Spinnablity and structure characterization of mullite fibers via sol–gel-ceramic route

Viscosity and rheology behaviors of the mullite sols prepared from aqueous solution of aluminum nitrate, aluminum isopropoxide and tetraethylorthosilicate has been investigated. Rheological measurement suggested that mullite sols exhibited good spinniabilty when the evaporation of the solvent is allowed during sol–gel process. Spinnable sols showed shear-thinning flow behavior with high viscosity to the time of gelation. By adjusting temperature, the gelation degree and viscosity of the sol could be stabilized at a certain value and the sol–gel transition could be transferred to the spinning line of a laboratory dry-spinning apparatus. Continuous mullite fibers were obtained from such sols using sol gel dry-spinning method. The final ceramic fibers had smooth surfaces with an average diameter of 50 μm. Structure evolution of mullite ceramic fibers were characterized by MAS NMR and specific surface area analyses.     

Rheology of sols and fiber drawing

The investigations on the viscosity of the metal alkoxide solutions in the course of hydrolysis and condensation reactions leading to the gelation of sols have been reviewed. All the solutions which gel finally show a continuous increase in viscosity with time until they gel at a certain viscosity higher than about 100 P which is dependent on the composition of the starting solution. When a metal alkoxide solution is catalyzed with an acid and its water content is small at less than 4 or 5 in the water to alkoxide mol ratio, the solution exhibits spinnability at viscosities above about 10 P and becomes drawable into gel fibers, whereas no spinnability appears when a solution contains a large amount of water or is catalyzed with an alkali like ammonia.

The investigations on the relation between the reduced viscosity and the concentration or the relation between the intrinsic viscosity and the number-averaged molecular weight have made it clear that the spinnable silicon alkoxide solutions have long-shaped siloxane particles and non-spinnable solutions have round particles.

It has been established that spinnable solutions exhibit Newtonian flow behavior up to high viscosities, where fibers can be drawn, while non-spinnable solutions exhibit marked structural viscosity and, sometimes, thixotropy. Similar behavior is confirmed in the alumina solutions prepared from inorganic salts. It has been reported that the viscosity measurements provide information on the rate of the reaction leading to gelation. It is also known that the viscosity of the alkoxide solution controls the thickness of the thin coating film made by the dip-coating technique using an alkoxide solution.     

Interaction between colloidal particles in SiO2 and TiO2 sols

The effect of the interaction between colloidal particles in SiO₂ and TiO₂ sols on the viscosity of the sols has been examined. It was found that the interaction was significantly influenced by the zeta potential of electrical double layers and terminal bonds associated with particle surfaces. Water/alkoxide mole ratio and pH affect the zeta potential and the terminal bonds and, consequently, change the interaction between the particles and the viscosity of the sols. In addition, the relationship of viscosity with the volume fraction of the colloidal particles in SiO₂ sol has been described by a Dougherty-Kreiger equation when the viscosity reached a stable value with time.     

Comparative study on hydrophobic anti-reflective films from three kinds of methyl-modified silica sols

Methyl-modified silica sols were prepared with the polymer of methyltriethoxysilane (MTES), the polymer of dimethyldiethoxysiloxane (DDS), and hexamethydisizane (HMDS) as mono-, di-, and tri-methyl modifiers respectively. By comparing the size and the shape of clusters in three different methyl-modified silica sols, the special nature of the sol was found to be the key to the property of films. Different modal modification of methyl to silica particles led to different cluster fractal structures that influenced the morphology, the porosity of films and consequently the anti-reflection characteristics. The contribution of methyls in or on clusters directly determined the hydrophobicity. Using mono- or di-methyl-modified silica sol, the film/water contact angles were less than 120°. But it could reach 165° when using tri-methyl-modified silica sol. The lowest reflectivity of film could reach 0.0% for all the three modified sols. As a result of the comparative study, tri-methyl-modified silica sol was more suitable to prepare hydrophobic anti-reflective film with required high optical performance.     

Preparation of extremely fine particles by vacuum evaporation onto a running oil substrate

A large amount of extremely fine particles of Ag were prepared by vacuum evaporation onto a running oil substrate. The size distribution of Ag particles thus prepared was studied by electron microscopy. It was found that the mean diameter of the particles strongly depends upon the deposition rate, and kind and viscosity of the oil. Particles of mean dimater from 30 to 80 Å can be produced. The sticking coefficient on the oil surface for arriving atoms is, in general, smaller than that on a glass surface. The coefficient increases with increasing viscosity for the same kind of oil. Coalescence growth of particles is also detected by heating the oil containing the particles. The heat treatment also enables us to control the practicle size between 30 to 80 Å.     

Growth of LaBGeO5 crystal fibers by the micro‐pulling down technique

The LaBGeO₅ compound (LBGO) was prepared by solid state reaction at 1050°C and characterized by XRD and DTA analysis. The direct growth of LBGO fibers from the melt using micro‐pulling down technique was unsuccessful because of its high viscosity. The study of the LBGO‐LiF phase diagram showed that LiF could be considered as a convenient flux to reduce viscosity of the melt during the growth process. Several crystal fibers were then grown and characterized by Raman spectroscopy. To decrease the high volatility of LiF, B₂O₃ was added to the melt. A white cloudy fiber was obtained from LiF‐B₂O₃ flux and checked by Raman spectroscopy and scanning electron microscopy.     

Low-temperature fabrication and photocatalytic activity of clustered TiO2 particles formed on glass fibers

Clustered anatase phase TiO₂ particles were uniformly formed on the surface of glass fibers by a liquid phase deposition (LPD) method at 60 °C using TiF₄ and H₃BO₃ as the precursors. The clustered TiO₂ particles deposited on the glass fibers and as a photocatalyst these particles not only have a larger surface area than TiO₂ thin films, but also can avoid the disadvantages of using TiO₂ powders encountered in air purification or water treatment. The photocatalytic activity of the sample was evaluated by the photocatalytic oxidation of nitrogen monoxide (NO) in the gaseous phase. The deposition conditions and chemical composition of the clustered TiO₂ particles were discussed. It was found that the clustered TiO₂ particles that formed on the glass fibers obviously showed photocatalytic activity without high-temperature calcination. A formation mechanism was proposed to account for the formation of TiO₂ clustered morphology on the glass fibers.     

Direct preparation of silica hollow spheres in a water in oil emulsion system: The effect of pH and viscosity

Silica hollow particles have been prepared in this work in using a water in oil (W/O) emulsion system at room temperature that employs a water-soluble amine as catalyst and tetraethylorthosilicate (TEOS) as the silica source. The pH value of the aqueous phase and the viscosity of external oil phase were found to be the key factors contributing to the formation of stable and regular spherical silica hollow particles. When the pH value of the aqueous phase was controlled between 8 and 9, silica hollow particles could be synthesized. The shell thickness of the hollow particles increased with the length of the hydrocarbon tail of the amine catalyst. The viscosity of the external oil phase determined the shape regularity of the spherical silica hollow particles. If viscous silicon oil was used as the external oil phase instead of kerosene, perfect silica hollow spheres could be fabricated. The kinetics of the formation of silica hollow particles was believed to be based on the difference between the hydrolysis rate and the condensation rate of TEOS, which can be adjusted by the pH value of the aqueous phase and the viscosity of external oil phase.     

Preparation of thick,crack-free germanosilicate glass films by polyvinylpyrrolidone and study of the UV-bleachable absorption band

With addition of polyvinylpyrrolidone (PVP) transparent, stable GeO₂–SiO₂ sols containing up to 60–80 mol% GeO₂ were synthesized using tetraethyl orthogermanate (TEOG) and tetraethylorthosilicate (TEOS) as precursors for germania and silica, respectively. It was shown by TEM analyses that the PVP can be absorbed onto the colloidal particles providing steric hindrance for the combination and aggregation of particles. These sols were observed showing rapid increase in viscosity within both the early period and the end period of sol aging time, but exhibiting a viscosity value of about 28 mPa s within the rest aging time (45–90 h) satisfying well the requirements for the deposition of thick films by cycles of dip-coating operation. It was determined by TG-DTA and SEM analyses that the densification of GeO₂–SiO₂ gel material with PVP was much more retarded than the gel without PVP resulting in crack-free germansilicate films with a thickness of 3 μm. The crystallization behavior of germansilicate films was enhanced with the increase of GeO₂ content but glass films with a composition of 60GeO₂ · 40SiO₂ was obtained by sintered at 700 °C for 1 h and annealed at 550 °C under a flowing H₂/N₂ atmosphere for 2 h. FT-IR analyse showed that the heat treatment at 700 °C for 60 min was effective to remove the organics and hydroxyl groups in the germansilicate film. An intense 5 eV absorption band was distinctly observed in films. The intensity of this absorption band was found to be effectively bleached by UV illumination. Weak photoluminescence emission bands which originated from the neutral oxygen di-vacancy (NODV) were detected near 375 and 276 nm. Therefore, the 5 eV absorption band observed in this work was mainly caused by the neutral oxygen monovacancy (NOMV). A saturated absorptivity change of the UV-bleachable band after prolonged illumination was found to be 256 cm⁻¹ for the 60GeO₂ · 40SiO₂ films implying the NOMV concentration in the films reached about 3.8 × 10¹⁸ cm⁻³.     

Interaction of vinyl trichlorosilane and water with E-glass surfaces

The objective of this work was to elucidate the nature of water vapor adsorption on E-glass fibers drawn under dry conditions and E-glass fibers drawn in a vinyl trichlorosilane (VTS) atmosphere. VTS was chosen as representative of trifunctional organosiliconchloride coupling agents. The long-range goal of this work is to provide guidance in the design of glass fiber and coupling agent compositions to reduce the influence of atmospheric water on the stability of glass reinforced plastics. The adsorption of VTS vapor on E-glass fibers was found to be time-dependent and to fit the Langmuir model. Water vapor was shown to have a lower affinity for E-glass surfaces drawn in dry air than for E-glass fibers drawn in a dry air/VTS atmosphere. The BET surface areas were found to vary with sequential adsorptions of water vapor. The adsorption of water vapor on E-glass surfaces was found to vary with the square root of time.     

Continuous preparation of decorated nano zinc oxide organic sols with intense luminescence

A sort of decorated nano ZnO organic sols have been successively prepared by pulsed laser ablation at the interface of ZnO target and a flowing liquid containing polymers. It is found that the decorated nano ZnO ethanol sols, the decorated nano ZnO-PS (polystyrene) cyclohexane sols and the decorated nano ZnO-PMMA (polymethyl methacrylate) ethyl butyrate sols all have strong fluorescence emission at 329 nm and 411 nm, 308 nm and 317 nm, and at 330 nm and 400 nm, respectively. The results show the decorating for nano ZnO will intensely affect their luminescence, and the wavelength and intensity of luminescence can be adjusted or controlled by the different decoration.     

A SAXS study of V2O5·nH2O sols and gels

Small-angle X-ray scattering (SAXS) measurements on V₂O₅·nH₂O sols and gels, prepared by dissolving V₂O₅ glass into water at room temperature, show that there are V₂O₅ polymeric fibers entangled like random coils in the sol of n 5000, while the deviation from the random coil behavior occurs in the dilute sol of n 6000.

A Bragg peak appears at the scattering vector h 0.02 Å⁻¹ to be superimposed on an asymptotic h⁻²-course in the SAXS curve of the concentrated sol of n 680. This means that the spatial correlation between V₂O₅ polymeric fibers takes place even in the fluid state.

V₂O₅·nH₂O sols completely lose fluidity at n 250 to transfer to the gel state, where V₂O₅ polymeric fibers begin to pile up in the parallel with a substrate surface. Such a layer structure is preserved up to the gel of n 4. However, V₂O₅ polymeric fibers are randomly oriented within each layer.     

Organic–inorganic hybrid coatings for corrosion protection

The corrosion resistance of sol–gel derived, organic–inorganic, silica-based hybrid coatings with various amounts of organic content was studied. Hybrid sols were prepared by copolymerizing tetraethylorthosilicate (TEOS) and 3-methacryloxypropyltrimethoxysilane (MPS) with a two-step acid-catalyst process. Hybrid coatings were dip-coated on 304 stainless steel substrates and annealed at 300 °C for 30 min. Such prepared hybrid coatings were found to be relatively dense, uniform and defect free. The adhesion and flexibility of the coatings were characterized. The influences of the amount of organic component incorporated into the coatings and the aging of sols on corrosion protection were studied. Electrochemical analyses showed that the relatively dense hybrid coatings provided excellent corrosion protection by forming a physical barrier, which effectively separated the anode from the cathode. Some preliminary biocompatibility tests were also conducted on the hybrid coatings.     

Co-solvent mediated fiber diameter and fiber morphology control in electrospinning of sol–gel formulations

Understanding the hydrolysis, condensation and solvent evaporation processes in precursor sols for electrospun ultrafine fibers permits good control over fiber diameter and morphology in a reproducible manner. The (CH₃CH₂CH₂O)₄Ti/AcOH/Poly(n-Vinyl Pyrrolidone)/Ethanol sol–gel system is used to illustrate this principle. Four different co-solvents (ether, DMF, DMSO and formamide) were also used to establish their impact on fiber diameter. Among other physical properties of solvents, the relative volatility of the solvent mixtures used in this study was found to influence the morphology of the electrospun fibers by affecting the stability and range of existence of the electrified jet.     

Optical properties of the optical fiber containing Co doped ZnO-Al2O3-SiO2 glass-ceramics

Optical characteristics of silica glass optical fibers containing Co²⁺ doped ZnO-Al₂O₃-SiO₂ (ZAS) glass-ceramics prepared by slurry-doping method were investigated. The absorption and emission bands of the fibers were found to be originated from the tetrahedral Co²⁺ in ZnAl₂O₄ crystals in ZAS glass-ceramics particles embedded in the core of the fibers. The crystal field strength of the Co²⁺ ions in the optical fiber was found to be smaller than that of the Co²⁺ ions in the bulk ZAS glass-ceramics.     

Application of a microrheology technique to measure the viscosity of disodium cromoglycate liquid crystal

Although the science of rheology is well established, some important challenges still persist to measure the viscoelastic properties of complex fluids, such as biological solutions and liquid crystals (LC). In this work, we present a method, based on the calculation of the step length of Brownian particles, to measure the effective local viscosity sensed by microscopic objects in the LC host. This approach allowed us to quantify the anisotropy of the viscosity, and it could also be extended to measure the local viscosity in other nonhomogeneous media. We also present a new guided light dark-field microscopy technique that was used to track particles during our experiments.     

High pressure effects on liquid viscosity and glass transition behaviour,polyamorphic phase transitions and structural properties of glasses and liquids

Since the pioneering work of Bridgman it has been known that pressure affects the glass transition of polymers and liquid state viscosities. Usually the T_g and viscosity both increase as a function of pressure as expected from ‘free volume’ theories. However, H₂O provided a notable exception in that the viscosity passes through a minimum at low temperature. It was thought that this might be linked to the anomalous thermal expansion behavior. However further research on geologically important aluminosilicate liquids revealed that they could show anomalous viscosity decreases with increasing pressure and this behavior is given a structural interpretation as five-fold coordinated Si⁴⁺ and Al³⁺ species are formed. Also the existence of polyamorphism or density-driven liquid–liquid phase transitions in certain systems can lead to anomalies in the T_g or η vs. P relations. This may be the case for H₂O, for example. Current research is focusing on investigating structural changes in liquids and glasses at high pressure as the rich variety of behavior is becoming recognized. Both experimental studies and computer simulations are important as the underlying phenomonology is linked to changes in the glass or liquid structure as a function of densification.     

High porosity silica xerogels prepared by a particulate sol–gel route: pore structure and proton conductivity

The lower cost and higher hydrophilicity of silica xerogels could make them potential substitutes for perfluorosulfonic polymeric membranes in proton exchange membrane fuel cells (PEMFCs). For that purpose, we need to obtain micro or micro and mesoporous silica xerogels with a high porosity. The preparation of micro (<2 nm) and micro and mesoporous silica xerogels (2<d_poresize10 nm) from particulate as oppossed to polymeric suspensions of silica using tetraethyl orthosilicate (TEOS) as precursor is used. Two techniques of varying packing density have been performed in this work: (1) Control of the aggregation degree in the sol by adjusting its pH before gelation (pH 5, 6 and 8) and (2) Mixture of sols with a different average particle size (particles formed under acid and base catalyzed reactions). Proton conductivity of the obtained xerogels was studied as a function of temperature and relative humidity (RH). High pore volume, high porosity and small pore size SiO₂ xerogels have been achieved in the calcination temperature range from 250 to 550 °C. The calcined xerogels showed microporosity or micro and mesoporosity in the whole range of calcination temperatures. By mixing sols (molar ratio: acid/base=4.8) porosities up to 54.7±0.1% are achieved, at 250 °C of firing temperature. According to EMF measurements, electrical transport is due to protons in this kind of materials. The proton conductivity of the studied xerogels increased linear with measured temperature. A S-shaped dependence of the conductivity with the RH was observed with the greatest increase noted between 58% and 81% RH. Xerogels with a low porosity (40.8±0.1%) and an average pore size less than 2.0 nm showed lower values of proton conductivity than that of xerogels with a higher porosity and a higher average pore size in the whole range of temperature and RH. When silica xerogels, with the highest conductivity, are treated at pH 1.5, that property increased from 2.84×10⁻³±5.11×10⁻⁵ S/cm to 4.0×10⁻³±7.2×10⁻⁵ S/cm, at 81% RH and 80 °C. It indicates that the surface site density of these materials has a strong effect on conductivity. Proton conductivity values achieved are less than one order of magnitude lower than that of Nafion, under the same conditions of temperature and RH.     

Influence of drawing temperature on the strength of fluoride glass fibers

The tensile strength of fluorozirconate glass fibers was measured as a function of fiber drawing temperature, and strengths ranging from 60–220 MPa were observed. Statistical analysis of the strength data points to a bimodal behavior in the fibers drawn at high temperatures, while the mode of fracture is essentially unimodal for fibers drawn at lower temperatures. SEM examination of the fibers drawn at high temperatures revealed the presence of crystals in the fibers, and these crystals were associated with flaws which gave rise to the observed low strength distribution.     

Studying the Molecular Mechanisms of Interaction of Nanoaerosol Particles with a Model Membrane

An X-ray study of the interaction of Langmuir monolayers with nanoparticles under the conditions when nanoparticles arrive at the monolayer from the side of the air medium has been performed for the first time. Measurements by the X-ray standing-wave method and two-dimensional diffraction have made it possible to investigate the transport of nanoaerosol particles through a lipid monolayer and observe the structural changes occurring in it. Nanoaerosol particles were formed by electrohydrodynamic sputtering of desalted solutions of ferritin. It was shown that nanoaerosol particles pass through the monolayer without disrupting the two-dimensional packing of lipid molecules. The ferritin molecules penetrating the aqueous subphase after sputtering are found to be adsorbed under the monolayer.