Abstract
This research aims to optimize the comprehensive performance of wood-based electromagnetic shielding interference (EMI) materials and master the effect of heat treatment on its coating density, interfacial morphology, conductivity, and hydrophobic and electromagnetic shielding. The results showed that the surface roughness of composite coatings was 11.0 μm when the wood was conducted via electroless two deposition Cu and one Ni and the heat treatment temperature was 150 °C. The composite coating’s surface gradually became more uniform with increasing temperature. The coating’s thickness via 120 °C heat treatment was 97.5 μm. Energy Dispersive Spectroscopy (EDS) spectra verified the existence of Cu and Ni particles. The heat treatment had an obvious influence on conductivity of composite materials and the pore network structure. The contact angle of composite materials reached 119°. The average electromagnetic shielding efficiency via 180 °C heat treatment was up to 91.4 dB in the frequency ranging from 300 to 3.0 GHz, which verified that the composite materials can shield 99.99% of the incident electromagnetic wave energy. The conductivity gradient structure can realize multi-dielectric interface loss and multiple reflection loss.
Download PDF
Full Article
Micro-nanoarchitectonics of Electroless Cu/Ni Composite Materials Based on Wood via Heat Treatment
Yanfei Pan,a,b,1,* Mayin Dai,a,b,1 Xin Zheng,a, b,1 Lei Yun,a, b,1 Fengqi Qiu,a,b,1 Dongbo Yang,a,b Caiyi Deng,a,b Qiang Guo,a,b and Jintian Huang a,b,*
This research aims to optimize the comprehensive performance of wood-based electromagnetic shielding interference (EMI) materials and master the effect of heat treatment on its coating density, interfacial morphology, conductivity, and hydrophobic and electromagnetic shielding. The results showed that the surface roughness of composite coatings was 11.0 μm when the wood was conducted via electroless two deposition Cu and one Ni and the heat treatment temperature was 150 °C. The composite coating’s surface gradually became more uniform with increasing temperature. The coating’s thickness via 120 °C heat treatment was 97.5 μm. Energy Dispersive Spectroscopy (EDS) spectra verified the existence of Cu and Ni particles. The heat treatment had an obvious influence on conductivity of composite materials and the pore network structure. The contact angle of composite materials reached 119°. The average electromagnetic shielding efficiency via 180 °C heat treatment was up to 91.4 dB in the frequency ranging from 300 to 3.0 GHz, which verified that the composite materials can shield 99.99% of the incident electromagnetic wave energy. The conductivity gradient structure can realize multi-dielectric interface loss and multiple reflection loss.
DOI: 10.15376/biores.17.4.6718-6739
Keywords: Wood; Electroless Cu-Ni; Conductivity; Hydrophobicity; Electromagnetic shielding
Contact information: a: College of Material Science and Art Design, Inner Mongolia Agricultural University, Hohhot, China, 010018; b: Inner Mongolia Key Laboratory for Sand Shrubs Fibrosis and Energy Development and Utilization; 1: These authors contributed equally to this work;
* Corresponding authors: panyanfeiz@imau.edu.cn; jintian_h@imau.edu.cn
GRAPHICAL ABSTRACT
INTRODUCTION
Electromagnetic pollution has become an inevitable social problem as communication technologies represented by emerging 5G wireless systems and modern electronics flourish (Chen et al. 2013; Meng et al. 2018; Srivastava and Manna 2022; Xiong et al. 2022). The elimination of harmful electromagnetic waves is necessary for the protection of electronic communication equipment and the maintenance of healthy human living circumstances (Thomassin et al. 2013; Abbasi et al. 2019; Wang et al. 2019). The matrix of commonly used electromagnetic shielding materials is mostly non-renewable and not easy to degrade; thus, large-scale use will bring environmental problems (Hu et al. 2021). Therefore, it will be an inevitable trend to prepare shielding materials based on environment-friendly materials with rich resources that are renewable and degradable (Zheng et al. 2020; Xiong et al. 2022).
In recent years, many researchers have focused on the preparation of lightweight and efficient biomass-based EMI shielding materials that take advantage of the green and sustainable nature of biomass resources (Qi et al. 2021; Jia et al. 2022). For example, Shen et al. prepared epoxy (EP)/carbon composites by filling epoxy into carbonized wood. The EP/ C-1200 composite exhibited a maximum SE value of 27.8 dB at a thickness of 3 mm (Shen et al. 2019). Liang et al. (2020) demonstrated an MXene aerogel/wood PC composite with excellent electrical conductivity, achieving a SE value of 69.4 dB at 3 mm. Gan et al. (2020) prepared conductive wood by delignification and chemical vapor deposition and obtained 58 decibels of SE at 3.5 cm. Generally, EMI shielding performance is contributed by two parts: reflection and absorption. Reflection is related to the dielectric properties of the material, while absorption is related to the multiple loss characteristics (Lou et al. 2021; Yu et al. 2022). Therefore, in order to further optimize the shielding performance of wood-based materials and improve the total SE of wood-based materials, heat treatment of wood-based materials is a useful method.
As a green physical modification method, wood heat treatment can increase the dimensional stability and biological durability of wood, which is an effective technical means to improve the utilization of wood resources (Fang 2019). During heat treatment, the components of wood are degraded. Compared with untreated wood, the compressive strength of heat-treated wood often shows a certain degree of improvement. After high temperature heat treatment, the moisture absorption, degradation resistance and dimensional stability of wood were improved (Ma 2019; Lin 2021; Yan 2021; Yu 2021). Moreover, after high-temperature heat treatment, the hydrophilic groups in the wood decrease and the surface wettability of the wood decreases (Hao et al. 2021). As with all solids, most of the properties of electroless Ni and Cu coatings are temperature-dependent. The thermal expansion coefficient affects the internal stress and bonding strength of electroless Ni and Cu coatings without high temperature heat treatment, which are thermodynamically metastable and tend to change from amorphous or microcrystalline state to crystalline state. Under certain conditions, when the composite coatings are heat-treated, atoms in the coatings will have mutual diffusion, resulting in recrystallization of amorphous or microcrystalline structure, and the formation of metallic Ni and intermetallic compounds, such as Ni2P, Ni3P, and Ni5P2 (Pan et al. 2020). Generally speaking, heat treatment will change the coating’s structure, and the structural change will affect the coating’s performance. However, there have been few reports on the relationship between heat treatment and electromagnetic shielding efficiency of Ni-Cu-P ternary wood-based composites.
In this study, the relationship between electromagnetic shielding effectiveness and heat treatment temperature of wood-based composite materials via electroless Cu and electroless Ni was analyzed. After the optimal heat treatment temperature, the electromagnetic shielding effectiveness of the composite material prepared by us can reach 90.40 dB in the L-band, which has a good application prospect in the aerospace and military fields.
EXPERIMENTAL
Reagents and Materials
Copper sulfate pentahydrate (CuSO4·5H2O), Seignette salt (NaKC4H4O6·4H2O), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), potassium ferrocyanide (K4Fe(CN)6·3H2O), formaldehyde, NaOH, 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 were analytically pure and purchased from Tianjin Beilian Fine Chemicals Development Co., Ltd. The base fluid was deionized water.
Poplar was selected as the base material, which was collected from Tumotezuoqi, Hohhot, China, with a tree age of 3 to 5 years. Poplar wood without growth defects was cut into round chips 9 cm in diameter. Thickness was 0.37 ±0.05 mm. The moisture content was about 11.2%.
Experimental Design
Six temperature gradients were designed. After electroless Cu and electroless Ni treatment, the wood was heat treated at 25 ℃, 100 ℃, 120 ℃, 150 ℃, 180 ℃, and 200 ℃. The experimental design is shown in Table 1. Specimens were labeled based on the types of electroless treatment. For example, when the wood was modified via two electroless Cu treatments and then one electroless Ni treatment, the sample was marked as 2CulNi.
Table 1. Experimental Design
Preparation of Specimens
Substrate preparation
Poplar chips without surface defects were screened from poplar wood. The poplar chips were rubbed with 600 mesh sandpaper to make them smooth. Then, the wood chips were cut into round sheets with a diameter of 9 cm, and then boiled in a water bath at 100 ℃ to remove the impurities inside the poplar.
Fig. 1. Schematic diagram of sample preparation
First, the specimen was placed in the activation solution A (CuSO4·5H2O 15 g/L, HCl 12 mL/L) and activated for 15 min. Then it was removed and placed into the activation solution B (NaBH4 15 g/L, NaOH 15 g/L) to activate for 90 s. CuSO4·5H2O (53 g/L), NaKC4H4O6·4H2O (13 g/L), EDTA-2Na (27 g/L), K4Fe(CN)6·3H2O (1.3 g/L), and HCHO (70 mL/L) were added to the beaker containing 150 mL distilled water. Electroless Cu was carried out in the prepared plating solution at pH =11.8 (it was adjusted with 25% NaOH) and a temperature of 60 °C. The electroless Cu was carried out, as shown in Fig. 2.
Fig. 2. Schematic diagram of electroless Cu on wood surface
Two electroless Cu samples were activated in electroless Ni activator A (NiSO4·6H2O 15 g/L, HCl 12 mL/L) for 15 min. The samples were taken out and exposed to the activation solution B (NaBH4 15 g/L, NaOH 15 g/L) for 90 s. NiSO4·6H2O (33 g/L), NaH2PO2·2H2O (28 g/L), Na3C6H5O7·H2O (30 g/L), and CH4N2S (10 mg/L) were added to the beaker containing 350 mL distilled water. Electroless Ni coating was carried out in the prepared plating solution at pH = 9 (adjusted with ammonia) and a temperature of 60 °C. The electroless Ni was carried out, as shown in Fig. 3.
Fig. 3. Schematic diagram of electroless Ni on wood surface
Heat treatment of 2Cu1Ni sample
The samples were put into a furnace, and the samples were clamped by fixtures. Six temperature gradients were designed. The wood was treated by electroless Cu-Ni, and then heat treated at 25 °C, 100 °C, 120 °C, 150 °C, 180 °C, and 200 °C, in turn, and then the performance was tested (Fig. 4).
Fig. 4. Schematic diagram of heat-treated wood-based composite
Testing Instrument and Operation Procedure
VK-X160 laser scanning confocal microscope test
A laser scanning confocal microscope (VK-X160, Keyence, Osaka, Japan) was used for observing the overall surface morphology of composite materials and the roughness of coating surface. The specific operation method is to open the laser scanning confocal microscope first, then open the computer and observation software in turn, and place each group of test samples in the center of the sample table. The largest objective was chosen 20 times the size of the observation.
RTS-8 four-probe
A four-probe tester (RST-8, Guangzhou Four-probe Technology Co., Ltd., Guangzhou, China) was used for testing the electrical conductivity of composites. The specific test method is to measure the samples at five different positions along the horizontal and grain lines, and take five different points at each position to test the resistance and take the average value.
Scanning electron microscopy (SEM)
A scanning electron microscope (Phenom, Thermo Fisher Scientific, Waltham, MA, USA) was employed to characterize the microstructure of composite materials.
Hydrophobic performance
A contact angle meter (JY-PHa, Chengde Youte Instrument Manufacturing Co., Ltd., Chengde, China) was used to characterize the hydrophobicity of composite materials. The specific test procedure is to measure a composite sample at five different positions after 20 s of dripping, select two similar values from five values, and take the average.
DR-S02A electromagnetic shielding effectiveness
The DR-S02A shielding effectiveness test device (Beijing DingrongShichuang Technology Co., Ltd., Beijing, China) was used to characterize the electromagnetic shielding performance of the composite material. The test frequency range is between 0.3×10-3 and 3.0×103 MHz. The composites were tested according to the ASTM 4935 (2010) international standard test. The sample thickness was ≤ 10 mm. The error was from +0.5 dB to -0.5 dB, and the maximum VSWR (voltage standing wave ratio) was less than 1.2. The insertion loss (IL) was < 0.5 dB.
According to Schelkunoffs theory (Pan et al. 2022), the detailed formulas are as follows:
RESULTS AND DISCUSSION
Heat Treatment Coatings Morphology
Figure 5 shows the surface morphology of wood without superheat treatment. Comparing Fig. 5a with 5b (3D image), it could be clearly observed that copper and nickel were deposited on the composite material surface via electroless Cu-Ni, which partially covered the whole area, and the morphology of wood grain pores disappeared.
Fig. 5. The wood surface laser microscope morphology 200× (a: Surface morphology; inset shows depth map; b: 3D image)
Figure 6 showed that the surface morphology of the wood was partially uneven. The maximum surface roughness of metal coatings on wood surface was 33.8 μm (Table 2). The 3D images after heat treatment (Fig. 7) showed that with the increase of heat treatment temperature, the surface roughness of the bonded metal coatings could be as low as 11.0 μm (Table 2).
The rate of metal ion deposition on smooth wood surface was affected by the catalysis of matrix and deposition (Pan et al. 2022). The metal coatings on the wood surface were gradually transformed with compact deposition with increasing temperature, which further verified that the heat treatment could accelerate metal Cu and Ni particle rearrangement and optimized the surface structure.
Fig. 6. The laser microscope morphology of 2Cu1Ni electroless plated wood before heat treatment is 200 × (a-k temperature treatment is 25 °C, 100 °C, 120 °C, 150 °C, 180 °C, 200 °C ; embedded well depth map ; the processing temperatures of 3D images from B to l are 25 °C, 100 °C, 120 °C, 150 °C, 180 °C and 200 °C, respectively
Table 2. Surface Roughness
Fig. 7. The laser microscope morphology of 2Cu1Ni electroless plated wood after heat treatment is 200 × (a-k temperature treatment is 25 °C, 100 °C, 120 °C, 150 °C, 180 °C, 200 °C ; embedded well depth map ; the processing temperatures of 3D images from B to l are 25 °C, 100 °C, 120 °C, 150 °C, 180 °C, and 200 °C, respectively
Surface Morphology
Figure 8 shows the SEM morphology of wood surface via electroless Cu-Ni. After electroless Cu-Ni and different heat treatments, the distribution of metal coating on the wood surface tended to be flat. There were obvious pore structures on the coating’s surface (Fig. 8a), and there were local agglomerations. The pore structures were obvious, and the number of pore structures clearly increased with increase in temperature (Fig. 8b). More pore structures resulted in smaller grains (Fig. 8f). The pore morphology of electroless Cu-Ni composite coatings were ideal when the heat treatment temperature was 180 ℃.
Figures 9 and 10 show the area scanning and local point energy spectra of composite coatings of different heat treatments. To verify the uniform dispersion of metal particles in the coatings after heat treatment, two different areas were selected in the composite coatings scanning image. The composite coatings mainly included the five elements Ni, Cu, P, C, and O. The main elements were Ni and Cu, which further verified the results of laser confocal microscopy. At the same time, surface scanning and local point energy spectra showed that the Ni and Cu particles were tightly embedded together. Therefore, when the heat treatment temperature was 120 ℃, the number of pore structure was less, indicating that the morphology of electroless Cu-Ni composite coatings on the wood surface were ideal.
Fig. 8. Electroless 2Cu1Ni wood surface morphology after heat treatment (a, b, c, d, e, and f)
Cross-section Morphology of Composites
Figure 11 shows the cross-section morphology via electroless Cu-Ni with different heat treatments. It shows that the coating’s thickness would also change with the increase of heat treatment temperature, showing a trend of first increase and then decrease. Cu and Ni particles were fully embedded in the pores of wood, and the thickness of metal coatings on the wood surface reached 97.48 μm when the electroless temperature reached 120 °C (Table 3). The appearance of the interfacial morphology of the composite material verified that the surface and interface morphology of the composite coatings were better at 120 °C. The bright white wood in the cross-sectional morphology showed that the metal Cu and Ni particles penetrated into the wood pores, which was conducive to the formation of dense and high-strength composites. The cross-sectional morphology showed that a bright white area in the wood pores, which further illustrated that the metal Cu and Ni particles entered the wood pores. The heat treatment could accelerate the rearrangement of Cu and Ni metal particles rearrangement, and an ideal pore structure could be obtained. A conclusion could be drawn that the Cu and Ni nanoparticles could enter the pore structure of wood, so the intrinsic permeable channel design of wood where the conductive network structure was constructed, which could realize various reflection losses of electromagnetic waves.