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.     

Effect of B2O3 Addition on the Thermal Stability of Barium Phosphate Glasses for Optical Fiber Devices

The effect of B₂O₃ addition on the thermal stability of BaO–P₂O₅ glasses is studied by differential thermal analysis (DTA), X-ray diffraction (XRD) analysis, scanning electron microscopy, and micro-Raman spectroscopy. The difference between glass-transition and onset-crystallization temperatures increases monotonically with increasing B₂O₃ concentration. The DTA result reveals that no exothermic peak due to surface crystallization exists in the BaO–P₂O₅ glass doped with 3 mol% B₂O₃. A single-mode BaO–P₂O₅-B₂O₃ glass fiber could be fabricated by a rod-in-tube technique. The modification of glass structure due to B₂O₃ addition is qualitatively discussed.     

In Situ Whisker-Reinforced AlPO4-Modified β-Eucryptite Glass-Ceramic: I, Morphology and Crystallization Kinetics

A whisker-reinforced glass-ceramic composite in the Li₂O-Al₂O₃-SiO₂-P₂O₅ system has been fabricated by a single-stage process that simultaneously forms the glass-ceramic material and whiskers of TiO₂ in situ . The method utilizes typical glass-ceramic processing techniques and requires precise addition of a TiO₂ nucleation agent during the glass-melting operation, followed by controlled nucleation and crystallization. The maximum nucleation temperature, 740°C, was determined by differential thermal analysis (DTA), and the result was confirmed by scanning electron microscopy (SEM) of the microstructures formed by nucleation/isothermal crystallization heat treatments. The apparent activation energy for crystallization of the material was determined to be 285 ± 3 kj/mol. The average aspect ratio of the TiO₂ whiskers depends on temperature and time during crystallization. The X-ray diffraction patterns of the in situ composite show that eucryptite(ss) and rutile exist as two different phases; no additional phases were observed. Elemental X-ray mapping by electron microprobe indicates that these highly crystallized composites consist of modified β-eucryptite glass-ceramic matrix and acicular grains of TiO₂.     

Coarsening of Lamellar Microstructures in Directionally Solidified Yttrium Aluminate/Alumina Eutectic Fiber

Coarsening of the fine lamellar structure of a directionally solidified Y₃Al₅O₁₂ (yttrium aluminum garnet, YAG)/Al₂O₃ eutectic fiber at elevated temperatures was investigated. The fibers were grown continuously by an edge-defined film-fed growth (EFG) technique. To study the thermal stability of the lamellar structure, the fibers were heat-treated in air at 1360°–1460°C for up to 200 h. X-ray diffractometry and scanning electron microscopy were used to characterize the microstructures of the fibers. Image analysis was used to measure the length of the interface line between Y₃Al₅O₁₂ and Al₂O₃ phases. The kinetics of coarsening and the rate-controlling mechanisms were investigated. Also, the Graham and Kraft model for describing the coarsening behavior of the lamellar Al-CuAl₂ eutectic alloy was used to explain the coarsening behavior of Y₃Al₅O₁₂/Al₂O₃ eutectic fiber.     

Synthesis of α-Alumina Hexagonal Platelets Using a Mixture of Boehmite and Potassium Sulfate

Single-crystal α-alumina (Al₂O₃) hexagonal platelets with a diameter of about 200 nm and 25 nm in thickness were synthesized by heating a mixture of boehmite and potassium sulfate at 1000°C for 2 h and washing with water. The potassium sulfate addition effects on the Al₂O₃ phase and morphology were investigated using differential thermal analysis (DTA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). It was found that potassium sulfate addition helps in the formation of single-crystal α-Al₂O₃ hexagonal platelets and promotes phase transformation from intermediate γ-Al₂O₃ to α-Al₂O₃.     

Stable and Metastable Phase Relations in the System Alumina–Zirconia–Yttria

The Al₂O₃ZrO₂Y₂O₃ system was studied in the range of temperatures 1600°–2800°C by methods of X-ray analysis at 20°C, petrography, DTA in He at temperatures to 2500°C, thermal analysis in air using a solar furnace at temperatures to 3000°C, and electron microprobe X-ray analysis. The stable and metastable phase diagrams were constructed. The liquidus and solidus projections, crystallization paths for the alloys, and polythermal sections are presented in the article. The structure of the restricting systems defines the phase equilibria in the ternary system. No ternary compounds were found. The metastable phase relations were caused by the ambivalent behavior of Y₃Al₅O₁₂ during crystallization.     

Alumina–Aluminide Alloys (3A) Technology: II, Modeling of TixAly–Al2O3 Composites Formation

The reaction sintering of Ti _x Al _y –Al₂O₃ composites from TiO₂/Al starting powder mixtures has been characterized by thermogravimetry and differential thermal analysis (TG/DTA), in situ temperature measurements, and predictions via a continuum model. In order to model the TiO₂/Al reaction system, it was necessary to first determine the postmill reactant concentrations and the dominant reaction. The postmill reactant concentrations were obtained from TG/DTA measurements in air, while X-ray diffraction (XRD) was used to gain insight into the reaction mechanisms. A continuum model of the process was fitted to in situ temperature measurements by adjusting two parameters. The model was then used to investigate the effects of various processing conditions on the reaction behavior.     

Rapid Crystallization of SiO2-Al2O3 Glasses

The crystallization of Al₂O₃-rich glasses in the system SiO₂-Al₂O₃ which were prepared by flame-spraying and/or splat-cooling was studied by DTA, electron microscopy, and X-ray diffraction. Over a wide range of compositions, the crystallization temperature ( T_x ) remained near 1000°C, changing smoothly with composition. In all cases crystallization of mullite was detected by X-ray diffraction. In the low-Al₂O₃ region, coarsening of the microstructure during crystallization was observed by electron microscopy. In the high-Al₂O₃ region mullite and γ-Al₂O₃ cocrystallized; this behavior may be interpreted as evidence of a cooperative process of crystallization at the respective T_x 's. The crystallite size of the mullite immediately after rapid crystallization increased continuously with increasing Al₂O₃ content. In light of the T_x data, the adequacy of the evidence for the proposed metastable miscibility gap in the SiO₂-Al₂O₃ system is questioned.     

Thermal Analysis of 3-mol%-Yttria-Stabilized Tetragonal Zirconia Powder Doped with Copper Oxide

Thermal analysis was performed upon 3-mol%-yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) which had been doped with CuO using an aqueous adsorption technique. Cyclic differential thermal analysis (DTA) scans indicated that the CuO present on the powder surfaces first transforms to Cu₂O and then melts. The molten Cu₂O then reacts with yttria at the powder surfaces to form a new phase containing Y, Cu, and O. Because Y takes time to diffuse to the particle surfaces, the apparent melting point of this new phase appears at higher temperatures in initial DTA scans than in subsequent scans. Vaporization of the molten copper-oxide-rich phase at the temperatures studied causes a gradual shift in composition from Y₂Cu₄O₅ to the less copper-rich Y₂Cu₂O₅ phase. The presence of the Y₂Cu₂O₅ phase in CuO-doped 3Y-TZP allows for previous sintering and superplasticity results to be explained.     

Mullite Transformation Kinetics in P2O5-, TiO2-, and B2O3-Doped Aluminosilicate Gels

Mullite transformation kinetics of sol-gel-derived diphasic mullite gels doped with P₂O₅, TiO₂, and B₂O₃ were studied using quantitative X-ray diffraction and differential thermal analysis (DTA). The mullite transformation temperature initially increased with P₂O₅ doping because of phase separation and formation of α-alumina and cristobalite. In TiO₂-doped samples, the mullite transformation temperature decreased with TiO₂ doping, and the transformation rate increased with decreasing TiO₂ particle size. Kinetic studies showed that titania reduced the activation energy for both nucleation and growth relative to pure diphasic mullite gels by lowering the glass viscosity and/or enhancing the solid-state mass transport through lattice defects. B₂O₃ doping decreased the mullite transformation temperature and lowered the activation energy for both nucleation and growth but especially affected the mullite nucleation process, as indicated by the much smaller grain size.     

Nucleation and Crystallization of a Lead Halide Phosphate Glass by Differential Thermal Analysis

The nucleation and crystallization mechanisms of a lead halide phosphate glass [40P₂O₅·30PbBr₂·30PbF₂ (mol%)] were investigated by differential thermal analysis (DTA) and X-ray diffraction analysis. There were two crystalline phases in the crystallized samples: the major phase was PbP₂O₄, and the minor phase was PbP₂O₆. The average activation energy for crystallization, E , for two different particle sizes of this glass was determined to be 119 ± 4 kJ/mol by the Kissinger method and 124 ± 4 kJ/mol by the Augis–Bennett method. The Avrami constants were determined to be 1.6 and 2.5 for particle sizes of 203 and 1040 μm, respectively, by the Ozawa equation, and 1.7 and 2.4 for particle sizes of 203 and 1040 μm, respectively, by the Augis–Bennett equation. The decrease in the crystallization peak height in the DTA curve with increasing particle size suggested that the particles crystallize primarily by surface crystallization. A nucleation-rate type curve was determined by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, 1/ T _p, or the height of the crystallization peak, (δ T )_p, as a function of nucleation temperature, T _n. The temperature where nucleation can occur for this glass ranges from 360°–450°C and the maximum nucleation rate is at 420°± 10°C.     

Reactions and Microstructure Development in Mullite Fibers

Microstructural and compositional changes during heat treatment of sol–gel-derived mullite fibers with additions of 2 wt% B₂O₃, 2 wt% P₂O₅, 2 wt% Cr₂O₃, and (1 wt% P₂O₅+ 1 wt% Cr₂O₃) were compared with those of undoped mullite fibers. For all compositions the sequence of phase development was the crystallization of a spinel phase (†-Al₂O₃ or Al–Si spinel) from amorphous material, followed by the formation of mullite at higher temperatures. Differential thermal analysis showed that additions of B₂O₃ and P₂O₅ increased the temperature of spinel formation and that B₂O₃ significantly decreased the temperature of mullite formation. After 1 h at 1200°C, the size of mullite grains in fibers that contained B₂O₃ was less than 1000 Å the grains in fibers of other compositions were 6000 to 12000 Å. After 60 h at 1400°C, fibers modified with B₂O₃ had a grain size less than 2000 to 3000 Å the grains in fibers of other compositions were 6000 to 12000 Å. B₂O₃ was the most volatile additive.     

Experimental Phase Diagram Study and Thermodynamic Optimization of the Ag-Bi-O System

The Ag-Bi-O system has been experimentally studied using differential thermal analysis (DTA) and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), and thermodynamically optimized. The temperatures of the eutectic, monotectic, and Bi₂O₃ allotropic transformations have been measured in N₂, in air, and in O₂ by DTA. There are no ternary phases stable at ambient pressure. Presently measured transformation temperatures have been combined with existing oxygen activity measurements in the metal liquid to optimize thermodynamic parameters describing the liquid phase. The resulting fit is excellent. EDX measurements of the composition in the oxide liquid have a rather low precision but confirm the thermodynamic optimization. However, some uncertainties remain concerning the liquid composition at the eutectic transformation and the shape of the miscibility gap at higher temperatures.     

Sc2O3 Nanopowders via Hydroxyl Precipitation: Effects of Sulfate Ions on Powder Properties

Hydroxyl-type Sc₂O₃ precursors have been synthesized via precipitation at 80°C with hexamethylenetetramine as the precipitant. The effects of starting salts (scandium nitrate and sulfate) on powder properties are investigated. Characterizations of the powders are achieved by elemental analysis, X-ray diffractometry (XRD), differential thermal analysis/thermogravimetry (DTA/TG), high-resolution scanning electron microscopy (HRSEM), and Brunauer-Emmett-Teller (BET) analysis. Hard-aggregated precursors (γ-ScOOH·0.6H₂O) are formed with scandium nitrate, which convert to Sc₂O₃ at temperatures ≥400°C, yielding nanocrystalline oxides of low surface area. The use of sulfate leads to a loosely agglomerated basic sulfate powder having an approximate composition of Sc(OH)_2.6(SO₄)_0.2·H₂O. The powder transforms to Sc₂O₃ via dehydroxylization and desulfurization at temperatures up to 1000°C. Well-dispersed Sc₂O₃ nanopowders (∼64.3 nm) of high purity have been obtained by calcining the basic sulfate at 1000°C for 4 h. The effects of SO₄²⁻ on powder properties are discussed.     

Non-Isothermal Crystallization Kinetics of a Blast Furnace Slag Glass

The capability of the blast furnace slag to be vitrified was investigated by means of differential thermal analysis (DTA), scanning electron microscopy, and X-ray diffraction (XRD) in samples prepared by melting without the addition of any other ingredients. Differential thermal analysis revealed the presence of two major exothermic peaks. XRD indicates the presence of more than one crystalline phase: gehlenite Ca₂Al₂SiO₇, BaAl₂Si₂O₈, and pyroxene Ca(Mg,Al)(Si,Al)₂O₆. The activation energy of crystallization was estimated on the basis of DTA carried out on particle size 312–500 μm at different heating rates. Analysis of non-isothermal DTA data yielded values of 457.5 KJ/mol and 2.21 for the activation energy of crystallization and the Avrami exponent, respectively. A value for the activation energy corresponding to structural relaxation at temperatures around the glass transition has also been determined for this glass.     

Sol–Gel Combustion Synthesis of Nanocrystalline YAG Powder from Metal-Organic Precursors

Yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) nanocrystalline powders were synthesized by a novel sol–gel combustion process. Yttrium acetate and aluminum sec-butoxide were used as the precursors and triethanolamine was used as the chelating agent and fuel. Thermal and crystallization behaviors of the YAG precursor powders were investigated by thermal gravimetric differential thermal analysis (DTA), Fourier transform infrared spectrum, and X-ray diffraction. The combustion-synthesized powders are amorphous and transform to a pure YAG crystalline phase at 900°C. The crystallization activation energy of amorphous YAG precursor was investigated by variable heating rate DTA. The calculated activation energy is 58.9 KJ/mol. The average crystalline size of heat-treated YAG powders at 900°C is ∼20 nm.     

The System Bi2O3-SiO2

The Bi₂O₃-rich side of the system Bi₂O₃-SiO₂ was studied with powder X-ray diffraction and differential thermal analysis. In the composition 6Bi₂O₃. x SiO₂, the metastable γ phase (bcc) was observed to exist over the range of 0 < x ≤ 1. In most of the compositions studied, metastable phases of water-quenched melts transformed into another metastable phase before reaching stable phases. A modification of the phase diagram is proposed.     

Phase Equilibria in the System HfO2–Y2O3–CaO

Phase equilibria in the system HfO₂–Y₂O₃–CaO were studied in the temperature range 1250° to 2850°C by both experimental methods (X-ray phase analysis at 20° to 2000°C, petrography, annealing and quenching, differential thermal analysis in He at temperatures to 2500°C, thermal analysis in air using a solar furnace at temperatures to 3000°C, and electron microprobe X-ray analysis) and theoretical means (development of a mathematical model for the liquidus surface by means of the reduced polynomial method). Phase equilibria were determined by the structure of the restricting binary systems. No ternary compounds were found. The liquidus was characterized by the presence of six four-phase, invariant equilibria. Solid solutions were based on monoclinic (M), tetragonal (T), and cubic (F) modifications of HfO₂; C and H forms of Y₂O₃; CaO; and CaHfO₃ that crystallized in two polymorphous modifications, namely, the cubic and rhombic perovskite-type structure.     

Temperature/Composition Phase Diagram of the System Bi2O3-PbO

The Bi₂O₃-PbO phase diagram was determined using differential thermal analysis and both room- and high-temperature X-ray powder diffraction. The phase diagram contains a single eutectic at 73 mol% PbO and 635°C. A body-centered cubic solid solution exists above ∼600°C within a composition range of 30 to 65 mol% PbO. The compounds α-Bi₂O₃, σ5-Bi₂O₃, and γ-PbO (litharge) have wide solubility ranges. Four compounds, 6Bi₂O₃·PbO, 3Bi₂O₃·2PbO, 4Bi₂O₃,5PbO, and Bi₂O₃·3PbO, are formed in this system and the previously unreported X-ray diffraction patterns of the latter three compounds are reported. Diffraction patterns for some of these mixed oxides have been observed in ZnO-based varistors grown using Bi₂O₃ and PbO as sintering aids.     

Effects of Impurities and Manufacturing Methods on the Devitrification of Silica Fibers

Devitrification resistance is one of the most important properties of silica fibers. The devitrification behavior of silica fibers made by the acid leaching method, the sol-gel method, and the high-purity silica fusing method, and the effects of small amounts of Al₂O₃ and B₂O₃, were studied by using X-ray diffraction. The results indicated that the devitrification tendency of silica fibers made by the high-purity silica fusing method is apparently greater than that of the fibers made by the other two methods. A small amount of B₂O₃ has the effect of retarding devitrification, while Al₂O₃ promotes devitrification of the fibers. The combined effect of Al₂O₃ and B₂O₃ is to strongly enhance the devitrification of silica fibers. The results are discussed in the context of structural analysis.