Sains Malaysiana 36(1): 65-75 (2007)
Electromagnetic and Absorption Properties of Some TPNR/Fe3O4/YIG Microwave Absorbers as Revealed by a Microwave Vector Network Analyser and Specular Absorber Method (Sifat Electromagnet dan Penyerapan Beberapa Penyerap Gelombang Mikro TPNR/Fe304/YIG yang diperlihatkan oleh Penganalisis Rangkaian Vector Gelombang Mikro dan Kaedah Penyerap Spekular)
Ahmad Nazlim Yusoff Diagnostic Imaging & Radiotherapy Programme Faculty of Allied Health Sciences, Universiti Kebangsaan Malaysia 50300 Jalan Raja Muda Abdul Aziz, Kuala Lumpur, Malaysia
Junaidah Mat Sani Physics Department, Faculty of Applied Sciences MARA University of Technology 26400 Jengka, Pahang Darul Makmur, Malaysia.
Mustaffa Hj. Abdullah Physics Programme, Faculty of Science & Technology Universiti Kebangsaan Malaysia 43600 Bangi, Selangor Darul Ehsan, Malaysia
Sahrim Hj. Ahmad & Nazree Ahmad Materials Science Programme, Faculty of Science & Technology Universiti Kebangsaan Malaysia 43600 Bangi, Selangor Darul Ehsan, Malaysia
The complex scattering parameters (S11* and S21*), relative dielectric permittivity (er* = er' - jer"), relative magnetic permeability (mr* = mr' - jmr") and absorption characteristics of some thermoplastic natural rubber (TPNR) – Fe3O4/YIG composites were investigated by means of a microwave vector network analyser and specular absorber method. The measurements were performed in the frequency range of 1 – 13 GHz with and without the presence of a transverse external magnetic field with magnitude of 1 T. The specular absorber method is used in determining the reflection loss (RL) of the composites and its dependence on material properties, thickness and the external magnetic field. In toroidal form, the composites under study seem to transmit more (Pt > 60%) but absorb and reflect less (Pa + Pr < 30%) microwave power in the frequency range used in this study and in both unmagnetised and magnetised states. The external magnetic field is seen to have the effects of reducing S11*, S21*, er', er", mr' and mr". The suppression of the relaxation and resonance behaviours on the mr' and mr" plots for all samples are observed for measurements performed in the presence of the external field. The composites, as revealed by the specular absorber method, show conditions of minimal reflection on RL versus frequency plot for all sample thickness where more than 95% of the microwave power is not reflected back. The location of the dips of minimal reflection on the frequency domain depend on the thickness, the dielectric and magnetic properties of the materials. The conditions of minimal reflection was found to occur when the thickness (t) of the sample equals the odd number multiple of a quarter wavelength in the material (lm), t = nlm/4 (n = 1, 3, 5, 7 …), where a geometrical cancellation took place at the surface of the absorber between the reflected waves, resembling a thin filem inteference phenomenon. The dips are suppressed and shifted to a higher frequency in the presence of the external magnetic field. With the knowledge of the dielectric permittivity and magnetic permeability of a material, the specular absorber method may provides a simple theoretical graphic aids in determining the absorption characteristics and location of the matching condition in the frequency domain.
Keywords : specular absorber method; thermoplastic natural rubber; yttrium iron garnet; reflection loss; quarter wavelength thickness ABSTRAK Parameter penyerakan (S11* dan S21*), ketelusan dielektrik relatif (er* = er' - jer") dan ketelapan magnet relatif (mr* = mr' - jmr") kompleks serta ciri-ciri penyerapan bagi beberapa komposit getah asli termoplastik (GATP) – Fe3O4/YIG diselidiki menggunakan penganalisis rangkaian vektor gelombang mikro dan kaedah penyerap spekular. Pengukuran dilakukan dalam julat frekuensi 1 – 13 GHz dengan dan tanpa kehadiran medan magnet luar melintang dengan magnitud 1 T. Kaedah penyerap spekular digunakan dalam menentukan kehilangan pantulan (RL) bagi komposit dan kebergantungannya terhadap sifat dan ketebalan bahan serta medan magnet luar. Dalam bentuk toroid, komposit yang dikaji didapati lebih banyak menghantar (Pt > 60%) tetapi kurang menyerap dan memantulkan (Pa + Pr < 30%) kuasa gelombang mikro dalam julat frekuensi yang digunakan dalam kajian ini untuk kedua-dua keadaan termagnet dan tidak termagnet. Medan magnet luar dilihat sebagai mempunyai kesan mengurangkan S11*, S21*, er', er", mr' dan mr". Penekanan kelakuan relaksasi dan resonans ke atas plot mr' and mr" untuk kesemua sampel dapat diperhatikan untuk pengukuran yang dilakukan dengan kehadiran medan magnet luar. Kesemua komposit, seperti yang diperolehi daripada kaedah penyerap spekular, menunjukkan keadaan pantulan minimum pada plot RL melawan frekuensi untuk semua ketebalan sampel dengan lebih daripada 95% daripada kuasa gelombang mikro tidak dipantulkan semula. Lokasi keadaan dengan pantulan minimum (puncak minimum) pada domain frekuensi bergantung kepada ketebalan, sifat dielektrik dan magnet bahan. Keadaan dengan pantulan minimum didapati berlaku apabila ketebalan (t) bahan bersamaan dengan gandaan ganjil bagi seperempat panjang gelombang di dalam bahan (lm), t = nlm/4 (n = 1, 3, 5, 7 …), pada mana suatu pembatalan secara geometri berlaku pada permukaan penyerap di antara gelombang-gelombang terpantul, menyerupai fenomena inteferens filem nipis. Puncak minimum didapati tertekan dan teranjak ke frekuensi yang lebih tinggi dengan kehadiran medan magnet luar. Dengan adanya maklumat mengenai ketelusan dielektrik dan ketelapan magnet bahan, kaedah penyerap spekular dapat menyediakan suatu bantuan grafik secara teori dalam menentukan ciri penyerapan dan lokasi keadaan kesepadanan dalam domain frekuensi.
Kata kunci: kaedah penyerap spekular; getah asli termoplastik; garnet itrium besi; kehilangan pantulan; ketebalan seperempat panjang gelombang
RUJUKAN/REFERENCESAhmad Nazlim Yusoff. 2000. Penyediaan, pencirian dan kegunaan bahan ferit Li-Ni-Zn. Ph.D Thesis, Universiti Kebangsaan Malaysia. Dyer, S. A. 2001. Survey of Instrumentation and Measurement, Canada, John Wiley & Sons. Kim, S. S., Jo, S. B., Gueon, K. I., Choi, K. K., Kim, J. M. and Churn, K. S. 1991. Complex permeability and permittivity and microwave absorption of ferrite rubber composites in X-band frequencies. IEEE Trans. Magn. 27(6): 5462-5464. Kim, D. Y., Chung, Y. C., Kang, T. W. and Kim, H. C. 1996 Dependence of microwave absorbing property on ferrite volume fraction in Mn-Zn ferrite rubber composites. IEEE Trans. Magn. 32(2): 555-558. McCann, J, Dietrich, F and Rafferty, C. 1998. The genotoxic potential of electric and magnetic fields : an update. Mutation Research 411: 45-86. Meshram, M. R., Agrawal, N. K., Sinha, B. and Misra, P. S. 2004. Characterization of M-type barium hexagonal ferrite-based wide band microwave absorber. J. Magn. Magn. Mater. 271(2-3): 207-214. Musal, H. M. and Hahn, H. T. 1989. IEEE Trans. Magn. 25(5): 3851. Musal, H. M. and Smith, D. C. 1990. Universal design chart for specular absorbers. IEEE Trans. Magn. 26(5): 1462-1464. Shin, J. Y. and Oh, J. H. 1993. The microwave absorbing phenomena of ferrite microwave absorbers. IEEE Trans. Magn. 29(6): 3437-3439. Srivastava, S. L. 1993. Microwave dielectric spectroscopy. In Recent Advances in Microwaves Ed. G. P. Srivastava, New Delhi: Narosa Publishing House. Wallace, J. L. 1993. Broadband magnetic microwave absorbers : Fundamental limitations. IEEE Trans. Magn. 29(6): 4209-4214. Yusoff, A. N., Abdullah, M. H., Ahmad, S. H., Jusoh, S. F., Mansor, A. A. and Hamid, S. A. A. 2002a. Electromagnetic and absorption properties of some microwave absorbers. J. Appl. Phys. 92(2): 876-882. Yusoff, A. N., Abdullah, M. H., Ahmad, S. H., Jusoh, S. F., Mansor, A. A and Hamid, S. A. A. 2002b. Microwave dielectric, magnetic and absorption properties of some (Li0.5Fe0.5)0.7-xNixZn0.3Fe2O4 ferrites. J. Phys. Sci. 13: 109-124. Yusoff, A. N. and Abdullah, M. H. 2004. Microwave electromagnetic and absorption properties of some Li-Zn ferrites. J. Magn. Magn. Mater. 269: 271-280. Truong, V. –T., Riddell, S. Z. and Muscat, R. F. (1998) Polypyrrole based microwave absorbers. J. Mater. Sci. 33: 4971-4976.
|
|||
|
