Nonaqueous sol‐gel synthesis of InTaO4 nanoparticles and their assembly into macroscopic aerogels

Due to their band gap energy, metal tantalates absorb both ultraviolet and visible parts of the solar spectrum, which is beneficial to applications in photocatalysis. However, tantalates are very challenging to synthesize. In comparison to traditional solid‐state reactions, which often lead to impurities, wet‐chemical approaches starting from molecular precursors offer better homogeneity. In this study, amorphous InTaO₄ nanoparticles were synthesised in a nonaqueous sol‐gel method. Subsequent annealing at 800°C yielded crystalline and phase‐pure nanoparticles. In addition, the amorphous nanoparticles could be used as building blocks for the assembly into macroscopic gels by careful centrifugation. After supercritical drying, the three‐dimensionally interconnected microstructure was preserved, resulting in highly porous aerogel monoliths with a large surface area of 357 m² g^?1. Upon calcination, crystallization and 46% shrinkage of the aerogel occurred and a decrease in surface area to 49 m² g^?1 was observed. Uniaxial compression tests revealed the mechanical stability of these nanoparticle‐based aerogels. Although the large surface area and the ability to absorb visible light, combined with the unique mechanical properties, are ideal prerequisites to make these aerogels promising for photocatalytic reactions, the degradation of methylene blue only showed limited success.     

Development of ZrO2‐MgO nanocomposite powders by the modified sol‐gel method

The ZrO₂‐MgO nanocomposites were synthesized using a new sol‐gel method with sucrose and tartaric acid as a gel agent. The samples were characterized by thermal analysis (TG/DTA), X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy‐dispersive X‐ray mapping (EDX mapping), and Ultraviolet‐visible spectroscopy (UV‐vis). The results showed that the cubic phase of ZrO₂‐MgO was formed in the presence of both gel agents. The average particle size of the samples synthesized with sucrose was lower (30 nm) than that of tartaric acid (50 nm). Finally, the formation mechanism and the optical properties of zirconia‐magnesia have been discussed.     

A new method for treatment of sol‐gel data in irradiated polymers

A log‐log plot of sol fraction versus dose has been found to be an effective graphical technique to treat sol‐gel analysis data. The treatment procedure is described in detail with an example of radiation crosslinking of linear low‐density polyethylene. Radiation chemical yields both for crosslinking and scission of macromolecules were determined using Monte Carlo simulation. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2621–2625, 2001     

Anticorrosion assessment of Al2O3‐TiO2 composite thin films derived from sol‐gel dipping technique

Metal surface protection by means of nanostructured ceramic coatings is considered an issue, which is variously scrutinized, and more efforts are made to improve steel performance in accordance with this method. In this respect, this study sought to examine the protective effects of a nanostructured coating containing TiO₂ and Al₂O₃. The optimization approach for obtaining a coating without any cracks with the highest efficiency of corrosion resistance is reported. The variables under assessment were as follows: weight ratio of the oxides, calcination temperature, and preparation parameters. The synthesized samples were analyzed utilizing XRD, FESEM, AFM, Tafel test, EIS, and microhardness measurement. The results revealed that the optimized conditions were as follows: (i) Ti:Al precursor mixture with a weight ratio of 75:25, (ii) heat treatment at 1000°C. This process yielded a uniform composite coating without any cracks with 5.1 nm roughness, containing crystalline rutile phase and amorphous alumina. Not only the intended coating improves the corrosion resistance of steel up to 97%, but it also increases its surface hardness up to 10 units.     

Effects of catalysts on polymerization and microstructure of sol‐gel derived bioglasses

This work investigates the effects of the type and concentration of acid catalysts on the microstructural features of silica‐rich bioactive glasses (75Si–16Ca–5Na–4P, in mol%) synthesized from acetate and nitrate precursor salts, using an innovative rapid sol‐gel method, followed by thermal stabilization at 550°C. The results of XRD, SEM, Fourier Transform Infrared Spectrometer (FTIR) and solid state MAS NMR analysis revealed that the lower degrees of polymerization and network connectivity were found for bioglass powders obtained in the presence of high acid concentrations. A low amount of citric acid gave a polymerization degree similar to that obtained in the absence of any catalyst, but the synthesis time was as short as that found for high acid concentrations. The XRD and FTIR results demonstrated amorphous glass powders free of any nitrate by‐products. The microstructure and degree of polymerization could be modified by changing the type and concentration of acid catalyst, enabling one to tailor the bioactivity of glasses even without changing the starting composition.     

High lithium ionic conductivity in the garnet‐type oxide Li7−2xLa3Zr2−xMoxO12 (x=0‐0.3) ceramics by sol‐gel method

Lithium garnet‐type oxides Li_7?2xLa₃Zr_2?xMo_xO₁₂ (x=0, 0.1, 0.2, 0.3) ceramics were prepared by a sol‐gel method. The influence of molybdenum on the structure, microstructure and conductivity of Li₇La₃Zr₂O₁₂ were investigated by X‐ray diffraction, scanning electron microscopy, and impedance spectroscopy. The cubic phase Li₇La₃Zr₂O₁₂ has been stabilized by partial substitution of Mo for Zr at low temperature. The introduction of Mo (x≥0.1) can accelerate densification. Li_6.6La₃Zr_1.8Mo_0.2O₁₂ sintered at lower temperature 1100°C for 3 hours exhibits highest total ionic conductivity of 5.09 × 10^?4 S/cm. Results indicate that the Mo doping LLZO synthesized by sol‐gel method effectively lowers its sintering temperature and improves the ionic conductivity.     

Multicycle investigation of a sol‐gel‐derived Fe2O3/ATP oxygen carrier for coal chemical looping combustion

Fe‐based oxygen‐carrier particles with attapulgite (ATP) as a support material for coal chemical looping combustion (CLC) have been prepared using a sol‐gel approach. The multiredox characteristics of the prepared Fe4ATP6 (Fe₂O₃ to ATP mass ratio of 40:60) were experimentally examined in a batch fluidized bed reactor at 900°C. The experimental results indicated that the synergistic reactions between ATP and Fe₂O₃ increased the coal conversion. Fe4ATP6 exhibited high reactivity, particularly for low‐rank coals, in the CLC process. The improved pore structure and surface area were responsible for the high reactivity of Fe4ATP6. In 60 redox cycles, H₂ was mainly generated in the outlet gas as the carbon conversion efficiency had reached 95%, and both the coal combustion efficiency and CO₂ capture efficiency were greater than 95%. © 2015 American Institute of Chemical Engineers AIChE J, 62: 996–1006, 2016     

The effect of agarose content on the morphology,phase evolution,and magnetic properties of CoFe2O4 nanoparticles prepared by sol‐gel autocombustion method

Hard‐magnetic CoFe₂O₄ nanoparticles were produced with the sol‐gel autocombustion route using agarose and citric acid as a neutral organic gel and combustion agent. For this purpose, the obtained gel is annealed at 800°C for 3 hours and then the spinel structure of the samples was confirmed by X‐ray diffraction (XRD). In addition, the existence of metal‐oxygen complexes in the gel and nanoparticles was investigated by Fourier transformation infrared (FTIR) spectra. Furthermore, FESEM images showed that semispherical and rod‐like particles were obtained by variation in agarose contents. For the rod‐like sample with 2.5 g agarose, the saturation magnetization and coercivity were measured equal to 71.7 emu/g and 10076.84 Oe, respectively.     

Transparent photo‐curable co‐polyacrylate/silica nanocomposites prepared by sol‐gel process

This work prepared the highly transparent photo‐curable co‐polyacrylate/silica nanocomposites by using sol‐gel process. The FTIR and ¹³C NMR analyses indicated that during the sol‐gel process, the hybrid precursors transform into composites containing nanometer‐scale silica particles and crosslinked esters/anhydrides. Transmission electron microscopy (TEM) revealed that the silica particles within the average size of 11.5 nm uniformly distributed in the nanocomposite specimen containing about 10 wt % of Si. The nanocomposite specimens exhibited satisfactory thermal stability that they had 5% weight loss decomposition temperatures higher than 150°C and coefficient of thermal expansion (CTE) less than 35 ppm/°C. Analysis via derivative thermogravimetry (DTG) indicated that the crosslinked esters/anhydrides might influence the thermal stability of nanocomposite samples. The UV‐visible spectroscopy indicated that the nanocomposite resins possess transmittance higher than 80% in visible light region. Permeability test revealed a higher moisture permeation resistance for nanocomposite samples, which indicated that the implantation of nano‐scale silica particles in polymer matrix forms effective barrier to moisture penetration. Adhesion test of nanocomposite samples on glass substrate showed at least twofold improvement of adhesion strength compared with oligomer. This evidenced that the silica and the hydrophilic segments in nanocomposite resins might form interchains hydrogen bonds with the ? OH groups on the surface of glass so the substantial enhancement of adhesion strength could be achieved. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Rheological evolution of a particular sol‐gel system

Rheological measurements and results discussed in this paper made on gelling mass containing thetraethoxysilane (TEOS or Si(OC₂H₅)₄) and calcium nitrate as sol‐gel precursors is important because of the interest of sol‐gel processes as a possible preparation route of glasses and amorphous body structures (sol‐gel route). The rheological measurements in our study, have been made to investigate the sol‐gel transition of Si(OC₂H₅)₄ by measuring the viscosity evolution at time intervals and temperature values established. The temperature dependence of the viscosity was used to determine the activation energies of the flow.     

Polarization behavior of sol‐gel‐derived relaxor Ba(Zr,Ti)O3 films

Films of the relaxor ferroelectric BaZr_0.25Ti_0.75O₃ (0.25‐BZT) were synthesized via a sol‐gel route to investigate the effect of film thickness on the dielectric properties and for comparison with normal ferroelectric BaTiO₃ (BT). The as‐prepared films on Nb‐doped SrTiO₃ (Nb–ST) displayed a (100) orientation; thinner films had stronger (100) orientations. Microwave dielectric measurements up to a few GHz quantified the polarizations, that is, the dipole contribution, ε_dipole, the combination of the ionic and electronic polarizations, ε_ionic+el., and the total contribution, ε_total. The ε_dipole in the relaxors at a film thickness of t=630 nm was 360, which was double that for the normal ferroelectric BT (ε_dipole=180) at t=735 nm. The larger apparent permittivity of the BZT therefore originated from the larger ε_dipole of the polar nanoregions (PNRs), while the nanograins of BT with few domain walls led to a comparably smaller ε_dipole. The volume ratio of the surface and film‐substrate interface lacking the dipole interactions increased with the reduction in the film thickness, leading to the significant depression in the permittivity for both specimens. The difference in the thickness dependence of the dielectric properties of the sol‐gel derived relaxor BZT and the normal ferroelectric BT films was attributed to the different origins of their dipole contribution, that is, the PNRs and ferroelectric domains, respectively.     

Computer simulation and synthesis of stimuli‐responsive polymer by sol‐gel for selective recognition of (4‐chloro‐2‐methylphenoxy)acetic acid

A novel photoresponsive functional monomer bearing a siloxane polymerizable group and azobenzene moieties was synthesized, and then photoresponsive molecularly imprinted sol‐gel polymers were successfully fabricated from the synthesized functional monomer, using (4‐chloro‐2‐methylphenoxy)acetic acid (MCPA) as a molecular template. The photoisomerization properties of the functional monomer are retained after incorporation into the rigid three‐dimensional crosslinked polymer matrix. The template is then removed from the resulting polymer to generate pores, which are complementary to the template in shape, size and functionality. The substrate affinity of the molecularly imprinted polymer (MIP) receptor sites is photoswitchable. This can be attributed to the photoisomerization of azobenzene chromophores within the MIP receptors, resulting in alteration of their geometry and the spatial arrangement of their binding functionalities. The binding affinity of the imprinted recognition sites was switchable by alternate irradiation with UV and visible light, suggesting that azobenzene groups located inside the binding sites could be used as chemical sensors and the trans–cis isomerization could regulate the affinity for MCPA. To study the hydrogen bond interactions between template molecules and functional monomer, computational molecular modeling was employed. The data indicate that the design of the MIP is rational. Copyright © 2012 Society of Chemical Industry     

Surface sedimentation and adherence of Nano‐SiO2 to improve thermal stability and flame resistance of melamine‐formaldehyde foam via sol‐gel method

Melamine‐formaldehyde foam possesses intrinsic flame retardance; however, relative poor thermal stability and a certain amount of heat release rate restrict its applications in heated environment to a degree. In the present research, sol‐gel method has been adopted to precipitate nano‐SiO₂ particles on the surface of the melamine‐formaldehyde foam's fibers to construct a protective inorganic gel layer. Taking advantages of the shielding effects of the gel layer, the thermal‐oxygen degradation of the foam can be greatly retarded during heating; hence, the thermal stability is remarkably improved, and the flame retardance is further enhanced. In addition, introducing a small amount of membrane‐forming agent in the sol‐gel system can make the depositional nano‐SiO₂ particles well adhered to avoid dusting.     

A new sol‐gel route alumina for selective oxidation of H2S to sulphur

In this study, a new synthesis method was developed for the production of modified sol‐gel alumina (SG‐M) for the selective oxidation of H₂S to elemental sulphur. The catalytic activity of this modified alumina without any active metal incorporation was then compared with the activity of commercial alumina (alumina‐com) for H₂S selective oxidation. The N₂ adsorption‐desorption isotherm showed that the SG‐M alumina synthesized in this work has a mesoporous structure with well‐defined hysteresis loops. Both alumina materials showed a γ‐Al₂O₃ crystalline phase with an amorphous structure in their crystal structure. The surface acidity of the alumina materials was determined using pyridine‐adsorbed FTIR analyses, and both alumina showed Lewis acid sites on their surfaces. The catalytic activity tests were performed at 250°C using a feed ratio of O₂/H₂S:0.5. The complete conversion of H₂S over SG‐M was achieved during 400 minutes of reaction time. However, the commercial alumina lost its activity at earlier reaction times. Lewis acid sites and surface hydroxyl groups caused the alumina to be active in H₂S selective catalytic oxidation, and the formation of Al‐S bonds, observed when the H₂S conversion fell, caused a decrease in the catalytic activity of the alumina materials. A high sulphur yield (≥95%) was obtained over SG‐M, even though there was no active metal incorporation and even in the presence of excess oxygen. Considering the catalytic activities, the new sol‐gel alumina synthesized in this work is superior to commercial alumina. It was concluded that, as a catalyst without any active metal, SG‐M is a promising catalyst in H₂S selective oxidation to sulphur.     

Organically modified inorganic sol‐gel materials for second‐order nonlinear optics

A series of the organically modified inorganic NLO sol‐gel materials based on the prepolymer of alkoxysilanes and an alkoxysilane dye (ASD) have been investigated. Optically clear samples exhibit large second‐order optical nonlinearity (d₃₃ = ∼10.8–54.0 pm/V at 1064 nm) after poling and curing at 220°C for 1 h. The thermal behavior of these NLO sol‐gel materials was studied by temperature‐dependent dielectric relaxation. The results indicate that the crosslinking density of cured NLO sol‐gel materials was increased with increasing alkoxysilane content. Subsequently, better temporal stabilities were obtained for the poled/cured NLO sol‐gel materials with a higher alkoxysilane content. Moreover, the structural influence of alkoxysilanes on the thermal behavior and second‐order nonlinearity was also studied for these NLO sol‐gel materials. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1852–1859, 2001     

Oxygen permeation characteristics of sol‐gel derived barium‐substituted strontium ferrite membranes

The pervoskite‐type oxides have received attention due to their potential applications in catalysis, solid oxide fuel cells, gas sensors, and gas separable membranes. In view of their importance in oxygen separation from air, Ba_xSr_1?xFeO_3?δ (0≤x≤1.0) samples have been synthesized by sol‐gel process and investigated with regard to phase(s), oxygen permeation, and electrical conductivity. These compounds possess at room temperature, a perovskite‐type cubic, mixture of rhombohedral and hexagonal, and hexagonal phase(s) depending upon the composition 0≤x≤0.94, x=0.96‐0.98, and x=1.0, respectively. The barium incorporation causes initially enhancement but decrease in electrical conductivity above x=0.94. Above 800°C, all the compositions exhibit a stable cubic phase. The compacts made in the form of discs serve as stable oxygen permeable membranes displaying flux density () of ~2.45‐3.58 mL/cm².min at 1000°C. A good correlation has been demonstrated between the oxygen permeation and the electrical conductivity data. The maximum values of and conductivity correspond to Ba_xSr_1?xFeO_3?δ (x=0.94) with a perovskite‐type cubic structure. Hence, this membrane is quite suitable for oxygen separation technology.     

Reversible sol–gel transitions in aqueous solutions of N‐isopropylacrylamide ionic copolymers

Ionic copolymers of N‐isopropylacrylamide (NIPA) exhibiting sol–gel transitions in aqueous solutions were investigated. The studies were aimed at understanding of the structure–property relationship in the design of injectable, in situ forming gels for potential biomedical applications in delivery of therapeutics and tissue engineering. Aqueous solutions of NIPA ionic copolymers were found to flow freely at ambient temperatures and formed soft gels with controlled syneresis above 32 °C, the lower critical solution temperature of NIPA. The sol–gel transitions and temperature‐dependent properties of the resulting gels were analyzed using dynamic rheometry and ultraviolet and infrared spectrometry, and were found to be controlled by the molecular weight and composition of copolymers, ionization state of comonomers and composition of aqueous solvent. Copyright © 2008 Society of Chemical Industry