Journal Pre-proof In-situ grown hollow Fe3O4 onto graphene foam nanocomposites with high EMI shielding effectiveness and thermal conductivity Haoming Fang, Haichang Guo, Yiran Hu, Yanjuan Ren, Po-Chun Hsu, Shu-Lin Bai PII:
S0266-3538(19)32599-0
DOI:
https://doi.org/10.1016/j.compscitech.2019.107975
Reference:
CSTE 107975
To appear in:
Composites Science and Technology
Received Date: 15 September 2019 Revised Date:
18 December 2019
Accepted Date: 26 December 2019
Please cite this article as: Fang H, Guo H, Hu Y, Ren Y, Hsu P-C, Bai S-L, In-situ grown hollow Fe3O4 onto graphene foam nanocomposites with high EMI shielding effectiveness and thermal conductivity, Composites Science and Technology (2020), doi: https://doi.org/10.1016/j.compscitech.2019.107975. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
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 and editing;
Shu-Lin Bai: Supervision; Writing – review and editing; Funding acquisition
In-situ Grown Hollow Fe3O4 onto Graphene Foam Nanocomposites with High EMI Shielding Effectiveness and Thermal Conductivity Haoming Fanga,b, Haichang Guoa, Yiran Hu , Yanjuan Rena,Po-Chun Hsub, Shu-Lin Baia* c
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, North Carolina 27708, USA
c College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China *Corresponding author. E-mail address:
[email protected]
Abstract 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 insitu 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 super-flexible performance with high electrical conductivity (84.02 ± 8.385 S cm-1). In order to reveal the mechanisms, the finite element method (FEM) simulation and theoretical modeling are carried out by considering the effect of content and morphology of Fe3O4 spheres with different preparation processing. 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.
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Keywords: compressed graphene foam; in-situ grown hollow Fe3O4; EMI shielding effectiveness; thermal conductivity.
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 devices are the two major issues that have raised wide and enormous interests 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 composites are transparent to the electromagnetic waves showing inefficiency in the electromagnetic interference shielding. The EMI shielding materials require the high electric conductivity for electromagnetic 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 composites. For better absorption attenuation and
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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 inorganic 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 magnetic fillers are insufficient in TC enhancement. Graphene with high TC 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 demonstrated 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 graphenebased composites is a feasible way [19]. The method to prepare Fe3O4/graphene 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 composites 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 investigation on the hollow Fe3O4 spheres/3D graphene composites for high EMI SE and thermal conductivity is meaningful.
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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/hFe3O4/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 networks of the GF/h-Fe3O4/PDMS composites show super-flexible performance 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 undertaken 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. Materials and Experiment 2.1 Materials Nickel foams (2mm 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
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Sinopharm Chemical Reagent Co., Ltd. The solid Fe3O4 spheres (diameter around 900 nm) were purchased from Shanghai Institute of Metallurgy, Chinese Academy of Sciences. 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 Figure 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 (Figure S8 and Table S1). Then the PDA modified GF was acquired via a similar approach as our previous work [25]. Subsequently, 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 150mL Teflon container with modified GF. The mixture was stirred for 10 min, sequentially adding 1.6 ml deionized water into the corresponding solution 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 temperature 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 = 10/1 in weight) at ambient temperature. Finally, the mixture was vacuumed and cured at 80 °C for 4 h to get the corresponding composites.
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Figure 1 Schematic of the fabrication procedure of GF/h-Fe3O4/PDMS 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, 5V) 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 = 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 Xray source at 225 W. The Raman spectra were tested on Micro Raman imaging spectrometer with He–Ne laser excited at 532 nm (Thermo Scientific, 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).
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2.3.2 Thermal properties measurement Thermal conductivity was calculated from the equation: =
×
× , where
, ,
, and
represent thermal conductivity, thermal diffusivity, 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 (Figure S18a and Figure 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 (Figure S18b and Figure S18d). 2.3.3 Mechanical, electrical and EMI shielding effectiveness measurement The in-plane electrical conductivity was characterized using a four-probe 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 was 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 mm3. Typically, two silver wires (0.25mm, Alfa) were fixed at the two ends of the sample by high purity silver paint (SPI Supplies) and connected to the digital source meter (Keithley 2635A). After that, the sample was mounted onto the testing 7
instrument and tested at a loading rate of 1.5 mm/min. The displacement direction is along with the in-plane direction of compressed graphene as shown in Figure. S10. The permittivity, dielectric loss and shielding effectiveness (SE) of each sample was measured by a vector network analyzer (VNA, E8363C, 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 rectangular 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 effectiveness can be calculated by transmittance (T), absorption (A) and reflectance (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.
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 (Figures 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 cubicphase h-Fe3O4 spheres are characterized via the selected-area electron diffraction (SAED). The SEM images of the interconnected pristine and highly-compressed 3D GF/h-Fe3O4 skeletons (Figures 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 Figure S2 shows five layers of the graphene sheet. Compared with pure h-Fe3O4 spheres (Figure S5), the obvious N peak in XPS spectra (Figure 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.
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Therefore, h-Fe3O4 spheres were synthesized onto the graphene surface with a relative high content of 9.72 wt.% (measured by TGA in Figure S6). The peaks located at 710.0 eV and 723.7 eV are assigned to Fe 2p3/2 and Fe 2p1/2, respectively (Figure 2e). The Fe 2p3/2 peak for h-Fe3O4 was deconvoluted into the Fe2+ (Blue area) and Fe3+ peaks (Red area), the ratio of the area in Fe2+ and Fe3+ 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 calcination treatments (Figure 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 reference (JCPDS card no. 63-3107) of the cubic Fe3O4 phase, all the identified peaks (Figures 2f and S7) verify the polycrystalline structure of h-Fe3O4 again.
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Figure 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 hFe3O4/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 Fe2+peak, the red area to the Fe3+peak, 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 hFe3O4. (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).
3.2 Electro-mechanical properties and EMI shielding efficiency of composites 10
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 (Figure 3a). In addition, uniaxial cyclic tension with insitu 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 (Figure 3b). The hysteresis loop is seen obviously after every tensile cycle under the strain of 20% (Figure 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. Figure 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 12wt.%, the composite exhibits ultra-high average EMI SE value of 70.37dB, i.e. 22 dB enhancement compared with GF/PDMS composites with the same loading as shown in Figure 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 electromagnetic wave. As shown in Figure 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 h-Fe3O4 (Figure 3h) as well as the inner/outer surfaces of h-Fe3O4 spheres (Figure 3i) will cause energy dissipation, which subsequently results in the absorption attenuation (the magnetic properties of GF/h-Fe3O4 are shown in
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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 composites is absorption attenuation and reflection. As shown in Figure 3d, with the loading increase of GF/h-Fe3O4, the SEref of composites enhances slowly when compared with SEabs. Further, the ratio of average SEabs and SEref rises from 2.82 to 5.03 (Figures S12), suggesting the dominating mechanism of GF/h-Fe3O4/PDMS composites is absorption attenuation which is different from the most graphene filled electromagnetic shielding composites. Compared with the reported flexible PDMS (the EC is lower than 5 S cm-1) based composites [20, 31-37] (Figure 3f and Table S3), the GF/h-Fe3O4/PDMS composites possess a superior EMI SE due to the excellent electrical conductivity and strong interfacial interaction.
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Figure 3 (a) In-plane electrical conductivity of GF/PDMS and GF/h-Fe3O4/PDMS composites. (b) Electromechanical properties of GF/h-Fe3O4/PDMS composites as a function of strain. (c) EMI SE of GF/hFe3O4/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. 13
To verify the effect of in-situ growth h-Fe3O4 spheres, the distribution (random and in-situ grown) and morphologies (solid and hollow) of Fe3O4 spheres have been investigated. Except for in-situ grown GF/h-Fe3O4/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 Fe3O4/PDMS),
(denoted
as
h-Fe3O4/PDMS),
GF/solid-Fe3O4/PDMS
GF/hollow-Fe3O4/PDMS (denoted as GF/r-h-Fe3O4/PDMS)
(denoted
as
GF/r-s-
and GF/in-situ grown h-
Fe3O4/PDMS composites (denoted as GF/in-situ-h-Fe3O4/PDMS) with same loading were prepared under same conditions as described in supporting information. 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 Figure S13). Compared with pure PDMS, the addition of two types of Fe3O4 spheres would not induce obvious change in SEtot, SEabs, SEref, εʹ, εʹʹ and uʹ (Figures S14 and S15) due to the nearly insulated properties and lack of magnetic resonance without electron transfer condition. However, 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 ε', ε'' and µ' (Figure 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 h-Fe3O4 in the composites (both in hFe3O4/PDMS and GF/r-h-Fe3O4/PDMS) show the higher ε', 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
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grown approach is able to induce stronger interfacial interaction between Fe3O4 spheres and graphene leading to higher electromagnetic attenuation. 3.3 Thermal transport performance of composites To investigate the thermal transport performance of GF and GF/h-Fe3O4, the in-plane (κ⊥) and outof-plane (κ??ǁ), TC values of composites with different loadings are measured (Figure 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, κ??ǁ 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 κ??ǁ over κ⊥ and presents the anisotropic degree of TC as shown in Formula S37. This slight difference 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 Figure 4b. When the maximum compression ratio is up to 95%, the κ??ǁ 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 inplane and out-of-plane directions, respectively. The obvious TC enhancement by the graphene is caused by four factors: high quality, interconnected networks, and high density and orientation. 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 skeleton
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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 (Figure 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 (Figure 4c, formula S20 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 composites [39-43] (Figure 4d and Table S3), the GF/h-Fe3O4/PDMS composites show an outstanding TC enhancement efficiency (formula S36) of 1300, which is near 10 times that of compared materials.
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Figure 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 M’ model in the tag is Maxwell–Eucken model. (d) Summary on TC of various PDMS-based composites. The heat transfer mechanism is further investigated by FEM simulation and infrared (IR) imaging detection. For the FEM simulation by homogenization model (Figures 5a and S19), heat flows from bottom to top during the whole procedure time from 1ms to 200s. On the other hand, the structure model shows the
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heat flow along the GF skeleton in less than 200ms (Figures 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 (Figure S21b). It is found that the actual temperature distribution and heat transfer properties 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 electronic 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 (Figure 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/h-Fe3O4/PDMS composite (Figure 5c). Besides, the temperature reaches a steady-state within only tens of seconds for GF/hFe3O4/PDMS composites under heating and cooling cycles, while pure PDMS shows very sluggish heat transfer (Figure S22). The corresponding FEM multi-physic field simulation was established via the coupling of heat conduction and natural airflow, suggesting the comparable different temperature values and distribution at chips (Figure 5d). Additionally, the FEM simulation is employed to predict the failure temperature of chips attached to different TIMs (Figure 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
18
of the mechanism of cooling efficiency for real application in TIMs are shown in supporting information (Note S7).
Figure 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 19
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.
4 Conclusions In conclusion, we proposed an approach to in-situ grow hollow Fe3O4 spheres onto the graphene surface. After fabrication into polydimethylsiloxane (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 mechanism of thermal conductivity has been investigated by FEM simulation 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.
Supporting information Details of preparation method and characterization. XRD, XPS, TGA and BET spectra of h-Fe3O4. Specific heat capacity of GF/PDMS and GF/Fe3O4/PDMS composites with different loading. The mechanisms of heat transfer in homogenization and structure models in more accurate resolution via FEM simulation. The mechanisms of cooling efficiency for real application in TIMs. The mechanism of real and imaginary parts of permittivity and real permeability part of various composites. Specific summary on TC and EMI SE of
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PDMS based composites. The magnetic properties of pristine and modified GF are shown in supplementary video. Acknowledgements The work is supported by NSFC (Grant No.11672002) and NSAF (Grant No. U1730103). Conflict of Interest The authors declare no conflict of interest. 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). [10] S.H. Lee, S. Yu, F. Shahzad, J. Hong, S.J. Noh, W.N. Kim, S.M. Hong, C.M. Koo, Low percolation 3D Cu and Ag shell network composites for EMI shielding and thermal conduction, Compos. Sci. Technol. 182 (2019). [11] J.-Q. Luo, S. Zhao, H.-B. Zhang, Z. Deng, L. Li, Z.-Z. Yu, Flexible, stretchable and electrically conductive MXene/natural rubber nanocomposite films for efficient electromagnetic interference shielding, Compos. Sci. Technol. 182 (2019).
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: