Preparation and Identification of BaFe₂O₄ Nanoparticles by the Sol–Gel Route and Investigation of Its Microwave Absorption Characteristics at Ku-Band Frequency Using Silicone Rubber Medium ^†

Abstract

In the last decade, spinel structures have been widely explored due to widespread applications in antibacterial nanocomposites, memory devices, catalysts, photocatalysts, high-frequency 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.

1. 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 (M²⁺ = Ba²⁺, Sr²⁺, Co²⁺, Mg²⁺, Zn²⁺, Cu²⁺, Mn²⁺, 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.

2. Experimental

2.1. 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). Silicone rubber was obtained from ELASTOSIL^® M4503, Wacker RTV-2 (Munich, 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 = 1.78897 Å. 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.

2.2. 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.

2.3. 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.

3. Results and Discussion

3.1. Phase Identification Analysis

Figure 1 depicts 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.

3.2. 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.

3.3. 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 Ba²⁺–O²⁻ and Fe³⁺–O²⁻ 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 = Ba²⁺ or Fe³⁺) 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].

3.4. Magnetic Properties

The magnetic properties of the BaFe₂O₄ nanoparticles were explored using VSM operated at a 25-Hz frequency, −15 < kO_e < 15 field, and room temperature. The saturation magnetization (M_s), remanent magnetization (Mr), and coercivity (Hc) were 0.5 emu/g, 0.2 emu/g, and 4471.0 Oe, respectively (Figure 4.). Numerous studies have investigated the magnetic parameters of M-type BaFe₁₂O₁₉ nanoparticles, exhibiting Ms = 41, 54.97, 75.54 emu/g, Hc = 5450, 4964.5, and 2800 Oe [9,10,11,12], which show more paramagnetic properties in comparison to synthesized BaFe₂O₄ nanoparticles.

3.5. 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. Comparison of presented study with some previously published research.

Particles	Max RL (dB)	Diameter (mm)	Absorption Bandwidth (GHz) < −10 dB	Ref.
BaFe12O19/Fe3O4	33.6	2.5	1.3	[21]
CoFe2O4	14	3	2	[22]
BaFe12O19/CoFe2O4	10	5	-	[23]
BaFe12O19	16.1	3	3.8	[24]
BaCu0.5Mg0.5ZrFe10O19	9	2.1	-	[25]
BaFe12O19	7	2.5	-	[11]
Ba0.25Sr0.75 Fe11(Ni0.5Mn0.5)O19	3.6	4	-	[26]
BaFe12O19	10.7	3	-	[12]
Ba0.2Sr0.2La0.6MnO3	22.36	2	2.67	[4]
BaFe2O4	51.67	1.75	<5.6	Presented study

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].

4. 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.