Large-Scale Synthesis and Mechanism of β-SiC ... - ACS Publications

Oct 18, 2016 - ABSTRACT: Silicon carbide (SiC) nanomaterials have many applications in semiconductor, refractories, functional ceramics, and composite...
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Large-Scale Synthesis and Mechanism of β‑SiC Nanoparticles from Rice Husks by Low-Temperature Magnesiothermic Reduction Jianjun Su,†,+ Biao Gao,†,+ Zhendong Chen,† Jijiang Fu,† Weili An,† Xiang Peng,§ Xuming Zhang,†,§ Lei Wang,‡ Kaifu Huo,*,†,‡ and Paul K Chu§ †

The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China Wuhan National Laboratory for Optoelectronics (WNLO) School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China § Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong China Downloaded via AUCKLAND UNIV OF TECHNOLOGY on January 28, 2019 at 22:28:38 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Silicon carbide (SiC) nanomaterials have many applications in semiconductor, refractories, functional ceramics, and composite reinforcement due to their unique chemical and physical properties. However, large-scale and cost-effective synthesis of SiC nanomaterials at a low temperature is still challenging. Herein, a lowtemperature and scalable process to produce β-phase SiC nanoparticles from rice husks (RHs) by magnesiothermic reduction (MR) at a relative low temperature of 600 °C is described. The SiC nanoparticles could inherit the morphology of biogenetic nano-SiO2 in RHs with a size of about 20−30 nm. The MR reaction mechanism and role of intermediate species are investigated. The result shows that SiO2 is first reduced to Mg2Si in the rapid exothermic process and the intermediate product, Mg2Si, further reacts with residual SiO2 and C to produce SiC. Moreover, the SiC shows considerable electromagnetic wave absorption with a minimum reflection loss of −5.88 dB and reflection loss bandwidth < −5 dB of 1.78 GHz. This paper provides a large-scale, cost-effective, environmental friendly, and sustainable process to produce high-quality βphase SiC nanoparticles from biomass at a low temperature, which is applicable to functional ceramics and optoelectronics. KEYWORDS: Silicon carbide, Rice husks, Magnesiothermic reduction, Thermodynamic calculation, Electromagnetic wave absorption

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INTRODUCTION Silicon carbide (SiC) is promising in various applications such as abrasives, refractories, functional ceramics, and composite reinforcement or filler because of the high Young’s modulus and hardness, large high-temperature strength, excellent corrosion resistance, and good thermal shock resistance. In addition to these traditional applications, SiC is also used in optoelectronic devices such as light-emitting diodes (LEDs) and power electronics on account of the wide bandgap and high breakdown field.1−3 The physical and chemical properties of SiC critically depend on the grain size and crystal structure.3 Nanostructured SiC such as nanoparticles (NPs) and nanowires have unique properties compared to the bulk counterpart, for instance, higher hardness, higher strength, lower sintering temperature, and better optoelectronic functionalities owing to the large surface area and size-dependent properties.4−9 Hence, there has been increasing interest and research in nanostructured SiC especially pertaining to applications in optoelectronics, functional ceramics, and composite reinforcement.10,11 SiC nanoparticles have been synthesized mainly by chemical vapor deposition,12 sol−gel processes,13 and thermal or laser pyrolysis of organic.14 For example, Lomello et al. synthesized pure SiC nanopowders by CVD using SiH4 and C2H2 as the precursors.15 Unfortunately, these techniques have some disadvantages, for example, the use of expensive raw materials © 2016 American Chemical Society

or toxic precursors (SiH4 or SiCl4), requiring special equipment, and heavy agglomeration of the final product. Carbothermal reduction of quartz sand and petroleum coke has been shown to be an effective method to synthesize commercial SiC powders.16 However, this process is usually carried out in an electrical resistance furnace at a temperature over 2000 °C and the as-synthesized SiC usually in the form of large chunks, which needs to be broken, sorted, crushed, milled, and classified into different sizes. Fine SiC nanopowders can be produced using nanometer-sized carbon and silica as the precursors. For instance, Hans-Peter et al. synthesized nanocrystalline SiC powders by carbothermal reduction with nanoSiO2 as the precursor.17 However, the high temperature and long processing time lead to big energy consumption, heavy agglomeration, and/or uneven morphology and size of the final SiC nanopowders. Nowadays, synthesis of nanomaterials using natural resources has drawn much attention because it is an environmental friendly, less-harmful, and cost-effective step toward the green synthesis of nanomaterial.18−21 Rice is the staple food of over half the world’s population and provides 20% of the world’s dietary energy supply. Rice husks (RHs) are major agricultural Received: June 29, 2016 Revised: September 25, 2016 Published: October 18, 2016 6600

DOI: 10.1021/acssuschemeng.6b01483 ACS Sustainable Chem. Eng. 2016, 4, 6600−6607

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ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Illustration of the Synthesis Procedures of SiC NPs from RHs

mechanism in the literature,32−35 the thermal dynamic calculation and experiments performed at different temperatures described in this paper confirm that the Mg2Si is the intermediate species in the MR process for the transformation from biogenic silica to SiC NPs. Our model indicates that SiO2 is first reduced to Mg2Si in the rapid exothermic process and the intermediate product, Mg2Si, further reacts with residual SiO2 and C to produce SiC NPs. Because the gas phase SiO intermediate is not produced at 600−650 °C, SiC nanostructures can be controllably prepared by taking advantage of the pseudomorphic transformation from nano-SiO2 to nano-SiC. Using this method, uniform SiC NPs can be produced from RHs on a large scale and at a low cost. In addition, the electromagnetic (EM) wave absorption performance of the assynthesized SiC NPs is assessed and considerable electromagnetic wave absorption ability with the minimum reflection loss (RL) of −5.88 dB is observed suggesting promising applications pertaining to electromagnetic wave absorption.

byproducts from rice production and contribute about 20−25 wt % of the total dry weight of paddy rice.22,23 Over 700 million tons of rice are harvested each year, and so RHs account for about 140 million tons. RHs are generally disposed by burning or land filling, resulting in waste of energy, air pollution, and greenhouse gas emission.24,25 RHs mainly contain lignin, cellulose, hemicellulose, as well as hydrated silica and the content of hydrated silica is about 15−28 wt % depending on the species, origin, climate, and geographic location.26 The hydrated silica retains nanoparticles morphology around the cellulose microcompartments in the tough external layer (epidermis) of RHs during biomineralization.27,28 Therefore, RHs are a natural reservoir for nanostructured silica and its derivatives. In recent years, extraction of silica functional materials from RHs has drawn much attention, but the lignocellulose in RHs is usually burnt and wasted.29 However, it is of great environmental and economic importance to utilize both the lignocellulose and silica in RHs to produce SiC. Synthesis of SiC powders from RHs by carbothermal reduction method has been proposed;30 however, in most of the cases, the SiC from RHs are prepared at 1200−1500 °C in an inert or reducing atmosphere.31 The high-temperature carbothermal reduction is not a pseudomorphic transformation process due to the melting of SiO2 and formation of gas phase SiO intermediates, so that the SiC products exhibit different final morphological forms such as particles, fibers, and whiskers rather than the inherent pristine morphology of SiO2 NPs precursors in the RHs. Moreover, the high reaction temperature requires a lot of energy and it is important to develop a low-cost, low-temperature, and commercially sustainable method to produce uniform SiC NPs from RHs. Herein, we report a low-temperature, cost-effective, and scalable process to produce β-SiC NPs from RHs by magnesiothermic reduction (MR). The SiC nanoparticles could inherit the morphology of biogenetic nano-SiO2 in RHs with a size of about 20−30 nm. The pseudomorphic transformation process from SiO2 to SiC is carried out at a temperature as low as 600 °C, which is approximately half of that in conventional carbothermal reduction, and the yield of SiC NPs from RHs is about 7−10 wt %. The overall reaction of MR can be described as SiO2 + C + 2Mg = SiC + 2MgO, but the detailed reaction mechanism and role of intermediate species are still unclear.32−34 For example, Shi et al. proposed that silica is reduced to silicon initially and then silicon reacts with carbon to form SiC.32 However, Dasog et al. reported that the formation of magnesium sesquicarbide (Mg2C3) intermediate is the necessary step to form SiC during MR of silica in the presence of carbon.35 Different from the reported reaction

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EXPERIMENTAL SECTION

The RHs were obtained from a local rice mill in Wuhan. Chlorhydric acid (HCl) and hydrofluoric acid (HF) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and magnesium powders (Mg, 99.5%) were obtained from Aladdin Industrial Corporation (China). All the reagents were used without further purification. The RHs were washed thoroughly with distilled water to remove soil and then dried at 80 °C for 2 h. After washing with distilled water several times and dried overnight, the RHs were annealed in a tube furnace at 600 °C for 1 h under Ar for carbonization and removal of small organic molecules. The carbonized RHs were further boiled in HCl (1 mol L−1) for 4 h to remove metal impurities and then dried at 80 °C for 3 h. Synthesis of SiC NPs from carbonized RHs was conducted using the MR process. In a typical process, the carbonized RHs and magnesium powders were mixed at the molar ratio of SiO2/Mg = 1:2.5 and sealed in a stainless steel container that was inserted into a tube furnace and heated to 600 °C at a heating rate of 5 °C min−1 under continuous argon flow (50 SCCM) for 3 h. The products were immersed in HCl (1 mol L−1) under stirring to remove the magnesia (MgO), washed with distilled water until the pH reached neutral, calcined in air at 700 °C for 1 h to remove residual carbon, and washed with HF to remove residual SiO2 to produce light-green SiC NPs powders The samples were characterized by X-ray diffraction (XRD, Philips X’ Pert Pro) with Cu Kα radiation (1.5416 Å) between 10° and 90° (2θ), X-ray photoelectron spectroscopy (XPS, ESCALB MK-II, VG Instruments, U.K.), field-emission scanning electron microscopy (FESEM, FEI Nova 400 Nano), transmission electron microscopy (TEM, FEI Titan 60−300 Cs), high-resolution TEM (HR-TEM, Titan), and energy-dispersive X-ray spectroscopy (EDS, Oxford INCA 200). The 6601

DOI: 10.1021/acssuschemeng.6b01483 ACS Sustainable Chem. Eng. 2016, 4, 6600−6607

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ACS Sustainable Chemistry & Engineering nitrogen adsorption and desorption isotherms were determined by the Brunauer−Emmett−Teller (BET) (Micrometrics, ASAP 2020) method after degassing the samples at 383 K for 5 h. The thermal stability of the SiC NPs was determined by thermogravimetric analysis (TGA, NETZSCH; TG 209 F3). The electromagnetic wave (EM) parameters were determined using a PNA-N5244A vector network analyzer (Agilent, USA) at EM frequencies of 2−18 GHz based on a thickness of 3 mm using paraffin as substrate. The filling rates were 50% in the measurement.

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RESULTS AND DISCUSSION Synthesis and Characterization of SiC NPs from RHs. Scheme 1 shows the procedures of the production of ultrafine SiC NPs from RHs. The RHs are boiled in HCl to remove metal impurity and alkali oxide.36 Thermogravimetric analysis and differential thermal analysis (TG-DTA) conducted on the acid-leached RHs (Figure S1) suggest that the SiO2 accounts for 20.6 wt %.37 Figure 1a depicts the SEM image of the acid-

Figure 2. (a) SEM image of the outer surface of the CRHs, (b, c) TEM images of the CRHs, and (d) EDS spectrum of the CRHs. The inset in panel a is the magnified image.

MR products after rinsing with HCl in air (Figure S3) suggest an optimal temperature of 700 °C to remove residual carbon. The elemental analysis of SiC final products (Figure S4) shows that the sample only comprise of Si, C, and O elements. The purity of as-synthesized SiC is about 98.6 wt % and the impurities are trace residual SiO2 (0.9 wt %) and C (0.5 wt %). The yield of SiC NPs from RHs is 7−10 wt %, indicating promising route to the large-scale synthesis of SiC NPs. Figure 3a depicts the SEM image of the as-synthesized SiC NPs inherit the morphology of the original SiO2 NPs in the RHs. The TEM image (Figure 3b) further shows that the RHs-

Figure 1. (a) SEM image of the HCl leached RHs and EDS elemental maps of (b) Si, (c) O, and (d) C in the HCl leached RHs. The inset in panel a is the magnified image.

leached RHs revealing dome-shape protrusions at linear ridges and furrows with a size of 30−40 μm on the outer surface of the RHs. The EDS elemental map in Figure 1b shows that Si is mainly distributed in the protrusions and adjoining slopes; however, O (Figure 1c) and C (Figure 1d), which are the main ingredients in organic matters, are uniform distributed in the RHs. The leached RHs are annealed under Ar at 600 °C for 1 h to convert the organic species like cellulose, semicellulose, and lignin into carbon. The yield of the carbonized RHs is 42% due to decomposition and volatilization of the organic species in the RHs.22 The TG-DTA plot of the carbonized RHs in air (Figure S2) shows the SiO2 mass fraction is about 44.8 wt %. The SEM image in Figure 2a discloses that the carbonized RHs inherit the morphology of the RHs except that the protrusions become smooth and slightly smaller. The TEM images (Figure 2b,c) of the carbonized RHs indicate that silica NPs with diameters of 20−40 nm are uniformly distributed on the surface of the protrusions. The EDS spectrum of carbonized RHs (Figure 2d) indicates that the carbonized RHs consist of Si, O, and C. The silica NPs are in good contact with the surrounding carbon on the nanoscale, thereby facilitating the formation of SiC NPs due to the shorter mass transfer distance compared to traditional carbon and silicon sources.38 The nano-SiO2 in the carbonized RHs can be reduced to SiC by MR at 600 °C due to the large amount of heat released from the reaction. Finally, the SiC NPs are obtained by removing the MgO byproduct by HCl etching and excessive carbon by annealing in air. The TG curves of the

Figure 3. (a) SEM image of the as-synthesized SiC NPs, (b) TEM image of the SiC NPs, (c) HR-TEM image showing the lattice fringes of the SiC nanocrystallites with the black and red circles indicating the SiC nanocrystallites and nanopores, respectively, and (d) XRD pattern of the as-synthesized SiC. The inset in panel b is EDS spectrum of the SiC NPs and inset in panel c is the magnified picture of the area enclosed by the black dashed circle. 6602

DOI: 10.1021/acssuschemeng.6b01483 ACS Sustainable Chem. Eng. 2016, 4, 6600−6607

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Figure 4. (a) XPS survey spectrum of the SiC NPs and high-resolution scans of (b) Si 2p and (c) and C 1s XPS spectra with the fitted results; (d) TG curve of the as-synthesized SiC NPs under flowing air.

information is obtained from the high-resolution spectra of Si 2p (Figure 4b) and C 1s (Figure 4c). The Si 2p peak can be deconvoluted into two subpeaks at 100.9 and 103.0 eV associated with SiC and SiO2, respectively.41,42 The presence of SiO2 arises from inevitable oxidation on the surface of SiC NPs and the trace SiO2 residual. The C 1s spectrum can be fitted with two peaks at 283.0 and 284.6 eV stemming from SiC NPs and trace residual carbon.43,44 To investigate the oxidation resistance and thermal stability, TG measurement is performed at room temperature to 1200 °C in air (Figure 4d). The SiC NPs exhibit high thermal stability with the initial oxidation temperature above 900 °C because the surface SiO2 protects the SiC NPs from reacting with external oxygen. However, the weight of the SiC NPs increases dramatically at 945 °C due to the rapid oxidation. Reaction Mechanism. MR is an effective method to prepare SiC nanostructures with SiO 2 and carbon as predecessors by the following reaction: SiO2(s) + 2Mg(g) + C(s) → SiC(s) + 2MgO(s). However, the mechanism and role of intermediate species are not well understood.32−35 Several reaction mechanisms have been proposed. For example, Shi et al. proposed that silica is reduced to silicon initially and then silicon reacts with carbon to form SiC in the presence of the metal catalyst Mg. This mechanism (I) is described by reactions 1 and 2:32,45

derived SiC powders have a uniform size of about 20−30 nm, and EDS spectrum of SiC NPs reveals the presence of Si and C (inset in Figure 3b in which the Cu signal stems from the Cu grid used in TEM examination). The weak oxygen element signal in the EDS spectrum can attribute to inevitable oxidation on the surface of SiC NPs. The lattice resolved HR-TEM image (Figure 3c) shows that the SiC NPs have good crystallinity with distinct lattice fringes of 0.25 nm corresponding to the SiC (110) planes of the cubic phase (3C). There are numerous nanopores by the removing the byproducts (mainly magnesia) by HCl as shown in Figure 3c. The XRD pattern acquired from the as-synthesized SiC (Figure 3d) indicates four salient diffraction peaks at 35.6, 41.4, 60.0, and 72.8° corresponding to the (110), (200), (220), and (311) planes of cubic (β phase) SiC (JCPDS: 29-1129), respectively. The weak peak at 33.6° marked with S.F. can be assigned to the stacking faults in the βSiC structure.39,40 The BET surface area of the SiC NPs determined from the adsorption/desorption curve is as large as 89.51 m2 g−1 (Figure S5). The direct band gap of RHs-derived SiC NPs calculated from the UV−vis absorption spectrum (Figure S6) is about 3.44 eV. We also investigate the influences of different temperature on the morphology of the SiC products. Figure S7 are the SEM images of the SiC NPs at different MR temperatures: (a) 575 °C, (b) 600 °C, (c) 650 °C, and (d) 700 °C. The morphology of SiC NPs could inherit the morphology of SiO2 precursors in RHs by MR at low temperature (575, 600, and 650 °C). However, when the reaction temperature is increased to 700 °C, the SiC NPs become severe agglomerate as shown in Figure S7d, which is caused by the aggregation of SiO2 and the intense exothermic reaction at a high temperature of 700 °C. The XPS survey spectrum in Figure 4a shows that the assynthesized SiC NPs are composed of Si, C, and O. More

SiO2 (s) + 2Mg(g) → Si(s) + 2MgO(s)

(1)

Si(s) + C(s) → SiC(s)

(2)

However, Dasog et al. proposed that Mg first reacts with C to form MgC2 or Mg2C3 and then MgC2 reacts with SiO2 to form SiC. The proposed mechanism (II) is shown as reactions 3 and 4:35 6603

DOI: 10.1021/acssuschemeng.6b01483 ACS Sustainable Chem. Eng. 2016, 4, 6600−6607

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ACS Sustainable Chemistry & Engineering

Figure 5. (a) XRD patterns of the reaction product at different MR temperature. (b) Mass fraction changes in different phases as a function of the reaction temperature. (c) TG-DTA results of the mixture of carbonized RHs and Mg powder heated from room temperature to 800 °C in argon atmosphere.

Mg(s) + 2C(s) → MgC2(s)

(3)

SiO2 (s) + MgC2(s) → SiC(s) + MgO(s) + CO(g)

(4)

together with reduced Mg2Si and increased MgO. Thus, formation of SiC can be ascribed to the reaction of intermediate product Mg2Si with residual SiO2 and C according to reaction 6. Therefore, we conclude that the MR mechanism consists of reactions 5 and 6 (mechanism (III)): (a) Mg2Si produced by the reaction between SiO2 and Mg, (b) Mg2Si and residual C, SiO2 reacting further to form SiC and MgO as shown in the following:

Other studies indicate that the intermediate species in SiC formation by MR is magnesium silicide (Mg2Si).33,38 In our experiments, the MR of carbonized RHs and Mg was performed at 400, 525, 550, 575, and 600 °C (denoted as SiC-400, SiC-525, SiC-550, SiC-575, and SiC-600, respectively). The XRD patterns of reaction products and mass fraction changes in different phases as a function of the reaction temperature are depicted in Figure 5a,b. The XRD pattern of the product prepared at 400 °C shows that the peaks are associated with Mg (JCPDS: 35-0821) with a concentration of 90.2 wt % and the remaining is MgO (JCPDS: 45-0946). The MgO may originate from surface oxidation of Mg. Other products are not shown, implying that MR cannot occur at a low temperature of 400 °C. When the temperature is increased to 525 °C, Mg (54.9%) and small amounts of MgO (24.1%) and Mg2Si (21%) (JCPDS: 35-0773) can be detected; however, no SiC can be detected by XRD, indicating that only a fraction of the Mg reacts with SiO2 according to reaction 5. No Si and MgC2 peaks could be indentified, indicating that the initial product of the MR is Mg2Si, which is agreement with the previous works.27,31 The Gibbs free energy changes (ΔG) as a function of the temperature for reactions 1, 3, and 5 are shown in Figure S8. Reaction 5 shows more negative ΔG between room temperature and 1000 °C, further demonstrating the possibility of formation of Mg2Si based on the thermodynamic theory. At 550 °C, most of the Mg metal is converted to Mg2Si and MgO with the molar ratio of 1:1 and the step first is confirmed (reaction 5). Figure 5a,b indicates that diffraction peaks from cubic phase SiC are observed up to 575 or 600 °C

SiO2 (s) + 4Mg(g) → Mg 2Si(s) + 2MgO(s)

(5)

Mg 2Si(s) + SiO2 (s) + 2C(s) → 2SiC(s) + 2MgO(s) (6)

Figure 5c shows the TG-DTA curves of the mixture of carbonized RHs and Mg from room temperature to 800 °C at a heating rate of 5 °C min−1 in Ar. The weight of the reactants does not change significantly indicating the absence of gaseous substance formation during the MR process. There is a wide exothermal zone at 400 °C in the DTA curve as a result of surface oxidization of Mg. Another sharp exothermic peak at 550−620 °C suggests a heat releasing process according to the enthalpy change calculation of reaction 5 (ΔH = −958.7 kJ mol−1). According to the TG-DTA results, reaction 5 commences at above 525 °C and the large amount of heat released triggers reaction 6, and so the actual reaction temperature is higher. The heat released from reaction 5 expedites formation of SiC (reaction 6) in spite of the low temperature of 600 °C. To confirm further the abovementioned reaction mechanism, SiC is directly prepared by heating the mixture of Mg2Si (the XRD pattern as showing in Figure S9), SiO2, and RHs derived carbon at 650, 850, and 1050 °C in Ar, respectively. The corresponding XRD patterns (Figure S10) reveal that β-SiC could be formed by thermal 6604

DOI: 10.1021/acssuschemeng.6b01483 ACS Sustainable Chem. Eng. 2016, 4, 6600−6607

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ACS Sustainable Chemistry & Engineering

Figure 6. Schematic illustration of the reaction mechanism between CRHs and Mg.

treatment of Mg2Si, SiO2, and carbon mixture; however, the formation of β-SiC by reaction of Mg2Si, SiO2, and carbon need higher temperature due to no intimate contact between these components. The EDS elemental map of Mg obtained from at reaction of 550 °C is shown in Figure S11. The result shows that the Mg element is mainly distribute on the outside of the nanoparticles. Combing with the XRD results in Figure 5a, the EDS mapping result further confirm our proposed reaction mechanism. The in situ formed Mg2Si could also react with the remaining SiO2 to produce Si by the reaction 7:46 Mg 2Si(s) + SiO2 (s) → 2Si(s) + 2MgO(s)

(7)

However, the Gibbs free energy changes versus the temperature for reactions 6 and 7 (Figure S8) indicate that the SiC is easier to generate than Si thermodynamically in the presence of the carbon reactants. Hence, the MR process involving SiO2, C, and Mg system can be described by reactions 5 and 6. On the basis of the above-mentioned analysis, the reaction mechanism of the formation SiC NPs from RHs is schematically depicted in Figure 6, which consists of two steps (reactions 5 and 6). The Mg2Si layer is first generated on the SiO2 nanoparticles by reduction of SiO2 with the surrounding Mg vapor. The unreacted SiO2 core further reacts with the Mg2Si and C to generate SiC. With the reaction proceeding, the SiO2 is consumed and the SiC NPs are finally produced. The carbon in carbonized RHs closely integrates with the SiO2 nanoparticle, resulting in large contact surface, short diffusion distance, and the low reaction temperature.38 Electromagnetic Wave Absorption Performance. SiC can absorb EM energy by causing dielectric loss.47,48 Because of the good thermal stability, inert chemical properties, and small density, SiC NPs derived from RHs is a good candidate for lightweight and high-temperature EM wave absorbers. Figure 7 displays the frequency dependence of RL of the SiC NPs at 2− 18 GHz. The minimum RL of SiC is −5.88 dB and the bandwidth of RL < −5 dB is 1.78 GHz. These results indicate that the SiC NPs are potentially useful in EM wave absorbers, especially at high-temperature environments due to high thermal conductivity of SiC.

Figure 7. Frequency dependence of reflection loss of the SiC NPs at 2−18 GHz.

layer based on the fast exothermic reaction and (2) reaction between the intermediate Mg2Si and residual SiO2 and diffused C to form SiC. The SiC NPs show the minimum RL is −5.88 dB and bandwidth RL< −5 dB is 1.78 GHz. In addition to clarification of the reaction mechanism, this low-cost and largescale process can be extended to a variety of materials and enables better utilization of other agricultural wastes such as corn and bamboo leaves, besides RHs.

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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/acssuschemeng.6b01483. Thermogravimetric analysis and differential thermal analysis (TG-DTA) results of the leached RHs, carbonized RHs heated from room temperature to 850 °C in the oxygen atmosphere, thermogravimetric (TG) results of the MR products after rinsing with HCl and heated from room temperature to 1200 °C in an oxygen atmosphere, elemental analysis of SiC final products, Brunauer−Emmett−Teller (BET) results of the synthesized SiC NPs, UV−vis absorption spectrum of the SiC NPs products, SEM images of the SiC NPs at different MR temperatures, X-ray diffraction (XRD) patterns of the SiC prepared directly from the mixture of RHs derived Mg2Si/MgO and carbonized RHs at 650, 850, and 1050 °C, TEM image and Mg EDS elemental map of the reaction product without further treatment after MR at 550 °C, Gibbs free energy changes as a function of the temperature in reactions 1−7, and thermodynamic values used in the calculation (PDF)

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CONCLUSIONS A facile, low-cost, and large-scale MR process is described to synthesize β-phase SiC NPs from the waste RHs and the yield β-SiC from RHs is 7−10 wt %. The as-synthesized SiC NPs have an average size of 20−30 nm, which are similar to the biogenetic SiO2 NPs in RHs. The reaction mechanism of the formation of SiC from the mixture of carbonized RHs and Mg is proposed. The reaction process consists of two steps: (1) reduction of the surface of the SiO2 nanoparticles to the Mg2Si 6605

DOI: 10.1021/acssuschemeng.6b01483 ACS Sustainable Chem. Eng. 2016, 4, 6600−6607

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AUTHOR INFORMATION

Corresponding Author

*K. Huo. E-mail address: [email protected]. Author Contributions +

These two authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Natural Science Foundation of China (NOs. 51504171, 51572100, and 31500783), Project of Hubei Provincial Education Office (B2015346), Outstanding Young and Middle-Aged Scientific Innovation Team of Colleges and Universities of Hubei Province (T201402), Applied Basic Research Program of Wuhan City (2013011801010598), Natural Science Foundation of Hubei Province (2015CFA116), HUST Key Interdisciplinary Team Project (2016JCTD101), and City University of Hong Kong Applied Research Grant (ARG) No. 9667122.

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