bluuebunny/crossref_metadata_2025 huggingface dataset:
Fabrication and Secondary‐Phase Crystallization of Rare‐Earth Disilicate–Silicon Nitride Ceramics
Authors: Cinibulk Michael K. , Thomas Gareth , Johnson Sylvia M.
Published: March 2005

The fabrication and intergranular‐phase devitrification of silicon nitride densified with rare‐earth (RE) oxide additives has been investigated. The additions of the oxides of Sm, Gd, Dy, Er, and Yb, having high melting points and behaving similarly to Y2O3, were compositionally controlled to tailor a microstructure with a crystalline secondary phase of RE2Si2O7. The lanthanide oxides were found to be as effective as Y2O3 in densifying Si3N4, resulting in identical microstructures and densities of 98–99% of theoretical density. The crystallization behavior of all six disilicates was similar, characterized by a limited nucleation and rapid growth mechanism resulting in large single crystals. Complete crystallization of the intergranular phase was obtained with the exception of a thin residual amorphous film which was observed at interfaces and believed to be rich in impurities, the cause of incomplete devitrification.
Sintering Behavior of Fine Aluminum Nitride Powder Synthesized from Aluminum Polynuclear Complexes
Authors: Hashimoto Noboru , Yoden Hiroyoshi , Deki Shigehito
Published: March 2005

The sintering behavior of fine AIN powder synthesized from an aluminum polynuclear complex was investigated. The focus of this work was to investigate the densification behavior of the AIN powder with different particle sizes (specific surface area: 3.2–22.8 m2/g). The AIN powder was synthesized from basic aluminum chloride and glucose mixed in a water solution. This powder was divided into two groups: one with 2 wt% Y2O3 added as the sintering aid and the other without such an additive. The AIN powder investigated possessed favorable densification potential. The density of the AIN powder with a surface area of 16.6 m2/g and without additives attained theoretical density at 1700°C. Adding Y2O3 further decreased the sintering temperature required for full densification to 1600°C. It is speculated that low‐temperature sintering of our fine AIN powder with Y2O3 proceeds in two steps: in the initial stage, sintering proceeds predominantly through interdiffusion between yttrium aluminates formed on the AIN powder surface; in the second stage, the densification may occur by the interdiffusion between solid phases formed by a reaction between the yttrium aluminates and AIN. To investigate the effect of oxygen on sintering, the content of oxygen in AIN powder was varied while the particle size was kept constant. In this study, the difference in surface oxygen content scarcely affected the sintering behavior of fine AIN powder.
Rheology of Porous Alumina and Simulation of Hot Isostatic Pressing
Authors: Besson Jacques , Abouaf Marc
Published: March 2005

Rheology of a porous alumina was studied using sinter‐forging, hot‐pressing, and sintering tests. The results are analyzed using constitutive equations for porous materials. The deformation and densification rates are found to follow Coble creep behavior with an eventual control by interface reactions. The effect of porosity on shear and densification behavior is studied and compared to results obtained on metal powders showing similar trends for shear and a stronger influence in the case of densification. Large pores are likely to buckle at low densities when external forces are applied. The sintering pressure is also estimated and lies in the range 0 to 3 MPa. Finally, the constitutive equations are used to simulate hot isostatic pressing of test shapes, showing that the proposed model correctly predicts the deformation of the ceramic preforms.
Low‐Pressure Chemical Vapor Deposition of α‐Si3N4 from SiF4 and NH3: Kinetic Characteristics
Authors: Lee Woo Y. , Strife James R. , Veltri Richard D.
Published: March 2005

Deposition of α‐Si3N4 from SiF4 and NH3 was systematically studied using an axisymmetric, vertical hot‐wall reactor in the temperature range of 1340° to 1490°C. The relationship between process variables and deposition behavior was identified. The deposition process was most strongly influenced by temperature. In general, deposition rate increased exponentially with increased deposition temperature, although reagent depletion in the axial direction caused a rapid decrease in the deposition rate. The deposition rate increased moderately with increased flow rate or decreased NH3/SiF4 molar ratio. The decomposition characteristic of pure NH3 and SiF4 were studied utilizing mass spectroscopy and compared to thermodynamic predictions in order to assess their influences on the Si3N4 deposition process. Finally, the crystallography of Si3N4 deposits was correlated as a function of temperature and deposition rate.
Preparation of MgAl2O4 Spinel Powders via Freeze‐Drying of Alkoxide Precursors
Authors: Wang Chuei‐Tang , Lin Lang‐Sheng , Yang Sheng‐Jenn
Published: March 2005

High‐sinterability MgAl2O4 powder has been produced from alkoxide precursors via a freeze‐drying method. Clear alumina sol and magnesium methoxide were used as starting materials in the process. The spinel powders were characterized by various techniques, such as thermal analysis, X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. The tap density and sinterability of the spinel power are affected by the ball‐milling techniques. Highly dense, transparent, polycrystalline MgAl2O4 has been obtained from these powders by sintering and hot isostatic pressing. Bimodal grain‐size microstructure is observed in a HIPed sample.
Creep of Beryllia Single Crystals
Authors: Corman Gregory S.
Published: March 2005

The compressive creep behavior of single‐crystal BeO was studied under inert atmosphere. The steady‐state creep behavior was highly anisotropic because of easily activated basal slip. Crystals compressed along a [1101] axis deformed at temperatures as low as 650°C, whereas [1100]‐aligned crystals showed comparable strain rates at temperatures 1650°C. The measured activation energies and stress exponents for creep were 371 kJ/mol and 4.65 for the [1101] alignment and 496 kJ/mol and 3.39 for the [1100] alignment, respectively. The precise mechanisms controlling creep behavior were not determined. Only one semiquantitative data point was obtained for the [0001] alignment, which indicates that the creep rate at 100 MPa and 1750°C is less than that for c‐axis sapphire.
Characteristics of the High‐Tc Superconducting Bi‐Pb‐Sr‐Ca‐Cu Oxides Derived from an Ethylenediaminetetraacetic Acid Precursor
Authors: Chen Fueng‐Ho , Koo Horng‐Show , Tseng Tseung‐Yuen
Published: March 2005

The modified sol‐gel method using ethylenediaminetetraacetic acid as a chelating agent to prepare high‐Tc superconducting Bi‐Pb‐Sr‐Ca‐Cu oxides is reported. Results revealed that the desirable Bi‐based compounds can be acquired by such a process with a high percentage of the 110 K phase and a Tc(zero) of 108 K. On the other hand, a new method is proposed to quantitatively evaluate the volume fraction of the 110 K phase in Bi‐based superconducting materials, mainly composed of 85 and 110 K phases. That is a lever‐rule type of relation, V= (Y– 29.07°)/ (28.81°– 29.07°), where V is the volume fraction of the 110 K phase and Y= 2 θ for the X‐ray diffraction peak emerging at about 29° with a CuKα radiation source and 28.81°Y 29.07°. The volume fraction of the 2223 phase estimated in this way is more reliable than that estimated by measuring the area ratios of XRD peaks and it is expected to be even more reliable than those derived from conventional methods based on perfect diamagnetism.
Microstructure of a Shock‐Consolidated Diamond Compact Consisting of Fine Particles
Authors: Kondo Ken‐ichi , Sawai Sumiichi
Published: March 2005

Transmission electron microscopy clearly demonstrates two types of grain boundaries in a shock‐consolidated diamond compact consisting of fine particles: one is thick (40 nm) and irregular, whereas the other is thin (less than a few nanometers) and straight. The former is formed during the first compression (21 GPa) while the latter is formed after passage of the first shock‐wave front. Numerical calculations describing thermal diffusion support the proposed mechanism under a multiple‐compression process.
Fractal Characteristics of Fracture Surfaces
Authors: Baran George R. , Roques‐Carmes Claude , Wehbi Dohloul , Degrange Michel
Published: March 2005

Quantitative fractography is often used to study material failure mechanisms. During calculation of surface or profile roughness parameters, the magnification used in obtaining fractographic data is found to influence the value of the parameters. Fractal geometry has been developed into a tool capable of defining surface and profile topography without sensitivity to magnification, and several studies have related fractal dimension (DF) to other physical or mechanical properties. In this study, we obtained the fractal dimension of profiled fracture surfaces of one glass and three proprietary dental porcelains. The fracture toughness (K1c) of these materials was also measured using the indentation‐strength method. Results show the surfaces to be fractal. No quantitative relationship between fractal dimension and toughness was found. Differences in K1c were demonstrated between some materials. It is postulated that the size range within which fractal dimension can be defined as constant is dependent on the toughening mechanism, and that the relationship between KIc and DF cannot be identical for all materials.
Effect of Nonlinear Elasticity on Measured Fatigue Data and Lifetime Estimations of Optical Fibers
Authors: Griffioen Willem
Published: March 2005

The compressive stress–strain relation in optical fiber, which is important for a fiber during bending, is shown experimentally to be an asymmetric continuation of the already known nonlinear relation in the tensile region. Measured strengths of optical fibers in fatigue experiments can be 50% higher because of nonlinear corrections. Consequently decreases in measured corrosion susceptibilities, up to 10%, result in estimated fiber lifetimes, which decrease by an order of magnitude. The elastic line of a fiber inserted in a two‐point‐bending testing apparatus, however, is hardly influenced by nonlinear elastic effects.
Effect of Crystal Orientation on Conductivity and Electron Mobility in Single‐Crystal Alumina
Authors: Will Fritz G. , DeLorenzi Horst G. , Janora Kevin H.
Published: March 2005

The electrical conductivity of high‐purity, single‐crystal alumina is determined parallel to and perpendicular to the c‐axis. The mean conductivity of four samples of each orientation is a factor 3.3 higher parallel to the c‐axis than perpendicular to it. The conductivity as a function of temperature is attributed to extrinsic electron conduction at temperatures from 400° to 900°C and intrinsic semiconduction at temperatures from 900° to 1300°C. In the high‐temperature regime, the slope on all eight specimens is 4.7 ± 0.1 eV. Hence, the thermal bandgap at 0 K is 9.4 ± 0.2 eV.
Effect of Composition on Superconducting Properties in the System Ba‐Y‐Gd‐Cu‐O
Authors: Hill Michael D. , Wong‐Ng Winnie Kawai , Chiang Chwan K. , Fuller Edwin R. , Paretzkin Boris , Blendell John E. , Lagergren Eric , Kacker Raghu
Published: March 2005

The superconducting properties of a solid‐solution region in the Ba‐Y‐Gd‐Cu‐O system were investigated as a function of composition. Phase relations, the peritectic decomposition temperature, the superconducting onset temperature (Tc), and the transport current density (Jc) were measured. The highestTcandJcare observed along the line between the stoichiometric Ba2YCu3O6+xand Ba2GdCu3O6+xcompositions.
Preparation and Ferroelectric Properties of SBN:50 Ceramics
Authors: Deshpande Subhash B. , Potdar Hari S. , Godbole Prakash D. , Date Sadgopal K.
Published: March 2005

Stoichiometric quantities of AR‐grade chemicals, e.g., Ba(NO3)2, Sr(NO3)2, and Nb2O5, were used to synthesize Sr0.5Ba0.5Nb2O6 (SBN:50) by the solid‐state reaction route. The reaction mixture was characterized by DTA/TG/DTG to locate the formation temperature of SBN:50. X‐ray diffrction was used to identify the major and any additional phases formed in the calcined as well as sintered materials. The materials were further characterized by electrical measurements, the temperature dependence of the dielectric constant (ɛ) and the loss (tan δ), FE hysteresis‐loop parameters (Ps and Ec), and room‐temperature dc resistivity. SBN:50 exhibited a broad maximum of ɛ in the temperature range 65° to 95°C. Moreover, the FE hysteresis loop persisted even at 130°C, which is much higher than the Curie range determined from an ɛ vs T plot. Microstructural photographs revealed varied grain‐size distribution between 5 and 20 μm. The presence of residual porosity (∼ 95% dense) resulted in opaque compacts.
Stress–Strain Behavior of Duplex Ceramics: I, Observations
Authors: Lutz Ekkehard H. , Swain Michael V.
Published: March 2005

The stress–strain behavior of a variety of duplex ceramics was measured with the aid of strain gauges attached to the tensile surface of beams in four‐point flexure. The duplex materials consisted of a matrix of alumina and, alternatively, of yttria–tetragonal‐zirconia composites without (2Y‐TZP) and with alumina (3Y‐TZP20A). Up to 20 vol% pressure zones of different sizes containing unstabilized zirconia were dispersed within the matrices. Different types of stress–strain behavior were distinguished. The results were compared with microcrack and shear band observations of indented and R‐curve specimens.
Calculated X‐ray Diffraction Data for Diamond Polytypes
Authors: Ownby P. Darrell , Yang Xi , Liu Jenq
Published: March 2005

Calculated X‐ray diffraction pattern data for diamond polytypes are presented, which provides the required parameters for the characterization of the proposed diamond polytypes. The literature on diamond polytypes contains some small but significant errors with respect to the details of the crystal structure, including space groups, the atom positions in the unit cell, and the atomic layer stacking. In this paper, the crystal structures of four different hexagonal diamond and two different rhombohedral diamond polytypes are theoretically calculated to provide correct crystallographic information with which the diamond polytypes in a given sample can be characterized.
Synthesis of High‐Tc Dual‐Phase Ag–YBa2Cu3O7–x Superconductor Composite Powders by Sol–Gel Process
Authors: Agarwala Mukesh K. , Bourell David L. , Persad Chadee
Published: March 2005

Fine, homogeneous, dual‐phase Ag–YBa2Cu3O7–x composite powders were prepared by a simple colloidal sol–gel coprecipitation technique that was previously used for synthesis of single‐phase YBa2Cu3O7–x. Samples containing up to 60 wt% silver were synthesized. Silver neither reacted with nor degraded the YBa2Cu3O7–x. The presence of silver was found to aid the oxygenation of the precursor during calcination to form orthorhombic YBa2Cu3O7–x. Measurements made by ac magnetic susceptibility showed no significant degradation in transition temperature for samples containing up to 40 wt% silver.
Origin of the Grain Growth Anomaly in Donor‐Doped Barium Titanate
Authors: Drofenik Miha
Published: March 2005

The grain growth of donor‐doped BaTiO3 prepared from powders with different initial specific surface areas was investigated, and a microscale model was proposed that relates the critical amount of donor‐dopant, which inhibits anomalous grain growth, to the critical free surface area at the phase boundary. The results show that the model proposed can explain the inhibition of anomalous grain growth in donor‐doped BaTiO3 and can predict well the ratio of critical concentration to surface area for the system studied.
Microstructure Development of Oxycarbide Composites during Active‐Filler‐Controlled Polymer Pyrolysis
Authors: Erny Tobias , Seibold Michael , Jarchow Otto , Greil Peter
Published: March 2005

The microstructure development of a ceramic composite material fabricated by active‐filler‐controlled polymer pyrolysis (AFCOP) was investigated. During heating of a polysiloxane precursor mixed with titanium powder in argon atmosphere up to 1400°C, thermally induced decomposition of the polymer phase is combined with simultaneous carburization of the transition metal filler. Precipitation of nanocrystalline titanium carbide at the filler particle surface starts above 400°C, and larger, faceted carbide particles have growth above 800°C. A skeleton of turbostratic carbon is formed above 800°C in the polymer‐derived silicon oxycarbide matrix from which b‐silicon carbide and cristobalite crystallize above 1000°C. A pronounced reduction in linear shrinkage involved in polymer–ceramic conversion is observed. The shrinkage reduction ranges from more than 25% for the filler‐free precursor to less than 10% in the presence of 30 vol% of the titanium filler. Thus, active‐filler‐controlled pyrolysis offers the possibility of controlling shrinkage and porosity formation during polymer–ceramic conversion in order to fabricate bulk components from organometallic polymer precursor systems.
Kinetics of Indentation Cracking in Glass
Authors: Cook Robert F. , Liniger Eric G.
Published: March 2005

The propagation of indentation radial cracks in soda—lime silicate glass is measured as a function of time after indentation. Rapid lift‐off of the indenter from the specimen surface causes a step‐function perturbation in the radial crack mechanical energy release rate, thus providing access to a large range of observable crack velocities in the indentation stress field. Analysis of the data shows distinct threshold, reaction‐limited, and transport‐limited behavior in the crack velocity responses, in agreement with measurements made using macroscopic crack geometries. Atomistic models of fracture kinetics in reactive environments are fit to the data and are deconvoluted to yield the underlying atomic‐scale, bond‐rupture parameters. These latter are used to calculate potential functions for activated fracture and predict crack velocity responses as a function of temperature and pressure.
Effect of the Manganese Valence State on the Electrical Conductivity of Barium Titanate
Authors: Osawa Shin‐ichi , Furuzawa Akira , Fujikawa Nobuyoshi
Published: March 2005

The effect of defect structure on electrical conductivity for Mn‐doped BaTiO3 at high temperatures was studied experimentally and theoretically. The measured conductivity was proportional to Po2−1/4 in the n‐type region, and to Po2+1/6 in the p‐type region in the temperature range between 900° and 1000°C. The results agree with calculations on the basis of the equilibrium equation for the valence change of Mn ions between tetravalent and trivalent. In the p‐type region, the concentration of trivalent Mn ions decreases with Po2−1/6.
Kinetics of Hexacelsian‐to‐Celsian Phase Transformation in SrAl2Si2O8
Authors: Bansal Narottam P. , Drummond Charles H.
Published: March 2005

The kinetics of hexacelsian‐to‐celsian phase transformation in SrAl2Si2O8 have been investigated. Phase‐pure hexacelsian was prepared by heat treatment of glass flakes at 990°C for 10 h. Hexacelsian flakes were isothermally heat‐treated at 1026°, 1050°, 1100°, 1152°, and 1200°C for various times. The amounts of monoclinic celsian formed were determined using quantitative X‐ray diffraction. Values of reaction rate constant, k, at various temperatures were evaluated from the Avrami equation. The Avrami parameter was determined to be 1.1, suggesting one‐dimensional growth with the interface rather than a diffusion‐controlled transformation mechanism. From the temperature dependence of k, the apparent activation energy for this reaction was evaluated to be 527 ± 50 kj/mol (126 ± 12 kcal/mol). This value is consistent with a mechanism involving the transformation of the layered hexacelsian structure to a three‐dimensional network celsian structure which necessitates breaking of the strongest bonds, the Si─O bonds.
Effects of Sputtering and Plasma Etching on the Surface Reactivity of Graphite
Authors: Nowakowski Marilyn J. , Vohs John M. , Bonnell Dawn A.
Published: March 2005

The surfaces of highly oriented pyrolytic graphite (HOPG) samples were treated using two different methods, exposure to an energetic oxygen ion beam and immersion in an oxygen ion plasma, and the reactions which occur during treatment were characterized using high‐resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), and scanning tunneling microscopy (STM). Both surface treatments result in similar oxidation species. The results of this investigation provide spectroscopic evidence for the presence of semiquinone functionalities on sputtered and oxidized HOPG. STM images are presented to quantify the increase in defect sites after oxygen ion sputtering and to correlate defect site density with reactivity.
Phase Transformation Introduced by Mechanical and Chemical Surface Preparations of Tetragonal Zirconia Polycrystals
Authors: Mitra Nibedita , Vijayan Kalyani , Bai B. N. Pramila , Biswas S. K.
Published: March 2005

Cutting of Y2O3‐doped TZP rods by a low‐speed diamond saw introduces an unidentified, metastable phase X (x‐ZrO2) coexisting with the tetragonal (t‐ZrO2) and the monoclinic (m‐ZrO2) phases initially present in the sample. Further mechanical deformation of the cut surface by indentation or polishing sustains the x‐ZrO2. Chemical etching removes the x‐ZrO2 and increases the m‐ZrO2content.
Silicon Carbide Platelet/Alumina Composites: II, Mechanical Properties
Authors: Chou Yeong‐Shyung , Green David J.
Published: March 2005

The mechanical properties, i.e., Young's modulus, fracture toughness, and flexural strength, of SiC‐platelet/Al2O3 composites with two different platelet sizes were studied. Both Young's modulus and the fracture toughness of composites using small platelets (12 μm) increased with increasing SiC volume fraction. Maximum values for toughness and Young's modulus of 7.1 MPa·m1/2 and 421 GPa were obtained for composites containing 30 vol% platelets. Composites fabricated using larger platelets (24 μm), however, showed spontaneous microcracking at SiC volume fractions of ≤0.15. The presence of microcracks decreased Young's modulus and the fracture toughness substantially. Two types of radial microcracks were identified by optical microscopy and found to be consistent with a residual stress analysis. Anisotropy in fracture toughness was identified with a crack length indentation technique. Cracks propagating in a plane parallel to platelet faces experienced the least resistance, which was the the lowest toughness plane in platelet composites with preferred orientation. Enhanced fracture toughness was found in the plane parallel to the hot‐pressing direction, but no anisotropy in toughness was observed in this plane. The flexural strength of alumina showed a decrease from 610 to 480 MPa for a 30 vol% composite and was attributed to the presence of the platelets.
Influence of Tertiary Alkanolamines on Portland Cement Hydration
Authors: Gartner Ellis , Myers David
Published: March 2005

The physical and chemical effects of small additions of two different tertiary alkanolamines to portland cement were investigated. The strengths of standard test mortars moist cured for more than 1 day were found to be enhanced in some cases by addition of triisopropanolamine, but not by similar amounts of triethanolamine. Thermogravimetric and X‐ray diffractometric data indicate that the increased mortar strengths resulted from an increased degree of hydration of the cement. Calorimetry and aqueous‐phase analysis show that the higher alkanolamine, triisopropauo‐lamine, remains in solution for a sufficient time to catalyze hydration of C4 AF after all of the free gypsum has been consumed to form calcium sulfoaluminate hydrates, In contrast, the lower alkanolamine, triethanolamine, is mostly adsorbed by the cement within the first hours of hydration. It is hypothesized that the catalytic mechanism involves facilitated transport of ferric ions through the aqueous phase in the form of ferric‐alkanolamine complexes.
Phase Relations in the System Al2O3─Ce2Si2O7 in the Temperature Range 900° to 1925°C in Inert Atmosphere
Authors: Tas A. Cuneyt , Akinc Mufit
Published: March 2005

Equilibrium relationships in the system Al2O3‐Ce2Si2O7 in inert atmosphere have been investigated in the temperature range 900° to 1925°C. A simple eutectic reaction was found at 1375°C and 51 mol% Ce2Si2O7. A high‐low polymorphic transformation in Ce2Si2O7 was observed at 1274°C. New XRD patterns are suggested for both polymorphs of cerium pyrosilicate. The melting point of Ce2Si2O7 was found to be 1788°C. A value for ΔH°m,Ce2Si2O7 of 36.81 kJ/mol was calculated from the initial slope of the experimentally determined liquidus in equilibrium with the pyrosilicate phase.
Crack Growth and Damage in Constrained Sintering Films
Authors: Bordia Rajendra K. , Jagota Anand
Published: March 2005

The constrained sintering of films on substrates leads to a reduction in densification rate and may lead to processing flaws. This paper reports on a study of damage and cracking in sintering films, with particular emphasis on the growth of preexisting cracks. Experiments have been conducted with glass and polycrystalline Al2O3 films on various substrates. The effect of important variables (viz., film thickness, crack length, and friction with the substrate) on crack growth is reported. The experiments with glass films show that cracking occurs above a critical film thickness which is in good quantitative agreement with a recent analysis for this problem. In the case of Al2O3 films, we observe a diffuse damage zone ahead of cracks. Crack growth occurs by the coalescence of microcracks with each other and with the main crack. Some possible reasons for this difference between the glass and Al2O3 films are presented. As a model for diffuse damage, the stability of a sintering film under spatial variations in constitutive parameters is analyzed. It is shown that the film is unstable to small perturbations only in the early stages of densification, and that for viscous sintering the films are usually kinetically stable.
Drying of Granular Ceramic Films: II, Drying Stress and Saturation Uniformity
Authors: Chiu Raymond C. , Cima Michael J.
Published: March 2005

Films composed of ceramic particles were observed during drying. The films were prepared from 20 vol% aqueous dispersions of α‐alumina and α‐quartz and were free of any organic binder. Conditions for uniform film saturation during drying were established by consideration of a liquid transport model and by direct observation of the drying films. Drying stresses were measured in situ by a substrate deflection method based on an optical interference technique. Simultaneous stress and weight measurements were used to determine stress as a function of saturation. The maximum stress occurred near 100% saturation and was approximately 2 and 1.1 MPa for films produced from 0.35‐and 0.68‐μm particles, respectively. The maximum stress decreased from 2 to 0.9 MPa for films produced from the 0.35‐μm particles when 0.005 wt% surfactant was added to the slurry. The surfactant decreased the liquid surface tension from 72 to 32 dyn/cm. These observations are direct evidence of the effects of capillary tension on the state of stress in a ceramic body. Mechanical properties of the green ceramic films were estimated by use of a linear elastic fracture model. Knowledge of the critical cracking thickness and maximum stress in the film was used to estimate the fracture resistance of the granular film. The fracture resistance values are approximately 0.02 and 0.007 MPa·m1/2 for films produced from alumina and silica, respectively. The difference in mechanical behavior of the silica and alumina films is similar to that expected by the difference in Hamaker constants between the two materials.
Influence of Oxygen Partial Pressure on the Quasi‐Ternary System Cr─Mn─Ti Oxide
Authors: García‐Rosales Carmen , Schulze Horst A. , Naoumidis Aristides , Nickel Hubertus
Published: March 2005

The quasi‐ternary system Cr─Mn─Ti oxide was investigated at 1000°C under oxygen partial pressures ranging from 0.21 bar to 10‐21bar (1 bar = 105Pa). X‐ray diffraction analysis was used to identify phases and determine lattice parameters. The positions of phase boundaries as a function of oxygen partial pressure were measured using the emf method. The spinel MnCr2O4may be regarded as the most interesting compound in this system. Part of the chromium can be replaced by trivalent titanium at low oxygen partial pressures and by trivalent manganese at high pressures, and the formation of a limited solid solution with the spinel Mn2TiO4is possible in all cases. As a result, a coherent single‐phase spinel region exists over the entire oxygen partial pressure range at 1000°C.
Scanning Acoustic Microscopy Observations of High‐Temperature Crack‐Bridging Mechanisms in Alumina
Authors: Marrow James , Luprano Vincenza , Roberts Steve
Published: March 2005

High‐purity, fine‐grained alumina showed a strong R‐curve behavior for long cracks propagated at 1200°C. R‐curve behavior was not observed at room temperature. A combined investigation using high‐frequency scanning acoustic microscopy and scanning electron microscopy of crack profiles demonstrated asperity contact in the crack wake at 1200°C. A microcrack zone was not observed. Crack bridging, resulting from intergranular subcritical crack propagation was considered to be responsible for the toughness increase with increased crack length.
Indentation Crack Initiation and Propagation in Tempered Glass
Authors: Tandon Rajan , Cook Robert E.
Published: March 2005

Indentation crack initiation and propagation in tempered glass surfaces are examined and the results compared to those in annealed and in ion‐exchange‐strengthened glasses. The presence of surface compression due to tempering inhibits median crack initiation during the loading cycle of the indentation and depresses the radial crack initiation load during unloading. However, the extent of lateral cracking is enhanced in tempered glass surfaces. In situ crack propagation experiments reveal that the compressive stress tends to weakly stabilize crack extension prior to failure. The degree of crack stabilization is considerably lower than expected from a theoretical analysis.
Tin(IV) Oxide Coating of Aluminum Borate Whiskers and Alumina Particles by Chemical Vapor Deposition
Authors: Tsugeki Keiichi , Yoshikawa Tomohisa , Maeda Hideaki , Kusakabe Katsuki , Morooka Shigeharu
Published: March 2005

Aluminum borate whiskers of 0.5–1.0 μ diameter and alumina particles of 10–20 μ diameter were coated with SnO2 by the reaction of SnCl4–H2O–N2 gas mixtures in a rotary kiln reactor. Prior to coating, the whiskers were slightly etched to ensure adhesion between the SnO2 layer and the whisker surface. The whiskers were coated at 100°C for 1 h, and then at 300°C for 2 h. This procedure was effective for covering the entire whisker surface with a uniform SnO2 layer. Precoating was not necessary for the alumina particles. A compressed disk of the coated whiskers had an electrical conductivity of 30–40 S/m.
Structural Relaxations in Alkali Silicate Systems by Brillouin Light Scattering
Authors: Masnik John E. , Kieffer John , Bass Jay D.
Published: March 2005

The Brillouin light scattering technique was used to investigate the viscoelastic relaxations in alkali silicate systems up to temperatures of 1500°C. The line shape analysis of the Brillouin spectra reveals information about the mechanical rigidity and the viscous dissipation in the scattering medium. Brillouin frequency shifts and the frequency‐dependent viscosity coefficients, v′ were measured as a function of temperature. The shapes of the v′ vs temperature curves, and the location of their maxima on the temperature scale, reveal the activation energies and the resonance frequencies of the underlying mechanisms. In binary alkali silicates, small dissipation maxima were observed between the glass transition and the melting temperature, which are attributed to the increased cation mobility as a result of the accelerated disintegration of the silica networks. In mixed alkali systems, multiple dissipation maxima are observed. These are spread over a range of temperatures, which are distinctly higher than in binary systems.
Alpha‐Decay‐Induced Amorphization in Complex Silicate Structures
Authors: Weber William J.
Published: March 2005

Alpha decay in Cm‐doped Ca2Nd8(SiO4)6O2, Pu‐doped ZrSiO4, and natural ZrSiO4 minerals results in the simultaneous accumulation of point defects and amorphous regions that both contribute to the macroscopic swelling. A composite model for the macroscopic swelling is used to determine the amorphous fraction as a function of cumulative alpha‐decay events. The dependence of the derived amorphous fraction in Ca2Nd8(SiO4)6O2 on cumulative alpha‐decay events is consistent with amorphization occurring directly within an individual displacement cascade. In ZrSiO4, on the other hand, the rate of amorphization is lower and the derived amorphous fraction as a function of cumulative alpha‐decay events is consistent with models for amorphization occurring as a result of the local accumulation of high‐defect concentrations from the multiple overlapping of displacement cascades.
Anisotropic Thermal Expansion of Tetragonal Zirconia Polycrystals
Authors: Sheu Tzer‐Shin
Published: March 2005

Thermal expansion coefficients (αa and αc) in two crystallographic axes (a and c) of the tetragonal phase are measured at 25°–1200°C in ZrO2–M2O3 (M = Sc, In, Yb) and in ZrO2–YTaO4. The difference between these two thermal expansion coefficients, αc–αa, decreases with M2O3 or YTaO4 composition even though the tetragonality (c/a) behaves differently in these two systems. The locus of αc=αa represents a maximum tetragonality for the tetragonal phase, but not the phase boundary for the cubic phase. The relationships among thermal expansion, temperature, and composition are discussed.
Impedance Spectroscopy of Hydrating Cement‐Based Materials: Measurement, Interpretation, and Application
Authors: Christensen Bruce J. , Coverdale Tate , Olson Rudolf A. , Ford Steven J. , Garboczi Edward J. , Jennings Hamlin M. , Mason Thomas O.
Published: March 2005

This work concerns the state of the art for use of impedance spectroscopy for studying the evolving microstructure of cement‐based materials during hydration. Features of the spectra are discussed and related to components of the microstructure with the assistance of pixel‐based computer modeling techniques. It is proposed that the enormously high relative dielectric constants (∼105) observed just after set are the result of dielectric amplification and are related to the distribution of pore sizes and the thickness of product C─S─H layers separating the pores. The conductivity is related to the volume fraction of porosity, the conductivity of the pore solution, and the interconnectivity of the porosity. The conductivity, when normalized by that of the pore solution, i.e., inverse formation factor, is a measure of this interconnectivity and can be used to predict such engineering properties as ionic diffusivity and water permeability. Composite mixing laws are employed to aid in explaining the behavior of the conductivity and to obtain a qualitative measure of the pore shape with hydration. Procedures for predicting the conductivity of the pore solution and for subtracting out electrode lead effects at high frequency are discussed.
Evaluation of Whisker Alignment in Axisymmetric SiCw‐Reinforced Al2O3 Composite Material
Authors: Park Hyoung‐Joon , Kim Hyoun‐Ee , Kim Doh‐Yeon
Published: March 2005

A quantitative method to evaluate the degree of whisker alignment in axisymmetric composite materials was developed. The angular distribution of the whiskers was analyzed by measuring the aspect ratios of the whiskers observed on a planar section. However, due to the large difference in the probability of whiskers being detected on the planar section (depending on whisker length and degree of alignment), the angular distribution of the whiskers observed on the planar section was significantly different from the actual distributions of the whiskers in three dimensions. Three‐dimensional angular distributions were evaluated by comparing the aspect ratios observed on the planar section with those calculated under the assumption that the whisker angle fell in a Gaussian distribution with average angle of 90°. By this method, the degree of preferential alignment is expressed as the standard deviation of the Gaussian distribution. This quantification of whisker alignment is useful in analyzing the mechanical behaviors of composite materials reinforced with elongated particles.
High‐Temperature Oxidation of Chemically Vapor‐Deposited Silicon Nitride in a Carbon Monoxide‐Carbon Dioxide Atmosphere
Authors: Narushima Takayuki , Goto Takashi , Hagiwara Jun , Iguchi Yasutaka , Hirai Toshio
Published: March 2005

Oxidation behavior of chemically vapor‐deposited silicon nitride (CVD‐Si3N4) in CO─CO2 atmospheres between 1823 and 1923 K was investigated using a thermogravimetric technique. Mass loss of Si3N4 (active oxidation) was observed in a region of PCO2/PCO < 1, while mass gain (passive oxidation) was observed at around PCO2PCO= 10. In the active oxidation region below PCO2PCO= 10 –4, carbon particles were formed on the Si3N4 surface as an oxidation product, and the mass‐loss rates were independent of PCO2/PCO In the active oxidation region above PCO2/PCO= 10–4 the mass‐loss rates decreased with increasing PCO2/Pco. The critical PCO2/PCO value from the active to passive oxidation was 2 orders of magnitude larger than the calculated value predicted from the Wagner model.
Ultra‐High Strain Ceramics with Multiple Field‐Induced Phase Transitions
Authors: Brodeur Russell P. , wa Gachigi Kamau , Pruna Philip M. , Shrout Thomas R.
Published: March 2005

The induced polarization and longitudinal strain behavior of lanthanum‐ and titanium‐modified antiferroelectric lead zirconate ceramics were measured as functions of electric field as high as 30 kV/mrn (300 kV/cm). Two field‐induced phase transitions were observed, both in polarization and strain. Polarization and strain levels greater than 0.5 C/m2and 0.75%, respectively, were achieved in this materials family.
Effects of La2O3/ZnO Additives on Microstructure And Microwave Dielectric Properties of Zr0.8Sn0.2TiO4 Ceramics
Authors: Kudesia Rajiv , McHale Anna E. , Snyder Robert L.
Published: March 2005

Dielectric ceramics of Zr0.8Sn0.2TiO4 containing La2O3 and ZnO as sintering aids were prepared and investigated for microstructure and microwave dielectric properties. Low‐level doping with La2O3 and ZnO (up to 0.30 wt%) is good for densification and dielectric properties. These additives do not affect the dielectric constant and the temperature coefficient. Dielectric losses increase significantly at additive levels higher than 0.15 wt%. The combined additives La2O3 and ZnO act as grain growth enhancers. With 0.15 wt% additives, a ceramic having a dielectric constant, a quality factor, and a temperature coefficient of frequency at 4.2 GHz of 37.6, 12 800, and –2.9 ppm/°C, respectively, was obtained. The quality factor was considerably improved by prolonged sintering.
Effect of Thermoelastic Anisotropy on theR‐curve Behavior of Monolithic Alumina
Authors: White Kenneth W. , Hay Jack C.
Published: March 2005

The postfracture tensile (PFT) test has previously been used to evaluate the wake stresses of both cubic and noncubic monolithic ceramics at room temperature. The present study applies this procedure to the evaluation of the characteristic stress–crack face separation relationships for alumina at temperatures ranging from room temperature through 600°C, which is slightly below the softening temperature for the glassy grain boundai y phase. For comparison with the PFT behaviors,R‐curves are obtained throughout this range of temperatures using a short double cantilever beam geometry. The average residual stresses arising from thermal expansion anisotropy diminish with increasing temperature, causing two effects evidenced by a general downward shift of the characteristic PFT stress–displacement curves. The resulting lower peak stresses and smaller limiting crack opening displacements (COD) manifest through a decrease of the COD–grain size ratio criteria for bridging. Also, the residual stresses contribute to a significant fraction of large grains fracturing transgrarularly. This fraction is modified somewhat by test temperatures of 800°C and above; however, no detectable change can be claimed for the present results, which are limited to 600°C.
Subcritical Crack Growth in CVI Silicon Carbide Reinforced with Nicalon Fibers: Experiment and Model
Authors: Henager Charles H. , Jones Russell H.
Published: March 2005

Subcritical crack growth measurements of ceramic‐matrix composites have been conducted on materials consisting of CVI SiC matrix reinforced with Nicalon fibers (SiC/SiCf) having C and BN fiber–matrix interfaces. Velocities of effective elastic cracks were determined as a function of applied stress intensity in pure Ar and in Ar plus 2000 ppm O2 at 1100°C. A stage‐II regime, where the crack velocity depends only weakly on the applied stress intensity, was observed in the V–K diagrams over a range of applied stress intensities that correspond to the R‐curve of the materials. This stage‐II behavior was followed by a stage‐III, or power‐law, regime at higher stress intensity values. Oxygen was observed to increase the crack velocity in the stage‐II regime and to shift the stage‐II‐to‐stage‐III transition to lower stress intensity values. A 2D micromechanics approach was developed to model the time dependence of observed crack‐bridging events and is able to rationalize the measured effective crack velocities, the time dependence of the crack velocity, and the stage‐II‐to‐stage‐III transition in terms of the stress relaxation of crack‐bridging fibers.
Toughness Properties of a Silicon Carbide with an in Situ Induced Heterogeneous Grain Structure
Authors: Padture Nitin P. , Lawn Brian R.
Published: March 2005

Toughness characteristics of a heterogeneous silicon carbide with a coarsened and elongated grain structure and an intergranular second phase are evaluated relative to a homogeneous, fine‐grain control using indentation–strength data. The heterogeneous material exhibits a distinctive flaw tolerance, indicative of a pronounced toughness curve. Quantitative evaluation of the data reveals an enhanced toughness in the long‐crack region, with the implication of degraded toughness in the short‐crack region. The enhanced long‐crack toughness is identified with crack‐interface bridging. The degraded short‐crack toughness is attributed to weakened grain or interface boundaries and to internal residual stresses from thermal expansion mismatch. A profound manifestation of the toughness‐curve behavior is a transition in the nature of mechanical damage in Hertzian contacts, from classical single‐crack cone fracture in the homogeneous control to distributed subsurface damage in the heterogeneous material.
In‐plane Shear Properties of 2‐D Ceramic Matrix Composites
Authors: Brøndsted Povl , Heredia Fernando E. , Evans Anthony G.
Published: March 2005

The in‐plane shear properties of a range of 2‐D ceramic matrix composites have been measured using the Iosipescu configuration. The nonlinear deformation is found to be associated primarily with matrix cracks. Consequently, the shear modulus decreases as strain proceeds. The flow strength in shear is found to be compatible with the stress at which multiple matrix cracking is expected to occur, without interface slip. Consequently, the shear strength scales with the shear modulus. The shear ductility is found to diminish as the matrix modulus increases. This effect is attributed to the influence of matrix modulus on the bending deformation of fibers between matrix cracks. The experimental observations are used to suggest a mechanism map that identifies (class III) materials which redistribute stress around notches by means of shear bands.
Phase Stability of Chemically Derived Enstatite (MgSiO3) Powders
Authors: Huang Chao M. , Kuo Dong H. , Kim Youn J. , Kriven Waltraud M.
Published: March 2005

A modified Pechini chemical preparation technique was used to produce enstatite (MgSiO3) powder. By this method, low‐temperature stable orthoenstatite (OE) was obtained at 850°C after 2 h of calcination. The effects of annealing temperature/time, initial particle size, chemical dopants, and shear stress on the conversion of protoenstatite (PE) to clinoenstatite (CE) on the powder have been studied. The results indicated that the transformation of orthorhombic PE to monoclinic CE was sensitive to the initial powder particle size as well as to the type and amount of chemical dopant used. Sodium ions (Na+), which were found to cause the formation of a glassy phase around the PE grains, destabilized the PE phase physically. In comparison, manganese ions (Mn2+), were found to preferentially substitute for the smaller Mg2+ ions in solid solution and stabilized the PE phase chemically. The powders with different chemical dopants were examined by TEM and EDS.
Effects of Solids Loading and Dispersion Schedule on the State of Aqueous Alumina/Zirconia Dispersions
Authors: Wernet Judith , Feke Donald L.
Published: March 2005

The effects of solids loading and dispersion order on the state of aqueous slips of either zirconia or alumina, and binary slips of these two powders, have been examined. Since these powders can acquire surface charge when they are dispersed in aqueous media, changes in the ionic composition of the slip can occur as a result of the addition of solids. At higher solids loading, a substantial fraction of the ionic activity within the slip may be attributed to the solid itself. As the solids loading within unary powder suspensions is increased, it is possible to drive the suspension pH toward the point of zero‐charge for the powder, resulting in diminished electrostatic stabilization and possible powder agglomeration. An analytical model was constructed to account for the observed results. This embodied a simple surface site complexation model which was augmented to include material and charge balance constraints on ionized surface sites as well as solvated ions. For binary powder dispersions of alumina and zirconia, it was found that the order of dispersing the two components could have a pronounced effect on the overall suspension behavior. The variability in results is attributed to the solubility of the alumina in the basic processing media.
Microstructure and Chemical Reaction in a Si3N4–TiC Composite
Authors: Huang Jow‐Lay , Chiu Hung‐Liang , Lee Ming‐Tung
Published: March 2005

The effects of TiC addition to Si3N4 on microstructure and the chemical aspects of Si3N4–TiC interphase reaction were investigated in samples hot‐pressed at 1800°C in Ar and N2. Composition of a TiC1–xNx solid solution was predicted based on thermodynamic calculation, with titanium carbonitride taken to be an ideal solid solution. The predicted value of x= 0.7 is slightly higher than that derived from the measured lattice parameter and Vegard's law (x = 0.67). Four distinguishable areas were observed in samples hot‐pressed in nitrogen atmosphere. They were identified as β‐Si3N4, mixtures of TiC and titanium carbonitride solid solution, SiC with twins, and iron silicide. As the duration of hot‐pressing increased, more titanium carbonitride was formed, while less TiC phase remained. Thermodymanic calculations indicate one source of nitrogen gas came from the decomposition of Si3N4. The TiC particles also became more irregular, and reactants were found inside or between TiC as the hot‐pressing time was extended. Silicon carbide was not detected in samples which were hot‐pressed in argon atmosphere; however, numerous pores were found around TiC.