Synthesis of Polymer Blend Ferrite Composite for Microwave Absorption at X-Band Frequency anisotropy, chemical

The microwave absorption properties of polymer composite PANI/PVA/NiFe 2 O 3 are investigated. The polymer composites of PANI/PVA and NiFe 2 O 3 are prepared in two steps. NiFe 2 O 3 is synthesized by modified sol gel method and PANI by chemical polymerization method. Microwave absorption parameters of polymer composite are measured at X-band. The microwave absorption is found to be −28 dB (99%) at 10.2 GHz. Different characterization techniques such as SEM-EDX, FTIR and XRD are done. The SEM result shows flakes like structure for PANI/PVA and crystalline structure for NiFe 2 O 3 . FTIR of the composite reveals the interaction between the PANI/PVA and NiFe 2 O 3 .


Introduction
Electromagnetic interference became very serious problem in the modern day microwaves communication. Microwave affects the neuron electrical conductivity leading to long term neurological disorder in humans [1]. In order to minimize these problems new materials are synthesized having the potential to absorb or shield the microwave. Microwave absorbing materials (MAM) also play an important role in the military defence system in application to RADAR. Various techniques are used for synthesizing the materials. These materials are magnetic, non-magnetic or composite of both with conducting polymer magnetic. Conducting polymer polyaniline can be synthesized by chemically or by electrochemical polymerization [2]. Ferrites based materials have high coercive force, reliable magnetization, large magneto-crystalline anisotropy, chemical gradation [7] [8] [9] [10]. Nickel Ferrite is considered to be n-types semiconductor. The complex part of the permittivity (ε") and permeability (µ") are directly associated with the microwave absorption. Microwave absorbing materials should be light weight, chemically and environmentally stable and low density. Jin et al. synthesized PPy/MMT polymer composite, which has the microwave absorbing power of −10 dB at X-band frequency [11]. Tiwari [12] et al. developed a simple differential bridge technique to measure the dielectric constant of thin polymer film of PPy and polystyrene at microwave range. The results are in good agreement with the reported data. PPy/TiO 2 (np)/CNT and PANI/TiO 2 (np)-Fe 3+ polymer nanocomposite have the microwave absorption of 99.99% [13] [14]. Composite of conducting polymer and magnetic material will give better absorption.
The synthesized polymer nanocomposite material has the minimum reflection loss of −28 dB which is 99% adsorption at 10.2 GHz. The magnetic loss inside the material was responsible for the microwave adsorption by the material.

Experimental Method
Nickel ferrite material is synthesized by using a modified sol gel method. Ana-

Characterization
The synthesized polymer nanocomposite is characterized by the SEM-EDX, FITR and XRD. The electromagnetic parameters such as permittivity, permeability and reflection loss are measured by using vector network analyzer (Agilent ENA 50 GHz) between frequencies 8 -12 GHz.

SEM-EDX, FTIR and XRD Study
The SEM image of the polymer nanocomposite in Figure 1 Table 1. The FTIR spectrum of polymer composite is shown in Figure 1(c). The characteristic peak at the 1540 cm −1 , 985 cm −1 , 1050 cm −1 , 1290 cm −1 are due to the C=C stretching, N=Q=N (Q = quionoid ring), C=C stretching mode in benzenoid ring and C-N stretching.
The peak at the 600 cm −1 shows the interaction between the PANI/PVA and NiFe 2 O 3 [15].   [16]. This confirmed the crystallinity of the sample which also confirmed by the SEM image in Figure 1(a). The crystallite size of the NiFe 2 O 3 can be calculated using the Scherrer formula [12]: where, τ is the mean size of the ordered (crystalline) domains, K is a dimensionless shape factor with a value close to unity, λ is the wavelength and β is the line broadening at half the maximum intensity (FWHM). The average crystallite size is found to be 5.33 nm.

Electromagnetic Parameter
For the passive medium permittivity and permeability is given by: The real part of the permittivity and permeability shows the capacity of storing electric and magnetic energy inside the medium, whereas the imaginary part shows loss inside the material. The microwave absorbing property by the material depends on the dielectric and magnetic loss.   The magnetic resonance inside the polymer composite may be due to natural and exchange resonance [11]. Figure 3(a) shows the graph for dielectric and magnetic loss. In our study it is found that the magnetic loss (tanδ M ) is dominating over the dielectric loss (tanδ E ). The magnetic loss in the material may be due to the gyromagnetic spin resonance [18]. The dielectric loss inside the material can be explained by the Debye dipole relaxation mechanism. If the plot between the ′  and ′′  is semi-circle then it is known as Cole-Cole semi-circle given by the relation [19].

Reflection Loss Measurement
Reflection loss by the polymer composite is measured by rectangular wave guide method in X-band frequency. The reflection loss measurement with metal back of the material is calculated by using the transmission line theory [22].
Here Z in is impedance of the material, Z 0 is the impedance of the free space, μ r relative permeability, ϵ r relative permittivity, f frequency, d is the thickness of the sample and R L is the reflection loss (absorption) by the medium. Figure 3 Table 2 shows the comparison of reflection loss of our composite along with other reported work.

Conclusion
The polymer composite is synthesized by the chemical oxidative polymerization method using APS as the oxidizing agent. The SEM image of the polymer nickel ferrite shows the formation of crystalline structure as supported by the XRD data. The PANI/PVA shows the formation of flake-like structure. The minimum reflection loss (metal back) by the polymer composite is found to be −28 dB with 99% absorbance by the material. Thus this polymer nanocomposite can be used as coating material for adsorbing the microwave frequency between 8 GHz to 12 GHz. Wencai et al. [25] Fe-Co-Ni −14.7 Duan et al. [26]