Migration of Cu ions in Cu2O---Al2O3---SiO2 glasses on heating in air

The behavior of copper ions in the Cu₂O·Al₂O₃·4SiO₂ (in moles) glass on heating in air at temperatures up to 500°C was studied. When the glass, in which about 90% of Cu was present as Cu⁺ ions, was heated in air above 300°C, a CuO layer was formed on the surface. The amount of CuO was increased with heating temperature and time, corresponding to the decrease in weight of the glass. Furthermore, the fraction of Cu²⁺ ions in the glass increased. These observations suggest that oxygens do not diffuse into the glass, but Cu⁺ ions migrate to the surface from the interior to balance the surplus positive charge produced by the oxidation of Cu⁺ to Cu²⁺ ions inside the glass. The following reaction scheme for the formation of the CuO layer was proposed; 2Cu⁺(interior) + ₂¹O₂(surface) → Cu²⁺(interior) + CuO(surface).     

The effect of mixed alkaline earths on the diffusivity and the incorporation of iron in 5Na2O · xMgO · (15−x)CaO · yAl2O3 · (80−y)SiO2 melts

Diffusion coefficients of iron were measured in glass melts with the basic compositions 5Na₂O · xMgO · (15−x)CaO · yAl₂O₃ · (80−y)SiO₂ with x=5, 10 and y=0, 5, 7.5, 10 and 15. The melts were doped with 0.25 mol% Fe₂O₃ and studied in the temperature range from 1000 to 1600 °C using square-wave voltammetry. The voltammograms exhibited distinct peaks attributed to the reduction of Fe³⁺ to Fe²⁺, from which peak currents mixed diffusion coefficients of iron were calculated. Diffusion coefficients in all melt compositions which did not show crystallization could be fitted to Arrhenius equation. The diffusivities measured in different melt compositions were related to the same viscosity, i.e. not the same temperature. Increasing the alumina concentration from 5 to 10 mol% resulted in an increase of the viscosity corrected diffusivities. At further increasing alumina concentrations, the diffusivities get smaller again. This can be explained by the stabilizing effect of Na⁺ and Ca²⁺ on FeO⁻₄ and AlO⁻₄-tetrahedra, which strengthens the incorporation of Fe³⁺ into the glass structure.     

Electronic traps in mixed Si1−xGexO2 films

Thermally stimulated luminescence (TSL) and infrared (IR) spectroscopy were measured in plasma grown Si_1−xGe_xO₂ (x=0, 0.08, 0.15, 0.25, 0.5) with different thicknesses (12–40 nm). A comparison with the TSL properties of thermally grown SiO₂ and GeO₂ was also performed. A main IR absorption structure was detected, due to the superposition of the peaks related to the asymmetric O stretching modes of (i) Si–O–Si (at ≈1060 cm⁻¹) and (ii) Si–O–Ge (at 1001 cm⁻¹). Another peak at ≈860 cm⁻¹ was observed only for Ge concentrations, x>0.15, corresponding to the asymmetric O stretching mode in Ge–O–Ge bonds. A TSL peak was observed at 70°C, and a smaller structure at around 200°C. The 70°C peak was more intense in all Ge rich layers than in plasma grown SiO₂. Based on the thickness dependence of the signal intensity we propose that at Ge concentrations 0.25x0.5 TSL active defects are localised at interfacial regions (oxide/semiconductor, Ge poor/Ge rich internal interface, oxide external surface/atmosphere). Based on similarities between TSL glow curves in plasma grown Si_1−xGe_xO₂, thermally grown GeO₂ and SiO₂ we propose that oxygen vacancy related defects are trapping states in Si_1−xGe_xO₂ and GeO₂.     

Cation site occupation by Ag/Na ion-exchange in R2O–Al2O3–SiO2 glasses

Ag⁺/Na⁺ ion-exchanged R₂O–Al₂O₃–SiO₂ glasses with uniform concentration profile of Ag⁺ and Na⁺ were prepared by heat treatment in molten silver salt followed by holding at the same temperature in an ambient atmosphere. Their glass transition temperature (T_g) and thermal expansion coefficient (TEC) were measured and structures were investigated using ²⁹Si-MAS NMR, ²⁷Al-MAS NMR, IR and Raman spectroscopies. Both T_g and TEC decreased with increase of the exchange ratio, but T_g was still above the ion-exchange temperature of 400°C even for the fully exchanged sample. The ²⁹Si- and ²⁷Al-MAS NMR spectra were mostly unchanged and no sign of the structural alteration of the glass network was observed. On the other hand, the vibrational spectra showed remarkable peak shifts depending on the exchange ratio. From these structural results, it was found that when the exchange ratio was low, the introduced Ag⁺ ions were stabilized at the non-bridging oxygen (NBO) site, and then Na⁺ ions in AlØ₄ site were exchanged by Ag⁺ ions after full replacement of NBO sites, where Ø represents the bridging oxygen.     

Colloidal sol/gel processing of ultra-low expansion TiO2/SiO2 glasses

A series of titania-silica glasses with 0–9% TiO₂ were fabricated using a sol/gel process. The sol was prepared by dispersing colloidal silica fume in an aqueous solution of titania which was synthesized through the acid-catalyzed hydrolysis of titanium isopropoxide. The sols gelled in 2–4 days, and then were dried for 6–8 days. The dry gels were sintered at 1450–1500°C to produce clear, dense, microstructure-free glasses. The gels underwent a total shrinkage of 50% to yield glass rods about 50 mm long and 5 mm in diameter, or glass discs about 4 cm in diameter and 5 mm thick. The drying step was most critical in the production of crack-free specimens.

In the gel, the transmission electron microscope (TEM) revealed the presence of 1–5 nm rutile microcrystallites uniformly distributed within a network of colloidal silica particles. After sintering to 1450–1500°C, though, a dense, transparent, microstructure-free glass was created. Fourier transform infrared spectroscopy (FTIR) verified the formation of an amorphous solid-solution of titania and silica after sintering.

The thermal expansion of the glasses was measured using a differential dilatometer. The average linear coefficients of thermal expansion (CTE @ 25–675°C) varied between +5 × 10⁻⁷ and −0.2 × 10⁻⁷°C⁻¹ in the range 0 to 9% TiO₂. The glass with 7.2% TiO₂ exhibited a zero thermal expansion coefficient at 150–210°C. The hysteresis in CTE on heating and cooling was of the order of 0.01–0.02 ppm.     

Mössbauer studies on some glasses and crystals in the Na2O---Fe2O3---SiO2 system

Mössbauer absorption measurements have been made at room temperature on ⁵⁷Fe in iron sodium silicate glasses containing 3–15 mol% Fe₂O₃ and various iron alkali silicate crystals in order to study the state of iron in these glasses. The spectra of all the glasses gave one doublet with a quadrupole splitting varying from 0.73–0.78 mm s⁻¹, while those of Na₂O · Fe₂O₃ · 4 SiO₂ and 5 Na₂O · Fe₂O₃ · 8 SiO₂ crystals showed much smaller quadrupole splitting, 0.28 mm s⁻¹ and 0.10 mm s⁻¹, respectively, and an asymmetrical doublet of much narrower linewidth. When sodium was replaced by other alkali metals of larger size, such as K and Cs, in MFeSi₂O₆ and MFeSi₃O₈ crystals, the quadrupole splitting became wider and approached to 0.73 mm s⁻¹. Such a variation was not observed for glasses. These results suggest that a larger number of non-identical sites exist in iron sodium silicate glasses than in the corresponding crystals.     

Structural transformation of the TiO2---SiO2 system gel during heat-treatment

The gel formation of the (100-x)TiO₂·xSiO₂ (x = 0–10 mol%) system has been studied. The progressive elimination of residues was followed by DTA and TGA curves. DTA curves showed that the formation of anatase during heat treatment could be sensibly slowed down with the increase of SiO₂. The relationship between the gel composition and crystallization temperature of anatase has been systematically investigated. The X-ray diffraction spectra demonstrated that the crystallization temperature of anatase is 400°C for TiO₂ gel and 430°C for 90TiO₂ - 10SiO₂ gel. The infrared absorption spectra were used to follow the structural transformation of gels heat-treated at different temperatures. With the help of EPR it is evident that titanium ions exist only in tetravalence.     

Toughened glass-ceramics containing ZrO2 and Al2O3 prepared by the sol-gel process from metal alkoxides

Glasses of compositions 5ZrO₂·5SiO₂(ZS), 5ZrO₂·Al₂O₃·4SiO₂(ZAS) and 5 5ZrO₂·0.5Al₂O₃·0.5Na₂O·4SiO₂(ZANS) were prepared by the sol-gel process from metal alkoxides and sintered to make glass-ceramics. Tetragonal ZrO₂ was precipitated by heat treatment at 900 to 1300°C. The activation energy for tetragonal ZrO₂ crystal growth was extremely high in Al₂O₃ containing glasses. ZAS and ZS were sintered to the near theoretical densities above 1200°C, at which the predominant phase was tetragonal ZrO₂. On the other hand, for ZANS, high densification was not attained owing to the large pores enclosed by the glass phase. Strength and fracture toughness increased with the densification and the crystal growth of tetragonal ZrO₂, reaching 450 MPa and 9 MN/m^1.5, respectively.     

Raman spectroscopic study on the structure of ZnCl2---ZnX2 and ZnCl2---KX (X = Br, I) glasses

Raman spectra have been measured for ZnCl₂---ZnX₂ and ZnCl₂---KX (X = Br, I) glasses to investigate the structure of the glasses with varying composition. The assignment of each band was made, and the change of the spectra with composition was explained in terms of the bridging and non-bridging states of halide ions and the change of the tetrahedral units, ZnX_nCl_4−n²⁻ (n = 0–4), formed in the glasses. As the content of ZnX₂ in ZnCl₂---ZnX₂ glasses increases (20 → 80 mol%), the peak frequency of the Zn---Cl stretching mode increases (238 → 248 cm⁻¹ in X = I glasses, 238 → 259 cm⁻¹ in X = Br glasses) while the Zn---I and Zn---Br stretching frequencies decrease (173 → 120 cm⁻¹ for Zn---I, 196 → 157 cm⁻¹ for Zn---Br). The decrease of the Zn---I and Zn---Br band frequencies was attributed to the increase of the number n of the ZnX_nCl_4−n²⁻ tetrahedra. The increase of the Zn---Cl frequency suggests the existence of the bonding state of Cl⁻ ions which is intermediate between the bridging and the non-bridging states. In ZnCl₂---KX glasses, the Zn---Cl_non-bridging band at about 300 cm⁻¹ was observed in addition to the bands observed in ZnCl₂---ZnX₂ glasses. The addition of KX produces non-bridging anions while the tetrahedral units, ZnX_nCl_4−n²⁻ are also formed.     

XAS study of lanthanum coordination environments in glasses of the system K2O---SiO2---La2O3

The La L₁ and L₃ XANES and L₃ EXAFS have been investigated for the series of glasses 10K₂O---50SiO₂---x La₂O₃ (x = 1, 5, 10) and (10 − x)K₂O---40SiO₂−(x/3)La₂O₃ (x = 7.5, 5, 2.5) and model compounds La₂O₃, LaAlO₃, LaPO₄, La₂NiO₄, La₂CuO₄ and La(OH)₃. An edge resonance at 25 eV above the L₁ edge in the glass spectra is concentration-dependent, decreasing in intensity with increasing lanthanum concentration. The 2s → nd forbidden transition increases with La₂O₃ concentration, indicating a reduction in the ‘average’ site symmetry of the first coordination shell of La. Mapping _X(k) space, which is a new and promising technique, was employed to extract bond distance, coordination number and thermal parameters from the EXAFS. By this method, one calculates the complete _X(k) space a function of all physically reasonable values of the adjusted parameters in all possible combinations. The advantage in this method is the assurance of a global minimum. Bond lengths were comparable to those obtained by Fourier transforming the phase corrected EXAFS. The values are 2.42 Å (± 0.03 Å) for La---O. The coordination numbers (N ≤ 7 ± 1.5) were derived by mapping and comparison to the published structures for other La compounds. _X(k) mapping is compared with least-squares fitting the data, and the correlation between the Debye-Waller factor and coordination number is also discussed.     

Electrical conductivities of mixed cation glasses

The electrical conductivities of (1−x) Li₂O · x BaO · 2 SiO₂, (1−x) Na₂O · x MgO ·2 SiO₂, (1−x) Na₂O · x CaO · SiO₂ and (1−x) Na₂O · x BaO · 2SiO₂ glasses were measured at temperature ranging from room temperature to 450°C. The transport numbers for Na⁺ ion in (1−x) Na₂O · x BaO · 2 SiO₂ glasses were measured. It was found that the alkali ion carried a significant part of the current in these glasses except one that had no alkali ions, and the conductivity decreased markedly as the alkali oxide was substituted by an alkaline earth oxide. The results of conductivity measurements combined with the data hitherto reported on mixed alkali glasses led to the proposal that the so-called mixed alkali effect could be explained on the basis of the independent path model in which it is assumed that cations can move only through vacant sites left by those of the same type.     

Surface modification of SiO2 glass by laser processing

Various oxide films on SiO₂ glass substrates were irradiated by a laser beam. A continuous CO₂ laser source (wavelength 10.6 μm) was used for this purpose; the composition change at the surface layer was determined by Rutherford backscattering spectrometry (RBS). All the alkaline-earth oxides as well as those of lanthanum and yttrium, entered the glass after treatment. ZrO₂ and CeO₂, however, did not enter the SiO₂ glass due to laser irradiation. It is interesting, however, that a film of ZrO₂ + Al₂O mixture easily entered into the SiO₂ glass by laser processing. The conditions and mechanism of laser-enhanced interaction of ZrO₂ or other oxide films with SiO₂ glass surfaces are discussed especially in view of their structural behaviour in glass.     

Growth of laser single-crystals Er:YVO4 by floating zone method

High optical quality Er³⁺:YVO₄ laser crystals have been grown by using the floating zone method (FZ). The spectroscopic properties and 3 μm lasing of Er³⁺:YVO₄ were investigated. It is found that the Er³⁺ concentration has a negative effect on the emission of the transition ⁴I_13/2→⁴I_15/2(1.55 μm ), and a positive effect on that of the ⁴I_11/2→⁴I_13/2 transition (2.68 μm). With direct upper-state pumping and a plane-concave cavity a self-terminating laser was achieved at the wavelength of 2.724 μm in the 30 at% Er³⁺ doped sample.     

The effect of annealing on the structure of Ge20Te80 glass: atomic-scale imaging using a scanning tunnelling microscope

The effect of annealing on the structure of Ge₂₀Te₈₀ glass has been examined in atomic-scale images obtained using a scanning tunnelling microscope (STM). The STM image has been able in part to answer the question as to how the atomic structure is changed by low-temperature (<T_g) annealing, whereas the effect of annealing on the structure was never observed in previous neutron diffraction study. Scanning tunneling microscope images as large as ≈ 100 nm² have provided atomic-resolution ridges ≈ 3 nm in length. The nearest neighbour distance between peaks in each alignment is equal to ≈ 0.5 nm; the alignments in parallel with each other are a distance of ≈ 0.7 nm apart. These ridges can generate a surface associated with pseudo flatness, the size of which is less than ≈ 10 nm². Thus, the effect of annealing on the structure of as-quenched Ge₂₀Te₈₀ glass has given rise to intermediate-range order in a region less than ≈ 10 nm². The atomic configuration of annealed Ge₂₀Te₈₀ glass is inhomogeneous in the range < 10 nm².     

Silicate groupings in glassy and crystalline 2PbO·SiO2

The type and the amount of silicate groupings existing in glassy and crystalline 2PbO·SiO₂ have been determined by direct chemical methods: paper chromatography, trimethylsilylation combined with gas-liquid partition chromatography and by the molybdate method. The results obtained by these three different methods are in good agreement and demonstrate, that glassy 2PbO·SiO₂ and each of the three main crystalline polymorphs are characterized by its own specific silicate anion distribution: the distribution in vitreous 2PbO·SiO₂ is of a polyanionic nature; in T---Pb₂SiO₄ dimetic groups [Si₂O₇]⁶⁻ prevail; M₁---Pb₂SiO₄ contains predominantly [Si₄O₁₂]⁸⁻ rings and H---Pb₂SiO₄ is a typical polysilicate with chain anions [SiO₃²⁻]_n. The results fit a structural model according to which glass is a random array of discrete polyatomic groupings; the gradual transition from the glassy state to the stablest crystalline structure is connected with degradation and polymerization of silicate anions.     

Li conductivity in lithium niobate: silica glasses

Glasses of composition 65 mol% LiNbO₃:: 35 mol% SiO₂ have been shown to be Li⁺ ion conductors with a conductivity at 200°C > 1 × 10⁻⁵ (η cm)⁻¹ and an activation energy of 0.54 eV. The addition of approximately 0.1 mol% Fe₂O₃ leads to an enhancement of conductivity to ≈10⁻³ (η cm)⁻¹ at 200°C and an activation energy of 0.67 eV. The effect of Fe is shown to be in the control of microstructure in the glass, with Fe₂O₃ concentrations < 1 mol% acting as a grain growth inhibitors and larger concentrations acting as a nucleating agents. A model for this process based on the expected stoichiometry of the melt and the effect of Fe²⁺ and Fe³⁺ in charge compensation is in excellent agreement with experimental data from electron spin resonance.     

Vibrational spectroscopy of LaBSiO5 glass and glass–crystal composites

Infrared (IR) reflectivity and Raman scattering spectra of LaBSiO₅ glass and glass–crystal composites were studied in the temperature range 25–260 °C. Using an analogy with the LaBGeO₅ crystal it was possible to assign the main spectral features of the LaBSiO₅ glass and glass–crystal composites. The BO₄ chain arrangement and the bending vibrations of SiO₄ are influenced by the loss of the long-range order in the glass whereas the stretching vibrations of the SiO₄ groups are practically unaffected. The structural disorder in LaBSiO₅ crystallites is caused by rotation of BO₄ tetrahedra.     

Characteristics of ZnO–B2O3–CaO–Na2O–P2O5 glass powders prepared by spray pyrolysis

Fine-sized ZnO–B₂O₃–CaO–Na₂O–P₂O₅ glass powders with spherical shape were directly prepared by high temperature spray pyrolysis. The ZnO–B₂O₃–CaO–Na₂O–P₂O₅ powders prepared by spray pyrolysis at temperatures above 1200 °C had broad peaks at around 30° in the XRD patterns. The glass transition temperatures (T_g) of the glass powders obtained by spray pyrolysis at preparation temperatures between 900 °C and 1400 °C were near 480 °C regardless of the preparation temperatures. The dielectric layers formed from the glass powders prepared by spray pyrolysis at preparation temperatures above 1300 °C had clean surface and dense inner structure at the firing temperature of 580 °C. The transmittance of the dielectric layer formed from the glass powders obtained by spray pyrolysis at preparation temperature of 1400 °C was 90% at the firing temperature of 580 °C, in which the thickness of the dielectric layer was 13 μm. The UV cutoff edges gradually shift towards longer wavelength with increasing the preparation temperature of glass powders and the firing temperature of dielectric layers.     

An infrared study of crystallization in sodium-disilicate glasses containing iron oxides

The infrared absorption spectra of sodium-disilicate glasses containing various amounts of Fe₂O₃ ([Na₂O · 2 SiO₂]_1−x [Fe₂O₃]_x, where X = 0.05, 0.1 and 0.2) were investigated in the wavenumber range from 200–2000 cm⁻¹. The addition of Fe₂O₃ to the sodium-disilicate glass does not seem to introduce any new absorption band as compared with the spectrum of a pure sodium-disilicate glass; nevertheless, a general shift of the existing absorption bands toward lower wavenumbers is observed. The amount of shift is, in fact, proportional to the content of Fe₂O₃ in the glass. This observation is consistent with the recently proposed structural model for the bonding of Fe³⁺ ions in the iron-sodium-silicate glass system.

Annealing of 20 mol% iron oxide glasses at 550 and 580°C produced an extra sharp infrared absorption peak at about 610 cm⁻¹ wavenumber. This new peak is believed to be related to the crystallized particles of the glass as concluded from both a scanning electron micrograph and an electron diffraction pattern.     

The effect of MgO on the thermodynamics of the Fe/Fe-redox equilibrium and the incorporation of iron in soda-magnesia-aluminosilicate melts

Melts with the basic compositions 10Na₂O · 10MgO · xAl₂O₃ · (80−x)SiO₂ (x=0, 5, 10, 15 and 20), 10Na₂O · xMgO · 10Al₂O₃ · (80−x)SiO₂ (x=5, 10, 15 and 20) and xNa₂O · 10MgO · 10Al₂O₃ · (80−x)SiO₂ (x=5, 10 and 15) all doped with 0.25 mol% Fe₂O₃ were studied using square-wave voltammetry. The temperatures applied were in the range of 1000–1600 °C. The square-wave voltammograms recorded show peaks caused by the reduction of Fe³⁺ to Fe²⁺. The attributed peak potentials measured decreased linearly with decreasing temperatures. Increasing the MgO-concentration led to more negative peak potentials. Introducing alumina in the melt first resulted in less negative peak potentials. If the molar Al₂O₃-concentration is equal to that of Na₂O (=10 mol%) the peak potentials are least negative. Further increase of the Al₂O₃-concentration led to more negative peak potentials. The variation of the Na₂O-concentration led to a maximum in the peak potentials at an Na₂O-concentration of 10 mol%. An empirical formula which allows the calculation of standard potentials from the chemical composition is proposed. Furthermore, a structural explanation for the effect of the chemical composition is given. Especially, the incorporation of Al₂O₃ as AlO₄⁻-tetrahedra at [Al₂O₃] < [Na₂O] and as network modifier at larger concentrations was structurally explained by the similarities of Fe²⁺ and Mg²⁺, with respect to cation radii and metal–oxygen bond lengths.