bluuebunny/crossref_metadata_2025 huggingface dataset:
Role of Steric Hindrance in the Performance of Superplasticizers for Concrete
Authors: Yoshioka Kazuhiro , Sakai Etsuo , Daimon Masaki , Kitahara Ayao
Published: July 2010

Interparticle potential energy calculations were performed to investigate the mechanisms by which a new class of concrete admixtures, generally referenced as poly(carboxylic acid)‐type (PC) superplasticizers, which aid in dispersing cement particles, are formed. These calculations consisted of long‐range Van der Waals, electrostatic, and steric interactions. The repulsive potential that resulted from electrostatic interactions was negligible, which would allow cement particles to flocculate in the absence of steric contributions. A model was developed to describe the adsorption behavior of these superplasticizers, which consisted of grafted polyethylene oxide (PEO) chains on a PC backbone on cement surfaces. Using this adsorption model, the influence of the length of the PEO molecular chain and the density per unit area on the steric contribution was quantified. Steric hindrance effects were the dominant stabilizing mechanism in this system. As expected, enhanced stability was observed with increasing adlayer thickness (and/or density). The results of this study may be useful in designing the molecular structure of this new and important class of dispersion aids for cement‐based systems.
Thermodynamic Analysis of the Cubic–Tetragonal Phase Equilibria in the System ZrO2–YO1.5
Authors: Katamura Junji , Sakuma Taketo
Published: July 2010

The cubic‐tetragonal (c‐t) phase equilibria in the system ZrO2‐YO1.5 are thermodynamically analyzed from Landau's phenomenological theory. The calculated c‐t two‐phase field is depicted as a miscibility gap with a sharp maximum and the spinodal region as originally predicted by Hillert and Sakuma. However, the observed c‐t two‐phase field and the spinodal region are better described by the present model. In addition, this model can be used to discuss the nature of the c‐t diffusionless transformation from the order parameter in contrast with the original model. The predicted change in the tetragonality of t‐ZrO2 with YO1.5 content is slightly different from that in the c/a axial ratio estimated from X‐ray diffraction analysis. The displacement of cations and anions may not take place simultaneously during the c‐t transformation.
Strength vs Stressing Rate and Indentation Crack Growth Behavior of Indium(III) Phosphide
Authors: White Grady S. , Braun Linda M.
Published: July 2010

Susceptibility of InP to environmentally enhanced crack growth behavior was investigated in water and in 50% relative humidity by strength vs stressing rate and indentation crack length measurements. Statistical analysis indicated that, at the 95% confidence level, no environmentally enhanced fracture was detected for InP in liquid or gaseous water.
Effect of Porosity and Microstructure on the Strength of Si3N4: Designed Microstructure for High Strength, High Thermal Shock Resistance, and Facile Machining
Authors: Kawai Chihiro , Yamakawa Akira
Published: July 2010

The flexural strength of porous Si3N4 ceramics with a variety of microstructures and porosities were evaluated, and the effect of microstructure on the flexural strength was investigated to obtain machinable Si3N4 ceramics having both high strength and high thermal shock resistance. Porous Si3N4 having three types of microstructure, consisting of (1) only spherical grains, (2) combinations of spherical and columnar grains, and (3) only columnar β‐grains connected randomly in three dimensions, were readied as specimens. Their mean pore diameters and porosities were 0.2 to 0.3 μm and 8% to 59%, respectively. The flexural strength of the porous Si3N4 (3) was much larger than that of the porous Si3N4 having the other microstructures, and the maximum flexural strength was 455 MPa at a porosity of 38.3%. The thermal shock resistance (ΔT), which was determined by a water quench test, of porous Si3N4 with such microstructure and a porosity of 50% was 980 K. All of the porous Si3N4 (3) was easily machined with cemented carbide drills.
Effects of Temperature, Environment, and Orientation on the Fracture Toughness of Single‐Crystal YAG
Authors: Mah Tai‐Il , Parthasarathy Triplicane A.
Published: July 2010

The fracture toughness of single‐crystal YAG was measured as a function of temperature in air and under vacuum using the SENB technique up to 1600°C. The measured KIC was 2.2 MPa·m 1/2 at room temperature and it increased with temperature above 1200°C, reaching a value of 4.5 MPa·m1/2at 1600°C in air. The effects of crystallo‐graphic orientation and oxygen partial pressure on tough‐ness were studied at 1600°C. The toughness increased further to 5.5 MPa·m 1/2when tested under vacuum (10‐5torr)at 1600°C. Both temperature and testing environments were found to influence the fracture toughness, but there was no significant effect of crystallographic orientation.
Validity of Using Average Diameter for Determination of Tensile Strength and Weibull Modulus of Ceramic Filaments
Authors: Petry M. Dennis , Mah Tai‐Il , Kerans Ronald J.
Published: July 2010

Strengths and Weibull moduli for alumina/yttrium aluminum garnet eutectic (AYE) filaments and for Si‐C‐O (Nicalon TM) filaments were calculated using measured and average filament diameters. The strengths agreed closely. Thus an average filament diameter could be used instead of the measured filament diameter in calculating strengths. The Weibull modulus obtained from an average filament diameter approximates the Weibull modulus obtained using the measured filament diameter.
Oxygen Tracer Diffusion in Polycrystalline Calcium‐Doped Lanthanum Chromites
Authors: Yasuda Isamu , Ogasawara Kei , Hishinuma Masakazu
Published: July 2010

Oxygen tracer diffusion in polycrystalline (La,Ca)CrO3−delta was studied by the gas‐solid isotope exchange reaction in combination with a depth‐profiling technique using secondary‐ion mass spectroscopy, which allowed determination of coefficients of lattice and grain‐boundary diffusion separately. A significant contribution of fast grain‐boundary diffusion was confirmed. The lattice diffusion coefficient increased with increased acceptor content and decreased oxygen partial pressure, which was consistent with the currently established point defect model and, thereby, suggested that the lattice oxide ions diffused by a vacancy mechanism.
Pressureless Sintering of Titanium Diboride with Nickel, Nickel Boride, and Iron Additives
Authors: Einarsrud Mari‐Ann , Hagen Eirik , Pettersen Gunnar , Grande Tor
Published: July 2010

The effect of relatively small additions (1‐5 wt%) of nickel, nickel boride (NiB), and iron to promote the liquid‐phase sintering of titanium diboride (TiB2) has been studied. Carbon also was added to some samples, to reduce the amount of oxygen impurities in the TiB2 ceramics. Green bodies that were formed by uniaxial pressing were sintered in a graphite furnace at 1300°–1700°C, both under vacuum and in a 500 mbar argon atmosphere, and high densities (>94% of theoretical density) were obtained at temperatures greater than or equal to 1500°C. The weight loss of the samples during sintering was shown to be dependent on the densification rate and the final density and was not governed only by the thermodynamics of the system. Significant exaggerated grain growth was observed in samples with nickel, NiB, and iron during sintering at 1700°C. The exaggerated grain growth was observed to be closely related to the oxygen content of the samples and to temperature. The addition of carbon strongly reduced the density and the oxygen content and, thereby, inhibited grain growth. We have proposed that the exaggerated grain growth is enhanced by surface diffusion in a titanium‐oxide‐rich layer on the TiB2 grains.
Thermodynamic Assessment of the Silver–Oxygen System
Authors: Assal Jérôme , Hallstedt Bengt , Gauckler Ludwig J.
Published: July 2010

An assessment of the silver–oxygen system has been made, and a consistent set of thermodynamic parameters has been optimized. The calculated thermodynamic properties and phase relations are in good agreement with the experimental data. Ag2O is the only phase that is commonly found within the system. In air, it decomposes to silver and oxygen gas at 420 K. There is a eutectic between silver and Ag2O at a temperature of 804 K, an oxygen partial pressure (PO2) of 526 bar (5.26 × 107 Pa), and an oxygen mole fraction in the liquid phase of 0.25. Uncertainties remain on the Ag2O liquidus for PO2 > 108 Pa. An ionic two‐sublattice model has been used to describe the liquid phase. This work is part of a study of interactions between compounds from the bismuth‐strontium‐calcium‐copper‐oxygen system and silver.
Effect of Direct‐Current Biasing on the Dielectric Properties of Barium Strontium Titanate
Authors: Liou Jih‐Wei , Chiou Bi‐Shiou
Published: July 2010

Ba1−xSrxTiO3 (BST, x= 0 to x= 1) ferroelectric ceramics doped with 1.0 mol% MgO and 0.05 mol% MnO2 were prepared with a rate‐controlled sintering profile. As the strontium molar fraction x increased, the average grain size decreased from 14 μm for x= 0 to 2 μm for x= 1. Temperature dependence of the dielectric properties was measured as a function of dc biasing. At 10°C above the Curie temperature, the changes of dielectric constant at 5000 V/cm were 28%, 17%, 26%, and 36% for x= 0, 0.25, 0.5, and 0.75, respectively. The reduction of dielectric constant by the applied dc biasing was fitted by a phenomenological equation that was based on Devonshire's theory. From this phenomenological equation, an anharmonic coefficient, which was an order parameter of the anharmonic interactions, was calculated for each composition. As the strontium molar fraction increased, these coefficients increased from 2.45 × 10−19 cm2V−2 for x= 0 to 5.90 × 10−19 cm2V−2 for x= 0.75. A similar trend was observed on the dc field dependence of the dielectric loss, except at high field for x= 0, of which a high loss was obtained. A two‐element circuit model was proposed to explain the effect of high dc biasing on the dielectric loss.
An Analysis of Nucleation‐Rate Type of Curves in Glass as Determined by Differential Thermal Analysis
Authors: Ray Chandra S. , Day Delbert E.
Published: July 2010

The nucleation rate (I) versus temperature type of curves for a Na2O2·CaO·3SiO2 (NC2S3) glass doped with 0.1 wt% platinum, 0.5 wt% Ag2O, and 2.0 wt% P2O5 were determined using a previously developed differential thermal analysis (DTA) technique. In this DTA technique, a constant amount of glass sample was nucleated at selected temperatures for a specific time, followed by a DTA scan at a fixed heating rate. The functional dependence of the maximum intensity of the exothermic DTA crystallization peak ((deltaT)p) or the inverse temperature at the DTA peak maximum (Tp−1) on the nucleation temperature (Tn) was used to determine the nucleation rate versus temperature type of curves. Calculations for qualitatively assessing the dependence of (deltaT)p on Tn were performed using I and crystal growth rate (U) curves for a hypothetical system. Values of (deltaT)p calculated for different degrees of overlap between the I and U curves were compared with those measured experimentally. The (deltaT)p vs Tn curves depended strongly on the overlap of I and U, whereas the Tp−1 vs Tn curves were unaffected by the overlap.
Synthesis of Celsian (BaAl2Si2O8) from Solid Ba‐Al‐Al2O3‐SiO2Precursors: I, XRD and SEM/EDX Analyses of Phase Evolution
Authors: Allameh Seyed M. , Sandhage Kenneth H.
Published: July 2010

An intimate Ba‐Al‐Al2O3‐SiO2powder mixture, produced by high‐energy milling, was pressed to 3 mm thick cylinders (10 mm diameter) and hexagonal plates (6 mm edge‐to‐edge width). Heat treatments conducted from 300° to 1650°C in pure oxygen or air were used to transform these solid‐metal/oxide precursors into BaAl2Si2O8. Barium oxidation was completed, and a binary silicate compound, Ba2SiO4, had formed within 24 h at 300°C. After 72 h at 650°C, aluminum oxidation was completed, and an appreciable amount of BaAl2O4had formed. Diffraction peaks consistent with hexagonal BaAl2Si2O8, BaAl2O4, β‐BaSiO3, and possibly β‐BaSi2O5were detected after 24 h at 900°C. Diffraction peaks for BaAl2O4and BaAl2Si2O8were observed after 35 h at 1200°C, although SEM analyses also revealed fine silicate particles. Further reaction of this silicate with BaAl2O4at 1350° to 1650°C yielded a mixture of hexagonal and monoclinic BaAl2Si2O8. The observed reaction path was compared to prior work with other inorganic precursors to BaAl2Si2O8.
GeO2‐Based Sol–Gel Films
Authors: Brusatin Giovanna , Guglielmi Massimo , Martucci Alessandro
Published: July 2010

This paper describes the preparation of new oxide host materials for Er3+ based on GeO2 by the sol‐gel method. The characterization of these new materials is reported, with particular regard to the crystallization behavior: the tendency of GeO2 gels to crystallize during thermal treatment, in fact, is a critical problem. The investigated systems, powders and films, are GeO2‐SiO2, with different compositions, and 80GeO220MxOy, with M = Nb, Sn, Ti, Te.
Formation and Structural Characterization of 1:1 Ordered Perovskites in the Ba(Zn1/3Ta2/3)O3–BaZrO3 System
Authors: Chai Liang , Davies Peter K.
Published: July 2010

The phase stabilities in the(1−x)Ba(Zn1/3Ta2/3)O3 (BZT)‐xBaZrO3(BZ)system have been investigated using samples prepared by the mixed oxide method. The substitution of Zr4+destabilizes the 1:2 cation ordering in BZT and pro‐motes the formation of a cubic, 1:1 ordered structure with a doubled perovskite repeat. The homogeneity range of the 1:1 phase extends from x = 0.04 to approximately x = 0.25; substitutions beyond this range stabilize a disordered perovskite. The limits of stability of the 1:1 ordering coin‐cide with compositions previously found to exhibit anoma‐lies in their dielectric loss. The range of homogeneity is consistent with a “random layer” model for the 1:1 ordered “Ba{β′;1/2β1/2}O3” structure. In this model the B× positions are assumed to be occupied exclusively by Ta5+, and the b× sites by a random distribution of Zn2+, Zr4+, and the remaining Ta 5+ cations. The validity of the model, where the ordered solid solutions can be represented by Ba{[Zn2−y/3Ta(1−2y)/3Zry]1/2[Ta]1/2}O3(y =2x)was con‐firmed by Rietveld refinements conducted using data col‐lected with a synchrotron X‐ray source.
Estimation of Phase Stability in Pb(Mg1/3Nb2/3)O3 and Pb(Zn1/3Nb2/3)O3 Using the Bond Valence Approach
Authors: Wakiya Naoki , Shinozaki Kazuo , Mizutani Nobuyasu , Ishizawa Nobuo
Published: July 2010

The structure stability of perovskite‐type compounds has been quantitatively estimated by applying bond valence calculations to Pb(Mg1/3Nb2/3)O3 (PMN) and Pb(Zn1/3Nb2/3)O3 (PZN). The bond valence calculations revealed that the bond strength between oxygen and cations in the pyrochlore‐type compounds is greater than that in the perovskite PMN. It is found that the absolute value of the bond valence sum of oxygen, |VO|, for a PZN single crystal is smallest in reported Pb‐containing perovskite‐type compounds, corresponding to the fact that it is impossible to synthesize PZN by solid‐state reaction under atmospheric pressure. The calculated amount of additives required for stabilizing PZN under atmospheric pressure agreed well with the experimental values.
Hydrothermal Corrosion of Alumina Ceramics
Authors: Oda Kohei , Yoshio Tetsuo
Published: July 2010

The corrosion behavior and strength degradation of alumina ceramics with 99%, 99.9%, and 99.99% Al2O3 were studied in water at 300°C and 8.6 MPa for 1 to 10 d. The weight loss in alumina ceramics was mainly attributed to the dissolution of SiO2 and Na2O grain‐boundary impurities. Intergranular corrosion proceeded in the alumina ceramics by preferential attack at the grain boundaries. The extent of the strength reduction for corroded alumina ceramics was related to the impurity level in the alumina ceramics.
Calcium‐Ion Selective Site Occupation at Ruddlesden‐Popper‐Type Faults and the Resultant Dielectric Properties of A‐Site‐Excess Strontium Calcium Titanate Ceramics
Authors: Fujimoto Masayuki , Suzuki Toshimasa , Nishi Yuji , Arai Katsuhiko , Tanaka Junzo
Published: July 2010

The microstructure and dielectric properties of A‐site‐excess and stoichiometric‐composition strontium calcium titanate ((Sr,Ca)TiO3) ceramics were investigated in the present study. Ruddlesden‐Popper‐type planar faults in A‐site‐excess specimens were observed using high‐resolution scanning transmission electron microscopy (HR‐STEM). STEM microchemical analysis, calculation of the high‐resolution image using a multislice method, and structural‐energy calculation further revealed that calcium ions selectively occupied the cation sites of the faults. An observed shift of the peak dielectric constant to low temperature and the expansion of the lattice parameter of the A‐site‐excess specimen were attributed to the selective occupation of the cation sites of the planar faults by calcium ions.
Coarsening in a Viscous Matrix
Authors: Scherer George W.
Published: July 2010

When a viscous material is three‐dimensionally constrained (as when a gel or slurry of glass is infiltrated into the interstices of a particle packing), heat treatment leads to coarsening of the pore‐size distribution, rather than densification. The smaller pores generate greater capillary pressure; therefore, they shrink and force the large pores to expand. In this article, the kinetics of coarsening are analyzed, and sample calculations are performed for a body with a lognormal distribution of pore sizes.
Kinetics of the Amorphous →γ→α Transformations in Aluminum Oxide: Effect of Crystallographic Orientation
Authors: Simpson Todd W. , Wen Qingzhe , Yu Ning , Clarke David R.
Published: July 2010

Time‐resolved reflectivity (TRR) measurements have been used to determine the growth kinetics of both the amorphous‐to‐γ‐phase and the γ‐to‐α phase transformations of alumina films that have been deposited on (0001), (1210), and (0112) sapphire substrates. The activation energies for the amorphous‐to‐γ‐phase and γ‐to‐α transformations respectively are 4.5 and 5.2 eV, irrespective of the crystallographic orientation of the underlying sapphire substrate. For both transformations, the relative transformation rates seem to reflect the symmetry of the sapphire substrates, with the fastest kinetics being on the lowest symmetry orientation and slowest kinetics being on the highest symmetry orientation. These relative rates are attributed to the differences in step mobility on the different substrate orientations.
Thermal Analysis of Amorphous Phases in Hydroxyapatite Coatings
Authors: Gross Karlis A. , Gross Volfgangs , Berndt Christopher C.
Published: July 2010

The amorphous phase in hydroxyapatite coatings has been examined by using X‐ray diffractometry, Fourier transform infrared spectroscopy, optical microscopy, and thermal analysis methods. The amorphous phase mostly consists of a dehydroxylated calcium phosphate. When heated, crystallization of hydroxyl‐rich areas produces hydroxyapatite, followed by diffusion of hydroxyl ions, thus increasing the amount of crystalline phase. Hydroxyl‐deficient amorphous areas crystallize to oxyapatite at 700°C. Thus, crystallization occurs over a range of temperatures and is dependent on the hydroxyl content of the amorphous phase and the partial water‐vapor pressure. The activation energies of crystallization to hydroxyapatite, diffusion of hydroxyl ions, and crystallization to oxyapatite are 274, 230, and 440 kJ/mol, respectively. Shrinkage from these processes leads to a crack network and decreases the mechanical strength of the coating.
Tensile Behavior of a Quasi‐Isotropic Carbon‐Carbon Composite
Authors: Namiki Fumiharu , Chou Tsu‐Wei
Published: July 2010

This paper reports an analytical and experimental investigation of the tensile behavior of a two‐dimensional woven carbon‐carbon composite with a quasi‐isotropic stacking sequence. Specimens in their as‐received condition were already saturated with transverse cracks due to thermal stress induced during fabrication. Further cracking under load was not detected. The completely unloaded specimen showed that the slight residual strain tended to increase with applied tensile strain. Damage progression as seen on specimen edges and elastic moduli degradation were recorded. The predicted Young's moduli were in good agreement with the measured data.
Microfabrication of Ceramics by Co‐extrusion
Authors: Hoy Charles Van , Barda Andrew , Griffith Michelle , Halloran John W.
Published: July 2010

Fine‐scale ceramic objects are fabricated by forcing a thermoplastic ceramic extrusion compound through a die with reduction ratio R. Objects with complex shapes are fabricated by assembling an extrusion feedrod from a shaped ceramic compound with space‐filling fugitive compound. After each reduction state, R2 extrudates are assembled into a feedrod and extruded again, reducing the size and multiplying the number of shaped objects. Several stages of extrusion produce arrays of objects in the size range of 10 µm.
In Situ Measurements of Frictional Bridging Stresses in Alumina Using Fluorescence Spectroscopy
Authors: Pezzotti Giuseppe , Sbaizero Orfeo , Sergo Valter , Muraki Naoki , Maruyama Ken , Nishida Toshihiko
Published: July 2010

Fracture of a large‐grained alumina polycrystal has been examined in situ by optical microscopy. Concurrently, local bridging stresses, as generated by friction or tension of unbroken ligaments in the wake of the crack path, were measured by piezospectroscopy. Stress measurements were performed both at fixed sites as a function of the external load and at a fixed external load along the crack profile. Frictional stresses were similar/congruent50 MPa, while unbroken ligaments between the crack faces were found to support tensile stresses up to similar/congruent100 MPa. The maximum bridging stress was dictated by the weak (intrinsic) interface bonding of the polycrystal. Average bridging stresses, either theoretically calculated from R‐curve data or experimentally measured by piezospectroscopy on frictional/bridging sites, were similar. Such a circumstance enables us to explain the fracture behavior of polycrystalline alumina by considering crack‐wake shielding as the main micromechanism contributing to toughening.
Nonisothermal Reaction Kinetics of SrNb2O6 and BaNb2O6 for the Formation of SrxBa1−xNb2O6
Authors: Lee Wen‐Jiung , Fang Tsang‐Tse
Published: July 2010

The solid‐state reaction of SrNb2O6 and BaNb2O6 to form SrxBa1−xNb2O6(SBN) at different temperatures and heating rates was investigated. The reaction kinetics were analyzed by X‐ray diffractometry for quenched samples, and the internal‐standard method was applied to quantify the extent of the reaction. A nonisothermal kinetic empirical model was proposed to evaluate the activation energy and rate constant of SBN with different Sr:Ba ratios. It was found that the kinetic form would change above and below a transition at a reaction fraction of ∼60%, which might be due to the change of the frequency factor. It was also verified that the model that was presented was more favorable to describe the nonisothermal reaction kinetics of SBN.
Electrochemical Determination of the Gibbs Energy of Formation of MgAl2O4
Authors: Jacob K. Thomas , Jayadevan Kampurath P. , Waseda Yoshio
Published: July 2010

The standard Gibbs energy of formation of the spinel MgAl2O4 from component oxides, MgO and α‐Al2O3, has been determined in the temperature range 900 to 1250 K using a solid‐state cell incorporating single‐crystal CaF2 as the solid electrolyte. The cell can be represented as—Pt,O2,MgO+MgF2|CaF2|MgF2+MgAl2O4+α‐Al2O3,O2,Pt—The standard Gibbs energy of formation from binary oxides, computed from the reversible emf, can be represented by the expression—capdeltaG°f,ox=−23600 − 5.91T(±150) J/mol—The ‘second‐law’ enthalpy of formation of MgAl2O4 obtained in this study is in good agreement with high‐temperature solution calorimetric studies reported in the literature.
Microwave Dielectric Study of Water Structure in the Hydration Process of Cement Paste
Authors: Miura Nobuhiro , Shinyashiki Naoki , Yagihara Shin , Shiotsubo Masami
Published: July 2010

Microwave dielectric relaxation measurements, via the time‐domain reflectometry method, were performed on portland cement paste for the first time, and the water structure during the hydration process was observed. The relaxation process due to the orientation of free water, which is independent of calcium silicate hydrate (C‐S‐H), was observed at ∼10 GHz. The relaxation strength, in proportion to the amount of free water, decreased rapidly as the curing time increased for the first three days. This change is in good agreement with that of a chemical reaction that was reported by measurements of the heat that is evolved during hydration. The free water is taken into C‐S‐H and is transformed to hydrated water by the hydration process. When hydration proceeds, the relaxation processes due to the orientation of the hydrated water in C‐S‐H occur at ∼100 MHz and 1 MHz.
Stress‐Induced Resistivity Anomaly in Semiconducting Barium Titanate Ceramic Wire
Authors: Matsuda Hirofumi , Kuwabara Makoto , Hamamoto Kohichi
Published: July 2010

Thin, semiconducting BaTiO3 ceramic wires prepared in the present study exhibited reversible stress‐induced, nonlinear current—;voltage characteristics across several grain boundaries. A remarkable change in resistance with the application (by the three‐point bending method) of only ∼1% tensile deformation indicated that the BaTiO3 wires may have potential as stress‐sensing devices. Resistance in the BaTiO3 wires bot increased and decreased with increasing tensile stress parallel to the electric fields, far below the ferroelectric transition temperature, Tc; in Sr‐substituted wires near Tc, on the other hand, resistance only increased. Detailed studies of the patterns and fluctuation of polarization at grain boundaries could be meaningful, because stress‐sensing characteristics may be induced by changes in the relative angle between polarization vectors of adjacent grains.
Endothermic Reactions between Mullite and Silicon Carbide in an Argon Plasma Environment
Authors: Park Youngsoo , McNallan Michael J. , Butt Darryl P.
Published: July 2010

Reactions between SiC and mullite in an Ar plasma were investigated using a model composite in which a free‐standing CVD SiC coupon was imbedded in mullite cement. After treatment in a radio frequency (RF) plasma, the Si content of the mullite in contact with SiC was found to be less than that in the starting material, and deposits were found on the walls of the plasma chamber due to the reaction of mullite with SiC as follows: Al6Si2O13(s)+ SiC(s)= 3Al2O3(s)+ 3SiO(g)+ CO(g). This reaction, which is endothermic (1405 kJ/mol at 1500 K), absorbs thermal energy and consequently prevents the rapid sintering which is observed for single‐phase mullite in similar environments. As a consequence, it is suggested that RF plasma sintering probably cannot be used to densify SiC‐reinforced mullite‐matrix composites because of the resulting energy consumption and damage to the SiC phase.
Humidity‐Sensing Characteristics of Divalent‐Metal‐Doped Indium Oxide Thin Films
Authors: Tahar Radhouane Bel Hadj , Ban Takayuki , Ohya Yutaka , Takahashi Yasutaka
Published: July 2010

Metal‐doped indium oxide thin films that have been prepared on glass substrates via a dip‐coating method from an aqueous sol show a rapid decrease in electrical conductivity at room temperature (by approximately three orders of magnitude) when brought into contact with moist air. This observation is in contrast to the conductivities of nondoped indium oxide films that are independent of moisture. Thus, the doped films can be applied as a humidity sensor. The film thickness has no substantial effect on the sensing properties, probably because of the porous nature of the films that are prepared by a sol‐gel process. To examine the effects of film morphology, a comparison between the humidity‐sensing properties of the films that have been prepared from organic and aqueous sols also has been conducted. The roles of the doped metal ions are also discussed.
Piezoresistivity in PTCR Barium Titanate Ceramics: II, Mathematical Modeling
Authors: Capurso Joseph S. , Schulze Walter A.
Published: July 2010

A modified grain boundary potential barrier model for positive temperature coefficient of resistance (PTCR) barium titanate is developed. It is based on Heywang's double Schottky barrier model, together with Devonshire's thermodynamic phenomenology of polarization behavior. Modifications are made to address the nonlinearity of the dielectric response with respect to electric field in the depletion region adjacent to the grain boundary, and the circular dependence of these quantities with the height of the barrier. Piezoresistivities are calculated for various PTCR compositions and sintering conditions, and these are compared to experimental results from the preceding paper.
Titanium Oxide Thin Films on Organic Interfaces through Biomimetic Processing
Authors: Baskaran Suresh , Song Lin , Liu Jun , Chen Yuan L. , Graff Gordon L.
Published: July 2010

Titanium oxide (TiO2) thin films have been deposited on silicon, glass, and plastic substrates by destabilization of an aqueous titanium lactate solution at low temperatures (<100°C). The process uses a commercially available, low‐cost precursor and is simple to perform; it involves only control of pH in aqueous, chelated titanium solutions. With this solution technique, high deposition rates (>50 nm/min), film thickness (>100 nm), and excellent film uniformity have been obtained. Uniform coatings can be applied on complex‐shaped polymeric substrates and porous membranes. Films can be formed on both sulfonated and untreated polymeric surfaces. As‐deposited films on plastic substrates consist of amorphous, hydrated TiO2. On sulfonated self‐assembled monolayers on silicon substrates, nanocrystalline TiO2 films have been formed. The deposited films exhibit strong ultraviolet (UV) absorption with excellent transmission in the visible wavelength range, which indicates that the coatings may be useful as protective UV blockers for polymeric materials.
Characteristics of Strontium Aluminate Crystals Used for Long‐Duration Phosphors
Authors: Katsumata Tooru , Sasajima Kazuhito , Nabae Takehiko , Komuro Shuji , Morikawa Takitaro
Published: July 2010

Sr3Al2O6, SrAl2O4, SrAl4O7, and SrAl12O19 that have been doped with Eu2+ and Dy3+ ions have been grown by a floating‐zone technique for application as long‐duration phosphors. Long‐duration phosphorescence with a variety of colors has been observed in SrAl2O4, SrAl4O7, and SrAl12O19 crystals that have been doped with Eu2+ and Dy3+ ions. The peak wavelength of the phosphorescence is 520 nm for SrAl2O4, 480 nm for SrAl4O7, and 400 nm for SrAl12O19. The phosphorescence is characterized by decay times that have been analyzed by a curve‐fitting technique.
Sintering of Chemically Synthesized Lead(II) Selenide Fine Powders
Authors: Unuma Hidero , Shigetsuka Naoki , Masui Kanji , Nagao Jiro , Takahashi Minoru
Published: July 2010

PbSe fine powders were synthesized from a liquid‐phase reaction of lead‐ascorbic acid complex and sodium selenosulfate. The resultant powders were sintered up to nearly theoretical density by controlling the sintering atmosphere and by optimizing the sintering temperature and time. The highest sintered density, 99.7% relative to the theoretical density of 8.27 103 kg/m3, was attained when a powder compact was sintered at 998 K for 6 h under a flow of argon containing 3% H2 gas. Powder compacts could be sintered to higher than 98% when the sintering temperature was 1073 K or lower. When the sintering temperature or time exceeded the optimum range, considerable volatilization of the component occurred, resulting in a decrease in the sintered density. The use of a powder bed was found to be effective in suppressing the volatilization of the component.
Surface Formation Energy for Intergranular Fracture in Two‐Dimensional Polycrystals
Authors: Holm Elizabeth A.
Published: July 2010

A global energy minimization method for calculating inter granular fracture surfaces which minimize surface formation energy was developed using standard graph algorithms. The intergranular fracture of equiaxed, single‐phase, polycrystalline grain structures was examined under a variety of energetic conditions. In energetically isotropic systems the minimum energy fracture surface has length 1.1 times the sample width independent of grain size. A criterion for fracture mode was derived for this case. Using a ball‐and‐spring model, local mechanical effects were found to increase fracture length by at most 5% in isotropic systems. The presence of typical low‐energy, low‐angle grain boundaries minimally affects intergranular fracture path, but may alter the preferred fracture mode. In systems with randomly dispersed, very weak boundaries (i.e., micro‐cracks), fracture surfaces veer to follow low surface formation energy boundaries, resulting in tortuous, low‐energy fracture paths, and intergranular fracture is strongly preferred.
Hyperfine Interactions of cis and trans Octahedral Fe2+ Sites in the Layered Silicate Annite
Authors: Terra Joice , Ellis Donald E.
Published: July 2010

Spin‐polarized first‐principles calculations based on density functional theory were performed for 23‐ and 101‐atom embedded clusters representing annite. Hyperfine interactions and magnetic moments were obtained and compared with experimental Mössbauer spectra and SQUID magnetometry measurements. It was found that the electric field gradients and the magnetic fields are profoundly affected by the position of the hydroxyls. The calculated magnetic moments are close to 4 μB in both octahedral sites.
Light‐Scattering Properties of Representative, Morphological Rutile Titania Particles Studied Using a Finite‐Element Method
Authors: Thiele Erik S. , French Roger H.
Published: July 2010

The light‐scattering performance of TiO2 pigment depends intimately upon particle size, size distribution, shape and dispersion quality. TiO2 particles exhibit complex shapes and have anisotropic optical constants, and particulate dispersions of TiO2 are characterized by complicated microstructures such as aggregates and agglomerates. Despite this complexity, the theoretical understanding of light scattering by white pigment has been based upon Mie theory, which is restricted to the case of a single, optically isotropic sphere. We utilize a finite‐element method which produces rigorous solutions to Maxwell's equations to determine computationally the light‐scattering properties of complex particulate microstructures. This represents a significant step beyond the restrictions of Mie theory, providing a method to determine quantitatively the effects of particle shape, optical anisotropy, and interactions between neighboring particles upon the light‐scattering properties of white pigments in coatings. In the present study, we use the finite‐element method first to compute the light‐scattering properties of a single, morphological rutile particle with a representative size and shape. These results are compared to the light‐scattering properties of the optically isotropic, equivalent volume sphere using Mie theory. Neither the average index nor the weighted sum approximation offers clear advantages in this case. Second, the far‐field light‐scattering properties of two such particles interacting at near field are determined as a function of the interparticle separation. The agglomerated pair of particles exhibits a 20% decrease relative to the single particle in the scattering parameter associated with the hiding power of a paint film. The basis for this decrease is the same as for the crowding effect observed in extensive paint films. The results of both sets of computations are compared to Mie theory to determine the sizes of spherical particles with equivalent scattering cross sections. These comparisons highlight the inherent difficulties in using Mie theory to evaluate particle size by light‐scattering methods.
Simulation of Grain Growth and Pore Migration in a Thermal Gradient
Authors: Tikare Veena , Holm Elizabeth A.
Published: July 2010

The Potts Monte Carlo simulation was used to simulate microstructural evolution in uranium dioxide fuel rods. During service, grain growth, pore migration, and thermal segregation of the pores and UO2 occur in the rods in a thermal gradient. In this investigation, we developed a model which simulates simultaneous grain growth, pore migration, and thermal segregation of the pores and UO2 in a temperature gradient. Grain growth in a thermal gradient was simulated using the Monte Carlo Potts model technique developed by Anderson, Srolovitz, and co‐workers. Pore migration was simulated using conserved dynamics with minimum‐energy exchanges at a finite temperature. A temperature gradient was introduced into the model via interfacial mobility gradient. Finally, thermal segregation of the pores and UO2 was achieved by introducing a heat of migration term, ΔEt, which biased the motion of porosity to the high‐temperature region. The development of this model is described and the incorporation of the proper physics of pore migration and thermal segregation is discussed.
Numerical Simulation of Zener Pinning with Growing Second‐Phase Particles
Authors: Fan Danan , Chen Long‐Qing , Chen Shao‐Ping P.
Published: July 2010

The Zener pinning effect with growing second‐phase particles in Al2O3‐ZrO2 composite systems were studied by two‐dimensional (2‐D) computer simulations using a diffuse‐interface field model. In these systems, all second‐phase particles are distributed at grain corners and boundaries. The second‐phase particles grow continuously, and the motion of grain boundaries of the matrix phase is pinned by the second‐phase particles which coarsen through the Ostwald ripening mechanism, i.e., long‐range diffusion. It is shown that both matrix grains and second‐phase particles grow following the power‐growth law, Rtm ‐ R0m = kt with m = 3. It is found that the mean size of the matrix phase (D) depends linearly on the mean size of the second‐phase particles (r) for all volume fractions of second phase from 10% to 40%, which agrees well with experimental results. It is shown that D/r is proportional to the volume fraction of the second phase (f) as f−1/2 for a volume fraction less than 30%, which agrees with Hillert and Srolovitz's predictions for 2‐D systems, while experimental results from 2‐D cross sections of three‐dimensional (3‐D) Al2O3‐rich systems showed that either a f−1/2 or a f−1/3 relation might be possible. It is also found that D/r is not proportional to f−1/3 and f−1 in 2‐D simulations, which suggests that the Zener pinning effect can be very different in 2‐D and 3‐D systems.
Adsorption Effects on the Rheological Properties of Aqueous Alumina Suspensions with Polyelectrolyte
Authors: Guo Lu‐Cun , Zhang Yao , Uchida Nozomu , Uematsu Keizo
Published: July 2010

The influence of adsorption coverage and free polymer on the rheological properties of aqueous alumina suspensions with polyelectrolyte was studied. The flow curves of the suspensions followed the Casson model very well in the observed range of shear rate. The Casson yield value, τc, was used to evaluate the stability. The coverage exerted a profound influence on the rheological properties by affecting the interaction between particles. The zeta potential increased and the value of τc decreased as the coverage increased. The free polymer could cause weak flocculation as its concentration was increased to a certain level. The opposite effects of the adsorbed polymer and free polymer on stability resulted in the presence of optimum coverage for stabilization. The concentration of free polymer increased rapidly as the adsorption coverage approached the saturation limit, and then the free polymer started to dominate the stability. Stabilization could be achieved at a condition of unsaturated adsorption and was related to the non‐high‐affinity adsorption at alkaline pH (9.2). The impact of adsorption affinity on stability was discussed.
Development of Low‐Toxicity Gelcasting Systems
Authors: Janney Mark A. , Omatete Ogbemi O. , Walls Claudia A. , Nunn Stephen D. , Ogle Randy J. , Westmoreland Gary
Published: July 2010

A series of low‐toxicity gelcasting systems has been developed. The reagents used in these systems have very low acute toxicity. The new systems perform at least as well as, and in some cases better than, the original acrylamide‐based system. The development of these systems is described herein, including the search for new gel compositions, the study of suspensions made with the new gel precursor solutions, and pyrolysis of the dried gels and gelcast parts. Applications of the new gelcasting systems include complex silicon nitride parts, large‐diameter rings, rapid prototyping by green machining, and metal‐powder gel casting.
Control of Interface Migration in Melt‐Infiltrated Niobium‐Doped Strontium Titanate by Solute Species and Atmosphere
Authors: Jeon Jae‐Ho , Je Jung Ho , Kang Suk‐Joong L.
Published: July 2010

The solid/liquid interface migration of niobium‐doped strontium titanate (SrTiO3) has been investigated by using barium and calcium solute sources in the form of a Cu‐Ba‐Ca‐O liquid. The solubilities of barium and calcium in SrTiO3 that is in equilibrium with Cu‐Ba‐Ca‐O melts can be determined by measuring the solute concentration in the migrated Sr(Ba,Ca)TiO3 regions of SrTiO3 grains. No interface migration is observed for a certain composition range of Cu‐Ba‐Ca‐O infiltrant. For SrTiO3 specimens that have been sintered in air, the zero driving force for the interface migration is found at a Ba:Ca ratio of ∼0.75 in Sr(Ba,Ca)TiO3 solid solution. This value is in good agreement with a predicted value of zero coherency strain in a thin diffusional Sr(Ba,Ca)TiO3 layer on bulk SrTiO3. For the specimens that have been sintered in 5H2‐95N2, however, a zero driving force results when the ratio is ∼0.84. The difference between the value for the specimens sintered in air and that for the specimens sintered in 5H2‐95N2 is attributed to the change in defect concentration via the change in atmosphere. The estimated difference in the lattice parameters of SrTiO3 sintered in air and those of SrTiO3 sintered in 5H2‐95N2 is ∼1.8 10−3Å (∼1.8 10−2 nm). The difference in lattice parameters has been further confirmed by using synchrotron X‐ray scattering experiment, which has revealed the lattice parameter to be 3.9053 Å (0.39053 nm) in air and 3.9070 Å (0.39070 nm) in 5H2‐95N2.
Optical Properties of Zinc Aluminate, Zinc Gallate, and Zinc Aluminogallate Spinels
Authors: Sampath Suresh K. , Cordaro James F.
Published: July 2010

The optical spectra of zinc aluminate (ZnAl2O4), zinc gallate (ZnGa2O4), and zinc aluminogallate (ZnAlGaO4) spinel powders were studied at wavelengths in the range of 250‐900 nm using reflectance spectroscopy. The ZnAl2O4 and ZnGa2O4 powders were synthesized by using conventional ceramic processing techniques and had systematic variations in the molar ratio of ZnO to M2O3 (M = Al or Ga). The cubic spinel crystal structure of each composition was confirmed via powder X‐ray diffractometry. The ZnAl2O4 powders showed optical properties in the ultraviolet wavelength region and had combined characteristics that were similar to that of ZnO (wurtzite structure) and Al2O3 (corundum structure), which result from the similar local environments of the zinc and aluminum cations within the cubic spinel crystal structure. A mechanically induced optical absorption (optomechanical effect) in the ultraviolet wavelength region was also observed in ZnAl2O4. The ZnGa2O4 powder followed a similar behavior, with the exception that the optomechanical effect did not occur in the gallate. The ZnAlGaO4 showed optical spectra that were intermediate to that of the endpoint compositions.
Kinetics of Thermal, Passive Oxidation of Nicalon Fibers
Authors: Zhu Yuntian T. , Taylor Seth T. , Stout Michael G. , Butt Darryl P. , Lowe Terry C.
Published: July 2010

The oxidation of Nicalon fibers is a concern, because of its potential as a reinforcement of high‐temperature composites, whose service conditions involve high‐temperature, oxidizing environments. Two limiting types of oxidation mechanisms are often used to describe the kinetics: chemical‐reaction‐controlled oxidation, at small oxide thicknesses, and diffusion‐controlled oxidation, at large oxide thicknesses. Neither mechanism can satisfactorily describe the intermediate region where the oxidation kinetics are controlled jointly by both the chemical reaction rate at the interface and the diffusion of oxygen through the oxide layer. To describe the entire oxidation process with a general relationship, one must consider all stages of the oxidation process, namely (i) adsorption of oxygen at the outer surface of the oxide, (ii) diffusion of oxygen from the outer surface toward the interface where oxidation occurs, and (iii) reaction at the interface to form a new layer of oxide. Previously, a very useful general relationship was derived for the oxidation kinetics for a flat plate, which could account for all three stages of oxidation. However, that equation is inadequate to describe the oxidation of cylindrical fibers, because the effective area for oxygen diffusion changes along the diffusion path and the oxidation interfacial area decreases as the oxide thickness increases for cylindrical fibers. In this paper, we have derived a general kinetic relationship for the oxidation of cylindrical fibers, which can account for all stages of oxidation. Comparison of the theory with experimental data of Nicalon fibers shows good agreement.
Ordering‐Induced Microstructures and Microwave Dielectric Properties of the Ba(Mg1/3Nb2/3)O3–BaZrO3System
Authors: Akbas Mehmet A. , Davies Peter K.
Published: July 2010

The structures of compositions across the Ba(Mg1/3Nb2/3)‐O3‐BaZrO3(BMN–BZ) system have been examined using X‐ray diffractometry and transmission electron microscopy, and their dielectric properties have been characterized in the microwave range. Although pure BMN adopts a 1:2 ordered structure, of space groupPm1, additions of 5–15 mol% BZ stabilize a cubic (Fmm), 1:1 ordered phase with a doubled perovskite repeat. At higher levels of substitution (>25 mol% BZ), the B‐site cations are disordered. After normal sintering, the niobates in the 1:1 phase field are comprised of nanometer‐sized ordered domains that are dispersed in a disordered matrix. However, by reducing the cooling rate to 10°C/h, a fully ordered microstructure is formed with domain sizes >100 nm in size. The structure of the 1:1 phases has been interpreted using a “random‐layer” model, in which one site is occupied by niobium, and the second is occupied by a random distribution of the remaining cations. The addition of small concentrations of BZ produces a 100% improvement in the dielectric‐loss properties of BMN, and aQ·fvalue of 82000 is obtained for a 5 mol% substitution.
Intrinsic and Extrinsic Size Effects in Fine‐Grained Morphotropic‐Phase‐Boundary Lead Zirconate Titanate Ceramics
Authors: Randall Clive A. , Kim Namchul , Kucera John‐Paul , Cao Wenwu , Shrout Thomas R.
Published: July 2010

The processing, electromechanical properties, and microstructure of lead zirconate titanate (PZT) ceramics over the grain‐size range of 0.1‐10 μm were studied. Using measurements over a large temperature range (15‐600 K), the relative role of extrinsic contribution (i.e., domain‐wall motion) was deduced to be influenced strongly by the grain size, particularly for donor‐doped PZT. Analytical transmission electron microscopy studies were conducted to investigate the trend in domain configurations with the reduction of grain size. The correlations between domain density, domain variants, domain configurations (before and after poling), spontaneous deformation, and the elastodielectric properties were qualitatively discussed, leading to new insights into the intrinsic and extrinsic effects and relevant size effects in ferroelectric polycrystalline materials.