Structure-Property Relations in Lanthanide Borate Glasses

Glass formation in the system Ln₂O₃-B₂O₃ (Ln = Nd, Sm) was studied. Glasses could be formed in the range from 0 to -28 mol% rare-earth oxide (Ln₂O₃), but liquid immiscibility in these systems limits the range of homogeneous glasses to 0 to 1.5 and 25 to 28 mol% Ln₂O₃. The infrared spectra indicate that the rare-earth-rich glasses are structurally similar to rare-earth metaborates (LnB₃O₆) which contain (B₃O₆)- chains. The variation in density, transformation temperature, thermal expansion coefficient, and transformation-range viscosity of these glasses with the size of the rare-earth ion is discussed. Glasses near the metaborate composition have a transformation temperature of =700°C, which is high for binary borate glasses. Glasses could not be formed in the systems EU₂O₃-, Gd₂O₃-, HO₂O₃-, and Er₂O₃-B₂O₃, even by quenching at =1300°C/s. The sudden lack of glass formation in the system Ln₂O₃-B₂O₃ with Ln³⁺ ions smaller than Sm³⁺ is explained on the basis of the size effect of the Ln³⁺ ion on the stability of (B₃O₆)- chains in these metaborates.     

Structure and Properties of Silver Borate Glasses

Clear glasses form in the system Ag₂O-B₂O₃ up to about 35 mol% (65 wt%) Ag₂O. Infrared absorption, thermal expansion, and density data indicated an analogy to the Na₂O-B₂O₃ system. Pentaborate-triborate group pairs appear to be formed on addition of Ag₂O to B₂O₃ up to 20 mol% Ag₂O and diborate groups from 20 to 33 mol% Ag₂O. This interpretation is supported by the comparison of the infrared absorption spectra of quenched and crystallized glasses. One crystallization product, Ag₂O-4B₂O₃, was identified previously. A new compound starts to appear at 28 mol% Ag₂O. The theory that silver is generally present as a network modifier like sodium was substantiated by the comparison of the molar volume of sodium and silver borate glasses. Above 27 mol% Ag₂O some atomic silver is assumed to be present; below 15 mol%, exploratory studies indicate a two-phase structure within an immiscibility gap. A low-temperature internal friction peak in the glasses up to 28 mol% Ag₂O corresponds to the alkali peak in other glasses; a high temperature peak appearing in the 34 mol% Ag₂O glass is associated with the appearance of nonbridging oxygen in the system.     

Effects of Doping Trivalent Ions in Bismuth Borate Glasses

Bismuth borate glasses from the system: 40Bi₂O₃–59B₂O₃–1Tv₂O₃ (where Tv=Al, Y, Nd, Sm, and Eu) and three glasses of composition: 40Bi₂O₃–60B₂O₃, 37.5Bi₂O₃–62.5B₂O₃ and 38Bi₂O₃–60B₂O₃–2Al₂O₃ were prepared by melt quenching and characterized by density, UV-visible absorption spectroscopy and differential thermal analysis (DTA) studies. Bismuth borate glasses exhibit a very strong optical absorption band just below their absorption edge. Glasses were devitrified by heat treatment at temperatures above their glass transition temperatures and the crystalline phases produced in them were characterized by Fourier transform infrared (FTIR) absorption spectroscopy and X-ray diffraction (XRD). Bi₃B₅O₁₂ was found to be the most abundant phase in all devitrified samples. DTA studies on glasses and FTIR and XRD analysis on crystallized samples revealed that very small amounts of trivalent ion doping causes significant changes in the devitrification properties of bismuth borate glasses; rare-earth ions promote the formation of metastable BiBO₃–I and BiBO₃–II phases during glass crystallization.     

Properties of BaO–R2O3– Ga2O3–GeO2 (R = Y, Al, La, and Gd) Glasses

Barium gallogermanate glasses were prepared with substitutions of Al₂O₃, Y₂O₃, La₂O₃, and Gd₂O₃ for Ga₂O₃. The effects of these substitutions on the glass transformation temperature, viscosity, thermal expansion, and molar volume have been determined. The changes in properties associated with each substitutional ion are consistent with structural roles reported for these ions in other glasses. Aluminum acts as an intermediate with [AlO₄^–] tetrahedra substituting directly for [GaO₄^–] tetrahedra. Yttrium and gadolinium act as "atypical" modifier ions because of their large field strengths. Finally, the properties of the La₂O₃-substituted glasses indicate a possible dual structural role for La³⁺ ions in these glasses.     

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.     

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 .     

Properties and Structure of Lanthanum Borate Glasses

Glass formation limits were determined for the lanthanum borate glasses. Stable immiscibility prevents the formation of clear glasses over the range 0 to 20 mol% La₂O₃, but excellent quality glasses could be formed between 20 and 28 mol% La₂O₃. Data are reported for the density, refractive index, thermal expansion coefficient, glass transformation and dilatometric softening temperatures, transformation range viscosity, helium permeability, and chemical durability of these glasses. A limited Raman and infrared spectroscopy study suggests that lanthanum plays a similar structural role in these glasses and in the related crystals.     

Infrared Spectra and Atomic Arrangement in Fused Boron Oxide and Soda Borate Glasses

The infrared absorption spectra of fused B₂O₃ and of a series of soda borate glasses are presented. These spectra were obtained using vacuum-pressed briquettes of the powdered glass and powdered KBr. The spectrum of fused B₂O₃ shows quite definitely that this glass does not consist of a completely continuous triangularly coordinative network. It is shown that hydrogen bonds play an important part in the atomic arrangement of the glasses of zero or low soda content. The B₂O₃ glass apparently consists of complexes of an approximate unit (B₉O₁₄)- held together by hydrogen bonds. One in nine borons is tetrahedrally coordinated. The glasses of low soda content are similar. The spectra for soda concentrations greater than 15% did not permit the determination of the atomic arrangement with exactitude, but it is shown to be quite different from that found in glasses with 10% Na₂O or less.     

Structure and Nonlinear Optical Properties of Lanthanide Borate Glasses

The third–order nonlinear optical susceptibility, χ⁽³⁾, of lanthanide (lanthanum, praseodymium, neodymium, and samarium) borate glasses has been measured by the third harmonic generation method. The structure of the present glass system has been studied by infrared and Raman spectroscopic methods. The network structures of the present Ln₂O₃–B₂O₃ glasses have been confirmed to be basically similar to each other. Praseodymium, neodymium, and samarium borate glasses exhibit χ⁽³⁾ values that are larger than lanthanum borate glasses, because of the optical resonance effect, in accordance with the f – f transition. Especially, the χ⁽³⁾ value for 30Pr₂O₃·70B₂O₃ glass is 1.8 × 10⁻¹² esu, which is a factor of ∼60 larger than that of SiO₂ glass. This striking enhancement of χ⁽³⁾ is mainly attributed to the large transition moment to the first excitation state.     

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.     

Properties of Sm2O3─Al2O3─SiO2 Glasses for In Vivo Applications

The compositional range for glass formation below 1600°C in the Sm₂O₃─Al₂O₃─SiO₂ system is (9–25)Sm₂O₃─(10–35)Al₂O₃─(40–75)SiO₂ (mol%). Selected properties of the Sm₂O₃─Al₂O₃─SiO₂ (SmAS) glasses were evaluated as a function of composition. The density, refractive index, microhardness, and thermal expansion coefficient increased as the Sm₂O₃ content increased from 9 to 25 mol%, the values exceeding those for fused silica. The dissolution rate in 1 N HCl and in deionized water increased with increasing Sm₂O₃ content and with increasing temperature to 70°C. The transformation temperature ( T _g ) and dilatometric softening temperature ( T _d ) of the SmAS glasses exceeded 800° and 850°C, respectively.     

Gamma-Induced Absorption and Structural Studies of Arsenic Borate Glasses

Gamma-induced coloration in borate glasses containing arsenic was studied to secure information on: ( a ) the effect of addition of As₂O₃ on the induced coloration of the alkali borate base glass, and ( b ) the possibility of using radiation-induced coloration in studying structural changes. At concentrations of less than 10 mole % As₂O₃, arsenic takes network-modifying positions and at concentrations greater than 10.0 mole % As₂O₃, it takes network-forming positions. Between 15.0 and 25.0 mole % As₂O₃, an additional change in the structure was detected. This change may be confined to the mode of arrangement of the structural units associated with the formation of groups or compounds. Additional studies based on infrared absorption, ultraviolet absorption edge, and density measurements, as well as the chemical analysis of the glasses, confirmed the structural changes suggested from irradiation studies.     

Bismuth Phosphate Glasses

Homogeneous glasses are formed in the Bi₂O₃-P₂O₅ system up to 35 mol% (63.8 wt%) Bi₂O₃. In property vs composition plots, the thermal expansion coefficient and tan δ exhibit minima, and hardness and activation energy for conduction show maxima at about 20 mol% Bi₂O₃. The deformation temperature of the glasses also increases abruptly at this composition. This anomalous behavior is interpreted in terms of a change in the function of Bi³⁺ ions from network formers to network modifiers.     

Ultraviolet Absorption of Pentavalent Vanadium in Binary Alkali Borate Classes

The ultraviolet absorption of vanadium(V) in R₂O-B₂O₃ glasses (R represents Li, Na, or K) and in aqueous buffer solutions was investigated. In glasses of variable R₂O:B₂O₃ ratio, a change in the absorption spectra similar to the [VO₄]³- → [VO₃(OH)]^2- change in aqueous solution was observed. It is suggested that vanadium(V) in these glasses may be present as either [VO₄]^3- or [VO₃O_1/2]^2- (O_1/2 indicates a bridging oxygen ion), depending on the basicity of the melt. The critical R₂O concentration, above which this change in vanadium(V) environment occurs with increasing alkalinity, is 24, 26, and ≅30 mol%, for K₂O, Na₂O, and Li₂O, respectively.     

Dielectric Studies of Some Borate and Phosphate Glasses

Some borate glasses, with sag points of 350° to 500°C., of high specific resistance and low power factor have been prepared. The power factor of the glasses has been measured to 250° C., and the resistivity has been measured up to temperatures approaching the transformation region. Borate glasses have higher resistivities and power factors than phosphate glasses. An increase in sag point of lead borate glasses by substitution of PbO with BaO, Al₂O₃, or SiO₂ improves their dielectric properties. Depending on the type of glass, the dielectric constant varies linearly with the density of the glass.     

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.     

Preparation of Inorganic Compounds at Near Room Temperature by the Direct Conversion of Borate Glass in Solutions of the Corresponding Anions

The preparation of alkaline-earth chromate, selenite, and stannate compounds at near room temperature by the direct conversion of borate glass in aqueous solutions of the corresponding anions was investigated. Borate glass particles (150–300 μm) with the composition 20Na₂O·20CaO·60B₂O₃ or 20Na₂O·20BaO·60B₂O₃ (mol%) were prepared by conventional methods and immersed in dilute solutions of K₂CrO₄, K₂SeO₃, or K₂SnO₃ at 37°C. The conversion of the glasses was monitored using weight loss and pH measurements, while X-ray diffraction (XRD), X-ray fluorescence, and scanning electron microscopy were used to characterize the structure and composition of the products. After a reaction for 140–320 h, porous crystalline products identified by XRD as CaSeO₃.H₂O, CaSnO₃.3H₂O, BaCrO₄, and BaSeO₃ were obtained. The conversion of fibers (0.5–1.0 mm in diameter) of the Na₂O–BaO–B₂O₃ glass in K₂CrO₄ solution was pseudomorphic. The kinetics and mechanisms of the conversion process, as well as the structure of the products, are discussed.     

Borate Volatility from SOFC Sealing Glasses

The volatility of borate species from glasses developed for solid oxide fuel cell seals was studied using thermodynamic calculations and compared with experimental results. Vapor pressure diagrams were used to identify the most volatile compounds under a range of expected operational conditions, e.g. oxidizing and reducing atmospheres with water vapor, at temperatures in the range of 700°–1000°C. The species with the highest vapor pressures were BO₂( g ) under dry conditions and B₃H₃O₆( g ) under wet, reducing conditions. The depletion of boron from glass surfaces to depths beyond 100 nm was characterized using Auger electron spectroscopy depth profile analysis. Weight loss experiments were conducted on several different glass compositions. The cumulative weight loss from a glass with 20 mol% B₂O₃ ("glass #59") was about 10 times greater than from a glass with 2 mol% B₂O₃ ("glass #27"), under the same conditions. The activation energy for volatilization from glass #59 was 371±86 kJ/mol and was 272±65 kJ/mol for "glass #27." The cumulative weight loss of each composition in forming gas with 30% water vapor was greater than in dry air at 800°C. Volatile species were collected in a water trap, and these results confirmed predictions about the effect of atmosphere and B₂O₃ content on volatilization behavior.     

Elastic Properties and Molar Volume of Rare-Earth Aluminosilicate Glasses

The elastic properties, molar volume, and glass transition temperature ( T _g) of rare-earth-containing aluminosilicate glasses were investigated in the compositions of SiO₂–LnAlO₃ and SiO₂–Ln_3/4Al_5/4O₃, where Ln is Y, La, Nd, Eu, or Yb. The molar volume decreased with decreased ionic size of the Ln³⁺ ion, and T _g and elastic moduli increased in the same order. The Yb-containing glasses showed the highest Young's modulus among all the oxide glasses, even higher than the highest value ever known for glass containing Y₂O₃, as expected from the smaller ionic radius of Yb³⁺ than that of Y³⁺. The bulk modulus was found to be almost proportional to the inverse four-thirds power of the molar volume of glasses in each composition, indicating that Ln³⁺ ions can substitute for each other without changing the glass structure except for the size of the local structure around themselves. From the comparison of these properties, the structural role of rare-earth ions in these glasses is discussed.     

Anomalous Behavior of Ultrasonic Velocity in Rubidium Borate Glasses

By use of the ultrasonic pulse echo overlap method, ultrasonic velocity in rubidium borate glasses is measured as a function of composition at a temperature of 298 K, which is found to exhibit a maximum, a minimum, and another maximum in succession as the rubidium oxide content increases. This behavior is remarkable among the borate anomalies. The elastic property of these glasses is analyzed in terms of the three structural units defined as BØ₃, Rb⁺BØ₂O⁻, and Rb⁺BØ⁻₄, where Ø represents a bridging oxygen and O⁻ a nonbridging oxygen, from which the cause of this anomalous behavior is elucidated.