Pei Hung,Yo Li,Gng Yng,Zheng-Xin Li,Yun-Qing Li,Ning Hu,Sho-Yun Fu,＊,Kosty S.Novoselov

Keywords:Graphene film Thermal conductivity Film assembly Defect repair Free-standing

ABSTRACT Thermal conductivity and thermal dissipation are of great importance for modern electronics due to the increased transistor density and operation frequency of contemporary integrated circuits.Due to its exceptionally high thermal conductivity,graphene has drawn considerable interests worldwide for heat spreading and dissipation.However,maintaining high thermal conductivity in graphene laminates(the basic technological unit)is a significant technological challenge.Aiming at highly thermal conductive graphene films(GFs),this prospective review outlines the most recent progress in the production of GFs originated from graphene oxide due to its great convenience in film processing.Additionally,we also consider such issues as film assembly,defect repair and mechanical compression during the post-treatment.We also discuss the thermal conductivity in in-plane and through-plane direction and mechanical properties of GFs.Further,the current typical applications of GFs are presented in thermal management.Finally,perspectives are given for future work on GFs for thermal management.

1.Introduction

Thermal management represents a major challenge in the state-ofthe-art electronics(such as battery,integrated circuit,high-frequency electronics and so on)because of the rapid increase of power densities.Efficient heat removal has become a critical issue for the performance and reliability of modern electronic,optoelectronic,photonic devices and integrated solutions[1-4].One way to solve this problem is to integrate heat spreading materials through which excessive heat is efficiently transported away from power devices,thus reducing the operating temperature of systems.To achieve this purpose,the heat spreading material needs to be highly thermally conductive in addition to being thin,flexible,and robust to match the complex and integrated nature of modern electronic components and systems[5,6].To date,however,most of commercially available thermal conductivity materials,e.g.copper(390±34 W m-1K-1),aluminum(214±17 W m-1K-1),and artificial graphite(18.5 W m-1K-1),cannot satisfy these demands[7,8].

Graphene-a single atomic plane of sp2-bound carbon-has attracted considerable interests in a variety of technologies.Apart from the advantages of high Young's modulus(1 TPa),high flexibility,strong chemical stability and high electron mobility(2.5?105cm2V-1s-1),graphene exhibits a recorded high in-plane thermal conductivity of 3000-5000 W m-1K-1at room temperatures[2,9].As a result,the stacking of graphene into layer-structure gives a flexible and robust graphene film(GFs),which is highly attractive for thermal management.So far,various types of materials,including graphene and graphite,etc.,have been utilized as the building block for the fabrication of GFs[10-12].Compared with graphene and graphite,graphene oxide(GO),a negatively charged and strongly functionalized graphene,can be formed into stable colloidal suspension,which is beneficial for the formation of laminated and highly orientated structure.Followed by the reduction process,the graphene produced has controllable structural and chemical properties in a scalable manner,making it one of the most important precursors for the fabrication of GFs.

Depending on the preparation method,the thermal conductivity of GFs can be reduced greatly(up to one order of magnitude compared to that of pristine graphene)due to the poor graphene alignment and structural defects,which calls for further improvement of the technology[13].In this review,various processes for production of highly thermal conductive yet robust GFs starting from GO are summarized.First,the general production approaches,including film assembly,defect repair and mechanical compression,are introduced.Then,detailed discussions are presented for the corresponding thermal conductivity of GFs made from various manufacturing processes.In addition,a brief discussion is given on the mechanical strength required to assure the integrity of GFs.Finally,typical applications and perspectives of GFs in thermal management are given.

2.Fabrication of free-standing GFs for thermal management

Because of the giant difference in thermal conductivity between the in-plane and through-plane direction,the orientation of graphene is the key to realize high thermal conductivity for GFs.Meanwhile,the abundant oxygen functional groups of GO and other structural defects introduced during film assembly deteriorate the thermal conductivity of GO films prepared,thus requiring the successive defect repair and mechanical compression to further increase the thermal conductivity of GFs.Therefore,the conventional protocol to produce large-area freestanding GFs involves film assembly,defect repair and mechanical compression.

2.1.Film assembly

Relying on high efficiency in the material processing,GO has been applied to fabricate GFs through several well-established techniques,such as electro-spray deposition,vacuum filtration,dip coating,spincoating,drop casting and electrophoretic deposition,etc.

2.1.1.Electro-spray deposition

Electro-spray deposition(ESD)is featured by mass scale capacity,negligible material loss and high precision for properties control(see Table 1),and has been widely studied for decades in the surface coating industry.During ESD process,mono-dispersed fine droplets feeding through a nozzle is atomized at the tip of the nozzle by applying high electric potential between the nozzle and substrate due to the repulsion forces between charges in the droplets.The GO sheets tend to bend when they land on the substrate because of surface contact stress.Once the moving speed of the substrate reaches a critical value,the GO sheets will be flat on the substrate.Along with the consecutive deposition of tiny droplets on the substrate,two-dimensional graphene sheets stack into a macroscopic film geometry(Fig.1a-b)[16].The thickness and degree of alignment of the deposited film can be controlled by adjusting the deposition parameters such as the graphene concentration,flow rate,applied potential moving speed of substrate and substrate temperature,etc.(Fig.1c-f)[14,15,17,18].A high manufacturing efficiency for fabricating large area GFs can be realized by integrating with a continuous roll-to-roll process,showing its the potential for the mass production of GFs[19].

Table 1Comparison of various GFs assembling methods.

Fig.1.(a-b)Schematic illustration of the process for fabricating GO films:(a)GO in the spraying zone and(b)depositing GO on the substrate[14].(c-f)SEM images of the samples prepared at various solution concentrations and flow rates during ESD:(c)2 mg/mL and 100μL/min(inset showing well-exfoliated graphene sheets),(d)1 mg/mL and 50μL/min,(f)0.5 mg/mL and 100μL/min,and(f)0.2 mg/mL and 50μL/min[15].

2.1.2.Vacuum filtration

Vacuum filtration is another extensively studied approach to assemble GFs.In the vacuum filtration process,the liquid of the GO suspension passes though the pores of membrane,sedimenting GO sheets into a dense laminated structure at the surface of the filter(Fig.2a)[20].The GO film after drying exhibited a highly aligned layer-structure through the entire cross-section(Fig.2b-c)[21],which is driven by several mechanisms,including steric hindrance,gravitational force,the“excluded volume”interactions among the GO sheets,π-πinteraction and van der Waals forces between adjacent GO sheets[20].It is suggested that larger GO sheet size leads to lower filtration rate but better alignment of GO,while smaller GO sheet size leads to higher filtration rate and more porous structure.Therefore,the GO films with different alignment degrees can be obtained by simply varying the GO size[22].In comparison,vacuum filtration is time consuming,which takes hours even days to produce GO film of limited size(see Table 1).

2.1.3.Dip coating

Characterized by low cost,simply processing procedure and uniform morphology,dip coating is a promising method to produce GFs of complex shape and large area via electrostatic self-assembly(Fig.3).Typically,GO sheets are deposited on hydrophilic substrates(such as pretreated quartz)by dip coating of an aqueous GO dispersion and subsequent temperature-controlled drying of the film.More complex structures can be formed,for instance bilayer with copolymer.To form such structures,an originated from polymer or oppositely charged nanoparticles is deposited on the GO monolayer by adsorption,and promotes the formation of the second GO monolayer(by dip-coating)[23].The thickness of GO film can be tuned by changing the temperature of GO dispersion as well as the dipping cycle[24].A free-standing GO film can be obtained by peeling off the substrate.The replacement of rigid substrate by flexible matrix also gives a free-standing flexible GO hybrid film of high thermal conductivity[25].

Fig.2.(a)A scheme presenting the evolution of GO film from GO suspension via vacuum filtration(ρ=density)[20];(b-c)digital image(b)and SEM image(c)of free-standing GFs[21].

Fig.3.Schematic representation of the manufacture of GO film[26].(a)Deposition of a monolayer of GO/copolymer sheets by dip-coating;(b)deposition of a polymer interlayer by adsorption;(c)deposition of the second GO layer by dip-coating;(d)final double layer system composed of GO.

2.1.4.Drop casting

Drop casting is a method of obtaining a macroscopic film by evaporating the solvent in a GO suspension.The evaporation of solvent rises GO concentration in the suspension,resulting in a significant increase in the sheet-to-sheet interactions and then making the sheets aligned on top of each other in the ever-growing deposit,thus forming a layer-by-layer nanostructure(Fig.4)[27].The selected solvent guarantees GO to be well dispersed without rapid precipitation as well as have a low boiling point and good volatility.In this case,the mono-dispersed GO in the dispersion will uniformly stack together to a GO film while removing the solvent under an appropriate temperature.

2.1.5.Electrophoresis deposition

Electrophoretic deposition(EPD)is a well-developed method to produce thin film,and has been successfully applied for the preparation of transparent films from carbon nanotubes and expanded graphite oxide particles.Generally,the deposition process involves two steps.Charged particles in suspension move toward an electrode of opposite charge due to the influence of an electric field and then deposit to form a compact film(Fig.5a-b)[28].Therefore,the EPD method requires the substrate to be conductive for the formation of electrical field.However,the EPD method has a number of advantages in the preparation of thin films from charged colloidal suspensions,such as high deposition rate,good thickness controllability,good uniformity,and large scale manufacturing on a variety of substrate morphologies(Fig.5c-d,Table 1)[29].

2.1.6.Other assembly methods

Other traditional methods,such as spray coating[30],blade coating[31],spin coating[32]and Langmuir-Blodgett assembly[33],have also been applied to assembly GO film.To simplify the production process,further,very specific methods have been developed recently.For instance,the combination of freezing-dry and mechanical compression results in a free-standing GO film(Fig.6a-c)[34,35].A synchronous reduction and assembly strategy,through which GO is reduced to graphene through an oxidation-reduction reaction between the metal substrate and GO,and further organized into highly ordered films in situ,also gives a flexible GFs(Fig.6d)[36].

Fig.4.(a)Schematic representation of a proposed self-assembly process of GO film by the evaporation of GO suspension;(b-c)SEM images of the surface(b)and cross-section(c)of GO film[27].

Fig.5.(a-b)Schematic diagram of the EPD process(a)and SEM image of GO free-standing film[28];(c-d)graphene sheets deposited onto nickel foam by EPD[29].

2.2.Defect repair

Although an improved alignment of GO film is achieved by film assembly process,the thermal conductivity maintains at very low level because of the large number of structural defects.Apart from oxygen functionalities,other structural defects,such as vacancies,adatoms,and grain boundaries,are inevitably generated either in synthetic procedure of GO or film assembly process,and deteriorates the thermal conductivity.Accordingly,chemical reduction,electrochemical reduction,thermal annealing and photothermal reduction,have been employed for the defect repair of GFs(Table 2).

2.2.1.Chemical reduction

The chemical reduction of GO to graphene is the most convenient approach to remove oxygen functional groups.Numerous reductants,such as hydrazine,hydriodic acid with acetic acid,sodium hydrogen sulfite,metals(e.g.Sn,Zn and Fe),cobaltocene,hydroquinone,sodium borohydride,and may other chemicals,have been used for this purpose in recent years[37,38,40-42,44,51-53].Generally,the reduction starts from the edges of GO nanosheets and proceeds into the basal planes.During the reduction,parts of the basal planes near the edges become reduced and subsequently snap together due toπ-πinteractions,thus narrowing the interlayer distance.Therefore,the reducing process is controlled by the diffusion of reactants within the GO film,leading to a much longer time to achieve the stable state for thicker GO film[54].The reduction degree varies depending on the chemical identity of the reducing agent,and the C/O ratio of the order of 15 can be generally achieved,indicating limited efficiency on the oxygen removing in GFs(see Table 2).Meanwhile,the reducing agents and byproducts are hard to eliminate after the reduction process(though it can be accomplished by the utilization of reductants vapor).It is worthwhile noting that heteroatoms from reductants could be doped into the graphene sheet during the de-oxygenation reactions due to the reorganization of unsaturated carbon[43,55,56].Last but not least,most of these agents,particularly hydrazine,are highly toxic,and strong acidic or alkaline conditions are commonly required in these reduction processes,thus limiting their applicability[57].

Fig.6.(a)Schematic illustration of the pressing process of the GO film;(b)SEM image of the GO aerogel with an inset that shows an optical photograph of the graphene aerogel;(c)SEM image of the graphene aerogel under 50%compression[34];(d)schematic drawings illustrating the procedures to the grown GFs on metal substrate and the detachment of the films from the substrate[36].

Table 2C/O ratio of GFs by varied reduction methods.

2.2.2.Electrochemical reduction

The removal of oxide groups can also be realized by electrochemical reduction,confirmed by the appearance of current peak during cyclic voltammetry in buffer solution(Fig.7a-c)[58,59].The possible mechanism(shown in Equations(1)-(3))is that electrons move away from the platelets upon applying the voltage,causing the oxidation of carboxylate groups on the periphery of the platelets.All of the unpaired electrons formed by the loss of CO2are then free to migrate until forming covalent bonds elsewhere[28].

Fig.7.(a)Cyclic voltammogram Au/cystamine/GO film in deaerated 0.1 M KNO3 at a scan rate of 10 mV/s(1 and 2 correspond to the first and second scans)[59];(b-c)XPS C1s spectrum of GO film before and after electrochemical reduction;(d)Time-of-Flight Secondary Ion Mass Spectrometry depth profiles of normalized secondary ion species within a 550 nm thick GFs on a quartz substrate[62].

Therefore,by coupling the electrochemical reduction with electrodeposition,the controllable synthesis of large-area GFs with thicknesses ranging from a single monolayer to several microns could be achieved on various conductive and insulating substrates[60].The reduction degree is governed by proton(H+),hence,the C/O ratio trend follows pH 12>pH 2>organic electrolyte.Although the presence of H+in the electrolyte favors the removal of oxygen groups via consecutive hydrogenation reactions,the acidic electrolyte gives a lowest sp2/sp3ratio due to the hydrogenations of edge zones and/or dangling bonds created after the removal of O-groups located both in the edges and basal planes[61].Additionally,due to the direct charge transfer between node and GO sheets,the reduction is found to be thickness dependent,as a higher degree of reduction is observed at the electrode/GO interface(Fig.7d)[62].

2.2.3.Thermal annealing

Although most of the oxygen-containing groups are erased by chemical or electrochemical reduction,the residual oxygen limits the thermal conductivity of GFs.In comparison,thermal annealing shows much higher efficiency on the removal of oxygen(see Table 2).The removal of oxygen is mainly in the form of CO2and H2O(Fig.8a-b),and the carbon atoms in the basal plane may also rearrange during annealing as the disorder related peaks(1350,1623 and 2939 cm-1)decrease(Fig.8c),leading to the restore of the sp2bonded carbon network.The reduction efficiency of thermal annealing increases as the increase of heating temperature[39,63].The thermal annealing atmosphere also determines the reduction efficiency,supported by the increased C/O ratio(Fig.8d).A considerably superior removal of oxygen containing functionalities is obtained under H2compared to the one using argon because C and O atoms in the GO are consumed to form CO/CO2in Ar,whereas only oxygen is utilized to form water in H2during thermal reduction[45,64].As a result,thermal annealing is usually carried out in vacuum,or an inert or reducing atmosphere.It is worth to mention that heteroatoms(such as N)in GO also can be eliminated during thermal annealing[56].

More profoundly,the inherent structural damages,such as vacancies and cracks,can also be repaired by high temperature annealing[65].The mechanism for the repair of vacancies is through attracting hydrocarbon contamination toward the hole,‘filling’occurs and the hole mends itself by non-hexagon arrangements.If no hydrocarbon contamination is present,healing can occur via reconstruction of the perfect graphene hexagon structure,especially for small vacancies(Fig.8e-f)[66].Higher temperature annealing gives rise to the increased grain size by forming interlayer bonding between neighboring graphene sheets via interlayer sliding of line defects,because of the enhanced reactivity of carbon atoms at the defect edge(Fig.8g-h)[67,68].However,with increasing level of structural damage,the self-healing becomes less effective,which could be overcome by the introduction of an external carbon source,such as carbon fiber[69],CNT[11,30],glucose[70],Vitamin C[71],polyimide[72],polydopamine[73]and polyaniline[74],etc.[75].Thermal reduction produces one of the highest quality of resulting graphene films.

2.2.4.Photothermal reduction

Photothermal reduction process by utilizing flash[47],laser[84],UV[85],and far infrared light[49],etc.have been proposed as a fast method of reduction.In photothermal reduction,energy bursting over a very short period causes instantaneous and localized heating of GO,and thus induces a deoxygenation reaction.However,the reduction efficiency is still limited while the C/O ratio maintains at a lower level(see Table 2)[86].Meanwhile,the pressure generated by the rapid release and escape of oxygen induces local mini-explosions,and causes the occurrences of such micropores,cracks,and voids in the film(Fig.9),which is visually absent in other types of reduction method[47].

Fig.8.(a-b)XPS spectra of a GFs before(a)and after(b)annealing at 750 °C;(c)Raman spectra for monolayer graphene samples annealed at different temperatures[65];d)C/O ratio as a function of annealing temperature(Black from Refs.[76],Red from Ref.[77],Green from Refs.[78],blue from Refs.[79],Cayan from Refs.[80],Magenta from Refs.[81],Yellow from Ref.[82],Dark yellow from Ref.[46],Navy from Refs.[27],Purple from Refs.[83]);e-f)atomic resolution HAADF images of suspended graphene with two holes(e)etched by laser etching and small hole repaired by thermal annealing(f)[66];g-h)molecular dynamics simulations of the defect-facilitated formation of interlayer bridging bonds between two defective neighboring graphene layers at 2500 K from top view(g)and side view(h)(Both insets in(g)and(h)clearly show that the left edge of the line defect in the top graphene layer is covalently bonded to the right edge of the neighboring line defect in the bottom graphene layer)[67].

2.2.5.Other defect repair methods

Besides the methods mentioned above,plasma reduction through which the energetic H+ions in plasma penetrate into the GO sheets,react with hydroxyl groups and form into water molecule[50],hydrothermal reduction[88,89],and so on,have been applied to eliminate oxygen containing groups.To further increase the reduction degree,the combination of several types of reduction methods has been developed.For instance,the consecutive reduction by hydrazine and thermal annealing could increase the C/O ratio up to 200 while only 8.3 and 125 of C/O ratio for hydrazine reduction and thermal annealing,respectively[90].

2.3.Mechanical compression

The defect repair process reduces the defect concentration within graphene sheets,however,the elimination of oxygen functionalities and reductants inevitably increases the interlayer space between graphene nanosheets and leaves voids,donating GFs with porous structure(Fig.10a-b)[91-93].To increase the contact between graphene sheets,mechanical compression is desirable to decrease the interlayer space and film thickness as well(Fig.10c-d).Moreover,the orientation of wrinkles can be controlled by directly applying external mechanical until plastic deformation happens.By changing direction and magnitude of tension strain,wrinkles with different spacing but all with orientations perpendicular to the tensile direction can be obtained[94].

3.Thermal conductivity and mechanical property of GFs

Unlike pristine monolayer graphene,the thermal transport and mechanical properties of GFs are governed not only by the structural parameters of GO used but also by the production method.In this section,we discussed the fundamental mechanisms of the thermal conductivity of GFs.Further,the improvement of and thermal conductivity and mechanical properties of GFs are presented as well.

3.1.Thermal transport modelling of graphene and GFs

The thermal transport occurs when there is a temperature gradient in a material,and thermal energy is carried by phonons and electrons.Because of the high values of in-plane phonon group velocities and phonon density caused by the strong sp2bonding,heat transport in graphene is inherently governed by phonons while the electron contribution to thermal conductivity,estimated from the Wiedemann-Franz law,is less than 1% at room temperature[99].However,the thermal conductivity of graphene is limited by its extrinsic parameters,such as grain size and structural defects,etc.(Fig.11a)[100].To probe the thermal transport in graphene,various types of numerical models,e.g.molecular dynamics simulations,density functional theory and Boltzmann-transport-equation approach,etc.,have been applied[101-103].

Taking the example of molecular dynamics simulations,they can directly model phonon thermal transport with the consideration of structure such as defects,interface and strain,etc.The fundamental idea for the molecular dynamic stimulation is based on Fourier's law,where a constant heat flux transports along the graphene in a specified direction is a result of a temperature gradient?T/?x in a steady state,thus the thermal conductivity can be expressed as:

Fig.9.(a-b)Typical SEM images of GO film before(a)and after(b)excimer laser irradiation;(c)Typical cross-sectional view of the excimer laser reduced GO;(d-e)close-up view of spots 1 and 2 in(b)shows the highly expanded nature of the excimer laser-reduced GO[87].

Fig.10.(a-c)SEM images of GO film(a),thermal annealed GFs(b),and thermal reduced GFs after mechanical compression(c);(d)XRD for GO film(curve 1),GFs(curve 2),and mechanical compressed GFs(curve 3)[91].

Fig.11.(a)Different types of structural defects,including vacancies,grain boundaries,substitutional and functionalization defects,Stone-Wales defects,and wrinkles or folds[100];(b)thermal conductivity as a function of grain size with a fit(red curve)[104];(c)thermal conductivity of defective graphene at different defect densities for single vacancy(SV),double vacancy(DV),and Stone-Wales(SW)defects[112];(d)thermal conductivity of G-OH,G-Epoxide,and G-Ether as a function of O/C ratio at 300 K[114].

whereτis the stimulation step time,A is the cross-sectional area of graphene whileΔE is the kinetic energy transported per unit time.The modelling results indicate that the thermal conductivity of graphene increases monotonically with the increase of the grain size(Fig.11b)[101,104],attributing to the extremely large phonon mean free path Λ~775 nm[105],which has been recently confirmed in the experimental study with sample size on the order of 1μm[106].As a result,the thermal conductivity of polycrystalline graphene has a limited value due to the additional phonon scattering associated with the grain boundaries.Moreover,the structural defects,including impurities,isotope,vacancies and oxygen-containing groups,etc.,degrade the thermal conductivity of graphene as well.Only 1% of nitrogen or 0.75% of boron atoms in graphene decreases the thermal conductivity of graphene by more than 50%[107,108].It is worth to note that the presence of isotope decreases the thermal conductivity of graphene as well due to the enhanced phonon-point defect(mass-difference)scattering.For instance,graphene composed of a 50:50 mixture of12C and13C has more than a factor of two decrease in thermal conductivity[109].Vacancies,including holes and cracks,increases the phonon scattering by the nanoscale hole edges,and decreases the phonon passage length-scale that became comparable to the phonon mean-free-paths,resulting in the significant reduction of the thermal conductivity of graphene(Fig.11c)[110-112].Also,the presence of oxygen-containing functional group increases the scatter of the intermediate and short wavelength flexural phonons by disrupting the sp2structure of graphene even at extremely low defect concentration(~0.1% defects).The thermal conductivity of graphene can be reduced by a factor ranging from 3 to 25,depending on the C/O ratio and the configuration of oxygen functional group.Compared with carboxyl,other types of defects such as hydroxyl,epoxy groups and nano-holes demonstrate much weaker effects on the reduction of the thermal conductivity,where the sp2nature of graphene is better preserved(Fig.11d)[113,114].Other factors,such as wrinkle[115],and edge roughness[116],etc.,also suppress the thermal transport of graphene.

When graphene evolves into GFs,the thermal conductivity is strictly limited not only by the phonon-defect and phonon-boundary scattering in graphene,but also by the phonon-interface scattering(Fig.12a).The increase of graphene layer,implying the transformation of 2D graphene into 3D graphite,leads to the reduction of thermal conductivity,which the value of thermal conductivity very close to that in graphite achieved already for 8 layers(Fig.12b)[118].This is mainly caused by the weak interlayer bonding of Van Der Waals force that breaks a phonon-phonon scattering selection rule in graphene and opens more scattering channels for acoustic phonons,flexural phonons in particular[120].Accordingly,the thermal conductivity of GFs can be manipulated by the construction of interlayer bonding,e.g.sp3bonding(Fig.12c)[119].

3.2.Thermal conductivity of GFs

Due to the highly orientated,laminated structure-the GFs demonstrate strongly anisotropic thermal conductivity for the in-plane and through-plane direction(see Table 3).

3.2.1.In-plane thermal conductivity

Attributed to the profound efficiency in heat dissipation,GFs exhibits a more uniform temperature distribution and lower center-line temperature than Al and Cu while a hot spot in the center(Fig.13a-d)[19].However,the thermal conductivity of GFs varies upon the production process and ranges from 600 to 3200 W/mK(Table 3).It is of high importance to evaluate the influence of production process on the thermal performance of GFs.Similar to graphene monolayer,the thermal conductivity of GFs is enhanced as the increase of C/O ratio due to the removal of oxygen functionalities and restore of sp2structure(Fig.13e)[81].The phonon transport in GFs is also limited by grain size(Fig.13f)[46].To be specific,an average GO area of 22.3μm2achieves an increase of 54%in thermal conductivity than that of average area of~1μm2[21].Notably,a thermal conductivity of 1102.62 W m-1K-1is obtained by optimizing the ratio of large-sized and small-sized graphene sheet because of the increased connectivity and compactness of GFs obtained[121].Whereas,the flexible characteristic of graphene sheet gives rise to the formation of wrinkles and folds during film assembly,especially for the large sized graphene sheet,hence harming the thermal conductivity of GFs[55].To that end,thermal annealing exhibits unique advantages of high C/O(Table 2)and damage repair compared with chemical reduction and thermal reduction.With the increase of annealing temperature,a higher thermal conductivity is obtained by GFs(Fig.13g).Nevertheless,voids and vacancies are generated during thermal annealing.For this reason,the combination of several types of reduction methods,stepwise thermal annealing could alleviate the expansion and decrease voids during reduction process[7].Furthermore,applying mechanical compression after reduction facilitates the removal of voids and close stacking of graphene sheets,thus providing more heat transport channels for photon and increasing the thermal conductivity as well[34].In addition,the thermal conductivity of GFs decreases with the increment of film thickness,caused by the increase of the total defect number in the fabricated microstructure(Fig.13h)[122,123].

Fig.12.(a)Illustration of thermal transport in GFs[117];(b)thermal conductivity of few-layer graphene with 1.7 nm in width vs layer number[118];(c)thermal conductivity of bilayer graphene vs the coverage of interlayer bonding[119].

3.2.2.Through-plane thermal conductivity

In comparison,the thermal conductivity of GFs in through-plane direction is much smaller than that in in-plane direction owing to the weak interlayer bonding(see Table 3)only carrying a small amount of heat energy via low frequency phonon vibration modes[93,124].The enhancement of through-plane thermal conductivity can be realized by the incorporation of graphitized filler between graphene sheets.For instance,the formation of nanoring from PMMA catalyzed by Ni increases the through-plane thermal conductivity up to 5.81 W m-1K-1(Fig.14a-b)[98].The graphitized GO/polyimide film with carbonized polyimide fiber as the skeleton shows a super-flexibility and ultrahigh thermal conductivity both in through-plane(150±7 W m-1K-1)and in-plane(1428±64 W m-1K-1)directions(Fig.14c-d)[25].

3.3.Mechanical property of GFs

There are two components of deformation which contribute to the total tensile deformation of GFs,including sheet deformation and displacement.The removal of oxygen-related group weakens the interlayer bonding of graphene,and the release of CO2and H2O induces the expansion of the interlayer space(Fig.15a),requiring the employment of mechanical compression to decrease the interlayer spacing and enhance the interlayer contact of graphene sheets.As a result,density,as an indicator for the compactness of GFs,is of great importance for the tensile strength of graphene.With the increase ofdensity,there ismore force transferbetween the interfacial regions,the folded graphene with sheets locked to each other increases the friction,and then increases the loading transfer between graphene sheets and the tensile strength as well(Fig.15b)[34].As stated above,the removal of oxygen functionalities decreases the interlayer bonding of graphene,leading to the significant drop of tensile strength of GFs compared to that of pristine GO film.As shown in Fig.15c,with the increase of annealing temperature,orderly stacks of graphene sheets upon the removal of oxygen groups is gradually restored.However,further increase of the annealing temperature from 2000 to 3000°C results in the decrease of tensile strength because of the decreased compactness of films obtained[95].Moreover,the larger graphene size donates GFs with much higher Young's modulus and tensile strength of more than 6 GPa and 60 MPa,respectively(see Table 3).Specifically,the GFs with graphene of average area of 23μm2has higher tensile strength and modulus than that with graphene of average area of 5μm2(Fig.15d)[125].Unlike thermal conductivity,the tensile strength of GFs is nearly unchanged with the increase of film thickness(Fig.15e),ascribed to the structural features GFs,including the large grain size,good alignment,and low interlayer binding energy[7].Besides,the introduction of carbon sources or crosslinker improves the tensile strength of GFs by increasing the grain size and establishing bridge between graphene sheet or/and skeleton[70,96,126].However,the higher Young's Modulus renders GFs brittleness and inconvenience to be handled for further treatment(Fig.15f).The existence of wrinkles degrades the thermal transports but improves the flexibility of GFs with elongation up to 16%(Fig.15g-h).

4.Application of GFs in thermal management

The exceptional high thermal conductivity facilitates GFs to spread heat efficiently from hot sources.More importantly,the flexible,light-weight and compact characteristics boosts applications of GFs in mobile devices[128],integrated circuits[129],electronic transistors[130,131],and flexible light-emitting devices[132],etc.(Fig.16).For instance,self-heating is a severe problem for the high-power semiconductor field-effect transistor.The temperature rise leads to the performance degradation and early thermal breakdowns.At the same time,the downscaling and higher circuit speeds lead to further increase in heat generation,power densities,and temperature rise.With the attachment of GFs,the temperature drop of AlGaN/GaN transistor operated at 3.3 W/mm is about 68°C,compared to that without GF heat spreader[131].The temperature of GF-capped photonic crystal cavity which creates compact and efficient light sources in dense chip-scale optical systems is 45 K lower than that without single layer graphene capping under an optical power of 100μW[133].GFs also can be used to reduce the cross-sectional heat diffusion in glass window by conducting heat away laterally and reducing near-infrared(NIR)transmittance,while commercial solar films focus only on blocking wavelengths in the solar spectrum but neglecting the consideration of the warming effect[134].

Table 3Thermal conductivity and tensile strength of GFs from various production methods.

Fig.13.(a-d)Infrared image of iron heat(a),Al(b),Cu(c),and GFs(d)[19];(e)thermal conductivity of GFs as a function of C/O ratio[81];(f)thermal conductivity of GFs versus the average size of graphene(data from Ref.[46];(g)thermal conductivity of GFs as function of density[34];thermal conductivity of GFs as function of film thickness(data from Ref.[123]).

Fig.14.(a-b)Molecular structure of optimized junction of(3,3)carbon nanoring and graphene sheet(a),and in-plane and through-plane thermal conductivities of different GFs(b)[98];(c-d)Cross-section SEM image of the GO/PI film(c)and scheme of the thermal conductive network of g-GO/PI film in through-plane direction and in-plane direction(d)[25].

Fig.15.(a)SEM image of the cross-section of the GFs[81];(b)tensile stress of the GFs versus density[34];(c)tensile strength of GFs treated annealed at varied temperature[81];(d)stress-strain curves of GFs with large and small sized GO[21];(e)tensile strength of GFs with different thicknesses[7];(f)cracks generated during sample handling[95];(g)SEM image of wrinkles in GFs[127];(h)schematic stretching and folding mechanism of GF[46].

5.Prospectives

Fig.16.Applications of GFs as a heat spreader in AlGaN/GaN transistor(a)[131],mobile(b)[128],integrated circuit(c)[132],smart window(d)[134].

Relying on outstanding thermal conductivity and high flexibility,free-standing GFs,which are easy to adopt the shape of electronic components that need to be cooled down,have drawn considerable efforts worldwide.The general protocol to produce free-standing GFs includes film assembly,defect repair and mechanical compression.According to the theoretical calculation,the thermal conductivity of GFs is governed by the structural parameters(e.g.grain size,impurities,vacancies and so on)of graphene.That is to say,the film assembly increases the alignment of graphene sheets,defect repair removes impurities and amends vacancies,and mechanical compression mainly decreases voids and increases density of GFs.Although progresses have been made on the improvement of the thermal conductivity and mechanical properties of GFs,some issues are needed to be addressed.

(1)In terms of defect repair,thermal annealing exhibits superior advantages in oxygen removing and damage repair over other methods,but the high time&energy consumption restricts its upscale production.To overcome this difficulty,the combinations of other reduction methods(e.g.chemical reduction)with thermal annealing have been developed,whereas these are still insufficient for the large-scale production from the industrial view.In addition,the high temperature condition restricts its applications in substrates with lower processing temperature.As a result,much more innovative techniques need be proposed to achieve the large-scale fabrication of GFs for industrial applications.

(2)The efficiency of heat dissipation from heat sink is not only determined by heat transport within GFs but also by the heat transfer from the heat source to GFs.Although some works have been devoted to improve the through-plane thermal conductivity,the thermal conductivity in through-plane direction is still one or two orders lower than that in in-plane thermal direction.More importantly,the interface between GFs and heat sources induces a giant thermal resistance;whereas seldom reports have been placed on the measurement of the interfacial thermal conductance of these heterogeneous materials to date.Hence,intensive works from the theoretical modelling and experimental aspects should be focused on heat transfer from power sources to GFs.

Declaration of competing interest

The authors have no conflict of interest to declare.

Acknowledgement

This work is financially supported by National Natural Science Foundation of China(Nos.51803016,11872132,U1837204 and 11672049)and the Start-up Funding of Chongqing University(Nos.0241001104454,0903005203352 and 0241001104417).