Microwave Engineering Laboratory
Effect of annealing temperature of nano-sized BaFe12O19 in
Novolac Phenolic Resin on microwave properties for use as EMI
shielding material in X-band
13-Apr-15, Slide 1
Outline
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Introduction
Theory of microwave absorption
Preparation of ferrite nanoparticles and magnetodilectric composite
Results and discussion
Conclussions
13-Apr-15, Slide 2
Introduction

Leakages of electromagnetic wave in various communication systems lead to electromagnetic interference
(EMI). Shielding materials minimize the external electromagnetic waves from interfering in functioning of
electronic devices.
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A good absorber should have two essential characteristics, low impedance mismatch at the air-absorber
interface to get low reflection and extent of the microwave entering into the materials and secondly, should
sufficiently attenuate and absorb the wave passing through it.

Microwave absorption characteristic of the absorbing material in a frequency range depends on complex
permittivity, (εr= εr´- jεr″) and complex permeability (µr =µr´- jµr" ).
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Ferrite is a metal oxide, which contains magnetic ions arranged in such a manner that it produces
spontaneous magnetization while maintaining good dielectric properties.
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Saturation magnetization and permeability depend strongly on the particle size, morphology, and
microstructure of the materials.
In the present investigation barium ferrite particles are annealed with varying temprature and used as the
magnetic inclusions in the Novolac Phenolic Resin (NPR) matrix for microwave absorption study.
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13-Apr-15, Slide 3
Electromagnetic Interference and its origin
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Vast use of this spectra leads to electromagnetic pollution in the
environment affecting proper functioning of electronic devices.
This interference leads to problems such as jamming of signal, inaccuracy
in target detection in warfare and also less camouflaging etc. Due to
electromagnetic interference, electromagnetic compatibility becomes a
prime concern.
13-Apr-15, Slide 4
How to minimize the electromagnetic interference (EMI)
Shields are used to isolate a region, to prevent
interference from outside sources and to avoid the
leakage of unwanted radiation due to internal
sources.
Shielding can be done by placing the device in an enclosure which will reflect all
the electromagnetic wave thus protecting the device from external electromagnetic
interference or it could be done by placing material on the device which will absorb
the electromagnetic wave incident on it.
13-Apr-15, Slide 5
Theory of microwave absorption
Microwave absorption mechanism require two conditions:
Low reflection at the air-absorber interface and
High attenuation within the bulk of the material.
Zd  Z1tanhγa
Z1  Zd tanhγa
(1)
μ 0μ r
μ
μ

 η0 r
ε
ε 0ε r
εr
(2)
Zin  Z1
Z1 
γ  jω εμ  j
2ππ
ε rμ r
c
(3)
ε r  ε'r  jε"r & μ r  μ'r  jμ"r
Z in  Z 0
Γ

(4)
r
tanh j 2fd / c   r  r
r

Zin  Z0
Zin  Z0
(5)
(6)
13-Apr-15, Slide 6
Material
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Magnetodielectric inclusion : BaFe12O19
Polymer matrix
: Novolac Phenolic Resin (NPR)
13-Apr-15, Slide 7
Synthesis of nano-sized barium ferrite (BaFe12O19) particles and
BaFe12O19/NPR composites
Ba(NO3)2
+ H2O
Fe(NO3)3.9H2O
+ H2O
Mixed
together
Stirring for 1 hr
NaOH
+Oleic
acid
Washed with distilled
water
Composite sample of size 10.38 mm
× 22.94mm × 4 mm was prepared by
compression moulding technique for
30 weight percent.
Barium
NPR
Mixed in dry form
Grinding
+ Ethanol
Fill in the die-mould
Dried at
700C
Annealed at 7000C/
8000C/ 9000C
Compressed
Composite sample after
natural cooling
13-Apr-15, Slide 8
X-Ray Diffraction Pattern
Figure 1. XRD pattern of barium ferrite annealed at (a)7000C, (b)8000C & (c)9000C
Table 1: crystallite size calculated from the XRD pattern
Ferrite
BaFe12O19
Crystallite sizes (nm)
annealing temperatures
at
different
700 0C
800 0C
900 0C
18.46
23.8
26.64
13-Apr-15, Slide 9
Transmission Electron Micrograph results
(a)
(b)
(c)
Fig. 2. TEM micrographs of barium ferrite annealed at (a)7000C, (b)800 0C & (c)9000C
With increasing temperature, the ferrite particles showing an elongated, rod shaped
structure:
 The nanoparticle growth occurs at unit cell level along preferential directions.
 The surface energy of barium ferrite is different along different directions of the unit cell,
particle growth would not occur in all directions equally.
 The growth of the nanoparticles along [0001] direction i.e. the c-axis, as it is energetically
favourable due to minimum surface energy at higher temperature and hence, the formation of
elongated nanostructure is observed at 9000C.
13-Apr-15, Slide 10
Nicolson Ross Technique (Agilent E8362C vector
network analyzer-85071E material measurement software)

Z  Z0

Z  Z0
r /  r  1
r /  r  1
T  exp[ j ( / c)  r  r d ]
(1)
(2)
(1   2 )T
S 21 ( ) 
1   2T 2
(1  T 2 )
S11 ( ) 
1   2T 2
V1  S 21  S11
(5)
V2  S 21  S11
( 6)
1  V1V2
X
V1  V2
(7 )
r  1   

  c1
 r 1  
  X  X 2 1
(8)
1 
 c
 r  r    ln( )  c2
 d T 
 1
(9)
T
V1  
1  V1
(3)
( 4)
2
(11)
2
(10)
r 
c2
c1
 r  c1c2
(12)
(13)
(14)
13-Apr-15, Slide 11
Figure3. Complex permittivity and Figure4. Complex permeability of 30 wt. %
BaFe12O19/NPR composites at annealing temperature of BaFe12O19, T=7000C,
8000C and 9000C
13-Apr-15, Slide 12
Figure5. (a) Dielectric and (b) magnetic loss of 30 wt. % BaFe12O19/NPR
composites at annealing temperature of BaFe12O19, T=7000C, 8000C and 9000C
13-Apr-15, Slide 13
Inferences
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The spectra show that the composite reinforced with barium ferrite at 9000C has
higher values for both real (εr´) and imaginary (εr´´) part of complex permittivity.
The grain size of 9000C barium ferrite as seen from the TEM images are also larger
and larger grain size leads to high polarizability, as the opposing effect to electric
field component, developed in the material reduces.
At higher annealing temperatures, the number of Fe2+ ions increases by conversion
of Fe3+ into Fe2+ leading to high polarisation.
The permeability spectra show higher value of permeability, µ´ and magnetic loss,
µ″ for higher annealing temperature.
As the size of the BaFe12O19 particles increases, the domain wall length increases
which lead to greater domain wall vibration and hence greater value of µ´ and µ″ is
obtained.
13-Apr-15, Slide 14
Microwave absorption results
 For all the three BaFe12O19/NPR
composite samples, with BaFe12O19
annealed at, T=7000C (S1), 8000C (S2) and
9000C (S3), two absorption peaks are
observed.
 S1 shows reflection loss of -26.22 dB at
9.66 GHz and -14.24dB at 11.68GHz, S2
shows -27.68 dB at 9.59 GHz and -15.71dB
at 11.08GHz and S3 shows -28.94 dB at 9.3
GHz and -15.6 dB at 11.49 GHz.
The maximum absorption peak shifts
towards left with increase in the annealing
temperature of the ferrite inclusions.
Figure.6 Reflection loss of BaFe12O19/NPR composite with
BaFe12O19 annealed at, T=7000C, 8000C and 9000C
The -10dB bandwidth for S1 is 0.64 GHz
and 0.65GHz, for S2 is 0.73GHz and
0.62GHz and for S3 is 0.49GHz and
0.83GHz
13-Apr-15, Slide 15
Conclusions
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The size of the ferrite particles can be controlled by changing the annealing
temperature.
The BaFe12O19/ NPR composite with 30wt. % of BaFe12O19 annealed at 9000C
shows good microwave characteristics over the X-band to use as an EMI shielding
material.
With increase in the annealing temperature of the ferrite particles the microwave
properties get improved.
13-Apr-15, Slide 16
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