Formation, Transformation, and Decomposition of Y2TeO6

A metastable modification of Y₂TeO₆ crystallizes at 770°C to 870°C at a heating rate of 10°C·min⁻¹ in air from an amorphous material prepared by the simultaneous hydrolysis of yttrium and tellurium alkoxides. It has a tetragonal unit cell with a=1.0428 nm and c=1.7524 nm. The tetragonal-to-hexagonal phase transformation occurs at 920° to 960°C. Hexagonal Y₂TeO₆ decomposes into Y₆TeO₁₂ and TeO₃↑ at 1220° to 1320°C; only Y₆TeO₁₂ is obtained as the decomposition product. Both modifications of Y₂TeO₆ are built up by octahedral TeO₆ groups.     

Reaction and Fired-Property Studies of Cordierite Compositions

Ceramic bodies approaching the theoretical composition of cordierite were prepared from mixtures of talc and clay; talc, clay, and varying amounts of MgCO₃; talc, clay, and alumina; and prochlorite and clay. Additions of BaCO₃ and PbSiO, were made. Differential thermal analysis and X-ray diffraction methods were used in reaction studies of each composition. Bodies had thermal expansions from 0.344 to 0.109% (25° to 900°C.). BaCO₃ and PbSiO₃ affected thermal expansion. Dielectric properties were improved by additions of BaCO₃ and were markedly improved by additions of both BaCO₃ and PbSiO_l. Cordierite started to crystallize at about 1250°. Additions of PbSiO₃ lowered this temperature about 50°C. With sufficient BaCO₃, the Ba-C phase, as described by Wisely, crystallized in place of cordierite. The crystallization temperature for the Ba-C phase was about 1200°C. With insufficient BaCO₃, both cordierite and Ba-C formed. Additions of PbSiO₃ reduced the crystallization temperature of the Ba-C phase by 50°C. and increased the glassy phase. Several of the compositions should haw considerable practical value.     

Oxidation Resistance of Alumina–Titanium Nitride and Alumina–Titanium Carbide Composites

The presence of TiC or TiN paritcles in an Al₂O₃ matrix affects the thermal stability of the composites in oxidizing environments. In isothermic oxidation tests at 700°, 800°, 900°, 1000°, and 1100°C for up to 20 h, two different oxidation regimes have been observed at T < 900°C and at 900°C ≤ T ≤ 1100°C. At low temperatures ( T < 900°C), the oxidation follows a phase-boundary reaction; the reaction product initially consists of aggregates of submicrometer needlelike TiO₂ rutile crystals that subsequently grow and coalesce. When a continuous TiO₂ rutile layer is formed ( T ≥ 900°C), the oxidation kinetics change to parabolic, and the diffusion of O₂ through a thick TiO₂ layer is proposed as the governing step.     

The System Magnesia-Silica-Water Below 300°C.: I, Low-Temperature Phases from 100° to 300°C. and Their Properties

Reactions in the ternary system MgO-SiO₂-H₂O were studied over the temperature range 100° to 300° C. and were found to produce only two phases. Under conditions of 100° to 200° C. and atmospheric pressure up to 20,000 lb. per sq. in., and regardless of the initial MgO/SiO₂ ratio, the predominant magnesium silicate product was found to have a MgO/SiO₂ ratio of 1.5. In the range 200° to 300°C. at 210 to 20,000 lb. per sq. in. two stable phases, 3MgO.2SiO₂.2H₂O (I) and 3MgO.4SiO₂.H₂O (II), were observed. In this case the phase that was favored was determined by the molar ratio of MgO/SiO₂ of the reaction mixture at the start of the run. The physical and chemical properties of phases (I) and (II) resembled those of the natural minerals serpentine and talc respectively. Two different morphologies were observed in electron micrographs of phase (I). From 100° to 160°C. it occurred as crumpled foils, and at 170°C. fibrous crystallites, which resembled the natural asbestos mineral chrysotile, appeared at the expense of some of the foils.     

Formation of TiO2(B) Nanocrystallites in Sol-Gel-Derived SiO2-TiO2 Film

TiO₂(B), one of the polymorphs of TiO₂, has been formed by annealing a sol-gel-derived SiO₂-TiO₂ amorphous film on a silicon wafer at 900°C in air. Transmission electron microscopy (TEM) revealed that nanocrystallites with a size of 5-10 nm were dispersed in the amorphous SiO₂ matrix in the film. The X-ray diffraction pattern and lattice fringe spacing in high-resolution TEM images corresponded to those of TiO₂(B). These TiO₂(B) nanocrystallites are probably stable with the presence of surrounding SiO₂ in the film at 900°C, because previous works reported that this phase should be converted to anatase at temperatures higher than 550-700°C.     

Effect of Hydrothermal Conditions on the Morphology of Colloidal Boehmite Particles: Implications for Fibril Formation and Monodispersity

The synthesis of colloidal boehmite (AlOOH) is studied by heating basic aluminum chloride solutions under constant stirring. The temperature and Al₂O₃: Cl molar ratio influence the product morphology. Synthesis at 140°C generates highly fibrous polycrystalline particles that are on average 360 nm long, 30 nm broad, and 8 nm thick. They contain 0.11 mol of excess H₂O per 1 mol of AlOOH. Synthesis at temperatures between 140° and 190°C produces broader fibrils and less excess H₂O. Preparation at 220°C eventually produces fully crystalline platelike boehmite particles about 260 nm long, 95 nm broad, and 14 nm thick, without excess H₂O. Fibril synthesis requires an Al₂O₃:Cl molar ratio exceeding 1.0 to yield noncoagulated particles. The fibrils are fairly monodisperse with 20% standard deviation in their length for an Al₂O₃: Cl molar ratio about 1.0.     

Kinetics of Hydration of Monocaldum Aluminate

Hydration of CaAl₂O₄ (CA) was studied by calorimetry, analysis of the liquid phase, measurement of the combined water, and electron microscopy. During the induction period, the solution remains almost unchanged and is equilibrated temporarily with both superficially intrusion-hydrated CA particles and Al(OH)₃ gel formed by dissociation of Al(OH)₄^– ions, the solubility of the Al(OH)₃ gel being 10^–4.24 molkg^–1 at 25°C, while the intrusion-hydrated layer on the CA particles grows following a nearly linear law to reach a critical thickness (∼3 nm at 10° to 20°C, or 12 nm at 30°C). At this point destruction of the layer occurs, nuclei of hydrous compounds are generated, and the induction period terminates. Subsequent reaction proceeds in accordance with the rate equation of Schiller based on the dissolution-crystallization mechanism.     

Abnormal Grain Growth During Rapid Annealing of Sol–Gel Alumina Thin Film Deposited on Ni-Based Superalloy

A ∼50 nm thick alumina layer was deposited on an Ni-based superalloy substrate by a sol–gel method. α-AlOOH particles presented in the layer after drying at 140°C transformed mostly to α-Al₂O₃ grains within ∼1 min at 1100°C under a low oxygen partial pressure annealing environment. During the same time period, the α-Al₂O₃ grains grew significantly in the lateral direction, resulting in the aspect ratio of grain diameter to thickness of ∼20. The presence of a preferred orientation in the α-Al₂O₃ layer suggested that the mechanism for the lateral growth was abnormal. The lateral growth mechanism appeared to become very slow when a critical thickness (∼100 nm) was reached.     

Preparation by a Sol-Gel Process of Borosilicate Glasses Containing Microcrystals of Tellurium and Zinc Telluride

Borosilicate glasses, 5B₂O₃· 95SiO₂ (mol%), containing TeO₂ and ZnO nominally equivalent to 10 wt% Te and ZnTe were prepared by a solgel method from Si(OC₂H₅)₄, B(OCH₃)₃, H₆TeO₆, and Zn(NO₃)₂. A study by electron spectroscopy for chemical analysis (ESCA) showed that glasses heated at high temperature (450°C) in air contained both Te⁶⁺ and Te⁴⁺ ions on the surface layer, but that mainly Te⁴⁺ ions occurred inside the bulk glass. When solgel-derived borosilicate glasses containing the TeO₂ compound were reduced at elevated temperature in a hydrogen atmosphere, Te crystallites ranging in size from 4 to 15 nm were produced at a lower temperature, between 200° and 250°C. The absorption edge moved from the infrared to the visible wavelength region as the particle size decreased to about 4 nm. For glasses containing both TeO₂ and ZnO, ZnTe crystallites formed at high temperature—over 300°C—and existed along with the Te phase.     

Synthesis, Properties, and Oxidation of Alumina-Titanium Nitride Composites

Al₂O₃-TiN composites varying from 60 to 66.6 mol% TiN were prepared by an in situ reaction between TiO₂ and AlN. N₂ or O₂ evolution takes place, depending on the composition selected. A pseudobrookite (PB) phase appears in the reaction product, the amount decreasing as the TiO₂:AlN ratio becomes poor in AlN. The in situ reaction product can be pressureless sintered to 94% to 97% theoretical density at 1600°C in N₂. The four-point flexural strength varies from 280 to 430 MPa at room temperature. The fracture toughness is 3 to 4.7 MPa.m^1/2. Oxidation of a 94% dense TiN-Al₂O₃ composite in the temperature range 710° to 1050°C was also studied. A layer of TiO₂ (rutile) protects the composite at 710°C from further oxidation with a weight gain of 0.08 mg/cm² in 90 min. In the temperature range 820° to 1050°C, the initial oxidation kinetics are parabolic, with an activation energy of 216.5 kJ/mol. Linear oxidation kinetics with an activation energy of 113.7 kJ/mol pertain at longer times.     

Synthesis of Titania Composite Membranes by the Pressurized Sol-Gel Technique

Crack-free titania composite membranes have been synthesized from colloidal titania sols (average particle size of 7–16 nm) by the pressurized sol-gel coating technique. The Knudsen permeability of N₂ through the titania membrane layer heat-treated at 300°C is about 21 × 10^-7–35 × 10^-7 mol/m²·s·Pa at room temperature. The permselectivity and the separation factor of He to O₂ in the temperature range of 25°-245°C are 2.49–2.74 and 1.66–1.84, respectively. The average pore diameters of these membranes estimated from the molecular weight cutoff data (6000–9000) using poly(ethylene glycol) are 3.0–4.0 nm. By doping zirconia into titania membranes, their thermal stability could be improved up to 800°C.     

Low-Temperature Synthesis of Cubic and Rhombohedral Y6WO12 by a Polymerized Complex Method

Cubic Y₆WO₁₂, which has a defect fluorite-type structure with disordered cation and anion sublattices, was synthesized for the first time at the low temperature of 600°C by a polymerized complex method. The crystallization process of Y₆WO₁₂ from an amorphous precursor has been investigated in detail using TG-DTA, XRD, and TEM. Above 400°C, the cubic phase began to crystallize upon decomposition of the organic materials in the precursor. The crystallite size of the cubic Y₆WO₁₂ increased from about 5 nm at 600°C up to about 35–40 nm at 1000°C. Above 1200°C, the cubic phase transformed to rhombohedral Y₆WO₁₂, which is a stable phase.     

Chemical Solution-Deposited BaTiO3 Thin Films on Ni Foils: Microstructure and Interfaces

The microstructure and interface quality of chemical solution-deposited BaTiO₃ films on Ni foil were investigated by transmission electron microscopy. The microstructures were found to consist of equiaxed and uniform grains, with average grain sizes for rapid thermal-annealed films of 12 nm (700°C) and 18 nm (750°C), respectively. Films furnace annealed at 1000°C after a rapid thermal anneal at 700°C showed a grain size of 42 nm. It is believed that the final grain size is limited by the highly reducing atmosphere and also by the existence of well-developed crystallites resulting from the rapid thermal annealing step. Spatially resolved electron energy loss spectroscopy identified the existence of residual carbon and variations in the oxygen content in BaTiO₃ films. High-resolution transmission electron microscopy revealed an interfacial layer of Ni–Ba alloy (5–10 nm thick) between the BaTiO₃ and Ni foil.     

Preparation of novel porous solids from alumina-pillared fluorine micas by acid-treatment

New porous solids from alumina-pillared fluorine micas (APMs), which were obtained from synthetic Na-tetrasilicic fluorine mica [NaMg_2.5Si₄O₁₀F₂], were prepared by sulfuric acid-treatment under mild conditions at 25 °C. The products were investigated by XRD, ICP, SEM, TEM and N₂ adsorption–desorption isotherm at 77 K. XRD measurements indicated that the interlayer pillared structure having a large basal spacing collapsed during the early stages of the acid-treatment. ICP analyses indicated that Al³⁺ and Mg²⁺ ions were leached out from the pillared micas during the acid-treatment. The pore properties of the leached products were found to differ from those of the mother pillared micas: the acid leaching of the pillared micas leads to the formation of mesopores around 3.2 nm in diameter. The correlation between the change in pore properties and cation elution behavior suggests that the mesopore formation results from the leaching of Mg²⁺ ions from the octahedral sheet of the pillared micas. The leached products thus obtained retained the flaky morphology of the mother pillared micas. These results show that the mild acid-treatment using APMs provides a novel route for obtaining unique mesopore solids having the large particle sizes of the mother micas.     

Nanosized PbZrO3 Powder from Oxalate Precursor: Microwave-Aided Synthesis and Thermal Characterization

Nanosized lead zirconate (PbZrO₃) powder was synthesized from its oxalate precursor, namely lead zirconyl oxalate (LZO). LZO heated in a microwave heating system for 1 h yielded the PbZrO₃ at 600°C. The same precursor (LZO), when heated in a resistance-heated furnace at 850°C for 3 h, does not give a pure product. Thermogravimetry, differential thermal analysis, and X-ray diffraction techniques were used to characterize the precursor and optimize the conditions for microwave processing. The particle size of PbZrO₃ powder prepared at 600°C using microwave heating was measured using transmission electron microscopy (TEM). The TEM images show that the particles of PbZrO₃ are spherical in shape and that the particle size varies between 20 and 22 nm.     

Phase Equilibrium Studies in the System Iron Oxide-Al2O3-Cr2O3

Subsolidus phase relations in the system iron oride-Al₂O₂-Cr₂O₃ in air and at 1 atm. O₂ pressure have been studied in the. temperature interval 1250° to 1500°C. At temperatures below 1318° C. only sesquioxides with hexagonal corundum structure are present as equilibrium phases. In the temperature interval 1318° to 1410°C. in air and 1318° to 1495° C. at 1 atm. O₂, pressure the monoclinic phase Fe₂O₃. Al₂O₃ with some Cr₂O₃ in solid solution is present in the phase assemblage of certain mixtures. At temperatures above 1380°C. in air and above 1445°C. at 1 atm. O₂ pressure a complex spinel solid solution is one of the phases present in appropriate composition areas of the system. X-ray data relating d- spacing to composition of solid solution phases are given.     

Oxidation of SiC/Porous Al2O3 Laminated Ceramic Composites

The oxidation behavior of SiC/porous Al₂O₃ interphase laminated composites was studied using oxidation experiments and mathematical modeling of the reaction/porous diffusion kinetics in this system. Oxidation at 800°C produced both closure of the interlayer porosity at the lateral ends of the laminate and a limited penetration of the oxidation product layer front from the laminate edges to its interior. Oxidation at 800°C resulted in a persistent product layer of nearly uniform thickness that is more suited to test the effects of oxidation on laminate properties. The modeling approach, which explicitly considers the porous microstructure of the interphase and its evolution upon oxidation, reproduces these experimental observations successfully. The model was extended to study the effect that the mixing of SiC grains with Al₂O₃ grains to form a two-phase porous interphase has on pore closure at the interface and oxide product front penetration into the interior of the laminate. Pore closure was found to be accelerated considerably with increasing SiC content, and was not accompanied by any significant decrease in the distance from the laminate edges upto which an oxidation product layer was formed.     

Crystal Chemistry of Hydrous Calcium Silicates: III, Morphology and Other Properties of Tobermorite and Related Phases

Three very similar hydrous calcium silicates that serve as binders for, or are closely related to the binding material of, autoclaved and moist-air-cured concrete and related products have been differentiated by electron microscopy, X-ray diffraction, differential thermal analysis, infrared absorption, degree of stability to the acetoacetic ester, and oxide composition. Tobermorite, 4–5CaO.5SiO₂.5H₂O, is the binder of properly autoclaved concrete and related products. It integrates uniformly into a dimension-ally stable solid of high strength. This phase is identified by X-ray diffraction lines at 11, 3.07, and 2.97 a.u. of strong intensity, by a basal spacing at 11 a.u., and by a flat platy crystal habit. The analogous hydrate of similar CaO/SiO₂ molar ratio (C/S) and designated the 0.8 to 1.33 C/S hydrate, generally shows a basal spacing at 14 a.u., occurs in crinkly foils, and exhibits a strong exothermic effect in differential thermal analysis at 835° C. This phase differs markedly from tobermorite, does not undergo any crystal-to-crystal binding, and shows some of the physical characteristics of clay. It is the low-lime intermediate in the formation of tobermorite. The binder of low-pressure-steam-cured concrete occurs in part in gel-like globules, sometimes in the shape of spheres. This phase, having a probable composition anywhere between 1.5 and 2.0 C/S, depending on the time of curing, is characterized by the gel-like morphological characteristic, a basal spacing of about 10 a.u., and variable but still characteristic differential thermal analysis effects. Apparently the same product is obtained by reacting lime and silicic acid, but it Occurs in the form of distinctly fibrous crystals.     

Interface Reactions Between Metals and Ceramics: IV, Wetting of Sapphire by Liquid Copper-Oxygen Alloys

The wettability of sapphire single crystals by liquid copper which contained oxygen added as cupric oxide was investigated using the sessile drop technique in vacuum at 1230°C. Additions of cupric oxide to copper, varying from 1 to 72% of copper weight, resulted in rapid chemical reaction at the solid-liquid interface with a significant reduction of the contact angle, the final value being dependent on the oxygen in the system. In all cases the interfacial product was CuAlO₂. A linear relation between the fourth power of the basal radius of the molten drop and the amount of oxygen present was observed. The initial stage of the reaction could be explained by the formation of a Cu₂O layer at the interface, followed by reaction between Cu₂O and Al₂O₃ to form CuAlO₂.     

Synthesis of Nanoparticulate Silica Composite Membranes by the Pressurized Sol–Gel Technique

A crack-free silica composite membrane has been synthesized from a nanoparticulate silica sol (particle diameter <10 nm) by a pressurized sol–gel coating technique developed in this study. The microporous silica layers with an estimated pore radius of 0.78 nm were deposited inside the pores (average pore size of 0.1 μm) of slip cast a-alumina support tubes. The microstructure of the coated layer was controlled by adjusting sol properties and pressurizing conditions. The room-temperature intrinsic permeability of N₂ through the silica membrane layer after heat treatment at 200°C is about 4.9 × 10⁻¹² mol·m/m²·s· Pa, and the mechanism of gas transport is Knudsen flow. The thermal stability of the silica composite membrane is excellent up to 500°C.