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In-situ grown hollow Fe3O4 onto graphene foam nanocomposites with high EMI shielding effectiveness and thermal conductivity
Composites Science and Technology 188 (2020) 107975
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
In-situ grown hollow Fe3O4 onto graphene foam nanocomposites with high EMI shielding effectiveness and thermal conductivity Haoming Fang a, b, Haichang Guo a, Yiran Hu c, Yanjuan Ren a, Po-Chun Hsu b, Shu-Lin Bai a, * a
Department of Materials Science and Engineering, HEDPS/CAPT/LTCS, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Engineering, Peking University, Beijing, 100871, China b Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC, 27708, USA c College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Compressed graphene foam In-situ grown hollow Fe3O4 EMI shielding effectiveness Thermal conductivity
With the increasing packaging density and multi-functionality, thermal management and electromagnetic pollution in electronic devices are crucial. In this work, we propose a novel method to in-situ grow hollow Fe3O4 sphere (h-Fe3O4, inner and outer diameter of 870 nm and 975 nm, respectively) onto three-dimensional graphene foam (GF) surface and then filled it with polydimethylsiloxane (PDMS) to fabricate nanocomposites with high electromagnetic interference shielding effectiveness (70.37 dB from 8.2 to 12.4 GHz) and thermal conductivity (28.12 � 1.212 W m 1 K 1) at room temperature. Moreover, conductive networks inside composites show superflexible performance with high electrical conductivity (84.02 � 8.385 S cm 1). The effect of in-situ growth hollow Fe3O4 spheres in the enhancement of EMI SE has been demonstrated via comparing with different con tents, morphologies and preparation processing. Besides, the mechanism of thermal conductivity has been investigated by FEM simulation and theoretical modeling. Finally, the usage of GF/h-Fe3O4/PDMS composites as thermal interface materials (TIMs) for chip cooling is proved to be successful, and the corresponding temperature under usage power density is accurately predicted. These comprehensive properties of GF/h-Fe3O4/PDMS composite open a potential application for next-generation TIMs in chip packaging.
1. Introduction The technology of 5G communication demands the chips with higher integration density, miniaturization as well as multi-functionalization. Among all the technological challenges, efficient thermal management and electromagnetic pollution prevention among various chips and de vices are the two major issues that have raised wide and enormous in terests in both academic and industrial fields [1]. Soft and flexible thermal interface materials (TIMs) with high thermal conductivity (TC) and electromagnetic interference shielding effectiveness (EMI SE) can potentially solve the two issues simultaneously and therefore reduce the overall weight and bulkiness. Generally, the traditional TIMs are based on inorganic particles (Al2O3 [2], AlN [3], BN [4] and their hybrid mixtures [5,6]) filled polymer composites. The widely used method for TIMs manufacturing is the mixture blending, and this process requires a high filler loading (commonly up to 50 vol%) to achieve a relatively high value of TC (>3 W m 1 K 1) [7]. However, the high volume of filler loading would be
detrimental to the softness of composites. Besides, most of these com posites are transparent to the electromagnetic waves showing in efficiency in the electromagnetic interference shielding. The EMI shielding materials require the high electric conductivity for electro magnetic wave reflection, large internal surface area and interfaces for absorption attenuation and multiple reflections as well as magnetic properties for the impedance matching [8]. Therefore, electrical conductive fillers [9–11] such as transition metal carbide/carbonitride (Mxene), Ag nanowire, Cu nanowire, and carbon materials have been employed to combine with polymers to fabricate EMI shielding com posites. For better absorption attenuation and impedance matching, it is necessary to corporate with magnetic fillers like Fe3O4 [12], BiFeO3 [13] due to their high dielectric loss and magnetic loss. For the sake of multifunctional TIMs, although the metal and inor ganic magnetic fillers would improve the EMI SE of composites, the softness, flexibility and high TC are also key requirements to be met. High loading metal-filled composites are too stiff to be used and mag netic fillers are insufficient in TC enhancement. Graphene with high TC
* Corresponding author. E-mail address: [email protected] (S.-L. Bai). https://doi.org/10.1016/j.compscitech.2019.107975 Received 15 September 2019; Received in revised form 18 December 2019; Accepted 26 December 2019 Available online 27 December 2019 0266-3538/© 2020 Elsevier Ltd. All rights reserved.
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and electrical conductivity (EC) of 5300 W m 1 K 1 and 106 S cm 1 respectively is a promising filler candidate, which has been demon strated in the study on thermal management and EMI SE performance (graphene film) [9,14]. Nevertheless, when graphene is incorporated into composites [15,16] via traditional stirring and blending, some disadvantages and challenges happen such as agglomeration, limited loading and inert surface with a huge specific surface area of 2630 m2/g and low surface energy of 46.7 mJ/m2. It has been demonstrated that three dimensional (3D) microstructure engineering of graphene [17] including aerogel, pyrolysis of polymer foam, CVD template framework can take advantage of graphene properties, then improve the thermal and electrical transfer behavior of composites. Therefore, the high EMI SE properties of 3D graphene-based composites have been also verified due to the high reflection of electromagnetic waves [18]. Further, to enhance the absorption attenuation of electromagnetic waves, the introduction of magnetic fillers like Fe3O4 into 3D graphene-based composites is a feasible way [19]. The method to prepare Fe3O4/gra phene composites via the physical blending of two independent particles [20] or synthesize the solid Fe3O4 spheres onto the graphene surface [21,22] was found to give an obvious improvement in EMI SE of com posites compared with only graphene ones. However, from the EMI shielding mechanism [23,24], the morphology (such as hollow structure with more interfaces) of the material may have some effects on the dipole polarization resulting in higher EMI SE. Therefore, the investi gation on the hollow Fe3O4 spheres/3D graphene composites for high EMI SE and thermal conductivity is meaningful. Herein, we propose a novel approach to grow in-situ h-Fe3O4 sphere onto 3D graphene networks via the assistance of polydopamine (PDA) layers. Due to the π-π interaction and self-polymerization of PDA layers [23,24], the chemical vapor deposition (CVD) grown GF is able to be functionalized and activated with few defects. Then the corresponding composites are made by infiltrating the polydimethylsiloxane (PDMS) into different compression-oriented GF/h-Fe3O4 skeleton. Due to the highly conductive interconnected GF networks and intensive interaction between h-Fe3O4 and graphene, the GF/h-Fe3O4/PDMS composites exhibit high in-plane TC of 28.12 W m 1 K 1 � 1.212 m 1 K 1, EC of 84 S � 8.385 cm 1 and EMI SE of 70.37 dB. Meanwhile, conductive net works of the GF/h-Fe3O4/PDMS composites show super-flexible per formance even bending, stretching and twisting. In order to study the phenomenon of these excellent performances, the finite element method (FEM) simulation based on Fourier’s law of heat transfer model is un dertaken to figure out the mechanisms of high TC. Besides, various contents and orientation of GF, morphologies of Fe3O4 spheres and different preparation processing are designed to optimize EMI SE properties of composites. Last, GF/h-Fe3O4/PDMS composites are employed as TIMs for chip cooling in practical application, showing a remarkable decrease in temperature. The conforming FEM simulation model is established to express the heat distribution inside chips and predict the usage power densities with various TIMs.
2.2. Fabrication of hollow Fe3O4 spheres/GF composite The details of growth and surface modification for GF are shown in supporting information (Note S1) and illustrated in Fig. 1. First, the nickel (Ni) foam was used as the templates for graphene growth and was compressed to increase the density from 0.167 � 0.002 to 1.912 � 0.049 g/cm3 (Fig. S8 and Table S1). Then the PDA modified GF was acquired via a similar approach as our previous work [25]. Subse quently, the in-situ hollow Fe3O4 sphere was growth onto the GF surface via a facile solvothermal method [26]. 1.6 mmol of Fe(NO3)3⋅6H2O, 12 ml of glycerol and 84 ml of isopropanol were added into a 150 mL Teflon container with modified GF. The mixture was stirred for 10 min, sequentially adding 1.6 ml deionized water into the corresponding so lution and stirring for 10 min again. Then the container was assembled into a stainless-steel autoclave and kept in oven at 190 � C for 12 h. After this hydrothermal reaction, the autoclave cooled down to room tem perature naturally. The h-Fe3O4 in-situ coated GF was obtained via washing with ethanol and deionized water separately for several times and lyophilizing. Finally, the GF/h-Fe3O4 was annealed at 350 � C at N2 atmosphere with a heating rate of 3 � C min 1. Then the GF/h-Fe3O4 samples with different densities were infiltrated with liquid PDMS (base agent/curing agent ¼ 10/1 in weight) at ambient temperature. Finally, the mixture was vacuumed and cured at 80 � C for 4 h to get the corre sponding composites. 2.3. Characterization 2.3.1. Structural characterization The microstructures and morphologies of GF and h-Fe3O4 were measured by scanning electron microscope (SEM, S-4800, HITACHI, 5 V) and transmission electron microscope (TEM, Tecnai F30, FEI). X-ray diffraction (XRD) patterns were collected on an X-ray diffractometer (Rigaku Dmax-2000, Japan) with Cu Kα radiation (λ ¼ 1.5406 Å) at a current of 10 mA and voltage of 30 kV. The scan rate was 4� /min from 20� to 70� . The X-ray photoelectron spectroscopy (XPS) measurements were conducted by Axis Ultra imaging photoelectron spectrometer (Kratos Analytical Ltd., Japan) with a monochromatic Al Ka X-ray source at 225 W. The Raman spectra were tested on Micro Raman im aging spectrometer with He–Ne laser excited at 532 nm (Thermo Sci entific, DXRxi). The specific surface area and pore distribution of solid and hollow Fe3O4 spheres were obtained by Accelerated Surface Area & Porosimetry System (ASAP 2020, MICROMETER, America). 2.3.2. Thermal properties measurement Thermal conductivity was calculated from the equation:λ ¼ α � Cp � ρ, where λ; α, Cp , and ρ represent thermal conductivity, thermal diffu sivity, specific heat capacity, and material density, respectively. The thermal diffusivity coefficient of samples with a diameter of 12.7 mm for out-of-plane TC and 25.4 mm for in-plane TC was obtained by laser-flash diffusivity instrument (DXF-500, TA Instruments, America). The specific heat capacity was obtained by differential scanning calorimetry (DSC) Q100 (TA Instruments, America) at a heating rate of 5 � C min 1 from 0 � C to 180 � C. The density of samples was measured using an automatic density analyzer (PEAB, XS105DU, METTLER TOLEDO, Switzerland). The thermal images were recorded by the thermal imager (SC7300M, Flir Systems USA). The thermal stability of GF/h-Fe3O4 was studied by dynamic thermogravimetric analysis (Q600 SDT, TA) at the nitrogen atmosphere from room temperature to 1000 � C at a heating rate of 10 � C min 1. Besides, we designed a testing platform to verify the heat transfer performance of our composites along both in-plane and out-of-plane directions (Fig. S18a and Fig. S18c). All the simulations of thermal performance were carried out by finite element method (FEM, COMSOL Multiphysics 5.4) and the FEM models for in-plane and out-of-plane TC were established via real testing for reference (Fig. S18b and Fig. S18d).
2. Materials and experiment 2.1. Materials Nickel foams (2 mm in thickness, density of 0:167 � 0:002 g cm 3 and porosity of 97.2%) were supplied by Shanghai Zhongwei Co., Ltd. Dopamine hydrochloride, tris(hydroxymethyl) aminomethane (Tris) and 3-aminopropyltriethoxysilane (APTS) were purchased from Aladdin. Polydimethylsiloxane (PDMS, Sylgard184) was purchased from Dow Corning. Fe(NO3)3∙6H2O and glycerol were from Sinopharm Chemical Reagent Co., Ltd. The solid Fe3O4 spheres (diameter around 900 nm) were purchased from Shanghai Institute of Metallurgy, Chinese Academy of Sciences.
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Fig. 1. Schematic of the fabrication procedure of GF/h-Fe3O4/PDMS composites.
mm3. Typically, two silver wires (0.25 mm, Alfa) were fixed at the two ends of the sample by high purity silver paint (SPI Supplies) and con nected to the digital source meter (Keithley 2635A). After that, the sample was mounted onto the testing instrument and tested at a loading rate of 1.5 mm/min. The displacement direction is along with the inplane direction of compressed graphene as shown in Fig. S10. The permittivity, dielectric loss and shielding effectiveness (SE) of each sample were measured by a vector network analyzer (VNA, E8363C,
2.3.3. Mechanical, electrical and EMI shielding effectiveness measurement The in-plane electrical conductivity was characterized using a fourprobe arrangement (SZT-A, Suzhou Jingge Electronic Co., Ltd, China). The dimension of the sample is 22.90 � 10.20 � 2.00 mm. The contact probe materials are stainless steel with a 1 mm gap distance. Uniaxial monotonic and cyclic tension were conducted on a single column testing instrument (Instron 5843) and electrical measurements were made simultaneously. The dimension of the sample is 22.90 � 10.20 � 2.00
Fig. 2. (a) TEM images of h-Fe3O4 (inserted image is the corresponding SAED patterns). The red and blue bars are the outer and inner diameter of h-Fe3O4. (b) XPS survey spectra of GF (black line) and h-Fe3O4/GF (red line). The inserted signal is the zoom-in N1s spectra of h-Fe3O4/GF. (c) Fe 2p spectra analysis of h-Fe3O4. The blue area is attributed to the Fe2þpeak, the red area to the Fe3þpeak, the purple line is the background signal and yellow line is the fitted line from the measured data. (d) SEM images of compressed GF/h-Fe3O4. The I shape is the width of compressed GF. The inserted image is the zoom-in image in the rectangle area. The substrate of this inserted image is the surface of GF and the particles are h-Fe3O4. (e) Raman spectrum of h-Fe3O4/GF (red line), h-Fe3O4 (black line) and GF (blue line). (f) XRD pattern of GF (black line) and h-Fe3O4/GF (red line). The inserted image is the zoom-in pattern from 28 to 50� . The blue labels are the standard XRD references (card no.65–3107 for graphene and card no.65–3107 for Fe3O4). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 3
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Agilent) at frequencies of X band (8.2–12.4 GHz). The dimension of each sample is 22.90 � 10.20 � 2.00 mm3, which fits exactly into the rect angular waveguide (WR90, the dimension is 22.90 � 10.20 mm2). The sample holder was positioned between the flanges of the waveguide connected between the two ports of VNA. The EMI shielding effective ness can be calculated by transmittance (T), absorption (A) and reflec tance (R) of waves. Moreover, these parameters can be represented via scattering parameters (S-parameters): S12 (or S21) and S11 (or S22) via Formulas (S1) to (S7) as shown in Note S2.
pristine and highly-compressed 3D GF/h-Fe3O4 skeletons (Fig. 2b and S3b) suggest that the h-Fe3O4 spheres were in-situ synthesized onto graphene surface uniformly. The typical TEM image of a graphene sheet from GF in Fig. S2 shows five layers of the graphene sheet. Compared with pure h-Fe3O4 spheres (Fig. S5), the obvious N peak in XPS spectra (Fig. 2c) suggests the formation of the PDA layer. Owing to the strong static interaction between the hydroxyl group of the PDA layer and ferric ion, the GF is provided with functional reaction sites for h-Fe3O4 growth. Therefore, h-Fe3O4 spheres were synthesized onto the graphene surface with a relative high content of 9.72 wt% (measured by TGA in Fig. S6). The peaks located at 710.0 eV and 723.7 eV are assigned to Fe 2p3/2 and Fe 2p1/2, respectively (Fig. 2e). The Fe 2p3/2 peak for hFe3O4 was deconvoluted into the Fe2þ (Blue area) and Fe3þ peaks (Red area), the ratio of the area in Fe2þ and Fe3þ is about 1:1.7, which again indicates that the sphere belongs to Fe3O4 phase [28]. In addition, the sharp G peak (1580 cm 1) and the absence of D peak (1350 cm 1) in the Raman spectrum of GF/h-Fe3O4 clearly reveal that there are very few defects on graphene surface after the solvothermal process and calci nation treatments (Fig. 2d). The identified peaks at 225 cm 1 (A1g mode), 280, 380, 494, 580, 670 cm 1 (Eg mode) and 1338 cm 1 (a hematite two-magnon scattering) elucidate the Raman shift of Fe3O4 clearly as reported [29]. Moreover, compared with the standard XRD
3. Results and discussion 3.1. Structure and elemental characterizations of GF/h-Fe3O4 After annealing in N2 gas at 350 � C, the h-Fe3O4 spheres possesses hierarchical structures that consist of the radially standing nanoparticles on the surface and the highly porous interior (Fig. 2a and S3c). The mechanism of porous interior formation is based on the self-templated/ ion-exchange strategies [26,27]. The outer and inner diameters of h-Fe3O4 are 975 nm and 870 nm, respectively. The polycrystalline cubic-phase h-Fe3O4 spheres are characterized via the selected-area electron diffraction (SAED). The SEM images of the interconnected
Fig. 3. (a) In-plane electrical conductivity of GF/PDMS and GF/h-Fe3O4/PDMS composites. (b) Electro-mechanical properties of GF/h-Fe3O4/PDMS composites as a function of strain. (c) EMI SE of GF/h-Fe3O4/PDMS composites as a function of frequency and loading. (d) SEabs and SEref of GF/h-Fe3O4/PDMS composites as a function of frequency. (e) Average SEtot of GF/PDMS and GF/h-Fe3O4/PDMS composites as a function of loading (inserted numbers are the SEtot contrast between two composites). (f) Comparison on EMI SE of various PDMS-based composites. Schematic EMI SE mechanism in GF/h-Fe3O4/PDMS composites including (g) interlayer interaction, (h) intrastratal interaction and (i) interfacial interaction. 4
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reference (JCPDS card no. 63–3107) of the cubic Fe3O4 phase, all the identified peaks (Fig. 2f and S7) verify the polycrystalline structure of h-Fe3O4 again.
composites (denoted as GF/in-situ-h-Fe3O4/PDMS) with same loading were prepared under same conditions as described in supporting infor mation. The diameters of solid- and hollow-Fe3O4 are about 900 nm and the specific surfaces are 93.65 m2/g and 136.65 m2/g, respectively (BET in Fig. S13). Compared with pure PDMS, the addition of two types of Fe3O4 spheres would not induce obvious change in SEtot, SEabs, SEref, ε’ , 0 ε00 and u (Figs. S14 and S15) due to the nearly insulated properties and lack of magnetic resonance without electron transfer condition. How ever, combining with GF (loading of 6.5 wt%), there is a remarkable change of morphologies and distributions of GF and h-Fe3O4 spheres inside the composites. Since the strong interfacial polarization via in-situ growth h-Fe3O4 spheres onto graphene surface and relatively high permeability from regular agglomeration of h-Fe3O4, the highest value of ε0 , ε’’ and μ’ (Fig. S16) would be obtained for GF/in-situ-h-Fe3O4/ PDMS composites. Subsequently, there is about 15 dB improvement of SEabs for GF/in-situ-h-Fe3O4/PDMS composites by contrast with GF/ PDMS composites. Concerning the morphology effect, the addition of hFe3O4 in the composites (both in h-Fe3O4/PDMS and GF/r-h-Fe3O4/ PDMS) show the higher ε0 , ε’’, μ’ and SEabs than s-Fe3O4 ones, suggesting the hollow microstructure of Fe3O4 spheres with higher specific area would introduce more dipole polarization and increase the SEabs. For the distribution case, the GF/r-h-Fe3O4/PDMS composites show a similar SEref but less SEabs than GF/in-situ-h-Fe3O4/PDMS composites, which indicates that the in-situ grown approach is able to induce stronger interfacial interaction between Fe3O4 spheres and graphene leading to higher electromagnetic attenuation.
3.2. Electro-mechanical properties and EMI shielding efficiency of composites The in-plane EC of composites reaches the value of 93.50 � 6.347 and 84.02 � 8.385 S cm 1 at the loading of 12 wt% for the GF/PDMS and GF/h-Fe3O4 PDMS composites respectively, due to the compressed GF that facilitates the charge transport (Fig. 3a). In addition, uniaxial cyclic tension with in-situ EC measurement was carried out. A linear part of the stress-strain curve extends to about 30% strain, then strain hardening appears, just as conventional elastoplastic materials (Fig. 3b). The hysteresis loop is seen obviously after every tensile cycle under the strain of 20% (Fig. S11 a,b), reflecting the viscoelasticity and plastic deformation of composites. In addition, the fracture strain of composite is up to 80.79%, which is also supported by the huge electrical resistance jump. The increase of electrical resistance under the strain from 60% to 80% reveals that the GF would break away and separate from each other remarkably. Fig. 3c presents EMI SE of GF/h-Fe3O4/PDMS composites as a function of filler loading. When the loading of GF/h-Fe3O4 is up to 12 wt %, the composite exhibits ultra-high average EMI SE value of 70.37 dB, i. e. 22 dB enhancement compared with GF/PDMS composites with the same loading as shown in Fig. 3e. According to Schelkunoff’s equation (Formula (S15) to (S19), there are three main contributing factors to the outstanding EMI SE (SEtot), including, reflection (SEref), absorption attenuation (SEabs) as well as multiple reflections (SEmul) of electro magnetic wave. As shown in Fig. 3g, the electric conductive GF networks endow an efficient charge transfer pathway, which leads to strong interaction between the electromagnetic waves and free charge on the surface of GF, thus improves the reflection. The electric capacity and permeability of composites play an important role in the absorption attenuation of electromagnetic energy. The localized currents generated in conductive interconnected GF networks, the polarization/relaxation process of dipoles and free charges at the interface of graphene and hFe3O4 (Fig. 3h) as well as the inner/outer surfaces of h-Fe3O4 spheres (Fig. 3i) will cause energy dissipation, which subsequently results in the absorption attenuation (the magnetic properties of GF/h-Fe3O4 are shown in videoS1 Supplemental Video 1). As for the multiple reflection of the electromagnetic wave, if the SEabs is higher than 10 dB, this factor can be neglected due to the high electromagnetic absorption attenuation [30]. Hence, the main EMI SE mechanism for GF/h-Fe3O4/PDMS com posites is absorption attenuation and reflection. As shown in Fig. 3d, with the loading increase of GF/h-Fe3O4, the SEref of composites en hances slowly when compared with SEabs. Further, the ratio of average SEabs and SEref rises from 2.82 to 5.03 (Fig. S12), suggesting the domi nating mechanism of GF/h-Fe3O4/PDMS composites is absorption attenuation which is different from the most graphene filled electro magnetic shielding composites. Compared with the reported flexible PDMS (the EC is lower than 5 S cm 1) based composites [20,31–37] (Fig. 3f and Table S3), the GF/h-Fe3O4/PDMS composites possess a su perior EMI SE due to the excellent electrical conductivity and strong interfacial interaction. Supplementary video related to this article can be found at http: //doi:10.1016/j.compscitech.2019.107975. To verify the effect of in-situ growth h-Fe3O4 spheres, the distribu tion (random and in-situ grown) and morphologies (solid and hollow) of Fe3O4 spheres have been investigated. Except for in-situ grown GF/hFe3O4/PDMS composite, all solid Fe3O4 and hollow Fe3O4 fillers are randomly distributed in the PDMS to prepare the composites. The randomly distributed solid Fe3O4/PDMS (denoted as s-Fe3O4/PDMS), hollow-Fe3O4/PDMS (denoted as h-Fe3O4/PDMS), GF/solid-Fe3O4/ PDMS (denoted as GF/r-s-Fe3O4/PDMS), GF/hollow-Fe3O4/PDMS (denoted as GF/r-h-Fe3O4/PDMS) and GF/in-situ grown h-Fe3O4/PDMS
3.3. Thermal transport performance of composites To investigate the thermal transport performance of GF and GF/hFe3O4, the in-plane (κ?) and out-of-plane (κk) TC values of composites with different loadings are measured (Fig. 4a) via transient laser flash apparatus (LFA). At 1.4 wt% pristine GF, the composites show κ? of 0.54 W m 1 K 1, κk of 0.83 W m 1 K 1 and ω value of 1.54. The dimensionless anisotropic factor ω is defined as the ratio of κk over κ? and presents the anisotropic degree of TC as shown in Formula (S37). This slight differ ence of TC in these two directions is caused by the marginal pure PDMS layer with high thermal resistance on top and bottom sides, as discussed previously [38]. The ω value of GF/PDMS and GF/h-Fe3O4/PDMS composites increases linearly when the compression ratio is lower than 83% while rising sharply at a high compression ratio of 95% with ω value of 13.93 and 11.58. This tendency reflects that the density and orientation of GF can be enhanced anisotropically and eventually improve the heat transfer in the in-plane direction considerably with axial compression as shown in Fig. 4b. When the maximum compression ratio is up to 95%, the κk and κ? of GF/PDMS and GF/h-Fe3O4/PDMS composites have the highest values of 1.99 � 0.004, 2.42 � 0.005, 27.76 � 1.304 and 28.12 � 1.212 W m 1 K 1, respectively. Compared with the pristine and modified GF, the addition of h-Fe3O4 did not increase the heat resistance but serves as the intermediate layer to enhance the phonon transfer between layers, which is reflected by TC increase of 21.61% and 1.30% in in-plane and out-of-plane directions, respectively. The obvious TC enhancement by the graphene is caused by four factors: high quality, interconnected networks, high density and orien tation. The high quality of graphene grown by CVD retains the high thermal transfer ability even after surface modification and fabrication process of composites. Because of the interconnected GF skeleton, phonons can transfer in ballistic scattering mode through the GF skel eton inside composites. The use of denser Ni templates with densities of 0.167 to 1.912 g/cm3 results in freestanding denser GF with densities of 0.017 to 1.093 g/cm3 (Fig. S8 and Table S1). Nearly 100 times increase in GF density results in a compact heat transport pathway. Besides, several analytical models including mixture rules, Maxwell model, Maxwell–Eucken model and effective medium theory were used to predict the relationship between loading and TC (Fig. 4c, Formula (S20) 5
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Fig. 4. (a) In-plane, out-of-plane TC and (b) anisotropic factor of GF/PDMS and GF/h-Fe3O4/PDMS composites with different loadings. (c) TC comparison among measured results and various theoretical ones. The insert image is the zoom-in image of the low boundary of Maxwell–Eucken model, the Maxwell model and effective medium theory. The M0 model in the tag is Maxwell–Eucken model. (d) Summary on TC of various PDMS-based composites.
to (S35) and Note S5). As these models are based on discontinuous particles and fillers filled composites, the predicted results are not well consistent with experimental ones. Only the result by mixture rule is close to TC of corresponding composites, suggesting that the heat transfer behavior is more like a parallel-plate model when augmenting the orientation of GF. In contrast to other reported PDMS based com posites [39–43] (Fig. 4d and Table S3), the GF/h-Fe3O4/PDMS com posites show an outstanding TC enhancement efficiency (Formula (S36)) of 1300, which is near 10 times that of compared materials. The heat transfer mechanism is further investigated by FEM simu lation and infrared (IR) imaging detection. For the FEM simulation by homogenization model (Fig. 5a and S19), heat flows from bottom to top during the whole procedure time from 1 ms to 200s. On the other hand, the structure model shows the heat flow along the GF skeleton in less than 200 ms (Fig. 5a and S20), followed by warming up the adjoining matrix until homogenous heat distribution in composites. Because of these different heat transfer mechanisms, heat transfer is facilitated in the composites with 3D interconnected GF. In addition, the timetemperature curves by the real testing are compared with that by FEM simulation in homogenization and structure models (Fig. S21b). It is found that the actual temperature distribution and heat transfer prop erties of GF/h-Fe3O4/PDMS composites lie between these two models. From the time-temperature curve by real testing and simulation, with increasing loading of filler, both the equilibrium temperature and the thermal diffusion of composites increase, reflecting the outstanding heat transfer performance consistent with Fourier’s law of heat conduction. To verify the application of the studied composites as TIMs in elec tronic devices and compare the heat dissipation capability of different TIMs, the testing platform for κ? was designed by supplying heat pulse through the TIMs to the heat sink (Fig. S18a and Note S6). The IR images
of the real surface temperature of ceramic heaters stuck with PDMS and GF/h-Fe3O4/PDMS composite as TIMs under power densities of 0.2, 0.8 and 1.8 W/cm2 reveal the excellent heat dissipation ability of GF/hFe3O4/PDMS composite (Fig. 5c). Besides, the temperature reaches a steady-state within only tens of seconds for GF/h-Fe3O4/PDMS com posites under heating and cooling cycles, while pure PDMS shows very sluggish heat transfer (Fig. S22). The corresponding FEM multiphysics field simulation was established via the coupling of heat conduction and natural airflow, showing the comparable different temperature values and distribution at chips (Fig. 5d). Additionally, the FEM simulation is employed to predict the failure temperature of chips attached to different TIMs (Fig. 5e). As reported [2], the operating temperature of chips is supposed to be lower than 120 � C. This means that GF/h-Fe3O4/PDMS composites can efficiently conduct the chip heat flux for more than 10 W/cm2. In contrast, pure PDMS can only handle lower than 3 W/cm2 of heat flux, which limits the chip functions such as overclocking and full loading. More discussion of the mechanism of cooling efficiency for real application in TIMs are shown in supporting information (Note S7). 4. Conclusions In conclusion, we proposed an approach to in-situ grow hollow Fe3O4 spheres onto the graphene surface. After fabrication into poly dimethylsiloxane (PDMS) matrix composites, the GF/h-Fe3O4/PDMS composites show an EC of 84 � 8.385 S cm 1, TC of 28.12 � 1.212 W m 1 K 1, EMI SE of 70.37 dB. The effect of in-situ growth hollow Fe3O4 spheres in the enhancement of EMI SE has been demonstrated via comparing with different preparation approaches. Besides, the mecha nism of thermal conductivity has been investigated by FEM simulation 6
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Fig. 5. (a) FEM simulated results of heat flux by homogenization and structure models. (b) Real infrared images of the surface temperature variation of GF/h-Fe3O4/ PDMS composites at different times. (c) Infrared images of real surface temperatures and (d) the corresponding simulated temperatures of ceramic heaters tested with PDMS and GF/h-Fe3O4/PDMS composites as TIMs. (e) Predicted surface temperature of ceramic as a function of power density.
and theoretical modeling. For the practical application, the composites are employed as TIMs by integrating with electronic devices and show remarkable cooling efficiency. The corresponding simulation to real circumstances can successfully predict the temperature of chips attached to different composites. Thus, these GF/h-Fe3O4/PDMS nanocomposites will endow promising opportunities for thermal management and EMI shielding materials.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compscitech.2019.107975. References [1] M.C. Hsieh, Advanced Flip Chip Package on Package Technology for Mobile Applications, International Conference on Electronic Packaging Technology, Wuhan, China, 2016, pp. 486–491. [2] Y.-X. Fu, Z.-X. He, D.-C. Mo, S.-S. Lu, Thermal conductivity enhancement with different fillers for epoxy resin adhesives, Appl. Therm. Eng. 66 (1–2) (2014) 493–498. [3] Y. Zhou, J. Hu, X. Chen, F. Yu, J. He, Thermoplastic polypropylene/aluminum nitride nanocomposites with enhanced thermal conductivity and low dielectric loss, IEEE Trans. Dielectr. Electr. Insul. 23 (5) (2016) 2768–2776. [4] H. Fang, S.L. Bai, C.P. Wong, “White graphene” – hexagonal boron nitride based polymeric composites and their application in thermal management, Composites Communications 2 (2016) 19–24. [5] L. Shao, L. Shi, X. Li, N. Song, P. Ding, Synergistic effect of BN and graphene nanosheets in 3D framework on the enhancement of thermal conductive properties of polymeric composites, Compos. Sci. Technol. 135 (2016) 83–91. [6] Y. Yao, X. Zeng, R. Sun, J.B. Xu, C.P. Wong, Highly thermally conductive composite papers prepared based on the thought of bioinspired engineering, ACS Appl. Mater. Interfaces 8 (24) (2016) 15645–15653. [7] D.D.L. Chung, Thermal interface materials, J. Mater. Eng. Perform. 10 (1) (2001) 56–59. [8] X. Shen, J.-K. Kim, Building 3D architecture in 2D thin film for effective EMI shielding, Matter 1 (4) (2019) 796–798. [9] L.X. Liu, W. Chen, H.B. Zhang, Q.W. Wang, F. Guan, Z.Z. Yu, Flexible and multifunctional silk textiles with biomimetic leaf-like MXene/silver nanowire nanostructures for electromagnetic interference shielding, humidity monitoring, and self-derived hydrophobicity, Adv. Funct. Mater. 29 (44) (2019).
Declaration of competing interest 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. CRediT authorship contribution statement Haoming Fang: Conceptualization, Methodology, Validation, Investigation, Resources, Writing - original draft, Project administration, Visualization. Haichang Guo: Investigation. Yiran Hu: Resources, Investigation. Yanjuan Ren: Investigation. Po-Chun Hsu: Writing review & editing. Shu-Lin Bai: Supervision, Writing - review & editing, Funding acquisition. Acknowledgements The work is supported by NSFC (Grant No.11672002) and NSAF (Grant No. U1730103). 7
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
In-situ grown hollow Fe3O4 onto graphene foam nanocomposites with high EMI shielding effectiveness and thermal conductivity Haoming Fang a, b, Haichang Guo a, Yiran Hu c, Yanjuan Ren a, Po-Chun Hsu b, Shu-Lin Bai a, * a
Department of Materials Science and Engineering, HEDPS/CAPT/LTCS, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Engineering, Peking University, Beijing, 100871, China b Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC, 27708, USA c College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Compressed graphene foam In-situ grown hollow Fe3O4 EMI shielding effectiveness Thermal conductivity
With the increasing packaging density and multi-functionality, thermal management and electromagnetic pollution in electronic devices are crucial. In this work, we propose a novel method to in-situ grow hollow Fe3O4 sphere (h-Fe3O4, inner and outer diameter of 870 nm and 975 nm, respectively) onto three-dimensional graphene foam (GF) surface and then filled it with polydimethylsiloxane (PDMS) to fabricate nanocomposites with high electromagnetic interference shielding effectiveness (70.37 dB from 8.2 to 12.4 GHz) and thermal conductivity (28.12 � 1.212 W m 1 K 1) at room temperature. Moreover, conductive networks inside composites show superflexible performance with high electrical conductivity (84.02 � 8.385 S cm 1). The effect of in-situ growth hollow Fe3O4 spheres in the enhancement of EMI SE has been demonstrated via comparing with different con tents, morphologies and preparation processing. Besides, the mechanism of thermal conductivity has been investigated by FEM simulation and theoretical modeling. Finally, the usage of GF/h-Fe3O4/PDMS composites as thermal interface materials (TIMs) for chip cooling is proved to be successful, and the corresponding temperature under usage power density is accurately predicted. These comprehensive properties of GF/h-Fe3O4/PDMS composite open a potential application for next-generation TIMs in chip packaging.
1. Introduction The technology of 5G communication demands the chips with higher integration density, miniaturization as well as multi-functionalization. Among all the technological challenges, efficient thermal management and electromagnetic pollution prevention among various chips and de vices are the two major issues that have raised wide and enormous in terests in both academic and industrial fields [1]. Soft and flexible thermal interface materials (TIMs) with high thermal conductivity (TC) and electromagnetic interference shielding effectiveness (EMI SE) can potentially solve the two issues simultaneously and therefore reduce the overall weight and bulkiness. Generally, the traditional TIMs are based on inorganic particles (Al2O3 [2], AlN [3], BN [4] and their hybrid mixtures [5,6]) filled polymer composites. The widely used method for TIMs manufacturing is the mixture blending, and this process requires a high filler loading (commonly up to 50 vol%) to achieve a relatively high value of TC (>3 W m 1 K 1) [7]. However, the high volume of filler loading would be
detrimental to the softness of composites. Besides, most of these com posites are transparent to the electromagnetic waves showing in efficiency in the electromagnetic interference shielding. The EMI shielding materials require the high electric conductivity for electro magnetic wave reflection, large internal surface area and interfaces for absorption attenuation and multiple reflections as well as magnetic properties for the impedance matching [8]. Therefore, electrical conductive fillers [9–11] such as transition metal carbide/carbonitride (Mxene), Ag nanowire, Cu nanowire, and carbon materials have been employed to combine with polymers to fabricate EMI shielding com posites. For better absorption attenuation and impedance matching, it is necessary to corporate with magnetic fillers like Fe3O4 [12], BiFeO3 [13] due to their high dielectric loss and magnetic loss. For the sake of multifunctional TIMs, although the metal and inor ganic magnetic fillers would improve the EMI SE of composites, the softness, flexibility and high TC are also key requirements to be met. High loading metal-filled composites are too stiff to be used and mag netic fillers are insufficient in TC enhancement. Graphene with high TC
* Corresponding author. E-mail address: [email protected] (S.-L. Bai). https://doi.org/10.1016/j.compscitech.2019.107975 Received 15 September 2019; Received in revised form 18 December 2019; Accepted 26 December 2019 Available online 27 December 2019 0266-3538/© 2020 Elsevier Ltd. All rights reserved.
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and electrical conductivity (EC) of 5300 W m 1 K 1 and 106 S cm 1 respectively is a promising filler candidate, which has been demon strated in the study on thermal management and EMI SE performance (graphene film) [9,14]. Nevertheless, when graphene is incorporated into composites [15,16] via traditional stirring and blending, some disadvantages and challenges happen such as agglomeration, limited loading and inert surface with a huge specific surface area of 2630 m2/g and low surface energy of 46.7 mJ/m2. It has been demonstrated that three dimensional (3D) microstructure engineering of graphene [17] including aerogel, pyrolysis of polymer foam, CVD template framework can take advantage of graphene properties, then improve the thermal and electrical transfer behavior of composites. Therefore, the high EMI SE properties of 3D graphene-based composites have been also verified due to the high reflection of electromagnetic waves [18]. Further, to enhance the absorption attenuation of electromagnetic waves, the introduction of magnetic fillers like Fe3O4 into 3D graphene-based composites is a feasible way [19]. The method to prepare Fe3O4/gra phene composites via the physical blending of two independent particles [20] or synthesize the solid Fe3O4 spheres onto the graphene surface [21,22] was found to give an obvious improvement in EMI SE of com posites compared with only graphene ones. However, from the EMI shielding mechanism [23,24], the morphology (such as hollow structure with more interfaces) of the material may have some effects on the dipole polarization resulting in higher EMI SE. Therefore, the investi gation on the hollow Fe3O4 spheres/3D graphene composites for high EMI SE and thermal conductivity is meaningful. Herein, we propose a novel approach to grow in-situ h-Fe3O4 sphere onto 3D graphene networks via the assistance of polydopamine (PDA) layers. Due to the π-π interaction and self-polymerization of PDA layers [23,24], the chemical vapor deposition (CVD) grown GF is able to be functionalized and activated with few defects. Then the corresponding composites are made by infiltrating the polydimethylsiloxane (PDMS) into different compression-oriented GF/h-Fe3O4 skeleton. Due to the highly conductive interconnected GF networks and intensive interaction between h-Fe3O4 and graphene, the GF/h-Fe3O4/PDMS composites exhibit high in-plane TC of 28.12 W m 1 K 1 � 1.212 m 1 K 1, EC of 84 S � 8.385 cm 1 and EMI SE of 70.37 dB. Meanwhile, conductive net works of the GF/h-Fe3O4/PDMS composites show super-flexible per formance even bending, stretching and twisting. In order to study the phenomenon of these excellent performances, the finite element method (FEM) simulation based on Fourier’s law of heat transfer model is un dertaken to figure out the mechanisms of high TC. Besides, various contents and orientation of GF, morphologies of Fe3O4 spheres and different preparation processing are designed to optimize EMI SE properties of composites. Last, GF/h-Fe3O4/PDMS composites are employed as TIMs for chip cooling in practical application, showing a remarkable decrease in temperature. The conforming FEM simulation model is established to express the heat distribution inside chips and predict the usage power densities with various TIMs.
2.2. Fabrication of hollow Fe3O4 spheres/GF composite The details of growth and surface modification for GF are shown in supporting information (Note S1) and illustrated in Fig. 1. First, the nickel (Ni) foam was used as the templates for graphene growth and was compressed to increase the density from 0.167 � 0.002 to 1.912 � 0.049 g/cm3 (Fig. S8 and Table S1). Then the PDA modified GF was acquired via a similar approach as our previous work [25]. Subse quently, the in-situ hollow Fe3O4 sphere was growth onto the GF surface via a facile solvothermal method [26]. 1.6 mmol of Fe(NO3)3⋅6H2O, 12 ml of glycerol and 84 ml of isopropanol were added into a 150 mL Teflon container with modified GF. The mixture was stirred for 10 min, sequentially adding 1.6 ml deionized water into the corresponding so lution and stirring for 10 min again. Then the container was assembled into a stainless-steel autoclave and kept in oven at 190 � C for 12 h. After this hydrothermal reaction, the autoclave cooled down to room tem perature naturally. The h-Fe3O4 in-situ coated GF was obtained via washing with ethanol and deionized water separately for several times and lyophilizing. Finally, the GF/h-Fe3O4 was annealed at 350 � C at N2 atmosphere with a heating rate of 3 � C min 1. Then the GF/h-Fe3O4 samples with different densities were infiltrated with liquid PDMS (base agent/curing agent ¼ 10/1 in weight) at ambient temperature. Finally, the mixture was vacuumed and cured at 80 � C for 4 h to get the corre sponding composites. 2.3. Characterization 2.3.1. Structural characterization The microstructures and morphologies of GF and h-Fe3O4 were measured by scanning electron microscope (SEM, S-4800, HITACHI, 5 V) and transmission electron microscope (TEM, Tecnai F30, FEI). X-ray diffraction (XRD) patterns were collected on an X-ray diffractometer (Rigaku Dmax-2000, Japan) with Cu Kα radiation (λ ¼ 1.5406 Å) at a current of 10 mA and voltage of 30 kV. The scan rate was 4� /min from 20� to 70� . The X-ray photoelectron spectroscopy (XPS) measurements were conducted by Axis Ultra imaging photoelectron spectrometer (Kratos Analytical Ltd., Japan) with a monochromatic Al Ka X-ray source at 225 W. The Raman spectra were tested on Micro Raman im aging spectrometer with He–Ne laser excited at 532 nm (Thermo Sci entific, DXRxi). The specific surface area and pore distribution of solid and hollow Fe3O4 spheres were obtained by Accelerated Surface Area & Porosimetry System (ASAP 2020, MICROMETER, America). 2.3.2. Thermal properties measurement Thermal conductivity was calculated from the equation:λ ¼ α � Cp � ρ, where λ; α, Cp , and ρ represent thermal conductivity, thermal diffu sivity, specific heat capacity, and material density, respectively. The thermal diffusivity coefficient of samples with a diameter of 12.7 mm for out-of-plane TC and 25.4 mm for in-plane TC was obtained by laser-flash diffusivity instrument (DXF-500, TA Instruments, America). The specific heat capacity was obtained by differential scanning calorimetry (DSC) Q100 (TA Instruments, America) at a heating rate of 5 � C min 1 from 0 � C to 180 � C. The density of samples was measured using an automatic density analyzer (PEAB, XS105DU, METTLER TOLEDO, Switzerland). The thermal images were recorded by the thermal imager (SC7300M, Flir Systems USA). The thermal stability of GF/h-Fe3O4 was studied by dynamic thermogravimetric analysis (Q600 SDT, TA) at the nitrogen atmosphere from room temperature to 1000 � C at a heating rate of 10 � C min 1. Besides, we designed a testing platform to verify the heat transfer performance of our composites along both in-plane and out-of-plane directions (Fig. S18a and Fig. S18c). All the simulations of thermal performance were carried out by finite element method (FEM, COMSOL Multiphysics 5.4) and the FEM models for in-plane and out-of-plane TC were established via real testing for reference (Fig. S18b and Fig. S18d).
2. Materials and experiment 2.1. Materials Nickel foams (2 mm in thickness, density of 0:167 � 0:002 g cm 3 and porosity of 97.2%) were supplied by Shanghai Zhongwei Co., Ltd. Dopamine hydrochloride, tris(hydroxymethyl) aminomethane (Tris) and 3-aminopropyltriethoxysilane (APTS) were purchased from Aladdin. Polydimethylsiloxane (PDMS, Sylgard184) was purchased from Dow Corning. Fe(NO3)3∙6H2O and glycerol were from Sinopharm Chemical Reagent Co., Ltd. The solid Fe3O4 spheres (diameter around 900 nm) were purchased from Shanghai Institute of Metallurgy, Chinese Academy of Sciences.
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Fig. 1. Schematic of the fabrication procedure of GF/h-Fe3O4/PDMS composites.
mm3. Typically, two silver wires (0.25 mm, Alfa) were fixed at the two ends of the sample by high purity silver paint (SPI Supplies) and con nected to the digital source meter (Keithley 2635A). After that, the sample was mounted onto the testing instrument and tested at a loading rate of 1.5 mm/min. The displacement direction is along with the inplane direction of compressed graphene as shown in Fig. S10. The permittivity, dielectric loss and shielding effectiveness (SE) of each sample were measured by a vector network analyzer (VNA, E8363C,
2.3.3. Mechanical, electrical and EMI shielding effectiveness measurement The in-plane electrical conductivity was characterized using a fourprobe arrangement (SZT-A, Suzhou Jingge Electronic Co., Ltd, China). The dimension of the sample is 22.90 � 10.20 � 2.00 mm. The contact probe materials are stainless steel with a 1 mm gap distance. Uniaxial monotonic and cyclic tension were conducted on a single column testing instrument (Instron 5843) and electrical measurements were made simultaneously. The dimension of the sample is 22.90 � 10.20 � 2.00
Fig. 2. (a) TEM images of h-Fe3O4 (inserted image is the corresponding SAED patterns). The red and blue bars are the outer and inner diameter of h-Fe3O4. (b) XPS survey spectra of GF (black line) and h-Fe3O4/GF (red line). The inserted signal is the zoom-in N1s spectra of h-Fe3O4/GF. (c) Fe 2p spectra analysis of h-Fe3O4. The blue area is attributed to the Fe2þpeak, the red area to the Fe3þpeak, the purple line is the background signal and yellow line is the fitted line from the measured data. (d) SEM images of compressed GF/h-Fe3O4. The I shape is the width of compressed GF. The inserted image is the zoom-in image in the rectangle area. The substrate of this inserted image is the surface of GF and the particles are h-Fe3O4. (e) Raman spectrum of h-Fe3O4/GF (red line), h-Fe3O4 (black line) and GF (blue line). (f) XRD pattern of GF (black line) and h-Fe3O4/GF (red line). The inserted image is the zoom-in pattern from 28 to 50� . The blue labels are the standard XRD references (card no.65–3107 for graphene and card no.65–3107 for Fe3O4). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 3
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Agilent) at frequencies of X band (8.2–12.4 GHz). The dimension of each sample is 22.90 � 10.20 � 2.00 mm3, which fits exactly into the rect angular waveguide (WR90, the dimension is 22.90 � 10.20 mm2). The sample holder was positioned between the flanges of the waveguide connected between the two ports of VNA. The EMI shielding effective ness can be calculated by transmittance (T), absorption (A) and reflec tance (R) of waves. Moreover, these parameters can be represented via scattering parameters (S-parameters): S12 (or S21) and S11 (or S22) via Formulas (S1) to (S7) as shown in Note S2.
pristine and highly-compressed 3D GF/h-Fe3O4 skeletons (Fig. 2b and S3b) suggest that the h-Fe3O4 spheres were in-situ synthesized onto graphene surface uniformly. The typical TEM image of a graphene sheet from GF in Fig. S2 shows five layers of the graphene sheet. Compared with pure h-Fe3O4 spheres (Fig. S5), the obvious N peak in XPS spectra (Fig. 2c) suggests the formation of the PDA layer. Owing to the strong static interaction between the hydroxyl group of the PDA layer and ferric ion, the GF is provided with functional reaction sites for h-Fe3O4 growth. Therefore, h-Fe3O4 spheres were synthesized onto the graphene surface with a relative high content of 9.72 wt% (measured by TGA in Fig. S6). The peaks located at 710.0 eV and 723.7 eV are assigned to Fe 2p3/2 and Fe 2p1/2, respectively (Fig. 2e). The Fe 2p3/2 peak for hFe3O4 was deconvoluted into the Fe2þ (Blue area) and Fe3þ peaks (Red area), the ratio of the area in Fe2þ and Fe3þ is about 1:1.7, which again indicates that the sphere belongs to Fe3O4 phase [28]. In addition, the sharp G peak (1580 cm 1) and the absence of D peak (1350 cm 1) in the Raman spectrum of GF/h-Fe3O4 clearly reveal that there are very few defects on graphene surface after the solvothermal process and calci nation treatments (Fig. 2d). The identified peaks at 225 cm 1 (A1g mode), 280, 380, 494, 580, 670 cm 1 (Eg mode) and 1338 cm 1 (a hematite two-magnon scattering) elucidate the Raman shift of Fe3O4 clearly as reported [29]. Moreover, compared with the standard XRD
3. Results and discussion 3.1. Structure and elemental characterizations of GF/h-Fe3O4 After annealing in N2 gas at 350 � C, the h-Fe3O4 spheres possesses hierarchical structures that consist of the radially standing nanoparticles on the surface and the highly porous interior (Fig. 2a and S3c). The mechanism of porous interior formation is based on the self-templated/ ion-exchange strategies [26,27]. The outer and inner diameters of h-Fe3O4 are 975 nm and 870 nm, respectively. The polycrystalline cubic-phase h-Fe3O4 spheres are characterized via the selected-area electron diffraction (SAED). The SEM images of the interconnected
Fig. 3. (a) In-plane electrical conductivity of GF/PDMS and GF/h-Fe3O4/PDMS composites. (b) Electro-mechanical properties of GF/h-Fe3O4/PDMS composites as a function of strain. (c) EMI SE of GF/h-Fe3O4/PDMS composites as a function of frequency and loading. (d) SEabs and SEref of GF/h-Fe3O4/PDMS composites as a function of frequency. (e) Average SEtot of GF/PDMS and GF/h-Fe3O4/PDMS composites as a function of loading (inserted numbers are the SEtot contrast between two composites). (f) Comparison on EMI SE of various PDMS-based composites. Schematic EMI SE mechanism in GF/h-Fe3O4/PDMS composites including (g) interlayer interaction, (h) intrastratal interaction and (i) interfacial interaction. 4
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reference (JCPDS card no. 63–3107) of the cubic Fe3O4 phase, all the identified peaks (Fig. 2f and S7) verify the polycrystalline structure of h-Fe3O4 again.
composites (denoted as GF/in-situ-h-Fe3O4/PDMS) with same loading were prepared under same conditions as described in supporting infor mation. The diameters of solid- and hollow-Fe3O4 are about 900 nm and the specific surfaces are 93.65 m2/g and 136.65 m2/g, respectively (BET in Fig. S13). Compared with pure PDMS, the addition of two types of Fe3O4 spheres would not induce obvious change in SEtot, SEabs, SEref, ε’ , 0 ε00 and u (Figs. S14 and S15) due to the nearly insulated properties and lack of magnetic resonance without electron transfer condition. How ever, combining with GF (loading of 6.5 wt%), there is a remarkable change of morphologies and distributions of GF and h-Fe3O4 spheres inside the composites. Since the strong interfacial polarization via in-situ growth h-Fe3O4 spheres onto graphene surface and relatively high permeability from regular agglomeration of h-Fe3O4, the highest value of ε0 , ε’’ and μ’ (Fig. S16) would be obtained for GF/in-situ-h-Fe3O4/ PDMS composites. Subsequently, there is about 15 dB improvement of SEabs for GF/in-situ-h-Fe3O4/PDMS composites by contrast with GF/ PDMS composites. Concerning the morphology effect, the addition of hFe3O4 in the composites (both in h-Fe3O4/PDMS and GF/r-h-Fe3O4/ PDMS) show the higher ε0 , ε’’, μ’ and SEabs than s-Fe3O4 ones, suggesting the hollow microstructure of Fe3O4 spheres with higher specific area would introduce more dipole polarization and increase the SEabs. For the distribution case, the GF/r-h-Fe3O4/PDMS composites show a similar SEref but less SEabs than GF/in-situ-h-Fe3O4/PDMS composites, which indicates that the in-situ grown approach is able to induce stronger interfacial interaction between Fe3O4 spheres and graphene leading to higher electromagnetic attenuation.
3.2. Electro-mechanical properties and EMI shielding efficiency of composites The in-plane EC of composites reaches the value of 93.50 � 6.347 and 84.02 � 8.385 S cm 1 at the loading of 12 wt% for the GF/PDMS and GF/h-Fe3O4 PDMS composites respectively, due to the compressed GF that facilitates the charge transport (Fig. 3a). In addition, uniaxial cyclic tension with in-situ EC measurement was carried out. A linear part of the stress-strain curve extends to about 30% strain, then strain hardening appears, just as conventional elastoplastic materials (Fig. 3b). The hysteresis loop is seen obviously after every tensile cycle under the strain of 20% (Fig. S11 a,b), reflecting the viscoelasticity and plastic deformation of composites. In addition, the fracture strain of composite is up to 80.79%, which is also supported by the huge electrical resistance jump. The increase of electrical resistance under the strain from 60% to 80% reveals that the GF would break away and separate from each other remarkably. Fig. 3c presents EMI SE of GF/h-Fe3O4/PDMS composites as a function of filler loading. When the loading of GF/h-Fe3O4 is up to 12 wt %, the composite exhibits ultra-high average EMI SE value of 70.37 dB, i. e. 22 dB enhancement compared with GF/PDMS composites with the same loading as shown in Fig. 3e. According to Schelkunoff’s equation (Formula (S15) to (S19), there are three main contributing factors to the outstanding EMI SE (SEtot), including, reflection (SEref), absorption attenuation (SEabs) as well as multiple reflections (SEmul) of electro magnetic wave. As shown in Fig. 3g, the electric conductive GF networks endow an efficient charge transfer pathway, which leads to strong interaction between the electromagnetic waves and free charge on the surface of GF, thus improves the reflection. The electric capacity and permeability of composites play an important role in the absorption attenuation of electromagnetic energy. The localized currents generated in conductive interconnected GF networks, the polarization/relaxation process of dipoles and free charges at the interface of graphene and hFe3O4 (Fig. 3h) as well as the inner/outer surfaces of h-Fe3O4 spheres (Fig. 3i) will cause energy dissipation, which subsequently results in the absorption attenuation (the magnetic properties of GF/h-Fe3O4 are shown in videoS1 Supplemental Video 1). As for the multiple reflection of the electromagnetic wave, if the SEabs is higher than 10 dB, this factor can be neglected due to the high electromagnetic absorption attenuation [30]. Hence, the main EMI SE mechanism for GF/h-Fe3O4/PDMS com posites is absorption attenuation and reflection. As shown in Fig. 3d, with the loading increase of GF/h-Fe3O4, the SEref of composites en hances slowly when compared with SEabs. Further, the ratio of average SEabs and SEref rises from 2.82 to 5.03 (Fig. S12), suggesting the domi nating mechanism of GF/h-Fe3O4/PDMS composites is absorption attenuation which is different from the most graphene filled electro magnetic shielding composites. Compared with the reported flexible PDMS (the EC is lower than 5 S cm 1) based composites [20,31–37] (Fig. 3f and Table S3), the GF/h-Fe3O4/PDMS composites possess a su perior EMI SE due to the excellent electrical conductivity and strong interfacial interaction. Supplementary video related to this article can be found at http: //doi:10.1016/j.compscitech.2019.107975. To verify the effect of in-situ growth h-Fe3O4 spheres, the distribu tion (random and in-situ grown) and morphologies (solid and hollow) of Fe3O4 spheres have been investigated. Except for in-situ grown GF/hFe3O4/PDMS composite, all solid Fe3O4 and hollow Fe3O4 fillers are randomly distributed in the PDMS to prepare the composites. The randomly distributed solid Fe3O4/PDMS (denoted as s-Fe3O4/PDMS), hollow-Fe3O4/PDMS (denoted as h-Fe3O4/PDMS), GF/solid-Fe3O4/ PDMS (denoted as GF/r-s-Fe3O4/PDMS), GF/hollow-Fe3O4/PDMS (denoted as GF/r-h-Fe3O4/PDMS) and GF/in-situ grown h-Fe3O4/PDMS
3.3. Thermal transport performance of composites To investigate the thermal transport performance of GF and GF/hFe3O4, the in-plane (κ?) and out-of-plane (κk) TC values of composites with different loadings are measured (Fig. 4a) via transient laser flash apparatus (LFA). At 1.4 wt% pristine GF, the composites show κ? of 0.54 W m 1 K 1, κk of 0.83 W m 1 K 1 and ω value of 1.54. The dimensionless anisotropic factor ω is defined as the ratio of κk over κ? and presents the anisotropic degree of TC as shown in Formula (S37). This slight differ ence of TC in these two directions is caused by the marginal pure PDMS layer with high thermal resistance on top and bottom sides, as discussed previously [38]. The ω value of GF/PDMS and GF/h-Fe3O4/PDMS composites increases linearly when the compression ratio is lower than 83% while rising sharply at a high compression ratio of 95% with ω value of 13.93 and 11.58. This tendency reflects that the density and orientation of GF can be enhanced anisotropically and eventually improve the heat transfer in the in-plane direction considerably with axial compression as shown in Fig. 4b. When the maximum compression ratio is up to 95%, the κk and κ? of GF/PDMS and GF/h-Fe3O4/PDMS composites have the highest values of 1.99 � 0.004, 2.42 � 0.005, 27.76 � 1.304 and 28.12 � 1.212 W m 1 K 1, respectively. Compared with the pristine and modified GF, the addition of h-Fe3O4 did not increase the heat resistance but serves as the intermediate layer to enhance the phonon transfer between layers, which is reflected by TC increase of 21.61% and 1.30% in in-plane and out-of-plane directions, respectively. The obvious TC enhancement by the graphene is caused by four factors: high quality, interconnected networks, high density and orien tation. The high quality of graphene grown by CVD retains the high thermal transfer ability even after surface modification and fabrication process of composites. Because of the interconnected GF skeleton, phonons can transfer in ballistic scattering mode through the GF skel eton inside composites. The use of denser Ni templates with densities of 0.167 to 1.912 g/cm3 results in freestanding denser GF with densities of 0.017 to 1.093 g/cm3 (Fig. S8 and Table S1). Nearly 100 times increase in GF density results in a compact heat transport pathway. Besides, several analytical models including mixture rules, Maxwell model, Maxwell–Eucken model and effective medium theory were used to predict the relationship between loading and TC (Fig. 4c, Formula (S20) 5
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Fig. 4. (a) In-plane, out-of-plane TC and (b) anisotropic factor of GF/PDMS and GF/h-Fe3O4/PDMS composites with different loadings. (c) TC comparison among measured results and various theoretical ones. The insert image is the zoom-in image of the low boundary of Maxwell–Eucken model, the Maxwell model and effective medium theory. The M0 model in the tag is Maxwell–Eucken model. (d) Summary on TC of various PDMS-based composites.
to (S35) and Note S5). As these models are based on discontinuous particles and fillers filled composites, the predicted results are not well consistent with experimental ones. Only the result by mixture rule is close to TC of corresponding composites, suggesting that the heat transfer behavior is more like a parallel-plate model when augmenting the orientation of GF. In contrast to other reported PDMS based com posites [39–43] (Fig. 4d and Table S3), the GF/h-Fe3O4/PDMS com posites show an outstanding TC enhancement efficiency (Formula (S36)) of 1300, which is near 10 times that of compared materials. The heat transfer mechanism is further investigated by FEM simu lation and infrared (IR) imaging detection. For the FEM simulation by homogenization model (Fig. 5a and S19), heat flows from bottom to top during the whole procedure time from 1 ms to 200s. On the other hand, the structure model shows the heat flow along the GF skeleton in less than 200 ms (Fig. 5a and S20), followed by warming up the adjoining matrix until homogenous heat distribution in composites. Because of these different heat transfer mechanisms, heat transfer is facilitated in the composites with 3D interconnected GF. In addition, the timetemperature curves by the real testing are compared with that by FEM simulation in homogenization and structure models (Fig. S21b). It is found that the actual temperature distribution and heat transfer prop erties of GF/h-Fe3O4/PDMS composites lie between these two models. From the time-temperature curve by real testing and simulation, with increasing loading of filler, both the equilibrium temperature and the thermal diffusion of composites increase, reflecting the outstanding heat transfer performance consistent with Fourier’s law of heat conduction. To verify the application of the studied composites as TIMs in elec tronic devices and compare the heat dissipation capability of different TIMs, the testing platform for κ? was designed by supplying heat pulse through the TIMs to the heat sink (Fig. S18a and Note S6). The IR images
of the real surface temperature of ceramic heaters stuck with PDMS and GF/h-Fe3O4/PDMS composite as TIMs under power densities of 0.2, 0.8 and 1.8 W/cm2 reveal the excellent heat dissipation ability of GF/hFe3O4/PDMS composite (Fig. 5c). Besides, the temperature reaches a steady-state within only tens of seconds for GF/h-Fe3O4/PDMS com posites under heating and cooling cycles, while pure PDMS shows very sluggish heat transfer (Fig. S22). The corresponding FEM multiphysics field simulation was established via the coupling of heat conduction and natural airflow, showing the comparable different temperature values and distribution at chips (Fig. 5d). Additionally, the FEM simulation is employed to predict the failure temperature of chips attached to different TIMs (Fig. 5e). As reported [2], the operating temperature of chips is supposed to be lower than 120 � C. This means that GF/h-Fe3O4/PDMS composites can efficiently conduct the chip heat flux for more than 10 W/cm2. In contrast, pure PDMS can only handle lower than 3 W/cm2 of heat flux, which limits the chip functions such as overclocking and full loading. More discussion of the mechanism of cooling efficiency for real application in TIMs are shown in supporting information (Note S7). 4. Conclusions In conclusion, we proposed an approach to in-situ grow hollow Fe3O4 spheres onto the graphene surface. After fabrication into poly dimethylsiloxane (PDMS) matrix composites, the GF/h-Fe3O4/PDMS composites show an EC of 84 � 8.385 S cm 1, TC of 28.12 � 1.212 W m 1 K 1, EMI SE of 70.37 dB. The effect of in-situ growth hollow Fe3O4 spheres in the enhancement of EMI SE has been demonstrated via comparing with different preparation approaches. Besides, the mecha nism of thermal conductivity has been investigated by FEM simulation 6
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Fig. 5. (a) FEM simulated results of heat flux by homogenization and structure models. (b) Real infrared images of the surface temperature variation of GF/h-Fe3O4/ PDMS composites at different times. (c) Infrared images of real surface temperatures and (d) the corresponding simulated temperatures of ceramic heaters tested with PDMS and GF/h-Fe3O4/PDMS composites as TIMs. (e) Predicted surface temperature of ceramic as a function of power density.
and theoretical modeling. For the practical application, the composites are employed as TIMs by integrating with electronic devices and show remarkable cooling efficiency. The corresponding simulation to real circumstances can successfully predict the temperature of chips attached to different composites. Thus, these GF/h-Fe3O4/PDMS nanocomposites will endow promising opportunities for thermal management and EMI shielding materials.
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Declaration of competing interest 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. CRediT authorship contribution statement Haoming Fang: Conceptualization, Methodology, Validation, Investigation, Resources, Writing - original draft, Project administration, Visualization. Haichang Guo: Investigation. Yiran Hu: Resources, Investigation. Yanjuan Ren: Investigation. Po-Chun Hsu: Writing review & editing. Shu-Lin Bai: Supervision, Writing - review & editing, Funding acquisition. Acknowledgements The work is supported by NSFC (Grant No.11672002) and NSAF (Grant No. U1730103). 7
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