Showing papers in "Journal of Non-crystalline Solids in 1988"

Hydrolysis and condensation of silicates: Effects on structure

Abstract: The hydrolysis and condensation reactions of monomeric alkoxysilanes and organylalkoxysilanes utilized in sol-gel processing are reviewed. Both reactions occur by acid or base-catalyzed bimolecular displacement reactions. The acid-catalyzed mechanisms are preceded by protonation of OH or OR substituents attached to Si, whereas under basic conditions hydroxyl or silanolate anions attack Si directly. Many of the observed structural trends are understood on the basis of the pH and [H2O] dependence of the hydrolysis, condensation, and dissolution reactions.

Preparation of monodisperse silica particles: control of size and mass fraction

Abstract: We extend the work of Stober, Fink and Bohn (J. Colloid Interfac Sci. 26 (1968) 62) to establish the ranges of reagent concentrations which result in the precipitation of monodisperse silica particles from ethanol solutions containing ammonia, water, and tetraethyl orthosilica. A correlation is presented which can be used to predict final particle sizes over concentrations of 0.1-0.5 M TEOS, 0.5–17.0 M H 2 O and 0.5-3 M NH 3 . The maximum particle size achievable for these conditions at 25°C is ⋍ 800 nm and near the maximum, monodispersity is difficult to maintain. The effects of reaction temperature are studied and particle size distributions are analyzed. A seeded growth technique for preparing larger particles and/or increasing solids mass fraction up to a theoretical limit of ⋍ 24 wt% of silica is described.

Chemical modification of alkoxide precursors

Abstract: 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.

Structural instability and relaxation in liquid and glassy phases near the fragile liquid limit

Abstract: We consider two sorts of structural instability of glassy substances, each of which may be released by a relaxation process with its own characteristic relaxation time, and specific kinetic features. The first of these is the instability against relaxation out of the amorphous state into the crystalline state, while the second is the instability against relaxation within the amorphous state itself. The latter may often involve relaxation out of a homogeneous amorphous phase into a two-phase amorphous structure, but we will not specifically consider this liquid-liquid phase separation process here. In most glasses, the former (which is no more than the characteristic nucleation time) is much longer than the latter time. However, there are important classes of glasses, for instance the metallic glasses, in which the former is in fact the shorter time, a fact which is responsible for the inability to observe the glass transition phenomenon in such substances. In this paper we will be considering the relation between these two times and the specific kinetics of each. The nucleation time has been the subject of theoretical developments over a number of decades, and details will be omitted in order to concentrate on experimental studies of this phenomenon. We will described briefly the recently developed DSC techniques for determining the classical time-temperature-transformation curves for a variety of supercooled liquids, and the relation of these to the nucleation curves. The relaxation process within the amorphous state, which can be observed for cases where the nucleation time is relatively long, has a number of features which currently lack a complete explanation. In most cases the relaxation process is non-Arrhenius in its temperature dependence, nonexponential in its time dependence, and nonlinear in its structural state dependence. Some examples taken from glasses at the “fragile” edge of the deduced viscosity-temperature pattern for glassforming liquids are dealt with in detail, and the distinction between shear stress relaxation and thermodynamic stress relaxation is made. The possibility that near T g the latter relaxation time remains Vogel-Fulcher in form with T 0 ≡ T K (the Kauzmann temperature), in contrast with the common observations for (the decoupled) shear relaxation, is raised. Strong support for this notion is found in the current “specific heat spectroscopy” results of Nagel and co-workers. Microscopic relaxation processes, as observed using spectroscopic probes and neutron scattering techniques, are reviewed, and the difference in non-exponentiality from macroscopic relaxation are examined in the light of current theories. Finally, secondary relaxations in ionic and molecular glasses, and their relation to the fastest of all glassy state relaxation processes, the tunnelling modes (TLS), are briefly considered.

Chemistry of material preparation by the sol-gel process

Abstract: 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.

Effect of disorder on diffusion and viscosity in condensed systems

Abstract: A simple theoretical model describing the influence of disorder on transport properties (viscosity, diffusion coefficients, etc.) in undercooled melts and crystals is suggested. The basic assumption is that structural disarray results in a random probability distribution of energy barriers for diffusion characterized by dispersion σ around some mean value 〈E〉. It is shown that the effect of σ on the mean jump frequency 〈v(E)〉 may lead to corrections of many order of magnitude as compared to the hopping frequency calculated traditionally in terms of the average activation energy 〈E〉 only. The temperature course of 〈v(E)〉 is then examined making use of the relation between σ and the entropy of the system S. Thus an analytical formula is obtained which properly describes molecular transport in both the crystalline and the amorphous state. Even in a simplified form, η=η0 exp(β/Tα), it reproduces well the existing data on temperature variation of viscosity η (or self-diffusion) in glassforming melts. In another aspect - in terms of the percolation theory - the model describes the diffusion of a foreign particle in a rigid host structure and yields also a qualitative estimate of the variation of the percolation threshold Ep with the degree of amorphisation σ.

NMR confirmation of strained “defects” in amorphous silica

Abstract: Solid state 29 Si magic angle sample spinning nuclear magnetic resonance spectroscopy and Raman spectroscopy were used to investigate the local silicon environment and siloxane ring vibrations in amorphous silica gels. Our results unambiguously relate the 608 cm −1 Raman “defect” in a-SiO 2 with reduced SiOSi bond angles indicative of strained 3-membered rings of silicate tetrahedra.

Sol-gel kinetics I. Functional group kinetics☆

Abstract: 1 H and 29 Si NMR were used to measure the concentrations of the various functional groups (alkoxy, silanol and SiOSi bond) in an acid-catalyzed Si(OCH 3 ) 4 (TMOS):CH 3 OH:H 2 O sol-gel solution. There are three kinds of functional group reactions: hydrolysis, water producing condensation and alcohol producing condensation. The hydrolysis rate constant was found to be greater than 0.2 1/(mol min). The water-producing condensation and alcohol-producing condensation rate constants are 0.006 and 0.001 1/(mol min), respectively. The rate of hydrolysis is much faster than either rate of condensation. Both condensation pathways are significant for typical sol-gel reaction conditions.

Effects of temperature and composition on silicate glass structure and dynamics: SI-29 NMR results

Abstract: The distribution of bridging and non-bridging oxygens is a fundamental aspect of the intermediate range order of silicate glasses and liquids. Disorder in this distribution can be characterized by the relative abundances of Qn species, which can be quantified by 29Si NMR on both spinning and static samples. We emphasize the latter here, and show that increasing the field strength of the network modifying cation and increasing temperature have similar randomizing effects on this distribution. These changes have a major influence on thermodynamic activities. In the liquid, exchange between sites is rapid, but the exchange rate may become slow enough with decreasing temperature to actually define Tg. In silicate liquids, some kind of high energy, low abundance, defects probably are present to allow this structural change to occur, to account for the bulk of the configurational heat capacity, and to explain the observed spin-lattice relaxation times.

Precursors for sol-gel preparations

Abstract: Precursors used for the synthesis of oxide systems by the sol-gel method are reviewed and their role in the various stages of the process is discussed. Emphasis is given to alkoxide precursors and to their physical and chemical properties. In particular, the following topics are discussed: degree of oligomerization, volatility, viscosity, reactions with alcohol, molecular association between alkoxides, hydrolysis, stabilization against hydrolysis. Some information about preparation methods, commercial products and availability is also given. Among non-alkoxide precursors, nitrates, carboxylates, acetylacetonates, chlorides and other inorganics are described from the point of view of their use in sol-gel preparations. They are compared, when possible, with the corresponding alkoxides.

Aging and drying of gels

Abstract: This review describes the chemical and physical processes occurring during the aging and drying of an inorganic gel. The chemical reactions that lead to gelation are shown to continue long after the solution gels, causing changes in the composition, structure, and properties of the gel. The formation of new crosslinks produces shrinkage (syneresis) and increases the modulus and viscosity of the gel, so that aging reduces the subsequent shrinkage during drying. If the solubility of the solid phase is drying, the small pores of the gel develop high capillary pressures that results in shrinkage. If the exterior surface shrinks much faster than the interior, the differential strain can cause warping and cracking; very slow drying is generally required to prevent such catastrophes. Fracture is most likely at the moment that shrinkage stops and the liquid/vapor meniscus moves into the gel; after that point, the meniscus may remain smooth or may become unstable and ragged. The theory of drying is reviewed and shown to account for the observed behavior. Several strategies are described that minimize the tendency of the gel to crack.

Applications of the sol-gel process

Abstract: 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.

Aerogels — highly tenuous solids with fascinating properties

Abstract: Aerogels are sol-gel derived, supercritically dried materials with porosities up to 98%. The pore structure extends from scale lengths of about 1 to 100 nm, showing micro- and macro-porosity as well. The solid thermal conductivity of aerogels is 2 to 3 orders of magnitude smaller than for massive silica glass. Infrared radiative heat transport in SiO2-aerogels is sufficiently suppressed at ambient temperatures. Translucent aerogels thus are suitable as evacuated superinsulating spacers for all kinds of window systems with thermal loss coefficients as low as 0.5 W/(m2 K) for thicknesses around 15 mm. Aerogels are exceptional acoustic material too, with a sound velocity in the order of 100 to 300 m/s, corresponding to an acoustic impedance of only 104 to 105 kg/(m2 s) and a Young's modulus in the range of 106 to 107 N/m2. Due to the large specific surface area of about 800 m2/g aerogels might be useful as catalytic substrates. Last but not least aerogels are considered extremely pure precursor materials for the production of glass fiber.

Synthesis and reactions of metal alkoxides

Abstract: 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.

NMR determinations of SiOSi bond angle distributions in silica

Abstract: The 29 Si NMR chemical shift in crystalline silicates is strongly dependant on the SiOSi bond angle, hence, if a suitable relationship between shift and angle can be obtained then the shape of the NMR line in glasses can be used to give information about the statistical distribution of bond angles and by transformation the SiSi partial pair correlation function in these materials. Several functional relationships between shift and angle have been given in the literature and these result in different SiOSi bond angle distributions deduced. This paper will try to review the current situation and comment on the prospect of an NMR alternative to X-ray diffraction structural determinations.

Section 1. Invited paperProspects of sol-gel processes

Abstract: 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.

Formation of LiNbO3 films by hydrolysis of metal alkoxides

Abstract: Homogeneous, crack-free, thin films of crystalline LiNbO3 were synthesized above 250°C on Si(100) substrates by the dip-coating method using a double alkoxide solution. The coating solution, which was prepared by the controlled partial hydrolysis of the double alkoxide, gave stoichiometric LiNbO3 crystalline films at temperatures as low as 250°C. The concentration of the alkoxide solution influenced both of thickness and quality of films. Crystallinity of thin film top-coated directly on the substrate affected the crystallization state of films coated on the film remarkably. Films crystallized on α-Al2O3(0001) showed preferred orientation along the c-axis, while the preferred orientation could not be observed on Si(100) substrates.

Mechanical strength of silica aerogels

Abstract: Pure silica aerogels are obtained by hypercritical evacuation of the solvent. The strength is measured by the three-point flexural test on monolithic parallelepipedic samples and by a diametral compression test on cylindrical samples. The stress-strain curve shows a perfect elastic behaviour and the “conchoidal” fracture morphology indicates that the material is as brittle as a conventional glass. The mechanical properties are followed as a function of the bulk density. Aerogels with the highest porosity ( P > 95%) reveal a maximum flexural strength lower than 10 −2 MPa. A model is proposed to account for the obtained mechanical properties.

The intensity of Raman scattering in glasses containing heavy metal oxides

Abstract: Exceptionally high Raman intensities have been observed in glasses containing major amounts of Tl 2 O, PbO, Bi 2 O 3 , and / or Sb 2 O 3 in combination with GeO 2 , SiO 2 , and / or As 2 O 3 . Raman peaks were more than 30× the strength of vitreous SiO 2 and 3× vitreous germania (based on spectra reduced to absolute zero and corrected for refractive index differences). The variation of peak intensities with composition were investigated in several systems and the observed variations were found to be consistent with calculations based on a theoretical approach developed by Lines and coworkers. Two compositions show particular promise for active fiber device applications: one based on Sb 2 O 3 ·GeO 2 , exhibiting a strong bridged anion mode with a frequency shift near 400 cm −1 , and one based on Bi 2 O 3 ·PbO·GeO 2 ·SiO 2 , with a superposition of strong acoustic Raman heavy metal modes with a frequency shift near 140 cm −1 .

A large-size silica glass produced by a new sol-gel process

Abstract: A large-size silica glass was fabricated with a new sol-gel process involving the following procedures: (a) hydrolysis of Si(OC2H5)4 with HCl, (b) addition of silica powders and their high dispersion, (c) adjustment of the pH value by adding ammonia solution, (d) gellation, (e) drying to dry-gel, and (f) sintering to silica glass. With this process a dry-gel plate as large as 520 x 360 mm2 was obtained, which was sintered to a 420 x 290 mm2 sized silica glass. The effect of pH on the gelation time of the sol, and the correlation between the weight percentage of silica powder and porosity of dry-gel were examined. In addition, fiber preforms were produced by this sol-gel process.

Neutron and X-ray amorphography

Abstract: A brief account is presented of the use of neutron and X-ray diffraction techniques in studying the structure of amorphous solids. An emphasis is placed on the exact nature of the information provided by diffraction measurements and on the correct methods of extracting structural parameters from experimental data. The use of modelling to investigate order beyond the basic structural unit is discussed, together with the need for accurate quantitative comparisons between models and experiment. Modern diffraction procedures are capable of yielding high quality data and it is concluded that, when used properly, these provide a stringent test of the various structural models proposed in the literature.

A transparent alumina film doped with laser dye and its emission properties

Abstract: Laser dye (Rhodamine 6G, Rhodamine B or Oxazine 4) was doped in a matrix of alumina gel glass by the sol-gel technique using aqueous sol solution. The undesirable dye aggregation usually observed in aqueous solution is largely reduced in the film, even at a concentration as high as 10 −2 M. When this film was irradiated with a N 2 laser, the film gave laser emission with a beam divergence of about 0.1 rad. The width of the dye laser emission is 10 nm which is typical of a dye laser. The calculated conversion efficiency is 2.1%. A decrease of power is linearly proportional to the shot number, which indicates a one-photon process.

Nd-doped silica glass I: Structural evolution in the sol-gel state

Abstract: Silica glass containing neodymium is a potentially important laser glass because of its low thermal expansion coefficient and high temperature stability. The sol-gel method is highly advantageous in that it permits the uniform distribution of relatively large concentrations of neodymium in a silica host with respect to conventional processing techniques. In this paper, the evolution of the Nd-doped silica gel as a function of heat treatment temperature, from the liquid state to the glassy state, is discussed. The fluorescence spectra for sol-gel derived neodymia-silica glass is presented. A tentative structural model of the local environment around the neodymium ions in silica glass is presented.

The role of N,N-dimethylformamide, a DCCA, in the formation of silica gel monoliths by sol-gel method

Abstract: In order to find the role of N,N-dimethylformamide (DMF), a newly found DCCA (drying control chemical additive), in producing silica gel monoliths by the sol-gel method, microstructures of the silica gel monolith prepared from tetramethoxysilane (TMOS) solutions containing DMF and formamide, respectively, have been examined. Dry gels prepared from the solutions containing DMF and formamide had larger pores, which was interpreted to be one of the effects of the DCCA in suppressing occurrence of cracks and fractures in the process of drying wet gels. When drying gel discs prepared from solutions containing DMF were exposed to the vapors and, subsequently liquids of DMF, formamide, methanol and water whose surface tensions are different from each other, cracks were generated for formamide and water having high-surface tensions and no cracks were produced for DMF and methanol. This was interpreted to indicate that the low surface tension of DMF may be one of the effects in protecting the drying gel from crack generation. It was assumed that DMF vaporizes at last during the drying process and weakens the capillary force exerting on the silica network structure.

Characterization of nitrogen containing phosphate glasses by X-ray photoelectron spectroscopy

Abstract: X-ray photoelectron spectroscopy has been applied to phosphorus oxynitride glasses NaPO3 − xN2x3(O < x ⩽ 1), prepared by melting sodium metaphosphate (NaPO3)n in flowing ammonia, in order to determine the mechanism of the nitrogen/oxygen substitution. The formation of phosphorus-nitrogen bonds in place of phosphorus-oxygen bonds involves two kinds of nitrogen atoms N (N1) and N (N2) in mixed tetrahedra P(O, N)4. They substitute bridging oxygen O and non-bridging oxygen =0 according to rules which have been defined from obvious chemical considerations, while another part of oxygen atoms carries the negative charges required for the electronegativity with respect to Na+ ions. This study also brings information about the kinetical aspect of nitridation. N1 and N2 are simultaneously involved; the N2 content, which is first predominant, reaches a limiting value which corresponds to one atom per tetrahedron.

Generation of fractal structures by negative pressure rupturing of SiO2 glass

Abstract: Recently there have been several studies of the dynamics of fractal structures utilizing SiO2 in low-density gel and ‘smoke’ forms (1,2). It is found that over a range of densities, down to a twentieth of the density of silica glass, mechanically stable structures can be formed whose characteristics satisfy the criteria of fractal structures and whose range of self-similarity increases with decreasing density. In this work we test the possibility of producing the fractal low-density silica aggregates by catastrophically rupturing a condensed amorphous silica phase utilizing the methods of molecular dynamics computer simulation. The isotropic expansion causes bond angle opening without rupture up to a tensile limit of −70 kbar. Beyond this point (instability or spinodal) the structure ruptures in a very specific way, viz., by developing a self similar void structure which can be well described by a fractal dimension changing linearly with density. The structure maintains the preferred bond distances and coordination numbers, but permits the density to change by formation of internal surface at a minimum cost in energy. Down to a threshold density, the resulting structures span the simulation box and form a continuous network by sharing oxygens between the basic SiO4 tetrahedra but gradually lose connectivity in higher dimensions. An analysis of the density correlation functions shows the structures to have fractal dimensions. The bond stretching vibrations and the density of states are monitored over a range of densities. The simulated density of states is interpreted by reference to theoretical studies of fracton dynamics (1,2) in aggregates near the percolation threshold and compared with data obtained from neutron and light scattering on the experimental low-density amorphous silicas (1) of the aerogel type.

The gel to glass transition: Chemical and microstructural evolution

Abstract: The chemical and microstructural changes occurring in the conversion of a died gel to fully dense glass are reviewed. The main emphasis is on gels prepared from alkoxide precursors, including silica and more complex silicate compositions. The gel to glass conversion in these systems is contrasted with that in colloidal systems. The processes occurring in the conversion are crucially dependent on the composition of the starting solution and the chemistry of the sol to gel transformation. Shrinkage is governed by four processes operating at different stages during the gel to glass transition: capillary contraction, condensation-polymerization, structural relaxation and viscous sintering. A variety of techniques have recently been applied to study the changes in the porous gel as a result of heat treatment, including dilatometry, gas adsorption, DTA, TGA, TEM, infra-red spectroscopy (to monitor OH content, in particular), Raman spectroscopy, resonance techniques and SAXS. The conversion of dried gels into monolithic glass samples using the slow drying and firing method is discussed, including removal of hydroxyl content and prevention of bloating. Other processing routes are also briefly reviewed including hypercritical drying and sintering, the use of drying control additives prior to sintering, and colloidal techniques.

Change in the structure of niobium pentoxide due to mechanical and thermal treatments

Abstract: The structure and properties of amorphous niobium pentoxide obtained from its H-phase by prolonged grinding were studied by X-ray diffractometry, IR and Raman spectroscopy, and thermal analysis. The short range order at distances less than 0.5 nm was always preserved as detected from the radial distribution function, even after prolonged grinding. The amorphized Nb 2 O 5 showed no peak corresponding to the stretching of collinear NbONb bonding around 850 cm −1 in both Raman and IR spectra. The mechanically amorphized niobium pentoxide has a structure similar to the TT-phase and not to the starting H-phase. The amorphous state obtained by grinding is an intermediate in the course of the transformation from H- to TT-phase, either with the insufficient shift of Nb or the incomplete construction of the bipyramidal hexagonal polyhedra necessary to form a stable TT-phase. The structural details for these processes are also discussed.

Effect of formamide additive on the chemistry of silica sol-gels II. Gel structure

Abstract: In the present work we studied the influence of formamide on the structural and textural characteristics of tetramethoxysilane-derived silica gels by Raman spectroscopy, small angle X-ray scattering, Mo acidic test, and N 2 adsorption-desorption isotherms It was shown that sols are made of primary particles of about 20 A in diameter These primary structural units agglomerate in secondary particles of about 60 A in diameter Gelation occurs when the secondary structural units agglomerate with each other and form a three-dimensional network throughout the sample The evolution of the relative Raman intensity of the SiOSi peak at 830 cm −1 confirms that polycondensation still takes place after gelation, until a reduced time (reaction time/gelation time) t / t G of about 2, after which no significant variation of the Raman intensity is observed The structural and textural characteristics were found to be dependent on the concentration of formamide The particle size, pore volume, and average pore radius increase when the concentration of formamide increases These results are in very good agreement with previous work which showed that formamide decreases the hydrolysis rate but increases the polycondensation rate of tetramethoxysilane

Field assisted transport of Na+ ions, Ca2+ ions and electrons in commercial soda-lime glass I: Experimental

Abstract: When suitable contact electrodes are used, the application of a DC potential to a commercial soda-lime glass causes the Na+ and Ca2+ ions to move toward the cathode. Using Rutherford backscattering, nuclear reaction analysis (for hydrogen profiles) and resonant backscattering (for oxygen profiles), the motion of these ions is quantitatively related to the charge that has passed through the glass. It is demonstrated that in the region depleted of positive ions, the current is carried by electrons, not by O− ions as previously suggested. Continuous recording of the current through the glass versus time shows that, for a fixed voltage, there is a general decrease (proportional to 1 T 1.47 ) in the current with time. However, the current is not always a monotonically decreasing function of time. On the contrary, while there are long periods of time (10 h) where the current varies smoothly with time, these periods are interrupted by shorter periods (a few hours in length) where there are large random fluctuations in the current.