Aluminum nitride filled flexible silicone rubber composites for microwave substrate applications

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


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


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


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


123

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Aluminum nitride filled flexible silicone rubber composites for microwave substrate applications

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