Influence of Thermal Oxidation on Complex
Permittivity and Microwave Absorbing Potential
of KD-I SiC Fiber Fabrics
Donghai Ding1, Fa Luo2, Yimin Shi2, Jin Chen1
1
College of Materials and Mineral Resources, Xi’an University of Architecture and Technology,
Xi'an, Shaanxi CHINA
2
State Key Laboratory of Solidification Processing, Northwester Polytechnical University, Xi'an, Shaanxi CHINA
Correspondence to:
Donghai Ding email: 290610692@qq.com
ABSTRACT
The complex permittivity and microwave absorbing
potential of KD-I SiC fiber fabrics at X band as
functions of thermal oxidation time and temperature
were investigated. The values of ε′ (real part) and ε″
(imaginary part) of the complex permittivity of the
fabrics decreased with increasing thermal oxidation
time and temperature. It was found that the
microwave absorbing potential of fabrics can be
optimized by thermal oxidation. The best reflection
loss was obtained for the sample oxidized for 30
min at 500℃. Its lowest reflection loss is about
-25.4 dB at 10.3 GHz, and its absorbing frequency
band is about 4 GHz. The oxidized KD-I SiC fiber
fabric is a good microwave absorbing material with
low density, high temperature resistance, and
flexibility.
Nowadays, the electric and microwave absorbing
properties of SiC fibers are attracting more and
more attention [6-12]. Shimoo et al. researched the
effect of heat-treatment in hydrogen on C/Si ratio
and specific resistivity of Hi-Nicalon SiC fibers [6].
Tan et. al. studied the electromagnetic wave
absorption potential of Tyranno SiC fiber woven
fabrics in the 17-40 GHz frequency range [7].
As microwave absorbing materials, the SiC fiber
fabrics have some advantages such as low density,
adjustable electrical property, and resistance to high
temperature.
As proposed in our previous
investigation [12], the rich carbon outer layer plays
a key role in the complex permittivity of KD-I SiC
fiber fabrics and the imaginary part is too high to
absorb microwave effectively. Haitao Liu studied
the effects of the oxidation parameters on the
flexural strength of KD-I fibers and found that the
strength retention ratio of the fibers was 89.6% after
60 min oxidation at 600 °C [13]. Therefore, the
thermal oxidation could be used to improve the
microwave absorbing properties of KD-I SiC fiber
fabrics without obvious degradation of strength.
Keywords: Complex permittivity; SiC fiber fabric;
Free carbon
INTRODUCTION
Increasing attention has been focused on
polycarbosilane derived SiC fibers due to its
excellent properties such as thermal resistance,
chemical stabilization, high tensile strength, and
low density [1]. Because it can be woven into fabric
with different structures including 2D, 2.5D and 3D,
SiC fibers are usually used as reinforcement for
high temperature ceramic matrix composites [2,3].
It has been recognized that the micro-structure and
mechanical properties of SiC fibers have been
investigated well in the past several decades [4,5].
Journal of Engineered Fibers and Fabrics
Volume 9, Issue 2 – 2014
In this paper, the influence of thermal oxidation on
complex permittivity and microwave absorbing
potential of KD-I fabrics was studied within the
frequency range of 8.2-12.4GHz (X band) at room
temperature. Due to the intimate relation between
complex permittivity and DC electrical conductivity,
we studied the effect of thermal oxidation time on
DC electrical conductivity. In addition, the chemical
compositions were characterized by EDS in order to
gain a better understanding from the microscopic
aspect.
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EXPERIMENTAL
Fabrics Preparation and Thermal Oxidation
The KD-I fibers were provided by National
University of Defense Technology (China). Table I
shows the parameters of SiC fiber. The fabrics were
braided by Nanjing Glass Fiber Institute (China).
The fabrics structure is the 2.5D shallow bending
joint shown by Figure 1. The fiber volume fraction
of the fabrics is 40%. The density of fabrics is about
1.0 g/cm-3 calculated on the basis of fiber volume
fraction and density of SiC fiber.
Because of the low complex permittivity of paraffin
wax, melted paraffin was cast into the fabrics to
obtain fabric/wax composites so that the fiber
bundles in fabric were fixed to facilitate the
measurement of permittivity. The measured samples
were cut into 22.86 mm×10.16 mm×2.0 mm. The
wax is insulator, and the ε′ and ε″ are 2.26 and 0 at
X band. So the fabric mostly determines the
complex permittivity of the fabric/wax composites.
Given the convenience of discussions later, the
influence of wax on the complex permittivity was
ignored.
TABLE I. Parameters of KD-I SiC fiber [8].
Fiber
type
Diameter
(μm)
Density
(g/cm3)
Tension strength
(MPa)
KD-I
14~16
2.54
1800~2200
Based on the measured complex permittivity and
permeability, the microwave absorbing properties
can be calculated by the following equations [14]:
RL = 20 log
Z in − Z 0
Z in + Z 0
(1)
where RL denotes the reflection loss in dB unit. Z0
is the characteristic impedance of free space. Zin is
the input characteristic impedance at the
absorber/free space interface, which can be
expressed as:
Z in = Z 0
(2)
where c is the velocity of light, t is the thickness of
an absorber. In this paper, all t values are 3 mm. εr
and μr are the measured relative complex
permittivity and permeability, respectively.
FIGURE 1. Schematic showing of KD-i SiC fibers fabric
structure.
The fabrics were oxidized in air within the
temperature range of 400 – 500 °C in a closed
box-type furnace.
The DC electric conductivity (σ) of SiC filaments
was calculated from the equation:
Analysis and Characterization Techniques
The ε′ and ε″ of complex permittivity are
macroscopical parameters that correlate with
polarization and dielectric loss of materials,
respectively; the real part (μ′) of complex
permeability represents the amount of energy stored
in the material from ac magnetic field, while the
imaginary part (μ″) is the energy loss to the
magnetic field. The complex permittivity of fabrics
was measured by the method of rectangle
waveguide using vector network analyzer (E8362B).
Journal of Engineered Fibers and Fabrics
Volume 9, Issue 2 – 2014

  2πft 
µr
tanh j 
 µ r ε r 
εr

  c 
σ = 1/ ρ =
L
RS
(3)
where σ is special conductivity, R is the resistance,
S is the cross section area of filament, and L is the
length of filament. The electrical resistance of SiC
filaments at room temperature was measured by two
probes direct current method. The filaments were
tin-lead bonded on alumina substrate. The length of
filaments was 10 mm. The average diameter of
KD-I filaments was both considered as 14 μm.
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RESULTS AND DISCUSSION
Microstructure Characterization
Figure 2 shows the chemical element intensity
profile of the KD-I fibers as received and oxidized
KD-I fiber diameter was characterized by EDS. It
can be observed that there is an excess carbon outer
layer on as received KD-I fibers while the carbon
element density along the oxidized fiber diameter is
approximately uniform. It can be concluded that the
carbon outer layer was vaporized after thermal
oxidation. Table II displays the chemical
composition of KD-I SiC fibers dependence on
thermal oxidation temperature for 30 min. Because
of the lower oxidation temperature, the oxidation of
SiC cannot occur. The decreasing of C/Si atomic
ratio after oxidation should be due to the oxidation
of free carbon. After oxidation at 500°C for 30 min,
the C/Si decreased to 1.09 %. In addition, the
decreased weight in the process of thermal
oxidation is shown in Table III which should be
attributed to the oxidation of free carbon.
Based on the above results, it was deduced that the
free carbon is obviously oxidized from about 400℃.
Besides, the weight loss also improved the
oxidation of free carbon.
TABLE II. The chemical components of KD-1 SiC fiber after
thermal oxidation at different temperature.
Chemical elements
(at %)
as received
400℃
470℃
500℃
C
Si
39.13
35.95
29.67
30.74
O
31.59
30.15
26.53
28.12
29.20
33.69
43.8
41.14
C/Si
1.24
1.19
1.12
1.09
Electrical Conductivity
In the present study, the electrical conductivity of
KD-I SiC fiber was measured, and the results were
shown in Table IV. The obvious electrical
conductivity decreased from 0.05 to 10-6 S/cm after
thermal oxidation at 500°C. Based on discussions of
previous investigation [12], a conceptual
microscopic explanation for the conductivity drop
of KD-I SiC fiber is the idea that the conductive
network formed by free carbon has been damaged
by vaporization of free carbon through thermal
oxidation. In the present investigation, the electrical
conductivity of SiC fiber decreased sharply when
the thermal oxidation time is in the range of 0-60
min shown in Table IV, which is agreement with the
data of weight loss of SiC fibers oxidation at 500°C
for different time shown in Table III.
TABLE III. Weight loss of SiC fiber oxidation at 500 °C for
different time.
Time/minute
Weight loss/%
0
0
30
0.30
60
0.62
90
0.96
120
1.12
TABLE IV. Conductivity of SiC filament after oxidation at
500℃for different time.
0
30
60
90
120
Conductivity/
(S/cm)
0.05
0.004
0.003
10-6
10-6
Complex Permittivity
As our next step, we studied the influence of
thermal oxidation on the complex permittivity of
fabrics. Figure 3 shows the variation of complex
permittivity of KD-I SiC fiber fabrics after thermal
oxidation for different time at 500°C at X band.
FIGURE 2. Chemical element intensity profile along fiber
diameter. (a) as received fiber; (b) after oxidized fiber.
Journal of Engineered Fibers and Fabrics
Volume 9, Issue 2 – 2014
Time/minute
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Overall, the ε′ and ε″ decreased with increasing of
the time. From the value of ε′=9.8 at 10 GHz for as
received sample, it drops to 7.5 for the sample after
oxidation for 30 min and followed by three obvious
decreases for the samples after oxidation for 60, 90
and 120 min. As to the ε″, the value decreased
significantly from 17.3 to 3.4 at 10 GHz after
oxidation for 30 min, but the decreases are much
more slight for the samples after oxidation for 60,
90 and 120 min. In addition, the frequency response
effect of ε′ and ε″ can be observed obviously only
for as received sample.
In addition, it can be observed from Figure 3 and
Figure 4 that the decreasing of the values of ε″ is
more obvious than that of ε′.
FIGURE 4. Complex permittivity of KD-1 SiC fiber fabrics after
thermal oxidation at different temperature for 30 min. (a) real
part; (b) imaginary part.
At X band, the main mechanisms for the
polarization in a dielectric material are relaxation
polarization and interfacial polarization [15, 16].
The weakly bond electron in dielectric materials can
induce the formation of electron relaxation
polarization, which is an energy consumption
process. This polarization form can increase the ε′
and ε″ of complex permittivity, which can be
expressed as follows:
FIGURE 3. Complex permittivity of KD-I SiC fiber fabrics after
thermal oxidation at 500 ℃ for different time. (a) real part; (b)
imaginary part.
Figure 4 shows the complex permittivity of KD-I
SiC fiber fabrics after thermal oxidation at different
temperature for 30 min. The values of the complex
permittivity decreased with the rising of
temperature. It is worth mentioning that the ε′
begins to decrease obviously at 435°C while the ε″
decreased obviously at 400°C. It can be concluded
that the decreasing rate of ε′ and ε″ increase as the
increasing temperature in the range of 400 -500 °C.
Journal of Engineered Fibers and Fabrics
Volume 9, Issue 2 – 2014
εs −ε∞
1 + (ωτ ) 2
(4)
εσ −ε∞
σ
ωτ +
2
ε 0ω
1 + (ωτ )
(5)
ε I '= ε∞ +
ε I ''=
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where εI′ and εI’’ are the increased parts of ε′ and ε″
induced by electron relaxation polarization, σ is
conductivity, ε0 is the dielectric constant in vacuum,
εs is the static permittivity, ε∞ is the relative
dielectric permittivity at the high frequency limit, ω
is the angular frequency and τ is the relaxation
polarization time [15].
the lowest RL value of about -25.4 dB at 10.3 GHz,
and its AFB is 4 GHz. It can be seen from Figure 5
that the microwave absorbing potential of as
received, 90 min and 120 min samples are all poor.
The reason is that the complex permittivity for as
received sample are too much higher, which cannot
match the characteristic impedance of free space;
and the complex permittivity for 90 min and 120
min samples are too much lower to dissipate
electromagnetic energy.
The excess carbon should play an important role in
the higher complex permittivity of KD-I fabrics at
X band. The free carbon with free electron can
induce the electron relaxation polarization.
Therefore, the values of ε′ and ε″ of as-received
sample are both high. In the process of thermal
oxidation, the free carbon should be gradually
vaporized, which was indicated by weight loss and
decreased C/Si ratio. Consequently, the relaxation
polarization induced by the free electron weakens
gradually. The conductivity decreasing approved
this on the other hand. As a result, the complex
permittivity decreased after thermal oxidation.
CONCLUSION
The influence of thermal oxidation on complex
permittivity and microwave absorbing potential of
KD-I SiC fibers fabric within the frequency range
of 8.2-12.4 GHz were investigated. The DC
electrical conductivity of KD-I SiC fiber decreased
in the process of thermal oxidation at 500°C. Both
the values of ε′ and ε″ of the fabrics decreased with
increasing thermal oxidation time and temperature.
The damage of conductive network due to
vaporization of free carbon through thermal
oxidation should mainly contribute to the
decreasing of complex permittivity of KD-I SiC
fiber fabrics. The best reflection loss was obtained
for sample oxidized for 30 min at 500 °C. Its lowest
reflection loss is about -25.4 dB at about 10.3 GHz,
and its absorbing frequency band is 4 GHz. Thermal
oxidation in air within the temperature range of 400
– 500 °C could be one useful method to improve the
microwave absorbing potential of KD-I fabrics.
Microwave Absorbing Potential
It was revealed that KD-I SiC fiber fabric has
potential as light microwave absorbing material
working at high temperature. In order to work at
higher temperature in air, KD-I fabrics after proper
thermal oxidation need anti-oxidation coatings to
avoid deterioration of their microwave absorbing
potential. In our next step, the anti-oxidation
coatings will be fabricated on fabrics oxidized for
30 min at 500 °C, and the high temperature
microwave absorbing property will be evaluated.
FIGURE 5. Reflection loss of KD-I SiC fiber fabrics after
thermal oxidation for different time at 500 °C.
To evaluate the microwave absorbing potential, the
reflection loss (RL) in dB unit can be calculated. If
the RL values of an absorber are less than -10 dB
(90% absorption), then we can say that the absorber
works very well. And, the microwave absorbing
frequency band (AFB) is defined as the frequency
range, in which the RL values are lower than -10 dB.
Figure 5 shows the calculated RL values of KD-I
fabrics after thermal oxidation for different time at
500 ℃ . It was observed that the microwave
absorbing potential of KD-I fabrics can be
improved by thermal oxidation. The best result is
obtained for sample oxidized for 30 min. It shows
Journal of Engineered Fibers and Fabrics
Volume 9, Issue 2 – 2014
ACKNOWLEDGMENTS
This work was supported by the Scientific Research
Program Funded by Shaanxi Provincial Education
Department (Grant No. 2013JK0922 and Grant No.
2013JK0921), by the fund of the State Key
Laboratory of Solidification Processing in NWPUC
(SKLSP201305)and by the basic research fund of
Xi’an University of Architecture and Technology
(JC1310,RC1210).
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AUTHORS’ ADDRESSES
Donghai Ding, PhD
Fa Luo
Yimin Shi
Jin Chen
College of Materials and Mineral Resources
Xi’an University of Architecture and Technology
No. 13 Yanta Road
Xi'an, Shaanxi 710055
CHINA
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