Graphene-based Electromagnetic Functional Materials

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(School of Material Science and Engineering,Beijing Institute of Technology,Beijing 100081,China)

ABSTRACT:Two-dimensional nanomaterials possess excellent electrical,thermal and mechanical properties,exhibiting great potential in the electromagnetic field.Among them,graphene has a large specific surface area and a high carrier concentration,which is the focus of contemporary technology.Herein,the electromagnetic response mechanism,wave absorption and shielding properties of graphene-based electromagnetic functional materials are demonstrated systematically.In the electromagnetic frequency band (2～18 GHz) investigated,electromagnetic losses include conduction loss,multiple relaxation,magnetic resonance and magnetic eddy current generally.Here,the physical formation mechanism and response characteristics of these four electromagnetic loss behaviors are introduced in detail,and the sources of electromagnetic loss of different graphene-based electromagnetic functional materials are summarized.The strategies for designing high-performance electromagnetic functional materials are also proposed.Subsequently,the application standards of high performance electromagnetic functional materials are presented.The response laws of microwave absorption and electromagnetic shielding are put forward.Two methods to improve the electromagnetic response performance are also proposed.For the electromagnetic functional material performances,the latest research progress of graphene-based electromagnetic functional materials in microwave absorption and electromagnetic shielding is introduced.The single-phase and heterogeneous graphene materials,high temperature dielectric properties and electromagnetic responses are covered.In addition,the key problems in the current development of graphene-based electromagnetic functional materials are systematically analyzed,and the research and development directions in the future are prospected.

KEY WORDS:electromagnetic pollution; graphene; electromagnetic property; electromagnetic loss; microwave absorption;EMI shielding

1 Introduction

With the popularization of Internet of Things and the wide application of electronic terminal equipment,the electromagnetic pollution of big data is increasingly serious.Excessive electromagnetic radiation will not only pollute the environment but also affect the normal work of equipment.Worst of all,it will damage the health of the human body.Specially,in the coming era of artificial intelligence,electromagnetic protection and confrontation are becoming the key issues.Hence,searching for an effective strategy to fight electromagnetic pollution has become a key topic for global researchers.Up to now,great effort has been devoted[1-10].

In order to fight the electromagnetic pollution,an effective method is to develop the high-performance electromagnetic functional materials,such as microwave absorption materials and electromagnetic interference (EMI) shielding materials.The commonly electromagnetic functional materials mainly include dielectric materials,magnetic materials and electromagnetic composites.Specially,low-dimensional nanomaterials with excellent physical and chemical properties attract favor,such as nanoparticles,nanowires,nanosheets,etc.Until now,many excellent absorption and shieding materials have been developed and applied in business.In low-dimensional materials,two-dimensional materials exhibit more prominent electromagnetic response than both zero-dimensional materials and one-dimensional materials,displaying a fantastic potential[11-20].

In recent years,colorful two-dimensional materials exhibit great application value in electromagnetic field,such as graphene,transition metal di-chalcogenides,MXene,etc.Among them,graphene possesses large specific surface area and high carrier concentration,as well as huge reserves in nature,which is becoming a shining star in electromagnetic field.To date,a large number of graphene-based electromagnetic functional materials have been prepared and applied widely.These research results demonstrate that graphene possesses huge potential in electromagnetic fighting,information protection,electromagnetic imaging[21-30].

In this article,the electromagnetic loss mechanism is systematically combed,and the reason for electromagnetic loss is elucidated in detail.Meanwhile,the response regulations of both microwave absorption and EMI shielding are comprehensively overviewed.Subsequently,the research development of graphene-based electromagnetic functional materials in microwave absorption and EMI shielding field is exhibited.Finally,the problems faced by graphene-based electromagnetic functional materials in electromagnetic field are briefly described,and the development and research direction are predicted in the future.

2 Electromagnetic attenuation mechanism of graphene-based electromagnetic functional materials

High-efficiency electromagnetic response is originated from excellent electromagnetic attenuation inside materials.In general,the attenuation behavior consists of two parts,dielectric loss and magnetic loss.Based on the Debye theory,dielectric loss is divided into conduction loss and relaxation loss further.Meanwhile,in the investigated gigahertz frequency band,magnetic loss mainly depends on the eddy currents and resonances.These attenuation behaviors harvest and dissipate electromagnetic energy,realizing materials outstanding microwave absorption and EMI shielding[31-35].

2.1 Dielectric loss

2.1.1 Conduction loss

Conduction loss plays an important role in electromagnetic response for graphene-based electromagnetic functional materials,as exhibited in Fig.1[36-38].The response regulation could be explained by Cao's model well.In 2009,Cao and his workers put forward the Electron-Hopping Model firstly,reveling electron tran-sport behavior in disordered graphite layers.Subsequently,Cao et al.deepened and highlighted the model further,proposing Aggregation-Induced Charge Transport (AICT) model.The low-dimensional carbon materials connect each other forming microcosmic network matrix,and the connection ways include contacting,stacking and the combination of both them.Under electromagnetic field,electrons transporting in the carbon materials network form conductive network,which brings about strong conduction loss (Fig.1).

Meanwhile,two modes for electrons transport in carbon materials are illustrated,namely electron migrating and electron hopping (Fig.1a,1b).On the complete carbon lattice,electrons transport in the form of migration.At defects and interfaces,electrons generate hopping due to the existing of energy barrier (Fig.1c—g).Hence,an effective approach to improve the conductivity (σ) of materials is to enhance the electrons hopping efficiency.In 2014,Wen et al.exhibits the temperature dependence ofσ.Theσincreases with raised temperature.The change is originated from the contribution of temperature to electron hopping.

2.1.2 Relaxation loss

According to the Debye theory,relaxation loss is another key factor for electromagnetic response,as exhibited in Fig.2[39-42].Under preparation process of graphene,abundant defects and groups are introduced,destroying the complete lattice structure of graphene(Fig.2f).It results in the redistribution of electrons on graphene.At defects and groups,many positive and negative charge centers gather to form dipoles.When electromagnetic wave propagates into materials,a builtin electromagnetic field is constructed.Under the electromagnetic field,these dipoles generate orientated rotation causing relaxation loss.It harvests and dissipates electromagnetic energy.Different from conduction loss,relaxation loss possesses strong frequency dependence.Besides defect and group,interface also has an important effect on relaxation loss.In general,interface structure is originated from the adjacent nanosheets and heterogeneous interfaces introduced (Fig.2b,2c).Under the electromagnetic field,electrons at interfaces gather and get rearranged,which causes strong relaxation loss.As shown in Fig.2a,2d,Cao et al.constructed CdS-Fe2O3hetero-structures,and reported the capacitor-like model and the equivalent circuit model firstly,which explains this electromagnetic response behavior at heterogeneous interface well.Subsequently,they prepared ZnO@MWCNTs hybrid structure,demonstrating the contribution of interface to dielectric loss (Fig.2e).

2.2 Magnetic loss

In order to improve electromagnetic response of graphene,magnetic particles are introduced into graphene to obtained magnetic graphene hybrid materials.In the electromagnetic field,commonly used magnetic particles include Fe3O4,Co3O4and NiFe2O4,etc.These magnetic particles possess strong magnetic response,bringing about great magnetic loss.Generally,the magnetic eddy currents and magnetic resonance play the main roles for electromagnetic loss in gigahertz frequency[43-45].

2.2.1 Magnetic resonances

Based on peers' research,magnetic resonance can be divided into two kinds,namely natural resonance and exchange resonance,which can be characterized by complex permeability (μ? andμ″),as exhibited in Fig.3a,3b.In the investigated frequency band (2～18 GHz),the natural resonance appears at low frequency (＜10 GHz),and the exchange resonance appears at high frequency(＞10 GHz).Fig.3d and 3e exhibit the physical models of both them,respectively[46-47].

2.2.2 Magnetic eddy currents

Magnetic eddy current also has an effect on electromagnetic loss,which is caused at magnetic particles and magnetic nanoclusters,as exhibited in Fig.3c and f.Unlike resonance behavior,eddy current is inevitable in magnetic system.The eddy current behavior is characterized by the eddy current coefficient (μ″(μ)-2f-1).Based on the previous research,if the magnetic loss in the material depends only on magnetic eddy current,the value ofμ″(μ)-2f-1is a constant[48-49].It doesn't change with frequency.As shown in Fig.3f,this state generally occurs only at high frequencies in investigated frequency of 2～18 GHz.

3 Electromagnetic response of graphene-based electromagnetic functional materials

3.1 Microwave absorption

The purpose of microwave absorption is to achieve high absorption and dissipation of incident electromagnetic wave.It demands appropriate impedance matching at the first incident interface and high attenuation inside materials.The evaluation standard includes two aspects generally,namely minimum reflection loss (RL) and bandwidth (≤-10 dB).Based on transmission line theory,the microwave absorption performance can be obtained as follows[46]:

WhereZinstands for the wave impedance of microwave absorption materials,calculated as follows:

whereεrandμrrepresent the complex permittivity and permeability of the materials,respectively.cstands for the light velocity,fstands for the electromagnetic wave frequency anddstands for the thickness of the absorber.

Besides reflection loss and bandwidth,the thickness is another important factor for microwave absorption materials,especially in modern production and life.High absorption,large bandwidth and low thickness are the current targets of absorption materials.

3.2 EMI shielding

Another research hotspot in electromagnetic field is EMI shielding materials.Compared with microwave materials,EMI shielding possesses larger application field.The shielding effectiveness (SE) is applied to evaluate EMI shielding of materials.Generally,the standard for commercial application of shielding materials is greater than or equal to 20 dB (≥20 dB),which can be calculated according to the following equation[50]:

WherePIandPTstand for the power of the incident wave and the wave that travels through the materials andTstands for the transmission coefficient.For EMI shielding,the reflection and absorption play important roles.

4 Advances in research on graphenebased electromagnetic functional materials

4.1 Graphene-based electromagnetic functional materials in microwave absorption

Two-dimensional graphene possesses large surface area and high carrier concentration,which exhibits great application potential in electromagnetic field.Up to now,great research has been devoted,and many excellent microwave absorption materials and EMI shie-lding materials have been developed.

In 2015,Cao et al.[40]reported the microwave absorption performance of graphene composite with ultrathin and lightweight,and investigated its high temperature dielectric properties,as shown in Fig.4.The obtained graphene composite exhibits high-efficiency absorption performance and excellent thermal-stability permittivity.The optimal reflection loss arrives at -42 dB and the effective bandwidth (≤-10 dB) covers the entire X-band (8.2～12.4 GHz).The outstanding dielectric property and absorption performance are originated from the greatσof graphene,which causes strong conduction loss.Meanwhile,reduced graphene oxide (rGO)possesses abundant defects and groups as polarization centers,which results in high relaxation loss.In addition,the synergistic effect of polarization andσalso plays an important role.The research results demonstrate that graphene composite prepared has an important potential as a high performance microwave absorbing material,which could be widely applied in microwave device,information security and electromagnetic pollution defense.

In order to improve the dielectric property and microwave absorption performance of graphene further,the magnetic medium is introduced to obtain magnetic graphene.In 2018,Zhang et al.[44]demonstrated a simple strategy to implant small NiFe2O4clusters on rGO(NiFe2O4-rGO).The prepared NiFe2O4clusters are distributed uniformly on rGO.Through the tailoring of magnetic clusters,electromagnetic property of NiFe2O4-rGO is optimized prominently and the outstanding absorption performance is acquired.The optimal absorption performance of composite reaches -58 dB,and corresponding bandwidth (at 10 dB) increases by ～12,comparing with rGO.Moreover,by tailoring the hybrid ratio of both NiFe2O4and rGO,the selective-frequency for electromagnetic wave is realized,and the maximal absorption peak shifts from 4.6 to 16.0 GHz,covering 72% effective bandwidth (2～18 GHz).Obviously,after introducing magnetic NiFe2O4clusters,the total microwave absorption is improved.Besides conduction loss and relaxation loss,the magnetic loss also has an important effect on electromagnetic response.The excellent microwave absorption benefits from the synergistic effect of conductance,relaxation and magnetic behavior.The findings open up a new way for the design of microwave absorption materials in the future,and prepared absorber could be widely used in various fields,including commerce,military,aerospace,etc.

Although magnetic graphene has been prepared and the microwave absorption performance of graphene-based materials is improved,accurately and controllably tailoring magnetic graphene remains a challenge.Hence,in order to solve this problem,a simple and effective tailoring strategy was proposed by Ma et al.[41]On nitrogen-doped graphene (NG),the confined sites are tuned controllably and Fe3O4small magnetic clusters are uniformly implanted.Finally,a new magnetic graphene is synthesized (Fe-NG).Fig.5a shows the TEM image of 50-Fe-NF sample (Fe3O4∶NG=1∶1).Fe3O4magnetic clusters are distributed uniformly,and Fe3O4particle exhibits a small size.As observed in Fig.5c,the mean size of Fe3O4particle only is 6.23 nm.After introducing the Fe3O4clusters,the synergy of dielectric and magnetic loss is realized successfully,and the impedance matching of composite is improved obviously.The maximal reflection loss reaches -53.6 dB,and the effective bandwidth covers 5 GHz with only 1.8 mm thickness.Meanwhile,through the tailoring of Fe3O4magnetic clusters,the absorption peak could be shifted from 4.6 GHz to 14.7 GHz,covering 60% investigated frequency (Fig.5b and 5d).The research result demonstrates that the excellent microwave absorption is originated from the synergy of conductance,multi-relaxation,resonance,eddy current and multi-scattering.The Fe-NG has high-efficiency and uniform frequency-selective microwave absorption characteristics,and broad application prospects in the fields of electromagnetic shielding,microwave absorption,medical treatment,information security,and military.

In microwave absorption field,in order to acquire high-performance graphene-based absorber,two effective strategies are applied commonly.One is to introduce the heterogeneous phase to improve electromagnetic loss source,such as NiFe2O4,ZnO,TiO2,etc[44,41,52].The other is to design the microstructure of graphene-based electromagnetic functional materials.By the optimization of microstructure,the electromagnetic loss is improved significantly.In 2018,Ma et al.[53]reported a new 3D hierarchical Co3O4-rGO hybrid architecture,namely,3D flower-like Co3O4-rGO hybrid architecture.The hybrid architecture is fabricated by a facile,green and highly tunable strategy.As shown in Fig.6a and 6b,flower-like Co3O4grows on the rGO,marked with green,bringing about abundant interfaces,which results in strong interface loss.Meanwhile,the introduced Co3O4could improve dipole polarization and conductive network,which tunes the hybrid architectures and electromagnetic properties highly,as showed in Fig.6c and 6d.Hence,the porous flower-like Co3O4-rGO hybrid-architecture shows outstanding microwave absorption (Fig.6e and 6f).The optimal reflection loss reaches -61 dB,and 10 dB bandwidth covers 4 GHz.In addition,the high temperature absorption performance of composites is investigated,and the hybrid architecture exhibits the excellent high-temperature stability at the temperature range of 353～473 K.The research demonstrates that flower-like Co3O4-rGO hybrids is a potential and efficient microwave absorption material,which provides a new way for microwave absorption design.

4.2 Graphene-based electromagnetic functional materials in EMI shielding

Graphene possesses excellent electronic,thermal and mechanism properties.Hence,besides high-efficiency microwave absorption performance,graphene also displays great EMI shielding performance.In 2014,Wen et al.[54]reported rGO excellent dielectric property and EMI shielding behavior with a low loading at high temperature.The complex permittivity of rGO composite exhibits temperature and loading dependence.The real permittivity (ε′) exhibits same change at different loading,which improves with increased temperature.However,imaginary permittivity (ε″) exhibits the opposite change at low and high loading with increased temperature.It decreases with increased temperature at low loading,and increases at high loading.This change could be ascribed to different temperature response for conductance and relaxation behavior.With the improvement of temperature,the relaxation loss decreases,while,the conduction loss increases.The increased conduction loss could compensate for the reduced relaxation loss.In addition,the dielectric property increases with the loading,which is attributed to gradually improved conductive network with loading.Hence,the maximal EMI shielding is attained at 20% loading and 473 K,reaching -38 dB.Excellent EMI SE performance indicates that chemically graphitized rGOs provides potential applications for shielding device.

Fig.7 demonstrates the comparison of rGO and graphite nanosheet (GN) in electromagnetic property,σand EMI shielding performance[55].As known,the graphene oxide (GO) is prepared from GNviaoxidationreduction method,and then,the GO is reduced to obtain rGO.The rGO and GN samples are prepared by the paraffin wax as host.As observed in Fig.7a,the dielectric loss tangent (tanδe) of rGO exhibits an obvious improvement than GN sample at the same loading.Fig.7b shows theσof rGO and GN samples.The enhancedσcould be ascribed to the reduced number of graphene layers.It effectively reduces carrier collisions between graphene layers.Hence,theσof GO sample is larger than GN sample.High dielectric loss andσresult in the enhanced EMI shielding performance at room temperature,as shown in Fig.7c and 7d.The maximal EMI shielding performance of GN sample only is ～8.5 dB,which can't meet commercial application.However,the shielding performance of rGO exhibits a significant improvement,and the optimal shielding performance attains 30 dB,increasing by 300%.Besides conduction loss,the relaxation and multi-scattering also have important effect on electromagnetic loss.These results demonstrate that rGO is a promising microwave attenuation filler with high-performance EMI shielding,which is expected to be applied in many fields.

In order to improve the EMI shielding performance of graphene-based electromagnetic functional materials,the microstructure is designed.Song et al.[20]proposed a new sandwich structure graphene-based composite,which possesses unique EMI shielding features of frequency selectivity (D-GN-D,GN-D-GN and GN-D-GN-DGN),as shown in Fig.8.Due to the application of electrical and magnetic loss spacers,the resultant sandwich structures exhibit the tunable EMI shielding performance.Meanwhile,compared with graphene,the composites exhibit an improved EMI shielding performance.The optimal EMI shielding attains 49 dB for GN-D-GN sample at 11 GHz.The research result demonstrates that high-efficiency EMI shielding is originated from the highσ,multiple interfaces and magnetic eddy current.Meanwhile,these response mechanisms demonstrate that multiple interfaces,electromagnetic loss media,and changes of representative EM wavelength could be the key factors for tuning the EMI shielding performance.The research provides a versatile strategy that can be extended to other frequency ranges and various types of sandwich structures,which provides an opportunity for the design and construction of advanced electromagnetic attenuation material and device.

5 Overview and outlook

In summary,graphene-based electromagnetic functional materials with unique physical and chemical property are becoming an enthusiastic sought-after topic in electromagnetic field.Recently,great development has been acquired in microwave absorption and EMI shielding field,and an amount of graphene-based absorption and shielding materials have been developed.However,the current graphene-based electromagnetic functional materials are not perfect,and some problems about it need to be solved urgently.Firstly,preparing a low-cost and high-performance electromagnetic functional material is still a key problem.Secondly,the single application function of graphene-based electromagnetic functional materials limits the scope of commercial applications.Thirdly,the cross-integration of multiple fields proposes a new demand for electromagnetic functional materials.The development of graphene-based devices is an urgent need in the future electromagnetic field.

Graphene with outstanding electric,thermal and mechanism property is the most shining star,which possesses great development potential in electromagnetic devices.The problem in the research process is a serious challenge,but it will be overcome eventually.It is believed that graphene-based electromagnetic functional materials will be an important pillar in electromagnetic field in the future.