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Article

A Novel Nano-Laminated GdB2C2 with Excellent Electromagnetic Wave Absorption Performance and Ultra-High-Temperature Thermostability

by
Longfei Jiang
1,2,
Gang Qin
2,
Pengxing Cui
2,
Guoqing Wang
2 and
Xiaobing Zhou
1,2,*
1
School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China
2
Zhejiang Key Laboratory of Data-Driven High-Safety Energy Materials and Applications, Ningbo Key Laboratory of Special Energy Materials and Chemistry, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China
*
Author to whom correspondence should be addressed.
Nanomaterials 2024, 14(12), 1025; https://doi.org/10.3390/nano14121025
Submission received: 9 May 2024 / Revised: 8 June 2024 / Accepted: 11 June 2024 / Published: 13 June 2024
(This article belongs to the Special Issue Advanced Nanomaterials for Electromagnetic Shielding and Absorption Applications)
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Versions Notes

Abstract

:
A novel nano-laminated GdB2C2 material was successfully synthesized using GdH2, B4C, and C via an in situ solid-state reaction approach for the first time. The formation process of GdB2C2 was revealed based on the microstructure and phase evolution investigation. Purity of 96.4 wt.% GdB2C2 was obtained at a low temperature of 1500 °C, while a nearly fully pure GdB2C2 could be obtained at a temperature over 1700 °C. The as-obtained GdB2C2 presented excellent thermal stability at a high temperature of 2100 °C in Ar atmosphere due to the stable framework formed by the high-covalence four-member and eight-member B-C rings in GdB2C2. The GdB2C2 material synthesized at 1500 °C demonstrated a remarkably low minimum reflection loss (RLmin) of −47.01 dB (3.44 mm) and a broad effective absorption bandwidth (EAB) of 1.76 GHz. The possible electromagnetic wave absorption (EMWA) mechanism could be ascribed to the nano-laminated structure and appropriate electrical conductivity, which facilitated good impedance matching, remarkable conduction loss, and interfacial polarization, along with the reflection and scattering of electromagnetic waves at multiple interfaces. The GdB2C2, with excellent EMWA performance as well as remarkable ultra-high-temperature thermal stability, could be a promising candidate for the application of EMWA materials in extreme ultra-high temperatures.

Graphical Abstract

1. Introduction

Ultra-high-temperature ceramics (UHTCs) are materials which are usually used at temperatures above 1800 °C. Most UHTCs, including transition-metal borides and/or carbides, are characterized by their strong covalent bonds [1,2,3]. Due to the robust covalent bonds between the transition metal and the boron and/or carbon, the family of UHTCs boasts an array of unparalleled characteristics, including elevated melting points, superior hardness, outstanding mechanical performance at high temperatures, remarkable thermal stability, and commendable resistance to both oxidation and corrosion [4,5,6,7]. Therefore, UHTCs are promising candidates for aerospace applications, including nose cone caps and leading edges, and coatings for the protection of high-temperature structural components in hypersonic vehicles, e.g., coatings of carbon fiber-reinforced ceramic matrix composites which improve the synergistic blend of strength and durability [8,9].
On the other hand, for aerospace applications, UHTCs not only should have high-temperature thermal stability but should also possess desirable functional capabilities, such as excellent electromagnetic wave absorption (EMWA) performance [10,11,12]. Numerous EMWA materials have been investigated, such as carbon-based materials [13,14,15,16], magnetic metal materials [17,18], ferrite, as well as its composites [19], and polymer matrix composites [20]. Magnetic materials such as ferrite exhibit excellent EMWA performance, but they lack dielectric loss capacity, and once they exceed the Curie temperature, their magnetic properties disappear, rendering the magnetic loss mechanism ineffective [19]. Polymer-based composites are lightweight and have high design flexibility. However, the materials in question possess relatively low melting points, rendering them ill-suited for high-temperature applications [20,21]. In addition, there are carbon-related materials such as carbon fiber, carbon nanotubes, and graphene that exhibit the characteristics of reduced weight, lower density, enhanced electrical conductivity, and superior mechanical properties [14,15,16,22,23,24,25,26,27,28,29]. Nevertheless, these materials often present a high dielectric constant coupled with low permeability, which compromises impedance matching and restricts the penetration of electromagnetic waves into the material. Additionally, their susceptibility to oxidation in high-temperature environments renders them unsuitable for applications at elevated temperatures [30]. The task of creating a material with a relatively broad effective absorption bandwidth and enhanced EMWA performance poses a formidable challenge. Additionally, these materials must exhibit low density, thin thickness, and exceptional thermal stability even under extreme temperatures, such as stealth materials used in high-speed military vehicles [31].
Rare earth diborocarbides (REB2C2, RE = Sc, Y, and lanthanide elements) are a group of laminated structure materials like MAX phases (M represents a transition metal; A denotes elements from groups IIIA, IVA, VA, or VIA; and X stands for carbon or nitrogen) [32,33,34,35]. Within the rare earth diborocarbides, GdB2C2 belongs to a tetragonal structure with the P4/mbm space group (No.127) [36]. Gd atoms are configured in alternating B-C layers along the z-axis, Gd-Gd bonds are metallic bonds, while B-C bonds are covalent bonds, leading to the formation of four-membered and eight-membered B-C rings. The anisotropy structures of chemical bonds may result in GdB2C2 showing strong anisotropy in physical properties [37,38]. For example, with regard to the magnetic properties of GdB2C2, it was demonstrated that it was antiferromagnetic in the c plane, while it was ferromagnetic along the c axis [39]. The resistivities of GdB2C2 were reported to decrease with decreasing temperature, which showed metallic conductivity and electron-type conductors [40].
On the other hand, most of the reported synthesis methods for REB2C2 have been related to YB2C2 [41,42]. There are few studies on fabrication methods for GdB2C2 [40]. Single-crystal GdB2C2 was synthesized employing Gd, B, and C as starting materials using repetitive arc-melting. The samples were then encapsulated in a vacuum-sealed silica tube for a duration of several days [36]. The other representative synthesis procedure for GdB2C2 was a two-step procedure, including GdB4 fabricated by induction heating mixtures of Gd and B at 1900 °C. Then, GdB4 and graphite were heated at 1900 °C for three hours to obtain GdB2C2 [40]. Additionally, to the best of the authors’ knowledge, there are limited reports on the one-step synthesis of GdB2C2 powders, their EMWA properties, and their thermal stability at ultra-high temperatures. Given the typical nano-laminated structure of GdB2C2, which is akin to MAX phases, it is anticipated to exhibit exceptional EMWA performance due to the potential multiple interface scattering losses and dielectric loss mechanisms inherent in GdB2C2 [32,33,43,44,45,46].
Therefore, the main aim of this work was to develop a facile one-step fabrication method for the nano-laminated GdB2C2 material and reveal the EMWA mechanism. In addition, the thermal stability at an ultra-high temperature of 2100 °C was investigated to determine the potential aerospace applications of GdB2C2.

2. Experimental Procedure

2.1. Materials

GdH2, B4C, and carbon black were employed as raw materials for synthesizing GdB2C2. The GdH2 powders (purity > 99.9%, average particle size: ~70 μm) were commercially supplied by Hunan Rare Earth Metal Materials Research Institute Co., Ltd., Changsha, China. The B4C powders, with a purity of 99% and a mean particle size of around 500 nanometers, were procured from the Suzhou Nutpool Materials Technology Co., Ltd., Jian, China. Carbon black powders, characterized by a purity of 99.9% and a mean particle size of 500 nanometers, were acquired from ENO High-Tech Material Co., Ltd., Qinhuangdao, China.

2.2. Fabrication of GdB2C2

The GdH2, B4C and carbon black powders were mixed in a glovebox under an argon atmosphere with a molar ratio of GdH2:B4C:C = 2:1:3. To investigate the in situ reaction process of GdB2C2, the mixed powders were fired at various temperatures ranging from 900 °C to 1800 °C for 4 h in a graphite furnace under an argon atmosphere. The heating and cooling rates were 5 °C/min. The as-obtained powders were ground in an agate mortar for 40 min. Figure 1 shows the schematics of the GdB2C2 powder synthesis procedures.

2.3. Characterizations

The compositions of phases and crystalline structures within the samples synthesized at different temperatures were evaluated using a Bruker AXS D8 Advance X-ray diffractometer, procured from Rheinstetten, Germany, which operates on Cu Kα radiation with a wavelength set at λ = 1.5406 Å. The power parameters for this instrument were set at 1600 W, equating to a current of 40 mA and a voltage of 40 kV, while utilizing a step scan methodology of 0.02°/2θ with a step duration of 0.2 s. The constituents of the phases and parameters of the lattice within the resulting materials were deciphered through the Rietveld refinement processing of the XRD patterns, facilitated by TOPAS-Academic v6 software. The microstructures of the samples synthesized at different temperatures were examined using a scanning electron microscope (SEM; Regulus 8230, Hitachi, Tokyo, Japan) equipped with an energy-dispersive spectroscopy (EDS) system. The average grain sizes were quantified by analyzing a selection of SEM micrographs, with a rigorous count of no less than 100 grains per sample. Transmission electron microscopy (TEM; Talos F200x, Thermo Fisher Scientific, Waltham, Massachusetts, USA) was employed to confirm the microstructure and phase composition of the GdB2C2 synthesized at 1500 °C. Thin foil of the sample for TEM analysis was fabricated using the focused ion beam (FIB) technique (Aurgia, Carl Zeiss, Thornwood, NY, USA).
To investigate the effects of the heat-treated temperatures on the EMWA performance, the complex permittivity and complex permeability of the samples fabricated at 1500 and 1800 °C were measured at a frequency range from 2 to 18 GHz using a Keysight E5063A Network Analyzer (Santa Rosa, California, USA). A toroidal ring was fabricated using the as-obtained GdB2C2 powders mixed with 60 vol.% paraffin for measurement of the complex permittivity and complex permeability. The geometry size of the toroidal ring was 3 mm (inner diameter), 7 mm (outer diameter), and 2 mm (thickness). A start frequency of 2 GHz, an intermediate frequency bandwidth of 300 Hz, and 201 sample points were set during measurement. Following port and path calibrations, system accuracy was verified using air and polytetrafluoroethylene standards. At least five samples were measured for each synthesized temperature, and the mean value was calculated.

3. Results and Discussion

3.1. Microstructure and Phase Composition of GdB2C2

Figure 2 presents the X-ray diffraction (XRD) patterns of the powders synthesized at various temperatures ranging from 900 to 1800 °C. The GdB4, Gd-C, and Gd2O3 phases were detected at the temperature of 900 °C, in addition to the residual un-reacted raw materials of GdH2, B4C, and C. As the synthesis temperature increased to 1100–1300 °C, GdB2C2 was formed, while the GdB4 and Gd-C phases were still detected, which implied that the reaction was not completed. When the temperature increased to 1400–1500 °C, GdB2C2 was the main phase, and just a small amount of the GdB4 impurity phase was detected. While the temperature increased to 1600–1800 °C, nearly fully pure GdB2C2 was obtained.
According to the Rietveld refinement, the content of GdB2C2 phase was increased from 95.47 wt. % to 100 wt % as the synthesis temperature increased from 1400 to 1700 °C, while the content of GdB4 impurity decreased from 4.53 wt% to 0 wt%. Figure 3 shows the typical Rietveld refinement of the XRD pattern of GdB2C2 fabricated at 1500 °C. The Rietveld fitting results (reliability factors Rwp = 9.4%) indicated that a tetragonal structure of GdB2C2 was obtained (the inset in Figure 3). An Rwp value is a critical metric for evaluating the fit between calculated and experimental diffraction patterns. An Rwp value that is lower than 10% indicates the reliability of the Rietveld fitting results. The lattice parameters a and c were determined to be approximately 3.78 Å and 7.27 Å (Table 1), which were almost equal to the reported lattice parameters of GdB2C2 [40]. The lattice parameter measurements were validated by the TEM analysis, which confirmed the c-axis value of 7.5612 Å in the GdB2C2 powders through selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HR-TEM) imaging. These observations are presented in Figure 4a,b. The HR-TEM image elucidates the atomic arrangement along the zone axis, revealing a lattice fringe spacing of 0.37806 nm, which corresponds to the (002) plane of GdB2C2. The experimentally determined spacing of the (002) plane (Figure 4b) thereby substantiated the successful synthesis of the GdB2C2. It is noted that there was some difference in the value of the lattice constant c determined from X-ray measurements (0.727 nm) and TEM analysis (0.75612 nm). This may due to the TEM image only providing localized measurements, which may be influenced by the localized stresses and/or lattice distortions.
According to the phase evolution analysis of the samples fabricated at various temperatures, the formation process of GdB2C2 via the in situ reaction among GdH2, B4C, and C could be concluded as follows: At the low temperature of 900 °C, GdH2 underwent dehydrogenation and decomposed to Gd and H2 (reaction 1) [47]. The generated Gd diffused on the surfaces of B4C and C and reacted with them to form GdB4 and Gd-C phases (reaction 2). At the temperature ranging from 1100 to 1300 °C, the intermediate phases of GdB4 and Gd-C further reacted with residual C, and GdB2C2 began to be formed (reaction 3). The grain size was only several hundred nanometers due to the low synthesis temperature (Figure 5a). Upon elevating the temperature to 1400 °C, most of the GdB4 and Gd-C intermediate phases underwent transformation into GdB2C2. The predominant size of the GdB2C2 particles ranged from 0.5 to 1 μm, while some nano-sized particles were also discerned (Figure 5b and Figure 6a). When the temperature increased to 1500 °C, the GdB2C2 grains growth along with the nano-sized particles disappeared (Figure 5c). The mean particle size was around 1.1 μm (Figure 6b). As the temperature increased to over 1600 °C, almost pure GdB2C2 was obtained. At this high temperature range, the GdB2C2 grains grew rapidly. The mean particle size increased from 4.76 μm to 16.44 μm as the temperature increased from 1600 to 1800 °C (Figure 6c–e). The typical nano-laminated structure of GdB2C2 was observed (Figure 5d–f). It is noted that the GdB2C2 grains preferred to grow along the c plane; as a result, the (002) diffraction peak was emphasized as the highest peak for the sample fabricated at 1800 °C (Figure 2b). This phenomenon was also observed in the single-crystal GdB2C2 [38].
GdH2 → Gd + H2
2Gd + B4C + C → GdB4 + GdC2
GdC2 + GdB4 + 2C → 2GdB2C2

3.2. Ultra-High-Temperature Thermal Stability of GdB2C2 at 2100 °C

Thermal stability at high temperature is a critical property in the application of UHTCs. Figure 7a–c show the morphology of GdB2C2 powders (prepared at 1500 °C) after heat treatment at 2100 °C for 20 min under argon atmosphere. Obvious grain growth was observed. The maximum length of GdB2C2 was around 30 μm, which was significantly increased and was up to 30 times larger than that of the original as-synthesized GdB2C2 sample (~1.1 μm). The SEM image of the fracture surface (Figure 7c) reveals a characteristic nano-laminated structure akin to that of MAX phases. The observed laminated fracture features, including delamination, slipping, and kink band formation (Figure 7c), indicate the ductile behavior of GdB2C2.
The phase composition of GdB2C2 after being heat-treated at 2100 °C is depicted in Figure 7d,e. No significant phase transition was observed. However, GdB2C2 post- heat-treated at 2100 °C exhibits strong diffraction peaks corresponding to the (002) and (004) crystallographic planes. Additionally, the characteristic diffraction peaks of the (100) and (002) planes for the heat-treated sample at 2100 °C exhibit a shift towards higher 2 theta angles when contrasted with the as-obtained GdB2C2 powders. This implied that the lattice parameters of dB2C2 after heat treatment at 2100 °C decreased compared to the as-obtained GdB2C2 powders.
The decrement in the lattice parameters after heat treatment can be attributed to the volatilization of Gd atoms, which resulted in the formation of Gd vacancies. This can be easily explained by the crystal structure of GdB2C2, which had Gd-B and Gd-C bonds with bond lengths of 276.4 and 272.0 pm, respectively, as well as shorter B-C bonds (162.2 pm and/or 151.2 pm) [36]. The bond energies of the Gd-B and Gd-C interactions were notably lower than those of the B-C bonds, rendering the former more susceptible to dissociation at the elevated temperature of 2100 °C. Consequently, Gd atoms preferentially evaporated from the surface of the material. The evaporation of Gd atoms and the consequent vacancies naturally resulted in a contraction of the lattice as the remaining atoms repositioned to maintain the integrity of the crystal structure. On the other hand, the robustly covalent four-membered and eight-membered B-C rings were preserved, resisting evaporation. This suggested that the high-covalence four-member and eight-member B-C rings play a critical role in the ultra-high-temperature thermal stability of GdB2C2. Comparable behavior has been noted in analogous YB2C2 materials, further supporting the significance of these high-covalence B-C rings in maintaining the thermal stability of the related compounds [42].

3.3. Electromagnetic Wave Absorption Properties of GdB2C2

The interaction of a material with EMW is determined by various factors, with the efficiency of impedance matching playing a crucial role. Effective impedance matching enhances the penetration and absorption of the EMW into the material, whereas inadequate matching results in EMW reflection. The complex dielectric constant and complex permeability reflect the aptitude of a material for absorbing EMW, which in turn determines its efficiency in transforming them into alternative forms of energy [48].
To investigate the effects of fabrication temperature on the EMWA performance of the GdB2C2 powders, the complex permittivity and complex permeability of the GdB2C2 powders synthesized at 1500 and 1800 °C were measured. It was observed that the real and imaginary parts of the complex permeability remained nearly constant at approximately 1 and 0 at the frequency range of 2–18 GHz, respectively. This indicates that the magnetic loss in these materials was negligible. Consequently, the EMWA properties of the GdB2C2 powders were predominantly governed by complex permittivity [49,50].
Figure 8 presents the real (ε′) and imaginary (ε″) parts of the complex permittivity of the GdB2C2 powders synthesized at 1500 and 1800 °C. Conventionally, ε′ is indicative of the storage capability of dielectric energy, whereas ε″ reflects the dissipation of dielectric energy [51,52]. Furthermore, tan δe (tan δe = ε′/ε″) is usually used to analyze the dielectric attenuation of a sample, which reflects the EMW attenuation capability of EMWA materials (Figure 8c). The dielectric loss tangent provides a quantitative measure of the efficiency with which the material can attenuate incident EMWs.
Both the ε′ and ε″ of the GdB2C2 powders synthesized at 1500 °C were higher than those of the samples synthesized at 1800 °C, confirming that the fine particle size could promote the dielectric properties of GdB2C2. The finer the grain size, the more interfaces were formed between GdB2C2 powders and the paraffin matrix. The improvement in the interaction between the GdB2C2–paraffin and the enhancement of interfacial polarization contributed to the higher ε′ of GdB2C2 powders synthesized at 1500 °C compared to the sample synthesized at 1800 °C. The increase in ε″ was primarily attributed to the rise in electrical conductivity. The electrical conductivity can be determined using Equation (4) as follows [53]:
σ = 2 π ε 0 ε ″
As presented in Figure 8d, the electrical conductivities of GdB2C2 powders synthesized at 1500 °C were higher than that of the sample synthesized at 1800 °C. This can be mainly attributed to the metallic conductivity characteristic of the laminated structural GdB2C2 powders, as the finer grain size more easily formed a conductive net structure, which increased the transmission channels of carriers. The diminutive grain size paved the way for the emergence of a conductive network, empowering more expeditious pathways for the conveyance of charge carriers [54]. Furthermore, the ε′ and ε″ of GdB2C2 presented a fluctuation corresponding to the resonance. The permittivity of the GdB2C2 showed typical nonlinear resonant characteristics, indicating the existence of polarization and relaxation behavior, which implied good dielectric loss performance in the corresponding frequency range. In addition, the enhanced number of heterogeneous interfaces caused by the smaller grain size also improved the relaxation process of GdB2C2, which generated the improvement in relaxation polarization.
Reflection loss (RL) and EAB were used to evaluate the EMWA properties of the as-obtained GdB2C2 powders synthesized at 1500 and 1800 °C. According to the classic transmission line theory, the RL values of the as-obtained GdB2C2 powders could be calculated by Equations (7)–(9) [55,56,57] as follows:
R L dB = 20 log ⁡ Z in − Z 0 / Z in + Z 0
Z in = Z 0 μ r / ε r tan h ⁡ j 2 π fd / c μ r ε r
Z 0 = μ r / ε r
Within the equation, Z0 stands for the impedance of free space, with Zin embodying the input impedance. The complex relative permeability, μr, unfolds as μ′–jμ″, and the complex relative permittivity, εr, is articulated by ε′–jε″. The symbol c embodies the speed of light, d signifies the thickness of the material, and f epitomizes the frequency. Figure 9 showcases the reflection loss (RL) values for GdB2C2 powders forged at 1500 °C and 1800 °C across the expansive frequency realm of 2–18 GHz with varying thicknesses. The minimum reflection loss (RLmin) value of the GdB2C2 synthesized at 1500 °C was −47.01 dB at the frequency of 15.92 GHz with a thickness of 3.44 mm. Meanwhile, for the sample synthesized at 1800 °C, the RLmin value was −29.51 dB at the frequency of 16.32 GHz with a large thickness of 4.62 mm. For the sake of comparison, the selected theoretical calculated RL of the as-obtained GdB2C2 powders synthesized at 1500 and 1800 °C in the frequency range of 2–18 GHz with different thicknesses is shown in Figure 9c,f. The EAB of GdB2C2 powders synthesized at 1500 °C was wider than that of GdB2C2 powders synthesized at 1800 °C. The widest EAB was around 1.76 GHz for a thin GdB2C2 sample with the thickness of 3.86 mm (Figure 9c). Meanwhile, the widest EAB of GdB2C2 powders synthesized at 1800 °C was merely 1.68 GHz for the sample even with a thickness of 4.96 mm (Figure 9f). This indicates that GdB2C2 powders synthesized at a low temperature of 1500 °C with a finer grain size can significantly improve the EMWA properties of GdB2C2.
In order to reveal the intrinsic reason for the improved EMWA performance for the GdB2C2 powders synthesized at 1500 °C, the impedance match (IM, Z) and attenuation ability were calculated. Figure 10 illustrates the IM (Z) values of these two samples, which were calculated using the following formula [58]:
Z = Z i n / Z 0 = μ r / ε r tanh ⁡   j 2 π f d / c μ r ε r    
IM is a critical factor in determining the entry of EMWs into a material’s surface. The optimal IM occurs when the input impedance (Zin) closely matches the air impedance (Z0). The closer the IM value Z is to 1, the more electromagnetic waves can enter the EMW absorber, and the better impedance it has. The frequency range with good impedance matching of the GdB2C2 powders synthesized at 1500 °C was larger than that of the sample synthesized at 1800 °C. Therefore, the EMWs could enter into the sample synthesized at 1500 °C, while most of the EMWs were reflected in the case of the sample synthesized at 1800 °C due to the poor IM.
In addition to evaluating the attenuation ability of the EMW energy of the samples, the attenuation constant (α) of the as-obtained GdB2C2 powders synthesized at 1500 and 1800 °C in the frequency range of 2–18 GHz was calculated by the following formula [59]:
α = 2 π f c ( μ ″ ε ″ − μ ′ ε ′ ) + ( μ ′ ε ″ + μ ″ ε ′ ) 2 + ( μ ″ ε ″ − μ ′ ε ′ ) 2
The elevation of the α value signifies a heightened capacity of the material to effectively attenuate electromagnetic waves [60]. As depicted in Figure 11, samples prepared at 1500 °C exhibited a higher attenuation constant than those prepared at 1800 °C within the frequency range of 12.64 to 18 GHz. Combined with the calculated IM value, the finer grain size of the sample synthesized at 1500 °C improved the IM as well as the attenuation ability compared to the sample synthesized at 1800 °C. As a result, the EMWA performance was augmented. On the other hand, high-performance EMWA materials, such as SiC, ferrite, polymer, and carbon-based materials could be introduced into GdB2C2 to construct a hybrid multi-phase composite to further improve the EMWA performance of GdB2C2 and broaden the EAB [61,62,63].
Figure 12 shows the possible EMWA mechanism of the nano-laminated GdB2C2. Firstly, the optimal IM facilitated the penetration of a substantial portion of EMWs into the GdB2C2 sample. This is the premise of the excellent EMWA performance of GdB2C2. Secondly, the metallic conductivity of GdB2C2 played a pivotal role in inducing conductance loss through electron transition pathways within the material. Thirdly, a large number of homogeneous interfaces and heterogeneous interfaces in the nano-laminated GdB2C2 sample, such as GdB2C2/GdB2C2, GdB2C2/paraffin, and GdB2C2/GdB4, remarkably increased the interfacial polarization as well as hopping electrons between GdB2C2 nano-sheets. This was beneficial to improving the dielectric loss of the GdB2C2. Finally, the two-dimensional nano-laminated GdB2C2 could form a three-dimensional microstructure and constructed an effective conductive network, resulting in the enhancement of multiple scattering and reflections. Therefore, the excellent EMWA performance of GdB2C2 was contributed to the synergistic effect of favorable IM, enhanced conductance loss, interfacial polarization, dipole polarization, and multiple scattering and multiple interface reflections.
Figure 13 shows the optimal EMWA performance of ternary-layered-structure Ti3SiC2, Ti3AlC2, Ti3C2Tx, and Ti3C2Tx-based materials reported in the relevant literature. Numerous layered-structure materials have been thoroughly investigated, and excellent EMWA properties have been achieved in terms of filler loading, thinner matching thickness, low RLmin value, and broader EAB. In this study, GdB2C2 powders fabricated at 1500 °C presented excellent EMWA properties along with remarkable thermal stability at an ultra-high temperature of 2100 °C. It is noted that all of the EMWA performances of the samples were measured at room temperature. The high-temperature EMWA performance of GdB2C2 materials will be investigated in future work.

4. Conclusions

In conclusion, a novel nano-laminated GdB2C2 was successfully synthesized using in situ solid-state reaction technology. The formation mechanism of GdB2C2 was determined based on the investigation of the microstructure and phase composition of the samples synthesized at temperatures ranging from 900 °C to 1800 °C. GdB2C2 can be initially formed at 1100 °C. GdB2C2 with purity of 96.4 wt.% was obtained at 1500 °C, while nearly fully pure GdB2C2 could be fabricated at a temperature over 1700 °C. It is noted that the GdB2C2 grains preferred to grow along the c plane at temperatures over 1800 °C. In addition, the as-obtained GdB2C2 showed excellent thermal stability at a high temperature of 2100 °C in Ar atmosphere. This can be ascribed to the high-covalence four-member and eight-member B-C rings of GdB2C2, which formed a stable framework. A comparison of EMWA performance in the GdB2C2 samples prepared at 1500 °C and 1800 °C was carried out. The minimum reflection loss value (RLmin) of −47.01 dB with an effective absorption bandwidth (EAB) of 1.76 GHz at a thickness of 3.44 mm was obtained for the GdB2C2 synthesized at 1500 °C. The main mechanisms responsible for the excellent EMWA performance of GdB2C2 were good impedance matching, remarkable conduction loss, interfacial polarization, as well as multiple interface reflection and scattering. The excellent EMWA performance as well as the remarkable ultra-high-temperature thermal stability of the as-obtained GdB2C2 with make it a promising candidate as a next-generation high-temperature EMWA material.

Author Contributions

Conceptualization, X.Z.; Methodology, L.J. and P.C.; Software, G.Q.; Validation, G.W.; Formal analysis, L.J., G.Q., P.C. and G.W.; Investigation, L.J.; Data curation, L.J.; Writing—original draft, L.J.; Writing—review & editing, X.Z.; Supervision, X.Z.; Project administration, X.Z.; Funding acquisition, X.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National Natural Science Foundation of China (Grant No. 12275337, U2330103, and U20B2010) and the Zhejiang Provincial Natural Science Foundation of China under Grant No. Z24A050005. We would like to recognize support from the Ningbo Youth Science and Technology Innovation Leading Talent Project (2023QL043).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic of GdB2C2 powder synthesis procedures and EMWA property test.
Figure 1. Schematic of GdB2C2 powder synthesis procedures and EMWA property test.
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Figure 2. XRD patterns of GdB2C2 fabricated at different temperatures: (a) 900–1400 °C, (b) 1500–1800 °C.
Figure 2. XRD patterns of GdB2C2 fabricated at different temperatures: (a) 900–1400 °C, (b) 1500–1800 °C.
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Figure 3. Rietveld refinement of XRD pattern of GdB2C2 synthesized at 1500 °C.
Figure 3. Rietveld refinement of XRD pattern of GdB2C2 synthesized at 1500 °C.
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Figure 4. (a) SAED pattern and (b) HR-TEM image of GdB2C2 synthesized at 1500 °C.
Figure 4. (a) SAED pattern and (b) HR-TEM image of GdB2C2 synthesized at 1500 °C.
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Figure 5. SEM images of GdB2C2 powders fabricated at varying temperatures: (a) 1300 °C, (b) 1400 °C, (c) 1500 °C, (d) 1600 °C, (e) 1700 °C, and (f) 1800 °C.
Figure 5. SEM images of GdB2C2 powders fabricated at varying temperatures: (a) 1300 °C, (b) 1400 °C, (c) 1500 °C, (d) 1600 °C, (e) 1700 °C, and (f) 1800 °C.
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Figure 6. Particle size distribution of GdB2C2 powders fabricated at varying temperatures obtained by SEM image analysis: (a) 1400 °C, (b) 1500 °C, (c) 1600 °C, (d) 1700 °C and (e) 1800 °C.
Figure 6. Particle size distribution of GdB2C2 powders fabricated at varying temperatures obtained by SEM image analysis: (a) 1400 °C, (b) 1500 °C, (c) 1600 °C, (d) 1700 °C and (e) 1800 °C.
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Figure 7. SEM images of GdB2C2 powders at (a) 1500 °C and heat treatment at (b,c) 2100 °C, (d) XRD pattern of the as-synthesized GdB2C2 at 1500 °C and the sample after heat treatment at 2100 °C, (e) partial XRD patterns showing a peak shift of (002).
Figure 7. SEM images of GdB2C2 powders at (a) 1500 °C and heat treatment at (b,c) 2100 °C, (d) XRD pattern of the as-synthesized GdB2C2 at 1500 °C and the sample after heat treatment at 2100 °C, (e) partial XRD patterns showing a peak shift of (002).
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Figure 8. Real (a) and imaginary (b) parts of the complex permittivity, dielectric loss angle (c), and electrical conductivity (d) of GdB2C2 synthesized at 1500 °C and 1800 °C.
Figure 8. Real (a) and imaginary (b) parts of the complex permittivity, dielectric loss angle (c), and electrical conductivity (d) of GdB2C2 synthesized at 1500 °C and 1800 °C.
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Figure 9. Three-dimensional and two-dimensional patterns of reflection loss values at frequency range of 2 to 18 GHz for different thicknesses of GdB2C2 samples synthesized at 1500 °C (a–c) and 1800 °C (d–f).
Figure 9. Three-dimensional and two-dimensional patterns of reflection loss values at frequency range of 2 to 18 GHz for different thicknesses of GdB2C2 samples synthesized at 1500 °C (a–c) and 1800 °C (d–f).
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Figure 10. Two-dimensional patterns of Z values of GdB2C2 samples synthesized at (a) 1500 °C and (b) 1800 °C.
Figure 10. Two-dimensional patterns of Z values of GdB2C2 samples synthesized at (a) 1500 °C and (b) 1800 °C.
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Figure 11. Attenuation constant of GdB2C2 samples synthesized at 1500 °C and 1800 °C at the frequency range from 2 to 18 GHz.
Figure 11. Attenuation constant of GdB2C2 samples synthesized at 1500 °C and 1800 °C at the frequency range from 2 to 18 GHz.
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Figure 12. Main EMWA mechanisms of GdB2C2.
Figure 12. Main EMWA mechanisms of GdB2C2.
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Figure 13. Comparison of EMWA performance of GdB2C2 with that of other absorbing materials [11,51,54,64,65,66,67,68,69,70,71,72,73,74].
Figure 13. Comparison of EMWA performance of GdB2C2 with that of other absorbing materials [11,51,54,64,65,66,67,68,69,70,71,72,73,74].
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Table 1. The Rwp, lattice parameters (a and c), and amounts of GdB2C2 and GdB4 phase in the as-obtained powders fabricated at various temperatures according to Rietveld refinement results.
Table 1. The Rwp, lattice parameters (a and c), and amounts of GdB2C2 and GdB4 phase in the as-obtained powders fabricated at various temperatures according to Rietveld refinement results.
Holding Temperature
(°C)		RwpExperimentalGdB2C2
(wt.%)		GdB4
(wt.%)
a (Å)c (Å)
14008.43.787.2895.474.53
15009.43.787.2796.383.62
16008.43.797.2799.240.76
17008.33.797.271000
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Jiang, L.; Qin, G.; Cui, P.; Wang, G.; Zhou, X. A Novel Nano-Laminated GdB2C2 with Excellent Electromagnetic Wave Absorption Performance and Ultra-High-Temperature Thermostability. Nanomaterials 2024, 14, 1025. https://doi.org/10.3390/nano14121025

AMA Style

Jiang L, Qin G, Cui P, Wang G, Zhou X. A Novel Nano-Laminated GdB2C2 with Excellent Electromagnetic Wave Absorption Performance and Ultra-High-Temperature Thermostability. Nanomaterials. 2024; 14(12):1025. https://doi.org/10.3390/nano14121025

Chicago/Turabian Style

Jiang, Longfei, Gang Qin, Pengxing Cui, Guoqing Wang, and Xiaobing Zhou. 2024. "A Novel Nano-Laminated GdB2C2 with Excellent Electromagnetic Wave Absorption Performance and Ultra-High-Temperature Thermostability" Nanomaterials 14, no. 12: 1025. https://doi.org/10.3390/nano14121025

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