DEVELOPMENT AND CHARACTERIZATION OF FERRITE BASED MICROWAVE ABSORBERS

Abstract

Microwave absorbing materials in the form of paints have been frequently coated on the surface of objects to reduce its radar cross section (RCS) effectively. In view of their wide spread applications, microwave absorbers have been the topic of intensive research for many years. Ferrites happen to be to an important class of magnetic material showing high microwave absorption properties. The present investigation concentrates mainly on the development and characterization of various types of microwave absorbing paints using hexagonal ferrites. Microwave absorber using hexagonal ferrite have been designed in the frequency range of 8GHz to 18GHz,with an aim of minimum absorption of 10 dB for a maximum coating thickness of 2 mm. In the present investigation eighteen samples of ferrites powders have been developed by varying dopants and through extensive attrition milling and sintering process. Average particle size of different ferrite powders have been monitored by Fisher sub -sieve sizer instrument. Synthesized ferrite powders have been mixed in 40% part ofan epoxy resin to form a microwave absorbing paint. These paints have been coated on the conducting aluminum sheet of size (94x72.5 mm) for Xband and (86x54.5 mm) for Ku band in different thicknesses (0.5 mm to 2 mm).Absorption has been measured experimentally. To characterize ferrite-absorbing paint, the effect of particle size as produced through attrition milling and sintering on microwave absorption has been studied. It was found that the maximum absorption is obtained when average particle size of ferrite powder lies in the range of 4 to 6 microns. Complex permeability and complex permittivitty of developed ferrite powders were also determined experimentally, since absorption coefficient depends on these parameters. Further, an empirical relation between 6 (fractional molecular weight) of (MnTi) and ferromagnetic resonance frequency(fr) for M-type hexagonal ferrite [Ba(MnTi)6Fe(i2.26)Oi<)] has been developed theoretically usingthe available data and curve fitting technique. Since no such relation is available in the literature, these relations are expected to help future researchers and scientists to develop microwave absorbers at desired resonant frequencies. As a next step, microwave absorption in single layer and multilayer absorbers were evaluated using Transmission Line Modeling (TLM) method . It was found that the two layer microwave absorber consisting of a combination of ferrites [Ba(MnTi)i.6Fe8.80i9] and [Ba(MnTi)i.8Fe8.40i9] , provide a broadband absorption characteristics with minimum absorption of9 dB in the frequency range of8.2GHz tol8 GHz, except at 11.8 GHz,12.1GHz and 14.9 GHz respectively for a coating thickness of 2 mmat normal incidence. Three-layer microwave absorber consisting of ferrites [Ba(MnTi)i.6Fe8.8019], [Ba(MnTi)i.7Fe8.60i9] and [Ba(MnTi)i.8Fe8.40i9] coated on aluminum sheet provides broadband absorption with minimum absorption of10 dB for overall coating thickness of1.5 mm in the frequency range of 12.4 GHz to 18 GHz .However a dip was observed at 14.9 GHz. It was also found that the optimum thickness for three-layer absorber is 1.8 mm with minimum absorption of 10.1 dB for a frequency range of 12.4 to 18 GHz. Some investigations have also been carried out for oblique incidence of electromagnetic wave on the M-type hexagonal ferrite [Ba(MnTi)1.6Fe8.8019] based microwave absorbing paint. The absorption for coating thickness of 1 mm of single layer paint has been evaluated for an angle of incidence of 30° in the frequency range of (8GHz to 18 GHz).It was found that the minimum absorption of 10.26 dB and 12 dB is obtained for parallel and perpendicular polarization respectively in the frequency range of 8.8 GHz to 10 GHz. Further,it was observed that as the angle of incidence increased to 60°,the minimum absorption decreases to 5.47dB and 8.3 dB at parallel and perpendicular polarization respectively in the frequency range of 8.8GHz to 10 GHz . A mathematical model [2][11] has been used for the evaluation of absorption for Wtype hexagonal ferrite [Ba(Mni.6Co6)2Fei6027] based resonant and matched absorbers at X and Ku band respectively. It was found that there is a good agreement between theoretical and experimental results for a coating thickness of 2 mm, validating the model. To see the effect of dielectric material, on microwave absorption, the developed material such as barium titanate (BaTiC>3) and commercially available silicon carbide (SiC) have been studied with ferrite. It was found that, if the ferrite paint is coated over the dielectric paint, the absorption reduces while if dielectric layer is applied over ferrite, the overall absorption properties improved and provide wider band absorption characteristics. Further it was observed that only ferrite based microwave absorber provides minimum absorption of 1.43 dB from 15.8 to 18 GHz for a coating thickness of 1 mm, whereas if it is mixed with SiC in the proportion of 25:75 minimum absorption increases to 8 dB. It was also found that the SiC-Ferrite combination in two-layer microwave absorber provides broadband characteristics with minimum absorption of 10.2 dB in the frequency range of 10.2 GHz to 12 GHz for a coating thickness of 1.84 mm. Effects of various structures of frequency selective surfaces (FSS)on the performance of microwave absorber have also been studied. It was found, that the double square loop array and circle inside the square loop array, are the promising candidates as regards the maximum absorption is concerned. A simple equivalent circuit approach was used for the design of the different FSS structures incorporated at the front end of the absorber. in