The effect of Ag nanoparticles content on dielectric and microwave absorption properties of β-SiC

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The effect of Ag nanoparticles content on dielectric and microwave absorption properties of β-SiC Bo Weia,b,c, Jintang Zhoua,b,c,∗, Zhengjun Yaoa, Azhar Ali Haidrya, Xinlu Guoa, Haiyan Linc, Kun Qiana, Wenjing Chena a

College of Materials and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 211100, China Key Laboratory of Material Preparation and Protection for Harsh Environment (Nanjing University of Aeronautics and Astronautics), Ministry of Industry and Information Technology, Nanjing, 211100, China c Research Institute of Aerospace Special Materials & Technology, Beijing, 100074, China b

ARTICLE INFO

ABSTRACT

Keywords: Microwave absorption Dielectric property β-SiC Nano Ag Electroless plating

In the present work, high purity cubic silicon carbide (β-SiC) was synthesized by using resorcinol-formaldehyde aerogel coated silica as precursor at high temperature. Subsequently, β-SiC was coated with various concentrations of silver nanoparticles (Ag NPs) by tin sensitization electroless plating. Thereafter, the effect of various Ag NPs contents on the dielectric and microwave absorption properties of β-SiC was investigated in detailed. It is found that the Ag NPs can significantly increase the overall complex permittivity, reduce the thickness of the absorber and increase the absorption bandwidth to some extent. Despite, the improvement of the attenuation ability of electromagnetic wave with increasing contents of Ag NPs also has an adverse effect on impedance matching. The improvement in the microwave absorption of β-SiC coated with Ag NPs mainly comes from the enhancement of dipole polarization, interface polarization and conductivity loss. In the 2–18 GHz band, Ag@SiC (1.0 g/L) can achieve an effective bandwidth of 4.99 GHz at a thickness of 1.6 mm, and it is a kind of lightweight, high-temperature microwave absorbent with excellent performance.

1. Introduction In recent years, the rapid development of communication devices working within electromagnetic (EM) radiation has a great impact on human life and environment as well. On the one hand, it is true that all kinds of communication products have brought great convenience to people [1,2], but On the other hand, electromagnetic pollution has shown detrimental effect on living species, especially on human health [3]. Meanwhile, the development of radar technology greatly threatens the survival environment of aircraft, other weapons and equipment in military war [4,5], the use of absorbing materials provides an effective way to solve several problems such as electromagnetic pollution. The research and development of absorbing agents has recently become a research hotspot as the core of absorbing materials is to realize improved absorbing functionality [6]. Silicon carbide (SiC) absorbents have large loss angle tangent and can attenuate electromagnetic wave effectively in a certain microwave band. Meanwhile, excellent physical properties and chemical corrosion resistance make silicon carbide a potential candidate for better microwave absorption under extreme conditions, such as high temperature

∗

and pressure, strong acid and strong alkali. However, the SiC absorbing frequency band is relatively narrow, which cannot meet the requirements of modern absorbing composite materials [7,8]. Therefore, it is necessary to do intensive research to improve its microwave absorption properties. At present, advanced researche on the doping of B, N and P elements is ongoing and until now the dielectric loss is increased by improving the dielectric performance, or depositing magnetic elements on the SiC surface to increase magnetic losses. For instant, Zhao et al. [9] prepared N-doped nano-SiC particles by using a laser-induced gasphase reaction. The found that adjusting the content of SiC could be used to control the complex dielectric constant of N–SiC particles with high dielectric constant imaginary part and electrical loss factor. The particles have an effective absorption bandwidth of 6 GHz at a thickness of 2.96 mm, with a maximum reflection loss of −63.41 dB at 12.17 GHz. Li et al. [10] plated nickel-cobalt film on the surface of SiC particles by chemical plating method and shown that adjusting the Ni and Co content ratios of Ni–Co can lead to obtain Ni–Co films with different conductive and ferromagnetic characteristics. When the initial atomic ratio of Ni and Co is 1.5 and the material thickness is 2.5 mm and the maximum reflection loss of −32 dB was obtained at 6.3 GHz.

Corresponding author. College of Materials and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 211100, China. E-mail address: [email protected] (J. Zhou).

https://doi.org/10.1016/j.ceramint.2019.11.029 Received 3 October 2019; Received in revised form 30 October 2019; Accepted 4 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Bo Wei, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.029

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Among other conductive materials (e.g., Ni, Cu and Fe), silver is widely used owing to its stable physical and chemical properties and low oxidation tendency [11,12]. However, Ag is rarely coated on SiC to obtain improved wave absorbers due to its high preparation cost. The strong skin effect of silver can also lead to form a conductive network, which results in the reflection of electromagnetic wave and thus poor absorption performance. To address such issues, Ag nanoparticles (Ag NPs) were coated on the β-silicon carbide instead of a single silver filler, which reduces the cost, improved the overall dielectric performance and hence improved microwave absorption performance. In this work, innovative Ag@SiC absorbent was prepared by carbon thermal reduction and subsequent electroless plating. The influence of the content of Ag NPs on the dielectric and absorbing properties of the SiC was investigated by changing the concentration of silver precursor in electroless plating. It is found that the Ag NPs coating significantly improves the real and imaginary parts of the overall complex permittivities, and the resistive loss mechanism changes to the dielectric loss mechanism. Ag NPs have an adverse effect on impedance matching performance, but significantly improves the attenuation performance, which we believe can be correlated with the enhancement of dipole polarization, interface polarization and conductivity loss.

2.3. Preparation of Ag NPs For the sensitization treatment, SnCl2 sensitizing solution (1 g SnCl2, 3 mL hydrochloric acid, 30 mL deionized water) was prepared and 0.5 g SiC powder was added into the sensitizing solution under rigorous stirring for 15 min at 60 °C. After centrifuging, washing and vacuum drying at 60 °C, Sn2+ coated sensitized SiC powder was obtained. Thereafter, two solutions were prepared, namely solution-A (Ag salt solution) and solution-B reducer solution, various compositions of these solutions is shown in Table 2. Both the solutions A and B were then mixed under rigorous stirring followed by the addition of sensitized SiC powder. The composite Ag@ SiC powder with silver-gray coating was obtained after stirring at a constant temperature of 30 °C for 90 min. 2.4. Characterization Analysis of the lattice structure and element composition of the products in each stage were performed by a powder X-ray diffractometry (XRD, Bruker, D8 Advance) and a fourier transform infrared spectrometer (FT-IR, Nicolet, Nexus 670) respectively. The thermodynamic stability, the microstructural morphologies were obtained by thermogravimetric analysis (TG, NETZSCH, TA), scanning electron microscopy (SEM, Hitachi, S4800) and transmission electron microscopy (TEM, JEOL, JEM-2100F). Moreover, the element composition of surface coating was observed by an X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCALAB 250XI). Last but not the least, the electromagnetic parameters were tested by a vector network analyzer (VNA, Ceyear, 3672B), during this testing a mold was used to mix the sample with paraffin in a 1 : 1 ratio of mass and compactness to get a annular specimen with a thickness of about 2 mm. We also used the coaxial method to test the electromagnetic performance of the annular specimen.

2. Experimental 2.1. Reagent and raw materials The related specifications of chemical reagents and raw materials used in this work are shown in Table 1. 2.2. Preparation of high-purity β-SiC 5 mL ethyl orthosilicate was weighed in a beaker and subsequently 40 mL ethanol, 40 mL deionized water, 2.5 mL ammonia and 1.5 g PVP (k-30) was added under rigorous stirring (150 rpm) at 30 °C for 20 h. Then, the solution was filtered and SiO2 was obtained after roasting at 650 °C for 3 h. The prepared SiO2 was mixed evenly with an appropriate amount of resorcinol/formaldehyde solution with a molar ratio of 1 : 2, the pH value was adjusted to 8–9 by ammonia solution. Then, 2.5 ml nickel chloride hexahydrate and 0.01 g anhydrous sodium carbonate was added for 5 h reaction at 60 °C followed by solidifying for 1 h at 85 °C and drying. The crude product was obtained after two step heatingtreatment of the mixture in argon atmosphere of, at 650 °C for 1 h and 1450 °C for 4 h. Thereafter, the powder was annealed in static-air at 650 °C for 3 h to remove carbon followed by pickling and drying to obtain high-purity β-SiC.

3. Results and discussion 3.1. Preparation mechanism and basic characterization The typical synthesis process of Ag@SiC is shown in Fig. 1. Firstly, SiO2 was prepared by ethyl orthosilicate-sol-gel method and the impurities were removed by calcining at high temperature to obtain evenly dispersed SiO2 powder. Then aerogel was prepared in the system containing resorcino-formaldehyde (RF) sodium carbonate and then coated on SiO2 to obtain the precursor of RF aerogel@SiO2. High purity SiC with β-SiC as the main crystal type was obtained by carbon thermal reduction in Ar atmosphere. The prepared SiC powder was then sensitized with Sn2+ ions to add target sites for Ag coating [13]. The following chemical reactions will occur during the plating process:

Table 1 Reactant properties.

CH2 OH (CHOH ) 4 CHO + 2Ag (NH3)2 OH

Reactant

Manufacture

Purity, %

Tetraethyl orthosilicate Ammonia solution Ethanol absolute Resorcinol Formaldehyde solution Sodium carbonate anhydrous Nickel chloride hexahydrate Polyvinyl Pyrrolidone(K-30)

Macklin, China Nanjing Reagent, China Sinopharm Chemical ReagentCo., Ltd Energy Chemical, China Nanjing Reagent, China Nanjing Reagent, China Macklin, China Sinopharm Chemical Reagent Co., Ltd Sinopharm Chemical ReagentCo., Ltd, China Energy Chemical, China Aladdin, China Sinopharm Chemical ReagentCo., Ltd, China Aladdin, China

99.0 99.7 99.7 99.0 99.7 99.7 99.0 99.7

D(+)-Glucose

Tin(Ⅱ) chloride Silver nitrate Ethanol absolute Hydrochloric acid

CH2 OH (CHOH ) 4

COONH4 + 2Ag

+ 3NH3

+ H2 O

The essence of electroless silver plating is the silver-mirror reaction of glucose solution and silver salt solution. Finally, SiC coated by Ag NPs were prepared by AgNO3-glucose electroless silver plating system by adjusting the Ag content (controlling the concentration of Ag) in the main salt. The advantage of SiC as a wave absorbent is not only due to its high dielectric loss, but also due to its excellent physical and chemical stability. Although the use of ordinary Ni and Cu metal coating can improve the microwave absorption performance to a certain extent, the properties of these metals are more active with poor thermal stability and chemical corrosion resistance, which will have an adverse effect on the comprehensive properties of SiC absorbent. As a stable metal, Ag can not only significantly change the overall dielectric property of SiC, but also have little influence on other properties of SiC. The crystal structure, surface functionality and thermally mass loss are firstly discussed, as can be seen in Fig. 2. The XRD patterns of the sample can be

99.0 98.0 99.0 99.7 99.5

2

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Table 2 Formulation composition. Ag salt solution/1L Reducer solution/1L

AgNO3(g) 0.25, 0.5, 0.75, 1 D(+)-Glucose(g) 15

ammonia solution 5 mL ethanol absolute 100 mL

seen in Fig. 2-(a) showing the characteristic absorption peaks of 3C–SiC (111), (200), (220), and (311) crystal planes appeared at 2 theta = 35.597°, 41.383°, 59.977°, and 71.777°, respectively. The obtained patterns are in good agreement with standard PDF. No. 04-291129, indicating that the synthesized SiC mainly existed in β-crystal. For Ag coated samples, with the increase of Ag content, clutter and fine peak shape became more and more prominant, which is probably due to the influence of partial poor crystallinity and defective Ag lattice. Meanwhile, (111), (200), (220), and (311) crystal planes of 3C–Ag appeared at 2 theta = 38.116°, 44.277°, 64.426°, and 77.472°, respectively, consistent with PDF. No. 04-04-0783. The peak intensity increased with the increase of Ag content, i.e., the Ag prepared by electroless plating was mainly 3C crystal. From the FT-IR spectra shown in Fig. 2-(b), one can notice the absorption peaks at 3423 cm-1 and 1628 cm-1 originating from the stretching vibration of the H–O bond, the characteristic absorption peak of SiC appeared at 928 cm-1 and 810 cm-1, and the curve has almost no stray peaks. No other absorption peaks were observed, which proves that the prepared samples are highly pure. The possible absorption peak of Ag–O bond at 753 cm-1 is almost unnoticeable in the figure, indicating that wet and warm electroless plating will not oxidize the generated Ag NPs. Moreover, from the Tg curves of samples obtained in the temperature range of 50–800 °C, as shown in Fig. 2-(c), at the quantity hardly changed; indicating that Ag has excellent thermal stability and will not affect the high temperature absorbent properties of SiC.

deionized water 50 mL deionized water 900 mL

pH 11.5–12.5 temperature 30 °C

in the sensitization process. With the increase of Ag concentration, the particle size and number of Ag particles increased significantly. For Ag@ SiC (1.0 g/L), Ag NPs adhered to each other and tended to become membranes. The EDS spectrum also shows that with the increase of Ag concentration the intensity of characteristic absorption peak at 2.98 keV increases, confirming the increase of Ag content on SiC surface. Fig. 4 shows the TEM image of Ag@SiC (0.75 g/L) and the SAED diffraction pattern. The crystal plane fringe has been calibrated in the HRTEM image. The coating of Ag NPs is basically uniform, and there is a certain space between the particles to avoid the reflection caused by skin effect, as shown in Fig. 4-(a). From SAED pattern in Fig. 4-(b), the polycrystalline diffraction rings of the (111) and (220) crystal planes from βSiC are clearly visible, while the (111) and (200) crystal planes from the Ag single crystal speckle group are present. From HRTEM images of Fig. 4(c) and Fig. 4-(d), the lattice fringes show the (111) crystal faces of SiC and Ag, respectively, the crystal grows mainly along the (111) crystal face. Fig. 5 shows the XPS spectrum of Ag@SiC (0.75 g/L). The composition of the sample elements is shown in Fig. 5-(a), mainly including C, Si and Ag elements. It can be seen from the high resolution Ag XPS spectrum in Fig. 5-(b) that the characteristic absorption peak of Ag0 appears in 368.10 eV and 374.27 eV. Ag is mainly in the form of Ag elementary substance in 3d orbit, reflecting the strong antioxidant performance of Ag. 3.3. Electromagnetic performance analysis It is aknown fact that Ag has very low contact resistance compared with the semiconductor properties of β-SiC. When Ag is coated on the surface of SiC, the overall dielectric property will change significantly, which will increase a series of dielectric loss and affect the absorption performance. However, excessive Ag content is easy to form a conductive network on the surface of the wave absorber, causing significant skin effect, skin depth reduction, and ultimately leading to a strong reflection of electromagnetic waves. Therefore, it is of great significance to find an appropriate Ag content to suppress the reflection effect of electromagnetic wave and increase the attenuation capacity at

3.2. Micro morphology and crystal lattice structure The microstructural morphology of the samples obtained by SEM is shown in Fig. 3. SiC prepared in this work mainly consists of irregular block with particle size ranging from 1 to 10 μm. These structures with different particle sizes and shapes can provide more reflection, scattering points and attenuation times for electromagnetic waves. When Ag concentration was low, nano-sized Ag particles were formed on SiC surface and dispersed evenly, which was related to the pretreatment of Sn2+ ions

Fig. 1. Preparation mechanism of Ag@SiC. Including four parts: (1) SiO2 prepared by hydrolysis of TEOS; (2) RF aerogel@SiO2 prepared by Sol-Gel method; (3) SiC prepared by high temperature calcination; (4) Ag@SiC prepared by electroless plating. 3

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the same time. Microwave absorbing properties of the absorbent depends on its electromagnetic parameters, including complex permittivities (εr, εr = ε' - jε'') and complex permeabilities (μr, μr = μ' - jμ''), where the real part represents its ability to store electromagnetic energy, and the imaginary part represents its ability to attenuate electromagnetic energy [14]. Reasonable combination of real part and imaginary part of electromagnetic parameters can obtain good absorbing performance. Fig. 6 shows the electromagnetic parameters of Ag@SiC prepared with different Ag NPs concentrations. Fig. 6-(a) shows the real part of the complex permittivities in the range of 2–18 GHz, the real part of the complex permittivities of SiC prepared in this work fluctuates with frequency from 9.2–6.1. With the increase of Ag content, the peak value of the real part increases significantly and shows a dependence on Ag content. The higher the Ag content leads the higher the real part, which indicates that Ag is beneficial to enhance the material charge storage property. Meanwhile, Ag@SiC shows a more obvious electrical dispersion effect, indicating that the dielectric property of Ag@SiC is more responsive to the change of frequency. Fig. 6-(b) shows the imaginary part of the complex permittivities and it can be seen that the value of the imaginary part after Ag coating is significantly improved in the whole frequency band. For Ag@SiC (1.0 g/L), the peak value increased from 3.1 to 4.6, indicating that the dielectric attenuation performance of electromagnetic wave has been significantly improved. Fig. 6-(c) and Fig. 6-(d) show the real part and imaginary part of the complex permeabilities of samples, respectively. SiC as a nonmagnetic material, μ′ is close to 1 and μ′′ is close to 0 in the whole frequency band. When the surface of SiC is coated with different content of Ag, μ′ and μ′′ hardly changes. Compared with dielectric loss, the contribution of magnetic loss is negligible. Therefor, this work mainly discusses the influence of Ag on the dielectric properties of SiC. A good absorber should meet the requirements of electromagnetic wave absorption and attenuation [15], which requires consideration of the impedance matching and attenuation performance of the absorber. Impedance matching rate can be expressed by equations (1)–(3) [16]:

R=

Z0 Zin Z0 + Zin µr µ 0

Zin = Z0 =

(1) (2)

r 0

µ0

(3)

0

here R is the reflection coefficient, Z0 is the impedance of air, Zin is the wave impedance at interface, μ0 and ε0 are vacuum permeability and vacuum permittivity, respectively. It is generally recognized that the impedance matching rate of the absorber should be greater than 0.3, so that more electromagnetic waves can enter into the absorber for better attenuation [17]. Fig. 7-(a) shows the relationship between impedance matching rate and frequency of samples. The impedance matching ratio of SiC is higher (~0.3) in the whole frequency band, which has a good ability to allow electromagnetic wave to enter. The impedance matching ratio reduces gradually with increasing contents of Ag NPs. For example, for Ag@SiC (1.0 g/L), the peak value decreased from 0.42 to 0.32, and the impedance matching ratio was less than 0.3 in a large range. This result indicates that good conductor Ag would have an adverse effect on the impedance matching performance of the absorber, which was more obvious in the low frequency range with lower electromagnetic energy. Further, the attenuation characteristics of the material can be evaluated by attenuation coefficient α, as shown in equation (4) [18]:

=

Fig. 2. Basic performance characterization: (a) XRD patterns, (b) FT-IR spectra and (c) Tg curves of samples.

2 f × c

(µ

µ

)+

(µ

2

µ ) + (µ

+µ

)

2

(4)

here c is the speed of light in a vacuum. Fig. 7-(b) shows the relationship between attenuation factor α and frequency of samples. The value 4

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Fig. 3. The micromorphology and element composition of samples. SEM images of (a) β-SiC; (b) Ag@SiC (0.25 g/L); (c) Ag@SiC (0.50 g/L); (d) Ag@SiC (0.75 g/L); (e) Ag@SiC (1.0 g/L). (f) EDS spectra of all samples.

Fig. 4. Tem analysis and lattice fringe calibration of Ag@SiC (0.75 g/L): (a) TEM images; (b) SAED pattern; (c) HRTEM image of SiC; (d) HRTEM image of Ag.

Fig. 5. Element analysis of surface coating of Ag@SiC (0.75 g/L): (a) XPS spectra and (b) high resolution Ag XPS spectrum. 5

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Fig. 6. Electromagnetic parameters of samples: (a) Real part of permittivity (ε′); (b) Imaginary part of permittivity (ε′′); (c) Real part of permeability (μ′); (d) Imaginary part of permeability (μ′′).

of the α peak of intrinsic SiC is about 156, which increases significantly specifically for higher Ag NPs contents. For example, for Ag@SiC (1.0 g/L) sample the peak value can reach 216. For nonmagnetic SiC and diamagnetic Ag, this attenuation of electromagnetic waves is due to dielectric losses. Due to the poor dielectric performance of ordinary SiC, the loss mechanism generally comes from resistance loss and the attenuation of electromagnetic wave is mainly caused by dielectric polarization and leakage conductivity loss [19,20]. The coating of Ag significantly improves dielectric properties, enhances dielectric polarization effect and conduction current intensity, and thus improves attenuation ability. The dielectric polarization effect can be divided into

interface polarization and dipole polarization [21]. Among them, the degree of dipole polarization can be characterized by Cole-Cole semicircle. According to debye relaxation model, the dielectric properties of materials are characterized in the plural form [22]: r

=

+

s

1 + j2 f

=

j

(5)

here f is the microwave frequency; εr is the static permittivity, ε∞ is the optical permittivity at the high frequency limit; τ is the relaxation cycle. The expression of ε′, ε'' and tan δε can be derived from equation (5):

Fig. 7. Analysis of absorption and attenuation of electromagnetic wave: (a) Impedance matching ratio and (b) attenuation constant α of samples. 6

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=

=

tan

+

s

1 + (2 f )2

2 f (s ) 1 + (2 f ) 2 2

=

=

(

s

s

+

Ag coating can be seen visually from Fig. 8-(f), with the increase of Ag content, the value of tanδε first increases and then decreases. Moreover, the peak value shifts towards the high frequency direction, indicating that there is a limit to the increase of dielectric loss caused by Ag, and is not that the higher Ag content is beneficial to dielectric loss. In addition to dipole polarization, Ag coating uniformity and Ag lattice defects or impurities will significantly affect the Ag@SiC interface, enhancing the interface polarization effect. In addition, due to the greatly different dielectric properties of Ag and SiC, Maxwell-Wagner effect [24] is significantly enhanced at the interface where the two are connected, leading to an increased degree of space charge accumulation. When the charge accumulates to a certain extent, an electric current will be formed, and the pyroelectric transducer process will occur to promote the loss of electromagnetic energy. At the same time, the good conductor property of Ag increases the overall conductivity, which provides favorable conditions for the conduction of current. In conclusion, the improvement of dielectric loss ability with Ag@SiC originates from the enhancement of dipole polarization, interface polarization and conductivity loss.

(6)

2

(7)

)2 f (2 f )2

2

(8)

Equations (6) and (7) are known as Debye equations. The tanδε in equation (8) represent the dielectric loss. The expression of Cole-Cole semicircle can be obtained by canceling 2πf in equation (6) and equation (7):

(

)2 + ( )2 = (

s

)

(9)

In the Cole-Cole model, each semicircle represents a debye relaxation process [23]. According to equations (8) and (9), tanδε and ColeCole semicircle of samples are plotted as shown in Fig. 8. As mentioned above, an increase in Ag content will lead to an increase in the real part of the complex permittivities. In the Cole-Cole model, an increase in the real part will cause the range appearing in the circular model to shift toward the high-frequency direction, as shown in Fig. 8-(a) ~ (e). The Cole-Cole model is similar in shape, indicating that the dependence of SiC on electromagnetic wave absorption and frequency does not change due to Ag coating. At the same time, the frequency range of Cole-Cole circle will increase significantly with the increase of Ag content, that is, Ag coating makes dipole polarization increase in the region of 2–18 GHz band. The effect on dielectric loss of

3.4. Reflection loss and absorption mechanism The properties of absorbing materials can be directly reflected by reflection loss value (RL). The reflection loss of single-layer absorbing materials can be calculated by equation (10), (11) [25]:

RL = 20 log (Zin

1)/(Zin + 1)

Fig. 8. Typical Cole-Cole semicircles (a) ~ (e) and dielectric tangent loss (tanδε) (f) of samples. 7

(10)

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Fig. 9. Three-dimensional representation and two-dimensional representation of the values of reflection loss for samples: (a) ~ (b): SiC; (c) ~ (d) Ag@SiC (0.25 g/L); (e) ~ (f) Ag@SiC (0.5 g/L); (g) ~ (h): Ag@SiC (0.75 g/L); (i) ~ (j) Ag@SiC (1.0 g/L).

Zin =

µr / r tan h [j (2 fd )/c ] µr

r

(a), (c), (e), (g) and (i) in Fig. 9, it can be found that Ag coating can expand the absorption region of SiC to a certain extent. Furthermore the maximum reflection loss (RL) and effective absorption bandwidth are also increased. At the same time, several peaks with excellent absorbing properties appear in the range of sufficient thickness. The higher the Ag content, the more the number of peaks appear, and the RL value increased slightly. For instance, RL value can reach to −58.2 dB for Ag@ SiC (1.0 g/L) sample.

(11)

where Zin is normalized input impedance of absorbing agent; d is the thickness of the absorbing layer. It can be seen from equations (10) and (11) that the reflection loss has a quantitative relationship with the complex permittivities and complex permeabilities. Fig. 9 shows two-dimensional as well as three-dimensional representation of the values of reflection loss for samples. By comparing 8

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Fig. 9. (continued)

effective bandwidth and corresponding thickness of each sample are shown in Table 3. For Ag@SiC prepared in this work, the increase of εr is beneficial to decrease the thickness of the absorber, which is of great significance for the lightweight of the absorber. Fig. 10-(a) shows the relationship between RL value and frequency of samples at a thickness of 2 mm. With the increase of Ag content, the peak value of RL moved towards the low-frequency direction. Among them, Ag@SiC (0.25 g/L) has the maximum effective absorption bandwidth of 4.68 GHz, and Ag@SiC (0.50 g/L) has the maximum RL value of −41.62 dB. Considering RL and effective absorption bandwidth, Ag@SiC (0.50 g/L) has the best performance at a thickness of 2 mm. Fig. 10-(b) shows the relationship between the RL value and frequency at different thickness of Ag@SiC (0.50g /L). In contrast to other reports [27,28], this work demonstrates a larger effective absorption bandwidth for lower the thickness of the wave absorber, which is due to the improvement of dielectric properties by Ag. It is easy to form a conductive network in the inner part when the absorber is more thick, leading to the increase of electromagnetic wave reflection effect. At the same time, according to the analysis of equation (12) and Fig. 9, the λ /4 wavelength of Ag@SiC appears at a lower thickness, which has a stronger attenuation effect on electromagnetic waves. Fig. 11 shows the microwave absorption mechanism of Ag@SiC structure. Under the electromagnetic field, Ag@SiC has a series of dielectric behaviors such as conductance loss, interface polarization and dipole polarization. The coating of Ag NPs improves the dielectric

Table 3 Maximum effective bandwidth and corresponding thickness of each sample. Ag@SiC /c (Ag) (g/L) Tickness (mm) Effective bandwidth (GHz)

0 2.5 5.01

0.25 2.4 5.33

0.50 2.3 4.97

0.75 1.8 4.61

1.0 1.6 4.99

By comparing (b), (d), (f), (h) and (j) parts in Fig. 9, Ag coating can increase the effective absorption bandwidth, while a narrow area of RL < −10 dB appears at the same time, which reflects the position of λ /4 wavelength. According to transmission line theory, a strong standing wave at λ /4 can form inside the absorbent resulting in interference and canceling out with the incident electromagnetic wave and thus creating a significant attenuation effect. In addition, it can be concluded from two-dimensional model that the general law of reflection loss changes with frequency and thickness. This suggests that when the thickness of absorbing layer increases, the peak value of reflection loss moves towards the low frequency band. This peak displacement can be explained by the λ /4 equation [26]:

tm = n /4 = nc /4fm ( r µr )1/2

(12)

where λ is the wavelength; tm is the absorbent's matching thickness at the best wave absorption performance; fm is the frequency corresponding to permittivity and permeability. According to equation (12), the frequency fm of the same absorbent material decreases with the increase of tm. In addition, the large values of εr is favorable for the decrease of absorbent thickness; the estimated values of maximum 9

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Fig. 10. Performance comparison between different samples with the same thickness: (a) Reflection loss of samples at a thickness of 2.0 mm; Performance comparison of the same sample with different thickness: (b) Reflection loss of Ag@SiC (0.50 g/L) at different thickness.

properties and further enhances these attenuation effects, so as to effectively attenuate the incident electromagnetic wave. By adjusting the content of silver, a balance between impedance matching and attenuation can be found to obtain the best absorption performance. In comparison to recent reports, the Ag@SiC (1.0 g/L) prepared in this work achieves an effective absorption bandwidth of 4.99 GHz at 1.6 mm, with optimal RL value of −36.3 dB, see Table 4. From this we can infer that Ag@SiC has obvious advantages on the matching thickness and it is a kind of light absorbent with good microwave absorption performance.

Table 4 Reported microwave absorption properties of some recent absorbing composites.

4. Conclusion In this work, we successfully prepare high purity β-SiC, Ag@SiC wave absorbers with different Ag contents aided with Sn2+ ion electroless silver plating. It was found that Ag NPs were mainly in 3C crystal form with excellent thermal stability and oxidation resistance.

Sample

Optimal RL value (dB)

Absorption bandwidth (GHz)

Thickness (mm)

Ref.

Co0.2Ni0.4Zn0.4Fe2O4/GN Ni(OH)2/biomass carbon rGO/Ni Fe@Ni Graphene/CoFe2O4/ Y3Fe5O12 NiFe2O4 rGO/Ni0.5Zn0.5Fe2O4 Carbon nanotube/Silica Ag@SiC

−53.5 −23.6 −59.7 −22.7 −36.1

4.8 2.1 4.8 5.5 2.0

3.1 6.0 2.7 2.0 3.5

[17] [29] [30] [31] [32]

−22.5 −37.71 22.0 −36.3

1.8 3.68 4.2 4.99

3.5 2.0 4.0 1.6

[33] [34] [35] This work

Fig. 11. Absorption mechanism of Ag@SiC. Ag coating leads to three improvements: Conductivity loss, interfacial polarization and dipole polarization. 10

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Meanwhile, Ag can improve the dielectric properties of SiC obviously. On the premise of controlling skin effect intensity and reflection effect, appropriate amount of Ag nanoparticles can enhance dipole polarization, interface polarization and conductivity loss, and improve microwave absorption performance. Ag coating can also significantly reduce the matching thickness of SiC absorbent and increase the bandwidth and RL to a certain extent, which is of great significance for preparing lightweight and high-temperature absorbent.

4399–4403. [13] M. Uysal, R. Karslioglu, A. Alp, H. Akbulut, Nanostructured core-shell Ni deposition on SiC particles by alkaline electroless coating, Appl. Surf. Sci. 257 (24) (2011) 10601–10606. [14] C. Wang, X. Han, X. Zhang, S. Hu, T. Zhang, J. Wang, Y. Du, X. Wang, P. Xu, Controlled synthesis and morphology-dependent electromagnetic properties of hierarchical cobalt assemblies, J. Phys. Chem. C 114 (35) (2010) 14826–14830. [15] T. Liu, P.H. Zhou, J.L. Xie, L.J. Deng, Electromagnetic and absorption properties of urchinlike Ni composites at microwave frequencies, J. Appl. Phys. 111 (9) (2012) 093905. [16] L. Wang, Q. Quan, L. Zhang, L. Cheng, P. Lin, S. Pan, Microwave absorption of NdFe magnetic powders tuned with impedance matching, J. Magn. Magn. Mater. 449 (2018) 385–389. [17] P. Liu, Z. Yao, J. Zhou, Z. Yang, L.B. Kong, Small magnetic Co-doped NiZn ferrite/ graphene nanocomposites and their dual-region microwave absorption performance, J. Mater. Chem. C 4 (2016) 9738–9749. [18] E.Q. Huang, J. Zhao, J.W. Zha, L. Zhang, R.J. Liao, Z.M. Dang, Preparation and wide-frequency dielectric properties of (Ba0.5Sr0.4Ca0.1)TiO3/poly (vinylidene fluoride) composites, J. Appl. Phys. 115 (19) (2014) 194102. [19] Q. Li, C. Zhang, J. Li, Photocatalysis and wave-absorbing properties of polyaniline/ TiO2 microbelts composite by in situ polymerization method, Appl. Surf. Sci. 257 (3) (2010) 944–948. [20] Z.C. Lou, H. Han, M. Zhou, J.Q. Han, J.B. Cai, C.X. Huang, J. Zou, X.Y. Zhou, H.J. Zhou, Z.B. Sun, Synthesis of magnetic wood with excellent and tunable electromagnetic wave-absorbing properties by a facile vacuum/pressure impregnation method, ACS Sustain. Chem. Eng. 6 (1) (2018) 1000–1008. [21] O. Konski, T. Chester, Electric properties of macromolecules. v. theory of ionic polarization in polyelectrolytes, J. Phys. Chem. 64 (5) (1960) 605–619. [22] J. Frenkel, J. Dorfman, Spontaneous and induced magnetisation in ferromagnetic bodies, Nature 126 (3173) (1930) 274–275. [23] P.H. Fang, Cole-Cole diagram and the distribution of relaxation times, J. Chem. Phys. 42 (10) (1965) 3411–3413. [24] S. Yilmaz, W. Wirges, S. Bauergogonea, S. Bauer, R. Gerhardmulthaupt, F. Michelotti, Dielectric, pyroelectric, and electro-optic monitoring of the crosslinking process and photoinduced poling of red acid magly, Appl. Phys. Lett. 70 (5) (1997) 568–570. [25] B. Lu, X.L. Dong, H. Huang, X.F. Zhang, X.G. Zhu, J.P. Lei, J.P. Sun, Microwave absorption properties of the core/shell-type iron and nickel nanoparticles, J. Magn. Magn. Mater. 320 (6) (2008) 1106–1111. [26] Z. Wang, H. Bi, P. Wang, M. Wang, Z. Liu, L. Shen, Magnetic and microwave absorption properties of self-assemblies composed of core-shell cobalt-cobalt oxide nanocrystals, Phys. Chem. Chem. Phys. 17 (5) (2015) 3796–3801. [27] P. Liu, L. Li, Z. Yao, J. Zhou, M. Du, T. Yao, Synthesis and excellent microwave absorption property of polyaniline nanorods coated Li0.435Zn0.195Fe2.37O4 nanocomposites, J. Mater. Sci. Mater. Electron. 27 (8) (2016) 7776–7787. [28] Z. Qiao, S. Pan, J. Xiong, L. Cheng, Q. Yao, P. Lin, Magnetic and microwave absorption properties of La-Nd-Fe alloys, J. Magn. Magn. Mater. 423 (2017) 197–202. [29] H. Guan, H. Wang, Y. Zhang, C. Dong, G. Chen, Y. Wang, Microwave absorption performance of Ni(OH)2 decorating biomass carbon composites from Jackfruit peel, Appl. Surf. Sci. 447 (2018) 261–268. [30] Q. Zeng, D.W. Xu, P. Chen, Q. Yu, X.H. Xiong, H.R. Chu, 3D graphene-Ni microspheres with excellent microwave absorption and corrosion resistance properties, J. Mater. Sci. Mater. Electron. 29 (2018) 2421–2433. [31] B. Zhang, J. Wang, H. Tam, X. Su, S. Huo, S. Yang, Synthesis of Fe@Ni nanoparticles-modified graphene/epoxy composites with enhanced microwave absorption performance, J. Mater. Sci. Mater. Electron. 29 (2018) 3348–3357. [32] Y. Wang, Y. Pu, Y. Shi, H. Cui, Excellent microwave absorption property of the CoFe2O4/Y3Fe5O12 ferrites based on graphene, J. Mater. Sci. Mater. Electron. 28 (17) (2017) 1–7. [33] X. Gu, W.M. Zhu, C.J. Jia, R. Zhao, W. Schmidt, Y.Q. Wang, Synthesis and microwave absorbing properties of highly ordered mesoporous crystalline NiFe2O4, Chem. Commun. 47 (18) (2011) 5337–5339. [34] X. Zhou, D. Chuai, D. Zhu, Electrospun synthesis of reduced graphene oxide (RGO)/ NiZn ferrite nanocomposites for excellent microwave absorption properties, J. Supercond. Nov. Magnetism 32 (2019) 2687–2697. [35] B. Wen, M.S. Cao, Z.L. Hou, W.L. Song, L. Zhang, M.M. Lu, H.B. Jin, X.Y. Fang, W.Z. Wang, J. Yuan, Temperature dependent microwave attenuation behavior for carbon-nanotube/silica composites, Carbon 65 (12) (2013) 124–139.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is supported by the National Natural Science Foundation of China (51702158), the Fundamental Research Funds for the Central Universities (NP2018111), Open Fund of Key Laboratory of Materials Preparation and Protection for Harsh Environment (Nanjing University of Aeronautics and Astronautics), Ministry of Industry and Information Technology (56XCA18159-3). References [1] P.J. Liu, Z.J. Yao, V.M.H. Ng, J.T. Zhou, L.B. Kong, Enhanced microwave absorption properties of double-layer absorbers based on spherical NiO and Co0.2Ni0.4Zn0.4Fe2O4 ferrite composites, Acta. Metall. Sin-Engl. 31 (2) (2017) 1–9. [2] Y.M. Lei, Z.J. Yao, H.Y. Lin, J.T. Zhou, P.J. Liu, T.B.H. Qi, Y. Ning, C. Shen, Synthesis and enhanced microwave absorption properties of urchin-like polyaniline/Ni0.4Zn0.4Co0.2Fe2O4 composites, Polym. Bull. 76 (2018) 3113–3125. [3] R.O. Becker, A.A. Marino, Electromagnetic pollution, Sciences 18 (1) (1978) 14–15. [4] Y. Qian, Z. Yao, H. Lin, J. Zhou, Mechanical and microwave absorption properties of 3D-printed Li0.44Zn0.2Fe2.36O4/polylactic acid composites using fused deposition modeling, J. Mater. Sci. Mater. Electron. 29 (22) (2018) 19296–19307. [5] J.S. Li, H. Huang, Y.J. Zhou, C.Y. Zhang, Z.T. Li, Research progress of graphenebased microwave absorbing materials in the last decade, J. Mater. Res. 32 (07) (2017) 1213–1230. [6] S. Zhang, Z. Qi, Y. Zhao, Q. Jian, X. Ni, Y. Wang, Core/shell structured composites of hollow spherical CoFe2O4 and CNTs as absorbing materials, J. Alloy. Comp. 694 (2017) 309–312. [7] X. Wang, H. Yan, R. Xue, S. Qi, A Polypyrrole/CoFe2O4/Hollow Glass Microspheres three-layer sandwich structure microwave absorbing material with wide absorbing bandwidth and strong absorbing capacity, J. Mater. Sci. Mater. Electron. 28 (2016) 519–525. [8] Z.J. Li, G.M. Shi, Q. Zhao, Improved microwave absorption properties of core-shell type Ni@SiC nanocomposites, J. Mater. Sci. Mater. Electron. 28 (8) (2017) 5887–5897. [9] D.L. Zhao, F. Luo, W.C. Zhou, Microwave absorbing property and complex permittivity of nano SiC particles doped with nitrogen, J. Alloy. Comp. 490 (2010) 190–194. [10] Y. Li, R. Wang, F. Qi, C. Wang, Preparation, characterization and microwave absorption properties of electroless N-Co-P-coated SiC powder, Appl. Surf. Sci. 254 (15) (2008) 4708–4715. [11] M. Pant, D.K. Kanchan, P. Sharma, M.S. Jayswal, Mixed conductivity studies in silver oxide based barium vanado-tellurite glasses, Mater. Sci. Eng. B-Adv. 149 (1) (2008) 18–25. [12] J.H. Kim, S.S. Kim, Microwave absorbing properties of Ag-coated Ni-Zn ferrite microspheres prepared by electroless plating, J. Alloy. Comp. 509 (2011)

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