Chemical modification of alkoxide precursors

The chemical reactivity of metal alkoxides offers a broad range of possibilities for chemical modification of these molecular precursors. The whole hydrolysis-condensation process may then be completely different leading to new products. An analysis is presented concerning some of the most common chemical additives used in the sol-gel process. Their role is explained in terms of chemical reactivity. The most important parameters appear to be the reactivity of the new ligand towards hydrolysis, the charge distribution in the new molecular precursor and the coordination numbers of the metal atom.     

Sol-gel processing of TeO2-TiO2-PbO thin films

TeO₂-TiO₂-PbO thin films were prepared by sol-gel processing from tellurium(IV) isopropoxide precursor and their transmittance spectrum was measured, from which the refractive index was calculated. The hydrolysis of tellurium(IV) isopropoxide and the decomposition process of its hydrolysis product were investigated. The discrepancy between the observed hydrolysis behavior of tellurium(IV) isopropoxide and the partial charge model on the reactivity of metal alkoxides toward hydrolysis, being calculated with the Pauling electronegativity, is explained by a combination of the inductive and steric effects of isopropoxide groups and the electronic configuration of the tellurium atom with lone pair electrons.     

Synthesis and reactions of metal alkoxides

A brief account of the methods generally employed for the synthesis of metal alkoxides has been presented. Successful elucidation of the structural features of simple alkoxides on the basis of a coordination model involving alkoxy bridges has prompted a detailed study of a variety of bimetallic alkoxides during the last two decades followed by the extension of the work to a novel series of ter- and tetra-metallic alkoxides during the last two years. Most of these polymetallic alkoxides (including derivatives of strongly electropositive alkali, alkaline earth metals and lanthanons) are stable to heat and can be volatilized unchanged.

After listing the typical properties and reactions of these alkoxy derivatives, their hydrolytic reactions are briefly discussed. The uses of metallic alkoxides for ceramic materials and the potential applications of bimetallic alkoxides in this direction are discussed.     

Preparation and characterization of transparent Eu doped Y2O3 aerogel monoliths, for application in luminescence

This paper focuses on a new sol-gel preparation method of Eu doped Y₂O₃ aerogels, for application in luminescence and their characterization. The preparation method is based on a sol-gel technique using metal salts, by controlling the hydrolysis of these precursors with an epoxide. The monoliths prepared in this manner are transparent in the visible radiation domain. They have a mesoporous texture, a specific surface area of ≈350 to 400 m² g⁻¹ and they are X-ray amorphous. The gel network could be clearly observed by Transmission Electron Microscopy and showed the presence of localized poorly crystalline nanodomains, with some Eu segregation. A first evaluation of the luminescence which they develop during crystallization, has been carried out as a function of the heat treatment schedule.     

Comparative study of the sol-gel processes starting with different substituted Si-alkoxides

In the present work a comparative study of the hydrolysis-polycondensation processes of different Si-substituted alkoxides, leading to hybrid materials with covalent -Si-C- bonds, was carried out. The following alkoxides were used: tetraethoxysilane (TEOS), methyltriethoxysilane (MTEOS) and vinyltriethoxysilane (VTEOS). Using gas chromatography coupled with mass spectrometry (CG-MS), nuclear magnetic resonance (²⁹Si-NMR) and infrared spectrometry (IR), information about the sol-gel process in the mentioned systems were obtained. The differences in the reactivity of the studied alkoxides are connected with the steric effect of the organic substituents. The reactivity of the alkoxides in the early stages of the hydrolysis-polycondensation process increased in the order TEOS < VTEOS < MTEOS.     

Preparation of gels to obtain glasses of high homogeneity by low temperature synthesis

The conditions for the preparation of gels in order to obtain oxide glasses of high homogeneity by low temperature synthesis are discussed with reference to the system TiO₂SiO₄. Three kinds of titanium alkoxides, ethoxide, iso-propoxide and ter-amyloxide, were mixed with silicon methoxide or ethoxide by various methods and were hydrolysed to form gels. Titanium ethoxide was added to an ethanol solution of partially hydrolysed silicon ethoxide. The mixture of titanium iso-propoxide and silicon ethoxide was hydrolysed very slowly by adding distilled water. Titanium ter-amyloxide, the rate of hydrolysis of which is supposed to be much lower than those of ethoxide and methoxide, was mixed with silicon methoxide to be hydrolysed with ammonia water of pH = 11.2 ～ 11.3. The uniformity of composition in the gels so formed was investigated by measuring the homogeneity of the glasses obtained by melting the mixtures of gels and sodium nitrate for a very short time, i.e., until immediately after the melt became batch free. The gel obtained from titanium ter-amyloxide and silicon methoxide was the most homogeneous, indicating that the employment of alkoxides with a close rate of hydrolysis is essential for the preparation of glass of high homogeneity by the low temperature process.     

Application of the sol-gel process to lower the melting temperature of glaze

Applications of the sol-gel process have been studied to lower the melting temperature of glaze in the system Na₂O---K₂O---CaO---ZnO---Al₂O₃---SiO₂. The high melting components such as SiO₂ and Al₂O₃ in the raw materials were replaced partly by the metal alkoxides, and the powders of raw materials were processed to be coated by the hydrolized metal alkoxides. It was found that the melting temperature lowered by 50–200°C depending on the amounts of the metal alkoxides used for replacement.     

Chemistry of material preparation by the sol-gel process

A survey over the role of chemistry in sol-gel processing is given. The basic chemistry of the sol-gel process is complex due to the different reactivities of the network forming and the network modifying components and the wide variety of reaction parameters. Despite the important progress in the investigations of the mechanisms of hydrolysis and condensation, a direct relation of reaction parameters to material properties is still very difficult.     

Physical and chemical evolutions occurring in glass formation from alkoxides of silicon,aluminum and sodium

A multicomponent glass was obtained on using the new method of glass preparation from gel. The reactions occurring in the ethanolic solution of the parent alkoxides give condensation of the monomers so that hydrolysis of the remaining alkoxides groups followed by polycondensation appears to proceed on preformed oligomers. DTA, TGA, density and chemical durability measurements indicate that the transition from gel to the final glass is a continuous process. During the gel to glass conversion, scanning electron microscopy results show considerable compacting of the material.     

Preparation and optical properties of titania-doped hybrid polymer via anhydrous sol-gel process

Incorporation of metal alkoxides (Ti, Zr, etc.) for tuning the optical properties of silica glasses by the sol-gel process is of significant interest for optical applications. In this paper, we report an anhydrous sol-gel process for preparation of photosensitive titania-doped hybrid glassy polymer with good homogeneity and high doping concentration (TiO₂ up to 40 mol%). The process consists of two steps: in the first step methacryloxypropyltrimethoxysilane (MPS) is hydrolyzed by boric acid through ligand exchange reaction (OH↔OR) under anhydrous conditions; and in the second step dimethyldimethoxysilane (DMDMS), diphenyldimethoxysilane (DPhDMS) and titanium ethoxide (TET) were added to condense with the silanols formed in the first step. The optical properties of the synthesized hybrid polymer were studied, and results showed that the hybrid material has low OH absorption, low optical losses (0.45 dB/cm at 1550 nm and 0.16 dB/cm at 1310 nm respectively), and good thermo-optical linearity with tuneable refractive index. The effect of TiO₂ doping in reducing the OH concentration of the hybrid material was observed, and the mechanism for this effect is discussed.     

Preparation of alumina gels by a non-hydrolytic sol-gel processing method

Monolithic alumina gels were prepared by a novel non-hydrolytic sol-gel route based on the condensation reaction between aluminum alkoxides Al(OR)₃ and aluminum halides AlX₃, through the formation of alkyl halide RX(X = Br, Cl; R = iso-propyl, sec- or ter-butyl), around 100°C. Samples were then calcined and were found to be amorphous up to about 750°C; they kept a high specific surface area to about 900°C. This delayed crystallization is correlated with the large number of five-coordinate aluminum sites measured in dried gels by ²⁷Al NMR spectroscopy.     

Glassy and crystalline systems from gels: Chemical basis and technical application

The sol-gel synthesis of glasses, glass ceramics and ceramics was successfull in many laboratories. Now it has the change of being scaled up for technological applications. Therefore, an overview is given about specific sol-gel products and sol-gel process advantages that are connected with the recent trends in the research and development in this field.     

Chemical processing of rare earth chalcogenides

Chalcogenides are compounds containing sulfur, selenium and tellurium and are well known for their semiconducting and optical properties. Their characteristic absorption at low frequencies render these materials excellent for long wavelength transmission. In the past few years, the need for far infrared transmitting materials to be used in target acquisition equipment formed the basis for identifying several new crystalline sulfide materials which transmit into the extended far infrared (IR) spectral region. Rare earth sulfides exhibiting excellent transmission into the far infrared (8-14 μm) emerged as a new group of infrared window materials. This paper reviews the status of binary and ternary rare earth sulfide compounds for optical window application. Conventional and non-conventional techniques used for processing these materials will be discussed in complete detail. The use of metallorganic precursors and modification of the precursor chemistry to tailor the composition of the final ceramic will be described. The potential of these emergent low temperature chemical processing techniques such as sol-gel, precipitation and co-precipitation for synthesis of non-oxide ceramics will also be addressed, particularly in light of their successful application in processing of novel electronic oxide ceramics, glasses and composites.     

Fluorescence properties of a ruthenium complex in silane sols and gels at different pH

Silane gels containing ruthenium Ru(bpy)₃²⁺ complex were obtained by the sol-gel method. The gelation of sols at different pH values was monitored by fluorescence stationary and time-resolved measurements. In all tested sols a blue shift of fluorescence emission maximum of the fluorophore was observed during the gelation. Changes in λ_max on subsequent days of sol-gel transition depended on starting pH of the sol. An increase of the excited state of fluorophore lifetime in the gelation process was observed. When the gelation was finished the lifetime τ₁ was constant. A strict correlation between the blue shift of fluorophore emission maximum and changes in the lifetime of ruthenium complex in this system on subsequent days of the sol-gel transition was observed. On this basis, changes in the fluorophore microenvironment in the sol induced by different rates of competing hydrolysis and condensation reactions were discussed.All observed effects were attributed to the interaction of molecules of the ruthenium complex with hydroxyl groups in the gel net.     

Preparation of silica and soda-silica glasses by the sol-gel process

Samples of silica glass and soda-silica glasses (containing 1 wt% and 13 wt% Na₂O) were prepared by hydrolysis of alkoxides to form gels, followed by heating the gels at low temperatures, below the glass transformation. The water necessary for hydrolysis was either taken from the atmosphere or added to the starting solution to accelerate the process. In the latter case the gelation time exhibited a minimum for a certain quantity of water added. The heat treatment process to form a glass from a gel was studied by DTA and TGA. At low temperatures (below 100°C) an endothermic DTA peak was observed due to the loss of organic volatiles and water. A large exothermic peak was also found at 276°C for the silica and at 365–395°C for the soda-silica gels. At higher temperatures the 13Na₂O87SiO₂ (wt%) composition gave a DTA trace very similar to the same composition conventionally melted which had been rapidly quenched from well above the transformation range (i.e. with a high fictive temperature). The preparation of bulk samples of the glasses is discussed.     

Laser damage of glycidoxypropyltrimethoxysilane based hybrid materials

The preparation of solid samples, having specific optical properties, is one of the main goals for the realization of devices in the photonic field. To this end, it is important to attain the best control of the optical properties in the final materials. The sol-gel technique is a powerful synthesis method allowing the preparation of matrices with high stability, mechanical resistance and high optical quality. The use of hybrid organic-inorganic precursors permits samples with different thicknesses, ranging from films of hundreds of nanometers to bulk samples of millimeters, to be obtained. The control of the synthesis protocol and the choice of precursors and catalyst allows the control of the final matrix microstructure, which is related to optical properties, like the laser damage threshold. In this work four different matrices, based on glycidoxypropyltrimethoxysilane and Zr alcoxide, have been prepared through sol-gel synthesis. An interpenetrating organic and inorganic network, controlled by the synthesis protocol, characterizes these matrices. The obtained materials show high resistance to the optical damage and long term stability.     

Prospects of sol-gel processes

The ultrastructural control of materials through sol-gel processes offers significant promise for the achievement of reliable performance in glass, glass-ceramics, ceramics and composites. This will be attainable only if the fundamental chemistry of the sol-gel process is understood. Several examples based on this approach will be presented for optical glass, structural ceramics and electromagnetic materials. New concepts, such as ceramic or glass molecular composites and optically active gels, will be discussed. It will be shown how these new concepts are derived from an understanding of polymer chemistry and chemical reactivity. Advanced sol-gel basic research directions and their prospects will be discussed.     

Multinuclear NMR study of phosphosilicate gels derived from POCl3 and Si(OC2H5)4

Solid state ¹H, ²⁹Si and ³¹P MAS NMR have been used to investigate the microstructure of phosphosilicate gels prepared by a modified sol-gel method involving hydrolysis of silicon precursors in a solely aqueous environment at 50 °C. Gels with molar compositions 5, 10, 20 and 30 mol% P₂O₅ in P₂O₅-SiO₂ were studied. After drying to 400 °C the gels have very similar structures formed by a siloxane framework containing silanol groups and trapped molecules of orthophosphoric acid together with a very small amount, of pyrophosphoric acid. Unlike the gel samples previously synthesized by the hydrolysis of the silicon precursor in alcoholic solution at room temperature, the co-polymerization of phosphorus and silicon is much reduced. Although co-polymerization increases with phosphorus content, it still represents less than 50% of the phosphorus in the 30 mol% P₂O₅ gel. Furthermore there is no evidence for six-coordinated silicon in the glassy matrix.     

Fluorescent porphyrins covalently bound to silica xerogel matrices

We study the influence of the nature and ring positioning of amino, hydroxy, and carboxy substituent groups on the fluorescence and some other physicochemical properties of free (H₂TPP) and metalled tetraphenylporphyrins (MTPP). Furthermore, we have studied the feasibility of covalently bonding TPP species on the surface of the pores of a silica matrix prepared by the sol-gel method through the bridging action of either one of the following functionalized alkoxides: 3-isocyanatopropyltriethoxysilane, (IPTES) or 3-aminopropyltriethoxysilane (APTES). Importantly, the red fluorescence observed in free porphyrin solutions is preserved even after the macrocyclic species is covalently bound to the silica matrix. The formation of a chemical union between the porphyrin substituting groups and the organic groups of the IPTES or APTES alkoxides render the H₂P-f precursory species has been proved successful by FTIR spectroscopy; likewise, the bond between Si-OH groups on the silica surface and the H₂P-f has been made evident. The best results, in terms of fluorescence preservation, are obtained by using a TPP substituted with -NH₂ groups in the ortho position of the phenyl rings, i.e. H₂T(o-NH₂)PP. A comparative fluorescence study of xerogels physically grafted or covalently bound with the latter TPP species confirm that, in the first case, the interaction of the molecule with the silanol groups of the silica surface of the gel inhibits fluorescence; while in the second case fluorescence is preserved.     

Monolithic copper oxide aerogel via dispersed inorganic sol-gel method

To avoid the use of rare copper alkoxides, copper oxide aerogels were prepared using copper chloride, polyacrylic acid and propylene oxide via the dispersed inorganic sol-gel method, a supercritical fluid drying process and a 500 °C thermal treatment. The morphology and composition of the aerogel without thermal treatment (copper-based aerogel) and copper oxide aerogel were both characterized and analyzed. Based on studies of the gelation mechanism, it was demonstrated that polyacrylic acid guides the sol formation including providing a steric effect. Analysis of the thermal treatment process shows that the copper-based aerogel has four primary thermal reactions and the major composition of samples treated above 420 °C is monoclinic copper oxide.