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Enhanced thermal conductivities of epoxy nanocomposites via incorporating in-situ fabricated hetero-structured SiC-BNNS fillers
Journal Pre-proof Enhanced thermal conductivities of epoxy nanocomposites via incorporating in-situ fabricated hetero-structured SiC-BNNS fillers Yixin Han, Xuetao Shi, Xutong Yang, Yongqiang Guo, Junliang Zhang, Jie Kong, Junwei Gu PII:
S0266-3538(19)33004-0
DOI:
https://doi.org/10.1016/j.compscitech.2019.107944
Reference:
CSTE 107944
To appear in:
Composites Science and Technology
Received Date: 28 October 2019 Revised Date:
4 December 2019
Accepted Date: 7 December 2019
Please cite this article as: Han Y, Shi X, Yang X, Guo Y, Zhang J, Kong J, Gu J, Enhanced thermal conductivities of epoxy nanocomposites via incorporating in-situ fabricated hetero-structured SiC-BNNS fillers, Composites Science and Technology (2020), doi: https://doi.org/10.1016/ j.compscitech.2019.107944. 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. © 2019 Published by Elsevier Ltd.
Enhanced thermal conductivities of epoxy nanocomposites via incorporating in-situ fabricated hetero-structured SiC-BNNS fillers
Yixin Hana#, Xuetao Shia#, Xutong Yanga, Yongqiang Guoa, Junliang Zhanga,b, Jie Konga, Junwei Gua*
a
MOE Key Laboratory of Material Physics and Chemistry under Extraordinary
Conditions, Shaanxi Key Laboratory of Macromolecular Science and Technology, Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’ an, Shaanxi, 710072, P. R. China. b
School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang, 471023, P. R. China.
Corresponding Authors: [email protected] or [email protected] (J. Gu).
#
The authors Yixin Han# and Xuetao Shi# contributed equally to this work and should be considered co-first authors.
Abstract
Novel hetero-structured silicon carbide-boron nitride nanosheets (SiC-BNNS) by sol-gel and in-situ growth method were performed as thermally conductive & insulating fillers, and the SiC-BNNS/epoxy thermally conductive nanocomposites were then prepared by blending-casting approach. Synthesized hetero-structured SiC-BNNS fillers have synergistic improvement effects on the thermal conductivities of the SiC-BNNS/epoxy nanocomposites. When the amount of hetero-structured SiC-BNNS fillers is 20 wt% (SiC-BNNS, 1/1, w/w), the thermal conductivity coefficient (λ) value of the SiC-BNNS/epoxy nanocomposites (0.89 W/mK) is 4.1 times that of pure epoxy resin (0.22 W/mK), and 2.1, 1.4, and 1.7 times of SiC/epoxy (0.43 W/mK), BNNS/epoxy (0.62 W/mK), and (SiC/BNNS)/epoxy thermally conductive nanocomposites (0.52 W/mK) with the same amount of fillers (20 wt% single BNNS, SiC, or SiC/BNNS hybrid fillers), respectively.
Meantime,
the
obtained
(SiC-BNNS)/epoxy thermally
conductive
nanocomposites also demonstrate favorable electrical insulating properties, and the breakdown strength, volume resistivity as well as surface resistivity is 22.1 kV/mm, 2.32×1015 Ω·cm, and 1.26×1015 Ω·cm, respectively.
Key words: A. Polymer-matrix composites (PMCs); B. Electrical properties; D. Scanning electron microscopy (SEM); E. Casting.
1. Introduction With the fast advancement of telecommunication technology, electronic devices are developing towards miniaturization and integration [1-3]. Thermal accumulation generated by rapid heating rate and high-speed operation together with increasing power consumption cause irreversible damage to electronic components [4-6]. For example, continuously excessive high temperatures will endanger the semiconductor junction, damage the connection interface of the circuit, and form mechanical stress damage [7-8]. Highly thermally conductive & insulating materials can effectively improve the heat conduction/dissipation of the electronic devices ensuring the electrical insulation performance, meanwhile guarantee safe use and prolong the service life [9-10]. Polymer matrix composites are extensively applied in electronic packaging field due to their good processibility, light weight, and low cost, etc., [11]. Researchers have focused on the preparation of highly thermally conductive & insulating polymer composites and their potential applications in the fields of electronic equipments, portable and wearable energy conversion & storage devices [12-14]. Epoxy resin has excellent electrical insulating and mechanical properties, wonderful heat and corrosion resistance, etc., and has been widely used in coatings, adhesives, and advanced composites fields [15-16]. However, its low thermal conductivity coefficient (λ, 0.22 W/mK) has limited further application in the heat conduction/dissipation field [17-18]. Researchers are ususally focusing on improving the λ value of epoxy matrix by introducing single or hybrid fillers. Huang [19] et al. obtained the λ value of 2.2 W/mK for 50 vol% AlN/epoxy composites, 11 times than that of pure epoxy (0.2 W/mK). In our previous work [20], the fabricated BN/epoxy composites with 60 wt% BN fillers presented the highest λ value of 0.98 W/mK, about 5 times that of pure epoxy. However, the improvement of λ values is limited. Researchers adopted surface functionalization of
thermally conductive fillers to further improve the λ values of the epoxy composites for a given fillers loading [21-23]. Yang [24] et al. prepared silane coupling agent functionalized nano-sized SiC particles (f-SiCnp)/epoxy nanocomposites. The λ value of the 30 wt% f-SiCnp/epoxy nanocomposites was 0.78 W/mK, higher than that of 30 wt% SiCnp/epoxy nanocomposites (0.61 W/mK). Investigations revealed that the surface functionalization would reduce the thermal resistances between thermally conductive fillers and epoxy matrix [25]. However, the increase of λ is still limited and insufficient. Under this background, researchers tried to improve the λ values of the polymers by blending two and/or three hybrid thermally conductive fillers (Al/SiCw [26], BN/AlN [27-28], BN/SiC [29], Al2O3/AlN [30], SiC nanowires/graphene [31], GNPs/MWCNTs [32], and AlN/BN/SiC [33], etc.,). Compared with single thermally conductive fillers, the introduction of hybrid fillers with different particle sizes and morphologies can further effectively increase the probability of forming thermally conductive paths inner polymer matrix [34]. However, it would also lead to the formation of more interfacial thermal resistances, against to take full advantage of the excellent thermal conductivity for fillers themselves. Phonon scattering due to the interfacial thermal resistances at the fillers-polymer matrix interfaces and the contact thermal resistances at fillers-fillers interfaces are inevitable, which is the main cause of the low thermal conductivities for obtained composites. More importantly, the bonding force between hybrid thermally conductive fillers only depends on physical deposition and adsorption, which will cause the agglomeration of fillers inner polymer matrix [35-36].When the fillers are incorporated in a high content, the interfacial thermal resistances can be overcome by forming contacted three-dimensional (3D) fillers networks. Relative investigations have indicated that the hetero-structured thermally conductive fillers could fully utilize the advantages of single thermally conductive fillers, finally to
achieve the synergistic improvement effects, thereby form more efficient heat conduction pathways, and effectively avoid introducing more interfacial thermal barriers [37-38]. In this work, the silicon carbide (SiC) precursor solution was firstly prepared by sol-gel method, and the hetero-structured SiC/BN nanosheets (SiC-BNNS) thermally conductive & insulating fillers were then synthesized by in-situ growth method. It was expected that the SiC nanoparticles would form SiC-SiC heat conduction pathways on the surface of the BNNS, and the BNNS-SiC-BNNS heat conduction paths would be further constructed
with
(SiC-BNNS)/epoxy
the
adjacent
thermally
BNNS conductive
fillers.
Afterwards,
nanocomposites
the were
corresponding prepared
by
blending-casting approach. Structures and surface morphologies of the hetero-structured SiC-BNNS fillers were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). Meantime, the amount of hetero-structured SiC-BNNS fillers affecting on the thermal conductivities, electrical insulation, mechanical and thermal properties of the SiC-BNNS/epoxy thermally conductive nanocomposites were also investigated. 2.Experimental section 2.1 Preparation of hetero-structured SiC-BNNS fillers Phenolic resin was dissolved in 50 mL ethanol and stirred at 45 oC for 1 hr. Silicon powder was then dispersed inner phenolic resin/ethanol solution. Carbon-coated silicon (Fig. S1) was then obtained by coating growth. Then the BNNS was placed in 50 mL ethanol, mixed with carbon-coated silicon solution and ultrasonic pulverized (300 W, 10 min). Hetero-structured SiC-BNNS precursor solution was prepared by heating and stirring for 2 hrs at 45 oC, followed by injected into 100 mL deionized water and stirred for 1 hr. The mixture was then centrifuged at 6500 rpm for 10 min, and the sludge was
dried at 60 oC under vacuum for 8 hrs. Three hetero-structured SiC-BNNS fillers with different mass ratio, SiC-BNNS-(I) (SiC-BNNS, 2/1, w/w), SiC-BNNS-(II) (SiC-BNNS, 1/1, w/w), and SiC-BNNS-(III) (SiC-BNNS, 1/2, w/w), were all prepared by grinding the dried samples, which were treated for 2 hrs at 800 oC and then another 2 hrs at 1600 oC. Actually, phenolic resin was carbonized at 800oC and the silicon covered by phenolic resin was Si/C binary system. When the thermal treatment temperature increased to 1600oC, the silicon powder transformed into liquid silicon. Meanwhile, the reaction between silicon and carbon resulted in silicon carbide on the surface of BNNS with excellent chemical stability, and the final product was SiC-BNNS fillers. 2.2 Fabrication of the (SiC-BNNS)/epoxy thermally conductive nanocomposites Hetero-structured SiC-BNNS fillers were mixed with epoxy resin, followed by mechanically stirred for 2 hrs at room temperature. Afterwards, the curing agent was then added and stirred for another 4 hrs at 70 oC. Subsequently, the mixtures were poured into a preheated mold, cured at 120 oC for 5 hrs, and then cooled to room temperature, finally to obtain the (SiC-BNNS)/epoxy thermally conductive nanocomposites. The information of "Main Materials" and the "Characterization" details were presented in the "Supplementary Material". 3. Results and discussion 3.1. Characterization on hetero-structured SiC-BNNS fillers Fig. 2(a) shows the XPS spectra of BNNS and SiC-BNNS. For BNNS, the characteristic peaks of B 1s, C 1s, N 1s, and O 1s are corresponding to 190, 284.6, 398, and 532.5 eV, respectively [39]. For SiC-BNNS, the characteristic peaks of B, C, N, and O remain unchanged, whereas the C peak intensity increases significantly. In addition, Si 2s and Si 2p peaks at 153 and 101.8 eV appear. Besides, there are no other impurities, indicating successful introduction of SiC on the surface of BNNS. Figs. 2 (a'-a'''') present the Si 2p,
C 1s, B 1s, and N 1s spectra of SiC-BNNS, respectively. In Fig. 2(a'), the peaks of 100.2, 101.5, and 102.3 eV are corresponding to Si-C, Si-N, and Si-CBN bonds, and the main bonding modes are Si3N4 and SiCBN. In Fig. 2(a''), the peaks at 283.5 and 286.3 eV correspond to the C-Si covalent bond and C-N bond, mainly attributed to the high temperature fracture of Si-C and B-N bonds, to generate C-N and C-Si bonds. In Fig. 2 (a'''), the peak at 189.9 eV is attributed to the formation of the intermediate for SiCBN between BN and SiCB. The peak at 188.8 eV corresponds to the SiC-B bond. The peak of 188.3 eV is attributed to the recombination of B and Si after Si-C and B-N bond cleavage, and a small amount of B4Si is formed. The peak of 191.2 eV corresponds to B-N bond. In Fig. 2 (a''''), the peak of 397.2 eV corresponds to the Si-N bond and the peak at 398.8 eV is corresponding to the C-N bond. Such two peaks can be ascribed to in-situ growth, causing the Si-C and B-N bonds broken to form Si-N and C-N bonds, respectively. In addition, the small peak of 400.3 eV corresponds to the N-O bond. In XRD (Fig. 2(b)), the peaks and crystal plane index of BNNS are 26.7° (002), 41.7° (100), 43.7° (101), 50° (102), 55° (004), 76° (110), and 82° (112). Peaks and crystal plane index of SiC correspond to 36° (111), 42° (200), 60° (220), 72° (311), and 76° (222), further prove the successful formation of SiC-BNNS. Fig. 3 shows the SEM images of the hetero-structured SiC-BNNS fillers. There are SiC nanoparticles existing on the surface of BNNS, and the number of SiC nanoparticles increases with the increasing amount of Si powder. It indicates that increasing amount of Si powder is beneficial to the in-situ formation of SiC nanoparticles on the surface of BNNS. When the mass ratio of BNNS to Si powder is 2:1 (SiC-BNNS-(II), equals to the mass ratio of BNNS to SiC of 1:1), the SiC-BNNS-(II) (Fig. 3(b)) fillers have the optimized hetero-structured morphology, more favorable to the synergistic thermal conductivity effects. Indeed, when the amount of Si powder is small, in-situ generated
SiC nanoparticles are relatively small (Fig. 3(a)), thereby SiC nanoparticles are scattered on the surface of BNNS sporadically, difficult to form SiC-SiC heat conduction pathways or to overlap BNNS adequately. With excessive amount of Si powder, too much SiC nanoparticles would completely cover the BNNS (Fig. 3(c)). Consequently, it is also difficult to exert the relatively better thermal conductivity of the BNNS and the synergistic improvement effects of hetero-structured SiC-BNNS fillers. TEM and AFM images of BNNS and SiC-BNNS-(II) are shown in Figs. 3 (d-g). Visibly, the surface of the pure BNNS is flat (Fig. 3(d)) and transparent (Fig. 3(f)). The size and the thickness of pure BNNS are around 1 um in diameter and 20 nm, respectively. As shown in Fig. 3(e), there are SiC nanoparticles on the surface of BNNS and the size of the sheet is still around 1 um in diameter, meaning the in-situ growth of SiC nanoparticles will not destroy the integrity of the BNNS. Meanwhile, the average thickness of the SiC-BNNS-(II) is around 70 nm (Fig. 3(g)), much higher than that of pure BNNS, mainly ascribed to the successful growth of the SiC nanoparticles on the BNNS surface. Figs. 3(e'-e'''') are the EDS analyses of SiC-BNNS-(II) fillers. Uniform distribution of Si, C, B, and N elements further illustrates that the SiC nanoparticles are uniformly distributed on the surface of the BNNS, further suggesting the desired hetero-structured SiC-BNNS fillers were successfully prepared. 3.2. Thermal conductivities of the (SiC-BNNS)/epoxy nanocomposites Fig. 4 presents the comparison of thermal conductivities for epoxy nanocomposites with different kinds and amounts of thermally conductive fillers. As shown in Fig. 4(a), the λ values of the epoxy nanocomposites are all elevated with the increasing addition of thermally conductive fillers. Under the same amount of fillers, the order of the λ values of the obtained epoxy nanocomposites is listed as following, (SiC-BNNS)/epoxy > BNNS/epoxy > (SiC/BNNS)/epoxy > SiC/epoxy. This is due to the fact that the intrinsic
λ value of BNNS is higher than that of SiC, so the effect of BNNS on improving λ value is better. In the premise of the same fillers concentration, the SiC/BNNS (1/1, w/w) hybrid fillers are easier to connect with each other than single BNNS and SiC, easier to form more effective thermally conductive pathways inner epoxy matrix. However, more interfacial thermal resistances are inevitably formed [40], difficult to exhibit the excellent bulk λ value of BNNS and SiC. Therefore, the (SiC/BNNS)/epoxy nanocomposites have slightly lower λ values than those of BNNS/epoxy nanocomposites. For hetero-structured SiC-BNNS fillers, the interfacial thermal resistances introduced by direct doping of BNNS and SiC can be reduced by strong bonding force based on chemical bonds. Meanwhile, the agglomeration of BNNS and SiC can be also avoided effectively, and the efficient thermally conductive paths of SiC-SiC and BNNS-SiC-BNNS can be constructed inner epoxy matrix more easily (Fig. 4(d)), to further accelerate the heat flow conduction. Therefore, the (SiC-BNNS)/epoxy nanocomposites have the optimal λ value. Furthermore, the comparison of thermal conductivities for SiC-BNNS/epoxy nanocomposites with different mass ratio of SiC to BNNS also indicate that the (SiC-BNNS-(II))/epoxy nanocomposites have the most excellent thermal conductivity (λ of 0.89 W/mK and α of 0.65 mm2/s) with 20 wt% SiC-BNNS-(II) fillers. This is because the SiC nanoparticles in SiC-BNNS-(II) are relatively uniformly distributed on the surface of BNNS in comparison to SiC-BNNS-(I) and SiC-BNNS-(III) in Figs. 3(a)-(c). Therefore, SiC can perfectly overlap with adjacent BNNS (Fig. 4(d)), which will promote the synergistic effect of the "point-surface" structure of SiC and BNNS, to enhance the thermal conductivities. Such hetero-structure of SiC-BNNS-(II) is more beneficial to reducing thermally conductive threshold, thereby improving the λ values. Meantime, the λ and α values of the (SiC-BNNS-(II))/epoxy nanocomposites increase with increasing amount of SiC-BNNS-(II) fillers (Fig. 4(b)). When the amount of
SiC-BNNS-(II) is 20 wt%, the λ and α values of the (SiC-BNNS-(II))/epoxy nanocomposites are up to 0.89 W/mK and 0.65 mm2/s, increased by 4.1 and 2.7 times compared to those of pure epoxy (0.22 W/mK and 0.17 mm2/s), respectively. The λ value of the (SiC-BNNS-(II))/epoxy nanocomposites is also significantly higher than that of BNNS/epoxy (0.61 W/mK), (SiC/BNNS)/epoxy (0.52 W/mK), and SiC/epoxy nanocomposites (0.43 W/mK) with the same mass fraction of fillers. It is mainly attributed to the fact that SiC-BNNS-(II) fillers are easier to overlap with each other inner epoxy matrix, thus forming more SiC-SiC, BNNS-BNNS, and BNNS-SiC-BNNS thermally conductive paths, which would effectively enhance the λ and α values of the (SiC-BNNS)-(II))/epoxy nanocomposites. Fig. 4(c) shows the infrared thermal images of pure epoxy resin and (SiC-BNNS-(II))/epoxy nanocomposites. Surface temperature of the pure epoxy resin reaches 78.7 oC after 75 s. Whereas, after the same heating time (75 s), the surface temperatures of the (SiC-BNNS-(II))/epoxy nanocomposites reach 81.7 oC (5 wt% SiC-BNNS-(II)), 84.2 SiC-BNNS-(II)), and 92.4 demonstrates
that
the
o
C (10 wt% SiC-BNNS-(II)), 88.0
o
C (15 wt%
o
C (20 wt% SiC-BNNS-(II)), respectively. It further
thermal
conductivities
of
the
(SiC-BNNS-(II))/epoxy
nanocomposites improve with the increasing amount of SiC-BNNS-(II) fillers, consistent with the results from Fig. 4(b). In addition, the uniform heat distribution shown in Fig. 4(c) further presents evidence of the relatively uniform dispersion of SiC-BNNS-(II) fillers inner (SiC-BNNS-(II))/epoxy nanocomposites. 3.3 Electrical insulation properties of the (SiC-BNNS-(II))/epoxy nanocomposites Fig. 5 demonstrates the electrical breakdown strength (Eb), surface resistivity (ρs), and volume resistivity (ρv) of the (SiC-BNNS-(II))/epoxy nanocomposites with different contents of SiC-BNNS-(II) fillers. With the increasing amount of SiC-BNNS-(II) fillers, the Eb, ρs, and ρv of the (SiC-BNNS-(II))/epoxy nanocomposites increase firstly and then
decrease. When the amount of SiC-BNNS-(II) fillers is 5 wt%, the corresponding Eb, ρs, and ρv of the (SiC-BNNS-(II))/epoxy nanocomposites reach the highest values of 31.2 kV/mm, 8.7×1015 Ω·cm, and 6.2×1015 Ω·cm, respectively. Fortunately, the obtained (SiC-BNNS-(II))/epoxy nanocomposites still preserve favorable electrical insulating properties with the amount of SiC-BNNS-(II) up to 20 wt%, as their Eb, ρs, and ρv maintain at 22.1 kV/mm, 2.32×1015 Ω·cm, and 1.26×1015 Ω·cm, respectively. This is because when the injected charge moving from the electrode to the inside of the (SiC-BNNS-(II))/epoxy nanocomposites, an appropriate amount of BNNS-SiC-BNNS can effectively disperse the charge accumulation, thus weakening the formation of charge packets, thereby enhance the Eb. As the amount of SiC-BNNS-(II) fillers increases, SiC-BNNS-(II) fillers will generate a large number of carriers and introduce more defects. Incorporated defects will enable the starting electrons move more easily and introduce more electrons as the starting electrons collide, which will cause the formation of charge packets inner epoxy matrix and reduce the Eb [41]. Electrical insulation of SiC-BNNS-(II) fillers is slightly poorer compared with that of epoxy matrix. This is because the ρs and ρv of the (SiC-BNNS-(II))/epoxy nanocomposites are mainly determined by epoxy resin, when the amount of SiC-BNNS-(II) is low. As the amount of SiC-BNNS-(II) fillers rises, mutual contact area between SiC-BNNS-(II) increases, and the conduction of carriers between SiC-BNNS-(II) fillers increases, resulting in the decrease of ρs and ρv [42]. 4. Conclusions Hetero-structured SiC-BNNS thermally conductive & insulating fillers were successfully fabricated, which can realize the synergistic effects of SiC and BNNS, and present better enhancement of thermal conductivities than single BNNS, SiC, and SiC/BNNS hybrid fillers for epoxy thermally conductive nanocomposites. When the amount of hetero-structured SiC-BNNS-(II) is 20 wt%, the λ value of the (SiC-BNNS-(II))/epoxy
nanocomposites is increased to 0.89 W/mK, 4.1 times that of pure epoxy resin (0.22 W/mK), and 2.1, 1.4, and 1.7 times that of SiC/epoxy (0.43 W/mK), BNNS/epoxy (0.62 W/mK), and (SiC/BNNS)/epoxy nanocomposites (0.52 W/mK) with the same amount of fillers (20 wt%), respectively. Meantime, the corresponding (SiC-BNNS)/epoxy nanocomposites with 20 wt% SiC-BNNS-(II) fillers also demonstrate favorable electrical insulating properties (breakdown strength, volume resistivity as well as surface resistivity of 22.1 kV/mm, 2.32×1015 Ω·cm, and 1.26×1015 Ω·cm, respectively). In-situ fabrication of hetero-structured SiC-BNNS thermally conductive & insulating fillers will bring new perspectives in the fields of electronic packaging. 5. Acknowledgements The authors are grateful for the support and funding from the Foundation of National Natural Science Foundation of China (Nos. 51973173 and 51773169); Natural Science Basic Research Plan for Distinguished Young Scholars in Shaanxi Province of China (No. 2019JC-11); Natural Science Basic Research Plan in Shaanxi Province of China (No. 2018JM5001); Open Fund from Henan University of Science and Technology; Fundamental Research Funds for the Central Universities (No. 310201911py010); X. T. Yang thanks for the Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (CX201920). We would like to thank the Analytical & Testing Center of Northwestern Polytechnical University for SEM, TEM, and AFM tests. References [1] F. Zhang, Y. Feng, M. Qin, L. Gao, Z. Li, F. Zhao, Z. Zhang, F. Lyu, W. Feng. Stress controllability in thermal and electrical conductivity of 3D elastic graphene-crosslinked carbon nanotube sponge/polyimide nanocomposites, Adv. Funct. Mater. 29 (2019) 1901383. [2] (a) L. Chen, N. Song, L. Shi, P. Ding. Anisotropic thermal conductive composite with
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Fig. 1 Schematic diagram for the fabrication of hetero-structured SiC-BNNS fillers and their (SiC-BNNS)/epoxy thermally conductive nanocomposites.
Fig. 2 (a) XPS spectra of BNNS and SiC-BNNS; Narrow-spectrum XPS of (a') Si 2p, (a'') C 1s, (a''') B 1s, (a'''') N 1s; (b) XRD of SiC-BNNS.
Fig. 3 SEM images of (a) SiC-BNNS-(I), (b) SiC-BNNS-(II) and (c) SiC-BNNS-(III); TEM images of (d) BNNS and (e) SiC-BNNS-(II); EDS of (e') Si, (e'') C, (e''') B, and (e'''') N; AFM images of (f)BNNS and (g) SiC-BNNS-(II).
Fig. 4 (a) λ values of the epoxy thermally conductive nanocomposites; (b) λ, α values and (c) Infrared thermal images of the SiC-BNNS-(II)/epoxy nanocomposites; (d) Schematic illustration of thermally conductive mechanism for hetero-structured SiC-BNNS fillers.
Fig. 5 (a) Breakdown strength and (b) Electrical resistivity of the SiC-BNNS-(II)/epoxy thermally conductive nanocomposites.
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. Junwei Gu 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: Yixin Han: [email protected] Xuetao Shi: [email protected] Xutong Yang: [email protected] Yongqiang Guo: [email protected] Junliang Zhang: [email protected] Jie Kong: [email protected] Junwei Gu: [email protected] & [email protected]
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:
S0266-3538(19)33004-0
DOI:
https://doi.org/10.1016/j.compscitech.2019.107944
Reference:
CSTE 107944
To appear in:
Composites Science and Technology
Received Date: 28 October 2019 Revised Date:
4 December 2019
Accepted Date: 7 December 2019
Please cite this article as: Han Y, Shi X, Yang X, Guo Y, Zhang J, Kong J, Gu J, Enhanced thermal conductivities of epoxy nanocomposites via incorporating in-situ fabricated hetero-structured SiC-BNNS fillers, Composites Science and Technology (2020), doi: https://doi.org/10.1016/ j.compscitech.2019.107944. 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. © 2019 Published by Elsevier Ltd.
Enhanced thermal conductivities of epoxy nanocomposites via incorporating in-situ fabricated hetero-structured SiC-BNNS fillers
Yixin Hana#, Xuetao Shia#, Xutong Yanga, Yongqiang Guoa, Junliang Zhanga,b, Jie Konga, Junwei Gua*
a
MOE Key Laboratory of Material Physics and Chemistry under Extraordinary
Conditions, Shaanxi Key Laboratory of Macromolecular Science and Technology, Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’ an, Shaanxi, 710072, P. R. China. b
School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang, 471023, P. R. China.
Corresponding Authors: [email protected] or [email protected] (J. Gu).
#
The authors Yixin Han# and Xuetao Shi# contributed equally to this work and should be considered co-first authors.
Abstract
Novel hetero-structured silicon carbide-boron nitride nanosheets (SiC-BNNS) by sol-gel and in-situ growth method were performed as thermally conductive & insulating fillers, and the SiC-BNNS/epoxy thermally conductive nanocomposites were then prepared by blending-casting approach. Synthesized hetero-structured SiC-BNNS fillers have synergistic improvement effects on the thermal conductivities of the SiC-BNNS/epoxy nanocomposites. When the amount of hetero-structured SiC-BNNS fillers is 20 wt% (SiC-BNNS, 1/1, w/w), the thermal conductivity coefficient (λ) value of the SiC-BNNS/epoxy nanocomposites (0.89 W/mK) is 4.1 times that of pure epoxy resin (0.22 W/mK), and 2.1, 1.4, and 1.7 times of SiC/epoxy (0.43 W/mK), BNNS/epoxy (0.62 W/mK), and (SiC/BNNS)/epoxy thermally conductive nanocomposites (0.52 W/mK) with the same amount of fillers (20 wt% single BNNS, SiC, or SiC/BNNS hybrid fillers), respectively.
Meantime,
the
obtained
(SiC-BNNS)/epoxy thermally
conductive
nanocomposites also demonstrate favorable electrical insulating properties, and the breakdown strength, volume resistivity as well as surface resistivity is 22.1 kV/mm, 2.32×1015 Ω·cm, and 1.26×1015 Ω·cm, respectively.
Key words: A. Polymer-matrix composites (PMCs); B. Electrical properties; D. Scanning electron microscopy (SEM); E. Casting.
1. Introduction With the fast advancement of telecommunication technology, electronic devices are developing towards miniaturization and integration [1-3]. Thermal accumulation generated by rapid heating rate and high-speed operation together with increasing power consumption cause irreversible damage to electronic components [4-6]. For example, continuously excessive high temperatures will endanger the semiconductor junction, damage the connection interface of the circuit, and form mechanical stress damage [7-8]. Highly thermally conductive & insulating materials can effectively improve the heat conduction/dissipation of the electronic devices ensuring the electrical insulation performance, meanwhile guarantee safe use and prolong the service life [9-10]. Polymer matrix composites are extensively applied in electronic packaging field due to their good processibility, light weight, and low cost, etc., [11]. Researchers have focused on the preparation of highly thermally conductive & insulating polymer composites and their potential applications in the fields of electronic equipments, portable and wearable energy conversion & storage devices [12-14]. Epoxy resin has excellent electrical insulating and mechanical properties, wonderful heat and corrosion resistance, etc., and has been widely used in coatings, adhesives, and advanced composites fields [15-16]. However, its low thermal conductivity coefficient (λ, 0.22 W/mK) has limited further application in the heat conduction/dissipation field [17-18]. Researchers are ususally focusing on improving the λ value of epoxy matrix by introducing single or hybrid fillers. Huang [19] et al. obtained the λ value of 2.2 W/mK for 50 vol% AlN/epoxy composites, 11 times than that of pure epoxy (0.2 W/mK). In our previous work [20], the fabricated BN/epoxy composites with 60 wt% BN fillers presented the highest λ value of 0.98 W/mK, about 5 times that of pure epoxy. However, the improvement of λ values is limited. Researchers adopted surface functionalization of
thermally conductive fillers to further improve the λ values of the epoxy composites for a given fillers loading [21-23]. Yang [24] et al. prepared silane coupling agent functionalized nano-sized SiC particles (f-SiCnp)/epoxy nanocomposites. The λ value of the 30 wt% f-SiCnp/epoxy nanocomposites was 0.78 W/mK, higher than that of 30 wt% SiCnp/epoxy nanocomposites (0.61 W/mK). Investigations revealed that the surface functionalization would reduce the thermal resistances between thermally conductive fillers and epoxy matrix [25]. However, the increase of λ is still limited and insufficient. Under this background, researchers tried to improve the λ values of the polymers by blending two and/or three hybrid thermally conductive fillers (Al/SiCw [26], BN/AlN [27-28], BN/SiC [29], Al2O3/AlN [30], SiC nanowires/graphene [31], GNPs/MWCNTs [32], and AlN/BN/SiC [33], etc.,). Compared with single thermally conductive fillers, the introduction of hybrid fillers with different particle sizes and morphologies can further effectively increase the probability of forming thermally conductive paths inner polymer matrix [34]. However, it would also lead to the formation of more interfacial thermal resistances, against to take full advantage of the excellent thermal conductivity for fillers themselves. Phonon scattering due to the interfacial thermal resistances at the fillers-polymer matrix interfaces and the contact thermal resistances at fillers-fillers interfaces are inevitable, which is the main cause of the low thermal conductivities for obtained composites. More importantly, the bonding force between hybrid thermally conductive fillers only depends on physical deposition and adsorption, which will cause the agglomeration of fillers inner polymer matrix [35-36].When the fillers are incorporated in a high content, the interfacial thermal resistances can be overcome by forming contacted three-dimensional (3D) fillers networks. Relative investigations have indicated that the hetero-structured thermally conductive fillers could fully utilize the advantages of single thermally conductive fillers, finally to
achieve the synergistic improvement effects, thereby form more efficient heat conduction pathways, and effectively avoid introducing more interfacial thermal barriers [37-38]. In this work, the silicon carbide (SiC) precursor solution was firstly prepared by sol-gel method, and the hetero-structured SiC/BN nanosheets (SiC-BNNS) thermally conductive & insulating fillers were then synthesized by in-situ growth method. It was expected that the SiC nanoparticles would form SiC-SiC heat conduction pathways on the surface of the BNNS, and the BNNS-SiC-BNNS heat conduction paths would be further constructed
with
(SiC-BNNS)/epoxy
the
adjacent
thermally
BNNS conductive
fillers.
Afterwards,
nanocomposites
the were
corresponding prepared
by
blending-casting approach. Structures and surface morphologies of the hetero-structured SiC-BNNS fillers were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). Meantime, the amount of hetero-structured SiC-BNNS fillers affecting on the thermal conductivities, electrical insulation, mechanical and thermal properties of the SiC-BNNS/epoxy thermally conductive nanocomposites were also investigated. 2.Experimental section 2.1 Preparation of hetero-structured SiC-BNNS fillers Phenolic resin was dissolved in 50 mL ethanol and stirred at 45 oC for 1 hr. Silicon powder was then dispersed inner phenolic resin/ethanol solution. Carbon-coated silicon (Fig. S1) was then obtained by coating growth. Then the BNNS was placed in 50 mL ethanol, mixed with carbon-coated silicon solution and ultrasonic pulverized (300 W, 10 min). Hetero-structured SiC-BNNS precursor solution was prepared by heating and stirring for 2 hrs at 45 oC, followed by injected into 100 mL deionized water and stirred for 1 hr. The mixture was then centrifuged at 6500 rpm for 10 min, and the sludge was
dried at 60 oC under vacuum for 8 hrs. Three hetero-structured SiC-BNNS fillers with different mass ratio, SiC-BNNS-(I) (SiC-BNNS, 2/1, w/w), SiC-BNNS-(II) (SiC-BNNS, 1/1, w/w), and SiC-BNNS-(III) (SiC-BNNS, 1/2, w/w), were all prepared by grinding the dried samples, which were treated for 2 hrs at 800 oC and then another 2 hrs at 1600 oC. Actually, phenolic resin was carbonized at 800oC and the silicon covered by phenolic resin was Si/C binary system. When the thermal treatment temperature increased to 1600oC, the silicon powder transformed into liquid silicon. Meanwhile, the reaction between silicon and carbon resulted in silicon carbide on the surface of BNNS with excellent chemical stability, and the final product was SiC-BNNS fillers. 2.2 Fabrication of the (SiC-BNNS)/epoxy thermally conductive nanocomposites Hetero-structured SiC-BNNS fillers were mixed with epoxy resin, followed by mechanically stirred for 2 hrs at room temperature. Afterwards, the curing agent was then added and stirred for another 4 hrs at 70 oC. Subsequently, the mixtures were poured into a preheated mold, cured at 120 oC for 5 hrs, and then cooled to room temperature, finally to obtain the (SiC-BNNS)/epoxy thermally conductive nanocomposites. The information of "Main Materials" and the "Characterization" details were presented in the "Supplementary Material". 3. Results and discussion 3.1. Characterization on hetero-structured SiC-BNNS fillers Fig. 2(a) shows the XPS spectra of BNNS and SiC-BNNS. For BNNS, the characteristic peaks of B 1s, C 1s, N 1s, and O 1s are corresponding to 190, 284.6, 398, and 532.5 eV, respectively [39]. For SiC-BNNS, the characteristic peaks of B, C, N, and O remain unchanged, whereas the C peak intensity increases significantly. In addition, Si 2s and Si 2p peaks at 153 and 101.8 eV appear. Besides, there are no other impurities, indicating successful introduction of SiC on the surface of BNNS. Figs. 2 (a'-a'''') present the Si 2p,
C 1s, B 1s, and N 1s spectra of SiC-BNNS, respectively. In Fig. 2(a'), the peaks of 100.2, 101.5, and 102.3 eV are corresponding to Si-C, Si-N, and Si-CBN bonds, and the main bonding modes are Si3N4 and SiCBN. In Fig. 2(a''), the peaks at 283.5 and 286.3 eV correspond to the C-Si covalent bond and C-N bond, mainly attributed to the high temperature fracture of Si-C and B-N bonds, to generate C-N and C-Si bonds. In Fig. 2 (a'''), the peak at 189.9 eV is attributed to the formation of the intermediate for SiCBN between BN and SiCB. The peak at 188.8 eV corresponds to the SiC-B bond. The peak of 188.3 eV is attributed to the recombination of B and Si after Si-C and B-N bond cleavage, and a small amount of B4Si is formed. The peak of 191.2 eV corresponds to B-N bond. In Fig. 2 (a''''), the peak of 397.2 eV corresponds to the Si-N bond and the peak at 398.8 eV is corresponding to the C-N bond. Such two peaks can be ascribed to in-situ growth, causing the Si-C and B-N bonds broken to form Si-N and C-N bonds, respectively. In addition, the small peak of 400.3 eV corresponds to the N-O bond. In XRD (Fig. 2(b)), the peaks and crystal plane index of BNNS are 26.7° (002), 41.7° (100), 43.7° (101), 50° (102), 55° (004), 76° (110), and 82° (112). Peaks and crystal plane index of SiC correspond to 36° (111), 42° (200), 60° (220), 72° (311), and 76° (222), further prove the successful formation of SiC-BNNS. Fig. 3 shows the SEM images of the hetero-structured SiC-BNNS fillers. There are SiC nanoparticles existing on the surface of BNNS, and the number of SiC nanoparticles increases with the increasing amount of Si powder. It indicates that increasing amount of Si powder is beneficial to the in-situ formation of SiC nanoparticles on the surface of BNNS. When the mass ratio of BNNS to Si powder is 2:1 (SiC-BNNS-(II), equals to the mass ratio of BNNS to SiC of 1:1), the SiC-BNNS-(II) (Fig. 3(b)) fillers have the optimized hetero-structured morphology, more favorable to the synergistic thermal conductivity effects. Indeed, when the amount of Si powder is small, in-situ generated
SiC nanoparticles are relatively small (Fig. 3(a)), thereby SiC nanoparticles are scattered on the surface of BNNS sporadically, difficult to form SiC-SiC heat conduction pathways or to overlap BNNS adequately. With excessive amount of Si powder, too much SiC nanoparticles would completely cover the BNNS (Fig. 3(c)). Consequently, it is also difficult to exert the relatively better thermal conductivity of the BNNS and the synergistic improvement effects of hetero-structured SiC-BNNS fillers. TEM and AFM images of BNNS and SiC-BNNS-(II) are shown in Figs. 3 (d-g). Visibly, the surface of the pure BNNS is flat (Fig. 3(d)) and transparent (Fig. 3(f)). The size and the thickness of pure BNNS are around 1 um in diameter and 20 nm, respectively. As shown in Fig. 3(e), there are SiC nanoparticles on the surface of BNNS and the size of the sheet is still around 1 um in diameter, meaning the in-situ growth of SiC nanoparticles will not destroy the integrity of the BNNS. Meanwhile, the average thickness of the SiC-BNNS-(II) is around 70 nm (Fig. 3(g)), much higher than that of pure BNNS, mainly ascribed to the successful growth of the SiC nanoparticles on the BNNS surface. Figs. 3(e'-e'''') are the EDS analyses of SiC-BNNS-(II) fillers. Uniform distribution of Si, C, B, and N elements further illustrates that the SiC nanoparticles are uniformly distributed on the surface of the BNNS, further suggesting the desired hetero-structured SiC-BNNS fillers were successfully prepared. 3.2. Thermal conductivities of the (SiC-BNNS)/epoxy nanocomposites Fig. 4 presents the comparison of thermal conductivities for epoxy nanocomposites with different kinds and amounts of thermally conductive fillers. As shown in Fig. 4(a), the λ values of the epoxy nanocomposites are all elevated with the increasing addition of thermally conductive fillers. Under the same amount of fillers, the order of the λ values of the obtained epoxy nanocomposites is listed as following, (SiC-BNNS)/epoxy > BNNS/epoxy > (SiC/BNNS)/epoxy > SiC/epoxy. This is due to the fact that the intrinsic
λ value of BNNS is higher than that of SiC, so the effect of BNNS on improving λ value is better. In the premise of the same fillers concentration, the SiC/BNNS (1/1, w/w) hybrid fillers are easier to connect with each other than single BNNS and SiC, easier to form more effective thermally conductive pathways inner epoxy matrix. However, more interfacial thermal resistances are inevitably formed [40], difficult to exhibit the excellent bulk λ value of BNNS and SiC. Therefore, the (SiC/BNNS)/epoxy nanocomposites have slightly lower λ values than those of BNNS/epoxy nanocomposites. For hetero-structured SiC-BNNS fillers, the interfacial thermal resistances introduced by direct doping of BNNS and SiC can be reduced by strong bonding force based on chemical bonds. Meanwhile, the agglomeration of BNNS and SiC can be also avoided effectively, and the efficient thermally conductive paths of SiC-SiC and BNNS-SiC-BNNS can be constructed inner epoxy matrix more easily (Fig. 4(d)), to further accelerate the heat flow conduction. Therefore, the (SiC-BNNS)/epoxy nanocomposites have the optimal λ value. Furthermore, the comparison of thermal conductivities for SiC-BNNS/epoxy nanocomposites with different mass ratio of SiC to BNNS also indicate that the (SiC-BNNS-(II))/epoxy nanocomposites have the most excellent thermal conductivity (λ of 0.89 W/mK and α of 0.65 mm2/s) with 20 wt% SiC-BNNS-(II) fillers. This is because the SiC nanoparticles in SiC-BNNS-(II) are relatively uniformly distributed on the surface of BNNS in comparison to SiC-BNNS-(I) and SiC-BNNS-(III) in Figs. 3(a)-(c). Therefore, SiC can perfectly overlap with adjacent BNNS (Fig. 4(d)), which will promote the synergistic effect of the "point-surface" structure of SiC and BNNS, to enhance the thermal conductivities. Such hetero-structure of SiC-BNNS-(II) is more beneficial to reducing thermally conductive threshold, thereby improving the λ values. Meantime, the λ and α values of the (SiC-BNNS-(II))/epoxy nanocomposites increase with increasing amount of SiC-BNNS-(II) fillers (Fig. 4(b)). When the amount of
SiC-BNNS-(II) is 20 wt%, the λ and α values of the (SiC-BNNS-(II))/epoxy nanocomposites are up to 0.89 W/mK and 0.65 mm2/s, increased by 4.1 and 2.7 times compared to those of pure epoxy (0.22 W/mK and 0.17 mm2/s), respectively. The λ value of the (SiC-BNNS-(II))/epoxy nanocomposites is also significantly higher than that of BNNS/epoxy (0.61 W/mK), (SiC/BNNS)/epoxy (0.52 W/mK), and SiC/epoxy nanocomposites (0.43 W/mK) with the same mass fraction of fillers. It is mainly attributed to the fact that SiC-BNNS-(II) fillers are easier to overlap with each other inner epoxy matrix, thus forming more SiC-SiC, BNNS-BNNS, and BNNS-SiC-BNNS thermally conductive paths, which would effectively enhance the λ and α values of the (SiC-BNNS)-(II))/epoxy nanocomposites. Fig. 4(c) shows the infrared thermal images of pure epoxy resin and (SiC-BNNS-(II))/epoxy nanocomposites. Surface temperature of the pure epoxy resin reaches 78.7 oC after 75 s. Whereas, after the same heating time (75 s), the surface temperatures of the (SiC-BNNS-(II))/epoxy nanocomposites reach 81.7 oC (5 wt% SiC-BNNS-(II)), 84.2 SiC-BNNS-(II)), and 92.4 demonstrates
that
the
o
C (10 wt% SiC-BNNS-(II)), 88.0
o
C (15 wt%
o
C (20 wt% SiC-BNNS-(II)), respectively. It further
thermal
conductivities
of
the
(SiC-BNNS-(II))/epoxy
nanocomposites improve with the increasing amount of SiC-BNNS-(II) fillers, consistent with the results from Fig. 4(b). In addition, the uniform heat distribution shown in Fig. 4(c) further presents evidence of the relatively uniform dispersion of SiC-BNNS-(II) fillers inner (SiC-BNNS-(II))/epoxy nanocomposites. 3.3 Electrical insulation properties of the (SiC-BNNS-(II))/epoxy nanocomposites Fig. 5 demonstrates the electrical breakdown strength (Eb), surface resistivity (ρs), and volume resistivity (ρv) of the (SiC-BNNS-(II))/epoxy nanocomposites with different contents of SiC-BNNS-(II) fillers. With the increasing amount of SiC-BNNS-(II) fillers, the Eb, ρs, and ρv of the (SiC-BNNS-(II))/epoxy nanocomposites increase firstly and then
decrease. When the amount of SiC-BNNS-(II) fillers is 5 wt%, the corresponding Eb, ρs, and ρv of the (SiC-BNNS-(II))/epoxy nanocomposites reach the highest values of 31.2 kV/mm, 8.7×1015 Ω·cm, and 6.2×1015 Ω·cm, respectively. Fortunately, the obtained (SiC-BNNS-(II))/epoxy nanocomposites still preserve favorable electrical insulating properties with the amount of SiC-BNNS-(II) up to 20 wt%, as their Eb, ρs, and ρv maintain at 22.1 kV/mm, 2.32×1015 Ω·cm, and 1.26×1015 Ω·cm, respectively. This is because when the injected charge moving from the electrode to the inside of the (SiC-BNNS-(II))/epoxy nanocomposites, an appropriate amount of BNNS-SiC-BNNS can effectively disperse the charge accumulation, thus weakening the formation of charge packets, thereby enhance the Eb. As the amount of SiC-BNNS-(II) fillers increases, SiC-BNNS-(II) fillers will generate a large number of carriers and introduce more defects. Incorporated defects will enable the starting electrons move more easily and introduce more electrons as the starting electrons collide, which will cause the formation of charge packets inner epoxy matrix and reduce the Eb [41]. Electrical insulation of SiC-BNNS-(II) fillers is slightly poorer compared with that of epoxy matrix. This is because the ρs and ρv of the (SiC-BNNS-(II))/epoxy nanocomposites are mainly determined by epoxy resin, when the amount of SiC-BNNS-(II) is low. As the amount of SiC-BNNS-(II) fillers rises, mutual contact area between SiC-BNNS-(II) increases, and the conduction of carriers between SiC-BNNS-(II) fillers increases, resulting in the decrease of ρs and ρv [42]. 4. Conclusions Hetero-structured SiC-BNNS thermally conductive & insulating fillers were successfully fabricated, which can realize the synergistic effects of SiC and BNNS, and present better enhancement of thermal conductivities than single BNNS, SiC, and SiC/BNNS hybrid fillers for epoxy thermally conductive nanocomposites. When the amount of hetero-structured SiC-BNNS-(II) is 20 wt%, the λ value of the (SiC-BNNS-(II))/epoxy
nanocomposites is increased to 0.89 W/mK, 4.1 times that of pure epoxy resin (0.22 W/mK), and 2.1, 1.4, and 1.7 times that of SiC/epoxy (0.43 W/mK), BNNS/epoxy (0.62 W/mK), and (SiC/BNNS)/epoxy nanocomposites (0.52 W/mK) with the same amount of fillers (20 wt%), respectively. Meantime, the corresponding (SiC-BNNS)/epoxy nanocomposites with 20 wt% SiC-BNNS-(II) fillers also demonstrate favorable electrical insulating properties (breakdown strength, volume resistivity as well as surface resistivity of 22.1 kV/mm, 2.32×1015 Ω·cm, and 1.26×1015 Ω·cm, respectively). In-situ fabrication of hetero-structured SiC-BNNS thermally conductive & insulating fillers will bring new perspectives in the fields of electronic packaging. 5. Acknowledgements The authors are grateful for the support and funding from the Foundation of National Natural Science Foundation of China (Nos. 51973173 and 51773169); Natural Science Basic Research Plan for Distinguished Young Scholars in Shaanxi Province of China (No. 2019JC-11); Natural Science Basic Research Plan in Shaanxi Province of China (No. 2018JM5001); Open Fund from Henan University of Science and Technology; Fundamental Research Funds for the Central Universities (No. 310201911py010); X. T. Yang thanks for the Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (CX201920). We would like to thank the Analytical & Testing Center of Northwestern Polytechnical University for SEM, TEM, and AFM tests. References [1] F. Zhang, Y. Feng, M. Qin, L. Gao, Z. Li, F. Zhao, Z. Zhang, F. Lyu, W. Feng. Stress controllability in thermal and electrical conductivity of 3D elastic graphene-crosslinked carbon nanotube sponge/polyimide nanocomposites, Adv. Funct. Mater. 29 (2019) 1901383. [2] (a) L. Chen, N. Song, L. Shi, P. Ding. Anisotropic thermal conductive composite with
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Fig. 1 Schematic diagram for the fabrication of hetero-structured SiC-BNNS fillers and their (SiC-BNNS)/epoxy thermally conductive nanocomposites.
Fig. 2 (a) XPS spectra of BNNS and SiC-BNNS; Narrow-spectrum XPS of (a') Si 2p, (a'') C 1s, (a''') B 1s, (a'''') N 1s; (b) XRD of SiC-BNNS.
Fig. 3 SEM images of (a) SiC-BNNS-(I), (b) SiC-BNNS-(II) and (c) SiC-BNNS-(III); TEM images of (d) BNNS and (e) SiC-BNNS-(II); EDS of (e') Si, (e'') C, (e''') B, and (e'''') N; AFM images of (f)BNNS and (g) SiC-BNNS-(II).
Fig. 4 (a) λ values of the epoxy thermally conductive nanocomposites; (b) λ, α values and (c) Infrared thermal images of the SiC-BNNS-(II)/epoxy nanocomposites; (d) Schematic illustration of thermally conductive mechanism for hetero-structured SiC-BNNS fillers.
Fig. 5 (a) Breakdown strength and (b) Electrical resistivity of the SiC-BNNS-(II)/epoxy thermally conductive nanocomposites.
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. Junwei Gu 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: Yixin Han: [email protected] Xuetao Shi: [email protected] Xutong Yang: [email protected] Yongqiang Guo: [email protected] Junliang Zhang: [email protected] Jie Kong: [email protected] Junwei Gu: [email protected] & [email protected]
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: