3.1 Surface morphology
The morphology of electroless composite coating on wood surface was shown in Fig. 2. Figure 2a showed that the wood surface was first treated by electroless Ni for one time deposition, and then treated by electroless Cu for two times deposition. A uniform red layer coated the surface, and the red layer was unevenly distributed in the well depth map. In Fig. 2b, the metal layer on the surface of wood was arranged along the inherent wood grain to form a banded metal layer and there was a certain height difference between the banded layers. At this time, the surface roughness of the composite coating was 12.9 µm, and the linear roughness was 4.61 µm, which was shown in Table 2. Figure 2c showed that the wood surface was firstly treated by two times electroless Cu, and then one time electroless Ni deposition. The surface was covered with a uniform silver-gray coating, which was evenly distributed in the well depth map. In Fig. 2d, the distribution of metal layer on the surface of wood was still arranged along the inherent wood grain to form a banded metal layer, and there was a certain height difference between the banded layers. At this time, the surface roughness of the coating was 9.99 µm and the line roughness was 3.45 µm (Table 2). After electroless Cu, fine Cu particles can fill the inherent porous structure of wood and promote the formation of uniform metal layer on wood surface. In the meantime, uniform Cu layer provided an ideal self-catalytic substrate for the deposition of Ni particles. Therefore, firstly electroless Cu on the wood surface and then electroless Ni, the formed composite coating had relatively flat surface, and the linear roughness and surface roughness were ideal.
3.2 SEM Analysis
The SEM morphology of electroless composite coating on wood surface was shown in Fig. 3. Figure 3a showed that many smaller particles were distributed on the surface of wood, and local particles aggregated together. The larger particles in Fig. 3b closely covered the wood surface and had a larger pore structure locally. Figure 3c was a local enlarged view of Fig. 3a. It can be seen that Cu particles grew closely among Ni particles and embedded in Ni particles, indicating that there was no exposed wood, which proved that the wood was completely coated by Cu and Ni. Figure 3d and 3e showed that after electroless Cu and Ni treatment on wood surface, the surface coating was not only uniform but also had a large thickness. The thickness of the former coating was 157 µm, while that of the latter coating was 84 µm (Table 3). Due to the large size of Ni particles, the surface of the deposited wood was not fully covered by the inherent defects of the wood, and some particles will fill this part of the defects in the process of Cu particle deposition, resulting in the uniformity of the composite coatings. In addition, the difference of coating thickness on wood surface proved that the uniformity of substrate surface would directly affected the deposition thickness and uniformity of coating, which further restricted the self-catalytic reaction rate of coating. At the same time, the corrosion degree of the composite substrate was relatively large when the wood was electroless Cu and then electroless Ni deposition. The difference of composite interface substrate verified that the acidity condition of electroless Ni was easier to dissolve wood components. However, the magnification of the coatings in Fig. 3e showed that the Cu coating was uniformly deposited on the Ni layer surface, and the deposition Ni followed by deposition Cu had no effect on the composite coatings.
Table 3
Surface roughness of wood based composites
Type
|
Wood thickness
(µm)
|
Thickness of composite
(µm)
|
Coatingsthickness (µm)
|
1–2
|
308
|
392
|
84
|
2 − 1
|
198
|
355
|
157
|
3.3 XRD Analysis
The relationship between the electroless Cu-Ni treatment on different wood surfaces and the crystalline properties of the coatings was shown in Fig. 4. The crystal patterns of the samples were obtained by electroless Ni once before electroless plating Cu twice and electroless Cu two times before electroless Ni one time deposition. Figure 4a showed that the diffraction peaks at 2θ = 43, 50.54 and 74.44° were face-centered cubic structures Cu (111), (200) and (220), respectively [19]. The diffraction peak at 2θ = 44.5° was the characteristic peak of the face-centered cubic structure Ni (111) [20];In Fig. 4b, the intensity of the diffraction peak at 2θ = 53 ° had a local change [21], which may be due to the mutual extrusion of the Cu and Ni coatings on the wood surface༛Fig. 4c showed that the diffraction peak of Ni (220) had disappeared at 2θ = 78°, and the diffraction peak of Cu (220) had split at 2θ = 74.44°, which further proved that the metal Cu and Ni were tightly embedded together to form a dense composite layer. It can be concluded that the metal Cu and Ni on the wood surface were tightly embedded together, and the dense composite layer would refine the grain size of the wood surface composite.
3.4 Hydrophobic performance Analysis
The comparison of hydrophobicity of electroless coatings on wood surface was shown in Fig. 5. The figure marked 1 was the hydrophobic of wood surface by electroless Ni one time before plating Cu two times, and the figure marked 2 was the hydrophobic by electroless Cu two times before plating Ni one time. The contact angle of the former was 107.8° by three-point method and the latter reached 122.5 °. The surface of Cu block was smooth, and the contact angle was generally below 90° [21]. Similarly, the contact angle of Ni surface was also less than 90° indicating that they were hydrophilicity [22]. However, the composites via electroless Ni and Cu were hydrophobicity, and the composites via two electroless Cu depositions and one electroless Ni deposition showed better hydrophobicity. The reason was that the surface of wood was deposited by Cu layer twice and then Ni layer was deposited. Because of the smaller Cu particles, the inherent defects of wood surface were well filled. The metal Cu and Ni were tightly embedded together to form a dense and thick composite metal layer, which can avoid the penetration of water molecules and improve the hydrophobic effect of the composite coating.
3.5 Conductivity Analysis
Figure 6 was the electrical conductivity of composites with different treatment conditions, the data showed that the conductivity of the composite increased gradually with the extension of electroless Ni time. The conductivity of labelled 1–2 curve changed obviously. When the electroless Ni time was 25 min and 35 min, the conductivity curve showed inflection points. The conductivity of labelled 2 − 1 curve fluctuated greatly with time. After two times of electroless Cu and one time of electroless Ni, the electrical conductivity of the composite was small and controllable, and the average conductivity along the grain reached 2370.76 S/cm. The conductivity of coatings via electroless Ni and electroless Cu was showed in Table 4. The possible reason was that the porous structure of wood, the metal Cu deposited on the surface of wood filled the inherent defects, and the deposition again of metal Ni on the surface of Cu layer would promote its distribution to be more uniform. Because Ni particles were large, there were gaps at the contact interface, and gas phase may exist in the gap, which affects its conductivity.
Table 4
The conductivity of coatings via electroless Cu and electroless Ni
Type
|
Conductivity (S/cm)
|
One time electroless Ni deposition
|
258.15
|
Two times electroless Ni deposition
|
316.51
|
One time electroless Cu deposition
|
509.14
|
Two times electroless Cu deposition
|
1984.95
|
3.6 Electromagnetic shielding effectiveness Analysis
Figure 7 and Table 5 showed that different electroless Ni and Cu treatments on the wood surface, the EMSE of the composites presented different trends ranging from 300 KHz to 3.0 GHz. After two times electroless Cu and one time Ni deposition, the EMSE of the composite increased firstly and then decreased. When the deposition Ni time was up to 25 min, the EMSE of the composite was better, with an average value of 93.8 dB. After one time electroless Ni and two times Cu deposition, the electromagnetic shielding effectiveness of the composite increases first and then decreases. When the electroless Nitime was 30 min, the electromagnetic shielding effectiveness of the composite was better, with an average value of 75.8 dB. The above analysis verified that the composite prepared by electroless Cu firstly and then Ni layer had good EMSE.
Table 5
The relation between the order of deposition and electromagnetic shielding effectiveness
Type of deposited coatings
|
electromagnetic shielding effectiveness (dB)
|
1–2 (20 min)
|
72.6
|
1–2 (25 min)
|
73.3
|
1–2 (30 min)
|
75.8
|
1–2 (35 min)
|
72.7
|
1–2 (40 min)
|
69.2
|
2 − 1 (20 min)
|
92.6
|
2 − 1 (25 min)
|
93.8
|
2 − 1 (30 min)
|
86.7
|
2 − 1 (35 min)
|
90.5
|
2 − 1 (40 min)
|
86.3
|
Figure 7 showed that the EMSE of composites was not proportional to the conductivity. Figure 6 showed that the conductivity of the standard 1–2 was twice that of the standard 2 − 1, but the electromagnetic shielding effectiveness was 20 dB different. The comparison between Fig. 8a and Fig. 8c showed that the type 2 − 1 of composites had stronger electromagnetic wave absorption ability and the type 1–2 of composites had stronger electromagnetic wave reflection ability (Fig. 8c and Fig. 8d). Due to the good impedance matching, the incident electromagnetic wave can easily penetrate the interior of the composite coating, and only a few electromagnetic waves can be reflected. The absorption layer with positive conductivity gradient and negative magnetic gradient achieved strong electromagnetic absorption through strong hysteresis loss and dielectric loss [23]. Strong hysteresis loss was the main electromagnetic wave loss mode.
Electroless Cu and Ni coatings on wood had synergistic effect on electromagnetic shielding effectiveness, mainly due to separation of conductive network and specific interface polarization mechanism of composite coatings [24–25]. The desired conductive network of composites would show superior charge storage capacity and promote absorption of more incident electromagnetic microwave energy by polarization of electric field [23–26].
It was obvious that the SET value was up to 93.8 dB and effective absorption value of 0.9999 at 3 GHz, indicating that the multilayer composites can block over 99.99% of incident EM waves [23–26].
It should be noticed that the SET increased first and then decreased with increasing the conductivity, which is different from the previous reports where a higher conductivity will cause a higher SET. On the contrary, both the values of SET and SEA exhibited an increasing trend with the increase of magnetic permeability [27].
3.7 Hydrophobic property of electroless Ni time
The above analysis verified that the composite coating with electroless Cu twice and electroless Ni once on the wood surface had good hydrophobicity. In order to further study the hydrophobic properties of the composite coating, which were compared by selecting six treatment times of electroless Ni for 35 min, 40 min, 45 min, 50 min, 55 min and 60 min. Figure 9 was the surface contact angle test diagram of wood-based metal composites after electroless Cut wice and then electroless Ni once. Figure 9a, b, c, d, e and f were the treated samples with different electroless Ni time, respectively, 35 min, 40 min, 45 min, 50 min, 55 min and 60 min. The data showed that the contact angles of wood-based composites were 112.2, 114.6, 121.6, 122.5, 123.0 and 114.0° (Table 6). When the deposition Ni time was up to 55 min, the hydrophobic property was the ideal. Here, the contact angle was 123.0°.
Table 6
The contact angle of electroless Cu/Ni composites
Electroless Ni time (min)
|
contact angle (°)
|
35
|
112.2
|
40
|
114.6
|
45
|
121.6
|
50
|
122.5
|
55
|
123.0
|
60
|
114.0
|
The reason was that the metal layer coated on the wood surface, which would promote the self-catalytic reaction rate and prolong the electroless Ni time. The roughness of the metal layer decreases gradually. The smoothness of the coating surface tended to be uniform and the particles tended to be close. The small Cu particles can make up for the inherent defects of the wood. The Ni particles grown up on the surface of the uniform Cu layer, and the metal layer structure on the wood surface was dense. Therefore, the hydrophobic property of the composites was best.
3.8 Formation mechanism of composite coatings
Microscopically, wood had inherent characteristics with micro/nano multi-scale pore structure, and its natural skeleton morphology can be used as matrix templates for other materials [28]. In this study, the surface pores of wood would increase after degreasing treatment. A part of Cu element was deposited on activated pores and wood surface during electroless Cu, then Ni layer was deposited on Cu layer by electroless again. The data of electromagnetic shielding effectiveness (Table 5) showed that the conductive composite obtained by firstly electroless Cu then electroless Ni had ideal electromagnetic shielding effectiveness [29]. Electromagnetic waves went through multilayer composites, and a large amount of free charge accumulated spontaneously between the metal layer and the wood [30], three Ni/Cu Cu/Cu Cu/Wood layers with different electrical–magnetic properties can induce multiple reflections at each interface, which promote to the absorption attenuation [23]. Figure 3b demonstrated that the microstructure of nickel grains is coral-like, and there are many space and pores between the nickel particles, which increased the multiple reflection and absorption effects of the incident electromagnetic waves between them [30]. In addition, due to the inherent porous structures, the Cu-Ni coatings were also endowed with 3D cell structure, as depicted in the Fig. 10 [30]. The electromagnetic waves was reflected and scattered repeatedly within the porous metal coatings and was significantly attenuated by reflection loss and dielectric loss [31–33]. At the same time, using this efficient multilayer structure, multiple reflection and absorption of incident electromagnetic microwave between Cu and Ni layers are carried out [30].With the extension of electroless Ni time, metal Cu and Ni were tightly embedded together. Cu layer and Ni layer squeezed each other to form a dense composite layer, which had good electromagnetic shielding effectiveness. The formation mechanism of electroless Cu-Ni composite coating on wood surface was shown in Fig. 10.