An ultrathin heat exchange plate based on an asymmetric capillary structure of wettability gradient

Abstract

This invention provides an ultrathin vapor chamber based on an asymmetric capillary structure with a wettability gradient, and relates to an improved method for an ultrathin vapor chamber with an asymmetric distribution of hydrophilic-phobic composite capillary structures. The vapor chamber includes a sealed, flat cavity filled with a working fluid. Its innovation lies primarily in the construction of the capillary structures distributed on the upper and lower shell plates of the cavity. The upper shell plate comprises a hydrophilic surface with superhydrophilic microstructures exhibiting strong capillary forces, while the lower shell plate comprises a smooth hydrophobic surface without capillary structures and a hydrophilic surface with superhydrophilic microstructures exhibiting strong capillary forces. The superhydrophilic microstructures with strong capillary forces are uniformly distributed on the surface of the upper shell plate, while those on the lower shell plate are distributed in the edge region of the plate surface, with the central region being a smooth hydrophobic surface without capillary structures, forming a wettability gradient structure. Compared to traditional ultrathin heat spreaders with uniformly symmetrical capillary structures, this invention designs a continuous wettability gradient in the lower shell plate, allowing steam to rise vertically without obstruction in the central region. Meanwhile, the condensate formed by steam condensation flows back laterally efficiently through the superhydrophilic structure at the edge of the lower shell plate. During the gradual change of capillary force, the paths of steam and liquid are separated, thus achieving the synergy of steam diffusion and liquid reflux within a single thin cavity.

Description

Technical Field

[0001] This invention relates to the field of heat dissipation device technology, and specifically to an ultrathin heat exchange plate with an asymmetric capillary structure. Background Technology

[0002] As electronic devices evolve towards higher performance, greater integration, and miniaturization, the heat flux density of their core processors is increasing dramatically. Heat sinks, as passive heat dissipation devices that utilize the phase change of the working fluid for efficient two-dimensional heat conduction, have become a key component in solving chip heat dissipation problems. To adapt to increasingly thinner device forms, heat sinks themselves must also evolve towards ultra-thin designs.

[0003] However, the ultra-thin design of vapor chambers introduces an inherent contradiction: the space inside the cavity for vapor diffusion and the capillary structure for liquid reflux are compressed against each other in the thickness direction. Existing vapor chambers typically have uniformly distributed wicks on the upper and lower shells, requiring vapor and condensate to flow counter-currently in the same narrow porous medium, resulting in significant flow resistance and easy mutual interference. Specifically, under high heat flux, the vapor pressure generated at the evaporation end may backflow and hinder the capillary reflux of liquid from the condensation end to the evaporation end, leading to localized drying at the evaporation end and a sharp deterioration or even failure of heat transfer performance. This is the main bottleneck for improving the performance of ultra-thin vapor chambers.

[0004] Therefore, there is an urgent need for a new type of ultrathin heat exchanger design that has a relatively simple structure, does not require complex physical separation, and can achieve efficient synergistic circulation of steam and liquid at an ultrathin scale. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides an ultrathin heat spreader based on an asymmetric capillary structure, which addresses the problem of mutual interference between the flow of steam and liquid inside existing ultrathin heat spreaders, leading to the drying of the evaporation surface. This not only effectively avoids the technical problem of mutual disturbance between steam and liquid but also improves the working efficiency of the heat spreader, making it both technically sound and economically viable.

[0006] To achieve the above-mentioned objectives, the technical solution of this invention is as follows: an ultrathin heat exchanger based on an asymmetric capillary structure using a wettability gradient. Its core lies in the precise control of the capillary structure within the shell, including the different distributions of capillary structures in the upper and lower shell plates. The surface of the lower shell plate is divided into three regions: an edge region, a transition region, and a central region. The edge region contains superhydrophilic capillary structures, exhibiting superhydrophilic properties, while the central region has no capillary structure distribution, exhibiting hydrophobic properties. This asymmetric distribution of capillary structures achieves the flow of the wettability gradient, completing the functional path optimization of steam flow and liquid reflux. Simultaneously, the surface of the upper shell plate contains symmetrical superhydrophilic capillary structures.

[0007] Because the central region of the lower shell plate has a smooth surface, the liquid working fluid is not easily strongly adsorbed. Upon heating, it can quickly form vapor molecules that detach from the evaporation wall and enter the central space of the upper shell plate with low resistance. After the vapor condenses in the central space of the upper shell plate, the condensate is adsorbed onto the condensation surface by the superhydrophilic capillary structure. Due to its hydrophilic properties, it rapidly diffuses outwards and flows vertically along the cavity to the edge of the lower shell plate. When the liquid reaches the edge region of the lower shell plate, the hydrophilic properties of the superhydrophilic capillary structure cause the condensate to flow laterally to the central region. The key is that there is a sudden change in capillary force between the hydrophilic structure in the edge region and the hydrophobic structure in the central region. This abrupt change is called the transition region. When the vapor diffuses vertically upwards, the presence of the transition region prevents it from flowing to the edge region of the lower shell plate, ensuring smooth liquid return. The entire process realizes a cycle of "vertical upward diffusion of vapor - condensation - liquid return replenishment - evaporation," and the flow path is spatially optimized by partitioning, greatly reducing flow conflicts.

[0008] This invention achieves natural separation of upward vapor diffusion and lateral liquid reflux by designing a wettability gradient from the center to the edge of the evaporation surface, i.e., a contact angle greater than 90° (hydrophobic) in the central region and less than 10° (superhydrophilic) in the edge region. The capillary force difference generated by this contact angle difference enables the separation of upward vapor diffusion and lateral liquid reflux. The hydrophobic central region reduces liquid retention and promotes rapid vapor escape; the superhydrophilic edge region enhances capillary suction, ensuring efficient liquid reflux.

[0009] Furthermore, the wettability and capillary force of the transition region of the lower shell plate change continuously and gradually, which makes the flow transition smoother and reduces stress concentration.

[0010] Furthermore, the lower shell plate has a superhydrophilic capillary structure that can form a continuous annular capillary force enhancement band on the sidewall near the edge region, with a sintered copper powder thickness of 75 μm, to enhance the collection and pumping capacity of the surrounding reflux liquid.

[0011] Furthermore, the superhydrophilic capillary structure at the edge region of the lower shell plate has a sintered copper powder thickness of 50 μm.

[0012] Compared with the prior art, the present invention has the following advantages: Compared with existing ultrathin heat exchange plates, this invention avoids the competition between the reverse flow of vapor and liquid in traditional heat exchange plates through wettability gradient design, fundamentally alleviates the problem of dry evaporation surface, and significantly improves the heat transfer limit of heat exchange plate.

[0013] Compared with existing ultrathin heat spreaders, all functions of this invention are achieved through surface modification of the inner wall of the cavity, without the need to introduce additional physical partitions or complex three-dimensional structures. It can be prepared by surface treatment, selective sintering or laser modification processes, which reduces manufacturing costs and is economical.

[0014] Compared to existing ultra-thin heat spreaders, this design facilitates the thinning of heat spreaders, enabling them to meet the heat dissipation needs of cutting-edge electronic products with a thickness of 1.5mm or even less. Attached Figure Description

[0015] Figure 1 This is a cross-sectional view of the ultra-thin heat spreader.

[0016] Figure 2 This is a schematic diagram of the unfolded planar development of the lower shell of the ultrathin heat spreader.

[0017] Figure 3 This is a schematic diagram illustrating the working principle of an ultra-thin heat spreader.

[0018] Figure 4 This is a schematic diagram showing the change in working contact angle of the lower shell of the ultrathin heat exchanger as it transitions from the central region to the edge region.

[0019] In the diagram: 1. Cavity; 2. Superhydrophilic capillary structure; 3. Wetting structure; 4. Heat source; 5. Edge region; 6. Central region; 7. Capillary force enhancement zone; 8. Transition region; 9. Steam flow direction; 10. Steam molecules; 11. Liquid flow direction; 12. Liquid working medium; 13. Inner wall of lower shell plate; 14. Inner wall of upper shell plate; 15. Contact angle of central region; 16. Contact angle of transition region; 17. Contact angle of edge region. Specific implementation methods

[0020] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0021] Example 1 (Refer to the attached figures): This example is an integral structure of an ultrathin heat spreader based on an asymmetric capillary structure with wettability gradient. The structure includes a cavity 1, a lower shell inner wall 13, and an upper shell inner wall 14. The cavity is sealed by vacuum brazing. The lower shell inner wall has a wettability structure 3, and the upper shell wall has a uniformly symmetrically distributed superhydrophilic capillary structure 2.

[0022] This embodiment constructs a sealed, flat cavity, providing physical support and a sealed space for the upper condensation surface and the lower evaporation surface, ensuring that the working fluid can stably undergo evaporation-condensation phase change cycles within it. Simultaneously, by setting the inner surface of the upper wall as a uniform superhydrophilic capillary structure, an efficient interface is provided for steam condensation and liquid collection.

[0023] Example 2 (Referring to the accompanying drawings): This example describes a lower shell structure of an ultrathin heat exchanger plate based on an asymmetric capillary structure using a wettability gradient. The lower shell plate wall is divided into three functional regions: a central region 6, a transition region 8, and an edge region 5. The central region, located directly above the heat source 4, occupies approximately 30% of the lower shell plate area. This region undergoes chemical polishing to create a smooth surface, followed by fluorosilane vapor deposition to achieve a surface contact angle of approximately 110°, without any additional porous capillary structures. The transition region surrounds the central region. By controlling the energy density and scanning interval of the laser texturing process, a micro-nano rough structure with gradually increasing porosity from the inside out is formed, with its surface contact angle continuously varying to approximately 10°. The edge region, located on the outermost side, uses a sintering process to attach a layer of spherical copper powder with an average particle size of 50 μm, forming a porous capillary structure. Oxygen plasma treatment further enhances its surface to exhibit superhydrophilicity, resulting in a contact angle of less than 10°. Specifically, the thickness of the sintered copper powder is increased by 50% on the inner side immediately adjacent to the central region, forming a capillary-enhancing band 7 for powerfully collecting the liquid flowing down from the condensation wall.

[0024] This embodiment designs the spatial layout of the central hydrophobic region, the transitional gradient region, and the edge superhydrophilic region to create differentiated surface characteristics on a single evaporation surface, thereby achieving active control over the phase change behavior of the working fluid.

[0025] Example 3, referring to the accompanying drawings: This example is a schematic diagram of the flow path of the working fluid inside an asymmetric capillary ultrathin heat exchanger based on a wettability gradient. The steam path is represented by a solid hollow arrow 9, which points vertically upward from the central area of ​​the lower shell plate's inner wall to the inner wall of the upper shell plate. Multiple arrows are arranged side by side, some slightly tilted, symbolizing the process of steam being generated from the central area of ​​the evaporation surface and then vertically diffusing into the central space of the cavity with low resistance. Small dots are used to represent the diffusion of steam molecules 10. The liquid return path is represented by dashed arrow 11, which starts from the condensation surface of the upper shell plate's inner wall, first flowing horizontally to both sides, then vertically downward, and finally turning horizontally towards the center when approaching the inner wall of the lower shell plate. Small black dots are distributed on the dashed arrows, representing 12 drops of liquid working fluid flowing along the path.

[0026] In this embodiment, the steam adopts a vertically upward diffusion path, directly reaching the upper wall condensation surface from the central area. The path is short and the resistance is low. The condensate adopts a path of "upper wall condensation - edge capture - lateral reflux - central replenishment", which does not intersect with the steam path. This fundamentally avoids the flow competition and evaporation end drying problems caused by the reverse flow of steam and liquid in traditional heat spreaders, thereby significantly improving the heat transfer limit and working stability of the heat spreader.

[0027] Example 4 (Referring to the accompanying drawings): This example is a schematic diagram illustrating the change in the working contact angle of the lower shell of an asymmetric capillary structure ultrathin heat exchanger based on a wettability gradient, transitioning from the central region to the edge region. From left to right, the figures show three independent sub-figures, corresponding to the central region, transition region, and edge region of the inner wall of the lower shell, respectively. The central region is represented by a horizontal straight line, with a nearly semi-circular liquid working fluid drawn above it. The liquid working fluid has a relatively large height and a small contact area with the surface at its bottom. The angle θ formed at the contact point between the liquid working fluid and the surface is the central region contact angle of 15°. The transition region is represented by a horizontal straight line, with a slightly flattened liquid working fluid drawn above it. The height of this liquid working fluid is approximately 2 / 3 the height of the liquid working fluid in the central region, and the contact area between its bottom and the surface is increased. The angle θ formed at the contact point between the liquid working medium and the surface is the contact angle 16 of the transition region; a horizontal straight line is drawn at the edge region to represent the solid surface, and an extremely flat liquid working medium is drawn above the surface, almost in a thin layer, closely attached to the surface, with a very low height and a large contact area between the bottom and the surface. The angle θ formed at the contact point between the liquid working medium and the surface is the contact angle 17 of the edge region.

[0028] This embodiment achieves active control over the phase change behavior of the working fluid by designing a contact angle that continuously varies from the center to the edge on the inner wall of the lower shell plate, i.e., a wettability gradient from hydrophobic to superhydrophilic. The large contact angle in the central region facilitates the rapid detachment of vapor from the wall surface, while the extremely small contact angle in the edge region generates a strong capillary adsorption force, driving efficient condensate reflux.

Claims

1. An ultrathin heat exchanger based on an asymmetric capillary structure using a wettability gradient, comprising an upper shell plate and a lower shell plate that are sealed together, wherein a sealed cavity is formed between the upper shell plate and the lower shell plate, characterized in that: The inner surface of the lower shell plate has a continuous wettability gradient, that is, the surface contact angle decreases continuously from the central region to the edge region. The inner surface of the upper shell plate has a uniform hydrophilic capillary structure.

2. The ultrathin heat spreader according to claim 1, characterized in that, The central region of the lower shell plate is a smooth hydrophobic surface without capillary structure, and its contact angle is greater than 90°.

3. The ultrathin heat spreader according to claim 2, characterized in that, The edge region of the lower shell plate is a superhydrophilic microstructure surface with strong capillary force and a contact angle of less than 10°.

4. The ultrathin heat spreader according to claim 3, characterized in that, A transition region is provided between the central region and the edge region of the lower shell plate. The capillary density or surface roughness in the transition region changes in a gradient, so that the surface contact angle decreases continuously from the center to the edge, and the contact angle of the transition region is between 10° and 90°.

5. The ultrathin heat spreader according to claim 1, characterized in that, The uniform hydrophilic capillary structure on the inner surface of the upper shell plate is a sintered copper powder layer with a surface contact angle of less than 10°.

6. The ultrathin heat spreader according to claim 1, characterized in that, The outermost edge region of the lower shell plate is provided with a capillary reinforcement band.

7. The ultrathin heat spreader according to claim 1, characterized in that, The thickness of the cavity is 0.8 mm to 3.0 mm.

8. The ultrathin heat spreader according to claim 6, characterized in that, The thickness of the sintered copper powder in the capillary reinforcement zone is greater than that in other parts of the edge region.

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