Journal of Magnetism and Magnetic Materials

(1)

Current Perspectives

Recent developments of smart electromagnetic absorbers based polymer-composites at gigahertz frequencies

Fadzidah Mohd. Idris

^a,n

, Mansor Hashim

^a

, Zulki ﬂ y Abbas

^b

, Ismayadi Ismail

^a

, Rodziah Nazlan

^a

, Idza Riati Ibrahim

^a

aInstitute of Advanced Technology, Universiti Putra Malaysia, Malaysia

bDepartment of Physics, Universiti Putra Malaysia, Malaysia

a r t i c l e i n f o

Article history:

Received 12 May 2015 Received in revised form 4 December 2015 Accepted 21 December 2015 Available online 22 December 2015 Keywords:

Radar absorbing materials High gigahertz frequencies Absorption mechanism Reviews

a b s t r a c t

The rapid increase in electromagnetic interference has received a serious attention from researchers who responded by producing a variety of radar absorbing materials especially at high gigahertz frequencies.

Ongoing investigation is being carried out in order tofind the best absorbing materials which can fulfill the requirements for smart absorbing materials which are lightweight, broad bandwidth absorption, stronger absorption etc. Thus, to improve the absorbing capability, several important parameters need to be taken into consideration such asfiller type, loading level, type of polymer matrix, physical thickness, grain sizes, layers and bandwidth. Therefore, this article introduces the electromagnetic wave absorption mechanisms and then reveals and reviews those parameters that enhance the absorption performance.

1. Introduction

In this new era of technology, many new devices involving high proliferations of electronic instruments especially in the tele- communication area are being developed in order to fulﬁll human needs and provide various facilities. These devices are being designed to function at higher and higher frequency (gigahertz).

Thus, the high frequency electromagnetic wave is drawing more attention due to the explosive growth in the utilization of tele- communication devices in industrial, medical and military applications [1]. Many devices being developed such as AC motors, digital computers, calculators, printers, modems, electromagnetic type writers, digital circuitry and cellular phones are capable of creating electromagnetic interference. Rapid development of gigahertz application devices may cause severe interruption to those applications and results in serious electromagnetic (EM) interference pollution. Furthermore, there are ongoing con- troversies worldwide over the potential health hazards to the human body associated with exposure to electromagneticﬁelds.

The capability to control the problems created by EMI either by eliminating or reducing the spurious electromagnetic radiation levels in different applications is in high demand. Thus, electromagnetic wave absorbers with the capability of absorbing

unwanted electromagnetic signals were investigated and these research and development efforts to produce radar absorbing materials (RAM) have increased. In addition, many extensive studies on microwave absorption properties of various materials have been carried out to look for the radar absorbing materials with high absorptive ability and wide band absorption. In fact, much attention has been paid to RAMs due to their unique microwave- energy absorption capability with effective reduction of electromagnetic backscatter so that they are expected to have promising applications in the stealth technology of aircrafts, television image interference of high-rise buildings, and microwave dark-room and protection[2,3]. These problems have stimulated research interest in microwave absorbing materials with strong absorption, wide- frequency band, small speciﬁc gravity and thin thickness [4–6].

Microwave absorbers with broadband and thin thickness have been developed to eliminate EMI [7–14]. The most current research focuses on the electromagnetic properties in the range of 8–18 GHz [15–17] and 18–40 GHz [65,66]. However, research on their electromagnetic and absorption properties is still being carried out.

2. Factors inﬂuence the electromagnetic wave absorption From the earliest development of absorbers, the composite materials were very signiﬁcant[18–20]. It is not surprisingly since composite materials allow convenient use towards application especially on surfaces, good control over mechanical properties Contents lists available atScienceDirect

journal homepage:www.elsevier.com/locate/jmmm

Journal of Magnetism and Magnetic Materials

nCorresponding author.

E-mail addresses:[email protected](F.Mohd. Idris), [email protected](M. Hashim).

Journal of Magnetism and Magnetic Materials 405 (2016) 197–208

(2)

and variation of electromagnetic properties. It can be achieved with proper selection of matrix material and different inclusions:

dielectric, conductive, or ferromagnetic. In addition, electromagnetic wave absorption characteristics of a material depend on its dielectric properties (through the permittivity,

ε

) and magnetic properties (through the permeability,

μ

⁾^[21–24]. By incorporation of dielectric and magneticﬁllers, the electromagnetic properties of such materials can be improved to obtain a maximum absorption of electromagnetic energy [25]. These composite materials are useful as microwave absorbers due to their advantages with re- spect to lighter weight, lower cost, designﬂexibility and adjustable microwave properties over intrinsic ferrites. Ferrite and ferrite composites backed with a conducting plate are usually used to achieve higher absorption[2,26,27].

In producing microwave absorbing materials, several parameters need to be taken into consideration in order to obtain a smart absorbing material. The parameters are weight, thickness, filler content, types offiller, loading level, particle distributions, size and shape, intrinsic conductivity of filler, frequency range, environmental resistance and mechanical strength[28,29]. With advances in nanotechnology, nano-microwave absorbers have played an important role in developing new microwave absorption materials with the properties of strong absorption, wide frequency range, low density and thin thickness. These are in good agree- ment with advanced applications whereby small thickness, wide band absorptions, light weight, high strength and high absorbing property are used to appraise the absorbing performance[30,31].

Therefore, the combination of polymers and nanomaterials is able to integrate the large electric/magnetic loss of inorganic materials and the easy tenability of polymer and is possibly an optimal strategy to design excellent EM wave absorption materials[32].

Several studies have been carried out to investigate the influ- ences of magnetic fillers and their volume percentages, particle sizes, and shapes in composite materials on the absorption of electromagnetic waves to produce absorbers which are thin, flexible, and have broad absorption ranges[33–37].

3. Developments of ferrite wave absorbers

Many reports have been given on the historical development of the science and technology of ferrites that has occurred since 1940 [38,39]. A ferrite wave absorber[40–43]has received considerable worldwide attention in response to an increasing demand of a counterplan for electromagnetic compatibility (EMC). This ferrite absorber can be expected to decay, for example, television ghosting, forged echoes in ships' radar signals, and electromagnetic wave leakage in various electric equipments. The typical electromagnetic wave absorber is a ferrite plate backed with a conductive metal plate. Various types that depart from this structure have been developed, as shown inTable 1 [44].

To date, considerable efforts have been made to design various materials to reach the ideal targets and various materials have been applied to EM wave absorption. As an ideal EM wave absorber, it should possess light weight, high EM wave absorption, broad width, tunable absorption frequency, and multi-functionality[45–47]. Moreover, nowadays, research interest is focusing on and towards higher gigahertz frequency, nano-particles, ferrite incorporated into polymers, over a single layer, wide bandwidth and to achieve higher and higher absorption.

4. Magnetic loss and dielectric loss mechanism

Electromagnetic radiations which have both dielectric and magnetic components are effective in order to absorb the

microwave radiation. According to Knott et al. and Vinoy and Jha [22,48], the designation of microwave absorbing materials typically consist of shaped materials with properties that allow electromagnetic waves to penetrate into regions where the electric and magneticﬁelds experience loss. The main factors dominating the microwave absorption properties of a material are the complex permittivity (ε* = ′ −ε jε) and complex permeability (μ* = ′ −μ jμ") other than electromagnetic impedance match[49]. The real parts (ε μ′, ′) of complex permittivity and permeability symbolize the storage capability of electric and magnetic energy, whereas, the imaginary parts(ε", "μ)represent the loss of electric and magnetic energy. Furthermore, there are two possible contributions for microwave absorption, namely dielectric loss (tanδε=

ε ε′

"

) and magnetic loss ( δμ=

μ μ′

tan ^"). As a microwave absorber, big imagin-

ary parts of complex permittivity and permeability are expected which enables great absorption of the incident radiation. The common features of the microwave behavior of magnetic and dielectric materials are precession, resonance and relaxation. For magnetic loss, the resonance phenomenon can happen when the frequency of the microwavefield applied at the right angles to the staticfield is the same as the precession frequency. The energy absorbed efficiently from the microwavefield leads to a magnetisation which precesses with a larger angle as inFig. 1(a). How- ever, if the microwavefield is then removed, the magnetic moment will spiral in as the precessional energy is dissipated until it is parallel with the staticfield as in Fig. 1(b). Absorption of an electromagnetic wave by magnetic materials is also considered to occurring via relaxation induced by the domain wall movement or alternatively by a natural spin resonance mechanism. The natural spin resonance depends highly on the magnetic anisotropy and thus on the structure and shape of magnetic particles. As for dielectric materials, when microwaves penetrate and propagate through the material, translational motions of free or bound charges such as electrons or ions are induced by the internalfield generated within the affected volume, thus, it rotates the dipoles.

Inertial, elastic and frictional forces resist these induced motions and this causes energy losses. The electrical energy is dissipated as heat due to the resistance of the material. Additional loss can also occur via molecular polarization phenomena, such as dipole ro- tation, space charge relaxation and hopping of conﬁned charges [50].Thus, great absorption may be obtained and attributed to the signal being sufﬁciently attenuated and dissipated as heat once the radiation in synchronized frequencies has entered the material [51].

Table 1

Typical applications of ferrite absorbers (After Kotsuka[44])

Example of absorber Type of absorber

Anechoic chamber Two-layer absorber consisting of carbon and ferrites.

Ferrite single-layered absorber.

Multilayered absorber consisting of resistiveﬁlm and ferrite (ﬂooring in anechoic chamber).

Wall panel of building (countermeasure for TV ghosting)

Absorber consisting of concrete (stone) panel and ferrite plate.

Counter absorber containing ferrite particles.

Countermeasures for suppressing un- necessary echo in ship's radar signals

Single rubber–ferrite absorber Absorber of thick paint plate.

Two-layered absorber consisting of metallicﬁber and rubber–ferrite.

Countermeasure for leakage wave from electric devices.

Rubber–ferrite absorber.

Plastic–ferrite absorber F.Mohd. Idris et al. / Journal of Magnetism and Magnetic Materials 405 (2016) 197–208

198

(3)

5. Quarter wave principle and skin depth effect

Absorption of an electromagnetic wave depends on the ability of materials to attenuate or absorb the electromagnetic radiation inside the materials. The electromagnetic attenuation offered by absorbing materials may depend on the three mechanisms which are reﬂection of the incoming wave, absorption of the wave as it passes through the material's thickness, and multiple reﬂections of the waves at various interfaces as shown inFig. 2.

The skin depth can be understood by a distance that evaluates the ability of the electromagnetic ﬁeld to propagate within a material. A material has zero skin depth when it is a perfect electric conductor. While for good conductor like copper at microwave frequencies (10 GHz), the skin depth is of the order of 0.65

μ

m. Thus, the lower the microwave electrical conductivity, the higher is the ability of the electromagneticﬁeld to propagate across the material. Skin depth depends on the frequency of the applied wave where it varies with the frequency. Thus, the frequency, electrical conductivity, magnetic permeability and skin depth can be related with the following equation:

δ

π μ μ σ

= f

1

r 0

whereμ₀is the permeability of free space (4

π

10⁷H/m) and μr

is the relative permeability (a dimensionless parameter) of the

material. As for the electromagneticﬁeld which propagates within the material, it can be expressed using the equation of propagating wavelength in the material,

λ

, as follows:

λ λ

= ε μ⁰

where

λ

0 is the wavelength in free space and ε and μ are the modulus of ε and μrespectively. The maximum reﬂection loss is associated with a quarter wavelength (0.25

λ

) thickness of the material. However, calculations show a variation between 0.25

λ

and 0.3

λ

which depends on the magnitude of ε and μ. According to Micheli et al.[52], the peak may shift to up to 0.5

λ

for the case of addition of a magnetic component where μ₄ε.

5.1. Filler type

The type offiller incorporated into a polymer matrix plays an important role in order to get better and higher absorption. By incorporating dielectric and magneticfillers, the electromagnetic properties of such materials can be improved to obtain a maximum absorption of electromagnetic energy [25]. Ferrites are considered to be the best magnetic material for electromagnetic wave absorbers due to their excellent magnetic and dielectric properties, but they are expensive and heavy. On the other hand, the use of polymers to protect the electronic devices from EMI is popular due to the light weight,flexibility and cost effectiveness.

In order to effectively suppress EMI, ferrite materials are incorporated into polymer matrices. Thus, research has been carried out by preparing ferrite-polymer composites to investigate the effects of ferrite materials and their volume fractions on the microwave absorbing properties [53]. Ferrites play the role as the absorption center of the incident radiation which favors energy and heat loss while the polymer acts as matrix. Thus, radar absorbing material technology includes the combination of materials in such way that the reflection loss be as small as possible in a wide frequency range by associating polymeric substrates loaded with microwave absorber centers, as microwave ferrites and / or conducting polymers and / or carbon / iron particles, this condition is usually satisfied. They are called hybrid absorbers when the magnetic and electric fields attenuate simultaneously[54,55]. In addition, materials such as Cu–Zn, Mn–Zn, Ni–Zn, and other substituted soft ferrites are being used for ferrite wave absorbers. For example, many works have been carried out on polymer-based composites filled with magnetic materials in micrometer-size, such as Ba-ferrite[56], iron-fibre[57], NiZn-ferrite[59]and Fe3O4/

YIG[59].

Furthermore, there are various ferrite composite materials for EMI absorption as follows:

1. Yusoff et al.[60]reported the effects of absorption characteristics by incorporating a ferrite into a thermoplastic natural rubber (TPNR) matrix.

2. Dosoudil et al. [61]reported ferrite polymer composites with Ni0.33Zn0.67Fe2O4 and several composite materials based on NiZn sintered ferrite and a polyvinylchloride (PVC) polymer as the nonmagnetic matrix.

3. Abbas et al.[62]studied the microwave absorption properties of ferrite-polymer composite whereby the M-type hexaferrite was incorporated into commercial polyurethane as the matrix.

4. Zheng et al.[63]reported the microwave magnetic properties of the composites with resin composites containing 20 vol% Fe3O4

nanoparticles.

5. Kong et al. [64] reported the magnetic, electromagnetic and microwave absorbing properties of polymer-based nanocomposites consisting of thermoplastic natural rubber (TPNR) as the matrix and Fe3O4nanoparticles as theﬁllers.

Fig. 1.Precessional motion of magnetisation; (a) precession maintained by a mi- crowaveﬁeld and (b) the moment spiralling into line withHas the precessional energy is dissipated.

Fig. 2.Graphical representation of multiple internal reﬂections.

F.Mohd. Idris et al. / Journal of Magnetism and Magnetic Materials 405 (2016) 197–208 199

(4)

Table 2

Types of polymer matrix,ﬁller,ﬁller's particle size and its absorbing properties from past to present.

YEAR Types of polymer (use as

matrix)

Filler Reﬂection loss (dB), thickness, working frequency (GHz) Ref.

1998 Natural rubber 2 wt% PPy powder Paints with PPy powder : RL¼ 22 dB. [74]

Mixture of PPy powder (2 wt%) and carbonyl iron (ca. 25% by volume) :t¼2.5 mm RL¼ 25 dB

Working frequency : 12–18 GHz

2002 TPNR 30 wt% Li–NiZn ferrite Pure ferrite experiment (37 dB, 5.09 mm) [60]

ferrite (computer simulation) –(21.5 dB, 1.3 GHz, 5 mm) –(38 dB, 12.7 GHz, 2.5 mm) TPNR ferrite (computer simulation) –(26 dB, 4.1 GHz, 30 mm) –(21.5 dB, 9 GHz, 15 mm)

2004 Polyester 7 wt% ZnO 7 wt% ZnO nanowire/polyester showed maximum reﬂection loss

of 12.28 dB at 10.9 GHz.

[143]

2006 Polyester 8 wt% Carbon nanotubes absorbing level reach 8 dB for thickness 1.4 mm measured from

8 to 40 GHz.

[144]

2007 Ethylene–propylene–diene monomer (EPDM 4045)

50 vol% of Carbonyl iron(p. size : 1 to 10μm,)þBarium ferrite (BaZn1.5Co0.5Fe16O27) (p. size : 1 to 20μm) –The double-layer microwave absorbers with reﬂection losso 13 dB over the range of 6–18 GHz (t¼3.6 mm) and reﬂection loss ofo8 dB over the range of 2–18 GHz (t¼3.7 mm)

[86]

2007 Polyurethane Barium hexaferrite (BaCo0.9Fe0.05Si0.95Fe10.1O19) For 80 % ferrite content (24.5 dB, 12 GHz with a20 dB

bandwidth from 11–13 GHz, 1.6 mm)

[62]

2008 Polyaniline Barium ferrite(particle size : 50–70 nm) Shielding effectiveness due to absorption¼28.9 dB(99.9 %) [121]

Working frequency: 12.4–18 GHz

2009 Epoxy resin 17/83 wt% (NiZn-ferrite/epoxy) NiZn-ferrite (3 dB, 11.7 GHz) [122]

(grain size NiZN-ferrite : 0.3μm) NiCuZn-ferrite (1 dB, 8–12.5 GHz)

17/83 wt%(NiCuZn-ferrite/epoxy) (grain size NiCuZn-ferrite : 1μm)

2009 parafﬁn 10 wt% Ni0.5Zn0.5Fe2O4þCarbonyl iron(particles size range of 1–3μm)(mixed with parafﬁn at ratio : 1 :4) r20 dB in ranges : [90]

–5.4–14.8 GHz, t : 1.6–3.3 mm –5.0–13.3 GHz, t : 1.7–3.6 mm Minimum RL values:

–NZCI6 h–44 dB at 8.2 GHz and NZCI8 h–44 dB at 7.8 GHz

2009 Epoxy resin Carbon nanoﬁbers (dielectric lossy material) and NiFe (magnetic lossy material) –0.5 wt% CNFþ30 wt% NiFe(X band-t¼2.68 mm, RL :25 dB,

10 GHz)(Ku band-t: 2 mm, RL:34 dB, 15 GHz)

–50 wt% NiFe þ 1 wt% CNF(X band-t¼2 mm, RL :22 dB, 10 GHz)(Ku band-t: 1.49 mm, RL:15 dB, 15 GHz)

[123]

2009 Polyaniline MWCNT (SE) of27.5 to39.2 dB, 2 mm [124]

2010 Thermoplastic natural rubber (TPNR)

4–12 wt% Fe3O4(particle size 20–30 nm) Pure Fe3O4 [64]

–(32.19 dB, 3.65 GHz) –(-10.77 dB, 11.65 GHz) 12 wt% Fe3O4

–(25.51 dB, 12.65 GHz RL less than 10 dB,bandwidth 2.7 GHz, 9 mm)

–(18.61 dB, 6 mm) –(29.85 dB, 10 mm) (PP:NR:LNR¼70:20:10)

2010 Epoxy silicone MWCNT(60–100 nm)þcarbonyl iron RL¼ 16.9 dB at 10.5 GHz and a bandwidth over the frequency

range of 3.4–18 GHz with reﬂection loss below10 dB can obtained for a compositeﬁlled with 0.5 vol% MWCNT and 50 vol% CI particles,t¼2 mm

[125]

2010 Epoxy resin 40 wt% mesoporous C–SiO2–Fe nanocomposites optimal reﬂection loss calculated from the measured permittivity

and permeability is34.4 dB at 13.1 GHz, 2 mm.

[126]

2011 Epoxy resin 40 wt% Fe3O4 47 dB, 3.1 GHz, 4.8 mm [127]

Crystallite size: 100–500 nm

2011 PANI PANI/NZF (particle size : 0.2–0.5μm) RL (2–18 GHz),t¼2 mm [65]

–1:1 (PANI/NiZn) :14 dB, 11.2 GHz –2:1 (PANI/NiZn) :16dB, 12.5 GHz

F.Mohd.Idrisetal./JournalofMagnetismandMagneticMaterials405(2016)197–208200

(5)

–3:1 (PANI/NiZn) :20 dB, 14 GHz RL (18–40 GHz),t¼2 mm

–1:1 (PANI/NiZn) :8.5 dB, 35 GHz –2:1 (PANI/NiZn) :9 dB, 37.5 GHz –3:1 (PANI/NiZn) :12 dB, 40 GHz

2011 Epoxy resin SrFe11Zn0.5Ni0.5O19(spherical and needle shapes have size in the range of 20–25 nm) 29.62 dB (99% power attenuation) at 10.21 GHz [106]

2011 Silicon rubber Mn0.66Zn0.34Fe2O4(Particle sizes :1–50μm) 3 mm, volume fraction : 0.20,–37.5 dB. 11 GHz [67]

2011 Wax urchin-like Fe3O4 29.96 dB and below20 dB in 3.76–8.15 GHz corresponding to

3–4 mm thickness

[128]

2011 Resin acrylic PANi-MnFe2O4 15.3 dB (10.4 GHz),t¼1.4 mm [31]

2012 Epoxy Resin 15%PANIþ10%Fe3O4 Thickness¼1 mm, [25]

–(42 dB, 16.3 GHz)(more than 99.99% microwave absorption) –(37.4 dB, 14.85 GHz)

–(11 dB , 18 GHz) 15% PANIþ25% Fe3O4

20 % PANI

2012 Polyurethane PU/CB/NZF0.5(49/1/50 wt%) –(4 dB) (60 % absorption) [129]

Average particle size:

NZF0.5 : 4.4μm CB : 12 nm

2012 Parafﬁn wax 70 % (Yttrium ion substituted NiZn ferrites) -thickness : 1.5–3.5 mm [130]

Ni0.7Zn0.3YxFe2-xO4 Ni0.7Zn0.3Fe2O4(24.5 dB, 8.5 GHz, 2.5 mm)

Ni0.7Zn0.3YFe2xO4(34.8 dB, 8 GHz, 2.5 mm,bandwidth less than20 dB is 2 GHz)

2012 Parafﬁn Fe3O4/parafﬁn¼2:1 Thickness¼5 mm [91]

(particle size : 10–20 nm (35.1 dB, 5.2 GHz)

(30.2 dB, 17.6 GHz)

2012 Resin acrylic Conductive polypyrrole-MnFe2O4 12 dB (11.3 GHz),1.5 mm [131]

(Crystallite size24.27 nm) 2012 Thermoplastic polyurethane

(TPU)

10 wt% Graphene, 10 wt% MWCNT) Thickness¼2 mm [132]

Graphene :12.56 dB (10.43 GHz) MWCNT :7.6 dB (10.73 GHz)

2013 Epoxy resin MnO2þMWNT For 2–18 GHz: [66]

–the powder prepared using an MnO2content of 25 wt% (M-5) had a pronounced absorption band at 7 GHz with a reﬂection loss of23 dB

(both are dielectricﬁllers)

For 18–40 GHz:

The maximum absorption was12 dB when the concentration of MnO2was 5 wt%

2013 Wax 20 wt% Fe3O4nanoparticles with a diameter of about 25 nm were uniformly dispersed over the surface of the graphene sheets.

RL¼ 40.36 dB at 7.04 GHz ,t¼5.0 mm. [133]

2013 Parafﬁn wax as binder 50 wt%(MWCNT/Fe3O4)hybrid RL¼ 41.61 dB at a frequency of 5.5 GHz,t¼3.5 mm [134]

material consisting of magnetite (Fe3O4) nanocrystals grown on multiwalled carbon nanotube (MWCNT)

-average crystallite sizes of the magnetite are 7.7 nm.

2013 Parafﬁn wax as binder Fe3O4/PANI core/shell hybrid microspheres RL¼ 37.4 dB at 15.4 GHz has been reached from the sample

with a PANI shell thickness of 100 nm. Only narrow bandwidth [135]

300 nm Fe3O4microspheres

2013 Parafﬁn wax 50 wt% polypyrrole-reduced graphene oxide–Co3O4(PPy–RGO–Co3O4) 33.5 dB at 15.8 GHz with a thickness of 2.5 mm and the ab-

sorption bandwidth with the reﬂection loss below10 dB is up to 11.4 GHz (from 6.6 to 18.0 GHz),t¼2–4 mm

[136]

Co3O4 nanoparticles with the sizes in the range of 10–30 nm

2013 Parafﬁn 25 wt%(graphene@Fe3O4@SiO2@SnO2) RL¼ 37.4 dB at 15.1 GHz and the absorption bandwidth with

the reﬂection loss below10 dB are 6 GHz(range from12 to 18 GHz),t¼2 mm.

[137]

2013 Silicone rubber 0.5 wt% MWCNTþ45 vol% Carbonyl iron The minimum RL decreased from10.4 dB to13.2 dB at

thickness 1 mm and from15.3 dB to25.1 dB at thickness 1.5 mm.

[138]

2013 TPU titanium dioxide (TiO2) coated MWCNT(15%) and magnetite (Fe3O4)(15%) 42.53 dB at 10.98 GHz, 2 mm [139]

2014 Ni–Zn–Co ferritefilm and iron nanofilm reflectivity was lower than10 dB (0.5–18 GHz)(37 dB at

3 GHz) for the Ni–Zn–Co, , ferriteﬁlm absorber(t¼0.68 mm composed of 620 units) and4 dB (2–18 GHz) for the ironﬁlm absorber(t¼0.26 mm composed of 1050 units)

[140]

2014 Tri-substituted- TPH/Fe3O4magnetic hybrid microspheres were regular spheres and the average particle size was about matching thickness was 5.0 mm, the effective RL values of [141]

F.Mohd.Idrisetal./JournalofMagnetismandMagneticMaterials405(2016)197–208201

(6)

6. Ting et al.[65]reported on NiZn ferrite coated with polyaniline at different aniline/NiZn ferrite weight ratio and the composite structure was further introduced into epoxy resin.

7. Kong et al.[89]reported on the epoxy resin composite prepared by mixing 40 vol% of nanocrystalline Fe3O4with epoxy resin.

8. Gama et al.[67]reported the fabrication of Mn0.66Zn0.34Fe2O4

which was mixed with silicone rubber as a polymeric matrix with different volume fractions of the ferrite in the silicone matrix.

9. Drmota et al. [68] designed a composite material used for microwave absorption based on magnetic ﬁller, composed of two phases within the SrO–Fe2O3system and embedded in a polyphenylene sulﬁde matrix with a concentration ratio of 80:20 by weight.

More materials being used and the amount being incorporated into the polymer are described in detail as inTable 2.

5.2. Loading level

Fillers are the element that usually improves the required design of a microwave absorber. They may contribute to its conductivity, absorption and even strength and other properties. They also can influence the material's basic response properties such as permeability and permittivity. The electromagnetic properties, absorption bandwidth and resonance frequency of the composites can be very effectively tuned, simply by varying the volume fractions of the constituents’ filler phases. Thus, by controlling the volume fractions of the composites, microwave absorbing response may be tuned in a broad frequency range with strong absorption and wide absorption bandwidth. In the case of ferrite- polymer composites, research has been carried out to investigate the effects of ferrite materials and their volume fractions on the microwave absorbing properties[53]. In a similar way, the influence of the addition of conducting materials has been investigated [60]. In fact, by changing the amount offiller content, it results in having minimum reflection loss either at lower frequency or higher frequency. For example, by increasing the ferrite content in the sample, it may enhance the absorption. However, too much ferrite content may reduce the mechanical properties of the sample. Therefore, researchers need to be sensitive in order to play against the ratio betweenfiller content and polymer matrix. It is because the amount of filler content is much related to the thickness in order to exhibit a low reflection loss over a wide frequency range and also to be suitable for application. According to the quarter wave principle, by increasing the ferrite content in composite, absorption in the sample can be increased; thus, it may help reduce sample thickness.

5.3. Type of polymer matrix (criteria of choosing the polymer as matrix)

In general, microwave absorbers are processed using different polymeric matrices and polymers have been traditionally considered as good carrier matrices for nanoparticles. In some cases, polymers also act as an absorbing component among the polymer- based nanocomposites. It is because, instead of the polymer ser- ving as matrix such as epoxy resin[70], polyurethane[71], and rubber, it also helps to improve the EM wave absorption properties, as in the case of polyaniline. Polymer-based nanocomposites combine both the high EM wave loss of nanoparticles, and easy processibility and multi-functionality of polymers. These materials are hoped to act as ideal EM wave absorber with low density, thin thickness, broad absorption band, high EM wave loss, and even other functionality. In addition, in polymeric composites, the polymer plays the role of reducing aggregation of particles and Table2(continued) YEARTypesofpolymer(useas matrix)FillerReﬂectionloss(dB),thickness,workingfrequency(GHz)Ref. bisphthalonitrile137nminanarrowdispersion.31dB,33dBand37dBwereobtainedinthewidemicro- wavefrequencyrangefrom2GHzto16GHzbyadjustingthe contentofTPH. 2014WaxCoFe2O4þcarbonyliron(CI)thicknessesof2.4mm(CoFe2O4)and0.5mm(CI),theachieved minimumpeakofreﬂectionlosswas38.2dB,andbandwidthat lessthan10dBreached9.4GHz.

[109] 5Feb2014PANI59%Ni0.5Zn0.5Fe2O4(weightratioPANI/NZF:2:1)hybridnanorodscontaining59wt%NZFOexhibitexcellentmi- crowaveabsorptionproperties,withamaximumreﬂectionloss (RL)of27.5dBobservedat6.2GHz.Andthewidestabsorption band(RLB10dB)is8.1GHz,correspondingtoamatchingthin thicknessof2mm

[142] 2015ParafﬁnCuOrice-coatedNicore-shellcompositesRL¼62.2dBat13.8GHzwiththickness1.7mm.Measuredfrom 2to18GHz[145] 2015BaZn2Fe16O27/carboncompositeRL¼27.10dBat11.4GHz[146] Thickness3.2mm.measuredfrom2to18GHz.ObtainedRL fromsimulation. 2015Polyester7.5wt%CNTs,7.5wt%CNT/Co,7.5wt%CNT/ZnORLCNT¼8dB,RLCNT/ZnO¼13dB,RLCNT/Co¼12dB. Thickness¼2.5mm,measuredfrom2to18GHz.[147] 2015PolyesterChiralmaterials(embedding3-turncopperwirehelices)RL¼23dB.Measuredfrom8to18GHz[148] 2015ParafﬁnwaxPolyaniline/grapheneoxide/Fe3O4RL¼53.5dBat7.5GHz,bandwidth2.8GHz, thickness¼3.91mm[149]

(7)

imposes an upper limit on the size of inorganic particles [72].

Many attractive properties of polymers like light weight, non- corrosiveness, mechanical strength and dielectric tunability can be utilized along with novel magnetic and optical properties of nanoparticles to make multi-functional materials. By incorporating ferrite into conducting polymer, it may improve the absorption capability since ferrite particles have an added advantage of reducing the eddy current losses due to their high electrical resistance. Inclusion of ferrite nano-particles in polymers is also important as magnetic nanoparticle gives various potential applications in microwave absorbers, sensors and electromagnetic shielding especially for higher frequency applications[73].

There are various conducting polymers and insulating polymers which can be used as a matrix. In the last three decades, conducting polymers have emerged as a new class of materials. It is because of their high conductivity, intriguing electrical properties, and ease of production. Potential applications such as microwave absorbers were seriously considered soon after the dis- covery of these materials [74]. Thefirst environmentally stable, conducting polymer was polypyrrole. Polypyrrole (PPy) is one of the most promising conducting polymers due to unique properties and excellent environmental stability[75]. Other than this, of all the conducing polymers, research attraction is going towards polyaniline which is also known as PANI. It has been studied ex- tensively in recent years as a suitable matrix because of its ease of synthesis and environmental stability. Moreover, polyaniline is perhaps the most versatile conducting polymer because it is in- expensive and has desirable properties, such as low specific mass, thermal and chemical stability, controllable conductivity and high conductivity at microwave frequencies [76,77]. It is well known that conducting polymers can effectively shield electromagnetic waves generated from an electric source, whereas electromagnetic waves from a magnetic source can be effectively shielded only by magnetic materials. Thus, the incorporation of magnetic constituents into the conducting polymers has become an interest in the field of microwave absorption studies especially at higher frequency range[78]. These multifunctional composites open new possibilities and unusual electromagnetic properties for the achievement of good shielding effectiveness for various electromagnetic sources[79]. For example PANI alone, as a microwave- absorbing material has only electrical loss. Thus, it will not be of any help in improving the microwave absorption property and widening the absorption bandwidth[80].

On the other hand, for polymers which are electrically insulating and transparent to electromagnetic wave, they also need ferrite materials to be incorporated into polymer matrices in order to effectively suppress the electromagnetic interference [81].

Among the broad polymer candidates suitable for incorporating ferrites for preparing the microwave absorbing materials, epoxy is one of the most attractive. This polymer is well established as thermosetting matrices for advanced structural composites, dis- playing a series of promising characteristics for a wide range of applications owing to their commercial availability, excellent mechanical properties, low cost, ease of processing, good adhesion to many substrates and good chemical resistance[82]. Pure epoxy is electrical insulator with d.c. electrical conductivity value of10¹⁵S/cm[83].

Briefly, there are several requirements for polymer based nanocomposites where these materials need to attend to which are low weight, good weather-resistance capability, goodflexibility, minimum thickness, wide operational band with low reflection, extreme temperature capability, low outgassing and low cost [28,53,67,84]. In a nutshell, a polymer matrix has an obvious effect on microwave absorbing properties. The details on type offiller content, polymer matrix used and the related properties are described in detail as inTable 2.

5.4. Physical thickness of the sample

According to Zou[85]the microwave absorbing property is not only related to the absorber, but also to the thickness of the material and the frequency. Since thickness is a critical parameter of an absorber, many attempt to change the optimal thickness which would deteriorate the absorption performance. Ideally, by increasing the thickness of the sample, it results in higher absorption; thus, less electromagnetic energy is reflected back while a thinner sample causes lower absorption. However, a sample which is too thick is not very suitable for application. According to Feng et al. [86]in certain applications, the absorbers should not only absorb incident plane waves over a wide range of frequency, but also should be as thin as possible, and the optimization restrictive condition of the maximum thickness of the microwave absorber is less than 4.0 mm. On the other hand, Liu et al.[87]reported that their ferrite achieved minimum reflection loss at the thickness of 3 mm. On the contrary, if the thickness increases continually, the reflection loss would decrease. In particular, it is an unrealistic to increase the weight of RAM on an aircraft. Ideally, the minimum reflection loss increases and subsequently decreases with additional thickness. The relationship between thickness, permeability and frequency of minimum reflection loss can be explained using the following equation:

= πμ

f c″

d

m 2

1. fm: matching frequency with minimum reﬂection loss 2.c: velocity of light

3.d: sample thickness

4.

μ

”: imaginary part of permeability

Thus, from this equation it is easy to tell that the matching frequency fm shifts towards lower frequency with increasing sample thickness[88]. More results have been reported on the fact that increasing thickness causes the resonance frequency to move towards lower frequency[74,85,89,90,91].

5.5. Grain sizes (nanometer vs micrometer and the effects of particle size upon the EM wave properties)

With advances in nanotechnology, nano-microwave absorbers have played an important role in developing new microwave absorption materials. These materials provide the properties of strong absorption, wide frequency range, low density and thin thickness. Recently, more and more researchers have devoted themselves to study nanocomposite materials. It is because by combining different components in the nanosize range, it may yield new materials which combine the advantages of each nanomaterials component. Nanomaterials which have a new wave absorption mechanism lead to an excellent electromagnetic wave absorber performance in the GHz range with the effects of small size, surface and shape anisotropy[92]. According to the natural– resonance equations [93], 2πf_r =rHa where Ha=4K1/3μ0Ms. Whereris the gyromagnetic ratio,H_ais the anisotropyfield andK₁ is the anisotropy coefficient. It demonstrates that the resonance frequency of the magnetic materials shifts to higher frequency whenHaincreases due to the small size effect. It is believed that the anisotropy energy of small size materials, especially on nanometer scale, would be remarkably increased due to the increased surface anisotropicfield induced by the small size effect.

Thus, by having smaller grain size, each grain size has shape- and-size anisotropy which plays an important role in creating different anisotropy ﬁelds inside the grain. Thus, it results in a

(8)

range of anisotropy fields and a range of absorptions to be sustained over a broad frequency range. A detailed explanation on the effects of the size towards the microwave absorption was explained by Idris et al.[94]. In addition, Wen et al.[95]and Zhang et al.[96]found that the small-size effect is the dominant factor concerning the magnetic resonance. Consequently, ferrites with nanoscale (submicron grain size) dimensions are expected to have potential in the GHz range and are some of the most promising materials in magnetic nanocomposites for the absorption of microwave radiation [97]. It is known that ferromagnetic nano- powders exhibit a higher absorption at lowfield strengths and a broader absorption range in the microwave region than multi- domain powders[68]. According to Huo et al.[32]a nanostructure material might be one of the most promising materials to com- pensate the aforementioned disadvantages. It may also enhance the EM wave absorption ability owing to its higher surface area, more surface atoms, multiple reflection, and thus larger dielectric or/and magnetic losses. They also reported that the surface area, number of dangling bond atoms and the unsaturated co-ordina- tion on surface are all increased 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 microwave. It is also being reported by Qiu et al. [98]that the quantity of the dangling bond atoms is great as nanosized particles have a tremendous surface area and a large number of atoms on surface. On the other hand, Kodama [99], Dormann et al. [100]

and Batlle and Labarta[101]reported that with diminishing diameter of particles the surface effects become increasingly important. It will primarily affecting the anisotropy coefﬁcient and damping parameter (intrinsic damping, surface effects damping Dormann et al.[100]and Coffey et al.[102]and interparticle interactions damping[103].

According to Cao, [104], materials in the micrometer scale mostly exhibit physical properties the same as that of bulk form.

However, materials in the nanometer scale may exhibit physical properties distinctively different form that of bulk. The nature of nanometer particles is different to that of bulk materials, such as quantum size effect, surface effect and tunneling effect. It can broaden the absorption bandwidth with the loss above 10 dB [98]. Nanometer crystallite has the surface state because of the great surface area. If the surface energy level interval is within the energy range of microwave, the electrons can absorb the energy and leap from ground state to excited states, which may increase the microwave absorption[98].

Moreover, microwave absorption is enhanced when the particle size is reduced from micron to nano-size [105]. This can be explained on the basis of quantum size effect. In nanocrystallite particles, the quantum size effect makes the electronic energy levels split and the spacing between adjacent energy states increase inversely with the volume of the particles. If the particle of absorber medium is small enough and the discrete energy level spacing is in the energy range of microwave, the electron can absorb the energy as it transits from one level to another, and lead to increase in attenuation [106]. Moreover, there will be more interfaces if the grain size is smaller, and there will be stronger exchange coupling interaction at the interface.

More researchers however have reported on the advantages of the nanometer size compared with the micron size. For example, Peng et al.[3]reported on nanocrystalline NiZn ferrite around 35 to 45 nm which exhibit higher reflection loss than both those of micron-sized powders and those with size less than 25 nm. Kolev et al. [109] reported on the role of nanosized magnetic filler in composite absorbers as compared with conventional micron-sized fillers and gave an importantfinding on the effect of both sizes especially at higher GHz frequency. Bregar[97]reported on their nanosize ferromagnetic particles have properties that are different

than those of the bulk materials or large particles. They also reported that the surface effects become increasingly important as the particle size diminishes which will affect primarily the loss factor. In addition, with diminishing particle size, volume fraction of the particles increases in the composite; correspondingly due to increased number of particles and their interaction, the loss factor was increased. Therefore, with small size absorbingﬁllers having diameters of a few nanometers, the loss factor in a composite can exceed a very high value as compared to conventional absorbing ﬁller of micron sizes. Abbas et al. [69] reported that a large bandwidth along with the high absorption is attributed to the nano-sized particles of synthesized polyaniline. Kolev et al.[109]

reported on differences in the properties between 30 nm and 3

μ

m used as theﬁller. In nanosized materials one observes not only a larger surface-to-volume ratio, but also a number of unique effects that make the study of their dynamic characteristics so attractive. As the particle volume diminishes, the number of in- complete molecules and defects on the surface grows con- siderably, which leads to a rise of the exchange anisotropy interaction and the average particles magnetic moment dynamics, which affects the electromagnetic wave attenuation due to the multiple scattering of the particles. According to experiments done by Ruan[105], the microwave attenuation between the particles with different sizes of 5

μ

m and 65 nm showed that the loss of nanosized particles was larger than that of micrometersized particles. Gazeau[107]and Kodama[99]remarked that surface spins of ferrite nanoparticles are disordered, and the exchange coupling between the surface and core gives rise to a variety of spin dis- tribution within a single domain particles. The surface spin can result in high hysteresis loss. Thus, the microwave absorption will improve. In fact, Wang et al.[133]stated that higher surface area from smaller particle size leads to the formation of more dipoles which the dipole polarization will contribute to the enhanced EM absorption properties.

5.6. Layers (single, double, multilayer)

Recently, many researchers are interested in having microwave absorbing materials with the ability of absorbing the electromagnetic wave within broad frequency range with lower reﬂec- tion loss. Thus, many researches have been carried out for single layer absorbers as well as for multilayer absorbers. However, it is usually hard for single layer absorbing materials to absorb in a wide frequency range[108]. It is because apart from a limited number of parameters, it also gives narrow absorption bandwidth.

Besides, it is quite difficult to improve the absorption efficiency and bandwidths of the absorbers by only adding optimized ab- sorbents. Liu et al.[109]reported that single-layer absorbing materials cannot easily satisfy the requirements for broad-band absorption. Thus, to fulfill the requirements of broad absorption bandwidth, many research works are being carried out by introducing multilayers as inFig. 3. By applying two or more layers into the microwave absorber design, the operating bandwidth can be extended from the narrowband condition to a broader bandwidth. He et al.[110]and Cao et al.[111]report that by introducing a double or multilayer structure, it is better than that of the single layer by reducing the reflection loss and it also results in a broad absorption bandwidth. In double- or multi-layers, researchers usually promote a design having a matching layer and an absorbing layer as reported by Feng et al.[86]and Qing et al.[112].

Choosing the right material that is going to be used as the matching layer and absorbing layer for absorbing materials would be an extra credit or advantages in order to have a thin absorbing layer. The microwave absorption properties of the double-layer structure are inﬂuenced by the coupling interactions between the absorbing and matching layers [109]. By having two different F.Mohd. Idris et al. / Journal of Magnetism and Magnetic Materials 405 (2016) 197–208

204

(9)

layers, it is important to satisfy the two keys: (i) the impedance matching characteristic and (ii) the attenuation characteristic can be well satisfied by the double-layer absorbers for achieving excellent microwave absorbing materials. It is because in the double- layer absorber, the matching layer with good transmission capa- cityfirstly achieved a good impedance matching, and then the absorption layer could achieve an excellent absorption capacity. In addition, it is proved that by introducing a double layer structure, the microwave absorption properties can be enhanced sig- nificantly and the total layer thickness may be reduced compared with a single layer absorber [113]. Thus, the development of multilayer absorbing materials is important in order to obtain absorbing materials which absorb wider absorption bandwidths and smaller reflection loss values[111,114,115].

The reﬂection loss for the single layer and the proposed double layer microwave absorbers can be calculated using the equation reported by Michielssen et al.[116].

5.7. Broad bandwidth versus narrow bandwidth

In the design of radar absorbing materials, the frequency bandwidths are taken into consideration in order to obtain better absorbing performances. The frequency bandwidths can be calculated by subtracting the higher frequency from the lower one at a given reflection loss. The importance of frequency bandwidths (wider or narrower) depends on the requirement of the application. Reflection loss (RL) as a parameter is commonly used to evaluate the microwave absorption capacity. According to Feng et al.[117]desirable radar absorbing materials properties should comply with wider frequency bandwidths of RLo10 dB. By having “10 dB absorbing bandwidth”, it is referring to the frequency bandwidth having reflection loss characteristics over 90%

[117]. This was also reported by Truong[74]and their co-workers in their research that at least 90% of the incident radiation being absorbed when the reﬂectivityo10 dB. However, Liu et al.[118]

reported that the 10 dB absorption bandwidth corresponds to 68%

EM wave amplitude attenuation or to 90% power attenuation, whereas a 20 dB absorption bandwidth corresponds to 90%

amplitude attenuation or to 99% power attenuation. In addition, Motojima et al.[119], Kong et al.[89], Michielssen et al.[116]and Yan et al. [90] reported that the effective EM wave absorption

occurs when the reﬂection loss iso20 dB(indicating more than 99% of the introduced EM wave being absorbed). Generally, by having“RL o20 dB”it is comparable to 99% of microwave absorption referring to Eqs.(1) and (2)[90]where the frequency (f) dependence of reﬂection loss (RL) at a certain absorber thickness (d) was calculated from complex permeability and permittivity

μ ε

π μ ε

= ( )

( )

⎛

⎝⎜ ⎞

⎠⎟ ⎡

⎣⎢

⎢

⎛

⎝⎜⎜ ⎞

⎠⎟⎟ ⎤

⎦⎥

Z Z j fd ⎥

tanh 2c

1

in r

r r r

0 1/2

1/2

= [ − ) ( + )] ( )

RL 20 log Zin Z0/Zin Z0 2

wherecis the velocity of light,Z₀is the impedance of free space andZinis the input impedance of absorber. The relationship between electromagnetic reﬂectivity reduction correlated with the percentage of absorbed energy is given inTable 3by Lee[120].

All in all, when the value of the reﬂection loss is less than 20 dB, it is equivalently to 99% of the electromagnetic wave being absorbed by the materials. When the value of the reﬂection loss is less than 10 dB, it is equivalent to 90% of the electromagnetic wave being absorbed by the materials. Therefore, this value is the target value to be attained for the EM absorbers application.

6. Conclusions

Development of electromagnetic absorbing materials should consider factors influencing the electromagnetic wave absorption to obtain stronger absorption and better performances for application. By aiming to have stronger absorption over wide frequency band, problems could be solved by reasonable choice of the materials (combination of particles with both magnetic and dielectric loss). In addition, by incorporating a dielectric and magneticfiller into polymer matrix, it gives an obvious effect on microwave absorbing properties. Thus many researchers tend to mix the magnetic absorber (soft and /or hard magnet) with the dielectric absorber so that it will absorb at different frequencies and func- tioning over a wide frequency band. On the other hand, a wide range of absorption can also be obtained by having a mixture of grain sizes in the material. It is referring to the concept of having different anisotropyfields as the grain size varies from smaller to bigger grain size. Thus this results in sustained absorption at wide frequency range. The most important step in designing a broadband absorber is to precisely characterize the frequency response of the constituent material properties so that the overall reflectivity, in addition to the absorption properties, can be tailored to bandwidth requirements.

Fig. 3.Structure of proposed double-layer microwave absorbing material.

Table 3

Relationship between reﬂectivity reduction and the absorbed energy (Lee[120]).

Reﬂectivity reduction, dB Absorbed energy, %

0 0

3 50

10 90

15 96.9

20 99

30 99.9

40 99.99