SiC Nanocrystals Research Articles

Influence of microstructure evolution on temperature-dependent dielectric and electromagnetic wave absorption properties of PDCs-SiC

Optimal microstructures for materials getting excellent high-temperature electromagnetic wave absorption (EMA) performance are still not clear since most research has been focused on room-temperature EMA performance. In this paper, amorphous ceramics embedded with isolated SiC nanocrystals or SiC network were obtained by taking advantage of microstructure evolution of polymer derived ceramics (PDCs). Relations between microstructure evolution and temperature-dependent dielectric loss/EMA properties were explored. EMA properties of amorphous PDCs-SiC and sample containing isolated SiC nanocrystals were improved as temperature increased. At 600 °C, reflection loss was smaller than −10 dB over the entire X band for sample thickness in the range of 3.03–3.11 mm, and the minimum value reached −41.8 dB. After formation of SiC nanocrystal network, EMA property of sample decreased with increasing temperatures, which was ascribed, primarily, to rapid increase of conduction loss and consequently deteriorating impedance matching condition. The results indicated that, to obtain excellent EMA property, percolation of EMA agents should be avoided to minimize strong reflection at high temperature. The mechanisms for temperature-dependent dielectric loss and EMA were discussed.

Structural and Optical Properties of Silicon Carbide Powders Synthesized from Organosilane Using High-Temperature High-Pressure Method.

The development of new strategies for the mass synthesis of SiC nanocrystals with high structure perfection and narrow particle size distribution remains in demand for high-tech applications. In this work, the size-controllable synthesis of the SiC 3C polytype, free of sp2 carbon, with high structure quality nanocrystals, was realized for the first time by the pyrolysis of organosilane C12H36Si6 at 8 GPa and temperatures up to 2000 °C. It is shown that the average particle size can be monotonically changed from ~2 nm to ~500 nm by increasing the synthesis temperature from 800 °C to 1400 °C. At higher temperatures, further enlargement of the crystals is impeded, which is consistent with the recrystallization mechanism driven by a decrease in the surface energy of the particles. The optical properties investigated by IR transmission spectroscopy, Raman scattering, and low-temperature photoluminescence provided information about the concentration and distribution of carriers in nanoparticles, as well as the dominant type of internal point defects. It is shown that changing the growth modes in combination with heat treatment enables control over not only the average crystal size, but also the LO phonon—plasmon coupled modes in the crystals, which is of interest for applications related to IR photonics.

Highly spherical SiC nanoparticles grown in nonthermal plasma

AbstractSilicon carbide (SiC) nanoparticles have excellent properties and varied possible applications. However, the synthesis of this material usually requires high temperatures due to its high melting point (2730°C). In this study, we report on a synthesis of highly spherical SiC nanocrystals (50–150 nm) using nonthermal plasma, a method not yet widely employed for the synthesis of SiC, but generally very suitable for high‐melting‐point materials. We perform the synthesis in a two‐stage reactor, where Si nanocrystals are synthesized in the first stage and these are used as growth seeds for SiC in the second stage. The Si nanocrystals remain inside the SiC nanoparticles after the synthesis. The produced SiC nanocrystals exhibit yellow‐orange naked‐eye‐visible photoluminescence.

Effects of Microwave-Assisted Heat Treatments on the Nanostructural Evolution of Amorphous Silicon Oxycarbide Thin Films.

This study investigated the effects of heat treatment on changes in the nanostructure of amorphous silicon oxycarbide thin films. Hydrogenated amorphous silicon oxycarbide (a-Si0.6C0.3O0.1:H) thin films were prepared via plasma-enhanced chemical vapor deposition. The films were subjected to post-deposition heat treatments via microwave-assisted heating, which resulted in the formation of nanocrystals of SiC and Si in the a-Si0.6C0.3O0.1:H matrix at temperatures as low as ~800 °C. The crystallization activation energies of SiC and Si were determined to be 1.32 and 1.04 eV, respectively lower than those obtained when the sample was heat-treated via conventional heating (CH). Microwaves can be used to fabricate nanocrystals at a temperature approximately ~300 °C lower than that required for CH. The optical and nanostructural evolutions after post-deposition heat treatments were examined using photoluminescence (PL) and X-ray diffraction. The position of the PL peaks of the nanocrystals varied from ~425 to ~510 nm as the annealing temperature was increased from 800 to 1000 °C. In this study the optical band gap of SiC and Si varied from ~2.92 to ~2.40 eV and from ~2.00 to ~1.79 eV, as the size of the SiC and Si nanocrystals varied with respect to the heating temperature and isothermal holding time, respectively.

Ionization-bombardment assisted deposition of MXene/SiC heterostructure for micro-supercapacitor with enhanced sodium storage

Improving the energy density of MXene micro-supercapacitors (MSCs) is still a challenge because the MXene nanosheets will easily agglomerate, leading to irreversible reduction of capacity. We present SiC/MXene heterostructure by ionization-bombardment assisted deposition synthesis. Under high voltage, C–C bond and Si-O bond are broken by electron bombardment. After arc sintering, C atom and Si atom form stable Si-C bond with lower bond energy, and then form SiC nanocrystals. Furthermore, SiC nanocrystals were patterned into mesoporous SiC nanonesh to enhance the specific surface area. The surface of SiC framework is full of charge, which makes the MXene nanosheets uniformly deposited and self-assembled to form heterostructure. SiC/MXene heterostructure has low barrier, strong bonding force and fast electron migration rate. SiC nanomesh provide a conductive network, and also serve as a framework to support the MXene film, increasing ion storage space, creating an interconnected path for the conduction of Na ions. The specific capacity of the MXene/SiC MSC is up to 97.8 mF cm−2 at a current density of 1 A cm−2. The fabrication of high-performance nanostructures leads to large-scale production and micro-energy storage devices.

SiBCN-reduced graphene oxide (rGO) ceramic composites derived from single-source-precursor with enhanced and tunable microwave absorption performance

SiBCN-reduced graphene oxide (SiBCN-rGO) ceramic composites with different rGO content derived from a single-source precursor were successful synthesized by polymer derived ceramics routes to tune their dielectric properties and electromagnetic wave absorption capacity. SiBCN-rGO ceramic composites with 6 wt% GO annealed at 1300 °C possessed the best microwave absorption performance obtaining the minimum reflection coefficient (RCmin) of −62.71 dB at 10.72 GHz, and the thin thickness of 2.17 mm. Adjusting its thickness to 1.58 mm could obtain the maximum effective absorption bandwidth (RC < −10 dB) of 4.32 GHz. The microwave absorption mechanisms of the as-prepared SiBCN-rGO ceramic composites were mainly attributed to the conductive loss and polarization loss induced by the synergistic effects of amorphous SiBCN matrix, rGO and SiC nanocrystals.

Investigation of Microstructure and Nanoindentation Hardness of C+ &amp; He+ Irradiated Nanocrystal SiC Coatings during Annealing and Corrosion

The microstructure and nanoindentation hardness of unirradiated, irradiated, annealed and corroded SiC coatings were characterized. Irradiation of 400 keV C+ and 200 keV He+ with approximately 10 dpa did not cause obvious amorphous transformation to nanocrystal SiC coatings and induced helium bubbles with 2–3 nm dimension distributed uniformly in the SiC matrix. High temperature annealing resulted in the transformation of SiC nanocrystals into columnar crystals in the irradiated region. Line-shaped bubble bands formed at the columnar crystal boundaries and their stacking fault planes and made the formation of microcracks of hundreds of nanometers in length. Meanwhile, some isolated helium bubbles distributed in SiC grains still maintained a size of 2–3 nm, despite annealing at 1200 °C for 5 h. The SiC coating showed excellent corrosion resistance under high-temperature, high-pressure water. The weight of the sample decreased with the increase of corrosion time. The nanoindentation hardness and the elastic modulus increased significantly with C+ and He+ irradiation, while their values decreased with high-temperature annealing. An increase in the annealing temperature led to an increased reduction in the values. Corrosion caused the decrease of nanoindentation hardness and the elastic modulus in the whole test depth range, whether the samples were irradiated or unirradiated.

Silicon-Carbide (SiC) Nanocrystal Technology and Characterization and Its Applications in Memory Structures.

Colloidal cubic silicon-carbide nanocrystals have been fabricated, characterized, and introduced into metal–insulator–semiconductor and metal–insulator–metal structures based on hafnium oxide layers. The fabricated structures were characterized through the stress-and-sense measurements in terms of device capacitance, flat-band voltage shift, switching characteristics, and retention time. The examined electrical performance of the sample structures has demonstrated the feasibility of the application of both types of structures based on SiC nanoparticles in memory devices.

Tunable EMW absorption properties of a novel SiC–SiO2/3Al2O3·2SiO2 composite ceramic

A kind of SiC–SiO2/3Al2O3·2SiO2 composite ceramic was successfully prepared by the technique of chemical vapor infiltration (CVI) for enhanced electromagnetic wave (EMW) absorption properties. Due to the weak EMW absorption performances of the pure SiO2/3Al2O3·2SiO2 ceramic, SiC nanocrystals as absorbers were introduced to improve the EMW absorption performances. The existences of SiC nanocrystals are beneficial for enhancing the EMW absorption performances, and the SiC-nanocrystal contents can be tailored in the SiO2/3Al2O3·2SiO2 ceramics. Although the SiC nanocrystals are helpful for the dielectric and EMW absorption properties, they are unfavorable for impendence matching owing to weak-absorption and strong-reflection if the SiC nanocrystals exceed the threshold value. According to the investigation, it can attain wonderful EMW absorption properties when the SiC-nanocrystal content is 77.1 wt% in the composite ceramic. The results show that the value of RC can reach −22.4 dB at 9.9 GHz, and the EAB is the complete X-band for the SiC–SiO2/3Al2O3·2SiO2 composite ceramics. And they can be tailored for the dielectric properties to obtain the excellent EMW absorption properties in the prepared SiC–SiO2/3Al2O3·2SiO2 composite ceramics. The mechanisms of EMW consumptions are Debye relaxation, dipole polarization, interfacial polarization, multi-reflection, multi-scattering and conductivity loss in the SiC–SiO2/3Al2O3·2SiO2 composite ceramics.

Role of in-situ formed free carbon on electromagnetic absorption properties of polymer-derived SiC ceramics

In order to enhance dielectric properties of polymer-derived SiC ceramics, a novel single-source-precursor was synthesized by the reaction of an allylhydrido polycarbosilane (AHPCS) and divinyl benzene (DVB) to form carbon-rich SiC. As expected, the free carbon contents of resultant SiC ceramics annealed at 1600 °C are significantly enhanced from 6.62 wt% to 44.67 wt%. After annealing at 900–1600 °C, the obtained carbon-rich SiC ceramics undergo phase separation from amorphous to crystalline feature where superfine SiC nanocrystals and turbostratic carbon networks are dispersed in an amorphous SiC(O) matrix. The dielectric properties and electromagnetic (EM) absorption performance of as-synthesized carbon-rich SiC ceramics are significantly improved by increasing the structural order and content of free carbon. For the 1600 °C ceramics mixed with paraffin wax, the minimum reflection coefficient (RCmin) reaches –56.8 dB at 15.2 GHz with the thickness of 1.51 mm and a relatively broad effective bandwidth (the bandwidth of RC values lower than –10 dB) of 4.43 GHz, indicating the excellent EM absorption performance. The carbon-rich SiC ceramics have to be considered as harsh environmental EM absorbers with excellent chemical stability, high temperature, and oxidation and corrosion resistance.

Boron-doped silicon carbide (SiC) thin film on silicon (Si): a novel electrode material for supercapacitor application

In this work, we report the synthesis of silicon carbide (SiC) thin film on silicon by modified chemical vapour deposition technique using boron-doped liquid polycarbosilane as a precursor. Subsequent microscopic and physical characterizations of this film show the presence of SiC nanocrystal along with boron in the SiC thin film. The electrochemical characterizations of the film shows a highest capacitance of 232F/g at 2.2A/g current density from Galvanostatic charging–discharging method with good cyclic stability upto 2000 cycles. The high capacitance value is attributed to the surface defect states and amorphous carbon which acts as the charge active sites. The result further infers that the (B)SiC/Si is a promising electrode material for high-performance energy storage application.

Interaction of Fluorocarbon with Silicon Monoxide and Processes of SiC Nanowire Formation

A study of the processes of the thermal carbonization of silicon monoxide in the presence of nonstoichiometric carbon monofluoride demonstrated that raising the annealing temperature of mixtures of SiO and CFx powders in a quasi-closed volume to 1000°C and higher leads to the formation of whisker-like SiC nanocrystals. It is found that, in parallel with the known crystallization of SiC nanowires as a result of the interaction of SiO vapor with carbon monoxide, the previously undescribed interaction of CO with gas-phase silicon difluoride SiF2 takes part in their formation. At temperatures below 1200°C, this reaction is dominant and makes the most pronounced contribution to the yield of SiC nanowires.

Excellent electromagnetic wave absorption of MOF/SiBCN nanomaterials at high temperature

Electromagnetic wave absorbing materials at high-temperature are urgently needed for stealth aircrafts or aero-engines worked in harsh environments. In this contribution, cobalt-containing siliconboron carbonitride (MOF/SiBCN) nanomaterials were prepared by pyrolyzing metal–organic framework, i.e. cobalt 2-methylimidazole (ZIF-67), and hyperbranched polyborosilazane. The rhombic dodecahedral ZIF-67 and cobalt element promoted in situ formation of dielectric loss phases, including SiC nanocrystals, CoSi nanocrystals and turbostratic carbons. The ZIF-67/SiBCN nanomaterials showed excellent microwave absorption both at room and elevated temperature. The minimum reflection coefficient (RCmin) was −51.6 dB and effective absorption bandwidth (EAB) is 3.93 GHz at room temperature. At an elevated temperature of 600 °C, the RCmin reached −30.29 dB and EAB covered almost the whole X-band (3.95 GHz, 8.45–12.4 GHz). The ZIF-67/SiBCN nanocomposites are promising and useful platform for microwave absorbing materials at high-temperature. It may shed light on the downstream applications in designing next generation areo-engines and stealth aircrafts.

Formation of SiC nanocrystals prepared by sol-gel processing of green carbon sources and DFT calculations

SiC nanocrystals are synthesized by sol-gel processing with tetraethoxysilane and green carbon sources (sugar, molasses, and stevia extract) as starting materials. The reactions of carbon precursors and silicon were investigated using density functional theory. To obtain the discrepancy between the energy levels of the interacting orbitals of precursors, molecules were optimized using B3LYP/6-31+G(d,p) method. XRD, FE-SEM, TG-DTA and FTIR analysis were implemented in order to compare the efficiency of different carbon sources. According to XRD experiments, SiC nanocrystals prepared by sugar and molasses had no contamination, while the sample prepared by stevia has impurity in the form of carbon and silica. TG-DTA results revealed that this difference is due to the fact that the carbon source in stevia did not react efficiently with silicon. Moreover, based on the DFT study and HOMO and LUMO analysis on the reactive energy of silicon and carbon precursors, it is revealed that sugar has the best reactivity among carbon sources for SiC formation.

Emerging One-/Two-Dimensional Heteronanostructure Integrating SiC Nanowires with MoS2 Nanosheets for Efficient Electrocatalytic Hydrogen Evolution.

MoS2 has emerged as a good application prospect in the electrocatalytic hydrogen evolution reaction (HER). Nevertheless, the catalytic activity of MoS2 is greatly restricted by its inferior electrical conductivity, inadequate exposure of active edge sites, and sluggish water dissociation dynamics. Herein, a 1D/2D heteronanostructure composed of SiC nanowires wrapped with MoS2 nanosheets was prepared via the hydrothermal synthesis of MoS2 on highly connected SiC nanowires (SiCnw). The nanocomposites exhibit an emerging tectorum-like morphology with interface connections of C-Mo bonds, which benefit the efficient interfacial transmission of electrons. Due to the synergetic catalytic effects of MoS2 nanosheets and SiC nanowires, the MoS2/SiCnw nanocomposites possess efficient catalytic performance with a low Tafel slope (55 mV/dec). SiC nanocrystals could reduce the activated water dissociation energy barrier, and the morphologies of connected nanowires could improve the active site exposure and charge transport. The nanocomposites possess favorable hydrogen adsorption free energy from density functional theory (DFT) calculations. The electrocatalytic performance of MoS2/SiCnw nanocomposites could be further improved by assembling the nanocomposites on a carbon fiber paper to enhance the electronic transmission efficiency.

Dissociation of tetramethylsilane for the growth of SiC nanocrystals by atmospheric pressure microplasma

AbstractWe report on mass spectrometry of residual gases after dissociation of tetramethylsilane (TMS) during the synthesis of silicon carbide (SiC) nanocrystals (NCs) by an atmospheric pressure microplasma. We use these results to provide details that can contribute to the understanding of the formation mechanisms of NCs. Mass spectrometry reveals the presence of high‐mass polymerization products supporting the key role of neutral fragments and limited atomization. On this basis, we found that the loss of methyl groups from TMS, together with hydrogen abstraction, represents important paths leading to nucleation and growth. The combination of TMS concentration and NC residence time controls the NC mean size and the corresponding distributions. For higher precursor concentrations, the reaction kinetics is sufficiently fast to promote coalescence.

Microstructure and mechanical properties of porous SiC ceramics by carbothermal reduction and subsequent recrystallization sintering

ABSTRACT Porous SiC ceramics were prepared by a new approach combination of carbothermal reduction and subsequent recrystallization sintering. Firstly, micro-sized SiC particles were used as the skeleton, and SiC spherical nanocrystals were in-situ synthesized by the vapor-solid reaction between carbon nanoparticles and silicon monoxide vapor. The shape and diameter of SiC nanocrystals are related to the pristine carbon nanoparticles, showing a shape memory effect. After recrystallization sintering, high purity α-SiC porous ceramics with tailored necking area were obtained by the evaporation-condensation of SiC nanocrystals. A linear relation was revealed between the flexural strength and the value of d/d0 of porous recrystallized SiC (RSiC) ceramics (the neck diameter to coarse micron-sized grains diameter ratio). The necking area and d/d0 value increased with the SiC nanocrystals content, due to the higher saturated vapor pressure resulted in high mass mobility to neck area of SiC nanoparticles. As a result, a remarkable value of d/d0 ~ 99% and outstanding flexural strength of 75.7 MPa could be achieved for the porous RSiC ceramics with ~42% porosity by adding 20 wt.% nano-sized SiC sintered at 1950°C for 2 h.

Controlling the Energy-Level Alignment of Silicon Carbide Nanocrystals by Combining Surface Chemistry with Quantum Confinement.

The knowledge of band edges in nanocrystals (NCs) and quantum-confined systems is important for band alignment in technologically significant applications such as water purification, decomposition of organic compounds, water splitting, and solar cells. While the band energy diagram of bulk silicon carbides (SiCs) has been studied extensively for decades, very little is known about its evolution in SiC NCs. Moreover, the interplay between quantum confinement and surface chemistry gives rise to unusual electronic properties and remains barely understood. Here, we report for the first time the complete band energy diagram of SiC NCs synthesized such that they span the regime from strong to intermediate to weak quantum confinement. The absolute positions of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals show clear size dependence. While the HOMO level follows the expected behavior for quantum-confined electronic states, the LUMO energy shifts below the bulk conduction band minimum, which cannot be explained by a simple quantum confinement caused by the size effect. We show that this effect is a result of the interplay between quantum confinement and the formation of surface states due to partial and site-selective oxygen passivation.

Room-Temperature Defect Qubits in Ultrasmall Nanocrystals.

There is an urgent quest for room-temperature qubits in nanometer-sized, ultrasmall nanocrystals for quantum biosensing, hyperpolarization of biomolecules, and quantum information processing. Thus far, the preparation of such qubits at the nanoscale has remained futile. Here, we present a synthesis method that avoids any interaction of the solid with high-energy particles and uses self-propagated high-temperature synthesis with a subsequent electrochemical method, the no-photon exciton generation chemistry to produce room-temperature qubits in ultrasmall nanocrystals of sizes down to 3 nm with high yield. We first create the host silicon carbide (SiC) crystallites by high-temperature synthesis and then apply wet chemical etching, which results in ultrasmall SiC nanocrystals and facilitates the creation of thermally stable defect qubits in the material. We demonstrate room-temperature optically detected magnetic resonance signal of divacancy qubits with 3.5% contrast from these nanoparticles with emission wavelengths falling in the second biological window (1000–1380 nm). These results constitute the formation of nonperturbative bioagents for quantum sensing and efficient hyperpolarization.

Lightweight and flexible 3D graphene microtubes membrane for high-efficiency electromagnetic-interference shielding

Ultralight, flexible and high-performance electromagnetic-interference (EMI) shielding performance materials are urgently required in the areas of aircraft/aerospace, portable and wearable smart electronics. Benefiting from the outstanding properties of graphene and the unique architectures, 3D assembled graphene structures have been widely applied in the field of EMI-shielding. Herein, for the first time, using Si3N4 nanowires as self-sacrificial templates with the thermal decomposition characteristics, 3D graphene microtubes (3DGMTs) membrane is fabricated by plasma enhanced chemical vapor deposition (PECVD) method, exhibiting the characteristics of free-standing and flexibility. 3DGMTs membrane assembled with silicon carbide (SiC) nanocrystals decorated edge-rich vertical graphene nanosheets (VGNs) exhibits superb EMI shielding effectiveness (SE) of around 38 dB in the frequency range of 8.2~12.4 GHz at a density of 0.0036 g cm−3 and a thickness of 1.5 mm. Considering the ultralow density and thickness, 3DGMTs membrane shows a high specific SE (SSE, defined as SE divided by mass density) of 10,556 dB cm3 g−1 in X-band, which far surpasses that of almost all the reported materials. Remarkably, our work not only provides a new idea for preparation of hollow tubular carbon materials for a wide range of applications, but also presents some fundamental insights for edge-rich VGNs applied in EMI shielding.