Kaolinite–Mullite Reaction Series: The Development and Significance of a Binary Aluminosilicate Phase

The semiquantitative estimations of 980°C exothermic reaction products of kaolinite by quantitative X-ray diffraction (QXRD) and chemical leaching techniques show the formation of a significant amount of amorphous aluminosilicate phase (∼ 30 to 40 wt%). The theoretically expected AlO₄/AlO₆ ratio in the 980°C reaction is in close agreement with the value measured by the X-ray fluorescence (XRF) technique and the experimental radial electron distribution (RED) profile agrees with the suggested 980°C formation of Si-Al spinel with mullite-like composition. Mullitization of kaolinite has been compared with a synthetic Al₂O₃—SiO₂ mixture. In synthetic mixtures development of an intermediate amorphous aluminosilicate phase is an essential step prior to mullitization. Kaolinite forms mullite in two ways: (i) by polymorphic transformation of cubic mullite at 1150° to 1250°C and (ii) by nucleation of mullite in the amorphous aluminosilicate phase and its subsequent growth above 1250°C. Thus chemical continuity is maintained throughout the reaction series and the intermediate spinel phase is silicon bearing and its subsequent transformation to mullite confirms the topotactic concept in the kaolinite transformation.     

Two-Stage Sintering of Nanosize Pure Zirconia

We report a two-stage pressureless heating schedule for the preparation of crack-free and dense pure zirconia (ZrO₂) monoliths with grain sizes close to the nanoscale. The use of nanosize ZrO₂ powders prepared by a modified microemulsion synthetic route was essential for the success of this two-stage sintering approach. Green compacts made from these powders present a narrow pore size distribution and a low-agglomeration state. Temperatures for the two-stage heating schedule were selected after analyzing the thermal evolution of green compacts by shrinkage, density, and high-temperature X-ray diffraction studies. During the first stage of the thermal processing, the green body was heated at 960°C–10 h to obtain a compact with a high density and near-nanosize grains. In the second stage, the compact was heated at 1150°C–2 h to obtain fully dense compacts. Microcracks were prevented because 1150°C is below the reversible but disruptive monoclinic–tetragonal phase transformation.     

Isothermal Section of a Cu2O–Al2O3–SiO2 Pseudo-Ternary System at 1150°C in Air

In this study, the isothermal section of a Cu₂O–Al₂O₃–SiO₂ pseudo-ternary phase diagram at 1150°C was analyzed by means of a scanning electronic microscope and powder X-ray diffraction of the quenched samples qualitatively, and the compositions of the tie-points of the tie-planes as well as their regions were determined by in situ high-temperature quantitative X-ray diffraction analysis and energy-dispersive X-ray spectroscopy. Then, the isothermal section of the Cu₂O–Al₂O₃–SiO₂ pseudo-ternary phase diagram at 1150°C was constructed; it was found that the isothermal section is composed of two single liquid-phase regions, five two-phase regions, and six three-phase regions.     

Formation of β-Si3N4 Coatings by Chemical Vapor Deposition

BetaSi₃N₄ coatings were obtained by chemical vapor deposition in a fused-silica reaction tube by outside heating of the system SiCl₄-NH₃-N₂ at a deposition temperature (reaction tube temperature) of 1300°C, whereas α- and α+β-phase coatings were obtained at depositon temperatures of 1150° and 1250°C, respecively. Formation of β-phase coatings at relatively low temperatures is explained in terms of the effect of a catalytic impurity, SiO vapor from the reaction tube. The X-ray diffraction patterns and sulface morphologies of the coatings were studied.     

Axial Thermal Expansion of TetragonaI ZrO, Between 1150° and 1700°C

The lattice constants for three samples of tetragonal ZrO₂ were measured in a high-temperature X-ray diffractometer in the range 1150° to 1700°C in an ambient air atmosphere. The unrefined lattice constants were the same for these materials only after each had been previously heat-treated at 1550° to 1750°C in an ambient air atmosphere. The thermal expansions of the two axes are linear and are given by: a value, A = 3.588₂+ 4.50 X 10+(T) , within 0.001₃ A; and c value, A = 5.188₂+ 7.57 × 10⁻⁵(T), within 0.002₂ A, where T is in °C for the range 1150° to 1700°C. The lattice constant values are a = 3.639₉ A and c = 5.275₈ A at 1150°C and a = 3.678₂ A and c = 5.339₇ A at 1700°C.     

Effects of CuO Doping on the Microstructural and Dielectric Properties of Ba0.6Sr0.4TiO3 Ceramics

This study investigates the effect of CuO on the sintering behavior, dielectric properties, and microstructures of Ba_0.6Sr_0.4TiO₃ (BST) ceramics. The ceramics were sintered in air at temperatures ranging from 1000° to 1230°C. It is found that a small amount of added CuO (0.6 mol%) can significantly increase the density and improve the dielectric properties of BST ceramics. Doped BST ceramics can be sintered to a density >95% of the theoretical density at 1150°C. scanning electron microscopic observations show that the BST grain sizes increase with increasing amounts of CuO. No secondary phases in the BST ceramics are observed using X-ray diffraction pattern for CuO additions up to 0.9 mol%. However, compositional analysis using transmission electron microscopy-EDX for the BST ceramics with 0.9 mol% CuO sintered at 1150°C showed that a small level of secondary phase formation is present. On the other hand, large dislocations are observed for BST with 0.6 mol% CuO addition as a result of lattice distortion, which creates the vacancy condensation because of the atomic mismatch in the solid solutions. Optimal CuO doping concentrations can reduce the loss tangents of BST that can also ensure a high dielectric constant. When the doping concentration of CuO is 0.6 mol% and the ceramic is sintered at 1150°C, the BST ceramic has the following properties at 1 MHz: dielectric constant=4094, tan δ=0.55%.     

The Crystal Structure of LaAlO3-Stabilized La2/3TiO3 Ceramics: An HRTEM Investigation

LaAlO₃-stabilized La_2/3TiO₃ (LT) ceramics were prepared by the conventional mixed oxide route. Small amounts of manganese oxide were added to eliminate Ti⁴⁺ reduction. The powders were calcined at 1150°C and sintered at 1400°–1500°C for 4 h and cooled at rates of 900°–15°C/h. The products were high density and single phase, with an average grain size of 6 μm. The LaAlO₃-stabilized LT ceramics exhibited a relative permittivity (ɛ_r) of 64, a positive temperature coefficient of resonant frequency (τ_f) of 84, and dielectric Q value × resonant frequency ( Q × f ) values of 16 400 GHz. The crystal structure and microstructures have been investigated using high-resolution transmission electron microscopy (HRTEM) in conjunction with X-ray diffraction (XRD). One candidate crystal structure, a ≈2 a _p (where a _p is the lattice parameter of the high-temperature form of the cubic perovskite), b ≈2 a _p, and c ≈2 a _p with a space group Cmmm (65), has been confirmed by XRD, electron diffraction, and lattice imaging techniques. Microtwins, with twin boundaries parallel to the {100} planes, were observed in the microstructures.     

Sol–Gel Preparation and Characterization of Lithium Disilicate Glass–Ceramic

A sol–gel process is described for preparation of crystalline lithium disilicate (Li₂Si₂O₅) from tetraethylorthosilicate and lithium ethoxide. The glass network structure and crystallinity resulting from heat treatment at temperatures from 150° to 900°C were investigated by nuclear magnetic resonance, X-ray diffraction, and differential scanning calorimetry/thermogravimetric analysis. Q³ structural units (SiO₄ tetrahedra with three bridging oxygen atoms) formed in the amorphous gel at a low temperature (≤150°C) persist to elevated temperature (≤500°C) and directly transform to crystalline Li₂Si₂O₅ at about 550°C. The heating schedule slightly affects the crystalline phase transformation.     

La2Ti2O7 Ceramics

La₂Ti₂O₇ powders were prepared using three different techniques. Single-phase material was obtained at 1150°C by calcination of mixed oxides, at 1000°C by molten salt synthesis, and at 850°C by evaporative decomposition of solutions. Particle sizes and morphologies of the powders differed substantially, as did the sintered microstructures and dielectric properties. Very dense (99%), translucent, grain-oriented lanthanum titanate was fabricated by hot-forging at 1300°C under a 200-kg load. Anisotropy was demonstrated by X-ray diffraction, scanning electron microscopy, thermal expansion, and dielectric measurements.     

Transformation and Densification of Nanocrystalline θ-Alumina during Sinter Forging

The sinter forging behavior of α-Al₂O₃ seeded and unseeded nanocrystalline θ-Al₂O₃ was investigated as a function of temperature, stress, and strain rate. Seeded samples exhibited the highest degree of plastic deformation during the θ- to α-AI₂O₃ phase transformation. As a result, microstructure control, increased densification, and a higher degree of transformation were obtained. A uniform microstructure of 150 nm α-Al₂O₃ grains developed, reaching 57% relative density after sintering 1.5 wt%α-Al₂O₃ seeded samples for 30 min at 1060°C. When sinter forged at 0.25 mm/min to 63 MPa and 1060°C for 30 min large deformations during the phase transformation increased the relative density to 74%. When the stress was increased to 235 MPa (1060°C, 30 min), 99.7% dense α-Al₂O₃ with a grain size of 230 nm was obtained. By increasing the sinter forging temperature to 1150°C, 99.5% relative density was achieved at 190 MPa for 30 min.     

Thermal Expansion of Some Cubic Refractory Compounds of Thorium

The thermal expansion of a number of cubic refractory compounds of thorium was measured by high-temperature X-ray diffraction. The coefficients of thermal expansion range from 7.5 to 8.2 × 10⁻⁶/°C for ThN, ThC_0.84, ThP_0.71, ThP_0.61, and Th₃P₄ between 900° and 1300°, 1450°, 1750°, 1750°, and 1150°, respectively; the coefficients for ThS, ThB₆, and ThO₂ are in the range of 11.2 to 12.0 × 10⁻⁶/°C between 900° and 1400°, 1700°, and 2100°, respectively. The results are discussed in relation to other properties of these compounds.     

High-Temperature Transmission Electron Microscopy and X-Ray Powder Diffraction Studies of Polymorphic Phase Transitions in Ba4Nb2O9

Polymorphic phase transitions in Ba₄Nb₂O₉ were studied by thermal analyses, high-temperature transmission electron microscopy and X-ray powder diffractometry. Two stable polymorphs were isolated, low-temperature α-modification and high-temperature γ-modification, with the endothermic phase transition at 1176°C. The α→γ transformation is accompanied by the formation of a 120° domain structure, which is a consequence of hexagonal→orthorhombic unit cell reconstruction. Reheating the presintered γ-Ba₄Nb₂O₉ results in the formation of a metastable γ'-modification (formerly known as β-polymorph) in the temperature range between 360° and 585°C, before the γ→α transformation at 800°C. Above ∼490°C Ba₄Nb₂O₉ becomes moderately sensitive to a loss of BaO. In air the surface of Ba₄Nb₂O₉ grains decomposes to nanocrystalline Ba₅Nb₄O₁₅ and BaO, which instantly reacts with atmospheric CO₂ to form BaCO₃. Surface reaction delays γ→α transformation up to 866°C in air. In vacuum the loss of BaO is even more enhanced and consequently the formation of minor Ba₃Nb₂O₈ phase is observed above 1150°C.     

Phase Transformation Kinetics in the System Bismuth Oxide-Zirconia

The effect of ZrO₂ addition on the phase transformation of Bi₂O₃ was studied with powder X-ray diffraction and differential thermal analysis. Samples containing higher than a few mole percent ZrO₂ were obtained as the metastable β phase (tetragonal) at room temperature when they were cooled at 20°C/min from 800°C where the δ phase (fcc) is stable. When the same samples were cooled at the same rate from 1000°C, which is above the solidus, they formed the stable α phase (monoclinic) at room temperature. Evidence for the relatively higher solubility of ZrO₂ in the α phase is discussed to explain this observation.     

Synthesis, Characterization, and Luminescent Properties of CaS:Eu Phosphor

Europium-doped calcium sulfide phosphor was synthesized by a co-precipitation method using calcium carbonate and ammonium sulfate as the sources for calcium and sulfide, respectively. Reduction of the precipitated CaSO₄ into CaS was carried out at 550°, 750°, 950°, and 1150°C at a flow rate of H₂ 5 mL/min and N₂ 95 mL/min. The formation of crystalline CaS phase was evident by the X-ray diffraction analysis of the fired products. The phosphor yielded red emission of wavelength 634 nm for the excitation wavelength of 465 nm. The emission intensity of the phosphors increased with increasing firing temperature from 550° to 950°C, but firing at 1150°C showed a dramatic decline in emission due to thermal quenching. The study of the effect of Eu loading level showed a decrease in emission with an increase in loading, and the optimum loading for maximum emission was found to be 0.5 mol%. And it can be possible to generate white light of light emitting devices by mixing with SrGa₂S₄:Eu green phosphor and this phosphor on the 460 nm InGaN blue chip.     

Metallographic Observation of the Monoclinic-Tetragonal Phase Transformation in ZrO2

The monoclinic-tetragonal phase transformation of ZrO₂ was examined with a vacuum hot-stage microscope. Polished sections of vacuum-sintered 99.7% pure ZrO₂ were observed from room temperature to 1300°C. The rapid formation of platelet substructure within ZrO₂ grains in the temperature range 1050° to 1150°C was associated with the heating transformation. Photomicrographs and motion pictures were taken of the specimen as the transformation progressed. The surface deformation was irreversible, preventing observation of the phase inversion on cooling. From these hot-stage studies, supplementary DTA, X-ray, and thermal expansion data, and other existing information, it is concluded that the transformation is of the diffusionless, athermal type, characteristic of Fe-Ni martensitic transformations.     

Synthesis and Mechanism of Nanosized AlN from an Aluminum Oleic Emulsion Using Carbothermal Reduction at Low Temperatures

Nanosized Al₂O₃ particles homogeneously dispersed in a matrix of amorphous carbon (a-C) were prepared by decomposition of an aluminum oleic emulsion at 600°C in Ar. Nanosized aluminum nitride (AlN) grains were prepared by carbothermal reduction and nitridation (CRN) of this Al₂O₃–a-C mixture in NH₃ using graphite, BN, and alumina crucibles or boats. The phases formed by CRN were identified by X-ray diffraction analysis. The morphology and grain size of the AlN were determined by transmission electron microscopy. The formation of single-phase AlN was achieved at temperatures as low as 1150°–1200°C in NH₃ using a cylindrical graphite crucible with holes in its two flat faces. Mass spectroscopy (MS) showed that a significant amount of HCN and a minor amount of C₂H₂ are formed at 500°C by reaction of NH₃ with carbon at the decomposition temperature of NH₃. A most probable formation mechanism of the AlN from nanosized Al₂O₃ and a-C in NH₃ is discussed on the basis of MS results and thermodynamic considerations.     

Kinetics and Microstructural Evolution of Heterogeneous Transformation of θ-Alumina to α-Alumina

Ultrafine (<0.1 μm) high-purity θ-Al₂O₃ powder containing 3–17.5 mol%α-Al₂O₃ seeds was used to investigate the kinetics and microstructural evolution of the θ-Al₂O₃ to α-Al₂O₃ transformation. The transformation and densification of the powder that occurred in sequence from 960° to 1100°C were characterized by quantitative X-ray diffractometry, dilatometry, mercury intrusion porosimetry, and transmission and scanning electron microscopy. The relative bulk density and the fraction of α phase increased with annealing temperature and holding time, but the crystal size of the α phase remained ∼50 nm in all cases at the transformation stage (≤1020°C). The activation energy and the time exponent of the θ to α transformation were 650 ± 50 kJ/mol and 1.5, respectively. The results implied the transformation occurred at the interface via structure rearrangement caused by the diffusion of oxygen ions in the Al₂O₃ lattice. A completely transformed α matrix of uniform porosity was the result of appropriate annealing processes (1020°C for 10 h) that considerably enhanced densification and reduced grain growth in the sintering stage. The Al₂O₃ sample sintered at 1490°C for 1 h had a density of 99.4% of the theoretical density and average grain size of 1.67 μm.     

Modified Phase Diagram for the Barium Oxide–Titanium Dioxide System for the Ferroelectric Barium Titanate

The ferroelectric phase transition behavior in BaTiO₃ was investigated for various annealing times, temperatures, and Ba/Ti ratios by means of a differential scanning calorimeter. Coupling these observations with powder X-ray diffraction and transmission electron microscopy allowed new insights into the barium oxide (BaO)–titanium dioxide (TiO₂) phase diagram. The transition temperature was varied systematically with the Ba/Ti ratio at annealing temperatures from 1200° to 1400°C in air. The transition temperature decreased with increasing concentrations of BaO and TiO₂ partial Schottky defects, and showed a discontinuous change at the phase boundaries. Beyond the solubility region, two peritectoid reactions were confirmed and revised; first around 1150°C for Ba_1.054Ti_0.946O_2.946→Ba₂TiO₄+BaTiO₃ and second 1250°C for BaTi₂O₅→Ba₆Ti₁₇O₄₀+BaTiO₃, respectively. All other regimes of the BaO–TiO₂ were found to be consistent with the reported diagrams in the literature.     

Crystalline Intermediate Phases in the Sol–Gel-Based Synthesis of La2NiO4+δ

X-ray diffraction and transmission electron microscopy were applied to investigate a sol–gel synthetic process for the mixed oxygen ion and electron conductor La₂NiO_4+δ with a K₂NiF₄ structure type. The development of the La₂NiO_4+δ is elucidated considering the influence of calcination temperatures and dwell times. Following the thermal decomposition of nitrate and organic precursors in an intermediate step, the lanthanum nickel oxide is obtained after a short dwell time above 750°C. This occurs by the transformation of an ultrafinely dispersed powder consisting of lanthanum oxycarbonate, lanthanum oxide, and nickel oxide. The pure La₂NiO_4+δ phase was obtained by similar solid-state reactions between nanocrystalline powder particles at just 950°C.     

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.