Showing papers in "Journal of Non-crystalline Solids in 1994"

Enthalpy relaxation and recovery in amorphous materials

Abstract: The field of enthalpy relaxation is reviewed. Current phenomenologies for dealing with the non-linear and non-exponential character of enthalpy relaxation are presented, and their successes and shortcomings are discussed. Qualitative experimental data and quantitative parameterizations are summarized, and some directions for future research are suggested.

Molecular mechanisms for corrosion of silica and silicate glasses

Abstract: Dissolution, selective leaching, and stress-corrosion cracking are all processes which can degrade the performance of silica and silicate glasses exposed to aqueous environments. Reactions which corrode glass in water include hydration, hydrolysis and condensation, and ion-exchange processes. Techniques such as solid-state nuclear magnetic resonance, Raman and Fourier transform infrared spectroscopies, pH stat titrations and elemental depth profiling have been used to establish the relative importance of the above reactions on glass degradation as a function of solution pH and composition, temperature and stress level. This paper reviews results obtained for silica, alkali silicate, and alkali boro- and aluminosilicate glasses. For most glasses, the rate at which water enters the glass structure controls the kinetics of the other glass-water reactions, explaining the corrosion characteristics of different glass compositions.

Neutron scattering from vitreous silica. V. The structure of vitreous silica: What have we learned from 60 years of diffraction studies?

Abstract: A quantitative assessment is presented of the structural information derived from X-ray and neutron diffraction and small angle scattering investigations of vitreous silica, starting from the classic studies of Warren and co-workers in the 1930s. Particular points that are addressed include the regularity of the SiO4 tetrahedral structural units, the SiOSi angle distribution, the interpretation of the first peak in the diffraction pattern and the extent of the longer range density fluctuations as revealed by X-ray and neutron measurements at small scattering vectors. The use of structural modelling in the interpretation of the diffraction data is discussed and representative models are compared with experiment. The extent of the (dis)agreement of each model with experiment is compared quantitatively and the paper concludes by identifying the most accurate structural model of vitreous silica known to the author at the present time.

The origin of the intrinsic 1.9 eV luminescence band in glassy SiO2

Abstract: The current controversy over the nature of the centers giving rise to the 1.9 eV photoluminescence (PL) band (the R-band), the suggested defect models and the relevant experimental data are briefly reviewed. The luminescence emission, excitation and polarization spectra of neutron-irradiated synthetic silica were studied between 6 and 300 K using site-selective dye-laser and Ar ion laser excitation. Resonant zero-phonon lines (ZPL) were observed below 80 K both in luminescence emission and excitation spectra in the 1.9–2.1 eV region. A vibration line in emission spectra 890 cm−1 below the ZPL energy is attributed to the symmetric stretching vibration of the silicon-non-bridging oxygen bond in the ground electronic state of the non-bridging oxygen hole center. A similar line 860 cm−1 above the ZPL in the excitation spectra corresponds to the same vibration in the excited state. The intensities of the resonant ZPLs are dependent on the excitation energy and show a nearly Gaussian distribution with the peak at 1.935 ± 0.01 eV and halfwidth 82 ± 7 meV. In the zero approximation, this distribution describes the concentration distribution of the PL centers with the respective energies of the excited electronic state. The 4.8 eV excitation band of the 1.9 eV PL is complex, due to different electronic transitions. No ZPLs or vibrational structures are observed under excitation with KrF excimer laser or Xe lamp in this band. The optical absorption in this region is due to overlapping bands of several different defect centers. The low-temperature luminescence bands at 2.05–2.1 eV and 2.35–2.4 eV, excited by the green (2.41 eV) and blue (2.71 eV) Ar ion laser lines, have a nature different from the 1.9 eV band. Several different defects contribute to the 2.0 eV optical absorption band in irradiated glassy SiO2.

Structural studies of complex systems using small-angle scattering: a unified Guinier/power-law approach☆

Abstract: A unified analysis method for small-angle scattering data is demonstrated by surveying complex systems that display multiple size-scale structures. Using this approach the relationship between micro- and nano-structures can be ascertained. The method uses a function that is general enough to adequately describe systems ranging from particulates with fractally rough interfaces to mass fractals such as polymer coils. Additionally multiple Guinier and power-law regimes can be treated. The unified method can distinguish Guinier regimes buried between two power-law regimes. Data from particulate filled systems, low crystallinity polymers and low density polymer foams are analyzed.

The dynamic structure model for ion transport in glasses

Abstract: A model is developed for ion transport in glass which involves the creation of fluctuating pathways within a dynamically determined structure. Key features include a site memory effect which introduces vacancies appropriate to each kind of mobile ion, and a mismatch energy which emerges whenever an ion attempts to enter a different kind of site. The exploration of this model by numerical methods leads (i) to a power law relationship between ionic conductivity and cation content (now confirmed in the literature) and (ii) to the elucidation of many facets of the mixed alkali effect. It is suggested that this ‘dynamic structure’ model could form the basis for a comprehensive theory of vitreous electrolytes.

Translational and rotational molecular motion in supercooled liquids studied by NMR and forced Rayleigh scattering

Abstract: It has been shown that translational diffusion coefficients, Dt, in the supercooled van der Waals liquids, orthoterphenyl, phenolphthaleindimethylether, and salol, have a weaker temperature dependence than the shear viscosity, η, at T ≲ 1.2Tg and can be described by Dt ∼ η−χ with χ < 1 whereas Dr ∼ η−1 applies for the mean rotational diffusion coefficients, Dr, down to the glass transition temperature, Tg. This apparent decoupling of translational and rotational motion has been discussed in relation with possible anomalous short time diffusion, spatial heterogeneity, and cooperative molecular motions close to Tg.

IR spectra and structure of V2O5GeO2Bi2O3 glasses

Abstract: This paper reports a study of the structure V2O5GeO2Bi2O3 glasses by IR spectroscopy to obtain information about the competitive role of each of V2O5, GeO2 and Bi2O3 in the formation of the glass network. The amorphous samples were obtained by the twin-roller technique. Bi2O3 leads to transformation of the layered and chain vanadate structure and depolymerization of three-dimensional germanate lattice. The bismuthate complexes can be viewed as deformed BiO6 groups. It is established that with a decrease in the V2O5 content VO4 groups are formed. GeO4 structural unit with different degrees of polymerization arer present in the network in the entire concentration range.

Raman spectra of MOTeO2 (M = Mg, Sr, Ba and Zn) glasses

Abstract: The structure of MOTeO 2 (M = Mg, Sr, Ba and Zn) glasses was investigated by Raman spectroscopy. The glasses with low BaO have a continuous network constructed by sharing corners of TeO 4 trigonal bipyramids and TeO 3+1 polyhedra having one non-bridging oxygen (NBO) atom. In the glasses, TeO 3 trigonal pyramids having NBO atoms are also formed in the continuous network. In 36BaO · 64TeO 2 glass, isolated structural fragments, such as TeO 3 2− and Te 3 O 8 4− ions, coexist with the continuous network. The structure of MgOTeO 2 glasses is different from that of the BaOTeO 2 glasses. In glasses containing about 40 mol% MgO, a (Te 3 O 8 4− ) n unit, in which n is relatively small, is formed. The fraction of tellurium atoms forming TeO 3 trigonal pyramids increases with MgO content. When MgO exceeds 40 mol%, isolated structural units, such as TeO 3 2− and Te 3 O 8 4− ions, will be formed in glasses. The SrOTeO 2 and ZnOTeO 2 glasses have the same structure as the BaOTeO 2 and MgOTeO 2 glasses, respectively.

Compatibility of polyimide-silicate ceramers induced by alkoxysilane silane coupling agents

Abstract: A hybrid material incorporating silica networks in polyimide films was produced using the sol-gel technique from solution mixtures of polyamic acid and partially polymerised tetraethoxysilane (TEOS) containing small amounts of a functionalised alkyl trimethoxysilane. These were converted into films by solvent evaporation, followed by imidisation and condensation reactions through temperature increases to a maximum of 350°C. Depending on the alkoxysilane solution composition and mixing parameters, different morphologies were obtained, varying from semi-interpenetrating networks of linear polyimide within highly crosslinked silica chains, to finely dispersed heterogeneous systems exhibiting either co-continuous or particulate microstructure. Gelation studies revealed that in the presence of amine catalysts, the tetraethoxysilane solutions modified with γ-glycidyloxypropyltrimethoxysilane had the shortest gelation times, indicating that the epoxy groups become involved in the formation of the network by reactions with the carboxylic acid groups of the polyamic acid and through homopolymerisation. At the maximum temperature allowable for the imidisation reactions (400°C) small amounts of residual SiOH groups were detected within the silicate network. The tensile strength of ceramers was found to go through a maximum with increasing curing time at 400°C, whereas the elongation at break decreased monotonically and then remained constant.

Quantum confinement of PbS microcrystals in glass

Abstract: A microcrystalline PbS phase has been thermally produced from an oxide glass host. X-ray linewidth measurements have shown that the average particle size can be controlled in the range 8–30 nm, depending on the thermal development schedule. Quantum confinement behavior has been observed based on the measurement of sub-band structure of the absorption edge. Spectral shifts of the sub-bands were found to be consistent with the inverse square of the particle size, consistent with a quantum confinement origin. A lower-concentration PbSe phase has also been obtained from the same base-glass composition showing similar optical effects.

The short-range structure of sodium ultraphosphate glasses

Abstract: Anhydrous sodium ultraphosphate glasses were prepared with Na2O contents between 0 and 50 mol% and were characterized by several structurally sensitive spectroscopic probes to determine the nature of the phosphate tetrahedra that constitute the short-range glass structure. Solid state 31P magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy reveals that Na2O depolymerizes the branched (Q3) P-O network of P2O5 to form metaphosphate (Q2) sites, in quantitative agreement with Van Wazer's ‘chemically simple’ model. X-ray photoelectron spectroscopy reveals that the concomitant increase in non-bridging oxygen with increasing Na2O content is also in quantitative agreement with this structural model. Raman spectroscopic analyses of glasses with approximately 40 mol% Na2O suggest that some intermediate-range order, perhaps associated with strained rings, also exists within the glass network. Strained sites are eliminated when the solid glass is heated to melt temperatures.

Multifunctional inorganic-organic composite sol-gel coatings for glass surfaces

Abstract: The sol-gel process allows the fabrication of ceramic colloidal particles in the presence of organo alkoxy silanes carrying various functions and the synthesis of multi-functional transparent inorganic-organic composites if the particle size can be kept in the lower nano range. In addition to this, these composites can be used as controlled release systems or designed as gradient systems. Using this approach, a coating with a very low surface free energy (antisoiling properties) and temperature stability up to 350°C, a controlled release system for permanent wettability (anti-fogging) and systems containing metal colloids for optical effects have been developed.

Coordination change of Te atoms in binary tellurite glasses

Abstract: Coordination changes of Te(+ IV) atoms were derived from Raman and X-ray photoelectron spectra of the binary tellurite glasses containing Li, Na, Ba and Zn. A structure model consistent with the experimental results was proposed. In this model, TeO4 units were converted mainly to O22Te=O and O12Te(=O) O− in the range of lower atomic ratio O/Te, and the rate of increase of TeO3/TeO4 decreased in the range of higher O/Te because of the equilibrium TeO42 ↔ O22TeO. The increase in the fraction of non-bridging oxygen atoms and three-coordinated Te(+ IV) atoms was confirmed by X-ray photoelectron spectra. The fraction of non-bridging oxygen estimated from X-ray photoelectron spectra agreed well with that from the Raman spectra and the model.

The many varieties of E′ centers: a review

Abstract: Three varieties of E′ centers with a spin state, S = 1 2 , and with a G - tensor , G x ∼ G y ∼ 2.0003, G z ∼ 2.0018 , were identified in the early reports on paramagnetic states in irradiated α-quartz. The atomic structure of two of these had an hydrogen ion (proton) in nearby sites and hence there are hyperfine interactions of the E′ electrons with the nuclear moments of the hydrogens, I = 1 2 . These three types have not yet been resolved in the spectra of silicas. In addition, two other variants on the oxygen vacancy configuration of the E′ center have been observed in silica. In α-quartz, four varieties of E″ centers have been observed for which S = 1. One of the these has been resolved in the spectra of some silicas. The progressive reduction of SiO2 to SiO(2 − x) for which 0 G - value increases from ∼ 2.0010 to ∼ 2.0055 and with a width of ∼ 0.05 mT. The 2.005 G - value is that observed for paramagnetic electrons in amorphous Si. The spectra of these states are resolved from the E′ spectrum when x > 1.38. This E′ type is observed in ion-implanted silica.

Glass formation and structure of glasses in the V2O5MoO3Bi2O3 system

Abstract: The glass formation region in the V2O5MoO3Bi2O3 system was investigated by the roller-quenching method. Low melting glasses were obtained in the MoO3- and V2O5-rich compositions. Characterization of the glasses was made by X-ray diffraction, differential thermal analysis (DTA) and IR spectroscopy. According to the DTA data, the glass transformation temperature, Tg, for the different compositions varied between 200 and 290°C and the cystallization temperature, TX, was within the interval of 225–330°C. Structural models for glasses of th MoO3Bi2O3 and V2O5Bi2O3 systems were suggested on the basis of IR spectral investigations, by comparing with known crystalline structures. It was shown that the glasses possess [MoO4], [MoO6], [VO4], and [BiO6] groups as basic structural units.

Direct singlet-to-triplet optical absorption and luminescence excitation band of the twofold-coordinated silicon center in oxygen-deficient glassy SiO 2

Abstract: A direct singlet-to-triplet optical absorption transition (S0→T1) in the twofold-coordinated silicon center (‘B2(Si) center’) has been detected by time-resolved photoluminescence techniques. The peak of the excitation band is at 3.15 eV, the peak of emission is at 2.75 eV, decay time is 10.5 ms at 293 K. The oscillator strength of this transition is ∼ 10−6. The newly detected band is similar to the low energy absorption/excitation bands in the Ge- and Sn-based isoelectronic analogues of the twofold-coordinated silicon center in doped silicas. The differences in the emission spectra obtained under this excitation and under excitation in the singlet-to-singlet ‘B2’-band at 5.03 eV are due to correlated effects of glassy disorder both on transition energy and on intercombinational singlet-to-triplet conversion. The spin conversion goes to the second excited triplet state (T2) and is thermally activated with an activation energy of 0.13 eV. Competing structural models for the B2(Si) center are discussed.

Boroxol groups in vitreous boron oxide: new evidence from neutron diffraction and inelastic neutron scattering studies

Abstract: Although the nuclear magnetic resonance (NMR) and neutron diffraction evidence for the majority of the boron atoms in vitreous boron oxide being involved in boroxol groups would seem to be irrefutable, nevertheless several recent papers have challenged this conclusion. The present paper, therefore, examines these claims in the light of neutron diffraction data, with much greater accuracy than those reported previously, and of neutron inelastic scattering results, optimised to give the highest possible energy transfer resolution in the region of the boroxol ring breathing mode. A fit to the interference function, Qi(Q) , indicates that the fraction, f, of boron atoms in boroxol groups is 0.80 ± 0.05, while the neutron weighted density of states exhibits a well resolved boroxol ring breathing mode of similar magnitude to the theoretical prediction for a structural model with f = 0.75. Both results confirm the conclusion of the earlier NMR and neutron studies.

Effect of modifier ions on fluorescence and absorption of Eu3+ in alkali and alkaline earth silicate glasses

Abstract: Fluorescence and absorption spectra of Eu3+ in silicate glasses have been measured. The fluorescence intensity ratio of 5D0 → 7F2 to 5D0 → 7F1 transitions of Eu3+ increases with increasing ionic radius in the order of Li < Na < K for binary alkali silicate glasses and with decreasing ionic radius in the order of Ba < Sr < Ca for binary alkaline earth silicate glasses. It was found for the first time that the intensity ratio exhibited a maximum with the addition of alkaline earth oxides in alkali alkaline earth silicate glasses. The variation of intensity ratio and the Judd-Ofelt parameter, Ω2, with composition depended on the structural change in the vicinity of Eu3+ ions and EuO covalency.

Multiple boron sites in borate glass detected with dynamic angle spinning nuclear magnetic resonance

Abstract: The first high-field boron nuclear magnetic resonance spectra of sufficient resolution to detect multiple sites in borate glass are reported. These results are obtained by using dynamic angle spinning nuclear magnetic resonance spectroscopy, in fields of 4.7, 7.1 and 8.4 Tesla, to measure the boron-11 spectrum in samples enriched in boron-10. Two boron sites, which differ in quadrupole interaction strength by 5%, in chemical shift by 5 ppm, and have relative population of 3:1, were resolved. The more populated site is assigned to boron in boroxol rings, and the less populated site is assigned to non-ring boron trioxide units. The chemical shift difference between these sites suggests that the BO bond in the boroxol rings is somewhat longer than that in the boron trioxide groups.

Structural and FTIR spectroscopic studies of gel-xerogel-oxide transitions of SnO 2 and SnO 2 : Sb powders and dip-coated films prepared via inorganic sol-gel route

Abstract: Sb doped and undoped SnO2 thin solid coatings were prepared by the dip-coating technique via the sol-gel route. Aqueous gels of undoped compound were made by using a SnCl4 precursor and doping was achieved through the addition of SbCl3 in the concentration range 1–10 mol%. Fourier transform infrared (FTIR), ATR and near normal reflectance spectroscopic techniques were utilized to determine the vibrational spectra of aqueous gels, xerogels and oxides of Sb doped and undoped compounds. Analysis of the corresponding FTIR spectra revealed the existence of Sb-O modes at 770 cm−1. The drying of both types of gel was accompanied by the formation of hydrogen bonds of medium strength wit the OH-O length about 2.7–2.8 A. Some properties of pure and Sb doped oxide powders formed in the temperature range ≤1000°C were measured by the thermogravimetry, X-ray diffraction and FTIR spectroscopic techniques. The doping brought about a broadening of the X-ray diffraction peaks. The reflectivity of the samples increased in the spectral range 4000-600 cm−1 up to 60%, due to the formation of plasma modes. Thin solids films were made by the dip-coating method. The efficiency of the dip-coating process was 0.02–0.1 μm per dipping, depending on the viscosity of the gel. The electrical resistivity of the doped SnO2 coatings was about 1.2 × 10−2 ω cm and thus comparable to the electrical resistivity of the alkoxide derived Sb doped SnO2. Doping decreased visible transmittance ∼5% but infrared reflectance increased up to 40%, depending on the film thickness.

The dynamics of strong and fragile glass formers: vibrational and relaxation contributions☆

Abstract: There are two contributions to the low-frequency excitation spectra (Raman and neutron scattering, specific heat, etc.) of glass formers: relaxations and vibrations. It is shown from the analysis of low-frequency Raman spectra that the relative weight of vibrational over relaxational excitations is larger for less fragile (in Angell's classification) glass formers. The spectra are compared with predictions of mode coupling theory (MCT) for relaxation processes. Qualitatively the predicted behaviour is observed in all analyzed systems. However, some quantitative disagreement due to significant vibrational contribution to the spectra is found for intermediate glass formers. The spectra are also analyzed using a model of vibrations coupled with relaxations. It is found that the temperature at which overdamping of the low-frequency vibrations happens is essentially the critical temperature of MCT. New details of a scenario for the liquid-glass transition are suggested.

Structural study of WO3TeO2 glasses

Abstract: Raman spectra of WO3TeO2 glasses and tungstate crystals were investigated. Properties of the glasses such as transition temperature, thermal expansion and density were measured. In the glasses, most tellurium atoms form TeO4 trigonal bipyramids and WO bonds are present. The glasses contain clusters composed of corner-shared WO6 octahedra. An increase in WO3 content promotes the growth of clusters. The coordination state of W6+ having a WO bond is discussed on the basis of crystal chemistry. Tungsten coordination polyhedra occupying the interface of the cluster have a WO bond.

Vibrational study of the role of trivalent ions in sodium trisilicate glass

Abstract: The structure of glasses in the system Na 2 O · x R 2 O 3 · (3 − 2 x )SiO 2 (RAl, B) has been studied by employing Raman and infrared reflectance spectroscopies. These particular glass compositions were chosen for elucidating the role of aluminum and boron substitution on glass structure and properties while keeping the Na/(Si + R) ratio fixed. In the presence of aluminum, the equilibrium 2Q 3 ⇔ Q 4 + Q 2 shifts to the left. The role of boron was found to be the opposite, since it induces the destruction of the Q 3 species and the creation of the highly charged Q 2 and Q 1 species, as well as the fully polymerized Q 4 silicate units. The combined far-infrared and XPS results on the same glasses suggest that the average charge density of sites populated by sodium ions is smaller in aluminosilicate than in borosilicate glasses.

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

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

Analysis of electrical relaxation in glasses and melts with large concentrations of mobile ions

Abstract: Various analyses of or fits to electrical relaxation data for a model Li 2 OAl 2 O 3 2SiO 2 glass s were tried. These included three-parameter fits using Kohlrausch-Williams-Watts (KWW) and Cole-Davidson (CD) distributions of electric field relaxation times, along with a four-parameter σ ′ ( ω n ) fit using the distribution implicitin the Jonscher expression for the frequency dependence of the real part of the complex conductivity, σ ′ = σ + Aω n . In the frequency range 10 −2 ≤ ω τ > ≤10 2 , where is the mean electric field relaxation time, the KWW firt was best, the σ ′ ( ω n ) fit second best and the CD fit the worst. Because the σ ′ ( ω n ) fit predicts qualitattively incorrect relaxation behavior at low frequencies, it is suggested that this method of data analysis not be used. It was noted that the dependence of σ ′ on ω 1 observed for ionically conducting solids at very high frequencies or very low temperatures make a nearly negligible contribution to the relaxation of the electric field.

Nature of the ac conductivity of ionically conducting crystals and glasses

Abstract: Many disordered systems, both crystalline and glasses, show ‘universal dynamic response’, in which the ac conductivity as a function of frequency is given by σ(ω) = σ(0) + Aωs. Here σ(0) is the dc conductivity and the exponent s (⪕ 1) generally decreases with increasing temperature, T. For various ionically conducting crystals and glasses, we have obtained two ranges in which s = constant, independent of T: at high temperatures s = s0 (a value near 0.6) called the ‘Jonscher regime’, and at low temperatures s = 1, called the constant-loss regime. For the Jonscher regime, the parameter A is activated with activation energy closely related to that of σ(0). On the other hand, for the constant-loss regime (which extends down to cryogenic temperatures), the parameter A varies only slowly with T and is not activated. For the cases of borate and silicate glasses as well as heavily doped crystalline CaTiO3 and CeO2, careful analysis was carried out over the intermediate regime in which the effective s falls from 1.0 to s0. The results show that this regime can simply be described as a superposition of the Jonscher and constant-loss behaviors. Work by others on silicate glass over a much wider frequency range than that of the present study falls into the same pattern. Finally, a brief review is given of possible mechanisms of both the Jonscher and constant-loss types of behavior.

1.3 μm emission of neodymium and praseodymium in tellurite-based glasses

Abstract: Tellurite glasses are examples of non-silica oxide glass hosts which are the subject of intensive materials research. Because their characteristic phonon energies are lower than those of silica glasses, praseodymium- and neodymium-doped tellurite glasses are considered for 1.3 μm fiber amplifiers. The glass formation, waveguide fabrication, optical properties and suitability for fiber drawing of sodium, zinc, tellurium oxide glasses doped with rare earth ions are described.

‘Window’ effect in the analysis of frequency dependence of ionic conductivity

Abstract: Usually the frequency dependence of ionic conductivity of solids in analyzed in the form of a power law or as the deviation of imaginary part of electric modulus from ideal Debye peak. The power law formalism is shown to be sensitive to the choice of frequency window employs in the analysis. The results demonstrate the inadequacies of either formalism and a method is suggested of combining the two formalisms for parameterizing non-ideal electric response in the vicinity of the electrical conductivity relaxation time.