cooling includes the steps of:

[0028] (a’ ’ ’) attaching a first multi-tiered carbon nanostructure composite onto a top surface of a heat spreading substrate;

[0029] (b’”) attaching a second multi-tiered carbon nanostructure composite onto a bottom surface of the heat spreading substrate to form the device; where at least the first multi-tiered carbon nanostructure composite on the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the second multi-tiered carbon nanostructure composite on the bottom is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0030] In some instances of the above methods, the vertically aligned carbon nanotubes on the supports can independently include one or more coatings formed from metals, metal oxides, nanoparticles, ceramics, and / or polymers. Exemplary metals, metal oxides, nanoparticles, ceramics, and / or polymers are described above.

[0031] In certain instances where a heat spreading substrate is included the resulting device can be stamped or rolled to provide a cooling device having a defined shape.

[0032] The carbon nanostructure -based devices described above can be used for cooling one or more heat sources. In an exemplary cooling method, the method includes the steps of:

[0033] (1) attaching a carbon nanostructure-based cooling device to one or more heat sources;

[0034] (2) contacting at least the nano- and / or micro- scale surface structures of the device to one or more cooling fluids; and

[0035] (3) optionally flowing or circulating the one or more cooling fluids during step (2); wherein the device increases at least one of the following: active boiling or condensing nucleation site density, turbulent mixing, critical heat flux, heat transfer coefficient(s), and / or wherein the device at least reduces: superheating required for boiling nucleation between the one or more heat sources and the one or more cooling fluids, and provides increased and / or enhanced cooling of the one or more heat sources, as compared to a method for cooling which does not include a carbon nanostructure-based cooling device having the nano- and / or micro-scale surface structures present.

[0036] Figures 3A and 3B show non-limiting illustrations of a carbon nanostructure -based cooling device during active cooling of a heat source where the nucleation sites on the device improve or enhance at least the boiling of the cooling fluids present.

[0037] The cooling devices used in the methods can accelerate bubble formation, increase bubble nucleation density, and / or accelerate release or departure of bubbles, when the one or more cooling fluids are boiling.

[0038] In some instances, the increase and / or enhancement may be defined by an increase in active boiling or condensing nucleation site density, mixing, critical heat flux, and / or heat transfer coefficients by at least about 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or higher, as compared to a cooling method which does not use a carbon nanostructure-based cooling device having the nano- and / or micro-scale surface structures present.

[0039] In some instances, the increase and / or enhancement may be defined by a decrease in superheating required for boiling nucleation by at least about 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or higher, as compared to a cooling method which does not use a carbon nanostructure -based cooling device having the nano- and / or micro-scale surface structures present. “Superheating,” as used herein, refers to the temperature above the saturation temperature of a liquid.

[0040] In the cooling methods, one or more cooling fluids can be, for example, selected from distilled water, deionized water, fluorocarbon-based fluids (such as HFE fluids (segregated hydrofluoroethers), perfluorinated compound fluids, NOVEC fluids (such as fluoroketones), hydrocarbon fluids (such as mineral oil), non-fatty saturated straight chain alcohols, Ci to C30 alkanes, fully or partly halogenated hydrocarbons (such as HCFC / CFC / HFC refrigerants), and combinations thereof. Such cooling fluids can be obtained from commercial sources. In some instances, the cooling fluids can have a boiling temperature in a range from between about 30 °C and 100 °C.

[0041] In certain instances of the cooling methods, the method includes immersing the entire device and the one or more heat sources into the one or more cooling fluids in step (2) and during optional step (3). In some cases, the cooling methods can be employed in data center cooling, which can involve convective or pool boiling. Convective boiling includes flow of one or more cooling fluids, whereas pool boiling does not include forced or pumped flowing of the cooling fluids. Pool boiling may include for example buoyancy driven flow.

[0042] BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1A shows a non-limiting side-view of a single-layered carbon nanostructure composite 100 having an array of vertically aligned carbon nanostructures (i.e., carbon nanotubes) 110 on opposite surfaces of a support 120.

[0044] Figure IB shows a non-limiting side-view of a multi-tiered carbon nanostructure composite 200 having 3 tiers of arrays of vertically aligned carbon nanostructures (i.e., carbon nanotubes) 210 on opposite surfaces of supports 220.

[0045] Figure 2A shows a non-limiting side-view of a carbon nanostructure-based device for cooling 300 having two single-layered carbon nanostructure composites 100 on top and bottom surfaces of a heat spreading substrate 310.

[0046] Figure 2B shows a non-limiting side-view of a carbon nanostructure-based device for cooling 400 having two multi-tiered carbon nanostructure composites 200 on top and bottom surfaces of a heat spreading substrate 410.

[0047] Figure 3A shows a non-limiting illustration of a carbon nanostructure-based cooling device 500 of the types described during cooling where the device is in contact with a heat source 510 and a cooling fluid 520 is present on one side of the device which contains nucleation sites on at least the surface contacting the cooling fluid and enhances boiling characteristics, such as bubble formation 525.

[0048] Figure 3B shows a non-limiting illustration of a carbon nanostructure-based cooling device 500 of the types described during cooling where the device is placed in contact with a heat source 510 forming a thermal interface 530 to the heat source. A cooling fluid 520 is present on all sides of the device and heat source and the device contain nucleation sites that enhance boiling characteristics, such as bubble formation 525.

[0049] Figure 4A shows a non-limiting illustration of a carbon nanostructure-based cooling device 600 of the types described in contact with a heat source 610 forming a thermal interface 630 to the heat source. A lid 620 confines the flow of a cooling fluid 640 through channels 605 patterned in the carbon nanostructures of the device. Figure 4B shows a non-limiting illustration of a carbon nanostructure-based cooling device 600 of the types described in contact with a heat source 610. A lid 625 having patterned passages confines the flow of a cooling fluid 640 through channels on the carbon nanostructures of the device.

[0050] Figure 5 shows a non-limiting top-down and cross-sectional view of a carbon nanostructure -based cooling device 700 of the types described on the outer surface of a tubeshaped heat source (i.e., condenser tube) 710. A cooling fluid 720 is present inside the tubeshaped heat source and when a vapor phase 730 is passed over the cooling device a condensate 740 forms thereon.

[0051] Figure 6A shows a non-limiting top-down view of a patterned array of carbon nanostructures defining channels 810 on a surface, where the carbon nanostructures 800 form rows of the carbon nanostructures defining channels in between.

[0052] Figure 6B shows a non-limiting side view of a patterned array of carbon nanostructures having defining channels 810 on a surface, where the carbon nanostructures 800 form rows of the carbon nanostructures defining channels in between and the surface is a heat spreading substrate 820.

[0053] Figure 6C shows a non-limiting top-down view of a patterned array of carbon nanostructures forming a grid pattern on a surface formed from rows of crossing carbon nanostructures 800 thereon.

[0054] Figure 6D shows a non-limiting top-down view of a patterned array of carbon nanostructures having a grid pattern formed from specific areas of carbon nanostructures 800 on a surface.

[0055] Figure 6E shows a non- limiting top-down view of a patterned array of carbon nanostructures having a plurality of pillars or islands formed from specific areas of carbon nanostructures 800 on a surface.

[0056] Figure 7 shows a non-limiting illustration of a testing set up and labeled components for evaluating carbon nanostructure-based cooling devices with a boiling enhancing surface.

[0057] Figure 8A is a graph of the boiling curve of the copper bar control tested in the Examples.

[0058] Figure 8B is a graph showing the heat transfer coefficient (HTC) as a function of the copper bar superheat of the copper bar control tested in the Examples.

[0059] Figure 9A is a graph of the boiling curve of the Boiling Surface Enhancement (BES) having a carbon nanotube array with 65 um thickness grown thereon tested in the Examples. Figure 9B is a graph showing the heat transfer coefficient (HTC) as a function of the Boiling Surface Enhancement (BES) superheat of the BES having a carbon nanotube array with 65 um thickness grown thereon tested in the Examples.

[0060] Figure 10A is a graph of the boiling curve of the Boiling Surface Enhancement (BES) having a carbon nanotube array with 305 um thickness grown thereon tested in the Examples.

[0061] Figure 1 OB is a graph showing the heat transfer coefficient (HTC) as a function of the Boiling Surface Enhancement (BES) superheat of the BES having a carbon nanotube array with 305 um thickness grown thereon tested in the Examples.

[0062] Figure 11 shows a picture of a Boiling Surface Enhancement (BES) having an etched pattern of square features of carbon nanotubes (CNTs), where the silver-colored squares are the underlying aluminum foil substrate after the removal of CNTs therefrom.

[0063] Figure 12A is a graph of the boiling curve of the Boiling Surface Enhancement (BES) having an etched pattern of square features of carbon nanotubes (CNTs) tested in the Examples.

[0064] Figure 12B is a graph showing the heat transfer coefficient (HTC) as a function of the Boiling Surface Enhancement (BES) superheat of the BES having an etched pattern of square features of carbon nanotubes (CNTs) tested in the Examples.

[0065] DETAILED DESCRIPTION OF THE INVENTION

[0066] Carbon nanostructure-based devices with improved cooling performance and methods of making and using thereof are described below.

[0067] I. Definitions

[0068] “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.

[0069] “Compliant” or “Compliance,” as used herein, refers to the ability of a material to conform when contacted to one or more surfaces such that efficient conformance to the asperities of the adjoining surface results in sufficient or high contact areas at the interfaces between the surfaces and the material.

[0070] “Interdigitation” or “Interdigitating”, as used herein, refers to the ability and or degree which one or more individual nanostructure elements or bundles or multiple nanostructure elements of an array to infiltrate or penetrate into the adjacent nanostructure elements of another array when the two different arrays are contacted or stacked. “Carbon Nanotube Array” or “CNT array” or “CNT forest”, 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.

[0071] “Carbon Nanotube Sheet” or “CNT sheet”, as used herein, refers to a plurality of carbon nanotubes which are aligned in plane to create a free-standing sheet. Carbon nanotubes are said to be “aligned in plane” when they are substantially parallel to the surface of the sheet that they form. Nanotubes are said to be substantially parallel when they are oriented on average greater than 40, 50, 60, 70, 80, or 85 degrees from sheet surface normal.

[0072] “Coating material” as used herein, generally refers to polymers and / or molecules that can bond to CNTs through van der Waals bonds, TI-TI stacking, mechanical wrapping and / or covalent bonds and bond to metal, metal oxide, or semiconductor material surfaces through van der Waals bonds, 7t-7i stacking, and / or covalent bonds.

[0073] “Conformal,” or “Conformally Coated,” as used herein means covering a surface topography of an object such that the covering follows the features or topology of the coated surface in all three dimensions, including following the features of individual nanostructures on the surface or the features of larger superstructures formed from bundles of discrete elements..

[0074] “Thermal contact resistance,” as used herein, refers to when two surfaces are in thermal contact and resistance to heat transfer between the surfaces exists.

[0075] “Thermal conductivity,” as used herein, refers to the transport of energy in the form of heat through a body of mass as the result of a temperature gradient and is determined as the heat energy transferred per unit of time and per unit of surface area divided by the temperature gradient. Thermal conductivity is typically expressed in units of W m1K1.

[0076] “Elastic recovery” as used herein, refers to the ability of a material to return to its original shape following compression, expansion, stretching, or other deformation.

[0077] “Compression set” as used herein, refers to the permanent deformation of a material which remains when a force, such as compression, was applied to the material and the force was subsequently removed.

[0078] Numerical ranges disclosed in the present application include, but are not limited to, ranges of temperatures, ranges of pressures, ranges of integers, ranges of conductance and resistance values, ranges of times, and ranges of thicknesses, etc. The disclosed ranges of any type, disclose individually each possible number that such a range could reasonably encompass, as well as any sub-ranges and combinations of sub-ranges encompassed therein. For example, disclosure of a thickness range is intended to disclose individually every possible thickness value that such a range could encompass, consistent with the disclosure herein.

[0079] Use of the term "about" is intended to describe values either above or below the stated value, which the term “about” modifies, in a range of approx. + / - 10%; in other instances the values may range in value either above or below the stated value in a range of approx. +1- 5%. When the term "about" is used before a range of numbers (i.e., about 1-5) or before a series of numbers (i.e., about 1, 2, 3, 4, etc.) it is intended to modify both ends of the range of numbers or each of the numbers in the series, unless specified otherwise.

[0080] IL Carbon Nanostructure-based Devices

[0081] Carbon nanostructure-based devices including carbon nanostructure composites therein, as described below, can provide for improved cooling of different heat source(s), such as chips, dies, and other heat generating sources.

[0082] In some instances, the carbon nanostructure-based devices for cooling are formed from single-layered or multi-tiered carbon nanostructure composites. See Figures 1 A and IB. For instance, in a non- limiting instance, a carbon nanostructure-based device for cooling can include: a single-layered carbon nanostructure composite, which is planar and includes a top surface and a bottom surface; where at least the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0083] In yet another non-limiting instance, a carbon nanostructure-based device for cooling can include: a multi-tiered carbon nanostructure composite, which is planar and includes a top surface and a bottom surface; wherein at least the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0084] In some other instances, single- layered or multi-tiered carbon nanostructure composites can combined with a heat spreading substrate, as described below. Typically, the heat spreader is a planar substrate. See Figures 2A and 2B. In some instances, the heat spreading substrate may include structures, such as channels, fins, pins, and / or hydrofoils. In one non-limiting instance, a carbon nanostructure-based device for cooling including a heat spreader therein can include: a heat spreading substrate with a top surface and a bottom surface; a first single-layered carbon nanostructure composite on the top surface; and a second single-layered carbon nanostructure composite on the bottom surface; where at least the first single- layered carbon nanostructure composite on the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the second single-layered carbon nanostructure composite on the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0085] In a second non-limiting instance, a carbon nanostructure-based device for cooling including a heat spreader therein can include: a heat spreading substrate with a top surface and a bottom surface; a first multi-tiered carbon nanostructure composite on the top surface; and a second multi-tiered carbon nanostructure composite on the bottom surface; where at least the first multi-tiered carbon nanostructure composite on the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the second single-layered carbon nanostructure composite on the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures. In certain instances of the cooling devices which include a heat spreading substrate, combinations of single-layered and multi-tiered carbon nanostructure composites may be used. For example, a single-layered carbon nanostructure composite can be placed on the top surface of the heat spreading surface and a multi-tiered carbon nanostructure composite may be placed on the bottom, or vice versa, while meeting the requirements specified for the respective devices defined above. In some instances, only the top surface which contacts one or more cooling fluids includes a single-layered and multi-tiered carbon nanostructure composite having nano- and / or micro- scale surface structures thereon to act as nucleation sites and / or which function to increase turbulent mixing and / or boundary layer heat transfer. In other words, the side of the heat spreading substrate which contacts the heat source does not need to include a composite thereon.

[0086] A non-limiting example of a heat spreading substrate in a cooling device is shown in Figure 2A which shows a non-limiting side-view of a carbon nanostructure-based device for cooling 300 having two single-layered carbon nanostructure composites 100 on top and bottom surfaces of a heat spreading substrate 310. Another non-limiting example of a heat spreading substrate in a cooling device is shown in Figure 2B which shows a non-limiting side-view of a carbon nanostructure-based device for cooling 400 having two multi-tiered carbon nanostructure composites 200 on top and bottom surfaces of a heat spreading substrate 410.

[0087] With respect to bubble departure for the cooling devices, these can be said to facilitate the bubble departure frequency, such as by reducing the bubble growth time. Regarding boundary layer heat transfer by the cooling devices, such nano- and / or micro- scale surface structures can interrupt the laminar boundary layer and facilitate turbulent mixing that increases convective heat transfer.

[0088] It is believed that for boiling enhancement, the nano- and / or micro- scale surface structures can increase or improve nucleation and enhance removal of nucleated bubbles from the surface preventing the formation of a vapor film at the surface (from coalesced bubbles, or because small discrete bubble cannot be formed fast enough). Such a vapor film has lower thermal conductivity than a liquid film, so in general heat transfer coefficient is reduced once the transition away from nucleate boiling occurs. Modifying the surface to include the nano- and / or micro- scale surface structures allows the nucleate boiling to continue for longer, at higher power densities or heat fluxes. This is typically referred to as an increase in critical heat flux. A similar phenomena can also occur in vapor condensation. With mixing (i.e. turbulent mixing) the cold on hot fluid at the thermal boundary layer are physically mixed (instead of effectively stratifying in a continuous gradient from hot to cold). The mixing of the fluids more uniformly distributes the heat in the fluid which effectively increases the energy transfer to the fluid. Even in laminar flow, when there is more surface area (due to for example the nanostructures), heat transfer increases, because heat transfer is directly proportional to surface area. Thus, the nano- and / or micro- scale surface structures can provide increase turbulent mixing and / or increase boundary layer heat transfer. Without limitation, a testing setup for evaluating boiling enhancement by nano- and / or micro- scale surface structures, such as an aligned CNT array, is shown in Figure 7 and testing and evaluation parameters and calculations, as discussed in the Examples below, are incorporated herein.

[0089] In some instances, for the surface of the cooling devices which contacts one or more cooling fluids the CNTs of the array exposed to the fluid(s) have a thickness which ranges from between about 5 to 500 microns, as well as individual values or sub-ranges within the aforementioned range. In some instances, the thickness is in a range from about 5 to 250, 5 to 200, 5 to 150, 5 to 100, 5 to 75, 5 to 50, or 5 to 25 microns, as well as individual values or subranges within the aforementioned ranges.

[0090] In certain cases, the heat spreading substrate is a planar substrate which can be made of copper, aluminum, steel, AlSiC, AlSilOMg, graphite, and combinations thereof. Such heat spreading substrates may be a metal plate or a meta foil of a suitable thickness. The heat spreading substrates may have any suitable shape, dimensions (i.e., length and width), and thickness needed for cooling applications. In some instances, the heat spreading substrate has a thickness ranging from about 50 pm to 5 mm, and ranges and values contained within. In some instances, the entirety or substantially all (i.e., at least about 80%, 90%, 95%, 96%, 97%, 98%, or 99%) of the surface area of the top and bottom surfaces of the heat spreading substrates are covered by the single-layered or multi-tiered carbon nanostructure composites described. In some instances, the surface of the heat spreading substrate is only covered in strategic locations, for example, in the area where a heat source contacts the heat spreading substrate, or in a structured arrangement such as strips, grids or arrays that align with the preferred cooling fluid pathways. In some instances, the heat spreading substrate is flexible and / or compliant.

[0091] In some cases, the heat spreading substrate can be stamped and provide a cooling device having a defined shape, such as including channels, fins, pins, tubes, or other suitable shapes. In some instances, the cooling devices described may further include a lid which is present on the device to cause the one or more cooling fluids to flow through channels, fins, pins, hydrofoils, or tubes, which may be patterned onto the surface of the composite and / or lid interfacing the cooling fluid(s). See Figures 4 A and 4B. As shown in Figure 4 A, a carbon nanostructure-based cooling device 600 of the types described can be placed in contact with a heat source 610 forming a thermal interface 630 to the heat source where a lid 620 confines the flow of a cooling fluid 640 through channels 605 patterned in the carbon nanostructures of the device. As shown in Figure 4B, a carbon nanostructure-based cooling device 600 of the types described can be placed in contact with a heat source 610 where a lid 625 having patterned passages confines the flow of a cooling fluid 640 through channels on the carbon nanostructures of the device. Thus, the lid may itself be patterned to define channels or passages when placed onto a cooling device such that the cooling fluid(s) are flowed through the channels / passages formed by the lid.

[0092] Typically, at least the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer are in contact with a cooling fluid. In some cases, both the top and surfaces include nano- and / or micro- scale surface structures and both surfaces may be contacted with one or more cooling fluids.

[0093] In some instances, at least one surface of the cooling devices described, such as the bottom surface which is attached to a heat source, includes an adhesive for purposes of attachment thereto. Exemplary adhesives are described below in terms of coatings which may be present on the composites.

[0094] In some instances, the cooling devices may be attached to a heat source(s) by bolts, screws, or other fastening means, and do not require the use of adhesives to be affixed to heat source(s).

[0095] In some instances, where the cooling devices do not include a heat spreading substrate, and are formed from single-layered or multi-tiered composites alone, the one or more supports can function as heat spreading substrates, such as when formed of a metal substrate or a metallic foil. Such supports onto which arrays of carbon nanostructures can be grown directly are described below.

[0096] In instances where the cooling devices do not include a heat spreading substrate, the composites themselves include a support which can be stamped and hold a defined shape, such as to provide a device having channels, fins, pins, tubes, or other suitable shapes. In some instances, the cooling devices described may further include a lid which is present on the device to cause the one or more cooling fluids to flow through channels, fins, pins, hydrofoils, or tubes, which may be patterned onto the surface of the composite and / or the lid interfacing the cooling fluid(s). See Figures 4A and 4B. As shown in Figure 4A, a carbon nanostructure-based cooling device 600 of the types described can be placed in contact with a heat source 610 forming a thermal interface 630 to the heat source where a lid 620 confines the flow of a cooling fluid 640 through channels 605 patterned in the carbon nanostructures of the device. As shown in Figure 4B, a carbon nanostructure-based cooling device 600 of the types described can be placed in contact with a heat source 610 where a lid 625 having patterned passages confines the flow of a cooling fluid 640 through channels on the carbon nanostructures of the device.

[0097] In some cases, the surface of the cooling devices which interface with one or more cooling fluids are self-cleaning, whereby “self-cleaning,” is enabled by the application of one or more coatings which are hydrophobic onto the carbon nanostructures of the array of the composite. The skilled person can select appropriate hydrophobic coatings to apply to the carbon nanostructures of an array of a composite render it self-cleaning.

[0098] In some instances, a cooling device may be formed that is tube-shaped and has an inner surface and an outer surface, which can be considered equivalent to the top and bottom surfaces discussed above. The heat source(s) may interface with either the outer or inner surface of the tube-shaped cooling device, as appropriate. The one or more cooling fluid(s) may interface with either the outer or inner surface of the tube-shaped cooling device, as appropriate.

[0099] In certain other instances, a cooling device may be formed with a heat spreading substrate and has a tube-shape which can be placed around the interior and / or exterior of a tubeshaped heat source, such as a condenser tube. In such instances, single-layered or multi-tiered carbon nanostructure composites may be placed on either surface of a heat spreading substrate in any suitable motif (i.e., two of the same types of composites on the inner and outer surfaces, or different types of composites, or a composite on only one surface). Cooling fluid may be flowed through the center of the tube-shaped heat source and / or outside of the tube-shaped heat source. In certain cases, as shown in Figure 5, a carbon nanostructure-based cooling device 700 of the types described can be placed on the outer surface of a tube-shaped heat source (i.e., condenser tube) 710 and cooling fluid 720 is present inside the tube-shaped heat source such that when a vapor phase 730 is passed over the cooling device a condensate 740 forms thereon.

[0100] In some instances, the cooling devices described may be contacted with a heat source and cooling fluid and additionally a vapor phase. When the vapor phase contacts a surface of the cooling device condensates of the vapor may form thereon. The shape of the cooling devices is not particularly restricted in such instances.

[0101] In some instances, the cooling devices described can be mounted to the heat source(s) (i.e., chip, direct or packaged die, lid, or a heated surface) with a loading mechanism including one or more fasteners, or can include a frame structure that has at least one opening to allow at least the nano- and / or micro- scale surface structures present on the composites to be in contact with one or more cooling fluids. An exemplary frame structure can include a mounting backplate which could be a rigid frame on the back side of a board and the cooling device could either be fastened directly with this frame, or clamped downward with another frame. In the latter case, the clamping frame would include an opening so that at least the nano- and / or microscale surface structures present on the composites would be in contact with the one or more cooling fluids.

[0102] A. Single-layered Carbon Nanostructure Composites

[0103] The single-layered carbon nanostructure composites which may form part of the cooling devices described are typically planar composites having a top and a bottom surface. See Figure 1 A which shows a non-limiting side-view of a single-layered carbon nanostructure composite 100 having an array of vertically aligned carbon nanostructures (i.e., carbon nanotubes) 110 on opposite surfaces of a support 120. These planar composites are not particularly restricted in terms of size / dimension and / or shape. Accordingly, the single-layered carbon nanostructure composites may be prepared, as described below, to the desired shape and dimensions (length and width) needed for forming a cooling device.

[0104] The single-layered carbon nanostructure composites are formed of a carbon nanotube array including vertically aligned carbon nanotubes which are grown from opposing surfaces of a support. Details of such arrays are described below.

[0105] 1. Carbon Nanostructure Arrays

[0106] In certain instances, the arrays may include carbon nanostructures formed of tubular structures (i.e., carbon nanotubes) which form vertically aligned forests or arrays. In certain other instances, the carbon nanostructures can form, but are not limited to, fibers, wires, horns, cones, or other tube-shaped structures which have a high aspect ratio (i.e., greater than 1).

[0107] Carbon nanotube arrays can be prepared using techniques known in the art. In one embodiment, the arrays are prepared as described in U.S. Publication No. 2014-0015158-Al, incorporated herein by reference. This method involves the use of multilayer substrates to promote the growth of dense vertically aligned CNT arrays and provide excellent adhesion between the CNTs and metal surfaces.

[0108] In most instances, the carbon nanostructure arrays are carbon nanotube (CNT) arrays which contain a plurality of carbon nanotubes supported on, or attached to, the surface of an inert support, such as a metallic (e.g., Cu, Al, or Au) foil, metal alloys (i.e., steel). In some instances, the support can be a flexible, electrically, and thermally conductive substrate, such as graphite or other carbon-based material. In yet other instances, the support can be an electrically insulating substrate, such as a flexible ceramic. The CNT arrays can be formed using the methods described herein. The CNTs are vertically aligned on the support. CNTs 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.

[0109] 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. 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. In some cases, the carbon nanotube density on the support surface ranges from about 1 x 107to 1 x 1011nanotubes per mm2, from about 1 x 108to 1 x IO10nanotubes per mm2, or from about 1 x 109to 1 x IO10nanotubes per mm2.

[0110] 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 substrate). The CNT 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 refer 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 instances, 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 CNTs in the arrays can range from about 1 to 5,000 micrometers, 5 to 5000 micrometers, 5 to 2500 micrometers, 5 to 2000 micrometers, 5 to 1000 micrometers. In some instances, the length of CNTs in the arrays can range from about 1-500 micrometers or about 1-100 micrometers.

[0111] The CNTs display strong adhesion to the support. In certain embodiments, the CNT array will remain substantially intact after being immersed in a solvent or fluid, such as ethanol or a cooling fluid, and sonicated for a period of at least five minutes. In some instances, the CNTs do not come off when exposed to boiling solvent(s) or fluid(s) and resist cavitation and mixing present in boiling conditions. In particular cases, at least about 90%, 95%, 96%, 97%, 98%, 99%, or 99.9% of the CNTs remain on the surface after sonication and / or boiling in solvent(s) or fluid(s), such as ethanol or a cooling fluid. In some cases, the CNT nanostructures of the array of the single-layered carbon nanostructure composites form into larger superstructures, such as, but not limited to, tube bundles, clumps, or rows. These superstructures may be formed through mechanisms, such as capillary clumping with solvent(s) or by way of application of coating(s).

[0112] 2. Coating(s) / Coating Materials

[0113] The CNT arrays can include a coating or coating material (terms can be used interchangeably) which adheres or is bonded to the CNTs. The coating / coating material can be applied as described herein. In some embodiments, the coating contains one or more oligomeric materials, polymeric materials, waxes, or combinations thereof. In other instances, the coating contains one or more non-polymeric materials. In some cases, the coating can contain a mixture of oligomeric, waxes, and / or polymeric material and non-polymeric materials.

[0114] In certain instances, the coating material(s) can act as a bonding or adhesive agent(s). Without limitation, such coating material(s) can be selected from adhesives (z.e., pressure sensitive adhesives, thermal adhesives), holt melts, hot glues, an epoxy, solders, a liquid-metal contacts, a phase change material (i.e., a wax or waxes), and combinations thereof. Such bonding or adhesive agent(s) are known in the art to the person of ordinary skill. Further, such bonding or adhesive agent(s) can be applied to CNT arrays using a variety of methods known in the art.

[0115] In certain cases, an adhesive can be applied to at least one surface of the CNT arrays present on the top and / or bottom surface of the single-layered composite. Such an adhesive can include, but are not limited to, adhesives (i.e., pressure sensitive adhesives, thermal adhesives), holt melts, hot glues, an epoxy, solders, a liquid- metal contacts, a phase change material (i.e., a wax or waxes), a hot glue, and combinations thereof. A hot melt adhesive combines wax, tackifiers, and a polymer base to provide improved adhesion properties to one or more surfaces to which the single-layered composite may be interfaced with. In some instances, the adhesive is a pressure sensitive adhesive. In yet other instances, the adhesive is a combination of a pressure sensitive adhesive polymer and a thermally activated (or activatable) adhesive polymer.

[0116] In some instances, the coating which adheres or is bonded to the CNTs of an array is applied before two or more CNT arrays are stacked while in other instances, the coating which adheres or is bonded to the CNTs of an array is applied following stacking of two or more CNT arrays, as described below to form multi-tiered composites. In yet other cases, the coating is infiltrated or backfilled into the CNT arrays and adheres or is bonded to the CNTs of the arrays. As used herein, “infiltration” or “infiltrated” refer to a coating material(s) which are permeated through at least some of the carbon nanotubes of the arrays. In some instances, the extent of infiltration is in the range of about 0.1-99.9%. In some instances, the infiltrated coating material at least partially fills the interstitial space between carbon nanotubes while in some other instances the infiltrated coating coats at least some of the surfaces of the carbon nanotubes, or both. In some embodiments, the infiltrated coating material fills all or substantially all (z.e. , of at least about 95%, 96%, 97%, 98%, or 99%) of the interstitial space between carbon nanotubes present in the CNT arrays.

[0117] A variety of materials can be coated onto the CNT arrays. In particular instances, the coatings can cause a decrease in the thermal resistance of the CNTs of arrays. The coatings can be applied conformally to coat the tips and / or sidewalls of the CNTs. It may also be desirable that the coating be reflowable using, for example, solvent, heat or some other easy to apply source. In some embodiments, the coating is readily removable, such as by heat or dissolution in a solvent, to allow for “reworking.” “Reworking,” as used herein, refers to breaking an interface ( / .<?., removing the coating) by applying solvent(s) and / or heat.

[0118] In some instances, the coating(s) described have a thickness ranging from about 1 nm to less than about 10 microns and are porous which allows for fluids to flow or wick through the coated CNT arrays, which may form bundles of CNTs. In certain instances, the CNTs of the arrays may be induced to be clumped or bundled, such as by exposure to solvent(s) and followed by drying. Clumping and bundling may be used to expose side-walls of the CNTs allowing them to be exposed to cooling fluid(s) and / or to coating materials, as described, which may be applied, coated, or infiltrated onto the arrays. a. Polymeric Coating Materials

[0119] In some instances, the coating is, or contains, one or more polymeric materials. The polymer coating can contain a conjugated polymer, such as an aromatic, heteroaromatic, or nonaromatic polymer, or a non-conjugated polymer.

[0120] Suitable classes of conjugated polymers include polyaromatic and polyheteroaromatics including, but not limited to, polythiophenes (including alkyl-substituted polythiophenes), polystyrenes, polypyrroles, polyacetylenes, polyanilines, poly fluorenes, polyphenylenes, polypyrenes, polyazulenes, polynaphthalenes, polycarbazoles, polyindoles, polyazepines, poly(3,4-ethylenedioxythiophenes), poly(p-phenyl sulfides), and poly(p-phenylene vinylene). Suitable non-aromatic, conjugated polymers include, but are not limited to, polyacetylenes and poly diacetylenes. The polymer classes listed above include substituted polymers, wherein the polymer backbone is substituted with one or more functional groups, such as alkyl groups. In some embodiments, the polymer is polystyrene (PS). In other embodiments, the polymer is poly(3-hexythiophene) (P3HT). In other embodiments, the polymer is poly(3,4- 3thylenedioxythiophene) (PEDOT) or poly(3,4-3thylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS).

[0121] In other instances, the polymer is a non-conjugated polymer. Suitable non-conjugated include, but are not limited to, polyvinyl alcohols (PVA), poly(methyl methacrylates) (PMMA), polydimethylsiloxanes (PDMS), polyurethane, silicones, acrylics, and combinations (blends) thereof.

[0122] In other instances, the polymer is a paraffin wax. In other embodiments, the polymer is a synthetic wax such as Fischer-Tropsch waxes or polyethylene waxes. In other instances, the polymer is a wax that has a melting temperature above about 30, 40, 50 , 60, 70, 80, 90, 100, 110, or 120 °C, or 130 °C.

[0123] In other instances, the polymer is an adhesive, such as, but not limited to, a hot glue or hot melt adhesive that combines wax, tackifiers, and a polymer base to provide improved adhesion properties to one or more surfaces. In some cases, the adhesive is a pressure sensitive adhesive. In certain other embodiments, the adhesive is a monomer that polymerizes upon contact with air or water such as a cyanoacrylate. In yet other cases, the adhesive is a combination of a pressure sensitive adhesive and a thermally activated (or activatable) adhesive polymers which enhances ease of adhesion by way of the pressure sensitive adhesive and additional and more permanent or semi-permanent adhesion by way of the thermal adhesive.

[0124] Polymers to be coated onto the CNT arrays can be dissolved in one or more solvents and spray or dip coated or chemically or electrochemically deposited onto the vertical CNT forests or arrays grown on a support. The coating materials can also be spray coated in powder form onto the top of vertical CNT forests or arrays grown on a substrate. The coatings can include polymers or molecules that bond to CNTs through van der Waals bonds, TT-TT stacking, mechanical wrapping and / or covalent bonds and bond to metal, metal oxide, or semiconductor material surfaces through van der Waals bonds, a- a stacking, and / or covalent bonds.

[0125] For spray or dip coating, coating solutions can be prepared by sonicating or stirring the coating materials for a suitable amount of time in an appropriate solvent. The solvent is typically an organic solvent or solvent and should be a solvent that is easily removed, for example by evaporation at room temperature or elevated temperature. Suitable solvents include, but are not limited to, chloroform, xylenes, hexanes, pyridine, tetrahydrofuran, ethyl acetate, and combinations thereof. The polymer can also be spray coated in dry form using powders with micron scale particle sizes, i.e., particles with diameters less than about 100, 50, 40, 20, 10 micrometers. In this instance, the polymer powder would need to be soaked with solvent or heated into a liquid melt to spread the powder particles into a more continuous coating after they are spray deposited.

[0126] The thickness of the polymer coatings is generally between about 1 and 1000 nm, between 1 and 500 nm, between 1 and 100 nm, between 1 and 50 nm. In some embodiments, the coating thickness is less than about 500, 450, 400, 350, 300, 250, 200, 150, 100, 90, 80, 70, 60, 50, 40, 30, 20 or 10 nm.

[0127] Spray coating process restricts the deposition of coating to the CNT tips and can limit clumping due to capillary forces associated with the drying of the solvent. The amount of coating visible on the CNT arrays increases with the number of sprays. Alternative techniques can be used to spray coat the coating materials onto the CNT arrays including techniques more suitable for coating on a commercial scale.

[0128] In another instance that demonstrates a coating process, CNT sheets can be dipped into coating solutions or melted coatings to coat CNTs throughout the array, increasing the thermal conductivity of the sheet in the cross-plane direction by greater than 20, 30, 50, or 70 %.

[0129] In other cases, the coating material can be deposited on the CNT array using deposition techniques known in the art, such as chemical deposition (e.g., chemical vapor deposition (CVD)), aerosol spray deposition, and electrochemical deposition.

[0130] In one instance, a polymer coating can be applied by electrochemical deposition. In electrochemical deposition, the monomer of the polymer is dissolved in electrolyte and the CNT array is used as the working electrode, which is opposite the counter electrode. A potential is applied between the working and counter electrode with respect to a third reference electrode. The monomer is electrooxidized on the CNT array tips or sheet sidewalls that face the electrolyte as a result of the applied potential. Controlling the total time in which the potential is applied controls the thickness of the deposited polymer layer.

[0131] 3. Other Coating Materials a. Metallic Nanoparticles or Metallic Layers

[0132] The CNT arrays can additionally be coated with or contain one or more metal-based nanoparticles or layers thereon. One or more metal-based nanoparticles may be adsorbed to the distal ends and / or sidewalls of the CNTs to bond the distal ends and / or sidewalls 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. Examples of suitable metal nanoparticles or layers can be made of or include palladium, gold, silver, titanium, iron, nickel, copper, tungsten, vanadium, molybdenum, niobium, tantalum, zirconium, oxides thereof, and combinations thereof.

[0133] In some cases, the thickness of the metallic layers, when formed as coatings on the CNT arrays, can be from between about 1 and 1000 nm, between 1 and 5000 nm, between 1 and 1000 nm, between 1 and 500 nm, between 1 and 100 nm, between 1 and 50 nm. In some embodiments, the coating thickness is less than about 10,000, 5,000, 2,500, 1,000, 500, 450, 400, 350, 300, 250, 200, 150, 100, 90, 80, 70, 60, 50, 40, 30, 20 or 10 nm. b. Flowable or Phase Change Materials

[0134] The CNT arrays can additionally be coated with or contain one or more flowable or phase change materials. In certain instances, where flowable or phase change materials are applied to the CNT arrays these may be added to the CNT array to displace the air between CNTs and improve contact between the distal ends and / or sidewalls 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.

[0135] 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. In some cases, the coating material, such as a non-polymeric coating material and the flowable or phase change material are the same material or materials.

[0136] The thickness of the flowable or phase change materials, when formed as coatings on the CNT arrays, can be from between about 1 and 30 micrometers, between 1 and 10 micrometers, between 1 and 1000 nm, between 1 and 500 nm, between 1 and 100 nm, between 1 and 50 nm. In some embodiments, the coating thickness is less than about 10,000, 5,000, 2,500, 1,000, 500, 450, 400, 350, 300, 250, 200, 150, 100, 90, 80, 70, 60, 50, 40, 30, 20 or 10 nm. c. Solders

[0137] The CNT arrays can additionally be coated with or contain one or more solder materials. Solders are made of a metal or metal alloy wherein the metal is selected from, but not limited to, aluminum, bismuth, copper, cobalt, chromium, gallium, zinc, tantalum, platinum, gold, nickel, iron, tin, lead, silver, titanium, indium, and alloys thereof. In some stances the solder material of the composite is a gold-tin solder, a tin-silver-copper solder, a tin-copper solder, a tin-lead solder, a tin-chrome solder a gallium solder, a gallium-indium-tin solder, or an indium solder. In some cases, the solder is a commercial solder material known in the art, such as a lead-free solder. Solders are typically formed primarily of tin optionally alloyed with other metals. The composition of the solder material can be chosen to have any suitable melting temperature or melting temperature range of about -19 to 300 °C. The composition of the solder materials can be chosen to have any suitable tensile strength. Solders may be coated on or infiltrated into the carbon nanostructures of an array in a molten solder state followed by cooling. d. Ceramics

[0138] The CNT arrays can additionally be coated with or contain one or more ceramic materials. Exemplary ceramics include, but are not limited to, AI2O3, AIN, BN, BeO, TiC, WC, S1O2, ZrCT. Hl'Ch, TiO , WO3, ZnO, and combinations thereof.

[0139] The thickness of the one or more ceramics, when formed as coatings on the CNT arrays, can be from between about 1 and 10,000 nm, between 1 and 5,000 nm, between 1 and 1,000 nm, between 1 and 500 nm, between 1 and 100 nm, between 1 and 50 nm. In some embodiments, the coating thickness is less than about 500, 450, 400, 350, 300, 250, 200, 150, 100, 90, 80, 70, 60, 50, 40, 30, 20 or 10 nm.

[0140] B. Multi-tiered Carbon Nanostructure Composites

[0141] The multi-tiered carbon nanostructure composites which may form part of the cooling devices described are typically planar composites having a top and a bottom surface. See Figure IB which shows a non-limiting side-view of a multi-tiered carbon nanostructure composite 200 having 3 tiers of arrays of vertically aligned carbon nanostructures (i.e., carbon nanotubes) 210 on opposite surfaces of supports 220 which are stacked. These planar composites are not particularly restricted in terms of size / dimension, thickness, and / or shape. Accordingly, the multi-tiered carbon nanostructure composites may be prepared, as described below, to the desired shape, thickness, and dimensions (length and width) needed for forming a cooling device.

[0142] The multi-tiered carbon nanostructure composites include at least a first layer or tier comprising a carbon nanotube array comprising vertically aligned carbon nanotubes grown from opposing surfaces of a first substrate, and at least a second layer or tier comprising a carbon nanotube array comprising vertically aligned carbon nanotubes grown from opposing surfaces of a second substrate, where the at least first layer or tier and the at least second layer or tier are stacked and the vertically aligned carbon nanotubes of the at least first layer or tier at least partially interdigitate the vertically aligned carbon nanotubes of the at least second layer or tier which are interfacing each other.

[0143] The CNT arrays described above, with respect to the single-layered carbon nanostructure composites, can be stacked according to the methods described below to afford such multi-tiered composites. In some instances, multi-tiered composites are formed by stacking CNT arrays according to a method including the steps of:

[0144] (1) providing at least two or more CNT arrays; and

[0145] (2) stacking the at least CNT arrays. where the stacking results in at least partial interdigitation of the nanostructures, CNTs, of the arrays. In some instances, the method of making the multi-tiered composite can further include a step of applying or infiltrating a coating, a coating of metallic nanoparticles, and / or a coating of flowable or phase change materials, which are described above. In some cases, the step of applying or infiltrating a coating, a coating of metallic nanoparticles, and / or a coating of flowable or phase change materials occurs prior to stacking, alternatively during stacking, or alternatively after stacking. In yet other instances, the method includes applying pressure during the stacking step. The applied pressure may be in the range of about 1-100 psi, 1-50 psi, 1-30 psi, more preferably about 1-20 psi, and most preferably about 1-15 psi. In some embodiments, the pressure is about 15 psi. Pressure may be applied continuously until the adjacent tiers are bonded, if a coating material(s) which can act as a bonding or adhesive agent, such as an adhesive or phase change material, is used. Pressure may be applied for any suitable amount of time. In some embodiments, only a short time is used, such as less than 1 minute, if no bonding agent is used.

[0146] For forming multi-tiered composites, at least two CNT arrays are stacked to form the multi-tiered composites. By using more CNT arrays the thickness of the multi-tiered composites can be increased, as needed. In some cases, up to 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, or more CNT arrays can be stacked according to the methods described above. The thickness of the resulting multi-tiered composites formed by stacking can be in the range 1-10,000 microns or more. In some instances, the thickness of the resulting multi-tiered composites formed by stacking can be about 1-3,000 micrometers or about 70-3,000 micrometers. In some instances, the number of layers and / or thickness is based on the thickness of the CNT forest formed on the arrays used in the stacking process.

[0147] In a non-limiting case, at least two vertically aligned arrays formed on supports / substrates are stacked / contacted such that the nanostructure elements, such as CNTs, of the arrays at least partially interdigitate on contact. In one instance, full interdigitation of carbon nanostructure elements of the arrays occurs within one another when stacked. In other instances, the CNT arrays may interdigitate only at the tips of the nanostructure elements, such as CNTs. In yet other embodiments, the individual nanostructures can navigate through the nanostructures of the adjacent array during the interdigitating process and the nanostructure elements of the individual arrays, such as the CNTs or some portion thereof, fully or substantially interdigitate within one another; “substantially,” as used herein, refers to at least 95%, 96%, 97%, 98%, or 99% interdigitation between the nanostructure elements of the individual arrays. In some instances, the extent of interdigitation is in the range of about 0.1% to 99% or at least about 1%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 95%.

[0148] In some cases, the CNT nanostructures of the stacked arrays, which interdigitate at least partially, may also form into larger superstructures, such as, but not limited to, tube bundles, clumps, or rows. These superstructures may be formed through mechanisms such as capillary clumping or by way of application of a polymer coating prior to, during, or following the stacking process.

[0149] In some embodiments, a polymer coating and / or adhesive, or other coating as described above, is applied to the CNT array(s) which are subsequently stacked. In such embodiments, the thickness of the coating and / or adhesive, or other coating as described above, is about 1-1000 nm, 1-500 nm, or 1-100 nm.

[0150] The CNT arrays of the multi-tiered composites can include a coating or coating material (terms can be used interchangeably) which adheres or is bonded to the CNTs, as described above regarding the coating materials suitable for single-layered composites. The coating / coating material can be applied as described. In some instances, the coating contains one or more oligomeric materials, polymeric materials, waxes, or combinations thereof. In other instances, the coating contains one or more non-polymeric materials. In some cases, the coating can contain a mixture of oligomeric, waxes, and / or polymeric material and non-polymeric materials. In still other instances, the coating can be formed of metallic nanoparticles, flowable or phase change materials, and / or solders, as described above regarding the coating materials suitable for single-layered composites. In certain instances, the CNTs of the arrays may be induced to be clumped or bundled, such as by exposure to solvent(s) and followed by drying. Clumping and bundling may be used to expose side- walls of the CNTs allowing them to be exposed to cooling fluid(s) and / or to coating materials, as described, which may be applied, coated, or infiltrated onto the arrays. In certain embodiments, the coating material(s) act as a bonding or adhesive agent(s). Without limitation, such coating material(s) can be selected from adhesives (z'.e., pressure sensitive adhesives, thermal adhesives), holt melts, hot glues, an epoxy, solders, a liquid-metal contacts, a phase change material (i.e., a wax or waxes), and combinations thereof. Such bonding or adhesive agent(s) are known in the art to the person of ordinary skill. Further, such bonding or adhesive agent(s) can be applied to CNT arrays using a variety of methods known in the art.

[0151] In certain cases of the above method, following the stacking step the method further includes a step of applying an adhesive to at least one surface of the CNT arrays on the top and / or bottom surface of the multi-tiered composite. Such an adhesive can include, but is not limited to, adhesives (i.e., pressure sensitive adhesives, thermal adhesives), holt melts, an epoxy, solders, a liquid-metal contacts, a phase change material i.e., a wax or waxes), a hot glue, and combinations thereof. A hot melt adhesive combines wax, tackifiers, and a polymer base to provide improved adhesion properties to one or more surfaces to which the multi-tiered composite may be interfaced with. In some instances, the adhesive is a pressure sensitive adhesive. In yet other instances, the adhesive is a combination of a pressure sensitive adhesive polymer and a thermally activated (or activatable) adhesive polymer.

[0152] C. Modification(s) of Single-layered and Multi-tiered Carbon Nanostructure Composites

[0153] 1. Surface Energy Modification

[0154] In certain instances, it may be desirable to modify the surface energies of at least a portion of the carbon nanostructures present in the single-layered or multi-tiered carbon nanostructure composites described. This may be done to modify the extent of hydrophobicity or hydrophilicity of carbon nanostructures present therein. “Hydrophobicity,” as used herein refers to composites that lack an affinity for water; tending to repel and not absorb water or aqueous solutions. In some instances, the as-formed CNT arrays of the composites are by themselves hydrophobic. “Hydrophilicity,” as used herein refers to composites that readily interact and do not repel water. In some instances, the surface energy may be tuned to affect the affinity for a cooling fluid other than water, such fluorocarbons, or hydrocarbons to produce similar phenomena as observed with hydrophilic or hydrophobic coatings, such as when water is the cooling fluid.

[0155] In some cases, the carbon nanostructures or clumped carbon nanostructures thereof can have a surface energy that wicks one or more cooling fluids back towards the nucleation sites (such as the microstructures). It is believed in some instances, that modifying the arrays can result in the carbon nanostructures acting as wicks which can replenish cooling fluid into the arrays of the device due to large capillary force. This increases the critical heat flux and delays dry-out. Accordingly, in some instances, it is possible to selectively tune the surface energies of the carbon nanostructures of the array from their as-formed hydrophobic (no modification) surface energy and increase their hydrophilicity.

[0156] It is also believed that modifying the arrays can help to reduce superheating required for the onset of nucleation. Increasing the hydrophobicity of a surface can limit bubble diameter, increase bubble departure frequency, and / or increase the availability of nucleation sites. Accordingly, in some instances, it is possible to selectively tune the surface energies of the carbon nanostructures of the array from their as-formed hydrophobic (no modification) surface energy and increase their hydrophobicity to a maximum possible extent, and any possible surface energies in between. This allows the degree or extent of hydrophobicity to be tuned, as needed. In general, surface(s) may be considered hydrophobic when the contact angle of a droplet of a fluid of interest (typically water) placed on the surface is greater than 90°. The surface is considered hydrophilic when the contact angle is <90°. A surface may be considered superhydrophobic if the contact angle is > 150°. In some instances, the hydrophobicity of the surface is further enhanced or maintained due to the nano-scale structure, micro-scale structure or hierarchal micro / nano structures of the surface or coating(s) applied to the surface.

[0157] Accordingly, in some instances, the carbon nanostructures of the single-layered or multitiered carbon nanostructure composites may be modified to be hydrophilic, hydrophobic, or a combination thereof. For instance, it may be desirable that only one side or surface of the composites be modified to be hydrophilic and that the other side is hydrophobic, or remains unmodified as the as-formed array. This may be accomplished by exposing only one side of the composites to modification treatment(s).

[0158] In some instances, the coatings previously described may be selected based on their ability to modify the hydrophilicity of the array onto which they are coated or infiltrated on. For instance, the carbon nanostructures of the single-layered or multi-tiered carbon nanostructure composites can be coated with a metal, a polymer, and / or a ceramic, such as those described above, to modify the surface energy of the nano- and / or micro- scale surface structures and modify the wetting characteristics of the coated carbon nanostructure composites to one or more cooling fluids.

[0159] In some instances, the carbon nanostructure arrays grown on the supports can be further exposed to conditions which create defect sites on the carbon nanostructures, as may be desired. For example, the carbon nanostructure arrays grown on the supports can be exposed to a plasmabased treatment (i.e., oxygen plasma), and / or an acid bath treatment (i.e., nitric acid bath) in order to create defect sites on the carbon nanostructures of the array and / or introduce hydrophilic functional groups thereon, as compared to arrays which were not treated with plasma and / or acid bath treatments.

[0160] Such plasma and / or acid bath treatments may be applied following formation of the CNT arrays and before the addition of any coating(s) and any stacking to form multi-tiered composites. However, there is no particular limitation on when such surface energy modification treatments may be performed. In some cases, such treatments may be applied to one surface of the composites (top or bottom), which is meant to be interfaced with a heat source(s) and / or cooling fluids. The person of ordinary skill in the art is familiar with plasma and / or acid bath treatments in the art of carbon nanotubes and the selection of conditions and parameters for modifying the carbon nanostructures therein.

[0161] As a non-limiting example, the hydrophilicity of carbon nanostructures of an array can be modified by selectively installing hydrophilic groups (such as carboxylic acid functional groups, or -OH groups) onto the carbon nanostructures. The degree of modification can be varied by parameters, such as exposure times. In some instances, depending on the properties of the cooling fluids, the surface energy of at least a portion of the composites may be modified to render it more compatible with the cooling fluids.

[0162] 2. Nucleation Sites and Optional Patterning Modification

[0163] As described above, the single-layered or multi-tiered carbon nanostructure composites of the heat spreaders include at least nano-scale surface structures due to the presence of carbon nanostructures (i.e., carbon nanotubes) of the array(s) present in the composites. Such nano-scale surface structures are believed to serve as nucleation sites for forming bubbles, when surfaces containing such structures are contacted to cooling fluids which boil during exposure to heat from heat source(s). The heat source(s) are contacted to one side / surface of the heat spreader and the cooling fluid(s) are contacted to the opposite side / surface of the heat spreader.

[0164] In certain instances, it may also be desirable to form pattern(s) on one or more surfaces of at least a portion of the carbon nanostructures present in the single-layered or multi-tiered carbon nanostructure composites described. This can be done to include additional nano- and / or microscale surface structures thereon. These may be present throughout the CNT arrays forming the composites or may be present at least on one or both of the outermost facing surfaces of the CNT arrays of the composites. Such nano- and / or micro- scale surface structures are also believed to serve as nucleation sites and can enhance nucleation and formation of bubbles, when surfaces containing such structures are exposed to cooling fluids which boil during exposure to heat from heat source(s).

[0165] The presence of such nano- and / or micro- scale surface structures is believed to increase active nucleation site density and accelerate bubble departure rates when contacted with a cooling fluid, resulting in boiling enhancement. These features can be used to control aspects including bubble departure diameter and location, reducing the likelihood of film coalescence. In some instances, bubble departure rates can be about every 10 ms to about every 100 ms, and ranges in between.

[0166] In some cases, the hydrophobicity or hydrophilicity of the composite surface having such nano- and / or micro- scale surface structures can affect the average bubble diameter formed thereon where hydrophobicity can induce larger diameters, such as up to 10 mm, and hydrophilicity can induce smaller diameters, such as down to 1 mm.

[0167] The presence of such additional nano- and / or micro- scale surface structures are believed to provide increased surface area and can also provide improved heat transfer coefficients, such as in single phase cooling applications.

[0168] Non-limiting micro-scale surface structures can be patterned onto the arrays of the composites and can form grids, channels, islands, hydrofoils, or combinations thereof thereon. In some instances, the micro-scale surface structures have feature sizes (such as width of a microscale channel, grid, or island, height of a CNT array, array pitch when grids, channels, and / or islands are present, etc.) ranging from between about 100 nm to about 500 pm, as well as sub-ranges and individual values contained within. “Array pitch,” as used herein, refers to the distance between two surface structures. For instance, array pitch is the distance between the centers of two adjacent posts or pins on a surface. In some instances, the micro-scale surface structures have feature sizes (such as width of a microscale channel, grid, or island, height of a CNT array, array pitch when grids, channels, and / or islands are present, etc.) ranging from between about 0.1 nm to about 1000 mm, as well as sub-ranges and individual values contained within. In some other instances, such channels can be microchannels optionally having average lengths of between about 1 pm to about 250 pm.

[0169] Non-limiting examples of patterns which may be formed onto the arrays are shown in Figures 6A-6E. Figure 6A shows a non-limiting top-down view of a patterned array of carbon nanostructures defining channels 810 on a surface, where the carbon nanostructures 800 form rows of the carbon nanostructures defining channels in between. Figure 6B shows a non-limiting side view of a patterned array of carbon nanostructures having defining channels 810 on a surface, where the carbon nanostructures 800 form rows of the carbon nanostructures defining channels in between and the surface is a heat spreading substrate 820. Figure 6C shows a nonlimiting top-down view of a patterned array of carbon nanostructures forming a grid pattern on a surface formed from rows of crossing carbon nanostructures 800 thereon. Figure 6D shows a non-limiting top-down view of a patterned array of carbon nanostructures having a grid pattern formed from specific areas of carbon nanostructures 800 on a surface. Lastly, Figure 6E shows a non- limiting top-down view of a patterned array of carbon nanostructures having a plurality of pillars or islands formed from specific areas of carbon nanostructures 800 on a surface.

[0170] In some instances, such as the square features and circular features shown in Figures 6D and 6E, these features have dimensions, such as length or diameter, which ranges from between about 5 to 500 microns, as well as individual values or sub-ranges within the aforementioned range. In some instances, the dimensions are in a range from about 5 to 250, 5 to 200, 5 to 150, 5 to 100, 5 to 75, 5 to 50, or 5 to 25 microns, as well as individual values or sub-ranges within the aforementioned ranges.

[0171] Suitable methods for patterning are known in the art. In some instances, the carbon nanostructure (i.e., carbon nanotube) arrays used to form the composites can be patterned by inducing capillary action to clump the nanostructures into bundles and creating channels between the clumps formed. This can be achieved by wetting the arrays with solvent(s) followed by removal of the solvents, such as by evaporation or drying. In some other instances, the arrays can be patterned using laser(s) or other focused energy sources. Patterning can be performed at any suitable time during fabrication of the cooling devices. In some instances, patterning is performed following attachment of the single-layered or multi-tiered carbon nanostructure composites to a heat spreading substrate.

[0172] D. Properties of Single-layered and Multi-tiered Carbon Nanostructure Composites

[0173] In some instances, the composites described exhibit thermal resistances of less than about 5.0, 2.5, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, or 0.1 cm2K / W. In certain instances, the thermal resistance value of a multi-tiered composite formed by stacking of CNT arrays is the same or substantially unchanged, as compared to the value(s) of the single-layered composite used to form the stack; “substantially,” as used herein refers to less than a 10%, 5%, 4%, 3%, 2%, or 1% change. In some cases, the composites function as thermal interface materials (TIMs). Such TIMs can exhibit thermal resistance hysteresis and stable operation over a wide pressure range of about 0 to 500 psi, 0 to 400 psi, 0 to 300 psi, 0 to 200 psi, or 0 to 100 psi, when increasing and decreasing the pressure on the TIM in the aforementioned ranges.

[0174] In one instance, the apparent thermal conductivity of the CNT arrays of the multi-tiered structures formed by stacking of such CNT arrays is increased by at least about 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70% or greater compared to single-layered structures.

[0175] In some cases, the composites described exhibit conductance values in the range of about 1-2500 W / m-K, 1-2000 W / m-K, 1-1500 W / m-K, 1-1000 W / m-K, 1-500 W / m-K, 5-500 W / m-K, 5-400 W / m-K, 5-300 W / m-K, 5-200 W / m-K, 5-150 W / m-K, 5-100 W / m-K, or 3-30 W / m-K.

[0176] Coating(s), which may be applied to the composites, can be an effective means for increasing the contact area and reducing the thermal resistance of CNT forest thermal interfaces. The bonding process added by inclusion of (nanoscale) coatings around individual CNT contacts includes, for example, pulling, through capillary action, of additional CNTs close to the interface to increase contact area.

[0177] In some instances, the one or more cooling fluids may also serve to increase contact area at the interface formed with the cooling devices, or reduce contact resistance at the interface, as compared to that observed when the interface is not exposed to the one or more cooling fluids. Sometimes this decrease in thermal resistance at the interface, when exposed to the cooling fluid(s), may be at least about 100%, 90%, 80%, 70%, 60%, 50%, 40%, 30% or 20% reduction in thermal resistance, as compared to the dry interface.

[0178] The composites demonstrate good compliance, i.e., the ability to conform when contacted to one or more surfaces of heat generating source(s) or material(s) (such as a direct or packaged die, chip, electronics or components thereof). Compliant TIMs formed from the composites described can have contact areas at interfaces between surface(s) of heat generating source(s) and the TIM, such that the compliance of the TIMs, expressed as a percentage value, is between about 1% to 50%, 1% to 40%, 1% to 30%, 1% to 25%, 1% to 20%, or 1% to 10% of the total thickness of the TIM.

[0179] The composites described can also exhibit excellent elastic recovery properties following one or more repeated deformations, typically compressions, at varying pressures up to about 50, 100, 200, 300, 400, 500 psi, or greater. Elastic recovery of the composites, expressed as a percentage value, following one or more compressions can be greater than about 50%, 60%, 70%, 80%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99%. In some instances, the composites described also demonstrate compression set properties following one or more repeated deformations, typically compressions, at varying pressures up to about 50, 100, 200, 300, 400, 500 psi, or greater. Compression set of the composites, expressed as a percentage value, following one or more compressions can be less than about 20%, 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.9%, 0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2%, or 0.1%.

[0180] III. Methods of Making Carbon Nanostructure-based Devices for Cooling

[0181] The carbon nanostructure -based device for cooling described above can be formed according to various methods.

[0182] In a first non-limiting example, a method of making a carbon nanostructure-based device for cooling includes the steps of:

[0183] (a) forming or providing a single-layered carbon nanostructure composite, which is planar and includes a top surface and a bottom surface, to form the device; where at least the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0184] In another non-limiting example, a method of making a carbon nanostructure-based device for cooling includes the steps of:

[0185] (a’) forming or providing a multi-tiered carbon nanostructure composite, which is planar and includes a top surface and a bottom surface, to form the device; where at least the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the bottom surface is a thermal interface for attachment for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0186] In yet another non-limiting example, a method of making a carbon nanostructure-based device for cooling includes the steps of: (a”) attaching a first single-layered carbon nanostructure composite onto a top surface of a heat generating substrate;

[0187] (b”) attaching a second single-layered carbon nanostructure composite onto a bottom surface of the heat generating substrate to form the device; where at least the first single-layered carbon nanostructure composite on the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the second single-layered carbon nanostructure composite on the bottom is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0188] In still another non-limiting example, a method of making a carbon nanostructure-based device for cooling includes the steps of:

[0189] (a’ ”) attaching a first multi-tiered carbon nanostructure composite onto a top surface of a heat spreading substrate;

[0190] (b’”) attaching a second multi-tiered carbon nanostructure composite onto a bottom surface of the heat spreading substrate to form the device; where at least the first multi-tiered carbon nanostructure composite on the top surface includes nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; where the second multi-tiered carbon nanostructure composite on the bottom is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

[0191] In certain instances of the methods above, which include a heat spreading substrate, combinations of single-layered and multi-tiered carbon nanostructure composites may be used. For example, a single-layered carbon nanostructure composite can be placed on the top surface of the heat spreading surface and a multi-tiered carbon nanostructure composite may be placed on the bottom, or vice versa, while meeting the requirements specified in the respective methods above.

[0192] Details for forming the single-layered and multi-tiered composites used in the methods are as described above. Details of the heat spreading substrates are also described above. Such heat spreading substrates may be formed according to art known methods or may be purchased from commercial sources. The shape, dimensions, and thickness of the heat spreading substrate can be selected or modified, as appropriate, for use in the methods including such substrates.

[0193] In instances of the methods where composites are attached to a heat spreading substrate this can be achieved by use of adhesive(s) or by way of fasteners or bolts / screws, which may be used to adhere / bond or held on the composite in place onto the appropriate surface of the heat spreading substrate. Suitable adhesives and fasteners (i.e., bolts) are known in the art. In some instances, the composites themselves may be adhesive and will adhere or bond to the appropriate surface of the planar substrate without the need for any additional adhesive needed. In instances where fasteners, such as bolts or screws, are used an adhesive is optional. The attaching steps of the methods, where an adhesive is used, may optionally include the application of pressure and / or heat, as appropriate. Once attached to a heat spreading substrate, the composites are firmly held onto the heat spreading substrate and are resistant to removal or peeling when exposed to a flowing of fluids, such as cooling liquids.

[0194] In some instances of the methods, one or more coatings, as defined above, can be applied to the one or more arrays present in the single-layered or multi-tiered composites. In some instances, the coating(s) are adhesive and are applied to at least one surface of the CNT arrays present on the top and / or bottom surface of the single-layered or multi-tiered composites. Such an adhesive can include, but are not limited to, adhesives (i.e., pressure sensitive adhesives, thermal adhesives), holt melts, hot glues, an epoxy, solders, a liquid-metal contacts, a phase change material (i.e., a wax or waxes), a hot glue, and combinations thereof. A hot melt adhesive combines wax, tackifiers, and a polymer base to provide improved adhesion properties to one or more surfaces to which the single-layered composite may be interfaced with. In some instances, the adhesive is a pressure sensitive adhesive. In yet other instances, the adhesive is a combination of a pressure sensitive adhesive polymer and a thermally activated (or activatable) adhesive polymer.

[0195] In certain instances of the above methods, a patterning step is included where micro-scale surface structures are patterned onto the arrays of the composites to form grids, channels, islands, hydrofoils, or combinations thereof thereon. In some instances, the micro-scale surface structures have feature sizes (such as width of a microscale channel, grid, or island, height of a CNT array, array pitch when grids, channels, and / or islands are present, etc.) ranging from between about 100 nm to about 500 pm, as well as sub-ranges and individual values contained within. In some instances, the micro-scale surface structures have feature sizes (such as width of a microscale channel, grid, or island, height of a CNT array, array pitch when grids, channels, and / or islands are present, etc.) ranging from between about 0.1 nm to about 10 mm, as well as sub-ranges and individual values contained within. In some other instances, such channels can be microchannels optionally having average lengths of between about 1 pm to about 250 pm.

[0196] Suitable methods for patterning are known in the art. In some instances, the carbon nanostructure (i.e., carbon nanotube) arrays used to form the composites can be patterned by a step of inducing capillary action to clump the nanostructures of the composites into bundles and creating channels between the clumps formed. This can be achieved by wetting the array(s) with solvent(s) followed by removal of the solvents, such as by evaporation or drying. In some other instances, the arrays can be patterned using laser(s) or other focused energy sources. Patterning can be performed at any suitable time during the methods of making the cooling devices. In some instances, patterning is performed following attachment of the single-layered or multitiered carbon nanostructure composites to a heat spreading substrate.

[0197] In some instances of the above methods, the vertically aligned carbon nanotubes on the supports can independently include one or more coatings formed from metals, metal oxides, nanoparticles, ceramics, and / or polymers. Exemplary metals, metal oxides, nanoparticles, ceramics, and / or polymers are described above.

[0198] In certain instances where a heat spreading substrate is included the resulting device can be stamped to provide a cooling device having a defined shape, such as including channels, fins, pins, tubes, or other suitable shapes. In certain other instances where the cooling devices do not include a heat spreading substrate, the single-layered or multi-tiered composites themselves include a support which can be stamped to hold a defined shape, such as to provide a device having channels, fins, pins, tubes, or other suitable shapes. Such a stamping step typically occurs following formation of the device. As an example, a device may formed from planar structures (such as heat spreading substrates) than after stamping have features that are U shaped. When stacking structures with the U shaped features, closed passages can be formed through which a fluid may flow, with the bottom surface of an upper layer serving as the lid or cap of the U shaped feature stamped into the structure.

[0199] In certain other instances, the cooling devices formed (with or without a heat spreading substrate) can be rolled to provide a cooling device have a tube shape. Such tubular devices may be placed on inner surface and / or outer surface of a tube-shaped heat source.

[0200] IV. Methods of Cooling Using Carbon Nanostructure-based Devices

[0201] The carbon nanostructure-based devices described above can be used for cooling one or more heat sources. Exemplary heat sources include, without limitation, one or more dies (such as semiconductor dies, direct dies, or packaged dies), microchips, batteries, battery cells, power transformers, passive electronics, lids, heated surfaces, lasers, detectors, other heat generating electronics, as well as combinations thereof.

[0202] In an exemplary cooling method, the method includes the steps of:

[0203] (1) attaching a carbon nanostructure-based cooling device to one or more heat sources;

[0204] (2) contacting at least the nano- and / or micro- scale surface structures of the device to one or more cooling fluids; and

[0205] (3) optionally flowing or circulating the one or more cooling fluids during step (2); wherein the device increases at least one of the following: active boiling or condensing nucleation site density, turbulent mixing, critical heat flux, heat transfer coefficient(s), and / or wherein the device at least reduces: superheating required for boiling nucleation between the one or more heat sources and the one or more cooling fluids, and provides increased and / or enhanced cooling of the one or more heat sources, as compared to a method for cooling which does not include a carbon nanostructure-based cooling device having the nano- and / or micro-scale surface structures present.

[0206] Figures 3A and 3B show non-limiting illustrations of a carbon nanostructure -based cooling device during active cooling of a heat source. Figure 3A shows an exemplary carbon nanostructure-based cooling device 500 of the types described during cooling where the device is in contact with a heat source 510 and a cooling fluid 520 is present on one side of the device which contains nucleation sites on at least the surface contacting the cooling fluid and enhances boiling characteristics, such as bubble formation 525.

[0207] Figure 3B shows an exemplary carbon nanostructure-based cooling device 500 of the types described during cooling where the device is placed in contact with a heat source 510 forming a thermal interface 530 to the heat source. A cooling fluid 520 is present on all sides of the device and heat source and the device contain nucleation sites that enhance boiling characteristics, such as bubble formation 525.

[0208] The cooling devices used in the methods can accelerate bubble formation, increase bubble nucleation density, and / or accelerate release or departure of bubbles, when the one or more cooling fluids are boiling. In some instances, the increase and / or enhancement may be defined by an increase in active boiling or condensing nucleation site density, mixing, critical heat flux, and / or heat transfer coefficients by at least about 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or higher, as compared to a cooling method which does not use a carbon nanostructure-based cooling device having the nano- and / or micro-scale surface structures present.

[0209] In some instances, the increase and / or enhancement may be defined by a decrease in superheating required for boiling nucleation by at least about 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or higher, as compared to a cooling method which does not use a carbon nanostructure -based cooling device having the nano- and / or micro-scale surface structures present. “Superheating,” as used herein, refers to the temperature above the saturation temperature of a liquid.

[0210] In the cooling methods, one or more cooling fluids can be, for example, selected from distilled water, deionized water, fluorocarbon-based fluids (such as HFE fluids (segregated hydrofluoroethers), perfluorinated compound fluids, NOVEC fluids (such as fluoroketones), hydrocarbon fluids (such as mineral oil), non-fatty saturated straight chain alcohols. Ci to C30 alkanes, fully or partly halogenated hydrocarbons (such as HCFC / CFC / HFC refrigerants), and combinations thereof. Such cooling fluids can be obtained from commercial sources. In some instances, the cooling fluids can have a boiling temperature in a range from between about 30 °C and 100 °C.

[0211] In certain instances of the cooling methods, the method includes immersing the entire device and the one or more heat sources into the one or more cooling fluids in step (2) and during optional step (3). In some cases, the cooling methods can be employed in data center cooling, which can involve convective or pool boiling. Convective boiling includes flow of one or more cooling fluids, whereas pool boiling does not include forced or pumped flowing of the cooling fluids. Pool boiling may include for example buoyancy driven flow.

[0212] The present invention will be further understood by reference to the following non-limiting examples.

[0213] EXAMPLES

[0214] Example 1: Two-Phase Immersion Cooling

[0215] This example relates to two (2) case studies, as described below:

[0216] Case Study 1:

[0217] Application tests and characterization of Carbice Pad as a boiling enhancement coating. Study of thin and thick carbon nanotube (CNT) arrays versus a bare copper surface.

[0218] Case Study 2:

[0219] Study on the impact of microstructure grid patterns on increasing the efficacy of the boiling enhancement coating.

[0220] Nomenclature and Abbreviations:

[0221] 2PIC: Two-phase immersion cooling

[0222] Novec 7000: Immersion cooling fluid with 34 °C boiling temperature Novec 7100: Immersion cooling fluid with 61 °C boiling temperature

[0223] Novec 649: Immersion fluid with 49 °C boiling temperature. Has zero global warming potential.

[0224] BEC: Boiling enhancement coating

[0225] ONB: On-set of nucleate boiling

[0226] CHF: Critical heat flux

[0227] More particularly, the goals and motivations for the case studies undertaken were to study a Carbice Pad boiling enhancement coating to enhance boiling efficiency in two phase immersion cooling. In addition, these studies also assess enhancement to the onset of nucleate boiling (ONB) point, and to the critical heat flux (CHF) and onset temperature of CHF.

[0228] Methods:

[0229] An exemplary test setup for evaluating carbon nanostructure-based cooling devices with a boiling enhancing surface is shown in Figure 7.

[0230] The pool boiling test setup is summarized as follows:

[0231] • An acrylic box for conducting the pool boiling tests. The box has a mounted sample holder at the bottom and is mounted on a stand to elevate the bath. The box is enclosed. And has a condenser coil mounted on top which flows cooling water. The condenser coil recondenses the vapor of the fluid bath, which is an immersion cooling fluid. There are also thermocouples (TC) inside the box to measure the fluid bath temperature. The sample holder has an outer insulating layer that is made of, for example, PEEK, which forms an insulating collar. Inside the sample holder is a heater bar, which serves as the reference interface for testing. The heater bar is made out of, for example, copper. The heater bar fits into the insulating collar with a gasket and four screws. The insulating collar assembly is mounted to the bottom of the boiling box. Aside from thermal insulation for the bar, this insulating collar assembly functions as a sample holder, wherein the sample can be modularly removed between the bar, the box, or both. At the base of the heater bar are inserted cartridge heaters which provide heating power. The heater bar has depth-probing thermocouples (TCs) inserted to different depths. Thermocouples are mounted from the bottom upwards. This design choice was required to mount the thermocouples close enough to the heater bar surface, since the thickness of the box walls get in the way of horizontal mounting. As a result, thermocouple depth accuracy is important in computing the heat flux and the surface temperature. These TCs are used to extrapolate the bar surface temperature and also to determine the heat flux traveling through the heater bar.

[0232] The cartridge heaters are powered using an analog power supply with the supplied voltage monitored with a multimeter.

[0233] Nominally a boiling enhancing coating is applied to the top of this copper bar to characterize the impact on the boiling phenomena / behavior.

[0234] In some tests, a boiling spreader plate is mounted to the copper bar using 4 comer screws at a pressure of about 30-60 psi. A thermal interface material (TIM) is used underneath this boiling surface to make thermal contact. The boiling plate itself is made of 2 mm copper and is 2” x 2”. The boiling plate is either a standard bare machined metal, having milled copper groves, and is sanded to roughen the surface for boiling, or has a boiling enhancement coating, such as a carbon nanostructure-based device described herein, applied to the surface.

[0235] Measurement Procedure:

[0236] 1. Turn on cartridge heater to desired power.

[0237] 2. Monitor all temperatures until steady state condition is reached, then record temperatures.

[0238] 3. Increase heater power gradually and repeat until system reaches critical heat flux (whereupon the temperature will not hit a steady state but continue to rise).

[0239] 4. After CHF is achieved, measure at descending powers. Computations:

[0240] Bar heat flux and the bar surface temperature were computed by interpolating the three

[0241] TCs that were mounted at different depths within the copper test bar:

[0242] Top of the bar (x = 0): Tsurf = m * (0 — xf) + TrHeat flux:

[0243] Boiling heat transfer coefficient (HTC), denoted h, was computed via the ratio between the heat flux q", and the copper bar’s superheat temperature (superheat temperature is the temperature of the bar relative to the saturation temperature of the bath fluid):

[0244] Results and Discussion:

[0245] Characterization of Boiling Surface Enhancements using Novec 649 immersion cooling fluid

[0246] The cooling fluid used here was Novec 649 which has a 49 °C boiling point.

[0247] Sample 1: Bare Copper

[0248] A bare copper bar was tested first to establish a control sample.

[0249] Figure 8A is a graph of the boiling curve of the copper bar, and Figure 8B is a graph showing the heat transfer coefficient (HTC) as a function of the bar superheat. The bare copper sample experiences on-set nucleate boiling (ONB) at about 59 °C, which is a superheat temperature of about 10 °C. The critical heat flux CHF (and denoted by the arrow in Figure 8A) was measured at a flux of 17.5 W / cm2and at a temperature of 76 °C (superheat of about 27 °C). The peak HTC that was observed was 6300 W / m2-K.

[0250] Sample 2: Boiling Surface Enhancement, 65 um thickness CNT array

[0251] A Boiling Surface Enhancement (BES) having a carbon nanotube array with 65 um thickness present thereon.

[0252] The 65 um thick BES experiences an on-set nucleate boiling (ONB) of about 52 °C, which is a superheat of 3 °C. This is substantially lower than the bare copper surface of Sample 1 , which shows the efficacy of the boiling surface at promoting bubble nucleation at lower superheat temperatures.

[0253] The CHF (denoted by the arrow in Figure 9A) was measured at a flux of 13.2 W / cm2and at a temperature of 71 °C. The peak HTC that was observed was 8000 W / m2-K. See Figure 9B.

[0254] Since the ONB is lower, and the HTC is higher than the bare copper surface, this shows that the BES indeed acts to enhance the boiling heat transfer. The nanostructure of the aligned carbon nanotubes of the array acts as a hydrophobic surface coating that facilitates bubble departure more efficiently than the natural roughness of the bare copper surface alone. It was also observed that for the sample before onset of nucleate boiling no bubbles, just film convection was seen. For Sample 2 at the onset of the CHF the boiling was heavily turbulent and there was formed a column of vapor that began to oscillate and chum.

[0255] Further, it was noted that the CHF is overall lower than that of the copper, which indicated that the sample experiences dry-out earlier with less heat flux than the copper.

[0256] Sample 3: Boiling Surface Enhancement, 305 um thickness CNT array

[0257] A Boiling Surface Enhancement (BES) having a carbon nanotube array with 305 um thickness present thereon.

[0258] This sample was thicker than Sample 2 to illustrate the effect of the aligned nanotube heights on the boiling physics.

[0259] For Sample 3, based on Figure 10A and Figure 10B graphs, the following was observed:

[0260] • ONB was observed at about 50 °C. In this sample, the nucleate boiling was observed suddenly at very near the saturation temperature. The nanostructure of the longer CNTs causes very small bubbles to form.

[0261] • CHF was found to be 11.2 W / cm2at 91 °C.

[0262] • Max HTC was found to be only 4400 W / m2-K.

[0263] It is noted that in this sample the thicker CNTs acted to promote bubble nucleation at a much lower superheat temperature. However, the increased CNT thickness also caused higher overall series thermal resistance in the system, which caused an overall poor HTC and overall lower CHF than the other samples.

[0264] Effect of Microstructuring on Boiling Enhancement Surface

[0265] The cooling fluid used here was Novec 649 which has a 49 °C boiling point.

[0266] A Boiling Enhancement Surface (BES) was etched into patterns to create a combination of micro and nano surface structures made of carbon nanotubes (CNTs). The CNTs act as a hydrophobic surface that facilitate bubble departure, and also the natural capillary wicking of the CNTs can promote cooling fluid replenishment. The microscale cavities that were etched into the sample (i.e., by removing the CNTs via laser etching to expose the foil carrier underneath) can act to isolate bubbles and control the bubble size. The overall goal was to delay the CHF temperature, lower the ONB, and increase the overall HTC.

[0267] Microstructured samples were etched to create square features. All microstructured samples included aligned CNT arrays having a 65 um nominal thickness. The sample dimensions that were investigated were 250 um and had 500 um feature sizes, which are the lengths of the square features etched in the samples. Figure 11 shows an example of the 250 um etched pattern, where the silver-colored squares are the aluminum foil substrate, on which the CNT array was grown, after the CNTs were removed therefrom to form the square features.

[0268] Comparison of the boiling curves for the 65 um thick boiling surface with various grid sizes was made versus no grid. In general, there was an overall improvement in the boiling characteristics by incorporating an etched-grid microstructure pattern. This was reflected in a slight, but consistent reduction in the ONB ; an increase in the CHF, and an overall improvement in the HTC, as shown in Figures 12A and 12B.

[0269] There appeared to be some correlation between the grid size and the amount of improvement observed. The grid pattern itself appeared to ensure bubble isolation and delayed the dry-out. The smaller grid pattern saw higher CHF and higher HTC, which shows that the grid size allows more rapid bubble departure with smaller bubble sizes.

[0270] Peak CHF realized was 18.0 W / cm2, which occurred at only 67 °C. The peak HTC of 13,500 W / m2-K was achieved. This is a 2x improvement in the cooling capacity of the boiling enhanced surface versus a standard machined copper surface.

[0271] Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of skill in the art to which the disclosed invention belongs. Publications cited herein and the materials for which they are cited are specifically incorporated by reference.

[0272] Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.

Claims

We claim:

1. A carbon nanostructure-based device for cooling comprising: a single-layered carbon nanostructure composite, which is planar and comprises a top surface and a bottom surface; wherein at least the top surface comprises nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; wherein the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

2. A carbon nanostructure-based device for cooling comprising: a multi-tiered carbon nanostructure composite, which is planar and comprises a top surface and a bottom surface; wherein at least the top surface comprises nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; wherein the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

3. A carbon nanostructure-based device for cooling comprising: a heat spreading substrate with a top surface and a bottom surface; a first single-layered carbon nanostructure composite on the top surface; and a second single-layered carbon nanostructure composite on the bottom surface; wherein at least the first single-layered carbon nanostructure composite on the top surface comprises nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures;wherein the second single-layered carbon nanostructure composite on the bottom is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

4. A carbon nanostructure-based device for cooling comprising: a heat spreading substrate with a top surface and a bottom surface; a first multi-tiered carbon nanostructure composite on the top surface; and a second multi-tiered carbon nanostructure composite on the bottom surface; wherein at least the first multi-tiered carbon nanostructure composite on the top surface comprises nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; wherein the second multi-tiered carbon nanostructure composite on the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

5. The device of any one of claims 3 or 4, wherein the heat spreading substrate is made from a material selected from copper, aluminum, steel, AlSiC, AlSilOMg, graphite, and combinations thereof.

6. The device of any one of claims 1 or 3, wherein the single-layered carbon nanostructure composites are hydrophilic, hydrophobic, or a combination thereof.

7. The device of any one of claims 1 or 3, wherein the carbon nanostructures of the singlelayered carbon nanostructure composites are coated with a metal, a polymer, and / or a ceramic to modify the surface energy of the nano- and / or micro- scale surface structures and modify the wetting characteristics of the coated carbon nanostructure composites to the cooling fluid.

8. The device of any one of claims 2 or 4, wherein the multi-tiered carbon nanostructure composites are hydrophilic, hydrophobic, or a combination thereof.

9. The device of any one of claims 2 or 4, wherein the carbon nanostructures of the multitiered carbon nanostructure composites are coated with a metal, a polymer, and / or a ceramic to modify the surface energy of the nano- and / or micro- scale surface structures and modify the wetting characteristics of the coated carbon nanostructure composites to the cooling fluid.

10. The device of any one of claims 1-9, further comprising an adhesive selected from the group consisting of a pressure sensitive adhesive, a holt melt, hot glue, an epoxy, a solder, a liquid-metal contact, reactive adhesives including crosslinked silicones, solvent based adhesives,and combinations thereof; wherein the adhesive is optionally present on the surface for attachment to the one or more heat sources.

11. The device of any one of claims 1-10, wherein the micro-scale surface structures form grids, arrays, channels, islands, hydrofoils, or combinations thereof.

12. The device of claim 11 , wherein the surface structures have average sizes ranging from between about 10 nm to about 250 pm.

13. The device of claim 11 , wherein the surface structures have average sizes ranging from between about 0.1 mm to about 1000 mm.

14. The device of any one of claims 1 or 3, wherein the single-layered carbon nanostructure composites comprise a carbon nanotube array comprising vertically aligned carbon nanotubes grown from opposing surfaces of a support.

15. The device of claim 14, wherein the vertically aligned carbon nanotubes grown from opposing surfaces of the support can independently comprise one or more coatings formed from metals, metal oxides, nanoparticles, ceramics, and / or polymers.

16. The device of claim 14, wherein the vertically aligned carbon nanotubes grown from opposing surfaces of the support can independently be clumped or bundled.

17. The device of any one of claims 2 or 4, wherein the multi-tiered carbon nanostructure composites comprise: at least a first layer or tier comprising a carbon nanotube array comprising vertically aligned carbon nanotubes grown from opposing surfaces of a first support, and at least a second layer or tier comprising a carbon nanotube array comprising vertically aligned carbon nanotubes grown from opposing surfaces of a second support, wherein the at least first layer or tier and the at least second layer or tier are stacked and the vertically aligned carbon nanotubes of the at least first layer or tier at least partially interdigitate the vertically aligned carbon nanotubes of the at least second layer or tier which are interfacing each other.

18. The device of claim 17, wherein the vertically aligned carbon nanotubes grown from opposing surfaces of the first and / or the second supports of the at least first and / or second layer or tier, respectively, can each independently comprise one or more coatings formed from metals, metal oxides, nanoparticles, ceramics, and / or polymers.

19. The device of claim 17, wherein the vertically aligned carbon nanotubes grown from opposing surfaces of the first and / or the second supports of the at least first and / or second layer or tier, respectively, can each independently be clumped or bundled.

20. The device of any one of claims 1-19, wherein the device has a shape comprising channels, fins, pins, hydrofoils, tubes, or combinations thereof.

21. A method of making a carbon nanostructure-based device for cooling comprising the steps of:(a) forming or providing a single-layered carbon nanostructure composite, which is planar and comprises a top surface and a bottom surface, to form the device; wherein at least the top surface comprises nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; wherein the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

22. A method of making a carbon nanostructure-based device for cooling comprising the steps of:(a’) forming or providing a multi-tiered carbon nanostructure composite, which is planar and comprises a top surface and a bottom surface, to form the device; wherein at least the top surface comprises nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; wherein the bottom surface is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

23. A method of making a carbon nanostructure-based device for cooling comprising the steps of:(a”) attaching a first single-layered carbon nanostructure composite onto a top surface of a heat generating substrate;(b”) attaching a second single-layered carbon nanostructure composite onto a bottom surface of the heat generating substrate to form the device;wherein at least the first single-layered carbon nanostructure composite on the top surface comprises nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; wherein the second single-layered carbon nanostructure composite on the bottom is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

24. A method of making a carbon nanostructure-based device for cooling comprising the steps of:(a’ ”) attaching a first multi-tiered carbon nanostructure composite onto a top surface of a heat spreading substrate;(b’”) attaching a second multi-tiered carbon nanostructure composite onto a bottom surface of the heat spreading substrate to form the device; wherein at least the first multi-tiered carbon nanostructure composite on the top surface comprises nano- and / or micro- scale surface structures, which function as nucleation sites and / or which function to facilitate bubble departure, and / or increase turbulent mixing and / or increase boundary layer heat transfer of a cooling fluid in contact with the surface structures; wherein the second multi-tiered carbon nanostructure composite on the bottom is a thermal interface for attachment to one or more heat sources and the interface has a thermal resistance which is less than that of an interface without the presence of carbon nanostructures.

25. The method of any one of claims 23 or 24, wherein the heat spreading substrate is made from a material selected from copper, aluminum, steel, AlSiC, AlSilOMg, graphite, and combinations thereof.

26. The method of any one of claims 21 or 23, wherein the single-layered carbon nanostructure composites are hydrophilic, hydrophobic, or a combination thereof.

27. The method of any one of claims 21 or 23, wherein the carbon nanostructures of the single-layered carbon nanostructure composites are coated with a metal, a polymer, and / or a ceramic to modify the surface energy of the nano- and / or micro- scale surface structures and modify the wetting characteristics of the coated carbon nanostructure composites to the cooling fluid.

28. The method of any one of claims 22 or 24, wherein the multi-tiered carbon nanostructure composites are hydrophilic, hydrophobic, or a combination thereof.

29. The method of any one of claims 22 or 24, wherein the carbon nanostructures of the multi-tiered carbon nanostructure composites are coated with a metal, a polymer, and / or a ceramic to modify the surface energy of the nano- and / or micro- scale surface structures and modify the wetting characteristics of the coated carbon nanostructure composites to the cooling fluid.

30. The method of any one of claims 21-29, further comprising an adhesive selected from the group consisting of a pressure sensitive adhesive, a holt melt, hot glue, an epoxy, a solder, a liquid-metal contact, reactive adhesives including crosslinked silicones, solvent based adhesives, and combinations thereof; wherein the adhesive is optionally present on the surface for attachment to the one or more heat sources.

31. The method of any one of claims 21-30, wherein the micro- scale surface structures form grids, channels, islands, hydrofoils, or combinations thereof.

32. The method of claim 31, wherein the surface structures have average sizes ranging from between about 10 nm to about 250 pm.

33. The method of claim 31, wherein the surface structures have average sizes ranging from between about 0.1 mm to about 1000 mm.

34. The method of any one of claims 21 or 23, wherein the single- layered carbon nanostructure composites each comprise a carbon nanotube array comprising vertically aligned carbon nanotubes grown from opposing surfaces of a support.

35. The method of claim 34, wherein the vertically aligned carbon nanotubes grown from opposing surfaces of the support can independently comprise one or more coatings formed from metals, metal oxides, nanoparticles, ceramics, and / or polymers.

36. The method of claim 34, wherein the vertically aligned carbon nanotubes grown from opposing surfaces of the support can independently be clumped or bundled.

37. The method of any one of claims 22 or 24, wherein the multi-tiered carbon nanostructure composites each comprise: at least a first layer or tier comprising a carbon nanotube array comprising vertically aligned carbon nanotubes grown from opposing surfaces of a first support, and at least a second layer or tier comprising a carbon nanotube array comprising vertically aligned carbon nanotubes grown from opposing surfaces of a second support,wherein the at least first layer or tier and the at least second layer or tier are stacked and the vertically aligned carbon nanotubes of the at least first layer or tier at least partially interdigitate the vertically aligned carbon nanotubes of the at least second layer or tier which are interfacing each other.

38. The method of claim 37, wherein the vertically aligned carbon nanotubes grown from opposing surfaces of the first and / or the second supports of the at least first and / or second layer or tier, respectively, can each independently comprise one or more coatings formed from metals, metal oxides, nanoparticles, ceramics, and / or polymers.

39. The method of claim 37, wherein the vertically aligned carbon nanotubes grown from opposing surfaces of the first and / or the second supports of the at least first and / or second layer or tier, respectively, can each independently be clumped or bundled.

40. The method of any one of claims 21-39, wherein the method further includes a step of stamping the formed heat spreader to have a shape comprising channels, fins, pins, tubes, or combinations thereof.

41. A method for enhanced cooling of one or more heat sources, the method comprising the steps of:(1) attaching the device of any one of claims 1-20 to the one or more heat sources;(2) contacting at least the nano- and / or micro- scale surface structures of the device to one or more cooling fluids; and(3) optionally flowing or circulating the one or more cooling fluids during step (2); wherein the device increases at least one of active boiling or condensing nucleation site density, mixing, critical heat flux, heat transfer coefficients, and / or reduces superheating required for boiling nucleation between the one or more heat sources and the one or more cooling fluids, providing increased cooling of the one or more heat sources, as compared to a method for cooling which does not include a device according to claims 1-20 having the nano- and / or micro-scale surface structures.

42. The method of claim 41, wherein the one or more heat sources is selected from one or more dies (such as semiconductor dies), microchips, batteries, battery cells, power transformers, passive electronics, lids, heated surfaces, lasers, detectors, heat generating electronics, and combinations thereof.

43. The method of any one of claims 41-42, wherein the one or more cooling fluids is a dielectric fluid selected from the group consisting of distilled water, deionized water, fluorocarbon-based fluids (such as HFE fluids (segregated hydrofluoroethers), perfluorinatedcompound fluids, NOVEC fluids (such as fluoroketones), hydrocarbon fluids (such as mineral oil), non-fatty saturated straight chain alcohols. Ci to C30 alkanes, HCFC / CFC / HFC refrigerants , and combinations thereof.

44. The method of any one of claims 41-43, wherein the device accelerates bubble formation, increases bubble nucleation density, and / or accelerates release or departure of bubbles, when the one or more cooling fluids are boiling.

45. The method of any one of claims 41-44, wherein the method comprises immersing the entire device and the one or more heat sources into the one or more cooling fluids in step (2) and during optional step (3).

46. The method of any one of claims 41-45, wherein the method is employed in data center cooling.

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