Improving the microwave absorption in Ni-coated fabrics by saccharin addition in plating bath

Abstract

Polyester fabrics are coated uniformly by nickel thin film nanostructure with 200 nm thickness by electroless plating method. This study investigates the effect of saccharin as an additive in plating solution, on the morphology, grain size, crystallinity, surface resistivity, and magnetic properties of the coated fabrics which influences the microwave shielding strongly. The morphology changes from spheroid to blade shape with an average grain size of 85 ± 10 nm and the surface resistivity reached to its lowest value in 5 g l⁻¹ saccharin concentration. It has been seen that the crystallinity is modified and the magnetic behavior becomes softer in the presence of the additive. The measurements on microwave shielding effect in 8.2–18 GHz frequency range exhibited 98–90 and 96–85% effective transmission loss (without and with the additive) by flexible and lightweight Ni-coated fabrics. The shielding effect becomes dominated by the absorption mechanism as higher eddy current loss, higher magnetic hysteresis loss, and shape anisotropy along blade shape nanostructure while saccharin is added to plating bath.

Introduction

Recently, electromagnetic (EM) radiation and microwave shielding is a topic of interest due to the worldwide electronic system applications such as cellular phones and wireless devices [1,2]. Among all of the shielding mechanisms, absorption attracts more attention for applications such as human protection, electromagnetic interference (EMI) shielding in sensitive electronic devices, and military applications. In the recent years, polymeric base nanocomposites with various dielectric and magnetic nanostructures as fillers are the most attractive and efficient microwave absorber materials that can be used in designing lightweight and flexible microwave shields [3–7]. On the other hand, reflection from the high conductive metallic surfaces is a traditional approach for EMI shielding.⁸ Also, flexible and lightweight surfaces coated with metal thin film with appropriate thickness can have similar shielding effectiveness [8–11]. Polyester fabric can be used as a flexible, lightweight, and low-cost substrate for metal deposition which has high resistance in chemical bath. There are some reports from different research groups in EMI shielding of metal thin film-coated fabrics in 100–1800 GHz frequency range [9–12]. Akman et al. [13] investigated the microwave absorption of Ni, Co, and various ratio of these metal nanoparticle-coated polyacrylonitrile fabrics in 8.2–18 GHz. In another research, Gupta et al. [14] studied the microwave absorption of cotton-coated fabrics with Ni–Zn and carbon formulation in X and Ku band. Nickel is one of the ferromagnetic metals which are good candidates for microwave shielding. Among various coating methods, electroless plating is one of the simplest and industrial methods to grow metal thin film from micrometer to nanometer grain size by controlling the deposition conditions and bath contents [15,16]. Some researchers have investigated the effect of additives in Ni electroless plating and electrodeposition on different substrate [17–19]. The main objective of these researches is to study the effect of additive on the grain size, crystallinity, hardness, roughness, and internal stress of Ni structure [17–21]. But the effect of saccharin on different characteristics of Ni thin film deposited on the polyester fabrics has not been investigated so far.

Here, a uniform nickel thin film on polyester fabrics which has obtained nanostructure by adding saccharin in electroless plating solution (EPS) is achieved.

The effect of saccharin as an additive in EPS on morphology, crystallinity, surface resistivity, and magnetic and microwave shielding properties of coated textile is investigated in this study. The main goal of this research is to relate the microwave absorption coefficient to the variation in some physical characteristics of Ni nanostructure thin film that is coated in the presence of the additive.

Experimental method

One hundred percent polyester plain-woven monofilament fabric with 45.5 g m⁻² area density and 120 mesh counts (per centimeter) was used as substrate. Polyester fabrics were uniformly coated with a thin film of nickel with nanostructure morphology on the surface by electroless plating method, which is a common technique in printed board circuit industry. Coating procedure contains five main steps as follows. At first, some microscopic holes were produced on the surface by immersing all the pristine fabric in 45℃ etching bath consisting of 8 vol% HCl (37%, Merck), 8 vol% H₂SO₄ (98%, Merck), and 4 vol% H₂O₂ (Merck) as strong oxidizing agents for 90 s and hence the substrate’s surface becomes hydrophilic. The substrates were plunged in to 20 vol% H₂SO₄ solution at 65–70℃ for 45 s as neutralizer bath. Then, immersing preetched fabric in PdCl₂/SnCl₂ (99%, Merck/99.9%, Samen Co., Iran) bath at 45–50℃ for 3 min to absorb catalyst on the substrate. In this method activation and sensitization are done at the same time. Palladium particles nucleate on the surface. To omit additional Sn(OH)₂, all the samples are immersed in accelerating bath (20 vol% H₂SO₄ solution) at 65–70℃ for 30 s. Substrates are washed thoroughly with distilled water after each step. Finally, uniform coating of Ni thin film over pretreated polyester fabric as cathode was carried out using EPS containing 50 g l⁻¹ NiSO₄·6H₂O (99.9%, Merck) as metal salt and 50 g l⁻¹ NaPO₂H₂·H₂O (99%, Sigma-Aldrich) as reducing agent. The pH value of plating bath was adjusted to 8.5 by adding NH₄OH (28%, Merck) solution. Nickel deposition time was 5 min for all the samples. In some cases saccharin (C₇H₅NO₃S, Merck) with different concentrations (2, 5, and 10 g l⁻¹) was added to coating bath as an organic additive.

Measurement technique

The surface morphology of the samples was observed using TESCAN SEM system. The crystal structure of the samples was analyzed by X-ray diffraction (XRD) with CuKα (1.541 Å) incident radiation by an X’pert PRO. The magnetic properties of nickel-coated polyester fabrics were carried out by a vibrating sample magnetometer (VSM) (AGFM/VSM 3886 Kashan, Iran) with a magnetic field strength of 10 kOe. The thickness of the sample was measured by SDL Atlas digital thickness gauge (M034A) according to ASTM D-1777 standard test method (4.14 kPa). Mass per unit area (g m⁻²) was calculated according to ASTM D-3776.

The surface resistivity was measured by using a homemade setup based on the AATCC 76-1995 test method. The resistance (R) between two electrodes of spacing (l) and width (w) was measured by a digital multimeter (APPA 305) and then using R = R_s $\frac{l}{w}$ the surface resistivity (R_s) was calculated in Ω sq⁻¹.

In order to evaluate the microwave properties, the complex scattering parameters (S₁₁ and S₂₁) were measured by a vector network analyzer (Agilent E8364B) and waveguides (in the frequency range of 8.2–18 GHz) [22]. Setup was calibrated before each measurement. Also all of the measurements have been carried out at room temperature. Each data was an average value of five measurements.

Reflection and transmission coefficients were calculated using measured scattering parameters as R=|S₁₁ (or S₂₂)|² and T=|S₂₁ (or S₁₂)|². Also the absorption coefficient was obtained as A=1−R−T.

Results and discussions

Some organic additives have influenced the metal deposition rate in electroless plating method. Higher cathodic overpotential in the presence of an additive improves the metal nucleation rate on the substrate. Here, the nickel nucleation rate increases via adding saccharin in EPS, effectively. As it has been mentioned in the following sections crystallinity, morphology, and some other physical characteristics change clearly by absorbing saccharin on the cathode surface.

Morphology

The SEM images indicate decrement in the Ni grain size on the surface and changing the morphology from spheroid to blade shape by adding and increasing the saccharin concentration.

In addition, the Ni grain size becomes smaller by increasing the saccharin concentration from 5 to 10 g l⁻¹, but the surface loses its continuity and uniformity due to very low growth rate which caused island growth on the surface that is obvious from the sample appearance. Figure 1(c) to (f) reveals that the sample that was coated in EPS with 5 g l⁻¹ saccharin is uniform with a good continuity in the structure. An average grain size in this sample is equal to 85 ± 10 nm.

Figure 1.

SEM images of (a) Ni-coated fabric, (b) scratched Ni-coated polyester fiber, (c) fiber surface coated without saccharin, (d) with 2 g l⁻¹, (e) 5 g l⁻¹, and (f) 10 g l⁻¹ saccharin in EPS.

Figure 2.

Average surface resistivity of nickel-coated fabric in different additive concentration in EPS.

As shown in Figure 1(b) the thicknesses of coated fabric and Ni thin layer deposited on it were about 50 µm and 200 nm, respectively.

Surface resistivity

Our measurements show the decrement in the surface resistivity (R_s) of Ni-coated polyester fabric by increasing saccharin concentration up to 5 g l⁻¹ (Figure 2). Moreover, an increment in the surface resistivity has been observed at higher concentration (10 g l⁻¹).

As shown in SEM images, this phenomenon can be as a result of evolution in morphology, high uniformity, continuity, and smooth Ni nanostructure on the surface in the presence of the additive that can improve the electron movement and electrical conductivity in the structure.

Furthermore, discontinuity was clear in the appearance of Ni thin film deposited on the fabric surface at higher additive concentration as low Ni growth rate and it caused further surface resistivity. As a result of our measurements, the sample that was coated in 5 g l⁻¹ additive in EPS has the lowest R_s. Therefore, we study the crystallinity, magnetic properties, and microwave absorption of the coated fabrics without saccharin in EPS and with concentration equal to 5 g l⁻¹ to determine the effect of the additive material in the structure and other physical properties.

Crystallinity

Figure 3 shows the XRD pattern of Ni-coated fabrics. The figure indicates the crystallinity of the deposited Ni structure, modified in the presence of saccharin in EPS.

Figure 3.

XRD pattern of the nickel thin film-coated fabric.

The reason for this effect is higher nucleation rate in the presence of saccharin where Ni atoms have enough time to form and make crystal structure. In addition, due to the substrate activation in Sn/Pd bath, some Sn and Pd atoms are inserted to the crystal structure and are replaced with Ni atoms as impurity. X’pertpro analyses predict the presence of Ni₁₇Sn₃ (JCPDS card 3-065-7194), Ni_0.525Pd_0.475 (JCPDS card 3-065-5788), and NiPd (JCPDS card 3-065-9444) compounds with the same space group (Fm3m). In our results, transition in the characteristic peaks to lower angles and an increment in the lattice constant occur due to higher atomic radius of impurity atoms with respect to pure Ni structure. The lattice constants for both of the samples, with and without saccharin in EPS, are 3.53 and 3.52 Å, respectively.

Magnetic properties

Figure 4 demonstrates the magnetization curve of two samples with respect to applied field at room temperature. This figure indicates the ferromagnetic nature of Ni nanostructure thin film deposited on the surface.

Figure 4.

Room temperature magnetization curve of the nickel-coated fabric.

In the presence of saccharin, the magnetic behavior becomes softer and consequently the magnetization saturation occurs in smaller field than the sample without any additive. Considering SEM images and XRD analysis, this phenomenon is due to the elongation of Ni grains on the surface and growth of the structure in (111) orientation that is an easy magnetization direction in nickel structure. Also, this phenomenon causes soft magnetic behavior for the sample that is coated in EPS with saccharin. Furthermore, as shown in this figure, the samples with saccharin possess higher remanent magnetization (M_r) value equal to 0.08 emu g⁻¹, coercivity (H_c, 13.7 Oe), and area of hysteresis loop compared to the sample without any additive (0.04 emu g⁻¹ and 6.0 Oe). Generally, crystalline and shape anisotropy are the main components that increase the H_c magnitude [23]. Based on the XRD results, both of the samples have the same crystal structure. Therefore, as confirmed in the presence of saccharin the morphology is different, shape anisotropy is strong, resulting in higher coercivity magnitude.

Microwave shielding

It appears that saccharin as an additive in EPS can change morphology, electrical and magnetic characteristics of nickel structure as mentioned above. Also, it can influence on the shielding effect (SE) of Ni-coated fabric in microwave region. Microwave absorption, reflection and transmission of both samples in the X and Ku bands frequency (8.2–18 GHz) are plotted in Figure 5(a) and (b).

Figure 5.

(a) Absorption and (b) reflection and transmission coefficients for nickel-coated polyester fabric in different EPS.

Based on the measurement results, reflection (0.72–0.55) is the dominant shielding mechanism for the sample that was coated in pure EPS. While 0.26–0.35 of the incident waves have been absorbed by this sample. For the Ni-coated sample in EPS that possess saccharin as an additive, both reflection (0.45–0.33) and absorption (≃0.52) contribute to shielding against microwave radiation. It is apparent that the absorption level improved by using saccharin as an additive is considerably due to increment in the conductivity and decrement of the skin depth $(\delta =\sqrt{\frac{2}{\omega \mu {\sigma }_{\mathit{tot}}}})$ . This phenomenon can be attributed to eddy current loss which is a loss mechanism in alternating magnetic field. Higher eddy current loss appears in the sample with higher conductivity and it can improve the microwave absorption effectively. In addition, magnetic hysteresis loss is another loss mechanism in magnetic media. According to Figure 4, coating in the presence of saccharin leads to larger hysteresis loop area and, consequently, higher microwave absorption. Moreover, shape anisotropy along nickel blade shape nanostructures, which have been grown in different direction, participates in the absorption coefficient and shift the resonance peak [24] of this sample effectively.

According to the results, both of the samples can be used as flexible, thin, and lightweight shields against incident EM field in 8.2–18 GHz frequency range. The total transmission loss of the sample without any additive was 98–90% (SE_tot = 17.0 − 10.0 dB) mainly via reflection. In the presence of additive 96–85% (SE_tot = 14.0 − 8.2 dB) of incident wave has been shielded by both absorption and reflection. Therefore, saccharin as an organic additive in EPS influences the SE of the Ni structure via absorption as well as some other physical properties.

Conclusion

In this study, a uniform thin film of nickel nanostructure with about 200 nm thickness was deposited on a polyester fabric using electroless plating method. We investigate the effect of saccharin as an additive on the EM properties of coated fabrics. Measurements show a decrement in the surface resistivity by increasing the amount of additive up to 5 g l⁻¹, which provides high eddy current loss in microwave region. Also, the morphology and crystallinity are evaluated via an increment in the nickel nucleation rate. The Ni grain’s shape varies into blade shape with an average grain size equal to 85 ± 10 nm, which has more shape anisotropy and microwave absorption loss than in the absence of saccharin. In addition, the additive affects the magnetization of Ni nanostructure by growing in easy magnetization direction (111). Higher coercivity and magnetic hysteresis loop area which are as a result of additive cause higher transmission loss. Although by applying additive, absorption is increased, the total SE is decreased in this case. Finally, lightweight, flexible, and uniform Ni-coated polyester fabric can shield successfully 98–90% (without any saccharin) and 96–85% (with saccharin 5 g l⁻¹) of the incident field.