bluuebunny/crossref_metadata_2025 huggingface dataset:
Zirconium Oxide Coating on Anodized Aluminum by the Sol‐Gel Process Combined with Ultraviolet Irradiation at Ambient Temperature
Authors: Hirai Shinji , Shimakage Kazuyoshi , Sekiguchi Motoi , Wada Kenji , Nukui Akihiko
Published: July 2010

Instead of the conventional sol‐gel process in which heat treatment is used to form a dense composite film after dipping/withdrawing a substrate into/from the sol, a new sol‐gel process incorporating ultraviolet (UV) irradiation at ambient temperature has been attempted to improve the alkaline corrosion resistance of aluminum. A sol‐gel film prepared by UV irradiation at ambient temperature indicated that alkaline corrosion resistance is superior in the range of 2.4 times to 2.6 times that of the conventional composite film heated at 573 K. The densification of the coating layer due to the elimination of residual organics induced by photoexcitation was responsible for the improvement in alkaline corrosion resistance. The high alkaline corrosion resistance of the irradiated film may be attributed to the formation of a dense coating layer and the self‐repairing action caused by repetition for preparation and dissolution of the reactant in the penetrating path of NaOH solution into the composite film.
Preparation of Portland Cement Components by Poly(vinyl alcohol) Solution Polymerization
Authors: Lee Sang‐Jin , Benson Elizabeth A. , Kriven Waltraud M.
Published: July 2010

The four components portland cement‐dicalcium silicate, C2S (Ca2SiO4); tricalcium silicate, C3S (Ca3SiO5); tricalcium aluminate, C3A (Ca3Al2O6); and tetracalcium aluminate iron oxide, C4AF (Ca4Al2Fe3O10)‐were formed using a solution‐polymerization route based on poly(vinyl alcohol) (PVA) as the polymer carrier. The powders were characterized using X‐ray diffraction techniques, BET specific surface area measurements, and scanning electron microscopy. This method produced relatively pure, synthetic cement components of submicrometer or nanometer crystallite dimensions, high specific surface areas, as well as extremely high reactivity at relatively low calcining temperatures. The PVA content and its degree of polymerization had a significant influence on the homogeneity of the final powders. Two types of degree of polymerization (DP) PVA were used. Lower crystallization temperatures and smaller particle size powders were obtained from the low‐DP‐type PVA at optimum content.
Varistor Behavior at Twin Boundaries in ZnO
Authors: Haskell Benjamin A. , Souri Shukri J. , Helfand Martin A.
Published: July 2010

The electrical behavior of commercial ZnO varistor devices has been examined with voltage contrast microscopy and point contact dc electrical measurements. Nonlinear voltage‐dependent behavior has been observed across both of the major crystalline boundary types present in the system: Bi2O3 layer containing ZnO grain boundaries (or grain boundaries) and antimony spinel layer internal ZnO inversion twin boundaries (or twin boundaries). Twin boundaries, which bisect practically every grain in a typical commercial device, possess potential barriers with higher average breakdown voltages than do grain boundaries. Certain zinc antimonate spinel (Zn7Sb2O12) grains are electrically isolated from the matrix, whereas others are conductive within the matrix.
Extrapolation of Fracture Mirror and Crack‐Branch Sizes to Large Dimensions in Biaxial Strength Tests of Glass
Authors: Quinn Janet B.
Published: July 2010

Annealed glass disks were fractured in biaxial tension under a variety of conditions; crack lengths prior to branching and the sizes of the mirror and mist regions were measured. The experimental results indicated the expected correlation between fracture stress and these fracture features, except for an extrapolation to a consistent, clearly defined y‐intercept. Neither environment, surface preparation, nor the rate of applied stress was a factor in the fracture stress versus branch, mirror, or mist size relationships.
Subsolidus Phase Diagram of the Bi2O3‐Gd2O3 System and the Morphology of Phase Separation
Authors: Su Pomin , Virkar Anil V.
Published: July 2010

The low‐temperature, subsolidus phase diagram for the binary system Bi2O3‐Gd2O3 was determined using X‐ray diffraction (XRD), electron microprobe analysis (EPMA), and conductivity measurements. The high‐temperature phase in Bi2O3‐rich compositions is cubic (CaF2 structure). A simple eutectoid reaction from cubic → monoclinic + rhombohedral phases was observed, leading to the formation of a cellular structure. The eutectoid temperature and composition are ∼683°C and 8.8% Gd2O3, respectively. The cubic phase containing ∼27.5% Gd2O3 congruently transforms into the rhombohedral phase at ∼810°C. The solubility of Gd2O3 in the monoclinic phase was determined to be negligible. The concentration of Gd2O3 in the rhombohedral phase in equilibrium with the monoclinic phase is ∼18% Gd2O3. Samples with Gd2O3 concentration greaterthan equal to10% undergo a composition‐invariant, thermally activated (massive) transformation to a metastable rhombohedral phase below a composition‐dependent critical temperature, Tc(2), followed by its decomposition into a rhombohedral (of equilibrium composition) + monoclinic two‐phase mixture upon further annealing. The resulting monoclinic phase appears to form by nucleation and growth. Samples containing ≤7% Gd2O3 undergo cubic → tetragonal displacive transformation upon cooling below another composition‐dependent critical temperature, Tc(1). A long‐term annealing leads to decomposition of the tetragonal phase into a mixture of rhombohedral and monoclinic phases. The resulting microstructure was extremely fine and typical of a cellular reaction.
Quantitative Evaluation of Glass‐Forming Impurities in Alumina: Equivalent Silica Concentration (ESC)
Authors: Yoo Jung Ho , Nam Jong Chul , Baik Sunggi
Published: July 2010

Equivalent silica concentration (ESC) is proposed as a quantitative measure of the total impurity concentration that contributes to intergranular glass formation in nominally pure alumina. The method involves, in its determination, the processing and sintering of the alumina under contamination‐free conditions, as well as measuremet of the average grain size at the moment that abnormal grains first appear. This latter value can be converted to an ESC value from the inverse relationship between grain size and silica concentration, which can be established via an identical method using ultrapure alumina. The ESC values for four different commercial alumina powders of varying purity have been measured and compared with their chemical‐analysis data.
Oxidation Behavior of NBD 200 Silicon Nitride Ceramics
Authors: Mukundhan Priya , Wu Jianqing , Du Henry H.
Published: July 2010

The oxidation behavior of NBD 200 Si3N4 containing 1 wt% MgO sintering aid was investigated in oxygen at 900°‐1300°C. The oxide growth followed a parabolic rate law with an apparent activation energy of 260 kJ/mol. The oxide layers were enriched with sodium and magnesium because of outward diffusion of intergranular Na+ and Mg2+ cations in the ceramics. The 2‐4 orders of magnitude higher oxidation rate for NBD 200 Si3N4 than for other Si3N4 ceramics with a similar amount of MgO could be attributed to the presence of sodium. The oxidation process was most likely rate limited by grain‐boundary diffusion of Mg2+.
A Ceramic Photonic Crystal Temperature Sensor
Authors: Ballato John , James Andrew
Published: July 2010

Photonic bandgap effects are observed in a disordered array of sol‐gel‐derived SiO2 particles prepared via centrifugation of a colloidal suspension. Tunability in the optical response is achieved by infiltrating an organic liquid (1‐methylnaphthalene) that possesses a moderately high temperature coefficient of refractive index (dn/dT) into the interparticle void space. The varying dn/dT value between SiO2 and 1‐methylnaphthalene is used to tune the contrast ratio, relative to temperature. The resulting optical temperature sensor is the first adaptive ceramic photonic crystal of its type, to our knowledge, and provides a proof‐of‐concept for simple, disposable optical devices that are based on photonic band engineering. Novel applications of optical structures that possess short‐range particulate order are discussed, in consideration of the direct analogy between photonic bandgaps and X‐ray diffraction.
Synthesis and Structural, Ferroelectric, and Piezoelectric Properties of SrBi4Ti4O15 Ceramics
Authors: Gelfuso Maria V. , Thomazini Daniel , Eiras José A.
Published: July 2010

SrBi4Ti4O15 ceramics were prepared via ordinary firing (OF) and hot forging (HF). Characterization using scanning electron microscopy and X‐ray diffractometry shows platelike grains that were highly oriented (F= 0.95) after hot forging. Ferroelectric, dielectric, and piezoelectric characterizations revealed that polarization reorientation was restricted to the ab plane of the orthorhombic structure, parallel to the (Bi2O2)2+ layers. The thickness coupling factor for OF samples was only half that for HF samples oriented parallel to the HF direction (in the ab‐plane), as a consequence of poling restrictions in randomly oriented grains.
Electronic Structure of Pristine and Solute‐Incorporated SrTiO3: I, Perfect‐Crystal Geometry and Acceptor Doping
Authors: Rodrigues Richard P. , Chang Hyunju , Ellis Donald E. , Dravid Vinayak P.
Published: July 2010

In Part I of this three‐part report, various aspects of the electronic structure of pristine and acceptor‐impurity‐incorporated perfect‐crystal SrTiO3 were investigated. An embedded‐cluster approach was used to perform first‐principles, self‐consistent‐field calculations. Within the framework of the local‐density functional approximation, one‐electron equations were evaluated using a discrete variational method to study pristine titanium‐ and strontium‐centered clusters of perfect‐crystal SrTiO3. Clusters with a single acceptor impurity substitution at the central titanium site also were considered. However, the relaxation of the atomic structure resulting from the incorporation of these single impurities were not considered in the calculations. Calculations involved determination of the densities of states and spin densities of states of the cluster atoms near the Fermi energy. Mulliken charge populations of atomic orbitals, valence of individual atoms, and nature of bonds between the atoms also were determined. Spatial distribution of charge density and spin density of valence band orbitals, which were included in the variational space, were determined for pristine and impurity‐centered clusters. The influence of impurity substitution at a titanium site on the local electronic structure was evaluated in terms of the variations in the densities of states and in the spatial distribution of charge densities. The role of local charge transfer and impurity‐induced changes in Mulliken populations also were investigated in connection with the electronic activity of SrTiO3. Possible mechanisms of electrical conductivity were studied. Cluster calculations revealed a mixed ionic‐covalent nature of the Ti‐O bond and purely ionic nature of the Sr‐O bond for the perfect‐crystal geometry of SrTiO3. The optical bandgap and the densities of states calculated using the cluster method were in good agreement with previous theoretical and experimental investigations. The methodology correctly predicted the acceptor nature and the trends therein of the transition‐metal impurities at the titanium sites. For pristine and impurity‐incorporated clusters, the densities of states, the charge‐ and spin‐density distributions, and the Mulliken charge population consistently converged to the same results, providing information regarding the electronic behavior of the SrTiO3.
Fabrication, Sinterability, and Mechanical Properties of Lead Zirconate Titanate/Silver Composites
Authors: Hwang Hae Jin , Yasuoka Masaki , Sando Mutsuo , Toriyama Motohiro , Niihara Koichi
Published: July 2010

High‐toughness and high‐strength lead zirconate titanate (PZT) composites that contain fine silver particles were successfully fabricated at low sintering temperatures. Addition of silver to a PZT matrix did not result in unwanted reaction phases; however, some silver diffused toward the perovskite crystal structure. A small quantity of silver accelerated the sinterability of the PZT composites. The formation of oxygen vacancies due to the partial substitution of silver appeared to enhance the sinterability of the PZT. Fracture toughness depended on the size and degree of sphericity of the silver particles, and SEM observations on crack propagation suggested that the toughening mechanism in the PZT/Ag composites involves crack bridging resulting from the ductile behavior of silver particles. It is proposed that high fracture strength in PZT/1 to 5 vol% Ag composites results from the relaxation of transformation‐induced internal stress by the silver particles.
Microstructure‐Property Relations of Hot‐Pressed Silicon Carbide‐Aluminum Nitride Compositions at Room and Elevated Temperatures
Authors: Lubis Ahmad H. , Hecht Norman L. , Jr. Graves George A. , Ruh Robert
Published: July 2010

A series of SiC‐AlN compositions of 0, 10, 25, 50, 75, 90, and 100 mol% AlN were hot pressed at 2100°C for a 1 h soak at a pressure of 35 MPa under vacuum. 2H‐wurtzite SiC‐AlN solid‐solution structures were formed for compositions with 25‐100 mol% AlN. The associated lattice parameters for these solid solutions followed Vegard's law. The microstructures varied with composition; the number of needlelike grains decreased for compositions up to 25 mol% AlN and the amount of equiaxed grains increased for compositions with 25–100 mol% AlN. Densities for all the specimens were >99% of the theoretical density. Coefficients of thermal expansion varied from 4.80 × 10‐6/°C to 6.25 × 10‐6/°C in the 20°‐1400°C range. Young's moduli varied from 451 GPa to 320 GPa at room temperature (RT) and retained 98%, 96%, and 94% of their RT values at 500°, 1000°, and 1250°C, respectively. These three properties correlated linearly with composition. RT microhardness varied from 21.6 GPa to 11.2 GPa and correlated linearly with composition within the solid‐solution range. Flexural strengths increased from 487 MPa to 604 MPa from 0 mol% AlN to 25 mol% AlN and then decreased to 284 MPa for 100 mol% AlN. At 1250°C, flexural strengths decreased from 90% to 65% of the RT values. Fracture toughness increased from 3.6 MPa·m1/2 to 4.2 MPa·m1/2 from 0 mol% AlN to 10 mol% AlN and then decreased to 2.5 MPa·m1/2 for 100 mol% AlN.
Influence of Microstructure and Temperature on the Interfacial Fracture Energy of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics
Authors: Trice Rodney W. , Halloran John W.
Published: July 2010

The microstructure and interfacial fracture energy of silicon nitride/boron nitride fibrous monoliths, GammaBN, were determined as a function of starting silicon nitride composition and temperature using the method described by Charalambides. The glassy phase created by the sintering aids added to the silicon nitride cells was shown to migrate into the boron nitride cell boundaries during hot‐pressing. The amount of glassy phase in the boron nitride cell boundaries was shown to strongly influence GammaBN at room temperature, increasing the fracture energy with increasing amounts of glass. Similar trends in the interfacial fracture energy as a function of temperature were demonstrated by both compositions of fibrous monoliths, with a large peak in GammaBN observed over a narrow temperature range. For silicon nitride cells densified with 6 wt% yttria and 2 wt% alumina, the room‐temperature interfacial fracture energy was 37 J/m2, remaining constant through 950°C. A sharp increase in GammaBN, to 60 J/m2, was observed between 1000° and 1050°C. This increase was attributed to interactions of the crack tip with the glassy phase in the boron nitride cell boundary. Measurements at 1075°C indicated a marked decrease in GammaBN to 39 J/m2. The interfacial fracture energy decreased with increasing temperature in the 1200° to 1300°C regime, plateauing between 17 to 20 J/m2. A crack propagation model based on linkup of existing microcracks and peeling/cleaving boron nitride has been proposed.
Effect of Alumina on the Strength, Fracture Toughness, and Crystal Structure of Fluorcanasite Glass‐Ceramics
Authors: Zhang Nai‐Zheng , Anusavice Kenneth J.
Published: July 2010

The influence of alumina content (0‐15 wt% Al2O3) on the indentation strength, fracture toughness (KIc), and crystal structure of fluorcanasite (Al2O3‐CaO‐F‐K2O‐Na2O‐SiO2) glass‐ceramics was analyzed. Increasing the Al2O3 content from 0 wt% (CAN0) to 8 wt% (CAN8) caused the mean indentation strength and KIc values to decrease from 213 ± 14 MPa and 2.7 ± 0.1 MPa·m1/2, respectively, for the CAN0 glass‐ceramic to 78 ± 16 MPa and 1.3 ± 0.2 MPa·m1/2, respectively, for the CAN8 glass‐ceramic. Increased Al2O3 concentrations (0‐15 wt%) significantly affected the crystal size, crystal shape, aspect ratio, and crystal aggregation characteristics of the fluorcanasite glass‐ceramics. The addition of greaterthan equal to8 wt% of Al2O3 to fluorcanasite glass caused a transformation from canasite to leucite.
Strong Sintered Miserite Glass‐Ceramics
Authors: Pinckney Linda R. , Beall George H. , Andrus Ronald L.
Published: July 2010

Strong and tough glass‐ceramic materials based on the complex chain silicate miserite (KCa5open square(Si2O7)(Si6O15)F) have been prepared using standard frit sintering and internal nucleation processes. The miserite may be accompanied by crystals of fluorite, cristobalite, xonotlite, a canasite‐like phase, and fluorapatite. The highly crystalline glass‐ceramics have a microstructure composed primarily of interlocked, lath‐ or log‐shaped miserite crystals with pronounced cleavage planes. This microstructure provides abraded flexural strength values as high as 235 MPa (34 000 psi) and fracture toughness values >3.0 MPa·m1/2. These strength and toughness values are quite high for a glass‐derived material. Miserite glass‐ceramics may be useful for many applications in which such strength and toughness are desired.
Phase Equilibria in Subsystems of the Quaternary Reciprocal System Na2O‐SiO2‐Al2O3‐NaF‐SiF4‐AlF3
Authors: Rutlin Jørn , Grande Tor
Published: July 2010

The phase relations in the pseudobinary system sodium fluoride‐mullite have been investigated as a model system for fluoride attack on fire clay refractories in aluminum electrolysis cells. The phase composition below the solidus temperature 857°C changes from sodium fluoride, cryolite, nepheline, and ß‐alumina to cryolite, albite, silica, and α‐alumina with increasing oxide content. Albite was not observed to crystallize and an increasing amount of glass was observed with increasing mullite content. The phase diagram of the ternary system sodium fluoride‐cryolite‐nepheline was also measured in order to describe the composition of the most stable melt along the sodium fluoride‐mullite composition join. The present findings are discussed in relation to the deterioration mechanism of fire clay refractories during fluoride attack. The formation of viscous albite‐based melts is suggested to increase the resistance toward fluoride attack due to the reduced diffusion rate of fluorides.
Deposition of Multifunctional Titania Ceramic Films by Aerosol Routes
Authors: Yang Guixiang , Biswas Pratim , Boolchand Punit , Sabata Ashok
Published: July 2010

Aerosol coating processes were developed to deposit titania ceramic films onto steel and silica substrates. In situ light‐scattering measurements were used to understand the deposition mechanisms in different system configurations. The conditions that were necessary to obtain uniform, nonporous, and well‐adhered titania films on steel substrates were established. The as‐coated films had excellent anticorrosion characteristics at room temperature, as established by the standard salt‐fog test. Film crystallinity and morphology were examined using X‐ray diffractometry and scanning electron microscopy; these analysis methods revealed an oriented, nanocrystalline anatase phase. Film composition was established, as a function of film thickness, using Auger electron spectroscopy and was confirmed to be stoichiometric (Ti:O = 1:2). The optical band gap and optical phonons of the deposited films were probed using spectrophotometry and Raman scattering, respectively; these analysis methods revealed a blue shift of the gap, relative to bulk anatase, and a localization of carriers in the nanometer‐sized crystallites.
Crystallization and Microstructure Development of Sol–Gel‐Derived Titanium Dioxide Thin Films with Single and Multiple Layers
Authors: Ohya Yutaka , Mishina Jyunji , Matsuda Tatsuya , Ban Takayuki , Takahashi Yasutaka
Published: July 2010

Sol–gel TiO2 thin films were fabricated at 650°C using sol that was derived from the Ti(OiPr)4‐diethanolamine‐H2O‐iPrOH system. Repeated deposition and rapid heating resulted in columnar grains, whereas slow heating—at a rate of 10°–20°C/min—caused equiaxed grains to form. Slow heating caused the film to crystallize as anatase. The anatase‐to‐rutile ratio of the rapidly heated film increased with increased deposition thickness and repeated depositions. The tendency of the thicker film to crystallize as rutile during rapid heating was attributed to the rapid grain growth of fine anatase grains.
Effects of Trace O2 Levels on the Nitriding Kinetics of High‐Purity Silicon Powders
Authors: Parikh Raju S. , Lightfoot Annamarie , Haggerty John S. , Sheldon Brian W.
Published: July 2010

The effects of trace O2 levels on the nitridation of compacts made from silane‐derived Si powders were studied in N2 atmospheres, with oxygen levels of either 5 ppm or 10 ppb (approximately). The nitriding kinetics were studied by thermogravimetric analysis as a function of temperature (1100–1200°C) and heating rate (5°C/min and 100°C/min). Reducing the O2 level in the nitriding gas enhanced conversion to Si3N4 at lower temperatures, reduced the composition variations within the samples, and decreased the α/β ratios. The results suggest that nucleation and rapid growth of Si3N4 at relatively low temperatures are possible only when the oxygen partial pressure in the system is below the threshold value for passive oxidation.
Synthesis of Oriented Ba2NaNb5O15 (BNN) Thin Films from an Alkoxy‐derived Precursor
Authors: Yogo Toshinobu , Sakamoto Wataru , Isaji Tadayuki , Ichida Masao , Nakamura Arao , Hirano Shin‐ichi
Published: July 2010

Ba2NaNb5O15 (BNN) thin films with a tungsten bronze structure were fabricated using a precursor solution that was synthesized from barium, sodium, and niobium alkoxides. Highly (002)‐oriented BNN films were prepared successfully on Pt(100)/MgO(100) substrates at 700°C, using a BNN underlayer. X‐ray pole‐figure measurement showed that the BNN films that crystallized on Pt(100)/MgO(100) substrates had two in‐plane orientations. The remanent polarization and coercive field of the BNN film (thickness of 1.0 µm) that was crystallized at 700°C were 12.3 µC/cm2 and 101 kV/cm, respectively, at –150°C (123 K). BNN films on fused silica substrates exhibited second harmonic generation upon irradiation with 1064 nm light.
In‐Plane Mechanical Properties of an All‐Oxide Ceramic Composite
Authors: Heathcote John A. , Gong Xiao‐Yan , Yang James Y. , Ramamurty Upadrasta , Zok Frank W.
Published: July 2010

The present article examines the in‐plane tensile properties of a two‐dimensional (2D) all‐oxide ceramic composite. The distinguishing characteristics of the material include fine‐scale porosity within the matrix and the absence of a fiber coating. The anisotropy in the elastic‐plastic properties has been studied through tension tests in the axial (fiber) direction and at 45° to the fiber axes, both in the presence and the absence of holes or notches. The notch sensitivity in the axial direction is comparable to that of conventional dense‐matrix, weak‐interface composites, demonstrating the effectiveness of the porous matrix in enabling crack deflection and damage tolerance. Furthermore, the notch sensitivity is rationalized using models that account for the effects of inelastic straining on the local stress distributions around notches and holes, coupled with a scale‐dependent failure criterion. In the off‐axis orientation, the tensile strength is dictated by a plastic instability, analogous to necking in metals. Following instability, deformation continues within a diffuse localized band, with a length comparable to the specimen width. Similar deformation and fracture characteristics are obtained both with and without holes. The off‐axis properties are discussed in terms of the comminution and rearrangement of matrix particles during straining.
Effect of Microstructure on the Mechanical Behavior of Continuous‐Fiber‐Reinforced Ceramic‐Matrix Composites
Authors: Jessen Todd L. , Greenhut Victor A. , II Lewis David , Friel John J.
Published: July 2010

The mechanical behavior of a unidirectional continuous‐fiber ceramic‐matrix composite (CFCMC) was correlated with matrix‐rich channels in the microstructure. A large population of CFCMCs was prepared via alkoxide infiltration, which incorporated either uncoated Nicalon fibers (64 samples) or BN‐coated fibers (118 samples). No structure/property correlation was observed for the uncoated composites, because of the uniformity of the microstructure. For the BN‐coated composites, both the flexure strength and work of fracture (WOF) were correlated with oriented matrix‐rich channels. The channels were <90 μm wide and were distributed throughout the cross section. The BN‐coated CFCMCs exhibited laminate‐like behavior: the strength was statistically higher when the channels were aligned parallel with the applied load and the WOF was statistically higher when the channels were perpendicular to the load. Grouping the specimens on the basis of channel orientation, relative to applied load, reduced the variance in strength and WOF. This categorization also resulted in consistent, predictable failure behavior. This observation implies that prior CFCMC data that do not consider microstructure orientation may show wider scatter in mechanical behavior than is warranted.
A New Model for Tensile Creep of Silicon Nitride
Authors: Luecke William E. , Wiederhorn Sheldon M.
Published: July 2010

The tensile creep rate of most commercial grades of Si3N4 increases strongly with stress. Although the usual power‐law functions can represent the creep data, the data often show curvature and systematic variations of slope with temperature and stress. In this article, we present a new approach to understanding the creep of ceramics, such as Si3N4, where a deformable second phase bonds a deformation‐resistant major phase. A review of experimental data suggests that the rate of formation and growth of cavities in the second phase controls creep in these materials. The critical step for deformation is the redistribution of the second phase away from the cavitation site to the surrounding volume. The effective viscosity of the second phase and the density of active cavities determine the creep rate. Assuming that the hydrostatic stresses in pockets of the second phase are normally distributed leads to a model that accurately describes the tensile creep rate of grades of Si3N4. In this model, the creep rate increases exponentially with the applied stress, is independent of Si3N4 grain size, is inversely proportional to the effective viscosity of the deformable phase, and is proportional to the cube of the volume fraction of the deformable phase.
A Hydrogen‐Permselective Silicon Oxycarbide Membrane Derived from Polydimethylsilane
Authors: Lee Lang‐Luen , Tsai Dah‐Shyang
Published: July 2010

A novel microporous membrane has been prepared by modifying an asymmetric SiC support using polydimethylsilane (PMS). One membrane, pyrolyzed at 573 K, possesses H2/N2 selectivity of ∼100 and H2 permeance of 8.9 × 10−8 mol·m−2·s−1·Pa−1 at 473 K. The other membrane, pyrolyzed at 873 K, has a lower H2/N2 selectivity of ∼40 and lower H2 permeance of 4.9 × 10−9 mol·m−2·s−1·Pa−1 at 473 K. Permeation in both membranes exhibits the characteristics of activated diffusion at elevated temperatures. A higher pyrolysis temperature results in a less‐permeable membrane. The support preparation and the thermal events of PMS are analyzed. The effects of pyrolysis temperature on membrane property are discussed.
Effect of Thermal Exposures on the Strengths of Nextel™ 550 and 720 Filaments
Authors: Petry M. Dennis , Mah Tai‐Il
Published: July 2010

The effects of thermal exposure on the strengths of Nextel™ 550 and 720 tows, bare and coated with carbon, were determined by room‐temperature tensile testing of single filaments extracted from tows that had been exposed to different thermal environments (i.e., air or vacuum) at temperatures from 550° to 1400°C. The results help define the allowable composite processing conditions when using these tows. A 28% drop in the strength of Nextel 550 filaments occurred after a thermal exposure at 1100°C for 2 h in air. After an exposure of 1300°C/2 h/air, a strength degradation of ∼47% resulted. Filaments exposed above 1100°C under vacuum showed more severe strength degradation than filaments exposed in air. The observed strength degradation may stem from a combination of phase transformations of the alumina, the onset of mullite crystallization, and/or exaggerated mullite grain growth. Strength after heat treatment under vacuum at 1050° and 1150°C did not deteriorate as rapidly as after heat treatment under vacuum between 950° and 1050°C or between 1150° and 1250°C. This may be a result of the competition between healing of flaws by the amorphous silica and its evaporation (leading to an increase in its viscosity or loss) and/or densification of the filaments. Nextel 720 filaments exhibited about 9% strength loss after an exposure at 1100°C/2 h/air. The filaments maintained 75% of their strength after a 1300°C/2 h/air heat treatment. The observed strength degradation may stem from thermal grooving, grain growth, and/or annealing of the mullite subgrain boundaries. Thermal exposure of >10 h at 1300°C was required to produce measurable grain growth. Strength loss between 1200° and 1300°C (air heat treatment) was not as great as between 1100° and 1200°C or 1300° and 1400°C.
Modeling of the Hydrolysis of α‐Tricalcium Phosphate
Authors: Ginebra Maria‐Pau , Fernández Enrique , Driessens Ferdinand C. M. , Planell Josep A.
Published: July 2010

Some of the formulations of apatitic calcium phosphate bone cements are based on the hydrolysis of α‐tricalcium phosphate (α‐Ca3(PO4)2, α‐TCP). In this work the hydrolysis kinetics of α‐TCP are studied, taking into account the particle‐size distribution of the initial powder, to identify the mechanisms that control the reaction in its successive stages. The temporal evolution of the depth of reaction is calculated from the degree of reaction data, measured by X‐ray diffractometry. A kinetic model is proposed, which suggests the existence of two rate‐limiting mechanisms: initially, the surface area of the reactants and, subsequently, the diffusion through the hydrated layer formed around the reactants. For the specific particle size and preparation used, the controlling mechanism changeover takes place after 16 h of reaction.
Phase Evolution during the Formation of α‐Tricalcium Phosphate
Authors: TenHuisen Kevor S. , Brown Paul W.
Published: July 2010

The formation of α‐tricalcium phosphate from monobasic ammonium phosphate and calcium carbonate was investigated using a number of complementary techniques. Differential thermal analysis showed five distinct thermal events attributed to melting of the ammonium phosphate, decomposition of acidic calcium orthophosphate into an amorphous calcium metaphosphate, crystallization of β‐calcium metaphosphate, crystallization of β‐calcium pyrophosphate, and calcination of calcium carbonate. X‐ray diffraction analyses as a function of temperature supported the evolution of these events and phases and also showed the formation of other intermediates, including an amorphous phase, apatite, lime, and β‐tricalcium phosphate. Thermal gravimetric analysis (TGA) showed a large weight loss occurring between 150° and 250°C due to reaction between the acidic phosphate liquid and the calcium carbonate. This resulted in an amorphous intermediate with a Ca/P ratio between 0.5 and 1.0 from which both β‐calcium metaphosphate and β‐calcium pyrophosphate crystallized. Mass spectroscopy indicated that the ammonia and carbon dioxide were evolved in four different steps, while water was evolved in at least five steps. These decomposition steps correlated with those observed by TGA. Scanning electron microscopy indicated the formation of an intermediate phase that coated the calcium carbonate by 250°C. The proposed mechanistic reaction path to alpha‐tricalcium phosphate involves the formation and consumption of the following sequence of intermediates: an acidic amorphous condensed phosphate; β‐calcium metaphosphate and β‐calcium pyrophosphate; lime, apatite, and β‐tricalcium phosphate.
Interdiffusion and Kirkendall Effect in Doped BaTiO3–BaZrO3 Perovskites: Effect of Vacancy Supersaturation
Authors: Gopalan Srikanth , Virkar Anil V.
Published: July 2010

Interdiffusion in Sc‐doped and Ta‐doped BaTiO3‐BaZrO3 perovskites was examined, both theoretically and experimentally. Analytical expressions for the lattice velocity, v, and the interdiffusion coefficient,D̃, were obtained. The v and the D̃ were numerically evaluated as a function of dopant type (trivalent‐acceptor or pentavalent‐donor) and concentration using literature values of oxygen vacancy diffusivity, DVO, the A‐site vacancy diffusivity, DVA, and assumed values for the B‐site vacancy diffusivities, DVB and DVB′. Regardless of the chosen values of DVB and DVB′, the calculated D̃ increased with the Ta concentration and decreased with the Sc concentration. The dependence (shape) of the calculated v vs dopant type and concentration, however, was a function of the magnitudes of DVA and DVB. For DVA < (DVB, DVB′) < DVO, the calculated v exhibited a maximum at ∼0.8% acceptor dopant. Diffusion couples in the BaTiO3‐BaZrO3 system doped with either Sc (acceptor) or Ta (donor) were annealed in air over a temperature range between 1300° and 1500°C. Extensive Kirkendall porosity was observed in several samples. The measured porosity vs dopant concentration (acceptor/donor) trace was similar to that of calculated v vs dopant concentration (acceptor/donor). However, the measured D̃ vs dopant concentration (acceptor/donor) was not in accord with the calculations. This difference is attributed to a nonequilibrium vacancy supersaturation and the accompanying porosity formation. To obtain further evidence of the deviation from equilibrium, experiments were also conducted on both coarse‐grained and fine‐grained samples. The coarse‐grained samples, which are expected to have fewer vacancy sinks and sources, exhibited greater Kirkendall porosity and greater D̃ values compared to the fine‐grained samples, consistent with a greater deviation from equilibrium.
Role of Nitrogen in the Crystallization of Silicon Nitride‐Doped Chalcogenide Glasses
Authors: Chen Guorong , Cheng Jijian
Published: July 2010

Glasses in the Ge‐S, Ge‐As‐Se, and Ge‐As‐Se‐Te systems, doped with Si3N4, were melted after sealing under reduced pressure, and their crystallization behavior was examined using differential thermal analysis/differential scanning calorimetry and X‐ray diffractometry. The effect of Si3N4 doping on the suppression of the crystallization of chalcogenide glasses was confirmed for all three systems and is attributed to the increased crosslinking upon substitution of the chalcogens by nitrogen atoms, presumably forming structural units that are similar to Ge3N4.
Electrolyte Effects on Nonionic Steric Layers: Bis‐hydrophilic PMAA‐PEO Diblock Copolymers Adsorbed on Barium Titanate
Authors: Sindel Jürgen , Bell Nelson S. , Sigmund Wolfgang M.
Published: July 2010

The steric layers of bis‐hydrophilic diblock copolymers formed of poly(methacrylic acid) (PMAA) and poly(ethylene oxide) (PEO) have been investigated through the direct examination of repulsive forces using the atomic force microscopy (AFM) colloidal probe technique. Increasing the KNO3 and Ba(NO3)2 concentrations causes a reduction in the extension of the PEO chains into the solution, but attractive forces are never observed. It is believed that the electrolyte level through the hydration of the polymer affects the conformation of the stabilizing PEO chains. Increasing electrolyte levels disrupt the hydrogen bonding between the polymer and solvent needed to create an extended polymer conformation.
Infiltration‐Growth Processing of NdBa2Cu3O7‐delta Superconductor
Authors: Babu Nadendla Hari , Rajasekharan Thankappan , Menon Latika , Malik S. K.
Published: July 2010

NdBa2Cu3O7‐delta (Nd‐123) superconductor has been processed to obtain a highly textured microstructure with micrometer‐sized spherical Nd4Ba2Cu2O10 (Nd‐422) inclusions through a process that involves the infiltration of liquid phases into a shaped preform followed by its slow cooling through the peritectic formation temperature of Nd‐123. The process was carried out in high‐purity argon (maximum of 4 ppm of oxygen) to reduce the formation of low‐Tc solid solutions. A major advantage of the process is that it minimizes the possibility of Nd‐422 coarsening in the liquid. The result is a very uniform distribution of nearly spherical Nd‐422 inclusions with 80% of the particles having a diameter less than 1 µm, in the highly textured Nd‐123 matrix. Since the present process eliminates shrinkage and large hollows within the interior of the samples, it can enable the fabrication of 3‐dimensional complex‐shaped objects with good dimensional tolerance and high Jc for applications. The critical current densities in the Nd‐123/Nd‐422 composites have been measured and the values obtained are discussed in the context of the microstructure of the material.
Preparation of a Barium Titanate Thin Film on a Porous Titanium Body by the Hydrothermal‐Electrochemical Method
Authors: Kajiyoshi Koji , Sakabe Yukio
Published: July 2010

The electrolysis profile of the hydrothermal‐electrochemical method that was used to grow BaTiO3 thin films uniformly on porous titanium bodies 5.0 mm in diameter and 1.0‐2.5 mm thick, with 32.7 vol% porosity, in an aqueous solution of Ba(OH)2 at 150°C, was investigated. The thin films could be grown only in a surface region that was limited to a depth of lessthan equal to0.25 mm into the porous titanium body via a “single‐step” electrolysis, in which an anodic potential was applied continuously to the porous titanium body. In contrast, the thin films could be grown uniformly on the entire surface of the porous titanium body using a “cyclic” electrolysis, in which an anodic potential and a rest potential were applied alternately. The BaTiO3 film growth inside the porous titanium body is considered to be dominated by the local solution pH.
Effect of Crystallization, Redox, and Waste Loading on the Properties of Several Glassy Waste Forms
Authors: Bloomer Patrick E. , Feng Xiangdong , Chantaraprachoom Nanthavan , Gong Meiling , McCready David E.
Published: July 2010

A laboratory study performed on three waste streams with high metal content (between 20 and 60 wt%) showed that waste form durability was strongly influenced by processing conditions and waste form compositions. The study involved varying the waste loading, melting conditions, and heat treatment of a total of 15 waste forms. The normalized sodium release obtained from the product consistency test (PCT) was the main criterion used for durability comparisons between each waste form. It was found that a reducing melt condition had no effect on the durability of some waste forms, yet it decreased the durability of others up to 700% when compared to the oxidized melts. One waste form composition tested showed a 140% increase in durability when devitrified (crystallized) relative to its glassy form. Some compositions showed no change in durability after devitrification, while others showed a 200% decrease. Increased waste loading was also found to have varying effects dependent on the composition of the waste form and the processing atmosphere. These responses to processing and composition variations were dictated by fundamental glass chemistry and can be adjusted to achieve maximum waste loading, acceptable durability, and desirable processing characteristics with optimized formulations.
Structural Stability and Conductivity of Phase‐Stabilized Cubic Bismuth Oxides
Authors: Jiang Naixiong , Wachsman Eric D.
Published: July 2010

Bismuth oxide, when stabilized in the face‐centered cubic phase by lanthanide doping, has one of the highest oxygen‐ion conductivities among all solid oxide electrolytes. We previously reported a time‐dependent aging phenomenon in phase‐stabilized bismuth oxides that involves ordering of the anion sublattice, as distinct from a crystallographic phase change, and results in decreased conductivity. In this paper, we report on the structural stability of the disordered lattice in terms of aging, as a function of both dopant species and dopant concentration. The aging rate decreases with both increased dopant cation radius and increased dopant concentration.
Nonlinear Current‐Voltage Characteristics with Negative Resistance Observed at ZnO‐ZnO Single‐Contacts
Authors: Nakamura Yoshinobu , Harada Takeshi , Kuribara Hitoshi , Kishimoto Akira , Motohira Naobumi , Yanagida Hiroaki
Published: July 2010

A couple of zinc oxide (ZnO) single crystals with single boundaries (ZnO‐ZnO single‐contacts) are fabricated by the traditional vapor reaction method and their electrical properties are characterized. The ZnO‐ZnO single‐contacts obtained show nonlinear current‐voltage (I‐V) characteristics without varistor‐forming constituents. Some of the ZnO‐ZnO single‐contacts show pronounced nonlinear I‐V characteristics with negative resistance. The I‐V characteristics of the ZnO‐ZnO single‐contacts are apparently similar to those of ZnO varistors; however, there are marked differences in the electric structure of the boundaries between the ZnO‐ZnO single‐contacts and ZnO varistors. The capacitance‐voltage (C‐V) relations of the ZnO‐ZnO single‐contacts are quite different from that of ZnO varistors and no evidence for the formation of double Schottky barriers at the boundary region are found. A very slow response to current stress is a feature of ZnO‐ZnO single‐contacts and it is suggested that any thermal processes including Joule heat would modify the carrier transport efficiency through the boundaries.
Mechanical Properties of Textured Bi2Sr2CaCu2O8+delta High‐Temperature Superconductors
Authors: Pastor Jose Ygnacio , Poza Pedro , LLorca Javier
Published: July 2010

The mechanical properties of Bi2Sr2CaCu2O8+delta fibers produced via laser‐induced directional solidification at different growth rates were determined through longitudinal and transverse tension tests, as well as flexure tests. In addition, polished sections of as‐received fibers and the fracture surfaces of the broken samples were examined using scanning electron microscopy to elucidate the relationship between the microstructure and the mechanical properties. The fibers were anisotropic, and the transverse fiber strength was very low, because of early failure via cleavage of the grains perpendicular to the c‐axis. The longitudinal strength and the degree of anisotropy increased as the fiber growth rate decreased, whereas the transverse strength followed the opposite trend. This behavior was due to changes in the porosity and the alignment of the crystals along the fiber axis.
Evidence for Structural Changes of Amorphous Carbon Coatings on Silicon Carbide During Tribological Tests
Authors: Berndt Florian , Kleebe Hans‐Joachim , Ziegler Günter
Published: July 2010

During tribological testing of hydrogenated amorphous carbon coatings (a‐C:H) on silicon carbide ceramics, a friction coefficient of 0.06 was observed under dry sliding conditions, which is thought to represent a lower boundary value for the given experimental setups. Based on chemical and thermal analyses of the coating material, a structural model of the as‐deposited amorphous coating was modified taking simplified quantum chemical and statistical arguments into consideration. The tribological behavior of the coating could be rationalized by this modified structure model: Tribological loading leads to a partial release of internal stresses stored in the coating. This process results in an increase of the local mobility of atoms, which can be seen as the driving force for initial structural rearrangements within the coating. Proof for the occurrence of such rearrangements has been provided by electron energy‐loss spectroscopy (EELS) measurements, monitoring the local sp3/sp2 hybridization ratio, of the coating before and after tribological tests.
Silicon‐29 Solid‐State MAS NMR Investigation of Selective Area Laser Deposition Silicon Carbide Material
Authors: Harrison Shay , Xie Xiangqun , Jakubenas Kevin J. , Marcus Harris L.
Published: July 2010

The microstructure of silicon carbide fabricated from the gas‐phase‐based, laser‐driven selective area laser deposition (SALD) process has been characterized by X‐ray diffraction and solid‐state 29Si NMR techniques with magic angle spinning (MAS). X‐ray patterns reveal the characteristic spectrum of pure ß, cubic silicon carbide polytype while NMR shows a ß‐SiC pattern with additional peaks. To determine the source of the extra peaks, a comparative evaluation of commercially available silicon carbide powders, both alpha‐ and ß‐SiC, was undertaken with NMR. Spin‐lattice relaxation time measurements (T1) were also carried out to determine the polytype morphology of the SALD deposit. The results showed an NMR spectrum that appears to be a combination of alpha and ß polytypes. Microtwinning is advanced as a possible explanation for this phenomenon.
Variation in the Composition of C‐S‐H Gel in Portland Cement Pastes Cured at Various Temperatures
Authors: Escalante‐Garcia José Ivan , Sharp John H.
Published: July 2010

Variation in the composition of C‐S‐H gel, the main hydration product of portland cement, cured at various temperatures, was studied using scanning electron microscopy with energy dispersive X‐ray spectroscopy. Samples of two cement pastes were cured isothermally under water at 10°, 30°, and 60°C for one year. For the inner‐product C‐S‐H gel of both cement pastes, increased curing temperature decreased the Ca/Si and Al/Ca ratios, but increased the S/Ca ratio. The Al/S and (Al + Fe)/S ratios decreased with increased curing temperature. Sulfur, probably in the form of sulfate ions, could be sorbed by the C‐S‐H gel at higher temperatures.
Spinel Electrodes for Lithium Batteries
Authors: Thackeray Michael M.
Published: July 2010

This article gives a historical account of the development of spinel electrodes for rechargeable lithium batteries. Research in the late 1970s and early 1980s on high‐temperature Li/Fe3O4 cells led to the evaluation of lithium spinels Li[B2]X4 at room temperature (B = metal cation). This work highlighted the importance of the [B2]X4 spinel framework as a host electrode structure and the ability to tailor the cell voltage by selection of various B cations. Examples of lithium‐ion cells that operate with spinel anode/spinel cathode couples are provided. Particular attention is given to spinels within the solid‐solution system Li1+xMn2‐xO4 (0 lessthan equal to x lessthan equal to 0.33).