Ping Song ,Zhonglei Ma ,Hua Qiu ,Yifan Ru ,Junwei Gu

ABSTRACT With the rapid development of fifth?generation mobile com?munication technology and wearable electronic devices,electromagnetic interference and radiation pollution caused by electromagnetic waves have attracted worldwide attention.Therefore,the design and development of highly efficient EMI shielding materials are of great importance.In this work,the three?dimensional graphene oxide (GO) with regular honeycomb structure (GH) is firstly constructed by sacrificial template and freeze?dry?ing methods.Then,the amino functionalized FeNi alloy particles (f?FeNi) are loaded on the GH skeleton followed by in?situ reduction to prepare rGH@FeNi aerogel.Finally,the rGH@FeNi/epoxy EMI shielding com?posites with regular honeycomb structure is obtained by vacuum?assisted impregnation of epoxy resin.Benefitting from the construction of regular honeycomb structure and electromagnetic synergistic effect,the rGH@FeNi/epoxy composites with a low rGH@FeNi mass fraction of 2.1 wt% (rGH and f?FeNi are 1.2 and 0.9 wt%,respectively) exhibit a high EMI shielding effectiveness (EMI SE) of 46 dB,which is 5.8 times of that (8 dB) for rGO/FeNi/epoxy composites with the same rGO/FeNi mass fraction.At the same time,the rGH@FeNi/epoxy composites also possess excellent thermal stability (heat?resistance index and temperature at the maximum decomposition rate are 179.1 and 389.0 °C respectively) and mechanical properties (storage modulus is 8296.2 MPa).

KEYWORDS Electromagnetic interference shielding;rGO@FeNi;Epoxy resins;Honeycomb structures

1 Introduction

With the rapid development of fifth?generation (5G) commu?nication technology and wearable electronic devices,electro?magnetic interference (EMI) and radiation pollution caused by electromagnetic waves have become prominent issues in consumer electronics,artificial intelligence and information security,etc.The leakage of electromagnetic waves will not only cause malfunctioning issues of electronic communica?tion equipment and its delicate components,but also pose serious threats to human health [1—4].Polymer?based EMI shielding composites have become an important research direction of efficient EMI shielding materials due to their advantages of light weight,corrosion resistance,stable EMI shielding performance and outstanding mechanical proper?ties [5—7].

Currently,polymer?based EMI shielding composites mainly achieve their excellent EMI shielding performances by incorporating conductive and/or magnetic functional fill?ers into polymer matrix.Commonly used polymer matrixes include natural rubber (NR),polyethylene (PE),polypro?pylene (PP),polyurethane (PU),polyvinylidene fluoride (PVDF),polyimide (PI) and epoxy resins,etc.[8—10].The conductive fillers usually include metals,conductive poly?mers and carbon materials.Among them,carbon materi?als such as carbon black,graphite,carbon fiber,carbon nanotubes (CNTs) and reduced graphene oxide (rGO) have attracted extensive attention due to their advantages of low density,high electrical conductivity (σ),corrosion resist?ance and easy processing [11—13].The rGO,especially,has been widely used in EMI shielding due to its unique two?dimensional lamellar structure,excellentσand outstand?ing mechanical properties [14—16].However,rGO tends to agglomerate in polymer matrix and has a large contact resistance,resulting in the relatively high conductive per?colation threshold of polymer?based composites [17,18].Therefore,high rGO mass fraction is usually required to obtain excellent EMI shielding performance,which greatly affects the mechanical properties and processabilities of the composites [19].

The construction of efficient three?dimensional (3D) conductive networks can effectively reduce the percolation threshold of rGO/polymer composites,so as to realize the synergistic improvement ofσ,EMI shielding performances and mechanical properties for polymer composites at low filler content [20,21].Current methods for constructing effi?cient 3D conductive networks in polymer?based composites mainly include foaming,surface coating?hot pressing and sol—gel methods,etc.[22].Eswaraiah et al.[23] reported the fabrication of functionalized rGO/PVDF foam composites using 2,2’?azobiso?butanitrile (AIBN) as the chemical foam?ing agent.The resultant rGO/PVDF foam composites had an EMI shielding effectiveness (EMI SE) of 28 dB when the rGO mass fraction was 7 wt%.Yan et al.[24] fabricated the rGO/PS EMI shielding composites with segregated struc?ture by dip?coating rGO onto the surface of PS particles followed by hot pressing them together.The EMI SE of rGO/PS composites reached 45 dB when the rGO volume fraction was 3.47 vol%.In our previous work,Gu et al.[25] firstly prepared the graphite nanoplatelets (GNPs)/rGO aerogel by sol—gel method,and then obtained the GNPs/rGO/epoxy EMI shielding composites by vacuum?assisted impregna?tion of epoxy resin.The results show that the EMI SE of GNPs/rGO/epoxy nanocomposites reached 51 dB when the rGO and GNPs mass fractions were 0.1 and 20.4 wt%,respectively.

Although the constructed 3D conductive networks can endow the polymer composites with excellentσand EMI shielding performances at low filler content,the conductive networks obtained by the above methods have poor struc?tural regularity and the distribution of nanofillers is not easy to control,leading to the poor stability ofσand EMI shield?ing performances [26,27].The rGO conductive networks with honeycomb structure based on alumina (Al2O3) hon?eycomb plates possess the advantages of regular morphol?ogy,and controllable size,shape and distribution of holes,as well as good repeatability and stability.In our previous work,Gu et al.[28] used Al2O3honeycomb plates as tem?plates to prepare the rGO with regular honeycomb structure (rGH)/epoxy EMI shielding composites by sacrificial tem?plate,freeze?drying and vacuum?impregnation methods.The obtained rGH/epoxy composites at a mass fraction of 1.2 wt% rGH exhibit an elevated EMI SE value of 38 dB,which was 6.3 times of that of the homogeneously blended rGO/epoxy composites at the same rGO mass fraction.

In addition,the EMI shielding mechanism of the existing polymer?based composites is mainly based on conduction loss,and the impedance mismatch between their surface and air results in the reflections of electromagnetic waves on the surface of composites,which is easy to cause the secondary electromagnetic wave pollution [29,30].Studies have shown that the introduction of magnetic materials can improve the impedance match between the composites and exter?nal space,thus reducing the reflections of electromagnetic waves and secondary contamination [31—33].The magnetic materials mainly include pure metal (iron,cobalt and nickel,etc.),metal oxides (iron oxide,ferrite and nickel oxide,etc.) and metal alloys (iron?cobalt alloy,iron?nickel alloy and cobalt—nickel alloy,etc.).FeNi alloy exhibits excellent initial permeability,relative permeability,as well as low coercivity and repeated magnetization loss,and shows great potentials in EMI shielding applications [34—36].

In this paper,Al2O3honeycomb plates were used as tem?plates to construct GO with regular honeycomb structure (GH) by sacrificial template and freeze?drying.Then,the surface functionalized FeNi alloy particles (f?FeNi) were loaded on the GH aerogel and reduced in situ to prepare magnetic and conductive rGH@FeNi aerogel Finally,rGH@FeNi/epoxy EMI shielding composites with regular honey?comb structure were obtained by vacuum?assisted impreg?nation of epoxy resin and high temperature curing.The chemical structures off?FeNi,rGH,and rGH@FeNi were characterized and analyzed by X?ray diffraction (XRD),Raman spectroscopy,X?ray photoelectron spectroscopy (XPS) and physical property measurement system (PPMS).The microstructures of rGH@FeNi aerogel and rGH@FeNi/epoxy composites were observed by scanning elec?tron microscope (SEM).On this basis,the effects of regular honeycomb structure andf?FeNi loading on electrical con?ductivities (σ),EMI SE,thermal stability and mechanical properties of rGH@FeNi/epoxy composites were detailedly investigated.

2 Experimental

2.1 Preparation of rGH@FeNi Aerogel

The GH aerogel was prepared according to our previous work [28].The microporous Al2O3honeycomb plate (pore size 500 μm) was firstly placed in 10 g L?1GO solution pre?pared by oxidative exfoliation of graphite flakes [37].Subse?quently,GO was completely filled in the Al2O3honeycomb lattice by vacuum degassing.After standing for 48 h,it was frozen in ?25 °C refrigerator,and then freeze?dried for 48 h to obtain GO?Al2O3honeycomb composite structure.Then,it was placed in dilute hydrochloric acid (HCl) to remove the Al2O3honeycomb plate,and the GH aerogel was obtained by freeze?drying for 48 h after repeated washing with deion?ized (DI) water to remove the residual dilute HCl.

FeNi alloy particles (2 g) and ammonium persulfate (APS,1 g) were added into 100 mL DI and mechanically stirred for 30 min.After stirring at 70 °C for 30 min,acrylic acid (AA,10 mL) was added into the FeNi disper?sion and stirred for 3 h.Ethylenediamine (EN,20 mL) was added into the above mixture drop by drop and con?tinuously stirred for 3 h.The product was washed for several times with DI water until the pH was about 7.After drying,thef?FeNi alloy particles were prepared.

f?FeNi was placed in DI water and mechanically stirred for 30 min to form a homogeneous dispersion.GH aerogel was placed in the obtained dispersion with slow stirring for 30 min,and then heated to 60 °C with continuous stir?ring for 2 h.GH@FeNi aerogel was obtained by freeze?drying for 48 h.The GH@FeNi aerogel was annealed under 5% hydrogen/argon (V/V) atmosphere at 600 °C for 2 h to obtain rGH@FeNi aerogel.

2.2 Preparation of rGH@FeNi/epoxy Composites

Epon 862 bisphenol F epoxy resin and EK3402 curing agent (100/26.5,W/W) were mixed and mechanically stirred at 70 °C for 30 min to obtain a uniform mixture.The mixture was then poured into a mold containing rGH@FeNi aerogel,followed by vacuum?assisted degas?sing to remove air bubbles.The rGH@FeNi/epoxy EMI shielding composites were obtained by curing at 120 °C for 5 h.Schematic diagram of the fabrication for rGH@FeNi/epoxy composites is shown in Fig.1.The mass frac?tions of fillers in rGH@FeNi/epoxy composites are shown in Table 1.For comparison,rGO/FeNi/epoxy EMI shield?ing composites with the same rGO/FeNi mass fraction were prepared by direct blending.

Table 1 Filler mass fraction of rGH@FeNi/epoxy composites

Fig.1 Schematic diagram of the fabrication for rGH@FeNi/epoxy composites

3 Results and Discussion

3.1 Characterizations of rGH,f?FeNi,GH@FeNi and rGH@FeNi

Figure S1a shows the magnetic variation curves of rGH,f?FeNi and rGH@FeNi aerogels.It can be seen thatf?FeNi shows excellent magnetic properties with saturation mag?netization of 113.6 emu g?1,while rGH is nonmagnetic.The hysteresis loop of rGH@FeNi aerogel exhibits typical superparamagnetism [38],with a saturation magnetization of 48.7 emu g?1,which is slightly lower than that off?FeNi.The calculated mass fraction off?FeNi accounts for 43% of the total filler mass,which is consistent with the experimen?tal design scheme.Figure S1b further shows that the rGH@FeNi aerogel has excellent magnetic properties.

Figure 2 is XRD (a),Raman (b) and XPS (c) spectra off?FeNi,GH@FeNi and rGH@FeNi aerogels.FeNi andf?FeNi both present the same characteristic diffraction peak positions in XRD spectra (Fig.S2),indicating that the amino functionalized modification did not change the crys?tal phase structure of FeNi.The diffraction peaks at 43.6° and 50.8° correspond to (111) and (200) crystal planes of α?Fe inf?FeNi cubic respectively,and the diffraction peak at 5.3° corresponds to (220) crystal plane off?FeNi face?centered cubic Ni [39].In XRD spectrum of GH@FeNi aerogel,the characteristic diffraction peak at 9.9° corre?sponds to (001) crystal plane of GO.No obvious GO dif?fraction peak was observed in XRD spectrum of rGH@FeNi aerogel,but a broad and weak characteristic diffrac?tion peak of rGO appears at 26.0°,implying that the GH has been completely reduced to rGH [40,41].The XRD pattern of rGH@FeNi aerogel also shows the characteristic diffraction peak off?FeNi,indicating that rGH@FeNi aero?gel can retain the crystal phase structure of rGO andf?FeNicompletely.All Raman spectra show the D?band around 1343 cm?1and G?band around 1589 cm?1.The intensity ratios of D?band and G?band (ID/IG) forf?FeNi,GH@FeNi and rGH@FeNi aerogels are 0.84,0.89,and 1.02,respec?tively,which is because that the introduction off?FeNi and reduction of GO will increase defect degree of GH.The proper defect and graphitization degree can improve the impedance matching with enhanced polarization relaxation and dipole relaxation,which is beneficial to the attenua?tion of electromagnetic waves [36].All XPS spectra show the characteristic absorption peaks of C 1 s,N 1 s,and O 1 s at 284.8,398.8,and 531.8 eV.The GH@FeNi aerogel presents characteristic absorption peaks of Fe 2p and Ni 2p at 710.8 and 854.8 eV,respectively,indicating that thef?FeNi is successfully grafted on the surface of GH.The C/O value of rGH@FeNi is increased significantly from 1.7 to 8.1,confirming the complete reduction of GH@FeNi [42],which is consistent with the XRD results.Figure 2c’ shows the high?resolution C 1 s spectra off?FeNi,GH@FeNi and rGH@FeNi aerogel.It can be seen that the high?resolution C 1 s spectrum off?FeNi presents four characteristic peaks at 284.6,285.6,286.6,and 288.6 eV corresponding to unoxi?dized carbon (C—C),carbon in hydroxyl and epoxy groups (C?O),carbon in carbonyl group (C=O),and carbon in carboxyl group (O?C=O),respectively.After grafting off?FeNi on GH surface,the high?resolution C 1 s spectrum of GH@FeNi aerogel shows a characteristic peak of C?N at 286.1 eV,which is attributed to the generated C?N bond by condensation reaction between ?NH2onf?FeNi and ?COOH on GH.The results demonstrate that thef?FeNi has been successfully grafted on the surface of GH skeleton.The peak intensity of oxygen?containing functional groups decreased significantly after thermal annealing,indicating that the GH@FeNi aerogel has been successfully reduced to rGH@FeNi aerogel [43,44].

Fig.2 XRD (a),Raman (b),XPS spectra (c) and high?resolution C 1 s (c′) of f?FeNi,GH@FeNi and rGH@FeNi aerogel (rGH 1.2 wt%,f?FeNi 0.9 wt%)

3.2 Morphologies of rGH@FeNi Aerogel and rGH@FeNi/epoxy Composites

Figure 3 shows SEM images of rGH@FeNi aerogel and rGH@FeNi/epoxy composites.As can be seen from Fig.3a,a′,the rGH@FeNi aerogel presents anisotropic structures with highly regular hexagonal honeycomb structure in cross section and highly oriented prismatic structure in longitudinal sections.The side length of honeycomb cells is about 500 μm.When the Al2O3tem?plate is immersed in GO solution,the surface of Al2O3template is ionized with released Al3+spontaneously diffused into the weakly acidic GO solution.This sig?nificantly increases the ionic strength near Al2O3tem?plate and causes the negatively charged GO nanosheets to aggregate on the Al2O3template and form GO hydro?gel with hollow hexagonal honeycomb structure.The distribution of C,Fe,and Ni elements in EDS diagram (Fig.3a′′) also shows that thef?FeNi has been success?fully grafted on the surface of rGH skeleton,and the rGH@FeNi aerogel present hexagonal honeycomb and prismatic structures in cross and longitudinal sections,respectively.Therefore,thef?FeNi and rGO construct a more effective 3D conductive and magnetic path.As shown in Fig.3b,b′,the rGH@FeNi aerogel can maintain its morphology without obvious damage after the back?filling of epoxy resin.The fracture surfaces of rGH@FeNi/epoxy composite still present the original regular hexagonal honeycomb and prismatic structures in cross and longitudinal sections,respectively.The construction of highly oriented hexagonal honeycomb structure is of great significance for the improvement of σ,EMI shield?ing and mechanical properties of the composites.

Fig.3 SEM images of rGH@FeNi aerogel (rGH 1.2 wt%,f?FeNi 0.9 wt%) along the cross and longitudinal sections (a &a′),elemental map?ping images of C,Fe and Ni for rGH@FeNi aerogel (a′′),SEM images of rGH@FeNi/epoxy composites (rGH 1.2 wt%,f?FeNi 0.9 wt%) along the cross and longitudinal sections (b &b′)

3.3 Electrical Conductivities and EMI Shielding Performances of rGH@FeNi/Epoxy Composites

Figure 4a isσof rGO/FeNi/epoxy and rGH@FeNi/epoxy EMI shielding composites.With the same mass fraction of rGO and rGH (1.2 wt%),the two kinds of composites both exhibit decreasedσas thef?FeNi mass fraction increases.When the mass fraction of rGH@FeNi is 2.1 wt% (the mass fraction off?FeNi is 0.9 wt%),theσof rGH@FeNi/epoxy composites is 38.7 S m?1,which is much higher than that of rGO/FeNi/epoxy composites with the same mass frac?tion.However,it is slightly lower than theσ(40.2 S m?1) of rGH/epoxy composites withoutf?FeNi.For rGO/FeNi/epoxy composites,the rGO tends to aggregate easily in the epoxy resin matrix during physical blending due to its high surface energy,resulting in the difficulty of forming perfect conduc?tive path and relatively lowσ.When rGO is constructed into rGH with the assistance of honeycomb template,the rGO forms an efficient 3D continuous conductive network inside the epoxy resin matrix,thus leading to the greatly increasedσ[27,45].As the mass fraction of magneticf?FeNi increases,the contact resistance between rGH conductive networks gradually increases due to the intrinsic electrical insulation off?FeNi,leading to the reducedσof rGH@FeNi/epoxy composites to a certain extent.

Figure 4 b,c shows the EMI SETvalues of rGO/FeNi/epoxy and rGH@FeNi/epoxy EMI shielding composites,respectively.With the same mass fraction of rGO and rGH (1.2 wt%),the EMI SETof rGO/FeNi/epoxy composites gradually increases as thef?FeNi mass fraction increases,while the that of rGH@FeNi/epoxy composites firstly increases and then decreases.When the mass fraction of rGH@FeNi is 2.1 wt% (the mass fraction off?FeNi is 0.9 wt%),the EMI SETof rGH@FeNi/epoxy composites reach the maximum value of 46 dB,which is 5.8 times of that of rGO/FeNi/epoxy composites (8 dB) with same mass frac?tion.On the one hand,the honeycomb structure in rGH@FeNi/epoxy composites constructs a highly effective 3D conductive rGH network inside the epoxy resin matrix,leading to the significantly increasedσand thus improved ohmic loss of incident electromagnetic waves [46,47].On the other hand,the continuous 3D conductive/magnetic rGH network can enhance the multiple reflection and scattering of electromagnetic waves in rGH@FeNi/epoxy composites and extend the transmission path of electromagnetic waves,which contribute to the attenuation of electromagnetic waves and EMI SETof the composites [48,49].Meanwhile,the increasing mass fraction off?FeNi can enhance the hyster?esis loss of electromagnetic waves in the rGH@FeNi/epoxy composites.Nevertheless,as the mass fraction off?FeNi fur?ther increases to 1.2 wt%,the excessivef?FeNi is detrimental to the gelation of rGH and brings damage to the construction of rGH@FeNi continuous network to some extent (Fig.S3).Therefore,the EMI SETof rGH@FeNi/epoxy composites decreases slightly when the mass fraction of rGH@FeNi is 2.4 wt%.

Figure 4d is the electromagnetic wave effective absorb?ance of rGH@FeNi/epoxy EMI shielding composites (cal?culated by Eq.S9).It can be seen that with a constant mass fraction of rGH (1.2 wt%),the electromagnetic wave effec?tive absorbance of rGH@FeNi/epoxy composites gradually increases as the mass fraction off?FeNi increases.When the mass fraction of rGH@FeNi is 2.1 wt% (the mass fraction off?FeNi is 0.9 wt%),the electromagnetic wave absorption rate of rGH@FeNi/epoxy composites is 89.6%,which is higher than that of rGH/epoxy composite withoutf?FeNi (84.5%).This is because that the loading off?FeNi enhances the hysteresis loss of rGH@FeNi/epoxy composites to elec?tromagnetic waves due to the improved impedance match?ing between rGH 3D conductive skeleton and epoxy resin matrix,thus leading to the significantly increased effective absorbance of the composites [36,39].

Figure 4e shows the absorpted shielding effective?ness (SEA) and reflected shielding effectiveness (SER) of rGH@FeNi/epoxy EMI shielding composites in X?band.As can be seen,with the increase off?FeNi mass frac?tion,the SERof rGH@FeNi/epoxy composites remain at about 5 dB with the constant rGH mass fraction (1.2 wt%).The SEAand SETshow the same variation tendency,both of which firstly increase and then decrease with the increase off?FeNi mass fraction.When the mass fraction of rGH@FeNi is 2.1 wt% (the mass fraction off?FeNi is 0.9 wt%),the SEAof rGH@FeNi/epoxy composites can reach 41 dB.This is mainly attributed to the fact that the incorporation off?FeNi can improve the impedance matching of composites and enhance the hysteresis loss of electromagnetic waves in composites,thus improving the SEAvalue.For practical application,the EMI shield?ing mechanism of rGH@FeNi/epoxy composites is evalu?ated by reflection (R),absorption (A) and transmission (T) coefficients (Eqs.S3?S5).As shown in Fig.4f,the rGH@FeNi/epoxy composites with variousf?FeNi mass fraction all show extremely low T coefficient.When the mass fraction of rGH@FeNi is 2.1 wt% (the mass fraction off?FeNi is 0.9 wt%),the T coefficient of rGH@FeNi/epoxy EMI shielding composites is as low as only 2 × 10—5,demonstrating that the incorporation off?FeNi can signifi?cantly improve the EMI shielding performances.At the same time,the R value of rGH@FeNi/epoxy composites gradually decreases with the constant rGH mass fraction (1.2 wt%) and increasedf?FeNi mass fraction.It is because that the incorporatedf?FeNi can reduceσand increase the magnetism,which can improve the impedance matching between rGH@FeNi cellular network and external space.This results in the increased absorption and reduced reflec?tion of electromagnetic waves on the surface of rGH@FeNi/epoxy composites,and is beneficial to reducing the secondary pollution of electromagnetic waves [50,51].

Fig.4 σ of rGO/FeNi/epoxy and rGH@FeNi/epoxy composites (a),EMI SET of rGO/FeNi/epoxy composites (b),EMI SET (c),effective absorbance (d),EMI SEA &SER (e) and R,A and T coefficients (f) of rGH@FeNi/epoxy composites

When the incident electromagnetic wave reaches the sur?face of rGH@FeNi/epoxy EMI shielding composites,some incident electromagnetic waves are reflected on the surface of rGH@FeNi/epoxy composites due to the impedance mismatch between external space and composites,while the rest electromagnetic waves enter the interior of rGH@FeNi/epoxy composites.First of all,when electromagnetic waves pass through the honeycomb wall of rGH,the highly con?ductive rGH will generate induced current and cause ohmic losses of electromagnetic waves [22,52].Secondly,the existence of the honeycomb conductive/magnetic structure lengthens the transmission path of electromagnetic waves in the composites,leading to the enhanced attenuation of electromagnetic waves by multiple reflection and scattering [53].Meanwhile,the hysteresis loss caused by the magneticf?FeNi also contribute to the absorption of incident electro?magnetic waves [54].In addition,as the σ of rGH is much higher than that of epoxy resin,there are different σ values on the two sides of interfaces.Under the external electric field,the positive charges and negative charges accumulate in the rGH and epoxy resin sides respectively,thus result?ing in the interface polarization loss [55,56].Therefore,the multiple loss mechanism composed of ohmic loss,internal multiple reflections,hysteresis loss and interface polariza?tion loss greatly improve the EMI shielding performance of rGH@FeNi/epoxy composites.The EMI shielding mecha?nism of rGH@FeNi/epoxy composites is shown in Fig.5.

Fig.5 EMI shielding mechanism of rGH@FeNi/epoxy composites

3.4 Thermal Stabilities of the rGH@FeNi/epoxy Composites

Figure 6a,a′ show the thermal gravimetric analysis (TGA) and derivative thermogravimetric (DTG) curves of pure epoxy and rGH@FeNi/epoxy EMI shielding composites,respectively [57],with the corresponding characteristic data shown in Table 2.The heat?resistance index (THRI) is calcu?lated by Eq.1 to evaluate the long?term service temperature of polymer nanocomposites.

Table 2 Thermal characteristic data of the rGH@FeNi/epoxy com?posites

Fig.6 TGA (a) and DTG (a′) curves of the rGH@FeNi/epoxy composites

whereT5andT30correspond to the decomposition tempera?tures of 5% and 30% weight loss,respectively.It can be seen that the heat?resistance index (THRI) and temperature at the maximum decomposition rate (Tdmax) of pure epoxy are 173.5 °C and 381.3 °C,respectively.TheT5,T30,THRI,andTdmaxof rGH@FeNi/epoxy composites are significantly improved with the increased FeNi mass fraction and con?stant rGH mass fraction (1.2 wt%).When the mass fraction of rGH@FeNi is 2.1 wt% (the mass fraction of FeNi is 0.9 wt%),theTHRIandTdmaxof rGH@FeNi/epoxy composites can reach 179.1 and 389.0 °C,respectively,which are 5.6 and 7.7 °C higher than those of pure epoxy,respectively.This is because that the incorporation off?FeNi with excel?lent thermal stability can improve the heat resistance of rGH@FeNi/epoxy composites.In addition,the 3D honey?comb network structure formed by rGH@FeNi can effec?tively prevent the thermal degradation of rGH@FeNi/epoxy composites,slow down the thermal degradation rate,thus resulting the improved thermal stability [58,59].

3.5 Dynamic Mechanical Analysis of the rGH@FeNi/epoxy Composites

Figure 7a,a′ shows the dynamic mechanical analysis (DMA) curves of rGH@FeNi/epoxy EMI shielding com?posites and their corresponding storage modulus (Es) at the temperature of 40 °C,respectively.It can be seen that theEsof pure epoxy is 5800.2 MPa.With the constant mass fraction of rGH (1.2 wt%),theEsof rGH@FeNi/epoxy composites first increase and then decrease with the increasingf?FeNi mass fraction.When the mass fraction of rGH@FeNi is 2.1 wt% (the mass fraction off?FeNi is 0.9 wt%),the rGH@FeNi/epoxy composites exhibit the largestEsof 8296.2 MPa,which is 43% higher than that of pure epoxy.The related reason is that in incorporation off?FeNi increases the surface roughness of rGH@FeNi aerogel,which can inhibit the activity of molecular chain segments of epoxy resin due to the interface binding effect,thus leading to the improvedEs[60,61].With the further increase off?FeNi mass fraction (2.4 wt%),the excessive FeNi destroys the 3D honeycomb network structure of rGH@FeNi aerogels to some extent,resulting in the slight decrease ofEs.

4 Conclusions

In conclusion,the rGH@FeNi/epoxy EMI shielding com?posites with regular 3D honeycomb structures were prepared by sacrificial template,freeze?drying and vacuum?assisted impregnation of epoxy resin.Benefitting from the construc?tion of 3D honeycomb structure and electromagnetic syner?gistic effect,the rGH@FeNi/epoxy composites with a low rGH@FeNi mass fraction of 2.1 wt% (rGH andf?FeNi are 1.2 and 0.9 wt% respectively) exhibit a high EMI shielding effec?tiveness (EMI SE) of 46 dB,which is 5.8 times of that (8 dB) for rGO/FeNi/epoxy composites with the same rGO/FeNi mass fraction.Meanwhile,the rGH@FeNi/epoxy composites also possess excellent thermal stability (THRIandTdmaxwere 179.1 and 389.0 °C,respectively) and mechanical properties (Eswas 8296.2 MPa).It is believed that the epoxy?based EMI shielding composites with comprehensive performances have great application potentials in consumer electronics,artificial intelligence and information security,etc.

Fig.7 DMA curves (a) and Es (a′) of the rGH@FeNi/epoxy composites

AcknowledgementsThe authors are grateful for the supports from the National Natural Science Foundation of China (U21A2093 and 51903145),Natural Science Basic Research Plan for Distin?guished Young Scholars in Shaanxi Province of China (2019JC?11) and Fundamental Research Funds for the Central Universi?ties (D5000210627).This work is also financially supported by Polymer Electromagnetic Functional Materials Innovation Team of Shaanxi Sanqin Scholars.

FundingOpen access funding provided by Shanghai Jiao Tong University.

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Supplementary InformationThe online version contains supplementary material available at https:// doi.org/ 10.1007/ s40820? 022? 00798?5.