Preparation, surface modification and microwave characterization of magnetic iron fibers

Article abstract of DOI:10.1016/j.jmmm.2006.02.233

In this paper, magnetic iron fibers of 3-10 μm diameter and an adjustable aspect ratio were synthesized successfully by a method involving pyrolysis of carbonyl under a magnetic field. A surface modification technology was also investigated. The electromagnetic parameters of the iron-fiber-wax composites were measured using the transmission/reflection coaxial line method in the microwave frequency range of 2-18 GHz. The results show that the prepared iron-fiber-wax composites exhibit high magnetic loss that can be further improved after phosphating. On the other hand, the complex permittivity was significantly decreased after phosphating. As a result, this kind of iron fiber may be useful for thin and lightweight radar-absorbing materials.

Full text of DOI:10.1016/j.jmmm.2006.02.233

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Journal of Magnetism and Magnetic Materials 306 (2006) 125–129
www.elsevier.com/locate/jmmm
Preparation, surface modification and microwave
characterization of magnetic iron fibers
Ã
Yan Niea, , Huahui Hea, Zhenshen Zhaoa, Rongzhou Gonga, Hongbin Yub
aDepartment of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China
bDepartment of optoelectronic Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
Received 17 September 2005; received in revised form 29 November 2005
Available online 23 March 2006
Abstract
In this paper, magnetic iron fibers of 3–10 mm diameter and an adjustable aspect ratio were synthesized successfully by a method
involving pyrolysis of carbonyl under a magnetic field. A surface modification technology was also investigated. The electromagnetic
parameters of the iron-fiber–wax composites were measured using the transmission/reflection coaxial line method in the microwave
frequency range of 2–18 GHz. The results show that the prepared iron-fiber–wax composites exhibit high magnetic loss that can be
further improved after phosphating. On the other hand, the complex permittivity was significantly decreased after phosphating. As a
result, this kind of iron fiber may be useful for thin and lightweight radar-absorbing materials.
r 2006 Elsevier B.V. All rights reserved.
PACS: 42.81.Cn; 84.40.ꢀx
Keywords: Magnetic iron fiber; Phosphating treatment; Complex permeability; Complex permittivity; Microwave
1. Introduction
methods are not suitable for producing fibers with a
diameter down to several micrometers, which can be
detrimental to the further increase in the permeability
according to the analysis in Ref. [9]. In this paper, a novel
technology based on pyrolysis of carbonyl and the
application of a magnetic field has been put forward to
produce magnetic iron fibers. The magnetic iron fibers with
diameter of 3–10 mm and an adjustable aspect ratio were
prepared. Such a method proved to have the ability of large
amount production. Furthermore, a surface modification
technology was also adopted to improve the electromag-
netic property and had been proved to be effective.
The application of radar-absorbing materials can reduce
the radar cross-section of the target effectively, thus
contributing to the stealthy defense system [1–3]. As a
result, many research groups all over the world have been
studying the preparation and the electromagnetic proper-
ties of absorbers. Up to now, the most commonly used
absorbers are magnetic materials (such as ferrite particles)
and dielectric materials (such as carbon black particles)
[4–6]. However, the expansion of applications was limited
due to their thickness and weight. Recently, some research
has been devoted to the study of the electromagnetic
properties of nonspherical magnetic materials [1,7–10].
Some kinds of metal fibers have been produced by bundle
drawing, shatter machining, and melt extracting [11–15].
All these works show that they exhibit higher permeability
and hence better absorption in the radar band. But all these
2. Experiments
2.1. Preparation technique
The assembled laboratory preparation set-up is shown in
Fig. 1.
Liquid Fe(CO)5 is used as the source of the iron element.
First, it is heated to 140 1C by an oil-bath and, hence is
converted into Fe(CO)5 vapor. Then, the produced vapor is
Ã
Corresponding author. Tel.:+86 27 87542494x808;
fax: +86 27 87547337.
E-mail address: nieyanko@yahoo.com.cn (Y. Nie).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2006.02.233

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Y. Nie et al. / Journal of Magnetism and Magnetic Materials 306 (2006) 125–129
126
Cooling water
Liquid Fe(CO)5
Nitrogen
Evaporator
Ammonia
Ammonia
Resistance
heater
Toroidal
foil
Reactor
Cooling
water
Cooling
water
Nitrogen
Exhaust
Fiber
collector
gas
collector
Fig. 1. The schematic diagram of iron fiber preparation set-up.
cobalt, permalloy, iron-coated multiwall carbon nano-
Table 1
Process parameters
tubes, etc.), which are lower than 100 over the whole
investigation band [16–18]. Too high values of e0 and e00
may bring about the mismatch of normalized input
impedance and reduce the absorption property in radar
band [19]. Thus, a phosphating process technique is put
forward to solve this problem. The treatment flow is shown
schematically in Fig. 2. The solution used is mainly made
up of sodium dihydrogen phosphate (NaH2PO4) and
phosphorous acid (H3PO4) with concentrations 49.5 g/l
and 2 ml/l, respectively, and the pH value of the solution
controlled between 4 and 5. At the beginning of the
process, the solution is heated to 28 1C, and then the fibers
are added accompanied by solution agitation. After 1 min
treatment, the fibers are extracted out of the solution by
using magnetic separating technology and then dried in a
vacuum.
Parameter
Value
Fe(CO)5 vapor flow
Nitrogen flow
500 ml/h
150 ml/h
150 ml/h
11000 A/m
350 1C
Ammonia flow
Magnetic field
Actor temperature
introduced into the reactor from the top using nitrogen as
the carrier gas. Meanwhile, ammonia is also added into the
reactor simultaneously through two different inlets lying in
the top part, diluting the vapor and controlling the
composition of the prepared fibers. The chamber tempera-
ture is raised to 350 1C by using the resistance heater
arranged around the side wall. In this hot environment, the
vapor decomposition takes place as shown in Eq. (1). After
the agglomeration and nucleation stages, the formed iron
elements adhere to one another under a constant magnetic
field along the vertical axis of the reactor, which is provided
by the toroidal coil. Finally, the chain-shaped magnetic
iron fibers are produced. Some specific parameters used in
the process are described in Table 1.
2.3. Characterization techniques
The morphology of magnetic iron fibers was observed by
Philips XL-30TMP scanning electron microscope (SEM).
The transmission/reflection method was adopted to deter-
mine the relative complex permeability m and permittivity e
of the magnetic material–wax composites using an
HP8722ES vector network analyzer system [20]. The
magnetic fillers were randomly dispersed in the wax with
a different volume fraction. The cylindrical toroidal
samples were fabricated. The samples were 3 mm in inner
diameter, 7 mm in outer diameter, and 3–3.5 mm in
thickness.
350 ꢁ
C
FeðCOÞ5ðgÞ ꢀ! FeðsÞ þ 5COðgÞ.
(1)
2.2. Surface modification technique
The complex permittivity of the fiber–wax composite
sample presents a high value (both the values of e0 and e00
higher than 100 at 2 GHz as discussed below), compared
with those of other magnetic inclusions (ferrite, iron,
3. Results and discussion
Fig. 3 shows the SEM of the typical fibers prepared by
the proposed technique. It can be seen from graphs (a) and

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Y. Nie et al. / Journal of Magnetism and Magnetic Materials 306 (2006) 125–129
127
Magnetic iron
fiber
Preparation of Sodium Dihydrogen Phosphate / Distilled
water solution
Treatment
solution
Sufficient
mixture
Preparation of Phosphorous acid / Distilled water solution
Magnetic seperation
Dried in vacuum
Agitation
Fig. 2. The phosphating process flow.
Fig. 3. Scanning electron micrographs of the prepared fibers.
(b) that the fibers appear to be chain-shaped with about
3 mm diameter, a narrow dimension distribution, and good
dispersion. The diameter and the aspect ratio can be
adjusted by changing the process parameters. Fig. 3(c)
shows the prepared iron fiber with a diameter of about
10 mm. Furthermore, the length of the fibers can be
controlled from micrometers to millimeters by changing
the actor temperature and the velocity of the vapor and
gas flow.
Although possessing the better magnetic property with
large filling fraction, the complex permittivity of composite
A1 is too high for radar absorption applications. Ref. [19]
states that too high values of e0 and e00, compared to the
values of m0 and m00, can reduce the absorption property in
radar band. This disadvantage can be overcome by
decreasing the filling fraction. But it always works at the
cost of decreasing the magnetic loss. This paper presents
the phosphating process technique to overcome this
disadvantage. The results for iron-fiber–wax composite
A2 and phosphated iron-fiber–wax composite A3 are
illustrated in Fig. 5. Both composites have the same
volume fraction of 30%.
Both the complex permittivity and permeability of
composite A2 are reduced, compared to those of composite
A1, as shown in Fig. 4, in the whole test frequency range
after reducing the filling fraction. Especially, the complex
permittivity has been reduced significantly. The decrement
of permittivity with cutting down the filling fraction is due
to the decreasing conductivity. Compared to reducing the
filling fraction, it can be seen from Fig. 5 that e0 and e00 can
be decreased further after phosphating. Furthermore, it is
very interesting that the enhancement of the value of m00 is
evident, and its value reaches 5.2 at 2 GHz and remains
above 1.0 over the 2–10 GHz frequency range, which
indicates higher magnetic loss after phosphating. The value
of m0 decreases slightly in the frequency range 2–7 GHz, but
exhibits a tendency to increase in the higher frequency
The coaxial sample A1 was prepared by mixing the iron
fibers with wax at a volume content of 50%. The diameters
of the fibers used here are about 3 mm.
Fig. 4 indicates that the value of m’’ is as high as 3.5 at
2 GHz indicating higher magnetic loss, while the real part
of the permeability (m0) of composite A1 also reaches 3.6 at
2 GHz over the whole investigated band and it can be
inferred that they present larger values in the lower
frequency range. Meanwhile, the complex permittivity
(e0,e00) is very high in the frequency range. It is also shown
in Fig. 4 that both the complex permeability and
permittivity of composite A1 decrease with the increasing
frequency, and the magnitude of change in lower frequency
range are larger than that in the higher frequency range.
The theoretical analysis of the complex permeability and
permittivity was carried out in Ref. [21] and similar
phenomena were reported in Ref. [22], where they were
considered to be due to eddy current loss and ferromag-
netic resonance.

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Y. Nie et al. / Journal of Magnetism and Magnetic Materials 306 (2006) 125–129
128
350
160
140
120
100
80
Composite A1
Composite A1
300
250
200
150
100
50
60
40
20
0
0
2
4
6
8
10
12
14
16
18
2
4
6
8
10
12
14
16
18
(a)
(b)
Frequency (GHz)
Frequency (GHz)
4
3
2
1
0
4
3
2
1
Composite A1
Composite A1
0
2
2
4
6
8
10
12
14
16
18
4
6
8
10
12
14
16
18
(c)
(d)
Frequency (GHz)
Frequency (GHz)
Fig. 4. Complex permittivity and permeability of composites A1 versus frequency: (a) the real part of the permittivity, (b) the imaginary part of the
permittivity, (c) the real part of the permeability, and (d) the imaginary part of the permeability.
140
Composite A3
Composite A2
Composite A3
Composite A2
40
30
20
10
0
120
100
80
60
40
20
2
4
6
8
10
12 14 16
18
2
4
6
8
10 12 14 16 18
(a)
Frequency (GHz)
(b)
Frequency (GHz)
2.0
1.5
1.0
0.5
0.0
Composite A3
Composite A2
5
4
3
2
1
0
Composite A3
Composite A2
2
4
6
8
10
12
14
16
18
2
4
6
8
10
12
14
16
18
(c)
Frequency (GHz)
(d)
Frequency (GHz)
Fig. 5. Complex permittivities and permeabilities of composites A2 and A3 versus frequency: (a) the real part of the permittivity, (b) the imaginary part of
the permittivity, (c) the real part of the permeability, and (d) the imaginary part of the permeability.

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129
range, which is beneficial to the improvement of the
absorbing performance in this frequency range.
According to the theory about phosphating, the reac-
tion, as expressed in Eq. (2), takes place in the solution
during the treatment [23].
permittivity and high magnetic loss can be used to achieve
high absorption.
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pyrolysis of carbonyl under a magnetic field. They possess
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amount production. The diameter and aspect ratio of the
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