The effect of solution chemistry on the preparation of MgAl2O4 by hydrothermal-assisted sol-gel processing

Preparation of magnesium aluminate spinel powder by hydrothermal-assisted sol-gel processing from MgAl₂(OCH₂CH₂OR)₈, RCH₃ (1), CH₂CH₂OCH₃ (2), MgAl₂[OCH(CH₃)₂]₈ (3) and MgAl₂(O-^sBu)₈ (4) in toluene and parent alcohol has been investigated. Coordination status of aluminum atom in precursors was determined by ²⁷Al NMR and correlation between coordination number of aluminum and development of spinel phase in hydrothermal-assisted sol-gel processing has been studied. The gels obtained from hydrothermal-assisted hydrolysis of magnesium-aluminum alkoxides that contain six-coordinated aluminum atoms in solution (1 and 2) after calcination at 700 °C resulted in the formation of pure spinel phase, whereas in similar hydrolysis and calcination processes of precursors that contain four-coordinated aluminum (3 and 4) spinel phase forms along with some Al₂O₃ and MgO. Selected powders obtained from hydrothermal-assisted sol-gel processing were characterized by thermal analysis (TGA/DSC), X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). Results indicate that the coordination status of aluminum in the precursor is very crucial for the formation of pure phase spinel. The morphology of prepared spinels was studied by SEM and the results showed that the solvent in hydrothermal-assisted sol-gel processing has a marked effect on the morphology of the resulting MgAl₂O₄. In hydrothermal-assisted sol-gel processing of aluminum-magnesium alkoxides in hydrophobic solvent, spherical particles are formed, while in the parent alcohol, non-spherical powders are formed.     

Sol-gel fabrication of compact, crack-free alumina film

Compact, crack-free alumina film was fabricated using an alumina sol with a high Al₂O₃ content. With the addition of ethylacetoacetate (CH₃COCH₂COOC₂H₅, EAcAc), the stable sol could be prepared with a molar ratio of aluminum sec-butoxide (Al(O-sec-Bu)₃, ASB) to water up to 1:25. It was found that EAcAc could notably decrease the surface tension of the liquid in the gel pores. The EAcAc modification layer on the colloidal particle retarded greatly the densification of the Al₂O₃ gel film and provided a long-lasting structural relaxation during heating. Therefore, the formation of cracks was effectively prevented in this alumina material. The alumina gel film contained a high Al₂O₃ content and there was a rather small mass loss during sintering. The critical thickness of Al₂O₃ sol-gel film was eight times higher than that could be achieved via the general sol-gel route and a film thicker than 0.8 μm was prepared by a single-step dipping operation.     

Variables in the synthesis of unusually high pore volume aluminas

This paper summarizes our successful efforts to obtain high pore volume aluminas (> 5.0 cm³ g^–1 ). The pore volumes of these aerogels, prepared by the hydrolysis of aluminium isopropoxide (AIP) in alcoholic media, are very sensitive to the amounts of water and alcohol employed in the synthesis. An unexpectedly large ratio of water to alkoxide yields the highest pore volumes. By suitable selection of initial levels of aluminium alkoxide, methanol and water, one obtains average pore volumes of 8.6 cm³ g^–1. A two-step formulation of AIP, methanol, and water followed by hypercritical solvent removal to form the alumina aerogel yields the highest pore volumes. Other alternative approaches also increase the yield of aerogel produced within the autoclave. Pore volumes of about 7 cm³ g^–1 result if the premix is concentrated twofold after formulation and before autoclave processing. Lower pore volumes result if the reactant charge is increased directly by simply increasing the concentration of AIR It appears that the reduction in pore volume results in part from the higher isopropanol content in the final reaction mixture (isopropanol is a co-product from AIP hydrolysis).     

Properties and catalytic activity of NOx reduction of alumina-titania catalysts prepared by sol-gel method

The alumina-titania catalysts were prepared from various alumina and titania sources by sol-gel method, which were metal alkoxide and metal alkoxide modified with organic groups. Specific surface area, pore size distribution, solid acidity and catalytic activity of NO reduction for the alumina-titania catalysts depended on the alumina and titania sources. The alumina-titania catalyst prepared from metal alkoxide for alumina source and metal alkoxide modified with organic groups for titania source exhibited higher activity of NO reduction than the other alumina-titania catalysts. Catalytic activity of NO reduction for the alumina-titania catalysts depended on specific surface area and solid acidity. It was suggested that solid acidity of the alumina-titania catalysts depended on the coordination structure of Al atoms and the homogeneity of alumina and titania components.     

Effect of ultrastructure on crystallization of mullite

The effect of pre-mullite structure on crystallization of mullite from aluminosilicate gels was investigated. The aluminosilicate gels were prepared by hydrolytic poly condensation of silicon alkoxide aluminium with alkoxide, with colloidal alumina and with aluminium nitrate. The structural differences in these materials were characterized by²⁷Al and²⁹Si nuclear magnetic resonance and X-ray diffraction. Their thermal behaviour was monitored by differential thermal analysis. These investigations show that the crystallization process in this system is fundamentally affected by the nature of the ultrastructure. A spontaneous crystallization of mullite at 980°C is promoted by a high degree of homogeneity and the network connectivity between silicon and aluminium. A method that provides condensation of such an ultrastructure is also given.     

Synthesis and characteristics of near-stoichiometric barium titanate powder by low temperature hydrothermal reaction using titanium tetra(methoxyethoxide)

Near-stoichiometric BaTiO₃ powders with ultrafine particle size and high crystallinity were prepared by low temperature hydrothermal reaction of Ba(OAc)₂ and Ti(OCH₂CH₂OCH₃)₄. BaTiO₃ particles were synthesized in the spherical, metastable cubic crystalline grains with size distribution between 60–90 nm in diameter. Ultrafine particle size was resulted from the control of the hydration rate and the decrease of Ti-O-Ti cross-linking extent of titanium precursor, Ti(OCH₂CH₂OCH₃)₄, which gives electronic, steric, and weakly chelating effect to titanium ion. Increasing the Ba/Ti mole ratio in reactant could not overcome the notorious Ba-deficiency but, improved stoichiometry and produced finer and less agglomerated particles. Interestingly, adding a slight pressure to autogeneous hydrothermal condition (total 4–10 atm) has yielded near-stoichiometric, highly crystalline, and less agglomerated BaTiO₃ particles. These particles, which were in metastable cubic form as synthesized, initiated phasetransition to tetragonal form by calcination at below 400 °C.     

Preparation of alumina films by the sol-gel method

This review describes our study on preparation of alumina films by a sol-gel process and their several applications that have been investigated since 1986. Alumina films were prepared from alkoxide or inorganic salt. Both as-prepared alumina films were transparent in ultraviolet, visible and near infrared regions. The alumina from inorganic salt (inorganic alumina) was structureless even after annealed at 300–700°C in air, while the alumina from alkoxide (alkoxide alumina) was in pseudo-boehmite at an annealing temperature lower than 400°C and was in - or -type at 400–700°C. Both alumina films became opaque after annealed at temperatures above 1000°C. The inorganic alumina film annealed at 800°C showed a gas permeability that was influenced by physico-chemical properties of penetrant and alumina. Composite films of alumina and poly(vinyl alcohol) (PVA) were hydrophilic but insoluble in water, and removal of PVA from the composite films by annealing at 600°C led to formation of transparent alumina films. Such properties enabled us to use a counter diffusion method for fabricating -Fe₂O₃-doped alumina films. Alumina films doped with organic dyes such as laser dyes, hole-burning dyes and non-linear optical dyes, which were fabricated by gelation of dye-added alumina sol, exhibited laser emission, hole-spectra and second- or third-harmonic generation properties, respectively. Hydrogenation of alkene was catalyzed by Ni nanoparticles doped alumina films that were prepared by gelation of Ni²⁺ solution-added alumina sol and annealing the Ni²⁺-doped alumina gel in hydrogen gas. Nonlinear optical properties were observed for alumina films doped with CdS, Au and Ag nanoparticles, which were fabricated by gelation of Cd²⁺, HAuCl₄ and AgNO₃ solution-added alumina sols and annealing the Cd²⁺-doped alumina gel in H₂S gas and the Ag⁺- and Au³⁺-doped alumina gels in H₂ gas. Rare earth metal ion-doped alumina films, which were prepared by gelation of rare earth metal ion solution-added alumina sol and annealed the ion-doped alumina gel, exhibited not only normal luminescence but also up-conversion emission, energy transfer type luminescence and long lasting luminescence.     

Significant reduction of upconversion emission in CaTiO3: Yb, Er inverse opals

Inverse opal photonic crystals of Yb³⁺, Er³⁺ co-doped CaTiO₃ (CaTiO₃: Yb, Er) were prepared using self-assembled polystyrene templates combined with the infiltration of sol-gel precursor. The influence of the photonic band gap on upconversion emission of Er³⁺ has been investigated in the CaTiO₃: Yb, Er inverse opals. Significant reduction of the upconversion emission was detected if the photonic band-gap overlaps with the Er³⁺ ions emission band.     

27Al and 29Si NMR study of sol-gel derived aluminosilicates and sodium aluminosilicates

Solid state ²⁷Al and ²⁹Si NMR was used to examine the structures of aluminosilicates and sodium aluminosilicates prepared by the sol-gel method from metal alkoxides. In contrast to the borosilicate system, where B-O-Si bonds are not formed until heat treatment above 150° C, Al-O-Si formation appears complete upon gelation. Aluminium occupies tetrahedral [AlO₄]^– sites in the polymer network and octahedral [Al(H₂O)₆]³⁺ (or similar) sites in the intersticies for charge balance. When sodium is added as a counter ion the octahedral aluminium is converted to tetrahedral aluminium in the oxide network. In gels of high aluminium content prepared from (Bu^sO)₂Al-O-Si(OEt)₃, some aluminium in five coordinate environments is also observed. All gels remain amorphous on heating to 800° C.     

Covalently bonded organosilicon and organoaluminium dispersants for silicon carbide

Covalently bonded organosilicon and organoaluminium compounds were used as dispersants for SiC powder and whiskers in hexane. The improved dispersion resulted in a threefold increase in whisker packing density in sedimented compacts. Comparison experiments with organosilanes having the formulas RSi (OCH₃)₃ and RSiCl₃ showed that the chlorosilanes were much more effective than their alkoxy counterparts in dispersing SiC powder. The aluminium alkoxy carboxylate aluminiumsec-butoxide stearate was also a much more effective dispersant for SiC than the alkoxysilanes. Fourier transform infrared spectroscopy and solid-state¹³C nuclear magnetic resonance were used to characterize the organometallic dispersants grafted to the surface of the SiC.     

Exploring the Interplay Between Ligand Derivatisation and Cation Type in the Assembly of Hybrid Polyoxometalate Mn‐Andersons

Herein a library of hybrid Mn‐Anderson polyoxometalates anions are presented: 1 , [(MnMo₆O₁₈)((OCH₂)₃‐C‐(CH₂)₇CHCH₂)₂]^3?; compound 2 , [(MnMo₆O₁₈)((OCH₂)₃C‐NHCH₂C₁₆H₉)₂]^3?; compound 3 , [(MnMo₆O₁₈)((OCH₂)₃C‐(CH₂)₇CHCH₂)₁((OCH₂)₃C‐NHCH₂C₁₆H₉)₁]^3?; compound 4 , [(MnMo₆O₁₈)((OCH₂)₃C‐NHC(O)CH₂CHCH₂)₂]^3? and compounds 5 – 9 , [(MnMo₆O₁₈)((OCH₂)₃C‐NHC(O)(CH₂)_xCH₃)₂]), where x = 4, 10, 12, 14, and 18 respectively. The compounds resulting from the cation exchange of the anions 1 – 9 to give TBA ( a ) and DMDOA ( b ) salts, and additionally for compounds 1 , 2 and 3 , tetraphenylphosphonium (PPh₄) ( c ) salts, are explored at the air/water interface using scanning force microscopy, showing a range of architectures including hexagonal structures, nanofibers and other supramolecular forms. Additionally the solid‐state structures for compounds 1c , 2c , 4a , 6a , 9a , are presented for the first time and these investigations demonstrate the delicate interplay between the structure of the covalently derivatised hybrid organo‐clusters as well as the ion‐exchange cation types.     

The synthesis of nanosized TiO2 powder using a sol-gel method with TiCl4 as a precursor

Nanosized TiO₂ powder with anatase structure was synthesized by a sol-gel method using TiCl₄ ethanol solution as a precursor. The grain size of TiO₂ powder was homogenous and was about 10 nm after the precursor was calcined at 500 °C for 1 hour. Anatase TiO₂ powder formed after the precursor was calcined at a temperature ranging from 300 °C to 550 °C. The gelatinizing mechanism of TiCl₄ in ethanol solution can be described as followings. When mixed with ethanol, TiCl₄ reacted with ethanol to form TiCl _x (OCH₂CH₃)_{4 – x} species and HCl gas. During gelatinizing process, TiCl _x (OCH₂CH₃)_{4 – x} species absorbed water from atmosphere to form Ti(OH)₄ precursor, which was polymerized to be an inorganic polymer. The formation of inorganic polymer of Ti(OH)₄ was intensified with gelatinizing time. In contrast, the organic component was removed from the precursor. The formation of anatase TiO₂ can also be promoted by increasing gelatinizing time. The influence of alcohol on the reacting progress and dispersivity was also studied. The size and activity of alcohol molecule were found to have influence on the polymerization and mineralization degree of the precursor and the dispersivity of TiO₂ powders.     

Diffusion of self-assembled monolayers of thiols on the gold surfaces covered with polydimethylsiloxane stamps

Thiols can diffuse and form self-assembled monolayers (SAMs) on the gold surfaces covered with polydimethylsiloxane (PDMS) stamps. For the first time, with cells as the indicator of how far alkanethiols had diffused to form SAM, we studied the growth dynamics of SAMs of HS(CH₂)₁₁(OCH₂CH₂)₃OH (EG₃) and HS(CH₂)₁₁(OCH₂CH₂)₆OH (EG₆) on the gold surfaces covered with PDMS stamps. The growth of SAMs is well described by one-dimensional diffusion from a line source of concentration, with surface diffusion coefficient of 193.4 ± 19.2 μm²/min (EG₃) and 95.8 ± 18.9 μm²/min (EG₆).     

Mechanical damping and dynamic modulus measurements in alumina and tungsten fibre-reinforced aluminium composites

Simultaneous measurements of mechanical damping, or internal friction (Q ^–1 ), and dynamic Young's modulus (E) were made near 80 kHz and at strain amplitudes () in the range 10^–8 to 10^–4 on small specimens of continuous or chopped fibre-reinforced metal matrix composites (MMCs): 6061 aluminium reinforced with alumina (Al/Al₂O₃) and 6061 aluminium reinforced with tungsten (Al/W). Baseline experiments were also done on 99.999% aluminium (pure Al). The strain amplitude dependence of damping and the temperature dependence of dynamic modulus were of particular interest in this study. The temperature (T) dependence of the modulus from room temperature up to 475° C was determined for the Al/Al₂O₃ and pure Al specimens and a highly linear decrease in modulus with increasing temperature was observed. The rate of modulus loss (dE/dT –80 M Pa° C^–1 ) was the same for both materials and the reduction in modulus of the Al/Al₂O₃ was attributed to the reduction in modulus of the alu minium matrix, not the alumina fibres. The size, type, and amount of fibre reinforcement were found to have a significant effect on the strain amplitude dependence of the damping in both MMCs. Unreinforced aluminium exhibited classical dislocation damping trends with a region of strain amplitude independent damping at low strains (less than 10^–5) followed by a non linear, strain amplitude dependent region at higher strains. The addition of alumina fibres (chopped or continuous), while increasing stiffness, resulted in a significant reduction in damping capacity for the MMC relative to that for aluminium and near complete suppression of the amplitude dependent response. The damping levels increased as the volume fraction of fibre, and therefore, the amount of fibre/matrix (FM) interface decreased, indicating that the matrix, not factors such as increased dislocation densities at the FM interface, was the dominant influence on the damping. Analysis of the Al/Al₂O₃ results by Granato-Lücke (GL) theory indicated that dislocation densities were increased relative to those in aluminium, but the dis locations were well pinned and unable to increase damping levels effectively. Analysis of the Al/W results by GL theory also revealed high dislocation densities, but, unlike the Al/Al₂O₃ specimens, the Al/W specimens (continuous fibres) exhibited strong amplitude dependent damping (starting near strain levels of 2 × 10^–6) with damping levels approximately twice those of pure aluminium. Trends showed increased damping with increased fibre diameter, not with increased FM interface area. There was some evidence that it was the tungsten fibre itself that dominated the damping behaviour in Al/W composites, not the aluminium matrix or the FM interface.     

Preparation of SiO2 films from Si(OC2H5)4-C2H5OH-H2O solutions without catalysts

Thin and homogeneous SiO₂ films were prepared on aluminium plates from Si(OC₂H₅)₄-C₂H₅OH-H₂O solutions which had controlled compositions and did not contain any acid as a catalyst. Aluminium with the coatings showed good corrosion resistance. However, SiO₂ films prepared from alkoxide solutions with HCI contained Cl^– ions. Aluminium with coatings including Cl^– ions corroded in a corrosion test. Alkoxide solutions without acids were necessary for use in the coating of aluminium. The transformation of gels prepared from solutions without acids to amorphous SiO₂ was investigated.     

Synthesis of aluminum nitride powder by carbothermal reduction of a combustion synthesis precursor

A new homogeneous mixing precursor of alumina and carbon (Al₂O₃ + C) was prepared by low temperature combustion synthesis (LCS) with aluminum nitrate (Al(NO₃)₃·9H₂O), urea (CO(NH₂)₂), and glucose (C₆H₁₂O₆·H₂O) as starting materials. The precursor prepared was a foamy and porous mass due to the large amount of gases liberated in the combustion reaction and composed of nanosize particles. XRD analysis showed that this precursor consisted of amorphous alumina and carbon. The amorphous alumina in the precursor first transformed into γ-Al₂O₃, and then γ-Al₂O₃ was directly nitrided to yield AlN during the calcination process. The reaction temperature needed for a complete conversion for the precursor was about 1400 °C, which is much lower than that when using alumina and carbon black as starting materials. The synthesized AlN powder was composed of very fine particles and had good dispersability.     

DeNO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> activity of Ag/γ–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposites prepared via the solvothermal route

Ag/γ–Al₂O₃ with silver loading of 3 wt.% were prepared by the solvothermal-calcination reaction of AgNO₃ in mixed water-alcohol solutions at 50–250 °C for 0–120 min, followed by calcinations at 550 °C for 2 h using γ-Al₂O₃ (SSA: 120–409 m² g⁻¹), γ-AlOOH (SSA: 270 m² g⁻¹), Al(OH)₃ (SSA 10 m² g⁻¹), Al(OCH(CH₃)₂)₃ and Al(NO₃)₃ as an aluminum source. The resultant product produced by the solvothermal reaction was Ag/γ–AlOOH even though a different aluminum source was used and Ag/γ–AlOOH was converted to Ag/γ–Al₂O₃ by the following calcinations. However, the characteristics of them changed greatly depending on the alumina source. The deNO _x catalytic performance of Ag/γ–Al₂O₃ also greatly changed depending on the aluminum precursor and solvothermal solvent in the order γ-AlOOH = γ-Al₂O₃ >> Al(OCH(CH₃)₂)₃ >> Al(NO₃)₃ > Al(OH)₃ and methanol = ethanol > 1-propanol > butanol >> hexanol, since the amount and size of silver particle impregnated and specific surface area of the product changed markedly. Ag/γ–Al₂O₃ prepared by solvothermal-calcination method consisted of homogeneously dispersed fine particles of silver and showed better performance for NO _x decomposition than that by conventional impregnation-calcination method.     

Enhancement of the corrosion resistance of substrates by thin SiO2 coatings prepared from alkoxide solutions without catalysts

The corrosion resistance of aluminium coated with SiO₂ films, prepared by the sol-gel method using alkoxide solutions without catalysts, was extremely enhanced. The corrosion resistance depended on the coating conditions. Only 0.02–0.05% of the surface of aluminium with the SiO₂ film, of which the thickness was submicrometre, was corroded after the corrosion test. Small etch pits were observed in surfaces of aluminium with the SiO₂ films, which showed good corrosion resistance, after the corrosion test.     

Single-step synthesis of nanoporous silica colloids

Nanoporous silica colloids were prepared by a convenient single-step sol-gel process. In this approach, acidic aluminum nitrate (Al(NO₃)₃) solution was added to the ethanol solution of tetraethoxy orthosilicate (TEOS). In the preliminary stage, alumina/silica core-shell particles were formed. Then the Al₂O₃ cores were dissolved subsequently with the decrease of pH value, and the nanoporous silica was formed. The porous silica particles were characterized by transmission electron microscopy (TEM). The formation mechanism of the porous silica was discussed.     

Synthesis of bulk and film YBa2Cu3O7−x high-temperature superconductor by the sol-gel method

Bulk YBa₂Cu₃O_7–x has been prepared by the sol-gel method using metal alkoxide and metal acetate as precursors and ammonia or ethylenediamine as complexing agent. The temperature of the transformation of the gel precursor to YBa₂Cu₃O_7–x depended on the type of precursor and the agent used for adjusting the pH. The gel synthesized from precursor alkoxide gives YBa₂Cu₃O_7–x at lower temperature (750C for 6 h) than with acetate. The gel synthesized by different metal acetates and ammonia produces YBa₂Cu₃O_7–x at a lower temperature than the sample prepared from acetate and ethylenediamine. From X-ray diffraction studies it appears that a relationship exists between the reactivity of the gel and the orientation of the corresponding pellet. The preliminary results from a film prepared by painting the gel on alumina and yttria-stabilized zirconia are presented. A high sintering temperature (940C for 1 h) was found to be necessary for obtaining superconductor film. The reactions between YBa₂Cu₃O_7–x and supports were reduced by using silver as buffer. The T _on of the film prepared on the modified alumina support was 91 K and T _off was 78 K.