Thermally Conductive, Electrical Insulating, Optically Transparent Bi

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A Thermally Conductive, Electrical Insulating, Optically Transparent Bi-Layer Nanopaper Lihui Zhou, Zhi Yang, Wei Luo, Xiaogang Han, Soo-Hwan Nathaniel Jang, Jiaqi Dai, Bao Yang, and Liangbing Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09471 • Publication Date (Web): 05 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016

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ACS Applied Materials & Interfaces

A Thermally Conductive, Electrical Insulating, Optically Transparent Bi-Layer Nanopaper

Lihui Zhou1, 2, Zhi Yang3, Wei Luo2, 3, Xiaogang Han2, Soo-Hwan Jang2, Jiaqi Dai2, Bao Yang3, Liangbing Hu2*

1

School of Chemistry and Molecular Engineering, East China University of Science and

Technology, Shanghai 200237, China

2

Department of Materials Science and Engineering, University of Maryland, College Park,

Maryland 20742, USA

3

Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20742,

USA

Email: [email protected]

ACS Paragon Plus Environment


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Abstract Cellulose nanofiber (CNF) from abundant and renewable wood is an emerging material with excellent mechanical, chemical and optical properties. Transparent nanopaper made of CNF (CNF-nanopaper) could potentially replace plastics in electronics due to its excellent optical transparency, mechanical strength and biodegradability. However, CNF-nanopaper normally has a low thermal conductivity and poor stability in increasing temperatures, which is not suitable for long term stability and reliability in devices. Herein, for the first time, we report a thermally conductive, electrically insulating, and optically transparent nanopaper using a bi-layer design where a thin layer of boron nitride (BN) nanosheets were coated on the CNF-nanopaper. An optical transparency (70 %) and a thermal conductivity (0.76 W/m/K) were successfully achieved through a solution-based process at room temperature. Such an optically transparent, electrically insulating, and thermally conductive bi-layer nanopaper can find applications in a range of electronic devices.

Key words: Cellulose nanopaper, Optically transparent, Thermally conductive, Electronically insulating, Bi-layer design, Solvent exfoliation


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1. INTRODUCTION Flexible electronics are emerging type of technologies due to their great potential to enable a range of new applications such as skin electronics, wearable devices, and conformal systems.1-3 Ubiquitously, flexible electronics are typically built on a certain type of substrate. Plastic substrates such as polyethylene terephthalate (PET),4 polycarbonate (PC),5 and polyimide (PI)6 are the most common choices due to their lightweight, flexibility, and low cost. Recently, flexible glass has been developed by Corning Inc., which shows excellent bendability and much higher processing temperature compared with plastics.7 On the other hand, transparent paper made of cellulose nanofibers (CNF), could also be an emerging substrate due to its excellent mechanical strength, high optical transmittance and potentially low cost.8-11 Furthermore, transparent paper is a biomass based biodegradable substrate, which is extremely attractive for green electronics toward a more sustainable future.12 Recently, a range of flexible electronics have been demonstrated on transparent paper, including thin film transistors,13,14 organic solar cells,15 light-emitting diodes and sensors.16 Thermal conductivity is important for integrated electronic systems on substrates, as large amounts of heat will be generated in such systems and poor thermal conductivity would cause local hot spots, which result in local failure of the devices.17,18 To increase the thermal conductivity, transparent coatings of carbon nanotubes and graphene have been applied.19-22 However, these types of thermally conductivity coatings are also electrically conductive, which is often not preferred from a system integration viewpoint, as the electrical isolation of various components on the device is essential. In this work, we report thermally conductive coatings on transparent CNF-nanopaper, which simultaneously electrically insulating. We used a bi-layer design, where boron nitride



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nanosheets (BNNSs) are aligned on the top layer of transparent CNF-nanopaper. The layered structure of BN leads to a high thermal conductivity in BN layer. The BN nanosheets are dispersed using an ethanol/water cross solvent through a scalable method. The scalable, optically transparent, electrically insulating and thermally conductive bi-layer nanopaper can potentially find a variety of applications in optoelectronics. 2. EXPERIMENTAL SECTION 2.1. Preparation of BNNSs BN powder was purchased from Sigma-Aldrich Co., Ltd (USA) (particle size: ∼1 µm). To prepare a large volume of BNNSs dispersion with good quality, 1.5 g of pristine BN powder was added into 500 mL of ethanol/H2O mixed solvent (v/v=9:1 vol%). The mixed solution was sonicated for 48 h in a water bath (FS 110D, Fisher Scientific, USA), and then settled at room temperature for 24 h. Then the top 3/4 of the supernatant was collected and the concentration of BNNSs dispersion was measured to be 1.9 mg/ml. 2.2. Fabrication of bi-layer nanopaper with 2.5 wt% of BN The CNF was disintegrated from wood pulp based on the method proposed in our previous work.23 In order to fabricate the bi-layer nanopaper, solution A was first prepared by pouring 58.3 g of 1.0 wt% CNF solution into 400 ml of deionized water under vigorous stirring at room temperature for 30 min. Similarly, solution B was prepared by pouring 1.7 g of 1.0 wt% CNF solution into 150 ml of deionized water. Secondly, 8.2 ml BNNSs suspension (1.9 mg/ml) was dropped into solution B during the stirring, which was labeled as solution C. The solution C was continuously stirred for 30 min, followed by sonicating in a water bath for another 30 min to form a uniform BNNSs/CNF suspension. Finally, the solution A was filtered with a Bucher funnel using a filter membrane (A DVPP/Cellulose acetate filter membrane of 9 cm in diameter,


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0.65 µm pore size, Milipore, USA). After solution A was drained, the solution C was added carefully to the filter. The as-prepared wet paper was placed between filter papers and followed by heating at 30 °C under mechanical pressing for 2 days, after which the free-standing bi-layer nanopaper could easily be peeled from the filter membrane. The total content of BN in the whole free-standing film is about 2.5 wt%. 2.3. Materials Characterization Scanning electron microscopy (SEM) images were taken on a Hitachi SU-70 Schottky field emission gun scanning electron microscope, while transmission electron microscope (TEM) images were taken from a JEOL 2100 TEM at 200 kV. X-ray diffraction (XRD) patterns were collected with a Bruker D8 Advance using Cu Kα radiation (λ = 1.5406 Å). The transmittance of CNF and bi-layer nanopaper was analyzed with a Lambda 35 UV-Vis Spectrometer (PerkInElmer, USA). The thermal conductivity along the BN nanosheets layer on the bi-layer nanopaper is measured by applying steady-state method outlined in our previous studies.24 3. RESULTS AND DISCUSSION A thermally conductive, flexible and optical transparent bi-layer nanopaper was designed and fabricated based on BNNSs and CNF. Figure 1a schematically shows the configuration of the bi-layer nanopaper, where a thin layer of BNNSs was uniformly coated on the pure CNFnanopaper. Figure 1b presents a closer view of the structure of the BNNSs layer and CNF layer, where the BNNSs are stacked closely on the surface of CNF-nanopaper. Due to the ultralow thickness of the BNNSs layer (1 µm), the transparency of the CNF-nanopaper does not decrease significantly. Furthermore, the high thermally conductivity of the BNNSs layer is due to a much improved in-plane thermal conductivity of the bi-layer nanopaper. Compared with the mixed design, the bi-layer structure design in this study leads to thermal conductivity only at the surface.



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Figure 1. (a) Schematic to illustrate the transparent bi-layer nanopaper conducts heat along the in-plane direction. (b) A detailed structure of bi-layer nanopaper. A liquid-exfoliation technique was employed to make BNNSs suspension from bulk BN, which is an efficient strategy to produce scalable BNNSs with remarkably stable chemical and thermal properties.25 Figure 2a shows the bulk BN powder in the ethanol/H2O (9:1 vol%) mixed solvent, where BN powder could not be uniformly and stably dispersed prior to sonication. As shown in Figure 2b, BN is comprised of alternating B and N atoms in a "honeycomb" lattice structure similar to graphite. In plane, sp2-hybridized B and N atoms are covalently bound, whereas neighboring cross planes are held together by Van der Waals forces. However, the Van der Waals forces between adjacent BN layers are stronger than that of graphite due to the electronegativity difference between B and N atoms.26 Based on the theory of Hansan solubility Parameters (HSP), the dispersion process of nanomaterials in liquid is one of adaption between the dispersive force, intermolecular force, and Hydrogen-bonding solubility parameters of solvents and solutes.27 An optimized ethanol/H2O ratio allows the solvent to penetrate into the interlayer of BN to form favorable interactions with BN molecules. Also, the solvent effect can weaken the Van der Waals forces between adjacent BN layers and slowly expands the inter-layer spacing, which assists the exfoliation of BN and form BNNSs.28 According to previous reports,


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ethanol/H2O with 9:1 volume ratio was selected in this work.29 After 48 h of sonication in the mixed solvent, a white "milky" BNNSs suspension can be obtained (Figure 2e), indicating that BNNSs were successfully peeled apart (Figure 2f). The highly stable suspension showed little precipitation after being stored for 2-3 weeks under ambient conditions (22 °C, RH 30%). The morphology of bulk BN powder and BNNSs were first investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). As displayed in Figure 2c, the bulk BN powder consisted of flakes with uniform thicknesses and shapes. The lateral sizes of BN flakes ranged from hundreds of nanometer to 1 µm with smooth surfaces (Figure 2d). After exfoliation, the lateral sizes of the majority of the BNNSs in the dispersion varied from 200-500 nm (Figure 2g). However, the thickness decreased significantly that some single BN nanosheets appears extremely thin and transparent (Figure 2h). The selected area electron diffraction (SAED) pattern (Figure 2h inset) reveals the crystalline nature and the hexagonal structure, characteristics of a single exfoliated BN nanosheet, which proves the liquid-exfoliation process using ethanol/H2O mixed solvent is a highly efficient method to obtain BNNSs.



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Figure 2. Photographs of BN powder in ethanol/H2O (a) before and (e) after sonication. Schematic of BN structure (b) before and (f) after sonication. SEM and TEM images of BN powder (c, d) before and (g, h) after sonication. The inset of h is an SAED pattern of BNNSs. Bi-layer nanopaper can be fabricated using the BNNSs solution and CNF solution. Pure nanopaper made from CNF solution was used as a control. Figure 3a and 3b show SEM images of the cross-section of pure CNF-nanopaper made from cellulose nanofibers with a diameter of 10-20 nm. The cellulose nanofibers tend to self-assemble and form a layered structure with layer-by-layer stacking, which provides CNF-nanopaper with good flexibility and transparency. Figure 3c shows the cross-section images of the bi-layer nanopaper with 2.5 wt% of BN, where the thickness of BNNSs layer is about 1 µm. When the BN layer is enlarged, BNNSs can be clearly observed (Figure 3d). Furthermore, the surface observations by SEM (Figure 3e-3f) and atomic force microscope (AFM, Figure 3g-3h) confirmed that BNNSs are successfully coated onto the surface of CNF-nanopaper.


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Figure 3. SEM images of (a, b) pure CNF-nanopaper and (c, d) bi-layer nanopaper. SEM images of (e) CNF surface and (f) BN surface of the bi-layer nanopaper. (g, h) AFM images of the BN surface of bi-layer nanopaper. The bi-layer nanopaper with 2.5 wt% of BN is relatively optical transparent, as shown in Figure 4a. Optical measurement (Figure 4b) of the bi-layer nanopaper exhibits 70%



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transmittance at 550 nm wavelength, which is comparable to that of pure CNF-nanopaper (~94%). X-ray diffraction (XRD) patterns of pure CNF-nanopaper and bi-layer nanopaper exhibit characteristic peaks around 2θ values of 16.5° and 22.5°, representing typical cellulose crystals structure indexing as the (110) and (210) planes, respectively (Figure 4c). Moreover, another two peaks at 2θ values of 26.8° and 55.2° appeared in the bi-layer nanopaper, which correspond to the (002) and (004) planes of BN. When the BNNSs are dispersed randomly on the surface of CNF-nanopaper, they prefer to lie on their preferential orientations, which are confirmed by the XRD patterns. The in-plane thermal conductivity of the pure CNF-nanopaper and the bi-layer nanopaper were further investigated using a steady-state method. The pure CNF-nanopaper shows a poor thermal conductivity of 0.04 W/mK, which is typical for cellulose based paper (Figure 4d).30 After covering with a 1 µm thickness of BNNSs layer, the thermal conductivity of whole bi-layer nanopaper increased to 0.76 W/mK, which is about 19 times higher than that of pure CNFnanopaper. When the content of BNNSs increased, the in-plane thermal conductivity increased sharply, following a percolation-like behavior. For example, the in-plane thermal conductivity of bi-layer nanopaper with 10 wt% and 90 wt% of BN were 11.3 and 30.0 W/mK, respectively.


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Figure 4. (a) Photograph of bi-layer nanopaper 2.5 wt% of BN. (b) Transmittance of pure CNFnanopaper and bi-layer nanopaper with 2.5 wt% of BN. (c) XRD patterns of pure CNFnanopaper and bi-layer nanopaper with 2.5 wt% of BN, (d) Thermal conductivity of bi-layer nanopaper with different total BN contents of 0, 2.5 wt%, 10 wt% and 90 wt%, respectively. In order to investigate the heat transfer performance of the bi-layer nanopaper, thermographs were captured by an infrared thermal camera using a laser as the heat source. The non-uniform temperature distribution revealed poor in-plane thermal conductivity of pure CNFnanopaper since the heat created by a laser pointer could not spread out easily, which resulted in a relative high temperature at the center of laser pointer (Figure 5b). In contrast, a more uniform temperature distribution can be observed on the bi-layer nanopaper (Figure 5c). Figure 5a shows the temperature distribution of the two samples with respect to the location of the laser pointer.



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For pure CNF-nanopaper, the temperature starts around 21°C while it starts around 23 °C for bilayer nanopaper with 2.5 wt% of BN. However the temperature distribution of pure CNFnanopaper is narrower than that of the bi-layer nanopaper, which indicates heat was not distributed along the pure CNF-nanopaper as compared to the bi-layer nanopaper. The better thermal distribution of the bi-layer nanopaper can be contributed to the higher in-plane thermal conductivity due to the introduction of the BNNSs layer.

Figure 5. (a) Temperature distribution of pure CNF-nanopaper and bi-layer nanopaper with 2.5 wt% of BNNSs. Thermograph of (b) pure CNF-nanopaper and (c) bi-layer nanopaper with 2.5 wt% of BNNSs.


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4. CONCLUSION In summary, we reported a bi-layer transparent paper with percolative, aligned BN nanosheets coated on the surface of CNF-nanopaper. The perfect in-plane alignment of the BNNSs on the nanopaper increase the thermal conductivity of nanopaper from 0.04 W/mK to 0.76 W/mK when the BN content increases from 0 wt% to 2.5 wt%, while maintaining the high optical transparency (up to 70%). Compared with well-developed thermally conductive coating with graphene and carbon nanotube, the BNNSs coating is electrically insulating, which is essential for system integrations on transparent and flexible substrates. ACKNOWLEDGEMENTS The project was funded by the Office of Naval Research under grant N000141410721. The authors acknowledge the support of the Maryland Nanocenter and its Fablab, Nisplab, and surface analysis center. The project was also funded by the China Scholarship Council (CSC).

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Thermally Conductive, Electrical Insulating, Optically Transparent Bi

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