bluuebunny/crossref_metadata_2025 huggingface dataset:
Host cell preference and variable transmission strategies in malaria parasites
Authors: Reece Sarah E , Duncan Alison B , West Stuart A , Read Andrew F
Published: March 2005

Malaria and other haemosporin parasites must undergo a round of sexual reproduction in their insect vector in order to produce stages that can be transmitted to vertebrate hosts. Consequently, it is crucial that parasites produce the sex ratio (proportion of male sexual stages) that will maximize the number of fertilizations and thus, transmission to new vertebrate hosts. There is some evidence to show that, consistent with evolutionary theory, the sex ratios of malaria parasites are negatively correlated to their inbreeding rate. However, recent theory has shown that when fertilization success is compromised, parasites should respond by increasing their investment in sexual stages or by producing a less female biased sex ratio than predicted by their inbreeding rate alone. Here, we show that two species of rodent malaria,
Plasmodium chabaudi
and
Plasmodium vinckei petteri
, adopt different strategies in response to host anaemia, a factor thought to compromise transmission success:
P. chabaudi
increases investment in sexual stages, whereas
P. vinckei
produces a less female biased sex ratio. We suggest that these different transmission strategies may be due to marked species differences in host cell preference.
Mechanism for the Dehydroxylation of Kaolinite, Dickite, and Nacrite from Room Temperature to 455°C
Authors: SUITCH P. R.
Published: March 2005

The mechanism for the dehydroxylation of Keokuk kaolinite, dickite, and nacrite (Al2Si2O5(OH)4) was investigated in detail with the aid of powder X‐ray diffraction, the Rietveld method, and infrared spectroscopy. It was determined that these polymorphic clay minerals dehydroxylate via an inhomogeneous mechanism in the range room temperature to 455°C. This result implies that when these samples dehydroxylate (up to 40% dehydroxylated at 455°C) the individual crystallites spontaneously react either completely, to form the metaphase of these minerals, or not at all, retaining all the structural details of the unheated crystalline material. These results amplify previous nuclear magnetic resonance results concerning this mechanism and are used to explain the appearance of the differential thermal analysis curves for these minerals.
Crystallography of the Directionally Solidified NiO‐Gd2O3 Eutectic
Authors: Dubois Béatrice , Dhalenne Guy , D' Yvoire Ferdinand , Revcolevschi Alexandre
Published: March 2005

Crystallographic relations between the two phases of the eutectic in the system NiO‐Gd2O3 were determined by X‐ray diffraction and optical microscopy. A model of the atomic arrangement at the phase interface is proposed.
Hot‐Pressing of β‐SiC Powder with Al‐B‐C Additives
Authors: Lin Bor‐Wen , Imai Masamitsu , Yano Toyohiko , Iseki Takayoshi
Published: March 2005

The sintering behavior of β‐SiC powders with additions of Al, B, and C was studied at 1600° to 1800°C with applied pressures of 20 to 60 MPa. Ceramics with densities of ∼3.08 g/cm3 were obtained by hot‐pressing at 1650°C and 50 MPa. The bending strength did not degrade up to 1200°C. A large amount of a second phase, which was apparently Al8B4C7, was observed as streaks in the microstructure. It is suggested that a liquid phase, which coexisted with this compound, enhanced densification.
Thermal Stress Fracture of Brittle Ceramics by Conductive Heat Transfer in a Liquid Metal Quenching Medium
Authors: Oguma Masaomi , Fairbanks Carolyn J. , Hasselman D. P. H.
Published: March 2005

Observed and calculated values of the critical quenching temperature difference for circular rod specimens of a soda‐lime‐silica glass and polycrystalline alumina quenched in a liquid Pb‐Sn alloy indicate that thermal stress fracture was caused by conductive heat transfer, in support of the original theory of Hencke, Thomas, and Hasselman.
Effect of Air Bubbling on Circulation and Heat Transfer in a Glass‐Melting Tank
Authors: UNGAN A. , VISKANTA R.
Published: March 2005

This paper reports a numerical methodology to simulate the effect of air bubblers on glass‐melt circulation and heat transfer in a glass‐melting tank. Considering the high velocity and short residence time of the rising air bubbles, heat transfer and mass transfer between air bubbles and the glass melt are neglected. Using existing empirical relations, the momentum exchange between a stream of air bubbles and the glass melt is calculated by considering bubble shape, size, and relative velocity and the thermophysical properties of the air and glass melt. Representative results of three‐dimensional numerical simulations for flow and heat transfer with and without air bubblers are presented for a container‐glass‐melting tank. The results show a significant effect of air bubblers on glass circulation patterns and the enhancement of heat transfer from the combustion space to the molten glass and from the molten glass to the batch.
Sintering of ZrO2 in HCl Atmospheres
Authors: READEY MICHAEL J. , READEY DENNIS W.
Published: March 2005

Pure zirconia fired in HCl shows greatly reduced shrinkage compared to the same material fired in air. One of the main microstructural differences is the much more rapid grain growth in the porous zirconia fired in HCl. The observed grain growth does not follow any simple growth law over the temperature range investigated. The grains grow up to a limiting particle size close to the initial agglomerate size arid appear to stop.
Stress and Fracture Behavior of Monolithic Fuel Cell Tapes
Authors: MAJUMDAR SAURIN , CLAAR TERRY , FLANDERMEYER BRIAN
Published: March 2005

This paper summarizes the stress and fracture analyses of a three‐layer ceramic composite which is a fundamental building block for the monolithic solid oxide fuel tell currently being developed at Argonne National Laboratory. A small difference in the coefficients of thermal expansion of different layers can result in large stresses and cause cracking parallel to the plane of the layers. A simple fracture model for a three‐layer composite has been developed. The cracking behavior predicted by the model is in reasonably good agreement with the fabrication experience to date.
Laser Synthesis of Si/C/N Powders from 1,1,1,3,3,3‐Hexamethyldisilazane
Authors: Rice Gary W.
Published: March 2005

Laser‐driven pyrolysis of the organosilicon compound 1,1,1,3,3,3‐hexamethyldisilazane ((Me3Si)2NH) yields amorphous fine powders (0.1 μm) containing Si, C, and N. Reactant Si and N retentions are 100% and 90%, respectively. The C content of the powder can be varied by changing the pyrolysis temperature, and the N content can be varied by adding NH3 to the reactant gas. Upon firing, SiC is produced, with unusually facile loss of nitrogen from the powder.
Nucieation and Growth of Cracks in Vitreous‐Bonded Aluminum Oxide at Elevated Temperatures
Authors: JAKUS K. , WIEDERHORN S. M. , HOCKEY B. J.
Published: March 2005

The nucleation and growth of cracks was studied at elevated temperatures on a grade of vitreous‐bonded aluminum oxide that contained ∼8 vol% glass at the grain boundaries. Cracks were observed to nucleate within the vitreous phase, close to the tensile surface of the flexural test specimens used in these experiments. Crack nucleation occurred at a strain of ∼0.08% to 0.12% which corresponded to a crack nucleation time of ∼35% of the time to failure by creep rupture. Once nucleated, cracks propagated along grain boundaries, as long as the stress for crack propagation was maintained. The crack velocity for cracks that were nucleated by the creep process was found to be linearly proportional to the apparent stress intensity factor, whereas for cracks that were nucleated by indentation, the crack velocity was proportional to the fourth power of the apparent stress intensity factor.
Effect of Second‐Phase Particles on Grain Growth in Calcite
Authors: Olgaard David L. , Evans Brian
Published: March 2005

Grain growth in polycrystalline caicite (CaCO3) at 800°C and 300‐MPa confining pressure is inhibited by the addition of Al2O3 particles; volume fractions of second phase (f) ranged from 0.02 to 10.0 vol%, and several powders with different average particle size were used. The stable grain size reached during heat treatment was inversely proportional to l/fm where m varied from 0.3 to 0.55 — agreeing with results from other grain growth experiments in ceramics and metals, and with models developed for particles located at grain boundaries and grain corners, but not agreeing with models developed for random dispersions. The dependence of the stable grain size on second‐phase particle size in these experiments was less systematic, possibly because of variations in the particle size distributions used, or because of particle agglomeration effects. In the single‐phase aggregates, the growth kinetics were consistent with a normal grain growth equation with n=3, although uncertainties were large.
Theory of Filtration of Ceramics: I, Slip Casting
Authors: TILLER FRANK M. , TSAI CHUN‐DAR
Published: March 2005

The theory of colloidal filtration in slip casting of ceramics as developed by Aksay and Schilling for incompressible cakes has been extended to compressible materials. The rate of cake deposition in the mold depends upon the capillary pressure and permeability of the mold. Assuming that the capillary pressure is inversely proportional to an average diameter typifying the mold and that the permeability is directly proportional to the square of the diameter, an optimum diameter exists for production of maximum pressure drop across the cake and maximum rate of deposition.
Effective Fracture Toughness of Microcracked Materials
Authors: ROSE L. R. F.
Published: March 2005

Explicit analytical formulas are derived for the stress intensity factors at the tips of a main crack and of a microcrack for the two‐dimensional case of a collinear microcrack. This configuration is used to derive an estimate of the toughness degradation due to microcracks linking up with an advancing main crack. The implications of this estimate for theoretical predictions of the toughening due to stress‐induced micro‐cracking are discussed.
Exaggerated Grain Growth in ZrO2‐Toughened Al2O3
Authors: KIBBEL B. , HEUER A. H.
Published: March 2005

Annealing of ZrO2‐toughened Al2O3 (ZTA) at elevated temperatures causes growth of both the intergranular ZrO2 particles and the Al2O3“matrix” grains. Exaggerated (“breakaway”) grain growth occurs in some, but not all, specimens. Analytical electron microscopy of two ZTA's, both of which contained a continuous amorphous (glassy) grain‐boundary phase, but only one of which showed breakaway grain growth, revealed that the occurrence of breakaway grain growth could be correlated with the chemistry of the ubiquitous glassy grain‐boundary phase.
Phase Equilibration in ZrO2‐Y2O3 Alloys by Liquid‐Film Migration
Authors: CHAIM R. , HEUER A. H. , BRANDON D. G.
Published: March 2005

The tetragonal (t) and cubic (c) ZrO2 solid solutions in two‐phase ZrO2‐8 wt% Y2O3 ceramics have low and high solute content, respectively. Annealing samples sintered at 1600°C between 700° and 1400°C requires a change in the volume fraction of the coexisting phases, as well as their equilibrium Y2O3 content. The enrichment in Y2O3 content of the c‐ZrO2 grains is accomplished by liquid‐film migration involving the ubiquitous silicate grain‐boundary phase, while the volume fraction of t‐ZrO2 increases by the nucleation and growth of cap‐shaped t‐ZrO2 lenses. The interfaces between the c‐ZrO2 matrix and the growing t‐ZrO2 lenses are semicoherent.
Processing and Microstructure of Mullite‐Zirconia Composites Prepared from Sol‐Gel Powders
Authors: YUAN QI‐MING , TAN JIA‐QI , SHEN JI‐YAO , ZHU XUAN‐HUI , YANG ZHENG‐FANG
Published: March 2005

A high‐purity stoichiometric mullite precursor was obtained by hydrolysis of the alkoxides Al(OC3H7)3 and Si(OC2H2)4. Fully sintered mullite ceramics can be prepared from sol‐gel powders by sintering them at 1600°C for 4 h in air with the addition of 15 to 20 Vol% ZrO2 or 1 to 3 mol% Y2O3 or both. Introduction of 1 to 3 mol% Y2O3 aids the retention of tetragonal ZrO2; the volume fraction of t‐ZrO2 retained increases with increasing Y2O3 content. The maximum t‐ZrO2 retained reaches 34% in a matrix of synthetic mullite with 3 mol% Y2O3, but most of this t‐ZrO2 does not undergo stress‐induced transformation during grinding.
Preparation of Mullite‐Zirconia Composites from Glass Powder
Authors: McPHERSON R.
Published: March 2005

Glass powders with the composition mullite/5 wt% ZrO2, prepared by rapid solidification, were used to prepare a poly‐crystalline ceramic by hot‐pressing to 1040°C. The as‐prepared structure consisted of a fine‐grain‐sized (∼0.1 μm) solid solution of ZrO2 in a tetragonal form of mullite. Heat treatment between 1300° and 1660°C resulted in a range of microstructures consisting of tetragonal ZrO2 particles dispersed in mullite. Transformable tetragonal ZrO2 was observed only after heat treatment at 1600°C.
Mathematical Model of Viscosity Measurements for Viscoelastic Solids
Authors: Hsueh Chun‐Hway
Published: March 2005

A methodology is described for determining the viscosity of viscoelastic solids. A three‐parameter model consisting of two springs and one dashpot is constructed to represent the viscoelastic material. With this model, the viscosity can be determined from the experimental stress‐time curve generated from short‐term loading at a constant strain rate, followed by holding the total strain. The feasibility of this model to represent viscoelastic ceramics is also addressed.
Thermal Stresses in Circular Ceramic Rod Specimens Subjected to Sudden Radiative Cooling
Authors: Welsh Theresa M. , Knight Charles E. , Hasselman D. P. H.
Published: March 2005

The thermal stresses which result from sudden radiation heat losses in a ceramic circular cylinder were calculated by the finite‐element method. It was shown that, for a given value of coefficient of thermal expansion and Young's modulus, the magnitude of the maximum stress increases with increasing emissivity and cylinder diameter and decreasing thermal conductivity. In terms of the initial temperature, T, the maximum stress was found to be proportional to Tx, with 3.3 ≤x≤ 3.5.
Mechanism of Dense Magnesia Layer Formation near the Surface of Magnesia‐Carbon Brick
Authors: Watanabe Akira , Takahashi Hlrokuni , Nakatani Fumio
Published: March 2005

After operation of a basic oxygen furnace with a MgO‐C brick lining, formation of a dense MgO layer is observed near the working surface. It is affected by soak atmosphere gas in the first stage of operation. As operation of the furnace proceeds, the MgO‐C brick being submitted to cyclical heating and cooling, internal oxidation‐reduction reactions of MgO and C occur. As a result, needlelike crystals of MgO grow at the interface between the dense MgO layer and the original layer, resulting in further growth of the densi MgO layer.
On the Equilibrium Thickness of Intergranular Glass Phases in Ceramic Materials
Authors: CLARKE DAVID R.
Published: March 2005

The fundamental question as to whether thin intergranular films can adopt an equilibrium thickness in polycrystalline ceramics is addressed. Two continuum approaches are presented, one based on interfacial energies and the other on the force balance normal to the boundary. These indicate that there will exist a stable thickness for the intergranular film and that it will be of the order of 1 nm. The origin of an equilibrium thickness is shown to be the result of two competing interactions, an attractive van der Waals‐disperson interaction between the grains on either side of the boundary acting to thin the film and a repulsive term, due to the structure of the intergranular liquid, opposing this attraction. As both of these interactions are of short range (<10 nm), it is a natural consequence that the equilibrium thickness is of the order of 1 nm, a value commensurate with that observed experimentally in a wide range of ceramics. Two further consequences of importance arc indicated. The first is that thin intergranular liquid phases can support a normal stress. The second is that the dielectric constants of the adjacent grains play an important role in determining the thicknesses of the intergranular phase. This leads to the conclusion, consistent with observations, that the thickness of the intergranular phase in polyphase ceramics is expected to be different at boundaries between dissimilar phases than that between like phases.
Sintering with Rigid Inclusions
Authors: SCHERERA GEORGE W.
Published: March 2005

Rigid inclusions retard the densification of a sintering body by creating a hydrostatic tensile stress in the matrix. Two models of this process are presented and compared with others from the literature. The composite can be represented by a composite sphere, with the core representing the inclusion. Alternatively, a self‐consistent (s‐c) calculation can be performed in which the sintering material is regarded as being surrounded by a composite matrix with a slower densification rate. The results differ only in that the shear modulus of the matrix is replaced by the shear modulus of the composite in the s‐c calculation. The stress in the inclusion cannot exceed twice the sintering pressure, unless the Poisson's ratio of the matrix is negative. Predictions of higher stresses by previous authors carry the erroneous implication that Poisson's ratio is negative. Since the predicted stresses are not large, models of this type cannot account for the experimentally observed retardation of densification of polycrystalline matrices by inclusions.
Effect of Condensed Silica Fume on the Microstructure of the Interfacial Zone in Portland Cement Mortars
Authors: BENTUR ARNON , COHEN M. D.
Published: March 2005

The development of the microstructure at the cement paste‐sand grain interfacial zone was studied, and the effect of condensed silica fume was evaluated. In portland cement mortars, the microstructure of the interfacial zone, extending to about 20 to 50 μm from the sand grain surface, was significantly different from that of the bulk paste, and it was characterized by a massive Ca(OH)2 layer engulfing the sand grain and by some channel‐type gaps. When 15% condensed silica fume was added, the interfacial zone had a homogeneous and dense microstructure, similar to that of the bulk paste, without the presence of a massive Ca(OH)2 layer or gaps. On the basis of observations at early ages, it is suggested that these characteristics were the result of differences in the nature of the fresh paste, which was more prone to bleeding in the portland cement mortar.
Creep and Densification During Sintering of Glass Powder Compacts
Authors: RAHAMAN MOHAMED N. , DE JONGHE LUTGARD C. , SCHERER GEORGE W. , BROOK RICHARD J.
Published: March 2005

The simultaneous creep and densification of glass powder compacts was studied as a function of low applied uniaxial stress, temperature, and particle size. The creep rate can be expressed as the sum of the contribution from the applied stress that varies linearly with stress, and a contribution due to anisotropic densification that varies linearly with the densification rate. For a constant applied stress, the ratio of the creep rate to the densification rate is almost independent of both termperature and density. While these observations are consistent with the model of Scherer for the viscous sintering of glass, other observations show significant deviations from the model. Both the densification rate and the creep viscosity, which has an exponential dependence on porosity, show much stronger dependence on density compared with theoretical predictions.
Fatigue Crack Propagation in Transformation‐Toughened Zirconia Ceramic
Authors: Dauskardt R. H. , Yu W. , Ritchie R. O.
Published: March 2005

Fatigue crack propagation under tension‐tension loading is observed in a transformation‐toughened partially stabilized zirconia (PSZ) ceramic containing 9 mol% MgO. Such subcritical crack growth behavior is demonstrated to be cyclically induced, based on a comparison with behavior under sustained loading (at the maximum load in the fatigue cycle) and at varying cyclic frequencies. Crack extension rates, which are measured as a function of the cyclic stress intensity range ΔK over the range 10‐10 to 10‐6 m/cycle, are found to be load ratio dependent and to show evidence of fatigue crack closure, similar to behavior in metals. Cyclic crack growth rates are observed at ΔK levels as low as 3 MPa m1/2 and are typically many orders of magnitude faster than reported data on environmentally assisted, subcritical crack growth in PSZ under sustained‐load conditions.
Effects of Elemental Additives on Electrical Resistivity of Silicon Carbide Ceramics
Authors: Takeda Yukio , Nakamura Kousuke , Maeda Kunihiro , Matsushita Yasuo
Published: March 2005

Effects of various elemental additives on the electrical resistivity of hot‐pressed SiC ceramics were studied. The electrical resistivity at room temperature of dense SiC ceramics varied greatly depending on the additives used. SiC ceramics with added Be had an extremely high electrical resistivity of 3 × 1012°.cm. On the other hand, SiC ceramics with added B and Al had electrical resistivities of 2 × 10 and 0.8 °cm, respectively. The differences in the electrical resistivity of the dense SiC ceramics were considered to be due to different solubilities of the additives in SiC grains. SiC ceramics with added Be had a low level of impurities in the SiC grains as a result of the low solubility of Be in these grains.
Dissolution Kinetics of TiO2 in HF‐HC1 Solutions
Authors: BRIGHT ERIC , READEY DENNIS W.
Published: March 2005

The dissolution kinetics of TiO2 powders in HF‐HCl solutions was studied as a function of time, temperature, and acid concentration. The rate of dissolution was linear with time, directly proportional to [HF], independent of [HCl], and exhibited an exponential temperature dependence with an activation energy of 74.9 kj/mol. The results are explained in terms of a surface reaction mechanism involving an activated complex at a cation surface site formed by the substitution of an HF molecule into the coordination sphere of the titanium ion.
Sintering TiO2 in HCl Atmospheres
Authors: Readey Michael J. , Readey Dennis W.
Published: March 2005

TiO2 was sintered in HCl atmospheres to enhance the effects of vapor transport. Little or no densification is observed for temperatures between 1000° and 1300°C. Particle coarsening occurs at temperatures above 1200°C. The apparent activation energy for particle growth is 114 kJ/mol. It is concluded that the primary mass‐transport mechanism is vapor transport while the particle growth rate is limited by grain‐boundary mobility.
Elastic Modulus‐Porosity Relation in Polycrystalline Rare‐Earth Oxides
Authors: Phani K. K. , Niyogi S. K.
Published: March 2005

Many equations have been previously proposed to describe the elastic modulus‐porosity relation of brittle solids. The equations arc not applicable to all materials over a wide range of porosity. A new equation representing elastic modulus‐porosity relation of brittle solids has been proposed. It has been shown that the proposed equation describes best the Young's modulus‐porosity data of six rare‐earth oxides over a wide range of porosity reported by previous investigators. The equation suggests that there was some kind of ordered packing in all the oxides.
Calcium as an Acceptor Impurity in BaTiO3
Authors: HAN YOUNG HO , APPLEBY JOHN B. , SMYTH DONALD M.
Published: March 2005

The equilibrium electrical conductivity of polycrystalline, calcium‐doped BaTiO3 was studied over the oxygen partial pressure range 10‐13 to 105 Pa and the temperature range 800° to 1000°C. There is little effect if CaO is substituted for a corresponding amount of BaO, i.e., Ba, 1‐xCaxTiO3. If CaO is substituted for a corresponding amount of the TiO2 content, i.e., BaTi1‐xCaxO3‐x, the equilibrium conductivity shows strong evidence of acceptor‐doped behavior. If the corresponding amount of excess CaO is added to stoichiometric BaTiO3, i.e., BaCaxTiO3+x, the conductivity profiles are very close to those for samples with TiO2 replaced by CaO, and show highly acceptor‐doped behavior. This is in agreement with the replacement of a small amount of Ti by Ca2+ on the octahedral B‐sites of BaTiO3, where it acts as an acceptor center, CaT
Temperature‐Dependent Iron Solubility in Mullite
Authors: Scheider Hartmut
Published: March 2005

Sample disks prepared from Al2O3 (61 wt%), SiO2 (28 wt%), and Fe2O3(II wt%) powders were sintered at 1270° and 1440°C and then annealed between 1300° and 1670°C. The annealed samples consisted of mullite as the main compound with minor amounts of glass and sometimes magnetite. The iron content of the mullites decreases strongly from ∼ 10.5 wt% Fe2O3 at 1300°C to ∼ 2.5 wt% Fe2O3 at 1670°C. A complex temperature‐controlled exsolution mechanism of iron from mullite is considered.
Constitutive Model for Predicting Ultimate Drying Shrinkage of Concrete
Authors: HANSEN WILL
Published: March 2005

A constitutive model is derived from theory of elasticity for predicting ultimate drying shrinkage of concrete. The model was extended by incorporating the semiempirical composite model proposed by Hirsch and Dougill for predicting Young's modulus of concrete. Their composite model is the geometric mean of Paul's upper and lower limit boundaries of a two‐phase composite. According to the shrinkage model the parameters needed for predicting ultimate drying shrinkage of concrete at any relative humidity of drying are the following: ultimate shrinkage of a paste of same water‐to‐cement (W/C; ratio and degree of hydration as the concrete, relative volume of aggregates and unhydrated cement, and the elastic properties of hydrated paste and the particles. The shrinkage model was tested on shrinkage results obtained in this study and by Pickett. Three different W/C ratios were covered together with a wide range in aggregate contents. Excellent agreement with the results was found.
Thermal Shock Behavior of an Alumina‐SiC Whisker Composite
Authors: Tiegs T. N. , Becher P. F.
Published: March 2005

Thermal shock testing of an alumina‐20 vol% SiC whisker composite showed no decrease in flexural strength with temperature differences up to 900°C. Alumina, on the other hand, normally shows a significant decrease inflexural strength with a temperature change of >400°C. The improvement in the thermal shock resistance of the composite is believed attributable to the increased fracture toughness of this material.
Formation and Transformation of δ‐Ta2O5 Solid Solution in the System Ta2O5‐Al2O3
Authors: Yamaguchi Osamu , Tomihisa Daijo , Uegaki Tatsuji , Shimizu Kiyoshi
Published: March 2005

In the system Ta2O3‐Al2O5 solid solutions of metastable δ‐Ta2O5 (hexagonal) are formed up to 50 mol% Al2O3 from amorphous materials prepared by the simultaneous hydrolysis of tantalum and aluminum alkoxides. The values of the lattice parameters decrease linearly with increasing Al2O3, content. The to β‐Ta2O5 (orthorhombic, low‐temperature form) transformation occurs at ∼950°C. The solid solution containing 50 mol% Al2O3 transforms at 1040° to 1100°C to orthorhombic TaAlO4. Orthorhombic TaAlO4 contains octahedral TaO6 groups in the structure.
Hydrothermal Dissolution of Perovskite: Implications for Synroc Formulation
Authors: Kastrissios Theodora , Stephenson Mark , Turner Peter S. , White Timothy J.
Published: March 2005

The results of a transmission electron microscope study of the hydrothermal alteration products formed by perovskite and two titantate‐based “synroc” formulations are reported. It was found that perovskite, the host phase for strontium‐90, dissolved at combined temperatures and pressures greater than 110°C and 180 kPa. Dissolution of perovskite was accompanied by an epitaxial crystallization of the titanium dioxide polymorphs, brookite and anatase. Hollandite, which incorporates cesium‐137, dissolved more slowly than perovskite, with only minor TiO2 crystallization. Intergrowths of zirconolite and zirkelite, the principal actinide‐containing phases, were highly resistant to hydrothermal treatments, maintaining their integrity under all experimental conditions.
Effect of Oxygen Contamination on Densification of TiB2
Authors: BAIK SUNGGI , BECHER PAUL F.
Published: March 2005

In order to identify the mechanisms behind the adverse effect of oxygen contamination on the densification of TiB2 ceramics, densification and grain growth behavior during pressureless sintering (1700° to 2050°C) were examined as a function of oxygen content of the powder and compared with those observed during hot‐pressing (1400° to 1700°C). Marked difference in the grain size‐density relations was observed between the two processing methods. Specific roles of oxygen in various kinetic processes were analyzed and the likely oxygen‐bearing species were also identified.
A New Modified Weibull Distribution Function
Authors: Phani K. K.
Published: March 2005

The tensile strength of two groups of fused silica optical fibers, reported by previous workers, has been analyzed in terms of a modified Weibull flaw distribution function. The function provides upper and lower strength limits and is characterized by two shape and two location parameters. The long‐length strengths predicted from this function are in close agreement with the experimental data.
Toughening Behavior Involving Multiple Mechanisms: Whisker Reinforcement and Zirconia Toughening
Authors: BECHER P. F. , TIEGS T. N.
Published: March 2005

Since the contribution of transformation toughening increases with the loca crack resistance (which is proportional to the toughness of the matrix), ZrO2 particles were added to a toughened, whisker‐reinforced ceramic matrix. Analysis revealed that the combination of these multiple toughening agents should result in ceramic composites tougher than (1) that achieved by either mechanism by itself or (2) the sum of the two processes. The toughness of mullite could be increased 1.8‐and 2.4‐fold with a 20 vol% addition of ZrO2 particles or SiC whiskers, respectively. However, when 20 vol% of both ZrO2 particles and SiC whiskers were added, the toughness was increased at least 3‐fold with monoclinic m‐ZrO2 and by >5‐fold with tetragonal t‐ZrO2. The differences in the toughening achieved when t‐ZrO2 vs m‐ZrO2 is present in the SiC‐whisker‐reinforced mullite are attributed to differences in their interdependencies upon the whisker reinforcement.
Analysis of the Sintering Pressure
Authors: Raj Rishi
Published: March 2005

When the sintering of a polycrystalline body is analyzed by coupling the change in interface energies and the change in total volume of the polycrystal, it is discovered that the sintering pressure is a sum of two terms, one deriving from the surface curvature of the pores and the other depending on the size of the grains. When pores are present at two‐grain, three‐grain, or four‐grain junctions, and when the number of pores of each type per particle remains constant, the sintering pressure is expressed by a simple equation. This result may suggest new approaches to the problem of grain growth during the sintering process. The result also highlights a fundamental difference between the sintering of ceramics and the sintering of glasses.
Microcracking in Ceramics Induced by Thermal Expansion or Elastic Anisotropy
Authors: TVERGAARD VIGGO , HUTCHINSON JOHN W.
Published: March 2005

The effect of crystal anisotropy on the formation of grain‐boundary microcracks is analyzed, by considering a planar array of hexagonal grains as a model of a polycrystalline ceramic. The stress singularities at triple‐grain junctions are analyzed by an asymptotic method as well as by a numerical solution, and the critical size of a grain‐boundary defect is investigated by a crack analysis. It is found that elastic anisot‐ropies can significantly increase the stress levels near triple points, which results in a smaller critical grain size for microcracking.
Rising Fracture Toughness from the Bending Strength of Indented Alumina Beams
Authors: KRAUSE RALPH F.
Published: March 2005

The analytical function of crack extension to a fractional power is used to represent the fracture resistance of a vitreous‐bonded 96% alumina ceramic. A varying flaw size, controlled by Vickers indentation loading between 3 and 300 N, was placed on the prospective tensile surfaces of four‐point bend specimens, previously polished and annealed. The lengths of surface cracks were measured by optical microscopy. Straight lines were fitted to the logarithmic functions of observed bending strength versus indentation load in two series of experiments: (I) including the residual stress due to indentation and (II) having the residual stress annealed out at an elevated temperature. Within the precision of measurement these lines have the same slope, being about 32% less than the ‐1/3 slope which a fracture toughness independent of crack extension would indicate. Considering the criteria for crack extension and specimen failure, the fracture mechanics equations were solved for the conditions of the two series of experiments. Approximately the same values of fracture toughness, rising as a function of indentation flaw size, were obtained from both series of experiments.
Evaluation of Bridging Stress from R‐Curve Behavior for Nontransforming Ceramics
Authors: Hsueh Chun‐Hway , Becher Paul F.
Published: March 2005

Bridging stresses at the crack interface behind the crack tip are calculated for nontransforming ceramics which show increasing toughness with crack extension (R‐curve behavior). With this approach, bridging stresses can be related analytically to the experimental R curve. Sample calculations for SiC‐whisker‐reinforced Al2O3, with a bridging‐zone length of >0.5 mm, are shown in the present study. The results indicate that the bridging stress has a maximum value within a short distance (<0.01 mm) behind the crack tip and then decreases to 0 or levels off to a finite value at the end of the bridging zone depending upon the nature of the R‐curve behavior.
Ammonolysis Products of the Dialkylamides of Titanium, Zirconium, and Niobium as Precursors to Metal Nitrides
Authors: BROWN GILBERT M. , MAYA LEON
Published: March 2005

The dialkylamides of titanium(IV), zirconium(IV), and niobium(V) react with liquid ammonia to produce solid products of high metal content. Infrared spectroscopy and elemental analysis indicate that these ammonolysis products are nitridoor imido‐bridged polymers. Based on metal content, pyrolysis at <800°C converts these materials quantitatively to the corresponding nitride or carbonitride. The only products trapped following vacuum pyrolysis are ammonia and dialkylamine. Temperature‐programmed pyrolysis with analysis of the evolved gases by mass spectroscopy shows that the composition of the gaseous products depends on whether helium or hydrogen is used as the carrier gas. The presence of a hydrogen atmosphere compared to a helium environment or a dynamic vacuum results in a lower carbon content of the titanium and niobium products. A general reaction scheme for conversion of the precursor compounds to refractory materials is formulated from the results of the various pyrolytic experiments.
Toughening Behavior in Whisker‐Reinforced Ceramic Matrix Composites
Authors: BECHER PAUL F. , HSUEH CHUN‐HWAY , ANGELINI PETER , TIEGS TERRY N.
Published: March 2005

The toughening behavior of whisker‐reinforced ceramics is analyzed in terms of a whisker bridging zone immediately behind the crack tip. This approach is consistent with microscopy observations which reveal that intact bridging whiskers exist behind the crack tip as a result of debonding of the whisker‐matrix interface. The theoretical results based on both the stress intensity and the energy change introduced by bridging whiskers reveal the dependence of toughening upon the composition and matrix, interface, and whisker properties. Furthermore, the analytical models of whisker bridging accompanied by very limited pullout accurately describe experimental observations of the toughening behavior in several SiC‐whisker‐reinforced ceramics. Such analytical descriptions also indicate that increases in whisker size and strength and modification of interface properties will result in further increases in toughness by whisker reinforcement.
Reactions of Silicon‐Based Ceramics in Mixed Oxidation Chlorination Environments
Authors: MARRA JOHN E. , KREIDLER ERIC R. , JACOBSON NATHAN S. , FOX DENNIS S.
Published: March 2005

The reaction of silicon‐based ceramics with 2% Cl2/Ar and 1% Cl2/1% to 20% O2/Ar at 950 °C was studied with thermogravimetric analysis and high‐pressure mass spectrometry. Pure Si, SiO2, several types of SiC, and Si3N4 were examined. The primary corrosion products were SiCl4(g) and SiO2(s) with smaller amounts of volatile silicon oxychlorides. The reactions appear to occur by chlorine penetration of the SiO2 layer, and gas‐phase diffusion of the silicon chlorides away from the sample appears to be rate limiting. Pure SiO2 shows very little reaction with Cl2. SiC with excess Si is more reactive than the other materials with Cl2, whereas SiC with excess carbon is more reactive than the other materials with Cl2/O2. Si3N4 shows very little reaction with Cl2. These diferences are explained on the basis of thermodynamic and microstructural factors.
Fast, Radiation‐Hard Scintillating Detector: A Potential Application for Sol‐Gel Glass
Authors: NOGUES J.‐L. , MAJEWSKI S. , WALKER J.K. , BOWEN M. , WOJCIK R. , MORESHEAD W.V.
Published: March 2005

To meet the present‐day rate and radiation dose requirements, one would like to have a scintillator with the high radiation resistance of pure silica glass and the short fluorescence decay time of organic fluors. The new technology of sol‐gel glass production permits the preparation of organic/inorganic composite materials by doping the silica gels with organic fluors. The procedure of production of monolithic organic/inorganic composite materials, such as 2‐(4‐tertbutylphenyl)‐5‐(4‐biphenylyl)‐1,3,4‐oxadiazole/SiO2, p‐terphenyl/SiO2, p‐quaterphenyl/SiO2, and 3‐hydroxyflavone/SiO2, and the associated characterization techniques are described. The efficiency of light production, the light attenuation, the time width of light pulse, and the radiation resistance of the scintillating sol‐gel glass are discussed. Preliminary results indicate that this technical advance will permit the creation of new materials with potential application such as particle detectors which are both radiation hard and fast.
Pulverization and Dispersion Technique for Agglomerated Particles of Alumina Powder in a Slurry
Authors: Ueyama T. , Wada H. , Kaneko N.
Published: March 2005

A technique was developed to pulverize agglomerates in a slurry into primary particles. Pulverization increased the powder packing density of alumina green sheets from 50 to 61 vol%. The sinterability of alumina was also increased. The internal voids in the sintered body were drastically reduced.
Mechanical Properties of a Uniaxially Reinforced Mullite‐Silicon Carbide Composite
Authors: Singh Raj N. , Gaddipati Achuta R.
Published: March 2005

Mechanical properties of a mullite ceramic composite uniaxially reinforced with silicon carbide filaments were investigated in flexure mode. Significant improvements in strength and toughness were observed for the composite in comparison to monolithic mullite ceramic. Flexure strength values between 646 and 855 MPa for the composite and between 202 and 271 MPa for the monolithic mullite were obtained. Influence of fiber–matrix interfacial shear stress on the strength, strain to failure, mode of failure, and toughness were studied.
Raman Spectroscopy of Higher Titanate Phases in the BaTiO3‐TiO2 System
Authors: JAVADPOUR JAFAR , EROR NICHOLAS G.
Published: March 2005

Polytitanates in the BaO‐TiO2 system with Ba: Ti ratios ranging from 1:2 to 1:5 were prepared employing the liquid mix technique. The samples were heated at 600° to 1300°C in oxygen, and Raman spectroscopy was used to investigate the phase relations in the system. The powders of various compounds were amorphous at temperatures less than 600°C. They crystallized into a single phase or a mixture of phases at 700°C. These mixtures underwent further phase transformation at higher temperatures to form single‐phase compounds. Details of the procedure for sample preparation and characterization of the products are described. The results from the present study are compared with previously published data.
Stability of Aqueous α‐Al2O3 Suspensions with Poly(methacrylic acid) Polyelectrolyte
Authors: CESARANO JOSEPH , AKSAY ILHAN A. , BLEIER ALAN
Published: March 2005

Stability of aqueous α‐Al2O3 suspensions with Na+ salt of poly(methacrylic acid) (PMAA‐Na) polyelectrolyte was studied as a function of pH. At a given pH, the transition from the flocculated to the dispersed state corresponded to the adsorption saturation limit of the powders by the PMAA. As the pH was decreased, the adsorption saturation limit increased until insolubility and charge neutralization of the PMAA was approached. The critical amount of PMAA required to achieve stability is outlined in a stability map.
Characterization of the Grain Boundary in Shock‐Compacted Diamond
Authors: Sawai Sumiichi , Kondo K.
Published: March 2005

Microscopic observations of shocked diamond compacts with an initial grain size of 2 to 4 μm have revealed that the grain boundaries consist of three phases: (1) extremely small, recrystallized diamond particles with a uniform size of 50 nm, (2) a mixture of the recrystallized andlor fragmented diamond and low‐density phase of carbon, and (3) low‐density phases of carbon only. These differences are caused by the pore‐size distribution of the green compact.
Phase Equilibria and Ordering in the System Zirconia‐Hafnia‐Yttria
Authors: TRUBELJA MLADEN F. , STUBICAN VLADIMIR S.
Published: March 2005

The phase equilibria in the system zirconia‐hafnia‐yttria were investigated at 1300° and 1600°C by precise lattice parameter measurements and the direct phase‐inspection method. Large fluorite solid solution regions were dominant at both investigated temperatures. The extent of ordering within the ternary system was studied in the samples containing 40 mol% yttria at 1300°C. The δ‐phase (Zr,Hf)3Y4O12 formation was established only in those samples with less than 15 mol% hafnia.
Sol‐Gel Synthesis of Magnesium Oxide‐Silicon Dioxide Glass Compositions
Authors: BANSAL NAROTTAM P.
Published: March 2005

MgO‐SiO2 glasses containing up to 15 mol% MgO, which could not have been prepared by the conventional glass‐melting method because of the presence of stable liquid‐liquid immiscibility, have been synthesized by the sol‐gel technique. The gelling time decreased with increase in magnesium content, water: TEOS ratio, and reaction temperature. The gels were subjected to various thermal treatments and characterized using DTA, TGA, IR spectroscopy, XRD, and surfacearea‐ and port‐size‐distribution measurements. No organic groups could be detected in the glasses after heat treatments to ∼800°C, but trace amounts of hydroxyl groups were still present. The overall activation energy for gel formation in the 10MgO·90SiO2 (mol%) system for the water: TEOS mole ratio of 7.5 was calculated to be 58.7 kJ/mol.
Grain Growth of the Lime and Periclase Phases in a Synthetic Doloma
Authors: BALDO JOAO B. , BRADT RICHARD C.
Published: March 2005

Simultaneous grain growth of the lime (CaO) and periclase (MgO) phases was studied in a synthetic, hydroxide‐derived doloma over the temperature range 1400° to 1700°C. The grain growth kinetic exponents were 5 and 6 and the activation energies 333±45 and 437±47 kJ/mol for the CaO and MgO phases, respectively. The large kinetic exponents are attributed to the topological restraints of the two phases on the growth of one another. The parameters suggest that the grain growth of the CaO may be governed by Ca2+ diffusion within the CaO phase for it is the continuous phase in the microstructure. In most instances for grain growth of the MgO phase, the Mg2+ must diffuse through the lime phase, and this process is believed to be limited by movement of associated defects involving Mg2+ within the CaO structure.
Radiation Effects on the Physical Properties of Low‐Expansion‐Coefficient Glasses and Ceramics
Authors: HIGBY PAIGE L. , FRIEBELE E. JOSEPH , SHAW CATHY M. , RAJARAM MOHAN , GRAHAM EARL K. , KINSER DONALD L. , WOLFF ERNEST G.
Published: March 2005

The effects of irradiation on the coefficient of thermal expansion (CTE), the density, and the elastic moduli of low‐CTE materials such as amorphous SiO2, ultra‐low‐expansion glass, β‐SiC, Astrositall, and Zerodur were surveyed. It was found that the properties of all of these materials were affected by radiation exposures up to 2 × 109 rd; SiO2 and SiC were the most radiation‐resistant of the materials studied.
Mechanism of Grain Growth and α‐β’Transformation During Liquid‐Phase Sintering of β′‐Sialon
Authors: LEE DONG‐DUK , KANG SUK‐JOONG L. , YOON DUK N.
Published: March 2005

The rates of grain coarsening and α‐β′transformation during the liquid‐phase sintering of Si3N4‐β′60‐YAG sialon have been measured at varying liquid fractions and z values in order to determine the rate‐controlling mechanism. The average β′‐grain size after sintering for 16 h at 1650°C shows no variation with the liquid‐matrix fraction if the z value is fixed and a marked increase with the z value if the liquid fraction is fixed. Similarly, the amount of untransformed α‐phase after sintering for 2.5 or 3.5 min at 1600°C shows no variation with the liquid‐matrix fraction if the z value is fixed and a marked decrease with the z value if the liquid fraction is fixed. These results show that the grain coarsening and the α‐β′transformation are controlled by the interface reaction. This conclusion is consistent with the observations in carbide‐Co systems and with the theoretical predictions that the growth of faceted grains is controlled by interface reaction and that of spherical grains by diffusion. A general rule between the shape and the growth mechanism of grains in a liquid matrix is thus proposed.
Determinations of Residual Thermal Stresses in a SiC‐Al2O3 Composite Using Neutron Diffraction
Authors: MAJUMDAR SAURIN , KUPPERMAN DAVID , SING JITENDRA
Published: March 2005

Residual thermal stresses and strains, developed during cooling of a SiC whisker‐Al2O3 composite, were determined by an experimental neutron diffraction technique and the results were compared with analytically determined values. High compressive residual thermal stresses were generated in the whiskers during cooldown after sintering. Analytical estimates of the residual stresses were obtained by using two self‐consistent models: (1) a plane strain composite cylinder model and (2) Eshelby's ellipsoidal inclusion theory. The two models gave almost identical estimates of the stresses and strains in the whiskers. The interpretation of the measured strains in the whiskers by neutron diffraction had some uncertainty because of the lack of a clearly defined crystal structure of SiC.