Significantly Enhanced Electromagnetic Interference Shielding Performances of Epoxy Nanocomposites with Long-Range Aligned Lamellar Structures

Highlights Ti3C2Tx@Fe3O4/CNF aerogels (BTFCA)/epoxy electromagnetic interference (EMI) shielding nanocomposites with long-range aligned lamellar structures were prepared by bidirectional freezing, freeze-drying and vacuum-assisted impregnation of epoxy resins. Successful construction of 3D long-range aligned lamellar structures and electromagnetic synergistic effect could significantly increase the EMI shielding effectiveness and reduce the secondary contamination. BTFCA/epoxy EMI shielding nanocomposites possessed outstanding EMI shielding effectiveness of 79 dB, and also presented excellent thermal stabilities and mechanical properties. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-022-00949-8.


Introduction
With the rapid development of modern electronic information technology, especially for aerospace weapons and equipment technology, electromagnetic interference (EMI) pollution problem caused by high-frequency and high-power electronic equipment is becoming increasingly serious. It poses serious threats to normal operation of precise electronic components and human health [1][2][3]. Polymer matrix EMI shielding composites have gradually become the most promising EMI shielding materials due to their advantages of lightweight, excellent specific strength, low cost, easy processing and adjustable performances [4][5][6].
To our knowledge, polymer matrix EMI shielding composites have achieved satisfactory EMI shielding performances by adding highly conductive and/or magnetic fillers [7-9]. The commonly used conductive fillers are metal, conductive polymers and inorganic nonmetallic materials [10,11]. Among them, inorganic nonmetallic materials such as the graphite [12], carbon nanotubes (CNT) [13,14], graphene [15][16][17] and MXene [18][19][20] are currently the focus of most attention, due to their advantages of high specific strength, low density, superior electrical conductivity (σ) and easy processing, etc. Herein, Ti 3 C 2 T x has been widely applied in the field of EMI shielding due to mature preparation technology and superior σ value [21][22][23]. However, Ti 3 C 2 T x nanosheets tend to agglomerate inner polymer matrix, and have large contact resistance between the nanosheets, leading to higher percolating threshold of the composites [24][25][26], which would cause machining difficulty and poor mechanical properties [27,28].
Researches show that construction of three-dimensional (3D) conductive networks is proved to be an effective way to synchronously realize the excellent σ, EMI shielding effectiveness (EMI SE) and mechanical properties of polymer composites at relatively low Ti 3 C 2 T x loadings [29][30][31]. Shi et al. [32] prepared Ti 3 C 2 T x aerogel by freeze-drying method, and further impregnated epoxy resins to prepare Ti 3 C 2 T x aerogel/epoxy composites. When the volume fraction of Ti 3 C 2 T x was 0.40 vol%, the EMI SE of Ti 3 C 2 T x aerogel/epoxy composites was 35 dB. Sun et al. [33] prepared PS@Ti 3 C 2 T x composites by electrostatic self-assembly and molding method. When the mass fraction of Ti 3 C 2 T x was 4.0 wt%, σ and EMI SE of PS@Ti 3 C 2 T x composites were 1081 S m −1 and 54 dB, respectively. In our previous work, Gu et al. [34] obtained cellulose-derived carbon aerogel@ reduced graphene oxide aerogels (CCA@rGO) by freezedrying and thermal reduction, and further prepared CCA@ rGO/polydimethylsiloxane (PDMS) composites by vacuumassisted impregnation of PDMS. When the mass fraction of CCA@rGO was 3.05 wt%, the σ and EMI SE of the obtained CCA@rGO/PDMS composites reached 75 Sm −1 and 51 dB, respectively.
Compared with the randomly dispersed 3D conductive networks, the aligned 3D conductive networks are not only conducive to further improving the σ [35][36][37], but also can make efficient utilization of the conductive fillers/polymer interfaces to enhance the reflection and reabsorption of electromagnetic waves [38][39][40]. Wu et al. [41] prepared Ti 3 C 2 T x foams by directional freezing and further impregnated PDMS to prepare Ti 3 C 2 T x foam/ PDMS composites. When the mass fraction of Ti 3 C 2 T x was 6.1 wt%, the σ and EMI SE of Ti 3 C 2 T x foam/PDMS composites were 2211 S m −1 and 54 dB, respectively. Zhao et al. [15] prepared Ti 3 C 2 T x /graphene hybrid aerogels (MGA) by directional freezing, and further impregnated epoxy resins to prepare MGA/epoxy composites. When the volume fraction of graphene and Ti 3 C 2 T x was 0.18 and 0.74 vol%, the σ and EMI SE of MGA/epoxy composites were up to 695.9 S m −1 and 50 dB, respectively. Compared with the directional aligned 3D conductive networks, the bidirectional aligned 3D conductive networks can further reduce the percolating threshold and enhance the attenuation of electromagnetic waves by taking advantage of more regular internal interfaces [42][43][44]. Han et al. [45] prepared Ti 3 C 2 T x foams by bidirectional freezing method. The density of Ti 3 C 2 T x foams was only 11.0 mg cm −3 , EMI SE and SE/density (SSE) reached 71 dB and 8818 dB cm 3 g −1 respectively, which exceeded the shielding performances of most reported foams. Sambyal et al. [46] prepared Ti 3 C 2 T x /CNT foams by bidirectional freezing method. Results showed that the EMI SE of Ti 3 C 2 T x /CNT foams (with density of only 2.5 mg cm −3 ) was up to 78 dB. Bidirectional aligned 3D Ti 3 C 2 T x foam and Ti 3 C 2 T x /CNT foam have been reported to exhibit excellent EMI shielding performances, but relatively poor mechanical properties have limited their 1 3 broader applications. Cellulose nanofibers (CNF) possess excellent mechanical properties [47,48], and could be employed to construct bidirectional aligned 3D conductive networks with Ti 3 C 2 T x via bidirectional freezing method by hydrogen bonds, which would be favor of enhancing the mechanical properties of Ti 3 C 2 T x foams [49][50][51].
In addition, for common polymer matrix EMI shielding composites, most electromagnetic waves are reflected at the interfaces between composites and air due to impedance mismatch, which would cause electromagnetic pollution to the service environment [52][53][54]. Researches show that the introduction of magnetic materials would improve the impedance matching of composites and air, weaken the reflection of electromagnetic waves, and absorb electromagnetic waves through magnetic loss [55][56][57]. Among commonly magnetic fillers, Fe 3 O 4 shows great application potential in EMI shielding materials due to excellent magnetism and low cost [58][59][60].
In this work, Ti 3 C 2 T x and Fe 3 O 4 were firstly assembled by electrostatic interaction, followed by combined with CNF through hydrogen bonding, and bidirectional aligned Ti 3 C 2 T x @Fe 3 O 4 /CNF aerogels (BTFCA) were then prepared by bidirectional freezing and freeze-drying technique. Finally, the BTFCA/epoxy nanocomposites were prepared by vacuum-assisted impregnation of epoxy resins. Structures and morphologies of BTFCA were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Furthermore, the effects of Fe 3 O 4 loadings on σ, EMI SE, thermal stabilities and mechanical properties of the BTFCA/epoxy nanocomposites were analyzed in detail, and the corresponding EMI shielding mechanism was investigated.

Fabrication of BTFCA
Fe 3 O 4 was positively modified by CTAB, followed by mixing with Ti 3 C 2 T x dispersion and freeze-dried (−30 °C, < 2 Pa) for 36 h to get Ti 3 C 2 T x @Fe 3 O 4 hybrid. Different contents of Ti 3 C 2 T x @Fe 3 O 4 were dispersed in 10 mL of 2.5 mg mL −1 CNF in a glass vessel by a probe sonication for 10 min in an ice bath, followed by vigorous stirring for 3 h. Then the dispersion was poured in a square mold (side length of 3 cm, height of 5 cm, PDMS wedge with a slope angle of around 15° at the bottom, copper as bottom, nylon as wall), and liquid nitrogen (− 196 °C) was used to freeze the bottom of the cylindrical mold through the intermediary of copper blocks. BTFCA was obtained by freeze-drying at −60 °C with pressure less than 5 Pa, followed by annealed at 400 °C for 2 h at a heating rate of 5 °C s −1 in an Ar + 5% H 2 ambient. Ti 3 C 2 T x content was fixed as 400 mg, the weight ratio of Ti 3 C 2 T x /Fe 3 O 4 was 4/1, 2/1, 1/1, and 1/2, respectively, and the corresponding samples were marked as BTFCA-1, BTFCA-2, BTFCA-3 and BTFCA-4. For comparison, the bidirectional Ti 3 C 2 T x / CNF aerogels (BTCA) were also prepared.

Fabrication of BTFCA/epoxy Nanocomposites
Epon 862 and diethyl methyl benzene diamine were firstly stirred at 70 °C for 1 h, and then filled into BTFCA via vacuum-assisted impregnation technique. Finally, BTFCA/epoxy nanocomposites were prepared by heating at 120 °C for 5 h. For comparison, BTCA/epoxy nanocomposites were prepared by the same process. Figure 1 is the schematic diagram of preparation for BTFCA/epoxy nanocomposites.

Characterization of BTCA and BTFCA
In Fig. S1, Ti 3 AlC 2 precursor with dense layered structure ( Fig. S1a) is exfoliated into few-layered Ti 3 C 2 T x nanosheets (Fig. S1b). In Fig. S2, Fe 3 O 4 presents (220), (311), (400), (422), (511), and (440) diffraction peaks [61], and corresponding saturation magnetization is 70 emu g −1 . After electrostatic assembly, Fe 3 O 4 is uniformly dispersed on Ti 3 C 2 T x nanosheets to obtain Ti 3 C 2 T x @Fe 3 O 4 (Fig. S3). Figure 2 shows XRD, Raman, XPS spectra and hysteresis loops of BTCA and BTFCA-2. From Fig. 2a, BTCA shows diffraction peaks at 6.2° and 23° corresponding to (002) lattice plane of Ti 3 C 2 T x and (002) lattice plane of CNF [62]. After Fe 3 O 4 is introduced, three new diffraction peaks appear at 36.7°, 43.6°, and 63.8° of BTFCA-2, corresponding to the (311), (400) and (440) crystal planes of Fe 3 O 4 , respectively. As observed in Fig. 2b, BTCA has D-band and G-band at 1355 and 1583 cm −1 , which are attributed to the graphitized structures [63,64] formed after thermal reduction of CNF, and characteristic peaks of Ti 3 C 2 T x appear within 100 ~ 700 cm −1 . Peak at 198 cm −1 is attributed to A1g symmetry out-of-plane vibration of Ti atom. Peaks at 374 and 583 cm −1 are ascribed to the Eg group vibration and in-plane (shear) modes of Ti, C, and surface functional groups [65]. After Fe 3 O 4 is introduced, a new peak appears in BTFCA-2 at 655 cm −1 , which is attributed to the vibration of Fe 3 O 4 in A1g mode. As shown in Fig. 2c, BTCA presents peaks of Ti 3p, Ti 3s, C 1s, Ti 2p, O 1s, Ti 2s, and F 1s at 35, 60, 287, 457, 531, 563, and 685 eV [66]. After Fe 3 O 4 is introduced, BTFCA-2 shows two new peaks at 709 and 723 eV, which are attributed to Fe 2p 3/2 and Fe 2p 1/2 , respectively. From Fig. 2d, the saturation magnetization of BTCA is 0, and the saturation magnetization of BTFCA-2 significantly improves to 16.7 emu g −1 . XRD, Raman spectroscopy, XPS and hysteresis loops indicate that BTFCA has been successfully prepared. Figure 3 is the SEM images of BTCA and BTFCA, as well as the photo of BTFCA-2 attracted on the magnet. From Fig. 3a, Ti 3 C 2 T x and CNF in BTCA support each  Fig. 3d-e). The reason is that, in bidirectional freezing process, the temperature difference makes ice crystals grow orderly in both radial and axial directions at the same time. between Ti 3 C 2 T x and CNF, which reduces the overlap between Ti 3 C 2 T x and CNF, resulting in great increase in cell size of BTFCA. In addition, BTFCA-2 can overcome its own gravity by magnetic force and attract to the magnet (Fig. 3f), indicating that the introduction of Fe 3 O 4 endows BTFCA with outstanding magnetism. Figure S4 shows the SEM images of BTCA/epoxy and BTFCA-2/epoxy nanocomposites. Both BTCA/epoxy and BTFCA-2/epoxy nanocomposites can well maintain the original long-range aligned lamellar structures, indicating that the mechanical properties are strong enough to resist the adhesion force generated by impregnation of epoxy resins and maintain the structural integrity. On the one hand, Ti 3 C 2 T x nanosheets and CNF possess numbers of polar functional groups such as -OH and -F on the surface to form abundant hydrogen bonds, which is conducive to enhancing the stiffness of BTFCA. On the other hand, the high rigidity of Ti 3 C 2 T x nanosheets and Fe 3 O 4 also endows BTFCA with great rigidity, which enables BTFCA/epoxy nanocomposites to maintain the integrity of the long-range aligned lamellar structures. Figure 4a shows the σ of BTCA/epoxy and BTFCA/epoxy nanocomposites, and the relevant values are shown in Tab S1. The σ of BTFCA/epoxy nanocomposites decreases gradually with increasing loadings of Fe 3 O 4 . When the mass fraction of Fe 3 O 4 is 1.48 wt%, the σ of BTFCA-2/ epoxy nanocomposites is 1235 S m −1 , lower than that of BTCA/epoxy (1306 S m −1 ) nanocomposites, and also significantly higher than that of blended Ti 3 C 2 T x @Fe 3 O 4 / epoxy (7.6 S m −1 , Tab. S1) nanocomposites with the same loadings of Ti 3 C 2 T x and Fe 3 O 4 . Highly conductive Ti 3 C 2 T x nanosheets are aligned along the axial and radial directions in BTCA and BTFCA to construct a bidirectional aligned 3D conductive network, which enhances the contact among Ti 3 C 2 T x nanosheets and forms abundant conductive paths, thus showing outstanding σ. The introduction of Fe 3 O 4 would affect the contact among Ti 3 C 2 T x nanosheets and hinder formation of Ti 3 C 2 T x -Ti 3 C 2 T x conductive paths, leading to slightly reduced σ of BTFCA/epoxy nanocomposites. The addition of excessive Fe 3 O 4 reduces the overlap among Ti 3 C 2 T x nanosheets and significantly increases the cell size of BTFCA, which severely restricts the formation of bidirectional aligned 3D conductive networks for BTFCA, resulting in significant decrease of σ. Figure 4b illustrates EMI SE of BTCA/epoxy and BTFCA/ epoxy nanocomposites. The EMI SE of BTFCA/epoxy nanocomposites increases first and then decreases with increasing loadings of Fe 3 O 4 . When the mass fraction of Fe 3 O 4 is 1.48 wt%, EMI SE of BTFCA-2/epoxy nanocomposites is 79 dB, 11.3% higher than that of BTCA/epoxy (71 dB) nanocomposites, and also about 10 times that of blended Ti 3 C 2 T x @ Fe 3 O 4 /epoxy (8 dB, Fig. S5) nanocomposites with the same Under alternating electric field, they can induce microcurrent to form electrical loss through tunneling effect and other ways, and convert the energy of electromagnetic waves into heat. At the same time, the internal complex heterogeneous interfaces in BTFCA/epoxy nanocomposites extend the transmission paths of electromagnetic waves, which are conducive to enhancing the scattering and reabsorption of electromagnetic waves and further dissipating electromagnetic waves. After introduction of Fe 3 O 4 , reduced overlaps among Ti 3 C 2 T x nanosheets lead to gradually decreased σ of BTFCA/epoxy nanocomposites and weakened electrical loss (such as ohmic loss) is not conducive to improvement of EMI SE. On the other hand, BTFCA/epoxy nanocomposites construct 3D magnetic networks, which enhances the multiple reflection and reabsorption of electromagnetic waves and strengthens the magnetic hysteresis loss and other magnetic losses of electromagnetic waves, so as to improve the dissipation ability of electromagnetic waves. In addition, the introduction of Fe 3 O 4 brings more heterogeneous interfaces. Due to interface polarization, there are a large number of dipoles at heterogeneous interfaces, which will cause polarization loss to electromagnetic waves and further enhance the attenuation of electromagnetic waves. As a result, BTFCA-2/epoxy nanocomposites present the best EMI shielding performances. And the corresponding schematic illustration of EMI shielding mechanism of BTFCA/ epoxy nanocomposites is shown in Fig. 5. Figure 4c is SE T , SE A and SE R of BTCA/epoxy and BTFCA/epoxy nanocomposites. With increasing loadings of Fe 3 O 4 , SE R of BTFCA/epoxy nanocomposites decreases gradually, and SE A increases first and then decreases. When the mass fraction of Fe 3 O 4 is 1.48 wt%, SE R and SE A of BTFCA-2/epoxy nanocomposites are 8 and 71 dB respectively. With increasing loadings of Fe 3 O 4 , the gradually decreased σ of BTFCA/epoxy nanocomposites improves the impedance matching, resulting in gradual decrease of SE R . Although the electrical loss of BTFCA/epoxy nanocomposites to electromagnetic waves is gradually weakened, the internal multiple reflection, magnetic loss and polarization loss are gradually enhanced. As a result, SE A of BTFCA-2/ epoxy nanocomposites is the maximum. Figure 4d shows the electromagnetic wave effective absorbance of BTCA/epoxy and BTFCA/epoxy  Figure 4e is the reflection (R), absorption (A) and transmission (T) coefficients of BTCA/epoxy and BTFCA/epoxy nanocomposites. With increasing loadings of Fe 3 O 4 , the R coefficient of BTFCA/epoxy nanocomposites decreases gradually, and the T coefficient decreases first and then increases. When the mass fraction of Fe 3 O 4 is 1.48 wt%, the T coefficient of BTFCA-2/epoxy nanocomposites is the lowest, only 4 × 10 -4 , and the R coefficient is 0.78. It demonstrates that the incorporation of Fe 3 O 4 can significantly improve the EMI shielding performances. With increasing loadings of Fe 3 O 4 , the σ of BTFCA/epoxy nanocomposites decreases gradually, which improves the impedance matching between BTFCA/epoxy nanocomposites and the air, and reduces the R coefficient gradually, thus weakening the secondary electromagnetic pollution. With introduction of Fe 3 O 4 , although the decreased σ leads to reduced electrical loss to electromagnetic waves, the internal multiple reflection, magnetic loss and interfacial polarization loss are enhanced. Under the comprehensive action, BTFCA-2/ epoxy nanocomposites show the optimal EMI shielding performances and the corresponding T coefficient is the lowest. Figure 4f demonstrates EMI SE of BTFCA-2/epoxy nanocomposites in different thicknesses, and the EMI SE of BTFCA/epoxy nanocomposites increases with increasing thicknesses. When the thickness increases from 1 to 2 mm, the EMI SE of BTFCA-2/epoxy nanocomposites increases from 34 to 79 dB. This is because the increased thickness is in favor of lengthening the propagation path of electromagnetic waves in BTFCA/epoxy nanocomposites, which is conducive to the scattering and reabsorption of electromagnetic waves to further enhance EMI shielding performances.  Figure 6 presents the thermogravimetric analysis (TGA) curves of BTCA/epoxy and BTFCA/epoxy nanocomposites, and the corresponding thermal data are shown in Tab. S2. T heat-resistance index (T HRI ) can reflect the heat resistance of the materials [67]. It can be seen that

Conclusion
Bidirectional aligned BTFCA and the corresponding BTFCA/epoxy nanocomposites with long-range aligned lamellar structures were successfully prepared. Benefitting from the successful construction of bidirectional aligned 3D Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.