bluuebunny/crossref_metadata_2025 huggingface dataset:
Dimensionless Parameters for Microstructural Pathways in Sintering
Authors: Nettleship Ian , Slaughter William S.
Published: July 2010

Dimensionless parameters have been developed to study microstructural pathways for the sintering of powders. These parameters are designed to facilitate the comparison of microstructural paths for any sintering experiments, independent of the characteristic length scales in the microstructures. Microstructural pathways constructed with the dimensionless parameters are found to be similar for three different alumina ceramics. The systematic differences between the experimental results and the predictions of models based on simplifying geometric assumptions are explained in terms of the packing disruptions in the green microstructures.
Strengthening and Toughening of Silicon Nitride by Superplastic Deformation
Authors: Kondo Naoki , Ohji Tatsuki , Wakai Fumihiro
Published: July 2010

Mechanical properties of silicon nitride which was superplastically deformed in plane‐strain compression were investigated. Superplastically deformed silicon nitride exhibited a highly anisotropic microstructure, where rod‐shaped grains tended to be aligned along the extruding direction. The bending strength and fracture toughness were increased substantially by the deformation process when a stress was applied in the extruding direction. It appears that these improvements were mainly due to effective operation of grain bridging and pull‐out by the grain alignment.
Multilayered Oxide Interphase Concept for Ceramic‐Matrix Composites
Authors: Lee Woo Y. , Lara‐Curzio Edgar , More Karren L.
Published: July 2010

Hi‐Nicalon/SiC minicomposite specimens containing three oxide interphase layers (amorphous SiO2, monoclinic ZrO2, and amorphous SiO2) were prepared by chemical vapor deposition. The minicomposites exhibited graceful composite failure behavior with reasonable load‐carrying capability in room‐temperature tensile tests. Much of the composite behavior and load‐carrying capability was retained even after matrix precracking and subsequent oxidation in air at 960°C for 10 h. In both the as‐prepared and oxidized specimens, crack deflection and fiber pull‐out occurred preferentially within the multilayered interphase region. The potential merits and uncertainties associated with this multilayered oxide interphase approach were discussed in the context of designing environmentally durable interfaces for ceramic‐matrix composites.
Correlation between Surface Texture and Chemical Composition in Undoped, Hard, and Soft Piezoelectric PZT Ceramics
Authors: Hammer Marianne , Monty Cecile , Endriss Axel , Hoffmann Michael J.
Published: July 2010

The electrical and mechanical properties of piezoelectric lead zirconate titanate (PZT) ceramics are strongly influenced by domain‐wall motion, which can be tailored via the substitution of ions within the perovskite structure. Different domain mobilities are achieved by lead and/or oxygen vacancies, according to the valency and ionic radius of the dopants. To quantify the surface near domain mobility, hard (silver‐doped), soft (lanthanum‐doped), and undoped PZT ceramics have been prepared. An applied mechanical stress after sintering causes texturing near the surface, because of the ferroelastic behavior of the 90° domains. The texture is quantified via X‐ray diffractometry (XRD) analysis of the tetragonal FT(002)‐ and FT(200)‐peak intensities, using least‐squares refinement with Gaussian profile functions. The samples are subsequently annealed to remove the surface texture and again characterized via XRD measurements. However, annealed samples still reveal a preferred domain orientation that can be removed only by a second annealing of the pulverized samples. A comparison of the tetragonal additive systems clearly reveals the greatest domain mobility for lanthanum‐doped PZT ceramics, whereas the silver‐doped and undoped samples have similar ferroelastic behavior. Furthermore, the surface texture of all the compositions is dependent on the applied mechanical stress and cannot be removed completely by heat treatment.
Influence of SO3 on the Phase Relationship in the System CaO–SiO2–Al2O3–Fe2O3
Authors: Uda Satoshi , Asakura Etsuro , Nagashima Masahisa
Published: July 2010

The influence of the additive SO3 on the phase relationships in the quaternary system CaO‐SiO2‐Al2O3‐Fe2O3 was investigated by observing the change of volume ratio of 3CaOSiO2 (C3S) to 2CaOSiO2 (C2S) + CaO (C) in the sintered material with the increase of SO3 content. The primary phase volume of C3S in the quaternary phase diagram shrank with the increase of SO3 and disappeared when the SO3 content exceeded 2.6 wt% in the sintered material. Changes in the peritectic reaction relationship between CaO (C), 2CaOSiO2 (C2S), 3CaOSiO2 (C3S), 3CaOAl2O3 (C3A), 4CaOAl2O3Fe2O3 (C4AF), and liquid were also observed and discussed.
Mechanically Enhanced Reactivity of Silicon for the Formation of Silicon Nitride Composites
Authors: Shaw Leon L. , Yang Zhenguo , Ren Ruiming
Published: July 2010

The chemical reactivity of silicon can be enhanced by mechanical activation. Ball milling elemental silicon powders with a small amount of carbon addition at ambient temperature has resulted in an enhanced nitridation of silicon at high temperatures. In comparison with powder mixtures without milling, the nitridation process at 1250°C has been accelerated by a factor of 9 and 23 by milling powder mixtures in N2 and NH3, respectively, before nitridation. The enhanced nitridation process for powders milled in N2 is primarily attributed to the mechanical activation, whereas for powders milled in NH3, the trapped nitrogen within the powder mixtures during milling also contributes to the enhanced nitridation process, in addition to the effect of the mechanical activation.
Perspectives on the History of Glass Composition
Authors: Kurkjian Charles R. , Prindle William R.
Published: July 2010

The 100th anniversary of The American Ceramic Society corresponds approximately with the 100th anniversary of what might be considered the start of the age of glass science, i.e., the publication, in Germany, in 1886, of the catalog of Schott und Genossen, containing 44 optical glass compositions. The American Ceramic Society centennial seems, accordingly, to be an appropriate occasion to examine the history of glass composition that both preceded and followed the seminal work of Schott and to survey some of the major discoveries and changes in glass composition as well as the reasons that led to them. Although it is certainly of interest to consider a more complete history of the glass industry, we have opted to attempt the more modest task just described. The history of glass structure theories is considered, particularly with regard to the effects of composition on structure, and how these relate to glass properties. The article then continues with a discussion of recent special glasses and concludes with a description of lightguide glasses, the discovery of which has changed the nature of glass science and the glass industry.
Technical Cost Framework for High‐Temperature Manufacturing of Small Components and Devices
Authors: Evans Anthony G. , Hutchinson John W. , Hutchinson Robert G. , Sugimura Yuki , Lu Tian Jian
Published: July 2010

The high‐temperature step that is required in a manufacturing process can contribute substantially to the cost. A technical cost framework (TCF) that allows trends in these costs to be determined as a function of the essential variables provides opportunities for their minimization. The basic variables are the furnace capacity, the operating temperature, and the furnace construction. The dependent variables are power, acquisition, and replacement. Information from thermal and process models as well as the furnace element life expectancy provides the functions needed to conduct the minimization through the TCF. The overall strategy is developed and illustrated with examples.
Lead Zirconate Titanate Fine Fibers Derived from Alkoxide‐Based Sol‐Gel Technology
Authors: Meyer Richard , Shrout Thomas , Yoshikawa Shoko
Published: July 2010

Fine‐scale fibers with diameters <100 µm were prepared using metal alkoxide precursors. Variables that influence fiber drawing and fiber properties in the lead zirconate titanate (PZT) compositional system were investigated to obtain the processing conditions necessary to fabricate high‐quality fibers. The gel‐to‐ceramic conversion was characterized. In addition to drying, firing cycles at temperatures >650°C were needed to achieve phase‐pure perovskite fibers. Dense fibers with diameters of 10‐70 µm and grain sizes <1 µm were found with sintering temperatures up to 1000°C. Increasing the firing temperature to 1200°C for 30 min resulted in grain growth up to 2.5 µm. Electrical properties of both lanthanum‐doped and undoped PZT fibers were measured and were comparable to bulk disks fabricated using powder derived from the same sol.
Effects of Crystal Orientation and Temperature on the Strength of Sapphire
Authors: Schmid Frederick , Harris Daniel C.
Published: July 2010

The flexure and compressive strengths of sapphire are dependent on crystal orientation and temperature. Most notably, the c ‐axis compressive strength decreases below the tensile strength at temperatures >400°C and falls to 2% of the room‐temperature compressive strength at 800°C. Loss of compressive strength complicates the interpretation of flexure tests. Four‐point flexure specimens with no component of c ‐axis compression increase in strength at temperatures >500°C; however, specimens that have c ‐axis compression decrease in strength. It has been observed that c ‐axis compression causes twinning on rhombohedral crystal planes. Intersection of twins on different rhombohedral planes causes fracture that leads to mechanical failure.
Effect of Incidence Angle on the Impact‐Wear Behavior of Silicon Nitride
Authors: Zanoria Elmer S. , Blau Peter J.
Published: July 2010

Experiments were conducted by repeatedly hammering a silicon nitride (Si3N4) ball on a polished different Si3N4 material at different incidence angles: 45°, 60°, 75°, and 90°. Material removal during impact was dominated by the delamination process, which involved abrasion and fracturing of the parent material, compaction of pulverized debris into a film, nucleation of cracks along the film/substrate interface, and lateral propagation of the interfacial cracks. The dependency of repeated impact wear on the incidence angle involved a combination of the effects of normal and tangential stresses on surface deformation. The contrasting influences of these stresses resulted in a minimization of impact wear of the polished Si3N4 material at incidence angles in the range of 60°‐75°.
Grain Growth Control and Dopant Distribution in ZnO‐Doped BaTiO3
Authors: Caballero Amador C. , Fernández José F. , Moure Carlos , Durán Pedro , Chiang Yet‐Ming
Published: July 2010

ZnO additions to BaTiO3 have been studied in order to determine the role of this dopant on sintering and microstructure development. As a consequence of a better initial dopant distribution, samples doped with 0.1 wt% zinc stearate show homogeneous fine‐grained microstructure, while a doping level of 0.5 wt% solid ZnO is necessary to reach the same effect. When solid ZnO is used as the dopant precursor, ZnO is redistributed among the BaTiO3 particles during heating. Since no liquid formation has been detected for temperatures below 1400°C in the system BaTiO3‐ZnO, it is proposed that dopant redistribution takes place by vapor‐phase transport and grain boundary diffusion. Shrinkage and porosimetry measurements have shown that grain growth is inhibited during the first step of sintering for the doped samples. STEM‐EDX analysis revealed that solid solubility of ZnO into the BaTiO3 lattice is very low, being strongly segregated at the grain boundaries. Grain growth control is attributed to a decrease in grain boundary mobility due to solute drag. Because of its effectiveness in controlling grain growth, ZnO appears to be an attractive additive for BaTiO3 dielectrics.
Oxidation Behavior of Chemically‐Vapor‐Deposited Silicon Carbide and Silicon Nitride from 1200° to 1600°C
Authors: Fox Dennis S.
Published: July 2010

The isothermal oxidation of pure CVD SiC and Si3N4 has been studied for 100 h in dry, flowing oxygen from 1200° to 1600°C in an alumina tube furnace. Adherent oxide formed at temperatures to 1550°C. The major crystalline phase in the resulting silica scales was alpha‐cristobalite. Parabolic rate constants for SiC were within an order of magnitude of literature values. The oxidation kinetics of Si3N4 in this study were not statistically different from that of SiC. Measured activation energies were 190 kJ/mol for SiC and 186 kJ/mol for Si3N4. Silicon oxynitride did not appear to play a role in the oxidation of Si3N4 under the conditions herein. This is thought to be derived from the presence of ppm levels of sodium impurities in the alumina furnace tube. It is proposed that sodium modifies the silicon oxynitride, rendering it ineffective as a diffusion barrier. Material recession as a function of oxide thickness was calculated and found to be low. Oxidation behavior at 1600°C differed from the lower temperatures in that silica spallation occurred after exposure.
Interfacial Bond Strength in SiC/C/SiC Composite Materials, As Studied by Single‐Fiber Push‐Out Tests
Authors: Rebillat Francis , Lamon Jacques , Naslain Roger , Lara‐Curzio Edgar , Ferber Mattison K. , Besmann Theodore M.
Published: July 2010

The interfacial characteristics of SiC/C/SiC composites with different fiber‐coating bond strengths have been investigated using single‐fiber push‐out tests. Previous studies have shown that weak or strong bonds can be obtained by using as‐received or treated fibers, respectively, and that the stress‐strain behavior is improved with the treated fibers. This effect results from multiple branching of the cracks within the interphase. The model used to extract interfacial characteristics from nanoindentation and microindentation tests does not consider the presence of an interphase. However, the results highlight the significant effect of the interphase on the interfacial parameters, as well as the effect of roughness along the sliding surfaces. For the composite with treated fibers, the uncommon upward curvature of the push‐out curves is related to different modes of crack propagation in the interphase. Different techniques are required to analyze the interfacial properties, such as nanoindentation and microindentation with push‐out and push‐back tests.
Synthesis and Sintering of Cordierite from Ultrafine Particles of Magnesium Hydroxide and Kaolinite
Authors: Sumi Katsuhiro , Kobayashi Yuichi , Kato Etsuro
Published: July 2010

Thermal reactions of magnesium compounds and kaolinite were investigated to obtain dense cordierite ceramics without additives. Magnesium hydroxide was precipitated from aqueous solution in the form of ultrafine hexagonal tabular particles of about 0.1 µm, and heating this mixture with kaolinite in a mole ratio of MgOAl2O32SiO2 resulted in formation of an amorphous state at about 900°C following thermal decomposition. µ‐Cordierite was then crystallized from the amorphous phase at about 950°C, and gradually transformed into alpha‐cordierite. Firing the pressed specimens yielded a dense alpha‐cordierite ceramic with a relative density of 97.7% at 1350°C.
Low‐Temperature Synthesis of Diamond‐like Carbon Films on Steel Substrates by Electron‐Cyclotron‐Resonance Plasma‐Enhanced Chemical Vapor Deposition
Authors: Zhu Shen , Shahedipour Fatemeh , White Henry W.
Published: July 2010

Using microwave electron‐cyclotron‐resonance plasma‐enhanced chemical vapor deposition, diamond‐like carbon films were directly grown at low temperatures (lessthan equal to400°C) on Fe‐based alloy substrates without diamond seeding or use of a template layer. A single, broad line in the Raman spectra was observed in the region of 1328‐1335 cm‐1 for films grown in gas mixtures with a ratio of CH4:H2 greaterthan equal to 2%. In contrast, disordered carbon and graphite phases appeared in the spectra for film grown with a concentration of 20% CH4 in hydrogen. Diamond nucleation with an amorphous carbon layer was observed in the initial growth stage, while many diamond particles with irregular morphological features were observed on the surface of thicker films. These growth features are a consequence of the catalytic nature of the Fe‐based substrate.
Permittivity and Permeability of Mullite‐SiC‐Whisker and Spinel‐SiC‐Whisker Composites
Authors: Ruh Robert , Chizever Hirsch M.
Published: July 2010

Real and imaginary permittivity (ɛ′ and ɛ″, respectively) values were determined on mullite‐silicon carbide whisker (mullite‐SiCw) composites, mullite‐ZrO2 composites, and spinel‐SiCw composites, in the frequency range of 2‐18 GHz, using a 14 mm coaxial air‐line measurement system. Depending on the frequency, the addition of 30 vol% of SiCw to mullite increases ɛ′ from 6.6‐6.9 to 16‐22 and increases ɛ′ from 0 to 16‐22. The addition of 32 vol% of SiCw to spinel increases ɛ′ from 7.5‐7.8 to 37‐44 and increases ɛ″ from 0 to 13‐20. The addition of 21 vol% of ZrO2 to mullite increases ɛ″ to ∼9 but does not change ɛ″. Real and imaginary permeability values were 1 and 0, which are within experimental error.
Morphology and Crystallization Behavior of Sol‐Gel‐Derived Titania
Authors: Slunecko Jaroslav , Kosec Marija , Holc Janez , Drazic Goran , Orel Boris
Published: July 2010

TiO2 sols were prepared by hydrolysis of titanium ethoxide. The sols were exposed for several days at room temperature to atmospheres of different humidity in order to study its influence during aging of sols on rutile formation, morphology, and crystal structure. Exposure of sols to high relative humidity resulted in nonagglomerated TiO2 powders with a rutile structure. TiO2 gels obtained from sols aged at low relative humidity were composed of gel fragments with a poorly crystallized anatase structure.
Synthesis and Fluorescence Properties of Eu2+‐Complex‐Doped SiO2 Gels
Authors: Tsuboi Hisanori , Soga Kohei , Inoue Hiroyuki , Makishima Akio
Published: July 2010

Eu2+‐complex‐doped SiO2 gels were prepared and their fluorescence properties were studied. SiO2 gels were obtained by the reactions of silicon tetrachloride, tert‐butyl alcohol, and methyl alcohol under nitrogen. Both to protect Eu2+ against oxidation and to enhance the emission intensity of Eu2+, Eu2+ was complexed with 15‐crown‐5 or its derivative. Samples showed blue emission. Fluorescence lifetimes of samples varied from 690 to 955 ns depending on the Eu2+ concentration at 290 K. After drying of samples, 50‐70% of Eu2+ remained in the divalent state without oxidation in the samples. It was found that Eu2+ complexes inside gels were stable against oxidation in air.
Ag‐Bi1.5Y0.5O3 Composite Cathode Materials for BaCe0.8Gd0.2O3‐Based Solid Oxide Fuel Cells
Authors: Wu Zhonglin , Liu Meilin
Published: July 2010

Ag‐Bi1.5Y0.5O3 composites exhibit high electronic and ionic conductivities when the volume fraction of the silver phase is properly chosen, indicating that these composites are candidate cathode materials for solid‐state ionic devices. In this study, the chemical and thermal compatibilities between Ag‐Bi1.5Y0.5O3 composites and BaCe0.8Gd0.2O3 electrolyte are studied at intermediate temperatures. Furthermore, the electrochemical properties of the interfaces between the composites and the electrolyte are characterized under various conditions using impedance spectroscopy. The stability of the electrodes is estimated from the time dependence of the interfacial resistances. Results indicate that Ag‐Bi1.5Y0.5O3 composites are promising cathode materials for solid oxide fuel cells based on BaCe0.8Gd0.2O3 electrolyte at intermediate temperatures.
X‐ray Computed Tomography for Evaluation of Density Gradient Formation during the Compaction of Spray‐Dried Granules
Authors: Deis Terry A. , Lannutti John J.
Published: July 2010

X‐ray computed tomography was used to quantify the internal structure of compacts made from poly(vinyl alcohol)‐containing spray‐dried agglomerates at specific levels of humidity. High humidities increased the average density but decreased the uniformity of the density distribution. Lower levels of humidity generally produced more diffuse high density zones. Internal uniformity is superior at lower humidities where agglomerate fracture is more common than gradual agglomerate deformation. SEM has been used to determine zone‐specific agglomerate behavior at 0% and 98% relative humidity. These results can be used to connect gradient development to compaction curve behavior.
Effects of Boron, Carbon, and Iron Content on the Stacking Fault Formation during Synthesis of β‐SiC Particles in the System SiO2‐C‐H2
Authors: Seo Won‐Seon , Koumoto Kunihito , Arai Shigeo
Published: July 2010

Through the systematic addition of B, C, and Fe, additive effects on the stacking fault formation and the morphology of the particles formed during reaction synthesis of β‐SiC were investigated in the present study. The whisker content of the synthesized product and the formation mechanism of whiskers were closely related to the stacking fault density. The addition of B inhibited whisker formation probably because of isotropic imperfection and the suppression of surface diffusion. Increase in the reduction force by using an active carbon source and also adding excess carbon led to an increase in stacking fault density through enhanced whisker formation. In the presence of Fe, the synthesized β‐SiC whiskers appeared to possess only a small amount of stacking faults. The growth mechanism was different with Fe; i.e., isotropic growth occurred via a dissolution‐precipitation reaction through a liquid phase in the Fe‐Si system.
Orientation Relationships of Reactively Grown Ba6Ti17O40 and Ba2TiSi2O8 on BaTiO3 (001) Determined by X‐ray Diffractometry
Authors: Senz Stephan , Graff Andreas , Blum Werner , Hesse Dietrich , Abicht Hans‐Peter
Published: July 2010

Fresnoite grows at 700° and 800°C, and Ba6Ti7O40 grows at 1200°C with definite orientations, which are determined by X‐ray diffraction pole figure analysis. Partially textured fresnoite is formed at higher temperatures. The SiO2 films react with the BaTiO3 crystals, forming the phases Ba2TiSi2O8 (fresnoite) and Ba6Ti17O40. At 700° and 800°C, both phases grow with definite orientations, which are determined by X‐ray diffraction pole figure analysis. Partially textured polycrystalline phases are formed at higher temperatures.
Newtonian Viscosity of Amorphous Silicon Carbonitride at High Temperature
Authors: An Linan , Riedel Ralf , Konetschny Christoph , Kleebe H.‐Joachim , Raj Rishi
Published: July 2010

The creep viscosity of chemical‐precursor‐derived silicon carbonitride (SiCN), which is known to remain predominantly amorphous at temperatures below 1400°C, was measured in the temperature range 1090‐1280°C. Experiments were done in uniaxial compression at constant loads in pure nitrogen atmosphere. The creep behavior exhibited three stages. In stage I the strain rate decreased rapidly with time and deformation was accompanied by densification. In stage II the samples exhibited a steady‐state creep rate. In stage III, which commenced after long‐term deformation, creep gradually declined to rates that were below the sensitivity of our apparatus. The relative density of the specimens during stage II and stage III remained constant at ≅2.3 g/cm3. The shear viscosity in stage II was nearly Newtonian and was measured to be 1.3 × 1013‐5.0 1013 Pa·s at 1280°C, which is approximately 103 times the value for fused silica. The creep‐hardened as well as uncrept specimens contained silicon nitride crystallites. The volume fraction of these crystals was variable but always less than 5%. Such a small volume fraction of crystals does not explain the dramatic creep‐hardening behavior in stage III, even if it is assumed that the crystals formed during creep deformation in stage II.
Reaction‐Bonded Three‐Layer Alumina‐Based Composites with Improved Damage Resistance
Authors: She Jihong , Scheppokat Sven , Janssen Rolf , Claussen Nils
Published: July 2010

Three‐layer alumina‐based composites with Al2O3‐containing mullite as an outer layer and ZrO2‐containing alumina as an inner layer were fabricated by die‐pressing and reaction‐bonding. The strength of samples with Vickers indents made at loads up to 300 N was investigated. When compared to monolithic ceramics, three‐layer composites exhibit excellent damage resistance due to compressive stresses in the outer layer. Hot isostatically pressed samples show a four‐point bend strength of 800 MPa after indentation at 300 N.
Sol‐Gel Synthesis of Amorphous Calcium Phosphate and Sintering into Microporous Hydroxyapatite Bioceramics
Authors: Layrolle Pierre , Ito Atsuo , Tateishi Tetsuya
Published: July 2010

A new route for preparing hydroxyapatite (Ca10(PO4)6(OH)2) bioceramic has been described. An amorphous, nanosized, and carbonate‐containing calcium phosphate powder that had a Ca:P ratio of 1.67 was synthesized from calcium diethoxide and phosphoric acid in ethanol via a sol‐gel method. The powder was pressed at 98 MPa into green specimens and then heated to a temperature range of 500°‐1300°C. At 600°C, the powder crystallized to a carbonated hydroxyapatite and a trace of ß‐tricalcium phosphate before converting to hydroxyapatite at 900°C. The thermal crystallization was associated with grain growth, shrinkage, and active surface diffusion. The activation energy of grain growth was 37 ± 2 kJ/mol. After sintering at 1100°C, the decomposition of carbonated hydroxyapatite generated a microporous ceramic with an average pore size of 0.2 µm and an open porosity of 15.5%. This microporous bioceramic can be used as a bone filler.
Controlled Crystallization in Lead Zirconate Titanate Glass‐Ceramics Prepared by the Sol‐Gel Process
Authors: Yao Kui , Zhang Liangying , Yao Xi , Zhu Weiguang
Published: July 2010

Lead titanate and lead zirconate titanate grains were grown in situfrom lead‐titanium‐boron‐silicon and lead‐zirconium‐titanium‐boron‐silicon gels prepared via the sol‐gel method. The lead zirconate titanate grains were much smaller and more uniform than lead titanate grains grown from gels of similar composition. The crystallization mechanisms of the two systems were studied via scanning electron microscopy, X‐ray diffractometry, Raman spectroscopy, and differential thermal analysis. Crystallization had a tendency to begin near the surface or at defects in the lead titanate system, whereas it was controlled by the original precipitated nuclei of tetragonal zirconia particles in the lead zirconate titanate system. Controlled bulk crystallization may explain the finer structure of the resultant lead zirconate titanate glass‐ceramic.
Dispersion of Aqueous Barium Titanate Suspensions with Ammonium Salt of Poly(methacrylic acid)
Authors: Jean Jau‐Ho , Wang Hong‐Ren
Published: July 2010

The colloidal stability of aqueous barium titanate (BaTiO3) suspensions with the ammonium salt of poly(methacrylic acid) (PMAA‐NH4), as a function of pH, has been investigated. The concentration of PMAA‐NH4 required to stabilize aqueous BaTiO3 suspensions decreases as the pH increases. A stability map that delineates the critical amount of PMAA‐NH4 necessary to achieve colloidal stability, as a function of pH, has been constructed.
Evaluation of the Tensile Interface Strength in Brittle‐Matrix Composite Systems
Authors: Majumdar Bhaskar S. , Gundel Douglas B. , Dutton Rollie E. , Warrier Sunil G. , Pagano Nicholas J.
Published: July 2010

Two test methods for determining the tensile (normal) interface bond strength in brittle‐matrix composites have been described: one geometry is the cruciform bend specimen, and the other is the embedded fiber specimen. Both specimens have the characteristic that free edge debonding is avoided, so that valid interface strength data can be obtained from the knee in the stress‐strain curve. The techniques are applied to a Sigma SiC/7040 glass composite, and a bond strength of ∼5 MPa has been obtained. The method of analysis is described. Normal interface separation has been observed at or very close to the SiC/carbon (primarily graphite) interface of the Sigma fiber and is consistent with the weak orientation of the graphitic layers. Two examples of interface separation are cited in the as‐fabricated composite, to show the application and validation of the test results. In one instance, tensile interface separation has been observed next to existing voids, and in the other instance, interface separation has occurred near radial matrix cracks. Analyses show that the debonds are consistent with the low normal strength of the interface.
Ordering of Octahedral Vacancies in Transition Aluminas
Authors: Wang Yuan Go , Bronsveld Paul M. , DeHosson Jeff Th. M. , Djuričić Boro , McGarry David , Pickering Stephen
Published: July 2010

The microstructure of transition aluminas obtained via the dehydration of boehmite has been characterized by using transmission electron microscopy (TEM). The presence of γ‐, δ‐, and θ‐aluminas was identified by using selected‐area electron diffraction. Modifications that resulted from the reordering of aluminum vacancies on octahedral sites in a cubic close‐packed oxygen network have been detected and analyzed by using high‐resolution transmission electron microscopy (HRTEM) combined with image simulations. A good correspondence of the observed and calculated images confirmed the ordering of vacant octahedral sites located on {011} and {011} planes that formed a zigzag configuration along the <010> direction. Two more arrangements of empty octahedral sites, but now concentrated on {001} planes, have been determined in the sintered powder‐gel agglomerates. Structure analysis suggested that the modifications are all associated with the rearrangement of vacant sites during the phase transformation from gamma‐alumina to δ‐alumina, and further to theta‐alumina, and may be driven by configuration entropy minimization.
Angle of Hertzian Cone Cracks
Authors: Kocer Cenk , Collins Richard E.
Published: July 2010

This paper discusses the difference between the experimentally observed angle of Hertzian cone cracks and the angle defined by the trajectories of the preexisting stress fields. It is argued that there is no reason why these angles should be the same, as has usually been assumed. A finite element method has been used to model the growth of cracks in the Hertzian stress fields. In this model, the crack is incrementally advanced along the direction of maximum strain energy release, as calculated by the evolving, rather than the preexisting, stress fields. For the modeled Hertzian indentation system, a cone crack is observed to grow, but at an angle which is significantly different from that defined by the normal to the maximum preexisting tensile stress. The angle of the cone crack, as grown in the model, is in excellent agreement with observations on experimentally grown cone cracks in glass, with the same Poisson's ration. It is proposed that, in general, cracks will grow along paths that result in the maximum release of strain energy. For asymmetric, nonuniform preexisting stress fields, such paths do not necessarily coincide with the normal to the maximum preexisting tensile stress.
Phase Relationships and Related Microstructural Observations in the Ca‐Si‐Al‐O‐N System
Authors: Hewett Catherine L. , Cheng Yi‐Bing , Muddle Barry C. , Trigg Mark B.
Published: July 2010

A systematic set of samples with compositions in the Ca alpha‐SiAlON (α′) system has been fabricated using pressureless sintering to evaluate the phase relationships on, and near, the Ca α′ plane. A phase behavioral diagram for the Ca α′ plane within the Jänecke prism has been determined and found to be similar to those for the Y and Sm α′ systems. However, distinct differences were also observed; two additional regions (α′+ 27R + liquid) and (α′+ 33R + liquid) have been identified. Investigation of the microstructures revealed elongated Ca α′ grains in the samples prepared by pressureless sintering. The aspect ratio of the elongated grains of α′ increased as the concentration of stabilizing cation was increased. Single crystalline phase Ca α′ samples of comparable density to hot‐pressed material were fabricated successfully using pressureless sintering.
Transient Thermal Gradients in Barium Titanate Positive Temperature Coefficient (PTC) Thermistors
Authors: Smith David S. , Ghayoub Noureddine , Charissou Isabelle , Bellon Olivier , Abélard Pierre , Edwards Arthur H.
Published: July 2010

Barium titanate positive temperature coefficient (PTC) ceramic disk thermistors can suffer major mechanical damage if inhomogeneous heating occurs under voltage. The steady‐state and transient temperature distributions for thin disk samples (radius of 5 mm, thickness of 2 mm) have been studied with an infrared microscope, using a spatial resolution of 35 µm. The transient temperature distribution is observed to be particularly sensitive to the electrical boundary conditions during the initial heating period after application of a voltage. Small variations in electrode symmetry can lead to axial asymmetric thermal gradients up to 25 K/mm across the entire rim when an ac voltage of 100 V is applied. A finite‐difference model in two dimensions, based on solution of the heat equation with a local‐temperature‐dependent Joule heating‐source term, has been developed to describe the axial and radial transient temperature distributions in the cylindrical geometry. The predictions reveal current concentration at the edge of an electrode when the metal layer coverage is slightly smaller than that of the opposite face. This phenomenon results in stronger localized heating in a ring that initiates the thermal gradient.
Evolution of Young's Modulus, Strength, and Microstructure during Liquid‐Phase Sintering
Authors: Ostrowski Thomas , Ziegler Alexander , Bordia Rajendra K. , Rödel Jürgen
Published: July 2010

This comparative study addresses the influence of microstructural evolution on mechanical properties of porous alumina prepared by partial densification with and without addition of a glassy phase. The composition used for liquid‐phase sintering is heated above and, in order to freeze the starting defect distribution, below the glass transition point. The ensuing densification rate is compared with measurements of the temperature and time‐dependent wetting angle. The strength of the alumina is correlated with microstructural characteristics, Young's modulus, fracture toughness, and crack tip toughness as a function of porosity. Linkage of the crack tip toughness to elastic modulus, in combination with fractography and a description of a linearly shrinking critical defect, yields a predictive capacity to determine strength of porous ceramics.
Extra‐Large Grains in the Silicon Nitride Ceramics Doped with Yttria and Hafnia
Authors: Park Dong‐Soo , Lee Soo‐Young , Kim Hai‐Doo , Yoo Byoung‐Ju , Kim Bu‐Ahn
Published: July 2010

Microstructural development of silicon nitride doped with Y2O3 and HfO2 was investigated to determine how extra‐large grains develop during gas pressure sintering. Grains as long as 200 µm and a few tens of micrometers wide developed at high temperatures (>2173 K) in a fine‐grained matrix containing a limited amount of liquid which had a poor propensity to spread. A small number of grains could grow quite large before their growth was halted by neighboring grains of comparable size.
Interface Properties in High‐Strength Nicalon/C/SiC Composites, As Determined by Rough Surface Analysis of Fiber Push‐Out Tests
Authors: Kerans Ronald J. , Parthasarathy Triplicane A. , Rebillat Francis , Lamon Jacques
Published: July 2010

Fiber push‐out curves generated on high‐strength, treated‐fiber Nicalon/C/SiC composites indicate behavior that is distinctly different from other composites. These composites have been analyzed using a model that explicitly includes two sets of debond‐crack‐surface topographies that correspond to two stages of crack propagation. The model successfully duplicates the observed unusual push‐out load‐deflection behavior by adjusting interfacial toughness and topographical parameters to fit the experimental data. Although the topographical parameters of the debonding cracks are largely unconfirmed, the fitted values are consistent with observations and expectations. This analysis has determined interfacial fracture energies that are unusually high for composites with acceptable mechanical behavior but are consistent with measurements on bulk pyrocarbons. The analysis provides a unifying rationalization of the apparent inconsistencies in results reported for such materials.
Cyclic Thermal Shock Resistance of Several Advanced Ceramics and Ceramic Composites
Authors: Schneibel Joachim H. , Sabol Stephen M. , Morrison Jay , Ludeman Evan , Carmichael Cecil A.
Published: July 2010

The cyclic thermal shock behavior of two Si3N4ceramics, two SiC‐whisker‐reinforced alumina composites (Al2O3/SiCw), a SiC‐particulate‐reinforced alumina (Al2O3/SiCp), and an alumina continuously reinforced with SiC fibers (Al2O3/SiCf) composite has been studied. Specimens were repeatedly quenched from 1473 K into a fluidized bed with a heat transfer coefficient of 1400 W/(Km2) [250 Btu/(hft2F)]. The thermal shock damage was assessed by room‐temperature flexure strength measurements. Si3N4and Al2O3/SiCpshowed no noticeable damage after 100 cycles, whereas Al2O3/SiCwand Al2O3/SiCfdegraded substantially. The experimental results are discussed and rationalized in terms of finite element simulations and microstructural observations. Our analysis suggests that the thermal shock performance of other materials may be estimated from comparisons with the present work.
Lifetime‐Applied Stress Response in Air of a SiC‐Based Nicalon‐Fiber‐Reinforced Composite with a Carbon Interfacial Layer: Effects of Temperature (300° to 1150°C)
Authors: Becher Paul F. , Lin Hua‐Tay , More Karren L.
Published: July 2010

The lifetimes in air as a function of applied flexure stress and temperature (300–1150°C) are described for a Si–O–C based (Nicalon) fiber plain‐weave cloth reinforced SiC‐matrix composite (∼7% closed porosity) with an ∼0.3 µm thick carbon interfacial layer. The measured lifetimes of both samples with and without an external SiC seal coating were similar and decreased with applied flexural stress (for stresses greater than ∼90 MPa) and with temperature. At temperatures of ≥600°C, the external CVD SiC coating had negligible effect on the lifetimes; however, at 425°C, a detectable improvement in the lifetime was observed with an external SiC coating. When the applied stress was decreased below an apparent “threshold stress” (e.g., ∼90 MPa) for tests conducted at temperatures ≤950°C, no failures were observed for times of ≥1000 h. Electron microscopy observations show that the interfacial carbon layer is progressively removed during tests at 425° and 600°C. In these cases, failure is associated with fiber failure and pullout. At 950° and 1150°C, the carbon interface layer is eliminated and replaced by a thick silica layer due to the oxidation of the Nicalon fiber and the SiC matrix. This results in embrittling the composite.
Composition‐Graded Solid Electrolyte for Determination of the Gibbs Energy of Formation of Lanthanum Zirconate
Authors: Jacob K. Thomas , Dasgupta Niladri , Waseda Yoshio
Published: July 2010

A composition‐graded solid electrolyte has been used to determine the standard Gibbs free energy of formation of lanthanum zirconate (La2Zr2O7) from the component oxides lanthana (La2O3) (A‐rare earth) and zirconia (ZrO2) (monoclinic) in the temperature range of 870–1240 K. The cell used for measurement can be represented as Pt, O2, CaO + CaF2∥ CaF2| (LaF3)x(CaF2)1‐x∥LaF3+ La2Zr2O7+ ZrO2, O2, Pt x=0 x=0.32 A composition‐graded electrolyte has been introduced to compensate the solubility effects of the electrode material (lanthanum fluoride, LaF3) in the solid electrolyte (calcium fluoride, CaF2). The ability of the graded electrolyte to gen‐erate a Nernstian response is demonstrated, using electrodes with known fluorine chemical potentials. For the reaction La2O3 (A‐rare earth) + 2ZrO2 (monoclinic) → La2Zr2O7 (pyrochlore), the Gibbs free energy change (ΔG°f,ox) is given by the formula −133800 −10.32T (±4500) (in units of J/mol). The enthalpy and entropy of formation of La2Zr2O7 obtained in this study are in good agreement with calorimetric data. The “third‐law” enthalpy of formation of La2Zr2O7, from the component oxides at 298.15 K, is −133.8 ± 5 kJ/mol.
Immobilization of Sodium‐Bearing High‐Level Radioactive Waste in Synroc Containing Na2Al2Ti6O16‐Type Freudenbergite
Authors: Li Liyu , Luo Shanggeng , Wang Dexi
Published: July 2010

Because sodium has a tendency to form less‐durable phases during the fabrication of synroc, it is very difficult to immobilize. This study shows that synroc that contains Na2Al2Ti6O16‐type freudenbergite can be used to incorporate sodium‐bearing high‐level radioactive waste. Synroc samples that contain 5.0 wt% Na2O (11.5 wt% waste oxide, accordingly) exhibit very good chemical durability and physical properties. The standard Synroc‐C formulation can incorporate only 2 wt% Na2O. Therefore, this study greatly improves the immobilization ability of sodium in synroc.
X‐ray Diffraction from Levitated Liquid Yttrium Oxide
Authors: Krishnan Shankar , Ansell Stuart , Price David L.
Published: July 2010

We report the first measurements of the structure factor, S(Q),and radial distribution function, G(r),of yttrium oxide in the normal and supercooled liquid states in the temperature range 2500–3100 K. Measurements were obtained by synchrotron X‐ray scattering on levitated, laser‐heated liquid specimens. At temperatures far in excess of the melting point, the first and second coordination shells begin to merge, indicative of increased ionicity in the liquid. As the temperature is lowered into the supercooled region, there is substantial sharpening and strengthening in the first peaks in both S(Q)and G(r).Supercooling causes a decreasein the first shell coordination and Qvalues together with an increasein the Y‐O interionic distance.
Fabrication of Transparent Yttria Ceramics at Low Temperature Using Carbonate‐Derived Powder
Authors: Saito Noriko , Matsuda Shin‐ichi , Ikegami Takayasu
Published: July 2010

A new preparation method for a highly sinterable yttria powder has been developed, the resultant powder characterized, and its sinterability studied. As a precursor of the yttria powder, a fine and needle‐shaped yttrium carbonate was prepared by a precipitation and aging method. A fine and low‐agglomeration yttria powder was obtained by calcining the carbonate precursor at 1100°C. The primary crystallites measured ∼0.1 μm and weakly agglomerated to a size of ∼0.3 μm. The powder had a very high sinterability, so that the powder compact could be sintered to transparency by normal sintering under vacuum without additives at low temperatures, over 1600°C. The sintered transparent ceramics exhibited a homogeneous microstructure with no abnormal grain growth.
TEM/EELS Characterization of a Sintered Polycrystalline Silicon Carbide Fiber
Authors: Tenailleau H. , Bourrat Xavier , Naslain Roger , Tressler Richard E. , Giannuzzi Lucille A.
Published: July 2010

A sintered, polycrystalline SiC fiber has been characterized by TEM and EELS. The fibers consist primarily of alpha‐SiC (6H‐polytype) with a mean grain size on the order of approximately 1 μm. Poorly organized turbostratic carbon is located within porous regions between some grain boundaries and triple junctions of the SiC. In addition, 100 nm spherical boron carbide precipitates are observed intragranularly within the SiC grains. These precipitates have a rhombohedral crystal structure with an approximate stoichiometry of B4C.
Grain‐Boundary Viscosity of Calcium‐Doped Silicon Nitride
Authors: Pezzotti Giuseppe
Published: July 2010

Internal‐friction data of calcium‐doped Si3N4 polycrystalline materials that otherwise contain only SiO2 at grain boundaries were examined and compared with those collected on the same polycrystal in the undoped state or doped with anions (i.e., fluorine and chlorine). Precise microstructural characterizations previously performed on these materials enabled us to quantitatively evaluate the inherent viscosity of the intergranular SiO2 film through the analysis of the anelastic internal‐friction‐peak components. The intergranular glass viscosity and its scaling with increasing calcium addition followed the same trend as bulk SiO2 glasses with the same chemical composition. Broadening of the internal‐friction peak with increased calcium content in the material has also been rationalized according to the reduction of the activation energy for the viscous flow of bulk glasses. The present analysis, which is in agreement with our previous studies on undoped and anion‐doped Si3N4, has demonstrated that the overall viscoelastic response of the polycrystal is mainly dictated by the chemistry of the intergranular glass.
R‐Curve Behavior of Silicon Nitride Ceramic Reinforced with Silicon Carbide Platelets
Authors: Choi Byung‐Jin , Kim Hyoun‐Ee
Published: July 2010

Si3N4 ceramics reinforced with SiC platelets were fabricated by hot pressing at 1800°C. The microstructure of the Si3N4 matrix itself was the same with or without the addition of the SiC platelets. However, the mechanical properties of the Si3N4 were changed remarkably by the SiC addition. The fracture toughness and the crack resistance with crack propagation (R‐curve behavior) were improved while the fracture strength was decreased slightly by the platelets. Improvement in crack resistance was attributed to the extensive interaction of cracks with the platelets. The reduction in strength, on the other hand, is believed to be due to cracks associated with weak platelet‐matrix interfaces.
Effects of Amorphous and Crystalline SiO2 Additives on γ‐Al2O3‐to‐alpha‐Al2O3 Phase Transitions
Authors: Saito Yoshitoshi , Takei Takahiro , Hayashi Shigeo , Yasumori Atsuo , Okada Kiyoshi
Published: July 2010

This paper focused on the effects of various phases of SiO2 additives on the γ‐Al2O3‐to‐α‐Al2O3 phase transition. In the differential thermal analysis, the exothermic peak temperature that corresponded to the theta‐to‐α phase transition was elevated by adding amorphous SiO2, such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO2, such as quartz and cristobalite. Amorphous SiO2 was considered to retard the γ‐to‐α phase transition by preventing γ‐Al2O3 particles from coming into contact and suppressing heterogeneous nucleation on the γ‐Al2O3 surface. On the other hand, crystalline SiO2 accelerated the α‐Al2O3 transition; thus, this SiO2 may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO2 additives, especially amorphous and crystalline phases, largely influenced the temperature of γ‐Al2O3‐to‐α‐Al2O3 phase transition.
Microporous Glass‐Ceramics in NASICON‐Type Copper(II) Titanium Phosphate
Authors: Yamamoto Kousuke , Abe Yoshihiro
Published: July 2010

A novel porous glass‐ceramic with a skeleton of a NASICON‐type copper(II) titanium phosphate was prepared via the controlled crystallization of a glass and the subsequent chemical leaching of the resulting dense glass‐ceramic. A volume‐crystallized dense glass‐ceramic comprised of CuTi2(PO4)3 and Cu3(PO4)2, whose surface was covered by a thin layer of CuO, was prepared by reheating a glass with a nominal composition of 50CuO20TiO230P2O5 (in mol%) in air. When the resulting glass‐ceramic was leached with dilute HCl, the Cu3(PO4)2 and CuO phases were dissolved out selectively, and a cuprous NASICON crystal of CuTi2(PO4)3 was converted to its cupric type, CuTi4(PO4)6, which was left as a skeleton of the porous materials. The specific surface area and the average pore radius of the porous glass‐ceramic obtained were ∼70 m2/g and ∼7 nm, respectively. The porous glass‐ceramic showed high catalytic activities for the dehydration of 2‐propanol.