Radar Absorbing Materials: Furthering
the Advancement of Stealth Technology
Francesco D. Marino, ME Student Northeastern University
Abstract:
This paper will discuss the different types of
cutting edge materials being designed as radar
absorbing materials (RAM). It will define the type of
RAM and its general performance. I will discuss the
plausible use of each material and provide details on
what aspects must be improved or further researched.
Overall a general path for future RAM will be
proposed and discussed.
Keywords: Radar absorbing material (RAM),
Radar absorbing structure (RAS), Reflection Loss
(RL)
Introduction:
Aircraft today are becoming faster and more
agile each and every year. Nations however are
designing methods to counter these newer, better
aircraft at the same rate. The way the military is
trying to beat out the counter measures built by the
enemies, such as surface to air missiles sites, anti-air
guns and air-to-air missiles, are by not letting them
see the aircraft at all. In today’s fast paced modern
world by the time you see an aircraft approaching it
is most likely too late to implement a defense or
escape; so aircraft defense systems rely of extremely
sensitive radar systems. Scientists today are trying to
develop ways to let aircraft slip through the radar net
undetected. The way radar works is a source emits a
wave at a known frequency as it travels out it reflects
off of things and bounces back to the receiver. By
knowing the frequency and velocity of the wave you
can locate the position and speed of the object
reflecting the wave. By designing a material that will
absorb those waves it will not be recognized by the
receiver, therefore giving the appearance that there is
nothing there. The image shows how when a radar
ream hits a RAM it reflects the wave internally and
does not let the wave escape.
How a RAM works
There are many different types of materials
that absorb wide ranges of frequencies however some
are not practical to use such as lead. The design of a
versatile, durable, high absorption material is greatly
needed to advance todays aircraft past the treats of
anti-aircraft systems. The standardized measuring
technique is by reflection loss. 10 dB absorbing
bandwidth means that the frequency bandwidth can
achieve 90% of reflection loss, whereas a 20 dB
absorbing bandwidth means that the frequency
bandwidth can achieve 99% of reflection loss.
RAM Types:
1 Polymer RAM
1.1 Carbonyl Iron/Rubber
Carbonyl iron can be used as microwave
absorption filler in the frequency range of 2.6–18
GHz and even at higher frequencies because it has
good temperature stabilization and high value of
microwave permeability and dielectric constant
which makes it a very good RAM. EthylenePropylene-Diene Monomer (EPDM) has excellent
durability, chemical resistance and, makes a good
insulator. It also is soluble with many types of
fillers.[1] This makes the two materials a good pair.
By doping the EPDM with carbonyl iron a strong
RAM is created.
Fig. 1) Effects of carbonyl iron volume fraction on the reflection
loss of the RAM (d = 3:0 mm). [1]
1.2 Polyaniline/rubber
The goal of RAM materials is to create a
substance that yields minimal scattering of waves
when irradiated by a power source. Acceptable
results were attained using the conducting polymer
polyaniline (PAni). The polymer compound was
produced using blends of EPDM (ethylenepropylenediene rubber) and the PAni. They were prepared in an
internal mixer for different processing time. Flat
sheets of 3mm thick and 15cm x 15cm dimension
were obtained by compression molding at 100°C.[2]
Four different samples were produced shown in
Table 1.
Out of the four RAMs produced only #2 and
#3 had a RL less then -10dB. They both provide a
sufficient RL across an equal frequency range.
However the performance of RAM #2 slightly
surpasses that of RAM #3. The use of this material
would not be too bad as a coating for the leading
edge of wing tips or even the entire wing and/or
fuselage.
Table 1) Parameters used in the RAM preparation[2]
Fig. 2) Affects of thickness of the RAM on the
microwave absorption properties. (a) Vf = 0.3 (b) Vf =
0.45. [1]
The reflection loss by the material is measured. The
volume fraction of the iron is varied along with the
thickness of the test sample. In figure 1 the sample
thickness is held constant at 3mm and the volume
fraction is varied from 0 to .45. In figure 2 the
volume fraction is held constant and the thickness of
the sample is varied. From these graphs you can see
that its effectiveness greatly depends on its
composition and the thickness of the sample. Only a
few of the tests resulted in a RL of 10dB over the Xband frequencies.
Fig. 3) RAM #1 reflectivity on X-Band [2]
2 Carbon Fiber RAM
Carbon fiber is one of the most widely used
materials in aviation due to its light weight and high
strength however, it happens to be a fantastic radar
reflector which is bad for its stealth capabilities. [3]
In order to masks the signature of the carbon fiber
components a few methods are being developed.
2.1 Nickel coated CF
Fig. 4) RAM #2 reflectivity on X-Band [2]
Fan et al.’s approach to this problem is to coat the
carbon fibers in a thin nickel film. They used
electroless plating in order to adhere the nickel to the
carbon fibers (CF). The nickel carbon fibers (NCF)
have a uniform and continuous coating with a
thickness of ca. 1.2 μm (Figure 7b). The nickel
carbon fibers are then tested for their reduction loss
across a range of frequencies by mixing the NCF into
cement with a 15% and 30% weight ratio. [4]
Fig. 5) RAM #3 reflectivity on X-Band [2]
Fig. 7) The surface morphology of (a) CF; (b) NCF [4]
The results of this test show the reflection loss is
good with a 30% weight ratio to the phenol–
formaldehyde cement above 13 GHz (Figure 8). This
material would perform well in aircraft if the NCF
could be integrated into other materials of the
aircraft. Phenol-formaldehyde cement would not be
on any use in an aircraft so the NCF should be tested
in other substrates to validate its plausible use.
Fig. 6) RAM #4 reflectivity on X-Band [2]
After dried in air, they were annealed at 350
and 400°C for 4 h in air to achieve the CNWCFs.
350°C produced very few nanowires however 400°C
produced very abundant long nanowires as shown in
figure 9. The RL was tested for samples of varying
thickness across a range of frequencies. (Figure 10)
Fig. 8) Reflection loss of the NCF *Disregard FG and NFG*[4]
2.2 Cupric oxide-nanowire (CNW) coated CF
In previous reports, cupric oxide nanowires
(CNWs) showed remarkable optical, electrical,
magnetic, and mechanical properties [5]. Jun et al.
synthesized the CNWCF by annealing copper coated
CFs in air. The Cu/CFs were created by electroless
plating.
Fig. 9) The low and high magnification FESEM images of CNWCFs
synthesized at different temperatures: (a) and (b) at 350 °C; (c)
and (d) at 400°C. [5]
Table 2) [5]
The carbon fibers were cut to 2–3 mm in
length before the surface treatment. The treatment
involves three steps:
(1) The carbon fibers were immersed in
nitric acid for 5 h and then washed with distilled
water.
(2) The carbon fibers were treated with
stannous chloride and ammoniacal silver solution.
(3) They were coated by electroless plating
at room temperature for 3 h.
Table 1 lists the electroless copper bath
conditions. SEM images show that the CFs were
completely coated with a thin copper film of 2μm. [5]
Fig. 10) Reflectivity curves of different thickness of CNWCFs. [5]
The microwave absorption mechanism of CNWCFs
has rarely been reported in the previous literature.
The results from this experiment offer a promising
future for CNWCF use as RAM. Since the thinner
samples induced reduction in the X-band and higher
it will prove useful for thin film coatings of aircraft
and wind turbines. Further advancements must be
done in order to achieve a higher reflectivity of the
thinner samples on the same scope as the think
samples.
3 Composite RAM
The use of ferrite materials for RAM
applications is the standard approach. However the
weakness of ferrite materials is that they lose their
effectiveness over large frequency ranges. [6] Also
the addition of ferrites increases the weight
drastically; composite RAMs can be engineered to
cover large frequency ranges and are exponentially
lighter.
3.1 Composite Carbon nanotube RAS
Zhang et al. integrated carbon nanotubes into the
composite carbon fiber structure so it would be able
to absorb radar while still maintaining its structural
integrity and not sacrificing its light- weight [7]. The
CNTs were dispersed in acetone solution in an
ultrasonic installation for 2h. After, the epoxy resin
had been compounded with the CNT in a self-made
die. Then, the glass/carbon fibers/epoxy composites
were prepared in a planar rectangular mold. Curing at
room temperature for 2h, RAS specimens which
composed with glass fibers, carbon fibers, epoxy
resin and CNT, were produced, while the pressure
was stabilized at 6atm. [7] Four different test samples
were created each with a different weight percentage
of CNT and one control sample (Table 3).
Table 3) Weight percentages of CNT to epoxy Composite
denotations
Fig. 11) Reflection loss of four kinds of double-layer RAS: A
curve, CNT1.2, 5.0mm,CNT0,4.0mm; B
curve,CNT1.2,4mm,CNT0,3mm; C curve,
CNT1.6,5mm,CNT0,3mm; D curve, CNT1.6,4 mm, CNT 0, 2mm
Different double layer configurations were
produced and tested shown in the above figure 11.
The materials produced excellent RL across a broad
range of frequencies; however these test samples
were all at least 6mm in thickness which is way too
thick to use as a practical coating a 6mm coating over
the entire aircraft would add hundreds of pounds.
The beauty about CF composites is that they are
extremely strong and can be used for structural
support. So these materials could be integrated into
parts of the aircraft that are already made from epoxy
composites and even perhaps make the skin of the
aircraft out of these composites. The only thing
needed to be done is to figure out how to synthesize
components with this material while not jeopardizing
structural integrity. A stealth plane doesn’t do any
good if its wings fall off when it takes flight.
3.2 Carbon Black composite
With weight being a huge concern Oh et al.
strived to create a RAS using a blend of lightweight
carbon black and a binder matrix of fiber reinforced
composite. The goal of their research was to
minimize thickness of the material while maximizing
the absorption throughout the X-band frequency.
The specimens were created by mixing the
epoxy matrix with carbon black and then uniformly
applying it to the glass fabric. To cure the specimen it
is baked in a autoclave for 30 min at 80° and then for
90 min at 130° while maintaining the pressure at 3
atm. [8] Multiple samples were created using
different weight percentages of carbon black. The
following table shows the denotations.
The test is conducted three times. Once without the
DZR to provide a control; the second test with nondispersive DZR, and the third with dispersive DZR.
Table 4) Specimen denotations of glass fabric/epoxy composites
[8]
Due to the high viscosity of the CB15 and
CB20 samples the epoxy matrix was able to be
uniformly applied to the glass fabric; so the test
results were inconclusive. The CB10 sample reflected
more than 45% of the incident waves at the surface
so this material would not serve as a suitable RAS.
The absorbing bandwidths of the CB5-CB8 change
with respect to the thickness. According to Oh et al.
the CB5 exhibits excellent characteristics of
reflection loss near 12GHz; however it has the
weakness of having a large total matching thickness
of 8mm. CB6 shows excellent characteristics at
8GHz and a matching thickness of 2.7mm. CB7 and
CB8 exhibited low reflection loss across all
frequencies. [8] As seen from the results, it is very
important to manufacture the RAS materials with the
right amount of additive along with a precise uniform
thickness.
Fig. 12) Reflected power from a PEC plane versus frequency (a)
without coating (b) with two layers of non-dispersive DZR
metamaterial coatings (c) with two layers of dispersive DZR
metamaterials. [10]
From figure 12 you can see that both the
DZR coatings produced a huge loss of reflected
power with the non-dispersive DZR coating having a
reflective power of 0 across the entire X-band.
Oraizi et al. also wanted to see the
performance of the metamaterial with a varying
incidence angle. The sample material had the
following parameters coating a PEC plate.
4 Double Zero (DZR) Metamaterials
A double zero Metamaterial is an artificially created
material engineered to have properties not found in
nature. A DZR metamaterial have the real parts of
both the permittivity and permeability equal to zero;
Re(ε)=0 and Re(μ)=0.[9] Oraizi at el. realized that
this material could be used for a number of different
applications, one of them being the use as a RAM.
They fabricated a two layer DZR to coat a perfectly
electric conductor (PEC) plate with and test at an
incident angle of 45° through the X-band range of
frequencies.[10]
The sample plate was rotated through incidence
angles from 0 to 45°. Figure 13 shows the test results
in comparison to a non-coated PEC plate. It is
observed that both the non-dispersive DZR coating
and dispersive coating have a good performance
throughout the frequency bandwidth and the
changing angle of incidence. If you look at the
parameters notice that the thickness of the material is
less than 2 mm. This is very good performance for
the thickness of the material.
performance throughout varying angles of incidence
enables it to works on all the surfaces of the aircraft.
Secondly the use of RAS in aircraft is an
important option to work towards for the future. By
making the internal components of the plane radar
absorbent it will further the stealth capabilities of the
plane as a whole. The important aspect to work on
would be the manufacturing process with these RAS
materials and increasing the strength of the RAS. If
all the major components of the aircraft could be
replaced with equally strong and light RAS materials
then there would be no negatives to adding stealth
capabilities to the aircraft.
Fig. 13) 3-D diagram of reflected power from a PEC plane versus
frequency and angle of incidence under oblique incidence for
three cases: (a) without coating. (b) With two layers of nondispersive DZR metamaterial coatings (c) With two layers of
dispersive DZR metamaterial coatings. [10]
Conclusion:
Even with all this data it is tough to say that
an of the materials discussed are ready for
application. A lot of work must be still done to
improve these materials. The goal should be to make
the materials thinner and more effective across a
wider frequency. Secondly each one of these test
were performed in different manners under different
test conditions. In order to receive a clearer palate if
information each of the materials should be tested in
the same machine under the same conditions. This
will enable to data not to be skewed by different
testing operations. The best solution would be to
standardize the test to measure the RAM’s
effectiveness. This will enable scientists all over the
world to perform their testing on their own and be
able to easily compare it to other results worldwide.
Judging from these results I feel that the
DZR RAM shows the most promising results. With
having such a good stable RL across such wide range
of frequencies as well across a wide range of incident
angles. The test was performed with a sample
thickness of only 2mm. This is outstanding
performance for such a small thickness. This material
seems like it could be used as a suitable coating for
the entire aircraft because it performs so well with a
thickness of only 2mm. Also its ability to maintain it
Overall a lot more work needs to be done to
improve thin coating RAM and RAS before they can
be applied to aircraft. The use of a standardized test
must be implemented and equal testing must be
performed in order to decide on the best material and
parameters to implement in aircraft for today and the
future.
References
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[9] Oraizi, Homayoon, A. Abdolali, and N.
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[10] Oraizi, H., A. Abdolali, and N. Vaseghi.
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