- Email: [email protected]
BNNSs microspheres
Composites Part B 188 (2020) 107882
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Preparation of highly thermally conductive and electrically insulating PI/ BNNSs nanocomposites by hot-pressing self-assembled PI/ BNNSs microspheres Lei Cao a, Jingjing Wang b, Jie Dong a, *, Xin Zhao a, Hai-Bei Li b, Qinghua Zhang a, ** a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, 201620, PR China School of Ocean, Shandong University, Weihai, 264209, PR China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyimide Boron nitride nanosheets Complex microspheres Hot-pressing Filler orientation
Traditional polymer-based thermally conductive composites with randomly distributed fillers always yield an undesired heat removal due to the lack of efficient heat transfer pathways. Thus, realization of rational and ordered distribution of thermally conductive nanofillers in polymer matrix is believed to be significant for obtaining a desirable thermal conductivity. Herein, a series of thermally conductive polyimide/boron nitride nanosheets (PI/BNNSs) composites with a highly ordered BNNSs network have been successfully prepared. For achieving an uniform dispersion and high orientation of BN nanosheets in PI matrix, self-assembled PI/BNNSs complex microspheres were firstly prepared via the van der Waals interaction, and then these complex micro spheres were further hot-pressed at the Tg of PI matrix, which rendered the alignment of BNNSs during the deformation of complex microspheres and built an efficient heat transfer pathway. As a consequence, the resultant composites possess a much higher in-plane thermal conductivity up to 4.25 W/mK with 12.4 vol% oriented BNNSs than those of pure PI and random distribution composite (0.85 W/mK for pure PI and 1.3 W/mK for the PI/random BNNSs-12.4). Meanwhile, these nanocomposites present excellent electrically insulating properties, improved dimensional stabilities and good thermal stabilities. This facile method provides a new way to design and fabricate highly thermally conductive PI-based composites for applying in heat dissipation of modern portable and collapsible electronic devices.
1. Introduction As modern electronic devices tend to be more integrated and mini mized with the increase of power density, efficient thermal management and effective heat removal have become a vital issue to ensure the reliability and lifetime of electronic devices [1,2]. Polyimide (PI)-based substrates are widely used as electronic packaging materials in advanced electronic systems, such as interlayer dielectric films and flexible printed circuits (FPC) because of their robustness, good thermal stability, low dielectric constant and loss and other exceptional properties [3,4]. However, the intrinsic thermal conductivity of pure PI only ranges from 0.1 to 0.4 W/mK [5], far below the requirement of thermally conductive materials, which limits its wide application in modern microelectronic industry. To address this heat dissipation issue, different types of highly
thermally conductive fillers have been incorporated into polymer matrix including metal oxides (Al2O3 [6], ZnO [7], etc.), carbon-based materials (carbon nanotube [8], graphene [9], etc.) and ceramic materials (BNNSs [10], AlN [11], etc.), to prepare thermally conductive polymer com posites. Generally, two main factors affect the thermal conductivity of resultant composites: one is thermal resistance at interface between polymer matrix and nanofiller, which should be minimized to facilitate smooth heat transfer in the materials; the second prominent factor lies in creating a thermally conductive network for efficient phonon transfer. Numerous works have revealed that improving affinity of nanofiller to polymer by modifying the surface of nanofillers could increase the heat transportation in polymer nanocomposites [12–14]. While, for the aro matic PI matrix, constructing thermally conductive network throughout the whole sample seems to be more difficult to be handled compared to
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Dong), [email protected] (Q. Zhang). https://doi.org/10.1016/j.compositesb.2020.107882 Received 29 November 2019; Received in revised form 5 February 2020; Accepted 13 February 2020 Available online 14 February 2020 1359-8368/© 2020 Published by Elsevier Ltd.
L. Cao et al.
Composites Part B 188 (2020) 107882
some flexible thermoplastic polymer matrixes. As a typical thermally conductive filler, BNNSs exhibit some attrac tive characters, including superhigh thermal conductivity, wide bad gap (around 5.9 eV) and large aspect ratio two-dimensional (2D) morphology, which have been regarded as the most ideal nanofiller and extensively explored in various polymer matrixes to create an improved heat conducting property and excellent electric insulation performance. However, it should be noticed that BNNSs exhibit an anisotropic char acter in thermal conductivity, namely, the in-plane thermal conductivity can reach as high as 2000 W/mK, while that declines to only a few W/ mK in the thickness direction. Accordingly, the heat transportation of BNNSs-based composites greatly depends on the alignment of BNNSs in the polymer matrix. For achieving rational control of alignment of BNNSs in polymer matrix, a verity of strategies including layer-by-layer [15], hot-pressing [16], electrical/magnetic field [17] and electro spinning [18] as well as strong shearing [19] have been devoted to regulate the orientation of this 2D nanofiller. For example, Chen et al. successfully constructed polyvinylidene fluoride (PVDF)/BNNSs ther mally conductive nanocomposites by electrospinning polymer/BNNSs nanocomposite fibers, vertically folding electrospun nanofibers and subsequent hot-pressing. The BNNSs could stack in sequence along the oriented direction of fibers, resulting high alignment of BNNSs in the resultant nanocomposites and endowing them with a high thermal conductivity of 16.3 W/mK with a 33 wt% filler loading [20]. Hu et al. fabricated an epoxy/BNNSs composite through a combination of ice-templating self-assembly and infiltration method for achieving a well-aligned BNNSs platelets, which possessed a thermal conductivity up to 4.42 W/mK at a filler loading of 34 vol% (gaining 2226% in thermal conductivity enhancement) [21]. The above studies fully indi cate that these methods can effectively induce the BNNSs orientation and construct thermally conductive network, resulting in a great enhancement in thermal conductivity. However, the operation of these methods is relatively complex and needs multiple processes; besides, high loading fractions are still required to achieve a desired thermal conductivity and the filler aggregation easily takes place at a high filler loading. As a simple alternative method, Wang et al. [22] proposed that realization of selective ordered distribution of nanofillers in an immis cible polymer blend with a cocontinuous structure could reduce the thermal percolation threshold to some extent and prepare thermally conductive polymer nanocomposites. Specially, Wang et al. demon strated that a selective distribution of nanofillers at the interface of composites was a more ideal approach for achieving a high thermal conductivity enhancement in relative to the continuous distribution of nanofillers in the polymer phase [22]. They successfully distributed the BNNSs at the interfacial area of polystyrene (PS) accurately via an initial formation of PS/BNNSs composite microspheres and a following hot-pressing. These two steps resulted in a homogeneous dispersion of thermally conductive nanofiller at the interface of deformed PS phases and meanwhile promoted them highly aligned along the in-plane di rection, thus forming a thermally conductive network at a low filler content. The prepared nanocomposite exhibited a high thermal con ductivity of 8.0 W/mK at a filler content of only 13.4 vol%. As we know, common methods for fabricating PI-based thermally conductive nano composites mainly include simple solution blending, electrospinning and freeze-drying approaches, etc. Preparing PI/BNNSs nanocomposites via the selective distribution of BNNSs nanofillers at the interface of polyimide blends and forming continuous thermally conductive net works has been rarely reported. In present work, we propose a facile method for preparing highly thermally conductive PI/BNNSs composites. Firstly, self-assembled PI/ BNNSs complex microspheres were directly prepared via the van der Waals interaction between polyimide matrix and nanofiller; subse quently, hot-pressing was adopted to prepare nanocomposite films and rendered BNNSs homogeneously distributed and aligned well at the interface of deformed PI microspheres, making the resultant composites
exhibit a high in-plane thermal conductivity of 4.2 W/mK at a 12.4 vol% filler loading. Moreover, the prepared nanocomposites have superior electrically insulating properties with the volume resistivity of ~1014 Ω cm, comparable to that of the pristine PI matrix. Comparatively, the present method has the advantages of simplicity and adaptability for a commercial scaleup, which may find utilization in preparing thermally conductive polyimide materials in modern electronic devices. 2. Experimental section Materials. BN powder was purchased from Alpha Aesar (China) Chemical Co. Ltd (Shanghai, China). Polyimide (PI, Ultem 1010®) was purchased from Saudi Basic Industries Corporation (SABIC). Polyvinyl alcohol (PVA, degree of hydrolysis ¼ 88%), N,N0 -dimethylacetamide (DMAc) and isopropanol (IPA) were all purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and used as received. Preparation of BNNSs: BNNS nanosheets were prepared via a method combining sonication-assisted liquid phase exfoliation and centrifugation. In detail, 2 g of BN powder was added into the 200 mL IPA. The dispersion was sonicated for 12 h with a frequency of 53 kHz. The resultant suspension was centrifuged at 2000 rpm for 15 min to remove nonexfoliated BN. A homogeneous BNNSs dispersion in IPA (0.4 mg/mL) could be obtained for further use. Fabrication of polyimide microspheres: PI microspheres were fabricated by a reprecipitation method. Typically, 10 g PI and 10 g PVA were dissolved in 200 mL DMAc in a 1000 mL round-bottom flask. 400 mL deionized water was added dropwise into the mixed DMAc solution under vigorous stirring at 500 rpm. The dispersion was stirred for another 2 h to ensure sufficient precipitation of PI microspheres after 400 mL deionized water was added into the DMAc solution. Finally, PI microspheres were obtained by centrifugation at a 8000 rpm and washed with deionized water followed drying in vacuum oven at 60 � C. Preparation of PI/BNNSs complex microspheres: PI microspheres (0.5 g) were redispersed in IPA (100 mL) by ultrasonication for 3 min. The PI/BNNSs complex microspheres were fabricated by dropwise adding the BNNSs/IPA dispersion into the PI microspheres/IPA disper sion under vigorous stirring. After 2 h, PI/BNNS complex microspheres were collected by vacuum filtration and dried at 60 � C. A series of complex microspheres were prepared with the BNNSs loading between 0 and 12.4 vol%。 Preparation of oriented/random PI/BNNSs composite films: The schematic illustration of preparing PI/BNNSs composite films is shown in Scheme 1. The as-prepared PI/BNNSs complex microspheres were hot-pressed at 230 � C for 20 min with a pressure of 20 MPa. For obtaining a dense microstructure, the composites were further coldpressed for 5 min at room temperature with a pressure of 20 MPa. In present work, the thickness of all composite films was controlled to ~250 μm. The obtained composites were denoted as PI/oriented BNNSsx, where x refers to volume fraction of fillers in the composite. For comparison, randomly dispersed BNNSs in polyimide composite films were also prepared by a solution blending method and denoted as PI/random BNNSs-x. As a typical example, the PI/random BNNSs-12.4 was synthesized as following: 8 g PI was thoroughly dissolved in DMAc in a three-necked flask equipped with a mechanical stirrer at room temperature under N2. 2 g exfoliated BNNSs were dispersed in DMAc with the assistance of ultrasonic vibration prior to mixing with the above PI solution. Then, these two solutions were mixed and continuously stirred at room for 6 h to obtain a homogeneous viscous PI/ BNNSs solution. The above mixture was cast on a glass substrate by a scalpel. The casted film was thermally treated in an oven at 80 � C, annealing for 6 h to remove the solvent, and finally at 230 � C for 20 min. The obtained solid composite film can be denoted PI/random BNNSs12.4 with the BNNSs volume content of 12.4 vol%. The thickness of the resultant films was kept to 200–250 μm. Other composites were prepared by a similar procedure. Characterization: The SEM images were observed in Hitachi S2
L. Cao et al.
Composites Part B 188 (2020) 107882
Scheme 1. Schematic illustration of preparing PI/oriented BNNSs composites.
4800. The microstructures of BNNSs were also analyzed by the trans mission electron microscope (TEM, JEM-2010F) at an acceleration voltage of 200 kV and atomic force microscope (AFM, Bruker Dimension Edge). Size distribution of polyimide microspheres was determined by the nanoparticle size analyzer (Malvern). Zeta potentials of BNNSs and PI in IPA were investigated on Zetasizer Nano-ZS90. The alignment structure of BNNSs in the composites films was measured by X-ray diffraction (XRD, Rigaku D/max-2550) and 16B1 Beamline in Shanghai Synchrotron Radiation Facility (SSRF). Thermal conductivity (K) of these composites was measured using the laser flash technique (NETZSCH, LFA 467 Nano-Flash) and calculated as: K ¼ α � Cp � ρ, where ρ is the density of the composites, Cp represents the specific heat capacity, α refers to the thermal diffusivity. The surface temperature of the composites was recorded by an infrared thermograph (Fotric 255s, China). Dielectric properties of the composites were measured by a broadband dielectric spectrometer (Novocontrol concept 40). The vol ume resistivity of the PI/BNNSs composites was obtained using a elec trometer (Keithley 6517B). The coefficient of thermal expansion (CTE) of prepared nanocomposites was measured using the dynamic thermo mechanical analyzer (DMA, TA Q800) with the control force mode at a heating rate of 5 � C/min from 40 to 200 � C. Thermal gravimetric anal ysis (TGA) was carried out in nitrogen atmospheres at a heating rate of 5 � C/min from 40 to 900 � C by a Netzsch-TGA instrument (NETZSCH TG 209 F1). The glass transition temperature of the composites was ob tained using differential scanning calorimetry (DSC, TA Q20) at a heating rate of 10 � C/min under nitrogen atmosphere. In modeling the interfacial interaction of the PI/BNNSs complex, an polyimide unit and a roughly 29 Å � 23 Å ribbon were firstly constructed and then were fully optimized using the all-electron density functional theory (DFT) equa tions implemented in the Gaussian 09 package with the B3LYP func tional of generalized gradient approximation and the 6-31G basis set.
ultrathin feature. The inserted high resolution TEM observed from the edge of a BN nanosheet illustrates that 5–7 layers of BNNSs are stacked together. AFM was further used to confirm the thickness of exfoliated BNNSs as revealed in Fig. 1(B), and the detailed height (Fig. S1) illus trates the thickness of ~3.5 nm for BNNSs, which agrees well with the TEM result (thickness of the single layer BN nanosheet is around 0.5 nm [23]). Additionally, the ζ-potential of exfoliated BNNSs in IPA is esti mated to 31 mV, which is consistent with the previously reported result [22]. Synthetic polymer microspheres are usually prepared during poly mer synthesis from a monomer, such as in suspension, emulsion, and dispersion polymerization. In this work, we adopted a method for pre paring polyimide microspheres from the commercial amorphous Ultem 1010®, in which the precipitant water was utilized for creating the liquid-liquid phase separation and the stabilizer PVA was added to so lution for shaping the precipitate into microspheres. Fig. 1(C) shows the monodisperse PI microspheres with a relatively uniform particle size of ~3.0 μm. Besides, a smooth surface morphology can be observed for the pure PI microspheres. According to Fig. 1(D), the statistical distribution of diameter of PI microspheres is mainly ranging from 2–5 μm. The ζ-potential value is around 18 mV for the synthesized PI microspheres dispersed in IPA (comparable to the reported value [24]), which seems indicating the inaccessible self-assembly with the negatively charged BN nanosheets. Interestingly, when mixing the BNNSs and PI microspheres dispersion solutions in IPA together, BNNSs can tightly absorb on the surface of PI microspheres. Fig. 1(E) and (F) shows SEM micrographs of PI/BNNSs complex microspheres with 5.9 vol% and 12.4 vol% BNNSs loadings. Clearly, the complex microspheres exhibits rough surfaces due to the wrapped BNNSs, and majority of PI microspheres is coated by the BNNSs as increasing the filler feeding to 12.4 vol%. The PI/BNNSs complex microspheres attaching 2.9 vol% and 9.1 vol% BN nanosheets also exhibit a uniform BNNSs dispersion, as revealed in Fig. S2. As a comparison, previously reported polystyrene (PS)/BNNSs [22] or PS/GO [25] complex microspheres were mainly prepared by the self-assembly process between polymer microspheres and nanofillers based on the electrostatic adsorption, in which the polymer micro spheres or nanofillers should be modified by surface pretreatment. However, in our case, both negatively charged BNNSs nanosheets and PI microspheres can directly self-assemble to form complex microspheres, indicating its convenience and simplicity. In addition, this fact also suggests that there exists some other factors driving the assembly pro cess of these two components. As illustrated in Fig. 2(A), BNNSs show a well-dispersed white colloidal suspension in IPA and can keep this stable state for a week.
3. Results and discussion The hexagonal boron nitride (h-BN) is a flake-shaped synthetic ceramic, which has a crystal structure analogous to graphite and consists of layers of six-membered rings of covalently bonded nitrogen and boron atoms weakly bound by van der Waals force. The thermal conductivity of unexfoliated h-BN with multi-layer stacks is around 200 W/mK in the planar direction. However, after exfoliation, the thermal conductivity can reach as high as 2000 W/mK for the single/few layer BNNSs [19]. Herein, the BNNSs were prepared using the liquid sonication technique. In Fig. 1(A), the TEM micrograph shows that exfoliated BNNSs have a lateral size around 500 nm and are slightly transparent, indicating their 3
L. Cao et al.
Composites Part B 188 (2020) 107882
Fig. 1. (A) TEM micrograph of BNNSs exfoliated from h-BN powder, and the inset is the high-resolution TEM image of BNNSs edge; (B) AFM image of BNNSs; (C) SEM image of pure PI microspheres; (D) size distribution of PI microspheres; (E, F) SEM micrographs of PI/BNNSs complex microspheres with the BNNSs loadings of 5.9 vol% and 12.4 vol%, respectively.
However, as adding PI microspheres, the PI/BNNSs complex micro spheres can quickly settle to the bottom after 24 h. For better under standing the reason for such self-assembly process between PI microspheres and BNNSs, we used density functional theory (DFT) to explore the adsorption behavior between them based on an optimized polyimide unit, boron unit and polyimide-BNNSs unit. Details of the interfacial chemistry extracted from the ab initio model are shown in Fig. 2, which can be assumed to be qualitatively valid at a larger scale equally. In Fig. 2(B), BNNSs nanosheets roll up to get closer to the hetero-atoms of polyimide unit and enhance their contact area. This deformation could be an artefact due to the BNNSs nanosheets were modelled by a cluster approach. The actual periodic systems will not present this deformation and previous report has illustrated that this structural deformity do not impact the calculated interaction energies between BNNSs and other chemicals [26]. Clearly, the polyimide unit prefers to orient itself almost perfectly parallel to the boron nitride ribbon, and the nearest atom distance and average ground state
interfacial distance between polyimide unit and BNNSs nanosheets are calculated to be 3.15 and 3.24 Å, respectively, suggesting that the adsorption is physical in nature (van der Waals (vdW) interaction). The calculated adsorption energy (ΔE) for BNNSs sheets on polyimide unit is around 59.8 kcal/mol, which is larger in comparison with the reported ΔE values for the poly-paraphenylene terephthalamide (PPTA)/BNNS [27] and graphene/BNNS [28] complexes. First, it is well known that h-BN powder can be effectively exfoliated via a sonication-assisted liquid phase exfoliation method. In this process, the BNNSs are easily edge-hydroxylated and polar O–H groups can form and attach on the surface of BN nanosheets [29]. It could be speculated that after mixing, a hydrogen bonding interaction could be formed between the abundant carbonyl groups in imide rings and the polar hydroxy groups in BNNSs, which benefits increasing the compatibility between BN nanosheets and PI polymer chains. In addition, the polar B–N bonds in BNNSs and carbonyl and imide groups in polyimide are beneficial for forming the strong adsorption by the dipole-dipole interaction. Furthermore,
Fig. 2. (A) Photographs of BNNSs in IPA before (1#) and after (2#) addition of PI microspheres; (B) polyimide unit and boron nitride sheet are brought together and optimized through density function theory (DFT). The calculated interfacial region and the interaction energy show van der Waals interaction at the interface. 4
L. Cao et al.
Composites Part B 188 (2020) 107882
abundant six-membered phenyl rings and rigid imide rings in PI back bone endow an interfacial chemistry similar to that of adjacent layers of BNNSs nanosheets, which are also in favor of increasing the vdW interaction. Therefore, this strong interfacial interaction accesses the self-assembly between BNNSs and PI microspheres. The as obtained PI/BNNSs complex microspheres were further pre pared into composite films by a hot-pressing technology at 230 � C, slightly higher than the glass transition temperature of Ultem 1010® (Tg ¼ 214 � C), under a pressure of 20 MPa. The microstructures of com pressed pure PI, PI/random BNNSs and PI/oriented BNNSs composites with different filler loadings are revealed in SEM images. As depicted in Fig. 3(A), the pure PI shows a dense and smooth cross-section morphology, while for the PI/random BNNSs-12.4, the microstructure is chaotic and BNNSs aggregation can be observed (as marked in the elliptical area). Comparatively, Fig. 3(C-E) illustrate that the hotpressing method imparts an advantageous BNNSs in-plane orientation in composites derived from the PI/BNNSs complex microspheres. Additionally, the magnified cross-section image of PI/oriented BNNSs containing 12.4 vol% BNNSs in Fig. 3(E) exhibits a homogeneous dispersion and directly connected BN nanosheets in the PI matrix, which is mainly attributed to the uniform hybridization of PI microspheres with BNNSs in advance. Actually, under the present hot-pressing tem perature, PI molecular chains have an increased mobility and micro spheres are easily to deform but can not be melted. With the deformation of microspheres, these BNNSs are directed to distribute along the interface of microspheres and can not penetrate into PI phase (as depicted in Fig. S3). Accordingly, a continuous heat transportation path can be constructed by the oriented and connected BNNSs nanosheets as the BNNSs content increasing to a certain value. Similar results have been reported in many polymer nanocomposites, such as PVDF/PS/ MWCNT [30], PI/r-GO [31] and PI/ZnO [7], etc. For further verifying the anisotropic alignment of BNNSs in these composites, X-ray charac terization was recorded as shown in Fig. 3(F). The pure PI exhibits a typical amorphous nature. While, for the nanocomposites, two evident peaks (2θ ¼ 26.5� and 43.1� ) representing the (002) and (100) lattice planes of BNNSs are easily found. Previous studies have manifested that the intensity ratio of I002/I100 elucidates the orientation of BN in poly mer composites [32]. It is noticed that the I002/I100 ratio of the PI/ran dom BNNSs-12.4 composite is only 5.2, whereas for PI/oriented BNNSs-12.4, this ratio sharply increases to 139, indicating a much higher in-plane orientation of BNNSs nanosheets perpendicular to
hot-pressing direction. Besides, by comparing different PI/oriented BNNSs samples, increasing the BNNSs content results in a little higher BNNSs orientation in the composites. Fig. 4 shows the thermal transportation properties of the resultant PI/BNNSs nanocomposites. The in-plane and through-plane thermal conductivity of composites were measured by a laser-flash system. As a comparison, PI/random BNNSs composites with different BNNSs load ings were also investigated. As shown in Fig. 4(A), the in-plane thermal conductivity of pure PI is only 0.85 W/mK. As adding BNNSs, the inplane thermal conductivities of these two series of composites both have obvious enhancements with increasing BNNSs content. Compara tively, the thermal conductivity of PI/oriented BNNSs composites grows much faster that of PI/random BNNSs samples, which mainly attributes from the well-aligned and continuous BNNSs network. In particular, the thermal conductivity is significantly increased to 4.25 W/mK with incorporation of oriented 12.4 vol% BNNSs into the PI matrix, which is around two times higher than that of the PI/random BNNSs-12.4. Meanwhile, the thermal conductivity enhancement (TCE¼(Kc-Kp)/Kp � 100%, where Kc and Kp represent the thermal conductivity of com posites and pure polymer matrix) has been introduced to quantitatively characterize the improving efficiency of BNNSs on the PI matrix. As depicted in Fig. 4(B), the PI/oriented BNNSs-12.4 reveals a remarkable enhancement of in-plane thermal conductivity of ~400% as compared to the pure PI, whereas the PI/random BNNSs-12.4 sample only exhibits an enhancement of 62.3%, further suggesting significant impact of orientation and internal connection on the in-plane thermal conduc tivity of the composites. Additionally, as shown in Fig. 4(B), the TCE values for PI/random BNNSs composites fabricated by the solution blending are all below 100%. Thus, in the conventional method, higher filler content is always required to construct a continuous heat trans portation network in composites so as to obtain a high in-plane thermal conductivity. As depicted in Fig. 4(C), even incorporating 21.2 vol% BNNSs into PI matrix by a conventional solution blending method in previous works, the thermal conductivity values are still below 1.5 W/ mK [33–35]. The previously reported thermal conductivities of BN-based polymer composites are summarized in Fig. 4(C). Clearly, the PI/oriented BNNSs-12.4 exhibits a higher thermal conductivity compared to the previous investigations focused on the EP/BN [36–39], NR/BN [19], PI/BN [40], EP/BN-rGO [41], PVDF/BN [42] and PDMS/BN [43] as well as HDPE/BN [44] nanocomposites, implying the aligned and continuous BNNSs network in present work has a prominent
Fig. 3. Microstructure characterization of PI/BNNSs composites. SEM fracture images of pure PI (A), PI/random BNNSs-12.4 (B), PI/oriented BNNSs-5.9 (C), PI/ oriented BNNSs-12.4 (D, E), and (F) XRD patterns of the PI/BNNSs composites with different BNNSs loadings. 5
L. Cao et al.
Composites Part B 188 (2020) 107882
Fig. 4. (A) In-plane thermal conductivity of the PI/oriented BNNSs and PI/random BNNSs nanocomposites with different BNNSs loadings; (B) comparison of thermal conductivity enhancement between the PI/oriented BNNSs and the PI/random BNNSs composites as a function of BNNSs loading; (C) comprehensive comparison of thermal conductivity versus nanofiller loadings between PI/oriented BNNSs-12.4 in this work and reported polymer/BN composites; (D) the diagram of in-plane transfer of heat flow in the PI/oriented BNNSs nanocomposites; (E) infrared thermal images of pure PI, PI/random BNNSs-12.4 and PI/oriented BNNSs-12.4 composites and (F) the surface temperature variations of nanocomposites versus time.
advantage in boosting heat transfer capability of PI-based nano composites. As illustrated in Fig. 4(D), the homogeneously attached BNNSs on surface of PI microspheres via the van der Waals interaction and subsequent hot-pressing can easily result in an uniform distribution and orientation of BNNSs along the in-plane direction in composites, which form a thermally conductive network throughout the whole
sample and heat can readily transfer along the shortest transfer chan nels. The effects of BNNSs on the through-plane thermal conductivity of PI/BNNSs composites were also investigated (Fig. S4). It is clear that the through-plane thermal conductivities of PI/oriented BNNSs composites are much lower than those along the in-plane direction. For example, even incorporating 12.4 vol% BNNSs nanosheets, the through-plane 6
L. Cao et al.
Composites Part B 188 (2020) 107882
thermal conductivity increases from 0.26 W/mK to 0.40 W/mK with a enhancement of only 54%, indicating an anisotropic character in ther mal transportation of PI/oriented BNNSs composites. Contrary to the thermal conductivity along in-plane direction, PI/random BNNSs com posites possess higher through-plane thermal conductivities in relative to PI/oriented BNNSs samples under all filler contents, which is due to the fact that there are many BNNSs nanosheets parallel to the through-plane direction in the PI/random BNNSs composites. As is well known, BNNSs exhibit an anisotropic character in thermal conductivity, namely, the in-plane thermal conductivity can reach as high as 2000 W/mK, while, it declines to only a few W/mK in the thickness direction. Therefore, more BNNSs nanosheets parallel to the through-plane di rection in PI/random BNNSs composites result in a higher through-plane thermal conductivity. To demonstrate the potential application of these materials in flex ible electronics, the prepared pure PI, PI/oriented BNNSs-12.4 and PI/ random BNNSs-12.4 composites were utilized as substrates to support light-emitting-diode (LED) chips using silver paste (Fig. 4(E)). Surface temperature variations of substrates in the center with LED working time were measured by a portable thermal imaging camera. It can be observed that after a steady-state (180 s) of the same flow caused by the equal standard LED chips, the temperature distribution is excessively concentrated to generate hot spots on pure PI and PI/random BNNSs12.4 specimens. However, the temperature distribution is uniform for the PI/oriented BNNSs-12.4 composite with a relatively lower center spot temperature of 48 � C, which has dropped by about 12 and 5 � C in comparison with the pure PI film and the composite containing
randomly dispersed BNNSs, respectively. As shown in Fig. 4(F), throughout the operation process, the PI/oriented BNNSs-12.4 displays an obvious slower temperature rise and lower stable temperature than those of other two samples, further illustrating a better heat dissipation in PI/BNNSs composites containing highly aligned BNNSs. For electrical and electronic applications, the reliable dielectric and electrical insulation performances are usually also crucial importance [45]. The frequency-dependent dielectric properties of composite films with various BNNSs loadings are presented in Fig. 5(A) and (B). The dielectric constant of pure PI is around 3.0 at 1 MHz under an ambient condition. As incorporating oriented BNNSs, these composites exhibit increased dielectric constant and dielectric loss mainly attributed to the higher intrinsic dielectric constant of BNNSs (~4.0 [33]). For the com posite containing 12.4 vol% oriented BNNSs nanosheet, the dielectric constant and tan δ are 3.48 and 0.02 at 1 MHz, respectively, which still can satisfy the requirements of most electronic applications. As depicted in Fig. 5(C), the PI/oriented BNNSs-12.4 composite exhibits thermally stable dielectric constants at temperatures ranging from 25 to 150 � C, which benefit to prevent charge transfer or “crosstalk” between con ducting materials. However, it presents a slight fluctuation at the low frequency region in the dielectric constant as rising the temperature to 200 � C. Fig. 5(D) presents the volume electrical resistivity of nano composites. Clearly, no obvious fluctuations in the volume resistivity have been detected after the addition of BNNSs. For example, the vol ume resistivity of PI/oriented BNNSs-12.4 still reaches as high as ~1014 Ω cm, which is far beyond the required resistance for electrical insu lation (~109 Ω cm). Overall, the prepared dielectric thermally
Fig. 5. Frequency-dependent dielectric constant (A) and dielectric loss tangent (B) of different PI/BNNSs nanocomposites, (C) dielectric constant of PI/oriented BNNSs-12.4 at different temperatures and (D) volume electrical resistivity of the nanocomposites with different BNNSs loadings. 7
L. Cao et al.
Composites Part B 188 (2020) 107882
conductive PI/oriented BNNSs composites show an excellent integrated electrical property, which will ensure their reliability and lifetime uti lized in modern electrical systems and electronic devices. It is well-known that, for the insulting substrate in electronic devices, a poor dimensional stability and low thermal stability will bring severe impacts on their functionality. Thus, herein, we examined the dimension and thermal reliability of these nanocomposites. As illustrated in Fig. 6 (A), the PI/oriented BNNSs composites exhibit a distinct reduce in displacement during the measurement from 100 to 160 � C under a preloaded force of 0.02 N, indicating the improved dimensional stabil ity. Detailed CTE values of prepared nanocomposites in different tem perature regions are listed in Fig. 6(B). Obviously, the composites with oriented BNNSs present lower CTE values in relative to the pure PI. For example, the PI/oriented BNNSs-12.4 exhibits a CTE of 20 ppm K 1 in 140–160 � C, while the value is as high as 56 ppm K 1 for the pure PI. It also demonstrates that increasing the BNNSs loading will continuously reduce the thermal expansion behavior of nanocomposites, which can be accounted by that well-aligned BNNSs structure is beneficial for stabi lizing the whole framework of composites. Effects of oriented BNNSs nanosheets on glass transition temperature (Tg) of PI matrix were also investigated. Fig. 6(C) presents the dependence of Tg on BNNSs loadings. It is interesting to find that all of prepared composites exhibit a decreasing trend on Tg as a function of BNNSs loading, which is kindly diverse from previous reports [46,47]. In detail, the pure PI exhibits a Tg at 214 � C, which gradually decreases to 198 � C as incorporating 12.4 vol
% BNNSs. Similar results have been reported in Hu’s work as they investigated the epoxy/BN nanocomposites, and they accounted this by a fact that polymer chains surrounding BN microplatelets could easily change their conformation and additional free volume could be intro duced into the composites as incorporating BNNSs nanosheets, both of which have a synergistic effect on reducing the Tg [21]. In our case, the reduced Tg may be caused by a similar reason. TGA analysis was carried out under nitrogen to investigate the thermal stability of PI/BNNSs composites with different filler loadings. The preliminary results in Fig. 6(D) show that all samples exhibit 5%-weight-loss temperatures (Td5) and maximum-weight-loss temperatures (Tmax) over 443 and 491 � C, respectively, indicating their excellent thermal stabilities, which have a significance for these materials applied as electronic substrates in high-temperature circumstances. Gu et al. [33,48,49] reported the “heat-resistance index” (THRI) was another parameter that could reflect and reveal the “thermal stability” of polymers or polymer composites. Based on their previous studies, THRI values of PI-based nanocomposites in this work were also calculated and results were listed in the inserted table, which ranged from 245.3 � C to 254.6 � C and seemed that the introduction of BN inclined to improve the stability of resultant composites. 4. Conclusions In summary, we have successfully fabricated highly thermally
Figure 6. (A, B) Displacement distance and CTE values of pure PI and PI/BNNSs nanocomposites in the heating process, (C, D) DSC and TGA heating curves of nanocomposites with different BNNSs loadings. 8
Composites Part B 188 (2020) 107882
L. Cao et al.
conductive PI/BNNSs composites by combining self-assembled PI/ BNNSs complex microspheres via van der Waals interaction and subse quent hot-pressing technology around the Tg of PI matrix. The formation of connected and aligned BNNSs structure accompanied by the defor mation of PI microspheres acts as an efficient heat transfer pathway. Thus, the obtained composites possess a favorable in-plane thermal conductivity of around 4.25 W/mK with a BNNSs loading of 12.4 vol%, which is more than four times as high as that of the pure PI and much more superior to that of random distribution composites. Moreover, the well-aligned BNNSs has been demonstrated to be beneficial for stabi lizing the whole framework of composites and led to the improved dimensional stability, excellent electrically insulating and thermalstable dielectric properties. It is hoped that this work can provide some guidelines for future designs of thermally conductive PI-based thermal management materials for application in advanced electronic devices, which can meet the increased needs of heat dissipation.
[10] [11] [12]
[13] [14]
[15] [16]
Declaration of competing interest
[17]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[18]
CRediT authorship contribution statement
[19]
Lei Cao: Conceptualization, Methodology, Writing - original draft. Jingjing Wang: Software. Jie Dong: Investigation, Resources, Visuali zation. Xin Zhao: Formal analysis, Data curation, Funding acquisition. Hai-Bei Li: Software, Writing - review & editing. Qinghua Zhang: Su pervision, Project administration, Funding acquisition.
[20] [21]
[22]
Acknowledgements
[23]
This work was supported by National Natural Science Foundation of China (No. 21774019, 51903038), the Program of Shanghai Academic Research Leader (18XD1400100), the Scientific Research Innovation Plan of Shanghai Education Commission (2019-01-07-00-03-E00001) and the Fundamental Research Funds for the Central Universities (2232019G-02).
[24] [25]
Appendix A. Supplementary data
[26]
Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2020.107882.
[27]
References
[28]
[1] Burger N, Laachachi A, Ferriol M, Lutz M, Toniazzo V, Ruch D. Review of thermal conductivity in composites: mechanisms, parameters and theory. Prog Polym Sci 2016;61:1–28. [2] Xu X, Chen J, Zhou J, Li B. Thermal conductivity of polymers and their nanocomposites. Adv Mater 2018;30(17):1705544. [3] Yuan C, Jin K, Li K, Diao S, Tong J, Fang Q. Non-porous low-κ dielectric films based on a new structural amorphous fluoropolymer. Adv Mater 2013;25(35):4875–8. [4] Kim JW, Lee AS, Yu S, Han JW. En masse pyrolysis of flexible printed circuit board wastes quantitatively yielding environmental resources. J Hazard Mater 2018;342: 51–7. [5] Tanimoto M, Yamagata T, Miyata K, Ando S. Anisotropic thermal diffusivity of hexagonal boron nitride-filled polyimide films: effects of filler particle size, aggregation, orientation, and polymer chain rigidity. ACS Appl Mater Interfaces 2013;5(10):4374–82. [6] Choi S, Kim J. Thermal conductivity of epoxy composites with a binary-particle system of aluminum oxide and aluminum nitride fillers. Compos B Eng 2013;51: 140–7. [7] Yorifuji D, Ando S. Enhanced thermal conductivity over percolation threshold in polyimide blend films containing ZnO nano-pyramidal particles: advantage of vertical double percolation structure. J Mater Chem 2011;21(12):4402–7. [8] Jiang Q, Wang X, Zhu Y, Hui D, Qiu Y. Mechanical, electrical and thermal properties of aligned carbon nanotube/polyimide composites. Compos B Eng 2014; 56:408–12. [9] Guo Y, Xu G, Yang X, Ruan K, Ma T, Zhang Q, et al. Significantly enhanced and precisely modeled thermal conductivity in polyimide nanocomposites with
[29]
[30] [31] [32]
[33]
[34] [35]
9
chemically modified graphene via in situ polymerization and electrospinning-hot press technology. J Mater Chem C 2018;6(12):3004–15. Guo Y, Lyu Z, Yang X, Lu Y, Ruan K, Wu Y, et al. Enhanced thermal conductivities and decreased thermal resistances of functionalized boron nitride/polyimide composites. Compos B Eng 2019;164:732–9. Xie SH, Zhu BK, Li JB, Wei XZ, Xu ZK. Preparation and properties of polyimide/ aluminum nitride composites. Polym Test 2004;23(7):797–801. Cho HB, Tokoi Y, Tanaka S, Suematsu H, Suzuki T, Jiang W, et al. Modification of BN nanosheets and their thermal conducting properties in nanocomposite film with polysiloxane according to the orientation of BN. Compos Sci Technol 2011;71(8): 1046–52. Lee HL, Kwon OH, Ha SM, Kim BG, Kim YS, Won JC, et al. Thermal conductivity improvement of surface-enhanced polyetherimide (PEI) composites using polyimide-coated h-BN particles. Phys Chem Chem Phys 2014;16(37):20041–6. Ma T, Zhao Y, Ruan K, Liu X, Zhang J, Guo Y, et al. Highly thermal conductivities, excellent mechanical robustness and flexibility, and outstanding thermal stabilities of aramid nanofiber composite papers with nacre-mimetic layered structures. ACS Appl Mater Interfaces 2019;12(1):1677–86. Song J, Chen C, Zhang Y. High thermal conductivity and stretchability of layer-bylayer assembled silicone rubber/graphene nanosheets multilayered films. Compos Part A Appl S 2018;105:1–8. Huang Y, Su Y, Li S, Ouyang Q, Zhang G, Zhang L, et al. Fabrication of graphite film/aluminum composites by vacuum hot pressing: process optimization and thermal conductivity. Compos B Eng 2016;107:43–50. Chung JY, Lee JG, Baek YK, Shin PW, Kim YK. Magnetic field-induced enhancement of thermal conductivities in polymer composites by linear clustering of spherical particles. Compos B Eng 2018;136:215–21. Guo Y, Ruan K, Yang X, Ma T, Kong J, Wu N, et al. Constructing fully carbon-based fillers with a hierarchical structure to fabricate highly thermally conductive polyimide nanocomposites. J Mater Chem C 2019;7(23):7035–44. Kuang Z, Chen Y, Lu Y, Liu L, Hu S, Wen S, et al. Fabrication of highly oriented hexagonal boron nitride nanosheet/elastomer nanocomposites with high thermal conductivity. Small 2015;11(14):1655–9. Chen J, Huang X, Sun B, Jiang P. Highly thermally conductive yet electrically insulating polymer/boron nitride nanosheets nanocomposite films for improved thermal management capability. ACS Nano 2019;13(1):337–45. Hu J, Huang Y, Yao Y, Pan G, Sun J, Zeng X, et al. Polymer composite with improved thermal conductivity by constructing a hierarchically ordered threedimensional interconnected network of BN. ACS Appl Mater Interfaces 2017;9(15): 13544–53. Wang X, Wu P. Preparation of highly thermally conductive polymer composite at low filler content via a self-assembly process between polystyrene microspheres and boron nitride nanosheets. ACS Appl Mater Interfaces 2017;9(23):19934–44. Kim G, Jang AR, Jeong HY, Lee Z, Kang DJ, Shin HS. Growth of high-crystalline, single-layer hexagonal boron nitride on recyclable platinum foil. Nano Lett 2013; 13(4):1834–9. Cheng JH, Xiao YC, Wu C, Chung TS. Chemical modification of P84 polyimide as anion-exchange membranes in a free-flow isoelectric focusing system for protein separation. Chem Eng J 2010;160(1):340–50. Yuan H, Wang Y, Li T, Wang Y, Ma P, Zhang H, et al. Fabrication of thermally conductive and electrically insulating polymer composites with isotropic thermal conductivity by constructing a three-dimensional interconnected network. Nanoscale 2019;11(23):11360–8. Singla P, Riyaz M, Singhal S, Goel N. Theoretical study of adsorption of amino acids on graphene and BN sheet in gas and aqueous phase with empirical DFT dispersion correction. Phys Chem Chem Phys 2016;18(7):5597–604. Rahman MM, Puthirath AB, Adumbumkulath A, Tsafack T, Robatjazi H, Barnes M, et al. Fiber reinforced layered dielectric nanocomposite. Adv Funct Mater 2019;29 (28):1900056. Sediri H, Pierucci D, Hajlaoui M, Henck H, Patriarche G, Dappe YJ, et al. Atomically sharp interface in an h-BN-epitaxial graphene van der Waals heterostructure. Sci Rep 2015;5:16465. Wang J, Wu Y, Xue Y, Liu D, Wang X, Hu X, et al. Super-compatible functional boron nitride nanosheets/polymer films with excellent mechanical properties and ultra-high thermal conductivity for thermal management. J Mater Chem C 2018;6 (6):1363–9. Cao JP, Zhao J, Zhao X, You F, Yu H, Hu G-H, et al. High thermal conductivity and high electrical resistivity of poly(vinylidene fluoride)/polystyrene blends by controlling the localization of hybrid fillers. Compos Sci Technol 2013;89:142–8. Xu L, Chen G, Wang W, Li L, Fang X. A facile assembly of polyimide/graphene coreshell structured nanocomposites with both high electrical and thermal conductivities. Composites A Appl S 2016;84:472–81. Zhang DL, Zha JW, Li WK, Li CQ, Wang SJ, Wen Y, et al. Enhanced thermal conductivity and mechanical property through boron nitride hot string in polyvinylidene fluoride fibers by electrospinning. Compos Sci Technol 2018;156: 1–7. Gu J, Lv Z, Wu Y, Guo Y, Tian L, Qiu H, et al. Dielectric thermally conductive boron nitride/polyimide composites with outstanding thermal stabilities via in-situ polymerization-electrospinning-hot press method. Compos Part A Appl S 2017;94: 209–16. Li TL, Hsu SLC. Enhanced thermal conductivity of polyimide films via a hybrid of micro-and nano-sized boron nitride. J Phys Chem B 2010;114(20):6825–9. Yang N, Xu C, Hou J, Yao Y, Zhang Q, Grami ME, et al. Preparation and properties of thermally conductive polyimide/boron nitride composites. RSC Adv 2016;6(22): 18279–87.
L. Cao et al.
Composites Part B 188 (2020) 107882 [43] Chen J, Huang X, Sun B, Wang Y, Zhu Y, Jiang P. Vertically aligned and interconnected boron nitride nanosheets for advanced flexible nanocomposite thermal interface materials. ACS Appl Mater Interfaces 2017;9(36):30909–17. [44] Zhang X, Zhang J, Xia L, Li C, Wang J, Xu F, et al. Simple and consecutive melt extrusion method to fabricate thermally conductive composites with highly oriented boron nitrides. ACS Appl Mater Interfaces 2017;9(27):22977–84. [45] Tang L, He M, Na X, Guan X, Zhang R, Zhang J, et al. Functionalized glass fibers cloth/spherical BN fillers/epoxy laminated composites with excellent thermal conductivities and electrical insulation properties. Compos Commun 2019;16: 5–10. [46] Chen J, Huang X, Sun B, Jiang P. Highly thermally conductive yet electrically insulating polymer/boron nitride nanosheets nanocomposite films for improved thermal management capability. ACS Nano 2018;13(1):337–45. [47] Yu J, Huang X, Wu C, Wu X, Wang G, Jiang P. Interfacial modification of boron nitride nanoplatelets for epoxy composites with improved thermal properties. Polymer 2012;53(2):471–80. [48] Yang X, Guo Y, Han Y, Li Y, Ma T, Chen M, et al. Significant improvement of thermal conductivities for BNNS/PVA composite films via electrospinning followed by hot-pressing technology. Compos B Eng 2019;175:107070. [49] Yang X, Guo Y, Luo X, Zheng N, Ma T, Tan J, et al. Self-healing, recoverable epoxy elastomers and their composites with desirable thermal conductivities by incorporating BN fillers via in-situ polymerization. Compos Sci Technol 2018;164: 59–64.
[36] Wang XB, Weng Q, Wang X, Li X, Zhang J, Liu F, et al. Biomass-directed synthesis of 20 g high-quality boron nitride nanosheets for thermoconductive polymeric composites. ACS Nano 2014;8(9):9081–8. [37] Hou J, Li G, Yang N, Qin L, Grami ME, Zhang Q, et al. Preparation and characterization of surface modified boron nitride epoxy composites with enhanced thermal conductivity. RSC Adv 2014;4(83):44282–90. [38] Zeng X, Yao Y, Gong Z, Wang F, Sun R, Xu J, et al. Ice-templated assembly strategy to construct 3D boron nitride nanosheet networks in polymer composites for thermal conductivity improvement. Small 2015;11(46):6205–13. [39] Huang T, Zeng X, Yao Y, Sun R, Meng F, Xu J, et al. Boron nitride@graphene oxide hybrids for epoxy composites with enhanced thermal conductivity. RSC Adv 2016; 6(42):35847–54. [40] Sato K, Horibe H, Shirai T, Hotta Y, Nakano H, Nagai H, et al. Thermally conductive composite films of hexagonal boron nitride and polyimide with affinityenhanced interfaces. J Mater Chem 2010;20(14):2749–52. [41] Yao Y, Sun J, Zeng X, Sun R, Xu JB, Wong CP. Construction of 3D skeleton for polymer composites achieving a high thermal conductivity. Small 2018;14(13): 1704044. [42] Li Q, Zhang G, Liu F, Han K, Gadinski MR, Xiong C, et al. Solution-processed ferroelectric terpolymer nanocomposites with high breakdown strength and energy density utilizing boron nitride nanosheets. Energy Environ Sci 2015;8(3):922–31.
10
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Preparation of highly thermally conductive and electrically insulating PI/ BNNSs nanocomposites by hot-pressing self-assembled PI/ BNNSs microspheres Lei Cao a, Jingjing Wang b, Jie Dong a, *, Xin Zhao a, Hai-Bei Li b, Qinghua Zhang a, ** a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, 201620, PR China School of Ocean, Shandong University, Weihai, 264209, PR China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyimide Boron nitride nanosheets Complex microspheres Hot-pressing Filler orientation
Traditional polymer-based thermally conductive composites with randomly distributed fillers always yield an undesired heat removal due to the lack of efficient heat transfer pathways. Thus, realization of rational and ordered distribution of thermally conductive nanofillers in polymer matrix is believed to be significant for obtaining a desirable thermal conductivity. Herein, a series of thermally conductive polyimide/boron nitride nanosheets (PI/BNNSs) composites with a highly ordered BNNSs network have been successfully prepared. For achieving an uniform dispersion and high orientation of BN nanosheets in PI matrix, self-assembled PI/BNNSs complex microspheres were firstly prepared via the van der Waals interaction, and then these complex micro spheres were further hot-pressed at the Tg of PI matrix, which rendered the alignment of BNNSs during the deformation of complex microspheres and built an efficient heat transfer pathway. As a consequence, the resultant composites possess a much higher in-plane thermal conductivity up to 4.25 W/mK with 12.4 vol% oriented BNNSs than those of pure PI and random distribution composite (0.85 W/mK for pure PI and 1.3 W/mK for the PI/random BNNSs-12.4). Meanwhile, these nanocomposites present excellent electrically insulating properties, improved dimensional stabilities and good thermal stabilities. This facile method provides a new way to design and fabricate highly thermally conductive PI-based composites for applying in heat dissipation of modern portable and collapsible electronic devices.
1. Introduction As modern electronic devices tend to be more integrated and mini mized with the increase of power density, efficient thermal management and effective heat removal have become a vital issue to ensure the reliability and lifetime of electronic devices [1,2]. Polyimide (PI)-based substrates are widely used as electronic packaging materials in advanced electronic systems, such as interlayer dielectric films and flexible printed circuits (FPC) because of their robustness, good thermal stability, low dielectric constant and loss and other exceptional properties [3,4]. However, the intrinsic thermal conductivity of pure PI only ranges from 0.1 to 0.4 W/mK [5], far below the requirement of thermally conductive materials, which limits its wide application in modern microelectronic industry. To address this heat dissipation issue, different types of highly
thermally conductive fillers have been incorporated into polymer matrix including metal oxides (Al2O3 [6], ZnO [7], etc.), carbon-based materials (carbon nanotube [8], graphene [9], etc.) and ceramic materials (BNNSs [10], AlN [11], etc.), to prepare thermally conductive polymer com posites. Generally, two main factors affect the thermal conductivity of resultant composites: one is thermal resistance at interface between polymer matrix and nanofiller, which should be minimized to facilitate smooth heat transfer in the materials; the second prominent factor lies in creating a thermally conductive network for efficient phonon transfer. Numerous works have revealed that improving affinity of nanofiller to polymer by modifying the surface of nanofillers could increase the heat transportation in polymer nanocomposites [12–14]. While, for the aro matic PI matrix, constructing thermally conductive network throughout the whole sample seems to be more difficult to be handled compared to
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Dong), [email protected] (Q. Zhang). https://doi.org/10.1016/j.compositesb.2020.107882 Received 29 November 2019; Received in revised form 5 February 2020; Accepted 13 February 2020 Available online 14 February 2020 1359-8368/© 2020 Published by Elsevier Ltd.
L. Cao et al.
Composites Part B 188 (2020) 107882
some flexible thermoplastic polymer matrixes. As a typical thermally conductive filler, BNNSs exhibit some attrac tive characters, including superhigh thermal conductivity, wide bad gap (around 5.9 eV) and large aspect ratio two-dimensional (2D) morphology, which have been regarded as the most ideal nanofiller and extensively explored in various polymer matrixes to create an improved heat conducting property and excellent electric insulation performance. However, it should be noticed that BNNSs exhibit an anisotropic char acter in thermal conductivity, namely, the in-plane thermal conductivity can reach as high as 2000 W/mK, while that declines to only a few W/ mK in the thickness direction. Accordingly, the heat transportation of BNNSs-based composites greatly depends on the alignment of BNNSs in the polymer matrix. For achieving rational control of alignment of BNNSs in polymer matrix, a verity of strategies including layer-by-layer [15], hot-pressing [16], electrical/magnetic field [17] and electro spinning [18] as well as strong shearing [19] have been devoted to regulate the orientation of this 2D nanofiller. For example, Chen et al. successfully constructed polyvinylidene fluoride (PVDF)/BNNSs ther mally conductive nanocomposites by electrospinning polymer/BNNSs nanocomposite fibers, vertically folding electrospun nanofibers and subsequent hot-pressing. The BNNSs could stack in sequence along the oriented direction of fibers, resulting high alignment of BNNSs in the resultant nanocomposites and endowing them with a high thermal conductivity of 16.3 W/mK with a 33 wt% filler loading [20]. Hu et al. fabricated an epoxy/BNNSs composite through a combination of ice-templating self-assembly and infiltration method for achieving a well-aligned BNNSs platelets, which possessed a thermal conductivity up to 4.42 W/mK at a filler loading of 34 vol% (gaining 2226% in thermal conductivity enhancement) [21]. The above studies fully indi cate that these methods can effectively induce the BNNSs orientation and construct thermally conductive network, resulting in a great enhancement in thermal conductivity. However, the operation of these methods is relatively complex and needs multiple processes; besides, high loading fractions are still required to achieve a desired thermal conductivity and the filler aggregation easily takes place at a high filler loading. As a simple alternative method, Wang et al. [22] proposed that realization of selective ordered distribution of nanofillers in an immis cible polymer blend with a cocontinuous structure could reduce the thermal percolation threshold to some extent and prepare thermally conductive polymer nanocomposites. Specially, Wang et al. demon strated that a selective distribution of nanofillers at the interface of composites was a more ideal approach for achieving a high thermal conductivity enhancement in relative to the continuous distribution of nanofillers in the polymer phase [22]. They successfully distributed the BNNSs at the interfacial area of polystyrene (PS) accurately via an initial formation of PS/BNNSs composite microspheres and a following hot-pressing. These two steps resulted in a homogeneous dispersion of thermally conductive nanofiller at the interface of deformed PS phases and meanwhile promoted them highly aligned along the in-plane di rection, thus forming a thermally conductive network at a low filler content. The prepared nanocomposite exhibited a high thermal con ductivity of 8.0 W/mK at a filler content of only 13.4 vol%. As we know, common methods for fabricating PI-based thermally conductive nano composites mainly include simple solution blending, electrospinning and freeze-drying approaches, etc. Preparing PI/BNNSs nanocomposites via the selective distribution of BNNSs nanofillers at the interface of polyimide blends and forming continuous thermally conductive net works has been rarely reported. In present work, we propose a facile method for preparing highly thermally conductive PI/BNNSs composites. Firstly, self-assembled PI/ BNNSs complex microspheres were directly prepared via the van der Waals interaction between polyimide matrix and nanofiller; subse quently, hot-pressing was adopted to prepare nanocomposite films and rendered BNNSs homogeneously distributed and aligned well at the interface of deformed PI microspheres, making the resultant composites
exhibit a high in-plane thermal conductivity of 4.2 W/mK at a 12.4 vol% filler loading. Moreover, the prepared nanocomposites have superior electrically insulating properties with the volume resistivity of ~1014 Ω cm, comparable to that of the pristine PI matrix. Comparatively, the present method has the advantages of simplicity and adaptability for a commercial scaleup, which may find utilization in preparing thermally conductive polyimide materials in modern electronic devices. 2. Experimental section Materials. BN powder was purchased from Alpha Aesar (China) Chemical Co. Ltd (Shanghai, China). Polyimide (PI, Ultem 1010®) was purchased from Saudi Basic Industries Corporation (SABIC). Polyvinyl alcohol (PVA, degree of hydrolysis ¼ 88%), N,N0 -dimethylacetamide (DMAc) and isopropanol (IPA) were all purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and used as received. Preparation of BNNSs: BNNS nanosheets were prepared via a method combining sonication-assisted liquid phase exfoliation and centrifugation. In detail, 2 g of BN powder was added into the 200 mL IPA. The dispersion was sonicated for 12 h with a frequency of 53 kHz. The resultant suspension was centrifuged at 2000 rpm for 15 min to remove nonexfoliated BN. A homogeneous BNNSs dispersion in IPA (0.4 mg/mL) could be obtained for further use. Fabrication of polyimide microspheres: PI microspheres were fabricated by a reprecipitation method. Typically, 10 g PI and 10 g PVA were dissolved in 200 mL DMAc in a 1000 mL round-bottom flask. 400 mL deionized water was added dropwise into the mixed DMAc solution under vigorous stirring at 500 rpm. The dispersion was stirred for another 2 h to ensure sufficient precipitation of PI microspheres after 400 mL deionized water was added into the DMAc solution. Finally, PI microspheres were obtained by centrifugation at a 8000 rpm and washed with deionized water followed drying in vacuum oven at 60 � C. Preparation of PI/BNNSs complex microspheres: PI microspheres (0.5 g) were redispersed in IPA (100 mL) by ultrasonication for 3 min. The PI/BNNSs complex microspheres were fabricated by dropwise adding the BNNSs/IPA dispersion into the PI microspheres/IPA disper sion under vigorous stirring. After 2 h, PI/BNNS complex microspheres were collected by vacuum filtration and dried at 60 � C. A series of complex microspheres were prepared with the BNNSs loading between 0 and 12.4 vol%。 Preparation of oriented/random PI/BNNSs composite films: The schematic illustration of preparing PI/BNNSs composite films is shown in Scheme 1. The as-prepared PI/BNNSs complex microspheres were hot-pressed at 230 � C for 20 min with a pressure of 20 MPa. For obtaining a dense microstructure, the composites were further coldpressed for 5 min at room temperature with a pressure of 20 MPa. In present work, the thickness of all composite films was controlled to ~250 μm. The obtained composites were denoted as PI/oriented BNNSsx, where x refers to volume fraction of fillers in the composite. For comparison, randomly dispersed BNNSs in polyimide composite films were also prepared by a solution blending method and denoted as PI/random BNNSs-x. As a typical example, the PI/random BNNSs-12.4 was synthesized as following: 8 g PI was thoroughly dissolved in DMAc in a three-necked flask equipped with a mechanical stirrer at room temperature under N2. 2 g exfoliated BNNSs were dispersed in DMAc with the assistance of ultrasonic vibration prior to mixing with the above PI solution. Then, these two solutions were mixed and continuously stirred at room for 6 h to obtain a homogeneous viscous PI/ BNNSs solution. The above mixture was cast on a glass substrate by a scalpel. The casted film was thermally treated in an oven at 80 � C, annealing for 6 h to remove the solvent, and finally at 230 � C for 20 min. The obtained solid composite film can be denoted PI/random BNNSs12.4 with the BNNSs volume content of 12.4 vol%. The thickness of the resultant films was kept to 200–250 μm. Other composites were prepared by a similar procedure. Characterization: The SEM images were observed in Hitachi S2
L. Cao et al.
Composites Part B 188 (2020) 107882
Scheme 1. Schematic illustration of preparing PI/oriented BNNSs composites.
4800. The microstructures of BNNSs were also analyzed by the trans mission electron microscope (TEM, JEM-2010F) at an acceleration voltage of 200 kV and atomic force microscope (AFM, Bruker Dimension Edge). Size distribution of polyimide microspheres was determined by the nanoparticle size analyzer (Malvern). Zeta potentials of BNNSs and PI in IPA were investigated on Zetasizer Nano-ZS90. The alignment structure of BNNSs in the composites films was measured by X-ray diffraction (XRD, Rigaku D/max-2550) and 16B1 Beamline in Shanghai Synchrotron Radiation Facility (SSRF). Thermal conductivity (K) of these composites was measured using the laser flash technique (NETZSCH, LFA 467 Nano-Flash) and calculated as: K ¼ α � Cp � ρ, where ρ is the density of the composites, Cp represents the specific heat capacity, α refers to the thermal diffusivity. The surface temperature of the composites was recorded by an infrared thermograph (Fotric 255s, China). Dielectric properties of the composites were measured by a broadband dielectric spectrometer (Novocontrol concept 40). The vol ume resistivity of the PI/BNNSs composites was obtained using a elec trometer (Keithley 6517B). The coefficient of thermal expansion (CTE) of prepared nanocomposites was measured using the dynamic thermo mechanical analyzer (DMA, TA Q800) with the control force mode at a heating rate of 5 � C/min from 40 to 200 � C. Thermal gravimetric anal ysis (TGA) was carried out in nitrogen atmospheres at a heating rate of 5 � C/min from 40 to 900 � C by a Netzsch-TGA instrument (NETZSCH TG 209 F1). The glass transition temperature of the composites was ob tained using differential scanning calorimetry (DSC, TA Q20) at a heating rate of 10 � C/min under nitrogen atmosphere. In modeling the interfacial interaction of the PI/BNNSs complex, an polyimide unit and a roughly 29 Å � 23 Å ribbon were firstly constructed and then were fully optimized using the all-electron density functional theory (DFT) equa tions implemented in the Gaussian 09 package with the B3LYP func tional of generalized gradient approximation and the 6-31G basis set.
ultrathin feature. The inserted high resolution TEM observed from the edge of a BN nanosheet illustrates that 5–7 layers of BNNSs are stacked together. AFM was further used to confirm the thickness of exfoliated BNNSs as revealed in Fig. 1(B), and the detailed height (Fig. S1) illus trates the thickness of ~3.5 nm for BNNSs, which agrees well with the TEM result (thickness of the single layer BN nanosheet is around 0.5 nm [23]). Additionally, the ζ-potential of exfoliated BNNSs in IPA is esti mated to 31 mV, which is consistent with the previously reported result [22]. Synthetic polymer microspheres are usually prepared during poly mer synthesis from a monomer, such as in suspension, emulsion, and dispersion polymerization. In this work, we adopted a method for pre paring polyimide microspheres from the commercial amorphous Ultem 1010®, in which the precipitant water was utilized for creating the liquid-liquid phase separation and the stabilizer PVA was added to so lution for shaping the precipitate into microspheres. Fig. 1(C) shows the monodisperse PI microspheres with a relatively uniform particle size of ~3.0 μm. Besides, a smooth surface morphology can be observed for the pure PI microspheres. According to Fig. 1(D), the statistical distribution of diameter of PI microspheres is mainly ranging from 2–5 μm. The ζ-potential value is around 18 mV for the synthesized PI microspheres dispersed in IPA (comparable to the reported value [24]), which seems indicating the inaccessible self-assembly with the negatively charged BN nanosheets. Interestingly, when mixing the BNNSs and PI microspheres dispersion solutions in IPA together, BNNSs can tightly absorb on the surface of PI microspheres. Fig. 1(E) and (F) shows SEM micrographs of PI/BNNSs complex microspheres with 5.9 vol% and 12.4 vol% BNNSs loadings. Clearly, the complex microspheres exhibits rough surfaces due to the wrapped BNNSs, and majority of PI microspheres is coated by the BNNSs as increasing the filler feeding to 12.4 vol%. The PI/BNNSs complex microspheres attaching 2.9 vol% and 9.1 vol% BN nanosheets also exhibit a uniform BNNSs dispersion, as revealed in Fig. S2. As a comparison, previously reported polystyrene (PS)/BNNSs [22] or PS/GO [25] complex microspheres were mainly prepared by the self-assembly process between polymer microspheres and nanofillers based on the electrostatic adsorption, in which the polymer micro spheres or nanofillers should be modified by surface pretreatment. However, in our case, both negatively charged BNNSs nanosheets and PI microspheres can directly self-assemble to form complex microspheres, indicating its convenience and simplicity. In addition, this fact also suggests that there exists some other factors driving the assembly pro cess of these two components. As illustrated in Fig. 2(A), BNNSs show a well-dispersed white colloidal suspension in IPA and can keep this stable state for a week.
3. Results and discussion The hexagonal boron nitride (h-BN) is a flake-shaped synthetic ceramic, which has a crystal structure analogous to graphite and consists of layers of six-membered rings of covalently bonded nitrogen and boron atoms weakly bound by van der Waals force. The thermal conductivity of unexfoliated h-BN with multi-layer stacks is around 200 W/mK in the planar direction. However, after exfoliation, the thermal conductivity can reach as high as 2000 W/mK for the single/few layer BNNSs [19]. Herein, the BNNSs were prepared using the liquid sonication technique. In Fig. 1(A), the TEM micrograph shows that exfoliated BNNSs have a lateral size around 500 nm and are slightly transparent, indicating their 3
L. Cao et al.
Composites Part B 188 (2020) 107882
Fig. 1. (A) TEM micrograph of BNNSs exfoliated from h-BN powder, and the inset is the high-resolution TEM image of BNNSs edge; (B) AFM image of BNNSs; (C) SEM image of pure PI microspheres; (D) size distribution of PI microspheres; (E, F) SEM micrographs of PI/BNNSs complex microspheres with the BNNSs loadings of 5.9 vol% and 12.4 vol%, respectively.
However, as adding PI microspheres, the PI/BNNSs complex micro spheres can quickly settle to the bottom after 24 h. For better under standing the reason for such self-assembly process between PI microspheres and BNNSs, we used density functional theory (DFT) to explore the adsorption behavior between them based on an optimized polyimide unit, boron unit and polyimide-BNNSs unit. Details of the interfacial chemistry extracted from the ab initio model are shown in Fig. 2, which can be assumed to be qualitatively valid at a larger scale equally. In Fig. 2(B), BNNSs nanosheets roll up to get closer to the hetero-atoms of polyimide unit and enhance their contact area. This deformation could be an artefact due to the BNNSs nanosheets were modelled by a cluster approach. The actual periodic systems will not present this deformation and previous report has illustrated that this structural deformity do not impact the calculated interaction energies between BNNSs and other chemicals [26]. Clearly, the polyimide unit prefers to orient itself almost perfectly parallel to the boron nitride ribbon, and the nearest atom distance and average ground state
interfacial distance between polyimide unit and BNNSs nanosheets are calculated to be 3.15 and 3.24 Å, respectively, suggesting that the adsorption is physical in nature (van der Waals (vdW) interaction). The calculated adsorption energy (ΔE) for BNNSs sheets on polyimide unit is around 59.8 kcal/mol, which is larger in comparison with the reported ΔE values for the poly-paraphenylene terephthalamide (PPTA)/BNNS [27] and graphene/BNNS [28] complexes. First, it is well known that h-BN powder can be effectively exfoliated via a sonication-assisted liquid phase exfoliation method. In this process, the BNNSs are easily edge-hydroxylated and polar O–H groups can form and attach on the surface of BN nanosheets [29]. It could be speculated that after mixing, a hydrogen bonding interaction could be formed between the abundant carbonyl groups in imide rings and the polar hydroxy groups in BNNSs, which benefits increasing the compatibility between BN nanosheets and PI polymer chains. In addition, the polar B–N bonds in BNNSs and carbonyl and imide groups in polyimide are beneficial for forming the strong adsorption by the dipole-dipole interaction. Furthermore,
Fig. 2. (A) Photographs of BNNSs in IPA before (1#) and after (2#) addition of PI microspheres; (B) polyimide unit and boron nitride sheet are brought together and optimized through density function theory (DFT). The calculated interfacial region and the interaction energy show van der Waals interaction at the interface. 4
L. Cao et al.
Composites Part B 188 (2020) 107882
abundant six-membered phenyl rings and rigid imide rings in PI back bone endow an interfacial chemistry similar to that of adjacent layers of BNNSs nanosheets, which are also in favor of increasing the vdW interaction. Therefore, this strong interfacial interaction accesses the self-assembly between BNNSs and PI microspheres. The as obtained PI/BNNSs complex microspheres were further pre pared into composite films by a hot-pressing technology at 230 � C, slightly higher than the glass transition temperature of Ultem 1010® (Tg ¼ 214 � C), under a pressure of 20 MPa. The microstructures of com pressed pure PI, PI/random BNNSs and PI/oriented BNNSs composites with different filler loadings are revealed in SEM images. As depicted in Fig. 3(A), the pure PI shows a dense and smooth cross-section morphology, while for the PI/random BNNSs-12.4, the microstructure is chaotic and BNNSs aggregation can be observed (as marked in the elliptical area). Comparatively, Fig. 3(C-E) illustrate that the hotpressing method imparts an advantageous BNNSs in-plane orientation in composites derived from the PI/BNNSs complex microspheres. Additionally, the magnified cross-section image of PI/oriented BNNSs containing 12.4 vol% BNNSs in Fig. 3(E) exhibits a homogeneous dispersion and directly connected BN nanosheets in the PI matrix, which is mainly attributed to the uniform hybridization of PI microspheres with BNNSs in advance. Actually, under the present hot-pressing tem perature, PI molecular chains have an increased mobility and micro spheres are easily to deform but can not be melted. With the deformation of microspheres, these BNNSs are directed to distribute along the interface of microspheres and can not penetrate into PI phase (as depicted in Fig. S3). Accordingly, a continuous heat transportation path can be constructed by the oriented and connected BNNSs nanosheets as the BNNSs content increasing to a certain value. Similar results have been reported in many polymer nanocomposites, such as PVDF/PS/ MWCNT [30], PI/r-GO [31] and PI/ZnO [7], etc. For further verifying the anisotropic alignment of BNNSs in these composites, X-ray charac terization was recorded as shown in Fig. 3(F). The pure PI exhibits a typical amorphous nature. While, for the nanocomposites, two evident peaks (2θ ¼ 26.5� and 43.1� ) representing the (002) and (100) lattice planes of BNNSs are easily found. Previous studies have manifested that the intensity ratio of I002/I100 elucidates the orientation of BN in poly mer composites [32]. It is noticed that the I002/I100 ratio of the PI/ran dom BNNSs-12.4 composite is only 5.2, whereas for PI/oriented BNNSs-12.4, this ratio sharply increases to 139, indicating a much higher in-plane orientation of BNNSs nanosheets perpendicular to
hot-pressing direction. Besides, by comparing different PI/oriented BNNSs samples, increasing the BNNSs content results in a little higher BNNSs orientation in the composites. Fig. 4 shows the thermal transportation properties of the resultant PI/BNNSs nanocomposites. The in-plane and through-plane thermal conductivity of composites were measured by a laser-flash system. As a comparison, PI/random BNNSs composites with different BNNSs load ings were also investigated. As shown in Fig. 4(A), the in-plane thermal conductivity of pure PI is only 0.85 W/mK. As adding BNNSs, the inplane thermal conductivities of these two series of composites both have obvious enhancements with increasing BNNSs content. Compara tively, the thermal conductivity of PI/oriented BNNSs composites grows much faster that of PI/random BNNSs samples, which mainly attributes from the well-aligned and continuous BNNSs network. In particular, the thermal conductivity is significantly increased to 4.25 W/mK with incorporation of oriented 12.4 vol% BNNSs into the PI matrix, which is around two times higher than that of the PI/random BNNSs-12.4. Meanwhile, the thermal conductivity enhancement (TCE¼(Kc-Kp)/Kp � 100%, where Kc and Kp represent the thermal conductivity of com posites and pure polymer matrix) has been introduced to quantitatively characterize the improving efficiency of BNNSs on the PI matrix. As depicted in Fig. 4(B), the PI/oriented BNNSs-12.4 reveals a remarkable enhancement of in-plane thermal conductivity of ~400% as compared to the pure PI, whereas the PI/random BNNSs-12.4 sample only exhibits an enhancement of 62.3%, further suggesting significant impact of orientation and internal connection on the in-plane thermal conduc tivity of the composites. Additionally, as shown in Fig. 4(B), the TCE values for PI/random BNNSs composites fabricated by the solution blending are all below 100%. Thus, in the conventional method, higher filler content is always required to construct a continuous heat trans portation network in composites so as to obtain a high in-plane thermal conductivity. As depicted in Fig. 4(C), even incorporating 21.2 vol% BNNSs into PI matrix by a conventional solution blending method in previous works, the thermal conductivity values are still below 1.5 W/ mK [33–35]. The previously reported thermal conductivities of BN-based polymer composites are summarized in Fig. 4(C). Clearly, the PI/oriented BNNSs-12.4 exhibits a higher thermal conductivity compared to the previous investigations focused on the EP/BN [36–39], NR/BN [19], PI/BN [40], EP/BN-rGO [41], PVDF/BN [42] and PDMS/BN [43] as well as HDPE/BN [44] nanocomposites, implying the aligned and continuous BNNSs network in present work has a prominent
Fig. 3. Microstructure characterization of PI/BNNSs composites. SEM fracture images of pure PI (A), PI/random BNNSs-12.4 (B), PI/oriented BNNSs-5.9 (C), PI/ oriented BNNSs-12.4 (D, E), and (F) XRD patterns of the PI/BNNSs composites with different BNNSs loadings. 5
L. Cao et al.
Composites Part B 188 (2020) 107882
Fig. 4. (A) In-plane thermal conductivity of the PI/oriented BNNSs and PI/random BNNSs nanocomposites with different BNNSs loadings; (B) comparison of thermal conductivity enhancement between the PI/oriented BNNSs and the PI/random BNNSs composites as a function of BNNSs loading; (C) comprehensive comparison of thermal conductivity versus nanofiller loadings between PI/oriented BNNSs-12.4 in this work and reported polymer/BN composites; (D) the diagram of in-plane transfer of heat flow in the PI/oriented BNNSs nanocomposites; (E) infrared thermal images of pure PI, PI/random BNNSs-12.4 and PI/oriented BNNSs-12.4 composites and (F) the surface temperature variations of nanocomposites versus time.
advantage in boosting heat transfer capability of PI-based nano composites. As illustrated in Fig. 4(D), the homogeneously attached BNNSs on surface of PI microspheres via the van der Waals interaction and subsequent hot-pressing can easily result in an uniform distribution and orientation of BNNSs along the in-plane direction in composites, which form a thermally conductive network throughout the whole
sample and heat can readily transfer along the shortest transfer chan nels. The effects of BNNSs on the through-plane thermal conductivity of PI/BNNSs composites were also investigated (Fig. S4). It is clear that the through-plane thermal conductivities of PI/oriented BNNSs composites are much lower than those along the in-plane direction. For example, even incorporating 12.4 vol% BNNSs nanosheets, the through-plane 6
L. Cao et al.
Composites Part B 188 (2020) 107882
thermal conductivity increases from 0.26 W/mK to 0.40 W/mK with a enhancement of only 54%, indicating an anisotropic character in ther mal transportation of PI/oriented BNNSs composites. Contrary to the thermal conductivity along in-plane direction, PI/random BNNSs com posites possess higher through-plane thermal conductivities in relative to PI/oriented BNNSs samples under all filler contents, which is due to the fact that there are many BNNSs nanosheets parallel to the through-plane direction in the PI/random BNNSs composites. As is well known, BNNSs exhibit an anisotropic character in thermal conductivity, namely, the in-plane thermal conductivity can reach as high as 2000 W/mK, while, it declines to only a few W/mK in the thickness direction. Therefore, more BNNSs nanosheets parallel to the through-plane di rection in PI/random BNNSs composites result in a higher through-plane thermal conductivity. To demonstrate the potential application of these materials in flex ible electronics, the prepared pure PI, PI/oriented BNNSs-12.4 and PI/ random BNNSs-12.4 composites were utilized as substrates to support light-emitting-diode (LED) chips using silver paste (Fig. 4(E)). Surface temperature variations of substrates in the center with LED working time were measured by a portable thermal imaging camera. It can be observed that after a steady-state (180 s) of the same flow caused by the equal standard LED chips, the temperature distribution is excessively concentrated to generate hot spots on pure PI and PI/random BNNSs12.4 specimens. However, the temperature distribution is uniform for the PI/oriented BNNSs-12.4 composite with a relatively lower center spot temperature of 48 � C, which has dropped by about 12 and 5 � C in comparison with the pure PI film and the composite containing
randomly dispersed BNNSs, respectively. As shown in Fig. 4(F), throughout the operation process, the PI/oriented BNNSs-12.4 displays an obvious slower temperature rise and lower stable temperature than those of other two samples, further illustrating a better heat dissipation in PI/BNNSs composites containing highly aligned BNNSs. For electrical and electronic applications, the reliable dielectric and electrical insulation performances are usually also crucial importance [45]. The frequency-dependent dielectric properties of composite films with various BNNSs loadings are presented in Fig. 5(A) and (B). The dielectric constant of pure PI is around 3.0 at 1 MHz under an ambient condition. As incorporating oriented BNNSs, these composites exhibit increased dielectric constant and dielectric loss mainly attributed to the higher intrinsic dielectric constant of BNNSs (~4.0 [33]). For the com posite containing 12.4 vol% oriented BNNSs nanosheet, the dielectric constant and tan δ are 3.48 and 0.02 at 1 MHz, respectively, which still can satisfy the requirements of most electronic applications. As depicted in Fig. 5(C), the PI/oriented BNNSs-12.4 composite exhibits thermally stable dielectric constants at temperatures ranging from 25 to 150 � C, which benefit to prevent charge transfer or “crosstalk” between con ducting materials. However, it presents a slight fluctuation at the low frequency region in the dielectric constant as rising the temperature to 200 � C. Fig. 5(D) presents the volume electrical resistivity of nano composites. Clearly, no obvious fluctuations in the volume resistivity have been detected after the addition of BNNSs. For example, the vol ume resistivity of PI/oriented BNNSs-12.4 still reaches as high as ~1014 Ω cm, which is far beyond the required resistance for electrical insu lation (~109 Ω cm). Overall, the prepared dielectric thermally
Fig. 5. Frequency-dependent dielectric constant (A) and dielectric loss tangent (B) of different PI/BNNSs nanocomposites, (C) dielectric constant of PI/oriented BNNSs-12.4 at different temperatures and (D) volume electrical resistivity of the nanocomposites with different BNNSs loadings. 7
L. Cao et al.
Composites Part B 188 (2020) 107882
conductive PI/oriented BNNSs composites show an excellent integrated electrical property, which will ensure their reliability and lifetime uti lized in modern electrical systems and electronic devices. It is well-known that, for the insulting substrate in electronic devices, a poor dimensional stability and low thermal stability will bring severe impacts on their functionality. Thus, herein, we examined the dimension and thermal reliability of these nanocomposites. As illustrated in Fig. 6 (A), the PI/oriented BNNSs composites exhibit a distinct reduce in displacement during the measurement from 100 to 160 � C under a preloaded force of 0.02 N, indicating the improved dimensional stabil ity. Detailed CTE values of prepared nanocomposites in different tem perature regions are listed in Fig. 6(B). Obviously, the composites with oriented BNNSs present lower CTE values in relative to the pure PI. For example, the PI/oriented BNNSs-12.4 exhibits a CTE of 20 ppm K 1 in 140–160 � C, while the value is as high as 56 ppm K 1 for the pure PI. It also demonstrates that increasing the BNNSs loading will continuously reduce the thermal expansion behavior of nanocomposites, which can be accounted by that well-aligned BNNSs structure is beneficial for stabi lizing the whole framework of composites. Effects of oriented BNNSs nanosheets on glass transition temperature (Tg) of PI matrix were also investigated. Fig. 6(C) presents the dependence of Tg on BNNSs loadings. It is interesting to find that all of prepared composites exhibit a decreasing trend on Tg as a function of BNNSs loading, which is kindly diverse from previous reports [46,47]. In detail, the pure PI exhibits a Tg at 214 � C, which gradually decreases to 198 � C as incorporating 12.4 vol
% BNNSs. Similar results have been reported in Hu’s work as they investigated the epoxy/BN nanocomposites, and they accounted this by a fact that polymer chains surrounding BN microplatelets could easily change their conformation and additional free volume could be intro duced into the composites as incorporating BNNSs nanosheets, both of which have a synergistic effect on reducing the Tg [21]. In our case, the reduced Tg may be caused by a similar reason. TGA analysis was carried out under nitrogen to investigate the thermal stability of PI/BNNSs composites with different filler loadings. The preliminary results in Fig. 6(D) show that all samples exhibit 5%-weight-loss temperatures (Td5) and maximum-weight-loss temperatures (Tmax) over 443 and 491 � C, respectively, indicating their excellent thermal stabilities, which have a significance for these materials applied as electronic substrates in high-temperature circumstances. Gu et al. [33,48,49] reported the “heat-resistance index” (THRI) was another parameter that could reflect and reveal the “thermal stability” of polymers or polymer composites. Based on their previous studies, THRI values of PI-based nanocomposites in this work were also calculated and results were listed in the inserted table, which ranged from 245.3 � C to 254.6 � C and seemed that the introduction of BN inclined to improve the stability of resultant composites. 4. Conclusions In summary, we have successfully fabricated highly thermally
Figure 6. (A, B) Displacement distance and CTE values of pure PI and PI/BNNSs nanocomposites in the heating process, (C, D) DSC and TGA heating curves of nanocomposites with different BNNSs loadings. 8
Composites Part B 188 (2020) 107882
L. Cao et al.
conductive PI/BNNSs composites by combining self-assembled PI/ BNNSs complex microspheres via van der Waals interaction and subse quent hot-pressing technology around the Tg of PI matrix. The formation of connected and aligned BNNSs structure accompanied by the defor mation of PI microspheres acts as an efficient heat transfer pathway. Thus, the obtained composites possess a favorable in-plane thermal conductivity of around 4.25 W/mK with a BNNSs loading of 12.4 vol%, which is more than four times as high as that of the pure PI and much more superior to that of random distribution composites. Moreover, the well-aligned BNNSs has been demonstrated to be beneficial for stabi lizing the whole framework of composites and led to the improved dimensional stability, excellent electrically insulating and thermalstable dielectric properties. It is hoped that this work can provide some guidelines for future designs of thermally conductive PI-based thermal management materials for application in advanced electronic devices, which can meet the increased needs of heat dissipation.
[10] [11] [12]
[13] [14]
[15] [16]
Declaration of competing interest
[17]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[18]
CRediT authorship contribution statement
[19]
Lei Cao: Conceptualization, Methodology, Writing - original draft. Jingjing Wang: Software. Jie Dong: Investigation, Resources, Visuali zation. Xin Zhao: Formal analysis, Data curation, Funding acquisition. Hai-Bei Li: Software, Writing - review & editing. Qinghua Zhang: Su pervision, Project administration, Funding acquisition.
[20] [21]
[22]
Acknowledgements
[23]
This work was supported by National Natural Science Foundation of China (No. 21774019, 51903038), the Program of Shanghai Academic Research Leader (18XD1400100), the Scientific Research Innovation Plan of Shanghai Education Commission (2019-01-07-00-03-E00001) and the Fundamental Research Funds for the Central Universities (2232019G-02).
[24] [25]
Appendix A. Supplementary data
[26]
Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2020.107882.
[27]
References
[28]
[1] Burger N, Laachachi A, Ferriol M, Lutz M, Toniazzo V, Ruch D. Review of thermal conductivity in composites: mechanisms, parameters and theory. Prog Polym Sci 2016;61:1–28. [2] Xu X, Chen J, Zhou J, Li B. Thermal conductivity of polymers and their nanocomposites. Adv Mater 2018;30(17):1705544. [3] Yuan C, Jin K, Li K, Diao S, Tong J, Fang Q. Non-porous low-κ dielectric films based on a new structural amorphous fluoropolymer. Adv Mater 2013;25(35):4875–8. [4] Kim JW, Lee AS, Yu S, Han JW. En masse pyrolysis of flexible printed circuit board wastes quantitatively yielding environmental resources. J Hazard Mater 2018;342: 51–7. [5] Tanimoto M, Yamagata T, Miyata K, Ando S. Anisotropic thermal diffusivity of hexagonal boron nitride-filled polyimide films: effects of filler particle size, aggregation, orientation, and polymer chain rigidity. ACS Appl Mater Interfaces 2013;5(10):4374–82. [6] Choi S, Kim J. Thermal conductivity of epoxy composites with a binary-particle system of aluminum oxide and aluminum nitride fillers. Compos B Eng 2013;51: 140–7. [7] Yorifuji D, Ando S. Enhanced thermal conductivity over percolation threshold in polyimide blend films containing ZnO nano-pyramidal particles: advantage of vertical double percolation structure. J Mater Chem 2011;21(12):4402–7. [8] Jiang Q, Wang X, Zhu Y, Hui D, Qiu Y. Mechanical, electrical and thermal properties of aligned carbon nanotube/polyimide composites. Compos B Eng 2014; 56:408–12. [9] Guo Y, Xu G, Yang X, Ruan K, Ma T, Zhang Q, et al. Significantly enhanced and precisely modeled thermal conductivity in polyimide nanocomposites with
[29]
[30] [31] [32]
[33]
[34] [35]
9
chemically modified graphene via in situ polymerization and electrospinning-hot press technology. J Mater Chem C 2018;6(12):3004–15. Guo Y, Lyu Z, Yang X, Lu Y, Ruan K, Wu Y, et al. Enhanced thermal conductivities and decreased thermal resistances of functionalized boron nitride/polyimide composites. Compos B Eng 2019;164:732–9. Xie SH, Zhu BK, Li JB, Wei XZ, Xu ZK. Preparation and properties of polyimide/ aluminum nitride composites. Polym Test 2004;23(7):797–801. Cho HB, Tokoi Y, Tanaka S, Suematsu H, Suzuki T, Jiang W, et al. Modification of BN nanosheets and their thermal conducting properties in nanocomposite film with polysiloxane according to the orientation of BN. Compos Sci Technol 2011;71(8): 1046–52. Lee HL, Kwon OH, Ha SM, Kim BG, Kim YS, Won JC, et al. Thermal conductivity improvement of surface-enhanced polyetherimide (PEI) composites using polyimide-coated h-BN particles. Phys Chem Chem Phys 2014;16(37):20041–6. Ma T, Zhao Y, Ruan K, Liu X, Zhang J, Guo Y, et al. Highly thermal conductivities, excellent mechanical robustness and flexibility, and outstanding thermal stabilities of aramid nanofiber composite papers with nacre-mimetic layered structures. ACS Appl Mater Interfaces 2019;12(1):1677–86. Song J, Chen C, Zhang Y. High thermal conductivity and stretchability of layer-bylayer assembled silicone rubber/graphene nanosheets multilayered films. Compos Part A Appl S 2018;105:1–8. Huang Y, Su Y, Li S, Ouyang Q, Zhang G, Zhang L, et al. Fabrication of graphite film/aluminum composites by vacuum hot pressing: process optimization and thermal conductivity. Compos B Eng 2016;107:43–50. Chung JY, Lee JG, Baek YK, Shin PW, Kim YK. Magnetic field-induced enhancement of thermal conductivities in polymer composites by linear clustering of spherical particles. Compos B Eng 2018;136:215–21. Guo Y, Ruan K, Yang X, Ma T, Kong J, Wu N, et al. Constructing fully carbon-based fillers with a hierarchical structure to fabricate highly thermally conductive polyimide nanocomposites. J Mater Chem C 2019;7(23):7035–44. Kuang Z, Chen Y, Lu Y, Liu L, Hu S, Wen S, et al. Fabrication of highly oriented hexagonal boron nitride nanosheet/elastomer nanocomposites with high thermal conductivity. Small 2015;11(14):1655–9. Chen J, Huang X, Sun B, Jiang P. Highly thermally conductive yet electrically insulating polymer/boron nitride nanosheets nanocomposite films for improved thermal management capability. ACS Nano 2019;13(1):337–45. Hu J, Huang Y, Yao Y, Pan G, Sun J, Zeng X, et al. Polymer composite with improved thermal conductivity by constructing a hierarchically ordered threedimensional interconnected network of BN. ACS Appl Mater Interfaces 2017;9(15): 13544–53. Wang X, Wu P. Preparation of highly thermally conductive polymer composite at low filler content via a self-assembly process between polystyrene microspheres and boron nitride nanosheets. ACS Appl Mater Interfaces 2017;9(23):19934–44. Kim G, Jang AR, Jeong HY, Lee Z, Kang DJ, Shin HS. Growth of high-crystalline, single-layer hexagonal boron nitride on recyclable platinum foil. Nano Lett 2013; 13(4):1834–9. Cheng JH, Xiao YC, Wu C, Chung TS. Chemical modification of P84 polyimide as anion-exchange membranes in a free-flow isoelectric focusing system for protein separation. Chem Eng J 2010;160(1):340–50. Yuan H, Wang Y, Li T, Wang Y, Ma P, Zhang H, et al. Fabrication of thermally conductive and electrically insulating polymer composites with isotropic thermal conductivity by constructing a three-dimensional interconnected network. Nanoscale 2019;11(23):11360–8. Singla P, Riyaz M, Singhal S, Goel N. Theoretical study of adsorption of amino acids on graphene and BN sheet in gas and aqueous phase with empirical DFT dispersion correction. Phys Chem Chem Phys 2016;18(7):5597–604. Rahman MM, Puthirath AB, Adumbumkulath A, Tsafack T, Robatjazi H, Barnes M, et al. Fiber reinforced layered dielectric nanocomposite. Adv Funct Mater 2019;29 (28):1900056. Sediri H, Pierucci D, Hajlaoui M, Henck H, Patriarche G, Dappe YJ, et al. Atomically sharp interface in an h-BN-epitaxial graphene van der Waals heterostructure. Sci Rep 2015;5:16465. Wang J, Wu Y, Xue Y, Liu D, Wang X, Hu X, et al. Super-compatible functional boron nitride nanosheets/polymer films with excellent mechanical properties and ultra-high thermal conductivity for thermal management. J Mater Chem C 2018;6 (6):1363–9. Cao JP, Zhao J, Zhao X, You F, Yu H, Hu G-H, et al. High thermal conductivity and high electrical resistivity of poly(vinylidene fluoride)/polystyrene blends by controlling the localization of hybrid fillers. Compos Sci Technol 2013;89:142–8. Xu L, Chen G, Wang W, Li L, Fang X. A facile assembly of polyimide/graphene coreshell structured nanocomposites with both high electrical and thermal conductivities. Composites A Appl S 2016;84:472–81. Zhang DL, Zha JW, Li WK, Li CQ, Wang SJ, Wen Y, et al. Enhanced thermal conductivity and mechanical property through boron nitride hot string in polyvinylidene fluoride fibers by electrospinning. Compos Sci Technol 2018;156: 1–7. Gu J, Lv Z, Wu Y, Guo Y, Tian L, Qiu H, et al. Dielectric thermally conductive boron nitride/polyimide composites with outstanding thermal stabilities via in-situ polymerization-electrospinning-hot press method. Compos Part A Appl S 2017;94: 209–16. Li TL, Hsu SLC. Enhanced thermal conductivity of polyimide films via a hybrid of micro-and nano-sized boron nitride. J Phys Chem B 2010;114(20):6825–9. Yang N, Xu C, Hou J, Yao Y, Zhang Q, Grami ME, et al. Preparation and properties of thermally conductive polyimide/boron nitride composites. RSC Adv 2016;6(22): 18279–87.
L. Cao et al.
Composites Part B 188 (2020) 107882 [43] Chen J, Huang X, Sun B, Wang Y, Zhu Y, Jiang P. Vertically aligned and interconnected boron nitride nanosheets for advanced flexible nanocomposite thermal interface materials. ACS Appl Mater Interfaces 2017;9(36):30909–17. [44] Zhang X, Zhang J, Xia L, Li C, Wang J, Xu F, et al. Simple and consecutive melt extrusion method to fabricate thermally conductive composites with highly oriented boron nitrides. ACS Appl Mater Interfaces 2017;9(27):22977–84. [45] Tang L, He M, Na X, Guan X, Zhang R, Zhang J, et al. Functionalized glass fibers cloth/spherical BN fillers/epoxy laminated composites with excellent thermal conductivities and electrical insulation properties. Compos Commun 2019;16: 5–10. [46] Chen J, Huang X, Sun B, Jiang P. Highly thermally conductive yet electrically insulating polymer/boron nitride nanosheets nanocomposite films for improved thermal management capability. ACS Nano 2018;13(1):337–45. [47] Yu J, Huang X, Wu C, Wu X, Wang G, Jiang P. Interfacial modification of boron nitride nanoplatelets for epoxy composites with improved thermal properties. Polymer 2012;53(2):471–80. [48] Yang X, Guo Y, Han Y, Li Y, Ma T, Chen M, et al. Significant improvement of thermal conductivities for BNNS/PVA composite films via electrospinning followed by hot-pressing technology. Compos B Eng 2019;175:107070. [49] Yang X, Guo Y, Luo X, Zheng N, Ma T, Tan J, et al. Self-healing, recoverable epoxy elastomers and their composites with desirable thermal conductivities by incorporating BN fillers via in-situ polymerization. Compos Sci Technol 2018;164: 59–64.
[36] Wang XB, Weng Q, Wang X, Li X, Zhang J, Liu F, et al. Biomass-directed synthesis of 20 g high-quality boron nitride nanosheets for thermoconductive polymeric composites. ACS Nano 2014;8(9):9081–8. [37] Hou J, Li G, Yang N, Qin L, Grami ME, Zhang Q, et al. Preparation and characterization of surface modified boron nitride epoxy composites with enhanced thermal conductivity. RSC Adv 2014;4(83):44282–90. [38] Zeng X, Yao Y, Gong Z, Wang F, Sun R, Xu J, et al. Ice-templated assembly strategy to construct 3D boron nitride nanosheet networks in polymer composites for thermal conductivity improvement. Small 2015;11(46):6205–13. [39] Huang T, Zeng X, Yao Y, Sun R, Meng F, Xu J, et al. Boron nitride@graphene oxide hybrids for epoxy composites with enhanced thermal conductivity. RSC Adv 2016; 6(42):35847–54. [40] Sato K, Horibe H, Shirai T, Hotta Y, Nakano H, Nagai H, et al. Thermally conductive composite films of hexagonal boron nitride and polyimide with affinityenhanced interfaces. J Mater Chem 2010;20(14):2749–52. [41] Yao Y, Sun J, Zeng X, Sun R, Xu JB, Wong CP. Construction of 3D skeleton for polymer composites achieving a high thermal conductivity. Small 2018;14(13): 1704044. [42] Li Q, Zhang G, Liu F, Han K, Gadinski MR, Xiong C, et al. Solution-processed ferroelectric terpolymer nanocomposites with high breakdown strength and energy density utilizing boron nitride nanosheets. Energy Environ Sci 2015;8(3):922–31.
10