Si Vapor Research Articles

Molten-salt-mediated synthesis of SiC nanowires for microwave absorption applications

Silicon carbide (SiC) nanowires were synthesized by a reaction of multiwall carbon nanotubes (MWCNTs) and silicon vapor from molten salt medium at 1250 °C. The phase, morphology, and microstructure of the nanowires were systemically characterized by X-ray diffraction, field emission scanning electron microscopy, and high resolution transmission electron microscopy. The results revealed that the nanowires were of single-crystalline β-SiC phase with the growth direction along [111] and had diameters of 20–80 nm and lengths up to several tens of micrometers. The molten salt introduced facilitated the evaporation of Si (vapor) onto MWCNTs (solid) and the growth of SiC nanowires followed the vapor–solid process. The investigation of microwave absorbability indicated that a minimum reflection loss of −17.4 dB at 11.2 GHz could be achieved with 30 wt% SiC nanowires as the filler in the silicone matrix. The attenuation of microwave could be attributed to the dielectric loss and a possible absorption mechanism was also discussed.

Si Vapor Chamber Integrated with Through Silicon Via for 3D Packaging

We demonstrate an etched silicon vapor chamber integrated with a through-silicon via (TSV) for 3D packaging. The Si vapor chamber chip enables low mismatch in the thermal expansion coefficient of a Si-LSI chip and provides a new heat dissipation path for 3D-LSI inter layer cooling. For the first prototype of the vapor chamber, an outside 33-mm × 33-mm chip consisting of a 25-mm × 25-mm area for the vapor chamber, a wick structure 30-μm high, and a vapor passage 100-μm high is developed. In-situ observation of the behavior of the working fluid through the cover glass and heat transfer enhancement is successfully demonstrated. The improvement rate of thermal resistance is 7.1% compared to a test chip without working fluid. Next, the fluid flow of a second vapor chamber prototype consisting of the first prototype integrated with a TSV structure using a Si pillar of 150-μm diameter is investigated. Thermal resistance and droplet observation conducted to evaluate the influence of the TSV. The operation of the vapor chamber is confirmed when a Si pillar is arranged to a coarse pitch of more than 500 μm. A droplet is generated and the vapor passage is partially obstructed. However, the droplet eventually degenerated and the performance of the vapor chamber is maintained. When the Si pillar is arranged to a fine pitch of 200 μm, the entire vapor passage is blocked during the liquid charging process, and no improvement is observed in the thermal resistance of the chip.

Spin–Spin Correlation Length in MnSi Thin Films

Thin MnSi films have been prepared on Si(111) by molecular beam epitaxy under simultaneous evaporation of Mn and Si. RHEED analysis in combination with TEM investigation reveals that this codeposition technique leads to smooth films. Resistivity measurements show a metallic behavior that is qualitatively the same as for bulk MnSi. The magnetic ordering temperature T ord of 43 K, which has been determined from resistivity data and by SQUID magnetometry, is considerably enhanced compared to the bulk value. For the thinnest films, however, a decrease of T ord is observed which is explained by the reduction of the spin–spin interaction near the surfaces of the film. Recently, it has been stated, that the spin–spin correlation length is much larger than known from other itinerant magnetic materials. In contrast, we show that the coupling range can be estimated as 7 monolayers and does not exceed the values of common magnets.

Infiltration mechanism of diamond/SiC composites fabricated by Si-vapor vacuum reactive infiltration process

Abstract Dense diamond/SiC composites were fabricated by Si vapor vacuum reactive infiltration of carbon-containing diamond porous preform at 1600 °C for 1 h. The microstructural evolution of the composites was investigated. The infiltration mechanisms during reactive infiltration were discussed. The composite consists of diamond, β-SiC and a small amount of Si. Epitaxial growth of nano-sized SiC on diamond and graphite surfaces occurred due to the diffusion-reaction mechanism in the initial stage of infiltration. Growth of micron-sized SiC with no preferential orientation was controlled by solution-precipitation mechanism in the final stage. The infiltration process was determined both by molecular diffusion and capillary effects. Explosive evaporation of molten Si, volume expansion of the solids and heat release during the reaction were the key factors contributing to the rapid densification of diamond/SiC composites. High thermal conductivity (580 W m −1 K −1 ) and low density (3.33 g cm −3 ) of the composites were beneficial to thermal management applications.

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.

Concentration gradient induced morphology evolution of silica nanostructure growth on photoresist-derived carbon micropatterns

The evolution of silica nanostructure morphology induced by local Si vapor source concentration gradient has been investigated by a smart design of experiments. Silica nanostructure or their assemblies with different morphologies are obtained on photoresist-derived three-dimensional carbon microelectrode array. At a temperature of 1,000°C, rope-, feather-, and octopus-like nanowire assemblies can be obtained along with the Si vapor source concentration gradient flow. While at 950°C, stringlike assemblies, bamboo-like nanostructures with large joints, and hollow structures with smaller sizes can be obtained along with the Si vapor source concentration gradient flow. Both vapor–liquid-solid and vapor-quasiliquid-solid growth mechanisms have been applied to explain the diverse morphologies involving branching, connecting, and batch growth behaviors. The present approach offers a potential method for precise design and controlled synthesis of nanostructures with different features.

Interface Oxidative Structural Transitions in Graphene Growth on SiC (0001)

The structural transition from a three-dimensional SiC lattice to a two-dimensional graphene sheet is a crucial element in the growth mechanism of graphene on SiC. An interfacial defective transition layer near the surface of the SiC substrate is believed to be an intermediate structure for graphene layer formation. The transition layer consists of SiOxCy, vacancies, and other defects in the SiC lattice, which result from Si evaporation via thermal degradation of the SiC lattice and oxidation reactions of residual oxygen and other oxygen containing molecules on the SiC surface at high temperatures. This partially oxidized and structurally degraded SiC lattice layer is formed at temperatures lower than the graphene growth temperature but then decomposes with increasing temperature, leading to graphene formation. Then, the growth mechanism for graphene on SiC (0001) in high vacuum consists of multiple steps, including Si removal by thermal decomposition and oxidation, collapsing of the near surface SiC latt...

Molecular dynamics simulation of Si nanoclusters in high rate and low temperature epitaxy

Si nanocluster formation during rapid cooling from high temperature Si vapors and its subsequent interaction with substrate was simulated by molecular dynamics. It was confirmed that several nm sized clusters with a loosely bound structure form during rapid cooling and their metastable structures are retained even at temperatures below the Si melting point. Such globular clusters tend to rearrange themselves upon impingement to be a similar structure to that of the substrate, at least near the cluster/substrate interface. This is suggestive of the role of clusters formed under the mesoplasma condition as growth precursors for fast rate epitaxy.

Thin silica films on Ru(0001): monolayer, bilayer and three-dimensional networks of [SiO4] tetrahedra

The atomic structure of thin silica films grown over a Ru(0001) substrate was studied by X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, low energy electron diffraction, helium ion scattering spectroscopy, CO temperature programmed desorption, and scanning tunneling microscopy in combination with density functional theory calculations. The films were prepared by Si vapor deposition and subsequent oxidation at high temperatures. The silica film first grows as a monolayer of corner-sharing [SiO(4)] tetrahedra strongly bonded to the Ru(0001) surface through the Si-O-Ru linkages. At increasing amounts of Si, the film forms a bilayer of corner-sharing [SiO(4)] tetrahedra which is weakly bonded to Ru(0001). The bilayer film can be grown in either the crystalline or vitreous state, or both coexisting. Further increasing the film thickness leads to the formation of vitreous silica exhibiting a three-dimensional network of [SiO(4)]. The principal structure of the films can be monitored by infrared spectroscopy, as each structure shows a characteristic vibrational band, i.e., ∼1135 cm(-1) for a monolayer film, ∼1300 cm(-1) for the bilayer structures, and ∼1250 cm(-1) for the bulk-like vitreous silica.

Dependence of the seed layer quality on different temperature ramp-up conditions for 3C-SiC hetero-epitaxy on Si (100)

Usually a waiting step at around 1000°C to 1100°C is implemented during the carbonization for 3C-SiC epitaxy on silicon in order to form a closed carbon layer which prevents the formation of voids by evaporation of Si. On the other hand, such a process step may lead to non-ideal nucleation conditions resulting in a low layer quality with high densities of defects and domain boundaries. Our investigations indicate that a continuous temperature ramp-up with the highest possible heating rate and no waiting step is preferable and results in an improved layer quality concerning domain sizes as confirmed by X-ray diffraction measurements of the full width at half maximum of the SiC(200) reflection. The result can be understood within the framework of the Tammann theory for the temperature dependence of nucleation and growth. The two thermal regimes, as proposed by Tammann, had been located at temperatures of around 1000–1100°C for nucleation and >1200°C for growth.

Large‐area fabrication of equidistant free‐standing Si crystals on nanoimprinted glass

AbstractBy combining nanoimprint lithography with the emerging high‐rate deposition technique electron‐beam evaporation of amorphous Si, we developed a low‐cost fabrication process for the design of periodic arrays of Si crystals on large areas of 50 cm2 in a solid phase crystallization and subsequent selective etch process. The method allows for precise control over the feature size of the crystals. The promising absorption properties of the features and the versatility and simplicity of the preparation process inspire the development of three‐dimensional solar cell architectures and tailored large‐area photonic crystals.magnified imageCross‐sectional SEM image of periodic arrays of Si crystals on nanoimprinted glass(© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of filament temperature and deposition time on the formation of tungsten silicide with silane

The effect of filament temperature and deposition time on the formation of tungsten silicide upon exposure to the SiH 4 gas in a hot wire chemical vapor deposition process was studied using the techniques of cross-sectional scanning electron microscopy and Auger electron spectroscopy. At a relatively low temperature of 1500 °C, the decomposition of WSi 2 phase and the diffusion of Si towards the silicide/W interface produce a thick W 5Si 3 layer. The diffusional nature leads to a parabolic rate law for silicide growth. An exponential decrease of silicide thickness with temperature between 1600 and 2000 °C illustrates the dominance of Si evaporation at higher temperatures (T ≥ 1600 °C) over the silicide formation.

PLASMA CHEMICAL SYNTHESIS OF FINE MoSi2 AND MoSi2-Si3N4 COMPOSITE POWDERS

The preparation of fine MoSi2 and MoSi2 − Si3N4 composite powders with specific surface area of 16−33 and 30−41 m2/g respectively by evaporation of Si and Mo raw powder mixtures or coarse grained MoSi2 in radio frequency inductively coupled nitrogen plasma flow was investigated. Specific surface area of obtained powders as well as content of MoSi2 modifications, Si3N4 and extra phases Mo5Si3, Si depends on the raw powder feeding rate, the cooling gas flow rate and the composition of raw mixtures. The shape of the product particles was spherical for MoSi2 and both spherical and whiskers for composites.

Glass-Forming Exogenous Silicon Contamination in Solid Oxide Fuel Cell Cathodes

Spatially resolved analyses, by energy-dispersive X-ray spectroscopy (EDS) scanning electron microscopy (SEM), allowed the quantification of exogenous Si contamination in a solid oxide fuel cell (SOFC) cathode after operation. The Si quantification, taking into account the endogenous Si impurity level, correlated well with the expectation from the condensation of Si(OH)4 vapor, originating from upstream alloy components and saturated in the hot inlet air. At higher resolution, EDS-transmission electron microscopy (TEM) pointed out the deposition of Si vapor in the form of amorphous SiO2, blocking oxygen incorporation into the electrolyte phase within a composite SOFC cathode.

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.

Atomic-Scale Investigation of Epitaxial Graphene Grown on 6H-SiC(0001) Using Scanning Tunneling Microscopy and Spectroscopy

Graphene was epitaxially grown on a 6H-SiC(0001) substrate by thermal decomposition of SiC under ultrahigh vacuum conditions. Using scanning tunneling microscopy (STM), we monitored the evolution of the graphene growth as a function of the temperature. We found that the evaporation of Si occurred dominantly from the corner of the step rather than on the terrace. A carbon-rich (6√3 × 6√3)R30° layer, monolayer graphene, and bilayer graphene were identified by measuring the roughness, step height, and atomic structures. Defect structures such as nanotubes and scattering defects on the monolayer graphene are also discussed. Furthermore, we confirmed that the Dirac points (ED) of the monolayer and bilayer graphene were clearly resolved by scanning tunneling spectroscopy (STS).

Growth kinetics of epitaxial graphene on SiC substrates

Optical absorption and Raman scattering studies of epitaxial graphene structures obtained by annealing of carbon terminated face of $4H\text{-SiC}(000\text{\ensuremath{-}}1)$ on-axis substrates using standard chemical-vapor deposition reactor are presented. Two series of samples grown at different argon pressures in the reactor and different annealing times were studied. Optical absorption and Raman scattering were used to determine the number of graphene layers formed on the substrate surface. The observed dependence of the number of graphene layers formed on annealing time and argon pressure strongly indicates that the growth kinetics of graphene is limited by Si evaporation and two-dimensional Si diffusion.

Fabrication and magnetic properties of Fe–6.5% Si alloys by magnetron sputtering method

Two-side deposition approach was applied to prepare Si-enriched FeSi alloys from 3% Si starting sheets with 0.35 mm thickness. FeSi layer was first deposited by direct current (DC) magnetron sputtering on the alloy, which was then subjected to vacuum annealing. It is found that reduction of the deposited FeSi layer thickness suppresses evaporation of Si during the annealing process, which is of great significance in enhancing efficiency of Si penetration into the low-Si starting alloy. After the Si-enriched process, the alloys demonstrate dense and uniform microstructure with 6.5% Si and exhibit α-Fe(Si) bcc structure revealed by X-ray diffractometer (XRD) and transmission electron microscope (TEM). Iron losses of the PVD-prepared Fe–6.5% Si alloy are as low as those of the alloys fabricated by CVD.

Vapor-condensed phase processes in the early solar system

Abstract– Equilibrium thermodynamic calculations of the sequence of condensation of phases from a cooling gas of solar composition at total pressures thought to have prevailed in the inner part of the solar nebula successfully predict the primary mineral assemblages of refractory inclusions in CM2 and CV3 chondrites. Many refractory inclusions in CM2 chondrites contain a relatively SiO2-poor assemblage (spinel, hibonite, grossite, perovskite, corundum) that represents a high-temperature stage of condensation, and some may be pristine condensates that escaped later melting. Compact Type A and Type B refractory inclusions, consisting of spinel, melilite, perovskite, Ca-rich clinopyroxene ± anorthite, in CV3 chondrites are more SiO2-rich and equilibrated with the solar nebular gas at a slightly lower temperature. Textures of many of these objects indicate that they underwent melting after condensation, crystallizing into the same phase assemblage as their precursors. The Ti3+/Ti4+ ratio of their pyroxene indicates that this process occurred in a gas whose oxygen fugacity () was approximately 8.5 log units below that of the iron-wüstite buffer, making them the only objects in chondrites known to have formed in a system whose composition was close to that of the sun. Relative to CI chondrites, these inclusions are uniformly enriched in a group of elements (e.g., Ca, REE, Zr, Ta, Ir) that are chemically diverse except for their high condensation temperatures in a system of solar composition. The enrichment factor, 17.5, can be interpreted to mean that these objects represent either the first 5.7 wt% of the condensable matter to condense during nebular cooling or the residue after vaporization of 94.3% of a CI chondrite precursor. The Mg and Si isotopic compositions of Types A and B inclusions are mass-fractionated by up to 10 and 4 ‰/amu, respectively. When interpreted in terms of Rayleigh fractionation during evaporation of Mg and Si from the inclusions while they were molten, the isotopic compositions imply that up to 60% of the Mg and up to 25% of the Si were evaporated, and that approximately 80% of the enrichment in refractory (CaO+Al2O3) relative to more volatile (MgO+SiO2) in the average inclusion is due to initial condensation and approximately 20% due to subsequent evaporation. The mineralogical composition, including the Ti3+/Ti4+ ratio of the pyroxene, in Inti, a particle sampled from Comet Wild 2 by the Stardust spacecraft, is nearly identical to that of a Type B inclusion, indicating that comets contain not only the lowest-temperature condensates in the form of ices but the highest-temperature condensates as well. The FeO/(FeO+MgO) ratios of olivine and pyroxene in the matrix and chondrules of carbonaceous and ordinary chondrites are too high to be made in a system of solar composition, requiring s only 1 or 2 log units below iron-wüstite, more than 105 times higher than that of a solar gas. Various ways have been devised to generate cosmic gases sufficiently oxidizing to stabilize significant FeO in olivine at temperatures above those where Fe-Mg interdiffusion in olivine ceases. One is by vertical settling of dust toward the nebular midplane, enriching a region in dust relative to gas. Because dust is enriched in oxygen compared to carbon and hydrogen relative to solar composition, a higher results from total vaporization of the region, but the factor by which theoretical models have so far enriched the dust is 10 times too low. Another is by transporting icy bodies from the outer part of the nebula into the hot, inner part where vaporization of water ice occurs. Not only does this method fail to make the needed by a factor of 30–1000 but it also ignores simultaneous evaporation of carbon-bearing ices that would make the even lower.

Effects of Some Light Alloying Elements on the Oxidation Behavior of Fe and Ni-Cr Based Alloys During Air Plasma Spraying

The oxidation behavior of iron binary powders with addition of Si (1, 4 wt.%) and B (1, 3 wt.%) and that of a Ni-Cr based alloy powder with Si (4.3 wt.%), B (3.0 wt.%), and C (0.8 wt.%) additions during atmosphere plasma spray (APS) have been investigated. Analysis of the chemical composition and phases of oxides in the captured in-flight particles and deposited coatings was carried out. The results show that the addition of Si and B to iron effectively reduced the oxygen contents in the coatings, especially during the in-flight period at higher particles temperature. Ni-Cr based alloy powder with Si, B, and C additions reduced the oxidation of the base alloys significantly. Preferential oxidation and subsequent vaporization of Si, B, and C from the surface of the sprayed particles are believed to play a major role in controlling oxidation in the APS process.