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

pm86@mail.gatech.edu; raj@ece.gatech.edu

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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