3.1 Materials Characterization
The composition of CW and [email protected] is confirmed by XRD and Raman spectra. As shown in Fig. 1A, the CW has two wide diffraction peaks at about 23° and 44°, which means that the CW obtained is amorphous. For the [email protected] samples, the peaks of CW disappear and the diffraction peaks of Ti2AlC, V2AlC and Cr2AlC appear, indicating the formation of corresponding MAX phase coatings. The Raman spectra are more sensitive to the surface change of materials and thus are employed to detect the formation of CW and [email protected] As shown in Fig. 2B, the CW has two strong characteristic peaks near 1350 cm− 1 and 1600 cm− 1, which represent the D peak and G peak of carbon. For the Ti2[email protected] sample, the peaks at 260–270 and 360 cm− 1 correspond to the ω2, ω3 and ω4 vibration mode of Ti2AlC is observed, indicating the formation of Ti2AlC. For the V2[email protected] and Cr2[email protected] samples, the characteristic peaks of V2AlC and Cr2AlC were also detected, which confirms their composition[27, 28].
The schematic of the fabrication process of [email protected] composites is shown in Fig. 2A. The synthesis route consists of two steps. The first step is preparing CW with porous structure by carbonizing natural linden wood at high temperature. The following step is in-situ growing of MAX phase coatings on CW template by high-temperature molten salt reaction. The metallic elements existing as ionic form in the molten salt media can infiltrate into the pores of carbonized wood and react with carbon matrix to form MAX phase, which is driven by the pressure difference between inside and outside of the pores of carbonized wood (i.e. the capillary effect)[29, 30].
The optical photo of the product is shown in Fig. 2B, we can see that the color of CW is black, while [email protected] has metallic luster, indicating that MAX phase coating is successfully prepared on the carbonized wood substrate. As shown in Fig. 2C-F, the CW has a rich pore structure, this is because that linden is a kind of natural organic polymer compounds, mainly composed of cellulose, hemicellulose and lignin. These substances will further decomposed and evaporated during the process of high temperature carbonization, leaving honeycomb pore structures of different length-scales from nanometer to micron. The pore shapes of CW are mainly oval with large pores of 30 ~ 60 µm and small pores of 10 ~ 15 µm. In addition, as shown in Fig. 2E-F, the channels of CW are long and straight, which is favorable for the impregnation and infiltration of MAX phase coating. Since the microstructure of Ti2[email protected], V2[email protected] and Cr2[email protected] is similar, we choose Ti2[email protected] as the representative to show their microstructure. As shown in Fig. 2G-J, owing to the atomic-level reaction in the molten salt, Ti2[email protected] inherits the natural porous structure of CW, without morphological distortion and pore blockage. In addition, through Fig. 2G-H, we can see that the surface of [email protected] samples is different from that of smooth carbonized wood substrate. The [email protected] with this porous structure has a large number of surfaces and interfaces, which can increase the transmission path of incident electromagnetic waves and result in multiple scattering and increasing the attenuation of electromagnetic waves.
3.2 EMI shielding performance
The electromagnetic interference shielding effectiveness (EMI SE) of CW and three kinds of [email protected] (Ti2[email protected], Cr2[email protected], V2[email protected]) are shown in Fig. 3. With the thickness of 1 mm, the average EMI SE in the frequency range of 0.6 ~ 1.6 THz of all the three kinds of [email protected] is higher than 45 dB, which have been improved comparing to the CW (~ 42 dB). Among them, V2[email protected] show the highest EMI shielding effectiveness with the average value of ~ 55 dB. Notably, as shown in Fig. 3B-3C, the dominant shielding mechanism of CW is EMW absorption with the average absorption coefficient of 79%, while that value of Ti2[email protected], Cr2[email protected], V2[email protected] is 36%, 42%, and 23%. On the contrary, Ti2[email protected], Cr2[email protected], V2[email protected] show high reflection coefficient of 64%, 58%, and 77%.
The shielding mechanism change is caused by to the structure evolution from CW to [email protected] For the CW, the highly porous structure allows electromagnetic waves to easily enter into the low reflection wood channel and be attenuated multi-reflection inner the structure . While when the MAX phase coating is fabricated on the CW channel, the pore size is reduced that decreased the ratio of incident electromagnetic waves. Another important reason is the impedance matching property is changed after the MAX phases coating fabrication. Owing to the porous structure and relatively low dielectric constant, CW has better impedance matching property with the free space than the [email protected], which allows more electromagnetic waves entering into the channel, and enhanced the ration of higher EMW absorption. In contrast, MAX phase with high dielectric constant greatly increased the impedance mismatching between the free space and [email protected], which result in more incident electromagnetic waves being reflected at the surface [24, 31].
In order to further understand the EMI shielding mechanism of the [email protected], the electrical conductivities of the materials were measured. As shown in in Fig. 3D, V2[email protected] shows the highest electrical conductivity among the three kinds of [email protected] As we know, the increased conductivity can result in higher impedance mismatch between the free space and [email protected] interface, which can contribute to higher reflection loss and increase its EMI shielding effectiveness. In addition, when the electromagnetic wave enters the body of the material, the higher conductivity will lead to a larger eddy current and converts EMW energy into Joule heat, which can improve the electromagnetic wave absorption loss. Therefore, the [email protected] show enhanced EMI shielding effectiveness than the CW, and V2[email protected] shows the best shielding effectiveness.