Behavior of Bubbles in Glassmelts: III, Dissolution and Growth of a Rising Bubble Containing a Single Gas

Finite difference solutions of the mass transport equations governing the dissolution (growth) of a rising gas bubble, containing a single gas, in a glassmelt were obtained. These solutions were compared with those obtained from an approximate procedure for a range of the controlling parameters. Applications were made to describe various aspects of O₂ and CO₂ gas-bubble behavior in a soda-lime-silicate melt.     

Influence of Batch Moisture and Atmosphere on Melting Behavior of Lead Oxide-Containing Glass

Melting behavior in the sand-potash-lead oxide system was found to depend strongly on the amount of H₂O added to the batch (between 0.1 and 9.0 wt%) and also on the melting atmosphere (air, N₂, and air loaded with 30 vol% H₂O). Pb₃O₄ and PbO (red), which were used as raw materials, behave similarly. The newly formed compounds are Pb₃O₄ (formed only when PbO (red) was used as a raw material), PbO (yellow), and K₂Pb₂Si₂O₇. The addition of 2 wt% As₂O₃ to the batch influences the processes of formation and dissolution of compounds only in some cases.     

Indirect Dissolution of (Al, Cr)2O3 in CaO—MgO—Al2O3—SiO2 (CMAS) Melts

The dissolution of (Al, Cr)₂O₃ into CaO—MgO—Al₂O₃—SiO₂ melts, under static and forced-convective conditions was investigated at 1550°C in air. With sufficient MgO in the melt, or sufficient Cr₂O₃ in (Al, Cr)₂O₃, a layer consisting of a spinel solid solution, Mg(Al, Cr)₂O₄, formed at the (Al, Cr)₂O₃/melt interface. The dissolution kinetics of 1.5 and 10 wt% Cr₂O₃ specimens were determined as a function of immersion time, specimen rotation rate, and magnesia content of the melt. Electron microprobe analysis was used to characterize concentration gradients in the (Al, Cr)₂O₃ sample, the Mg(Al, Cr)₂O₄ spinel, or in the melt after immersion of specimens containing 1.5 to 78 mol% Cr₂O₃. The dissolution kinetics and microprobe analyses indicated that a steady-state condition was reached during forced-convective, indirect (Al, Cr)₂O₃ dissolution such that spinel layer formation was rate limited by solid-state diffusion through the spinel layer and/or through the specimen, and spinel layer dissolution was rate limited by liquid-phase diffusion through a boundary layer in the melt. This is consistent with a model previously developed for the indirect dissolution of sapphire in CMAS melts.     

Indirect Dissolution of Sapphire into Calcia-Magnesia-Alumina-Silica Melts: Electron Microprobe Analysis of the Dissolution Process

Spinel, MgAl₂O₄, has been observed to form on sapphire during sapphire dissolution into CaO-MgO-Al₂O₃-SiO₂ (CMAS) melts at 1450°. and 1550°C. Electron microprobe analysis was used to characterize the sapphire/melt interface for cases in which spinel did (indirect dissolution) or did not (direct dissolution) form on the sapphire during dissolution into CMAS melts. The concentrations of Al₂O₃, MgO, CaO, and SiO₂ were determined as a function of position within the spinel reaction product and in the adjacent melt. The rate-limiting steps for direct and indirect sapphire dissolution into CMAS melts are discussed.     

Dissolution Rate of Silicon Nitride Ceramics into Molten CaO–Al2O3–SiO2 Slags

The dissolution rates of silicon nitride (Si₃N₄) ceramics into CaOAl₂O₃SiO₂ slags were investigated by using a rotating specimen method in the temperature range of 1773–1873 K. Dissolution rates in the present study increased as the revolution speed and temperature increased and decreased as the SiO₂ content of the slags increased. The nitrogen content of the slags increased after the Si₃N₄ ceramics had been immersed into them. The slags contained two types of nitrogen ions—N³⁻ and CN^-—because a graphite crucible was used for the experiment. N³⁻ ions were confirmed in all the slags that were used in the present work; the CN^- content was much lower than that of the N³⁻ ions, except in the slag without SiO₂. Based on those results, Ficks law of diffusion was used to analyze the dissolution rates. The dissolution mechanism of the Si₃N₄ ceramics into CaO–Al₂O₃SiO₂ slags has been discussed in this paper.     

Critical Microstructures for Microcracking in Al2O3-ZrO2 Composites

The internal strains asSociated with the martensitic phase transformation of zirconia were used to introduce microcracks into Al₂O₃/ZrO₂ composites. The degree of transformation was found to be dependent on the volume fraction of ZrO₂ and its size, the latter of which could be controlled by suitable heat treatments. The microstructural changes that occurred during the heat treatments were studied using quantitative microscopy and X-ray diffraction. For materials containing more than 7.5 vol% Zr0₂, the ZrO₂ particles were found to pin the Al₂O₃ grain boundaries, thus limiting the Al₂O₃ grain growth. The limiting grain size was found to be dependent on size and volume fraction of ZrO₂. Heat treatments for the higher volume fraction materials (>7.5 vol% ZrO₂) caused micro-structural changes which resulted in increased amounts of monoclinic ZrO₂ at room temperature; elastic modulus measurements indicated that this was occurring concurrently with microcracking. By combining the ZrO₂ grain-size distributions with the X-ray analysis it was possible to calculate the critical ZrO₂ size required for the transformation. The critical size was found to decrease with increasing amounts of ZrO₂. Hardness and indentation fracture toughness were measured on the composites. Grain fragmentation was observed at the edge of the indentations and microcracks were observed directly, using an AgNO₃ decoration technique, near the indentations.     

Processing Impurities as Phase Assemblage Modifiers in Titanate Nuclear Waste Ceramics

The tolerance of titanate nuclear waste ceramics to fluctuations in the concentrations of processing contaminants was monitored using X-ray diffraction, electron microscopy, secondary ion mass spectrometry, and analysis of dissolution liquors. Several waste forms were fabricated, all of which contained idealized Purex waste simulant and, in addition, varying quantities of the common waste stream impurities P₂O₅, MgO, Fe₂O₃, Na₂O, and SiO₂. Incorporation of the oxides individually stabilized new phases including monazite (P₂O₅), pseudobrookite (MgO), loveringite (Fe₂O₃), freudenbergite (Na₂O), and pollucite (SiO₂)—only the latter phase deleteriously affected waste form performance by promoting cesium dissolution. However, when the processing contaminants were added simultaneously, a number of synergetic effects, particularly the stabilization of a soluble glassy phase, resulted in elemental losses which were an order of magnitude greater for some matrix and radwaste species. It was found that up to 25 wt% of the idealized Purex waste could be incorporated in the waste without diminution of its properties.     

Preparation of a Calcium Titanium Phosphate Glass–Ceramic with Improved Chemical Durability

A calcium titanium phosphate glass–ceramic for use as a dental material with excellent chemical durability was derived from a mother glass with a small amount of fluorine. The laser Raman spectroscopic analysis showed that 35CaO–10CaF₂–30P₂O₅–25TiO₂ glass, as the nominal composition, consists of ortho-, pyro-, and meta-phosphate groups. On heating the glass at 865°C, orthophosphate crystals, such as fluorine-containing oxyapatite (Ca₁₀(PO₄)₆(O,F₂)) and the Nasicon-type phase (CaTi₄(PO₄)₆), were preferentially precipitated; the apatite particles of several tens of nanometers in size were embedded in the CaTi₄(PO₄)₆ phase. The pale bluish color of the glass–ceramic indicated that titanium ions were included in the residual glassy phase. When the glass–ceramic was treated with dilute hydrochloric acid, only the apatite particles at the surface were leached out, while no CaTi₄(PO₄)₆ phase was etched; the dissolution of the glass–ceramic was effectively controlled. Almost no dissolution of ions from the glass–ceramic occurred in water. It was suggested that the behavior is a result of the microstructure of the glass–ceramic, which consists of crystalline and glassy phases with excellent chemical durability.     

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.     

Corrosion Behaviors of Zirconia Refractory by CaO–SiO2–MgO–CaF2 Slag

In this work the corrosion behaviors of zirconia refractory (partially MgO-stabilized zirconia) was investigated in CaO–SiO₂–MgO–CaF₂ slag with varying CaF₂ content at 1873 K. To figure out the corrosion mechanism, the characteristics of present slag at high temperature were examined in terms of melting temperature and vaporization behaviors. Corrosion experiment and melting temperature measurement were carried out by heating microscope (HM) and the vaporization phenomenon was investigated by thermo gravimetry–differential scanning calorimetry. After experiment, the corroded interfaces of zirconia refractory by slag were analyzed by scanning electron microscope-energy dispersive spectroscopy and electron probe microanalysis. With an addition of CaF₂, three different layers were formed at the interface of slag and zirconia refractory. Furthermore, the corrosion behaviors of zirconia refractory were found to be continuously accelerated with an increase of CaF₂ which facilitated the dissolution of intermediate compound. On the other hand, melting temperature of CaO–SiO₂–MgO–CaF₂ slag showed no continuous decrease with an increase of CaF₂. Also, considerable vaporization of fluoride gas was occurred in high CaF₂ containing slag during HM experiment which might cause a gradual change of slag composition and also environmental pollution. From the results, present study suggested that a proper amount of CaF₂ should be added when it is used for enhancing refining capacity of slag in order not to cause any severe damage of zirconia-based refractory by slag.     

Reaction Densification of α'-SiAION: II, Densification Behavior

Reaction hot-pressing behavior of α-Si₃N₄, Al₂O₃, A1N, and M₂O_Z powder mixtures (M = Li, Mg, Ca, Y, Nd, Sm, Gd, Dy, Er, and Yb) forming α'-SiAlON has been studied. Five characteristic temperatures are found to control the densification behavior of these materials. The densification proceeded in three major stages. The first two stages were formation of ternary oxide eutectic and wetting of majority nitride powder. The third stage involved dissolution/melting of intermediate phase. Variation from this behavior sometimes occurs due to localization of wetting liquid at A1N, extremely high melting/dissolution temperature of Mg₂Al₄Si_sO₁₈ and Nd and Sm melilite, and secondary precipitation of Dy-α'-SiAlON. The dominant densification mechanism was found to be massive particle rearrangement, irrespective of the wetting and dissolution/melting behavior. The efficiency of this mechanism is mostly affected by the amount of available liquid and less by its viscosity. Fully dense, single-phase ceramics were obtained in all cases except Mg when hot-pressed at a constant heating rate to 1750°C, and considerably lower temperatures for Li, Ca, Gd, Dy, Er, and Yb-SiAlON when held isothermally.     

Aqueous Colloidal Characterization and Forming of Multimodal Barium Titanate Powders

Aqueous suspensions of two barium titanate (BaTiO₃) powders, with 100 and 500 nm nominal size, were found to be stabilized by a predominantly steric mechanism when using a polyelectrolyte surfactant, an ammonium salt of poly(methacylate) (PMA-NH₄). Considerable amounts of barium ions were shown to readily dissolve from BaTiO₃ powders in an aqueous environment, even without the addition of acid or base. This barium dissolution obscured the accurate measurement of the isoelectric point of the two powders. Suspension stabilization was observed to occur at basic pH when using PMA-NH₄. Concentrated suspensions of each individual powder exhibited shear thinning behavior, with the onset of shear thickening occurring at relatively high shear rates. Suspensions containing 85 vol% coarse/15 vol% fine powders demonstrated the lowest apparent viscosity for a given solids loading, while the highest sintered density was obtained with a mixture of 70 vol% coarse/30 vol% fine, when sintering at 1300°C for 2 h (97.4% of theoretical, slip cast from a suspension containing 50 vol% solids).     

Microstructure of Ti(CN)–WC–NbC–Ni Cermets

An investigation of the microstructural evolution and dissolution phenomena in a Ti(C_0.7N_0.3)– x WC– y NbC–20Ni system is reported. In Ti(C_0.7N_0.3)– y NbC–20Ni systems, a phase separation occurs between the Ti(CN) core and the (Ti,Nb)(CN) rim phases when the system contains >15 wt% NbC. This phase separation results from the increased misfit between the cores and the solid-solution rim phases with the addition of NbC. Based on data obtained from a previous study and compositional analyses of the rim structure of the Ti(C_0.7N_0.3)– y NbC–20Ni system, the average dissolution rates of WC and NbC appear to be approximately the same with respect to that of Ti(CN), under given sintering conditions (1510°C for 1 h). In addition, compositional changes in the rim structure of the Ti(C_0.7N_0.3)– x WC– y NbC–20Ni system are compared with those for a Ti(C_0.7N_0.3)– x WC–20Ni system to explain the effect of NbC on WC dissolution in the Ti(C_0.7N_0.3)–WC–NbC–Ni system. The presence of NbC in the Ti(C_0.7N_0.3) –x WC–20Ni system is found to suppress the dissolution of WC.     

Melting and Interface Reaction of Mn–Si–O Glass on BaTiO3

A glass with the eutectic composition 3MnO_1.5–2SiO₂ was used to simulate the formation of a liquid phase during sintering of BaTiO₃. Two oxide additives (Mn₂O₃ and SiO₂) performing various functions of the properties of BaTiO₃ were investigated for their crystallization and thermal characteristics at temperatures ≤1400°C. The wetting behavior of the glass, the dissolution of BaTiO₃ in glass melt, the identification of newly formed phases, and the sequential reaction kinetics of the glass/BaTiO₃ system, especially when isothermally treated at 1150°C, were investigated by electron microscopy with quantitative X-ray energy dispersive spectroscopic (Q-EDS) analysis. The evolution of the interfacial reaction of the glass/BaTiO₃ at 1150°C is reported and discussed.     

Rapid Hardening of Cement by the Addition of a Mechanically Activated Al(OH)3–Ca(OH)2 Mixture

Rapid hardening of cement was achieved in the present study by adding a mechanically activated Al(OH)₃–Ca(OH)₂ mixture to the starting cement paste. Among the dominant parameters for hardening were the mechanical treatment time for the Al(OH)₃ powder and the Al(OH)₃/Ca(OH)₂ ratio. The hardening mechanisms are discussed here in terms of the ionic concentration of the solution and the hydration products created when the Al(OH)₃–Ca(OH)₂ mixture was added to water. Mechanical activation of the Al(OH)₃ powder accelerated dissolution into an aqueous alkaline solution and induced the formation of calcium aluminate hydration products. Those hydration products increased the compressive strength of the cement paste at a very early stage of hardening.     

Acid Leaching of SHS Produced Magnesium Oxide/Titanium Diboride

The stoichiometric self-propagating high-temperature synthesis (SHS) thermite reaction involving magnesium (Mg), titanium dioxide (TiO₂), and boron oxide (B₂O₃) forms MgO and titanium diboride (TiB₂) as final products. Selective acid leaching is used to remove the MgO leaving TiB₂ powder. This study investigates the acid leaching of SHS-produced MgO/TiB₂ powders and a stoichiometric mixture of commercially obtained MgO and TiB₂ powders. Leaching was conducted at pH levels of 4.0, 2.5, and 1.0 by the introduction of concentrated aliquots of HNO₃. This method maintains a minimum pH target throughout the leaching process, thereby sustaining a dynamic concentration to remove the oxide. The optimal leaching conditions were determined to be at 90°C at a minimum pH target of 2.5 for the SHS-produced product. At these conditions, conversion percentages of 83%–84% of MgO were measured with only trace amounts of TiB₂ measured in the solution (<100 μg/L). Conversion percentages for each leaching condition and dissolution mass of solid MgO and TiB₂ at each pH are also reported. Results from powder X-ray diffraction confirm the removal of MgO and minimal dissolution of TiB₂, and indicate the formation of unidentified compounds. Inductively coupled plasma mass spectrometry (ICP) was used to analyze the ionic composition and extent of leaching. Scanning electron microscopy was used to observe the particle morphology of the leached powders.     

Grain-Boundary-Phase Identification of a Lead-Based Relaxor by X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy was used to study a lead-based relaxor dielectric ceramic to identify the grain-boundary phase most likely to be responsible for the insulation degradation of relaxor dielectric ceramics under humid loading conditions. The grain-boundary phase consisted mainly of lead and oxygen. The binding energies of Pb4f_7/2 and O1s in the grain-boundary phase were found to be 137.3 eV and 528.8 eV, respectively, and these values agreed well with those for Pb₃O₄ and PbO₂. In addition, the broadness of the Pb4f_7/2 peak suggested the presence of PbO. Measurements were made of lead dissolution from dielectric ceramics in hot water; specimens with a grain-boundary phase gave a 30 to 50 times greater lead concentration in the hot water than specimens without a grain-boundary phase. This demonstrated that the grain-boundary phase easily dissolved in hot water. Thus, it was concluded that the grain-boundary phase contained water-soluble PbO₂ and PbO. The results strongly supported the resistance degradation mechanism of relaxor dielectric ceramics under humid loading conditions as previously proposed by the authors.     

Particle Size Control and Dependence on Precursor pH: Synthesis Uniform Submicrometer Zinc Aluminate Particles

Zinc aluminate (ZnAl₂O₄) particles have been synthesized by the hydrothermal method using NH₃·H₂O as a pH adjustment mineralizer. Experimental results showed that ZnAl₂O₄ particle size was dependent on the precursor pH, and could be controlled through pH adjustment. It was 5.5, 11.5, and 27 nm when the precursor pH was 8.2, 9.3, and 10.5, respectively. On the other hand, the particle size distribution changed broader with increase in pH. These differences were attributable to the different NH₃·H₂O function. NH₃·H₂O was mainly used as a base at lower pH (<9.0), while its complex function predominated at higher one (>9.5). From thermodynamic viewpoint, the rate-limiting steps were dissolution of Al(OH)₃ and γ-AlO(OH) to Al(OH)₄⁻ at lower and higher pH, respectively. The newly formed γ-AlO(OH) with high reactivity was the critical factor in the synthesis of bimodal particles. Higher temperature treatment of γ-AlO(OH) could decrease the reactivity, and could be used as an aluminum source for synthesis uniform ZnAl₂O₄ particles.     

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.     

Characterization of Corrosion Mechanisms Occurring in a Sintered Sic Exposed to Basic Coal Slags

The high-temperature reactions occurring between an a-Sic and a basic coal slag were examined. The resulting corrosion behavior was found to involve at least three mechanisms, including passivation of the Sic via SiO₂ formation, dissolution of the SiO₂ by the coal slag, and formation of a localized Fe-Ni silicide. This mechanism, which dominated when the slag thickness exceeded 100 μm, was also responsible for the degradation in high-temperature strength.