Reduction of Ions in Glass by Hydrogen

The reduction by hydrogen of Fe³⁺, Ce⁴⁺, and Sn⁴⁺ in soda-lime-silica glass was found to be diffusion-limited in the glass transition temperature range (500°C). The reduction was studied in fibers by chemical analysis and in bulk samples by optical absorption. Reduction to Fe²⁺, Ce³⁺, and Sn²⁺ proceeds by a reaction of the type H₂+2(ɛSi-O)^-+2Fe³⁺→2Fe²⁺+2(ɛSi–OH). Since the rate of reduction by hydrogen is quite similar for these systems and since reduction cannot be accomplished with CO, it is concluded that hydrogen is the diffusing species. A mechanism is developed in which hydrogen diffuses through the glass until it is trapped by a reducible ion.     

Kinetics of Nonisothermal Crystallization of Bi2Sr2CaCu2Ox Glasses with Different Copper Valence States

The kinetics of the nonisothermal crystallization process in Bi₂Sr₂CaCu₂O_x glasses with different copper valance states, R(Cu⁺) = Cu⁺/(Cu⁺+ Cu²⁺) = 0.79–0.99, were examined by using differential scanning calorimetry. Four different kinetic equations were used for the data analyses. It was found that the values of activation energy for crystal growth, E _a, decreased with increasing Cu⁺ content. The value of E _a, which was estimated from the modified Ozawa equation, for the glass with R(Cu⁺) = 0.79 was 483 kJ·mol^-1 and for the glass with R(Cu⁺) = 0.99 was 353 kJ·mol^-1. In the former glass, the number of crystal nuclei was almost independent of the heating rate. But, in the latter glass, the number of crystal nuclei varied significantly with the heating rate. The crystallization mechanism in the glasses was closely related to the thermal stability and viscosity of glasses: the glass with R(Cu⁺) = 0.99 showing a high thermal stability and a low activation energy for viscous flow had a small value of E _a. The plot of In ( T _p²/α) against 1/ T _p, where T _p and α are crystallization peak temperature and heating rate, respectively, gave a reasonable value of E _a for the glass with R(Cu⁺) = 0.79 but was unsuitable for the evaluation of E _a for crystal growth of the glass with R(Cu⁺) = 0.99.     

Influence of Atmosphere on the Final-Stage Sintering Kinetics of Ultra-High-Purity Alumina

The influence of sintering atmosphere on the final-stage sintering of ultra-high-purity alumina has been investigated. Model final-stage microstructures were tailored via a latex sphere impregnation and burnout technique. Critical experiments have been conducted to quantitatively examine the influence of the oxygen partial pressure on the final-stage sintering kinetics. Samples were sintered at 1850°C in either dry hydrogen ( P o₂∼ 3 × 10⁻¹⁷ atm) or wet hydrogen P o₂∼ 5 × 10⁻¹⁰ to 2 × 10⁻¹¹ atm), and their microstructures were characterized as a function of sintering time, Sintering in dry hydrogen decreased the susceptibility of the final-stage microstructure to pore/boundary breakaway. In the kinetic analysis, the variation in the number of pores per grain, N _g, was taken into account. It was found that in both atmospheres, the densification rate was controlled by grain boundary diffusion, and that sintering in dry hydrogen increased the densification rate by a factor of 2.25. In addition, it was determined that the grain growth rate in both atmospheres was controlled by the rate of surface diffusion of matter around the pores and that sintering in dry hydrogen enhanced the grain growth rate by a factor of 5.6. The overall effect of the dry hydrogen atmosphere was that it enhanced the coarsening rate relative to the densification rate by a factor of 2.5, and consequently shifted the grain size-density trajectory to much lower densities for a given grain size.     

Oxidation States of Chromium Dissolved in Glass Determined by X-ray Photoelectron Spectroscopy

The oxidation states of chromium dissolved in binary sodium silicate glasses were analyzed by X-ray photoelectron spectroscopy (XPS). The measured "equilibrium" Cr⁶⁺ fractions in glasses with varying Na₂O contents are shown to be in good agreement with results obtained by other techniques reported in the literature. XPS was then used to analyze the interface of a sodium silicate glass/chromium alloy (Inconel 718) seal. It is shown that Cr²⁺ and Cr³⁺ were the major chromium species dissolved in the glass interfacial region.     

Extrinsic OH− Absorption in Transparent Polycrystalline Lanthana-Doped Yttria

Transparent lanthana-doped yttria fabricated by transient solid second-phase sintering under wet hydrogen typically has a broad absorption band with a peak at 3.08 μm. The absorption band shift observed in samples treated in wet deuterium indicated that the 3.08-μm absorption was due to OH⁻ ions. The diffusion rates of hydrogen defects in lanthana-doped yttria were determined in the temperature range from 1000° to 1400°C. The changes in the concentrations of OH⁻ ions upon anneals were determined by measuring infrared absorbance at 3.08 μm. The diffusion coefficient is 1.3 × 10⁻⁷, 9.9 × 10⁻⁷, and 4.1 × 10⁻⁶ cm²/s at 1000°, 1200°, and 1400°C, respectively, with an activation energy of 140 kJ/mol. Annealing in a controlled oxygen partial-pressure environment can remove the OH⁻ absorption band and bring the total absorption in the 3- to 5-μm range closer to the intrinsic values.     

Study of Devitrification of Lithium Glass

An etching technique has been found which allows the devitrification phenomena in glass to be followed. Using this technique a study was made on 30 mole % Li₂O-SiO₂ glass at temperatures of 520°, 560°, and 600°C. By studying the average crystal size versus time at a given temperature, the growth rate was determined. It was found to be 7.5 × 10⁻⁴, 8 × 10⁻³, and 3 × 10⁻² cm. per hour for the three temperatures. The nucleation of crystals appeared to be linear with time at each temperature. An activation energy of 49 kcal. per mole was found for the growth process. Crystals were identified as Li₂Si₂O₅.     

Reactions and Bonding of Sodium Disilicate Glass with Chromium

Reactions and reaction mechanisms occurring at sessile drop interfacial regions of sodium disilicate glass on chromium were identified at 1000°C and an ambient atmosphere of 2.7 × 10⁻⁴Pa with a p(O₂) of 1 × 10⁻¹⁵ and 1 × 10⁻⁵ Pa. Depending on the experimental conditions, the principal reactions were redox reactions of Cr⁰ with Na⁺ to form Cr²+ and Na⁰, Cr²+ with Na⁺ to form Cr³+ and Na⁺, Cr⁰ with Si⁴+ to form CrSi_x, alloy dendrites and Cr²⁺, and Cr²⁺ with Si⁴⁺ to form Cr³⁺ and SiO(gas). Adherence was developed when the interfacial region was saturated with Cr³⁺, i.e., Cr₂O₃.     

Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy Study of Nonstoichiometric Silicon-Carbon-Oxygen Glasses

Crystallization behavior of Si-C-O glasses in the temperature range of 1000°–1400°C was investigated using transmission electron microscopy (TEM) in conjunction with electron energy-loss spectroscopy (EELS). Si-C-O glasses were prepared by pyrolysis of polysiloxane networks obtained from homogeneous mixtures of triethoxysilane, T^H, and methyldiethoxysilane, D^H. Si-C-O glass composition depended on the molar ratio of the precursors utilized. At a ratio of T^H/D^H= 1, the formation of a carbon-rich glass was observed, whereas a ratio of T^H/D^H= 9 yielded a Si-C-O glass with excess free silicon. Both materials were amorphous at 1000°C, but showed a distinct difference in crystallization behavior on annealing at high temperature. Although T^H/D^H= 1 revealed a small volume fraction of SiC precipitates in addition to a very small amount of residual free carbon at 1400°C, T^H/D^H= 9 showed, in addition to SiC crystallites, numerous larger silicon precipitates (20–50 nm), even at 1200°C. Both materials underwent a phase separation process, SiC _x O_2(1-x)→ x SiC + (1 - x )SiO₂, when annealed at temperatures exceeding 1200°C.     

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.     

Interdiffusion and Phase-Boundary Reaction Between Silicate Glass Surface and Aqueous CsCl Solution

Technical silicate glass samples with fresh fracture surfaces or HF-etched surfaces were treated in aqueous CsCl at 90°C for 10 to 85 min. Reaction profiles were determined quantitatively using a SIMS technique. The Cs₂O concentration at the glass surface increased with increasing time, followed by penetration of Cs⁺ ions into the glass. The profiles can be described assuming a model which contains both a surface phase-boundary process and an interdiffusion process in the glass for which D=5.10⁻¹⁷ cm² s⁻¹.     

Outgassing of Glass Caused by Thermal Neutrons and Gamma Radiation

Prebaked evacuated glass bulbs were exposed to gamma radiation and thermal neutrons. The gases initially present in the bulb plus those evolved in a subsequent baking were measured and analyzed with a mass spectrometer. Several glasses were exposed to a gamma dosage of more than 10⁷ rads. Small quantities of hydrogen as well as some CO, CO₂, and water were detected. Several boron-containing glasses were exposed to more than 10 ¹⁷ thermal neutrons/cm². The gas evolved was almost entirely helium, formed as a result of alpha-particle emission in the reaction The total quantity of helium formed can be predicted from the thermal neutron dose and the boron content of the glass. The role of helium permeability is also discussed.     

Microstructure and Properties of Co2+: ZnAl2O4/SiO2 Nanocomposite Glasses Prepared by Sol–Gel Method

Transparent bulk Co²⁺: ZnAl₂O₄/SiO₂ nanocomposites containing nanocrystalline Co²⁺: ZnAl₂O₄ dispersed in silica glass matrix were obtained by the sol–gel method. The gels of composition 89SiO₂–6Al₂O₃–5ZnO− x CoO ( x =0.2, 0.4, 0.6, 0.8, 1.0) (mol%) were prepared at room temperature by using two different aluminum salts, aluminum nitrate and aluminum alkoxide (aluminum-iso-propoxide, Al(OPrⁱ)₃), as starting materials. The transparent gels were converted to the crystalline phase of gahnite by heating above 900°C. The microstructural evolution of gels was characterized. The effect of Co²⁺ concentration on spectroscopic properties was also discussed. Co²⁺: ZnAl₂O₄ nanocrystals dispersed in the SiO₂-based glass are formed at lower heat-treatment temperature and shorter heating time by using Al(OPrⁱ)₃ as raw material.     

Gamma Radiation Effects on the Strength of a Borosilicate Glass

The effects of ⁶⁰Co gamma radiation on the strength-related mechanical properties of a borosilicate glass were examined. Although the glass darkened considerably, only a very slight densification was observed after irradiation to levels of 10⁸ rads. The strength distributions were not appreciably changed by the irradiation, nor was the calculated slow crack growth parameter, or N value. Neither did radiation affect the elastic modulus or the fracture toughness of the glass. Gamma radiation does not affect the strength below 10⁸ rads.     

Sodium Diffusion in SiO2 Glass

Diffusion of the radioactive tracer ²²Na in a commercial SiO₂ glass was investigated from 170° to 1000°C. The temperature dependence curve had discontinuities at about 573° and 250°C. The resulting Arrhenius equations are D = 3.44 × 10² exp(-21.1 kcal/RT) cm²/sec between 1000° and 573°C, D = 0.398 exp(-25.8 kcal/RT) cm²/sec between 573° and 250°C, and D = 2.13 exp(-28.3 kcal/RT) cm²/sec between 250° and 170°C. The two anomalies are discussed in terms of "quartz-like" and "cristobalite-like" precrystalline elements in the structure of the glass. Comparison of the Na diffusion in SiO, glass with that in soda-silica and soda-lime-silica glasses shows that SiO₂ glass occupies a boundary position with respect to these systems. A possible diffusion mechanism is discussed.     

Effect of Network Modifier on Spontaneous Emission Probabilities of Er3+ in Oxide Glasses

Compositional dependence of spontaneous emission probabilities of Er₃₊ was studied for silicate, borate, and phosphate glasses using the Judd–Ofelt theory. Through all of the glass systems, spontaneous emission probabilities of the ⁴ I _13/2→⁴ I _15/29→⁴ I _9/2→⁴ I _J, and ⁴ S _3/2→⁴ I _J ( J = 9/2, 11/2,13/2,15/2) transitions increased with increased ionic packing ratio of the glass host, which varied markedly with the type of network modifier. The effect of Er–O covalency on the transition probabilities was discussed from the spectral profile of the ⁴ I _15/2→⁴ I _3/2transition in terms of nephelauxetic effect.     

Effect of Alkali Ions on the Crystallization of 3BaO·5SiO2 Glass

In this study we have investigated the effects of alkali additions on the spherulitic crystallization process in 3BaO·5SiO₂ glass. Both Li⁺ and Na⁺ increased the growth rate of the spherulites and reduced the crystallization temperature from that in the base glass. Li⁺ and Na⁺ also reduced the temperature at which the microstructural transformation from spherulites to laths took place. Dielectric loss measurements suggest that the effect of the alkali additions on crystallization temperature is due to their concentration in grain boundaries, thereby lowering the viscosity of the remaining glass, and increasing mobility rates.     

Proton Conductivity in Zr4+-Ion-Doped P2O5–SiO2 Porous Glasses

The effect of zirconium ions on glass structure and proton conductivity was investigated for sol-gel-derived P₂O₅–SiO₂ glasses. Porous glasses were prepared through hydrolysis of PO(OCH₃)₃, Zr(OC₄H₉)₄, and Si(OC₂H₅)₄. Chemical bonding of the P⁵⁺ ions was characterized using ³¹P-NMR spectra. The phosphorous ions, occurring as PO(OH)₃ in the ZrO₂-free glass, were polymerized with one or two bridging oxygen ions per PO₄ unit with increased ZrO₂ content. The chemical stability of these glasses was increased significantly on the addition of ZrO₂, but the conductivity gradually decreased from 26 to 12 mS/cm at room temperature for 10P₂O₅·7ZrO₂·83SiO₂ glass. A fuel cell was constructed using 10P₂O₅·5ZrO₂·85SiO₂ glass as the electrolyte; a power of ∼4.5 mW/cm² was attained.     

Space-Charge Development in Glass

Data are given that graphically demonstrate space-charge build-up in a glass containing alkali and in a glass relatively free of alkali. The observed potential distributions show a decided asymmetry and have large potential drops near the electrodes with a linear potential change near the center of the sample. The observations qualitatively fit the expected theoretical distributions, according to Proctor and Sutton, for material having cations mobile and anions essentially immobile. Crude estimates arising from comparison with the theory yield carrier concentration near 2 × 10¹⁶ per cm³. mobility of the cation near 3 × 10^-6 cm.² per volt second, and diffusion constant near 2 × 10^-6 cm.² per second. These mobility and diffusion values are two orders of magnitude larger than values computed from conductivity data or computed from measurements of the diffusion coefficient for sodium cations, both in simple alkali silicates. According to the theory, these higher values are perhaps reasonable, since they are associated with only the more mobile portion of the cation population.     

Oxidation Equilibrium of Iron in Borosilicate Glass

Ferrous/ferric equilibria were determined in alkali-alkaline-earth borosilicate glass as a function of temperature, oxygen partial pressure, and glass composition. Expected linear relations are found between log(Fe²⁺/Fe³⁺) and log( p O₂) or 1/ T . The slopes of the correlation with 10g( p O₂) are near the expected value of -0.25, but are found to decrease with increasing temperature. Reaction enthalpies determined from the correlation with 1/ T of –100 to –116 kJ/mol are similar to those reported for other silicate glasses. The ferroudferric equilibrium is not dependent on total iron content in the range 0.5 to 0.09% Fe₂O₃. More reducing conditions are required at lower temperatures to stabilize the amber chromophore. The ferroudferric equilibria are correlated to the number of bridging and nonbridging oxygen ions in the glass. The results suggest that the oxidation-reduction reaction can be written as: Fe²⁺+ (¹/₄)O₂+ (³/₂)O^2-= FeO₂⁻