1.1489092

Electromagnetic and absorption properties of some microwave absorbersA. N. Yusoff, M. H. Abdullah, S. H. Ahmad, S. F. Jusoh, A. A. Mansor, and S. A. A. Hamid

Citation: Journal of Applied Physics 92, 876 (2002); doi: 10.1063/1.1489092 View online: http://dx.doi.org/10.1063/1.1489092 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/92/2?ver=pdfcov Published by the AIP Publishing

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Electromagnetic and absorption properties of some microwave absorbersA. N. Yusoffa)Diagnostic Imaging and Radiotherapy Programme, Faculty of Allied Health Sciences,Universiti Kebangsaan Malaysia, Jalan Raja Muda Abdul Aziz, 50300 Kuala Lumpur, MalaysiaM. H. Abdullah, S. H. Ahmad, and S. F. JusohSchool of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia,43600 Bangi, Selangor Darul Ehsan, Malaysia

A. A. Mansor and S. A. A. HamidDepartment of Physics, Universiti Teknologi MARA, 26400 Jengka, Pahang Darul Makmur, Malaysia~Received 29 June 2001; accepted for publication 6 May 2002!

Electromagnetic properties of a thermoplastic natural rubber ~TPNR!, a lithiumnickelzinc ~LiNiZn! ferrite and a TPNRferrite composite subjected to transverse electromagnetic ~TEM! wavepropagation were investigated. The incorporation of the ferrite into the matrix of the TPNR wasfound to reduce the dielectric loss but the magnetic loss increased. The absorption characteristics ofall the samples subjected to a normal incidence of TEM wave were investigated based on a modelof a single-layered plane wave absorber backed by a perfect conductor. It is evident from a computersimulation that the ferrite is a narrowband absorber, whereas the polymeric samples show broadbandabsorption characteristics. Minimal reflection of the microwave power or matching condition occurswhen the thickness of the absorbers approximates an odd number multiple of a quarter of thepropagating wavelength. This is discussed as due to cancellation of the incident and reflected wavesat the surface of the absorbers. The LiNiZn ferrite exhibits another matching condition at lowfrequency when the magnitude of the complex relative dielectric permittivity (er*) equals that of thecomplex relative magnetic permeability (mr*). The specular absorber method provides a simpletheoretical graphic aid for determining the absorption characteristics and the location of thematching conditions in the frequency domain. The result for the ferrite sample was tested andconfirmed directly from terminated one-port measurements. 2002 American Institute of Physics.@DOI: 10.1063/1.1489092#

I. INTRODUCTION

The increase in electromagnetic pollution due to therapid development of gigahertz ~GHz! electronic systemsand telecommunications has resulted in a growing and in-tense interest in electromagnetic-absorber technology. Elec-tromagnetic interference ~EMI! can cause severe interruptionof electronically controlled systems. It can cause device mal-functions, generate false images, increase clutter on radarand reduce performance because of system-to-system cou-pling. These are some of the reasons why the use of self-generated electromagnetic radiation apparatuses, which in-clude cellular telephones, wireless computer and pagers, arestrictly prohibited in certain areas, for example, in hospitals,banks, petrol stations and inside airplanes. To overcome theproblems created by EMI, electromagnetic wave absorberswith the capability of absorbing unwanted electromagneticsignals are used, and research on their electromagnetic andabsorption properties is still being carried out.1,2 Recent de-velopments in microwave absorber technology have resultedin materials that can effectively reduce the reflection of elec-tromagnetic signals, on the one hand, and have good physicalperformance and lower production cost on the other.3,4 There

are a variety of absorber materials that can be used to sup-press EMI depending on whether they are suitable fornarrow- or broadband absorption and for low- or high-frequency applications. In the microwave region, commonlyused dielectric materials are foams, plastics, rubbers, thermo-plastics, natural rubbers and polypyrroles. These nonmag-netic, environmentally resistant absorbers often contain mag-netic materials such as ferrites, iron or cobaltnickel alloysas fillers. By incorporation of the magnetic fillers, the valuesof the dielectric permittivity and magnetic permeability ofthe materials can be altered to achieve maximal absorption ofthe electromagnetic energy. An ideal absorber must fulfill therelation5 that er*5mr* , where er*5er82 jer9 is the complexrelative dielectric permittivity, with er8 the real part or dielec-tric constant and er9 the imaginary part or dielectric loss, andmr*5mr82 jmr9 is the complex relative magnetic permeabil-ity, with mr8 the real magnetic permeability and mr9 the mag-netic loss.

The specular absorber method has been widely used byseveral workers as a theoretical approach in explaining thepropagation characteristics of a transverse electromagnetic~TEM! wave in a single-layer absorber backed by a perfectconductor.6,7 This method is based on the assumption that thedielectric permittivity and magnetic permeability are intrin-sic properties of the material. For a wave normally incidenton the surface of a single-layer absorber backed by a perfect

a!Author to whom correspondence should be addressed; electronic mail:[email protected]

JOURNAL OF APPLIED PHYSICS VOLUME 92, NUMBER 2 15 JULY 2002

8760021-8979/2002/92(2)/876/7/$19.00 2002 American Institute of Physics [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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conductor, the input impedance (Z in! at the airmaterial in-terface is given by Z in5Z0(mr*/er*)1/2 tanh(gt), where Z05A(m0 /e0)5377 V is the intrinsic impedance of free space,g5@ jv(mr*er*)1/2#/c is the propagation factor in the mate-rial, v is the angular frequency, c is the speed of light and tis the thickness of the sample.8,9 The reflection coefficient~G! is defined as G5(Z in /Z021)/(Z in /Z011)5@(mr*/er*)1/2 tanh(gt)21#/@(mr*/er*)1/2 tanh(gt)11#. Thepower reflectivity or the reflection loss (RL), in decibels~dB!, can be written as RL520 log10uGu. The dip in RL indi-cates the occurrence of absorption or minimal reflection ofthe microwave power. The intensity and the frequency at thereflection loss minimum, therefore, depend on the propertiesand thickness of the materials.

In this article, we report the microwave dielectric, mag-netic and absorption properties of a thermoplastic naturalrubber ~TPNR! that is composed of 70 wt % polypropylene~PP!, 20 wt % natural rubber ~NR! and 10 wt % liquid natu-ral rubber ~LNR!, a LiNiZn ferrite and a composite thatconsist of 70 wt % of the TPNR and 30 wt % of the LiNiZn ferrite. The effects of incorporating the ferrite into thematrix of the TPNR on the absorption characteristics of thematerial are examined and discussed.

II. MATERIALS AND METHODS

A polycrystalline Li0.2Ni0.3Zn0.3Fe2.2O4 ~LiNiZn! fer-rite was prepared by a double sintering method in air. Pow-ders of high purity Li2O ~99.5%!, NiO ~99.995%!, ZnO~99.999%! and Fe2O3 ~99.998%! were weighed, mixed andground thoroughly for 2 h in the desired stoichiometric com-position together with 0.5 wt % Bi2O3 ~99.999%!. The mix-ture was presintered at 800 C for 6 h and subsequently fur-nace cooled to room temperature. The prereacted mixturewas then reground for another 2 h. A cylindrically shapedferrite sample 5.0 mm in diameter and 5.0 mm in thicknesswas molded under pressure of about 300 MPa. A small quan-tity of polyvinyl alcohol ~PVA! was used as a binding agent.The powder and the cylinder were then sintered at 1050 Cfor 15 h and furnace cooled to room temperature. A toroidsample of 3.5 mm outside diameter and 1.6 mm inner diam-eter was machined from the ferrite cylinder for the micro-wave measurements.

The TPNR matrix was prepared by melt blending PP,NR, and LNR in a weight ratio10,11 of 70:20:10 with the LNRas the compatibilizer. The LNR, which was prepared by pho-tosynthesized degradation of NR in visible light, wasblended with NR and PP in a laboratory cam mixer ~modelBrabender Plasticorder PL 200! at 170 C at a rotating speedof 50 rpm. The NR and LNR were allowed to mix for about2 min before the PP was introduced. After 12 min of mixing,the homogeneous TPNR mixture was removed from themixer and ground in a granulator machine ~model Ph 400SS!. The desired amounts of the TPNR mixture ~70 wt %!and of ferrite powder ~30 wt %! were mixed in the samplemachine and blended in a similar manner. The TPNR and thecomposite were molded into a thin sheet 5.0 mm thick usingcompression molding under pressure of about 700 MPa at

175 C. Toroidal samples of 3.5 mm outside diameter and1.6 mm inner diameter were prepared from the TPNR andTPNRferrite sheets.

The scattering parameters of the toroidal samples thatcorrespond to the reflection (S11* and S22* ! and transmission(S21* and S12* ! of a TEM wave were measured using a HewlettPackard 8719D microwave vector network analyzer. Themeasurement was performed in the frequency range of 113GHz. The toroids tightly fit into a 3.5 mm coaxial measure-ment cell. Full two-port calibration was initially performedon the test setup in order to remove errors due to the direc-tivity, source match, load match, isolation and frequency re-sponse in both the forward and reverse measurements. Figure1~a! shows the coaxial fixture used in measuring the scatter-ing parameters. The real and the imaginary components ofthe complex dielectric permittivity and magnetic permeabil-ity were determined from the complex scattering parametersusing the NicolsonRoss12 ~for magnetic! and precision13~for nonmagnetic! models. The dependences of the absorp-tion characteristics on the frequency, thickness and both thedielectric permittivity and magnetic permeability were ob-tained based on a model in which an electromagnetic wave isincident normal to the surface of the material backed by aperfect conductor. RL was also measured by the terminatedone-port technique using a short S11* test fixture, as shown in

FIG. 1. ~a! Two-port coaxial fixture used in measuring the complex scatter-ing parameters (S11* and S21* ), and ~b! one-port terminal short fixture formeasuring the reflection scattering parameter (S11* ).

877J. Appl. Phys., Vol. 92, No. 2, 15 July 2002 Yusoff et al.


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Fig. 1~b!. In this case RL520 log10uS11u. The simulated dataare compared to those from the experimental one-porttechnique.

III. RESULTS AND DISCUSSIONThe frequency dependences of the complex scattering

parameters, S11* and S21* , for all samples are shown in Fig. 2.The TPNR and TPNRferrite composite show a lower S11*than S21* . The S parameters for the LiNiZn ferrite sampleare different from the other two polymeric samples withhigher S11* and lower S21* . This indicates that the ferrite re-flects more but transmits less microwave energy than theother two samples over the whole frequency range. The dif-ference in reflectivity and transmittivity among the samplescan be suggested to be due to the differences in their micro-structures, dielectric and magnetic properties. The LiNiZnferrite is crystalline with high magnetic moment whereasTPNR is nonmagnetic and semicrystalline in nature. The ef-fects of microstructure, dielectric and magnetic properties ofthe ferrite materials on microwave propagation was brieflydiscussed elsewhere.6,7 The incorporation of the ferrite intothe TPNR matrix is found to have only a small effect on thereflection and transmission properties, but there is a tendencyfor the TPNRferrite composite to behave like the ferrite.The plots of S22* and S12* are similar to those of S11* and S21*but are not shown. The similarity between S11* and S22* and

between S12* and S21* for all the samples indicates their reci-procity in the absence of an external magnetic field.

Figure 3 shows er8 and er9 values for the three samples inthe frequency range of 113 GHz. It can be clearly seen thatall samples show an almost constant er8 value throughout thewhole frequency range used in this work. The most probablemechanism in this frequency range is orientationalpolarization.14 This is supported by the fact that neither re-laxation nor resonant type behavior is present in the er8 plot.Furthermore, the atomic and electronic polarizations occur ata period shorter than the period of a microwave. The ferriteshows the highest er8 value followed by the TPNRferritecomposite and the TPNR matrix. The mechanism of polar-ization in the ferrite at microwave frequencies is dependenton the availability of ions of different valences and it is be-lieved that the orientational polarization in the ferrite ismainly a result of the process of electron transfer betweenferrous (Fe21) and ferric (Fe31! ions.15,16 The dielectric loss,however, is not constant over the whole frequency range.The polymeric samples show a gradual decrease of er9 to-wards high frequencies, but er9 for the TPNR is alwayshigher than that for the TPNRferrite composite. The fre-quency variation of the dielectric loss for the ferrite is differ-ent from that of the two polymeric samples. The loss if al-most constant between 2 and 8 GHz, but increases slightly

FIG. 2. Frequency dependence of the complex scattering parameters ~S11*and S21* ! of the TPNR ~s!, the TPNRferrite composite ~,! and theLi0.2Ni0.3Zn0.3Fe2.2O4ferrite ~h! toroids.

FIG. 3. Frequency dependence of the real (er8) and the imaginary (er9) partsof the complex dielectric permittivity of the TPNR ~s!, TPNRferrite com-posite ~,! and the Li0.2Ni0.3Zn0.3Fe2.2O4ferrite ~h! samples.

878 J. Appl. Phys., Vol. 92, No. 2, 15 July 2002 Yusoff et al.


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for frequencies below 2 and above 8 GHz. The dielectric lossin the samples can be described as due to the contributionsfrom both the dc conductivity and the ac conductivity or ionjump and dipole relaxation based on the expression2,17 er95@sdc /(ve0)1eac9 # , where sdc is the dc conductivity, v isthe angular frequency, e0 is the permittivity of free space andeac9 is the ac loss contribution at high frequencies. The ex-pression shows that dc conduction loss is inversely propor-tional to the frequency, hence, the reason for the increase iner9 for the materials with a decrease in frequency in the low-frequency regime. Similar behavior has also been observedfor a MnZn ferriterubber composite.8 For the ferrite, ionjump and relaxation between two equivalent Fe21 and Fe31ion positions are responsible for the dielectric loss at highfrequencies.

Figure 4 shows the real and imaginary parts of the com-plex magnetic permeability ~mr8 and mr9! for the threesamples. The values of mr8 and mr9 are, respectively, unityand zero in the whole frequency range for the nonmagneticTPNR sample, while a strong decrease with an increase infrequency for both quantities is observed at low frequenciesfor the ferrite. However, mr8 for the ferrite shows a gradualincrease with an increase in frequency for frequencies above7.5 GHz. The plot for the ferrite also shows that mr8.1 forfrequencies below 3 GHz and 0,mr8,1 for frequencies

higher than 3 GHz. The effects of incorporating the ferriteinto the matrix of the TPNR matrix is to raise mr8 above unityat low frequencies and lower mr8 at high frequencies, whilemr9 is slightly increased above zero throughout the wholefrequency range. The magnetic permeability for the TPNRmatrix is as expected since it is nonmagnetic. A sharp de-crease in mr8 and mr9 with the frequency from 1 GHz for theferrite constitutes a part of the resonance peak due to domainwall resonance which is supposed to occur at lower fre-quency. The wall resonance nearly vanishes in the TPNRferrite composite sample because the particles are too smallto support a multidomain structure.18 The pure TPNR sampleexhibits no wall resonance as expected. Ferrimagnetic orspin resonance, on the other hand, is hardly observed for theferrite due to the large influence of wall resonance but acloser look at the logarithmic plot for the TPNRferrite com-posite sample ~diagram in inset! indicates a small peak thatcould be due to spin resonance which occurs at approxi-mately 3.5 GHz. The randomness in the internal fieldscaused by variation of the spontaneous magnetization andanisotropy field at different points in the unmagnetizedsamples18,19 explains the suppression of the resonance peakobserved in this study.

Figure 5~a! shows the frequency dependence of the re-flection loss for the TPNR at various sample thicknesses ~t52.5, 5, 15, and 30 mm!. The curves are obtained by assum-

FIG. 4. Frequency dependence of the real (mr8) and the imaginary (mr9)parts of the complex magnetic permeability of the TPNR ~s!, TPNRferritecomposite ~,! and the Li0.2Ni0.3Zn0.3Fe2.2O4ferrite ~h! samples. The insetshows the occurrence of ferrimagnetic or spin resonance in the TPNRferrite composite sample.

FIG. 5. Reflection loss plot at several thicknesses ~t! of ~a! the TPNR and~b! the TPNRferrite composite. ~s!: 2.5 mm, ,: 5 mm, h: 15 mm and L:30 mm.!



ing normal incidence of the electromagnetic field on thesurface of a specular TPNR absorber backed by a perfectconductor. RL is calculated from a computer simulation us-ing the values of mr* and er* previously obtained, as shownin Fig. 7. The reflection loss minimum or the dip in RL isequivalent to the occurrence of minimal reflection of the mi-crowave power for the particular thickness. The plots for theTPNR show that the number of dips increases with an in-crease in sample thickness. It can be seen that there is onlyone shallow dip for t55 mm and almost no reflection lossfor t52.5 mm, however, the loss starts to appear at higherfrequencies, where two and four complete dips can be ob-served for t515 and 30 mm, respectively. The occurrence ofthe dips is found to be due to a successive odd number mul-tiple of the quarter wavelength thickness of the material ort5nl/4 ~n51, 3, 5, 7, 9, ...!, where n51 corresponds to thefirst dip at low frequency. The propagating wavelength in amaterial (lm) is expressed by lm5l0 /(umr*uuer*u)1/2 wherel0 is the free space wavelength and umr*u and uer*u are themoduli of mr* and er* , respectively. At that particular thick-ness, the incident and reflected waves in the material are outof phase 180, resulting in total cancellation of the reflectedwaves at the airmaterial interface. For t530 mm, it can beshown that the dips occur when the thicknesses equal to1.2~l/4!, 3.1~l/4!, 5.1~l/4! and 7.1~l/4!. Calculations per-formed on other thicknesses give similar results, but the lo-cation of each consecutive dip is shifted towards a higherfrequency for a smaller value of t.

The results for the TPNRferrite composite appear to besimilar to those for the TPNR. The plots for t52.5, 5, 15 and30 mm are shown in Fig. 5~b!. For t52.5 mm, the reflectionloss over a wide frequency range is small. For t55 mm, abroad reflection loss dip starts to appear. A similar calcula-tion shows that the dip occurs at a thickness of 1.1l/4. Com-pared with the result for the 5 mm thick TPNR, the inclusionof the ferrite filler has lowered the frequency of the quarterwavelength dip. For higher values of t, the occurrence ofreflection loss for the TPNRferrite composite is similar tothat of the TPNR but the magnitude or intensity varies in acomplicated manner. This is believed to be due to impedancemismatch at the airmaterial interface. This is the reasonwhy the microwave power is not totally absorbed by thematerials. The dips for each thickness from left to right canalso be shown to occur at t5nl/4 ~n51, 3, 5, 7, 9, ...!.Apparently, the number increases as the thickness is in-creased and the dip for the same n of different thicknesses isshifted towards a lower frequency region.

Figure 6~a! shows the frequency dependence of the re-flection loss of the ferrite at sample thicknesses of 2.5, 5, 15and 30 mm. Clearly demonstrated is that the intensity andfrequency of the reflection loss minimal for the ferrite alsodepend on the materials thickness. The dips of minimal re-flection for the ferrite in the low-frequency region are alsoshifted towards a lower frequency with an increase in thick-ness. However, by using a similar computer simulation, twofrequency-thickness configurations of the ferrite where mini-mal reflection of microwave power occurs can be deter-mined, as depicted in Fig. 6~b!. This behavior was also foundin NiZnCo ferrite composites1 and in polypyrrole-based

microwave absorbers.2 The matching frequencies are 1.2 and12.6 GHz, with corresponding matching thickness values of6.7 and 2.9 mm, respectively. The first matching at low fre-quency in some ferrites has been related to the spin rotationalresonance frequency.9 However, the spin rotational reso-nance frequency of the (Li0.5Fe0.5!0.4Ni0.3Zn0.3Fe2O4 ferritecould not be determined due to the absence of a resonancepeak in the magnetic loss spectrum. It is suggested that themaximal absorption at this frequency is simply due to umr*u5uer*u regardless of the existence of resonance or not. The

FIG. 6. ~a! Frequency dependence of the reflection loss of the ferrite atvarious sample thicknesses ~s: 2.5 mm, ,: 5 mm, h: 15 mm and L: 30mm!, ~b! frequency dependence of the reflection loss of the ferrite showingtwo matching conditions at low and high frequencies from the simulatedtwo-port technique and ~c! the results for the simulated two-port and theexperimental one-port techniques of the pure ferrite ~thickness 5 5.09 mm!.



simulation used in this study agreed very well with the ex-perimental result using the S11 short technique. Figure 6~c!shows an example of simulated and experimental plots of RLfor a 5.09 mm thick ferrite sample. The slight deviation inthe magnitude and in the location between the two dips couldbe due to some experimental errors, such as the existence ofthe air layer between the sample and the termination anddimensional inaccuracies of the sample.

Figure 7 shows that the crossing of the modulus of mr*and er* in the low-frequency region in the ferrite sampleoccurs at the same frequency of maximal absorption. Figure7 also shows plots of mr* and er* for the TPNR and TPNRferrite composite. However, the crossing of mr* and er* is

absent for both samples and the parameters are separatedthroughout the whole frequency range. This shows that thedips observed for the TPNR and TPNRferrite composite aredue to geometrical factors whereas for the ferrite, materialproperties play an important role in the low-frequency ab-sorption. As depicted in Fig. 6, maximal absorption aroundthis frequency also occurs at other thicknesses, but the mag-nitude decreases with an increase or decrease in thickness.Maximal absorption for t515 and 30 mm is expected tooccur at lower frequencies. A slight variation in the fre-quency is due to the variation of the factor tanh~gt! in theequation for reflection loss. The second matching frequencyis associated with the quarter wavelength ~l/4! thickness ofthe material as discussed for the TPNR and TPNRferritecomposite. The wavelength in the material at the secondmatching condition is 11.9 mm. Hence l/452.9 mm, whichis equal to the thickness of the sample at that frequency. InFig. 6~a!, it can be observed that the matching condition isabout to occur at thickness of 2.5 mm. The absorption ofmicrowaves has been shown to depend on the polarizabilityof the materials.20,21 Polar molecules are known to stronglyabsorb microwavesin comparison to nonpolar molecules. This explains thepresent results that the microwave power absorption by theferrite, which possess a higher polarizability or dielectricconstant, is higher than that of the TPNR or the TPNRferrite composite.

IV. CONCLUSION

The dependence of the absorption characteristics of aspecular absorber ~backed by a perfect conductor! on thethickness, dielectric and magnetic properties of the materialsare discussed. The ferrite was found to impose only a smallchange on the microwave electromagnetic properties of theTPNR. The dielectric loss in the samples at low frequenciesis very much influenced by dc conductivity, whereas the lossat higher frequencies is attributed to ac conductivity. Theabsorption properties of the ferrite analyzed by a specularabsorber method reveal two matching conditions in the low-and high-frequency regimes. The first matching condition isdue to the material properties where umr*u5uer*u, while thesecond matching condition is due to geometrical cancellationof the incidence and reflected waves in the absorber when thethickness is equal to nl/4, where n is an odd integer and l isthe wavelength of the microwave in the materials. The re-flection loss or the dips of the TPNR and the TPNRferritecomposite are due only to geometrical factors. The numberof dips increases with an increase in thickness. It was ob-served from the study that the ferrite can be used as a nar-rowband absorber, whereas polymeric samples are good inbroadband absorption. Apart from that, the polymericsamples can also be used for narrowband applications basedon selective band absorption especially for thicker samples.The specular absorber provides a simple theoretical graphicaid for determining the absorption characteristics and the lo-cation of the matching conditions in the frequency domain.

FIG. 7. Variation of mr* and er* as a function of the frequency for allsamples. The crossing of the modulus of mr* and er* can be seen for theferrite at low frequency.



ACKNOWLEDGMENTThis work was supported by Research and Development

Grant Nos. IRPA 09-02-02-0005 and IRPA 09-02-02-0074,from the Ministry of Science, Technology and Environmentof Malaysia.

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