Duo Pan, Gui Yang, Hala M. Abo?Dief, Jingwen Dong, Fengmei Su?,Chuntai Liu, Yifan Li, Ben Bin Xu?, Vignesh Murugadoss, Nithesh Naik,Salah M. El?Bahy, Zeinhom M. El?Bahy, Minan Huang, Zhanhu Guo

ABSTRACT With the innovation of microelectronics technology, the heat dissipation problem inside the device will face a severe test. In this work, cellulose aerogel (CA) with highly enhanced thermal conductiv?ity (TC) in vertical planes was successfully obtained by constructing a vertically aligned silicon carbide nanowires (SiC NWs)/boron nitride(BN) network via the ice template?assisted strategy. The unique net?work structure of SiC NWs connected to BN ensures that the TC of the composite in the vertical direction reaches 2.21 W m-1 K-1 at a low hybrid filler loading of 16.69 wt%, which was increased by 890%compared to pure epoxy (EP). In addition, relying on unique porous network structure of CA, EP?based composite also showed higher TC than other comparative samples in the horizontal direction. Meanwhile, the composite exhibits good electrically insulating with a volume electrical resistivity about 2.35 × 1011 Ω cm and displays excellent electromagnetic wave absorption performance with a minimum reflection loss of - 21.5 dB and a wide effective absorption bandwidth (＜ - 10 dB) from 8.8 to 11.6 GHz. Therefore, this work provides a new strat?egy for manufacturing polymer?based composites with excellent multifunctional performances in microelectronic packaging applications.

KEYWORDS Epoxy; Ice template; Vertical alignment; Thermal conductivity; Multifunctionality

1 Introduction

With the innovation of the third?generation semiconduc?tor technology, electronic equipment has shown a develop?ment trend of multi?function, miniaturization and integra?tion [1—4]. As a result, a large amount of heat generated inside the equipment continue to accumulate, which seri?ously affects its reliability and service life [5]. Polymer materials have been widely used in the sealing and interface bonding of various electronic devices due to their excellent mechanical properties, good insulation and unique chemical stability [6—8]. However, the thermal conductivity (TC) of polymer materials is generally low (usually no more than 0.5 W m-1K-1), which limits their wide?ranging applica?tions in the microelectronic packaging industry [9—11]. How to improve the TC of polymer materials and make them bet?ter apply to the field of thermal management has become an important problem that needs to be solved urgently [12].

Compared with the difficulty to change the molecular chain structure of the intrinsic polymer to improve TC, it is simple and practical to add a filler with high TC into the polymer [13—15]. Generally, such thermally conductive fill?ers mainly include metal oxides (e.g., Al2O3, ZnO and MgO)[16], metal particles (e.g., Cu, Zn and Ag) [17], aluminum nitride (AlN) [18], boron nitride (BN) [19], silicon carbide(SiC) [20], graphene [21] and carbon nanotubes [22], etc.Among them, BN is considered to be the most promising two?dimensional material because of its low density, excel?lent electrical insulation, oxidation resistance and chemical stability properties [23, 24]. Unfortunately, a large amount of BN are often needed in the composite to achieve a satis?factory heat dissipation effect due to the existence of inter?face thermal resistance between polymer matrix and fillers,which is bound to sacrifice the excellent toughness of the matrix and the mechanical properties of the composite [25,26]. Therefore, it is of great significance to use less BN fill?ers to obtain composites with a better TC in high power density electronic devices.

In recent years, compared with simple blending, two strategies of driving the horizontally orientation of BN[27] and building a three?dimensional (3D) BN thermally conductive network [28] can effectively construct heat conduction paths to improve the TC at a low BN content.In the former orientation applications, composites mostly appear in the form of films. For example, Yang et al. [29]fabricated polyvinyl alcohol/boron nitride composite film with high in?plane TC (19.99 W m-1K-1) via the combina?tion of electrostatic spinning and hot?pressing technique.Wu et al. [30] reported a BN nanosheet/polymer composite film with superior in?plane TC of around 200 W m-1K-1and extremely low through?plane TC of 1.0 W m-1K-1.Although these film?like composites have considerable in?plane TC, the heat dissipation of microelectronic devices is mainly through the rapid transfer of accumulated heat energy from the heat source to the heat sink in a short verti?cal direction [31, 32]. The TC of the film?like composites in the vertical direction is quite low, which hinders their large?scale use in actual production. The latter uses fillers to establish a spatially interconnected thermally conductive network structure to improve the overall TC of the compos?ite [33]. Chen et al. [34] prepared BN?polyvinylidene dif?luoride (PVDF) 3D scaffold by removing the sodium chlo?ride salt template method and found that the TC of the final epoxy/BN?PVDF was 1.227 W m-1K-1. Zhou et al. [35]synthesized a 3D interconnective cross?linking polystyrene(c?PS)/BN composite foam with a TC of 1.28 W m-1K-1and found that the composite foams exhibited low density and dielectric constants. Although the 3D heat conduction paths constructed by BN in the matrix improves the overall TC of the bulk composite to a certain extent, the TC in the vertical direction cannot be effectively improved by relying on BN alone [36].

Therefore, on one hand, one?dimensional materials (e.g.,carbon nanotubes, silver nanowire and SiC nanowire) as a thermally conductive bridge to connect BN were introduced[37], and on the other hand, various strategies (e.g., suction filtration method, magnetic field method and electric field method) were adopted to achieve vertical alignment of fill?ers [38—40]. One?dimensional SiC nanowire overcomes the defects of traditional SiC materials and has been widely used in aerospace, chemical, electronics and other industrial fields due to its excellent high?temperature strength, good ther?mally conductive performance, high wear resistance and cor?rosion resistance [41]. Xiao et al. [31] successfully prepared epoxy?based composites with excellent through?plane TC(4.22 W m-1K-1) by constructing a free?standing and verti?cally aligned SiC nanowires/BN framework through modi?fied filtration strategy. Kim et al. [42] fabricated a directional thermally conductive BN?Fe3O4/SiC binary filler epoxy composite by introducing Fe3O4particles on the BN surface via magnetic alignment technology. The obtained composite not only has excellent thermal management performance,but also has a high storage modulus. Unfortunately, these methods have high technical requirements, and many factors such as density mismatch, electric field distribution, interac?tion between magnetic ions and uncontrollable grafting sites need to be considered [43].

In addition, with the rapid development of the 5G era,microelectronic devices bring us convenience while carrying potentially serious electromagnetic pollution [44—46]. Effi?cient microwave absorbing materials have attracted a lot of attention in recent years; however, few studies have combined excellent thermal management and microwave absorption properties simultaneously [47—50]. In this work, based on the previous research work [51, 52] about the cellulose aero?gel obtained by the ice template method, the unique structure with a small amount of SiC nanowires (SiC NWs) vertically connected to BN was obtained by modifying the fillers and combining with directional freezing technology. The finally obtained epoxy composite not only exhibits excellent ther?mal management capability in the vertical direction, but also displays excellent electromagnetic wave absorption perfor?mance, which is attributed to the good synergistic effects of SiC NWs and BN in both function and structure.

2 Experimental

2.1 Materials

Hexagonal boron nitride powder (h?BN, ～ 10 μm, 99.9%) was purchased from Hefei AVIC Nano Technology Development Co., Ltd., China. SiC nanowires (SiC NWs, diameters:0.1—0.5 μm, length: 20—50 μm) were supplied by Xuzhou Hongwu Nano Material Co., Ltd., China. Sodium hydroxide(NaOH, AR, 96%), Urea (H2NCONH2, AR, 99%) and cel?lulose with a length of ≤ 25 mm were obtained from Shang?hai Aladdin Reagent Co., Ltd., China. Boric acid (H3BO3,AR, ≥ 99.5%) and epichlorohydrin (ECH, C3H5ClO,1.183 g cm-3) were supplied by Shanghai McLean Bio?chemical Technology Co., Ltd., China. Epoxy (EP, E?44)and amine curing agent 593 were provided by Evergreen Chemicals Technology Co., Ltd., China. All reagents were of analytical grade and used without any further purification.

2.2 Preparation of Cellulose/m?SiC NWs/m?BN Aerogel(CA/m?SiC/m?BN)

Before synthesizing aerogels, the surface ofh?BN and SiC NWs needs to be modified for better connection and dis?persion. Specifically, the originalh?BN was exfoliation and modified by putting BN and boric acid (BA) in a planetary ball mill at a mass ratio of m(BN)/m(BA) = 1:10 for 36 h at 400 rpm and then subjected to a series of operations of centrifugation, washing and drying to obtain the modified BN, marked as m?BN. For the modification of SiC NWs(m?SiC NWs), commercial SiC NWs were oxidized at a high temperature of 1300 °C with an air atmosphere in a tube furnace for 5 min.

The cellulose/m?SiC NWs/m?BN aerogel was obtained by the corresponding hydrogel through directional freezing followed with freeze drying steps. Firstly, the alkaline solu?tion was prepared by mixing NaOH/urea/deionized water at a weight ratio of 7:14:79. Then, 5 g cellulose was added into 50 mL above solution and stirred evenly and put it in a refrigerator at - 10 °C for 1 h. After that, different masses of m?BN (0.5, 1.0, 1.5 and 1.8 g) followed with a small amount of m?SiC NWs were successively added and magnetically stirred for 2 h to obtain a cellulose/m?SiC NWs/m?BN solu?tion. Next, 10 mL cross?linker ECH is introduced and the obtained mixed paste solution was poured into a mold with an internal diameter of 20 mm and a Cu disk substrate, and cross?linked for regeneration at 60 °C for 5 h in an oven.After that, the formed cellulose/m?SiC NWs/m?BN hydro?gels were soaked in deionized water to remove the residual reactants (NaOH, urea and ECH) and followed by direc?tional freezing treatment with liquid nitrogen in the mold for 10 min. Finally, the frozen cellulose/m?SiC NWs/m?BN hydrogels were then transferred to a freeze drier at - 80 °C for 72 h to get cellulose/m?SiC NWs/m?BN aerogels. To simplify writing, cellulose/m?SiC NWs/m?BN aerogel was marked as CA/m?SiC/m?BN.

2.3 Preparation of CA/m?SiC/m?BN/EP Composites

The CA/m?SiC/m?BN/EP composites were fabricated via a vacuum?assisted impregnation method. In brief, 10 g EP and 2 g curing agent 593 were magnetically stirred in a water bath (80 °C) to obtain a homogeneous solution. The CA/m?SiC/m?BN aerogels were immersed into above solu?tion in a vacuum system and cured in an oven at 60 °C for 3 h and 80 °C for 2 h. Eventually, the composites denoted as CA/xm?SiC/ym?BN/EP (x and y correspond to the mass (g) of m?SiC and m?BN) were obtained. In addition,CA/m?SiC/EP and CA/m?BN/EP with the same synthesis method, and CA/m?SiC/m?BN/EP composites by a simple blending (CA/m?SiC/m?BN/EPblend) were also prepared for comparison.

2.4 Characterization

The scanning electron microscopy (SEM, JSM?6380, Japan)was applied to observe the morphologies of m?BN, m?SiC NWs and the microstructure of the CA/m?SiC/m?BN net?work. Atomic force microscopy was conducted (AFM,Bruker MultiMode 8) to investigate the lateral size and thickness of m?BN. Fourier transform infrared spectroscopy(FT?IR) was tested in the range of 500—4000 cm-1using Nicolet NEXUS 870 spectrometer. Elemental scanning was achieved by X?ray photoelectron spectroscopy (XPS,ESCALAB 250 Xi). The filler loading of different samples was estimated by a thermal gravimetric analyzer (TGA,209 F3, Netzsch). Contact angle was obtained from contact angle measurement (CD?100D, innuo?instruments, Shang?hai). Volume resistivity was measured by an Electrometer(Tektronix, 6517B, America) at room temperature. TheKwas measured by a Hot Disk Thermal Constant Analyzer(Hot Disk TPS 2500S, Sweden), and an infrared thermal camera (E60, FLIR) was employed to measure the change of sample surface temperature over different heating and cooling time. A vector network analyzer (VNA, Keysight N5222B, USA) was used to measure the electromagnetic parameters of different samples with the coaxial method in the frequency range of 2—18 GHz.

3 Results and Discussions

3.1 Characterization of m?BN, m?SiC NWs

The original BN has chemical stability and large thickness,which limits its dispersibility in the organic matrix and the improvement of the overall TC [53]. To illustrate the suc?cessful stripping and modification of BN, the microscopic morphology and element bond analysis were tested by using SEM, AFM and FT?IR, respectively. Figure 1a shows the platelet morphology of the original BN, the uneven lateral size (about 5—10 μm) and the thickness of about 500 nm can be clearly observed. After modified by ball milling with boric acid (BA), the obtained m?BN presents a thin and transparent morphology (Fig. 1b). Taking 50 pieces of m?BN flakes for further observed with an atomic force microscope, from Fig. 1c—e, it is found that the m?BN with an average lateral size of 2—3 μm and a thickness of about 20 nm.

The mechanism of BN modification can be found in FT?IR spectra as shown in Fig. 1f. Except for two char?acteristic peaks of B?N stretching vibration (1375 cm-1)and B?N?B bending vibration (819 cm-1) about original BN, m?BN exhibits two other different broad absorptions at 3420 and 3230 cm-1, which correspond to the —OH and N—H stretching vibration [54, 55]. The acquisition of hydrophilic groups is due to the high?speed mechanical shearing action that makes the N atoms and B atoms in an active state to react with BA [56]. The increase in hydro?philicity of m?BN is verified by the fact that the contact angle of m?BN is significantly smaller than that of BN in the illustration.

In order to reduce the interface thermal resistance between SiC NWs and BN, SiC NWs are treated by high?temperature oxidation. According to the SEM image in Fig. 2a, the untreated SiC NWShave a rod?like structure with a length of about 25 μm and have a smooth surface without impurities. After calcination at 1300 °C, the sur?face of the m?SiC NWSbecomes rough and has granular aggregates (Fig. 2b). As shown in Fig. 2c, the FT?IR char?acteristic peaks at around 810 and 918 cm-1are attributed to the C—Si stretching vibration of SiC NWs [57]. The peak at around 3421 cm-1belongs to the —OH group in the water absorbed on the surface of the sample. After modi?fication, the new peaks of Si—O—Si and C—O—Si groups at 1095 and 1219 cm-1appear on m?SiC NWs [58]. Fur?thermore, a series of peaks located at around of 1500 cm-1(inside the blue dashed circle) are ascribed to the stretch?ing vibration of C—O and C=O [59].

Fig. 1 Characterizations of original BN and m?BN. a, b are SEM images of pristine BN and m?BN. c AFM image, d diameter length and e thickness distribution of m?BN. f FT?IR analysis of BN and m?BN. Insets are the corresponding contact angle test results

Figure 2d—f presents the XPS high?resolution spectra of C, Si and O elements to further confirm the linking of functional groups on m?SiC NWS. From Fig. S1, the C 1sspectrum of commercial SiC NWSshows three fit?ting peaks at 283.4, 284.9 and 286.4 eV corresponding to C—Si, C=C/C—C and C?O, respectively [60]. While the C 1sspectrum of m?SiC NWSnot only contains the above fitting peaks, but also emerges a C=O bond at 287.1 eV.More importantly, after modification, the peak attributed to C=C/C—C gets stronger. In addition, except for the Si—C bond at 102.8 eV in the Si 2pspectrum, an obvi?ous fitting peak found in m?SiC NWSat 100.3 eV cor?responds to Si—O bond. [61, 62]. Only one main fitting peak of O 1sbelongs to the O—Si bonding at 532.2 eV.From the above FT?IR and XPS results, it can be inferred that multi?oxygen?containing functional group (Si—O—Si,C—O—Si and C=O) were formed on the surface of m?SiC NWSafter calcination treatment [63]. These valence bond structures not only enhance the dispersion of m?SiC NWSin the matrix (from the illustration in Fig. 2c, compared to the commercial SiC NWS, the aqueous dispersion of m?SiC NWSdid not show delamination after standing for 24 h), but also provide a theoretical support for m?SiC NWSconnecting m?BN to form thermally conductive paths.

3.2 Fabrication and Characterization of CA/m?SiC/m?BN and CA/m?SiC/m?BN/EP

As shown in Fig. 3, the preparation of CA/m?SiC/m?BN/EP can be summarized in the following four processes: firstly,the preparation of a paste dispersion of cellulose/m?SiC/m?BN; then, the mixed dispersion is cross?linked in a mold to form m?SiC/m?BN cellulose hydrogel, which undergoes a directional freezing in a liquid nitrogen environment [64];Next, the m?SiC/m?BN cellulose hydrogel is freeze?dried to become m?SiC/m?BN cellulose aerogel (CA/m?SiC/m?BN);Finally, CA/m?SiC/m?BN/EP is obtained by impregnating the CA/m?SiC/m?BN aerogel in EP with the aid of a vacuum system. In addition, it can be seen from the vignette that CA/m?SiC/m?BN is lightweight, and even the final CA/m?SiC/m?BN/EP composite can also be placed steadily on the tiny grass branches.

Fig. 2 Characterizations of commercial SiC NWS and m?SiC NWS. a, b are SEM images of commercial SiC NWS and m?SiC NWS. c FT?IR spectra of SiC NWS and m?SiC NWS. High?resolution XPS analysis of d C 1s, e Si 2p and f O 1s of m?SiC NWS

Fig. 3 Schematic diagram of the preparation of CA/m?SiC/m?BN/EP

Fig. 4 a XPS and b FT?IR spectra of CA, CA/m?SiC NWS, CA/m?SiC NWS/m?BN and CA/m?SiC NWS/m?BN/EP

Figure 4 presents the XPS and FT?IR spectra of cel?lulose aerogel (CA), CA/m?SiC NWS, CA/m?SiC/m?BN and CA/m?SiC/m?BN/EP to further confirm the covalent connection. Specifically, the binding energy peaks of dif?ferent elements contained in CA, SiC NWSand BN were observed on the hybrid spectrum of CA/m?SiC/m?BN and CA/m?SiC/m?BN/EP (Fig. 4a). As shown in Fig. 4b, the observed peak for pure CA centered at 3421, 1375, 1165 and 1053 cm-1correspond to the —OH stretching vibration,—OH bending vibration, —CO antisymmetric bridge stretch?ing vibration and —CO—C vibration of pyranoid ring skel?eton, respectively [37, 65]. In addition, the bands at 2896 and 1425 cm-1belong to the absorption peak of the —CH stretching vibration on the pyranoid ring and the branched chains, respectively. After the introduction of m?SiC NWSand m?BN into cellulose, their characteristic peaks succes?sively appeared on the infrared spectra of CA/m?SiC NWSand CA/m?SiC/m?BN, which corresponded to the results in Figs. 1f and 2c. Furthermore, the FT?IR spectrum of the final product CA/m?SiC/m?BN/EP not only contains all the above characteristic peaks, but also adds the obvious absorption peaks of the EP matrix. The bands at 2830—3000,1452 and 940 cm-1correspond to the stretching vibration of—CH/—CH2/—CH3, the stretching vibration peak of benzene ring and the characteristic absorption peak of epoxy group(—CH(O)CH—), respectively. It is worth mentioning that the cellulose with polyhydroxy group (inset in Fig. 4b), the m?SiC NWSwith multi?oxygen?containing functional group and m?BN with N—H group are tightly combined together relying on the hydrogen bond between these groups. Thus,a stable and continuous thermally conductive structure is formed in CA/m?SiC/m?BN/EP [66].

The microstructure of the thermally conductive filler has a significant effect on the TC of the composite. In this work,the microstructure of CA/m?SiC/m?BN is affected by the content of m?SiC NWSand m?BN. From Fig. S2, pure cel?lulose presents a unique porous network structure with a pore diameter of about 10 μm [67]. The influence of m?SiC NWSon the overall microstructure was studied by chang?ing its content (0.00, 0.03, 0.06, 0.10 and 0.13 g) when the mass of m?BN in the aerogel was controlled at 1.8 g. As shown in Fig. 5a, when m?SiC NWSis not introduced, m?BN in CA/1.8 m?BN is embedded on the hole wall of the CA.With the addition of m?SiC NWSin different masses and assisted by directional freezing, the CA/m?SiC/m?BN skel?eton presents a vertically oriented structure in thex—zplane(Fig. 5b—d, f). Especially, when the mass of m?SiC NWSis 0.10 g, CA/0.10 m?SiC/1.8 m?BN shows a high orienta?tion along the ice growth direction. From the enlarged SEM image of Fig. 5e, it can be clearly seen that the m?SiC NWSare arranged in a vertical direction. However, as the mass of SiC NWs continues to increase to 0.13 g, SiC NWs are entangled with each other to limit the vertical growth of ice crystals, resulting in a porous network structure with differ?ent pore sizes.

Fig. 5 SEM images of CA/m?SiC NWS/m?BN with different contents of m?SiC NWS and GO and m?BN. a-d and f are the section mor?phologies of CA/1.8 m?BN, CA/0.03 m?SiC/1.8 m?BN, CA/0.06 m?SiC/1.8 m?BN, CA/0.10 m?SiC/1.8 m?BN and CA/0.13 m?SiC/1.8 m?BN skeleton in x—z plane. e is local enlarged drawing of d. g, i-l are the section morphologies of CA/0.10 m?SiC, CA/0.10 m?SiC/0.5 m?BN,CA/0.10 m?SiC/1.0 m?BN, CA/0.10 m?SiC/1.5 m?BN and CA/0.10 m?SiC/1.8 m?BN skeleton in x—y plane. h is local enlarged drawing of g

The influence of m?BN on the overall microstructure was studied by changing its content (0.0, 0.5, 1.0, 1.5 and 1.8 g)when the mass of SiC NWs in the aerogel was controlled at 0.10 g. From Fig. 5g—h, CA/0.10 m?SiC shows a network structure of cellulose and m?SiC NWSconnected without the participation of m?BN. In order to more intuitively show the influence of different contents of m?BN on the overall mor?phology, the structural characterizations of CA/m?SiC/m?BN skeleton in thex—yplane are shown in Fig. 5i—l. It can be seen that the embedded m?BN in the cellulose increases with increasing its mass. When the mass of m?BN reaches to 1.8 g, the m?BN starts to connect to each other, which is the best content for the construction of thermal conduction path,because excessive BN will aggregate and then increase the interface thermal resistance. More importantly, combined with Fig. 5d, l, it is easy to observe that m?SiC NWSand m?BN form a continuous thermally conductive network structure on the pore wall of CA, which is conducive to the rapid transfer of phonons between the two fillers.

In order to explore the influence of the microscopic morphology of the composite on the thermally conduc?tive properties, the morphology analysis of the EP?based composites is further carried out. As can be seen from Fig.S3a, the cross?sectional morphology of CA/0.10 m?SiC/EP without m?BN shows a stripe structure with single crack direction and vertical extension, and m?SiC NWScan be evenly distributed in EP, which is attributed to their good interfacial compatibility. Similarly, from Fig. S3b, the cross?sectional morphology of CA/1.8 m?BN/EP without m?SiC NWSshows that m?BN is uniformly distributed in EP. For the CA/0.10 m?SiC/1.8 m?BN/EPblendobtained by simple blending, the cross?sectional appearance has no regular and fixed orientation (Fig. S3c). For the optimal sample CA/0.10 m?SiC/1.8 m?BN/EP, an obvious vertical strati?fication can be observed (Fig. S3d). These results further confirm the successful acquisition of EP?based composites with a vertically oriented thermally conductive network. In addition, the EP can penetrate into the 3D network since the modification of the fillers enhances the interfacial interaction. Therefore, this well?arranged vertical structure can provide an efficient heat transfer channel in the direc?tion of the through?plane, resulting in excellent anisotropic TC [68].

3.3 Anisotropic Thermal Properties of CA/m?SiC/m?BN/EP

Combined with the thermal gravimetric analyzer, the den?sity and filler loadings of different samples are presented in Tables S1 and S2. Figure 6a, b presents the TC of CA/m?SiC/m?BN/EP composites in both vertical plane and hori?zontal plane directions with different m?SiC NWSand m?BN contents. It can be seen that the TC of CA/m?SiC/m?BN/EP and CA/m?SiC/m?BN/EPblendcomposites increased with increasing the content of m?SiC NWSand m?BN, and the existence of the unique thermally conductive network results in a significant enhancement of TC in both direc?tions. For example, whether it is CA/1.8 m?BN/EP and CA/0.10 m?SiC/EP with a single filler, or CA/m?SiC/m?BN/EP with a composite filler, they all perform higher TC than pure EP (0.22 W m-1K-1) in both vertical plane and hori?zontal plane. Moreover, at a specific content of m?SiC NWSand m?BN, the TC in both planes for the composite with unique network structure is better than that of the compos?ites with a random dispersion [69]. It is worth noting that in Fig. 6a, the TC of the EP?based composite has basically not changed with the content of m?SiC NWSincreased from 0.88 to 1.25 wt% (i.e., 0.10—0.13 g). This is mainly attrib?uted to excessive m?SiC NWSwhich are entangled with each other (Fig. 5f) and prevent further improvement of TC. In order to visually highlight the superior thermally conductive properties of the composite under the optimal filler content,thermal conductivity enhancement (TCE) was adopted using Eq. (1) [70]:

whereKcandKmare the TC of EP?based compos?ites and pure EP. After calculation, the TCE of the CA/0.10 m?SiC/1.8 m?BN/EP (Kc= 2.21 W m-1K-1) com?posite reached 890.9% at the total filler loading of 16.69 wt%, exhibiting excellent TC enhancement performance.In summary, the appropriate contents of m?SiC NWSand m?BN in CA/m?SiC/m?BN/EP are 0.88 wt% (0.10 g) and 15.81 wt% (1.8 g), respectively.

The TC of the EP?based composites is theoretically described by classic Agari and Hashin—Shtrikman (HS)models, which are often used to test the filler distribution in the specific network. In view of the relatively low con?tent of m?SiC NWS (about 0.24—0.88 wt% concluded from Tables S1 and S2) in CA/m?SiC/m?BN/EP, the filler m?BN contributes significantly better to the TC of the composite than m?SiC as shown in Fig. 6a, b. Therefore, in this work,m?SiC/EP is regarded as a matrix and m?BN is used as a thermally conductive filler to simplify the simulation struc?ture. The Agari model is shown as Eq. (2) [71]:

whereKc,KfandKmrepresent the TC of EP?based compos?ites, m?BN and matrix (m?SiC/EP), respectively. Here,Kfis 350 W m-1K-1in horizontal and 600 W m-1K-1in the vertical direction, and 30 W m-1K-1in random dispersion.Kmis 0.35 W m-1K-1in horizontal and 0.52 W m-1K-1in the vertical direction, and 0.32 W m-1K-1in random dispersion.φis the weight fraction of m?BN filler.C1is the influence parameter of filler on the secondary structure of polymer matrix.C2measures how easily the filler can form a thermally conductive network [72]. From Fig. 6c,compared with CA/m?SiC/m?BN/EPblendthrough a simple blending (random dispersion) method, CA/m?SiC/m?BN/EP obtained by the ice template method has largerC1andC2after simulation calculation on the vertical and horizon?tal planes. This indicates that an ideal crystal structure and heat conduction paths are formed inside CA/m?SiC/m?BN/EP [73], especially in the vertical direction.

In order to further highlight the unique vertical net structure formed inside CA/m?SiC/m?BN/EP, the Hashin—Shtrikman(HS) model is introduced as Eqs. (3, 4) [74]:

Fig. 6 Anisotropic thermal properties analysis of CA/m?SiC/m?BN/EP. a Thermal conductivities and thermal conductivity enhancements of CA/m?SiC/m?BN/EP and CA/m?SiC/m?BN/EPblend composites with different m?SiC NWS contents at the m?BN mass of 1.8 g. b Thermal con?ductivities and thermal conductivity enhancements of CA/m?SiC/m?BN/EP and CA/m?SiC/m?BN/EPblend composites with different m?BN con?tents at the m?SiC NWS mass of 0.10 g. c Agari model fitting lines, d Hashin—Shtrikman (HS) model fitting lines, e EMT model fitting lines and f Foygel’s theory fitting lines of CA/m?SiC/m?BN/EP and CA/m?SiC/m?BN/EPblend composites

where theKHS+(upper boundary) refers to the matrix that is completely surrounded by filler;KHS+(lower boundary)indicates that the fillers are completely separated by matrix.KfandKmrepresent the TC of m?BN and matrix (m?SiC/EP), In that case,Kftakes a value of 600 W m-1K-1andKmtakes a value of 0.52 W m-1K-1.φmandφfare the weight fraction of matrix and filler. Figure 6d presents the simulation results of anisotropic TC in accordance with HS model. Obviously, the measured TC stays between the upper and lower boundaries, which meets the fitting result of the HS model. The appearance of the approximate logarithmic fitting curve of the HS + upper boundary shows that the construction of the heat conduction paths in this experiment can achieve a greater impact on the overall TC with a low m?BN content.

In practical application, the interfacial thermal resistance between the filler/matrix and filler/filler in the thermally con?ductive composites is the key factor affecting the heat transfer process [26]. Therefore, the effective medium theory (EMT)and the Foygel theory are used to model and analyze the inter?facial thermal resistance in the samples. EMT is used to fit the relationship between the interface thermal resistance from filler/matrix and the TC as shown in Eq. (5) [75]:

whereKc,KfandKmare the TC of composite, m?BN and matrix (m?SiC/EP), respectively.φfis the weight fraction of m?BN,Ritris the interfacial thermal resistance at m?BN/matrix interface,Landaare the average diameter length and diameter?thickness ratio of m?BN, respectively. The corresponding fitting results are shown in Fig. 6e and the values ofRitrfor CA/m?SiC/m?BN/EP in both vertical and horizontal directions and randomly dispersed CA/m?SiC/m?BN/EPblendcomposites are 1.48 × 10—6, 4.67 × 10—6, and 5.55 × 10—6m2KW-1, respectively. This result clearly proves that the successful modification of m?BN enhanced its inter?action with the matrix. In addition,Ritrin vertical direction is lower than horizontal direction, which indicates that m?BN can also be tightly bonded to m?SiC (as the matrix) in the vertical direction.

The Foygel model is applied to simulates and calculate the thermal boundary resistance between fillers, as shown by Eqs.(6, 7) [76]:

whereKcandKmare the TC of composite and matrix,K0is a pre?exponential factor related to the filler.φfis the weight fraction of m?BN,φcis the critical permeability content of the filler,Lis the average diameter length of m?BN, β is related to the conductivity index of the filler, andRBdis the thermal boundary resistance at filler/filler interface. It can be seen from Fig. 6f that theRBdof CA/m?SiC/m?BN/EP in vertical direction is as low as 8.31 × 10—7m2·K/W, which is nearly one order of magnitude lower than the results of hori?zontal direction (2.94 × 10—6m2K W-1) and random distri?bution (4.65 × 10—6m2K W-1). Furthermore, the appearance of an approximate exponential fitting curve in the vertical direction indicates that m?BN is arranged in an orderly man?ner and no stacking occurs in this direction. This further confirmed the advantages of CA/m?SiC/m?BN/EP with high?speed heat conduction channel in the vertical direction.

To demonstrate the advantage of CA/m?SiC/m?BN/EP obtained in this work in terms of thermally conductive per?formance, a comparison with previous similar studies is shown in Table S3. After comprehensive comparison, the CA/m?SiC/m?BN/EP exhibits the excellent TC at low con?tent among the reported bulk composites.

Infrared thermal imager was used to intuitively estimate the thermal management capability of different EP?based composites. Specifically, as shown in Fig. 7a, different sam?ples with a diameter of 2 cm and a thickness of 3 mm are placed simultaneously on a hot plate set at 80 °C, and then,the infrared thermal imager is used to record the surface temperature changes over time. In order to verify that the TC of CA/m?SiC/m?BN/EP is anisotropic in two directions,the custom metal bases are used as the point heat source for the sample in the experimental design. It is worth noting that two test points A and B are selected on the sample surface during the experiment, as shown in Fig. 7b; point A is used to test the temperature change of the sample center with the point heat source in the vertical direction, while point B is used to reflect the thermal conduction capability of the sample in the horizontal direction. It is found that the sur?face temperature of different samples in the following order:CA/m?SiC/m?BN/EP ＞ CA/m?SiC/m?BN/EPblend＞ CA/m?BN/EP ＞ CA/m?SiC/EP ＞ EP, this result is consistent with the order of TC obtained in Fig. 6a, b. Obviously, the surface temperature of CA/m?SiC/m?BN/EP at point A shows a fast?est increasing with time compared to other samples. The specific temperature change with time of point A recorded by a computer is shown in Fig. 7c. After 120 s, the center temperature of CA/m?SiC/m?BN/EP is as high as 76.7 °C,which is close to the temperature of hot plate. Not only that,as shown in Fig. 7d, the sample CA/m?SiC/m?BN/EP also showed excellent heat transfer efficiency at point B close to the edge (the temperature is about 67.2 °C after 120 s),which shows that CA/m?SiC/m?BN/EP has high thermal conductivity along both vertical and horizontal directions,thereby forming a heat dissipation area with a larger radius[77].

3.4 Thermal Conduction Mechanism of Different EP?Based Composites

Fig. 7 a Optical photographs of different EP?based composites and schematic design for thermal imaging test. b Infrared thermal images of dif?ferent EP?based composites variation with heating time and c-d the temperature changes of A and B on the surface of EP?based composites with heating time

According to the morphology characterization and TC test results of different samples, the heat flow transmission path can be reasonably simulated. In addition, the packaging material is generally coated on the surface of the micro?electronic device, so here we focus on the comparison and analysis of the thermally conductive mechanism of different samples in the vertical direction. As we all know, phonons are the collective modulus of the vibrations of mutually cou?pled atomic lattice systems and are the main thermal energy carriers of polymer?based materials. Therefore, constructing efficient phonon transport channel is the key to improve the TC of thermal interface composites. As shown in Fig. 8,for CA/m?SiC/EP and CA/m?BN/EP with a single filler, the phonon transmission channel mainly relies on the vertically oriented m?SiC NWs and m?BN network structures, respec?tively. Since the in?plane TC of BN (600 W m-1K-1)) is higher than the in?line thermal conductivity of m?SiC NWs(100 W m-1K-1), CA/m?BN/EP exhibits a faster heat flow transfer rate. At the optimal filler content, the TC of CA/m?SiC/m?BN/EPblendwith dual fillers randomly dispersed is higher than that of CA/m?BN/EP and CA/m?SiC/EP, which is mainly due to the fact that more phonon transmission paths can be formed inside the structure of CA/m?SiC/m?BN/EPblend[78]. When m?SiC NWs and m?BN form a syn?ergistic vertical network structure in CA/m?SiC/m?BN/EP,resulting in a faster heat flow transfer compared to other samples. Hence, the vertically aligned networks composed of interconnected m?SiC NWs and m?BN heat conductive paths play a critical role in the phonon transmission, which can significantly improve the TC of the composite in the vertical direction [79].

Fig. 8 Schematic diagram of heat flow of different EP?based composites along the vertical direction

3.5 Electrical Insulative Properties of Different EP?Based Composites

Electrical insulation is the basic feature of electronic pack?aging materials [4]. During the experiment, conductive glue was applied on both sides of the tested sample and the experimental results are shown in Fig. 9. It can be seen that compared with pure EP (8.97 × 1014Ω·cm), the resistiv?ity of CA/m?SiC/EP (8.87 × 108Ω·cm) decreases sharply,while the resistivity of CA/m?BN/EP (9.47 × 1015Ω·cm)increases slightly, which is mainly attributed to the linear semiconductor characteristics of m?SiC NWSand the high electrical insulation of BN [80]. For the CA/m?SiC/m?BN/EPblendobtained by random dispersion, there is no consist?ent linear direction despite the participation of m?SiC NWS,so its resistivity is mainly influenced by EP, which is about 2.42 × 1013Ω·cm. It is worth mentioning that the resistivity of CA/m?SiC/m?BN/EP (2.35 × 1011Ω·cm) with a vertical network structure decreases compared with that of pure EP and CA/m?SiC/m?BN/EPblend, but it is much higher than the theoretical critical resistivity (1010Ω·cm) [25], com?bined with the high resistivity value of CA/m?BN/EP, this is mainly due to the high insulation of m?BN playing an important role in this special structure. It is convinced that the thermal management composite CA/m?SiC/m?BN/EP obtained in this work has a great potential for application in the field of microelectronic packaging.

Fig. 9 The volume resistivity of pure EP and different EP?based composites in the vertical direction

3.6 Dielectric and Electromagnetic Wave Absorption Performances of Different Samples

SiC NWs are a very important wide?bandgap semiconduc?tor material and have a high dielectric loss capability, so it shows excellent electromagnetic wave absorption potential[81]. In addition, EP with high resistivity (8.97 × 1014Ω·cm)has almost no dielectric response, so the electromagnetic wave absorption capacity of different EP?based compos?ites (CA/m?BN/EP, CA/m?SiC/EP and CA/m?SiC/m?BN/EP) mainly depends on the composition and structural framework of the internal filler. Generally speaking, the electromagnetic wave absorption capacity of non?magnetic dielectric materials is directly determined by the real (ε′)and imaginary (ε″) parts of the complex permittivity,ε′ andε″ represent the ability to store and dissipate electromag?netic waves, respectively [82, 83]. The obtainedε′,ε″ and the corresponding dielectric loss tangents (tanδε=ε″/ε′) are displayed in Fig. 10a—c. According to dielectric theory, the dielectric loss parameter ε′ for dielectric materials, while ε″ is attributed to the electrical conductivity. And the fast movement of electrons in high?conductivity fillers can also promote polarization. Based on the conductivity of m?SiC is much higher than that of insulating m?BN, therefore, the CA/m?BN/EP exhibits lowε′,ε″, and tanδ values, namely,2.6—3.3, 0.8—1.3, and 0.3—0.4, respectively. Compared with CA/m?BN/EP, theε′,ε″, and tanδεvalues of CA/m?SiC/EP show significant enhancement to 8.5—9.9, 7.0—7.3, and 0.7—0.8, respectively. In addition, tanδεindicates the abil?ity of material to convert electromagnetic waves into other forms of energy (most of which are converted into heat),thereby achieving the dissipation of microwave energy, and a high tanδεvalue indicates the strong ability to absorb electromagnetic waves [84]. It can be seen from Fig. 10c that CA/m?SiC/m?BN/EP not only has excellent microwave absorption capacity, but also it can convert the absorbed electromagnetic wave energy into heat for effective evacua?tion through the vertical heat conduction paths according to the aforementioned analysis of thermally conductive prop?erties [85].

To evaluate the microwave absorption properties of differ?ent composites, the reflection loss (RL) values were calcu?lated according to the transmit line theory by the following Eqs. (8, 9) [86]:

Fig. 10 a Real parts and b imaginary parts of the complex permittivities. c dielectric loss tangents and d reflection loss of the CA/m?SiC/EP,CA/m?BN/EP and CA/m?SiC/m?BN/EP

whereZinis the input impedances of the composite,μrandεrdenote the complex permeability and complex permittiv?ity, respectively,frepresents the frequency of microwaves,drefers to the thickness of the composites andcis the velocity of light in vacuum.

Figure 10d shows the electromagnetic wave reflection loss curve of the three systems with a thickness of 3 mm.For the CA/m?SiC/m?BN/EP, the minimum reflection loss is - 21.5 dB at 10.4 GHz, and the effective absorption bandwidth (＜ - 10 dB) is from 8.8 to 11.6 GHz. Accord?ing to Maxwell—Wagner polarization theory, this is mainly attributed to the vertical network structure formed by CA/m?SiC/m?BN in composite, which not only strengthens the effective interface (between m?SiC and m?BN) of interfa?cial polarization, but also enhances the microwave reflec?tion paths [87]. In order to verify the excellent microwave absorption properties of the composites, the impedance matching between the materials and the incident micro?waves was further explored. From Fig. S4, compared with CA/m?BN/EP, the microwave impedance of CA/m?SiC/EP and CA/m?SiC/m?BN/EP increases greatly because of their lower complex permittivity. In addition, CA/m?SiC/m?BN/EP exhibits better microwave absorption performance than CA/m?SiC/EP due to higher microwave impedance and almost the same dielectric loss.

4 Conclusions

In summary, vertically aligned m?SiC NWs/m?BN cellulose aerogel (CA/m?SiC/m?BN) networks have been success?fully constructed by ice template combined with directional freezing technology. CA/m?SiC/m?BN/EP composites were prepared by infiltrating the vertical networks with EP. In particular, the high?temperature calcination treatment of SiC NWs and the combined boric acid ball milling modification of BN not only improve the compatibility between the filler and matrix, but also reduce the thermal boundary resistance between the filler and filler, thereby effectively improving the thermal conductivity of the composite material. The CA/m?SiC/m?BN/EP composites exhibit a significantly enhanced TC of 2.21 W m-1K-1in vertical plane at a low filler loading of 16.69 wt%, which is increased by 890%compared to pure EP. In addition, from the analysis results of infrared thermal imaging, it is found that CA/m?SiC/m?BN/EP in the horizontal direction also exhibits a better thermally conductive performance than pure EP, CA/m?SiC/EP, CA/m?BN/EP and CA/m?SiC/m?BN/EPblendwith random dispersion. Furthermore, CA/m?SiC/m?BN/EP also has a superior volume resistivity of 2.35 × 1011Ω·cm and a minimum reflection loss of - 21.5 dB. Therefore, the new EP?based composite synthesized in this work will have good application prospects in the fields of electronic packaging.

AcknowledgementsWe acknowledge for the financial support from National Natural Science Foundation of China (21704096,51703217) and the China Postdoctoral Science Foundation (Grant No. 2019M662526). The authors also gratefully acknowledge financial support from Taif University Researchers Supporting Project Number (TURSP?2020/135), Taif University, Taif, Saudi Arabia.

FundingOpen access funding provided by Shanghai Jiao Tong University.

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Supplementary InformationThe online version contains supplementary material available at https:// doi. org/ 10. 1007/s40820? 022? 00863?z.