Significant Enhancement of Thermal Conductivity in Nanofibrillated

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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 40766-40773

Significant Enhancement of Thermal Conductivity in Nanofibrillated Cellulose Films with Low Mass Fraction of Nanodiamond Na Song,*,†,§ Siqi Cui,†,‡,§ Xingshuang Hou,† Peng Ding,† and Liyi Shi*,† †

Research Center of Nanoscience and Nanotechnology and ‡School of Materials Science and Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China S Supporting Information *

ABSTRACT: High thermal conductive nanofibrillated cellulose (NFC) hybrid films based on nanodiamond (ND) were fabricated by a facile vacuum filtration technique. In this issue, the thermal conductivity (TC) on the in-plane direction of the NFC/ND hybrid film had a significant enhancement of 775.2% at a comparatively low ND content (0.5 wt %). The NFC not only helps ND to disperse in the aqueous medium stably but also plays a positive role in the formation of the hierarchical structure. ND could form a thermal conductive pathway in the hierarchical structures under the intermolecular hydrogen bonds. Moreover, the hybrid films composed of zero-dimensional ND and onedimensional NFC exhibit remarkable mechanical properties and optical transparency. The NFC/ND hybrid films possessing superior TC, mechanical properties, and optical transparency can open applications for portable electronic equipment as a lateral heat spreader. KEYWORDS: nanodiamond, nanofibrillated cellulose, hierarchical structure, thermal conductivity, optical transparency

1. INTRODUCTION Nowadays, with the further development of portable electronic equipment, lateral heat spreader has been widely used.1−3 Polymeric nanocomposites are a promising selection for solving heat dissipation issues owing to their excellent characteristics such as low specific weight and so forth. Nevertheless, preparation of desired materials with high thermal conductivity (TC) is an issue that should be solved instantly. Carbon-based nanomaterials, such as graphene nanosheets and carbon nanotubes, which have extremely high TC with lightweight, show a certain enhancement in the TC of polymer composites.4−6 However, an electrical insulating property of materials is also concerned. Diamond has a high feasibility because of its excellent properties, such as favorable mechanical properties, high TC (∼2000 W·m−1·K−1), chemical inertness, and especially electrical insulating properties.7 Additionally, nanodiamond (ND) also exhibits most of the remarkable performances of bulk diamond even on nanoscale.8,9 In a report, polyvinyl alcohol (PVA)/ND nanocomposites were prepared, which achieved a 2.5 times improvement in Young’s modulus and 25% improvement in the TC with 1 wt % ND.10 In a separate study, Branson et al. reported that ethylene glycol nanofluids exhibit 12% TC enhancement with a ND loading of 0.9 vol %.11 Because of its attractive thermal properties, ND has caused great concern in the area of effective thermal management materials.10−12 One-dimensional (1D) nanofibrillated cellulose (NFC), a new biological material with a highly crystalline nanofiber, shows remarkable mechanical properties which can be used to fabricate highly ordered layered composites. Therefore, NFC© 2017 American Chemical Society

based nanocomposites have drawn more and more attention in expectation of high mechanical properties, lightweight, transparency, and so forth.13,14 NFC has a natural tendency on the formation of a continuous nanofiber network, in which NFC serves as a continuous matrix around fillers to achieve a homogeneous dispersion effect and adapt their basal plane parallel to the surface.15 Up to now, multipurpose NFC-based composites with excellent mechanical properties have been prepared by various materials, such as boron nitride,16 graphene nanosheets,6 montmorillonite clay platelets,17 and talc.15 In our previous studies, NFC hybrid films were prepared with graphene nanosheets by the vacuum filtration process18 and the layer-by-layer assembly process,19 which show regular layered structures and remarkable performance. Moreover, because of the hierarchical structure, the composites, with gas barrier properties, can be used as a water barrier and flameretardant.17 On the basis of the above-mentioned reasons, NFC was selected as the reinforced phase in hybrid films to produce transparent composite substrates.20 Herein, hierarchical NFC/ND hybrid films were fabricated for lateral heat spreader applications through a facile vacuum filtration technique. The combination of 1D NFC and zerodimensional (0D) ND resulted in the three-dimensional (3D) hierarchical nanocomposites with excellent performances. In this contribution, the 1D NFC was selected as the adhesive, not just to enhance the tensile strength and toughness of the hybrid Received: June 27, 2017 Accepted: October 31, 2017 Published: November 10, 2017 40766

DOI: 10.1021/acsami.7b09240 ACS Appl. Mater. Interfaces 2017, 9, 40766−40773

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic representation of the vacuum filtration fabrication of a NFC/ND hybrid film from the solution.

Figure 2. (a) FTIR spectrum of ND particles. (b) FTIR spectrum of NFC/ND hybrid films. (c) Schematic illustration of the intermolecular hydrogen bond between NFC and ND. Suspension test (d): the comparison between the ND in deionized water and in NFC suspension after ultrasonic treatment (the left) and after sitting overnight (the right). After ultrasonic treatment, the ND was suspended homogeneously in both deionized water and NFC suspension. However, after sitting overnight, some sediment can be found in the bottom of the bottle with deionized water. In addition, the ND with the NFC suspension was still well-suspended.

films but also to separate the ND particles from compact restacking. Incorporation of 0.5 wt % ND can multiply the TC by 7.75 ± 0.53 times. Although many papers have reported the applications of ND in many aspects, this is the first known report on such an efficiently enhanced in-plane TC of hybrid films filled with ND as a spherical filler. The above NFC/ND hybrid films exhibited excellent TCs, high mechanical proper-

ties, and favorable transparency which could expand the application in electronic equipment.

2. EXPERIMENTAL SECTION 2.1. Fabrication of NFC/ND Hybrid Films. The NFC suspension was manufactured through the method described by Saito (for the concrete process, see the Supporting Information). The NFC/ND 40767


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ACS Applied Materials & Interfaces hybrid films were fabricated through a vacuum filtration process. A variety of ND powders (Figure S1c) were added in a controlled manner to NFC suspensions at ordinary temperature and then strongly agitated for about 1 h to yield a uniform dispersion with an ND content of 0−10 wt %. Then, the dispersions were vacuum-filtered on the membrane (mixed cellulose ester membrane, 47 mm in diameter, 0.45 μm pore size) (Figure 1). The hybrid films were dried in a vacuum oven at 40 °C overnight before removing from the membranes. The hybrid films with an NFC/ND weight ratio of 100:0, 99.5:0.5, 99:1, 97:3, 95:5, and 90:10 were named CND-0, CND-05, CND-1, CND-3, CND-5, and CND-10, respectively. 2.2. Characterization. The Fourier transform infrared (FTIR) spectra were obtained on an Avatar 370 FTIR spectrometer using potassium bromide pellets. All of the samples were dried at 40 °C overnight under vacuum before the FTIR analysis. The morphology and microstructures of hybrid films were examined by a JSM-6700F emission scanning electron microscopy (SEM) instrument. Transmission electron microscopy (TEM) images of NFC and ND were acquired using a 200CX 178 transmission electron microscope. The TCs of all samples were measured using a laser flash apparatus (Netzsch LFA 447 NanoFlash) at 25 °C. Each TC test was repeated six times, and the values with large errors were excluded. The stress− strain curves were obtained using a dynamic thermomechanical analyzer, from TA Instruments (Q800), with a film tension mode. Rectangular strips of 2 × 20 mm with varying thicknesses were cut from the films and tested at a rate of 0.5 N·min−1 at room temperature. The ultraviolet−visible (UV−vis) spectra were collected at an ordinary temperature using a UV−vis spectrophotometer (UV-2501PC).

detected by the broad vibration of −OH in the range of 3100− 3500 cm−1,24,25 which leads to a shift to lower wavenumbers of the peak. The −OH peaks (Figures 2b and S1d) distinctly exhibit a shift to lower wavenumbers in the NFC/ND hybrid films, demonstrating the intermolecular hydrogen bonding between the NFC and ND. Consequently, NFC not only formed the matrix phase of hybrid films but also played an important role in the stable dispersion of the ND in aqueous medium (Figure 2d). The NFC/ND hybrid films were prepared by the selfassembly process with NFC and ND under vacuum. The vacuum-assisted self-assembly is a facile method which has advantage in the rapid technology and scaling-up of the preparation of the hybrid films, compared with layer-by-layer assembly,26,27 freeze-casting,28,29 spray-coating,30 and solventcasting methods.24,31 The alignment of the NFC appeared to be driven under the vacuum pressure by the compressive force caused by the lowering of the air−water interface during solvent removal.32 In this issue, the well-aligned NFC network was formed by accumulating nanofibers.6,15,33,34 As for the detailed mechanism and methodology of the method, it has been elaborated in our previous research and not summarized it here.6 Because of intermolecular hydrogen bonds between ND and NFC, this alignment can also be extended to NFC/ND systems. In the cross-sectional view of the samples, the significant hierarchical structure formed by well-aligned NFC (Figure 3c−f) and ND particles between NFC can be easily

3. RESULTS AND DISCUSSION 3.1. Hydrogen Bonding and Self-Assembly. In a typical experiment, because of the repulsive forces that were generated by the surface carboxyl groups, NFC was homogeneously dispersed in deionized water without forming aggregates (Figure S1a).21 The diameter of the NFC varied over the range of 20−50 nm (Figure S1b, Table S1) and had a large aspect ratio, which was significant for the formation of a continuous network. As shown in the FTIR spectrum of the ND particles (Figure 2a), the absorbance peaks at 2922 and 1320 cm−1 were attributed to the C−H stretching and deformation vibration of the alkyl group, respectively, which coincide with other reported studies.10,22 The absorption peak at 1629 cm−1 corresponds to the stretching vibration of the aromatic sp2 carbon bond which can be connected with graphite around the ND particles. The absorption peaks at 1726 and 1117 cm−1 can be ascribed to the CO stretching vibration in the carboxylic acid and carboxyl groups and the epoxy C−O stretching vibration, respectively.23 Strong absorbance peaks assigned as hydroxyl groups appeared at 3426 cm−1 (O−H stretching vibration) and 1629 cm−1 (O−H bending vibration).23 As mentioned above, the oxygen-containing functional groups on the surface of the ND particles are deemed to conduce not only to the dispersion of ND particles in aqueous medium but also to the strong interaction, including intermolecular hydrogen bonding, with NFC. For the hybrid films, the dominating contributor to the interactions was considered as the intermolecular hydrogen bonds between −OH/−COOH groups on ND particles and −OH groups on NFC nanofibers (Figure 2c).10 There have been reports about hydrogen bond interaction between ND and highly polar polyalcohols, for example, PVA, in which the −OH groups on PVA played an essential part in participating in intermolecular hydrogen bonding with oxygen-containing groups of the ND.10 Intermolecular hydrogen bonds in polymer composites can be

Figure 3. (a,b) SEM images of the surface of CND-1 and CND-10, respectively. (c) Cross-sectional SEM image of CND-1. (d−f) Crosssectional SEM images of CND-10.

found (Figure 3f). This unique structure could provide a pathway for lateral thermal conduction. Meanwhile, in the SEM images of the top view of CND-1 (Figure 3a), ND particles were almost uniformly distributed in NFC. In addition, in the image of CND-10 (Figure 3b), the agglomerates 100−200 nm in size could be found easily. 3.2. Thermal Conductivities. The TCs of the NFC/ND hybrid films were tested by a laser flash system. Round test disks with a diameter of 25.0 mm and square test disks with a 40768


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Figure 4. (a) In-plane and through-plane TCs of NFC/ND hybrid films. (Inset: Partial enlarged drawing of in-plane TCs of CND-3, CND-5, and CND-10.) (b) Comparisons on the values of TC enhancement per wt. (c) Schematic diagram of TCs of hybrid films.

Table 1. TCE in Previous Literatures filler

matrix type

Km (W·m−1·K−1)

type

composite content (wt %)

K (W·m−1·K−1)

TCE (%)

TCE/wtc

epoxy epoxy epoxy epoxy

0.12 0.2 0.2 0.255

MWCNT Py-PGMA−GNS GPMMA Ag−GNS

0.5 0.5 0.5 0.5

0.21 0.5 1.25 ∼0.6

75 150 525 135

150 300 1050 270

epoxy epoxy

0.144 0.17

MWCNT@SiO2 Al2O3−CNTs

0.5 0.75

0.213 0.4

48 135.3

96 180.4

epoxy epoxy epoxy epoxy PI PI PI PEEK PEEK PVDF-HFP PVA PE PCM PPS NFC

∼0.13 0.2 0.22 0.2 0.188 0.15 0.177 0.23 0.23 0.22 0.13 0.2 0.22 0.243 0.034b

GD400-MWCNTs/MGPs TRGO−silica NS SWCNT f-GF MWCNT 3D-C MWCNT SWCNT/IF-WS2 HPEEK−CNT rLGO ND TRGO annealed graphene MWNT-7/MIP-2 ND

1 1 1 1 0.1 0.35 1 1 0.5 0.25 1 1 1 1 0.5

∼0.32 0.322 0.5 0.3 0.31 1.7 ∼0.47 ∼0.529 0.414 3.2a 0.17 0.325 0.32 0.401 0.118

146 61 125 50 62.77 1033 165 130 80 1354.5 30.8 62.5 45.5 65 247.1

146 61 125 50 627.7 2952 165 130 160 5418.2 30.8 62.5 45.5 65 494.2

1 0.5 1

0.135 9.820 10.754

297.1 775.2 858.5

297.1 1550.4 858.5

1.122a

testing method comparative method hot disk thermal analyzer steady-state method modified transient plane source technique steady-state heat flow technique transient plane source technique hot disk thermal analyzer laser flash comparative method laser flash laser flash laser flash hot disk thermal analyzer steady-state method steady-state method laser flash steady-state method steady-state method laser flash steady-state method laser flash

refs 39 40 41 42 43 44 45 46 47 48 4 49 50 51 52 53 10 54 55 56 this work

a

The thermal conductivities for the in-plane direction. bThe thermal conductivities for the through-plane direction. The directions of TC without superscript were not shown in the original. cThe values are the ratio of TCE and filler content.

length of 10.0 mm were cut out from the as-prepared films for in-plane and through-plane TC measurements, respectively.

The detailed methodology and mechanism of the measurement have been reported in our previous studies.5,6 40769


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Figure 5. (a) Stress−strain curves of CND-05 and CND-10. Inset in (a) shows the digital images of CND-05. (b) UV−vis spectra of NFC/ND hybrid films with different contents of ND from 0 to 10 wt %. (c) Optical images of NFC/ND hybrid films with different contents of ND. (d) UV− vis transmittance of NFC/ND hybrid films at 550 nm.

It was obvious in Figure 4a that the TC of hybrid films has significant enhancements with the increase of ND content. In particular, the in-plane TC is significantly increased from 1.122 to 9.820 W·m−1·K−1, with only 0.5 wt % ND loading. Meanwhile, the thermal conductivity enhancement (TCE) was introduced to quantitatively characterize the efficiency of the ND in the NFC matrix. It can be defined as TCE = (K − Km)/Km, where K and Km are the TC of the hybrid films and the pure NFC film, respectively. The CND-05 hybrid film revealed a remarkable enhancement of the in-plane TC by 775.2 ± 53.0% as compared to the pure NFC film, whereas that of the CND-1 hybrid film increased as high as 858.5 ± 65.3% of the pure NFC film, which showed better thermal conductive properties than that of our previous works.6,18 This is the first study that presents such a significant enhancement of the inplane TC filled with ND. Table 1 summarizes TC and the calculated TCE values for polymeric composites with the content of 1 wt % or less than in previous literatures. The comparison highlights that the measured TCE and TC in the NFC/ND are indeed at a high level. Compared with previous investigations about polymeric composites, similar TC values often need higher wt % filler content. Moreover, the TCE of the hybrid film per 1 wt filler loading (Figures 4b and S2)35,36 was used to further analyze the efficiency of the ND in the NFC matrix. If measured by this, the calculated values of CND-05 and CND-1 were found to be more than 1550 ± 108 and 859 ± 65 (Table 1), respectively. As compared with the polymer composites in previous research studies (Figures 4b and S2), these NFC/ND hybrid films are also at high levels at the same filler contents focused on the thermal conductive composites. It is supposed that such an enhancement in hybrid films was attributed to the enhanced interfacial thermal transport abilities arising from the addition content of ND. The 1D NFC

possesses the inherent property of anisotropy, which could easily form the hierarchical films.37 The hierarchical structure could help the ND, which dispersed in the NFC network, to form a thermal conductive pathway (Figure 3f yellow lines, 4c red arrows). Compared with CND-05 and CND-1, CND-10 shows lower TCE, which may be caused by the agglomerates of ND particles (Figure 3f red circles with dotted lines). In addition, this condition could be improved by the modification of ND particles in follow-up works. At the same time, the orientation of the hierarchical structure could also influence the TCE of hybrid films. The higher the orientation of the hierarchical structure is, the more obvious the improvement of the TC is.19,38 The degree of orientation of the hierarchical structure could be quantitatively identified by the fast Fourier transform approach (Figure S3). Compared with the CND-1 hybrid film, the CND-10 hybrid film exhibits a similar ΦH/V of 1.95 (Figure S3), revealing a similar degree of orientation of the hierarchical structure.19,33 This factor could also influence the TCE of hybrid films. Besides the in-plane TC, the through-plane TC of NFC/ND hybrid films was in the range of 0.118−0.329 W·m−1·K−1, which was obviously larger than that of the pure NFC film (0.034 W·m−1·K−1). Similarly, CND-05 revealed a high enhancement of the through-plane TC by 247.1 ± 12.3% (Table 1) as compared to the pure NFC film. When measured by the TCE of per 1 wt of filler, the value is 494 ± 25, which exhibit a certain level. However, the TCs on the through-plane and in-plane directions are quite different. This could be attributed to the hierarchical structure of NFC and hybrid films. The structure built a more efficient thermal conductive pathway on the in-plane direction, which provides more remarkable enhancement.6,18,19 After simple calculation, it can be seen that the hierarchical NFC/ND hybrid films exhibited anisotropic 40770


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introduction of the ND which will create light scattering when the light transmits through these films, and the increasing ND content results in more severe light scattering.60,61 In addition, the CND-05 hybrid film has 65.29% transmittance at 550 nm wavelength, which reached a high transparency level close to that of CND-0. In addition, the NFC/ND hybrid films show high optical transparency, which exhibited multifunction compared to our previous work.6 Electrical insulating films with controllable optical transparency and TCs could extend their applications in a variety of fields, for example, CND-05 can be used as a transparent substrate for flexible electronics applications. Hybrid films with higher ND loading exhibit remarkable TC and mechanical properties, which possess the potential application prospect in military.

TCs, and the anisotropy index (AI) (Figure S4), which is defined as AI = TCin‑plane/TCthrough‑plane, presents a downward trend with the increase of the through-plane TC. The enhancement of the TCE and the decrease of the AI may probably be caused by the increased content of the 0D filler.57 3.3. Mechanical Properties. Nevertheless, favorable mechanical performances of thermal management materials play a vital role in applications in portable and flexible modern electronics. Therefore, to evaluate the potential applications for NFC/ND hybrid films, the mechanical performances of NFC/ ND hybrid films were explored by the tensile test. Figure 5a exhibits the typical stress−strain curves of CND-05 and CND10. In addition, the tensile strengths of CND-05 and CND-10 were up to 99.07 and 86.16 MPa, respectively, which are a little lower than that of CND-0 (103 MPa) (Figure S5, Table S2). Although the addition of ND disturbs the ordered structure (Figure S3) and destroys the continuous nanofiber network of the NFC, which lead to deterioration of mechanical properties,16,18 the hybrid films still exhibit a decent mechanical property. Toughness could be obtained by the area under the stress−strain curve. In addition, the toughnesses of CND-05 and CND-10 were up to 3.37 and 1.32 MJ/m3 (Table S2), respectively, which show an outstanding performance. Also, the mechanical performances of the hybrid films exhibit a downward trend with the increase of ND contents. These indicate that the hybrid films inherited the mechanical performances of the NFC. That is, in these hierarchical hybrid films, the NFC played the role of an adhesive to provide tensile strength and toughness. Compared with previously reported NFC-based materials, the NFC/ND hybrid films do not exhibit superior mechanical properties.58 The mechanical properties of NFC-based materials are based on the NFC. In addition, the mechanical properties of the NFC are influenced by many factors, such as preparation method, resource of raw material, and so forth.59 Different raw materials exhibit different properties such as polymerization degree, crystallinity, and crystal form.59 The above factors also play an important role in the mechanical properties of NFC-based materials. This could be the reason why the NFC/ND hybrid films are far from the optimum mechanical properties than other reported NFCbased materials.58 That is to say, the layered structure of the film, oriented nature of the fibrils, and the strong binding between ND and NFC play a positive role in the mechanical properties. However, from the mechanical properties obtained at last, it can be observed that the mechanical properties of the NFC matrix and the negative influence of the filler on the structure play a more important role in this system. More systematic and specialized research on the relationship between the film structure and mechanical properties (including debonding) is needed in the future. 3.4. Optical Transparency. The optical transparency of NFC/ND hybrid films was monitored by UV−vis spectra (Figure 5b). The thicknesses of the films are all from 50 to 60 μm. The CND-0 film revealed more than 60% transparency in the visible region (Figure 5b). As shown in Figure 5c, the background underneath the NFC/ND hybrid films is distinct through the optical images, which shows that all of the hybrid films were transparent and their colors gradually deepen with increasing ND contents from 0 to 10 wt %. In other words, an increasing content of ND resulted in a decrease of transparency of hybrid films. Similarly, the light transmittance of NFC/ND hybrid films at 550 nm wavelength also decreased with the increasing ND content (Figure 5d). This is due to the

4. CONCLUSIONS In summary, hierarchical NFC/ND hybrid films are fabricated by the self-assembly process with 1D NFC and 0D ND. The resulting films have excellent combined TCs and mechanical properties, which is attributed to the hierarchical structure formed by 1D NFC. The transparency and TC of the hybrid films can be tuned by adjusting the ND content. The hybrid film containing 0.5 wt % ND shows a high in-plane TC of 9.820 W·m−1·K−1, which is 7.75 ± 0.53 times better than that of the pure NFC film. This is the first study of ND being used in the NFC which shows an extremely high enhancement on the inplane TC. All in all, flexible transparent electrical insulating NFC/ND hybrid films with high TCs can open applications for portable electronic equipment as a lateral heat spreader.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09240. Detailed synthesis method of NFC suspension; photograph and TEM image of NFC dispersion; TEM image of ND; FTIR spectrum of NFC/ND hybrid films; size distribution of NFC; comparisons on the values of TCE per wt; FFT frequency domain images of CND-1 and CND-10; AI of NFC/ND hybrid films; stress−strain curves of CND-0, CND-05, and CND-10; and mechanical properties of hybrid films (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.S.). *E-mail: [email protected] (L.S.). ORCID

Na Song: 0000-0002-3343-3000 Author Contributions §

N.S. and S.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51703122), PetroChina Innovation Foundation (no. 2016D-5007-0508), the Natural Science Foundation of Shanghai (no. 17ZR1440700), and the Program of Shanghai Academic/Technology Research Leader 40771


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(no. 17XD1424400). The authors thank Dr. Yanyan Lou for help with the SEM measurements.

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Significant Enhancement of Thermal Conductivity in Nanofibrillated

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