silicon nitride metacomposites

Journal Pre-proofs Tunable negative permittivity behavior and electromagnetic shielding performance of silver/silicon nitride metacomposites Chuanbing Cheng, Yuliang Jiang, Xiao Sun, Jianxing Shen, Tailin Wang, Guohua Fan, Runhua Fan PII: DOI: Reference:

S1359-835X(19)30502-0 https://doi.org/10.1016/j.compositesa.2019.105753 JCOMA 105753

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

28 September 2019 18 December 2019 26 December 2019

Please cite this article as: Cheng, C., Jiang, Y., Sun, X., Shen, J., Wang, T., Fan, G., Fan, R., Tunable negative permittivity behavior and electromagnetic shielding performance of silver/silicon nitride metacomposites, Composites: Part A (2019), doi: https://doi.org/10.1016/j.compositesa.2019.105753

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Tunable negative permittivity behavior and electromagnetic shielding performance of silver/silicon nitride metacomposites Chuanbing Cheng,*a Yuliang Jiang,b Xiao Sun,a Jianxing Shen,a Tailin Wang,a Guohua Fan,b Runhua Fanbc

a

Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, School of Materials Science and Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China b Key

Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China c College

of Ocean Science and Engineering, Shanghai Maritime University, Shanghai 201306, China

To whom correspondence should be addressed: E-mail: [email protected]

1

Abstract The recent rise of metacomposites offered a new research strategy for electromagnetic shielding materials owing to their negative electromagnetic parameters, such as negative permittivity. Herein, we prepared silver/silicon nitride (Ag/Si3N4) metacomposites with tunable negative permittivity by a facile impregnation-calcination process, and explored their electrical conductivity, permittivity and electromagnetic shielding properties. As the Ag content increased, formative metal networks in the composites rendered their conductivity characteristic changing from a hopping conductivity to a metal-like conductivity. Tunable negative permittivity behavior combined with enhanced shielding effectiveness (SE) was observed at 2-18 GHz in the metacomposites with high Ag contents. The plasma-like negative permittivity was accounted for by a low frequency plasmonic state of free electrons in the inductive Ag networks, and the frequency band and absolute magnitude of negative permittivity could be adjusted by controlling Ag content, which was well described by Drude model. The average total SE of our obtained metacomposite could reach ⁓30 dB, and the reflection was the primary shielding mechanism, which was attributed to the intense impedance mismatching stemmed from the negative permittivity. Our work opens up the possibility of designing metacomposites for promising electromagnetic shielding materials, promoting their application in microwave field.

Keywords: Metacomposite, Metamaterial, Negative permittivity, Electromagnetic shielding

2

1. Introduction Electromagnetic (EM) metamaterials, as a new member of EM media and with negative permittivity and/or permeability, have stimulated extensive interest with unique properties, including negative refraction, perfect lens, reversed Doppler Effect and inverse Cherenkov Effect.[1-4] They have shown great potential in communication, medicine, aerospace and military fields, especially in microwave and electronic devices.[5-10] In general, conventional metamaterials

were

artificially

engineered

materials,

which

were

prepared

using

micro-/nano-machining processes, and their EM properties mainly rely on their periodic structure, such as geometry and arrangement of structural units.[11, 12] Nevertheless, the negative EM parameters were recently achieved in some composites without periodical structure by manipulating their chemical composition and microstructure.[13-17] For instance, Tsutaoka's group reported the negative permittivity at the microwave region by building resin composites with randomly dispersed metal Cu, Ni or Fe53Ni47 particles.[18-20] Qiu et al.[15] obtained the negative parameters in carbon nanotube/polyaniline composites at MHz frequency range. The negative permittivity also was observed at KHz and lower frequency bands in polymer composites consisting of carbon nanofibers or graphene.[21, 22] Moreover, the tuning of negative permittivity was explored in some percolation composites by designing functional components.[23-26] For example, Fan et al.[25, 27] reported adjustable negative permittivity in pyrolysis carbon/ceramic composites, and the magnitude of negative permittivity was successfully adjusted by controlling the heat treatment temperature and carbon content. Liu et al.[28] prepared ternary epoxy composites containing with Fe and SiO2-coated Fe particles, and the dispersion of negative permittivity could be precisely regulated by changing the Fe/coated-Fe ratio. Those composites, referred to as metacomposites or intrinsic metamaterials, possess good 3

designability, tunable negative parameters and isotropic EM response, which are beneficial for their applications in many fields. For instance, carbon/hafnium nickel oxide metacomposites with small absolute values of negative permittivity had excellent absorption performance, which made them better apply in the microwave absorbing field.[6] Moreover, the percolative metacomposites with suppressed negative permittivity were recently used to prepare multi-layer composites with low loss and high dielectric constant, facilitating their application in energy-storage capacitors.[8, 29] It is well-known that EM shield to radiation pollution derived from electrical devices and equipment is vitally important for protecting human beings and sensitive circuits in high precise apparatuses.[30, 31] A variety of materials have been designed and prepared to explore for this application. It has been widely accepted that constructing conductor/insulator composites is a favorite and effective strategy for designing shielding materials. For instance, Park et al.[32] fabricated graphene nanoplatelet/polyethylene composites, and a maximum K-band shielding effectiveness (SE) of 31.6 dB was achieved in the composite with 19 vol% content of graphene nanoplatelet. He et al.[33] incorporated multi-walled nanotube into waterborne polyurethane to obtain excellent shielding performance, and the SE value of composites could reach 24.7 dB in X-band. Dinakaran et al.[34] reported the shielding property of Ag-graphite/poly-vinylidene difluoride composites, and the SE was 29.1 dB at 12.4 GHz. It should be noted that most of those shielding materials generally possessed positive EM parameters.[35, 36] Interestingly, the excellent EM shielding performance was found in some metacomposites with negative permittivity lately, which provided a new research idea for the design and constructing of EM shielding materials. For example, the negative permittivity was found in the graphene nanosheet/alumina ceramics at the whole X-band, and the total SE could reach 22 dB.[37] Kim et 4

al.[38] reported graphene foam/conductive polymer composites with negative permittivity, which delivered outstanding EM shielding performance. Furthermore, excellent microwave absorption and shielding properties were observed in polythiophene thin films at the Ku-band region, along with the appearance of negative permittivity behavior.[39] Whereas, in those researches, there is little attention paid to the negative permittivity behavior, and the internal relation between the negative permittivity and EM shielding performance also is not clearly explained. Herein, the metacomposites consisting of silver (Ag) particles and films dispersed in porous silicon nitride (Si3N4) matrix were fabricated by an impregnation-calcination process. The Ag metal with excellent chemical stability and high conductivity is one of preferred conductive fillers to improve the EM properties of composites, while the Si3N4 ceramic is selected as a structural matrix material owing to its electric insulation, high strength, good thermostability and excellent chemical durability.[40-43] These advantages would make the hybrid composites better adapt to an adverse electromagnetic environment, such as high temperature.[44] This liquid impregnation method can easily tailor the composites' microstructure to optimize their EM properties, and also avoid the traditional high-temperature sintering process of ceramic composites. The effect of Ag content on the microstructure, electrical and EM shielding properties of the metacomposites was studied in detail. It was found that the increasing Ag content brought about the formation of conducting networks and the change of conductivity mechanism in the metacomposites. Negative permittivity and high SE were observed in the metacomposites with high Ag contents. Drude model was applied to analyze the plasma-like negative permittivity behavior, and the detailed EM shielding performance and the corresponding shielding mechanism also were discussed. Besides, the relevance between the 5

negative permittivity and shielding behavior was illustrated. 2. Experimental 2.1 Sample preparation The porous Si3N4 ceramics with the porosity of about 50% were synthesized by protective atmosphere sintering, and the details regarding their experimental procedure can be found elsewhere.[27] The obtained Si3N4 ceramics were machined into square pieces (the length and width are both 15 mm, and the thickness is about 2.3 mm) and ring specimens (the inside diameter is 3 mm, the outside diameter is 7 mm, and the thickness is 2.3 mm), respectively. The as-prepared samples were dipped into 3 mol/ml silver nitrate (AgNO3) alcohol solutions and vacuumized for 30 min to make the solution infiltrate into the porous ceramics. The specimens were then fetched from the alcohol solution, and the excess solution on their surfaces was wiped up. The impregnated specimens were dried in a circulation oven at 80 oC for 1h and 120 oC for 1h to remove alcohol. Subsequently, the samples were calcined at 500 oC for 1 h in air, and Ag particles were imbedded into the porous Si3N4 ceramics (2AgNO3=2Ag+2NO2 ↑+O2 ↑). After repeating impregnation-carbonization process, the Ag/Si3N4 composites with Ag content of 34 wt% (7.0 vol%), 38 wt% (8.1 vol%), 44 wt% (9.9 vol%) and 50 wt% (12.0 vol%) were fabricated and denoted as AS34, AS38, AS44, and AS50, respectively. For comparison purpose, the porous Si3N4 ceramic without impregnation process was referred as AS0. 2.2 Characterization For the Ag/Si3N4 composites, the weight content (WC) and volume content (VC) of Ag were determined using the following expressions, respectively: 6

m1  m0 100% m1

(1)

1 (m1  m0 ) 100%  Ag m1

(2)

WC(%) 

VC(%) 

where m0 is the weight of porous Si3N4 ceramic, m1 represents the weight of composites, ρAg is the theoretical density of metal Ag (10.49 g/cm³), and ρ1 represents the relative density of composite, which is measured by Archimedesʹ method. The phase constituents of the porous Si3N4 ceramic and Ag/Si3N4 composites were examined using an X-ray diffractometer (XRD, LabX XRD-6100, Shimadzu, Japan) with Cu Kα radiation source. The diffraction patterns were recorded over a 2θ angle range of 10-80° at a sweep speed of 5°/min at room temperature in the air. The fracture surface morphologies of the samples were observed using a field emission scanning electron microscope (FESEM, Gemini500, Zeiss, Germany). The measurements of alternating current (AC) electrical conductivity σac and impedance (Z′, Z′′) for the square samples were carried out by a parallel plate capacitor method using an Agilent E4991A Precision Impedance Analyzer with a working frequency of 10 MHz-1 GHz. The σac was calculated according to the following equation:

 ac 

d Rs S

(3)

where d is the thickness of specimen, S is the electrode plate area (3.85×10-5 m2), Rs is real part of impedance (Rs=Z′). Agilent 8722ES vector network analyzer was applied to determine the microwave dielectric property and EM shielding effectiveness for the cylindrical specimens. The data of electromagnetic S parameters (S11 and S21) and complex permittivity were measured using a coaxial airline method at room temperature in the frequency range of 2-18 GHz. The power coefficients of reflection (R) and transmission (T) were calculated by equation (4)-(5), 7

respectively:[45]

R  S11

2

T  S 21

2

(4) (5)

The total shielding effectiveness (SE) and absorption efficiency (AE) were determined by the relational expressions, respectively: [45, 46]

SE   10 lg T 

AE  A / 1  R  100%

(6) (7)

3. Results and Discussions 3.1 Microstructure and composition characterization The XRD patterns of porous Si3N4 ceramic and its Ag composites are shown in Figure 1. The main crystalline phases of the porous ceramic matrix are β-Si3N4 and α-Si3N4. There are four intense diffraction peaks in the XRD patterns of Ag/Si3N4 composites, locating at about 2θ=38.6°, 44.6°, 64.9° and 77.9°, which are corresponding to Ag (111), Ag (200), Ag (220) and Ag (311), respectively.[47, 48] That is, Ag was successfully introduced into the porous ceramics. Some small Si3N4 diffraction peaks also are found in the composites, and their intensity is much lower in comparison with those of Ag peaks. Other impurity phases are not observed in the composites, indicating that there was no other reaction during the preparation process, and the AgNO3 was totally decomposed into metallic Ag. Moreover, the average crystallite size of Ag metal in the composites is evaluated using Scherrer formula, D = Kλ/βcosθ, where D represents the crystallite size (nm), β is the full width at half-maximum height, θ signifies the angle of the diffraction peaks (°), λ is the wavelength of Cu Kα radiation (0.15 nm), and K is the Scherrer 8

constant (0.89).[49, 50] For the composites with increasing Ag content, the crystallite size of Ag is 28.84, 28.43, 30.50 and 32.74 nm, respectively. More times of immersion and calcination lead to a slight increase in the crystallite size of Ag metal. Figure 2 shows the FESEM images of Si3N4 ceramic and its composites with different Ag contents. As seen, many clubbed β-Si3N4 grains overlap with each other in the Si3N4 ceramic (Figure 2a), and a great deal of three-dimensional network pores also are observed, which is beneficial to the subsequent impregnation process. The Ag particles or clusters (bright areas) randomly disperse in the composites (Figure 2b-h), which form and adhere to the surface of Si3N4 grains, especially to the junction connecting grains. Meanwhile, a similar distribution of Ag particles is also observed on the surface of composites (Figure S1). More Ag clusters are found in the composites with the increase of metal content, and their size also become greater. Besides, some ultrathin Ag films spreading out on the grains are also detected in magnified FESEM image of the composite with 50 wt% Ag content (Figure 2i), which is further corroborated by the EDX analysis (Figure S2). From the FESEM and EDX-mapping images of the other composites (Figure S3), the existence of Ag films on the grain surface also is confirmed. 3.2 Electrical conductivity Figure 3a shows the electric conductivity (σac) as a function of frequency for the Si3N4 ceramics with various Ag contents. Two types of dispersion characters for σac curves are observed, which indicates that the conductivity mechanism changes and a percolation behavior appeared in the ceramics with increasing Ag contents.[13] The difference of dispersion character is related to the microstructure of ceramics. As the Si3N4 ceramic is a class of good insulating materials, its σac increased almost linearly with increasing the frequency, exhibiting a typical insulating behavior.[51] The composite with 34 wt% Ag content (AS34) shows a clearly 9

increasing trend of σac in the low frequency region and then exhibits a slightly decreasing conductivity at the high frequencies. Most of Ag particles and films were distributed in isolation in the composite, and the free electrons can jump between the adjacent metal phases as an alternate electric field was applied. When the frequency of external electric field increased, more electrons can participate in the jumping movement, resulting in the increasing σac at the low frequency band. The hopping conduction behavior usefully conforms to Jonscher power law:[51, 52]

 ac  A  2 f



n

(8)

where A is the pre-exponential factor and n is the fractional exponent (0
(9)

where ZC is capacitive reactance and ZL is inductive reactance. With regard to our obtained 10

percolative composites, ZC is closely associated with isolated Ag clusters, while ZL mainly originates from conductive Ag networks. For the Si3N4 ceramic and the composite with low Ag content, the values of Z′′ are negative in the whole test frequency band (Z′′ < 0). That is, the ZC is larger than the ZL and is dominant in the reactance response. Hence, AS0 and AS34 samples can be regarded as a capacitor, showing a capacitive behavior.[54, 55] On further increasing the Ag content, the values of Z′′ shift to positive (Z′′ > 0), and ZL plays a primary role in the reactance due to the formation of conducting circuits in the composites. Thus, these composites manifest an inductive behavior.[13] The inductive characteristic stems from induced current in the Ag networks of composites under external high-frequency electric field. As mentioned above, with the increase of Ag content, the composites undergo a shift of conductive mechanism from hopping conductivity to metal-like conductivity, accompanied by a capacitive-inductive transition. 3.3 Negative permittivity behavior Figure 4 represents frequency dependences of permittivity (ε* = ε′ - jε″) for the Si3N4 ceramic and Ag/Si3N4 composites. The real part of permittivity ε′ for the porous Si3N4 ceramic shows little fluctuation at the total frequency range, and its values are positive (⁓ 3.3) (Figure 4a). The ε′ of AS34 sample remarkably increases to contrast with that of the Si3N4 ceramic, owing to the enhanced polarization in the composite.[37, 56] Further increasing the Ag content, the ε′ of composites is turned into negative at the low frequency (Figure 4a and b), and the negative ε′ also is observed at the frequency below 1 GHz (Figure S4). This plasma-like negative ε′ behavior, which is similar to that of bulk metals, is attributed to the low-frequency plasmonic state stemmed from the plenty of unbound electrons in the formed inductive Ag networks. The negative ε′ spectra are analyzed by the Drude model:[57, 58] 11

    

2  2   2

(10)

where ωp (ωp = 2πfp) is effective plasma frequency of free electrons, ωτ is the electron scattering rate, and ε∞ is the optical dielectric constant. The fitting ε′ curves (solid lines), which are achieved using iterative method in OriginPro 9.0 software according to equation (10), are well consistent with the testing data (dotted lines), and the reliability factor R2 is 0.9418 (AS38), 0.9907 (AS44) and 0.9966 (AS50), respectively. The obtained fitting ωp for the composites becomes larger as the Ag content increases, and is 293.56, 698.77 and 724.66 GHz, respectively. The ωp is a natural frequency of oscillations to describe collective motions of free electrons in materials.[59] According to the Drude model, the negative ε′ will not be obtained unless the frequency is below ωp, and the ωp mainly relies on the effective density (neff) and mass (meff) of unbound electron in the material, which can be expressed as: [57, 58]

 

n eff e 2 m eff  0

(11)

where e is the electronic charge (1.6×10-19 C), and ε0 is the vacuum dielectric constant (8.85×10-12 F/m). For our obtained composites, the increasing Ag loading generates more free electrons in Ag networks, leading to the increase of neff, while the formation of more Ag networks results in the decrease of meff owing to the smoother movement of free electrons in the composites. Thus, the ωp shifts toward a higher frequency when the metal content is raised in the composites. As can be noted, the larger ωp will bring about the stronger dispersion of negative ε′ combining with the Drude model, and the absolute values of negative ε′ generally enlarge.[27, 60] The similar tendency of negative ε′ behavior is observed in our synthesized composites. Besides, the values of ε′ for AS38 and AS44 samples are negative at the low frequency, and 12

change to positive at around 7.4 and 14.8 GHz, respectively. The negative-positive transition frequency point of ε′ is known as screened plasma frequency (f0), which can be given by: [57]

f0 =

p 2



(12)

From the above formula, the larger ωp will arouse an increasing f0. Hence, the f0 of composites moves to a higher frequency on increasing the Ag content. For the AS50 sample, the ε′ is negative in entire test frequency band, and the f0 will be more than 18 GHz. As stated above, there are grounds for believing that the magnitude and frequency band of negative ε′ can be adjusted by manipulating inductive Ag networks and changing Ag content of composites. Table 1 shows the comparison of the negative ε′ in various metacomposites. The negative ε′ was achieved in some metacomposites consisting of metal, carbon, conducting ceramic or polymer component. Whereas, most investigations mainly focused on the negative ε′ at MHz and lower frequency bands, and little attention was given to the regulating characteristics of negative ε′. The tunable negative ε′ behavior was realized in our obtained metacomposites at the microwave region (2-18 GHz), which could promote the application of metacomposites in microwave field, and it was found that the construction of inductive conductive networks was vital for effectively tailoring the negative permittivity ε′.

13

Table 1. Comparison of negative permittivity property of the Ag/Si3N4 metacomposites with some representative metacomposites Materials

Filler content

Test frequency

Negative permittivity

Refs

Ni/Al2O3

31-35 wt%

10-1000 MHz

⁓-105

[61]

Ag/yttrium iron garnet

37-45 wt%

10-1000 MHz

⁓-104

[62]

Carbon/hafnium nickel oxide

-

10-1000 MHz

⁓-102

[6]

Carbon/yttrium iron garnet

13 wt%

10-1000 MHz

⁓-103

[17]

TiN/Al2O3

40-70 wt%

10-1000 MHz

⁓-105

[26]

CNT/polyaniline

20-30 wt%

1-1000 MHz

⁓-103

[15]

Graphene/polypyrrole

10-70 wt%

1-1000 MHz

⁓-104

[63]

SiC/polyaniline

10-60 wt%

100 Hz-10 MHz

⁓-103

[58]

Ni/poly(vinylidene fluoride)

6-12 wt%

0.1-100 kHz

⁓-104

[5]

Al2O3/polyaniline

1-5 wt%

1 Hz-10 kHz

⁓-105

[64]

Cu/resin

16.4-23.3 vol%

2-18 GHz

⁓-104

[65]

Ag/Si3N4

12 vol%

2-18 GHz

⁓-103

This work

The imaginary part of permittivity ε′′ is an indicator to describe dielectric loss of materials. Normally, the process of conduction and polarization is obligated to the dielectric loss.[52] According to Debye theory, the ε′′ is deemed to the joint contributions from the conduction loss (εc″) and polarization relaxation loss (εr″), which can be expressed as: [66]

 dc  0

(13)

s    1   2 2

(14)

 c ''   r '' 

 ''   c ''+ r '' 

 dc  s       0 1   2 2

(15)

where σdc is the direct current conductivity, εs is the static dielectric constant, and τ is relaxation time. In our obtained percolation composites, the conduction loss originates from the charges flowing through the neighboring Ag particles or films, while polarization relaxation loss stems 14

from the charge unbalance existing at the Ag-Si3N4 interfaces (interfacial polarization) and their interior (defect dipole).[52] For the Si3N4 ceramic, the ε″ is rather small (around 0.01) in the test frequency (Figure 4c), and is exclusively attributed to polarization relaxation process, while the conductive loss can be neglected due to the good insulation of Si3N4 ceramic[66]. The incorporation of Ag component into the Si3N4 ceramics leads to the considerable increment in ε′′, especially at the low frequency. As the Ag loading increases in the composites, the enlarged Ag-Si3N4 interface areas result in enhanced relaxation loss, and the higher conductivity brings about a remarkable increase of conduction loss.[67] Consequently, the ε′′ becomes larger in the composites with increasing Ag content. For the composites with high Ag contents, the conduction loss will play a leading role in the dielectric loss owing to the formation of conductive networks, so the ε′′ roughly decreases on increasing the frequency according to equ (13). [68, 69] 3.4. Electromagnetic Shielding Performance EM shielding mainly involves to the reflection and absorption of EM waves interacted with materials. The EM shielding effectiveness (SE) is devoted to characterize the total ability of the shielding material to attenuate incoming EM waves, which generally is the sum of the shielding by reflection and absorption.[40, 45] The EM shielding performance of the Si3N4 ceramic and its composites with various Ag contents is plotted at a frequency band of 2-18 GHz. As the Ag loading is raised in the ceramics, the SE has an obvious enlargement (Figure 5a), and their average SE increases from 1.18 to 30.73 dB (Figure 5b). For the composites with the Ag contents more than 38 wt%, the SE values are greater than 20 dB in the whole test frequency band (2-18 GHz), which can satisfy the typical requirement of commercial application in EM shielding devices.[70, 71] As the Ag content is 50wt% (12 vol%), the SE value of composite can reach the 15

maximum of 43.6 dB at the low frequency. Table 2 shows the comparison of EM shielding performance of the Ag/Si3N4 composites with some representative composites, which mainly include metal-polymer, carbon-polymer, metal-ceramic, carbon-ceramic and multiphase composites. It is found that the EM shielding performance of our obtained metacomposites is moderate comparing with those composites in consideration of the sample thickness (2.3 mm). As is well-known, the formation of conductive networks is the essential factor to enhance SE values.[46, 72] The three-dimensional Ag networks are formed in our obtained composites with high metal contents, which not only leads to the enhancement of SE but also brings about the appearance of negative permittivity. Thus, the higher SE values are achieved along with the negative permittivity behavior in the metacomposites, especially when the absolute value of negative permittivity is comparatively large. Table 2. Comparison of EM shielding performance of the Ag/Si3N4 metacomposites with some representative composites reported in recent literatures Filler

Test frequency

Thickness

Total SE

content

(GHz)

(mm)

(dB)

Ni chain/PVDF

12 wt%

18-26.5

0.5

Ag nanowire/WPU

28.6 wt%

8.2-12.4

Ag nanowire/Cellulose

9.57 wt%

MWCNT/WPU

Materials

Permittivity

Refs

35.4

Positive

[5]

2.3

64.0

-

[40]

0.5-1

0.15

48.6

-

[47]

10.6 wt%

8.2-12.4

0.4

24.7

-

[33]

Graphene/PEDOT:PSS

-

8-12

1.5

91.9

Negative

[38]

FeSiAl/Al2O3

40 wt%

8.2-12.4

2.0

36.4

Positive

[44]

MWCNT/BaTiO3

8 wt%

12-18

1.5

28.0

Positive

[73]

2.0 vol%

8.2-12.4

1.5

22.0

Negative

[37]

15 wt%

8-12

-

29.1

Positive

[34]

15 wt%

8.2-12.4

2.0

35.0

-

[41]

12 vol%

2-18

2.3

30.7

Negative

Graphene nanosheet/Al2O3 Ag-graphite/PVDF MWCNT-Fe3O4@Ag/Ep oxy Ag/Si3N4

16

This work

In order to illuminate the internal relation between negative permittivity and shielding performance, power data gathered from the EM shielding characterization set-up are further analyzed. It is widely recognized that the ratio of reflected and absorbed to incident power can best represent the reflection and absorption abilities of the shielding materials. In this study, the reflected power (R) and transmitted power (T) of Ag/Si3N4 composites are directly obtained by the microwave measurement (according to equ 4 and 5), and the absorbed power (A) can be calculated as: A = 1 – (R + T), as the total incident power is a constant (1 mW).[74] The average reflected, absorbed and transmitted powers are plotted for the Si3N4 ceramics as a function of Ag content in Figure 6a. It is observed that the T monotonously minifies with raising the Ag content, and the composites with negative permittivity shows the very low transmitted powers, which is 0.0188, 0.0021 and 0.0008 mW, respectively. That is, there are few EM waves passing through the composites, manifesting an excellent shielding property. Most of incident EM waves will penetrate through the porous Si3N4 ceramic, which is an outstanding candidate for wave-transparent materials, so its value of T is high (⁓ 0.77 mW), and the low A also is observed.[75] For the AS34 sample, the A, R and T each contributes about a third of incident power. In the composites with higher Ag contents, the R is much larger than A, indicating that the quantity of power blocked by reflection is much higher than that blocked by absorption. Hence, the reflection, rather than absorption, makes a dominant contribution to the shielding performance in this intrinsic metamaterials. Reflection loss (RL) curves of composites with high silver contents also are given in Figure S5, and their values are more than -2 dB in the testing frequency range, which further proves our judgment. The strong reflection stems largely from enhanced impedance mismatch, which should be related to the negative permittivity in the metacomposites.[5, 38] In order to deeply understand impedance characteristic of the 17

composites, relative input impedance (|Zin/Z0|) is proposed according to the transmission line theory, which can be expressed by:[76]

Z in / Z 0 

*  2 fd   * *  tanh  j *  c  

(16)

where Zin is the input impedance of the EM wave incident on the material interface, Z0 is the impedance of air (Z0 = (μ0/ɛ0)1/2), μ* is the complex permeability (μ* =μ′ - jμ″), d is the thickness of shielding materials, and c is the velocity of light. The |Zin/Z0| signifies the matching degree between input and air impedance. The closer that the |Zin/Z0| value is to 1, the better the impedance matching is, and the more incident EM waves entering the materials.[77] Figure 6b shows the |Zin/Z0| curves of composites with different silver contents. As silver content increases in the composites, the values of |Zin/Z0| gradually move away from 1. For the composites with negative permittivity, their |Zin/Z0| values are far less than 1 (more than an order of magnitude), indicating the appearance of strong impedance mismatch. Hence, the negative permittivity leads to the intense impedance mismatching on the basis of the equ (16), and then arouses the strong reflection in the metacomposites. In addition, the larger magnitude of negative permittivity leads to stronger impedance mismatching (smaller |Zin/Z0| values), so the total SE values of metacomposites get larger (Figure 5). That is, the metacomposites with considerable negative permittivity at broad frequency range can be better applied in the EM shielding field. The absorption efficiency (AE), which represents the absorption capacity to EM waves that entered into material interior, is also given in Figure 6c. It is observed that the addition of Ag leads to a remarkable increase in the AE, and the AS50 sample is found to demonstrate a high AE close to 96.5%. As far as know, the absorption is mainly connected with the long-range induced 18

current (free electrons) and polarization (interfacial polarization and electric dipole movement) in the shielding materials.[78, 79] The incremental Ag loading will result in the augment of conducting loops, Ag-Si3N4 interface areas and electric dipoles, and the massive incident waves can be rapidly decayed and converted into thermal energy.[38, 80] Thus, it is more difficult for entered EM waves to escape from the composites with higher Ag content, giving rise to the larger AE values. To more clearly demonstrate the EM shielding mechanism, the schematic illustration of shielding mechanism for the metacomposites is presented in Figure 6d. According to the previous analysis, the negative permittivity behavior combined with high conductivity, which results from abundant unbound electrons in the inductive Ag networks, deliver the strong reflection and relatively large SE. Moreover, the absorption mainly originates from the induced currents in the Ag networks and polarization relaxation.[81-83] The contribution of absorption to the overall SE should be based on the ability of the shielding material to decay the waves that are not reflected.[84] Although the obtained metacomposites have high AE, a great amount of EM waves is reflected by reason that the reflection occurs before the absorption process. Therefore, we concluded that the reflection is the dominant shielding mechanism for our obtained metacomposites with high Ag contents. 4. Conclusion In this manuscript, the metacomposites made of porous Si3N4 ceramic integrated with Ag metal was prepared by the solution impregnation and calcination process. The percolation phenomenon and transformation of conductivity mechanism appeared in the Ag/Si3N4 metacomposites with increasing metal content. The metacomposites with high Ag contents 19

exhibited tunable negative permittivity and excellent EM shielding performance. The low frequency plasmonic state, which was provided by unbound electrons in the inductive Ag networks, caused the negative permittivity behavior. The frequency band and value of negative permittivity were closely related with the Ag content, and the Drude model was used in analyzing such dielectric response. The inductive conductive network in the metacomposites was the decisive building block to adjust the negative permittivity. The high value of total SE (⁓ 30 dB) was dominated by the reflection phenomenon, which was due to the intense impedance mismatching derived from the negative permittivity behavior. Moreover, the larger magnitude of negative permittivity brought about stronger impedance mismatching, which made the SE value get larger in the metacomposites. Our work offers new capabilities for creating new EM shielding materials by constructing metacomposites with the tunable negative permittivity. Acknowledgements This work is financially supported by National Natural Science Foundation of China (No. 51901109 and 51871146) and Innovation Program of Shanghai Municipal Education Commission (Grant No. 2019-01-07-00-10-E00053). References [1] Smith DR, Pendry JB, Wiltshire MC. Metamaterials and negative refractive index. Science. 2004;305(5685):788-92. [2] Fang N, Lee H, Sun C, Zhang X. Sub-diffraction-limited optical imaging with a silver superlens. Science. 2005;308(5721):534-7. [3] Liu Y, Zhang X. Metamaterials: a new frontier of science and technology. Chemical Society Reviews. 2011;40(5):2494-507. 20

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Figure 1. XRD patterns of the Si3N4 ceramic and Ag/Si3N4 composites with different silver contents.

31

Figure 2. Fracture section SEM micrographs of porous Si3N4 ceramic (a) and Ag/Si3N4 composites with different silver contents of (b-c) 34, (d-e) 38 and (f-g) 44 and (h-i) 50 wt%.

32

Figure 3. Frequency dependences of AC conductivity (a) and reactance (b) for the Si3N4 ceramics with various silver contents. Microstructural evolution schematic of the composites with increasing silver contents (c).

33

Figure 4. Frequency dependences of dielectric permittivity for the Si3N4 ceramic and its composites with different Ag contents.

34

Figure 5. EM total SE of the Si3N4 ceramic and its composites with various Ag contents.

35

Figure 6. The reflected, transmitted, absorbed powers (a), relative input impedance (b) and absorption efficiency (c) of the Si3N4 ceramic and its composites with various Ag contents. Schematic illustration of EM shielding mechanism for the composites with high Ag contents (d).

36

Supporting Information for

Tunable negative permittivity behavior and electromagnetic shielding performance of silver/silicon nitride metacomposites Chuanbing Cheng,*a Yuliang Jiang,b Xiao Sun,a Jianxing Shen,a Tailin Wang,a Guohua Fanb, Runhua Fan bc

a

Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, School of Materials Science and Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China

b Key

Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China

c College

of Ocean Science and Engineering, Shanghai Maritime University, Shanghai 201306, China

To whom correspondence should be addressed: E-mail: [email protected] (C. Cheng)

37

Figure S1 EFSEM images of the surface for Ag/Si3N4 composite with Ag contents of 34 wt% (a), 38 wt% (b), 44 wt% (c) and 50 wt% (d).

From the EFSEM images of the composites (Figure S1), it was found that the Ag particles or clusters randomly distributed on the surface of composites with various Ag contents.

38

Figure S2 EDX analysis of the Ag/Si3N4 composite with 50 wt% Ag content.

In the EFSEM and EDX-mapping images of the composite with 50 wt% Ag content (Figure S2 a and b), many Ag particles or clusters (bright areas) were observed at the junction connecting Si3N4 grains. It’s noteworthy that the Ag element was still detected at the grain surface where Ag particles were not clearly observed in the EFSEM image.

39

Figure S3 EFSEM and EDX-mapping images of the Ag/Si3N4 composites with Ag contents of 34 wt% (a-b), 38 wt% (c-d) and 44 wt% (e-f).

40

Figure S4. Frequency dependences of permittivity for the Si3N4 ceramic and its composites with different Ag contents at the frequency below 1 GHz.

The relative complex permittivity of the samples was measured by an Agilent E4991A Precision Impedance Analyzer at the frequency of 10 MHz-1 GHz. The real part (ε′) and imaginary part (ε′′) of permittivity were determined by the equations:

'

 '' 

Cp d

0S d

2 f  0 SRP

(S1)

(S2)

where d is the thickness of specimen, S is the electrode plate area (3.85×10-5 m2), ε0 is the permittivity of free space (8.85×10-12 F·m-1), f is the frequency, and Cp and Rp are the parallel capacitance and resistance, respectively. The data of Cp and Rp can be directly measured by the Impedance Analyzer. 41

Figure S5. Frequency dependent reflection loss curves of Ag/Si3N4 composites with high Ag contents (≥ 38 wt%).

According to the transmission-line theory, reflection loss (RL) values were calculated by following formulas:[S1]

Z in  Z 0

*  2 fd   * *  tanh  j *  c  

(S3)

Z in  Z 0 Z in  Z 0

(S4)

RL  20 log

where Zin is the input impedance of samples, Z0 is the impedance of air (Z0 = (μ0/ɛ0)1/2), ɛ* is the complex permeability (ɛ* = ɛ′ - jɛ″), μ* is the complex permeability (μ* =μ′ - jμ″), c is the velocity of light, f is the frequency, and d is specimen thickness.

References [S1] Gao Y, Wang C, Li J, Guo S. Adjustment of dielectric permittivity and loss of graphene/thermoplastic polyurethane flexible foam: Towards high microwave absorbing performance. Composites Part A: Applied Science and Manufacturing. 2019;117:65-75. 42

Highlights: 1. A plasma-like negative permittivity was observed in the metacomposites, which resulted from the low frequency plasmonic state provided by formed silver networks. 2. The frequency band and absolute magnitude of negative permittivity could be adjusted by controlling Ag content, which was well described by Drude model. It was found that the construction of inductive conductive networks was vital for effectively tailoring the negative permittivity permittivity. 3. The average total SE of our obtained metacomposite with high Ag content could reach ⁓30 dB. The reflection was the primary electromagnetic shielding mechanism, which was attributed to the intense impedance mismatching originated from the negative permittivity. 4. The larger magnitude of negative permittivity resulted in stronger impedance mismatching, which made the SE value get larger in the metacomposites.

No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication.

Author Contributions 1. Chuanbing Cheng conceived and designed the experiments and wrote the manuscript. 2. Runhua Fan and Jianxing Shen helped to perform the analysis with constructive discussions. 3. Yuliang Jiang and Guohua Fan helped for electrical measurement. 5. Xiao Sun and Tailin Wang helped for XRD and SEM measurement.

43