Showing papers in "Journal of the American Ceramic Society in 1999"

Ferroelectric ceramics : History and technology

Abstract: Ferroelectric ceramics were born in the early 1940s with the discovery of the phenomenon of ferroelectricity as the source of the unusually high dielectric constant in ceramic barium titanate capacitors. Since that time, they have been the heart and soul of several multibillion dollar industries, ranging from high-dielectric-constant capacitors to later developments in piezoelectric transducers, positive temperature coefficient devices, and electrooptic light valves. Materials based on two compositional systems, barium titanate and lead zirconate titanate, have dominated the field throughout their history. The more recent developments in the field of ferroelectric ceramics, such as medical ultrasonic composites, high-displacement piezoelectric actuators (Moonies, RAINBOWS), photostrictors, and thin and thick films for piezoelectric and integrated-circuit applications have served to keep the industry young amidst its growing maturity. Various ceramic formulations, their form (bulk, films), fabrication, function (properties), and future are described in relation to their ferroelectric nature and specific areas of application.

Templated Grain Growth of Textured Bismuth Titanate

Abstract: Textured bismuth titanate, Bi4Ti3O12 (BiT), was produced by templated grain growth (TGG). Molten-salt-synthesized BiT platelets were dispersed in a matrix of 200 nm BiT powder and aligned by tape casting. Highly textured BiT was obtained with the use of only 5 vol% template particles by sintering at 1000°C for 1 h. The uniformity of the through-thickness texture is much higher than reported in the literature for BiT tapes cast with 100% platelets. Initial platelet alignment is shown to increase because of frequent interaction with the fine powder particles during tape casting. By avoiding pressure densification techniques and using only a small portion of anisometric particles, TGG is a low-cost option for fabricating textured ceramics.

SiC Recession Caused by SiO2 Scale Volatility under Combustion Conditions: I, Experimental Results and Empirical Model

Abstract: A high-pressure burner rig was developed to evaluate the response of chemical-vapor-deposited SiC material during exposure to simulated gas turbine combustor conditions. Linear weight loss and surface recession rates of SiC were observed in both fuel-lean and fuel-rich gas mixtures. This response was shown to result from SiO2 scale volatility. Arrhenius-type temperature dependence was demonstrated. In addition, the effects of pressure and gas velocity were defined in terms of a gaseous-diffusion-controlled process for volatile reaction products (such as SiO, Si(OH)4, and iO(OH)x). Accordingly, multiple linear regression was used to develop empirical recession relationships of the form exp(-DeltaQ/RT)Pxvyfor both lean and rich combustion conditions. Part II of this paper discusses the thermodynamics and gaseous-diffusion model of this recession. The empirical models discussed here enable prediction of SiC recession for any combination of T, P, and vin turbine environments. For typical combustion conditions, recession of 0.2-2 µm/h was predicted at 1200°-1400°C. Thus, long-term, high-temperature, high-velocity exposure may degrade silicon-based or SiO2-forming material by recession in combustion gas environments.

Homogeneous Precipitation of TiO2 Ultrafine Powders from Aqueous TiOCl2 Solution

Abstract: Crystalline TiO2 powders were prepared by the homogeneous precipitation method simply by heating and stirring an aqueous TiOCl2 solution with a Ti4+ concentration of 0.5M at room temperature to 100°C under a pressure of 1 atm. TiO2 precipitates with pure rutile phase having spherical shapes 200-400 nm in diameter formed between room temperature and 65°C, whereas TiO2 precipitates with anatase phase started to form at temperatures >65°C. Precipitates with pure anatase phase having irregular shapes 2-5 µm in size formed at 100°C. Possibly because of the crystallization of an unstable intermediate product, TiO(OH)2, to TiO2xH2O during precipitation, crystalline and ultrafine TiO2 precipitates were formed in aqueous TiOCl2 solution without hydrolyzing directly to Ti(OH)4. Also, formation of a stable TiO2 rutile phase between room temperature and 65°C was likely to occur slowly under these conditions, although TiO2 with rutile phase formed thermodynamically at higher temperatures.

Characterization of Barium Titanate Powders: Barium Carbonate Identification

Abstract: A range of methods for the detection of barium carbonate contaminant in barium titanate powder has been assessed, namely: X-ray diffraction (XRD), scanning electron microscopy (SEM), with EDS-X-ray microanalysis, Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), gas chromatography (GC) for analysis of carbon, and X-ray photoelectron spectroscopy (XPS). The most satisfactory procedure for the detection of the small amounts of BaCO3 commonly present is FT-IR. Surface analyses by XPS show that the carbonate is present as a discrete phase and is not a surface film on barium titanate particles.

Microengineering of Ceramics by Direct Ink‐Jet Printing

Abstract: A modified piezoelectric drop-on-demand ink-jet printer was used to build 3-D ceramic structures with cavities and overhangs by direct ceramic deposition. Inks in the form of zirconia and carbon suspensions were prepared with suitable viscosities and surface tensions. The multinozzle printer was used to microengineer small components in which support structures were created from the carbon ink and the carbon was removed after pyrolysis of the organic vehicle but before sintering the zirconia. The viability of this process is discussed in terms of machine limitations and ink drying problems.

Influence of Slurry Parameters on the Characteristics of Spray‐Dried Granules

Abstract: A systematic study has been performed to determine how the characteristics of granules prepared by spray drying aqueous alumina slurries are influenced by processing parameters: binder type (PEG Compound 20M or PEG-8000), solids loading (30 or 40 vol%), ammonium polyacrylate deflocculant level (0.35-1.00 wt%), and spray-dryer type. Correlations between slurry rheology and granule characteristics have been made, and a model for granule formation is presented. The packing density of the primary particles within the granules is lower for slurries with higher yield stress and is dependent on the slurry solids loading. Granules prepared using 0.35 wt% deflocculant (0.14 mg/m2), which correspond to high slurry yield stress, are of solid morphology, whereas higher deflocculant levels result in hollow granules that contain a single large open pore or crater. The degree to which particles are able to rearrange during drying influences the final granule density and is determined by the strength of the floc structure, as indicated by the slurry yield stress. When the yield stress is low, a crater may form from the inward collapse of the surface of a forming granule when the particle packing density in a droplet continues to increase after the droplet size becomes fixed by the formation of a rigid shell, leaving an internal void with internal pressure lower than that of the surrounding atmosphere.

Origin of Solid‐State Activated Sintering in Bi2O3‐Doped ZnO

Abstract: Activated sintering in Bi2O3-doped ZnO has been studied with emphasis on the mechanistic role of intergranular amorphous films. The atomic-level microstructures and bismuth solute distributions in doped powders have been investigated using high-resolution electron microscopy and scanning transmission electron microscopy. Densification is observed to be significant below the bulk eutectic temperature in the presence of Bi2O3 concentrations as low as 0.58 mol%. Transmission electron microscopy of as-calcined and sintered powders shows that significant neck growth and particle coarsening occur in the solid state. Intergranular amorphous films of ∼1 nm thickness, terminating in wetting menisci at sinter-necks, are observed to form concurrently with the onset of activated sintering. In a few instances, amorphous films are also observed at surfaces of the ZnO particles. These films appear to be the free-surface counterpart to equilibrium-thickness intergranular films. Activated sintering in this binary system is attributed to rapid mass transport through subeutectic, equilibrium-thickness intergranular films, with the amorphous phase also providing capillary pressure.

Mechanochemical Synthesis of Lead Zirconate Titanate from Mixed Oxides

Abstract: High-density lead zirconate titanate (PZT) ceramics have been successfully prepared by using a novel mechanochemical fabrication technique, which skips the phase-forming calcination at an intermediate temperature that is always required in the industrial processes currently in use. The fabrication technique starts with mixing of the low-cost industrial oxide powders, and the designed PZT perovskite phase is formed by reacting the oxide constituents in a mechanochemical chamber that consists of a cylindrical alumina vial and one stainless-steel ball inside it. The solid-state reaction among constituent oxides is activated via mechanical energy instead of high temperature. When mechanochemically activated for 20 h, an ultrafine PZT powder of perovskite structure with a minimized degree of particle agglomeration is obtained. The resulting PZT powder sinters to 99.0% of theoretical density at 1100°C for 1 h. The sintered PZT ceramic exhibits a dielectric constant of 1340 and a dielectric loss of 0.6% at a frequency of 1 kHz at room temperature.

Preparation of Dense BaTiO3 Ceramics with Submicrometer Grains by Spark Plasma Sintering

Abstract: Dense BaTiO3 ceramics consisting of submicrometer grains were prepared using the spark plasma sintering (SPS) method. Hydrothermally prepared BaTiO3 (0.1 and 0.5 µm) was used as starting powders. The powders were densified to more than similar/congruent95% of the theoretical X-ray density by the SPS process. The average grain size of the SPS pellets was less than similar/congruent1 µm, even by sintering at 1000-1200°C, because of the short sintering period (5 min). Cubic-phase BaTiO3 coexisted with tetragonal BaTiO3 at room temperature in the SPS pellets, even when well-defined tetragonal-phase BaTiO3 powder was sintered at 1100° and 1200°C and annealed at 1000°C, signifying that the SPS process is effective for stabilizing metastable cubic phase. The measured permittivity was similar/congruent7000 at 1 kHz at room temperature for samples sintered at 1100°C and showed almost no dependence on frequency within similar/congruent100-106 Hz; the permittivity at 1 MHz was 95% of that at 1 kHz.

Crystallization Kinetics and Mechanism of Low-Dielectric, Low-Temperature, Cofirable CaO-B2O3-SiO2 Glass-Ceramics

Abstract: The crystallization kinetics and mechanism of low-dielectric, low-temperature, cofirable CaO-B2O3-SiO2 glass-ceramics were investigated. Crystalline phases formed during firing included calcium silicates (CaSiO3, Ca3Si2O7, Ca2SiO4) and calcium borate (CaB2O4), with crystalline wollastonite (CaSiO3) the major phase. The crystallization kinetics of wollastonite followed an Avrami equation. The results of the present study showed an apparent activation energy of 200-260 kJ/mol. Combined with the results of reduced growth rate (growth rate × viscosity) and thermal analysis, the rate-controlling mechanism of crystallization appeared to be a two-dimensional surface nucleation growth. As the amount of crystalline wollastonite increased, the dielectric constant decreased, but the thermal expansion coefficient remained relatively unchanged.

Control of Microstructure and Orientation in Solution-Deposited BaTiO3 and SrTiO3 Thin Films

Abstract: Columnar and highly oriented (100) BaTiO{sub 3} and SrTiO{sub 3} thin films were prepared by a chelate-type chemical solution deposition (CSD) process by manipulation of film deposition conditions and seeded growth techniques. Randomly oriented columnar films were prepared on platinum-coated Si substrates by a multilayering process in which nucleation of the perovskite phase was restricted to the substrate or underlying layers by control of layer thickness. The columnar films displayed improvements in dielectric constant and dielectric loss compared to the fine-grain equiaxed films that typically result from CSD methods. Highly oriented BaTiO{sub 3} and SrTiO{sub 3} thin films were fabricated on LaAlO{sub 3} by a seeded growth process that appeared to follow a standard two-step growth mechanism that has been previously reported. The film transformation process involved the bulk nucleation of BaTiO{sub 3} throughout the film, followed by the consumption of this matrix by an epitaxial overgrowth process originating at the seed layer. Both BaTiO{sub 3} and PbTiO{sub 3} seed layers were effective in promoting the growth of highly oriented (100) BaTiO{sub 3} films. Based on the various processing factors that can influence thin film microstructure, the decomposition pathway involving the formation of BaCO{sub 3} and TiO{sub 2} appeared tomore » dictate thin film microstructural evolution.« less

Microstructure and Fracture Toughness of Si3N4 Ceramics: Combined Roles of Grain Morphology and Secondary Phase Chemistry

Abstract: Silicon nitride materials that contained different mixtures of sintering aids were investigated with respect to microstructure development and resulting fracture toughness. Postsintering annealing at 1850°C for various times was adopted in order to coarsen the respective microstructures. Although constant processing conditions were used, a marked variation in fracture toughness of the Si3N4 materials was evaluated. With a larger grain diameter of the Si3N4 grains, an increase in fracture resistance was generally observed. However, a correlation between fracture toughness and apparent aspect ratio could not be established. The observed changes in microstructure were in fact caused by the difference in secondary-phase chemistry. Si3N4 grain growth was dominated by diffusion-controlled Ostwald ripening and was hence affected by the viscosity of the liquid at processing temperature. In addition, crystallization at triple pockets also depends on the sintering additives employed and was found to influence fracture toughness by altering the crack-propagation mode as a consequence of local residual microstresses at grain boundaries. The stress character (compressive vs tensile) is governed by the type of crystalline secondary phase formed. Moreover, a variation in interface chemistry changes the glass network structure on the atomic level, which can promote transgranular fracture, i.e., can result in a low fracture resistance even in the presence of favorable large Si3N4 matrix grains. Therefore, secondary-phase chemistry plays a dominant role with respect to the mechanical behavior of liquid-phase-sintered Si3N4. Fracture toughness is, in particular, influenced by (i) altering the residual glass network structure, (ii) affecting the secondary-phase crystallization at triple pockets, and (iii) changing the Si3N4 grain size/morphology by affecting the diffusion rate in the liquid. The first two effects of secondary-phase chemistry are superimposed on the merely structural parameters such as grain diameter and apparent aspect ratio.

Engineered Porosity via Tape Casting, Lamination and the Percolation of Pyrolyzable Particulates

Abstract: Sintered ceramic preforms with open pore volumes from 20% to 80% (with no alteration in sintering shrinkage) were developed by adding pyrolyzable pore-forming agents (PFAs) to a tape-casting colloidal suspension. Sintered porous characteristics were directly controlled by the amount, size, and distribution of the PFA added to the green tape as well as adjustments made to the tape formulation. A conceptual model of the green tape microstructure was used to explain the influence of PFA and tape formulation on retained porosity and sintering shrinkage. The creation of a connected, open, porous network within the preform was the result of PFA particle percolation within the green body.

Fracture of Glass/Poly(vinyl butyral) (Butacite®) Laminates in Biaxial Flexure

Abstract: Glass-polymer laminates designed as safety glazing for automotive and architectural applications demonstrate a rich variety of deformation and failure modes due to the complex stress fields developed on loading and the statistical nature of glass fracture. This complexity in stress development results from the large modulus mismatch between float glass and typical polymers used in safety glazing (E glass /E polymer ≃10 3 -10 5 ). We investigate stress development and the sequence of glass-ply fracture in model two-ply glass-poly(vinyl butyral) (PVB; Butacite®) laminates during loading in biaxial flexure using a circular (upper) punch on three-point (lower) support. The experiment is analyzed using a three-dimensional finite-element model with a viscoelastic constitutive model of plasticized PVB deformation. The stress analysis shows that the maximum biaxial stress shifts location from one glass ply to the other as a function of loading rate and/or temperature and the loading-support dimensions. We identify two primary modes for the initiation of failure associated with changes in maximum stress location: (1) first crack initiated in upper, ring-loaded, glass ply (at the internal glass-polymer interface) and (2) first crack initiated in lower, supported, glass ply (outer glass surface). The sequence of glass ply fracture is seen to depend strongly on loading rate and temperature: high temperatures, relative to the polymer-glass transition temperature, and/or slow loading rates bias first cracking to the upper ply; low temperatures and/or high loading rates promote lower ply first cracking. We present a method to compute the probability of first cracking by combining the finite-element-based stress analysis with a Weibull statistical description of glass fracture. The test protocol and stress analysis presented can form the basis of a laboratory-scale test for laminates and can be readily extended to describe load-bearing capacity of laminate plates used in large-scale commercial applications.

Factors Affecting the Electrical Conductivity of Donor‐Doped Bi4Ti3O12 Piezoelectric Ceramics

Abstract: High-temperature piezoelectric ceramics based on W6+-doped Bi4Ti3O12 (W-BIT) were prepared by both the conventional mixing oxides and the chemical coprecipitation methods. Sintering was carried out between 800° and 1150°C in air. A rapid densification, >99% of the theoretical density (rhoth) at 900°C/2 h, took place in the chemically prepared W6+-doped Bi4Ti3O12 ceramics, whereas conventionally prepared BIT-based materials achieved a lower maximum density, ∼94% of rhoth, at higher temperature (1050°C). The microstructure study revealed a platelike morphology in both materials. Platelike grains were larger in the conventionally prepared W-BIT-based materials. The sintering behavior could be related both to the agglomeration state of the calcined powders and to the enlargement of the platelets at high temperature. The W6+-doped BIT materials showed an electrical conductivity value 2-3 orders of magnitude lower than undoped samples. The electrical conductivity increased exponentially with the aspect ratio of the platelike grains. The addition of excess TiO2 produced a further decrease of the electrical conductivity.

Electromechanical Behavior of PZT-Brass Unimorphs

Abstract: We have examined the static and dynamic electromechanical responses of PZT‐brass (piezoelectric‐nonpiezoelectric) plate unimorphs of various brass/PZT thickness ratios. The study was performed both experimentally and theoretically. The static measurements showed that, given a PZT layer thickness, there exists a brass/PZT thickness ratio that gives the unimorph the highest static displacement under an applied field. The effects of geometric shape and external loading on the displacement of the unimorphs were also examined. The dynamic measurements showed that, given a PZT layer thickness, the bending-mode resonance frequencies increase with an increasing brass/PZT thickness ratio. These results were in good agreement with the theoretical predictions that were obtained with the plate geometry.

Geometrical Origin and Theory of Negative Thermal Expansion in Framework Structures

Abstract: By framework structures are meant materials consisting of relatively stiff units such as octahedra or tetrahedra, joined by shared oxygen (or other) atoms at the corners. Examples are ZrW2O8 and many aluminosilicates. Rigid rotation of the units often gives a reduction of the volume or of some lattice constant as a purely geometrical effect. The theory of this effect is developed and shown to give a negative contribution to the thermal expansion coefficient. This is in addition to the usual positive contribution from anharmonicity of the interatomic forces. The negative effect varies through the phonon spectrum, being strongest for low frequencies, but the sign of the temperature coefficient may be reversed above a soft mode phase transition.

Defect reactions and the controlling mechanism in the sintering of magnesium aluminate spinel

Abstract: Pressureless sintering studies have been conducted for excess Al2O3, stoichiometric, and excess MgO compositions of MgAl2O4 at 1500-1625°C. Initial powders of various compositions are prepared by solid-state reaction of MgO and Al2O3. A Brouwer defect equilibrium diagram is constructed that assumes intrinsic defects of the Schottky type. The densification rate derived from sintering kinetics is compared with the compositions investigated when the concentration is converted to the activity of the two oxide components in MgAl2O4. The grain-size exponent of p similar/congruent 3 suggests that densification takes place by a lattice-diffusion mechanism in the solid state. Determined activation enthalpies of 489-505 kJmol-1 are close to those obtained from oxygen self-diffusion derived in previous sintering studies. It is, therefore, proposed that oxygen lattice diffusion through vacancies is the rate-controlling mechanism for the sintering of nonstoichiometric MgAl2O4 compositions. The discrepancy between densification-rate ratios in experimental results and oxygen vacancy concentration in the Brouwer diagram is accounted for by the defect associates formed in the nonstoichiometric compositions.

High-Resolution Transmission Electron Microscopy of Ti4AlN3, or Ti3Al2N2 Revisited

Abstract: The structure and chemistry of what initially was proposed to be Ti3Al2N2 are incorrect. Using high-resolution transmission electron microscopy, together with chemical analysis, the stoichiometry of this compound is concluded to be Ti4AlN3-delta (where delta = 0.1). The structure is layered, wherein every four layers of almost-close-packed Ti atoms are separated by a layer of Al atoms. The N atoms occupy ∼97.5% of the octahedral sites between the Ti atoms. The unit cell is comprised of eight layers of Ti atoms and two layers of Al atoms; the unit cell is hexagonal with P63/mmc symmetry (lattice parameters of a= 0.3 nm and c= 2.33 nm). This compound is machinable and closely related to other layered, ternary, machinable, hexagonal nitrides and carbides, namely M2AX and M3AX2 (where M is an early transition metal, A is an A-group element, and X is carbon and/or nitrogen).

Aggregation effects on the compressive flow properties and drying behavior of colloidal silica suspensions

Abstract: The influence of aggregation phenomena on the compressive flow properties and drying behavior of nonaqueous and aqueous silica (SiO 2 ) suspensions of varying electrolyte (NH 4 Cl) concentrations were studied. Compressive rheology measurements, including sedimentation and centrifugal consolidation, were first conducted to investigate consolidation behavior in the absence of solvent evaporation. The volume-fraction-dependent osmotic pressure and compressive yield stress were determined for dispersed and flocculated SiO 2 suspensions, respectively. Consolidation behavior then was studied in situ by simultaneously measuring stress evolution and solvent loss as a function of drying time. The observed drying stress histories of the films were complex, consisting of several characteristic regions. First, there was an initial period of stress rise to a maximum drying stress. These measured stress values exhibited good agreement with the osmotic pressure and compressive yield stress at equivalent SiO 2 volume fractions for the dispersed and flocculated systems, respectively. Beyond the maximum drying stress there was a subsequent region of stress decay, which coincided with the draining of liquid-filled pores. No residual drying stress was detected for films prepared from salt-free SiO 2 suspensions, whereas salt-containing films exhibited residual drying stresses likely due to salt-bridging effects. Microstructural characterization of dried films prepared from aqueous SiO 2 suspensions revealed nonuniformities in the spatial distribution of colloidal particles and precipitated salt, with the highest concentrations located at the outer edges of the films. Such features result from capillary-induced transport of these species during drying, and they have important implications on colloidal processing of ceramic thick films and bulk forms.

Nanocrystallization and Phase Transformation in Monodispersed Ultrafine Zirconia Particles from Various Homogeneous Precipitation Methods

Abstract: Monodispersed ultrafine (nano- to micrometer) zirconia precursor powders were synthesized by three different physicochemical methods: (I) forced hydrolysis, (II) homogeneous precipitation in inorganic salt solutions, and (III) hydrolysis/condensation of alkoxide. The forced hydrolysis method produced monoclinic nanocrystalline particles (cube shaped) of nanometer scale, which depended largely on the initial salt concentration. Methods II and III, both involving the use of alcohol as a solvent, exhibited a faster particle formation rate and generated amorphous ultrafine (submicrometer) monodispersed microspheres, indicating that the presence of alcohol may have stimulated particle nucleation due to its low dielectric property (and, thus, the low solubility of nucleus species in mixed water-alcohol solutions). Nucleation and growth of the particles in solutions are discussed based on the measurements obtained by small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS). High-temperature X-ray diffraction (HTXRD) and TGA/DTA studies elucidated the differences in phase transformation for different types of powders. The most interesting finding was the nonconventional monoclinic nanocrystal nucleation and growth that occurred prior to transformation to the tetragonal phase (at 1200°C) during the heat treatment of the nanocrystalline powders produced by the forced hydrolysis.

Effect of Seeding and Water Vapor on the Nucleation and Growth of α‐Al2O3 from γ‐Al2O3

Abstract: Isothermal transformation kinetics and coarsening rates were studied in unseeded and alpha-Al2O3-seeded γ-Al2O3 powders heated in dry air and water vapor. Unseeded samples heated in dry air transformed to alpha-Al2O3 with an activation energy of 567 kJ/mol. Seeding with alpha-Al2O3 increased the transformation rates and reduced incubation times by providing low-energy sites for nucleation/growth of the alpha-Al2O3 transformation. The activation energy for the transformation was reduced to 350 kJ/mol in seeded samples heated in dry air. Seeded samples completely transformed to alpha-Al2O3 after 1 h at 1050°C when heated in dry air compared to 1 h at 925°C when heated in saturated water vapor. The combined effects of a lower nucleation barrier due to seeding and the increased diffusion due to water vapor reduced the activation energy for the transformation by 390 kJ/mol and the transformation temperature by ∼225°C compared to the unseeded samples heated in dry air. The accelerated kinetics is believed to be due to increased surface diffusion.

Stoichiometry Control and Phase Selection in Hydrothermally Derived BaxSr1–xTiO3 Powders

Abstract: Ba x Sr 1-x TiO 3 (BST) powders were processed at temperatures <100°C by reacting nanosized TiO 2 powders in alkaline, aqueous solutions of BaCl 2 , SrCl 2 , and NaOH. The effects of processing variables (NaOH concentration, time, temperature, and the ratios of barium, strontium, and titanium initially in solution) on the resultant BST powder stoichiometry and solid solubility were examined. In all cases, strontium was more readily incorporated into the BST powders than barium, and the extent varied systematically with the processing variables. BST powders that were processed in solutions with a large initial excess of barium and strontium, relative to titanium, consisted of a single-phase solid solution. In contrast, BST powders that were processed in solutions with a small initial excess of barium and strontium, relative to titanium, contained a biphasic solid solution which corresponded to separate barium-rich and strontium-rich phases.

Superfast Densification of Oxide/Oxide Ceramic Composites

Abstract: Superfast densification of ceramic composites in the pseudobinary system Al2O3-Y3Al5O12 was carried out by using a new technique called spark plasma sintering (SPS). Dense ceramic composites were obtained by heating appropriate powder mixtures to 1573 K in an SPS unit at a rate of 600 K/min. No holding time at 1573 K was applied. Scanning electron microscopy studies showed the compacted materials to contain submicrometer-sized grains of the same sizes as the precursor powder mixtures; i.e., no significant grain growth had occurred.

Effects of Interface Roughness on Residual Stresses in Thermal Barrier Coatings

Abstract: Using a newly developed object-oriented finite-element analysis method, both an actual microstructure and model microstructures of a plasma-sprayed thermal barrier coating system were numerically simulated to analyze the full-field residual stresses of this coating system. Residual stresses in the actual microstructure were influenced by both the irregular top-coat/bond-coat interface and cracks in the top coat. By treating the microcracked top coat as a more-compliant solid microstructure, the effects of the irregular interface on residual stresses were examined. These results then could be compared to results that have been obtained by analyzing a model microstructure with a sinusoidal interface, which has been considered by some earlier investigators.

Preparation of Porous Anatase Coating from Sol-Gel-Derived Titanium Dioxide and Titanium Dioxide-Silica by Water-Vapor Exposure

Abstract: The effects of water vapor on the crystallization behavior of sol-gel-derived titanium dioxide (TiO2) thin films that contained 0-50 mol% silica (SiO2) were investigated. Anatase formed on exposure to water vapor at 60°-180°C, with a simultaneous decrease in the concentration of OH groups. An increase in the SiO2 content of the exposed films led to an increase in the average crystalline size. Because crystallization of the exposed films of the films was not accompanied by shrinkage, porous anatase coatings were obtained via exposure at a relatively low temperature. Phase separation of the immiscible TiO2-SiO2 system was induced with water vapor, which resulted in acceleration of the crystallization of the sol-gel films.

Synthesis and Structural, Ferroelectric, and Piezoelectric Properties of SrBi4Ti4O15 Ceramics

Abstract: SrBi 4 Ti 4 O 15 ceramics were prepared via ordinary firing (OF) and hot forging (HF). Characterization using scanning electron microscopy and X-ray diffractometry shows platelike grains that were highly oriented (F = 0.95) after hot forging. Ferroelectric, dielectric, and piezoelectric characterizations revealed that polarization reorientation was restricted to the ab plane of the orthorhombic structure, parallel to the (Bi 2 O 2 ) 2+ layers. The thickness coupling factor for OF samples was only half that for HF samples oriented parallel to the HF direction (in the ab-plane), as a consequence of poling restrictions in randomly oriented grains.

Hi‐Nicalon/SiC Minicomposites with (Pyrocarbon/SiC)n Nanoscale Multilayered Interphases

Abstract: SiC/SiC minicomposites that comprise different pyrocarbon/silicon carbide ((PyC/SiC)n) multilayered interphases and a tow of SiC fibers (Hi-Nicalon) have been prepared via pressure-pulsed chemical vapor infiltration. Pyrocarbon and SiC were deposited from propane and a CH3SiCl3/H2 mixture, respectively. The microstructure of the interphases has been investigated using transmission electron microscopy. The mechanical tensile behavior of the minicomposites at room temperature exhibits the classical features of tough composites, regardless of the characteristics of the (PyC/SiC) sequences. The interfacial shear stress has been determined from the width of hysteresis loops upon unloading/reloading and from the crack-spacing distance at saturation. All the experimental data indicate that the strength of the fiber/interphase interfaces is rather weak (∼50 MPa).

Continuous Coating of Oxide Fiber Tows Using Liquid Precursors: Monazite Coatings on Nextel 720™

Abstract: Seven different aqueous or ethanolic precursors were used to continuously coat monazite (LaPO4) on Nextel 720™ fiber tows. Immiscible liquid displacement was used to minimize bridging of coating between filaments. Precursor viscosities, densities, and concentrations were measured, and solids were characterized by DTA/TGA and X-ray. Coatings were cured in-line at 1200°-1400°C and characterized for thickness, microstructure, and composition by optical microscopy, SEM, and TEM. Tensile strengths of the coated fibers varied with the precursor used and were 25% to >50% lower than those of the as-received fiber. The coating stoichiometry and coating thickness of a particular precursor did not correlate with tensile strength. Median coating thicknesses were typically ∼50–100 nm for precursors with 40–80 g/L monazite, much larger than thicknesses predicted by theory for 12 mm diameter monofilaments. There were significant thickness variations from filament to filament, but usually little variation in composition or microstructure. Amorphous AlPO4 layers formed from phosphate-rich precursors. Factors that could affect coating characteristics and tensile strength reduction were discussed.