Preparation of alumina fibers by sol-gel method

The shape of particles present in the alumina sols prepared from inorganic salts was examined by transmission electron microscopic observation and related to the rheological property and fiber drawing behavior of the sols. It was found that fibers could be drawn in the viscosity range of 1–100 Pa·s from the sols in which long-shaped particles were found, while no fibers could be drawn from the sols containing round particles. It was also found that non-spinnable sols showed relatively large structural viscosity, whereas spinnable sols showed Newtonian flow or very little structural viscosity.     

陶瓷纤维/莫来石晶须原位增强堇青石-莫来石轻质隔热材料

以蓝晶石、粘土、氧化镁粉为原料,以淀粉为造孔剂和固化剂,引入适量的硅酸铝陶瓷纤维或多晶莫来石纤维和AlF3,通过莫来石晶须在陶瓷纤维表面的原位形成,制备了具有陶瓷纤维/莫来石晶须互锁结构的堇青石-莫来石轻质隔热材料.研究了陶瓷纤维在AlF3的作用下对材料显微结构观察、常温力学性能和导热系数的影响.研究表明:在AlF3的作用下,莫来石晶须在硅酸铝纤维表面垂直生长,部分穿插在发育良好的堇青石晶粒中;这种具有陶瓷纤维/莫来石晶须互锁结构的的陶瓷材料,其力学性能得以提高并降低了材料的导热系数.     

Rheology of the gelation of fluorine-doped silica sols

Dynamic oscillatory shear measurements are used to probe the gelation kinetics of aqueous sols composed of either particulate silica or silicon alkoxide solution. Unlike steady shear measurements, these dynamic tests do not alter the structure and kinetics of the sol-gel process. The dynamic storage moduli of both systems show sharp transitions at the onset of gelation. However, the gelation kinetics of the two systems are very different; the modulus of alkoxide system remains unchanged until the gel point is reached, whereas that of the particulate system increases with time even below the gel point as the colloid cluster grow. Unlike the alkoxide gel, the particulate gel reverses to a sol upon shearing. The gelation kinetics of a resheared sample is slower than that of a fresh sample. The structures associated with the sol-gel transition for this particulate system have been monitored using freeze fracture microscopy and correlate with rheological observations. The gelation times for both sols are varied by changing the fluoride ion content.     

Fluorescence properties of a ruthenium complex in silane sols and gels at different pH

Silane gels containing ruthenium Ru(bpy)₃²⁺ complex were obtained by the sol-gel method. The gelation of sols at different pH values was monitored by fluorescence stationary and time-resolved measurements. In all tested sols a blue shift of fluorescence emission maximum of the fluorophore was observed during the gelation. Changes in λ_max on subsequent days of sol-gel transition depended on starting pH of the sol. An increase of the excited state of fluorophore lifetime in the gelation process was observed. When the gelation was finished the lifetime τ₁ was constant. A strict correlation between the blue shift of fluorophore emission maximum and changes in the lifetime of ruthenium complex in this system on subsequent days of the sol-gel transition was observed. On this basis, changes in the fluorophore microenvironment in the sol induced by different rates of competing hydrolysis and condensation reactions were discussed.All observed effects were attributed to the interaction of molecules of the ruthenium complex with hydroxyl groups in the gel net.     

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.     

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.     

Investigation on sol–gel silica coatings for the protection of ancient glass: Interaction with glass surface and protection efficiency

To hinder the phenomenon of weathering of ancient stained glass, the present work proposes the application of sol–gel coatings to the glass surface. Previous investigations [1], [2], [3], [4], [5] and [6], in fact, show that sol–gel silica coatings do not change the appearance of artistic glasses when deposited on their surface. Moreover, the film thickness is so small (around 200 nm) and its composition and structure so compatible with that of the original glass, that the characteristics of the coating and original glass are not distinguishable. In this work, several recipes used to produce sol–gel coatings have been tested in order to understand their behavior when adopted for covering ancient weathered glass. The coatings are made of sol–gel silica prepared with two different catalysts: H⁺, Pb²⁺ and without catalyst. All the investigated samples show a good adhesion of the coating to the glasses used to simulate the behavior of ancient artefacts. The sol–gel silica coatings have been studied before and after accelerated ageing to test the resistance of the protective coatings to weathering. Another important index to test of the efficiency of the sol–gel coatings for the protection of an ancient glass is the lead ion mobility. In ancient stained glass, in fact, this element is present in the metallic lead network, in the grisaille paintings and constitutes a main component of many glass tesserae. The action of water on this highly mobile ion involves the degradation of the glass itself and the release of the ion in the rain solution. Ageing tests show the efficiency of H⁺ and Pb²⁺ catalyzed coatings and the inefficiency of the non-catalyzed sol–gel layers.     

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.     

Rheological characterization of the gel point in sol–gel transition

The gelation of hydrolyzed alcoholic solutions of acetic acid modified titanium isopropoxide has been analyzed through rheological measurements. A new method is proposed for determining the chemical gel point of these systems that fail to obey the Chambon–Winter criterion. The suggested method is based on defining the gel point as the instant at which the variation of the viscoelastic function tan δ with frequency is minimum. The influence of the sol composition on the gel point has also been investigated. The correlation between the gelation time and the examined compositional parameters, i.e. the modification and hydrolysis molar ratios, is reported. It can be concluded that the initial sol composition has a strong effect on the kinetics of the gelation process, but it does not significantly affect the gelation mechanism.     

The effect of aluminum sources on synthesis of low expansion glass–ceramics in lithia–alumina–silica system by sol–gel route

This paper investigated the effect of different aluminum sources on the crystallization behaviors, thermal expansion and morphology of lithium aluminosilicate glass–ceramics sintered at different temperatures. Specimens were prepared by sol–gel technology. The crystalline phase in the specimen with Al(NO₃)₃9H₂O aluminum source sintered at 1300 °C has shown a major phase of Li₂Al₂Si₃O₁₀ accompanied by minor phase of LiAl₅O₈ and trace amount of SiO₂. The β-eucryptite appeared as the only phase in the specimens with aluminum isopropoxide or aluminum isopropoxide mixed with Al(NO₃)₃9H₂O aluminum sources sintered at 1300 °C. The sintering temperature has significant effect on the thermal expansion behavior of all the specimens. SEM indicated that microstructures were spherulitic crystals and no distinct differences were observed in the specimens with different aluminum sources.     

New approach for measuring gelation rate during synthesis of ZnO/SiO2 gel

The synthesis of mixed ZnO/SiO₂ oxides has been carried out using sol gel technique. Gelation time of the produced oxides gel has been measured experimentally by using turbidity change with time using Turbidimeter. In addition, gelation time was estimated visually. It is found that the gelation time is decreased by increasing the concentration of ZnO and SiO₂. Correlations between gelation times and concentrations of products are also discussed. Surface energy between the formed gel and solution is calculated as 12.1 mJ/m². The gelation rate is increased and the Gibbs free energy for the formation of critical nucleus is decreased by increasing the concentration product of ZnO · SiO₂. The critical radius of the nucleus is decreased from 5.75 to 5.02 Å when the concentration product is increased from 6.42% to 15.72%. On the other hand, the number of molecules in the critical nucleus is decreased from 11 to 8 when the concentration product changed under the same conditions. This approach can be used as a model to discuss the effect of any additives on the enhancing or inhibition the gelation rate for any gel.     

Raman and ESR study of sol–gel materials from ZnO–TiO2–B2O3 system

The paper studies the materials from ZnO–TiO₂–B₂O₃ ternary system, obtained by sol–gel method [1] and [2], starting from organic and inorganic precursors. The obtained samples are investigated by FTIR and Raman spectroscopy, which provide structural information, at molecular level. FTIR absorption maxima are identified and discussed according to literature data. Raman spectra are acquired by a Raman Jasco NRS-3100 spectrometer, at 532 nm wavelength and put in evidence characteristic vibration modes for all three oxide components. ESR spectra were plotted with the aid of a JES-FA 100-JEOL Japan spectrometer and titanium surrounding is investigated.     

A new chemical strategy: Mineralizer-free synthesis of iron zircon ultrafine particles by an epoxide assisted sol–gel route

Ultrafine iron zircon particles were prepared by a sol–gel route using propylene oxide as a gelation agent. The unique chemistry of this method produces highly homogeneous gel intermediate, resulting in the substantial lowering of heat treatment temperature to 1000 °C without using of mineralizer. This calcination temperature is lower than that in solid-state chemistry and other sol–gel routes, leading to the minimizing of the particles aggregation and growth. The non-mineralizer synthesis process guarantees the preparation of pure phase and high quality iron zircon with ultrafine particle size. This epoxide assisted sol–gel route shows great potential as a general procedure for the large scale production of ultrafine particles of zircon based pigments.     

Thermosoftening phenyl siloxane glasses prepared via sol concentration

Thermosoftening phenyl polysiloxane glasses have been prepared by concentration of the corresponding sol above the melting temperature of the resultant materials, which we call the ‘sol concentration method’ (SCM). Because the glass melt is directly obtained without a gelation process, a remarkable shortening of the process time and suppression of crack formation in the sol–gel transition region have been realized. That is the whole process time of the SCM (about 24 h) is much shorter than that of the conventional sol–gel process (about 10 days). Thermosoftening phenyl polysiloxane glasses with high transparency in the UV region were obtained by the SCM.     

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.     

Er-doped silica–hafnia films for optical waveguides and spherical resonators

In this paper we present some result on sol–gel derived silica–hafnia systems. In particular we focus on fabrication, morphological and spectroscopic assessment of Er³⁺-activated thin films. Two examples of silica–hafnia-derived waveguiding glass ceramics, prepared by top–down and bottom–up techniques are reported, and the main optical properties are discussed. Finally, some properties of activated microspherical resonators, having a silica core, obtained by melting the end of a telecom fiber, coated with an Er³⁺-doped 70SiO₂–30HfO₂ film, are presented.     

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.     

Preparation of zirconia mullite flakes via a plasma rapid solidification process using starting materials derived from a sol–gel technique

Flakes of zirconia–mullite with different zirconia contents varying from 3 to 24 wt% were produced from sol–gel derived raw materials via a plasma melting method followed by a rapid solidification process using a rotating copper roll. The morphology, phase constitution and microstructure development of the as-prepared flakes and of the flakes after various heat treatments were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). It was found that the starting materials could be transformed from the initial irregular-shaped powders into flakes which consisted of mullite, zirconia phase, a relatively large amount of glassy phase and pores. Using TEM, it was shown that the crystalline phases consisted of zirconia polymorphs and mullite. The glassy phases consisted of Al₂O₃–SiO₂ solid solution supersaturated with zirconia. Firing at 1500 °C or 1700 °C resulted in full crystallisation of the flakes and a fairly homogeneous distribution of zirconia particles in terms of size and shape dispersed in the mullite matrix, which contained both intra-granular and inter-granular precipitates. The microstructural characteristics of the flakes may provide promising physical properties for applications in high temperature thermal insulation materials.     

Observation of the aggregation behavior of silica sols using laser speckle contrast measurements

We report on a new optical method to observe the onset of aggregation in alcoholic tetraethoxysilane (TEOS) sols using laser speckle contrast measurements. The contrast in a speckle image produced by coherent light provides information about the internal contrast of the medium being studied. For silica sols, changing the amount of acid or base catalyst was the most important factor in determining the aggregation behavior of the sol. We investigated this effect by varying the TEOS/base ratio by a factor of 6. This shifted the onset of aggregation as determined by speckle contrast from 46% of the gel time for the lowest amount of base to 74% for the highest. Conversely, varying TEOS/acid ratio by a factor of 3, shifted the onset of aggregation from 74% of the gel time for the lowest amount of acid to 64% for the highest. Measurements of this type provide information that can be used to test models of sol aggregation and gel formation.     

Preparation and characterization of nickel-modified carbon cryogel beads with uniform particle size

Nickel-modified carbon cryogel beads (Ni/CCBs) with uniform particle size were successfully prepared using the syringe injection method, employing sol–gel polycondensation of a resorcinol–formaldehyde (RF) solution containing Ni acetate as a catalyst. This process was followed by solvent exchange, freeze-drying and carbonization in a nitrogen atmosphere. The effect of viscosity, which increased as the sol–gel polycondensation progressed, of the RF solution on both the particle size distributions of the Ni/CCB and the porous properties was investigated. The influence of Ni content in the initial reactants on the porous properties of the Ni/CCBs was also studied. Monodisperse Ni/CCBs could be obtained by injection during the initial reaction period, before the steep increase in the viscosity of the RF solution. The porous properties of the Ni/CCBs were independent of the viscosity of the RF solution. The BET surface area, micropore volume and mesopore volume of Ni/CCBs decreased with increased Ni content, whereas the macropore volume increased with increased Ni content. The size of the Ni nanoparticles dispersed in the carbon matrix of Ni/CCBs was confirmed to range from 30 to 60 nm.