SiO Vapor Research Articles

Conversion Process of Amorphous Si-Al-C-O Fiber into Nearly Stoichiometric SiC Polycrystalline Fiber

Tyranno SA (SiC-polycrystalline fiber, Ube Industries Ltd.) shows excellent heat-resistance up to 2000℃ with relatively high mechanical strength. This fiber is produced by the conversion process from a raw material (amorphous Si-Al-C-O fiber) into SiCpolycrystalline fiber at very high temperatures over 1500℃ in argon. In this conversion process, the degradation reaction of the amorphous Si-Al-C-O fiber accompanied by a release of CO gas for obtaining a stoichiometric composition and the subsequent sintering of the degraded fiber proceed. Furthermore, vaporization of gaseous SiO, phase transformation and active diffusion of the components of the Si-Al-C-O fiber competitively occur. Of these changes, vaporization of the gaseous SiO during the conversion process results in an abnormal SiC-grain growth and also leads to the non-stoichiometric composition. However, using a modified Si-Al-C-O fiber with an oxygen-rich surface, vaporization of the gaseous SiO was effectively prevented, and then consequently a nearly stoichiometric SiC composition could be obtained.

Synthesis of SiC/SiO2 nanochains by carbonthermal reduction process and its optimization

SiC/SiO2 nanochains have been successfully synthesized by carbonthermal reduction process with economical Silicon sources and Carbon sources in crucible furnace, which can largely promote the industrialization of the SiC synthesis process. The important effect of Si/SiO2 ratio, catalysts, gas atmosphere, Carbon source, reduction temperature on growth and morphology of the nanochains has been analyzed to optimize the synthesizing conditions, and so on. The characterization and performance of nanochains have been studied by IR, XRD, SEM, and TEM. The results indicate that the evenly dispersed SiC/SiO2 nanochains with uniform surface morphology and better crystallization can be obtained by calcination at 1500°C in N2 with activated carbon as Carbon sources, Si/SiO2=4 and Fe2O3 as catalyst. The synthesis mechanism of the nanochains has also been studied by TG and TEM, which verifies that the growth process obeys the VLS mechanism. The SiC nanochains are grown out from the Si–Fe–O systems and SiO vapors play an important role in the development of the reduction process.

Instantaneous formation of SiOx nanocomposite for high capacity lithium ion batteries by enhanced disproportionation reaction during plasma spray physical vapor deposition

Nanocomposite SiOx particles have been produced by a single step plasma spray physical vapor deposition (PS-PVD) through rapid condensation of SiO vapors and the subsequent disproportionation reaction. Core-shell nanoparticles, in which 15 nm crystalline Si is embedded within the amorphous SiOx matrix, form under typical PS-PVD conditions, while 10 nm amorphous particles are formed when processed with an increased degree of non-equilibrium effect. Addition of CH4 promotes reduction in the oxygen content x of SiO x , and thereby increases the Si volume in a nanocomposite particle. As a result, core-shell nanoparticles with x = 0.46 as anode exhibit increased initial efficiency and the capacity of lithium ion batteries while maintaining cyclability. Furthermore, it is revealed that the disproportionation reaction of SiO is promoted in nanosized particles attaining increased Si diffusivity by two orders of magnitude compared to that in bulk, which facilitates instantaneous composite nanoparticle formation during PS-PVD.

Experimental investigation of hydrogen adsorption in doped silicon-carbide nanotubes

Silicon-carbide nanotubes were synthesized by the reaction between SiO vapor as the Si source and carbon nanotubes as the carbon precursor. The resulting nanotubes were then purified with a hot, concentrated NaOH solution in order to remove the amorphous silica from their surface. The purified silicon-carbide nanotubes were doped with either potassium (K) or titanium (Ti). The hydrogen adsorption isotherms of the nanotubes were measured gravimetrically with the aid of a magnetic suspension microbalance. The results indicate that the K-doped silicon-carbide nanotubes exhibit promise as a hydrogen adsorption material, storing three times more hydrogen than the carbon nanotubes.

Dust in brown dwarfs and extra-solar planets

Context.Clouds form in atmospheres of brown dwarfs and planets. The cloud particle formation processes, seed formation and growth/evaporation are very similar to the dust formation process studied in circumstellar shells of AGB stars and in supernovae. Cloud formation modelling in substellar objects requires gravitational settling and element replenishment in addition to element depletion. All processes depend on the local conditions, and a simultaneous treatment is required.Aims.We apply new material data in order to assess our cloud formation model results regarding the treatment of the formation of condensation seeds. We look again at the question of the primary nucleation species in view of new (TiO2)N-cluster data and new SiO vapour pressure data.Methods.We applied the density functional theory (B3LYP, 6-311G(d)) using the computational chemistry package Gaussian09 to derive updated thermodynamical data for (TiO2)Nclusters as input for our TiO2seed formation model. We tested different nucleation treatments and their effect on the overall cloud structure by solving a system of dust moment equations and element conservation for a prescribed Drift-Phoenixatmosphere structure.Results.Updated Gibbs free energies for the (TiO2)Nclusters are presented, as well as a slightly temperature dependent surface tension forT= 500...2000 K with an average value ofσ∞= 480.6 erg cm-2. The TiO2seed formation rate changes only slightly with the updated cluster data. A considerably larger effect on the rate of seed formation, and hence on grain size and dust number density, results from a switch to SiO nucleation. The question about the most efficient nucleation species can only be answered if all dust/cloud formation processes and their feedback are taken into account. Despite the higher abundance of SiO over TiO2in the gas phase, TiO2remains considerably more efficient at forming condensation seeds by homogeneous nucleation. The paper discusses the effect on the cloud structure in more detail.

Reaction bonded silicon nitride foams: The influence of iron disilicide on microstructure and mechanical strength

In this study open-cell reaction bonded silicon nitride (RBSN) foams with high and controlled level of porosity have been produced based on the wet processing of Si-PMMA suspensions in the presence of iron disilicide (FeSi2) as an additive. The investigation focuses on the effect of FeSi2 on the mechanical strength, nitriding mechanisms, and microstructure of the RBSN foam. The nitriding reactions and the foam microstructures and properties were investigated under various nitriding conditions and for different FeSi2 contents.It was observed that 1wt% FeSi2 significantly increases the strength of the RBSN foams regardless of the nitriding temperature and atmosphere. The nitriding conditions have a strong influence on RBSN foam strength. The maximum strength was achieved under a N2 atmosphere at 1390°C with a foam porosity of 71vol%. The high porosity level of the foam significantly affects the nitriding mechanisms and consequently the foam microstructure, compared to “dense” RBSN ceramics. FeSi2 promotes the formation of both α- and β-Si3N4 phases depending on its concentration and its addition also increases the whisker content of the foam microstructure due to an increase in SiO and Si vapor formation.

Hydrogen sorption hysteresis and superior storage capacity of silicon-carbide nanotubes over their carbon counterparts

We report for the first time the results of an extensive experimental study of hydrogen sorption in silicon-carbide nanotubes (SiCNTs), which were synthesized using the reaction between SiO vapor and carbon nanotubes (CNTs) in an argon atmosphere in the temperature range 1200 °C–1500 °C. The as-synthesized SiCNTs were then purified using a sodium hydroxide solution in order to remove the side products of the synthesis reaction. The hydrogen sorption characteristics of the as-synthesized SiCNTs, as well as those of the purified SiCNTs were then measured at 25 °C and for pressures of up to 100 bars. The results reveal hysteresis between the adsorption and desorption isotherms, which we attribute to the presence of metal impurities and/or the multilayer structure of the nanotubes. The hydrogen storage capacity of the as-synthesized SiCNTs is similar to that of the CNTs, whereas for the purified SiCNTs it is 50% higher than that of the CNTs, in agreement with the results of molecular simulations reported previously. In addition, the hydrogen uptake rate in the SiCNTs is about five times faster than that in the CNTs and, in contrast with the CNTs, its desorption from SiCNTs is completely reversible under vacuum.

Surface modification of carbon nanofibers with SiC by heating different SiO vapor sources in argon atmosphere

Surface modification of carbon nanofibers with SiC by heating different SiO vapor sources in argon atmosphere

Synthesis of silicon carbide whiskers using reactive graphite as template

Abstract Silicon carbide whiskers have been synthesized by using reactive graphite as a template. Natural graphite flake was firstly activated using chemical oxidation and thermal oxidation methods. After that, the reactive graphite sources were mixed with silicon powder and heated in the coke bed at 1200 and 1400 °C. The structural evolution of graphite and morphologies of SiC whiskers were studied with the aids of XRD, SEM, TEM and EDS techniques. The results showed that natural graphite flake can be activated into reactive graphite such as oxidized graphite and expanded graphite with much more defects using thermal and chemical oxidation methods. The expanded graphite with a great deal of defects has higher reactivity than natural graphite flake and oxidized graphite and accelerates the formation of long and thick SiC whiskers. It is proposed that the vapor–solid mechanism is predominant for the growth of β-SiC whiskers in this system. During heating-up, Si or SiO vapors meet with the activated carbon atoms on graphite substrate to form SiC nucleus. Then these vapors continually deposit on the SiC nucleus following the SiC whiskers which grow along the 〈111〉 direction.

A SELF-CONSISTENT MODEL OF THE CIRCUMSTELLAR DEBRIS CREATED BY A GIANT HYPERVELOCITY IMPACT IN THE HD 172555 SYSTEM

Spectral modeling of the large infrared excess in the Spitzer IRS spectra of HD 172555 suggests that there is more than 10^19 kg of sub-micron dust in the system. Using physical arguments and constraints from observations, we rule out the possibility of the infrared excess being created by a magma ocean planet or a circumplanetary disk or torus. We show that the infrared excess is consistent with a circumstellar debris disk or torus, located at approximately 6 AU, that was created by a planetary scale hypervelocity impact. We find that radiation pressure should remove submicron dust from the debris disk in less than one year. However, the system's mid-infrared photometric flux, dominated by submicron grains, has been stable within 4 percent over the last 27 years, from IRAS (1983) to WISE (2010). Our new spectral modeling work and calculations of the radiation pressure on fine dust in HD 172555 provide a self-consistent explanation for this apparent contradiction. We also explore the unconfirmed claim that 10^47 molecules of SiO vapor are needed to explain an emission feature at 8 um in the Spitzer IRS spectrum of HD 172555. We find that unless there are 10^48 atoms or 0.05 Earth masses of atomic Si and O vapor in the system, SiO vapor should be destroyed by photo-dissociation in less than 0.2 years. We argue that a second plausible explanation for the 8 um feature can be emission from solid SiO, which naturally occurs in submicron silicate "smokes" created by quickly condensing vaporized silicate.

The Gas Phase Reaction Crystallization Mechanisms of Amorphous SiCN Ceramic Annealed in Vacuum

Microstructures of precursor-derived amorphous SiCN ceramics under conditions of vacuum and high temperature (1300°C, 1900°C) were observed by transmission electron microscopy. The gas phase reaction crystallization mechanisms of amorphous SiCN in vacuum was investigated based on thermodynamic calculation. The results indicate vacuum heat treatment can promote decomposition, induces gaseous species (SiO, CO and Si vapor) production. Gas phase reaction between Si(g), SiO(g) and C induces SiC nucleation inside amorphous ceramic network. Free Si- and N- can not form Si-N-enriched regions and N- is discharged in the form of N2. The crystallization of amorphous SiCN in vacuum displays significant difference to that in N2 and Ar.

Synthesis, structure and photoluminescence of cubic silicon carbide nanowires

Large quantities of 3C-SiC nanowires (NWs) are synthesised under atmospheric pressure from the ball-milled SiO powders and activated carbon. The samples on the stack of carbon are determined to be pure 3C-SiC. 3C-SiC NWs synthesised at 1250 and 1300°C have a length of tens to hundreds of micrometres and a diameter of 40–80 nm. A higher synthesising temperature of 1350°C results in NWs together with nanoparticles. The nucleation of 3C-SiC NWs relates to vapour–solid reaction between SiO vapour and activated carbon. The residual oxygen in the chamber leads to the generation of CO gas, whose reaction with SiO vapour relates to the growth of the nanowires. There are two peaks at 416 and 439 nm in the photoluminescence line of the nanowires, which are ascribed to size confinement effect, defects and the oxygen vacancy in the amorphous SiO2 layer.

Synthesis, structural and photoluminescence properties of SiO x nanospheres prepared by evaporation of silicon monoxide

We report synthesis of amorphous silicon oxide nano-spheres in bulk quantity by using a simple non-catalytic process based on thermal evaporation of silicon monoxide. Structural characteristics of the nano-spheres were investigated using high resolution scanning and transmission electron microscopes equipped with energy dispersive X-ray spectroscopy (EDAX). Selected area electron diffraction and EDAX analyses revealed that nano-spheres were amorphous and comprised of silicon oxide only with Si and O in an atomic ratio of ~ 1:2. Photoluminescence measurements showed that the SiO x nano-spheres had strong emission band in the blue region. Possible growth mechanism based on vapor-solid model has been discussed. The SiO vapors generated at high temperature react with O 2 to form SiO x vapors which subsequently condense on the substrates (placed in the appropriate temperature zone) to form SiO x nano-spheres.

Simple synthesis of ZrO 2/SiO x core/shell nanofibers using ZrSi 2 with gallium

ZrO 2/SiO x core/shell nanofibers with diameter ~ 50 nm were synthesized by the thermal oxidation of ZrSi 2 substrates with gallium. The crystalline ZrO 2 cores were grown with amorphous SiO x shells. It is proposed that the growth of crystalline ZrO 2 core was guided by the prior supersaturation of Zr species in the molten gallium film, whereas the amorphous SiO x shell could be attributed to the deposition of SiO vapor on the surface of ZrO 2 core. In addition, the ZrO 2/SiO x core/shell nanofibers show a wide visible photoluminescence (PL) emission at 480 nm, which should originate from the SiO x shells.

Crystal Growth of Silicate Phosphors from the Vapor Phase

Well-crystallized Ba2SiO4:Eu powders were grown on a substrate by the vapor phase reaction between a mixed powder (barium carbonate and europium oxide) and SiO gas. The vaporization of SiO occurs at 1400–1600◦C from the SiO2 source (or SiO powder) in a reducing atmosphere. The formed SiO gas was transported by 95 vol% Ar 5 vol% H2 gas and reacted with the raw material powders. The emission intensity of the Ba2SiO4:Eu phosphor synthesized by the new vapor phase technique is about 2.6 times higher than that of a conventional solid-state reaction sample. key words: phosphor, green emission, silicon monoxide, Ba2SiO4:Eu

Reaction Mechanism of SrSi2O2N2:Eu2+ Phosphor Prepared by a Direct Silicon Nitridation Method

Highly efficient yellow–green-emitting SrSi2O2N2:0.02Eu2+ phosphor with a well-dispersed and uniform morphology was synthesized by a direct silicon nitridation method. The related synthesis parameters including soaking temperature, holding time, silicon source, gas species, and flow rate, have been studied, respectively, in terms of phase compositions and luminescent properties. The possible reaction mechanism was also proposed based on the results obtained. Silicon is assumed to be transported to the surface of decomposed SrO particles in the form of SiO or Si vapor, and nitridation reaction occurs here to form SrSi2O2N2:0.02Eu2+ phosphor directly. The particle size and morphology of phosphors obtained are controlled by the raw oxide materials instead of silicon. CO2 from the decomposition of SrCO3 plays an important role in the transportation of Si, which reacts with Si to form SiO vapor and introduces excessive oxygen into the phosphor obtained, resulting in an O/N ratio close to SrSi2O2N2. Direct silicon nitridation is proved to be a simple, cost-effective, facile, and potential morphology-controlling method to synthesize (oxo)nitridosilicate phosphors of excellent properties.

Growth of amorphous silica nanowires using nickel silicide catalyst by a thermal annealing process

We report on the growth of NiSi 2-catalyzed amorphous SiO 2 nanowires by rapid-thermal-annealing of Ni(40 nm)/poly-Si(60 nm)/SiO 2(110 nm)/undoped Si substrate structures at 900 °C in N 2 ambient. The diameter of the nanowires is dependent on the diameter of the NiSi 2 catalyst particles; the former is about 16–45% smaller than the later. Considering the presence of the nanoparticles located at the tip of the nanowires, the growth behavior of the a-SiO 2 nanowires is described in terms of the generation of SiO vapor and the VLS mechanism.

Formation and analysis of SiC coating layer on carbon short fiber

AbstractSiC‐coated carbon fibers for aluminum–carbon composites were synthesized with carbothermal reduction. For the SiC layer formation on the surface of carbon fiber, silica sol was coated and heat treated at a temperature of 1460 and 1500 °C. SiC layer formation by carbothermal reduction complete at above 1500 °C for 1 h. From TGA in atmosphere, SiC coating effectively protected against the oxidation of graphite. The thickness of the SiC coating layer ranged from 150 to 300 nm and the layer was formed homogenously along the surface of carbon fiber. As the first step of reaction for SiC formation, it is generally accepted that SiO is formed and vaporized in interface between fiber and silica. It is analogized in this work that the SiO vapor reacts directly with carbon fiber, resulting in formation of SiC phase. From the TEM results, the SiC layer was confirmed as a ZnS‐type FCC structure. Copyright © 2010 John Wiley & Sons, Ltd.

탄화규소 나노섬유의 고온 대기 및 SO2가스분위기에서의 부식물성

The SiO vapor that was generated from a mixture of Si and was reacted at for 2 h under vacuum with carbon nanofibers to produce SiC nanofibers having an average diameter of 100~200 nm. In order to understand the gas corrosion behavior, SiC nanofibers were exposed to air up to . SiC oxidized to amorphous , but its oxidation resistance was inferior unlike bulk SiC, because of high surface area of nanofibers. When SiC nanofibers were exposed to Ar-1% atmosphere, SiC oxidized to amorphous , without forming , owing to the thermodynamic stability of .

Manufacturing aspects of oxide nanowires

Exploring the mass manufacturing aspects of nanostructures can enable the transition from laboratory-based research into a commercial product. Among the several one-dimensional nanostructures, oxide nanomaterials have a wide variety of applications including energy harvesting, photonics and biosensing applications. In this article, mass manufacturing aspects of bottom-up grown silica nanowires on silicon (Si) by metal thin film catalysis have been detailed. The investigation reports on (a) a growth model derived from studying nanowire nucleation as a function of heating time, (b) nanowire growth rate estimation via weight differential of the Si substrate before and after growth, and (c) reusability of the Si substrate for nanowire growth. Silica nanowires were found to grow on Pd coated Si substrate in an open tube furnace at 1100 °C with Ar as a carrier gas and a Si support wafer. Nanowires nucleated following a combination of Vapor Liquid Solid (VLS) and Oxide Assisted Growth (OAG) mechanisms conducive for mass manufacturing. The role of SiO vapor was found to be critical in the growth of the wires. Further, five distinct growth regimes were identified while estimating the growth rate. Experimental observations indicated the non-reusability of the Si substrate after one time growth due to depletion of catalyst.