Abstract
A wood-based composite exhibiting excellent electromagnetic shielding performance was prepared by electroless Ni plating. The properties of the material were characterized by a series of tests. The results showed that the growth route of Ni particles was first arranged along the inherent grain of the wood to form a banded metal layer. With extension of the duration of electroless plating, the growth of Ni particles gradually extended around and filled the pores between wood fibers, and finally formed flake-shape structure. The metal coatings formed a strip along the inherent grain of the wood surface and then changed into a sheet until it covered the entire wood surface. The coatings resistance was from 12 Ω to 0.5 Ω with the increase in duration of electroless plating. When the duration was 20 min, the composite coating resistance was 0.5 Ω. Here, the contact angle of composite coatings was 98.3° when the plating time was 15 min. When the wood surface was modified via two depositions of Ni, the average electromagnetic shielding value of the composites was over 80 dB in the frequency ranging from 0.3 kHz to 3.0 GHz.
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Performance and Preparation of the Electroless Ni Wood-Based Composites
Yanfei Pan,a,b,1 Dingwen Yin,a,b,1 Surigala,a,b Yinan Hao,a,b Dong Xing,b Sufen Hao,a,b Xiaofang Yu,a,b Haibiao Yu,a,b and Jintian Huang a,b,*
A wood-based composite exhibiting excellent electromagnetic shielding performance was prepared by electroless Ni plating. The properties of the material were characterized by a series of tests. The results showed that the growth route of Ni particles was first arranged along the inherent grain of the wood to form a banded metal layer. With extension of the duration of electroless plating, the growth of Ni particles gradually extended around and filled the pores between wood fibers, and finally formed flake-shape structure. The metal coatings formed a strip along the inherent grain of the wood surface and then changed into a sheet until it covered the entire wood surface. The coatings resistance was from 12 Ω to 0.5 Ω with the increase in duration of electroless plating. When the duration was 20 min, the composite coating resistance was 0.5 Ω. Here, the contact angle of composite coatings was 98.3° when the plating time was 15 min. When the wood surface was modified via two depositions of Ni, the average electromagnetic shielding value of the composites was over 80 dB in the frequency ranging from 0.3 kHz to 3.0 GHz.
Keywords: Electroless Ni; Wood; Resistance gradient; Electromagnetic shielding
Contact information: a: College of Material Science and Art Design, Inner Mongolia Agricultural University, Hohhot, 010018, China; b: Inner Mongolia Key Laboratory for Sand Shrubs Fibrosis and Energy Development and Utilization, Hohhot, 010018, China; 1: These authors contributed equally to this work; * Corresponding author: jintian_h@163.com
INTRODUCTION
As a material with light weight, easy preparation, and excellent shielding efficiency, wood-metal composite materials have developed rapidly in preparation technology. Covering a layer of metal on the wood surface can improve the electromagnetic shielding effectiveness, temperature resistance, wear-resistance, and strength of wood (Zhou 2011). In general, there are many methods for surface metallization of non-metallic materials, mainly vacuum thermal evaporation deposition, vacuum evaporation, sputtering, chemical vapor deposition, physical vapor deposition, electrophoresis, electroplating, chemical plating, and replacement plating. Electroless plating is widely used due to its simple operation and low cost (Zhou 2011).
In the 1980s, Japanese scholars carried out research on the chemical plating of wood. The research demonstrated the relationship between the volume resistivity of the particle board and the electromagnetic shielding effect. With a decreased volume resistivity of the particle board, the electromagnetic shielding becomes better. Li (2019) prepared a Cu film and a ZnO film on the surface of the wood veneer by magnetron sputtering on Pinus sylvestris. Shi (2016) added nanoparticles Al2O3, SiO2, and SiC to the plating solution to prepare an excellent Ni-P-nanoparticle composite coatings, using birch as the plating substrate. A wood-copper electromagnetic shielding material was developed with SE up to 60 dB (Wang et al. 2010). Qin (2015) chemically plated the wood of an artificial forest, which provided wood decoration, electric heat conduction, and electromagnetic shielding. He (2018) prepared wood/polyaniline electromagnetic shielding composite materials via in-situ polymerization. Chen (2016) used magnetic expanded graphite and wood fiber to prepare an electromagnetic shielding composite board. Zhu (2016) used waste wood chips to reprocess and use it to explore the work of replacing metal materials in certain corrosive environments, which has the effect of electrical conductivity and electromagnetic shield. Chen et al. (2019) used wood as a substrate to prepare wood-based composites with excellent elasticity and electrical conductivity by using delignification and electroless Ni plating. Wood chemical plating has been used to prepare electromagnetic shielding particle board (Wang 2017), wear-resistant materials (Pan et al. 2016), and synthetic wood reflection absorption integrated electromagnetic shielding materials (Guo 2017) by chemically plating wood with Cu and Ni.
During the period of application, the surface of the wood has been chemically plated with Cu for several times (Guo et al. 2017; Wang et al. 2010). The electromagnetic shielding effectiveness of the composite material in the frequency range of 300 kHz to 1.5 GHz can reach up to 60 dB (Guo 2017), which confirmed that the wood-based conductivity gradient structure could improve the absorption efficiency of electromagnetic waves. Therefore, if a controllable electromagnetic gradient multilayer structure can be developed, it is expected to break through the limitation of high absorption of electromagnetic waves by wood-based metal composite materials. However, due to the anisotropy of the anatomical structure of wood, its electrical properties are closely related to the texture (Liu and Zhao 2012). The resistivity of wood in the grain direction is greater than that in the grain direction. After electroless treatment on the wood surface, the growth rate of the surface coating in the transverse direction is faster than that in the direction following the texture (Pan et al. 2016, 2017). According to skin depth theory, the intensity of electromagnetic waves attenuates according to an exponential relationship with the increase of the incident depth of electromagnetic waves (Liang et al. 2017). The magnitude of material resistance and electrical conductivity directly affects the electromagnetic shielding effectiveness of materials (Wang et al. 2009; Liu et al. 2014; Lee et al. 2016; Sudesh et al. 2017; He et al. 2018; Li et al. 2018; Poothanari et al. 2018). Dense, continuous metal plating is very important for the preparation of high-efficiency chemically plated wooden shielding materials. Therefore, a profound understanding of the resistance gradient structure of chemical plating on the surface of the wood is the key to the preparation of efficient electromagnetic shielding materials that need to operate in a wide frequency range. At present, the electromagnetic shielding effectiveness to prepare light electromagnetic shielding materials based on wood is between 30 and 60 dB. It is convenient and controllable to self-assemble to build a light and controllable conductivity gradient multilayer structure. The relationship between the resistance gradient of the coating and the duration of electroless plating has received insufficient research attention.
In this study, the time duration of electroless deposition of Ni plating was considered as an independent variable. The change rule and optimal of the resistance gradient in the process of electroless plating of Ni on the wood surface were analyzed relative to the performance according to the changes in the wood surface and internal micro-morphology of each time period. In addition, scanning electron micrographs were obtained as a function of deposition time, making it possible to track the progress of development of electrically conductive material.
EXPERIMENTAL
Preparation of Specimens
Materials
The fast-growing poplar wood (hardwood) was procured from Saihan district, Hohhot, China. It was later cut into 11 cm (diameter) × 0.34 mm (thickness) circular pieces after the wood had been soaked in water for 30 min. Then, the wood was treated with #600 sandpaper.
Substrate pretreatment
The circular pieces treated with #200 sand paper were placed in a beaker containing distilled water at 100 °C for 120 min. Then they were taken out and placed in a beaker containing distilled water (900 mL).
Electroless Ni on wood surface
Wood was activated with activation solution A (15 g/L nickel sulfate hexahydrate, 12 mL/L hydrochloric acid) for 15 min. Next, the wood was activated in solution B (15 g/L sodium borohydride, 15 g/L sodium hydroxide) for 90 s and removed when no liquid was dripping. The electroless Ni was conducted. Electroless Ni solution included 33 g/L nickel sulfate hexahydrate, 28 g/L sodium hypophosphite, 30 g/L trisodium citrate dihydrate, and 10 mg/L thiourea. The process was conducted at pH=9 (adjusted with 30 mL/L ammonia) and 60 °C. After the plating solution was reconstituted, Ni was continuously electrolessly plated under the same conditions, covered with a 0.34 mm thick round poplar wood board, compacted, and dried at 60 °C for 4 h.
Fig. 1. Preparation of electroless Ni composite materials on wood surface
Surface Characterization
RTS-8 four-probe tester
A thin, Ni-coated wood chip was placed on the test platform of the RTS-8 four-probe tester (Guangzhou Four Probe Technology Co. Ltd, GuangZhou, China; the probe was slowly adjusted to contact the sample. After confirming that the probe contacted the sample, the sheet resistance was checked in the main interface of the software to start the measurement. The wood chip sample was measured at five different positions for horizontal and straight lines, and five different points for each position. The average value for resistance was recorded.
Scanning electron microscopy (SEM)
The surface morphology of the material was characterized by scanning electron microscope (Phenom, PW-100-516, Eindhoven, Holland).
Confocal microscopy
A VK-X160 laser confocal microscope (Keyence, Osaka, Japan) was used. The samples were placed in the center of the square sample table. The settings “multiple of 20” and “surface measurement method” were selected. The surface roughness and “3D” model were measured. These steps were repeated for five different positions for each sample. The average value was calculated.
Hydrophobic test
After dripping water on a dry sample for 30 s, a JY-PHa contact angle measuring instrument (Chengdeyoute Testing Instrument Manufacturing Co. Ltd., Cheng De, China) was used to characterize the hydrophobic properties of the material.
Electromagnetic shielding effectiveness test
The DR-S01 shielding effectiveness test device (DR Beijing Technology Co., Ltd., Beijing, China) was used to measure the electromagnetic shielding effectiveness of the composite material at a frequency from 0.3×103 to 3.0×103 MHz. The thickness of the sample used for electromagnetic shielding measurement was 1.5 mm.
RESULTS AND DISCUSSION
Morphology of Electroless Ni Plating on Wood Surface
Figure 2 shows the surface morphology of the wood surface treated with different duration of electroless plating. Figure 2a shows the pit structure of the wood. The scale of roughness of the wood was quite large, as shown in Fig. 2b. Figure 2c shows the surface morphology of electroless Ni on the wood surface following 1 min of deposition. With the extension of duration of electroless Ni plating, the white strip area on the wood surface gradually decreased, while the black strip gradually increased (Figs. 2e, 2g, 2i, and 2k). The black material was generally distributed along with the wood texture, and the morphology gradually became flat in the 3D image with the extension of duration of plating (Figs. 2d, 2f, 2h, 2j, and 2l). Previous experimental results determined that the black material is Ni particles. Ni particles grow quickly on higher wood surface and are gradually deposited in areas, which makes the wood surface smooth and flat (Pan et al. 2016). Table 1 showed that as the time of electroless Ni was prolonged, the surface roughness of the wood surface gradually decreased (Fig. 2), further verifying the effect of Ni content on the rate of electroless plating reaction. As the duration of electroless plating increased, Ni particles on the wood surface gradually increased and filled the defects of the wood, making the originally uneven wood surface smooth and flat. When the electroless plating time was 20 min, the surface roughness of the metal coating on the wood surface was 8.014 μm. The wood surface tended to be flat, which demonstrated that the rate of metal ion deposition on the wood surface is determined by the catalysis of the substrate and the deposition layer (Nagasawa et al. 1999). With the extension of the duration of plating, the substrate surface was gradually coated with a layer of metal Ni. The deposited Ni particles autocatalyze Ni2+. With the extension of the duration of plating, the catalytic ability of the deposited Ni matrix was enhanced.
Fig. 2. The shape of the laser copolymerization microscope on the surface of the wood at different electroless plating times is 200× (a: 0 min, c:1 min, e:5 min, g:10 min, i:15 min, k:20 min inset shows a 3-D picture, a to k depth diagrams from different durations of electroless plating)
Table 1. Roughness of Metal Coating on Wood Surface
SEM Morphology of Composite Materials with Different Electroless Ni Plating Time
Figure 3 shows the surface morphology of the electroless Ni layer at different durations of electroless plating. It is evident that the pit structure of the wood was clearly visible when the wood was treated without electroless Ni (Fig. 3a). When the wood was treated with electroless Ni for 1 min, the wood surface was uneven, and the metal layer was unevenly distributed (Fig. 3b). With the extension of the duration of plating, the surface of the wood tended to be flat, and the size of the surface particles gradually increased. The pit structure of the wood tended to be covered, and the surface brightness gradually increased. The growth route of the Ni particles was first arranged on the strip metal layer along with the inherent pit structure of the wood (Figs. 3c and 3d). The growth gradually extended to the surroundings, and then it filled the pores between the fibers, eventually forming a lamellar structure, and then forming a lamellar shape until covering the entire wood surface (Figs. 3e and 3f).