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Showing papers in "Journal of Non-crystalline Solids in 1991"


Journal ArticleDOI
TL;DR: In this article, an overview of relaxational phenomenology is given in a manner intended to highlight a number of the important problems which, notwithstanding much recent sophisticated investigation, continue to confront the field.
Abstract: An overview of relaxational phenomenology is given in a manner intended to highlight a number of the important problems which, notwithstanding much recent sophisticated investigation, continue to confront the field. The rapidly lengthening timescale for diffusional and/or reorientational motion, which provokes the glass transition, is examined within the framework of the ‘strong’ and ‘fragile’ classification of both liquids and plastic crystals. The behavior patterns observed are related to the topological features of the potential energy hypersurfaces which may characterize each extreme. In view of the implication that the observed glass transition is the kinetically obscured reflection of an underlying higher order thermodynamic transition which could be associated with a diverging length scale (at least for fragile systems), the problem of the basic diffusional length scale at the glass transition, using a probe molecule approach, is considered. Then, details of the kinetics of relaxation under isothermal conditions are reviewed to decide on the range of deviations from Debye behavior which may be encountered. A correlation with fragility is strongly indicated. The phenomena of serial decoupling of relaxational modes from the main structural relaxation as T g is approached is outlined and, finally, the additional phenomena that may be encountered in experiments that explore the state-dependence (or non-linearity) of relaxation are briefly examined.

1,815 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated 29 Si nuclear magnetic resonance (MAS-NMR) spectroscopy experiments with Li, sodium and potassium silicate glasses containing 20−56 mol% alkali oxide and determined the distribution of SiO 4 structural units as a function of composition.
Abstract: Lithium, sodium and potassium silicate glasses containing 20–56 mol% alkali oxide were investigated by 29 Si nuclear magnetic resonance (MAS-NMR) spectroscopy. In the spectrum of each sample, at least two to four distinct peaks were identified. The distributions of SiO 4 structural units, Q n , where n is the number of bridging oxygen atoms bound to other Si atoms, were determined as a function of composition. The equilibrium constants of the reactions, 2Q n ⇌ Q n −1 + Q n +1 ( n = 3, 2, 1), were determined. The reaction proceeds to the right direction as cationic power of alkali ion ( Z / r ) increases (Li + >Na + >K + ) at the same alkali oxide concentration. The apparent equilibrium constants of the above reactions are discussed along with a proposed thermodynamic model. The 29 Si chemical shifts assigned to each structural unit increase linearly with alkali oxide contents. The slope of these lines decreases as the numbers of attached bridging oxygen (BO) atoms decrease. The average chemical shifts also increase linearly with an increase of alkali content. A close relationship between the average chemical shifts and the theoretical optical basicity was observed.

601 citations


Journal ArticleDOI
TL;DR: In this article, the glass transition temperatures of organic liquids confined to small pore sizes were measured as a function of pore size in controlled pore glasses (CPG) having pore diameters in the range of 40-730 A. The surface of the glass was treated with hexamethyldisilazane to promote wetting by the organic liquids studied.
Abstract: The glass-transition temperatures, Tg, of organic liquids confined to small pores were studied by differential scanning calorimetry (DSC). The Tg was measured as a function of pore size in controlled pore glasses (CPG) having pore diameters in the range of 40–730 A. The surface of the glass was treated with hexamethyldisilazane to promote wetting by the organic liquids studied (o-terphenyl and benzyl alcohol). Glasses formed in the pores had a lower Tg than in the bulk and the reduction in Tg increased as the pore size decreased. For example, the depression of the glass transition temperature, ΔTg, of benzyl alcohol in 40 A and 85 A pores was 7.2 K and 3.1 K, respectively. The magnitude of ΔTg also depends on the material; e.g. for o-terphenyl in the 85 A pores, ΔTg was 8.8 K versus 3.1 K for benzyl alcohol. In general, it was noted that ΔTg was considerably less than for the depression of the crystalline melting point, ΔTm, studied in related work. For example, for benzyl alcohol in the 85 A pores, ΔTm was ∼ 25 K and ΔTg was ∼ 3 K.

399 citations


Journal ArticleDOI
TL;DR: In this article, the structure of glasses in the CaO�Al2O3SiO2 (CAS) and MgOAl 2O3-6-1 SiO2 systems have been studied using Raman and infrared (IR) reflectance spectroscopies.
Abstract: The structure of glasses in the CaOAl2O3SiO2 (CAS) and MgOAl2O3SiO2 (MAS) systems have been studied using Raman and infrared (IR) reflectance spectroscopies. Kramers-Kronig analysis was performed on the IR data. Two compositional series were chosen in each system, one which has a constant average polymerization of 1 non-bridging oxygen per tetrahedral cation, and the other in which silica content is constant at 50 mol%, but polymerization varies. The results indicate that the presence of alkaline earth network modifiers favors the formation of species with high negative charge density. In the NBO/T = 1 series this leads to the equilibrium 2Q3 = Q4 + Q2 (where the superscript is the number of bridging oxygens per tetrahedron) shifting to the right at low Al content and to the left at high Al/Si. In the fully polymerized, 50% silica Mg glass, there is evidence of clustering into Al-rich and Si-rich domains. Transformation of the IR data yields a well-resolved low-frequency band in the CAS spectra whose frequency depends on polymerization and whether Ca is acting as a network modifier or charge balancer. The analogous Mg band is apparently buried in the mid-frequency network mode.

233 citations


Journal ArticleDOI
TL;DR: The main vibrational band in the Raman spectrum of GeO2, which is associated with symmetric stretch of the GeO4 bonds, is observed to contain fine structure.
Abstract: Raman spectra of glasses with compositions along the Na2OGeO2 join and corresponding crystalline phases have been investigated using micro-Raman techniques. The main vibrational band in the Raman spectrum of GeO2, which is associated with symmetric stretch of the GeOGe bonds, is observed to contain fine structure. These weak vibrational features may be indicative of several distinct 4-membered ring populations within the network. Changes in the intensity of the fine structure with addition of Na2O indicate that the ring statistics change systematically with composition. No evidence is observed along the join for the formation of 6-fold coordinated germanium atoms. The ‘germanate anomaly’ exhibited by these glasses appears to result from the formation of 3-membered rings of GeO4 tetrahedra. The maximum (or minimum) in the anomaly occurs when the network becomes saturated in 3-membered GeO4 rings. Beyond this point, the continued formation of Q3 tetrahedra (tetrahedra containing 1 NBO) is responsible for the decline (increase) in some properties. The glass structure begins to resemble the crystalline digermanate structure at ≥ 30–35 mol% added Na2O. Crystallization occurs at ∼ 40 mol% Na2O compositions at which large numbers of Q2 tetrahedra form and there is total breakdown of the glass network.

180 citations


Journal ArticleDOI
TL;DR: In this paper, the light-induced red-shift in the optical absorption edge in arsenic-chalcogen glasses is reviewed and details of the changes in the atomic structure that accompany the shift in the absorption edge are of particular importance.
Abstract: Illumination with bandgap light induces changes in physical properties of many chalcogenide semiconductors. Fundamental aspects of strike reversible photodarkening (the light-induced red-shift in the optical absorption edge) in arsenic-chalcogen glasses are critically reviewed. For understanding photodarkening at the microscopic level, details of the changes in the atomic structure that accompany the shift in the absorption edge are of particular importance. Study of the structural changes by a variety of techniques has revealed a phenomenon rich in basic physics but has not led a coherent picture of the underlying microscopic mechanism. Application of advanced experimental probes providing more detailed structural information has clarified some of the fundamental changes in the atomic structure and their relation to changes in the electronic and mechanical properties. Modifications in short-range and intermediate-range order accompany photodarkening. The changes in short-range order in the form of increased AsAs bonding are very small and probably do not play a predominant role in the changes in the electronic structure. Evidence suggests that the primary effect of the light-induced changes is the modification of intermediate-range correlations.

178 citations


Journal ArticleDOI
TL;DR: In this article, a mechanism for the photodissolution of metals in amorphous chalcogenides is proposed, based on the combined transport of ionic and electronic charge carriers during the process.
Abstract: A mechanism for the phenomenon of photodissolution of metals in amorphous chalcogenides is proposed which is based on the combined transport of ionic and electronic charge carriers during the photodissolution process. A parabolic law for the kinetics of this reaction is derived and the photoconductivity of the reaction product is the controlling factor for the rate. It is suggested that the site of the absorption of the actinic light is determined by the sign of the dominant electronic (photo) carriers in the reaction product.

171 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that the observed power-law behavior of the frequency-depend ent conductivity, o-(w), is probably not fundamental, and that the Ngai relation between dc and ac conductivity activation energies follows from independent experimental facts, and there are interesting mechanical analogies to the observed ac electrical behavior.
Abstract: Alternating current conduction in disordered solids is discussed, from a general point of view As regards experiment, it is argued that the observed power-law behavior of the frequency-depend ent conductivity, o-(w), is probably not fundamental, that the Ngai relation between dc and ac conductivity activation energies follows from independent experimental facts, that the shape of the modulus peak has no fundamental significance, and that there are interesting mechanical analogies to the observed ac electrical behavior As regards hopping models for ac conduction, it is shown that three commonly used arguments against the existence of a distribution of activation energies are all incorrect Also, it is shown that o-Qo) =~ o-(0) only if there are correlations in the directions of different charge carrier jumps; in particular this result implies o-Qo) = o-(0) for all frequencies in the continuous time random walk (CTRW) model In the final section a number of open problems are listed, and suggestions are made for future work

163 citations


Journal ArticleDOI
TL;DR: In this paper, the properties and spectra of a series of calcium aluminosilicates (Al2O3: 0−39 mol; SiO2: 10−75 mol%; CaO: 13−65 mol%) as functions of composition were studied.
Abstract: The structural role of aluminum in alkali silicate glasses has been a question of interest for some time. In spite of this interest, the question remains an unsettled one. The corresponding alkaline earth glasses have been much less studied, although it is probable that an understanding of the similarities and dissimilarities of these two families of glasses will provide some illumination of the structure and behavior of the alkali silicates. Here, a step is taken in this direction with a systematic study of the properties and spectra of a series of calcium aluminosilicates (Al2O3: 0–39 mol%; SiO2: 10–75 mol%; CaO: 13–65 mol%) as functions of composition. Results of measurements of density, refractive index, and microstructure are presented. No abrupt changes are observed in density or refractive index as functions of the molar ratio of Al2O3 to CaO. This may mean that aluminum ions are four-fold coordinated only in calcium aluminosilicate glasses. Minima in the molar volume of oxygen appear at lower silica content as the ratio of Al2O3 to CaO increases. TEM studies showed phase separation. FTIR spectra showed that the amount of non-bridging oxygens increased with the amount of excess lime. Again, no apparent evidence was found for the existence of six-fold coordinated aluminum ions.

158 citations


Journal ArticleDOI
TL;DR: In this paper, the measurement of the absorption coefficient at E = 1.2 eV, with a new range for the calibration factor, is suggested as an easy and generally usable procedure for the determination of deep defect density.
Abstract: Values for the deep defect density determined from PDS, CPM or ESR may vary significantly on the same sample depending on the method of data analysis used. Experimental conditions under which they yield comparable results are discussed. Procedures for determination of deep defect density from optical spectra are reviewed and compared on samples in light-saturated states. The measurement of the absorption coefficient at E=1.2 eV, with a new range for the calibration factor, is suggested as an easy and generally usable procedure for the determination of deep defect density.

141 citations


Journal ArticleDOI
TL;DR: In this article, LiNbO3 glasses were prepared by using a conventional melt-quenching method and some properties such as thermal stability, density and refractive index and crystallization behaviors of glasses were measured.
Abstract: (100−x)TeO2−xLiNbO3 (x = 10−60) glasses were prepared by using a conventional melt-quenching method, and some properties such as thermal stability, density and refractive index and crystallization behaviors of glasses were measured. The thermal stability of the glasses with x = 20 and 30 was high. The density of glasses decreased monotonically with increasing LiNbO3 content from 5.50 g/cm3 to 4.58 g/cm3. The values of the refractive index of glasses were 2.04-2.11 and showed an anomalous compositional dependence. It was found from Raman scattering spectra that the structure of glasses was composed of the structural units of TeO4 trigonal bipyramid, TeO3 trigonal pyramid and NbO6 octahedra. In the glasses with x = 10 and 20, α-TeO2 (paratellurite) crystals were precipitated by heat treatment above 450°C. In the glasses with x ≧ 40, LiNbO3 crystals were formed through the transformation of a metastable pyrochlore-type compound at temperatures above 500°C. From the present study, it is proposed that the fabrication of transparent TeO2-based glasses containing ferroelectric LiNbO3 microcrystallites may be possible.

Journal ArticleDOI
TL;DR: In this article, a model for the photodissolution of metals in amorphous chalcogenides is proposed, based on the simultaneous transport of metal ions and electronic charge carriers.
Abstract: A model for the photodissolution of metals in amorphous chalcogenides is proposed, based on the simultaneous transport of metal ions and electronic charge carriers. A parabolic law for the kinetics of the process if derived; the photoconductivity of the reaction product is the controlling factor. It is suggested that the absorption site of the actinic light is determined by the sign of the dominant electronic (photo) carriers in the reaction product. The effect of pressure on the photodissolution rate can be readily understood within the framework of this model.

Journal ArticleDOI
George H. Beall1
TL;DR: In this paper, the relationships between composition, microstrcuture, and properties of glass-ceramics based on chain silicates are discussed, and potential applications are described.
Abstract: Glass-ceramics based on chain silicates: enstatite (MgSiO 3 ), potassium fluorrichterite (KNaCaMg 5 Si 8 O 22 F 2 ), and fluorcanasite (K 2 Na 4 Ca 5 Si 12 O 30 F 4 ), display a combination of high flexural strength ( > 200 MPa) and high fracture toughness (> 3 MPa rmm 1 2 ). The relationships between composition, microstrcuture, and properties are discussed. Recent and potential applications are described.

Journal ArticleDOI
TL;DR: In this paper, the metastability and reversibility of photo-induced phenomena in amorphous semiconductors have been reviewed from a phenomenological point of view, with an emphasis towards the structural randomness, network connectivity, and the nature of chemical bonds.
Abstract: Metastability and reversibility of a variety of photo-induced phenomena in amorphous semiconductors have been widely researched. In the introductory part of this talk, these metastable effects are briefly reviewed from a phenomenological point of view. The main part is focused on photo-induced ESR centers with g-values equal to 2.005, 2.004 and 2.01 in undoped hydrogenated- and deuterated-amorphous silicons. It has been shown from electron-spin-echo-modulation experiments using pulsed ESR techniques that (1) deuterium (or hydrogen) is separated from a photo-created dangling bond defect (as well as a native one) by at least 5 A, (2) Si-D-Si three-center bonds were not observed in the vicinity of a dangling bond defect, and (3) 29 Si hyperfine structure does exist in the light-induced ESR with g=2.004 and g=2.01 and its splitting width is close to that of dangling bond centers (g=2.005). These results are discussed with an emphasis towards the structural randomness, network connectivity, and the nature of chemical bonds.

Journal ArticleDOI
TL;DR: In this article, it was found that there is a correlation between the chalcogenide glass forming ability and the number of lone-pair electrons for a chalcanogenide or chalkogenide system.
Abstract: The nature of chemical bonds of some chalcogenide glasses is discussed in terms of the chemical bond approach. It was found that there is a correlation between the chalcogenide glass forming ability and the number of lone-pair electrons for a chalcogenide or chalcogenide system. The stable vitreous state can be obtained only if enough lone-pair electrons exist in the structure of the chalcogenide system. The cation in the chalcogenide glasses may interact with the lone-pair electrons of a bridging chalcogen atom and influence the glass forming ability. A simple criterion for computing the ability of a chalcogenide system to retain its vitreous state is proposed. Experimental results support the above analysis.

Journal ArticleDOI
TL;DR: In this paper, the Gibbs-DiMarzio equation for glass transition of a cross-linked polymer as a function of cross-link density was modified to obtain the average coordination number.
Abstract: High purity chalcogenide glasses incorporating Ge, Sb, Se, As and Te were prepared by vacuum melting of previously distilled 5 N to 6 N pure raw materials from which the surface oxide was also removed in some instances. Glass transition temperatures, Tg, of several chalcogenide glasses prepared by similar processing techniques were determined using differential scanning calorimetry (DSC). An emperical relationship between the glass transition temperature and average coordination number is proposed by modifying the Gibbs-DiMarzio equation for glass transition of a cross-linked polymer as a function of cross-link density. The results for the Tg of three ‘iso-structural’ systems are reported and discussed in light of presumed structural arrangements in these glasses.

Journal ArticleDOI
TL;DR: In this paper, the authors measured the refractive index of silica glass in the UV and visible regions from −165.4 to 83.3°C and fitted the wavelength dispersion at each temperature by the three-term Sellmeier equation.
Abstract: The refractive index of silica glass has been measured in the UV and visible regions from −165.4 to 83.3°C. The wavelength dispersion at each temperature was fitted by the three-term Sellmeier equation. Analysis of the Sellmeier equation shows that the temperature dependence of the refractive index originates from the temperature shift of the observed shift of the exciton peak at around 10.4 eV.

Journal ArticleDOI
TL;DR: In this article, high purity chalcogenide glasses incorporating Ge, Sb, Se, As and Te were prepared by vacuum melting of previously distilled 5 N to 6 N pure raw materials from which the surface oxide was also removed in some cases.
Abstract: High purity chalcogenide glasses incorporating Ge, Sb, Se, As and Te were prepared by vacuum melting of previously distilled 5 N to 6 N pure raw materials from which the surface oxide was also removed in some cases. Molar volume and elastic properties of several isostructural chalcogenide glasses prepared by similar processing techniques were obtained from the measured values of densities and the velocities of transverse and longitudinal acoustic waves respectively. Property variations with the average coordination number, 〈r〉, were examined. Molar volume displayed a distinct minimum at 〈r〉 = 2.4, coincident with Phillips' ‘percolation threshold’ for each system studied. Elastic moduli, however, failed to show any dramatic changes at this threshold. Near the stoichiometric tie-line, a chemically ordered covalent network (COCN) model for the atomic arrangement in these glasses was found to be preferred over the chance coordination predicted by the random covalent network (RCN) model.

Journal ArticleDOI
TL;DR: Extended X-ray absorption fine structure (EXAFS) and photoluminescence (PL) measurements have been performed on three different Er-doped glasses: silica and sodium silicate doped with Er in the molten phase, and silica doped by MeV ion implantation as discussed by the authors.
Abstract: Extended X-ray absorption fine structure (EXAFS) and photoluminescence (PL) measurements have been performed on three different Er-doped glasses: silica and sodium silicate doped with Er in the molten phase, and silica doped by MeV ion implantation. All samples show a luminescent transition centered around a wavelength of λ = 1.54 μ m, corresponding to an intra-4f transition of Er 3+ . EXAFS data show that the Er ions have about six O first neighbors, at a distance of 2.26 A in all systems. Both silica glasses show a second-neighbor ErSi shell at 3.11 A which is not seen in the sodium silicate glass. This difference is reflected in a difference in the PL spectra. The ErO shell in the silica glass shows 0.01 A 2 more disorder than that in the Na-loaded material. These findings are rationalized in terms of the network-modifying effect of Na. Both EXAFS and PL show no significant differences between silica samples prepared in the molten phase or by implantation. At an Er concentration of ∼1 at.%, no direct ErEr bonds are observed.

Journal ArticleDOI
TL;DR: The Tool-Narayanaswamy (TN) model for structural relaxation associated with the glass transition is discussed and critiqued in this article, where the authors show that the model fails to account for the non-exponential character of structural relaxation by effectively invoking a distribution of relaxation times and allowing the relaxation times to depend both on temperature and structure.
Abstract: The widely applied Tool-Narayanaswamy (TN) model for structural relaxation associated with the glass transition is discussed and critiqued. The TN model accounts for the non-exponential character of the structural relaxation by effectively invoking a distribution of relaxation times and for the non-linear character by allowing the relaxation times to depend both on temperature and structure. The model successfully explains qualitatively all of the at first glance unusual features of experimental structural relaxation results and usually also gives a good quantitative fit to such data, provided that the system is not extremely far out of equilibrium. Analysis of some rate cooling/rate heating enthalpy structural relaxation experiments, however, reveals some deficiencies in the TN model. In particular, the model appears to fail a critical test in which the TN analysis of non-linear relaxation experiments on glycerol cannot be reconciled with the results of linear ac calorimetry relaxation experiments on the same material.

Journal ArticleDOI
TL;DR: Fluoride phosphate glass with low impurity content can be UV-transmitting optical materials that are complementary to silica glass and fluoride crystals as discussed by the authors, but their UV transmittance, luminescence and solarization properties depend on the melting conditions and trace impurities from raw materials and crucible materials.
Abstract: Fluoride phosphate glasses having special optical properties are attractive candidates for applications in high performance optics and laser technology. Their potentially good UV-transmittance, luminescence and solarization properties depend on the melting conditions and trace impurities from raw materials and crucible materials. Glasses melted under reducing conditions using normal quality batch materials yield glasses with higher UV-transmittance near 250 nm but with stronger luminescence and solarization effects than glasses melted under normal conditions. Fluoride phosphate glass with low impurity content can be UV-transmitting optical materials that are complementary to silica glass and fluoride crystals.

Journal ArticleDOI
TL;DR: In this paper, the effects of methanol, ethanol, 1-propanol and 2-Propanol on the kinetics and mechanisms of the hydrolysis-condensation reactions of the silanes TPOS, TEOS, and TMOS were studied.
Abstract: The effects of methanol, ethanol, 1-propanol and 2-propanol on the kinetics and the mechanisms of the hydrolysis-condensation reactions of the silanes TPOS, TEOS and TMOS were studied. Also, the influence of the amount of water on the hydrolysis-condensation processes of TMOS and TPOS was investigated. Accordingly, hydrolysis time versus condensation time curves were recorded and 29 Si-NMR investigations were performed. The hydrolyzability of the silanes in an acidic environment was found to decrease in the sequence TMOS > TEOS > TPOS. The effect of the alcohols, in the sequence methanol > ethanol, 1-propanol > 2-propanol, upon the hydrolysis rate of each of the silanes is explained by differences in degree of dissociation of HCl in the different alcohols. Also, the exchange of alkoxy groups, observed when an alcoholic solvent was used with an alkoxy group different from the alkoxy group of the silane, may influence the hydrolysis rate of the silane. Differences in dimerization are ascribed to differences in base strength of the activated silanol complexes with regard to those of the protonated alcohols.

Journal ArticleDOI
Jae Chul Ro1, Jae Chung In1
TL;DR: In this paper, a study of structures and properties of gels prepared by the acid- and base-catalyzed reactions of TEOS was carried out and the results of volume vs. thickness data showed that acid-and base-calyzed gels have slit-shaped micropores.
Abstract: Apparent density, nitrogen adsorption, SEM and IR spectroscopy have been employed in a study of structures and properties of gels prepared by the acid- and base-catalyzed reactions of TEOS. The gel made from a low water content sol contains residual organic groups caused by incomplete hydrolysis, which contributes to the formation of micropores during heat treatment. According to the results of volume vs. thickness data, the acid-catalyzed gels have slit-shaped micropores. The base-catalyzed gels, however, have cylindrical pores. An acid-catalyzed particle has shown fibrous or plate-like structure in the SEM, whereas base-catalyzed gels have shown spherical particles distributed over the gel. The acidic gel made from higher water content (molar ratio of water to TEOS, R > 4) sol has shown coarse microstructures when compared with that from lower water content ( R

Journal ArticleDOI
TL;DR: In this paper, the amorphographic relationships between the structures of a number of single component amorphous network solids are discussed and the minimum size of model required to accurately predict the shape of the first diffraction peak is estimated.
Abstract: The amorphographic relationships between the structures of a number of single component amorphous network solids are discussed New neutron diffraction data are introduced for several of the materials including vitreous P 2 O 5 which has not previously been studied by neutron diffraction These data suggest that network connectivity is more important than structural unit stereochemistry in determining intermediate range order Previous neutron data for amorphous Ge and vitreous SiO 2 are compared with bith random network and crystal based models and criteria are proposed for the minimum size of model required to accurately predict the shape of the first diffraction peak, which for a number of amorphous network solids is found to be Lorentzian It is concluded that new computer modelling techniques are capable of generating models in good agreement with modern diffraction data and should considerably enhance present understanding of the structure of amorphous solids

Journal ArticleDOI
TL;DR: In this article, the structure of PbOBiO1.5 glasses has been investigated by IR and Raman spectroscopy, and the peak deconvolution was made on Raman spectrum of the glasses.
Abstract: The structure of PbOBiO1.5GaO1.5 glasses has been investigated by IR and Raman spectroscopy. An infrared absorption band involving four-coordinated Ga3+ ions was observed at about 610–650 cm−1. The peak of this band shifts toward higher wavenumbers with increasing GaO1.5 content, indicating that covalency of GaO bond increases with increasing GaO1.5 content. The peak deconvolution was made on Raman spectra of the glasses. The Raman bands at about 130 cm−1, 400 cm−1, 550 cm−1 and 650 cm−1, obtained by deconvolution, have been assigned to PbO or BiO, GaOPb or GaOBi, GaOGa, and GaOnon-bridging vibrations, respectively. The presence of 550 cm−1- and 650 cm−1-band indicates that Ga3+ ions are coordinated by four oxygens. The much higher peak wavenumber of the 550 cm−1-band in the spectrum of BiO1.5GaO1.5 glass compared with that in the spectrum of PbOGaO1.5 glass is attributed to the much higher ionic field strength of Bi3+ ion compared with that of Pb2+ ion. The fraction of non-bridging oxygens in GaO4 tetrahedra decreases with increasing PbO and BiO1.5 contents. It is assumed that most Pb2+ and Bi3+ ions are network formers.

Journal ArticleDOI
TL;DR: In this article, a phenomenological description of the property-composition relationship provides a tool for the formulation of bioactive glass compositions, and the complexation of phosphate by the initially formed silica gel is proposed as the mechanism responsible for the bioactivity.
Abstract: In order to improve the properties of bioactive glasses, one must be able to control the bioactivity. This requires a good description of the bone bonding mechanism or, preferably, a good understanding of it. In this paper the bioactivity is approached both phenomenologically and deterministically. The phenomenological description of the property-composition relationship provides a tool for the formulation of bioactive glass compositions. The complexation of phosphate by the initially formed silica gel is proposed as the mechanism responsible for the bioactivity.

Journal ArticleDOI
TL;DR: In this article, the surface crystallization kinetics of several glasses were determined in a wide range of temperatures and time, and it was shown that surface nucleation rates are very high and that surface condition (e.g., fire polished versus mechanically polished or as-received; clean versus dirty), on the chemical composition of the parent glass and also on the nature of the crystallizing phase.
Abstract: A review of previous research on surface nucleation in glasses demonstrates that these are mostly qualitative and that strong discrepancies exist regarding the nucleation mechanism. In this article, the surface crystallization kinetics of several glasses — a Na20.3Ca0.6SiO2 (devitrite), a non-stoichiometric devitrite, and two commercial soda-lime-silica (a float and a microscope slide) glasses — were determined in a wide range of temperatures and time. An analysis of the average number of crystals per crystallization arises from a fixed number of special sites, Ns. The number of crystals nucleated strongly depends on the unit area, Ns, crystal growth rates and viscosity data indicates that the surface nucleation rates are very high and that surface condition (e.g., fire polished versus mechanically polished or as-received; clean versus dirty), on the chemical composition of the parent glass, and also on the nature of the crystallizing phase. However, Ns does not depend on time or temperature. The experimental evidence indicates that the surfaces ‘per se’ do not alter the thermodynamic barrier for nucleation (the interfacial energy or the chemical potential). The enhanced nucleation rates at the external surfaces are the result of the catalytic effect of some (unknown) solid impurity particles and faster surface diffusion rates.

Journal ArticleDOI
TL;DR: In this article, the mechanisms of a-Si:H film deposition from SiH4 glow discharges are analyzed, and the authors show that SiH3 radicals combine a low sticking probability and a high surface mobility on the H-covered film surface.
Abstract: The mechanisms of a-Si:H film deposition from SiH4 glow discharges are analyzed. Recent progress in modeling and diagnostics of electrical power dissipation in DC and RF discharges revealed the importance of electron attachment and powder formation from negative ions. In conventional deposition conditions of optoelectronic-grade a-Si:H, the plasma chemistry favors SiH3 radicals which combine a low sticking probability and a high surface mobility on the H-covered film surface. Low SiH4 pressure discharges involves a large fraction of non mobile species such as SiHm≤2 radicals, but also a high flux of positive ions which can enhance surface mobility and structural rearrangement by momentum transfer (ion kinetic energy). Surface reactions can be also controlled by electronic energy transfer (neutralization of ions, deexcitation of metastables, photon irradiation, radical recombination). H atoms induce microcristalline silicon growth by combining the effects of chemical etching and annealing of the growth zone and H-coverage of the surface.

Journal ArticleDOI
TL;DR: The atomic structure of liquid-quenched amorphous Al 90 Fe x Ce 10− x (x = 5, 7) was studied by diffraction method using X-rays from a synchrotron source as mentioned in this paper.
Abstract: The atomic structure of liquid-quenched amorphous Al 90 Fe x Ce 10− x ( x = 5, 7) was studied by diffraction method using X-rays from a synchrotron source. The diffraction data were Fourier-transformed to obtain the atomic pair-density function, and were compared with those obtained by the pulsed neutron scattering to determine the AlFe and AlCe coordination and the extent of compositional short range ordering. The accuracy of the pair-density function, limited by both statistical and systematic errors, is discussed. It was found that both Fe and Ce atoms show strong compositional short range ordering, amounting to 70–80% of the maximum ordering possible. While Ce atoms form a dilute dense random packing substructure, the substructure of Fe atoms and the surrounding Al was found to be substantially different from the random packing.

Journal ArticleDOI
TL;DR: An efficient algorithm is presented for the automatic determination of rings in finite as well as in infinite networks and a sketch is given of the proof that there is always an upper limit to the length of Rings in any crystal.
Abstract: Different definitions of rings in polyhedral networks are considered and some of their properties are discussed. An efficient algorithm is presented for the automatic determination of rings in finite as well as in infinite networks. A sketch is given of the proof that there is always an upper limit to the length of rings in any crystal.