In-situ deposition and comparative study of electromagnetic absorption performance of MXene (Ti₃C₂T_x)@nano-Fe₁Co_0.8Ni₁ composites with different compositions

1.   Introduction

With the rapid development of modern technology, electronic devices have brought increasing convenience to people’s lives and become an indispensable part of modern life. However, electromagnetic radiation pollution has also become increasingly severe, causing electromagnetic interference between electronic devices and posing a significant threat to human health [1–3]. Therefore, high-performance electromagnetic wave absorption materials have garnered widespread attention to effectively mitigate the impact of electromagnetic radiation and interference on human life.

According to microwave absorption theory, electromagnetic waves generally undergo reflection, absorption, and transmission processes on an object’s surface [4]. The key to design of absorption materials is to enhance the absorption of electromagnetic waves and lessen the reflection on the material’s surface [5–6]. Absorbing materials can be classified into two major categories according to different electromagnetic loss mechanisms: electric loss type and magnetic loss type [7]. Electric loss absorbing materials have a highly complex dielectric constant, mainly relying on the mechanisms of ohmic loss and dielectric polarization relaxation loss to attenuate electromagnetic wave energy. Magnetic loss absorbing materials have a high complex magnetic permeability, mainly relying on mechanisms such as magnetic hysteresis loss, ferromagnetic resonance, and eddy current loss to absorb electromagnetic waves [8–12]. However, a single electromagnetic wave absorption material has difficulty in simultaneously considering both loss mechanisms. Therefore, combining dielectric loss materials with magnetic loss materials has emerged as an important direction for the development of electromagnetic wave absorption composites.

MXene, 2D material, comprises transition metal carbides or nitrides, with a 2D layered structure represented by the structural formula M_n+1X_nT_x, where M is a transition metal such as Ti, V, and Mo; X represents C or N (n = 1, 2, 3); T_x represents surface terminal groups such as –OH, =O, or –F [13–14]. Owing to its unique layered physical and chemical structure, MXene has exhibited remarkable electromagnetic wave absorption performance. Qing et al. [15] synthesized Ti₃C₂ nanosheets with a typical MXene structure and achieved a minimum reflection loss (RL_min) of −17 dB at 14.6 GHz with a sample thickness of 1.4 mm. Li et al. [16] prepared a 2D Ti₂CT_x MXene with commendable electromagnetic interference (EMI) shielding performance in the X-band. Jin et al. [17] tested the absorption performance of HF-etched Nb₂CT_x MXene and attained RL_min of −52.2 dB with a sample thickness of 2.90 mm. Zhang et al. [18] obtained RLmin of −34.3 dB using a paraffin and Ti₃C₂T_x mixture at a thickness of 1.7 mm.

These MXene materials have demonstrated excellent electromagnetic wave absorption performance and potential for engineering applications. However, most MXene materials only have a dielectric loss due to their lack of magnetism [13]. They commonly encounter challenges such as single loss mechanism, excessive coating thickness, and narrow adjustable absorption frequency range, which significantly impede their effective engineering applications as high-performance absorption materials.

Fe, Co, and Ni are typical magnetic metal materials with high magnetic permeability, high saturation, high temperature resistance, corrosion resistance, and low magnetic loss [19]. As magnetic absorption materials, pure Fe, Co, Ni, and their alloys have been widely utilized in various technological fields, such as electromagnetic pollution control, electromagnetic information leakage protection, and electromagnetic radiation protection [20–22]. The effective absorption of electromagnetic waves depends on the coordination of dielectric and magnetic losses. As magnetic materials, the absorption mechanism of Fe, Co, Ni, and their alloys mainly relies on magnetic loss. However, the excessive aggregation of magnetic metal particles and the resulting skin effects often lead to impedance mismatch [23], further limiting the effective enhancement of absorption performance.

Although MXene, Fe, Co, and Ni all exhibit significant electromagnetic wave absorption capabilities, their primary electromagnetic loss mechanisms are complementary. Hence, extensive research has been conducted on MXene/Fe, Co, and Ni composites. Zhou et al. [24] developed sandwich-like FeCo@Ti₃C₂T_x composites using in-situ reduction method, achieving RL_min of −36.29 dB at a frequency of 8.56 GHz and a sample thickness of 2.0 mm. Li et al. [25] synthesized MXene–FeCo films with various morphologies through electrostatic self-assembly and vacuum-assisted filtration. The films were composed of alternating magnetic FeCo alloy and multilayered MXene nanosheets, which reduced the inherent high electrical conductivity of MXene while significantly enhancing the magnetic loss. The samples with a thickness of approximately 3.5 mm achieved RL_min of −43.70 dB at 3.76 GHz. Zhang et al. [26] prepared Fe_0.64Ni_0.36/MXene/CNF composites by embedding FeNi nanoparticles into MXene nanosheets and attained RL_min of –54.1 dB at 2.7 mm thickness. Wu et al. [27] achieved the effective absorption bandwidth (EAB) of 4.3 GHz for the prepared Ti₃C₂T_x@Fe₂O₃ at 2.25 mm thickness.

Numerous studies have demonstrated the excellent electromagnetic wave absorption performance of nanomaterials [28–30]. However, research on the absorption performance of MXene/nanomagnetic composites, particularly MXene/nano-FeCoNi at different composition ratios, remains relatively limited. Understanding the impact of the preparation process and structure components on the electromagnetic wave absorption performance is of great theoretical significance and practical value for advancing the development of MXene/magnetic metal composites as absorption materials.

In light of these considerations, chemical liquid-phase deposition was employed in this study to investigate the preparation and electromagnetic absorption performance of MXene/nano-FeCoNi composites. Variations in the microstructure, magnetic properties, and absorption characteristics of MXene@nano-Fe₁Co_0.8Ni₁ composites with different amounts of MXene were explored, and the influence of process parameters and material structure on their absorption performance and electromagnetic wave loss mechanism was investigated. We hope that this research will furnish valuable insights into the optimization of the performance of MXene/FeCoNi composites as absorption materials.

2.   Experimental

2.1   Materials

In-situ liquid-phase reduction was employed to prepare MXene@Fe₁Co_0.8Ni₁ composites with varying MXene (Ti₃C₂T_x) contents. All experimental materials used were of AR purity. The chemical reagents and raw materials utilized in the experiment are detailed in Table 1.

Table  1.  Chemical reagents and raw materials used in the experiment

Reagents	Chemical formula
Ferrous sulfate heptahydrate	FeSO4·7H2O
Nickel sulfate hexahydrate	NiSO4·6H2O
Cobalt sulfate heptahydrate	CoSO4·7H2O
Trisodium citrate dihydrate	Na3(C6H5O7)·2H2O
Sodium potassium tartrate tetrahydrate	C4H4O6KNa·4H2O
Sodium hydroxide	NaOH
Hydrazine hydrate	N2H4·H2O
MXene	Ti3C2Tx

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2.2   Preparation of MXene@Fe₁Co_0.8Ni₁ composites

0.5 mol/L solution of Fe²⁺, Ni²⁺, and Co²⁺ ions was prepared with 100 mL of deionized water, followed by the addition of 0.4 mol/L trisodium citrate and sodium potassium tartrate. The ion solution was incorporated with 1, 3, and 6 mL of 15 mg/mL MXene dispersion, corresponding to 15, 45, and 90 mg of MXene and labeled as 100% MXene, 300% MXene, and 600% MXene, respectively. The mixture was thoroughly stirred and then subjected to ultrasonication at 60°C. A mixed solution of sodium hydroxide and hydrazine hydrate was added to the reaction mixture under ultrasonication and stirring conditions, and the reaction proceeded for 30 min. The samples were then washed with deionized water and freeze-dried to obtain the corresponding composites.

2.3   Characterization

The microstructure of the composites was analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM). The crystal structure of the composites was characterized by X-ray diffractometer (XRD). The static magnetic properties of the prepared composites were tested with vibrating sample magnetometer (VSM). For the study of electromagnetic absorption performance, the sample was mixed with paraffin in the mass ratio of 1:1, heated in a water bath, and processed into a ring sample through the mold. The electromagnetic absorption performance in the frequency range of 2–18 GHz was evaluated with vector network analyzer by controlling the incident electromagnetic wave frequency.

3.   Results and discussion

3.1   Microstructure characteristics of MXene@nano-Fe₁Co_0.8Ni₁ composites

The different MXene@nano-Fe₁Co_0.8Ni₁ samples with varying MXene (Ti₃C₂T_x) contents were prepared by in-situ liquid-phase deposition. The MXene solution concentration was 15 mg/mL, and the addition amounts were 15, 45, and 90 mg. The molar ratio of alloy elements in the nano-Fe₁Co_0.8Ni₁ medium-entropy magnetic particles was 1:0.8:1. Fig. 1 shows the SEM and TEM images of MXene, Fe₁Co_0.8Ni₁, and MXene@nano-Fe₁Co_0.8Ni₁ composites. Fig. 1(a) illustrates the microstructural morphology of the multilayered MXene material. MXene (Ti₃C₂T_x) exhibits a multilayered structure with single layer thickness of approximately 40 nm and interlayer spacing of 400–900 nm, displaying typical 2D structural features. Fig. 1(b) shows the SEM image of spherical Fe₁Co_0.8Ni₁ alloy particles, indicating an average particle size of approximately 100 nm. Fig. 1(c)–(e) is the TEM images of 100% MXene@Fe₁Co_0.8Ni₁ (15 mg), 300% MXene@Fe₁Co_0.8Ni₁ (45 mg), and 600% MXene@Fe₁Co_0.8Ni₁ (90 mg) composites, respectively. As can be seen, the nano-Fe₁Co_0.8Ni₁ alloy particles are uniformly distributed on the MXene surface, exhibiting good dispersion without noticeable aggregation. This phenomenon is attributed to the in-situ nucleation and growth of the Fe, Co, and Ni ions around MXene during the reduction, effectively increasing the nucleation rate of the alloy particles.

Fig. 1.  (a, b) SEM image of MXene Fe₁Co_0.8Ni₁ and (c–e) TEM image of 100% MXene@nano-Fe₁Co_0.8Ni₁, 300% MXene@nano-Fe₁Co_0.8Ni₁, and 600% MXene@nano-Fe₁Co_0.8Ni₁, respectively.

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Compared with the Fe₁Co_0.8Ni₁ alloy particles without MXene addition, the average particle size of Fe₁Co_0.8Ni₁ particles in the composites is approximately 150–200 nm, which is slightly larger than that of pure Fe₁Co_0.8Ni₁ alloy particles. This finding is due to the in-situ deposition of Fe₁Co_0.8Ni₁ alloy particles around MXene in the composites. Under the same conditions, additional Fe, Co, and Ni ions are reduced, which enhances the nucleation rate and the growth of the alloy particles by combining large numbers of fine grains.

Fig. 2 presents the XRD patterns of different MXene@nano-Fe₁Co_0.8Ni₁ composites. The diffraction peaks at 44.2°, 51.5°, and 75.8° correspond to the characteristic peaks of the face-centered cubic Fe, Ni, and Co and the (111), (200), and (220) crystal planes, respectively. The diffraction peak at 10° is assigned to the characteristic diffraction peak of the MXene (200) crystal plane. Corresponding characteristic peaks are observed in all three sets of samples, and no other hybrid peaks are found. This finding indicates the high purity of the prepared samples and suggests that the high-purity MXene@nano-Fe₁Co_0.8Ni₁ composites can be effectively prepared via chemical liquid-phase in-situ deposition.

Fig. 2.  XRD patterns of different MXene@nano-Fe₁Co_0.8Ni₁ composites

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3.2   Static magnetic properties of MXene@Fe₁Co_0.8Ni₁ composites

The static magnetic properties of the composites were initially analyzed with a VSM to investigate the impact of MXene incorporation on the electromagnetic wave absorption performance. Fig. 3 shows the hysteresis loops of MXene@nano-Fe₁Co_0.8Ni₁ composites with varying MXene additions. The prepared MXene@nano-Fe₁Co_0.8Ni₁ composites exhibit soft magnetic behavior. Compared with that of the Fe₁Co_0.8Ni₁ alloy particles, the specific saturation magnetic intensity of the MXene@Fe₁Co_0.8Ni₁ composite displays a trend of decreasing first and then increasing with the rise in MXene content. In particular, the specific saturation magnetic intensity decreases from 127.3 emu/g for Fe₁Co_0.8Ni₁ to 90.2 emu/g and subsequently increases to 101.1 emu/g. The coercivity (inset of Fig. 3) also demonstrates a declining phase followed by an increase, starting from 120 Oe, decreasing to 80.4 Oe, and then increasing to 200.5 Oe. This trend can be attributed to the nonmagnetic property of MXene, resulting in a reduction in the specific saturation magnetic intensity of the composites due to the dilution effect caused by the nonmagnetic MXene at high contents. With further increase in MXene content, MXene acts as an in-situ deposited nucleation core and provides increased nucleation opportunities for the alloy ions in the solution, leading to an enhanced in-situ reduction nucleation rate of Fe, Co, and Ni ions. As a consequence, the reaction becomes comprehensive, and the specific saturation magnetic intensity of the MXene@Fe₁Co_0.8Ni₁ composites subsequently increases. Table 2 lists the specific saturation magnetic intensity, remanence, and coercivity of Fe₁Co_0.8Ni₁ alloy particles and different MXene@nano-Fe₁Co_0.8Ni₁ composites.

Fig. 3.  Hysteresis loops of Fe₁Co_0.8Ni₁ and different MXene@Fe₁Co_0.8Ni₁ composites.

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Table  2.  Specific saturation magnetization, remanence, and coercivity of Fe₁Co_0.8Ni₁ and different MXene@nano-Fe₁Co_0.8Ni₁ composites

Samples	Specific saturation magnetic intensity, Ms / (emu·g−1)	Remanence, Mr / (emu·g−1)	Coercivity, Hc / Oe
Fe1Co0.8Ni1	127.3	7.26	120.3
100% MXene@Fe1Co0.8Ni1	104.1	3.78	80.4
300% MXene@Fe1Co0.8Ni1	90.2	6.66	180.4
600% MXene@Fe1Co0.8Ni1	101.1	8.81	200.5

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3.3   Electromagnetic wave absorption performance

3.3.1   Dielectric constant and permeability

Electromagnetic wave dissipation involves dielectric loss and magnetic loss, which can be represented by the dielectric constant (ε_r) and permeability (μ_r), respectively, as shown in Eqs. (1) and (2). The properties consist of real parts (ε′, μ′) and imaginary parts (ε″, μ″). The real part indicates the material’s ability to store electromagnetic wave energy, and the imaginary part represents its ability to dissipate electromagnetic wave energy.

Fig. 4 shows the complex dielectric constant and permeability of Fe₁Co_0.8Ni₁ alloy particles and different MXene@nano-Fe₁Co_0.8Ni₁ composites. As can be seen from Fig. 4(a), MXene addition enhances the dielectric energy storage capacity of the composites, and the real part of the complex dielectric constant increases with the MXene content. In the frequency ranges of 2–6 and 15–18 GHz, the real part of the dielectric constant of 100% MXene@nano-Fe₁Co_0.8Ni₁ and 300% MXene@nano-Fe₁Co_0.8Ni₁ composites is approximately 70% higher than that of Fe₁Co_0.8Ni₁ alloy particles. The 600% MXene@nano-Fe₁Co_0.8Ni₁ composite exhibits significant decrease in the real part of the dielectric constant in the 7.5–8.5 GHz frequency while remaining relatively stable in other frequency ranges. In addition, the dielectric storage performance of the 100% MXene@nano-Fe₁Co_0.8Ni₁ composite exhibits higher sensitivity to electromagnetic wave frequency than those of the other composites, which is attributed to the tendency of Fe₁Co_0.8Ni₁ magnetic alloy particles to agglomerate on the MXene surface, forming a conductive structure when a relatively small amount of MXene is added.

Fig. 4.  Complex dielectric constant and permeability of Fe₁Co_0.8Ni₁ and different MXene@nano-Fe₁Co_0.8Ni₁ composites: (a) ε′; (b) ε″; (c) μ′; (d) μ″.

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Fig. 4(b) shows the imaginary part of the dielectric constant of the composite is higher than that of Fe₁Co_0.8Ni₁ magnetic alloy particles, with distinct absorption peaks observed in the frequency range of 3–13 GHz for all the composite. The 100% MXene@nano-Fe₁Co_0.8Ni₁ composite shows a significantly higher imaginary dielectric constant than the other two materials and reaches its peak at about 10 GHz, indicating its improved dielectric loss capability due to MXene addition. However, the dielectric loss performance is nonlinearly related to the MXene addition amount and shows sensitivity to the frequency of electromagnetic waves. Therefore, specific electromagnetic wave frequencies are essential in the design of absorption materials.

Fig. 4(c) shows the real part of the permeability of the composite is lower than that of Fe₁Co_0.8Ni₁ magnetic alloy particles at low frequency but becomes comparable at high frequency. It also exhibits a continuous decrease with the increasing electromagnetic wave frequency. It is noting that MXene addition leads to increased fluctuations and steep increases in specific frequency ranges. For instance, the 100% MXene@nano-Fe₁Co_0.8Ni₁ composite demonstrates the most pronounced increase in the real part of permeability in the frequency range of 10–14.5 GHz, significantly surpassing the other materials.

In terms of the imaginary part of permeability, the composites with MXene addition show lower values than the Fe₁Co_0.8Ni₁ magnetic alloy particles, as shown in Fig. 4(d). Given that MXene is a nonmagnetic material, its addition dilutes the electromagnetic performance. Nevertheless, the imaginary part of the permeability of the 100% MXene@nano-Fe₁Co_0.8Ni₁ composite exhibits significant fluctuations in the frequency range of 8–14 GHz, indicating heightened sensitivity to electromagnetic waves relative to that of the Fe₁Co_0.8Ni₁ magnetic particles. The added MXene alters the interaction of the composite with electromagnetic waves at specific frequencies.

3.3.2   Dielectric loss and magnetic loss

Dielectric loss (tanδ_ε) and magnetic loss (tanδ_μ) are commonly used to characterize the dielectric and magnetic loss capabilities and can be quantified using Eqs. (3) and (4):

As shown in Fig. 5(a), 100% MXene@Fe₁Co_0.8Ni₁ composites displays distinct dielectric loss peak in the frequency range of 8–14 GHz. With MXene addition amount increases, two dielectric loss peaks are observed in the 5–7 and 8–14 GHz frequency range of 300% MXene@Fe₁Co_0.8Ni₁ composites. A dielectric loss peak emerges in the 7–12 GHz frequency range at 600% MXene@Fe₁Co_0.8Ni₁ composites. The dielectric loss increases first and then decreases with the rise in MXene content. In addition, the dielectric loss of the composites is greater than that of the Fe₁Co_0.8Ni₁ magnetic particles without MXene primarily due to the high conductivity of MXene that helps enhance the material’s dielectric loss capabilities [25].

Fig. 5.  (a) Dielectric loss and (b) magnetic loss of Fe₁Co_0.8Ni₁ and different MXene@Fe₁Co_0.8Ni₁ composites.

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Fig. 5(b) shows that the magnetic loss of the composites decreases first and then increases with the addition of MXene. The magnetic loss of the MXene composites is consistently lower than that of the Fe₁Co_0.8Ni₁ alloy. The 100% MXene@Fe₁Co_0.8Ni₁ composites exhibits magnetic loss peak in the 12–18 GHz frequency range. Meanwhile, a peak is observed at 10–18 GHz for 300% MXene@Fe₁Co_0.8Ni₁ composites and at the 4–8 and 8–18 GHz ranges for 600% MXene@Fe₁Co_0.8Ni₁ composites. Given that MXene is nonmagnetic and incapable of contributing to magnetic loss, the composites can only demonstrate a certain level of magnetic loss performance through the introduction of ferromagnetic Fe₁Co_0.8Ni₁ particles. This performance is evidently lower than that of the Fe₁Co_0.8Ni₁ magnetic particles alone [25].

The fluctuation in the dielectric loss and magnetic loss of the 100% MXene@Fe₁Co_0.8Ni₁ composites is significantly greater than that of the other materials, indicating that the MXene addition amount does not exhibit a simple linear relationship with the dielectric loss and magnetic loss. Proper control of the addition amount is essential to achieve optimal absorption performance.

3.3.3   Loss mechanism

Dielectric loss primarily arises from electrical conduction and polarization. Electrical conduction loss occurs when free electrons move within the material under the influence of an electromagnetic field, resulting in inductive currents that dissipate the energy of electromagnetic waves and convert it into thermal energy. This loss is positively correlated with the electrical conductivity (σ), as shown in Eq. (5). Meanwhile, polarization loss occurs through the conversion of electromagnetic energy during polarization relaxation, which can be attributed to the interface polarization at hetero-interfaces and the dipole polarization induced by surface charge, polar molecules, and functional groups. The Debye law expresses the relationship between the real and imaginary parts of the dielectric constant as shown in Eq. (6), and the resulting relationship curve is known as the Cole–Cole curve [31]. The semicircle and upward tail on the curve represent the polarization relaxation and electrical conduction loss processes, respectively. Here, ε_s represents the static dielectric constant, ε_∞ is the dielectric constant at infinite frequency, ε₀ denotes the dielectric constant in vacuum, σ represents the electrical conductivity, and f is frequency.

Fig. 6 illustrates the Cole–Cole semicircles of Fe₁Co_0.8Ni₁ alloy and different MXene@Fe₁Co_0.8Ni₁ composites. Each semicircle represents Debye relaxation process. All the Fe₁Co_0.8Ni₁ magnetic particles and their respective composites with varying MXene addition amounts exhibit Debye semicircles, indicating polarization relaxation losses as the dominant dielectric loss mechanism within the materials under the influence of the electromagnetic field. However, significant electrical conduction loss is not observed in all the semicircles, suggesting that the dielectric loss of the materials is primarily governed by the polarization relaxation.

Fig. 6.  Cole–Cole semicircles of Fe₁Co_0.8Ni₁ alloy and different MXene@Fe₁Co_0.8Ni₁ composites: (a) Fe₁Co_0.8Ni₁ alloy; (b) 100% MXene@Fe₁Co_0.8Ni₁ composite; (c) 300% MXene@Fe₁Co_0.8Ni₁ composite; (d) 600% MXene@Fe₁Co_0.8Ni₁ composite.

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The magnetic loss of the absorptive material mainly includes hysteresis loss, eddy current loss, natural resonance, and exchange resonance [10]. Hysteresis loss arises from the displacement and rotation of domain walls during magnetization and can be neglected in weak magnetic fields [12]. Natural resonanee occurs at low frequencies, exchange resonance occurs at high frequencies, and eddy current loss (C₀) can be determined using Eq. (7) [32]. When the value does not change with frequency, the magnetic loss is considered to be caused by the eddy current loss.

Fig. 7 shows the relationship between C₀ and frequency of Fe₁Co_0.8Ni₁ alloy and different MXene@Fe₁Co_0.8Ni₁ composites. The C₀ of Fe₁Co_0.8Ni₁ alloy shows decreasing overall trend within the frequency, indicating that their magnetic loss mechanism is primarily governed by exchange resonance. Within the 2–6 GHz frequency, all the composites show a similar C₀ trend without significant eddy current loss characteristics. Furthermore, the C₀ of the 100% MXene@Fe₁Co_0.8Ni₁ composite fluctuates significantly, with a substantial decrease in the 6–10 GHz range and a large increase in the 11–14 GHz range. This finding indicates that the eddy current loss is not the predominant magnetic loss mechanism within the frequency ranges for the 100% MXene@Fe₁Co_0.8Ni₁ composite. In the 10–14 GHz frequency range, the C₀ of the 300% MXene@Fe₁Co_0.8Ni₁ composite tends to stabilize, indicating the presence of a certain eddy current loss within this frequency. The C₀ of the 600% MXene@Fe₁Co_0.8Ni₁ composite shows continuous fluctuations with no distinctive eddy current loss characteristics.

Fig. 7.  Relationship between C₀ and frequency of Fe₁Co_0.8Ni₁ alloy and different MXene@Fe₁Co_0.8Ni₁ composites.

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3.3.4   Attenuation constant and impedance matching

The attenuation constant and impedance matching are two critical parameters that can characterize the absorption properties of microwave-absorbing materials. The attenuation constant (α) quantifies the dissipation capacity of the electromagnetic wave as expressed in Eq. (8), where c is velocity of light. Impedance matching (Z) determines the number of microwaves entering the absorber as represented in Eq. (9), where Z_in and Z₀ denote the impedance of the absorbing material and the free space impedance, respectively.

The effective absorption of electromagnetic waves relies on a well-coordinated interaction between dielectric and magnetic losses, achieving impedance matching and dynamic equilibrium in electromagnetic wave absorption. The impedance matching value close to or equal to 1 demonstrates superior impedance matching and electromagnetic wave absorption properties [33].

Fig. 8(a) illustrates the attenuation constant displays overall increasing trend within the range of 2–18 GHz. Compared with the Fe₁Co_0.8Ni₁ alloy, the composites exhibit higher attenuation constant in the 9–16 GHz frequency range upon MXene addition. When 100% MXene is added, the composite illustrates a significant attenuation peak in the 12–15 GHz frequency range, maintaining consistency with the original Fe₁Co_0.8Ni₁ alloy in the 16–18 GHz frequency range. Moreover, MXene additions of 300% and 600% induce similar trends in the attenuation constant, with a notable attenuation peak found in the 10–14 GHz frequency range and a stable attenuation constant maintained in the 14–18 GHz frequency range. These findings suggest that MXene addition enhances the dissipation capacity of Fe₁Co_0.8Ni₁ magnetic particles for electromagnetic waves, particularly in relatively high-frequency ranges.

Fig. 8.  (a) Attenuation constant and (b) impedance matching of Fe₁Co_0.8Ni₁ alloy and different MXene@Fe₁Co_0.8Ni₁ composites.

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Fig. 8(b) shows the impedance matching of Fe₁Co_0.8Ni₁ alloy and MXene@Fe₁Co_0.8Ni₁ composites with different amounts of added MXene. Impedance matching is mainly used to represent the ability of incident microwaves to penetrate and enter the absorbing material. When Z is equal to 1, the internal impedance of the material is the same as the spatial free impedance; that is, no reflection occurs when the electromagnetic wave enters the absorbing material. In this study, the impedance matching of the absorbing material is close to 1, which is highly conducive to improving the absorbing performance. Fig. 8(b) shows the overall impedance matching decreases with the increase in MXene addition, signifying the enhancement of the ability to reflect electromagnetic waves. In particular, 100% MXene addition demonstrates a sharp increase in impedance matching in the 10–15 GHz frequency range, suggesting that the appropriate addition of MXene may enhance the impedance matching of absorbing materials at specific frequencies.

3.3.5   Absorption performance

RL and EAB (RL < −10 dB) are important indicators for evaluating the performance of absorbing materials (Eq. (10)) [31,34]. RL values less than −10, −20, and −30 dB represent electromagnetic wave loss rates of 90%, 99%, and 99.9%, respectively.

Fig. 9 shows the variation of RL with frequency for different samples with different coating thicknesses. Within the thickness range of 1–5 mm, the main RL interval of samples tends to shift toward the lower frequency range with increasing MXene content. This finding indicates that MXene addition effectively enhances the material’s ability to absorb low-frequency electromagnetic waves. Moreover, the EAB tends to decrease gradually with the increase in MXene content. When the MXene content is 600%, a promising electromagnetic wave absorption performance is observed at 1.0 mm coating thickness, indicating that MXene addition helps optimize the thickness of the absorbing coating. For the same material, the absorption peak tends to shift toward lower frequencies with the increasing sample thickness. This phenomenon is attributed to the quarter wavelength rule followed by electromagnetic wave-absorbing materials, as shown in Eq. (11) [35]:

Fig. 9.  Relationship between RL and frequency of (a) Fe₁Co_0.8Ni₁ alloy, (b) 100% MXene@Fe₁Co_0.8Ni₁ composite, (c) 300% MXene@Fe₁Co_0.8Ni₁ composite, and (d) 600% MXene@Fe₁Co_0.8Ni₁ composite.

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where t_m and f represent the sample thickness and electromagnetic wave frequency, respectively; |ε_r| and |μ_r| represent the modulus of ε_r and μ_r at f, respectively; n denotes the odd positive integers.

Fig. 10 is the 3D graph of the RL of the Fe₁Co_0.8Ni₁ alloy and different MXene@Fe₁Co_0.8Ni₁ composites. The black contour lines in the projection plane represent electromagnetic wave absorption reaching −10 dB. With the increase in MXene content, the material’s RL_min shows decreasing trend from −32.5 to −46.9 dB. Moreover, the EAB decreases from 5.36 (Fe₁Co_0.8Ni₁ alloy) to 3.60 GHz (600% MXene@Fe₁Co_0.8Ni₁ composite).

Fig. 10.  3D graph of the reflection loss of (a) Fe₁Co_0.8Ni₁ alloy, (b) 100% MXene@Fe₁Co_0.8Ni₁ composite, (c) 300% MXene@Fe₁Co_0.8Ni₁ composite, and (d) 600% MXene@Fe₁Co_0.8Ni₁ composite.

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Table 3 summarizes the RL of the Fe₁Co_0.8Ni₁ alloy and different MXene@Fe₁Co_0.8Ni₁ composites. A comparison reveals that the coating thickness of MXene@Fe₁Co_0.8Ni₁ composites for effective electromagnetic wave absorption gradually decreases with the addition of MXene, from 1.5 to 1 mm, representing a maximum decrease of 50%. This phenomenon is attributed to the unique layered structure of MXene, causing multiple reflections and scattering of electromagnetic waves during the incident process, and the presence of numerous local defects and functional groups in MXene, further contributing to electromagnetic wave attenuation. The MXene@Fe₁Co_0.8Ni₁ composites also contain a large number of interfaces between MXene and magnetic metals, enhancing the materials’ polarization effect. With the increase in MXene content, the electromagnetic wave absorption performance of the composites at a thin coating thickness is effectively enhanced. Compared with the electromagnetic wave absorbing properties of MXene-based magnetic MAM composites in previous studies, the MXene (Ti₃C₂T_x)@nano-Fe₁Co_0.8Ni₁ composites prepared by liquid-phase reduction in the present work achieved RL_min of −46.9 dB at the thickness of 1.1 mm. This finding provides a valuable reference for the design and preparation of thin, light, and good absorbent materials.

Table  3.  RL of Fe₁Co_0.8Ni₁ alloy and different MXene@ Fe₁Co_0.8Ni₁ composites

Samples	RLmin (thickness) / dB	EABmax (thickness) / GHz
Fe1Co0.8Ni1	−32.5 (1.5 mm)	5.36 (1.5 mm)
100% MXene@Fe1Co0.8Ni1	−34.1 (1.2 mm)	4.56 (1.1 mm)
300% MXene@Fe1Co0.8Ni1	−34.6 (1.2 mm)	3.86 (1.2 mm)
600% MXene@Fe1Co0.8Ni1	−46.9 (1.1 mm)	3.60 (1.0 mm)

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4.   Conclusions

In this study, MXene@nano-Fe₁Co_0.8Ni₁ composites were fabricated by in-situ liquid-phase deposition. The impact of MXene addition on the microstructure, static magnetic, and electromagnetic wave absorption properties of the MXene@Fe₁Co_0.8Ni₁ composites was investigated and discussed. The following conclusion can be drawn:

(1) The thickness of MXene is approximately 40 nm, with inter-layer spacing of 400–900 nm. Spherical Fe₁Co_0.8Ni₁ particles are uniformly distributed on the surface of the multilayered MXene. The alloy particles have an average particle size of approximately 100 nm, exhibiting excellent dispersion without noticeable particle aggregation. The composite primarily consists of face-centered cubic metal crystal particles and MXene.

(2) As the MXene content increases, the specific saturation magnetic intensity and coercivity of the composite material initially decrease from 127.3 to 90.2 and then increase to 101.1 emu/g. Meanwhile, the coercivity decreases from 120 to 80.4 before increasing to 200.5 Oe, indicating the soft magnetism.

(3) Compared with Fe₁Co_0.8Ni₁ magnetic alloy, the 100% MXene@Fe₁Co_0.8Ni₁ composite show significant fluctuations in the dielectric constant and magnetic permeability MXene addition increases the material’s dielectric loss but decreases its magnetic loss. Furthermore, the dielectric and magnetic loss performance of the composite does not exhibit a linear relationship with the amount of MXene added.

(4) The Fe₁Co_0.8Ni₁ magnetic alloy and MXene@Fe₁Co_0.8Ni₁ composites exhibit polarization relaxation losses. Eddy current losses are not the primary mechanism of the magnetic loss. With MXene addition, the material’s attenuation constant increases, and the impedance matching decreases.

(5) With increasing MXene addition, the RL_min of the composites increases, and the EAB_max decreases. When the MXene content is 90 mg, the composite exhibits RL_min of −46.9 dB at a thickness of 1.1 mm and EAB_max of 3.60 GHz at a thickness of 1.0 mm.