bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Machining Damage on the Strength of a Glass‐Ceramic
Authors: Cook R.F. , Lawn B. R. , Dabbs T. P. , Chantikul P.
Published: June 2006

A study has been made of the strength characteristics of machined glass‐ceramic surfaces using an indentation flaw technique. The strength is found to decline or increase with progressive removal of the machining damage layer by a polishing procedure, depending on whether the indentation is made after or before the polishing. The results are interpreted in terms of a residual compressive stress in the initial machined surface. It is argued that failure to allow for the contribution of this stress in the fracture mechanics description of flaw growth could lead to significant errors in the evaluation of crack propagation parameters.
The Determination of Surface Development in Cement Pastes by Nuclear Magnetic Resonance
Authors: BARBIC̆ L. , KOCUVAN I. , BLINC R. , LAHAJNAR G. , MERLJAK P. , ZUPANCIC I.
Published: June 2006

Pulsed proton NMR was used to determine the specific surface of hydrating cement pastes. The method is based on the fact that the measured proton spin‐lattice relaxation rate (l/T1) of water in cement pastes is (in view of the fast exchange between water molecules in the adsorbed and bulk phase) proportional to the fraction of water molecules covering the solid surface and thus proportional to the NMR surface of the newly grown hydration products. In general, the method can be used for powders, fibrous and porous materials in contact with liquid water, or other fluids containing protons.
Ordering Mechanisms in Li Ferrites
Authors: KIMURA TOSHIO , KOHMURA KUNIYOSHI , YAMAGUCHI TAKASHI
Published: June 2006

This paper describes the ordering mechanisms observed in Li ferrite powder samples with different particle sizes and aggregation states, as well as in sintered pellets. Measurement of ordering rates in the powder samples and observation of chemically etched surfaces of partially ordered pellets indicate that the ordering proceeds by a nucleation‐growth mechanism and that the ordering initiates at grain boundaries and prevails into the inner part of the grains. In powder samples, surfaces as well as grain boundaries contribute to the initiation of ordering but their temperature dependences are different. The activation energy of the ordering is calculated to be 192 kj/mol.
Microstructure and Property Characterization of Sintered Si3N4, SiC, and SiAlON
Authors: Dutta Sunil
Published: June 2006

Commercially produced pressureless sintered Si3N4, SiC, and SiAlON were characterized with respect to density, phases present, bend strength, and oxidation resistance. The room‐temperature bend strengths of sintered Si3N4, SiC, and SiAlON are comparable. However, the room‐temperature strengths are much lower (=40 to 50%) than the room‐temperature strength of hot–pressed Si3N4 (NC‐132). The strength loss in Si3N4 and SiAlON materials at high temperature was attributed to a viscous grain‐boundary phase retained during cooling from the sintering temperature. The oxidation resistance of sintered a‐SiC was the best of any materials tested.
Formation of Oxynitride Glasses by ArnmonoJysis of Gels
Authors: Brinker C.J.
Published: June 2006

Porous borosilicate gels were heated in ammonia at 460° C. The ammonia treatments reduced the carbon and hydrogen content and increased the gel sintering temperature. The increased sintering temperature was attributed to an increase in Tg due to chemically dissolved nitrogen.
Morphological Changes of an Intergranular Thin Film in a Poly crystalline Spinel
Authors: LANGE F. F. , CLARKE D. R.
Published: June 2006

An intergranular second phase, present as a film in a large‐grained MgAl2O4 densified with LiF, was observed, on sequential heat treatment at 1500° C, to progressively reduce its area by first forming slender fingers which then break up into spheroid‐shaped islands. This “beading” process is an example of a Rayleigh instability driven by capillarity processes. Electron microscopy revealed that the islands consist of two phases (presumably LiF and MgAl2O4), characteristic of a solidified melt. The observed morphologies are more than likely characteristic of many intergranular phases which form nonzero contact angles with the major phase. An analysis is presented showing that perturbations on the edge of a receding plate can grow by surface diffusion.
Sluggish Pansition Between Tetragonal and Rhombohedral Phases of Pb(Zr,Ti)03 Prepared by Application of Electric Field
Authors: KAKEGAWA K. , MOHRI J. , SHIRASAKI S. , TAKAHASHI K.
Published: June 2006

Tetragonal and rhombohedral phases always coexist in Pb(ZrxTil1−x)03 near the morphotropic phase boundary compositions (0.52≤x≤0.55) when prepared by solid‐solid reaction among the constituent oxides. A special technique was developed to obtain perovskite ceramics exhibiting no co‐existence in the boundary compositions, in which no compositional fluctuation occurred in the B site of the perovskite structure. Even these monophasic perovskite ceramics of the morphotropic phase boundary compositions were two‐phase with the lapse of time after poling. This phenomenon is interpreted in terms of thermodynamics. It is suggested that this phenomenon may closely relate to the origin of aging in piezoelectrics.
Relation Between Power and Exponential Laws of Slow Crack Growth
Authors: Gupta P. K.
Published: June 2006

The coefficients V̂0Q̂, and N of the power law of slow crack growth, V̂=V̂0 exp [–Q̂/(RT)](K/KC)N, are evaluated in terms of the fundamental parameters V0. Q0. and B of the exponential law, V=V0 exp [(‐Q0+BK)/(RT)]. It is shown that N =θ(BKC)/(RT),Q̂=Q̂0−θBKC, and V̂0=V0θ−N, where θ is a dimensionless coefficient with a value ranging from 0.2858 to 1.0, depending on the ratio of stress intensity at the fatigue limit to the critical stress intensity factor.
Micromechanical Model for Fibrous Ceramic Bodies
Authors: GREEN D. J. , LANGE F.F.
Published: June 2006

A micromechanical model is presented to relate the mechanical properties of low‐density fibrous ceramic bodies to their micro‐structures and the properties of the fibers. It is found that properties such as the elastic moduli, fracture toughness, and strength depend on the fiber spacing. In particular, it is shown that the critical stress intensity (Kc) depends on the density of the body with respect to the fiber density, the degree of preferred orientation, the fiber strength, and the fiber radius, suggesting ways of increasing Kc. Furthermore, it is predicted that Kc will be related to the sonic velocity in the material, reflecting variations in the degree of preferred orientation. The model is found to be consistent with the experimental observations on a silica‐based fibrous ceramic, which is being used in the thermal protection system of the Space Shuttle.
Gaseous Fission Product Release Predictions in TRISO HTGR Fuel Particles
Authors: KOVACS W. J.
Published: June 2006

Structural integrity of SiC layers on TRISO‐coated HTGR fuel particles is a critical property necessary to ensure fission product retention. Characterization and detection of microcracks in SiC layers by high‐pressure (69 MPa) mercury intrusion is discussed. Particular emphasis is placed on relating metallic and gaseous fission‐product release to the level of defective SiC layers. Gaseous fission‐product release during irradiation is shown to be more accurately predicted when the fraction of defective SiC layers as determined by mercury intrusion is taken into account.
Crack Propagation in Irradiated B4C Induced by Swelling and Thermal Gradients
Authors: HOLLENBERG G. W. , BASMAJIAN J. A.
Published: June 2006

Boron carbide pellets, used as the neutron absorber materials in fast breeder reactors, were observed to crack in two distinctly different ways. Radial thermal gradients are produced as a result of internal heat generation. In larger‐diameter pellets, these thermal expansion gradients are large and it was observed that pellet cracking occurred early in operation. It was also observed that swelling gradients were established at high burnup levels. These pellets were also cracked but in a different manner.
Effect of Water Content on the Chemical Durability of Na2O 3SiO2 Glass
Authors: TOMOZAWA M. , ERWIN C. Y. , TAKATA M. , WATSON E. B.
Published: June 2006

Na2O 3SiO2 glasses with high water content were prepared under high‐pressure, hydrothermal conditions and their chemical durabilities in water were measured at atmospheric pressure and 30°C. Chemical durability decreases with increasing water content, which is probably due to a positive dependence of H+ (or H3O+) diffusivity on water content.
Segregation and Near‐Surface Diffusion for Undoped and Cr‐Doped CoO
Authors: NOWOTNY J. , SIKORA I. , WAGNER J. B.
Published: June 2006

Work function measurements were used to monitor the re‐equilibration kinetics for the system CoO‐Cr2O3 involving undoped CoO, Cr‐doped CoO, CoCr2O4 spinel phase, and undoped Cr2O3. The activation energy of chemical diffusion for the near‐surface layer was determined for these materials. Combined work function, X‐ray, and ESCA studies indicate that Cr segregates in CoO‐CrzO3 solid solutions, which leads to the formation of a new “surface phase” of spinel‐like structure.
Glass Formation in the System GeO2‐Bi2O3‐Tl2O
Authors: NASSAU K. , CHADWICK D. L.
Published: June 2006

The occurrence of glass in the three binary and one ternary oxide systems containing Ge, Bi, and Tl was studied by air‐ and water‐quenching melted samples. Bulk glass occurred widely in these systems and was obtained with as little as 10 mol% GeO2, combined with 68 mol% BiO1.5 and 22 mol% TIO0.5. Some glass occurred in all compositions except those with the highest Bi2O3 or TI2O contents. Glass specimens were studied by differential thermal analysis to yield glass transition and crystallization exotherm temperatures; the thermal expansion coefficients and densities were also measured. Diagrams are presented to show the variation of these properties as well as λ0, the wavelength for zero material dispersion, in the binary and ternary systems. Based on these data, it is demonstrated that ultralow‐loss optical waveguides can be constructed in the 3‐3.5‐μm regionb.
Surface Diffusion in MgO and Cr‐Doped MgO
Authors: LYTLE S.A. , STUBICAN V.S.
Published: June 2006

The surface diffusion of MgO and Cr‐doped MgO was examined using grain‐boundary grooving. Measurements were taken at T=1100° to 1400°C for a concentration range of 0 to 0.442 at.% Cr. The activation energy for surface diffusion of MgO was found to be 239± kj/mol. The surface diffusivity did not change markedly with chromium concentration. How‐ever, an abrupt change in activation energy from ∼240 to ∼180 kj/mol occurred between 0.038 and 0.254 at % Cr, which may be associated with a change in surface structure.
Effect of LiCl on the Rate of Calcite Decomposition
Authors: KNUTSEN GARY F. , SEARCY ALAN W. , BERUTO DARIO
Published: June 2006

The rate of decomposition of calcite powder in vacuum was measured by the Langmuir and Knudsen methods. Measured pressures in effusion cells in the absence of additives approach the equilibrium value. The Langmuir experiments gave an apparent equilibrium CO2 pressure of =3×10−4 times the known pressure for the reaction. It is suggested that the CO2 is in equilibrium with the calcite but the CaO is not. In Langmuir experiments, 10 wt% LiCl does not increase the initial decomposition rate. In the Knudsen cells, 10 wt% LiCl can increase the initial rate by >2 times. LiCl reduces the surface area of the product CaO under either condition from ≥30 to ∼1 m2/g. Probably an LiCl‐CaO eutectic provides a solution path for CaO recrystallization and, at high CO2 pressures, also for CaCO3 decomposition.
Polarization and Conduction Mechanism in Mixed‐Alkali Glasses
Authors: AGARWAL ANIL K. , DAY DELBERT E.
Published: June 2006

Thermally stimulated polarization and depolarization current (TSPC/TSDC) measurements on mixed alkali (Li‐Na) silicate glasses show two TSDC peaks along with a dc conduction current at high temperatures, similar to single‐alkali silicate glasses. The activation energies for the two TSDC peaks and for dc conduction increase with the addition of a second alkali. The first TSDC peak does not change in size but the second becomes substantially smaller when the second alkali is added. The reduction in size of the second TSDC peak correlates well with the reduction in the dielectric loss hi mixed‐alkali glasses. A model is proposed to account for the behavior of the two TSDC peaks.
Analysis of Liquid‐State Interdiffusion in the System CaO‐Al2O3‐SiO2 Using Multiatomic Ion Models
Authors: OISHI Y. , NANBA M. , PASK JOSEPH A.
Published: June 2006

Concentration profiles after liquid‐state interdiffusion were determined using an electron probe microanalyzer for diffusion couples of ternary oxide systems with an average composition ‐40CaO2OAl2O3‐40SiO2 (wt%) at r=1370° to 1550°C. Multicomponent diffusion equations were derived for three multiatomic ion models in terms of tracer diffusion coefficients of the constituent elements. The sizes of the diffusing units for silicate and aluminate ions were estimated by determining the structure parameters in the diffusion equations so as to provide the best fit of the theoretical calculations to the experimentally determined concentration profiles. The results were compared with the theoretical calculations for the monoatomic ion model and the oxide molecule model as diffusing species.
Influence of MgO Additiions on the Microstructure and Mechnical Properties of Al2O3‐ZrO2 Composites
Authors: Kosmač Tomaž , Wallace Jay , Claussen Nills
Published: June 2006

The effect of MgO and ZrO2 dopants, added separeately or simultaneously, on the grain size, denisity, and toughness of Al2O3 was studied. Small ZrO2 addotoopms (<100 ppm) had little effect, whereas larger amounts decreased the sintered density. Additions of Mgo Up to the solubility limit (∼ 300 ppm) increased both density and grain size; further additions had little effect on the density but strongly reduced grain size.
Fracture Toughness of Metaphosphate Glasses
Authors: Ashizuka M. , Bradt R. C.
Published: June 2006

The fracture toughnesses and Young's elastic moduli of several metaphosphate glasses were measured by the indentation technique. The results show that the normal glasses lie along a KIc vs E line between (0.4, 40) and (0.7, 60) MPa‐m112 and GPa, respectively. The abnormal ZnO and MgO metaphosphates appear to be tougher and are above the line of the normal glasses.
Bubble Rise in Glassmelts
Authors: JUCHA R. B. , POWERS D. , McNEIL T. , SUBRAMANIAN R. S. , COLE R.
Published: June 2006

Bubble‐rise experiments in borate glassmelts were conducted in a transparent furnace at T=800° to 1000°C using video techniques to record and analyze the bubble rise. The results of these experiments over a range of Reynolds numbers from 10 −7 to 10° clearly indicate that the bubble motion is governed by the Rybezynski‐Hadamard equation and hence that the glassmelts are not subject to interfacial contamination.
Analytical Phase Diagram Calculation of the System MnO‐NiO from Experimental Activity Data
Authors: CHO SEUNG‐AM , LEóN‐SUCRE CARLOS E.
Published: June 2006

Venezuelan Institute of Scientific Research and School of Metallurgical Engineering and Materials Science, Central University of Venezuela (UCV), Apartado 51717, Caracas 1050‐A, Venezuela The complete phase diagram of the binary system MnO‐NiO was analytically calculated using the pseudosimpler thermo‐dynamic solution model based on the available experimental activity data. The system exhibits a symmetrical subsolidus miscibility gap with its critical temperature at Tc=386.8°C. The possible spinodal curve is also calculated. The solidus and liquidus reveal complete mutual solubilities between the end components at high temperature. The solidus is a continuously sloping monotonic curve but the liquidus is skewed and concave upward near the MnO end. An explanation of the effect of the cationic size differences on the characteristics of phase relations of similar binary systems formed between the NaCl‐type divalent monoxides is proposed.
Effects of Oxidation and Oxidation Under Load on Strength Distributions of Si3N4
Authors: EASLER T. E. , BRADT R. C. , TRESSLER R. E.
Published: June 2006

The room‐temperature strength distributions of a sintered and a hot‐pressed Si3N4 were examined in the as‐machined condition, after oxidation at 1370°C and after oxidation under load at 1370°C. The strength‐controlling flaw populations were highly transient in nature. Both the duration of oxidation and the magnitude of the applied load were observed to effect changes in strength. This dynamic situation is related to both strengthening and weakening processes, which at times may occur simultaneously in the same strength distribution.
Preparation and Electric Field Alignment of Acicular SbSI Crystals
Authors: Morgan P. E. D.
Published: June 2006

Ferroelectric needles of SbSI may be aligned in an electric field, just as magnetic particles are aligned in magnetic fields, prior to consolidation into a highly oriented polycrystalline ceramic. The discovery, in 1978, should have wide applicability.
Coordination and Bond Character of Silicon and Aluminum Ions in Amorphous Thin Films in the System SiO2‐Al2O3
Authors: Hanada T. , Soga N.
Published: June 2006

The coordination and bond character of the cations in amorphous SiO2‐Al2O3 films prepared by rf sputtering were examined by studying chemical shifts in the SiK and AlK X‐ray emission spectra. The coordination number of Si ions in these films was always 4, regardless of composition, whereas the average coordination number of Al ions changed from 4 to 5, depending onAl2O3 content. TheSi‐O‐Al bond type seemed to appear as Al2O3 was introduced into amorphous SiO2 films or vice versa.
Pressureless Sintering of SiC
Authors: Omori Mamoru , Takei Humihiko
Published: June 2006

Pressureless sintering of SiC was accomplished at 2100°C with oxide additives. These additives were the products of the reaction of Al(OH)3 with HCl and of Y(OH)3 with HCOOH. These reaction products were dissolved in water and mixed with submicrometer β‐SiC. A mixture of equal weights of these additives was effective for the sintering of SiC.
Model of Structural Relaxation in Glass with Variable Coefficients
Authors: Scherer George W. , Rekhson Simon M.
Published: June 2006

The model of structural relaxation by Narayanaswamy deals only with the constant temperature coefficients of the glass property, p. The assumption is seldom valid for real glasses. In this work, equations are obtained for the fictive temperature of glass if the temperature coefficient, dp/dT, is a linear function of T or of 1 /T2. The temperature dependence of p has no significant influence on the calculated fictive temperatures.
Crack Blunting of High‐Silica Glass
Authors: ITO S. , TOMOZAWA M.
Published: June 2006

The mechanical strength of abraded high‐silica glass was measured after immersion in water and silicic acid solution at room temperature and 88°C. The strength increase was observed at 88°C. This phenomenon is usually explained by crack blunting. From an observation of the dissolution rate of a porous glass with a similar composition and a comparison of the strength increase in water and silicic acid, it was concluded that the dissolution and subsequent precipitation of the high–silica glass is the mechanism of the crack blunting.
Volatilization Associated with the Sintering of Polyphase Si3N4 Materials
Authors: Lange F. F.
Published: June 2006

Weight loss which occurs while sintering composited Si3N4 powders requires the loss of one or more of the end‐member constituents through a volatilization reaction. By plotting the direction of the compositional change on the appropriate equivalence phase diagram, the principal volatilization reaction can be determined. For a particular composition in the system Si‐Y‐N‐O sintered at 1750°C, the principal volatilization reaction was Si3N4(s) +3SiO2(s)→6SiO(g) +2N2(g).
Ordered Fluorite‐Related Phases in the Systems ZrO2(HfO2)‐MgO‐Nb2O5 (Ta2O5)
Authors: Michel D. , Perezy Jorba M.
Published: June 2006

New phases in the ternary systems ZrO2 (HfO2)‐MgO‐Nb2O5 (Ta2Os) were identified. These M7O12 compositions have fluorite superstructures isomorphous with the rhombohedral Y6UO12 type.
Effect of γ Radiation on High‐Water‐Content‐Glasses
Authors: ACOCELLA J. , TAKATA M. , TOMOZAWA M. , WATSON E.B. , WARDEN J.T.
Published: June 2006

Na2O−3SiOz‐glasses containing up to ∼11 wt% water were prepared under high‐pressure, hydrothermal conditions. The samples were exposed to γ radiation of 5 × 107 rad. Optical and ESR measurements were made before and after exposure to radiation and it was found that the induced absorption in the visible region decreased drastically with increasing H2O concentration. The induced ESR signal‐integrated intensity also decreased with increasing water content. After irradiation an increase was observed in the fundamental OH absorption bands in the ir region of the spectra of the water‐containing specimens.
Surface Crystal Formation During Acid Corrosion of Phosphate‐Doped Lead Silicate Glass
Authors: LEHMAN RICHARD L. , GREENHUT VICTOR A.
Published: June 2006

The effect of small P2O5 additions on the acid corrosion behavior of lead silicate glass was studied at 22°C. Crystals of penta‐lead phosphosilicate, a phase heretofore synthesized only by high‐temperature melt techniques, formed on the surface of the phosphate‐doped glass during the corrosion process. No crystals were observed to form on the pure lead silicate glass. Crystallization appears to result from the controlled release of phosphorus and lead at the glass surface and/or from the reactive catalytic properties of a silica gel surface film. Apparent lead diffusion rates into the acid solution for the phosphate‐doped lead silicate glass are much less (by a factor of 11,3) than for the pure lead silicate glass. Application of these findings is suggested in two areas: formulation of low cation‐release glass and glaze compositions and the synthesis of high‐temperature phases in aqueous solution at ambient temperatures.
Alpha Alumina Formation in Alum‐Derived Gamma Alumina
Authors: DYNYS F.W. , HALLORAN J. W.
Published: June 2006

The formation of alpha alumina in gamma alumina obtained from ammonium alum was studied at 1150°C. The transformation occurs by a nucleation and growth process, preceded by an incubation time. Mechanical treatments of the gamma alumina markedly increase the density of nuclei and hence the transformation rate. The alpha alumina forms in porous colonies consisting of a single crystal with an interconnected wormy morphology.
Vibrational Spectra of HfO2–ZrO2 Solid Solutions
Authors: Krebs M. A. , Condrate Robert A.
Published: June 2006

Vibrational spectra were measured and analyzed for HfO2–ZrO2 solid solutions. Some Raman bands in the high–wave–number region shift almost linearly with changes in ZrO2 concentration between the pure end–members; their band locations can be used to determine ZrO2 concentrations in annealed solid solutions. The Raman bands of pure ZrO2 and HfO2 can be correlated using the band shifts of the solid solutions. Induced stress causes band shifts for the solid solutions.
Development of Low–Thermal Expansion Mullite Bodies
Authors: Leipold Martin H. , Sibold Jack D.
Published: June 2006

A series of ceramic compositions based on variations in the crystal–glass ratio of a mullite body were developed. The thermal expansion of these compositions varies from 3.7 to 5.0×10–6/°C to 800°C. The materials are particularly useful for applications involving silicon, in that an identical thermal expansion is available. The high–temperature creep data for the lower–expansion compositions are inferior as a result of their higher glass contents. Raw material sources and fabrication procedures for specific compositions are given.
Hydrothermal Reaction‐Sintering of Monoclinic HfO2
Authors: Toraya Hideo , Yoshimura Masahiro , Somiya Shigeyuki
Published: June 2006

Sintered monoclinic HfO2 bodies were fabricated below the transformation temperature from Hf metal and water by hydrothermal reaction‐sintering. Sintering was observed above 900°C under 100 MPa for 3 h. Generally, both the bulk density and the crystallite size of the sintered bodies increased with increasing temperature. Bodies with the maximum relative density (0.98) were obtained by treatment above 1000°C.
Elastic/Plastic Indentation Damage in Ceramics: The Lateral Crack System
Authors: MARSHALL D.B. , LAWN B.R. , EVANS A.G.
Published: June 2006

The mechanics of lateral crack propagation in a sharp‐indenter contact field are described. The driving force for fracture has its origin in the residual component of the elastic/plastic field, which becomes dominant as the indenter is unloaded. Expressions for equilibrium crack evolution are derived, with due allowance for the close proximity of crack plane and specimen free surface. As with the median/radial crack system considered in an earlier paper, the ratio hardness‐to‐modulus complements toughness in the fracture relations. The basic predictions of the theory are examined in terms of experimental measurements of lateral crack dimensions in materials with a wide range of mechanical properties. The prospects of predicting the extent of lateral fracture in other ceramics, and thence of establishing a base for analyzing such important practical properties as surface erosion, are discussed.
Dependence of Fracture Toughness of Alumina on Grain Size and Test Technique
Authors: MUSSLER BERNHARD , SWAIN MICHAEL V. , CLAUSSEN NILS
Published: June 2006

The fracture toughness (KIc) of sintered alumina was measured using notched beam (NB) and indentation/strength‐in‐bending (ISB) techniques. KIc (NB) decreases with increasing grain size. For fine‐grained materials (<5 μm), NB results overestimate KIc, and exhibit a substantial notch‐radius sensitivity. A stress‐intensity‐derived model is used to explain this notch sensitivity. The ISB results are very similar to those obtained using the double‐cantilever‐beam (DCB) method and show an increasing fracture toughness with increasing grain size. The differences between the NB and ISB (DCB) results for coarser‐grained materials are thought to be related to R‐curve behavior.
Heat Capacities of Heavy‐Metal Fluoride Glasses
Authors: Gavin D. L. , Chung K.‐H. , Bruce A. J. , Moynihan C. T. , Drexhage M. G. , El Bayoumi O. H.
Published: June 2006

The heat capacities of a series of heavy‐metal fluoride glasses were measured by differential scanning calorimetry from −23°C to above the glass transition. On a per mole of atoms basis all the heat capacities, Cp, at ambient temperature were ∼22 J/mol·K and near the glass transition they approached the Dulong‐Petit limit of 3R. The differences, ΔCp between liquid and glass heat capacities per mole of atoms at the glass transition were large compared to those for oxide glasses.
Thermal Conductivity of Mn2SiO4 in the [100] Direction from 80 to 400 K
Authors: Moore J. P. , Finch C. B.
Published: June 2006

The thermal conductivity, λ, of synthetic single‐crystal tephroite (Mn2SiO4) was Measured from 80 to 400 K with heat flow in the [100] direction. Although the results do not show a strong influence of the antiferromagnetic‐paramagnetic transition at 163 K, analysis of the data indicates that the high‐temperature λ may be limited by a combination of 3‐phonon processes plus scattering by the disordered magnetic moments.
Physics of Fracture
Authors: LAWN BRIAN R.
Published: June 2006

The underlying physical bases of present‐day fracture theory are examined. It is proposed that the atomically sharp crack should be taken as the cornerstone for modeling propagation processes at the fundamental level. lkansmission electron microscopy evidence is presented in support of this contention. Linear continuum fracture mechanics is shown to have intrinsic limitations in its capacity to describe crack‐tip phenomena; a more realistic description is provided by lattice statics, incorporating the picture of a crack as a narrow slit terminated by nonlinear linkage bonds. This description establishes a powerful starting point for understanding and predicting the effects of important crack‐tip interaction processes.‘pwo such processes, chemically enhanced slow crack growth and process‐zone toughening, are discussed in this light. Finally, the nature of strength‐controlling flaws in brittle ceramics is considered, with particular reference to the validity of the widely adopted hypothesis that such flaws may be regarded as true micro‐cracks.
A Comparison of Reducing and Oxidizing Heat Treatments of Hot‐Pressed Silicon Nitride
Authors: CLARKE D.R.
Published: June 2006

The effects of postfabrication heat treatments in argon and air atmospheres at 1400°C on the microstructure of a hot‐pressed silicon nitride are compared. Heating in argon leads to a disappearance of the intergranular glass phase (with the consequent formation of interconnected porosity) and a reduction in the content of silicon oxynitride, and results in a soft, easily abraded surface. On subsequent heating in air, an oxide scale, similar to that produced by oxidation, rich in impurity and additive cations, is formed on the surface. In addition, silicon oxynitride and silica are formed below the scale. On cooling, macroscopic cracking, attributed to the volume increase of SO2 on transformation to cristobalite, occurs. Material heated first in air and then in argon resembled that of material heated in argon alone.
Viscoelastic Analysis of the Split Ring Seal
Authors: SCHERER GEORGE W.
Published: June 2006

A viscoelastic analysis is provided that describes the change with temperature and time of the radius of curvature of a split ring seal. The setting temperature of the split ring depends strongly on the ratio of the thicknesses of the constituent layers. It is possible to predict that dependence and to relate the setting temperature of the split ring to that of a sandwich seal of the same materials. Thus, the change in radius of curvature of a split ring can be used to predict the stresses in a sandwich seal.
Conductivity of Strongly Reduced and Quenched Ceramic La‐Doped SrTiO3 Oregon Graduate Center, Beaverton, OR 97006
Authors: Odekirk Bruce , Balachandran U. , Eror N. G. , Blakemore J. S.
Published: June 2006

Polycrystalline LaxSr1–xTiO3 (0x0.01) was quenched following equilibration at 1325 K in oxygen activities within the range 10−20a0210−8. Electrical conductivity at 10 to 500 K supports an Sr vacancy model previously indicated by high‐temperature equilibrium measurements, with excess oxygen compensating the La donors for ao210−18 and Ti site‐controlled valency conduction for stronger prequench reduction. Deductions about the reduced state from quenched‐condition conductivity are less straightforward than those obtained from high‐temperature measurements because of impurity scattering and impurity banding for x0.002.
Microstructure in Hydrated Silicate Glasses
Authors: Tomozawa Minoru , Capella Steven
Published: June 2006

A microstructure ranging from 5 to 20 nm in size was detected by small‐angle X‐ray scattering in Na2O·3SiO2 and 4EaO·28Na2O·68SiO2 glasses hydrated at ∼100° and 80°C, respectively. This observation suggests the presence of silica gel‐solvent (water) phase separation.
Statistical Analysis of Bending Strengths for Brittle Solids: A Multiaxial Fracture Problem
Authors: LAMON JACQUES , EVANS ANTHONY G.
Published: June 2006

A general approach for the statistical analysis of brittle fracture under tensile multiaxial stress states was evaluated for a fracture criterion based on recent concepts of noncoplanar crack extension. The resultant failure expressions were applied to bend tests and used to interpret experimental results obtained with porcelain cylinders. Implications for the interpretation of bending tests and some important trends in the failiire strength with specimen size are discussed.
Grain‐Boundary Diffusion of 51Cr in MgO and Cr‐Doped MgO
Authors: OSENBACH JOHN W. , STUBICAN VLADIMIR S.
Published: June 2006

The grain‐boundary diffusion product, D'δ, of 51Cr in MgO and Cr‐doped MgO as a function of grain‐boundary orientation and point‐defect concentration was determined at T=1200° to 1450°C. A large degree of anisotropy was found in the grain‐boundary diffusion behavior in MgO. The ratio of D'δ|| parallel to D'δ‖ perpendicular to the growth direction, D'||/D'‖, is 102 for a 5° (100) tilt boundary, decreased to ∼2 in boundaries with tilt angles > 10°. The decrease in D'||/D'‖ is due to a large increase in D'‖ with increasing tilt angle. The results indicate that grain‐boundary diffusion in MgO is connected to the orientation of dislocations and the mechanism is one of dislocation pipe diffusion. The grain‐boundary diffusion product D'δ increases with increasing Cr concentration in MgO and is ∼4 times larger for MgO containing 0.56 at. % Cr than for the undoped MgO. For all bicrystals studied, the activation energies are within 180 ± 20 kJ/mol which is 60% of the activation energy for 51Cr diffusion in undoped MgO.
Effect of Thermal History on the Properties of a Willemite Glass‐Ceramic
Authors: SHELBY J. E. , NICHOLS M. C.
Published: June 2006

The effect of time and temperature on the properties and structure of a willemite glass‐ceramic were measured. ‐0 polymorphs of willemite were found to exist in most of the glass‐ceramics. Those properties of the glass‐ceramic which depend primarily on the composition of the glassy phase (helium permeation and diffusion, and chemical durability) were found to be strong functions of the thermal history of the samples, whereas those properties which depend on both the compositions and the relative amounts of both the glassy and crystalline phases (thermal expansion coefficient, density, and refractive index) were found to be relatively insensitive to thermal history. The properties of the glass‐ceramics could be described by simple mixing models using the properties measured for willemite and for glasses having theoretical residual glass compositions corresponding to varying degrees of crystallinity in the glass‐ceramic.
Positron Annihilation in Phase‐Separated BaO‐SiO2 Glasses
Authors: Zanotto Edgar Dutra
Published: June 2006

The kinetics of amorphous phase separation in BaO‐SiO2 glasses was studied by small‐angle X‐ray scattering and positron annihilation. Results show that the annihilation of positrons depends primarily on the average composition of the glasses; it therefore does not depend on compositional fluctuations caused by phase separation and cannot be used to determine the kinetics of phase separation in glasses.
Effect of Hot‐Pressing Temperature on the Thermal Diffusivity/Conductivity of SiC/AlN Composites
Authors: Bentsen Larry D. , Haselman D. P. H. , Ruh Robert
Published: June 2006

Data are presented for the thermal diffusivity/conductivity of hot‐pressed mixtures of SiC and AlN. Results indicate that hot‐pressing at higher temperatures, which permits solid‐solution formation, results in significantly lower thermal diffusivity/conductivity than is obtained by hot‐pressing at lower temperatures, at which discrete phases exist.
Elastic Properties of Hafnium and Zirconium Oxides Stabilized with Praseodymium or Terbium Oxide
Authors: Dole Stephen L. , Hunter Orville
Published: June 2006

Elastic moduli of polycrystalline HfO2 and ZrO2 stabilized with 33 mol% Praseodymium oxide or terbium oxide were studied as functions of porosity and temperature. Thermal expansion was measured to 1200°C for each composition. The modulus‐vs‐porosity and thermal expansion curves were typical; however, modulus‐vs‐temperature relations were abnormally steep below 400°C, a phe nomenon observed previously for stabilized HfO2 compositions. All compositions exhibited obvious stoichiometric changes resulting from oxidation‐reduction annealing, but only the Tb4O7‐stabilized ZrO2 showed any significant effect of oxidation state on the modulus values; this effect was very pronounced.
Gamma Radiation Effects on the Strength of a Borosilicate Glass
Authors: ZDANIEWSKI W.A. , EASLER T.E. , BRADT R.C.
Published: June 2006

The effects of60Co gamma radiation on the strength‐related mechanical properties of a borosilicate glass were examined. Although the glass darkened considerably, only a very slight densification was observed after irradiation to levels of 108rads. The strength distributions were not appreciably changed by the irradiation, nor was the calculated slow crack growth parameter, orNvalue. Neither did radiation affect the elastic modulus or the fracture toughness of the glass. Gamma radiation does not affect the strength below 108rads.
Thermodynamic Measurements and Modeling of
Authors: BESMANN THEODORE M.
Published: June 2006

Measurements were made of (CO) pressure over  at 1823, 1773, 1673, 1573, and 1473 K for various values of x, in the presence of <UC1.86). The data were used to model  as a solid solution of  and , where ΔGxs> is represented by an expansion in two terms, the first of which is temperature‐dependent. The model allows for the accurate prediction of chemical activities and phase equilibria in regions also containing , ,  ‐ , and  ‐ (U).
Empirical Dependence of Elastic Moduli on Porosity for Ceramic Materials
Authors: DEAN E. A. , LOPEZ J. A.
Published: June 2006

Twenty‐seven sets of elastic modulus‐porosity data spanning ranges > 20% in porosity were fitted by linear, exponential, Hasselman, and 2/3‐power relations. The fits were tested for goodness of fit, for extrapolation to zero‐porosity moduli, and for consistency in determining zero‐porosity Poisson's ratios and bulk moduli. On the whole, the linear relation appears to give a superior fit. This result is seen as supportive of self‐consistent theoretical calculations of moduli in porous media. Nonlinear data sets are attributed to a tendency for the spheroidicity of pores to increase with porosity.
Effect of Particle Size on the Kinetics of the Reaction Between Magnesium and Aluminum Oxides
Authors: BERETKA JULIUS , BROWN TREVOR
Published: June 2006

The kinetics of the reaction MgO+A12O3+MgAl2O4 was studied using both unsized “small” (∼1 to 3 pm) and sized “large” (∼50 and 100 μm) particles of the reactants within narrow size distributions and uniform shapes. The data were fitted to various equations commonly used in kinetic analyses of isothermal reactions of solids, analyzed by graphical analysis and linear regression analysis, and compared in terms of the predictive mean square error (MSE) criterion. Graphical and linear regression analysis showed that the reaction could be described almost equally well by first‐order kinetics, the equation derived by Avrami‐Erofe'ev for the nucleation and growth model, and the equations of Jander, Carter, and Ginstling‐Brounshtein for diffusion models. The MSE method gave similar results, except that the model of Avrami‐Erofe'ev was marginally better than the other four equations. The results are compared with those of the systems BaCO3+Fe2O3 and MgO+Fe2O3 studied previously.
Effect of MgO Solute on the Kinetics of Grain Growth in A12O3
Authors: Bennison Stephen J. , Harmer Martin P.
Published: June 2006

The kinetics of grain growth in fully dense Al2O3 with and without MgO solute additions were measured. The MgO was found to retard boundary migration and to rkvelop more uniform microstructures. A grain‐growth mechanism involving solute partitioning of segregated ions (calcium and magnesium) between different boundary types is proposed. Implications regarding the sintering of Al2O3 are discussed.
Properties and Structures of RO‐GeO2 Glasses
Authors: SHELBY J. E.
Published: June 2006

Glass‐formation limits were determined for the PbO, CaO, SrO, and ZnO germanate systems. All of these systems exhibit phase separation over at least a portion of the glass‐formation range. Data are reported for the thermal expansion coefficient, glass‐transformation temperature, and helium permeability of these glasses. The results appear to be primarily determined by the morphology of the glasses.
Experimental Assessment of Pore Breakaway During Sintering
Authors: SAKARCAN METIN , HSUEH C. H. , EVANS ANTHONY G.
Published: June 2006

Experimental measurements of intragranular pore‐size distributions and of the shapes of distorted pores attached to grain boundaries were used to examine the podgrain‐boundary separation problem. In situ evaluation of the ratio of the boundary mobility‐to‐surface diffusivity from pore distortion measurements, coupled with prior analytic expressions for the critical pore breakaway condition, permitted specific intragranular pore sizes generated by separation to be predicted. The predictions lie within the distribution of measured intragranular pore sizes.
Formation of α‐Si3N4 Solid Solutions in the System Si3N4‐A1N‐Y2O3
Authors: Huang Zhen‐Kun , Greil Peter , Petzow Günter
Published: June 2006

The subsolidus phase diagram of the quasiternary system Si3N4‐AlN‐Y2O3 was established. In this system α‐Si3N4 forms a solid solution with 0.1Y2O3: 0.9 AIN. The solubility limits are represented by Y0.33Si10.5Al1.5O0.5N15.5 and Y0.67Si9A13ON15. At 1700°C an equilibrium exists between β‐Si3N4 and this solid solution.
Compressive Creep and Oxidation Resistance of an Si3N4 Material Fabricated in the System Si3N4‐Si2N2O‐Y2Si2O7
Authors: Lange F. F. , Davis B.I. , Graham Henry C.
Published: June 2006

The compressive creep behavior and oxidation resistance of an Si3N4/Y2Si2O7 material (0.85Si3N4+0.10SiO2+0.05Y2O3) were determined at 1400°C. Creep re sistance was superior to that of other Si3N4 materials and was significantly in creased by a preoxidation treatment (1600°C /120 h). An apparent parabolic rate constant of 4.2 × 10−11 kg2·m‐4·s−1 indicates excellent oxidation resistance.
Measurement and Interpretation of Thermopower in Oxides
Authors: TRESTMAN‐MATTS A. , DORRIS S.E. , MASON THOMAS O.
Published: June 2006

The influence of point‐defect gradients on thermoelectric coefficient measurements in oxides is quantified. This permits corrections to be made for data collected by conventional means and suggests alternative techniques of thermopower measurements which maintain the average sample temperature while imposing thermal gradients. Experimental results on CoO, Fe3O4, and FeO in equilibrium with various atmospheres are considered.
Mechanical Properties of Fine‐Grained Substoichiomebic Titanium Carbide
Authors: MIRACLE DANIEL B. , LIPSITT HARRY A.
Published: June 2006

An attempt was made to lower the ductile‐to‐brittle transition temperature (DBTT) of polycrystalline TiC and to prevent the premature intergranular fracture noted in stoichiometric Tic at high temperatures by producing four substoichiometric compositions chosen from the single‐phase TiC phase field. Each desired composition was prepared by blending and vacuum hot‐pressing the appropriate mixture of Ti powder and stoichiometric TiC powder. Each billet was characterized for density, hardness, lattice parameter, and microstructure. The actual bulk compositions were determined by averaging electron probe microanalysis data collected randomly from a polished section of each billet. Specimens cut from the billets were strength‐tested in four‐point bending from room temperature to 1400°C and in compression from room temperature to 1200°C. A qualitative determination of the material's ductility was obtained from a load vs deflection plot and by optical microscopy of polished surfaces after deformation. Both the hardness and strength dropped with decreasing C/Ti atom ratios. Billets produced at the lower C/Ti atom ratios showed a significant deviation from linearity of the load/ deflection curve at temperatures as low as 1200°C in bending, with little or no drop in strength, and as low as 600°C in compression.
Enhanced Oxygen Diffusion at 1400°C in Deformed Single‐Crystal Magnesium Oxide
Authors: Henriksen Anders F. , Chiang Yet‐Ming , Kingery W. David , Petuskey William T.
Published: June 2006

The rate of oxygen difusion at 1400°C into pure, deformed, and scandium‐doped MgO was measured using a secondary‐ion mass spectrometer with 16O‐ primary ions as sputtering agents to monitor the penetration of 18O into the sample from an 18O‐enriched surface layer. In samples doped with scandium, oxygen diffusivity did not differ significantly from the value, 1.3 × 10‐15 cm2/s, found for the pure sample. In deformed samples, the diffusion coefficient was 5.8 × 10‐15 cm2/s.
Preparation of High‐Silica Glasses from Colloidal Gels: I, Preparation for Sintering and Properties of Sintered Glasses
Authors: RABINOVICH E.M. , JOHNSON D. W. , MacCHESNEY J.B. , VOGEL E.M.
Published: June 2006

A new method for preparing fused silica and high‐silica glasses through the sougel process is described. Ordinarily, fused silica is formed by fusion of silica at 1900° to 2000°C. The method described consists of preparation of a porous body by mixing colloidal silica with water, drying, redispersion, molding by casting, followed by gelation, drying, and sintering at 1450° to 1500°C. The sintering is facilitated by addition of 3 to 5 wt% B2O3. The process of dispersion‐drying‐redispersion (so‐called “double process”) results in formation of a two‐mode pore structure which permits drying of cast tubes and rods without breakage. The preparation of the porous bodies, study of pore structure, and properties of resulting glasses are described in Part I, the sintering is considered in Part II, and the mechanism of gel formation is interpreted from infrared spectroscopy studies in Part III.
Analysis of Thermal Stress Failure of Segmented Thick‐Walled Refractory Structures
Authors: CHANG W. S. , KNIGHT C. E. , HASSELMAN D. P. H. , MITCHINER R. G.
Published: June 2006

A finite‐element analysis was conducted of the thermal stresses in rectangularly shaped components of segmented thick‐walled refractory structures with length in the direction of heat flow much larger than the height or width. Heating was assumed to occur uniformly at a linear rate over the front face defined by the smaller dimensions. Most calculations were carried out for two‐dimensional conditions of plane stress, which assumes that the magnitude of stress is independent of height. A limited number of three‐dimensional calculations permitted analyzing the effect of height. For moderate values of heating rates, corresponding to those possibly encountered in practice, a biaxial tensile stress was found to exist some distance behind the hot face with the maximum value of stress along the center‐line parallel to the component length, in accordance with the fracture mode observed in practice. At high heating rates, however, the maximum tensile stress component was found to occur parallel to the face being heated.

For conditions of plane stress under moderate heating rates, the magnitude of stress was found to be proportional to heating rate and an inverse function of the thermal diffusivity. Maximum values of stress were encountered for intermediate values of width. This latter result suggests that the incidence of thermal stress failure may be reduced by either reductions or increases in the values of width commonly used in practice. The three‐dimensional calculations indicated that the magnitude of stress rises rapidly with height. This result suggests that improvements in thermal stress resistance can be obtained by reductions in the values of height from the current values in practice. By means of a fracture‐mechanical analysis it is demonstrated that cracks oriented perpendicularly to the direction of heat flow formed as the result of the above stresses could be unstable and are expected to propagate across the total cross section to lead to total separation, in accordance with observations. The stress values obtained in the present study were compared with those predicted by the Kienow theory. Reasonable agreement was found for the peak stresses at the lower values of time. However, with increasing duration and the higher values of height the Kienow theory underestimates the magnitude of stress considerably.