Preparation and Identification of BaFe 2 O 4 Nanoparticles by the Sol – Gel Route and Investigation of Its Microwave Absorption Characteristics at Ku-Band Frequency Using Silicone Rubber Medium †

In the last decade, spinel structures have been widely explored due to widespread applications in antibacterial nanocomposites, memory devices, catalysts, photocatalysts, highfrequency devices, and electromagnetic absorbing materials. In this study, BaFe2O4 spinel structures were synthesized through the sol–gel method using a low sintering temperature and were identified by vibrating sample magnetometer (VSM), X-ray powder diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FE-SEM), and vector network analyzer (VNA) analysis. Results showed that uniform and pure crystal structures of BaFe2O4 nanoparticles were prepared based on the sol–gel method. Finally, BaFe2O4 nanoparticles were blended by silicone rubber to characterize the microwave absorption properties of the nanocomposite at the ku-band frequency. According to the VNA results, the BaFe2O4/silicone rubber nanocomposite with 1.75 mm thickness absorbed more than 94.38% of microwave irradiation along the ku-band frequency and the maximum reflection loss of the BaFe2O4/silicone rubber nanocomposite was 51.67 dB at 16.1 GHz.


Introduction
The magnetic materials of normal spinel ferrites with the general chemical formula MFe2O4 have various applications owing to a type of M cation, for which M is the divalent metal cation (M 2+ = Ba 2+ , Sr 2+ , Co 2+ , Mg 2+ , Zn 2+ , Cu 2+ , Mn 2+ , etc.).The intrinsic properties of BaFe2O4 nanoparticles, such as high magnetic saturation and coercivity, high chemical and mechanical resistance, and high curie temperature, have indicated that it as a good candidate for microwave devices, radar-absorbent materials, permanent magnets, drug deliveries, photocatalytic catalysts, credit cards, etc.The methods of synthesizing spinel ferrites greatly affect their properties and applications.In recent decades, extensive research has been performed to improve the synthesis methods to increase crystal purity, decrease size, and control the morphology of the nanostructures.Diverse methods have been used to prepare of BaFe2O4 nanoparticles, such as spray pyrolysis, co-precipitation, microemulsion, ball milling, and hydrothermal approaches [1][2][3].The crystallinity, size, and shape of the nanostructures are the most influential factors on the properties of nanomaterials [4].Most of methods require a high calcination temperature of about 800-1000 °C [2,5,6].In this research, a single phase of ferrite nanoparticles was prepared by the sol-gel method with a low sintering temperature.Moreover, the microwave absorption of the BaFe2O4 nanoparticles was investigated using a silicone rubber polymeric matrix.

Materials and Instruments
Barium nitrate was obtained from Sigma-Aldrich (St. Louis, MO, USA) and citric acid, iron (III) nitrate nonahydrate, and ammonia solution were purchased from Merck (Darmstadt, Germany).
Tescan Mira2 (Brno, Czech Republic)presented SEM micrographs of the nanoparticles.The crystal structure of the nanostructures was investigated using a Philips X'Pert MPD (Amsterdam, Netherlands) instrument operating on 40 mA and 40 kV current with a Co tube and a wave length of λ = 1.78897Å. Shimadzu 8400 S FTIR (Kyoto, Japan) revealed the chemical structure of the sample.The magnetic hysteresis loop was obtained using IRI Kashan VSM.Microwave absorption properties were investigated by Agilent technologies (Santa Clara, CA, USA), E8364A.

Synthesis of BaFe2O4 Nanoparticles
Barium ferrite nanoparticles were prepared by the conventional sol-gel method.Firstly, metal salts and citric acid with stoichiometric ratios were dissolved in distilled water, and then the pH of the solution was raised to an alkaline medium by the ammonia solution.Finally, the solution was dried and calcined at 450 or 650 °C for 4 h to compare the results.

Preparation of BaFe2O4/Silicone Rubber Nanocomposite
The BaFe2O4 nanoparticles were blended with silicone resin and then a hardener was added with 20 wt% to mold a BaFe2O4/silicone rubber nanocomposite and study the microwave absorption of the nanocomposite.BaFe2O4 [00-046-0113] BaCO3

FE-SEM Morphology
The morphology of BaFe2O4 nanostructures at 650 °C was investigated using FE-SEM micrographs, as shown in the Figure 2. BaFe2O4 nanoparticles have a polycrystalline structure with an average size of about 70 nm.

FTIR Spectroscopy
The FTIR analysis was used to determine the structure and measurement of chemical species.According to the results shown in Figure 3, the peaks at 497.12, 618.30, and 764.16 cm −1 are related to stretching vibrations of Ba 2+ -O 2− and Fe 3+ -O 2− in the octahedral and tetrahedral sites, and the peaks at 1053.63 and 1111.98 cm −1 are associated with vibrations of M-O-M (M = Ba 2+ or Fe 3+ ) in the finger print region corresponding to the orthorhombic crystalline structure of the prepared BaFe2O4 nanoparticles [2,7,8].The peak at 1630.34 cm −1 and broadband absorption at 3434.51 cm −1 are attributed to the bending and stretching vibration of the O-H bond associated with adsorbed water as well as the remaining hydroxyl functional groups on the surface of the nanoparticles [5,6].

Microwave Absorption Properties
The transmission line theory equation indicates that the microwave absorption properties of the materials are generally related to the permittivity and permeability of the absorbers [13][14][15][16].According to the results, the BaFe2O4/silicone rubber nanocomposite with 1.75 mm thickness absorbed more than 94.38% of microwave irradiation at the ku-band frequency, and the maximum reflection loss of the BaFe2O4/silicone rubber nanocomposite was 51.67 dB at 16.1 GHz (Figure 5).Table 1 compares the results of this study with some previously published data.The broadband and intense microwave absorption of the BaFe2O4/silicone rubber nanocomposite originated from proper impedance matching, multiple scattering, and interfacial polarization, which led to more microwave attenuation [17][18][19][20].

Conclusions
The obtained results demonstrate that BaFe2O4 nanoparticles were prepared through the sol-gel method using a low sintering temperature, which confirms that the heat treatment had a significant effect on the crystal purity of the nanostructures.According to the XRD patterns, phase impurities of nanoparticles disappeared when the temperature increased.The FE-SEM micrograph exhibited uniform morphology for BaFe2O4 nanostructures.The FTIR curve demonstrated that the metal-oxide bonds of BaFe2O4 nanoparticles had been synthesized at a low temperature.Finally, VNA results illustrated that the maximum reflection loss of the BaFe2O4/silicone rubber nanocomposite was 51.67 dB at 16.1 GHz and that the nanocomposite absorbed more than 94.38% of microwave irradiation along the ku-band frequency with a thickness of 1.75 mm.The results suggest that BaFe2O4 nanoparticles can be a promising microwave absorbing material.

Figure 1
Figure1depicts the XRD patterns of the samples synthesized by the sol-gel method and calcined at 450 or 650 °C for 4 h.The pattern of BaFe2O4 calcined at 650 °C exhibits that all the obtained peaks correspond with the JCPDS number [00-046-0113].The XRD patterns indicate that by increasing the calcination temperature from 450 to 650 °C, the BaCO3 (JCPDS: [00-005-0378]) crystalline phases disappeared and a pure phase of BaFe2O4 nanoparticles was synthesized.The size of the BaFe2O4 nanoparticles was 10.2 nm based on the Scherrer equation.

Figure 5 .
Figure 5.The reflection losses of BaFe2O4/silicone rubber nanocomposite at various thicknesses.

Table 1 .
Comparison of presented study with some previously published research.