plla

Mala

7014

niver

Received 27 April 2010

Received in revised form

Available online 15 June 2010

Keywords:

Magnetite

Thermoplastic natural rubber

Microstructure

abso

(Fe3O4) nanocomposites were investigated. The TPNR matrix was prepared from polypropylene (PP),

of absorbing unwanted electromagnetic signals were investigated.

icallyer tointomer-eter-

with conventional magnetic particles have difculty in meeting

Contents lists available at ScienceDirect

.els

Journal of Magnetism an

Journal of Magnetism and Magnetic Materials 322 (2010) 34013409microwave absorbing properties of polymer-based nanocompositesE-mail address: kong_ing_2005@yahoo.com (I. Kong).the criterion in thin and light weight microwave absorber due tohigh ller content and material thickness are needed to exhibit alow reection coefcient over a wide frequency range.

In this paper, we report the magnetic, electromagnetic and

0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmmm.2010.06.036

n Corresponding author. Tel.: +60 3 89215891(ofce), +60 12 5731025(mobile);

fax: +60 3 89213777.electromagnetic eld. With the aim of controlling the problemscreated by EMI, electromagnetic wave absorbers with the capability

size, such as Ba-ferrite [6], iron-bre [7], NiZn-ferrite [8] andFe3O4/YIG [9]. However, these polymer-based composites lledelectronically controlled devices such as system malfunctions,generating false images, increase clutters on radar and reduceperformance because of system-to-system coupling [2,3]. Further-more, there are ongoing controversies world wide over the potentialhealth hazards to the human body associated with exposure to

cost effectiveness. However, polymers are electrinsulating and transparent to electromagnetic wave. In ordeffectively suppress EMI, ferrite materials are incorporatedpolymer matrices [5]. Many works have been done on polybased composites lled with magnetic materials in micromThe high frequency electromagnetic wave is drawing moreattention, due to the explosive growth in the utilization oftelecommunication devices in industrial, medical and militaryapplications [1]. Many devices such as AC motors, digital compu-ters, calculators, point-of-sale terminals, printers, modems, electro-nic typewriters, digital circuitry and cellular phones are capable ofcreating electromagnetic interference (EMI) that may cause inter-ruption to those applications. EMI can cause severe interruption on

the weight, thickness, ller content, types of ller, environmentalresistance and mechanical strength [4].

Ferrites are considered to be the best magnetic material forelectromagnetic wave absorbers due to their excellent magneticand dielectric properties, but they are expensive and heavy. Onthe other hand, the use of polymers to protect the electronicdevices from EMI is popular due to the light weight, exibility andMagnetic property

Microwave absorbing property

1. Introductioncompatibilizer. TPNR-Fe3O4 nanocomposites with 412 wt% Fe3O4 as ller were prepared via a Thermo

Haake internal mixer using a melt-blending method. XRD reveals the presence of cubic spinel structure

of Fe3O4 with the lattice parameter of a8.395 A. TEM micrograph shows that the Fe3O4 nanoparticlesare almost spherical with the size ranging 2050 nm. The values of saturation magnetization (MS),

remanence (MR), initial magnetic susceptibility (wi) and initial permeability (mi) increase, while thecoercivity (HC) decreases with increasing ller content for all compositions. For nanocomposites, the

values of the real (er0) and imaginary permittivity (er0 0) and imaginary permeability (mr0 0) increase, whilethe value of real permeability (mr0) decreases as the ller content increases. The absorption or minimumreection loss (RL) continuously increases and the dip shifts to a lower frequency region with the

increasing of both ller content in nanocomposites and the sample thickness. The RL is 25.51 dB at12.65 GHz and the absorbing bandwidth in which the RL is less than 10 dB is 2.7 GHz when the llercontent is 12 wt% at 9 mm sample thickness.

& 2010 Elsevier B.V. All rights reserved.

Research on their electromagnetic and absorption properties is stillbeing carried out. In producing microwave absorbing materials,several parameters needed to be taken into consideration, such as7 June 2010natural rubber (NR) and liquid natural rubber (LNR) in the ratio of 70:20:10 with the LNR as theMagnetic and microwave absorbing pronatural rubber nanocomposites

Ing Kong a,n, Sahrim Hj Ahmad a, Mustaffa Hj AbduAhmad Nazlim Yusoff c, Dwi Puryanti a

a School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaanb Department of Mechanical Engineering, University of New Orleans, New Orleans, LAc Diagnostic Imaging and Radiotherapy Programme, Faculty of Allied Health Sciences, U

a r t i c l e i n f o

Article history:

a b s t r a c t

Magnetic and microwave

journal homepage: wwwerties of magnetitethermoplastic

h a, David Hui b,

ysia, 43600 Bangi, Selangor Darul Ehsan, Malaysia

8, USA

siti Kebangsaan Malaysia, 50300 Jalan Raja Muda Abdul Aziz, Kuala Lumpur, Malaysia

rbing properties of thermoplastic natural rubber (TPNR) lled magnetite

evier.com/locate/jmmm

d Magnetic Materials

3.5 mm, inner diameter 1.5 mm).

increasing of Fe3O4 ller. This was due to the added crystalline

I. Kong et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3401340934022.3. Measurements

The microstructure of the nanocomposites was studied using theX-ray diffraction (XRD) technique (Siemens D5000 diffractometer)with CuKa1 radiation (l1.541 A) in the 2y range 10601, in stepsof 0.021. The magnetic properties were measured using a vibratingsample magnetometer (VSM model LDJ 9600) at room tempera-tures (25 1C). The measurements were carried out in a maximumeld of 5 kOe. Magnetic parameters such as saturation magnetiza-tion (MS), remanence (MR), coercivity (HC), initial magneticsusceptibility (wi) and initial permeability (mi) were determined.The scattering parameters of the toroidal samples corresponding tothe reection (S11

* and S22* ) and transmission (S21

* and S12* ) of a

transverse electromagnetic (TEM) wave were measured using aconsisting of thermoplastic natural rubber (TPNR) as the matrix andFe3O4 nanoparticles as the llers. TPNR exhibits imtermediateproperties between those of natural rubber and thermoplastics. It isthe carrier of microwave absorbents. It can make the microwaveabsorber soft, exible and easy to be clipped. Fe3O4 nanoparticle, amember of spinel-type ferrite, was selected mainly because of itsunique and novel physiochemical properties that can be attainedaccording to their particle size (quantum size effect), shape,morphology and engineering form (lm/self-assembled nanocrys-tals and ferrouids) [10]. TPNR-Fe3O4 nanocomposites with variousller content were prepared by the melt-blending method and theyare expected to show better microwave absorption properties withlower ller content and material thickness than those of theconventional composites. The microstructure and morphology ofthe nanocomposites were also studied.

2. Experimental methods

2.1. Materials

Fe3O4 nanoparticles, with the particle size ranging 2030 nm,were obtained from commercial suppliers in powder form(Nanostructured & Amorphous Materials Inc., USA). Naturalrubber (NR) and polypropylene (PP) were supplied by RubberResearch Institute of Malaysia (RRIM) and Mobile (M) Sdn. Bhd.,respectively. Liquid natural rubber (LNR) was prepared by thephotosynthesized degradation of the NR in visible light.

2.2. Preparation of the composites samples

TPNR lled Fe3O4 nanocomposites with 412 wt% of Fe3O4were prepared by melt-blending technique using laboratorymixer (Model Thermo Haake 600p). The weight ratio of PP, NRand LNR is 70:20:10 with the LNR as the compatibilizer for themixture. Blending was carried out with a mixing speed of 100 rpmat 180 1C for 13 min. The NR was initially melted in an internalmixer. The LNR, which was previously mixed with Fe3O4nanoparticles was then added into the internal mixer 3 min afterthe blending started. NR, LNR and the nanoparticles were allowedto mix for 4 min before PP was charged into the internal mixer.Once a homogeneous mixture is assumed after 13 min, the blendwas removed from the internal mixer and subsequently pressedat 185 1C under 45 MPa of pressure for about 2 min using a hotpress (Carver Laboratory Press) into thin sheets of about 3 mmthick from which test specimens were cut. Toroidal-shapedsamples were prepared using injection moulding (Model Ray-Run) to t closely into a coaxial measurement cell (outer diameterHewlett Packard 8720D microwave vector network analyzerFe3O4 phase that migrates into the amorphous phase of TPNR,thus, reducing the amorphous domains of the TPNR sample [11].The diffractogram also indicates that the structure of Fe3O4 in thenanocomposites is maintained.

3.2. Morphology observation

Fig. 2 shows the TEM micrograph of magnetite nanoparticles.The particles are almost spherical with diameter ranging 2050 nm.Particles were polydisperse and some of them agglomerated due tomagneto-dipole interactions between particles.

Fig. 3 shows the SEM micrograph of the cross section ofnanocomposites containing 12 wt% of Fe3O4. There are largeamounts of Fe3O4 nanoparticles throughout the TPNR matrix andthe nanoparticles are well dispersed in the matrix. The Fe3O4nanoparticles are visible as white spots inside or outside the TPNRmatrix.

3.3. Magnetic properties

The hysteresis of pure Fe3O4 nanoparticles and nanocompo-(MVNA). The measurements were performed at the frequencyrange 120 GHz. The toroids were tightly t into a 3.5 mm coaxialmeasurement cell. The inner dimension of the air-line is 1.5 mm.Each sample was positioned exactly in the middle of the sampleholder by means of a tiny metal rod. After the sample was insertedinto one end of the airline, it was pushed until it reached the desiredposition indicated by a displacement marker on the rod. A full two-port calibration was initially performed on the test setup, in order toremove errors due to the directivity, source match, load match,isolation and frequency response in both the forward and reversemeasurements. The real and imaginary components of the complexdielectric permittivity and magnetic permeability were determinedfrom the complex scattering parameters using the NicolsonRossmodel for magnetic material and the precision model for non-magnetic material. The instrument was calibrated by measuring thecomplex permittivity and permeability of air using the air-lledsample holder, where the results show that er0 mr0E0 ander00mr00 1. The dependence of the absorption characteristics onthe frequency and thickness were obtained based on a model, inwhich an electromagnetic wave is normally incident on the surfaceof the materials backed by a perfect conductor.

3. Results and discussion

3.1. Microstructure

The X-ray diffractogram of TPNR, pure Fe3O4 nanoparticles andnanocomposites with different ller content are shown in Fig. 1.For pure Fe3O4 nanoparticles, there are characteristic peaks at2y30.341, 35.621, 43.181, 53.661 and 57.221 which can beassigned to (2 2 0), (3 1 1), (4 0 0), (4 2 2) and (5 1 1) planes ofFe3O4, respectively (JCPDS 01-1111). The d values calculated fromthe XRD patterns are well indexed to the cubic spinel phase ofFe3O4 with the lattice parameter of a8.395 A with no impurityphases detected. The uncorrected crystallite size, D, calculatedfrom the XRD peak broadening using DebyeScherrers formula(Dkl/b cos y. k0.9, l is the X-ray wavelength, b is the FWHMof the (3 1 1) peak and y is its peak position) is 22 nm. The X-raydiffraction patterns of the nanocomposites comprise of twophases, which are the crystalline and amorphous phases. It canbe seen that the crystallinity of the composites increased withsites with different ller content was measured at room

10 20 30

Inte

nsity

(a.u

.)

2 theta (d

(220

)

Fe3O4

12 wt%

8 wt%

4 wt%

TPNR

Fig. 1. X-ray diffraction patterns of pure TPNR, pure Fe3O4 nano

Fig. 2. TEM photograph of the Fe3O4 nanoparticles.

Fig. 3. SEM micrograph of the nanocomposites with 12 wt% Fe3O4 nanoparticles.

I. Kong et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 34013409 3403temperature. Fig. 4 exhibits the loops for the four samples with 4,8, 12 and 100 wt% of Fe3O4. The corresponding results from VSMfor all samples are listed in Table 1. It can be inferred from theloops that the Fe3O4 nanoparticles and nanocomposites aremagnetically soft at room temperature as their coercivities arein the range of 7590 Oe [12]. The saturation magnetization (MS)and remanence (MR) increase, while the coercive force (HC)decreases with increasing ller content. The results obtained inthis study agreed well with that reported by Ahmad et al. [13] onbarium ferrite polymeric composites and Stefcova et al. [14] onmagnetic silicon rubber. For samples with lower ller content, theincrease in HC indicates that the TPNR matrix is resistive towards

40 50 60egree)

(511

)

(311

)

(422

)

(400

)

particles and nanocomposites with different ller contents.the alignment of the magnetic moment of the ller. Therefore, thenanocomposites with lower ller content are hardlydemagnetized compared to those nanocomposites with higherller loading. The initial magnetic susceptibility (wi) and initialpermeability (mi) of the samples also increased with increasingller content. This result is consistent with previous reports onother magnetic polymers [1517].

At lower ller content (o10 wt%), the ller can be modeled asisolated non-interacting spheres. In this regime, the effectivemagnetic permeability of the composites with isolated particlesshows a linear relationship with ller concentration demon-strated by Wagners equation

mi 1A, 1where mi() is the initial magnetic permeability of granularcomposites, is the weight fraction of the ller and A is acoefcient dependent on magnetic properties, geometry andvolume of the ller [16]. The measured initial permeability valuesare plotted in Fig. 5 to compare with the values given by Eq. (1).The measured values were very nearly equal to those predicted byEq. (1) at low weight fraction of o0.1.

3.4. Microwave electromagnetic properties

The complex dielectric permittivity and magnetic permeabilityrepresent the dynamic dielectric and magnetic properties ofelectromagnetic materials. The real components (er0 and mr0) of

46

8

0

I. Kong et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3401340934041

u/g)the complex dielectric permittivity and magnetic permeabilitysymbolize the storage capability of electric and magnetic energy.The imaginary components (er00 and mr00) represent the loss of theelectric and magnetic energy. The mechanisms of energy loss inmagnetic materials are due to dielectric and magnetic properties,which depend on the imaginary part of the complex permittivityand complex permeability. The average dielectric energy loss isgiven by /WES1/2oer00Eo2, where er00 [sdc/(oeo)+eac00], sdc isthe direct current conductivity, o is the angular frequency, eo isthe permittivity of free space and eac00 is the alternating current

-10

-8

-6

-4

-2

0

2

-5 -4 -3 -2 -1 0

Mag

netiz

atio

n (e

m

Field

Fig. 4. Hysteresis loops for pure nanoparticles and

Table 1Magnetic properties of nanocomposites with increasing ller content.

Samples wi (70.001) mi (70.01) HC (7

4 wt% 0.006 1.08 85.93

8 wt% 0.013 1.16 83.12

12 wt% 0.018 1.22 80.46

Fe3O4 0.032 1.40 73.02

1.00

1.05

1.10

1.15

1.20

1.25

1.30

2184

Initi

al P

erm

eabi

lity

(i)

Filler Content (wt%)

i (Experimental) i (Wagner's Equation)

Fig. 5. The experimental and Wagners equation predicted initial magneticpermeability values as a function of the weight fraction of the Fe3O4 nanoparticles.1 2 3 4 5

(kOe)

Fe3O4 (x 8)

12 wt%

8 wt%

4 wt%

nanocomposites with different ller contents.loss contribution at high frequencies [3,18,19]. The averagemagnetic energy loss is given by /WMS1/2omr00Ho2. Formicrowave absorbers, high imaginary components of the complexdielectric permittivity and magnetic permeability are expected[20].

Fig. 6(a) and (b) shows the real (er0) and imaginary (er00)components of the relative complex dielectric permittivity (er*) forpure TPNR, pure Fe3O4 nanoparticles and nanocompositescontaining 4, 8 and 12 wt% of Fe3O4 in 120 GHz frequencyrange. It can be seen that the er0 for pure TPNR is nearly constantat 2.6 and the er00 is almost zero, indicating that the dielectriclosses are very small. For the nanocomposites and the pure Fe3O4,the real values er0 of the samples are almost constant in the wholefrequency range, but the imaginary values er00 decreasedramatically from 1 to 5 GHz before being gradually decreasedfor frequencies above 5 GHz. The values er0 and er00 of thenanocomposites increase as the ller content increases. All thesamples showed a constant er0 value throughout the wholefrequency range used in this work. The constancy in the valuesof er0 indicates that there was a domination of one type ofpolarization process, where the oscillation of the electric dipolemoments was in phase or slightly out of phase with themicrowave frequency. The most possible mechanism in thisfrequency range is orientational polarization. This interpretationis supported by the fact that neither relaxation nor resonant typebehavior is present in the er0 versus frequency plot. Furthermore,the atomic and electronic polarizations occur at a period shorterthan the period of a microwave. The dielectric loss in the samplescan be described as due to the contributions from both the direct

0.05 Oe) MS (70.05 emu/g) MR (70.05 emu/g)

2.38 0.30

4.50 0.64

6.09 0.84

63.79 11.57

I. Kong et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 34013409 34055.0current conductivity and the alternating current conductivity orion jump and dipole relaxation base on the expression er00 [sdc/(oeo)+eac00]. The expression shows that direct current conductionloss is inversely proportional to the frequency, hence, the reason

2.5

3.0

3.5

4.0

4.5

1 2 3 4 5 6 7 8 9 10

TPNR 4 w

r

Frequ

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1 2 3 4 5 6 7 8 9 10

TPNR 4w

r

Frequ

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

1 2 3 4 5 6 7 8 9 1

TPNR 4 w

r

/r

Freque

Fig. 6. (a) Real, (b) imaginary and (c) tan loss permittivity curves plotted againfor the increase in er for the materials with decreasing frequencyin the low-frequency regime.

The dissipation factors represented by dielectric loss (tandeer00/er0) are illustrated in Fig. 6(c). Tan de Increased as the ller

11 12 13 14 15 16 17 18 19 20

t% 8 wt% 12 wt% Fe3O4

ency (GHz)

11 12 13 14 15 16 17 18 19 20

t% 8 wt% 12 wt% Fe3O4

ency (GHz)

0 11 12 13 14 15 16 17 18 19 20

t% 8 wt% 12 wt% Fe3O4

ncy (GHz)

st frequency for pure TPNR, pure Fe3O4 nanoparticles and nanocomposites.

I. Kong et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 340134093406content increased. Tan de initially decreased sharply beforereaching an almost constant value as the frequency was furtherincreased.

Fig. 7(a) and (b) shows the real and imaginary components ofthe relative complex magnetic permeability (mr0 and mr00) for pureTPNR, pure Fe3O4 nanoparticles and nanocomposites containing 4,8 and 12 wt% of Fe3O4 in 120 GHz frequency range. The values ofmr0 and mr00 are, respectively, unity and zero in the wholefrequency range for the nonmagnetic TPNR sample, while astrong decrease with an increase in frequency for both quantitiesis observed at the whole frequency range for the pure Fe3O4nanoparticles. For the nanocomposites, the values of mr0 decreaseswhile mr00 increases as the ller content increases. Both the mr0 andmr00 for the nanocomposites decrease in the whole frequency rangeused in this study. The effects of incorporating the Fe3O4nanoparticles into the matrix of TPNR is to raise mr0 above unityat low frequencies and lower mr0 at high frequencies, while mr00 isslightly increased above zero throughout the whole frequencyrange. The magnetic permeability for the TPNR matrix is asexpected since it is non-magnetic. A sharp decrease in mr0 and mr00with the frequency from 1 GHz for the ferrite constitutes a part forthe resonance peak due to domain wall resonance, which issupposed to occur at lower frequency [3]. The pure TPNR sampleexhibits no wall resonance as expected.

The magnetic dissipation factors, tan dm (mr00/mr0), are shown inFig. 7(c). The loss tangent of the magnetic permeability increases asthe ller content increases. The value, however, decreased slowly asthe frequency increases. Generally, microwave absorption proper-ties of the ferrite are determined by the dielectric and magneticlosses [2123]. The dielectric and magnetic losses increase whenthe concentration of Fe3O4 nanoparticles increase, which may resultin an improvement of the microwave absorption properties.

3.5. Microwave absorption properties

According to the transmission line theory [24], for a micro-wave absorbing material backed by a perfect conductor, the inputimpedance (Zin) at the airmaterial interface is given by

Zin Z0m*re*r

stanh gt

, 2

where mr*mr0 jmr00, er*er0jer00, Zo(mo/eo)1/2376.7 O, is theintrinsic impedance of the free space, g[jo(mr*er*)1/2]/c is thepropagation factor in the material, o is the angular frequency, c isthe speed of light and t is the thickness of the sample. Thereection coefcient (G) is dened as G(Zin/Zo1)/(Zin/Zo+1)[(mr*/er*)1/2 tanh(gt)1]/[(mr*/er*)1/2 tanh(gt)+1]. The powerreectivity or the reection loss (RL), in decibel (dB) of thenormally incident electromagnetic wave at the absorber surfacecan be calculated using the following equation:

RL 20log109G9: 3The matching condition for a perfect absorption is given by

ZinZo. According to Eqs. (2) and (3), the matching condition hasbeen found to be determined by the combination of the sixparameters er0, er00, mr0, mr00, o and t.

The variation of the minimum reection loss (RL) for pure TPNR,pure Fe3O4 nanoparticles and nanocomposites containing 4, 8 and12 wt% Fe3O4 at the thickness of 9 mm are shown in Fig. 8. Here,the bandwidth is dened as the frequency width, in which thereection loss is less than 10 dB. The RL curves were obtained byassuming a normal or nearly perpendicular incident of themicrowave eld onto the surface of a specular absorber backedby a perfect conductor, where the absorber is assumed to have a

constant thickness. RL was calculated from a computer simulationusing the values of mr* and er* previously obtained. The reectionloss minimum or the dip in RL is equivalent to the occurrence ofminimal reection of the microwave power for the particularthickness or destructive interference between the reected waves.

From Fig. 8, it is found that there are only two shallow dips forpure TPNR at 5.25 and 14.15 GHz. TPNR almost does not absorbmicrowave when there is no ller in it and it can be considered asweak microwave absorber. For nanocomposites, the results revealthe existence of two matching conditions, at low and highfrequencies, for the same thickness of the samples. These twomatching conditions are associated with an odd number multipleof a quarter wavelength thickness (t) of the material, i.e., tnl/4(n1, 3, 5, 7, 9,y), where n1 corresponds to the rst dip at lowfrequency. The propagating wavelength in a material (lm) isexpressed by lmlo/(9mr*99er*9)1/2, where lo is the free spacewavelength and 9mr*9 and 9er*9 are the moduli of mr* and er*,respectively. At the particular thickness, the incident and reectedwaves in the material are out of phase by 1801, resulting in a totalcancellation of the reected waves in the material. It can be seenthat the rst and second matching conditions are associated withl/4 and 3l/4, respectively. The dips in RL versus frequency plotimply low reectivity or good absorption. The dips are shifted to alower frequency with increasing ller content. The frequency ofthe high frequency dip decreases from 14.35 GHz for sample with4 wt% ller to 12.65 GHz for sample with 12 wt% ller. It wasreported in the literatures that the values of er0 and er00 increasewith increasing ller concentration, resulting in the position of thedips moving to a lower frequency [25]. This shows that the dipfrequency of the nanocomposites can be manipulated easilyby changing the ller concentration. As the ller content increases,the reection loss increases from 10.79 dB for sample with4 wt% ller to 25.51 dB for sample with 12 wt% ller. Theabsorbing bandwidth of the nanocomposites is also proportionalto the ller content. The bandwidth for sample with 4 wt% llerwith a thickness of 9 mm is only 0.6 GHz, while the one for12 wt% ller with the same thickness is 2.7 GHz. It implies that theincrease of ller content in the nanocomposites can shift themaximum attenuation to a lower frequency, increasemaximum absorbing effect and also enlarge the absorbingbandwidth. The pure Fe3O4 nanoparticles showed a minimumreection loss of 32.19 dB at 3.65 GHz and 10.77 dB at11.65 GHz, respectively. The rst matching condition at lowfrequency in some ferrites has been interpreted as due to thespin rotational resonance frequency. Nevertheless, in this study,the spin rotational resonance frequency for the ferrites could notbe determined due to the absence of the resonance peak on themagnetic loss spectrum. It has also been suggested that maximumabsorption or minimum reection of microwave power by anabsorber backed by a perfect conductor would occur when9mr*99er*9 regardless of whether there exists resonance or other-wise [3,18]. The maximum absorption at 9mr*99er*9, depends on themagnitudes of both the magnetic and dielectric losses. Themaximum absorption at that particular frequency is consistentwith the reection coefcient

G ZinZoZinZo Zin=Zo1

Zin=Zo 1 0 or Zin Zo: 4

This is achievable only if tanh(gt)E1 or gt must be largeror either g or t is large so that tanh(gt) remains unity. Sinceg is proportional to (mr*er*)1/2, the higher values of the twoquantities will contribute to the higher absorption.Table 2 shows the calculated and experimental matchingfrequencies for both f1(n1) and f2(n3). The results indicatethat the experimental and the calculated values are in good

agreement.

I. Kong et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 34013409 34071.0Fig. 9 shows the frequency dependence of the reection lossfor the nanocomposites with 12 wt% ller at various thickness(t610 mm). The dips of the minimum reection loss increases

1 2 3 4 5 6 7 8 9 1

TPNR 4 w

r

Freque

1 2 3 4 5 6 7 8 9 1

TPNR 4 w

r

Freque

0.6

0.7

0.8

0.9

0.0

0.1

0.2

0.3

0.4

0.5

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

1 2 3 4 5 6 7 8 9 10

TPNR 4 wt%

r

/r

Freque

Fig. 7. (a) Real, (b) imaginary and (c) tan loss permeability curves plotted againfrom 18.61 dB for t6 mm to 29.85 dB for t10 mm and thefrequency of the maximum absorption also shifts from 16.95 GHzfor t6 mm to 11.55 GHz for t10 mm with increasing thickness

0 11 12 13 14 15 16 17 18 19 20

t% 8 wt% 12 wt% Fe3O4

ncy (GHz)

0 11 12 13 14 15 16 17 18 19 20

t% 8 wt% 12 wt% Fe3O4

ncy (GHz)

11 12 13 14 15 16 17 18 19 20

8 wt% 12 wt% Fe3O4

ncy (GHz)

st frequency for pure TPNR, pure Fe3O4 nanoparticles and nanocomposites.

-35

-30

-25

-20

-15

-10

-5

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

RL

(dB

)

Frequency (GHz)

TPNR 4 wt% 8 wt% 12 wt% Fe3O4

Fig. 8. Frequency dependences of RL for pure TPNR, pure Fe3O4 nanoparticles and nanocomposites at the thickness of 9 mm.

Table 2Microwave absorption properties of nanocomposites with increasing ller content at the thickness of 9 mm.

Samples Calculated matching frequency (70.05 GHz) Experimental matching frequency (70.05 GHz) Reection loss (70.01 dB) Bandwidth (70.05 GHz)

f1 f2 f1 f2 RL1 RL2 BW1 BW2

TPNR 4.62 14.05 5.25 14.15 1.99 2.29 4 wt% 4.63 14.23 4.85 14.35 7.81 10.79 0.68 wt% 4.12 13.02 4.25 13.15 9.37 16.10 2.012 wt% 3.93 12.53 3.95 12.65 11.26 25.51 0.8 2.7Fe3O4 3.58 11.50 3.65 11.65 32.19 10.77 1.6 1.1

-35

-30

-25

-20

-15

-10

-5

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

RL

(dB

)

Frequency (GHz)

6 mm 7 mm 8 mm 9 mm 10 mm

Fig. 9. Frequency dependences of RL for the nanocomposites with 12 wt% Fe3O4 at different thicknesses.

I. Kong et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 340134093408

[1,20]. As mentioned above, the dip in RL indicates the occurrenceof absorption or minimal reection of the microwave power. Theintensity and the frequency at the reection loss minimal,therefore, depend on the properties and thickness of the

direct current conductivity, whereas the loss at higher frequenciesis attributed to alternating current conductivity. The ferritesexhibited domain wall resonance in the low-frequency regime

from School of Applied Physics, Universiti Kebangsaan Malaysiafor the revision in English writing and Mr. Mohd. Razali fromScience and Technology Research Institute for Defense (STRIDE)

References

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Acknowledgments

This work was supported by the Scientic Advancement FundAllocation (SAGA), STGL-010-2006, of the Academy of ScienceMalaysia and Science Fund 03-01-02-SF0059 from the Ministry ofScience, Technology and Innovation, Malaysia (MOSTI). Theauthors would like to acknowledge Professor Roslan Abd. Shukormatching frequency is increased.

4. Conclusions

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[25] Y.J. Chen, M.S. Cao, T.H. Wang, Q. Wan, Microwave absorption of the ZnOnanowire-polyester composites, Appl. Phys. Lett. 84 (17) (2004) 33673369.is thought that the matching thickness is reduced while the J. Appl. Phys. 92 (2002) 876882.materials. It is evident that the dips correspond to n1, 3, 5,yare suppressed and shifted to a higher frequency. Since theoccurrence of the dip reects the geometrical cancellation ofwaves at the surface of an absorber with a particular thickness, it

Magnetic and microwave absorbing properties of magnetite-thermoplastic natural rubber nanocompositesIntroductionExperimental methodsMaterialsPreparation of the composites samplesMeasurements

Results and discussionMicrostructureMorphology observationMagnetic propertiesMicrowave electromagnetic propertiesMicrowave absorption properties

ConclusionsAcknowledgmentsReferences