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Highly thermal conductive and electrical insulating polymer composites with boron nitride
Journal Pre-proof Highly thermal conductive and electrical insulating polymer composites with boron nitride Meng Li, Mengjie Wang, Xiao Hou, Zhaolin Zhan, Hao Wang, Hui Fu, Cheng-Te Lin, Li Fu, Nan Jiang, Jinhong Yu PII:
S1359-8368(19)35252-7
DOI:
https://doi.org/10.1016/j.compositesb.2020.107746
Reference:
JCOMB 107746
To appear in:
Composites Part B
Received Date: 7 October 2019 Revised Date:
3 December 2019
Accepted Date: 2 January 2020
Please cite this article as: Li M, Wang M, Hou X, Zhan Z, Wang H, Fu H, Lin C-T, Fu L, Jiang N, Yu J, Highly thermal conductive and electrical insulating polymer composites with boron nitride, Composites Part B (2020), doi: https://doi.org/10.1016/j.compositesb.2020.107746. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.
Graphical Abstract Table of contents entry:
Highlight: BNNS fillers was a good choice for improving high thermal conductivity and electrical insulating of PDMS matrix composites compared to sphere BN.
Highly Thermal Conductive and Electrical Insulating Polymer Composites with Boron Nitride Meng Li1, 2, Mengjie Wang2, Xiao Hou2, Zhaolin Zhan*1, Hao Wang2, Hui Fu2, Cheng-Te Lin 2, 3, Li Fu4, Nan Jiang, *2, 3 and Jinhong Yu*2, 3 1
Faculty of Materials Science and Engineering, Kunming University of Science and Technology,
Kunming, 650093, China. 2
Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine
Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo,315201, China. 3
Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of
Sciences, Beijing 100049, China. 4
College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018,
China. *Corresponding author, E-mail: [email protected]; [email protected]; [email protected].
Abstract: The heat accumulation has become a serious problem due to electronic devices towards high power and intelligence. Highly thermal conductive and electrical insulating polymeric composites are potential to solving the overheating problem of electronic devices. Herein, boron nitride nanosheets (BNNSs) were successfully exfoliated by shear forces generated high pressure. Then, BNNSs and sphere BN (S-BN) were added into the polydimethylsiloxane (PDMS) matrix to prepare composites. The thermal conductivity of the BNNSs/PDMS composites achieved 1.16 W m-1 K-1 at 35 wt% fillers, which
is almost 5 times that of pure PDMS and higher than that of S-BN/PDMS composites. The efficient thermal paths were constructed by BNNSs in PDMS matrix leading to enhancing the thermal transport in polymer matrix. Meanwhile, the volume resistances and breakdown strength of BNNSs/PDMS composites reach 7.5×1013 Ω·cm and 39.8 V/µm, which are attributed to the superior electrical insulating properties of BNNSs. The result shows that the excellent thermal conductivity and electrical insulation properties of BNNSs/PDMS make it a promising application in the field of thermal management. keywords: Polymer-matrix composites (PMCs); Electrical properties; Microstructures; Thermal properties;
1. Introduction Nowadays, the electronic devices are rapidly developing toward the miniaturization and multi-functional direction [1-3]. Quantities of electronic components are concentrated in a narrow space, which posing a serious challenge to the heat dissipation of electronic devices. As a result, whether the electronic device can dissipate heat in time is the key factor for the normal operation of the electronic device [4, 5]. However, most polymer materials have low thermal conductivity (less than 0.5 W m -1 K-1) [6-8]. In order to increase the thermal conductivity of polymer, the addition of highly thermal conductive fillers is an effective solution, such as metals, carbon materials, ceramics and so on [9-15]. Especially, hexagonal boron nitride (h-BN) is also known as "white graphite". Boron nitride related nanofillers such as boron nitride nanosheets (BNNSs) and boron nitride nanotubes (BNNTs) have attracted to attention due to their potential application for thermal dissipation [16-18]. Boron nitride possesses an extremely highly thermal conductivity, in which the theoretical calculation of thermal
conductivity of hexagonal boron nitride nanoribbons is as high as 1700 to 2000 W m-1 K-1 [19]. Most importantly, h-BN owns excellent insulation properties are 5.2 eV band gap and 35 V/µm breakdown strength [20]. Meanwhile, h-BN possesses passivity to reactions with acids and melts, superb oxidation resistance and low coefficient of friction. This allows h-BN to be used under conditions that require insulation and heat dissipation, which is different with graphene. Different shapes of BN, such as zero-dimensional (0D, nanoparticle), one-dimensional (1D, nanotube) and two-dimensional (2D, nanosheet) can enhance the thermal conductivity of the polymer to a certain extent [21-28]. Polydimethylsiloxane (PDMS) has excellent compatibility, flexibility, chemical stability and thermal stability due to its unique molecular chain structure, which makes PDMS play an important role in some conditions [29]. However, low thermal conductivity of the PDMS limits its use in the advanced thermal management fields. Therefore, h-BN is a kind of ideal heat conductive fillers to improve the thermal conductivity of the PDMS composites with great application value and prospects in electronic industry. Like graphene [30], many preparation methods of BNNSs were reported. Although the lateral scale will destroyed [31, 32], liquid phase exfoliation by mechanical is the preferred method in preparation of BNNSs [33]. In this work, a jet cavitation method was proposed to prepare BNNSs. The thermal and electrical properties of BNNSs/PDMS composites were investigated systematically in comparison with that of the s-BN/PDMS composites. The BNNSs/PDMS composite presents preferable thermal conductivity and excellent electrical insulation and shows the strong potential application for thermal management field. 2. Experimental 2.1. Materials
The hexagonal boron nitride (h-BN, with lateral size of 6 µm) material was obtained by ESK Ceramics GmbH & Co (Germany). The sphere boron nitride (S-BN, with lateral size of 50 µm) material was purchased from Suzhou nutpool materials Technology Co., Ltd. (Suzhou, China). PDMS (Sylgard 184) was purchased from Corning Co., Ltd. (Shanghai, China). All materials were used as supplied without further purification. Deionized water was used to prepare all h-BN suspensions (20 mg/ml). 2.2. Exfoliation of h-BN The BNNSs suspension is obtained by the device which quickly passing the h-BN through a small hole. With the effect of the shearing, orifice and impact, h-BN are crushed, emulsified and dispersed in DI water. And a few layers BNNSs would be kept the suspension state at the crash which achieved in circulation cooling device at 4 °C water bath. The exfoliation scheme is shown in Fig. 1. The h-BN suspension was exfoliated 100 times to obtain the BNNSs. The pressure was in 100 MPa by letting the piston force through the nozzle. Finally, the supernatant was collected and filtered. The BNNSs sample was collected and dried at 50 °C for 10 h. 2.3. Preparation of composites Polydimethylsiloxane (PDMS) composites with different BNNSs loadings were fabricated by the following procedures. First, the PDMS and curing agent were mixed with a mass ratio of 10:1 at room temperature. Subsequently, different loadings of BNNSs was added into PDMS. The above mixture was mixed utilizing a speed mixer at a speed of 2500 rpm for 5 min. Then, the samples were put into vacuum oven at room temperature for 8 h to remove the air and cured at 85 °C for 1 h. Finally, the composites were prepared in square shape of about 1 mm in thickness and 10 mm in lateral size. The PDMS composites with S-BN were also prepared with the same ways as described above. For convenience, the
PDMS containing BNNSs and S-BN were denoted as BNNSs/PDMS and S-BN/PDMS, the pure PDMS is without any other substances. When measured the insulation properties, the process of preparing the samples is similar with above, but the size of the samples should change to 100 mm in lateral size. 2.4 Characterization The morphology of h-BN, BNNSs, S-BN, BN/PDMS and S-BN/PDMS composites were investigated using a scanning electron microscopy an accelerating voltage of 10 kV. The crystal structure of h-BN, BNNSs and S-BN were analyzed by a transmission electron microscopy JEM-2100 (TEM, Jeol, Japan) operating at 10 kV accelerating voltage. The Atomic force micrographs (AFM) of BNNSs was obtained using Dimension 3100 scanning probe microscope (Veeco, USA) in tapping mode. The X-ray diffraction (XRD), Raman spectra, Fourier transformed infrared (FTIR) spectra and X-ray photoelectron spectroscopy (XPS) were performed by D8 Advanced (Bruker, Germany), Reflex Raman System (Renishaw. UK) at laser wave-length of 532 nm, Nicolet 6700 (Thermo, USA) and Kratos AXIS Ultra DLD spectrometer, respectively. Thermal conductivity was measured by LFA467 Nanoflash (NETZSCH, Germany). The infrared photos were recorded by infrared camera (Fluke, Ti400, USA). The volume resistivity was measured by Keithley 6517B Electrometer. The breakdown voltage was investigated by voltage breakdown tester HT-100 (Guilin Electrical Equipment Scientific Research Institute Ltd, China). 3. Results and discussion Initially, as shown in Fig. 1 (a), the raw h-BN dispersion was poured into the exfoliation device. Under the pressure of 100 MPa, the dispersion was quickly pressed out from the nozzle. After 100 cycles of exfoliation, filter and drying. BNNSs powder and dispersion are shown in the inset to Fig. 1 (b)
and Fig. 1 (c). The yield of BNNSs with different cycles was shown in Fig. 1 (b). The yield gradually increases as the cycle number of exfoliation, Particularly, the exfoliation efficiency (42.3%) is much higher than the other liquid exfoliation work [34-36].
Fig. 1 (a) Scheme of the exfoliation process of BNNSs; (b) Yield of BNNS with different cycle number. inset to (b) Photos of as-prepared BNNSs powder; (c) Photos of as-prepared BNNSs suspension.
Fig. 2 (a-c) show the morphologies of h-BN, S-BN and BNNSs respectively, which were obtained by SEM. In order to accurately measure the size of the fillers, the size distribution was counted from 120 sheets and shown in the inset to Fig. 2 (a-c). The result indicates the majority of h-BN, S-BN and BNNSs were approximately 6, 47, and 0.8 µm respectively. In addition, as shown in Fig. 2 (d), the thickness of BNNSs were reduced to 57.8 nm, which characterized by AFM. So, the result can be assumed that this is an efficient way to prepare BNNS.
The TEM images of BNNSs were shown in Fig. 3 (a-c). As shown in Fig 3 (a), It was observed that BNNS still maintains sheet shape with a size of around 0.8 µm after exfoliation, this is consistent with the result of Fig 2 (c). Meanwhile, the SAED pattern of inset to Fig. 3 (a) indicated that the obvious six-fold symmetry characteristic diffraction for BNNSs, implying that the well-crystallized feature of BNNSs. In Fig. 3 (b), the thickness of BNNSs was almost 45 nm, which is consistent with the statistics of thickness
Fig. 2 SEM images of (a) h-BN, (b) S-BN, and (c) BNNSs; AFM image of (d) BNNSs; (inset: histograms or the lateral size distributions of the h-BN, S-BN, BNNSs, and height of BNNSs).
of AFM measurement. Fig. 3 (c) shows the high-resolution TEM image of BNNSs. The distances between each two adjacent white dots is the practical distances between any two nearest B or N atoms.
They are also equal to the (100) lattice constant. As shown in the inset to Fig. 3 (c), the TEM contrast-intensity profiles achieved along the marked blue line indicates that the stripe distance is ~0.25 nm. The Fig. 3 (d) shows the geometrical relationship of BNNSs. By calculation, the distance between the N and B atoms is approximately 1.43 Å, this is very close to the theoretical length of B-N bond of 1.44 Å.
Fig. 3 (a) Low-magnification TEM images of BNNSs. inset to (a) corresponding electron-diffraction pattern; (b) TEM images displaying BNNSs with a thickness of around 45 nm; (c) High resolution TEM images of BNNSs. inset to (c)
TEM contrast intensity profile recorded along the marked blue circle reveals that the fringe separation is～0.25 nm; (d) Structure model of BNNSs.
The exfoliation effect of h-BN and the differences among the three kinds of boron nitride were further compared by spectroscopic experiments, which is shown in Fig. 4. Fig.4 (a) displays the XRD patterns of h-BN, S-BN and BNNS. The distinct characteristic peak at 2θ are 26.80°, 40-45°, 50.20° and 55.18°, respectively. The corresponding peak can be assigned to (002), unresolved (100) and (101), (102), and (004) crystallographic planes of hexagonal phase of BN. Meanwhile, the BNNSs powders show the same distinct characteristic peaks as compared h-BN above, this indicates that the exfoliation process has no influence on the crystal structure of h-BN. However, it is interesting to find that the (002) peak of BNNS, which exfoliated via jet cavitation, becomes broader regarding pristine h-BN, this result points that the crystallite size might gradually decrease during the exfoliation process. According to the reported in the literature, the peak becomes broad due to the crystallite size, which are inversely proportional to the FWHM: as the peak becomes broader, the crystallite size becomes smaller [37]. As shown in Fig. 4 (b), The same XPS spectrum of h-BN and BNNS further prove that no impurities were introduced during the exfoliation process. In addition, the XPS of S-BN was noticed that there are two peaks at 117 and 72 eV, which might be attributed to the binder in S-BN. Fig. 4 (c) exhibits the Raman spectrums of three kinds of boron nitride, the characteristic peak can be attribute to the typical B-N stretching mode (E2g). h-BN show the peak at 1364 cm-1. However, the difference is that the peak of BNNSs is at 1367 cm-1. The peak shift to higher wavenumbers after exfoliation can be related to the strain changes within the layers [38-40]. So the result is consistent with the effective exfoliation of h-BN into thinner sheets, which leads to a weaker interlayer interactions and higher in-plane strain [41-43]. Furthermore, the peak of S-BN is
also at 1367 cm-1. According to the SEM images of S-BN, this mainly might because S-BN is made up many small BNNSs which bonded with each other. Similarly, according to FTIR result, which is shown in the Fig. 4 (d), it was characterized whether other impurities can be introduced during the exfoliation process. The data displays that there are two FTIR bands at 815 and 1375 cm-1, which is ascribed to the B-N-B bending (out-of-plane vibration, A2u mode) and the B-N stretching (in-plane ring vibration, E1u mode) respectively [44-46]. In addition, another absorption peak at ~3200 cm-1, which is attributed to hydroxyl group (-OH) vibration. This might be due to large amounts of defects such as vacancy defects.
Fig. 4 (a) XRD patterns, (b)XPS, (c) Raman spectra and (d) FTIR spectra of h-BN, s-BN and BNNSs.
In order to investigate the effect of different boron nitride shapes on the thermal conductivity, a series of composites with different shape fillers were prepared. The fracture morphologies of S-BN/PDMS and BNNSs/PDMS composites, as shown in Fig. 5 (a, b), exhibits the connection status between matrix and filler. Shown in Fig. 5 (a), it can be clearly found that most of the BNNSs are directly connected to each other, it is easy for composites to form thermal paths. However, for S-BN/PDMS, As shown in Fig. 5(b), S-BN fillers are dispersed in the PDMS matrix randomly and in an unconnected state. As a result, it is difficult to form the thermal paths. In order to further directly describe the thermal paths, the heat flow models of the composites were constructed as shown in the Fig.5 (c, d). The Fig. 5 (c) shows the heat flow can efficiently transfer along BNNSs pathways. Conversely, from Fig. 5 (d), it shows that a lot of randomly dispersed S-BN particles cannot form thermal pathways. In addition, heat flow would be interrupted at the interfaces between the PDMS matrix and the S-BN.
Fig. 5 (a, b) SEM images of brittle fractured surfaces of BNNSs and S-BN composites; (c, d) Schematic of thermal mechanism inside the BNNSs/PDMS and S-BN/PDMS composites.
As shown in Fig 6 (a), the thermal conductivity of the composites shows monotonous increase with the addition of fillers, and the thermal conductivity of BNNSs/PDMS composites with 35 wt% BNNSs fillers reach to 1.16 W m-1 K-1. However, the thermal conductivity of S-BN/PDMS composites are only 0.77 W m
-1
K-1. When the amount of filler is 35 wt%, the thermal conductivity of BNNSs/PDMS
composite is about 5 times that of pure PDMS, while S-BN/ PDMS is less than 4 times. What is more, when the fraction of fillers increased from 30 to 35 wt%, a sharp increasing on thermal conductivity can be seen. It is inferred
Fig. 6 (a) Thermal conductivity of S-BN/PDMS and BNNSs/PDMS composites; (b) Temperature-dependent thermal conductivity;(c) Thermal conductivity of heating and cooling cycle; (d) Optical and heating infrared thermal images and (e) surface temperature variation with time upon heating and cooling event of pure PDMS, h-BN/PDMS and BNNSs/PDMS composites with 35 wt% filler.
that the continuous BNNSs network can be formed at this time. BNNSs in contact with each other reduces the interface thermal resistance and allows more heat to be transferred along the paths. Fig. 6 (b) shows the thermal conductivity of pure PDMS and PDMS composites with 35 wt% fillers as a relationship of temperature changes. The thermal conductivity of samples is nearly stable, which indicates that the materials can maintain outstanding thermal transport properties from 25 to 100 °C. Similarly, Fig. 6 (c) shows the change on thermal conductivity of composites during multiple heating and cooling processes between 25 and 100 °C. The thermal conductivity of the three composites only changed slightly during this process, indicating that the composites exhibit a stable thermal conductivity
performance in this temperature range. Fig. 6 (d) is the optical and infrared thermal images of pure PDMS, BNNSs/PDMS, S-BN/PDMS, and it was used to further compare the thermal conductivity of the samples [47]. The color of the samples gradually changed with the temperature. At the beginning, the surface temperatures of the three samples were all 29 °C. As the temperature increasing, three samples began to be different from each other. After the samples were heated for 90 s, the surface temperature of BNNSs/PDMS, S-BN/PDMS and pure PDMS were 91, 86 and 83 °C, respectively. The surface temperature of BNNSs/PDMS is 5 and 8 °C higher than S-BN/PDMS and pure PDMS respectively, which indicates that the heat transfer capacity of BNNSs/PDMS is the best among the three samples. Moreover, Fig. 6 (e) records the surface temperature of three sample samples during heating and cooling process, which further reflects the thermal conductivity of samples. In heating process, BNNSs/PDMS always has the highest surface temperature and the fastest cooling rate in the cooling process among the three samples. In a word, the thermal conductivity of BNNSs/PDMS is better than that of S-BN/PDMS and pure PDMS. Fig. 7 show that the electrical insulating properties of pure PDMS, BNNSs/PDMS and S-BN/PDMS with various contents of fillers. Fig. 7 (a) and (c) are the test schematic model of volume resistivity and breakdown strength, respectively. Volume resistivity is the impedance of a material per unit volume to current, and it is used to characterize the electrical properties of a material. In other words, A flat sample
Fig. 7 the test schematic model of (a) volume resistivity and (c) breakdown strength of pure PDMS, S-BN/PDMS and BNNSs/PDMS; (b) the volume resistivity and (d) breakdown strength of pure PDMS, S-BN/PDMS and BNNSs/PDMS.
is placed between two electrodes, according to measure the direct-current voltage applied to the electrode and the current flowing through the volume of the sample. And then, dividing the two values, the result is the volume resistivity. Similarly, breakdown strength is also a parameter of the insulation properties of materials. It is the maximum electric field strength that a material can withstand, by measuring the ratio of the breakdown voltage and thickness of samples, the breakdown strength can be obtained. Meanwhile, Fig. 7 (b) and (d) shows the volume resistivity and breakdown strength of BNNSs/PDMS and S-BN/PDMS and pure PDMS, respectively. As shown in Fig.7 (b), the volume
resistivity of thee samples are all higher 1012 Ω·cm, which means that the composites remain highly electrical insulating performance after addition of fillers. Besides, the volume resistivity of BNNSs/PDMS and S-BN/PDMS enhances slightly with the increase of filler, this can be attributed to the addition of BNNSs or S-BN fillers. And when the content of fillers is 10 wt%, the volume resistances of the BNNSs/PDMS and S-BN/PDMS are reached to 7.5×1013 and 2.3×1014 Ω·cm, respectively. However, the volume resistivity of S-BN/PDMS is higher than BNNSs/PDMS that may be due to the existence of the binder in S-BN and continuous BNNSs network. The increased mobility of charge carriers along the BNNSs network can decrease of volume electrical resistivity of the BNNSs/PDMS. Fig. 7 (d) shows that the breakdown strength of pure PDMS, BNNSs/PDMS and S-BN/PDMS. The breakdown strength of the composites can increase to some extent with the increase of the fillers content, and it reached to 39.8 V/µm with 30 wt% BNNSs loading. The addition of BNNSs provides a thicker insulating layer and better insulation for the BNNSs/PDMS composite. The sudden increase of breakdown strength at 30 wt% is attributed to the percolation effect [48, 49]. Subsequently, the decrease in the breakdown strength of BNNSs/PDMS with the 35 wt% BNNSs fillers may be due to the agglomeration of BNNSs. In addition, the breakdown voltage of BNNSs/PDMS is superior to S-BN/PDMS. Therefore, the electrical insulation properties of the composites which added fillers can achieve a promoting effect relative to pure PDMS. 4. Conclusion In summary, BNNSs were successfully exfoliated by effective and green jet cavitation method. Subsequently, the effect of fillers shape on thermal transfer performance were also investigated, and result shows that the BNNSs/PDMS composites display more effective thermal transportation properties,
achieving 1.16 W m-1 K-1 with 35 wt% filler loading. It is mainly because the connection of adjacent BNNSs and forms thermal transfer channels, which can effectively avoid interface phonon scatter. Furthermore, the volume resistance and breakdown strength of composites were also enhanced with fillers loading. An effective and environment friendly method was proposed to realize the BNNS preparation and the thermal conductive composites with high insulation can be fully utilized the potential application value in thermal management field. Conflicts of interest There are no conflicts to declare. Acknowledgement The authors are grateful for the financial support by the National Natural Science Foundation of China (51573201), NSFC-Zhejiang Joint Fund for the Integration of Industrialization and Informatization (U1709205), Public Welfare Project of Zhejiang Province (2016C31026), The Scientific Instrument Developing Project of the Chinese Academy of Sciences (YZ201640), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA22000000), and the Science and Technology Major Project of Ningbo (2016S1002 and 2016B10038). We also thank the Chinese Academy of Sciences for the Hundred Talents Program, the Chinese Central Government for the Thousand Young Talents Program, 3315 Program of Ningbo.
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Author Statement Dear editor, We wish to draw the attention of the editor to the following facts which may be considered as potential conflicts of interest and to significant financial contributions to this work. We warrant that the article is the authors' original work, hasn't received prior publication and isn't under consideration for publication elsewhere. We also confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We understand that the corresponding author is the sole contact for the editorial process. Jinhong Yu is responsible for communicating with the other authors about progress, submission of revisions and final approval of proofs. Finally, the authors declare no competing financial interests. Signed by all authors as follows: Meng Li
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Mengjie Wang
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Xiao Hou
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Zhaolin Zhan
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Hao Wang
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Hui Fu
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Cheng-Te Lin
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Li Fu
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Nan Jiang
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Jinhong Yu
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. □The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S1359-8368(19)35252-7
DOI:
https://doi.org/10.1016/j.compositesb.2020.107746
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JCOMB 107746
To appear in:
Composites Part B
Received Date: 7 October 2019 Revised Date:
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Accepted Date: 2 January 2020
Please cite this article as: Li M, Wang M, Hou X, Zhan Z, Wang H, Fu H, Lin C-T, Fu L, Jiang N, Yu J, Highly thermal conductive and electrical insulating polymer composites with boron nitride, Composites Part B (2020), doi: https://doi.org/10.1016/j.compositesb.2020.107746. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.
Graphical Abstract Table of contents entry:
Highlight: BNNS fillers was a good choice for improving high thermal conductivity and electrical insulating of PDMS matrix composites compared to sphere BN.
Highly Thermal Conductive and Electrical Insulating Polymer Composites with Boron Nitride Meng Li1, 2, Mengjie Wang2, Xiao Hou2, Zhaolin Zhan*1, Hao Wang2, Hui Fu2, Cheng-Te Lin 2, 3, Li Fu4, Nan Jiang, *2, 3 and Jinhong Yu*2, 3 1
Faculty of Materials Science and Engineering, Kunming University of Science and Technology,
Kunming, 650093, China. 2
Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine
Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo,315201, China. 3
Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of
Sciences, Beijing 100049, China. 4
College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018,
China. *Corresponding author, E-mail: [email protected]; [email protected]; [email protected].
Abstract: The heat accumulation has become a serious problem due to electronic devices towards high power and intelligence. Highly thermal conductive and electrical insulating polymeric composites are potential to solving the overheating problem of electronic devices. Herein, boron nitride nanosheets (BNNSs) were successfully exfoliated by shear forces generated high pressure. Then, BNNSs and sphere BN (S-BN) were added into the polydimethylsiloxane (PDMS) matrix to prepare composites. The thermal conductivity of the BNNSs/PDMS composites achieved 1.16 W m-1 K-1 at 35 wt% fillers, which
is almost 5 times that of pure PDMS and higher than that of S-BN/PDMS composites. The efficient thermal paths were constructed by BNNSs in PDMS matrix leading to enhancing the thermal transport in polymer matrix. Meanwhile, the volume resistances and breakdown strength of BNNSs/PDMS composites reach 7.5×1013 Ω·cm and 39.8 V/µm, which are attributed to the superior electrical insulating properties of BNNSs. The result shows that the excellent thermal conductivity and electrical insulation properties of BNNSs/PDMS make it a promising application in the field of thermal management. keywords: Polymer-matrix composites (PMCs); Electrical properties; Microstructures; Thermal properties;
1. Introduction Nowadays, the electronic devices are rapidly developing toward the miniaturization and multi-functional direction [1-3]. Quantities of electronic components are concentrated in a narrow space, which posing a serious challenge to the heat dissipation of electronic devices. As a result, whether the electronic device can dissipate heat in time is the key factor for the normal operation of the electronic device [4, 5]. However, most polymer materials have low thermal conductivity (less than 0.5 W m -1 K-1) [6-8]. In order to increase the thermal conductivity of polymer, the addition of highly thermal conductive fillers is an effective solution, such as metals, carbon materials, ceramics and so on [9-15]. Especially, hexagonal boron nitride (h-BN) is also known as "white graphite". Boron nitride related nanofillers such as boron nitride nanosheets (BNNSs) and boron nitride nanotubes (BNNTs) have attracted to attention due to their potential application for thermal dissipation [16-18]. Boron nitride possesses an extremely highly thermal conductivity, in which the theoretical calculation of thermal
conductivity of hexagonal boron nitride nanoribbons is as high as 1700 to 2000 W m-1 K-1 [19]. Most importantly, h-BN owns excellent insulation properties are 5.2 eV band gap and 35 V/µm breakdown strength [20]. Meanwhile, h-BN possesses passivity to reactions with acids and melts, superb oxidation resistance and low coefficient of friction. This allows h-BN to be used under conditions that require insulation and heat dissipation, which is different with graphene. Different shapes of BN, such as zero-dimensional (0D, nanoparticle), one-dimensional (1D, nanotube) and two-dimensional (2D, nanosheet) can enhance the thermal conductivity of the polymer to a certain extent [21-28]. Polydimethylsiloxane (PDMS) has excellent compatibility, flexibility, chemical stability and thermal stability due to its unique molecular chain structure, which makes PDMS play an important role in some conditions [29]. However, low thermal conductivity of the PDMS limits its use in the advanced thermal management fields. Therefore, h-BN is a kind of ideal heat conductive fillers to improve the thermal conductivity of the PDMS composites with great application value and prospects in electronic industry. Like graphene [30], many preparation methods of BNNSs were reported. Although the lateral scale will destroyed [31, 32], liquid phase exfoliation by mechanical is the preferred method in preparation of BNNSs [33]. In this work, a jet cavitation method was proposed to prepare BNNSs. The thermal and electrical properties of BNNSs/PDMS composites were investigated systematically in comparison with that of the s-BN/PDMS composites. The BNNSs/PDMS composite presents preferable thermal conductivity and excellent electrical insulation and shows the strong potential application for thermal management field. 2. Experimental 2.1. Materials
The hexagonal boron nitride (h-BN, with lateral size of 6 µm) material was obtained by ESK Ceramics GmbH & Co (Germany). The sphere boron nitride (S-BN, with lateral size of 50 µm) material was purchased from Suzhou nutpool materials Technology Co., Ltd. (Suzhou, China). PDMS (Sylgard 184) was purchased from Corning Co., Ltd. (Shanghai, China). All materials were used as supplied without further purification. Deionized water was used to prepare all h-BN suspensions (20 mg/ml). 2.2. Exfoliation of h-BN The BNNSs suspension is obtained by the device which quickly passing the h-BN through a small hole. With the effect of the shearing, orifice and impact, h-BN are crushed, emulsified and dispersed in DI water. And a few layers BNNSs would be kept the suspension state at the crash which achieved in circulation cooling device at 4 °C water bath. The exfoliation scheme is shown in Fig. 1. The h-BN suspension was exfoliated 100 times to obtain the BNNSs. The pressure was in 100 MPa by letting the piston force through the nozzle. Finally, the supernatant was collected and filtered. The BNNSs sample was collected and dried at 50 °C for 10 h. 2.3. Preparation of composites Polydimethylsiloxane (PDMS) composites with different BNNSs loadings were fabricated by the following procedures. First, the PDMS and curing agent were mixed with a mass ratio of 10:1 at room temperature. Subsequently, different loadings of BNNSs was added into PDMS. The above mixture was mixed utilizing a speed mixer at a speed of 2500 rpm for 5 min. Then, the samples were put into vacuum oven at room temperature for 8 h to remove the air and cured at 85 °C for 1 h. Finally, the composites were prepared in square shape of about 1 mm in thickness and 10 mm in lateral size. The PDMS composites with S-BN were also prepared with the same ways as described above. For convenience, the
PDMS containing BNNSs and S-BN were denoted as BNNSs/PDMS and S-BN/PDMS, the pure PDMS is without any other substances. When measured the insulation properties, the process of preparing the samples is similar with above, but the size of the samples should change to 100 mm in lateral size. 2.4 Characterization The morphology of h-BN, BNNSs, S-BN, BN/PDMS and S-BN/PDMS composites were investigated using a scanning electron microscopy an accelerating voltage of 10 kV. The crystal structure of h-BN, BNNSs and S-BN were analyzed by a transmission electron microscopy JEM-2100 (TEM, Jeol, Japan) operating at 10 kV accelerating voltage. The Atomic force micrographs (AFM) of BNNSs was obtained using Dimension 3100 scanning probe microscope (Veeco, USA) in tapping mode. The X-ray diffraction (XRD), Raman spectra, Fourier transformed infrared (FTIR) spectra and X-ray photoelectron spectroscopy (XPS) were performed by D8 Advanced (Bruker, Germany), Reflex Raman System (Renishaw. UK) at laser wave-length of 532 nm, Nicolet 6700 (Thermo, USA) and Kratos AXIS Ultra DLD spectrometer, respectively. Thermal conductivity was measured by LFA467 Nanoflash (NETZSCH, Germany). The infrared photos were recorded by infrared camera (Fluke, Ti400, USA). The volume resistivity was measured by Keithley 6517B Electrometer. The breakdown voltage was investigated by voltage breakdown tester HT-100 (Guilin Electrical Equipment Scientific Research Institute Ltd, China). 3. Results and discussion Initially, as shown in Fig. 1 (a), the raw h-BN dispersion was poured into the exfoliation device. Under the pressure of 100 MPa, the dispersion was quickly pressed out from the nozzle. After 100 cycles of exfoliation, filter and drying. BNNSs powder and dispersion are shown in the inset to Fig. 1 (b)
and Fig. 1 (c). The yield of BNNSs with different cycles was shown in Fig. 1 (b). The yield gradually increases as the cycle number of exfoliation, Particularly, the exfoliation efficiency (42.3%) is much higher than the other liquid exfoliation work [34-36].
Fig. 1 (a) Scheme of the exfoliation process of BNNSs; (b) Yield of BNNS with different cycle number. inset to (b) Photos of as-prepared BNNSs powder; (c) Photos of as-prepared BNNSs suspension.
Fig. 2 (a-c) show the morphologies of h-BN, S-BN and BNNSs respectively, which were obtained by SEM. In order to accurately measure the size of the fillers, the size distribution was counted from 120 sheets and shown in the inset to Fig. 2 (a-c). The result indicates the majority of h-BN, S-BN and BNNSs were approximately 6, 47, and 0.8 µm respectively. In addition, as shown in Fig. 2 (d), the thickness of BNNSs were reduced to 57.8 nm, which characterized by AFM. So, the result can be assumed that this is an efficient way to prepare BNNS.
The TEM images of BNNSs were shown in Fig. 3 (a-c). As shown in Fig 3 (a), It was observed that BNNS still maintains sheet shape with a size of around 0.8 µm after exfoliation, this is consistent with the result of Fig 2 (c). Meanwhile, the SAED pattern of inset to Fig. 3 (a) indicated that the obvious six-fold symmetry characteristic diffraction for BNNSs, implying that the well-crystallized feature of BNNSs. In Fig. 3 (b), the thickness of BNNSs was almost 45 nm, which is consistent with the statistics of thickness
Fig. 2 SEM images of (a) h-BN, (b) S-BN, and (c) BNNSs; AFM image of (d) BNNSs; (inset: histograms or the lateral size distributions of the h-BN, S-BN, BNNSs, and height of BNNSs).
of AFM measurement. Fig. 3 (c) shows the high-resolution TEM image of BNNSs. The distances between each two adjacent white dots is the practical distances between any two nearest B or N atoms.
They are also equal to the (100) lattice constant. As shown in the inset to Fig. 3 (c), the TEM contrast-intensity profiles achieved along the marked blue line indicates that the stripe distance is ~0.25 nm. The Fig. 3 (d) shows the geometrical relationship of BNNSs. By calculation, the distance between the N and B atoms is approximately 1.43 Å, this is very close to the theoretical length of B-N bond of 1.44 Å.
Fig. 3 (a) Low-magnification TEM images of BNNSs. inset to (a) corresponding electron-diffraction pattern; (b) TEM images displaying BNNSs with a thickness of around 45 nm; (c) High resolution TEM images of BNNSs. inset to (c)
TEM contrast intensity profile recorded along the marked blue circle reveals that the fringe separation is～0.25 nm; (d) Structure model of BNNSs.
The exfoliation effect of h-BN and the differences among the three kinds of boron nitride were further compared by spectroscopic experiments, which is shown in Fig. 4. Fig.4 (a) displays the XRD patterns of h-BN, S-BN and BNNS. The distinct characteristic peak at 2θ are 26.80°, 40-45°, 50.20° and 55.18°, respectively. The corresponding peak can be assigned to (002), unresolved (100) and (101), (102), and (004) crystallographic planes of hexagonal phase of BN. Meanwhile, the BNNSs powders show the same distinct characteristic peaks as compared h-BN above, this indicates that the exfoliation process has no influence on the crystal structure of h-BN. However, it is interesting to find that the (002) peak of BNNS, which exfoliated via jet cavitation, becomes broader regarding pristine h-BN, this result points that the crystallite size might gradually decrease during the exfoliation process. According to the reported in the literature, the peak becomes broad due to the crystallite size, which are inversely proportional to the FWHM: as the peak becomes broader, the crystallite size becomes smaller [37]. As shown in Fig. 4 (b), The same XPS spectrum of h-BN and BNNS further prove that no impurities were introduced during the exfoliation process. In addition, the XPS of S-BN was noticed that there are two peaks at 117 and 72 eV, which might be attributed to the binder in S-BN. Fig. 4 (c) exhibits the Raman spectrums of three kinds of boron nitride, the characteristic peak can be attribute to the typical B-N stretching mode (E2g). h-BN show the peak at 1364 cm-1. However, the difference is that the peak of BNNSs is at 1367 cm-1. The peak shift to higher wavenumbers after exfoliation can be related to the strain changes within the layers [38-40]. So the result is consistent with the effective exfoliation of h-BN into thinner sheets, which leads to a weaker interlayer interactions and higher in-plane strain [41-43]. Furthermore, the peak of S-BN is
also at 1367 cm-1. According to the SEM images of S-BN, this mainly might because S-BN is made up many small BNNSs which bonded with each other. Similarly, according to FTIR result, which is shown in the Fig. 4 (d), it was characterized whether other impurities can be introduced during the exfoliation process. The data displays that there are two FTIR bands at 815 and 1375 cm-1, which is ascribed to the B-N-B bending (out-of-plane vibration, A2u mode) and the B-N stretching (in-plane ring vibration, E1u mode) respectively [44-46]. In addition, another absorption peak at ~3200 cm-1, which is attributed to hydroxyl group (-OH) vibration. This might be due to large amounts of defects such as vacancy defects.
Fig. 4 (a) XRD patterns, (b)XPS, (c) Raman spectra and (d) FTIR spectra of h-BN, s-BN and BNNSs.
In order to investigate the effect of different boron nitride shapes on the thermal conductivity, a series of composites with different shape fillers were prepared. The fracture morphologies of S-BN/PDMS and BNNSs/PDMS composites, as shown in Fig. 5 (a, b), exhibits the connection status between matrix and filler. Shown in Fig. 5 (a), it can be clearly found that most of the BNNSs are directly connected to each other, it is easy for composites to form thermal paths. However, for S-BN/PDMS, As shown in Fig. 5(b), S-BN fillers are dispersed in the PDMS matrix randomly and in an unconnected state. As a result, it is difficult to form the thermal paths. In order to further directly describe the thermal paths, the heat flow models of the composites were constructed as shown in the Fig.5 (c, d). The Fig. 5 (c) shows the heat flow can efficiently transfer along BNNSs pathways. Conversely, from Fig. 5 (d), it shows that a lot of randomly dispersed S-BN particles cannot form thermal pathways. In addition, heat flow would be interrupted at the interfaces between the PDMS matrix and the S-BN.
Fig. 5 (a, b) SEM images of brittle fractured surfaces of BNNSs and S-BN composites; (c, d) Schematic of thermal mechanism inside the BNNSs/PDMS and S-BN/PDMS composites.
As shown in Fig 6 (a), the thermal conductivity of the composites shows monotonous increase with the addition of fillers, and the thermal conductivity of BNNSs/PDMS composites with 35 wt% BNNSs fillers reach to 1.16 W m-1 K-1. However, the thermal conductivity of S-BN/PDMS composites are only 0.77 W m
-1
K-1. When the amount of filler is 35 wt%, the thermal conductivity of BNNSs/PDMS
composite is about 5 times that of pure PDMS, while S-BN/ PDMS is less than 4 times. What is more, when the fraction of fillers increased from 30 to 35 wt%, a sharp increasing on thermal conductivity can be seen. It is inferred
Fig. 6 (a) Thermal conductivity of S-BN/PDMS and BNNSs/PDMS composites; (b) Temperature-dependent thermal conductivity;(c) Thermal conductivity of heating and cooling cycle; (d) Optical and heating infrared thermal images and (e) surface temperature variation with time upon heating and cooling event of pure PDMS, h-BN/PDMS and BNNSs/PDMS composites with 35 wt% filler.
that the continuous BNNSs network can be formed at this time. BNNSs in contact with each other reduces the interface thermal resistance and allows more heat to be transferred along the paths. Fig. 6 (b) shows the thermal conductivity of pure PDMS and PDMS composites with 35 wt% fillers as a relationship of temperature changes. The thermal conductivity of samples is nearly stable, which indicates that the materials can maintain outstanding thermal transport properties from 25 to 100 °C. Similarly, Fig. 6 (c) shows the change on thermal conductivity of composites during multiple heating and cooling processes between 25 and 100 °C. The thermal conductivity of the three composites only changed slightly during this process, indicating that the composites exhibit a stable thermal conductivity
performance in this temperature range. Fig. 6 (d) is the optical and infrared thermal images of pure PDMS, BNNSs/PDMS, S-BN/PDMS, and it was used to further compare the thermal conductivity of the samples [47]. The color of the samples gradually changed with the temperature. At the beginning, the surface temperatures of the three samples were all 29 °C. As the temperature increasing, three samples began to be different from each other. After the samples were heated for 90 s, the surface temperature of BNNSs/PDMS, S-BN/PDMS and pure PDMS were 91, 86 and 83 °C, respectively. The surface temperature of BNNSs/PDMS is 5 and 8 °C higher than S-BN/PDMS and pure PDMS respectively, which indicates that the heat transfer capacity of BNNSs/PDMS is the best among the three samples. Moreover, Fig. 6 (e) records the surface temperature of three sample samples during heating and cooling process, which further reflects the thermal conductivity of samples. In heating process, BNNSs/PDMS always has the highest surface temperature and the fastest cooling rate in the cooling process among the three samples. In a word, the thermal conductivity of BNNSs/PDMS is better than that of S-BN/PDMS and pure PDMS. Fig. 7 show that the electrical insulating properties of pure PDMS, BNNSs/PDMS and S-BN/PDMS with various contents of fillers. Fig. 7 (a) and (c) are the test schematic model of volume resistivity and breakdown strength, respectively. Volume resistivity is the impedance of a material per unit volume to current, and it is used to characterize the electrical properties of a material. In other words, A flat sample
Fig. 7 the test schematic model of (a) volume resistivity and (c) breakdown strength of pure PDMS, S-BN/PDMS and BNNSs/PDMS; (b) the volume resistivity and (d) breakdown strength of pure PDMS, S-BN/PDMS and BNNSs/PDMS.
is placed between two electrodes, according to measure the direct-current voltage applied to the electrode and the current flowing through the volume of the sample. And then, dividing the two values, the result is the volume resistivity. Similarly, breakdown strength is also a parameter of the insulation properties of materials. It is the maximum electric field strength that a material can withstand, by measuring the ratio of the breakdown voltage and thickness of samples, the breakdown strength can be obtained. Meanwhile, Fig. 7 (b) and (d) shows the volume resistivity and breakdown strength of BNNSs/PDMS and S-BN/PDMS and pure PDMS, respectively. As shown in Fig.7 (b), the volume
resistivity of thee samples are all higher 1012 Ω·cm, which means that the composites remain highly electrical insulating performance after addition of fillers. Besides, the volume resistivity of BNNSs/PDMS and S-BN/PDMS enhances slightly with the increase of filler, this can be attributed to the addition of BNNSs or S-BN fillers. And when the content of fillers is 10 wt%, the volume resistances of the BNNSs/PDMS and S-BN/PDMS are reached to 7.5×1013 and 2.3×1014 Ω·cm, respectively. However, the volume resistivity of S-BN/PDMS is higher than BNNSs/PDMS that may be due to the existence of the binder in S-BN and continuous BNNSs network. The increased mobility of charge carriers along the BNNSs network can decrease of volume electrical resistivity of the BNNSs/PDMS. Fig. 7 (d) shows that the breakdown strength of pure PDMS, BNNSs/PDMS and S-BN/PDMS. The breakdown strength of the composites can increase to some extent with the increase of the fillers content, and it reached to 39.8 V/µm with 30 wt% BNNSs loading. The addition of BNNSs provides a thicker insulating layer and better insulation for the BNNSs/PDMS composite. The sudden increase of breakdown strength at 30 wt% is attributed to the percolation effect [48, 49]. Subsequently, the decrease in the breakdown strength of BNNSs/PDMS with the 35 wt% BNNSs fillers may be due to the agglomeration of BNNSs. In addition, the breakdown voltage of BNNSs/PDMS is superior to S-BN/PDMS. Therefore, the electrical insulation properties of the composites which added fillers can achieve a promoting effect relative to pure PDMS. 4. Conclusion In summary, BNNSs were successfully exfoliated by effective and green jet cavitation method. Subsequently, the effect of fillers shape on thermal transfer performance were also investigated, and result shows that the BNNSs/PDMS composites display more effective thermal transportation properties,
achieving 1.16 W m-1 K-1 with 35 wt% filler loading. It is mainly because the connection of adjacent BNNSs and forms thermal transfer channels, which can effectively avoid interface phonon scatter. Furthermore, the volume resistance and breakdown strength of composites were also enhanced with fillers loading. An effective and environment friendly method was proposed to realize the BNNS preparation and the thermal conductive composites with high insulation can be fully utilized the potential application value in thermal management field. Conflicts of interest There are no conflicts to declare. Acknowledgement The authors are grateful for the financial support by the National Natural Science Foundation of China (51573201), NSFC-Zhejiang Joint Fund for the Integration of Industrialization and Informatization (U1709205), Public Welfare Project of Zhejiang Province (2016C31026), The Scientific Instrument Developing Project of the Chinese Academy of Sciences (YZ201640), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA22000000), and the Science and Technology Major Project of Ningbo (2016S1002 and 2016B10038). We also thank the Chinese Academy of Sciences for the Hundred Talents Program, the Chinese Central Government for the Thousand Young Talents Program, 3315 Program of Ningbo.
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Author Statement Dear editor, We wish to draw the attention of the editor to the following facts which may be considered as potential conflicts of interest and to significant financial contributions to this work. We warrant that the article is the authors' original work, hasn't received prior publication and isn't under consideration for publication elsewhere. We also confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We understand that the corresponding author is the sole contact for the editorial process. Jinhong Yu is responsible for communicating with the other authors about progress, submission of revisions and final approval of proofs. Finally, the authors declare no competing financial interests. Signed by all authors as follows: Meng Li
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Mengjie Wang
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Xiao Hou
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Zhaolin Zhan
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Hao Wang
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Hui Fu
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Cheng-Te Lin
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Li Fu
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Nan Jiang
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Jinhong Yu
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. □The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: