bluuebunny/crossref_metadata_2025 huggingface dataset:
Activated Sintering of ThO2 and ThO2‐Y2O3 with NiO
Authors: HALBFINGER GEORGE P. , KOLODNEY MORRIS
Published: June 2006

The effect of additives on the sintering of ThO2 and ThO2‐Y2O3 compacts and loose powders was studied by isothermal shrinkage measurements and by scanning electron micrography. Small amounts of the oxides of Ni, Zn, Co, and Cu reduced the sintering temperature. The behavior of NiO at a concentration of 0.8 wt% (2.5 mol%) was studied in detail and found to yield high‐density bodies at temperatures below 1500°C. The presence of Y2O3 as a separate phase increases the rate of sintering of ThO2, but smaller amounts of NiO are much more potent. The major portion of the densification occurs very rapidly and is followed by a much slower sintering process typical of volume diffusion. The fast early shrinkage may be caused by the capillary forces of a liquid, but since no evidence of melting was found, a solid‐state mechanism may be responsible.
Cold‐Pressing and Low‐Temperature Sintering of Alkoxy‐Derived PLZT
Authors: BROWN L. M. , MAZDIYASNI K. S.
Published: June 2006

Multiorganometallic compounds were used to prepare high‐purity submicron‐size lead lanthanum zirconate‐titanate (PLZT) powders which can be consolidated at low sintering temperatures to high‐density piezoelectric and electrooptic ceramic bodies. Simultaneous hydrolytic decomposition of lead isoamyloxide, lanthanum isopropoxide, and zirconium and titanium tertiary amyloxides produced quaternary titanate powders with particle sizes of 75 to 300 Å and analyzed purity of 99.92%; the major contaminant was Na from an intermediate reaction product. Homogeneity and stoichiometry of the powders before and after processing were demonstrated by electron microscopy and chemical analysis. High‐temperature X‐ray diffraction indicated an amorphous material up to ∼600°C, where the crystalline phase was first observed. Powders prepared in this manner sintered to high‐density fine‐grained transparent bodies with uniform microstructure. The experimental results suggest that less rigorous consolidation methods are required with high‐purity homogeneous precursor materials with fine particle size.
Grain‐Boundary Segregation in MgO‐Doped Al2O3
Authors: MARCUS H. L. , FINE M. E.
Published: June 2006

Magnesia‐doped alumina was fractured in a high‐vacuum Auger electron spectrometer, and the resulting intergranular fracture surface was analyzed for surface composition. Calcium was present on the fracture surface in amounts of ∼2 to 5 at.%. There was no indication of Mg on the fracture surface. The bulk Ca concentration was 5 to 15 ppm, whereas MgO was present at the 1000‐ppm level. The presence of excess Ca at the grain boundary can be explained by the fact that its ionic radius is relatively large compared to those of Al and Mg. Sputtering profiles normal to the fracture surface indicate that the segregated Ca is concentrated in a region near tha intergranular fracture surface.
Thermodynamic Properties of the System Indium‐Oxygen
Authors: CHATTERJI D. , VEST R. W.
Published: June 2006

The free‐energy change for the reaction 2In(l)+3NiO(s) = In2O3(s)+3Ni(s) was determined from 550° to 800°C from emf measurements on solid‐oxide galvanic cells. The results were used to develop an equation for the standard molar free energy of formation of In2O3, i.e. ΔG°ln2O3= ‐215,550+72.63T±450 cal/ mol. The standard molar enthalpy and entropy of formation of In2O3 at 298°K were calculated to be ‐214,000±1500 cal/mol and ‐69.03±o0.22 eu, respectively, using the available thermo‐chemical data. The absolute entropy of In2O3 at 298°K was calculated to be 32.23±0.22 eu. The free‐energy results of this study were used in conjunction with literature data to calculate partial pressures of the gaseous species over In2O3 for different experimental conditions.
Subsolidus Equilibria in the System BaOCeO2‐TiO2
Authors: GUHA J. P. , KOLAR D.
Published: June 2006

Subsolidus phase relations in the system BaO‐CeO2‐TiO2 were investigated using X‐ray powder diffraction. The existence of the binary compound CeTi2O5 was confirmed, and its limited stability was studied before the ternary system was investigated. Six compatibility triangles were established in the ternary system at 1200°C. No ternary compound was detected. Extensive mutual solid solubility was observed between BaTiO3 and BaCeO3, whereas little or no solid solubility occurred between the other compounds in the system. The effects of small additions of CeO2 on the tetragonal→Cubic and cubic→hexagonal transition temperatures of BaTiO3 were investigated.
Strengthening of Hot‐Pressed Al2O3 by Quenching
Authors: KIRCHNER H. P. , GRUVER R. M. , WALKER R. E.
Published: June 2006

Compressive surface stresses were formed in hot‐pressed Al2O3 by quenching. Flexural strength was improved substantially, up to a maximum of 223,000 psi. The variation of strength with grain size of the quenched bodies was similar to that of the unquenched material, but the strengths were shifted to higher values. The presence of a glassy intergranular phase is not necessary to obtain compressive surface stresses by quenching.
Effect of Surface Damage on the Strength of Al2O3 Ceramics with Compressive Surface Stresses
Authors: GRUVER R. M. , KIRCHNER H. P.
Published: June 2006

In fully dense fine‐grained Al2O3 ceramics, fracture originates at surface flaws. The severity of these flaws depends on the methods used to form the surface and any subsequent damage that may have occurred. It is desirable to find ways to minimize the severity of these flaws. Both untreated and quenched Al2O3 rods were scratched by rotating them under a diamond point with various loads. The strengths of the quenched rods decreased less than those of the untreated rods. Microscopic examination showed that the compressive surface stresses induced by quenching reduced the penetration of surface damage.
Method of Studying Subcritical Cracking of Opaque Materials
Authors: MCKINNEY K. R. , SMITH H. L.
Published: June 2006

Since subcritical cracking often precedes catastrophic failure, it is an important engineering consideration, especially if a structure is to be under stress for a long time. A specimen has been developed for laboratory study of subcritical cracking, but it is assumed that crack growth can be observed visually through the specimen. With opaque materials, such observation is generally impractical or impossible. A compliance method of obtaining crack‐growth data is proposed. Experimental examples using polymethyl methacrylate, soda‐lime glass, and an opaque glass‐ceramic show that the method works very well. The usefulness of the method is illustrated for 3 commercial ceramics. It is shown that subcritical cracking occurs over a range of fracture energies for each of these materials.
Effects of Titanium Oxide on Equilibria Among Refractory Phases in the System CaO‐MgO‐Iron Oxide
Authors: SHULTZ R. L.
Published: June 2006

The role of titanium oxide in some important refractory systems was elucidated by studying selected equilibria in the system CaO‐MgO‐iron oxide‐titanium oxide at O2 pressures of 0.21 atm (air) and 10−9 atm and under the extreme reducing conditions imposed by the presence of metallic Ti in contact with the oxide phases. Solidus relations were determined for the system CaO‐MgO‐TiO2 in air; 6 composition triangles were delineated, within each of which 3 crystalline phases coexist in equilibrium with liquid at a constant solidus temperature. The solidus temperatures range from 1407° to 1670°C. There is also a composition area within which MgO coexists with a Ca4Ti3O10‐Ca3Ti2O7 solid solution, with solidus temperatures varying continuously from 1659° to 1670°C. Studies of reactions between MgO and titanium oxide in contact with metallic Ti in a closed system indicate that the mutual solubility between MgO and TiO at 1400°C is very small. Addition of 5 wt% TiO2 to the system CaO‐MgO‐iron oxide at 1500°C in air and in 10−9 atm O2 decreases the amount of iron oxide which can be absorbed by a CaO‐MgO body without formation of a liquid phase; hence, titanium oxide has a strong deleterious effect on the refractoriness of such bodies.
Effect of Rare‐Earth Ions on Electrical Conductivity of BaTiO3 Ceramics
Authors: MURAKAMI TOSHIAKI , MIYASHITA TADASHI , NAKAHARA MOTOHIRO , SEKINE EIJI
Published: June 2006

Lattice constants, grain size, electrical conductivity, and luminescence were measured for sintered BaTiO3 ceramics doped with 0 to 1.2 at.% rare‐earth ions. BaTiO3 doped with low levels of rare‐earth ions contains grains 10 μm in size and has lattice constants nearly equal to those of undoped ceramics. In this case, rare‐earth ions occupy Ba2+ sites and yield donors. When grain growth is inhibited by high doping levels or by insufficient sintering, the lattice constants change, the rare‐earth ions occupy both Ba2+ and Ti4+ sites, and, consequently, BaTiO3 becomes insulative because of charge compensation.
Crack Healing in UO2
Authors: ROBERTS J. T. A. , WRONA B. J.
Published: June 2006

Isothermal annealing of thermally shocked UO2 bars (O/M= 2.00 ±0.01) at 1600°, 1800°, and 2000°C caused crack healing, which was determined from recovery of room‐temperature fracture strength. The activation energy for crack healing was ∼0.5 of that for volume diffusion, and healing occurred at the same rate as grain growth. This result has important implications with respect to crack healing in oxide fuels during in‐reactor restructuring.
Undulating Structure in Complex Silicate Glasses
Authors: OHLBERG S. M. , GOLOB HELEN R.
Published: June 2006

Complex silicate glasses (e.g. commercial plate, sheet, and ophthalmic compositions), mica, and fused SiO2 were carefully examined in the electron microscope using Pt‐C replicating techniques. The complex glasses show an undulating structure at 300 to 500 Å, whereas the mica and fused SiO2 do not. The low contrast observed between peak and valley is consistent with a small difference in chemical composition. The observed structure is explained in terms of an extension of the anionic model for molten silicates whereby the peaks and valleys result from clustering of silicate anions accompanied by partial expulsion of cations to the periphery.
Depolymerization of GeO2 and B2O3‐GeO2 Network Glasses by Sb2O3
Authors: RIEBLING E. F.
Published: June 2006

Binary Sb2O3‐GeO2 glasses containing 45 mol% Sb2O3 and ternary Sb2O3‐B2O3‐GeO2 glasses containing 50 mol% GeO2 were prepared. Their densities (volumes), refractive indices, and infrared spectra were determined, and their colors and high‐temperature viscosities were estimated visually. Small amounts of Sb2O3 (∼10 mol%) appear to perturb neither the Ge‐O‐Ge network nor those B‐O‐Ge networks with small B/Ge ratios (∼0.2). The B‐O‐Ge networks with larger B/Ge ratios (∼1.0) depolymerize in the presence of even less Sb2O3. Amounts of Sb2O3 >10 mol% appear to depolymerize the Ge‐O‐Ge and Ge‐O‐B networks progressively, possibly with the formation of chains. A structurally sensitive ir isofrequency contour technique developed for ternary glass systems was applied successfully to these Sb2O3‐B2O3‐GeO2 glasses. These contours can thus readily detect significant network depolymerization in the absence of the usual network modifiers.
Effect of Lime on Hydration of Pastes Containing Gypsum and Calcium Aluminates or Calcium Sulfoaluminate
Authors: MEHTA P. K.
Published: June 2006

Chemical reactions responsible for ettringite formation in the commercial expansive cements are reviewed. X‐ray diffraction analyses of the hydrated expansive compounds showed that in the presence of Ca(OH)2 and CaSO4 the tricalcium aluminate hydrated very slowly and the monocalcium aluminate quickly, whereas the calcium sulfoaluminate hydrated at a uniform rate during the early hydration period. Scanning micrographs are presented which show that ettringite formed in the presence of Ca(OH)2 is colloidal. It is proposed that colloidal ettringite can adsorb large quantities of water, thereby causing slump loss in fresh concrete and expansion in hardened concrete.
Elastic Properties of Polycrystalline Monoclinic Gd2O3
Authors: HAGLUND J. A. , HUNTER O.
Published: June 2006

The elastic properties of polycrystalline monoclinic Gd2O3 were determined by the sonic‐resonance method. Volume‐fraction porosity varied from 0.025 to 0.367 and temperature from room temperature to 1400°C. The Young's and shear moduli are linear functions of volume‐fraction porosity, but the rate of their decrease with increasing porosity is less than that expected. The moduli decreased more rapidly than expected with increasing temperature. The Debye temperature is 362°K. With increasing temperature, the first Grueneisen constant, γ, decreases, whereas the second Grueneisen constant, δ, increases.
Creep of Mixed‐Oxide Fuel Pellets at High Stress
Authors: ROUTBORT J. L. , VOGLEWEDE J. C.
Published: June 2006

The rate of steady‐state compressive creep in (U,Pu)O2‐ε was investigated in the power‐law creep region (at a constant stress of 69 MN/m2 between 1600° and 1500°C) as a function of the oxygen‐to‐metal (O/M) ratio (1.88 to 1.995). The creep rate over this range is independent of the starting material and decreases with increasing O/M ratio. The apparent activation energy for creep and the preexponential structure factor are sensitive functions of the O/M ratio, with approximately the same dependence on this ratio as these parameters measured at low stresses. These results imply that diffusion of the same defect species controls creep deformation in both the stress‐assisted diffusion region (ε∞σ) and the dislocation‐motion region (ε∞σ4.4).
Electrical and Mechanical Relaxations in a Mixed‐Alkali Silicate Glass
Authors: HIGGINS T. J. , BOESCH L. P. , VOLTERRA V. , MOYNIHAN C. T. , MACEDO P. B.
Published: June 2006

Electrical relaxation measurements from 0.04 Hz to 2.5 MHz between 80° and 283°C and mechanical relaxation measurements from 120 to 3600 Hz between ‐44° and 305°C were conducted for 0.5Na2O·0.5K2O·3SiO2 glass. The average time constant for the electric‐field relaxation, which is controlled by the diffusion of the more mobile Na+ ion, was smaller by a factor of ∼30 to 50 than that for the mechanical relaxation associated with the mixed‐alkali internal friction peak. This result supports the theory of Day and co‐workers that the time constant for mixed‐alkali mechanical relaxation is controlled by the less mobile alkali ion (K+). The results indicate that a distinct mixed‐alkali electrical relaxation analogous to mixed‐alkali mechanical relaxation is not observed because the electric field in the glass decays rapidly to zero via diffusion of the more mobile cation, rather than because the mixed‐alkali mechanical‐relaxation mechanism is inherently electrically inactive.
Effect of Microstructure on Strength of Si3N4‐SiC Composite System
Authors: LANGE F. F.
Published: June 2006

The Si3N4‐SiC composite system was investigated to better understand the effect of microstructure on the strength‐controlling factors, i.e. fracture energy, elastic modulus, and crack size. Silicon carbide dispersions with average particle sizes of 5, 9, and 32 μm were used to form 3 composite series within this system, each containing 0.10, 0.20, 0.30, and 0.40 vol fraction of the dispersed phase. These composites were fabricated by hot‐pressing. Fracture energy and strength values were measured for each composite. A linear relation between the elastic modulus of the two phases was assumed. The crack size was calculated for each composite using the appropriate property values. The strength behavior of the 9‐ and 32‐μm series was controlled by the crack size, which, in turn, was controlled by the particle size and volume fraction of the SiC phase. Particle size and volume fraction did not affect the crack size of the 5‐μm series, in which strength was controlled by both fracture energy and elastic modulus. Strengths measured at 1400°C and thermal conductivity measurements indicate that several of these composites are promising as high‐temperature structural materials.
Systems of Na‐Fe‐SiO2 Glasses and Steel: II, Moessbauer Spectroscopy*
Authors: BHAT VASANTH K. , MANNING CHARLES R. , BOWEN L. H.
Published: June 2006

Moessbauer effect spectroscopic studies were made on Na‐Fe‐SiO2 glasses before and after wetting experiments on steel in an Ar atmosphere. The glasses prepared under reducing conditions contained Fe3+ ions. The Fe2+/Fe3+ ratio for all glasses increased as the time at wettability test temperature was increased. Iron in the glasses (according to isomer‐shift data) was possibly octahedrally coordinated. The Fe environment in glasses very nearly saturated with Fe oxide was most symmetrical. The glasses prepared under reducing conditions exhibited more symmetrical Fe environments than those prepared under oxidizing conditions. The structural symmetry in the glasses very nearly saturated with Fe oxide facilitated the electron transfer reaction, 2Fe3++Fe0=3Fe2+; the conversion to Fe2+ occurred to the greatest extent in these glasses.
Highlights of Progress in the Science of Fracture of Ceramics and Glass
Authors: WACHTMAN JOHN B.
Published: June 2006

Rapid, catastrophic propagation of cracks is described in terms of stress concentration and energy balance conditions from a fracture‐mechanics viewpoint. Slow crack propagation by stress corrosion or other mechanisms is also presented in fracture‐mechanics terms. Data on slow crack propagation and accompanying acoustic emission in a variety of ceramics and glasses are reviewed. A survey is presented of the application of these results to the quantitative treatment of strength, dependence of strength on loading rate, time to failure, proof testing, acoustic emission monitoring, thermal shock, impact damage, and erosive machining. Some limitations in the application of fracture mechanics to ceramics are discussed.
Properties and Structure of Glasses in the System Bi2O3‐SiO2‐GeO2
Authors: TOPPING J. A. , CAMERON N. , MURTHY M. K.
Published: June 2006

In the system Bi2O3‐SiO2‐GeO2, good glasses can be formed only from limited compositional regions consisting of 2 narrow strips along the lines xBi2O3‐(100‐:t) GeO2 (x≤40) and 40Bi2O3ySiO2 (60‐y)GeO2 (mol%); such glass is dark brown. Compositions from a large region (Bi2O3 content <40 mol%) showed immiscibility. In the binary system Bi2O3‐GeO2, density and refractive index vary linearly with composition (mol%). Negative deviations of molar volume from ideality suggest that the coordination of a significant number of Ge ions is changing from 4‐fold to 6‐fold. Thermal expansion and electrical resistivity data are also reported.
Evaluation of Lattice Site and Valence of Mn and Fe in Polycrystalline PbTiO3 by Electron Spin Resonance and Thermogravimetry
Authors: HENNINGS D. , POMPLUN H.
Published: June 2006

The oxides of Mn and Fe or combinations of La2O3 with these oxides were incorporated into PbTiO3 under defined thermodynamic conditions. The substitution reaction of these additives was examined gravimetrically. At low partial pressures of O2 and PbO vapor, Mn2+ and Fe2+ are formed at the A sites, and Mn3+ and Fe3+ enter both A and B sites. At higher partial pressures of PbO, Mn4+ is most probably formed at the B sites.

The Mn2+, Mn4+, and Fe3+ were determined by ESR.
Internal Friction of Glasses with Low Water Contents
Authors: DAY DELBERT E.
Published: June 2006

Internal friction was measured for NaPO3 glasses containing from 0.0004 to 0.146 wt% H2O. In glasses containing ≤ 0.001 wt% H2O, the only internal friction peak observed was that resulting from the Na+ ions, i.e. the alkali peak; it was ∼ 30% larger than in as‐melted glasses. The second peak normally observed in as‐melted NaPO3 glass was completely absent when the water content was < 0.001 wt%, consistent with the interpretation that it is caused by a reorientation of proton‐Na+ elastic dipoles. The concept that the second peak in as‐melted alkali phosphate and silicate glasses results from nonbridging oxygen ions is not substantiated by these studies of NaPO3 glasses, which contain a high percentage of these ions but do not exhibit this peak when the proton concentration is sufficiently low.
Sintering of CoO: II, Intermediate Stage*
Authors: KUMAR P. , JOHNSON D. L.
Published: June 2006

Dilatometry and stereology were used to study the intermediate‐stage sintering of CoO as a function of O2 partial pressure and temperature. Volume‐diffusion sintering kinetics were observed, but the diffusion coefficients calculated from the sintering data were between the values previously determined for Co and O using tracer techniques. The apparent diffusion coefficient is independent of po2 below 10−3 atm and increases with increasing po2 above this pressure. Carbon dioxide retards mass transport. Grain growth and sintering are correlated; the grain‐growth rate constant is approximately proportional to the sintering volume‐diffusion coefficient.
Model for Strength of Brittle Porous Materials
Authors: HRMA PAVEL , ŠATAVA VLADIMÍR
Published: June 2006

Many porous bodies form by the growth and joining of individual solid particles into a network‐like structure. Assuming that the distribution of particles in the porous body is random, the relation between strength and porosity was derived and compared with commonly used empirical relations.
Plausible Concepts Necessary and Sufficient for Interpretation of Ceramic Grain‐Boundary Phenomena: II, Solute Segregation, Grain‐Boundary Diffusion, and General Discussion*
Authors: KINGERY W. D.
Published: June 2006

Solute segregation and phase separation at and near grain boundaries are analyzed in terms of possible modes of compositional variation and applicable driving forces, several of which are of practical importance. The importance of compositional variations, including both equilibrium and transient phenomena, and a range of “grain‐boundary widths” is discussed as related to grain‐boundary diffusion and to the current status of grain‐boundary concepts.
Fracture of Hardened Cement Paste in Relation to Surface Forces and Porosity
Authors: RADJY FARIBORZ
Published: June 2006

The fracture surface energy, γ, is interpreted for two limiting fracture mechanisms. In the first mechanism, all the gel particles encountered by the propagating crack break. In the second case, the gel particles are assumed to be whiskerlike and do not break. Only in the first model is the comparison of γ, with the surface energy of tobermorite relevant. Both models take the effect of porosity on γ, and the fracture stress into account.
Transition from Ionic to Electronic Conduction in Pure ThO2 Under Reducing Conditions
Authors: CHOUDHURY N. S. , PATTERSON J. W.
Published: June 2006

Open‐circuit emf and ac conductivity studies were conducted on two batches of dense polycrystalline ThO2. The open‐circuit emf data were used to delineate the low‐po2 ionic domain boundary for “pure” ThO2, which is presented as a log Pθ line on a log Po2‐1/T diagram. In addition the ionic conductivity, σion, and the high‐Po2 log Pθ boundary were also determined, mainly from ac conductivity measurements, which also confirmed the Po2I/4 dependence of σp, the p‐type electronic conductivity, shown by other investigators. The main results are, for the first batch, log Pθ= 12.7−220.2 × 103/4.575T, log σion= 1.9−44.3×103/4.575T, and log Pθ=−1.0−31.4 × 103/4.575T; for the second batch, log Pθ=11.2−219.7 × 103/4.575T, log σion= 1.7−41.6 × 103/4.575T, and log Pθ=0.6−40.4 × 103/4.575T. The oxygen permeability of ThO2 tubes and the oxidation rate constant of Th were predicted from the conductivity and emf data and compared with direct measurements previously reported. The calculated and previously measured permeabilities agreed very well; however, the correlation between the predicted and previously measured oxidation kinetics was somewhat less satisfactory.
Vibrational Spectra of Spodiosite Analogs
Authors: KOWALCZYK L. N. , CONDRATE R. A.
Published: June 2006

Spodiosite analogs are a class of compounds related to the apatite family. The ir spectra (4000 to 200 cm−1) and the laser‐Raman spectra were obtained for several analogs (Ca2(XO4)Cl in which X=P, As, or V) prepared by a reaction flux technique. Fundamental modes were identified, and empirical band assignments were made. Unquestionable band assignment of the v2 modes can be made using Raman spectra. Effects of solid‐state interactions were noted in the spectra and interpreted by factor‐group analysis using the factor group D2h. The spectra of chlorspodiosite (Ca2(PO4)Cl) were compared with the spectra of synthetically prepared chlorapatite.
Viscous Flow in Glass During Phase Separation
Authors: SIMMONS JOSEPH H. , MILLS SUSAN A. , NAPOLITANO ALBERT
Published: June 2006

The isothermal viscosities of two borosilicate glasses, one a commercial glass widely used for chemical glassware, increase by 4 to 5 orders of magnitude with heat‐treatment time near the annealing point. The glasses are basically sodium borosilicates but differ greatly in phase‐separation characteristics. Electron micrographs were used to analyze the development of microstructure during the suspected phase separation. In both glasses, structure development is primarily responsible for the viscosity increase. An analysis of the data and a theoretical interpretation of the effect are presented.
Precipitation Studies in the System WC‐ZrC
Authors: BRUN M. , STUBICAN V. S.
Published: June 2006

The solubility of WC in ZrC varied from 25 mol% at 1800°C to 65 mol% at 2550°C. There was no detectable solubility of ZrC in WC in the same temperature range. Tungsten carbide precipitates from supersaturated WC‐ZrC solutions as rods. The crystallographical relation between the precipitated phase and the matrix established by electron diffraction techniques is: [100]wc|[110]ZrC and [010]wc|[011]zrc. The interfacial planes were (001) for WC and (111) for ZrC. The hardness of the solid solution of 55 mol% WC and 45 mol% ZrC increased ∼ 10% after it was annealed at 1900°C.
Plausible Concepts Necessary and Sufficient for Interpretation of Ceramic Grain‐Boundary Phenomena: I, Grain‐Boundary Characteristics, Structure, and Electrostatic Potential
Authors: KINGERY W. D.
Published: June 2006

Analogies between grain boundaries in metals and nonmetals, direct observations of grain‐boundary characteristics, and inferences about the atomic structure of grain boundaries are reviewed. The electrostatic potential and associated space charge are discussed, and new experimental measurements showing the existence and sign of the boundary charge are reported for NaCl and Al2O3.
Yielding and Flow of c‐Axis Sapphire Filaments
Authors: TRESSLER R. E. , BARBER D. J.
Published: June 2006

Yielding and flow in uniaxial, constant‐strain‐rate tensile tests of single‐crystal sapphire filaments oriented with the tensile axis parallel to the c axis were studied between 1760° and 1875°C. The existence of a strain‐aging effect leading to non‐reproducible upper yield points was established. The flow stress exhibited a power‐law dependence on strain rate, with stress exponents ranging from 8.50 at 1775° to 12.4 at 1875°C. The apparent activation enthalpies calculated from constant‐temperature experiments reflect a dependence of flow stress on temperature which includes both substructure and deformation‐mechanism temperature dependencies. The preliminary activation enthalpies calculated from differential temperature and differential strain‐rate (constant‐structure) experiments are comparable to the value for defect diffusion in sapphire, 80 kcal/mol. From direct evidence, the {101} planes are identified as the dominant slip planes with the 1/3 〈101〉 Burgers vectors inferred from the testing geometry.
Raman Scattering Study of the Crystallization and Phase Transformations of ZrO2
Authors: KERAMIDAS VASSILIS G. , WHITE WILLIAM B.
Published: June 2006

X‐ray diffraction, electron diffraction, and Raman scattering measurements are presented for dehydrated amorphous ZrO2. Although the material lacks sufficient crystallinity to diffract X rays, electron diffraction patterns indicate a microcrystallinity with a grain size of 15 to 30 A. Raman spectra characteristic of the metastable tetragonal polymorph were obtained from these materials. Heating the amorphous materials induces recrystallization into first a metastable tetragonal phase and then the stable monoclinic phase.
Hot‐Pressing Behavior of Si3N4
Authors: TERWILLIGER G. R. , LANGE F. F.
Published: June 2006

The densification behavior of Si3N4 containing MgO was studied in detail. It was concluded that MgO forms a liquid phase (most likely a magnesium silicate). This liquid wets and allows atomic transfer of Si3N4. Evidence of a second‐phase material between the Si3N4 grains was obtained through etching studies. Transformation of α‐ to β‐Si3N4 during hot‐pressing is not necessary for densification.
Influence of Oxygen Activity on the Sintering of MgCr2O4
Authors: ANDERSON H. U.
Published: June 2006

The sintering behavior of MgCr2O4 powder compacts was investigated as a function of temperature, time, and oxygen activity. The results show that MgCr2O4 cannot be densified to >70% of theoretical density at temperatures up to 1700°C if the oxygen activity exceeds 10−6 atm. The oxygen activity must be decreased to <10−10 atm before densities exceeding 90% of theoretical can be achieved. Weight loss and X‐ray data indicated that maximum density occurred at an oxygen activity just above that where MgCr2O4 becomes unstable.
Directional Solidification of the ZrO2‐MgO Eutectic
Authors: KENNARD F. L. , BRADT R. C. , STUBICAN V. S.
Published: June 2006

The eutectic between ZrO2 and MgO was located at 27 wt% MgO. Directional solidification of this composition yields MgO rods in a cubic ZrO2 matrix. Plane front growth occurred at solidification rates of <2.0 cm/h. The interfiber spacing was proportional to the inverse square root of the solidification rate. In samples where cellular growth occurred, the colony size was not affected by the solidification rate. The room‐temperature fracture strength, ∼23,000 psi, which was not affected by the MgO fiber size, was controlled by the colony size. Annealing for 24 h at 1200°C increased the fracture strength by 50% as a result of a compressive stress in the matrix caused by the cubic‐to‐monoclinic transformation in the ZrO2.
Effect of Slow Crack Growth on the Thermal‐Stress Resistance of an Na2O‐CaO‐SiO2 Glass
Authors: BADALIANCE R. , KROHN D. A. , HASSELMAN D. P. H.
Published: June 2006

The effect of slow crack growth on the thermal‐shock resistance of an Na2O‐CaO‐SiO2 glass was studied. An analytic and numerical technique was developed to calculate the critical quenching temperature difference, ΔTc, of circular rods quenched in water from known crack velocity data. For rods of a specific radius and crack depth, ΔTc, was calculated to be 147°C, in favorable correspondence with experimentally observed values of 155° to 160°C. In the absence of crack growth, ΔTc was estimated to be 238°C, well in excess of the observed value and indicative of the significant effect of slow crack growth on thermal‐stress resistance. It is also shown that crack growth significantly extends the time‐to‐failure to a value much greater than the time of maximum thermal stress. A form of subcritical crack instability is predicted at stress intensity factors well below the critical stress intensity factor, at which catastrophic failure becomes inevitable over the duration of the transient thermal stress. It is suggested that, when all other factors are equal, the effect of slow crack growth on thermal‐stress resistance can be minimized by maximizing thermal diffusivity. It is also argued that surface‐compression strengthening will be more effective than reduction in flaw size in increasing thermal‐stress resistance. Recommendations are made for the design and selection of brittle materials subjected to thermal stress in stress‐corrosive environments.
Light Scattering from Phase‐Separated Glass
Authors: KERWAWYCZ JOHN , TOMOZAWA MINORU
Published: June 2006

The intensity of the light scattered from phase‐separated glasses, rather than being constant as a function of angle as expected, exhibits increased backscattering. When interference effects can be neglected, Goldstein's diffusion‐zone model, proposed to explain the scattering data, has been supported by experimental results. Although the refractive‐index profile predicted by Goldstein seems to agree well with existing data, the present workers believe that the source of this profile is not a diffusion zone but rather the stress that results from the differing thermal‐expansion characteristics of the 2 phases. By measuring depolarized scattering as a function of temperature in various scattering planes, the 2 models were compared directly. The diffusion model is shown to be inappropriate, whereas the qualitative predictions of the proposed stress model are confirmed, and the model is established.
Subsolidus Equilibria in the System LiF‐AlF3‐Na3AIF6
Authors: BAYLOR RICHARD , STINTON DAVID P. , BROWN JESSE J.
Published: June 2006

Subsolidus equilibrium relations in the system LiF‐AlF3‐Na3AlF6 were investigated in air by solid‐state reaction techniques. The system contains 2 ternary compounds, Na2LiAlF6 and Na3Li3Al2F12, and extensive cryolite solid solution on the Na3AlF6 ‐ Li3AlF6 binary join. Both ternary compounds decompose between 500° and 600°C. At and below 500°C, seven 3‐phase regions and two 2‐phase regions exist; the 2‐phase regions result from the extensive cryolite solid‐solution series. Phase relations for the 500°C isothermal section are presented, and relations above and below this temperature are described.
Para‐to‐Ortho H2 Conversion on Gd, Y, Gd2O3, and Y2O3
Authors: ROSSINGTON D. R. , CAPOZZI V. F.
Published: June 2006

The mechanism of parahydrogen conversion was studied on Gd2O3 and Y2O3 powders and on Gd and Y evaporated metal films at low and high temperatures (77° to 90°K and 298° to 418°K). Absolute rates of conversion are compared to theoretical values for 3 possible reaction mechanisms, and it is concluded that a paramagnetic vibrational mechanism is operative on Gd2O3, Gd, and Y. On Y2O3 the reaction rate is enhanced by additional surface paramagnetic sites. The portion of the surface which is active is ∼1 for the metals and ∼0.01 for the oxides.
Mass Spectrometric Studies of the Nitridation of Silicon
Authors: LIN SIN‐SHONG
Published: June 2006

The nitridation of silicon was examined by analyzing the nitriding atmosphere with a mass spectrometer. Silicon monoxide vapor was found throughout the entire temperature range of the process in amounts of the order of 10−3 of the volume of the nitriding agent. Controversial results obtained in previous investigations of nitridation kinetics are explained in terms of the SiO vapor and the O2 partial pressure of the system. The effects of H2 and H2O on nitridation are also discussed.
Compressive Deformation of Polycrystalline UO2
Authors: RADFORD KENNETH C. , TERWILLIGER G. R.
Published: June 2006

The deformation of polycrystalline stoichiometric UO2 in compression exhibits a low‐temperature (<1200°C) behavior that is distinct from its high‐temperature behavior. The data for both temperature regions fit either an Arrhenius equation, =ν exp [‐ΔH(τ)/RT], or the relation =Aτn/T exp [−ΔH0/RT]. At low temperatures, the activation energy and volume, the shape of the yield‐stress‐temperature curve, and the grain size‐strength relation suggest a Peierls mechanism as rate‐controlling in the deformation process. At high temperatures (≳ 1300°C), a different dislocation mechanism becomes rate‐controlling for coarse‐grained material, whereas very fine‐grained (1 μm) material exhibits Nabarro‐Herring deformation.
Review of Synthesis and Properties of Tobermorite, C‐S‐H(I), and C‐S‐H Gel
Authors: SNELL D. S.
Published: June 2006

The investigation and characterization of tobermorites and the synthetic hydrates C‐S‐H(I) and C‐S‐H gel are reviewed, with particular attention to the application of X‐ray diffractometry, ir spectrophotometry, electron microscopy, and differential thermal analysis.
Study of Composition of Leached Glass Surfaces by Photoelectron Spectroscopy
Authors: ESCARD JACQUELINE H. , BRION DOMINIQUE J.
Published: June 2006

The effects of water attack on the surfaces of 2 bulk glasses, including changes with time of exposure to air after water exposure, were determined using X‐ray photoelectron spectroscopy. Although the layer explored was extremely thin (∼30 Å), this nondestructive technique has potential for quantitative analysis. The results obtained for an industrial glass and an experimental nonresistant one are compared, i.e. the rapidity of ion exchange at the glass‐water interface, fluctuation of Na content in the glass surface during water exposure, migration of Na through the SiO2 surface layer, and nonmigration of Ca under the same conditions. The present results agree with those obtained by other workers from solution analysis and other surface‐sensitive physical techniques. The surface of lead crystal glass exposed to dilute acetic acid was also studied.
Thermal Conductivities of As‐Se Glasses
Authors: KURIYAMA MASAAKI
Published: June 2006

The thermal conductivities of As‐Se glasses (0 to 50 at.% As) were measured by a comparative continuous‐heating method from room temperature to above the glass transition. Thermal conductivity increased slowly with increasing temperature, then, in the transition region, decreased suddenly, reaching minimum values. Thermal conductivity at 40°C, as a function of composition, exhibited a maximum at 40% As.
Hot‐Pressing of Silicon Carbide with 1% Boron Carbide Addition
Authors: BIND J. M. , BIGGERS JAMES V.
Published: June 2006

The densification behavior of pressure sintered cubic silicon carbide containing 1 wt% boron carbide was studied as a function of temperature (1750° to 1950°C). Specimens of theoretical density were obtained at 1950° with a pressure of 3000 psi. Experimental results showed that densification proceeded by a plastic flow mechanism. Interpretation of the data in terms of Murray's equation yields an activation energy of (11S ± 18) kcal/mol. At 1850C and above, tabular grains of 6H and 2H SiC were observed in a matrix of fine grains of 3C SiC.
Creep Deformation of 0° Sapphire
Authors: FIRESTONE R. F. , HEUER A. H.
Published: June 2006

Creep deformation of 0° sapphire was studied between 1600°and 1800°C at stresses up to 114 MN/m2. Microscopical evidence (dislocation structures observed by transmission electron microscopy (TEM) and by etch pits) suggested that Nabarro climb was the predominant deformation mechanism. Both the experimental creep rates and stress exponents were in good agreement with those predicted by this model. Although non‐basal dislocations with a ½〈101〉 Burgers vector were present, the good creep resistance of 0° sapphire was attributed to the difficulty of activating pyramidal slip.
Effect of Curing Conditions on the Capillary Porosity of Hardened Portland Cement Pastes
Authors: AUSKERN A. B. , HORN W. H.
Published: June 2006

The capillary pore structure of hardened portland cement pastes cured by high‐pressure steam, chemical acceleration, high‐pressure steam with reactive SiO2, water immersion, water immersion and high‐pressure steam, and hot‐pressing was measured using mercury porosimetry to 50,000 psi. Differences of > 2 orders of magnitude exist in the average capillary pore diameters of the cement pastes studied. The largest pores (∼1 to 3 μm in diameter) are associated with high‐pressure steam‐cured pastes. The smallest average capillaries observed were 0.02 μm for pastes steam‐cured with reactive SiO2. Hot‐pressed pastes had essentially no porosity accessible to mercury. The application of pore size control to problems of polymer‐impregnated concrete is discussed.
Flow Stress of MgO Single Crystals Compressed Along [100] at High Hydrostatic Pressures
Authors: AUTEN THOMAS A. , RADCLIFFE S. VICTOR , GORDON ROBERT B.
Published: June 2006

The compressive yield strength of the soft orientation of MgO single crystals (the [100] loading axis) was measured under hydrostatic pressures up to 1000 MPa in two devices. An initial set of monotonic loading experiments indicated that the yield strength under pressure was decreased by 2.1 to 3.1×10−4MPa−1 in units of fractional decrease (dσ/σ) per increase in chamber pressure. Individual crystals were deformed in increments at various pressures in a series of interrupted tests; the results indicated that pressure had no effect on yield strength. (The interrupted tests would be more likely than the monotonic experiments to demonstrate intrinsic material behavior since they avoided variations in strength among crystals and had more uniform platen friction conditions.) All specimens were tested without protective jackets, leading to slip‐band observations that contradict a previous theory of the effect of jackets on the flow stress of MgO single crystals during high‐pressure tests. It is concluded that hydrostatic pressure up to 1000 MPa has a negligible effect on the yield strength of the soft orientation of MgO single crystals, in accordance with predictions made from earlier models by Davis and Gordon.
Finite Element Analysis of Stresses in Ion‐Exchanged Glass
Authors: VARSHNEYA A. K. , PETTI R. J.
Published: June 2006

Anomalous tensile stress maxima were observed in rectangular specimens of unidirectionally ion‐exchanged glasses. Two‐dimensional finite element analysis was performed to reveal all stress components in these specimens with or without hypothetical variations of specimen size and material properties. Results show that the end effects could be responsible, in part, for the anomalous maxima observed in some of the specimens. Not allof the observations could, however, be accounted for in this manner.
Ferrimagnetic Resonance Study of the Growth Rate of Super‐paramagnetic Precipitates of Magnesioferrite in MgO
Authors: DE BIASI R. S. , DEVEZAS T. C.
Published: June 2006

Superparamagnetic magnesioferrite particles were precipitated from single‐crystal MgO containing °2.2 cation% Fe, by aging between 700° and 1000°C. The anisotropy field of the precipitates was measured as a function of aging time and aging temperature. Superparamagnetic analysis was used to calculate the precipitate growth rate for several aging temperatures. It was found that from the earliest times studied, the particles grow according to the law of diffusion‐controlled coarsening. An activation energy of 72.5 kcal/mol was calculated for this growth process.
Fatigue Resistance of a Lithium Aluminosilicate Glass‐Ceramic
Authors: RITTER JOHN E. , CAVANAGH M. S.
Published: June 2006

The resistance to delayed failure of a lithium aluminosilicate glass‐ceramic was measured by dynamic fatigue experiments on abraded samples in the as‐formed glassy state and after various stages of heat treatment. Heat treatment was varied in a controlled manner to give microstructures of high quartz and keatite with small and large grain size. The glassy state was most susceptible to delayed fatigue, and the keatite state the least. The increase in fatigue resistance of the ceramic phases is attributed to the crystalline structure having a lower reactivity with water.
Application of Load‐Relaxation Techniques to Study Subcritical Crack Growth in Brittle Materials
Authors: VIRKAR ANIL V. , GORDON RONALD S.
Published: June 2006

The load relaxation technique was applied to double‐ cantilever‐beam specimens for the determination of diagrams of the stress intensity factor (K1) vs crack velocity (V) for soda‐lime glass. The effect of machine compliance was explicitly incorporated into the velocity equation. The data were in good agreement with published data from different techniques. It was shown that a hard machine (small compliance) is best suited for the load‐relaxation technique. The use of a soft machine (high compliance) will lead to catastrophic failure during load relaxation. A single‐edge notched‐beam specimen in bending is unsuitable for the relaxation technique since catastrophic failure will occur during relaxation regardless of machine compliance.
Effect of Internal Stress on the Strength of BaTiO3
Authors: POHANKA R. C. , RICE R. W. , WALKER B. E.
Published: June 2006

The strength of BaTiO3 which had been hot‐pressed to ∼99% of theoretical density and tested above the Curie temperature was—9000 psi greater than at room temperature. This measured difference in strength is attributed to internal stress from the cubic→tetragonal phase transformation. Correction of measured strength to zero porosity predicts internal stresses of ∼11,000 psi, in good agreement with the dielectric theory for fine‐grain BaTiO3. The internal stress is nearly independent of grain size in the range ∼1.5 to 150 μm, showing that 90° domains do not reduce internal stresses causing failure. Lower internal stress in commercial BaTiO3 is attributed to greater porosity, large flaw sizes, and possibly the effects of additives.
Estimation of Gibbs Free Energy Functions
Authors: LINDEMER T. B.
Published: June 2006

A computational method has been derived for the estimation of the molar heat capacity contribution to the Gibbs free energy function (fefTO) above 298°K. This method, which is based on the observed temperature invariance of certain ratios of the integrals of these molar heat capacity contributions, has been applied to the analysis of fefTOdata for metal carbides, nitrides, fluorides, borides, and silicides in the crystalline state. The estimated fefTOfor a given compound is more accurate than that obtained by other known estimation techniques and permits a more accurate calculation of ΔG TO than is possible from the commonly used approximation ΔG TO=ΔG 298O‐ ΔG 298O.
Crystallization of Ca(PO3)2 Glass Below the Glass Transition Temperature
Authors: ABE YOSHIHIRO , ARAHORI TADAHISA , NARUSE AKIRA
Published: June 2006

Block specimens of Ca(PO3)2 glass crystallize easily at temperatures considerably lower than the glass transition temperature, Tg; thin blown films of the glass do not exhibit this behavior. The crystals, which are β‐Ca(PO3)2 with a trace of the γ form, are identical to those formed above Tg; they exhibit “hemispherulitic” growth from the glass surface. The activation energy for crystal growth below Tg is 30 kcal/mol, which indicates that the growth mechanism differs from that in blown film samples above Tg (activation energy = 110 kcal/mol). A model for crystallization below Tgis proposed; the phenomenon is explained in terms of induced tension in the molecular chains at the glass‐ crystal interface.
Thermal‐Shock Resistance and Fracture‐Strength Behavior of Two Tool Carbides
Authors: MAI Y. W.
Published: June 2006

The thermal‐stress resistance and fracture‐strength behavior of TiC and WC after severe thermal shocking were investigated. The damage‐resistance parameter, (XIc/δf)2, for both carbides was determined over a wide range of loading rates and temperatures. The fracture‐strength behavior of these two carbides in the quenching temperature range of 25° to 800°C follows Has‐selman's model for an instantaneous strength loss at a critical quenching temperature. When WC was shocked at temperatures > 800°C, it exhibited higher retained strengths because of its higher (KIc/δf)2 values at these temperatures. The effects of specimen size and repeated thermal shocks on retained strength and critical quenching temperature for these carbides were also investigated.
An Experimental Test of Weibull Scaling Theory
Authors: LEWIS DAVID , OYLER S. M.
Published: June 2006

The validity of the Weibull scaling theory for the modulus of rupture was tested for high‐purity alumina ceramics fabricated by 3 processes: extrusion, slip casting, and dry‐pressing. The Weibull parameters were determined by 2 techniques which produced comparable results for the Weibull modulus, m. Greatly varying results were obtained for the lower limiting strength, σu. The Weibull parameters were found to be dependent on the particular fabrication process. Microstructural variations with specimen size produced large variation in the calculated parameters and made the correct assessment of the theory difficult.