Flat heat pipe with multi-scale wick

Abstract

The invention provides a flat heat pipe with a multi-scale wick. The flat heat pipe comprises a lower bottom plate, the lower surface of the multi-scale liquid absorption core is fixedly connected with the upper surface of the lower bottom plate, the multi-scale liquid absorption core is foamy copper internally provided with a honeycomb-shaped V-shaped channel, and a hydrophilic modification layer is formed on the surface of the multi-scale liquid absorption core; and the edge of the lower surface of the upper cover plate is connected with the edge of the upper surface of the lower bottom plate so as to seal the multi-scale wick.

Description

Technical Field

[0001] The invention belongs to the field of high heat flux density heat transfer, and in particular relates to a flat heat pipe with a multi-scale liquid wick. Background Art

[0002] With the miniaturization and high integration of electronic devices, the high heat flow has brought great challenges to thermal management. Heat accumulates in a narrow space. When the operating temperature exceeds the rated operating temperature, the performance, reliability and service life of electronic components will be greatly reduced. At the same time, electronic devices also have high requirements for temperature uniformity. Uneven temperature distribution will form local high temperature, generate thermal stress and strain, and lead to device fatigue failure.

[0003] A flat heat pipe is a device that uses the phase change principle to efficiently transfer heat. It can quickly transfer the heat flow of electronic products to a specific part for dissipation. Due to the small heat dissipation space of electronic products, it is urgent to develop ultra-thin flat heat pipes to meet the heat dissipation needs of wearable and micro-electronic devices.

[0004] The invention patent application with publication number CN109378552A discloses a cross-type flat plate heat pipe, including a plurality of flat plate heat pipe units arranged in the same plane and in strip shape, each of the flat plate heat pipe units has at least one end connected to the end or edge of at least one other flat plate heat pipe unit among the plurality of flat plate heat pipe units, and an angle is formed between each two connected flat plate heat pipe units. The cross-type flat plate heat pipe provided by the present invention is provided with a plurality of flat plate heat pipe units which can be cross-connected according to demand, and can dissipate heat in multiple directions when dissipating heat from a heat source, thereby increasing the heat transfer at the heat source position, and effectively ensuring the temperature uniformity of a planar heat source with a hot spot while enhancing the heat exchange capacity.

[0005] The invention patent application with publication number CN108362144A discloses a composite flat plate heat pipe, including: a liquid cooling plate, whose cross section is U-shaped, at least one serpentine cooling liquid channel is provided in the two opposite wall plates of the liquid cooling plate, at least one liquid inlet hole and at least one liquid outlet hole are provided on the side of the liquid cooling plate bottom plate connected to the two opposite wall plates of the liquid cooling plate away from the wall plate, one end of each serpentine condensation channel is connected to one of the liquid inlet holes, and the other end of each serpentine condensation channel is connected to one of the liquid outlet holes; a flat plate heat pipe, whose central area is an evaporation area, and the areas near the two ends of the flat plate heat pipe are condensation areas, and each condensation area is embedded in a recessed part of the liquid cooling plate. The present invention adopts a U-shaped liquid cooling plate installed in the condensation area of ​​the flat plate heat pipe, which increases the heat exchange area with the condensation area of ​​the flat plate heat pipe, quickly takes away the heat of the condensation section, improves the liquefaction rate of the gas phase working medium, promotes the gas-liquid circulation, and finally greatly improves the heat exchange performance of the entire heat pipe.

[0006] However, the reflux rate and reflux volume of the flat heat pipe wick disclosed in the above two patents have room for further improvement. The flat heat pipe mainly relies on the wick to provide capillary force to drive the condensed working fluid to flow back to the evaporator to ensure good spontaneous circulation inside the flat heat pipe. The heat dissipation space of electronic products is limited. It is difficult to achieve a good balance between the reflux rate and reflux volume of the working fluid in ultra-thin flat heat pipes, resulting in the problem of thick flat heat pipes or limited heat transfer performance of ultra-thin flat heat pipes. Summary of the invention

[0007] The present invention provides a flat heat pipe with a multi-scale liquid wick. The multi-scale liquid wick has a large working medium reflux speed and reflux amount during operation, thereby having good heat transfer performance.

[0008] The present invention provides a flat heat pipe with a multi-scale liquid wick, comprising:

[0009] Lower base plate;

[0010] A multi-scale liquid absorbent core, wherein the lower surface of the multi-scale liquid absorbent core is fixedly connected to the upper surface of the lower base plate, and the multi-scale liquid absorbent core is a foam copper having a honeycomb V-shaped groove and a hydrophilic modification layer on the surface;

[0011] An upper cover plate, wherein the lower surface edge of the upper cover plate is connected to the upper surface edge of the lower base plate to seal the multi-scale liquid absorbent core.

[0012] Preferably, the aspect ratio of the honeycomb V-shaped groove is 0.25-0.7. The honeycomb V-shaped groove increases the disturbance of the fluid, thereby improving the heat transfer performance of the flat heat pipe. In addition, if the aspect ratio of the groove is too large, the reflux rate of the fluid will be reduced; if the aspect ratio of the groove is too small, the reflux amount of the fluid will be reduced.

[0013] Preferably, the porosity of the copper foam is 50%-85%, and the pore size is 20μm-100μm. Appropriate porosity and pore size can lay a foundation for subsequent hydrophilic treatment and micro-groove processing, so that the prepared multi-scale liquid absorbent core has better performance.

[0014] Preferably, a hydrophilic modified layer is formed on the surface of the copper foam and the honeycomb V-shaped groove.

[0015] Preferably, the material of the hydrophilic modification layer is copper hydroxide or copper oxide, and the morphology is needle-shaped, petal-shaped, or fluffy nanofibers. By controlling the morphology of the hydrophilic modification layer, the surface of the foam copper and the honeycomb V-shaped groove has good hydrophilicity, which increases the surface wettability. This allows the evaporation zone of the flat heat pipe to be replenished in time during operation, resulting in better heat transfer performance.

[0016] Preferably, the hydrophilic modification layer has a petal-shaped nanofiber morphology, which has the best hydrophilic property and can achieve full wetting in a short time.

[0017] Preferably, the thickness of the multi-scale wick is 150 μm-2000 μm. Appropriate thickness of the multi-scale wick can ensure the heat transfer performance of the flat heat pipe while ensuring structural stability and avoiding structural collapse.

[0018] Preferably, the multi-scale wick is prepared by the following method, including: using femtosecond laser to process honeycomb V-shaped grooves on the copper foam, and constructing petal-shaped nanofibers on the surface of the wick by hydrophilic treatment to obtain the multi-scale wick. The femtosecond laser processing can better control the ablation size and the structural integrity of the porous copper foam, and the macroscopic, mesoscopic, and microscopic composite structures and reasonable distribution greatly improve the working medium reflux speed and reflux amount of the multi-scale wick.

[0019] Preferably, the lower surface of the multi-scale wick is connected to the upper surface of the lower base plate by welding;

[0020] The lower surface edge of the upper cover plate is connected to the upper surface edge of the lower base plate by welding.

[0021] The multi-scale liquid wick and the lower base plate are welded to form good contact, thereby reducing the contact thermal resistance of the flat heat pipe; the upper cover plate and the edges of the lower base plate are welded to form a closed cavity, and the liquid filling pipe is connected in the cavity to fill the working fluid required by the flat heat pipe.

[0022] Preferably, the upper cover plate and the lower base plate are made of copper sheets, and the contact surfaces of the lower base plate and the multi-scale wick, and the upper cover plate and the lower base plate are evenly plated with solder with a thickness of 5-10 μm by electrochemical deposition to achieve good contact and packaging between the lower base plate and the multi-scale wick, and the upper cover plate and the lower base plate, without increasing the overall thickness of the heat pipe.

[0023] Preferably, the edges of the upper cover plate and the lower base plate are slightly larger than the multi-scale liquid absorbent core by 0.5-1 mm, and are welded together to ensure their sealing.

[0024] Preferably, it also includes a liquid filling pipe, which is located between the lower base plate and the upper cover plate and connected to the upper cover plate and the lower base plate by welding, and is used to fill the working fluid into the flat plate heat pipe.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] Since the multi-scale liquid wick provided by the present invention has a composite of microscopic, mesoscopic and macroscopic multi-scale structures, and each structure is reasonably distributed, it has excellent working fluid reflux speed and reflux amount, can timely replenish the liquid working fluid in the evaporation zone, and improve the heat transfer performance of the flat heat pipe.

[0027] During the reflux process of the working fluid in the liquid absorbent core provided by the present invention, the working fluid in the channel diffuses to the foam copper matrix through the pores on the channel wall. Since the honeycomb channel supplies liquid on six sides, while the ordinary parallel channel supplies liquid on two sides, the liquid supply speed of the honeycomb channel is faster. In addition, the honeycomb channel is more evenly distributed on the surface of the liquid absorbent core, and there is a greater probability of cutting open the closed cavity generated during the sintering of the foam copper during the femtosecond laser processing process, thereby increasing the liquid storage volume of the liquid absorbent core. In summary, the multi-scale liquid absorbent core has an excellent working fluid reflux speed and reflux amount, can replenish the working fluid in the evaporation zone in time, and improve the heat transfer performance of the flat heat pipe.

[0028] Due to the branching structure of the honeycomb V-shaped groove provided by the present invention, the working fluid forms eddies when flowing in the groove, which increases fluid disturbance and destroys the thermal boundary layer, thereby enhancing the heat transfer performance of the flat heat pipe.

[0029] Since the honeycomb V-shaped groove provided by the present invention is wide at the top and narrow at the bottom, when the working medium flows in the flat heat pipe, the liquid working medium is located at the bottom of the groove and in the pores of the foam copper, and the gaseous working medium is located at the top of the groove, thereby achieving the purpose of gas-liquid working medium separation. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 A schematic diagram of the structure of a flat heat pipe with a multi-scale liquid wick provided in a specific embodiment of the present invention, wherein 1 is an upper cover plate; 11 is a contact surface of the upper cover plate; 2 is a multi-scale liquid wick; 21 is a gap between hexagons; 22 is a foam copper; 3 is a lower bottom plate; 31 is a contact surface of the lower cover plate; 4 is a liquid filling tube;

[0031] Figure 2 Schematic diagram of gas-liquid working medium separation in the honeycomb V-shaped groove of the multi-scale liquid wick prepared in Example 1 of the present invention;

[0032] Figure 3 This is a real picture of the liquid wick after the honeycomb grooves are processed by femtosecond laser in Example 1 of the present invention;

[0033] Figure 4-6 The morphology of the nanoscale microstructure provided by Examples 1 to 3 of the present invention, wherein Figure 4 This is the petal-shaped nanofiber morphology provided in Example 1. Figure 5 This is the morphology of the needle-shaped nanofibers provided in Example 2. Figure 6 The morphology of the fluffy nanofibers provided in Example 3;

[0034] Figure 7 This is a comparison chart of the surface wettability of nanofiber liquid-absorbing cores with different morphologies provided in Examples 1 to 3 of the present invention. Figure 7 (a) is the surface wettability result of needle-shaped nanofibers. Figure 7 (b) is the surface wettability result of petal-shaped nanofibers. Figure 7(c) is the wettability result of the fluffy nanofiber surface;

[0035] Figure 8 The chemical composition analysis results of nanofibers with different morphologies provided in Examples 1 to 3 of the present invention are shown in FIG. Figure 8 (a) is the O1s spectrum analysis diagram of nanofibers with different morphologies. Figure 8 (b) is the Cu 2p spectrum analysis diagram of nanofibers with different morphologies;

[0036] Fig. 9 The curve diagram of the liquid level rise height versus time provided in Example 1 and Comparative Examples 1 to 3 of the present invention;

[0037] Fig.10 The water absorption curve of the absorbent core provided in Example 1 and Comparative Examples 1 to 3 of the present invention varies with time;

[0038] Fig.11 The steady-state temperature of the heat source of the liquid absorbent core provided in Example 1 and Comparative Examples 1 to 3 of the present invention varies with the heating power;

[0039] Fig.12 This is a comparison chart of the steady-state temperature of the heat source using the multi-scale wick at 300s provided in Example 1 of the present invention and Comparative Examples 1 to Comparative Examples 3 and the heat source temperature using the untreated foam copper wick. DETAILED DESCRIPTION

[0040] The present invention is further described in detail below in conjunction with the embodiments and drawings. It should be pointed out that the embodiments described below are intended to facilitate the understanding of the present invention and do not have any limiting effect on the present invention.

[0041] The specific embodiment of the present invention is based on the fact that the wick is the core component of the flat heat pipe, and the balance between the reflux rate and reflux amount of the working fluid in the wick is the key to the excellent heat transfer performance of the flat heat pipe, and a multi-scale wick structure is designed. The advantages of the small and controllable pore size of the foam copper are used to form basic micron-level pores, and the honeycomb structure grooves are processed on the foam copper by combining the means of femtosecond laser processing to form micron-level channels. At the same time, by hydrophilic treatment of the foam copper, highly hydrophilic nano-scale oxide short fibers are formed on the surface of the foam copper and the honeycomb grooves. The multi-scale structural composite of micro, meso and macro scales and the reasonable distribution of each structure make the wick have a large reflux rate and reflux amount of the working fluid, which can replenish the working fluid in the evaporation zone in time and improve the heat transfer performance of the flat heat pipe.

[0042] In order to obtain a flat heat pipe with a larger working medium reflux velocity and reflux amount, a specific embodiment of the present invention provides a flat heat pipe with a multi-scale liquid wick, such as Figure 1As shown, it includes a lower base plate 3, a multi-scale liquid absorbent core 2, an upper cover plate 1 and a liquid filling tube 4, wherein the upper surface 31 of the lower base plate 3 is fixedly connected to the lower surface of the multi-scale liquid absorbent core 2, and the edge of the lower surface 11 of the upper cover plate 1 is connected to the edge of the upper surface 31 of the lower base plate 3 to seal the multi-scale liquid absorbent core 2. The liquid filling tube 4 is located between the lower base plate 3 and the upper cover plate 1, and is used to fill the multi-scale liquid absorbent core 2 with a working medium (the working medium in this embodiment is deionized water).

[0043] The multi-scale wick 2 provided in the specific embodiment of the present invention is a porous copper foam 22 with a honeycomb V-shaped groove inside and a hydrophilic nanofiber deposition surface. Multiple copper foams 22 are separated by the honeycomb V-shaped grooves to form gaps 21 between the hexagons.

[0044] The preparation process of the honeycomb V-shaped groove provided in the specific embodiment of the present invention is as follows: first, a honeycomb V-shaped groove is processed on the porous copper foam by using a femtosecond laser. The honeycomb V-shaped groove is wide at the top and narrow at the bottom. The liquid medium flows in the lower part of the groove and the gas medium flows in the upper part of the groove to achieve gas-liquid separation. Figure 2 As shown, the thickness of the flat heat pipe is further reduced. In addition, the structure of the honeycomb grooves causes the liquid to generate eddies during the flow process, enhances disturbances, and destroys the thermal boundary layer, thereby improving the heat transfer performance of the flat heat pipe. And due to the multi-scale structure of micro, meso, and macro and the reasonable distribution of each structure, the multi-scale liquid wick has excellent working fluid reflux speed and reflux volume, which can replenish the working fluid in the evaporation zone in time, further improving the heat transfer performance of the flat heat pipe.

[0045] like Figure 3 As shown, in Example 1, the porous copper foam provided in this embodiment has a thickness of 280μm, an average pore size of 40μm, and a porosity of 75%; the laser speed used is 100mm / s, the step is 0.01mm, the laser power is 12W, the side length of the processed hexagonal copper foam unit is 1.2mm, the width of the V-shaped groove between the hexagons is about 50μm, and the depth is about 120μm. By adjusting the parameters of the femtosecond laser, the size of the hexagon, the gap between them, and the depth of the honeycomb groove can be easily adjusted, and the porosity can be controlled.

[0046] After processing the honeycomb V-shaped grooves in this embodiment, the liquid absorbent core is subjected to super-wetting and hydrophilic treatment. Before the treatment, the sample is cleaned in an ultrasonic bath of ethanol, electrode cleaning liquid, and deionized water for 60 seconds in sequence. After the sample is taken out, it is dried with compressed nitrogen. Then it is placed in a solution of 2.5 mol / L sodium hydroxide and 0.13 mol / L ammonium persulfate for a quantitative reaction time, during which a water bath is used to maintain a constant temperature of about 36.5°C. After taking it out, it is washed with deionized water and dried in a low-temperature vacuum oven for 1 hour. The hydrophilic treatment can improve the wettability of the surface of the liquid absorbent core, so that the flat heat pipe can be replenished with water in time during operation, and the heat transfer is more uniform.

[0047] By adjusting the hydrophilic treatment time, the microstructure morphology generated on the surface can be controlled. Table 1 shows the corresponding microstructure morphology under different reaction conditions. Figure 4 As shown in Figure 2, petal-shaped nanofibers were formed on the surface of the multi-scale wick after 2 h of hydrophilic treatment, and needle-shaped nanofibers and villi-shaped nanofibers were obtained after 1 h and 3 h of hydrophilic treatment, respectively. Figure 5 and Figure 6 shown.

[0048] Table 1 Microstructure morphology under different reaction conditions

[0049]

[0050] Figure 7 The surface wettability comparison diagram of nanofiber liquid absorbent cores with different morphologies provided in Examples 1 to 3 of the present invention is as follows: Figure 7 (a)- Figure 7 (b) shows the comparison of the surface wettability of the wick samples with needle-shaped, petal-shaped, and villi-shaped surface morphologies. The results show that when using 4 μL of deionized water for contact angle testing, the petal-shaped nanostructure has the best surface wettability, and the droplet is completely wetted in about 109 ms.

[0051] like Figure 8 (a) and Figure 8 (b) shows the XPS analysis of nanofibers with different morphologies provided in Examples 1 to 3 of the present invention. In this example, the XPS results of nanofibers with different morphologies were subjected to peak fitting and compared with the standard spectrum. Comprehensive analysis showed that the chemical composition of the needle-shaped nanofibers was copper hydroxide, and the chemical composition of the petal-shaped and fuzz-shaped nanofibers was copper oxide.

[0052] Comparative Example 1

[0053] Compared with Example 1, there are no grooves in the copper foam.

[0054] Comparative Example 2

[0055] Compared with Example 1, parallel channels are provided in the copper foam.

[0056] Comparative Example 3

[0057] Compared with Example 1, thicker parallel grooves are provided in the copper foam.

[0058] Comparative Example 4

[0059] Compared with Example 1, there is no channel in the copper foam, and the wick is not subjected to hydrophilic treatment.

[0060] The groove structure dimensions and surface microstructure morphologies of Example 1 and Comparative Examples 1 to 4 are shown in Table 2.

[0061] Table 2 Main parameters of different structure wicks

[0062]

[0063] Fig. 9 The curves of the liquid level rise of the liquid absorbent core over time in Example 1 and Comparative Examples 1-3 are as follows: Fig.10 The water absorption curves of the absorbent cores of Example 1 and Comparative Examples 1-3 are as a function of time. Combined with the analysis of the liquid level rise curve and the water absorption test results, the multi-scale absorbent core with honeycomb channels (fine) in Example 1 has the best suction speed and water absorption, and its maximum suction rate can reach 17.5 mm / s, and the water absorption within 20 seconds reaches 64.8 mg, that is, the absorbent core has excellent working medium reflux rate and reflux amount; the parallel channel (fine) absorbent core in Comparative Example 3 has a suction rate comparable to that of the honeycomb structure absorbent core, but its water absorption is lower; the parallel channel (coarse) absorbent core in Comparative Example 2 has a lower maximum suction rate, but a higher water absorption. Fig.11 The steady-state temperature of the heat source of the absorbent core of Example 1 and Comparative Examples 1-3 changes with the heating power. It can be seen that the multi-scale absorbent core with a honeycomb structure in Example 1 has the best cooling performance. When the heating power is 5W, the heat source temperature is only 79.47°C. Combined with the experimental results in Comparative Examples 1-3, the change law of the cooling performance of the absorbent core is analyzed, and it can be obtained that: under low heating power, due to the small evaporation amount of the working fluid in the evaporation zone, the demand for liquid replenishment is also small, so the working fluid reflux rate is the determining factor of the cooling performance of the absorbent core; under high heating power, due to the large demand for liquid replenishment, the working fluid reflux rate becomes the determining factor of the cooling performance of the absorbent core. Based on this, we can roughly infer the cooling performance of different absorbent cores according to their suction speed and water absorption. In addition, within 300s, the heat source temperature of the multi-scale absorbent core with a honeycomb structure in Example 1 gradually reaches a steady state, while the heat source temperature of the absorbent core in Comparative Example 4 continues to rise. Fig.12 The heat source temperature of the honeycomb-like structure wick at 300s was compared with the heat source temperature of the wick in Comparative Example 4. When the heating power was 5W, the stable heat source temperature of the honeycomb-like multi-scale wick was 18.96°C lower than the heat source temperature of the wick in Comparative Example 4, further demonstrating the excellent cooling performance of the multi-scale wick.

[0064] The multi-scale liquid wick provided by the present invention uses femtosecond laser processing to reduce the thermal effect and ensure the integrity of the porous structure in the copper foam; the honeycomb structure increases the disturbance in the flow of the liquid working medium, destroys the thermal boundary layer, and strengthens the heat transfer performance of the flat heat pipe; the petal-shaped nanofibers formed after the hydrophilic treatment of the copper foam enhance the surface wettability of the copper foam. In summary, the liquid wick achieves the balance of the reflux velocity and reflux amount of the working medium in the liquid wick through the composite of macroscopic, mesoscopic and microscopic multi-scale structures and the reasonable distribution of each structure, which is conducive to the timely replenishment of the evaporation zone, thereby improving the heat transfer performance of the flat heat pipe. In addition, a specific thickness of tin is formed on the surface of the cover plate by electrochemical deposition and welding is performed, which reduces the contact thermal resistance between the liquid wick and the lower base plate, and ensures connection and sealing. The gas and liquid working mediums each go their own way and run smoothly, and the quantitative control of solder during welding greatly reduces the thickness of the flat heat pipe to between 0.2-0.4mm, realizing the ultra-thinness of the flat heat pipe.

Claims

1. A flat heat pipe with a multi-scale wick, characterized in that: include: Lower base plate; A multi-scale liquid absorbent core, wherein the lower surface of the multi-scale liquid absorbent core is fixedly connected to the upper surface of the lower base plate, the multi-scale liquid absorbent core is a foam copper with a honeycomb V-shaped groove inside, and a hydrophilic modified layer is formed on the surface of the multi-scale liquid absorbent core; An upper cover plate, wherein the lower surface edge of the upper cover plate is connected to the upper surface edge of the lower base plate by welding to seal the multi-scale liquid absorbent core.

2. The multi-scale liquid wick flat heat pipe according to claim 1, characterized in that: The width-to-depth ratio of the honeycomb V-shaped groove is 0.25-0.

7.

3. The multi-scale liquid wick flat heat pipe according to claim 1, characterized in that: The porosity of the foam copper is 50%-85%, and the pore size is 20-100 μm.

4. The multi-scale liquid wick flat heat pipe according to claim 1, characterized in that: The material of the hydrophilic modification layer is copper hydroxide or copper oxide, and the morphology is needle-shaped, petal-shaped or fluff-shaped nanofibers.

5. The multi-scale liquid wick flat heat pipe according to claim 4, characterized in that: The morphology of the hydrophilic modified layer is petal-shaped nanofibers.

6. The multi-scale liquid wick flat heat pipe according to claim 1, characterized in that: The thickness of the multi-scale liquid-absorbing core is 150-2000 μm.

7. The multi-scale liquid wick flat heat pipe according to claim 1, characterized in that: The multi-scale liquid-absorbing core is prepared by the following method: using femtosecond laser to process honeycomb V-shaped grooves on foam copper, and constructing a hydrophilic modification layer on the surface by chemical treatment, so as to obtain the multi-scale liquid-absorbing core.

8. The multi-scale liquid wick flat heat pipe according to claim 1, characterized in that: The lower surface of the multi-scale wick is connected to the upper surface of the lower base plate by welding; The lower surface edge of the upper cover plate is connected to the upper surface edge of the lower base plate by welding.

9. The multi-scale liquid wick flat heat pipe according to claim 1, characterized in that: It also includes a liquid filling pipe, which is located between the lower base plate and the upper cover plate and is connected to the edges of the lower base plate and the upper cover plate by welding, and is used to fill the working medium into the flat heat pipe of the multi-scale liquid wick.

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