Primary Silica Research Articles

Organic/inorganic nanocomposites prepared by mechanical smashing of agglomerated silica ultrafine particles in molten thermoplastic resin

AbstractA new approach for the preparation of organic/inorganic nanocomposites was investigated in which nano‐sized silica particles without surface modification were dispersed by the mechanical smashing of strength‐controlled porous silica agglomerates in molten resin during direct melt‐compounding. The method consists of two stages. The first stage involves preparation of the strength‐controlled agglomerated silica nanoparticles with pore structure, and the second stage involves melt‐compounding of a thermoplastic resin with the silica agglomerates prepared in the first stage. In the first stage, on the basis of the theory of stability of an aqueous colloidal silica solution, the pore structure and the fracture strength of the silica agglomerate could be controlled by pH‐control and electrolyte ion‐addition to the colloidal solutions of silica nanoparticles. In the next stage, primary silica nanoparticles could be dispersed in some matrix resins such as a poly(ethylene‐ran‐vinyl alcohol) and polystyrene uniformly by the proposed mechanical approach by the use of the high shear stress acting on porous silica agglomerates inside these melt‐compounded resins. Copyright © 2006 John Wiley & Sons, Ltd.

Rapid precipitation of amorphous silica in experimental systems with nontronite (NAu-1) and Shewanella oneidensis MR-1

Nanometer-size (<50 nm) precipitates of amorphous silica globules were observed in laboratory systems containing nontronite NAu-1, Shewanella oneidensis strain MR-1, and lean aqueous media. Their formation was attributed to the release of polysilicic acids at the expense of dissolving NAu-1, and subsequent polymerization and stabilization mediated by biomolecules. Rapid (<24 h) silica globule formation was confirmed in the immediate vicinity of bacterial cells and extracellular polymeric substances in all experimental systems that contained bacteria, whether the bacteria were respiring dissolved O 2 or Fe(III) originating from NAu-1, and whether the bacteria were viable or heat-killed. Silica globules were not observed in bacteria- and biomolecule-free systems. Thermodynamic calculations using disilicic acid, rather than monomeric silica, as the primary aqueous silica species suggest that the systems may have been supersaturated with respect to amorphous silica even though they appeared to be undersaturated if all aqueous silica was assumed to be monomeric H 4SiO 4. The predominant aqueous silica species in the experimental systems was likely polysilicic acids because aqueous silica was continuously supplied from the concurrent dissolution of aluminosilicate. Further polymerization and globule formation may have been driven by the presence of polyamines, a group of biologically produced compounds that are known to drive amorphous silica precipitation in diatom frustules. Globules were likely to be positively charged in our systems due to chemisorption of organic polycations onto silica surfaces that would have been otherwise negatively charged. We propose the following steps for the formation of nanometer-size silica globules in our experimental systems: (i) continuous supply of polysilicic acids due to NAu-1 dissolution; (ii) polysilicic acid polymerization to form <50 nm silica globules and subsequent stabilization mediated by microbially produced polyamines; (iii) charge reversal due to chemisorption of organic polycations; and (iv) electrostatic attraction of positively charged silica globules to net negatively charged bacterial cells. Rapid, biogenic precipitation of silica may be common in soil and sediment systems that appear to be undersaturated with respect to amorphous Si.

Structural Characteristics of Silica Sonogels Prepared with Additions of Isopropyl Alcohol

Wet silica gels with approximately 1.4 x 10(-3) mol SiO2/cm3 and approximately 92 vol % liquid phase were obtained from sonohydrolysis of tetraethoxysilane (TEOS) with different additions of isopropyl alcohol (IPA). The IPA/TEOS molar ratio R was changed from 0 to 4. Aerogels were obtained by supercritical CO2 extraction. The samples were analyzed by small-angle X-ray scattering (SAXS) and nitrogen adsorption. The wet gels exhibit mass fractal structure with fractal dimension increasing from D approximately 2.10 to D approximately 2.22, characteristic length xi decreasing from approximately 9.5 to approximately 6.9 nm, as R increases from 0 to 4, and an estimated characteristic length for the primary silica particles lower than approximately 0.3 nm. The supercritical process apparently eliminates a fraction of the porosity, increasing the mass fractal dimension and shortening the fractality domain in the mesopore region. The fundamental role of isopropyl alcohol on the structure of the resulting aerogels is to decrease the porosity and the pore mean size as R changes from pure TEOS to R = 4. A secondary structure appearing in the micropore region of the aerogels can be described as a mass/surface fractal structure, with correlated mass fractal dimension Dm approximately 2.7 and surface fractal dimension Ds approximately 2.3, as inferred from SAXS and nitrogen adsorption data.

An optimized sol–gel synthesis of stable primary equivalent silica particles

An optimized synthesis of nanometer silica particles via hydrolysis and condensation of tetraethylorthosilicate (TEOS) is described. At the optimum experimental conditions, homogeneous and stable silica nanoparticles with mean particles size of 7.1 ± 1.9 nm were obtained. The particle size is in a good agreement with primary particles. The size, size distribution (SD) and the yield of silica were controlled by the concentration of the reactants, ammonia feed rate, temperature and mixing mode. The increase in TEOS concentration resulted in bigger and multi-model distributed powder, while high temperature and magnetic agitation produced a highly aggregated powder. However, higher H 2O/TEOS ratio and lower ammonia concentration at slower feed rate produced particles in the range of 10–14 nm. It was also found that the concentration of silanol group increased significantly with the decrease in particle size, especially below 40 nm. The optimized technique developed is simple and reproducible, affording a high yield of ∼75% of nanometer silica in a primary size range.

Adsorption and Migration of Poly(Vinyl Pyrrolidone) at a Fumed Silica Surface

A series of poly(vinyl pyrrolidone) (PVP)/fumed silica powder samples were studied using adsorption, FT-IR and AFM methods. Mild treatment (293 K for several hours) of PVP/A-300 powder with an added additional portion of A-300 in a glass reactor (2 dm3) using a mixer (> 500 rpm which provided the powder as a pseudo-liquid state, PLS) led to the redistribution (migration) of the PVP molecules between silica particles previously covered by PVP and free of PVP. This time-dependent rearrangement of the adsorbed PVP molecules caused the formation of a denser polymer layer at the silica surface. The aggregate size of the primary silica particles with adsorbed PVP molecules decreased after treatment. The volume of macropores with a pore radius Rp > 25 nm increased due to the decrease in the distances between aggregates in agglomerates because of the adhesive effect of the PVP molecules. However, the volume of micropores at Rp < 1 nm decreased because of the penetration of the PVP molecules into the primary silic...

Nanoporous Silica from the Spray Reaction of Metal Solution Microdroplets with Silicon Tetrachloride

Nanopore formation in silica, in which nanoparticles of metal compounds were embedded, was studied using analytical techniques such as SEM, XRD, XRF, XPS and nitrogen adsorption. Such metal compound nanoparticles formed between primary silica particles, providing micropores and mesopores caused by the formation of Si—O—M bonds (M = Al, Ni). Acid treatment dissolved the metal compound nanoparticles and destroyed the Si—O—M bonds, thereby increasing the micropore volume. The mesopore volume of the aluminium system increased on acid treatment, whereas that of the nickel system decreased.

Zeolite Beta nanosized assemblies

Nanosized zeolite Beta assemblies are prepared by a steam assisted conversion (SAC) method from micron-sized porous amorphous silica grains soaked in clear solutions containing the alumina source and organic template. The zeolite Beta assemblies are built of closely packed uniform nanocrystals (100 nm) and retain the size and morphological features of the primary silica grains. The crystallinity and the phase purity depend strongly on the temperature and time of SAC treatment as well as on the initial aluminum content. For comparison, colloidal zeolite Beta samples with similar Si/Al ratio were prepared by a hydrothermal treatment (HT). The Raman and NMR spectroscopic data reveal that the method of preparation (SAC or HT) does not affect the local structure of Al-rich samples, while for high-silica samples the degree of disorder is higher in the ones obtained via the SAC approach. The adsorption/desorption isotherms of zeolite Beta assemblies and colloidal Beta powders indicate the presence of both micro- and mesoporosity. The catalytic behavior of the zeolite Beta assemblies and colloidal Beta powders in pentane hydroisomerization reaction is studied.

Preparation of Carboxyl‐Functionalized Polystyrene/Silica Composite Nanoparticles

AbstractSummary: This paper describes the synthesis of carboxyl‐functionalized polystyrene/silica (PS/SiO2) composite nanoparticles with various contents of poly(methacrylic acid) (PMAA) on the surface by post‐addition of methacrylic acid (MAA) via emulsion polymerization. High yields and binding efficiencies (around 90%) are achieved by an optimal procedure mainly involving the appropriate addition of MAA and amounts of surfactant and silica. The kinetics investigated indicates that the polymerization follows a mechanism different than that found in some earlier studies. The amount of grafted PMAA was determined by titration and FT‐IR, and altered in a wide range (1–40 wt.‐% to PS). Transmission electron microscopy (TEM) photographs show that the composite nanoparticles are about 60 nm in spherical shape and have a multi‐layered core‐shell structure with a cluster of primary silica beads as the core and PMAA as the outmost shell. There are approximately 4 to 21 primary silica beads in one composite nanoparticle, depending on the amount of silica added.Preparation of carboxyl‐functionalized polystyrene(PS)/silica composite nanoparticles.magnified imagePreparation of carboxyl‐functionalized polystyrene(PS)/silica composite nanoparticles.

Synthesis of SiO 2 xerogels and Pd/SiO 2 cogelled xerogel catalysts from silylated acetylacetonate ligand

SiO 2 xerogels and Pd/SiO 2 cogelled xerogel catalysts have been prepared in a mixture of tetrahydrofurane (THF) and ethanol containing tetraethoxysilane (TEOS), and an aqueous ammonia solution of 0.18 mol/l, from synthesized new silylated acetylacetonate ligands, respectively, 3-[3-(trimethoxysilyl)propyl]-2,4-pentanedione (MS-acac-H), 2,2,6,6-tetramethyl-4-[3-(trimethoxysilyl)propyl]-3,5-heptanedione (MS-dPvM), and 1,3-diphenyl-2-[3-(trimethoxysilyl)propyl]-1,3-propanedione (MS-dBzM), able to form a chelate with a metal ion such as Pd 2+. All samples form homogeneous colored gels. The resulting catalysts are composed of palladium crystallites with a diameter of about 3.5 nm, located inside primary silica particles exhibiting a monodisperse microporous distribution as well as large palladium particles from 20 to 50 nm, situated outside the silica aggregates. The silylated organic ligand has a strong influence on the textural properties of xerogels and catalysts, both before and after calcination and reduction steps. Changing the nature of the silylated ligand permits tailoring textural properties such as pore volume, pore size and surface area. Although small palladium crystallites are located inside the silica particles, their complete accessibility, via the micropore network, has been shown. 1,2-Dichloroethane hydrodechlorination over Pd/SiO 2 catalysts mainly produces ethane and the reaction rate increases linearly with palladium dispersion. Hydrodechlorination over Pd/SiO 2 cogelled xerogel catalysts is a structure insensitive reaction compared to the ensemble size concept.

Evaluating Formation and Growth Mechanisms of Silica Particles Using Fluorescence Anisotropy Decay Analysis

At present, there is no direct experimental evidence that primary silica particles, which exist only transiently for a few seconds during the Stöber silica synthesis, can be stable in aqueous solutions. In the present work, we show that primary silica particles are formed spontaneously after the dissolution of diglycerylsilane (DGS) in aqueous solutions and remain stable for prolonged periods of time. By using time-resolved fluorescence anisotropy (TRFA), we demonstrate that this unique property of DGS is ascribed to the slow kinetics of silica particle growth in diluted sols at pH approximately 9.0. The anisotropy decay of the cationic dye rhodamine 6G (R6G), which strongly adsorbs to silica oligomers and nanoparticles in DGS sols, could be fit to three components: a fast (picosecond) scale component associated with free R6G, a slower (nanosecond) rotational component associated with R6G bound to primary silica particles, and a residual (nondecaying) anisotropy component associated with R6G that was bound to secondary or larger particles that were unable to rotate on the time scale of the R6G emission lifetime (4 ns). The data show that, under conditions where fast hydrolysis is obtained, the initial size of the nuclei depends on the silica concentration, with larger nuclei being present in more concentrated sols, while the rate of growth of primary particles depends on both silica concentration and solution pH. At low silica concentrations and high pHs, it was possible to observe the growth of stable, nonaggregating primary silica particles by a mechanism involving rapid nucleation followed by monomer addition. The presence of stable primary particles was confirmed by atomic force microscopy (AFM) imaging. At higher silica concentrations and lower pHs, there was an increase in the initial size of the nuclei formed, which subsequently grew to a larger radius (> 4.5 nm) or aggregated with time, and in such cases, nucleation and aggregation occurred simultaneously in the early stage of silica formation. The data clearly show the power of time-resolved fluorescence anisotropy decay measurements for probing the growth of silica colloids and show that this method is useful for elucidating the mechanism of particle formation and growth in situ.

Synthesis of mesostructured silica with tailorable textural porosity and particle size

Mesostructured silica was prepared on three length scales (i.e., nanometric, sub-micrometric and micrometric), with the aid of a non-ionic poly(ethylene oxide) (PEO) surfactant as structure-directing agent under strong acid pH reaction conditions. The size of the primary silica particles can be tuned to between 10 nm and 10 μm by simply adjusting the stirring time during synthesis. The micrometric silica shows a perfectly spherical morphology and a unimodal pore system made up of structural mesopores of ∼6 nm. The other silicas synthesised at the sub-micrometric and nanometric length scales exhibit, in addition to the framework-confined porosity (∼6 nm), a textural porosity arising from the interparticle voids between the primary particles.

Evolution of Sodium Silicate Sols through the Sol-to-Gel Transition Assessed by the Fluorescence-Based Nanoparticle Metrology Approach

The fluorescence-based nanosize metrology approach, proposed recently by Geddes and Birch (Geddes, C. D.; Birch, D. J. S. J. Non-Cryst. Solids 2000, 270, 191), was used to characterize the evolution of primary silica particles through the sol-to-gel transition and during aging of sodium silicate (SS) derived silica. In this study, the evolution of silica particles within SS derived silica was examined as a function of pH and glycerol doping through the sol-to-gel transition and up to 10 months after gelation. Time-resolved anisotropy decays were measured for the cationic dye rhodamine 6G, which was strongly adsorbed to the silica nanoparticles, and for the anionic probe pyranine, which provided accurate data on the microviscosity of the internal aqueous solution within sols and gels. The data provide evidence for the presence of nonaggregated primary particles far beyond the gelation point and even after prolonged aging of the resulting silica when aging is done either at low pH or in the presence of glycerol. Both the fraction of stable primary particles and the final size of the primary particles within the aged silica were dependent on the pH and the presence of glycerol. In general, lower pH values (pH 3.5) or the presence of glycerol increased the fraction of primary particles relative to samples at prepared at pH 6.5. Gelation pH did not have an affect on the final particle size, which was 1.5 ± 0.4 nm at both pH 3.5 and 6.5. On the other hand, a smaller particle size (0.9 ± 0.2 nm) was observed for SS sols and hydrogels containing 50% glycerol.

Effect of the temperature on the TMOS sono-hydrolysis and a study of the structure of the resulting aged sonogels

The effect of temperature on the oxalic acid catalyzed sono-hydrolysis of tetramethoxysilane (TMOS) was studied by means of a heat flux calorimetric method. The activation energy of the process was measured as (24.5 ± 0.8) kJ/mol in the temperature range between 10 and 50 °C. The structural characteristics of the resulting sonogels, after long period of aging in saturated conditions, were studied by means of small angle X-ray scattering. The structure can be described as formed by ∼2.7 nm mean size mass fractal-like aggregates (clusters) of primary silica particles of ∼0.3 nm mean size, all imbibed in a liquid phase. The average mass fractal dimension of the clusters was found to be 2.58. The primary particle density was estimated as 2.23 g/cm 3, in good agreement with the value frequently quoted for fused silica. The volume fraction of the clusters in the saturated sonogels was estimated as about 28%. The moment in which the meniscus of the liquid phase penetrates into the clusters under rapid evaporation process has been detected by an inflection in the first derivative of the curve of weight loss in a simple thermogravimetric test.

Structural study of aged saturated silica gels obtained from tetramethoxysilane sonohydrolysis with different water/tetramethoxysilane molar ratio

The structural characteristics of saturated silica sonogels were studied by means of small-angle x-ray scattering (SAXS) and thermogravimetric analysis (TG), after a long time of aging in saturated conditions. The sonogels were obtained by a sol-gel routine from ultrasound stimulated tetramethoxysilane (TMOS) hydrolysis carried out with the initial water/TMOS molar ratio (r) ranging from 2 to 10. The saturated sonogel structure can be described as composed by mass fractal-like aggregates (clusters) of primary silica particles, all imbibed in a liquid phase. The values of the mass fractal dimension (D) of the clusters was found all around 2.5, while the characteristic size of the clusters $(\ensuremath{\xi})$ was found generally increasing with r, going from approximately 2.3 nm $(r=2)\mathrm{to}4.5\mathrm{nm}$ $(r=10).$ The volume fraction of the clusters was estimated from the SAXS data. The results were compared to the values of weight loss fraction at the inflection point that has been found in the derivative of the TG curve, which should accounts for the instant in which the meniscus of the liquid phase penetrates into the clusters under a rapid evaporation process as in a TG test.

Alteration and geochemical patterns in the 3.7–3.8 Ga Isua greenstone belt, West Greenland

Volcanic rocks of the 3.7–3.8 Ga Isua greenstone belt have undergone heterogeneous, poly-phase post-magmatic alteration, resulting in mobilisation of many elements including LREE and Th. In contrast, Nb, Ta, Ti, Zr, Y and HREE were relatively immobile during alteration processes. The degree of element mobility appears to have been controlled by chemical composition, primary volcanic structures, carbonate and silica enrichment, and proximity to ultramafic units and shear zones. On the basis of the geochemical characteristics of the least altered samples, two distinct associations have been recognised in structurally separated lithotectonic sequences. These magmatic associations are: a low-HFSE association (TiO 2 =0.20–0.40 wt.%; Zr=12–30 ppm; Nb=0.13–0.80 ppm; Y=6–14 ppm) in the central arc sequence, resembling Tertiary boninites; and a high-HFSE association (TiO 2 =0.50–1.14 wt.%; Zr=34–77 ppm; Nb=1.2–2.7 ppm; Y=11–28 ppm) in the outer and inner arc sequences, comparable to Phanerozoic island arc basaltic to picritic volcanic rocks. These two geochemically distinct sequences were likely to have been juxtaposed as a consequence of Phanerozoic-style plate tectonic processes operating in the early Earth. The Isua volcanic rocks are characterised by sub-chondritic Nb/Ta (10–16) ratios. Post-magmatic alteration can be ruled out as the cause of these low ratios. Rather, these ratios reflect a compositional characteristic of the Paleoarchean mantle source(s). The sub-chondritic Nb/Ta ratios may have resulted from either preferential sink of Nb into the core as a weakly siderophile element , or preferential retention of Nb in the subducted oceanic crust during subduction zone petrogenetic processes. Alternatively, the building blocks of the Earth were not composed of CI-like chondritic material.

Structural characteristics of pyrocarbon-fumed silica

Abstract Two series of pyrocarbon/fumed silica (CS) samples at different carbon concentrations CC=0.5–64 wt.% (first series, CSI) and 2.6–53 wt.% (second series, CSII) synthesised by means of pyrolysis of CH2Cl2 at fumed silica substrate (SBET=297 (CSI) and 232 (CSII) m2 g−1) under slightly different conditions were studied by using TEM, FTIR-PAS, DTG, and nitrogen adsorption–desorption methods. On methylene chloride carbonisation, disordered carbon deposits can form mainly in the inter-particle volume of secondary particles (aggregates of primary particles and agglomerates of aggregates) covering the surfaces of primary silica particles; therefore, marked reduction of the pore (gaps between primary particles) volume and the specific surface area is observed with increasing CC. Estimation of distributions of the pore fSCD(Rp) and particle f(a) sizes using a self-consistent method with binary regularisation with respect to both fSCD(Rp) and f(a) shows that the average size of particles increases (silica particles are covered by pyrocarbon) and individual pyrocarbon particles (up to 50 nm according to TEM) also appear. Structural parameters of carbosils are characterised by nonlinear changes with increasing carbonisation time. Surface functionalities on CS samples correspond to aromatic and twinned CC bonds with contribution of oxygen-containing groups.

Optical properties of dense and porous spheroids consisting of primary silica nanoparticles

The light scattering by granular and macroporous silica spheroids consisting of nanometer-sized primary particles was systematically investigated using a laser particle counter coupled with a pulse height analyzer. The shape- and porosity-controlled spheroids as model particles were prepared using spray drying method by changing the particle size of colloidal suspension. The effect of shape and porosity of dense and porous spheroidal particles on electrical mobility was also studied using a differential mobility analyzer and an electron microscope. The electrical mobility equivalent diameter of particles classified by the differential mobility analyzer was estimated by measuring Feret diameter and the projected area equivalent diameter from the SEM micrographs. The electrical mobility diameter of the spheroids was in good agreement with the projected area equivalent diameter regardless of the primary particle size and porosity. The measured partial scattering cross section of dense and porous silica particles with same mobility diameter showed significant differences. As the primary particle size of granules and the porosity of porous particles increased at parity of electrical mobility diameter, the scattering intensity decreased. The effective refractive indices of dense and porous particles were computed by best fitting of the scattering intensity measurements. The porosities of dense and porous spheroids were calculated using the effective refractive indices as obtained by the effective medium theory. The porosities were also measured by a comparison of particle size before and after annealing at 1700°C. By comparing these porosities, the effective refractive indices of the spheroidal particles were confirmed.

1- and 2-Photon Fluorescence Anisotropy Decay in Silicon Alkoxide Sol−Gels: Interpretation in Terms of Self-assembled Nanoparticles

We have studied the one- and two-photon induced fluorescence anisotropy decay of rhodamine 6G (R6G) during polymerization of tetramethyl orthosilicate (TMOS) approaching the sol-to-gel transition, a time denoted tg, using time-correlated single-photon counting and femtosecond Ti:sapphire laser excitation. A biexponential decay of fluorescence anisotropy is observed at all times. We propose a different interpretation to the widely accepted view, that fluorescence anisotropy reports solely on molecular viscosity in sol−gels. We think our results are consistent with the presence of both free dye and dye bound to nm-size silica particles rather than just the coexistence of different discrete viscosity domains as reported previously. A corollary of our interpretation is that the microviscosity changes very little from that of the initial bulk sol throughout the sol−gel polymerization. Nanometer-size particles are known from small angle scattering studies to be precursors to gelation in sol−gels over a wide range of conditions and our interpretation might prove to be an important step toward understanding the self-assembly mechanisms of silicon alkoxide based materials at the molecular level. According to our measurements and interpretation, for TMOS at pH 2.3 for example, primary silica particles of ≈0.8-nm mean radius grow by monomer−monomer or monomer−cluster addition to produce larger structures ≈1.1-nm mean radius after one month.

Behavior of silica particles of different microstructure on RF thermal plasma treatment

Hollow ceramic microspheres are facing continuously growing applications as was described by Cochran [1]. Thus, it is not surprising at all that research on them has been extended more recently. Various processing techniques for making ceramic hollow spheres of 5–200 micron diameter have been developed. One promising method is based on sol-gel processing [2–5]. Particular method involves the dispersion of an aqueous colloidal sol of metal oxide in a dehydrating liquid which extracts water from the interior of sol droplets. A second method comprises the coating of a pre-processed polymer or glass microsphere with ceramic gel followed by firing [6]. In other methods, atomized liquid droplets or spray dried powders passing through a furnace or a thermal plasma flame give rise to hollow microspheres [7–9]. Thermal plasma ensures adequately high temperature for melting the ceramic particles. Water vapor or other gases liberated due to the decomposition of binder or blowing agent, which is usually blended to the agglomerates, are supposed to form internal cavity. By controlling the operating conditions, hollow particles can be formed even during the spray drying process [10, 11]. The present report describes the preparation of hollow silica microspheres by inductively coupled RF thermal plasma. The research was initiated and supported by PREMIS Technologies, France. The effect of process gases and the type of the starting materials on the microstructure of the formed particles was studied in the experiments. The experimental apparatus consisted of a TEKNAtype induction plasma torch (PL-035LS) with a quartz confinement tube of 25 mm and a water cooled steel chamber connected to a cyclon. The plasma plate power of 21 kW was provided by a three turn, water cooled induction coil from an RF generator operating at an oscillator frequency of 3 MHz. High purity argon was used both as plasma and sheath gas with flow rates of 20 and 60 l min−1, respectively. Hydrogen was mixed into the sheath gas with a proportion of 10 V/V% to improve the enthalpy and the heat transfer coefficient of argon. In some experiments, air was substituted for argon. The powders were fed with a PRAXAIR powder feeder unit through an injection probe to the top of the plasma flame by argon carrier gas (3 l min−1) with a constant feed rate of 10 g min−1. The outlet of the quartz injection tube has an inner diameter of 2 mm. It was located 10 mm below the top of the induction coil. Three different raw materials were investigated in the experiments. Powder Type 1 of negligible porosity (from Degussa) is made by precipitation of pure silica. Powder Type 2 is an agglomerated sample, in which the diameter of primary silica particles (from SIFRACO) is ∼5 μm. Besides silica, NaNO3 and Na-silicates were also blended into Powder Type 2 for different purposes. Na-silicates were used as binding agent of primary particles, while the blowing agent, NaNO3 was devoted to release different gases (O2, N2) at higher temperatures by decomposition. Powder Type 3 is also an agglomerated silica sample (from SIFRACO). It differs from Powder Type 2 in that it contains only polyvinyl alcohol (PVA) as binder about 1% by weight. Both the raw materials and the produced powders were characterized for density, specific surface area, phase composition (XRD) and microstructure (SEM). Density was determined by means of a helium picnometer (Micromeritics AccuPyc 1300). The specific surface areas were measured with a conventional volumetric instrument by BET method. The microstructural (SEM) investigations were performed on a Jeol JSN50A apparatus. Cross-section images of the particles were made by embedding them in resin media followed by their cutting with diamond blade. During the in-flight process of plasma spraying silica ceramic grains are melted. As a result, one anticipates formation of solid particles of reduced size. It is reasoned by the better space filling of melts as compared to solids. In addition particles can also evaporate to some extent. Spheroidization by plasma flame takes the advantage of this phenomenon [12, 13]. Particles, however, can also become hollow. Formation of hollow particles can be attributed to the release of gases in the interior of particles, if they cannot break through the outer melted layer. The freezing temperature of the

Processes of Silica Network Structure Formation in Reverse Micellar Systems

The processes by which the silica-network structure is formed in reverse micelles were elucidated. The reverse micellar system was composed of didodecyldimethylammonium bromide (DDAB), cyclohexane, and HCl− aqueous solution, and the silica source was tetraethoxysilane (TEOS). The hydrolysis reactions for TEOS were completely different from those of usual homogeneous sol−gel processes. The rate of hydrolysis was affected by the curvature of the reverse micelles. The reverse micelles also promoted the formation of network bonds of silica even under acidic conditions. Primary silica particles spherical in shape were generated first. DDAB and partially hydrolyzed TEOS cooperatively formed the primary spherical particles, which were stable against coagulation and did not grow to large particles. The primary particles formed clusters, which were converted to linear silica rods with some branches upon neutralization of HCl with NaOH, and the system became a transparent gel. The diameter of the silica rods was si...