Carbon 125 (2017) 103e112 Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon Tunable and weakly negative permittivity in carbon/silicon nitride composites with different carbonizing temperatures ** Chuanbing Cheng a, c, d, Runhua Fan a, b, , Zhongyang Wang b, d, Qian Shao e, Xingkui Guo e, Peitao Xie b, d, Yansheng Yin a, Yuliang Zhang a, Liqiong An a, Yanhua Lei a, * Jong Eun Ryu f, Akash Shankar f, Zhanhu Guo c, a College of Ocean Science and Engineering, Shanghai Maritime University, Shanghai, 201306, PR China b Dezhou Meta Research Center of Innovative Materials, Dezhou, 253000, PR China c Integrated Composites Laboratory (ICL), Department of Chemical & Biomolecular Engineering, University of Tennessee, Knoxville, TN, 37996, USA d Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials(Ministry of Education), Shandong University, Jinan, 250061, PR China e College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao, 266590, PR China f Department of Mechanical Engineering, Indiana University-Purdue University Indianapolis, Indianapolis, IN, 46202, USA article info abstract Article history: Despite the exotic electromagnetic properties have been demonstrated in metamaterials to date, how to Received 3 July 2017 effectively adjust negative electromagnetic parameters remains a challenge. Tunable negative permit- Received in revised form tivity is essential for the metamaterials to satisfy a variety of practical applications, such as capacitor, 30 August 2017 microwave absorbing and shielding. Here, we fabricated a random metamaterial, carbon/silicon nitride Accepted 10 September 2017 (C/Si N ) composite, using a feasible impregnation-pyrolysis method. The microstructure and dielectric Available online 13 September 2017 3 4 property of the composites with different heat treatment temperatures (HTTs) and carbon contents were investigated. The amorphous carbon membrane adhered on the rod-like Si N grains. The negative Keywords: 3 4 Amorphous carbon permittivity behavior combined with inductive character was obtained in the composites, which was Metamaterial attributed to the low frequency plasmonic state generated from the formative conducting carbon net- Negative permittivity works. The magnitude of negative permittivity is demonstrated to be successfully adjusted by controlling Dielectric property the HTT and carbon content. The result is in good agreement with the analysis of Drude model. Inter- Silicon nitride estingly, a weakly negative permittivity behavior was observed in the measured frequency, showing small negative values of permittivity between À50 and À10, which was ascribed to a moderate carrier concentration provided by the carbon networks. This work provides an effective way to achieve the tunable and weakly negative permittivity in random metamaterials. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction precisely control negative parameters as well as to achieve the desired functional properties [12,13]. Until now, two technical Electromagnetic metamaterials with negative permittivity or/ routes have been developed to prepare metamaterials: ordered and permeability have been the subject of intense research due to metamaterials and random metamaterials [14]. Ordered meta- their unusual physical properties and exciting potential applica- materials are artificially structured media with periodic unit cells, tions, such as negative refraction [1,2], optical imaging [3,4], elec- in which the negative parameters can be tuned by changing the tromagnetic cloaks [5e7], novel antenna [8,9] and sensor [10,11].In design of the periodic structures, such as shape, size and geometric the past few decades, although fascinating achievements of the arrangement [15,16]. Whereas, the constructing process of the pe- metamaterials have been reported, great effort is still required to riodic structures is complex, and these metamaterials also show an anisotropic electromagnetic response, causing an adverse impact on the applications [17]. Random metamaterials without clear pe- riodic array were proposed to overcome these limitations, which * Corresponding author. can be prepared using typical processing of material with ease ** Corresponding author. College of Ocean Science and Engineering, Shanghai [14,18]. Their electromagnetic properties can be feasibly tuned by Maritime University, Shanghai, 201306, PR China. E-mail addresses: [email protected] (R. Fan), [email protected] (Z. Guo). tailoring the chemical compositions and microstructures, offering http://dx.doi.org/10.1016/j.carbon.2017.09.037 0008-6223/© 2017 Elsevier Ltd. All rights reserved. 104 C. Cheng et al. / Carbon 125 (2017) 103e112 an efficient way to construct metamaterials with more freedom used as starting materials. Firstly, the mixtures of Si3N4, 3 wt% Y2O3 [19]. and 6 wt% Yb2O3 powder were milled in ethanol for 48 h using the In the studying of random metamaterials, considerable atten- Si3N4 grinding media. After completely drying, the mixed powders tion has been paid to the development of metal composites to were sieved with 100-mesh sieve and then cold isostatic pressed at achieve tunable negative parameters. For instance, some metal 40 MPa. The green bodies were sintered at 1650 C for 2 h in ni- particles (such as, Ni, Fe, Ag, Cu or Co) were randomly dispersed in trogen atmosphere. Finally, the sintered ceramics were machined insulating matrixs, and the tunable negative parameters were into square discs (15 mm  15 mm  2 mm) and then ultrasonically demonstrated in the obtained composites by tailoring their cleaned in ethanol. composition and microstructure [14,20e24]. Negative permittivity For the preparation of C/Si3N4 composites, the obtained Si3N4 also was obtained in some metal alloy composites containing specimens were dipped into 1.5 g/ml or 1 g/ml sucrose aqueous Fe50Ni50, FeNiMo or Fe78Si9B13 particles [25e27]. Meanwhile, solutions and vacuumized for 12 h to make the sucrose solution several recent studies were devoted to realize the negative infiltrate into the Si3N4 discs. The specimens were then removed permittivity in carbon composites with different carbon nano- from the sucrose solution and dried in a ventilated oven at 80 C for structures, including graphene, carbon nanotube or nanofiber 12 h. After that, the as-received specimens were calcined at 500 C [18,19,28,29]. However, in those researches, there is little attention for 1 h under flowing nitrogen-gas. After repeating impregnation- given to the magnitude of negative parameters, and negative value calcination process, the specimens with carbon content of 4.9, of permittivity is generally enormous. In fact, a weakly negative 8.3, 10.1, and 12.5 wt% were prepared. Finally, the as-prepared permittivity behavior with small negative value of permittivity in samples were calcined at 500, 850, 1100, and 1300 C respec- broad frequency range is desired for many applications. For tively, for 1 h in the nitrogen atmosphere to get the C/Si3N4 com- example, the small negative magnitude of permittivity endowed a posites. For comparing and analyzing structural evolution, the carbon based metamaterial with good impedance matching and carbon derived from sucrose powders was fabricated under the improved absorption performance, benefiting its application in the same calcination process. microwave absorbing field [22,30]. High-permittivity was obtained in bilayer composites, which were constituted by the meta- materials with a suppressed negative permittivity, making them well apply in capacitors [31]. In addition, the weakly negative 2.2. Characterization permittivity behavior can facilitate the applications of meta- materials in solar energy harvesting [7,32], sensor [33] and an- X-ray photoelectron spectroscopies (XPS) for chemical and tennas [30]. compositional analysis were performed using an ESCALAB 250 Theoretical analysis indicates that the negative permittivity photoelectron spectrometer (Thermo Fisher Scientific, USA). The arises from the low frequency plasmonic state of free electrons in phase structures of C/Si3N4 composites and pyrolysis carbon the random metamaterials, and the magnitude of negative derived from sucrose carbonization were characterized by X-ray permittivity closely associated with their free carrier concentration diffractometer (XRD, XD2/3, Beijing Purkinje General Instrument [14,23]. It is widely believed the low or moderate free carrier Co., Ltd., Beijng, China) with Cu Ka radiation. The diffraction pat- density is more beneficial to offer the weakly negative permittivity terns were collected in the diffraction angle ranging from 10 to 80 behavior in a wider frequency band [30,34,35]. Compared with the at room temperature in the air. The Raman spectra were recorded metals and nanostructured carbon materials, amorphous carbon using a micro-Raman spectrometer (Jobin Yvon HR800, France) materials with a relatively low carrier concentration can be regar- with 532.05 nm incident radiation. The microstructure of com- ded as a good candidate for constructing metamaterials, to attain posites were observed using a field emission scanning electron the tunable and weakly negative permittivity behavior [36e38]. microscope (FESEM, Hitachi SU-70, Tokyo, Japan), equipping with Besides, the amorphous carbon is easily synthesized from pyrolysis energy dispersive X-ray spectroscopy (EDX). Pore size distribution of organic precursors, and its electrical and microstructural prop-