bluuebunny/crossref_metadata_2025 huggingface dataset:
Internal Friction, Dielectric Loss, and Ionic Conductivity of Tetragonal ZrO2‐3% Y2O3 (Y‐TZP)
Authors: WELLER M. , SCHUBERT H.
Published: March 2005

The defect structure in 3 mol% Y‐TZP was studied by correlated internal friction, dielectric loss, and ionic conductivity experiments. A prominent mechanical and dielectric loss peak occurs in the temperature range between 380 and 550 K that depends on the frequency of measurement. The relaxation parameters were determined as Hm= 90 ± 3 kJ·mol−1, τ∞= (1.0+1.5−0.6) × 10−14 s for the mechanical relaxation and Hd= 84 ± 3 kJ·mol−1, τ∞= (1.6+1.7−0.9) × 10–13 s for the dielectric relaxation. The ionic conductivity below 790 K is controlled by an activation enthalpy of Hσ= 89 ± 3 kJ·mol−1; at higher temperatures Hσ= 60 ± 3 kJ·mol–1. An atomistic model is presented which assumes that oxygen vacancies are trapped by yttrium ions forming anisotropic complexes which—by reorientation—cause anelastic and dielectric relaxation. At higher temperatures (>790 K) these complexes are dissociated, which leads to the reduced activation enthalpy for ionic conductivity.
Dissolution and Reaction of Yttria‐Stabilized Zirconia Single Crystals in Hydrothermal Solutions
Authors: YOSHIMURA MASAHIRO , HIUGA TAKEHIRO , SOMIYA SHIGEYUKI
Published: March 2005

Dissolution and reaction of yttria‐stabilized zirconia (YSZ) single crystals were investigated in various solutions at 600° to 780°C under 100 MPa. YSZ crystals were not corroded in pure H2O and neutral solutions such a LiF, LiCl, NaNO3, KCl, KBr; K2SO4, and Na2SO4 even under severe conditions at 600°C, 100 MPa. They were, however, dissolved and reprecipitated in basic solutions such as NaF, K2CO3, KOH, NaOH, and LiOH with partial decompositon (destabilization in the last three solutions. YSZ crystals were completely decomposed into m‐ZrO2 in acidic solutions of Li2SO4, H2SO4, and HCl, whereas they reacted with the solution and formed other compounds in KF, NH4F, and H3PO4 solutions.
Mechanical properties of toughened ZrO2‐Y2O3 Ceramics
Authors: MASAKI TAKAKI
Published: March 2005

Tetragonal zirconia polycrystals were produced from high‐purity powders containing 1.5 to 5.0 mol% Y2O3 by cold isostatic pressing and sintering, hot‐pressing, and hot isostatic pressing. The mechanical properties and microstructures of these resulting materials were examined, with emphasis on the relation between strength and fracture toughness.
Crystallization and Transformation of Distorted Cubic PbTiO3
Authors: Yamaguchi Osamu , Narai Akira , Komatsu Toshikatsi , Shimizu Kiyoshi
Published: March 2005

Distorted cubic PbTiO3 crystallizes from an amorphous material prepared by the simultaneous hydrolysis of lead and titanium alkoxides. It can be preserved at room temperature by quenching and has a unit cell with a =0.3978 nm. The structure contains octahedral TiO6, groups.
Direct‐Current Conductivity of Y2O3 as a Function of Water Vapor Pressure
Authors: NORBY TRULS , KOFSTAD PER
Published: March 2005

The dc conductivity of a sintered sample of Y2O3 containing an excess of lower‐valent metal (Ml) impurities has been measured at 600° to 1100°C in air as a function of the water vapor pressure (0.4 to 3000 Pa). The logarithm of the p‐type conductivity vs log PH2o has a slope of – 1/2 in certain regions of FH2o and temperature. This reflects a defect situation where interstitial protons from water vapor balance the excess of lower‐valent cation impurities: [H˙] = [M1′]. The activation energy for the p‐type conductivity under these conditions is evaluated and interpreted in terms of the enthalpy needed to dissolve interstitial protons in yttria.
Rearrangement During Sintering in Two‐Dimensional Arrays
Authors: WEISER MARTIN W. , DE JONGHE LUTGARD C.
Published: March 2005

Densification of two‐dimensional arrays of nearly monosized copper spheres was examined by hot‐stage optical microscopy. The evolution of the particle arrangement was studied with computer methods, and statistical correlations were sought. Color graphics were found useful for displaying the spatial relationship of the local sintering parameters. It was found that differential densification was the major cause of rearrangement, rather than asymmetric neck formation.
Low‐Temperature Phase Relationships in the System ZrO2‐TiO2
Authors: McHALE ANNA E. , ROTH ROBERT S.
Published: March 2005

The phase relationships in the system ZrO2‐TiO2 near the compound ZrTiO4 have been clarified through an experimental study involving the characterization of both single‐crystal and powder specimens, the latter prepared through conventional solid‐state reaction and also by low‐temperature co‐precipitation methods. Zr1+xTi1‐xO4 (1/10 >x>‐1/6), having the α‐PbO2‐type structure, is found to transform on cooling between ∼1100° and ∼1150°C. Below this temperature there is an unusual, continuous phase transition leading to the formation of the stable low‐temperature phase ZrTi2O6. Low‐level doping with Y2O3 was found to enhance apparent cation ordering in intermediate compositions in the temperature range just below the phase transition.
Strength Characteristics of Transformation‐Toughened Zirconia
Authors: MARSHALL D. B.
Published: March 2005

Mechanisms of failure in a transformation‐toughened MgO‐partially‐stabiltzed ZrO2are identified using in situ observations. The observations are related to measured stress‐strain curves, strengths under various loading conditions, strength degradation from surface damage, and comparative strengths and toughnesses of toughened and overaged materials. Both reversible and irreversible tetragonal‐to‐monoclinic transformations, as well as microcracking, are associated with nonlinear stress‐strain curves. The influence of this nonlinear response on stresses produced by flexural loading is evaluated. Failure is preceded by stable growth of microcracks, resulting inR‐curve behavior and damage‐tolerant strength characteristics. The implications ofR‐curve response for strengthening and toughening characteristics are discussed.
Implications of Transformation Plasticity in ZrO2‐Containing Ceramics: I, Shear and Dilatation Effects
Authors: CHEN I‐WEI , MOREL P. E. REYES
Published: March 2005

Transformation plasticity in ZrO2‐containing ceramics generally exhibits shear and dilatation effects of comparable magnitude. The coupling between external stresses and crystallographic strains assists the tetragonal‐monoclinic transformation, which, via shear localization, gives rise to macroscopic shear and dilatant deformation. Application of a yield criterion based on both shear and dilatation effects correctly correlates deformation data from tension, compression, bending, and indentation, and further delineates a crack‐tip process zone comparable to the one observed experimentally. Similar shear and dilatation effects in microcracking due to transformation plasticity are explored. These findings suggest that the strength of the ultimate transformation‐toughened structural ceramics should be yield limited and sensitive to the stress state. Strategies for fracture control are recommended.
Transformation and Microcrack Toughening as Complementary Processes in ZrO2‐Toughened Al2O3
Authors: RÜHLE M. , CLAUSSEN N. , HEUER A. H.
Published: March 2005

Both tetragonal (t) and monoclinic (m) ZrO2 particles in ZrO2‐toughened Al2O3 can give rise to toughening. In the stress field of propagating cracks, the t‐ZrO2 particles can undergo the stress‐induced t→m transformation, and the residual stresses around already‐transformed m‐ZrO2 particles can cause microcracking. The t‐ZrO2 particles transformed in crack tip stress fields do not, however, also cause appreciable microcracking. The toughening increments via these distinct mechanisms are comparable. It appears that optimally fabricated Zr02‐toughened Al2O3's should contain a mixture of t‐ and m‐ZrO2.
Degradation During Aging of Transformation‐Toughened ZrO2‐Y2O3 Materials at 250°C
Authors: LANGE F. F. , DUNLOP G. L. , DAVIS B. I.
Published: March 2005

The detrimental aging phenomenon observed in ZrO2‐Y2O3 materials, which causes tetragonal ZrO2 to transform to its monoclinic structure at temperatures between 150 and 400°C, was investigated with respect to the gaseous aging environment and the Y2O3 and SiO2 content of the material. It is shown that the aging phenomenon is caused by water vapor and that inter‐granular silicate glassy phases play no significant role. Transmission electron microscopy of thin foils, before and after aging, showed that the water vapor reacted with yttrium in the ZrO2 to produce clusters of small (20 to 50 nm) crystallites of α‐Y(OH)3. It is hypothesized that this reaction produces a monoclinic nucleus (depleted of Y2O3) on the surface of an exposed tetragonal grain. Monoclinic nuclei greater than a critical size grow spontaneously to transform the tetragonal grain. If the transformed grain is greater than a critical size, it produces a microcrack which exposes subsurface tetragonal grains to the aging phenomenon and results in catastrophic degradation. Degradation can be avoided if the grain size is less than the critical size required for microcracking.
High‐Temperature Precipitation Hardening of Two‐Phase Y2O3‐Partially‐Stabilized ZrO2 Single Crystals: A First Report
Authors: DOMINGUEZ‐RODRIGUEZ A. , LANTERI V. , HEUER A. H.
Published: March 2005

Precipitation hardening was observed in two‐phase (cubic plus tetragonal) Y2O3‐partia1ly‐stabilized ZrO2 single crystals deformed at 1400°C. Slip was activated on (001) 〈110〉, primarily in Luders bands.
Spinel‐Corundum Phase Equilibria in the Systems Mn‐Cr‐Al‐O and Co‐Cr‐Al‐O at 1373 K
Authors: JACOB K.T. , IYENGAR G.N.K. , KIM W.K.
Published: March 2005

The tie lines delineating ion‐exchange equilibria between MCr2O4‐MAl2O4 spinel solid solution, where M is either Mn or Co, and Cr2O3‐Al2O3 solid solution with the corundum structure were determined at 1373 K by electron microprobe and E0AX point count analysis of the oxide phases equilibrated with metallic Co and Au‐5% Mn. The component activities in the spinel solid solutions are derived from the tie lines and the thernodynamic data for Cr2O3‐Al2O3 soiid solutions available hi the literature. The Gibbs free energies of mixing calculated from the experimental data are discussed in relation to the values derived from the cation distribution a.odel based on the site preference energies and assuming random mixing on both tetrahedral and octahedral sites. Positive deviations from ideality observed in this study suggest a miscibility gap for both series of spinel solid solutions at low temperatures in the absence of oxidation.
Oxidation Kinetics of Silicon Carbide Crystals and Ceramics: I, In Dry Oxygen
Authors: COSTELLO JOHN A. , TRESSLER RICHARD E.
Published: March 2005

The oxidation kinetics of several single‐crystal and polvcrystalline silicon carbide materials and single‐crystal silicon in dry oxygen over the temperature range 1200° to 1500°C were fitted to the linear‐parabolic model of Deal and Grove. The lower oxidation rates of silicon carbide compared to silicon can be rationalized by additional consumption of oxidant in oxidizing carbon to carbon dioxide. The (000J) Si face of the silicon carbide platelets exhibited lower parabolic oxidation rates than the (0001) C face, by a factor of 10 at 1200°C. Apparent activation energies increased from a value of ∼120 kJ/mol below 1400°C to a value of ∼300 kJ/mol above this temperature. The (0001) Si face exhibited this high activation energy over the entire temperature range. The controlled nucleation thermally deposited material exhibited the highest oxidation rates of the polycrystalline materials followed by the hot‐pressed and sintered α‐silicon carbides. In general, the oxidation rates of the polycrystalline materials were bracketed by the oxidation rates of the basal planes of the single‐crystal materials. Higher impurity concentrations and higher density of nucleation sites led to a greater susceptibility to crystallization of the scale which significantly complicated the oxidation behaviors observed. When crystallization of the oxide scale occurred in the form of a layer of spherulitic cristobalite crystals, a retardation of the oxidation rates was observed. An accelerated oxidation behavior was found when this coherent layer was superseded by the formation of fine mullite crystals.
Solid‐state NMR Evidence of 4‐, 5, and 6‐Fold Aluminum Sites in Roller‐Quenched SiO2‐A12O3 Glasses
Authors: Risbud Subhash H. , Kirkpatrick R. James , Taglialavore Antoni P. , Montez B.
Published: March 2005

The structures of roller‐quenched SiO2‐Al2O3 glasses containing 10 to 50 wt% Al2O3 were investigated by TEM and high‐resolution 27Al and 29Si magic angle sample spinning nuclear magnetic resonance (MASS‐NMR) spectroscopy. The chemical shifts observed in the NMR spectra of these phase‐separated (on the TEM scale) glasses provide evidence, for the existence of all three (4‐, 5‐, and 6‐jold) Al‐coordination units in these glasses and for the presence of A1 in both the Si‐rich and Si‐poor phases.
Crystal Structure of Silicon Powders Produced from Laser‐Heated Silane
Authors: Marras Robert A. , Haggerrty John S.
Published: March 2005

The crystallinity and microstructure of fine silicon particles produced by the decomposition of laser‐heated silane gas were studied in as‐synthesized and annealed states. Amorphous, partially crystalline, and fully crystalline particles can be produced. With annealing at 550°C, partially crystalline particles become fully crystalline; amorphous particles remain amorphous. The results agree with previous interpretations of formation, growth, and crystallization processes in the reaction zone based on light scattering and transmission diagnostics.
Mechanism of the Internal Reduction of (Mg, Cu)O
Authors: Backhaus‐Ricoult M. , Carter C. B.
Published: March 2005

Single crystals of (Mg, Cu)O have been internally reduced to produce metallic copper precipitates inside the oxide matrix. Scanning electron microscopy, transmission electron microscopy, and scanning transmission electron microscopy investigations of the composite materials yielded information on the morphology and composition of the precipitates and on the orientation relationship between the two phases. The structure of dislocation networks formed during the reaction and the presence of Cu2O particles in short‐time reduction experiments allow the internal reduction of(Mg, Cu)O to be modeled by a two‐step reduction mechanism.
Finite‐Element Calculation of Stresses in Glass Parts Undergoing Viscous Relaxation
Authors: SOULES THOMAS F. , BUSBEY ROBERT F. , REKHSON SIMON M. , MARKOVSKY ALEX , BURKE MICHAEL A.
Published: March 2005

Using the finite‐element program MARC developed by MARC Analysis Corporation, thermal stresses are calculated for a simple sandwich seal and a bead seal under conditions of uniform cooling, reheating, and an isothermal hold and for a tempered glass plate. Results of the viscoelastic calculations are compared with previously obtained analytical solutions and with experimental stress measurements. The formalism for including viscoelastic material properties in the program is developed.
Effect of Surface Flaw Size on Fracture Strength of Alumina Ceramics
Authors: Ikeda Kiyohiko , Igaki Hisashi
Published: March 2005

Semielliptical surface flaws of different sizes were introduced into Al2O3by Knoop microhardness indentation. The specimens were fractured by four‐point bending and the profiles of the indentation flaws were determined by observing the fracture surfaces with a scanning electron microscope. The relation between the indentation flaw size and the fracture strength could be well explained by applying the fracture‐mechanics analysis for semielliptical surface flaw in bending. The calculated values of the as‐indented critical stress intensity factor, KIC, were lower than previously reported presumably because of the influence of the residual stresses produced by the indenter.
Crack‐Interface Grain Bridging as a Fracture Resistance I, Mechanism in Ceramics: I, Experimental Study on Alumina
Authors: SWANSON PETER L. , FAIRBANKS CAROLYN J. , LAWN BRIAN R. , MAI YIU‐WING , HOCKEY BERNARD J.
Published: March 2005

Direct microscopic evidence is presented in support of an explanation of R‐curve behavior in monophase ceramics by grain‐localized bridging across the newly formed crack interface. In situ observations are made of crack growth in tapered cantilever beam and indented flexure specimens of a coarsegrained alumina. The fractures are observed to be highly stable, typical of a material with a strongly increasing resistance characteristic, but are discontinuous at the microstructural level. Associated with this discontinuity is the appearance of overlapping segments in the surface fracture trace around bridging grains; the mean spacing of such “activity sites” along the trace is about 2 to 5 grain diameters. These segments link up with the primary crack beneath the specimen surface, and continue to evolve toward rupture of the bridge as fracture proceeds. The bridges remain active at large distances, of order 100 grain diameters or more, behind the crack tip. Scanning electron microscopy of some of the bridging sites demonstrates that secondary (interface‐adjacent) microfracture and frictional tractions are important elements in the bridge separation process. Evidence is sought, but none found, for some of the more popular alternative models of toughening, notably frontal‐zone microcracking and cracktip/internal‐stress interaction. It is suggested that the crackinterface bridging mechanism may be a general phenomenon in nontransforming ceramics.
Crack‐Interface Grain Bridging as a Fracture Resistance Mechanism in Ceramics: II, Theoretical Fracture Mechanics Model
Authors: MAI YIU‐WING , LAWN BRIAN R.
Published: March 2005

A fracture mechanics model is developed for nontransforming ceramics that show an increasing toughness with crack extension (R‐curve, or T‐curve, behavior). The model derives from the observations in Part I, treating the increased crack resistance as the cumulative effect of grain bridging restraints operating behind the advancing tip. An element of discreteness is incorporated into the formal distribution function for the crack‐plane restraining stresses, to account for the primary discontinuities in the observed crack growth. A trial forceseparation function for the local bridge microrupture process is adopted, such that an expression for the microstructureassociated crack driving (or rather, crack closing) force may be obtained in analytical form. The description can be made to fit the main trends in the measured toughness curve for a coarse‐grained alumina. Parametric evaluations from such fits conveniently quantify the degree and spatial extent of the toughening due to the bridging. These parameters could be useful in materials characterization and design. It is suggested that the mechanics formulation should be especially applicable to configurations with short cracks or flaws, as required in strength analysis.
Drying Shrinkage Mechanisms in Portland Cement Paste
Authors: HANSEN WILL
Published: March 2005

The shrinkage mechanisms of portland cement paste were investigated from shrinkage, weight loss, and pore structure measurements using nitrogen sorption and mercury intrusion porosimetry (MIP). Thin samples (2.3 mm) of well‐hydrated (165 d) pastes of 0.4 and 0.6 water‐to‐cement (W/C) ratios were dried directly from saturated surface dry state to 75%, 50%, 11%, and 0% relative humidity (rh). From equilibrium shrinkage vs calculated increase in surface free energy curves two active stress mechanisms were identified. The Gibbs‐Bangham (surface free energy) effect is the major stress mechanism, which is active in the entire rh range investigated, whereas the capillary stress effect is active above 25% rh. From elastic modulus calculations it can be concluded that true Gibbs‐Bangham shrinkage accounts for only 33% of total first drying shrinkage. Thus nearly 67% of first drying shrinkage may be due to a decrease in interlayer spacing caused by Gibbs‐Bangham and capillary induced stresses. Further, nitrogen measures the true external surface area, and total external pore volume can be obtained from combined measurements using nitrogen sorption and MIP.
Comparison of Ba(OH)2, BaO, and Ba as Starting Materials for the Synthesis of Barium Titanate by the Alkoxide Method
Authors: Phule Pradeep P. , Raghavan Srini , Risbud Subhash H.
Published: March 2005

A discussion of the relative merits and problems of using Ba(OH)2, BaO, and Ba as starting materials for the synthesis of barium titanate by the alkoxide method is presented. The Ba(OH)2 process is promising because it does not involve the synthesis of barium alkoxide, whereas the BaO (93% pure) process suffers from complex chemical reactions; the impurities in BaO cause unknown effects on the composition and properties of the final ceramic. Use of high‐purity Ba metal (99.99%) for the synthesis of barium alkoxide is more desirable since it results in high‐purity, ultrafine barium titanate powders.
Criteria for Crack Branching in Cylindrical Rods: I, Tension
Authors: KIRCHNER H.P. , CONWAY J.C.
Published: March 2005

The angular and radial dependencies of the mirror‐mist boundaries predicted by the stress intensity and Johnson and Holloway (JH) criteria were compared with experimentally determined shapes of the boundaries in cylindrical specimens fractured in tension. At the front surface, for relative mirror radii up to at least 0.2, boundary correction factors based on a recently available finite‐element analysis provided the best predictions, showing among other things that the so‐called mirror constant is a variable. In this size range, the stress intensity criterion provided better predictions of the boundary shapes than the Johnson and Holloway criterion. For large relative mirror sizes, neither the JH criterion nor the stress intensity criterion assuming semicircular cracks provided accurate predictions.
Criteria for Crack Branching in Cylindrical Rods: II, Flexure
Authors: KIRCHNER H.P. , CONWAY J.C.
Published: March 2005

The angular and radial dependencies of the mirror‐mist and mist‐hackle boundaries predicted by the stress intensity criterion for crack branching were compared with those of experimentally determined boundaries in cylindrical specimens fractured in flexure. The analysis models the reduction of branching radii near the surface caused by reduced constraint and the elongation toward the neutral axis caused by the stress distribution except that it underestimates the elongation in a range of angles for specimens fractured at relatively low stresses. This difference is believed to result from changes in crack shape that are not accounted for in the analysis. As in the case of rectangular bars fractured in tension and flexure and cylindrical rods fractured in tension, the stress intensity criterion continues to provide better predictions than the Johnson and Holloway criterion.
Diffusional Creep and Kinetic Demixing in Yttria‐Stabilized Zirconia
Authors: DIMOS D. , KOHLSTEDT D. L.
Published: March 2005

The creep behavior of fine‐grained yttria‐stabilized zirconia with 25 mol% Y2O3 has been characterized as part of an investigation of kinetic demixing in solid‐solution oxides which are subjected to a nonhydrostatic state of stress. At temperatures between 1400° and 1600°C, the steady‐state strain rate of (Zr0.6Y0.4)O1.8 samples with average grain sizes between 2.5 and 14.5 μm can be summarized by the flow law ɛ= 6.5 × 10−7σ1.2 exp[−550 (kJ/mol)/RT]d−2.2 (s−1) for stresses in the range 8 to 60 MPa, where σ is in pascals and d is in meters. This flow law indicates that deformation occurs by a Nabarro‐Herring creep mechanism in which the creep rate is limited by cation lattice diffusion. Kinetic demixing was not observed in deformed polycrystalline samples even though diffusional creep was rate limited by cation lattice diffusion. This result can be explained if the cation diffusivities are approximately equal or if extensive grain rotation occurs during diffusional creep.
Heterogeneous‐Nucleation and Glass‐Formation Studies of 56Ga2O3· 44CaO
Authors: ETHRIDGE EDWIN C. , CURRERI PETER A. , PLINE DAVID
Published: March 2005

Glass formation and heterogeneous crystallization are described for the reluctant‐glass‐forming 56Ga2O3· 44CaO eutectic composition. The times and temperatures for nucleation at various cooling rates and experimental conditions were measured and empirical continuous‐cooling‐crystallization boundaries were constructed for various heterogeneous nucleation processes. A definition for an empirical critical cooling rate to form a glass from reluctant borderline glass formers is proposed, i.e., the cooling rate that results in glass formation in 95% of the quenching experiments.
Mixed‐Mode Fracture in Soda‐Lime Glass Using Indentation Flaws
Authors: GLAESEMANN G. SCOTT , RITTER JOHN E. , JAKUS KARL
Published: March 2005

Mixed‐mode failure of soda‐lime glass under inert and fatigue test conditions was studied using Knoop indentation flaws. For annealed cracks (residual stress‐free) crack extension (catastrophic or subcritical) is by an abrupt transition from the initial crack plane to a noncoplanar crack plane followed by a reorientation of the crack normal to the applied stress. Although fatigue strength of these inclined flaws increased linearly with respect to orientation of the flaws to the applied stress up to an angle of 60°, this increase was considerably less than what was predicted by existing theories. It is believed that subcritical crack growth causes the crack to be realigned perpendicular to the applied stress before failure for all orientations; hence, fatigue strength does not show the dramatic increase at orientation angles as predicted by theory. For as‐indented cracks the contact residual stress causes the crack extension to be less inclined to the initial crack plane than for annealed cracks, but in this case also, the crack realigns itself perpendicular to the applied stress. Again, fatigue strength is relatively insensitive to the orientation angle as predicted by theory and subcritical crack growth is believed to play a primary role in determining this strength dependency.
Aging Behavior of Ceria‐Stabilized Tetragonal Zirconia Polycrystals
Authors: Matsumoto Roger L. K.
Published: March 2005

A commercial ceria‐stabilized tetragonal zirconia polycrystal is shown not to degrade at low temperatures. Slight strength decrease is found for elevated‐temperature treatments, which is not related to formation of the monoclinic phase. A possible mechanism to explain the behavior based upon the ferroelastic nature of the tetragonal phase is proposed.
Degradation Mechanisms of Silicon Carbide Fibers
Authors: Johnson S.M. , Brittain R.D. , Lamoreaux R.H. , Rowcliffe D.J.
Published: March 2005

The effects of high temperatures on polymer‐derived Nicalon and chemically vapor‐deposited (CVD) SiC fibers have been investigated by mass‐spectrometric vapor pressure studies and by SEM observations of exposed fibers. Vaporization is kinetically limited for both materials; the predominant species vaporizing from Nicalon is SiO, whereas CO predominates in vaporization of CVD SiC fibers. The inherent instability of Nicalon and other SiC fibers in contact with matrices must be considered in ceramic composite fabrication and use.
Transformation Plasticity of CeO2‐Stabilized Tetragonal Zirconia Polycrystals: I, Stress Assistance and Autocatalysis
Authors: REYES‐MOREL PATRICIO E. , CHEN I‐WEI
Published: March 2005

Transformation plasticity in CeO2‐stabilized tetragonal zirconia polycrystals due to the tetragonal‐to‐monoclinic transformation was studied by inducing volumetric and shear deformation under compression and bending between the burst temperature of martensite (monoclinic) formation (Mb) and the burst temperature of austenite (tetragonal) formation (Ab). The stress‐strain curve features a load drop, a perfect plastic regime, and an extended strain‐hardening regime before the exhaustion of transformation. Macroscopic shear bands formed in the perfect plastic regime. The yield stress has a strong, positive pressure and temperature sensitivity but is strain‐rate sensitive only in the last stage of deformation. These results are rationalized in terms of stress assistance to the transformation which, in a homogeneous tetragonal polycrystal, may propagate autocatalytically. Autocatalysis can be impeded by a second phase, such as monoclinic ZrO2 or Al2O3, and is suppressed at higher temperature. Flow localization is found to precede and precipitate crack formation. As a result, the actual fracture energy is much less than the total plastic work. The implications of stress‐assisted, autocatalytic transformation on strength and toughness are explored.
Shape and Structure of Zinc Oxide Particles Prepared by Vapor‐Phase Oxidation of Zinc Vapor
Authors: SUYAMA YOKO , TOMOKIYO YOSHITSUGU , MANABE TAKESHI , TANAKA EISHI
Published: March 2005

The shape and structure of granular and tetrapod‐like ZnO particles prepared by a vapor‐phase method were analyzed by electron microscopy, and the growth mechanism is discussed. Only the wurtzite structure exists in ultrafine particles less than 50 nm, indicating that the ZnO particles have the wurtzite structure from the early stages of growth. The legs of the tetrapod‐like particles are hexagonal prisms grown along the <001> direction. With regard to the geometrical orientation relationship between any two of the four legs, it was found that the (1¯0) planes of the two legs are parallel to each other.
Phase Transformation and Thermal Conductivity of Hot‐Pressed Silicon Carbide Containing Alumina and Carbon
Authors: Sakai Tosikazu , Aikawa Toshihiko
Published: March 2005

Phase transformation and thermal conductivity of hot‐pressed β‐SiC with Al2O3 and carbon additions were studied. Densification rate was a complex function of both Al2O3 and carbon. Simultaneous additions of Al2O3 and carbon accelerated the 3C → 4H phase transformation. Carbon additions lowered the thermal conductivity of the compact as did the high‐temperature hot‐pressing. The 3C → 4H transformation and the thermal conductivity were deduced to be related to each other.
Creep Parameters of Alumina Containing a Glass Phase Determined in Bending Creep Tests
Authors: FETT T. , KELLER K. , MISSBACH M. , MUNZ D. , PINTSCHOVIUS L.
Published: March 2005

An alumina ceramic containing a glass phase is investigated in bending creep tests. It is shown how the constitutive equation describing the material behavior can be derived from outer fiber strain measurements.
Room‐Temperature Thermal Conductivity of Cemented Transition‐Metal Carbides
Authors: PERECHERLA ANJIRAJU , WILLIAMS WENDELL S.
Published: March 2005

Measurements are reported of the room‐temperature thermal conductivity of cemented multicarbides (WC‐TiCx‐NbCx‐TaCx/Co) and straight tungsten carbide (WC/Co), which are widely used tool materials. The thermal conductivity of cemented titanium carbide was found to be lower than that of cemented tungsten carbide. The difference is attributed to strong phonon and electron scattering from carbon atom vacancies in the nonstoichiometric cubic carbide TiCx; these defects are absent in stoichiometric hexagonal WC. Higher binder contents in tungsten carbide samples lowered the overall thermal conductivity. Scattering of electrons and phonons by C and W atoms in solid solution in the binder phase presumably reduces its thermal conductivity. No dependence on grain size was detected.
Magnetoresistance of CuxCo1‐xFe2O4 Ferrites
Authors: Ghani Ahmed A. , Eatah Ali I. , El‐Faramawy El‐Mozamel
Published: March 2005

Magnetoresistance measurements (Δ/R) were carried out on CuxCo1‐xFe2O4 samples with x=1, 0.75, 0.50, 0.25 and cobalt ferrite in the range of magnetic intensity (H) from 1.0 to 4.5 kG. Linear plots were obtained between log (Δ/R) and log H for all compositions except that of cobalt ferrite. The values of n at x=0.5 do not agree with the results of some authors. The discrepancy may be attributed to the value of magnetic field intensity at which such measurements were carried out. The similarity of features of n and μD with composition leads us to believe that the magnetoresistance may arise from the scattering of conduction electrons by localized electrons involving the s‐d exchange interaction.
Shear‐Lag Analysis of Fiber Push‐Out (Indentation) Tests for Estimating Interfacial Friction Stress in Ceramic‐Matrix Composites
Authors: Shetty Dinesh K.
Published: March 2005

A shear‐lag analysis is presented for estimating sliding friction stress at fiber‐matrix interfaces in ceramic‐matrix composites using the single‐fiber push‐out test. The analysis includes an approximate correction for the increased interfacial compression and, therefore, the interfacial friction stress arising from the transverse (Poisson) expansion of the fibers subjected to the compressive load. An exponential decrease of the interfacial shear stress along the fiber length is predicted. This result is similar to the results of a finite‐element analysis reported in the literature. The analysis also provides a basis for the experimental determination of a coefficient of interfacial friction (μ) and a residual interfacial compression (σ0). It is shown that the sliding friction stress (τf=μσ0) can be overestimated if the transverse expansion of the fibers is not taken into account.
Elastic Anisotropy in Zirconia Single Crystals
Authors: INGEL ROBERT P. , III DAVID LEWIS
Published: March 2005

Results are presented for the measured single‐crystal elastic constants of yttria‐stabilized zirconia, for yttria contents of 1.7 to 20 mol%. The results cover a range of materials which vary from a mixture of monoclinic, tetragonal, and cubic to those which are fully cubic. These single‐crystal measurements are used to calculate the bounds on the elastic moduli for polycrystalline materials. Comments are made on the elastic anisotropy of zirconia relative to a number of other single‐crystal ceramics, with graphical comparisons of the anisotropy of Young's moduli of these ceramics.
Kinetics and Mechanisms of High‐Temperature Creep in Silicon Carbide: III, Sintered α‐Silicon Carbide
Authors: LANE J.E. , CARTER C.H. , DAVIS R.F.
Published: March 2005

The operative and controlling mechanisms of steady‐state creep in sintered α‐SiC have been determined both from kinetic data within the ranges of temperature and constant compressive stress of 1670 to 2073 K and 138 to 414 MPa, respectively, and from the results of extensive TEM and other analytical analyses. Dislocations in glide bands, B4C precipitates, and the interaction of these two entities were the dominant microstructural features of the crept material. The stress exponent increased from 1.44 to 1.71 with temperature; it was not a function of stress at a given temperature. The curves of In ɛ vs 1/T showed a change in slope at 1920 ± 20 K. The respective activation energies below and above this temperature interval were 338 to 434 and 802 to 914 kJ/mol. A synthesis of all this information leads to the conclusion that the controlling creep mechanism at low temperatures is grain‐boundary sliding accommodated by grain‐boundary self‐diffusion; at high temperatures, the controlling mechanism becomes grain‐boundary sliding accommodated by lattice diffusion. The parallel mechanism of dislocation glide contributes increasingly to the total strain as the number/volume of precipitates declines as a result of progressive coalescence with increasing temperature.
Hydrolysis Reactions at the Surface of Fluorozirconate Glass
Authors: PANTANO CARLO G. , BROW RICHARD K.
Published: March 2005

The surface reaction products associated with the hydrolysis of ZBLA glasses have been characterized using X‐ray photoelectron spectroscopy. The predominant surface reaction product in water and basic solutions is a zirconium (hydr)oxy‐fluoride species. The compositional analyses indicate that the (hydr)oxyfluoride which forms in water has an average oxygen‐to‐fluorine ratio of ∼2/5. The Zr 3d binding energy of this (hydr)oxyfluoride species was interpreted on the basis of its Pauling charge; this verified the compositional analyses and suggested a seven‐coordinate surface species of the form ZrO2F5 or Zr(OH)2F5. It is proposed that zirconium (hydr)oxyfluoride is created in all aqueous solutions because of attack of Zr–F bonds by molecular water. The extent of reaction in water influences the surface concentration of the oxyfluoride species, but not their composition. In very acidic solutions, the surface concentration of (hydr)oxyfluoride is minimal, whereas in highly basic solutions the (hydr)oxy‐fluoride species exhibits an increased oxygen‐to‐fluorine ratio.
Damage‐Enhanced Creep in a Siliconized Silicon Carbide: Phenomenology
Authors: WIEDERHORN SHELDON M. , ROBERTS D. ELLIS , CHUANG TZE‐JER , CHUCK LEON
Published: March 2005

The creep behavior of a commercial grade of reaction‐bonded silicon carbide was characterized at a temperature of 1300°C. Creep occurred more easily in tension than in compression. At a given applied stress, the steady‐state creep rate in tension was found to be at least 20 times that obtained in compression. In both tension and compression, the stress exponent for steadystate creep was found to increase with increasing applied stresses. At low applied stresses, the stress exponent was ∼4, suggesting some kind of dislocation mechanism operating in the two‐phase composite. At high stresses, the stress exponent was ∼11 in tension. The increase in the stress exponent was attributed to damage accumulation in the form of cavities. An effective threshold stress for cavitation of less than 100 MPa was suggested. In compression, the cause of the increase of stress exponent with stress cannot be attributed to cavitation.
Shape and Size of Crystalline MgO Particles Formed by the Decomposition of Mg(OH)2
Authors: Kim M.G. , Dahmen U. , Searcy Alan W.
Published: March 2005

Decomposition of Mg(OH)2 at 300° to 400°C yields MgO crystals with often unequal edge lengths which, from counting of crystal planes in high‐resolution transmission electron micrographs, range from 0.8 to 2.4 nm, in agreement with conclusions of Moodie and Warble. Optical diffractograms and electron diffraction patterns yield concordant results. An origin for discordant X‐ray diffraction estimates of particle size for MgO produced from Mg(OH)2 in the same temperature range is suggested.
Proposed Liquidus Surface of the CaSiO3‐BaSiO3‐SrSiO3 System Constructed from Literature Data
Authors: DeVries Robert C.
Published: March 2005

In the absence of a published diagram, two versions of the liquidus surface of the ternary system CaSiO3‐BaSiO3‐SrSiO3 were constructed from binary data. This information was adequate for a prediction of the temperature and composition of a ternary minimum or eutectic. This speculative interpolation, a more feasible approach than calculation under the circumstances, resulted in a useful working diagram which ultimately needs the test of experimental work.
Low‐Temperature Sintering of Aluminum Oxide
Authors: YEH TSUNG‐SHOU , SACKS MICHAEL D.
Published: March 2005

A fine‐sized (∼0.1 μm), agglomerate‐free Al2O3 dispersion was used to prepare homogeneous green bodies with ∼69% relative density and ∼10‐nm median pore radius. Samples could be sintered at 1150°C to a relative density >99.5% and an average grain size of 0.25 μm.
Kinetics of Barium Titanate Synthesis
Authors: HERTL W.
Published: March 2005

Reaction curves were obtained at various temperatures and concentrations for the formation of BaTiO3 from particulate titania in Ba(OH)2 solution. Kinetic analyses were performed by constructing mathematical models which took into account the particle size distribution of the reactant titania for both the topochemically‐rate‐controlled and the diffusion‐rate‐controlled reactions. At [Ba(OH)2] > ca. 0.1M the rate‐controlling step is the Ba reaction with TiO2 at the interface. The measured activation energy is 105.5 kJ/mol. The rates are independent of Ba(OH)2 concentration, indicating that the TiO2 interface is saturated. At [Ba(OH)2] < ca. 0.1M the rate‐determining step shifts to diffusion through the product BaTiO3 layer, the rates are concentration dependent, and the BaTiO3 particle sizes are inversely proportional to the Ba(OH)2 concentrations used.
Particle and Granule Parameters Affecting Compaction Efficiency in Dry Pressing
Authors: Zheng Jingmin , Reed James S.
Published: March 2005

The slope of density versus log (pressure) represents the efficiency of compaction forming. A high maximum particle packing density of powders increased the pressing efficiency and the compact density. A high granule density favors a high compact density, whereas, a low granule density favors an increase in the pressing efficiency and the elimination of the intergranular pores; an optimum clearly existed. At a very low forming pressure, the compact density using granules containing a binder was lower; but at a higher forming pressure, the binder acted as a lubricant and assisted particle sliding and rearrangement which increased the pressing efficiency and compact density.
Predicting Failure Stress of Silicon Nitride Ceramics Using Microfocus Radiography
Authors: Cotter Daniel J. , Koenigsberg William D. , Pasto Arvid E. , Bowen Leslie J.
Published: March 2005

Microfocus projection radiography was used to evaluate nondestructively a large quantity of silicon nitride modulus‐of‐rupture test bars. Quantitative data (size, shape, and location) on major naturally occurring voids in rejected bars were determined from radiographs. Failure stress prediction was attempted using a fracture mechanics model and nondestructive evaluation data and compared to actual failure stress.
Powder Processing Science and Technology for Increased Reliability
Authors: Lange Fred F.
Published: March 2005

Issues concerning powder consolidation methods compatible with the colloidal approach and issues associated with other powder processing steps, viz., densification and microstructural control, are presented with regard to research directions leading to more reliable ceramics.
Mechanism of Grain Orientation During Hot‐Pressing of Bismuth Titanate
Authors: Kimura Toshio , Yoshimoto Takeshi , Iida Nobuyoshi , Fujita Yoshiaki , Yamaguchi Takashi
Published: March 2005

Grain‐oriented Bi4Ti3O12 ceramics have been prepared by hot‐pressing. Platelike grains align during hot‐pressing; the long‐axis direction perpendicular to the crystal c axis is oriented perpendicular to the pressing direction. The effects of hotpressing conditions such as temperature, pressure, time, and type of filler on the density, orientation degree, and deformation of hot‐pressed compacts were examined. Sharp increases in the density, deformation, and grain orientation occurred during the early stage; the density decreased, and the latter two incresed on prolonged hot‐pressing. The grain orientation is closely related to the deformation in the radial direction. The grain‐orientation mechanism is discussed referring to mechanisms proposed for high‐temperature deformation in ceramics. The compact is deformed by two processes: one accompaines grain orientation, and the other does not. Grain‐boundary sliding is proposed for the former process. The contribution of grain‐boundary sliding to the deformation increases with increasing temperature and pressure. The filler also influences the contribution. The decrease in density is caused by cavity formation during prolonged hot‐pressing.
Formation of Mullite from Pyrophyllite by Mechanical and Thermal Treatments
Authors: Sanchez‐Soto Pedro J. , Perez‐Rodriguez José L.
Published: March 2005

When pyrophyllite is submitted to mechanical and thermal treatments, significant changes take place, according to XRD and DTA results. With grinding, the DTA endothermic peak is shifted to 540°C and becomes sharper and similar to kaolinite. A sharp exothermic peak is also produced at 1000°C, which is not present in the unground material. Increasing the mullite is associated with the formation of the latter peak. Thermal treatment causes the formation of mullite at higher temperatures.
Sinterable β‐Spodumene Glass‐Ceramics
Authors: Knickerbocker Sarah , Tuzzolo Michelle R. , Lawhorne Samuel
Published: March 2005

β‐Spodumene glass‐ceramic compositions were melted and studied. Compositional variations were made in the three major components as well as through minor additions of other oxides. Sintering characteristics and microstructures were studied and values for crystallization temperature, melting temperature, and crystallized thermal expansion coefficient were recorded. It was found that sinterable β‐spodmene glass‐ceramics could be made with a wide range of properties. Selection of an appropriate composition would be based on desired properties.
High‐Resolution Electron Microscopy Observations of Stacking Faults in β‐SiC
Authors: Koumoto Kunihito , Takeda Shunji , Pai Chul Hoon , Sato Takayori , Yanagida Hiroaki
Published: March 2005

Structural images of the stacking faults in β‐SiC were obtained with a high‐resolution electron microscope. Stacking faults initially present in β‐SiC powder particles were eliminated as grain growth proceeded at elevated temperatures.
Investigation of Plasma‐Sprayed Dysprosia Coatings
Authors: Venkatachari Koththavasal R. , Kriven Waltraud M.
Published: March 2005

Plasma‐sprayed coatings of dysprosia were made and characterized. The coatings consisted mostly of the metastable, monoclinic phase and minor amounts of the cubic phase. Grinding did not transform the high‐temperature monoclinic phase, but, on annealing in air at 800° for 5 h, 50% reversion to the cubic phase occurred. Commercial yttria‐stabilized zirconia thermal barrier coatings out‐performed unannealed plasma‐sprayed dysprosia coatings in terms of wear resistance.
“Nose Method” of Calculating Critical Cooling Rates for Glass Formation
Authors: Weinberg Michael C. , Uhlmann Donald R. , Zanotto Edgar D.
Published: March 2005

In the past, it has been shown that reasonably good estimates of critical cooling rates for glass formation could be obtained by computing the cooling curve which just passes through the nose of a T‐T‐T (time—temperature—transformation) diagram. Also, it has been noted that critical cooling rates found by this procedure are generally greater than those obtained by other methods. Herein, we provide an explanation of these observations. The nose method of computing critical cooling rates is compared with others, and specific illustrations are given for the glass‐forming systems SiO2, GeO2, and P2O5.
In Situ 27Al NMR Studies of Cement Hydration: The Effect of Lithium‐Containing Setting Accelerators
Authors: Luong Tim , Mayer Hermann , Eckert Hellmut , Novinson Thomas I.
Published: March 2005

Hydration of calcium aluminate cements is known to cause phase transformations during which the coordination number of aluminum by oxygen changes from 4 to 6. Systematic 27Al magic angle spinning and wide‐line nuclear magnetic resonance (NMR) experiments at 78.3 MHz with detailed attention to the quantitative aspects of quadrupolar NMR spectroscopy show that the fraction of Al(6) is a monotonic function of the water/solid (w/s) ratio and does not depend on the type and concentration of setting accelerators. In situ 27Al wide‐line NMR measurements during the hydration process can provide good estimates of the induction period and the rate of the phase conversion. Using this approach, lithium carbonate at a concentration of 0.5 wt% of the mixing solution has been shown to eliminate the induction period but to leave the rate of the phase conversion essentially unchanged.
Modified Specimen Preparation for Applied‐Moment Double‐Cantilever‐Beam Testing
Authors: Jessen Todd L. , Lewis David
Published: March 2005

A method is discussed for preparing applied‐moment double‐cantileverbeam specimens used in fracture toughness (KIc) testing. This procedure eliminates the need to precrack the specimens, which often leads to specimen failure before testing. In addition to yielding toughness values which are in agreement with literature values, this method allows the experimenter greater control in selecting the location of failure initiation.