SiCl4 Vapor Research Articles

Structural features of fumed binary and ternary nanooxides with silica, alumina, and titania

Complex fumed nanooxides SiO2/Al2O3 (SA), SiO2/TiO2 (ST), and Al2O3/SiO2/TiO2 (AST) were studied using nuclear magnetic resonance spectroscopy, transmission electron microscopy, X-ray diffraction, infrared spectroscopy, adsorption, and quantum chemistry. Interior and surface contents of alumina in SA and AST are nonmonotonic functions of total Al2O3 content in contrast to that of titania in ST and AST. This is due to a larger difference in the precursor reactivity of sublimated Al2Cl6 and SiCl4 vapor than that for TiCl4 and SiCl4. Titania tends to form cores and silica (with alumina in AST) forms NPNP shells. Partially crystalline titania phase forms at lower content (∼9 wt%) than alumina remaining mainly amorphous even at CAl2O3 = 30 wt% (SA) or 22 wt% (A22S28T50). Chemical shift peak position in 29Si NMR spectra does not depend on CAl2O3 in contrast to CTiO2 due to different effects of dielectric alumina and semiconducting titania on magnetic shielding of the Si nuclei.

Experimental and numerical approaches on the effect of gas flow on the formation of tens-square-cm-scale mat of silicon nanowires from SiCl4 and zinc vapor

We have synthesized a mat of silicon nanowires (approx. 10 mg in weight and 20 cm2 in size) via the zinc reduction of silicon tetrachloride. The mat consisted of smooth and long nanowires with a median diameter (d50) of 40 nm. The flow pattern of gas inside the reactor that was used to form the mat significantly affected the formation of nanowires, even though conditions such as temperature or flow rates were fixed. In the best flow pattern, nanowires preferentially formed on a fixed position in the reactor and the mat gradually thickened as the reaction time increased. Computational fluid dynamic (CFD) simulations revealed that both a slow circulation flow of gases and a reaction front formed around the best position in the reactor, which resulted in the largest yield. This suggested that the reaction gases required a longer residence time for the preferential formation of the nanowires. The mechanism of this selective nanowire formation is discussed.

Tubular alumina as a key component of new thermally stable ceramic materials

Using scanning electron microscopy and X-ray diffraction, we have examined processes that occur in tubular alumina during heating in air to temperatures in the range 1100–1200°C. The results demonstrate that, starting at temperatures between 600 and 700°C, α-Al2O3 (corundum) is formed in the solid material. Heating the tubular material to 1000°C converts some of the starting alumina into χ-Al2O3 and some into α-Al2O3. The samples calcined for 2–10 h at 1100°C contain α-Al2O3 and ae-Al2O3. Heating to 1000–1300°C has no effect on the shape of the tubular particles. The ceramics obtained in this way are stable in contact with water. When tubular alumina with a SiCl4 vapor hydrolysis product deposited on its surface is heat-treated at temperatures in the range 1100–1300°C, we observe the formation of mullite (Al2O3 · 2SiO2) on the surface of the tubular particles.

Synthesis of SiOC Base Fibers from Silicone Resin with Low Carbon Content and Control of Surface Functionality by Metal Chloride Treatment in Vapor

Melt spinnable silicone resin with a low carbon content was spun to fiber form with an averaged diameter of 16.8 m. When the resin fiber was cured by SiCl4 vapor and pyrolyzed at 1273K in inert atmosphere, Si-O-C fiber with smooth surface was obtained. The measured tensile strength was relatively low. The fiber, however, showed oxidation resistance during high temperature exposure under an air flow. When the fiber was cured by TiCl4 with an increased vapor pressure at 313K, 40% mass gain was observed after the curing. SiO2-TiO2 fiber was obtained by pyrolysis in an air flow, while SiOC-TiO2 fiber was obtained by pyrolysis in an inert atmosphere. Structure of TiO2 and the resulting fiber surface morphology strongly depended on the temperature and the atmosphere during the pyrolysis.

Evaluation of oxidation resistance of thin continuous silicon oxycarbide fiber derived from silicone resin with low carbon content

Si–O–C ceramic fiber was synthesized from a kind of silicone resin with low carbon content. The melt-spun resin fiber was exposed to SiCl4 vapor under a nitrogen gas flow, and the fiber was heated at 373 K for 2 h to complete the curing process. The cured fiber was pyrolyzed at 1273 K in an inert atmosphere to be converted to Si–O–C fiber. The entire chemical composition of the pyrolyzed fiber was almost identical to that of a previously reported resin which was pyrolyzed without curing. Auger spectrum analysis indicated an increase in silicon content near the fiber surface. The Si–O–C fiber thus obtained was heat-treated at 1511 or 1603 K in an air flow to evaluate oxidation resistance. Elemental analysis, XRD measurement, and SEM image observations were carried out on the oxidized Si–O–C fibers. Even with such thin fiber diameters, the oxidation process under these conditions was slow and the formation of a thin oxide layer on the fiber surface was confirmed. The existence of a residual Si–O–C core surrounded by a crystallized silica layer was observed in fractured fiber cross-sections after severe treatment conditions of 24 h oxidation at 1511 K or 3 h oxidation at 1603 K.

Fluorine incorporation in silica glass by the MCVD process: Study of fluorine incorporation zone, evaluation of optical properties and structure of the glass

Abstract A theoretical approach was made to find out a complete fluorine incorporation zone on a ternary diagram which serves as a useful graphical representation to select the flows of the supplied reagents for incorporation of the suitable amount of fluorine into cladding glass of optical fiber preform made by the MCVD process using CCl2F2 as a source of fluorine under oxygen abundance, oxygen deficiency and intermediate oxygen state conditions. The possible mechanism for incorporation of fluorine into cladding glass of optical fiber is also evaluated on the basis of the thermodynamical data. The fluorine incorporation mechanism in silica glass by the MCVD process is found to be dependent on the CCl2F2/SiCl4 ratio in the input gas mixture. Fluorine doping is found to be effective for removing the strained Si–O–Si bonds, which govern the optical transparency in deep ultra-violet (DUV) and vacuum ultra-violet (VUV) regions. The maximum refractive index depression of −0.5 × 10−3 is obtained with incorporation of fluorine into silica cladding glass by the MCVD process using CCl2F2 as a dopant precursor with suitable flow of SiCl4 vapor along with O2 through backward deposition pass. The structure of fluorine doped silica glass preform samples containing 1.70–1.79 mol% fluorine incorporated by the MCVD process based on the analyses of 19F MAS spectra done by high-resolution 19F NMR spectroscopy reveal the presence of two distinct types of fluorine environments. The majority of the fluorine environments are formed in SiO1.5F polyhedral and less abundant species is observed to be highly unusual, yielding a fivefold coordinated silicon of the type SiO2F polyhedral which become increased with increasing the fluorine content.

Synthesis of two‐end‐functionalized copolymer of styrene and methyl methacrylate via living radical polymerization

AbstractThe random copolymers (HO‐P(St‐r‐MMA)‐COOH) of styrene (St) and methyl methacrylate (MMA) with hydroxyl group at one end and carboxyl group at another end were synthesized by nitroxide‐mediated living radical polymerization initiated by 4,4′‐azobis(4‐cyanovaleric acid) (ACVA) and 4‐hydroxyl‐2,2,6,6–tetramethylpiperidineoxyl (TEMPO‐OH). The experimental results have shown that all synthesized copolymers have narrow molecular weight distribution. The conversion of monomers and the molecular weight of copolymer increase with polymerization time. Thus, a copolymerization mechanism containing living radical polymerization is suggested. The use of this method permits the copolymer with two functional chain ends and controllable molecular weight as well as low molecular weight distribution. X‐ray photoelectron spectroscopy result shows that the synthesized copolymers can be tethered on the surface of silicon wafer through the reaction between the hydroxyl end of the copolymer and native oxide layer on the wafer. In addition, an organic/inorganic hybrid surface has achieved by treating copolymer tethered Si‐substrates with SiCl4 vapor. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3118–3122, 2006

Stability of dye loaded faujasites against organic solvents: effect of SiCl4 treatment

Several dyes (quinizarin, indigo, 1,4-diketo-2,5-dimethyl-3,6-diphenylpyrrolo[3,4-c]pyrrole) were loaded within faujasites and mordenite. In the presence of organic solvents, leaching was observed. However, after treatment with SiCl4 vapour the pores of the zeolites were sealed and no leaching could be observed. Furthermore, samples of NaY zeolite loaded with 1,4-diketo-2,5-dimethyl-3,6-diphenylpyrrolo[3,4-c]pyrrole were characterised with diffuse reflectance UV/VIS spectroscopy, diffuse reflectance infrared spectroscopy, thermogravimetry/differential scanning calorimetry, XRD and nitrogen adsorption, both before and after the SiCl4 treatment. In comparison with the untreated dye loaded zeolite, the thermal stability of the dye inside the sealed zeolite increased. The treatment does not have any influence on the framework composition or the dye.

Adsorption of pyridine on dealuminated zeolite HY

Infrared spectra showed that over 80% of aluminum atoms on the surface of zeolite HY were extracted by a exposure of SiCl4 vapor at elevated temperatures. The dealumination of the HY surface resulted in an appreciable change in its acidic properties. Infrared spectra of adsorbed pyridine indicated the formation of various kinds of Lewis acid sites on the delauminated HY surfaces. Although the total number of the surface Bronsted acid site decreased in the SiCl4‐induced dealumination process, the nature of these remaining Brø nsted acid sites was unchanged spectroscopically.

Gas-phase coating of TiO2 with SiO2 in a continuous flow hot-wall aerosol reactor

The feasibility of in situ coating of titania particles with silica using a high-temperature, gas-phase process was demonstrated. Titania was produced from the reaction of TiCl4 and O2 in a hot-wall, tubular, aerosol reactor and directly coated in the gas phase via the reaction of O2 with SiCl4 vapor. Rough SiO2 coatings were obtained at 1300 °C while uniform, dense coatings were obtained at all conditions examined for 1500 °C. The presence of water in the reactor significantly influenced the morphology of the coatings and resulted in smooth, dense, and uniform coatings at 1300 °C. Coating thicknesses could be controlled from 5 nm to roughly 100 nm, corresponding to growth rates on the order of 10–100 nmys. The characteristics of the coatings depended upon the concentration of SiCl4 and the coating temperature. These process variables influenced the coating mechanism, growth rate, and densification which directly influenced the coating uniformity and thickness.

An Inversion Technique to Determine the Aerosol Size Distribution in Multicomponent Systems from In Situ Light Scattering Measurements

An inversion technique based on angular dissymmetry to determine the size distribution of a nonabsorbing aerosol formed in a multicomponent system is developed. The parameters of the lognormal distribution function and an effective refractive index are determined by fitting the computed Mie scattering intensities to measured scattering intensities at multiple angles. The technique is used to determine the evolution of the aerosol size distribution formed by oxidation of silicon tetrachloride, phosphorous oxy-chloride, and germanium tetrachloride. The addition of varying quantities of POCl3 and GeCl4 to a SiCl4 vapor feed on the alteration of the size distribution and morphology of particles is established.

Synthesis of silicon and silicon nitride powders by vapour-phase reactions

The Bureau of Mines is investigating vapour-phase reactions for syntheses of high-purity, nanometre-sized silicon and Si3N4 powders suitable for industrial applications. In this study, submicrometre-sized silicon powder was formed by a vapour-phase reaction of SiCl4, NH3, and magnesium vapor at temperatures ranging from 950–1050°C. The silicon was subsequently nitrided to form Si3N4 at 1150°C.

ChemInform Abstract: Dealumination of Zeolites with Silicon Tetrachloride Vapor. Part 6. Zeolites Li,Na‐X and Li,Na‐Y.

AbstractThe dealumination of Li,Na‐X and Li,Na‐Y with SiCl4 vapor is studied by IR, 29Si and 27Al MAS NMR, XRD, and AAS.

Dealumination of zeolites with silicon tetrachloride vapour. Part 6.—Zeolites Li, Na-X and Li, Na-Y

The reaction of zeolites Li, Na-X and Li, Na-Y with SiCl4 vapour has been examined by IR spectroscopy, 29Si and 27Al magic-angle-spinning NMR (MAS NMR), X-ray diffraction and atomic absorption spectrophotometry. The aim was to prepare a fully crystalline and highly siliceous polymorph of silica with the faujasite structure. This can be achieved in single step when the starting material is the lithium form of zeolite Y. However, zeolite Li, Na-X cannot be similarly dealuminated with SiCl4 vapour. The reason for this is not the ‘screening’ of the framework by the charge-balancing cations, as previously suggested, but the presence in the framework of zeolite X of six-membered rings containing three aluminium atoms.

Surface deacidification of ZSM5 by SiCl4 treatment: Assessment of surface specificity by methylene blue adsorption

A study has been made of ZSM5 zeolite deacidification (dealumination) by high temperature treatment in SiCl4 vapour, monitoring total acidity by Na+-exchange, and external surface acidity by methylene blue adsorption (exchange), with comparative measurements of theNSi/NAl ratio (nominally external surface) by XPS.

Induction of mesoporosity in ALPO-5. Treatment with silicon tetrachloride

Reaction of microporous AlPO4(ALPO-5) with SiCl4 vapour in a nitrogen atmosphere at 870 K results in the partial isomorphous substitution of P predominantly by Si. The product has fewer but stronger acid sites than the parent phase, whilst crystallinity is retained. Isothermal adsorption studies with H2O and N2 probe molecules indicates that some adsorption sites are lost via reaction with SiCl4, and there is evidence for the development of mesoporosity in addition to the micropores already present. It is argued that reaction occurs preferentially at the intercrystallite regions in the aggregates, and at intracrystalline defects, i.e. defects in the pore structure. Comparisons with SAPO-5 are also made.

Vapogel-A new Glass Formation Technique

ABSTRACTSiCI4 was tested in both the liquid and vapor state for its suitability as a monolithic gel precursor. Samples with molar ratios of H2O/SiCl4 ranging from 4 to 400 were investigated. Several advantages of using SiCl4 over the classic organometallics such as tetraethylorthosilicate and tetramethylorthosilicate were observed. Gelation of the SiCl4-H2O sol is more rapid. Also, greater purity is achieved with SiCl4 vapor as the starting material. In addition to the SiCl4 experiments, GeCl4 was also investigated as both a dopant and in the pure state for its gelation characteristics. Higher dopant concentrations were achieved using this technique than are possible with other monolithic gel methods, but pure GeO2 gels were not formed. The gel made by the vapor phase method has been named vapogel.

ChemInform Abstract: Aluminium Distribution in the Bulk and on the Surface of Y Zeolites Dealuminated with SiCl4 Vapour. Influence of Conditions of Dealumination.

AbstractThe dealumination of zeolite Y with SiCl4 is not only dependent on the reaction temp. but also depends on subsequent washing; the Si/Al ratio both in the bulk and on the surface increases with efficiency of washing (H2O > EtOH > EtOH and NH4OH/NH4Cl buffer).

Aluminium distribution in the bulk and on the surface of Y zeolites dealuminated with SiCl4 vapour. Influence of conditions of dealumination

Zeolites Y, dealuminated by reaction with SiCl4 at 530 and 630 K followed by washing with water, ethanol or ethanol and buffer before ammonium exchange (resulting Si/Al ratio: 3.8–10.6), have been examined using X-ray photoelectron spectroscopic measurements combined with chemical analysis. Subsequent extraction with H2Na2 EDTA allowed preferential removal of non-framework Al species which helped in the discussion of the Al location. It was found that, besides the increase of reaction temperature, the efficiency of washing (water > ethanol > ethanol and buffer) increases the Si/Al ratio both in the bulk and on the surface. Temperature increase and milder washing enhance the Al-enrichment of the surface with respect to the bulk. Non-framework Al species are responsible for the surface enrichment.

ChemInform Abstract: Zeolites Treated with Silicon Tetrachloride Vapour. Part 1. Preparation and Characterisation.

AbstractTreatment of the wide‐pore Zeolites zeolite Yand ZSM‐5 with SiCl4 vapour at elevated temp. causes Substitution of framework aluminium by silicon.