Patent Application
20200340757
The present disclosure relates to the art of heat transfer devices and, in particular, to an expanded graphite-enabled vapor chamber or heat pipe for heat spreading.
Advanced thermal management materials are becoming more and more critical for today's microelectronic, photonic, and photovoltaic systems. As new and more powerful chip designs and light-emitting diode (LED) systems are introduced, they consume more power and generate more heat. This has made thermal management a crucial issue in today's high performance systems. Systems ranging from active electronically scanned radar arrays, web servers, large battery packs for personal consumer electronics, wide-screen displays, and solid-state lighting devices all require high thermal conductivity materials that can dissipate heat more efficiently. Furthermore, many microelectronic devices (e.g. smart phones, flat-screen TVs, tablets, and laptop computers) are designed and fabricated to become increasingly smaller, thinner, lighter, and tighter. This further increases the difficulty of thermal dissipation. Actually, thermal management challenges are now widely recognized as the key barriers to industry's ability to provide continued improvements in device and system performance.
Heat sinks are components that facilitate heat dissipation from the surface of a heat source, such as a CPU or battery in a computing device, to a cooler environment, such as ambient air. Typically, heat transfer between a solid surface and the air is the least efficient within the system, and the solid-air interface thus represents the greatest barrier for heat dissipation. A heat sink is designed to enhance the heat transfer efficiency between a heat source and the air mainly through increased heat sink surface area that is in direct contact with the air. This design enables a faster heat dissipation rate and thus lowers the device operating temperatures.
A vapor-based heat transfer apparatus comprises a hollow structure and a working liquid within the hollow structure. A vapor chamber is a closed structure having an empty space inside within which a liquid is provided. Vapor chambers are typically passive, two-phase (liquid-vapor) heat transport loops that are used to spread heat from relatively small, high heat-flux sources to a region of larger area where the heat can be transferred elsewhere at a significantly lower heat-flux.
A standard heat pipe transfers heat along the axis of the pipe and, thus, is well-suited to cooling discrete heat sources. Vapor chambers are suitable for collecting heat from larger-area heat sources and then spreading the heat or conducting the heat to a heat sink for cooling. Vapor chambers are useful for heat spreading in two dimensions, particularly where high powers and heat fluxes are applied to a relatively small evaporator area. During operation of a vapor chamber, the heat transferred from a heat source to the evaporator can vaporize the liquid within the evaporator wick. The vapor can flow throughout the chamber, serving as an isothermal heat spreader. The vapor then condenses on the condenser surfaces, where the heat may be removed by forced convection, natural convection, liquid cooling, etc. (e.g. through a heat sink). The condensed liquid is transported back to the evaporator via capillary forces in the wick.
Materials for vapor-based heat transfer apparatus (e.g. heat pipe or vapor chamber) must be thermally conducting. Typically, a heat pipe or vapor chamber is made from a metal, especially copper or aluminum, due to the ability of metal to readily transfer heat across its entire structure. However, there are several major drawbacks or limitations associated with the use of metallic chambers. One drawback relates to the relatively low thermal conductivity of a metal (<400 W/mK for Cu and 80-200 W/mK for Al alloy). In addition, the use of copper or aluminum heat transfer apparatus can present a problem because of the weight of the metal, particularly when the heating area is significantly smaller than that of the heat sink. For instance, pure copper weighs 8.96 grams per cubic centimeter (g/cm3) and pure aluminum weighs 2.70 g/cm3. In many applications, several vapor chambers or heat pipes need to be arrayed on a circuit board to dissipate heat from a variety of components on the board. If metallic pipes or chambers are employed, the sheer weight of the metal on the board can increase the chances of the board cracking or of other undesirable effects, and increases the weight of the component itself. Many metals do not exhibit a high surface thermal emissivity and thus do not effectively dissipate heat through the radiation mechanism.
Thus, there is a strong need for a heat pipe or vapor chamber that contains a reduced amount of metal and is effective for dissipating heat produced by a heat source such as a CPU and battery in a device. The heat transfer system should exhibit a higher thermal conductivity and/or a higher thermal conductivity-to-weight ratio as compared to metallic heat sinks. These heat transfer apparatus must also be mass-producible, preferably using a cost-effective process.
The present disclosure provides a vapor-based heat transfer apparatus (e.g. a heat pipe or a vapor chamber), comprising (a) a hollow structure made of a thermally conductive material having a thermal conductivity no less than 5 W/mK, more preferably no less than 10 and further more preferably no less than 20 W/mK (e.g. most preferably greater than 50 W/mK or even >100 W/mK, such as Cu, Al, etc.), (b) a wick structure in contact with one or a plurality of walls of the hollow structure, and (c) a working liquid within the hollow structure and in contact with the wick structure, wherein the wick structure comprises flakes of exfoliated graphite worms or expanded graphite.
In certain preferred embodiments, a plurality of walls of the hollow structure comprise an evaporator wall having a first surface plane, and a condenser wall, having a second surface plane wherein the flakes of exfoliated graphite worms or expanded graphite are aligned to be substantially parallel to one another and perpendicular to at least one of the first surface plane and the second surface plane.
In some embodiments, the flakes of exfoliated graphite worms or expanded graphite comprise flakes that are bonded together or bonded to the one or a plurality of hollow structure walls by a binder or adhesive. In some embodiments, the flakes of exfoliated graphite worms or expanded graphite comprise a composite having flakes of exfoliated graphite worms or expanded graphite dispersed in a matrix selected from a polymer, carbon, glass, ceramic, organic, or metal.
In some embodiments, the flakes of exfoliated graphite worms or expanded graphite comprise a coating or paint comprising flakes of exfoliated graphite worms or expanded graphite dispersed in an adhesive and the adhesive is bonded to the interior or exterior surface (or both) of one or a plurality of hollow structure walls.
In some preferred embodiments, the flakes of exfoliated graphite worms or expanded graphite comprise a layer of expanded/exfoliated graphite foam having pores and pore walls containing flakes of exfoliated graphite worms or expanded graphite and the foam has a physical density from 0.001 to 1.8 g/cm3. In some embodiments, the flakes of exfoliated graphite worms or expanded graphite comprise a layer of paper, film, mat, or membrane of flakes of exfoliated graphite worms or expanded graphite.
In some preferred embodiments, the flakes of exfoliated graphite worms or expanded graphite comprise expanded/exfoliated graphite flake-coated metal particles, ceramic particles, carbon particles, glass particles, polymer particles, metal-decorated expanded/exfoliated graphite flakes, ceramic-decorated expanded/exfoliated graphite flakes, or a combination thereof.
The working fluid may contain a fluid selected from water, methyl alcohol, acetone, propylene glycol, refrigerant, ammonia, or alkali metal selected from cesium, potassium or sodium.
Preferably, the thermally conductive material used to construct the hollow structure has a thermal conductivity no less than 100 W/mK. In some embodiments, the thermally conductive material contains a material selected from Cu, Al, steel, Ag, Au, Sn, W, Zn, Ti, Ni, Pb, solder, boron nitride, boron arsenide, diamond, gallium arsenide, aluminum nitride, silicon nitride, or a combination thereof.
The apparatus may further comprise one or more extended structures configured to dissipate heat from the apparatus to an ambient environment, wherein the extended structure has a finned heat sink structure. The apparatus may be physically connected to a heat sink or a cooling system.
In some embodiments, the thermally conductive material that constitutes the hollow structure contains flakes of exfoliated graphite worms or expanded graphite. In some embodiments, the flakes of exfoliated graphite worms or expanded graphite comprise a composite having flakes of exfoliated graphite worms or expanded graphite dispersed in a matrix selected from polymer, carbon, glass, ceramic, organic, or metal.
The disclosure also provides a vapor-based heat transfer apparatus, comprising (a) a hollow structure made of a thermally conductive material, comprising flakes of exfoliated graphite worms or expanded graphite and having a thermal conductivity no less than 5 W/mK, (b) a wick structure in contact with one or a plurality of walls of said hollow structure, and (c) a working liquid within said hollow structure and in contact with said wick structure.
In this apparatus, the thermally conductive graphene material of the hollow structure and/or the wick structure comprises flakes of exfoliated graphite worms or expanded graphite.
In some preferred embodiments, the thermally conductive material that constitutes the hollow structure comprises flakes of exfoliated graphite worms or expanded graphite in a form of a paper, film, membrane, coating, or a composite having flakes of exfoliated graphite worms or expanded graphite dispersed in a matrix selected from polymer, carbon, glass, ceramic, organic, or metal. The metal matrix is preferably selected from Cu, Al, steel, Ag, Au, Sn, W, Zn, Ti, Ni, Pb, solder, or a combination thereof. The ceramic matrix is preferably selected from boron nitride, boron arsenide, diamond, gallium arsenide, aluminum nitride, silicon nitride, or a combination thereof.
The apparatus may further comprise an adhesive that hermetically seals the expanded/exfoliated graphite flake-based paper, film, membrane, or composite.
The disclosure also provides a microelectronic, photonic, or photovoltaic system containing the invented vapor-based heat transfer apparatus as a heat dissipating device.
Also provided is a process for producing the wick structure in the invented heat transfer apparatus, the process comprising: (a) preparing a graphite flake dispersion having multiple flakes of exfoliated graphite worms or expanded graphite dispersed in a liquid; (b) subjecting the graphite flake dispersion to a forced assembly procedure, forcing the multiple graphite flakes to assemble into a liquid-impregnated laminar graphite structure, wherein the multiple graphite flakes are alternately spaced by thin layers of said liquid (preferably less than 10 nm in thickness); and (c) removing the liquid or solidifying the liquid to become a solid wick structure, wherein the flakes of exfoliated graphite worms or expanded graphite in the wick structure are aligned to be substantially parallel to one another and perpendicular to at least one of the first surface plane and the second surface plane.
In certain embodiments, the step of solidifying the liquid comprises polymerizing and/or curing a reactive monomer or resin to form a polymer or a cured resin solid, or cooling the liquid to below a melting point to form a solid.
The disclosure also provides a process for producing a hollow structure element in the invented heat transfer apparatus, the process comprising: (a) preparing a graphite flake dispersion having multiple flakes of exfoliated graphite worms or expanded graphite dispersed in a liquid; (b) subjecting the graphite flake dispersion to a forced assembly procedure, forcing the multiple graphite flakes to assemble into a liquid-impregnated laminar graphite structure, wherein the multiple graphite flakes are alternately spaced by thin layers of the liquid (preferably less than 10 nm in thickness); and (c) removing the liquid or solidifying the liquid to become a solid hollow structure element, wherein the flakes of exfoliated graphite worms or expanded graphite in the hollow structure element are aligned to be substantially parallel to one another and parallel or perpendicular to a surface plane of the hollow structure element. In some embodiments, the step of solidifying the liquid comprises polymerizing and/or curing a reactive monomer or resin to form a polymer or a cured resin solid, or cooling the liquid to below a melting point to form a solid.
The present disclosure provides a vapor-based heat transfer apparatus (e.g. a heat pipe or a vapor chamber, as schematically shown in
In certain preferred embodiments, as illustrated in
The heat coming from a heat source (below the hollow structure in
A first type of wick structure may contain a sintered body of particles (e.g. flakes of exfoliated graphite worms or expanded graphite or graphite flake-coated Cu particles) having some surface pores or internal pores. This type of wick structure offers the highest degree of versatility in terms of power handling capacity and ability to work against gravity. A second type of wick structure may contain a mesh screen, which is less expensive to manufacture and allows the heat pipe or vapor chamber to be thinner relative to a sintered wick. However, due to the capillary force of the screen being significantly less than that of a sintered wick, its ability to work against gravity or handle higher heat loads is lower. The third type of a wick structure is a grooved wick whose cost and performance is the lowest of the three. The grooves may act as an internal fin structure aiding in the evaporation and condensation.
In addition to or alternatively, the thermally conductive material used in the hollow structure may also comprise flakes of exfoliated graphite worms or expanded graphite. In the presently invented vapor-based heat transfer apparatus, either the wick structure or the hollow structure (or both) may comprise flakes of exfoliated graphite worms or expanded graphite.
In some embodiments, the flakes of exfoliated graphite worms or expanded graphite may be in a form of paper, film, membrane, coating/paint, or a composite having flakes of exfoliated graphite worms or expanded graphite dispersed in a matrix selected from polymer, carbon, glass, ceramic, organic, or metal.
The production of flakes of exfoliated graphite worms or expanded graphite, graphite flake-reinforced composites, paper, film membrane, or foam of flakes of exfoliated graphite worms or expanded graphite, each as a material, will be briefly described as follows:
Exfoliated graphite may be obtained by immersing powders or filaments of a starting graphitic material (e.g. natural graphite powder) in an oxidizing liquid medium (e.g. a mixture of sulfuric acid, nitric acid, and potassium permanganate) in a reaction vessel at a desired temperature for a period of time (typically from 0.5 to 96 hours, depending upon the nature of the starting material and the type of oxidizing agent used). The resulting graphite oxide particles (sulfuric acid-intercalated graphite or graphite intercalation compound, GIC) may then be subjected to thermal exfoliation to produce exfoliated graphite worms. Graphite worms are composed of exfoliated graphite flakes that remain weakly interconnected. Graphite worms may be broken up by using mechanical shearing, air jet milling, ultrasonication, etc.
The aforementioned features are further described and explained in detail as follows: As illustrated in
A highly ordered graphite particle can consist of crystallites of a considerable size, having a length of La along the crystallographic a-axis direction, a width of Lb along the crystallographic b-axis direction, and a thickness Lc along the crystallographic c-axis direction. The constituent graphene planes of a crystallite are highly aligned or oriented with respect to each other and, hence, these anisotropic structures give rise to many properties that are highly directional. For instance, the thermal and electrical conductivity of a crystallite are of great magnitude along the plane directions (a- or b-axis directions), but relatively low in the perpendicular direction (c-axis). As illustrated in the upper-left portion of
Due to the weak van der Waals forces holding the parallel graphene layers, natural graphite can be treated so that the spacing between the graphene layers can be appreciably opened up so as to provide a marked expansion in the c-axis direction, and thus form an expanded graphite structure in which the laminar character of the carbon layers is substantially retained. The process for manufacturing flexible graphite is well-known in the art. In general, flakes of natural graphite (e.g. 100 in
These worms of graphite flakes which have been greatly expanded can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as “flexible graphite” 106) having a typical density of about 0.04-2.0 g/cm3 for most applications.
The upper left portion of
The exfoliated graphite (or mass of graphite worms) may be re-compressed by using a calendaring or roll-pressing technique to obtain flexible graphite foils (26 in
Alternatively, the exfoliated graphite may be subjected to high-intensity mechanical shearing/separation treatments using a high-intensity air jet mill, high-intensity ball mill, or ultrasonic device to produce separated nanographene platelets 33 (NGPs) with all the graphene platelets thinner than 100 nm, mostly thinner than 10 nm, and, in many cases, being single-layer graphene (also illustrated as 112 in
Further alternatively, with a low-intensity shearing, graphite worms tend to be separated into the so-called expanded graphite flakes (108 in
The present disclosure provides a process for producing a highly oriented (aligned), adhesive-impregnated laminar graphite flake structure for use as a wick electrode or as a vapor chamber/heat pipe hollow structure. This adhesive may be initially in a liquid state (e.g. uncured resin, metal melt, pitch, etc.), but becomes solidified after the wick structure or hollow structure element is made. In some embodiments, the process comprises: (a) preparing a graphite flake dispersion having multiple flakes of exfoliated graphite worms or expanded graphite dispersed in (or impregnated with) a liquid; (b) subjecting the graphite flake dispersion to a forced assembly procedure, forcing the multiple graphite flakes to assemble into the liquid-impregnated laminar graphite structure, wherein the multiple graphite flakes are alternately spaced by thin liquid layers, less than 10 nm (preferably <5 nm) in thickness, and the multiple graphite flakes are substantially aligned along a desired direction, and wherein the laminar graphite flake structure has a physical density from 0.5 to 1.6 g/cm3 (more typically 0.7-1.3 g/cm3) and a specific surface area from 50 to 3,300 m2/g, when measured in a dried state of the laminar structure without the presence of the liquid; and (c) removing/drying the liquid or solidifying the liquid to become a solid (e.g. polymerizing and/or curing a reactive monomer or resin to form a polymer or cured resin solid; or cooling the liquid to below the melting point for solidification).
In some desired embodiments, the forced assembly procedure includes introducing a graphite flake dispersion, having an initial volume V1, in a mold cavity cell and driving a piston into the mold cavity cell to reduce the graphite flake dispersion volume to a smaller value V2, allowing excess liquid to flow out of the cavity cell (e.g. through holes of the mold cavity cell or of the piston) and aligning the multiple graphite flakes along a direction at an angle from 0° to 90° relative to a movement direction of the piston. The liquid may be intended to be an adhesive or simply a fluid medium to facilitate the flow of graphite flakes.
In this dispersion, if so desired, practically each and every isolated graphite flake is surrounded by the liquid (e.g. adhesive) that is physically adsorbed on or chemically bonded to graphite flake surface. During the subsequent consolidating and aligning operation, isolated graphite flakes remain isolated or separated from one another through liquid (e.g. adhesive). Upon removal of the excess liquid, graphite flakes remain spaced apart by liquid adhesive and this liquid adhesive-filled space can be as small as 0.4 nm.
Shown in
Thus, in some desired embodiments, the forced assembly procedure includes introducing dispersion of graphite flakes in a mold cavity cell having an initial volume V1, and applying a suction pressure through a porous wall of the mold cavity to reduce the graphite dispersion volume to a smaller value V2, allowing excess liquid to flow out of the cavity cell through the porous wall and aligning the multiple graphite flakes along a direction at an angle from approximately 0° to approximately 90° relative to a suction pressure direction; this angle depending upon the inclination of the bottom plane with respect to the suction direction.
Thus, in some preferred embodiments, the forced assembly procedure includes introducing a first layer of the graphite flake dispersion onto a surface of a supporting conveyor and driving the layer of graphite flake suspension supported on the conveyor through at least a pair of pressing rollers to reduce the thickness of the graphite dispersion layer and align the multiple graphite flakes along a direction parallel to the conveyor surface for forming a layer of liquid-impregnated laminar graphite structure.
The process may further include a step of introducing a second layer of the graphite dispersion onto a surface of the layer of liquid-impregnated laminar structure to form a two layer laminar structure, and driving the two-layer laminar structure through at least a pair of pressing rollers to reduce a thickness of the second layer of graphite flake dispersion and align the multiple graphite flakes along a direction parallel to the conveyor surface for forming a layer of liquid-impregnated laminar structure. The same procedure may be repeated by allowing the conveyor to move toward a third set of pressing rollers, depositing additional (third) layer of graphite flake dispersion onto the two-layer structure, and forcing the resulting 3-layer structure to go through the gap between the two rollers in the third set to form a further compacted, liquid-impregnated laminar graphite flake structure.
The above paragraphs about
There are many feasible ways of producing the invented vapor-based heat transfer device. For instance, as schematically illustrated in
A first type of wick structure may contain a sintered body of particles having some surface pores or grooves. This type of wick structure offers the highest degree of versatility in terms of power handling capacity and ability to work against gravity. A second type of wick structure may contain a mesh screen, which is less expensive to manufacture and allows the heat pipe or vapor chamber to be thinner relative to a sintered wick. However, due to the capillary force of the screen being significantly less than a sintered wick, its ability to work against gravity or handle higher heat loads is lower. The third type of a wick structure is a grooved wick whose cost and performance is the lowest of the three. The grooves may act as an internal fin structure aiding in the evaporation and condensation. Any suitable wick structure could be used. Further, a graphene-filled adhesive can be employed, and may be used in a coating or paint.
During operation of a vapor chamber, the heat transferred from a heat source to the evaporator can vaporize the liquid within the evaporator wick. The presence of a graphite flake-based chamber wall structure and/or graphite flake-based wick structure enables significantly faster heat transfer from the heat source to the evaporator portion of the wick structure, allowing for more efficient evaporation of the working fluid. The vapor can flow throughout the chamber, serving as an isothermal heat spreader. The vapor then condenses on the condenser surfaces, where the heat may be removed by forced convection, natural convection, liquid cooling, etc.
[e.g. through a heat sink (such as is shown in
We have observed that the presently invented expanded/exfoliated graphite flake-based wick structure enables a vapor chamber to deliver 1.5-3.5 times higher maximum heat flux in comparison with a vapor chamber of the same dimensions but featuring a conventional Cu-based wick structure. For instance, one can easily achieve a maximum heat flux of >>1,500 W/cm2 (over an area of 4 cm2) for a vapor chamber having an optimized graphite flake-based wick. The heat flux value is even significantly higher if a graphite flake-reinforced Cu hollow chamber wall is implemented. Any microelectronic, photonic, or photovoltaic system may be made to contain the invented vapor-based heat transfer apparatus as a heat dissipating device to help keep the system cool.
The following examples serve to provide the best modes of practice for the presently disclosed process and should not be construed as limiting the scope of the process:
Meso-carbon microbeads (MCMBs) were supplied from China Steel Chemical Co., Kaohsiung, Taiwan. This material has a density of about 2.24 g/cm3 with a median particle size of about 16 μm. MCMB (10 grams) were intercalated with an acid solution (sulfuric acid, nitric acid, and potassium permanganate at a ratio of 4:1:0.05) for a period of time from 4 hours up to 48 hours. Upon completion of the reaction, the mixture was poured into deionized water and filtered. The intercalated MCMBs were repeatedly washed in a 5% solution of HCl to remove most of the sulfate ions. The sample was then washed repeatedly with deionized water until the pH of the filtrate was no less than 4.0. The slurry was then dried in a vacuum for 24 hours to obtain a graphite intercalation compound (GIC).
The GIC was then thermally exfoliated at 650° C. for 1 minute to produce exfoliated graphite worms, which were divided into two portions. One portion was subjected to chemical activation by mixing the graphite worms with KOH at a 1:1 weight ratio and then heated the mixture to 800° C. for 2 hours to produce activated graphite worms, having a specific surface area of 1,770 m2/g. The other portion, having a specific surface area of 341 m2/g, was used as a control sample. The activated graphite worms having a high specific surface area, when used as a wick structure, behave like a piece of sponge, being significantly more capable of transporting condensed liquid from the condenser region back to the evaporator region via capillary forces.
The graphite worms, with or without chemical activation, were then made into wick structures using both the presently invented processes (wick structure containing oriented expanded graphite flakes, perpendicular to the evaporator plane; prepared according to a procedure as illustrated in
Chopped graphite fibers with an average diameter of 12 μm and natural graphite particles were separately used as a starting material, which was immersed in a mixture of concentrated sulfuric acid, nitric acid, and potassium permanganate (as the chemical intercalate and oxidizer) to prepare graphite intercalation compounds (GICs). The starting material was first dried in a vacuum oven for 24 h at 80° C. Then, a mixture of concentrated sulfuric acid, fuming nitric acid, and potassium permanganate (at a weight ratio of 4:1:0.05) was slowly added, under appropriate cooling and stirring, to a three-neck flask containing fiber segments. After 4 hours of reaction, the acid-treated graphite fibers or natural graphite particles were filtered and washed thoroughly with deionized water until the pH level of the solution reached 6. After drying the product at 100° C. overnight, we obtained a graphite intercalation compound (GIC) or graphite oxide fiber.
The GIC was then submitted to a thermal exfoliation treatment at 800° C. for 45 seconds to obtain exfoliated graphite worms. Some of these worms were submitted to low-intensity shearing using a kitchen-scale food processor to produce expanded graphite flakes.
A portion of the expanded graphite flakes was dispersed in a UV-curable liquid adhesive to form a dispersion. Part of the dispersion was compressed and consolidated into a layer of adhesive-impregnated, compacted and highly oriented graphite flakes (adhesive-impregnated laminar graphite flake structure) according to the process illustrated in
Graphite intercalation compound or graphite oxide was prepared by oxidation of natural flake graphite with an oxidizer liquid consisting of sulfuric acid, sodium nitrate, and potassium permanganate at a ratio of 4:1:0.05 at 30° C. When natural graphite flakes (particle sizes of 25 μm) were immersed and dispersed in the oxidizer mixture liquid for 4 hours, the suspension or slurry remains optically opaque and dark. After this, the reacting mass was rinsed with water 3 times to adjust the pH value to at least 3.0. The mass was then dried in a vacuum oven at 80° C. for 24 hours to obtain a GIC. The GIC was thermally exfoliated at 900° C. for 60 seconds to obtain exfoliated graphite worms. Some of the worms were subjected to low-intensity shearing using a food processor to obtain powder of expanded graphite flakes. Some amount of the worms and some amount of the expanded graphite flakes were subjected to chemical activation (using NaOH melt at 800° C. for 6 hours) to obtain activated graphite worms and activated expanded graphite flakes, respectively.
Some of these graphite flakes (activated or non-activated) and some of these graphite worms (activated or non-activated) were then dispersed in water to form several dispersion samples, which were then made into wick structures using the presently invented process (roll-pressing-based as illustrated in
Some amount of the dried expanded graphite flake powder prepared in Example 3, along with Cu particles, was poured into a ball-milling pot chamber and then ball-milled in a plenary ball milling device for 30 minutes to obtain expanded graphite-coated Cu particles. Certain amounts of the expanded graphite-coated Cu particles were compacted, using a compression press, to form layers of compacted expanded graphite-coated Cu particles. Some of compacted layers were used as a wick structure in a vapor chamber. Other layers were melted and solidified to make expanded graphite flake-reinforced Cu composite-based hollow structures for vapor chambers and heat pipes.
