Bioinspired porous Ti2CTX/Si3N4 composites with aligned lamellar structure for efficient microwave absorption
0 Abstract
With the ever-deepening understanding of nano-electromagnetic interactions and the advancements of fabrication methodologies of nanomaterials, diverse electromagnetic platforms utilizing nanomaterials have been developed for next-generation electromagnetic safeguarding applications. This study presents the design and fabrication of bioinspired porous Ti2CTX/Si3N4 composites featuring an aligned lamellar structure, aimed at facilitating the effective absorption and dissipation of electromagnetic radiation. The layered configuration of Ti2CTX/Si3N4 composites facilitates the repeated reflection of electromagnetic waves between neighboring Ti2CTX layers, hence enhancing the energy dissipation of these waves. At a Ti2CTX concentration of merely 0.21 wt.%, the effective absorption bandwidth of Ti2CTX/Si3N4 composites encompasses the whole X-band (8.2-12.4 GHz), with a minimum reflection loss of -53 dB achievable at a sample thickness of 5 mm. Simultaneously, the fabricated
Keywords
INTRODUCTION
Over the past 20 years, the integration of nanomaterials with dielectrics and electromagnetic fields has flourished[1,2], achieving remarkable accomplishments across various domains. Among these, nano-functional materials applied to protect against gigahertz (GHz) electromagnetic wave pollution have emerged as a hot topic in research and applications[3-6]. With the continuous advancement of technology[7], the increasing popularity of electronic devices and their higher operating frequencies and power levels have led to an increase in the severity of GHz electromagnetic wave pollution[8,9]. This poses a significant threat to both human health and the normal functioning of electronic devices[10-14]. Electromagnetic wave absorbing materials could eliminate electromagnetic pollution based on absorption, and the electromagnetic energy can be directly converted into non-polluting energy, such as heat[15], thus avoiding the secondary pollution of electromagnetic waves. To meet the diverse needs of practical applications, electromagnetic wave-absorbing materials must have good mechanical properties and functionality.
Natural materials owe their unique performance not merely to their intrinsic chemistry but to ingenious structural design that orchestrates interactions among constituents. Today, macro-, micro-, and nano-scale architectures are engineered to unlock a material’s full potential[16-20]. For example, the human skull consists of two layers: a dense layer of bone and a loose porous cancellous bone layer[21]. This layer has a three-dimensional (3D) porous sandwich structure and good compressive strength[22-24]. This is sufficient to protect the brain from external loads during daily human activities. The porous structure of plants plays a vital role in transporting nutrients[25], providing mechanical support and regulating the transpiration process[26] and replicating moth eyes to introduce gradual transitions that prevent abrupt impedance jumps and enable broad-band impedance matching[27]. By sculpting intricate 3D geometries - porous networks, honeycombs, foams, and labyrinthine channels - electromagnetic waves are forced to undergo multiple reflections, scattering, and diffraction inside the material, greatly extending their propagation paths and dissipating their energy[28,29]. The integrative nature of natural materials - simultaneously providing electromagnetic absorption, mechanical load-bearing, and lightweight characteristics - offers a blueprint for architecting materials across micro- and macro-scales, opening new avenues for electromagnetic-wave-absorbing materials.
MXene is a new type of two-dimensional (2D)[30,31], layered, transition-metal-based material with a unique physical structure that can be used to produce electromagnetic wave-absorbing composite materials[32-34]. First, the surface of MXene is abundant in functional groups (e.g., -OH, -O, -F)[35]. These functional groups generate dipole polarization in the presence of an electromagnetic field, enhancing the dielectric loss capability of the material[36]. Furthermore, MXene can self-assemble into 3D porous structures under the influence of electrostatic and van der Waals forces[37]. Secondly, MXenes have high electrical conductivity[38], enabling the formation of an effective conductive network and improving the conductive loss. Third, its 2D layered structure can effectively enhance the multiple reflections and scattering of electromagnetic waves within the material[39], thereby improving its absorption efficiency[40-42]. Therefore, assembling 2D MXenes into 3D structures can effectively enhance electromagnetic wave absorption performance. Zhao et al. blended graphene oxide with an MXene suspension and coated MXene nanosheets onto the surface of a reduced graphene oxide (rGO) skeleton[43]. This process yielded MXene/rGO hydrogels that were freeze-dried to create a 3D porous structure, with shielding effectiveness of more than -50 dB in the X-band at a low Ti3C2TX content of 0.74 vol.%. However, the large specific surface area and high number of surface functional groups make MXenes highly susceptible to oxidation in air. The poor compressive ability and insufficient mechanical properties of MXene aerogels are greatly limited by their inherent brittleness, 2D lamellar structure, and lack of a protective layer. The researchers found that the oxidation of MXenes could be effectively prevented, and their stability improved through compositing with other materials and structural design. Wu et al. reported on an MXene/MoS2 composite coated with carbon to protect its structure[44]. This carbon layer reduces oxidation by external oxygen and the formation of oxygen-containing groups during annealing at high temperatures. Zhao et al. reported that an MXene/GO-based aerogel possesses structural diversity and an outstanding electromagnetic interference (EMI) shielding effectiveness of > -50 dB[43]. All of the abovementioned second-phase materials are highly conductive[45]. The introduction of these materials affects the dielectric properties of the substrate, making it difficult to regulate these properties. This increases the reflection of electromagnetic waves on the surface of the material, thereby reducing its absorption efficiency. In contrast, introducing electromagnetic wave transparent materials with lower dielectric constants does not affect the dielectric properties of the substrate material but can improve its stability to a certain extent. Si3N4 is characterized by its low density (3.18-3.44 g/cm3), excellent microwave transparent dielectric properties[46], and good mechanical and chemical stabilities[47,48]. Therefore, it can maintain structural integrity in extremely harsh environments and is not easily deformed or damaged. Its low dielectric loss helps reduce the reflection of electromagnetic waves on the material surface[49-51], allowing the electromagnetic waves to pass through effectively and reach deep inside the material. Amorphous Si3N4 prepared by Chemical Vapor Infiltration (CVI) has several unique advantages[52,53], including (1) Structural homogeneity: Amorphous Si3N4 has no long-range ordering and does not have structural defects such as grain boundaries, twins, and dislocations; (2) Excellent mechanical properties: amorphous Si3N4 are stronger and harder than crystalline Si3N4, making them suitable for high-strength and high-toughness applications; (3) Chemical stability: amorphous Si3N4 is chemically stable and has good antioxidant properties, enabling it to remain stable in high-temperature and oxidative environments. Therefore, depositing amorphous Si3N4 on the surface of MXene using the CVI process results in a uniformly distributed Si3N4 layer, forming a unique Si3N4-MXene-Si3N4 sandwich structure[54]. This effectively improved the mechanical properties and chemical stability of the aerogel. Overall, a Si3N4-MXene composite aerogel with a layered porous structure is expected to enhance both the mechanical and electromagnetic wave absorbing properties simultaneously; however, there are currently few related studies.
Here, we provide new ideas for nanomaterials in the field of electromagnetic wave absorption by introducing insulating electromagnetic wave transparent materials in combination with MXene’s properties. In this study, MXene aerogel with lamellar structure was fabricated via a bidirectional freeze-drying method. Then we deposited amorphous Si3N4 onto the surface of MXene nanosheets of Ti2CTX aerogels using the CVI technique[55]. The resulting Ti2CTX/Si3N4 composites retained the pristine lamellar structure of MXene aerogel. The layered porous structure of the Ti2CTX/Si3N4 aerogel promotes multiple reflections and scattering of electromagnetic waves within the material, which significantly increases the propagation path of electromagnetic waves, thereby improving the opportunity for absorption. In addition, the combination of the high electrical conductive MXenes and the low dielectric Si3N4 produces interfacial polarization, the abundance of functional groups on the surface of MXene brings sufficient dipole polarization, the high electrical conductivity of MXene given conductive loss, the synergistic effect of the multiple reflections and scattering, interfacial polarization, dipole polarization, and conductive loss results in an efficient electromagnetic wave absorption system[56]. With a Ti2CTX content of only 0.21 wt.%, the composites have an effective absorption bandwidth (EAB) that covers the entire X-band (8.2-12.4 GHz). The lowest reflection loss of −53 dB was achieved at a sample thickness of 5 mm. Therefore, this study proposes the design and fabrication of biomimetic porous Ti2CTX/Si3N4 composites with a laminated structure to effectively absorb and dissipate electromagnetic radiation.
METHODS
Materials
Ti2AlC powders were purchased from Lianlixin Technology Co., Ltd., China. Lithium fluoride (LiF) and hydrochloric acid (HCl) solution were obtained from Aladdin Reagent. Silicon tetrachloride (SiCl4 ≥ 99.99%), ammonia (NH3 ≥ 99.99%), Hydrogen (H2 ≥ 99.99 %), and argon (Ar ≥ 99.9%) were obtained from Xian Wei Guang Gas Co., Ltd. All compounds were utilized immediately without additional purification unless otherwise indicated.
Preparation of few-layered Ti2CTX nanosheets
Few-layered Ti2CTX nanosheets were synthesized by selectively removing aluminum atoms from Ti2AlC powders using an etching solution. The etchant was formulated by dissolving 1.0 g of LiF in 30 mL of 10 M HCl, resulting in a homogeneous acidic solution for subsequent etching reactions. The preparation procedure commenced with the gradual addition of Ti2AlC powder into the etchant solution under constant magnetic stirring. The resultant mixture was subjected to continuous agitation at ambient temperature for 72 h to facilitate complete etching. Subsequently, the etched powder underwent multiple centrifugation cycles (5 min per cycle at 6,000 rpm) using deionized (DI) water until a neutral pH of 6.5 was attained. The purified sediment was vacuum-dried for 72 h, and the final dried powder was kept until further use.
Preparation of porous Ti2CTX/Si3N4 composites
The porous Ti2CTX aerogel with an aligned lamellar structure was prepared via a bidirectional freezing method. The homemade Teflon tubes (23 mm × 11 mm × 30 mm) were sealed with a copper plate and a polydimethylsiloxane (PDMS) wedge titled to an angle of 20°. Ti2CTX slurry with a concentration of 10
Simulation calculation
The model has a thickness of 5 mm in the Z-axis using the Microwaves & RF/Optical module in CST Studio Suite 2020, with the material set to MXene from the material library. Periodic boundary conditions are set in the XY-axis. A perfectly matched layer is set in the Z-axis direction and a perfectly conductive layer is set in the Z-axis. The incident electromagnetic wave is directed along the Z axis, and calculations are performed at 8.2, 10.3, and 12.4 GHz to determine current density, E, power flow, and power loss density.
Characterization
The X-ray diffractometer (X’Pert Pro, Philips, Netherlands) confirmed the microstructure of samples, while morphology was characterized on a scanning electron microscope (SEM, HITACHI S-4700, Japan) and a transmission electron microscope (TEM, G20, FEI Tecnai, USA). The thermal stability of Ti2CTX and
Ti2CTX and Ti2CTX/Si3N4 composites for microwave absorption testing were tailored and refined into samples of 22.86 mm × 10.16 mm × x mm in dimension, and their dielectric constant was measured by the waveguide method on a vector network analyzer (VNA, MS4644A, Anritsu, Japan) through the waveguide method in a frequency range corresponding to the X-band (8.2-12.4 GHz). The calculation of reflection coefficient (RC) in decibels (dB) was based on the measured EM parameter values referring to a metal back-panel model as follows:
where c is the speed of light in vacuum, f is the frequency, ε is the dielectric constant, μ is permeability, and d is the thickness of samples.
RESULTS AND DISCUSSION
Fabrication and characterization of Ti2CTX/Si3N4 composites
Simulation tests have shown that the layered biomimetic structure designed in this study offers significant advantages for electromagnetic wave absorption. Firstly, two different structural models were designed: a bidirectionally oriented biomimetic multilayer structure [Figure 1A(1)] and a unidirectionally oriented porous structure [Figure 1B(1)]. When electromagnetic waves are incident along the Z-axis, we can observe the distribution of power loss density within a material, and assess its absorption performance of electromagnetic waves. The greater the power loss density, the faster the electromagnetic waves are lost. Through simulation calculations, the maximum values of the layered biomimetic structure at 8.2, 10.3, 12.4 GHz are 1.0 × 108, 1.3 × 108, 1.4 × 108 [Figure 1A(2)-A(4)]. The maximum values of the porous structure at 8.2, 10.3, and 12.4 GHz are 1.1 × 107, 1.5 × 107, and 1.7 × 107, respectively [Figure 1B(2)-B(4)]. Compared with unidirectional porous structures, layered biomimetic structures can dissipate electromagnetic waves more effectively. To further illustrate the advantages of layered biomimetic structures, comparisons were made between current density, E and power flow [Figure 1C-E]. The images show that bidirectional layered biomimetic structures have significant advantages, as they can dissipate electromagnetic waves more efficiently and reduce electromagnetic wave reflection. Therefore, we chose a bidirectional, layered, biomimetic structure for our design and prepared Ti2CTX/Si3N4 composite materials.
Figure 1. Structure design of Ti2CTX/Si3N4 composites. (A1) The Bidirectional structure; (A2-A4) CST simulation results for Power loss density of Bidirectional structure at 8.2-12.4 GHz; (B1) The Unidirectional structure; (B2-B4) CST simulation results for Power loss density of Unidirectional structure at 8.2-12.4 GHz; (C) Current density in CST simulations of different structures; (D) E in CST simulations of different structures; (E) Power flow in CST simulations of different structures.
The preparation process is shown in Figure 2 preparation of Ti2CTX aerogel: (1) Ti2CTX nanosheets were prepared by selectively removing Al atoms from Ti2AlC powders using an etching solution. The etched powders were then subjected to multiple centrifugation cycles using DI water. Then, the purified precipitates were vacuum-dried for 72 h, and the dried powders were stored for next use; (2) involved the preparation of the Ti2CTX aerogel: Ti2CTX solution with a concentration of 10 mg/mL was poured into a mold, which was then frozen in liquid nitrogen. The frozen samples were placed in a vacuum freeze dryer, and the ice template was subjected to a pressure of 0.1 Pa for 36 h, resulting in the preparation of porous Ti2CTX aerogels with an ordered lamellar structure [Figure 2A]. Preparation of Ti2CTX/Si3N4 composites
Figure 2. Schematic illustration of the fabrication route for Ti2CTX/Si3N4 composites.
Figure 3. Microstructure characterization of Ti2CTX and Ti2CTX/Si3N4 aerogel. (A) Image of Ti2CTX/Si3N4 sample; (B) SEM image of pure Ti2CTX; (C) SEM image of Ti2CTx/Si3N4; (D) Cross-sectional SEM image of Ti2CTX/Si3N4; (E) SEM image of Ti2CTX; (F) SEM image of Ti2CTX/Si3N4; (G-N) EDS maps of Ti2CTX/Si3N4. SEM: Scanning electron microscope; EDS: energy dispersive X-ray spectroscopy; HAADF: high-angle annular dark-field.
An optical image of the Ti2CTX/Si3N4 aerogel used for dielectric testing is presented in Figure 3A; the aerogel dimensions were 22.86 mm in length and 10.16 mm in width. The microstructure of the freeze-dried Ti2CTX aerogel showed a lamellar structure with orderly stacking of layers and an interlamellar spacing of about 30 μm [Figure 3B and Supplementary Figure 1]. A closer view of the lamellar structure revealed that the spacing between the Ti2CTX lamellae decreased to approximately 23 μm after Si3N4 deposition
In the TEM image of the Ti2CTX/Si3N4 aerogel [Figure 3G], a lattice spacing of 0.45 nm can be observed. From the elemental distribution characteristics of the Ti2CTX/Si3N4 aerogel surface, which correspond to the high-angle annular dark-field (HAADF) image [Figure 3H], it can be concluded that the main elements are Si, N, Ti, C, O, and F [Figure 3I-N, Supplementary Figure 4 and Supplementary Table 2]. The Ti and C elemental signals can still be detected following the introduction of Si3N4, suggesting that the original Ti2CTX was not destroyed by the addition of Si3N4. The weak signals of Ti and C elements are due to the masking of the elemental signals by the Si3N4 layer. The elements O and F are derived from the surface functional groups of Ti in Ti2CTX: O from the -OH group and F from the LiF used in the etching process. Based on the above analyses, it is shown that Si3N4 is successfully deposited by the CVI process on the surface of Ti2CTX, which is uniformly distributed and tightly wrapped around the Ti2CTX nanosheets.
The successful preparation of the Ti2CTX/Si3N4 aerogel was further demonstrated by X-ray diffraction (XRD) [Figure 4A], at 7.07° and 39.6°, corresponding to Ti2CTX (002) and (103) crystal planes, respectively. A comparison of the two XRD curves reveals that, following the introduction of Si3N4, a wide, flat scattering peak appears at 27.5°. The presence of the ‘bun peak’ confirms the successful introduction of amorphous Si3N4 through CVI. The XPS spectra indicate the presence of Si, N, C, Ti, O elements [Figure 4B]. There are two peaks in the Si 2p curve, at 100.1 eV and 101.01 eV, attributed to Si-O and Si-N bonds, respectively [Figure 4C]. There are two peaks in the N 1s curve, at 398.4 eV and 400.61 eV, which correspond to N-Si and N-C bonds, respectively, and are mainly in the form of N-Si bonds [Figure 4D]. We can see four peaks clearly in the C 1s curve, 284.85 eV corresponds to C-C, 286.75 eV corresponds to C-O-C, 288.54 eV corresponds to C=C and 290.06 eV corresponds to C-C=O [Figure 4E]. There are four peaks in the Ti 2p curve, 453.72 eV corresponds to Ti3+2p3/2, 457.02 eV corresponds to Ti4+2p3/2, 459.8 eV corresponds to
Figure 4. Microstructure of Ti2CTX and Ti2CTX/Si3N4. (A) The XRD images of Ti2CTX and Ti2CTX/Si3N4; (B)The XPS images of
Dielectric properties and microwave absorption performance
The properties of electromagnetic wave-absorbing materials are described using electromagnetic parameters, including complex permittivity (εr = ε’ - jε’’) and complex permeability (μr’ = μ’ - jμ’’)[59,60]. The real and imaginary parts of the dielectric constant reflect the stored and lost energy of the electromagnetic wave, respectively. We tested and compared the electromagnetic parameters of two structures: a biomimetic layered structure [Figure 5A(1)] and a porous structure [Figure 5B(1)] at 8.2-12.4 GHz. The dielectric constant (εr = ε’ - jε’’) was measured to demonstrate dielectric properties within the frequency range of 8.2-12.4 GHz [Figure 5A(2)]. The ε’ and ε’’ of Ti2CTX/Si3N4 aerogel with layered biomimetic structure exhibited a similar trend at 8.2-10.2 GHz. In this frequency range, ε’ decreases from 2.6 to 2.3, while ε’’ decreases from 2.2 to 1.8. In the 9.6-10.2 GHz frequency range, ε’ increases slightly, and εr shows an overall decreasing trend with increasing frequency, which is related to the frequency dispersion effect and caused by polarization relaxation. When the electric field frequency is too high, the dipole cannot align with the electric field for a short period. This leads to weak polarization relaxation and consequently a decrease in the dielectric constant. The μ’ and μ’’ of Ti2CTX/Si3N4 aerogel are 0 and 1, respectively [Supplementary Figure 5], indicating that the aerogel is non-magnetic. Generally, excellent electromagnetic wave absorption performance depends primarily on the dielectric and magnetic losses of the material. For non-magnetic Ti2CTX/Si3N4 aerogels, electromagnetic wave absorption mainly depends on dielectric loss. The ε’ and ε’’ of aerogels with unidirectional porous structure gradually decrease between 8.2 and 12.4 GHz [Figure 5B(2)]. The real part decreases from 1.2 to 0.6, and the imaginary part decreases from 0.9 to 0.3. Comparing the dielectric constants of the two structures reveals that the real and imaginary parts of the layered biomimetic Ti2CTX/Si3N4 aerogel structure are greater than those of the porous structure.
Figure 5. Dielectric and microwave absorbing properties of Ti2CTX/Si3N4. (A1) Bidirectional structure: electromagnetic waves are incident along the Z-axis; (A2) The ε’ and ε’’ of Bidirectional structure; (A3) The 3D RL values of Bidirectional structure; (A4) RL of Bidirectional structure at different thicknesses; (B1) Unidirectional structure: electromagnetic waves are incident along the Z-axis; (B2) The ε’ and ε’’ of Unidirectional structure; (B3) The three-dimensional RL values of Unidirectional structure; (B4) RL of Unidirectional structure at different thicknesses; (C) Three-dimensional radar wave scattering signals of Ti2CTX/Si3N4. 3D: three-dimensional; RL: reflection loss.
Reflection loss (RL) is an important parameter that directly reflects wave absorption performance. RL less than -10 dB usually indicates that more than 90% of the incident electromagnetic wave can be absorbed, with the corresponding frequency range being the EAB. RL less than -20 dB usually indicates that more than 99% of the electromagnetic wave can be absorbed. RL can be less than -10 dB within a thickness range of 4.7-8 mm, frequency in the range of 8-12.4 GHz, enabling effective electromagnetic wave absorption
To evaluate the dielectric loss capability, the tangent of the dielectric loss was calculated based on the dielectric parameters [Figure 6A]. Within the 8.2-12.4 GHz frequency range, the tangent loss of the layered biomimetic structure ranged from 0.82 to 0.63, and that of the porous structure ranged from 0.69 to 0.56. Distinct peaks in the dielectric loss tangent of the bidirectional structure appear at 9.5 GHz and 10.5 GHz [Figure 6A]. These maxima arise from a synergistic resonance between dipolar polarization (from -OH, -F, and other functional groups on Ti2CTX) and interfacial polarization at the low-dielectric Si3N4 interface[61]. The attenuation coefficient refers to the ability of electromagnetic waves to be attenuated when entering a material[62,63]. Ti2CTX/Si3N4 aerogel with a biomimetic layered structure has a better attenuation coefficient from 100-120 at 8.2-12.4 GHz [Figure 6B], and the attenuation coefficient increases overall with increasing frequency. Aerogel with a porous structure has an attenuation coefficient about 60 at 8.2-12.4 GHz, this indicates that electromagnetic waves are effectively dissipated in the Ti2CTX interlayer. Excellent electromagnetic wave absorption performance can only be achieved by having suitable impedance matching and strong attenuation capability at the same time. Impedance matching (Z = Zi/Z0) determines whether the electromagnetic wave can enter the material system; impedance matching is better when Z is closer to 1[64,65]. Introducing electromagnetic wave transparent Si3N4 allows electromagnetic waves to enter the interior of the material rather than being reflected at the surface, effectively optimizing the impedance matching of Ti2CTX. For the bidirectional structure, Z remains within 0.4-1.2 across the entire 8.2-12.4 GHz band and stays very close to the ideal value of 1 for all investigated thicknesses [Figure 6C]. By contrast, the unidirectional structure exhibits Z in the range 0.8-3 over the same frequency range, progressively deviating from 1 as thickness increases [Supplementary Figure 9]. The superior impedance matching in the bidirectional structure originates from the graded-index transition enabled by the low-permittivity Si3N4 outer layers, which markedly mitigates the abrupt impedance mismatch between air and the highly conductive Ti2CTX layers.
Figure 6. Ti2CTX/Si3N4 microwave-absorption mechanism. (A) The dielectric loss tangent of different structures; (B) The attenuation coefficient of different structures, (C) The Z values of Ti2CTX/Si3N4; (D) The cole-cole curve of Ti2CTX/Si3N4; (E) The calculated conductive loss and polarization loss; (F) Comparison of EAB and RL of Ti2CTX/Si3N4 aerogel with the reported literature. EAB: Effective absorption bandwidth; RL: reflected loss.
According to the Debye theory, the dielectric behavior of Ti2CTX/Si3N4 aerogels can be evaluated using Cole-Cole curves[66]. The relationship between ε’ and ε’’ is semicircular, with each semicircle representing a Debye relaxation process. The relationship between ε’ and ε’’ of Ti2CTX/Si3N4 aerogel shows multiple distorted semicircles [Figure 6D], implying the existence of multiple modes of polarization loss. The curves in Figure 6D show multiple twisted semicircles, which implies the existence of multiple modes of polarization loss. These multiple relaxation processes may originate from dipole and interface polarization. Functional groups, structural defects, and a disordered lattice can act as dipole centers, inducing the generation of dipole polarization in an alternating electromagnetic field. Additionally, the heterogeneous junction surface between Ti2CTX and Si3N4 facilitates polarization loss at the interface. The Cole-Cole curve of the
When compared with other MXene-based electromagnetic wave absorbing materials, the Ti2CTX/Si3N4 aerogel produced in this study exhibits significant advantages [Figure 6F], including a broad EAB and robust absorption properties.
The electromagnetic wave absorbing mechanism of the prepared Ti2CTX/Si3N4 aerogel mainly includes the following [Supplementary Figure 10]: (1) Interfacial polarization loss: The abundant surface functional groups on Ti2CTX, combined with the low dielectric constant of Si3N4 [Supplementary Table 3], work synergistically at the Ti2CTX/Si3N4 interface. The significant disparity in electrical conductivity and dielectric constant between the two materials leads to intense charge accumulation and rearrangement under an electric field, resulting in the formation of macroscopic electric dipole moments[68]. This loss mechanism converts electromagnetic wave energy into thermal energy via polarization relaxation, thereby achieving effective absorption of electromagnetic waves. Additionally, the interfacial polarization between Ti2CTX and Si3N4 significantly enhanced the electromagnetic wave-absorbing properties of the composite; (2) Multiple reflections and scattering: the layered porous structure of the Ti2CTX/Si3N4 aerogel promotes multiple reflections and scattering of electromagnetic waves within the material[69]. This structural design significantly increases the propagation path of electromagnetic waves[70], thereby improving their energy dissipation efficiency; (3) Impedance matching: The low-dielectric-constant Si3N4 outer layers form a graded-index transition zone[71], which significantly mitigates the abrupt impedance mismatch between air and the highly conductive Ti2CTX layers, thereby markedly reducing surface reflection and increasing the depth of electromagnetic wave penetration and absorption efficiency[72]; (4) Synergistic effect: Ti2CTX exhibits high electrical conductivity. The conductive network formed by Ti2CTX nanosheets provides pathways for electron migration, generating conduction currents that dissipate electromagnetic wave energy through a conductive loss mechanism[33]. When Ti2CTX is compounded with Si3N4, this mechanism is retained and enhanced by the structural optimization of Si3N4. The combination of the high electrical conductivity of Ti2CTX and the low dielectric constant of Si3N4 results in an efficient electromagnetic wave absorption system[73-75].
Mechanical properties of Ti2CTX/Si3N4 aerogel with bidirectional structure
The freeze-dried Ti2CTX/Si3N4 aerogel possesses a porous structure that results in a low density. This allows it to be easily placed on plant leaves without significantly bending them [Supplementary Figure 11A]. The combination of the aerogel's anisotropic porous structure and the deposited Si3N4 imparts considerable strength to the Ti2CTX/Si3N4 aerogels. This enables the aerogel to sustain multiple times its own weight without fracturing [Supplementary Figure 11B]. When a maximum compressive force of 15.69 kN was applied, the compressive modulus of elasticity was found to be 14.56 GPa, while the compressive strength was found to be 315 MPa. This indicates that the Ti2CTX/Si3N4 aerogel has excellent mechanical properties
CONCLUSIONS
In summary, Ti2CTX aerogel with aligned lamellar porous structure was prepared using a bidirectional freeze-drying process. Subsequently, amorphous Si3N4 was infiltrated into the Ti2CTX using the Chemical Vapor Infiltration technique to produce a bionic, porous Ti2CTX/Si3N4 aerogel with an intercalated lamellar structure. The experimental results show that, with a Ti2CTX concentration of just 0.21 wt.%, the
DECLARATIONS
Acknowledgments
We are grateful to Gao Qianwen (Analytical & Testing Center of NPU) for her help in the microstructure characterization.
Authors’ contributions
Wrote the original draft: Xu, H.; Jing, C.; Xu, Z.; Zhan, H.
Supervised, reviewed, and revised the manuscript: Ye, F.; Chen, Q.; Zhu, M.; Kong, L.; Li, X.; Chai, X.; Qing, Y.; Fan, X.; Luo, F.
Availability of data and materials
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Financial support and sponsorship
This work was supported by the National Natural Science Foundation of China (Grants Nos. 52302367, 52203094), the National Key Laboratory of Electromagnetic Information Control and Effects Open Fund (Grants No. SYS1W2023010304), and the State Key Laboratory of Solidification Processing in NPU (Grant No. 2025-TS-08).
Conflict of Interest
Xia Chai is affiliated with Shaanxi Huaqin Technology Industry Co., Ltd, while the other authors have declared that they have no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
Supplementary Materials
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