Defect-mediated self-diffusion in calcium aluminosilicate glasses: A molecular modeling study

The mechanism of self-diffusion in calcium aluminosilicate glasses is investigated at the atomistic level using molecular dynamics (MD) simulations. We study nine glass compositions having the fixed ratio R = [CaO]/[Al₂O₃] = 1 and the concentration of SiO₂ varied from 11.8 to 76.5 mol%. The diffusion coefficient is calculated for each composition at different temperatures from 300 to 6000 K in steps of 300 K. The self-diffusivities of the various elements are found to be close to each other in magnitude, signifying the cooperative nature of the atomic movement. Network “defects” such as miscoordinated cations, non-bridging oxygen, and oxygen triclusters are also studied as a function of temperature and composition. We find that the behavior of self-diffusion correlates well with the concentration of network defects. A model of self-diffusion in calcium aluminosilicate glasses is proposed where diffusion is considered as a defect-mediated process resulting from bond-switching reactions at high temperature.     

Phase separation and crystallization in the system SiO2-Al2O3-P2O5-B2O3-Na2O glasses

The phase separation and crystallization behavior in the system (80 − X)SiO₂ · X(Al₂O₃ + P₂O₅) · 5B₂O₃ · 15Na₂O (mol%) glasses was investigated. Glasses with X = 20 and 30 phase separated into two phases, one of which is rich in Al₂O₃-P₂O₅-SiO₂ and forms a continuous phase. Glasses containing a larger amount of Al₂O₃-P₂O₅ (X = 40 and 50) readily crystallize and precipitates tridymite type AlPO₄ crystals. It is estimated that the phase separation occurs forming continuous Al₂O₃-P₂O₅-SiO₂ phase at first, and then tridymite type AlPO₄ crystals precipitate and grow in this phase. Highly transparent glass-ceramics comparable to glass can be successfully obtained by controlling heat treatment precisely. The crystal size and percent crystallinity of these transparent glass-ceramics are 20-30 nm and about 50%, respectively.     

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.     

Raman spectroscopy of calcium aluminate glasses and crystals

Solar furnace melting and fast-quench techniques have been used to prepare calcium aluminate glasses from 75 mol% CaO to 82 mol% Al₂O₃, which have been studied by Raman spectroscopy. The CaAl₂O₄ glass spectrum may be interpreted in terms of a fully-polymerized network of tetrahedral aluminate units, which is depolymerized on addition of CaO component analogous to binary silicate systems. The spectra of glasses with higher alumina content than CaAl₂O₄ may not be simply interpreted and a structural model is proposed which would be consistent with the glass spectra and with observed crystal structures along the CaAl₂O₄Al₂O₃ join. This model suggests formation of highly condensed aluminate tetrahedral on initial addition of alumina, with the appearance of aluminate polyhedra of higher average coordination at higher alumina content. Similar high coordination polyhedral are also suggested for a limited composition range along the CaOCaAl₂O₄ join. These interpretations are compared with the results of a previous study in the SiO₂Al₂O₃ glass system.     

Effect of pressure on the refractive index and relative density of the CaO · Al2O3 · 6SiO2 glass

The refractive index for glass of the CaO · Al₂O₃ · хSiO₂ system with х = 6 in the range of pressures up to 6.0 GPa was measured using a polarization-interference microscope and an apparatus with diamond anvils. The changes in the relative density characterizing the compressibility of glass were estimated from the measured refractive indices within the framework of the theory of photoelasticity. The data were compared with the previous data for glasses of the same system with х = 2 and 4. The most compressible of the three glasses in the range 2.0-6.0 GPa was the CaO · Al₂O₃ · 6SiO₂ glass. For glasses with х = 2, 4 and 6 we calculated the degrees of polymerization of silicon-aluminum-oxygen network, NBO/T (NBO - non-bridging oxygen), which are determined as the ratio of the number of gram-ions of non-bridging oxygen atoms to the total number of gram-ions of network formers. The structure-chemical parameter NBO/T was calculated with due regard for the formation of triclusters and highly coordinated aluminum. The degree of polymerization of the CaO · Al₂O₃ · хSiO₂ glasses increases with increasing х, which agrees with the change of their relative density under pressure.     

Iron redox equilibrium, structure and properties of zinc iron phosphate glasses

Iron redox equilibrium, structure and properties were investigated for the 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glasses melted at different temperatures. The structure and valence states of the iron ions in these glasses were investigated using Mössbauer spectroscopy, Raman spectroscopy and differential thermal analysis. Mössbauer spectroscopy indicated that the concentration of Fe²⁺ ions increased in the 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glass with increasing melting temperature. The Fe²⁺/(Fe²⁺ + Fe³⁺) ratio increased from 0.18 to 0.38 as the melting temperature increased from 1100 to 1300 °C. The measured isomer shifts showed that both Fe²⁺ and Fe³⁺ ions are in octahedral coordination. It was shown that the dc conductivity strongly depended on Fe²⁺/(Fe²⁺ + Fe³⁺) ratio in glasses. The dc conductivity increases with the increasing Fe²⁺ ion content in these glasses. The conductivity arises from the polaron hopping between Fe²⁺ and Fe³⁺ ions which suggests that the conduction is electronic in nature in zinc iron phosphate glasses.     

A glass with high crack initiation load: Role of fictive temperature-independent mechanical properties

The crack initiation load of a series of calcium aluminosilicate glasses and selected commercial glasses were evaluated using Vickers indentation. The results showed that a calcium aluminosilicate glass containing 80 mol% SiO₂, 10 mol% Al₂O₃ and 10 mol% CaO exhibited a high crack initiation load comparable to that of the less-brittle glass (LB glass) developed by Asahi Glass Co., Ltd. It has previously been determined that glasses experience a fictive temperature increase by indentation. The indented region of a glass, therefore, acquires, in general, different mechanical properties, such as hardness and elastic moduli, from the original, unindented glass. The extent of these mechanical property changes depends upon the glass composition and a certain glass composition with fictive temperature-independent mechanical properties can have the deformed region with matching mechanical properties to those of the undeformed region of the glass. It was found that the calcium aluminosilicate glass having no fictive temperature dependence on elastic moduli gave the highest crack initiation load. However, this composition did not coincide with fictive temperature-independence of hardness or density.     

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.     

Structural characterization of ionomer glasses by multinuclear solid state MAS-NMR spectroscopy

In the present study we report the results of ²⁹Si, ²⁷Al, ³¹P and ¹⁹F magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) of 4.5SiO₂-3Al₂O₃-1.5P₂O₅-(5−z)CaO-zCaF₂ glasses with z = 0-3 to elucidate the effect of fluoride content on the glass structure. The ²⁹Si MAS-NMR spectra gave a chemical shift of about −90 ppm corresponding to Q³(3Al) and Q⁴(3Al). The ²⁷Al MAS-NMR showed a large broad central peak around 50 ppm, which is assigned to four-coordinated Al linked via oxygen to P. A shoulder around 30 ppm and a small peak at about 0 to −10 ppm appeared in the ²⁷Al MAS-NMR spectra of the glasses on increasing the fluoride content assigned to five-coordinated and six-coordinated Al species, respectively. The ³¹P MAS-NMR spectra indicated the presence of Al-O-P bonds. The ³¹P chemical shift decreased with increasing fluoride content as a result of calcium being complexed with fluoride. This resulted in a reduction of the number of available cations to charge balance non bridging oxygens in phosphorus and an increase in the number of Al-O-P bonds being formed, instead. The ¹⁹F spectra indicated the presence of Al-F-Ca(n) and F-Ca(n) species in all the glasses containing fluoride as well as an additional Si-F-Ca(n) species in the glasses with higher fluoride content.     

The glass transition temperature and infrared absorption spectra of: (70−x)TeO2 + 15B2O3 + 15P2O5 + xLi2O glasses

Glasses of the system: (70−x) TeO₂ + 15B₂O₃ + 15P₂O₅ + xLi₂O, where x = 5, 10, 15, 20, 25 and 30 mol% were prepared by melt quench technique. Dependencies of their glass transition temperatures (T_g) and infrared (IR) absorption spectra on composition were investigated. It is found that the gradual replacement of oxides, TeO₂ by Li₂O, decreases the glass transition temperature and increases the fragility of the glasses. Also, IR spectra revealed broad weak and strong absorption bands in the investigated range of wave numbers from 4000 to 400 cm⁻¹. These bands were assigned to their corresponding bond modes of vibration with relation to the glass structure.     

The structure and optical properties of GeO2-GeS2 glasses

Oxy-chalcogenide glasses with compositions of xGeO₂-(100 − x)GeS₂, where 0 ? x ? 100 mol%, have been prepared and studied in terms of their structures and optical properties. X-ray fluorescence spectroscopy shows that Ge:S ratio can deviate from GeS₂ by ∼10 at.%, depending critically upon the preparation conditions. Raman scattering spectroscopy suggests that stoichiometric GeO₂-GeS₂ glasses have a heterogeneous structure in the scale of 1-100 nm. The optical gaps are nearly constant at 3.0-3.5 eV for glasses with 0 ? x ? 80 mol% and abruptly increase to ∼6 eV in GeO₂. This dependence suggests that the optical gap is governed by GeS₂ clusters, which are isolated and/or percolated. Composition-deviated glasses appear as orange and brown, and these glasses seem to have more inhomogeneous structures.     

Deep-UV Raman spectroscopic analysis of structure and dissolution rates of silica-rich sodium borosilicate glasses

As part of ongoing studies to evaluate relationships between structure and rates of dissolution of silicate glasses in aqueous media, sodium borosilicate glasses of composition Na₂O·xB₂O₃·(3 − x)SiO₂, with x ≤ 1 (Na₂O/B₂O₃ ratio ≥ 1), were analyzed using deep-UV Raman spectroscopy. Results were quantified in terms of the fraction of SiO₄ tetrahedra with one non-bridging oxygen (Q³) and then correlated with Na₂O and B₂O₃ content. The Q³ fraction was found to increase with increasing Na₂O content, in agreement with studies on related glasses, and, as long as the value of x was not too high, this contributed to higher rates of dissolution in single pass flow-through testing. In contrast, dissolution rates were less strongly determined by the Q³ fraction when the value of x was near unity, and appeared to grow larger upon further reduction of the Q³ fraction. Results were interpreted to indicate the increasingly important role of network hydrolysis in the glass dissolution mechanism as the BO₄ tetrahedron replaces the Q³ unit as the charge-compensating structure for Na⁺ ions. Finally, the use of deep-UV Raman spectroscopy was found to be advantageous in studying finely powdered glasses in cases where visible Raman spectroscopy suffered from weak Raman scattering and fluorescence interference.     

FTIR and ultrasonic investigations on modified bismuth borate glasses

Studies on xRO · 30Bi₂O₃ · (70−x)B₂O₃ glasses have been carried out (0 ? x ? 30 mol%, R = Zn, Ba). Elastic properties and Debye temperature have been investigated using sound velocity measurements at 4 MHz. The ultrasonic parameters along with the IR spectroscopic studies have been employed to explore the role of divalent cations in the structure of the studied glasses. Analysis of infrared spectra indicates that RO is preferentially incorporated into the borate network, forming BO₄ units. It is assumed that Bi₂O₃ enters the structure in the form of BiO₆ only. The change of density and molar volume with RO content reveals that BO₄ units linked to R²⁺ cations are denser than those linked to positive sites in the Bi₂O₃ network. Predicted values of four co-ordinated boron put forward questions about the reliability of assignment of structural units that Bi₂O₃ may form.     

Structure study of multi-component borosilicate glasses from high-Q neutron diffraction measurement and RMC modeling

A neutron diffraction structure study has been performed on multi-component borosilicate glasses with compositions (65 − x)SiO₂ · xB₂O₃ · 25Na₂O · 5BaO · 5ZrO₂, x = 5-15 mol%. The structure factor has been measured up to a rather high momentum transfer value of 30 Å⁻¹, which made high r-space resolution available for real space analyses. Reverse Monte Carlo simulation was applied to calculate the partial atomic pair correlation functions, nearest neighbor atomic distances and coordination number distributions. The Si-O network consists of tetrahedral SiO₄ units with characteristic first neighbor distances at r_Si-O = 1.60 Å and r_Si-Si = 3.0 Å. The boron environment contains two well-resolved B-O distances at 1.40 and 1.60 Å and both 3- and 4-fold coordinated B atoms are present. A chemically mixed network structure is proposed including ^[4]B-O-^[4]Si and ^[3]B-O-^[4]Si chain segments. The O-O and Na-O distributions suggest partial segregation of silicon and boron rich regions. The highly effective ability of Zr to stabilize glassy and hydrolytic properties of sodium-borosilicate materials is interpreted by the network-forming role of Zr ions.     

The effect of composition on UV-vis-NIR spectra of iron doped glasses in the systems Na2O/MgO/SiO2 and Na2O/MgO/Al2O3/SiO2

Glasses with the compositions xNa₂O · 10MgO · (90 − x)SiO₂, 10Na₂O · xMgO · (90 − x)SiO₂, 5Na₂O · 15MgO · xAl₂O₃ · (80 − x)SiO₂, xNa₂O · 10MgO · 10Al₂O₃ · (80 − x)SiO₂, 10Na₂O · 10MgO · xAl₂O₃ · (80 − x)SiO₂, 10Na₂O · 5MgO · 10Al₂O₃ · (80 − x)SiO₂ were melted and studied using UV-vis-NIR spectroscopy in the wavenumber range from 5000 to 30 000 cm⁻¹. At [Al₂O₃] > [Na₂O], the UV-cut off is strongly shifted to smaller wavenumbers and the NIR peak at around 10 000 cm⁻¹ attributed to Fe²⁺ in sixfold coordination gets narrower. Furthermore, the intensity of the NIR peak at 5500 cm⁻¹ increases. This is explained by the incorporation of iron in the respective glass structures.     

Effect of P2O5 content in two series of soda lime phosphosilicate glasses on structure and properties - Part I: NMR

The effect of the variation in phosphate (P₂O₅) content on the structure of two series of bioactive glasses in the quaternary system SiO₂-Na₂O-CaO-P₂O₅ was studied. The first series (I) was a simple substitution of P₂O₅ for SiO₂ keeping the Na₂O:CaO ratio fixed (1.00:0.87). The second series was designed to ensure charge neutrality in the orthophosphate , therefore as P₂O₅ was added the Na₂O and CaO content was varied to provide sufficient Na⁺ and Ca²⁺ cations to charge balance the orthophosphate present. The glass network connectivity (NC) was calculated for each glass and a modification for the presence of a separate P₂O₅ phase was included (NC′). ³¹P and ²⁹Si magic-angle-spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was performed on glass series I and II to determine the structural units present and their relation to glass properties. ³¹P MAS-NMR spectra of series I resulted in a broad resonance around 9 ppm corresponding to orthophosphate in an amorphous environment. The 9.25 mol% P₂O₅ glass shown to be partially crystalline by X-ray diffraction was heat treated, and the ³¹P MAS-NMR spectrum showed a sharp peak around 3 ppm corresponding to calcium orthophosphate or sodium pyrophosphate and overlapping broader peaks at 8.5, 10.5 and 14 ppm possibly corresponding to two mixed calcium-sodium orthophosphate phases and amorphous sodium orthophosphate respectively. ³¹P MAS-NMR spectra of series II resulted in a broad resonance around 10.5 ppm corresponding to orthophosphate in an amorphous environment. ²⁹Si MAS-NMR spectra of glasses from series I showed a shift in the resonance peak from around −78 to −86 ppm indicating an increase in Q³ species in the glass and a reduction in Q² with phosphate addition confirming the presence of orthophosphate. The heat treated sample showed a sharp ²⁹Si-NMR resonance at −88 ppm, indicating a crystalline Q² six-membered combeite (Na₂O · 2CaO · 3SiO₂) silicate-type phase, which was confirmed by powder X-ray diffraction. ²⁹Si MAS-NMR spectra of glasses from series II showed no shift in the resonance at around −78 ppm across the series, confirming an orthophosphate environment.     

Crystallization and dielectric properties of low temperature dielectrics containing Li2O filler

Crystallization and dielectric properties of typical low temperature co-fired ceramics (LTCC) consisting of calcium zinc aluminoborosilicate glass and Al₂O₃ filler were investigated by substituting the Al₂O₃ filler partially with Li₂O at the levels of 2-10 wt%. Depending on the content of Li₂O, densification was found significantly affected by early crystallization that resulted from the formation of unexpected crystalline phases including LiAlSiO₄, Ca₂SiO₄, LiAlO₂, and LiAlSi₃O₈. The effect of hindering sintering via earlier crystallization became enormous regardless of firing temperature when >5 wt% Li₂O substitution occurred. It was observed that the substitution of 2 wt% Li₂O for Al₂O₃ was beneficial in producing promising performance at the low temperature of 750 °C, which can be highlighted with k ∼ 8.7 and tan δ ∼ 0.009 at 1 MHz.     

Vibrational spectral analysis of structural modifications of Cr2O3 containing oxyfluoroborate glasses

Infrared and Raman spectra of oxyfluoroborate glasses of the compositions 20CaF₂-20 ZnO-(60-x) B₂O₃-x Cr₂O₃ with 0 ≤ × ≤ 0.10 mol% (x = 0, 0.02, 0.04, 0.05, 0.06, 0.07, 0.08 and 0.1) have been analyzed in order to explain the role of Cr₂O₃ on the structure of these glasses. Density, molar volume, oxygen molar volume and oxygen packing density are determined to explain the packing of the atoms in the glasses. From these studies, the existence of different borate groups like di-, tri-, tetra-, penta-, ortho-, pyro- and metaborates in these glasses is established. With the increase in the concentration of Cr₂O₃ up to 0.06 mol%, conversion of BO₃ units into BO₄ units and above this mole fraction the reconversion of BO₄ units into BO₃ units is observed. The breaking and reforming of the boroxol ring is explained from the Raman spectral studies of these glasses.     

The comparative structure, properties, and ionic conductivity of LiI + Li2S + GeS2 glasses doped with Ga2S3 and La2S3

The well known and characterized fast ion conducting (FIC) LiI + Li₂S + GeS₂ glass-forming system has been further optimized for higher ionic conductivity and improved thermal and chemical stability required for next generation solid electrolyte applications by doping with Ga₂S₃ and La₂S₃. These trivalent dopants are expected to eliminate terminal and non-bridging sulfur (NBS) anions thereby increasing the network connectivity while at the same time increasing the Li⁺ ion conductivity by creating lower basicity [(Ga or La)S_4/2]⁻ anion sites. Consistent with the finding that the glass-forming range for the Ga₂S₃ doped compositions is larger than that for the La₂S₃ compositions, the addition of Ga₂S₃ is found to eliminate NBS units to create bridging sulfur (BS) units that not only gives an improvement to the thermal stability, but also maintains and in some cases increases the ionic conductivity. The compositions with the highest Ga₂S₃ content showed the highest T_gs of ∼325 °C. The addition of La₂S₃ to the base glasses, by comparison, is found to create NBS by forming high coordination octahedral LaS₆³⁻ sites, but yet still improved the chemical stability of the glass in dry air and retained its high ionic conductivity and thermal stability. Significantly, at comparable concentrations of Li₂S and Ga₂S₃ or La₂S₃, the La₂S₃-doped glasses showed the higher conductivities. The addition of the LiI to the glass compositions not only improved the glass-forming ability of the compositions, but also increased the ionic conductivity glasses. LiI concentrations from 0 to 40 mol% improved the conductivities of the Ga₂S₃ glasses from ∼10⁻⁵ to ∼10⁻³ (Ω cm)⁻¹ and of the La₂S₃ glasses from ∼10⁻⁴ to ∼10⁻³ (Ω cm)⁻¹ at room temperature. A maximum conductivity of ∼10⁻³ (Ω cm)⁻¹ at room temperature was observed for all of the glasses and this value is comparable to some of the best Li ion conductors in a sulfide glass system. Yet these new compositions are markedly more thermally and chemically stable than most Li⁺ ion conducting sulfide glasses. LiI additions decreased the T_gs and T_cs of the glasses, but increased the stability towards crystallization (T_c − T_g).     

Thermal properties and crystallization of iron phosphate glasses containing up to 25 wt% additions of Si-, Al-, Na- and U-oxides

The thermal properties (expansion, T_g and T_SOFT.) of glasses, having 56-66% P₂O₅, 14.8-34.2% Fe₂O₃ and 2-25 wt% additions of SiO₂, Al₂O₃, Na₂O and UO₂, were comparatively estimated from dilatometric measurements in similar conditions. The T_g reversibility was clearly verified by varying the heating rates between 1 and 5 °C min⁻¹. From linear equations fits of the various glass properties as functions of the six components it is suggested the iron, sodium and uranium oxides decrease the thermal expansion (for 50 < T ? 300 °C), T_g and T_SOFT. From DTA/XRD analysis of three glasses it was confirmed the crystallization tendency decreased with increasing the UO₂ level in the glasses. Leaching test data for two compositions containing Na₂O suggest addition of UO₂ increases the chemical durability of the related glass. The roles of UO₂, Na₂O and Fe-oxide species as structural components of the glass network are discussed.