Fabrication and characterization of porous mullite ceramics with ultra-low shrinkage and high porosity via sol-gel and solid state reaction methods

Porous mullite ceramics with ultra-low shrinkage and high porosity were prepared by solid state reaction between MoO₃ and mullite precursor powders which were synthesized from tetraethylorthosilicate and aluminium nitrate nonahydrate via sol-gel methods. The synthetic process of mullite precursor powder and effects of MoO₃ amount on the phase composition, microstructure, physical properties such as firing shrinkage, open porosity, bending strength, water absorption and bulk density of porous mullite ceramics were investigated. The results indicated that the addition of MoO₃ not only lowered the mullite forming temperature from 985.4 to 853.3 °C, but also restrained densification behavior of samples due to the formation of mullite and Al₂O₃–MoO₃ solid solution, besides, MoO₃ also improves the formability, open porosity and bending strength of samples. The optimal amounts of MoO₃ is 8 wt%, and the resultant samples exhibit outstanding properties, including a low shrinkage rate of 1.86 ± 0.07%, an open porosity of 61.91 ± 0.16% and a bending strength of 9.35 ± 1.11 MPa.     

Poly(styrene-b-isobutylene-b-styrene) block copolymer ionomers (BCPI), and BCPI/silicate nanocomposites. 1. Organic counterion: BCPI sol-gel reaction template

Silicate structures were inserted along the cylindrical polystyrene (PS) domains in an ionomer form of elastomeric poly(styrene-b-isobutylene-b-styrene) tri-block copolymers, via in situ sol-gel reactions. Environmental scanning electron microscopy/energy dispersive X-ray spectroscopy studies indicated that silicate structures do in fact grow within the interior of ca. 0.8 mm thick films rather than forming undesirable silica precipitates on the surface. The combination of a domain-selective swelling solvent (DMAc) and the attachment of large organic counterions (benzyltrimethylammonium) along the styrene blocks facilitated the preferential migration of hydrolyzed Si(OEt)₄ monomers to these ionic domains where the sol-gel reactions are apparently seeded. Differential scanning calorimetry and dynamic mechanical studies indicated that T_g for the polyisobutylene (PIB) phase is essentially unaffected, but the PS phase T_g shifts to higher values with ionomer formation, and to even higher values with subsequent silicate phase insertion. These two methods provide indirect evidence that the silicate component is mainly incorporated in the PS rather than PIB domains. Combined with the results of earlier atomic force microscopy studies that demonstrated that the basic morphology of the unmodified block copolymer is unchanged despite the insertion of a silicate phase, the data presented here reinforce the concept of a robust sol-gel reaction template. Also, the rubbery plateau storage modulus was elevated as a result of ionomer formation and more so after the ionomer was imparted with a silicate phase, which illustrates mechanical reinforcement.     

Polyamide-silica gel hybrids containing metal salts: Preparation via the sol-gel reaction

The hydrolysis and condensation of tetramethoxysilane in a DMF solution of polyamides containing LiCl, CaCl₂ or ZnCl₂, both in presence and absence of polyoxazoline, resulted in the facile formation of polyamide-silica gel hybrids. Films were cast from the resulting mixtures and evaporation of the solvent resulted in the formation of clear, transparent hybrids with the salts dispersed at the molecular level. Pyrolysis of hybrids at 600 °C gave porous silica. Pore size and surface characteristics of these silica gel samples indicated a porous nature with a pore radius of 1.1 nm for silica gels obtained from hybrids HPA-6 (containing no salt) and HPA-9 (containing ZnCl₂) and a surface area of 213 m² g⁻¹ and 310 m² g⁻¹, respectively. Silica gel from hybrid HPA-7 (containing LiCl) had a pore radius of 1.9 nm and a surface area of 15 m² g⁻¹. The silica gel samples obtained from hybrids HPA-6, HPA-7 and HPA-9 exhibited narrow slit-like pores with a pore volume of 0.68 cm³ g⁻¹. Received: 7 January 1997/Accepted: 6 March 1997     

Synthesis of modified SiO2 networks: Polydimethylsiloxane-SiO2 organic-inorganic hybrids via sol-gel chemistry

Poly(dimethylsiloxane)-SiO₂ organic-inorganic hybrid networks were synthesized by a two-step, catalyst-free sol-gel process. Reactive PSX (polydimethylsiloxane) oligomers with methoxysilyl end-groups were used as the organic component, and tetramethylorthosilicate (TMOS) as the inorganic component of the hybrid. Our studies show that uncatalyzed sol-gel reactions based on TMOS can reach gelation in 7.5 h when reacted at room temperature, with a stoichiometric amount of water necessary for complete hydrolysis in the initial stages, and at a high concentration of reacting species. The relatively neutral reaction environment of this catalyst-free process is important in the synthesis of PSX-SiO₂ hybrid networks because it prevents redistribution reactions of PSX modifiers, especially when higher temperatures are used for the drying of the gels. The PSX-SiO₂ hybrid networks have reduced polarity, increased water resistance, and improved toughness compared to inorganic SiO₂ networks produced under similar conditions.     

Preparation and properties of organic-inorganic hybrids based on poly(methyl methacrylate) and sol-gel polymerized 3-glycidyloxypropyltrimethoxysilane

New organic-inorganic hybrid materials were prepared by the sol-gel process. Simultaneous polymerizations of methyl methacrylate (MMA) and an organically modified silicon alkoxide, 3-glycidyloxypropyltrimethoxysilane (GLYMO), with varying MMA/GLYMO molar ratios, were performed. Poly(oxypropylene)diamine was used as an epoxy opening agent, as basic catalyst for GLYMO condensation and as poly(methyl methacrylate) (PMMA) crosslinking agent. Chemical reactions and the structure of prepared hybrids were studied by means of Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA) and by scanning electron microscopy (SEM). Depending on the ratios of organic and inorganic components hybrids showed either a discrete microstructure or interpenetrating polymer network (IPN) structure. Silsesquioxane (SSQO) structures formed as a result of GLYMO hydrolysis and condensation in the sol-gel process influence the glass-transition temperature of PMMA. The hybrids have much better thermal stability than PMMA, their surfaces are more hydrophilic than PMMA and they may have potential as damping materials.     

Nafion-clay hybrids with a network structure

Nafion-clay hybrid membranes with a unique microstructure were synthesized using a fundamentally new approach. The new approach is based on depletion aggregation of suspended particles - a well-known phenomenon in colloids. For certain concentrations of clay and polymer, addition of Nafion solution to clay suspensions in water leads to a gel. Using Cryo-TEM we show that the clay particles in the hybrid gels form a network structure with an average cell size in the order of 500 nm. The hybrid gels are subsequently cast to produce hybrid Nafion-clay membranes. Compared to pure Nafion the swelling of the hybrid membranes in water and methanol is dramatically reduced while their selectivity (ratio of conductivity over permeability) increases. The small decrease of ionic conductivity for the hybrid membranes is more than compensated by the large decrease in methanol permeability. Lastly the hybrid membranes are much stiffer and can withstand higher temperatures compared to pure Nafion. Both of these characteristics are highly desirable for use in fuel cell applications, since a) they will allow the use of a thinner membrane circumventing problems associated with the membrane resistance and b) enable high temperature applications.     

Synthesis of colloidal polyoxazoline/silica hybrids prepared in an aqueous solution

New colloidal polymer hybrids with polyoxazolines such as poly(2-methyl-2-oxazoline) (PMeOZO) and poly(2-ethyl-2-oxazoline) (PEtOZO) were obtained in an aqueous solution as a sol-gel solvent. The polyoxazoline segment was incorporated into the siliceous particle and formed strong hydrogen bonds with silanol moieties, judging from TGA and FT-IR studies. Colloidal polymer hybrids exhibited no glass transition temperature of the polyoxazoline and showed excellent thermal stability and solvent-resistant property. Porous silica obtained by charring the colloidal polymer hybrids at 600 °C exhibited the peak at 3.5 Å. These results strongly indicate the homogeneous molecular scale dispersion of polyoxazoline in the colloidal silica gel matrix.     

Synthesis and characterization of nanocrystalline NiO-GDC via sodium alginate-mediated ionic sol-gel method

In this study, nanocrystalline nickel oxide gadolinium-doped ceria (NiO-GDC) powder was synthesized in-situ using Na-Alginate as the template via ionic sol-gel technique. The effects of calcination time and temperature on the particle size and the physiochemical properties of nanocrystalline NiO-GDC are presented in this paper. Using this method, gel beads were formed by contacting sodium alginate solution as the gelling template and metal (gadolinium/cerium/Ni) nitrates as the precursor. The obtained nanocrystallites were characterized using Field Emission Scanning Electron Microscopy, powder X-ray diffraction, energy dispersive X-ray spectroscopy, thermo gravimetric analysis, nitrogen adsorption/desorption analysis, and Fourier transform infrared spectroscopy. It was observed that the increasing calcination temperature had affected both the particle size and the surface area of the NiO-GDC, whereas the increasing calcination time had only impacted the size of the particles. The smallest mesoporous nanocrystalline NiO-GDC powder (12.1225 ± 0.005 m²/g surface area), composed of cubic GDC (5.18 nm crystallite size) and cubic NiO (7.99 nm crystallite size) were synthesized at a calcination temperature of 500 °C for 2 h. This study hopes to inspire more researches on the ionic-gelation method for synthesizing other metal nanostructures as well as other reaction parameters.     

Preparation and characterization of multilayer anti-reflective coatings via sol-gel process

Single layer and multilayer films consisting of SnO₂, Ta₂O₅, SiO₂, TiO₂, indium tin oxide (ITO) and antimony tin oxide (ATO) have been prepared by sol-gel dip coating technique. All of the multilayer films contained a SiO₂ top layer, which was composed of SiO₂ nanoparticles. The other films had polymeric character. Obtained films were characterized by ellipsometry, XRD, AFM and SEM. Light transmittance values of the films were compared. Films other than SiO₂ and Ta₂O₅ were found to have crystalline structure. Thickness values of the films were in the range of 30–115 nm and roughness values were in 1.2–23 nm range. Single layer porous silica provided 95% light transmittance, whereas ITO-TiO₂-SiO₂ multilayer film provided a light transmittance of 97.2%.     

Synthesis and characterization of benzoxazine resin-SiO2 hybrids by sol-gel process: The role of benzoxazine-functional silane coupling agent

Sol-gel process has been used to incorporate silica nanoparticles into polybenzoxazine matrix to form polybenzoxazine-silica hybrids. The hybrids have been synthesized by the reaction of bisphenol A/aniline type benzoxazine monomer (BA-a) and tetraethoxysilane as precursor for silica nanoparticles. Triethoxysilane-functional benzoxazine monomer was also prepared from γ-aminopropyltriethoxysilane, phenol and paraformaldehyde, and has been used as silane coupling agent to improve the adhesion between polybenzoxazine matrix as organic domain and silica nanoparticles as inorganic domain. Differential scanning calorimetry (DSC) and FT-IR are used to study the polymerization behavior of benzoxazine monomer in the presence of silica nanoparticles over different temperature range. Dynamic mechanical analysis indicates that the coupling agent is very effective to increase the T_g and the storage modulus of the hybrids. Thermogravimetric analysis also shows the thermal stability of neat polybenzoxazine matrix is improved by the inclusion of silica nanoparticles.     

Synthetic aspects and characterization of polypropylene-silica nanocomposites prepared via solid-state modification and sol-gel reactions

A new route is developed by combining solid-state modification (SSM) by grafting vinyl triethoxysilane (VTES) with a sol-gel method to prepare PP/silica nanocomposites with varying degree of adhesion between filler and matrix. VTES was grafted via SSM in porous PP particles. Bulk polymerization under similar experimental conditions as in SSM resulted in homopolymerization of VTES. However, SEC and NMR experiments showed that VTES was grafted as a single monomeric unit in the amorphous phase of PP with the possibility of VTES-polymer grafting during SSM. Silica-like nano-particles were synthesized in-situ by the sol-gel method. Magic-angle spinning (MAS) ²⁹Si NMR spectra showed that the chemical building blocks of the silica-like clusters are of Q³ and Q⁴ type. MAS ²⁹Si NMR and FT-IR spectroscopy showed that the grafted VTES becomes part of the in-situ formed silica particles. No decrease in molecular weight of PP was observed, indicating that chain scission is marginal compared to melt modification. The morphology of the nanocomposites as observed by ATR-FTIR microscopy showed a uniform dispersion of grafted VTES as well as in-situ formed silica. TEM and SEM demonstrated that the in-situ formed silica particles are nearly spherical and have sizes in the range of 50-100 nm.     

A comparative study of the structure,defects, optical,dielectric, and magnetic properties of GdMnO3 multiferroic ceramics synthesized by solid-state reaction and sol-gel methods

In this work, GdMnO₃ ceramics were synthesized by solid state reaction and sol-gel methods, and the structure, defects and optical, dielectric and magnetic properties of the synthesized samples were comparatively investigated. The samples synthesized by different methods show a single phase structure without any detectable impurities. The SEM results suggest that the particle size of the specimen obtained by the solid phase route is on the micron scale, while that of the specimen fabricated by the sol-gel route is on the nanometer scale. Compared with the ceramic fabricated by solid-state reaction technology, the specimen synthesized by sol-gel technique possesses lower oxygen vacancies and Mn²⁺ concentration, and Mn³⁺ concentration. The positron annihilation analyses show that the cation vacancy concentration of the specimen synthesized by the solid phase approach is higher than that of the specimen synthesized via the sol-gel approach. The compound obtained by the solid phase reaction has better dielectric properties than that obtained with the sol-gel method. The magnetic transition temperature and the effective magnetic moment are influenced by the Mn ion valence state in GdMnO₃. The stronger magnetization of the ceramic synthesized via the sol-gel approach is associated with the lower concentration of cation vacancies.     

Growth mechanisms of MgO nanocrystals via a sol-gel synthesis using different complexing agents

In the preparation of nanostructured materials, it is important to optimize synthesis parameters in order to obtain the desired material. This work investigates the role of complexing agents, oxalic acid and tartaric acid, in the production of MgO nanocrystals. Results from simultaneous thermogravimetric analysis (STA) show that the two different synthesis routes yield precursors with different thermal profiles. It is found that the thermal profiles of the precursors can reveal the effects of crystal growth during thermal annealing. X-ray diffraction confirms that the final products are pure, single phase and of cubic shape. It is also found that complexing agents can affect the rate of crystal growth. The structures of the oxalic acid and tartaric acid as well as the complexation sites play very important roles in the formation of the nanocrystals. The complexing agents influence the rate of growth which affects the final crystallite size of the materials. Surprisingly, it is also found that oxalic acid and tartaric acid act as surfactants inhibiting crystal growth even at a high temperature of 950°C and a long annealing time of 36 h. The crystallite formation routes are proposed to be via linear and branched polymer networks due to the different structures of the complexing agents.     

Morphological and thermal characterization of zirconia/hydroxyapatite composites prepared via sol-gel for biomedical applications

The thermal behavior of pure ZrO₂ and hydroxyapatite (denoted as Z and HAp, respectively), as well as three composites with different content of Z and HAp (Z90HAp10, Z70HAp30 and Z50HAp50) prepared via sol-gel method has been studied by thermogravimetry (TG) and first-order derivative of TG up to 1200?°C under inert gas atmosphere. Dehydration, loss of alcohol and acetylacetone and a multi-step thermal decomposition processes has been identified by analyzing the gases evolved in each step by Fourier transform infrared spectroscopy (FTIR). Fresh samples of Z-rich composites undergo an abrupt ejection of material from the crucible around 200?°C with noticeable increase of the sample temperature. During the occurrence of this phenomenon FTIR spectra demonstrated the evolution of gases (CO, CO₂, acetone and ethylene) due to the simultaneous decomposition of acetylacetone and ethanol, not present in the samples calcined at 120?°C. As far as the structural study is concerned, pure Z crystallizes at 1000?°C in the monoclinic system, but the presence of HAp in the composite materials enables the crystallization of Z in the tetragonal phase. Finally, the amorphization degree increases with increasing the content of Z in all the composites treated at 600 and 1000?°C.     

Polymer hybrids of functionalized silsesquioxanes and organic polymers utilizing the sol-gel reaction of tetramethoxysilane

The ternary polymer hybrids were prepared by organic polymers such as poly(2-methyl-2-oxazoline) (POZO) or poly(N-vinylpyrrolidone) (PVP) and aminopropylsilsesquioxane (Cube-aminopropyl) utilizing the sol-gel reaction of tetramethoxysilane (TMOS). The prepared polymer hybrids were characterized by IR, thermogravimetric analysis (TGA), X-ray diffraction (XRD), ¹H NMR, scanning electron microscopy (SEM), etc. In this hybrid system, hydrogen-bonding interactions played a critical role in the formation of the transparent polymer hybrids. Polymer hybrids using POZO showed high transparency and homogeneity in a wide range of the feed ratios of POZO to Cube-aminopropyl. On the other hand, in case of polymer hybrids using PVP, higher Cube-aminopropyl ratio brought about the phase separation, indicating the aggregation of the Cube-aminopropyl itself. The homogeneity of ternary polymer hybrids was found to be closely dependent on the difference between strength of hydrogen bonding interaction of polymer and residual silanol groups of silica gel and strength of that of polymer and Cube-aminopropyl. It is also observed that initial decomposition temperature of polymer hybrids was increased with increasing the Cube-aminopropyl ratio.     

Synthesis of modified SiO2 networks: Polydimethylsiloxane-SiO2 organic-inorganic hybrids via sol-gel chemistry

Poly(dimethylsiloxane)-SiO₂ organic-inorganic hybrid networks were synthesized by a two-step, catalyst-free sol-gel process. Reactive PSX (polydimethylsiloxane) oligomers with methoxysilyl end-groups were used as the organic component, and tetramethylorthosilicate (TMOS) as the inorganic component of the hybrid. Our studies show that uncatalyzed sol-gel reactions based on TMOS can reach gelation in 7.5 h when reacted at room temperature, with a stoichiometric amount of water necessary for complete hydrolysis in the initial stages, and at a high concentration of reacting species. The relatively neutral reaction environment of this catalyst-free process is important in the synthesis of PSX-SiO₂ hybrid networks because it prevents redistribution reactions of PSX modifiers, especially when higher temperatures are used for the drying of the gels. The PSX-SiO₂ hybrid networks have reduced polarity, increased water resistance, and improved toughness compared to inorganic SiO₂ networks produced under similar conditions.     

The influence of polyelectrolyte on the synthesis of B4C/BN nanocomposite powders via sol-gel method

In the present study, B₄C-BN nanocomposite powders were synthesized by using the sol-gel method. To investigate the effects of polyelectrolyte on phase content, particle size, and final morphology of synthesized powders different amounts of ammonium polycarboxylate were used as a gel dispersing agent and a nitrogen source. Highly crystalline, sub-micron/micron-sized boron carbide particles with varying morphologies including polyhedral-equiaxed, belt-like, needle-like, and complex-shaped hierarchical structures were produced from the polymeric gel containing glycerine, tartaric acid, and citric acid as carbon sources, and boric acid as boron source. With the addition of ammonium polycarboxylate as a polymeric gel network modifier, nanocomposite powders containing micron-sized polyhedral-equiaxed boron carbide particles and boron nitride nanoflakes were obtained. The results indicated that the particles dimensions, crystallinity, and B₄C to BN phase ratio of the synthesized powders are directly related to the preliminary formation of borate-ammonium and/or amine complexes in the polymeric gel. The SEM inspections revealed that the size of boron carbide particles tends to increase from 2 μm up to 40 μm as a function of ammonium polycarboxylate content. It was also observed that the average size and thickness of boron nitride nanoflakes within the range of 80 nm to 3 μm and 10–150 nm, respectively. B₄C/BN nanocomposite powders were synthesized with up to 32% BN content using a 43 wt% ammonium polycarboxylate additive.     

Ferroelectric and dielectric behaviors of sol-gel derived perovskite PMN-PT/PZT heterostructures via compositional development: An interface-dependent study

Phase-pure perovskite 0.67Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃/Pb(Zr_0.52Ti_0.48)O₃ (PMN-PT/PZT) heterostructures were prepared via compositional development using a sol-gel route. Interface-dependent microstructure, insulating, ferroelectric and dielectric performance were investigated. Relatively enhanced ferroelectricity (P_r?=?21.81 μC/cm², E_c?=?61.88?kV/cm) and dielectricity (ε_r?=?1959, tanδ?=?0.0152) were obtained for the heterostructure with the maximum number of interfaces (7 interfaces, namely F-7 type). Presumably, this behavior is due to the reduced leakage current density (10^?9–10^?8 A/cm² at ±400?kV/cm), which arises from columnar grain growth mode with more depletion layers generated between different compositions acting as a potential barrier for the movement of free carriers. A simplified equivalent circuit is used to provide a comprehensive explanation of the enhanced performance mechanism. These results highlight inherent issues in designing structures with specific interfaces and provide a new approach for designing high-performance PMN-PT-based thin film.     

Enhanced ferroelectric properties of (Zn,Ti) equivalent co-doped BiFeO3 films prepared via the sol-gel method

High-quality BiFe_1-2xZn_xTi_xO₃ (BFZTO with x = 0, 0.01, 0.02, 0.03, 0.04, and 0.05) films were successfully prepared on fluorine-doped tin oxide (FTO)/glass substrates via the sol-gel method. The influence of (Zn, Ti) equivalent co-doping on the structure, surface morphology, and ferroelectric properties of BFZTO films was investigated systematically. X-ray diffraction (XRD) and Raman spectra analysis indicate that co-doping results in structural transformations. Scanning electron microscope (SEM) images show that BFZTO films with x = 0.02 exhibit uniform fine grains and higher density, which is instrumental for the development of ferroelectric properties. X-ray photoelectron spectroscopy (XPS) analysis reveals that BiFe_0.96Zn_0.02Ti_0.02O₃ film can inhibit the conversion of Fe³⁺ into Fe²⁺, thereby greatly reducing oxygen vacancy concentration. Therefore, under the electric field strength of 150 kV/cm, BiFe_0.96Zn_0.02Ti_0.02O₃ film was found to have the lowest leakage current density, J = 1.13 × 10^?6 A/cm², which is five orders of magnitude lower than that of pure BiFeO₃ (BFO) film. Furthermore, this film exhibits the largest remnant polarization at room temperature, P_r = 131.9 μC/cm², which is more than twice as large as that of pure BFO (P_r = 52.6 μC/cm²). Additionally, by comparing P-E hysteresis loops of different regions on the surface of BiFe_0.96Zn_0.02Ti_0.02O₃ film, it was found that the film has high uniformity and stable overall performance. Dielectric and magnetic properties were also enhanced via (Zn, Ti) co-doping.     

Preparation and characterization of high-surface-area titanium dioxide by sol-gel process

One of the important ways to improve photocatalytic efficiency is to prepare catalyst with enhanced surface area. In this work, titanium dioxide (TiO₂) nanoparticles having enhanced surface area were synthesized under the interference of SiO₂. The mixed oxide, SiO₂-TiO₂ (10% mol% Si), was prepared by a sol-gel procedure using titanium tetra-n-butoxide as Ti-precursor. The commercial SiO₂ nanoparticles were added into the TiO₂ sols after hydrolysis. After condensation and calcination heat treatment, the SiO₂-TiO₂ nanoparticles were obtained. To achieve the purpose of obtaining the high-surface-area TiO₂, the SiO₂ was removed subsequently by aqueous NaOH solution. The TiO₂ products were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), electron spectroscopy for chemical analysis (ESCA), and by N₂ adsorption-desorption isotherm. A fine mesoporous structure was formed for as-prepared TiO₂ after calcination at 400^∘C and the average pore diameter was about 7 nm. The porous TiO₂ products possess mixing phases of anatase and rutile. Phase transformation from anatase to rutile occurred when the samples were calcined. The phase transition temperature is sensitive to the silicon content. The particle size of ∼43 nm remained constant upon calcinations from 500 to 700^∘C. The specific surface area was increased up to 66% compared to regular TiO₂ samples that were prepared by the similar sol-gel procedure. The porous TiO₂ nanostructures exhibited enhanced photocatalytic performance to decompose methylene blue under UV irradiation.