bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Speed and Pressure on Dry Sliding Interactions of Alumina against Steel
Authors: Ravikiran A. , Nagarajan V. S. , Biswas S. K. , Bai B. N. Pramila
Published: July 2005

Sliding of alumina (87%) pins against a hardened steel disk over a range of pressures (3.3–30.0 MPa) and speeds (0.1–12.0 ms‐1) has been studied. Four different regions (R1, R2, R3, and R4) of friction as a function of speed have been identified. R1 and R3 exhibit single‐valued friction while in R2 and R4 the friction exhibits dual behavior. The speed range over which these regions prevail is sensitive to the pressure. R1 and R2 are low‐speed and low‐temperature regions, and in both, metal transfer and formation and compaction of y‐Fe2O3 occur. R3 and R4 are associated with high speeds and high interface temperatures. Formation of FeO, FeAl2O4, and FeAlO3 has been observed. The implications of the tribochemical interactions on friction and wear characteristics are discussed.
Segregation in Single‐Crystal Fully Stabilized Yttria‐Zirconia
Authors: Hughes Anthony E.
Published: July 2005

X‐ray photoelectron spectroscopy (XPS) has been used to study surface segregation in 9.5 mol% single‐crystal Y2O3‐ZrO2 which has been subjected to air anneals at temperatures ranging from 900° to 1500°C. The kinetics of segregation reveal an enrichment of Y, Si, Na, and Fe at short times; however, at longer times surface equilibrium is reached. The heats of segregation for Y, Si, Na and Fe, determined from Arrhenius plots, were 9.3 ± 3.0, 59.0 ± 7.0, –23.5 ± 11.5, –18.0 ± 6.0 kJ/mol, respectively. XPS analysis of the equilibrium states at all temperatures revealed a distinct surface layer which, given the positive heats of segregation for Si and Y, is quite stable.
Structural Characterization and High‐Temperature Behavior of Silicon Oxycarbide Glasses Prepared from Sol‐Gel Precursors Containing Si‐H Bonds
Authors: Sorarù Gian Domenico , D'Andrea Gennaro , Campostrini Renzo , Babonneau Florence , Mariotto Gino
Published: July 2005

Silicon oxycarbide glasses have been synthesized by inert atmosphere pyrolysis at 1000°C of gel precursors obtained by cohydrolysis of triethoxysilane, HSi(OEt)3, and methyl‐diethoxysilane, HMeSi(OEt)2. The oxycarbide structures have been carefully characterized by means of different techniques such as 29Si magic angle spinning nuclear magnetic resonance (MAS‐NMR) and Raman spectroscopies, X‐ray diffraction (XRD), and chemical analysis. Experimental results clearly indicate that, depending on the composition of the starting gels, the resulting oxycarbide glass either is formed by a pure oxycarbide phase or contains an extra carbon or silicon phase. By increasing the temperature up to 1500°C, the oxycarbide glasses display compositional and weight stability; however, the amorphous network undergoes structural rearrangements that lead to the precipitation of nano‐sized β‐SiC crystallites into amorphous silica. Crystallization of metallic silicon is also clearly observed at 1500°C for the samples in which the presence of Si‐Si bonds was postulated at 1000°C.
Failure Modes of SiC‐Fiber/Si3N4‐Matrix Composites at Elevated Temperatures
Authors: Xu Hockin H. K. , Braun Linda M. , Ostertag Claudia P. , Krause Ralph F. , Lloyd Isabel K.
Published: July 2005

An investigation of composite failure modes as a function of temperature and fiber‐volume fraction was carried out in SiC‐fiber‐reinforced Si3N4‐matrix composites fabricated in our laboratories. Mechanical testing was carried out at temperatures from 25° to 1500°C. Matrix‐cracking stress and ultimate strength of the composites were measured from load‐displacement curves. They were both found to decrease with increasing temperature, but their temperature sensitivity decreased with increasing fiber‐volume fraction. The tendency for noncatastrophic failure increased with fiber‐volume fraction, while the tendency for catastrophic failure increased with temperature. The failure mode was demonstrated experimentally to be determined by the fiber bundle strength, Sfb, and the matrix cracking stress, σc. These two parameters, in turn, were shown to be controlled by the fiber‐volume fraction, f, and the temperature. Failure at various temperatures was noncatastrophic when Sfb > σc, and catastrophic when Sfb < σc. Transition in composite failure mode between noncatastrophic and catastrophic failure was controlled via the variation of fibervolume fraction and testing temperature. Catastrophic failure at high temperatures was found to be mainly a result of fiber strength loss.
Preparation and Characterization of Single‐Phase β‐SiAlON
Authors: Ramesh Peelamedu D. , Rao Kalya J.
Published: July 2005

A simple method for the preparation of monophasic β‐SiAlON using nitridation of Si and AIN with an oxygen partial pressure of 10 ‐4 atm is described. The effect of the AIN/Si ratio in the initial mixture on the formation of β‐SiAlON is discussed. The likely mechanism of the formation of β‐SiAlON is outlined.
Microcomposite Test Procedure for Evaluating the Interface Properties of Ceramic Matrix Composites
Authors: Lamon Jacques , Rebillat Francis , Evans Anthony G.
Published: July 2005

A microcomposite test procedure for evaluating the constituent properties of CMCs produced by chemical vapor infiltration is described. The analysis and experimental results demonstrate that the interface sliding resistance to can be obtained from the unload/reload hysteresis, after one (or more) matrix crack has been induced. The nonlinear strain can also be used to provide an independent determination of to, as well as give values for the misfit strain and the interface debond energy. Results obtained on SiC/C/SiC and SiC/BN/SiC materials are evaluated.
Fatigue Crack Growth from Small Surface Cracks in Transformation‐Toughened Ceramics
Authors: Andreasen Jens H. , Moller Carsten V. , Karihaloo Bhushan L.
Published: July 2005

A theoretical model based on the theory of complex potentials and dislocation formalism is used to simulate the fatigue crack growth of small cracks in a transformation‐toughened ceramic. Assuming power‐law crack growth in which the growth rate depends on the effective stress intensity at the crack tip instead of the applied stress intensity, it is shown that the crack growth rate decreases with the applied stress intensity in the initial stage of fatigue crack growth. This is in agreement with existing experimental evidence for the growth of small cracks in Mg‐PSZ. New experimental results obtained by in situ observation in a scanning electron microscope of a similar material confirm this behavior. The numerical results also confirm the plausibility of using the steady‐state toughness value obtained from quasi‐static crack growth as a normalizing parameter in the power‐law for fatigue crack growth.
Effects of Temperature on Conduction and Polarization in Portland Cement Mortar
Authors: McCarter William J.
Published: July 2005

The electrical impedance of Portland cement mortar was measured over the temperature range 10–80°C. Both the real and imaginary components of the complex impedance were found to be temperature dependent and linked through an Arrhenius relationship over the range 10–50°C. Both conduction and polarization processes are thermally activated with an activation energy of, respectively, 19.4 and 4.6 kj/mol. At temperatures in excess of 50°C, sample impedance was almost independent of temperature.
Anomalous Electromechanical Behavior of Portland Cement: Electro‐osmotically‐lnduced Shape Changes
Authors: Li Jie‐Fang , Ai Hua , Viehland Dwight
Published: July 2005

The electromechanical behavior of hardened portland cement paste has been investigated as a function of measurement frequency and dc electrical bias, using an inter‐ferometric technique. Large field‐induced shape changes on the order of 100 A have been found in millimeter‐thick specimens exposed to moisture. In addition, strong hysteresis effects were found on cycling a large ac field. Dry samples were found to exhibit no field‐induced deformations. The mechanism underlying this anomalous behavior is believed to be an electro‐osmotically induced swelling of the pore structures. Evidence in support of this hypothesis was found by corresponding investigations of porous silica gels.
Computer Simulation Studies of Strontium Titanate
Authors: Akhtar M. Javed , Akhtar Zeb‐Un‐Nisa , Jackson Robert A. , Catlow C. Richard A.
Published: July 2005

We present the results of computer simulation studies of SrTiO3. After deriving a reliable potential model, we concentrate on the properties of defects and dopants. Our calculations are used to propose a defect model that is consistent with experimental observations. For vacancy disorder, the calculations show that SrTiO3 (Schottky) and SrO (Schottky‐like) disorder have very similar energy, with a small tendency for the development of Sr deficiency at higher temperatures. All mono‐ and divalent cations prefer to substitute at strontium sites. For trivalent cations, three types of mechanism (Sr and Ti substitution and self‐compensation) are proposed, whereas for tetravalent cations substitution on both A and B sites may occur, depending on the ionic radius.
Dehydration and Crystallization Kinetics of Zirconia‐Yttria Gels
Authors: Ramanathan Srinivasan , Muraleedharan Ramayi V. , Roy Sujit Kumar , K. Padmanabha , Nayar Krishnan
Published: July 2005

Zirconia and zirconia‐yttria gels containing 4 and 8 mol % yttria were obtained by coprecipitation and drying at 373 K. The dehydration and crystallization behavior of the dried gels was studied by DSC, TG, and XRD. The gels undergo elimination of water over a wide temperature range of 373–673 K. The peak temperature of the endotherm corresponding to dehydration and the kinetic constants for the process were not influenced by the yttria content of the gel. The enthalpy of dehydration observed was in good agreement with the heat of vaporization data. The dehydration was followed by a sharp exothermic crystallization process. The peak temperature of the exotherm and the activation energy of the process increased with an increase in yttria content, while the enthalpy of crystallization showed a decrease. The “glow effect” reduced with increasing yttria content. Pure zirconia crystallizes in the tetragonal form while the zirconia containing 4 and 8 mol% yttria appears to crystallize in the cubic form.
Lattice Parameters and Densities of Rare‐Earth Oxide Doped Ceria Electrolytes
Authors: Hong Seong Jae , Virkar Anil V.
Published: July 2005

Lattice parameters and densities of various rare‐earth oxide doped ceria were measured. For the calculation of lattice parameters, the concept of the oxygen vacancy radius was introduced. The estimated oxygen vacancy radius was 1.164 A for trivalent, rare‐earth oxide doped ceria and 0.993 A for zirconia. The modeling of the lattice parameter and density based on the oxygen vacancy radius generally agreed with experimental results. The density measurements illustrate that oxygen vacancy formation is the predominant defect reaction. Evidence of some interstitial contribution was also noted for smaller‐sized dopant cations such as Yb3+ and Er3+.
Phase Relations in the System La‐Rh‐O and Thermodynamic Properties of LaRhO3
Authors: Jacob K. Thomas , Waseda Yoshio
Published: July 2005

Phase relations in the system La‐Rh‐O at 1223 K have been determined by examination of equilibrated samples by optical and scanning electron microscopy, powder X‐ray diffraction (XRD), and energy‐dispersive analysis of X‐rays (EDAX). Only one ternary oxide, LaRhO3, with distorted orthorhombic perovskite structure (Pbnm, a = 0.5525, b= 0.5680, and c= 0.7901 nm) was identified. The alloys and intermetallics along the La‐Rh binary are in equilibrium with La2O3. The thermodynamic properties of LaRhO3 were determined in the temperature range 890 to 1310 K, using a solid‐state cell incorporating yttria‐stabilized zirconia as the electrolyte. A new four‐compartment design of the emf cell was used to enhance the accuracy of measurement. For the reaction

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The compound decomposes on heating to a mixture of La2O3, Rh and O2. The calculated decomposition temperatures are 1843 (± 5) K in pure O2 and 1728 (± 5) K in air at a pressure of 1.01 × 105 Pa. The phase diagrams for the system La‐Rh‐O at different partial pressures of oxygen are calculated from the thermodynamic information.
Crystal Structure and Equation of State of Cotunnite‐Type Zirconia
Authors: Haines Julian , Léger Jean Michel , Atouf Abdeltif
Published: July 2005

Cotunnite‐type zirconia was studied by angular‐dispersive X‐ray diffraction in a diamond anvil cell after laser heating at 18 and 26.7 GPa. The structure, space group Pnam, Z= 4, was refined in situ at several different pressures on decompression and at ambient using the Rietveld method. The nine polyhedral Zr‐O distances range from 2.10 to 2.56 A at ambient pressure, which represents both an increase in the average and the minimum Zr‐O distance relative to the monoclinic and orthorhombic lower‐pressure forms. In addition, data obtained on an unheated sample indicate that the irreversible transition to the cotunnite phase began above 25 GPa and 70% conversion was achieved by 48.5 GPa. Tne compressibility of cotunnite‐type zirconia was found to be slightly anisotropic and a Birch‐Murnaghan equation‐of‐state fit of the p‐V data yielded a bulk modulus of 332(8) GPa with a first derivative of 2.3(4), which is in good agreement with the value predicted by previous ab ini‐tio calculations.
Relation between Strength, Microstructure, and Grain‐Bridging Characteristics in In Situ Reinforced Silicon Nitride
Authors: Li Chien‐Wei , Lui Siu‐Ching , Goldacker Jeffrey
Published: July 2005

The grain size of in situ Si3N4 is varied, and its effects on strength‐flaw size relations are related to the behavior of a bridging zone behind the crack tip. The bridging‐zone properties are calculated from a Dugdale model assuming that the bridging zone has a constant bridging stress (p*) and length (Db) at the moment of the critical fracture. The results show that as grain size increases, p* decreases while Db and the critical bridging zone opening (u*) first increase and then decrease, resulting in a maximum for short‐crack fracture toughness at an intermediate grain size. The initial increase of u* and Db with grain size is attributed to an increase in debonding length, while the decrease of p* is attributed to a decrease in strength for bridging grains due to a statistical effect which also causes Db and u* to drop in the large‐grain regime. Implications on microstructure design are discussed.
Crystallization Kinetics of α‐Agl in Ag I‐Based Silver Orthoborate Glasses
Authors: Taniguchi Akihiro , Tatsumisago Masahiro , Minami Tsutomu
Published: July 2005

Kinetics for nucleation and growth of α‐AgI crystals in AgI‐based silver orthoborate glasses, in which only α‐AgI crystals were found to crystallize in the heating process of the glasses, were studied by isothermal and nonisothermal measurements using differential scanning calorimetry and by observations of the microstructure of the glasses using scanning electron microscopy. The values of both n and m, dependent on the crystallization mechanism, were found to be 3, indicating that a constant number of nuclei of α‐AgI precipitated in a glass matrix grew three‐dimensionally. The preannealing of the glasses at temperatures between glass transition and crystallization did not affect the DSC isothermal curves in the crystal growth process, which suggested that the number of α‐Agl nuclei would have been saturated in the glass when the glasses were prepared by quenching the AgI‐based melts.
Mechanical Properties of Alumina–Zirconia–Silver Composites
Authors: Tuan Wei‐Hsing , Chen Wai‐Rue
Published: July 2005

Either ceramic inclusions or metallic inclusions can be used to enhance the mechanical properties of ceramics. In the present study, both silver inclusions and zirconia agglomerates have been added to alumina. The presence of the inclusions inhibits the grain growth of the alumina matrix. The strength of AI2O3‐ZrO2‐Ag composites is increased by microstructural refinement. Together with the plastic deformation of silver inclusions and the phase transformation of tetragonal zirconia agglomerates, the toughness of the composites is enhanced. Because silver inclusions and zirconia agglomerates are attached after sintering, the toughness increase for the Al2O3‐ZrO2‐Ag composites is less than the sum of the toughness increments for Al2O3‐Ag and Al2O3‐ZrO2 composites.
Dependence of Crystallization Behavior of Sodium Diborate (Na2O·2B2O3) on Its Glass Structure and the Characteristics of Phase Transformation
Authors: Li Hua , Liang Jingkui
Published: July 2005

The crystallization and phase transformation of glassy Na2O·2B2O3 have been studied by X‐ray diffraction, infrared spectroscopy, thermal analysis, and density measurement. By employing proper thermal treatment methods, three metastable and two relatively stable polymorphs of Na2O·2B2O3 are obtained, as well as two other compounds, Na2O·B2O3 and 2Na2O·5B2O3. The possible structural groups of these phases are deduced, which are found to be similar to that existing in the glass. Current glass models which imply the existence of structural “granularity” satisfactorily describe the crystallization behavior. The phase transformation is observed to depend not only on temperature, but also on the density of the phase.
Combustion Synthesis of MoSi2–SiC Composites
Authors: Subrahmanyam Jandhyala , Rao Renangi Mohan
Published: July 2005

MoSi2, SiC, and MoSi2–SiC composites were prepared by the thermal explosion mode of self‐propagating, high‐temperature synthesis (SHS), from elemental powders Mo, Si, and carbon. The products were characterized using chemical analysis, X‐ray diffraction, and scanning electron microscopy. The morphology of MoSi2 in the product points out that it is in the molten state at the combustion temperature. SiC in the composite shows a very fine particle morphology. These results are supported by the earlier thermochemical calculation carried out on this system.
Submicrometer Transparent Alumina by Sinter Forging Seeded γ‐Al2O3 Powders
Authors: Kwon Oh‐Hun , Nordahl C. Scott , Messing Gary L.
Published: July 2005

Seeding boehmite with α‐Al2O2, followed by calcination at 600°C, results in an agglomerated alumina powder (<53 μm) that can be sinter forged to full density at 1250°C. Compressive strains as high as ɛx=−0.9, and radial flow (ɛx= 1.0) during sinter forging remove large, interagglomerate pores. The fully dense alumina has a grain size of 0.4 pm and is visually transparent. It is proposed that deformation of dense agglomerates is the primary mecha‐ nism responsible for large pore elimination and compact densification. The sinter forging of sol‐gel‐derived alumina powders offers a new technology to prepare highly transparent, optical ceramics at lower temperatures than conventional routes.
Direct Synthesis and Sintering of Silicon Nitridenitanium Nitride Composite
Authors: Hillinger Gerald , Hlavacek Vladimir
Published: July 2005

Electrical discharge machining is an attractive machining technique which requires electrically conductive ceramic materials. Silicon nitride based composites with 35–40% of TiN possess electrical conductivity high enough to use this machining technique. The composite powder was prepared by combustion synthesis and densified by pressure sintering (sintering aids 6 wt% of Y2O3 and 4 wt% of A1,03). High fracture toughness, Kc 10.5 MPa.m1/2, is typical for the composite. It can be expected that this material can compete with pure silicon nitride in applications requiring complex machining.
Scratching and Grinding of a Machinable Glass‐Ceramic with Weak Interfaces and Rising T‐Curve
Authors: Xu Hockin H. K. , Jahanmir Said
Published: July 2005

The scratching and grinding response of a machinable mica‐containing glass‐ceramic is examined relative to the base glass. The influence of weak interfaces and a rising toughness curve (T‐curve) on machinability and damage modes is investigated. A profound manifestation of these characteristics is a change in the nature of subsurface damage from macroscopic fracture in the base glass to distributed microfracture in the glass‐ceramic. As a result, the mechanism of material removal changes from extension of macrocracks in the base glass to microfracture and dis‐lodgement of the mica grains in the glass‐ceramic. High removal rates in scratching compare with low forces in grinding of the glass‐ceramic. The removal rates and the grinding forces are found to correlate with the short‐crack toughness instead of the conventional long‐crack toughness.

Abrasive machining of ceramics possessing weak grain boundaries and interfaces and associated rising T‐ curve behavior is discussed in light of these results.
Formation and Properties of a Lead‐Barium‐Aluminum Phosphate Glass
Authors: Gongyi Guo , Yuli Chen
Published: July 2005

A lead‐barium‐aluminum phosphate glass has been prepared by a wet chemical process. The phosphate glass exhibits high transmission in the visible region of the spectrum and into the mid‐infrared and can strongly absorb in the ultraviolet at wavelengths of less than 344 nm. In addition, the glass has a relatively high index of refraction and a good chemical durability. Therefore, the phosphate glass can be used for general‐purpose optical applications.
Contact Fatigue of a Silicon Carbide with a Heterogeneous Grain Structure
Authors: Padture Nitin P , Lawn Brian R.
Published: July 2005

A comparative study of cyclic fatigue damage from Hertzian contacts in silicon carbide ceramics with homogeneous microstructure (fine, equiaxed grains, strong grain boundaries) and heterogeneous microstructure (coarse, contiguous elongate grains, weak interphase boundaries) is presented. Observations of the surface and subsurface damage patterns using optical microscopy reveal fundamentally different cyclic fatigue mechanisns: in the homogeneous material, by slow growth of a well‐developed cone crack outside the contact area; in the heterogeneous material, by progressive mechanical degradation within a distributed damage zone below the contact area. Scanning electron micrographs of the latter material show copious fine debris in the damage zone, consistent with a degradation mechanism by frictional attrition by forward‐reverse sliding at the weak interphase boundaries. Acoustic emission is recorded during both load and unload half‐cycles, confirming hysteresis in the sliding process. Flexure tests indicate initially slight strength losses from the cyclic contact damage in both microstructures, followed by accelerated losses at higher numbers of cycles. The underlying basis for establishing an analytical model of damage accumulation in the heterogeneous microstructure in terms of shear‐fault sliding, and for designing micro‐structures for optimal properties in fatigue and wear applications, is foreshadowed.
Low‐Temperature Oxidation, Hydrothermal Corrosion, and Their Effects on Properties of SiC (Tyranno) Fibers
Authors: Gogotsi Yury G. , Yoshimura Masahiro
Published: July 2005

The effects of oxidation in air and corrosion in high‐temperature, high‐pressure water on the mechanical properties of three commercially available amorphous Si‐Ti‐C‐O (Tyranno) fibers with different oxygen contents (12%–18%) and diameters (8–11 μm) were investigated. The fibers were exposed to isothermal treatments at elevated temperatures and subsequently tested at room temperature. Structural changes in the fibers after oxidation and corrosion were also studied in order to understand better the degradation mechanisms of the fibers. Oxidation resulted in the formation of vitreous silica films and decreases of strength and Young's modulus of the fibers. Hydrothermal corrosion under 100 MPa water pressure started above 300°C and resulted in the formation of a carbon layer on the surface of the fibers. Dissolution of silica in water during the treatment was observed. Corrosion at temperatures above 400°C led to the formation of relatively thick carbon films which delaminated easily. It caused a decrease of strength and Young's modulus of the fibers. The hydrothermal method can be used for producing carbon coatings with thickness up to 2 μm on the surface of silicon carbide fibers. The degrading of the mechanical properties after oxidation and corrosion was controlled by the thickness of the oxide or carbon layer. Based on this fact, it is possible to predict changes in the mechanical properties from the oxidation data.
Electric and Kinetic Properties of Screen‐Printed Strontium Titanate Films at High Temperatures
Authors: Gerblinger Josef , Hausner Martin , Meixner Hans
Published: July 2005

By eliminating the admixture of glass components to the original powder when making the paste, screen‐printed structures can be produced from strontium titanate with reproducible properties. The ways that the film structure and the temperature profile affect the dependence of the electric conductivity on the oxygen partial pressure during the burn‐in process are examined. The consequences of the surface/volume ratio of the grain structures on the response behavior of SrTiO3 layers during rapid changes in oxygen partial pressure are investigated. Possible influences of the composition of grain boundaries on conductivity characteristics and the time‐governing steps of the interaction between oxygen vacancies in the lattice and the oxygen molecules of the gaseous environment are also discussed.
Synthesis and Properties of Dielectric Ceramics from Metallo‐Organic Precursors through the Solid‐State Reaction of Mixed Gels
Authors: Nakamura Yoshikazu , Aoi Tsuneharu , Aruga Atsushi , Watanabe Yoshihisa , Hirayama Shigekazu , Fujimoto Shirou
Published: July 2005

A perovskite structure of 0.4Pb(Mg1/3Nb2/3)O3·0.3Pb(Mg1/2W1/2)O3·0.3PbTiO3 was prepared from metallo‐organic precursors through the solid‐state reaction of the mixed gels. Three types of mixed gels were crystallized to obtain PbTiO3, MgNb2O6, and MgWO4 powders. These powders were calcined at 900°C after mixing with a stoichiometric amount of Pb(CH3COO)2·3H2O. The dielectric constant of the ceramic fired at 900°C was improved by adding an excess of 10 mol% Mg(OC2H5)2, and the ceramic achieved X7T specification of the Electric Industries Association standard. The dielectric loss was reduced by adding an excess of 5 mol% Pb(CH3COO)2·3H2O.
Crystallization of a PbO‐BaO‐TiO2‐Al2O3‐SiO2 Glass
Authors: Shyu Jiin‐Jyh , Yang Yeong‐Song
Published: July 2005

The phase development, microstructure development, and Curie temperature of PbO‐BaO‐TiO2‐Al2O3‐SiO2 glass‐ ceramics were investigated by X‐ray diffractometry, electron microscopy, and dilatometry. The primary crystalline phase was perovskite titanate, which exhibited bulk nucle‐ ation. When the samples were heated at low temperatures, the Curie inversion was hindered and cubic perovskite was obtained. Increased heating temperature/time resulted in tetragonal perovskite. Growth of a surface layer due to the crystallization of the secondary crystalline phase PbO‐ Al2O3‐2SiO2 resulted in the coexistence of coarse and fine perovskite particles. The increase in the relative number of coarse‐to‐fine perovskite particles with the heating temperature/time was the major factor responsible for increased spontaneous deformation and higher Curie temperature of the perovskite phase. Substitution of barium for lead in the lead titanate phase probably occurred and is explained by the use of SiO2 instead of B2O3 as the glass former in the present study.
Effect of Neutron Irradiation on Knoop Microhardness Anisotropy in MgO·3Al2O3 Single Crystals
Authors: Yano Toyohiko , Ikari Masanori , Iseki Takayoshi , Farnum Eugene H. , Clinard Frank W. , Mitchell T. E.
Published: July 2005

MgO·3Al2O3 single crystals were irradiated with neutron fluences of 8.3 × 1022 n/m2 at 100°C and 2.4 × 1024 n/m2 at 470°C (E > 1.0 MeV) in the Japan Materials Testing Reactor. The Knoop microhardness of several orientations on the (100) plane of both the irradiated and unirradiated crystals were measured with different indentation loads. The change in hardness profile of the crystals was almost the same after the two irradiation conditions. The hardness increased by 4–15% because of the irradiations depending on the crystallographic orientation, the larger change being observed at orientations between the (001) and (011) directions. While both the {111} and {110} slip systems are simultaneously active in the unirradiated MgO·3Al2O3, the {111} system may be the dominant slip system in the neutron‐irradiated crystals. It is concluded that the restriction of the {110} slip system is caused by irradiation‐induced interstitial ions.
Fracture Criteria for Piezoelectric Ceramics
Authors: Park Seungbae , Sun Chin‐Teh
Published: July 2005

Fracture criteria for piezoelectric materials were investigated. Mode I and mixed mode fracture tests were performed on PZT‐4 piezoelectric ceramics to verify the validity of the mechanical strain energy release rate as a fracture criterion. Experimental results indicated that crack extension could be aided or impeded by an electric field, depending on the field direction. Further, the direction of crack extension was studied. A crack closure method, together with finite element analysis, was introduced to calculate the mechanical strain energy release rate. The maximum mechanical strain energy release rate was used to predict fracture loads under combined mechanical and electrical loads. It was found that the mechanical strain energy release rate criterion is superior to other fracture criteria and predicts fracture loads fairly accurately.
Transformation Toughening and Fracture Behavior of Molybdenum Disilicide Composites Reinforced with Partially Stabilized Zirconia
Authors: Soboyejo Wole , Brooks Douglas , Chen Long‐Ching , Lederich Richard
Published: July 2005

The effects of yttria stabilization (0–6 mol%) on the fracture toughness of molybdenum disilicide composites reinforced with 20‐vol%‐yttria‐stabilized zirconia particles are elucidated. Fracture toughness tests were conducted under three‐point bend loading using single‐edge‐notched specimens. The stress‐assisted martensitic phase transformation of zirconia associated with the fracture process was then studied using optical interference microscopy and laser Raman spectroscopy. Stress‐induced martensitic transformation from tetragonal to monoclinic phase was observed only in the plastic wake of the material stabilized with 2 mol % yttria. The degree of toughening in this composite was also predicted using micromechanical models that assess the combined effects of transformation toughening and crack deflection. However, the fracture toughness of the 2‐mol%‐yttria‐stabilized composite was in the same range as those of the other composites that did not exhibit evidence of transformation toughening. The toughening in the other composites is explained by considering the effects of crack deflection, residual stresses, and microcracking induced by residual stresses that occur as a result of the thermal expansion mismatch between the matrix and the reinforcements.
Development of Nano‐Composite Microstructures in ZrO2‐Al2O3 via the Solution Precursor Method
Authors: Balmer Mari Lou , Lange Fred F. , Jayaram Vikram , Levi Carlos G.
Published: July 2005

Aqueous mixtures of zirconium acetate and aluminum nitrate were pyrolyzed and crystallized to form a metastable solid solution, Zr1‐xAlxO2−x/2 (x < 0.57). The initial, metastable phase partitions at higher temperatures to form two metastable phases, viz., t−ZrO2+γ‐Al2O3 with a nano‐scale microstructure. The microstructural observations associated with the γ‐ →α‐Al2O3 phase transformation in the t‐ZrO2 matrix are reported for compositions containing 10, 20, and 40 mol% A12O3. During this phase transformation, the α‐Al2O3 grains take the form of a colony of irregular, platelike grains, all with a common crystallographic orientation. The plates contain ZrO2 inclusions and are separated by ZrO2 grains. The volume fraction of A12O3 and the heat treatment conditions influence the final microstructure. At lower volume fractions of A12O3, the colonies coarsen to single, irregular plates, surrounded by polycrystalline ZrO2. Interpenetrating microstructures produced at high volume fractions of A12O3 exhibit very little grain growth for periods up to 24 h at 1400°C.
Effect of Ammonium Citrate on the Rheology and Particle Packing of Alumina Slurries
Authors: Luther Erik E , Yanez Joesph A. , Franks George V , Lange Fred F. , Pearson Dale S.
Published: July 2005

The valence of the citrate ion changes from ‐1 to ‐3 as the ammonium counterions of tribasic ammonium citrate dissociate with increasing pH. The citrate anion is potential determining because it strongly adsorbs to the surface of oxide particles in aqueous suspensions, changing the surface charge. At pH 8, close to the “natural” isoelectric point of Al2O3, the fully dissociated, trivalent citrate ions produce a negative surface charge. At pH 3, where the natural surface charge of Al2O3 is positive, citrate ions can neutralize the surface. The greatest number density of citrate ions is adsorbed at pH 3 because they are only partially dissociated and the natural surface of Al2O3 contains a high density of positive sites. Attractive particle networks are produced at both the isoelectric point and when excess salt is added to a dispersed suspension. Rheological measurements are used to compare the strength of attractive, Al2O3 particulate networks produced with and without added citrate ions. The attractive networks produced at pH 3, the isoelectric point in the presence of citrate ions, are slightly weaker relative to those produced at pH 9, the natural isoelectric point of Al2O3. Much weaker networks are produced by dispersing slurries at pH 8 with small amounts of tribasic ammonium citrate and then adding excess salt (either tribasic ammonium citrate or NH4Cl). The strength of the weakly attractive networks produced at pH 8 appears identical to those produced when an indifferent electrolyte is added to Al2O3slurries dispersed at pH 4, reported previously. These observations suggest that although the citrate ion is known to adsorb to the surface of Al2O3, as evidenced by its potential‐determining effect, it does not produce a short‐range repulsive potential of any significance.
Effect of Nb Doping on the Microstructure of Sol—Gel‐Derived PZT Thin Films
Authors: Klissurska Radosveta D. , Brooks Keith G. , Reaney Ian M. , Pawlaczyk Czezlaw , Kosec Marija , Setter Nava
Published: July 2005

Aliovalent Nb doping (<10 at.%) of sol—gel‐derived lead zirconate titanate (PZT) thin films was investigated with the intention of improving the ferroelectric properties. Nb addition was found to significantly alter the perovskite crystallization by stabilizing the transient pyrochlore phase, resulting in the retention of pyrochlore second phases and an increase in the perovskite lateral grain size and columnarity. The occurrence and composition of Zr‐rich (surface) pyrochlore phases were found to depend on Nb concentration, annealing temperature, and Pb content. The observed changes in ferroelectric and dielectric properties as a function of Nb dopant addition were found to be strongly influenced by microstructural effects and the occurrence of pyrochlore, and hence the intrinsic effects of Nb incorporation in the perovskite lattice could not be directly ascertained.
Short‐Circuit Diffusion of Ba, Sr, and O during ATiO3 (A = Ba, Sr) Thin‐Film Growth by the Hydrothermal–Electrochemical Method
Authors: Kajiyoshi Koji , Tomono Kunisaburo , Hamaji Yukio , Kasanami Toru , Yoshimura Masahiro
Published: July 2005

The mass transport mechanism during the hydrothermal‐electrochemical growth of polycrystalline thin films of ATiO3 (A = Ba, Sr) perovskite onto Ti electrodes in A(OH)2solutions has been studied by a tracer technique, using Ba and 18O atoms as respective tracers for A‐site and oxygen atoms in SrTiO3 host films. It was found that the ATiO3film grows at the film/electrode interface by transport of both A‐site and oxygen atoms from the solution to the interface. A‐site and oxygen atoms are considered to diffuse as constituents of solution species, such as A2+, OH‐, and H2O, through open short‐circuit paths existing at grain boundaries.
Microstructural Shape Factors: Relation of Random Planar Sections to Three‐Dimensional Microstructures
Authors: Laraia Vincent J. , Rus Ioan L. , Heuer Arthur H.
Published: July 2005

The relation of random planar sections of anisometric microstructures to the actual three‐dimensional anisometry is considered for oblate and prolate spheroids and cylinders. Simple formulae are obtained relating the two‐dimensional (2‐D) to three‐dimensional (3‐D) aspect ratios; for oblate systems with large aspect ratios, the area‐weighted average 2‐D aspect ratio varies linearly with the actual aspect ratio, whereas for prolate systems of large aspect ratio, the 2‐D aspect ratio varies logarithmically with actual aspect ratio. Extension to polydisperse systems yields a shape factor, R, which gives greatest weight to grains (or particles) having the highest volume fraction. This not only preserves the linear and logarithmic functionalities found in monodisperse systems, but favorably affects the sensitivity of R to visually apparent differences among microstructures.
Stress‐Induced Phase Transformation in Single Crystal Titanium Carbide
Authors: Chien Fen‐Ren , Clifton Rodney J. , Nutt Steven R.
Published: July 2005

Single‐crystal ingots of TiC were cut into wafer substrates for heteroepitaxial growth of SiC films. During wafer sectioning by abrasive machining, a subsurface damage zone was created. TEM observations of the damage zone revealed a stress‐induced martensitic transformation from cubic TiC to rhombohedral and hexagonal structures, both involving an 8.0% contraction normal to the {111} habit plane and a shear along . It is proposed that the transformed structure is governed by the Ti sublattice, and that carbon atoms remain in octahedral coordination. The transformed phase coarsened to form a lamellar structure consisting of periodic quadlayer twins. The twins imparted hexagonal symmetry, and the lamellar phase was designated 8H. A strain gradient tensor was employed to describe the mapping of atom coordinates between the two phases. The present account constitutes the first report of this transformation in TiC.
Fabrication and Properties of Ceramic Composites with a Boron Nitride Matrix
Authors: Kim Dong‐Pyo , Gofer Cameron G. , Economy James
Published: July 2005

Boron nitride (BN) matrix composites reinforced by a number of different ceramic fibers have been prepared using a low‐viscosity, borazine oligomer which converts in very high yield to a stable BN matrix when heated to 1200°C. Fibers including Nicalon (SiC), FP (A12O3), Sumica and Nextel 440 (Al2O3‐SiO2) were evaluated. The Nicalon/BN and Sumica/BN composites displayed good flexural strengths of 380 and 420 MPa, respectively, and modulus values in both cases of 80 GPa. On the other hand, FP/BN and Nextel/BN composites exhibited very brittle behavior. Nicalon fiber with a carbon coating as a buffer barrier improved the strength by 30%, with a large amount of fiber pullout from the BN matrix. In all cases except for Nicalon, the composites showed low dielectric constant and loss.
Continuous Fabrication of Silicon Carbide Fiber Tows by Chemical Vapor Deposition
Authors: Lackey W. Jack , Hanigofsky John A. , Freeman Garth B. , Hardin Regina D. , Prasad Ajit
Published: July 2005

The feasibility of preparing small‐diameter, high‐strength, thermally stable silicon carbide fiber tows by the continuous chemical vapor deposition (CVD) of SiC onto carbon fiber tows was experimentally evaluated. Calculations of bending stresses and stresses caused by thermal expansion mismatch between the substrate and coating were used to evaluate the influence of coating thickness and substrate fiber diameter and type. Statistically designed and analyzed processing studies quantitatively showed the influence of key CVD process variables (temperature, pressure, and flow rates of CH3SiCl3 and H2) on fiber attributes such as coating thickness and uniformity, surface roughness, percent agglomeration, and strength. Emphasis was given to conceiving and evaluating various fiber spreading devices in order to enhance coating uniformity and to minimize filament agglomeration within a tow. Uniform coatings and fiber tensile strengths as high as 4 GPa were achieved.
Microstructure Comparison of Transparent and Opaque CVD SiC
Authors: Kim Yongil , Zangvil Avigdor , Goela Jitendra S. , Taylor Raymond L.
Published: July 2005

Transparent, translucent, and opaque regions of high‐ purity bulk SiC, produced by CVD, have been characterized for physical properties as well as microstructure and chemical purity to correlate degree of transparency with other material characteristics. A good correlation was obtained between SiC vis‐a‐vis IR transmission and its microstructure. The transparent material is highly oriented toward the (111) direction and is characterized by pure, essentially defect‐free, cubic β‐SiC columnar grains of size 5–10 μm. The translucent material of various colors is mostly cubic in structure but contains large amounts of twins, usually as complex mixtures of several twinning variants and secondary twinning within a single “grain.” Opaque CVD SiC is randomly oriented, does not exhibit columnar grains, and contains one directional disorder with hexagonal (α‐SiC) symmetry in a majority of grains and high density of dislocations elsewhere.
Strength of Tape Cast and Laminated Ceramics
Authors: Ham‐Su Rosaura , Wilkinson David S.
Published: July 2005

Monolithic A12O3 ceramic laminates were fabricated via a tape casting process. The strength of single tapes was compared with that of laminates, using biaxial flexure tests. The fracture stress was similar. However, the laminates presented a lower Weibull modulus. The feasibility of eliminating or diminishing void‐type flaws present in the green tapes was also assessed. To this end, tapes were first punctured, then laminated and sintered, and the effect of these known flaws in the final ceramic was assessed in four‐point flexure tests. The thermocompression of green tapes during laminate fabrication was found to modify the flaws to a more forgiving morphology.
Electron Cyclotron Resonance Plasma‐Enhanced Metalorganic Chemical Vapor Deposition of Tantalum Oxide Thin Films on Silicon near Room Temperature
Authors: Nagahori Atsushi , Raj Rishi
Published: July 2005

Films of tantalum oxide, ranging in thickness from 25 to 2000 nm, were successfully deposited on silicon by the electron cyclotron resonance plasma‐enhanced metalorganic chemical vapor deposition (ECR‐MOCVD) process at deposition rates of up to 33 nm/min. The films were produced by decomposition of tantalum pentaethoxide into tantalum oxide in an oxygen ECR microwave plasma. The films were amorphous (but fully dense) since the temperature of the substrate, which was unheated, was estimated to be 76–120°C. The growth of the tantalum oxide films was accompanied by the formation of an interlayer of silicon dioxide at the silicon interface. The tantalum oxide films were oxygen rich (relative to Ta2O5). The effective relative dielectric constant of these dual‐layer dielectrics ranged from ɛef= 8.5 to 24; this range was explained in terms of a bilayer consisting of a 13‐nm‐thick silicon dioxide layer of ɛsx= 4, and a tantalum oxide overgrowth of varying thickness having ɛtx∼ 25. A maximum breakdown strength of the films was 6.9 MV/cm and the leakage current at a field of 1 MV/cm was 3.0 × 10‐8. The breakdown properties were related to the presence of stray particles on the substrate; these properties are expected to improve considerably if the processing is carried out in a clean‐room environment. The silicon dioxide layer is believed to have grown by the diffusion of oxygen through the tantalum oxide overlayer despite the low substrate temperature.
Pressureless Sintering of a Ceramic Matrix with Multiple Rigid Inclusions: Finite Element Model
Authors: Hong Wei , Dharani L. R.
Published: July 2005

Interaction among the dispersed second phase of rigid inclusions in a ceramic matrix undergoing densification is investigated by a viscoelastic finite element method with a two‐dimensional multiple‐inclusion model. The interaction is negligible when the volume fraction of inclusions is low (<20%) and/or when the inclusions are uniformly distributed. The interaction becomes significant when the volume fraction of inclusions is high or the inclusions are agglomerated. The densification rates in the region between the inclusions are enhanced to some degree when the inclusions are closely packed. This enhancement will, however, cease when the inclusions get much closer, eventually coming in contact with each other. In general, the agglomeration or close packing of inclusions results in retardation in the overall densification of the sintering matrix.
Nonlinear Optical Properties of B2O3‐Based Glasses: M2O‐B2O3(M = Li, Na, K, Rb, Cs, and Ag) Binary Borate Glasses
Authors: Terashima Kentaro , Kim Sae‐Hoon , Yoko Toshinobu
Published: July 2005

The third‐order nonlinear optical susceptibilities χ(3) of M2O‐B2O3 (M = Li, Na, K, Rb, Cs, and Ag) binary borate glasses have been measured by the third harmonic generation (THG) method. It is found that the enhancement of χ(3)by the structural change of BO3 units to BO4 units is small, while the enhancement of χ(3) due to the formation of non‐ bridging oxygen is rather significant. The effects of alkali cations on the χ(3) of alkali borate glasses are discussed in terms of the M‐O bond character, focusing on the covalency of Li2O‐B2O3 glasses. Comparison of the χ(3) values for Cs2O‐B2O3 and Ag2O‐B2O3 glasses which contain cations of comparable polarizability reveals that the χ(3) value is much greater for Ag2O‐B2O3 glasses than for Cs2O‐B2O3glasses, which is possibly due to the great contribution of Ag(4dz2+ 5s+ 5pz) hybrid orbitals to the nonlinear optical response.
Effect of SiC on Interfacial Reaction and Sintering Mechanism of TiB2
Authors: Torizuka Shiro , Sato Kaoru , Nishio Hiroaki , Kishi Teruo
Published: July 2005

Compacts of TiB2 with densities approaching 100% are difficult to obtain using pressureless sintering. The addition of SiC was very effective in improving the sinterability of TiB2. The oxygen content of the raw TiB2 powder used in this research was 1.5 wt%. X‐ray photoelectron spectroscopy showed that the powder surface consisted mainly of TiO2 and B2O3. Using vacuum sintering at 1700°C under 13–0.013 Pa, TiB2 samples containing 2.5 wt% SiC achieved 96% of their theoretical density, and a density of 99% was achieved by HIPing. TEM observations revealed that SiC reacts to form an amorphous phase. TEM‐EELS analysis indicated that the amorphous phase includes Si, O, and Ti, and X‐ray diffraction showed the reaction to be TiO2+ SiC → SiO2+ TiC. Therefore, the improved sinterability of TiB2 resulted from the SiO2 liquid phase that was formed during sintering when the raw TiB2 powder had 1.5 wt% oxygen.
Lime Refractories with Limestone and Synthetic Calcium Hydroxide Additions
Authors: Wong Lana L. , Bradt Richard C.
Published: July 2005

The effects of fine, natural limestone and synthetic calcium hydroxide additions on the sintering and microstructure of two natural, dead‐burned limes were investigated. Density increases were achieved and microstructural changes were observed. The high reactivities of the decomposition products of the carbonates and hydroxides enhanced the sintering processes to yield refractory bodies with densities in excess of 90% of theoretical. Microstructurally, the additions promoted grain growth and simultaneously modified the porosity of these high‐density refractory bodies.