Shun Dong

In Situ Growth of Core–Sheath Heterostructural SiC Nanowire Arrays on Carbon Fibers and Enhanced Electromagnetic Wave Absorption Performance

Large-scale core-sheath heterostructural SiC nanowires were facilely grown on the surface of carbon fibers using a one-step chemical vapor infiltration process. The as-synthesized SiC nanowires consist of single crystalline SiC cores with a diameter of ∼30 nm and polycrystalline SiC sheaths with an average thickness of ∼60 nm. The formation mechanisms of core-sheath heterostructural SiC nanowires (SiCnws) were discussed in detail. The SiCnws-CF shows strong electromagnetic (EM) wave absorption performance with a maximum reflection loss value of -45.98 dB at 4.4 GHz. Moreover, being coated with conductive polymer polypyrrole (PPy) by a simple chemical polymerization method, the SiCnws-CF/PPy nanocomposites exhibited superior EM absorption abilities with maximum RL value of -50.19 dB at 14.2 GHz and the effective bandwidth of 6.2 GHz. The SiCnws-CF/PPy nanocomposites in this study are very promising as absorber materials with strong electromagnetic wave absorption performance.

Ultra-long SiC nanowires synthesized by a simple method

Ultra-long SiC nanowires with lengths ranging from several millimeters to one centimeter were successfully prepared from graphite, silicon, silica and alumina raw materials via a simple carbon thermal reduction method in a tube furnace at 1300 °C. Scanning electron microscopy (SEM), electron energy scattering (EDX), X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the morphology and microstructure of the obtained products. The results showed that the nanowires mostly consisted of 3C–SiC and exhibited mainly a straight-line shape with diameters in the range of 30–150 nm. Alumina may be a novel and highly effective mediator playing an important role in controlling the concentration of SiO during the growth of ultra-long SiC nanowires and an alumina-assisted growth of the vapor–solid (VS) mechanism was proposed for the growth mode of ultra-long SiC nanowires.

Facile synthesis of silicon nitride nanowires with flexible mechanical properties and with diameters controlled by flow rate

Ultralong Si3N4 nanowires (NWs) were successfully synthesized with size controlled in N2 gas by using an efficient method. The diameters of the Si3N4 NWs increased when the flow rate of N2 gas increased, with average diameters of 290 nm from flow rates of 100 ml/min, 343 nm from flow rates of 200 ml/min and 425 nm from flow rates of 400 ml/min. Young’s modulus was found to rely strongly on the diameters of the Si3N4 NWs, decreasing from approximately 526.0 GPa to 321.9 GPa; as the diameters increased from 360 nm to 960 nm. These findings provide a promising method for tailoring these mechanical properties of the NWs in a controlled manner over a wide range of Young’s modulus values. Vapour-liquid-solid (VLS) mechanisms were used to model the growth of Si3N4 NWs on the inner wall of an alumina crucible and on the surface of the powder mixture. Alumina may be an effective mediator of NW growth that plays an important role in controlling the concentrations of Si-containing reactants to support the growth of NWs on the inner wall of the alumina crucible. This approach offers a valuable means for preparing ultralong Si3N4 NWs doped with Al with unique properties.

Synthesis and characterization of ultralong SiC nanowires with unique optical properties, excellent thermal stability and flexible nanomechanical properties

Several-millimeter long SiC nanowires (NWs) with unique optical properties, excellent thermal stability and flexible nanomechanical properties were synthesized using a simple method with silicon and phenolic resin as the raw materials. The SiC NWs displayed special optical properties that were attributed to their large size and Al-doping. They displayed broad green emission at 527.8 nm (2.35 eV) and purple emission concentrated at 438.9 nm (2.83 eV), in contrast to the other results, and the synthesized SiC NWs could also remain relatively stable in air up to 1000 °C indicating excellent thermal stability. The Young’s moduli of the SiC NWs with a wide range of NW diameters (215–400 nm) were measured using an in situ nanoindentation method with a hybrid scanning electron microscopy/scanning probe microscopy (SEM/SPM) system for the first time. The results suggested that the values of the Young’s modulus of the SiC NWs showed no clear size dependence, and the corresponding Young’s moduli of the SiC NWs with diameters of 215 nm, 320 nm, and 400 nm were approximately 559.1 GPa, 540.0 GPa and 576.5 GPa, respectively. These findings provide value and guidance for studying and understanding the properties of SiC nanomaterials and for expanding their possible applications.

Preparation and characterization of high-performance ZrB2–SiC–Cf composites sintered at 1450 °C

ZrB2–SiC–Cf composites containing 20–50 vol% short carbon fibers were hot pressed at low sintering temperature (1450 °C) using nanosized ZrB2 powders, in which the fiber degradation was effectively inhibited. The strain-to-failure values of such composites increased with increasing fiber content, and the value for the composite with 50 vol% Cf was even more than 3 times higher than that of the composite with 20 vol% Cf. Furthermore, the composite exhibited non-brittle fracture mode when the fiber content was above 30 vol%, and the thermal shock critical temperature difference of the composite with 30 vol% Cf was up to 727 °C, revealing excellent thermal shock resistance of this composite. Additionally, ZrB2–SiC–Cf composites displayed good oxidation resistance when the fiber content was below 40 vol%, suggesting that this method provides a promising way for preparation of high-performance ZrB2–SiC–Cf composites at low temperature.

Simultaneous in situ and ex situ growth of ultra-long Si3N4 nanobelts with different optical properties

In situ and ex situ growth of ultra-long Si3N4 nanobelts (NBs) was simultaneously achieved via an effective method with the raw materials of graphite, nanosilicon and nanosilica. In situ growth of ultra-long Si3N4 NBs on the surface of the powder mixture resulted in a cross-section of 30–50 nm in thickness and 100–250 nm in width, and a length that can grow up to several millimeters. The width and thickness of the NBs obtained on the inner walls of the crucible, namely ex situ growth of Si3N4 NBs, are in the range of 200–500 nm and 50–200 nm, respectively. A vapor–solid (VS) mechanism was proposed for the growth mode of the in situ growth of Si3N4 NBs, while VS and vapor–liquid–solid (VLS) mechanisms simultaneously existed in the growth of the Si3N4 NBs obtained on the inner walls of the crucible. This method provides an effective way of preparing Si3N4 NBs on an industrial scale. The room-temperature photoluminescence (PL) spectra show that the synthesized α-Si3N4 NBs both had two strong emissions peaks, but the PL spectrum of the ex situ NBs shows an obvious red-shift compared to that of the in situ NBs, making it a potential material for applications in special optoelectronic nanodevices.

Size dependence of optical and mechanical properties of Si3N4 nanobelts controlled by flow rates

Single-crystalline Si3N4 nanobelts (NBs) were successfully synthesized in a size-controlled manner using graphite, nanosilicon and nanosilica powders in N2 gas. The average width of Si3N4 NBs increased with an increase in the flow rate of N2 gas, with 264 nm at 50 ml min−1, 295 nm at 100 ml min−1 and 341 nm at 200 ml min−1. The room-temperature photoluminescence (PL) spectra showed that the synthesized α-Si3N4 nanobelts with three different sizes all had two strong emission peaks located in the yellow and red spectral range, which could be mainly attributed to the incorporation of a small amount of Al in the NBs, and the larger size could also lead to a slight red shift. The Youngs modulus of Si3N4 NBs was measured with a hybrid scanning electron microscope/scanning probe microscope (SEM/SPM) system by means of the modulated nanoindentation method. The Youngs modulus of Si3N4 NBs strongly depended on their size, decreasing from about 548.6 to 455.1 GPa, which can be explained by surface effects and defect-related effects arising due to their nanometer size. These findings provide a method to tailor the optical and mechanical properties of the NBs in a controlled manner over a wide range of Youngs modulus values.

Strong contribution of in situ grown nanowires to enhance the thermostabilities and microwave absorption properties of porous graphene foams under different atmospheres

Free-standing and high-performance electromagnetic (EM) absorbing materials of three-dimensional (3D) hierarchical graphene foams (GFs) decorated with in situ grown Si3N4 nanowires (Si3N4nws) and SiC nanowires (SiCnws) were prepared using a one-step carbothermal reduction process under flowing N2 and Ar, respectively. The as-obtained Si3N4nws consist of single crystalline Si3N4 cores ∼200 nm in diameter and amorphous SiOx layers ∼2 nm in thickness, and the SiCnws are composed of single crystalline SiC cores about 200 nm in diameter and amorphous SiOx sheaths with an average thickness of 25 nm. By incorporating Si3N4nws and SiCnws into GFs, the thermostability and EM absorption of the composites were simultaneously improved effectively. The Si3N4nws-GFs and SiCnws-GFs were thermostable in an air atmosphere beyond ∼640 °C with above 98% and 94% weight retention, respectively. Compared with those of pure GFs, the Si3N4nws-GFs exhibit a stronger EM microwave absorption performance with a minimum reflection loss (RL) value of −48.7 dB at 6.4 GHz with a thickness of 2.36 mm, and the minimum RL value of the SiCnws-GFs could be as low as −67.8 dB at 5.9 GHz with 2.6 mm thickness. The multiscale Si3N4nws-GFs and SiCnws-GFs in the present study are very promising as absorber materials with strong EM wave absorption performance in critical environments.

Carbon Nanofiber Arrays Grown on Three-Dimensional Carbon Fiber Architecture Substrate and Enhanced Interface Performance of Carbon Fiber and Zirconium Carbide Coating

Carbon nanofibers (CNFs) were grown around the carbon fiber architecture through a plasma enhanced chemical vapor deposition method to enhance the interface performance between CF architecture substrate and ZrC preceramic matrix. The synthesized 3D CF hierarchical architectures (CNFs-CF) are coated with zirconium carbide (ZrC) ceramic to enhance their antioxidant property and high temperature resistance. The composition and the crystalline phase structure of the composite were detected with the X-ray photoelectron spectroscopy and X-ray diffraction. The results of scanning electron microscopy show that, the as-prepared CNFs and consistent ZrC ceramic coating are uniformly covered on the surface of carbon fiber architecture substrate. The ZrC ceramic products with excellent crystallinity were got from the pyrolysis of preceramic polymer at 1600 °C in inert atmosphere. Comparing with the untreated CF, the loading of ZrC ceramics around the CNFs-CF architecture surface are significantly increased. The thermal stability and mechanical property of CNFs-CF/ZrC nanocomposites have been promoted obviously compared with the CF/ZrC ceramic nanocomposite. The prepared CNFs-CF/ZrC ceramic nanocomposite is one of the potential candidate materials for the thermal protection application.

A multiscale hierarchical architecture of a SiC whiskers–graphite nanosheets/polypyrrole ternary composite for enhanced electromagnetic wave absorption

A core–shell SiC whiskers–graphite nanosheets/polypyrrole (SiCw–GNs/PPy) heterostructure with excellent electromagnetic (EM) absorption performance was successfully prepared by a simple combined method using glucose as the carbon precursor. The content of GNs could be tailored by the mass fraction of glucose, while a substantial decrease of EM absorption performance of SiCw–GNs is achieved as the GN content increases, which could be attributed to the inferior impedance matching degree and small attenuation constant. Impressively, on coating with the conductive polymer PPy by a simple chemical polymerization method, the SiCw–GNs/PPy heterostructures exhibited superior EM absorption abilities with a minimum reflection loss (RLmin) value of −64.2 dB and a maximum effective bandwidth of 7.9 GHz, and the matched characteristic impedance and improved loss ability of the SiCw–GNs/PPy heterostructures account for the significant enhancement. The core–shell SiCw–GNs/PPy heterostructures in the present study could be considered as a promising candidate for the oncoming generation of microwave absorbing materials.