Wood-Based Composites with High Electromagnetic Interference Shielding Effectiveness and Ultra-Low Reflection

Abstract

With the aggravation of electromagnetic radiation pollution, it is urgent to develop green, lightweight, ultra-thin and high-performance electromagnetic interference shielding materials to eliminate unnecessary electromagnetic interference; however, the construction of wood-based high-performance electromagnetic shielding materials by simple methods remains a challenge. Based on the layer-by-layer assembly strategy, a lightweight Ni/Wood/Ni composite (NWNC) with an interlayer structure was constructed by a simple electroless plating method using natural wood as a substrate for electromagnetic interference shielding. The synthesized NWNC has a smooth surface, and its minimum surface roughness is only 8.34 μm. After 15 min of electroless nickel plating, the contact angle (CA) of NWNC with an ultra-thin nickel layer (65 μm) was 118.3°. When the thickness of the nickel layer is only 0.102 mm, the conductivity can reach 1659.59 S/cm when the three electroless nickel plating time is 15 min. In the L-band, the electromagnetic shielding effectiveness can reach 94.1 dB after three times electroless nickel plating for 20 min. This is due to the conductive loss, magnetic loss and interface polarization loss generated by the electromagnetic network constructed by the nickel layer, which makes the composite material produce an electromagnetic shielding mechanism dominated by absorption. The L-band absorption efficiency can reach 39.01 dB, and due to the porous structure of the original wood, the multiple reflection and absorption inside the wood further lose the electromagnetic wave. This study provides a low-cost and simple method for the design of light, ultra-thin and efficient controllable wood-based electromagnetic shielding materials and has broad application prospects in the fields of construction and aerospace.

1. Introduction

The rapid development of the fifth generation (5G) wireless system, satellite communication and portable electronic equipment makes our life more intelligent and convenient; however, they cause serious electromagnetic pollution to the environment by releasing a large number of electromagnetic waves and endanger human health [1]. Electromagnetic interference (EMI) shielding materials can inhibit and eliminate the inevitable radiation and pollution caused by electronic equipment and the communication industry [2]. Electromagnetic interference shielding is an important and effective technology to suppress destructive electromagnetic radiation [3,4]. In order to solve the pollution problem caused by electromagnetic radiation, researchers have been committed to developing materials with lightweight, low density, thin thickness, sustainability, low cost and good electromagnetic shielding effectiveness [5,6,7].

In recent years, the use of wood and wood-derived porous carbon materials to prepare electromagnetic shielding materials has become a trend [8]. Moreover, due to the sustainability of wood and porous carbon materials, there are a large number of porous pipelines inside them, which make them have specific porous structures and lightweight characteristics. These characteristics enable these materials to become a matrix template for the preparation of electromagnetic interference shielding materials [9,10]. For example, Zheng et al. uniformly and firmly embedded magnetic Ni nanoparticles into porous carbon (PC) matrix to prepare Ni/PC composites, which further improved the electromagnetic attenuation ability. When the thickness is 2 mm, the EMI SE of Ni/PC material is 50.8 dB in the X band (8.2–12.4 GHz) [11]. The performance summary of some electromagnetic shielding materials was as shown in

Table 1. Performance summary of some electromagnetic shielding materials.

Materials	Density (g/cm3)	EMI Shielding Performance (SE)	Refs.
Ni/PC	0.288	50.8	[11]
WCM@N-G@AgNWs	-	60	[12]
MXene aerogel/wood	-	69.4	[13]
Cellulose/rGO/Fe3O4 aerogels	-	52.4	[14]
Ti3C2Tx film	-	92	[15]
Graphene/PDMS	0.06	30	[16]
MWCNT/WPU	0.02	20	[17]
MWCNTs/Fe3O4 foam	0.48	27.5	[18]
Ni/poplar/Ni composite	0.618	94.1	This work

. Yuan et al. reported that the AgNWs@N-G@wood-derived carbon composite, which is obtained by impregnating AgNW solution into the carbonized wood, exhibited excellent EMI shielding and thermal and mechanical performance. In particular, the shielding effectiveness of a 5 mm thick composite could reach 60 dB [12]. Liang et al. prepared carbonized wood as a skeleton to prepare MXene aerogel/wood porous carbon composites. The EMI shielding effectiveness of the composite at 3 mm is 69.4 dB, which has flame retardant properties [13]. Based on the above studies, it can be seen that wood-based porous carbon and its composites can exhibit excellent EMI shielding performance.

In addition to the good electromagnetic shielding performance discussed above, such as graphene, MXene and their composites also exhibit good electromagnetic shielding performance. Chen et al. prepared cellulose-reduced graphene oxide (rGO)/Fe₃O₄ aerogels by co-precipitation method. When the rGO content was 8 wt% and the Fe₃O₄ content was about 15 wt%, the EMI SE of 0.5 mm thick cellulose/rGO/Fe₃O₄ aerogels reached 32.4–40.1 dB in X-band. With the increase in sample thickness (0.5–2 mm), the EMI shielding performance is greatly improved. When the thickness is 2.0 mm, the EMI SE value is 49.4–52.4 dB [14]. Shahzad et al. prepared high flexible MXene (Ti₃C₂T_x, Mo₂TiC₂T_x, and Mo₂Ti₂C₃T_x) films and layered Ti₃C₂T_x-SA (sodium alginate) composite films by vacuum-assisted filtration. The EMI SE of the Ti₃C₂T_x film with a thickness of 45 μm can reach 92 dB [15]. Chen et al. prepared lightweight and flexible graphene/PDMS (polydimethylsiloxane) foam composites. When the density of the material is 0.06 g/cm³, the shielding effectiveness is 30 dB [16]. Zeng et al. prepared a lightweight, flexible and anisotropic porous multi-walled carbon nanotube (MWCNT)/waterborne polyurethane (WPU) composite by a one-way freeze-drying process. When the density was as low as 20 mg/cm⁻³, the electromagnetic shielding effectiveness was greater than 20 dB [17]. Yang et al. first prepared high-performance MWCNTs/Fe₃O₄ silicone rubber nanocomposite foam material by introducing Fe₃O₄ magnetic particles and multi-walled carbon nanotubes. When the foam density is 0.48 g/cm³, the EMI SE of the whole X band is 27.5 dB [18]; therefore, the porous conductive composite can simultaneously improve electromagnetic absorption and reduce material density. The porous structure can generate an electromagnetic field-induced current through multiple internal scattering and in the pore wall, which provides an effective way to enhance electromagnetic wave shielding ability [19].

In this study, a simple electroless plating method was used to design the sandwich structure by electroless nickel plating on the surface of porous wood and a wood-based electromagnetic shielding material with a controllable electromagnetic gradient was prepared. Based on the loss mechanism, the prepared wood-based electromagnetic shielding material absorbs the incident electromagnetic wave by using magnetic loss, conductivity loss and interfacial polarization loss. By making full use of the synergistic effect between the three loss mechanisms, the impedance matching is optimized and the dissipation ability is improved. Good microwave absorption performance is successfully achieved with very low reflection loss [20,21,22,23,24].

The wood-based composites designed in this study are based on the absorption layer of nickel layer and the porous layer of wood. Under the combined effect of surface absorption loss and interface polarization loss, the reflection of electromagnetic waves of the composites is almost zero, and the ideal electromagnetic shielding effect is achieved; therefore, this study introduces a simple method to develop lightweight and efficient low reflection electromagnetic interference shielding materials, which have broad application prospects in the multi-functional utilization of wood and military and aerospace.

2. Materials and Methods

2.1. Materials

Nickel sulfate hexahydrate (NiSO₄·6H₂O), sodium hypophosphite (NaH₂PO₂·H₂O), sodium citrate (Na₃C₆H₅O₇·H₂O), thiourea (CH₄N₂S), borohydride sodium (NaBH₄), hydrochloric acid (HCl) and ammonia water (NH₃·H₂O). All are analytical pure, purchased in Tianjin Beilian Fine Chemical Development Co., Ltd. (Tianjin, China); the base fluid is deionized water. Poplar was selected as the base material, which was collected from Tumotezuoqi, Hohhot, with a tree age of 5 years, spin cut into veneer; thickness 0.35 ± 0.01 mm; the moisture content is about 11.2%.

2.2. Preparation of Sample

Poplar sheets were soaked in water for 30 min and cut into round sheets of 9 cm in diameter. The round poplar slices were placed in a beaker with distilled water, boiled at 100 °C for 2 h, removed and dried and polished with 600 mesh sandpaper. NiSO₄·6H₂O (15 g/L) and HCl (12 mL/L) were added to the beaker with 200 mL distilled water and then activator A was obtained by continuous stirring and dissolving. Then, 200 mL of deionized water was added to another beaker and NaBH₄ (15 g/L) and HCl (12 mL/L) were added to the beaker to obtain the activator B. The round poplar flakes were activated with activator A for 15 min and then removed to the absence of liquid drop and placed in activator B for 90 s and then removed to the absence of active drop and then placed for 10 s. NiSO₄·6H₂O (33 g/L, main salt), NaH₂PO₂·2H₂O (28 g/L, reducing agent), Na₃C₆H₅O₇·H₂O (30 g/L, complexing agent) and CH₄N₂S (10 mg/L, stabilizer) were added to the beaker containing 350 mL distilled water. Electroless Ni plating was carried out in the prepared plating solution at pH = 9 (adjusted with 30 mL/L ammonia) and a temperature of 60 °C.

2.3. Characterization

The electrical conductivity of the composites was tested by a four-probe tester (RST-8, Guangzhou Four-probe Technology Co., Ltd., Guangzhou, China). A sample of wood chips should be measured at five different positions horizontally and along the grain, and five different points should be taken at each position to test the resistance and take the average value. Microstructure images of samples were obtained by scanning electron microscopy (Phenom, Thermo Fisher Scientific, Waltham, MA, USA). Turn on the laser confocal microscope (VK-X160, Keyence, Osaka, Japan), turn on the computer and observation software in turn, and place each group of test samples in the center of the square sample table. Select the maximum objective lens as a multiple of 20 for observation. After the image is focused, click on more measurement methods, select surface measurement method, select the whole plane in the box, measure its surface roughness and save the table data. Then click the 3D test renderings to observe the smoothness of the coating on the wood chip and select the metal mode. Adjust the observation angle so that it can see the surface morphology of the coating and save the picture. Repeat the observation steps above, measuring five different locations for each sample, saving all required measurements and averaging them. The hydrophobicity of the material was characterized by a contact angle meter (JY-PHa, Chengde Yote Instrument Manufacturing Co., Ltd., Chengde, China). After 20 s of water dripping, a sample of composite materials should be measured at five different positions, select two similar values from five values and take the average value. Agilent HP8720ES vector network analyzer (VNA, Agilent Santa Clara, CA, USA) was used to collect S parameters (S₁₁ and S₂₁) of each sample in the frequency range of 0.3 × 10³–3.0 × 10³ MHz (L band). The error is from plus 0.5 dB to minus 0.5 dB, and the maximum standing wave ratio is less than 1.2. Insertion loss (IL) is less than 0.5 dB. Where S₁₁ is the forward reflection coefficient and S₂₁ is the forward transmission coefficient.

According to Schelkunoff’s theory, shielding effectiveness (${\mathrm{SE}}_T$) of total electromagnetic interference (EMI) can be composed of reflection (${\mathrm{SE}}_R$), multiple reflection (SE_M) and absorption (${\mathrm{SE}}_A$). The detailed formulas are as follows [25,26]:

$${\mathrm{SE}}_T\left(\mathrm{dB}\right)={\mathrm{SE}}_R+{\mathrm{SE}}_M+{\mathrm{SE}}_A$$

(1)

$${\mathrm{SE}}_R\left(\mathrm{dB}\right)=-10\log \left(1-{\mathrm{S}}_{21}^2\right)$$

(2)

$${\mathrm{SE}}_A\left(\mathrm{dB}\right)=-10\log \left[{\mathrm{S}}_{21}^2/(1-{\mathrm{S}}_{11}^2\right)]$$

(3)

When the multiple reflections are less than 15 dB (${\mathrm{SE}}_T$), Formula (1) can be simplified as [27,28]:

$${\mathrm{SE}}_T\left(\mathrm{dB}\right)={\mathrm{SE}}_R+{\mathrm{SE}}_A$$

(4)

3. Results

3.1. Microstructure and Morphology

The growth mechanism of Ni layers on wood and corresponding SEM images at different reaction times and numbers are presented in Figure 1. Initially, Ni²⁺ reacted with sufficient ammonia water to form Ni(OH)₂. Then, in the presence of sodium hypophosphite, Ni(OH)₂ was reduced to Ni core on the surface of the wood to form Ni particles (Figure 1). When the reaction time was 1 min, a small amount of Ni nanoparticles was deposited on the wood surface (Figure 1a). When the reaction time was extended to 10 min (Figure 1b), Ni nanoparticles gradually grew along the direction of wood texture to form nickel chains and spread outwards. With the reaction time further extended to 25 min (Figure 1c), Ni nanoparticles have been completely deposited on the surface of wood, forming Ni layer. Figure 1d–f is electroless Ni again on the basis of the first electroless Ni. It can be observed from the electron microscope that the nickel layer formed by the first deposition of nickel nanoparticles was continuously deposited again, gradually covered the first nickel layer and gradually formed the second nickel layer. Similarly, Figure 1g–i is the third electroless Ni on the basis of the second electroless Ni. Under the above preparation and design, wood-based electromagnetic shielding materials with different electromagnetic shield performance can be obtained by simple electroless Ni design of different electroless Ni times and different deposition Ni, which can be controlled and selected according to the appropriate electromagnetic shielding demand.

In order to further understand the distribution of elements on the surface of the material, the EDS diagram of the first electroless Ni for 15 min was tested. Figure 2a,b measure the element distributions in two different regions of the first electroless Ni 15 min, respectively. It can be seen that there are nickel, oxygen, phosphorus and carbon elements on the surface of the composites, but almost all of them are nickel, and other elements are only distributed in a small amount. It shows that the distribution of nickel nanoparticles in the surface layer of the composite is uniform and there are no large amounts of impurities; thus, the magnetism of nickel can absorb electromagnetic waves to the maximum extent. Figure 3a–c measures the element distribution at different points of the sample, further proving that almost all of the surface of the sample is nickel.

Figure 4a–f show the interface morphology of wood in a different time of the electroless Ni process. Figure 4a shows the cross-section of electroless Ni for 1 min. It can be seen that the outer layer of the wood interface is almost only attached to a very thin nickel layer, and the thickness of the nickel layer is only 28 μm (

Table 2. Section thickness of wood-based composites.

Type (min)	Wood Thickness (μm)	Thickness of Composite (μm)	Coatings Thickness (μm)
1–1	437	465	28
1–5	323	378	55
1–10	269	326	57
1–15	303	368	65
1–20	219	290	71
1–25	257	338	81
2–20	322	430	108
3–5	296	432	136

). It can be seen from Figure 4b–f that with the continuous extension of electroless Ni time, the thickness of the coating is increasing. Up to 25 min after electroless Ni, the reaction of electroless Ni almost stopped and the wood surface has deposited 81 μm (Table 2) thick nickel layer. Obviously, with the extension of electroless Ni plating time, the coating thickness increases. Figure 4g,h are the interface morphology of the second electroless Ni time of 20 min and the third electroless Ni time of 5 min, respectively. The thickness of the coating is 108 and 136 μm, respectively. After the first electroless Ni, twice and three times of electroless Ni, the thickness of the coating will continue to increase, but the increase rate of the coating will slow down, which is caused by the mutual extrusion of nickel nanoparticles during the growth and deposition. In the interface diagram of Figure 4g,h, it can be seen that part of the coating has entered the porous interior of the wood. Especially, after three times of electroless Ni, the composite has nearly excellent electromagnetic shielding effectiveness.

3.2. Micromorphology of Composite

Figure 5, Figure 6 and Figure 7 show the surface morphology and well depth of the material after one, two and three electroless Ni on the wood surface and different electroless Ni times. Figure 2a shows the morphology of the wood surface after one-time electroless Ni deposition for 1 min. It can be clearly observed that some metal nickel particles are gradually deposited on the wood surface and the characteristic morphology of the wood surface gradually disappears while the nickel layer gradually begins to form. From the laser copolymerization morphology of the material surface (Figure 5a,c,e,g,i,k), it can be seen that with the increase in electroless Ni time, the nickel particles deposited on the wood surface increase and the deposited nickel particles continue to gather in the texture direction and gradually grow up. The 3D diagram (Figure 5b,d,f,h,j,l) shows that with the increase in electroless Ni time, the surface morphology of wood gradually tends to be gentle and the uneven area has gradually covered the metal nickel particles. With the increase in time, the surface roughness of metal coating on the wood surface gradually decreases and the surface roughness can reach 8.34 μm after 25 min of electroless Ni (

Table 3. Roughness of electroless Ni coating on wood surface via deposition Ni.

Type		Area 1 (μm)	Area 2 (μm)	Area 3 (μm)	Area 4 (μm)	Area 5 (μm)	Mean Value (μm)
1 min	1 deposition Ni	13.3	18.34	12.59	12.92	15.38	14.51
	2 deposition Ni	11.16	12.47	10.02	12.7	15.36	12.34
	3 deposition Ni	14.98	10.82	11.37	9.58	13.95	12.14
5 min	1 deposition Ni	11.66	18.69	10.52	7.16	9.72	11.55
	2 deposition Ni	14.17	13.01	8.21	10.96	12.89	11.85
	3 deposition Ni	15.5	13.1	16.29	11.16	14.78	14.17
10 min	1 deposition Ni	11.93	8.61	13.06	8.14	18.41	12.03
	2 deposition Ni	12.68	9.46	14.44	11.68	8.45	11.34
	3 deposition Ni	21.01	15.87	20.51	22.79	12.67	18.57
15 min	1 deposition Ni	14.35	16.64	7.62	9.26	9	11.37
	2 deposition Ni	20.17	15.53	13.47	12.87	10.85	14.58
	3 deposition Ni	11.75	12.89	22.59	11.91	10.79	13.99
20 min	1 deposition Ni	12.49	8.38	8.17	8.63	14.02	10.34
	2 deposition Ni	20.5	17.23	19.99	9.79	12.86	16.07
	3 deposition Ni	13.5	15.84	14.47	14.59	14.89	14.66
25 min	1 deposition Ni	8.52	7.46	7.52	8.06	10.16	8.34
	2 deposition Ni	13.73	13.98	13.97	15.3	13.12	14.02
	3 deposition Ni	21.97	16.34	13.16	14.49	9.45	15.08

).

Figure 6a shows the surface morphology of the second electroless Ni on the wood surface for 1 min. It can be clearly observed that the wood surface has been completely covered by nickel particles. It can be seen from the surface morphology of Figure 6c,e,g,i,k that the wood surface has been completely metalized and the coating is thicker and thicker. At this time, the minimum surface roughness of metal coating on the wood surface can reach 11.34 μm (Table 3).

Figure 7a is the third electroless Ni time of 1 min on the wood surface, and the increase in the metal coating can be observed in the figure. The 3D figure (Figure 7b) shows that with the increase in the number of electroless Ni, the surface morphology of wood becomes smoother and smoother and the formation of coating becomes closer and closer. The surface roughness of metal coating on the wood surface after three times electroless for 25 min was 15.08 μm (Table 3). The surface of wood tends to be smooth, which proves that the chemical rate of metal ion deposition on the surface of the wood is catalyzed by the substrate and the deposition layer. With the extension of electroless time, the surface of the substrate will gradually cover a layer of metal Ni and the deposited Ni particles will self-catalytic Ni²⁺. With the extension of electroless plating time, the catalytic ability of the deposited Ni substrate is significantly enhanced.

3.3. Analysis of Hydrophobic Performance

Figure 8 shows the contact angles of the composites treated with different times and times of electroless Ni. Figure 8a–c is the contact angle diagram of electroless Ni once, twice and three times. It can be seen from the figure that the hydrophobic angles of all samples were greater than 90°, and the range of hydrophobic angles fluctuated between 95° and 120° (

Table 4. Contact angle of electroless Ni coating on wood surface.

Type	Contact Angle (°)
1–1	116.0
1–5	111.4
1–10	115.1
1–15	118.3
1–20	105.8
1–25	115.8
2–1	105.0
2–5	108.9
2–10	97.8
2–15	108.6
2–20	116.0
2–25	98.4
3–1	105.6
3–5	103.0
3–10	96.5
3–15	100.9
3–20	115.0
3–25	105.7

). The measurement results of contact angle show that water is not wetting the composite, and the composite exhibits hydrophobic properties. When the electroless Ni time was 15 min, the hydrophobic property of the composite was the best and the contact angle was 118.3°. The results further verified the correctness of the conclusion that the nickel layer was tightly embedded with wood. The coating formed by electroless Ni can improve the hydrophobic properties of the composites, which proves that the metal Ni and wood surface are tightly embedded together to form a dense composite coating [29].

3.4. Electrical Conductivity and EMI Shielding Performance

Figure 9a depicts the changes in electrical conductivity of wood-based composites with the time and frequency of electroless Ni, and the electrical conductivity develops slowly from 0 to 1 min, indicating that a small amount of Ni particles were deposited on the wood surface. With the electroless time extended to 15 min, the conductivity was 53.48 S/cm. At this time, the nano nickel particles formed a nickel chain and grew up step by step, which was consistent with the EDS characterization results. The nickel particles were uniformly distributed on the surface of the composite, and there were no large number of impurities.

Figure 9k exhibits that the electroless time is 20 min and electromagnetic shielding effectiveness can reach 84.9 dB. Moreover, the conductivity is 99.18 S/cm when the time is prolonged to 25 min. During the process, associated with the lengthening of electroless time, the electrical conductivity is gradually enhanced. This is because the nano-Ni particles were completely deposited on the wood surface to form a Ni layer, and the surface roughness steadily decreased. The thickness of the deposited layer on the wood surface was 81 μm. After two consecutive electroless, the conductivity was raised by 40 S/cm from 0 to 25 min and the interface morphology clearly proved that the wood surface was completely covered by nickel particles. Along with the extension of time to 15 min, the Ni particle deposition layer progressively becomes thicker and the conductivity is 170.15 S/cm. At this time, the electromagnetic shielding efficiency is improved; however, with the continuation of electroless time, the conductivity declined first and then stabilized. It can be observed that the conductivity was 157.39 S/cm at 25 min. After three times of electroless, the conductivity added quickly to 1194.72 S/cm from 0 to 1 min and the conductivity reached the maximum value of 1659.59 S/cm when the nickel electroless time was extended to 15 min. This is due to the smooth surface morphology of wood; the deposition layers are becoming tighter and some Ni particles enter the hierarchical porous structure of wood.

From the data of Figure 9m, the nickel electroless time was 20 min and the electromagnetic shielding efficiency reached the maximum of 94.1 dB. It can be seen that the electrical conductivity of the composite strengthens with the improvement of electroless time and the number of electroless layers in 0–15 min. In the second and third electroless, the electrical conductivity indicates an inflection point at 15 min and then diminishes. As the improve of electroless time, the coating thickness continues to increase, but the rate will be slowed down, which is due to the mutual extrusion of nickel nanoparticles during the growth and deposition process. In the frequency range of 0.3 × 10³–3.0 × 10³ MHz, the maximum conductivity of the composite can reach 1659.59 S/cm, and the optimal electromagnetic shielding value is 94.1 dB, which is demonstrated that the composite has an ideal electromagnetic shielding effect [25,30]. It is worth noting that natural wood is rich in porous structure and has a large specific surface area, which helps other small particles embed in the wood. The existence of pores can not only improve the impedance matching performance but also promote the multiple reflections of electromagnetic waves in the pores and enhance the absorption efficiency of electromagnetic waves. Finally, only a small number of electromagnetic waves can be reflected on the surface of the composite material [31,32,33,34].

3.5. Electromagnetic Shielding Mechanism of Wood-Based Composites

When the electromagnetic wave is incident on the surface of the shield material, due to the mismatch between its inherent impedance and the impedance of the propagation medium, a small part of the electromagnetic wave will first reflect on the material interface, which reduces the electromagnetic wave energy passing through the interface and the resulting energy loss is called the reflection loss. The electromagnetic wave into the shielding body part of the energy in the process of transmission into heat, resulting in electromagnetic wave energy again being reduced; the loss of this part of the energy is called absorption loss; the other part of the electromagnetic wave decays gradually through the multiple reflection–absorption effect inside the shield, and finally, there is a small amount of electromagnetic wave transmitted from the other end of the shield.

In this paper, an electromagnetic wave shielding network composed of a Ni–Wood–Ni sandwich structure with nickel coating and wood was established (Figure 10). Wood is an environmentally friendly material with lightweight and sustainable development rich in the hierarchical porous structure. The surface of wood contains abundant active hydroxyl groups to provide an effective substrate for the attachment of inorganic particles [35,36]. At the same time, it helps to improve electromagnetic wave absorption [37,38] and promote microwave attenuation to obtain composite materials with high electromagnetic shielding performance. As an incident absorbing shielding layer, the electromagnetic wave in the shielding body decreases gradually along the penetration direction. After experiencing the interface reflection–absorption and multiple reflection–absorption in the wood, the shielding network realizes both high absorption and low reflection of electromagnetic waves.

When the third electroless Ni was carried out for 20 min, the composite material with high absorption efficiency of 94.1 dB was obtained. Its high absorption was due to the fact that the interface polarization was taken as the main polarization mode in the dielectric loss process of the microwave absorption material. On the interface between wood and nickel coating, a large number of free electrons are spontaneously gathered on the Ni–Wood heterogeneous interface, resulting in macroscopic dipole moment and Debye relaxation, leading to the attenuation of electromagnetic waves [39]. The shielding network in the Ni–Wood layer is gradually refined, and the interface contact is gradually increased, resulting in more heterogeneous interfaces and free charges and enhancing the polarization relaxation loss [40]. In addition, conductive loss is another main way of microwave attenuation. The conductive nickel-plating layer can convert some electromagnetic energy into heat energy [41], which promotes the improvement of conductivity and the conductivity of the composite can reach 1659.59 S/cm.

4. Conclusions

(1) The wood-based composites designed in this study are based on the absorption layer of nickel layer and the porous layer of wood. Under the combined effect of surface absorption loss and interface polarization loss, the reflection of electromagnetic waves of the composites is almost zero and the ideal electromagnetic shielding effect is achieved.

(2) There are nickel, oxygen, phosphorus and carbon elements on the surface of the composites, but almost all of them are nickel, and other elements are only a small amount of the distribution. The thickness of the coatings was up to 136 μm. After the first electroless Ni, twice and three times of electroless Ni, the thickness of the coatings will continue to increase, but the increase rate of the coating will slow down.

(3) When the electroless Ni time was 15 min, the hydrophobic property of the composite was the best and the contact angle was 118.3°. The results further verified the correctness of the conclusion that the nickel layer was tightly embedded with wood. The coating formed by electroless Ni can improve the hydrophobic properties of the composites, which proves that the metal Ni and wood surface are tightly embedded together to form a dense composite coating.

(4) In the L band, the maximum conductivity of the composite can reach 1659.59 S/cm, and the optimal electromagnetic shielding value is 94.1 dB, which demonstrates that the composite had an ideal electromagnetic shielding effect.

(5) A large number of free electrons are spontaneously gathered on the Ni–Wood heterogeneous interface, resulting in macroscopic dipole moment and Debye relaxation, leading to the attenuation of electromagnetic waves.