Preparation of mullite monoliths with well-defined macropores and mesostructured skeletons via the sol–gel process accompanied by phase separation

Mullite monoliths with well-defined macropores and mesostructured skeletons have been prepared via the sol–gel process accompanied by phase separation in the presence of poly(ethylene oxide) (PEO). Gelation of Al₂O₃–SiO₂ binary system with chloride salts as an additional precursor has been mediated by propylene oxide (PO) as an acid scavenger, while PEO worked as a phase-separation inducer. The dried gel and that heat-treated at 800 °C are amorphous, and γ-Al₂O₃ or Si–Al spinel phase nanocrystals are crystallized at 900–1000 °C. After heat-treated at and above 1100 °C for 5 h, the complete crystalline mullite is generated, and the macroporous monoliths in large dimensions of more than 15 mm × 15 mm × 10 mm are obtained. Heat-treatment at 200–1400 °C does not basically spoil the macroporous structure of monoliths, while decreases the macropore size and significantly alters the phase compositions and micro-mesoporous structure.     

Microstructure and property of porous mullites with a whiskers framework obtained by a sol–gel process

Porous mullites with a whiskers framework and high porosities were fabricated by the reaction sintering (1100 to 1600 °C, 1 h, in an airtight container) of an aerogel block shaped by the sol–gel transition of a mullite precursor composed of SiO₂ sol, Al₂O₃ and AlF₃ powders (as reaction catalyst). The effect of heating temperatures on porosity, whisker formation, microstructure feature and compressive strength of the porous mullites was determined by XRD, SEM and compressive test. The results indicate that after heating at temperatures from 1100 to 1600 °C, the porosities of the mullites varied within the range of 84.1–80.2%. The whiskers in the framework well lap-jointed each other to form the large space and became elongated and smooth at high temperatures due to the accelerated vapor–solid reaction rate. A maximum compressive strength of 16.1 MPa was obtained for the whiskers framework heated at 1600 °C; this strength was attributed to the strong bonding among the smooth whiskers.     

Preparation of macroporous cordierite monoliths via the sol–gel process accompanied by phase separation

Monolithic cordierite with a cocontinuous macroporous structure has been successfully prepared by the sol–gel process accompanied by phase separation in the presence of poly(acrylamide) (PAAm). Propylene oxide (PO) acts as an acid scavenger to mediate the gelation of MgO–Al₂O₃–SiO₂ ternary system, while PAAm works as a phase-separation inducer as well as a network former. The dried gel and that heat-treated at 800 °C are amorphous, and the sapphirine begins to precipitate at 900 °C, then transforming to orthorhombic β-cordierite at 1100 °C. After heat-treated at and above 1200 °C, the resultant β-cordierite further transforms to stable hexagonal α-cordierite. Heat-treatment changes the macroporous structure of cordierite monoliths such as macropore size and its distribution. The macroporous cordierite monolith after heat-treated at 1200 °C is found to possess a total porosity of 54%, interconnected macropores and dense solid skeletons.     

Preparation of hierarchically porous spinel CoMn2O4 monoliths via sol–gel process accompanied by phase separation

Cobalt manganite-based hierarchically porous monoliths (HPMs) with three-dimensionally (3D) interconnected macropores and open nanopores have been prepared via the sol–gel process accompanied by phase separation. The controlled hydrolysis and polycondensation of the brominated metal alkoxides, which are generated from an incomplete reaction between epichlorohydrin and MBr₂ (M = Co and Mn) in N,N-dimethylformamide (DMF), form a monolithic gel based on the two divalent metal cations. The dual-polymer strategy using polyvinylpyrrolidone (PVP) and poly(ethylene oxide) (PEO) effectively induces the spinodal decomposition, where PVP and PEO are preferentially distributed to the gel phase and fluid phase, respectively, resulting in a porous gel characterized by the co-continuous structure. The effects of DMF and PVP on the porous morphology derived from the phase separation have been systematically studied. Calcination of the as-dried gels allows for the crystallization into the spinel phase yielding hierarchically porous CoMn₂O₄ monoliths, which have been examined in detail by the structural and compositional analyses.     

A comparative study between three different methods of hydrogel network characterization: effect of composition on the crosslinking properties using sol–gel,rheological and mechanical analyses

Polymer Bulletin - Previously, different techniques were used to identify the crosslinking density of hydrogels. In this study, we aimed to compare three different methods of network structure...     

Investigation of the tetraethoxysilane-derived sol–gel process by rheological oscillation test and the effect of different synthetic parameters

The tetraethoxysilane (TEOS)-derived sol–gel process was systematically investigated by the rheological oscillation test and the effect of different synthetic parameters, including TEOS concentration, water content, base catalyst concentration and temperature, on the viscoelastic properties of the formed gel was analyzed. It is found that the rheological oscillation test provides an efficient and accurate way to determine the gelation time in the sol–gel process. The time at which the storage modulus and loss modulus intersect can be regarded as the gelation time in the sol–gel process and it matches well with the time observed with the naked eyes. Furthermore, both the increased TEOS and base catalyst concentration can be beneficial to the improvement of the gel elasticity, which could be related to the increased concentration of silica primary particles and its condensation reaction rate. However, the increased water content and temperature lead to a first increase and then a decrease in the elasticity of the gel. The proper molar ratio of TEOS to water is 0.67:4.01 and the suitable reaction temperature is 318 K in this study.     

Effect of synthesis methods and aging on synthesis of uvarovite garnet by ceramic and sol–gel processes

Abstract

The uvarovite garnet (CaO)₃ (Cr₂O₃ )(SiO₂ )₃ has been synthesised (mineralised with borax to facilitate diffusion of precursors) by several sol–gel methods. Two routes for uvarovite formation have been observed: where CaCrO₄ forms as an intermediate phase; and where metastable pseudowollastonite (α-CaSiO₃ ) forms as an intermediate phase. Synthesis via CaCrO₄ appears to be more reactive in unaged samples. The reactivity of samples can be directly related to the chemical homogeneity of raw powders, and two methods of synthesis, Pechini and alkoxide, were found to stand out for their reactivity. Aging of raw powders for 6 months inhibits nucleation. Consequently, samples without nucleating agents (i.e. chlorides), such as gels from alkoxides, lead to the stabilisation of amorphous material or metastable phases (α-CaSiO₃ ) and, as a result, hinder uvarovite formation. In contrast, aged samples involving heterogeneous nucleation agents, as in the Pechini method with CaCl₂ as precursor, enhance reactivity.     

Synthesis and microstructural characterization of Bi12SiO20 (BSO) thin films produced by the sol–gel process

We have produced Bi₁₂SiO₂₀ (BSO) thin films using the sol–gel process. The stable sol was synthesized using Bi(NO₃)₃·5H₂O and Si(OC₂H₅)₄ (TEOS) as the precursors, acetic acid and 2-ethoxyethanol as the solvents, and ethanolamine as the stabilizer. The stability of the solution, which depends on the concentration and the R_h value (R_h = [H₂O]/[M]), directly affects the microstructure of the BSO thin film. We determined that the optimal concentration for the preparation of BSO thin films is 0.76 M. The influences of the substrates, the annealing temperature, the concentration and the R_h = value of the solution on the microstructure of the Bi₁₂SiO₂₀ thin films were investigated. X-ray diffraction (XRD) showed that the Bi₁₂SiO₂₀ starts to form at 500 °C and that single-phase Bi₁₂SiO₂₀ polycrystalline thin films are formed at 700 °C. The coated films were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM).     

Synthesis and modification of silica-based epoxy nanocomposites with different sol–gel process enhanced thermal and mechanical properties

This research article describes the results of nano-silica composites filled with different epoxy contents containing nano-SiO₂ particles from (5–25 wt%). Reinforcing hybrid composites enhance thermal and mechanical properties to achieve vital and sustainable products. Silica-based nanocomposites with high purity were prepared and used for the surface modification of nanosized silica particles. The surface structure's composition and physical properties of modified nano-SiO₂ particles were characterized through Fourier transferred infrared spectrometer, X-ray photoelectron spectroscopy, thermogravimetric analyzer, and scanning electron microscopic. Silica-based nanocomposites were prepared by incorporating of modified nano-SiO₂ as an enhancing filler. The morphology of fracture surface and dynamic mechanical properties were investigated. Results showed that the silica-based epoxy nanocomposites are bearing a long chain structure that could improve the compatibility of silica nanocomposites with epoxy resin and contribute to a better dispersion state in the matrix, which enhanced the overall performance of epoxy-cured products.     

Entrap-immobilization of invertase on composite gel fiber of cellulose acetate and zirconium alkoxide by sol–gel process

We prepared a composite gel fiber by the gel formation of cellulose acetate and zirconium tetra-n-butoxide. Gel fiber is stable in common solvents, phosphate solution, and electrolyte solution. Invertase was entrap-immobilized on the gel fiber. The immobilization was easily performed under the mild conditions. The apparent Michaelis constant (K_m) and maximum reaction velocity (V_max) were estimated from Eadie–Hofstee plot for immobilized invertase. The K_m of immobilized invertase was larger than that of native invertase, while the opposite tendency was observed for the V_max. The activity for the immobilized invertase became higher with increasing fiber diameter. It indicates that the hydrolysis of sucrose occurs in the neighborhood of the fiber surface. The thermal stability of the immobilized invertase was higher than those of its native counterpart. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2084–2088, 2001     

Microwave-assisted sol–gel synthesis of alpha alumina nanopowder and study of the rheological behavior

In the present work, alpha alumina nanopowder was synthesized via a sol–gel route. After preparation of bohemite (AlOOH) sol, carbon black was added and the resultant sol was dried and calcined in microwave furnace for 10 min. XRD results showed that alpha alumina was the only crystalline phase with specific surface area, mean diameter and crystallite size of 51 m² g^?1, 100 and 25 nm, respectively. Rheological measurements revealed that the optimal content of Tiron at pH=10 is 1 and 0.1 g per 100 g nano- and micron-alumina (1.5 m² g^?1), respectively. Furthermore, the optimum solid content of the slips was determined as 35–45 and 70 wt.% for nano- and micron-alumina, respectively.     

Surface properties and moisture behaviour of pine and heat-treated spruce modified with alkoxysilanes by sol–gel process

The surface morphology and moisture behaviour of pine (Pinus sylvestris) sapwood and heat-treated spruce (Picea abies) deposited with two types of silane-based sol–gel coatings were studied by atomic force microscopy (AFM) and water contact angle measurement. The chemical composition and distribution of sol–gel coatings on wood surfaces were investigated by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The AFM images revealed that the sol–gel coatings applied by spreading covered the fine structure of the wood substrates. The surface roughness analysis of the AFM topographical images indicated that the sol–gel coatings, especially the one with short aliphatic chain, had a tendency to smooth the wood surface. The XPS results confirmed that the sol–gel coatings had successfully deposited onto pine sapwood and heat-treated spruce changing their surface chemistries. ToF-SIMS images showing Si ion distribution on treated surfaces revealed that the coatings fully covered pine sapwood surfaces. The thin coating layers formed on heat-treated spruce surfaces followed the original wood surface structure. The contact angle measurements indicated that the water repellent properties of both pine and heat-treated spruce were improved to certain extent by the sol–gel coatings.     

Influence of the synthesis process on the features of Y2O3-stabilized ZrO2 powders obtained by the sol–gel method

Y₂O₃-stabilized ZrO₂ (YSZ) powders have been prepared by the sol–gel method using different synthesis parameters. Specifically, zirconium n-propoxide was dissolved in propanol at pH 0.5 or 5 (provided by HNO₃), with or without acetic acid in the hydrolysis medium. Subsequently, the YSZ powders obtained by gelation and drying of these solutions was characterized using scanning and transmission electron microscopies, X-ray diffractometry, and N₂-adsorption. Compacts made from these YSZ powders which were then sintered were also analyzed. It was found that the pH of the hydrolysis medium has a notable influence on the microstructure, morphology, color, crystallinity, and sintering behavior process of these YSZ sol–gel powders. It was also found that the use of acetic acid also affects the YSZ powder features, and results in compacts with higher residual porosity after sintering. Finally, the compacts prepared from the YSZ powders obtained at pH 5 and without acetic acid exhibit the greatest sinterability.     

A study on the growth mechanism and gas diffusion barrier property of homogeneously mixed silicon–tin oxide by atomic layer deposition

SiO₂ and SnO₂ films were deposited using plasma-enhanced atomic layer deposition (PEALD) at low temperature (100 °C), and homogeneously mixed structure (HMS) films consisting of Si, Sn, and O were deposited through a “1st precursor dose – 2nd precursor dose – oxidation”, a new ALD process method for mixing two elements. For a deep consideration of the growth mechanism during the HMS film deposition process, density functional theory (DFT) calculations of the adsorption reactions of the precursors on the surface were conducted. The properties of the thin films such as density, atomic composition, crystallinity, surface roughness, optical transmittance and the water vapor diffusion barrier property were analyzed by XRR, XPS, XRD, AFM, UV-VIS and the electrical Ca test.By changing the dose sequence of the two precursors in the HMS process, various physical/chemical characteristics of the films could be controlled. Also, by adjusting the appropriate amount of Sn in the HMS films, the shortcomings of SnO₂ were compensated by the mixed SiO₂; and through this process, excellent gas diffusion barrier properties of WVTR ∼ 1.33 × 10⁻⁴ g/m²day were secured.     

A comparative study of the effect of Pd-doping on the structural,optical, and photocatalytic properties of sol–gel derived anatase TiO2 nanoparticles

Pd-doped anatase TiO₂ nanoparticles were synthesized by a modified sol–gel deposition technique. The synthetic strategy is applicable to other transition and post-transition metals to obtain phase-pure anatase titania nanoparticles. This is important in the sense that anatase titania forms the most hydroxyl radicals (compared to other polymorphs like rutile, brookite, etc.) for better photocatalytic performance. XRD and Raman data confirm the phase-pure anatase formation. Doping of Pd²⁺ into Ti⁴⁺ sites (for substitutional doping) or interstitial sites (for interstitial doping) creates strain within the nanoparticles and is reflected in the XRD peak broadening and Raman peak shifts. This is because of the ionic radii difference between Ti⁴⁺(∼68 pm) and Pd²⁺(∼86 pm). XPS data confirm the formation of high surface titanol groups at the nanoparticle surface and a large number of loosely bound Ti³⁺–O bonds, both of which considerably enhance the photocatalytic activity of the doped nanoparticles. A comparative study with other metal doping (Ga) shows that TiO₂: Pd nanoparticles have more Ti³⁺–O bonds, which enhance the charge transfer rate and hence improve the photocatalytic activity compared to other transition and post-transition metal-doped titania nanostructures.     

Low-temperature synthesis and microwave dielectric properties of trirutile-structure MgTa2O6 ceramics by aqueous sol–gel process

Trirutile-structure MgTa₂O₆ ceramics were prepared by aqueous sol–gel method and microwave dielectric properties were investigated. Highly reactive nanosized MgTa₂O₆ powders were successfully synthesized at 500 °C in oxygen atmosphere with particle sizes of 20–40 nm. The evolution of phase formation was detected by DTA–TG and XRD. Sintering characteristic and microwave dielectric properties of MgTa₂O₆ ceramics were studied at different temperatures ranging from 1100 to 1300 °C. With the increase of sintering temperature, density, ?_r and Q · f values increased and saturated at 1200 °C with excellent microwave properties of ?_r ～ 30.1, Q · f ～ 57,300 GHz and τ_f ～ 29 ppm/°C. The sintering temperature of MgTa₂O₆ ceramics was significantly reduced by aqueous sol–gel process compared to conventional solid-state method.     

Fabrication of chemically bonded polyacrylate/silica hybrid films with high silicon contents by the sol–gel method

Polyacrylate/silica hybrid latexes (PAES) with high silicon contents (up to 21%) were prepared by directly mixing colloidal silica with polyacrylate emulsion (PAE) modified by a silane coupling agent. Sol–gel-derived organic/inorganic thin films were obtained by addition of hydrophilic co-solvents to PAES and subsequent drying at room temperature. The effects of co-solvents and γ-methacryloxypropyltrimethoxysilane (KH570) content on the properties of PAES films were investigated. Dynamic light scattering (DLS) data indicated that the average diameter of PAES (96 nm) was slightly larger than that of PAE (89 nm). TEM photo revealed that colloidal silica particles were dispersed uniformly around polyacrylate particles and that some of the colloidal silica particles were adsorbed on the surface of PAE particles. The data of crosslinking degree and FT-IR spectra confirmed that the chemical structure of the PAES changed to form Si–O–Si-polymer crosslinking networks during the film formation. AFM photos, contact angle for water, and XPS analysis showed that the polyacrylate/inorganic hybrid films with high silicon contents were formed by the co-solvent-mediated, sol–gel method and that the Si-based polymers were uniformly distributed on the surface of the dried films. TGA data demonstrated that the PAES films display much better thermal stability than the PAE counterpart.     

Effect of yttria on crystallization and microstructure of an alumina–YAG fiber prepared by aqueous sol–gel process

Continuous fiber development is needed for high performance and high temperature composites. Various methods have been used to make ceramic fibers. In this research, composite fibers (yttrium aluminum garnet (YAG)/Al₂O₃) were prepared by a sol–gel method using aqueous solution. They were synthesized from aluminum salt, aluminum metal, yttrium oxide and water used as solvent. Transparent gel fibers were obtained by immersing a thin wire into the viscous sol, then pulling it out by hand. The obtained fibers contained very fine grains with diameter ranging from 10 to 80 μm after heat treatment. When yttria content was increased, the crystallization of YAG shifted to a lower temperature, whereas the transformation temperature to α-Al₂O₃ shifted to a higher temperature. Nevertheless, the fibers with different amounts of yttria contained alumina and YAG after heat treatment at 1400 °C. The composite fibers had vermicular structure and were denser than alumina fibers. The yttria percent concerning the limits of this study (≤10 wt%) effected on fiber diameter. As the yttria content was increased, the fiber diameter increased, whereas grain size and densification of the composite fibers decreased.     

Synthesis and microwave dielectric properties of CaSiO3 nanopowder by the sol–gel process

The synthesis and microwave dielectric properties of CaSiO₃ nanopowder by sol–gel method have been investigated in this paper. CaSiO₃ nanoparticles with an average grain size of 50–60 nm were obtained by calcining the CaO–SiO₂ xerogel that was prepared from Calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O) and tetraethylortho silicate (TEOS). Calcining the CaO–SiO₂ xerogel at 1150 °C, the pseudowollastonite-CaSiO₃ phase was completely formed. However, the main phase is not CaSiO₃ or CaSi₂O₄ but SiO₂ when calcining the mixture of SiO₂ and CaCO₃ at 1150 °C. Comparing with CaO–SiO₂ ceramics prepared by solid-state process, the CaSiO₃ ceramics made from nanopowders calcined at 1000 °C achieved more compact structure at the sintering temperature of 1320 °C, and then had excellent microwave dielectric properties: ?_r = 6.69, Qf = 25398 GHz.     

Facile production of porous bioactive glass scaffolds by the foam replica technique combined with sol–gel/electrophoretic deposition

3D porous bioactive glass (BG) scaffolds were fabricated by the foam replication technique combining sol–gel and electrophoretic deposition (EPD) processes. A special EPD set-up was built to optimize the scaffolds fabrication, which results in the rapid production of BG-based scaffolds in comparison to the traditional time-consuming foam replication technique normally used for the same kind of BG scaffolds. The influences of parameters related to the BG sol (aging time) as well as to the electrically driven process (deposition voltage) were studied and discussed in terms of the final scaffolds microstructure. Bioactivity of the samples, in terms of hydroxycarbonate apatite (HCA) formation in simulated body fluid (SBF), was determined after 1 day of immersion in SBF, confirming the suitability of the new scaffolds for bone regeneration applications.