DONG Zhi-jun,SUN Bing,ZHU Hui,YUAN Guan-ming,LI Bao-liu,GUO Jian-guang,LI Xuan-ke,CONG Ye,ZHANG Jiang

（Key Laboratory of Hubei Province for Coal Conversion and New Carbon Materials, Wuhan University of Science and Technology, Wuhan 430081, China）

Abstract：The development of modern technology has posed greater and more urgent needs for thermal management materials.Aligned carbon nanotube arrays and carbon/carbon composites have aroused extensive interest as ideal lightweight and stable thermal management materials because of their low thermal expansion coefficient,excellent thermal conduction and high-temperature resistance.Here,we first review the thermal conducting mechanism of carbon materials.We then describe the general fabrication methods,the main factors affecting the thermal conductivity of aligned carbon nanotube arrays and carbon/carbon composites as well as their use in thermal management.The preparation-structure-performance relationships are outlined and the strategies for achieving high thermal conductivity are summarized.Finally,a critical consideration of the challenges and prospects in the thermal management applications of aligned carbon nanotubes and carbon/carbon composites is given.

Key words:Carbon nanotubes；Carbon fibers；Chemical vapour deposition；Thermal conductivity；Thermal interface resistance

1 Introduction

Thermal management system is based on thermodynamics and heat transfer processes and widely used in the national economy and national defense and other fields.It controls dissipation,storage and conversion of heat in the equipment and devices system[1].Advanced thermal management materials (TMMs) not only realize the function of thermal management of the equipment and device system,but also protect the equipment and devices from overheating damage.For example,with the development of miniaturization and high frequency conversion of electronic components,multi-function integrated circuits with ultra-high integration are developed and applied constantly,which makes the power consumption of integrated circuit increase sharply,and accumulates a lot of heat in a short time period.The heat accumulated induces the rapid temperature increase of the electronic components.Generally,electronic devices are designed to work within a certain temperature range,so as to ensure optimal performance and stability for a long duration.If the heat accumulated is not quickly and effectively dissipated,it will lead to the early aging,be abnormal or damage of the electronic components.In addition,large-scale equipment,such as high-power devices of communication satellites and nuclear fusion apparatus,is prone to generate and accumulate large amounts of heat during operation.If the heat cannot be discharged from the inside in time,it will affect or even destroy the stable operation of the equipment[2].In the case of the hypersonic aircraft flying in the adjacent space over a long period,the high temperature of stagnation point and the prominent thermal stress caused by aerodynamic heating,requires high thermal conductive materials to transfer the heat of the nose cone,leading edge and other parts in time,so as to streamline the thermal protection design and increase the stability of the aircrafts[3].Therefore,the development of advanced TMMs with light weight and high thermal conductivity (TC) has become an extremely important part of the advanced thermal management system.

Traditional metal thermal conductive materials,such as aluminum,copper and silver,have limitations and shortcomings such as high mass density,easy oxidation,low specific TC (ratio of TC to bulk density),and high coefficient of thermal expansion (CTE),making it difficult to meet the increased performance requirements for high-efficiency thermal management,especially in some extreme service environments[4].Owing to their remarkable properties such as high TC,low CTE and high temperature resistance,carbon nanotube (CNT)-based and carbon fiber-based materials have caused extensive concerns as ideal light-weight and stable TMMs[5].They have wide application prospect in the field of thermal management of electronic devices.The room-temperature TC of the individual CNT and individual high thermal conductive carbon fiber are～3 000 and～1 000 W·m?1·K?1,respectively,which are nearly one order of magnitude larger than that of metals.These individual CNT or carbon fiber,however,cannot be employed directly as TMMs.Instead,they have been assembled into vertically aligned CNT (VACNT) arrays and carbon fiberreinforced carbon (C/C) composites,by chemical vapor deposition (CVD),plasma-enhanced chemical vapor deposition (PECVD),hot press molding,and so on[6].For instance,dense vertically aligned multiwalled carbon nanotube (VA-MWCNT) arrays have been prepared on Si wafer via CVD process using Fe as a catalyst,and their TC reaches 267 W·m?1·K?1[7].Besides,C/C composites with high TC have also been developed by hot press molding[8].

VACNT arrays and high thermal conductive C/C composites can be used as carbon-based TMMs in various fields.Fig.1 is a pictorial representation of carbon-based TMMs and their practical or potential applications.VACNT arrays have shown great potential as thermal interface materials (TIMs) to enhance heat dissipation for the electronic devices.For example,VACNT-based TIMs can be used for heat dissipation of solid state disk memory,the graphic processing unit and the central processing unit in the supercomputer.In 5G phones,VACNT-based TIMs can be used as thermal pads placed adjacent to the electromagnetic interference shielding cover to dissipate the heat produced by the chips.In the thermal control system of spacecraft,VACNT-based TIMs can be used in high power electronic devices,playing the role of device heat dissipation,waste heat transfer and reducing thermal interface resistance.In electric vehicles,VACNT-based TIMs can provide superior thermal management for the battery packs to maintain the safe operation and consistent performance[9].As for C/C composites with high thermal conductivity,they can be divided into unidirectional (1D),two-dimensional(2D),three-dimensional (3D) and multi-dimensional composites,according to the difference in the alignment and orientation of carbon fibers.High thermal conductive C/C composites have been used to manufacture heat dissipation plate of satellite electronic device and heat dissipation components in spacecraft cockpit.Besides,high thermal conductive C/C composites have also been applied in the nose cone and leading edge of wing of the hypersonic vehicles to transfer heat from a high temperature area to a low temperature zone[10].

Fig.1 Carbon-based TMMs and their applications.

In spite of the high TC of individual CNT and carbon fiber,the assembled VACNT arrays and C/C composites generally show a relatively low TC.For instance,the VACNT arrays have a TC varying from 15 to 267 W·m?1·K?1,which is almost one or two orders of magnitude smaller than that of the individual CNT[11].C/C composites with high thermal conductivity exhibit a TC ranging from 200 to 900 W·m?1·K?1,much lower than that of individual carbon fiber with high thermal conductivity[12].The significant difference is caused by several factors like defects,alignment,thermal interface resistance,and so on.To improve the heat transport of VACNT arrays and C/C composites,several techniques have been developed to control their structure such as increasing the volume fraction of CNTs via mechanical densification[13,14],reducing defect density by annealing at high temperature[15,16],and improving CNT alignment through strong magnetic field processing[17].

Over the past two decades,there have been many investigations on the thermal transfer properties of VACNT arrays and C/C composites.The issue of preparation-structure-performance relationship,however,has not been adequately outlined.In order to get more insight into this issue,so as to effectively improve the TC of VACNT arrays and C/C composites,the influences of the preparation conditions on micro-structure and the thermal conductivity are highlighted in this review.Firstly,we briefly introduce the thermal conducting mechanism of carbon materials.Then,we present the general fabrication methods,the main factors affecting TC of VACNT arrays and C/C composites as well as their applications in thermal management.The key structures closely correlated high TC and their control strategies are given.Finally,critical consideration on the challenges and prospects in thermal management applications of aligned CNTs and C/C composites are presented.

2 Thermal conducting mechanism of carbon materials

It is believed that thermal conduction is implemented by electrons and lattice vibration (phonons) in solids[18].The contribution from phonons and electrons is largely dependent on the type of materials.For metallic solids,such as Cu,Ag and Al,there are abundant of free electrons and the thermal conduction is mainly carried out by electrons.The thermal conduction in metallic solids is resulted from the impact of metal cations and free electrons[19].In case of nonmetallic solids,however,there is no free electrons and thermal conduction is mainly carried out by the phonon[20].The relative contributions to heat transfer of both electrons and phonons can be evaluated by calculation of the Wiedmann-Franz ratio.A heat transport governed by electrons has a typical characteristic of a constant Wiedmann-Franz ratio.But carbon materials have Wiedmann-Franz ratios with a wide scope,which indicates a phonon-dominated thermal conduction.It was reported that the contribution from phonons is up to 99% for the TC of carbon materials[21].The TC of a material with phonon-dominated thermal conduction can be discribed by the Debye equation[22]:

whereCis the specific heat per unit volume,υ is the phonon velocity andLis the mean free path of phonons.

At room temperature,the TC of carbon materials is mainly dependent on the average speed of phonon motion υ and the mean free path of phononsL.The magnitude ofLdepends on two processes.One is the scattering caused by collision with other phonons,and the other is the scattering caused by collision with impurities,defects,grain boundaries and crystal surfaces.The contribution of these two types of collisions to the mean free path of phonons can be quantitatively expressed as[21]:

whereLeis the phonon-phonon scattering path length,andLdis the interval of inhomogeneous phases,defects,grain boundaries,etc.

The diversity of the structure of carbon materials leads to different contributions of the two types of collisions to the TC of the materials.For carbon materials with high crystallinity,Leplays a leading role in the overall phonon scattering because of the fewer defects and grain boundaries.For carbon materials with low crystallinity,however,phonon scattering caused by uneven struct ure becomes predominant,and as a result the overall phonon scattering is mainly dependent on theLd.For carbon materials with medium crystallinity,the contributions fromLeandLdcannot be ignored[21].

3 VACNT arrays

3.1 Fabrication methods

VACNT arrays,which are also called CNT forests,have shown great potential for many applications including electrochemical capacitors,electrical interconnects in nanoscale devices,advanced TIMs and membranes for waste water treatment owing to their unique structures and remarkable mechanical,chemical,and physical properties[23,24].Various fabrication methods such as CVD[24,25],PECVD[26–29],thermal CVD (TCVD)[30],floating catalytic CVD(FCCVD)[31]and aerosol-assisted catalytic CVD (aerosol-assisted CCVD)[32]have been developed for growing VACNT arrays.Among these methods,CVD and PECVD have been identified as the suitable and efficient techniques to grow high quality VACNTs.Both methods offer better control over the alignment,length,diameter,and density of the CNTs by adjusting growth parameters.Particularly,the PECVD process has a relatively lower deposition temperature compared with the other CVD technique,and as a result the growth temperature of CNTs is slightly reduced because of the elastic interaction of electrons and dissociating molecules during PECVD[33].The growth of the VACNTs typically involves catalytic cracking of a carbon feed stock (hydrocarbons or carbon oxygen compounds) at high temperature on metal catalyst nanoparticles supported on an oxide buffer layer,which was deposited onto the surface of a substrate.Previous researches have demonstrated that the metal catalysts,buffer layer and substrate have significant impacts on the growth of the VACNTs.

(1) Catalyst layer:Transition metals including Fe,Co,Ni and Cu are widely used as catalysts in the CVD method.They are deposited in the form of single-component or multi-component layer on the buffer layer by physical vapor deposition (PVD),CVD from the transition metal-containing solution,magnetron sputtering,ion beam sputtering or thermal evaporation[34].In most cases,monometallic catalysts,such as pure Fe and Co,are used to produce vertically aligned MWCNT arrays (VA-MWCNTs),and it is seldomly effective to yield vertically aligned SWCNT arrays (VA-SWCNTs).Recently,it has been reported that VA-MWCNT arrays can also be formed on the thin layer of Ct-Me-N-(O) alloys (where Ct=Fe,Co,Ni,Pd;Me=Nb,Mo,Ta,Zr,Cr,Ti,V)[34].For the synthesis of VA-SWCNT arrays,three bimetallic combinations,Co-Mo,Fe-Al and Fe-Mo are proved to be effective[35–37].In these three bimetallic catalysts,Co (or Fe) is used as the active site,and uniformly dispersed Mo (or Al) acts as a stabilizing agent to stablize the active sites.Apart from the composition,the size and density of a catalyst are very important for the growth of CNTs,as they determine the density,diameter and alignment of CNTs.For the growth of MWCNTs,the catalyst particle size commonly exceeds 10 nm,but that is only 1–3 nm for SWCNT growth[38].Increasing the catalyst particle density as high as possible is crucial for the synthesis of high-quality VACNTs[39].

(2) Buffer layer:Before depositing the catalyst,a buffer layer is commonly deposited onto the substrate,which is used to prevent catalyst ripening and the diffusing into substrates.The type and deposition process of the buffer layer largely determine the effective growth of VACNTs.The most efficient techniques that are commonly used to deposit the buffer layer onto the substrate are by sputtering,electronbeam evaporation (EB) and atomic layer deposition(ALD).As far as the composition of buffer layer is concerned,nonmetallic films,e.g.,oxide films,have shown more advantage for the preparation of VACNTs as compared to metallic films.Furthermore,among the oxide buffer layers,including SiO2,TiO2,ZrO2,ZnO and Al2O3,it is believed that Al2O3is the best buffer layer material for the growth of VACNTs in the case that Fe is employed as the catalyst[40,41].Another critical factor controlling the growth of VACNTs is the preparation process of the buffer layer.It has been verified that the surface roughness and activation energy for nucleation and initial growth largely depend on the deposition method,which have a close relationship with the size and density of the catalyst nanoparticles resulted from annealing[41].

(3) Substrate:A variety of substrates have been utilized in the CVD or PECVD synthesis of VACNT arrays,such as quartz glass[42],planar silicon[24],silica/silicon[30],mesoporous silica[43],copper[44],aluminum[45–47]and stainless steel[27,28].Non-metallic Si,SiO2/Si or quartz glass substrates can withstand high growth temperature,and are widely utilized for the VACNT growth.

Among various metallic substrates,Al has received much attention owing to its high thermal and electrical conductivity,small mass density,as well as low cost[45–47].But the attempts to grow VACNTs on it have had limited success due to its relatively low melting point of 660 °C,which is much lower than the temperature (800–900 °C) normally required for the VACNT growth[45–47].What's amazing is that substrate crystallinity has a close relationship with the structure,density and the alignment of synthesized aligned CNTs.Guellati et al.[48]fabricated CNT arrays using the FCCVD method over amorphous Al2O3,Si/SiO2and crystalline Al2O3(sapphire).Their results showed that the sapphire substrate provided a more homogenous VACNTs with small diameter and few number walls (Fig.2).Besides the above factors,the CVD or PECVD conditions also have significant effects on the growth of the VACNTs.It has been proved that an appropriate growth temperature,low pressure,specific carbon sources as well as the assistance of weak etchants like H2O and CO2are crucial to synthesize VACNT arrays with high-quality[11].Hata et al.developed a H2O-assisted CVD method and successfully fabricated SWCNT arrays with a height of millimeter (2.5 mm) in 10 min[36].Sato et al.fabricated millimeter-tall (～ 1.2 mm) VA-SWCNT arrays on SiO2/Si substrate at 800 °C for 30 min via the CO2-assisted CVD,and compared the effects of CO2to that of H2O on the growth of the VA-SWCNT arrays[49].Miura et al.also confired the effectiveness of CO2-assisted CVD,and prepared VACNT arrays on Al substrates,with a height of 1.1 mm in 12 h[45].

Fig.2 FESEM micrographs of aligned CNTs synthesized on three kinds of substrate with 7 vol% H2 at 870 °C for 1 h:non crystalline substrate (a,e) Si/SiO2 and (b,f) Al2O3 and crystalline one,(c,g,d) sapphire.[48].(Reprinted with Permission).

3.2 Main factors affecting the thermal conductivity

For VACNT arrays used as TIMs,a high intrinsic TC is essential to assure the heat conduction performance.Unfortunately,the reported TC of VACNT arrays at room temperature varies from 15 to 267 W·m?1·K?1,which are not as high as expected[11].Many researches demonstrated that the TC of VACNT arrays is largely dependent on the tube diameter,defect density and packing density,which can be controlled by tuning the growth conditions[9,11,19,39].In addition to the controlling of growth conditions,post-treatment process like acid treatment and annealing is also effective for the improvement of the crystallinity and purity of CNTs,and thus enhances the thermal conduction performance[11,39].

3.2.1 Impact of the tube diameter

The TC of the VACNT array increases with the decrease of tube diameter.This is because that the increase in tube diameter leads to an increase of wall number of CNTs,which further leads to the increase of the defects and phonon scattering[50].Both experiments and simulation studies have verified that the TC of individual CNT increases with the reduction in the tube diameter[51–53].Pettes et al.[53]measured the roomtemperature TC of three MWCNT samples.It was found that the thermal conductivities at room temperature were 42–48,178–336,and 269–343 W·m?1·K?1for MWCNTs with outer diameters of 14.0,11.4 and 10.3 nm,respectively.They attributed this trend to the observed increase in the defect concentration rather than the enhanced phonon scattering with number of walls.Zhang et al.[50]found that the diameter of CNTs had an obvious influence on the thermal interface resistance.As the average diameter of CNTs increased from 19 to 29 nm,the thermal interface resistance increased from 88 to 148 mm2·K·W?1,due to the increase in the density of the VACNT arrays and the phonon scattering with number of walls.Xu et al.[54]also reported that the reduction in the diameter of the CNTs caused the decrease of thermal interface resistance,under the condition that the density of VACNT arrays was close,which could be resulted from the decline of the contact area.The controlling of the tube diameter of CNTs can be realized by regulating the size of catalysts.Normally,smaller catalyst nanoparticles lead to smaller tube diameter.Ji et al.[55]discovered that reducing the thickness of the catalyst layer could result in an obvious decrease in CNT diameter.The growth model is illustrated in Fig.3.

Fig.3 Model describing the formation of catalytic particles and the VACNT growth process[55].(Reprinted with Permission).

In addition,the CNTs with small diameter can also be obtained by improving the surface roughness of buffer layers,which can be realized by suitable deposition process.Li et al.[56]reported that EB Al2O3showed a rougher surface compared to the sputtered Al2O3,and as a result the rough surface of EB Al2O3facilitated the formation of catalyst nanoparticles with a small diameter and high density.Another approach to control the tube diameter is to use bimetallic catalysts.For instance,VA-SWCNT arrays with a mean tube diameter of 1.4 and 1.0 nm was obtained by CVD using Co-Mo and Cu-Co catalysts[57,58],respectively.

3.2.2 Impact of the packing density

Increasing packing density is helpful for the improvement of the TC of the VACNT arrays.The higher the packing density,the more effective pathways for thermal conduction of the VACNT arrays.Kang et al.[37]designed an effective Trojan-Mo catalyst,and then it was successfully used to synthesize large-area SWCNTs on sapphire substrate,which had a ultrahigh density of 160 tubes μm?1.

Parveen et al.[29]used Al as the barrier layer to prevent the diffusion of Fe in Si substrate,which remarkably improve the density and vertical alignment of SWCNTs (Fig.4a–h).Cha et al.[59]proposed a mechanical densification method to enlarge the volume fraction of VA-SWCNT arrays via squeezing along the direction of both sides,and the vertically aligned structure was not changed because of this(Fig.4i).They found that the TC increased from～ 5 to～ 9 W·m?1·K?1with the volume fraction of VASWCNT forests increasing from 20 vol% to 25 vol%(Fig.4j).Kong et al.[60]proposed an original 3D CNT network structure to enhance the heat conduction properties of VACNT arrays.In this 3D network structure,randomly aligned secondary CNTs crosslinked the primary CNTs in the arrays.The secondary CNTs were grown directly from the walls of the primary VACNTs.The 3D CNT network had an inplane TC of 5.40 W·m?1·K?1,which was at least 50 times larger than that of the VACNT array(0.10 W·m?1·K?1).

Fig.4 Schematic diagram of growth of highly dense and VA-SWCNTs using Al as barrier layer (a-d).SEM micrographs of (e) Fe catalyst,(f) as-grown SWCNTs on Fe catalyst,(g) Fe/Al catalysts,and (h) as-grown highly dense and VA-SWCNTs on Fe/Al catalysts by PECVD method[29].(i) Schematic image of the biaxial mechanical densification method to enhance the volume fraction of VASWCNT forests.(j) Dependence of the thermal diffusivity (black closed triangles for left axis) and thermal conductivity (red opened circles for right axis) of VASWCNT forests on the volume fraction[59].(Reprinted with Permission).

3.2.3 Impact of the defect density

High TC of CNTs is always related with the few defects and high degree of graphitization for mean long-range phonon transport.Lin et al.[14]confirmed that the few defects in SWCNTs resulted in high TC and that the annealing process improved the degree of graphitization and reduced the defects present.Ivanov et al.[15]found an increase in TC by 400% for VACNT arrays after annealed at 2 800oC,which once again confirms the importance of decreasing the defect density of VACNT arrays.

3.3 Applications of VACNT arrays for thermal management

To make full use of the thermal conduction performance,VACNT arrays should be encapsulated as TIMs between electronic device and heat sink.Hence the thermal interface resistance between the VACNT array and the mating surfaces is one of the crucial prameters determining the heat dissipation ability of the TIMs[39].Generally,there is a large thermal interface resistance between VACNT arrays and other substrates that are in direct contact.For example,Tong et al.[61]measured the thermal interface resistance between VACNT arrays and direct-contacting glass substrate and found it to be about 11 mm2·K·W?1,about one order of magnitude higher than that of the VACNT-silicon substrate interface.Previous studies[62]indicated that the thermal interface resistance between a VACNT array and heat sink has a close relationship with the top roughness of VACNT array and their effective contact area.The top roughness of the VACNT array raised from the varying CNT's height,which led to the incomplete contact,and as a result only part of CNTs participated in the thermal conduction.Qiu et al.[63]reduced the thermal interface resistance from 43 to 17 mm2·K·W?1by finely tuning the growth conditions to increase array height uniformity.Increasing the contact area can be realized by coating the VACNT array with a low thermal resistance layer.Qiu et al.[62]deposited diamond-like carbon/TiN nanofilms onto VACNT arrays by PECVD,which brought about a 50 times decrease in thermal interface resistance from 15 to 0.3 mm2·K·W?1(Fig.5a).In addition,the incomplete contact at the atomic or molecular scale between individual CNT and substrate significantly increases the macroscopic interface resistance[64].Bonding free-end VACNT tips with Ti/Au/In/Sn/Ni was demonstrated as a feasible strategy to improve the interfacial conduction(Fig.5b).Experimental measurements showed that thermal interface resistance was reduced to 1–3.5 mm2·K·W?1,which was almost an order of magnitude samller than that of the non-bonding counterpart[62,65].Chemical functionalization of the VACNT array interfaces is another approach to improve the interfacial conduction.Kaur et al.[66]reported a reduction up to 600% in the thermal interface resistance between VA-MWCNT arrays and metal surfaces through bridging the interface with short,covalently bonded organic molecules.The thermal interface resistance between the VACNT array and the growth substrate is another critical prameter influencing the heat conduction performance of VACNT-based TIMs.Generally,VACNT arrays are grown on substrates with low TC,which hinders the thermal conduction along the length direction of CNTs.Therefore,it is necessary to remove VACNT arrays from substrates,where they are grown,to various desirable substrates to enhance the heat transfer.To achieve this,many methods have been developed,such as two-side growth,the wet chemical etching method,press transfer,two-sided bonding,and peeling off[11,23].All these techniques promote the application of VACNT arrays as TIMs as they allow the VACNT arrays to be removed to other substrates that cannot withstand the high temperatures required for CNT growth.However,further treatment for the VACNTs-substrate interface is indispensable,otherwise,higher thermal interface resistance are almost unavoidable[67].Therefore,a new tight interface contact between VACNT array and substrate is still required to improve the heat conduction peroperty of the TIMs.Ping et al.[23]transferred a 20 μm tall VACNT array (act as a TIM) onto a desktop computer CPU cooling system.In order to investigate the heat transport performance of the VACNT-based TIMs,they measured the temperature of the CPU surface (Tc) as well as the environmental temperature close to the heat sink surface(Ts).It was found that the transferred VACNT arrays performed a better thermal conduction performance(Tc?Ts ≈ 4.8 °C) than commercial thermal pad (Tc?Ts ≈ 6.2 °C) when the heat sink with an RMS roughness of 0.88 mm was employed (Fig.5c-f).Zhang et al.[68]utilized VACNT arrays for high brightness lightemitting-diode (HB-LED) packaging.They found that the output light power of LED packages maintained a linear relationship with input current up to 900 mA without achieving saturation.This indicates that the VACNT arrays are promising TIMs used in HB-LED packages.Moreover,Chen et al.[69]applied VACNT arrays as TIMs in vibrational devices such as piezoelectric transformers.

Fig.5 (a) Coatings remarkably reducing thermal contact resistance (Rc) for VACNT arrays as TIMs[62].(b) Cu-Solder-Ti/Ni/Au-MWNTs-Ti/Ni/Au-Solder-Cu configuration and approximate thicknesses[65].(c) Thermal resistance of transferred VACNT arrays.(d) Optical images of a CPU on a motherboard and a heat sink covered by four VACNT arrays.Temperature differences (DT) between the CPU and the heat sink as a function of time using heat sinks with contact surface roughness of (e) 2.66 and (f) 0.88 mm[23].(Reprinted with Permission).

In addition,due to the low self-supporting strength of the VACNT arrays,they are prone to be damaged by external environment,such as complex mechanical environment and humid environment.Once the VACNT fragments fall on the motherboard when the VACNT arrays are applied,they may cause damage to other electronic devices because of their good electrical conductivity.Hence the VACNT arrays need to be protected in many cases to maintain the stability of their structure and morphology.At present,VACNT arrays are mainly composited with copper and polymers to increase their mechanical strength.There are several reports about Cu filled VACNT arrays fabricated by electro-deposition and PECVD[70–72].Ngo et al.found that the Cu filler significantly improved the mechanical stability and thermal conductive property of the VACNT array[70].Although the combination of VACNT array and copper can enhance the mechanical stability of the VACNT array,it cannot take advantage of the light weight and low density of the VACNT array,and there is also the problem of CTE mismatch.The composite of the VACNT arrays with polymers like epoxy resin,polydimethylsiloxane,bismaleimide and silicon elastomer is conducive to improve the flexibility of the vertical array of CNTs,and at the same time facilitates the VACNT arrays to be stripped from the growth substrate.However,the introduction of polymers largely reduces the intrinsic thermal conductivity of the VACNT arrays because of the polymer’s low thermal conductivity,which has a great negative impact on heat conduction[73].Wang et al.[74]found that the TC of the prepared VACNT arrays dropped from 17.76 to 13.15 W·m?1·K?1after they were impregnated with epoxy resin.If dibutyl phthalate (toughening agent) was added to the epoxy resin,the TC of the prepared VACNT arrays would further reduce to 8.23 W·m?1·K?1.

4 C/C composites

4.1 Fabrication methods

The preparation of high thermal conductive C/C composites mainly involves three steps:preform molding,densification and high-temperature graphitization,among which the preform densification process is the key technology.Precursor impregnation and pyrolysis (PIP) and/or chemical vapour infiltration (CVI) are usually used for densification.At present,research reports on high thermal conductive C/C composites mainly focus on 1D and 2D materials,and 3D materials are rarely reported.

4.1.1 1D C/C composites

1D high thermal conductive C/C composites are usually prepared by hot press moulding-carbonization-graphitization process or one-step high-temperature hot press moulding process (Fig.6a),which integrates with molding,carbonization,densification and graphitization as a whole.The in-plane (along the fiber length direction) TC of 1D composites is between 600 and 900 W·m?1·K?1at room temperature,which is significantly higher than that of 2D and 3D C/C composites (Table 1),but the lower mechanical properties limit its wide applications.The self-reinforced thermalgraph panels were produced by hot press moulding-carbonization-graphitization process in Amoco Corporation.The oxidized mesophase pitch-based fibers were used as the raw material(without adding binder) and the graphitization temperature was 3 000 oC.When the volume content of carbon fibers was 82%,Thermalgraph panels exhibited an in-plane TC of 746 W·m?1·K?1at room temperature[75].Zhang[76]used the same process to prepare a self-adhesive sheet.Firstly,the mesophase pitch fibers were pre-oxidized at 260 oC,and then laminated in one direction.Afterward,the sheet was formed by hot pressing at 25 MPa and 400 oC.Finally,the obtained sheet was carbonized at 1 000 oC and graphitized at 2 600 oC.The in-plane TC reached 852 W·m?1·K?1.Ma et al.[77]prepared self-adhesive bulks by one-step high-temperature hot press moulding using ribbonshape fibers oxidized at 260 oC as the reinforcement.After graphitized at 2 600 oC,the self-adhesive bulks had a density of 2.18 g cm?3and an in-plane TC of 717 W·m?1·K?1at room temperature.The same approach was utilized by Gao et al.[78]to fabricate carbon bulks with large size.They used mesophase pitch as the binder,which was quite different from the preparation of self-reinforced thermalgraph panels and self-adhesive sheets/bulks.Large-size carbon bulk with a density of 2.2 g cm-3was obtained after 2 800 oC graphitization,and its in-plane TC was 602 W·m?1·K?1at room temperature.Yuan et al.[79,80]obtained two types of carbon bulks via hot press moulding at 500 oC followed by carbonization at 1 000 oC and graphitization at 2 000–3 100 oC.One type obtained from round-shaped carbon fibers with large diameter possessed an in-plane TC of 650–734 W·m?1·K?1at room temperature,and the other type obtained from ribbon-shaped carbon fibers exhibited a much higher thermal conductivity of 862–896 W·m?1·K?1(Fig.6c-e).Yao et al.[81]fabricated a porous C/C composites via hot press moulding at a low temperature firstly.Then,the porous C/C composites was densified by CVI and PIP.Finally,carbon bulks with a density of 1.90 g cm?3were obtained after 3 000 oC graphitization,and it had an in-plane TC of 667 W·m?1·K?1at room temperature.Zhang et al.[82]developed a technology without hot pressing to fabricated two types of carbon rod through densification of large-diameter graphite fiber bundles.One type was prepared by infiltrating carbon fiber bundles with phenolic resin,followed by curing,carbonization,graphitization as well as CVI densification (ICCG +CVI).The other type was fabricated via CVI treatment of the carbon bundles.Further densification was carried out for both types of carbon rods by a second CVI treatment.After graphitization,the density of the carbon rods prepared by the ICCG+CVI and CVI processes were 1.65 and 1.72 g cm?3,respectively(Fig.6f-h).The axial TC of these two carbon rods were as high as 569 and 675 W·m?1·K?1,respectively.

Fig.6 (a) Schematic diagrams of 1D and 2D C/C composites and their thermal diffusion coefficient test directions[81].(b) Weaving process of preform architecture for 3D C/C composites[85].(c) SEM images of a whole ribbon-shaped carbon fibers after graphitization at 2 800 °C.(d) SEM images of transverse section of the highly oriented C/C composite block graphitized at 3 000 °C.(e) Optical photograph of one-dimensional C/C composite block[79].(f) SEM images of graphite fibers heat-treated at 3 000 °C.(g) SEM images of transverse section of the one-dimensional cylindrical C/C composites graphitized at 3 000 °C.(h) The optical photographs of the graphite fiber rods after heat treatment at 3 000 °C for 15 min and peeling off the T300-3k fiber shell[82].(Reprinted with Permission).

4.1.2 2D C/C composites

The fabrication methods for 2D C/C composites are similar to those for 1D composites,and their inplane TC at room temperature are between 300 and 500 W·m?1·K?1(Fig.6a and Table 1).Golecki et al.[83]fabricated 2D C/C composites by preform densification and high-temperature annealing.The 2D woven preforms were densified through both PIP (resin) and CVI,which had different types of weaves,for example,plain weave or different kinds of satin weave.After high-temperature annealing at 1 800–3 000 oC,2D C/C composites were obtained with in-plane TC of 340–460 W·m?1·K?1at room temperature,much higher than that in the vertical plane (20–70 W·m?1·K?1).Feng et al.[84]and Yao et al.[81]prepared 2D C/C composites by hot pressure molding,PIP or PIP+CVI densification,followed by heat treatment at 2 300–3 000 oC.Pitch-based carbon fiber fabric and mesophase pitch were used as the reinforcement and binder,respectively.The obtained 2D C/C composites exhibited in-plane TC of 345–443 W·m?1·K?1at room temperature.

Table 1 Comparison of TC of C/C composites reinforced with mesophase pitch-derived carbon fibers.

4.1.3 3D C/C composites

3D high thermal conductive C/C composites are usually prepared by PIP and/or CVI densification of fine-woven puncture fabric and high-temperature graphitization.The room-temperature TC of the 3D C/C composites are usually in the scope of 200–400 W·m?1·K?1(Table 1).In terms of mechanical properties,3D C/C composites are much better than 1D and 2D C/C composites,but their production cost is much higher than 1D and 2D C/C composites.Fiber Materials Inc.and Hercules Inc.manufactured products with models FMI-222 and Hercules 3D using P120 pitch-based carbon fibers as the reinforcement,whose room-temperature TC was 200 and 345 W·m?1·K?1,respectively.In addition,Fiber Materials Inc.also produced products with model FMI A27-130 using P130 pitch-based carbon fibers as reinforcement,and the products had a TC of 309 W·m?1·K?1at room temperature[86].The European Union Soci?t? Europ?enne de Propulsion Inc.produced 3D C/C composites with models N11 and N112 from PAN-based carbon fiber preforms.There was 2D braid in the in-plane direction (X-Ydirection) of the preforms,and carbon fibers were pierced in the vertical direction (Zdirection).The preforms were firstly densified by CVI,and then carbonized,and followed by pitch PIP densification and graphitization at 2 200 oC.The resulting 3D C/C composites (N112)had a room-temperature TC of 248 W·m?1·K?1.In contrast,the C/C composites without pitch PIP densification (N11) had a room-temperature TC of 220 W·m?1·K?1[87].Due to the lack of high-performance pitch and carbon fibers,domestic research on C/C composites with high thermal conductivity is still at the laboratory stage.

There are few domestic research reports on 3D high thermal conductive C/C composites.Feng et al.[12]fabricated a C/C composite via pitch PIP and high-temperature graphitization,using 3D woven preform with mesophase pitch-based carbon fibers as the reinforcement.The 3D C/C composite had a density of 1.95 g cm?3and an in-plane TC of 340 W·m?1·K?1.Li et al.[85]used continuous mesophase pitch-based carbon fibers and commercial PAN-based carbon fibers to prepare 3D composites via preform densification and high-temperature graphitization.The preforms were obtained through orthogonally weaving continuous mesophase pitch-derived fibers in theX-Yplane and puncturing commercial PAN-derived carbon fibers along theZdirection (Fig.6b).The resulted preform was then densified to a density of 1.58 g cm?3through 3 cycles of CVI and graphitization (3CVI).Afterward,it was further densified to a density of 1.84 g cm?3through 4 cycles of liquid pressure impregnation (LPI),carbonization as well as graphitization (3CVI+4LPI).The C/C composites fabricated via 3CVI and 3CVI+4LPI processes exhibited TC of 116 and 235 W·m?1·K?1in theX-Yplane,respectively,while the TC in theZdirection were only 19 and 42 W·m?1·K?1.

4.2 Main factors affecting the TC of C/C composites

C/C composites are composed of carbon fibers and matrix carbon,and the respective properties of carbon fibers and carbon matrix have important influence on their TC.Besides,the TC of C/C composites also depends on the structure of the preform,interface bonding between carbon matrix and carbon fibers,porosity,heat-treatment temperature,etc.[8].

4.2.1 Impact of carbon fibers

Carbon fiber is an important carrier of heat transfer,and its thermal conductivity,orientation,distribution and filling ratio significantly affect the overall TC of the composites (Table 2).Polyacrylonitrile (PAN)-derived carbon fibers have remarkable mechanical properties and are widely used as the reinforcement in the composites,but their TC are not high enough.For example,the axial TC of Thornel T300 and T650 PAN-based carbon fibers produced by Amoco Corporation are 5 and 14 W·m?1·K?1,respectively.The axial thermal conductivity of PAN-based carbon fibers manufactured by Toray corporation are between 7 and 150 W·m?1·K?1[89].Mesophase pitch-based carbon fibers are synthesized from mesophase pitch via spinning,pre-oxidation stabilization,carbonization and graphitization processes.After spinning and preoxidation stabilization,the inherent orientation of liquid crystal molecules is retained,and after carbonization and graphitization treatment,the crystal structure of the pitch is highly oriented along the fiber axis.As a result,the mesophase pitch-based carbon fibers haveextremely high TC,and they are often used as an ideal reinforcements for high thermal conductive C/C composites[90].

Table 2 Comparison of thermal conductivity of C/C composites with different carbon fiber reinforcements.

At present,mesophase pitch-derived carbon fibers have been mass produced by Cytec Inc.,Mitsubishi Chemical Inc.and Graphite Fiber Inc.P100,P120,P130 and K1100 are 4 types of mesophase pitch-derived carbon fibers,which possess round cross sections and are produced by Cytec Inc.,whose axial TC at room temperature are 520,640,1 100 and 1 100 W·m?1·K?1[91],respectively.The TC of K13D2U produced by Mitsubishi Chemical Inc.is up to 800 W·m?1·K?1,and the TC of YS-95A produced by Graphite Fiber Inc.reaches 600 W·m?1·K?1[92].Klett et al.[93]prepared 1D C/C composites using AR mesophase pitch (Mitsubishi AR-120) and carbon fibers as the matrix precursor and reinforcement,respectively.They used 3 different carbon fibers,which were PANbased carbon fibers (T300),pitch-based fibers (P55),and an experimental pitch-based ribbon fibers with high TC.The graphitized P55/AR-120 and T300/AR-120 composites exhibited thermal conductivities(along the fiber length direction) of 135.5 and 80.5 W·m?1·K?1at room temperature,respectively.In contrast,the composite reinforced by ribbon fibers showed an obvious 3D anisotropy,and it possessed a room-temperature TC (transverse to the fibers) of 213.5 W·m?1·K?1,larger than that along the fiber length direction (145 W·m?1·K?1).This result implied that fiber shape has significant influence on the thermal properties of matrix in carbon/carbon composites.A Bowers et al.[94]tested several 1D C/C composites prepared with petroleum pitch and different types of carbon fibers.The composites prepared from P120,P130 and K321 fibers possess room-temperature TC of 526,851 and 691 W·m?1·K?1along the fiber length direction,respectively.Manocha et al.[95]used T-300,F-180 (medium modulus pitch-based carbon fibers) and P100 as reinforcements and pitch with low quinoline insoluble content as matrix precursors to prepare 1D C/C composites.Among the resulting composites,the C/C composites reinforced by P100 carbon fibers exhibited the highest diffusivity and TC along both radial and axial directions of the fibers,due to the highly graphitic structure and high TC of P-100 carbon fibers.It is worth noting that the C/C composites reinforced by T300 carbon fibers showed high diffusivity and conductivity along the radial direction of the fibers as compared to C/C composites reinforced by F-180 carbon fibers.This was ascribed to the typical wavy radial structure of F-180 carbon fibers that impeded the thermal conduction along the radial direction of the fiber.Ma et al.[96]used ribbon-shaped carbon fibers and round-shaped carbon fibers to fabricate 1D C/C composites by one-step high-temperature hot press moulding.The C/C composite prepared from ribbon-shaped carbon fibers had a higher TC(837 W·m?1·K?1) than that from round-shaped carbon fibers (649 W·m?1·K?1) along the axial direction of fibers (Table 2).The improved TC was ascribed to the compact stacking,which promotes the thermal conduction.

4.2.2 Impact of carbon matrix

There are 3 main types of carbon matrix in C/C composites:pyrolytic carbon,resin-derived carbon and pitch-derived carbon.The structure and orientation of the carbon matrix have remarkable influence on the TC of the C/C composites (Table 3).Pyrolytic carbons are usually obtained by CVI process,which have microstructure called rough-laminar (RL),smooth laminar (SL) and isotropic (ISO).The RL microstructure has a high degree of orientation and is easy to be graphitized.The ISO is amorphous and is not graphitizable[83].The differences in the degrees of orientation and graphitization ability leads to a large divergence in heat transfer for the three kinds of pyrolytic carbon textures,and as a result the thermal conductive performance gradually degrades in the sequence of RL>SL>ISO[97].Resin-derived carbon and pitch-derived carbon are generally obtained by PIP process.The former generally has an isotropic structure and is difficult to be graphitized,while the latter is usually obtained from medium-temperature pitch or mesophase pitch.The residual carbon ratio of mesophase pitch is higher than that of medium temperature pitch,and the obtained carbon has a higher orienta-tion of graphite microcrystalline,which is beneficial for the heat transfer in the composites[98,99].Michalowski et al.[100]developed 2D C/C composites with pyrolytic and glass-like carbon matrices.Pyrolytic carbon matrix was obtained by CVI using propane as a precursor,and glass-like carbon matrix was deposited by PIP using phenolic resin as a precursor.They found that the C/C composites with pyrolytic carbon matrix exhibited a higher TC (～ 42 W·m?1·K?1) than that (～ 37 W·m?1·K?1) of the counterpart with glasslike carbon matrix.The significant contribution from pyrocarbon matrix to thermal conductivity could be attributed to the better structural ordering (alignment)in the presence of carbon fibers.Manocha et al.[95]fabricated 2D C/C composites using 3 different carbon matrix precursors,namely,coal tar pitch with low quinoline content (Pitch A),coal tar pitch wih high quinoline content (Pitch B) and synthetic AR mesophase pitch (Pitch C).It was found that C/C composites prepared with Pitch C exhibited higher TC along both axial and radial directions of the fibers (204,113 W·m?1·K?1),as compared to composites prepared with Pitch A (197,71 W·m?1·K?1) and Pitch B (181,44 W·m?1·K?1).The higher TC in both directions was due to highly oriented graphitic structure of the carbon derived from Pitch C,which was beneficial for thermal conduction with less scattering.The quinoline insoluble (QI) particles formed in Pitch B interrupted the large graphitic plane structure and thus reduced the mean free path of phonons.Therefore,the composite prepared with Pitch B showed a lower TC.The orientation and structure of resin-derived carbon,pitch-derived carbon and pyrolytic carbon and their effects on the TC of C/C composites were also reported by researchers such as Chen et al.[99],Lin et al.[80]and Liu et al.[101].Lin et al.[80]prepared 1D C/C composites using AR mesophase pitch,SC isotropic pitch and MP naphthalene-derived mesophase pitch as carbon matrix precursors.They found that composites prepared with AR mesophase pitch showed the highest TC in the direction parallel to fibers,whereas composites prepared with SC isotropic pitch exhibited the lowest thermal conductivity.Besides,it was also found that the carbon matrix precursors had no obvious influence on TC in the other two directions for 1D C/C composites.Liu et al.[101]investigated the TC of four C/C composites with carbon matrix derived from coal tar pitch,furfural acetone resin,natural gas and xylene.It was found that the variation of TC with the matrix type was positively correlated with the graphitization degree and apparent crystallite height.The C/C composites with xylenederived carbon matrix had the highest in-plane TC(148 W·m?1·K?1) and out-of-plane TC (75 W·m?1·K?1),which could be attributed to its carbon layers with the highest oriented structure.In contrast,the relatively low TC of C/C composites was achieved with resinderived carbon matrix resulted from the chaotic structure and large number of defects from the carbon matrix.

Table 3 Comparison of thermal conductivity of C/C composites with different carbon matrices.

4.2.3 Impact of the preform structure

Carbon fiber is a one-dimensional material that can be designed and woven into unidirectional,two dimensional,three dimensional and multi-dimensional fabric preforms.The preform structure directly affects the TC of the C/C composites.Generally,graphitized pitch-based carbon fibers have ultra-high modulus and cannot be woven into fabric preforms,but pitch-based carbon fibers obtained via low-temperature carbonization have good flexibility and are suitable for weaving.Luo et al.[97]studied the effects of three kinds of preform structure on the TC of 2D C/C composites,including needled carbon fiber felt,pressmoulding chopped carbon fibers and the carbon cloth laminate.Their results indicated that the C/C composites prepared from the needled carbon fiber preform had the highest TC inX-Ydirection,but the C/C composites prepared from laminated carbon cloth had the lowest TC inZdirection due to no carbon fibers in this direction.The C/C composites prepared from press-moulding chopped carbon fibers possessed the lowest TC inX-Ydirection,and itsZ-direction TC is between that of needled carbon fiber preform and that of the laminated carbon cloth.Zaman et al.[102]fabricated 3D-four directional C/C composites with a high density via hot isostatic pressure impregnation of coal tar pitch and carbonization.The composites showed a quasi-isotropic heat capacity,whereas the TC was dependent on the fiber volume fraction in the test direction.Araki et al.[103]designed the preform by controlling the fiber ratio in the three-dimensional directions to be 5∶1∶1,and successfully prepared 3D C/C composites with a TC of 500 W·m?1·K?1along the specific direction.Cao et al.[104]prepared 4D C/C composites by pitch impregnation under high pressure and carbonization.4D performs were used as the reinforcement,which was obtained by mix weaving carbon fibers and axial carbon rod.The axial thermal diffusivity of the composites decreased with the increase of temperature,but the thermophysical properties tended to be stable at high temperature.

4.2.4 Impact of other factors

In addition to the structure and thermal conductive property of carbon fibers,carbon matrix and preforms,other factors such as the fiber volume fraction,porosity and heat-treatment temperature also have important influence on the TC of the C/C composites.For the C/C composites with remarkable anisotropic thermal conduction property,the difference in the TC along a specific direction is primarily caused by different fiber volume fractions in this direction.Carbon fiber with high TC provides the heat transfer channel,so increasing the volume fraction of continuous long carbon fibers along a specific direction leads to an increase of TC in this direction[97].As for porosity,the higher the porosity is,the more discontinuous carbon matrix becomes,which leads to more obstruction to heat transport.Hence the TC decreases with the increase of porosity,and the lower the TC of the C/C composites,the greater the influence of porosity[8].In addition,the TC of C/C composites is largely dependent on the heat-treatment temperature.As the heattreatment temperature increases,the thermal diffusivity and TC of C/C composites gradually increase.When the heat treatment temperature is low,both the thermal diffusivity and TC are relatively small.This is mainly because there are many defects,pores and impurities presented in the C/C composites,which results in a relatively short phonon mean free path.With the increase of heat-treatment temperature,the degree of graphitization and the size of graphite crystallites rise,meanwhile,the crystal defects and content of impurities decrease.These changes cause the increase of phonon mean free path and the phonon velocity[105,106].Gao et al.[78]studied the relationship between TC and the heat-treatment temperature of 1D C/C composites.They observed that when the heat-treatmenttemperature increased from 2 300 to 2 800 oC,the TC increased from 341 to 581 W·m?1·K?1along with an increase ofLafrom 45.86 to 67.62 nm.The dependence of TC on heat-treatment temperature was also reported by Yuan et al.[79],and Feng et al.[84].Yuan et al.[79]reported that the TC of the prepared 1D C/C composites increased from 40 to 862 W·m?1·K?1,andLaincreased from 5.82 to 78.30 nm with the heattreatment temperature increasing from 1 000 to 3 000 oC.The lamellar stacking structure and high orientation formed in the composites during heat treatment at 3 000 oC significantly increased their TC.Feng et al.[84]observed a linear relationship between TC and heat-treatment temperature for 2D C/C composites.As the heat-treatment temperature increased from 2 300 to 3 000 oC,graphitization degree increased from 80.3% to 90.7%,andd002reduced from 0.337 1 to 0.336 2 nm.After heat treated at 3 000 oC,the C/C composites exhibited the highest TC of 443 W·m?1·K?1.

Based on the above review,we can draw four crucial principles involved in the fabrication of C/C composites with high thermal conductivity:(1) choosing mesophase pitch-based carbon fibers with high TC as a reinforcement,(2) selecting mesophase pitch as a carbon matrix precursor,(3) optimal designing of the preform structure by controlling the fiber volume fraction and alignment along specific directions,(4) increasing the degree of graphitization and orientation as well as decreasing porosity of the C/C composites as much as possible.

4.3 Applications of C/C composites in thermal management

Although C/C composites possess high thermal conductivity along specific direction,the large anisotropy in thermal conduction limits their wide-spread applications.In order to improve the thermal conduction of C/C composites,several researchers have introduced some fillers with high thermal conductivity into the carbon matrix,including CNTs,carbon nanofibers (CNFs),and SiC nanowires (SiCNWs).Chen et al.[107]prepared unidirectional C/C composites via impregnation-carbonization cycle using CNT doped furan resin as the carbon precursor.They found that the introduce of CNTs led to a certain orientation of the resin carbon around the carbon fibers along the radial direction of the fiber during carbonization,and as a result,the TC of the CNTs modified C/C composites was increased by more than 2 times along the direction perpendicular to the fiber length and the TC along the fiber length direction was also enlarged by 15% as compared to that of C/C composites without CNTs introduced.In the other work[108],they firstly synthesized CNTs on the surface of the carbon fibers in unidirectional preform,and then CVD was adopted to densify the preform so as to obtain CNTs modified C/C composites.It was observed that the CNTs promoted the deposition of pyrolytic carbon with a RL structure around the carbon fibers.In addition,this pyrolytic carbon layer had good interfacial bonding with the carbon fibers.As a result,the CNTs modified C/C composites exhibited significant improvement of TC along the fiber length direction and the direction perpendicular to the fiber length,which was 2.5 and nearly 5 times higher than that of C/C composites without CNTs introduced,respectively.Similar to the CNTs,the CNFs can also play an inductive role in the formation of pyrolytic carbon with a RL structure during CVI process.Li et al.[109]introduced CNFs into the preforms of C/C composites via spreading layers of carbon cloth and then densified the preforms by CVI in order to improve the thermal conduction properties of C/C composites.It was found that the CNF reinforced C/C composites with a CNF loading of 5 wt.% showed a 5.5%–24.1% and 153.8%–251.3% enhancement in TC in theX-YandZdirections,respectively,as compared to those of C/C composites without the CNF reinforcement.They attributed the increase in TC to the formation of a percolation network of CNFs on the carbon fibers,which provided the continuous pathways for the phonon transmission.Apart from the aforementioned CNTs and CNTs,SiCNWs also displayed an obvious inductive effect on the formation of a pyrolytic carbon layer with a RL structure in C/C composites,as reported by Lin et al.[110].They obtained SiCNW-C/C preforms,firstly,in which SiCNWs were grown inside the carbon felt via CVI followed by heat treatment.Then the SiCNW-C/C preforms were densified by CVI to obtain SiCNW-C/C composites.They found that the SiCNW-C/C composites exhibited a 31% and 52%enhancement in TC in theX-YandZdirections,respectively,as compared to those of C/C composites without the SiCNW reinforcement.This can be ascribed to the formation of interconnected thermal conduction network composed of SiCNWs and carbon matrix as well as the formation of a pyrolytic carbon layer with a RL structure.The above-mentioned researches focus on the improvement of thermal conductive properties of C/C composites reinforced with PAN-based carbon fibers.Unfortunately,there are very few reports about the enhancement of thermal conductive properties of C/C composites reinforced with mesophase pitch-based carbon fibers.In order to improve the thermal conduction of the C/C composites reinforced with mesophase pitch-based carbon fibers,Yuan et al.[111,112]introduced fillers like largediameter flake graphite,chopped carbon fibers and CNTs into the unidirectional C/C composites.The results indicated that the introduction of such fillers resulted in an increase of TC along the direction perpendicular to the fiber length from about 25.4 to 40–60 W·m?1·K?1.However,the TC along the fiber length direction was decreased significantly from 800 to 600 W·m?1·K?1,which resulted from the agglomeration or stacking of the fillers in carbon matrix.

During last few decades,a lot of efforts have been made to develop of C/C composites with excellent thermophysical properties for advanced thermal management systems,such as heat sinks,electronic packaging,spacecraft thermal doublers and radiators,and plasma facing components of fusion devices[106].NASA developed a C/C composites-based radiator panel with an aluminum honeycomb core and it was successfully applied on the Earth Orbiter-1 (EO-1)spacecraft[113].This radiator was constructed of two thin C/C face sheets bonded to both sides of an aluminum honeycomb core,which displayed excellent thermal and mechanical properties (Fig.7a).Apart from the C/C composite-based radiator panel,NASA has also developed C/C finned heat pipe radiators for nuclear power systems such as the SP-100 and the JIMO program (Fig.7c).This kind of heat pipe radiator has excellent high temperature resistance and significant weight reduction effect.It can be used for nuclear-powered spacecraft to dissipate tens of kilowatts or even hundreds of kilowatts of waste heat,effectively decreasing the volume and weight of the radiation heat dissipation system[114].Kowbel et al.[115,116]developed 2D C/C composites with low-cost for Sibased and GaAs-based electronics.The in-plane and out-plane TC of the composites was 350 and 40 W·m?1·K?1,respectively,and its thermal conductive performance greatly exceeded state-of-the-art Cu-W,AlN and BeO heat spreaders.High thermal conductive C/C composites have also been applied in thermal protection system of spacecrafts.The hypersonic aircraft X-43A was part of the X-plane series and specifically of NASA's Hyper-X program.The flight conducted in November 2004 was successful and the vehicle broke the speed record,close to Mach 10 (12 000 km/h)[117].High thermal conductive 2D C/C composites coated with oxidation resistance layers were utilized in the nose cone,leading edge of wing and flight control panel of X-43A (Fig.7b),and no oxidation and ablation occurred on the C/C composites[118].This is a pioneering and milestone engineering application of C/C composites with high TC.

Fig.7 (a) Edge of C-C radiator panel prior to installation[113].(b) Leading edges produced by Materials Research Corporation (MER) mounted on X-43A test vehicle[117].(c) Finned C/C heat pipe with an Nb-1Zr evaporator liner[114].(Reprinted with Permission).

5 Conclusions and outlook

In the present work,we review the mechanisms of thermal conduction of carbon materials,fabrication methods and main factors affecting the thermal conduction property of VACNT arrays and C/C composites.The TC of VACNT arrays highly depends on the tube diameter,packing density and defect density of CNTs,whereas the TC of C/C composites is largely dependent on the thermal conduction properties of carbon fibers and carbon matrix coupled with the structure of the preforms.The annealing/sintering and mechanical densification have been proved to effectively enlarge the TC of VACNT arrays.Besides,controlling growth conditions like the growth temperature,the surface roughness of the buffer layer,thickness of the catalyst layer were also applied to improve the heat transfer performance.VACNTs-substrate thermal interface resistance is one of the key factors determining the heat dissipation ability of the VACNT-based TIMs.Strategies such as finely tuning the growth conditions to increase array height uniformity,coating the VACNT array to enlarge the contact area,as well as metallic bonding and chemical functionalization to enhance VACNT-substrate interaction,can reduce the thermal interface resistance.For C/C composites,mesophase pitch-based carbon fibers and mesophase pitch should be used as the reinforcement and carbon matrix precursor,respectively,in order to achieve a better thermal conduction property.Moreover,optimal designing of the preform structure by controlling the fiber volume fraction and alignment along specific directions is also used in the preparation of C/C composites with excellent heat transfer performance in the required directions.Based on the results reviewed herein,VACNT arrays and C/C composites have great potential in thermal management because of their exceptional anisotropic TC,low density and CTE and chemical stability.However,a considerable amount of research is still required to meet the demand of thermal management of electronic devices.Some perspectives on VACNT arrays and C/C composites used in thermal management systems are proposed here.

(1) Higher thermal conductive VACNT arrays and C/C composites are expected to be developed via innovative methods because the TC of VACNT arrays and C/C composites is still much lower than individual CNT and carbon fiber.

(2) Future studies of VACNT-based TIMs should be focused on how to effectively transfer the VACNT arrays from their growing substrates to the substrates used to fabricate devices and further decrease the thermal interface resistance between VACNT arrays and substrates like metal or polymer.

(3) Many studies highlight the enhancement of thermal conduction performance of the VACNT arrays and C/C composites,but neglect their mechanical properties,which are also very important for their applications in thermal management.Further efforts in this direction are expected to improve their mechanical properties.

(4) It is urgent to explore low-cost and largescale manufacturing technology for industrial applications,especially for C/C composites with high TC.Their commercial and aerospace applications largely depend on major manufacturing cost reductions.

Acknowledgements

This work was financially supported by Natural Science Foundation of China (No.U1960106,52072275,U1864207 and 52002296).