Aluminum nitride filled flexible silicone rubber compositesfor microwave substrate applications
L. K. Namitha • S. Ananthakumar •
M. T. Sebastian
Received: 1 October 2014 / Accepted: 1 November 2014 / Published online: 8 November 2014
� Springer Science+Business Media New York 2014
Abstract Mechanically flexible aluminum nitride–sili-
cone rubber composites (SAN) were prepared by hot
pressing technique for different filling fractions. The effects
of filler content on the dielectric, thermal and mechanical
properties as well as on moisture absorption were investi-
gated. The relative permittivity and dielectric loss of the
composite were found to vary linearly with filler content.
The variation in relative permittivity of SAN composites
with temperature was also investigated at a frequency of
1 MHz. Theoretical modelling of relative permittivity of the
composites was performed and the results were correlated
with the experimental data. Among the theoretical models
effective medium theory is in good agreement with exper-
imental values of relative permittivity. The coefficient of
thermal expansion and specific heat capacity of the com-
posite were found to decrease with filler content and ther-
mal conductivity, thermal diffusivity and the moisture
absorption increased with filler loading. The SAN com-
posite is found to be a good candidate for a thermally
conductive flexible microwave substrate application.
1 Introduction
Flexible electronics are enabling a wide variety of
emerging technologies such as flexible displays, flexible
antenna, electronic paper, low-cost radio frequency iden-
tification tags and electronic textiles. Flexible electronics is
expected to make incredible inroads into consumer elec-
tronics markets in the coming years. The iSuppli Flexible
Display Report predicts the flexible display market to grow
from $1.1 billion in 2015 to $3.89 billion by 2020 [1].
Microwave substrate materials require not only the flexi-
bility but also low relative permittivity, low dielectric loss,
high thermal conductivity and low coefficient of thermal
expansion [2, 3]. Several ceramics have good desired
dielectric properties but the high processing temperature
and brittle nature greatly limits their practical use. Elasto-
mers have a significant role in electronic packaging
applications due to their ease of processing, low cost, low
relative permittivity, adhesive properties etc., but low
thermal conductivity and high coefficient of thermal
expansion limit their use in substrate application. Polymer-
ceramic composites are finding increased application in
electronic and microwave devices, since they combine the
dielectric and electrical properties of the ceramic fillers and
the low-temperature processability and mechanical prop-
erties of the polymer matrix. In the present scenario the
elastomer–ceramic composites are getting wide attention
due to its flexibility and hence they can be successfully
used as substrates for flexible microwave circuit. The
objective of the present study is to develop a flexible
microwave substrate material for electronic applications.
Highly elastic silicone rubber (SR) is attracting tre-
mendous attention of researchers worldwide because of
their high flexibility, good stability in a wide range of
temperatures, weather and chemical resistant, low tem-
perature toughness, electrically insulating properties etc.
The most interesting property of silicone rubber for sub-
strate application is its low relative permittivity
(er = 3–3.5 at 1 MHz), low dielectric loss (tan d = 10-3 at
L. K. Namitha � S. Ananthakumar
Materials Science and Technology Division, National Institute
for Interdisciplinary Science and Technology,
Trivandrum 695019, India
M. T. Sebastian (&)
Microelectronics and Materials Physics Lab, University of Oulu,
P. O. Box 4500, 90014 Oulu, Finland
e-mail: [email protected]; [email protected]
123
J Mater Sci: Mater Electron (2015) 26:891–897
DOI 10.1007/s10854-014-2479-9
1 MHz) and high stretchability [4, 5]. Some applications
like elastomeric thermal pads, electronic packaging etc.,
require high thermal conductivity and electrical insulation.
Many researches have improved the thermal conductivity
of silicone rubber by adding thermally conductive inor-
ganic fillers into it. Aluminum nitride is a ceramic filler
with high intrinsic thermal conductivity (370 W/mK), high
electrical resistivity. It also has excellent dielectric prop-
erties, low CTE and low cost [6]. These unique properties
led to considerable research on AlN as reinforcing agent in
recent years. Research on various polymer/AlN composite
have been performed and reported in the literature [6–9].
Zhou et al. [6] prepared AlN–PMMA composites and
reported the microwave dielectric properties. Although
considerable amount of work was reported on silicone
rubber–ceramic composites, very few of them deal with
their microwave dielectric properties. Silicone rubber filled
with thermally conductive, electrically insulating Al2O3,
AlN and ZnO for elastomeric thermal pads and their
thermal properties were studied by Sim et al. [10]. Chiu
et al. [11, 12] reported recently the surface modification of
AlN filler and its effect on the mechanical, thermal prop-
erties of the silicone rubber composites. Gunasekaran et al.
[13] reported the dielectric properties of different types of
rubber at microwave frequency range. The present author’s
group has recently reported the effect of particle size of
alumina filler on the microwave dielectric properties of
silicone rubber and butyl rubber composites [14, 15].
Recently Namitha and Sebastian [5] reported the micro-
wave dielectric characterization of silicone rubber–BZT
composites for flexible substrate application.
Among the available literature microwave dielectric
characterization of silicone rubber-aluminum nitride com-
posites are not yet reported. In the present paper we report
the development of flexible as well and stretchable, ther-
mally conductive, low dielectric constant and low dielec-
tric loss SAN composites for microwave substrate
application.
2 Experimental
In the present study we have used methyl end blocked
silicone rubber provided by Jyothi rubbers, Thrissur, India
is the matrix. Dicumyl peroxide (DCP) was used as curing
agent. Aluminum nitride (\50 lm) were obtained from
Sigma Aldrich, USA. Silicone rubber–AlN (SAN) com-
posites were prepared according to the formulations given
in Table 1 using a kneading machine (sigma mixing) at
room temperature. The composites thus obtained were then
hot pressed at 200 �C for 90 min under a pressure of
2 MPa with appropriate dies. The compositions of AlN
filled silicone rubber are represented as SAN0, SAN1,
SAN2, SAN3 and SAN4 as given in Table 1. The mixing
was difficult at higher filler loadings due to agglomeration.
The density of the composite was measured using Archi-
medes method. The microstructures of the composites were
examined using a scanning electron microscope (SEM)
(Jeol Model, JSM 5600LV). The tensile measurements
were carried out in a Universal Testing Machine (Houns-
field Model, H5K-S UTM) with a rate of grip separation of
500 mm/min. The moisture absorption characteristic of the
composite was measured by following the ASTM D 570-98
procedure. The volume % of water absorption was calcu-
lated using the relation,
Volume % of moisture absorption ¼ ðWf �WiÞ=qwWf�Wi
qwþ Wi
qc
� 100
ð1Þ
where Wf and Wi are the final and initial weights of the
sample and qw and qc are the densities of water and
composite respectively. The radio frequency dielectric
properties were measured by LCR meter (Hioki 3532-50).
The microwave dielectric properties of the composites
were measured by a Split Post Dielectric Resonator
(SPDR) technique at 5 GHz and 15 GHz using an ENA
series network analyzer (Agilent E5071C). The variation in
relative permittivity of SAN composites with temperature
was studied in the temperature range between 25 and 80 �C
at 1 MHz. Coefficient of thermal expansion (CTE) was
measured by a dilatometer (Netzsch Model, DIL 402 PC)
in the temperature range between 25 and 150 �C, at a
heating rate of 2 �C/min. CTE was determined from the
slope of thermal expansion versus temperature plots. The
thermal conductivity (Tc) studies were carried out in a laser
thermal property analyzer (Flash Line 2000, Anter Cor-
poration, Pittsburgh, USA) using alumina as the reference
sample by the relation
Tc ¼ k� Cp � q ð2Þ
where k is the thermal diffusivity, Cp is the specific heat
capacity at room temperature and q is the density of the
sample. Thermal conductivity, specific heat capacity and
thermal diffusivity studies were done at room temperature.
Table 1 Formulations of silicone rubber–AlN composites
Ingredients
in phraSAN0 SAN1 SAN2 SAN3 SAN4
Silicone
rubber
100 100 100 100 100
Dicumyl
peroxide
2 2 2 2 2
AlN 0 [0]b 10 [0.03] 50 [0.15] 100 [0.26] 200 [0.41]
a Parts per hundred of rubberb Volume fractions of filler given in brackets
892 J Mater Sci: Mater Electron (2015) 26:891–897
123
3 Results and discussion
Figure 1 shows the variations of experimental density of
SAN composites with volume fraction of filler. The density
of composites are measured by Archimedes method. It is
observed from the figure that, the density of the composite
is increased with filler loading. This is due to the higher
density of the filler.
Figure 2a shows the microstructure of AlN particles.
Figure 2b–d shows the fractogram of SAN0, SAN1 and
SAN4 composites respectively. It is clear from the figure
that at lower filler loading the ceramic particles are uni-
formly dispersed within the silicon rubber matrix. At
higher volume fractions (0.41 Vf) the filler particles tend to
agglomerate and form non-homogeneous dispersions
which in turn increase the porosity.
The stress–strain curve of SAN composites are shown
in Fig. 3. It can be seen from the figure that stress for
elongation increases with filler loading. The mechanical
properties of composites with thermally conductive filler
rely on the efficiency of stress transfer at the filler–
matrix interface and on their adhesion strength [16]. The
incorporation of filler particles enhanced the stiffness but
reduces the flexibility due to poorer mobility of molec-
ular chains [17]. Zhou et al. [18] reported that higher
filler loading (40 vol%) in silicone rubber, leads to dif-
ficulty in processing and the mechanical property is
degraded. The flexural strength decreased with increasing
amount of filler loadings. This is because at higher filler
loadings the filler–filler interaction is much more domi-
nant than the filler–matrix interaction. Hence, the filler–
filler interaction will result on filler agglomeration and it
will subsequently reduce the flexural strength of the
composite system.
Fig. 1 Variation of experimental density of SAN composites with
filler loading
Fig. 2 SEM images of a AlN
filler, b–d fractogram of SAN0,
SAN1 and SAN4 composites
J Mater Sci: Mater Electron (2015) 26:891–897 893
123
Moisture absorption of the composites is an important
parameter since it adversely affects the dielectric proper-
ties of substrate for practical applications [19]. It was
reported by Laverghetta [20] that the upper limit of
moisture absorption for electronic packaging applications
is 0.1 %. The variation in moisture absorption of SAN
composites as a function of filler loading is shown in
Fig. 4. It is clear from the figure that the moisture
absorption of SAN composites shows an increasing trend
with filler loading. The slight increase of moisture
absorption may be due to the increase in porosity with
filler loading. However the highest value obtained for the
moisture absorption for maximum filler loading was
around 0.1 %, hence, all these composites are suitable for
the practical applications.
Polymer–ceramic composites with low relative permit-
tivity are needed for microwave substrate applications
since low relative permittivity will increase the signal
transmission speed. Figure 5 shows the relative permittiv-
ity and dielectric loss of SAN composite at 1 MHz. From
the figure it is clear that both the relative permittivity (er)
and dielectric loss (tan d) increases with filler addition. At
lower filler loading the dielectric properties of the com-
posite depend mainly on the properties of the matrix (SR
have er of 3–3.5 at 1 MHz). As the filler loading increases
the er of the composite gradually increases due to the rel-
atively high er of the filler (AlN have er of 9 at 1 MHz) and
also the dipole–dipole interaction of the ceramic filler [6].
At 1 MHz, the relative permittivity of SAN composites
varies from 3.27 to 4.86 with increase in filler loading from
0 to 0.41 Vf. Dielectric loss also increases as the filler
content increases. The dielectric loss of SAN composite
increases from 2.8 9 10-3 to 9.8 9 10-3 as the filler
loading increases from 0 to 0.41 Vf.
Figure 6a, b shows the variation of relative permittivity
and dielectric loss of SAN composites with filler loading
measured at 5 and 15 GHz. It is evident from the figure that
the relative permittivity and dielectric loss increase with
increase in filler content. The increased relative permit-
tivity of the composite at higher filer loading is due to the
high er of the filler and the increased connectivity among
the filler particles and also between the polymer and filler
[21, 22]. The relative permittivity of the samples at 5 GHz
and 15 GHz do not show appreciable differences since
there is hardly any change in the polarization mechanism
[6]. The dielectric loss of a polymer–ceramic composite
depends on the factors such as porosity, moisture content
and interface between two components in the composite.
The small variation of dielectric loss may be due to the
dipole relaxation of water at microwave frequency [23]. As
Fig. 3 The stress–strain curve of SAN composites
Fig. 4 Variation of moisture absorption of SAN composites with
filler loading
Fig. 5 Variation of relative permittivity and loss tangent of SAN
composites at 1 MHz
894 J Mater Sci: Mater Electron (2015) 26:891–897
123
filler loading increases from 0 to 0.41 Vf the er varies from
3.0 to 4.79 at 5 GHz and 2.8–4.07 at 15 GHz. Dielectric
loss increases from 1.47 9 10-2 to 1.72 9 10-2 at 5 GHz
and 2.54 9 10-2–2.92 9 10-2 at 15 GHz respectively.
The following theoretical models are used to calculate
the effective relative permittivity of SAN composite at
5 GHz frequency
Maxwell Garnet [24]
eeff � em
eeff þ 2em
¼ Vf
ef � em
ef þ 2em
ð3Þ
Jayasundere–Smith equation [25]
eeff ¼emð1� vf Þ þ ef vf
3em
efþ2em
h i1þ 3vf ðef�emÞ
eiþ2em
� �h i
1� vf þ vf3em
efþ2em
h i1þ 3vf ðef�emÞ
efþ2em
� �h i ð4Þ
Effective medium theory (EMT) [26]
eeff ¼ em 1þ vf ðef � emÞem þ nð1� vf Þðef � emÞ
� �ð5Þ
where, eeff, em and ef are relative permittivity’s of com-
posite, matrix and filler respectively and Vf is the volume
fraction of the filler. Figure 7 shows the comparison of
experimental and theoretical values of relative permittivity
obtained from various models at 5 GHz for SAN com-
posites. Maxwell Garnet relations predict lower er com-
pared to the experimental data. This relation is suitable for
very low volume fraction of ceramic particles. The Max-
well Garnet relations consider only the dipolar excitation,
but the correlations between the excitations are not taken
into consideration. The multi polar contributions to the
local field are neglected here. In Jayasundere–Smith model
all filler particles are assumed to be spherical. Jayasundere
Smith model show good agreement with experimental
results at lower filler loading but at higher volume fraction
this model shows deviation, which may be due to the
imperfect dispersion of filler and entrapped air in the
composites. EMT model fits well with the experimental
results. In EMT model composites are treated as an
effective medium whose relative permittivity is obtained
from the relative permittivity of the constituents. The
complex effective permittivity will depend on the homo-
geneity of distribution of the filler, shape and size of filler;
also interface between filler and matrix. EMT model
includes morphology factor ‘n’ which is obtained from
empirical calculations (n = 0.135). The correction factor
compensate for the shape of the filler used in the polymer-
ceramic composite.
The temperature dependence of relative permittivity of
SAN composites at 1 MHz is shown in Fig. 8. The
Fig. 6 a Variation of relative permittivity of SAN composites at 5
and 15 GHz and b variation of dielectric loss of SAN composites at 5
and 15 GHz
Fig. 7 Comparison of experimental and theoretical permittivity of
SAN composites at 5 GHz
J Mater Sci: Mater Electron (2015) 26:891–897 895
123
temperature dependence of permittivity in polymers mainly
depends two factors such as the glass transition tempera-
ture, Tg, and the frequency dependence of material [27]. It
is clear from the figures that the temperature variation of
relative permittivity is relatively small for the composites.
SAN composites show a slight decrease in relative per-
mittivity with increase in temperature for all the compo-
sitions. The large difference in CTE of the filler and matrix
is responsible for the disturbance of aggregation of polar
components and thus the decrease in relative permittivity
with temperature [28, 29]. The silicone rubber aluminum
nitride composites show reasonably good temperature sta-
bility of relative permittivity.
The variation in CTE is an important factor for elec-
tronic packaging applications. The variation in CTE of AlN
filled silicone rubber with volume fraction of filler is shown
in Fig. 9. The CTE decreases with increase in volume
fraction of filler. CTE value of AlN and pure silicone
rubber are 4.5 and 266 ppm/�C respectively. The CTE of
SAN4 composite is 137 ppm/�C compared to the unfilled
matrix (266 ppm/�C). A mechanical interaction between
filler and matrix exists in the composites, which will con-
strain the expansion of the matrix. The reduction in CTE of
composite is due to restriction in mobility of silicone
molecule because of the entanglement if AlN particles by
the rubber matrix [22].
Figure 10 shows the variation of thermal conductivity of
SAN composites with the volume fraction of the filler. The
effective thermal conductivity of the composite is strongly
affected by its composition, structure, intrinsic thermal
conductivities, filler particle size, shape and interfacial
thermal resistance etc. [3]. Thermal conductivity of AlN
(140–180 W/mk) is higher than that of silicone rubber
(0.2 W/mk) hence the addition of AlN into silicone rubber
increases its thermal conductivity. It is evident from the
figure that thermal conductivity of the composites is
increased with increase in filler content. At lower filler
loading the heat conductive filler particles are surrounded
by the matrix and cannot touch each other and hence
thermal conductivity increases very slowly. At higher filler
loading the filler particles begin to touch each other and
form more compact structure. When loading of AlN is
[15 vol% thermal conductivity increases remarkably. At
40 % filler loading, thermal conductivity increases almost
6 times than that of pure silicone rubber.
The variation of specific heat capacity and thermal dif-
fusivity of SAN composites with volume fraction of filler is
shown in Fig. 11. As the ceramic content increases the
specific heat capacity of SAN composites decreases which
Fig. 8 Variation of relative permittivity with temperature for SAN
composites at 1 MHzFig. 9 Variation of CTE of SAN composites with ceramic content
Fig. 10 Variation of thermal conductivity of SAN composites with
the filler content
896 J Mater Sci: Mater Electron (2015) 26:891–897
123
is due to the low specific heat capacity of the ceramic filler
(740 J/kg/K), where as the thermal diffusivity increased
with filler volume fraction. Recently Namitha and Sebas-
tian reported the similar findings in silicone rubber-Ba
(Zn1/3 Ta2/3) O3 composite system [5].
4 Conclusions
Silicone rubbe–AlN flexible composites with varying
volume fractions of filler were prepared in a kneading
machine followed by hot pressing. The dielectric proper-
ties at different frequencies, mechanical flexibility, coef-
ficient of thermal expansion, temperature dependence of
relative permittivity, thermal conductivity and moisture
absorption of AlN filled silicone rubber matrix were
investigated. Both the relative permittivity and dielectric
loss increases with increase in filler volume fraction.
Relative permittivity (er) of 4.79 and tan d of 1.72 9 10-2
is obtained for 0.41 volume fraction of SAN composites at
5 GHz and er of 4.07 and tan d of 2.94 9 10-2 at
15 GHz. Experimental values of relative permittivity were
compared with theoretical models. Among the different
theoretical models Jayasundere Smith model and EMT
model are in good agreement with experimental values of
er. The variation of relative permittivity with temperature
showed that the SAN composites have good temperature
stability of relative permittivity. Thermal conductivity,
thermal diffusivity and moisture absorption of SAN
composite increases with filler loading while coefficient of
thermal expansion and specific heat capacity decreases
with filler loading. The SAN composites are possible
candidates for microwave substrate applications in flexible
electronics.
Acknowledgments Mrs. Namitha. L.K is grateful to Council of
Scientific and Industrial Research (CSIR), New Delhi for the award of
senior research fellowship. Dr. Prabhakar Rao for recording XRD
pattern, Mr. Chandran for SEM, Miss. Gayathri for thermal expansion
measurements and Mr. Brahmakumar for tensile measurements.
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Fig. 11 Variation of specific heat capacity and thermal diffusivity of
SAN composite with filler loading
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