1. Enhanced thermal conductivity of polymer composites filled with
three-dimensional brushlike AlN nanowhiskers
Zhongqi Shi,1,2,a͒
Mohamed Radwan,2
Soshu Kirihara,2
Yoshinari Miyamoto,2
and
Zhihao Jin1
1
School of Materials Science and Engineering, State Key Laboratory for Mechanical Behavior of Materials,
Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China
2
Department of Nano/Micro Structure Control, Smart Processing Research Center, Joining and Welding
Research Institute, Osaka University, Osaka 567-0047, Japan
͑Received 7 September 2009; accepted 5 November 2009; published online 3 December 2009͒
The thermal conductivity of polymer composites was improved by loading three-dimensional ͑3D͒
brushlike AlN nanowhiskers fillers synthesized by simple combustion method. Through filling 47
vol % of the synthesized AlN nanowhiskers, the thermal conductivity of the composite was
significantly increased to 4.2 W m−1
K−1
, which was 2.3 times higher than that of the composite
filled with the same content of commercial AlN equiaxed particles. According to Agari model
analysis and microstructure observation, the thermal conductivity enhancement can be ascribed to
the 3D brushlike AlN nanowhiskers promoted the formation of a more effect percolating network in
the matrix with lower thermal resistance. © 2009 American Institute of Physics.
͓doi:10.1063/1.3271028͔
Polymer-matrix composites have been used as one of
the most common packaging materials for encapsulating a
variety of electronic components for dissipating heat.1,2
Due to the low thermal conductivity of the polymer matrix
͑ϳ0.2 W m−1
K−1
͒, different kinds of inorganic fillers have
been added into the polymer to enhance the thermal
properties.3–5
However, the thermal conductivity of the com-
posites is still lower than expected due to the high thermal
interface resistance ͑TIR͒ between the filler and the
polymer.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 in
the matrix5,7
or decreasing the TIR at the polymer/filler in-
terface by minimizing the interfacial flaws.8
Recently, many
efforts have focused on dispersing one-dimensional5,9,10
or
two-dimensional11,12
micro- or nanostructured fillers into the
polymer or combining them together13
to improve the heat
transport as a result of their excellent crystal structure, high
thermal conductivity, and high aspect ratio. However, no lit-
erature has been reported on the tailor of three-dimensional
͑3D͒ nanostructured fillers into the polymer matrix, which
would further promote the thermal conductivity by increas-
ing the junctions among 3D fillers and achieving more effect
percolating 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, as
reported in our previous literature.14
The use of AlN as a
filler candidate to enhance the thermal conductivity of the
polymer is attributed to its attractive properties, such as high
thermal conductivity, high electrical resistivity, and good
chemical stability with polymers.15
To explore the promoting
effect of the 3D brushlike AlN fillers on thermal conductiv-
ity, three types of AlN fillers with different brushlike filler
aspect ratio were added into polymer matrix to fabricate a
series of composites and their thermal conductivities were
measured. The results demonstrated that the 3D brushlike
AlN nanowhiskers fillers can effectively enhance the thermal
conductivity of the polymer composite.
Three types of AlN fillers were used and their morpholo-
gies are shown in Fig. 1. One was commercially available
AlN powders ͑equiaxed particle, Ͼ99.9%, ϳ0.5 ␮m,
Tokuyama K.K., Japan͒, as shown in Fig. 1͑a͒. The other two
types of AlN fillers with different aspect ratio of 3D brush-
like particles ͑40 and 80 vol %, respectively͒ were fabricated
by 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 combusted
in 1 MPa N2 atmosphere. After the combustion reaction, uni-
form quasi-aligned AlN nanowhiskers ͑QANs͒ grown in the
reacting Al particles ͑like an anemone͒ have been obtained
with 40 vol % in the product ͑same as the mol content of
starting Al powders͒. By repeating the combustion reaction
for 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 about
80 vol % in the final product were achieved. In order to open
the anemonelike AlN particles and utilize the QAN, the two
products with different aspect ratio of anemonelike particles
were ground by ball milling for 20 min in alcohol and then
dried. Then, 3D brushlike AlN particles with 40 and 80
vol % in the synthesized AlN powders were obtained ͓Figs.
1͑b͒ and 1͑c͔͒. Figure 1͑d͒ shows a transmission electron
microscopy ͑TEM͒ image of a typical 3D brushlike AlN
particle. The particle is covered with a thin AlN crust
͑ϳ200 nm͒ and numerous QAN ͑diameter of 80–170 nm͒
epitaxially grown on the crust, which can be seen as a
growth substrate. For simplicity, according to their morpho-
logical characteristics, the three types of AlN fillers are
named as EP, BP40, and BP80. The properties of these fillers
are listed in Table I.
The polymer/AlN composites were fabricated according
to the following steps. First, the epoxy resin ͑O-Cresol No-a͒
Electronic mail: szqxjtu@gmail.com.
APPLIED PHYSICS LETTERS 95, 224104 ͑2009͒
0003-6951/2009/95͑22͒/224104/3/$25.00 © 2009 American Institute of Physics95, 224104-1
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2. volac͒, curing agent ͑Phenol-Novolac͒ and catalyst ͑triph-
enylphosphine͒ 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., Japan͒ to form a homogeneous
slurry. Then the slurry was dried in a vacuum oven at 80 °C
and ground to pass through a 50-mesh sieve. Finally, the
powder 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 the
thermal diffusivity, specific heat, and density of the compos-
ites. ␣ of the composites was measured by laser flash method
͑TC-7000, Sinku-Riko, Japan͒. ␳ was calculated by the den-
sity of AlN of 3.26 g cm−3
and the measured density of the
polymer matrix ͑1.31 g cm−3
͒. Cp was determined by the
specific 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 of
the 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, the
thermal 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 polymer
composite filled with the different content of EP is matched
with the predicted value by the Bruggeman model,16
which
was thought to be the Bruggeman model was based on
spherical particles suspended in a diluent matrix, similar in
this composite. The Bruggeman model can be given by
1 − Vf =
͑␭f − ␭͒͑␭m/␭͒1/3
␭f − ␭m
, ͑1͒
where ␭, ␭f, ␭m and Vf represent thermal conductivities of
the composite, filler, matrix, and volume fraction of the filler
in the composite, respectively. The value of 200 and
0.25 W m−1
K−1
were used for ␭f and ␭m, respectively.
Remarkably, the thermal conductivities of the composite
filled with BP40 and BP80 are much higher than the predic-
tion of Bruggeman equation. This demonstrates that the
brushlike AlN particles enhance the thermal conductivity of
the polymer matrix significantly. The intrinsic reason can be
explored by Agari model,17
which considers the effect of
dispersion state by introducing factors C1 and C2:
log ␭ = VfC2 log ␭f + ͑1 − Vf͒log͑C1␭m͒, ͑2͒
where ␭, ␭m, ␭f, and Vf are defined as same as before, C1 is
a factor relating to the effect of the filler on the secondary
structure of the polymer, and C2 is a factor relating to the
ease in forming conductive chains of the filler. The values of
C1 and C2 should be between 0 and 1, the closer C2 values
are to 1, the more easily conductive chains are formed in
composite. So, if the dispersion system is different, the ther-
mal conductivity of the composites may be different even if
the components in the composites are the same. Through
data fitting, C1 and C2 for the composites containing 47
vol % of the three different types of AlN fillers are obtained
and shown in Table II. The AlN fillers with different aspect
ratio of 3D brushlike particles affect the C2 values more than
the C1 values. This indicates that brushlike particles do not
change the secondary structure of the polymer significantly.
FIG. 1. ͑Color online͒ ͓͑a͒–͑c͔͒ SEM images for the three types of AlN fillers such as EP, BP40, and BP80. ͑d͒ TEM image of a typical 3D brushlike AlN
nanowhiskers particle and corresponding SAED pattern ͑inset͒.
TABLE I. The properties of three fillers.
Items
AlN fillers
EP BP40 BP80
Specific surface area/m2
g−1
2.60 2.58 3.11
Oxygen content /mass% 0.85 1.14 1.26
Brushlike particles in the AlN fillers /vol % 0 40 80 FIG. 2. ͑Color online͒ Thermal conductivity of polymer/AlN composites as
a function of filler content.
224104-2 Shi et al. Appl. Phys. Lett. 95, 224104 ͑2009͒
Downloaded 03 Dec 2009 to 117.32.153.178. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

3. However, the C2 value increases with the aspect ratio of
brushlike particles, which means the formation of thermal
conductivity paths in the composites strongly enhanced by
3D brushlike AlN particles. This result can also be approved
by their scanning electron microscopy ͑SEM͒ images ͑Fig.
3͒. In the case of the composites filled with EP, each particle
could be insulated easily, which prevents the formation of
thermal conductivity paths. However, with 3D brushlike par-
ticles filled in and increase of their aspect ratio, the thermal
conductivity paths can be formed and enhanced significantly.
In conclusion, the thermal conductivity of the polymer
composites was successfully enhanced by filling 3D brush-
like AlN nanowhiskers fillers obtained from the economical
combustion synthesis route. Through loading 47 vol % of
AlN 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 than
that of the composite filled with same content of commercial
AlN equiaxed particles. The results demonstrate that the 3D
brushlike AlN fillers are effective for packaging materials
with high thermal conductivity.
Z.S. would like to thank China Scholarship Council for
financial support during his stay at Osaka University for car-
rying out this research work ͑CSC No. ͓2007͔ 3020͒.
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TABLE II. C1 and C2 of Agari model for the composites containing 47
vol % of different types of AlN fillers.
Filler type C1 C2
Thermal conductivity
͑W m−1
K−1
͒
EP 1.046 0.540 1.8
BP40 1.012 0.797 3.3
BP80 1.004 0.889 4.2
FIG. 3. ͑Color online͒ SEM images and corresponding schematic morpholo-
gies of the polymer/AlN composites filled with 47 vol % of ͑a͒ EP, ͑b͒
BP40, and ͑c͒ BP80 fillers.
224104-3 Shi et al. Appl. Phys. Lett. 95, 224104 ͑2009͒
Downloaded 03 Dec 2009 to 117.32.153.178. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp