Preparation method of tubular capillary core with high length-diameter ratio and high porosity

Abstract

The invention provides a preparation method of a tubular capillary core with high length-diameter ratio and high porosity, which is mainly characterized in that the tubular capillary core is formed at one time through a loose sintering process, a specially designed forming die is used in the preparation process, the forming process is simple, and the yield is high; the prepared capillary wick is high in porosity and good in liquid absorption performance. The tubular capillary core with the high length-diameter ratio and the high porosity can be applied to the fields of aerospace, high-power LED and power battery thermal management systems, mixture distillation and separation and the like, and has huge market application value and potential.

Description

Technical Field

[0001] The invention relates to a method for preparing a tubular capillary wick with a high aspect ratio and high porosity. The tubular capillary wick prepared by the method has high porosity and excellent liquid absorption performance and can be used as a core component of a tubular loop heat pipe evaporator. Background Art

[0002] Nowadays, the improvement of electronic device performance is accompanied by an increase in power and energy consumption, which often leads to excessive heat generation and insufficient heat dissipation of components. It is urgent to design and prepare reliable, stable, flexible, long-distance transmission, and gravity-resistant high-performance heat dissipation equipment. Loop Heat Pipe (LHP) is an efficient and reliable two-phase heat exchange device that uses the vaporization of the working fluid in the evaporation zone and the liquefaction of the condensation zone to achieve heat transfer. It has the advantages of strong heat transfer capacity, good isothermal properties, long heat transfer distance, light weight, and small heat exchange temperature difference. It can effectively solve the heat dissipation problem under high heat flux density and has become a core heat dissipation component in the fields of aerospace, high-power LEDs, and power battery thermal management systems.

[0003] As the key core component of LHP, the pores of the inverted structure capillary wick can generate capillary force, thereby promoting the transfer of heat in the evaporator and the efficient circulation of the working medium in the heat transfer loop. The performance of the capillary wick is mainly characterized by porosity, permeability, pore structure, and suction performance. The performance of the capillary wick determines the operation effect of the LHP: generally, the high porosity, large permeability and small pore size of the capillary wick mean high heat transfer efficiency of the loop heat pipe. However, the capillary wick is also the most difficult and most expensive component to manufacture among all the accessories of the LHP. Today, electronic cooling systems conform to the development trend of electronic devices, that is, they will consume more power and cause higher heat flux density. This trend puts higher requirements on the manufacture of LHP. At present, the types of capillary wicks used in loop heat pipes are mainly: wire mesh capillary wicks, grooved capillary wicks, polymer capillaries, and metal sintered powder capillary wicks. In comparison, the performance of metal sintered capillary wicks is better than that of other types of capillary wicks. The main preparation methods include cold press demoulding sintering, loose sintering and slurry casting sintering, among which loose sintering technology is the most convenient and simple. The loose sintering method is often used to prepare flat evaporator capillary wicks, but due to technical limitations, it is rarely reported to be used to prepare tubular capillary wicks. The purpose of the present invention is to propose a preparation method for a high aspect ratio and high porosity tubular capillary wick using a loose sintering process, and the prepared capillary wick can achieve the purpose of optimizing the performance of tubular LHP.

[0004] The invention patent (CN103528409B) "A method for preparing a loop heat pipe capillary core" discloses a method for preparing a loop heat pipe capillary core, which adopts a process of cold pressing first and then atmosphere sintering to produce a cylindrical capillary core with a steam removal channel and a central channel whose length can vary with the mold closing height. The disadvantage of this preparation method is that the preparation process requires cold pressing first and then atmosphere sintering, and the steps are relatively cumbersome; the capillary core is easy to break during the demoulding process, and the inner wall flatness of the prepared capillary core is not high, and it needs to be precisely processed later before it can be used; finally, due to cold pressing, the aspect ratio of the capillary core is not high. Based on the above problems, the present invention provides a method for preparing a tubular capillary core with a high aspect ratio and high porosity. The method innovatively solves the problem of difficult core rod removal through a loose sintering process and a technology for removing the forming core rod during the sintering process, thereby realizing the overall loose sintering and one-time forming of the tubular capillary core, avoiding the difficulties in demolding in the later stage of sintering and damage to the inner wall of the capillary core during the demolding process, shortening the preparation process of the traditional tubular capillary core, improving the porosity of the capillary core while improving the product production efficiency, thereby optimizing the suction performance. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a simple method for preparing a powder sintered high aspect ratio and high porosity tubular capillary wick with excellent performance.

[0006] In the present invention, the preparation process of the high aspect ratio and high porosity tubular capillary wick is as follows:

[0007] Step 1, preparing a mold: according to the inner and outer diameters and length of the tubular capillary core to be prepared, prepare a graphite mold (1), a core rod (3), a base (4), and a pressing block (5); after inserting the core rod (3) into the base (4), fix the graphite mold (1) and the base (4) with a high-temperature tape;

[0008] Step 2, powder filling: according to the value of the porosity of the capillary core to be prepared, the mass of the required metal powder is calculated in combination with the mold size and the loose density of the powder, and the fixed mass of the metal powder is weighed and filled into the graphite mold. The outer surface of the graphite mold is continuously and lightly tapped to make the metal powder loaded into the mold evenly distributed. During this process, it is necessary to ensure that the core rod is always located at the axial center of the graphite mold;

[0009] Step 3, furnace loading: placing a briquette on top of the metal powder in the graphite mold (1) obtained in step 2, and then placing it in a sintering furnace for sintering;

[0010] Step 4, sintering: the room temperature is raised to 380°C and then kept warm for 30 minutes at a heating rate of 2°C / min; after the end of the heat preservation, the temperature is continued to rise to the sintering target temperature of 700°C for 60 minutes at a heating rate of 3°C / min; the sintering atmosphere is vacuum, and the vacuum degree is better than 5×10 -2 Pa; finally cooled with the furnace;

[0011] Step 5, demoulding: remove the pressing block on the top of the mold, and demould to obtain the tubular capillary core.

[0012] As an improvement, the preparation process adopts a loose sintering process, which avoids the problem of low porosity of the capillary wick caused by the molding process in the traditional preparation process.

[0013] As an improvement, the material of the core rod (3) is PMMA (polymethyl methacrylate), which can be completely removed during the high-temperature sintering process of the capillary core, thus solving the problem of difficulty in demoulding the tubular capillary core with a high aspect ratio.

[0014] As an improvement, during the sintering process, a pressing block is placed on top of the capillary core powder in the loose mold to provide sintering compressive stress, and the sintering compressive stress range is 0.1 to 10 kPa; the pressing block is made of ceramic and its size is consistent with the internal size of the graphite mold (1).

[0015] As an improvement, the prepared capillary core can be a single-aperture uniform pore structure or a double-aperture mixed pore structure, wherein the double-aperture pore structure can be achieved by adding pore-forming agents of different contents or different particle sizes to the tubular capillary core powder; the pore-forming agent powder can be PMMA (polymethyl methacrylate), NH4HCO3 (ammonium bicarbonate), PVA (polyvinyl alcohol), NaCl (sodium chloride); the mass fraction of the pore-forming agent ranges from 3 to 20%, and the particle size ranges from 13 to 50 μm.

[0016] As an improvement, the capillary wick sintering atmosphere can be vacuum: the vacuum degree is better than 5×10 -2 Pa, or gas protection sintering: such as hydrogen, argon, nitrogen, etc.; the sintering heating rate is 1-5°C / min, the sintering temperature is 600-1000°C, and the insulation time is 30-180min; during the heating process, the specific temperature in the range of 300-450°C must be kept to remove the PMMA core rod (3).

[0017] As an improvement, the prepared tubular capillary core has a porosity range of 35% to 92%, an outer diameter range of 10 to 45 mm, and a length range of 50 to 350 mm.

[0018] As an improvement, the capillary core is made of metal, which may be pure nickel, nickel-based alloy, pure copper or stainless steel, and the particle size of the capillary core metal powder ranges from 0.3 to 6.0 μm.

[0019] The performance of the capillary wick prepared by the technical route involved in the present invention can meet the use requirements of the tubular LHP, and the preparation process is short, the manufacturing cost is low, and the production efficiency is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 :(1) graphite mold, (2) micron-sized nickel powder, (3) PMMA core rod, (4) mold base, (5) ceramic press.

[0021] Figure 2 : Photo of the finished tubular capillary wick.

[0022] Figure 3 : Photograph of the microscopic pore structure of the capillary core.

[0023] Figure 4 : Comparison of porosity data of capillary wicks under different nickel powder filling masses and different sintering temperatures.

[0024] Figure 5 : Capillary wick absorption performance curves after sintering at 700℃ under different nickel powder filling mass conditions.

[0025] Figure 6 : Comparison of stress-strain curves of capillary wicks sintered at 700℃ under different nickel powder filling mass conditions. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical scheme and advantages of the present invention more clear, the present invention is further explained in detail in conjunction with the following drawings and embodiments. In addition, it should be understood that the specific embodiments described herein are only used to explain the present invention, not to limit the present invention, and the technical features involved can be combined with each other as long as they are not contradictory.

[0027] The mold structure used in the embodiment of the present invention is as follows Figure 1 As shown, it includes a graphite mold (1), micron-sized nickel powder (2), a PMMA core rod (3), a mold base (4), and a ceramic pressing sheet (5). The tubular capillary wick prepared in this embodiment has the following dimensions: a length of 130 mm, an outer diameter of 20 mm, and a center hole diameter of 6 mm. The specific implementation scheme is as follows:

[0028] Step 1, prepare the mold: prepare the graphite mold (1), the core rod (3), the base (4), and the pressing block (5); first insert the core rod (3) into the central positioning hole of the base (4), and then fix the graphite mold (1) and the base (4) with high-temperature tape.

[0029] Step 2, powder loading: First, the nickel powder used is ball-milled to ensure that the powder is not agglomerated; second, the mass of the required metal powder is calculated based on the value of the porosity of the capillary core to be prepared, combined with the mold size and the loose density of the powder, and the fixed mass of the metal powder is weighed and filled into the graphite mold (1). The powder filling mass range involved in this embodiment is 40 to 70 g; finally, the outer surface of the graphite mold (1) is continuously and lightly tapped to evenly distribute the metal powder loaded into the mold. During this process, it is necessary to ensure that the core rod is always located at the axial center of the graphite mold.

[0030] Step 3: Loading the furnace: Place a compact on top of the metal powder in the graphite mold (1) obtained in step 2, and then put it into a sintering furnace for sintering.

[0031] Step 4, sintering: the room temperature is raised to 380°C and then kept warm for 30 minutes at a heating rate of 2°C / min; after the end of the heat preservation, the temperature is continued to rise to the sintering target temperature of 700°C for 60 minutes at a heating rate of 3°C / min; the sintering atmosphere is vacuum, and the vacuum degree is better than 5×10 -2 Pa; finally cool with the furnace.

[0032] Step 5, demoulding: remove the pressing block on the top of the mold, demoulding to obtain a tubular capillary core. The actual photo of the prepared high aspect ratio capillary core is shown in Figure 2 As shown; Figure 3 The SEM photos of the microscopic pore structure of the capillary core; the porosity of the capillary core prepared under different sintering temperatures and different powder filling mass conditions ranges from 36.9% to 91.1% ( Figure 4 ); Figure 5 and Figure 6 They are respectively the suction performance curves and stress-strain curves of the six tubular capillary wicks prepared in this embodiment when the sintering temperature is 700°C.

[0033] In view of the cumbersome steps of the existing preparation technology, which requires cold pressing and then high-temperature sintering, as well as the shortcomings of difficult product demolding, low porosity and low aspect ratio, the present invention adopts the overall loose sintering technology of the tubular capillary core to achieve one-time forming of the product, avoiding the difficulties in demolding in the later stage of sintering and damage to the inner wall of the capillary core during the demolding process. It shortens the preparation process of the traditional tubular capillary core, improves the porosity of the capillary core while improving the product production efficiency, thereby optimizing the suction performance.

Claims

1. A method for preparing a tubular capillary wick with a high aspect ratio and high porosity, the preparation process of which is as follows: Step 1, preparing a mold: according to the inner and outer diameters and length of the tubular capillary core to be prepared, prepare a graphite mold (1), a core rod (3), a base (4), and a pressing block (5); after inserting the core rod (3) into the base (4), fix the graphite mold (1) and the base (4) with a high-temperature tape; the material of the core rod (3) is PMMA (polymethyl methacrylate); Step 2, powder filling: according to the value of the porosity of the capillary core to be prepared, the mass of the required metal powder is calculated in combination with the mold size and the loose density of the powder, and the fixed mass of the metal powder is weighed and filled into the graphite mold. The outer surface of the graphite mold is continuously and lightly tapped to make the metal powder loaded into the mold evenly distributed. During this process, it is necessary to ensure that the core rod is always located at the axial center of the graphite mold; Step 3, furnace loading: placing a briquette on top of the metal powder in the graphite mold (1) obtained in step 2, and then placing it in a sintering furnace for sintering; Step 4, sintering: the room temperature is raised to 380°C and then kept warm for 30 minutes at a heating rate of 2°C / min; after the end of the heat preservation, the temperature is continued to rise to the sintering target temperature of 700°C for 60 minutes at a heating rate of 3°C / min; the sintering atmosphere is vacuum, and the vacuum degree is better than 5×10 -2 Pa; finally cooled with the furnace; Step 5, demoulding: remove the pressing block on the top of the mold, and demould to obtain the tubular capillary core.

2. The method for preparing a high aspect ratio and high porosity tubular capillary wick according to claim 1, characterized in that The preparation process adopts a loose sintering process, which avoids the problem of low porosity of the capillary wick caused by the molding process in the traditional preparation process.

3. The method for preparing a high aspect ratio and high porosity tubular capillary wick according to claim 1, characterized in that The material of the core rod (3) is PMMA (polymethyl methacrylate), which can be completely removed during the high-temperature sintering process of the capillary core, thus solving the problem of difficulty in demoulding the tubular capillary core with a high aspect ratio.

4. The method for preparing a high aspect ratio and high porosity tubular capillary wick according to claim 1, characterized in that During the sintering process, a pressing block is placed on the top of the capillary core powder in the loose mold to provide sintering compressive stress, and the sintering compressive stress ranges from 0.1 to 10 kPa. The pressing block is made of ceramic and its size is consistent with the internal size of the graphite mold (1).

5. The method for preparing a high aspect ratio and high porosity tubular capillary wick according to claim 1, characterized in that The prepared capillary core can be a single-aperture uniform pore structure or a double-aperture mixed pore structure, wherein the double-aperture pore structure can be achieved by adding pore-forming agents of different contents or different particle sizes to the tubular capillary core powder; the pore-forming agent powder can be PMMA (polymethyl methacrylate), NH4HCO3 (ammonium bicarbonate), PVA (polyvinyl alcohol), NaCl (sodium chloride); the mass fraction of the pore-forming agent ranges from 3 to 20%, and the particle size ranges from 13 to 50 μm.

6. The method for preparing a high aspect ratio and high porosity tubular capillary wick according to claim 1, characterized in that The capillary wick sintering atmosphere can be vacuum: the vacuum degree is better than 5×10 -2 Pa, or gas protection sintering: such as hydrogen, argon, nitrogen, etc.; the sintering heating rate is 1-5°C / min, the sintering temperature is 600-1000°C, and the insulation time is 30-180min; during the heating process, the specific temperature in the range of 300-450°C must be kept to remove the PMMA core rod (3).

7. The method for preparing a high aspect ratio and high porosity tubular capillary wick according to claim 1, characterized in that The prepared tubular capillary core has a porosity range of 35% to 92%, an outer diameter range of 10 to 45 mm, and a length range of 50 to 350 mm.

8. The method for preparing a high aspect ratio and high porosity tubular capillary wick according to claim 1, characterized in that The material of the capillary core is metal, which may be pure nickel, nickel-based alloy, pure copper or stainless steel, and the particle size of the capillary core metal powder ranges from 0.3 to 6.0 μm.

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