Mullite fibers prepared from an inorganic sol–gel precursor

Mullite fibers were prepared by sol–gel process using alumina sol and silica sol. Alumina sol was synthesised from dissolution of aluminum powder in aluminum chloride hexahydrate solution. The optimal spinning alumina sol could be obtain in the composition range of Al/AlCl₃·6H₂O molar ratio 3.4–3.8. The Al and Si components were mixed at the molecular level and linear molecules were formed in the mullite precursor sol. The dried mullite precursor gel fibers completely transformed to mullite fibers at 1,000 °C with a smooth surface and uniform diameter.     

Silica-Zirconia Sol–Gel Coatings Obtained by Different Synthesis Routes

Homogeneous xSiO₂-(1−x)ZrO₂ coatings have been prepared onto glass-slides, monocrystalline Si and stainless steel (AISI 304) using sols prepared via acid and basic catalysis. Zirconium tetrabutoxide (TBOZr), zirconium n-propoxide (TPZ), tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) were used as precursors of zirconia and silica, respectively. The different parameters involved in the synthesis procedure, as molar ratios H₂O/alkoxides, NaOH/alkoxides, and sintering temperature have been analysed, correlating the stability and rheological properties of the sols. The evolution and structure of the sols and coatings have been studied by FTIR. Coatings have been prepared by dipping from acid and basic sols. Electrophoretic Deposition (EPD) technique has also been used to prepare coatings onto stainless steel from basic particulate sols in order to increase the critical thickness. A maximum thickness of 0.5 μ m was reached by both dipping and EPD process for 75SiO₂: 25 ZrO₂ composition. The critical thickness decreases with ZrO₂ amount depending strongly of the drying conditions. Si–O–Zr bonds have been identified by FTIR, indicating the existence of mixed network Si–O–Zr in the coatings obtained by the different routes. Crystallisation of ZrO₂(t) was only observed at high sintering temperature (900^∘C) by FTIR and confirmed by DRX.     

The interaction of dibutyltin dilaureate with tetraethyl orthosilicate in sol-gel systems

Sols composed of dibutyltin dilaureate (DTL) and tetraethyl orthosilicate (TEOS) were prepared using a mixture of methyl ethyl ketone (MEK) and acetone as the solvent in order to study the interaction between the oligomeric Sn and Si species. The hydrolysis molar ratio r (r=nH₂O/nM (M: Si, Sn or Si+Sn) was 2. The use of an acid or basic catalyst was avoided, as the sols are intended to be used in the formulation of potential stone consolidants. The sols were studied by several spectroscopic techniques including Small Angle X-ray Scattering (SAXS), ²⁹Si and ¹¹⁹Sn NMR, Fourier Transform Infrared (FTIR) spectroscopy and X-ray diffraction (XRD). According to the spectroscopic results the lauric acid produced by the hydrolysis of DTL modifies the condensation path of the Si species, leading to the formation of two types of oligomeric chains: linear swollen and multiparticle diffusion-limited aggregates, depending on the molar ratio Sn/Si. The ²⁹Si NMR results indicated that the hydrolysis of DTL catalizes the condensation of the Si species, giving as a result higher condensation extents of the Si species in the Sn-Si sols compared to a pure Si sol. Based on the Radial Distribution functions (RDF) and the FTIR results, heterocondensation occurred.     

Preparation of Polycrystalline Antimonic Acid Films by Electrophoretic Deposition

Cubic antimonic acid (Sb₂O₅·nH₂O) films were successfully prepared on stainless steel and Si(100) substrates by electrophoretic deposition (EPD) using two types of sols. The sols were prepared by reacting an H₂O₂ aqueous solution with Sb(O-i-C₃H₇)₃ or metallic Sb powder. The resulting films were found to consist of fine particles of cubic Sb₂O₅·nH₂O single crystals with uniform particle sizes of 30 nm and 150 nm. The weight of the Sb₂O₅·nH₂O deposit on the anode Si(100) substrate by EPD increased linearly with the current density in the range of 0–0.67 mA cm^–2, when the sol pH was over 7. The proton conductivity of the polycrystalline Sb₂O₅·nH₂O discs, formed from the two types of sols, was evaluated by an ac impedance method at room temperature under controlled levels of relative humidity.     

Characterization of the hydrolysis and polymerization processes of methacryloxypropyltrimethoxysilane

The sol-gel synthesis of hybrid methacrylate-silica materials using methacryloxypropyltrimethoxysilane (MPS) as precursor, has been analyzed using ¹H and ²⁹Si liquid NMR as well as ²⁹Si and ¹³C MAS-NMR. Under the present experimental conditions (H₂O/MPS=3; pH=2), hydrolysis of methoxy groups is fast. However, 5% of unreacted alkoxy groups are still present in the sol after 14 days aging. Condensation reactions lead mainly to cyclic or short linear species. The number of cross-linking points never exceeds 20% of the Si units, which seems to prevent the formation of a gel. Polymerization of methacrylate groups occur easily under short times of U.V. irradiation. A quantitative analysis shows about 20% of residual unreacted groups in the polymerized materials. Addition of free methylmethacrylate in the hydrolyzed sols helps in getting a fully polymerized organic network.     

Preparation and Spectroscopy of Aluminosilicate Films Deposited on Graphite Surfaces via an Anhydrous Sol-Gel Route

Anhydrous aluminosilicate sols with Al : Si molar ratio ranging from 1 : 10 to 1 : 5 were prepared by a two-step anhydrous sol-gel process, in which tetraethoxysilane (TEOS) is pre-hydrolysed by formic acid, followed by addition of aluminium ethoxide. Unlike the case of aqueous sol-gel routes, where the Si–O–Al network homogeneity is greatly restricted by precipitation of Al(OH)₃, this anhydrous route yields clear, homogeneous sols. The sol formation and densification processes were investigated by infrared spectroscopy and X-ray powder diffraction (XRD) techniques, and the existence of Al–O–Si linkages was confirmed. To demonstrate an application of the anhydrous sol-gel process, aluminosilicate films were deposited onto graphite surfaces by dip coating and densified at 800°C under nitrogen, and their protective effect was evaluated.     

Sol-Gel Synthesis of Protoenstatite and a Study of the Factors That Affect Crystallization

Three different magnesium silicate sols were prepared from Mg(OMe)₂ and TEOS for which hydrolysis with H₂O₂ and H₂O was under stoichiometric, stoichiometric, or over stoichiometric. Xerogels were prepared from the sols by simple evaporation, spray-drying, or freeze-drying. The freeze-dried precursor formed mostly protoenstatite, a high-temperature polymorph of enstatite (MgSiO₃) that is generally not stable at room temperature; the other precursors formed mixtures of protoenstatite and clinoenstatite. The three xerogels and their calcined products were studied with XRD, HTXRD, BET, carbon analysis, TG/DTA and²⁹ Si-NMR. Residual carbon was found to be an important factor in the crystallization, and ²⁹Si-NMR spectroscopy was found to be a better tool than XRD to assess the phase purity of protoenstatite.     

Influence of Sol-Gel Synthesis Parameters on the Microstructure of Particulate Silica Xerogels

The influence of key sol-gel synthesis parameters on the pore structure of microporous silica xerogels was investigated. The silica xerogels were prepared using an acid-catalyzed aqueous sol-gel process, with tetraethoxysilane (TEOS) as the silicon-containing precursor. At high H2O : TEOS ratios, sols synthesized at pH 2–3 yielded minimum values of mean micropore diameter and micropore volume. Analysis of the resulting Type I nitrogen adsorption isotherms and the equilibrium adsorption of N(C4F9)3 indicated micropore diameters for these xerogels of less than approximately 10 Å.Xerogel micropore volumes corresponding to sols prepared at pH 3 and an H2O : TEOS ratio of r = 83 were consistent with nearly close packing of silica spheres in the xerogel. Xerogel microstructure was only weakly dependent upon H2O : TEOS ratio during sol synthesis for r > 10. Xerogel micropore volume increased rapidly with sol aging time during an initial induction period of particle formation. However, the xerogel microstructure changed only slowly with time after this initial period, suggesting potential processing advantages for the particulate sol-gel route to porous silica materials.Surface adsorption properties of the silica xerogels were investigated at ambient temperature using N2, SF6, and CO2. CO2 adsorbed most strongly, SF6 also showed measurable adsorption, and N2 adsorption was nearly zero. These results were consistent with the surface transport of CO2, and to a lesser extent SF6, observed in gas permeation studies performed through thin membrane films cast from similarly prepared silica sols.     

Structure of hybrid organic–inorganic sols for the preparation of hydrothermally stable membranes

A procedure for the preparation of hybrid sols for the synthesis of organic–inorganic microporous materials and thin film membranes is reported. We describe silane reactivity and sol structure for acid-catalysed colloidal sols from mixtures of either tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES), or bis(triethoxysilyl)ethane (BTESE) and MTES. Early-stage hydrolysis and condensation rates of the individual silane precursors were followed with ²⁹Si liquid NMR and structural characteristics of more developed sols were studied with Dynamic Light Scattering. Condensation was found to proceed at more or less similar rates for the different precursors. Homogeneously mixed hybrid colloids can therefore be formed from precursor mixtures. The conditions of preparation under which clear sols with low viscosity could be formed from BTESE/MTES were determined. These sols were synthesised at moderate water/silane and acid/silane ratios and could be applied for the coating of defect-free microporous membranes for molecular separations under hydrothermal conditions.     

苯分子与Si6O18H12和Al6O24H30团簇模型相互作用的理论研究

粘土矿已经被广泛用来去除有机物,修复和净化被石油碳氢化合物污染的土壤和地下水. 我们选择高岭石作为研究对象,构造了Si₆O₁₈H₁₂和Al₆O₂₄H₃₀两个团簇模型分别代表高岭石的硅氧层表面和铝氧层表面,在MP2/6-31G(d,p)//B3LYP/6-31G(d,p)的理论水平上系统地研究了气态下苯分子和高岭石团簇模型的相互作用. 并进一步分析了苯分子和高岭石表面相互作用的各种气态性质,比如：优化的几何构型、结构参数、吸附能、自然键轨道电荷分布、振动频率变化、静电势、电子密度性质(次级氢键的电子密度和拉普拉斯算符值)和电子密度差分等. 优化的几何构型表明苯分子吸附在高岭石表面的本质可能是次级氢键的形成. 其他性质的结果进一步验证了次级氢键的存在,并指出苯更倾向于吸附在高岭石的铝氧层表面,且苯环和铝氧层表面形成近似90°的夹角.     

Fabrication and characterization of composition-gradient CuO/SiO2 composite aerogel

With tetramethoxysilane as the silica precursor, CuCl₂·2H₂O as the copper–oxide precursor, acetonitrile as the solvent and gelled by PO via a sol–gel process, the CuO/SiO₂ composite aerogel was fabricated. By adjusting the amount of CuCl₂·2H₂O, CuO/SiO₂ composite aerogels with different molar ratio of Cu/Si such as 1, 5, 10, 20, 30 and 35 % was prepared. Finally, via a self-built device and sol-co-gelation technic, a continuous formation process was developed to fabricate the composition-gradient CuO/SiO₂ composite aerogel. Density of these aerogels was about 200 mg/cm³, the composition-gradient CuO/SiO₂ composite aerogel was cylindrical and about 2.5 cm in height. Scanning electron microscope was used to characterize its microstructure at different position. X-ray diffraction, energy dispersive spectrometer and Fourier transform infrared spectrometer were used to characterize its composition and composition distribution, the results showed that the cylindrical CuO/SiO₂ composite aerogel’s molar ratio of Cu/Si changed from 31.06 to 4.43 % as the measure point from the bottom up, the whole sample displayed obvious composition-gradient.     

Formation of sol-gel nanostructured mullite by additions of fluoride ion

Summary The subject of this study was to investigate the effect of fluoride ions addition on the temperature of sol gel mullite formation based on the hypotheses that the presence of fluoride ions can decrease the temperature of mullite formation (in respect to common 980°C, in sol-gel processing). Polymeric sols were prepared by mixing TEOS and aluminum nitrate nanohydrate and by adding fluoride ions (from 2 to 5 mass%). DTA, TG, XRD and SEM were used for characterisation of mullite gel and crystalline mullite. The experimental results confirmed that the addition of fluoride ions decrease the temperature of mullite formation up to 890°C for the fluorine concentration of 3.5 mass%. Experimental results showed that the temperature of mullite formation is not a simple function of the fluoride ion content. The mechanism of fluorine effect was discussed in terms of the gelling process, gel structure and the phase separation before the mullite formation.     

Surface topography control of NiS/Ni<Subscript>3</Subscript>S<Subscript>4</Subscript> nanosheets for the promotion of electrochemical performance

One of the promising candidates to replace the chromate conversion coatings for corrosion protection of aluminium alloy AA7075 are the hybrid sol–gel coatings. In the present work hybrid silica sol–gel coatings doped with cerium nitrate were prepared and characterized. Tetraethoxysilane (TEOS) and 3-glycidoxypropyl-trimethoxysilane (GPTMS) were used as precursors. Silica SiO₂ (Ludox) particles were added to achieve a barrier properties of coating, while Ce(NO₃)₃·6H₂O was added in order to obtain an active corrosion protection. Optimization of sol synthesis was based on the results of ATR-FTIR spectroscopy and UV–vis–NIR spectroscopy. Opening of epoxy rings and completion of hydrolysis and the condensation reactions during the synthesis process were confirmed. Coatings were characterized through thickness, water contact angle, roughness, adhesion, electrochemical properties (potentiodynamic and electrochemical impedance spectroscopy) and the response to prolonged immersion time in 0.1?M NaCl. The high degree of cross-linking of Si–O–Si network structure and high density was achieved during the synthesis of the sol. Moreover, the results showed that the curing process and the incorporation of cerium nitrate into the hybrid sol–gel coating affected to the corrosion properties of the coating. The observed enhancement in corrosion protection properties is attributed to the combination of the barrier properties of the silica matrix with the active protection of the cerium nitrate.

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Studies of Lead Zirconate Titanate Sol Ageing Part I: Factors Affecting Particle Growth

The formation and growth of polymeric particles during the hydrolysis and condensation of PbZr_0.3Ti_0.7O₃ (PZT 30/70) precursor solutions have been studied as functions of time by using photon correlation spectroscopy (PCS). Particle shape was deduced by measuring the rheological properties of the sols and was found to remain chain-like during sol ageing. Various factors that affect the ageing of PZT sols have been investigated. Ethylene glycol (EG) was found to greatly accelerate the hydrolysis and condensation of the acid-modified Pb-free Zr/Ti solutions but had little effect on the particle growth for the Pb-containing PZT sols. Temperature and H₂O concentration were found to have major effects on the ageing behaviors. The higher the temperature and H₂O concentration, the higher the particle growth rates and the faster the sol gels.     

Photophysical Properties of a Novel Organic–Inorganic Hybrid Material: Eu(III)-β-Diketone Complex Covalently Bonded to SiO2/ZnO Composite Matrix

In this article, dibenzoylmethane (DBM) was first grafted with the coupling reagent 3-(triethoxysilyl)-propyl isocyanate (TESPIC) to form precursor DBM–Si, and ZnO quantum dot was modified with 3-mercaptopropyltrimethoxysilane (MPS) to form SiO₂/ZnO nanocomposite particle. Then the precursor DBM–Si and the terminal ligand 1,10-phenthroline (phen) were coordinated to Eu³⁺ion to obtain ternary hybrid material phen–Eu–DBM–SiO₂/ZnO after hydrolysis and copolycondensation between the tetraethoxysilane (TEOS), water molecules and the SiO₂/ZnO network via the sol–gel process. In addition, for comparison, the binary hybrid material with SiO₂/ZnO network and ternary hybrid material with pure Si–O network were also synthesized, denoted as Eu–DBM–SiO₂/ZnO and phen–Eu–DBM–Si, respectively. The results reveal that hybrid material with SiO₂/ZnO network phen–Eu–DBM–SiO₂/ZnO exhibits the stronger red light, the longer lifetimes and higher quantum efficiency than hybrid material with pure Si–O network phen–Eu–DBM–Si, suggesting that SiO₂/ZnO is a favorable host matrix for the luminescence of rare earth complexes.     

A morphological study of mullite long fiber prepared using polyvinyl butyral as spinning aids

Morphologies of aluminosilicate gel fiber and mullite long fiber prepared from spinning sol of Al(NO₃)₃·9H₂O, tetraethylorthosilicate and ethanol using polyvinyl butyral as a spinning aids have been studied experimentally. The weight loss of fibers as function of drying time and calcination temperature was summarized and the fiber morphologies at different temperature were also discussed. The results indicated that fiber sample lost the main weight at the first several minutes at 40°C. A main axial crack was observed due to the heat stress if gel fiber was not pre-dried before calcination. Accompanied by weight loss during heat treatment, “black” fibers were shown at the temperature range of 300–500°C and some attachment was observed among fiber surface attributed by the decomposition of organic materials. The actual elimination temperature of organic materials was different from our former heat analysis measurement result, and it could be explained by the pre-treating procedure and high heating rate of heat analysis. The morphological study of mullite fiber calcined at different temperature could help to understand and obtain the mullite fiber with smooth surface.     

New data on the thermal analysis of diphasic mullite gel

A diphasic mullite gel has been synthesized by using Ludox and Al(NO₃)₃-9H₂O in ammoniacal solution. Both DTA and DDTA studies have been performed with a sensitive differential thermal analyzer system. Besides usual exotherm due to mullite formation, a pronounced exotherm between 800–1300°C has been noted. Considering previous XRD data, it is explained as due to formation of Si-Al spinel phase, which is subsequently transformed exothermally at 1320°C to mullite.     

Effect of formulation of silica‐based solution on corrosion resistance of silicate coating on hot‐dip galvanized steel

Several silica‐based solutions with 50 g/l of SiO₂ were prepared from sodium silicate solutions and silica sol; the silicate conversion coatings were obtained by immersing hot‐dip galvanized steel sheets in these solutions. These solutions were characterized using high‐resolution transmission electron microscopy and ²⁹Si nuclear magnetic resonance; the morphology of the coatings was observed by SEM and atomic force microscopy while the corrosion resistance was evaluated by electrochemical measurements as well as neutral salt spray tests. The results show that the coatings obtained from the single silica sol solution had poor adhesion and the coating obtained from the sodium silicate solution with low SiO₂/Na₂O molar ratio was uneven. By adding the silica sol to the silicate solution with low molar ratio, uniform coatings with better protection property were obtained. According to the results of ²⁹Si nuclear magnetic resonance spectra, the effects of the distribution of silicate anions with various polymerization degrees in the silica‐based solutions on the microstructure and corrosion resistance of the silicate coatings are discussed. Copyright © 2016 John Wiley & Sons, Ltd.     

The antibacterial finish of cotton via sols containing quaternary ammonium salts

The sols containing quaternary ammonium salts were prepared via sol-gel process. The effects of the molar rate of HCl, H₂O and EtOH to TEOS on the sol viscosity were investigated in detail. Cetyltrimethylammonium bromide (CTAB), Octadecyl dimethyl benzyl ammonium chloride (DC) and Ethylene-Bis (Octadecyl trimethyl ammonium chloride) [E-Bis(OTAC)]were added in the sols and applied to cotton samples by treated. The antibacterial activities of the samples were assessed against both Escherichia coli and Staphylococcus aureus bacteria. The samples treated by E-Bis(OTAC) sol exhibited the satisfactory antibacterial activity that resulted from the more microorganism adsorption and hydrophobicity. The antibacterial activities were still excellent after 10 times washings comparison with the control samples.     

27Al NMR Study of Al-Speciation in Aqueous Alumina-Sols

A new method of preparing concentrated alumo-sols, the hydrolysis of Al-tri-sec-butylate in acidic aqueous media at 85°C, was studied in detail by varying the H₂O/Al- and NO₃ ⁻/Al-ratios in wide ranges. The components of the sols were characterized by ²⁷Al NMR spectroscopy. The pH-value depends on both the chosen H₂O/Al- and NO₃ ⁻/Al-ratio and on the aging time of the sols and reflects the composition of the sols. Al₁₃ polycations were detected in sols with a pH-value between 3.0 and 3.7. As a new result its presence was shown by NMR below 3.4. The Al₁₃ content of the sols increased with pH and the maximum fraction of Al₁₃ polycations was detected in the sol with the highest pH (3.7). Nearly 65% of the entire aluminium content of this solution is bound in the Al₁₃ polycations. Hence, a new synthetic method for the preparation of Al₁₃ ions containing sols was developed. Aging studies of the sols showed, that the Al₁₃ polycations were more stable in solutions with higher pH-value. Al₁₃ polycations were detected after an aging time of four months only in sols with a pH-value of 3.7. Tempering the aged sols at 40° to 80°C caused formation of Al₁₃ and also of Al₃₀ polycations.