bluuebunny/crossref_metadata_2025 huggingface dataset:
Transient Effects of Nuclear Radiation on the Dielectric Properties of Refractory Low‐Loss Ceramics at Microwave Frequencies
Authors: EDELSON D. , JAEGER R. E. , WILLIAMS J. C.
Published: June 2006

Selected ceramic materials were exposed to an intense burst of nuclear radiation to determine the transient effects of such an environment on their dielectric properties at frequencies in the L, S, and X bands. The radiation source was a pulse reactor of the swimming pool type which delivers a maximum dose of 2 × 1014/cm2 and 5 × 105 rd gamma during a 13 ms pulse. The dielectric constant and loss of the materials under test were measured before, during, and after irradiation using a resonant cavity technique successfully adapted to the unusual conditions of the experiment. The time resolution in the dielectric measurements was 10 ms. Results are reported for polycrystalline alumina, beryllia, magnesia, boron nitride, clear and opaque fused silica, and, for comparison, four organic dielectrics. Among the materials which exhibited radiation effects, changes in dielectric constant ranged from 0.001 to 0.008 and in tan δ from a factor of 2 to several orders of magnitude.
Critical Evaluation of the Literature Data on Beta Alumina and Related Phases: I, Phase Equilibria and Characterization of Beta Alumina Phases
Authors: DE VRIES R. C. , ROTH W. L.
Published: June 2006

From a critical evaluation of the available data on the system Na2O‐Al2O2 and particularly on β‐Al2O3 some of the discrepancies in the literature were resolved, and plausible phase diagrams for the system were constructed. The diagrams define needed equilibrium data and are useful in putting into perspective the problems that have arisen with regard to synthesis and characterization of the 3–block and 3–block forms of β‐Al2O3.
Mobility of Cation Vacancies in the Solid Solution KCl‐RbCl
Authors: HOLT J. B. , SOCKEL H. G. , SCHMALZRIED H.
Published: June 2006

Of theoretical interest is the way in which the jump frequencies of cations A+ and B+ vary in the solid solution (A+ B+)X‐ as a function of composition. To provide experimental data for analysis, the electrical conductivity of various compositions of pure and doped KCI‐RbCI was measured, and from these measurements the cation vacancy mobility μv(c+) was calculated. In this system μv(c+) was constant. The relation of this finding to mass transport properties is discussed.
Thermal Expansion Anisotropy of Oxides and Oxide Solid Solutions
Authors: KIRCHNER HENRY P.
Published: June 2006

The thermal expansion properties of the oxides stishovite (the rutile form of SiO2), tetragonal vanadium dioxide, and zincite (ZnO) were measured. Solid solutions of rutile‐, corundum‐, and zincite‐structure oxides were prepared, and the thermal expansion properties were measured in an effort to find compositions with more isotropic thermal expansion coefficients. The changes in the observed lattice constants and the changes expected on the basis of the cation radii were compared for both pure oxides and the oxide solid solutions. The difference in these changes indicates the variations to be expected in the thermal expansion anisotropy. A composition range containing the thermally isotropic composition in the system A12O2,‐Cr2O3 was determined, and the isotropic composition was estimated to be 75% Al2O3‐25% Cr2O3.
Ion‐Selective Properties of a Polished Porous Glass
Authors: HAIR MICHAEL L. , ALTUG INCI
Published: June 2006

Some samples of porous glass were polished on one side and their properties as ionic membranes determined. It is shown that polishing forms a layer on the surface of the glass which reduces porosity. The charge density of the resultant membrane is increased and its selective response to K+ and Na+ ions in the presence of Ca++ ions is increased. Electron micrographs of the polished surface and charge density data both indicate reduction of the pore size in the polished surface layer as compared to the original glass. It is suggested that these results indicate that the polishing process is primarily one of viscous flow caused by frictional heating.
Phototrogy of Alkaline Earth Tungstates Doped with Bismuth
Authors: SAKKA SUMIO
Published: June 2006

Calcium, strontium, and barium tungstates and their solid solutions, doped with a small amount of bismuth, color remarkably on exposure to ultraviolet light and return to the original white by thermal fading or optical bleaching. The factors affecting the darkening and fading processes were studied experimentally. Under the same irradiation conditions the intensity of uv‐induced coloration depended on the Bi content and preparation temperature of the tungstates. Bi contents higher than 0.1 molyo and preparation temperatures higher than 1250°C gave strong coloration. The rate of fading of uv‐induced color increased with increasing Bi content. The fading was accelerated by heating; almost all the uv‐induced color disappeared at 230°C in 30 min. The fading rate was also increased by visible light exposure. The uv‐induced absorption spectra varied systematically with the composition of the tungstates.
Kinetics and Mechanism of High‐Temperature Reactions of Kaolinite Minerals
Authors: DUNCAN J. F. , MACKENZIE K. J. D. , FOSTER P. K.
Published: June 2006

The kinetics of mullite growth from kaolinite and halloysite at high temperatures were studied by quantitative X‐ray diffraction analysis. The evidence in both cases is consistent with a nucleation‐controlled model.
Fracture Surface Energy of Glass
Authors: WIEDERHORN S. M.
Published: June 2006

Fracture surface energies of six glasses were measured using the double‐cantilever cleavage technique. Values ranged from 3.5 to 5.3 J/m2 depending on the chemical composition of the glass and the temperature of the test. The fracture surface energy increased with decreasing temperature and increasing Young's modulus; however, exceptions to this behavior were noted. The magnitude of the values obtained is discussed with respect to the theoretical strength of glass and possible irreversible effects at the crack tip such as stress corrosion and plastic deformation are considered.
Crystallization and Melting Kinetics of Cristobalite
Authors: WAGSTAFF F. E.
Published: June 2006

By measuring internal crystallization in vitreous silica, the kinetics of crystallization and melting of cristobalite were determined near the melting point. Both processes followed linear growth kinetics and could be completely represented by the equation μ=AΔT/η. Analyses of the crystallization data and observations of the crystal morphology indicate that crystallization occurs by a continuous growth mechanism. Melting was heterogeneous, occurring only at the glass‐crystal interface and along grain boundaries.
Ammonium Iodate—A Potentially Useful Nonlinear Optical Material
Authors: CRANE G. R. , BERGMAN J. G. , GLASS A. M.
Published: June 2006

The space group of NH4IO3 was determined, from precession photographs and observations of symmetry‐related characteristics, to be Pna21. The cell constants are a0= 6.3740 ± 0.0005, b0= 6.4115±0.0005, and c0=9.1706±0.0005 Å. The crystal is piezoelectric along all three axes and pyroelectric with dP/dT⋍(3±1)×10−9 C/cm2°C along the c0 axis at room temperature. At 85°C a first‐order transition to a nonpolar piezoelectric phase is observed.
Strengthening of Glass Fibers: I, Cladding *
Authors: KROHN† D. A. , COOPER A. R.
Published: June 2006

Experiments on development of internal stress in fibers by cladding borosilicate glass with 96% silica glass are reported. The axial compression in the cladding, determined photo‐elastically, agreed well with the increased strength determined in axial tension fracture experiments. In turn, these results agreed well with the axial compressive stress predicted by analysis when proper allowance was made for stresses which can develop when the core is fluid and the cladding rigid.
Characterization of Freeze‐Dried Al2O3and Fe2O3
Authors: JOHNSON D. W. , SCHNETTLER F. J.
Published: June 2006

Oxide crystallite formation and growth from freeze‐dried sulfates were studied for the representative materials Al2O3and Fe2O3. Transmission and scanning electron micrographs showed the formation and growth of chainlike aggregates of crystallites. Aggregation occurred as part of the nucleation and growth of the oxide, and discrete oxide particles were never present. Orientation of the chain aggregates was related to the ice structure formed during freezing. X‐ray line broadening data showed that crystallite size is a function of the 1/5 to 1/7 power of time for isothermal treatments. A qualitative analysis of material transport favored the surface diffusion mechanism.
Deformation of Polycrystalline MgO Under Pressure
Authors: PATERSON M. S. , WEAVER C. W.
Published: June 2006

Polycrystalline MgO appears to deform ductilely when it is strained under confining pressures greater than 2 kbars at room temperature. Although the {110}〈110〉 slip systems in MgO do not provide the five independent slip systems required for homogeneous deformation, the lack is apparently met in part by the activity of additional slip systems and in part by microfracture. The strong pressure dependence of the stress‐strain curves probably arises from the pressure sensitivity of microfracture, which becomes less important as the temperature is raised; high‐temperature experiments at 2 and 5 kbars confining pressure show that the pressure dependence virtually disappears at 750°C and that there is a simultaneous trend toward wavy slip. High‐temperature experiments on single crystals oriented for cube slip produced only kinking below 300°C; above 300° there is some cube slip, but it is not clear that this is a major deformation mechanism. The pressure dependence of the fracture and flow stresses is interpreted in terms of recent theories of crack propagation under pressure.
Relation of Dissolved Nitrogen to Phototropy of Reduced Silicate Glasses
Authors: SWARTS E. LOWELL
Published: June 2006

Soda‐silica glasses melted under strong reducing conditions (graphite crucible) contain the precursor of a 570‐nm color center. Glasses thus prepared under an N2 atmosphere are readily colored by exposure to ultraviolet light; the uv‐generated color fades rapidly at room temperature, giving phototropic response. However, with glasses melted under N2‐free atmospheres (for example, Ar), this phototropic response is greatly attenuated or absent. The precursor of the 570‐nm center evidently results from strong reduction of the glass independent of the covering atmosphere, since X irradiation generates the same characteristic optical absorption in glasses melted under both N2 and Ar. For compositions melted under N2, the amount of N dissolved correlates positively with increasing SiO2 content and phototropic response. The dissolution of N markedly affects the uv absorption of otherwise “clean” glass. An N‐free reduced soda‐silica glass exhibits essentially the same uv absorption as a similar oxidized glass. However, as N dissolves in the glass, the uv cutoff moves to longer wavelengths. The phototropic response of reduced glasses under long‐wavelength uv light is greatly enhanced by incorporation of europium. Again, an N2 atmosphere during melting yields improved phototropic response. Dissolved N lowers the dissolved H2O content and increases the concentration of Eu in the lower valence state (Eu2+).
Stress Corrosion and Static Fatigue of Glass
Authors: WIEDERHORN S. M. , BOLZ L. H.
Published: June 2006

Stress corrosion cracking of six glasses was studied using fracture mechanics techniques. Crack velocities in water were measured as a function of applied stress intensity factor and temperature, and apparent activation energies for crack motion were obtained. Data were consistent with the universal fatigue curve for static fatigue of glass, which depended on glass composition. Of the glasses tested, silica glass was most resistant to static fatigue, followed by the low‐alkali aluminosilicate and borosilicate glasses. Sodium was detrimental to stress corrosion resistance. The crack velocity data could be explained by the Charles and Hillig theory of stress corrosion. It is probable that stress corrosion of glass is normally caused and controlled by a chemical reaction between the glass and water.
Formation, Microstructure, and Properties of Barium Zirconium Sulfide Ceramics
Authors: NITTA TSUNEHARU , NAGASE KANEOMI , HAYAKAWA SHIGERU
Published: June 2006

The preparation, microstructure, and properties of BaZrS3 ceramics are described. The compound was prepared by heating either an equimolar mixture of BaS and ZrS2 or BaZrO3 alone in CS2 gas. The reaction of the BaZrO3−CS2 system was extremely rapid, as would be expected in an autocatalytic reaction caused by very strong reduction by CS2. Well‐sintered bodies were obtained only by the BaZrO3−CS2 reaction. The time dependence of grain growth was affected considerably when reaction and sintering occurred above 1250° C. Non‐uniform grain growth which occurs when an excess of Ba is present can be interpreted in terms of secondary recrystallization resulting from segregation of BaS at the grain boundary during sintering. The BaZrS3 ceramic was stable in air up to 550°C and was oxidized above that temperature.
Elastic Properties of Polycrystalline Ytterbium Oxide
Authors: POWELL BOB R. , HUNTER ORVILLE , MANNING WILLIAM R.
Published: June 2006

The sonic resonance technique was used to determine the elastic moduli of polycrystalline Yb2O3 samples of rectangular cross section ranging in pore fraction from 0.058 to 0.27 at temperatures from 25° to 1000°C. The data showed that the Young's and shear moduli are best related to pore fraction by a linear equation and to temperature by the Wachtman equation. The Debye temperature for Yb2O3 was calculated from the Young's and shear moduli to be 385°K.
A Model of Structural Relaxation in Glass
Authors: NARAYANASWAMY O. S.
Published: June 2006

Study of the time dependence of physical properties in the transformation range of glass is complicated by the “memory effect” and the inherent nonlinearity which are characteristic of structural relaxation. A multiparameter model of structural relaxation is presented that differs from earlier models in that it takes account of both these effects. This model fits available experimental data well; these data were obtained for the most part by observing the evolution of properties (such as density or refractive index) following a step change in temperature. The present model also permits prediction of the physical properties of glass subjected to arbitrary and more complex temperature‐time histories. It should, therefore, also be useful in the rational design of heat treating processes such as annealing.
Lattice Parameters of (U,Pu)N Solid Solutions
Authors: TENNERY V. J. , BOMAR E. S.
Published: June 2006

The lattice parameters of the cubic solid solutions from the system (U1‐∞Pu∞)N were measured at 25°C. The lattice constant, a0, of UN was 4.88918±0.00017 Å and that of PuN was 4.90476±0.00008 Å. The parameter of the solid solution does not depend linearly on the composition parameter, x. The X‐ray specimens contained from 600 to 1200 ppm total O+C as impurities; thus, these samples had much lower impurity levels than those of previous investigators. The compositions of the X‐ray specimens were corroborated by quantitative chemical analysis. The significant deviation from Vegard's relation and the small variation observed for a0, in this system are discussed in terms of the strongly hybridized electron f–d bands that have been calculated for these compounds.
Polymorphic Transformation of PbO by Isothermal Wet Ball‐Milling
Authors: SENNA MAMORU , KUNO HIROSHI
Published: June 2006

Mechanochemical polymorphic transformation of massicot to litharge was achieved by isothermal wet ball‐milling. A first‐order relation was observed between the fractional conversion and the milling time; the apparent activation energies for various milling conditions were obtained by kinetic treatments. The fact that the Arrhenius equation can be applied to the mechanochemical transformation suggests that the reaction cannot be explained by a simple local heating mechanism and that a mechanical effect has a fundamental role. The effects of the number, volume, and density of the milling balls on the rate constant agreed fairly well with those predicted by Matsui's equations for impact comminution in ball‐milling, indicating that the action of the milling balls may be similar to that in impact comminution of granular materials.
Improved Hot‐Pressed Electrooptic Ceramics in the (Pb,La)(Zr,Ti)O3 System
Authors: HAERTLING G. H.
Published: June 2006

Physical, electrical, and optical properties of selected compositions within the lead lanthanum zirconate‐titanate (PLZT) system are presented, including chemically and thermally etched microstructures, examples of residual porosity defects, temperature dependence of remanent polarization and coercive field, lateral switching strains, and thermal expansion and optical transmission data. Optical‐quality ceramics were produced by stringent raw material selection and improved ceramic processing and hot‐pressing. Optimum results were obtained from starting oxides with higher purity (99.9%) and smaller particle size (<2 μm) than those used in previous investigations. These oxides were processed by methods developed to minimize segregation and promote chemical homogeneity and hot‐pressed at temperatures (1050° to 1250°C) higher than those used previously to remove residual porosity.
Oxygen Concentration Cell and Electrical Conductivity Measurements on PZT Ferroelectrics
Authors: BURT J. G. , KRAKOWSKI R. A.
Published: June 2006

The dependence of an averaged ionic transport number and the ac conductivity on oxygen activity was determined at 500° and 600°C for Nb2O5‐doped Pb(Zr1‐∞Ti∞)O3 (x=0.07, 0.10, 0.20, and 0.35). The electrical conductivity has a p‐character and indicates a complex dependence on [Ti] and Po2. For Po2<10‐3 atm the electrical conductivity increases linearly with [Ti]. For oxygen activities from 1 to 10‐8 atm all PZT compositions showed some degree of electronic conduction.
Hydration of Tricalcium Silicate
Authors: COLLEPARDI MARIO , MASSIDDA LUIGI
Published: June 2006

The kinetics of paste, bottle, and ball‐mill hydration of 3CaO SiO2 and the effects of additions of electrolytes and alcohols were studied. Paste and bottle hydrations proceed through periods of induction, acceleration, and decay. If 3CaO SiO2 is hydrated in an excess of H2O, as in bottle hydration, the reaction rate is lower than that for paste hydration. The ball‐mill hydration rate is much the highest and is controlled by the removal of the hydrate layer coating the 3CaO SiO2 particles. Electrolytes always accelerate and alcohols retard the reaction rate. Experimental results are discussed with reference to modern theories of the 3CaO SiO2 hydration mechanism.
Effect of Crystallization on the Mechanical Properties of Li2O‐SiO2Glass‐Ceramics
Authors: FREIMAN S. W. , HENCH L. L.
Published: June 2006

Variations in the thermal expansion coefficients, elastic moduli, and fracture strengths of Li2O‐SiO2glass‐ceramics were determined as a function of nucleation treatment and volume fraction of crystals present. Strength enhancement was attributed to a decrease in the mean free path between crystals as crystallization proceeds. It is postulated that the eventual reduction in strength in some glass‐ceramics is caused by the development of localized cracks at the crystal‐glass interface as a result of the volumetric changes which occur during crystallization.
Deformation, Recrystallization, Strength, and Fracture of Press‐Forged Ceramic Crystals
Authors: RICE ROY W.
Published: June 2006

Sapphire and ruby were very difficult to press‐forge because they deformed without cracking only in a limited temperature range before they melted. Spinel crystals were somewhat easier and MgO, CaO, and TiC crystals much easier to forge. The degree of recrystallization that occurred during forging (which was related to the ease and type of slip intersections) varied from essentially zero in Al2O3 to complete (i.e. random polycrystalline bodies were produced) in CaO. Forging of bi‐and polycrystalline bodies produced incoherent bodies as a result of grain‐boundary sliding. Strengths of the forged crystals were comparable to those of dense polycrystalline bodies of similar grain size. However, forged and recrystallized CaO crystals were ductile at lower temperatures than dense hot‐pressed CaO. This behavior is attributed to reduced grain‐boundary impurities and porosity. Fracture origins could be located, indicating that fracture in the CaO occurs internally as a result of surface work hardening caused by machining.
Nonisothermal Techniques for Studying Initial‐Stage Sintering
Authors: WOOLFREY J. L. , BANNISTER M. J.
Published: June 2006

Nonisothermal techniques for analyzing initial‐stage sintering are reviewed. It is shown that in principle they permit the determination of all the sintering parameters (rate law, activation energy, and diffusion coefficients) normally obtained from isothermal experiments. Comparison with results from isothermal experiments performed on the same materials (UO2 and ThO2 gel) demonstrates the validity and accuracy of the nonisothermal techniques.
Diffusion in the System K2O‐SrO‐SiO2: IV, Mobility Model, Electrostatic Effects, and Multicomponent Diffusion*
Authors: VARSHNEYA A. K. , COOPER A. R.
Published: June 2006

Experimentally determined coefficients to describe multicomponent diffusion in the system K2O‐SrO‐SiO2 are compared with predictions of a simple model assuming ideal solution behavior. The agreement is satisfactory overall. On the basis of this model, it is suggested that the differences in mobilities of various ions cause deviations from the condition ΣZ4j4=0 (Z=valence, j=flux) of no space‐charge buildup. Although these deviations are small, e.g. after diffusion for 1 s, (charge flux)/(ionic flux) ≤ 10−7, the electric field caused by this deviation from electrical neutrality influences the diffusion kinetics markedly.
Secondary Phase Separation in Lead Borate Glasses
Authors: SHAW R. R. , BREEDIS J. F.
Published: June 2006

The effect of secondary phase separation on the apparent volume fraction of second phase was examined in the immiscible region of the PbO‐B2A3 system. The volume fraction, V2, of the PbO‐rich phase agreed well with theoretical expressions relating V2 to density and composition, except for 0.2<V2<0.5. The discrepancy in this region was explained in terms of the peculiar shape of the immiscible phase boundary and the resulting mutual supersaturation of the immiscible phases with respect to each other.
Machinable Glass‐Ceramics Based on Tetrasilicic Mica
Authors: GROSSMAN DAVID G.
Published: June 2006

Tetrasilicic‐mica glass‐ceramics are formed from glasses in the simple quaternary system K2O‐MgF2‐MgO‐SiO2. The mica phase crystallizes as small (400‐Å) spherical grains at #625°C and recrystallizes into larger booklets at 940°C. The mica platelets grow by a dissolution and reprecipitation mechanism, thereby increasing the aspect ratio during growth. Two regions of strength dependence on grain size are found. When the mica platelets are very small, the strength increases as the 1/5 power of the mica‐plate diameter. Above a critical plate diameter, the strength decreases, being inversely proportional to the first power of the mica‐plate diameter. In the second region, the strength appears to be controlled by the interfacial shear stresses developed as a result of the thermal expansion mismatch between the mica platelets and the residual glass.
Acoustic Detection of Ferroelectric Phase Transitions in PLZT Ceramics
Authors: KRAUSE J. T. , O'BRYAN H. M.
Published: June 2006

Lanthanum‐doped lead zirconate‐titanate ferroelectric ceramics (PLZT) were tested acoustically using the pulse‐echo technique at 20 MHz. Phase transitions occurring within the range –269° to 300°C were detected in poled and unpoled Pb1.02‐xLax(Zr0.65Ti0.35)O3 with x=0.00 to 0.15. For x=0.00 to 0.08 a phase transition observed between 0° and 100°C was defined by changes in the longitudinal sound velocity as great as 5% accompanied by pronounced absorption peaks. Measurements of resonance and electromechanical coupling in poled samples indicate that this transition involves 2 ferroelectric phases. The acoustic techniques reported made possible the detection of the subtler phase transitions in PLZT ceramics with a degree of sensitivity much higher than that available from the more conventional techniques. Both poled and unpoled samples may be tested by this technique, so that the influence of domain orientation on the transitions observed can be demonstrated.
Diffusion and Reaction Studies in the System Al2O3‐SiO2
Authors: DAVIS ROBERT F. , PASK JOSEPH A.
Published: June 2006

High‐temperature diffusion kinetics and phase relations between couples of fused SiO2or cristobalite and sapphire or mullite were investigated in air and in helium. Subsolidus liquid formation between sapphire and cristobalite indicates the existence of a metastable system without mullite. A liquid phase is considered to be essential for the nucleation of mullite. The growth rate of mullite exceeded its dissolution rate in semi‐infinite fused‐SiO2‐sapphire couples above 1634°C. The inter‐facial liquid compositions provided data for a minor revision of the mullite liquidus curve. Diffusion coefficients calculated from the Al profiles vary greatly with concentration and temperature, resulting in a large range of values for apparent activation energy, which decreases with increasing Al2O3content (∼310 to ∼60 kcal/mol for ∼4 to ∼22 wt% Al2O3). The diffusion process in the liquid is considered to be cooperative movement of oxygen‐containing Al and Si complexes whose nature changes with composition and temperature; this change in the diffusing species contributes to the range in the values of experimental apparent activation energies.
Synthesis and Mechanical Properties of Stoichiometric Aluminum Silicate (Mullite)
Authors: MAZDIYASNI K. S. , BROWN L. M.
Published: June 2006

Highly dense translucent polycrystalline bodies of stoichiometric aluminum silicate (mullite) were obtained by vacuum‐hot‐pressing of high‐purity submicron mixed oxide powders. The powders were prepared by the hydrolytic decomposition of mixed metal alkoxides. X‐ray diffraction and electron microscopy indicated that the initially amorphous needlelike fine particulates transform into highly crystalline orthorhombic mullite at ∼1200°C. Optimum hot‐pressing conditions were 5 kpsi and 1500°C for 30 min. Densities within experimental error of the theoretical value of 3.19 g/cm3 were obtained. A typical microstructure consisted of fine interlocking needlelike grains arranged in an overall mosaic or “jigsaw” pattern. Microprobe traverses across the samples indicated homogeneous dispersion of SiO2 in the AlO3 matrix. Room‐temperature mechanical properties were measured and correlated with the microstructure and crystal structure of the ceramic compact. A slightly higher melting temperature than has been previously reported was observed for the 3Al2O3·2SiO2 studied. All specimens exhibited a smooth surface finish and excellent thermal‐shock resistance from 1200°C to room temperature.
Equilibria of SiF4 with SiO2, Be2SiO4, and BeO in Molten Li2BeF4
Authors: BAMBERGER C. E. , BAES C. P.
Published: June 2006

Equilibrium partial pressures of SiF4 were measured for the reactions 2SiO2(c)+2BeF2(d)⇋SiF4(g)+Be2SiO4(c) (log PsiF4(mm) = [8.790 ‐ 7620/T] ±0.06(500°–640°C)) and Be2SiO4(c) +2BeF2(d)⇋SiF4(g) +4BeO(c)(log PsiF4(mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF2 was present in solution with LiF as molten Li2BeF4. The solubility of SiF4 was low (∼0.04 mol kg‐1 atm‐1) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved ΔHf and ΔGf values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO2 and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF4 generated showed an increasing positive deviation from the expression given for the equilibrium involving Be2SiO4 and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO2.
Creep of CaO‐Stabilized ZrO2
Authors: ST‐JACQUES R. G. , ANGERS R.
Published: June 2006

The compressive creep of 18 mol% CaO‐stabilized ZrO2 was studied at 1200° to 1400°C and 500 to 4000 psi. The specimens were polycrystalline with grain diameters from 7 to 29 μm. The activation energy for creep is 94 kcal/mol, and the creep rates are linearly proportional to the stress and to the inverse of the grain size. These results lead to the conclusion that creep in 18 mol% CaO‐stabilized ZrO2 may be controlled by cation diffusion associated with grain‐boundary sliding.
Gas Solubility in Relation to the Structures of Glasses and Liquids
Authors: FAILE S. P. , ROY D. M.
Published: June 2006

Alkali borate and B2O3 glasses containing large concentrations of gaseous noncondensed compounds (including Ar and H2) were synthesized at high temperatures and pressures; the solubilities of the gases were determined. These materials quenched from the liquid state had glass networks with densities lower than those typical of normal glasses not containing such gases. This result is consistent with dilution in the liquid state, where the gas dissolves by generating space for itself in the melt rather than by filling interstices. For Henry's law to be obeyed, as it was in the present work, the gas could not fill large interstices only, since high pressures (up to 9 kbars) tend to destroy these interstices. The solubility data obtained are consistent with the trends predicted by a simplified theory that makes use of the surface tensions of the liquids and the molecular sizes of the gases in a system.
Microstructural Changes During Heat Treatment of Sintered Al2O3
Authors: MOCELLIN A. , KINGERY W. D.
Published: June 2006

Samples of both high‐purity and Mg‐doped Al2O3 sintered in H2, N2, O2, or vacuum were annealed at 1650° to 1850°C for times up to 64 h. In pore‐free systems, grain growth is limited by the mobility of Mg‐rich second‐phase inclusions; in samples annealed in H2, grain growth is limited by pore dragging with a transition toward limitation by solid‐inclusion dragging at high dopant levels; in samples annealed in N2 and O2, grain growth is characterized by a transition from an “anchoring effect” of the pores toward a combination of pore dragging by and unpinning from the grain boundaries. Time‐dependence of grain growth is insufficient to determine the mechanisms and provide an adequate foundation for model‐based calculations. Observations of microstructure and its change with time, together with the rate of grain growth as a function of composition, allow elimination of alternate hypotheses and determination of the process which controls the rate of grain growth and change in pore size.
Relaxation of Volume and Index of Refraction in Pressure‐Compacted B2O3 Glass
Authors: MIZOUCHI N. , COOPER A. R.
Published: June 2006

Relaxation processes were studied in bulk B2O3 glasses compacted under hydrostatic pressures up to 21 kbars at room temperature. Measurements of room‐temperature density and refractive index at 1 atm revealed that (1) more than one ordering parameter is required to describe observed time‐dependent volumetric behavior and (2) the refractive index is not a single‐valued function of density, so that the Lorentz‐Lorenz equation must be modified to account for changes in molecular configuration.
Hertzian Fracture of Vitreous Carbon
Authors: NADEAU J. S.
Published: June 2006

Hertzian fracture produced by steel ball indenters on vitreous carbon is qualitatively similar to Hertzian fracture of soda‐lime glass. The bulk material obeys Auerbach's law, i.e. the critical fracture load is proportional to the ball radius. However, a surface layer of material, which is denser than the bulk, is stronger and does not obey Auerbach's law. The fracture energy calculated from Auerbach's law and the theory of Frank and Lawn is more than an order of magnitude lower than that measured by conventional methods. Part of this discrepancy arises from the fact that the ring crack is 17% larger than the circle of contact. The fracture surface energy of vitreous carbon as determined by a somewhat empirical application of Auerbach's law is ∼3.5 J/m2. The Hertzian fracture stress of vitreous carbon, like that of soda‐lime glass, approaches the transverse rupture strength (TRS) when large indenters are used.
Improvements in Atmosphere Sintering of Transparent PLZT Ceramics
Authors: SNOW GARY S.
Published: June 2006

An improved atmosphere sintering process was developed for fabricating large transparent PLZT plates for electrooptic applications. Cold‐pressed 9/85/35 PLZT slugs were sintered in O2 in Pt crucibles for ∼45 min at 1180°C and were then heat‐treated in air for 60 h at 1200°C in Al2O3 crucibles containing PbZrO3 atmosphere powder. Transparent plates as large as 8.4 cm in diameter and I cm thick were thus fabricated. A mechanism is proposed which qualitatively accounts for the success of this process.
Kinetics of Nitridation of Si Powder Compacts
Authors: MESSIER DONALD R. , WONG PHILIP
Published: June 2006

The kinetics of the reaction of Si powder compacts and N2 to form Si3N4 were investigated thermogravimetrically from 1300° to 1450°C. The results indicate that trace amounts of O2 inhibit the rate of nitridation by forming a thin protective layer of SiO2, whereas Fe impurities catalytically accelerate the rate of reaction by liquid‐phase formation. Complicating factors that account for difficulties in interpreting the reaction kinetics and for the poor agreement among previous investigations are discussed.
Strength Degradation by Projectile Impacts
Authors: EVANS A. G.
Published: June 2006

The impacting of ceramic components by small projectiles can lead to strength degradation caused by the formation of Hertzian cracks. The conditions which produce degradation are analyzed in terms of the momentum and elastic properties of the projectile. A critical momentum must be exceeded before strength loss can occur, and the critical condition depends on the surface condition of the ceramic. Comparison of the analytical predictions with data for SiC confirms the reliability of the analysis.
Synthesis‐Fabrication of an Al2O3‐Tungsten Carbide Composite
Authors: CRAYTON PHILIP H. , GREENE EDWARD E.
Published: June 2006

A composite of 70 vol% Al2O3 and 30 vol% tungsten carbide was formed by hot‐pressing. Simultaneously carbon reacts with an intimate mixture of WO3 and Al2O3 to form a dense body. The composite approached theoretical density; a 1‐to‐10‐μm carbide phase was uniformly dispersed in a 2‐μm Al2O3 matrix. Maximum density and fine‐grained microstructure were obtained when pressure was applied during heating from 1200° to 1600°C and temperature and pressure were then maintained for 20 min. At an initial ratio of 2.8 and 2.9 mol C/mol WO3, the tungsten appeared as free W, WC, and W2C. For C/WO3=3.0 to 3.6, mixtures of W2C and WC were present, whereas for C/WO3>3.6, free C appeared with WC. The effects of the hot‐pressing parameters are discussed.
The System K2SO4‐Cs2SO4
Authors: LEVIN ERNEST M. , BENEDICT JOSEPH T. , SCIARELLO JAMES P. , MONSOUR S.
Published: June 2006

The phase diagram for the system K2SO4‐Cs2SO4 was determined by using DTA for melting relations and DTA and high‐temperature X‐ray diffractometry for subsolidus relations. At the solidus the system shows complete solid solubility, with a minimum at 940°C and 50 mol% Cs2SO4. Orthorhombic K2SO4 and Cs2SO4, the stable low‐temperature forms, show mutual solid solubility and form a eutectoid at 50 mol% Cs2SO4 and 430°C, the lowest temperature of stability of the high‐temperature hexagonal solid‐solution phase. Isothermal plots of the a and c dimensions of this hexagonal phase vs composition show large positive deviations from linearity for c. These deviations are interpreted on the basis of the crystal structure of KNaSO4 with a similar unit cell.
Raman Spectrum of PbZrO3
Authors: PASTO A. E. , CONDRATE R. A.
Published: June 2006

Laser‐induced Raman spectra were obtained for powders and multidomain single crystals of PbZrO3 at room temperature. Factor‐group analysis was used to interpret the spectra on the basis of the space group C2v8‐Pba 2. Raman spectra were also observed for single crystals of PbZrO3 at several other temperatures between 76° and 520°K and related to crystal structural changes. No first‐order Raman bands were noted above the Curie temperature, indicating a phase transformation to a cubic zirconate structure. The interpretation of Raman spectra for thermally decomposed PbZrO3 is discussed.
Crystallization of Metastable Aluminas in the Presence of V2O5
Authors: BYE G. C. , GAVIN D. G.
Published: June 2006

In investigating possible effects of high temperatures on a V2O5/γ‐Al2O3 catalyst, it was found that metastable aluminas with unusually well‐developed crystallinity can be prepared in the presence of V2O5. With control of firing temperature, time, and atmosphere, δ‐, θ‐, and k‐Al2O3 could be obtained in this state. X‐ray powder diffraction patterns containing many more lines than usually observed were indexed, unit‐cell dimensions calculated, and values compared with previous data. All preparations contained small amounts of V which could not be removed by H2O2; ESR revealed V4+ ions in them.
Interpretation of the Kaolinite‐Mullite Reaction Sequence from Infrared Absorption Spectra
Authors: PERCIVAL H. J. , DUNCAN J. F. , FOSTER P. K.
Published: June 2006

The phases in the kaolinite‐mullite reaction sequence were reexamined by ir absorption spectrophotometry. Particular attention was paid to the controversial intermediate Al‐containing phases. Amorphous materials were leached from fired kaolinite samples with NaOH to help identify crystalline phases. Metakaolinite partially decomposes, releasing amorphous γ‐Al2O3 and SiO2, before the “950°C” exothermic reaction in which metakaolinite is completely decomposed. The resulting spinel‐type phase, which is associated with amorphous SiO2 and some poorly crystalline “primary” mullite, is γ‐Al203 (crystalline) rather than an Al‐Si spinel. There is some evidence, however, that a fraction of the γ‐Al2O3, may be an Al‐Si spinel. At ≥1100°C secondary mullite therefore forms primarily from the γ‐Al2O3/amorphous SiO2 reaction and the recrystallization of primary mullite, whereas excess amorphous SiO2 eventually crystallizes as cristobalite.
Sintering of CoO: I, Initial Stage
Authors: KUMAR P. , JOHNSON D. L.
Published: June 2006

The initial‐sintering kinetics of compacts of uniform spheres of CoO were investigated at various temperatures and O2 partial pressures. Volume‐diffusion kinetics were exhibited initially, followed by retardation of sintering and deviation from the sintering model as a result of faceting of the spheres. The calculated volume diffusivities lie between those measured for Co and O in CoO.
Effect of Milling on the Intrinsic Coercivity of Barium Ferrite Powders
Authors: HANEDA KOICHI , KOJIMA HIROSHI
Published: June 2006

The effect of milling on the intrinsic coercive force of Ba ferrite powders has been ascribed to (1) thermal fluctuations caused by superparamagnetic fine particles and (2) lattice defects introduced by milling. To test these interpretations, the Moessbauer absorption spectra and angular variation of coercive force for ball‐milled powders were determined. With increasing milling time, the Moessbauer absorption spectrum becomes ambiguous, and the absorption lines are broadened; the quadrupole‐split center line corresponding to superparamagnetic BaFe12O19 cannot be detected. The angular variation of coercive force shows that milling considerably lowers either the nucleation field of the reverse domain or the effective anisotropy field or both. It is concluded that lattice defects, rather than superparamagnetic fine particles introduced by milling, greatly affect the intrinsic coercive force of Ba ferrite powders.
High‐Temperature Strength Behavior of Hot‐Pressed Si3N4: Evidence for Subcritical Crack Growth
Authors: LANGE F. F.
Published: June 2006

The high‐temperature strength of commercial hot‐pressed Si3N4 was obtained for (1) two materials with different impurity contents, (2) the weak and strong material directions, (3) air and Ar ambients, and (4) different stressing rates. Strength degradation occurred at a lower temperature for the less pure material; both material directions exhibit the same rate of strength degradation. The testing ambient did not affect strength. The strength at temperatures ∼1200°C depended strongly on stressing rate. The presence of rough, crack‐shaped topographical features on the fracture surface and the observation of large cracks that formed during stressing are reported as evidence for subcritical crack growth at high temperatures. It is hypothesized that accelerated creep caused by grain‐boundary sliding at preexisting crack fronts is the mechanism responsible for the observed subcritical crack growth.
Limits on the Gibbs Free Energy of Formation of Intermetallic Compounds Containing Pt‐Group Elements
Authors: WENGERT P. R. , SPANOUDIS L.
Published: June 2006

Upper limits on the standard‐state Gibbs free energy of formation (δF°f) are presented for intermetallic compounds AnBm. The A elements include Li, Be, Mg, Ca, Sr, Ba, Al, Si, Y, Sc, La, Ti, V, Nb, Ta, Cr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and the B elements include Pt, Pd, Ir, Rh, Ni, Co, Ru, and Fe. Many of the A elements are used as glass constituents and many of the B elements as container materials for glass at high temperatures. Experiments performed by Bronger and co‐workers are the data source.
Characterization and Stabilization of Metastable Tetragonal ZrO2
Authors: MITSUHASHI T. , ICHIHARA M. , TATSUKE U.
Published: June 2006

Fine single‐domain and polydomain particles of tetragonal ZrO2 were prepared by hydrothermal and heat treatment of ZrO2·nH2O. The particles were characterized by X‐ray diffraction, electron microscopy, NMR, mass spectrometry, and ir spectroscopy. The main impurity in the samples was 1 to 2 wt% OH− ions, most of which were concentrated on the particle surfaces or at domain boundaries; some were also distributed in the lattice. Fine single‐domain tetragonal particles were strain‐free, but polydomain particles had large strains. The single‐domain tetragonal particles were transformed much more easily than the polydomain particles by mechanical treatment. The stablization of metastable tetragonal ZrO2 cannot be explained adequately by the surface‐energy theory. An explanation based on the concept of a martensitic transformation may be more reasonable.
Miscibility Gaps in Alkali‐Silicate Binaries—Data and Thermodynamic Interpretation
Authors: HALLER WOLFGANG , BLACKBURN DOUGLAS H. , SIMMONS JOSEPH H.
Published: June 2006

Measurements of the miscibility gaps in the Li2O‐ and Na2O‐SiO2 binaries are reported and compared to data of other workers. Heat treatments by brief immersion in liquid Sn were used for measuring the properties of glasses with a strong devitrification tendency. The miscibility‐gap boundaries of both systems are successfully described by modified regular‐mixing equations based on the concept that the thermodynamic mixing processes are controlled by complex but discrete molecular structures which exist in the melt. The assumptions and implications of the model are discussed.
Thermal Diffusivity of Be4B, Be2B, and BeB6
Authors: HOLT J. B.
Published: June 2006

The thermal diffusivities of polycrystalline Be4B, Be2B, and BeB6 were measured by the flash method. Generally, the thermal diffusivities at a given temperature decrease with increasing boron content. The thermal diffusivities of Be4B, Be2B, and BeB6 varied from 0.13 to 0.072 to 0.031 cm2/s, respectively, at 200°C and from 0.068 to 0.038 to 0.007 cm2/s at 1000°C. Heat transport in BeB6 is expected to occur almost entirely by phonon conduction, whereas electronic conduction probably plays a major role in Be4B and Be2B. Analytical expressions for the thermal diffusivities (α) of Be4B and Be2B at 200° to 1000°C and of BeB6 at 25° to 1500°C are: α(Be4B)=1/(5.83+9.05×10 3 T), α(Be2B)=1/(10.92+1.40×10 2T), and α(BeB6)=5.60×10 4+5.72/T+77.3/T2‐4.09×104/T3 cm2/s.
Crystallization of Na2B4O7 from Its Melt
Authors: NAGEL SUZANNE R. , BERGERON C. G.
Published: June 2006

The temperature dependence of the rate of growth of Na2B4O7 from its melt was determined between 1° and 192°C of undercooling. The maximum rate of growth was 2000 μm/min at 57°C of undercooling. Analysis of the growth data indicated that growth could occur by a screw dislocation mechanism over the entire range of undercooling. When this mechanism was assumed, there was good correlation between the experimental data and the predictions of the Turnbull and Cohen equation.
Transmission Electron Microscopy of Hydrated Dicalcium Silicate Thin Films
Authors: LAWRENCE F. V. , REID D. A. , De CARVALHO A. A.
Published: June 2006

A method of preparing calcium silicate hydration products for transmission electron microscopy is presented. Thin films of dicalcium silicate are evaporated onto substrates and hydrated in a vacuum chamber. Since the resulting hydration products are undisturbed, the interrelations of the species can be studied. The technique provides specimens suitable for high‐resolution microscopy. This method produces hydration products like those reported when conventional specimen preparation techniques are used. Calcium silicate hydrate gel, two forms of C‐S‐H II (fans and fiber bundles), afwillite, and Ca(OH)2 morphologies were observed. Coevaporated CaCl2 additions improve the electron diffraction patterns obtained from the C‐S‐H II fan structures but increase shrinkage of these structures on drying.
Thermal‐Shock Structures in a Partially Stabilized ZrO2 in the System CaO‐ZrO2
Authors: EL‐SHIEKH AHMED M. , NICHOLSON PATRICK S.
Published: June 2006

The stress‐relieving mechanisms in two batches of thermal‐shock‐resistant ZrO2 partially stabilized by CaO were investigated by electron‐optical replication techniques. In one batch of material, thermal shock produces a high density of micro‐cracks; this behavior is interpreted as resulting from the brittle nature of the matrix cubic phase. In the other batch of material, thermal shock produces coherent “bumps,” suggesting that the material acts in a ductile fashion.
Viscosity and Thermal Expansion of Alkali Germanate Glasses
Authors: SHELBY J. E.
Published: June 2006

Transformation‐range viscosities are reported for Na2O‐, K2O‐, Rb2O‐, and Cs2O‐GeO2 glasses. A minimum in viscosity occurs between 1 and 5 mol% alkali oxide and a maximum between 17 and 22 mol% alkali oxide. The thermal‐expansion coefficients of the Na2O‐GeO2 glasses also exhibit a minimum at < 5 mol% Na2O. The accepted structural model for alkali germanate glasses does not predict the minimum at low R2O concentrations. A modification of this model is suggested to explain these data.
Prediction of Fracture Energy and Flaw Size in Glasses from Measurements of Mirror Size
Authors: MECHOLSKY J. J. , RICE R. W. , FREIMAN S. W.
Published: June 2006

Fracture strengths (δ), fracture‐initiating flaw sizes, and mirror radii (r), as outlined by either the mist or the hackle boundary, were measured for silicate and nonsilicate glasses (e.g. As2S3 and glassy carbon). For all glasses, δr1/2= constant. The average ratios of inner and outer mirror radii to flaw size were ∼10:1 and ∼13:1, respectively, for most of the glasses. Critical fracture energies calculated from either flaw or mirror size agreed very well with those obtained by double‐cantilever‐beam measurements.