NMR studies on rapidly solidified SiO2Al2O3 and SiO2Al2O3Na2O-glasses

²⁷Al and ²⁹Si MAS NMR studies were performed on roller-quenched SiO₂Al₂O₃-glasses with Al₂O₃ contents ranging from 10 to 60 mol% and on SiO₂Al₂O₃Na₂O glasses containing 10 mol% Al₂O₃ and 2.5 to 10 mol% Na₂O. Pure aluminium silicate glasses show NMR peaks at 0, 30 and 60 ppm. The frequency distribution of the different Al-sites is not affected by the glass composition. In glasses of the system SiO₂Al₂O₃Na₂O the 30 ppm peak decreases to zero as the Na₂O content increases. The 30 ppm peak is assigned to distorted triclustered AlO-tetrahedra, rather than to fivefold coordinated Al. Triclustering of tetrahedra may provide for charge neutrality in glasses with molar excess of Al₂O₃ over Na₂O. As charge balance is increasingly achieved by addition of alkali ions, the tendency of tetrahedral triclustering is reduced, reflected by the disappearance of the 30 ppm peak in glasses containing ≥ 7.5 mol% Na₂O.     

Formation and optical properties of (R2O or R′O)Nb2O5Ga2O3 glasses

Glass-forming tendencies of melts in the systems (alkali oxide or alkaline earth oxide)-Nb₂O₅Ga₂O₃ were examined by an ordinary crucible-melting technique. The glass-forming tendency increased with increasing radius of alkali or alkaline earth ion in the respective groups. Clear glasses were obtained on a practically useful scale in the systems (K₂O or Cs₂O)Nb₂O₅Ga₂O₃ and (SrO or BaO)Nb₂O₅Ga₂O₃. The infrared absorption spectra indicated that the Ga³⁺ ions in the glasses are tetrahedrally coordinated with oxygen ions. The glasses showed high optical transmissions from the ultraviolet region of 0.3 μm in wavelength to the infrared region of 7 μm, except for a region near 3 μm. The absorption near 3 μm, which is attributed to OH vibration, could be eliminated by replacing part of the carbonate in the raw materials with a fluoride and melting the mixture of raw materials in a dry N₂ gas atmosphere. The glass-forming tendencies of the melts and the optical transmissions of the glasses were discussed in terms of the glass structure.     

Spinodal decomposition during continuous cooling of a PbOB2O3Al2O3 glass

Spinodal decomposition during continuous cooling of the PbOB₂O₃Al₂O₃ quasi-binary glass system was analysed by numerical integration of Cook's differential equation (which includes the contribution of random density fluctuations) for small angle X-ray scattering (SAXS) intensity. The SAXS curves derived from the calculations have a wide range of k-Fourier components (0 < k < k_c) for which a positive amplification factor occurs and they show a “crossover” point at $\text{k}{}_{\mathrm{c}}\text{= 0.155}\text{A}\text{?}{}^{\mathrm{?1}}$. The wavenumber which receives maximal amplification, k_m, increases with the cooling rate, Q, as $\text{Q}{}^{\mathrm{<mtext>1</mtext><mtext>n</mtext>}}$, with n = 10.9. This Q dependence of k_m is similar to that predicted by Huston et al., however our results show a higher value of n. The dependence on Q and k_m of the SAXS intensity I(k_m) was also deduced. The measurements of SAXS curves were performed on glass samples prepared by the splat-cooling technique. Because of the difficulties which arise in the determination of the cooling rate of the samples, the only experimental results that could be compared with the theory are the k_m dependence of I and the value of k_c. These results are satisfactorily understood in terms of the present analysis.     

X-ray diffraction study of Na2OGeO2 melts

The structure of Na₂OGeO₂ melts in the temperature range from 1100 to 1150°C has been investigated with the high temperature X-ray diffraction technique. Comparing the radial distribution functions obtained for the melts with those for the corresponding glasses, the first peak due to the GeO interatomic distance is invariant upon melting, although it becomes broader due to thermal vibration. The second peak for the GeGe interatomic distance for melts shifts toward the large distance, which is explained by broadening of the GeOGe bond angle, not by the thermal expansion of the GeOGe bond. The composition dependences of GeO distances and coordination numbers of the Ge⁴⁺ ion of the melts are found to be almost the same as the corresponding glasses, indicating that even in melts at such high temperatures 6-fold coordinated Ge⁴⁺ ions are present and their content changes with the Na₂O content as in the case of the corresponding glasses.     

Diffusion-controlled crystal growth in K2OSiO2 compositions

The growth of cristobalite dendrites in two K₂OSiO₂ glasses containing 10.3 and 15.0 mol% K₂O has been studied over a wide range of temperature. The kinetics of crystallization were linear; i.e. the growth rates were independent of time. The observed crystallization rates were consistent with a diffusion-controlled mechanism. Measurements of the tip radii of curvature of the dendrites are consistent, at low undercoolings, with the predictions of Horvay and Cahn for the growth of an isolated dendrite. However, at lower temperatures at which the dendrites are closely spaced, the predicted radii are seriously in error. A model is described which takes account of the overlapping diffusion fields of neighboring dendrites and gives improved agreement with the observed growth rates.The presence of a maximum in each of the crystal growth rate versus temperature curves has not been explained. This phenomenon is apparently related to changes in the size, shape and spacing of the dendrites, which occur in the same temperature range.     

DC conductivity of ZnOV2O5 glasses

This work presents for the first time results regarding the DC conductivity of glasses belonging to ZnOV₂O₅ system. It is shown that the conduction can be described by a small polaron hopping model. The high temperature activation energy and conductivities are close to the values determined for V₂O₅P₂O₅ glasses of similar composition. The analysis of the pre-exponential factor shows strong evidence for a non-adiabatic conduction regime.This fact agrees with the conclusion drawn for the majority of vanadate glasses. The strong variation of α suggests the presence of fluctuations in the V⁵⁺ surroundings.     

Ionic conductvity and NMR investigation of quaternary glasses in the ZrF4BaF2ThF4LiF system

The existence of glasses involving large amounts of LiF (up to 60%) within the ZrF₄BaF₂ThF₄LiF quaternary systems has allowed the authors to study the evolution of transport properties with varying LiF content.A minimum of ionic conductivity bound to a maximum of activation energy has been detected when the atomic Li/F ratio is equal to ≅ 0.07. In the Li-low concentration domain, σ increases regularly and ΔE decreases simultaneously when the BaF₂ concentration increases; on the contrary in the Li-high concentration region log σ and ΔE are quasi-linear functions, increasing and decreasing respectively, of the LiF rate.A ⁷Li and ¹⁹F NMR investigation has shown that Li⁺ and F⁻ ions are simultaneously mobile and the temperature dependence of the number of mobile F⁻ ions has been determined. In the Li-low concentration domain transport properties result from mixed contributions of mobile Li⁺ and F⁻ ions, for high Li concentrations they depends only on the Li⁺ rate.Glasses with high Li-content have good electrical performnces (e.g. σ_175°C ≅ 2.10⁻⁴ω⁻¹ cm⁻¹ for Zr_0.20Ba_0.10Li_0.60Th_0.10F₂).     

Formation of glass in the GeSiS system

The formation of glass in the GeSiS system was investigated. After synthesis of material with the general formula Ge_1?xSi_xS_y, where x was chosen to be 0.05, 0.1, 0.2, 0.3 and y was in the range 1.28–3.6, cylindrical samples were prepared and used for the characterization of glass by means of DTA. It was found that the substitution of germanium with silicon does not lead to any expressive change of the glass transition temperature, crystallization and the onset of melting.     

Thermodynamic and thermokinetic characteristics of the glass transition in a GeSe2GeTeSb2Te3 alloy

Glassy alloys of (GeSe₂)₇₀ (GeTe)₁₅ (Sb₂Te₃)₁₅ were prepared by water-quenching and subjected to several thermal treatments through the glass transition region. The thermodynamic and thermokinetic characteristics of the glass were inferred from heat capacity measurements by differential scanning calorimetry. It was demonstrated that the undercooled liquid obtained by heating the glass is in equilibrium, and what is more, that not only each particular cooling process through the glass transition produces a given glass, but also that any trance of the glass may be suppressed by reheating above glass transition. The enthalpy and entropy differences between each glass and the undercooled liquid used to obtain that particular glass were taken as properties sensitive to the relaxation inherent to the formation of the glass. The activation energy spectrum characterizing the relaxation processes on cooling through the glass transition has been obtained. It has a peak energy of 1.43 eV which may be related to the bonds between the constituent atoms of the sample with weaker interaction energy. Therefore, the relaxation may be due to a breaking and rearrangement of these bonds in the glassy structure.     

Electrical and optical properties of the AsTeIn and GeSeIn chalcogenide systems

The influence of indium on the optical and properties of As_2−xTe_3−xxIn_2x, As_20−xTe_80−xIn_2x and Ge_20−xSe_80−xIn_2x is described. In Te-containing glasses the Fermi level is shifted by 0.05 eV and in Se-containing glasses by 0.2 eV towards the valence band.     

Self-diffusion in the chalcogenide glass system SeGeAs

Se, As and Ge self-diffusion were investigated in three different glasses of the chalcogenide system SeGeAs by means of the radioactive tracers ⁷⁵Se, ⁷³As and ⁷¹Ge. All D values (Se between 200 and 290°C, As between 240 and 290°C and Ge between 280 and 295°C) lay between 10^?14 and 5 × 10^?16 cm² s^?1. The diffusion profiles were analyzed using a chemical micro-etching technique. Roles of glass structure and possible diffusion mechanism are discussed.     

X-ray diffraction analysis on the structure of the glasses in the system PbOSiO2

The structures of PbO·SiO₂ and 2PbO·SiO₂ glasses have been analyzed by use of X-ray diffraction data and pair function method. For PbO·SiO₂ glass, a model consisting of chains of PbO₃ pyramids and silicate chains showed good agreement with the observed RDF. For 2PbO·SiO₂ glass, the present authors reported previously a model in which chains of PbO₃ pyramids are connected with SiO₄ tetrahedra, while the chromatographic analyses of silicate anions by Götz et al. and Smart et al. showed that silicate anions are distributed from monomer to polymer in the glass. We reexamined the structure of this glass referring to these results. Three representative models containing isolated SiO₄, Si₄O₁₂ rings and (SiO₃)_n chains respectively as well as PbO₃ chains were constructed and the RDFs were calculated with changing structure parameters. These three models showed satisfactory agreement with the observed data, showing that silicate anions are distributed from monomer to polymer in 2PbO·SiO₂ glass and an increase of SiO₂ content leads to polymerization of silicate anions to longer chains up to PbO·SiO₂ composition, while the chains of PbO₃ pyramids remain unchanged.     

Raman scattering study of BaF2ZrF4FeF2 glasses

Several BaF₂ZrF₄ glasses containing 5 mol% FeF₂ have been studied to elucidate the fraction of nonbridging fluorine atoms (−F⁻) in a ZrF₆ octahedron constituting a zigzag chain. A continuous increase in the fraction of nonbridging fluorine atom in a ZrF₆ octahedron is deduced from the increase in the integrated peak intensity of the symmetric stretching-vibrational mode (ν^s) for the Zr-nonbridging fluorine atom when the BaF₂ (+ FeF₂) content is lower than 38 mol%, i.e., in the composition region where the average coordination number of Zr is regarded as six. This result is well consistent with the previous results obtained from Raman studies of binary BaF₂HfF₄ glasses, and also with those of Mössbauer and Raman studies of ternary BaF₂ZrF₄FeF₂ glasses. A drastic decrease in the symmetric-stretching vibrational mode (ν^s) in 42BaF₂·53ZrF₄·5FeF₂ glass suggests that a friction of ZrF₆ octahedron is reduced when BaF₂ (+FeF₂) content is close to 50 mol%. The zigzag chain is therefore considered to be constituted of ZrF₇ units in such glasses.     

Composition dependence of the glass transition temperatures of PdNiP and PtNiP glasses

Thermal properties of glassy PdNiP and PtNiP alloys have been measured as a function of the concentration of transition metals. The glass transiion temperature, T_g, of these alloys glasses exhibits a negative linear deviation with transition metal content - which is in contrast to the increasing T_g of binary glassy alloys with increasing metalloids.It is suggested that the suppression of the glass transition temperature of these glassy alloys may be attributed to the excess configurational entropy of disorder associated with a mixture of hard spheres differing in radius. In contrast, the increasing T_g of binary glassy alloys with the metalloid content may be associated with the short-range order resulting from strong interactions between metal and metalloid atoms.     

Sensitivity of the photo-darkening effect in CuAsSe glasses

When CuAsSe glasses are irradiated, they exhibit higher concentrations of darkening than AsSe glasses. Since darkening depends on the composition, the darkening centers in CuAsSe glasses to be of the same kind as those in AsSe glasses, i.e. arsenic clusters. Concerning the kinetics of erasing, it was found that the activation energy and the rate constant of erasing in CuAsSe are almost equal to those in AsSe glasses, but for the kinetics of darkening, it was found that the activation energy of darkening is equal to that of AsSe but α₀, which is proportional to the number of latent darkening centers, and the darkening rate constant k₁ are about twice as high as the corresponding constants of AsSe glasses. This may be the reason for the greater darkening in CuAsSe glasses. The high value of α₀ was attributed to the generation of more AsAs bonds on the addition of Cu to the AsSe glass network. The high value of k₁ was attributed to the increase in efficiency of photo-decomposition because of the many impurity levels in the band gap and also because of the narrow optical energy gap in the CuAsSe glasses.     

Sol-gel preparation of wavelength-selective reflecting coatings in the system ZrO2SiO2

Multilayer, wavelength-selective reflecting coatings were prepared by alternate stackings of amorphous zirconia and silica by the sol-gel spinning technique; alkoxide-derived, polymeric sols were used. A detuned, i.e. non-λ/4 stacking system was applied. The spectrum showed a transmission minimum (∼ 4%) at a wavelength of 430 nm, with a bandwidth of 72.5 nm after deposition of the 17th layer.     

Glass transition and specific heats in the systems PS,PSe,AsS and AsSe

The glass transition temperatures were measured in the systems AsS, As_0.5P_0.5S, PSe, AsSe and PAsSe. Heat capacities of the glasses in the selenium systems were obtained by differential scanning calorimetry. As shown by the residual entropies departures from ideality are high in the chalcogen glasses. The results are discussed in terms of the structure of glasses in these systems. The thermodynamic data of glasses and liquids in these systems indicate a balance of intra- and intermolecular saturation of bonds. The amount of polymerization increases with increasing average molecular weight in the glass and with increasing temperature in some of the investigated liquids.     

Optical absorption of glassy semiconductors of the GeSbSe system

The optical transmittance of chalcogenide glasses Ge₂SbSe₇ (I), Ge₃SbSe₆ (II), GeSb₂Se₇ (III) and GeSbSe₃ (IV) was studied in the near infrared spectral region, 0.7–25 μm. The longwavelength tail of the absorption edge can be described by Urbach's rule. At higher absorption levels the absorption coefficient K depends quadratically on the energy of incident radiation. The temperature dependence of the absorption edge is discussed and the optical gaps 1.77 eV (I), 1.67 eV (II), 1.66 eV (III), 1.57 eV (IV) together with the corresponding temperature coefficients are also determined. The studied glasses are quite transparent in the 600–5000 cm^?1 wavenumber range.     

Nuclear magnetic resonance studies of the glasses in the system Na2OB2O3SiO2

The ¹¹B NMR spectra have been used to study the structure of glasses in the system Na₂OB₂O₃SiO₂. The fraction of BO₄ units, and the fraction of BO₃ units with one or two nonbridging oxygens, are measured and analyzed according to a structural model. The results indicate that: (1) for a sodium oxide to boron oxide ratio of 0.5 or less, the Na⁺¹ ions are attracted primarily by the borate network; therefore, the ternary glasses can be viewed as binary sodium borate glasses diluted by SiO₂; (2) when the sodium oxide to boron oxide ratio exceeds 0.5, the additional Na₂O results in the formation of [BSi₄O₁₀]^?1 units at the expense of diborate and SiO₄ units. In this process, Na⁺¹ ions are still taken up only by the borate network. After all the available SiO₄ units are consumed to form [BSi₄O₁₀]^?1 units, additional Na⁺¹ ions are proportionally shared between the borate and silicate networks.