Visible Spectroscopy of Irradiated High-Alkali Borate and Mixed-Alkali Phosphate Glasses

Expectations of increased stability for trapped electrons in high-alkali glasses, based on extrapolations from observations on low-alkali borate glasses, are not borne out. In 69Na₂O-31B₂O₃ glass, electron centers have approximately the same thermal stability as in Na₂O·2B₂O₃ glass. In Na₂Oplus;P₂O₅ glasses the lifetimes, 3 · 0.5 μS, of transiently trapped electrons as well as their absorption spectra prove to be independent of increase of Na₂O content from 50 to 60 mol%. The same composition change destabilizes "permanent" hole centers. Exchange of Na₂O with K₂O in the metaphosphate glass also has no effect on the trapped electron lifetime. Small linear shifts in the trapped-hole absorption peak wavelengths are observed in the latter case. The most important positive finding in the phosphate glasses is a pronounced mixed-alkali effect on the yield of transiently trapped electrons and holes and of permanently trapped holes. The yield is a minimum at Na:K=1:1, due either to the elimination of trap sites or to the reduction of alkali ion mobilities which play a role in trap formation.     

Dependence of Crystallization Behavior of Sodium Diborate (Na2O·2B2O3) on Its Glass Structure and the Characteristics of Phase Transformation

The crystallization and phase transformation of glassy Na₂O·2B₂O₃ have been studied by X-ray diffraction, infrared spectroscopy, thermal analysis, and density measurement. By employing proper thermal treatment methods, three metastable and two relatively stable polymorphs of Na₂O·2B₂O₃ are obtained, as well as two other compounds, Na₂O·B₂O₃ and 2Na₂O·5B₂O₃. The possible structural groups of these phases are deduced, which are found to be similar to that existing in the glass. Current glass models which imply the existence of structural "granularity" satisfactorily describe the crystallization behavior. The phase transformation is observed to depend not only on temperature, but also on the density of the phase.     

Glass-Forming Ability and Crystallization Tendency Evaluated by the DTA Method in the Na2O–B2O3–Al2O3 System

In order to evaluate the crystallization tendency of glasses, the ratio of the crystallization temperature to the liquidus temperature ( T _c/ T _L) was obtained by DTA measurement for the Na₂O–B₂O₃ and Na₂O–B₂O₃–Al₂O₃ systems. The critical cooling rate for glass formation ( Q *) was also measured. The measurements were performed in the composition range of (100 − x )Na₂O–( x )B₂O₃, ( x = 25–35 and 60–100 mol%), and (100 − y )0.5Na₂O·0.5B₂O₃−( y )Al₂O₃, ( y = 6–34 mol%). The relationship between T _c/ T _L and Q * was discussed. A linear relationship between T _c/ T _L and log Q * for these systems was found. Furthermore, the relationship between T _c/ T _L and Q * was verified by computer simulation based on the crystallization kinetics of glass or supercooled liquid.     

Comparison of the Formation of Calcium and Barium Phosphates by the Conversion of Borate Glass in Dilute Phosphate Solution at near Room Temperature

The formation of phosphates of calcium or barium at near room temperature by the direct conversion of borate glass in dilute phosphate solution was investigated. Borate glass particles (150–300 μm), with the composition 20Na₂O·20AO·60B₂O₃ (mol%), where A is the alkali-earth metal Ca or Ba, were prepared by conventional processing, and immersed in 0.25 M K₂HPO₄ solution at 37°C and with a starting pH value of 9.0 or 12.0. The effects of the borate glass composition and the solution pH on the rate of formation, the chemical composition, and the structure of the phosphate products formed in the conversion reaction were investigated using weight loss and pH measurements, X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and Brunauer–Emmett–Teller surface area. At both pH values, the calcium borate glass particles were completely converted to hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂, with a high surface area (160–170 m²/g). At pH=9.0, the barium borate glass particles converted completely to a barium phosphate product that was isostructural with BaHPO₄, whereas at pH=12.0, the barium phosphate product was isostructural with Ba₃(PO₄)₂, with both products having much lower surface areas (4–8 m²/g). The formation of the phosphate products was pseudomorphic, retaining the external shape of the original glass particles.     

High-Temperature Energy Relations in the Alkali Borates: Binary Alkali Borate Compounds and Their Glasses

The energy relations existing between the congruently melting compounds Li₂O -2B₂O₃, Na₂O–2B₂O₃, Na₂O-4B₂O₃, and K₂O-4B₂O₃ and their glasses have been determined for the temperature range 25° to 1100°C. High-temperature heat-content, entropy, and heat of solution data are given for both the glasses and the corresponding devitrified materials. A comparison of the heats of fusion of the alkali borates on a gram atom of oxygen basis shows that they follow the order Li > Na > K. The entropy differences between the glass and the corresponding crystalline material have been determined at 25°C. The free-energy change at 25°C. for the reaction crystal → glass has been calculated for the four compounds.     

Synthesis of Titanate Derivatives Using Ion-Exchange Reaction

Two types of titanate derivatives, layered hydrous titanium dioxide (H₂Ti₄O₉· n H₂O) and potassium octatitanate (K₂Ti₈O₁₇) with a tunnellike structure, were synthesized using an ion-exchange reaction. Fibrous potassium tetratitanate (K₂Ti₄O₉· n H₂O) was prepared by calcination of a mixture of K₂CO₃ and TiO₂ with a molar ratio of 2.8 at 1050°C for 3 h, followed by boiling-water treatment of the calcined products for 10 h. The material then was transformed to layered H₂Ti₄O₉· n H₂O through an exchange of K⁺ ions with H⁺ ions using HCl. K₂Ti₈O₁₇ was formed by a thermal treatment of KHTi₄O₉· n H₂O. Pure KHTi₄O₉· n H₂O phase was effectively produced by a treatment of K₂Ti₄O₉ with 0.005 M HCl solution for 30 min. Thermal treatment at 250°–500°C for 3 h resulted in formation of only K₂Ti₈O₁₇.     

Growth of α-Al2O3 Crystals by Na2O Evaporation

A technique for growing α-Al₂O₃ crystals is described in which Na₂O·11Al₂O₃ is dissolved in a liquid of composition Na₂O·4TiO₂·3Al₂O₃. Alpha Al₂O₃ is precipitated as Na₂O evaporates from the system; Na₂O·11Al₂O₃ serves as a source of Al₂O₃, and Na₂O in the liquid. The content of solids in the mixture is always such that it does not melt completely. The size of the α-Al₂O₃ crystals grown is related to the Na₂O content of the composition. Crystals as large as 4000 by 3000 μm in the α-axis direction and 500 μm in the c -axis direction have been grown.     

Viscosity of Molten Na2O─B2O3 Slags

The viscosity of sodium borate slags at high Na₂O concentrations (37.3 to 49.4 mol%) and high temperatures (1000° to 1300°C) follows an Arrhenius-type relationship. This relationship was also observed for sodium borate slags (mass% Na₂O/mass% B₂O₃= 0.86) containing CaO and CaF₂ for the same temperature range. There has been a reduction in viscosity of the sodium borate slags (mass% Na₂O₃mass% B₂O = 0.53 to 0.86) with increase in Na₂O concentration. On adding CaO (10 to 50 mass%) to the sodium borate slag (mass% Na₂O/mass% B₂O₃= 0.86), the viscosity increased considerably, while an addition of CaF₂ (S to 15 mass%) to the slag (30.9 mass% Na₂O₃ 35.8 mass% B₂O₃, 33.3 mass% CaO) decreased the viscosity. The average activation energies of Na₂O─B₂O₃, Na₂O─B₂O₃─CaO₃ and Na₂O─B₂O₃─CaO─CaF₂ slag systems have been estimated as 14.6, 124.7, and 41.4 kJ/mol, respectively, for the given composition ranges and 1000° to 1300°C temperature range.     

Conductivity Activation Energy Relations in High-Sodium-Content Borate and Aluminoborate Glasses

Glasses were prepared having compositions x Na₂O + (1 − x )B₂O₃ and x Na₂O + (1 − x )( n B₂O₃·Al₂O₃) where n = 4.0 and 6.7, and their conductivities and activation energies were determined up to 70 mol% Na₂O. Below 50 mol% Na₂O, the conductivity activation energy decreases with Na₂O content, which is attributed to a decreasing cation jump distance. Above 50 mol% Na₂O, however, the activation energy increases with Na₂O content. These measurements are the first to clearly show a conductivity activation energy increasing with alkali oxide content, an effect which is not yet explained by any of the current models of ionic conduction in glass.     

Studies in Lithium Oxide Systems: VII, Li2O–B2O2–SiO2

Phase relations in the system Li₂O–B₂O₃–SiO₂ were studied by quenching and solid-state reactions. No ternary compounds were detected in the portion of the system containing less than 53% Li₂O. Compatibility triangles were formed from the binary borate and silicate compounds. Liquidus data obtained by quenching are reported for four joins, Li₂O·2SiO₂–Li₂O·2B₂O₃, Li₂O·SiO₂-Li₂O·2B₂O₃, Li₂O·SiO₂-Li₂O·B₂O₃, and Li₂O·2B₂O₃-SiO₂. The last join cuts across the two-liquid region and is not a true binary system. Some probable ternary invariant points were located in the portion of the system which was quenchable to glass and adjacent to the two-liquid region. Further data on the previously reported immiscible liquid formation are given and the significance is discussed. Data on the thermal expansion behavior of certain glasses are presented.     

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.     

Purification of Mullite by Reduction and Volatilization of Impurities

Mullite materials usually contain a residual glassy phase rich in SiO₂, which concentrates impurities as Na₂O, K₂O, Fe₂O₃, and other minority compounds. A suitable way to minimize this glassy phase is the reduction and volatilization of its components by calcination at high temperatures (1300–1450°C) in atmospheres with a very low partial pressure of O₂. Over 95% of the Na₂O, K₂O, and Fe₂O₃ in mullite can be removed in this way, leaving concentrations lower than 0.02% by weight. To avoid the degradation of mullite that occurs when the partial pressure of O₂ is too low, the material to be purified is covered with TiO₂ plates.     

Energy Relations in Binary Alkali Borates

The heats of solution of glasses and devitrified alkali borates in the systems Li₂O–B₂O₃, Na₂O—B₂O₃, and K₂O–B₂O₃ were measured in 2 N nitric acid with a simple vacuum-bottle calorimeter. The standard deviation was 0.35 cal. per gm. for approximately 130 determinations. From these data, together with available thermodynamic data, heats of formation were calculated for alkali borate glasses and devitrified alkali borates from two combinations of possible reactants. All the curves for the heats of solution of glasses showed minima at 20 mole % alkali oxide, but similar curves for devitrified alkali borates showed minima at 20 mole % for potassium and sodium borates only. The curve for devitrified lithium borates showed a minimum at 25 mole % alkali. The conclusion was drawn that there is probably no congruently melting Li₂O.4B₂O₃ compound.     

Solubilities of Ar, N2, CO2, and He in Glasses at Pressures to 10 Kbars

Glasses containing up to several percent of Ar, N₂, and CO₂ were prepared at 2000 to 150,000 psi and up to 950°C and retained under ambient conditions. The solubilities are presented as a function of pressure, temperature, and composition of glass. Solubilities of O, He, and H₂O were also investigated in various glass compositions, especially K₂O–4SiO₂ and B₂O₃. The evolution of the gases at atmospheric pressure was followed by electron microscopy and density measurements.     

Influence of Al2O3, ZnO, and K2O on the Opacity of Fluoride-Opacified Glasses

In the glass SiO₂ 71, Na₂O 17, CaO 12% with 7.5 parts per hundred of fluorine added to the batch, substitution of up to 6% ZnO for CaO produced a great increase in the opacity; substitution of Al₂O₃ for SiO₂ or of K₂O for Na₂O produced much smaller effects, which were dependent on the composition and were inappreciable in the presence of 6% ZnO. Despite the differences in opacity, no differences in fluorine content were detected. No support was found for the belief that Al₂O₃ is essential to the successful opacification of a glass by means of fluorides.     

CO2 Retention in High-Alkali Borate Glasses

A study of the high-alkali region of glass formation in the system Na₂O +B₂O₃ reveals that retention of CO₂ from carbonate starting materials can become a serious preparative problem at the high-alkali extreme. Results presented for glasses prepared using both Na₂O and Na₂CO₃ show that residual CO₂ can lead to major differences in physical properties which in this work are represented by the viscosity-related glass transition temperature .     

Time–Temperature–Transformation Diagrams for Borosilicate Glasses and Preparation of Chemically Durable Porous Glasses

To clarify the effects of glass composition and heat-treatment conditions on phase separation and crystallization, time–temperature–transformation (TTT) diagrams for the system (65 − x )SiO₂·25B₂O₃·(10 − y )Na₂O· y CaO· x ZrO₂, which is used for fabricating alkali-durable porous glasses, were determined. The microstructures and properties, such as resistance to alkaline attack as a function of ZrO₂ content, of the porous glasses fabricated on the basis of the developed TTT diagrams were examined. CaO and ZrO₂ additions were shown to affect the locations of the noses in the TTT curves for phase separation and crystallization. The substituting of CaO for Na₂O was effective for retaining ZrO₂ in the skeleton of the porous glasses. The alkali resistance of ZrO₂-containing porous glasses was 8–10 times superior to that of the porous glass without ZrO₂. The results demonstrate that TTT diagrams can be used as guides for heat-treatment scheduling in processing or predicting of glass-forming ability and the onset of phase separation and crystallization in glass-forming systems.     

Reduction of Eu3+ to Eu2+ in Aluminoborosilicate Glasses Prepared in Air

Eu₂O₃-doped aluminoborosilicate glasses were prepared in air at high temperature. Luminescence measurements were used to investigate a valence change from Eu³⁺ to Eu²⁺ ions in the aluminoborosilicate glasses. The results showed that the doped Eu³⁺ ions were partially reduced to Eu²⁺ in the Eu₂O₃:RO–Al₂O₃–B₂O₃–SiO₂ (RO=CaO, SrO, BaO, Li₂O) glasses, but not in the Eu₂O₃:RO–Al₂O₃–B₂O₃–SiO₂ (RO=Na₂O, K₂O) glasses. The changes of Eu reduction with different RO components were discussed with the variation of optical basicity of RO and with different valency of R cations. The effects of co-doping BaO and ZnO in aluminoborosilicate glasses on Eu reduction were also investigated and discussed.     

Prominent Nanocrystallization of 25K2O·25Nb2O5·50GeO2 Glass

Some K₂O-Nb₂O₅-GeO₂ glasses are prepared, and their crystallization behaviors are examined. 25K₂O·25Nb₂O₅·50GeO₂ glass with the glass transition temperature T _g= 622°3C and crystallization onset temperature T _x= 668°3C shows a prominent nanocrystallization. The crystalline phase is K_3,8Nb₅Ge₃O_20,4 with an orthorhombic structure. The sizes of crystals in the crystallized glasses heat-treated at 630° and 720°3C for 1 h are °10 and 20–30 nm, respectively, and the crystallized glasses obtained by heat treatments at 620°-850°3C for 1 h maintain good transparency. The density of crystallized glasses increases gradually with increasing heat-treatment temperature, and the volume fraction of crystals in the sample heat-treated at 630°3C for 1 h is estimated to be ∼35%. The usual Vickers hardness and Martens hardness (estimated by nanoindentation) of 25K₂O·25Nb₂O₅·50GeO₂ glass change steeply by heat treatment at T _g, i.e., at around 35% volume fraction of nanocrystals. The present study demonstrates that the composite of nanocrystals and the glassy phase has a strong resistance against deformation during Vickers indenter loading in crystallized glasses.     

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.