Effect of peptiser species on crystallisation of alumina gel produced by sol–gel process

The present work reports a study on the effect that a peptiser species has on the crystallisation of alumina gel produced by a sol–gel process to help develop a method for producing α-Al₂O₃ at low temperature. The white precipitate of aluminium hydroxide, which was prepared with a homogeneous precipitation method using aluminium nitrate and urea in an aqueous solution, was peptised using various peptisers at room temperature to form a transparent alumina sol. The alumina gel obtained from the alumina sol, which was produced using formic acid as the peptiser, was most dominantly crystallised into α-Al₂O₃ by annealing at 900°C. The optimal [peptiser]/[Al³⁺] (P/A) molar ratio for the crystallisation into α-Al₂O₃ was 0.2. The alumina gel began to crystallise into α-Al₂O₃ with annealing at as low as 500°C when formic acid and a P/A ratio of 0.2 were used.     

Processing and characterization of sol–gel titania membranes

In this work we present a structural characterization of sol–gel titania membranes obtained in both supported and unsupported forms. We used two commercial grade alumina supports obtained from Whatman and Rojan Advanced Ceramics. The unsupported membranes were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), differential scanning calorimetry (DSC), nitrogen sorption, and X-ray powder diffraction (XRD). Morphological studies were performed in both supported and unsupported membranes using a field emission scanning electron microscope (FE-SEM). In order to evaluate the performance of the supported membranes, single-gas permeation experiments were carried out at room temperature with nitrogen, helium, and carbon dioxide. We concluded from nitrogen sorption experiments that increasing the membrane heat treatment temperature leads to samples with lower specific surface areas and greater pore sizes. Close packed titania particles of uniform size were observed in SEM micrographs of unsupported membranes. The SEM analyses also revealed the presence of titania coatings on supported membranes. Some of the obtained membranes showed a separation capacity for He/CO₂ and He/N₂ larger than that expected for the Knudsen mechanism in the investigated pressure range. However, a good part of the analyzed samples showed an improvement of their separation capacity with increasing the feed pressure.     

Synthesis of nanocrystalline gadolinium doped ceria via sol–gel combustion and sol–gel synthesis routes

Gadolinium doped ceria (GDC) has been investigated as a promising material for application as an electrolyte in intermediate temperature solid oxide fuel cells (IT-SOFC). In this work, 10GDC powders (Gd_0.1Ce_0.9O_1.95) were synthesized by sol–gel combustion and sol–gel synthesis routes using the same complexing agents in both procedures. The thermal behavior of Gd–Ce–O precursor gels was investigated by TG–DSC measurements. X-ray diffraction (XRD) analysis was used for the characterization of phase purity and crystallinity of synthesized samples. Scanning electron microscopy (SEM) was employed for the estimation of surface morphological features. Nitrogen adsorption–desorption (BET model) was used for evaluation of specific surface area. The surface composition was determined by X-ray photoelectron spectroscopy (XPS). Electrical properties of synthesized ceramic samples were studied by means of impedance spectroscopy.     

Study of the degradation of hybrid sol–gel coatings in aqueous medium

The design and development of suitable multilayered functional coatings for delaying corrosion advance in metals and become controlled-release vehicles requires that the properties of the coatings are known. Coatings prepared by the sol–gel method provide a good approach as protective layers on metallic surfaces. This kind of coating can be prepared from pure chemical reagents at room temperature and atmospheric pressure, with compositions in a very wide range of environmentally non-aggressive precursors. Sol–gel coatings based on siloxane bonded units were prepared, starting with an organic–inorganic hybrid system. Synthesis procedures included acid-catalysed hydrolysis, sol preparation, and the subsequent gelation and drying. The alkoxide precursors used were methyl-triethoxysilane (MTMOS) and tetraethyl-orthosilicate (TEOS) in molar ratios of 10:0, 9:1, 8:2 and 7:3. After determination of the optimal synthesis parameters, the materials were characterised by solid ²⁹Si nuclear magnetic resonance (²⁹Si NMR), Fourier transform infrared spectroscopy (FTIR), contact angle measurement and electrochemical impedance spectroscopy (EIS) test. Finally, the materials were assayed by controlling their weight in contact with water, to determine their ability to degrade by hydrolysis. Electrochemical analysis reveals the formation of pores and water uptake during the degradation. The quantity of TEOS is one of the principal parameters that determine the kinetics of degradation. There is a correlation between the degradation process obtained for long periods and the electrochemical parameters obtained by EIS in short times. The study tries to incorporate knowledge that can be used for designing the degradation process of the functional coatings and to control their properties in short times.     

Sol–gel synthesis and catalytic properties of vanadium phosphates

Vanadium phosphates with variable specific surface areas were synthesized from vanadium alkoxides and phosphoric acid by precipitation in various solvents. The catalytic properties were studied in the reaction of methanol oxidation. Although the specific activities increase with the specific surface areas (BET), the intrinsic activities seem to depend on the particle morphology. The selectivities do not vary within a large range. The same method of precipitation was applied to prepare vanadium phosphates on a silica support. It is shown that the supported phase is highly dispersed, and that these supported catalysts are more stable than silica‐supported vanadium oxide for methanol oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sol–gel synthesis and characterization of Ce doped-BaTiO3

Barium titanate (BaTiO₃) have been doped “in situ” with 5.5 mol% cerium by a sol–gel method using barium acetate, titanium (IV) isopropoxide, and cerium (III) acetylacetonate as starting materials. The dried gel showed a microstructure consisting of nano-sized grains (∼140 nm) with great tendency to agglomeration. Several thermal analysis techniques were used to study the decomposition process of the gel. The presence of hydroxyls up to 720 °C suggests a strong bonding TiOH that is responsible for the existence of aggregates even at high temperatures. The as-prepared gel powder was found to be amorphous, and then decomposes through oxides and barium carbonate around 500 °C and crystallizes on the perovskite structure of tetragonal BaTiO₃ at 1100 °C for 3h in air. A small influence of the frequency on the dielectric properties of the Ba_0.945Ce_0.055TiO₃ ceramics was observed in 100 Hz to 1 MHz domain. At the Curie temperature point (22 °C) the dielectric constant was 10130 at 100 Hz while the dielectric loss (tan δ) was 0.018.     

Nontemplate sol–gel synthesis of catalytically active mesoporous titanosilicates

A method for the nontemplate sol–gel synthesis of micro-mesoporous and mesoporous titanosilicates is developed using commercially available raw materials (a mixture of oligomer esters of orthosilicic acid (Ethylsilicate-40) and a water–alcohol solution of titanium(IV) ethoxide). The effect of the pH values of the reaction mixture on sol–gel synthesis is studied using the characteristics of the porous structure of titanosilicates. Samples of amorphous structure with different textural characteristics are obtained by varying the pH in the range of 3–10: specific surface area, 320–740 m²/g; micropore volume, 0.04–0.15 cm³/g; and mesopore volume, 0–0.92 cm³/g. It is shown that a sample of mesoporous titanosilicate obtained with alternating pH values has maximum catalytic activity in the oxidation reaction of 4-tert-butylphenol (TBP) in aqueous solutions of hydrogen peroxide. (Conversion of 4-tert-butylphenol, 45 wt %; selectivity of the formation of 4-tert-butylcatechol, 66 wt %. Conditions: 10 wt % of titanosilicate; TBP/H₂O₂ = 1/2; 75°C; 1 h).     

Enhancement of corrosion protection of 3-glycidoxypropyltrimethoxysilane-based sol–gel coating through methylthiourea doping

Protection of aluminum metal and its alloys from corrosion is a key requirement for many engineering applications. Nowadays, sol–gel coating technology is recognized as the ideal replacement for chromate conversion coatings. The present work makes use of 3-glycidoxypropyltrimethoxysilane (GPTMS) as a precursor for sol–gel coating. GPTMS was subjected to hydrolysis and subsequent condensation reaction to get a three-dimensional network and methylthiourea (MTU) was incorporated into the sol–gel matrix. MTU-doped GPTMS-based sol–gel coatings were applied over aluminum metal by dip coating method. The resultant coating was studied by FTIR, XRD and SEM. MTU-doped GPTMS-based sol–gel coatings increased the hydrophobic nature of the coating and were stable up to a temperature of 450°C. The protective nature of the coatings was evaluated in a 1% NaCl environment using electrochemical impedance and polarization studies. The study has revealed that doping of MTU enhanced the protection ability of doped GPTMS-based sol–gel coating to a significant extent.     

Effect of annealing temperature on ferroelectric electron emission of sol–gel PZT films

In this work, the influence of annealing temperature on the ferroelectric electron emission behaviors of 1.3-μm-thick sol–gel PbZr_0.52Ti_0.48O₃ (PZT) thin film emitters was investigated. The results revealed that the PZT films were crack-free in perovskite structure with columnar-like grains. Increasing annealing temperature led to the growth of the grains with improved ferroelectric and dielectric properties. The remnant polarization increased slightly from 35.3 to 39.6 μC/cm² and the coercive field decreased from the 56.4 to 54.6 kV/cm with increasing annealing temperature from 600 to 700 °C. The PZT film emitters exhibited remarkable ferroelectric electron emission behaviors at the threshold voltage above 95 V. The film annealed at 700 °C showed a relatively lower threshold voltage and higher emission current, which is related to the improved ferroelectric and dielectric properties at higher annealing temperature. The highest emission current achieved in this work was around 25 mA at the trigger voltage of 160 V.     

Effects of formulation properties on sol–gel behavior of chitosan/glycerolphosphate hydrogel

Chitosan solution containing glycerolphosphate disodium salt (Gp) is an injectable thermosensitive in situ gel-forming system which undergoes sol–gel transition under certain physiological pH and temperature conditions. When a drug-incorporated chitosan/Gp solution is injected into the body, it forms a three-dimensional gel at 37 °C, which allows the drug to be released in a sustained manner. This hydrogel can be used as a drug delivery system for prolonged release of peptides and glycopeptides. The objective of this work was to investigate the effect of different excipients on the sol–gel behavior of this thermosensitive hydrogel. Chitosan polymeric solutions (2 % w/v) containing Gp and different excipients, such as hydroxypropyl methyl cellulose (HPMC), polyethylene glycol (PEG) with two different molecular weights (PEG200 and PEG1000), and poloxamer (F127) in various concentrations, were prepared, and the pH, sol–gel transition time, and syringeability of the final solutions were evaluated. The obtained results point to HPMC as the best additive for chitosan/Gp solutions in developing an in situ gel-forming drug delivery system with optimum gelling time. A significant decrease was noted in the sol-to-gel transition time (from 90 to 60 s) when HPMC was added to the system. This may have been due to the HPMC structure which acted as a viscosity-enhancing and gel-promoting agent. The in vitro release of vancomycin hydrochloride from chitosan/Gp/HPMC hydrogel was also studied. Vancomycin release studies showed a sustained release profile for over 20 days. It can be concluded that combining chitosan/Gp and HPMC is a promising strategy for preparing a thermally reversible in situ gel-forming delivery system with an optimized gelation time.     

Sol–gel preparation of titania multilayer membrane for photocatalytic applications

The main goal of the present study is to prepare a titania membrane with high permeability and photocatalytic activity for environmental applications. In this investigation a mesoporous titania multilayer membrane on alumina substrate is successfully fabricated via the sol–gel processing method. The prepared titania polymeric sol for the membrane top layer has an average particle size of 11.7 nm with a narrow distribution. The resulting TiO₂ multilayer membrane exhibits homogeneity with no cracks or pinholes, small pore size (4 nm), large specific surface area (83 m²/g), and small crystallite size (10.3 nm).The permeability and photocatalytic properties of the titania membrane were measured. The photoactivity of the titania membrane was examined to be 41.9% after 9 h UV irradiation based on methyl orange degradation. This measurement indicates high photocatalytic activity per unit mass of the catalyst. Through multilayer coating procedure, the photocatalytic activity of the membrane improved by 60% without sacrificing the membrane permeation. The prepared TiO₂ photocatalytic membrane has a great potential in developing high efficient water treatment and reuse systems due to its multifunctional capability such as decomposition of organic pollutants and physical separation of contaminants.     

Hydrophilization by coating of silylated polyoxazoline using sol–gel process

Hybrid films prepared from TEOS and polyoxazolines (Si–POx–Si) crosslinking agents were coated on different substrates in order to modify their surface properties. The film cohesion and adhesion on substrates were expected through the hydrogen bonding of the polyoxazoline crosslinked network. Low molecular-weight α,ω-unsaturated polyoxazolines (DA-PMOx)s were synthesized by a one step cationic ring-opening polymerization (CROP) of 2-methyl-2-oxazoline (MOx) with a good control over the molecular weight. Based on double thiol-ene coupling (d-TEC) a post-functionalization of DA-PMOx end chains gave in good yield polyoxazoline cross linker (Si–POx–Si). Glass and various polymer substrates (PP, PEI, POM, etc.) were spin coated by the organic–inorganic hybrid films through sol–gel process. AFM, SEM, visible reflectance spectroscopy and contact angle experiments allowed the full characterization of targeted surfaces and demonstrated the efficiency of the polyoxazoline coating.     

Sol–gel synthesis and infrared radiation property of Li-substituted cordierite

The Mg_2?xAl_4+1/2xLi_1/2xSi₅O₁₈ (0.1≤x≤1) ceramics with the substitution of (Li_1/2Al_1/2)²⁺ for Mg²⁺ were synthesized by the sol–gel method. The characterization of the modified cordierite included X-ray diffraction, SEM, EDS and infrared radiation. The crystal structure of Mg₂Al₄Si₅O₁₈ with the substitution of (Li_1/2Al_1/2)²⁺ for Mg²⁺ changed and the amount of secondary phase increased with increasing the x value from 0.1 to 1. High infrared emissivity over 0.9 in the band of 8–14 μm at room temperature was obtained in Mg_2?xAl_4+1/2xLi_1/2xSi₅O₁₈ (x=0.1). The material based on cordierite with x=0.1 sintered at 1200 °C maintained a single phase, compact microstructure and good infrared emissivity with potential use in infrared heating.     

Bond strength of hybrid sol–gel coatings with different additives

The hybrid sol–gel coating on Al 2024-T3 was modified by adding polyaniline, TiO₂, or γ-Al₂O₃ nanoparticles in the formulation separately. The coating was then used as an adhesive to bond Al 2024-T3 alloys, forming a single lap joint. The bond strength of the sol–gel coating was investigated using a universal tensile test machine. The lap shear strength of the original sol–gel coating was about 1.38 MPa and it was increased up to 2.26 MPa after the modification by adding 0.05 wt% PANI microparticles in the sol–gel coating. The small increase in strength was attributed to an improvement in its adhesive flexibility because of incorporation of the long-chain organic polymer in its structure. Furthermore, the addition of different amounts of TiO₂ nanoparticles in the unmodified sol–gel coating also led to an increase in shear strength compared to the undoped sol–gel coating. Typically, a sol–gel coating containing 2.0 wt% of TiO₂ recorded the highest adhesive strength of about 4.0 MPa. A similar increase in strength was observed when doping γ-Al₂O₃ nanoparticles into the original hybrid sol–gel coating. Adding 0.5 wt% of γ-Al₂O₃ in the sol–gel coating increased the adhesive bonding strength up to 4.48 MPa. The fracture surface of the specimen was separately observed by SEM and Optical Microscopy in order to examine potential evidences of mechanism and nature of failure. The reason why the adhesive strength increased after the modification of the sol–gel coating is discussed in this article.     

The effect of ceramic nanoparticles on tribological properties of alumina sol–gel thin coatings

Thin alumina coatings containing zirconia or alumina nanoparticles having diameter of ～20–30 nm were deposited by the sol–gel dip-coating process on silicon wafers. The mass content of nanoparticles in the alumina coating was fixed at 15% in relation to the theoretical mass of alumina matrix resulted from the amount of the applied precursor. Atomic force microscopy (AFM) was used to image the surface topography of as-made coatings and find out the wear level after frictional tests. Tribological tests were performed with the use of a microtribometer operating in the load range of 30–100 mN. It was found that the presence of α-alumina (corundum) or zirconia nanoparticles enhances the tribological performance of alumina layers annealed at 100 °C by decreasing the average wear rate by 20% and 63% for zirconia and corundum nanoparticles, respectively. No wear was observed for samples containing both types of nanoparticles annealed at 500 °C.     

Effect of carbon nanotubes on sintering behavior of alumina prepared by sol–gel method

Alumina (Al2O3)/carbon nanotube (CNT) (99/1 by weight) composite was prepared by mixing CNT dispersion with AlCl₃-based gel, followed by high temperature sintering at a temperature up to 1150 °C in argon. Composite alumina precursor showed phase transition order from amorphous to γ-Al₂O₃ after sintered at 900 °C for 2 h, partially to θ-Al₂O₃ after sintered at 1000 °C for 2 h, and then partially to α-Al₂O₃ after sintered at 1150 °C for 2 h. By comparison, control alumina precursor directly transformed from amorphous to α-Al₂O₃ after sintered at a relatively low temperature of 600 °C for 2 h. Composite alumina showed porous structure with pore diameter ranging from 100 nm to 2 µm, whereas control alumina was relatively pore-free. The elevated alumina-crystal phase transition temperatures and the formation of porous structure were ascribed to the presence of CNTs in alumina precursor. The composite alumina sintered at 900 °C for 2 h containing only γ-Al₂O₃ had a BET surface area of 138 m²/g, which was significantly higher than that of control alumina sintered at 1150 °C for 2 h containing only α-Al₂O₃, ~15 m²/g.     

The effect of zirconia ions on corrosion performance of sol–gel coated galvanized steel

Currently, galvanized steel is treated with hexavalent chrome passivation. Sol–gel coating has been found to be a potential replacement for the hazardous hexavalent chrome passivation treatment. The aim of this work is to study the effect of zirconyl nitrate on corrosion behavior of sol–gel coating. Aminopropyl-trimethoxysilane and 3-glycidoxypropyltrimethoxysilane were employed as precursors to prepare the sol–gel-based silane coating. The sol–gel film was deposited on galvanized steel sheet by dip coating method. The chemical properties of sol–gel solution and coated films were analyzed by infrared spectroscopy. Morphology of the film was characterized by scanning electron microscope. The corrosion resistance of the coated samples was evaluated by electrochemical impedance spectroscopy, potentiodynamic polarization curve, and salt spray test. The results indicated that zirconia-doped coatings have better corrosion resistance in comparison with their undoped counterparts. The coating doped with 0.5% zirconyl nitrate provides better corrosion protection due to the inhibitive action of zirconia ion.     

Sol–gel incorporation of silica nanofillers for tuning the anti-corrosion protection of acrylate-based coatings

The aim of the present work is to synthesize through sol–gel approach new hybrid polymeric nanocomposites to be used as coating materials. An acrylic-based polymer was prepared by free-radical copolymerization of two monomers widely used for coatings, namely 2-ethylhexylacrylate (EHA) and glycidyl methacrylate (GMA) bearing epoxy moieties, in which silica nanoparticles were incorporated by in situ acid hydrolysis and subsequent condensation of tetraethoxysilane (TEOS). Glycidoxypropyl trimethoxysilane (GPTS) was used as coupling agent to fine-tune the compatibility between organic and inorganic phases. The morphology, mechanical properties and corrosion resistance of thin films applied on aluminum alloys were optimized by varying the content of silica nanoparticles whose properties were strongly affected by the TEOS/GPTS ratios. Performances of the obtained hybrid materials were scrutinized by atomic force microscopy (AFM), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and electrochemical impedance spectroscopy (EIS). Thus it was evidenced that an optimum amount of silica nanoparticles with a precise morphology and composition in term of TEOS/GPTS ratio is needed to maintain good coating barrier properties. Outstanding anti-corrosion protection was reached by using optimized hybrid films.     

Preparation of scratch resistant superhydrophobic hybrid coatings by sol–gel process

Organic–inorganic hybrid coatings on glass substrates with superhydrophobic properties and with improved scratch resistance were obtained by means of applying a multilayer approach including multiple sol–gel processes. The coatings exhibited a water contact angle (WCA) higher than 150°. Ultraviolet (UV)-curable vinyl ester resins and vinyltriethoxysilane (VTEOS) as coupling agent were employed to increase the adhesion between substrate and the inorganic layers. The surfaces were characterized by means of dynamic contact angle and roughness measurements. Indeed, the occurrence of superhydrophobic behavior was observed. The scratch resistance of the hybrid coatings was tested to evaluate the adhesion of the coatings to the glass substrate. The proposed preparation method for scratch resistant, mechanically stable, superhydrophobic coatings is simple and can be applied on large areas of different kinds of substrates.     

Preparation of nanocrystalline SrTiO3 powder by sol–gel combustion method

In this research a sol–gel combustion route has been presented to synthesize strontium titanate (SrTiO₃:ST) nanocrystalline, using citric acid as fuel. The synthesis procedure was optimized by systematically varying the molar ratios of total metal nitrate to citric acid (MN:CA) from 1:1 to 1:3. The effect was investigated through XRD, SEM and TEM analysis. Analysis of XRD spectrum shows the complete of SrTiO₃ nanocrystalline, however, a minor phase of SrCO₃ impurity was found. Hence, an acid treatment process, with 1 mol/l HNO₃ solution and deionized water, was applied to remove the impurity. The results show that the appropriate condition to prepare the single phase nanocrystalline SrTiO₃ powders is MN:CA molar ratio of 1:3, coupled with an acid treatment process and at the lower calcination temperature of 500 °C. The particle size of powders was in nanometer ranges. The average crystallite size calculated from FWHM was about 23 nm. Morphology of powders was identified by SEM analysis. However, TEM estimated the average particle size about 7.5 nm after applying an acid treatment technique at 600 °C.