Facile synthesis of net-like Fe₃O₄/MWCNTs decorated by SnO₂ nanoparticles as a highly efficient microwave absorber

Abstract

Net-like SnO₂/Fe₃O₄/MWCNTs were prepared through a simple two-step hydrothermal process. The as-synthesized composites were characterized by X-ray diffraction, vibrating sample magnetometer, transmission electron microscopy (TEM), and vector network analyses. In the process of Fe₃O₄ ripening, Fe₃O₄ particles play a role of bridge to connect MWCNTs to form a real net-like structure. TEM images indicate that Fe₃O₄ microspheres have size ranging from 100 to 200 nm, which can be seen as a scaffold for binding MWCNTs. The electromagnetic parameter results show that SnO₂ nanocrystals were introduced into the system for tuning the parameters of SnO₂/Fe₃O₄/MWCNTs composites to improve impedance match and SnO₂ nanocrystals can supply space charge and interfacial polarization. The SnO₂/Fe₃O₄/MWCNTs composites exhibit highly efficient microwave absorption capacity within the tested frequency range (2–18 GHz). The optimal reflection loss of electromagnetic waves is −42.0 dB at 10.9 GHz with an absorber thickness of 1.9 mm. The composites are potential excellent microwave absorbers with a special net-like structure for synergistic interaction between Fe₃O₄ microspheres with MWCNTs and semiconductors for tuning of electromagnetic parameters. The fabricated SnO₂/Fe₃O₄/MWCNTs composites are promising materials for high-performance microwave absorption and satisfy the highly efficient absorption capability, thin thickness, and light weight requirements for electromagnetic absorption.

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1 Introduction

Magnetic–dielectric composites have been increasingly used for microwave absorption (MA). Composite materials not only retain the inherent properties of their components but also provide enhanced functionality and multifunctional properties because of the interaction among their components.^1–4 As a class of one-dimensional carbon nanostructure, carbon nanotubes (CNTs) have received much attention due to their unique chemical and physical properties; these materials are suitable for various applications, including coatings and films,⁵ batteries,^6–8 environment and energy storage,⁹ and biotechnology.^10–12 Studies have investigated the application of MWCNTs in MA; these composites include ZnO@MWCNTs,¹³ WMCNTs/Fe₃O₄ (ref. 14 and 15), SnO₂@MWCNTs.¹⁶ Ferroferric oxide, a conventional microwave absorbing material, has also been investigated as a promoter for MA enhancement because of its excellent magnetic properties.^17–21

Dielectric and magnetic loss materials are combined to obtain an excellent microwave absorber.^22–36 Wang et al.¹⁴ report a hybrid material consisting of magnetite (Fe₃O₄) nanocrystals with a core size of 8 nm grown on multiwalled carbon nanotubes (MWCNTs). The composites exhibit a surprisingly high-performance capability for MA with minimum reflection loss reaching −41.61 dB at a thickness of 3.4 mm. Cao et al.¹⁵ fabricated a novel dielectric–magnetic nanostructure by hybridizing 3D Fe₃O₄ nanocrystals and multi-walled carbon nanotubes minimum RL values of the composites with a thickness of 6.8 mm reach −23.0 dB and −52.8 dB at 4.08 GHz and 12.8 GHz, respectively. Besides, they fabricated grape-like Fe₃O₄-MWCNTs through co-precipitation. It could be seen that the samples with 3.2 mm possess the best MA performance.³⁷ Huang et al.³⁸ prepared boron and nitrogen doped carbon nanotubes/Fe₃O₄ composite. Nano-scale Fe₃O₄ particles with the diameter about 30 nm are uniformly distributed on the nanotubes and the composite have a minimum reflection loss is −51.2 dB at 11.7 GHz with a thickness of 5 mm. Du et al.³⁹ prepared core–shell Fe₃O₄@C composites through in situ polymerization of phenolic resin followed by high-temperature carbonization. Fe₃O₄@C shows dielectric loss, interfacial loss, and improved matching impedance after carbon coating. Hence, the capacity of these composites for MA can be significantly improved by assigning appropriate impedance from the combination of dielectric and magnetic losses. MWCNTs were decorated with Fe₃O₄ nanoparticles with a size distribution of 7–18 nm using the wet chemical method, which could be used as the microwave absorber.⁴⁰ As mentioned above, Fe₃O₄ nanocrystals and carbon composites have been invested in microwave absorber because they combine magnetic loss ability and dielectric loss ability effectively, which is contribute to improve MA. In comparison with the Fe₃O₄ nanocrystals decorate the MWCNTs, there are few correlative literatures report Fe₃O₄ microspheres combine with MWCNTs.

In this study, we demonstrate the synthesis of Fe₃O₄ microspheres decorate the MWCNTs with a net-like structure through the hydrothermal process. SnO₂ nanocrystals were introduced into the system though the other hydrothermal process. Fig. 1 shows the synthesis process of SnO₂/Fe₃O₄/MWCNTs composites.

Fig. 1 Schematic of the synthesis procedure of SnO₂/Fe₃O₄/MWCNTs.

Small Fe₃O₄ nanoparticles were adhered to the surface of MWCNTs and assembled to obtain large Fe₃O₄ microspheres with size ranging from 100 nm to 200 nm. In the process of Fe₃O₄ ripening, Fe₃O₄ particles play a role of bridge to connect MWCNTs to form a real net-like structure. SnO₂ nanocrystals were then introduced to the system to tune the complex permittivity of Fe₃O₄/MWCNTs composites and improved impedance match. This study aims to fabricate high-performance microwave absorbers and discuss the synergistic interaction between magnetic microspheres and MWCNTs for MA.

2 Experimental sections

2.1 Materials

MWCNTs were provided by Chengdu Organic Chemicals Co., Ltd. (China). Raw MWCNTs (∼1 g) were refluxed at 120 °C for 12 h in the HNO₃ solution (65–68 wt%). Ethylene glycol, sodium acetate trihydrate (CH₃COONa·3H₂O), iron(III) nitrate nonahydrate [Fe(NO₃)₃·9H₂O], and tin(IV) chloride pentahydrate (SnCl₄·5H₂O) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ammonium water was supplied by Yantai Shuangshuang Chemical Factory. All of the chemical regents used in this work were analytically pure and were not subjected to further purification.

2.2 Synthesis of Fe₃O₄/WMCNTs

Fe₃O₄/WMCNTs were fabricated by a simple hydrothermal process. Briefly, the as-treated MWCNTs (80 mg) were dispersed in 70 mL of ethylene glycol solution and then added with Fe(NO₃)₃·9H₂O (1, 2, 3 and 4 mmol) and CH₃COONa·3H₂O. The mixture was sonicated for 2 h. The mixed solution was then transferred into a Teflon-lined stainless steel autoclave with a capacity of 100 mL and heated at 200 °C for 20 h. The resulting sample was collected by washing with distilled water several times and drying in an oven at 60 °C.

2.3 Synthesis of SnO₂/Fe₃O₄/WMCNTs

SnO₂/Fe₃O₄/MWCNTs composites were prepared by a hydrothermal process. All of the as-prepared Fe₃O₄/MWCNTs products were dispersed in 70 mL of distilled water and then added with 4 mmol SnCl₄·5H₂0. The mixture suspension was sonicated for 2 h. Next, NH₃·H₂O (25 wt%) was added drop-wise into the reaction mixture to adjust the pH equal to 10. The mixed solution was transferred into a Teflon-lined stainless steel autoclave and heated at 160 °C for 18 h. The final products were washed by distilled water several times until pH = 7 and dried in an oven at 60 °C. The SnO₂/Fe₃O₄/MWCNTs composites with different mole ratio of Fe₃O₄ and SnO₂ nanocrystals (δ_Fe3O4 : δ_SnO2) were obtained and labeled as S-1 (1 : 4), S-2 (2 : 4), S-3 (3 : 4), and S-4 (1 : 1). The SnO₂/MWCNTs composite was prepared with the same method.

2.4 Characterization

The X-ray powder diffraction (XRD) patterns of the SnO₂/Fe₃O₄/MWCNTs were tested on PANayltical Empyrean powder X-ray diffractometer using monochromatic Cu–Kα radiation (λ = 0.154178 nm, 40 kV, 40 mA) and a scanning speed of 4° min⁻¹. Magnetic properties were determined using HH-20 vibrating sample magnetometer (VSM). TEM images were recorded on a FEI Tecnai G20 TEM system. The sample for microwave measurement was prepared by uniformly blending SnO₂/Fe₃O₄/MWCNTs absorbents with paraffin wax at a mass fraction of 70% and manufactured into a coaxial ring (3.04 mm inner diameter, 7.00 mm outer diameter). The complex permittivity and permeability of the SnO₂/Fe₃O₄/MWCNTs were measured by a vector network analyzer (VNA, AV3629D) through transmission/reflection method.

3 Results and discussion

3.1 Formation mechanism of SnO₂/Fe₃O₄/MWCNTs

The synthesis of SnO₂/Fe₃O₄/MWCNTs is shown in Fig. 1. After 2 h of ultrasonic treatment, acid-treated MWCNTs and Fe(NO₃)₃·9H₂O were dispersed in ethylene glycol solution (Fig. 1a). The suspension was added with sodium acetate trihydrate and heated at 200 °C. Fe³⁺ first formed Fe(OH)₃ under weak base environment and further reduced to Fe₃O₄ nanoparticles in the absence of ethylene glycol. Binding of Fe₃O₄ could be induced by magnetic dipole–dipole attraction (Fig. 1c).⁴¹ After that, the Fe₃O₄/MWCNTs composites were dispersed in distilled water through ultrasonic treatment (Fig. 1d). These functional groups and defects provide a location for Sn⁴⁺, which forms nuclei and converts into SnO₂ nanoparticles. Electrostatic interactions play a key role in this process (Fig. 1e).

Fig. 2a–c show the SEM images of the Fe₃O₄/MWCNTs composites without the addition of SnO₂, the synthesized Fe₃O₄ with size of 100–200 nm in the Fe₃O₄/MWCNTs composites, which partly are cross-linked with the CNTs. The surface of MWCNTs looks smooth because of the small Fe₃O₄ particles self-assembled into microspheres. With addition of 4 mmol SnO₂, the obtained SnO₂/Fe₃O₄/MWCNTs sample consists of net-like Fe₃O₄/MWCNTs and the SnO₂ crystals. At the same time, the surfaces of MWCNTs looks rough due to the surfaces of MWCNTs were decorated by the SnO₂ nanoparticles. Further increasing the Fe³⁺ amount to 2 mmol (Fig. 3b), 3 mmol (Fig. 3c), and 4 mmol (Fig. 1d), the more Fe₃O₄ microspheres were obtained, but the size have a slight decrease and the microsphere stricture was damaged to a certain extent. Clearly, the less the added Fe³⁺, the complete stricture of Fe₃O₄ microspheres formed. Thus, it is possible to tune the network structure or morphology by varying the content of Fe³⁺.

Fig. 2 the SEM images of Fe₃O₄/MWCNTs (a–c) and SnO₂/Fe₃O₄/MWCNTs with 1 mmol Fe³⁺ (d–f).

Fig. 3 SEM images of SnO₂/Fe₃O₄/MWCNTs obtained at different amounts of Fe³⁺ added: (a) 1 mmol, (b) 2 mmol, (c) 3 mmol, and (d) 4 mmol.

3.2 Characterization of SnO₂/Fe₃O₄/MWCNTs

Fig. 4 shows the crystal structure of the final SnO₂/Fe₃O₄/MWCNTs samples identified using XRD patterns. MWCNTs have its peaks, which appeared at 26.1° and 42°. The peaks at 26.6°, 33.9°, 37.9°, and 51.8° are assigned to the (110), (101), (200), and (211) planes of tetragonal rutile SnO₂ (JCPDS no. 41-1445), with lattice parameters of a = b = 4.738 nm and c = 3.187 nm. Notably, there are no obvious diffraction peaks for the Fe₃O₄ in S1 sample, which might be duo to the relatively low content in the SnO₂/Fe₃O₄/MWCNTs system and covered by the peaks of SnO₂. The diffraction peaks of Fe₃O₄ gradually emerged as the content of Fe³⁺ added to the system was increased. Diffraction peaks at 30.1°, 35.4°, 43.0°, 56.9°, and 62.5° represent the Bragg reflection from the (220), (311), (400), (511), and (440) planes of the cubic spinel crystal structure of Fe₃O₄ (JCPDS no. 19-0629), with lattice parameters of a = b = c = 8.396 nm. The diffraction peaks of MWCNTs were covered by the peaks of SnO₂ and Fe₃O₄. Based on calculation using Scherrer's formula, the average crystallite sizes of SnO₂ and Fe₃O₄ are about 2 and 20 nm, respectively. No peaks corresponding to impurities were detected; hence, the composites are composed of SnO₂, Fe₃O₄, and MWCNTs.

Fig. 4 XRD of MWCNTs and SnO₂/Fe₃O₄/MWCNTs composites.

The magnetization of four SnO₂/Fe₃O₄/MWCNTs composites was measured at room temperature by a vibrating sample magnetometer (HH-20). Hysteresis loops of sample S-1 with a small coercivity (H_c) 18.2 emu g⁻¹ indicated the presence of the soft ferromagnetic behavior.⁴² The values of saturation magnetization (M_s) increased from 1.83 to 15.59 emu g⁻¹ in Fig. 5. These results demonstrated that the composites are ferromagnetic and thus may be used to absorb microwave after introducing Fe₃O₄ particles.

Fig. 5 Magnetic properties of SnO₂/Fe₃O₄/MWCNTs composites.

The TEM images in Fig. 6a shows that Fe₃O₄ microspheres are cross-linked by MWCNTs or decorated on the MWCNTs. The Fe₃O₄ microspheres with size of 100–200 nm are regarded as scaffold for binding MWCNTs to form a real net-like structure. After introducing the SnO₂ nanocrystals, there is a clearly decrease about the size of Fe₃O₄ microspheres. Fig. 6c shows that Fe₃O₄ nanoparticles decorated on the MWCNTs and SnO₂ nanocrystals uniformly covered the surface of the nanotubes, thereby allowing incident electromagnetic waves to enter into the interior of the composites. Fig. 6d shows the HRTEM image of SnO₂/Fe₃O₄/MWCNTs. The Fe₃O₄ microspheres consist of nanocrystals with size of 16–22 nm. Fig. 6e presents the diffraction profile generated by the inserted SAED pattern and confirms the structure of SnO₂ and Fe₃O₄. The results of EDS analysis confirmed the presence of C, O, Fe, and Sn in the composites. The valence states of Fe and Sn were determined through XPS analysis. Fig. 7a shows the binding energies of Fe 2p at 711.6 and 724.7 eV, but no satellite peak was observed at 720 eV; this findings and the XRD result can demonstrate that the microspheres are Fe₃O₄. For the spectrum of Sn in Fig. 7b, the peaks of Sn were found at 487.5 and 495.8 eV, which are assigned to the lattice of Sn⁴⁺ ions in tin oxide.

Fig. 6 (a) TEM images of Fe₃O₄/MWCNTs; (b and c) TEM images, (d) HRTEM image, (e) SAED pattern, and (f) EDS pattern of SnO₂/Fe₃O₄/MWCNTs sample with Fe³⁺ contents 1 mmol.

Fig. 7 XPS spectra: (a) Fe 2p and (b) Sn 3d spectra of SnO₂/Fe₃O₄/MWCNTs sample with Fe³⁺ contents 1 mmol.

3.3 Microwave absorbing properties

Generally, the electromagnetic wave absorption properties of absorbers were evaluated by reflection loss (RL), which is calculated using relative complex permittivity and permeability according to transmit line theory. RL is defined in the following equation:^14,46,47

RL (dB) = 20 log₁₀|(Z_in − Z₀)/(Z_in + Z₀)| (1)

where Z_in is the normalized input impedance of absorber, Z₀ is the impedance of free space, ε_r is the complex permittivity, μ_r is the complex permeability, f is the frequency, c is the light velocity, and d is the thickness of the SnO₂/Fe₃O₄/MWCNTs composites. The electromagnetic parameters of the as-synthesized material were confirmed using a vector network analyzer through coaxial method. The electromagnetic parameters of four SnO₂/Fe₃O₄/MWCNTs composites were tested and the results were shown in Fig. 8.

Fig. 8 Frequency dependence of (a) the real part (ε′) and (b) imaginary part (ε′′) of complex permittivity; (c) the real part (μ′) and (d) imaginary part (μ′′) of the complex permeability of four SnO₂/Fe₃O₄/MWCNTs samples.

The complex permittivity (ε_r = ε′ − jε′′) and complex permeability (μ_r = μ′ − jμ′′) of SnO₂/Fe₃O₄/MWCNTs composites were observed in 2–18 GHz. The real permittivity (ε′) and real permeability (μ′) represent the storage ability of microwave energy, whereas the imaginary permittivity (ε′′) and imaginary permeability (μ′′) indicate the dissipation capacity of microwave energy.^23,33,43 Fig. 8a shows that the real permittivity (ε′) value decreased with increasing frequency in the investigated region. Sample S-1 exhibited maximum changes, particularly ε′ decreased from 17.6 to 12.2. The ε′ values of sample S-1 are the highest among the four SnO₂/Fe₃O₄/MWCNTs samples; in other words, this sample features high energy storage and polarization.⁴³ Fig. 8b presents the imaginary part (ε′′) of the four SnO₂/Fe₃O₄/MWCNTs composites, which show a similar trend between different samples. S-1 exhibits the highest ε′′ value, which indicating its superior dielectric loss. The curves of the samples present a complicated frequency-dependence behavior. For example, the ε′′ value of sample S-1 first decrease from 5.7 at 2 GHz to 4.4 at 7.1 GHz with a slight peak at 5.4 GHz, then increase gradually within 7.1–18 GHz with multiple peaks. This finding indicates the presence of multiple resonances behavior, which is related to highly conductivity and skin effects, electronic spin, and charge polarization because of point effects and polarized centers.^26,44,45 It mostly comes from the synergistic influence of Fe₃O₄ microspheres, SnO₂ nanocrystal, and the interfaces between Fe₃O₄ and dielectric MWCNTs. Moreover according to the free electron theory, ε′′ ≈ 1/πε₀ρf, which ρ is the resistivity. The conductivity of S-1 samples is higher than others samples, the Fe₃O₄ microspheres can connect with MWCNTs to form a conductivity work, which will induce conduction.

Fig. 8c and d show the μ′ and μ′′, respectively, of all SnO₂/Fe₃O₄/MWCNTs composites within 2–18 GHz. Both the μ′ and μ′′ of different Fe³⁺ loading concentrations indicate a similar decreasing trend with increasing frequency. For example, the μ′ value of sample S-1 decreased from 1.17 to 1.00 with several small fluctuant peaks. The μ′′ value increased from 0.15 at 2 GHz to a maximum of about 0.25 at 2.5 GHz, sharply decreased to 0 at 9 GHz, and maintained negative within 9–18 GHz. In the band 2–8 GHz and the band 14–18 GHz, all the samples exhibit two distinct peaks. Theoretically, the probable reasons of these peaks contribute to domain wall resonance, eddy current resonance and natural resonance.

The simulated RL curves of the Fe₃O₄/MWCNTs, SnO₂/MWCNTs and SnO₂/Fe₃O₄/MWCNTs composites are shown in Fig. 9. The Fe₃O₄ microspheres decorate MWCNTs have a very poor performance in the MA, because the majority of the surface of MWCNTs is naked to space (Fig. 4a) and most incident EM wave will be reflected on the surface due to high surface resistance. For SnO₂/MWCNTs composites exhibit a low EM wave ability and the optimal RL of EM wave is −16.5 dB at 5.04 GHz with an absorber thickness of 3.5 mm. The poor EM wave absorption performance may cause by the relatively low characteristic impedance and signal dielectric loss action. The impedance properties of the composites have greatly improved after introducing the SnO₂ nanoparticles into the system (Fig. 8c), which is beneficial for microwave penetration into the system and the SnO₂/Fe₃O₄/MWCNTs composites exhibit an effective absorption ability. For example, S-1 exhibited significantly enhanced electromagnetic wave absorption compared with the other three SnO₂/Fe₃O₄/MWCNTs in 2–18 GHz. Absorber thickness of 1.9 mm and mass fraction of Fe₃O₄ is 12.9%, the relevant Z_in/Z₀ values of sample S1 almost close to 1. Therefore, the EM wave can incident in the interior of absorber to be attenuated rather than reflect at the surfaces. So the S-1 sample have an optimal RL of electromagnetic wave is −42.0 dB at 10.9 GHz, the effective bandwidth (RL ≤ −10 dB) could reach 2.8 GHz (9.4–12.2 GHz) and when the thickness of the absorber is 1.5 mm, the microwave absorber shows the maximum effective absorption bandwidth of 3.8 GHz (12.4–16.2 GHz). The testing data indicate that the thickness of absorber can be easily adjusted to meet the need of absorption toward different frequency microwaves. Table 1 gives the information of other SnO₂/Fe₃O₄/MWCNTs composites.

Fig. 9 Frequency dependence of simulated reflection loss value of Fe₃O₄/MWCNTs (a), SnO₂/MWCNTs (b) and SnO₂/Fe₃O₄/MWCNTs composites: (c) S-1, (d) S-2, (e) S-3, and (f) S-4 samples.

Table 1 Reflection information of SnO₂/Fe₃O₄/MWCNTs composites^a

Sample	Mass fraction of Fe3O4 (%)	RL(Min) (dB)	f (GHz) (RLMin)	Thickness (mm)
a WFe3O4 = (MSnO2/Fe3O4/MWCNTs − MSnO2 − M MWCNTs)/(MSnO2/Fe3O4/MWCNTs + Mparaffin).
MWCNTs/Fe3O4	65.2	−3.2
MWCNTs/SnO2		−19.1	7.68	2.5
S-1	12.9	−42.0	10.9	1.9
S-2	15.5	−22.2	6.7	3.5
S-3	18.6	−21.7	4.6	4.5
S-4	22.4	−38.9	4.2	5.0

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In Fig. 10b, the maximum absorption peak shifted from high frequency to low frequency with increasing absorber thickness. According to the quarter-wavelength match principle, the relationship between absorber thickness (t_m) and peak frequency (f_m) can be described by the following equation:^18,38,52

Fig. 10 (a) RL curves for sample 1, (b) relationship between simulation thickness and peak frequency, (c) the relationship between Z_in/Z₀ and frequency.

Fig. 10b shows the variations in the RL curve of the SnO₂/Fe₃O₄/MWCNTs (S-1) with different thicknesses at 2–18 GHz. When the matching thickness of samples satisfies eqn (3), then microwave will be reflected from various interfaces with opposite phases, resulting in a counter act of one another at the interfaces. Based on the quarter-wavelength condition, absorber thickness (t_m) versus peak frequency (f_m) for the SnO₂/Fe₃O₄/MWCNTs composites is simulated and displayed in Fig. 10b. The red dots on the λ/4 curve are the match thickness (denoted as t^exp_m) of absorption peaks. The relationship between the experimental match thickness and peak frequency is in good agreement with the simulations using the quarter-wavelength principle for SnO₂/Fe₃O₄/MWCNTs composites. In Fig. 10c, the frequency dependence of Z_in/Z₀ for composite S-1 is almost close to 1 at the absorber thickness of 1.9 mm, therefore the EM wave can easily permeate into the absorber to be attenuated and the composite exhibit maximum reflection loss at the 10.6 GHz. It can conclude that S-1 has a better MA property at identical coating thickness (1.9 mm).

SnO₂/Fe₃O₄/MWCNTs composites exhibited the improved MA and satisfied the highly efficient absorption capability, thin thickness, and light weight requirements for electromagnetic absorption. Microwave absorption properties may be ascribed to several factors, such as impedance match, dielectric loss, magnetic loss, and interface relaxation. According to Debye theory, ε′′ represents dielectric loss, which consists of polarization loss and conductivity losses. The ε′′ is mainly enhanced by the conductivity of the SnO₂/Fe₃O₄/MWCNTs as well as polarization at the Fe₃O₄-MWCNTs, Fe₃O₄–Fe₃O₄, and SnO₂-MWCNTs interfaces (Fig. 11a). The plenty of interfaces in SnO₂/Fe₃O₄/MWCNTs composites offer more chances of multi-reflection and diffusion scattering to form microwave dissipation paths. The specific structure of SnO₂/Fe₃O₄/MWCNTs could build a conductive network and increases the conductive pathways between Fe₃O₄ and MWCNTs. In this heterostructure, Fe₃O₄ microspheres play at least two roles in ameliorating MA properties. First, the Fe₃O₄ microspheres can provide a microwave magnetic loss action as a soft ferromagnetic material. The natural resonance and the eddy current effect play a pivotal role in magnetic loss process. Natural resonance generally appears at frequency below 8.2 GHz (Fig. 8d). At same time, the steady values of C₀ (C₀ = μ′′(μ′)⁻²f⁻¹) for samples with different Fe₃O₄ concentrations at 10–18 GHz, which suggests that the composites have an obvious eddy current loss.^18,20,37,43 Second, the Fe₃O₄ microspheres are considered as a scaffold for binding two individual MWCNTs to form a real network. This makes it possible for electron hopping and migrating, which is benefit for MA (Fig. 11b).

Fig. 11 (a and b) Main polarization and conductivity pathways.

In Table 2, we listed the reflection loss properties of some Fe₃O₄ based composites. The poor impedance matching properties caused the unbalancing of the complex permittivity and the permeability. No matter Fe₃O₄ nanospheres or Fe₃O₄ micro-spheres, the single Fe₃O₄ cannot show the efficient MA. Besides, Fe₃O₄ presents several limitations, including poor impedance matching, large thickness, and high loading content, which restrict their practical applications. When Fe₃O₄ combine with some dielectric loss materials, such as MWCNTs, SnO₂ and PANI, those Fe₃O₄ based composites got an efficient MA with a higher impedance matching and the synergy effect between dielectric and magnetic loss materials. But the large thickness of those materials will limit their practical applications. In this work, SnO₂ nanocrystals were introduced to the system, which not only tune the complex permittivity and complex permeability of SnO₂/Fe₃O₄/MWCNTs composites to improve the impedance match, but also supply more space charge polarization and interfacial polarization. The optimal reflection loss (RL) of electromagnetic wave is −42.0 dB with the absorber thickness of only 1.9 mm. This is an attractive candidate for high performance MA materials, which satisfies the current requirements of electromagnetic absorbing materials, which include high-efficiency absorption capability, thin thickness and light weight.

Table 2 EM wave absorption performance of relative absorber

Sample	RL(Min) (dB)	Thickness (mm)	Ref
Fe3O4 nanospheres	−9	1.5	48
Fe3O4 micro-spheres	−15	1.5	49
Fe3O4-MWCNTs	−24.8	3.2	37
Fe3O4/SnO2 nanorods	−27.4	4.0	53
Fe3O4 micro-spheres/PANI	−31.3	3.0	19
Fe3O4/SWCNHs	−38.8	5.8	50
Fe3O4/GCs	−32.0	3.5	51
SnO2/Fe3O4/MWCNTs	−42.0	1.9	This work

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4 Conclusions

Net-like SnO₂/Fe₃O₄/MWCNTs were prepared through hydrothermal process. Fe₃O₄ microspheres decorated on the MWCNTs can connect MWCNTs form a conductive network and provide a microwave magnetic loss action as a soft ferromagnetic material. SnO₂ nanocrystal can not only tune the electromagnetic parameters to improve the impedance match, but also supply more space charge polarization and interfacial polarization. When the mass fraction of Fe₃O₄ is 12.9% with an absorber thickness of only 1.9 mm the SnO₂/Fe₃O₄/MWCNTs composites shown an optimal microwave reflection loss is −42.0 dB at 10.9 GHz. The excellent electromagnetic absorption capacity is attributed to the well impendence match, dielectric and magnetic loss, and special structure.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (Grant No. 51477002; 51173002).

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