Synergistic Effects of Boron Nitride (BN) Nanosheets and Silver (Ag) Nanoparticles on Thermal Conductivity and Electrical Properties of Epoxy Nanocomposites

polymers

Article Synergistic Eﬀects of Boron Nitride (BN) Nanosheets and Silver (Ag) Nanoparticles on Thermal Conductivity and Electrical Properties of Epoxy Nanocomposites

Yunjian Wu 1,2, Xiaoxing Zhang 1,3,* , Ankit Negi 2, Jixiong He 2, Guoxiong Hu 1, Shuangshuang Tian 3 and Jun Liu 2,*

1 School of Electrical Engineering and Automation, Wuhan University, Wuhan 430072, China; [email protected] (Y.W.); [email protected] (G.H.) 2 Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695, USA; [email protected] (A.N.); [email protected] (J.H.) 3 School of Electrical and Electronic Engineering, Hubei Key Laboratory for High-eﬃciently Utilization of Solar Energy and Operation Control of Energy Storage System, Hubei University of Technology, Wuhan 430068, China; [email protected] * Correspondence: [email protected] (X.Z.); [email protected] (J.L.)

Received: 13 January 2020; Accepted: 8 February 2020; Published: 12 February 2020

Abstract: Polymer composites, with both high thermal conductivity and high electrical insulation strength, are desirable for power equipment and electronic devices, to sustain increasingly high power density and heat flux. However, conventional methods to synthesize polymer composites with high thermal conductivity often degrade their insulation strength, or cause a significant increase in dielectric properties. In this work, we demonstrate epoxy nanocomposites embedded with silver nanoparticles (AgNPs), and modified boron nitride nanosheets (BNNSs), which have high thermal conductivity, high insulation strength, low permittivity, and low dielectric loss. Compared with neat epoxy, the composite with 25 vol% of binary nanofillers has a significant enhancement (~10x) in thermal conductivity, which is twice of that filled with BNNSs only (~5x), owing to the continuous heat transfer path among BNNSs enabled by AgNPs. An increase in the breakdown voltage is observed, which is attributed to BNNSs-restricted formation of AgNPs conducting channels that result in a lengthening of the breakdown path. Moreover, the effects of nanofillers on dielectric properties, and thermal simulated current of nanocomposites, are discussed.

Keywords: polymer nanocomposites; thermal conductivity; electrical properties; silver nanoparticles/boron nitride nanosheets; PACS:

1. Introduction Miniaturization of power equipment and electronic devices, coupled with a high voltage electromagnetic environment, demands superior thermal conductivity and electrical insulation strength in insulating materials [1–3]. An ideal insulating material is expected to possess high thermal conductivity, high breakdown strength, low permittivity, and low dielectric loss, simultaneously [4]. Polymers are widely used as insulating materials for their excellent electrical insulation properties [5,6], 1 1 but their low thermal conductivity (~0.2 W m− K− at room temperature) reduces the lifetime and reliability of power equipment and electronic devices [7–9]. To improve the thermal conductivity of polymers, many kinds of nanomaterials with high thermal conductivity have been added as ﬁllers in the literature [10–12]. However, obtaining signiﬁcant

Polymers 2020, 12, 426; doi:10.3390/polym12020426 www.mdpi.com/journal/polymers Polymers 2020, 12, 426 2 of 13 improvement in thermal conductivity without undermining insulation properties is challenging [13]. Nanofillers, such as carbon nanotubes [14], graphene [15], and metals (such as copper [16]) leads to the decrease of insulation strength. Some ceramic fillers, including aluminum oxide [17], aluminum nitride [18], silicon carbide [19], and strontium titanate [20] induce a sharp rise in dielectric properties, while others, such as silicon dioxide, limit thermal conductivity enhancement [21]. Boron nitride nanosheets (BNNSs) are promising for thermally conductive insulating materials, not only because they have intrinsically high thermal conductivity, but because they have a wide band gap of 5.9 eV, with excellent electrical insulating properties, such as high breakdown strength, low electrical conductivity, and low dielectric parameters [22,23]. However, BNNSs are difficult to form a thermal transfer network in polymer matrix alone. If the BNNSs were connected by nanoparticles to form an inter-filler thermal network in polymer composites, the thermal conductivity would be effectively improved at relatively low content [24–26]. Silver nanoparticles (AgNPs) are potentially used as the connecting nanoparticles, owing to their outstanding thermal conductivity and simpler distribution on the surface of BNNSs by silver nitrate reduction [27,28]. Furthermore, the AgNPs on the surface of BNNSs can be electrically blocked by BNNSs, effectively [29,30]. In this work, we chose epoxy as the polymer matrix that is widely used in insulating material, and BNNSs and AgNPs as the nanofillers. By placing AgNPs on the surface of BNNSs, and then adding the hybrid nanofiller into the epoxy matrix, we successfully synthesized an epoxy nanocomposite (EP-AgBN) with superb thermal conductivity, high insulation strength, low permittivity, and low dielectric loss. The synergistic effects of BNNSs and AgNPs in epoxy nanocomposites on thermal conductivity and electrical properties are studied. It is expected that the EP-AgBN exhibits strong potential as an insulating material for power equipment and electronic devices, such as insulators, flexible substrates, and electronic packages.

2. Experimental Section

2.1. Materials Hexagonal boron nitride (h-BN) (size of 1~2 µm) was purchased from Aladdin, China. N, N-dimethylformamide (DMF, purity 99.5%), silver nitrate (purity 99.8%), alcohol (purity 99.9%), ≥ ≥ ≥ and acetone (purity 99.95%) were purchased from Sinopharm Chemical Reagent, China. E-51 epoxy ≥ resin (2,20-[(1-methylethylidene)bis(4,1-phenyleneoxymethylene)]bis-oxiranhomopol) and 593 curing agent (adduct of diethylenetriamine and butyl glycidyl ether) were purchased from Shengshi Chemical, wuhan, China. All materials were used as received.

2.2. Preparation of AgNPs-BNNSs Liquid-phase exfoliation of h-BN was used to prepare BNNSs. In the exfoliation process, 1.5 g h-BN was dissolved in a suﬃcient amount of DMF (300 mL), then the mixture was under continuous sonication for 72 h. During this period, h-BN was gradually exfoliated into BNNSs. Seventy-two hours later, a solution of 1.5 g silver nitrate and 15 mL DMF was dropped slowly into the exfoliation solution that was stirred and sonicated synchronously. This process would last for 2 h, and the mixture was kept without any operation, in room temperature for 24 h, to ensure the uniform distribution of AgNPs on the surface of BNNSs. Finally, the solution was ﬁltered, and repeatedly washed with alcohol and acetone to obtain the AgNPs-BNNSs. In this method, the DMF acts as both the exfoliation liquid and reducing agent.

2.3. Preparation of EP-AgBN The AgNPs-BNNSs was added into acetone (1:15 mass ratio) and sonication was used to mix them uniformly. After the addition of E-51 epoxy (20 g), the mixture was stirred continuously under 70 ◦C for 7 h to volatilize all the acetone. Next, the mixture was subjected to sonication, and stirring simultaneously with the addition of the 593 curing agent at 30 ◦C for 2 h, the mass ratio of epoxy and Polymers 2020, 12, 426 3 of 13 curing agent was 4:1. Finally, the mixture was degassed in a vacuum box for 1 h. The curing process was in a mold at 100 ◦C under high pressure for 5 h to obtain composites of EP-AgBN. By controlling the content of AgNPs-BNNSs, we obtained composites with diﬀerent nanoﬁller content of 5, 10, 15, 20, and 25 vol%, respectively.

2.4. Morphology Characterization And Performances Measurement X-ray diffraction (XRD) was used to determine the elements, impurity, and crystal planes, with the scanning angle and scanning speed of X-ray diffractometer (Xpert Pro, Netherlands) as 2θ = 10◦–80◦ and 0.02◦/s, respectively. The morphology of AgNPs-BNNSs was taken by transmission electron microscopy (TEM) (JEM-2100, Tokyo, Japan) and the acceleration voltage was 200 kV. The morphology of EP-AgBN was obtained by scanning electron microscope (SEM) (QUANTA, Eindhoven, the Netherlands), and the accelerating voltage was 30 kV. The X-ray photoelectron spectroscopy (XPS) was obtained by X-ray photoelectron spectrometer (ESCALAB250Xi, Waltham, Massachusetts, USA), and the applied voltage was 15 kV. The thermal conductivity was measured by a fully automatic thermal conductivity meter (DRL-III, Xiangtan, China), which uses the steady-state heat flow method, according to the ASTM D5470-2006 standard. The dielectric properties were measured by a broadband dielectric spectrometer (BDS) (Concept 80, Montabaur, Germany) at room temperature from 1 Hz to 1 MHz. The electric breakdown voltage of all samples was tested using the plate electrode, with a radius 1 cm, and the frequency of the AC was 50Hz. Eight samples were tested repeatedly for each composite. The thermal simulated current (TSC) was measured by a thermal stimulated current analyzer (SETARAM II, Lyon, France). Firstly, the temperature was heated to the polarization temperature of 40 ◦C. Next, the polarization voltage of 300 V was applied and kept for 20 min. After that, the temperature was rapidly decreased to 100 C at a rate of 30 C/min. The polarization voltage was − ◦ ◦ removed at 100 C and kept for 10 min. Finally, temperature slowly increased to 120 C at a rate of − ◦ ◦ 2 ◦C/min, and the current recorded in this step was the TSC of the sample.

3. Results and Discussion

3.1. Morphology Characterization of AgNPs-BNNSs Figure1a,b show the optical images of h-BN and AgNPs-BNNSs. The h-BN is white and the prepared AgNPs-BNNSs is yellow. Figure1c shows the XRD patterns of BNNS by liquid-phase exfoliation and AgNPs-BNNSs. There are no impurity peaks in the spectrum, which indicates that the nanomaterials are highly purified. There exists diffraction peaks at 2θ 26.76 , 41.58 , 43.78 , ≈ ◦ ◦ ◦ 50.05◦, 55.02◦, 71.31◦, and 75.86◦ for both BNNSs and AgNPs-BNNSs, which correspond to the crystal planes (002), (100), (101), (102), (004), (104), and (110) of h-BN, respectively [30,31]. Also, there are three diffraction peaks at 2θ 38.16 , 64.48 , and 77.42 of AgNPs-BNNSs, which are the planes (111), ≈ ◦ ◦ ◦ (220), and (311) of silver crystals [32,33], which prove that the prepared AgNPs-BNNSs contain silver nanoparticles. Figure1d,e show the morphology of AgNPs-BNNSs. In Figure1d, h-BN is exfoliated to regular sheet-like BNNSs and the black spots distributed on the surface of BNNSs are AgNPs. Figure1e shows the atomic image of the composites by TEM. The lattice spacing of the sheet is 3.6 Å and the black spots are 2.3 Å, corresponding to the crystal plane (002) of BNNSs and the crystal plane (111) of AgNPs, respectively. All of these features further prove that the prepared composite products are AgNPs modified BNNSs. PolymersPolymers 20202020, 12, 12, x, 426FOR PEER REVIEW 4 4of of 13 13

(a) (c) 26.76° BNNSs (002) AgNP-BNNS

41.58° 50.05° 77.42° (100) (102) Ag(311) 43.78° 55.02° 64.48° (b) 38.16° 71.31°75.86° Intensity(arb. unit) Ag(111) (101) (002) Ag(220) (104) (110)

10 20 30 40 50 60 70 80 2θ

(e) (d)

23Å 36Å d2=2.3Å d1=3.6Å

Figure 1. Optical images of (a) hexagonal boron nitride (h-BN) and (b) silver nanoparticles-boron Figure 1. Optical images of (a) hexagonal boron nitride (h‐BN) and (b) silver nanoparticles‐boron nitride nanosheets (AgNPs-BNNSs); (c) XRD spectrum of BNNSs and AgNPs-BNNSs; (d) morphology nitride nanosheets (AgNPs‐BNNSs); (c) XRD spectrum of BNNSs and AgNPs‐BNNSs; (d) of AgNP-BNNS by TEM; (e) The TEM atomic image of BNNSs and AgNPs with lattice spacing labeled. morphology of AgNP‐BNNS by TEM; (e) The TEM atomic image of BNNSs and AgNPs with lattice 3.2.spacing Morphology labeled. Characterization of EP-AgBN

3.2 MorphologyFigure2a showsCharacterization the fracture of EP surface‐AgBN image of epoxy composite with a nanoﬁller content of 25 vol% by SEM. Owing to the vacuum degassing and high-pressure curing, the nanoﬁllers are homogeneously dispersedFigure and 2a shows maintain the strong fracture adhesion surface with image the of epoxy epoxy matrix. composite Figure with2b shows a nanofiller that the content nanoparticles of 25 vol%are attached by SEM. to Owing the BNNSs to the surface. vacuum Several degassing nanoparticles and high are‐pressure observed curing, with relatively the nanofillers larger size are in homogeneouslycomparison to Figure dispersed1d, as and small maintain particles strong combine adhesion into with larger the ones epoxy to reduce matrix. surface Figure energy2b shows during that thecomposite nanoparticles preparation. are attached The XPS to spectrumthe BNNSs can surface. reveal theSeveral composition nanoparticles of elements are observed contained with in a relativelysubstance larger surface size accurately. in comparison Figure to2 c,d,eFigure show 1d, as the small XPS particles spectrum combine of EP-AgBN. into larger The peak ones at to 190.4 reduce eV surfacerepresents energy the B–Nduring bond composite and the absorptionpreparation. curves The ofXPS C–C spectrum/C–H (284.7 can eV), reveal C–O the (286.2 composition eV) and C–N of elements(288.1 eV) contained are obtained in a substance by peak separation surface accurately. at 284.7 eV, Figure which 2c,d,e are theshow chemical the XPS bonds spectrum in the of epoxy EP‐ AgBN.matrix The [34 ,35peak]. Moreover, at 190.4 eV energy represents bandwidth the B–N between bond and Ag3d5 the /absorption2 (366.6 eV) curves and Ag3d3 of C–C/C–H/2 (372.6 (284.7 eV) is eV),6 eV, C–O meaning (286.2 that eV) theand AgNPs C–N (288.1 are distributed eV) are obtained on the surfaceby peak of separation BNNSs in at a form 284.7 of eV, granular which [are34]. the chemical bonds in the epoxy matrix [34,35]. Moreover, energy bandwidth between Ag3d5/2 (366.6 eV) and Ag3d3/2 (372.6 eV) is 6 eV, meaning that the AgNPs are distributed on the surface of BNNSs in a form of granular [34].

Polymers 2020, 12, 426 5 of 13 Polymers 2020, 12, x FOR PEER REVIEW 5 of 13

(a) (b)

original curve B-N C-C/C-H Ag3d5/2 (c) background (e) 284.7eV Ag3d fitting curve 3/2 background B-N C-C/C-H 190.4eV C-N C-O (d) 6eV C-O 286.2eV Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) C-N 288.1eV 372.6 366.6

194 192 190 188 291 288 285 282 380 375 370 365 360 Binding Energy (eV) Binding Energy (eV) Binding Energy (eV) Figure 2. (a,b) Morphology of epoxy (EP)-AgBN with a nanofiller content of 25 vol% by SEM with Figure 2. (a,b) Morphology of epoxy (EP)‐AgBN with a nanofiller content of 25 vol% by SEM with different scales; XPS spectra of EP-AgBN; (c) narrow B 1s scans of EP-AgBN; (d) narrow C 1s scans of different scales; XPS spectra of EP‐AgBN; (c) narrow B 1s scans of EP‐AgBN; (d) narrow C 1s scans of EP-AgBN; (e) narrow Ag 3d scans of EP-AgBN. EP‐AgBN; (e) narrow Ag 3d scans of EP‐AgBN. 3.3. Thermal Conductivity of EP-AgBN 3.3 Thermal Conductivity of EP‐AgBN Figure3a shows the e ffect of nanofiller content on thermal conductivity for EP-BN and EP-AgBN. 1 1 TheFigure measured 3a shows thermal the conductivityeffect of nanofiller of neat content epoxy ison 0.18 thermal W m conductivity− K− . Improvement for EP‐BN in and thermal EP‐ AgBN.conductivity The measured is realized thermal for both conductivity the nanofiller of neat composites. epoxy is 0.18 The W magnitude m−1 K−1. Improvement of rise is similar in thermal in both conductivitycases for low is nanofillerrealized for content both (the<10 nanofiller vol%). This composites. is primarily The due magnitude to insuffi cientof rise amount is similar of AgNPs in both to casessignificantly for low nanofiller influence thermalcontent (<10 conductivity. vol%). This Beyond is primarily 10 vol% due of nanofillers, to insufficient however, amount EP-AgBN of AgNPs shows to significantlygreat thermal influence conductivity thermal improvement conductivity. compared Beyond to 10 EP-BN. vol% Moreover,of nanofillers, the thermal however, conductivity EP‐AgBN of 1 1 showsEP-AgBN great reaches thermal to 2.14conductivity W m− K −improvementwith a filler loading compared of 25 to vol%, EP‐BN. which Moreover, is nearly twicethe thermal to that of 1 1 −1 −1 conductivityEP-BN (1.13 of W EP m‐−AgBNK− ). reaches This indicates to 2.14 thatW m AgNPs-BNNSs K with a filler are loading superior of to 25 BNNSs vol%, forwhich improving is nearly the Polymers 2020, 12, x FOR PEER REVIEW 6 of 13 twicethermal to that conductivity of EP‐BN (1.13 of epoxy. W m−1 K−1). This indicates that AgNPs‐BNNSs are superior to BNNSs for improving the thermal conductivity of epoxy. Figure 3b shows2.5 the thermal conductivity enhancement1200 (TC‐E) for EP‐AgBN and EP‐BN 1089 according to the formula TC‐E=（Ki EP-BN‐Kneat_epoxy/Kneat_epoxy, where KiEP-BN is the thermal conductivity 1000 of composite and Kneat_epoxy2.0 is the EP-AgBN TC of pure epoxy. Limited TC‐E is EP-AgBN achieved in both composites for low nanofiller loading (<10 vol%). Noticeable difference800 in TC‐E between728 the composites is observed at higher nanofiller1.5 contents.

600 Figure 4a,b illustrate the synergistic effect of BNNSs and AgNPs on thermal528 conductivity of 1.0 461 nanocomposites. The BNNSs formed the main thermalTC-E(%) transfer400 paths in nanocomposites356 and AgNPs connect the BNNS as a “thermal bridge”, which builds the inter‐filler thermal network in polymer 0.5 200 178 composites. Nanocomposites with inter‐filler network possess a72 higher thermal conductivity than those without a network0.0 as the inter‐filler contact resistance0 is much less than the interfacial thermal Thermal Conductivity (W/mK) resistance between matrix0 and 5 filler. 10 15 20 25 0 5 10 15 20 25 (a) Content of nano-filler(vol%) (b) Content of nano-filler(vol%)

Figure 3. (a) Thermal conductivity of EP-BN and EP-AgBN; (b) Thermal conductivity enhancement of EP-BNFigure and3. (a EP-AgBN.) Thermal conductivity of EP‐BN and EP‐AgBN; (b) Thermal conductivity enhancement of EP‐BN and EP‐AgBN.

(a) (b) Interfacial Inter-filler thermal contact resistance resistance

Polymer Matrix Thermal Transfer Path

Boron Nitride Silver Nano-particles

Figure 4. Thermally conductive path of polymer nanocomposites (a) without inter‐filler neatwork; (b) with inter‐filler network.

In Table 1, we summarized the thermal conductivity of nanocomposites with different BN‐based fillers. Compared with others, the thermal conductivity of EP‐AgBN is preferable.

Table 1. Thermal conductivity of composites with different BN‐based fillers.

Matrix Fillers TC(m‐1 K‐1) TC‐E Reference epoxy 60wt% micro BN 1.052 420% [3] polypropylene 30vol% micro BN ~2 ~900% [12] epoxy 25wt% BNNSs 0.65 180% [36] epoxy 25vol% BNNSs 1.13 528% this paper epoxy 30wt% h‐BN 1.178 514% [37] polymethylmethacrylate 10wt% BNNTs 0.5 194 [38] epoxy 22.5vol% micro+nano BN 1.4 250% [39] epoxy 25wt%BN +7.5wt%Al2O3 1.182 700% [40] epoxy 25.1vol%BNNS/AgNPs 3.05 1123% [29] epoxy 25vol% AgNPs‐BNNSs 2.14 1089% this paper

3.4 Electrical Conductivity and Breakdown Voltage of EP‐AgBN Insulation materials are expected to possess low electrical conductivity and high breakdown voltage in engineering applications. Figure 5a shows the linear electrical conductivity behavior for different composites (10−10–10−17 Scm−1) over a broad frequency range. The electrical conductivity is within the desired range for insulating materials (10−9–10‐20 Scm−1). Figure 5b shows the electrical conductivity of EP‐AgBN for three different frequencies: low frequency (100 Hz), intermediate Polymers 2020, 12, x FOR PEER REVIEW 6 of 13

2.5 1200 1089 EP-BN EP-BN 1000 Polymers 2020, 12, 4262.0 EP-AgBN EP-AgBN 6 of 13

800 728 1.5 Figure3b shows the thermal conductivity enhancement (TC-E) for EP-AgBN and EP-BN according

600 528 to the formula TC-E1.0 =(Ki-Kneat_epoxy/Kneat_epoxy, where Ki is the thermal461 conductivity of composite TC-E(%) and Kneat_epoxy is the TC of pure epoxy. Limited400 TC-E is achieved in356 both composites for low nanofiller loading0.5 (<10 vol%). Noticeable difference200 in TC-E between178 the composites is observed at higher nanofiller contents. 72 Figure4a,b0.0 illustrate the synergistic e ffect of BNNSs0 and AgNPs on thermal conductivity of Thermal Conductivity (W/mK) 0 5 10 15 20 25 0 5 10 15 20 25 nanocomposites. TheContent BNNSs of formed nano-filler(vol%) the main thermal transferContent paths of nano-filler(vol%) in nanocomposites and AgNPs connect the BNNS as a “thermal bridge”, which builds the inter-filler thermal network in polymer composites. Nanocomposites with inter-filler network possess a higher thermal conductivity than thoseFigure without 3. (a) aThermal network conductivity as the inter-filler of EP‐BN contact and EP resistance‐AgBN; (b) is Thermal much less conductivity than the enhancement interfacial thermal resistanceof EP‐BN between and EP‐AgBN. matrix and filler.

(a) (b) Interfacial Inter-filler thermal contact resistance resistance

Polymer Matrix Thermal Transfer Path Boron Nitride Silver Nano-particles Figure 4. Thermally conductive path of polymer nanocomposites (a) without inter-filler neatwork; Figure 4. Thermally conductive path of polymer nanocomposites (a) without inter‐filler neatwork; (b) (b) with inter-filler network. with inter‐filler network. In Table1, we summarized the thermal conductivity of nanocomposites with di fferent BN-based In Table 1, we summarized the thermal conductivity of nanocomposites with different BN‐based fillers. Compared with others, the thermal conductivity of EP-AgBN is preferable. fillers. Compared with others, the thermal conductivity of EP‐AgBN is preferable. Table 1. Thermal conductivity of composites with different BN-based fillers. Table 1. Thermal conductivity of composites with different BN‐based fillers. Matrix Fillers TC (m-1 K-1) TC-E Reference Matrix Fillers TC(m‐1 K‐1) TC‐E Reference epoxyepoxy 60wt%60wt% micro micro BN BN 1.052 1.052 420% 420% [3] [3] polypropylenepolypropylene 30vol%30vol% micro micro BN BN ~2 ~2 ~900% ~900% [12] [12] epoxyepoxy 25wt%25wt% BNNSs BNNSs 0.65 0.65 180% 180% [36] [36] epoxy 25vol% BNNSs 1.13 528% this paper epoxy 25vol% BNNSs 1.13 528% this paper epoxy 30wt% h-BN 1.178 514% [37] epoxy 30wt% h‐BN 1.178 514% [37] polymethylmethacrylate 10wt% BNNTs 0.5 194 [38] polymethylmethacrylateepoxy 22.5vol% 10wt% micro+ BNNTsnano BN 0.5 1.4 194 250% [38] [39] epoxy 22.5vol% micro+nano BN 1.4 250% [39] epoxy 25wt%BN +7.5wt%Al2O3 1.182 700% [40] epoxyepoxy 25.1vol%BNNS25wt%BN +7.5wt%Al/AgNPs2O3 1.182 3.05 700% 1123% [40] [29] epoxyepoxy 25vol%25.1vol%BNNS/AgNPs AgNPs-BNNSs 3.05 2.14 1123% 1089% [29] this paper epoxy 25vol% AgNPs‐BNNSs 2.14 1089% this paper 3.4. Electrical Conductivity and Breakdown Voltage of EP-AgBN 3.4 Electrical Conductivity and Breakdown Voltage of EP‐AgBN Insulation materials are expected to possess low electrical conductivity and high breakdown Insulation materials are expected to possess low electrical conductivity and high breakdown voltage in engineering applications. Figure5a shows the linear electrical conductivity behavior for voltage in engineering applications. Figure 5a shows the linear electrical conductivity behavior for different composites (10 10–10 17 Scm 1) over a broad frequency range. The electrical conductivity is −10− −17− −1 − different composites (10 –10 Scm ) over a broad frequency9 -20 range.1 The electrical conductivity is within the desired range for insulating materials (10− –10 Scm− ). Figure5b shows the electrical within the desired range for insulating materials (10−9–10‐20 Scm−1). Figure 5b shows the electrical conductivity of EP-AgBN for three different frequencies: low frequency (100 Hz), intermediate conductivity of EP‐AgBN for three different frequencies: low frequency (100 Hz), intermediate frequency (1000 Hz), and high frequency (10,000 Hz). As expected, the electrical conductivity increases

with the addition of conducting nanoﬁllers; however, this increase is limited due to the presence of highly insulating BNNSs, which restricts the aggregation of AgNPs. Polymers 2020, 12, x FOR PEER REVIEW 7 of 13 frequency (1000 Hz), and high frequency (10,000 Hz). As expected, the electrical conductivity increases with the addition of conducting nanofillers; however, this increase is limited due to the presence of highly insulating BNNSs, which restricts the aggregation of AgNPs. Figure 5c shows the Weibull distribution of the breakdown voltage. The breakdown voltage corresponding to a breakdown probability of 63.2% is taken as the breakdown voltage of the composite, as shown in Figure 5d. For EP‐BN, the breakdown voltage grows at a higher rate initially from 25.81 kV mm−1 and nearly saturates at around 30.51 kV mm−1at 25 vol% nanofiller. In the case of EP‐AgBN, the breakdown voltage achieves a maximum value, and then decays little due to electricity conductivity of silver nanoparticles. The breakdown voltage magnitude is still 4.02 kV mm−1 higher as compared to neat epoxy. The breakdown voltage of EP‐BN is marginally higher than that of EP‐ AgBN for the same nanofiller content due to the presence of silver nanoparticles. ComparedPolymers 2020 to neat, 12, 426 epoxy, the EP‐AgBN has a superior insulation strength for the nanofiller content7 of 13 of 25 vol%.

10-10 10-11 (a) 100Hz 10-11 (b) 1000Hz 10000Hz 10-12 10-12

10-13 -13 neat epoxy 10 -14 10 5vol% 10-15 10vol% 10-14 Conductivity(S/cm) 15vol% Conductivity(S/cm) 10-16 20vol% 25vol% 10-17 10-15 100 101 102 103 104 105 106 0 5 10 15 20 25 Frequency (Hz) Content of nano-filler(vol%) 31 95 (c) 30 (d) neat epoxy 5vol% 70 29 EP-BN 10vol% EP-AgBN 15vol% 28 40 20vol% 25vol% 27

10 26 Breakdown probability (%) Breakdown voltage(kV/mm) Breakdown 25 20 22 24 26 28 30 32 0 5 10 15 20 25 Breakdown Voltage (kV/mm) Content of nano-filler(vol%)

Figure 5. (a) Conductivity of EP and EP-AgBN at room temperature; (b) conductivity under 100 Hz, Figure 5. (a) Conductivity of EP and EP‐AgBN at room temperature; (b) conductivity under 100 Hz, 1000 Hz, and 10,000 Hz; (c) Weibull distribution of the breakdown voltage; (d) breakdown voltage of 1000 Hz, and 10,000 Hz; (c) Weibull distribution of the breakdown voltage; (d) breakdown voltage of EP-BN and EP-AgBN. EP‐BN and EP‐AgBN. Figure5c shows the Weibull distribution of the breakdown voltage. The breakdown voltage correspondingFigure 6a and to b aillustrates breakdown the probability synergistic of effect 63.2% of is BNNSs taken as and the AgNPs breakdown on electric voltage breakdown of the composite, of nanocomposites.as shown in Figure In Figure5d. For 6a, EP-BN, the polymer the breakdown will directly voltage breakdown grows at without a higher nanofillers. rate initially In from Figure 25.81 6b, kV the AgNPs1 are blocked by BNNSs, which makes it difficult1 to form an electrically conductive channel. mm− and nearly saturates at around 30.51 kV mm− at 25 vol% nanofiller. In the case of EP-AgBN, As thea result, breakdown the breakdown voltage achieves path is a forced maximum into value, a “zig and‐zag” then shape decays in EP little‐AgBN due to and electricity the breakdown conductivity distance is elongated, thereby intensifying the insulation strength. 1 of silver nanoparticles. The breakdown voltage magnitude is still 4.02 kV mm− higher as compared to neat epoxy. The breakdown voltage of EP-BN is marginally higher than that of EP- AgBN for the same nanofiller content due to the presence of silver nanoparticles. Compared to neat epoxy, the EP-AgBN has a superior insulation strength for the nanofiller content of 25 vol%. Figure6a and b illustrates the synergistic e ffect of BNNSs and AgNPs on electric breakdown of nanocomposites. In Figure6a, the polymer will directly breakdown without nanofillers. In Figure6b, the AgNPs are blocked by BNNSs, which makes it difficult to form an electrically conductive channel. As a result, the breakdown path is forced into a “zig-zag” shape in EP-AgBN and the breakdown distance is elongated, thereby intensifying the insulation strength. PolymersPolymers 20202020, 12,,12 x ,FOR 426 PEER REVIEW 8 of8 13 of 13

(a) (b)

Electrical Electrical Breakdown Breakdown Directly Zigzag

Polymer Matrix Electric Breakdown Path Boron Nitride Silver Nano-particles

Figure 6. Electric breakdown path of (a) neat polymer; (b) polymer nanocomposites. Figure 6. Electric breakdown path of (a) neat polymer; (b) polymer nanocomposites. 3.5. Permittivity and Dielectric loss of EP-AgBN 3.5. Permittivity and Dielectric loss of EP‐AgBN Improvement in thermal conductivity and insulation strength is often accompanied by undesirable dielectricImprovement properties in [thermal41]. Low conductivity permittivity andand dielectric insulation loss strength are typical is requirements often accompanied for electrically by undesirableinsulating dielectric materials. properties Figure7a shows [41]. Low low permittivity (~3.6–5.5)and dielectric in EP-AgBN loss are overtypical a broad requirements frequency forrange. electrically For pure insulating epoxy, thematerials. polarization Figure is 7a dipole shows polarization low permittivity under the (~3.6–5.5) electric in field. EP‐AgBN The pure over epoxy a broadcreates frequency a 3D network range. For structure pure epoxy, which the prevents polarization polarization is dipole inpolarization certain polar under groups, the electric thus leadingfield. Theto pure low permittivityepoxy creates [42 a ].3D Addition network ofstructure AgNP-BNNSs which prevents triggers polarization an increase inin permittivitycertain polar duegroups, to the thuspresence leading of to interfacial low permittivity polarization [42]. andAddition electron of AgNP polarization‐BNNSs [12 triggers,43,44]. an The increase decrease in ofpermittivity permittivity duewith to the frequency presence is of also interfacial obtained. polarization This can be and explained electron by polarization the decrease [12, of 43,44]. dipole The polarization decrease of and permittivityinterface polarization with frequency because is also they bothobtained. have aThis small can relaxation be explained frequency by the and decrease cannot keep of dipole up with polarizationthe change and of frequency interface atpolarization the high frequency because [12they]. both have a small relaxation frequency and cannot Figurekeep up7b with shows the the change permittivity of frequency enhancement at the high under frequency three di ff[12].erent frequencies. Enhancement in permittivityFigure 7b shows is fairly the low permittivity with a maximum enhancement value ofunder 30.6%. three This different behavior frequencies. is expected Enhancement due to the low in permittivity of is BNNSfairly low itself. with In addition a maximum to that, value composites of 30.6%. prepared This behavior in this workis expected are uniform due to with the stronglow permittivitynanofillers-matrix of BNNS bonding itself. In which addition only causesto that, mild composites enhancement prepared in the in interfacial this work polarization are uniform strength. with strong nanofillersFigure7c shows‐matrix the bonding room temperature which only causes dielectric mild loss enhancement angle (tan inδe) the for interfacial EP-AgBN polarization as a function strength.of frequency. Low dielectric loss (<0.2) for composites indicates that superior dielectric properties of neatFigure epoxy 7c are shows still the preserved. room temperature With the addition dielectric of loss nanofillers, angle (tan theδe) dielectric for EP‐AgBN loss of as EP-AgBN a function grows of frequency.due to the Low increase dielectric of interfacial loss (<0.2) polarization, for composites electron indicates polarization that superior loss, and dielectric loss of properties the conduction of neatcurrent epoxy formation. are still preserved. The spectral With variation the addition of tan δofe isnanofillers, bifurcated the into dielectric low (1–100 loss Hz) of andEP‐AgBN high frequency grows due(> 100to the Hz) increase bands owingof interfacial to significantly polarization, different electron behaviors. polarization Reduction loss, inand the loss dipole of the polarization conduction and currentinterface formation. polarization The explainsspectral thevariation slightly of decreasing tanδe is tanbifurcatedδe trend into in the low former (1–100 band Hz) [41 ].and The high rapid frequencyrise in the (>100 high Hz) frequency bands owing region to is assignedsignificantly to the different orientation behaviors. movement Reduction acceleration in the ofdipole dipole, polarizationand increase and of interface heat generation polarization by friction explains between the slightly the epoxy decreasing segments tanδ [e45 trend]. in the former band [41]. TheFigure rapid7 drise shows in the increasinghigh frequency tan δregione enhancement is assigned with to the nanofiller orientation content movement for three acceleration different of frequencies.dipole, and increase For low of nanofiller heat generation content, by the friction tanδe enhancement between the epoxy is relatively segments small, [45]. as the majority of spaceFigure charges 7d shows are localized increasing in deep tanδ trapse enhancement of nanoparticles, with nanofiller making them content diffi cultfor three to separate, different thus frequencies.limiting charge For low transfer nanofiller and leakage content, current the tan [δ46e ].enhancement Successive addition is relatively of nanofillers small, as causesthe majority significant of spacegrowth charges in tan areδe duelocalized to enhanced in deep charge traps transfer,of nanoparticles, leading to making more leakage them difficult current. to Furthermore, separate, thus a gap limitingbetween charge nanofiller transfer and and matrix leakage also current augments [46]. dielectric Successive loss addition [40]. of nanofillers causes significant growth in tanδe due to enhanced charge transfer, leading to more leakage current. Furthermore, a gap between nanofiller and matrix also augments dielectric loss [40].

Polymers 2020, 12, 426 9 of 13 Polymers 2020, 12, x FOR PEER REVIEW 9 of 13

6.0 35 neat epoxy 5vol% (a) (b) 30.6 10vol% 15vol% 30 2 5.5 10 Hz 27.5 20vol% 103Hz 25 23.7 25vol% 104Hz 5.0 20.7 20

4.5 15 12.8 Permittivity 10 4.0 5 Permittivity Enhancement (%) Enhancement Permittivity 3.5 0 100 101 102 103 104 105 106 0 5 10 15 20 25 Frequency (Hz) Content of nano-filler(vol%) 0.20 (c) (d) 120 107.6

neat epoxy (%)

0.15 e 102Hz 5vol% 90 3 83.7 10vol% 10 Hz 80.6 e 4 0.10 15vol% 10 Hz

tanδ 20vol% 60 25vol% 0.05 30 Enhancement of tanδ 0.00 0 100 101 102 103 104 105 106 0 5 10 15 20 25 Frequency (Hz) Content of nano-filler (vol%)

Figure 7. (a) Permittivity of EP and EP-AgBN at room temperature; (b) permittivity enhancement of EP-AgBNFigure 7. under (a) Permittivity 100, 1000, and of EP 10,000 and EP Hz;‐AgBN (c) tangent at room of dielectrictemperature; loss ( angleb) permittivity of EP and enhancement EP-AgBN; of (d) enhancementEP‐AgBN under of tan 100,δe of 1000, EP-AgBN and 10,000 under Hz; 100, (c) 1000, tangent and of 10,000 dielectric Hz. loss angle of EP and EP‐AgBN; (d) enhancement of tanδe of EP‐AgBN under 100, 1000, and 10,000 Hz. 3.6. Thermal Simulated Current of EP-AgBN 3.6. Thermal Simulated Current of EP‐AgBN TSC is a pivotal means to obtain the trap parameters and polarization properties of solid materials. Trap depthTSC represents is a pivotal the ability means to boundto obtain charge, the andtrap the parameters polarization and charges polarization can reﬂect properties the degree of of solid polarizationmaterials. [47 Trap,48]. Figuredepth 8represents shows the the TSC ability of pure to epoxybound and charge, EP-AgBN. and the Table polarization2 shows the charges calculated can reflect Polymerstrap parametersthe 2020 degree, 12, x FOR of according polarizationPEER REVIEW to the [47,48]. semi-peak Figure method 8 shows [49 the]. TSC of pure epoxy and EP‐AgBN. Table10 of 13 2 shows the calculated trap parameters according to the semi‐peak method [49]. An increase is observed in the temperature of peak value form 4.76 pA of pure epoxy to 12.16 12 pure epoxy pA at a filler loading of 25 vol%. However, 5vol% the temperature corresponding to peak value of TSC grows first with the content of Ag‐BN 10vol% below 20 vol% and then decreases slightly. The pure epoxy has a large trap depth of 3.285 eV, meaning 15vol% that it possesses a strong capacity to bind charges. The 8 addition of BNNSs‐AgNPs leads to a decrease 20vol% in trap depth of EP‐AgBN, indicating that the ability 25vol% to bind charges is weakened, which coincides with the increase in permittivity. The trap depth gets a TSC (pA) TSC minimum value of 1.966 eV that4 belongs to deep trap (shallow trap is less than 1 eV), this can be used to explain why there is no sharp rise in dielectric properties of EP‐AgBN. Moreover, the nanofillers fabricated in this paper intensifies the polarization of EP‐AgBN, resulting in an increase of the polarization charges, which coincides0 with the increase in dielectric loss. 40 60 80 100 120 Temperature (℃ )

Figure 8. Thermal simulated current of pure epoxy and EP-AB. Figure 8. Thermal simulated current of pure epoxy and EP‐AB.

Table 2. Calculated trap parameters.

Peak Temperature of Trap Polarization Sample Value Peak Value (°C) Depth(eV) Charges (pC) (pA) Pure 4.76 89.77 3.285 329.34 epoxy 5vol% 8.04 90.61 2.770 663.24 10vol% 8.85 92.43 2.664 766.02 15vol% 9.73 92.96 2.353 956.04 20vol% 10.13 96.34 1.980 1205.28 25vol% 12.16 94.62 1.966 1441.92

4. Conclusion To sum up, we successfully achieved nanocomposites with high thermal conductivity, high insulation strength, low permittivity, and low dielectric simultaneously in EP‐AgBN, solving the problem that polymer composites with high thermal conductivity always have low insulation strength or high dielectric parameters. The intrinsic excellent insulating properties of BNNSs and the well‐designed surface modification by AgNPs are proposed to be the key factors for obtaining outstanding insulation strength. Moreover, the nanofillers with electrical conductivity do not reduce the insulation properties of composites if they are effectively blocked by electrically insulating nanofillers. A nanofiller with better thermal conductivity can be obtained by placing nanofillers (such as graphene) with a small particle size on the surface of BNNSs. The excellent thermal and insulation properties of the present nanocomposites may pave the way for power equipment and electronic devices under a high energy density and complex electro‐thermal operation environment, such as insulators, electronic packages, and solar power generation devices. Author Contributions: Conceptualization, Y.W.; Data curation, G.H.; Funding acquisition, X.Z. and J.L.; Investigation, Y.W.; Methodology, G.H.; Resources, X.Z. and S.T.; Software, J.H. and S.T.; Writing – original draft, Y.W.; Writing – review & editing, A.N., J.H. and J.L.

Fundings: The authors acknowledge financial support by the National Key Research and Development Plan “Smart Grid Technology and Equipment” (NO.2017 YFB0903805). The authors also acknowledge financial support by the Faculty Research and Professional Development Fund at North Carolina State University.

Conflicts of Interest: The authors declare no conflict of interest.

References

Polymers 2020, 12, 426 10 of 13

Table 2. Calculated trap parameters.

Temperature of Polarization Sample Peak Value (pA) Trap Depth(eV) Peak Value (◦C) Charges (pC) Pure epoxy 4.76 89.77 3.285 329.34 5vol% 8.04 90.61 2.770 663.24 10vol% 8.85 92.43 2.664 766.02 15vol% 9.73 92.96 2.353 956.04 20vol% 10.13 96.34 1.980 1205.28 25vol% 12.16 94.62 1.966 1441.92

An increase is observed in the temperature of peak value form 4.76 pA of pure epoxy to 12.16 pA at a filler loading of 25 vol%. However, the temperature corresponding to peak value of TSC grows first with the content of Ag-BN below 20 vol% and then decreases slightly. The pure epoxy has a large trap depth of 3.285 eV, meaning that it possesses a strong capacity to bind charges. The addition of BNNSs-AgNPs leads to a decrease in trap depth of EP-AgBN, indicating that the ability to bind charges is weakened, which coincides with the increase in permittivity. The trap depth gets a minimum value of 1.966 eV that belongs to deep trap (shallow trap is less than 1 eV), this can be used to explain why there is no sharp rise in dielectric properties of EP-AgBN. Moreover, the nanofillers fabricated in this paper intensifies the polarization of EP-AgBN, resulting in an increase of the polarization charges, which coincides with the increase in dielectric loss.

4. Conclusions To sum up, we successfully achieved nanocomposites with high thermal conductivity, high insulation strength, low permittivity, and low dielectric simultaneously in EP-AgBN, solving the problem that polymer composites with high thermal conductivity always have low insulation strength or high dielectric parameters. The intrinsic excellent insulating properties of BNNSs and the well-designed surface modification by AgNPs are proposed to be the key factors for obtaining outstanding insulation strength. Moreover, the nanofillers with electrical conductivity do not reduce the insulation properties of composites if they are effectively blocked by electrically insulating nanofillers. A nanofiller with better thermal conductivity can be obtained by placing nanofillers (such as graphene) with a small particle size on the surface of BNNSs. The excellent thermal and insulation properties of the present nanocomposites may pave the way for power equipment and electronic devices under a high energy density and complex electro-thermal operation environment, such as insulators, electronic packages, and solar power generation devices.

Author Contributions: Conceptualization, Y.W.; Data curation, G.H.; Funding acquisition, X.Z. and J.L.; Investigation, Y.W.; Methodology, G.H.; Resources, X.Z. and S.T.; Software, J.H. and S.T.; Writing – original draft, Y.W.; Writing – review & editing, A.N., J.H. and J.L. All authors have read and agreed to the published version of the manuscript. Funding: The authors acknowledge financial support by the National Key Research and Development Plan “Smart Grid Technology and Equipment” (NO.2017 YFB0903805). The authors also acknowledge financial support by the Faculty Research and Professional Development Fund at North Carolina State University. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Li, Y.; Zhang, X.; Zhang, J.; Xie, C.; Shao, X.; Wang, Z.; Xiao, S. Study on the thermal decomposition

characteristics of C4F7N-CO2 mixture as eco-friendly gas insulating medium. High Volt. 2019.[CrossRef] 2. Zhi, C.; Bando, Y.; Terao, T.; Tang, C.; Kuwahara, H.; Golberg, D. Towards Thermoconductive, Electrically Insulating Polymeric Composites with Boron Nitride Nanotubes as Fillers. Adv. Funct. Mater. 2009, 19, 1857–1862. [CrossRef] Polymers 2020, 12, 426 11 of 13

3. Wattanakul, K.; Manuspiya, H.; Yanumet, N. Thermal conductivity and mechanical properties of BN-filled epoxy composite: Effects of filler content, mixing conditions, and BN agglomerate size. J. Compos. Mater. 2011, 45, 1967–1980. [CrossRef]

4. Chen, D.; Zhang, X.; Xiong, H.; Li, Y.; Tang, J.; Xiao, S.; Zhang, D. A first-principles study of the SF6 decomposed products adsorbed over defective WS2 monolayer as promising gas sensing device. IEEE Trans. Device Mater. Reliab. 2019, 19, 473–483. [CrossRef] 5. Li, S.; Yu, S.; Feng, Y. Progress in and Prospects for Electrical Insulating Materials. High Volt. 2016, 1, 122–129. [CrossRef] 6. Zhang, X.; Wu, Y.; Wen, H. The influence of oxygen on thermal decomposition characteristics of epoxy resins cured by anhydride. Polym. Degrad. Stab. 2018, 156, 125–131. [CrossRef] 7. Zhang, Y.; Choi, J.R.; Park, S.J. Interlayer polymerization in amine-terminated macromolecular chain-grafted expanded graphite for fabricating highly thermal conductive and physically strong thermoset composites for thermal management applications. Compos. Part A Appl. Sci. 2018, 109, 498–506. [CrossRef] 8. Chen, Z.; Zhang, X.; Xiong, H.; Chen, D.; Cheng, H.; Tang, J.; Tian, Y.; Xiao, S. Dissolved gas analysis in transformer oil using Pt-doped WSe2 monolayer based on first principles method. IEEE Access 2019, 7, 72012–72019. [CrossRef] 9. Zhang, Y.; Park, S.J. In Situ Shear-induced MercaptoGroup-activated Graphite Nanoplatelets for Fabricating Mechanically Strong and Thermally Conductive Elastomer Composites for Thermal Management Applications. Compos. Part A Appl. Sci. 2018, 112, 40–48. [CrossRef] 10. Mu, M.; Wan, C.; McNally, T. Thermal Conductivity of 2D Nano-structured Graphitic Materials and Their Composites with Epoxy Resins. 2D Mater. 2017, 4, 042001. [CrossRef] 11. Hao, W.; Zhang, X.; Rong, X. Thermodynamic simulations of SrTiO3/epoxy nanocomposites with different mass fractions. Sn Appl. Sci. 2019, 1, 354. 12. Cheewawuttipong, W.; Fuoka, D.; Tanoue, S. Thermal and mechanical properties of polypropylene/boron nitride composites. Energy Procedia 2013, 34, 808–817. [CrossRef] 13. Pan, C.; Kou, K.; Zhang, Y.; Li, Z.; Ji, T.; Wu, G. Investigation of the dielectric and thermal conductive properties of core–shell structured HGM/hBN/PTFE composites. Mater. Sci. Eng. B-Adv. 2018, 238, 61–70. [CrossRef] 14. Spitalsky, Z.; Tasis, D.; Papagelis, K.; Galiotis, C. Carbon Nanotube–polymer Composites: Chemistry Processing Mechanical and Electrical Properties. Prog. Polym. Sci. 2010, 35, 357–401. [CrossRef] 15. Zhang, X.; Fang, R.; Chen, D.; Zhang, G. Using Pd-Doped γ-Graphyne to Detect Dissolved Gases in Transformer Oil: A Density Functional Theory Investigation. Nanomaterials 2019, 9, 1490. [CrossRef] 16. Barani, Z.; Mohammadzadeh, A.; Geremew, A.; Huang, C.Y.; Coleman, D.; Mangolini, L.; Balandin, A.A. Thermal Properties of the Binary-Filler Hybrid Composites with Graphene and Copper Nanoparticles. Adv. Funct. Mater. 2019, 1904008. [CrossRef] 17. Wang, J.; Qiao, J.; Wang, J.; Zhu, Y.; Jiang, L. Bioinspired Hierarchical Alumina–Graphene Oxide–poly (vinyl alcohol) Artificial Nacre with Optimized Strength and Toughness. Acs Appl. Mater. Inter. 2015, 7, 9281–9286. [CrossRef] 18. Xu, Y.; Chung, D. Increasing the Thermal Conductivity of Boron Nitride and Aluminum Nitride Particle Epoxy-matrix Composites by Particle Surface Treatments. Compos. Interface 2000, 7, 243–256. [CrossRef] 19. Zhou, T.; Wang, X.; Gu, M.; Xiong, D. Study on Mechanical Thermal and Electrical Characterizations of Nano-SiC/Epoxy Composites. Polym. J. 2009, 41, 51. [CrossRef]

20. Zhang, X.; Wen, H.; Chen, X.; Wu, Y.; Xiao, S. Study on the Thermal and Dielectric Properties of SrTiO3/Epoxy Nanocomposites. Energies 2017, 10, 692. [CrossRef] 21. Huang, X.; Jiang, P.; Tanaka, T. A Review of Dielectric Polymer Composites with High Thermal Conductivity. IEEE Electr. Insul. Mag. 2011, 27, 8–16. [CrossRef] 22. Meng, W.; Huang, Y.; Fu, Y.; Wang, Z.; Zhi, C. Polymer Composites of Boron Nitride Nanotubes and Nanosheets. J. Mater. Chem. C 2014, 2, 10049–10061. [CrossRef] 23. Zhang, X.; Wu, Y.; Chen, X.; Wen, H.; Xiao, S. Theoretical Study on Decomposition Mechanism of Insulating Epoxy Resin Cured by Anhydride. Polymers 2017, 9, 341. [CrossRef][PubMed] 24. Huang, C.; Qian, X.; Yang, R. Thermal Conductivity of Polymers and Polymer Nanocomposites. Mat. Sci. Eng. R 2018, 132, 1–22. [CrossRef] Polymers 2020, 12, 426 12 of 13

25. Pan, C.; Kou, K.; Zhang, Y. Enhanced through-plane thermal conductivity of PTFE composites with hybrid fillers of hexagonal boron nitride platelets and aluminum nitride particles. Compos. Part B 2018, 153, 1–8. [CrossRef] 26. Wang, X.; Yu, Z.; Jiao, L.; Bian, H.; Yang, W.; Wu, W.; Dai, H. Aerogel Perfusion-Prepared h-BN/CNF Composite Film with Multiple Thermally Conductive Pathways and High Thermal Conductivity. Nanomaterials 2019, 9, 1051. [CrossRef] 27. Kandare, E.; Khatibi, A.A.; Yoo, S.; Wang, R.; Ma, J.; Olivier, P.; Wang, H. Improving the Through-thickness Thermal and Electrical Conductivity of Carbon fibre/Epoxy Laminates by Exploiting Synergy between Graphene and Silver Nano-inclusions. Compos. Part A Appl. Sci. 2015, 69, 72–82. [CrossRef] 28. Pashayi, K.; Fard, H.R.; Lai, F.; Iruvanti, S.; Plawsky, J.; Borca-Tasciuc, T. Self-constructed Tree-shape High Thermal Conductivity Nanosilver Networks in Epoxy. Nanoscale 2014, 6, 4292–4296. [CrossRef] 29. Wang, F.; Zeng, X.; Yao, Y.; Sun, R.; Xu, J.; Wong, C.P.Silver Nanoparticle-Deposited Boron Nitride Nanosheets as Fillers for Polymeric Composites with High Thermal Conductivity. Sci. Rep. 2016, 6, 19394. [CrossRef] 30. Pullanchiyodan, A.; Nair, K.; Surendran, K.P. Silver-decorated Boron Nitride Nanosheets as an Effective Hybrid Filler in PMMA for High-thermal-conductivity Electronic Substrates. Acs Omega 2017, 2, 8825–8835. [CrossRef] 31. Zhi, C.; Bando, Y.; Tan, C.; Golberg, D. Effective Precursor for High Yield Synthesis of Pure BN Nanotubes. Solid State Commun. 2005, 135, 67–70. [CrossRef] 32. Shen, J.; Shi, M.; Li, N.; Yan, B.; Ma, H.; Hu, Y.; Ye, M. Facile Synthesis and Application of Ag-chemically Converted Graphene Nanocomposite. Nano Res. 2010, 3, 339–349. [CrossRef] 33. Liu, K.; Chen, S.; Luo, Y.; Jia, D.; Gao, H.; Hu, G.; Liu, L. Noncovalently Functionalized Pristine Graphene/metal Nanoparticle Hybrid for Conductive Composites. Compos. Sci. Technol. 2014, 94, 1–7. [CrossRef] 34. Zhan, Y.; Ren, Y.; Wan, X.; Zhang, J.; Zhang, S. Dielectric Thermally Conductive and Stable Poly (arylene ether nitrile) Composites Filled with Silver Nanoparticles Decorated Hexagonal Boron Nitride. Ceram Int. 2018, 44, 2021–2029. [CrossRef] 35. Jang, I.; Shin, K.H.; Yang, I.; Kim, H.; Kim, J.; Kim, W.H.; Kim, J.P. Enhancement of Thermal Conductivity of BN/Epoxy Composite through Surface Modification with Cilane Coupling Agents. Colloids Surf. A 2017, 518, 64–72. [CrossRef] 36. Xue, Y.; Jin, X.; Fan, Y.; Tian, R.; Xu, X.; Li, J.; Lin, J.; Zhang, J.; Hu, L.; Tang, C. Large-scale synthesis of hexagonal boron nitride nanosheets and their improvement in thermal properties of epoxy composites. Polym. Compos. 2014, 35, 1707–1715. [CrossRef] 37. Hou, J.; Li, G.; Yang, N.; Qin, L.; Grami, M.E.; Zhang, Q.; Wang, N.; Qu, X. Preparation and characterization of surface modified boron nitride epoxy composites with enhanced thermal conductivity. Rsc. Adv. 2014, 4, 44282–44290. [CrossRef] 38. Zhi, C.Y.; Bando, Y.; Wang, W.L. Mechanical and thermal properties of polymethyl methacrylate-BN nanotube composites. J. Nanomater. 2008, 2008, 642036. [CrossRef] 39. Zhu, B.L.; Ma, J.; Wu, J. Study on the properties of the epoxy-matrix composites filled with thermally conductive AlN and BN ceramic particles. J. Appl. Polym. Sci. 2010, 118, 2754–2764. [CrossRef] 40. Bian, W.; Yao, T.; Chen, M.; Zhang, C.; Shao, T.; Yang, Y. The Synergistic Effects of the Micro-BN and

Nano-Al2O3 in Micro-nano Composites on Enhancing the Thermal Conductivity for Insulating Epoxy Resin. Compos. Sci. Technol. 2018, 168, 420–428. [CrossRef] 41. Huang, X.; Zhi, C.; Jiang, P.; Golberg, D.; Bando, Y.; Tanaka, T. Polyhedral Oligosilsesquioxane-Modiﬁed Boron Nitride Nanotube Based Epoxy Nanocomposites: An Ideal Dielectric Material with High Thermal Conductivity. Adv. Funct Mater. 2013, 23, 1824–1831. [CrossRef] 42. Wang, Z.; Cheng, Y.; Yang, M.; Huang, J.; Cao, D.; Chen, S.; Wu, H. Dielectric Properties and Thermal

Conductivity of Epoxy Composites Using Core/Shell Structured Si/SiO2/Polydopamine. Compos. Part B 2018, 140, 83–90. [CrossRef] 43. Singha, S.; Thomas, M.J. Dielectric Properties of Epoxy Nanocomposites. IEEE Trans. Dielectr. Electr. Insul. 2008, 15, 12–23. [CrossRef] 44. Zhang, Y.; Zhang, X.; Li, Y.; Li, Y.; Chen, Q.; Zhang, G.; Xiao, S.; Tang, J. AC breakdown and decomposition

characteristics of environmental friendly gas C5F10O/Air and C5F10O/N2. IEEE Access 2019, 7, 73954. [CrossRef] Polymers 2020, 12, 426 13 of 13

45. Luo, H.; Zhang, D.; Jiang, C.; Yuan, X.; Chen, C.; Zhou, K. Improved Dielectric Properties and Energy Storage Density of Poly (vinylidene ﬂuoride-co-hexaﬂuoropropylene) Nanocomposite with Hydantoin Epoxy Resin

Coated BaTiO3. Acs Appl. Mater. Inter. 2015, 7, 8061–8069. [CrossRef] 46. Dang, Z.M.; Yu, Y.F.; Xu, H.P.; Bai, J. Study on Microstructure and Dielectric Property of the BaTiO3/Epoxy Resin Composites. Compos. Sci. Technol. 2008, 68, 171–177. [CrossRef]

47. Kuo, D.H.; Chang, C.; Su, T.Y.; Wang, K.; Lin, B.Y. Dielectric Behaviours of Multi-doped BaTiO3/epoxy Composites. J. Eur. Ceram Soc. 2001, 21, 1171–1177. [CrossRef] 48. Tanaka, T. Dielectric nanocomposites with insulating properties. IEEE Trans. Dielectr. Electr. Insul. 2005, 12, 914–928. [CrossRef] 49. Wang, W.; Min, D.; Li, S. Understanding the Conduction and Breakdown Properties of Polyethylene Nanodielectrics: Eﬀect of deep traps. IEEE Trans. Dielectr. Electr. Insul. 2016, 23, 564–572. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).