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 Articles you may be interested in Electromagnetic and microwave absorbing properties of SmCo coated single-wall carbon nanotubes/NiZn-ferrite nanocrystalline composite J. Appl. Phys. 115, 174101 (2014); 10.1063/1.4873636 Electromagnetic and microwave absorbing properties of raw and milled FeSiCr particles J. Appl. Phys. 115, 17B536 (2014); 10.1063/1.4869064 Electromagnetic and microwave absorption properties of magnetic stainless steel powder in 2–18 GHz J. Appl. 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The result for the ferrite sample was tested and confirmed directly from terminated one-port measurements. whereas the polymeric samples show broadband absorption characteristics. Jalan Raja Muda Abdul Aziz. wireless computer and pagers.my are a variety of absorber materials that can be used to suppress EMI depending on whether they are suitable for narrow.4 There a兲 Author to whom correspondence should be addressed. It can cause device malfunctions. with ␮ r⬘ the real magnetic permeability and ␮ r⬙ the magnetic loss.org/termsconditions.or highfrequency applications. Mansor and S. Universiti Teknologi MARA. iron or cobalt–nickel alloys as fillers. S. F.2 Recent developments in microwave absorber technology have resulted in materials that can effectively reduce the reflection of electromagnetic signals. electronic mail: nazlim@medic. plastics. for example. H.1489092兴 I. a lithium–nickel–zinc 共Li– Ni–Zn兲 ferrite and a TPNR–ferrite composite subjected to transverse electromagnetic 共TEM兲 wave propagation were investigated. To overcome the problems created by EMI. environmentally resistant absorbers often contain magnetic materials such as ferrites.1. and have good physical performance and lower production cost on the other. Downloaded to ] IP: 103. Jusoh School of Applied Physics. in hospitals. By incorporation of the magnetic fillers. generate false images. This is discussed as due to cancellation of the incident and reflected waves at the surface of the absorbers. A. thermoplastics. and S. A.246. The incorporation of the ferrite into the matrix of the TPNR was found to reduce the dielectric loss but the magnetic loss increased. on the one hand.00 876 © 2002 American Institute of Physics [This article is copyrighted as indicated in the article. commonly used dielectric materials are foams. the values of the dielectric permittivity and magnetic permeability of the materials can be altered to achieve maximal absorption of the electromagnetic energy. Universiti Kebangsaan Malaysia. These nonmagnetic. Reuse of AIP content is subject to the terms at: http://scitation. and ␮ r* ⫽ ␮ r⬘ ⫺ j ␮ r⬙ is the complex relative magnetic permeability. Minimal reflection of the microwave power or matching condition occurs when the thickness of the absorbers approximates an odd number multiple of a quarter of the propagating wavelength. Selangor Darul Ehsan. accepted for publication 6 May 2002兲 Electromagnetic properties of a thermoplastic natural rubber 共TPNR兲. INTRODUCTION The increase in electromagnetic pollution due to the rapid development of gigahertz 共GHz兲 electronic systems and telecommunications has resulted in a growing and intense interest in electromagnetic-absorber technology. The Li–Ni–Zn ferrite exhibits another matching condition at low frequency when the magnitude of the complex relative dielectric permittivity ( ⑀ r* ) equals that of the complex relative magnetic permeability ( ␮ r* ). banks. The absorption characteristics of all the samples subjected to a normal incidence of TEM wave were investigated based on a model of a single-layered plane wave absorber backed by a perfect conductor.106. 24 Feb 2015 03:37:25 . electromagnetic wave absorbers with the capability of absorbing unwanted electromagnetic signals are used.1063/1. Ahmad. Yusoffa) Diagnostic Imaging and Radiotherapy Programme. Pahang Darul Makmur.6. Universiti Kebangsaan Malaysia.or broadband absorption and for low. It is evident from a computer simulation that the ferrite is a narrowband absorber. 43600 Bangi. These are some of the reasons why the use of selfgenerated electromagnetic radiation apparatuses. Malaysia M. The specular absorber method has been widely used by several workers as a theoretical approach in explaining the propagation characteristics of a transverse electromagnetic 共TEM兲 wave in a single-layer absorber backed by a perfect conductor. rubbers. where ⑀ r* ⫽ ⑀ r⬘ ⫺ j ⑀ r⬙ is the complex relative dielectric permittivity. 26400 Jengka. which include cellular telephones. © 2002 American Institute of Physics. petrol stations and inside airplanes. The specular absorber method provides a simple theoretical graphic aid for determining the absorption characteristics and the location of the matching conditions in the frequency domain. and research on their electromagnetic and absorption properties is still being carried out. Electromagnetic interference 共EMI兲 can cause severe interruption of electronically controlled systems. increase clutter on radar and reduce performance because of system-to-system coupling.7 This method is based on the assumption that the dielectric permittivity and magnetic permeability are intrinsic properties of the material. NUMBER 2 15 JULY 2002 Electromagnetic and absorption properties of some microwave absorbers A. natural rubbers and polypyrroles. Hamid Department of Physics.aip. For a wave normally incident on the surface of a single-layer absorber backed by a perfect 0021-8979/2002/92(2)/876/7/$19.ukm. are strictly prohibited in certain areas. Faculty of Science and Technology. 关DOI: 10. with ⑀ r⬘ the real part or dielectric constant and ⑀ r⬙ the imaginary part or dielectric loss. In the microwave region. NR. ZnO 共99. isolation and frequency response in both the forward and reverse measurements.0 mm thick using compression molding under pressure of about 700 MPa at FIG.5 mm outside diameter and 1. The prereacted mixture was then reground for another 2 h. and 共b兲 one-port terminal short fixture for * ). 2. where Z 0 ⫽ 冑 ( ␮ 0 / ⑀ 0 )⫽377 ⍀ is the intrinsic impedance of free space.6 mm inner diameter were prepared from the TPNR and TPNR–ferrite sheets. The scattering parameters of the toroidal samples that * and S 22 * 兲 and transmission correspond to the reflection (S 11 * and S 12 * 兲 of a TEM wave were measured using a Hewlett (S 21 Packard 8719D microwave vector network analyzer. and LNR in a weight ratio10. measuring the reflection scattering parameter (S 11 175 °C. A cylindrically shaped ferrite sample 5. The powder and the cylinder were then sintered at 1050 °C for 15 h and furnace cooled to room temperature. we report the microwave dielectric.3Fe2.2O4 共Li–Ni–Zn兲 ferrite was prepared by a double sintering method in air. 20 wt % natural rubber 共NR兲 and 10 wt % liquid natural rubber 共LNR兲. ␻ is the angular frequency. R L was also measured by the terminated * test fixture. A toroid sample of 3.5 mm outside diameter and 1. Vol. MATERIALS AND METHODS A polycrystalline Li0.org/termsconditions. The TPNR matrix was prepared by melt blending PP. 92. in decibels 共dB兲. load match. A small quantity of polyvinyl alcohol 共PVA兲 was used as a binding agent.999%兲 and Fe2O3 共99. which was prepared by photosynthesized degradation of NR in visible light. 24 Feb 2015 03:37:25 . No.2Ni0. Figure 1共a兲 shows the coaxial fixture used in measuring the scattering parameters.995%兲. Reuse of AIP content is subject to the terms at: http://scitation. The measurement was performed in the frequency range of 1–13 GHz.5 mm coaxial measurement cell. ␥ ⫽ 关 j ␻ ( ␮ r* ⑀ r* ) 1/2兴 /c is the propagation factor in the material. Downloaded to ] IP: 103. thickness and both the dielectric permittivity and magnetic permeability were obtained based on a model in which an electromagnetic wave is incident normal to the surface of the material backed by a perfect conductor. can be written as R L ⫽20 log10兩 ⌫ 兩 . The dip in R L indicates the occurrence of absorption or minimal reflection of the microwave power. Appl. The LNR. Toroidal samples of 3. The dependences of the absorption characteristics on the frequency.aip. In this article.8.J. NiO 共99. was blended with NR and PP in a laboratory cam mixer 共model Brabender Plasticorder PL 200兲 at 170 °C at a rotating speed of 50 rpm. source match.5 wt % Bi2O3 共99. mixed and ground thoroughly for 2 h in the desired stoichiometric composition together with 0.106. The effects of incorporating the ferrite into the matrix of the TPNR on the absorption characteristics of the material are examined and discussed. Phys. The intensity and the frequency at the reflection loss minimum. II. as shown in one-port technique using a short S 11 [This article is copyrighted as indicated in the article. 877 conductor. 共a兲 Two-port coaxial fixture used in measuring the complex scattering parameters (S * 11 and S * 21).998%兲 were weighed. therefore. Full two-port calibration was initially performed on the test setup in order to remove errors due to the directivity.999%兲.9 On: Tue. magnetic and absorption properties of a thermoplastic natural rubber 共TPNR兲 that is composed of 70 wt % polypropylene 共PP兲.0 mm in diameter and 5.246.. a Li–Ni–Zn ferrite and a composite that consist of 70 wt % of the TPNR and 30 wt % of the Li– Ni–Zn ferrite.9 The reflection coefficient 共⌫兲 is defined as ⌫⫽(Z in /Z 0 ⫺1)/(Z in /Z 0 ⫹1) The ⫽ 关 ( ␮ r* / ⑀ r* ) 1/2 tanh(␥t)⫺1兴/关(␮r*/⑀r*)1/2 tanh(␥t)⫹1兴. power reflectivity or the reflection loss (R L ). The toroids tightly fit into a 3. The real and the imaginary components of the complex dielectric permittivity and magnetic permeability were determined from the complex scattering parameters using the Nicolson–Ross12 共for magnetic兲 and precision13 共for nonmagnetic兲 models. c is the speed of light and t is the thickness of the sample. 1. After 12 min of mixing. The NR and LNR were allowed to mix for about 2 min before the PP was introduced.6 mm inner diameter was machined from the ferrite cylinder for the microwave measurements. The mixture was presintered at 800 °C for 6 h and subsequently furnace cooled to room temperature.5%兲. The desired amounts of the TPNR mixture 共70 wt %兲 and of ferrite powder 共30 wt %兲 were mixed in the sample machine and blended in a similar manner. depend on the properties and thickness of the materials.0 mm in thickness was molded under pressure of about 300 MPa. the homogeneous TPNR mixture was removed from the mixer and ground in a granulator machine 共model Ph 400 SS兲. Powders of high purity Li2O 共99.11 of 70:20:10 with the LNR as the compatibilizer. 15 July 2002 Yusoff et al.3Zn0. the input impedance (Z in兲 at the air–material interface is given by Z in⫽Z 0 ( ␮ r* / ⑀ r* ) 1/2 tanh(␥t). The TPNR and the composite were molded into a thin sheet 5. * and S 12 * are similar to those of S * * The plots of S 22 11 and S 21 * but are not shown. Phys.aip.. dielectric and magnetic properties.15.6. The loss if almost constant between 2 and 8 GHz.org/termsconditions. * and S 21 parameters.2O4 –ferrite 共䊐兲 samples. dielectric and magnetic properties of the ferrite materials on microwave propagation was briefly discussed elsewhere.16 The dielectric loss. No. TPNR–ferrite composite 共䉮兲 and the Li0. Downloaded to ] IP: 103. The difference in reflectivity and transmittivity among the samples can be suggested to be due to the differences in their microstructures. The Li–Ni–Zn ferrite is crystalline with high magnetic moment whereas TPNR is nonmagnetic and semicrystalline in nature. but ⑀ r⬙ for the TPNR is always higher than that for the TPNR–ferrite composite. 1共b兲.3Zn0. 2. Furthermore.3Zn0. The simulated data are compared to those from the experimental one-port technique. RESULTS AND DISCUSSION The frequency dependences of the complex scattering * . S 11 * The TPNR and TPNR–ferrite composite show a lower S 11 * . This indicates that the ferrite rehigher S 11 flects more but transmits less microwave energy than the other two samples over the whole frequency range. 3. is not constant over the whole frequency range.106. however. The frequency variation of the dielectric loss for the ferrite is different from that of the two polymeric samples.3Fe2. Frequency dependence of the complex scattering parameters 共S * 11 * 兲 of the TPNR 共䊊兲. It can be clearly seen that all samples show an almost constant ⑀ r⬘ value throughout the whole frequency range used in this work.3Fe2. 2. The most probable mechanism in this frequency range is orientational polarization. 92.9 On: Tue. The ferrite shows the highest ⑀ r⬘ value followed by the TPNR–ferrite composite and the TPNR matrix. but increases slightly [This article is copyrighted as indicated in the article.2Ni0. 24 Feb 2015 03:37:25 .7 The incorporation of the ferrite into the TPNR matrix is found to have only a small effect on the reflection and transmission properties. III. Figure 3 shows ⑀ r⬘ and ⑀ r⬙ values for the three samples in the frequency range of 1–13 GHz.878 J. * and S 21 * for all the samples indicates their recibetween S 12 procity in the absence of an external magnetic field. Appl.14 This is supported by the fact that neither relaxation nor resonant type behavior is present in the ⑀ r⬘ plot. Frequency dependence of the real ( ⑀ r⬘ ) and the imaginary ( ⑀ r⬙ ) parts of the complex dielectric permittivity of the TPNR 共䊊兲.2Ni0. the TPNR–ferrite composite 共䉮兲 and the and S 21 Li0. but there is a tendency for the TPNR–ferrite composite to behave like the ferrite. In this case R L ⫽20 log10兩 S 11兩 . The effects of microstructure.246. The polymeric samples show a gradual decrease of ⑀ r⬙ towards high frequencies. The S parameters for the Li–Ni–Zn ferrite sample than S 21 are different from the other two polymeric samples with * and lower S 21 * . 2. Vol. The similarity between S * and S and 11 22 Yusoff et al. FIG. the atomic and electronic polarizations occur at a period shorter than the period of a microwave.2O4 –ferrite 共䊐兲 toroids. for all samples are shown in Fig. 15 July 2002 FIG. Fig. Reuse of AIP content is subject to the terms at: http://scitation. The mechanism of polarization in the ferrite at microwave frequencies is dependent on the availability of ions of different valences and it is believed that the orientational polarization in the ferrite is mainly a result of the process of electron transfer between ferrous (Fe2⫹) and ferric (Fe3⫹兲 ions. The wall resonance nearly vanishes in the TPNR– ferrite composite sample because the particles are too small to support a multidomain structure. 䉮: 5 mm. Frequency dependence of the real ( ␮ r⬘ ) and the imaginary ( ␮ r⬙ ) parts of the complex magnetic permeability of the TPNR 共䊊兲. ␻ is ⫽ 关 ␴ dc /( ␻ ⑀ 0 )⫹ ⑀ ac the angular frequency. The values of ␮ r⬘ and ␮ r⬙ are. 15 July 2002 FIG.5 GHz. The magnetic permeability for the TPNR matrix is as expected since it is nonmagnetic. while ␮ r⬙ is slightly increased above zero throughout the whole frequency range. 4. Vol. Figure 4 shows the real and imaginary parts of the complex magnetic permeability 共␮ r⬘ and ␮ r⬙ 兲 for the three samples. for frequencies below 2 and above 8 GHz. No. The curves are obtained by assum- [This article is copyrighted as indicated in the article. ␮ r⬘ for the ferrite shows a gradual increase with an increase in frequency for frequencies above 7.aip.106. 䊐: 15 mm and 〫: 30 mm. Appl.5 mm. TPNR–ferrite composite 共䉮兲 and the Li0. the reason for the increase in ⑀ r⬙ for the materials with a decrease in frequency in the lowfrequency regime.9 On: Tue. and 30 mm兲. Reuse of AIP content is subject to the terms at: http://scitation. 15.19 explains the suppression of the resonance peak observed in this study.兲 higher than 3 GHz. The dielectric loss in the samples can be described as due to the contributions from both the dc conductivity and the ac conductivity or ion jump and dipole relaxation based on the expression2.org/termsconditions. ⑀ 0 is the permittivity of free space and ⑀ ⬙ac is the ac loss contribution at high frequencies. is hardly observed for the ferrite due to the large influence of wall resonance but a closer look at the logarithmic plot for the TPNR–ferrite composite sample 共diagram in inset兲 indicates a small peak that could be due to spin resonance which occurs at approximately 3. on the other hand. 92. However.5.2Ni0.18 The pure TPNR sample exhibits no wall resonance as expected. 共䊊兲: 2. 24 Feb 2015 03:37:25 .3Zn0. The randomness in the internal fields caused by variation of the spontaneous magnetization and anisotropy field at different points in the unmagnetized samples18. The inset shows the occurrence of ferrimagnetic or spin resonance in the TPNR– ferrite composite sample. where ␴ dc is the dc conductivity. 879 FIG. 5. The plot for the ferrite also shows that ␮ r⬘ ⬎1 for frequencies below 3 GHz and 0⬍ ␮ r⬘ ⬍1 for frequencies Yusoff et al. Reflection loss plot at several thicknesses 共t兲 of 共a兲 the TPNR and 共b兲 the TPNR–ferrite composite..17 ⑀ r⬙ ⬙ 兴 . The effects of incorporating the ferrite into the matrix of the TPNR matrix is to raise ␮ r⬘ above unity at low frequencies and lower ␮ r⬘ at high frequencies.246. Downloaded to ] IP: 103. Ferrimagnetic or spin resonance. A sharp decrease in ␮ r⬘ and ␮ r⬙ with the frequency from 1 GHz for the ferrite constitutes a part of the resonance peak due to domain wall resonance which is supposed to occur at lower frequency. The expression shows that dc conduction loss is inversely proportional to the frequency.3Fe2. respectively. 2. unity and zero in the whole frequency range for the nonmagnetic TPNR sample.2O4 –ferrite 共䊐兲 samples.5 GHz.J. while a strong decrease with an increase in frequency for both quantities is observed at low frequencies for the ferrite. ion jump and relaxation between two equivalent Fe2⫹ and Fe3⫹ ion positions are responsible for the dielectric loss at high frequencies. Phys.8 For the ferrite. 5. hence. Figure 5共a兲 shows the frequency dependence of the reflection loss for the TPNR at various sample thicknesses 共t ⫽2. Similar behavior has also been observed for a Mn–Zn ferrite–rubber composite. 106. 5共b兲. the incident and reflected waves in the material are out of phase 180°. Apparently.org/termsconditions.5 mm. This is the reason why the microwave power is not totally absorbed by the materials.5. 92. The dips for each thickness from left to right can also be shown to occur at t⫽n␭/4 共n⫽1.1共␭/4兲. 5.246. This is believed to be due to impedance mismatch at the air–material interface. 6.5兲0. a broad reflection loss dip starts to appear. the spin rotational resonance frequency of the (Li0. resulting in total cancellation of the reflected waves at the air–material interface.6 GHz.9 However. 3. The [This article is copyrighted as indicated in the article.1␭/4.7 and 2. respectively. Compared with the result for the 5 mm thick TPNR. 9. however. 9. 24 Feb 2015 03:37:25 .5 mm.9 On: Tue. 7.1共␭/4兲 and 7.5Fe0. 䉮: 5 mm. the loss starts to appear at higher frequencies. 5.1共␭/4兲. However.兲.2 and 12. where n⫽1 corresponds to the first dip at low frequency. . the inclusion of the ferrite filler has lowered the frequency of the quarter wavelength dip. the number increases as the thickness is increased and the dip for the same n of different thicknesses is shifted towards a lower frequency region. It is suggested that the maximal absorption at this frequency is simply due to 兩 ␮ r* 兩 ⫽ 兩 ⑀ r* 兩 regardless of the existence of resonance or not. The plots for t⫽2. 5.2 The matching frequencies are 1. Downloaded to ] IP: 103. Phys. two frequency-thickness configurations of the ferrite where minimal reflection of microwave power occurs can be determined. The first matching at low frequency in some ferrites has been related to the spin rotational resonance frequency. it can be shown that the dips occur when the thicknesses equal to 1.. 共a兲 Frequency dependence of the reflection loss of the ferrite at various sample thicknesses 共䊊: 2.aip.. A similar calculation shows that the dip occurs at a thickness of 1. For t⫽30 mm. 7. as depicted in Fig.5. The results for the TPNR–ferrite composite appear to be similar to those for the TPNR. 2. FIG. The plots for the TPNR show that the number of dips increases with an increase in sample thickness. 䊐: 15 mm and 〫: 30 mm兲. For t⫽5 mm. The dips of minimal reflection for the ferrite in the low-frequency region are also shifted towards a lower frequency with an increase in thickness. 3. 15 and 30 mm are shown in Fig.3Zn0. 共b兲 frequency dependence of the reflection loss of the ferrite showing two matching conditions at low and high frequencies from the simulated two-port technique and 共c兲 the results for the simulated two-port and the experimental one-port techniques of the pure ferrite 共thickness ⫽ 5.4Ni0. Calculations performed on other thicknesses give similar results. At that particular thickness. 7. 5. The reflection loss minimum or the dip in R L is equivalent to the occurrence of minimal reflection of the microwave power for the particular thickness. The propagating wavelength in a material (␭ m ) is expressed by ␭ m ⫽␭ 0 /( 兩 ␮ r* 兩兩 ⑀ r* 兩 ) 1/2 where ␭ 0 is the free space wavelength and 兩 ␮ r* 兩 and 兩 ⑀ r* 兩 are the moduli of ␮ r* and ⑀ r* .. Reuse of AIP content is subject to the terms at: http://scitation. where two and four complete dips can be observed for t⫽15 and 30 mm..9 mm. 15 and 30 mm. Appl.2共␭/4兲. . but the location of each consecutive dip is shifted towards a higher frequency for a smaller value of t.880 J. respectively. It can be seen that there is only one shallow dip for t⫽5 mm and almost no reflection loss for t⫽2.3Fe2O4 ferrite could not be determined due to the absence of a resonance peak in the magnetic loss spectrum. For higher values of t.. by using a similar computer simulation. with corresponding matching thickness values of 6. as shown in Fig. R L is calculated from a computer simulation using the values of ␮ r* and ⑀ r* previously obtained. microwave absorbers. Vol. 15 July 2002 ing normal incidence of the electromagnetic field on the surface of a specular TPNR absorber backed by a perfect conductor. the occurrence of reflection loss for the TPNR–ferrite composite is similar to that of the TPNR but the magnitude or intensity varies in a complicated manner. Clearly demonstrated is that the intensity and frequency of the reflection loss minimal for the ferrite also depend on the material’s thickness. This behavior was also found in Ni–Zn–Co ferrite composites1 and in polypyrrole-based Yusoff et al. The occurrence of the dips is found to be due to a successive odd number multiple of the quarter wavelength thickness of the material or t⫽n␭/4 共n⫽1. the reflection loss over a wide frequency range is small.5 mm. respectively. 3. Figure 6共a兲 shows the frequency dependence of the reflection loss of the ferrite at sample thicknesses of 2. No. 6共b兲. 5. For t⫽2.09 mm兲.兲. aip. while the second matching condition is due to geometrical cancellation of the incidence and reflected waves in the absorber when the thickness is equal to n␭/4. but the magnitude decreases with an increase or decrease in thickness.246. 24 Feb 2015 03:37:25 . As depicted in Fig. Reuse of AIP content is subject to the terms at: http://scitation. The absorption of microwaves has been shown to depend on the polarizability of the materials.9 mm. Figure 6共c兲 shows an example of simulated and experimental plots of R L for a 5. Figure 7 also shows plots of ␮ r* and ⑀ r* for the TPNR and TPNR– ferrite composite. The crossing of the modulus of ␮ r* and ⑀ r* can be seen for the ferrite at low frequency.9 mm. material properties play an important role in the low-frequency absorption. 2. 6共a兲. The dielectric loss in the samples at low frequencies is very much influenced by dc conductivity. is higher than that of the TPNR or the TPNR– ferrite composite. Maximal absorption for t⫽15 and 30 mm is expected to occur at lower frequencies.09 mm thick ferrite sample. which is equal to the thickness of the sample at that frequency. 15 July 2002 881 absent for both samples and the parameters are separated throughout the whole frequency range. This shows that the dips observed for the TPNR and TPNR–ferrite composite are due to geometrical factors whereas for the ferrite. Variation of ␮ r* and ⑀ r* as a function of the frequency for all samples. Downloaded to ] IP: 103.org/termsconditions. dielectric and magnetic properties of the materials are discussed. simulation used in this study agreed very well with the experimental result using the S 11 short technique. The wavelength in the material at the second matching condition is 11. In Fig.106. It was observed from the study that the ferrite can be used as a narrowband absorber. However. Hence ␭/4⫽2. which possess a higher polarizability or dielectric constant. such as the existence of the air layer between the sample and the termination and dimensional inaccuracies of the sample. 6. The first matching condition is due to the material properties where 兩 ␮ r* 兩 ⫽ 兩 ⑀ r* 兩 . Figure 7 shows that the crossing of the modulus of ␮ r* and ⑀ r* in the low-frequency region in the ferrite sample occurs at the same frequency of maximal absorption.21 Polar molecules are known to strongly absorb microwaves in comparison to nonpolar molecules.5 mm. 7.Yusoff et al. CONCLUSION FIG. The second matching frequency is associated with the quarter wavelength 共␭/4兲 thickness of the material as discussed for the TPNR and TPNR–ferrite composite. it can be observed that the matching condition is about to occur at thickness of 2. the crossing of ␮ r* and ⑀ r* is The dependence of the absorption characteristics of a specular absorber 共backed by a perfect conductor兲 on the thickness. whereas the loss at higher frequencies is attributed to ac conductivity.. The reflection loss or the dips of the TPNR and the TPNR–ferrite composite are due only to geometrical factors. The specular absorber provides a simple theoretical graphic aid for determining the absorption characteristics and the location of the matching conditions in the frequency domain.20. maximal absorption around this frequency also occurs at other thicknesses. [This article is copyrighted as indicated in the article. Appl. Apart from that. The number of dips increases with an increase in thickness. Vol. No. The slight deviation in the magnitude and in the location between the two dips could be due to some experimental errors. IV. J. 92. the polymeric samples can also be used for narrowband applications based on selective band absorption especially for thicker samples. whereas polymeric samples are good in broadband absorption.9 On: Tue. The ferrite was found to impose only a small change on the microwave electromagnetic properties of the TPNR. where n is an odd integer and ␭ is the wavelength of the microwave in the materials. This explains the present results that the microwave power absorption by the ferrite. A slight variation in the frequency is due to the variation of the factor tanh共␥t兲 in the equation for reflection loss. The absorption properties of the ferrite analyzed by a specular absorber method reveal two matching conditions in the lowand high-frequency regimes. Phys. 6 S. T. p. Y. 92. D. Raman. 67. K. IEEE Trans. Y. Y. V. Energy 1. S. Sci. E. Sci. J. Magn. E. 12 A. W. 1039 共2000兲. Appl. K. 15 July 2002 10 S. Ramakrishna. Chaki. I. 58. 25 共1990兲. Patton. C. Ross. R. in Dielectric Relaxation in Solids 共Chelsea Dielectric. Meas. Kwon. 30. 2. Magn. 6109 共1994兲. Abdullah. 21 R. Appl. London. J. 1990兲. J. Z. Boey. Cho. Truong. and J. J. Shin. Sci. Yusoff. 31. London. J. Composites. edited by D. [This article is copyrighted as indicated in the article. Khastgir. 18 E. p. IEEE Trans. L. E. N. N. and K. and C. Gedye. p. 5462 共1991兲. H. J. and R. 5231 共1995兲. 32. T. Riddell. 555 共1996兲. and K. Kim. IEEE Trans. Churn. 32. Polym. 17 共1988兲. E.882 ACKNOWLEDGMENT This work was supported by Research and Development Grant Nos. Muscat. 75. 4 N. Oh. 17 A. J.246. J. Abdullah. part 2. Phys. Instrum. Chakraborty. K. Jonscher. J. Y. Sulaiman. H. 66. Nicolson and G. No. IEEE Trans. Ahmad. Appl. Kohijiya. Mater. 4971 共1998兲. 8 D. Y. p. Smith. 29. 1125 共1995兲. J. 9 H. Ahmad. B. from the Ministry of Science. Craik 共Wiley. S. Wallace. Appl. Phys. 33. 14 J. Polym. 5 J. C. New Delhi. H. IM-19. F. and K. 19 C. R. K. M. 575. Jo. 1069 共2000兲. 3 K. and F. S. 51. Oh. 15 M. Chia. 13 Herlett Packard HP 8719D Microwave Vector Network Analyzer manual. Reuse of AIP content is subject to the terms at: http://scitation. Composites. S.9 On: Tue. C. Mater. F. 1 2 Yusoff et al. 29. London.aip. J. C. L. 5817 共1997兲. Kim. 51. B. B. Abdullah and A. 46. Y. Sci. Jacob. M. Ferrites: A Review of Materials and Applications 共Mills and Boon. 1357 共1994兲. Chung. Downloaded to ] IP: 103. C. J. Magn. and J. N. Choi. 27. Sulaiman. Technology and Environment of Malaysia. K. 24 Feb 2015 03:37:25 . K. J. Microwave Power Electrmagn. and K. Cheng. C. Kim. Lee. Smith. 11 S. C. IEEE Trans. Magn. and A. Sobhanadri and R. Shin and J. L. S. Riches.. Mater. Gueon. Chem. Part A 31. and J. F. 16 J. Westaway. F. 1972兲. C. T. Part A 31. Vol. H.org/termsconditions. Mater. Oh. Westaway. and H. Viswanathan and V. S. 377 共1970兲. I. Sci. Kang. S. 20 R. Gedye. edited by B. J. Kang. Can. Sci. 1983兲.106. 1975兲. 7 J. I. Yoon. IRPA 09-02-02-0005 and IRPA 09-02-02-0074. S. 4719 共1996兲. 3437 共1993兲. H. Das. D. Kim. 4209 共1993兲. in Ferrite Materials: Science and Technology. Microwave Resonance and Relaxation in Magnetic Oxides. Murthy 共Narosa. S.