1.   Introduction

With the continuous development of communication engineering and electronic technology in the civil and military fields [1–4], electromagnetic (EM) wave pollution caused by electronic devices has become a problem that interferes with equipment operation [5–8]. Metal sulfides (MoS₂, Cu_xS_y, V_xS_y, and WS₂) exhibit excellent electronic activity [9–10]. Cu₉S₅ is a semiconductor material with a natural size effect and diverse morphologies [11–12]. Magnetic loss can be introduced into Cu₉S₅ materials to achieve component synergy by increasing loss pathways. ZnFe₂O₄ possesses paramagnetism and high magnetic loss capability [13–14]. The poor dielectric properties of ZnFe₂O₄ are complementary to those of Cu₉S₅. The application of magnetized composites faces great challenges, which center on the modulation of magnetic properties and the construction strategy of structures.

Research on the use of Cu₉S₅ in microwave absorption is lacking. Tao et al. [15] utilized a hydrothermal method to prepare a Cu₉S₅ nanoweb that presented a high reflection loss (RL) of −55.03 dB. Using a flower-like Cu₉S₅/rGO composite synthesized through a one-step solvothermal method, Liao et al. [16] obtained an RL_min of −51.9 dB and an effective absorption bandwidth (EAB) of 3.92 GHz. ZnFe₂O₄ is more widely used than Cu₉S₅. Zhou et al. [17] prepared a 3D flower-like composite of ZnFe₂O₄/rGO through chemical precipitation and heat treatment. This composite achieved an EAB of 5.56 GHz. Studies have focused on composites made from high-dielectric materials combined with magnetic materials to introduce magneto–electric coupling losses. However, the introduction of magnetoelectric coupling losses while retaining sulfur vacancies and various defective modes is superior to other strategies. This approach would have great importance for improving wave absorption [18–19].

Herein, for the construction of magnetized 3D functional materials for applications in EM wave absorption, the morphology of Cu₉S₅ was controlled by using a simple solvothermal method, and ZnFe₂O₄ was introduced to endow magnetic loss capability. The final magnetized 3D flower-like Cu₉S₅/ZnFe₂O₄ composites were thus prepared. ZnFe₂O₄ was introduced in the form of small polyhedral particles on the surface of flower-like Cu₉S₅ to enhance its dissipative ability. The radar-scattering cross-sectional and EM absorption properties of the Cu₉S₅/ZnFe₂O₄ composites were analyzed.

2.   Experimental

2.1   Materials

Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 99%), iron chloride hexahydrate (FeCl₃·6H₂O, 99%), copper chloride dihydrate (CuCl₂·2H₂O, 99%), thiourea (H₂NCSNH₂, 99%), ethanol (CH₃CH₂OH, 99%), urea (CO(NH₂)₂, 99%), and ammonium fluoride (NH₄F) were purchased from Macklin Biochemical Technology Co. (Shanghai, China). All chemicals were of analytical grade and purity.

2.2   Preparation of flower-like Cu₉S₅

Flower-like Cu₉S₅ was prepared by using a simple hydrothermal method. First, 2.5 mmol CuCl₂·2H₂O and 1.25 mmol H₂NCSNH₂ were placed in 60 mL of deionized water and sonicated for 1 h. The solutions were transferred to a Teflon-lined stainless steel autoclave (100.0 mL) and kept in an oven at 180°C for 6 h. A black product was collected through centrifugation, washed several times with ethanol, and placed in a vacuum oven at 50°C overnight.

2.3   Preparation of magnetized flower-like Cu₉S₅/ZnFe₂O₄ composites

The prepared flower-like Cu₉S₅ (0.5 mmol) was fully dissolved in deionized water (25 mL) and sonicated for 0.5 h. Subsequently, 1.5 mmol Zn(NO₃)₂·6H₂O and 3 mmol iron chloride hydrate (FeCl₃·6H₂O) were dissolved in 50 mL of deionized water with magnetic stirring until fully dissolved. The mixture was added with 7.5 mmol CO(NH₂)₂ and 3 mmol NH₄F, stirred continuously at room temperature for 15 min, and mixed with Cu₉S₅ suspension to prepare the precursor solution. The precursor solution was transferred to a 100 mL stainless steel reactor with a polytetrafluoroethylene (PTFE) liner. The reactor was covered and placed in an oven at 150°C for 10 h. The reaction solution was transferred to a centrifuge tube and washed three times with deionized water and anhydrous ethanol at 8000 r/min. The resulting precipitate was dried under vacuum at 50°C for 36 h to obtain the product in powder form. The molar ratios of Zn²⁺ and Cu₉S₅ added in the control experiments were 3:1, 2:1, and 1:1. Three sets of samples were obtained and named CZ-1, CZ-2, and CZ-3. Fig. 1 shows the specific process.

Figure  1.  Schematic of the synthesis of Cu₉S₅/ZnFe₂O₄ composites.

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2.4   Characterization

The crystal information of the composites was obtained through X-ray diffractometer (XRD, Bruker, D8-Advance). The morphology of the composites was analyzed via scanning electron microscope (SEM, Zeiss Sigma300 [Gemini lens]) and transmission electron microscope (TEM, JEOL JEM-F200). The magnetic properties of the samples were characterized by vibrating sample magnetometer (VSM, LakeShore 7404). The surface chemical information of the composites was acquired via X-ray photoelectron spectroscopy (XPS, Thermo Fisher). EM parameters were obtained with a vector network analyzer (VNA AV3629D) by using the coaxial method (see the supplementary information).

3.   Results and discussion

3.1   Characterizations of the Cu₉S₅/ZnFe₂O₄ composites

3.1.1   Reaction mechanism of the Cu₉S₅/ZnFe₂O₄ composites

The chemical equations of the reaction are as follows [15]:

During preparation, NH₄F can first form masked particles with Fe³⁺ ([FeF₆]³⁻) [17]. At this time, Zn²⁺ reacts with OH⁻ produced through the decomposition of urea to form ${\text{Zn(OH)}}_{\text{4}}^{\text{2}\text{–}}$, which promotes the reaction on the negative surface of crystals and inhibits the growth rate of positive surfaces. These changes weaken the selectively oriented growth of crystals. Favorable conditions are provided for the formation of polyhedral ZnFe₂O₄. Under alkaline conditions, the coprecipitation of ${\text{Zn(OH)}}_{\text{4}}^{\text{2}\text{–}}$ with Fe³⁺ leads to the formation of ZnFe₂O₄ ferrite. Driven by the tendency of the surface free energy to decrease, Cu²⁺ is first reduced into Cu⁺ due to the slow decomposition of thiourea. Thiourea continues to decompose to yield H₂S and binds preferentially to Cu⁺ to form CuS nuclei. Subsequently, the nucleus continues to grow until it is stable. It crystallizes and gradually grows into flower-like Cu₉S₅ through electrostatic self-assembly. Cu, Zn, and Fe are all periodic metal elements, among which Fe is the most active. The electronegativity values of Cu, Zn, and Fe are 1.9, 1.65, and 1.83, respectively. The difference in the abilities of the three metal atoms to attract electrons in molecules can lead to metal atom substitution during material compounding. Together with the electrostatic effect, ZnFe₂O₄ can be tightly bonded with Cu₉S₅.

3.1.2   Crystalline phase and chemical bond composition analysis

XRD patterns provide information on crystal structure homogeneity. Fig. 2(a) shows the XRD patterns of the composites. In reference to the Cu₉S₅ standard comparison card (JCPDS card No. 47-1748) [15], the characteristic peaks of Cu₉S₅ at 27.8°, 29.2°, 32.2°, 46.2°, 49.4°, and 54.7° corresponded to the (0 0 15), (1 0 7), (1 0 10), (1 1 0), and (1 1 15) crystal planes, respectively. The characteristic peaks at 29.9°, 35.3°, 42.8°, 56.6°, and 62.2° could be assigned to the (2 2 0), (3 1 1), (4 0 0), (5 1 1), and (4 4 0) crystal planes, respectively, in accordance with the standard No. 22-1012 for ZnFe₂O₄ [20] (Fig. 2(b)). The diffraction peaks of Cu₉S₅ in ZC-3 were the most pronounced in terms of the intensity of the diffraction peaks of the magnetic spheres. Likewise, the diffraction peak of ZnFe₂O₄ in CZ-1 was intense, even at 53.1°, where the (4 2 2) crystal plane was observed, because the ZnFe₂O₄ content of CZ-1 was higher than that of other samples. In the two other samples, this diffraction peak was masked due to the gradual increase in Cu₉S₅ content. These results, in combination with the diffraction pattern of the pure material, showed that the synthesized flower-like Cu₉S₅/ZnFe₂O₄ composites were pure in the physical phase.

Figure  2.  XRD patterns of (a) flower-like Cu₉S₅/ZnFe₂O₄ composites and (b) Cu₉S₅ and ZnFe₂O₄.

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XPS tests were performed on CZ-2 to further reveal the elemental composition and phenochemical information of the flower-like Cu₉S₅/ZnFe₂O₄ composites [21–22]. The energy spectra confirmed the presence of Cu, S, Zn, Fe, O, and C elements in the samples, as shown in Fig. 3(a). In the Cu 2p spectrum (Fig. 3(b)), one pair of peaks at 932.3 and 951.9 eV corresponded to Cu 2p_3/2 and Cu 2p_1/2 in Cu⁺ and the other pair at 933.6 and 953.5 eV was attributed to Cu²⁺. Cu²⁺ and Cu⁺ together formed the valence state of copper in Cu₉S₅ [23–24]. The peaks at 161.4 and 162.2 eV were attributed to S 2p_1/2 and S 2p_3/2 in Cu₉S₅, respectively, and the peak at 168.6 eV corresponded to S–O (Fig. 3(c)) [25]. The peaks at 1021.8 and 1044.8 eV in Zn 2p corresponded to Zn 2p_1/2 and Zn 2p_3/2, respectively (Fig. 3(d)). This result indicated that Zn²⁺ was present. Similarly, the peaks at 711.2 and 713.6 eV were attributed to Fe 2p_3/2 and those at 725.4 and 719.5 eV were assigned to Fe 2p_1/2, as shown in Fig. 3(e) [26–27]. They originated from Fe³⁺ in ZnFe₂O₄ and Fe²⁺ during the hydrothermal process or through contact with oxygen on the surfaces of the flower-like structures. The O 1s spectrum (Fig. 3(f)) was divided into three peaks corresponding to metallic oxygen (Fe–O/Zn–O), defective oxygen (O–C=O), and adsorbed oxygen (O–C) [28–29]. By using the peak area of the XPS spectra, the atomic ratio of Cu/S was calculated to be 1.86, which was consistent with the actual value [24]. This finding, in combination with the analysis of XRD results, revealed that the composites were synthesized. The presence of three metal elements and the large difference in the electronegativity of each metal element could effectively contribute to the generation of polarization centers, which help enhance dipole and interfacial polarizations [30].

Figure  3.  XPS spectra of (a) CZ-2 and (b–f) Cu 2p, S 2p, Zn 2p, Fe 2p, and O 1s.

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3.1.3   Morphology and magnetic properties

The morphology of the composites has a considerable effect on the capacity for EM wave absorption [31]. Fig. 4 provides the SEM and TEM images of the magnetized flower-like Cu₉S₅/ZnFe₂O₄. Fig. 4(a)–(f) shows that the substrate of the composites comprised stacked Cu₉S₅ sheets with a certain thickness. Its size was approximately 2.5 μm. ZnFe₂O₄ polyhedrons were grown on the surface of the composites through the hydrothermal method to finally form the magnetized Cu₉S₅/ZnFe₂O₄ composites. Incident EM waves scattered at a certain angle on the surfaces of the flower-like Cu₉S₅/ZnFe₂O₄ clusters, resulting in the multiple attenuation of EM waves. Fig. 4(g)–(i) shows the TEM images of the flower-like Cu₉S₅/ZnFe₂O₄ composites. The light-colored muslin-like area in Fig. 4(g) can be attributed to the Cu₉S₅ sheet (the area marked by a yellow ellipse in the figure), whereas the rectangular marked area can be ascribed to ZnFe₂O₄ polyhedrons, which could combine with Cu₉S₅. The lattice spacings in the CZ-2 sample image (Fig. 4(h)) obtained through high-resolution TEM were calculated to be 0.211, 0.321, and 0.339 nm, which were attributed to the (4 0 0) crystal plane of ZnFe₂O₄ and the (0 0 15) and (1 0 1) crystal planes of Cu₉S₅, respectively. Distinct phase interfaces (regions delineated by green lines) and lattice distortion (yellow circles), which could generate stresses at numerous interfaces, are present in Fig. 4(i). The interaction of different components could produce a large number of lattice defects. Energy-dispersive spectroscopy (EDS) was conducted to characterize and obtain the elemental distribution of the five elements Cu, S, Zn, Fe, and O (Fig. 4(j)). Uniform distribution of the elements and good material bonding were achieved [32].

Figure  4.  SEM images of (a, d) CZ-1, (b, e) CZ-2, and (c, f) CZ-3; TEM images of (g–i) CZ-2 and (j) corresponding EDS analysis.

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High-precision VSM was performed to obtain the magnetization degree of the materials and related parameters and to further understand the magnetic properties of the magnetized materials [33].

where H and M represent the strength and magnetization of the applied magnetic field, respectively; μ′ and μ″ represent the real and imaginary parts of the complex permeability, respectively; σ represents the lag angle of the magnetization phase after the external magnetic field [17]. In general, the magnetization strength of a material is related to permeability [34]. Hysteresis loss can also be introduced to increase the loss paths of EM waves. Fig. 5 shows the magnetic information of the magnetized flower-like spheres. Pure polyhedral ZnFe₂O₄ had a saturation magnetization strength (M_s) of 92.4 emu/g, which proved that it could be used as a good magnetization medium. The characterization results demonstrated that Cu₉S₅ exhibited typical ferromagnetic hysteresis properties due to the presence of certain orbital magnetic moments and spin magnetic moments inside ZnFe₂O₄ molecules after ZnFe₂O₄ was composited [35]. In addition, the squareness ratio (M_r (residual magnetization intensity)/M_s) of all samples was less than 0.5, indicating that the composites presented a multidomain structure that facilitated their irreversible domain wall displacement to generate magnetic losses. The flower-like Cu₉S₅/ZnFe₂O₄ composites could be easily magnetized due to the introduction of ZnFe₂O₄, which effectively improved their magnetic properties. The introduced magnetic losses also helped dissipate EM waves, resulting in high absorption.

Figure  5.  (a–b) Magnetization hysteresis loops of ZnFe₂O₄ and the Cu₉S₅/ZnFe₂O₄ composites at room temperature and (c) schematic of magnetization.

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3.2   Properties of the Cu₉S₅ and Cu₉S₅/ZnFe₂O₄ composites

3.2.1   EM and microwave absorption properties

The properties of EM wave absorption need to be analyzed on the basis of the real and imaginary parts of the dielectric constant and magnetic permeability (ε′, ε″, μ′, and μ″) [36–37]. Fig. 6(a)–(c) shows that the variation in the EM parameters of the composite materials in the whole frequency band conformed to the dispersion effect. At a paraffin loading ratio of 55wt%, the composites showed a gradual increase in ε′ from 8.1 to 18.9 and in ε″ from 1.7 to 5.3 with the increase in Cu₉S₅ content. This behavior was in accordance with the effective medium theory. Comparing the Cu₉S₅ monomaterial with the best absorption performance and the magnetized flower-like Cu₉S₅/ZnFe₂O₄ composites, it revealed that consistent with the change in the paraffin ratio and the high-dielectric constant of Cu₉S₅ itself, the EM parameters of both materials increased. The magnetized Cu₉S₅/ZnFe₂O₄ composites exhibited μ′ values greater than 1, as shown in Fig. 6(d), indicating that they all presented sufficient magnetic energy storage capacity for EM waves. Among the composites, CZ-2 performed well in terms of μ″ and demonstrated the best magnetic loss capability as proven by the combined tanδ_μ analysis (Fig. 6(f)) [38]. When materials with a high-dielectric constant and strong ability to jump electronically are placed in an EM wave environment, their surfaces become susceptible to microcurrents that generate a micromagnetic field, which interferes with the change in magnetic flux inside the materials; this phenomenon, in turn, affects the change in the dielectric response characteristics of the materials and finally leads to the fluctuation of the dielectric peak [39]. The Cu₉S₅/ZnFe₂O₄ composites performed as expected in terms of magnetic permeability (Fig. 6(d)–(e)). Specifically, fluctuations were caused by the introduction of magnetic materials that can effectively change and modulate the magnetic and EM wave absorption properties of the composites under the action of an external EM field [40–41].

Figure  6.  Dielectric constant and magnetic permeability of Cu₉S₅ and Cu₉S₅/ZnFe₂O₄ composites: (a) ε′; (b) ε′′; (c) μ′; (d) μ′′; (e) tanδ_ε; (f) tanδ_μ.

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RL and EAB are the most intuitive indicators for evaluating the performance of EM wave–absorbing materials [42]. Fig. 7 shows the 2D/3D images of the RL of the Cu₉S₅ and magnetized flower-like Cu₉S₅/ZnFe₂O₄ composites. As can be seen, CZ-2 exhibited the best performance among all the tested composites. Fig. 8(a) illustrates that CZ-2 showed RL of −54.38 dB at 1.78 mm and high EAB of 5.92 GHz at 1.6 mm. CZ-1 exhibited EAB of 5.92 GHz at 2.1 mm, whereas CZ-3 had poor performance. ZnFe₂O₄ loading considerably improved the performance of the composites relative to that of the single Cu₉S₅ material. The excellent absorbing material (MA) properties of CZ-2 were due to the synergistic effect of the magnetized Cu₉S₅/ZnFe₂O₄ composite, which presented good matching impedance and attenuation capability at the optimal composite ratio [43–44]. The growth of polyhedral ZnFe₂O₄ on the surface of Cu₉S₅ not only introduced hysteresis losses to increase the loss paths in the composite but also enabled the formation of a large number of heterojunction interfaces at the junction of the Cu₉S₅ surface and ZnFe₂O₄ phases [45–46].

Figure  7.  2D/3D reflection loss curves of Cu₉S₅ and Cu₉S₅/ZnFe₂O₄ composites.

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Figure  8.  (a) EM wave absorption performance of CZ-2 and (b) performance comparison chart.

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Cu₉S₅ has a high EAB value. However, its RL value does not meet performance requirements. The magnetized Cu₉S₅/ZnFe₂O₄ composites exhibited excellent EM wave absorption because the introduction of magnetic materials conferred them with magnetic properties and modulated their EM parameters. Compared with similar materials, Cu₉S₅/ZnFe₂O₄ composites have excellent microwave properties [16–17,24–25,32]. Fig. 8(a)–(b) shows that the RL and EAB values of the composites prepared in this work were better than those of most previously reported ZnFe₂O₄-related composites (RL_min = −54.38 dB and EAB = 5.92 GHz).

Debye theory can be introduced to analyze the dielectric loss caused by the repeated turning of electric dipoles. It is expressed as follows [47]:

where ε_s and ε_∞ denote the static dielectric constant and dielectric constant at infinite frequencies, respectively, and are plotted in the form of Cole–Cole semicircles in accordance with the relationship between ε'' and ε'. One semicircle corresponds to one Debye relaxation process [48]. The introduction of ZnFe₂O₄ could increase material loss (Fig. 9). The semicircle diagrams showed that the material demonstrated remarkable hesitation behavior and dielectric loss. In particular, the semicircle of CZ-2 was in a nearly ideal state. The chirality in each sample was mainly due to the uneven charge accumulation distribution at the phase interface of the composite, lattice defects, and dipolar behavior.

Figure  9.  Cole–Cole curves of Cu₉S₅ and Cu₉S₅/ZnFe₂O₄ composites: (a) Cu₉S₅; (b) CZ-1; (c) CZ-2; (d) CZ-3.

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Eddy current losses were introduced to evaluate the contribution to magnetic loss to further analyze the magnetic loss characteristics of the magnetized flower-like Cu₉S₅/ZnFe₂O₄ composites, and expressed as Eq. (10) [49]. When eddy current losses occur in magnetic materials, the eddy current coefficient (C₀) does not vary with frequency (f). Moreover, the eddy current effect dominates magnetic losses. Fig. 10(a) plots the variation in the C₀ of the composite materials with f. CZ-1 and CZ-2 presented considerable eddy current losses at 10–14 GHz, whereas CZ-3 showed an overall rapid downward trend.. These results were due to the high conductivity of Cu₉S₅, which enhanced the overall skin-effect loss. Natural and exchange resonances are the main forms of magnetic resonance present in the composites. The distinct peaks present in CZ-2 at 3–4 GHz could be attributed to natural resonance [27].

Figure  10.  C₀ value and attenuation constant (α) of Cu₉S₅/ZnFe₂O₄ composites.

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where δ is the electrical conductivity, d represents the thickness of the material, and μ₀ is the vacuum permeability.

The attenuation constant can be used as an evaluation index for the overall loss capability of the composites for EM waves [35]. As shown in Fig. 10(b), the loss capability of CZ-3 was strong in the low-frequency band, and that of CZ-2 was the strongest in the high-frequency band, which matched the RL to some extent. The evaluation of the material’s ability to absorb EM waves needs to be combined with that of another important parameter, namely, impedance matching.

The attenuation constant of the materials and impedance matching (|Z_in/Z₀|) were combined to analyze the materials’ performance [50]. The closer the impedance is to 1, the closer the impedance between the absorber and the atmosphere is such that EM waves can enter the absorber. Excellent impedance matching is a prerequisite for ensuring the performance of absorbers. |Z_in/Z₀| must be evaluated on the basis of the modulus of normalized input impedance. As shown in Fig. 11(a), the impedance matching plot for pure Cu₉S₅ was less than 1, whereas that of CZ-2 was close to 1, illustrating the excellent performance of CZ-2.

Figure  11.  Impendence matching and 1/4 wavelength values of (a) Cu₉S₅ and (b) CZ-2.

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where μ_r and ε_r are the complex permittivity and complex permeability, respectively, and c is the speed of light in a vacuum.

The λ/4 model was introduced to explain the interference of EM waves when they are attenuated [51]. Given ${t}_{\rm m}=\dfrac{n\lambda }{4}=\dfrac{nc}{4{f}_{\mathrm{m}}{(\mid {\varepsilon }_{\mathrm{r}}\mid \mid {\mu }_{\mathrm{r}}\mid )}^{1/2}}$, where n represents an integer multiple, which is used to represent the wavelength of a specific multiple of a light or electromagnetic wave in the material, and λ is the wavelength. The interference was generated when the matching thickness (t_m) and absorber peak (f_m) met the appropriate conditions, thereby attenuating EM waves. Fig. 11(b) shows that when the matching thickness was 1.79 mm, CZ-2 satisfied the λ/4 model and the impedance matching condition, resulting in the RL value of −54.38 dB.

3.2.2   Microwave absorption mechanism and simulated radar-scattering cross-sectional (RCS) of composites

The relevant EM wave absorption mechanism of the magnetized flower-like Cu₉S₅/ZnFe₂O₄ composites is shown in Fig. 12. First, based on the special structure of the Cu₉S₅/ZnFe₂O₄ composite, the incident wave could scatter incident EM waves at different angles when in contact with the Cu₉S₅/ZnFe₂O₄ EM wave absorber, thereby achieving multiple attenuation. Second, numerous polyhedral ZnFe₂O₄ grew on the surface region of the flower-like Cu₉S₅ [52]. The surface of the semiconductor Cu₉S₅ sheet could be tightly connected to the polyhedral ZnFe₂O₄, forming numerous heterojunction Cu₉S₅–ZnFe₂O₄ interfaces at the phase interface. Cu₉S₅ could promote electron transport behavior. After the introduction of ZnFe₂O₄, the composites presented excellent magnetic properties [53]. In turn, the magnetic loss properties of the composites were enhanced. When EM waves entered the interior of the material, eddy currents were generated due to the change in magnetic flux and induction strength inside the magnetized flower-like Cu₉S₅/ZnFe₂O₄ composites, which converted EM waves into Joule heat through eddy current loss. The Cu₉S₅/ZnFe₂O₄ composites presented magneto–crystalline anisotropic equivalent fields, which could generate natural resonance. Standing waves formed in the samples, which could effectively absorb EM waves. Lattice distortion in Cu₉S₅ led to local electron deficiencies, resulting in dipole polarization. In addition, dipole polarization in the flower-like Cu₉S₅/ZnFe₂O₄ composites could improve impedance matching and effectively enhance EM wave performance. The magnetoelectric coupling effect present within the composites also facilitated the generation of electrodepolarization behavior that enhanced the loss of incident EM waves by the absorber [54].

Figure  12.  Schematic of the microwave absorption mechanism of Cu₉S₅/ZnFe₂O₄ composites.

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The composite model of perfect electrical conductor (PEC) and Cu₉S₅/ZnFe₂O₄ composites was used for simulation, and the RCS values of the model can be used to evaluate the attenuation ability of the Cu₉S₅/ZnFe₂O₄ composites for EM waves under far-field conditions. The model consisted of an upper layer of composite for the absorber layer (thickness: 1.78 mm) and a lower layer of PEC layer (thickness: 1 mm). Its dimensions were 180 mm × 180 mm (see the supplementary information) [55]. As shown in Fig. 13(a), three groups of models showed different degrees of reduction in RCS values compared with the pure PEC plate. The models presented a remarkable attenuation effect after being coated with the composite material. The specific RCS values at 0°–60° are shown in Fig. 13(b), and the RCS reduction value of CZ-2 at 0° was 20.1 dB·m², which is indicative of the strongest attenuation effect. This value also demonstrated that the Cu₉S₅/ZnFe₂O₄ composite can effectively enhance EM absorption through the synergistic effect of multiple losses, thus reducing the radar-scattering intensity. Fig. 13 (c)–(f) is the 3D result of simulation, which is consistent with the above results, and it can be intuitively seen that CZ-2 has the best radar scattering ability.

Figure  13.  (a) Simulated RCS of the Cu₉S₅/ZnFe₂O₄ composites at different incidence angles, (b) reduction in RCS from composite removal with PEC, and (c–f) computer simulation technology (CST) results of PEC, CZ-1, CZ-2, and CZ-3, respectively.

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4.   Conclusion

Flower-like Cu₉S₅/ZnFe₂O₄ composites with the synergistic effects of multiple loss pathways were prepared through a simple hydrothermal method. The unique 3D flower-like Cu₉S₅/ZnFe₂O₄ composites were constructed by growing ZnFe₂O₄ in the form of small polyhedral particles on the surface of flower-like Cu₉S₅ to enhance the polarization loss mode for EM waves. The introduction of ZnFe₂O₄ can effectively modify the magnetization of Cu₉S₅ and improve the impedance matching characteristics of the composites. It also improved the drawback of the high-dielectric constant of Cu₉S₅. Multi-polarization behavior and lattice defects contributed to EM wave absorption by the composites. CZ-2 presented RL_min of −54.38 dB at 1.79 mm and EAB of 5.92 GHz at 1.6 mm, indicating that the excellent EM absorption can be used to achieve high-performance microwave absorption.