In Situ Grown 1D/2D Structure of Dy3Si2C2 on SiCw for Enhanced Electromagnetic Wave Absorption

To improve electromagnetic wave (EMW) absorption performance, a novel nano-laminated Dy3Si2C2 coating was successfully in situ coated on the surface of SiC whisker (SiCw/Dy3Si2C2) using a molten salt approach. A labyrinthine three-dimensional (3D) net was constructed by the one-dimensional (1D) SiCw coated with the two-dimensional (2D) Dy3Si2C2 layer with a thickness of ~100 nm, which significantly improved the EMW absorption properties of SiCw. Compared to pure SiCw with the minimum reflection loss (RLmin) value of −10.64 dB and the effective absorption bandwidth (EAB) of 1.04 GHz for the sample with a thickness of 4.5 mm, SiCw/Dy3Si2C2 showed a significantly better EMW absorption performance with RLmin of −32.09 dB and wider EAB of 3.76 GHz for thinner samples with a thickness of 1.76 mm. The enhancement of the EMW absorption performance could be ascribed to the improvement of impedance matching, enhanced conductance loss, interfacial polarization as well as multiple scattering. The SiCw/Dy3Si2C2 can be a candidate for EMW absorber applications due to its excellent EMW absorption performance and wide EAB for relatively thin samples, light weight, as well as potential oxidation and corrosion resistance at high temperatures.


Introduction
Electromagnetic wave (EMW) radiation pollution seriously endangers human health, as a consequence of the widespread applications of the high frequency electronic devices [1][2][3][4][5]. In recent years, numerous EMW absorption materials have been developed to solve these problems [6,7], including carbon-based materials [8,9], magnetic metal materials [10][11][12], ferrite and its composites [13][14][15], and polymer matrix composites [16][17][18]. However, the poor oxidation resistance of carbon-based materials and polymer matrix composites at high temperatures has impeded their applications, despite their excellent EMW absorption properties [19]. Magnetic materials also cannot be used at high temperatures due to the demagnetization [20]. Furthermore, a relatively high density of ferrite materials also hinders their applications in some special fields, such as aerospace. Therefore, the development of high performance EMW absorption materials with high absorption capability, broad effective absorption bandwidth (EAB), low density as well as small thickness, and excellent oxidation resistance at high temperatures is a critical challenge in this field to minimize EMW radiation pollution.
SiC has been considered a promising candidate for EMW absorbers because it has excellent dielectric properties, high temperature stability, as well as outstanding oxidation and corrosion resistance [21,22]. Most of the works on SiC-based EMW absorption materials in the molten salt furnace. The holding time was set 5 h. The heating and cooling rate was 5 • C/min. The as-obtained samples were washed and filtered using deionized water several times. The in situ coated SiC w /Dy 3 Si 2 C 2 powder can be obtained after drying 12 h at 60 • C in a vacuum oven.

Characterizations
The phase compositions of the as-obtained SiC w /Dy 3 Si 2 C 2 were detected using an X-ray diffractometer (XRD: D8 Advance, Bruker AXS, Karlsruhe, Germany) using Cu Kα radiation (λ = 1.5406 Å). The operating current and voltage were 40 mA and 40 kV, respectively. The step scan and step time was 0.02 • 2θ and 0.2 s, respectively. The microstructure of the SiC w /Dy 3 Si 2 C 2 powders was observed using a scanning electron microscope (SEM, 8230, Hitachi, Tokyo, Japan). The microstructure and phase compositions of the Dy 3 Si 2 C 2 coating were further investigated using a transmission electron microscope (TEM, Talos F200X, Thermo Fisher Scientific, Waltham, MA, USA) equipped with an energy dispersive spectroscopy (EDS) system. The samples for TEM observations were prepared using the focused ion beam (FIB, Auriga, Carl Zeiss, Jena, Germany) technique. The complex permittivity and complex permeability were measured at a frequency range from 2 to 18 GHz using a Network Analyzer of Agilent N5230A. In order to measure the complex permittivity and complex permeability, SiC w /Dy 3 Si 2 C 2 powder was mixed with 50 wt.% paraffin with a size of an inner and outer diameter of 3 and 7 mm as well as a thickness of 2 mm, respectively. For the sake of comparison, the electromagnetic properties of the pure SiC whiskers were detected using the same method.

Results and Discussion
3.1. Microstructure and Phase Composition of SiC w /Dy 3 Si 2 C 2 Figure 1 presents the XRD patterns of the pure SiC whiskers and the as-obtained SiC w /Dy 3 Si 2 C 2 whiskers. rate was 5 °C/min. The as-obtained samples were washed and filtered u water several times. The in situ coated SiCw/Dy3Si2C2 powder can be obtain 12 h at 60 °C in a vacuum oven.

Characterizations
The phase compositions of the as-obtained SiCw/Dy3Si2C2 were detect ray diffractometer (XRD: D8 Advance, Bruker AXS, Karlsruhe, Germany radiation (λ = 1.5406 Å). The operating current and voltage were 40 mA and tively. The step scan and step time was 0.02° 2θ and 0.2 s, respectively. The of the SiCw/Dy3Si2C2 powders was observed using a scanning electron mic 8230, Hitachi, Tokyo, Japan). The microstructure and phase compositions coating were further investigated using a transmission electron microscop F200X, Thermo Fisher Scientific, Waltham, MA, USA) equipped with an ene spectroscopy (EDS) system. The samples for TEM observations were prep focused ion beam (FIB, Auriga, Carl Zeiss, Jena, Germany) technique. Th mittivity and complex permeability were measured at a frequency range fro using a Network Analyzer of Agilent N5230A. In order to measure the co tivity and complex permeability, SiCw/Dy3Si2C2 powder was mixed with 50 with a size of an inner and outer diameter of 3 and 7 mm as well as a thic respectively. For the sake of comparison, the electromagnetic properties o whiskers were detected using the same method.   The XRD pattern of pure SiC whiskers indicated that they are formed by the 3C-SiC phase (JCPDS No. 75-0254). A small peak at approximately 33.5 • corresponds to the stacking faults, which spontaneously formed during the growing process of SiC whiskers. The XRD pattern of the SiC w /Dy 3 Si 2 C 2 powder revealed that besides SiC w , it also contained characteristic peaks of Dy 3 Si 2 C 2 (JCPDS No. 97-005-1299) along with some impurities of Dy 2 O 3 (JCPDS No. 97-018-5606). This confirmed that the Dy 3 Si 2 C 2 modification of SiC whiskers was successfully obtained. Figure 2 shows the SEM images of both SiC w and SiC w /Dy 3 Si 2 C 2 whiskers. The XRD pattern of pure SiC whiskers indicated that they are formed by the 3C-SiC phase (JCPDS No. 75-0254). A small peak at approximately 33.5° corresponds to the stacking faults, which spontaneously formed during the growing process of SiC whiskers. The XRD pattern of the SiCw/Dy3Si2C2 powder revealed that besides SiCw, it also contained characteristic peaks of Dy3Si2C2 (JCPDS No. 97-005-1299) along with some impurities of Dy2O3 (JCPDS No. 97-018-5606). This confirmed that the Dy3Si2C2 modification of SiC whiskers was successfully obtained. Figure 2 shows the SEM images of both SiCw and SiCw/Dy3Si2C2 whiskers. The diameter of the pure SiC whiskers was ~500 nm. A dense 2D structure Dy3Si2C2 coating with a structure of randomly oriented nano-laminated sheets was in situ coated on the surface of SiC whiskers (Figure 2b,c). The thickness of Dy3Si2C2 coating was around 100 nm, as shown in the SEM image of the fracture surface of the SiCw/Dy3Si2C2 whisker ( Figure 2d). The corresponding elemental distribution of Si and Dy indicated that most of the Dy3Si2C2 was homogenously coated on the surface of SiC whisker (Figure 2e,f).

Microstructure and Phase Composition of SiCw/Dy3Si2C2
To further confirm the microstructure and phase composition of the Dy3Si2C2 coating, semi-quantitative EDS and high-resolution transmission electron microscope (HR-TEM) analysis along with selected-area electron diffraction (SAED) were performed. Figure 3 presents a high-angle annular dark-field (HAADF) image of the as-synthesized SiCw/Dy3Si2C2 and the corresponding Dy, Si, C, and O elemental distributions, respectively. The diameter of the pure SiC whiskers was~500 nm. A dense 2D structure Dy 3 Si 2 C 2 coating with a structure of randomly oriented nano-laminated sheets was in situ coated on the surface of SiC whiskers (Figure 2b,c). The thickness of Dy 3 Si 2 C 2 coating was around 100 nm, as shown in the SEM image of the fracture surface of the SiC w /Dy 3 Si 2 C 2 whisker (Figure 2d). The corresponding elemental distribution of Si and Dy indicated that most of the Dy 3 Si 2 C 2 was homogenously coated on the surface of SiC whisker (Figure 2e,f).
To further confirm the microstructure and phase composition of the Dy 3 Si 2 C 2 coating, semi-quantitative EDS and high-resolution transmission electron microscope (HR-TEM) analysis along with selected-area electron diffraction (SAED) were performed. Figure 3 presents a high-angle annular dark-field (HAADF) image of the as-synthesized SiC w / Dy 3 Si 2 C 2 and the corresponding Dy, Si, C, and O elemental distributions, respectively. Materials 2023, 16, x FOR PEER REVIEW 5 of 6 The semiquantitative EDS analysis results of areas 1-3 are shown in Table 1, suggesting the presence of SiC, Dy3Si2C2, and/or Dy2O3. Table 1. EDS results collected from points 1-3 in Figure 3a. Furthermore, the HR-TEM image of the interface between SiCw and Dy3Si2C2 coating is shown in Figure 3f. The lattice fringe spacing was 0.2878 nm, which can be assigned to the (041) planes of Dy3Si2C2. Therefore, taking into account all the results obtained by XRD, EDS, and HR-TEM analysis, it can be concluded that a dense ~100 nm Dy3Si2C2 coating was successfully fabricated on the surface of SiCw using the molten salt approach.

No. Composition in Atomic % Probable Phases
The formation process of the Dy3Si2C2 coating using the molten salt approach is similar to the formation mechanism of Y3Si2C2 and Pr3Si2C2 powders [45,48]. First, DyH2 decomposed to Dy and released H2 [49]. The Dy element diffused to the surface of SiC The semiquantitative EDS analysis results of areas 1-3 are shown in Table 1, suggesting the presence of SiC, Dy 3 Si 2 C 2 , and/or Dy 2 O 3 . Table 1. EDS results collected from points 1-3 in Figure 3a. Furthermore, the HR-TEM image of the interface between SiC w and Dy 3 Si 2 C 2 coating is shown in Figure 3f. The lattice fringe spacing was 0.2878 nm, which can be assigned to the (041) planes of Dy 3 Si 2 C 2 . Therefore, taking into account all the results obtained by XRD, EDS, and HR-TEM analysis, it can be concluded that a dense~100 nm Dy 3 Si 2 C 2 coating was successfully fabricated on the surface of SiC w using the molten salt approach.

No. Composition in Atomic % Probable Phases
The formation process of the Dy 3 Si 2 C 2 coating using the molten salt approach is similar to the formation mechanism of Y 3 Si 2 C 2 and Pr 3 Si 2 C 2 powders [45,48]. First, DyH 2 decomposed to Dy and released H 2 [49]. The Dy element diffused to the surface of SiC whiskers via the liquid molten salt, and then the Dy 3 Si 2 C 2 coating was formed. The main reactions can be summarized as follow: On the other hand, the potential formation barrier of the Dy 3 Si 2 C 2 could decline because the surface energy of both DyH 2 and SiC w could be remarkably promoted by polarization effect of the molten salt [50,51]. In addition, the diffusion rate of the Dy, Si, and C atoms can be obviously promoted in the liquid molten salt reaction medium. Therefore, the Dy 3 Si 2 C 2 coating can be in situ formed on the surface of SiC whiskers at a relatively low temperature (1000 • C) and adhered well to the surface of SiC w .

Dielectric Properties of SiC w /Dy 3 Si 2 C 2
The EMW absorption property of materials is mainly confirmed by their complex permittivity and permeability. Meanwhile, good impedance matching between absorbing materials and free space can make EMW incident into materials with less reflection. While SiC w and SiC w /Dy 3 Si 2 C 2 are nonmagnetic materials, the real (µ ) and imaginary (µ") parts of the complex permeability is around 1 and 0, respectively (not shown here). Therefore, the EMW absorption capability of SiC w and SiC w /Dy 3 Si 2 C 2 is highly dependent on their complex permittivity. The real (ε ) and imaginary (ε") parts of complex permittivity of the pure SiC w and SiC w /Dy 3 Si 2 C 2 whiskers are shown in Figure 4.
On the other hand, the potential formation barrier of the Dy3Si2C2 could decline because the surface energy of both DyH2 and SiCw could be remarkably promoted by polarization effect of the molten salt [50,51]. In addition, the diffusion rate of the Dy, Si, and C atoms can be obviously promoted in the liquid molten salt reaction medium. Therefore, the Dy3Si2C2 coating can be in situ formed on the surface of SiC whiskers at a relatively low temperature (1000 °C) and adhered well to the surface of SiCw.

Dielectric Properties of SiCw/Dy3Si2C2
The EMW absorption property of materials is mainly confirmed by their complex permittivity and permeability. Meanwhile, good impedance matching between absorbing materials and free space can make EMW incident into materials with less reflection. While SiCw and SiCw/Dy3Si2C2 are nonmagnetic materials, the real (µ′) and imaginary (µ′′) parts of the complex permeability is around 1 and 0, respectively (not shown here). Therefore, the EMW absorption capability of SiCw and SiCw/Dy3Si2C2 is highly dependent on their complex permittivity. The real (ε′) and imaginary (ε′′) parts of complex permittivity of the pure SiCw and SiCw/Dy3Si2C2 whiskers are shown in Figure 4. Most of the real (ε′) and imaginary (ε′′) parts of the complex permittivity of SiCw/Dy3Si2C2 were higher than that of pure SiCw, indicating that the Dy3Si2C2 coating could promote the dielectric properties of SiCw.
According to the Debye theory, ε′ and ε′′ can be calculated by the following equations [52]: where ε0, εs, and ∞ represent free space dielectric constant, the permittivity in static state, and light frequency, respectively. ω and τ are angular frequency and polarization relaxation time, respectively. σ is electric conductivity. εp′′ and εc′′ correspond to the contributions to ε′′ from polarization loss and conductance loss, which are associated with σ. Generally, the real part of the permittivity signifies the storage capability of the dielectric energy, while the imaginary part of the permittivity stands for the loss of dielectric energy [53]. Thus, the improvement of ε′ can be ascribed to the interfacial polarization caused by the improved heterogeneous interfaces in SiCw/Dy3Si2C2 whiskers, which were generated Most of the real (ε ) and imaginary (ε") parts of the complex permittivity of SiC w / Dy 3 Si 2 C 2 were higher than that of pure SiC w , indicating that the Dy 3 Si 2 C 2 coating could promote the dielectric properties of SiC w .
According to the Debye theory, ε and ε" can be calculated by the following equations [52]: where ε 0 , ε s , and ε ∞ represent free space dielectric constant, the permittivity in static state, and light frequency, respectively. ω and τ are angular frequency and polarization relaxation time, respectively. σ is electric conductivity. ε p and ε c correspond to the contributions to ε from polarization loss and conductance loss, which are associated with σ. Generally, the real part of the permittivity signifies the storage capability of the dielectric energy, while the imaginary part of the permittivity stands for the loss of dielectric energy [53]. Thus, the improvement of ε can be ascribed to the interfacial polarization caused by the improved heterogeneous interfaces in SiC w /Dy 3 Si 2 C 2 whiskers, which were generated by the incorporation of nano-laminated (2D) Dy 3 Si 2 C 2 coating on the surface of SiC w . The enhancement of ε was mainly decided by the increasing of the electrical conductivity (σ), where σ can be confirmed by the follow equation [54]: where ε 0 represents the permittivity in a vacuum. The electrical conductivity of SiC w /Dy 3 Si 2 C 2 was higher than that of pure SiC w , as shown in Figure 5a.
where ε0 represents the permittivity in a vacuum. The electrical conductivity of SiCw/Dy3Si2C2 was higher than that of pure SiCw, as shown in Figure 5a. This can be mainly attributed to the metallic conductivity characteristic of the 2D structural Dy3Si2C2 coating, as the coating formed a net structure, increasing the transmission channels of carriers [55]. In addition, both ε′ and ε′′ of SiCw/Dy3Si2C2 showed a fluctuation corresponding to the resonance, while this was not observed for the pure SiCw. The permittivity of the SiCw/Dy3Si2C2 whiskers showed typical nonlinear resonant characteristics, indicating the existence of polarization and relaxation behavior, which implied better dielectric loss performance in the corresponding frequency range. The Cole-Cole semicircle was used to investigate the relaxation polarization process. According to the Debye theory, the relationship between ε′ and ε′′ can be expressed by Equation (6) The Cole-Cole curves of pure SiCw and SiCw/Dy3Si2C2 are shown in Figure 5b. Each Deby relaxation process is manifested by one Cole-Cloe semicircle [55,56]. There was only one Cole-Cole semicircle observed in pure SiCw, indicating one relaxation process, while four semicircles were observed in SiCw/Dy3Si2C2, confirming the improvement of dielectric loss capacity in SiCw/Dy3Si2C2. The improvement of the relaxation process of SiCw/Dy3Si2C2 was mainly caused by the significantly increased interface relaxation, which resulted from the improved number of heterogeneous interfaces in SiCw/Dy3Si2C2.

Electromagnetic Wave Absorption Performance
Reflection loss (RL) and effective absorption bandwidth (EAB, the corresponding frequency range of RL < −10 dB, which presents more than 90% EMW energy absorbed) are usually used to evaluate the EMW absorption performance of materials. According to the transmission line theory, the RL values of SiCw and SiCw/Dy3Si2C2 can be calculated by the following equations [57-59]: This can be mainly attributed to the metallic conductivity characteristic of the 2D structural Dy 3 Si 2 C 2 coating, as the coating formed a net structure, increasing the transmission channels of carriers [55]. In addition, both ε and ε" of SiC w /Dy 3 Si 2 C 2 showed a fluctuation corresponding to the resonance, while this was not observed for the pure SiC w . The permittivity of the SiC w /Dy 3 Si 2 C 2 whiskers showed typical nonlinear resonant characteristics, indicating the existence of polarization and relaxation behavior, which implied better dielectric loss performance in the corresponding frequency range. The Cole-Cole semicircle was used to investigate the relaxation polarization process. According to the Debye theory, the relationship between ε and ε" can be expressed by Equation (6) [56]: The Cole-Cole curves of pure SiC w and SiC w /Dy 3 Si 2 C 2 are shown in Figure 5b. Each Deby relaxation process is manifested by one Cole-Cloe semicircle [55,56]. There was only one Cole-Cole semicircle observed in pure SiC w , indicating one relaxation process, while four semicircles were observed in SiC w /Dy 3 Si 2 C 2 , confirming the improvement of dielectric loss capacity in SiC w /Dy 3 Si 2 C 2 . The improvement of the relaxation process of SiC w /Dy 3 Si 2 C 2 was mainly caused by the significantly increased interface relaxation, which resulted from the improved number of heterogeneous interfaces in SiC w /Dy 3 Si 2 C 2 .

Electromagnetic Wave Absorption Performance
Reflection loss (RL) and effective absorption bandwidth (EAB, the corresponding frequency range of RL < −10 dB, which presents more than 90% EMW energy absorbed) are usually used to evaluate the EMW absorption performance of materials. According to the transmission line theory, the RL values of SiC w and SiC w /Dy 3 Si 2 C 2 can be calculated by the following equations [57][58][59]: where Z 0 and Z in is space free impedance and input impedance, respectively. c, d, and f are speed of light, thickness, and frequency, respectively. µ r = µ − jµ and ε r = ε − jε represent the complex permeability and permittivity of material. Figure 6 shows the 3D and 2D plots of RL values at the frequency range of 2 to 18 GHz at different thicknesses of the SiC w and SiC w /Dy 3 Si 2 C 2 samples.
R PEER REVIEW 8 of 9 0 = � ⁄ (9) where Z0 and Zin is space free impedance and input impedance, respectively. c, d, and f are speed of light, thickness, and frequency, respectively. µr = µ′ − jµ′′ and εr = ε′ − jε′′ represent the complex permeability and permittivity of material. Figure 6 shows the 3D and 2D plots of RL values at the frequency range of 2 to 18 GHz at different thicknesses of the SiCw and SiCw/Dy3Si2C2 samples.  The minimum RL (RL min ) value of the pure SiC w is −10.64 dB at the frequency of 5.52 GHz with the 4.5 mm sample thickness. After coating of SiC w with 2D Dy 3 Si 2 C 2 sheets, the RL min value was improved to −32.09 dB at the frequency of 14.48 GHz for the 1.54 mm sample thickness. For convenience of comparison, the selected theoretical calculated RL of pure SiC w and SiC w /Dy 3 Si 2 C 2 with different thicknesses in the frequency range of 2 to 18 GHz is shown in Figure 7a,b.
The minimum RL (RLmin) value of the pure SiCw is −10.64 dB at the frequency of 5.52 GHz with the 4.5 mm sample thickness. After coating of SiCw with 2D Dy3Si2C2 sheets, the RLmin value was improved to −32.09 dB at the frequency of 14.48 GHz for the 1.54 mm sample thickness. For convenience of comparison, the selected theoretical calculated RL of pure SiCw and SiCw/Dy3Si2C2 with different thicknesses in the frequency range of 2 to 18 GHz is shown in Figure 7a,b. It is obvious that the EAB of SiC w /Dy 3 Si 2 C 2 is much wider than that of SiC w for the samples with the thickness range of 1 to 4.5 mm at the frequency ranging from 2 to 18 GHz. The widest EAB can be as high as 3.76 GHz for thin SiC w /Dy 3 Si 2 C 2 samples with the thickness of 1.76 mm (Figure 7c). However, the widest EAB of pure SiC w is only 1.04 GHz for the sample with a thickness of 4.5 mm (Figure 7c). This indicates that the Dy 3 Si 2 C 2 coating can significantly improve the EMW absorption properties of SiC w .
In order to reveal the intrinsic reason for the improved EMW absorption performance for SiC w /Dy 3 Si 2 C 2 , the impedance match (Z) as well as the attenuation constant (α) were calculated. Z was confirmed by the following equation [60]: A favorable impedance match is the basic requirement to obtain an excellent EMW absorption performance, which ensures the EMW can enter materials instead of being reflected [61][62][63][64]. According to Equation (10), when the input impedance (Z in ) is infinitely close to the air impedance (Z 0 ), the ideal impedance matching can be obtained. Figure 8a,b presents the calculated Z values of the pure SiC w and SiC w /Dy 3 Si 2 C 2 samples with the thickness of 1-4.5 mm at the frequency ranging from 2 to 18 GHz. It is obvious that the EAB of SiCw/Dy3Si2C2 is much wider than that of SiCw for the samples with the thickness range of 1 to 4.5 mm at the frequency ranging from 2 to 18 GHz. The widest EAB can be as high as 3.76 GHz for thin SiCw/Dy3Si2C2 samples with the thickness of 1.76 mm (Figure 7c). However, the widest EAB of pure SiCw is only 1.04 GHz for the sample with a thickness of 4.5 mm (Figure 7c). This indicates that the Dy3Si2C2 coating can significantly improve the EMW absorption properties of SiCw.
In order to reveal the intrinsic reason for the improved EMW absorption performance for SiCw/Dy3Si2C2, the impedance match (Z) as well as the attenuation constant (α) were calculated. Z was confirmed by the following equation [60]: A favorable impedance match is the basic requirement to obtain an excellent EMW absorption performance, which ensures the EMW can enter materials instead of being reflected [61][62][63][64]. According to Equation (10), when the input impedance (Zin) is infinitely close to the air impedance (Z0), the ideal impedance matching can be obtained. Figure 8a  The frequency range with good impedance match (Z-value is close to 1) of SiCw/Dy3Si2C2 was much larger than that of pure SiCw, which indicates that the impedance match of the SiCw was well improved by the Dy3Si2C2 coating. Therefore, the EMW can enter the SiCw/Dy3Si2C2 sample, while most of the EMW was reflected in the case of pure SiCw due to the poor impedance matching.
Furthermore, to evaluate the attenuation ability of EMW energy of the samples, the α (Figure 9) was evaluated by the following formula [53]: The frequency range with good impedance match (Z-value is close to 1) of SiC w /Dy 3 Si 2 C 2 was much larger than that of pure SiC w , which indicates that the impedance match of the SiC w was well improved by the Dy 3 Si 2 C 2 coating. Therefore, the EMW can enter the SiC w /Dy 3 Si 2 C 2 sample, while most of the EMW was reflected in the case of pure SiC w due to the poor impedance matching.
Furthermore, to evaluate the attenuation ability of EMW energy of the samples, the α (Figure 9) was evaluated by the following formula [53]: Materials 2023, 16, x FOR PEER REVIEW Figure 9. Attenuation constant of pure SiCw and SiCw/Dy3Si2C2 at the frequency ra GHz.
A larger value of α implies a stronger attenuation ability [65]. The sole loss of SiCw at a low frequency resulted in a high attenuation constant, wh most of the EMW was reflected. This is in good agreement with the p matching of the SiCw sample. On the other hand, the introduction of the n (2D) Dy3Si2C2 coating significantly improved the impedance match as wel ation ability of the SiCw/Dy3Si2C2. As a result, the EMW absorption prope cantly improved.
The possible EMW absorption mechanism of SiCw/Dy3Si2C2 is illustrat A larger value of α implies a stronger attenuation ability [65]. The sole high dielectric loss of SiC w at a low frequency resulted in a high attenuation constant, which meant that most of the EMW was reflected. This is in good agreement with the poor impedance matching of the SiC w sample. On the other hand, the introduction of the nano-laminated (2D) Dy 3 Si 2 C 2 coating significantly improved the impedance match as well as the attenuation ability of the SiC w /Dy 3 Si 2 C 2 . As a result, the EMW absorption property was significantly improved.
The possible EMW absorption mechanism of SiC w /Dy 3 Si 2 C 2 is illustrated in Figure 10. Firstly, the favorable impedance matching suggests that the majority of the EMW can enter the SiC w /Dy 3 Si 2 C 2 sample, while just a small part of the EMW is reflected. This is the premise of excellent EMW absorption performance of the material. Secondly, the metallic conductivity characteristic of Dy 3 Si 2 C 2 coating improved the electrical conductivity of SiC w /Dy 3 Si 2 C 2 , which enhanced the conductance loss by improving the electron transition channel in SiC w /Dy 3 Si 2 C 2 . Thirdly, a large number of heterogeneous interfaces in the SiC w /Dy 3 Si 2 C 2 sample, such as Dy 3 Si 2 C 2 /Dy 3 Si 2 C 2 , SiC w /Dy 3 Si 2 C 2 , and SiC w /SiC w , significantly increased the interfacial polarization and hopping electrons between Dy 3 Si 2 C 2 nanosheets. This is beneficial for the improvement of the dielectric loss of the material. Finally, the high aspect ratio of SiC w with the 2D nano-laminated Dy 3 Si 2 C 2 coating constructed a 3D microstructure and formed an effective conductive network, resulting in the enhancement of multiple scattering and reflections. Therefore, the excellent EMW absorption performance of SiC w /Dy 3 Si 2 C 2 was attributed to the synergistic effect of fa-vorable impedance matching, enhanced conductance loss, interfacial polarization, dipole polarization, and multiple scattering and reflections.
A larger value of α implies a stronger attenuation ability [65]. The sole high dielectric loss of SiCw at a low frequency resulted in a high attenuation constant, which meant that most of the EMW was reflected. This is in good agreement with the poor impedance matching of the SiCw sample. On the other hand, the introduction of the nano-laminated (2D) Dy3Si2C2 coating significantly improved the impedance match as well as the attenuation ability of the SiCw/Dy3Si2C2. As a result, the EMW absorption property was significantly improved.
The possible EMW absorption mechanism of SiCw/Dy3Si2C2 is illustrated in Figure 10. Firstly, the favorable impedance matching suggests that the majority of the EMW can enter the SiCw/Dy3Si2C2 sample, while just a small part of the EMW is reflected. This is the premise of excellent EMW absorption performance of the material. Secondly, the metallic conductivity characteristic of Dy3Si2C2 coating improved the electrical conductivity of SiCw/Dy3Si2C2, which enhanced the conductance loss by improving the electron transition channel in SiCw/Dy3Si2C2. Thirdly, a large number of heterogeneous interfaces in the SiCw/Dy3Si2C2 sample, such as Dy3Si2C2/Dy3Si2C2, SiCw/Dy3Si2C2, and SiCw/SiCw, Figure 10. The EMW absorption mechanism of SiC w /Dy 3 Si 2 C 2 .
The EMW absorption property of SiC w /Dy 3 Si 2 C 2 is better when compared to most of the previously reported materials, as shown in Figure 11. significantly increased the interfacial polarization and hopping electrons betwee Dy3Si2C2 nanosheets. This is beneficial for the improvement of the dielectric loss of th material. Finally, the high aspect ratio of SiCw with the 2D nano-laminated Dy3Si2C2 coa ing constructed a 3D microstructure and formed an effective conductive network, resul ing in the enhancement of multiple scattering and reflections. Therefore, the excellen EMW absorption performance of SiCw/Dy3Si2C2 was attributed to the synergistic effect o favorable impedance matching, enhanced conductance loss, interfacial polarization, d pole polarization, and multiple scattering and reflections. The EMW absorption property of SiCw/Dy3Si2C2 is better when compared to most o the previously reported materials, as shown in Figure 11. It can be concluded that the as-obtained SiCw/Dy3Si2C2 whiskers could be a promisin candidate for EMW absorbers for aerospace applications due to their excellent EMW ab sorption performance and wide EAB for thin samples, light weight, and potential oxida tion resistance at high temperatures. It can be concluded that the as-obtained SiC w /Dy 3 Si 2 C 2 whiskers could be a promising candidate for EMW absorbers for aerospace applications due to their excellent EMW absorption performance and wide EAB for thin samples, light weight, and potential oxidation resistance at high temperatures.

Conclusions
In summary, a novel nano-laminated Dy 3 Si 2 C 2 coating was in situ fabricated on the surface of SiC w using the molten salt method to improve EMW absorption performance. A randomly stacked 2D Dy 3 Si 2 C 2 nanosheet coating with a thickness of~100 nm was uniformly coated on the surface of 1D SiC w , which further formed a 3D microstructure. The EMW absorption performance of the as-obtained 3D structural SiC w /Dy 3 Si 2 C 2 sample was significantly improved when compared to the pure SiC w sample. The minimum RL value increased from −10.64 dB for the pure SiC w to −32.09 dB for the SiC w /Dy 3 Si 2 C 2 . At the same time, the corresponding thickness of 1.54 mm was much thinner than that of the pure SiC w (4.5 mm). The possible EMW absorption mechanism of the as-obtained SiC w /Dy 3 Si 2 C 2 sample was ascribed to the synergic effect of favorable impedance matching, enhanced conductance loss, interfacial polarization, dipole polarization, and multiple scattering. The as-obtained 3D structural SiC w /Dy 3 Si 2 C 2 could be a candidate for EMW absorber applications due to its excellent EMW absorption performance and wide EAB for relatively thin samples, light weight, as well as potential oxidation and corrosion resistance at high temperatures.