A microwave absorber based on strontium ferrite–carbon black–nitrile rubber for S and X-band applications

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Abstract

Flexible and thin single layer microwave absorbers based on strontium ferrite–carbon black–nitrile rubber composites have been fabricated employing a specific recipe and their reflection loss characteristics were studied in the S (2–4 GHz) and X-bands (8–12 GHz). The incorporation of carbon black not only reinforces the rubber by improving the mechanical properties of the composite but also modifies the dielectric permittivity of the composite. Strontium ferrite when impregnated into a rubber matrix imparts the required magnetic permeability to the composite. The combination of strontium ferrite and carbon black can then be employed to tune the microwave absorption characteristics of the resulting composite. The complex dielectric permittivity and permeability were measured by employing a cavity perturbation technique. The microwave absorption characteristics of composites were modelled in that an electromagnetic wave incident normally on the metal terminated single layer absorber. The influence of filler volume fraction, frequency, absorber thickness on the bandwidth of absorption are discussed and correlated.

Introduction

Microwave absorbing materials are of vital importance in solving the increasing menace of electromagnetic interference problems caused by the proliferation of wireless communication system and other high-frequency devices in the S and X-band frequencies. They are in demand for reducing radiation reflection as well as providing interference shielding (EMI electromagnetic interference shielding) and protection [1], [2], [3], [4], [5]. Flexibility, mouldability and environmental resistance are the main factors to be considered in designing an absorber along with a large absorption capability and wide band width. Rubber Ferrite Composites (RFCs) in the form of sheets are flexible, lightweight and possesses good environmental resistance and has the required mechanical strength. Furthermore, absorptivity and bandwidth of absorption can be tailored by a judicious choice of fillers. The dielectric permittivity and magnetic permeability of the composite is greatly influenced by the volume fraction of fillers in the matrix [6]. This multi-functionality of RFCs makes it a potential candidate for microwave absorbing applications.

Spinel ferrites do not perform well in the Giga Hertz range of spectrum due to a drop in the complex permeability as given by Snoek’s limit [7], [8], [9]. Compared to spinel and garnets, hexagonal ferrites in the pristine form have been found to be good microwave absorbers in the frequency range of 1–100 GHz because of their high saturation magnetization, coercivity, chemical stability, corrosion resistance and adjustable anisotropy through appropriate ion substitution [9]. Hence incorporation of M-type strontium hexaferrite (SrF) in the rubber matrix enhances the magnetic permeability of the composite and in turn modifies the microwave absorption properties. Carbon and different forms of carbon are being used along with other fillers for enhancing the microwave absorption characteristics. Different forms of carbon, namely, Carbon Nano Tube, Carbon Nano Fibre, Carbon Nano Onion, graphene, reduced graphene oxide, carbonyl iron, etc. are increasingly finding its way as a filler in an appropriate matrix [5], [10], [11], [12], [13], [14]. Carbon black (CB) is a perfect choice because it not only reinforces the rubber matrix but also modifies the microwave absorption characteristics.

The incorporation of CB in the matrix along with SrF is aimed at achieving two objectives. Primarily, CB is a reinforcing agent and imparts the necessary mechanical strength. The secondary objective is to employ CB as a lossy dielectric wherein it contributes to the enhancement of microwave absorption by virtue of imparting a large permittivity to the composite. The strategy has been to choose a composite containing an optimum amount of magnetic filler and having the appropriate magnetic characteristics. Since we intend to incorporate CB further into this optimally synthesized composite, we thought it fit to choose the maximum loading of magnetic filler at around 80 phr. It has been assumed that further addition of CB into this will not go beyond the percolation threshold. Moreover incorporation of 30, 40, 50 phr (parts per hundred weights of rubber) of CB into this will not result in inhomogeneity. This is based on past experience. CB is less expensive when compared to CNT, graphene and other carbonaceous materials.

Excellent microwave absorbing materials in the desired frequency range with wide band width must satisfy two fundamental conditions [15], [16], (i) the incident electromagnetic wave penetrates the absorber by the greatest extent (impedance match) and (ii) the wave entering into the materials is entirely attenuated and absorbed within the finite thickness of the absorber. The level of absorptivity of an absorber is measured quantitatively using reflection loss in decibels. The reflection loss of −20dB is adopted to be a standard for typical electromagnetic wave absorbers [17]. It means that 99% of the total energy of electromagnetic wave incident on it is absorbed. Percentage of absorption is calculated using the following equation:P=(1-Pr/P0)×100where Pr and P0 are the reflected and incident radiation powers respectively.

Composite based on SrF and rubber samples containing various loadings of CB were prepared and the microwave absorption of these composite samples was studied using a network analyser in the S and X bands. The results were then modelled based on reflected power of a plane wave from a metal terminated one layer absorber system.

Section snippets

Acrylonitrile–butadiene rubber (Nitrile rubber) (NBR)

Nitrile rubber used in this study was Aparene – N 553 supplied by Apar Polymers Ltd., Gujarat, India. This NBR contains 34% acrylo nitrile content by weight.

Carbon black (CB)

High abrasion furnace black (HAF N-330) used in this investigation was supplied by M/S Philips Carbon Ltd., Cochin, India.

Strontium ferrite (SrF)

M-type strontium hexaferrite were prepared by a ceramic technique. Ferrous oxalate dehydrate (FOD) precursors were used for the synthesis. α-Fe2O3 was prepared by the decomposition of freshly prepared FOD. Appropriate

Structure and morphology

X-ray diffraction (XRD) patterns of composite samples were recorded using Rigaku Dmax-C and the structural parameters were evaluated. The structure and planes (h, k, l) of the composites were determined by comparing the measured XRD pattern with standard powder diffraction files (ICDD) [27]. Fig. 2 depicts the XRD patterns of representative samples. XRD reveals that SrF do not undergo any structural change during the different stages of processing of rubber. In Fig. 2, A and Z corresponds to

Conclusions

Hybrid type, S and X-band flexible and broadband microwave absorbing materials were synthesized and the influence of these magnetic–dielectric materials on the microwave absorbing properties have been investigated in detail. CB added composites exhibit enhanced microwave absorption and wider band widths than those for pure SrF rubber composites at its minimum thicknesses. Moreover, by the combination of two fillers (both CB and SrF) in the rubber matrix sample coded 50CBSrF is identified as the

Acknowledgments

This work was supported by All India Council for Technical Education (File No. 8023/RID-73/2004-05 dated: 29-03-2005), Government of India. SV acknowledges University Grants Commission (U.O.No. UGC(S) CELL (1)14515/RGNF/SC/08-09 dated 17/12/2011), for financial assistance in the form of a fellowship.

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    On deputation from Department of Physics, Govt. Victoria College, Palakkad 678 001, Kerala, India.

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