Enhanced thermal conductivity of polymer composites filled withthree-dimensional brushlike AlN nanowhiskers

Zhongqi Shi,1,2,a� Mohamed Radwan,2 Soshu Kirihara,2 Yoshinari Miyamoto,2 andZhihao Jin1

1School of Materials Science and Engineering, State Key Laboratory for Mechanical Behavior of Materials,Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China2Department of Nano/Micro Structure Control, Smart Processing Research Center, Joining and WeldingResearch Institute, Osaka University, Osaka 567-0047, Japan

�Received 7 September 2009; accepted 5 November 2009; published online 3 December 2009�

The thermal conductivity of polymer composites was improved by loading three-dimensional �3D�brushlike AlN nanowhiskers fillers synthesized by simple combustion method. Through filling 47vol % of the synthesized AlN nanowhiskers, the thermal conductivity of the composite wassignificantly increased to 4.2 W m−1 K−1, which was 2.3 times higher than that of the compositefilled with the same content of commercial AlN equiaxed particles. According to Agari modelanalysis and microstructure observation, the thermal conductivity enhancement can be ascribed tothe 3D brushlike AlN nanowhiskers promoted the formation of a more effect percolating network inthe matrix with lower thermal resistance. © 2009 American Institute of Physics.�doi:10.1063/1.3271028�

Polymer-matrix composites have been used as one ofthe most common packaging materials for encapsulating avariety of electronic components for dissipating heat.1,2

Due to the low thermal conductivity of the polymer matrix��0.2 W m−1 K−1�, different kinds of inorganic fillers havebeen added into the polymer to enhance the thermalproperties.3–5 However, the thermal conductivity of the com-posites is still lower than expected due to the high thermalinterface resistance �TIR� between the filler and thepolymer.6 In order to decrease the TIR of the composites,several methods have been developed such as forming con-ductive networks through appropriate packing of the filler inthe matrix5,7 or decreasing the TIR at the polymer/filler in-terface by minimizing the interfacial flaws.8 Recently, manyefforts have focused on dispersing one-dimensional5,9,10 ortwo-dimensional11,12 micro- or nanostructured fillers into thepolymer or combining them together13 to improve the heattransport as a result of their excellent crystal structure, highthermal conductivity, and high aspect ratio. However, no lit-erature has been reported on the tailor of three-dimensional�3D� nanostructured fillers into the polymer matrix, whichwould further promote the thermal conductivity by increas-ing the junctions among 3D fillers and achieving more effectpercolating network with significantly reduced TIR.

In this letter, 3D AlN nanowhiskers with brushlike struc-ture were filled into the polymer matrix to enhance its ther-mal conductivity. The 3D brushlike AlN fillers were fabri-cated by economical combustion synthesis method, asreported in our previous literature.14 The use of AlN as afiller candidate to enhance the thermal conductivity of thepolymer is attributed to its attractive properties, such as highthermal conductivity, high electrical resistivity, and goodchemical stability with polymers.15 To explore the promotingeffect of the 3D brushlike AlN fillers on thermal conductiv-ity, three types of AlN fillers with different brushlike filleraspect ratio were added into polymer matrix to fabricate a

series of composites and their thermal conductivities weremeasured. The results demonstrated that the 3D brushlikeAlN nanowhiskers fillers can effectively enhance the thermalconductivity of the polymer composite.

Three types of AlN fillers were used and their morpholo-gies are shown in Fig. 1. One was commercially availableAlN powders �equiaxed particle, �99.9%, �0.5 �m,Tokuyama K.K., Japan�, as shown in Fig. 1�a�. The other twotypes of AlN fillers with different aspect ratio of 3D brush-like particles �40 and 80 vol %, respectively� were fabricatedby combustion synthesis, as reported in our previous paper.14

Briefly, the starting mixture consisted of Al and AlN diluent�4/6 mol %� with 6 wt % of NH4Cl additive were combustedin 1 MPa N2 atmosphere. After the combustion reaction, uni-form quasi-aligned AlN nanowhiskers �QANs� grown in thereacting Al particles �like an anemone� have been obtainedwith 40 vol % in the product �same as the mol content ofstarting Al powders�. By repeating the combustion reactionfor two times with similar conditions just using the synthe-sized AlN powders obtained from the former product as di-luent, anemonelike AlN nanowhiskers particles with about80 vol % in the final product were achieved. In order to openthe anemonelike AlN particles and utilize the QAN, the twoproducts with different aspect ratio of anemonelike particleswere ground by ball milling for 20 min in alcohol and thendried. Then, 3D brushlike AlN particles with 40 and 80vol % in the synthesized AlN powders were obtained �Figs.1�b� and 1�c��. Figure 1�d� shows a transmission electronmicroscopy �TEM� image of a typical 3D brushlike AlNparticle. The particle is covered with a thin AlN crust��200 nm� and numerous QAN �diameter of 80–170 nm�epitaxially grown on the crust, which can be seen as agrowth substrate. For simplicity, according to their morpho-logical characteristics, the three types of AlN fillers arenamed as EP, BP40, and BP80. The properties of these fillersare listed in Table I.

The polymer/AlN composites were fabricated accordingto the following steps. First, the epoxy resin �O-Cresol No-a�Electronic mail: szqxjtu@gmail.com.

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volac�, curing agent �Phenol-Novolac� and catalyst �triph-enylphosphine� with a weight ratio of 60:40:0.5 were dis-solved in acetone and then mixed with each of the AlN fillers�0–47 vol %� by using a centrifugal mix-defoaming machine�AR-250, THINKY Co., Japan� to form a homogeneousslurry. Then the slurry was dried in a vacuum oven at 80 °Cand ground to pass through a 50-mesh sieve. Finally, thepowder mixture was cured at 180 °C for 2 h under a pres-sure of 10 MPa to obtain the composites.

The thermal conductivities � of the composites were cal-culated by equation �=� ·Cp ·�, where �, Cp, and � are thethermal diffusivity, specific heat, and density of the compos-ites. � of the composites was measured by laser flash method�TC-7000, Sinku-Riko, Japan�. � was calculated by the den-sity of AlN of 3.26 g cm−3 and the measured density of thepolymer matrix �1.31 g cm−3�. Cp was determined by thespecific heat of AlN of 0.74 J g−1 K−1 and the measured spe-cific heat of the polymer matrix �1.27 J g−1 K−1�.

Figure 2 shows the thermal conductivity as a function ofthe volume fraction of different AlN fillers. The thermal con-ductivities increase with the filler content. For the compos-ites filled with 47 vol % �70 wt %� of BP40 and BP80, thethermal conductivities are 3.3 and 4.2 W m−1 K−1, respec-tively. These are 1.8–2.3 times higher than that of the poly-mer composite filled with the same content of EP. In addi-tion, the measured thermal conductivity of the polymercomposite filled with the different content of EP is matchedwith the predicted value by the Bruggeman model,16 whichwas thought to be the Bruggeman model was based onspherical particles suspended in a diluent matrix, similar inthis composite. The Bruggeman model can be given by

1 − Vf =�� f − ����m/��1/3

� f − �m, �1�

where �, � f, �m and Vf represent thermal conductivities ofthe composite, filler, matrix, and volume fraction of the filler

in the composite, respectively. The value of 200 and0.25 W m−1 K−1 were used for � f and �m, respectively.

Remarkably, the thermal conductivities of the compositefilled with BP40 and BP80 are much higher than the predic-tion of Bruggeman equation. This demonstrates that thebrushlike AlN particles enhance the thermal conductivity ofthe polymer matrix significantly. The intrinsic reason can beexplored by Agari model,17 which considers the effect ofdispersion state by introducing factors C1 and C2:

log � = VfC2 log � f + �1 − Vf�log�C1�m� , �2�

where �, �m, � f, and Vf are defined as same as before, C1 isa factor relating to the effect of the filler on the secondarystructure of the polymer, and C2 is a factor relating to theease in forming conductive chains of the filler. The values ofC1 and C2 should be between 0 and 1, the closer C2 valuesare to 1, the more easily conductive chains are formed incomposite. So, if the dispersion system is different, the ther-mal conductivity of the composites may be different even ifthe components in the composites are the same. Throughdata fitting, C1 and C2 for the composites containing 47vol % of the three different types of AlN fillers are obtainedand shown in Table II. The AlN fillers with different aspectratio of 3D brushlike particles affect the C2 values more thanthe C1 values. This indicates that brushlike particles do notchange the secondary structure of the polymer significantly.

FIG. 1. �Color online� ��a�–�c�� SEM images for the three types of AlN fillers such as EP, BP40, and BP80. �d� TEM image of a typical 3D brushlike AlNnanowhiskers particle and corresponding SAED pattern �inset�.

TABLE I. The properties of three fillers.

Items

AlN fillers

EP BP40 BP80

Specific surface area /m2 g−1 2.60 2.58 3.11Oxygen content /mass% 0.85 1.14 1.26Brushlike particles in the AlN fillers /vol % 0 40 80 FIG. 2. �Color online� Thermal conductivity of polymer/AlN composites as

a function of filler content.

224104-2 Shi et al. Appl. Phys. Lett. 95, 224104 �2009�

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However, the C2 value increases with the aspect ratio ofbrushlike particles, which means the formation of thermalconductivity paths in the composites strongly enhanced by3D brushlike AlN particles. This result can also be approvedby their scanning electron microscopy �SEM� images �Fig.3�. In the case of the composites filled with EP, each particlecould be insulated easily, which prevents the formation ofthermal conductivity paths. However, with 3D brushlike par-ticles filled in and increase of their aspect ratio, the thermalconductivity paths can be formed and enhanced significantly.

In conclusion, the thermal conductivity of the polymercomposites was successfully enhanced by filling 3D brush-

like AlN nanowhiskers fillers obtained from the economicalcombustion synthesis route. Through loading 47 vol % ofAlN fillers contained 80 vol % of the 3D brushlike particles,the thermal conductivity of the composite was strongly in-creased to 4.2 W m−1 K−1, which was 2.3 times higher thanthat of the composite filled with same content of commercialAlN equiaxed particles. The results demonstrate that the 3Dbrushlike AlN fillers are effective for packaging materialswith high thermal conductivity.

Z.S. would like to thank China Scholarship Council forfinancial support during his stay at Osaka University for car-rying out this research work �CSC No. �2007� 3020�.

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TABLE II. C1 and C2 of Agari model for the composites containing 47vol % of different types of AlN fillers.

Filler type C1 C2

Thermal conductivity�W m−1 K−1�

EP 1.046 0.540 1.8BP40 1.012 0.797 3.3BP80 1.004 0.889 4.2

FIG. 3. �Color online� SEM images and corresponding schematic morpholo-gies of the polymer/AlN composites filled with 47 vol % of �a� EP, �b�BP40, and �c� BP80 fillers.

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