CoY hexaferrite-PEEK composites for integratedand miniaturized RF components
K. P. Murali1 • P. Markondeya Raj2 • Himani Sharma2 • Rao Tummala2
Received: 19 December 2015 / Accepted: 7 March 2016 / Published online: 21 March 2016
� Springer Science+Business Media New York 2016
Abstract CoY hexaferrite-filled PEEK (poly ethyl ether
ketone) compositeswere synthesized to characterize the effect
of hexaferrites on their dielectric, magnetic and thermal
properties for wireless sensing and communication applica-
tions. Fillers were synthesized from solid-state reaction route
and blendedwith the thermoplastic polymermatrix.XRDwas
used to study the phase purity of the synthesised fillers.
Impedance measurement showed permeability of*2 with a
loss tangent of 0.04 and frequency stability of permeability up
to 800 MHz for higher filler loading. Dielectric property
measurements using parallel-plate capacitance showed that
the composites can attain amaximumdielectric constant up to
8 and a loss tangent of 0.005. Thermo mechanical Analyser
was used to characterize the coefficient of linear thermal
expansion (CTE) of the composites. The measured CTE clo-
selymatches that of organic substrates and copper, resulting in
minimal CTE mismatch issues during processing and opera-
tion.VSMstudies revealed softmagnetic characteristics of the
composites. The results suggest the potential of this polymer
composite substrate for integrated RF modules with minia-
turized embedded passive components.
1 Introduction
The demand for multimode multiband (MMMB) commu-
nication systems with thinner formfactors, smaller size and
improved performance is driving new integrated RF front-
end module architectures with higher component densities.
To achieve such densities, there is an increasing trend to
embed components in the package core or build-up instead
of the traditional 2D assembly onto the package surface
which invariably increases the module size and degrades
the system performance. These components are either
manufactured separately and embedded in the module
substrate, or embedded as thinfilms. The size of RF com-
ponents (ex. filters, diplexers, matching networks and
antennas) scale inversely with the permittivity and per-
meability of the dielectrics. However, materials with high
permittivity and permeability invariably show frequency
instabilities and losses in the microwave frequencies. There
is a continuous need to innovate dielectric materials with
higher permittivities, permeabilities and low losses with
frequency stability in the GHz range [1, 2].
Emerging Internet of Things (IoT) applications require
RF dielectrics that are optimized not only for size and
performance, but are also manufacturable as flexible sub-
strates for easy deployment in ubiquitous sensing and
communication applications. These composites should also
stand high temperatures and harsh environment for these
applications. Traditional epoxy composites are not expec-
ted to meet these requirements because of their low thermal
stability, high losses, moisture absorption. Thermoplastics
such as PEEK are ideally suited for these applications
because of their low loss and high thermal stability [3].
Such composites can serve as conformal antennas for
optimized 3D shapes.
Antennas are one of the largest components in any
communication system. Hence, significant attention has
been paid for their miniaturization in recent years. Unlike
other passives that are generally based on inductor-capac-
itor networks, antenna integration and miniaturization
places additional material requirements for not only
& P. Markondeya Raj
[email protected]; [email protected]
1 Center for Materials for Electronics Technology,
Thrissur 680 581, India
2 Packaging Research Center, Georgia Institute of Technology,
Atlanta, GA 30332-0560, USA
123
J Mater Sci: Mater Electron (2016) 27:7010–7017
DOI 10.1007/s10854-016-4657-4
reducing the physical size and enhancing efficiency but
also on achieving wider operating bandwidth with easy
impedance matching. Important properties of antenna such
as bandwidth and wave impedance matching are propor-
tional to ratio of relative permeability (lr) and dielectric
constant (er) whereas miniaturization is proportional to the
square-root of the product of er and lr. Common techniques
such as using high-permittivity substrates (high er with
lr = 1) for antenna miniaturisation leads to narrow band-
width, high field confinement resulting in low radiation
efficiency and difficulty in impedance matching between
antenna and surrounding free space [2, 4, 5].
Magneto-dielectric (MD) materials exhibiting both erand lr can be effectively employed for enhancing the
performance of antenna and better miniaturisation by cir-
cumventing the aforementioned disadvantages. With MD
materials exhibiting higher values of er and lr, a smaller
antenna having wider bandwidth can be designed and also
a better impedance matching (Z/Z0 * 1) can be obtained
with close values of er and lr. These materials need to have
low loss to enhance high power efficiency. Further, due to
overcrowding of low frequency bands, high frequency
stability (in GHz) of electric and magnetic parameters is
required to meet the upcoming demand especially in
wireless communication applications [2, 5]. Hence the
ultimate goal is to develop low loss and preferably
matching er and lr materials exhibiting frequency-
stable magnetic and electric properties.
Polymer composite materials are preferred over other
commonly used ceramic RF dielectrics because of their
easy processability in larger size and thickness. Their
properties can be tailored by varying filler loading in the
polymer matrix. Since most of the low-loss polymers are
nonmagnetic, the magnetic parameters of the composite are
decided by the filler dispersed in the polymer matrix.
Compared to metal particulate-loaded composites, mag-
netic ceramic filler loaded composites exhibit low loss and
stable electric and magnetic properties at higher frequency.
Processability and properties such as quality factor of these
fillers can be further enhanced by dispersing their partic-
ulates in low loss polymer matrices [6–12]. Out of the
ceramic magnetic materials, hexaferrites show high reso-
nance frequency, good resistivity and hence can be used for
high-frequency applications.
Hexagonal ferrites are not as widely investigated as
spinel magnetic material as fillers for polymer composites
[12–14]. Magnetic properties of Y-type hexagonal ferrites
has been studied by substituting Zn and Cu showing fre-
quency stability of permeability nearly up to 400 MHz
[14]. Although ceramic composites or substituted ana-
logues of CoY type hexaferrites and their composites were
reported, a systematic study of the effect of the ferrite
loading on the properties of thermoplastic polymer
composites for miniaturised RF passives and antenna
applications has not been reported. The scarcity of research
reports on hexaferrites-organic medium composites has
been mentioned in a very recent review on low-loss mag-
netodielectric materials [1].
In this work, phase-pure Y type planar hexaferrites
(Ba2Co2Fe12O22) were prepared through solid state reac-
tion route and finely dispersed in Polyetheretherketone
(PEEK matrix) using a high speed mixer. PEEK is selected
as the matrix because of its good operating temperature,
excellent mechanical properties, outstanding chemical
resistance and fairly good electrical properties. The effect
of CoY hexaferrite filler loading in PEEK matrix on its
electrical, magnetic and thermal properties was systemati-
cally studied to assess their suitability as substrates and
build-up dielectrics for miniaturised antenna and RF
component applications.
2 Experimental
Materials used: BaCO3, 99.9 % pure, M/s Sigma Aldrich;
Co3O4 & Fe2O3 99.9 % M/s Merck; (PEEK), M/s Vesta-
keep, Germany; Dipropylene Glycol (DPG), Sigma
Aldrich.
Preparation of phase-pure Y type hexaferrites (Ba2Co2Fe12O22): BaCo3, Co3O4 and Fe2O3 were stoichiometri-
cally mixed in an agate mortar using deionised water as
media. The mix was heat-treated initially at 1150 �C for
5 h, reground and then crystallized at different tempera-
tures to find out the optimum temperature to form phase-
pure CoY type hexaferrite.
Preparation of PEEK: Ba2Co2Fe12O22 composites:
Phase-pure ceramic powders prepared through solid-state
route was dispersed in PEEK using a high-speed blender
for half an hour with the help of a lubricant (Dipropylene
Glycol). Filler concentration in PEEK matrix was varied
from 60 to 85 wt% in 5 wt% intervals to find out the
optimum filler loading to obtain the best properties to use
as base substrate for RF circuits. Samples above 85 wt%
filler loading were discarded because of their low
mechanical strength (high porosity) and related difficulty in
handling. Toroids of 13 mm outer diameter and 4 mm
inner diameter were pressed out of the mix with a uniaxial
pressure of 300 MPa using a hydraulic press (#3925,
Carver Inc.), and heat-treated at 340 �C. Density, electri-cal, magnetic and thermal properties were characterized to
study the suitability of the composites as miniaturised
circuit substrates for passives and antenna applications.
Characterisation studies: XRD (M/s Bruker) was used to
elucidate the structure of the CoY hexaferrites. Density
measurements were carried out by measuring mass and
volume. Volume was estimated from the geometry.
J Mater Sci: Mater Electron (2016) 27:7010–7017 7011
123
Scanning Electron Microscopy (Field Emission SEM-LEO
1530) was used to identify the microstructure details.
Parallel-plate capacitance was measured with HP 4294A
Precision Impedance Analyser to estimate the dielectric
constant and loss tangent. The pressed disks were metal-
lized with e-beam evaporated copper on either sides to act
as electrodes for probing. Impedance analysis (Agilent
4291B Impedance Analyzer) was performed to find the real
and imaginary parts of permeability. Vibration Sample
Magnetometry (VSM, Lakeshore 736 Series, Westerville,
OH) was employed to study hysteresis parameters of the
composite samples. Coefficient of linear thermal expansion
of the samples were measured using thermomechanical
analyser (TMA) (SI-model SS6100).
3 Results and discussion
3.1 XRD
XRD of Ba2Co2Fe12O22 filler prepared through conven-
tional solid-state ceramic route and calcined at 1150 �C for
5 h and then reheated at 1190 �C for 10 h is shown in
Fig. 1. The peaks match with that of the standard ICDD
pattern (044-0206) for the CoY hexaferrite corroborating
its pure phase. From the figure, it is clear that the samples
show CoY hexaferrite structure with (space group R-3m)
and exhibits a good crystalline state.
3.2 Density measurements
The heterogeneous system in the present study comprises
of PEEK and Y ferrite having densities 1.3 and 5.4 g/cc
respectively [8, 12]. Hence, the bulk density of the
composites largely depends on the amount of the particu-
late filler in the polymer matrix. Theoretical density of the
composite samples with different filler loading was calcu-
lated using the rule of mixtures. The variation of experi-
mental and calculated density with weight% of CoY
hexaferrite is given in Fig. 2. As expected, density
increases with increase in filler loading in PEEK matrix.
Experimental densities deviated more from the theoretical
values at higher loading. At 60 wt% (or *26.5 vol% fil-
ler), the estimated porosity is 7.5 %, while at 80 wt% (or
*49 vol% filler), the porosity is 11 %. At 85 wt%
(57 vol%) filler, the porosity is above 26 % and hence the
samples become fragile. The difference in the theoretical
and experimental values is due to the creation of micro-
voids or porosity in the composites with higher loading.
The microvoids affect the CTE, magnetic and electrical
properties of the composite.
3.3 Scanning electron microscopy
SEM micrographs were obtained from samples with dif-
ferent filler loadings to observe the microstructure and filler
distribution of the composites. The micrographs, shown in
Fig. 3, suggest that the filler is uniformly distributed in the
PEEK matrix. Although not evident from the images, the
samples with 85 wt% filler loaded sample has more
porosity (26 %) than those with lower filler wt% (*8 %),
which is attributed to the lower volume fraction of PEEK to
completely fill the voids.
3.4 Coefficient of thermal expansion (CTE)
The CTE of the dielectric films plays a critical role in the
mechanical design the RF module substrate. The magnetic
Fig. 1 XRD pattern of CoY hexaferrite
7012 J Mater Sci: Mater Electron (2016) 27:7010–7017
123
film should have a CTE close to that of the substrate and
copper in order to avoid interfacial and dielectric stresses
from thermal expansion mismatch. Since most of electronic
packaging materials are made of polymer or polymeric
composites and the CTE of a polymer is high, the CTE of
the composite substrates is usually lowered by controlling
the volume fraction and shape of inorganic fillers which
generally have smaller CTEs. In the particulate-filled
PEEK composites, where the CTE and elastic modulus of
the filler are lower and higher than those of matrix
respectively, the CTE mismatch strain gives rise to com-
pressive and tensile stress fields in the matrix and particles
during heating, respectively.
The composite CTE as a function of volume fraction is
shown in Fig. 4. The dominance of the matrix, which has a
larger CTE, is suppressed at higher filler fraction. The
overall CTE of the composite is reduced with the increase
in volume fraction of the filler. However, beyond 80 wt%
filler, not much reduction is seen.
Analytical models are used to predict the coefficient of
thermal expansion (CTE) of particle composites as a
function of particle volume fraction. The curve depicted by
the rule of mixture is shown as the straight line in the
figure. The bottom curves is obtained by Turner’s model,
which considers the mechanical interactions between the
composite phases. Assuming both phases have the same
dimensional change, the governing equation is given as
[15, 16]:
ac ¼ð1� /ÞKmam þ /Kpapð1� /ÞKm þ /Kp
ð1Þ
Kerner’s model takes both the normal and shear stresses
into account to estimate the CTE as [17]:
/c ¼/p Vpþ /m Vm
þ VpVm /p � /m
� � Kp � Km
KpVp þ KmVm þ 3KpKm
4Gm
ð2Þ
where a refers to the Coefficient of Thermal Expansion
(CTE), K refers to the bulk modulus, V is the volume
fraction and G is the shear modulus. The subscripts c, p and
m refer to the composite, particle and matrix respectively.
The values are compiled in Table 1. Schapery’s upper and
lower bounds [18] are illustrated as dashed lines the figure.
The experimental measurements do not agree well with
any of these models. As seen in the SEMs and density
plots, the composite samples have porosity or microvoids
which also have profound influence in controlling the CTE
of the composites, but are not captured by the models.
Therefore, as reported [20, 21], the void compression
mechanism plays a key in altering composite CTE seen for
85 wt% filler loaded sample having more porosity and
deviating the composite CTE from the models. It can be
seen that composites with filler loading above 75 wt% filler
have CTE close to 20 ppm/�C, which almost matches the
Fig. 2 Variation of experimental and theoretical density with filler
loading in PEEK matrix
Fig. 3 SEM images of composite surfaces with 65 wt% (*31 vol%) (a) and 85 wt% (*57 vol%) (b)
J Mater Sci: Mater Electron (2016) 27:7010–7017 7013
123
CTE of copper conductor layer (18 ppm/�C) commonly
used for substrates.
3.5 Dielectric properties
Variation of Dielectric constant and loss tangent with
respect to filler loading is given in Fig. 5.
From the figure, it is clear that the variation of dielectric
constant with filler loading is not linear and especially at
higher filler fraction the variation is not monotonous. The
permittivity variation as a function of filler loading was
fitted with the Bruggeman’s Effective Medium Theory
Model (EMT) [22] as shown in Fig. 6. The EMT equation
is represented as:
caea � eeffea þ 2eeff
þ cbeb � eeffeb þ 2eeff
¼ 0 ð3Þ
where ea and eb refer to the permeabilities of the filler and
matrix, ca and cb refer to the volume fraction of the filler
and matrix, and eeff is the effective nanocomposite per-
meability. PEEK is a non-magnetic material having a
permeability of *3.2. An effective particle permittivity
was extracted by mapping the experimental measurements
with permeability plots. Best fit was obtained when the
60 65 70 75 80 850.000010
0.000015
0.000020
0.000025
0.000030
0.000035
0.000040
0.000045
CTE
(ppm
/°C
)
YHF wt%
0
0.00001
0.00002
0.00003
0.00004
0.00005
0.00006
0 0.2 0.4 0.6 0.8 1
Kerner’smodel
Schapery’supper and lower bounds
Turner’s model
Volume Frac�on
CTE
Fig. 4 Variation of CTE with filler loading in PEEK matrix
Table 1 Thermomechanical properties of the filler and matrix
Ferrite [19] PEEK (Manuf. data)
CTE (ppm/C) 8 55
Youngs modulus (GPa) 138 3.6
Poisson’s ratio 0.35 0.45
Bulk modulus (GPa) 153 5.3
Shear modulus (GPa) 51 3.45
60 65 70 75 80 855.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Loss
tang
ent @
40 M
Hz
Die
lect
ric c
onst
ant @
40 M
Hz
Wt% of YHF
0.0035
0.0040
0.0045
0.0050
0.0055
0.0060
Fig. 5 Variation of dielectric constant and loss tangent with filler
loading in PEEK matrix
0
2
4
6
8
10
12
14
0 0.2 0.4 0.6 0.8 1
Nan
ocom
posi
te P
erm
i�vi
ty
Filler Volume Frac�on
2.5
5
8
12
17
5030
Fig. 6 Permittivity as a function of effective metal volume fraction.
The curves derived from Effective Medium Theory (EMT) using
different particle permittivities are also shown. Best fit is obtained
when permittivity of particles is 17
7014 J Mater Sci: Mater Electron (2016) 27:7010–7017
123
particle permittivity is *17 for EMT model, as shown in
the curves in Fig. 6.
The composite samples can be considered as biphasic
mixtures comprising of PEEK and filler with connectivity
between these phases. The non-linear variation of dielectric
constant can also be explained with respect to the con-
nectivity of the phases of the composite. Connectivity
describes the interspatial relationships in a multiphase
material, and is important because it controls electrical,
mechanical and thermal properties between the phases by
determining the coupling between the ceramic and poly-
mers. Since the matrix volume is predominant, at very low
filler fraction, composite phases retain 0–3 connectivity
with practically no filler–filler connectivity but with high
matrix–matrix connectivity. As the filler volume increases
in the composite system, it loses its 0–3 connectivity. This
-800
-600
-400
-200
0
200
400
600
800
-3000 -2000 -1000 0 1000 2000 3000
MA
GN
ETIZ
ATI
ON
(G)
FIELD (OERSTED)
80
85
60
-400
-300
-200
-100
0
100
200
300
400
-400 -200 0 200 400
MA
GN
ETIZ
ATI
ON
(G)
FIELD (OERSTEDS)
8085
60
Fig. 7 VSM results of 60, 80 and 85 wt% filler loading in PEEK matrix
0 200 400 600 800 10001.00
1.25
1.50
1.75
2.00
2.25
2.50
85 wt%80 wt%
75 wt%
70 wt%
65 wt%
60 wt%
µ'
Frequency (MHz)
Fig. 8 Variation of real part of permeability with filler loading in
PEEK matrix
0 200 400 600 800 10000.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07 85 wt%80 wt%75 wt%
70 wt%65 wt%60 wt%
µ'
Frequency (MHz)
Fig. 9 Variation of imaginary part of permeability with filler loading
in PEEK matrix
0
1
2
3
4
5
6
7
0 0.2 0.4 0.6 0.8 1
PERM
EABI
LITY
VOLUME FRACTION
2
3
4
610
1520
Fig. 10 Permeability (at 100 MHz) as a function of effective metal
volume fraction. The curves derived from Effective Medium Theory
(EMT) using various particle permeabilities are also shown
J Mater Sci: Mater Electron (2016) 27:7010–7017 7015
123
results in high filler–filler interaction at higher filler frac-
tion resulting in the non-linear change of dielectric prop-
erties. Still higher filler–filler interaction invariably reduces
the packing density and causes porosity into the composite
and hence reduction in dielectric constant is seen beyond
80 wt% loading.
It can be seen that the loss tangent also increases with
filler loading in the same fashion and the variation is
monotonous. When an electromagnetic signal passes
through a heterogeneous media, depending on the nature of
the media through which it travels, absorption, reflection
and scattering can occur. In the PEEK composites, the
signal has to pass through the filler, filler-matrix interface
and matrix. As the filler loading increases in the matrix,
interface volume also increases attributing to more and
more heterogeneous phases in the composite for the pas-
sage of electromagnetic signal, resulting in anharmonicity
and subsequent increase in loss. Another reason for the
non-linear increase in tand at higher filler loading is
increased porosity and associated moisture absorption.
Similar trends in dielectric constant and loss tangent of
ceramic filled polymer composites have been explained in
prior research in microwave composites [23, 24].
3.6 Magnetic properties
3.6.1 Magnetization curves
VSM results for composite samples with 60, 80 and
85 wt% are shown in Fig. 7. From the figure, it can be seen
that Y-type hexaferrite-filled PEEK composites exhibit soft
magnetic properties such as low coercive force Hc and
good saturation magnetization Ms (450–750 G). It can be
seen that Hc remains almost constant while MS increases
with increase in filler loading in PEEK matrix.
3.6.2 Permeability spectra
Variation of real and imaginary part of permeability (l0 andl00) with respect to frequency for different filler loadings is
shown in Figs. 8 and 9 respectively. The permeability
variation as a function of filler loading was fitted with the
Bruggeman’s Effective Medium Theory Model (EMT) [22]
as shown in Fig. 10. The EMT equation is represented as:
cala � leffla þ 2leff
þ cblb � lefflb þ 2leff
¼ 0 ð4Þ
where la and lb refer to the permeabilities of the filler and
matrix, ca and cb refer to the volume fraction of the filler
and matrix, and leff is the effective nanocomposite per-
meability. PEEK is a non-magnetic material having a
permeability of *1. The permeabilities for submicro- and
nano-ferrite particles are strongly dependent on the size,
shape, surface state and internal coupling between the
particles. An effective hexaferrite particle permeability was
extracted by mapping the experimental measurements with
permeability plots derived from EMT with various particle
permeabilities. Best fit was obtained when the particle
permeability is *3 for EMT model, as shown for the
curves in Fig. 10.
Spinel ferrite composites have a frequency-stability of
permeability up to a few hundreds of MHz [6, 9] whereas
CoY type hexaferrite composites exhibit higher frequency
stability of nearly up to 800 MHz as seen in the figure be-
cause of their strong planar magnetic anisotropy. Com-
monly, magnetic ceramic materials show two distinct
resonances in their permeability spectra, viz a viz one
corresponding to domain wall and the other one for gyro-
magnetic spin resonance. However, frequency spectra
beyond 1 GHz needs to be measured to investigate the
peak due to domain wall and spin resonances.
From the figures, it is seen that the 80 wt% CoY hex-
aferrite-loaded PEEK can be used for applications up to
800 MHz with a maximum permeability of 2 and a low
loss of 0.025. The composites also exhibited a dielectric
constant and loss tangent of 5.5 and 0.02 respectively.
Hence, RF components can be designed with better
miniaturization and easy impedance matching than that of
commonly used epoxy-glass dielectric substrates.
4 Conclusions
Phase-pure CoY hexaferrite composites were synthesised
through solid-state routes to use as fillers for polymer
composites as RF magnetodielectric substrates. A system-
atic study of dielectric, magnetic and thermal properties
with filler loading in PEEK matrix was performed. The
investigations showed that the composite substrates
exhibited frequency stability of permeability up to
800 MHz with permeability and loss ranging from 1.5 to 2
and 0.01 to 0.045 respectively. Dielectric constant and loss
tangent varied from 5.3 to 8.1 and 0.0038 to 0.0058
respectively. CTE was close to that of copper for com-
posites with high filler loading. The composites exhibited
simultaneous high permittivity and permeability and hence
can be effectively used for miniaturized and embedded RF
components and substrates.
Acknowledgments The authors are grateful to Department of
Electronics and Information Technology (DeitY), New Delhi for
funding to carry out this work.
7016 J Mater Sci: Mater Electron (2016) 27:7010–7017
123
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