bluuebunny/crossref_metadata_2025 huggingface dataset:
Solidification of Glass Powder with Simulated High‐Level Radioactive Waste During Hydrothermal Hot‐Pressing
Authors: Nishioka Mamoru , Hirai Shoji , Yanagisawa Kazumichi , Yamasaki Nakamichi
Published: March 2005

Borosilicate glass powder with simulated high‐level radioactive waste (HLW) was solidified by a hydrothermal hotpressing method. A leach test of the solid bodies was performed. Twenty‐one elements composing the simulated HLW in the leachant were analyzed quantitatively by neutron activation analysis. The addition of Al(OH)3, up to 7.5% of the starting powder, reduced the leachability. The first densification of the starting powder during hydrothermal hot‐pressing takes place by the rearrangement of particles, followed by solution‐precipitation. The densification fits two types of rate equations. Activation energies obtained from two rate equations agree and are smaller than those of the glass without simulated HLW. The results indicate that simulated HLW in the starting powder acts to reduce the activation energy for densification during hydrothermal hot‐pressing.
Preparation of Silicon Carbide/Aluminum Nitride Ceramics Using Organometallic Precursors
Authors: Czekaj Corinna L. , Hackney Michael L. J. , Hurley William J. , Interrante Leonard V. , Sigel Gary A. , Schields Paul J. , Slack Glen A.
Published: March 2005

Solid solutions of 2H‐SiC/AlN can be prepared at temperatures less than 1600°C by rapid pyrolysis (“hot drop”) of mixtures of [(Me3Si)0.80((CH2=CH)MeSi)1.0(MeHSi)0.35]n (VPS) or [MeHSiCH2]n (MPCS) with [R2AlNH2]3, where R=Et, i‐Bu or simply by slow pyrolysis of the precursor mixture in the case of [Et2AlNH2]3. In contrast, slow pyrolysis of mixtures of VPS or MPCS with [i‐Bu2AlNH2]3 yields a composite of 2H‐AlN and 3C‐SiC at 1600°C, which transforms into a single 2H‐SiC/AlN solid solution on heating to 2000°C. The influences of the nature of the precursor and processing conditions on the structure, composition, and purity of the SiC/AlN materials are discussed.
Preparation and Characterization of 17O‐Enriched Alumina for Nuclear Fusion Damage Experiments
Authors: Kim Hyoun‐Ee , Zinkle Steven J. , Allen William R.
Published: March 2005

Alumina enriched in 17O was successfully fabricated from aluminum isopropoxide and water containing the 17O isotope. This material was necessary for an experiment to study the radiation damage expected in alumina exposed to a nuclear fusion reactor environment. The enrichment levels of specimens subjected to different preparation schedules were measured using a nuclear reaction analysis technique. Replacement of the 17O isotope in the ceramic by atmospheric oxygen occurred readily. Therefore, successful fabrication of suitably enriched alumina specimens required that all processing steps be performed under vacuum or inertgas environments. The optimum fabrication procedure produced enriched alumina specimens of >99.5% of theoretical density, ∽10‐μm grain size, and a flexural strength of 280 MPa.
Infrared Detection by Si‐Ti‐C‐O Fibers
Authors: Muto Norio , Miyayama Masaru , Yanagida Hiroaki , Kajiwara Teijiro , Mori Norihisa , Ichikawa Hiroshi , Harada Hirobumi
Published: March 2005

Electrical resistivity at 20° to 90°C and infrared (IR) responses (thermal time constant and responsivity of electrical resistance changes) were measured on Si‐Ti‐C‐O fibers. The fibers showed negative temperature coefficient resistance‐temperature characteristics, and the thermistor constant B was 890 to 4760 K. A fast and steady output (resistance change) was observed in response to IR radiation from a 600 K blackbody furnace. The IR response was affected markedly by detector structure; the thermal time constant decreased with decreasing length of the fibers, and the responsivity increased with decreasing number of the fibers. In an IR detector with a separate arrangement of a few (n=10) fibers with a short length (1 mm) and a low resistivity (80 Ω·cm), a thermal time constant of 3.5 ms and a responsivity of 63 V/W were obtained. A fast and steady IR response of Si‐Ti‐C‐O fibers suggests the possibility of detecting both stationary and moving IR sources.
Analysis of Powder Compaction Using a Compaction Rate Diagram
Authors: Matsumoto Roger L. K.
Published: March 2005

Compaction response diagrams are useful tools for characterizing the powder pressing operation. It is shown that taking the first derivative of the curve contained in a compaction response diagram yields a compaction rate diagram which reveals nuances in the pressing operation. These additional details can be quite important in the design of powder systems.
Fracture Resistance Behavior of Silicon Carbide Whisker‐Reinforced Alumina Composites with Different Porosities
Authors: Krause Ralph F. , Fuller Edwin R. , Rhodes James F.
Published: March 2005

Fracture resistance behavior was characterized for SiC‐whisker‐reinforced alumina composites with porosities ranging from 0.6% to 11.5% The composites were hot‐pressed from an Al2O3 powder with 25 wt% SiC whiskers. Strengths of individual specimens were measured in four‐point flexure either for natural flaws or for Vickers‐indentation flaws as a function of radial crack size. Indentation crack sizes were controlled with indentation loads which varied between 2 and 200 N. A novel method of analysis of these measurements indicates that the fracture resistance of these composites increases as a function of crack extension, a rising R curve. This behavior is interpreted in terms of tractions from both crack‐bridging whiskers and interlocking grains, which develop in the wake of the crack tip as it extends. A decrease in porosity raises the level of fracture resistance, but has a negligible effect on the relative steepness of the R curve. The sizes of natural flaws which causes failure in flexure testing were also estimated from analysis of the data.
Laser‐Induced Calcite—Aragonite Transition
Authors: Alam Mansoor , DebRoy Tarasankar , Roy Rustum
Published: March 2005

High‐pressure phases of CaCO3, namely aragonite, calcite II, and possibly calcite III, were synthesized in air by exposing 10‐ to 20‐μm‐size particles of CaCO3 (calcite I phase) to a CO2 laser radiation at short pulse lengths (≤0.1 ms). The process, therefore, has the same effect as exposing the particles to at least several hundred megapascals pressure. Processing at higher pulse lengths resulted in the decomposition of CaCO3 to CaO and CO2. The extent of decomposition increased with increasing pulse length.
Neutron Diffraction Studies of Phase Transformations between Tetragonal and Orthorhombic Zirconia in Magnesia‐Partially‐Stabilized Zirconia
Authors: Howard Christopher J. , Kisi Erich H. , Roberts Ronald B. , Hill Roderick J.
Published: March 2005

Neutron powder diffraction and conventional dilatometry have been used to investigate the tetragonal‐to‐orthorhombic phase transformation and the orthorhombic‐to‐tetragonal reversion in a high‐toughness magnesia‐partially‐stabilized zirconia. For this material, the onset temperature on cooling for the tetragonal‐to‐orthorhombic transformation (determined by dilatometry) was 192 K, and the reversion on subsequent heating occurred between 500 and 620 K. Neutron diffraction patterns were recorded at temperatures down to 19 K then up to 664 K, and analyzed by the multiphase Rietveld method to determine the amounts of different phases as well as their lattice parameters and unit‐cell volumes. It is notable that, at its maximum, the orthorhombic phase amounted to 45% of the sample by weight. Length changes were measured, using pushrod dilatometers, in the temperature range 80 to 700 K. Length changes calculated from the neutron diffraction determinations of the proportions and unit‐cell volumes of the different phases are in very good agreement with the directly measured values.
Computer Simulation of Final‐Stage Sintering: II, Influence of Initial Pore Size
Authors: Chen I‐Wei , Hassold Gregory N. , Srolovitz David J.
Published: March 2005

A two‐dimensional Monte Carlo simulation procedure has been used to investigate the effect of the initial pore size on the microstructural evolution and the kinetics of final‐stage sintering. The sintering time scales with r40/Dgb and the grain‐growth time scales with r2O/Dm. Pores are found to effectively pin the grain boundaries from the beginning of final‐stage sintering at a porosity of Φ= 0.09 until Φ= 0.03. For Φ 0.03, the remaining pores do not effectively retard grain‐boundary migration and normal grain growth occurs. Small pores were found to be less effective at retarding grain growth than expected on the basis of a simple grain‐growth pinning model. The mean pore size was found to be nearly constant throughout the simulations.
Glass Formation and Properties in the System Calcia‐Gallia‐Silica
Authors: Angel Paul W. , Ray Chandra S. , Day Delbert E.
Published: March 2005

The glass‐formation region of the calcia‐gallia‐silica system was determined. The glasses within this region were measured to have a density of 3 to 4 g/cm3, a refractive index of 1.6 to 1.73, an Abbe number between 35 and 58, a thermal expansion coefficient of 6.5 × 10−7/°C to 11.5 × 10−7/°C, softening temperatures between 730° and 790°C, and a Vickers microhardness of 5.2 to 7.3 GPa. Crystalline phases were identified along the glass‐formation boundary. Infrared transmission spectra were used to explain glass structures and their effect on glass properties. The results suggest that the role of calcia in the glass structure is similar to that for calcia in calcium aluminosilicate glasses.
Metastable‐Phase Formation During the Crystallization of Magnesium Metaphosphate Glass
Authors: Saltzberg Michael A. , Hu Yung‐Haw
Published: March 2005

We have observed a previously unreported metastable intermediate phase during the crystallization of magnesium metaphosphate (Mg(PO3)2) glasses to the equilibrium ring phase, Mg2P4O12. Our current hypothesis is that the metastable phase contains metaphosphate chains, as in other crystalline alkaline‐earth metaphosphates. This type of phase could form by low‐energy rearrangement of the polyphosphate chains in the glass structure. Thus, its formation might be favored at low temperatures where there is insufficient thermal energy to break the metaphosphate chains and form tetrametaphosphate rings.
Collection and Characterization of Soot from an Optical Fiber Preform Torch
Authors: Clark Harold R. , Stawicki Robert P. , Smyth Imelda P. , Potkay Eugene
Published: March 2005

To follow the development of particle morphology, silica soot is extracted from four axial positions in the flame of a torch used to manufacture optical fiber preforms by the vaporphase axial deposition method. The soot, collected by three extraction techniques, is analyzed by transmission electron microscopy and gas adsorption to identify which characteristics of the soot morphology are dependent on the sampling technique and which upon the position in the flame. Comparison is also made to other work in which silica soot was extracted from a nondiffusion flame.
Cation Distributions in Rapidly Solidified Cobalt Ferrite
Authors: De Guire Mark R. , Kalonji Gretchen , O'Handley Robert C.
Published: March 2005

The cation distributions in two rapidly solidified cobalt ferrites have been determined using Mössbauer spectroscopy at 4.2 K in an 8‐T magnetic field. The samples were obtained by gas atomization of a CoO–Fe2O3–P2O5 melt. The degree of cation disorder in both cases was greater than is obtainable by cooling unmelted cobalt ferrite. The more rapidly cooled sample exhibited a smaller departure from the equilibrium cation distribution than did the more slowly cooled sample. This result is explained on the basis of two competing effects of rapid solidification: high cooling rate of the solid, and large undercooling.
High‐Temperature Gaseous Corrosion of Si3N4 in H2–H2O and Ar–O2 Environments
Authors: Kim Hyoun‐Ee , Moorhead A. J.
Published: March 2005

Samples of Si3N4 made by chemical vapor deposition (CVD) and hot isostatic pressing (HIP) were exposed for 10 h at 1400°C to H2–H2O and Ar–O2 environments. Variations in weight and surface morphology of the CVD‐Si3N4 samples were monitored to determine the corrosion behavior of ‘pure’ Si3N4 as baseline information. Additionally, the room‐temperature flexural strengths of the HIP‐Si3N4 were determined and correlated with observed changes in weight and microstructure. The corrosion behavior of both materials depended on the level of oxidant (H2O or O2) in the atmosphere, but the magnitudes of the weight changes observed in the HIP‐Si3N4 were significantly greater. When the PH2O in the H2 atmospheres was low (PH2O < × 10−5 MPa), significant weight loss and surface degradation occurred in both ceramics, accompanied by reductions in strength of the HIP‐Si3N4. However, in atmospheres with PH2O 1 × 10−4 MPa, the weight loss was smaller and the strength of the HIP‐Si3N4 was greater than that of the as‐received material. Somewhat different behavior was observed in samples exposed to Ar–O2 environments. Weight loss of both silicon nitrides increased with increasing Po2 until a level of about 2 × 10−5 MPa was reached. However, contrary to the results for H2–H2O exposures, even under these conditions the strength of the HIP‐Si3N4 material was found to have increased above that of the as‐received material.
Si3N4/SiC‐Whisker Composites without Sintering Aids: II, Fracture Behavior
Authors: Pezzotti Giuseppe , Tanaka Isao , Okamoto Taira
Published: March 2005

The fracture behavior of an Si3N4/SiC‐whisker composite fabricated without sintering aids is investigated using a double approach based on the examination of R‐curve behavior and a statistical analysis of crack propagation. In the composite with 20 vol% whisker, a 30% increase in toughness over the matrix value can be attributed to crack‐tip phenomena. Strong interfacial bonding prevents any contribution to toughening by mechanisms operating in the wake region of the crack. Based on experimental observations of microfracture in both SiC whiskers and Si3N4 grains, toughening caused by crack‐tip phenomena is quantitatively discussed in terms of fracture energy and whisker‐distribution parameters.
Growth Mechanism of Silicon Carbide Films by Chemical Vapor Deposition below 1273
Authors: Tanaka Satoru , Komiyam Hiroshi
Published: March 2005

SiC films were prepared from the reaction of Si2H6 with C2H4 or C2H2 at relatively low temperatures ranging from 873 K to 1273 K by low‐pressure chemical vapor deposition. The deposition rate profiles determined by gravimetry and the alloy composition (carbon content, x, for Si1‐xCx) profiles determined by X‐ray photoemission spectroscopy in the reactor were mainly investigated. The results revealed that the carbon content, x, was a function of the temperature, ratio of partial pressures of source gases, and source gas species (C2H4, C2H2). From these results we deduced that SiC formation occurred by two major competing reaction processes: (1) the silicon deposition processes, SiH2 Si (wall) and Si2H6 Si (wall), and (2) the carbon deposition process, C2H4+ SiH2 vinylsilane or C2H2+ SiH2 ethynylsilane.
Fracture and Stiffness Characteristics of Particulate Composites of Diamond in Zinc Sulfide
Authors: Farquhar Donald S. , Raj Rishi , Phoenix S. Leigh
Published: March 2005

Diamond particles have been embedded in hot‐pressed zinc sulfide (ZnS) ceramic to improve various mechanical properties while preserving special optical properties. Roomtemperature mechanical tests on small specimens have shown that adding 10 wt% diamond to ZnS has no effect on the yield stress, but increases the tensile strength and the elastic moduli ∼20%, and increases the fracture toughness ∼100%. The doubling in fracture toughness can be explained by elastic interaction of the diamond particles with the crack‐tip stress field. The results and the interpretation presented here are believed to represent a class of composite materials where both constituents are brittle but the dispersed phase has a much higher elastic modulus than the matrix.
Silicon Carbide/Silicon and Silicon Carbide/Silicon Carbide Composites Produced by Chemical Vapor Infiltration
Authors: Kmetz Michael , Suib Steven , Galasso Francis
Published: March 2005

Composites of SiC/Si and SiC/SiC were prepared from single yarns of SiC. The use of carbon coatings on SiC yarn prevented the degradation normally observed when chemically vapor deposited Si is applied to SiC yarn. The strength, however, was not retained when the composite was heated at elevated temperatures in air. In contrast, the strength of a SiC/C/SiC composite was not reduced after this composite was heated at elevated temperatures, even when the fiber ends were exposed.
‘Polymorph Method’ Determination of Monoclinic Zirconia in Partially Stabilized Zirconia Ceramics
Authors: Howard Christopher J. , Kisi Erich H.
Published: March 2005

The polymorph method, which provides phase analysis from a small number of integrated intensities in a powder diffraction scan, is adapted for the determination of monoclinic zirconia in a mixture with cubic, tetragonal. and orthorhombic zirconias and the γ‐phase (Mg2Zr5O12). Such a mixture is representative of Mg‐PSZ after subeutectoid aging. The quantitative determination of the monoclinic depends in principle on a knowledge of the relative amounts of the other phases present in the mixture. It is demonstrated, however, that without this knowledge, even in complex mixtures, the traditional polymorph method analysis gives an acceptable estimate of the monoclinic fraction in the sample.
Nonaqueous Cold Extrusion as a Route for Yttrium Barium Copper Oxide Wires
Authors: Varma Harikrishna , Mukundan Poothayil , Warrier Krishna Gopakumara , Darmodoran Alathoor D. , Balachandran Utham
Published: March 2005

Yttrium barium copper oxide (YBCO) superconducting wires of diameter 0.2 to 1 mm in the form of spirals up to 50 cm long were made by a novel nonaqueous cold extrusion technique. Finely ground YBCO powder having a set particle size distribution was intimately mixed with a polypropylene compound and extruded manually into wires which were heat‐treated at controlled rates followed by oxygen annealing at 940°C for 10 h. These wires are superconducting with a Tc of 89.5 K, the XRD pattern exhibiting an orthorhombic single phase. Current density at zero field was measured to be 223 A/cm2. The surface morphology of the extruded wires and microstructure of the polished cross section of the sintered wires are also presented.
Microstructure and Composition of Alumina/Aluminum Composites Made by Directed Oxidation of Aluminum
Authors: Breval Else , Aghajanian Michael K. , Luszcz Stan J.
Published: March 2005

Two Al2O3/Al composites, grown by the directed oxidation of molten Al alloys at 1400 and 1600 K, were investigated by X‐ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, and wet chemical analysis. The materials were found to contain a continuous network of Al2O3, which was predominantly free of grain‐boundary phases and was made up of nanometer‐ to micrometer‐sized crystallites, a continuous network of Al alloy, and isolated inclusions of Al alloy. No crystallographic orientation was observed in the metallic phase, whereas the Al2O3 was oriented with its c axis parallel to the growth direction. The higher process temperature yielded a lower metal content and less connectivity of the metallic consituent.
Effects of Plasma‐Sprayed Ceramic Coatings on the Strength Distribution of Silicon Carbide Materials
Authors: Butt Darryl P. , Mecholsky John J. , van Roode Mark , Price Jeffrey R.
Published: March 2005

Sintered α‐silicon carbide and siliconized silicon carbide tubular materials were plasma‐spray coated with various ceramic oxides using conventional air plasma‐spraying technology. The strength distributions of the coated tubes were evaluated by fracturing samples at room temperature using a c‐ring test configuration. Changes in strength are explained using statistical treatments of the data, including a Weibull approach. Failure origins are characterized using fractography via scanning electron microscopy. The stength of plasma‐spray coated SiC was equivalent or superior to that of as‐received material, despite a surface pretreatment which induced surface flaws in the SiC substrate.
Temperature Dependence of Electrical Condutivity in Polycrystalline Tin Oxide
Authors: Lee Jong‐Heun , Park Soon‐Ja , Hirota Kazushi
Published: March 2005

The electrical conductivity of polycrystalline SnO2 in air was measured between 400° and 1400°C using samples prepared by hot isostatic pressing and normal sintering. The apparent activation energy for conduction above 800°C was 1.3 to 1.4 eV in the hot isostatically pressed sample and 1.9 to 2.0 eV in the normally sintered sample. This difference can be interpreted as a result of the pore contributing an additional blocking effect to conduction. Also we found that the electrical conductivity above 1150°C bent down slightly from the linear regime when using a cumulative slope model.
Effect of Diamond Dispersion on the Superplastic Rheology of Zinc Sulfide
Authors: Xue Liang A. , Raj Rishi
Published: March 2005

The dispersion of diamond particles in superplastic zinc sulfide produces flow softening at low diamond concentrations, and hardening at high concentrations. The minimum in the flow stress is observed at 5 wt% (5.5 vol%) diamond content. The softening can be explained in terms of the grain refinement in the ZnS in the presence of diamond particles. The smaller grain size is shown to reduce the flow stress by enhancing the rate of diffusional creep. The hardening is believed to have been caused by the formation of a network among the diamond particles at high concentrations. The fact that the diamond particle size was approximately onethird the grain size of ZnS may have contributed to the network formation.
Quantitative Electron Microscopic Investigation of the Pore Structure in 10:90 Colloidal Silica/Potassium Silicate Sol‐Gels
Authors: Kerch Helen M. , Gerhardt Rosario A. , Grazul John L.
Published: March 2005

Transmission electron microscopy (TEM), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and nitrogen sorption technique (BET) were utilized to characterize the microstructure of a 10:90 wt% colloidal silica/potassium silicate gel as first described by Shoup. Gels in the unsintered state (15% theoretical density) were prepared for microscopy by the techniques of ultramicrotomy, Pt/C replication, and pore casting. Electron microscopic images of the ultramicrotomed thin sections (70 nm) show that the unfired gel possesses three distinct species of pores which are referred to as the micropores, mesopores, and macropores. The average micropore diameter was found to be 4 nm as determined by nitrogen desorption. Quantitative stereological analysis of the ultramicrotomed sections indicated that the average circular and lengthwise dimensions of the cylindrical mesopores were 0.15 and 0.39 μm, respectively. Similarly, this same analysis determined the average spherical macropore diameter to be 0.83 μm. In contrast, MIP results suggested that these gels possessed a unimodal pore size distribution centered around the 0.2‐μm pore size. The discrepancy between MIP and microscopy can be explained by viewing the void space as a pore‐throat network. Experimental evidence for this type of pore geometry was obtained from stereo pairs of Pt/C replicas and thick microtomed sections (0.5 μm) which gave information about particle connectivity and pore casts which depicted the pore connectivity in three dimensions.
Lubrication Studies with Alumina‐on‐Alumina, Steel‐on‐Steel, and Steel‐on‐Alumina Bearing Systems
Authors: Wu Wei‐Te , Duda J. L. , Klaus Elmer E.
Published: March 2005

In metal bearing systems, good lubricants for boundary lubrication generally show chemical interaction with the bearing surface under the conditions of temperature and pressure generated in the sliding contact. In this study, tests in a four‐ball wear tester have been used to compare lubricants under similar loadings using steel‐on‐steel, steel‐on‐alumina, and alumina‐on‐alumina bearing systems. In general, the alumina appears to be far less reactive with lubricants than steel. Alumina‐on‐alumina bearing systems do show a sensitivity to lubricant chemical composition with generally higher wear values than the other two bearing systems. Steel‐on‐alumina bearing systems generally show better wear characteristics than the all‐alumina systems which indicates the improved chemical interactions between the steel and the lubricant. This study shows that the chemical reactivity of the lubricant with the bearing surface is important in the wear process.
Isotropic Constitutive Model for Sintering Particle Packings
Authors: Jagota Anand , Mikeska Kurt R. , Bordia Rajendra K.
Published: March 2005

This paper presents an isotropic model for the effective viscosities of sintering particle packings. The relationship between the macroscopic stress and deformation rate is based on the behavior of individual interparticle contacts, represented by contact viscosities, and the statistics of the packing. The contact viscosities depend on the contact area, which is the primary variable used to describe the state of the packing. A direct consequence of this choice of state variable is that the effective viscosities are identically zero for an undeformed and unsintered packing. Specific results are presented for the case of Newtonian viscous materials. The model is compared with existing models for porous Newtonian viscous materials. Experiments conducted with spherical glass powder packings show good agreement with the model. Finite‐element simulations using the model have been used to study forging of a pellet.
Simultaneous Determination of Thermal Conductivity and Specific Heat for Refractory Materials
Authors: Ho Kai , Pehlke Robert D.
Published: March 2005

The present work describes a transient method for determining thermal properties of refractory materials. Specific to this technique are (1) the experimental measurement of heat flux on the heated surface and (2) a method of data analysis based on the numerical techniques of nonlinear least squares and finite difference. Several mathematical formulations are proposed to minimize computing time requirements. Silica molding sand is used in two examples to illustrate the applicability of the technique to refractory materials. Owing to the measurement of heat flux on the heated surface, both thermal conductivity k and volumetric specific heat C can be determined if their temperature dependencies are not considered. In the case when temperature‐dependent thermal properties are considered, it is found that the inherent accuracy of the experiment does not allow the determination of both temperature‐variable k and C at the same time. It is shown that the present method has advantages in the ease and directness of the required experiment and in greater computational efficiency. Thermal properties are reported for a bonded silica and an unbonded silica sand.
Effect of Hydrothermal Conditions on the Morphology of Colloidal Boehmite Particles: Implications for Fibril Formation and Monodispersity
Authors: Buining Paul A. , Pathmamanoharan Chellappah , Bosboom Monique , Jansen J. Ben H. , Lekkerkerker Hendrik N. W.
Published: March 2005

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.
Faceting Behavior of Alumina in the Presence of a Glass
Authors: Simpson Yonn K. , Carter C. Barry
Published: March 2005

The faceting of alumina interfaces in the presence of a glass affects both grain growth and grain‐boundary mobility during liquid‐phase sintering. The geometry and movement of facets that form during this sintering process are expected to play an essential role in the development of the final microstructure, in particular, by their influence on the topology of the grain boundaries which ultimately control the properties of Al2O3 compacts. A new method for studying the interaction between Al2O3 and a glass has been developed. A thin sample of Al2O3 suitable for examination in a transmission electron microscope is prepared and examined and then reacted with SiO2 and CaO via the vapor phase. This experimental approach allows the faceting behavior of glass/Al2O3 interfaces to be studied systematically without introducing unnecessary complications during subsequent sample preparation. Faceting occurs almost exclusively on the (0001) and {1102} planes. The interaction between glass and certain structured grain boundaries in alumina has been studied using polycrystalline thin films.
Silicon Nitride Derived from an Organometallic Polymeric Precursor: Preparation and Characterization
Authors: Schmidt Wayde R. , Sukumar Vijay , Hurley William J. , Garcia Roberto , Doremus Robert H. , Interrante Leonard V. , Renlund Gary M.
Published: March 2005

Partially crystalline Si3N4, with nanosized crystals and a specific surface area greater than 200 m2/g, is obtained by pyrolysis of a commercially available vinylic polysilane in a stream of anhydrous NH3 to 1000°C. This polymer does not contain N initially. Crystallization to high‐purity α‐Si3N4 proceeds with additional heating above 1400°C under N2. The changes in crystallinity, powder morphology, infrared spectra, and elemental compositions, for samples annealed from 1000° to 1600°C under N2, are consistent with an amorphous‐to‐crystalline transformation. Although macroscopic consolidation and local densification occur at 1400°C, volatilization and accompanying weight loss limit bulk densification. The effect of temperature on specific surface area is examined and related to the sintering process. These results are applicable to pyrolysis, decomposition, and crystallization studies of ceramics synthesized by polymeric precursor routes.
Effects of External Stress on Defect Annihilation and Bubble Swelling During Annealing of Neutron‐Irradiated Silicon Carbide
Authors: Suzuki Tetsuya , Yano Toyohiko , Iseki Takayoshi , Mori Tsutomu
Published: March 2005

Silicon carbide ceramics fabricated by three different methods were neutron‐irradiated in the Japan Materials Testing Reactor and were subsequently annealed free from stresses or under compressive external stresses. The macroscopic length monotonically decreased with annealing above the irradiation temperature, when annealing was performed below ∼1300°C. This decrease was not affected by the external stresses. However, annealing above ∼1300°C led to an increase in length in B‐containing SiC. The expansion was caused by the formation and growth of He bubbles at grain boundaries. The growth occurred by flow of vacancies into bubbles. The compressive stress retarded the expansion along the loading direction. This retardation was compensated by a length incrase along the lateral direction. The effect of external stresses was discussed by considering differently oriented He bubbles. One bubble was stress favored and the other stress unfavored. The stress determined the diffusional flow of vacancy‐He atom complexes between the favored and unfavored bubbles which were caused by anisotropic expansion under a directional stress.
Synthesis and Characterization of a SiAION Glass
Authors: Sato Robert K. , Bolvin Jacques , McMillan Paul F.
Published: March 2005

Homogeneous SiAION glasses containing up to 1 wt% nitrogen were synthesized via a pressureless method with a controlled quench rate and structurally investigated using 27Al and 29Si magic‐angle spinning nuclear magnetic resonance (MAS NMR), Raman, and infrared (IR) spectroscopies. Minor changes occur with the incorporation of nitrogen into the aluminosilicate glass structure as evidenced by modifications to spectra of a nitrogen‐free aluminosilicate glass. The 27Al MAS NMR spectrum of the SiAION glass shows the existence of aluminum in 4‐,5‐, and 6‐coordination to oxygen. The 29Si MAS NMR spectra show a distribution of silicon sites in 4‐coordination to oxygen. Raman and IR spectra of the SiAION glass show additional features due to incorporation of nitrogen in the structure compared with spectra of nitrogen‐free aluminosilicate glasses.
Coarsening‐Resistant Dual‐Phase Interprenetrating Microstructures
Authors: French Jonathan D. , Harmer Martin P. , Chan Helen M. , Miller Gary A.
Published: March 2005

Grain growth in a 50:50 (vol%) dual‐phase mixture of Al2O3 and c‐ZrO2 (cubic zirconia) is severely limited compared with that for either of the single phases. At 1650°C, the growth rates in the duplex composition are 160 and 3500 times lower than that for single‐phase Al2O3 and c‐ZrO2, respectively. The restriction of the grain growth is attributed to the limited mutual solubility and the physical constraint provided by the interpenetrating geometry of the two phases. Grain coordination number and dihedral angle are also considered as factor affecting grain stabilization in two‐phase systems. A potentially important practical application of this work is the fabrication of grain‐stabilized fibers for use as reinforcement in composite structures at elevated temperatures.
Microwave Sintering of Spinel‐Type Oxides
Authors: Aliouat Mohamed , Mazo Louis , Desgardin Gilbert , Raveau Bernard
Published: March 2005

Microwave sintering of spinel‐type oxides, and more particularly of the ferrite LiFe5O8, was performed in a resonant cavity without use of any absorbent support. The behavior of these materials in a microwave field is similar to those observed for perovskites. However, the quality factor of the cavity is largely dependent on the presence of the ferrite, and modification of the coupling of the cavity is necessary to reach high temperatures. Thus, the difficulty in sintering such material is due more to a drastic decrease of the Q factor than to a cut‐off temperature, as generally believed.
Durability of Fluoride Glasses in Water
Authors: Tick Paul , Mitachi Seiko
Published: March 2005

The durability of eight fluoride glasses was tested in liquid water from 25° to 75°C. The studies were done by first growing a corrosion layer and then examining structure and chemistry in cross section with an optical microscope and an electron microprobe. In this way the details of the corrosion behavior could be directly determined. The results indicate that the corrosion process proceeds by a moving boundary, whose penetration rate is diffusion limited.
Plasma Spraying of Porous Electrodes for a Planar Solid Oxide Fuel Cell
Authors: Tai Lone‐Wen , Lessing Paul A.
Published: March 2005

Porous electrodes of Sr‐doped lanthanum manganite in a planar solid oxide fuel cell were made by the plasma spray technique. Spherical carbon particles were utilized as an effective pore former. Spraying a mixture of annealed oxide powders sized between 50 to 200 μm and 15 wt% solid Carbospheres resulted in a uniform coating of 40% porosity. Porous coatings were also obtained when rigid resin intermediates were sprayed followed by annealing at 1000°C in air.
Correlating the Mechanical Properties of a Continuous Fiber‐Reinforced Ceramic‐Matrix Composite to the Sliding Resistance of the Fibers
Authors: Weihs Timothy P. , Sbaizero Orfeo , Luh Ellice Y. , Nix William D.
Published: March 2005

The sliding resistance of NICALON fibers (SiC) in a lithium aluminosilicate (LAS III) glass‐ceramic matrix has been correlated with the mechanical properties of the composite. Push‐down measurements of the sliding resistance for individual fibers show an order of magnitude increase when the ceramic composite is annealed in air for 4 h at 800°C. Such increases are correlated with a loss of toughness, a reduction in the ultimate strength, and an increase in the matrix cracking stress of the composite, as measured using tension tests and bend tests of bulk samples.
Effect of Crystallization on Thermal Conductivity and Specific Heat of Two Corning Glass‐Ceramics
Authors: Olson Jeffrey R. , Fischer Henry E. , Pohl Robert O.
Published: March 2005

A magnesium aluminosilicate glass‐ceramic (Corning Code 9606) has been heat‐treated at 900°, 1010°, 1200°, and 1260°C for up to 90 min., and the effect on the thermal conductivity between 30 and 300 K has been studied. It has been shown that the large increase of the thermal conductivity which results from the ceramming occurs predominantly when crystalline cordierite forms. At 1260°C, heat treatment for only 20 min leads to a thermal conductivity identical to that measured for commercial 9606, which has been cerammed for 8 h at the same temperature. This observation provides further evidence for the usefulness of commercial 9606 as a thermal conductivity standard. Measurements of the specific heats of Code 9606 before and after ceramming have been reviewed and have been found to be close to that of crystalline SiO2 above ∼ 30 K, regardless of the state of ceramming. A review has also been made of thermal conductivity and specific heat of several ZrTiO4 nucleated lithium aluminosilicate glass‐ceramics at various states of ceramming. The thermal conductivity of these glass‐ceramics seems to be sufficiently independent of chemical composition as well as of the degree of crystallization to warrant a recommended curve for this class of glass‐ceramics.
Strength and Phase Stability of Yttria‐Ceria‐Doped Tetragonal Zirconia/Alumina Composites Sintered and Hot Isostatically Pressed in Argon–Oxygen Gas Atmosphere
Authors: Hirano Masanori , Inada Hiroshi
Published: March 2005

Yttria‐ceria‐doped tetragonal zirconia (Y,Ce)‐TZP)/alumina (Al2O3) composites were fabricated by hot isostatic pressing at 1400° to 1450°C and 196 MPa in an Ar–O2 atmosphere using the fine powders prepared by hydrolysis of ZrOCl2 solution. The composites consisting of 25 wt% Al2O3 and tetragonal zirconia with compositions 4 mol% YO1.5–4 mol% CeO2–ZrO2 and 2.5 mol% YO1.5–5.5 mol% CeO2–ZrO2 exhibited mean fracture strength as high as 2000 MPa and were resistant to phase transformation under saturated water vapor pressure at 180°C (1 MPa). Postsintering hot isostatic pressing of (4Y, 4Ce)‐TZP/Al2O3 and (2.5Y, 5.5Ce)‐TZP/Al2O3 composites was useful to enhance the phase stability under hydrothermal conditions and strength.
Preparation of Aluminum Nitride Powder from Aluminum Polynuclear Complexes
Authors: Hashimoto Noboru , Sawada Yasushi , Bando Takashi , Yoden Hiroyoshi , Deki Shigehito
Published: March 2005

AIN powder was synthesized from aluminum polynuclear complexes. Basic aluminum chloride and basic aluminum lactate were used as the aluminum polynuclear complexes. These starting materials and glucose were dissolved in water and mixed homogeneously. AIN powder was obtained by calcining after drying and precalcining at 800°C under nitrogen gas flow. Then excess carbon was removed by firing in air. Nitridation in the system was investigated and compared with that in the alumina–carbon black system. It was found that in our reaction system nitridation began and proceeded at lower calcination temperatures above 1200°C than in the alumina–carbon black system. Using aluminum polynuclear complexes, AIN was synthesized through the nitridation of γ‐alumina and produced in a very fine and sharp particle size distribution.
Electrostatic Adsorption of a Colloidal Sintering Agent on Silicon Nitride Particles
Authors: Lidén Eva , Persson Michael , Carlström Elis , Carlsson Roger
Published: March 2005

Pressureless sintering of silicon nitride requires addition of sintering agents. The main part of this study was done in order to homogenize the distribution of sintering agents, in this case Y2O3, in a silicon nitride matrix. Colloidal 10‐nm Y2O3 Particles were electrostatically adsorbed on Si3N4 particle surfaces. The adsorption was studied by X‐ray fluorescence analysis and electrophoretic measurements. Addition of Y2O3 sol to a Si3N4 suspension decreased the viscosity of the suspension. The slip casting properties of Si3N4 suspensions with added Y2O3 sol were examined, and the homogeneity of Y2O3 in the green compacts was compared with conventionally prepared samples. An improved microstructural homogeneity was obtained when Y2O3 sol particles were adsorbed on the Si3N4 particle surfaces.
Corrosion and Strength of SiC‐Whisker‐Reinforced Alumina Exposed at High Temperatures to H2—H2O Atmospheres
Authors: Kim Hyoun‐Ee , Moorhead Arthur J.
Published: March 2005

The flexural strengths and corrosion behavior of a SiC‐whisker‐reinforced alumina composite material (Al2O3—SiCw) were studied as functions of time, temperature, and PH2O in H2—H2O environments. Experimental conditions included temperatures of 1300° and 1400°C, exposure times up to 30 h, and water vapor levels ranging from very dry (PH2O= 4.6 × 10−7 MPa) to relatively wet (PH2O= 2.2 × 10−3 MPa). The study showed that the weight, strength, composition, and microstructure were strongly dependent on the PH2O level in the environment. When the PH2O in the H2 atmosphere was low, active oxidation of the SiC whiskers in the composite occurred, causing severe reductions in the weight and strength of the samples. At 1400°C, the most severe degradation was observed following exposure to an atmosphere with PH2O= 1.3 × 10−5 MPa. Exposure for 10 h to such an environment reduced the room‐temperature flexural strength of the material to less than 50% of that of the unexposed samples. At higher water vapor levels, the reductions in weight and strength became less severe because of the formation of aluminosilicate glass and mullite at the sample surface. Finally, when the composite was exposed at 1400°C to H2 atmospheres containing water vapor pressures of 5 × 10−4 MPa and above, weight gains were observed; and the strength of the material was significantly higher than that of the unexposed material. The increase in strength was attributed to crack healing or blunting as a result of the formation of aluminosilicate glass and mullite on the sample surface. Similar dependencies of strength and weight on the level of oxidant (H2O) in the atmosphere were noted following the exposures at 1300°C; however, the effects were smaller in magnitude.
Nitrogen Adsorption onto Silane‐Derived Silicon Powders
Authors: Sheldon Brian W. , Haggerty John S.
Published: March 2005

Nitrogen chemisorption onto silane‐derived silicon powder was studied at atmospheric pressure in N2, using thermogravimetric analysis (TGA) to examine the reaction kinetics and using Fourier transform infrared (FTIR) spectroscopy to characterize the surface chemistry. Above 700°C the initial Si—H surface is transformed to an Si—N surface; the latter is characterized by a broad infrared absorption band at 830 to 840 cm−1. Isothermal TGA data exhibit a sigmoidal shape which is apparently caused by simultaneous hydrogen desorption and nitrogen adsorption. By holding samples under vacuum at 900°C before nitridation the adsorption reaction was isolated. The kinetics of this reaction are well described by a simple first‐order model with an activation energy of 140 kJ/mol, and with very low sticking probabilities (between 10−13 and 10−10).
Effect of Crystalline Leucite on Porcelain Bonding on Titanium
Authors: Hautaniemi Jarmo Antero , Herø Håkon
Published: March 2005

The effect of elimination of crystalline leucite on porcelain bonding to Ti was studied. The thermal expansion coefficient of low‐fusing dental porcelain was decreased by eliminating the crystalline leucite phase. As a result, minimal thermal mismatch and better bonding between Ti and porcelain were obtained.
Tensile Stress Relaxation of Alumina at High Temperature
Authors: Woodford David A. , VanSteele Donald R. , Hyder Mohammed J.
Published: March 2005

The development of a tensile testing methodology for ceramics which enables a stress vs strain‐rate response to be measured at high temperature is described. The test involves a carefully controled stress relaxation test at constant total strain using an experimental procedure and phenomenological analysis previously developed for metallic materials. It is demonstrated here with preliminary tests on alumina at 1050° and 1150°C. This offers, with further development, the possibility of establishing design stresses associated with low strain‐rate behavior for structural applications. The results demonstrate that data covering four decades of strain rate may be generated in tests lasting a few hours. The inelastic strain consists of substantial anelastic recoverable strain in addition to a permanent creep strain.
Sol‐Gel Process for the Preparation of Ba2Ti9O20 and BaTi5O11
Authors: Lu Hsin‐chun , Burkhart Lawrence E. , Schrader Glenn L.
Published: March 2005

Barium titanate precursors with Ba/Ti ratio 2:9 and 1:5 were prepared by first hydrolyzing titanium alkoxide and then mixing the resulting titania sol with a barium alkoxide‐methanol solution. After drying, the xerogels of the precursors of barium titanates were sintered at temperatures from 700°C (4 h) to 1200°C (110 h or longer). Characterization of the product was performed using X‐ray diffraction and laser Raman spectroscopy. At 700°C, BaTi5O11 was formed from the 1:5 precursor and a two‐phase mixture of BaTi2O5 and BaTi5O11 was formed from the 2:9 precursor. After prolonged heating at 1200°C, the latter mixture converted to a single‐phase material, Ba2Ti9O20.
Solid‐Solution Ranges of the n= 2 and n= 3 Superconducting Phases in Bi2(SrxCa1 −x)n+1CunOy and the Effect on Tc
Authors: Hong Byungsun , Mason Thomas O.
Published: March 2005

The cation solubility limits of the n= 2 and n= 3 superconducting phases in the Bi2(SrxCa1 −x)n+1CunOy system were established along tie lines with compatible phases via electron probe microanalysis on bi‐ (or poly‐) phasic samples prepared at 860°C. Pb additions (15 mol% of the Bi content) were used to facilitate formation of the n= 3 phase. In each case football‐shaped volumes in composition space were established as the solubility limits which bordered on the nominal compositions 2212 or 2223 (Bi:Sr:Ca:Cu) with the long axis parallel to the Sr‐Ca side of the quaternary (i.e., Sr‐to‐Ca intersolubility) but also extending toward Bi and Cu. This means that, for the most part, the superconducting phases are alkaline‐earth deficient relative to the ideal 2212 and 2223 compositions. The Pb content in the 2223 phase is typically 10% of the Bi content. Tc variations could be correlated with variations in Sr or (Sr + Bi) content and the length of the c‐axis parameter.
Influence of Thermal History on the Crystallization Behavior and Hardness of a Glass‐Ceramic
Authors: Roesky Rainer , Varner James R.
Published: March 2005

The glass system SiO2─Al2O3─Li2O─TiO2 was examined to determine the dependence of hardness on heat‐treatment temperature. In addition to hardness measurements, X‐ray diffraction (XRD) was used to determine the type and amount of phases present and to relate this to the hardness results. Hardness increased with heat‐treatment temperature. This correlated with the amounts of crystals present, as determined by XRD. For a particular heat‐treatment temperature, when hardness was plotted versus load on the indenter, there was a load dependence. There was at first a rapid decrease as the load on the indenter increased, with a constant value being approached at higher loads.
Benefits of High‐Resolution Electron Microscopy for the Structural Characterization of Mullites
Authors: Epicier Thierry
Published: March 2005

The structure of 3:2 mullites is investigated by high‐resolution electron microscopy. Attention is paid to the identification of isolated defects, i.e., oxygen vacancies in pure 3:2 mullite and chromium interstitials in chromium‐doped mullite. Models for such defects, including lattice reconstruction effects, are analyzed with the help of image simulations.