Layer-by-layer assembled carbon nanotube-polyethyleneimine coatings inside copper-sintered heat pipes for enhanced thermal performance

Accepted Manuscript Layer-by-layer assembled carbon nanotube-polyethyleneimine coatings inside copper-sintered heat pipes for enhanced thermal performance Seunghyeon Lee, Jaemin Lee, Hayoung Hwang, Taehan Yeo, Howon Lee, Wonjoon Choi PII:

S0008-6223(18)30803-0

DOI:

10.1016/j.carbon.2018.08.069

Reference:

CARBON 13424

To appear in:

Carbon

Received Date: 16 July 2018 Revised Date:

21 August 2018

Accepted Date: 30 August 2018

Please cite this article as: S. Lee, J. Lee, H. Hwang, T. Yeo, H. Lee, W. Choi, Layer-by-layer assembled carbon nanotube-polyethyleneimine coatings inside copper-sintered heat pipes for enhanced thermal performance, Carbon (2018), doi: 10.1016/j.carbon.2018.08.069. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

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Layer-by-Layer assembled carbon nanotube-

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polyethyleneimine coatings inside copper-sintered heat

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pipes for enhanced thermal performance

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Seunghyeon Lee1‡ , Jaemin Lee 1‡ , Hayoung Hwang 1 , Taehan Yeo 1 , Howon Lee 2 and

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Wonjoon Choi 1 *

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713, Republic of Korea

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School of Mechanical Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul, 136-

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Piscataway, NJ, 08854, USA

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Department of Mechanical and Aerospace Engineering, Rutgers University, 98 Brett RD,

* Author to whom any correspondence should be addressed: Wonjoon Choi

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E-mail: [email protected], Phone: +82 2 3290 5951, Fax: +82 2 926 9290.

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‡ These authors contributed equally to this work.

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Keywords: heat pipe; layer-by-layer; carbon nanotubes; boiling heat transfer; biporous structure

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Abstract

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Biporous structures at the nano–microscale are promising candidates for controlling phase

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change heat transfer, through their enhanced capillary wicking and fluid transportation. However,

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existing methods for fabricating biporous structures involve complex process which is not

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suitable for small-scale thermal devices such as heat pipes, owing to their confined and non-flat

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inner structures. Herein, we report the biporous structures inside copper-sintered heat pipes,

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enabled by layer-by-layer (LbL) assembled multi-walled carbon nanotube (MWCNT)-

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polyethyleneimine (PEI) coating for enhanced thermal performance. The repetitive filling and

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removing of the oppositely charged solutions with MWCNT-PEI and carboxylic-functionalized

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MWCNTs assembled the nanoporous MWCNT-PEI coatings (10, 20, and 40 bilayers) on the

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microporous copper-sintered inner surfaces. The fiber-like MWCNT networks structurally

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manipulated morphology and thickness of biporous structures, while the hydrophilic PEI shells

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chemically optimized wettability. A reduced thermal resistance (~14.3 %) was observed for

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MWCNT-PEI coating in 10 bilayers, due to the enhanced capillary wicking, interfacial contact

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areas, and bubble dynamics, whereas the 40 bilayers did not exhibit improved thermal

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performance owing to the redundant nanoporous layers causing reduced volume of microporous

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structures and increased thermal resistance. The LbL-assembled MWCNT-PEI coatings would

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act as functional layers to improve the performance of miniaturized and thin-film-based thermal

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devices.

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1. Introduction

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Thermal management is essential for improving or limiting the performance of various small-

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and large-scale platforms including micro-electronic devices, smartphones, laptops, drones, and

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vehicles. In particular, due to the enhanced performance of integrated circuits and miniaturized

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portable electronics, the control of thermal energy is becoming increasingly important in locally

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confined spaces and thin film-based devices. [1] For instance, rising temperature due to restricted

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structures in micro/nanoscale platforms has emerged as a significant obstacle for further

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enhancing overall functions and operational stability.

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Liquid-vapor phase change heat transfer enables the effective management of heat flux in

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restricted and confined spaces because it is superior to single-phase heat transfer due to the

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utilization of latent and sensible heat. Its outstanding capability has allowed the development of

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unique heat-transfer devices that can be integrated into small-scale platforms and large-scale

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systems. [2, 3] One of the most crucial factors in liquid-vapor phase change heat transfer is the

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development of optimal interfacial structures that contact the working fluids. The

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physicochemical properties of the interfaces including wettability [4], roughness [5], and

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porosity [6] have been manipulated to adjust liquid-vapor phase change heat transfer as they

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control bubble formation and departure as well as transportation of the working fluids. Porous

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structures have been intensively explored to obtain desirable surface properties because they

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intrinsically affect the working fluid transport-circulation and nucleation site density, by

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introducing the capillary wicking force and atomized bubble formation-departure at the pores. [7,

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8] Micro–nanostructured materials at the interfaces have been considered as promising

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candidates to modify the porous structures, in terms of the surface morphologies and

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physicochemical properties. [9] Various interfacial structures composed of ZnO [10], SiO2 [11],

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TiO2 [12, 13], CuO [14], and carbon nanotubes (CNTs) [15, 16] have allowed for the exploration

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of the underlying mechanisms of enhanced heat transfer characteristics and property

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optimization. For instance, vertically aligned nanowires [17] and flower-like structures [18]

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achieved the enhancement of the heat transfer coefficient (HTC) and critical heat flux (CHF).

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Recently, biporous structures have been investigated for the effective enhancement of phase

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change heat transfer. Biporous structures exhibit distinct pore sizes at different scales, whereas

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monoporous structure is comprised of a single pore size. Because of the strengthened capillary

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wicking forces in multiscale pores and the increased number of available pores [19], biporous

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structures have exhibited significant improvement in terms of HTC and CHF values. [20, 21]

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The enhanced capillary wicking force in the biporous structure extends the interfacial contact

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surfaces of the working fluid with the substrate, promotes working fluid transportation, and

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enables heat flux amplification from the heat source and departure of the bubbles attached to the

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substrate. [22] The small pores in the biporous structures facilitate working fluid transportation

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and escaping vapors on a different scale and their synergistic effects with the larger pores result

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in higher capillary wicking force and improved vapor separation. [23] The increased number of

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different sized pores reduces the threshold for bubble formation and controls the initiation of

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superheated status as well as the transition from free convection to nucleate boiling. [20]

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Among the various platforms incorporating liquid-vapor phase change heat transfer, heat pipes

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are effective for controlling the thermal energy distribution in a limited and confined space. [24]

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Heat pipes are composed of sealed metal containers containing working fluids such as deionized 4

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(DI) water, acetone, ethanol, or other refrigerants. Phase changes of the working fluid occur at

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both ends of the pipe (evaporation and condensation) and the intrinsic pressure difference

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between the two phases facilitates the transport of a large amount of heat along the inner channel

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without additional driving forces and with no significant heat loss. [25] Thus, heat pipes have

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been developed as core components of thermal energy controls in various applications such as

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portable electronic devices including laptops, smartphones [26], air conditioning systems [27],

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and solar heaters. [28] In particular, the inner structures which are directly exposed to the

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working fluids exert significant effects on the thermal characteristics and overall performance of

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the heat pipes. While the inner morphologies of heat pipes are usually fabricated as wicking

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surfaces formed by metal-sintered or periodically arranged structures such as groove or mesh-

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type structures, [29] the sintered metal structure inner morphology have exhibited the best

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thermal performance to date due to their superior porosity and number of nucleation sites

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compared to others. [30] However, sintering for the fabrication of heat pipe inner surfaces can

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only achieve randomly isolated and monoporous structures, whereas bi- or triporous structures

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have shown potential for further enhancement of thermal performance.

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Recent studies on optimizing surface characteristics in heat pipes have focused on the application

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of various nanofluids. [31] The use of nanofluids inside heat pipes enables the gradual deposition

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of nanomaterials in suspension onto the inner surfaces. [32, 33] According to these mechanisms,

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the raw surfaces of the heat pipes were modified using micro-nanostructured coatings that

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transformed the wettability and porosity during liquid-vapor phase change heat transfer. [34]

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However, the sedimentation or slipping of nanofluids inevitably causes variation in the thermal

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characteristics of the pipe, which prevents consistent performance. [35] In addition, the

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nanostructures deposited by the nanofluids are not sufficiently strong to maintain uniform and

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stable performance owing to their weak adhesion depending on the physical contact with the

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inner surface of the pipe. [15] Furthermore, since the precise and accurate pre-characterization of

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inner morphology is limited for nanofluids, it is difficult to conduct fundamental studies to

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investigate the correlation between the inner micro-nanostructures and thermal performance.

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Layer-by-Layer (LbL) deposition is a facile method for the fabrication of uniform thin-film

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micro-nanostructure layers on various substrates. [36, 37] The working principle of LbL

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deposition is the electrostatic adhesion between differently charged micro-nanomaterials in a

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single bilayer formed through repetitive alternating immersion of the substrate into a

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positively/negatively charged solutions. The solid electrostatic force and repetitive cleaning

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process with a non-charged solution leads to stable and uniform layer formation on the target

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substrate. [38] Moreover, the LbL technique is low-cost and solution-based [39], while

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conventional methods require the application of bulky chambers, high temperature, and vacuum

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conditions. [40] In addition, the LbL method is generally applicable regardless of surface

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morphology or space confinement by means of immersion [41], spin-coating [42], or spray-based

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deposition [43]. The optimal tuning of processing parameters such as pH of the charged solutions

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[44], contact duration to adjust layer thickness [45], and the types of LbL technique (immersion,

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spray, or spin-coating [46]) offers precise control of the resulting physicochemical properties. It

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has been demonstrated that CNT-based bilayers exhibit highly uniform and well-defined

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structures owing to their functionalization in the charged solution and surface characteristics. [16]

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Furthermore, CNT-based interfaces between the working fluid and metal heat transfer substrates

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have exhibited outstanding thermal performance. CNTs formed ideal fiber-like structures at the

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nanoscale for the biporous structures on micro-porous surfaces, while their wettability can be

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modified from hydrophobic to hydrophilic by combination with other materials in a single

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bilayer. [47] In addition, the superior thermal conductivity of CNTs minimizes thermal resistance

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at the contact interfaces with the heat transfer substrate. [48] The use of CNTs was effective in

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changing both the HTC and CHF in pool [49, 50] and flow boiling heat transfer. [51, 52]

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Herein, we report the facile fabrication of biporous structures composed of LbL assembled

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multiwalled carbon nanotube (MWCNT)-polyethyleneimine (PEI) on the inner surfaces of

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copper-sintered heat pipes and their enhanced thermal performance (Figure 1). To form the

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MWCNT-PEI coatings, negatively and positively charged solutions were prepared with

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MWCNTs and PEI. The functionalization of the MWCNTs with carboxylic groups provided the

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negative charges, whereas PEI is a polyelectrolyte with inherent positive charge. The repetitive

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filling and removal of the prepared solutions allowed the fabrication of fine and uniform coatings

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of the nanoporous MWCNT-PEI layers on the microporous copper-sintered inner surfaces of

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circular heat pipes. The processing conditions were designed to only deposit the MWCNT-PEI

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on the evaporator section facilitating the liquid-vapor phase change heat transfer as its effects

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from the biporous structures could be easily evaluated. The MWCNT-PEI coating was deposited

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in 10, 20, and 40 bilayer thicknesses to investigate the influence of physicochemical properties

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including morphology, thickness and wettability on the thermal characteristics of the heat pipes.

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The as-prepared and modified heat pipes with copper microstructures and nanoporous MWCNT-

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PEI coatings of 10, 20, and 40 bilayers were compared in terms of their thermal performances

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such as wall temperature distribution, thermal resistances and HTC. The developed LbL method

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using repetitive filling and removing inside the circular heat pipes offers the versatile utilization

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for forming the biporous structures in narrowly confined spaces which have critical limitations in

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terms of the inner coatings. Furthermore, the enhanced thermal performances using the

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MWCNT-PEI coatings inside the inner circular surfaces confirm the contribution of the biporous

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structures for facilitating the working fluid transportation between the evaporator and condenser

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sections in the sealed heat pipes.

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2. Experimental

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2.1 Preparation of copper-sintered heat pipes

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Copper-based heat pipes were selected for the as-prepared heat pipes because they exhibit high

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thermal performance compared to pipes constructed from other structures and materials. The as-

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prepared heat pipes were purchased from Skycares. All heat pipes were cylindrical, having a

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diameter of 8 mm and a length of 350 mm. A working fluid is DI water and the operating

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temperature range is from 5 ℃ to 250 ℃. The inner morphologies of the heat pipes were

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composed of copper-sintered particles with a diameter of a few tens of micrometers. Before

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subsequent processing, the internal structures of the heat pipes were rinsed with deionized (DI)

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water to remove impurities and clean the copper-sintered particles.

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2.2 Preparation of positively and negatively charged solutions of MWCNTs and PEI

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The MWCNTs were purchased from JEIO, Korea (95 wt% purity, 5–20 µm in length, 20–40 nm

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in outer diameter), and used to prepare the negatively and positively charged MWCNT solutions

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for the LbL-based deposition. To prepare the negatively charged MWCNT solution, the

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MWCNTs were first refluxed in an acidic solution of H2SO4/HNO3 (3/1 v/v, 95–98%/70%

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concentration) at 70 °C for 2 h. [53] After refluxing, filtration was performed to obtain the

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MWCNTs, which were then washed with DI water. The obtained MWCNTs were dried for 24 h

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to remove excess solvent under open-air and at room temperature. As a result, the surfaces of the

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MWCNTs were functionalized with carboxylic groups (-COOH) with negative charges. The

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carboxylic-functionalized MWCNTs (0.1 mg/mL) were sonicated for 1 h in DI water to form a

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stable dispersion, and the negatively charged MWCNT solution was generated. To prepare the

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positively charged MWCNT-PEI solution, polyethyleneimine (PEI, 50% w/v in water) was

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purchased from Aldrich. The carboxylic-functionalized MWCNTs (0.1 mg/mL) were mixed with

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PEI (0.5 mg/mL) and sonicated for 1 h in DI water in a cold bath. During this process, the PEI

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with intrinsic positive charges adhered to the surfaces of the carboxylic-functionalized

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MWCNTs through electrostatic attraction. The excess PEI was removed via centrifugation and

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the MWCNT-PEI sediments were sequentially sonicated in DI water, affording the positively

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charged MWCNT-PEI solution. [54]

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2.3 Fabrication of LbL-assembled MWCNT-PEI layers inside the circular heat pipes

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The carboxylic-functionalized MWCNT solution, MWCNT-PEI solution, and DI water were

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prepared as the positively/negatively charged and rinsing solutions, respectively, for the

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repetitive LbL deposition of MWCNT-PEI bilayers. To form MWCNT-PEI shells on the copper-

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sintered particles inside the circular heat pipes, the MWCNT-PEI, rinsing, and carboxylic-

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functionalized MWCNT solutions were sequentially injected into the circular heat pipes and

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equilibrated for 5, 1, and 5 min, respectively. One injection cycle formed a single bilayer of the

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LbL-assembled MWCNT-PEI/MWCNT shell on the copper-sintered microstructures inside the

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heat pipes. The positively and negatively charged solutions were replaced with fresh solutions

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after the deposition of every 5 bilayers. During the deposition process, 4 mL solutions were

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injected into the circular heat pipes to contact the evaporator section. The injection, removal, and

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rinsing processes were repeated to form the desired numbers of bilayers inside the heat pipes.

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After electrostatic adherence and rinsing, the MWCNTs weakly attached by physical contact

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were selectively removed, leaving the solid and uniform LbL-assembled MWCNT-PEI shells on

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the copper-sintered microstructures. Three MWCNT-PEI shells composed of 10, 20, and 40

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bilayers were fabricated to investigate the effects of biporous structure thickness on the thermal

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performances of the heat pipes.

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2.4 Characterization of the biporous structures of MWCNT-PEI shells and copper-sintered

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particles

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The zeta potentials of the positively and negatively charged solutions containing MWCNT-PEI

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and carboxylic-functionalized MWCNTs, respectively, were measured using a zeta potential and

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particle size analyzer, ELSZ-1000. The surface morphology of the biporous structures of the

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LbL-assembled MWCNT-PEI and copper-sintered microstructures were characterized using field

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emission scanning electron microscopy (FE-SEM) under low vacuum (Quanta 250 FEG). The

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thicknesses of the MWCNT-PEI bilayer coatings on the bare copper substrates were measured by

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Alpha Step (AS-IQ Kla-tencor). Static contact angle measurements were performed using a

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Krüss EasyDrop contact angle measurement system.

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2.5 Measurement of the thermal performances of as-prepared and LbL-assembled heat

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pipers with different MWCNT-PEI bilayer thicknesses

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The thermal performances of the as-prepared and LbL-assembled MWCNT-PEI-coated heat

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pipes with 10, 20, and 40 bilayers were tested by measuring thermal resistance, wall temperature

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distribution, and HTC. All heat pipes were evacuated by vacuum pump and filled with 3.5 mL of

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DI water as a working fluid. All experimental measurements were conducted under open-air and

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room temperature conditions. The mounting platform fixed and maintained all heat pipes at 45°

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angles. Thermal performance was measured from three sections of the heat pipes; evaporator,

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adiabatic, and condenser sections. K-type thermocouples were attached to the surfaces of each

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section and a DC power supply (UDP-1500 30 V-50 A, UNICORN) provided direct heating of

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the wires to supply controlled heat and power for the evaporator section. A water jacket

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connected to a water circulation device at a constant temperature (~20 °C) removed the

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transported heat from the condenser. The adiabatic section was surrounded by glass wool to

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minimize heat loss. The initial heating power was set to 20 W and was gradually increased to 100

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W. The wall temperature of the heat pipes was recorded in real-time at the saturated states using

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a data acquisition system (cDAQ NI 9213, National Instruments) at the saturated states. In the

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heat pipes, thermal resistance is a significant parameter which indicates the overall thermal

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performance.

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The average wall temperatures of the evaporator and condenser are denoted as respectively. Input heating power at the evaporator,

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equation.

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can be estimated through the following

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V and I are the input voltage and electric current. The uncertainty of voltage and current

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measured by the digital multimeter were ±1.8 % and ±2 %, respectively. The accuracy of

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temperature measurement system was ±1.3%. According to the propagation of error method, the

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following equation was obtained for the uncertainty analysis including heat loss.

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=

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The uncertainty of the experimental setup and heat loss is summarized in Table 1 with reference

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to the instruction of equipment.

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3. Results and Discussion

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3.1. LbL-assembled MWCNT-PEI coatings for biporous structures

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The negatively and positively charged solutions of carboxylic-functionalized MWCNTs and

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MWCNT-PEI were prepared for the LbL deposition. The functionalization of carboxylic groups

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(-COOH) with intrinsic negative charge on the MWCNTs were stably dispersed in DI water. On

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the other hand, mixing and sonicating the carboxylic-functionalized MWCNTs and PEI with its

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intrinsic positive charge enabled the formulation of the positively charged MWCNT-PEI solution.

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The optimal surface charges of the negatively and positively charged MWCNTs are essential to

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form a stable dispersion in DI water and fabricate the LbL-assembled coatings, which adhere to

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the substrate by electrostatic force. Depending on the degree of ionization of the materials in the

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solution, the physicochemical characteristics of the LbL-assembled coatings, such as thickness,

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composition, wettability, and surface charge can be tuned. [53] As a mismatch of the degree of

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ionization between the positively and negatively charged solutions increases, more material

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would be needed to form LbL-assembled coatings to balance the electrostatic charges of the

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oppositely charged layer. [55] After the prepared solutions were optimized, Zeta potentials

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indicating the degrees of the ionization of the MWCNTs were determined as -61.58 and +49.43

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mV for the negatively and positively charged solutions of carboxylic-functionalized MWCNTs

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and MWCNT-PEI, respectively. The balanced Zeta potential magnitudes allowed the fabrication

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of an evenly distributed carboxylic-functionalized MWCNTs and MWCNT-PEI bilayer via LbL

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deposition.

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Copper-sintered, circular heat pipes were prepared as base devices for LbL deposition due to

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their superior thermal performance compared to other structures and materials. Their diameters

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and lengths were 8 and 350 mm, respectively. The carboxylic-functionalized MWCNT solution,

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MWCNT-PEI solution, and DI water were used as the positively/negatively charged and rinsing

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solutions, respectively, for the repetitive LbL deposition of MWCNT-PEI bilayers. The solutions

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were sequentially injected into the internal space of the circular heat pipes. The carboxylic-

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functionalized MWCNT and MWCNT-PEI solutions were kept in the tubes for 5 min, whereas

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the rinsing with DI water was performed for 1 min. The rinsing process was required to remove

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weakly attached materials through the physical contact with the copper pipe, resulting in solid

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and uniform bilayers maintained via electrostatic forces. One cycle of sequentially filling and

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removing the pipe with MWCNT-PEI, DI water, carboxylic-functionalized MWCNTs, and DI

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water resulted in formation of a single bilayer of MWCNT-PEI (Figure 1). Repetition of this

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cyclic process enabled the successive deposition of multiple bilayers inside the narrowly

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confined structures of the circular heat pipes having intrinsic limitations of the formation of the

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coatings, whereas other LbL methods using immersion[16, 41], spin-coating[42], or spray-based

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deposition[43] were not capable to form the uniform bilayers owing to structural restrictions.

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Through experimental observation, the optimal amount of the charged solutions required to fill

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the inner space of the circular heat pipes and deposit MWCNT-PEI on the copper-sintered

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microstructures was 4 mL. The 10, 20, and 40 bilayer MWCNT-PEI coatings were applied to the

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inner copper-sintered structures of the circular heat pipes and, as a result, a distinct color change

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after LbL deposition was observed, as shown in Figure 1. The entire LbL deposition process was

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conducted simply under open-air and room temperature conditions without the need for bulky

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equipment setups and high-temperature processing. This unique LbL processing technique

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represents a facile route to achieve fine and uniform coatings on copper heat pipes.

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Physical characterization of the biporous structures of the LbL-assembled MWCNT-PEI coatings

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was performed by scanning electron microscopy (SEM). The bare and MWCNT-PEI-coated heat

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pipes were cut and treated to expose the inner structures for SEM measurements. The internal

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surface morphology of the bare heat pipes exhibited randomly agglomerated structures,

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comprised of copper-sintered particles of a few tens of micrometers in size (Figure 2a). This

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indicated that the original inner surface morphology consisted of a monoporous microstructure

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and the individual copper microparticles exhibited a smooth surface without any other porous

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structure. The inner surface morphology of the MWCNT-PEI-coated circular heat pipes treated

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by LbL deposition clearly showed nanoporous shells, which were composed of the fiber-like

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MWCNTs, while preserving the original copper-sintered microstructures (Figure 2b and 2c).

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The nanoporous structures contained randomly oriented and entangled fiber-like structures

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originating from the MWCNT networks. In other words, the biporous structure at the nano- and

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microscales was observed in the inner surface morphology of the pipe. The multiscale biporous

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structures can facilitate capillary wicking to transport and circulate the working fluid in the

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liquid-vapor phase. In addition, the structure increased the number of active sites for heat transfer

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and increases nucleation density, thereby enhancing its capability for the effective heat transfer.

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Furthermore, the MWCNTs and PEI manipulate the physicochemical features of the biporous

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structure and enhance the thermal performance of the heat pipes due to their intrinsic properties.

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In the LbL-assembled MWCNT-PEI coatings, the MWCNTs offer excellent thermal properties

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for phase change heat transfer [56, 57], superb chemical-mechanical stability [58], and

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nanoporous fiber-like morphology [59]. The PEI changes the wettability from hydrophobic to

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hydrophilic in combination with the hydrophilic MWCNTs and provides solid adhesion between

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the bilayers and copper-sintered surface through the difference in electrostatic charge.

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The thickness of the nanoporous coating on the copper-sintered microstructures can significantly

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affect the overall thermal performance because of its influence on capillary wicking and working

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fluid circulation. As the number of MWCNT-PEI bilayers in the LbL coating increased, a clear

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increase in overall thicknesses should be observed in the coated shells. [60] However, direct

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measurement of the MWCNT-PEI coating thickness on the copper-sintered microstructures such

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as SEM, profilometers, and atomic force microscopy was not feasible owing to the extremely

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high surface roughness. Other methods using the refractive indexes were not suitable because the

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randomly aggregated MWCNTs on the surfaces and the multiple layers of the copper particles in

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depth interrupted the accurate measurements. Energy dispersive X-ray spectroscopy could only

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provide the overall increasing trend of the MWCNT-PEI coating thickness, as the number of

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bilayers increased (Figure S1). The high-curvature surfaces of microstructures caused

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defocusing of the interfacial boundaries and the quantitative measurements of the MWCNT-PEI

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coating thicknesses were not feasible. Therefore, an alternative LbL deposition of the MWCNT-

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PEI coatings was conducted on a flat and polished copper substrate, which offered an

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environment similar to the heat pipes, since the LbL deposition has been proven that the coating

18

thickness of the bilayer depends on the types of substrate materials and charge differences of

19

solutions rather than structural variation. The uniformity of the nanoporous MWCNT-PEI

20

coatings were confirmed by the SEM measurement in Figure 2 and Figure S1. The average

21

thicknesses of the MWCNT-PEI coatings on multiple samples were 310, 600, and 820 nm for the

22

10, 20, and 40 bilayer coatings, respectively (Figure 3). The same immersion and rinsing

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conditions for the relevant solutions for the copper pipes were applied to the flat copper

2

substrates in terms of duration and solution sequence/composition. The growth of LbL coating

3

thickness was linearly proportional to the number of bilayers, while the electrostatic force

4

dominated the interaction between the bilayers. [60] However, when the electrostatic force is

5

saturated, it is insufficient to form new bilayers with the same thickness. Therefore, the growth

6

rate saturates at a critical point, and the thickness of a single bilayer gradually decreases. [61]

7

This trend is clearly observed in the overall coating thicknesses of the MWCNT-PEI on the

8

copper substrates, as shown in Figure 3. The MWCNT-PEI coating exhibited a linearly

9

proportional thickness from 10 to 20 bilayers. However, between 20 and 40 bilayers, the increase

10

in thickness reduced by less than two-fold, indicating a saturation of the formation of the

11

MWCNT-PEI layers via electrostatic forces.

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The wettability of the inner surface morphology in the heat pipes is crucial for manipulating the

14

contact interfaces between the solid and liquid-vapor phase which affects the phase change in the

15

evaporator when thermal energy is supplied. To characterize the MWCNT-PEI coatings, the

16

wettability transitions were evaluated on the copper substrates for the 10, 20, and 40 bilayer

17

coatings (Figure 4). The static contact angles of the MWCNT-PEI coatings in the 10, 20, and 40

18

bilayer coatings were 75, 67.9, and 51.8°, respectively. Since MWCNTs are hydrophobic in

19

nature, [62] the intrinsic contact angle of the CNTs reached 167 ± 3°. [63] The nanoporous

20

structure formed by the MWCNT network might cause capillary wicking to imbibe the

21

contacting water. [7] In contrast, since PEI has an intrinsically hydrophilic nature, [64] its

22

addition to the MWCNTs resulted in a significant decrease in the contact angles, as shown in

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Figure 4. Moreover, as the thickness of the MWCNT-PEI coating increased with increasing

2

numbers of bilayers, the contact angles decreased. This indicated that the hydrophilicity was

3

amplified by the increasing amount of PEI and thickness of the nanoporous structures. Because

4

the hydrophilic nature promotes capillary wicking during boiling heat transfer, it can improve

5

liquid circulation at contact interfaces and promote the continuous formation of atomized

6

bubbles. [65] Simultaneously, these properties can delay drying inside the heat pipes.

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3.2. Thermal performance of heat pipes with LbL-assembled MWCNT-PEI coatings

11

Experimental measurements of the key characteristics of the heat pipes, including wall

12

temperature distribution, HTC, and thermal resistance were performed to evaluate various

13

aspects of thermal performance of the bare and LbL-assembled MWCNT-PEI-coated heat pipes.

14

All heat pipes were evacuated by vacuum pump and filled with 3.5 mL of DI water as a working

15

fluid in test environments featuring open-air and room temperature conditions. Thermal

16

performance was measured from three sections of the heat pipes; evaporator, adiabatic, and

17

condenser sections (Figure 5a). K-type thermocouples were attached to the surfaces of the three

18

sections of the heat pipes. The overall features of the experimental setup are described in Figure

19

5b. All heat pipes were fixed at a 45° angle on the mounting platform and a DC power supply

20

provided direct heating of the wires to control the heat and power applied to the evaporator

21

section. The water jacket operated with water circulation at a constant temperature (~20 °C)

22

removed the transported heat from the condenser section. The adiabatic section was protected

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and surrounded by glass wool to minimize heat loss. The heating power was initially 20 W and

2

was gradually increased to 100 W. The wall temperature of the heat pipes was recorded in real-

3

time at saturated states using a cDAQ data acquisition system from National Instruments.

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The wall temperature of the heat pipes as a function of input power was investigated to estimate

6

the thermal resistances and HTC values. Instruments including the power supply, water pump,

7

chiller, and data acquisition systems were optimized for the thermal performance measurements.

8

Initially, the evaporator section of the heat pipes was heated with 20 W input power from the DC

9

power supply. Gradual increases up to 100 W by 20 W increments were applied to the bare and

10

LbL-assembled 10, 20, and 40 bilayer MWCNT-PEI-coated heat pipes (Figure 6a–6e). The wall

11

temperatures of the heat pipes reflect the overall thermal performance, which indicates the

12

capability for effective thermal energy transport. [66] As shown in Figure 6, the wall

13

temperatures of the LbL-assembled 10, 20, and 40 bilayer MWCNT-PEI-coated heat pipes with

14

biporous structures were lower than that of the bare heat pipes with monoporous structure at the

15

evaporator. These results confirm that the nanoporous structures improved thermal energy

16

transport inside the heat pipes by promoting boiling heat transfer in the evaporator section. The

17

internal structures of the bare heat pipes exhibited monoporous structure on the order of a few

18

tens of micrometers, whereas the additional coating of the LbL-assembled MWCNT-PEI formed

19

a biporous structure with nano and microscale pores which significantly changed the overall

20

properties of boiling heat transfer. The nanoporous percolation networks on the microstructures

21

exerted a stronger capillary force and a larger number of pores were available compared to that

22

of the monoporous structure. In terms of the nucleate boiling, the nucleate site density was

23

increased in the coated samples, which was directly related to heat removal capability from the

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surface. [67] In addition, the coated samples exhibited a lower threshold for the initiation of

2

nucleate boiling and more stably sustained nucleation through the smaller nucleate sites and

3

promoted bubble departure. [20] Meanwhile, the nanocavity created by the nanoporous structure

4

could promote the generation of the nanobubbles which facilitate the growth and formation of

5

the bubble in the microcavity. [64] Furthermore, the fiber-like structures of the MWCNTs guided

6

the dynamic circulation of the working fluid inside the porous structures resulting an increased

7

number of active sites. These physical mechanisms enabled by the biporous structure led to the

8

higher heat flux and increased heat removal via the LbL-assembled MWCNT-PEI shells

9

surrounding the copper-sintered microstructures, resulting in an overall reduction in the wall

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temperatures in the evaporator.

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The transition of the wettability is a crucial factor in reducing the wall temperatures in the LbL-

13

assembled MWCNT-PEI-coated heat pipes. The PEI is hydrophilic in nature, whereas the

14

MWCNTs are hydrophobic. Regarding the role of the hydrophobic and hydrophilic properties in

15

the phase change heat transfer, the formation of porous hydrophilic percolation networks has

16

generally led to enhanced performance. During the preparation of the positively charged solution

17

for LbL deposition, PEI wrapped the MWCNTs forming a core-shell structure. This indicated

18

that the surfaces of the LbL-assembled MWCNT-PEI layer might change the hydrophobic

19

natures of the MWCNTs, as shown in Figure 4. Hydrophobic surface properties in the porous

20

network impede working fluid transportation, and changes in wettability to a more hydrophilic

21

state could alleviate this limitation. [16] In the biporous structure, the hydrophilic surface

22

properties of the MWCNT-PEI facilitated the penetration of the working fluid through the

23

percolation networks and increased the contact area. [65] Moreover, the improved circulation of

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the working fluid promoted active bubble departure from the surface and delayed vapor film

2

formation.

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The condenser section exhibited different properties depending on the numbers of LbL-

5

assembled MWCNT-PEI bilayers. The 10 and 20 bilayer nanoporous MWCNT-PEI coatings

6

resulted in remarkably higher temperatures at the condenser compared to the bare heat pipes,

7

especially at high power inputs ranging from 60 to 100 W. The evaporated working fluid should

8

move from the evaporator section to the condenser section by means of the pressure difference

9

between both sides. Because the nanoporous LbL-assembled MWCNT-PEI facilitated the phase

10

change and boiling heat transfer through increased surface area and number of nucleation sites,

11

more working fluid evaporated in the confined structures. The hydrophilic nature of the

12

MWCNT-PEI likely resulted in the increase of interfacial areas between the working fluid and

13

the biporous structure, promoting working fluid departure from the contact surface compared to

14

the copper-sintered monoporous structures. From these mechanisms, a larger pressure difference

15

between the evaporator and condenser sections was generated, resulting in enhanced transport of

16

the working fluid inside the heat pipes coated with MWCNT-PEI layers. On the other hand, the

17

excessive nanoporous coating in the 40 MWCNT-PEI bilayer samples showed no distinct

18

improvement compared to the other samples. The temperature of the 40 MWCNT-PEI bilayer-

19

coated heat pipes at the condenser was similar to that of the bare heat pipes. This indicates that a

20

moderately thick nanoporous structures enabled the complementary relations of capillary

21

wicking and departure of the working fluid from the evaporator to the condenser. Excessively

22

thick nanoporous layers could cause excessive capillary force that may trap the working fluid

23

inside the biporous layer and prevent bubbles departure. [23]

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1

Determination of the thermal resistance of the heat pipes can be used to objectively evaluate

3

changes in thermal performance in terms of the heat transfer before and after the application of

4

the 10, 20, and 40 bilayer LbL-assembled MWCNT-PEI coatings (Figure 7). The thermal

5

resistance, , can be defined as the ratio of the temperature difference at the evaporator and

6

condenser and heat input,

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Where

8

evaporator and condenser sections, respectively. The heat input,

9

multiplying the input voltage and current. When the low thermal loads were supplied, the bare

10

and LbL-assembled MWCNT-PEI-coated heat pipes maintained low thermal resistances and

11

showed no significant difference because the phase change heat transfer and working fluid

12

transportation were not fully activated. [66] As the thermal loads increased, the difference in the

13

thermal resistance as a function of LbL-assembled coating thickness was observed (Figure 7a).

14

The LbL-assembled MWCNT-PEI in 10 and 20 bilayers showed significantly lower thermal

15

resistances in the range of 20 to 100 W compared to the bare heat pipes, whereas the 40 bilayer

16

coating did not exhibited enhanced performance as the thick coating increased the thermal

17

resistance. This indicates that excess nanoporous layers caused excessive capillary force that

18

trapped the working fluid inside the biporous layer and prevented bubble departure from the

19

porous surfaces. On the other hand, the optimal nanoporous coating layer thickness of 10 and 20

20

bilayers reduced the thermal resistance and resulted in higher thermal capacity and effective

21

thermal transport between the evaporator and condenser compared to the bare heat pipes with

and

are the average temperatures of the can be estimated by

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is the heat input and

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monoporous structure. The reduction rate of thermal resistance increased by 14.3% in the 10

2

bilayer MWCNT-PEI-coated samples, indicating outstanding performances, whereas the thicker

3

20 and 40 bilayer coating was saturated in terms of the positive effects induced by the

4

nanoporous layers. The higher number of bilayers might adopt denser structures and thicker

5

layers than the 10 bilayer coating. However, optimization of the porosity is especially important

6

in the evaporator. As the number of bilayers increases over 20 bilayers, the fraction of the

7

copper-sintered microstructure porosity is reduced, resulting in a critical decrease in the number

8

of micro-sized pores. Furthermore, the excessive thickness of the nanoporous layers could

9

prohibit the transportation of the working fluid and increase thermal resistance between the

10

heater and working fluid along the axial direction. Because the inner microstructure of the heat

11

pipes was fixed, redundant nanoporous layers which are not involved in the interaction with the

12

working fluid decrease the proportion of copper-sintered particle microstructures and increase

13

the thermal resistance of the heat pipes. Moreover, excessive decrease in the number of

14

microscale porous structures could cause bubbles to be trapped between the pores and prevent

15

bubble departure from the biporous structure.

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The overall thermal performances can be described by the HTC values of the heat pipes. The

18

HTC, ℎ, can be defined by the following equation:

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h =

1

19

where

20

assembled MWCNT-PEI-coated heat pipes are shown in Figure 7b. The HTC values of all tested

21

heat pipes were proportional to the heat input. Similar to the thermal resistance, the HTC value

is the area of the evaporator which is heated. The HTCs of the bare and LbL-

23

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1

of the 10 bilayer MWCNT-PEI coating showed the best performance, whereas the 40 bilayers

2

coating reduced the overall HTC.

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Copper-sintered heat pipes were selected for the as-prepared heat pipes because they exhibit high

5

thermal performance compared to pipes constructed from other structures and materials. The

6

applications of the LbL-assembled MWCNT-PEI coatings for other types of the heat pipes

7

having smooth surfaces or mesh-like wick structures would achieve the higher enhancement ratio

8

in terms of thermal performances, because it can maximize the contribution of the nanoporous

9

layers on the relatively flat surfaces. Furthermore, the developed nanoporous coatings can be

10

adequate to provide the effective heat transfer inside the thin-film based platforms, such as

11

miniaturized electronic devices because they only occupy small spaces in nanoscale. Finally,

12

since the LbL deposition is based on solution-processing and does not require the specialized

13

equipments within vacuum processing or high temperature conditions, the LbL-assembled

14

nanoporous coatings can be developed as functional layers for next-generation heat pipes.

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4. Conclusions

2

In summary, we reported the facile fabrication of nanoporous LbL-assembled MWCNT-PEI

3

coatings of copper-sintered microstructures inside circular heat pipes for the enhancement of

4

thermal performance enabled by the biporous structures at the nano/microscale. The as-prepared

5

copper-sintered, circular heat pipes were used as controls and the positively and negatively

6

charged MWCNT-PEI and carboxylic functionalized MWCNTs in DI water were prepared as

7

basic solutions for LbL deposition. One cycle of injecting-rejecting-cleaning of the prepared

8

solution in the inner space of the bare heat pipes formed a single nanoporous MWCNT-PEI layer,

9

which was denoted as one bilayer. The repetitive cycles of LbL deposition of the MWCNT-PEI

10

coating resulted in 10, 20, and 40 bilayers on the inner microstructures of the evaporator section

11

inside the heat pipes. While preserving the copper-sintered inner microstructure, the LbL-

12

assembled MWCNT-PEI coatings independently formed nanoporous shells and a biporous

13

surface morphology from the nano to microscale was obtained. The biporous structure of the

14

evaporator resulted in the significant enhancement of thermal performances. The LbL-assembled

15

MWCNT-PEI coating on the copper-sintered microstructure structurally induced enhanced

16

capillary wicking and active bubble formation because of the additional nanoporous fiber-like

17

percolation networks that created paths for working fluid transportation, extended interfacial

18

surface area, and nucleation sites. Furthermore, the hydrophilic nature originating from the PEI

19

shells surrounding the MWCNTs chemically contributed to the enhanced interfacial area

20

between the working fluid and heated surfaces, and promoted bubble departure at the active sites.

21

The outstanding thermal performance with a 14.3% reduction in thermal resistance was observed

22

in the 10 bilayer LbL-assembled MWCNT-PEI coating, whereas thicker nanoporous layers did

23

not improve thermal performance. Because redundant nanoporous layers reduced the porosity of

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the microporous structures, added interfacial thermal resistance between the working fluid and

2

heater surfaces, and hindered working fluid transportation, the 10-bilayer LbL-assembled

3

MWCNT-PEI coating was more effective than the 40-bilayer-coated heat pipes. The LbL-

4

assembled MWCNT-PEI coatings of the inner surfaces of the heat pipes provide a scalable

5

processing method to significantly improve the thermal performance of miniaturized and thin-

6

film-based devices and demonstrate the potential use in practical thermal applications. Moreover,

7

the LbL-assembled nanoporous coatings enable the development of functional interfaces for a

8

wide range of applications requiring phase change heat transfer.

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Acknowledgments

13

This work was supported by the Technology Development Program to Solve Climate Change of

14

the National Research Foundation (NRF) grant funded by the Korea government (Ministry of

15

Science and ICT, Grant No. NRF-2017M1A2A2044986), and the Korea Institute of Energy

16

Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy

17

(MOTIE) of the Republic of Korea (Grant No. 20173010032170).

19

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Figures and legend

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Figure 1. Layer-by-layer (LbL)-assembled multiwalled carbon nanotube (MWCNT)-

4

polyethyleneimine (PEI) coatings inside a copper-sintered heat pipe. Copper-sintered,

5

circular heat pipes were used as-prepared. The negatively charged MWCNT solution, deionized

6

(DI) water, and positively charged PEI solution were sequentially injected inside the circular heat

7

pipes and removed from the inner spaces to enable successive deposition of the MWCNT-PEI

8

layers. The monoporous inner structure of the as-prepared heat pipes on the order of a few tens

9

of micrometers was transformed to a biporous structures exhibiting nanoporous MWCNT-PEI

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layers on a microporous copper-sintered surface.

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Figure 2. Surface morphologies of LbL-assembled MWCNT-PEI coatings with different

4

bilayer thicknesses on the inner surfaces of copper-sintered heat pipes. Scanning electron

5

microscopy (SEM) images of the inner structures of copper-sintered heat pipes (a) before

6

application of LbL-assembled coating and after the application of (b) 10 bilayers and (c) 20

7

bilayers of the LbL-assembled MWCNT-PEI coatings. The 40 bilayer coating exhibited identical

8

surface characteristics as the 20 bilayer coating.

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Figure 3. Coating thickness of LbL-assembled MWCNT-PEI coatings with 10, 20, and 40

3

bilayers on copper substrates.

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Figure 4. Wettability of the LbL-assembled MWCNT-PEI coatings with different numbers

8

of bilayers. Static contact angles of the LbL-assembled MWCNT-PEI coatings with (a) 10, (b)

9

20, and (c) 40 bilayers on copper substrates.

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Figure 5. Experimental setup for the measurement of heat transfer characteristics of the

4

circular heat pipes. (a) Scheme of the specific locations for the real-time temperature

5

measurements monitored by thermocouples. The white circles indicate the locations of

6

thermocouples on the wall surfaces of the circular heat pipes. (b) Scheme of the experimental

7

setup for controlling the applied power and measuring the corresponding temperatures of the

8

heat pipes.

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Figure 6. Wall temperature distribution of the circular heat pipes with LbL-assembled

3

MWCNT-PEI coatings. Transition of the wall temperature distribution of the copper-sintered

4

heat pipes with LbL-assembled MWCNT-PEI coatings: bare, 10, 20, and 40 bilayers at various

5

input powers of (a) 20 W, (b) 40 W, (c) 60 W, (d) 80 W, and (e) 100 W.

6

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Figure 7. Heat transfer characteristics of the circular heat pipes with LbL-assembled

3

MWCNT-PEI coatings. (a) Thermal resistances and (b) heat transfer coefficients of the bare, 10,

4

20, and 40 bilayer LbL-assembled MWCNT-PEI-coated copper-sintered heat pipes as a function

5

of applied power.

6

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1

Table 1. Uncertainty of components of experimental setup and heat loss for performances

2

evaluation of heat pipes. Tcond

Tevap

V

I

R

Uncertainties (%)

1.3%

1.3%

1.8%

2.0%

2.75%

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Parameters

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