Introduction

Electromagnetic interference (EMI) and electromagnetic compatibility (EMC) are critical issues associated with microwave radiation, commonly observed in modern devices such as satellite communication systems, mobile devices, automated teller machines (ATMs), wireless networks, and local area networks (LANs)1,2,3,4. Ensuring effective EMI shielding and achieving EMC are crucial in our increasingly tech-centric world, as they help prevent signal disruptions and protect device functionality. Additionally, radar-absorbing coatings play a crucial role in defense applications, notably for camouflage on military platforms like aircraft and ships5. The ideal absorbent coatings should combine desirable physical and chemical characteristics, including superior mechanical strength, lightweight and low-density, minimal thickness, and reliable performance at elevated temperatures and across diverse frequency ranges6,7.

Recently, researchers have concentrated on preparing advanced microwave absorbing materials by integrating various polymer matrices with different fillers8,9. Because a microwave absorber cannot be molded, an absorbent medium is necessary. The inherent characteristics of the absorbing matrix strongly influence the microwave performance of microwave absorbing materials. To date, a variety of media have served as matrices including paraffin, polystyrene (PS), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), epoxy, silicone rubber, polyurethane, polyvinyl chloride (PVC), polymethyl methacrylate (PMMA), brick, polyurethane foam (PUF), cellulose, polyester, cement, and concrete. These matrices have enhanced microwave absorption and shielding capabilities10,11. Among them, epoxy resin offers strong interfacial bonding and uniform dispersion with microwave absorbing materials, moderate to high dielectric loss compatibility, good mechanical strength and processability, excellent thermal stability, and tunable dielectric properties that balance magnetic and dielectric losses for broadband microwave absorption12. Fillers that effectively convert microwave energy into heat by interacting with electromagnetic waves, must be compatible with the host polymer13,14. Nano-sized fillers are especially promising because they exhibit quantum properties that enhance their microwave absorption capabilities15. Carbon nanotubes (CNTs), owing to their exceptional quantum characteristics, impressive mechanical properties, good corrosion and thermal stability, high electrical conductivity, and lightweight have emerged as ideal fillers in these composites16. CNTs, with their larger surface area and increased number of dangling bonds, can induce interfacial polarization and a macroscopic quantum tunneling effect, and have demonstrated promising performance as microwave absorbers17,18,19. Peyravy et al. employed multi-walled carbon nanotubes (MWCNTs) to improve the microwave absorption of Dowex Optipore V503, a commercial hypercrosslinked polymeric adsorbent with low dielectric loss factor20. Dielectric properties results exposed a severe rise in the loss factor from 0.05 to 1.30 after coating the adsorbent with 0.3 wt% of MWCNTs. Surface modification of CNTs with magnetic metals such as Ni0.5Zn0.5Fe2O421, CuFe2O422, CaFe2O423, Zn0.25Co0.75Fe2O424, MgFe2O425 and etc. has further enhanced their dispersion within the polymer matrix and improved their electrical and magnetic interactions, leading to better electromagnetic wave absorption and shielding26. Ni0.5Zn0.5Fe2O4 offers large magnetic permeability, high resistivity, good mechanical hardness, and a relatively high Curie temperature, along with reduced eddy current losses and minimized dielectric losses at high frequencies, outperforming many other spinel ferrites27. Cao et al. developed a magnetic nanocomposite by coating Ni0.5Zn0.5Fe2O4 nanoparticles on MWCNTs using chemical precipitation–hydrothermal process21. They found that this approach offered advantages such as high purity, uniform chemical composition, consistent particle size, and cost-effectiveness, making it a promising avenue for scalable production. Similarly, Ma and colleagues prepared polyurethane-based composites embedded with different types of carbon nanotubes, including unmodified MWCNTs, silver-modified (Ag@MWCNTs), nickel-modified (Ni@MWCNTs)16. Their investigations revealed that the composite containing Ni@MWCNTs exhibited highest microwave absorption capacity. Despite Ni’s relatively high electrical resistivity compared to silver, its magnetic properties, combined with the electrical characteristics of carbon nanotubes, enhance the interaction with microwave energy, leading to improved EMI shielding. In another study, Feng et al. incorporated both MWCNTs and Ni@MWCNTs into poly(phthalazinone ethersulfone ketone) (PPESK), demonstrating that the Ni@MWCNTs/PPESK composite provided a broader absorption frequency range and a lower minimum reflection loss (RL) compared to the MWCNTs/PPESK composite28. Previous research has highlighted the unique significance of Ni nanoparticles as effective electromagnetic wave absorbers. However, a challenge is that nanoscale Ni readily oxidizes to form nickel oxide on the surface of CNTs, which can diminish its electromagnetic properties29.

While Ni nanoparticles are recognized as effective electromagnetic wave absorbers, a challenge persists: at the nanoscale, Ni easily oxidizes to form nickel oxide on the surface of the nanotubes, which can reduce its electromagnetic effectiveness. To address this issue, the study investigated Ni- and Zn-doped Fe3O4 ferrite, aiming to enhance magnetic properties and mitigate Ni oxidation within a unified ferrite-based system. The resulting Ni0.5Zn0.5Fe2O4 nanoparticles were then integrated with MWCNTs within an epoxy resin matrix to yield nanocomposites with enhanced characteristics. These materials underwent comprehensive characterization using techniques such as Fourier transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM), mechanical and thermal tests. The microwave absorption behavior of the samples were evaluated within the X-band frequency range using a network analyzer.

Experimental

Materials

Sodium lignin sulfonate (SLS), Ni (II) nitrate hexahydrate (Ni(NO3)2.6H2O), zinc (II) nitrate hexahydrate (Zn(NO3)2.6H2O), iron (III) nitrate nonahydrate (Fe(NO3)3.9H2O), sodium hydroxide, tetrahydrofuran (THF) and ethanol were sourced from Sigma-Aldrich. Epiran epoxy-06 was obtained from Khouzestan Petrochemical Company. Hardener f-205 was purchased from Kian Resin Company and MWCNTs (Purity ~ 95%, specific surface area 110 m2/g <, inner diameter 5–10 nm, outer diameter 20–30 nm, and length 1–2 μm) were supplied from Research Institute of Petroleum Industry of Iran. All chemicals used were of reagent grade and employed as received, without further purification.

Preparation of Ni0.5Zn0.5Fe2O4@MWCNTs

Ni(NO3)2.6H2O (0.99 g, 3.4 mmol), Zn(NO3)2.6H2O (1.01 g, 3.4 mmol), and Fe(NO3)3.9H2O (5.49 g, 13.6 mmol) were added to 10 mL of deionized water and stirred continuously with a stirrer until completely dissolved. Separately, 0.08 g of MWCNTs was added to 70 mL of deionized water in a different container, then subjected to probe ultrasound irradiation at moderate power for 15 min. 8 mg of SLS was introduced into the MWCNT suspension and ultrasonically dispersed for an additional 10 min. The MWCNT solution was then combined with the metal nitrate solution in a reaction vessel and stirred further. The pH was adjusted to approximately 11 ± 0.3 using a 1 M NaOH solution. This adjustment triggered the co-precipitation of nickel, zinc, and iron hydroxides, resulting in a brown, viscous slurry, which was transferred into a polytetrafluoroethylene (PTFE) vessel and sealed inside an autoclave. In order to synthesize the ferrite-coated MWCNTs, the autoclave was heated to 220 °C and maintained at this temperature for 3 h. After cooling, the Ni0.5Zn0.5Fe2O4@MWCNTs were isolated using a neodymium magnet, washed multiple times with deionized water and ethanol, and then dried under vacuum overnight.

In-situ Preparation of epoxy/Ni0.5Zn0.5Fe2O4@MWCNTs nanocomposite

2.1 g of epoxy resin was placed into a 50 mL beaker, after which 2 mL of acetone and 1 mL of THF were added. The mixture was stirred 40 min at 40 °C. Subsequently, various amounts of nanoparticles were incorporated into the resin, followed by an additional 45 min of sonication to promote uniform dispersion. Afterward, 0.8 g of hardener was added, and the mixture was stirred until it acquired a viscous consistency. To remove entrapped air and improve phase homogeneity, the mixture was degassed under vacuum. When the mixture approached gelation, it was poured into a mold measuring 22.86 × 10.16 × 5 mm for X-band electromagnetic measurements. The mold was then placed in an oven at 70 °C and left to cure overnight, ensuring complete polymerization and strong bonding in the final nanocomposite (Scheme 1).

Scheme 1
figure 1

Schematic representation of epoxy/Ni0.5Zn0.5Fe2O4@MWCNT nanocomposite preparation. Created in https://BioRender.com.

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Characterization

FTIR spectroscopy (Bruker, Germany) was utilized to characterize chemical structure of the nanocomposite. Additionally, the XRD pattern was obtained using a Philips instrument (PW1730, Netherlands) with Cu Kα radiation (wavelength = 1.53059 Å) to assess phase composition. The microstructure of Ni0.5Zn0.5Fe2O4@MWCNTs and epoxy/Ni0.5Zn0.5Fe2O4@MWCNTs nanocomposites was examined using TEM (Zeiss LEO 906, Germany) and SEM (HITACHI S–4160, Japan) analysis. To study the mechanical property of nanocomposites, uniaxial tensile tests were conducted via a universal testing machine (Z020, Zwick/Roell, Germany). For this, the dumbbell-shaped pieces were prepared based on the ASTM D638. The samples were placed between the grips of the machine and pulled apart in the longitudinal direction at a constant crosshead speed of 1.0 mm/min at room temperature. The tensile strength was subsequently calculated based on the maximum stress detected from the stress–extension curves. Thermal stability of the optimum sample was assessed with a thermogravimetric analyzer (TGA, Q600, TA instrument, USA) under air atmosphere in the range of 25–700 °C at a rate of 10 °C/min. To evaluate their microwave absorption properties within the 8–12 GHz frequency range, a vector network analyzer (VNA, HP8510C, USA) was employed. The magnetic characteristics of the nanocomposites were studied using a VSM (Meghnatis Daghigh Kavir Company, Iran). The zeta potential of the samples was measured using a zeta potentiometer (Malvern, ZEN3600, UK).

Results and discussion

In this study, the electromagnetic wave absorption capabilities of CNTs were enhanced by coating them with magnetic nanoparticles. The incorporation of magnetic metals with CNTs creates a synergistic effect that significantly improves their ability to absorb electromagnetic radiation. However, a common issue is that Ni metal easily oxidizes to nickel oxide when exposed to ambient oxygen (O), which results in the deterioration of its intrinsic magnetic and electrical properties. To address this, Ni must be protected from oxidation to maintain its functionality. One of the most effective strategies is doping Ni into ferrite spinel structures, as this approach prevents oxidation and helps preserve the desirable magnetic and electrical characteristics of Ni.

The nanocomposites were fabricated by embedding MWCNTs coated with Ni0.5Zn0.5Fe2O4 magnetic nanoparticles into an epoxy resin matrix. To synthesize the magnetic nanoparticles, solutions of nickel nitrate, zinc nitrate, and iron nitrate were added to an aqueous suspension containing MWCNTs and the surfactant SLS, which facilitated uniform dispersion of the nanotubes. After the synthesis, the Ni0.5Zn0.5Fe2O4@MWCNTs were mixed with epoxy resin in a beaker, subjected to ultrasonication, and then combined with a hardening agent. The resulting mixture was poured into molds to produce the final composite samples. A schematic illustration of nanocomposite is presented in Scheme 2.

Scheme 2
figure 2

Schematic representation of MWCNT decoration and epoxy/Ni0.5Zn0.5Fe2O4@MWCNT nanocomposite.

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FTIR spectrum of Ni0.5Zn0.5Fe2O4@MWCNTs nanoparticle

The FTIR spectrum of the Ni0.5Zn0.5Fe2O4@MWCNTs nanohybrid is presented in Fig. 1. The spectrum shows a broad stretching vibration band around 3434 cm[‒ [1, corresponding to hydroxyl groups typically associated with the MWCNTs. Several characteristic peaks are also present at 1637, 1384, 1628, and 1083 cm[‒ [1, which are attributed to the vibrational modes of different functional groups: the carbonyl (C = O) stretch, the carboxylate groups, the carbon–carbon double bonds (C = C), and the carbon–carbon single bonds (C–C), respectively30. Additionally, in spinel ferrite structures, vibrational absorption bands are generally observed near 575 cm[‒ [1 and 469 cm[‒ [1, corresponding to the tetrahedral and octahedral sites, respectively31.

Fig. 1
figure 1

FTIR spectrum of Ni0.5Zn0.5Fe2O4@MWCNTs nanohybrid.

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EDS analysis of Ni0.5Zn0.5Fe2O4@MWCNTs nanoparticle

Figure 2 displays the EDS spectrum of the Ni0.5Zn0.5Fe2O4@MWCNTs nanohybrid. As observed, the primary elements detected are Ni, Zn, Fe, O, and C. Their approximate atomic concentrations are 2.08% (6 wt%) for Ni, 2.08% (6.5 wt%) for Zn, 10.61% (29.9 wt%) for Fe, 39.6% (29.9 wt%) for O, and 43.75% (25.7 wt%) for C. Small amounts of sodium (1.7%, 1.9 wt%) are also present, likely resulting from SLS from the synthesis process. Notably, the atomic ratio of Ni to Zn is close to 1:1, indicating successful formation of Ni0.5Zn0.5Fe2O4 nanoparticles on the surface of the MWCNTs. Overall, elemental analysis suggests a significant presence of Ni, Zn, and Fe on the nanotube surface, implying a substantial coating layer of Ni0.5Zn0.5Fe2O4 nanoparticles.

Fig. 2
figure 2

EDS spectrum of Ni0.5Zn0.5Fe2O4@MWCNTs.

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X–Ray diffraction of Ni0.5Zn0.5Fe2O4@MWCNTs nanoparticle

Figure 3 displays the XRD pattern for Ni0.5Zn0.5Fe2O4@MWCNTs nanohybrid. The identified hkl planes, including (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (5 3 1), and (5 3 3), verified the formation of Ni0.5Zn0.5Fe2O4 nanocrystals on MWCNTs, consistent with JCPDS reference (Card No. 8-234). The Debye–Scherrer formula (Eq. 1) was employed to evaluate the average crystallite size of nanoparticles anchored to MWCNTs32:

$$\:\text{d}=\frac{\text{K}{\uplambda\:}}{\sqrt{{{\upbeta\:}}_{\text{m}}^{2}-{{\upbeta\:}}_{\text{s}}^{2}\:\text{c}\text{o}\text{s}{\uptheta\:}}}$$
(1)

where K denotes the Debye–Scherrer constant (0.89 for spherical particles), λ represents the incident X-ray wavelength (in this context, the Cu Kα line), β stands for the full width at half maximum (FWHM) that includes instrumental broadening βS, and θ is the diffraction angle corresponding to the (3 1 1) reflection, identified as the most intense peak for nanoparticles. The average size of the crystallites was estimated to be approximately 15 nm, which is very close to the values reported in the literature32.

Fig. 3
figure 3

XRD patterns of Ni0.5Zn0.5Fe2O4@MWCNTs nanohybrid.

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SEM, TEM and zeta potential study

Figure 4 presents the SEM images of the Ni0.5Zn0.5Fe2O4@MWCNTs nanohybrid. The Ni0.5Zn0.5Fe2O4 nanoparticles, exhibiting a spherical shape, were successfully distributed on the surface of the MWCNTs. Due to the agglomeration of Ni0.5Zn0.5Fe2O4 nanoparticles, TEM examination was used to further analyze the as-synthesized nanohybrid morphology. TEM was employed to examine the arrangement of Ni0.5Zn0.5Fe2O4 nanocrystals on MWCNTs, as shown in Fig. 5. The TEM images reveal that the Ni0.5Zn0.5Fe2O4 nanocrystals, with an average size of 15 nm, completely coat the external surface of the MWCNTs (Fig. 5b). These results indicate that this method offers a promising approach for functionalizing MWCNTs with Ni0.5Zn0.5Fe2O4 nanocrystals. During the surfactant coating stage, SLS enveloped the MWCNTs like a wrapper, covering their entire surface. This process significantly improves the dispersion of MWCNTs in aqueous environments. SLS, being an anionic surfactant with a negative charge, adsorbs onto the surface of MWCNTs through hydrogen bonding and π-π stacking interactions, forming structures akin to micelles. The zeta potential measurements for pristine MWCNTs, SLS, and MWCNTs dispersed in SLS (SLS@MWCNTs) confirmed this phenomenon (Table 1). The zeta potential of MWCNTs, SLS, and SLS@MWCNTs are ‒9, ‒35, and ‒4 mV, respectively. The presence of SLS on the MWCNTs surface reduces the negative surface charge, shifting the zeta potential towards positive values. In other words, the negatively charged (anionic) portion of SLS interacts with the hydrophilic regions of the MWCNTs, leading to an overall reduction in negative surface charge. Meanwhile, Ni, Zn, and Fe ions can readily adsorb onto the negatively charged surface of SLS@MWCNTs. This facilitates uniform nucleation of the magnetic nanoparticles on the MWCNT surface, resulting in a consistent coating of magnetic nanoparticles (Fig. 5c). The zeta potential measurements and TEM images collectively validate this mechanism.

Fig. 4
figure 4

SEM micrographs of Ni0.5Zn0.5Fe2O4@MWCNTs at different magnifications (5 KX, 10 KX, and 21 KX for a, b, and c, respectively).

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Fig. 5
figure 5

TEM images of a) MWCNTs, b) Ni0.5Zn0.5Fe2O4@MWCNTs nanohybrids, c) schematic pattern of the assembly of nanoparticles.

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Table 1 Zeta potential of MWCNTs, SLS, and SLS@MWCNTs.
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SEM evaluation of epoxy/Ni0.5Zn0.5Fe2O4@MWCNTs nanocomposite

To examine the morphology of the epoxy/Ni0.5Zn0.5Fe2O4@MWCNTs (10 wt%) nanocomposite, SEM analysis was conducted. The as-synthesized nanocomposite features a continuous epoxy matrix with a dispersed phase of Ni0.5Zn0.5Fe2O4@MWCNTs nanoparticles. Based on the SEM micrographs in Fig. 6a, the nanocomposites exhibit a uniform, continuous morphology with particles evenly distributed within the matrix. Additionally, no cracks or defects are evident, suggesting the as-prepared nanocomposite absorbent is suitable for microwave absorption applications. To further confirm the homogeneous and uniform distribution and dispersion of nanoparticles inside the matrix, EDX-mapping of Ni for epoxy/Ni0.5Zn0.5Fe2O4@MWCNTs (10 wt%) sample was also carried out as shown in Fig. 6b. The results clearly suggested that Ni0.5Zn0.5Fe2O4@MWCNTs nanoparticles were well evenly dispersed inside the whole matrix without any significant aggregation.

Fig. 6
figure 6

a) SEM image of epoxy/Ni0.5Zn0.5Fe2O4@MWCNTs (10 wt%). b) EDX-mapping of Ni for 10 wt% Ni0.5Zn0.5Fe2O4@MWCNTs.

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Magnetic properties of nanocomposite

VSM was utilized to generate hysteresis loops for Ni0.5Zn0.5Fe2O4@MWCNTs at ambient temperature. As displayed in Fig. 7, the magnetization-coercivity curve of Ni0.5Zn0.5Fe2O4@MWCNTs indicates a saturation magnetization of approximately 37 emu/g, accompanied by minimal remanence and coercivity. These results demonstrate that Ni0.5Zn0.5Fe2O4@MWCNTs display ferromagnetic behavior characteristic of nanocrystalline materials.

Fig. 7
figure 7

The magnetization graph of Ni0.5Zn0.5Fe2O4@MWCNTs.

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Mechanical properties of nanocomposite

Microwave absorbents often operate under elevated temperatures due to dielectric heating and ohmic losses. Composites must maintain stiffness and strength to prevent deformations that would alter the absorber geometry and, hence, its impedance matching and absorption performance33. Figure 8 show the illustrative stress–extension curves of samples at different nanoparticle loadings. From the results, increasing the nanoparticle content up to 10 wt% leads to higher tensile strength and greater elongation, after which both properties decline. Therefore, the nanocomposite with 10 wt% of Ni0.5Zn0.5Fe2O4@MWCNTs selected as an optimum sample. At low to moderate loadings (up to 10 wt%), the load is transferred from the matrix to the well-dispersed nanoparticles via interfacial bonding, increasing the overall stiffness (Young’s modulus) and tensile strength. As nanoparticle content increases beyond ~ 10 wt%, the particles tend to agglomerate due to van der Waals forces and insufficient surfactant/mixing. Agglomerates create stress concentrators that initiate microcracks, lowering both strength and elongation34.

Fig. 8
figure 8

Mechanical behavior of samples.

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Thermal study of nanocomposite

Thermal stability is a critical property for materials employed as microwave absorbers35. Therefore, the thermal behavior of the optimum sample along with the 0 wt% composite was investigated, as shown in Fig. 9. The initial thermal degradation occurring around 350–430 °C was ascribed to the breakdown of the polymer network, while the subsequent degradation between 500 and 700 °C was attributed to the oxidation of the char moiety36. The neat epoxy showed essentially no char yield (about 0 wt%), whereas the 10 wt% composite exhibited char yields that correlate well with the Ni0.5Zn0.5Fe2O4@MWCNTs content (about 9 wt%).

Fig. 9
figure 9

TGA thermograms of epoxy and eopxy/Ni0.5Zn0.5Fe2O4@MWCNTs (10 wt%).

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Microwave absorption properties

The RL for Ni0.5Zn0.5Fe2O4@MWCNTs can be calculated using the formula shown in Eq. 2, which depends on several factors such as material thickness, relative permittivity, relative permeability, frequency, and complex relative parameters:

$$\:\text{R}\text{L}=-20\text{l}\text{o}\text{g}\left|\frac{{Z}_{in}-1}{{Z}_{in}+1}\right|$$
(2)

The normalized impedance (Zin) for Ni0.5Zn0.5Fe2O4@MWCNTs is defined by the expression in Eq. 3:

$$\:{Z}_{in}=\sqrt{\frac{{\mu\:}_{r}}{{\epsilon\:}_{r}}}\text{t}\text{a}\text{n}\text{h}\left(j\frac{2\pi\:}{c}\sqrt{{\mu\:}_{r}{\epsilon\:}_{r}}fd\right)$$
(3)

where d represents the sample thickness, εr denotes the complex relative permittivity, c is the speed of light, f indicates the electromagnetic wave frequency, and µr signifies the complex relative permeability. As evident from the equation, the RL of epoxy/Ni0.5Zn0.5Fe2O4@MWCNTs composites is influenced by both magnetic and electrical properties, leading to variations and potential increases in RL values.

In this study, the sample thickness and frequency range were kept constant, while the relative permittivity and permeability were adjusted by increasing the nanoparticle content in the composites. Figure 10 illustrates the RL performance of epoxy/Ni0.5Zn0.5Fe2O4@MWCNTs composites with different nanoparticle concentrations over the X-band frequency range. The results show that the maximum RL values increase with nanoparticle content up to 10 wt%, after which there is a slight decrease at 30 wt%. Specifically, 10 wt% sample gained the maximum absorption loss with RL − 12.64 dB with a bandwidth of 0.315 GHz at a frequency of 10.87 GHz on the thickness of 5 mm which manifests optimum ability of microwave absorption. At 30 wt%, aggregation of nanoparticles within the composite results in lower RL values. Moreover, increasing the Ni0.5Zn0.5Fe2O4@MWCNTs content causes the EMI shielding effectiveness to shift toward lower frequencies. The total EMI shielding effectiveness is given by Eq. 437:

$$\:SE={SE}_{R}+{SE}_{A}$$
(4)

where SER and SEA are due to reflection and absorption, respectively. The reflection part of SE is given by Eq. 537:

$$\:{SE}_{R}=\left[\frac{{\left(\frac{{\sigma\:}_{ac}}{{\omega\:}_{H}{\epsilon\:}_{0}{\mu\:}_{r}}\right)}^{\frac{1}{2}}}{4}\right]$$
(5)

and the absorption part of SE is given by Eq. 637:

$$\:{SE}_{A}=20\text{log}\left[{exp}\left(\frac{t}{\delta\:}\right)\right]$$
(6)

where t is the thickness of the shield, µr is the magnetic permeability, δ is the skin depth, \(\:\delta\:=[{\frac{2}{{\omega\:}_{H}\mu\:{\sigma\:}_{ac}}]}^{\frac{1}{2}}\) defined as the depth to which the radiation penetrates into the material, while its intensity decreases to e‒ 1 of its original strength and ωH is angular frequency (\(\:{\omega\:}_{H}=2\pi\:{f}_{H}\)) and ε0 is the permittivity of the free space. From the above equations, it is clear that the SE depends mainly on the conductivity of the material37. Additionally, a comparison of the microwave absorption performance of the modified spinel-based structures is shown in Table 2. These results highlight the potential of Ni–Zn ferrite fillers in epoxy matrices for high-frequency electromagnetic applications.

Fig. 10
figure 10

RL of the epoxy/Ni0.5Zn0.5Fe2O4@MWCNTs composite in the different amount of nanoparticles.

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Table 2 Microwave absorbing performance of recently reported Ni0.5Zn0.5Fe2O4@MWCNTs-based nanocomposite.
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Conclusion

This study developed epoxy-based nanocomposites incorporating Ni- and Zn-doped Fe3O4 coated onto MWCNTs for microwave absorption applications. XRD confirmed the formation of Ni0.5Zn0.5Fe2O4 nanocrystals on the MWCNT surfaces. TEM images demonstrated that Ni0.5Zn0.5Fe2O4 nanoparticles, with an average diameter of 15 nm, uniformly coat the MWCNT surfaces. The epoxy/Ni0.5Zn0.5Fe2O4@MWCNTs sample with 10 wt% of nanoparticle showed higher mechanical property compared to the other samples and exhibited high thermal stability. RL measurements indicated that the maximum RL values for epoxy/Ni0.5Zn0.5Fe2O4@MWCNTs composites increase with nanoparticle content up to 10 wt%, beyond which RL values decline. These findings suggest that the synthesized epoxy/Ni0.5Zn0.5Fe2O4@MWCNTs nanocomposites are promising candidates for next-generation lightweight microwave-absorbing nanomaterials.