Indi an Journal of Engineering & Materials Sciences Vol. 7. October-December 2000. pp. 422-425 Microwave absorption studies of BaCoTi ferrite composites Archana Razdan". V K Babbar" & Praveen Singh b* "Department of Applied Physics. Guru Nanak Dev University. Amritsar 143005. Indi a bDepartment of Physics. Indian Institute of Technology. New Delhi 110016. Indi a Received 25 FebruGlY 2000; accepted 5 Septelllber 2000 A ferrit e series of the compos iti on BaFeI2_2,CoxTixOI9. wherein x varies from 0.0 to 0.9 in steps of 0.2 . has been prcpared by the standard ceramic technique. The microwave absorption studies of ferrite-polymer composites with different till-factors have been made in the freq uency range from 8.6- 12.4 GHz. A maximum return loss of 14.7 dB has been obtained for compos ites with x equ al 0.3 and absorber thickness of 4.0 mm at 9.2 GH z. The dependence of matching frequency on compositio n is appa rent for this thickness. The maximum values of dc res istivity and th e Curie tem perature have been found to be 234.8 kQ and 469°C respectively. These values correspond to the same composition for which the return loss is the maximum. A dependence of return loss on complex dielectric co nstant and permeability has been investigated in the X-band. The magnetic dielectrics, such as M-type hexaferrites, are known to be effective microwave absorbers due to their lossy interaction with the electric and magnetic fields of microwaves, which transforms the microwave energy to thermal energyl.5. The complex permeability and permittivity affect the reflection and atten uation characteristics of these materials. The composites of such ferrites with suitable polymers are quite useful as they can be easily shaped and converted into a paste or a paint that can be applied on the objects of interest. The absorption characteristics of ferrite-po lymer composites depend on the composition of ferrite, nature of polymer, fill factor and thickness of composite layer and can be tailormade by altering these parameters. Barium hexaferrite has been extensively used for producing permanent magnets and microwave devices 6 . The substitution of C02+Ti 4+ ions for Fe 3+ ions in Ba-ferrite is expected to change the anisotropy constant and modify the properties so as to make them effective microwave absorbers. In the present work, we report the microwave absorpt ion characteristics of substituted BaCoTi hexaferrites obtained fro m the observed variations of permeability and permittivity as a function of C02+Ti 4 +-content at different frequencies. It helps to determine the optimum amount of C0 2+Ti 4 + ions in Ba-ferrite, which yields a minimum reflection loss over a wide freq uency range. *Present address : Indian Veterinary Research Institute. Izatnagar. Bareilly 243 122. India Experimental Procedure A ferrite series of composition BaFe I2-2,CoxTi xOI 9, wherein x varies from 0-0.9 in steps of 0.2 was prepared by the conventional ceramic method. The samples were synthesised fro m stoichiometric mixtures of BaC0 3 , CaC0 3 , Ti0 2 and Fe203 of 99.9% purity. The mixture was wet-milled in di stilled water and calcined in air at 1000a C for 12 h. The material was again wet-milled and granulated using a small quantity of polyvinyl alcohol as binder. It was then cold pressed into pellets followed by sintering at l300 a C for 24 h. The ferrite-epoxy composites with 50% of ferrite by weight were prepared by homogeneously mixing the ferrite powder with 90% epoxy resin (EPG 280) and 10% hardener (Number 10). The hexaferrite composites of differe nt thickness were prepared by using rectangular perspex dies. These samples were cured at 80 a C for 2 h and coated on aluminium substrate. The variations of return loss versus frequency were studied by measuring VSWR using X-band microwave bench. In order to measure the complex relative permeability (J.1' - jJ.1') and permittivity (E' - jE'), the toroidal-shaped samples of 3.5 mm outer diameter and 1.5 mm inner diameter were prepared. The measurements of E', E'~ J.1' and J.1" as a function of frequ ency were performed by a reflection/transmission technique using a Network Analyser (HP 851OB). For this, the toroidal-shaped samples were tightly inserted into the standard coaxial line of 15 cm air length and the reflection and RAZDAN el al.: MICROWAVE ABSORPTION OF BaCoTi FERRITE COMPOSITES transmission scattering parameters (SII, S21) were determined. These parameters were employed to 7 calculate the values of E', E'~ f-l' and f-l'~ The dc resistivity of these samples was measured by a two terminal method using spring-loaded copper electrodes. The gravity method 8 was used to determine the Curie temperature. Results and Discussion X-ray diffraction analysis revealed the magnetoplumbite structure for all the compositions of BaCoTi ferrites. The variations of return loss and percentage absorption with frequency have been studied for Ba-ferrite composites as a function of C02+Ti 4+-content, sample thickness and fillfactor. The return loss is found to be maximum when the fillfactor is 50% as shown in Fig. I for a typical composition, BaCoo.3Tio.3FeI1.401 9. The remaining data is, therefore, reported for 50% fill-factor only. The lines in figures are not theoretical curves but are drawn only to guide the eye. Fig. 2 shows the return loss and the percentage absorption versus frequency for different compositions and the thickness of 4.0 mm. A maximum absorption of 14.7 dB was obtained at frequency of 9.2 GHz for C02+Ti 4+-content, x, equal to 0.3. A lower absorption of 13.9 dB was observed at a frequency of 9.l GHz for the composition with x equal to 0.5. The return loss variation for composites of 3.0 mm thickness is shown in Fig. 3. It shows 16 ---x=O .O -"-·x=0.1 .. . .. x=0.3 -o-·x=0.5 --6--x=0.7 ___ x=0.9 14 12 - . m 10 ~ II) <J) 0 ...J 8 - C .... :::J Qi 6 · 0:: 4 - Or----.----,----,----~==~~~~ 8.5 9.5 9.0 10.0 11 .5 9-r------------------------------. 8 I , I. I , I I ---x=O.O -"-·x=0.1 .. . .. x=O .3 II I I 14 \ I • I m 12 ~ <J) II) 6 ~. 5 ,~ ," I Qi 8 I 0:: C :; . I I E 4 :::J II) II) I \ I' ...J ~ I ,; : o CD 10 0::: 11.0 Fig. 2 - Variation of return loss with frequency for different compositions of BaCoxTi xFel2_2xOl9 hexaferrite composites of thickness 4.0 mm 7 iii 10.5 Frequency (GHz) 16~-----------------------------. 0 -l 423 I .. I 3 I I I I l, \ ,, , I ,, *' ". I 6 \ ... I 2 I I 4 I 1 2 .. .. ;;'."" I '" "'.... .,ro'ji." " A O-~--~--~--~--~~~~~ 0 8,5 8.5 9.0 9.5 10.0 10.5 Frequency (GHz) Fig. I - Variation of return loss as a function of frequency and fill -factor for BaCoo.3Tio.3FeI1.4019 hexaferrite composites 9.0 9.5 10.0 10.5 11 .0 11.5 Frequency (GHz) Fig. 3 - Variation of return loss with freq uency for different compositions of BaCoxTixFe l2_2xOl9 hexaferrite composites of thickness 3.0 mm INDIAN J. ENG. MATER. SCI., OCTOBER-DECEMBER 2000 424 absorption of less than 8.0 dB at 9.2 and 9.6 GHz for compositions with x equal to 0.0 and 0.1 respectively. Such a small absorption indicates a high impedance mismatch at air-sample interface. Other composites with x equal to 0.5, 0.7 a nd 0.9 show a very small absorption of about 2.0 dB and are not shown. It is apparent that the return loss is greater for the sample thickness of 4.0 mm than that of 3.0 mm. For other values of thickness, the return loss is further low and is not shown. Fig. 2 shows two absorption maxima for compositions with x equal to 0.7 and 0.9 and sample thickness of 4.0 mm. Fig. 3 shows similar maxima for x equal to 0.0 and 0.3 with sample thickness of 3.0 mm. These may be attributed to the resonance phenomena due to domain wall displacement at lower frequencies and spin rotation at higher frequencies. The dependence of matching frequency on composition is apparent particularly for sample thickness of 4.0 mm. The dc resistivity is found to increase from 172.9 kQ to 234.8 kQ as x increases from 0 to 0.3 followed by a continuous decrease to 16.5 kQ as x approaches 0.9 in barium ferrite. The conduction in ferrites is attributed to the simultaneous presence of Fe2+ and Fe 3+ ions on equivalent lattice sites, which are mainly 4fvl and 2b sites present in the R-block of the M-type barium ferrite 9 . The electrons can hop between Fe2+ and Fe3+ ions on the neighbouring equivalent sites and cause conduction. As C02+ and Ti4+ ions have site preference for 4fvl and 2b sites9, the substitution of these ions in barium ferrite forces some of the Fe3+ ions to migrate to 12k sites, thus weakening the hopping mechanism and increasing the resistivity. The decrease in resistivity for x>O.3 may be due to the contribution of C0 2+ and Ti 4+ ions to hopping and is being investigated. The Curie temperature, Te, increases marginally from 425°C to 469°C when x increases from 0 to 0.3, and decreases slightly to 457°C for x equal to 0.9. All the measurements have an inaccuracy of ±5°C. It is, thus, apparent that the dc reststtvtty, Curie temperature and microwave absorption all show their peak values for x equal to 0.3. The variation of complex permittivity, (E' - jE''), and complex permeability, (J..l'-jJ..l''), with frequency for BaFel2019 ferrite is shown in Fig. 4. The real part of complex permittivity, E', shows insignificant decrease from 3.2 to 2.8 in the X-band as shown in Fig. 5. However, with C0 2+Ti 4 + substitution, as shown in Fig. 5, E' exhibits a significant variation. It increases to 4.1 at 8.6 GHz and then decreases to 3.2 at 12.4 GHz. The dielectric properties of ~------------------------~ 4.5 4.0 - 4.0 3.5 . 3.5 3.0 . -+--;:- 2.5 2.5 -l E ~ -k-E' -:1. ~ E" -=: 1.5 . -:l. --~" ...... E' w.. 2.0 ---~l" ·w 2.0 · 'w 1 w -+- fl' ....... e" 1.5 1.0 - 1.0 0.5 - 0.5 . 0.0 .1 - - - - - . - - - - . - - - - . - - , - - - - - 1 8.0 9.0 10.0 11 .0 12.0 13.0 0.0 +--,----,----r---.,---r----j 8.0 Variation of E', E'~ 11' and 11" with frequency for BaFel2019 hexaferrite composite 10.0 11.0 12.0 13.0 Frequency (GHz) Frequency (GHz) Fig. 4 - 9 .0 Fig. 5 - Variation of E~ E'~ 11' and l1"with frequency for BaCoo.3Tio.3Fe11.4019 hexaferrite composite RAZDAN et al.: MICROWA VE ABSORPTION OF BaCoTi FERRITE COMPOSITES polycrystalline ferrite-polymer composites arise mainly due to the interfacial pol arisation and intrinsic electric dipole polarisation. The interfacial polarisation results from the heterogeneous structure of ferrites comprising low conductivity grains separated by high resistivity grain boundaries as proposed by KOOpSID. The higher values of E' for composites with x equal to 0.3 may be due to significant contributions of C0 2+Ti 4+ ions in addition to Fe 2+ and Fe3+ ions to interfacial polarisation . Besides these, the epoxy resin has also got some degree of electronic conductivity due to a large number of polar groups present in the polymer chains. The conductivity of epoxy-resin contributes to dielectric loss in these composites. The permittivity originated from electronic polarisation, ion polarisation and intrinsic electric polarisation is given by E = E.,., + (Eo - E.,.,)/(l + jon) where Eo and £= respectively represent permittivity for angular frequency U)--10 and U)--1oo, and r is the relaxation time for polarisation of intrinsic electric dipole. The permeability (p'-jp") of these composites shows only a small variation in the X-band. The real part of permeability varies between 1.1 and 1.3. The imaginary part of permeability shows a significant 425 increase with frequency from 8.6 to 12.4 GHz. The effective magnetic spectra of composites have been described in a model!!, which attempts to predict resonance-relaxation phenomenon by taking into account the relative values of internal wavelength. The lower values of permeability and weak dispersion effects observed in these samples may be attributed to the presence of nonmagnetic epoxy-resin between the neighbouring crystallites which weakens the intergranular magnetic interaction. References I Naito Y & Suetake K. IEEE Tran s Microwave Theory Tech. 19 (197 I) 65. 2 Diishovaski N. Petkov A. Nedkov 1 & Razkazov I. IEEE Trans Magll. MAG-3~ (1994) 969. 3 Grunberger W. Spreingmann B. Brusberg M. Schmid! M & Jahnke R. J Magll Magll Mater. 10 1 (1991) 173 . 4 Nedkov I. 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