Background

[0001] Devices and systems often generate heat during their operation. Such devices and systems include, among many others, computer systems and other electrical devices. Removing excess heat can improve

performance and/or minimize or eliminate physical damage. One device that can remove such heat is a heat pipe. The heat pipe is connected at one end to a thermally-conductive heat source, and at the other end to a lower- temperature thermally-conducted element such as, for example, a heat sink. In operation, the heat pipe transports the heat from the higher-temperature end to the lower-temperature element, thus reducing the temperature at the heat source. As systems and devices which generate more heat are developed and employed, it is advantageous to likewise improve the heat transfer capability of heat pipes so as to keep the temperature within the systems and devices and/or at their "hot spots" to a safe operational level.

Brief Description of the Drawings

[0002] FIG. 1 is a schematic representation of a heat pipe in accordance with an example of the present disclosure.

[0003] FIG. 2 is a schematic representation of a wick in accordance with an example of the present disclosure usable with the heat pipe of FIG. 1.

[0004] FIG. 3A is a schematic representation of a portion of the wick of FIG. 2 that includes a first combination of carbon nanotube aerogel and graphene in accordance with an example of the present disclosure.

[0005] FIG. 3B is a schematic representation of a portion of the wick of FIG. 2 that includes a second combination of carbon nanotube aerogel and graphene in accordance with an example of the present disclosure.

[0006] FIG. 3C is a schematic representation of a portion of the wick of FIG. 2 that includes a third combination of carbon nanotube aerogel and graphene in accordance with an example of the present disclosure.

[0007] FIG.4 is a flowchart in accordance with an example of the present disclosure of a method of making a heat pipe. Detailed Description

[0008] Referring now to the drawings, there is illustrated an example of a heat pipe which provides improved thermal conductivity, and improved heat transfer (heat dissipation, or latent heat exchange) performance. The heat pipe has a wick that is formed, at least in part, of carbon nano tubes (CNTs) and graphene in aerogel form.

[0009] Considering now a heat pipe, and with reference to FIG. 1 , a heat pipe 100 includes a sealed casing 110, a wick 120 disposed on the interior walls of the casing 110, and a cavity 130 within the heat pipe 100 which is at least partially filled with a working fluid 140.

[0010] The casing 110 is a thermally conductive material. In various examples, the casing 110 is copper, aluminum, stainless steel, or another metal. The casing 110 is of a material which is compatible with the working fluid 140, in order to prevent or inhibit corrosion, generation of non- condensable gases, and other effects which could reduce the useful lifetime of the heat pipe 100 if the casing material were incompatible with the working fluid 140. In some examples, the casing 110 is tubular with a cylindrical or elliptical cross-section, but the casing 110 may alternatively be of a different shape. The casing 110 encloses the wick 120, cavity 130, and working fluid 140, and defines the size of the heat pipe 100. In one example, the casing 110 has a length 112 in the range of 80 to 450 millimeters, and a maximum diameter 114 in the range of 0.3 to 3 millimeters.

[0011] The working fluid 140 is disposed at least partly within the cavity 130. In examples, the working fluid 140 is a saturated fluid. In examples, the working fluid 140 exists in both a liquid form and a vapor form over the range of operating temperatures of the heat pipe 100. In some examples, the vapor form of the working fluid 140 is in the cavity 130, while the liquid form of the working fluid is in the wick 120. The working fluid 140 may be water, alcohol, ammonia, a solvent, or another fluid. Some other suitable fluids include methoxy-nonafluorobutane (C4F9OCH3), acetone, alcohol, heptane, and pentane. The working fluid 140 may also include CNT aerogel in suspension. The type of working fluid 140 may be chosen based on the operating temperature range intended for the heat pipe 100.

[0012] The wick 120 is a porous structure disposed on the interior walls of the casing 110. The interior surface of the wick 120 defines the cavity 130. The wick 120 exerts a capillary force on the liquid form of the working fluid 140 which comes into contact with the wick 120. This capillary force tends to draw the liquid working fluid 140 from the cavity 130 into the wick 120, within which it is absorbed. The wick 120 includes carbon nanotubes and graphene in aerogel form. The structure and composition of the wick 120 is discussed subsequently in greater detail with reference to FIGS. 2 and 3A-3C.

[0013] In operation, a first end 102 of the casing 110 of the heat pipe 100 is thermally coupled to a heat source (not shown). In some examples, the heat source is an electronic component such as a CPU which generates a significant amount of heat during operation, and the first end 102 of the casing 110 of the heat pipe 100 contacts a thermally conductive surface of the component. The heat source is hotter than the temperature at an opposite second end 104 of the heat pipe 100. The casing 110 may be connected at the second end 104 to a thermally conductive element such as a heat sink (not shown) that can conduct heat away from the second end 104 and dissipate it into the environment.

[0014] The heat pipe 100 transfers heat from the higher temperature first end 102 to the lower temperature second end 104 via evaporative cooling. This involves a two-phase transition of the working fluid 140. As heat is conducted from the heat source into the first end 102, the working fluid 140 at the first end evaporates from its liquid form (indicated in FIG. 1 by solid arrows) into its vapor form (indicated in FIG. 1 by outlined arrows). The latent heat (thermal energy) absorbed at the first end 102 during evaporation of the working fluid 104 lowers the temperature at the hot end 102. As a result of the evaporation, a pressure gradient occurs in the cavity 130 - the vapor pressure at the hot end 102 is higher than the vapor pressure at the cool end 104. This vapor pressure gradient causes the vapor to rapidly migrate from the hot end 102 to the cool end 104. At the cool end 104, the vapor condenses into liquid form, releasing the latent heat (thermal energy). This thermal energy released during condensation is conducted away from the cool end 104 by the heat sink in contact with the cool end 104.

[0015] At the cool end 104, the condensed liquid is drawn by capillary action from the cavity 130 into the wick 120 through pores therein. Capillary action within the wick 120 further causes the liquid to migrate from the cool end 104 back to the hot end 102, where it fills the voids in the wick 120 left by evaporation at the hot end 102. The cycle of evaporation and condensation continuously repeats while a temperature difference exists between the first end 102 and second end 104 of the heat pipe 100.

[0016] In one example, the working fluid 140 fills the cavity 130 and saturates the wick 120. In other examples, lesser amounts of the working fluid 140 are used, and the cavity 130 may not be completely filled. In some examples, the quantity of working fluid 140 is determined by various design parameters of the heat pipe 100, such as for example the thickness of the wick 120 and the heat dissipation requirements of the environment in which the heat pipe 100 is expected to be deployed.

[0017] Considering now a heat pipe wick in greater detail, and with reference to FIG. 2, in one example a tube of a heat pipe has a tubular cross- section 200. A wick 220 is disposed on the interior walls of a casing 210 having a diameter 212. In some examples, the casing 210 is the casing 110 (FIG. 1), and the wick 220 is the wick 120 (FIG. 1).

[0018] In some examples, the wick 220 has a grooved or fluted profile 225 along its length on the interior walls of the casing 210. Such a profile increases the surface area of the wick 220 which is exposed to the cavity 230. This increases the efficiency of working fluid transfer between the cavity 230 and wick 210, and thus increases the heat transfer capability of the heat pipe. However, in another example the wick 220 is not grooved or fluted.

[0019] The interior of the casing has a diameter 212. In one example, the diameter 212 is in the range of 0.3 to 3 millimeters. The wick 220 has a minimum dimension 222 in the range of 0.2 to 0.3 millimeters, and a maximum dimension 224 in the range of 0.4 to 1.1 millimeters.

[0020] The wick 220 includes CNTs and graphene, in aerogel form. A carbon nanotube is an arrangement of carbon molecules which has a cylindrical three-dimensional nanostructure. A carbon nanotube may be a single-walled nanotube 240. The walls are single-atom sheets of carbon rolled into cylinders around an axis. Another carbon nanotube may be a multiple-walled nanotube (not shown), with the various walls disposed concentrically around the axis. In one example, the carbon nanotube 240 has a length 242 In the range of about 1 to 3 micrometers, and a widest diameter 244 in the range of about 8 to 12 nanometers. In one example, the aspect ratio (diameter to thickness) is between 50 and 5000. Carbon nanotubes have high thermal conductivity along their length, in one example about 3500 W/mK (watts per meter - degree Kelvin) at room temperature. In some examples, the carbon nanotube is curved into the shape of an arc, rather than being straight.

[0021] Graphene is a layered arrangement of carbon molecules having a two-dimensional hexagonal lattice nanostructure one atom thick. Graphene may have a single such layer, or multiple layers stacked together. In one example, the thickness of a stack of graphene 250 is between about 5 and 100 nanometers, and the length 252 and width 254 is between 0.1 to 5 micrometers. In one example, the aspect ratio (length to thickness, or width to thickness) is between 100 and 4500. Graphene has high thermal conductivity, in one example between about 4,840 and 5,300 W/mK at room temperature. While the graphene sheet 250 is illustrated as rectangular, in other examples it can have an arbitrary shape. Also, while the graphene sheet 250 is illustrated as flat, in other examples the sheet can be bent or curved such that the graphene sheet has a three-dimensional shape.

[0022] Aerogel is a low density, lightweight, high porosity, and high surface area material. In some examples, it is formed using a sol-gel process, by removing a liquid component of a gel and replacing it by a gas, in a manner which prevents the solid matrix from collapsing as the liquid is removed. The resulting aerogel is a solid which has extremely low density. An aerogel formed at least in part of CNT and graphene includes nanometer-scale particles of the CNT and graphene which are bonded together. The aerogel has a random arrangement of the various CNT and graphene nanostructures. Individual ones of CNT nanostructures, and individual ones of the graphene nanostructures, may be of different sizes and shapes from others.

[0023] In various examples, the aerogel includes 0.1 % to 30% graphene, by weight. In various examples, the density of the CNT and graphene aerogel is between 0.03 and 0.5 g/cm ³ (grams per cubic centimeter). In various examples, the porosity of CNT and graphene aerogel, defined as the ratio of void to material, is between 5% and 90%. In various examples, the pore diameter of the aerogel is between 3 and 50 nanometers. In various examples, the surface area of the aerogel is between 250 and 300 rrftg (square meters per gram). For a wick 220 or wick 120 (FIG. 1 ) formed of CNT and graphene aerogel, these characteristics collectively determine, at least in part, the capillary force that the wick exerts on the working fluid 140 (FIG. 1), which in turn determines, at least in part, the heat transfer performance of the heat pipe 100 (FIG. 1). The higher the porosity and/or the greater the surface area of the aerogel in the wick, the higher the capillary force exerted on the working fluid to draw liquid into the wick at the cod end 104 (FIG. 1) of the heat pipe, and transport it through the wick along the length of the heat pipe to the hot end 102 (FIG. 1); and the more rapidly this liquid transport occurs. Thus the high capillary force exerted by a CNT and graphene aerogel wick increases the vapor conversion cycle rate performance of the heat pipe.

[0024] The capillary force exerted by the wick is one factor that influences the heat transfer performance of the heat pipe. Another factor is the thermal conductivity of the wick. A wick formed of CNT and graphene aerogel advantageously improves the heat transfer performance of the heat pipe. Considering now in greater detail the aerogel of a heat pipe wick, and with reference to FIGS. 3A-3C, the percentage by weight of the graphene in the aerogel determines, at least in part, the thermal conductivity of the wick, and in turn the heat transfer performance of the heat pipe. FIGS. 3A-3C each schematically illustrate a portion (310, 330, 350 respectively) of the aerogel of a heat pipe wick, magnified to depict the CNT and graphene nanostructures in the aerogel. Carbon nanotubes 302 and/or graphene sheets 304 are depicted in the aerogel portions 310, 330, 350. For clarity of illustration, each individual carbon nanotube 302 is depicted as a linear rod, and each individual graphene sheet 304 as a planar sheet. In addition, and also for clarity of illustration, representative ones of the carbon nanotubes 302 and graphene sheets 304 are labeled with a reference designator.

[0029] Each aerogel portion 310, 330, 350 has a different proportion of graphene to CNT. Aerogel portion 310 (FIG. 3A) has a low proportion of graphene to CNT. For example, the aerogel may have about 0.1% graphene by weight and, as a consequence, no graphene sheets 304 appear in the portion 310. The individual CNTs 302 are dispersed in the aerogel. The aerogel 310 has CNT-to-CNT contact points 315 between some pairs of the CNTs 302. Each CNT-to-CNT contact point 315 is indicated by a dashed circle; for clarity of illustration, however, representative ones of the dashed circles are labeled with a reference designator. Because carbon nanotubes 302 are highly thermally conductive, heat is easily transferred between the contacting CNTs 302 at the contact points 315.

[0026] Aerogel portion 330 (FIG. 3B) has a moderate proportion of graphene to CNT. For example, the aerogel may have about 15% graphene by weight. Graphene sheets 304 are dispersed in the aerogel. Two graphene sheets 304 are illustrated in the portion 330, which has substantially the same number and arrangement of CNTs 302, and CNT-to-CNT contact points 315, as the aerogel 310 (however, for clarity of illustration, the contact points 315 are not identified in FIG. 3B). Various ones of the CNTs 302 contact at least one graphene sheet 304 at CNT-to-graphene contact points 335. Each CNT- to-graphene contact point 335 is indicated by a dashed circle; however, for clarity of illustration, representative ones of the dashed circle are labeled with a reference designator. Because graphene 304 is highly thermally

conductive, heat is easily transferred between a CNT 302 and a graphene sheet 304 at contact points 335. The aerogel portion 330 has higher thermal conductivity than the aerogel portion 310, due to the CNT-to-graphene contact points 335 present in the aerogel 330, but not in the aerogel 310. These additional contact points 335 allow heat to be more readily transferred from the hot end to the cool end of a heat pipe having a wick of the aerogel 330 than with a wick of the aerogel 310.

[0027] Aerogel portion 350 (FIG. X) has a high proportion of graphene to CNT. For example, the aerogel may have about 30% graphene by weight. Four graphene sheets 304 are illustrated in the portion 350, which has substantially the same number and arrangement of CNTs 302, and CNT-to CNT contact points 315, as the aerogel 310, 330. The aerogel 350 also has substantially the same number and arrangement of CNT-to-graphene contact points 335 as the aerogel 330 (however, these contact points 335 are not identified in FIG. 3C for clarity of illustration). In addition, various ones of the CNTs 302 contact at least one graphene sheet 304 at additional CNT-to- graphene contact points 355. Each contact point 355 is indicated by a dashed circle; however, for clarity of illustration, representative ones of the dashed circle are labeled with a reference designator. The aerogel portion 350 has higher thermal conductivity than the aerogel portion 330, because of the additional CNT-to-graphene contact points 355 between the CNTs 302 and graphene 304 present in the aerogel 350 but not in the aerogel 330.

These additional CNT-to-graphene contact points 355 allow heat to be more readily transferred from the hot end to the cool end of a heat pipe having a wick of the aerogel 350 than with a wick of the aerogel 330.

[0028] A wick aerogel that includes both CNT and graphene provides improved heat transfer over a wick aerogel of CNT alone. When the aerogel does not include graphene, there can be a suboptimal number of contact points between and among the various carbon nanotubes, reducing heat transfer effectiveness. For example, an aerogel using CNT alone can have a maximum thermal conductivity of about 2,000 W/mK at room temperature. By adding graphene to the aerogel, the maximum thermal conductivity of the aerogel can be increased to about 3,000 W/mK at room temperature. This higher thermal conductivity provides increased heat transfer capability for a heat pipe having a wick using CNT plus graphene aerogel. This increased heat transfer capability can reduce the temperature at the hot end of the heat pipe by 10% to 15% as compared to heat pipes having other wick types.

[0029] While the proportion of graphene to CNT in the aerogel 350 has been increased by increasing the amount of graphene, in other examples the proportion of graphene to CNT in the aerogel can be increased by decreasing the amount of carbon nanotubes in the aerogel. In some examples, the amount of CNT, graphene, or both in the aerogel can be adjusted so as to produce an aerogel having a thermal conductivity in the range of about 1 to about 3,000 W/mK at room temperature. By varying the composition of the aerogel of the wick in this manner, a heat pipe can be tailored to provide an optimal amount of heat transfer capability for a particular application.

[0030] Considering now a method of making a heat pipe having a wick of CNT plus graphene aerogel, and with reference to FIG. 4, a method 400 begins at 410 by applying a carbon nanotube and graphene aerogel coating to interior walls of a heat pipe tube. In some examples, the applying 410 includes stretching, cutting, and cleaning 412 the tube. The tube (casing) material may be stretched so as to form a tube with the desired tube diameter and casing thickness. After the stretching, the tube may be cut to the desired length of the heat pipe. The tube is then cleaned. In one example, the tube is given an acid wash, washed in ionized water, washed in a weak alkaline solution, again washed in ionized water, and dried. At 414, one end of the tube is then sealed. In some examples, the sealing is performed by squeezing the tube end and welding it closed. Other methods of stretching, cutting, cleaning, and sealing may be employed.

[0031] In some examples, the applying 410 further includes applying 416 carbon nanotube and graphene aerogel in sol-gel form to the interior walls of the heat pipe tube. In some examples, the applying 416 further includes spraying 418 the sol-gel through a nozzle arrangement which corresponds to the cross-sectional shape of the wick structure. The nozzle arrangement may include a set of orifices through which the sol-gel is ejected into the tube. The orifices may define the grooves or fluting of the wick cross-section.

[0032] At 430, the applied coating is sintered to form the heat pipe wick. In one example, the sintering is performed at 500 to 650 degrees C. The sintering removes the solvent from the sol-gel solution and solidifies the aerogel on the interior walls of the tube. The sintering forms a good chemical bond of the carbon nanostructures within the aerogel to form a ceramic-like structure which can exert high capillary force on working fluid which contacts it.

[0033] At 440, the heat pipe is at least partially filled with the working fluid. In some examples, at 442, after the working fluid has been injected, an end of the tube through which the working fluid was injected is sealed.

[0034] From the foregoing it will be appreciated that the heat pipe, wick, aerogel, and methods provided by the present disclosure represent a significant advance in the art. By more effectively conducting heat away from electronic components in contact with the hot end of heat pipes having wicks according to the present disclosure, the lifetime of components such as LCD panels, LEDs, CPUs and similar processors, and batteries can be extended, and the throughput and power efficiency of devices employing them can be increased. The risk of battery explosion in such devices can also be reduced. As such, the disclosed heat pipes can find wide applicability in electronic devices such as for example, notebook computers, tablet computers, smart phones, televisions, and more.

[0035] Although several specific examples have been described and illustrated, the disclosure is not limited to the specific methods, forms, or arrangements of parts so described and illustrated. Terms of orientation and relative position (such as "top," "bottom," "side," and the like) are not intended to indicate a particular orientation of any element or assembly, and are used for convenience of illustration and description. This description should be understood to include all combinations of elements described herein, and claims may be presented in this or a later application to any combination of these elements. The foregoing examples are illustrative, and different features or elements may be included in various combinations that may be claimed in this or a later application. Unless otherwise specified, operations of a method claim need not be performed in the order specified. Similarly, blocks in diagrams or numbers (such as (1), (2), etc.) should not be construed as operations that proceed in a particular order. Additional blocks/operations may be added, some blocks/operations removed, or the order of the blocks/operations altered and still be within the scope of the disclosed examples. Further, methods or operations discussed within different figures can be added to or exchanged with methods or operations in other figures. Further yet. specific numerical data values (such as specific quantities, numbers, categories, etc.) or other specific information should be interpreted as illustrative for discussing the examples. Such specific information is not provided to limit examples. The disclosure is not limited to the above- described implementations, but instead is defined by the appended claims in light of their full scope of equivalents. Where the claims recite "a" or "a first" element of the equivalent thereof, such claims should be understood to include incorporation of at least one such element, neither requiring nor excluding two or more such elements. Where the claims recite "having", the term should be understood to mean "comprising".