SiO2/Fe2O3 Mesoporous Composite Prepared With an Activated Carbon Template in Supercritical Carbon Dioxide

SiO₂/Fe₂O₃ mesoporous composites have been prepared with a nanoscale casting process using an activated carbon (AC) template in supercritical carbon dioxide (SC CO₂). The composite precursor and acetone solvent (for Fe₂O₃ precursor) were dissolved in SC CO₂, and then coated on the AC in the desired supercritical condition. After removal of the AC template by calcinations in air at 600°C, SiO₂/Fe₂O₃ mesoporous composite were obtained. Temperature, pressure, and composite precursor ratio effects were studied. Scanning electron microscopy result shows that the porous structure of AC template had been well replicated by the composite product. Transmission electron micrograph indicates that nano iron oxides were well dispersed in the composite product.     

Criteria for Flame-Spray Synthesis of Hollow, Shell-Like, or Inhomogeneous Oxides

The influence of metal precursor and solvent composition on the morphology of SiO₂, Bi₂O₃, and other oxide particles made by flame spray pyrolysis (FSP) was investigated. Silica precursors with boiling points T _bp=299–548 K dissolved in xylene were used as well as different solvents ( T _bp=308–557 K) with tetraethyl-orthosilicate (TEOS) as the silica precursor. For Bi₂O₃, nonvolatile bismuth nitrate pentahydrate was dissolved in solvents with T _bp=338–468 K. Product powders were characterized by nitrogen adsorption, X-ray diffraction, and scanning and transmission electron microscopy. From these data as well as from the literature of FSP synthesis of Bi₂O₃, CeO₂, MgO, ZnO, Fe₂O₃, Y₂O₃, Al₂O₃, and Mg–Al spinel, it is inferred that hollow/inhomogeneous particles are formed at low combustion enthalpy densities and when the solvent boiling point is comparable or smaller than the precursor melting or decomposition point.     

Formation of SiO2, Al2O3, and 3Al2O3. 2SiO2 Particles in a Counterflow Diffusion Flame

SiO₂, Al₂O₃, and 3Al₂O₃.2SiO₂ powders were synthesized by combustion of SiCl₄ or/and AlCl₃ using a counterflow diffusion flame. The SiO₂ and Al₂O₃ powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al₂O₃.2SiO₂ powder produced with a low-temperature flame was also amorphous and had a similar morphology. However, those produced with high-temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame.     

Preparation of TiO2 Nanoparticles on Different SiO2 Supports by the Adsorption Phase Technique

The influence of supports on the preparation of TiO₂ nanoparticles by the adsorption phase technique is studied in detailed. Series temperature experiments of two types of supports (named as SiO₂ A and B) were used. Energy-dispersive analysis by X-ray indicates that the concentration of TiO₂ on both supports decreases with temperature increasing. TiO₂ quantity on SiO₂ A decreases sharply between 40° and 60°C, whereas the temperature range for SiO₂ B is between 30° and 50°C. X-ray diffraction (XRD) shows that grain size of TiO₂ particles on two SiO₂ surfaces is all below 7 nm. It is also shown by XRD that particles on SiO₂ A decrease sharply as in the quantity curve of TiO₂, but particles on SiO₂ B all change gradually and TiO₂ particles on SiO₂ B are more uniform in transmission electron spectroscopy. The similarly of both supports is considered to be the reason for the similar changes in Ti concentration, and the different characteristics of the internal/external surface lead to variant quantity and grain size, as well as characteristics of TiO₂.     

Protective Silica Coatings on Zinc-Sulfide-Based Phosphor Particles

A spray drying approach was used to apply 15-nm-thick SiO₂ continuous coatings onto ZnS:Ag phosphor particles. A prehydrolyzed TEOS dip coating formula was used as the SiO₂ precursor. The phosphor was mixed together with the precursor then atomized without allowing gelation to occur until the droplets containing the particles were in flight. The coating gelled and dried while falling through a graded heat zone. The dried coated particles were captured in a cyclone separator and heat-treated to further densify the SiO₂ coating. The coatings protected the phosphors while in service in field emission display (FED) devices.     

Bismuth Oxide Nanoparticles by Flame Spray Pyrolysis

Bismuth oxide nanostructured particles were made via the flame spray pyrolysis (FSP) of bismuth nitrate that had been dissolved in a solution of ethanol/nitric acid or in acetic acid. These self-sustaining spray flames produced tetragonal β-Bi₂O₃. The use of ethanol/nitric acid solutions resulted in a mixture of hollow, shell-like, and solid nanograined particles. The particle homogeneity was improved as the content of acetic acid in the precursor solution increased. Solid bismuth oxide nanoparticles were prepared consistent with percolation theory, accounting for the specific volume of the product and the precursor. Using pure acetic acid as the solvent, the effect of FSP variables on spray flame and product powder characteristics was investigated. The specific surface area of the Bi₂O₃ particles could be controlled over a range of 20–80 m²/g by the liquid feed and oxygen gas flow rates for powder production rates of 6–46 g/h.     

Deposition of Nano-Size Titania—Silica Particles in a Hot-Wall CVD Process

The deposition of nano-size titania—silica particles is carried out in a hot-wall CVD reactor by using two premixed precursors, titanium tetraisopropoxide (TTIP) and tetraethyl orthosilicate (TEOS). The deposition occurs mostly in two regions, one near the reactor entrance and the other near the reactor exit. Deposits collected near the reactor entrance are densely packed micrometer-size aggregates/particles of good adhesion to the substrate, while those near the reactor exit are loosely packed nano-size particles with a poor adhesion to the substrate. It is conjectured that the more reactive TTIP reacts first and produces TiO₂ particles later covered by SiO₂ formed via a catalytic surface reaction of the less reactive TEOS on TiO₂ particle surfaces. The presence of surface SiO₂ retards the growth of TiO₂ particles, leading to deposits formed by micrometer-size aggregates containing nano-size primary particles of 30 to 40 nm in the first deposition region. With less or no SiO₂ present on TiO₂ particle surfaces, the deposits formed in the first deposition region are densely packed micrometer-size particles. The Ti/Si ratio of the produced particles, for a furnace temperature of 750°C, increases with increasing TTIP/TEOS concentration ratio, and is lower than the Ti/Si ratio of the incoming reactant stream. The Ti/Si ratio of the particles is also found to decrease with increasing furnace temperature.     

Chromium-Doped Forsterite Nanoparticle Synthesis by Flame Spray Pyrolysis

Synthesis of chromium-doped forsterite (Mg₂SiO₄:Cr) nanoparticles by flame spray pyrolysis (FSP) was investigated. The morphology, crystalline phase, and photoluminescence of the products were evaluated. Crystalline Mg₂SiO₄:Cr nanoparticles of several 10 nm in diameter were obtained, although a small amount of the submicrometer-sized particles and the unreacted MgO phase existed. The product powder showed electron-spin resonance signals from Cr⁴⁺ and photoluminescence typical for Cr⁴⁺ in Mg₂SiO₄, suggesting that a part of the Cr⁴⁺ ions were incorporated into Si⁴⁺ sites by FSP. On the other hand, the effects of excess SiO₂ addition on the structural and optical characteristics of Mg₂SiO₄:Cr were examined. Addition of excess SiO₂ up to 20 mol% did not influence these characteristics of the products. Further addition of excess SiO₂ (60–100 mol%) enhanced the formation of the amorphous phase and resulted in the emission from Cr³⁺ in the amorphous phase in addition to an emission from Cr⁴⁺ in Mg₂SiO₄.     

Structural Control of Phase Formation in Low-Tem perature AIPO4—Sio2 Reactions

The phase relation along the binary join of AIPO₄-SiO₂ were investigated up to 400°C using starting materials made by a solution route. Precursor structures used were boehmite (AIOOH), H₃PO₄, noncrystalline silica, and quartz. The silica precursors acted as structural seeds for the epitaxial growth of AIPO₄. Studies showed that SiO₂ and AIPO₄ were the only crystalline and noncrystalline phases present along the binary join, and no substantial crystalline solution or any ternary phase was observed. Three polymorphic forms of AIPO₄, i.e., berlinite, tridymite, and cristobalite, coexisted as low as 200°C. The nature of the silica precursor greatly influenced the development of the polymorphic phases of AIPO₄. The low-quartz precursor suppressed the formation of the cristobalite form of AIPO₄ and favored berlinite (AIPO₄ quartz) production. On the other hand, noncrystallin silica with a cristobalite-like broad XRD peak suppressed the formation of berlinite and enhaned that of the cristobalite form of AIPO₄. These precursor effects indicate that heteroepitaxy is very significant during the nucleation and growth of AIPO₄ phases on the surface of SiO₂ particles even in these low-temperature reactions.     

Zircon Synthesis via Sintering of Milled SiO2 and ZrO2

The formation of zircon (ZrSiO₄) via sintering of milled SiO₂ and ZrO₂ powders was studied, and the effects of slurry vs dry milling, sintering time, and particle size on zircon yield were examined. It was found that very high zircon yields could be obtained via slurry milling, cold pressing, and sintering of the oxide precursors. The controlling factor in determining zircon yield was found to be the particle size of the SiO₂ and ZrO₂ powders. Zircon yield as a function of sintering time was examined, and found to be similar to previous studies in which sol-gel precursors seeded with zircon were used. SEM studies reveal a homogeneous product with particle sizes on the order of 1–5 µm. It was found that complete reaction to zircon can be achieved from a once-through milling, pressing, and sintering process of SiO₂-ZrO₂ powders.     

Formation of a High Conductivity Fuel Cell Electrolyte by Pressing Diphenylsiloxane-Based Inorganic–Organic Hybrid Particles

An inorganic–organic nanohybrid particle with a two-dimensional chain structure, diphenylsiloxane-silica (Ph₂SiO–SiO₂), was prepared by sol–gel process using diphenyldiethoxysilane (DPDES) and tetraethoxysilane (TEOS), and a high proton conducting polymer of poly(2-acrylamido-2-methyl-1-propane sulfonic acid) (PAMPS) was deposited on the Ph₂SiO–SiO₂ particles via layer-by-layer assembly. A flexible sheet-like electrolyte was successfully obtained from the resulting PAMPS-deposited particles by pressing. The proton conductivity of the sheet prepared using unmodified Ph₂SiO–SiO₂ particles was lower than 10⁻⁹ S/cm at 80°C. On the other hand, the PAMPS-deposited samples showed proton conductivities ∼7 orders of magnitude higher than those of the sample prepared using unmodified particles, and their conductivity reached about 1 × 10⁻² S/cm at 80°C and 80% relative humidity. This is ascribed to the PAMPS layer being concentrated at the particle interfaces, which percolated throughout the monolithic sample.     

Bulk Concentration Effects on the Structure and Orientation of Adsorbed Silane on the Surface of Nanosized SiO2 Particles

In this study, the concentration effects of 3-methacryloxypropyltrimethoxysilane (MEMO) on the surface coverage extent, orientation, and structures of MEMO silane grafted onto the surface of SiO₂ particles are investigated using Fourier transform infrared (FTIR) and ²⁹Si-NMR spectroscopy. The MEMO silane concentration effects on the rheological behaviors of the SiO₂ particles–1,6-hexanedioldiacrylate (HDDA) suspensions are discussed in terms of the surface coverage extent and the MEMO silane structures grafted onto the surface of SiO₂ particles. At low MEMO concentrations, many free silanol SiOH groups were observed for the MEMO grafted onto the SiO₂ surface and the adsorbed MEMO molecules tended to orient parallel to the SiO₂ surface due to the hydrogen bonding of the MEMO-carbonyl and the hydroxyl group of the oxides. At high MEMO concentrations, the condensation reactions between neighboring grafted MEMO molecules result in the predominance of a T² and T³ silicon atom structure and complete coverage of the SiO₂ surface by the grafted MEMO. The enhanced steric hindrance and compatibility between the MEMO-modified SiO₂ particles and HDDA monomer, which originate from the complete coverage of the SiO₂ surface by MEMO silane and a higher proportion of free carbonyl groups on the grafted MEMO, may improve the dispersibility of nanosized SiO₂ in acrylate suspensions.     

Dopants in Flame Synthesis of Titania

The effect of dopants on the characteristics of titania particles made by oxidation of TiCl₄ in a laminar diffusion flame reactor is presented. Introduction of dopant SiCl₄ inhibits the transformation of anatase to rutile, due to the formation of interstitian solid solution of SiO₂ and TiO₂. Silica decreases the sintering rate of titania and decreases the primary particle size, and, as a result, the specific surface area increases. Intruduction of SnCl₄ enhances the transformation of anatase to rutile, due to the similar crystalline structure of SnO₂ and rutile titania. However, the presence of SnO₂ and rutile titania. However, the presence of SnO₂ does not affect the primary particle size or the specific surface area of titania particles. Introduction of AICI₃ enhances the transformation of anatase to rutile, due to the formation of excess oxygen vacancies as Al₂O₃ and TiO₂ form a substitutional solid solution.     

Fugitive Diffusion Barrier for Integration of Sol-Gel-Derived Lead Titanate with Oxidized Silicon Substrates

The crystalline phase development and microstructural changes with heating of sol-gel-derived lead titanate (PT) particles and films on silica with and without a fugitive (or removable) diffusion barrier layer were investigated. Amorphous gel-derived PT particles were deposited on SiO₂-coated TEM grids with and without polyimide (PI) or carbon barrier layers between SiO₂ and PT. TEM analysis showed that PI or carbon barriers prevented reaction between the gel-derived PT particles and SiO₂. PT particles crystallize and then the PI or carbon film decomposes. Sol-gel-derived PT films were deposited on oxidized Si substrates (Si/SiO₂) with and without a PI barrier layer. Perovskite PT films were prepared on Si/SiO₂ substrates with a PI barrier; however, some porosity remained in the films. Identically prepared films without the PI barrier formed a mixture of pyrochlore and perovskite. X-ray photoelectron spectroscopy results indicate that the PI film pre- vents the diffusion of Si into the PT film.     

Structure of Borosilicate and Borogermanate Melts at 1300°C; a Viscosity and Density Study

An improved counterbalanced sphere viscometer-densitometer was used to obtain information at temperatures between 1000° and 1400°C for (a) molten B₂O₃, (b) a series of borosilicate melts containing up to 55 mole % SiO₂, and (c) a series of borogermanate melts containing up to 65 mole % GeO₂. The dependence on composition of these structure-sensitive parameters was used to develop a model for the alteration of molten B₂O₈ by other network-forming species. The B/Si or B/Ge ratio is shown to be a significant factor in determining melt structure. The silicon (or germanium) atoms appear to be widely separated in the B₂O₂ solvent for compositions in the 0 to 10-20 mole % SiO₂ (or GeO₂) region. The evidence suggests that a gradual microclustering of SiO₂ (or GeO₂) accompanies moderate departures from ideality for B₂O₃ in the 10-20 to 60 mole % SiO₂ (or GeO₂) region. Extensive micro-clustering of SiO₂ (or GeO₂), approaching network formation, appears to occur in the 60 to 100 mole % SiO₂ (or GeO₂) region; some of the boron atoms remain in clusters and/or are forced to adopt a tetrahedral configuration in this region.     

Preparation of High-Purity ZrSiO4 Powder Using Sol–Gel Processing and Mechanical Properties of the Sintered Body

Effects of the concentration of ZrOCl₂, calcination temperature, heating rate, and the size of secondary particles after hydrolysis on the preparation of high-purity ZrSiO₄ fine powders from ZrOCl₂.8H ₂ O (0.2 M to 1.7 M ) and equimolar colloidal SiO₂ using sol–gel processing have been studied. Mechanical properties of the sintered ZrSiO₄ from the high-purity ZrSiO₄ powders have been also investigated. Single-phase ZrSiO₄ fine powders were synthesized at 1300°C by forming ZrSiO₄ precursors having a Zr–O–Si bond, which was found in all the hydrolysis solutions, and by controlling a secondary particle size after hydrolysis. The conversion rate of ZrSiO₄ precursor gels to ZrSiO₄ powders from concentrations other than 0.4 M ZrOCl₂.8H₂O increased when the heating rate was high, whereupon the crystallization of unreacted ZrO₂ and SiO₂ was depressed and the propagation and increase of ZrSiO₄ nuclei in the gels were accelerated. The density of the ZrSiO₄ sintered bodies, manufactured by firing the ZrSiO₄ compacts at 1600° to 1700°C, was more than 95% of the theoretical density, and the grain size ranged around 2 to 4 μm. The mechanical strength was 320 MPa (room temperature to 1400°C), and the thermal shock resistance was superior to that of mullite and alumina, with fairly high stability at higher temperatures.     

Characterization and Sintering Behavior of Alkoxide-Derived Aluminosilicate Powders

Submicrometer SiO₂-Al₂O₃ powders with compositions of 46.5 to 76.6 wt% Al₂O₃ were prepared by hydrolysis of mixed alkoxides. Phase change, mullite composition, and particle size of powders with heating were analyzed by DTA, XRD, IR, BET, and TEM. As-produced amorphous powders partially transformed to mullite and Al-Si spinel at around 980°C. The compositions of mullite produced at 1400° and 1550°C were richer in Al₂O₃ than the compositions of stable mullite solid solutions predicted from the phase diagram of the SiO₂-Al₂O₃ system. Particle size decreased with increasing Al₂O₃ content. The sintered densities depended upon the amount of SiO₂-rich glassy phase formed during sintering and the green density expressed as a function of particle size.     

Novel Low-Temperature Processing Route of Dense Mullite Ceramics by Reaction Sintering of Amorphous SiO2-Coated γ-Al2O3 Particle Nanocomposites

Dense mullite ceramics were successfully produced at temperatures below 1300°C from amorphous SiO₂-coated gamma-Al₂O₃ particle nanocomposites (AS-gammaA). This method reduces processing temperatures by similar/congruent300°C or more with respect to amorphous SiO₂-coated alpha-Al₂O₃ particle microcomposites (AS-alphaA) and to other Al₂O₃-SiO₂ reaction couples. The good densification behavior and the relatively low mullite formation temperature make AS-gammaA nanocomposites an excellent matrix raw material for polycrystalline aluminosilicate fiber-reinforced mullite composites.     

Sol–Gel Synthesis and Characterization of Ag and Au Nanoparticles in SiO2, TiO2, and ZrO2 Thin Films

Silver and gold nanoparticles were synthesized by the sol–gel process in SiO₂, TiO₂, and ZrO₂ thin films. A versatile method, based on the use of coordination chemistry, is presented for stabilizing Ag⁺ and Au³⁺ ions in sol–gel systems. Various ligands of the metal ions were tested, and for each system it was possible to find a suitable ligand capable of stabilizing the metal ions and preventing gold precipitation onto the film surface. Thin films were prepared by spin-coating onto glass or fused silica substrates and then heat-treated at various temperatures in air or H₂ atmosphere for nucleating the metal nanoparticles. The Ag particle size was about 10 nm after heating the SiO₂ film at 600°C and the TiO₂ and ZrO₂ films at 500°C. After heat treatment at 500°C, the Au particle size was 13 and 17 nm in the TiO₂ and ZrO₂ films, respectively. The films were characterized by UV–vis optical absorption spectroscopy and X-ray diffraction, for studying the nucleation and the growth of the metal nanoparticles. The results are discussed with regard to the embedding matrix, the temperature, and the atmosphere of the heat treatment, and it is concluded that crystallization of TiO₂ and ZrO₂ films may hinder the growth of Ag and Au particles.     

Synthesis of Submicrometer Mullite Powder via High-Temperature Aerosol Decomposition

High-purity mullite powders (3Al₂O₃.2SiO₂) have been prepared using a high-temperature aerosol decomposition technique yielding submicrometer (0.6 μm average) particles of spherical morphology with no hard agglomerates using aluminum nitrate and fumed silica as precursors. Depending upon the reaction conditions used, the powders range from amorphous to crystalline with no evidence of secondary-phase formation. This mullite synthesis approach has the advantages of not requiring postsynthesis milling, the ability to use a wide range of precursor systems, and enhanced control over chemical homogeneity and particle size/shape.