Structure of Glasses and Melts in the Na2O·GeO2·B2O3 System

Density and viscosity results are presented for ternary Na₂O·GeO₂·B₂O₃ melts (∼600° to 1300°C) and glasses containing as much as 35 mole % Na₂O. Synthetic partial molar volume models indicate a fairly broad stability region for BO₄ tetrahedra in the B₂O₃-rich melts. Similar models for GeO₂-rich melts reveal a more limited stability region for GeO₆ octahedra. The expansion coefficient contours and viscosity isotherms confirm the volume-based conclusions for the liquid state. The high-temperature volume models were used to develop glass volume models that agree to within several percent of experiment. It has been concluded that the melts and glasses possess similar structures. The relatively greater compositional stability of GeO₆ octahedra in the presence of B₂O₃ (compared to Al₂O₃) can be related to the smaller average number of oxygens around boron (III), at a fixed O/Ge ratio, compared to aluminum (III). Evidence is presented for a slight decrease of the thermal stability of GeO₆ octahedra in the GeO₂-rich melts above about 1000°C.     

Crystallization Behavior of Potassium Niobium Silicate Glasses

Crystallization behavior of potassium niobium silicate (KNS) glasses having compositions expressed by the general formula x K₂O· x Nb₂O₅·(1 - 2 x )SiO₂, with x = 0.167, 0.182, 0.200, 0.220, and 0.250, has been studied by DTA, X-ray diffraction, second harmonic optical generation (SHG), and electron microscopy. Bulk crystallization of potassium niobates in glasses with compositions near K₂O·Nb₂O₅·2SiO₂, as well as surface crystallization of KNbSi₂O₇ phase, has been established. Transparent glass-ceramics, based on potassium niobates with remarkable SHG signal values, can be obtained from glasses with the lowest silica content, by heat treatment at temperatures just above T _g, while at higher temperatures from all of the glasses under investigation the main crystallizing phase is KNbSi₂O₇ ferroelectric. Applying a dc electric field, grain-oriented crystallization is produced in KNS glasses with development of significantly anisotropic arrangements of KNbSi₂O₇ crystallites.     

Structure and Properties of Low Phonon Antimony Glasses in the K2O–B2O3–Sb2O3–ZnO System

The monolithic glass-forming region of the low phonon and low softening point antimony glasses containing high Sb₂O₃ (40–75 mol%) in the novel quaternary K₂O–B₂O₃–Sb₂O₃–ZnO system has been found with the help of X-ray diffraction (XRD) analysis. The structure of a series of glasses with the general composition of (mol%) 15K₂O–15B₂O₃–(70− x )Sb₂O₃– x ZnO (where x =5–25) has been evaluated by infrared reflection spectral (FT-IRRS) analyses. All the glasses are found to possess a low phonon energy of around 600 cm⁻¹, as revealed by FT-IRRS. Their softening point ( T _s), glass transition temperature ( T _g), and coefficient of thermal expansion (CTE) have been found to vary in the ranges of 351°–379°C, 252°–273°C, and 195–218 × 10⁻⁷ K⁻¹, respectively. These properties are found to be controlled by their fundamental property, like the covalent character of the glasses, which is found to increase with an increase in Sb₂O₃ content. In addition, the devitrified glasses have been characterized by XRD and field emission scanning electron microscopy, which manifests the presence of nanozinc antimony oxide crystals with sizes of 21–43 nm. The exhibited properties have revealed that they are a new class of versatile materials.     

The Influence of Sb2O3 Addition on the Properties of Low-melting ZnO–P2O5 Glasses

The influence of 0–16 mol% Sb₂O₃ substitution for P₂O₅ on the properties of ZnO–P₂O₅ glasses has been investigated. It was shown that Sb₂O₃ could participate in the glass network and thermal stability of the glasses decreased with increasing Sb₂O₃ content. Glass transition temperature T _g, softening temperature T _s, and water durability all decreased firstly (up to 6 mol% Sb₂O₃ added) and then increased. Substitution of 12 mol% Sb₂O₃ led to a 16°C decrease in T _g and 30°C decrease in T _s, and weight loss of the glass was only 0.42 mg/cm², which is ∼11 times lower than that of the glass without Sb₂O₃ after immersion in deionized water at 90°C for 1 day. The glass containing 12 mol% Sb₂O₃ might be a substitute for Pb-based glasses in some applications.     

Ionically Conducting Composite Membranes from the Li2O–Al2O3–TiO2–P2O5 Glass–Ceramic

This paper reports processing of lithium ion-conducting, composite membranes comprised of 14Li₂O·9Al₂O₃·38 TiO₂·39P₂O₅ glass–ceramic and polyethylene. The processing involved tape casting of 14Li₂O·9Al₂O₃·38TiO₂·39P₂O₅ glass powder with organic additives into tapes, subjecting the green tape to binder burnout and thermal soaking in the temperature range of 950°–1100°C, and finally infiltrating the porous tape with polyethylene solution. The ionic conductivity and microstructure of 150–350 μm thick membranes were characterized and are discussed in this paper. The crystallites of the glass–ceramic show liquid-like conductivity at ambient temperature, whereas the grain boundary conductivity is lower by a factor of five. The lower grain boundary conductivity is explained on the basis of crystallographic mismatch and the existence of AlPO₄ at the grain boundary. The polyethylene infiltration in the porous membrane improved mechanical resilience with a minor adverse effect on conductivity.     

Crystallization of Monoclinic 2TiO2·5Nb2O5

Monoclinic 2TiO₂·5Nb₂O₅ crystallizes at 810° to 835°C from an amorphous material prepared by the simultaneous hydrolysis of titanium and niobium alkoxides. Crystallization isotherms are described by the contracting cube equation 1 − (1 − f)¹¹³= k(t − t₀); the activation energy is 315 kJ·mol⁻¹. Monoclinic 2TiO₂·5Nb₂O₅ transforms to the orthorhombic modification at ∼1200° to 1300°C.     

Studies in Germanium Oxide Systems: II, Phase Equilibria in the System Na2O—GeO2

Phase equilibrium relations in the system Na₂O-GeO₂ have been determined using standard quenching techniques supplemented by differential thermal analysis. Two congruently melting compounds, Na₂O·GeO₂ and 2Na₂O·9GeO₂, exist; the melting points are 1103°± 15°C and 1073°± 3°C, respectively. The eutectic temperature between GeO₂ and 2Na₂O·9GeO₂ is 950°±f 10°C at 94.5 wt GeO₂. The eutectic temperature between 2Na₂O · 9GeO₂ and Na₂O·GeO₂ is 790° f 10°C at about 75 wt% GeO₂. Both the refractive index and the density of glasses in the system Na₂O-GeO₂ exhibit maximum values at about 16 to 18 mole % Na₂O. The Ge-O-Ge absorption band at 890 cm⁻¹ shifts toward lower wave numbers with the addition of Na₂O.     

Subsolidus Phase Equilibria in the System Bi2O3-TiO2-Nb2O5

The subsolidus phase equilibria in the system Bi₂O₃-TiO₂-Nb₂O₅ at 1100°C were determined by solid-state reaction techniques and X-ray powder diffraction methods. The system was found to contain 4 ternary compounds, i.e. Bi₃TiNbO₉, Bi₇Ti₄NbO₂₁, a cubic pyrochlore solid solution having a compositional range of 3Bi₂O₃· x TiO₂ (7– x )Nb₂O₅ where x ranges from 2.3 to 6.75, and an unidentified phase, 4Bi₂O₃·11TiO₂·5Nb₂O₅.     

Studies in Germanium Oxide Systems: I, Phase Equilibria in the System Li2O—GeO2

Phase equilibrium relations in the system Li₂O-GeO₂ were determined using standard quenching techniques. In contrast to published literature five congruently melting compounds were found to exist. They are Li₂O·7GeO₂, 3Li₂O O·8GeO₂, Li₂O O·GeO₂, 3Li₂O O·2GeO₂, and 2Li₂O.-GeO₂. The melting points, respectively, are 1033°± 5°C, 953°± 5°C, 1245°± 15°C, 1125°± 15°C, and 1280°± 15°C. Simple binary eutectic relations exist among the compounds. The eutectic temperature between 1:7 and GeO₂ is 1025°± 1h0°C at about 96.8 wt% GeO₂; the eutectic temperature between the 1:7 and 3:8 compounds is 935°± 10°C at about 90.9 wt% GeO₂; the eutectic temperature between the 3:8 and 1:1 compounds is 930°± 10 °C at about 89.8 wt% GeO₂. Liquidus data for compositions richer in lithia than the 1:1 compound are only approximate because of the difficulty of quenching them; the phase relations between the 1:1 and 3:2 and between the 3:2 and 2:l compounds, however, are found to be of the simple binary eutectic type. The glass–forming region was also determined. Melts allowed to cool in air crystallized. When, however, the melts were quenched, glasses containing as much as 8 wt% GeO₂ could be prepared in 5–g quantities. Both the refractive index–composition and density–composition curves for the glasses showed maxi–mums at about 6 to 8 wt% Li₂O.     

Sintering, Crystallization, and Properties of B2O3/P2O5-Doped Li2O·Al2O3·4SiO2 Glass-Ceramics

Sintering, crystallization, microstructure, and thermal expansion of Li₂O·Al₂O₃·4SiO₂ glass-ceramics doped with B₂O₃, P₂O₅, or (B₂O₃+ P₂O₅) have been investigated. On heating the glass powder compacts, the glassy phase first crystallized into high-quartz s.s., which transformed into β-spodumene after the crystallization process was essentially complete. The effects of dopants on the crystallization of glass to high-quartz s.s. and the subsequent transformation of high-quartz s.s. to β-spodumene were discussed. The major densification occurred only in the early stage of sintering time due to the rapid crystallization. All dopants were found to promote the densification of the glass powders. The effect of doping on the densification can fairly well be explained by the crystallization tendency. All samples heated to 950°C exhibited a negative coefficient of thermal expansion ranging from about −4.7 × 10^-6 to −0.1 × 10^-6 K^-1. Codoping of B₂O₃ and P₂O₅ resulted in the highest densification and an extremely low coefficient of thermal expansion.     

The System K2O-BaO-SiO2

Phase equilibrium studies of compound formation and liquidus and solidus surfaces of the system K₂O-BaO-SiO₂ are presented. The system contains 3 ternary compounds: K₄BaSi₃O₉, K₈BaSi₁₀O₂₅, and K₂Ba₃Si₈O₂₀. Both high and low polymorphs of the third have fields on the ternary liquidus surface. Solid solution with SiO₂ depresses the high-low inversion from 1030°C at K₂Ba₃Si₈O₂₀ to 835°C at 70.2 mol% SiO₂. Data for 20 liquidus invariant points were found; 8 are thermal maxima and 12 are eutectics or peritectics. The isofracts of quenched glasses were determined.     

Crystallization of Li2O · Al2O3· 6SiO2 Glasses Containing Niobium Pentoxide as Nucleating Dopant

Niobium pentoxide (T form, orthorhombic system) was utilized to promote devitrification in Li₂O · Al₂O₃· 6SiO₂ glasses. Two or more mole percentage of this nucleating dopant enhanced crystallization in these glasses. Glasses containing 4.0 and 8.0 mol% T-Nb₂O₅ exhibited a high tendency to form dispersed TT-Nb₂O₅ (monoclinic system) precipitates during the glass quenching process. The crystallization process in glasses containing 2.0 or 4.0 mol% T-Nb₂O₅ occurred as microphase separation, followed by the formation of dispersed TT-Nb₂O₅ crystalline precipitates (760°C), followed by β-quartz solid-solution ( ss ) formation (850° to 900°C) heterogeneously nucleated from the precipitates. β-quartz( ss ) transformed to β-spodumene( ss ), along with a polymorphic transition from the TT-Nb₂O₅ to M-Nb₂O₅ (tetragonal system) crystalline phase.     

Phase Equilibria in the System Na2O – Nb2O5

Phase equilibria data, obtained both by differential thermal analysis and by quenching, are presented for the system Na₂O-Nb₂O₅. Five compounds corresponding to the formulas 3Na₂O.1Nb₂0₆, lNa₂O. 1Nb₂O₅, lNa₂O 4Nb₂O₆, lNa_zO.7Nb₂O₅, and lNa₂O. 10Nb₂O₆ have been found. The compound 3Na_z0.lNb₂O₅ melts congruently at 992°C. The compounds 1Na₂O. 4Nb₂O₆, lNa₂O.7Nb₂O, and 1Na₂O. 1Onb₂O₅ melt incongruently at 1265°, 1275°, and 1290°C., respectively. The well-known perovskite structure phase NaNbO₃ was found to melt congruently at 1412°C. The transition temperatures in NaNbO₅ were checked by thermal analysis and only the major structural changes at 368° and 640°C. could be detected. A new disordered form of NaNbO₃ could be preserved to room temperature by very rapid quenching.     

Internal Friction of Proton-Exchanged Li2O·Al2O3·2SiO2 Glass

Protons were introduced into the surface of an Li₂O·Al₂O₃·2SiO₂ glass fiber (0.5 mm in diameter) by ion exchange in NH₄HSO₄ at 366°C for 21 h. Infrared absorption measurements established that the protons were associated with bridging oxygen ions. After ion exchange, the magnitude of the alkali internal friction peak decreased and a new peak appeared at ∼220°C. This new peak is attributed to the interaction of alkali and hydrogen ions, independent of the presence of nonbridging oxygen ions.     

The Binary System Nb2O5— SiO2

The binary system Nb₂O₅— SiO₂ has been shown to include an extensive two-liquid region over the range 5 to 80% Nb₂O₅. The minimum temperature of the two-liquid area is 1695°C. A eutectic composition occurs at 95% Nb₂O₅ and 1448°C. and another at approximately 5% Nb₂O₅ and 1695°C. The experimental results were obtained by the cone-fusion method.     

Phase Diagram and Oxygen-Ion Conductivity in the Y2O3-Nb2O5 System

The phase equilibria in the Y₂O₃-Nb₂O₅ system have been studied at temperatures of 1500° and 1700°C in the compositional region of 0-50 mol% Nb₂O₅. The solubility limits of the C-type Y₂O₃ cubic phase and the YNbO₄ monoclinic phase are 2.5 (±1.0) mol% Nb₂O₅ and 0.2 (±0.4) mol% Y₂O₃, respectively, at 1700°C. The fluorite (F) single phase exists in the region of 20.1-27.7 mol% Nb₂O₅ at 1700°C, and in the region of 21.1-27.0 mol% Nb₂O₅ at 1500°C, respectively. Conductivity of the Y₂O₃- x mol% Nb₂O₅ system increases as the value of x increases, to a maximum at x = 20 in the compositional region of 0 ≤ x ≤ 20, as a result of the increase in the fraction of F phase. In the F single-phase region, the conductivity decreases in the region of 20-25 mol% Nb₂O₅, because of the decrease in the content of oxygen vacancies, whereas the conductivity at x = 27 is larger than that at x = 25. The conductivity decreases as the value of x increases in the region of 27.5 ≤ x ≤ 50, because of the decrease in the fraction of F. The 20 mol% Nb₂O₅ sample exhibits the highest conductivity and a very wide range of ionic domain, at least up to log p _O2=−20 (where p _O2 is given in units of atm), which indicates practical usefulness as an ionic conductor.     

Influences of Zirconia and Silicon Nucleating Agents on the Devitrification of Li2O · Al2O3· 6SiO2 Glasses

The effects of Si and ZrO₂ dopants on the crystallization and phase transformation process in Li₂O · Al₂O₃· 6SiO₂ glasses were investigated using differential thermal analysis, X-ray powder diffractometry (XRD), and high-resolution transmission electron microscopy (TEM) interactively. Phase separation was observed in the studied glasses prior to substantial crystallization. Elemental Si (1 mol%) significantly aided in glass devitrification. Dropletlike phase-separated regions in the as-quenched or heat-treated glass devitrified at ∼760°C, which in turn provided sites for the heterogeneous nucleation and growth of β-quartz(ss) (solid solution), which transformed to β-spodumene(ss) at higher temperature. Low-temperature surface crystallization in these glasses occurred as low as 760°C. ZrO₂ has limited solubility in this glass system. Small ZrO₂ crystallites (·5 nm) in the as-quenched glass acted as sites for the heterogeneous nucleation and subsequent growth of large (<5 μm) β-quartz(ss) crystals in glasses containing 1.0 mol% or more ZrO₂. The transformation from β-quartz(ss) to β-spodumene(ss) was increasingly inhibited with ZrO₂ additions. The nucleating efficiency of Si was significantly greater than that of ZrO₂ in this glass system.     

Mullite Crystallization from SiO2-Al2O3 Melts

SiO₂-Al₂O₃ melts containing 42 and 60 wt% A1₂O₃ were homogenized at 2090°C (∼10°) and crystallized by various heat treatment schedules in sealed molybdenum crucibles. Mullite containing ∼78 wt% A1₂O₃ precipitated from the 60 wt% A1₂O₃ melts at ∼1325°± 20°C, which is the boundary of a previously calculated liquid miscibility gap. When the homogenized melts were heat-treated within this gap, the A1₂O₃ in the mullite decreased with a corresponding increase in the Al₂O₃ content of the glass. A similar decrease of Al₂O₃ in mullite was observed when crystallized melts were reheated at 1725°± 10°C; the lowest A1₂O₃ content (∼73.5 wt%) was in melts that were reheated for 110 h. All melts indicated that the composition of the precipitating mullite was sensitive to the heat treatment of the melts.     

Energetics of La2O3–HfO2–SiO2 Glasses

A series of La₂O₃–HfO₂–SiO₂ glasses, approximately along the join 0.73SiO₂–0.27( x HfO₂–(1− x )La₂O₃), 0< x <0.3), was prepared using containerless processing techniques (aerodynamic levitation combined with laser heating in oxygen). The enthalpy of formation and enthalpy of vitrification at 25°C were obtained from drop solution calorimetry of these glasses and appropriate crystalline compounds in a molten lead borate (2PbO–B₂O₃) solvent at 702°C. The enthalpy of formation from crystalline oxides was exothermic and became less exothermic with increasing HfO₂ content. Heat contents were measured by transposed temperature drop calorimetry and depended linearly on the HfO₂ content. Differential scanning calorimetry showed that both the onset glass transition and the onset crystallization temperature of these glasses increased with increasing HfO₂ content. Upon slow cooling in air, the glasses crystallized to a mixture of baddeleyite, cristobalite, lanthanum disilicate, and hafnon.     

Spectroscopic Properties of Er3+-Doped Na2O–La2O3–Al2O3–SiO2 Glasses

Er³⁺-doped sodium lanthanum aluminosilicate glasses with compositions of (90− x )(0.7SiO₂·0.3Al₂O₃)· x Na₂O·8.2La₂O₃· 0.6Er₂O₃·0.2Yb₂O₃·1Sb₂O₃ (in mol%) ( x = 12, 20, 24, 40, 60 mol%) were prepared and their spectroscopic properties were investigated. Judd–Ofelt analysis was used to calculate spectroscopic properties of all glasses. The Judd–Ofelt intensity parameter Ω _t ( t = 2, 4, 6) decreases with increasing Na₂O. Ω₂ decreases rapidly with increasing Na₂O while Ω₄ and Ω₆ decrease slowly. Both the fluorescent lifetime and the radiative transition rate increase with increasing Na₂O. Fluorescence spectra of the ⁴ I _13/2 to ⁴ I _15/2 transition have been measured and the change with Na₂O content is discussed. It is found that the full width at half-maximum decreases with increasing Na₂O.