Influence of zirconium on the structure of pristine and leached soda-lime borosilicate glasses: Towards a quantitative approach by O MQMAS NMR

¹⁷O MQMAS NMR was used to characterize the influence of zirconium on the structural organization of soda-lime borosilicate glasses. A new method of quantitative analysis of the ¹⁷O MQMAS spectra is presented, by a direct fit of the two-dimensional MQMAS spectrum which provides the resolution of all the structural groups in glasses containing up to five oxides. Additional data were also obtained from the quantitative deconvolution of the ¹¹B MAS NMR spectra, with the help of the direct fit of MQMAS data as well. Excess of non-bridging oxygen is clearly identified in these glasses. Sixfolded zirconium is preferentially compensated rather than the tetrahedral boron and calcium only partially compensate the tetrahedral boron. Alteration gels arising from glass leaching were probed by oxygen-17 supplied by the alteration solution. Most of the zirconium is inserted in the silicate network forming Si-O-Zr bonds with the same configuration in the glass and in the gel. During leaching, calcium clearly remains in the alteration gel, either near non-bridging oxygen or as a zirconium charge compensator. This quantitative approach applied to ¹⁷O MQMAS spectra demonstrates its potential for investigating the structure of increasingly complex glass and gel compositions.     

Structural characterization of phosphate glasses doped with silver

The present paper reports the influence of silver oxide addition on the local structure of 2P₂O₅ · CaO · 0.05ZnO glass matrix. The glass samples were investigated through several methods: X-ray powder diffraction (XRD), scanning electron microscopy (SEM), infrared absorption (FT-IR) and Raman scattering. The X-ray diffraction patterns confirm the vitreous character of these samples over the explored compositional range and the SEM pictures confirm this information. The phosphate structural units of the network former are assessed from FT-IR and Raman spectra as ultra-, meta-, pyro- and orthophosphate units. A slight structural depolymerization process of these phosphate-based glasses was evidenced for higher silver oxide content. In vitro behavior of the bulk glass sample with the highest silver oxide content was tested by immersion in simulated body fluid (SBF). X-ray diffraction and SEM measurements made on the SBF treated sample revealed growth of a crystalline phase on the surface sample.     

Crystallization of monoclinic WO3 in tungstate fluorophosphate glasses

Tungstate fluorophosphate glass compositions with high WO₃ concentration were prepared in the ternary system (80−0.8x)NaPO₃-(20−0.2x)BaF₂-xWO₃ with x = 40,50 and 60 mol%. Transparency decreases as WO₃ concentration increases but can be improved by addition of oxidizing systems such as CeO₂ or Sb₂O₃/NaNO₃. Characterizations by thermal analysis (DSC) point out that an increase in the amount of WO₃ results in a higher glass transition temperature. In addition, such compositions are very stable against devitrification since samples containing 40% and 50% of WO₃ donot even exhibit the expected crystallization event. In these samples, the stability against crystallization decreases with the WO₃ content and vitreous sample containing 60% of WO₃ exhibits an endothermic event around 620 °C due to crystallization of monoclinic WO₃ phase. In these glasses, it was shown that the nucleation stage can be induced by thermal-treatment when external nucleating agents such as Ti or Sb are used. Finally, gold-doped samples exhibit a higher crystallization tendency and monoclinic WO₃ phase can be grown in such glasses.     

Atomic structure of beryllium boroaluminate glasses: A multi-nuclear NMR spectroscopic study

The short-range structure and network speciation have been studied in a series of beryllium boroaluminate glasses using ¹¹B, ²⁷Al and ⁹Be NMR spectroscopy. All glasses are characterized by a coexistence of BeO₄, BO₃, BO₄, AlO₄, AlO₅ and AlO₆ species. The concentrations of BO₃, AlO₅ and AlO₆ species in these glasses are significantly higher and the geometry of the B-O and Al-O coordination polyhedra are unusually disordered compared to those in other alkali and alkaline-earth boroaluminate glasses reported previously in the literature. These results indicate that Be atoms may not play the typical role of a network-modifying cation in these glasses. This structural scenario is consistent with the highest field strength and electronegativity of Be among all alkali and alkaline-earth metals.     

Electrical conductivity studies in single and mixed alkali doped cobalt-borate glasses

The glasses of the type (Li₂O)_x-(CoO)_0.2-(B₂O₃)_0.8−x and (Li₂O)_0.2-(K₂O)_x-(CoO)_0.2-(B₂O₃)_0.6−x were prepared by melt quench technique and their non-crystallinity has been established by XRD studies. The glasses were investigated for room temperature density and dc electrical conductivity in the temperature range 300-550 K. Molar volumes were estimated from density data. Composition dependence of density and molar volume in both the sets of glasses has been discussed. Conductivity data has been analyzed in the light of Mott’s Small Polaron Hopping (SPH) Model and activation energies were determined. Variation of conductivity and activation energy with Li₂O content in single alkali glasses indicated change over conduction mechanism from predominantly electronic to ionic, at 0.4 mole fraction of Li₂O. In mixed alkali glasses, the conductivity has passed through minimum and activation energy has passed through maximum at x = 0.2. This has been attributed to the mixed alkali effect. It is for the first time that a change over of predominant conduction mechanism in lithium-cobalt-borate glasses and mixed alkali effect in lithium-potassium-cobalt-borate glasses has been observed. Various physical and polaron hopping parameters such as polaron hopping distance, polaron radius, polaron binding energy, polaron band width, polaron coupling constant, effective dielectric constant, density of states at Fermi level have been determined and discussed.     

Structural investigation of bismuth borate glasses and crystalline phases

Glasses of the system: xBi₂O₃-(100−x)B₂O₃ (x = 20 to 66 mol%) were prepared and characterized by density, DSC, UV-visible absorption and ¹¹B MAS-NMR spectroscopy. Glass molar volume increases while the glass transition temperature decreases with Bi₂O₃ concentration. Densities of some bismuth borate glasses are found to be greater or very close to those of single crystal phases with equal composition. B¹¹ MAS-NMR studies determined that the fraction of tetrahedrally coordinated borons (N₄) is maximum at 42 mol% of Bi₂O₃ and that there is a local maxima in N₄ at Bi₂O₃ concentration of 50 mol%. Glasses containing Bi₂O₃ concentration of 33 mol% and higher show an unusual, intense absorption band just below the optical band gap. Two crystalline phases: Bi₃B₅O₁₂ and Bi₄B₂O₉ were prepared by devitrification of glasses and characterized by X-ray diffraction, FTIR and ¹¹B MAS-NMR studies. Both crystalline phases contained significantly lower N₄ than glasses with equal composition.     

Development and characterizations of BaO-CaO-Al2O3-SiO2 glass-ceramic sealants for intermediate temperature solid oxide fuel cell application

Several compositions based on BaO-CaO-Al₂O₃-SiO₂ (BCAS) glass system have been studied in this investigation to see their applicability as sealant for solid oxide fuel cell (SOFC). The glasses as well as the corresponding glass-ceramics have been systematically characterized by differential thermal analysis, dilatometry, X-ray diffractometry, electron microscopy and impedance analysis to examine their suitability as sealant. While the glass transition temperature (T_g) determined from DTA are within 600-665 °C, the coefficient of thermal expansion (CTE) can be tailored between 9.5 and 13.0 × 10⁻⁶ K⁻¹. These glasses are found to be well adhered with metallic interconnects, such as commercial ferritic steel (Crofer22APU), at an optimum sealing temperature of 850 °C. The shrinkage behavior of the developed glasses in their pellet form has also been investigated. The resistivities of the glass-ceramics, as obtained from impedance analysis, are found to be within 10⁴-10⁶ Ω cm at 800 °C. Under sandwiched condition between two metals, some of the developed compositions are found to maintain this high resistivity even after 100 h of operation. One of the glass compositions has shown a low leak-rate of the order of ∼10⁻⁷ Pa m² s⁻¹.     

Sol-gel deposition of silica films on silicate glasses: Influence of the presence of lead in the glass or in precursor solutions

Many lead silicate historical glasses suffer degradation phenomena often observed as color changes and iridescence caused by lead ions leaching from the outer layers of the glass. In order to repair and to prevent these phenomena, glasses with large amounts of lead (6.7 and 14.3 at.% of lead) have been coated with silica films at neutral pH by dipping them in a precursor solution of TEOS (tetraethyl orthosilicate), ethyl alcohol and deionized water without any other acid or basic catalyst. Experiments with long dipping times (24 h) and temperatures around 20 °C have been performed to evaluate the role of lead ions of the glass as a catalyst. Silica films of very good quality and optical transparency have been also obtained on lead-free, soda-lime glasses by adding catalytic amounts of Pb(NO₃)₂ instead of HCl to the precursor solution. The films have been characterized by optical microscopy, AFM (Atomic Force Microscopy), XPS (X-ray Photoelectron Spectroscopy) and SIMS (Secondary Ion Mass Spectrometry).     

The epitaxial growth of sputtered silicon layers on differently oriented substrates at low temperature - An electron microscopic study

We investigated epitaxial silicon films deposited on differently oriented substrates by pulsed magnetron sputtering at temperatures of 500-550 °C. Our scanning and transmission electron microscopic as well as electron backscattering investigations show that epitaxial films grow not only on (1 0 0)-oriented substrates, but also on (2 1 0)-, (4 1 1)- and (3 1 1)-oriented ones. A change to the (1 0 0) orientation is found for the growth on (1 1 1)- , (3 2 1)- and close to (1 1 0)-oriented substrates. For these orientations transmission electron microscopic investigations show stacking faults, microtwins and small amorphous inclusions in a region starting at the substrate-film interface up to thicknesses of 150-200 nm. With increasing film thickness above 200 nm the crystalline perfection of the epitaxial layers improves.     

Structures of lanthanum and yttrium aluminosilicate glasses determined by X-ray and neutron diffraction

We have measured the structures of lanthanum and yttrium aluminosilicate glasses by X-ray and neutron diffraction and determined the interatomic distances and nearest-neighbor coordination numbers. The results obtained with the two techniques are in good agreement with each other and with recent NMR studies. The Si-O and Al-O coordination numbers are found to be 4 and 4.5, respectively. All the glasses show pronounced intermediate-range order that exhibits a reduced length scale with increasing La or Y content.     

Studies of Er doped germanate-oxyfluoride and tellurium-germanate-oxyfluoride transparent glass-ceramics

An erbium doped germanate-oxyfluoride glass 60GeO₂ · 20PbO · 10PbF₂ · 10CdF₂ (GPOF) and a tellurium-germanate-oxyfluoride glass 30TeO₂ · 30GeO₂ · 20PbO · 10PbF₂ · 10CdF₂ (TGPOF) were prepared in the bulk form. By appropriate heat treatment of the as-prepared glasses above, transparent glass-ceramics were obtained with the formation of β-PbF₂ nanocrystals in the glass matrix confirmed by X-ray diffraction. Optical absorption and photoluminescence measurements were performed on as-prepared glass and glass-ceramics. The luminescence of Er³⁺ ions in transparent glass-ceramics revealed sub-band splitting generally seen in a crystal host. The intensity of red and near infrared luminescence significantly increased in transparent glass-ceramic compared to that in as-prepared glass. Two luminescence bands at 758 nm from ⁴F_7/2 → ⁴I_13/2 and at 817 nm from ²H_11/2 → ⁴I_13/2 transitions were observed from transparent glass-ceramic but cannot be seen from the corresponding as-prepared glass. These results are attributed to the change of ligand field of Er³⁺ ions and the decrease of effective phonon energy when Er³⁺ ions were incorporated into the precipitated β-PbF₂ nanocrystals.     

Effect of BaO on the crystallization kinetics of glasses along the Diopside-Ca-Tschermak join

We report on the effect of BaO on the crystallization kinetics of glasses in the diopside (CaMgSi₂O₆)-Ca-Tschermak (CaAl₂SiO₆) system. Partial substitution (i.e. 5%, 10% and 20%) of Ba²⁺ for Ca²⁺ was attempted in composition CaMg_0.8Al_0.4Si_1.8O₆, in three different glasses while partial substitution of B³⁺ for Al³⁺ was made in the fourth glass. Structural investigations on the glasses have been made by density measurements, molar volume and Infra-red spectroscopy (FTIR). Non-isothermal crystallization kinetic studies have been employed to study the mechanism of crystallization in all the four glasses. The Avrami parameter for the glass powders is ∼2, indicating the existence of intermediate mechanism of crystallization. Crystallization sequence in the glasses has been followed by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and FTIR. Augite crystallized out being the dominant phase in all the glass-ceramics, while different polymorphs of BaAl₂Si₂O8 were present as secondary or minor phases.     

Medium range order and the radial distribution function

We have studied the subtle differences between the radial distribution functions g(r) of several models of disordered silicon, which contain differing amounts of paracrystalline medium range order. Due to the inherent averaging of diffraction data at medium range length scales r, the differences are indeed small. We find, however, that the residual function G(r) = r[g(r) − 1] exhibits an oscillatory decay, with discernibly different decay lengths. The decay lengths are found to be proportional to the radial extent of the medium range order in the model as determined by several other computational methods. Our results indicate that the extent of medium range order could be measured by diffraction experiments. However, to discern the nature of the ordering, fluctuation microscopy is needed, and for models, improved methods for modeling the oscillatory part of G(r) are essential.     

Quantitative measurement of Q species in silicate and borosilicate glasses using Raman spectroscopy

Raman spectroscopy has been used to measure the fraction of tetrahedral silicate units connected at three corners into the network (Q³) in binary lithium silicate glasses and also in the more complex borosilicate glasses used for waste immobilization. Agreement within experimental error was obtained with ²⁹Si MAS NMR measurements of the same samples. Raman provides an alternative method of structural determination for silicon-containing glasses with a high content of paramagnetic species where NMR loses resolution. Analysis was performed on borosilicate glasses containing up to 11.98 mol% Fe₂O₃ and the Q³ values obtained by Raman spectroscopy agree within error with the published ²⁹Si NMR results from borosilicate glasses containing the equivalent quantity of Al₂O₃.     

Structure and lattice dynamics of binary lead silicate glasses investigated by infrared spectroscopy

The polar lattices dynamics of seven binary lead silicate glasses have been studied by infrared spectroscopy. The analysis of the reflectivity spectra with a dielectric function model, based on a modified Gaussian profile, allows a quantitative evaluation of the presence of lead cations within different structural sites. From the role of the lead cations versus the degree of polymerization of the silicate network and the comparison with literature results, we may to give a scenario for explaining the observed structural evolution of the glass matrix and more particularly the drastic change occurring around 45% of lead content. Below this threshold, lead cations act only as modifiers of the silicate network. Above, the glass structure is deeply modified; a lead network involving around 10% of the lead content appears in glasses whose composition is just above the threshold and progressively grows at the expense of the silicate network with the increase of lead content. For high lead content, lead cations can act as modifiers of the silicate network or as network formers. Results also show that the analysis of far infrared measurements combined with the knowledge of the UV edge optical response is very promising to characterize the local disorder around cations in glasses.     

Structure and properties of glasses in the MI + M2S + (0.1Ga2S3 + 0.9GeS2), M = Li, Na, K and Cs, system

The structure and properties of glasses in the MI + M₂S + (0.1Ga₂S₃ + 0.9GeS₂), M = Li, Na, K and Cs, system were studied using Raman, IR spectroscopy, DSC and density measurements to help better understand the ionic transport in these glasses. The glass forming ranges of these ternary glasses were compared to those of the binary alkali sulfide and germanium sulfide systems. The more extensive glass forming range in the Na₂S system was used to examine the more extensive changes of structure and properties of these glasses as a function of Na₂S content. As expected, non-bridging sulfurs (NBS) form with the addition of alkali sulfide. Unlike their oxide counterparts, however, the alkali sulfide doped glasses appear to support longer-range super-structural units. For example, evidence that the adamantine-like structure exists in the K₂S and Cs₂S modified glasses is found in the Raman spectra of the glasses. The structural role of the alkali iodide addition was also explored since the addition of alkali iodide helps to improve the conductivity. For most of these glasses, as observed in many other oxide glasses, the added MI dissolves interstitially into the glass structure network without changing the alkali sulfide network structure. In 0.6Na₂S + 0.4(0.1Ga₂S₃ + 0.9GeS₂) glasses, however, the added NaI may affect the glass structure as it causes systematic changes in the frequency of the Ge-S network mode as seen in the Raman spectra.     

Fabrication and properties of novel low-melting glasses in the ternary system ZnO-Sb2O3-P2O5

Glasses in the ternary system ZnO-Sb₂O₃-P₂O₅ were investigated as potential alternatives to lead based glasses for low temperature applications. The glass-forming region of ZnO-Sb₂O₃-P₂O₅ system has been determined. Structure and properties of the glasses with the composition (60 − x)ZnO-xSb₂O₃-40P₂O₅ were characterized by infrared spectra (IR), differential thermal analysis (DTA) and X-ray diffraction (XRD). The results of IR indicated the role of Sb³⁺ as participant in glass network structure, which was supported by the monotonic and remarkable increase of density (ρ) and molar volume (V_M) with increasing Sb₂O₃ content. Glass transition temperature (T_g) and thermal stability decreased, and coefficient of thermal expansion (α) increased with the substitution of Sb₂O₃ for ZnO in the range of 0-50 mol%. XRD pattern of the heat treated glass containing 30 mol% Sb₂O₃ indicated that the structure of antimony-phosphate becomes dominant. The improved water durability of these glasses is consistent with the replacement of easily hydrated phosphate chains by corrosion resistant P-O-Sb bonds. The glasses containing ?30 mol% Sb₂O₃ possess lower T_g (<400 °C) and better water durability, which could be alternatives to lead based glasses for practical applications with further composition improvement.     

Laser-induced crystallization of β′-RE2(MoO4)3 ferroelectrics (RE: Sm, Gd, Dy) in glasses and their surface morphologies

The glasses with the compositions of 21.25RE₂O₃-63.75MoO₃-15B₂O₃ (RE: Sm, Gd, Dy) were prepared and the formation of β′-RE₂(MoO₄)₃ ferroelectrics was confirmed in the crystallized glasses obtained through a conventional crystallization in an electric furnace. The features of the glass structure and crystallization behavior were clarified from measurements of Raman scattering spectra. Continuous-wave Nd:YAG laser with a wavelength of 1064 nm (laser power: 0.6-0.9 W, laser scanning speed: S = 1-16 μm/s) was irradiated to 10.625Sm₂O₃-10.625Gd₂O₃ (or Dy₂O₃)-63.75MoO₃-15B₂O₃ glasses, and the structural modification was induced at the glass surface. At the scanning speed of S = 10 μm/s, crystal lines consisting of β′-Gd_2−xSm_x(MoO₄)₃ or β′-Dy_2−xSm_x(MoO₄)₃ crystals were patterned on the glass surface. It was found that those crystal lines have the surface morphology with periodic bumps. At S = 1 μm/s, it was found that crystal lines consist of the mixture of paraelectric α-Gd_2−xSm_x(MoO₄)₃ and ferroelectric β′-Gd_2−xSm_x(MoO₄)₃ crystals, indicating the phase transformation from the β′ phase to the α phase during laser irradiation. Homogeneous crystal lines with β′-RE₂(MoO₄)₃ ferroelectrics have not been written in this study, but further research is continuing.     

Local structure and its effect on the oscillator strengths and emission properties of Ho in chalcohalide glasses

The formation of structural units in Ge-Ga-S glass with CsBr addition and its effect on the optical band gap and several spectroscopic properties of Ho³⁺ were investigated. The optical band gap of the host glasses and the oscillator strengths of Ho³⁺ absorption decreased sharply when the ratio of CsBr/Ga became close to unity. These property changes were associated with the formation of structural units of [GaS_3/2Br]⁻ and they have a crucial effect on the changes of the local structure around Ho³⁺ ions. The local structure was not sensitive to the As or Br addition but was critically dependent on the concentration of CsBr.     

Optical properties of nanocomposites with iron core-iron oxide shell structure

Iron nanoparticles of diameter ∼5 nm were produced within a gel-derived silica glass by reducing a suitable gel composition. By heating these composites in the temperature range 573-973 K, Fe₃O₄ shells of a few nanometer thickness were grown around the iron nanoparticles. Three peaks were observed in the optical absorption spectra of the nanocomposites when they were dispersed in ethyl alcohol. The first one around 300 nm was caused by plasma resonance absorption of unoxidized iron particles; the second was shown to be due to the core-shell structure with different permittivities of the two regions and the third one was ascribed to a d-d transition. Detailed analyses of the second peak showed that the extracted values of electrical conductivity were below Mott’s minimum metallic conductivity for iron in the case of particles with diameters below ∼2.5 nm.