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Copper flake-coated cellulose scaffold to construct segregated network for enhancing thermal conductivity of epoxy composites
Accepted Manuscript Copper flake-coated cellulose scaffold to construct segregated network for enhancing thermal conductivity of epoxy composites Vu Chi Doan, Minh Canh Vu, Nhat Anh Thieu, Md Akhtarul Islam, Pyeong Jun Park, Sung-Ryong Kim PII:
S1359-8368(18)34560-8
DOI:
https://doi.org/10.1016/j.compositesb.2019.02.015
Reference:
JCOMB 6601
To appear in:
Composites Part B
Received Date: 30 December 2018 Revised Date:
1 February 2019
Accepted Date: 7 February 2019
Please cite this article as: Doan VC, Vu MC, Thieu NA, Islam MA, Park PJ, Kim S-R, Copper flakecoated cellulose scaffold to construct segregated network for enhancing thermal conductivity of epoxy composites, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.02.015. 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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Copper flake-coated cellulose scaffold to construct segregated network for enhancing thermal conductivity of epoxy composites Vu Chi Doana, Minh Canh Vua, Nhat Anh Thieua, Md. Akhtarul Islamb, Pyeong Jun Parkc and Sung-Ryong Kima*
Department of Polymer Science and Engineering, Korea National University, Chungju
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a
27469, Korea.
Department of Chemical Engineering and Polymer Science, Shahjalal University of
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b
Science and Technology, Sylhet, Bangladesh. c
School of Liberal Arts and Sciences, Korea National University, Chungju 27469, Korea.
*Corresponding author.
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Email address: [email protected] (S.R. Kim).
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Abstract This study reports about the enhanced thermal conductivity of epoxy composites fabricated by introducing relatively aligned structure of cellulose fibers with copper flakes embedded in their
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surface-layer. The obtained composite shows anisotropic thermal conductivity; in the parallel direction being higher than that in the perpendicular direction. With varying preparation conditions, a thermal conductivity of as high as 1.4 W m-1 K-1 has been achieved at a low filler
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concentration of 36.44 wt%. A theory available in literature is modified by the introduction of resistivity factor to account for the loose connectivity at the flake-flake junctions and the
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effective thermal conductivity of epoxy composites filled by copper flakes coated cellulose fiber has been investigated. The theoretical prediction shows that the heat conduction along the fiber direction is mostly influenced by the thermal conductivities of cellulose fibers and copper flakes. Meanwhile, the thermal conductivity of polymer matrix and packing fraction of fiber affect
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strongly on the perpendicular thermal conductivity of the composites. The contact resistivity factor of at the flake-flake junction has been estimated to be 0.07 m K W-1. This report gives potential guidance exploiting recyclable cellulose fiber for further study in designing high
thermal conductivity; aligned structure; cellulose fiber; copper flakes
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Keywords:
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thermal conductive composites for heat management applications.
coating; segregated network
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1.
Introduction The modern trend in design of electronic devices toward miniaturization, multi-task
responses and higher integration accompanies with high internal heat accumulation during their
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operation. This leads to decline of the longevity of the electronic packages or even causes serious problems due to the thermal failure [1]. Therefore; the development of high thermal conductive materials for the thermal management in electronic components has been under continuous
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progress to meet the growing demand. Polymer-based composites, with their exclusive combination of desired properties such as chemical resistance, light weight, low production cost,
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easy processing and so on, have been very appealing to be employed in thermal management as well. [2]
The use of conductive metal particles such as silver, copper, alumina and so on, as filler in polymeric materials have been intensively studied [3-7]. Conventionally, the conductive particles
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are piled up in the polymer medium until certainly achieving the desired thermal conductivity. Although the methods are simple with large-scale manufacturing possibility, the high concentration of metal fillers causes deterioration of the mechanical properties and processability
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of the composites. To minimize the adverse effect, researchers have employed conductive particles in various sizes and shapes [8-10]. The incorporation of a conductive filler in two
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different sizes as a hybrid system have been proved to be an effective method for enhancing thermal conductivity [8,9]. Choi et al. embedded a filler mixer of micro-AlN particles and nanoAl2O3 particles in epoxy composition and achieved a thermal conductivity of the composition as high as 3.402 W m-1 K-1, which is about 1600% enhancement compared to the neat polymer [8]. The authors attributed their achievement to the unique architecture where the small Al2O3 wedge between the large AlN particles increased the contact area of these large AlN particles and thus
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formed a conductive network throughout the composite mass. Another model to increase thermal conductivity of composite is to apply a couple of conductive fillers capable of showing synergetic effect in a polymer matrix. One such couple is carbon fibers and graphite, which
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shows synergetic enhancement of the thermal conductivity of Nylon 6-based composites; the achievement was 5.09 W m-1 K-1 for a total filler content of 60 wt% [10]. In summary, the introduction of conductive fillers into polymeric matrix significantly improves the thermal
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conduction of the polymeric compositions. However, use of high concentration of expensive fillers would increase the production cost and simultaneously sacrifice the process ability of the
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final compositions.
Copper is cheap and highly conductive to be used in composite industry. However, the present approaches of using copper nanoparticles in thermally conductive polymer composite do not bring desired results [11,12]. Currently, various approaches aiming to introduce copper
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nanowires and copper continuous networks have been proven to be effective in improving thermal properties of composite materials [13,14]. Yu et al. introduced a copper shell layer on polystyrene (PS) particles by electro-plating technique [13]. The thermal conductivity of the
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PS/Cu composite prepared by hot pressing the Cu@PS particles reaches as high as 26.14 W m-1 K-1 at 23 vol% of the copper shell (which is 143-fold higher than that of PS matrix). The authors
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attributed their achievement to the continuous networks of copper shell throughout the composition [13]. Although the thermal conductivity of PS composite increased remarkably, the electro-plating technique, however, is too expensive to transfer the developed technology from laboratory to industry for large scale productions. Yuan et al. proposed on a new method that covers copper nanowires with polydopamine (PDA), and the epoxy composite prepared with these high aspect ratio fillers shows high thermal conductivity, while the electrical insulating
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properties was retained [14]. The thickness of the PDA layer on the Cu wire surface also affected on the heat transfer of the composites, the highest thermal conductivity being realized at the thickness of 25 nm. Obviously, use of high aspect ratio filler promotes high enhancement in
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thermal conductivity of the composite, as the high aspect ratio fillers increases the connectivity among conductive filler-particles. Additionally, the effect of aspect ratio on thermal conductivity of composites was discussed and proven in several papers [12,15]. Similar to the boron nitride
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(hBN) and graphite, copper flake is a high aspect ratio material but cheaper and easier in manufacturing, which could be advantage of using them in enhanced thermal conductive
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composites. Recently, Xu et al. introduced copper flakes into the epoxy medium, and by simple embedding of 70 wt% of copper flakes into epoxy resin, the authors increased the thermal conductivity to 2.05 W m-1 K-1 [16]. Instead of the point-to-point contacts as in the case of using copper particles, the face-to-face contact pattern realized in the copper flakes endowed the
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contact area of the conductive filler and constructed effective thermal pathways for heat dispersion in the composite. The enhancement again is significant but the over-use issue still remains in place. This is because the copper flakes themselves have high density (8.96 g cm-3)
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while the epoxy density is normally in the range of 0.7-1.6 g cm-3, the big discrepancy in density may cause easy dispersion in the matrix and precipitation of the flakes in the bottom. Thus, the
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dispersion of high-density copper flakes in the polymeric matrix remains challenging. Cellulose fibers have been realized as a promising alternative for synthetic fibers in reinforcing physical properties of polymer composites owing to their biodegradability and availability from renewable sources. The consolidated hybrid system of conductive materials and cellulose fibers have been proven as an alternative filler-system in reinforcing composite materials [17-19]. Chen et al. introduced a hybrid scaffold, fabricated from cellulose fiber-
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assisted boron nitride nanosheets (BNNS), to epoxy matrix [22]. The interconnected conductive networks were formed by the BNNS layers, assembling on the cellulose fibers skeleton, and thus the thermal conductivity of the composition reached to 3.13 W m-1 K-1 at a concentration of
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BNNS (24 wt%). Zeng et al. developed non-covalently functionalized boron nitride nanotube (BNNTs) using cellulose chains. The resultant composites showed a thermal conductivity of the cellulose polymer as high as 21.39 W m-1 K-1 at 25 wt% of BNNTs, and this outstanding thermo-
hydrophobic BNNTs and hydrophobic cellulose medium.
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conduction was obtained by strong dispersive interactions between the alike components,
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Cigarette filter is a relatively vertical scaffold made from cellulose acetate fibers, the use of which in reinforced polymer composites has been reported recently [24-26]. Being inspired with the vertical alignment of cellulose fibers to form the continuous filler network at reduced filler content, we have made a facile approach to construct a cellulose-assisted copper flakes scaffold
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to improve the thermal conductivity of epoxy composite. Owing to the aligned arrangement of cellulose fibers, the copper flakes are vertically distributed in the epoxy matrix resulting in a
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significant enhancement in thermal conductivity of the composite-bulk.
Experimental
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2.1. Materials
Cigarette filters were collected from the used cigarettes (Bohem Cigar no.3) purchased from Bohem Cigar Co. Ltd (South Korea). The epoxy bisphenol-A (YD-114) and the curing agent (HN-2200) were provided by Kukdo Chemical Co. Ltd (South Korea). The latent catalyst N,N-dimethylbenzylamine (99%) was obtained from Acros Organics (USA). The copper flakes with average diameter of 10 µm and thickness of 65 nm were purchased from CN Vision Co. Ltd
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(South Korea). Other solvents are used from laboratory store as available without further purification.
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2.2. Preparation of the epoxy composite with Cu@Cell scaffold The fabrication process of the epoxy/Cu@Cell composites is schematically illustrated in the Fig. 1. The copper flake suspension was prepared by sonicating 1 g of copper flakes in 10 ml of ethanol. A dried cellulose scaffold was slowly immersed into the above suspension and held
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for 2 min. Then, the scaffold was taken out and dried from ethanol in a vacuum box at room
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temperature. After few dip-coating cycles, the cellulose-supported copper scaffold (Cu@Cell) turned reddish providing evidence for the presence of copper flakes on the surface. The Cu@Cell scaffolds were treated with acetone vapor in a closed container at varying conditions to find an optimum procedure. Typically, 200 mL of acetone solution (99%) was poured in a beaker and a Cu@Cell scaffold was hanged over the beaker. The container was then
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made air-tight to prevent the acetone vapor from leaking, and the whole system was placed in an oven at 50 oC for 6 h to bring the upper layer of the cellulose fibers into solution phase. The liquified (solution phase) cellulose fiber-surface served as a bed for the placement of the copper
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flakes. The condition for acetone vapor treatment was investigated by varying time and
Fig. 4.
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temperature and its effect on the thermal conductivity of the final product has been presented in
The Cu@Cell/Epoxy composites were prepared by infiltrating the filler into the epoxy resin. First, the epoxy resin and the curing agent (10:7 by weight ratio) were mixed by mechanical stirring for 10 min at room temperature. Then N,N-dimethylbenzylamine was added to the mixture in an amount 1 wt% of the curing agent, and the mixture was stirred for 30 min. The dried Cu@Cell scaffold was completely immersed into the epoxy mixture at 30 oC in the vacuum
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oven for 2 h to remove the trapped air. The composite sample was cured at 80 oC for 4 h and at 120 oC for 2 h. The composite-specimens were prepared in a form of disks of approximately 10
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mm in diameter and 1 mm in thickness for characterization studies. 2.3. Characterization
The morphology of the samples were studied by field-emission scanning electron microscope FESEM (6700F, JEOL Co., Japan). The composite-specimens were fractured in
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liquid N2 before taking micrographs. The elemental maps of Cu@Cellulose/epoxy composites
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are taken by an energy-dispersive X-ray spectrometer (EDS) detector (XMAX, Oxford Instruments, USA).
Thermal diffusivity of the epoxy composites was measured at room temperature by a laser flash thermal constant analyzer (TC-7000, ULVAC Co., Japan) with the samples of approximately 10 mm in diameter and 1 mm in thickness. For each composition, the
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measurement was repeated several times and the average value was taken to be the diffusivity of the composition. The thermal conductivity of the composites was calculated by the following formula:
K = α ⋅ ρ ⋅ Cp
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(1)
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where K, α, ρ, and Cp were the thermal conductivity (W m-1 K-1), the thermal diffusivity (m2 s-1), the density (kg m-3), and the specific heat capacity (J kg-1 K-1), respectively. The thermal degradation of the epoxy composites was measured by the thermo-gravimetric analyzer (TGA, TA 1000, TA Instrument Co., USA) from 30 to 800 oC at the heating rate of 10 o
C/ min in a nitrogen environment.
3.
Results and discussion
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3.1. Morphology observation The morphology of bare cellulose scaffold is shown in the Figs. (2a,b). As can be seen in the Figs. 2(a,b), the cellulose scaffold obtained from used cigarette filters shows a relatively
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vertical alignment of the fibers, which from its own part endows a line-up arrangement of the copper flakes in the epoxy composite. The inset shows the top view of the cellulose scaffold. The cellulose fibers are smooth and directionally assembled in the scaffold. After being coated by the
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copper flakes, the Cu@Cell scaffold still preserves its original shape, but exhibits a rough surface texture due to the presence of copper flakes on its surface (Fig. 2(b)). The acetone vapor
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partially dissolved the surface of the fibers allowing the copper flakes to stick to their surface during the fabrication steps. The inset in the Fig. 2(b) demonstrates the assembled layer of copper flakes along the cellulose fiber-axis. The continuous pathways of the copper flakes have been revealed in Fig. 2(b), and it is presumed that they are the very pathways by which the
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effective phonon transfer networks are formed and consequently, the face-to-face conductive contact area is optimized to realize improved thermal conductivity. Fig. 2(c) shows that the continuity of the conductive path by copper flakes is preserved even after impregnation with the
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epoxy resin. It can be explained with the fact that the acetone vapor partially dissolves the surface of cellulose fibers which acts as an adhesive layer to hold the copper flakes on its surface.
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On the contrary, the composite without acetone vapor treatment showed a detachment of Cu flakes from the cellulose fiber as shown in Figure S1 in the supporting information. When the cellulose fiber with the copper flakes adhered to its liquefied surface is exposed to air at ambient temperature, it is solidified and consequently integrating the copper flakes network as part of the fiber. Thus, the network remains intact in the Cu@Cell/Epoxy composites. Additionally, the EDS element map of Cu in Fig. 2(d) clearly reveals the decorated copper flakes on the surface of
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the cellulose fiber. Along the cellulose fiber-axis, the phonon transfer occurs mainly among the copper flakes by face-to-face pattern that is considered to be the key factor of the enhanced thermal conductivity of the final composition.
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3.2. Thermal properties of the composites
The thermal conductivity of epoxy based composites as a function of the numbers of dipcoating cycle (see section 2.2) is depicted in Fig. 3. The neat epoxy shows the thermal
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conductivity as poor as 0.19 W m-1 K-1 while the thermal conductivity of the Cu@Cell scaffold
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based epoxy composites increases with the increase of the dipping cycles of the cellulose scaffold into the copper-flake suspension. From Fig. 3 (a), with 5 dipping-cycles, but without acetone treatment of the fiber, the thermal conductivity of the composite increases up to 0.65 W m-1 K-1, which is 242% enhancement as compared to that of the neat epoxy composite (0.19 W m-1 K-1). In contrast, with 36.44 wt% of copper flakes dispersed in the hardened epoxy (without
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support of cellulose scaffold), the composite shows a conductivity of 0.51 W m-1 K-1, which is about 168% improvement as compared to the bare epoxy. The higher enhancement is attributed to continuous network structure of the copper-flakes supported by cellulose scaffold, while
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without cellulose assistance; the copper flakes would tend to precipitate at the bottom of the composition and could not contribute to the formation of conductive networks effectively.
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The thermal conductivity obtained from acetone vapor treated Cu@Cell scaffold supersedes those of the untreated ones. After 5-time dipping and 6 hours of acetone exposure, the thermal conductivity of Cu@Cell/Epoxy composite reaches 1.35 W m-1 K-1 (Fig. 3 (a) - the blue column) which is 107 % higher than that of the composites with untreated Cu@Cell scaffold samples (0.65 W m-1 K-1). This difference in thermal conductivity (with or without acetone vapor treatment) is attributed to the physical adherence of the copper flakes on the scaffold surface,
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when there is no acetone vapor treatment, and during impregnation with epoxy resin, portion of the adhered flakes is detached out from the fiber and passes in the epoxy matrix phase. The detached flakes do not contribute to the flake-to-flake contact pattern of the conductive network
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structure.
The copper flakes embedded in the upper layer of the cellulose fiber-surface form the conductive segregated networks accounted for the higher performance of the composites. In
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comparison with the thermal conductivity achieved by using spherical copper particles, e.g. 0.74 W m-1 K-1 at 68.4 wt% of functionalized Cu particles [3] and 0.52 W m-1 K-1 at 70 wt% of Cu
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nanoparticles [27], the flake-to-flake interaction pattern of copper flakes in the present work gained a competitive result (1.35 W m-1 K-1) at lower content of copper filler (36.44 wt%). The high achievement can be explained with the presumption that the flake-to-flake interaction of copper flakes fillers results in the higher possibility of heat transport through the filler networks
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rather than dispersing it in the poorly conductive matrix. Similar results were obtained in one of our recent work, in which flake-like sintered copper nanoparticles were dispersed among the (Polymethyl methacrylate (PMMA) particles [28]. The hybrid system of conductive-
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nonconductive fillers in epoxy resin gained a high thermal conductivity value (1.05 W m-1 K-1 at 30 wt% Cu content) due to the segregated network of copper filler which endowed the heat
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transfer capability throughout the composite mass. The effect of temperature and time-duration of the acetone vapor treatment of the Cu@Cell scaffold filler on the conductivity of the final scaffold-filled epoxy composites has also been investigated. The thermal conductivities at ~36.44 wt% of copper flakes are presented in the Fig. 3 (b). The composites with Cu@Cell scaffold filler were produced by 5-time dipping cycles. Treatment of the Cu@Cell scaffold with acetone vapor at 25 oC for 6 h did not realize significant
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enhancement (the thermal conductivity reached 0.78 W m-1 K-1). This may be due to the inadequacy of acetone vapor at 25 oC to bring the upper layer of the fiber into solution phase. The significant improvements were recorded when the Cu@Cell scaffold were exposed to the
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acetone vapor at 50 oC, when the acetone vapor was adequate and the diffusivity of the vapor in cellulose phase was higher. For 2 hours’ treatment, the thermal conductivity reached 1.07 W m-1 K-1 and after 12 hours’ exposure, the conductivity value was recorded to be as high as 1.46 W mK-1. However, further increase of the temperature (even at 80 oC for 6 h) did not show any
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appreciable improvement in thermal conductivity of the composite. This implies that the
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dissolved cellulose mixed copper layer becomes dense enough to inhibit the acetone vapor from interacting further deep into the cellulose core beyond a certain thickness. The direction-dependency of the thermal conductivity of the composites has been tested. The thermal conductivities of composites in the axial and radial direction are plotted in Fig. 4. As
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seen in the schematic illustration, the Cu@Cell fibers are continuous from the top the bottom of a given composite mass through flake-to-flake contact in the parallel direction. Meanwhile, the flake-to-flake contacts are interrupted in the perpendicular direction. Along the parallel direction,
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the epoxy mass constitutes the major portion of the transport path, and consequently, the conductivity is lower in the radial direction than that in the axial direction. Thus, in terms of
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thermal conductivity, the composite is anisotropic in nature. Table 1 compares the thermal conductivity achieved by the present approach with that in previous reports. As seen in the Table, the thermal conductivity of the epoxy composite has been effectively improved at lower copper loading [3,28-30]. The anisotropic thermal conductivity is obtained which would endow composites for versatile applications compared to that of isotropic material. The competitive results would encourage further study on constructing connected
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structure of metallic fillers for enhanced thermal conductive composites due to its low production cost and large-scale producibility. The thermo-gravimetric analysis (TGA) was employed to investigate the thermal stability
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of the composites (Fig. 5). As can be seen from the thermogram, the thermal degradation mainly occurs in the temperature range of 300-500 oC due to the decomposition of epoxy matrix. However, there is slight weight loss in the range of 100- 300 oC. This may be due to the removal
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of absorbed moisture from the samples and the decomposition of epoxy monomers. The T50, corresponding to the 50 wt% loss of the samples, of bare epoxy composites is at around 407 oC.
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In contrary, the T50 increases by adding the high thermally stable copper flakes to the medium. At 13.28 wt% copper flakes content, the T50 of the composite enhances to 417 oC while the temperature is up to 427 oC by loading 36.44 wt% copper flakes. The TGA results prove that introducing the copper flake fillers to the composition slightly enhanced the thermal stability.
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3.3. Effective thermal conductivity of composites
In a combined system of a non-metallic core fiber and a metallic coating layer, the heat is transported by both phonon and electron carriers, which was investigated theoretically [31]. The
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TC of the coating layer is determined by the ability to spread the electron (ke) and phonon (kp) in the coating body. The effective TC of coating layer (kc) was determined by the essential
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parameters shown in Table 2 [31]. The copper flake coated system developed in this work, however, differs a bit with the distribution of metal particles (complete coverage of the surface) assumed in the theory [31]. The copper flakes are decorated on the surface of cellulose fibers by dip-coating process and these flakes are not completely interconnected. Therefore, the gaps between the flakes reduce the thermal conduction ability of the coating layer. Another factor that should be considered is the probable contamination of the particle-surface that could affect the
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overall conductivity e.g. presence of the copper oxide layer on copper flake surface, which has significant low TC compared to the pure copper [32]. Thus, the resistance at flake-flake junctions and the contamination of the filler surfaces limits the thermal conductivity of the coating layer.
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The effect of the contact resistance between two conductive particles on the TC of a composition is also investigated by Xu et al. [33]. They considered the discontinuous system of copper flake decorated on the cellulose fiber as a continuous system of the new element on
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cellulose fibers. By adapting their results, the effective TC of the new element, kc' , which
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combines the contact resistivity Rc and the TC of copper flake layer kc , is given by [33]
1 1 = + Rc kc' kc
(2)
where kc' is the effective TC of new elements. Denoting the volume fraction of the coated fibers
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as f = f p + f c , where ƒp and ƒc are the volume fractions of the core fiber and copper coating layer, respectively, the thermal conductivities of the parallel direction (ke||) and perpendicular direction (ke⊥) are given by
2 β1 ( f p + f c ) = km exp 1 − fo ( + ) f f ( f + f ) 2 − β1 1 + p c p c fo2
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ke ⊥
(3)
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ke = (k 'c − km ) f c + kc (α 3 + 1) f p + km (1 − f p )
(4)
where km is the thermal conductivity of the matrix medium and ƒo is the maximum packing volume fraction of the fiber. Considering the morphology of cellulose scaffold as shown in Fig. 2, we assume that ƒo = 0.52 as in the random packing state of the fiber. Here, α3 and β1 represent
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the dependencies of effective TC on the Kapitza resistance and the directional thermal conductivity of the fibers. The parameters are given by [31].
1 + Rkm a
(2 + α 1 ) f p (2 − α 1 ) f p
fp k + m k1 f p + fc + 2 fc + 2 fc
;
k −1 k 'c
(5)
k ⊥ − k 'c k ⊥ + k 'c
(6)
α3 =
;
α1 = 2
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k1 =
fp − km k f p + fc 1
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β1 = 2
1 − Rkm a
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where R is the interfacial thermal resistance between copper coating layer and epoxy matrix. a is the radius of the core fibers, and k|| and k⊥ are the TCs in the perpendicular and parallel direction of the fiber, respectively. Diaz et al. investigated the TC of cellulose fibers and claimed that the heat flows faster along the fiber direction than that of perpendicular direction [34]. The thermal conductivity by parallel direction lays in the range of 0.53- 5.7 W m-1 K-1
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while that of perpendicular direction is in the range of 0.22- 0.72 W m-1 K-1. Figure 6 compares effective thermal conductivity from the theoretical prediction and our experimental results. To investigate the effect of resistivity on the effective TC of the composites,
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the theoretical TC by axial direction has been compared to that of the experimental results. The
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original TC of copper are fixed at ke = 261.4 W m-1 K-1 (thermal conductivity by electron carriers), kp = 15 W m-1 K-1 (thermal conductivity by phonon carriers) and that of cellulose fibers is k|| = 0.93 W m-1 K-1; k⊥ = 0.48 W m-1 K-1, respectively. Meanwhile, the resistivity Rc is varied from 0.01 (insignificant resistance by assuming the continuity of the coated copper layer) to 0.25 (high resistivity by assuming isolated dispersion of Cu flakes). The results are plotted in Fig. 6 (a), which shows that the interface resistance between two neighboring copper flakes plays a critical role in the thermal conductivity of the composites. When the resistance is insignificant (Rc =
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0.01), the TC of composites increases sharply with an increase in copper content in the composition. Meanwhile, with a high resistivity between the two flakes as represented by Rc = 0.25, the TC of the composites shows almost inappreciable increase in the effective TC with the
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increase in the copper content. In our system, the theoretical curve at the flake-to-flake interfacial contact resistance (Rc) of 0.07 m K W-1 results in fairly good fit to the experimental data.
Figure 6 (b) shows the effects of the parallel (axial) thermal conductivity of cellulose fiber
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on the composite’s parallel thermal conductivity. The parallel thermal conductivity of cellulose fibers strongly affects on the effective parallel TC of the composites. When the parallel thermal
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conductivity of cellulose fiber is lower than 1 W m-1 K-1, the theoretical TC curves fit well with our experimental results. However, the high parallel TC such as 5.7 W m-1 K-1 shows big discrepancy with the experimental results. A high thermal conductivity of 5.7 W m-1 K-1 has been observed for 5 nm long nanocrystal cellulose [34]. In this study, 1 mm-long cellulose fiber
from our analysis.
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was used, and k|| is expected to be much smaller, which is indeed obtained as 0.93 W m-1 K-1
Our experimental results are compared with the theoretical curves from the theoretical
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model (Eqs. (3) and (4)) in Fig. 6 (c). The contact resistivity Rc, perpendicular TC of cellulose fiber and measured TC of epoxy are 0.07 m K W-1, 0.48 W m-1 K-1 and 0.19 W m-1 K-1,
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respectively, for the theoretical prediction. Additionally, the maximum packing fraction of Cu@Cellulose fibers is assumed to be 0.52 and other parameters are listed in Table 2. In the parallel direction, the theoretical curve matches well with the experimental data while a discrepancy is observed in the perpendicular direction. The theoretical curves from the previous theory without crowding factor [31] and our model at two values of Rc (0.01 and 0.07) are plotted for a comparison. The experimental data for TC in the perpendicular direction, however, lie
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much than those predicted. It could be explained by the assumption employed in the theory that the isolated straight fibers are aligned along the axis and a conductive path is not formed in the perpendicular direction, leading to a poor thermal conductivity. In our experimental case, the
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curved cellulose fibers could form irregular links between fibers and form, at different region, a continuous perpendicular heat path in the composite medium as shown in Fig. 2 (b).
4.
Conclusions
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Copper flakes are adhered on the surface of cellulose fibers, and these fibers have been aligned vertically in epoxy composites. The acetone treatment is optimized at the temperature of
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50 oC for a period of 6 h, and along the parallel direction, a thermal conductivity of 1.35 W m-1 K-1 has been achieved, while that along the perpendicular direction is 0.65 W m-1 K-1. The composites show anisotropic behavior in terms of thermal conductivity and a modified theoretical model has been applied to investigate the effects of components on the thermal
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conductivity of composites. The theoretical prediction proves that the contact resistivity among these copper flakes plays a significant role in the effective thermal conductivity of composites. The thermal conductivities of cellulose fibers, k|| = 0.93 W m-1 K-1; k⊥ = 0.48 W m-1 K-1, well
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describe our experimental results. Our approach introduces a facile method to apply metallic flakes in fabrication of thermal conductive composites, and the thermal resistivity factor that has
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been introduced in the prediction model would provide information about the degree of realization of the flakes-flakes contact in the conductive coating layer.
Acknowledgements The support of The Leading Human Resource Training Program of Regional Neo Industry (No. 2016H1D 5A1908330) and The Basic Science Program (No. 2017R1A2B4005200) through the
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National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning is highly appreciated.
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coated copper nanowire, Compos. Sci. Technol. 164 (2018) 153-159. [15] V.C. Doan, M.C. Vu, M.A. Islam, S.R. Kim, Poly(methyl methacrylate)-functionalized reduced graphene oxide-based core–shell structured beads for thermally conductive epoxy
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composites. J. Appl. Polym. Sci. 136 (9) (2018) 47377 (1-7). [16] L. Xu, H. Wen, F. Zhang, M.M.F. Yuen, X.Z. Fu, R. Sun, C.P. Wong, Enhancement of
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thermal conductivity of epoxy adhesives by ball milling copper flakes, 18th International Conference on Electronic Packaging Technology (2017). [17] N.K. Kim, S. Dutta, D. Bhattacharyya, A review of flammability of natural fibre reinforced polymeric composites, Compos. Sci. Technol. 162 (2018) 64-78.
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[21] K. Uetani, K. Hatori, Thermal conductivity analysis and applications of nanocellulose materials, Sci. Technol. Adv. Mater. 18 (2017) 877-892.
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Funct. Mater. 27 (2017) 1604754.
[23] Z. Zeng, J. Sun, Y. Yao, R. Sun, J.B. Xu, C.P. Wong, A Combination of BNNTs and cellulose nanofibers for the preparation of a nanocomposite with high TC, ACS Nano 11
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(2017) 5167-5178.
[24] C. Wang, Y. Ding, Y. Yuan, X. He, S. Wu, S. Hu, M. Zou, W. Zhao, L. Yang, A. Cao, Y.
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Li, Graphene aerogel composites derived from recycled cigarette filters for electromagnetic wave absorption, J. Mater. Chem. C 3 (2015) 11893-11901. [25] K. Liu, H. Takagi, R. Osugi, Z. Yang, Effect of physicochemical structure of natural fiber on transverse thermal conductivity of unidirectional abaca bamboo fiber composites, Composites Part A 43 (2012) 1234-1241.
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[26] Z. Liu, Y. Chen, W. Dai, Y. Wu, M. Wang, X. Hou, H. Li, N. Jiang, C.T. Lin, J. Yu, Anisotropic thermal conductive properties of cigarette filter-templated graphene epoxy composites, RSC Adv. 8 (2018) 1065-1070.
fillers for epoxy, Appl. Therm. Eng. 66 (2014) 293-298.
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[27] Y.X. Fu, Z.X. He, D.C. Mo, S.S. Lu, Thermal conductivity enhancement with different
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[29] Y.H. Bae, M.J. Yu, M.C. Vu, W.K. Choi, S.R. Kim, Synergistic effects of segregated network by polymethylmethacrylate beads and sintering of copper nanoparticles on thermal and electrical properties of epoxy composites, Compos. Sci. Technol. 155 (2018) 144-150. [30] L. Ren, Q. Li, J. Lu, X. Zeng, R. Sun, J. Wu, J.B. Xu, C.P Wong, Enhanced thermal for
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nanofiber–dielectric matrix composites, Compos. Sci. Technol. 109 (2015) 18-24. [32] S. Yu, B.I. Park, C. Park, S.M. Hong, T.H. Han, C.M. Koo, RTA-treated carbon
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fiber/copper core/shell hybrid for thermally conductive composites, ACS Appl. Mater. Interfaces 6 (2014) 7498-7503. [33] J.Z. Xu, B.Z. Gao, F.Y. Kang, A reconstruction of Maxwell model for effective thermal conductivity of composite materials, Appl. Therm. Eng. 102 (2016) 972-979.
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[34] J.A. Diaz, Z. Ye, X. Wu, A.L. Moore, R.J. Moon, A. Martini, D.J. Boday, J.P. Youngblood, Thermal conductivity in nanostructured films: from single cellulose nanocrystal to bulk
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SC
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films, Biomacromolecules 15 (2014) 4096−4101.
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Table 1. Thermal conductivity enhancement for some polymer composites with different fillers
Method
68.3
0.74
335
CuNPs/Epoxy
70
0.52
190
Cu/pPMMA/Epoxy
30
1.05
Al2O3/AgNPs/Epoxy
70
1.30
624
36.4
1.35 (k||)
610
0.65 (k⊥)
242
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Cu@Cell/Epoxy
518
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Cu/Epoxy
Filler content Thermal Conductivity Enhancement -1 -1 (W m K ) (wt%) (%)
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Materials
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and methods.
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Reference
Mixing
Fu et al. [3]
Mixing
Zhang et al. [28]
Sintering
Bae et al. [29]
Bar coating
Ren et al. [30]
Dip coating
This work
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Table 2. The properties of copper-coated layers on the cellulose fibers.
Mean Free Path (MFP)
Coupling Factor
(nm)
(W m-3 K)
(W m-1 K-1) Phonon
Electron
Phonon
261.4
15
2.7
8.2
2.7 x 1016
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Electron
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Interface Thermal Resistance 2 (m K W-1)
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Thermal Conductivity
5.2 x 10-9
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Figure 1. Schematic description of fabrication process of Cu@Cellulose/Epoxy composites.
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Figure 2. SEM images of (a) the bare cellulose scaffold, (b) Cu@Cellulose scaffold (The insets
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in Figs. 2(a,b) show the top-view of the scaffold), (c) the morphology of the Cu@Cellulose scaffold in the epoxy composites along with (d) the EDS elemental map of copper.
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Figure 3. Thermal conductivity and copper content of composites as a function of dipping cycles. (a) The composites of with and without acetone treatment are composite with Cu flakes only ( )) contains 36.4 wt% of copper without cellulose fiber, and (b) the thermal conductivities of composites at various acetone vapor treatment conditions. The abbreviation “w/o treatment”
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represents for those composites of coating scaffold without acetone vapor treatment.
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Figure 4. Thermal conductivity of Cu@Cellulose/Epoxy composites by parallel direction and
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perpendicular direction.
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Figure 5. TGA results of composites as a function of dipping cycles.
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Figure 6. Theoretical predictions on effective thermal conductivity by varying (a) contact resistance (Rc), (b) thermal conductivity of cellulose fiber, (c) and comparison of the thermal
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conductivities between theoretical predictions with experimental results.
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S1359-8368(18)34560-8
DOI:
https://doi.org/10.1016/j.compositesb.2019.02.015
Reference:
JCOMB 6601
To appear in:
Composites Part B
Received Date: 30 December 2018 Revised Date:
1 February 2019
Accepted Date: 7 February 2019
Please cite this article as: Doan VC, Vu MC, Thieu NA, Islam MA, Park PJ, Kim S-R, Copper flakecoated cellulose scaffold to construct segregated network for enhancing thermal conductivity of epoxy composites, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.02.015. 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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Copper flake-coated cellulose scaffold to construct segregated network for enhancing thermal conductivity of epoxy composites Vu Chi Doana, Minh Canh Vua, Nhat Anh Thieua, Md. Akhtarul Islamb, Pyeong Jun Parkc and Sung-Ryong Kima*
Department of Polymer Science and Engineering, Korea National University, Chungju
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a
27469, Korea.
Department of Chemical Engineering and Polymer Science, Shahjalal University of
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b
Science and Technology, Sylhet, Bangladesh. c
School of Liberal Arts and Sciences, Korea National University, Chungju 27469, Korea.
*Corresponding author.
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Email address: [email protected] (S.R. Kim).
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Abstract This study reports about the enhanced thermal conductivity of epoxy composites fabricated by introducing relatively aligned structure of cellulose fibers with copper flakes embedded in their
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surface-layer. The obtained composite shows anisotropic thermal conductivity; in the parallel direction being higher than that in the perpendicular direction. With varying preparation conditions, a thermal conductivity of as high as 1.4 W m-1 K-1 has been achieved at a low filler
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concentration of 36.44 wt%. A theory available in literature is modified by the introduction of resistivity factor to account for the loose connectivity at the flake-flake junctions and the
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effective thermal conductivity of epoxy composites filled by copper flakes coated cellulose fiber has been investigated. The theoretical prediction shows that the heat conduction along the fiber direction is mostly influenced by the thermal conductivities of cellulose fibers and copper flakes. Meanwhile, the thermal conductivity of polymer matrix and packing fraction of fiber affect
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strongly on the perpendicular thermal conductivity of the composites. The contact resistivity factor of at the flake-flake junction has been estimated to be 0.07 m K W-1. This report gives potential guidance exploiting recyclable cellulose fiber for further study in designing high
thermal conductivity; aligned structure; cellulose fiber; copper flakes
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Keywords:
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thermal conductive composites for heat management applications.
coating; segregated network
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1.
Introduction The modern trend in design of electronic devices toward miniaturization, multi-task
responses and higher integration accompanies with high internal heat accumulation during their
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operation. This leads to decline of the longevity of the electronic packages or even causes serious problems due to the thermal failure [1]. Therefore; the development of high thermal conductive materials for the thermal management in electronic components has been under continuous
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progress to meet the growing demand. Polymer-based composites, with their exclusive combination of desired properties such as chemical resistance, light weight, low production cost,
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easy processing and so on, have been very appealing to be employed in thermal management as well. [2]
The use of conductive metal particles such as silver, copper, alumina and so on, as filler in polymeric materials have been intensively studied [3-7]. Conventionally, the conductive particles
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are piled up in the polymer medium until certainly achieving the desired thermal conductivity. Although the methods are simple with large-scale manufacturing possibility, the high concentration of metal fillers causes deterioration of the mechanical properties and processability
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of the composites. To minimize the adverse effect, researchers have employed conductive particles in various sizes and shapes [8-10]. The incorporation of a conductive filler in two
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different sizes as a hybrid system have been proved to be an effective method for enhancing thermal conductivity [8,9]. Choi et al. embedded a filler mixer of micro-AlN particles and nanoAl2O3 particles in epoxy composition and achieved a thermal conductivity of the composition as high as 3.402 W m-1 K-1, which is about 1600% enhancement compared to the neat polymer [8]. The authors attributed their achievement to the unique architecture where the small Al2O3 wedge between the large AlN particles increased the contact area of these large AlN particles and thus
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formed a conductive network throughout the composite mass. Another model to increase thermal conductivity of composite is to apply a couple of conductive fillers capable of showing synergetic effect in a polymer matrix. One such couple is carbon fibers and graphite, which
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shows synergetic enhancement of the thermal conductivity of Nylon 6-based composites; the achievement was 5.09 W m-1 K-1 for a total filler content of 60 wt% [10]. In summary, the introduction of conductive fillers into polymeric matrix significantly improves the thermal
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conduction of the polymeric compositions. However, use of high concentration of expensive fillers would increase the production cost and simultaneously sacrifice the process ability of the
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final compositions.
Copper is cheap and highly conductive to be used in composite industry. However, the present approaches of using copper nanoparticles in thermally conductive polymer composite do not bring desired results [11,12]. Currently, various approaches aiming to introduce copper
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nanowires and copper continuous networks have been proven to be effective in improving thermal properties of composite materials [13,14]. Yu et al. introduced a copper shell layer on polystyrene (PS) particles by electro-plating technique [13]. The thermal conductivity of the
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PS/Cu composite prepared by hot pressing the Cu@PS particles reaches as high as 26.14 W m-1 K-1 at 23 vol% of the copper shell (which is 143-fold higher than that of PS matrix). The authors
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attributed their achievement to the continuous networks of copper shell throughout the composition [13]. Although the thermal conductivity of PS composite increased remarkably, the electro-plating technique, however, is too expensive to transfer the developed technology from laboratory to industry for large scale productions. Yuan et al. proposed on a new method that covers copper nanowires with polydopamine (PDA), and the epoxy composite prepared with these high aspect ratio fillers shows high thermal conductivity, while the electrical insulating
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properties was retained [14]. The thickness of the PDA layer on the Cu wire surface also affected on the heat transfer of the composites, the highest thermal conductivity being realized at the thickness of 25 nm. Obviously, use of high aspect ratio filler promotes high enhancement in
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thermal conductivity of the composite, as the high aspect ratio fillers increases the connectivity among conductive filler-particles. Additionally, the effect of aspect ratio on thermal conductivity of composites was discussed and proven in several papers [12,15]. Similar to the boron nitride
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(hBN) and graphite, copper flake is a high aspect ratio material but cheaper and easier in manufacturing, which could be advantage of using them in enhanced thermal conductive
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composites. Recently, Xu et al. introduced copper flakes into the epoxy medium, and by simple embedding of 70 wt% of copper flakes into epoxy resin, the authors increased the thermal conductivity to 2.05 W m-1 K-1 [16]. Instead of the point-to-point contacts as in the case of using copper particles, the face-to-face contact pattern realized in the copper flakes endowed the
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contact area of the conductive filler and constructed effective thermal pathways for heat dispersion in the composite. The enhancement again is significant but the over-use issue still remains in place. This is because the copper flakes themselves have high density (8.96 g cm-3)
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while the epoxy density is normally in the range of 0.7-1.6 g cm-3, the big discrepancy in density may cause easy dispersion in the matrix and precipitation of the flakes in the bottom. Thus, the
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dispersion of high-density copper flakes in the polymeric matrix remains challenging. Cellulose fibers have been realized as a promising alternative for synthetic fibers in reinforcing physical properties of polymer composites owing to their biodegradability and availability from renewable sources. The consolidated hybrid system of conductive materials and cellulose fibers have been proven as an alternative filler-system in reinforcing composite materials [17-19]. Chen et al. introduced a hybrid scaffold, fabricated from cellulose fiber-
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assisted boron nitride nanosheets (BNNS), to epoxy matrix [22]. The interconnected conductive networks were formed by the BNNS layers, assembling on the cellulose fibers skeleton, and thus the thermal conductivity of the composition reached to 3.13 W m-1 K-1 at a concentration of
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BNNS (24 wt%). Zeng et al. developed non-covalently functionalized boron nitride nanotube (BNNTs) using cellulose chains. The resultant composites showed a thermal conductivity of the cellulose polymer as high as 21.39 W m-1 K-1 at 25 wt% of BNNTs, and this outstanding thermo-
hydrophobic BNNTs and hydrophobic cellulose medium.
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conduction was obtained by strong dispersive interactions between the alike components,
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Cigarette filter is a relatively vertical scaffold made from cellulose acetate fibers, the use of which in reinforced polymer composites has been reported recently [24-26]. Being inspired with the vertical alignment of cellulose fibers to form the continuous filler network at reduced filler content, we have made a facile approach to construct a cellulose-assisted copper flakes scaffold
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to improve the thermal conductivity of epoxy composite. Owing to the aligned arrangement of cellulose fibers, the copper flakes are vertically distributed in the epoxy matrix resulting in a
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significant enhancement in thermal conductivity of the composite-bulk.
Experimental
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2.1. Materials
Cigarette filters were collected from the used cigarettes (Bohem Cigar no.3) purchased from Bohem Cigar Co. Ltd (South Korea). The epoxy bisphenol-A (YD-114) and the curing agent (HN-2200) were provided by Kukdo Chemical Co. Ltd (South Korea). The latent catalyst N,N-dimethylbenzylamine (99%) was obtained from Acros Organics (USA). The copper flakes with average diameter of 10 µm and thickness of 65 nm were purchased from CN Vision Co. Ltd
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(South Korea). Other solvents are used from laboratory store as available without further purification.
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2.2. Preparation of the epoxy composite with Cu@Cell scaffold The fabrication process of the epoxy/Cu@Cell composites is schematically illustrated in the Fig. 1. The copper flake suspension was prepared by sonicating 1 g of copper flakes in 10 ml of ethanol. A dried cellulose scaffold was slowly immersed into the above suspension and held
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for 2 min. Then, the scaffold was taken out and dried from ethanol in a vacuum box at room
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temperature. After few dip-coating cycles, the cellulose-supported copper scaffold (Cu@Cell) turned reddish providing evidence for the presence of copper flakes on the surface. The Cu@Cell scaffolds were treated with acetone vapor in a closed container at varying conditions to find an optimum procedure. Typically, 200 mL of acetone solution (99%) was poured in a beaker and a Cu@Cell scaffold was hanged over the beaker. The container was then
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made air-tight to prevent the acetone vapor from leaking, and the whole system was placed in an oven at 50 oC for 6 h to bring the upper layer of the cellulose fibers into solution phase. The liquified (solution phase) cellulose fiber-surface served as a bed for the placement of the copper
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flakes. The condition for acetone vapor treatment was investigated by varying time and
Fig. 4.
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temperature and its effect on the thermal conductivity of the final product has been presented in
The Cu@Cell/Epoxy composites were prepared by infiltrating the filler into the epoxy resin. First, the epoxy resin and the curing agent (10:7 by weight ratio) were mixed by mechanical stirring for 10 min at room temperature. Then N,N-dimethylbenzylamine was added to the mixture in an amount 1 wt% of the curing agent, and the mixture was stirred for 30 min. The dried Cu@Cell scaffold was completely immersed into the epoxy mixture at 30 oC in the vacuum
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oven for 2 h to remove the trapped air. The composite sample was cured at 80 oC for 4 h and at 120 oC for 2 h. The composite-specimens were prepared in a form of disks of approximately 10
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mm in diameter and 1 mm in thickness for characterization studies. 2.3. Characterization
The morphology of the samples were studied by field-emission scanning electron microscope FESEM (6700F, JEOL Co., Japan). The composite-specimens were fractured in
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liquid N2 before taking micrographs. The elemental maps of Cu@Cellulose/epoxy composites
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are taken by an energy-dispersive X-ray spectrometer (EDS) detector (XMAX, Oxford Instruments, USA).
Thermal diffusivity of the epoxy composites was measured at room temperature by a laser flash thermal constant analyzer (TC-7000, ULVAC Co., Japan) with the samples of approximately 10 mm in diameter and 1 mm in thickness. For each composition, the
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measurement was repeated several times and the average value was taken to be the diffusivity of the composition. The thermal conductivity of the composites was calculated by the following formula:
K = α ⋅ ρ ⋅ Cp
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(1)
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where K, α, ρ, and Cp were the thermal conductivity (W m-1 K-1), the thermal diffusivity (m2 s-1), the density (kg m-3), and the specific heat capacity (J kg-1 K-1), respectively. The thermal degradation of the epoxy composites was measured by the thermo-gravimetric analyzer (TGA, TA 1000, TA Instrument Co., USA) from 30 to 800 oC at the heating rate of 10 o
C/ min in a nitrogen environment.
3.
Results and discussion
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3.1. Morphology observation The morphology of bare cellulose scaffold is shown in the Figs. (2a,b). As can be seen in the Figs. 2(a,b), the cellulose scaffold obtained from used cigarette filters shows a relatively
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vertical alignment of the fibers, which from its own part endows a line-up arrangement of the copper flakes in the epoxy composite. The inset shows the top view of the cellulose scaffold. The cellulose fibers are smooth and directionally assembled in the scaffold. After being coated by the
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copper flakes, the Cu@Cell scaffold still preserves its original shape, but exhibits a rough surface texture due to the presence of copper flakes on its surface (Fig. 2(b)). The acetone vapor
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partially dissolved the surface of the fibers allowing the copper flakes to stick to their surface during the fabrication steps. The inset in the Fig. 2(b) demonstrates the assembled layer of copper flakes along the cellulose fiber-axis. The continuous pathways of the copper flakes have been revealed in Fig. 2(b), and it is presumed that they are the very pathways by which the
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effective phonon transfer networks are formed and consequently, the face-to-face conductive contact area is optimized to realize improved thermal conductivity. Fig. 2(c) shows that the continuity of the conductive path by copper flakes is preserved even after impregnation with the
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epoxy resin. It can be explained with the fact that the acetone vapor partially dissolves the surface of cellulose fibers which acts as an adhesive layer to hold the copper flakes on its surface.
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On the contrary, the composite without acetone vapor treatment showed a detachment of Cu flakes from the cellulose fiber as shown in Figure S1 in the supporting information. When the cellulose fiber with the copper flakes adhered to its liquefied surface is exposed to air at ambient temperature, it is solidified and consequently integrating the copper flakes network as part of the fiber. Thus, the network remains intact in the Cu@Cell/Epoxy composites. Additionally, the EDS element map of Cu in Fig. 2(d) clearly reveals the decorated copper flakes on the surface of
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the cellulose fiber. Along the cellulose fiber-axis, the phonon transfer occurs mainly among the copper flakes by face-to-face pattern that is considered to be the key factor of the enhanced thermal conductivity of the final composition.
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3.2. Thermal properties of the composites
The thermal conductivity of epoxy based composites as a function of the numbers of dipcoating cycle (see section 2.2) is depicted in Fig. 3. The neat epoxy shows the thermal
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conductivity as poor as 0.19 W m-1 K-1 while the thermal conductivity of the Cu@Cell scaffold
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based epoxy composites increases with the increase of the dipping cycles of the cellulose scaffold into the copper-flake suspension. From Fig. 3 (a), with 5 dipping-cycles, but without acetone treatment of the fiber, the thermal conductivity of the composite increases up to 0.65 W m-1 K-1, which is 242% enhancement as compared to that of the neat epoxy composite (0.19 W m-1 K-1). In contrast, with 36.44 wt% of copper flakes dispersed in the hardened epoxy (without
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support of cellulose scaffold), the composite shows a conductivity of 0.51 W m-1 K-1, which is about 168% improvement as compared to the bare epoxy. The higher enhancement is attributed to continuous network structure of the copper-flakes supported by cellulose scaffold, while
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without cellulose assistance; the copper flakes would tend to precipitate at the bottom of the composition and could not contribute to the formation of conductive networks effectively.
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The thermal conductivity obtained from acetone vapor treated Cu@Cell scaffold supersedes those of the untreated ones. After 5-time dipping and 6 hours of acetone exposure, the thermal conductivity of Cu@Cell/Epoxy composite reaches 1.35 W m-1 K-1 (Fig. 3 (a) - the blue column) which is 107 % higher than that of the composites with untreated Cu@Cell scaffold samples (0.65 W m-1 K-1). This difference in thermal conductivity (with or without acetone vapor treatment) is attributed to the physical adherence of the copper flakes on the scaffold surface,
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when there is no acetone vapor treatment, and during impregnation with epoxy resin, portion of the adhered flakes is detached out from the fiber and passes in the epoxy matrix phase. The detached flakes do not contribute to the flake-to-flake contact pattern of the conductive network
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structure.
The copper flakes embedded in the upper layer of the cellulose fiber-surface form the conductive segregated networks accounted for the higher performance of the composites. In
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comparison with the thermal conductivity achieved by using spherical copper particles, e.g. 0.74 W m-1 K-1 at 68.4 wt% of functionalized Cu particles [3] and 0.52 W m-1 K-1 at 70 wt% of Cu
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nanoparticles [27], the flake-to-flake interaction pattern of copper flakes in the present work gained a competitive result (1.35 W m-1 K-1) at lower content of copper filler (36.44 wt%). The high achievement can be explained with the presumption that the flake-to-flake interaction of copper flakes fillers results in the higher possibility of heat transport through the filler networks
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rather than dispersing it in the poorly conductive matrix. Similar results were obtained in one of our recent work, in which flake-like sintered copper nanoparticles were dispersed among the (Polymethyl methacrylate (PMMA) particles [28]. The hybrid system of conductive-
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nonconductive fillers in epoxy resin gained a high thermal conductivity value (1.05 W m-1 K-1 at 30 wt% Cu content) due to the segregated network of copper filler which endowed the heat
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transfer capability throughout the composite mass. The effect of temperature and time-duration of the acetone vapor treatment of the Cu@Cell scaffold filler on the conductivity of the final scaffold-filled epoxy composites has also been investigated. The thermal conductivities at ~36.44 wt% of copper flakes are presented in the Fig. 3 (b). The composites with Cu@Cell scaffold filler were produced by 5-time dipping cycles. Treatment of the Cu@Cell scaffold with acetone vapor at 25 oC for 6 h did not realize significant
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enhancement (the thermal conductivity reached 0.78 W m-1 K-1). This may be due to the inadequacy of acetone vapor at 25 oC to bring the upper layer of the fiber into solution phase. The significant improvements were recorded when the Cu@Cell scaffold were exposed to the
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acetone vapor at 50 oC, when the acetone vapor was adequate and the diffusivity of the vapor in cellulose phase was higher. For 2 hours’ treatment, the thermal conductivity reached 1.07 W m-1 K-1 and after 12 hours’ exposure, the conductivity value was recorded to be as high as 1.46 W mK-1. However, further increase of the temperature (even at 80 oC for 6 h) did not show any
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appreciable improvement in thermal conductivity of the composite. This implies that the
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dissolved cellulose mixed copper layer becomes dense enough to inhibit the acetone vapor from interacting further deep into the cellulose core beyond a certain thickness. The direction-dependency of the thermal conductivity of the composites has been tested. The thermal conductivities of composites in the axial and radial direction are plotted in Fig. 4. As
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seen in the schematic illustration, the Cu@Cell fibers are continuous from the top the bottom of a given composite mass through flake-to-flake contact in the parallel direction. Meanwhile, the flake-to-flake contacts are interrupted in the perpendicular direction. Along the parallel direction,
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the epoxy mass constitutes the major portion of the transport path, and consequently, the conductivity is lower in the radial direction than that in the axial direction. Thus, in terms of
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thermal conductivity, the composite is anisotropic in nature. Table 1 compares the thermal conductivity achieved by the present approach with that in previous reports. As seen in the Table, the thermal conductivity of the epoxy composite has been effectively improved at lower copper loading [3,28-30]. The anisotropic thermal conductivity is obtained which would endow composites for versatile applications compared to that of isotropic material. The competitive results would encourage further study on constructing connected
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structure of metallic fillers for enhanced thermal conductive composites due to its low production cost and large-scale producibility. The thermo-gravimetric analysis (TGA) was employed to investigate the thermal stability
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of the composites (Fig. 5). As can be seen from the thermogram, the thermal degradation mainly occurs in the temperature range of 300-500 oC due to the decomposition of epoxy matrix. However, there is slight weight loss in the range of 100- 300 oC. This may be due to the removal
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of absorbed moisture from the samples and the decomposition of epoxy monomers. The T50, corresponding to the 50 wt% loss of the samples, of bare epoxy composites is at around 407 oC.
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In contrary, the T50 increases by adding the high thermally stable copper flakes to the medium. At 13.28 wt% copper flakes content, the T50 of the composite enhances to 417 oC while the temperature is up to 427 oC by loading 36.44 wt% copper flakes. The TGA results prove that introducing the copper flake fillers to the composition slightly enhanced the thermal stability.
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3.3. Effective thermal conductivity of composites
In a combined system of a non-metallic core fiber and a metallic coating layer, the heat is transported by both phonon and electron carriers, which was investigated theoretically [31]. The
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TC of the coating layer is determined by the ability to spread the electron (ke) and phonon (kp) in the coating body. The effective TC of coating layer (kc) was determined by the essential
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parameters shown in Table 2 [31]. The copper flake coated system developed in this work, however, differs a bit with the distribution of metal particles (complete coverage of the surface) assumed in the theory [31]. The copper flakes are decorated on the surface of cellulose fibers by dip-coating process and these flakes are not completely interconnected. Therefore, the gaps between the flakes reduce the thermal conduction ability of the coating layer. Another factor that should be considered is the probable contamination of the particle-surface that could affect the
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overall conductivity e.g. presence of the copper oxide layer on copper flake surface, which has significant low TC compared to the pure copper [32]. Thus, the resistance at flake-flake junctions and the contamination of the filler surfaces limits the thermal conductivity of the coating layer.
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The effect of the contact resistance between two conductive particles on the TC of a composition is also investigated by Xu et al. [33]. They considered the discontinuous system of copper flake decorated on the cellulose fiber as a continuous system of the new element on
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cellulose fibers. By adapting their results, the effective TC of the new element, kc' , which
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combines the contact resistivity Rc and the TC of copper flake layer kc , is given by [33]
1 1 = + Rc kc' kc
(2)
where kc' is the effective TC of new elements. Denoting the volume fraction of the coated fibers
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as f = f p + f c , where ƒp and ƒc are the volume fractions of the core fiber and copper coating layer, respectively, the thermal conductivities of the parallel direction (ke||) and perpendicular direction (ke⊥) are given by
2 β1 ( f p + f c ) = km exp 1 − fo ( + ) f f ( f + f ) 2 − β1 1 + p c p c fo2
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ke ⊥
(3)
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ke = (k 'c − km ) f c + kc (α 3 + 1) f p + km (1 − f p )
(4)
where km is the thermal conductivity of the matrix medium and ƒo is the maximum packing volume fraction of the fiber. Considering the morphology of cellulose scaffold as shown in Fig. 2, we assume that ƒo = 0.52 as in the random packing state of the fiber. Here, α3 and β1 represent
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the dependencies of effective TC on the Kapitza resistance and the directional thermal conductivity of the fibers. The parameters are given by [31].
1 + Rkm a
(2 + α 1 ) f p (2 − α 1 ) f p
fp k + m k1 f p + fc + 2 fc + 2 fc
;
k −1 k 'c
(5)
k ⊥ − k 'c k ⊥ + k 'c
(6)
α3 =
;
α1 = 2
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k1 =
fp − km k f p + fc 1
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β1 = 2
1 − Rkm a
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where R is the interfacial thermal resistance between copper coating layer and epoxy matrix. a is the radius of the core fibers, and k|| and k⊥ are the TCs in the perpendicular and parallel direction of the fiber, respectively. Diaz et al. investigated the TC of cellulose fibers and claimed that the heat flows faster along the fiber direction than that of perpendicular direction [34]. The thermal conductivity by parallel direction lays in the range of 0.53- 5.7 W m-1 K-1
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while that of perpendicular direction is in the range of 0.22- 0.72 W m-1 K-1. Figure 6 compares effective thermal conductivity from the theoretical prediction and our experimental results. To investigate the effect of resistivity on the effective TC of the composites,
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the theoretical TC by axial direction has been compared to that of the experimental results. The
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original TC of copper are fixed at ke = 261.4 W m-1 K-1 (thermal conductivity by electron carriers), kp = 15 W m-1 K-1 (thermal conductivity by phonon carriers) and that of cellulose fibers is k|| = 0.93 W m-1 K-1; k⊥ = 0.48 W m-1 K-1, respectively. Meanwhile, the resistivity Rc is varied from 0.01 (insignificant resistance by assuming the continuity of the coated copper layer) to 0.25 (high resistivity by assuming isolated dispersion of Cu flakes). The results are plotted in Fig. 6 (a), which shows that the interface resistance between two neighboring copper flakes plays a critical role in the thermal conductivity of the composites. When the resistance is insignificant (Rc =
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0.01), the TC of composites increases sharply with an increase in copper content in the composition. Meanwhile, with a high resistivity between the two flakes as represented by Rc = 0.25, the TC of the composites shows almost inappreciable increase in the effective TC with the
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increase in the copper content. In our system, the theoretical curve at the flake-to-flake interfacial contact resistance (Rc) of 0.07 m K W-1 results in fairly good fit to the experimental data.
Figure 6 (b) shows the effects of the parallel (axial) thermal conductivity of cellulose fiber
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on the composite’s parallel thermal conductivity. The parallel thermal conductivity of cellulose fibers strongly affects on the effective parallel TC of the composites. When the parallel thermal
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conductivity of cellulose fiber is lower than 1 W m-1 K-1, the theoretical TC curves fit well with our experimental results. However, the high parallel TC such as 5.7 W m-1 K-1 shows big discrepancy with the experimental results. A high thermal conductivity of 5.7 W m-1 K-1 has been observed for 5 nm long nanocrystal cellulose [34]. In this study, 1 mm-long cellulose fiber
from our analysis.
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was used, and k|| is expected to be much smaller, which is indeed obtained as 0.93 W m-1 K-1
Our experimental results are compared with the theoretical curves from the theoretical
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model (Eqs. (3) and (4)) in Fig. 6 (c). The contact resistivity Rc, perpendicular TC of cellulose fiber and measured TC of epoxy are 0.07 m K W-1, 0.48 W m-1 K-1 and 0.19 W m-1 K-1,
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respectively, for the theoretical prediction. Additionally, the maximum packing fraction of Cu@Cellulose fibers is assumed to be 0.52 and other parameters are listed in Table 2. In the parallel direction, the theoretical curve matches well with the experimental data while a discrepancy is observed in the perpendicular direction. The theoretical curves from the previous theory without crowding factor [31] and our model at two values of Rc (0.01 and 0.07) are plotted for a comparison. The experimental data for TC in the perpendicular direction, however, lie
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much than those predicted. It could be explained by the assumption employed in the theory that the isolated straight fibers are aligned along the axis and a conductive path is not formed in the perpendicular direction, leading to a poor thermal conductivity. In our experimental case, the
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curved cellulose fibers could form irregular links between fibers and form, at different region, a continuous perpendicular heat path in the composite medium as shown in Fig. 2 (b).
4.
Conclusions
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Copper flakes are adhered on the surface of cellulose fibers, and these fibers have been aligned vertically in epoxy composites. The acetone treatment is optimized at the temperature of
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50 oC for a period of 6 h, and along the parallel direction, a thermal conductivity of 1.35 W m-1 K-1 has been achieved, while that along the perpendicular direction is 0.65 W m-1 K-1. The composites show anisotropic behavior in terms of thermal conductivity and a modified theoretical model has been applied to investigate the effects of components on the thermal
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conductivity of composites. The theoretical prediction proves that the contact resistivity among these copper flakes plays a significant role in the effective thermal conductivity of composites. The thermal conductivities of cellulose fibers, k|| = 0.93 W m-1 K-1; k⊥ = 0.48 W m-1 K-1, well
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describe our experimental results. Our approach introduces a facile method to apply metallic flakes in fabrication of thermal conductive composites, and the thermal resistivity factor that has
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been introduced in the prediction model would provide information about the degree of realization of the flakes-flakes contact in the conductive coating layer.
Acknowledgements The support of The Leading Human Resource Training Program of Regional Neo Industry (No. 2016H1D 5A1908330) and The Basic Science Program (No. 2017R1A2B4005200) through the
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National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning is highly appreciated.
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films, Biomacromolecules 15 (2014) 4096−4101.
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Table 1. Thermal conductivity enhancement for some polymer composites with different fillers
Method
68.3
0.74
335
CuNPs/Epoxy
70
0.52
190
Cu/pPMMA/Epoxy
30
1.05
Al2O3/AgNPs/Epoxy
70
1.30
624
36.4
1.35 (k||)
610
0.65 (k⊥)
242
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Cu@Cell/Epoxy
518
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Cu/Epoxy
Filler content Thermal Conductivity Enhancement -1 -1 (W m K ) (wt%) (%)
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Materials
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and methods.
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Reference
Mixing
Fu et al. [3]
Mixing
Zhang et al. [28]
Sintering
Bae et al. [29]
Bar coating
Ren et al. [30]
Dip coating
This work
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Table 2. The properties of copper-coated layers on the cellulose fibers.
Mean Free Path (MFP)
Coupling Factor
(nm)
(W m-3 K)
(W m-1 K-1) Phonon
Electron
Phonon
261.4
15
2.7
8.2
2.7 x 1016
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Electron
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Interface Thermal Resistance 2 (m K W-1)
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Thermal Conductivity
5.2 x 10-9
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Figure 1. Schematic description of fabrication process of Cu@Cellulose/Epoxy composites.
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Figure 2. SEM images of (a) the bare cellulose scaffold, (b) Cu@Cellulose scaffold (The insets
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in Figs. 2(a,b) show the top-view of the scaffold), (c) the morphology of the Cu@Cellulose scaffold in the epoxy composites along with (d) the EDS elemental map of copper.
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Figure 3. Thermal conductivity and copper content of composites as a function of dipping cycles. (a) The composites of with and without acetone treatment are composite with Cu flakes only ( )) contains 36.4 wt% of copper without cellulose fiber, and (b) the thermal conductivities of composites at various acetone vapor treatment conditions. The abbreviation “w/o treatment”
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represents for those composites of coating scaffold without acetone vapor treatment.
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Figure 4. Thermal conductivity of Cu@Cellulose/Epoxy composites by parallel direction and
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perpendicular direction.
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Figure 5. TGA results of composites as a function of dipping cycles.
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Figure 6. Theoretical predictions on effective thermal conductivity by varying (a) contact resistance (Rc), (b) thermal conductivity of cellulose fiber, (c) and comparison of the thermal
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conductivities between theoretical predictions with experimental results.
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