Epoxy nanocomposites for electromagnetic interference shielding application

Composites Part B 171 (2019) 111–118

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Fabrication on the annealed Ti3C2Tx MXene/Epoxy nanocomposites for electromagnetic interference shielding application Lei Wang, Lixin Chen **, Ping Song, Chaobo Liang, Yuanjin Lu, Hua Qiu, Yali Zhang, Jie Kong, Junwei Gu * MOE Key Laboratory of Material Physics and Chemistry Under Extraordinary Conditions, Shaanxi Key Laboratory of Macromolecular Science and Technology, Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’ an, Shaanxi, 710072, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Ti3C2Tx MXene Thermal reduction Epoxy electromagnetic shielding nanocomposites

Few-layered Ti3C2Tx MXene was fabricated by ionic intercalation and sonication-assisted method, followed by thermal reduction at medium-low temperature. Then annealed Ti3C2Tx/epoxy electromagnetic interference (EMI) shielding nanocomposites were obtained by solution casting method. XRD, SEM, AFM and TEM indicated the successful preparation of few-layered Ti3C2Tx. FTIR, XPS and XRD showed that thermal reduction removed partial polar groups on the surface of Ti3C2Tx with no by-product. For a fixed Ti3C2Tx loading, compared with Ti3C2Tx/epoxy EMI shielding nanocomposites, the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites exhibited relatively higher electrical conductivity and excellent EMI shielding effectiveness (SE). When the mass fraction of annealed Ti3C2Tx was 15 wt%, the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites presented the optimal electrical conductivity of 105 S/m and EMI SE of 41 dB, 176% and 37% higher than that of 15 wt% Ti3C2Tx/epoxy EMI shielding nanocomposites. Furthermore, the 5 wt% annealed Ti3C2Tx/epoxy EMI shielding nanocomposites exhibited the optimal Young’s modulus of 4.32 GPa and hardness of 0.29 GPa, respectively.

1. Introduction With the electronic equipments wildly used in modern society, the detriment of electromagnetic pollution to people’s health and safety is becoming more and more severe [1]. Therefore, the researches on the electromagnetic interference (EMI) shielding materials have aroused extensive attention [2,3]. In comparison to traditional metal-based EMI shielding materials, polymer EMI shielding composites are vast potential [4] due to the ad­ vantages of lightweight, corrosion resistance and excellent processibility [5]. By means of one-time processing and adjustable EMI shielding performance, simply blending resin and conductive fillers is considered as an effective way. In comparison to other conductive fillers (such as carbon black [6], carbon fibers [7], mesocarbon microbeads [8], poly­ pyrrole [9]), graphene [10,11] and carbon nanotubes [12,13] have become mainstream of conductive fillers, due to their high specific surface area [14] and electrical conductivity [15]. Liang et al. [16] prepared 15 wt% graphene/epoxy nanocomposites with EMI SE value of 21 dB at X-band (8.2–12.4 GHz). Huang et al. [17] reported that the EMI

SE value of 15 wt% single-walled carbon nanotubes/epoxy nano­ composites at X-band reached 24 dB. Since 2011, Naguib et al. [18] prepared a class of two-dimensional (2D) nanomaterials–transition metal carbide/carbonitride (MXene). It possesses extraordinary electrical conductivity and specific surface area [19] comparable to graphene, as well as hydrophobicity characteristics [20], being a research hotspot in the EMI shielding field [21,22]. Ti3C2Tx (T represents the functional groups, including –OH, -O and –F groups) with mature preparation technology is commonly explored [23–25]. Shahzad et al. [26] reported that Ti3C2Tx film (45 μm in thickness) had a staggering EMI SE value of 92 dB. Liu et al. [27] fabricated Ti3C2Tx/poly (3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) film with EMI SE value of 42 dB, much higher than that of pure PEDOT: PSS film. In addition, the removal of polar groups on the surface of Ti3C2Tx can further improve the electrical conductivity and EMI SE of Ti3C2Tx. The methods for removing the surface groups of Ti3C2Tx mainly include chemical and thermal reduction. Liu et al. [28] reduced Ti3C2Tx film by hydrazine hydrate. After the removal of moiety of polar groups on the surface of Ti3C2Tx, the EMI SE value of the Ti3C2Tx film was improved to

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Chen), [email protected], [email protected] (J. Gu). https://doi.org/10.1016/j.compositesb.2019.04.050 Received 25 March 2019; Received in revised form 25 April 2019; Accepted 30 April 2019 Available online 1 May 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. SEM images of (a) Ti3AlC2 and (b) Ti3C2Tx nanosheets; (c) XRD patterns of Ti3AlC2 and Ti3C2Tx; (d) AFM image of Ti3C2Tx nanosheets; (e) TEM image and (f) SEAD pattern of crumpled Ti3C2Tx nanosheets (marked by red circle). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

70 dB, 32% higher than that of pristine Ti3C2Tx film. Wang et al. [29] reported that partial polar groups such as -F and -OH on the surface of Ti3C2Tx were removed after thermal reduction at high temperatures, but produced by-products such as TiO2. The use of chemical reducing agents often contaminates the environment. Under this background, thermal reduction has become a common method [30,31] due to its adjustable reaction temperature and time. Visibly, MXene presents great potential in EMI shielding films due to the excellent electrical conductivity. However, the EMI SE of the MXene/polymer EMI shielding composites are relatively lower, and the correlative investigations have hardly been reported, mainly ascribed to high volume fraction of insulating polymer matrix and difficulty in forming effective conductive pathways with random distributed MXene. Due to excellent chemical stability [32], wonderful mechanical properties [33] and low cost [34], epoxy resin has been widely used in the electronic devices, machinery manufacturing, aerospace and other fields. However, the intrinsic EMI SE value of pure epoxy resin is only about 2 dB, which cannot meet the requirements of EMI shielding. Therefore, it is expected to fabricate epoxy EMI shielding nano­ composites with large-scale production and adjustable properties by blending highly conductive Ti3C2Tx and epoxy matrix. MXene shows oxidation resistance at medium-low temperature to some extent, and it can be partially oxidized even under the protection of an inert environment at high temperatures, leading to generation of amorphous carbon, TiO2 and other by-products, which would reduce the electrical conductivity and EMI SE [35]. Hence, it is expected to ther­ mally reduce Ti3C2Tx at medium-low temperature, to improve the electrical conductivity while maintaining original structure with no by-products. In our present work, on the basis of ionic intercalation and sonication-assisted method to prepare few-layered Ti3C2Tx with high electrical conductivity, partial polar groups on the surface of Ti3C2Tx were also removed by thermal reduction at medium-low temperature, and the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites were then fabricated by solution casting method. The crystal form and mor­ phologies of the obtained Ti3C2Tx nanosheets were characterized by Xray diffraction (XRD), scanning electron microscope (SEM), atomic force

microscope (AFM) and transmission electron microscopy (TEM). And the structure and performance of the Ti3C2Tx nanosheets before and after thermal reduction were also investigated by Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS) and XRD. Furthermore, the mass fraction of annealed Ti3C2Tx influencing on the electrical conductivities, EMI shielding performances and mechanical properties of the annealed Ti3C2Tx/epoxy EMI shielding nano­ composites were analyzed and investigated in detail. 2. Experiment section 2.1. Main materials Ti3AlC2 powder (38 μm, 98% purity) was bought from 11 Technol­ ogy Co., Ltd. (Jilin, China). Lithium fluoride (LiF, 99% purity) and concentrated HCl were both received from Macklin Inc. (Shanghai, China). Bisphenol F epoxy (Epon 862) was provided by Hexion Inc. (Columbus, USA) and the curing agent of diethyl methyl benzene diamine was supplied by Baiduchem Co., Ltd. (Hubei, China). 2.2. Fabrication of annealed Ti3C2Tx Ti3C2Tx was fabricated by etching Ti3AlC2 powder with LiF/HCl. Generally, 2.0 g of LiF was completely dissolved in 40 mL of 9 M HCl. Then 2.0 g of Ti3AlC2 powder was slowly added into the above mixed solution in an ice bath within 10 min. Subsequently, the reaction was kept at 35 � C for 24 h under stirring. Resulting products were washed repeatedly with deionized water via centrifugation (3500 rpm for 5 min per cycle) until the pH reached about 7. Ti3C2Tx sediment was then dispersed in 100 mL of ethanol and sonicated by a probe sonicator (300 W) for 60 min to delaminate the multilayered Ti3C2Tx, followed by centrifugation at 10000 rpm for another 20 min. Whereafter, Ti3C2Tx slurry was dispersed in 100 mL of deionized water and further sonicated for 20 min. Finally, the few-layered Ti3C2Tx was obtained by centrifu­ gation at 3500 rpm for another 1 h and dried by lyophilization of the supernatant. Finally, the annealed Ti3C2Tx was fabricated by thermally annealing Ti3C2Tx at 200 � C for 2 h in an Ar þ 5% H2 atmosphere. 112

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Fig. 2. Ti3C2Tx and annealed Ti3C2Tx. (a) FTIR; (b) XRD. Inset: the enlarged local area between 5 and 10� ; (c) XPS.

2.3. Fabrication of annealed Ti3C2Tx/epoxy EMI shielding nanocomposites

indentations were conducted for each sample. 3. Results and discussion

Epon 862 and curing agent were mixed in acetone solution, followed by adding annealed Ti3C2Tx suspention, and mechanically stirred for 1 h at 70 � C. After that, the mixtures were poured into a mould to bring about the complete solvent evaporation at 70 � C under vacuum, and then cured at 120 � C for 5 h to fabricate the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites. Herein, the Ti3C2Tx/epoxy EMI shielding nanocomposites were prepared by the same process.

3.1. Ti3AlC2 and Ti3C2Tx Fig. 1(a) and Fig. 1(b) showed the SEM images of the Ti3AlC2 and Ti3C2Tx nanosheets, respectively. Ti3AlC2 had a dense lamellar struc­ ture, and the achieved Ti3C2Tx presented regular two-dimensional nanosheets. As shown in Fig. 1(c), the (104) peak at 39� of the Ti3AlC2 completely disappeared, indicating that Al element had been successfully etched [36]. The (002) peak moved from the original 9.70� to 6.58� , evincing that the layer spacing increased from 1.10 nm to 1.47 nm, due to the intercalation of water and Liþ. In addition, there were likewise five peaks located at 13.58� , 18.42� , 28.52� , 35.32� and 42.58� , corresponded to the ((004), (006), (008), (010) and (012)) crystal planes [37]. According to the AFM image (Fig. 1(d)), the size and thickness of the Ti3C2Tx nanosheets was respectively around 500 nm and 2.48 nm, indicating few-layered Ti3C2Tx nanosheets (2–3 layers). Fig. 1 (e) depicted the TEM image of the Ti3C2Tx nanosheets. Ultra-thin Ti3C2Tx nanosheets were crumpled, mainly ascribed to its flexibility. In sharp contrast to copper grid, it meant that the Ti3C2Tx nanosheets were ultra-thin and highly transparent. The corresponding selected-area electron diffraction (SEAD) pattern image (Fig. 1(f)) showed highly crystal structure of the Ti3C2Tx nanosheets. All above analyses proved successful fabrication of the Ti3C2Tx nanosheets.

2.4. Characterizations SEM images of the samples were captured on a VEGA3-LMH equip­ ment (TESCAN Co., Czech Republic). TEM images of the samples were performed on a Talos F200X/TEM microscope (FEI Co., USA) operated at 200 kV. AFM images of the samples were collected by a Dimension Fast Scan AFM (Bruker Co., USA). FTIR spectra of the samples were measured on a Bruker Tensor 27 device (Bruker Co., Germany) with thin films on KBr. XPS analyses of the samples were investigated on a PHI5400 apparatus (PE Corp., England). XRD of the samples was carried out on a Shimadzu-7000 type X-ray diffraction (λ ¼ 0.154 nm, Shi­ madzu, Japan). Electrical conductivities of the samples were measured using RTS-8 (Guangzhou Four Probes Technology Corp., China). EMI shielding performances of the samples were measured by an MS4644A Vector Network Analyzer instrument (Anritsu Corp., Japan), which used the wave-guide method in the X-band frequency range according to ASTMD5568-08, and the corresponding specimen dimension was 22.86 mm � 10.16 mm � 2 mm. Nanoindentation tests of the samples were performed by a Hysitron TI-980 TriboIndenter (Hysitron Co., USA). The peak indentation load was fixed at 9 mN and the unloading rate was 300 and 450 mN/s, respectively. The maximum load dwell time was set to 5 s. In order to get statistically significant results, at least 10

3.2. Annealed Ti3C2Tx FTIR spectra of the Ti3C2Tx and annealed Ti3C2Tx were illustrated in Fig. 2(a). In the FTIR spectrum of the Ti3C2Tx, the absorption peaks at 3440 cm 1 and 550 cm 1 were attributed to the stretching vibration of -OH group. The peak at 1710 cm 1 and 1400 cm 1 was caused by the 113

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fraction of Ti3C2Tx was 1 wt%, the electrical conductivity was increased to 0.119 S/cm (dramatic increase by several orders of magnitude), indicating the formation of conductive networks. With further increasing addition of Ti3C2Tx, the increasing trend of the electrical conductivities gradually strained. When the mass fraction of Ti3C2Tx was 15 wt%, the electrical conductivity of the Ti3C2Tx/epoxy EMI shielding nanocomposites reached the maximum value of 38 S/m. The change trend of the electrical conductivities for annealed Ti3C2Tx/epoxy EMI shielding nanocomposites was similar to that of Ti3C2Tx/epoxy EMI shielding nanocomposites, but increased more rapidly, which presented relatively slower percolation threshold and higher electrical conduc­ tivity. When the mass fraction of annealed Ti3C2Tx was 15 wt%, the electrical conductivity of the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites was up to 105 S/m. This was because that the lower addition of Ti3C2Tx was randomly distributed inner epoxy matrix, and Ti3C2Tx was commonly separated or wrapped by epoxy matrix, hardly to contact with each other (Fig. 4(a), (f)) [42]. Thus, it was difficult to form effective conductive networks, and the corresponding electrical con­ ductivities grew slowly. With increasing mass fraction of Ti3C2Tx, more contact of Ti3C2Tx could gradually form the conductive networks (Fig. 4 (b)-(c), (g)-(h)), so that the electrical conductivities increased rapidly. With further increasing addition of Ti3C2Tx, the conductive networks of Ti3C2Tx-Ti3C2Tx became more efficient (Fig. 4(d)-(e), (i)-(j)), and the ability to conduct electrons was further enhanced, leading to the optimal electrical conductivity. After thermal reduction, partial polar groups (such as the -F and -OH groups) on the surface of Ti3C2Tx were removed, so that the resistance of electrons in conduction was reduced and the final electrical conductivity was further improved. Total EMI SE (SET), absorption SE (SEA) and reflection SE (SER) at Xband were compared in Fig. 5(a–c). SET of the Ti3C2Tx/epoxy EMI shielding nanocomposites was improved with increasing addition of Ti3C2Tx, consistent with the change trend of electrical conductivity. The SET of the Ti3C2Tx/epoxy EMI shielding nanocomposites was increased from 4 dB (1 wt% Ti3C2Tx) to 30 dB (15 wt% Ti3C2Tx). The change trend of the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites was similar to that of Ti3C2Tx/epoxy EMI shielding nanocomposites. The SET was increased from 6 dB (1 wt% annealed Ti3C2Tx) to 41 dB (15 wt% annealed Ti3C2Tx). In addition, the SET of 5 wt% annealed Ti3C2Tx/ epoxy EMI shielding nanocomposites was 24 dB, exceeding the SET of 10 wt% Ti3C2Tx/epoxy EMI shielding nanocomposites (21 dB), which was mainly ascribed to the higher electrical conductivity. Definitely, with increasing addition of Ti3C2Tx, the Ti3C2Tx/epoxy EMI shielding nanocomposites presented higher electrical conductivity and more complex interfaces in favor of orientation polarization, endowing the Ti3C2Tx/epoxy nanocomposites with improved EMI SE.

Fig. 3. Electrical conductivities of the Ti3C2Tx/epoxy and annealed Ti3C2Tx/ epoxy EMI shielding nanocomposites.

– O and C–F groups [38,39], respectively. After thermal absorbance of C– reduction, the intensity of peak at 3440 cm 1 decreased and the peak at 1400 cm 1 disappeared, indicating that the content of -F and -OH groups on the surface of the annealed Ti3C2Tx nanosheets was decreased. Fig. 2 (b) showed the XRD patterns of the Ti3C2Tx and annealed Ti3C2Tx. After thermal reduction, the (002) peak of the annealed Ti3C2Tx shifted from 6.58� to 6.40� . Besides, there was no by-product diffraction peaks (e.g. TiO2), which indicated that thermal reduction was beneficial to the delamination of the Ti3C2Tx and the corresponding layer spacing was increased. Fig. 2(c) depicted the XPS spectra of the Ti3C2Tx and annealed Ti3C2Tx. The peak observed at 287 eV, 531 eV and 685 eV was assigned to O 1s, C 1s and F 1s, respectively [40], and the peak at 35 eV, 60 eV, 457 eV and 563 eV was corresponded to the characteristic peaks of Ti 3p, Ti 3s, Ti 2p and Ti 2s, respectively, consistent with the previous litera­ ture report [41]. The C/O ratio increased from 3.14 to 5.18 after thermal reduction, showed that the thermal reduction had removed partial oxygen-containing groups of Ti3C2Tx. Results confirmed that Ti3C2Tx underwent a certain degree of reduction with no by-product produced. 3.3. Ti3C2Tx/epoxy EMI shielding nanocomposites Fig. 3 depicted the electrical conductivities of the Ti3C2Tx/epoxy and annealed Ti3C2Tx/epoxy EMI shielding nanocomposites. The electrical conductivity of the pure epoxy resin was only 2.2 � 10 10 S/m. With the addition of Ti3C2Tx, the electrical conductivities of the Ti3C2Tx/epoxy EMI shielding nanocomposites increased significantly. When the mass

Fig. 4. SEM images of epoxy EMI shielding nanocomposites with (a) 1 wt% Ti3C2Tx, (b) 3 wt % Ti3C2Tx, (c) 5 wt% Ti3C2Tx, (d) 10 wt% Ti3C2Tx, (e) 15 wt% Ti3C2Tx; with (f) 1 wt% annealed Ti3C2Tx, (g) 3 wt % annealed Ti3C2Tx, (h) 5 wt% annealed Ti3C2Tx, (i) 10 wt% annealed Ti3C2Tx, (j) 15 wt% annealed Ti3C2Tx. 114

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Fig. 5. EMI shielding performances of (a) Ti3C2Tx/epoxy and (b) annealed Ti3C2Tx/epoxy EMI shielding nanocomposites; (c) SET, SEA and SER of the Ti3C2Tx/epoxy (Open symbol) and annealed Ti3C2Tx/epoxy EMI shielding nanocomposites (Solid symbol).

Fig. 6. Schematic illustration of shielding mechanism for annealed Ti3C2Tx/epoxy EMI shielding nanocomposites.

To further investigate the shielding mechanism, the SET, SEA and SER of the Ti3C2Tx/epoxy EMI shielding nanocomposites were depicted in Fig. 5(c). With increasing addition of Ti3C2Tx, the SET and SEA of the Ti3C2Tx/epoxy EMI shielding nanocomposites improved obviously, while SER increased slowly and was far less than SEA, illustrating that absorption played a main role in shielding mechanism. The enhanced SEA mainly arose from strengthened conductive networks with increasing addition of Ti3C2Tx, which improved the electrical loss of the Ti3C2Tx/epoxy EMI shielding nanocomposites to the electromagnetic

waves, dissipated as thermal energy [43]. SER usually depended on the interaction among the charge carriers from highly conductive Ti3C2Tx and entered electromagnetic waves. With further increasing addition of Ti3C2Tx, the increasing number of charge carriers promoted higher SER. It was also noticed that the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites showed higher SEA and SER than those of Ti3C2Tx/e­ poxy EMI shielding nanocomposites. This was attributed to the abun­ dant dipoles formed by removing functional groups after annealing Ti3C2Tx and stronger electron transport capability. 115

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Table 1 EMI shielding performance of previously reported polymer nanocomposites with similar composition. Nanocomposites

Filler Content

Thickness (mm)

Conductivity (S/m)

EMI SE (dB)

Frequency (GHz)

Refs

SWCNTs/epoxy RGO/epoxy MWCNT-Fe3O4@Ag/epoxy Ti3C2Tx/PEDOT:PSS Ti3C2Tx/MWCNTs Multilayered Ti3C2Tx/wax Annealed multilayered Ti3C2Tx/wax Annealed few-layered Ti3C2Tx/epoxy

15 wt% 15 wt% 15 wt% 88 wt% 50 wt% 60 wt% 90 wt% 15 wt%

2 2 2 0.011 0.049 2 1 2

20 10 28 34052 13000 – 1 105

25 21 35 42 3 39 32 41

8.2–12.4 8.2–12.4 8.2–12.4 8.2–12.4 8.2–12.4 2.0–18.0 8.2–12.4 8.2~12.4

[44] [45] [46] [27] [47] [48] [49] This work

Fig. 7. (a), (c) Representative load-displacements, (b), (d) Hardness and Young’s modulus of Ti3C2Tx/epoxy and annealed Ti3C2Tx/epoxy EMI shielding nanocomposites.

Fig. 6 presented the schematic illustration of electromagnetic waves transferring across the annealed Ti3C2Tx/epoxy EMI shielding nano­ composites. When incident waves stroked the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites, a portion of waves was reflected in the interfaces due to impedance mismatch and absorbed by interaction with charge carriers. The rest waves would be further reflected and reab­ sorbed for multiple times. With increasing addition of annealed Ti3C2Tx, the strengthened conductive networks gradually improved the ability to conduct electrons, further promoted the electrical loss of the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites to the electromagnetic waves. On the other hand, more interfaces extended the transmission path of electromagnetic waves in the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites, resulting in increased frequency of the mul­ tiple reflection and reabsorption of the electromagnetic waves by interaction with the abundant dipoles, which boosted the reabsorption

of the electromagnetic waves. Therefore, the electromagnetic waves were sufficiently attenuated and absorption was dominant in the shielding mechanism. Table 1 showed the comparison of EMI shielding performance of previously reported polymer nanocomposites with similar composition and the latest research progress of MXene based composites. The ob­ tained 15 wt% annealed Ti3C2Tx/epoxy EMI shielding nanocomposites in our work showed relatively higher EMI shielding performance (41 dB) with the same or lower thickness, or less MXene content. Fig. 7(a) and Fig. 7(c) was the representative load-displacement curves of the Ti3C2Tx/epoxy and annealed Ti3C2Tx/epoxy EMI shield­ ing nanocomposites, respectively. It could be apperceived that with increasing addition of Ti3C2Tx and annealed Ti3C2Tx, the indentation depth of both Ti3C2Tx/epoxy and annealed Ti3C2Tx/epoxy EMI shielding nanocomposites increased firstly and then decreased, revealing the 116

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corresponding nanocomposites’ ability to resist indentation. Fig. 7(b) and (d) presented the mass fraction of Ti3C2Tx and annealed Ti3C2Tx influencing on the Young’s modulus and hardness of the Ti3C2Tx/epoxy and annealed Ti3C2Tx/epoxy EMI shielding nanocomposites, respec­ tively. With increasing mass fraction of Ti3C2Tx, the Young’s modulus and hardness of the Ti3C2Tx/epoxy EMI shielding nanocomposites increased firstly from 3.62 GPa to 0.27 GPa (1 wt% Ti3C2Tx) to 4.37 GPa and 0.30 GPa (5 wt% Ti3C2Tx), and then decreased to 3.42 GPa and 0.26 GPa (15 wt% Ti3C2Tx). The change trend of the Young’s modulus and hardness for annealed Ti3C2Tx/epoxy EMI shielding nano­ composites was similar to that of Ti3C2Tx/epoxy EMI shielding nano­ composites. When the mass fraction of annealed Ti3C2Tx was 5 wt%, the Young’s modulus and hardness of the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites reached the maximum of 4.32 GPa and 0.29 GP, respectively. Appropriate loading of Ti3C2Tx could improve the stress transfer and hinder the crack propagation, leading to the improved mechanical properties. With further addition of Ti3C2Tx, the viscosity of epoxy matrix increased, and the defects were more likely to emerge. Under the action of external force, more stress concentration points would occur in the Ti3C2Tx/epoxy EMI shielding nanocomposites, which injured the mechanical properties. Meantime, excessive addition of Ti3C2Tx would destroy the crosslinking network structure of epoxy matrix and reduce the mechanical properties. Thermal reduction removed partial -F and -OH groups on the surface of Ti3C2Tx, resulting in reduced affinity of Ti3C2Tx with epoxy resin, which slightly decreased the mechanical properties of the Ti3C2Tx/epoxy EMI shielding nanocomposites.

[3] [4] [5] [6] [7]

[8] [9] [10]

[11]

[12]

[13]

4. Conclusion XRD, SEM, AFM and TEM analyses proved that the few-layered Ti3C2Tx was successfully fabricated. FTIR, XPS and XRD analyses indi­ cated that thermal reduction removed partial polar groups on the sur­ face of Ti3C2Tx with no by-product. For a fixed Ti3C2Tx loading, compared with that of Ti3C2Tx/epoxy EMI shielding nanocomposites, the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites exhibited relatively higher electrical conductivity and excellent EMI shielding effectiveness (SE). When the mass fraction of annealed Ti3C2Tx was 15 wt%, the obtained annealed Ti3C2Tx/epoxy EMI shielding nano­ composites presented the optimal electrical conductivity of 105 S/m and EMI SE of 41 dB, 176% and 37% higher than that of 15 wt% Ti3C2Tx/ epoxy EMI shielding nanocomposites. When the mass fraction of annealed Ti3C2Tx was 5 wt%, the annealed Ti3C2Tx/epoxy EMI shielding nanocomposites exhibited the optimal Young’s modulus of 4.32 GPa and hardness of 0.29 GPa, respectively.

[14]

[15] [16] [17] [18] [19] [20]

Acknowledgments [21]

This work is supported by Space Supporting Fund from China Aerospace Science and Industry Corporation (Nos. 2019-HT-XG and 2018-HT-XG); Natural Science Basic Research Plan for Distinguished Young Scholars in Shaanxi Province of China (No. S2019-JC-11); Foundation of Aeronautics Science Fund (No. 2017ZF53071); Natural Science Basic Research Plan in Shaanxi Province of China (No. 2018JM5001); C.B. Liang thanks for the School-enterprise Collaborative Innovation Fund for Graduate Students of Northwestern Polytechnical University (XQ201913); We would like to thank the Analytical & Testing Center of Northwestern Polytechnical University for TEM and AFM tests.

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