SiC Nanohybrid for Enhanced Microwave Absorption

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Functional Nanostructured Materials (including low-D carbon)

High-temperature Oxidation Resistant ZrN0.4B0.6SiC Nanohybrid for Enhanced Microwave Absorption Xian Jian, Wei Tian, Jinyao Li, Longjiang Deng, Zuowan Zhou, Li Zhang, Haipeng Lu, Liangjun Yin, and Nasir Mahmood ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22448 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019

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ACS Applied Materials & Interfaces

High-temperature Oxidation Resistant ZrN0.4B0.6SiC Nanohybrid for Enhanced Microwave Absorption Xian Jian†, ‡, *, Wei Tian†, Jinyao Li†, Longjiang Deng‡, Zuowan Zhou§, Li Zhang‡, Haipeng Lu‡, Liangjun Yin†, ‡, Nasir Mahmood‖, *

†School

of Materials and Energy, Center for Applied Chemistry, University of Electronic

Science and Technology of China, Chengdu, 611731, China ‡National

Engineering Research Center of Electromagnetic Radiation Control Materials, State

Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China §Key

Laboratory of Advanced Technologies of Materials, School of Materials Science and

Engineering, Southwest Jiaotong University, Chengdu 610031, China ‖School

of Engineering, RMIT University, 124 La Trobe Street, 3001 Melbourne, Victoria,

Australia

KEYWORDS: ZrN0.4B0.6, silicon carbide, catalytic chemical vapor deposition, chemical vapor infiltration, microwave absorption ABSTRACT: Most microwave absorbers lose their function under harsh working conditions, such as a high temperature and an oxidative environment. Here, we developed a 1 ACS Paragon Plus Environment

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heterogeneous ZrN0.4B0.6-SiC nanohybrid via combined catalytic chemical vapor deposition (CCVD) and chemical vapor infiltration (CVI) processes using ZrB2 as the starting material. The composition and structure of the ZrN0.4B0.6-SiC nanohybrid were controlled by tuning the CCVD and CVI parameters, such as reaction temperature, time, and reactant concentration. The optimal heterogeneous ZrN0.4B0.6-SiC nanohybrids were obtained initially by preparing ZrB2@C via the CCVD process at 650 °C for 30 min and the subsequent CVI at 1500 °C, where the ZrB2@C reacted with Si under N2. The ZrN0.4B0.6-SiC nanohybrid exhibited enhanced microwave absorption ability with a minimum reflection loss (RL) value of approximately -50.8 dB at 7.7 GHz, a thickness of ~3.05 mm, and anti-oxidation features at a high temperature of 600 °C. The heterogeneous ZrN0.4B0.6-SiC nanohybrid possessed reasonable conductivity leading to dielectric loss, while SiC nanofibers formed a 3D network that brought higher dipole moments, whereas a small part of the ZrN0.4B0.6-SiC nanohybrid structure generated an effective interface for higher attenuation of microwaves. Therefore, these material features synergistically resulted in a well-defined Debye relaxation, MaxwellWagner relaxation, dipole polarization and the quarter-wavelength cancellation, which accounted for the enhanced microwave absorption. 1. INTRODUCTION The increasing demand for electronic devices due to a modernized lifestyle has caused the serious issue of electromagnetic interference (EMI) pollution.1-5 Therefore, microwave absorption research has recently attracted intense attention to sustain life on Earth and keep our planet safe for future generations. For this purpose, a variety of absorbent materials have been invented and utilized, but the inherent limitations of the materials, such as improper combinations of electric and thermal conductivities, limit their applications. Moreover, traditional absorbent materials are also mainly focused on microwave absorption at room temperature.6-7 However, the extensive utilization of electronic devices and required 2 ACS Paragon Plus Environment

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performance in harsh circumstances, such as high temperatures, brings additional hurdles and requires additional specifications, such as well-controlled heterogeneous structures that provide multiple benefits. Traditionally used magnetic loss-type absorbers lose their magnetic properties over the Curie temperature, which limits their applications in a high temperature environment.8-9 However, dielectric loss-type absorbers absorb electromagnetic waves through the interaction with the electric field and have a high dielectric loss tangent.10-11 In particular, ceramic materials are potential candidates for dielectric loss-type absorbers applied in high temperature environments due to their advantages of high temperature and corrosion resistance, high strength and low thermal expansion coefficient.12-13 However, the oxidation resistance of dielectric loss-type absorbers is poor at high temperatures. Therefore, it is desirable to develop novel dielectric loss-type absorbers that can work at high temperatures for a long time and also have excellent oxidation resistance. It is well known that microwave absorption performance is mainly determined by two aspects. The first includes the intrinsic properties of matter, such as relative complex permeability and permittivity, which are associated with electromagnetic impedance matching.14 Generally, single absorbers fail to obtain good impedance matching, thus leading to undesirable microwave absorption performance. For example, Gao et al. prepared Ni nanowire-based absorbers that showed a minimum reflection loss (RL) of -8.5 dB at 10 GHz with a thickness of 3 mm.15 Similarly, Zhu et al. reported BaTiO3 nanotubes with a RL of 21.8 dB at 15 GHz, and the absorption bandwidth was approximately 1.7 GHz.16 Wu et al. synthesized SiC nanowires by the reaction between multiwall carbon nanotubes (MWCNTs) and silicon, indicating a RL of -17.4 dB at 11.2 GHz.17 Therefore, to address the challenge of poor impedance matching, designing nanohybrids with good impedance matching for better microwave absorption properties is highly desirable. The second aspect includes the microstructure and morphology, generally resulting in interfacial polarization, anisotropy and multiple scattering to promote an excellent microwave absorption property.18-19 Thus, the 3 ACS Paragon Plus Environment


particle size and morphology of the absorbents greatly influence the absorption performance in addition to the composition, specifically as the absorber size decreases from micro- to nanosize. Nanoabsorbers with high surface area have a large number of atoms on the surface with many dangling bonds, thereby increasing the particle activity and interfacial polarization with multiple scattering events and resulting in a high microwave absorbing performance.20-21 In addition, based on the transmission line theory, the microwave absorption performance of the absorber can be enhanced by multiple reflections or scattering inside the absorber. Recently, Zhao et al. found that core-shell structures Al2O3- and ZnO-coated carbon nanofibers with 3D networks possessed enhanced attenuation of microwaves due to numerous interfaces for multiple reflection/scattering.22 Liu et al. developed CoNi@SiO2@TiO2 and CoNi@Air@TiO2 core-shell nanostructures, and the unique design of the nanostructures had a significant influence on the microwave absorption property due to the well-matched impedance and multiple reflection or scattering at void spaces.23 A typical conducting ceramic material has a high melting point (~3245 °C), electrical (~107 S/m) and thermal conductivities (60 J·mol-1·K-1).24 However, ZrB2 can easily be oxidized at relatively higher temperatures, such as above 600 °C; therefore, an effective solution is needed to prevent its oxidation and keep it stable at high temperatures. Adding a surface coating as a protective cover from oxygen and doping of heteroatoms to strengthen the oxidation resistance were found to be effective ways to improve the efficiency of ZrB2. The ceramic ZrB2 modified with borosilicate glass showed good oxidation resistance due to the formation of an anti-oxidant layer on the surface of ZrB2.25-26 In another example, Bellosi and Monteverde reported that the oxidation resistance of ZrB2 increased to higher temperatures of 1350 °C ~ 1600 °C by adding 5 vol% Si3N4 to the ZrB2 ceramic matrix.27 On the other hand, ZrB2 possesses high conductivity (~107 S/m), which is too high for a good impedance mismatch and consequently leads to poor microwave absorption performance.28 Therefore, it is necessary to design absorbers with sufficient electrical conductivity to achieve enhanced 4 ACS Paragon Plus Environment

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microwave absorption performance. Thus, along with surface modification for oxidation resistance, tuning of the composition, microstructure and morphology of nanoabsorbers are also very effective ways to address the goal of a “wide effective bandwidth and strong microwave absorption at high temperature”.29-30 Herein, we prepared a unique heterogeneous ZrN0.4B0.6-SiC nanohybrid using catalytic chemical vapor deposition (CCVD) and chemical vapor infiltration (CVI) methods. The resulting heterogeneous ZrN0.4B0.6-SiC nanohybrid possessed enhanced high-temperature oxidation resistance up to 600 °C and enhanced microwave absorption properties due to the reasonable matching between the complex permittivity and permeability, the 3D network originating from the SiC nanofibers, the nanosize effect and a large number of interfaces. As a result, the nanohybrid showed enhanced microwave absorption performance with a minimum RL value of approximately -50.8 dB at 7.7 GHz for a 3.05 mm absorber thickness. Thus, we believe that tuning both the composition and structure to create a unique heterogeneous hybrid with a nanohybrid structure extended to a 3D network will be an effective approach to yield a unique microwave absorber for high-temperature applications. 2. EXPERIMENTAL SECTION Synthesis of carbon encapsulated ZrB2: Carbon encapsulated ZrB2 (ZrB2@C) was prepared by the CCVD method. Two grams of ZrB2 (~0.8 μm) nanoparticles were placed into the center of a quartz tube in a horizontal furnace heated to 600 °C under Ar at a flow rate of 50 mL/min. Then, C2H2 was introduced into the quartz tube for 30 min at 50 mL/min while the reaction temperature was maintained for 10 min. The samples were named ZrB2@C-60030 based on the reaction conditions (the reaction temperature was 600 °C, and the reaction time was 30 min). Different samples were prepared for a comparative study by tuning the reaction temperature and time, such as ZrB2@C-650-10, ZrB2@C-650-30 and ZrB2@C-65060. 5 ACS Paragon Plus Environment


Synthesis of ZrN0.4B0.6-SiC nanohybrid: One hundred milligrams of ZrB2@C-600-30 was mixed with 20 mg Si nanoparticles in a mortar and ground for over 30 min. Afterward, the well-mixed samples were moved into a pyrolytic boron nitride crucible, covered and heated at 1500 °C for 3 h with a N2 flow rate of 80 mL/min. The above as-prepared sample was denoted as ZS-1. The ZS-2, ZS-3 and ZS-4 samples correspond to the reactions of ZrB2@C650-10, ZrB2@C-650-30 and ZrB2@C-650-60 with Si nanoparticles and nitrogen, respectively. Characterization: The crystal structure of the nanohybrid was determined by powder X-ray diffraction (XRD). The morphology of the nanohybrids was determined using field-emission scanning electron microscopy (FESEM, JSM-7600F) and high-resolution transmission electron microscopy (HRTEM, Fei-F200 and FEI Tecnai G2 F20). The thermal stability was tested using an STA-8000 analyzer at a heating rate of 20 °C/min with a range of 25-1500 °C. In addition, the chemical composition and element valence state of the samples were determined by X-ray photoelectron spectroscopy. The carbon content of ZrB2@C-650-30 was measured by a high frequency infrared carbon sulfur analyzer (LECOCS230). The electrical conductivity was tested using a ST2722-SD four-point probe resistivity measurement system. The microwave absorption measurements of the composites were prepared by mixing paraffin wax with 80 wt% of the as-prepared ZrB2-based products. Then, the mixtures were pressed into coaxial rings (inner diameter: 3 mm, outer diameter: 7 mm, thickness: 2.0 mm). An N5234A vector network analyzer was applied to measure the complex permittivity and permeability of the composites in the frequency range of 2 GHz-18 GHz for analysis of the microwave absorption properties.

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Figure 1. Schematic presentation of the synthesis route of the ZrN0.4B0.6-SiC nanohybrid based on the CCVD and CVI methods.

3. RESULTS AND DISCUSSION The ZrN0.4B0.6-SiC nanohybrids were synthesized by CCVD and CVI techniques as presented schematically in Figure 1. Initially, carbon was coated on the surface of ZrB2 to obtain the core-shell structure of ZrB2@C with the CCVD method using C2H2 as the carbon source. Afterwards, to develop a heterogeneous ZrN0.4B0.6-SiC nanohybrid, the reaction among ZrB2@C, Si nanoparticles and N2 was took place at 1500 °C during the CVI process, resulting in the formation of the heterogeneous ZrN0.4B0.6-SiC nanohybrid composed of SiC nanofibers and ZrN0.4B0.6-SiC nanohybrid particles. The XRD patterns of the pure ZrB2 powder, ZrB2@C and ZrN0.4B0.6-SiC nanohybrids prepared under different reaction conditions were recorded to determine the structural features of the as-synthesized products. The diffraction peaks of pure ZrB2 were assigned to the single 7 ACS Paragon Plus Environment


phase of ZrB2 without the detection of any impurities (Figure 2). The XRD patterns of ZrB2@C indicate the main pure phase of ZrB2 and some weak peaks of ZrO2 attributed to the slight oxidation of ZrB2 during the CCVD process, as shown in Figure S1. In addition, the XRD peak for carbon was not detected due to the amorphous carbon on the surface of ZrB2. Furthermore, the carbon content for ZrB2@C-650-30 was approximately 13.42 wt%. However, the diffraction peaks of ZrB2 were replaced with ZrN0.4B0.6 and SiC in the nanohybrid that was formed during the CVI process at 1500 °C. The peaks present at 2θ of 33.5°, 38.8°, 56.1°, 66.9° and 70.2° were assigned to the ZrN0.4B0.6 (111), (200), (220), (311) and (222) lattice planes, respectively, according to JCPDS card No. 01-089-3791. The peaks located at 35.5°, 41.5°, 59.9° and 71.8° were attributed to the SiC (111), (200), (220) and (311) lattice planes, respectively, according to JCPDS card No. 00-001-1119. Therefore, XRD analysis confirmed that ZrB2@C completely reacted with the Si powders in the nitrogen atmosphere to form ZrN0.4B0.6 and SiC, which resulted in the depletion of ZrB2@C in the nanohybrid. Two main reactions occurred: carbon reacted with Si to generate SiC, and nitrogen reacted with ZrB2 to form ZrN0.4B0.6 at the high temperature of 1500 °C. Interestingly, the intensity ratio of the SiC (111) XRD peak at 35.5° to the ZrN0.4B0.6 (111) peak at 33.5° decreased with increasing reaction temperature, as seen for ZS-1 (at 600 °C) and ZS-3 (at 650 °C). Furthermore, in the case of 650 °C, the intensity ratio of the SiC (111) peak to ZrN0.4B0.6 (111) peak initially decreased with increasing reaction time from 0 to 30 min and then increased from 30 to 60 min (Table S1 of SI). It is suggested that the relative quantity change of ZrN0.4B0.6 and SiC in the nanohybrid was adjusted by tuning the CCVD and CVI parameters, such as reaction time and temperature.


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Figure 2. XRD patterns of pure ZrB2 and ZrN0.4B0.6-SiC nanohybrid corresponding to the reaction of ZrB2@C-600-30 (ZS-1), ZrB2@C-650-10 (ZS-2), ZrB2@C-650-30 (ZS-3), and ZrB2@C-650-60 (ZS-4) with Si nanoparticles and nitrogen at 1500 °C for 3 h. Furthermore, a clear change in morphology was observed after ZrB2 was converted to the ZrN0.4B0.6-SiC nanohybrid, as shown in Figure 3. The pure ZrB2 had an irregular particle shape with an average size of 1.05 µm; however, few very small size (180 nm) particles were observed (Figure 3a-b). On the conversion of ZrB2 to ZrN0.4B0.6-SiC nanohybrids through CCVD and CVI processes, hybrid particles coupled with the network of nanofibers were formed as marked by box and circles, respectively, Figure 3c-d. Such spherical particles with a fibrous network resulted in highly rich interfaces among the heterogeneous components of the ZrN0.4B0.6-SiC nanohybrid that was highly feasible for enhanced microwave absorption. The formation of spherical particles resulted in an overall reduction in the particle size to ~82 nm and average fiber diameter of 22 nm.



Figure 3. (a-b) SEM images of pure ZrB2 and (c-d) typical ZrN0.4B0.6-SiC nanohybrid of ZS-3 obtained from the reaction of ZrB2@C prepared at 650 °C for 30 min and Si powders under N2 at 1500 °C for 3 h. EDS analysis was carried out, and elemental maps were recorded to analyze the chemical composition and element distribution in the ZrN0.4B0.6-SiC nanohybrid, as shown in Figure S2. The EDS of the ZrN0.4B0.6-SiC nanohybrid (Figure S2a) confirmed the existence of Zr, B, C, Si, N and O species in the nanohybrid and demonstrated the high purity and fine composition of the as-prepared nanohybrid. It was clearly observed that Zr, C, N and Si elements were distributed evenly with a low content of O and B elements. The detected O element might be from surface adsorption, ZrO2 in the precursor of ZrB2@C or the testing environment. 10 ACS Paragon Plus Environment

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The morphology and structure of the ZS-3 nanohybrid were further investigated by TEM and HRTEM, respectively, as presented in Figure 4. Figure 4a-c confirmed the formation of a nanofiber with average diameter of 22 nm having a lattice spacing of 0.23 nm well-matched with the (200) plane of SiC and were in accordance with the XRD results. Further confirmed by the TEM elemental maps showing that the nanofibers are mainly composed of Si and C elements and the Zr, N and B elements are absent in fibers as shown in Figure S9. Figure 4d-i proved that the ZrN0.4B0.6-SiC nanohybrid existed in the samples, where ZrN0.4B0.6/SiC and ZrN0.4B0.6/C interfaces were clearly distinguished. The lattice spacing of the core was approximately 0.23 nm, corresponding to the ZrN0.4B0.6 (200) crystal plane, and the shell lattice spacing of 0.25 nm was assigned to SiC (111). To confirm the composition, the TEM elemental maps of nanoparticles for ZS-3 nanohybrids are recorded (Figure S10), suggesting that the Zr, N and B elements were detected in the particles regions along with C and Si, which indicating the nanoparticles were composed of ZrN0.4B0.6. In addition, a small amount of carbon coating on the core of ZrN0.4B0.6 remained from the precursor of ZrB2@C, as shown in Figure 4h. The presence of redundant carbon on the surface indicated that SiC was grown by the chemical reaction between carbon and Si nanoparticles at 1500 °C for 3 h, resulting in the formation of SiC on the surface of ZrN0.4B0.6 particles that generated ZrN0.4B0.6-SiC hybrid particles. In addition, ZrB2 reacted with N2 to produce a ZrN0.4B0.6 core at a sufficiently high temperature. Therefore, the formation mechanism might be explained as ZrB2 reacted with N2 to produce a ZrN0.4B0.6 core at a sufficiently high temperature, then the large ZrN0.4B0.6 particles split into small ZrN0.4B0.6 nanoparticles as a catalytic seed to induce the generation of finer SiC nanofibers during the CVI process, as shown in Figure 1.



Figure 4. HRTEM images of ZrN0.4B0.6-SiC nanohybrid of ZS-3: (a-c) SiC nanofibers, (d-f) ZrN0.4B0.6-SiC nanohybrid and (g-i) ZrN0.4B0.6-C nanohybrid. ZS-3 nanohybrids were obtained from the reaction of ZrB2@C at 650 °C for 30 min with Si nanoparticles and nitrogen at 1500 °C for 3 h. X-ray photoelectron spectroscopy (XPS) was used to further study the chemical composition and element valence state of different elements in the ZrN0.4B0.6-SiC nanohybrid. The XPS spectrum from the full scan showed the existence of core levels of B, C, Si, N, O and Zr elements, which confirmed the high purity of the as-prepared nanohybrid (Figure 5a). Furthermore, high-resolution XPS analyses were also applied to confirm the chemical state of each element in the nanohybrid. As shown in Figure 5b, the B 1s XPS spectrum had two 12 ACS Paragon Plus Environment

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peaks at 190.3 eV and 190.7 eV corresponding to sp2-bonded B-N and sp3-bonded B-N, respectively, which explained the presence of ZrN0.4B0.6 in the hybrid.31 The C 1s peak located at 284.9 eV is due to the C-C bond, demonstrating that there was a small amount of carbon in the hybrid. Moreover, there was also a peak centered at 283.2 eV, indicating a certain content of SiC on the surface of the hybrid (Figure 5c).32 The N 1s spectrum in Figure 5d was resolved into three peaks located at 397.7 eV, 398.2 eV and 398.9 eV, attributed to NSi,33 N-B,34 and N-Zr,35 respectively. The Si-N bond indicated a strong connection between the core and shell to create a good heterointerface. The existence of N-Zr and N-B bonds evident that the ZrB2 matrix was transformed into the ZrN0.4B0.6 substrate at high temperatures under N2. In addition, Si 2p peaks centered at 100.7 eV, 102.2 eV and 103.1 eV were assigned to Si-C, Si-N and Si-O bonds, respectively, as shown in Figure 5e.36 The high resolution Zr 3d spectrum was resolved into four peaks, where the strongest peak at 182.5 eV agreed well with the Zr-N bond, 35 while the other three peaks centered at 180.1 eV, 184.7 eV and 185.8 eV were attributed to Zr 3d5/2 (Zr-B), Zr 3d5/2 (Zr-O) and Zr 3d3/2 (Zr-O),37 respectively, as shown in Figure 5f. The presence of the Zr-O bond might be due to partial oxidation of ZrB2 grains during the CVI process at 1500 °C. The above analyses confirmed the formation of a unique heterogeneous structure of ZrN0.4B0.6-SiC with the morphology of nanofibers and nanohybrid particles for enhanced interfacial areas to make nanohybrids highly suitable for microwave absorbers.



Figure 5. (a) XPS spectrum full scan spectrum and (b) high resolution XPS spectra of B 1s, (c) C 1s, (d) N 1s, (e) Si 2p and (f) Zr 3d core level peak regions for the ZrN0.4B0.6-SiC nanohybrid ZS-3 obtained from the reaction of ZrB2@C prepared at 650 °C for 30 min with Si nanoparticles and nitrogen at 1500 °C for 3 h. Furthermore, thermogravimetric analysis (TGA) was carried out to verify the stability of the as-synthesized nanohybrid against oxidation at high temperature, Figure S3. The oxidization 14 ACS Paragon Plus Environment

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of ZrB2 began at 600 °C under air; an increase of 57 wt% below 900 °C might be attributed to the oxidization of ZrB2, while a weight decrease might be the result of volatilization of B2O3 up to 1200 °C. In the case of ZrN0.4B0.6-SiC nanohybrid, initially, the oxidization of ZrN0.4B0.6 occurred from 600-950 °C and then the oxidization of SiC from 950-1200 °C. A less weight gain of the ZrN0.4B0.6-SiC nanohybrid than that of pure ZrB2 clearly indicated higher resistance of SiC to oxidation that improved the stability of the ZrN0.4B0.6-SiC nanohybrid. To achieve better microwave absorption properties at high temperature, stability in terms of oxidation resistance, thermolysis and a phase-transition of the absorbers is necessary.38 Therefore, ZrN0.4B0.6-SiC nanohybrid with better oxidation resistance even at 600 °C might be a potential candidate for high-temperature microwave absorption. The microwave absorption property of the absorber is mainly determined by the electromagnetic parameters, including complex permittivity (εr = ε′- iε′′) and complex permeability (μr = μ′- iμ′′). Here, the real part of complex permittivity (ε′) and permeability (μ′) represent the storage capability of electric and magnetic energy, respectively. The imaginary parts of ε′′ and μ′′ reflect the loss capability of electric and magnetic energy, respectively.39 Therefore, the electromagnetic parameters of the ZrN0.4B0.6-SiC nanohybrid were investigated to explore the microwave absorption performance. It is well known that the ZrB2 and ZrN0.4B0.6-SiC nanohybrids are typical dielectric materials lacking magnetic constituents; therefore, the complex permeability is approximately 1 (μr=1), as shown in Figure S5. Figure 6 displayed the real and imaginary part of the permittivity (ε′ and ε′′) and dielectric loss tangent (tanδe) of samples/paraffin wax with 80% filler loadings in the frequency range of 218 GHz. Compared with the five samples, the real part of the permittivity mostly demonstrated a downward trend at 2-18 GHz in the sequence of ε′ (pure ZrB2) > ε′ (ZS-4) > ε′ (ZS-2) > ε′ (ZS-1) > ε′ (ZS-3). Notably, pure ZrB2 showed a high real part of the permittivity, where ε′ decreased in the frequency range of 2-14 GHz and increased gradually from 14 GHz 15 ACS Paragon Plus Environment


to 18 GHz, as shown in Figure 6a. The ε′ values of hybrids ZS-1, ZS-2, ZS-3 and ZS-4 declined with increasing frequency in the range of 2-18 GHz. Based on Weston’s theorem, 4041

when the value of the complex permittivity is equal to the complex permeability, the plane

electromagnetic wave enters into the interior of the absorber, and the zone back-scattered field is zero. For ZrN0.4B0.6-SiC nanohybrids, reducing the ε value approximately equal to μ favors the balance between εr and μr, leading to a decrease in the reflection coefficient of the absorbers. In addition, the variation law of the imaginary part (ε′′) of the permittivity of pure ZrB2 was similar to that of the ε′ values with the downward trend in Figure 6b. However, there were two peaks for the ε′′-f curves located at 8-12 GHz and 14-17 GHz for samples ZS1, ZS-2, ZS-3 and ZS-4. As shown in Table S2, compared to pure ZrB2 with a relatively high conductivity, the ZrN0.4B0.6-SiC nanohybrid had a decline in the ε′ and ε′′ values due to the decrease in the electrical conductivity.42-43 Furthermore, two resonance peaks existed in the frequency range of 8-12 GHz and 14-17 GHz in the ε′′-f curves for ZrN0.4B0.6-SiC nanohybrids because of the multiple interfaces due to existence of SiC nanofibers and ZrN0.4B0.6/SiC nanohybrid particle in the hybrid as shown in Figure 6b. By comparing the ε′′-f curves of pure ZrB2 and that of ZrN0.4B0.6-SiC nanohybrids, it was evident that the presence of hetero-interfaces was the main reason as no obvious peak was observed for pure ZrB2 (Figure 6). On the other hand, it was assumed that the ZrN0.4B0.6/SiC and ZrN0.4B0.6/C interfaces would produce a heterojunction capacitor, which contributed in the appearance of resonance peaks in the range of 8-12 GHz and 14-17 GHz. Similar, nonlinear resonant behavior and heterojunction capacitor contributions were also observed previously by Cao et al. for heterostructures.44-45 In addition, the microwave dissipation capabilities of the hybrid were unstable, which was ascribed to severe fluctuation of the values of ε′′.46 In general, the high value of the dielectric loss tangent means that the absorber has a high dielectric loss capability not considering the impedance matching. Figure 6c-d showed that the tanδe value of the ZrN0.4B0.6-SiC nanohybrid was smaller than that of pure ZrB2, indicating that pure ZrB2 16 ACS Paragon Plus Environment

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exhibited a relatively higher storage and loss capability for electromagnetic wave energy.

Figure 6. Electromagnetic parameters of five samples in the frequency range of 2-18 GHz: (a) real parts (ε′) and (b) imaginary parts (ε′) of the complex permittivity and (c, d) dielectric loss tangents. In practical applications, the microwave absorption performance is mainly determined by two factors. One factor is the dielectric and magnetic loss capabilities, and the other factor is the impedance-matching characteristic.47 Although a high permittivity represents a relatively high dielectric loss capability, a permittivity that is too high leads to the emission of incident microwaves and poor impedance-matching features. To understand the microwave absorption properties of the as-prepared samples, the RL values were calculated using the complex 17 ACS Paragon Plus Environment


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permittivity and permeability in the frequency range of 2-18 GHz on the basis of transmission line theory, as described in Eq. (1) and Eq. (2).48-49 Zin = Z0

μr

2πf εr tanh⁡(j c d

μr.εr)

(1)

Zin - Z0

(2)

RL = 20lg⁡|Zin + Z0|

where f is the microwave frequency, d is the thickness of the absorber, c is the velocity of light and Zin is the input impedance of the absorber. Figure 7 displayed the three-dimensional (3D) RL values of the absorber with samples and paraffin wax with a mass ratio of 8:2 in the frequency range of 2-18 GHz at a thickness of 1-5 mm. The attenuation peaks of the ZS hybrids moved to a lower frequency with increasing thickness, which was in agreement with the quarter wavelength cancellation model of d =

nc 4f |εr||μr|

(n = 1,2,3, …).50 The reflection loss

curves and λ/4 values of ZS-3 and ZS-4 nanohybrids were shown in Figure S8. The experimental results were in good agreement with those predicted using the quarterwavelength relationship model, indicating that the quarter-wavelength cancellation was one of the significant microwave absorption pathway. The RL values of pure ZrB2 hardly reached -5 dB within the thickness of less than 5 mm over 2-18 GHz, which means that the microwave absorption property of pure ZrB2 is not good for practical applications, as shown in Figure 7a. However, after the introduction of ZrN0.4B0.6 and SiC in this work, the ZS nanohybrid exhibited enhanced microwave absorption performance, possessing a wide effective bandwidth with RL values lower than -10 dB, as shown in Figure 7b-e. In the case of all reaction conditions during the CVD process, the as-prepared ZS nanohybrid also presented better microwave absorption performance because of the introduction of a rich interface and dipole polarization.50 Among them, the ZS nanohybrid ZS-3 with a filler loading of 80 wt% showed a relatively high microwave absorption capability. In detail, the minimal RL value of -50.8 dB was achieved at 7.7 GHz with an absorber thickness of 3.05 mm, and the effective microwave absorption bandwidth was 2.8 GHz from 6.5 GHz ~ 9.3 GHz. In addition, the ZS18 ACS Paragon Plus Environment

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4 nanohybrids can achieve the minimum reflection loss (RL) of -36.9 dB at 15.9 GHz with a thickness of only 1.24 mm and filler content of 80 wt.%. Moreover, its bandwidth over -10 dB was 3.7 GHz from 14.3 GHz to 18 GHz, as shown in Figure S6. Furthermore, we performed a comparative study with other materials reported in the literature, as shown in Table 1. It is inspiring that the ZrN0.4B0.6-SiC nanohybrid presented a much stronger EM absorption performance with a minimum RL value of approximately -50.8 dB at 7.7 GHz at a thickness of ~3.05 mm than other materials, making it the most suitable one for the mentioned application.



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Figure 7. 3D Microwave RL curves of samples calculated using the relative complex permeability and permittivity according to the transmission line theory in the frequency range of 2–18 GHz: (a) pure ZrB2 and ZrN0.4B0.6-SiC nanohybrid, (b) ZS-1, (c) ZS-2, (d) ZS-3, and (e) ZS-4.

Table 1. Performance comparison of current work with recently reported materials. Samples

RL

Thickness Weight fraction Frequency Bandwidth

Ref

(dB)

(mm)

(wt %)

(GHz)

≤ -10dB(GHz)

Ni nanowires

-8.5

3

65

10

--

15

BaTiO3 nanotube

-21.8

2

70

15

1.7

16

SiC nanowires

-17.4

3

30

11.2

2.5

17

1.8

10

16.2

5.3

22

(Al2O3+ZnO)/CNFs -58.5 Ti3SiC2

-16.4

1.6

15

11.1

2.7

51

C@C

-34.8

2

50

15

5.4

52

SiCnw-Cf

-21.5

2

30

7.7

2.45

53

NR-ZnO/PVDF

-25.44

3

10

16.48

--

54

Edge-rich graphene

-26.7

3.75

SiC Nanohybrid for Enhanced Microwave Absorption

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