Polymeric Precursors for Inorganic Materials

Standard ceramic processing centers around the growth, compaction, and sintering of compact colloidal particles. Recently, however, it has become clear that numerous glass and ceramic processing techniques cannot be understood in terms of standard colloid physics. The purpose of this talk is to demonstrate several areas where polymer concepts are not only necessary for the understanding of ceramic processing, but also provide the insights necessary to tailor the properties of ceramic materials to achieve specific goals. The talk will center on the polymerization of silicates in solution. By control of the growth conditions such as catalysis, pH, and reaction sequence, it is possible to create a variety of structures from dense colloidal particles to randomly branched polymers. By mapping the polymerization process onto simple fractal models, it is possible to identify the essential factors that control the structure.     

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.     

Precipitation of Catalyst Precursors Theoretical Fundamentals

Very high supersaturations, low stability of highly supersaturated solutions, marked secondary changes in the solid phase formed, chemical changes during the ageing of precipitates — this are reasons why our contemporary knowledge on precipitation is far more limited and is also inconsistent compared with mass crystallization of readily soluble substances. In view of this situation, it seems to be highly desirable to show the general fundamentals which could lead us to systematic understanding of the various cases apparently unrelated with each other.     

Stress measurements in sol-gel films

Thin solid films of a wide variety of materials have received increased attention during the past decade. These films have been instrumental in the growth of numerous technologies. Until recently, “thin films” have primarily described layers of metallic or dielectric materials deposited onto substrates by evaporation, electron beam or ion beam techniques.

Advances in sol-gel technology have extended film research to include “glassy” materials of either crystalline, or amorphous nature. Sol-gel films can be tailor-made to accommodate a diverse range of applications due primarily to flexibility in chemical make up which determines the respective film's structure. One important characteristic of such films is their inherent residual stress. This inherent stress, and the stress the film introduces to the substrate as it is deposited, can result in a complex stress profile. While “thin” in the case of sol-gel films generally means <1 μm in thickness, large (10–100s of nm of retardance) inherent stress per unit thickness can severely limit a film's performance and subsequent application.

We describe our efforts to quantify the residual stress in silica-based sol-gel films as a function of several processing parameters.     

Hot melt adhesives for glass containers by the sol-gel process

Based on organically modified silicates prepared by the sol-gel process, a sealing medium for glass surfaces was developed which overcomes basic difficulties of a hot sealing process performed with conventional thermoplastic organic polymers. The new polymers form a moisture resistant bond to the glass surface and the sealing strength is not affected by water layers usually present on those surfaces. The mechanical and thermal properties can be tailored by composition of the multicomponent system and by reaction conditions. A technical procedure based on the new material can be performed.     

Applications of the sol-gel process

The advantages and disadvantages of the sol-gel process are reviewed. Successful applications are dependent on sufficient scientific understanding of the various stages of the process. The many factors governing the development of structure and microstructure during gelation and subsequent heating are mentioned. Examples are described for applications in the areas of composites, porous solids and coatings. Opportunities for many new applications are presently available for exploitations.     

A novel sol-gel method for the synthesis of γ-aluminium oxide: development of the sol-gel transformation and characterization of the xerogel

A novel method for the synthesis of aluminium oxide gel has been developed, whereby the sol-gel transformation was investigated. Aluminium tri-sec-butoxide was used as precursor while acetone was chosen as solvent. The synthesis was carried out in a special reactor, which allowed the dosing of steam. ²⁷Al NMR spectroscopy showed that during the sol-gel process the signal at δ∼3 ppm increases strongly corresponding to the formation of hexacoordinated aluminium species. Beside hydrolysis and condensation reactions, the coordination of acetone to a strong Lewis acid aluminium site occurs, which was shown by FTIR and ²⁷Al NMR spectroscopy. Viscosimetric analysis showed that at the beginning of the sol-gel process short polymers are observed while before the gelation a three-dimensional polymer network is formed. After pyrolyzing the gel a high surface area γ-aluminium oxide xerogel was formed. The effect of heating on the morphology and structure was examined by nitrogen physisorption (BET and pore size distribution), XRD and ²⁷Al MAS NMR spectroscopy.     

Photochemistry of azobenzene in sol-gel systems

The photophysical and photochemical behavior of azobenzene incorporated into sol-gel systems was studied. Sols doped with azobenzene were prepared by the hydrolysis of tetramethoxysilane. The absorption spectra of azobenzene in sols with and without HCl as a catalyst showed photochromism; UV irradiation changes trans-azobenzene to cis-azobenzene, which returns to trans-azobenzene by successive visible light irradiation. The results indicate that azobenzene is not protonated in the sol prepared by the present conditions. The absorption spectra showed that the azobenzene-doped xerogel prepared without HCl is mostly adsorbed on silica surfaces by the hydrogen bonding between the azo groups and silanol and/or water molecules. The adsorption did not affect photochromic behavior and photo-reversible changes were observed in the xerogels. UV photolysis of the azobenzene-doped xerogel prepared with HCl so produced protonated benzo[c]cinnoline that photochromic behavior was deteriorated. Surface modified xerogels were prepared from the mixture of tetraethoxysilane and methyltriethoxysilane in order to make clear the effect of the surface silanol groups. It was shown that the formation of the protonated benzo[c]cinnoline is suppressed by the introduction of Si-CH₃. These present results confirm that the acidic sites of Si-OH₂⁺ of xerogels play an important role in the photochemical reaction of azobenezene in silica xerogels.     

Colour of silicate sol-gel glasses containing CuO

Glasses with compositions xCuO·(100 − x)SiO₂ and 5R_nO·xCuO. (95−x)SiO₂, where R = Li, Na, Ca and X = 0.25–10, were prepared by the sol-gel method. Samples were thermally treated between 60 °C and 1000 °C in oxidizing and reducing atmospheres. Copper incorporation was studied by spectrophotometry, X-ray diffraction and TEM.

All the samples are transparent and present a bluegreen colour at 600 °C in oxidizing atmosphere. The obtained results prove that copper is mainly incorporated as Cu²⁺.

At higher temperatures all the samples present -cristobalite, the samples containing Li₂O or CaO also show -quartz as crystalline phases.

In reducing atmosphere particle segregation takes place, producing in some cases opaque materials. Under specific conditions, transparent ruby glasses were obtained.     

Preparation of alumina fibers by sol-gel method

The shape of particles present in the alumina sols prepared from inorganic salts was examined by transmission electron microscopic observation and related to the rheological property and fiber drawing behavior of the sols. It was found that fibers could be drawn in the viscosity range of 1–100 Pa·s from the sols in which long-shaped particles were found, while no fibers could be drawn from the sols containing round particles. It was also found that non-spinnable sols showed relatively large structural viscosity, whereas spinnable sols showed Newtonian flow or very little structural viscosity.     

Silica glass tubes by new sol-gel method

Silica glass tubes were prepared by following a new sol-gel method. The pH of the sol containing hydrolyzed Si(OC₂H₅)₄ with HCl and colloidal silica was adjusted by adding ammonia solution. Sol was poured into a cylindrical tube and gelled while rotating cylinder at 1000 rpm to form a tube-shaped gel. Then the gel was dried carefully to be tube dry-gel and finally sintered to become a clear silica glass tube.

The effects were examined of the speed of rotation before and after gelation, to form the tubular gel, upon the shape and appearance of the derived tubes. The best speed of rotation was about 1000 rpm, and an adequate long period of rotation after gelation was necessary to maintain the tube shape. One of the large silica tubes had dimensions of 26 mm outside diameter, 13 mm internal diameter and 1000 mm length, and the ovality of the silica glass tube was 0.02%.

The derived tube can be applied in a silica glass tube for an optical fiber fabrication process or in a furnace tube for IC production.     

Er photoluminescence enhancement in Ag-doped sol-gel planar waveguides

This paper reports on the study of the effects of silver (Ag^o) nanoparticles on the optical and spectroscopic properties of Er³⁺-doped silica-based gels and glasses, including active bulk materials and planar waveguides for integrated optics. Two different procedures for silver and erbium ion incorporation into the glassy matrices have been investigated: the direct incorporation of a metal salt (AgNO₃ and/or Er(NO₃)₃) into the sol-gel solution, as well as a modified sol-gel process, based on pore-doping of a precursor gel with AgNO₃ and/or Er(NO₃)₃ solutions. The study of the parameters determining the average size and size distribution of the nanoparticles, together with their influence on the sol-gel material densification and Er³⁺ photoluminescence at 1.5 μm, has been performed by means of transmission electron microscopy, plus ultra-violet/visible and photoluminescence spectroscopies. The Ag^o colloidal nanoparticles, obtained by thermal precipitation, were approximately spherical, homogeneously distributed and they exhibited an average size between ∼2 and 15 nm, depending on the silver content and heat treatment performed. They are shown to be responsible for a remarkable enhancement of the Er³⁺ photoluminescence intensity, which is mainly due to the increase of the local electric field around the Er³⁺ ions, due to the surface plasmon resonance of the Ag^o nanoparticles.     

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.     

Gd-incorporation and luminescence properties in sol-gel silica glasses

A thorough study was performed on Gd-doped sol-gel silica glasses, in the concentration range 0-8 mol% Gd. The analyses were carried out as a function of Gd content, before and after a post-densification thermal treatment. Different results concerning optical, vibrational, magnetic, and structural features were gathered in correlated experiments. The presence of Gd-rich nanoclusters was revealed. The size of nanoclusters increases by increasing the dopant concentration and by performing a rapid thermal treatment (RTT) at 1800 °C in air, which causes also a remarkable intensity increase of the ⁶P_7/2 → ⁸S_7/2 radioluminescence transition of Gd³⁺. Nanoclusters are amorphous, possibly close to a Gd₂SiO₅ stoichiometry as suggested by fast Fourier transform infrared spectroscopy (FTIR). FTIR studies revealed also the presence of Gd-dimers interacting with OH⁻ groups. Moreover, the presence of isolated Gd³⁺ ions was detected by electron paramagnetic resonance investigations. Based on the results obtained with different techniques, the Gd-incorporation in silica glass host and the resulting optical properties are discussed.     

Sodium borosilicate glasses prepared by the sol-gel process

A sodium borosilicate gel of composition 80SiO₂·15B₂O₃·5Na₂O (wt%) was prepared from tetraethyl orthosilicate, trimethyl borate, sodium methylate, H₂O, and HCl as the catalyst. Variation of specific surface area and porosity as a function of heating temperature indicated that closed pores were opened at temperatures lower than 400°C and collapsed above 450°C. From TG and DTA curves, about 19% Si and B atoms are evaluated to have −OH bonds. X-ray diffraction patterns indicated crystallization of low-cristobalite out of the gel when it was heated at 700°C for 5 h, showing a difference from a melt-quenched glass of the same composition.     

Microstructural and optical properties of sol-gel silica-titania waveguides

Two micron silica-titania coatings made by single step sol-gel dip-coating were prepared as planar waveguides. Acid catalyzed solutions of methyltriethoxysilane (MTES) mixed with tetraethoxysilane (TEOS) and tetrabutoxytitanate were used as precursors. Purely inorganic and crackfree silica-titania coatings were obtained after annealing at 500°C. The waveguides had propagation losses, 0.3 dB/cm, of the same order as thin silica-titania films prepared without MTES. Films were studied by infrared spectroscopy. Rutherford backscattering spectrometry, nuclear reactions analysis and elastic recoil detection analysis. The waveguide structural composition after annealing at 500°C was found to be similar to MTES and TEOS derived silica-titania coatings. The waveguides were characterized by measuring refractive index, porosity and shrinkage, with thermal treatment. The MTES derived films showed a higher shrinkage during annealing but the same refractive index and porosity as the TEOS derived waveguides.     

Fine-patterning on glass substrates by the sol-gel method

A novel process has been developed to form fine patterns on glass substrates. The process consists of the following steps: (1) coating of gel film on a substrate, (2) patterning on the gel film, and (3) heat-treatment. The key point in this process is the control of hardening of the gel films by the addition of organic polymers such as polyethylene glycol (PEG). The addition of PEG increased the viscosity of the coating solutions and delayed the gelation time, but had little effect on the optical properties of the resultant heat-treated glass films. The newly developed process is expected to be applicable to the formation of the optical devices such as diffraction gratings and pre-groovesof optical memory disks.     

Kinetics of Indium Phosphide Epitaxial Growth Using Metal Organic Precursors

The deposition kinetics of InP using metal-organic precursors is presented. The proposed chemical mechanism involves both gas phase and surface reactions. The fundamental hypothesis adopted in deriving the mechanism was a dual site dissociative adsorption of the precursors on the growing surface. In any case, all the rate constants either were taken from the literature or estimated through thermochemical methods. In addition, the deposition reactor was simulated by means of a monodimensional model that accounts for the main reactor features through the boundary layer theory.     

Silica and silica-titania glasses prepared by the sol-gel process

Clear monolithic samples of silica and silica-titania glasses were prepared by the sol-gel process from alkoxides as starting materials. The effects of the composition of the initial alcoholic solutions on the gelation of the silica materials and the effects of using different titanium compounds on the formation of silica-titania gels and glasses were investigated. DTA and TGA revealed losses of water and organic volatiles during heat treatment of the gels at lower temperatures (up to 400°C) and the glass transformation and crystallization behaviour at higher temperatures (up to 1500°C). The effects of using atmospheres with varying oxygen contents on the DTA peaks caused by oxidation reactions were also studied.Structural changes occurring during heat treatment were monitored by infra-red spectroscopy which indicated that the water contents of the glasses after heat treatment to 900°C were about 1000 ppm. Transmission electron microscopy of ion beam thinned foils of a 80 SiO₂20TiO₂ composition showed a microstructure of extremely fine pores for heat treatments up to 1000°C. However, after extended heat treatments above 950–1000°C, the porosity appeared to decrease and a high concentration of fine crystallites of anatase (approximately 100 Å in diameter) embedded in a silica-rich glass matrix were obtained.