Effect of fabrication parameters on capillary pumping

Applied Thermal Engineering 143 (2018) 621–629

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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Research Paper

Effect of fabrication parameters on capillary pumping performance of multiscale composite porous wicks for loop heat pipe ⁎

T

⁎

Hui Lia, , Shengjuan Fua, Gongfa Lib,c, Ting Fub,c, Rui Zhoua, , Yong Tangd, Biao Tanga, Yong Denga, Guofu Zhoua a Guangdong Provincial Key Laboratory of Optical Information Materials and Technology & Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, PR China b Key Laboratory of Metallurgical Equipment and Control Technology, Ministry of Education, Wuhan University of Science and Technology, Wuhan 430080, Hubei, PR China c Hubei Key Laboratory of Mechanical Transmission and Manufacturing Engineering, Wuhan University of Science and Technology, Wuhan 430080, Hubei, PR China d Key Laboratory of Surface Functional Structure Manufacturing of Guangdong Higher Education Institutes, School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China

H I GH L IG H T S

composite porous wick was proposed for two-phase heat transfer device. • Multi-scale were fabricated on the sintered copper powders. • Nanostructures pumping performance was studied based on the IR thermal imaging method. • Capillary • Effects of wick fabrication parameters were investigated to optimize the design.

A R T I C LE I N FO

A B S T R A C T

Keywords: Porous wick Nanostructures Dealloying Capillary performance

In this study, a new multi-scale composite porous wick (MCPW) is proposed for the loop heat pipe to guarantee the thermal reliability of the microelectronics packages. The MCPW, which is featured with the nanostructures distributed on the sintered copper powders, can effectively enhance the capillary performance through modifying the properties of the copper powders. In this study, a number of MCPWs were developed by the sintering and alloying-dealloying treatment. Based on the infrared radiation (IR) thermal imaging method, the capillary rate-of-rise tests were used to the evaluate the capillary pumping performance, and the effects of the porous substrate and nanostructures were investigated in detail. The results indicated that morphologies of the copper powders, including powder size and powder type, would influence the capillary performance. The larger powder size and irregular type were better for liquid rise. Meanwhile, nanostructures on the powder surface played a dominant role in forming the hydrophilic surface on the copper powders, which could achieve the higher capillary height and rising velocity of working fluid for the wick. The optimum choice for the nanostructures formation was NaOH solution under the corrosive time 24 h.

1. Introduction Thermal management of microelectronics packages, such as light emitting diode, CPU and hard disk drive, are becoming increasingly challengeable due to the large heat flux and high heat dissipation. Loop heat pipe (LHP), as a two-phase heat transfer device, can provide high capability of transferring heat efficiency passively between heat source and heat sink with little temperature difference. Moreover, in contrast with the traditional heat pipe, LHP presents several significant ⁎

advantages, including long distance heat transferring, flexible configuration, various working orientations and so on. In 1972, Gerasimov and Maydanik [1] first created LHP to satisfy the demand of satellites and aerospace technology for highly-efficient heat transfer devices. Maydanik et al. [2], Peterson et al. [3] and Launay et al. [4] presented the details of the operational mechanism of LHP, respectively. Nowadays, LHP is widely used in the thermal management of spacecraft as well as the cooling of electronic devices, such as solar photovoltaic [5], PC [6], LED [7] and air-cooled heat exchanger [8], etc.

Corresponding authors. E-mail address: [email protected] (H. Li).

https://doi.org/10.1016/j.applthermaleng.2018.07.143 Received 4 January 2018; Received in revised form 21 June 2018; Accepted 30 July 2018 Available online 31 July 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.


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thermal insulation owing to the composite wick. Furthermore, carbon nanotubes were incorporated into the wick structures by Weibel et al. [15]. The resulted hydrophilic wicking surface improved the wick performance and reduced the surface superheat up to 72% through the capillary-fed boiling test. In view of the above situation, it is essential to conduct experiments to access the capillary performance. One of the potential methods is to measure the transient rate-of-rise of liquid [16]. Chen et al. [17] investigated the capillary limits of micro-wick structures experimentally. CCD camera and optical microscope were used to capture the liquid flow characteristics in the inclined wick. Zhou et al. [18] also combined the camera with the experimental setup to measure the capillary suction of porous wick. Moreover, the dynamic liquid in the wick was also visualized with high speed camera by Byon and Kim [10]. However, the rising height of working fluid is difficult to record accurately due to the transparent and colorless working fluid. Thus, some studies were presented concerning on the rate-of-rise test, including mass changing curve recorded by electronic balance [19], fluorescent visualization method [20], and mass flow rate at wick’s dryout threshold [21]. Due to the different radiation energies for wick and working fluid, the infrared radiation (IR) thermal imaging method was proposed by Tang et al. [22] to record the rise of meniscus. During the measurement, ethanol was used as the working fluid, and a measuring line with a location point was drawn accurately along the wick based on the color. Then the movement process of the point could be obtained to present the rising velocity of the wetted height. Based upon the analysis above, the present work developed a multiscale composition porous wick (MCPW) for the loop heat pipe. The novel wick, featured with nanoporous structures distributed on the sintered copper powders was manufactured through the alloying-dealloying (AD) process. In the present study, the capillary performance of the wick was assessed by means of the rate-of-rise test with IR camera. To better understand the influence the fabrication parameters on the MCPW and achieve the best capillary performance, it is necessary to perform investigations to get comprehensive information for the design optimization. The effects of the nanostructures on the wick, powder size, powder type, corrosive time and corrosive solutions were studied, respectively. It is expected that the data can provide useful information for its application in the heat dissipation area.

When LHP works normally, heat is removed by means of the circulation of working fluid enclosed inside the pipe. The vapor with latent heat is delivered from the evaporator to the condenser. Then the liquid is pumped back to the heat source under the action of the wick inside the evaporator. During this operation process, the wick, usually composed of porous media, mesh or micro groove, is considered as an important component that provides capillary force to drive the liquid circulation and supplies flow channel for the working fluid. In the porous wick, the flow rate of vapor releasing the porous media is generally determined by the following equation [9],

ṁ =

π ⎛ ρv σlv ⎞ ⎛ εde3 ⎞ ⎟ ⎜ ⎟⎜ 128 ⎝ μ v ⎠ ⎝ t ⎠

(1)

where ṁ is the mass flow rate of vapor; ρv , σlv , and μ v are vapor density, surface tension and dynamic viscosity, respectively; ε , de and t are the wick properties, namely porosity, effective pore diameter and thickness of the porous media. It can be seen that large pores can lead to large vapor mass release. Nevertheless, the wick capillary force ΔPcap can be written as,

ΔPcap =

4σ de

(2)

The pores have opposite effect on the capillary pressure. There exists a dilemma in the wick between the vapor release and liquid suction. While the capillary pumping of wick is not adequate for the required liquid to the evaporator, there will be dry-out phenomenon in the evaporator. Therefore, various modified wick structures have been proposed and studied to address the above problem. Byon and Kim [10] investigated in the capillary performance of a bi-porous wick experimentally and presented the liquid flow characteristics inside the wick with high speed camera. They found that capillary regimes in the wick was decided by the relationship between particle and cluster, and the 125/675 μm bi-porous wick was 11 times larger than that of a 125 μm mono-porous sample. Huang et al. [11] treated axially grooved aluminum wick based on the method of alkaline corrosion and carried out capillary rate-of-rise test to measure the capillary performance. The results showed that the corroded aluminum wick could achieve the better capillary performance parameter by about 155% higher in contrast with the conventional wick. Deng et al. [12,13] developed a novel composite wick by covering the copper powders on the V-grooves, and they proposed the capillary performance parameter (ΔPc K ) to evaluate the capillary characteristics of the composite wick. The results showed that the composite wick could effectively enhance the capillary performance because of the additional rising channels provided by the gaps between grooves and powders. Recently, researchers have also been devoted to the study of the multiscale porous wicks, allowing to different pore sizes to induce various characteristics. Xu et al. [14] designed and investigated in the loop heat pipe with composite multiscale porous wick. A groove multiscale wick was manufactured by sintered copper powder, with other size powders sintering above as the second layer. The LHP could operate properly under the anti-gravity condition and achieve a synergy between thermal conduction and

2. Experimental details 2.1. Sample preparation As shown in Fig. 1, the fabrication of the MCPW consists of the two main procedures: powder sintering treatment of porous copper substrate and alloying-dealloying treatment for nanostructures. Firstly, the copper powders were filled into the sintering mode and sintered by the loose sintering method, as shown in Fig. 2(a). During solid-sintering process, the mode was heated under the protection of the hydrogen atmosphere. The furnace (17300-30, Haoyue furnace Co. Shanghai, China) temperature was maintained at 900 ± 10 °C for one hour. In

Fig. 1. Schematic illustrations of fabrication the MCPW. 622


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Fig. 4. Schematic diagram of experimental setup to measure liquid rate-of-rise. Fig. 2. (a) Graphite module and (b) prepared MCPW sample used in the experiment.

5 min. Subsequently, the sample was immersed into 120 g/L NaOH and 5 wt% H2SO4 to remove the grease and oxide on the surface, respectively. After drying by the nitrogen gas, the porous copper substrate was coated with zinc layer through electroplating method. Afterwards, the sample was heated in the furnace with the temperature of 150 °C for 120 min, and Cu-Zn alloy was formed outside the copper powder surface at this moment. Hydrogen was used as the protective atmosphere in this process. Lastly, to remove zinc on the surface, the selective dealloying process was carried out by the 120 g/L NaOH solution in room ambient. It can be seen that the sample color changed into black after withdrawing through the alkaline liquor, as shown in Fig. 2(b). This phenomenon indicated that the nanostructures were presented outside the powder surface.

this study, it should be noted that two types of copper powders, supplied by Beijing Xinrongyuan Powder Tech. Co., China with purity of 99.5%, were used and compared, including spherical type and irregular type. Then three kinds of powder dimensions were sieved in order to optimize the wick design, < 50, 50–100 and 100–125 μm, respectively. Moreover, graphite block was manufactured into the sintering module to make demolding process smoothly, while the size of the cavity inside was 100 × 10 × 2 mm. The nanostructures were fabricated on the above sintered powders by the AD treatment. This process consists of three steps, electroless zinc, heat treatment and corrosion. Initially, the porous substrate was handled by ultrasonic cleaning machine with the deionized water for

Fig. 3. SEM images and EDS pattern of MCPW. (a) Sintered spherical copper powder as substrate, and (b) sintered irregular copper powder as substrate (insets show the sintered spherical and irregular copper powders, respectively); (c) continuous nanostructures on the surface; (d) EDS patterns for MCPW dealloyed by 5 wt% H2SO4 for 30 min. 623


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Table 1 List of sample tested and manufacturing properties. Test code

No.

Effective wick dimensions, length × width × thickness (mm × mm × mm)

Sintering treatment

AD treatment

Wick, powder type

Powder size (μm)

Porosity

Corrosive Solution

Corrosive time (h)

1

P1 P2 P3* P4**

85 × 15 × 2 85 × 15 × 2 85 × 15 × 2 85 × 15 × 2

MCPW, spherical MCPW, spherical MCPW, spherical Sintered, spherical

< 50 50–100 100–125 100–125

0.415 0.425 0.431 0.423

NaOH, 120 g/L NaOH, 120 g/L NaOH, 120 g/L –

12 12 12 –

2

D1* D2**

85 × 15 × 2 85 × 15 × 2

100–125 100–125

0.431 0.423

NaOH, 120 g/L –

12 –

D3 D4

85 × 15 × 2 85 × 15 × 2

MCPW, spherical Sintered, spherical MCPW, Irregular Sintered, Irregular

100–125 100–125

0.648 0.635

NaOH, 120 g/L –

12 –

3

C1* C2

85 × 15 × 2 85 × 15 × 2

MCPW, spherical MCPW, spherical

100–125 100–125

0.431 0.431

NaOH, 120 g/L 5 wt% H2SO4

12 0.5

4

T1 T2* T3 T4 T5 T6**

85 × 15 × 2 85 × 15 × 2 85 × 15 × 2 85 × 15 × 2 85 × 15 × 2 85 × 15 × 2

MCPW, spherical MCPW, spherical MCPW, spherical MCPW, spherical MCPW, spherical Sintered, spherical

100–125 100–125 100–125 100–125 100–125 100–125

0.431 0.431 0.431 0.431 0.431 0.423

NaOH, NaOH, NaOH, NaOH, NaOH, –

4 12 24 36 48 –

120 g/L 120 g/L 120 g/L 120 g/L 120 g/L

* The same MCPW. ** The same sintered wick.

recorded by an IR-camera (FLIR ThermaCAM SC300, thermal sensitivity of 0.02 °C). The test lasts for 75 s. Then the capillary height in the IR thermal images is extracted and analyzed. Furthermore, it should be noted that water and acetone were both used as the working fluid to evaluate the capillary performance accurately. The environment temperature was 25 °C. In this study, the uncertainties of the wicks were estimated to be 1.5% for the porosity. The measurement uncertainty of the capillary rise height was controlled within 1.6%. The uncertainty of the capillary rise test was about 2.5%. In order to optimize the fabrication parameters of MCPW, both of the two procedures, sintering treatment and AD treatment, were classified and researched. Thus, Table 1 illustrates the list of wicks, especially the manufacturing properties tested in the present study, including different powder sizes, powder types, corrosive solutions and corrosion time. All the samples have the same effective dimensions (85 × 15 × 2 mm), as shown in Fig. 2(b).

Fig. 3 shows the SEM images and EDS pattern of resulted MCPW. As can be seen in Fig. 3(a), it is found that the powder size is nearly unchanged after the AD treatment, and the liquid flow channels formed between the powders still exist without blocking as well. However, owing to the nanostructures on the powders, the surface of MCPW with sintered spherical copper powder is dramatically coarser than the traditional sintered porous wick, as shown in the SEM image inset in Fig. 3(a). The above phenomena are also supported by the SEM results of irregular copper powder substrate, as shown in Fig. 3(b). Although the sintered irregular copper powders contain plenty of complicated ravines and gullies on the surface, the nanostructures can still cover the all the powder surface. Fig. 3(c) illustrate the morphology of nanostructures. Then the component analysis for the powder surface of MCPW is characterized by the energy dispersive spectrometer (EDS), as shown in Fig. 3(d). After dealloying with 120 g/L NaOH solution for 12 h, the MCPW is mainly consisted of Cu and Zn. The weight present of copper is about 65.52%, while the zinc content is 34.48%.

2.3. Data analysis for capillary performance parameter 2.2. Experimental setup for capillary rate-of-rise test The wick is of vital importance to the LHP, which determines the heat and mass transfer for the whole loop. Particularly, the capillary limit of the wick generally depends on the following equation [23],

To characterize the capillary performance of the MCPW, the rising liquid front or the intake mass in the pores as the function of time can be taken into consideration [19]. In the present study, the IR thermal imaging method was used to conduct the capillary rate-of-rise experiment, owing to the different emissivity for the working fluid and wick during the capillary flow [22]. Therefore, the liquid meniscus could be captured dynamically as the liquid moving. Fig. 4 depicts the experimental setup for the capillary flow. In detail, the working fluid was filled inside the liquid pool, while the wick was hung vertically below the cantilever support. The moving stage was used to precisely control the movement of the liquid pool. To reduce the environmental effect, a box with observation window was covered on the experimental setup, with black cloth wrapping outside to prevent light influence. During the measurement, the stage was moved upward to make the liquid level touch the wick bottom, and wick length immersed into the working fluid was 3 mm approximately for each sample. Subsequently, the capillary force, produced between the powders, absorbed the working fluid filling into pores and thus the meniscus location went up gradually. The dynamic procedure of the working fluid movement was

Q=

ρl Aw h fg μl l w

(ΔPw K )

(3)

whereQ is the heat flux, ρl and μl are the density and dynamic viscosity of the working fluid, Aw and l w are the cross-sectional area and effective length of the wick, h fg is the latent heat of vaporization, ΔPw is the pressure loss in the wick during liquid flow, and K is the permeability. According to the momentum balance, the capillary pressure is equal to the pressure loss, which can be written as,

ΔPw = ΔPc

(4)

where ΔPc is the capillary pressure. Therefore, ΔPc K can be used to evaluate the heat transfer property of LHP. For the wick operates under the gravity condition, the followed assumptions can be given, (1) incompressible flow inside the wick region; (2) the working fluid has constant properties; (3) the wetted region maintains in uniform saturation condition. Thus, the capillary can 624


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Fig. 7. Effect of powder size on capillary performance of wicks.

Fig. 5. Effect of powder size on the capillary rise processes of MCPW with working liquid. (1) Water; (2) Acetone. Fig. 8. Effect of powder type on the capillary rise processes of MCPW.

be calculated as,

ΔPc =

μl hṁ + ρgh ρKA

Therefore, regarding to the above equations (5) and (6) yields,

dh ΔP K 1 ρgK = c − dt μl ε h μl ε

(5)

where μl is the liquid viscosity, K is the permeability, A is the sectional area of the wick. ṁ is the mass flow rate, which can be expressed by,

ṁ = ρAε

dh dt

(7)

where the capillary performance parameter (ΔPc K ) can be obtained by the method proposed by Deng et al. [13]. The linear fitting between dh/ dt and 1/ h can be performed from the experiment, and ΔPc K is part of the slope for the fitting line.

(6)

Fig. 6. Contact angle of the (a) pure copper plate and (b) copper plate with nanostructure. 625


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reduced correspondingly. Thus, the liquid penetrated into the pores is decreased, thereby inducing the lower capillary rise height and velocity. By contrast, the capillary rate-of-rise character for the sintered wick (P4) is shown in Fig. 5 as well. As expected, the sample exhibits a quick capillary rise velocity initially, but reaches the equilibrium height (19.5 mm) at about 20 s. The P4 result is obviously lower than the MCPW sample with the same powder size (P3). The possible reason may be the difference of contact angle between the solid powder and working fluid. Fig. 6 shows the contact angle between the copper plate and water droplet. It can be seen that the contact angle of copper plate is significantly larger than that of the surface treated by AD treatment. The contact angle of the pure copper plate is 91.5°, while the nanostructure surface is comparatively smaller (22.2°). The capillary pressure can be expressed as,

ΔPc =

2σ cosθ r

(8)

where θ is the contact angle and r is the pore radius. The lower the contact angle is, the higher the capillary force can be presented. Therefore, it can be concluded that the MCPW with better hydrophility is more capable of absorbing water. Fig. 5(b) shows the capillary rateof-rise result with acetone. It can be seen that the variation trends of the curves are almost the same in general. However, compared with the results of water, the capillary height rise fast for acetone. In particular, the meniscuses reach the highest point during the time of 40 s for P2 and P3. To evaluate the capillary performance, the parameters are shown in Fig. 7. The data show upward trend with the increase of the copper powder size. MCPW sample P3 has the highest value (3.01 × 10−8 N), corresponding to the variation of capillary height. Furthermore, it is interesting to find that the capillary performance parameters of the acetone are slightly lower than the those of water. This difference is caused by to the difference of the liquid viscosity. Water viscosity is 0.89 × 10−3 Pa·s, while acetone has the 31% lower viscosity. Hence, it is beneficial for acetone to climb spontaneously along the wick. In comparison, Deng et al. [25] also tested the spherical copper powder with the particle size of 75–100 μm, but they found that the capillary performance was merely 2.0 × 10−8 N. This result is due to the existence of the nanostructures on the powder surface, which can provide both extra capillary force and enough flow paths.

Fig. 9. Effect of powder type on capillary performance of wicks.

Fig. 10. Effect of corrosive solution on the capillary rise processes of MCPW.

3. Results and discussion

3.2. Effect of powder type

3.1. Effect of powder size

Fig. 8 shows the capillary rise process for spherical and irregular copper powders. Provided that the irregular MCPW sample (D3) has the ability to take in more working fluid as the time goes on. It costs about 32 s for the irregular MCPW to reach 77.1 mm, while the spherical MCPW needs more time to achieve the same height (62 s). It can be deduced that the large pores in the irregular sample may be easy to be filled by the capillary flow. The capillary performance parameters are presented in Fig. 9. The capillary performance parameter for irregular MCPW is 3.93 × 10−8 N, which is 27.2% higher than that of the spherical based sample. This distinct difference demonstrates that the powder type plays an important role in the capillary characteristics. Two reasons can be used to explain the above results. Firstly, owing to the complicated shapes and surface topography, the pores formed among the irregular powders was difficult to be blocked, compared with the spherical powders [26]. Secondly, micro and nanostructures on the irregular powder surface, forming tons of tiny pores, could provide extra flow paths for liquid flow. Therefore, MCPW with irregular powder substrate presents a better capillary rise performance. On the other hand, the maximum mass intake of the merely sintered copper powders stays in the same level, approximately 20 mm in height. There is little difference for the capillary performance parameters between sintered irregular and spherical powders, as can be seen

To investigate the effect of powder size on the capillary performance of MCPW, the capillary heights at given times for three kinds of spherical copper powders, spherical < 50, 50–100, 100–125 μm, are compared, as shown in Fig. 5. It can be seen that the capillary height and velocity of all these three samples grow rapidly in the early times of the capillary rise process, and then over time the difference occurs gradually. In Fig. 5(a), the MCPW sample P3 with spherical 100–125 μm yields the largest capillary height, and the slope of the curve is higher than the sample P1 and P2. For example, at t = 62 s, the height is 77.1 mm for P3, which is 136.5% and 33.4% higher than those of P1 and P2, respectively. Hence, as the size of copper powders increases, the capillary performance of MCPW can be enhanced effectively. The result is consistent with the findings of Chan and Kim [10] for the porous wick. They found that when the powder size was lower than 125 μm, the wick with larger powder size could exhibit higher rise capillary height and velocity. This phenomenon is attributed to the fact that there should be conflict between the capillary force and permeability for the wick [24]. For the smaller copper powders, plenty of pores besides the solid power probably tends to close, resulting in the complexity and tortuosity of the liquid flow channels, and the number of channels is 626


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Fig. 11. SEM images of the MCPW with AD treatment by (a) NaOH, 120 g/L; (b) 5 wt% H2SO4; (c) and (d) show the corresponding high-magnification images.

5 wt% H2SO4 solution. It can be seen that the rise height and velocity of NaOH solution sample (C2) is significantly larger than the H2SO4 sample for both water and acetone. The main reason can be attributed to the nanostructures formed after the corrosion process. The overall reaction for the dealloying with NaOH and H2SO4 can be formulated by the following equations respectively: Zn + H2SO4 = ZnSO4 + H2↑

(9)

and Zn + 2NaOH = Na2ZnO2 + H2↑

(10)

In fact, the chemical reaction between zinc and H2SO4 solution is obviously stronger and faster than that for NaOH, especially accompanied with lots of bubbles. Fig. 11 shows the SEM images of the powder surface of MCPW after AD treatment. It can be seen that the sizes of pores and ligaments are apparently different. In detail, the surface of the samples treated with NaOH solution possess continuous nanostructures, composed of nanopores with size of 50 nm and ligament with the size of 30 nm approximately. By contrast, the nanopores and ligaments of the sample treated by H2SO4 solution both distribute randomly. The size of nanopores varies from 50 to 900 nm. SEM study conducted by Lee et al. [27] proved that the dealloying treatment could effectively produce rough porous structure with pore size from 100 nm to 500 nm for Cu-Zn films, except for the difference that the corrosion solution was HCl. Thus, considering about relationship between capillary and contact angle in Eq. (8), it can be deduced that the alkali corrosion can induce a smaller contact angle between the liquid and copper powder. Besides, the comparison of the capillary performance between the alkali and acid solutions also confirm the abovementioned results. As can be seen in Fig. 12, the capillary performance parameters for MCPW with alkali solution are about 20% better than that of acid solution sample.

Fig. 12. Effect of corrosive solutions on capillary performance parameter of wicks.

in Fig. 9. This is not consistent with the findings of Deng et al. [12]. They found that the within gravity condition, capillary height of the sintered irregular wick could achieve a higher position than the spherical wick. The main influential factor is that the working fluid used was ethanol in the reference, but we chose to use water as working fluid.

3.3. Effect of corrosive solution Fig. 10 depicts the effect of various corrosive solutions for the capillary rise processes of MCPW, including 120 g/L NaOH solution and 627


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3.4. Effect of corrosive time To further research the effect of corrosive time for the capillary performance, we also compared the capillary heights and capillary performance parameters for the five different MCPWs (T1, T2, T3, T4, T5). Two types of working liquid, water and acetone, were used in the experiment as the working fluid, as can be seen in Fig. 13. Fig. 13(a) shows the evolution of the recorded capillary heights for the above samples with water. After 40 s test, the meniscus locations for the T1 ∼ T5 climb up to the height 59.37, 62.37, 75.03, 52.81 and 44.91 mm, respectively. Obviously, the MCPW that dealloyed by 24 h has the highest value, and thus there exhibits an optimal solution for the samples. Compared to the sintered wick (T6), nearly 275% increase in the cumulative capillary height is achieved for the T3 sample. Fig. 13(b) also indicates that the sample T3 (24 h) yields the optimal solution, followed by the sample of T4, T2, T5 and T1. It is found that the capillary rise velocity of the acetone is evidently quicker than that of the water, which can reach the maximum value in 30 s. This is due to the fact that the acetone presents the lower dynamic viscosity and surface tension than the water. The capillary performance parameters for the corresponding samples are shown in Fig. 14. It reflects the fact that the curves show upward trend initially, reach the maximum (about 2.25 × 10−8 N) in 24 h, and then decrease. The relative discrepancies of the capillary performance parameters for MCPW are all less than 10% between the water and ethanol, implying a good agreement. Surface morphology studies conducted by Tuan et al. [27] proved that the as with the increase of the corrosive time, the size of pores and ligaments both gradually got larger. Similarly, Kiani et al. [28] studied the influence of densities and distributions of nanoparticles for the contact angle of Cu, and revealed that the presence of more empty spaces between the ligaments could enhance the free energies of the solid-liquid and the solid-gas interfaces. However, when the spacing of the ligament exceeds the certain rang, the nanostructures may induce the opposite variation trend of contact angle, consistent with the findings of Chen et al. [29]. Therefore, it is reasonable to come to the conclusion that the corrosive time 24 h is the favorable time with respect to the enhanced capillary characteristic. Fig. 13. Effect of corrosive time on the capillary rise processes of MCPW with working liquid. (a) Water; (b) Acetone.

4. Conclusions In summary, a novel type of multi-scale composite porous wick, featured with nanostructures on the sintered powder surface, was developed. The fabrication process of the wick mainly included two steps, sintering treatment and AD treatment. The capillary performance of the wick, influenced by the substrate structure and nanostructures, is characterized by the rate-of-rise experiment based on the IR thermal image method. The effects of the copper powder type, powder size, and the morphology of the nanostructures are analyzed and compared in the study to optimize the design. The main conclusions can be summarized as follows: (1) The copper powder substrate was fabricated by the sintering method, and then the nanostructures were coated on the powder surface through the alloying-dealloying treatment. These processes resulted in a hydrophilic wicking surface for the working fluid incorporation into the wick. (2) The powder size and powder type of the substrate both had effect on the capillary performance for the MCPW. The larger powder size (100–125 μm) could yield better capillary character. Additionally, the irregular powder showed higher capillary performance parameter than the spherical copper powder. (3) The AD treatment played the dominant role for the liquid transportation in the wick. The NaOH solution was a more appropriate choice for the zinc corrosion and formation of the nanostructures, when the optimum corrosive time was about 24 h.

Fig. 14. Effect of corrosive time on capillary performance of MCPW.

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(4) Water and acetone were both used in the study as the working fluid for the rate-of-rise experiment. Even though acetone could conduct higher capillary height and velocity for the wick, but the capillary performance parameter results indicated a good agreement for these two liquids.

[10] [11]

[12]

Furthermore, some other parameters are also important for the MCPW, so more researches need to be further investigated in the future to further study the connections between the fabrication parameters and the capillary performance.

[13]

[14]

Acknowledgements

[15]

The authors gratefully acknowledge the project supported by the open fund of Hubei key laboratory of mechanical transmission and manufacturing engineering in Wuhan University of Science and Technology (2017A05), and the Innovation Project of Graduate School of South China Normal University. The authors also acknowledge financial support from the National Natural Science Foundation of China (Nos. 51405166, 201711530647), Natural Science Foundation of Guangdong Province (No. 2016A030310438), Guangdong Innovative Research Team Program (No. 2013C102), Science and technology project of Guangdong Province (No. 2016B090909001), Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (Grant No. 2017B030301007), Guangzhou Key Laboratory of Electronic Paper Displays Materials and Devices (201705030007), MOE International Laboratory for Optical Information Technologies and the 111 Project.

[16]

[17]

[18]

[19] [20]

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