Energy and Power En gi neering, 2011, 3, 580-584
doi:10.4236/epe.2011.34072 Published Online September 2011 (http://www.SciRP.org/journal/epe)
Research Progress on Nanostructured Radar Absorbing
Materials
Yanmin Wang, Tingxi Li, Lifen Zhao, Zuwang Hu, Yijie Gu
College of Materials Science and Engineering, Shandong University of Science and Technology,
Qingdao Economic & Technical Development Zone, Qingdao, China
Received December 14, 2010; revised February 20, 2011; accepted March 7, 2011
Abstract
Nanostructured radar absorbing materials (RAMs) have received steadily growing interest because of their
fascinating properties and various applications compared with the bulk or microsized counterparts. The in-
creased surface area, number of dangling bond atoms and unsaturated co-ordination on surface lead to inter-
face polarization, multiple scatter and absorbing more microwave. In this paper, four types of nanostructured
RAMs were concisely introduced as follows: nanocrystal RAMs, core-shell nanocomposite RAMs, nano-
composite of MWCNT and inorganic materials RAMs, nanocomposite of nanostructured carbon and poly-
mer RAMs. Their microwave properties were described in detail by taking various materials as examples.
Keywords: Nanostructured, Radar Absorbing Materials, Nanocrystal, Nanocomposite
1. Introduction
Much attention has been paid to RAMs due to their
unique absorbing microwave energy and effectively re-
ducing electromagnetic backscatter so that they are ex-
pected to have promising applications in the stealth
technology of aircrafts, television image interference of
high-rise buildings, and microwave dark-room and pro-
tection [1,2]. They are specially designed material to
suppress the reflected electromagnetic energy incident on
the surface of the absorber by dissipating the magnetic
and/or electrical fields of the wave into heat. The excel-
lent RAMs should have certain properties as follows: 1)
exhibit strong microwave absorption properties over a
wide frequency range; 2) need to be thin and lightweight,
especially for aircraft; 3) have simple coating-layer
structure and spend less working time during the urgent
process. Extensive study has been carried out to develop
new microwave absorbing materials with a high mag-
netic and electric loss [3-5]. Nanostructured RAMs have
received steadily growing interest because of their fasci-
nating properties such as absorbing more microwave
compared with the bulk or microsized counterparts.
Nanostructured RAMs mainly consist of the following
four types: nanocrystal RAMs, core-shell nanocomposite
RAMs, nanocomposite of MWCNT and inorganic mate-
rials RAMs, nanocomposite of nanostructured carbon
and polymer RAMs.
2. Nanocrystal RAMs
The surface area, the number of dangling bond atoms
and the unsaturated co-ordination on surface are all in-
creased due to the particle sizes of nanocrystals in the
range of nanometer. These lead to interface polarization
and multiple scatter, which is useful to absorb more mi-
crowave. La0.8Ba0.2MnO3 nano-particles (about 80 nm)
have microwave absorbing properties both in low and
high frequency band in range of 2-18 GHz [6]. The value
of microwave absorption in low frequency band is larger
than that in high frequency. The microwave absorbing
peak is 13 dB at 6.7 GHz and the effective absorbing
bandwidth above 10 dB reaches 1.8 GHz for the sample
with the thickness of 2.6 mm. The microwave absorption
can be attributed to both the dielectric loss and the mag-
netic loss from the loss tangents of the sample, but the
former is greater than the latter. The morphology and
size of the nanocrastals play very important roles in the
microwave reflection loss (RL) of the nanocrystals. For
the nanocrystal BaFe12O19 RAMs synthesized under cy-
clic microwave irradiation [7] from 160 to 760 watts, the
degree or extent of the crystallinity of the product in-
creases and the systematic increment in RL appears dur-
ing irradiation from −4.21 to −14.45 dB and −15.20 to

Y. M. WANG ET AL.581
−53.69 dB at minimum and maximum frequencies of Ku
band respectively. Furthermore, the position of minimum
RL peak moves towards higher frequency region and the
strongest RL of −53.69 dB takes place at 14.75 GHz for
complete grown nano crystals of pyramidal face.
3. Core-Shell Nanocomposite RAMs
To overcome EMI problems, RAMs should have the
capability of absorbing unwanted electromagnetic signals
so that they should have electric and/or magnetic dipoles
which interact with the electromagnetic fields in the ra-
diation. Pure dielectric or magnetic materials are insuffi-
cient for absorbing radiation energy. The magnetic–di-
electric absorbers of core-shell nanocomposite with
suitable dielectric and magnetic properties possess high
efficiency because of the complex permittivity and per-
meability that differ from zero.
The RL of the electroless (Ni-P)/BaNi0.4Ti0.4Fe11.2O19
nanocomposite powder [8] in Ku band (12.4 - 18 GHz) is
evidently enhanced to −28.70 dB, as compared to the
electroless Ni-P nanoglobules (−16.20 dB) and nanocry-
stalline BaNi0.4Ti0.4Fe11.2O19 powder (−24.20 dB). After
annealing at 400˚C for 4 h, the RL and bandwidth of
electroless (Ni-P)/BaNi0.4Ti0.4Fe11.2O19 nanocomposite
powder is further improved from −24.20 to −35.90 dB
and 1.50 to 4.00 GHz respectively. The RL of the an-
nealed (Ni-P)/BaNi0.4Ti0.4Fe11.2O19 nanocomposites pow-
der is improved due to the better match between the di-
electric loss and magnetic loss because the combination
of amorphous electroless Ni-P and BaNi0.4Ti0.4Fe11.2O19
gives the widest bandwidth above −12 dB of 4.00 GHz.
The proposed growth mechanism on the bases of charac-
terization results indicates that the deposition onto the
nanoparticulate BaNi0.4Ti0.4Fe11.2O19 powder of elec-
troless Ni-P layer consists of amorphous electroless Ni
matrix, having Ni and Ni3P nanocrystalline particles to
form electroless (Ni-P)/BaNi0.4Ti0.4Fe11.2O19 nanocom-
posite powder.
When the MnFe2O4 is coated with the TiO2 completely,
the composites have good compatible dielectric and
magnetic properties and hence the microwave absorbing
properties show the maximum value. The representative
MnFe2O4–TiO2 nano-composites [9] exhibit super-para
magnetic behavior resulting from MnFe2O4 nanopar-
ticles and the enhanced imaginary parts of permeability
due to the eddy loss of semiconductor TiO2 nanoparticles.
The complex permittivity and permeability of MnFe2O4
and MnFe2O4–TiO2 composites measured in the micro-
wave frequency range of 2 - 10 GHz show that the mi-
crowave absorption properties of the MnFe2O4–TiO2
composites are higher than that of MnFe2O4.
Additionally, the tanδ of the cupric oxide-nanowire-
covered carbon fibers (CNWCFs) shows two peaks when
the frequency ranges are 4 - 13 and 14 - 18 GHz, respect-
tively, which indicates wide range microwave absorption
in both frequency ranges according to the microwave
absorption theory. The absorption frequency and band-
width of CNWCFs decrease with the thickness increase-
ing and the optimum thickness of CNWCFs is 1 - 1.3
mm [10]. The reflectivity of CNWCFs (1mmin thickness)
is less than −4 dB over the range of 11.8 - 18 GHz and
−10 dB over the range of 13.5 - 16 GHz, while the re-
flectivity of CNWCFs (1.3mm in thickness) is less than
−4 dB over the range of 8.6 - 15 GHz and −10 dB over
the range of 9.8 - 13 GHz.
4. Nanocomposite of MWCNT and
Inorganic Materials RAMs
Although εr״ of the purified MWCNTs is larger than 40
between 2 and 18 GHz and even exceeds 100 at lower
frequencies, the RL still remains rather small, because
another important parameter relating to RL is the concept
of matched characteristic impedance, where the charac-
teristic impedance of the absorbing material should be
made nearly equal to that of the free space (377 Ω·sq−1)
to achieve zero-reflection at the front surface of the ma-
terial [11]. Nanocomposite of MWCNT and inorganic
materials could be good candidate because the combina-
tion of both materials brings about better matched char-
acteristic impedance and improved RL.
Although the RL of purified MWCNTs keeps con-
stantly at 1 dB except for a small peak of 1.23 dB at 2.8
GHz, the RL of MWCNT filled and surface decorated
with γ-Fe2O3 is larger than 3 dB between 5 and 18 GHz
with a maximum of 5.32 dB at 7.0 GHz. When γ-Fe2O3
is transformed to Fe/Fe3C by heat-treatment in H2 at-
mosphere at 950˚C [12], the microwave absorption of the
Fe/Fe3C–MWCNT composite is enhanced greatly at all
frequencies between 2 and 18 GHz so that the RL is lar-
ger than 3 dB between 4.6 and 18 GHz with a maximum
of 12.57 dB locating at 9.2 GHz. This result from en-
hanced magnetic loss and better matched characteristic
impedance, rather than electric loss, as shown by the
complex relative permeability and permittivity. The fre-
quency corresponding to the maximum RL of Fe/Fe3C–
MWCNT composite shows an inverse relationship with
the increase in its thickness and the maximum attenua-
tion of the incident wave is increased from 11.00 (12.40
GHz) to 13.88 (4.60 GHz) when the thickness is in-
creased from 1.5 to 3.5mm. The microwave absorbing
properties can be modulated simply by manipulating the
thickness of the prepared Fe/Fe3C–MWCNT composite
for application in different frequency bands.
The Er2O3 nanoparticles encapsulated in the cavities of

Y. M. WANG ET AL. 582
MWCNTs could also modulate the electromagnetic pa-
rameters of MWCNTs, and thus affect the microwave
absorbing properties. The Er2O3-filled MWCNTs possess
much broader absorbing bandwidth and larger reflective-
ity, complex permeability and magnetic loss tangent than
unfilled MWCNTs [13]. The maximum absorbing peak
of raw MWCNTs is about −21.58 dB at 9.4 GHz in the
range of X wave band with the thickness (dm) of 2.0 mm,
the bandwidth of the reflectivity below −5 dB is 3.50
GHz and the bandwidth of the reflectivity below −10 dB
is 1.58 GHz. In contrast, under the same matching thick-
ness (dm = 2.0 mm), the maximum absorbing peak of the
Er2O3-filled MWCNTs increases to −27.96 dB and shifts
to 10.0 GHz, the bandwidth of the reflectivity below −5
dB is 4.65 GHz and the bandwidth of the reflectivity
below −10 dB is 2.30 GHz. With the increase of thick-
ness, the peak value of reflectivity shifts to lower fre-
quencies and multiple absorbing peaks appear.
There are three reasons for the improved absorbing
performance of Er2O3 modified MWCNTs as follows:
firstly, the specific location of the Er ion in MWCNTs
could generate a charge effect [14] and RE oxide located
in MWCNTs cavities could change the microenviron-
ment of the resonators MWCNTs; secondly, the energy
levels of the nanosized Er2O3 crystals encapsulated in
one-dimensional MWCNTs are not continuous but dis-
crete because of quantum confinement effect according
to Kubo theory; finally, the Er3+ ion has the [Xe] 4f con-
figuration, the 5d shell is empty and there are three un-
paired 4f electrons interacting with the crystalline envi-
ronment. The electron magnetic moment may cause a
large magnetic loss in the composite.
5. Nanocomposite of Nanostructured Carbon
and Polymer RAMs
Carbon nanotubes (CNTs) as conductive filler have been
widely studied in RAMs due to their physical and
chemical properties such as light weight and strong mi-
crowave absorption properties in the GHz frequency
range [15]. For CNTs/polymer (PET, PP, PE and varnish)
nanocomposites [16], the position of reflectivity peak
moves to a lower frequency and the loss factors of com-
posites increase with increasing CNTs concentrations. At
the CNTs concentration of above 4 wt%, loss tangent of
the composites sharply increases because this behavior
corresponds to a phase transition from an insulator to a
conducting composite at this concentration and a drastic
change in the electrical resistivity with a corresponding
change in the behavior of its electromagnetic characteris-
tics. Polymer matrix has an obvious effect on microwave
absorbing properties. CNTs/PET composite achieves a
maximum absorbing value of 17.61 dB and RL of over 5
dB between 5 GHz and 18 GHz, while the maximum
absorbing value of CNTs/varnish composite is 24.27 dB.
However, CNTs/PE composite has a maximum absorb-
ing value of 8.01 dB when CNTs concentration is up to 8
wt%. In addition, the frequencies range for absorbing
values exceeding 5 dB of CNTs/(PET, PP, varnish) com-
posites are 13 GHz, 10 GHz and 6 GHz, respectively.
The microwave absorption of CNTs composites can be
mainly attributed to the dielectric loss rather than mag-
netic loss because the value of dielectric loss is much
higher than that of magnetic loss especially in frequent-
cies ranging from 6 GHz to 18 GHz.
Compared to the raw GFR (Glass Fiber–Reinforced)
composites (G8, G16) with low absorbing efficiency, the
GFR nanocomposites consisting of glass fiber, epoxy
and nanosized carbon materials such as CB (carbon
black) 16, MWCNT8 and MWCNT16 show outstanding
absorbing efficiency over 10 dB between 8 and 12 GHz
frequency range (as shown in Figure 1). Especially, the
GFR–MWCNT composite with 16 plies (MWCNT16)
provides three times higher efficiency than those of CB
composites (CB16) [17].
When the onion-like carbon (OLC) is dispersed in dif-
ferent polymer matrix such as PMMA and epoxy [18],
the EM reflection provided by OLC is higher when em-
bedded into PMMA host matrix because the higher ad-
hesion of PMMA and OLC agglomerates provide forma-
tion of more homogeneous distribution of OLC within
polymer matrix. OLC content essentially influences the
EM absorption for OLC/PMMA films: the EM attenua-
tion increases as much as 2.5 times with the increasing of
OLC concentration from 2 to 20 wt% for whole fre-
quency range. More efficient shielding properties have
been observed for OLC aggregates of the smallest sizes
as compared to larger-sized aggregates at the same OLC
loading, as is attributed to the better dispersion and for-
mation of a continuous conductive network by smaller
aggregates (reaching the significantly lower percolation
threshold) [19].
Figure 1. Comparison of radar absorbing efficiency for raw
GFR and GFR nanocomposite.

Y. M. WANG ET AL.583
For E-glass/epoxy composite laminates containing
three different types of carbon nano materials such as
carbon black (CB), carbon nanofiber (CNF) and multi-
wall carbon nanotube (MWNT) [20], the real and imagi-
nary parts of the complex permittivities of the compos-
ites are proportional to the filler concentrations. De-
pending on the types of fillers and frequency band, the
increasing rates of the real and imaginary parts with re-
spect to the filler concentrations are all different with the
order of CNF > MWNT » CB. These different rates can
have great effect on the thickness in designing the sin-
gle-layer microwave absorbers and the order of thickness
of composite materials at their optimums is CNF <
MWNT « CB. The excellence of CNF originates from its
high conductivity and straightness offering the possibil-
ity of big electric dipoles and inducing higher dielectric
constant of composites.
The mixed type absorbers employing two fillers for
both dielectric and magnetic characteristics are possible
candidate materials for overcoming the narrow absorp-
tion of dielectric RAMs and heavy weight of magnetic
RAMs. The mixed RAMs containing carbon nanofibers
(CNFs) as dielectric lossy materials to increase permit-
tivity and NiFe particles as magnetic lossy materials [21]
show improved absorbing characteristics with thinner
matching thicknesses. The present mixed RAMs show
the 10 dB absorbing bandwidth of 4.0 GHz in the X-band
(2.00 mm thickness) and 6.0 GHz in the Ku-band (1.49
mm thickness).
6. Conclusions
Nanostructured RAMs possess enhanced absorbing
property due to the nanometer size. The morphology and
size of the nanocrastal RAMs play very important roles
in the microwave RL of the nanocrystals and, as the de-
gree or extent of the crystallinity increases, the system-
atic increment in RL appears. The magnetic-dielectric
absorbers of core-shell nanocomposite RAMs with suit-
able dielectric and magnetic properties possess high effi-
ciency because of the complex permittivity and perme-
ability that differ from zero. Nanocomposite of MWCNT
and inorganic materials RAMs combines better matched
characteristic impedance and improved reflection loss of
both materials. For nanocomposite of CNTs and polymer
RAMs, the position of reflectivity peak moves to a lower
frequency and the loss factors of composites increase
with increasing CNTs concentrations. More efficient
shielding properties had been observed for OLC aggre-
gates of the smallest sizes as compared to larger-sized
aggregates at the same OLC loading. The increasing
rates of the real and imaginary parts for nanocomposite
of nanostuctured carbon and polymer RAMs with respect
to the filler concentrations depend on the types of fillers
and frequency band.
7. Acknowledgements
The authors wish to thank SRF for ROCS, SEM for fi-
nancial support for this work.
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