Tunable Negative Permittivity with Fano-like Resonance and Magnetic

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Tunable Negative Permittivity with Fano-like Resonance and Magnetic Property in Percolative Silver/ Yittrium Iron Garnet Nanocomposites Kai Sun, Runhua Fan, Yansheng Yin, Jiang Guo, Xiao-Feng Li, Yanhua Lei, Liqiong An, Chuanbing Cheng, and Zhanhu Guo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02036 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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The Journal of Physical Chemistry

Tunable Negative Permittivity with Fano-like Resonance and Magnetic Property in Percolative Silver/Yittrium Iron Garnet Nanocomposites

Kai Sun,a,b Runhua Fan,*, a Yansheng Yin,a Jiang Guo, b Xiaofeng Li,a Yanhua Lei, a Liqiong An, a Chuanbing Cheng,b,c and Zhanhu Guo*, b

a

College of Ocean Science and Engineering, Shanghai Maritime University, Shanghai 201306, P. R. China

b

Integrated Composites Laboratory (ICL), Department of Chemical & Biomolecular Engineering, University of Tennessee, Knoxville, TN 37996, USA

c

Dezhou Meta Research center of Innovative Materials, Dezhou 253000, P. R. China

*Corresponding author [email protected] (R. F.); [email protected] (Z. G.)

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ACS Paragon Plus Environment


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Abstract Composites with negative electromagnetic parameters can be promising candidates for metamaterials. In this paper, the impedance, permittivity and permeability were investigated in the silever/yrrium irongarnet (Ag/YIG) composites, which was fabricated by in-situ synthesis process. In the vicinity of percolation threshold, the dielectric loss is dominated by conduction and polarization. When reached the percolative state, the negative permittivity with Fano-like renonance (an asymmetric resonance) was observed. Furthermore, the negative permittivity behavior was attributed to inductive character, and the LC resonance was responsible for the Fano-like transition from negative to positve permittiivty. In addition, it was demonstrated that the permeability of Ag/YIG composites presented frequency dispersion due to the domain wall motion and the gyromagnetic spin rotation. The percolative Ag/YIG composites with tunable negative permittivity have great potential in the field of electromagnetic attenuation, shielding and antenna.

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1. Introduction

The percolation phenomenon, which is a classic and significant behavior in physics, is widespread in heterogeneous multicomponents of materials.1-4 When the concentration of functional fillers approaches a critical value (i.e., percolation threshold), the filler-particles come into contact with each other and establish a continuous network throughout the system. Along with the change of microstructure, the physical properties of the composites also undergo an abrupt shift and bring about attractive performances, such as high dielectric constant, high electrical or thermal conductivity.5-8 The percolation behaviors especially near percolation threshold have drawn intensive attentions due to their fascinating properties and potential applications in the field of thermal storage,9 light-emitting diodes10 and charge-storage capacitors,11 etc. It has been proved that conductive fillers can dramatically increase the permittivity of polymer-metal composites by taking the advantage of percolation threshold.12 Accordingly, extensive investigations13-19 have been particularly focused on the composites with high dielectric constant and low dissipation, due largely to their promising applications in microelectronics, electrical engineering and even biomedical engineering.12

When the fraction of conductive filler exceeds percolation threshold, the negative permittivity can be achieved in the composites,20-26 which have great potential for electromagnetic interference shielding,23 near field amplifying27, antenna,28 and sensors,29-31 etc. Zhong et al.32 obtained negative permittivity at KHz region in percolative polymer nanocomposites, due to the dielectric resonance of polarization. Guo et al.33-34 also achieved 3



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negative permittivity at lower frequency region in polymer composites, when carbon nanotube or graphene was beyond percolation threshold. Moreover, Wang et al.35 reported negative permittivity with a low frequency plasmonic oscillation of delocalized electrons in metallic cobalt networks formed in the alumina matrix.

As an alternative to metamaterials, which are composed of periodic structure units and possess peculiar property, Chui et al.36 theoretically investigated that the negative permittivity and negative permeability can be sulmatenously attained by incorporating metallic magnetic nanoparticles into insulating matrix. Shi et al.37-38 initially observed double negative property in ceramic matrix composites with nickel or iron particels in radio frequency region, which expanded the scope of metamatrials. In addition, Tsutaoka et al.39-40 obtained negative permittivity and negative permeability at microwave frequency regime in percolative polymer (polyphenylene sulfide) composites combined with conductive copper and magnetic yttrium iron garnet (YIG). Consequently, the composites with tunable negative permittivity are expected to be promising candidates for double negative materials, which provides a novel avenue to design metamaterials.

Metal-ceramic composites are excellent candidates to fabricate multifunctional devices owing to the combined different properites of metal and ceramic, which have huge contrast especially in electrical and magnetic properties.41-43 Compared with the reported ceramics including alumina35, 44 and silicon nitride,4, 45 YIG was widely used for high frequency electronic devices such as resonators,46 filters,47 and phase shifters48 owing to its unique physicochemcial 4


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proeprties. Meanwhile, compared with the reported metals including copper,6 iron49 and alloy,50 silver was selected due to its excellent electrical conductivity and oxidative stability.51 Hence, Ag/YIG composites can be promising candidates to achieve tunable electrical and magnetic property by tuning their compositions and tailoring their microstructures. In the previous research,52 the negative permittivity was achieved in porous Ag/YIG composites; however, the porosity was detrimental to their mechanical and electromagnetic properties, limiting their development and applications. Additionally, it was difficult to effectively control the content of functional phases via a chemical impregnation method.

Herein, an effective and versatile strategy (i.e., in-situ synthesis) was used to prepare percolative Ag/YIG composites. The impedance, permittivity and permeability of Ag/YIG composites were investigated. There was an obvious percolation phenomenon observed with the increase of silver content. With the geometric transition of the silver particles microstructure, the capacitive–inductive character transition and positive-negative permittivity transition occured in Ag/YIG composites. The nature of the negative permittivity was studied and attributed to the inductive character by equivalent circuit analysis. In addition, the dielectric loss mechanism of heterogeneous composites in the vicinity of percolation threshold was further clarified. The dependences of permeability on frequency and silver content in percolative Ag/YIG composites were also explored. 2. Experiment 2.1 Preparation Process 5



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The yttrium iron garnet Y3Fe5O12 (YIG) powders were prepared by a conventional solid state reaction.27 The raw materials, i.e., Y2O3 and Fe2O3 powders, were mixed by the 3 : 5 stoichiometric ratios and sintered at 1573 K for 6 hours. The prepared YIG powders added with different mass fractions of silver oxide (5 wt. %, 15 wt. %, 25 wt. %, 30wt. % and 40 wt. %, which were denoted as samples AY5, AY15, AY25, AY30, and AY40, respectively), were dispersed in absolute ethanol and uninterruptedly milled for 10 h. After ball-milling and sieving, the powders were pressed into green bodies at 30 MPa pressure. The compact bulks were sintered at 1323 K for 1 h in a resistance furnace. During the sintering process, the silver oxide was decomposed into metallic silver and YIG could maintain its chemical stability. It meant that the Ag/YIG composites with different silver contents were fabricated by one-step in situ synthesis process. 2.2 Characterization and Measurement

The phase identifications of the composites were investigated by X-ray diffraction (XRD; Tokyo, Japan) using the Rigaku D/max-rB X-ray with Cu Ka radiation. The fracture surface morphologies of the composites were observed by SU-70 Field Emission Scanning Electron Microscope (FESEM; Tokyo, Japan).

The electromagnetic parameters measurements were carried out at room temperature using an impedance analyzer E4991A (Agilent Technologies Co. Ltd) in the frequency range from 10 MHz to 1 GHz. The samples were machined into disks with a dimension of 12.5 mm in diameter and approximately 2.0 mm in thickness for impedance and permittivity measurement. The silver 6


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paste was painted between samples and electrodes to eliminate the contact resistance. After calibration and compensation for the analyzer, the samples were put between the two planar electrodes of 16453 A dielectric test fixture for permittivity measurement, under a 100 mV ac voltage.53 The impedance data ( Z＇and Z＂) were converted to capacitance C and resistance R for permittivity calculation ε′r =Cd/Aε0 and εr＂= d/2πfAε0, where C is capacitance, d is the thickness of sample, f is the test frequency, R is the resistance, A is the area of the electrode and ε0 is the permittivity of vacuum.

The 16454A test fixture was used to measure the permeability under 100 mA ac current. The samples were processed into toroidal form (inner diameter is 6.5 mm and outer diameter is 19.0 mm). The permeability µ′r=2π(Z-Z0)/[jωµ0dln(c/b)]+1, where Z and Z0 are the impedances of the test fixture with and without the sample mounted; d, c and b are the thickness, outer diameter and inner diameter of the sample, ω is the test angular frequency and µ0 is the permeability of vacuum.

3. Results & Discussion

Figure 1 shows the XRD patterns and SEM images of Ag/YIG composites. It can be seen that no impurity phase was observed in the sintered samples, except for silver (JCPDS card #04-0783) and yttrium iron garnet (JCPDS card #43-0507). In other words, the silver oxide was totally decomposed into metallic silver and there was no other reaction during the sintering process. Therefore, the Ag/YIG cermet composites were successfully fabricated. In the SEM

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images, the silver particles were randomly distributed in the matrix; with an increase in silver content, the isolated silver particles enhanced their interconnection and aggregated together to form clusters, eventually establishing a percolating network (in the red area of Figure 1e). In addition, partial island-like silver particles were still randomly distributed in YIG matrix (in the blue area of Figure 1e). Further increasing the silver loading level, the isolated silver particles were hardly observed and gradually formed networks (Figure 1f).

Figure 2 shows the frequency dispersion of the complex permittivity for Ag/YIG composites with different silver contents. The dielectric constant of the resultant composites was gradually enhanced with the increase of silver particles (shown in Figure 2a). Additionally, the real permittivity spectra presented frequency-independent behavior, which demonstrated that the frequency response from the matrix became dominant when the content of silver filler was relatively low.54-55

Further improving silver content, the spectra of the complex permittivity exhibited relaxation behavior; the dielectric constant markedly reached up to several hundreds, because of the charge accumulation at the interfaces between silver and YIG particles. The interfacial polarization, also known as the Maxwell-Wagner-Sillars effect,56 is responsible for the enhancement of dielectric constant. It is worth noting that the spectrum of imaginary part manifested a special shape, which can be divided into two parts (Figure 2b).

In the vicinity of percolation threshold, partial silver particles were gradually agglomerated

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and brought about the leakage current.57 Hence, the dielectric loss ( εr＂) was ascribed to the combined contributions of conduction and polarization,58 which could be described as equation (1),59

ε r′′=

ε −ε σ + s 2 ∝2 ωτ = ε c′′ + ε p′′ ωε 0 1 + ω τ

(1)

where ω is angular frequency, σ is dc conductivity, τ is relaxation time, ε0, εs and ε∝ is the vacuum, static and high-frequency dielectric constant, respectively. εc＂and εp＂represent the dissipation in the form of conductivity and polarization, respectively. The former refers to electrical leakage, which makes a great contribution in lower frequency. The latter means energy loss determined by polarizing dipoles under the action of ac electric field. When the alternative frequency of external field is too slow or too fast, the dielectric loss is so weak as to be neglected; the maximal dielectric loss appears at the critical frequency.27 Therefore, in the lower frequency region (the part I of Figure 2b), there was an apparently linear correlation between εr＂and frequency, which suggested conductive carriers played a primary role in dielectric loss.6 With the increase of frequency, the energy loss in the composites was gradually dominated by polarization rather than conduction, so the permittivity exhibited relaxation characteristic and the εr ＂ spectrum showed a loss peak at the critical frequency (the part II of Figure 2b). For heterogeneous composites with conductive fillers, the conductive carriers and polarizing dipoles make a combined effect on the energy loss especially near the percolation threshold.58

Further

improving the conductive silver content (sample AY40), there also exists leakage current due to the conductive silver clusters. Hence, the conduction electrons made a primary contribution to 9



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the dielectric loss in lower frequency, resulting in the rapid decrease of initial imaginary permittivity with increasing of the frequency.

When silver fraction exceeded the percolation threshold, the negative permittivity behavior was observed, which indicated that there existed a percolation phenomenon between samples AY25 and AY30. Compared with the negative permittivity behavior in previous investigations,60-62 interestingly, the real part of permittivity with a Fano-like resonance went across the zero-point (Figure 2c), where the dielectric constants of samples AY30 and AY40 switched from negative to positive at nearly 309 MHz and 40.3 MHz, respectively. Meanwhile, there was a huge loss peak obtained at the zero crossing point in Figure 2d. Similarly, the dielectric resonance with negative permittivity was also reported in Fe/Al2O3 composites.38, 63 Further, investigations demonstrate that the external factors including the sample thickness, size and sample-electrode contact area do not affect the Fano-like resonance behavior. Hence, it reveals that the resonance behavior stems from the intrinsic property rather than the dimensional resonance.63 In order to clarify the mechanism of negative permittivity behavior and Fano-like resonance, the impedance property of Ag/YIG composites was further investigated in the following sections.

The frequency dependence of reactance Z＂for Ag/YIG composites is presented in Figure 3. In the samples with lower silver content, the value of Z＂was negative throughout the test frequency region (Figure 3a), which means the voltage phase lags behind current phase. It was demonstrated that the composites exhibited capacitive character.27 With increasing the silver 10


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loading level, the absolute of reactance was reduced. When the resultant composites reached to percolative state, the reactance became positive and presented inductive character, which meant that there was a capacitive–inductive transition near percolation threshold. The similar phenomenon was also achieved in perovskite La1-xSrxMnO3 materials.64 In addition, with the increase of frequency, the reactance of samples with high silver content (AY30 and AY40) changed from positive to negative at nearly 309 MHz and 40.3 MHz respectively, which was corresponding to the Fano-like resonances frequency with a negative- positive permittivity transition (Figure 2c). Combined the variation trend of the real permittivity and reactance in Ag/YIG composites, it suggested that the reactance played an important role in the permittivity. It was indicated that the complex impedance and complex permittivity can be expressed as equation,65

ε* = 1

ε ′ = 2 fπC r

−i 2 fπC 0 Z *

 − Z ′′   2  ′ + Z ′′ 2  0 Z

(2)

(3)

where f is the test frequency, Z*, Z＇, Z＂is the complex, real and imaginary impedance and C0 is the capacitance of vacuum. It is shown that the imaginary impedance (i.e., reactance ) determines the real permittivity. Namely, when Z＂

Tunable Negative Permittivity with Fano-like Resonance and Magnetic

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