Synthesis and Microwave Absorption Properties of Novel Bi_1/2(Na_0.8K_0.2)_1/2TiO₃/Fe₃O₄ Composite

Abstract

The combination of two dielectric-magnetic components in the same composite has been shown to significantly improve the effectiveness of electromagnetic (EM) shielding and microwave absorption (MWA) because they have both a combination of high dielectric and magnetic losses and good impedance matching. The novel Bi_1/2(Na_0.8K_0.2)_1/2TiO₃/Fe₃O₄ (BNKT/Fe₃O₄) composite has been successfully synthesized by a two-step method with wide effective absorption bandwidth (EAB = 16 GHz) in the high-frequency (2–18 GHz). It was evident that the MWA efficiency of the BNKT/Fe₃O₄ composite has been significantly improved compared with pure Bi_1/2(Na_0.8K_0.2)_1/2TiO₃ or Fe₃O₄ materials. In addition, the BNKT/Fe₃O₄ composite could achieve reflection loss (RL = −39.41 dB, ∼99.99% at 10.16 GHz) with a sample thickness optimal (d = 4.7 mm). This work shows that the novel BNKT/Fe₃O₄ composite has excellent MWA properties, all contributing to a potential candidate in the electromagnetic wave absorption and shielding fields.

Fig. 5 The 3-D projection (magnitude) of scattering coefficients (S₁₁) and (S₂₁) of the BNKT/Fe₃O₄ composite with different thicknesses (2.9–5.4 mm) (a), (b); The relationship between reflection loss (dB) and the MWA percentage (%) (c); the RL (d) 2-D contour map (e) and 3-D reflection loss (f) of BNKT/Fe₃O₄ composite with different thicknesses (2.9–5.4 mm).

1. Introduction

In the fourth industrial revolution (Industry 4.0), along with the development of 6 G or Internet of Things (IoT) technology, mobile devices and digital information transmission devices are increasingly popular, with fast transmission speeds towards a more convenient, smarter life.^1,2) However, the rapid uncontrolled proliferation of these technological devices can lead to potential dangers with severe consequences.^3,4) EM wave pollution is inevitable because many electromagnetic waves (EMW) are still being generated by electronic technology equipment, military radar etc., works around us. Electromagnetic interference has become a severe problem for human health, the operation of e-commerce devices, and national security.^1,4,5) To solve the above problem, the attenuation and shielding of EMW are essential, and MWA materials are a research direction that has received great attention.

In the past, there have been many traditional MWA materials, but most of them have not met the requirements of practical applications such as strong absorption, wide absorption bandwidth, ease of manufacture in large quantities, and low cost.²⁾ Fortunately, highly efficient MWA composites have been recently developed based on a combination of complex permeability and permittivity components or impedance matching, microstructure, and interface polarization of the absorbers.^6,7) In particular, the fabrication of multiferroic composites (including dielectric and magnetic phases) is an excellent way to improve the shielding of EMW and MWA.^8,9) In these composites, the dielectric phase acts as a center of polarization, leading to an increase in dielectric loss. On the other hand, the magnetic phase acts as a magnet, which increases the permeability loss. As a consequence, both the magnetic and electrical losses in the multiferroic composites contribute to enhanced microwave absorption.⁸⁾

Among lead-free ferroelectric materials perovskite-based type, bismuth sodium titanate Bi_0.5Na_0.5TiO₃ (BNT), bismuth potassium titanate Bi_0.5K_0.5TiO₃ (BKT), and solid solutions based on these compounds seem to be considered as the choice of the most promising material for energy harvesting applications.^10,11) It has been reported that BNT ceramics modified with BKT, Bi_1/2(Na_1−xK_x)_1/2TiO₃ with x = 0.16–0.20, showed improved dielectric properties, due to a rhombohedral–tetragonal morphotropic phase boundary (MPB). This material has a high relative dielectric constant (ε_r = 1030) and excellent ferroelectric and piezoelectric properties (d₃₃ = 151 pC N⁻¹, d₃₁ = 46.9 pC N⁻¹) were obtained at the optimal composition of x = 0.20.¹⁰⁾ So, Bi_1/2(Na_0.8K_0.2)_1/2TiO₃ (BNKT) has been widely applied in the fields of energy storage and conversion.^10,11) Co et al. prepared BNKT by sol-gel method with RL = −21.72 dB (99.9%) found at 13.66 GHz and a thickness of 3.2 mm. The BNKT has been shown able to be used as EM wave shielding, or MWA materials.¹²⁾ The characterization studies primarily focus on the improvement of the electrical properties as the effect of dopant and co-dopants at A-, B-site of BNKT or dopant BNKT with different perovskite A′B′O₃.¹¹⁾ Modifying the BNKT material is a great way to improve dielectric loss and MWA properties, but it does not seem to meet the requirements of practical applications. In the high-frequency range, the permeability of BNKT is relatively low, which is also the limitation of the material and affects its ability to absorb microwaves.¹²⁾ To optimize the MWA efficiency and expand the absorption bandwidth, combining BNKT with a magnetic component to improve the MWA properties of the composite is necessary.

Ferromagnetic Fe₃O₄ is a typical magnetic material that has attracted more attention than other similar candidates as iron oxides or ferrite spinel oxides (MFe₂O₄ with M = Co, Ni, Mg, etc.) because of its superior magnetic properties, electronic conductivity, and biocompatibility.^13,14) Moreover, superparamagnetic properties and other intrinsic properties, such as a high surface area to volume ratio, make Fe₃O₄ material and Fe₃O₄-based composites are used extensively in the research on EM wave shielding and MWA in recent scientific advances, as a component contributing to the magnetic loss of electromagnetic radiation in the GHz range.^13,15–18)

Herein, Fe₃O₄ ferromagnetic material was proposed to combine with Bi_1/2(Na_0.8K_0.2)_1/2TiO₃ ferroelectric material by a two-step method to improve the magnetic loss, widen the absorption bandwidth and effective MWA of the fabricated novel composite. Accordingly, this article is considered the first report on synthesising multiferroic material in the manner mentioned above. The MWA efficiency expressed by reflection loss mapping was also compared and showed a difference with other material samples. In addition, the MWA mechanism of the BNKT/Fe₃O₄ composite is also clearly shown.

2. Experiments

2.1 Materials

The chemicals used here are commercial products supplied by Sigma - Aldrich: bismuth nitrate pentahydrate [Bi(NO₃)₃·5H₂O, ≥98%], acetic acid (CH₃COOH, ≥99%), titanium (IV) isopropoxide (C₁₂H₂₈O₄Ti, ≥97%), ferrous chloride tetrahydrate (FeCl₂·4H₂O, ≥99%), potassium nitrate (KNO₃, ≥99%), acetylacetone (CH₃COCH₂COCH₃, ≥99%), potassium hydroxide (KOH, ≥99%), sodium nitrate (NaNO₃, ≥99%), and ferric chloride hexahydrate (FeCl₃·6H₂O, ≥99%).

2.2 Preparation of Bi_1/2(Na_0.80K_0.20)_1/2TiO₃ powders

The Bi_1/2(Na_0.80K_0.20)_1/2TiO₃ particles were prepared by chemical method (sol-gel).^19,20) First, the starting materials: 2.425 g Bi(NO₃)₃·5H₂O, 0.132 g KNO₃, and 0.510 g NaNO₃ were added to a beaker containing 20 mL acetic acid and 5 mL DI water until completely dissolved. An excess amount of KNO₃; NaNO₃ was added to compensate for the loss during heating and annealing, as shown in previous works.¹⁹⁾ After that, 5 mL acetylacetone and 3 mL titanium isopropoxide were added to the solutions. Using a magnetic stirrer with heating support, the precursor solution mixture was stirred continuously for 5 h at 70°C. Then, the beaker containing the above solution was dried overnight in a vacuum oven at 80°C. The obtained product continues to be calcined at 700°C for 1 h and then finely ground with a mortar.

2.3 Preparation of BNKT/Fe₃O₄ composites

The BNKT/Fe₃O₄ composite was prepared by chemical method (two-step method). First, the prepared BNKT powder was added to 20 mL of DI water before stirring for 30 min. Next, 5.46 g FeCl₃·6H₂O and 2.03 g FeCl₂·4H₂O were added to 50 mL of DI water. After that, the salt solution mixture (Fe²⁺/Fe³⁺) was magnetically stirred at room temperature for 2 h before adding the BNKT solution. And then, the above solution mixture was adjusted to pH = 12 by slowly adding KOH solution (2 M) with continuous magnetic stirring. The black precipitated product was collected by magnetic separation (permanent magnet), washed several times with DI water, ethanol and then centrifuged. Finally, dry the product at 60°C for 24 h in a vacuum oven, and grind it finely with a mortar to obtain BNKT/Fe₃O₄ composite.

In addition, to ensure more accurate quantification of the mass fraction of each component (BNKT and Fe₃O₄) present in the composites, we separately fabricated pure Fe₃O₄ by simple co-precipitation method before making the samples combination. The process of making pure Fe₃O₄ samples is similar to that of composite samples, but BNKT is not added during the reaction. To ensure that the formation of Fe₃O₄ is pure and free of other components, thermal annealing and sample storage are important. The process of incubation of samples in a vacuum drying oven at a low temperature of 60°C and storing samples in a vacuum bag is to minimize the oxidation of Fe₃O₄ to Fe₂O₃ in the air. The resulting mass of the pure Fe₃O₄ product is practically completely determined. From there, the actual quantification of BNKT needs to be added during the assembly process, ensuring the correct component ratio.

2.4 Measurements

The crystalline structures of BNKT, Fe₃O₄ and BNKT/Fe₃O₄ composite were characterized by XRD (D5005 Diffractometer, Bruker, Cu Kα). Scanning electron microscope (SEM, S-4800, Hitachi) and energy-dispersive X-ray spectroscopy (EDX) analysis were used to investigate the morphology and reveal the prepared samples’ elemental composition and chemical properties, respectively. The magnetic hysteresis loops of BNKT, Fe₃O₄ and BNKT/Fe₃O₄ composite were analyzed by a vibrating sample magnetometer (VSM, Lakeshore 7404, US). The MWA properties of the BNKT/Fe₃O₄ composite were determined by the MWA measurement system using the transmission line method. This system consists of a vector network analyzer (VNA, PNA-X N5242A, Keysight) connected to the coaxial waveguide and the prepared sample (coaxial ring type) placed inside. Specifically, the fabricated BNKT/Fe₃O₄ composite material is mixed with paraffin wax (by the mass ratio of 50%) and then heat pressed to form coaxial rings. These coaxial rings’ outer and inner diameters are ϕ_out = 7.0 mm and ϕ_in = 3.05 mm, respectively. The collected data parameters are S matrices (S₁₁ = S₂₂, S₂₁ = S₁₂) in the high-frequency range from 2 to 18 GHz. Use the Nicolson-Ross-Weir (NRW) method to extract the input impedance and calculate the MWA efficiency.

3. Results and Discussion

3.1 Crystal structure and morphology of samples

Figure 1 shows the XRD patterns of pure Bi_1/2(Na_0.8K_0.2)_1/2TiO₃, Fe₃O₄ and their composites powder in the 2θ ranges of 20°–75°. The fabricated Bi_1/2(Na_0.8K_0.2)_1/2TiO₃ powder sample has characteristic peaks for perovskite structure, consistent with PDF:01-072-8109. The fabricated BNKT sample did not appear any strange peak picks, this result indicated that the sample did not have any secondary phase or other impurities, which is said to be consistent with previously published studies.^21,22) The formation of the MPB (tetragonal - rhombohedral phases) of Bi_1/2(Na_0.8K_0.2)_1/2TiO₃ sample was demonstrated when the characteristic diffraction peaks (002)/(200) were observed (2θ = 46°–47°).^22–25) Figure 1 shows nine diffraction peaks: (111), (220), (311), (222), (400), (422), (511), (440) and (533) at 2θ: 18.4°, 30.3°, 35.7°, 37.35°, 43.4°, 53.9°, 57.5°, 63.1°, and 74.62°, respectively. According to PDF: 01-071-6339, they are believed to be the planes of the Fe₃O₄ sample.

Fig. 1

XRD patterns of samples: pure Bi_1/2(Na_0.8K_0.2)_1/2TiO₃, Fe₃O₄ powders and BNKT/Fe₃O₄ composite.

The diffraction peaks of the BNKT/Fe₃O₄ composite, which can be indexed as Bi_1/2(Na_0.8K_0.2)_1/2TiO₃ (denoted by ◆) or Fe₃O₄ (denoted by ●) without any additional phase detected. This proves that BNKT/Fe₃O₄ composites have been successfully prepared by a simple co-precipitation method. No significant difference was found in the XRD spectrum of the BNKT component of the composite sample compared with the pure BNKT sample. This means that during the fabrication process of the composite, the crystal structure of BNKT is stable enough to meet the fabrication requirements. According to calculations, the amount of Fe₃O₄ is more than that of BNKT in composite (3 times), so the intensity of characteristic peaks of BNKT component also has a weakening.²⁶⁾

Figure 2 shows the SEM images of Fe₃O₄ and BNKT/Fe₃O₄ composite samples. The pure Fe₃O₄ has a spherical shape, quite uniform size, and tends to agglomerate into a cluster of particles (Fig. 2(a)). For the BNKT/Fe₃O₄ composite, the small spherical particles indicated as Fe₃O₄ are scattered around the surface of the BNKT particles with larger sizes. It can be seen that the BNKT powder used in the co-precipitation process has significantly reduced the aggregation of Fe₃O₄ particles. As a result, the interface between the dielectric and magnetic phases is improved dramatically (Fig. 2(b)). From the XRD, and SEM results, that can be confirmed that the Fe₃O₄ particles were successfully prepared and adhered to the surface of the BNKT particles. For SEM images, Fiji software was used to estimate the corresponding grain size distributions of Fe₃O₄ and BNKT/Fe₃O₄ composite.^2,27) The pure Fe₃O₄ sample showed a relatively narrow particle size distribution and about 20.91 nm of average grain size (D), as shown in the insets Fig. 2(a). After combining the two components BNKT and Fe₃O₄, the particle size distribution has widened (the insets in Fig. 2(a)) and the average particle size has also increased to about 57.73 nm. The BNKT/Fe₃O₄ composite material showed significantly improved particle dispersion. Compared with Fe₃O₄, the composite sample shows a closer contact between the BNKT and Fe₃O₄ phases, which increases the polarization interface loss.²⁾

Fig. 2

SEM images of samples: pure Fe₃O₄ powders (a) and BNKT/Fe₃O₄ composite (b). The insets are the corresponding grain size distributions of the Fe₃O₄ powders and BNKT/Fe₃O₄ composite.

The elemental composition and chemical properties of the pure Bi_1/2(Na_0.8K_0.2)_1/2TiO₃ and the Bi_1/2(Na_0.8K_0.2)_1/2TiO₃/Fe₃O₄ composite are shown by the EDX analysis integration as shown in Fig. 3. Figure 3(a) showed that all the expected elements such as Bi, K, Na, Ti and O were found. Furthermore, in addition to the chemical elements mentioned above, the element Fe is also found in the EDX spectrum of the Bi_1/2(Na_0.8K_0.2)_1/2TiO₃/Fe₃O₄ composite (Fig. 3(b)). It proves that the EDX results also provide evidence for the successful synthesis of Bi_1/2(Na_0.8K_0.2)_1/2TiO₃/Fe₃O₄ mixture, which is consistent with the results of XRD and SEM.

Fig. 3

EDX spectra of samples: pure Bi_1/2(Na_0.8K_0.2)_1/2TiO₃ (a) and BNKT/Fe₃O₄ composite (b).

3.2 Magnetic properties of materials

The magnetic hysteresis (M-H) loops of pure Bi_1/2(Na_0.8K_0.2)_1/2TiO₃, Fe₃O₄ particles, and BNKT/Fe₃O₄ composite were measured, shown in Fig. 4. The magnetization curves in Fig. 4(a)–(c) indicated the saturation magnetization (M_s), residual magnetization (M_r) and coercivity (H_c) of samples. The large M_s of the sample is beneficial for magnetic storage and magnetic loss during microwave energy dissipation. Therefore, it can be said that the MWA efficiency of the materials is closely related to their M_s value of them. The M_s value of BNKT/Fe₃O₄ (about 35.56 emu.g⁻¹) is lower than that of pure Fe₃O₄ (46.25 emu.g⁻¹). The decrease in M_s of the BNKT/Fe₃O₄ composite is attributed to two main reasons: (i) the reduced mass ratio of Fe₃O₄ particles (the main component) and (ii) the amount of BNKT powder which has an almost negligible magnetic contribution compared to Fe₃O₄ - the main component. However, it is clear that BNKT mixed with Fe₃O₄ significantly improved the M_s value, which is expected to strengthen the MWA efficiency of the composite. In addition, the H_c of the BNKT/Fe₃O₄ composite is higher than that of Fe₃O₄, which may be due to the sizeable magneto-crystalline anisotropy in the composite. The increase in H_c can be traced back to the residual stresses, the amount of BNKT introduced, and defects created during the synthesis of composite materials.⁸⁾

Fig. 4

Magnetic hysteresis loops of pure Bi_1/2(Na_0.8K_0.2)_1/2TiO₃ (a); BNKT/Fe₃O₄ composite (b) and M_s, H_c, and M_r values of pure Bi_1/2(Na_0.8K_0.2)_1/2TiO₃; Fe₃O₄ powders and BNKT/Fe₃O₄ composite (c), (d). The insets are samples Bi_1/2(Na_0.8K_0.2)_1/2TiO₃, BNKT/Fe₃O₄ composite with magnet, and the local magnification image of M-H loops.

3.3 Microwave absorption properties of BNKT/Fe₃O₄ composite

The MWA efficiency of the BNKT/Fe₃O₄ mixture was determined by the wire method through a vector network analyzer in the high frequency (2–18 GHz). From the data parameters of the matrix S (S₁₁ and S₂₁) measured by the instrument, the RL of the composite samples with different thicknesses was calculated by the NRW method, using the following equations:^13,28)   

\begin{equation} RL_{\text{(dB)}} = 20\log |(Z_{\textit{in}} - Z_{0})/(Z_{\textit{in}} + Z_{0})| \end{equation} (1)

  

\begin{equation} \Gamma = (Z_{\textit{in}} - Z_{0})/(Z_{\textit{in}} + Z_{0}) \end{equation} (2)

  

\begin{equation} Z_{\textit{in}} = Z_{0}\sqrt{\mu_{r}/\varepsilon_{r}} \tanh [j(2\pi fd)/c\sqrt{\mu_{r}\varepsilon_{r}}] \end{equation} (3)

Where Z_in is the normalized input impedance of the absorber, and Z₀ (= 376.7 Ohm) is the impedance of free space. μ_r and are the relative complex permeability and permittivity, j is the imaginary unit, f is the frequency of an incident wave, d is the thickness of the sample, c is the speed of the EMW in free space and Γ is the reflection coefficient. Writing the sum and the difference of the scattering coefficients as follows:²⁸⁾   

\begin{equation} \text{V}_{1} = \text{S}_{21} + \text{S}_{11};\ \text{V}_{2} = \text{S}_{21} - \text{S}_{11}; \end{equation} (4)

And if:   

\begin{equation} \text{X} = (1 - \text{V}_{1}\text{V}_{2})/(\text{V}_{1} - \text{V}_{2}) \end{equation} (5)

Then it can be shown that Γ may be obtained from the scattering coefficients since:   

\begin{equation} \Gamma = \text{X} \pm \sqrt{\text{X}^{2} - 1} \end{equation} (6)

The appropriate sign is chosen so that |Γ| ≤ 1. Figure 5(a)–(b) shows the 3-D projection (magnitude) of scattering coefficients (S₁₁) and (S₂₁) of the BNKT/Fe₃O₄ composite with different thicknesses (2.9–5.4 mm).

Fig. 5

The 3-D projection (magnitude) of scattering coefficients (S₁₁) and (S₂₁) of the BNKT/Fe₃O₄ composite with different thicknesses (2.9–5.4 mm) (a), (b); The relationship between reflection loss (dB) and the MWA percentage (%) (c); the RL (d) 2-D contour map (e) and 3-D reflection loss (f) of BNKT/Fe₃O₄ composite with different thicknesses (2.9–5.4 mm).

The relationship between reflection loss (dB) and the microwave absorption percentage (%) is given in Fig. 5(c).²⁹⁾ Accordingly, at the reflection loss value of −10 dB and −20 dB, the rate of MWA is 90% and 99%, respectively. Usually, we are interested in RL values less than −10 dB because 90% is considered a criterion for evaluating a material that can be a microwave absorber or not. The material with a lower RL below −20 dB and a wide absorption bandwidth is a candidate for excellent wave absorbers. The thickness of the sample is a factor of interest because it also greatly affects the reflection loss of the material.^3,13) Figure 5 shows the RL (d) 2-D contour map (e) of BNKT/Fe₃O₄ composite with different thicknesses (2.9–5.4 mm). It can be seen that all samples of different thicknesses have a maximum RL of less than −10 dB, and many samples have achieved RL values lower than −20 dB. That indicates the ability to MWA excellent of composite samples with wide effective absorption bandwidth. In the X-band region, the samples exhibit higher EMW absorption efficiency than other bands, such as samples with thicknesses of 4.35 mm and 5.40 mm, the value of the maximum RL is −37.29 dB (at 11.20 GHz) and −36.88 dB (at 8.39 GHz) (>99.9%), respectively. Especially with the BNKT/Fe₃O₄ composite sample, which has a thickness of 4.70 mm, the maximum reflected loss value (RL_max) it can achieve is −39.41 dB (∼99.99%) at 10.16 GHz and EAB of 16 GHz (2–18 GHz). As the thickness increases, the RL increases peak absorption intensity, and the peak position moves to a lower frequency. Thus, it is possible to change the thickness of the composite sample to change the ability to absorb radar waves. However, increasing the thickness too much will affect of the material impedance, consume it, and possibly reduce its ability to absorb EMW. Therefore, the optimal absorption layer thickness of a wave absorbing material must always satisfy two conditions: phase condition and shift-length effect.^1,3) Figure 5(f) shows the 3-D reflection loss of BNKT/Fe₃O₄ composite with different thicknesses (2.9–5.4 mm). It can be concluded that BNKT/Fe₃O₄ had a better microwave absorbing performance than other reports with the single phase Fe₃O₄, Bi_0.5(Na_0.8K_0.2)_0.5TiO₃, BaTiO₃ or composites such as Fe₃O₄/BaTiO₃, Fe₃O₄@BaTiO₃/RGO and BaFe₁₂O₁₉@Fe₃O₄.^{9,12,17,30–33)} The MWA capacity of some materials and composites is listed in Table 1.

Table 1 Microwave absorption capacity of some materials and composites.

The combination of oscillations of two components of electric and magnetic fields perpendicular to each other has created EMW that propagates through space.³⁴⁾ In this work, the BNKT/Fe₃O₄ composite material absorbs EMW generated by interacting with not only one but two fields (electrical and magnetic fields) to promote light-matter interactions in the region 2–18 GHz of the EM spectrum, which is an effective method to create EMW absorbing materials. Based on the above discussion and analysis, conclusions can be drawn about the main possible EMW absorption mechanisms of BNKT/Fe₃O₄ composites. Firstly, the resonance results from dielectric loss of BNKT and magnetic loss of Fe₃O₄ play an integral part in enhancing the MWA ability. The magnetic loss of Fe₃O₄ nanoparticles came from the eddy current loss (frequency: 8–18 GHz) and the natural resonance (frequency: 2–8 GHz). Secondly, the unique structure, nanometer size of the BNKT, Fe₃O₄ component phases or the anchoring of the Fe₃O₄ particles on the BNKT surface leads to impedance matching, multiple reflections, scattering and refraction of the EMW in the BNKT/Fe₃O₄ composite material, which results in improved EM attenuation. In addition, the dielectric loss also derived from the defects causing dielectric polarization and interface polarization (BNKT and Fe₃O₄). The increased dielectric loss resulted in improving the EM absorption efficiency of the material synthesis of BNKT/Fe₃O₄.

4. Conclusions

The novel Bi_1/2(Na_0.80K_0.20)_1/2TiO₃/Fe₃O₄ composite was prepared by a chemical method (two-step), a combination of sol-gel and co-precipitation methods. The X-ray diffraction measurement results show that the characteristic diffraction peaks with a distinct two-phase structure attributed to BNKT and Fe₃O₄, confirming that the composite has been fabricated successfully. The Fe₃O₄ particles with a grain size of about 20.91 nm were scattered around the BNKT particles. The combination of two electrical and magnetic components (corresponding to BNKT and Fe₃O₄) leads to changes in structure and morphology but also in the properties of Bi_1/2(Na_0.80K_0.20)_1/2TiO₃/Fe₃O₄ composite compared with the original pure components. The significantly enhanced MWA efficiency of composites is attributed to a combination of large dielectric loss, magnetic loss, and good impedance matching, as well as an improvement in interface polarization between components. In particular, when stuffed Bi_1/2(Na_0.80K_0.20)_1/2TiO₃/Fe₃O₄ with 50% paraffin to create a mixture of BNKT/Fe₃O₄/paraffin, it has proven itself to be a good candidate for applications in EM shielding or MWA. With thickness d = 4.7 mm, the Bi_1/2(Na_0.80K_0.20)_1/2TiO₃/Fe₃O₄ composite sample showed the maximum reflection loss RL_max = −39.41 dB (>99.9%) at the frequency f = 10.16 GHz. More importantly, the sample can cover almost the entire measuring frequency from 2 to 18 GHz with effective absorption bandwidth EAB = 16 GHz. Therefore, the Bi_1/2(Na_0.80K_0.20)_1/2TiO₃/Fe₃O₄ composite with a simple fabrication process, large quantity, and low cost are said to have excellent prospects for developing superior microwave absorption devices and technologies, which can be applied to solve big and really urgent problems in reality.

Acknowledgments

Nguyen Dang Co, ID VNU.2021.NCS.06, thanks The Development Foundation of Vietnam National University, Hanoi for sponsoring this research. Nguyen Dang Co was funded by the Master, PhD Scholarship Programme of Vingroup Innovation Foundation (VINIF), code VINIF.2022.TS.016. This research has been done under the research project QG.21.31 “Study and fabrication equipment to support monitoring, warning, and control of river water salinity for agriculture and response to climate change” of Vietnam National University, Hanoi.

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