Showing papers in "Journal of Non-crystalline Solids in 1990"

Chemical processes in glass polishing

Abstract: Chemical processes which occur during glass polishing are reviewed within the context of current mechanical models for the polishing process. The central chemical process which occurs is the interaction of both the glass surface and the polishing particle with water. A detailed mechanico-chemical model for the polishing process is proposed.

Surface chemistry of bioactive glass-ceramics

Abstract: General principles governing the bioactivity of glass-ceramics are discussed on the basis of surface chemical studies of glass-ceramic A–W containing crystalline apatite and wollastonite, and related materials. The apatite phase in the glass-ceramics did not play an important role in forming chemical bond of the glass-ceramic to the bone. An apatite layer formed on the surface of the glass-ceramic in vivo was responsible for the bonding of the glass-ceramic to the bone. It was shown that a chemical bond between apatite crystals in the body environment can give a fairly strong bond. The surface apatite layer was shown to be formed by a chemical reaction of calcium and silicate ions dissolved from the glass-ceramic with the surrounding body fluid. A P2O5-free CaO·SiO2 glass also formed the surface apatite and bonded to the bone. The same type of apatite layer as the surface apatite was formed even on the surfaces of various kinds of ceramics, metals and polymers, when they were placed near a glass dissolving the calcium and silicate ions in a simulated body fluid. These results show that bioactive materials having various functions can be obtained using glasses and glass-ceramics based on the simple components of CaO and SiO2.

Infrared reflectance spectra of lithium borate glasses

Abstract: The infrared reflectance spectra of lithium borate glasses (0–73 mol% Li 2 O) have been measured over a broad and continuous spectral range (30–4000 cm −1 ) in an attempt to study systematically their structure. The reflectance data have been analyzed by Kramers-Kronig inversion to obtain the optical and dielectric properties of the materials. The study of the deconvoluted mid-infrared spectra has revealed a glass network built up of various boron-oxygen groups, with structure and concentration strongly dependent on the Li 2 O content. Of particular interest was the far-infrared asymmetric absorption profile, which could be well simulated by two broad bands attributed to vibrations of lithium cations in two distinct network environments. The presence of these two Li + -host site distributions was found to be independent of the details of the borate network structure and has been taken to imply a more general characteristics of ionic conducting oxide glasses.

Science and technology of laser glass

Abstract: There is an interplay between developments in glass technology leading to advances in lasers and the application of lasers to expand our knowledge and understanding of glass science. The current status and future prospects of various ions and host glasses for lasers and the extent to which laser parameters can be tailored by changing the chemical composition of the glass are surveyed first. The interplay between glass science and technology is illustrated further by considering the spectroscopic properties of rare earths in fused silica, where additions of co-dopants and ion clustering have large effects on local fields and optical spectra, and the operation of laser-pumped fiber lasers, where selective excitation due to site-to-site variations in the local field at the lasing ion can affect lasing characteristics. Current requirements for glasses for high power lasers, such as those used for inertial confinement fusion research, are also reviewed.

Neutron scattering from vitreous silica IV. Time-of-flight diffraction☆

Abstract: The optimum configuration for a neutron time-of-flight diffractometer to study amorphous solids is discussed and it is concluded that measurements should be performed at relatively low scattering angles using a cold moderator. Good reciprocal space resolution is also important. Time-of-flight data for vitreous silica are combined with twin-axis data from part II (P.A.V. Johnson et al., J. Non-Cryst. Solids 58 (1983) 109) to give an accurate composite interference function and excellent real space resolution on Fourier transformation. The maximum scattering vector, Q max , is 45.2 A −1 . A peak fit to the resulting correlation function yields a mean SiO bond length of 1.608±0.004 A .

Organic aerogels: microstructural dependence of mechanical properties in compression

Abstract: Aerogels are a unique class of ultrafine cell size (<1000 A), low-density foams. These materials have continuous porosity and a microstructure composed of interconnected colloidal-like particles or chains with characteristic diameters of 100 A. Traditional aerogels are inorganic, made via the hydrolysis and condensation of metal alkoxides (e.g. tetraisopropoxy titanate). Recently, the authors reported the development of organic aerogels from the sol-gel polymerization of resorcinol with formaldehyde. Because these new aerogels are composed of a highly crosslinked aromatic polymer, they can be pyrolyzed in an inert atmosphere to form vitreous carbon aerogels. This work describes how the microstructure of these organic aerogels can be manipulated and controlled. The microstructural dependence of the compressive mechanical properties of both resorcinol-formaldehyde and carbon aerogels is examined in detail.

The short range structure of sodium phosphate glasses I. MAS NMR studies

Abstract: A series of x(Na2O + H2O)·(1 − x)P2O5 glasses have been characterized by 31P and 23Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. High-resolution 31P NMR spectra reveal the presence of Q2-(2 bridging oxygen/tetrahedron) and Q3-(3 bridging oxygen/tetrahedron) tetrahedral sites in glasses with x 0.5. Quantitative measurements of the respective NMR site populations are in excellent agreement with Van Wazer's predictions for the structures of ionic phosphates and illustrate the depolymerizing effects of residual H2O on the glass structure. The systematic change in the Q231P chemical shift is consistent with an increase in the average π-bond character of the phosphorus-nonbridging oxygen bond as x decreases.

Raman study of lead borate glasses

Abstract: Glasses in the system $xPbO:(100- x)B_2O_3 (22\leq x\leq 85$ mol%) have been studied by Raman spectroscopy with a view to understand the effect of PbO on the borate network. The conversion of three-fold to four-fold coordinated boron takes place primanly by incorporation of $BO_4$ into boroxol rings in the form of pentaborate and diborate units. For high lead content, back conversmn of four to three coordinatmn of borons occurs and the network consists primardy of pyroborate, orthoborate and metaborate groups.

Doped sol-gel glasses as chemical sensors

Abstract: It is reported that by trapping suitable analytical reagents, porous sol-gel glasses can be used for the preparation of a wide variety of chemical-sensing materials. The key to the successful preparation of these materials is based on the observation that a substantial fraction of the trapped molecules is exposed to the liquid or gaseous intra-pore volume. The glass matrix thus serves as a solid support for analytical reagents which interact with analyzates present in the adjacent phase. The phenomenon is quite general, and has been demonstrated for a variety of color tests for metal cations, proton (pH indication), anions, and organic molecules.

Organically modified ceramics and their applications

Abstract: ORMOCERs ( or ganically moddified ramics ) are inorganic-organic composites on a molecular or nano level. The inorganic backbone can be synthesize by sol-gel techniques from inorganic molecular precursors, for example alkoxides or soluble oxides. Organic components can be introduced by SiC bonds coordinative bonds, or ionic bonds. Introduction of organic polymeric chains leads to a second type of network. This network can be chemically linked to the inorganic backbone or act as an interpenetrating network. Properties of the materials can be varied in a wide range due to composition, reaction conditions and processing. They can reach from thermoplastic materials (e.g. sealings) to brittle coatings (hard coatings). Various functions such as photocuring, scratch resistance, special dielectric properties, or barrier functions can be introduced. Especially low permittivity constant in combination with thermal stability and low conductivities lead to the development of dielectric coatings for electronic applications.

IR study on the structural evolution of sol-gel derived SiO2 gels in the early stage of conversion to glasses

Abstract: The structure of alkoxysilane-derived SiO 2 gels which were prepared under the partial hydrolysis condition and contained a considerable amount of unhydrolyzed alkoxy groups, and the structure change of the gels in the early stage of gel to glass conversion were investigated as a function of the behavior of a small IR band peaking at 550–600 cm −1 . The 550–600 cm −1 peak was assigned to the skeletal vibration of the 4-fold siloxane rings by reference to various cyclic siloxanes and silicate minerals. The presence of unhydrolyzed alkoxy groups was important for the formation or retention of 4-fold rings in the gels. On heating the gels in air, it was found that the 4-fold siloxane rings were broken to form SiOH bonds with release of alkoxy groups around 200°C, at which temperature the siloxane network was degraded, followed by healing through the condensation of SiOH at a higher temperature.

Characterization of silica gels by infrared reflection spectroscopy

Abstract: The FTIR specular reflection spectra of porous silica gels, either dried at room temperature or subjected to partial densification at 400°C, are significantly different from the spectrum of bulk vitreous silica. Namely, the high frequency SiOSi stretching vibrational peak near 1100 cm −1 exhibits shifts in the gels of up to 40 cm −1 towards lower frequencies, whereas the relative intensity of the companion high frequency shoulder simultaneously increases. The origin and possible structural meaning of the observed changes were investigated by Kramers-Kronig analysis of the experimental near-normal infrared reflectivity spectra of a series of silica xerogels, porous Vycor and bulk vitreous silica. Major differences were found for the energy loss function Im(−1/ϵ ∗ ), the peaks of which have been associated with the LO components of the dominant vibrational modes, and the largest red shifts corresponded to the highest frequency mode. Simple calculations show that the observed behavior of the infrared spectra could in part be due to a certain degree of strain in the SiOSi bridging bonds at the surface of the gel pores.

Theories of relaxation

Abstract: The phenomenology of the glass transition is briefly reviewed. Four classes of theories for the transition are discussed: rheological theories that account for the temperature dependence of the viscosity and/or relaxation time τ p ; kinetic theories that predict the form of the relaxation function M p ; relaxation theories that explain both the form of M p and the temperature dependence of τ p ; and phenomenological theories that describe the kinetics of relaxation without reference to a microscopic model.

Calcium phosphate formation at the surface of bioactive glass in vivo

Abstract: Bioactive glasses are glasses which can adhere to bone through chemical bonding. This paper discusses the bonding to bone of phosphate-free glasses, and the formation of calcium phosphate at the surface of these glasses in vivo. The bone bonding was proved by a push-out test. In the bonding to bone, the glass surface initially transforms into a silica gel. By SEM/EDX analysis it was shown that phosphate ions from the physiological solution penetrate far into the gel. Calcium accumulation occurs only in the presence of phosphate. The results suggest that the initial stage of the calcium phosphate build-up is complexation of phosphate by the silica gel.

Molecular dynamics computer simulations of silica surface structure and adsorption of water molecules

Abstract: Molecular dynamics computer simulations have been used to study the structure of vitreous silica surfaces and adsorption of water molecules onto the silica surface. Results of simulations of bulk vitreous silica using pair potentials and multibody potentials, as well as simulations of the interaction between H4SiO4H2O molecules are presented as the background to the application of these surfaces. The simulations of water adsorption follow the expected trends of dissociation of adsorbed water molecules, silanol formation, siloxane bond rupture, and preferential association of adsorbed water molecules. Visualization of these surface reactions via dynamic graphics enabled observation of the specific mechanisms of adsorption and bond rupture. Most importantly, this visualization indicates the significant complexity involved in this adsortion process.

Upconversion fluorescences of TeO2- and Ga2O3-based oxide glasses containing Er3+

Abstract: Upconversion-pumped fluorescence was observed for the first time in Er2O3-doped oxide glasses even at room temperature without any sensitizing ions such as Yb3+ in the systems of TeO2- and Ga2O3-based oxide glasses. A diode laser operating at 798 nm was used as the pumping source for stepwise excitation. The upconversion-pumped fluorescence was confirmed by the quadratic changes of fluorescence intensity against the pumping power. It is suggested that these oxide glasses can become one candidate for a high temperature upconversion device.

Protective glass coatings on metallic substrates

Abstract: Sol-gel glass coatings of SiO2 were prepared on different metallic substrates (stainless steels AISI 304, 310 and 316) using the dip-coating technique. The characteristics of the solutions and the films produced were evaluated for each type of substrate and certain properties measured. Coatings were given standard tests relating to chemical and mechanical resistance: dry and acid corrosion and microhardness. Sol-gel SiO2 coatings act as a protective barrier against oxidation and acid corrosion and enhances the scratch resistance of these substrates.

Chemical reactions at silica surfaces

Abstract: Chemical modification of the silica surface can provide a powerful method for probing the nature of the surface hydroxyl groups and how these might be modified via thermal activation. In the present paper, it is shown how vibrational spectroscopic methods (infrared and Raman) can be used to study silanol groups which have been modified after reaction with a variety of very reactive hydrogen sequestering agents (D2O, ZnMe2, BCl3, TiCl4, AlMe3, GaMe3, BEt3 and (SiMe3)2NH), having differing steric dimensions and it is demonstrated that a nearly complete vibrational spectrum for some of the corresponding new surface species can be obtained. With the use of fast scanning FTIR spectroscopy, spectra were able to be recorded in less than a second and the application of these time-resolved methods has been used to probe differences in the reactivity of isolated and H-bonded silanol groups. The spectroscopic evidence suggests that the ability of a reactant to react bifunctionally, i.e. to react with more than one SiOH group, is important in determining the initial reactivity of the H-bonded silanols relative to those that are isolated or non-H-bonded. Further, the number of inaccessible and H-bonded silanols that do not react increases as the apparent size of the reactant molecule increases, regardless of whether the reactant can react bifunctionally. Finally, for the larger probe molecules used ((SiMe3)2NH and BEt3), the new chemisorbed surface species block other H-bonded silanols and prevent them from reacting with either of these probes.

Synthesis of an alumina coating from chelated aluminium alkoxides

Abstract: Chelating agent, such as acetylacetone (AcAcH) and ethylacetoacetate (EAA) are used to avoid the undersirable precipitation of gelatinuous aluminum hydroxides during hydrolysis of aluminum-sec-butoxides, Al(OBus). This procedure leads to alumina sols with particle size distributions in the range of 1 to 15 nm which depend on the type of chelating agent as well as on the molar Al(OBus)3/chelating agent ratio. Generally, EAA leads to particles smaller than AcAcH. Because of the small particle size, gel powders are X-ray amorphous and do not crystallize at temperature < 750°C. Crystallization of α-alumina starts between 950 and 1000°C. Complete transformation to α-alumina is achieved between 1050 and 1100°C. Coating experiments with different sols on silica glass lead to nearly crack-free dried gel films with AcAcH system. An ultrafine, dense microstructure is obtained.

A high-resolution 29Si and 27Al NMR study of alkaline earth aluminosilicate glasses

Abstract: The abundance of chain- (Q2), sheet- (Q3), and framework-like (Q4) tetrahedral units, and the distribution of Al among those units in CaOAl2O3SiO2 and MgOAl2O3SiO2 glasses, has been investigated by 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS-NMR). In each system two compositional series were studied: (i) average polymerization is held constant at one non-bridging oxygen per tetrahedron (Q3), and (ii) silica content held constant at 50 mol% (average polymerization varying from Q2.67 to Q4). The results indicate that (1) Al is entirely tetrahedrally coordinated and is largely in Q4 sites, except in Al-rich Q3 glasses where it is also in Q3 sites, and (2) at constant polymerization of Q3 the ‘reaction’ Q2 + Q4 = 2Q3 goes to the right with increasing Al content. 29Si chemical shifts calculated from 27Al peak positions assuming that all Al is tetrahedrally coordinated and in Q4 sites, and that only Q2, Q3, and Q4 sites are present in the glass, give reasonable agreement with measured values (±2 ppm), but this model applies only to sufficiently silicon-rich compositions in which there is negligible Al in Q3 sites.

Sol-gel thin film formation

Abstract: The overlap between the drying stage and the aggregation/gelation and aging stages of sol-gel film formation establishes a brief time for further condensation reactions to occur. For this reason, the structures of films are often considerably more compact than those of the corresponding bulk gels or xerogels prepared from identical precursors. Experimental techniques to study film formation and the structure of the deposited film in situ have been developed. These techniques include imaging ellipsometry, infrared microscopy and gas sorption on surface acoustic wave substrates.

Third harmonic generation from Li2OTiO2TeO2 glasses

Abstract: Third harmonic generation from high refractive index Li 2 OTiO 2 TeO 2 glasses was investigated. The maximum content of TiO 2 in the glass forming region was 10 mol% by conventional melt-quenching technique, and gc (3) was dependent on TiO 2 /(Li 2 O+TiO 2 ). The highest χ (3) , 8.0 × 10 −13 esu, was observed in 10Li 2 O·10TiO 2 ·80TeO 2 (mol%). In these glasses, χ (3) is 40 times larger than that of SiO 2 glass.

The base-catalyzed hydrolysis and condensation reactions of dilute and concentrated TEOS solutions

Abstract: The synthesis of submicron silica particles by the hydrolysis and condensation of dilute and concentrated solutions of tetraethylorthosilicate (TEOS) has been studied in low molecular weight alcohols (C 1 –C 4 ). A base (ammonia) was used to catalyze the reaction. Raman spectroscopy, gas chromatography and the molybdate method were used to establish the hydrolysis and condensation kinetics. Dynamic and classical light-scattering techniques were employed to monitor particle growth and particle number concentration kinetics, and particle size distribution. The effects of solvent and TEOS concentration on the degree of monodispersity of the particles are discussed. Further, the chemical and particle growth data were used to test theories of homogeneous nucleation and aggregative growth, which have been proposed as mechanisms that govern the growth of submicron monodisperse silica particles by TEOS hydrolysis.

Prospects for 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.

Switching behavior in II-IV-V2 amorphous semiconductor systems

Abstract: The electrical characteristics and switching behavior of amorphous ternary semiconducting CdGeAs 2 were studied under DC and AC conditions. The samples tested were fabricated as as-quenched bulk compounds, roller splat-quenched ribbons, and Ar-sputtered thin films. Vapor deposited Ag, Au and Al behaved similarly as electrical contacts while the commercial Ag-paste electrodes revealed variable contact resistance. The threshold electric field of ribbon sample was ∼ 5 × 10 3 V/cm, smaller than ∼ 1.5 × 10 4 V/cm for the sputtered thin films. The electrical band gap of amorphous CdGeAs 2 was determined to be 1.2 eV (n-type). The “forming” process was believed to occur during the first switching cycle whereby highly conductive amorphous channels formed through the width of the samples. The effects of current on switching were investigated and shown not to be significant. The response time for switching decreased approximately exponentially with the applied voltage. The thermal assisted electronic model was postulated for the OFF-ON transition in these materials.

Spectroscopy and applications of molecules in glasses

Abstract: A variety of organic colorant species, showing strong absorption bands in the visible and high-yield broad-band luminescence can be incorporated at ambient temperature in the homogeneous precursor solution of sol-gel glasses which then vitrifies by moderate heating. The spectroscopic properties of such doped sol-gel glasses are reviewed, and their applications in tunable dye lasers, materials for non-linear optics, luminescent solar concentrators, and other devices are outlined.

Dip-coated TiO2CeO2 films as transparent counter-electrode for transmissive electrochromic devices☆

Abstract: The dip-coating process is an attractive way for the preparation of thin films used in the field of electrochromism. The scope of the present paper is focused on the TiO 2 CeO 2 compounds since they exhibit a reversible electrochemical insertion of lithium ions maintaining a high optical transmissivity. These films can be used as transparent counter-electrode in an all solid state electrochromic transmissive device with, for example, WO 3 as electrochromic material and a lithium conductive polymer as electrolyte.

A MAS-NMR investigation of lithium silicate glasses and glass ceramics

Abstract: MAS-NMR of binary lithia-silica glasses, containing 25–39 mol% Li2O, shows that the Qn distribution largely follows that predicted by the binary (constrained) model but with some deviation when the lithia content exceeds 29 mol%. The deviation consists of disproportionation of Q3 to Q4 + Q2 to an extent dependent on the lithia concentration, decreasing from 33 mol% Li2O onwards. Heat treatment of the glasses produces two polymorphs of lithium disilicate as the devitrification products. The structural changes during the early stages of crystallisation are reflected in variations in both 29Si peak halfwidth and chemical shift but with these variations occurring at different temperatures. The static 7Li spectra for the crystallised samples exhibit a Pake splitting indicative of the presence of 7Li pair of separation ∼ 2.1 A .

Electronic properties of mixed valence oxide gels

Abstract: Transition metal ions exhibit several valence states. Redox reactions occur during the sol-gel synthesis of transition metal oxides. Mixed valence compounds are obtained. Their electronic properties arise from electron exchanges between metal ions in different valence states. Thermally activated electron hopping leads to semiconducting materials. Optical absorption arising from intervalence transfers gives rise to reversible optical switching. They can be used for making electrochromic display devices. Electron and ion transfers can occur at the oxide-water interface. They seem to be typical of mixed valence oxides having a spinel structure and lead to chemical modifications of both the oxide network and the solution.

Xps study of leached glass surfaces

Abstract: By use of a special deconvolution algorithm, it was possible to decompose the XPS O 1s signal into several components reflecting different oxygen bonds. The influence of exposure in vacuo, environmental atmosphere, distilled water, and in a polish solution as corrosive media on the surface of different glasses has been studied. The O 1s signals of fused silica, Na 2 O· n SiO 2 glasses, a BaO·SiO 2 and a BaOB 2 O 3 SiO 2 glass (Schott SK 16) were analyzed. On the glass surfaces, leached layers were formed and analyzed qualitatively and semiquantitatively by XPS and optical and infrared methods. As a reference, virgin surfaces of samples broken in ultrahigh vacuum have been used. The absolute binding energies as well as the shifts relative to the bridging oxygen in ≡SiOSi≡ have been determined for the first for silanol groups ≡SiOH in the surface layer (532.9±0.2 eV and 0.56±0.10 eV, respectively) and for water and/or hydronium bound oxygen (533.6±0.2eV and 1.25±0.10 eV, respectively). Silanol groups are formed in atmosphere as well as by the residual gas in vacuo. Details of the growth of the leached layer were studied, e.g. the dissolution of Ba ++ ions and the growth of the thickness of the leached layer.