Characterization of SiC nanowires prepared on C/C composite without catalyst by CVD

SiC nanowires were prepared on C/C composite surface without catalyst by chemical vapor deposition（CVD） using CH3 SiCl3 as precursor.SEM images of the CVD-product reveal that some long nanowires have grown to tens of micrometers with some gathered as a ball.Some short nanowires agglomerate like chestnut shell with many thorns accompanied by some deposited nano-particles.XRD,Raman-spectrum and FTIR patterns indicate that the product is a typical β-SiC.TEM images show that the nanowires have a wide diameter range from 10 to 100 nm,and some thin nanowires are bonded to the thick one by amorphous CVD-SiC.A SiC branch generates from an amorphous section of a thick one with an angle of 70° between them,which is consistent with the [111] axis stacking angle of the crystal.SAED and fast Fourier transform（FFT） patterns reveal that the nanowires can grow along with different axes,and the bamboo-nodes section is full of stacking faults and twin crystal.The twisted SiC lattice planes reveal that the screw dislocation growth is the main mechanism for the CVD-SiC nanowires.

Constructing orthogonally structured graphene nanointerface on SiC nanowires reinforced carbon/carbon composites to boost mechanical strength and strength retention rate

Carbon/carbon composites with higher mechanical strength and better reliability at elevated tempera-tures are urgently needed to satisfy the practical applications requirements.SiC nanowires(SiCNWs)modified C/C(SC-CC)composites have attracted an abundance of attention for their excellent mechanical performance.To further boost the mechanical strengths of composites and maximize the reinforcing efficiency of SiCNWs,we introduce orthogonally structured graphene nanosheets(OGNs)into SC-CC composites,in which OGNs are grafted on the SiCNWs via chemical vapor deposition(CVD)method,forming SC-G-CC composites.Benefiting from the nano-interface effects,uniform stress distribution,strong SiCNWs/PyC interfacial bonding and elevated stress propagation efficiency in the PyC matrix are achieved,thus SC-G-CC composites accomplish brilliant mechanical properties before and after 1,600℃ heat treatment.As temperature rises to 2,100℃,SiCNWs lose efficacy,whereas OGNs with excellent thermal stability continue to play the nano-interface role in the PyC matrix.Therefore,SC-G-CC com-posites show better mechanical performance after 2,100℃ heat treatment,and the mechanical strength retention rate(MSR)of interlaminar shear strength,out-of-plane and in-plane compressive strength of SC-G-CC composites reach 61.0%,55.7%and 55.3%,respectively.This work proposes an alternative thought for maximizing the potentiality of nanomaterials and edifies the mechanical modification of composites.

Ultralight and compressive SiC nanowires aerogel for high-temperature thermal insulation

Aerogels with excellent properties combination of ultralow density and great thermal insulation are drawing attention to applications in harsh conditions.Common aerogels,however,are usually constructed with nanoparticles with a weakness in physical combination.The silicon carbide nano wire(SiC_(NW)) is a kind of one-dimensional(1D) nano wire possessing the promising properties of flexibility,great thermal insulation,and stability at high temperatures.An aerogel constructed by the SiC_(NW) will produce a great material with a promising material,the amazing SiC_(NW) aerogel.Here,a novel SiC_(NW) aerogel was fabricated consisting of quantities of β-SiC_(NW) of15-40 nm in diameter and tens to hundreds of micrometers in length.This SiC_(NW) aerogel possessed an ultralow density of 5.82 mg·cm^(-3),high-temperature resistance,and great thermal insulation with its thermal conductivities of0.063 W·m^(-1)·K^(-1) at 100℃ and 0.243 W·m^(-1)·K^(-1) at900℃ in He.Furthermore,the thermal insulation applicability of this aerogel was simulated.This study provides a promising way for designing and fabricating other multifunctional nanowire aerogels for high-temperature thermal insulation.

SiC nanowires with thickness-controlled SiO2 shells： Fabrication, mechanism, reaction kinetics and photoluminescence properties

SiC nanowires with thickness-controlled SiO2 shells have been obtained by a simple and efficient method, namely treatment of SiC/SiO2 core-shell nanowires in NaOH solution. The products were characterized by transmission electron microscopy （TEM）, high-resolution transmission electron microscopy （HRTEM）, X-ray diffraction （XRD）, Raman spectroscopy, infrared （IR） spectroscopy, and photoluminescence spectroscopy. The thickness of the SiO2 shell can be effectively controlled by selecting the appropriate processing time, and pure SiC nanowires were also obtained by alkaline cleaning in 1 mol-L-1 NaOH solution for 40 min at 70 ~C. A mechanism for the removal of the SiO2 shells has been proposed, and a two-phase reaction kinetic equation was derived to explain the rate of the removal of the SiO2 shells. The validity of this equation was verified by experiment. This work not only describes an effective experimental method for achieving SiC nanowires with thickness-controlled SiO2 coatings but also provides a fundamental theoretical equation with a certain level of generality. In addition, photoluminescence （PL） measurement results showed that the SiC nanowires sheathed with an optimum SiO2 thickness （3.03 nm） have better photoluminescence properties than either the bare SiC nanowires or SiC nanowires with thicker coatings of SiO2.

3D printing“wire-on-sphere”hierarchical SiC nanowires/SiC whiskers foam for efficient high-temperature electromagnetic wave absorption

Electromagnetic wave absorption(EWA)materials for use in extreme environments are of great interest.Herein,SiC nanowires/SiC whiskers(SiC_(nw)/SiC_(w))foam with unique“wire-on-sphere”hierarchical structure was developed for efficient high-temperature EWA.SiC_(w)were assembled to porous SiC_(w)spheres via spray drying and then SiC_(w)spheres were 3D printed to SiC_(w)foam.The catalyst-free precursor infiltration pyrolysis process was first developed to ensure that SiCnwcan intentionally grow in situ on the surface of SiC_(w)spheres,thereby achieving“wire-on-sphere”hierarchical structure foam.At room-temperature,the maximum electromagnetic wave effective absorption bandwidth(EAB_(max))and minimum electromagnetic wave reflection coefficient(RC_(min))of SiC_(nw)/SiC_(w)foam can reach 4 GHz and-57 dB,respectively.At600℃,the EAB_(max)and RC_(min) were 3 GHz and-15 dB,respectively.Furthermore,even oxidized at 1000-1500℃,SiC_(nw)/SiC_(w)foam can still retain EAB_(max)ranging from 2.7 to 3.9 GHz and RC_(min)ranging from-16 dB to-64 dB,because the formation of SiO_(2)layer with appropriate thickness can boost interfacial polarization and regulate impedance matching.The SiC_(nw)/SiC_(w)foam also shows the flexural strength as high as 17.05 MPa.All results demonstrate that the SiC nw/SiC_(w)foam is a promising EWA material for applications of harsh environments.And the material-independent“wire-on-sphere”hierarchical structure and its novel preparation process should find the widespread use in the design and fabrication of high-efficiency EWA micro-nano materials.

High-performance supercapacitors based on free-standing SiC@PEDOT nanowires with robust cycling stability

Conductive polymers as one of the candidate materials with pseudocapacitor behavior have inspired wide attentions,because of their high conductivity,fexibility,low cost and excellent processability.However,the intrinsically poor cycling stability induced by the volume change over the doping/dedoping redox process limits their practical applications.Herein,we report the exploration of electrodes with robust cycling capacity for supercapacitors(SCs),which are rationally designed by coating conductive poly(3,4-ethylenedioxythiophene)(PEDOT)around free-standing SiC nanowires using an all-dry oxidative chemical vaper deposition(oCVD)method.The as-constructed SiC@PEDOT nanowire architecture enables a specific capacitance of 26.53 m F/cm^(2)at 0.2 m A/cm^(2),which is~370%to that of SiC nanowire counterpart(7.04 m F/cm^(2)).Moreover,their aqueous-based SCs exhibit robust cycling stability with104%capacity retention after 10000 cycles,which is among the highest values achieved for PEDOTbased SCs.

Fabrication and properties of ultraviolet photo-detectors based on SiC nanowires

A new type of ultraviolet photo-detectors （UVPDs） based on a bundle of highly aligned SiC nanowires was fabricated and the photo-electric properties of the UVPDs including 1-V characteristics and time response were studied in this work. SiC nan- owires were prepared by pyrolysis of a polymer precursor with ferrocene as the catalyst by a CVD route. The diameters of SiC nanowires varied from 100 to 200 nm while they were some centimeters long and the SiC nanowires were with zinc blended cubic form （β-SiC） tested by X-ray diffraction. A bundle of nanowires was fixed onto two legs＇ base by conductive silver paste to form the UVPDs. The electrical measurement of the device showed a significant increase of current when the device was exposed to 254 nm UV light, and the rising time of the device is very short, but the falling time is relatively long. Our results show that the UVPDs based on SiC nanowires have excellent electrical and optical properties which can be potentially applied.

Morphologies evolution and mechanical behaviors of SiC nanowires reinforced C/(PyC-SiC)_(n)multilayered matrix composites

SiC nanowires reinforced C/(PyC-SiC)_(n)multilayered matrix composites(SM-CS for short)were prepared by combined with sol-gel and chemical vapor infiltration(CVI)method.Firstly,(PyC-Si OC);multilayered structure was formed by cycles of impregnation and deposition.Then SiOC was transformed into SiC by heat-treatment,and(PyC-SiC)_(n)multilayered structure would be obtained.At the same time,the PyC layer which was designed as the outmost layer could decrease gas supersaturation to form in-situ tubular SiC nanowires on the surface of multilayered structure.The results of three-point bending test showed that the maximum force of SM-CS composites was increased by the number of cycles of the preparation process,which were up to enhanced by 74.38%compared with C/C composite materials.The fracture surface showed that the improvement was due to the multiscale reinforcing system of(PyC-SiC)_(n)multilayered structure and SiC nanowires.Multilayered structure can protect carbon fibers and release stress concentration by induction of cracks.And the mechanical interlocking effect of SiC nanowires could reinforce bonding force of the remaining matrix.

Electromagnetic wave absorption and mechanical properties of SiC nanowire/low-melting-point glass composites sintered at 580°C in air

Si C nanowires are excellent high-temperature electromagnetic wave (EMW) absorbing materials. However, their polymer matrix composites are difficult to work at temperatures above 300℃, while their ceramic matrix composites must be prepared above 1000℃ in an inert atmosphere. Thus, for addressing the abovementioned problems, SiC/low-melting-point glass composites were well designed and prepared at 580℃ in an air atmosphere. Based on the X-ray diffraction results, SiC nanowires were not oxidized during air atmosphere sintering because of the low sintering temperature. Additionally, SiC nanowires were uniformly distributed in the glass matrix material. The composites exhibited good mechanical and EMW absorption properties. As the filling ratio of SiC nanowires increased from 5wt%to 20wt%, the Vickers hardness and flexural strength of the composite reached HV 564 and 213 MPa, which were improved by 27.7%and 72.8%, respectively, compared with the low-melting-point glass. Meanwhile, the dielectric loss and EMW absorption ability of SiC nanowires at 8.2–12.4 GHz were also gradually improved. The dielectric loss ability of low-melting-point glass was close to 0. However, when the filling ratio of SiC nanowires was 20wt%, the composite showed a minimum reflection loss (RL) of-20.2 dB and an effective absorption (RL≤-10 dB) bandwidth of2.3 GHz at an absorber layer thickness of 2.3 mm. The synergistic effect of polarization loss and conductivity loss in SiC nanowires was responsible for this improvement.

Quantum confinement effect in β-SiC nanowires

The quantum confinement effect is important in nanoelectronics and optoelectronics applications; however, there is a discrepancy between the theory of quantum confinement, which indicates that band-gap widening occurs only at small sizes, and experimental observations of band-gap widening in large-diameter nanowires （NWs）. This paper reports an obvious blue shift of the absorption edge in the UV-visible absorption spectra of SiC NWs with diameters of 50-300 nm. On the basis of quantum confinement theory and high-resolution transmission electron microscopy images of SiC NWs, band-gap widening in SiC NWs with diameters of up to hundreds of nanometers is fully explained; the results could help to explain similar band-gap widening in other NWs with large diameters.

Metal-organic framework derived hierarchical NiCo_(2)O_(4) triangle nanosheet arrays@SiC nanowires network/carbon cloth for flexible hybrid supercapacitors

To overcome the disadvantages of traditional powder electrodes,such as the insufficient performance,the aggregation of active materials,and the complex fabrication process,rationally constructing free-standing electrode materials with hierarchical architecture is an effective and promising method,which could further improve the electrochemical properties.Herein,using metal-organic framework nanoarrays(MOFNAs)as self-sacrificial templates and SiC nanowires(SiCNWs)network as nanoscale conductive skeletons,we successfully fabricated the hierarchical core-shell SiCNWs@NiCo_(2)O_(4)NAs on carbon cloth(CC)substrate.Taking advantages of structural merits,such as hierarchical porous triangle-like NiCo_(2)O_(4)NAs,the interwoven SiCNWs network and conductive CC substrate,when evaluated as a binder-free supercapacitor electrode,the CC/SiCNWs@NiCo_(2)O_(4)NAs shows a high specific capacitance of 1604.7 F g^(-1)(specific capacity of 222.9 mA h g^(-1))at 0.5 A g^(-1),good rate performance,and excellent cycling stability.Significantly,the hybrid supercapacitor assembled with CC/SiCNWs@NiCo_(2)O_(4)NAs as the cathode and MOF derived CC/SiCNWs@CNAs as the anode,could deliver a high specific density of 49.9 W h kg^(-1) at a specific power of 800 W kg^(-1),stable cycling performance,and good flexibility.Impressively,this feasible strategy for fabricating hierarchical structure displays great potential in the field of energy storage.

Selective growth of SiC nanowires in interlaminar matrix for improving in-plane strengths of laminated Carbon/Carbon composites

β-SiC nanowires(SiCNWs) were selectively grown in the interlaminar matrix with a volume fraction of0.65% by applying a pyrocarbon coating on carbon fibers, which realizes the proper reinforcement of C/C composites. The thickness of the pyrocarbon is optimized to 0.5 μm based on the analysis of in-situ fiber strengths with the fracture mirror method. The pyrocarbon coating increased the in-situ fiber strength by^7% and prevent brittle fracture of the composites. Compared with C/C, the interlaminar shear and flexural strength of SiCNW-C/C(10.06 MPa and 162.44 MPa) increase by 158% and 57%. Incorporating SiCNWs changes the crystallite orientations and refines the crystallite size of pyrocarbon matrix. The functions of SiCNWs vary with their loading density. When SiCNWs are sufficient in the matrix, they help reinforcing and improving the critical failure stress of the matrix. When their density decreases to a certain degree, SiCNWs help changing the crystallite orientations of pyrocarbon and toughening the matrix.

Photocatalytic C–X borylation of aryl halides by hierarchical SiC nanowire-supported Pd nanoparticles

Hierarchical Si C nanowire-supported Pd nanoparticles showed high photocatalytic activity for the C–X(X = Br, I) borylation of aryl halides at 30 °C. The Si C/Pd Mott-Schottky contact enhances the rapid transfer of the photogenerated electrons from Si C to the Pd nanoparticles. As a result, the concentrated energetic electrons in the Pd nanoparticles can facilitate the cleavage of C–I or C–Br bonds, which normally requires high-temperature thermal processes. We show that the present Pd/Si C photocatalyst is capable of catalyzing the transformation of a large variety of aryl halides to their corresponding boronate esters under visible light irradiation, with excellent yields.

SiC nanowire-based SU-8 with enhanced mechanical properties for MEMS structural layer design

In addition to being used for pattern transfer,the negative photoresist SU-8 iswidely used as a structural material in microelectromechanical systems(MEMS).Due to its good photopatternability,SU-8 has lower manufacturing costs than many other materials,but its mechanical properties are relatively weak to some extent,which limits its performance.The mechanical properties of epoxy-like SU-8 can be enhanced by addingmicro-or nano-fillers such as carbon nanotube,clay,and SiC nanowire,which have superior elastic modulus.In this study,SiC nanowires were used to improve the mechanical properties of SU-8 while the SU-8 retains its photopatternability.The SiC nanowires were uniformly dispersed in SU-8 by stirring and ultrasonication.SU-8 materials with different SiC nanowire contents were fabricated into dog bone samples by lithography.The elastic modulus,storage modulus,and damping factor of the samples were measured by the Dynamic mechanical analysis(DMA)Q800.The experiment result shows that the rigidity and toughness increased,and the damping reduced.The 2 wt%SiC nanowires-reinforced SU-8 had a 73.88%increase in elastic modulus and a 103.4%increase in elongation at break.Furthermore,a spring component made by SiC-doped SU-8 could withstand greater acceleration.The SiC nanowires-reinforced SU-8 has the potential tomeet higher requirements in the design andmanufacture of MEMS and greatly reduce the manufacturing costs of MEMS devices.

Layered Foam/Film Polymer Nanocomposites with Highly Efficient EMI Shielding Properties and Ultralow Reflection

Lightweight,high-efficiency and low reflection electromagnetic interference(EMI)shielding polymer composites are greatly desired for addressing the challenge of ever-increasing electromagnetic pollution.Lightweight layered foam/film PVDF nanocomposites with efficient EMI shielding effectiveness and ultralow reflection power were fabricated by physical foaming.The unique layered foam/film structure was composed of PVDF/SiCnw/MXene(Ti_(3)C_(2)Tx)composite foam as absorption layer and highly conductive PVDF/MWCNT/GnPs composite film as a reflection layer.The foam layer with numerous heterogeneous interfaces developed between the SiC nanowires(SiCnw)and 2D MXene nanosheets imparted superior EM wave attenuation capability.Furthermore,the microcellular structure effectively tuned the impedance matching and prolonged the wave propagating path by internal scattering and multiple reflections.Meanwhile,the highly conductive PVDF/MWCNT/GnPs composite(~220 S m^(−1))exhibited superior reflectivity(R)of 0.95.The tailored structure in the layered foam/film PVDF nanocomposite exhibited an EMI SE of 32.6 dB and a low reflection bandwidth of 4 GHz(R<0.1)over the Kuband(12.4-18.0 GHz)at a thickness of 1.95 mm.A peak SER of 3.1×10^(-4) dB was obtained which corresponds to only 0.0022% reflection efficiency.In consequence,this study introduces a feasible approach to develop lightweight,high-efficiency EMI shielding materials with ultralow reflection for emerging applications.

Boosted electromagnetic wave absorption performance from synergistic induced polarization of SiC_(NWs)@MnO_(2)@PPy heterostructures

In the last decade,electromagnetic pollution has caused people’s considerable attention.Developing absorbing material with low cost,lightweight,simple preparation,and high electromagnetic attenuation efficiency has become a feasible means to deal with this problem.In this work,core–shell SiC_(NWs)@MnO_(2)@PPy(NWs:nanowires,PPy:polypyrrole)heterostructures composed of SiC nanowires core,MnO_(2)nanosheets inter-layer,and PPy coating were successfully prepared through chemical vapor deposition and two-step electrodeposition process.Taking advantage of the interfacial polarization and dipole polarization,the obtained product displays excellent electromagnetic wave absorption performances with the minimum reflection loss(RLmin)of−50.59 dB when the matching thickness is 2.41 mm,and the optimal effective absorption bandwidth(EAB)value reaches to 6.64 GHz at a matching thickness of 2.46 mm,revealing that the SiC_(NWs)@MnO_(2)@PPy nanocomposite could be served as a promising electromagnetic wave absorbing material.On the basis of systematic analysis concerning the electromagnetic parameters,the dissipation process of the incident electromagnetic wave was demonstrated reasonably,which may provide a referable preparation strategy for novel heterostructures,especially nonmagnetic lightweight absorbing material.

Bifunctional SiC/Si_(3)N_(4) aerogel for highly efficient electromagnetic wave absorption and thermal insulation

SiC ceramics are attractive electromagnetic(EM)absorption materials for the application in harsh environment because of their low density,good dielectric tunable performance,and chemical stability.However,the performance of current SiC-based materials to absorb EM wave is generally unsatisfactory due to poor impedance matching.Herein,we report ultralight SiC/Si3N4 composite aerogels(~15 mg·cm^(−3))consisting of numerous interweaving SiC nanowires and Si3N4 nanoribbons.Aerogels were prepared via siloxane pyrolysis and chemical vapor reaction through the template method.The optimal aerogel exhibits excellent EM wave absorption properties with a strong reflection loss(RL,−48.6 dB)and a wide effective absorption band(EAB,7.4 GHz)at a thickness of 2 mm,attributed to good impedance matching and multi attenuation mechanisms of waves within the unique network structure.In addition,the aerogel exhibits high thermal stability in air until 1000℃and excellent thermal insulation performance(0.030 W·m^(−1)·K^(−1)).These superior performances make the SiC/Si_(3)N_(4) composite aerogel promising to become a new generation of absorption material served under extreme conditions.

Microwave induced in-situ formation of SiC nano wires on SiCNO ceramic aerogels with excellent electromagnetic wave absorption performance

Electromagnetic absorption(EMA)materials with light weight and harsh environmental robustness are highly desired and crucially important in the stealth of high-speed vehicles.However,meeting these two requirements is always a great challenge,which excluded the most attractive lightweight candidates,such as carbon-based materials.In this study,SiC_(nw)-reinfbrced SiCNO(SiC_(nw)/SiCNO)composite aerogels were fabricated through the in-situ growth of SiC_(nw) in polymer-derived SiCNO ceramic aerogels by using catalyst-assisted microwave heating at ultra-low temperature and in short time.The phase composition,microstructure,and EMA property of the SiC_(nw)/SiCNO composite aerogels were systematically investigated.The results indicated that the morphology and phase composition of SiC_(nw)/SiCNO composite aerogels can be regulated easily by varying the microwave treatment temperature.The composite aerogels show excellent EMA property with minimum reflection loss of -23.9 dB@13.8 GHz,-26.5 dB@10.9 GHz,and -20.4 dB@14.5 GHz and the corresponding effective bandwidth of 5.2 GHz,3.2 GHz,and 4.8 GHz at 2.0 mm thickness for microwave treatment at 600℃,800℃,and 1000℃,respectively,which is much better than that of SiCN ceramic aerogels.The superior EMA performance is mainly attributed to the improved impedance matching,multireflection,multi-interfacial polarization,and micro current caused by migration of hopping electrons.

Fabrication of SiC nanowire thin-film transistors using dielectrophoresis

The selection of solvents for SiC nanowires（NWs） in a dielectrophoretic process is discussed theoretically and experimentally.From the viewpoints of dielectrophoresis force and torque,volatility,as well as toxicity, isopropanol（IPA） is considered as a proper candidate.By using the dielectrophoretic process,SiC NWs are aligned and NW thin films are prepared.The densities of the aligned SiC NWs are 2μm^（-1）,4μm^（-1）,6μm^（-1）,which corresponds to SiC NW concentrations of 0.1μg/μL,0.3μg/μL and 0.5μg/μL,respectively.Thin-film transistors are fabricated based on the aligned SiC NWs of 6μm^（-1）.The mobility of a typical device is estimated to be 13.4cm^2/（V·s）.

Microstructural Evolution and Oxidation Resistance of Multi-walled Carbon Nanotubes in the Presence of Silicon Powder at High Temperatures

The microstructural evolution and oxidation resistance of multi-walled carbon nanotubes （MWCNTs） by di- rectly heating silicon powder and MWCNTs in a coke bed from 1000 to 1500 ~C are investigated with the aid of X-ray diffraction （XRD）, scanning electron microscopy （SEM）, high resolution transmission electron microscopy （HRTEM） and thermogravimetry-differential scanning calorimetry （TG-DSC）. The results showed that the morphology and microstructure of MWCNTs did not change much after being treated from 1000 ~C to 1200 ~C. An obvious SiC coating was formed on the surface of MWCNTs from 1300 to 1400 ~C. Up to 1500 ~C, nearly all the MWCNTs transformed into SiC nanowires. The oxidation resistance of the treated MWCNTs was improved compared with as-received ones. Non-isothermal kinetics showed that the oxidation activation energy of the treated MWCNTs reached 208 kJ/mol, much higher than 264 k J/tool of as-received ones.