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Thermally Conductive Boron Nitride Nanosheet Composite Paper as a Flexible Printed Circuit Board Tun Wang, Daohui Ou, Huanhuan Liu, Shusen Jiang, Wanying Huang, Xiao-liang Fang, Xinyi Chen, and Miao Lu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00160 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 24, 2018

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ACS Applied Nano Materials

Thermally Conductive Boron Nitride Nanosheet Composite Paper as a Flexible Printed Circuit Board Tun Wang, Daohui Ou, Huanhuan Liu, Shusen Jiang, Wanying Huang, Xiaoliang Fang, Xinyi Chen*, Miao Lu* Pen-Tung Sah Institute of Micro-Nano Science & Technology, Xiamen University, Xiamen, Fujian, P.R. China, 361005 KEYWORDS. BN; fiberglass; thermal conductivity; tensile strength; transmission loss

ABSTRACT The development of portable and wearable electronic devices has substantially increased the demand for printed circuit boards with high thermal conductivity, optimal mechanical flexibility, electrical insulativity and minimal high-frequency transmission loss. Herein, we demonstrate the fabrication of a thermally conductive, flexible composite paper, for electronic and microwave devices based on hexagonal boron nitride nanosheets, polyvinyl alcohol (PVA) and fiberglass mesh (FGM). The prepared composite paper exhibits in-plane thermal conductivity of 22.51 W/(m·K), and, the FGM induced, the high mechanical strength of 27.92 MPa. The transmission loss, of the grounded coplanar waveguide lines, was 0.10 dB/mm at 7.0 GHz, and shows negligible variation while bending, indicating the high flexibility of

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circuit board. These results demonstrate potential application of BN-based composite paper in flexible electronic devices.

1. INTRODUCTION The on-chip integration of microelectronic devices has shown significant side effects, such as heat accumulation on the printed circuit board (PCB), which can result in malfunction or failure of the devices.1,2 Therefore, effective heat dissipation has become an important challenge in the longterm operation of microelectronic devices, particularly, the high radio frequency (RF) devices. In conventional flexible electronics, such as radio frequency identifications (RFIDs), sensors, memory devices, displays and power sources, the electrically insulative polymers, such as polyimide (PI) or polyethylene terephthalate (PET), are applied as substrates.3-6 These materials offer high mechanical strength (> 100 MPa),7-9 however, suffer from the low intrinsic thermal conductivity (0.1-0.2 W/(m·K)).10-13 Therefore, an external cooling system is required for most the devices, which is not suitable for the portable and wearable applications. An individual hexagonal boron nitride (h-BN) nanosheet (point group = D6h; space group = P63/mmc) has a layered structure,14,15 similar to graphene, and offers high theoretical thermal conductivity (~ 2000 W/(m·K)).16,17 Moreover, unlike graphene the h-BN is electrically insulative and provides a wide direct bandgap (5.2-5.9 eV)18,19 and comparatively high relative dielectric constant(3-4).18 Therefore, the h-BN is considered favorable, as thermal interface material (TIM) and the electrically insulating substrate, in the field of high power flexible electronics. Furthermore, the composite of h-BN nanosheets (BNNs), with other adhesive materials, have shown practical value20,21. However, the random distribution of BNNs and


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thermal insulating adhesive materials hindering phonon transport in the composite, thus lead to low thermal conductivity.22, 23 Recently, many efforts have been made to increase the thermal conductivity of h-BN based composites.24-26 Several research groups have reported that compatible matrix mixing with BNNs could significantly enhance the thermal conductivity (Table S1).27-31 For instance, BN and graphene nanosheets composites in polystyrene (PS) and polyamide (PA) matrix have shown the thermal conductivity of 0.66 W/(m·K) and 1.76 W/(m·K), respectively.32 Also, polyvinyl alcohol (PVA) and BNNs composites in polydimethylsiloxane (PDMS) matrix, could attain a throughplane thermal conductivity of 2.0 W/(m·K), at 30 wt. % BN in PDMS.33 Besides the selection of matrix, the optimization of the composite structure is another approach to enhance the thermal conductivity. Otherwise, randomly distributed BNNs and the epoxy in the composite paper would block the phonon transport.34 For example, magnetic fields were employed to arrange the h-BN platelets in composites, which increased the thermal conductivity by 40%.28 In addition, h-BN scaffolds with PDMS by thermal curing achieved the through-plane thermal conductivity of 1.4 W/(m·K).35 However, the thermal conductivity of these composites is usually still comparably low i.e. less than 10 W/(m·K) for a good heatsink (Table S1).27,32,35-37 Moreover, most reported works focused on utilizing thermally conductive h-BN filled composites as thermal interface material,12,30,32,33,38,39 not directly used as a substrate for flexible PCB. A good substrate for flexible PCB should have high thermal conductivity and mechanical strength. With that in mind, we have fabricated a BNNs based flexible composite paper with high in-plane thermal conductivity. PVA was introduced to increase the adhesion and FGM was


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applied to reinforce the mechanical strength. The composite paper exhibited a high thermal conductivity of 22.51 W/(m·K), and a mechanical strength of 27.92 MPa. For the practical application as a PCB, the composite paper served as a good heatsink for LED, and a substrate with low transmission loss for grounded co-planar waveguide (CPW). 2. EXPERIMENTAL SECTION 2.1. Materials. The h-BN flakes were purchased from Alfa Aesar, Shanghai, China. Polyvinyl alcohol (PVA, 95% hydrolyzed, average M.W. 95,000) was purchased from J&K Scientific Ltd. China. Fiberglass mesh was purchased from Kunshan Green Cycle Chemical Industry, China. 2.2. Synthesis of composite paper. The BNNs were exfoliated from the purchased h-BN flakes by using ionic liquid, magnetic stirring, sonication and thermal treatment.40 The resulting stable milk-like dispersion was centrifuged at 11000 rpm for 5 min. Then the as-derived white powder was sequentially sonicated and centrifuged in deionized water, ethanol, dichloromethane and ethanol. Then final BNNs were dispersed in isopropanol and a low speed centrifugation (3000 rpm,15 min) was applied to filter out the aggregated thick flakes. The similar exfoliation process can be found in published literature.14,41 The PVA powder of 200 mg was sufficiently dissolved in 20 mL deionized water for 1 h at 90 °C under magnetic stirring. The fabrication of BNNs/PVA@FGM composite paper is illustrated in Fig. 1. Firstly, the PVA solution was mixed with the exfoliated BNNs solution by magnetic stirring for 4 h and sonication for 2 h. The mixed solution with different BNNs content was sprayed homogeneously on FGM by using an airbrush. Then, the composite paper was heated in an oven at 80 °C for 5 min. The cycle of spraying and heating was repeated until the thickness of composite paper reached to 100 µm. For references, h-BN/PVA (using the commercial BN flakes) and BNNs/PVA without FGM were prepared as


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follows: h-BN or BNNs were added into the PVA solution under stirring for 1 h and then sonication for 2 h. Then the mixture was heated to adjust the concentration and dropped on the cellulose membrane, dried at 65 °C for 1.5 h. Then acetone was used to remove the cellulose membrane and the referencing composite papers were obtained. Before characterization and device fabrication, the composite papers were cut into 30 x 30 mm pieces and rolled under the twin-roll squeezer to improve the density and smoothness. 2.3. Materials characterization. Scanning electron microscopy (SEM, Carl Zeiss, Germany) and atomic force microscopy (AFM, OXFORD Cypher S, UK) were employed to characterize the morphology of h-BN, BNNs and prepared composite papers. Further, the structure of the exfoliated BNNs was confirmed via Raman spectroscopy (IDSpec ARCTIC, excited by 532 nm laser) and X-ray diffraction (XRD, Shimadzu, Japan). The specific heat capacity was measured by using differential scanning calorimetry (DSC 214 Polyma, Netzsch, Germany). The small circular pieces (diameter=2 mm) of prepared composite papers were put into the metal crucible, while as a reference, the sapphire standard sample was put into another metal crucible. The samples were heated from room temperature to 120 °C, at 5 °C/min and cooled down to 0 °C by using liquid nitrogen. To eliminate experimental error due to moisture presence, each sample went through two heating and cooling cycles. The laser flash diffusivity instrument (LFA467, Netzsch, Germany) was used to measure the in-plane thermal diffusivity of the composite papers. A special sample holder was used to accommodate the free-standing circular composite paper with a diameter of 22.5 mm. Then, the circular paper was spray coated, with graphite paint, to avoid undesirable laser reflections from the instrument.39 The thermal conductivity was calculated using Eqn. (1): K=Cp·Td·ρ

(1)


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where Cp and Td correspond to the specific heat capacity and thermal diffusivity of the sample, respectively. ρ represents the density of the composite BNNs film, which can be calculated from the m/Sh relationship, where m, S and h correspond the mass, area and thickness of the composite sample, respectively. The out-of-plane thermal conductivity cannot be measured directly because the composite paper was only 100 µm thick, much less than the requirement (1 to 5 mm) of laser flash diffusivity instrument LFA467 requirements. As an alternative, the composite paper was used as the thermal interface material (TIM), between a 5 W light-emitting diode (LED) chip and the heatsink, to exhibit the out-of-plane thermal performance. The temperature variation of the LED chips was measured by the thermocouples (Model GG-K-36-SLE. Precision: 0.4%; diameter: 0.127 mm) and recorded by Agilent 34972A. The mechanical property was measured by electronic universal material testing machine (WDW5000N-M, WEIDU, China). The sample for mechanical testing was prepared by cutting composite paper into rectangular shape. The sample was tightly clamped in machine jaws and elongated at the rate of 0.5 mm/min. To demonstrate the flexibility of the fabricated composite paper, we have fabricated the CPW lines, 20 mm in length, on the surface of the composite paper. We have also fabricated the similar CPW lines on PI, which is a well-known material applied in flexible electronics, for comparison purposes.42-44 The signal transmission performance, of the CPW line, was assessed by a vector network analyzer (ZNB 8, R&S) and a designed PCB was used for fixing the fabricated CPW line. The sweeping frequency was set from 1 MHz to 7 GHz and the parameter of S21 was selected to characterize the transmission loss of the RF signal.


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3. RESULTS AND DISCUSSION Figure 2a shows the SEM image of stacked h-BN platelets. The thickness of a single h-BN platelet assessed by AFM is around 60 nm (Fig. 2b). Figure 2c and 2d show that the hexagonal BNNs have been excellently exfoliated and dispersed with a lateral dimension of 0.5-3 µm and the thickness of 1.2 nm. Considering the absorbed water or contamination layer, the exfoliated BNNs were about 3 layers.45 The structure and surface properties of the exfoliated BNNs were characterized by Raman spectroscopy and X-ray diffraction (XRD) and results are presented in Fig. 2(e-f). In Raman spectrum, the intense and sharp peak at 1367 cm-1 corresponds to the E2g vibration mode of hBN.46, 47 The full width at half-maximum (FWHM) of the E2g mode peak was found to 19.2 cm-1, which much lower than previous reports (27 cm-1 and 22.9 cm-1).14,48 This indicates a good quality of the exfoliated BNNs,20,46 corresponding to the FWHM of monolayer and few-layer BNNs in reported literature.49, 50 In addition, the XRD pattern confirmed the hexagonal structure (JCPDS file no. 34-0421) of BNNs. The strong diffraction peak, at 2θ = 26.6°, corresponds to the (002) planes of BN, indicating a well-aligned stacking of the ultra-thin BNNs. Moreover, the ionic liquid assisted exfoliation of BNNs resulted in a very stable suspension in isopropanol alcohol (IPA). The inset in Fig. 2e presents the digital image of BNNs suspension after 30 days shelf time, indicating the homogenous and stable nature of exfoliated nanosheets. Figure 3a illustrates the cross-sectional SEM image of the BNNs/PVA composite paper without FGM addition. The BNNs were well dispersed in the PVA matrix without obvious aggregation, indicating potential of highly uniform and continuous thermal conduction. The inset in Fig. 3a shows the photos of the as-synthesized composite paper. The zoom-in region in Fig. 3a


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is presented in Fig. 3b, which indicates the alignment and dispersion of BNNs in the PVA matrix. The inset of Fig. 3c illustrates the fabrication of larger composite paper (≈140 mm in length) by spraying BNNs/PVA solution onto the FGM. Figure 3c and 3d presents the crosssectional SEM image of BNNs/PVA@FGM, in which BNNs, PVA and the framework of FGM can be identified. Most BNNs were aligned horizontally, as shown in Fig. 3d, which might be due to the applied pressure during roller pressing. The fibers have been surrounded by the wellaligned BNNs, which was expected to reduce the thermal contact resistance, and thus guarantee the high in-plane thermal conductivity. Figure 4a presents the relationship between the specific heat at 25 °C and mass content of BNNs in the composite paper. The h-BN/PVA composite paper exhibited a specific heat of 1.19 J/(g·K) when the BN content was 20 wt. %. As the specific heat of PVA is larger than h-BN,51-53 it is found that the specific heat of the composite paper could be improved by increasing the content of PVA. However, the content of PVA cannot be too high because of its low thermal conductivity. With the increase of BN mass content, all the samples showed the decrease of specific heat, while BNNs/PVA and BNNs/PVA@FGM composite papers were more stable in specific heat than the bulk h-BN/PVA with the change of BN mass content. It should be noticed that the presence of FGM had reduced the specific heat of BNNs/PVA by about 10% (1.10 to 0.98 J/(g·K)), which was corresponding to the small specific heat of FGM.54-56 Figure 4b presents the temperature-dependent (25 °C to 100 °C in 20 min) increase of specific heat, in which BNNs/PVA and BNNs/PVA@FGM showed higher specific heat and h-BN/PVA when the BN content was as high as 60 wt. % (change of specific heat with other BN contents shown in Fig. S1). It can be concluded that BNNs based composite papers have shown better specific


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heat with high BN mass content at high temperature than the bulk h-BN based composite paper, which might imply a potentially good heatsink of BNNs. The in-plane thermal conductivity of the composite paper at 25 °C is presented in Fig. 5a. The h-BN/PVA composite did not show significant increase of thermal conductivity with the increase of BN mass content, whereas the thermal conductivity of BNNs/PVA had remarkably increased from 4.61 W/(m·K) to 22.75 W/(m·K) with the increasing BNNs content. The tremendous increase of in-plane thermal conductivity can be attributed to the horizontal alignment and homogeneous dispersion of BNNs in PVA matrix as indicated in Fig. 3a. The BNNs/PVA@FGM composite paper has shown the thermal conductivity of 22.51 W/(m·K), which is close to that of BNNs/PVA composite paper. This further proves that presence of FGM does not affect the thermal conductivity, despite its poor intrinsic thermal conductivity.57 Figure 5b shows the change in thermal conductivity with respect to temperature and BNNs content. The thermal conductivity of the BNNs/PVA and BNNs/PVA@FGM has exhibited similar trend by varying temperature and BNNs content. In most of the cases, the measured thermal conductivity slightly increased with an increase in the temperature. To demonstrate the high thermal conductivity of the BNNs/PVA@FGM composite paper, we have compared our results with the previously published literature and summarized in Fig. 5c.27,30,32,33,35-37It can be clearly observed that the as-prepared BNNs/PVA@FGM composite paper has exhibited exceptional thermal conductivity, among h-BN composites. The thermal diffusivity has also been measured based on Eqn. (1), as shown in Fig. S2 and S3. The practical heat dissipation property of the as-prepared BN based composite papers, was evaluated by using LED with the configuration described in Fig. 6a. A copper plate (10 x10 x 1 mm) was used as the heatsink, while thermal grease was used as an adhesive to fill up the gap


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between LED chip, the composite paper and the copper plate. As a control, another LED chip without BN based composite paper was also prepared (described in the Supporting Information). Figure 6b illustrates the temperature variation of the working LED chip. For the control, the temperature of LED chip went up to 63.8 °C in 30 min. For h-BN/PVA embedded LED device, the temperature of LED chip reached 57.3 °C. On the other hand, the BNNs/PVA and BNNs/PVA@FGM composite papers exhibited significantly improved thermal dissipation property. After 30 minutes, the BNNs/PVA@FGM based LED stabilized at 51.4 °C and BNNs/PVA based LED stabilized at 50.5 °C. The ambient temperature (AT) remained stable at 25 °C during the experiment. These results confirm the good thermal conductivity of BNNs, which would not be obviously affected by the presence of FGM. A high thermal conductivity is one of the prime requirements in wearable and flexible electronic devices. However, the effect of mechanical strength should not be underestimated58. Figure 7a presents the stress-strain curves of the BNNs/PVA and BNNs/PVA@FGM composite papers, while the inset photos demonstrate the excellent flexibility. The tensile strength of the BNNs/PVA and BNNs/PVA@FGM composite papers with 40 wt. % BNNs was 9.86 MPa and 27.92 MPa, respectively. It should be noticed that the tensile strength of the as-prepared BNNs were too low to form a freestanding film by using conventional method.29 The high tensile strength was contributed by the presence of FGM. Moreover, the BNNs/PVA@FGM composite has also shown a higher strain than the BNNs/PVA composite, which can be attributed to the higher stiffness of FGM.59 Figure 7b summarizes the tensile strength between the BNNs/PVA@FGM and reported BN-composites.21,29,35,38,46,60 It could be concluded that our BNNs/PVA@FGM exhibited very competitive mechanical performance. The high thermal


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conductivity and tensile strength of the BNNs/PVA@FGM composite paper makes it an promising candidate for microelectronics61,62 and high RF devices. To investigate the RF characteristics of microwave circuits, CPW transmission lines were fabricated via copper bonding on the BNNs/PVA@FGM composite paper. A PI-based device was also fabricated for reference. The CPW lines parameters were designed by using transmission line calculator (TXLINE). The signal transmission performance through the designed CPW line, with a length of 20 mm, was evaluated by quantifying the transmission loss. Thus, a vector network analyzer (ZNB 8, R&S) was used to measure the transmission coefficient, S21, of the CPW line with a copper thickness of 15 µm. As seen from Fig. 8a, the transmission loss of two CPW lines increased with the sweeping frequency. The inset of Fig. 8a shows the designed CPW lines and PCB support layer. The transmission loss of the CPW line, with BNNs/PVA@FGM composite paper substrate, reached at 0.10 dB/mm, when the frequency was 7 GHz. This transmission loss is smaller than that of PI-substrate based CPW lines. The superior performance of the synthesized BNNs/PVA@FGM, in terms of transmission loss of CPW lines, indicates that despite the lower tensile strength, the BNNs/PVA@FGM composite should be preferred over PI in high-frequency systems. To investigate the influence of bending CPW lines, at different curvatures, the hard PCB was removed. Figure 8b shows the frequency-dependent transmission loss of designed CPW lines under various levels of bending. The inset shows the digital photographs of the bent CPW devices with different bending radii (r=∞ or flat for NO1, r = 10.44 mm for NO2, r = 5.97 mm for NO3). The measured transmission loss has shown negligible variation with bending, indicating the stability of CPW lines and flexibility of substrate. It should be noted that the


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transmission loss has reached the value of 0.30 dB/mm (Fig. 8b), which might be due to the poor connection between the CPW and Sub-Miniature-A (SMA) connector. The measured transmission loss of the designed CPW line, based on prepared composite paper, is consistent with the previous reports.6,63,64 This demonstrates possibility of RF signal transmission and shows great potential for the prepared composite paper as flexible substrate in the application of portable electronic devices. 4. CONCLUSION In summary, we have demonstrated the fabrication of a BN-based composite paper with high thermal conductivity, adequate mechanical strength and excellent flexibility. The composite was fabricated by spraying the BNNs and PVA solution on FGM. The thermal conductivity of BNNs/PVA@FGM composite paper reached up to 22.51 W/(m·K) and presence of FGM enhanced the tensile strength. The investigation of the CPW lines, built on the prepared composite paper, demonstrate its outstanding performance, as a flexible substrate, for highfrequency signal transmission. Based on these results, we believe that the BNNs composite paper has potential to be used in wearable electronics and RF signal processing.

ASSOCIATED CONTENT Supporting Information. Explanation of experimental details; specific heat of the composite papers; thermal diffusivity at 25 °C; comparison of BNNs/PVA with and without FGM in thermal diffusivity; Summary of BN based composites from literatures. AUTHOR INFORMATION


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Corresponding Author *(X.Y.C.) E-mail: [email protected] *(M.L.) E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the Graphene Technology and Industry Development Project of Fujian Provincial Development and Reform Commission, and the NSFC under project No. 61071010, and the Department of Education of Fujian Province (No. JAT170012). We thank Ms. Lingling Zheng in our institute for the technical support on electron microscopy.

ABBREVIATIONS BNN, boron nitride nanosheets; FGM, fiberglass mesh; PCB, printed circuit board; RF, radio frequency; RFIDs, radio frequency identifications; PI, polyimide; PET, polyethylene terephthalate; TIM, thermal interface material; PS, polystyrene; PA, polyamide; PVA, polyvinyl alcohol; PDMS, polydimethylsiloxane; CPW, co-planar waveguide; SEM, scanning electron microscopy; XRD, X-ray diffraction; LED, light-emitting diode; FWHM, full width at halfmaximum; AFM, atomic force microscopy; SMA, Sub-Miniature-A.

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Figure 1. Scheme of BNNs/PVA@FGM composite paper: fabrication, CPW substrate and LED heatsink.


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Figure 2. The SEM images of (a) as-received h-BN platelets and (c) exfoliated BNNs nanosheets. The AFM images of (b) as-received h-BN platelets and (d) exfoliated BNNs; (e) Raman spectrum of the exfoliated BNNs and inset shows the digital image of BNNs suspension in IPA after 30 days shelf life; (f) XRD pattern of the exfoliated BNNs.


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Figure 3. Cross-sectional SEM images of (a) (b) BNNs/PVA composite paper and (c) (d) BNNs/PVA@FGM composite paper at low and high magnification, respectively. The insets of (a) and (c) show the photos of BNN/PVA and BNNs/PVA@FGM composite papers, respectively.


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Figure 4. The specific heat of the composite papers varied with (a) BN mass content and (b) temperature.


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Figure 5. (a) The thermal conductivity of the composite papers (a) at 25 °C and (b) varied with BN content and temperatures; (c) the comparison of the thermal conductivity between the composite paper in this work and previously published literature.


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Figure 6. (a) The configuration of composite paper (BN content: 40 wt.%) based heatsink for LED; (b) the temperature variation of the working LED chip.


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Figure 7. The mechanical performance of the BNNs/PVA and BNNs/PVA@FGM composite papers: (a) stress-strain curves, the inset presents the photos to exhibit the flexibility; (b) comparison of the tensile strength between reported BN-composites and BNNs/PVA@FGM in this work.


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Figure 8. The RF characterization: (a) transmission loss of CPW lines with PI and BNNs/PVA@FGM

as

the substrates;

(b)

transmission

loss

of CPW

lines

using

BNNs/PVA@FGM composite paper as the substrate with different levels of bending.


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Thermally Conductive Boron Nitride Nanosheet ... - ACS Publications

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