Microwave-Absorbing Polymer-Derived Ceramics from Cobalt

Article pubs.acs.org/JPCC

Microwave-Absorbing Polymer-Derived Ceramics from CobaltCoordinated Poly(dimethylsilylene)diacetylenes Chunjia Luo,† Wenyan Duan,‡ Xiaowei Yin,‡ and Jie Kong*,† †

MOE Key Laboratory of Space Applied Physics and Chemistry, Shaanxi Key Laboratory of Macromolecular Science and Technology, School of Science and ‡Science and Technology on Thermostructure Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, 710072, People’s Republic of China S Supporting Information *

ABSTRACT: The microwave absorption materials in the X-band with high-temperature resistance are an important and urgent topic in functional materials. In this article, we present a useful and promising strategy to prepare cobalt-containing microwave absorption ceramics with high-temperature resistance. The polymer-derived ceramics were obtained via pyrolysis of carbon-rich poly(dimethylsilylene)diacetylenes coordinated with octacarbonyldicobalt. XRD and Raman spectra revealed that introduction of cobalt led to in situ formation of cobalt silicide nanocrystals and crystallized carbons (graphitic and tubular carbons), resulting in an enhanced microwave absorption property. The microwave absorption property and bandwidth could be tuned by controlling the cobalt content and annealing temperature of ceramics. When the average real permittivity, imaginary permittivity, and loss tangent of materials increased to 8.06, 3.10, and 0.39, the reflection coefficient (RC) value of as-synthesized ceramics was lower than −10 dB almost across the whole X-band (8.46−12.4 GHz). The minimum RC value of −42.43 dB at 10.55 GHz showed over 99.99% absorption of electromagnetic waves. The highperformance cobalt-containing microwave absorption ceramics possess promising potential in the field of electromagnetic interference protection.

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Co ferrite,14 are suitable to be used in this field. The metal oxide or alloy nanoparticles, such as carbonyl iron (Fe(CO)5) and Fe−Co−Ni alloy nanoparticles,15,16 are also employed to prepare microwave absorption composites in combination with the polymer matrix.17−19 The carbon materials including graphites, graphenes, carbon nanofibers, and nanotubes are the other major candidates due to their high specific surface area, low density, and high conductivity. Also, they were usually combined with the polymer matrix to prepare microwave absorption composites.1,20−25 Recently, the carbon materials are combined with ferrites to prepare advanced microwave absorption composites.26−28 However, since the magnetic materials may lose their magnetism and the carbon materials may be oxidized, new high-temperature-resistant microwave absorption materials are urgently required. The advanced structural ceramics, such as silicon carbide (SiC) and silicon carbonitride (SiCN), show great potential as microwave absorption materials especially in the extreme environment because they possess high strength, low expansion coefficient, high melting temperature, and good oxidation resistance.9,29 A polymer-derived ceramics (PDCs) route

INTRODUCTION Electromagnetic interference has become a serious pollution problem due to extensive utilization of electronic devices and communication facilities in commerce and military affairs, which is harmful to either devices or human health.1−3 The microwave-absorbing materials efficiently absorb or dissipate electromagnetic waves and convert electromagnetic energy into thermal energy and consequently protect workspace and environment from the radiation emitted by telecommunication apparatus.4−6 On the other hand, microwave absorption materials can be used for stealth of aircraft against radar surveillance.7,8 Nowadays, desirable microwave absorption materials should have strong absorption properties, broad effective absorption bandwidth, low density, excellent thermal stability, and antioxidant properties. X-band (8.2−12.4 GHz) is one of the bandwidths widely used for microwave-absorbing materials. Doppler, weather radar, TV picture transmission, and telephone microwave relay systems all lie in the X-band range.9,10 Thus, preparation of microwave absorption materials in the X-band with desirable properties is an important and essential topic. Ferrites, metal oxides, and alloy nanoparticles are essential magnetic microwave absorption materials.11 The hexagonal ferrites with high saturation magnetization and good chemical stability, such as Ni−Zn ferrite,12 Mn−Zn ferrite,13 and Nd− © 2016 American Chemical Society

Received: April 20, 2016 Revised: June 29, 2016 Published: August 9, 2016 18721

DOI: 10.1021/acs.jpcc.6b03995 J. Phys. Chem. C 2016, 120, 18721−18732

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The Journal of Physical Chemistry C Table 1. Composition of Co-Containing Ceramics Derived from Polymeric Precursors stom composition (%)a sample

precursor

Co2(CO)8 (mol %)

ceramics

P1 P2 P3

PDSDA PDSDA PDSDA PDSDA PDSDA PDSDA PDSDA PDSDA

0 2.5 5 5 5 5 7.5 10

C1 C2 C3 C4 C5 C6 C7 C8

P4 P5

temperature 1100 1100 1000 1050 1100 1200 1100 1100

°C °C °C °C °C °C °C °C

C

Si

Co

Ob

Si−C−Co formula

79.81 74.51 66.20 74.33 77.87 79.56 66.65 75.50

10.68 11.11 9.77 9.77 9.58 9.12 8.56 7.79

0 1.24 2.22 2.77 2.25 2.28 3.84 4.98

9.51 13.14 21.81 13.13 10.30 9.04 20.95 11.73

Si1C7.47 Si1C6.71Co0.11 Si1C6.78Co0.23 Si1C7.61Co0.28 Si1C8.13Co0.23 Si1C8.72Co0.25 Si1C7.78Co0.45 Si1C9.69Co0.64

a Atomic composition was determined using SEM-EDS. bThe introduction of oxygen is mainly due to the absorption of air in ceramic powders and oxygen-induced during pyrolysis before SEM-EDS measurement.

flask equipped with a Teflon stir bar and high-vacuum stopcock was charged with THF (60 mL) and n-BuLi (42.51 g, 0.14 mol) and cooled to −78 °C in an acetone/ice bath. The hexachloro1,3-butadiene (9.456 g, 0.035 mol) was added through an argon-purged syringe. After stirring about 12 h at room temperature, dichlorodimethylsilane (4.578 g, 0.035 mol) was dripped into the flask, which had been cooled down to −78 °C in an acetone/ice bath, to initiate the formation of PDSDA. After keeping the mixture overnight, excess chlorotrimethylsilane (2 mL) was added as a capping agent. Then the mixture was dissolved in a sufficient amount of toluene, and the lithium chloride was filtered out. After evaporation of the solvent, the polymer was precipitated from a concentrated solution of toluene into methanol, filtered out, and dried. Synthesis of Cobalt-Coordinated PDSDA (Co− PDSDA). The Co−PDSDA was synthesized according to refs 55 and 56. PDSDA and Co2(CO)8 were mixed under an argon atmosphere using a Schlenk tube. As shown in Table 1, Co2(CO)8, which was 2.5, 5, 7.5, and 10 mol % of the amount of alkynyl groups in PDSDA according to its structural unit weight, was dissolved in anhydrous THF and added dropwise into PDSDA solution. After the mixture sat overnight, most of the solvent was evaporated, and the concentrated solution was added to acetonitrile under stirring. The precipitate of Co− PDSDA was washed three times with acetonitrile and dried under vacuum. Preparation of Polymer-Derived Ceramics. The Co− PDSDA was transferred into the tube furnace (GSL-1700X, Kejing New Mater, Ltd., Hefei, China) for pyrolysis under an argon atmosphere. The cross-linking was performed at 400 °C (heating rate, 5 K/min; holding time, 2 h), followed by pyrolysis process up to the pyrolyzed temperature (heating rate, 5 K/min; holding time, 4 h). As shown in Table 1, the ceramics were designated as C1−C8 according to the pyrolyzed and annealing temperatures and Co2(CO)8 contents for convenience. Characterization. Nuclear magnetic resonance (NMR) measurement was carried out on a Bruker Avance 400 NMR spectrometer (Bruker BioSpin, Switzerland) to collect the 1H and 13C spectra. Fourier transform infrared spectroscopy (FTIR) measurement was carried out on a FT-IR spectrophotometer (PerkinElmer, USA). Raman spectroscopy studies were performed using a Raman Microprobe Instrument (Invia, Renishaw, USA) with 514.5 nm Ar+ laser excitation. Size exclusion chromatography (SEC) measurement was performed on a system equipped with a Waters 515 pump, an autosampler, and two MZ gel columns (103 and 104 Å) with a flow rate of 0.5 mL/min in THF (HPLC grade) at 25 °C. The

exhibited remarkable advantages to fabricate the functional ceramics with diverse shapes compared with conventional hot isostatic pressing,30 spark plasma sintering,31 liquid-phase sintering,32 or chemical vapor deposition.33 It was widely used in fibers,34−36 highly porous components,37−39 microelectromechanical systems,40,41 and Li-ion battery anodes.42−44 Owing to the advantages of PDCs route, transition metals, such as iron, cobalt, and nickel, can be conveniently introduced into preceramic precursors at an atom level, subsequently to control the element composition and crystalline behaviors of pyrolyzed ceramics.45−50 Therefore, the PDCs could be promising microwave absorption ceramics with high-temperature resistance. Recently, the electromagnetic absorption properties of PDCs-Si3N4/SiC,51 PDCs-SiOC,52 and PDCs-SiC53 ceramics have been thoroughly investigated. In particular, if the polymeric precursors are carbon rich, the transition-metal atoms will induce in situ generation of graphite carbons, carbon nanowires, and carbon nanotubes uniformly distributed in the ceramic matrix. Considering microwave absorption capability and impedance-matching characteristics, the transition-metalinduced hierarchical structures of carbon-rich ceramics are helpful to improve the microwave absorption property. In this article, we report cobalt-containing microwave absorption ceramic materials in situ pyrolyzed from a carbonrich poly(dimethylsilylene)diacetylene with alkynyls on backbone coordinated with octacarbonyldicobalt (Co2(CO)8). By controlling their hierarchical structures, the high-temperatureresistant ceramics exhibited an excellent electromagnetic microwave absorption property with a low reflection coefficient in almost the whole X-band. The in situ pyrolysis of cobaltcoordinated poly(dimethylsilylene)diacetylene precursor gives a convenient strategy to obtain ceramics with enhanced electromagnetic microwave absorption properties and hightemperature resistance.

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EXPERIMENTAL SECTION Materials. n-BuLi (2.2 mol), hexachloro-1,3-butadiene (97%), dichlorodimethylsilane (98%), and trichloroethylene (>98%) were purchased from Alfa Aesar China (Tianjin, China). Co2(CO)8 (stabilized with 1−5% hexane) was purchased from TCI (Shanghai, China). Anhydrous tetrahydrofuran (THF) was freshly distilled for use under reflux by using sodium/benzophenone. All other reagents were analytical grade and used as received. Synthesis of Poly(dimethylsilylene)diacetylenes (PDSDA). The PDSDA was synthesized by ourselves according to ref 54. The reactions were carried out using standard Schlenk techniques. Under an argon atmosphere, a 250 mL flame-dried 18722


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The Journal of Physical Chemistry C

Scheme 1. Synthetic Route for PDSDA and Co−PDSDA from n-BuLi, Hexachloro-1,3-butadiene, Dichlorodimethylsilane, and Octacarbonyldicobalt

mm was measured by a vector network analyzer (VNA, MS4644A, Anritsu, Atsugi, Japan) using the waveguide method in the X-band. On the basis of the metal backplane, the reflection coefficient (RC) can be calculated using the measured relative complex permeability and permittivity according to the following equations5

detectors included a differential refractometer (Optilab rEX, Wyatt) and a multiangle light-scattering detector (MALS, Wyatt) equipped with a 632.8 nm He−Ne laser (DAWN EOS, Wyatt). The refractive index increments of the polymers in THF were measured at 25 °C using an Optilab rEX differential refractometer. The carbon, hydrogen, and oxygen contents of polymers were determined on a Cario Elementar (Vario EL III, Germany). The cobalt and silicon content was determined using inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700, USA). Thermogravimetric analysis (TGA) and mass spectrometry (MS) analysis were performed on a simultaneous thermal device (STA, 449C Jupiter, Netzsch, Gerätebau GmbH, Selb, Germany) coupled with a quadrupole mass spectrometer (QMS, 403C Aëolos, Netzsch, Germany). The measurement was done under a steady flow of argon (40 mL/min) with a heating rate of 10 K/min at a range from ambient temperature to 1400 °C. Powder X-ray diffraction (XRD) measurement was conducted on the X’Pert Pro powder diffractometer from PANalytical (Cu Kα radiation, 40 kV, 40 mA) (D/ Max2550VB+/PC). The X’Celerator Scientific RTMS detection unit was used for detection. Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses were performed using a IE250X-Max50 (Oxford Instruments) energy-dispersive spectrometer equipped with a field emission scanning electron microscope (FESEM, JSM-6700F, JEOL, Japan). The ceramic powders were measured without sputtering a thin layer of gold. Transmission electron microscopy (TEM, FEI Tecnai G2 F30) was operated at 200 kV, coupled with electron diffraction analysis. A 5 μL amount of a droplet of an ultrasonically dispersed mixture of milled sample and alcohol (0.02 mg/mL) was dropped onto a copper grid (200 mesh) coated with carbon film and dried at ambient temperature for 30 min. Magnetism Measurement. The ceramic samples were studied using a magnetic property measurement system (MPMS, Quantum Design Inc., San Diego, USA) utilizing a superconducting quantum interference device (SQUID). Magnetic behavior was studied with a constant applied magnetic field. Furthermore, the magnetization as a function of applied field (from −20 to 20 kOe) was recorded at 25 °C. Microwave Absorption Measurement. The samples were prepared by uniformly mixing the 33 wt % ceramics powders with paraffin. The relative complex permittivity (ε = ε′ − jε″) of the samples with dimensions of 22.86 × 10.16 × 2.65

Z in − 1 Z in + 1

(1)

⎡ 2πfd ⎤ με tanh⎢j r⎥ r ⎣ c ⎦ εr

(2)

RC = 20 log10

Zin =

μr

where Zin, μr, and εr are the normalized input impedance, permittivity, and permeability of the materials, respectively, and f, d, and c represent the microwave frequency, thickness (m), and velocity of the electromagnetic wave in vacuum, respectively. Direct-current electrical conductivities of the samples were measured through a four-point probe technique setup (ET9000, Eastchanging, China), which mainly consisted of a high impedance current source (6220, Keithley, USA) and a high-impedance voltmeter (2182A, Keithley, USA).

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RESULTS AND DISCUSSION Synthesis of Cobalt-Coordinated Polydiacetylene(dimethyl)silanes. The synthetic routes of poly(dimethylsilylene)diacetylenes (PDSDA) and cobalt-coordinated PDSDA (Co−PDSDA) are illustrated in Scheme 1. The Co−PDSDA was prepared through the coordination reaction between alkynyl groups of PDSDA on backbone and Co2(CO)8. The 1H and 13C NMR spectra in Figure 1 clearly show the protons on methylsilyl groups and carbons on backbone alkynyl groups of PDSDA. The SEC measurement gives the number-average molecular weight of 3280 Da with a polydispersity index of 1.73. Unfortunately, the magnetic Co− PDSDA cannot be characterized by NMR. Thus, FT-IR spectroscopy was employed to identify the Co−PDSDA. As shown in Figure 2, the stretching vibration of alkynyl groups was observed at 2080 cm−1. The height decreased with the increasing addition content of Co2(CO)8 and completely disappeared when the addition was 10 mol %. Also, the three absorption bands appearing at 2035, 2065, and 2097 cm−1 became stronger with increasing addition content. The three peaks were typical absorption peaks of [−C−C−Co2(CO)6],45 which confirmed that Co2(CO)8 had reacted with alkynyl groups by coordination. The other two absorption peaks at 1606 and 1410 cm−1 were attributed to the cyclotrimerization 18723


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Figure 2. FTIR spectra of PDSDA and Co−PDSDA with different content of Co2(CO)8: (a) 0, (b) 2.5, (c) 5, (d) 7.5, and (e) 10 mol %.

confirmed the coordination between Co2(CO)8 and the alkynyl groups of PDSDA. It should be pointed out that the PDSDA and Co−PDSDA are soluble in aliphatic and aromatic solvents, which are not sensitive to air and moisture and are convenient for the forming of green bodies to prepare pyrolyzed ceramics in the future. Pyrolysis Behavior of PDSDA and Co−PDSDA. The pyrolysis behavior of the preceramic polymers was analyzed by simultaneous thermogravimetric analysis coupled with mass spectrometry in an argon atmosphere. From the TGA curves in Figure S1 (Supporting Information), the pure PDSDA underwent a thermolytic degradation in the temperature range from 500 to 800 °C, after which the TGA curves almost leveled off. The ceramic yield was 88% at 1100 °C. After the alkynyl groups of PDSDA were coordinated with Co2(CO)8, the first degradation before 400 °C was [−C−C−Co2(CO)6] groups, where they were decomposed to form cobalt nanoparticles. The pyrolysis occurred in the temperature range from 400 to 800 °C. Because there were still a lot of alkynyl groups on the backbone of Co−PDSDA, the ceramic yield pyrolyzed at 1100 °C with different contents of [Co2(CO)6] on the backbone maintained at 84%, 82%, 71%, and 51% for P2, P3, P4, and P5, respectively. The simultaneous TGA and mass spectrometry results are shown in Figure 3. The thermolysis of PDSDA (Figure 3a) was mainly accompanied by the decomposition products of H2 (m/ z = 2), hydrocarbons of CHx+ (x = 0−3, m/z = 12−15), CH4 (m/z = 16), and C2Hx+ (x = 1, 2, 5, and 6, m/z = 25, 26, 29, and 30) as well as other oligomer fragments (m/z = 45). The evolution of H2, CH4, and hydrocarbons in the range from 400 to 900 °C was ascribed to decomposition of Si−CH3 groups and cleavage of C−C bonds. In addition to the oligomer fragments mentioned above, another decomposition product of CO2 (m/z = 44) was observed during the thermolysis of Co− PDSDA (Figure 3c) before 400 °C because of the introduction of oxygen. Also, the temperature of the evolution of oligomer fragments decreased with the increase of the [Co2(CO)6] content in Co−PDSDA. Ceramics Pyrolyzed from Co−PDSDA Precursors. As shown in Table 1, after pyrolysis of Co−PDSDA (P1−P5) under an argon atmosphere at different temperatures, the bulk

Figure 1. 1H NMR spectrum (a) and 13C NMR spectrum (b) of PDSDA dissolved in CDCl3; SEC elution curve (c) of PDSDA at a flow rate of 0.5 mL/min in THF at 25 °C.

of alkynes.57 As is well known, a kind of two-coordinate FeI complex can catalyze the formation of alkyne trimerization.58 In this work, the introduction of [−C−C−Co2(CO)6] also has the effect similar to two-coordinate FeI complex. The elemental composition of polymers of P1 (PDSDA) and P3 (Co− PDSDA) is shown in Table S1 (Supporting Information). There is only a little oxygen in P1 from oxygen or moisture absorption before measurement. In comparison to P1, the oxygen and cobalt content of P3 was increased from 0.59 to 8.42 wt % and from 0 to 6.34 wt %, respectively. It further 18724


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existence of oxygen was also from O2 or moisture absorption of the milled ceramic powders before measurement of SEM-EDX. The powder XRD analysis of C1−C8 of ceramics pyrolyzed Co−PDSDA precursors was further conducted to elucidate the fine-phase structure of ceramics. Figure 4a shows the powder

Figure 4. Powder XRD patterns of Co-containing ceramics (a) pyrolyzed from Co−PDSDA containing 5 mol % contents of Co2(CO)8 at different annealing temperatures (C3, 1000 °C; C4, 1050 °C; C5, 1100 °C; C6, 1200 °C) and (b) annealed at 1100 °C with different contents of cobalts (C1, 0%; C2, 1.24%; C5, 2.25%; C7, 3.84%; C8, 4.98%).

Figure 3. TGA−mass curves of (a) P1, (b) P2, and (c) P4 measured at a scanning rate of 10 K/min under an argon atmosphere.

ceramic products were obtained and labeled as C1−C8. SEMEDS was used to analyze the chemical composition of the Cocontaining ceramics. The atom composition and possible ceramic formula of ceramics were an average value (Table 1). From the SEM-EDS (Table 1 and Figure S2) analyses, the contents of cobalt in the ceramics were increased with the increase of [Co2(CO)6] content in the precursors (Co− PDSDA). Also, the contents of carbons in the ceramics proved that the Co-containing PDC ceramics were a carbon-rich system. Although most of the oxygen introduced by [Co2(CO)6] was volatilized as H2O and CO2 under high temperature during the pyrolysis process as illustrated in Figure 3, there was still a little oxygen in the ceramics. In addition, the

XRD patterns of ceramics pyrolyzed at different temperatures from the Co−PDSDA precursor with 5 mol % content of [Co2(CO)6]. The ceramic pyrolyzed at 1000 °C is only composed of cobalt crystals and a few graphite carbons. The three peaks at 2θ = 44.2°, 51.6°, and 75.8° are indexed as the crystal face of the Co crystal (111), (200), and (220) (JCPDS 15-0806). When annealed at 1100 or 1200 °C, cobalt crystals transformed into cobalt silicide (CoSi) crystals and graphite carbons and silicon carbide (SiC) crystals were also generated. The peak at 2θ = 26.1° was indexed as the crystal face of graphite carbons (002) (JCPDS 41-1487). The peaks at 2θ = 28.3°, 34.9°, 40.5°, 45.6°, and 50.2° were indexed as the crystal 18725


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The Journal of Physical Chemistry C face of the CoSi crystal (110), (111), (200), (210), and (211) (JCPDS 50-1337), respectively. With the increased annealing temperature, the crystal peaks became stronger in intensity, indicating a higher degree of crystallization. The crystallization behavior of Co-containing ceramics pyrolyzed at 1100 °C from Co−PDSDA precursors with different content of [Co2(CO)6] is shown in Figure 4b. For the precursor with no cobalt (P1), the derived ceramic C1 showed an amorphous-like structure with only a few SiC crystals and amorphous carbon structures. For C2 and C5, which was pyrolyzed from the precursor with a low content of [Co2(CO)6] (P2 and P3), respectively, the intensity of SiC crystals peak was enhanced. At the same time, a new kind of crystal CoSi and graphite carbons were generated. For C7 and C8 pyrolyzed from precursors with a high content of [Co2(CO)6] (P4 and P5), the crystals of CoSi, Co2Si, and graphite carbons were all generated and the SiC crystal peaks disappeared. The peaks at 2θ = 32.7°, 39.52°, 42.4°, 44.1°, 44.8°, 45.3°, 46.0°, 48.7°, 53.7°, 66.6°, 68.4°,70.2°, and 75.6° were well indexed as the crystal face of the Co2Si crystal (111), (211), (310), (021), (220), (301), (121), (002), (320), (420), (312), (222), and (322) (JCPDS 040847), respectively. The XRD results indicate that introduction of cobalt atoms promotes the formation of SiC, CoSi, and Co2Si crystals and graphite carbons. As previously reported,59 the transition metals, such as cobalt, iron, and nickel, can reduce the crystal temperature of a silicon system through forming a eutectic liquid with silicon. In this system, the cobalt atoms in [Co2(CO)6] react with Si−C phase and form a eutectic liquid. When the liquid phase is saturated, cobalt silicide nuclei will be generated and solidified, which promotes the formation of SiC crystals. With the increased content of cobalt, amorphous carbons start regular arrangement. Lots of graphite carbons, carbon turbostratic structure, and carbon nanowires were in situ generated, which could be proven in the TEM images as described later. Here, some CoSi crystals convert into Co2Si as shown in Figure 4b, and the SiC crystals become less and less. According to XRD results,32,60 the dominant crystalline phases of cobalt in ceramics are CoSi and Co2Si. The formation of cobalt silicide can be described as follows

Figure 5. Raman spectra of Co-containing ceramics (a) pyrolyzed from Co−PDSDA containing 5 mol % contents of Co2(CO)8 at different annealing (C3, 1000 °C; C4, 1050 °C; C5, 1100 °C; C6, 1200 °C) and (b) annealed at 1100 °C with different contents of cobalts (C1, 0%; C2, 1.24%; C5, 2.25%; C7, 3.84%; C8, 4.98%).

2Co + SiC → Co2Si + C

carbons to nanocrystalline graphites in the pyrolyzed Cocontaining ceramics. TGA analysis from ambient temperature to 1400 °C was conducted to study the effect of Co on the thermal stability of ceramics. As shown in Figure 6, the Co ceramics barely show a mass loss (less than 5%) before 1100 °C. From 1100 °C, the Co ceramics start to be degraded under the argon atmosphere. Further, their thermal stability in air was analyzed by using a tube furnace. After 2 h oxygenation at 800 °C, the ceramics weight was 98.2% and 70.3% for C1 and C7, respectively. Thus, these ceramics show good thermal stability below 1100 and 800 °C under argon and air atmosphere, respectively. The hysteresis loop of the ceramics (C1, C2, and C7) monitored by a SQUID is presented in Figure S3. The development tendency of the saturation magnetization and coercivity depended on the content of CoSi and Co2Si in ceramic. In a magnetic field at a temperature of 300 K, the saturation magnetization of ceramic C7 was determined to be 0.046 emu/g. From the enlarged magnetization curves in the low-field region, an ultralow hysteresis loop can be identified with a remanence of 0.00014 emu/g and a coercivity of 11.33 Oe.

Co2Si + SiC → 2CoSi + C Co + SiC → CoSi + C

Moreover, the two typical cobalt silicons of Co2Si and CoSi are known as promising intermetallic compounds exhibiting low electrical resistivity, chemical and thermal stabilities, and excellent oxidation resistance.61,62 The nanocrystals uniformly distributed in the ceramic matrix are beneficial to give a contribution to enhance the microwave absorption property. Raman spectroscopy was employed to demonstrate the fine structure of carbons in carbon-rich ceramics. As shown in Figure 5, all spectra exhibited the most prominent features of the disorder-induced D band at 1350 cm−1 and the G band at 1600 cm−1 caused by in-plane bond stretching of sp2 carbons. All Raman curves had been fitted by the Gaussian−Lorentzian curve to determine the widths, positions, and ID/IG intensity ratios.63−67 With the increase of annealing temperature and cobalt content, the ID/IG ratio was apparently decreased. It indicated that the annealing temperature and the introduction of cobalt both promoted the conversion from amorphous 18726


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Figure 6. TGA curves of Co-containing ceramics (Co content: C1, 0%; C2, 1.24%; C5, 2.25%; C7, 3.84%; C8, 4.98%) in an argon atmosphere.

The direct-current electrical conductivity (σdc) of materials is shown in Figure S4. It is clear that the electrical conductivities are increased with increasing annealing temperature and cobalt contents. The σdc increased from 1.31 × 10−9 to 2.74 × 10−6 S/ cm and from 8.39 × 10−9 to 3.40 × 10−6 S/cm by increasing annealing temperature and cobalt contents. It is mainly attributed to the nanocrystals of SiC, CoSi, and Co2Si and crystallized carbons (graphitic and tubular carbons) formed at high temperature and cobalt contents. These nanocrystals and crystallized carbons connect with each other to form current flowing paths, reducing the barriers for the current flowing and leading to the increase of σdc.51,52

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MICROWAVE ABSORPTION PROPERTIES AND DIELECTRIC PROPERTIES OF CO-CERAMICS The relative complex permittivity and dielectric loss tangent (tan δ = ε′/ε″) are two main parameters for characterization of the dielectric properties and prediction and design of the microwave absorption properties. The real part (ε′) of the permittivity represents the polarization, and the imaginary part (ε″) is related to the dielectric loss ability. The dielectric loss tangent can forecast the electromagnetic wave-absorbing property of materials. Normally, the higher dielectric loss is helpful to the electromagnetic wave absorbing property.5 Figure 7 shows the permittivity of ceramics (C3−C6) with 5 mol % content of [Co2(CO)6] pyrolyzed at different temperatures. The real permittivity of ceramic (Figure 7a) was increased with increasing annealing temperature, which displays the same trend for the imaginary permittivity (Figure 7b) and the dielectric loss tangents (Figure 7c). With the increased temperature from 1100 to 1200 °C, the real permittivity of various ceramic is in the range of 5.35−5.23, 6.50−6.30, 8.40− 7.70, and 11.41−10.17 as well as the imaginary permittivity in the range of 0.61−0.52, 0.87−0.73, 2.84−1.81, and 4.90−4.16, respectively. The increase of the real and imaginary permittivity is attributed to formation of CoSi, SiC, and crystallized carbon. As mentioned in the results of XRD and the Raman spectrum, some small nanocrystals were embodied in the ceramic C3. When the annealing temperature increased, the CoSi and SiC crystals and graphitic carbons were found and the degree of crystallinity was increased. Dielectric loss of ceramics mainly

Figure 7. Real permittivity (a), imaginary permittivity (b), and loss tangent (c) of the Co-containing ceramics pyrolyzed from Co− PDSDA containing 5 mol % contents of Co2(CO)8 at different annealing temperatures (C3, 1000 °C; C4, 1050 °C; C5, 1100 °C; C6, 1200 °C) with a thickness of 2.65 mm.

depended on dipolar reorientation processes and interfacial polarization relaxation effect. The nanocrystals can induce the formation of nanograin boundaries between crystal and amorphous phase. According to the Maxwell−Wagner effect, 18727


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reflective coefficient of −24.33 dB at 9.8 GHz with an effective absorption bandwidth from 8.8 to 11.53 GHz. When ceramic was annealed at 1200 °C, the RC minimum value was −20.9 dB and the effective absorption bandwidth range was from 8.2 to 10.2 GHz. In principle, too high real permittivity is harmful to the impedance match and results in strong reflection and weak absorption. Therefore, the real and imaginary permittivity of C5 annealed at 1100 °C was close to the optimal value and exhibited better microwave absorption property. Therefore, we focused on the annealing temperature of 1100 °C to study the effect of Co content on the dielectric properties and microwave absorption properties of the ceramics (Figure 9). In Figure 9a and 9b, the real part and the imaginary part of permittivity increased with the increase of cobalt content. With the increase of cobalt content, the values of ε′ are in the range of 5.78−5.75, 7.67−7.30, 8.40−7.70, 8.78−7.39, and 9.95−9.03, respectively. Also, the values of ε″ are in the range of 0.19− 0.18, 1.61−1.18, 2.84−1.81, 3.16−2.57, and 3.53−3.25, respectively. The increase of permittivity is attributed to the dipolar reorientation and interfacial polarization due to the formation of SiC, CoSi, Co2Si, and crystallized carbons. As mentioned above, the formation of crystals generate grain boundaries. The introduction of cobalt leads to crystallization of SiC and carbons and the formation of CoSi and Co2Si crystals. Therefore, more dipolar relaxation, interfacial polarization, and resonances enhance the dielectric loss. Figure 9c shows the dielectric loss tangents of samples (C1, C2, C5, C7, C8). The C7 possesses the highest dielectric loss tangents. At the same time, both the real and the imaginary parts of permittivity were close to the optimal value. As a result, the C7 displayed the best microwave absorption property as illustrated in Figure 10a. The minimum RC value was −42.43 dB at 10.55 GHz, which means >99.99% electromagnetic waves can be absorbed with an effective absorption bandwidth almost across almost the whole X-band (8.46−12.4 GHz). The minimum reflection coefficient of C8 was −42.78 dB at 8.99 GHz, which was even better than C7, but the effective absorption bandwidth (8.4−10.77 GHz) of C8 was smaller. The reflection coefficient of C7 with different thickness is shown in Figure 10b. The specimen’s thickness plays an important role in the microwave absorption properties. When the sample thickness is 2.65 mm, the C7 possess the best microwave absorption property. In contrast, the minimum RC value of Co-containing ceramic fibers derived from polycarbosilanes doped with Co-colloids was about −4.8 dB with a thickness of 2.5 mm.36 The minimum RC value of FeCo alloy particles/graphite nanoflakes was −30.6 dB at 7.4 GHz with a thickness of 2.0 mm, but their effective absorption bandwidth was narrow.16 The average reflectivity of the PDC-SiC ceramics annealed at 1400 °C was 9.9 dB with a thickness of 2.75 mm, which actually showed no microwave absorption property.53 Thus, the as-prepared Co-containing ceramics with a minimum RC value of −42.43 dB and an absorption bandwidth of 8.4− 10.77 GHz possess excellent microwave absorption property. To analyze the mechanism of microwave absorption, the SET, SEA, and SER of the Co-containing ceramics pyrolyzed at 1100 °C with different content of cobalt are determined and shown in Figure S7. Obviously, both SEA and SER increase with the increase of the cobalt content. The increase of SEA and SER is ascribed to the contribution of crystallization of CoSi, Co2Si, and carbon, which leads to the SET increase from 2.08 to 6.09 dB. Nevertheless, the shielding effectiveness including SEA, SER, and SET is still at a lower level. Moreover, the C7 with 3.84%

the interfacial polarization and associated relaxation can be generated through the charges accumulated at the heterogeneous interfaces under microwave irradiation, which increase the dielectric loss and are favorable for the enhancement of microwave absorption.68−71 In addition, the nanograin boundaries and interfaces possess a large number of defects and dangling bonds due to the imperfect crystals and graphite structure. The defects can act as polarized centers and generate polarization relaxation under electromagnetic field to absorb electromagnetic wave. The dangling bonds, as well as the vacancies and positive interfacial charges, can generate electric dipole polarization.5 The electron motion hysteresis in the dipole under alternating electromagnetic field induces additional polarization relaxation and dielectric loss. Furthermore, according to free electron theory9

ε″ = δ /2πε0f where δ is the electrical conductivity, ε0 is the dielectric constant in vacuum, and f is the frequency of electromagnetic wave. The electrical conductivities are obviously increased with the formation of CoSi, Co2Si, and crystallized carbons. Therefore, the imaginary permittivity and dielectric loss are increased with the increase of the annealing temperature and cobalt content of ceramics. The reflection coefficient (RC) can be calculated according to the eqs 1 and 2. In this study, μr was taken as 1 because of the negligible magnetic properties of the Co-ceramics as shown in Figures S3 and S5. The lower RC value means the better microwave absorption properties of the Co-ceramics. The RC less than −10 dB means more than 90% microwave energy can be absorbed, which is an important baseline for microwave absorption materials. According to the previous report,5 the RC value is a minimum at a frequency of 10 GHz when the optimum real and imaginary parts of permittivity are equal to 7.3 and 3.3, respectively. The reflection coefficient of ceramics (C3−C6 with a thickness of 2.65 mm) is shown in Figure 8. The RC values of C3 and C4 were all higher than −10 dB in the whole X-band, indicating a bad microwave absorption property. The C5 annealed at 1100 °C possessed a minimum

Figure 8. Reflection coefficient of the Co-containing ceramics pyrolyzed from Co−PDSDA containing 5 mol % contents of Co2(CO)8 at different temperatures (C3, 1000 °C; C4, 1050 °C; C5, 1100 °C; C6, 1200 °C) with a thickness of 2.65 mm. 18728


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Figure 10. Reflection coefficient of the Co-containing ceramics pyrolyzed at 1100 °C with different contents of cobalts (C1, 0%; C2, 1.24%; C5, 2.25%; C7, 3.84%; C8, 4.98%) with a thickness of 2.65 mm (a), and reflection coefficient of C7 specimens with different thickness (b).

Microstructures of Co-Ceramics with Microwave Absorption. Why do the ceramics with high Co content (C7, C8) possess excellent microwave absorption property? The microstructures of ceramics are identified by TEM. From the TEM image in Figure 11a and the electron diffraction pattern in Figure 11b, ceramic C1 is almost an amorphous structure in correspondence with the XRD results. The TEM images in Figure 11c and 11d show the microstructure of ceramic C2 with a low content of cobalt. Some turbostratic carbons were formed around the cobalt silicide crystals. For the ceramics with high cobalt contents, much more nanocrystals were formed as shown in Figures 11e−g and 12. In addition to turbostratic carbons, lots of tubular carbons or nanowires can be observed in Figure 11g for ceramic C7. At the same time, the CoSi crystals in Figure 12 with a calculated interplanar spacing of 0.2 nm and Co2Si crystals with a calculated interplanar spacing of 0.214 nm can be observed around cobalt silicides in correspondence with the XRD results. The position of crystals and turbostratic carbons are around the cobalt silicide crystals,; it is confirmed that cobalt can promote the formation of crystals. As shown in Figure 12, a lot of grain boundaries are

Figure 9. Real permittivity (a), imaginary permittivity (b), and loss tangent (c) of the Co-containing ceramics pyrolyzed at 1100 °C with different contents of cobalts (C1, 0%; C2, 1.24%; C5, 2.25%; C7, 3.84%; C8, 4.98%) with a thickness of 2.65 mm.

content of cobalt annealed at 1100 °C exhibits the best microwave absorption property. Since the Co-ceramics show scarcely a mass loss (less than 5%) before 1100 °C in Figure 6, the PDC ceramics possess both excellent microwave absorption properties and high-temperature resistance. 18729


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tuned by addition of cobalt and changing annealing temperature. Both the crystals and the grain boundaries cause a great increase of microwave absorption property. The minimum RC value was −42.43 dB at 10.55 GHz, which means over 99.99% electromagnetic wave can be absorbed, with an effective absorption bandwidth almost across the whole X-band (8.46−12.4 GHz). Since the Co-ceramics show thermal stability at 1100 and 800 °C under Ar and air atmosphere, respectively, the PDC ceramics possess both excellent microwave absorption properties and high-temperature resistance. The in situ pyrolysis of cobalt-coordinated polymers gives a useful and promising strategy to prepare microwave absorption ceramic materials with enhanced microwave absorption properties and high-temperature resistance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03995. TGA, SEM-EDX results, hysteresis loop monitored by SQUID, σdc, SET, SEA, and SER, magnetic property of Co-containing ceramics (PDF)

Figure 11. Morphology of Co-containing ceramics: (a) TEM image and (b) electron diffraction pattern of ceramic C1, (c and d) TEM image of ceramic C2, (e and g) TEM image, and (f) electron diffraction pattern of ceramic C7.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone (fax): +86-2988431621. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (21174112), Fundamental Research Funds for the Central Universities (3102015BJ(II)JGZ026), and Aerospace Science and Technology Innovation Foundation. The help of Dr. G. Motz (CME, Universität Bayreuth) for the discussion is gratefully acknowledged.

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Figure 12. TEM micrograph (a) and enlarged TEM image (b, c) of representative Co-containing ceramic C7.

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formed in the ceramics between crystals phase and an amorphous phase or between free carbon nanodomains and an amorphous phase. Under the electric field, the space charge polarization and interface polarization will occur in grain boundaries. The dipoles in SiC, CoSi, and Co2Si will be formed under the applied electric field, leading to the increase of polarizations. The crystallized carbons also increase the polarizations by the high motion of electrons. All polarizations are helpful to high dielectric loss of ceramics, resulting in the enhancement of their microwave absorption properties.

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CONCLUSIONS The cobalt-containing microwave absorption ceramic materials were successfully prepared via in situ pyrolysis of carbon-rich poly(dimethylsilylene)diacetylene with alkynyls on the backbone coordinated with Co2(CO)8. The introduction of cobalt leads to the formation of CoSi and Co2Si nanocrystals and crystallized carbons (graphitic and tubular carbons). The hierarchical structures and dielectric properties could be 18730


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