Heat pipes featuring matching thermal expansion coefficients and heat dissipation using the same

Abstract

The heat pipe may be tailored to match the coefficient of thermal expansion (CTE) of a heat-generating component, such as an electronic component, in thermal contact therewith. The copper nanoparticles may be consolidated under mild conditions with a CTE adjuster to form a copper composite that defines a sealed outer shell of the heat pipe, which may contact the heat-generating component to promote effective heat transfer and a strong bond between the two. A working fluid for promoting heat transfer may be present within an interior space defined within the sealed outer shell. The working fluid may transfer heat from a first location to a second location within the heat pipe. Heat may enter the heat pipe from a heat source in contact with the first location, and heat may exit the heat pipe at the second location through discharge to a suitable heat sink.

Description

[Background technology]

[0001] Ineffective thermal communication between a heat source and a heatsink can hinder the dissipation of excess heat from a system. For example, heat-generating electronic components such as high-power LEDs (Light Emitting Diodes) and high-power circuits are constantly shrinking in size and becoming more powerful, thereby generating an excessive heat load concentrated in an increasingly small space. As the generation and concentration of excess heat increases, effective heat removal becomes crucial, but can be particularly problematic. Failure to remove excess heat from an electronic system can lead to serious consequences, such as overheating, reduced conductivity, higher-than-normal power requirements, and / or the need for clock-down operation to avoid burnout of the circuit board and device failure due to the presence of hot spots.

[0002] Ineffective heat conduction can be particularly prevalent in various types of circuit boards, especially printed circuit boards (PCBs). PCBs and similar circuit boards are insulators precisely because of the nature of their structure. Specifically, PCBs may use insulating substrates, such as glass fiber epoxy composites like FR4 with a thermal conductivity of approximately 0.25 W / m·K, on ​​which appropriate electronic circuits and various board components are placed. The low thermal conductivity of PCB substrates can make it difficult to remove excess heat from electronic systems, as such PCB substrates cannot transfer a considerable amount of heat to a heat sink on their own. Due to their small size, lead wires or embedded metal traces can remove very little excess heat. Conventional lead solder is not particularly thermally conductive (for example, less than 1 / 10th the thermal conductivity of more thermally conductive metals such as copper). For example, package substrates containing heat-generating devices such as GaN and SiC systems, monolithic microwave integrated circuits (MMICs), and phased arrays, as seen in 5G base stations and power converters, may experience similar thermal conduction problems.

[0003] Thermal vias are one method for removing excess heat generated by electronic components associated with printed circuit boards. However, the method of directly liquid casting high-melting-point metals into vias is incompatible with currently used substrate materials (substrates) (the metalworking temperature is >1000°C compared to the much lower polymer melting points of materials typically used as PCB substrates). Therefore, vias are often filled with rosin or similar fillers and then either galvanically end-protected at the ends with only a thick metal plating (e.g., copper) formed on the via wall (i.e., via barrel) to facilitate electrical communication through the PCB substrate, or left open. Galvanic end protection is a fairly slow process and may result in suboptimal thermal communication due to the relatively small metal area in contact with the heat source on the PCB substrate surface. Furthermore, the galvanic via filling method may leave gaps within the metal plugs extending through the PCB, thereby further affecting thermal conductivity. An alternative method for filling vias using metal nanoparticles is described in U.S. Patent No. 10,616,994, incorporated herein by reference, which may promote more complete filling of via holes and result in higher thermal conductivity. Larger diameter vias may be suited to such processes to provide more effective removal of excess heat. However, even thermal vias may be insufficient to remove a significant amount of excess heat.

[0004] Thermal marcoins are an alternative method of heat dissipation that may be used when higher heat conduction than that provided by thermal vias is required. Thermal marcoins are typically metallic bodies 3-4 mm in diameter that are pressed into and extend through the plane of a PCB or similar substrate. While this can result in increased heat conduction compared to thermal vias, thermal marcoin size mismatches are common, and the thickness of the thermal marcoins and / or PCB substrate can vary, which can lead to assembly problems when stacking multiple PCB layers together.

[0005] Heat pipes are an exceptional alternative heat transfer medium that can facilitate the transfer of large amounts of excess heat. While high thermal conductivity metals such as copper can only have thermal conductivity values ​​in the range of several hundred W / m·K, heat pipes can offer much higher effective thermal conductivity values, ranging from several thousand W / m·K to even higher, such as approximately 10,000 W / m·K to approximately 100,000 W / m·K. Heat pipes have traditionally been used in applications where passive heat dissipation in harsh operating environments is desirable. Examples include satellite and spacecraft applications. Because heat pipes can operate independently of gravity and orientation, they can be advantageous in these and other zero-gravity applications. Miniaturized heat pipes have recently been used to dissipate excess heat from printed circuit boards and similar systems, including small heat-generating electronic components where footprint (size) is critical.

[0006] A heat pipe functions through direct heat transfer to a working fluid contained within the internal space of a sealed container, which may be under atmospheric pressure or preferably near-atmospheric pressure (partial vacuum). Heat conduction may be further supplemented by the liquid-vapor phase transition and subsequent condensation of the working fluid. In short, the outer surface of the heat pipe is in thermal contact with the heat source. Heat from the heat source is carried into the sealed container and transferred to the working fluid contained within it. When heated, the working fluid and its vapor then move through the heat pipe to a cooler position (cool end), where excess heat is dissipated from the heat pipe to a heat sink or similar heat reservoir. The movement of the working fluid facilitates the direct transfer of heat to the cooler position. Having released heat, the working fluid and its condensed vapor then move through the heat pipe to a hotter position (heated end) to perform additional heat transfer. Furthermore, the incoming heat may cause the working fluid to evaporate, which then undergoes subsequent condensation at a lower temperature to release the stored latent heat. The evaporation and subsequent condensation of the working fluid can significantly increase the amount of heat conducted with it. After releasing the excess heat, and preferably after evaporation and condensation, the working fluid returns to the higher temperature portion of the heat pipe by capillary action or another suitable transport means.

[0007] The difficulty in using heat pipes to facilitate heat transfer lies in the fact that ineffective thermal communication can exist between the heat-generating component and the outer shell of the heat pipe due to mismatches in coefficients of thermal expansion (CTE). For example, copper, a highly thermally conductive metal often used to form the outer shell of heat pipes, has a significantly different CTE from ceramic materials commonly used in heat-generating components of printed circuit boards or similar components that generate excess heat. CTE mismatches can lead to the heat-generating component detaching from the heat pipe when heating occurs, thereby negating the heat pipe's ability to dissipate excess heat from the heat-generating component. Furthermore, the material used to bond the heat pipe to the heat-generating component can further contribute to the CTE mismatch problem. [Brief explanation of the drawing]

[0008] The following figures are included to illustrate specific aspects of the disclosure and should not be considered exclusive embodiments. The disclosed subject matter is subject to substantial modifications, alterations, combinations, and equivalents in form and function, as can be recalled by those skilled in the art and those interested in the disclosure. [Figure 1] A diagram showing the presumed structure of metal nanoparticles with a surfactant coating is shown above. [Figure 2] A diagram showing the presumed structure of metal nanoparticles with a surfactant coating is shown above. [Figure 3A] This is a cross-sectional view of an exemplary heat pipe, featuring a wicking structure adjacent to a hollow core. [Figure 3B] This is a photograph showing a cross-section view of an exemplary heat pipe, which has a metal mesh or foam wicking structure. [Figure 3C] This figure shows a cross-section view of an exemplary heat pipe with grooves. [Figure 4]This is a cross-sectional view of an exemplary heat pipe, which has a wicking structure adjacent to a hollow core with multiple conductive fibers extending from one end of the heat pipe. [Figure 5A] This is an example of a oscillating heat pipe, shown in a top cross-sectional view. [Figure 5B] This is a photograph showing a perspective view of an exemplary copper block with defined internal channels. [Figure 5C] This is a side cross-sectional view of an exemplary vibrating heat pipe. [Figure 6] This figure shows a heat pipe bonded to the upper surface of a heat-generating component. [Figure 7] This figure shows a heat pipe bonded to the bottom surface of a heat-generating component. [Figure 8] This figure shows heat pipes joined to the top and bottom surfaces of a heat-generating component. [Figure 9] This figure shows multiple heat pipes joined to the side of a heat-generating component. [Figure 10A] This figure shows a side view of heat pipes positioned at various locations on an electrically insulating substrate having high thermal conductivity. [Figure 10B] This figure shows a side view of heat pipes positioned at various locations on an electrically insulating substrate having high thermal conductivity. [Figure 10C] This figure shows a side view of heat pipes positioned at various locations on an electrically insulating substrate having high thermal conductivity. [Modes for carrying out the invention]

[0009] The present disclosure relates generally to thermal management and, more specifically, to heat pipes having an outer shell with an improved coefficient of thermal expansion (CTE) that matches a heat generating component, such as those employed in printed circuit boards (PCBs) and related electronic systems that include copper clad boards (CCBs) and substrates employing emerging ceramics such as AlN (aluminum nitride) and SiN (silicon nitride). The heat generating component may use an electrically insulating substrate and a thermal insulation substrate such as FR4 or other polymer substrates, but in some cases, a substrate that is electrically insulating but thermally conductive, such as AlN, may be used. Effective heat dissipation is found to be a problem in any case. Through the application of the disclosure herein, the heat pipe may be manufactured in such a manner that the CTE of the outer shell of the heat pipe can be adjusted to match the CTE of a given heat generating component. The heat pipe may be easily incorporated within a PCB to facilitate effective and robust heat transfer from internal heat generating components to an external heat sink.

[0010] As described above, the removal of excessive heat from heat generating components within a circuit board and related electronic assemblies can be problematic due to the prevalence of thermal insulation materials therein. Heat pipes can be effective in dissipating large amounts of excessive heat, but the CTE mismatch between the metal components of the heat pipe and the ceramic materials of the electronic components can often be significant. Where excessive CTE mismatch exists, the heat pipe may become disengaged from the heat generating component, thereby disabling the heat pipe's ability to dissipate excessive heat. The CTE mismatch can also be a problem even when there are substrates that are electrically insulating but highly thermally conductive, such as AlN and SiN, because it can still be difficult to remove excessive heat from the heat generating electronic components thereon due to the heat pipe's strong connection.

[0011] The present disclosure provides a heat pipe that can provide more effective CTE matching between the heat pipe and a heat generating component to which the heat pipe can be connected. Alternatively, the heat pipe disclosed herein may be effectively CTE matched to a thermally conductive substrate on which the heat generating component is disposed. More specifically, the present disclosure provides a metal composite material including a CTE modifier such as a copper composite material, and can easily change the addition amount of the CTE modifier in the metal composite material to promote more effective CTE matching with a given ceramic material in a heat generating component such as SiC, GaN, AlN, etc. The metal composite material may form at least a part of the sealed outer shell of the heat pipe in the disclosure of this specification. The metal composite material can be easily formed from a composition including metal nanoparticles such as copper nanoparticles, whereby the metal composite material and the heat pipe may be formed at a low temperature sufficiently lower than the melting point of the molten metal. Further details regarding metal nanoparticles such as copper nanoparticles and various characteristics that can facilitate their low temperature processing will be described below.

[0012] As will be described in more detail below, the metal nanoparticles may form a bulk metal matrix when compacted together. Various additives may be included in the bulk metal matrix to form the metal composite material. The CTE modifier can change the CTE of the bulk metal matrix. For example, the CTE modifier may reduce the CTE of the bulk metal matrix formed from copper nanoparticles to about 11 ppm at room temperature, or in some cases to about 3 ppm, compared to a value of about 17 ppm typically found in bulk copper. These features can significantly enhance the system design and assembly of the PCB, provide significant performance improvement while reducing the overall product cost. An exemplary explanation of how various CTE modifiers can change the CTE of the copper composite material is provided below.

[0013] In addition to facilitating improved CTE matching between the outer shell of the heat pipe and the ceramic material of the electronic components, the metal nanoparticle composition may also promote direct adhesion (bonding) between the electronic components and the heat pipe through a bonding layer. For example, the metal nanoparticle composition may be applied on the electronic components and in contact with the outer shell of the heat pipe, and subsequent compaction of the metal nanoparticles in the bonding layer may promote direct metallurgical bonding to the outer surface of the heat pipe. Direct metallurgical bonding significantly reduces the possibility of separation of the heat-generating electronic components and the heat pipe from each other as a result of thermomechanical stress. Since the outer shell of the heat pipe and the bonding layer may be formed from similar materials, the possibility of CTE mismatch and separation resulting from thermomechanical stress when the heat pipe contacts the electronic components is also reduced. At large electronic component sizes and high operating temperatures (e.g., up to about 350°C), even small CTE differences can result in high thermomechanical stress values, which can lead to delamination and ultimately device failure. This disclosure may mitigate this difficulty.

[0014] A further advantage of having a heat pipe bonding layer and / or outer shell formed from compacted metal nanoparticles is the porosity in the metal matrix obtained after compaction of the metal nanoparticles. The porosity in the metal matrix can impart flexibility to the bonding layer and / or outer shell, which can facilitate enhanced resistance to thermomechanical stress resulting from any CTE mismatches that remain present.

[0015] Furthermore, an even greater advantage provided by this disclosure is that metal composites containing CTE modifiers may be applicable to heat pipes of various designs. In particular, metal composites such as copper composites may be used to form both conventional heat pipes having a wicking structure extending into the interior of the heat pipe between a first end (hot end) and a second end (cold end), and oscillating heat pipes in which the working fluid moves within a loop having a defined flow path on the inner surface of the heat pipe. Depending on operational considerations, the loop may be open or closed. The structure housing the flow path may also be formed from metal nanoparticles as a starting material in this disclosure.

[0016] The heat pipes of this disclosure may be used in conjunction with printed circuit boards and similar architectures in which heat-generating components are concerned with heat dissipation. Heat-generating components may be located at various locations within the PCB. For example, the location requiring heat dissipation may be located on the front surface of the printed circuit board and within a heat-generating component oriented away from the non-conductive substrate of the PCB, on one or more sides of the heat-generating component, or on the underside of the heat-generating component. In the first two cases, the heat pipe may be in direct contact with the location requiring heat dissipation (for example, directly on the front surface of the PCB or on the front surface of a given PCB layer, whereas in the latter case, the heat pipe may extend further through the electrically insulating substrate to contact the underside of the location requiring heat dissipation). In the case of lateral contact with the heat-generating component, one or more heat pipes may extend laterally across a given PCB layer. Collectively, these configurations for connecting heat pipes to heat-generating components may be used to facilitate the stacking of multiple PCB layers on top of each other. Any combination of configurations for connecting heat pipes to electronic components may be utilized within a multilayer PCB to facilitate heat dissipation. Furthermore, in some layered configurations, oscillating heat pipes may be positioned between adjacent PCB layers, and wicking heat pipes may be positioned on the outermost (top or bottom) of the stacked PCB layers. Heat pipes may be thermally connected to structures (or arrangements) for dissipating excess heat, such as liquid reservoirs, radiators, or similar structures that function as heat sinks.

[0017] Similarly, in the case of heat-generating components housed on an electrically insulating but highly thermally conductive substrate such as AlN or SiN, the heat pipes of this disclosure may be positioned on any surface of the substrate or internally within the substrate. Here again, efficient heat transfer and a robust connection of the heat pipe to locations requiring heat removal may be achieved. In some cases, AlN or SiN may be deposited as a thin film (e.g., about 300 to about 500 micrometers thick) on the surface of the electrically insulating substrate to transfer thermal conductivity. A relatively thin layer may limit the thermal resistance when transferring heat to the heat pipe. Further details regarding how the heat-generating components may be positioned and how the heat pipes may be connected to the heat-generating components are provided herein. Alternatively, when an electrically insulating but thermally conductive substrate is used, the heat pipes may reside on the substrate rather than in direct contact with the heat-generating components, thereby receiving excess heat from the heat-generating components via the thermally conductive substrate.

[0018] In further embodiments, heat-generating components not associated with a substrate may be in direct contact with the heat pipe of the Disclosure, in which case an intermediate layer formed at least partially from compacted metal nanoparticles may be present between the heat pipe and the heat-generating component. The intermediate layer may provide electrical insulation between the heat pipe and the heat-generating component. For example, a thin AlN film may be bonded to the heat-generating component by a first bonding layer formed from metal nanoparticles and to the heat pipe by a second bonding layer formed from metal nanoparticles, and the AlN film provides electrical insulation between the heat pipe and the heat-generating component. CTE matching can be achieved in any of these cases.

[0019] Metal nanoparticles are uniquely qualified to form at least a portion of a heat pipe according to the disclosure herein, and to form a bonding layer between the heat pipe and heat-generating components. Both of these functions may be facilitated as a result of moderate processing conditions necessary to compact the metal nanoparticles to form a bulk metal (e.g., bulk copper) in a metal composite (e.g., a copper composite containing a CTE modifier). As will be described in more detail below, metal nanoparticles may be compacted (fused) together with the corresponding bulk metal under a range of mild processing conditions significantly lower than the melting point of the metal itself. Due to copper's high thermal conductivity and relatively low cost, copper nanoparticles may be a particularly desirable type of metal nanoparticle for use in various embodiments of this disclosure. When metal nanoparticles are compacted into a bulk metal in combination with a CTE modifier, they may effectively form a well-dispersed composite. Suitable CTE modifiers include, for example, carbon fibers, diamond particles, boron nitride particles, aluminum nitride particles, carbon nanotubes, graphene, W and / or Mo (molybdenum) particles, and any combination thereof. W and / or Mo particles may also impart oxidation resistance to copper as an additional benefit. In addition to promoting CTE matching within the heat pipe, CTE modifiers and micrometer-sized metal particles can limit the shrinkage during compaction of metal nanoparticles, which may otherwise exceed 20% in other metal nanoparticle systems. This limited shrinkage can further mitigate the thermomechanical stresses experienced during high-temperature-low-temperature cycles in operation.

[0020] In addition to the aforementioned advantages, metal nanoparticles can facilitate the manufacture of heat pipes having a more enhanced structure for heat dissipation. For example, a heat pipe having a wicking structure within an outer shell formed from metal nanoparticles may further include a plurality of thermally conductive fibers (e.g., metal fibers, ceramic fibers, carbon fibers, etc.) extending from the end portion (low-temperature end) of the heat pipe. The thermally conductive fibers can facilitate the easy dissipation of excess heat to a heat sink, such as in an ambient atmosphere, a marine environment (e.g., ocean, lake, or river water), or a radiator for space applications, by providing a large surface area for heat dissipation. The thermally conductive fibers may be bonded to the heat pipe using a metal nanoparticle composition effective for facilitating CTE matching, as described below. Bonding of the thermally conductive fibers may be achieved during the manufacture of the heat pipe without a separate bonding step, for example, by incorporating the thermally conductive fibers into a suitable metal nanoparticle paste composition prior to the compaction of metal nanoparticles that is carried out to form at least a portion of the heat pipe. Furthermore, if necessary, the thermally conductive fibers may extend into the internal space (cavity) of the heat pipe to promote enhanced thermal communication with the working fluid inside.

[0021] As used herein, the term “metal nanoparticles” refers to metal particles having a size of approximately 200 nm or less, without specifically mentioning the shape of the metal particles.

[0022] As used herein, the term “micrometer-scale metal particles” refers to metal particles that are at least 200 nm in size in at least one dimension.

[0023] The terms “consolidate,” “consolidation,” and other variations thereof are used interchangeably herein with the terms “fuse,” “fusion,” and other variations thereof.

[0024] As used herein, the terms “partially fused” and “partial fusion,” as well as other derived terms and grammatical equivalents thereof, refer to the partial bonding of metal nanoparticles. While fully fused metal nanoparticles retain only the minimal structural morphology of the original unfused metal nanoparticles (i.e., they resemble high-density bulk metal but have grain boundaries in the range of 100–500 nm), partially fused metal nanoparticles retain at least some of the structural morphology of the original unfused metal nanoparticles, such as a higher level of porosity, a smaller average grain size, and a greater number of grain boundaries. The properties of partially fused metal nanoparticles may be intermediate between those of the corresponding bulk metal and those of the original unfused metal nanoparticles. In some embodiments, a sufficiently dense (non-porous) bulk metal may be obtained after metal nanoparticle compaction to obtain a metal composite. In other embodiments, the metal composite may have a porosity of less than about 10%, or less than about 20%, or less than about 30%, with an amount exceeding complete densification (i.e., >0% porosity). Thus, in certain embodiments, the metal composite may have a porosity in the range of about 2% to about 30%, or about 2% to about 5%, or about 5% to about 10%, or about 10% to about 15%, or about 15% to about 20%, or about 20% to about 25%, or about 25% to about 30%. As described above, porosity can enhance resistance to thermomechanical stress.

[0025] Before describing more specific embodiments of this disclosure in further detail, we first provide an additional brief description of metal nanoparticles and their processing conditions, particularly copper nanoparticles. Metal nanoparticles exhibit many properties that can differ significantly from those of the corresponding bulk metal. One property of metal nanoparticles that may be particularly important for processing according to this disclosure is nanoparticle fusion (densification) that occurs at the melting temperature of the metal nanoparticles. As used herein, the term “melting temperature” refers to the temperature at which the metal nanoparticles liquefy and thereby give the appearance of moltenness. As used herein, the terms “fusion” and “densification” are synonymous with the metal nanoparticles joining or partially joining together to form a larger mass, such as a bulk metal defining the outer shell of a heat pipe or a bonding layer in contact with the heat pipe. The melting temperature may be about 80% lower than the melting point of the corresponding bulk metal. Thus, after heating above the melting temperature and then cooling, at least partial connectivity exists between the metal nanoparticles. Following the compaction of metal nanoparticles, the resulting nanoporosity may adapt to the thermal stresses generated during heating and cooling cycles while still maintaining the hermetically sealed structure of the heat pipe.

[0026] As the size decreases, especially when the equivalent sphere diameter is less than approximately 20 nm, the temperature at which metal nanoparticles liquefy drops dramatically from the temperature of the corresponding bulk metal. For example, copper nanoparticles with a size of approximately 20 nm or less may have a melting temperature of approximately 235°C or less, or approximately 220°C or less, or approximately 200°C or less, compared to the melting point of bulk copper, which is 1083°C. Therefore, compaction of metal nanoparticles performed at the melting temperature can make it possible to manufacture structures containing bulk metal at significantly lower processing temperatures than when the bulk metal itself is directly processed as the starting material. Processing conditions for compacting metal nanoparticles are typically within the range of normal PCB manufacturing parameters of approximately 375°F, or up to approximately 450°F, and 275-400 psi, although pressure is not necessarily required for the fusion of metal nanoparticles to occur. Higher density bulk metal may be obtained by applying pressure to accelerate the compaction of metal nanoparticles. For example, in the case of copper nanoparticles, the melting temperature is lower than the temperature at which commonly used PCB substrates melt or deform. Therefore, metal nanoparticles such as copper nanoparticles provide an easy material for forming bulk metal within heat pipes or bonding layers connected to heat-generating components within a PCB.

[0027] Numerous weighable processes have been developed for producing large quantities of metal nanoparticles within a target size range. Most typically, such processes for producing metal nanoparticles are carried out by reducing a metal precursor in the presence of one or more surfactants. The metal nanoparticles may then be isolated and purified from the reaction mixture by common isolation techniques and processed into formulations suitable for partitioning.

[0028] Any suitable technique may be used to form the metal nanoparticles used in the disclosures herein. Particularly easy metal nanoparticle production techniques are described in U.S. Patents 7,736,414, 8,105,414, 8,192,866, 8,486,305, 8,834,747, 9,005,483, 9,095,898, and 9,700,940, each of which is incorporated herein by reference in whole. As described therein, metal nanoparticles can be produced in a narrow size range by reducing a metal salt in a solvent in the presence of a suitable surfactant system which may comprise one or more different surfactants. A target size distribution of metal nanoparticles, including a bimodal size distribution, may be obtained by combining metal nanoparticles of different sizes together. Further descriptions of suitable surfactant systems are given below. While not bound by any theory or mechanism, it is thought that surfactant systems can mediate the nucleation and growth of metal nanoparticles, limit surface oxidation of metal nanoparticles, and / or inhibit widespread aggregation of metal nanoparticles prior to at least partial fusion. Suitable organic solvents for solubilizing metal salts and forming metal nanoparticles include, for example, formamide, N,N-dimethylformamide, dimethyl sulfoxide, dimethylpropylene urea, hexamethylphosphoramide, tetrahydrofuran, glycyrm, diglycyrm, triglycyrm, tetraglycyrm, proglycyrm, or polyglycyrm. Suitable reducing agents for reducing metal salts and promoting the formation of metal nanoparticles include, for example, alkali metals (e.g., lithium naphthalide, sodium naphthalide, or potassium naphthalide) in the presence of a suitable catalyst, or borohydride reducing agents (e.g., sodium borohydride, lithium borohydride, potassium borohydride, or tetraalkylammonium borohydride).

[0029] Figures 1 and 2 show diagrams of the presumed structure of metal nanoparticles having a surfactant coating thereon. As shown in Figure 1, the metal nanoparticle 10 comprises a metal core 12 and a surfactant layer 14 coating the metal core 12. The surfactant layer 14 may contain any combination of surfactants, as will be described in more detail below. The metal nanoparticle 20 shown in Figure 2 is similar to that shown in Figure 1, except that the metal core 12 grows around a nucleus 21, which may be the same metal as the metal core 12 or a different metal. The nucleus 21 is deeply embedded within the metal core 12 in the metal nanoparticle 20 and is so small in size that it is not considered to significantly affect the overall nanoparticle properties. The nucleus 21 may contain a salt or a metal, and the metal may be the same as or different from that of the metal core 12. In some embodiments, the metal nanoparticles may have an amorphous morphology. Although the metal nanoparticles 10 and 20 in Figures 1 and 2 are shown to be substantially spherical, at least a portion of the metal nanoparticles may have a non-spherical shape.

[0030] As described above, metal nanoparticles have a surfactant coating containing one or more surfactants on their surface. The surfactant coating can be formed on the metal nanoparticles during synthesis. The surfactant coating is generally lost during the compaction of the metal nanoparticles when heated above the melting temperature, resulting in the formation of bulk metal, which may have uniform nanoporosity inside. Forming a surfactant coating on the metal nanoparticles during synthesis can preferably limit the ability of the metal nanoparticles to fuse prematurely with each other, limit aggregation of the metal nanoparticles, and promote the formation of a group of metal nanoparticles with a narrow size distribution. The porosity value may be in the range of about 2-30% or about 2-15% after compaction, but this may be adjusted based on many factors, including the type of surfactant(s) present. At a nanoporosity of approximately 2% to approximately 15%, the copper composite may contain approximately 85% to 98% high-density fused copper nanoparticles having closed-pore nanoporosity with pore sizes in the range of approximately 50 nm to approximately 500 nm, approximately 100 nm to approximately 300 nm, or approximately 150 nm to approximately 250 nm.

[0031] The types of metal nanoparticles suitable for use in conjunction with the various embodiments of this disclosure are not considered to be particularly limited. Suitable metal nanoparticles include, but are not limited to, tin nanoparticles, copper nanoparticles, aluminum nanoparticles, palladium nanoparticles, silver nanoparticles, gold nanoparticles, iron nanoparticles, cobalt nanoparticles, nickel nanoparticles, titanium nanoparticles, zirconium nanoparticles, hafnium nanoparticles, tantalum nanoparticles, molybdenum nanoparticles, tungsten nanoparticles, and the like. Combinations of these metal nanoparticles may also be used. Micrometer-scale particles of these metals may also be present in metal nanoparticle paste compositions containing metal nanoparticles. Copper may be a particularly desirable metal for use in the embodiments of this disclosure due to its low cost, strength, and excellent electrical and thermal conductivity values. Copper nanoparticles may be used in combination with other types of metal nanoparticles and / or micrometer-scale metal particles containing metals other than copper.

[0032] In various embodiments, the surfactant system present within the metal nanoparticles may contain one or more surfactants. The properties of the metal nanoparticles can be adjusted by using the different properties of various surfactants. Factors that may be considered when selecting a surfactant or combination of surfactants to be included on the metal nanoparticles include, for example, the ease of surfactant release from the metal nanoparticles during nanoparticle fusion, the nucleation and growth rate of the metal nanoparticles, and the metal components of the metal nanoparticles.

[0033] In some embodiments, an amine surfactant or a combination of amine surfactants, particularly an aliphatic amine, may be present on the metal nanoparticles. Amine surfactants may be particularly desirable for use with copper nanoparticles. In some embodiments, two amine surfactants may be used in combination with each other. In other embodiments, three amine surfactants may be used in combination with each other. In more specific embodiments, a primary amine, a secondary amine, and a diamine chelating agent may be used in combination with each other. In even more specific embodiments, the three amine surfactants may include a long-chain primary amine, a secondary amine, and a diamine having at least one tertiary alkyl group nitrogen substituent. Further disclosures regarding preferred amine surfactants follow below.

[0034] In some embodiments, the surfactant system may include a primary alkylamine. In some embodiments, the primary alkylamine is C2-C 18 It may be an alkylamine. In some embodiments, the primary alkylamine is C7-C 10 It may be an alkylamine. In other embodiments, C5-C6 primary alkylamines may also be used. Although not bound by any theory or mechanism, the exact size of the primary alkylamine can be balanced between being long enough to provide an effective inverse micelle structure during synthesis and being highly volatile and / or easy to handle during nanoparticle compaction. For example, primary alkylamines having more than 18 carbon atoms may also be suitable for use in this embodiment, but they may be more difficult to handle due to their waxy properties. In particular, C7-C 10 Primary alkylamines can offer a good balance of desired properties for ease of use.

[0035] In some embodiments, C2~C 18Primary alkylamines may be, for example, n-hexylamine, n-heptylamine, n-octylamine, n-nonylamine, or n-decylamine. These are all linear primary alkylamines, but in other embodiments, branched primary alkylamines may also be used. For example, branched primary alkylamines such as 7-methyloctylamine, 2-methyloctylamine, or 7-methylnonylamine may be used. In some embodiments, such branched primary alkylamines may be sterically hindered if they are bonded to an amine nitrogen atom. Non-limiting examples of such sterically hindered primary alkylamines include, for example, t-octylamine, 2-methylpentane-2-amine, 2-methylhexane-2-amine, 2-methylheptane-2-amine, 3-ethyloctane-3-amine, 3-ethylheptane-3-amine, and 3-ethylhexane-3-amine. Additional branching may also be present. Although not bound by any theory or mechanism, it is thought that primary alkylamines can function as ligands in the metal coordination sphere, but can readily dissociate from the metal coordination sphere during the compaction of metal nanoparticles.

[0036] In some embodiments, the surfactant system may include a secondary amine. Suitable secondary amines for forming metal nanoparticles are linear, branched, or cyclic C4-C4 atoms bonded to the amine nitrogen atom. 12 It may contain alkyl groups. In some embodiments, branching may occur on carbon atoms bonded to the amine nitrogen atom, thereby causing significant steric hindrance at the nitrogen atom. Suitable secondary amines include, but are not limited to, dihexylamine, diisobutylamine, di-t-butylamine, dieopentylamine, di-t-pentylamine, dicyclopentylamine, and dicyclohexylamine. C4~C 12 Secondary amines outside this range may also be used, but such secondary amines may have undesirable physical properties, such as low boiling points or waxy viscosity, which can complicate their handling.

[0037] In some embodiments, the surfactant system may include a chelating agent, particularly a diamine chelating agent. In some embodiments, one or both nitrogen atoms of the diamine chelating agent may be substituted with one or two alkyl groups. If the two alkyl groups are on the same nitrogen atom, they may be the same or different. Furthermore, if both nitrogen atoms are substituted, the same or different alkyl groups may be present. In some embodiments, the alkyl group may be a C1-C6 alkyl group. In other embodiments, the alkyl group may be a C1-C4 alkyl group or a C3-C6 alkyl group. In some embodiments, alkyl groups of C3 or higher may be linear or branched. In some embodiments, alkyl groups of C3 or higher may be cyclic. Although not bound by any theory or mechanism, it is thought that diamine chelating agents may facilitate the formation of metal nanoparticles by promoting nanoparticle nucleation.

[0038] In some embodiments, suitable diamine chelating agents include N,N'-dialkylethylenediamines, particularly C1-C4N,N'-dialkylethylenediamines. Corresponding methylenediamine, propylenediamine, butylenediamine, pentylenediamine, or hexylenediamine derivatives may also be used. The alkyl groups may be the same or different. Possible C1-C4 alkyl groups include, for example, methyl, ethyl, propyl, and butyl groups, or branched alkyl groups such as isopropyl, isobutyl, s-butyl, and t-butyl groups. Exemplary N,N'-dialkylethylenediamines that may be suitable for inclusion on metal nanoparticles include, for example, N,N'-di-t-butylethylenediamine and N,N'-diisopropylethylenediamine.

[0039] In some embodiments, suitable diamine chelating agents may include N,N,N’,N’-tetraalkyl ethylenediamine, particularly C1-C4 N,N,N’,N’-tetraalkyl ethylenediamine. Corresponding methylenediamine, propylenediamine, butylenediamine, pentylenediamine, or hexylenediamine derivatives may also be used. The alkyl groups may be the same or different and include those described above. Exemplary N,N,N’,N’-tetraalkyl ethylenediamines that may be suitable for use in the formation of metal nanoparticles include, for example, N,N,N’,N’-tetramethyl ethylenediamine, N,N,N’,N’-tetraethyl ethylenediamine, and the like.

[0040] Surfactants other than aliphatic amines may also be present in the surfactant system. In this regard, suitable surfactants may include, for example, pyridine, aromatic amines, phosphines, thiols, or any combination thereof. These surfactants may be used in combination with aliphatic amines including those described above or may be used in a surfactant system in which no aliphatic amine is present. Further disclosure regarding suitable pyridine, aromatic amines, phosphines, and thiols follows.

[0041] Suitable aromatic amines may have the formula ArNR 1 R 2 wherein Ar is a substituted or unsubstituted aryl group and R 1 and R 2 are the same or different. R 1 and R 2 may be independently selected from H or an alkyl or aryl group containing from 1 to about 16 carbon atoms. Exemplary aromatic amines that may be suitable for use in the formation of metal nanoparticles include, for example, aniline, toluidine, anisidine, N,N-dimethylaniline, N,N-diethylaniline, and the like. Other aromatic amines that may be used with metal nanoparticles may be envisioned by those skilled in the art.

[0042] Suitable pyridines may include both pyridine and its derivatives. Examples of pyridines that may be suitable for use on metal nanoparticles include pyridine, 2-methylpyridine, 2,6-dimethylpyridine, colidine, and pyridazine. Chelated pyridines, such as bipyridyl chelators, may also be used. Other pyridines that may be used with metal nanoparticles can be imagined by those skilled in the art.

[0043] A suitable phosphine may have the formula PR3, where R is an alkyl or aryl group containing 1 to about 16 carbon atoms. The alkyl or aryl group bonded to the phosphorus center may be the same or different. Exemplary phosphines that may be present on metal nanoparticles include, for example, trimethylphosphine, triethylphosphine, tributylphosphine, tri-t-butylphosphine, trioctylphosphine, and triphenylphosphine. Phosphine oxides may also be used in a similar manner. In some embodiments, surfactants containing two or more phosphine groups configured to form a chelate ring may also be used. Exemplary chelated phosphines include, for example, bisphosphines such as 1,2-bisphosphine, 1,3-bisphosphine, and BINAP. Other phosphines that may be used with metal nanoparticles can be imagined by those skilled in the art.

[0044] Suitable thiols may have the formula RSH, where R is an alkyl or aryl group having about 4 to about 16 carbon atoms. Exemplary thiols that may be present on metal nanoparticles include, for example, butanethiol, 2-methyl-2-propanethiol, hexanethiol, octanethiol, and benzenethiol. In some embodiments, surfactants containing two or more thiol groups configured to form a chelate ring may also be used. Exemplary chelated thiols include, for example, 1,2-dithiol (e.g., 1,2-ethanethiol) and 1,3-dithiol (e.g., 1,3-propanethiol). Other thiols that may be used with metal nanoparticles can be imagined by those skilled in the art.

[0045] As described above, a distinguishing feature of metal nanoparticles is their low melting temperature, which may facilitate densification for forming bulk metal within metal composites as disclosed herein. To facilitate their deposition at specific locations, metal nanoparticles may be incorporated into pastes or similar formulations. Further disclosures relating to metal nanoparticle paste compositions and similar formulations follow below.

[0046] Metal nanoparticle paste compositions or similar formulations may be prepared by dispersing as-formed or isolated metal nanoparticles in an organic matrix containing one or more organic solvents and various other optional components. As used herein, the terms “nanoparticle paste formulation,” “nanoparticle paste composition,” and their grammatical equivalents are interchangeable and synonymous with a fluid composition containing dispersed metal nanoparticles suitable for distribution using the desired technique. The use of the term “paste” does not necessarily imply the adhesive function of the paste alone. Favorable distribution of metal nanoparticles and formation of bulk metal may be achieved under advantageous conditions through the sensible selection of organic solvents and other additives, including the filling of metal nanoparticles.

[0047] During the compaction of metal nanoparticles, cracks may occasionally occur. One way in which the nanoparticle paste of this disclosure may promote a reduction in the degree of cracking and void formation after compaction of metal nanoparticles is by maintaining a high solids content. More specifically, in some embodiments, the paste composition may contain at least about 30 wt% of metal nanoparticles, in particular about 30 wt% to about 98 wt% of the paste composition, or about 50 wt% to about 95 wt% of the paste composition, or about 70 wt% to about 98 wt% of the paste composition. Furthermore, in some embodiments, small amounts (e.g., about 0.01 wt% to about 15 wt%, or about 35 wt%, or about 60 wt%) of micrometer-scale particles, in particular micrometer-scale metal particles, may be present in addition to the metal nanoparticles. Such micrometer-scale metal particles may preferably promote the fusion of the metal nanoparticles into a continuous mass of bulk metal, further reducing the rate of cracking. Instead of being liquefied and directly compacted as in the case of metal nanoparticles, micrometer-scale metal particles may simply bond together when they come into contact with liquefied metal nanoparticles whose melting temperature has risen above their own. These factors can reduce the porosity that results after the metal nanoparticles have been fused together. Micrometer-scale metal particles may contain the same or different metals as the metal nanoparticles, but suitable metals for micrometer-scale metal particles include, for example, copper, silver, gold, aluminum, tin, molybdenum, and tungsten. In some embodiments, micrometer-scale graphite particles may also be included. Carbon nanotubes, carbon fibers, boron nitride, diamond particles, and / or graphene may be included as micrometer-scale particles, and in some embodiments, all of them may function as CTE modifiers to adjust the CTE according to the disclosure herein. According to some embodiments, carbonaceous additives may increase the thermal conductivity that results after the compaction of metal nanoparticles has been performed. Any of the aforementioned micrometer-scale particles can further function as crack deflectors to limit the propagation of cracks during use, thereby increasing mechanical strength.

[0048] A wise selection of the solvent(s) forming the organic matrix can also promote a reduction in crack and void formation during metal nanoparticle compaction. A carefully chosen combination of organic solvents can desirablely reduce the incidence of crack and void formation. More specifically, an organic matrix containing one or more hydrocarbons (saturated, monounsaturated, polyunsaturated (two or more double bonds), or aromatic), one or more alcohols, one or more amines, and one or more organic acids may be particularly effective for this purpose. In some embodiments, one or more esters and / or one or more anhydrides may be included. In some cases, alkanolamines such as ethanolamine may be present. While not bound by any theory or mechanism, this combination of organic solvents is thought to promote the removal and separation of surfactant molecules surrounding the metal nanoparticles during compaction, allowing the metal nanoparticles to fuse more easily with each other. More specifically, hydrocarbon and alcohol solvents are thought to passively solubilize surfactant molecules released from the metal nanoparticles by Brownian motion, reducing their ability to reattach to the metal nanoparticles. In conjunction with the passive solubilization of surfactant molecules, amines and organic acid solvents can actively separate the surfactant molecules through chemical interactions, so that the surfactant molecules are no longer available for recombination with metal nanoparticles.

[0049] Further adjustments to the solvent composition may be made to reduce the abrupt volume contraction that occurs during surfactant removal and metal nanoparticle compaction. Specifically, two or more members of each class of organic solvents (i.e., hydrocarbons, alcohols, amines, and organic acids) may be present in the organic matrix in combination with one or more alkanolamines, esters, or anhydrides, as desired, but the members of each class have boiling points separated from each other to a set degree. For example, in some embodiments, the various members of each class may have boiling points separated from each other by about 20°C to about 50°C. By using such a solvent mixture, the various components of the solvent mixture can be gradually removed over a wide range of boiling points (e.g., about 50°C to about 200°C), thus minimizing abrupt volume changes caused by rapid solvent loss during metal nanoparticle compaction.

[0050] In various embodiments, at least a portion of one or more organic solvents may have a boiling point of about 100°C or higher. In other various embodiments, at least a portion of one or more organic solvents may have a boiling point of about 200°C or higher. In some or other embodiments, one or more organic solvents may have a boiling point in the range of about 50°C to about 200°C, or about 50°C to about 250°C, or about 50°C to about 300°C, or about 50°C to about 350°C. The use of high-boiling point organic solvents can preferably increase the pot life of the metal nanoparticle paste composition and limit rapid solvent loss, otherwise it may lead to crack and void formation during nanoparticle compaction. In some embodiments, at least one of the organic solvents may have a higher boiling point than the boiling point of the surfactant(s) associated with the metal nanoparticles. Thus, the surfactant(s) can be removed from the metal nanoparticles by evaporation before the removal of the organic solvent(s) is carried out.

[0051] In some embodiments, the organic matrix may contain one or more alcohols, which in more specific embodiments may be C2-C 12 , C4~C 12 , or C7~C 12This may also be the case. In various embodiments, the alcohol may include a monohydric alcohol, a diol, or a triol. In certain embodiments, one or more glycol ethers (e.g., diethylene glycol and triethylene glycol), alkanolamines (e.g., ethanolamine, triethanolamine, etc.), or any combination thereof may be present, either alone or in combination with other alcohols. In some embodiments, various glycans may be present with one or more alcohols. In some embodiments, one or more hydrocarbons may be present in combination with one or more alcohols. As described above, the alcohols (and optionally glycans and alkanolamines) and hydrocarbon solvents are thought to passively promote the solubilization of surfactants as they are removed from metal nanoparticles by Brownian motion and to limit their reassociation with metal nanoparticles. Furthermore, since the hydrocarbon solvents and alcohol solvents coordinate only slightly with the metal nanoparticles, they do not simply replace the substituted surfactants in the nanoparticle coordination sphere. Exemplary but non-limiting examples of possible alcohol and hydrocarbon solvents include, for example, light aromatic petroleum distillates (CAS Registry No. 64742-95-6), hydrogenated light petroleum distillates (CAS Registry No. 64742-47-8), tripropylene glycol methyl ether, and ligroin (CAS Registry No. 68551-17-7, C 10 ~C 13 Examples include mixtures of alkanes, diisopropylene glycol monomethyl ether, diethylene glycol diethyl ether, 2-propanol, 2-butanol, t-butanol, 1-hexanol, 2-(2-butoxyethoxy)ethanol, and terpineol. In some embodiments, polyketone solvents may also be used.

[0052] In some embodiments, the organic matrix may contain one or more amines and one or more organic acids. In some embodiments, the one or more amines and one or more organic acids may be present in the organic matrix, which may also contain one or more hydrocarbons and one or more alcohols. As described above, the amines and organic acids can actively separate surfactants that have been passively solubilized by the hydrocarbon and alcohol solvents, thereby preventing the surfactants from being used for reassociation with the metal nanoparticles. Therefore, organic solvents containing a combination of one or more hydrocarbons, one or more alcohols, one or more amines, and one or more organic acids may offer synergistic benefits for promoting the compaction of metal nanoparticles. Exemplary but non-limiting examples of possible amine solvents include, for example, taloamine (CAS registry number 61790-33-8), alkyl (C8~C 18 ) Unsaturated amine (CAS Registry No. 68037-94-5), di(hydrogenated)amine (CAS Registry No. 61789-79-5), dialkyl (C8~C 20 )amine (CAS registry number 68526-63-6), alkyl (C 10 ~C 16 )Dimethylamine (CAS Registry Number 67700-98-5), alkyl (C 14 ~C 18 )Dimethylamine (CAS Registry No. 68037-93-4), dihydromethylamine (CAS Registry No. 61788-63-4), and trialkyl (C6~C 12 ) Amine (CAS Registry No. 68038-01-7) is an example. Exemplary but non-limiting examples of organic acid solvents that may be present in the nanoparticle paste composition include, for example, octanoic acid, nonanoic acid, decanoic acid, caprylic acid, pelargonic acid, undecylic acid, lauric acid, tridecylic acid, myristic acid, pentadecanoic acid, palmitic acid, margaric acid, stearic acid, nonadecylic acid, α-linolenic acid, stearidonic acid, oleic acid, and linoleic acid.

[0053] In some embodiments, the organic matrix may comprise two or more hydrocarbons, two or more alcohols, optionally two or more glycosides (glycol ethers), two or more amines, and two or more organic acids. For example, in some embodiments, each class of organic solvents may have two or more members, or three or more members, or four or more members, or five or more members, or six or more members, or seven or more members, or eight or more members, or nine or more members, or ten or more members. Furthermore, the number of members in each class of organic solvents may be the same or different. Specific advantages of using multiple members in each class of organic solvents are described below. Organic solvents with higher boiling points may offer safety advantages.

[0054] One particular advantage of using multiple members within each class of organic solvents may include the ability to provide a wide range of boiling points in the metal nanoparticle paste composition. By providing a wide range of boiling points, the organic solvent can be gradually removed as the temperature rises, while affecting the compaction of the metal nanoparticles, thereby limiting volume shrinkage and making cracking less likely. Gradually removing the organic solvent in this way requires less temperature control to affect slow solvent removal than when using a single solvent with a narrow boiling point range. In some embodiments, members within each class of organic solvents may have a boiling point range of about 50°C to about 200°C, or about 50°C to about 250°C, or about 100°C to about 200°C, or about 100°C to about 250°C, or about 150°C to about 300°C, or about 150°C to about 350°C. In more specific embodiments, various members of each class of organic solvents may each have boiling points separated from each other by at least about 20°C, specifically, about 20°C to about 50°C. More specifically, in some embodiments, each hydrocarbon may have a boiling point about 20°C to 50°C different from other hydrocarbons in the organic matrix, each alcohol may have a boiling point about 20°C to 50°C different from other alcohols in the organic matrix, each amine may have a boiling point about 20°C to 50°C different from other amines in the organic matrix, and each organic acid may have a boiling point about 20°C to 50°C different from other organic acids in the organic matrix. The more members of each class of organic solvent present, the smaller the difference between boiling points becomes. By making the difference between boiling points smaller, solvent removal can be performed more continuously, thereby limiting the degree of volume contraction that occurs at each stage. When four or more types of organic solvents of each class are present (for example, four or more hydrocarbons, four or more alcohols, four or more amines, and four or more organic acids, or five or more hydrocarbons, five or more alcohols, five or more amines, and five or more organic acids), a decrease in the degree of cracking may occur, but their respective boiling points are separated from each other within the above range.

[0055] In various embodiments, the metal nanoparticles used in the metal nanoparticle paste composition may be about 20 nm or smaller in size. In other various embodiments, the metal nanoparticles may be up to about 75 nm in size. As described above, metal nanoparticles in this size range have a melting temperature significantly lower than the melting temperature of the corresponding bulk metal, and as a result, readily undergo compaction with each other. In some embodiments, metal nanoparticles with a size of about 20 nm or smaller may have a melting temperature of about 220°C or smaller (e.g., a melting temperature in the range of about 140°C to about 220°C) or about 200°C or smaller, which may provide the above-mentioned advantages. In some embodiments, at least a portion of the metal nanoparticles may be about 10 nm or smaller in size, or about 5 nm or smaller. In more specific embodiments, at least a portion of the metal nanoparticles may be in the size range of about 1 nm to about 20 nm, or about 1 nm to about 10 nm, or about 1 nm to about 5 nm, or about 3 nm to about 7 nm, or about 5 nm to about 20 nm. In some embodiments, substantially all metal nanoparticles may be present within these size ranges. In some embodiments, larger metal nanoparticles may be combined with metal nanoparticles of a size of about 20 nm or less in the metal nanoparticle paste composition. For example, in some embodiments, metal nanoparticles in the size range of about 1 nm to about 10 nm may be combined with metal nanoparticles in the size range of about 25 nm to about 50 nm, or metal nanoparticles in the size range of about 25 nm to about 100 nm, or metal nanoparticles in the size range of about 25 nm to about 150 nm. Furthermore, as will be discussed later, in some embodiments, micrometer-scale metal particles and / or nanoscale particles may also be included in the metal nanoparticle paste composition. Larger metal nanoparticles and micrometer-scale metal particles may not be liquefiable at the low temperatures of their smaller counterparts, but they can still be compacted when in contact with smaller metal nanoparticles that are liquefied above their melting temperature, as generally described above.

[0056] In addition to metal nanoparticles and organic solvents, other additives may also be present in the metal nanoparticle paste composition. Such additional additives may include, for example, rheology control aids, thickeners, micrometer-scale conductive additives, nanoscale conductive additives, and any combination thereof. Chemical additives may also be present. Inclusion of micrometer-scale conductive additives, such as micrometer-scale metal particles, may be particularly advantageous, as described below. In some cases, it may be desirable to include nanoscale or micrometer-scale diamond or other thermally conductive additives to promote more efficient heat transfer and, similarly, to modulate the CTE. Suitable CTE modifiers, which may be in the micrometer-scale or nanoscale size range, may include, but are not limited to, carbon fibers, diamond particles, boron nitride particles, aluminum nitride particles, carbon nanotubes, graphene, and the like.

[0057] In some embodiments, the metal nanoparticle paste composition may contain about 0.01% to about 15% by weight of micrometer-scale metal particles, or about 1% to about 10% by weight of micrometer-scale metal particles, or about 1% to about 5% by weight of micrometer-scale metal particles, or about 0.1% to about 35% by weight of micrometer-scale metal particles. By including micrometer-scale metal particles in the metal nanoparticle paste composition, the rate of crack occurrence that occurs during the compaction of metal nanoparticles when forming bulk metal can be desirablely reduced. Although not bound by any theory or mechanism, it is thought that micrometer-scale metal particles can be compacted together as the metal nanoparticles are liquefied and form a transient liquid coating on the surface of the micrometer-scale metal particles. In some embodiments, the micron-scale metal particles may range in size from about 500 nm to about 100 microns in at least one dimension, or from about 500 nm to about 10 microns in at least one dimension, or from about 100 nm to about 5 microns in at least one dimension, or from about 100 nm to about 10 microns in at least one dimension, or from about 100 nm to about 1 micron in at least one dimension, or from about 1 micron to about 10 microns in at least one dimension, or from about 5 microns to about 10 microns in at least one dimension, or from about 1 micron to about 100 microns in at least one dimension. The micrometer-scale metal particles may contain the same metal as the metal nanoparticles or may contain different metals. Therefore, copper composites may be formed in this disclosure by combining copper nanoparticles and CTE modifiers with each other, and optionally by further combining them with micrometer-scale copper particles. Similarly, metal alloys can be produced by including micrometer-scale metal particles in a paste composition having a different metal than the metal of the metal nanoparticles. Metal alloys may also be formed by combining different types of metal nanoparticles with each other.Suitable micrometer-scale metal particles include, for example, particles of Cu, Ni, Al, Fe, Co, Mo, W, Ag, Zn, Sn, Au, Pd, Pt, Ru, Mn, Cr, Ti, V, Mg, or Ca. Non-metallic particles, such as micrometer-scale particles of the Si, B, and C systems, can also be used. In some embodiments, the micrometer-scale metal particles may be in the form of metal flakes, such as high-aspect-ratio copper flakes. Thus, in some embodiments, the metal nanoparticle paste compositions described herein may contain a mixture of copper nanoparticles and high-aspect-ratio copper flakes or other types of micrometer-scale copper particles in combination with a CTE modifier. Specifically, in some embodiments, the metal nanoparticle paste compositions may contain about 30% to about 90% by weight of copper nanoparticles and about 0.01% to about 15% by weight, or 1% to 35% by weight, of high-aspect-ratio copper flakes.

[0058] Other micrometer-scale metal particles that can be used in conjunction with high aspect ratio metal flakes include, for example, metal nanowires and other high aspect ratio particles, which may be up to about 300 micrometers in length. The ratio of metal nanoparticles to metal nanowires may range from about 10:1 to about 40:1, depending on various embodiments. Suitable nanowires may have, for example, a length of about 5 micrometers to about 50 micrometers and a diameter of about 100 nm to about 200 nm.

[0059] In some embodiments, nanoscale conductive additives may also be present in the metal nanoparticle paste composition. These additives may preferably provide further structural stabilization and reduce shrinkage during compaction of the metal nanoparticles. Furthermore, the inclusion of nanoscale conductive additives may increase the electrical and thermal conductivity values ​​after nanoparticle compaction, which may approach or exceed those of the corresponding bulk metal, and this may be desirable to facilitate heat transfer as disclosed herein. Nanoscale conductive additives may exhibit at least one size of nanoscale dimensions, such as at least one dimension in the range of about 5 nm to about 500 nm, or about 10 nm to about 200 nm. In some embodiments, nanoscale conductive additives may further have sizes in at least one dimension in the range of about 1 micrometer to about 50 micrometers, or about 50 micrometers to about 100 micrometers, or about 100 micrometers to about 300 micrometers. Suitable nanoscale conductive additives include, for example, carbon nanotubes, boron nitride, boron carbide, graphene, nanodiamond, and nanographite, any of which may function as a CTE modifier. If present, the metal nanoparticle paste composition may contain about 1% to about 20% by weight of the nanoscale conductive additive, or about 1% to about 10% by weight of the nanoscale conductive additive, or about 1% to about 5% by weight of the nanoscale conductive additive, or about 5% to about 15% by weight of the nanoscale conductive additive.

[0060] Additional substances that may optionally be present in the metal nanoparticle paste composition include, for example, flame retardants, UV protectants, antioxidants, carbon black, graphite, fibrous materials (e.g., shredded carbon fiber materials), and diamond.

[0061] In some more specific embodiments, the preferred nanoparticle paste composition may further contain diamond particles. The preferred size of the diamond particles may be as large as possible to limit the grain boundaries that need to be traversed by phonons during heat transfer, while remaining small enough not to impair the dispersibility of the metal nanoparticle paste composition.

[0062] In more specific embodiments, diamond particles suitable for use in metal nanoparticle paste compositions may have a size in the range of about 1 micrometer to about 1000 micrometers, or about 0.5 micrometers to about 500 micrometers, which can provide good particle dispersion and acceptable paste dispersibility. Diamond particles having a size in the range of about 200 micrometers to about 250 micrometers, or about 1 micrometer to about 10 micrometers, may represent a good compromise between providing effective dispersion and minimized grain boundaries to suppress phonon scattering. Other suitable size ranges for diamond particles may be in the range of about 25 micrometers to about 150 micrometers, or about 50 micrometers to about 250 micrometers, or about 100 micrometers to about 250 micrometers, or about 100 micrometers to about 200 micrometers, or about 150 micrometers to about 250 micrometers, or about 1 micrometer to about 100 micrometers, or about 10 micrometers to about 50 micrometers, or about 5 micrometers to about 25 micrometers.

[0063] In exemplary embodiments, the composite material may contain about 10% to about 75% by volume of diamond particles after the metal nanoparticles have been compacted to form a monolithic metal body (a metal composite such as a copper composite containing a CTE modifier). Other conductive particles may be present in a similar compositional range.

[0064] Mixing copper nanoparticles and diamond particles to form copper composites can be desirable for several reasons. Copper is less expensive than most other metals, impedance-matches relatively well with diamond, and has high thermal conductivity on its own. In some embodiments, impedance matching can be further improved by including carbide-forming additives to form a thin layer of carbide (a layer <10 nm thick from a single atom) on the diamond particles. Thus, the combination of copper nanoparticles and diamond particles can provide very effective heat transfer in various embodiments of this disclosure. To establish electronic communication between various substrate layers, copper also provides high conductivity. Depending on the specific composition used, for example due to the amount of non-conductive additives, the conductivity may be about 30-50% IACS, or about 35-60% IACS, or about 50-75% IACS, or about 55-90% IACS, or about 60-98% IACS (International Annealed Copper Standard).

[0065] A nanoparticle paste composition suitable for use in this disclosure may be formulated according to any of the above disclosures. According to some embodiments, multiple metals may be present in the metal nanoparticle paste composition. In some or other embodiments, a suitable metal nanoparticle paste composition may comprise a mixture of metal nanoparticles and other nano-sized particles (i.e., particles having dimensions of about 200 nm or less), and / or micrometer-scale particles including micrometer-scale metal particles. According to a more specific embodiment, the metal nanoparticle paste composition may also comprise copper nanoparticles.

[0066] Various heat pipes and printed circuit boards utilizing heat pipes may be formed, at least partially, from copper nanoparticles and copper nanoparticle paste compositions. It should be understood that alternative metal nanoparticles may be used to form heat pipes containing different metals, as may be required to facilitate the matching of CTEs to certain ceramic materials within heat-generating components. Therefore, it should be understood that any embodiments utilizing copper or copper nanoparticles in the following disclosure may utilize alternative metals or metal nanoparticles depending on the application-specific requirements.

[0067] The heat pipes of this disclosure may include a structure comprising a sealed outer shell comprising a copper composite material containing a CTE modifier, and a working fluid movable within an internal space defined within the sealed outer shell. The working fluid may move freely within the internal space of the heat pipe, or there may be defined channels through which the working fluid can move. Various heat pipe configurations are described below with reference to the drawings. Both wicking and vibrating heat pipe configurations are contemplated in this disclosure. That is, the heat pipes of this disclosure may include a wicking structure interposed between the sealed outer shell and a hollow core, or the flow path may be defined on the surface of the sealed outer shell.

[0068] In various embodiments, copper composites may be formed by compacting copper nanoparticles with micrometer-sized copper particles and a CTE modifier. Copper nanoparticles, micrometer-sized copper particles, and a CTE modifier may define a copper nanoparticle paste composition as described in more detail above. In some embodiments, a suitable copper nanoparticle paste composition may contain about 30% to about 60% by weight of copper nanoparticles, or about 5% to about 50% by weight of micrometer-sized copper particles, and an effective amount of a CTE modifier to target a specific CTE. The CTE modifier may be present in amounts ranging from about 1% to about 35% by weight, or about 4% to about 8% by weight, or about 5% to about 15% by weight, or about 10% to about 20% by weight. Exemplary guidance on how to select a specific CTE modifier and its amount to achieve a specific CTE value in a copper composite is provided below. Micrometer-sized copper particles may be omitted in some embodiments.

[0069] Suitable CTE modifiers include, but are not limited to, diamond particles, graphite / pitch-based carbon fibers (e.g., having a diameter of about 10 micrometers), W particles, Mo particles, diamond particles, boron nitride particles, boron carbide particles, aluminum nitride particles, carbon nanotubes, graphene, etc., and any combination thereof. For example, when a carbon-based additive is added at about 16 volume%, a thermal expansion of about 2-3 ppm can be achieved; when added at about 9 volume%, a thermal expansion of about 7 ppm can be achieved; and when added at about 11 volume%, a thermal expansion of about 6 ppm can be achieved.

[0070] Adding approximately 45 volume percent of diamond can achieve a thermal expansion of about 5–6 ppm, depending on the density (82%). At a loading of approximately 37 volume percent and a density of 93%, the thermal expansion provided by the diamond may be about 6 ppm. At diamond additions exceeding approximately 50 volume percent, the thermal expansion decreases to less than about 5 ppm.

[0071] The compacted copper nanoparticles themselves exhibit a thermal expansion of approximately 7–12 ppm, depending on the processing conditions and density. As density increases, the thermal expansion approaches that of bulk copper (17 ppm). At a density of approximately 91%, the thermal expansion is about 7–8 ppm, and at a density of approximately 93%, it increases to about 10–11 ppm. At a density of approximately 98%, the thermal expansion reaches about 12 ppm. Even at such high densities, the thermal expansion is still lower than that of bulk copper, which is presumed to be due to the nanoporosity present in the copper nanoparticles after compaction.

[0072] The addition of micrometer-scale metal particles to metal nanoparticles (e.g., these nanoparticles) increases thermal expansion, which can reach over 17 ppm depending on the specific metal. For example, the addition of Al particles with a bulk CTE of approximately 23–24 ppm can increase the CTE of the resulting composite to a value exceeding that of bulk copper. The addition of approximately 55% micrometer-scale copper powder results in a thermal expansion of approximately 14 ppm at a density of 96%.

[0073] Carbon nanotubes may increase the thermal conductivity of copper from the low value of 400 s W / m·K for bulk copper alone to approximately 600 W / m·K. The degree of thermal conductivity change achievable with carbon nanotubes may depend on the length of the carbon nanotubes, with longer carbon nanotubes exceeding a thermal conductivity value of approximately 600 W / m·K. Such a change in thermal conductivity may be carried out in conjunction with a change in CTE, as described above.

[0074] In the absence of a surface modifier, a suitable CTE modifier may reduce the thermal conductivity from the initial value of approximately 400 W / m·K of bulk copper to approximately 150 W / m·K, again depending on the porosity. This reduction in thermal conductivity may be offset, at least to some extent, by including metal powder, such as micrometer-scale metal particles or diamond particles. With approximately 25 volume percent diamond, the thermal conductivity of the copper-diamond composite may be approximately 240 W / m·K. By including a wetting agent in the copper-diamond composite, a thermal conductivity exceeding 1000 W / m·K may be achieved with a diamond filler of 50 volume percent. A thermal conductivity of approximately 300 W / m·K can be achieved with a filler of approximately 10 volume percent of carbonaceous additives. Thus, this disclosure may facilitate balancing the CTE and thermal conductivity of copper composites to promote robust heat transfer.

[0075] The preferred working fluid is not considered particularly limited and may include any liquid capable of effectively transferring heat from a first position to a second position within the heat pipe. Further characteristics of the preferred working fluid may include the suitability of the material(s) defining the inner surface of the heat pipe. Suitable working fluids may include, but are not limited to, liquid helium, liquid ammonia, liquid nitrogen, water, methanol, ethanol, mercury, liquid sodium, liquid indium, glycol (e.g., ethylene glycol or glycol-water mixtures), etc. Fluorocarbon refrigerants may also be used. The expected operating temperature range may determine the suitability of a given working fluid for inclusion in the heat pipe. In more specific embodiments, the working fluid may be selected to undergo vaporization (at the high-temperature end of the heat pipe) and condensation (at the low-temperature end of the heat pipe) at the expected operating temperature.

[0076] Figure 3A is a cross-sectional view of an exemplary heat pipe having a wicking structure adjacent to a hollow core. The heat pipe 300 includes a sealed outer shell 302 adjacent to the wicking structure 304 and optionally penetrating into the wicking structure 304. The sealed outer shell 302 is formed from a copper composite containing a CTE modifier as disclosed herein, although the copper composite may be formed during the compaction of a plurality of copper nanoparticles. Ends 310 and 312 may also be end-protected (sealed) with a copper composite to seal a working fluid (not shown) within the hollow core 306. Although not evident from Figure 3A, the copper composite at ends 310 and 312 may, at least partially, collide with the hollow core 306. Other techniques for sealing ends 310 and 312 are also possible, as described below. The wicking structure 304 is inserted between the hollow core 306 and the sealed outer shell 302 and may be in contact with the working fluid contained within the hollow core 306. The working fluid (not shown) moves within the hollow core 306 from the hot end (e.g., the end of the heat pipe 300 that is thermally in contact with a heat-generating component) to the cold end (e.g., the end of the heat pipe 300 that is thermally in communication with a heat sink such as a radiator to the ambient atmosphere, marine environment, or external space). Vaporization of the working fluid occurs at the hot end, followed by condensation of the working fluid at the cold end during latent heat release. After the working fluid vapor condenses at the cold end, the condensed working fluid may move back to the hot end via the wicking structure 304, with or without the assistance of gravity, depending on the design of the heat pipe 300, to facilitate further heat transfer. Details for constructing the heat pipe 300 using metal nanoparticles are provided below.

[0077] In various embodiments, the wicking structure 304 may comprise a foam, a metal mesh, multiple grooves, or any combination thereof. Suitable foams include, for example, Al foam, SiC foam, Cu foam, and any of these may be mesh foams. Suitable foams may be open-cell mesh foams, sponge-like, or similar structures. Figure 3B is a photograph showing a cross-section view of an exemplary heat pipe having a wicking structure of metal mesh or foam. The heat pipe in Figure 3B is partially flattened to facilitate placement at a specific location between the heat source and the heat sink. Figure 3C shows a cross-section view of an exemplary heat pipe having grooves as the wicking structure. Although shown in a large, rounded configuration, the heat pipe in Figure 3C may alternatively be at least partially flattened, as in the heat pipe shown in Figure 3B, or may have a different geometric shape.

[0078] The heat pipe 300 or similar heat pipe may be formed by providing a tubular wicking structure 304 and penetrating a copper nanoparticle paste composition into the outer portion of the wicking structure 304 to a depth of several micrometers. In non-limiting embodiments, the penetration depth may be about 100 micrometers or less, or about 75 micrometers or less, or about 50 micrometers or less, or about 25 micrometers or less, for example, about 10 to about 50 micrometers, or about 25 to about 75 micrometers, or about 50 to about 100 micrometers. A layer of the copper nanoparticle paste composition may remain on the outer surface of the tubular morphology, which is also adjacent to the copper nanoparticle paste composition penetrated into the wicking structure 304. During the compaction of the copper nanoparticles, the sealed outer shell 302 may be formed on the tubular morphology and, optionally, may at least partially penetrate into the tubular morphology. The wicking structure 304 may be provided by any continuous or substantially continuous manufacturing line (e.g., a continuous extrusion process) that gives it an elongated tubular shape suitable for contact with the working fluid inside the heat pipe 300. The penetration of the copper nanoparticle paste composition into and within the wicking structure 304 may be carried out in conjunction with the production of the elongated tubular shape, or the penetration may be carried out separately in one or more processing operations. The outer surface of the wicking structure 304 may be further electroplated following the compaction of the copper nanoparticles to ensure hermetically sealed heat pipe 300. The porosity of the wicking structure 304, the particle filling amount and density in the copper nanoparticle paste composition, and the application technique of the copper nanoparticle paste composition may affect the depth to which the copper nanoparticle paste composition penetrates into the wicking structure 304 and the thickness of the resulting sealed outer shell 302. In exemplary embodiments, the incorporation of the copper nanoparticle paste composition onto and into the wicking structure 304 may be carried out by spreading or applying the copper nanoparticle paste composition onto the outer surface of an elongated tubular shape, or by injection molding or hot-pressing the copper nanoparticle paste composition thereon.Following the application of the copper nanoparticle paste composition, the wicking structure 304 may be exposed to conditions that promote the compaction of the copper nanoparticles. For example, according to various embodiments, the copper nanoparticles may be heated above their melting temperature or subjected to pressure.

[0079] In non-limiting examples, the copper nanoparticle paste composition may be applied to the wicking structure 304 in a continuous process using a doctor blade-like technique, such as by supplying the wicking structure 304 through an orifice having a conical shape that pushes the paste composition into the structure. The penetration depth may be controlled by the viscosity and density of the paste composition, as well as the size and amount of the CTE modifier and thermal conductive additives. Higher filling amounts of any component may reduce the penetration depth. The wicking structure 304 is then placed under pressure using a clamp shell or wrapped in a suitable material such as Kapton or other commercially available shrink packaging material capable of handling processing temperatures of 220-240°C. A thin layer of copper nanoparticles may then be applied to close any remaining pores or voids. Pressure is not required for this step, but can be used again in a similar manner if desired. Finally, the resulting elongated structure may be electroplated to close any remaining pores or voids and provide a smooth surface finish. In the final structure, a solid wall structure is obtained in which all layers are fused together.

[0080] The compaction of metal nanoparticles provides a continuous matrix of bulk copper on at least a portion of the wicking structure 304 and penetrates at least a portion of the wicking structure 304 to provide a sealed outer shell 302. If necessary, electroplating may be performed after the compaction of metal nanoparticles to complete the formation of the sealed outer shell 302 and to provide an hermetically sealed heat pipe 300 when the ends 310 and 312 are closed.

[0081] After compaction of the metal nanoparticles and, if desired, further electroplating, the thickness of the sealed outer shell 302 may be in the range of about 100 micrometers to about 1000 micrometers, or about 100 micrometers to about 300 micrometers, or about 200 micrometers to about 300 micrometers, or about 300 micrometers to about 500 micrometers. The wicking structure 304 may be in the range of about 0.1 micrometers to about 3000 micrometers, or about 500 micrometers to about 3000 micrometers, or about 500 micrometers to about 1000 micrometers, or about 1000 micrometers to about 3000 micrometers in thickness.

[0082] The ends 310 and 312 of the wicking structure 304 remain open, but the working fluid has not yet filled inside. In non-limiting embodiments, the ends 310 and 312 may be closed, preferably by welding or spot welding, threaded or unthreaded plugs or caps, compression (pinch-off), or valve operation, while applying a vacuum. The copper nanoparticle paste composition may also be filled into one or both of the ends 310 or 312 and compacted to facilitate sealing in another manner. The closing of the ends 310 and 312 may occur separately or simultaneously. Filling the heat pipe 300 with working fluid may occur before either the end 310 or the end 312 is closed, or after at least one of the ends 310 or 312 is closed. For example, the end 310 may be closed first, then the working fluid may be filled into the hollow core 306, and then the end 312 may be closed to seal the working fluid into the hollow core 306. The heat pipe 300 may be bonded to a heat source, such as a printed circuit board or similar heat-generating component, before filling with working fluid and closing the end 312. Alternatively, the working fluid may be filled inside the heat pipe 300, and the bonding to the heat source may occur in conjunction with the closing end 312. Still alternatively, the heat pipe 300 may be filled with working fluid, and both end 310 and end 312 may be closed sequentially or simultaneously, and the bonding to the heat source may be performed separately thereafter (e.g., using a metal nanoparticle paste composition to form a bonding layer). If the heat pipe 300 is already fully assembled, spot heating may be performed to facilitate the bonding of the heat pipe 300 to the heat source or heat sink via a bonding layer formed from metal nanoparticles, because otherwise, more widely dispersed heat may be transferred by the heat pipe 300 away from the desired heating location and / or the heat pipe 300 may be damaged. Localized rapid heating to facilitate bonding to a heat source or heat sink via the bonding layer may be performed, for example, using a laser or a Xe lamp.

[0083] The heat source or heat sink may be bonded to the ends 310 and 312 of the heat pipe 300. Alternatively, the heat source or heat sink may be bonded to the side wall of the heat pipe 300 (for example, on a sealed outer shell 302). Another option is for the heat pipe 300 to be bonded to a thermally conductive substrate that is in thermal communication with the heat generating components.

[0084] When a copper nanoparticle paste composition is used to close at least one of the ends 310 or 312, multiple conductive fibers may extend from one end of the heat pipe. The conductive fibers may provide a large surface area to facilitate heat dissipation, thus further facilitating heat dissipation from the heat pipe to the heat sink. Figure 4 is a cross-sectional view of an exemplary heat pipe having a wicking structure adjacent to a hollow core from which multiple conductive fibers 402 extend from one end of the heat pipe. Except for the conductive fibers 402 extending from end 312, the heat pipe 400 is similar to the heat pipe 300 (Figure 3) and can be better understood by referring to it. Elements having similar configurations and functions are denoted by the same reference numerals. In Figure 4, the conductive fibers 402 are shown with their ends terminating at the end 312 of the heat pipe 400. However, it should be understood that the ends of the conductive fibers 402 may also extend into the interior of the heat pipe 400 (i.e., into the hollow core 306) so that they can come into contact with the working fluid inside. In this configuration, direct contact between the conductive fibers 402 and the working fluid can still promote more effective heat transfer. Furthermore, the conductive fibers 402 may provide a large surface area in which condensation of the working fluid can occur within the hollow core 306.

[0085] Suitable conductive fibers that may be present in the heat pipe disclosed herein include, but are not limited to, graphite fiber bundles that may exhibit a thermal conductivity value of more than twice that of bulk copper (e.g., 800-1100 W / m·K). Other suitable types of conductive fibers may include, but are not limited to, metal fibers (e.g., Al fibers or Cu fibers), diamond fibers, carbon nanotubes or carbon nanotube fibers, or any combination thereof. Suitable fiber lengths may be about 2-8 inches, depending on the flexibility of the fibers. Suitable fiber diameters may be about 5-50 micrometers, or about 5-10 micrometers, or about 5-20 micrometers, or about 30-50 micrometers. The fibers may be branched as desired so that more fiber ends are outside the heat pipe than are embedded in the end of the heat pipe. The conductive fibers may also be in the form of a porous foam extending from the heat pipe, in which case a cooling fluid such as air or liquid may pass through the pores of the foam outside the heat pipe to help remove the heat transferred through the conductive fibers. When conductive fibers in the form of a porous foam are used, the conductive fibers may be positioned such that the working fluid remains sealed within the hollow core of the heat pipe.

[0086] To introduce the conductive fibers 402 into the heat pipe 400, the end 312 may be sealed with a copper nanoparticle paste composition during a hot-press or injection molding operation. The copper nanoparticle paste composition may be applied to the end 312 first, and then the conductive fibers 402 may be inserted into the uncompacted copper nanoparticle paste composition. Following the compaction of the copper nanoparticles, the conductive fibers 402 may be firmly fixed to the end 312 of the heat pipe 400 in a matrix of bulk copper formed from the copper nanoparticle paste composition.

[0087] Figure 5A is a top cross-sectional view of an exemplary vibrating heat pipe that may be formed from a copper composite according to the disclosure herein. The heat pipe 500 includes a channel 504 arranged in a non-restrictive meandering pattern within the upper surface of a copper block 502. The channel 504 may be defined within a closed loop, and at least one portion, usually multiple portions, of the closed loop extends between the hot end and the cold end of the heat pipe 500. In a vibrating heat pipe such as the vibrating heat pipe 500 shown in Figure 5A, the working fluid (not shown) moves as multiple fluid slugs within the closed loop defined by the channel 504 as heat is absorbed at the hot end and moves to the cold end. Gaps may exist between each fluid slug in the channel 504. The fluid slugs may at least partially evaporate near the hot end of the heat pipe 500 and condense near the cold end, resulting in pulsating motion of the fluid slugs through the closed loop.

[0088] Referring further to Figure 5A, the copper block 502 may be formed from a copper composite, such as an exemplary copper composite formed from copper nanoparticles and a CTE modifier, as described above. In various embodiments, the copper block 502 may be formed by compaction of copper nanoparticles in a copper nanoparticle paste composition, which is molded to form a monolithic structure having patterned channels 504 on its upper surface, following the compaction of metal nanoparticles. In other embodiments, injection molding or metal casting may be used to produce a net-shaped portion (e.g., a hemisphere) in which all or part of the channels 504 are defined inside. Once the copper block 502 is formed, a complementary portion (e.g., a plate of the same composition as the copper block 502) may be applied onto the copper block 502 and filled with working fluid to hermetically seal the channels 504. Metal nanoparticles may be used to facilitate bonding between the copper block 502 and the complementary portion, thereby facilitating the sealing of the channels 504, which in turn allows the working fluid to flow through them. A second portion of copper nanoparticles, separate from those used to form the sealed outer shell of the heat pipe, may be used for this purpose. Alternatively or additionally, the second portion of copper nanoparticles may be used to facilitate bonding of the copper block 502 to the heat-generating components.

[0089] Figure 5B is a photograph showing a perspective view of an exemplary copper block having a defined channel inside. Although not shown in Figures 5A and 5B, the upper copper block (complementary portion) surrounds the channel 502 and maintains the working fluid inside. Figure 5C is a side section view of the heat pipe 500, where the channel 504 is defined within the copper block 502, and the complementary portion 506 seals the channel 504 and confines the working fluid inside. Similar to the copper block 502, the complementary portion 506 may be formed from a copper composite, and if desired, the copper composite may be further CTE-matched to the copper block 502. Otherwise, the complementary portion 506 may be formed by conventional machining techniques. In various embodiments, the complementary portion 506 may be formed by compaction of copper nanoparticles in a copper nanoparticle paste composition. A metallurgical joint may be formed between the copper block 502 and the complementary portion 506 using the copper nanoparticle paste composition. Valve 510 may be present to fill the flow path 504 with working fluid when the flow path 504 is sealed by the complementary portion 506. At least partial vacuum may be applied after sealing and before filling the flow path 504 with working fluid. The vacuum may facilitate the entry of working fluid into the flow path 504.

[0090] The heat pipes disclosed herein may have any particular shape. Without limiting them, heat pipes may be circular, prismatic, oval, triangular, rectangular, flat, partially flat, etc. Heat pipes may be bent or substantially straight, particularly after being connected between a heat source and a heat sink. In some embodiments, heat pipes may be bent to contact the top or bottom surface of a heat-generating component and to conform to the surface of a substrate on which the heat-generating component is placed. Except for adapting to a specific working environment, the dimensions of the heat pipes are not considered particularly limited.

[0091] The heat pipes disclosed herein may be used to dissipate heat from heat-generating components associated with a printed circuit board. The heat-generating components may include, for example, ceramics such as Si (CTE=3.5), SiC (CTE=4.2 ppm), GaN (CTE=5.6 ppm), or AlN (CTE=4.5 ppm). The CTE modifier combined with copper nanoparticles and its amount may be adjusted to match the CTE of the heat-generating component within a desired tolerance range. In non-limiting embodiments, the CTE of the heat pipe may match the CTE of the heat-generating component within tolerance ranges of about ±50%, or about ±25%, or about ±20%, or about ±15%, or about ±10%, or about ±5%, or about ±4%, or about ±3%, or about ±2%, or about ±1%.

[0092] The printed circuit boards of this disclosure may comprise heat-generating components disposed on or at least partially embedded within an electrically insulating substrate, and at least one heat pipe thermally communicating with the heat-generating components, wherein the at least one heat pipe is CTE-matched to the heat-generating components. The electrically insulating substrate may also be thermally insulating, such as FR4, or thermally conductive, such as containing AlN or SiN. In various embodiments, the at least one heat pipe comprises a sealed outer shell comprising a copper composite containing a CTE modifier, and a working fluid movable within an internal space contained within the sealed outer shell. Preferred heat pipes may include a wicking structure or sealed channel through which the working fluid moves, as described in more detail above. The at least one heat pipe may be bonded to the heat-generating components via a bonding layer comprising a copper composite, which is also CTE-matched to the copper composite comprising the heat-generating components and the sealed outer shell. Alternatively, the heat pipe may be bonded to an electrically insulating substrate having sufficient thermal conductivity rather than to the heat-generating components.

[0093] The heat-generating components may be positioned on the upper surface of the electrical insulating substrate, or at least partially embedded within the electrical insulating substrate. At least one heat pipe may be bonded to the upper or lower surface of the heat-generating components, and one or more heat pipes may be bonded to the sides of the heat-generating components, or any combination thereof. If bonded to the lower surface of the heat-generating components, at least one heat pipe may extend through the electrical insulating substrate. Further details regarding how one or more heat pipes may be connected to the heat-generating components are provided below.

[0094] Figure 6 shows a side view of a heat pipe bonded to the upper surface of a heat-generating component. Such a configuration may be used, for example, in a flip-chip package. As shown, the PCB 600 includes an electrical insulating substrate 602 and a heat-generating component 604 on the same. The heat pipe 606 is bonded to the upper surface of the heat-generating component 604 and oriented to conduct excess heat from there. The heat pipe 606 may be a heat pipe of the present disclosure described in more detail above. Although the heat pipe 606 is shown in Figure 6 in a substantially linear and unsupported configuration, it should be understood that the heat pipe 606 may alternatively be bent or curved and / or conform to at least a portion of the surface of the PCB 600. In yet another embodiment, the heat-generating component 604 may be at least partially embedded into the surface of the electrical insulating substrate 602, in which case the heat pipe 606 may be oriented similarly to that shown in Figure 6.

[0095] Figure 7 shows a side view of a heat pipe bonded to the bottom surface of a heat-generating component. In this configuration, the heat pipe 606 extends through a via 702 (i.e., a through-plane via) defined within the electrical insulation substrate 602 of the PCB 700. The via 702 may be sized appropriately so that the heat pipe 606 extends through it so that the surface of the heat pipe 606 can contact the bottom surface of the heat-generating component 604. The heat pipe 606 may again be the heat pipe of the present disclosure described in more detail above. Similarly, the heat-generating component 604 may be at least partially embedded in the surface of the electrical insulation substrate 602, and the heat pipe 606 may be oriented as shown in Figure 7. In Figure 7, the via 702 is shown to be substantially the same width as the heat-generating component 604, but it should be understood that the via 702 may be narrower so that the heat pipe 606 contacts less than the entire surface of the heat-generating component 604. It should also be understood that, alternatively, the heat pipe 606 may be bent or curved after passing through via 702 and / or conform to at least a portion of the bottom surface of PCB 700.

[0096] The heat pipes may also be bonded to the top and bottom surfaces of the heat-generating components, as shown in Figure 8. Figure 8 is a side view of heat pipes bonded to both the top and bottom surfaces of the heat-generating components. In PCB 800, heat pipe 606a is bonded to the top surface of heat-generating component 604 in a manner similar to that shown in Figure 6, and heat pipe 606b extends through the electrical insulation substrate 602 and is bonded to the bottom surface of heat-generating component 604 in a manner similar to that shown in Figure 7. Thus, this disclosure facilitates the attachment of multiple heat pipes to heat-generating components so that heat dissipation can occur from multiple surfaces of the PCB.

[0097] Figure 9 shows a top view of multiple heat pipes bonded to the side of a heat-generating component. In Figure 9, the heat-generating component 604 is placed on or at least partially embedded within an electrical insulating substrate 602. Heat pipes 606a and 606b are optionally contacted and bonded to the side of the heat-generating component 604 such that heat pipes 606a and 606b extend along the surface of the electrical insulating substrate 602. While two heat pipes (606a and 606b) are shown with lateral bonding to the heat-generating component 604 in the PCB 900, it should be understood that one or more heat pipes may optionally be bonded in combination with heat pipes bonded to the top and / or bottom of the heat-generating component 604 (Figures 6-8). Lateral bonding of one or more heat pipes to the heat-generating component 604 can facilitate the lamination of multiple PCB layers, with one or more heat pipes sandwiched between adjacent PCB layers, facilitating lateral heat removal from the PCB. By removing excess heat laterally via heat pipes extending between two PCB layers, PCB manufacturing may be simplified by eliminating the need to machine an electrically insulating substrate to accommodate heat pipes extending from the top or bottom surface of the heat-generating component 604 through multiple PCB layers. It should be understood that upper and bottom heat pipes (not shown) may be present, particularly for the upper and bottom layers of a multilayer PCB.

[0098] If the electrical insulating substrate has high thermal conductivity, the heat pipe of the present disclosure may be placed on at least one surface of the electrical insulating substrate. Thus, instead of heat being removed directly from the heat-generating components, heat may be transferred from the heat-generating components through the electrical insulating substrate, and then the heat may be removed via the heat pipe of the present disclosure. In other words, in the case of an electrical insulating substrate with high thermal conductivity, heat removal from the heat-generating components may occur indirectly.

[0099] Figures 10A to 10C show side views of a heat pipe placed on an electrically insulating substrate having high thermal conductivity. In PCB 1000A in Figure 10A, the heat pipe 606 is placed on the top surface 1001 of the electrically insulating substrate 602. The heat generating component 604 is also placed on the top surface 1001, but remains separated from the heat pipe 606 so that heat transfer occurs through the electrically insulating substrate 602. PCB 1000B in Figure 10B is similar to PCB 1000A in Figure 10A, except that the heat pipe 606 is placed on the bottom surface 1002, i.e., facing the heat generating component 604 on the top surface 1001. Similarly, PCB 1000C in Figure 10C incorporates the heat pipe 606 inside the electrically insulating substrate 602 to achieve a similar result.

[0100] Accordingly, the PCBs including the heat pipes of this disclosure may be single-layer or multilayer according to various embodiments. A multilayer PCB may contain individual layers that are stacked together to define vias and other substrate features. Bonding between multiple PCB layers may be achieved, for example, using a suitable adhesive during the deposition process. The number of layers in a multilayer PCB is not considered particularly limited and may be, for example, up to about 10 individual PCB layers, each of which may contain a heat-generating component that requires heat removal by the use of heat pipes as disclosed herein.

[0101] In various embodiments, the bonding layer connecting the sealed outer shell and heat pipe to the heat-generating components may be formed from a copper nanoparticle paste composition containing copper nanoparticles, more specifically, additives suitable for adjusting the CTE of the bonding layer to match the CTE of the heat-generating components. The CTE modifier may include carbon fibers, diamond particles, boron nitride, aluminum nitride, carbon nanotubes, graphene, and the like. The specific CTE modifier and its amount may be selected to provide the desired degree of CTE adjustment. As described in more detail above, other additives may be present in the copper nanoparticle paste composition to facilitate distribution and handling.

[0102] Accordingly, the method of the present disclosure may include providing an elongated wicking structure having an outer surface and an inner surface defining a hollow core, applying a copper nanoparticle paste composition to the outer surface, wherein the copper nanoparticle paste composition comprises a plurality of copper nanoparticles, a plurality of micrometer-sized copper particles, and a CTE modifier; compacting the copper nanoparticles to form a sealed outer shell on the outer surface of the elongated wicking structure, optionally electroplating to complete the formation of the sealed outer shell; partially filling the hollow core with a working fluid; and closing at least one end of the sealed outer shell. In various embodiments, at least one end may be closed with the copper nanoparticle paste composition, and then the copper nanoparticles inside are compacted as described in more detail above. Suitable copper nanoparticle paste compositions, CTE modifiers, and heat exchanger configurations are described in more detail above.

[0103] When forming a heat pipe that includes multiple fibers extending from the heat pipe, the method may further include arranging the multiple thermally conductive fibers within a second portion of the copper nanoparticle paste composition on the end of the heat pipe. When compacting the copper nanoparticles in the copper nanoparticle paste composition, the multiple fibers may adhere to the end of the heat pipe. Optionally, at least a portion of the conductive fibers may extend into the hollow core of the heat pipe and come into contact with the working fluid inside.

[0104] Embodiments disclosed herein include the following: A. Heat pipe. A heat pipe comprises a structure having a sealed outer shell made of a copper composite material containing a coefficient of thermal expansion (CTE) modifier, and a working fluid effective for moving within an internal space contained within the sealed outer shell. B. A printed circuit board comprising: a heat-generating component disposed on an electrically insulating substrate; at least one heat pipe that is in thermal communication with the heat-generating component, wherein the at least one heat pipe has a structure in which the sealed outer shell comprises a copper composite material containing a coefficient of thermal expansion (CTE) modifier; and a working fluid effective for moving within an internal space contained within the sealed outer shell.Optionally, the electrically insulating substrate may be thermally insulating such as FR4 and similar epoxy substrates, or thermally conductive such as aluminum nitride or silicon carbide. C. Method for manufacturing a heat pipe. The method comprises providing an elongated wicking structure having an outer surface and an inner surface defining a hollow core; applying a copper nanoparticle paste composition to the outer surface, wherein the copper nanoparticle paste composition comprises a plurality of copper nanoparticles, a plurality of micrometer-sized copper particles, and a coefficient of thermal expansion (CTE) modifier; compacting the copper nanoparticles to form a sealed outer shell on the wicking structure; partially filling the hollow core with a working fluid; and closing at least one end of the sealed outer shell.

[0105] Each of Embodiments A, B, and C may have one or more of the following additional elements in any combination.

[0106] Element 1: The copper composite is formed by compacting copper nanoparticles with micrometer-sized copper particles and a CTE modifier.

[0107] Element 2: The internal space includes a wicking structure adjacent to the hollow core.

[0108] Element 3: The wicking structure includes foam, wire mesh, multiple grooves, or any combination thereof.

[0109] Element 4: The internal space comprises a channel defined on the inner surface of the sealed outer shell.

[0110] Element 5: The copper composite material has a uniform nanoporosity of approximately 2% to 30%.

[0111] Element 6: The CTE modifier includes carbon fibers, W particles, Mo particles, diamond particles, boron nitride, carbon nanotubes, or any combination thereof.

[0112] Element 7: The heat pipe further comprises a plurality of thermally conductive fibers extending from the ends of the structure, and optionally, at least a portion of the thermally conductive fibers extending into the internal space and in contact with the working fluid.

[0113] Element 7A: The PCB further comprises a plurality of thermally conductive fibers extending from the ends of the structure, wherein, optionally, at least a portion of the thermally conductive fibers extend into the internal space and come into contact with the working fluid.

[0114] Element 7B: The method further comprises arranging a plurality of thermally conductive fibers in a copper nanoparticle paste composition and extending them from at least one end, wherein, optionally, at least a portion of the thermally conductive fibers extends into a hollow core and comes into contact with a working fluid.

[0115] Element 8: At least one heat pipe is bonded to a heat-generating component via a bonding layer containing a CTE-matched copper composite to a copper composite containing a sealed outer shell.

[0116] Element 9: The heat generating component is positioned on the surface of an electrically insulating substrate, with at least one heat pipe bonded to the top surface of the heat generating component, one or more heat pipes bonded to the side surface of the heat generating component, at least one heat pipe bonded to the bottom surface of the heat generating component, and at least one heat pipe extending through the electrically insulating substrate, or any combination thereof.

[0117] Element 10: At least one end is closed by applying a copper nanoparticle paste composition thereto and compacting the copper nanoparticles inside.

[0118] As non-limiting examples, typical combinations applicable to A, B, and C include: 1 and 2, 1-3, 1 and 4, 1 and 5, 1 and 6, 1 and 7, 7A or 7B, 1 and 8, 1 and 9, 1 and 10, 2 and 4, 2-4, 2 and 5, 2 and 6, 2 and 7, 7A or 7B, 2 and 8, 2 and 9, 2 and 10, 4 and 5, 4 and 6, 4 and 7, 7A or 7B, 4 and 8, 4 Examples include, but are not limited to, 9, 4 and 10, 5 and 6, 5 and 7, 7A or 7B, 5 and 8, 5 and 9, 5 and 10, 6 and 7, 7A or 7B, 6 and 8, 6 and 9, 6 and 10, 7, 7A or 7B, and 8, 7, 7A or 7B, and 9, 7, 7A or 7B, and 10, 8 and 9, 8 and 10, 8-10, 7B, and 8-10, as well as 9 and 10.

[0119] Additional embodiments disclosed herein include: A': Heat pipe. The heat pipe comprises a structure having a sealed outer shell made of a copper composite material containing a coefficient of thermal expansion (CTE) modifier, and a working fluid that is movable within an internal space defined within the sealed outer shell, the internal space including a wicking structure inserted between the sealed outer shell and a hollow core, or a flow path defined on the surface of the sealed outer shell. B': Printed Circuit Board (PCB). A PCB comprises heat-generating components arranged on or at least partially embedded within an electrically insulating substrate, and at least one heat pipe thermally communicating with the heat-generating components, wherein the at least one heat pipe comprises a structure having a sealed outer shell made of a copper composite material containing a coefficient of thermal expansion (CTE) modifier, and a working fluid movable within an internal space defined within the sealed outer shell, the internal space comprising a wicking structure inserted between the sealed outer shell and a hollow core, or a flow path defined on the surface of the sealed outer shell. C': A method for manufacturing a heat pipe. The method provides an elongated wicking structure having an outer surface and an inner surface defining a hollow core; and applies a copper nanoparticle paste composition to the outer surface, wherein the copper nanoparticle paste composition comprises a plurality of copper nanoparticles, a plurality of micrometer-sized copper particles, and a coefficient of thermal expansion (CTE) modifier; compacts the copper nanoparticles to form a sealed outer shell on the outer surface of the elongated wicking structure; partially fills the hollow core with a working fluid; and closes at least one end of the sealed outer shell to confine the working fluid within the hollow core.

[0120] Each of Embodiments A', B', and C' may have one or more of the following additional elements in any combination.

[0121] Element 1': The copper composite is formed by compacting copper nanoparticles with micrometer-sized copper particles and a CTE modifier.

[0122] Element 2': The wicking structure includes foam, wire mesh, multiple grooves, or any combination thereof.

[0123] Element 3': The sealed outer shell penetrates into at least a portion of the wicking structure.

[0124] Element 4': The complementary portion contacts the sealed outer shell and seals the upper surface of the flow path.

[0125] Element 5': The copper composite material has a uniform nanoporosity of approximately 2% to 30%.

[0126] Element 6': The CTE modifier includes carbon fibers, W particles, Mo particles, diamond particles, boron nitride, aluminum nitride, carbon nanotubes, graphene, or any combination thereof.

[0127] Element 7': The heat pipe further comprises a plurality of thermally conductive fibers extending from the ends of the structure, and optionally, at least a portion of the thermally conductive fibers extending into the internal space and in contact with the working fluid.

[0128] Element 8': At least one heat pipe is bonded to a heat-generating component via a bonding layer containing a CTE-matched copper composite to a copper composite containing a sealed outer shell.

[0129] Element 9': At least one heat pipe is bonded to the top surface of the heat generating component, one or more heat pipes are bonded to the side surface of the heat generating component, at least one heat pipe is bonded to the bottom surface of the heat generating component, and at least one heat pipe extends through an electrical insulating substrate, or any combination thereof.

[0130] Element 10': At least one end is closed by applying a second portion of the copper nanoparticle paste composition to at least one end and compacting the copper nanoparticles inside it.

[0131] Element 11': The method further comprises arranging a plurality of thermally conductive fibers within a second portion of a copper nanoparticle paste composition and extending from at least one end, wherein, optionally, at least a portion of the thermally conductive fibers extends into a hollow core and comes into contact with a working fluid.

[0132] As non-limiting embodiments, representative combinations applicable to A', B', and C' include, but are not limited to, 1' and 2', 1' and 3', 1' and 4', 1' and 5', 1' and 6', 1' and 7', 2' and / or 3' and 5', 2' and / or 3' and 6', 2' and / or 3' and 7', 4' and 5', 4' and 6', 4' and 7', 5' and 6', 5' and 7', and 6' and 7'. Any of the above, or any one of 1' to 7', may be further combined with 8' and 9', 8', 9', 10' and 11', 10', or 11'.

[0133] Unless otherwise indicated, all figures used in this specification and related claims to represent quantities, molecular weights, and other properties of components, reaction conditions, etc., should be understood in all cases to be modified by the term "about." Therefore, unless specifically objected, the numerical parameters described in the following specification and appended claims are approximations that may vary depending on the desired properties to be obtained by embodiments of the present invention. Each numerical parameter should be interpreted, at least in accordance with the usual rounding technique, taking into account the number of significant figures reported, and not as an attempt to limit the application of the doctrine of equivalents to the claims.

[0134] One or more exemplary embodiments incorporating the features of this disclosure are presented herein. For clarity, not all features of physical implementations are described or illustrated in this application. It is understood that in developing physical embodiments incorporating this disclosure, numerous implementation-specific decisions, which vary by implementation and from time to time, must be made to achieve the developer's objectives, including compliance with system-related, business-related, government-related, and other constraints. While the developer's efforts may be time-consuming, such efforts are routine work for those skilled in the art and will be of interest to this disclosure.

[0135] Accordingly, this disclosure is well adapted to achieve the purposes and benefits mentioned and those inherent thereto. The specific embodiments disclosed above are illustrative only, as this disclosure can be modified and implemented in different but equivalent ways that will be obvious to those skilled in the art who are interested in the teachings of this specification. Furthermore, it is not intended to limit the details of the configuration or design shown herein beyond those set forth in the following claims. Accordingly, it is clear that the specific exemplary embodiments disclosed above may be modified, combined, or altered, and all such variations are considered to be within the scope and spirit of the invention. The disclosures herein can be suitably implemented even without any elements not specifically disclosed herein and / or desired elements disclosed herein. While compositions and methods are described using terms such as “comprising,” “containing,” or “including” various components or processes, compositions and methods may also “consist essentially of” or “consist of” various components and processes. All the numbers and scopes disclosed above may vary to some extent. Whenever a numerical range with lower and upper limits is disclosed, any number and any range that falls within that range are specifically disclosed. In particular, any range of values ​​disclosed herein (in the form of "from about a to about b," or equivalently "from approximately a to b," or equivalently "from approximately ab") should be understood to indicate any number and range that are encompassed within a broader range of values. Similarly, terms in the claims have their obvious and ordinary meanings unless explicitly and clearly defined by the patentee. Furthermore, the indefinite articles "a" or "an" used in the claims are defined herein to mean one or more of the elements they introduce.

Claims

1. It is a heat pipe, A structure having a sealed outer shell containing a copper composite material comprising copper nanoparticles, micrometer-sized copper particles, and a coefficient of thermal expansion (CTE) modifier, A working fluid that is movable within the internal space defined within the sealed outer shell, Includes, The internal space includes a wicking structure inserted between the sealed outer shell and the hollow core, or a channel defined on the surface of the sealed outer shell. The copper composite material is formed by compaction of the copper nanoparticles with the micrometer-sized copper particles and the CTE modifier, and is a heat pipe.

2. The heat pipe according to claim 1, wherein the wicking structure includes foam, wire mesh, a plurality of grooves, or any combination thereof.

3. The heat pipe according to claim 1, wherein the sealed outer shell penetrates into at least a portion of the wicking structure.

4. The heat pipe according to claim 1, wherein a complementary portion contacts the sealed outer shell and seals the upper surface of the flow path.

5. The heat pipe according to claim 1, wherein the copper composite material has a uniform nanoporosity of about 2% to about 30%.

6. The heat pipe according to claim 1, wherein the CTE modifier comprises carbon fiber, W particles, Mo particles, diamond particles, boron nitride, aluminum nitride, carbon nanotubes, graphene, or any combination thereof.

7. The heat pipe according to claim 1, further comprising a plurality of thermally conductive fibers extending from the end of the structure, wherein, optionally, at least a portion of the thermally conductive fibers extends into the internal space and comes into contact with the working fluid.

8. A printed circuit board (PCB), A heat-generating component arranged on or at least partially embedded within an electrically insulating substrate, At least one heat pipe that is in thermal communication with the heat generating component, Equipped with, The at least one heat pipe is A structure having a sealed outer shell comprising a copper composite material containing copper nanoparticles, micrometer-sized copper particles, and a thermal expansion coefficient (CTE) modifier, A working fluid that is movable within the internal space defined within the sealed outer shell, Includes, The internal space comprises a wicking structure inserted between the sealed outer shell and the hollow core, or a channel defined on the surface of the sealed outer shell. The copper composite material is formed by compaction of the copper nanoparticles with the micrometer-sized copper particles and the CTE modifier, and is a printed circuit board (PCB).

9. The PCB according to claim 8, wherein the wicking structure includes foam, wire mesh, a plurality of grooves, or any combination thereof.

10. The PCB according to claim 8, wherein the sealed outer shell penetrates into at least a portion of the wicking structure.

11. The PCB according to claim 8, wherein the complementary portion contacts the sealed outer shell and seals the upper surface of the flow path.

12. The PCB according to claim 8, wherein the copper composite material has a uniform nanoporosity of about 2% to about 30%.

13. The PCB according to claim 8, wherein the at least one heat pipe is bonded to the heat generating component via a bonding layer comprising a copper composite material CTE-matched to the copper composite material comprising the sealed outer shell.

14. The at least one heat pipe is bonded to the upper surface of the heat generating component, One or more heat pipes are joined to the side surface of the heat generating component, The at least one heat pipe is bonded to the bottom surface of the heat generating component, and the at least one heat pipe extends through the electrical insulating substrate, The PCB according to claim 8, or any combination thereof.

15. The PCB according to claim 8, wherein the CTE modifier comprises carbon fibers, W particles, Mo particles, diamond particles, boron nitride, aluminum nitride, carbon nanotubes, graphene, or any combination thereof.

16. The PCB according to claim 8, further comprising a plurality of thermally conductive fibers extending from the ends of the structure, wherein, optionally, at least a portion of the thermally conductive fibers extends into the internal space and comes into contact with the working fluid.

17. It is a method, To provide an elongated wicking structure having an outer surface and an inner surface that define a hollow core, The application of a copper nanoparticle paste composition to the outer surface, wherein the copper nanoparticle paste composition comprises a plurality of copper nanoparticles, a plurality of micrometer-sized copper particles, and a thermal expansion coefficient (CTE) modifier. To form an outer shell sealed on the outer surface of the elongated wicking structure, the copper nanoparticles are compacted together with the micrometer-sized copper particles and the CTE modifier, The hollow core is partially filled with working fluid, Closing at least one end of the sealed outer shell to confine the working fluid within the hollow core, Methods that include...

18. The method according to claim 17, wherein the at least one end is closed by applying a second portion of the copper nanoparticle paste composition to the at least one end and compacting the copper nanoparticles inside it.

19. The method according to claim 18, further comprising arranging a plurality of thermally conductive fibers within the second portion of the copper nanoparticle paste composition and extending from at least one end thereof, wherein optionally at least a portion of the thermally conductive fibers extends into the hollow core and into contact with the working fluid.

20. The method according to claim 17, wherein the wicking structure includes a foam, a wire mesh, a plurality of grooves, or any combination thereof.

21. The method according to claim 17, wherein the copper composite material has a uniform nanoporosity of about 2% to about 30%.

22. The method according to claim 17, wherein the CTE modifier comprises carbon fiber, W particles, Mo particles, diamond particles, boron nitride, aluminum nitride, carbon nanotubes, graphene, or any combination thereof.

23. The method according to claim 17, wherein the sealed outer shell penetrates into at least a portion of the wicking structure.

24. The heat pipe according to claim 1, wherein the copper nanoparticles have a surfactant coating containing one or more surfactants on their surface.

25. The heat pipe according to claim 1, wherein the copper composite material comprises about 30% to about 98% by weight of copper nanoparticles and about 0.1% to about 15% by weight of micrometer-scale particles.

26. The heat pipe according to claim 1, wherein at least a portion of the copper nanoparticles are in the size range of about 1 to about 10 nm, and the remaining copper nanoparticles are in the size range of 25 to about 50 nm.

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