In-situ growth of MAX phase coatings on carbonised wood and their terahertz shielding properties

Electromagnetic interference (EMI) shielding materials have received considerable attention in recent years. The EMI shielding effectiveness (SE) of materials depends on not only their composition but also their microstructures. Among various microstructure prototypes, porous structures provide the advantages of low density and high terahertz wave absorption. In this study, by using carbonised wood (CW) as a template, 1-mm-thick MAX@CW composites (Ti2AlC@CW, V2AlC@CW, and Cr2AlC@CW) with a porous structure were fabricated through the molten salt method. The MAX@CW composites led to the formation of a conductive network and multilayer interface, which resulted in improved EMI SE. The average EMI SE values of the three MAX@CW composites were > 45 dB in the frequency of 0.6–1.6 THz. Among the composites, V2AlC@CW exhibited the highest average EMI SE of 55 dB.


Introduction
With the rapid development of terahertz (THz) technology, especially the use of THz band as the next-generation (6G) communication band, THz shielding materials are urgently required to prevent electronic device malfunctions resulting from signal crosstalk and to protect people from electromagnetic pollution. Moreover, the development of lightweight and environment-friendly THz shielding materials is preferred [1][2][3][4]. Typically, the effectiveness of THz shielding materials is highly related to their microstructures [5,6]. For example, structures with specific interface contacts (such as core-shell, sandwich-like, and porous foam structures) can be used to induce interfacial polarisation to dissipate THz waves, and thus, to improve electromagnetic interference shielding effectiveness (EMI SE) [7][8][9][10][11]. Among shielding materials, materials with porous structures have attracted considerable attention due to their special structure, large specific surface area, and light weight.
As a porous material, carbonised wood (CW) obtained through the high-temperature carbonisation of natural wood is widely studied because of its light weight, multistage pore structure, and anisotropy [12][13][14][15][16]. The natural threedimensional (3D) porous conductive network of CW renders it a promising candidate for high-effectiveness EMI shielding [17][18][19][20]. However, the single-shielding mechanism of CW limits its EMI shielding potential. Therefore, various conductive materials are integrated into a CW template to obtain functional composites with an improved EMI SE [21]. Therefore, in this study, we used CW as the template to design a wood-based conductive composite with a 3D porous structure as a THz shielding material. MAX (Ti 2 AlC, V 2 AlC, and Cr 2 AlC) phases are a family of layered ternary transition metal carbides/nitrides, which exhibit ceramic and metallic attributes, such as superior oxidation and corrosion resistance, eminent electrical/ thermal conductivity, and high dielectric loss. Therefore, MAX phases are proposed as ideal EMI shielding materials in harsh environments [22][23][24]. In this study, a MAX@CW composite (Ti 2 AlC@CW, V 2 AlC@CW, and Cr 2 AlC@CW) with a porous structure was fabricated through the in-situ growth of MAX phase coatings on CW in a molten salt bath. The MAX@CW composites retained the porous structure characteristics of the CW template. Meanwhile, the CW template guides the directional assembly of the MAX phase by making it filling into the porous structure of the CW, showing a multi-interface. This unique structure of the MAX@CW composites provided paths for the dissipation of THz waves within the porous network structure. This dissipation led to a considerable increase in transmission loss and the improvement of EMI SE. Thus, the MAX@CW composites exhibit a considerably higher SE than CW and can be used as a highly effective EMI shielding material.

1 Synthesis of CW
First, natural linden wood was cut into pieces of dimensions 2.5 cm × 3.5 cm × 3 mm. These pieces were placed into an alumina crucible, and high-temperature carbonisation was performed at 1000 ℃ for 6 h in an argon atmosphere. The temperature was increased from room temperature to 1000 ℃ at a rate of 4 ℃/min. The carbonised wood slices were carefully polished with a 1000-mesh sand tray to obtain 1-mm-thick CW pieces. Subsequently, carbon debris that has been polished was removed after several rounds of ultrasonic washing with deionised water and ethanol. Finally, the pieces were dried in an ordinary oven to obtain the final CW samples.

2 Synthesis of MAX@CW
The MAX@CW composites were prepared using the molten salt method. CW was used both as the template and carbon source. The M-site metal powder (Ti, V, and Cr) and A-site metal powder (Al) with the particle size of approximately 1 μm were used as raw materials to synthesize the target MAX@CW. Furthermore, analytical grade NaCl/KCl purchased from Shanghai Aladdin Industrial Co., Ltd., China, was selected for the molten salt bath.
The MAX@CW composites were prepared in-situ on a porous CW matrix in the salt bath as follows: First, the staring materials were mixed in a molar ratio of M-site (i.e., Ti, V, and Cr): Al : CW : NaCl : KCl = 2 : 1.2 : 0.8 : 4 : 4 to cover CW completely. Next, the mixed materials were placed in an alumina crucible and packaged in a tube furnace. The tube furnace was heated from room temperature to 800 ℃ at the rate of 4 ℃/min under argon atmosphere, and this temperature was maintained for 3 h. The furnace was then heated to a sintering temperature at the rate of 4 ℃/min for 3 h (the sintering temperatures for Ti 2 AlC@CW, V 2 AlC@CW, and Cr 2 AlC@CW were 1100, 1100, and 900 ℃ , respectively). Subsequently, the tube furnace was cooled to room temperature at the rate of 4 ℃/min. Next, the product was washed many times with deionised water at room temperature to remove salt. Finally, the MAX@CW composites were obtained after drying.

3 Characterisation
The derived samples were analysed using a Bruker D8 Discovery X-ray diffractometer (irradiated by Cu Kα, λ = 1.5406 Å), with the acceleration voltage of 40 kV, filament current of 40 mA, and 2θ range of 10°-70° at a step size of 0.02°. The structure and vibration characteristics of CW and MAX@CW were analysed using a confocal Raman microscope (Renishaw Invia Reflex, UK) with an excitation wavelength of 532 nm. A scanning electron microscope (Quanta FEG 250, FEI, USA) was used to characterise the morphology and microstructure of the CW and MAX@CW composites. The electrical conductivity of the material was measured using a four-probe detector (Cresbox, Napson, Japan). Terahertz time-domain spectroscopy (THZ-TDS) was employed at room temperature (22 ℃) and a humidity of 5% (FiCO, USA). The total spectral range was 0.2-3.0 THz (effective spectrum range: 0.3-1.65 THz), with a repetition frequency of 1 kHz, and the dimensions of the samples tested were 2 cm × 3 cm × 1 mm.

1 Material characterisation
The composition of CW and MAX@CW was confirmed through X-ray diffraction (XRD) and Raman spectra. CW exhibits two wide diffraction peaks at approximately 23° and 44°, which denote that the obtained CW is amorphous ( Fig. 1(a)) [25]. For the MAX@CW samples, the peaks of CW disappeared and the diffraction peaks of Ti 2 AlC, V 2 AlC, and Cr 2 AlC appeared, indicating the formation of corresponding MAX phase coatings. The Raman spectra are highly sensitive to the surface change of materials, and thus, can be used to detect the formation of CW and MAX@CW. CW shows two strong characteristic peaks near 1350 and 1600 cm −1 , which represent the D and G peaks of carbon, respectively ( Fig. 1(b)) [26]. For the Ti 2 AlC@CW sample, the peaks at 260-270 and 360 cm −1 correspond to the ω 2 , ω 3 , and ω 4 vibration modes of Ti 2 AlC, indicating the formation of Ti 2 AlC. For V 2 AlC@CW and Cr 2 AlC@CW, the characteristic peaks of V 2 AlC and Cr 2 AlC were detected, which confirmed their composition [27,28]. Figure 2(a) schematically illustrates the fabrication of the MAX@CW composites. The process is a two-step synthesis. In the first step, porous structure CW is prepared by carbonising natural linden wood at high temperature. In the next step, MAX phase coatings are grown in-situ on the CW template through a high-temperature molten salt reaction. The metallic elements existing in an ionic form in the molten salt media can infiltrate into the pores of CW and react with the carbon matrix to form the MAX phases, which is driven by the pressure difference between the inside and outside of CW pores (i.e., the capillary effect) [29,30]. Figure 2(b) presents the optical photograph of the product. The colour of CW is black, and MAX@CW exhibits a metallic lustre, indicating that the MAX phases are successfully coated on the carbonised wood substrate. CW has a rich pore structure because linden is a natural organic polymer compound, mainly composed of cellulose, hemicellulose, and lignin (Figs. 2(c)-2(f)).  These substances further decompose and evaporate during high-temperature carbonisation, resulting in a honeycomb pore structure of various length scales ranging from nanometer to micron. The CW pores were mainly oval, with large and small pores of 30-60 and 10−15 μm, respectively. The channels of CW are long and straight, which is favourable for the impregnation of the MAX phase coating (Figs. 2(e) and 2(f)). Because the microstructure of Ti 2 AlC@CW, V 2 AlC@CW, and Cr 2 AlC@CW is similar, we selected Ti 2 AlC@CW as a representative composite to show their microstructure. Because of the atomic-level reaction in molten salt, Ti 2 AlC@CW exhibits the natural porous structure of CW without morphological distortion or pore blockage (Figs. 2(g)-2(j)). Furthermore, Figs. 2(g)-2(h) show that the surface of MAX@CW samples differs from that of the smooth CW substrate. MAX@CW with this porous structure exhibits numerous surfaces and interfaces, which can lead to an increase in the transmission path of incident electromagnetic waves, resulting in multiple scattering, and an increase in electromagnetic wave attenuation.

2 EMI SE measurements
Typically, materials with large electrical conductivity are required to obtain high EMI SE values. Figure 3(a) illustrates the electrical conductivity of CW and three MAX@CW composites (Ti 2 AlC@CW, V 2 AlC@CW, and Cr 2 AlC@CW). The results revealed that the electrical conductivity of the three MAX@CW composites was higher than that of CW, and V 2 AlC@CW exhibited the highest conductivity. The high conductivity of MAX@CW is mainly realised by free electron carriers provided by the MAX phase particles. When the concentration of the MAX phases reaches a certain critical value, the MAX phases in the system line up to form a conductive infinite network chain. The "queue" of the conductive phase acts as a bridge, causing the free electron carriers to pass through the bridge from one end to the other, thus improving its conductivity. Furthermore, the interface effect between CW and MAX phase coatings in MAX@ CW contributes to the formation of efficient conductive networks in MAX@CW [31].
To investigate the EMI SE of CW and MAX@CW, we compared the SE of 1 mm CW and the three MAX@CW composites (Fig. 3(b)). The three MAX@CW composites exhibited an SE of > 45 dB, which was considerably higher than that of CW (~42 dB), in the frequency range of 0.6-1.6 THz. Among the composites, V 2 AlC@CW with the highest electrical conductivity exhibited the the highest EMI SE value of approximately 55 dB. This phenomenon occurred because the conductor material can reflect and guide THz waves, and then, produce current or magnetic polarisation opposite to the source electromagnetic field in the conductor, thus leading to a decrease in the radiation effect of the source electromagnetic field. Therefore, the energy of THz waves attenuates rapidly in an excellent conductor. Furthermore, the increased conductivity can result in a higher impedance mismatch between the free space and MAX@CW interface, which can contribute to higher internal multiple reflection and lead to an increase in its EMI SE [32]. Moreover, when THz waves enter the MAX@CW composites, the higher conductivity leads to a larger eddy current, which converts THz wave energy into Joule heat, thus improving the absorption loss of THz waves [33]. Because of numerous interfaces between the CW and MAX phases in MAX@CW, effective interface polarisation can be induced, which improves the dissipation of THz wave energy. Therefore, the MAX@CW composites exhibited enhanced EMI SE compared to CW, and V 2 AlC@CW exhibited the highest SE value.
The dominant shielding mechanism of CW is the THz wave absorption. The average absorption coefficient of CW is 79% and reflection coefficient is 21%. And the average absorption coefficients of Ti 2 AlC@CW, V 2 AlC@CW, and Cr 2 AlC@CW are 36%, 23%, and 42%, respectively. By contrast, Ti 2 AlC@CW, V 2 AlC@CW, and Cr 2 AlC@CW exhibit high reflection coefficients of 64%, 77%, and 58%, respectively (Figs. 3(c) and 3(d)). Shielding due to reflection was the dominant mechanism in the three MAX@CW composites. The change in the shielding mechanism can be understood from several proposed mechanisms (Fig. 4). When the THz waves are incident on the CW surface, the impedance mismatch decreases because CW has a large open hole. In this case, the incident THz waves can easily enter the channel structure of CW with low reflection, which results in high absorption. Thus, many THz waves are absorbed and subsequently attenuated [34]. The MAX@CW composites mainly reflect THz waves. The mechanism can be explained as follows: When THz waves strike the surface of a MAX@CW composite, these waves interact strongly with a high electron density of MAX-phase coatings on the surface, resulting in a decrease in energy of the THz waves through the immediate reflection of THz waves. Furthermore, the numerous interfaces between CW and the MAX phase in the MAX@CW composites can induce effective interface polarisation, thereby enhancing the loss ability of the THz waves. In addition, the surviving THz waves enter the channel of MAX@CW, thus traversing through a long transmission path, which causes further energy dissipation of THz waves after multiple reflections inside the channel [35,36]. This phenomenon is in stark contrast to pure CW, which has only a single interface and no interlayer reflecting surfaces to provide multiple internal reflection phenomenon.

Conclusions
MAX@CW composites were prepared on CW templates through the in-situ reaction in the molten salt bath. The as-obtained MAX@CW composites inherit the specific microstructure of the CW template, exhibiting a 3D interpenetrating pore structure and high specific surface area. MAX@CW with the porous structure causes the formation of a continuous conductive network structure, which can lead to an increase in the transmission paths for incident THz waves, thereby achieving multiple reflections and increasing electromagnetic wave attenuation. Therefore, MAX@CW exhibits excellent electromagnetic shielding performance in the THz band. In the frequency of 0.6-1.6 THz, the total electromagnetic SE of the three MAX@CW composites is > 45 dB, and the V 2 AlC@CW exhibits the highest EMI SE of 55 dB. The results indicated the potential of developing functional materials by using natural biological structures as templates. 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.