Highly Anisotropic Carbonized Wood as Electronic Materials for Electromagnetic Interference Shielding and Thermal Management

Lightweight carbonized wood has a hierarchically 3D porous framework and orthotropic channels along the growth direction, which demonstrate great potential for multifunctional electronic materials. Herein, carbonized wood with outstanding electromagnetic interference (EMI) shielding effectiveness (SE), high thermal conductivity, and excellent Joule heating property is fabricated. It also exhibits a highly anisotropic EMI shielding performance in the cross‐ and tangential‐sections. In the tangential‐section, the shielding performance of carbonized wood is tuned by monitoring the angle between the wood grain and the electric field vibration direction of the electromagnetic waves, and the SE value ranges from 29 to 77 dB when the angle changes from 90° to 0°. Benefiting from the high SE obtained from structural optimization and the low density, carbonized balsa wood displays a high SSE (the ratio of SE to density) and SSE/t (the ratio of SSE to thickness) of 1069 dB cm3 g−1 and 3263 dB cm2 g−1, respectively. The thermal conductivity of carbonized wood is also angle‐dependent, which ranges from 0.865 to 1.897 W m−1 K−1. With the improvement in thermal conductivity, carbonized wood can be used as a heat sink material. Meanwhile, excellent Joule heating properties are achieved due to the effective conductive pathway in carbonized wood.


Introduction
The proliferation of various electronic devices inevitably brings a series of electromagnetic pollution, affecting the working life of electronic components and even impacting human health. [1][2][3] Thus, there is an emergent demand for efficient electromagnetic interference (EMI) shielding materials to inhibit the endangerment of electromagnetic radiation. Traditional metals display DOI: 10.1002/aelm.202300162 excellent EMI shielding performance, while their high density and poor corrosion resistance limit practical utilization. [4] To comply with the trend of EMI shielding materials with lightweight and stable, emerging nanomaterials such as carbon nanotube, [5,6] graphene, [7,8] and MXene [9,10] have attracted wide attention.
As an abundant renewable material, wood possesses a unique hierarchically porous structure stretching from the nanoscale of the cell wall to the macroscale of the tree stem. Carbonized wood well maintained the porous skeleton of the wood, with the aperture size of ≈100 μm for vessels and 20 μm for wood fibers. Although the aperture size is smaller than wavelength of X-band, multiple reflections of electromagnetic waves are assumed to be the shielding mechanism as reported in many porous materials. [11][12][13] To further enhance the shielding performance of carbonized wood, conductive materials were added to prepare carbonized wood composites, such as MXene [14] and polyisoprene. [15] Nevertheless, high conductivity tended to cause reflection of most electromagnetic waves, thus magnetic particles were also hybrid with carbonized wood to give microwaves absorption properties, such as iron oxide, [16] ferroferric oxide, [17] nickel, [18,19] and carbon nanotubes/nickel. [20,21] The structure of the material largely affects its shielding performance, porous materials are usually used as they can improve the internal multi-reflection. Currently, wood species with low density and porous structures were usually chosen, such as balsa and poplar, and better EMI shielding performance of the carbonized poplar wood was achieved with a higher compression ratio. [22] This work systematically studies the correlation between EMI shielding properties and the cellular structure of carbonized wood, providing the basic information on the optimization of wood species. In addition, wood is highly anisotropic along the grain direction, causing anisotropy in physical and mechanical properties. Zhao et al. reported shielding mechanism in the cross-section was 52 dB, while for the tangential section was 44 dB. [23] Furthermore, we also found that EMI shielding performance could be monitored by changing the angle between the wood grain and the electric field of the incident waves in MXenecoated natural wood. [24] It is anticipated that angular modulation of shielding properties occurs in carbonized wood but is not been reported yet. Hence the effects of the orthotropic structure of wood on the EMI shielding performance are also carefully investigated in this work.
The anisotropic structure also affects thermal conductivity. It is reported that the highest thermal conductivity is achieved when the fiber orientation direction is parallel to the heat transfer direction. [25] The thermal conductivity of carbonized wood is predicted to be related to the orientation angle. Meanwhile, carbonized wood has the capacity to be used as a heat sink element or thermoelectric energy converter owing to the effective conductive pathway. Applications of carbonized wood in thermal management are reported. [26] This study aims to explore the effects of carbonized wood's anisotropic cellular structure on the EMI shielding and thermal properties. Carbonized woods made from three wood species (balsa, poplar, and linden) were fabricated and compared. The shielding mechanism was explored in detail and the EMI shielding effectiveness (SE) was regulated by changing the angle between the wood grain and the electric field in the tangentialsection. The heat dissipation and Joule heating performance of carbonized balsa wood were also analyzed. Figure 1a depicts the preparation process of carbonized wood. Balsa CW has the highest porosity of 93.35%, which is 30.96% higher than that of poplar CW (Figure 1b). Furthermore, the density of all samples decreased after carbonization due to the Adv. Electron. Mater. 2023, 9,2300162 www.advancedsciencenews.com www.advelectronicmat.de removal of carbohydrates. In particular, the density of balsa wood is only 0.072 g cm −3 after carbonization, which is the lowest density among the different species of carbonized wood ( Figure 1c). As seen in Figure 1d, balsa CW can be supported by a flower. The crystal structure of natural wood and carbonized wood are examined. Figure 1e shows the XRD of natural wood, the diffraction peaks at 15.5, 22.3, and 34.4°are attributed to the (1-10)/(110), (200), and (004) planes of type II cellulose, respectively. [27] In XRD of carbonized wood (Figure 1f), two broad peaks at 21.7 and 43.6°are owed to the (002) and (101) planes of graphite, implying the partial graphitization of the carbon materials. In Figure 1g, Raman spectra of the carbonized wood include the D-band (≈1332 cm −1 ) associated with a defective domain and the G-band (≈1596 cm −1 ) associated with a crystalline graphite domain. [28] The I D /I G value of the three carbonized woods varies slightly, implying that they have a similar degree of graphitization. Furthermore, the chemical composition of linden, poplar, and balsa wood is shown in Figure S5, Supporting Information, referring to previous works, [29][30][31] which is slightly different among the three wood species.

Characteristics and Cellular Structure of Carbonized Wood with Different Species
The cellular structures of carbonized wood and natural wood are shown in Figure 2 and Figure S3, Supporting Information. A large diversity in microstructures among tree species is observed. As shown in Figure 2a,b, the cross-section of linden and poplar carbonized wood contains porous structure with majority of wood fibers (2 ≈ 20 μm) and vessels (50 ≈ 100 μm). Comparatively, poplar carbonized wood has more wood fibers than linden carbonized wood. In contrast, fewer vessels (≈ 120 μm) are observed in balsa carbonized wood (Figure 2c). In tangentialsection, some narrow gaps corresponding to the open area of fiber cells and vessels are observed. After 1000°C treatment, the carbonized wood retained the integrity of the wood skeleton.

EMI Shielding Performance of Carbonized Wood with Different Cellular Structure
The EMI SE values in the cross-section of carbonized wood are depicted in Figure 3a. The average EMI SE T of linden C-CW, poplar C-CW, and balsa C-CW is 24.60, 37.60, and 57.67 dB, respectively, all meeting the commercial application requirement (>20 dB). [32] The average EMI SE T of poplar C-CW is 52.85% higher than linden C-CW, owing to more wood fibers that could dissipate electromagnetic waves inside the pores and increase the SE A . [33] Furthermore, balsa C-CW has the most wood fibers and the highest porosity, which contributes to the highest average EMI SE T and SE A of balsa C-CW (Figure 3b). In the tangentialsection, the EMI SE T values (when wood grain is perpendicular to electric field) are much smaller than the values obtained in the cross-section, and the difference between wood species is not significant (Figure 3d). The SE R values are similar in the two sections, but the SE A values all decrease from cross-section to tangential-section (Figure 3e). The power coefficients in Figure 3c,f indicate the reflection dominated shielding mechanism in both sections with R > A, and tangential-section reveals a higher R-value owing to the better electrical conductivity (lower sheet resistance, Figure 3g). Moreover, balsa CW has a higher A value than poplar CW and linden CW, as the more porous property of balsa CW facilitates the internal reflection of electromagnetic waves. The high electrical conductivity of linden CW and poplar CW also results in the reflection of electromagnetic waves and a high R-value.
The EMI shielding performance in cross-section is better than in tangential-section, and the shielding mechanism is also different. When incident electromagnetic waves arrive at the cross-section, they are firstly reflected because of the impedance mismatch, and the remaining electromagnetic waves are attenuated as thermal energy during multiple reflections within the wood fibers and vessels (Figure 3h). Regarding the tangentialsection, more incident waves are reflected due to the higher conductivity and absence of through-pores. Whereas the gaps on T-CW surface offer chances for multiple reflections, which facilitate to further dissipate electromagnetic waves (Figure 3i). [23]

Turning the EMI Shielding Performance by Rotating Grain-Electric Field Angle
In terms of the high EMI SE of balsa wood, it is chosen for the further studies on the electromagnetic response when alter-ing the angle between the wood grain and the electric field. According to Maxwell, the electromagnetic waves consist of electric and magnetic field at right angles to each other, and the direction of waves propagation. Thus, the electric field is vibrating along the direction normal to the propagation of electromagnetic waves. Figure 4a exhibits the angles (90°, 45°, and 0°) between the wood grain and the electric field vibration direction. When the angle turns from 90°(balsa T-CW-90°) to 0°(balsa T-CW-0°), the electrical conductivity increases from 0.75 to 1.55 S cm −1 (Figure 4b). As the electrons move faster along the fiber direction when electric field is parallel to the wood grain than perpendicular to the grain, the highest conductivity is obtained by balsa T-CW-0°. In addition, impedance matching of balsa T-CW-0°is reduced as Z is much smaller than 1 (Figure 4c) due to the incompatibility between high conductivity and impedance matching. [34] The high electrical conductivity also increases the attenuation constant ( ) of balsa T-CW-0° (Figure 4d). Correspondingly, the average EMI SE T of balsa T-CW ascends from 29.0 to 77.0 dB when the angle changes from 90°to 0°, it is worth mentioning that EMI SE T difference between 90°and 0°i s up to 48.0 dB (Figure 4e). This substantial raise in the EMI SE T can be ascribed to the improved electrical conductivity. A high impedance mismatch can be realized at the interface due to the high conductivity, resulting in the enhanced SE R [35] and reflection coefficient (R) (Figure 4f,g). Meanwhile, the high conductivity increases the , which leads to the rapid attenuation of incoming electromagnetic waves as heat energy, causing the high SE A .
The theoretical EMI SE can be calculated based on the electrical conductivity according to traditional shielding theory: [36] SE R (dB) = 39.5 + 10log 2 * * f * (1) where μ is the magnetic permeability, and equal to 4 × 10 −7 H/m. [37] The theoretical SE R values are in good agreement with the experimental results, due to the electrical conductivity is the main factor causing the reflection by the impedance mismatch (Figure 5a). On the contrary, there is a significant discrepancy in the SE A between theoretical values and experimental results (Figure 5b), particularly when the electrical field is perpendicular to the grain (T-CW-90°). Thus, the experimental SE A and SE T are lower than predicted (Figure 5b,c). The traditional shielding theory is proposed based on the shielding performance of homogenous material. However, the energy attenuation in T-CW is more complicated due to its aligned cellular structure along the growth direction. Hence, both the electrical conductivity and cellular structure of carbonized wood needed to be considered when discussing the shielding mechanism.

Shielding Mechanism with Different Grain-Electric Field Angles
Furthermore, the FEA is applied to simulate the attenuation process with different angles between wood grain and electric field, which takes the cellular structure of balsa T-CW into account (Figure 6a). All balsa T-CW specimens are placed in the same physical field, i.e., the input electric field strength at the surface is equal (Figure 6b). The intensity and distribution of surface current density differ when the angle adjusts from 90°to 0° (Figure 6c). The surface current generates a magnetic field perpendicular to it, which would yield an electromotive force to counter the external electric field and attenuate microwaves energy. The surface current density is stronger when the wood grain is parallel to the electric field than perpendicular to the electric field, which results in the improved resistance loss and electromagnetic loss for CW-0° (Figure 6d,e). [13,38] Carbonized wood is highly conductive with merely no magnetic loss ( Figure S4, Supporting Information). Therefore, the resistance loss and electromagnetic loss of balsa T-CW are similar and the electric field strength of output port is obviously reduced after passing the balsa T-CW-0° (  Figure 6f). In a word, both experimental and simulation results demonstrate the anisotropic structure of carbonized wood has significant effects on the EMI shielding performance and mechanism. Therefore, tunable EMI shielding performance can be achieved by simply rotating the carbonized wood in tangentialsection.

Demonstration of the EMI Shielding Performance
Tesla coil with frequencies from 50 kHz to 1 MHz is applied as a demonstration to show the shielding performance of carbonized wood. [12,39] As seen in Figure 7a, the bulb is on when the Tesla coil and the bulb are separated by balsa NW. The brightness of the bulb becomes weaker when the direction of the electric field generated by the Tesla coil is perpendicular to the direction of the grain of the carbonized wood ( Figure 7b). However, the bulb turns off when balsa T-CW-45°or balsa T-CW-0°are placed between the Tesla coil and the bulb (Figure 7c,d). The practical application illustrates that when the electric field is parallel to the wood grain, the EMI shielding performance is enhanced.

Comparison of SSE and SSE/t with Other Wood-Based EMI Shielding Materials
In the pursuit of an excellent SE value for EMI shielding material, the thickness and density of the material also should be considered. The performance of balsa T-CW-0°is compared with other wood-based EMI shielding materials in terms of SSE (the ratio of SE to density) and SSE/t (the ratio of SSE to thickness). [40,33,[41][42][43]14,16,44,45,21,46] Benefiting from the high SE obtained from structural optimization and the low density, the balsa T-CW-0°displays the highest SSE (1069 dB cm 3 g −1 ) and SSE/t (3263 dB cm 2 g −1 ) with a thickness of 3.3 mm in the X-band, outperforming other reported wood-based EMI shielding materials (Figure 8). In view of the low density and high EMI SE of carbonized wood, it has great potential to be the future EMI shielding material.

Thermal Management Performances of Carbonized Balsa Wood
To evaluate the effect of orientation angle on thermal conductivity, carbonized woods were prepared with different orientation angles of 90°, 45°, and 0°(The angle between heat transfer direction and wood grain), which are named as CW-90°, CW-45°, and CW-0°, respectively. As shown in Figure 9a, the thermal conductivity of CW-90°, CW-45°, and CW-0°reaches 0.865, 1.543, and 1.897 W m −1 K −1 , respectively. The thermal conductivity of CW-0°is enhanced by 119.31% compared to CW-90°owing to fewer impediments of heat flux. [25] The samples are placed on a heat plate at 100°C, and the surface temperature is determined by an IR thermal imager (Figure 9b,c). The temperature of CW-0°rises fastest, demonstrating its high thermal conductivity. Carbonized wood with different orientation angles is used as a thermal interface material to connect the heat sink and LED chip (Figure 9d). The temperature of the LED chip at 9.0 V operating voltage is recorded using an IR thermal imager (Figure 9e,f). At 40 s, LED chip with CW-90°as thermal interface material exceeds 80°C, however, LED chip with CW-0°as thermal interface material maintains a stable temperature below 60°C after 20 s, which can meet the practical application requirement for the LED chip. [47,48] Figure 10a shows the temperature variation of the carbonized wood with time under a direct current voltage (0.5 V ≈ 2.0 V, the current direction is parallel to wood grain). The surface temperature of the carbonized wood increases with the increasing voltage, and the steady-state temperature can be linearly fitted to the square of the voltage, proving that carbonized wood has a controllable thermoelectric energy conversion (Figure 10b). Meanwhile, IR thermal images display the uniform heat distribution of carbonized wood at different steady-state temperatures (Figure 10d). Figure 10c,e illustrates the Joule heating stability test of carbonized wood, periodically results were achieved under different voltages, and the steady-state temperature can be stabilized at ≈60°C for 3600 s under 1.5 V. Based on these results, carbonized wood has the potential to be used as a heating element owing to its precise temperature control and stability.

Conclusion
In this work, the effects of cellular structure on the EMI shielding performance of carbonized wood were first studied. Benefiting from the high SE obtained from structural optimization and the low density, carbonized balsa wood displays a high SSE and SSE/t of 1069 dB cm 3 g −1 and 3263 dB cm 2 g −1 , respectively, and EMI shielding performance in the cross-section is higher than that in the tangential-section due to more through-pores enhancing the multiple reflections of electromagnetic waves. Furthermore, the average EMI SE of carbonized wood in the tangential-section increases from 29.0 to 77.0 dB when the angle between the wood grain and the electric field rotates from 90°to 0°. The highest thermal conductivity is also achieved when the wood grain is parallel to the heat transfer direction. The carbonized wood exhibits excellent EMI shielding, heat dissipation, and Joule heating properties, demonstrating the great potential for application in the electronic devices.
where Z in , Z 0 , d, r , μ r , f, and c are the input impedance, free space impedance, sample thickness, relative complex permittivity ( r = ′ − j ′′), relative complex permeability (μ r = μ′ − jμ′′), frequency, and velocity of light, respectively. Simulation: The attenuation process of electromagnetic waves in balsa T-CW was simulated by COMSOL Multiphysics software via the finite element method. Simplified models of balsa T-CW (22.86 × 10.16 × 3.3 mm) with different angles (90°, 45°, and 0°) between the wood grain and the electric field were established. In addition, the ′, ′′, μ′, μ′′, and of balsa T-CW were set based on the experimental results at 10.0 GHz.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.