Microwave absorption coating based on assemblies of magnetic nanoparticles for enhancing absorption bandwidth and durability

Progress in Organic Coatings 141 (2020) 105538

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Microwave absorption coating based on assemblies of magnetic nanoparticles for enhancing absorption bandwidth and durability

T

Yibing Maa, YaYa Zhoua, Chunhong Qia, Youyi Suna,*, Yinghe Zhanga,b, Yaqing Liua,* a b

Shanxi Province Key Laboratory of Functional Nanocomposites, North University of China, Taiyuan, 030051, PR China Nanotechnology Department, Helmholtz Association, Hamburg, 21502, Germany

ARTICLE INFO

ABSTRACT

Keywords: Microwave absorption Superhydrophobic Core-shell magnetic nanoparticles Absorption bandwidth Durability

A new class microwave absorption coating based on assemblies of core-shell magnetic nanoparticles was facile preparation to improve absorption bandwidth and durability. The microwave absorption coating with a thickness of ca.1.5mm exhibited a wide bandwidth (ca.9.2GHz). Furthermore, the durability of microwave absorption coating was further evaluated. It showed good water-shedding ability and mechanical stability. The excellent performance was mainly attributed to special surface rough structure and low surface energy of coating, resulting from assemblies of core-shell magnetic nanoparticles. These provide a new idea to design and preparation of microwave absorption coating with high performance for application in stealth technology.

1. Introduction Microwave absorption materials (MAMs) have attracted lots of interesting due to absorb electromagnetic waves effectively and dissipate microwaves by interference for application in military stealth technology and electronic devices [1,2]. Over the past decade, lots of MAMs have been developed and reported for these purposes, such as multilayer blocks, thin films and coating [3–5]. Among, development of coating was one of the most effective strategies due to be low cost, lightweight and easy process [6–9]. As well-known, the light mass, thin thickness, wide bandwidth and strong wave absorption performance are the key parameters for real application of microwave absorbing coatings. Up to now, there were some works reporting the structural design of microwave absorbing coating to optimum these parameters, such as synthesis of new microwave absorption particles, the preparation of multi-layer coating and enhancing thickness of coatings. For example, a new microwave absorption coating based on ternary system of graphene oxide, nickel oxide nanoparticle and barium hexaferrite nanoparticle have been successfully prepared and had a reflection loss less than -20.0 dB in 8.0∼12.0 GHz frequency range [6]. A double-layer coating based on NiFe2O4-MWCNTs/epoxy nanocomposite was prepared by in-situ polymerization method. A narrow bandwidth of 1.6 GHz (RL≤-10 dB) was obtained at a total thickness of 2.0 mm [7]. A double-layer flexible coating based on carbon-coated nickel nanoparticles (Ni@C), carbon nanotubes (CNTs) and silicone resin was prepared. The narrow bandwidth of 3.7 GHz (RL≤-10 dB) was achieved

⁎

for the optimum double-layer coating with a total thickness of 2.0 mm [8]. In a comparison, the double-layer coating based on graphene nanoplatelets (GNPs)/BaTiO3/epoxy nanocomposite was designed and prepared. A wide bandwidth of 13.9 GHz (RL≤-8.0 dB) was obtained for the double-layer coating with a total thickness of 5.0 mm [9]. From these respects, enhancing thickness was still most effective method to improve absorption bandwidth of microwave absorption coating. Therefore, it is still high challenge to obtain absorption bandwidth at a thin thickness. In addition, in practical applications, it was inevitable for microwave absorption coating to encounter rainy, ice, wet-snow and other adverse environments, leading to a decrease or loss of the microwave absorption performance [10,11]. It made stealth fighters to be often retired [12]. Therefore, the good durability of microwave absorption coatings should be highly demanded for its real applications [13,14]. However, durability of microwave absorption coating has been few considered and reported. (Table 1) Here, a new class microwave absorption coating based on assemblies of core-shell magnetic nanoparticles was designed and prepared. The microwave absorbing coating had well-defined surface rough structures. This microwave absorbing coating did not only exhibit a high microwave absorption performance, but also showed good durability, which is a promising candidate for stealth materials.

Corresponding authors. E-mail addresses: [email protected] (Y. Sun), [email protected] (Y. Liu).

https://doi.org/10.1016/j.porgcoat.2020.105538 Received 27 September 2019; Received in revised form 1 January 2020; Accepted 1 January 2020 0300-9440/ © 2020 Published by Elsevier B.V.

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Table 1 Microwave absorption properties of microwave absorption coatings reported in present and previous works. Absorption coating

Thickness /mm

Reflection Loss/dB

Frequency /GHz

Bandwidth [RL < -10 dB]

NCP Fe3O4@SiO2@OTS/NCP GO/NiO/BaFe12O19/EP NiFe2O4-MWCNTs/EP Ni@C/CNTs/silicone resin GNPs/BaTiO3/EP

1.5 1.5 2.0 2.0 2.0 5.0

−29.1 −36.1 −24.0 −19.0 −25.3 −29.4

14.0 13.5 – 10.6 12.7 16.2

5.0[11.5–16.5] 9.2[8.8–18.0] 4.0[8.0–12.0] 1.6[9.9–11.5] 3.7[11.2–14.9] 13.9[4.1–18.0]

This work 6 7 8 9

Scheme 1. Synthesis process of Fe3O4@SiO2@ OTS nanoparticles.

Scheme 2. Preparation process of microwave absorption coating based on assemblies of core-shell magnetic nanoparticles.

2. Experimental section

stirring. After the reaction, the dark green solution was filtered and washed with water and methanol to remove excess ammonium sulfate and aniline. The stable rGO dispersion solution was prepared as shown in following. 0.5 g PVP was dissolved in 400.0 mL GO dispersion solution (2.5 mg/mL). 10.0 mL hydrazine hydrate (80.0 wt%) and 20.0 mL ethanol was mixed, and then was dropped into above rGO/PVP mixing solution at 80.0℃ under vigorous stirring. The reaction was continued for 3.0 h. Above three dispersion solutions were mixed under ultrasound stirring for 30.0 min to form Fe3O4/rGO/PANI nanoparticles (Fe3O4:rGO:PANI = 7:1.5:1.5) by the electrostatic adsorption.

2.1. Materials FeCl2·4H2O, FeCl3·6H2O and NaOH were purchased from Shanghai Chemical Reagent Co., Ltd. Ammonium per sulfate (APS), aniline, ethanol (C2H5OH) and hydrochloric acid (HCl) were purchased from Nanjing Chemical Reagent Co. Ltd. Graphene oxide was purchased from Tang Shan Jianhua Graphite Co. Ltd. Ethyl silicate (TEOS), urea, cetyltrimethyl ammonium bromide (CTAB), 1-amyl alcohol, cyclohexane and toluene were purchased from Tianjin Guang Fu technology development Co., Ltd. N-octyl triethoxysiloxane was purchased from Aladdin. Epoxy and curing agent were purchased from Shanghai Yue Ke Composite Co., Ltd. Q235 was purchased from the local chemical market.

2.3. Synthesis of Fe3O4@SiO2 modified with N-octyl triethoxysiloxane (Fe3O4@SiO2@OTS nanoparticles) Fe3O4@SiO2@OTS nanoparticles were synthesized by a simple method as shown in Scheme1. Firstly, Fe3O4@SiO2 nanoparticles were synthesized as shown in following. 1.0 g Fe3O4 nanoparticles were dispersed in a mixing solution (100.0 mL ethanol, 25.0 mL water, 5.0 mL ammonia and 2.0 mL TEOS) under mechanically stirring for 60.0 min. The above mixture was reacted at 120.0℃ for 5.0 h. Then, the Fe3O4@SiO2 nanoparticles were separation by the magnet and washed by ethanol solution. Secondly, 10.0 g Fe3O4@SiO2 nanoparticles and 20.0 mL N-octyl triethoxysiloxane were dispersed in 50.0 mL toluene solution. The mixing solution was reacted at 120.0℃ for 20.0 h. The Fe3O4@SiO2@OTS was separation by the magnet, washed by ethanol solution and dried at 60.0℃ for 12.0 h.

2.2. Synthesis of Fe3O4/rGO/PANI nanoparticles Fe3O4/rGO/PANI nanoparticles were synthesized by a facile solution mixing method as shown in following. Fe3O4 nanoparticles were firstly synthesized by chemical co-precipitation method according to previous works [13–15]. 3.6 g ferrous chloride (FeCl2·4H2O) and 6.1 g ferric chloride hexahydrate (FeCl3·6H2O) were dissolved in 90.0 mL water. The 92.5 mL NaOH aqueous solution (53.6 mg/mL) was dropped into above mixing solution at 50.0℃ under vigorous stirring. The mixture was reacted at 50.0℃ for 90.0 min. The Fe3O4 nanoparticles were separated with a magnet and washed for several times with water to remove excess materials. And then the Fe3O4 nanoparticles were dispersed in 120.0 mL water to form stable Fe3O4 dispersion solution. The PANI was synthesized as shown in following. 0.93 g aniline and 2.3 g ammonium persulfate were dissolved in 10.0 mL HCl solution (1.0 M). The mixing solution was reacted at 20.0℃ under vigorous

2.4. Preparation of microwave absorption coating based on assemblies of core-shell magnetic nanoparticles The microwave absorption coating was facile preparation by a spraying method as shown in Scheme 2. 20.0 g epoxy resin, 4.0 g amine 2

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Fig. 1. (A) XRD, (B) IR spectrum, (C) SEM image, (E) TEM image and (F) VSM of Fe3O4@SiO2@OTS. The inset of F is the optical photograph of Fe3O4@SiO2@OTS dispersed in ethanol solution.(D)TEM image of Fe3O4 nanoparticles.

curing agent and 4.0 g Fe3O4/rGO/PANI nanoparticles were mixed under mechanical stirring to form micro-wave absorption paints. Then, the micro-wave absorption paints were coated on surface of rubber substrate by the spraying method. The above coating was treated at 80.0℃ for 45.0 min. Finally, the Fe3O4@SiO2@OTS dispersion solution was sprayed on surface of polymer composites based on Fe3O4/rGO/ PANI nanoparticles and epoxy resin, and then was further completely cured at 80.0℃ for 4.0 h.

Nanovea ST400 non-contact white light profilometer (Nanovea, Irvine, CA, USA). The magnetic properties of the products were characterized by a sample vibrating magnetometer (VSM, Verso lab, Quantum Design, USA) at room temperature with the applied magnetic field of -12.0 kOe–12.0 kOe. Fourier-transform infrared spectroscopy (FT-IR) study was performed using KBr pelletson a Perkin Elmer FTIR spectrophotometer (Perkin-Elmer, USA). The Raman spectroscopy was characterized by a LabRAM HR-800 Raman spectrometer (Horiba Scientific), with an incident laser beam of 532.4 nm. Self-cleaning test was carried out as shown in following. The tilt angle of samples was less than 10°. The soil and sand (< 80.0 μm) were deposited on the surface of samples. The self-cleaning process by rolling water droplets was recorded by a high definition video camera (Nikon D 7200). Anti-icing test was carried out as shown in following. The samples were stored in the -20.0℃ freezer and were tilted at an angle of about 5.0° during the experiments. After 30.0 min, 500.0 mL super-cooled

2.5. Characterization The structure and phase of the product were identified by powder Xray diffraction (XRD, Smartlab (3), Rigaku, Japan) with Cu Kα radiation diffraction (λ = 0.154 nm, operating voltage 35.0 kV, current 40.0 mA) over the scan range 5-80°, scanning speed 4.0° min−1. The micro-structure was characterized by scanning electron microscopy (SEM, Su-8010) and transmission electron microscopy (TEM, JEM-2010). The micro-structure of materials was observed by Field emission scanning electron microscopy (FE-SEM, Hitachi SU8010) and a

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Fig. 2. (A)XRD and (B)IR spectrum of Fe3O4/rGO/PANI. (C)Raman spectra of (a) Fe3O4/rGO/PANI, (b) PANI and (c)rGO. (D)SEM image and (E)TEM image of Fe3O4/rGO/PANI.

water was poured or sprayed onto coating. Occurrence of icing was determined by eye inspection. The complex permittivity (ε=iε'-jε”), complex permeability(μ=iμ'jμ”) and microwave absorption properties of the samples were measured using an Agilent vector network analyzer E5071C by the transmission/reflection method in the frequency range of 2−18 GHz.

[14]. Fig.1B shows the IR spectrum of Fe3O4@SiO2@OTS. The absorption peak at 576.0 cm−1 was attributed to the Fe-O bond of Fe3O4 [17]. In addition, the strong absorption peaks at 1096.0 cm−1 and 1632.0 cm−1 were attributed to the Si-O-Si stretching and HeOeH bending vibrations of N-octyl triethoxysiloxane, respectively [18]. The absorption bands at 2920.5 cm−1 and 2849.0 cm−1 were attributed to the −CH2 asymmetric stretching mode of N-octyl triethoxysiloxane [18]. These results confirmed the formation of Fe3O4@SiO2@OTS. The micro-structure of Fe3O4@SiO2@OTS nanoparticles was further characterized by the SEM and TEM image as shown in Fig.1C. It clearly showed uniformly spherical shape with a diameter of ca.80.0 nm for the Fe3O4@SiO2@OTS. The above result was further confirmed by the TEM image as shown in Fig. 1D and E. As can be seen from Fig. 1D, the particle size of pure Fe3O4 was about 60 nm. It clearly showed larger nanospheres with an approximate diameter of 85.0 nm and homogeneous core-shell structure as shown in Fig. 1E. The size of core (Fe3O4) and shell (SiO2) was about 65.0 nm and 20.0 nm, respectively.

3. Results and discussion The formation of Fe3O4@SiO2@OTS nanoparticles was confirmed by the XRD, IR, VSM, SEM and TEM image as shown in Fig.1. Fig.1A shows the XRD characterization of Fe3O4@SiO2@OTS. A broad characteristic diffraction peak at 22.0° was observed, which was assigned to amorphous silica oxide [16]. At the same time, it also clearly showed some strong diffraction peaks at 29.9°、35.4°、43.0°、53.3°、56.9° and 62.6°, which were assigned to the (220), (311), (400), (422), (511) and (440) planes of Fe3O4 (JCPDS card, file no. 79-0418), respectively 4

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Fig. 3. (A) Description of coating hydrophobic property schematic diagram, (B)XRD and (C)VSM of (a)NCP and (b) Fe3O4@SiO2@OTS/NCP.

422.5 cm-1, 559.5 cm-1 and 771.5 cm-1 were assigned to Fe3O4 nanoparticles [21]. These results confirmed formation of Fe3O4/rGO/PANI nanoparticles. Micro-structure of Fe3O4/rGO/PANI nanoparticles was characterized by the SEM image and TEM image as shown in Fig. 2D and E, respectively. They both showed a typical film-like structure due to present rGO. In addition, there was some spherical nanoparticles and few aggregations as shown in Fig. 2E. These results indicated that the Fe3O4 nanoparticles and PANI were uniformly decorated and uniformly anchored on the rGO nanosheets. The microwave absorption coating based on assemblies of core-shell magnetic nanoparticles was prepared by spraying method. It was composed of topcoat and ground coat as shown in Fig. 3A. The topcoat resulted from assemblies of Fe3O4@SiO2@OTS nanoparticles, providing roughness and low surface energy. The ground coat was the polymer nanocomposite coating (NCP) based on Fe3O4/rGO/PANI and epoxy resin, which was acted as the adhesive agent of Fe3O4@SiO2@OTS nanoparticles. Fig. 3B shows the XRD of NCP and Fe3O4@SiO2@OTS/ NCP. It clearly presented some diffraction peaks at 2θ = 30.1°, 35.5°, 43.1°, 53.7°, 57.3° and 63.0° for all samples, corresponding to (220), (311), (400), (422), (511) and (440) reflections of Fe3O4 (JCPDS card, file No.19-0629), respectively [17]. In addition, the diffraction peak at 2θ = 22.0° was assigned to EP matrix [22]. These results indicated the formation of microwave absorption coating based on Fe3O4 and EP. Fig. 3C shows VSM of NCP and Fe3O4@SiO2@OTS/NCP, in which the MS was 25.0 emu/g and 29.2 emu/g, respectively. It could be found that the Fe3O4@SiO2@OTS/NCP coating showed better magnetic properties comparing to single NCP coating. The result was attributed to that the Fe3O4@SiO2@OTS/NCP loaded more magnetic particles. The high magnetic property was benefit to solve the impedance matching problem and enhance microwave absorption properties. Surface structure of NCP and Fe3O4@SiO2@OTS/NCP was characterized and compared by the SEM images and non-contact white light profilometer as shown in Fig. 4. As shown in Fig. 4A, the single NCP

These results indicated the formation of Fe3O4@SiO2@OTS nanoparticles with core-shell structure. Fig. 1F shows VSM of Fe3O4@SiO2@ OTS, in which the saturation magnetization (MS) was about 16.7 emu/ g. The result indicated that the Fe3O4@SiO2@OTS nanoparticles showed good magnetic property. In addition, the Fe3O4@SiO2@OTS nanoparticles showed good dispersion and stability in ethanol solution, which could be still separated entirely from the suspension solution in 10.0 s by an external magnet as shown in inset of Fig. 1F. It was worth pointing out that neither the mechanical mixture of Fe3O4 nanoparticles nor SiO2 responded to an external magnet, and Fe3O4 nanoparticles would be separated from the suspension while the SiO2 would remain in suspension. These results further confirmed the formation of Fe3O4@ SiO2@OTS nanocomposite. The formation of Fe3O4/rGO/PANI nanoparticles was confirmed and determined by the XRD, IR and Raman as shown in Fig. 2A, Fig. 2B and C, respectively. As shown in Fig. 2A, some strong diffraction peaks at 30.2°, 35.6°, 43.3°, 57.3°, 53.8°, 63.5° and 74.2° were observed, corresponding to (220)、(311)、(400)、(422)、(511)、(440) and (622) reflections of Fe3O4 (JCPDS card, file No.19-0629), respectively [14]. In addition, a broad weak diffraction peak around 23.0° was assigned to rGO and PANI [19,20]. As shown in Fig.2B, the absorption peak at 1731.0 cm−1 was assigned to carbonyl (C]O) group of rGO [19]. The strong absorption bands at 3433.0 cm−1 and 1639.5 cm−1 were ascribed to the stretching vibration of NeH and quinonoid units of PANI, respectively [20]. The absorption peak at 576.0 cm-1 was assigned to Fe-O of oxide iron [17]. As shown in Fig. 2C, the absorption peaks at 1590.5 cm-1 and 1335.5 cm-1 were attributed to G band and D band of rGO, respectively [19]. Other peaks at 1099.0 cm-1, 1207.5 cm1 , 1377.5 cm-1 and 1518.0 cm-1 were assigned to C–H stretching vibration of the quinoid/phenyl group, the CeN stretching vibration of benzenoid ring and the semiquinone radical cation structure of PANI, respectively [20]. Above absorption peaks all presented in the Raman spectrum of product. At the same time, some new absorption bands at

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Fig. 4. SEM images of (A)NCP and (B) Fe3O4@SiO2@OTS/NCP. Non-contact white light profilometer of (C)NCP and (D)Fe3O4@SiO2@OTS/NCP. The cross section optical photographs of (E)NCP and (F) Fe3O4@SiO2@OTS/NCP. Fig. 5. The sequential digital images of a water droplet during the bouncing test on the Fe3O4@SiO2@OTS/NCP (the arrows indicate the directions of the water droplet moving).

showed a flat and smooth surface. As shown in Fig. 4B, the Fe3O4@ SiO2@OTS/NCP showed rough surface, resulting from assemblies of Fe3O4@SiO2@OTS nanoparticles on surface of NCP. Above results were further confirmed by 3D optical profilometry surface image as shown in Fig. 4C-D. The average roughness of NCP and Fe3O4@SiO2@OTS/NCP was about 0.07 μm and 0.33 μm, respectively. The cross-section structure of NCP and Fe3O4@SiO2@OTS/NCP was further characterized and compared by the optical microscope as shown in Fig. 4E-F. The NCP showed a single layer structure with a thickness of 1.5 mm for the NCP (in Fig. 4E). In a comparison, a double-layer structure was clearly observed for Fe3O4@SiO2@OTS/NCP, in which the thickness of topcoat and ground coat was about 1.3 mm and 0.2 mm, respectively. These results indicated the formation of Fe3O4@SiO2@OTS/NCP with doublelayer structure. As shown in inset of Fig. 4A and C, a low water contact angle of ca. 81.6° and flat-spread water was observed, indicating the hydrophilicity of NCP [23]. Contrarily, the large water contact angle of ca.168.9° and a spherical-shaped water droplet was observed for

Fe3O4@SiO2@OTS/NCP as shown in inset of Fig. 4B and D, respectively. The result indicated the excellent superhydrophobicity of Fe3O4@SiO2@OTS/NCP, which was attributed to rough surface and low surface energy. The superhydrophobicity of Fe3O4@SiO2@OTS/NCP was further confirmed by a more realistic study of the bounce phenomenon [24]. A high-speed camera was used to record the bounce of 10.0 μl droplet falling to the Fe3O4@SiO2@OTS/NCP as shown in Fig. 5. As wellknown, when water droplets were fallen to surface of Fe3O4@SiO2@ OTS/NCP, the water droplets tended to bounce instead of wetting the surface. As shown in Fig.5, it displayed the approach, contact, deformation and departure of the water droplet from the surface of Fe3O4@SiO2@OTS/NCP in the bouncing test. In the process, the droplet did not fragment into small pieces, and no anchoring points with residual water was observed. The result confirmed the formation of Fe3O4@SiO2@OTS/NCP with excellent water-shedding property due to excellent superhydrophobicity.

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Fig. 6. (A) Optical photograph of (a) sandpaper abrasion test and (b) a water droplet on Fe3O4@SiO2@OTS/NCP after 100th abrasion cycle. (B)Water contact angle of Fe3O4@SiO2@ OTS/NCP as function of abrasion cycles. (C) SEM image and (D)non-contact white light profilometer of Fe3O4@SiO2@OTS/NCP after 100th abrasion cycle.

through the Fe3O4@SiO2@OTS/NCP with excellent superhydrophobicity, the contaminants were carried away smoothly [27]. This result confirmed the good self-cleaning performance of Fe3O4@ SiO2@OTS/NCP. The anti-icing property of NCP and Fe3O4@SiO2@OTS/NCP was further investigated and compared as shown in Fig. 8. As shown in Fig. 8A, the water droplet could not remain on surface of Fe3O4@SiO2@ OTS/NCP at a dip-angle of 10°, while the water drop easily adhered to the surface of single NCP. After 30.0 min, the Fe3O4@SiO2@OTS/NCP showed still a clean and water-free image, while the NCP obviously showed an ice drop as shown in Fig. 8B. When the water was sprayed on NCP and Fe3O4@SiO2@OTS/NCP at -20.0℃ as shown in Fig. 8C. After 30.0 min, the Fe3O4@SiO2@OTS/NCP still showed cleaning surface, while there were more ice blocks on surface of single NCP as shown in Fig. 8D. The different result was attributed to the good and weak watershedding property for Fe3O4@SiO2@OTS/NCP and NCP, respectively [28,29]. The matrix of NCP coating was epoxy resin, which was hydrophilicity due to its surface containing hydroxyl and carboxyl groups. Therefore, water droplets tended to adhere to its surface. However, due to a layer of hydrophobic nanoparticles on the surface of NCP coating, the composite coating showed superhydrophobic properties and small slide angle. So, the water droplets rolled away the surface of Fe3O4@ SiO2@OTS/NCP before it frozen, indicating a good anti-icing performance. The permittivity real part (ε'), permittivity imaginary part (ε”), permeability real part (μ') and permeability imaginary part (μ”) of NCP and Fe3O4@SiO2@OTS/NCP were characterized and compared as shown in Fig. 9. As shown in Fig. 9A, the ε' of NCP decreased from 8.7 to 6.8 and the ε” values also slightly decreased from 3.7 to 3.0. As shown in Fig. 9B, the ε' of Fe3O4@SiO2@OTS/NCP slightly decreased from 9.1–7.0 in the range of 2.0∼18.0 GHz, respectively. In a comparison, the ε' of Fe3O4@SiO2@OTS/NCP was higher than that of NCP.

Fig. 7. Optical photographs of self-cleaning process of Fe3O4@SiO2@OTS/NCP.

The mechanical stability of Fe3O4@SiO2@OTS/NCP was also evaluated by the mechanical abrasion test as shown in Fig. 6A. Fig. 6B shows the water contact angle as function of abrasion cycle. The water contact angle was almost no change around 167.5° within 100 abrasion cycles. Fig. 6C and D shows the SEM image and non-contact white light profilometer of Fe3O4@SiO2@OTS/NCP after 100 abrasion cycles, respectively. It was found that the Fe3O4@SiO2@OTS/NCP showed similar surface morphology before and after 100 abrasion cycles, and the surface roughness slightly decreased from 0.33 μm to 0.25 μm after 100 abrasion cycles. These results confirmed that the Fe3O4@SiO2@OTS/ NCP possessed good mechanical stability. The self-cleaning property of Fe3O4@SiO2@OTS/NCP was evaluated as shown in Fig. 7. The contaminant particles were easily removed from Fe3O4@SiO2@OTS/NCP by the rolled water droplets, leading to self-cleaning process [25]. The self-cleaning property of Fe3O4@SiO2@OTS/NCP was attributed to its good water-shedding and small slid angle of ca.1.0° [26]. As well-known, the air film was easily formed on superhydrophobic surface, reducing the direct contact between the contaminant and the coating. So, when water droplets passed

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Fig. 8. Optical photographs of water drop attaching on (a) Fe3O4@SiO2@OTS/NCP and (b) NCP for (A) 0.0 min and (B) 30.0 min at -20.0℃. Optical photographs of water spraying on (a)Fe3O4@SiO2@OTS/NCP and (b)NCP for (C) 0.0 min and (D)30.0 min at -20.0℃.

Fig. 9. The permittivity real part, Permittivity imaginary part, Permeability real part and Permeability imaginary part of (A)NCP and (B)Fe3O4@SiO2@OTS/NCP.

The ε” values also slightly decreased from 4.3 to 2.8 in the range of 2.0∼18.0 GHz for Fe3O4@SiO2@OTS/NCP. Contrarily, the ε” of Fe3O4@SiO2@OTS/NCP was lower than that of NCP at relatively high frequency range of 8∼18 GHz. As shown in Fig. 9B, the μ' and μ” of Fe3O4@SiO2@OTS/NCP were obviously higher than that of NCP (in Fig. 9A). This result was attributed to that Fe3O4@SiO2@OTS/NCP loaded more magnetic particles comparing to single NCP. Fig. 10A shows the dielectric loss (tanδε) and magnetic loss (tanδμ) of NCP and Fe3O4@SiO2@OTS/NCP, respectively. The tanδε of Fe3O4@ SiO2@OTS/NCP was lower than that of NCP, while its tanδμ was larger than that of NCP. At the same time, Fe3O4@SiO2@OTS/NCP showed similar tanδε and tanδμ, indicating a good matching of impedance [21]. The result also indicated that the electromagnetic attenuation mechanism of Fe3O4@SiO2@OTS/NCP was combination of magnetic loss and dielectric loss. In a comparison, the tanδε of NCP was clearly larger than its tanδμ. The result suggested that the electromagnetic attenuation mechanism of NCP was mainly attributed to dielectric loss [17]. The attenuation constant (a) [30–32], which reflects the EM wave attenuation ability of an absorber, is estimated from the following

equation [33].

a=

2 f c

µ

"

µ

+

(µ

2

+ µ " 2) (

2

+

2 " )

From Fig. 10B, it was found that the Fe3O4@SiO2@OTS/NCP displayed a larger attenuation constant comparing to NCP. The result suggested that this Fe3O4@SiO2@OTS/NCP could achieve the better attenuation of the entered EM wave comparing to NCP. The relation between ε' and ε” is a semicircle, usually defined as a Cole-Cole semicircle, and each semicircle corresponds to a Debye relaxation process [34,35]. The Cole-Cole semicircle of composite coating was shown in Fig. 10C. There were at least three overlapping Cole-Cole semicircles in all samples, indicating the presence of multiple dielectric relaxation processes. With sufficient electromagnetic fields, electrons could obtain enough energy to transcend the interface between graphene and Fe3O4 nanoparticles, so the interface behavior of the composite was the accumulation of space charges. In addition, the interfacial polarization between the composite particle and the epoxy may lead to other relaxation processes. Therefore, the dielectric loss of composite coating

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Fig. 10. (A)Dielectric loss factors and Magnetic loss factors of (tanδε1, tanδμ1) NCP and (tanδε2, tanδμ2) Fe3O4@SiO2@OTS/NCP, (B)attenuation constants, (C)ColeCole plots and (D) C0 value of (a)NCP and (b) Fe3O4@SiO2@OTS/NCP.

resulted from interfacial polarization and Debby dipole relaxation [36,37]. For most magnetic absorption materials, the magnetic loss could originate from the magnetic hysteresis, domain wall resonance, natural resonance, exchange resonance and eddy current effect. The magnetic hysteresis loss was negligible in weak field. The domain wall resonance usually occurs at a much lower frequency range in multidomain materials. The eddy current loss is another important factor for electromagnetic microwave absorption. It is related to the electric conductivity (σ) and thickness (d) of the samples, which can be expressed by C0:

Co = µ (µ ) 2f

2

in absorbing wave performance. The transmission line theorem (below) was used to characterize the absorbing property of the material [11–16].

RL (dB ) = 201g

Zin 1 Zin + 1

Zin refers to the normalized input impedance of a metal-backed microwave absorbing layer and is given by [34–38].

Zin =

= 2 µo d2

Where μ0 is the permeability in a vacuum, σ is the electric conductivity of the material. If C0 is a constant that tends to stabilize with frequency, the magnetic losses are caused by eddy current losses.C0 decreases with the increase of frequency and fluctuates seriously in the whole frequency range, indicating that eddy current effect has no significant effect on electromagnetic microwave absorption. Fig. 10D shows the curve of C0 value of composite coating as function of frequency. It could be seen from the figure that C0 value of the two composite coatings tended to be stable after 12.0 GHz. The result indicated that there was eddy current loss in the magnetic loss of composite coating. The microwave RL curves of composites were calculated from the transmission line theory through the complex permittivity and permeability at a given frequency and absorber thickness. Reflection loss, absorption bandwidth and matching thickness were important factors

µ

tanh j

2 c

fd µ

Where ε (ε=ε′-jε″) and μ (μ=μ′-jμ″) are the complex permittivity and permeability of the composites, respectively, c is the velocity of electromagnetic waves in free space, f is the frequency of microwave, and d is the thickness of an absorber. Theoretical refection loss of NCP and Fe3O4@SiO2@OTS/NCP was further characterized and compared as shown in Fig. 11. As expected, it could be observed that the thickness of NCP and Fe3O4@SiO2@OTS/NCP had a great influence on the microwave absorbing properties. In addition, the maximum RL absorption position and strength gradually appeared to shift toward a lower frequency and increased with increasing in thickness. For the NCP (Fig. 11A), the maximum RL reached 21.6 dB at 16.5 GHz with a thickness of 1.0 mm, and a bandwidth of RL less than -10.0 dB could reach up to 2.4 GHz (from 15.6–18.0 GHz). As shown in Fig. 11B, the maximum RL reached 21.2 dB at 15.8 GHz for Fe3O4@SiO2@OTS/NCP

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Fig. 11. Reflection loss curves of (A)NCP and (B)Fe3O4@SiO2@OTS/NCP as function of thickness in the frequency range of 2∼18 GHz. (C)The microwave absorption mechanism of Fe3O4@SiO2@OTS/NCP.

with a thickness of only 1.0 mm, and a bandwidth of RL less than -10.0 dB could reach up to 3.2 GHz (from 14.8–18.0 GHz). With a thickness of 1.5 mm, the maximum RL was about 36.1 dB at 13.5 GHz, and a bandwidth of RL less than 10.0 dB could reach up to 9.2 GHz (from 8.8–18.0 GHz). By comparing to NCP and other microwave absorption coatings reported in previous works [22,38,39], present Fe3O4@SiO2@OTS/NCP showed larger absorption bandwidth (ca.9.2 GHz) at similar thickness of 1.5 mm. These results were attributed to the special structure as shown in Fig. 11C. Firstly, the Fe3O4/rGO/PANI showed good microwave absorption properties due to good matching of impedance and good matching between dielectric loss/magnetic loss. Secondly, the microwave absorption coating showed a certain surface roughness due to assembly of Fe3O4@SiO2@OTS nanoparticles. The electromagnetic wave was multiple reflection and scattering on the surface, enhancing microwave absorption ability [40–43]. Thirdly, the SiO2 particle and EP matrix was a transparent material for electromagnetic wave, reducing the reflection of electromagnetic wave on surface of coating. The durability of Fe3O4@SiO2@OTS/NCP was further evaluated by measuring reflection loss curves for application in microwave absorption properties as shown in Fig. 12. It clearly showed similar reflection loss curves for the Fe3O4@SiO2@OTS/NCP before and after mechanical abrasion and cooling as shown in Fig. 12A. These results indicated good durability of Fe3O4@SiO2@OTS/NCP for application in microwave absorption materials under mechanical damage and ice environments. NCP and Fe3O4@SiO2@OTS/NCP were both kept out room (Location: North University of China, N37.998, E112.4726) for 7 days and there

were lots of dusts deposited on surface of coating as shown in inset of Fig. 12B and C, respectively. As shown in Fig.12B and C, the NCP and Fe3O4@SiO2@OTS/NCP clearly both showed different reflection loss curves before and after deposition dusts. The bandwidths (5.0 GHz and 9.2 GHz) of NCP and Fe3O4@SiO2@OTS/NCP were reduced to 2.8 GHz and 6.6 GHz, respectively. It was interesting that the dusts were easily removed from surface of Fe3O4@SiO2@OTS/NCP by rainy, which showed similar reflection loss curves with Fe3O4@SiO2@OTS/NCP before deposition dusts (in Fig. 12C). In a comparison, the dusts were difficult to be removed from surface of NCP, and the microwave absorption properties was not recovered (in Fig. 12B). These results confirmed that although the NCP showed similar microwave absorption properties with Fe3O4@SiO2@OTS/NCP, yet the durability of NCP was far lower comparing to Fe3O4@SiO2@OTS/NCP. 4. Conclusion In summary, a new class microwave absorption coating of Fe3O4@ SiO2@OTS/NCP was proposed and prepared by a simple spraying method. It did not only exhibit wide bandwidth (ca.9.2 GHz), but also showed good water-shedding performance and mechanical durability. These excellent characters were mainly attributed to its rough surface, resulting from assemblies of core-shell magnetic nanoparticles. The Fe3O4@SiO2@OTS/NCP was expected to apply in the field of military stealth technology and electromagnetic shielding of electronic devices.

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Fig. 12. (A)Reflection loss curves of Fe3O4@SiO2@OTS/NCP (a)without treatment, (b)after 100 mechanical abrasion cycles and (c) after cool for 60.0 min. Reflection loss curves and optical photograph of (B)NCP and (C)Fe3O4@SiO2@OTS/NCP, (a)without dusts, (b)with dusts, (c)with dusts after water washing.

CRediT authorship contribution statement

References

Yibing Ma: Investigation, Methodology. YaYa Zhou: Data curation. Chunhong Qi: Visualization, Writing - review & editing. Youyi Sun: Supervision, Resources. Yinghe Zhang: Software, Writing - review & editing. Yaqing Liu: Conceptualization, Validation.

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Declaration of Competing Interest All authors have and declare that: (i) no support, financial or otherwise, has been received from any organization that may have an interest in the submitted work ; and (ii) there are no other relationships or activities that could appear to have influenced the submitted work. Acknowledgments The authors are grateful for the support of the National Natural Science Foundation of China under grants (51773184 and U1810114), and the Shanxi Provincial Natural Science Foundation of China (201701D121046 and 201803D421081). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.porgcoat.2020. 105538. 11

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