bluuebunny/crossref_metadata_2025 huggingface dataset:
Relations Among Particle Size, Shape, and Surface Area of Mg(OH)2 and Its Calcination Product
Authors: PHILLIPS V. A. , OPPERHAUSER H. , KOLBE J. L.
Published: June 2006

Precipitated Mg(OH)2 particles 0.05 to 1 μm in diam. and of various shapes were calcined in air for 1/2 h at 704°, 816°, 954°, 1093°, or 1232°C. At 704°C, BET surface areas, initially 11 to 32 m2/g, increased 1.4‐ to 7.6‐fold, and TEM observations showed pseudomorphs. Depending on the parent crystal size and shape, these particles contained either single layers or multilayers of MgO crystallites 40 to 260 Å in diam., roughly perpendicular to the original c axis. Pseudomorph stability varied greatly, even between particles of different shape in the same sample. Coalescence into single or linked angular particles occurred at 1093° and 1232°C, but one sample retained single‐layer pseudomorphs containing cubic crystallites. One observation indicated that sodium cation impurity decreased the surface area of oxide formed at 704°C.
Spectroscopic Investigations of the Kaolinite‐Mullite Reaction Sequence
Authors: BULENS M. , LEONARD A. , DELMON B.
Published: June 2006

The changes of Al coordination in the kaolinite‐mullite thermal sequence in the presence of mineralizers were investigated. Small amounts of mineralizers added to kaolinite promoted, selectively (9007deg;), either (a) direct mullite formation or (b) spinel. X‐ray fluorescence measurements of the AlKα aL shift showed that, in case (a), the Al(IV) content decreases from the metakaolinite level to values found in 3:2 mullite (without intermediary spinel formation); in case (b) A1(IV) content decreases to values corresponding to pure γ‐Al2O3 (lower than in mullite). The formation of a mixed Al‐Si spinel as a major product is excluded. Results are discussed in the light of complementary ESCA measurements.
Material Loss and High Temperature Phase Transition in Lithium Ferrite
Authors: MISHRA RAJA K. , VAN DER BIEST OMER O. , THOMAS G.
Published: June 2006

The morphology and kinetics of the phase transformations accompanying oxygen and lithium loss in LiFe5O8 were studied at 1200°C in air, in oxygen (P O2= 760 torr), and in vacuum (P= 5×10−5torr), using high‐voltage electron microscopy. LiFeO2 nucleated and grew in the LiFe5O8 matrix and finally transformed to spinel in all three atmospheres; however, the kinetics of the transformation depended on the atmobphere. The dependence of these phase transformations on material loss is discussed.
A Statistical, Micromechanical Theory of the Compressive Strength of Brittle Materials
Authors: ADAMS MARC , SINES GEORGE
Published: June 2006

A general theory of the compressive strength of brittle materials is presented. This theory proposes that failure is brought about by structural weakening from accumulated crack damage which increases with the stress level. The statistics of the flaw distribution and the mechanism of crack initiation and extension are important. A sample calculation using the theory is given to demonstrate its application.
Stress and Structural Relaxation in Tempering Glass
Authors: NARAYANASWAMY O. S.
Published: June 2006

Temper stresses are brought about, primarily, by a partial relaxation of transient stresses generated by rapid cooling of the glass. Stress relaxation under nonisothermal conditions is competently handled by a mathematical tempering model, in which glass is treated as a simple viscoelastic material. However, this model proved inadequate in some respects since the properties of glass depend not only on its instantaneous temperature but also on its prior thermal history. A tempering model was therefore developed that incorporates both stress and structural relaxation. Predictions of this structural model are compared with experimental data on tempering and contrasted with predictions of the viscoelastic model. Such comparisons revealed that, typically, structural relaxation accounts for approximately 24% of the total residual temper stresses.
Carbon Content of High‐Purity Alkaline Earth Oxide Single Crystals Grown by Arc Fusion
Authors: FREUND F. , DEBRAS G. , DEMORTIER G.
Published: June 2006

Supposedly high‐purity MgO, CaO, and SrO single crystals grown by the submerged arc fusion method were found to contain up to 10,000 ppm carbon, probably atomically dissolved in the oxide lattice. The carbon was nondestructively analyzed by means of the nuclear reaction 12C (d,p) 13C using 1.0 or 1.7 MeV deuterons. Concentration profile analysis indicated that the carbon content near the surface is higher than in the bulk. Heating the crystals in air to ∼800°C increases the carbon content in the upper 1 to 5 μm surface layer and decreases it in the bulk. Heating in vacuum leads to soot formation at the surface and carbon depletion inside the crystals.
Electrical Conductivity in Lead Zirconate‐Titanate Ceramics
Authors: DIH J. J. , FULRATH R. M.
Published: June 2006

Electrical conductivity was investigated in polycrystalline lead zirconate‐titanate ceramics, with and without various dopants and stoichiometry, fabricated from chemical solutions and mixed‐oxide powders. A multiphase packing‐powder technique was used to control the stoichiometry of the system during sintering. The dc resistivity was measured by using a guard‐ring method. Electrical conduction follows typical semiconductor behavior. Thermoelectric power measurements showed that hole conduction was dominant. Impurity ions which can serve as acceptors or donors significantly affect the conduction process. For undoped PZT the activation energy was 3.6 eV from 650° to 800°C and 1.4 eV at <650°C.
Electrochemical Degradation of Ceramic Electrodes
Authors: MIZUSAKI J. , CANNON W. R. , BOWEN H. K.
Published: June 2006

Calculations of oxygen chemical potentials in the interior of ceramic electrodes with ion blocking interfaces indicate that phase changes and/or decomposition will occur as a result of dc polarization. For electrodes with reversible interfaces, unacceptably thick oxidation layers can form. Results are applied to nonslag and slag‐covered MHD electrodes.
Direct Examinations of PTC Action of Single Grain Boundaries in Semiconducting BaTiO3 Ceramics
Authors: NEMOTO H. , ODA I.
Published: June 2006

R‐T and I‐V characteristics of single grains and grain boundaries in large‐grained BaTiO3 PTC ceramics were studied with a two‐probe technique using a micromanipulator and fine Al wire. The PTC originates in the grain boundary only and behaves differently in each boundary. Even below Tc, the ceramic resistance depends almost entirely on the boundary. I‐V characteristics in the boundary follow Ohm's law and conduction by a space‐charge‐limited current with a trap, using different applied voltages. The PTC anomaly relates to activation of the trap in the boundary, not to barrier height. A band model in the intergranular layer, with dielectric BaTiO3 and the trap, is proposed.
Subcritical Crack Growth in Dense Alumina Exposed to Physiological Media
Authors: FERBER M. K. , BROWN S. D.
Published: June 2006

Dense alpha alumina plates were fabricated using a slip‐casting process. These samples were used to evaluate the subcritical crack‐growth characteristics of the alumina in various physiological media. Specifically, the double‐torsion method was used to obtain crack velocity vs stress intensity factor plots in distilled water, saline solution, and medium 199 at 37°C. The resulting data suggested that for the particular alumina samples analyzed,K1was not independent of the crack length and thus this method was not entirely valid. Despite these problems, there was a tendency for the stress corrosion to be greatest in the distilled water and least in the medium 199. Finally, the experimental crack‐growth parameters were within the range reported for similar biomaterials.
Characterization of the Porous CaO Particles Formed by Decomposition of CaCO3 and Ca(OH)2 in Vacuum
Authors: BERUTO DARIO , BARCO LUIGI , SEARCY ALAN W. , SPINOLO GIORGIO
Published: June 2006

The solid products of decomposing CaCO3 powder in vacuum at 510°C (sr‐CaO) and of decomposing Ca(OH)2 powder at 320°C in vacuum (h‐CaO) are particles which have approximately the same exterior dimensions as the parent CaCO3 or Ca(OH)2 particles. N2 adsorption and desorption isotherms show that sr‐and h‐CaO have high internal surface areas which for sr‐CaO have cylindrical symmetry, with the most common diameters being ∼ 10 nm, and for h‐CaO are slit‐shaped, with the most common slit width being ∼ 2.7 nm. The conclusions reached in earlier investigations, i.e that these decomposition reactions in vacuum initially yield a form of CaO which has the same unit cell dimensions as the parent solid, were in error, probably because water vapor converts much of the CaO to poorly crystalline Ca(OH)2 before XRD measurements can be completed in air. From the volume of N2 adsorbed by the porous powders, the porosity of h‐CaO is calculated to be 36±5% and of sr‐CaO 41.5±5%. These porosities imply that the linear dimensions of the 1 to 20 μm particles of h‐CaO and sr‐CaO are ∼5% smaller than those of the parent particles. XRD measurements made as a function of time show that particle shrinkage must occur by cooperative, diffusionless movement of crystallites of sr‐CaO or h‐CaO as they form.
Grain Growth in Fe3O4
Authors: YAN M. F.
Published: June 2006

Sintering and microstructural evolution were studied in Fe3O4 as a model system for spinel ferrites. Fe3O4 powder, purified by the salt‐crystallization method, was sintered to ∼99.5% density in a CO‐CO2 atmosphere. The pO2 Of the sintering atmosphere drastically affects the microstructure (grain size) of sintered Fe3O4 without significantly affecting density. The measured grain‐boundary mobilities, M, of Fe3O4 fit the equation M=M0(T) pO2−1/2 with M0(T) = 2.5×105 exp[‐(609kJ·mol‐1/RT](m/s)(N/m2)−l. The grain‐boundary migration process appeared to be pore‐drag controlled, with lattice diffusion of oxygen as the most likely rate‐limiting step.
Analytical Electron Microscopy of Cement Pastes: II, Pastes of Portland Cements and Clinkers
Authors: LACHOWSKI E. E. , MOHAN K. , TAYLOR H. F. W. , MOORE A. E.
Published: June 2006

Three hundred sixty‐five particles of C‐S‐H, Ca(OH)2, AFm phase, and AFt phase from pastes of normally ground portland cements and of finely ground cements and clinkers were analyzed. All the phases, except the Ca(OH)2, showed significant variation in composition among paste specimens and among particles within each specimen. The C‐S‐H contains significant amounts of Al, Fe, and S; for that of a normally ground portland cement paste, cured for 28 days, the median Si:AI, Si:Fe, and Si:S ratios were 11, 43, and 15, respectively, whereas the mean Ca:Si ratio for all the particles analyzed was 2.0. The AFm phase in cement pastes is not pure monosulfate but has a mixture of sulfate, hydroxide, and Al‐ and Si‐bearing ions in its interlayer sites; the AFt phase is not pure ettringite but contains Si and its sulfate is probably partly replaced by hydroxide. The Al and Fe contents in the C‐S‐H and the Si contents in the AFm and AFt phases are greater when finely ground starting materials are used. This fact, together with the marked variation among particles, emphasizes the difficulty of ionic transport in cement pastes.
Evaporation of Zinc Oxide from Spinel Solid Solutions in Vacuum
Authors: KOUMOTO KUNIHITO , YANAGIDA HIROAKI , MIZUTA SUSUMU
Published: June 2006

The mechanism for the evaporation of ZnO from powders of (Zn02Co0.8)O·Al2O3 (ZCA) and (Zn0.2Ni0.8)O·Al2O3 (ZNA) spinels was studied at 1335° to 1500°C in vacuum of 3×10−5 to 10−4 torr. The evaporation of ZnO occurred in two stages: at a constant rate in the first and a decreasing rate in the second. The rate‐determining process was analyzed as a decomposition reaction at the first stage and as the conjugated process of decomposition reaction and diffusion in the solid at the second. The evaporation rate constant and surface emissivity showed a similar dependence on temperature. The ratio of the concentration of ZnO at the surface to the initial concentration was =0.7 when the first stage of evaporation changed to the second.
Thermal Stress Analysis of Partially Absorbing Brittle Ceramics Subjected to Symmetric Radiation Heating
Authors: HASSELMAN D. P. H. , THOMAS J. R. , KAMAT M. P. , SATYAMURTHY K.
Published: June 2006

An analysis is presented of the thermal stresses in partially absorbing ceramics, in the form of flat plates symmetrically heated by thermal radiation and cooled by convection. The absorption coefficient is introduced as a material property, which affects thermal stress resistance. For zero heat transfer coefficient and optical thickness μa→0 the tensile thermal stresses are proportional to (μa)3. For h=∞ and μa→0, the tensile thermal stresses are a linear function of μa. Appropriate thermal stress resistance parameters were derived.
Plastic Deformation of Fine‐Grained Alumina (Al2O3): II, Basal Slip and Nonaccommodated Grain‐Boundary Sliding
Authors: HEUER A. H. , TIGHE N. J. , CANNON R. M.
Published: June 2006

Plastic deformation was studied in fine‐grained alumina polycrystals (grain sizes 1 to 5 μm); several deformation mechanisms occur simultaneously. This paper is concerned with basal slip and unaccommodated grain‐boundary sliding (GBS). The basal slip can give rise to a deformation texture. Cavitation occurs due to the occurrence of both unaccommodated GBS and basal slip, because of the marked plastic anisotropy of alumina.
REVIEW—Transformation Toughening in Ceramics: Martensitic Transformations in Crack‐Tip Stress Fields
Authors: EVANS A. G. , HEUER A. H.
Published: June 2006

The toughness of ceramics can be substantially enhanced by inducing a martensitic transformation in the stress field of the dominant crack. The specific characteristics of martensitic transformations pertinent to this toughening process are examined in this paper. A generalized model of the toughening is developed, which indicates the specific roles of the transformation parameters on the expected magnitude of the toughening. Finally, the implications of the analysis for the development of high‐toughness ceramics are discussed.
Directionally Crystallized Sodium Lead Silicate Glass‐Ceramic
Authors: MELLING P. J. , DUNCAN J. F.
Published: June 2006

The iron‐nucleated devitrification of a sodium lead silicate glass was studied; it produces a glass‐ceramic consisting of sodium metasilicate crystals in a lead silicate glass matrix. The nucleating agent is not incorporated in the crystal phase, Anisotropic devitrification of the glass in a thermal gradient produces an aligned microstructure with needles of sodium metasilicate oriented with the c axis parallel to the direction of crystallization.
Reaction of Clay and Carbon to Form and Separate Al2O3 and SiC
Authors: BECHTOLD BRYANT C. , CUTLER IVAN B.
Published: June 2006

Alumina and SiC can be recovered from clay by carbon reduction. Kinetic experiments showed that the solid‐solid reaction proceeds through gaseous intermediates of CO and SiO. By mixing part of the carbon with the clay and using the balance to form SiC, Al2O3 and SiC were physically separated as products. This separation is represented by

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Optimum conditions for ease of separation of Al2O3 and SiC and the high rate of reaction require pelletization of both the clay‐carbon mixture and the carbon of reaction (b) and the reaction of these pellets at >1500°C. By examining the effect of transition metal catalysts, SiC formation was determined to be rate‐controlling.
Scattering Study of Microstructurally Birefringent Glasses
Authors: TOMOZAWA MINORU , TAKAMORI TAKESHI
Published: June 2006

Microstructurally birefringent, phase‐separated borosilicate glasses were investigated by X‐ray small‐angle scattering and light scattering. X‐ray small‐angle scattering data confirmed the presence of ellipsoidal particles and the light‐scattering data indicated the presence of anisotropic distribution of particles. These observations support the previous contention that the positive birefringence is due to the form birefringence and the negative birefringence is due to the anisotropic distribution of spherical particles.
A Novel Technique for Producing Fine Metal Fibers for Enhancing Mechanical Properties of Glass‐Matrix Composites
Authors: LUCAS J. P. , TOTH L. E. , GERBERICH W. W.
Published: June 2006

Micrometer‐sized refractory metal fibers of Nb were produced by an unconventional method of mechanically reducing arc‐cast ingots of Nb‐Cu in which the original microstructure contains ductile Nb precipitates. During mechanical reduction the precipitates were elongated into fibers, which were extracted by chemically dissolving the Cu matrix. Composite specimens of Nb fibers in a glass matrix were fabricated by dry‐blending a mixture of fibers and powdered glass, then hot‐pressing into disks. Fracture toughness,KIC, was measured using microin‐dentation and three‐point bending methods. The fracture toughness increased with increasing volume fraction of fibers with the greatest improvement noted at 0.10≤vf≤0.20. The values ofKICincreased as the interfiber spacing and the fiber diameter decreased. Both the effective fracture surface energy and the fracture strength were enhanced by increasingvf. The small fibers embedded in the brittle matrix showed virtually no evidence of microcracking; Nb fibers and glass matrices also demonstrated good interfacial adhesion. Fractographs suggested that the fibers were very effective in resisting crack propagation. Experimental observations were explained using the concept of crack‐front bowing and fiber bridging.
Effect of a Microheterogeneous Structure on the Fracture Toughness of Glasses
Authors: VERNAZ E. , LARCHE F.
Published: June 2006

A model is presented that describes the variation of the critical stress intensity factor,Kk., of glasses as a function of composition. It is based on a microheterogeneous structure that produces nonannealable stresses in the material. The effect of these internal stresses on the measured values ofKICis expressed as a function of the mechanical and thermal parameters of the material. Application to several borosilicate glasses is presented and discussed.
Optical Absorption of Silver in Photochromic Glasses: Optically Induced Dichroism
Authors: NOLAN D. A. , BORRELLI N. F. , SCHREURS J. W. H.
Published: June 2006

The absorption spectrum of silver halide‐based photochromic glasses darkened by uv light is shown to vary with glass composition and photochromic heat treatment. The nature of the absorption is explained in terms of the anisotropic geometry of the photolytically produced silver specks. A correlation is established between the uv‐induced spectrum and the dichroic behavior that can be induced by bleaching with polarized light. A specific model is proposed for the silver‐speck geometry and calculations show goad agreement with experimental data.
Preparation and Unit‐Cell Parameters of Single Crystals of Ba2Ti9020
Authors: O'BRYAN H. M. , GRODKIEWICZ W. H. , BERNSTEIN J. L.
Published: June 2006

Ba2Ti9O20 crystallizes in the monoclinic system with α= l.4818(5) nm, b= 1.4283(6), and c= 0.7109(2) with β= 98.37°±0.07°. The most likely space group is P21/m, Z = 4 with a calculated density 4.58 g/cm3. The powder pattern was indexed. The Ba2Ti9O20 crystals form as stellated groups when melts of BaCl2+ 20 to 50% TiO2 cool from 1275°C.
Examination of Grain Boundaries of Mn‐Zn Ferrites by AES and TEM
Authors: FRANKEN P. E. C. , STACY W. T.
Published: June 2006

AES analysis of grain boundaries of Mn‐Zn ferrites, combined with ion etching, shows that CaO and SiO2 dopants are enriched in a layer of ∼2 nm; TEM showed no separate phase at the grain boundaries. Using TEM combined with X‐ray microanalysis three types of secondary phases, one crystalline and two amorphous, were found at the multiple‐grain junctions. Although the absence of a planar second phase has not been proved, these results argue against the presence of a high‐resistivity glassy layer and support the hypothesis that the high resistivity is caused by segregation of Ca during cooling, causing oxidation of the grain boundaries.
Preparation of Ultrafine MgO by the Spray‐ICP Technique
Authors: Kagawa Masahiro , Kikuchi Masae , Ohno Reiichi , Nagae Tsuneki
Published: June 2006

Ultrafine MgO with uniform shape and particle size was prepared by spraying a solution of magnesium nitrate into an inductively coupled plasma (ICP) of Ar. The formation of the uniform particles seems to result from the complete decomposition of the nitrate to its component atoms in the ICP flame, followed by the precipitation of the oxide occurring just outside the flame.
Free Volume of Phosphate Glasses
Authors: Abe Yoshihiro , Suzuki Suguru
Published: June 2006

Free volume (Vf), defined by Simha and Boyer, was measured for glasses in the system MO‐P2O5 (M=Be, Mg, Ca, Ba, Zn) and (1‐x)MO·P2O5‐xAl2O3·3P2O5 (M = Mg, Ca, Ba). Experimental Vf of the glasses was ∼0 to 0.3. These results indicate that Vf is not a universal constant (∼0.1) for all glasses as previously reported, but that Vf depends on the glass backbone structure.
Composites of PZT and Epoxy for Hydrostatic Transducer Applications
Authors: KLICKER K. A. , BIGGERS J. V. , NEWNHAM R. E.
Published: June 2006

Composites with 3–1 connectivity for transducer applications were made by embedding extruded PZT rods in an epoxy matrix. The effects of rod diameter, volume fraction of PZT, and composite thickness on the hydrostatic properties of the composites were determined. Due to decoupling of the d̄33 and d̄31 coefficients in a composite with 3–1 connectivity, the d̄n may be enhanced, even in composites of low volume fractions of PZT. Such composites alsojiave a low dielectric permittivity. The combination of high d̄n values and low ɛ33 value results in a greatly enhanced ḡn. Composites with 10 vol% PZT were made with values of d̄h and ḡh which are, respectively, two times (>80×10 −12 C/N) and 25 times (>70×10−3 (V·m/N) the solid PZT values.
Structure of High‐Angle Grain Boundaries in Metals and Ceramic Oxides
Authors: BALLUFFI R. W. , BRISTOWE P. D. , SUN C. P.
Published: June 2006

The current knowledge of the structure of high‐angle grain boundaries in metals and ceramic oxides is critically reviewed. Particular attention is given to effects due to differences in the bonding and crystal structure in these solid types. The results of recent experimental work and efforts to model grain‐boundary structure using computer simulation methods are described. Important characteristic features of boundaries in these materials are discussed. Difficulties which are presently being encountered in efforts to determine their structure are pointed out.
Residual Stress in Singly and Doubly Ion‐Exchanged Glass
Authors: SHAISHA E. E. , COOPER A. R.
Published: June 2006

Photoelastic measurement on rings sliced from soda‐lime glass tubes, ion‐exchanged with electric field assistance, were used to calculate the residual stress in this material. Account was taken of the effect of sample distortion due to slicing. Well below the glass transition the measured surface stresses were independent of time of exchange but increased markedly with decreasing temperature of exchange for both singly and doubly exchanged specimens.
Transient Thermal Stress Behavior in ZrO2‐Toughened Al2O3
Authors: BECKER PAUL F.
Published: June 2006

The initial strength of (σi) and thermal shock resistances (ΔTc and σr/σi), as determined by quench tests, of Al2O3‐ZrO2 composites are increased by increasing amounts of tetragonal ZrO2 second phase for contents of up to ∼15 vol%. For composites with ≤9 vol% ZrO2 the increases in σr and ΔTc reflect the increase in γIC with addition of ZrO2 However, for ZrO2contents >9 vol%, the thermal shock resistances (ΔTc and σr/σi) and σi are also affected by machining‐induced microcracking in the surface of the samples. For ZrO2 contents >14 vol%, bulk microcracking can become extensive and result in a degradation of σi and ΔTc.
Weak‐Electrolyte Models for the Mixed‐Alkali Effect in Glass
Authors: MOYNIHAN C. T. , LESIKAR A. V.
Published: June 2006

A weak electrolyte model for the mixed‐alkali effect on electrical conductivity and ionic mobility in glass was developed for the dilute foreign alkali region. The basic assumptions are (a) that alkali transport in single‐alkali glasses is due to a small concentration of mobile species and (A) adding small amounts of foreign alkali suppresses, by mass action, the mobile species concentration. The model was developed explicitly for the case where the mobile species are interstitial cations or cation pairs and was applied to analysis of conductivity data for 0.242(K2O +Na2O)–0.758SiO2 glasses in the dilute Na+ region. The analysis allows the fraction of mobile cations in the single‐alkali glass to be calculated. This varies from 0.5% at 25° to 6% at 500°C for 0.242K2O–0.758SiO2 glass and is in good agreement with estimates of the extent of dissociation from internal friction data and an ionic jump model of conductivity.
Glass Surface Cracks Caused by Alkaline Solution Containing an Alkaline‐Earth Element
Authors: Tomozawa M. , Oka Y. , Wahl J. M.
Published: June 2006

When glass was treated in a hot alkaline solution containing a small amount of heavy alkaline earth elements such as Ba or Sr, sharp cracks formed on the glass surface. The severity of the crack formation was greater, the heavier the alkaline earth element. The cracks caused a drastic reduction in the mechanical strength of the glass.
Fabrication of High‐Density Homogeneous ThO2‐UO2 Fuel Pellets
Authors: Matthews R. B. , Davis N. C.
Published: June 2006

Thorium oxide and uranium oxide pellet fuel development activities leading to a process fohd fabrication of high density, homogeneous fuel pellets are described. Conventional dry ball‐milling was used to comminute commercially available powders. After separate milling, these ThO2‐UO2mixtures were co‐milled, pressed, and sintered to 95 % of theoretical density with a homogeneous distribution of the components. An intimate mix of highly active powders promotes solid solution formation and yields high density pellets.
The β→αTransformation in Poly crystalline SiC: III, The Thickening of α Plates
Authors: OGBUJI L. U. , MITCHELL T. E. , HEUER A. H.
Published: June 2006

The final stages of the β→α polymorphic phase transformation in conventionally sintered and hot‐pressed SiC were studied by optical and transmission electron microscopy. The interface between the α and β regions of partially transformed grains invariably contains thin α lamellae adjacent and parallel to coherent β twins (or β stacking faults). These planar defects serve as nucleation sites for the transformation, with growth of α occurring by the motion of partial dislocations nucleated at the intersection of coherent and incoherent twin boundaries. Various structural phenomena in SiC, including faulting, mi‐crosyntaxy, and long‐period polytypism, are discussed in terms of this transformation mechanism.
The β→α Transformation in Polycrystalline SiC: IV, A Comparison of Conventionally Sintered, Hot‐Pressed, Reaction‐Sintered, and Chemically Vapor‐Deposited Samples
Authors: OGBUJI L. U. , MITCHELL T. E. , HEUER A. H. , SHINOZAKI S.
Published: June 2006

Microstructural characterization of conventionally sintered, hot‐pressed, reaction‐sintered, and chemically vapor‐deposited polycrystalline SiC shows that the initial stages of the β→α transformation in the first three materials are similar, the process being dominated by the elimination of high‐energy β→α interfaces. In the final stage of transformation, all four materials transform by nucleation and growth of α lamellae at {111}β twin boundaries or at β stacking faults.
Dehydration Behavior of Aluminum Sulfate Hydrates
Authors: GANCY A. B. , RAO J. M. , WENNER W. M.
Published: June 2006

An exothermic transition is observed near 400°CC on thermal dehydration of highly crystalline AI2(SO4)3.16H2O, Al2(S04)3 14H2O, and Al2(S04)3 9H2O when the early stages of heating are carried out in vacuum. Amorphous or partially crystalline hydrates do not show the exotherm. No systematic relation is apparent between the decomposition behavior and the pore volume distribution of the various anhydrous A12(SO4)3 products.
Defect Chemistry and Electrical Properties of Thallium Oxide Single Crystals
Authors: WIRTZ G. P. , YU C. J. , DOSER R. W.
Published: June 2006

Ehdectrical resistivity and Hall voltage were measured between 4.2 and 300 K on T12O3 crystals annealehd at 550°C for 24 h under oxygen pressures of 2×104 to 107 Pa. The carrier concentration varied from 7.97×1020 to 5.08×1020 cm−3, the low‐temperature Hall mobility from 131 to 189 cm2/V.s, and the Fermi level from 7.1×104 to 5.05×104 J/mol above the bottom of the conduction band as P02 was increased from 2×104 to 107 Pa. The dependence of Fermi level on carrier concentration and P0l was consistent with a parabolic density‐of‐states function describing the conduction band. Over the entire region of oxygen pressure investigated, Fermi‐Dirac statistics were required to describe the dependence of carrier concentration on P02.
Effect of Oxygen Potential on the Thermal Creep of Niobia Doped UO2
Authors: AINSCOUGH J. B. , RIGBY F. , MORROW S. A.
Published: June 2006

Compressive and tensile creep were measured on UO2 doped with 0.4 wt % niobia at controlled oxygen potentials. Kinetic results were almost identical, irrespective of the nature of the applied stress, but differences in after‐test microstructures were observed. Formation of “plastic” UO2 with increases in strain rate of more than two orders of magnitude were recorded when oxygen potential changed from −560 to −410 kJ/mol; this was associated, primarily, with the formation of the Nb4+ ion. Over the same range of oxygen potential, creep activation energy decreased linearly from −425 to −225 kJ/ mol. In contrast, strain rate and activation energy for undoped UO2 remain almost constant under these conditions.
Effect of Burnup on the Response of Stainless Steel‐Qad Mixed‐Oxide Fuels to Simulated Thermal Transients
Authors: FENSKE G. R. , BANDYOPADHYAY G.
Published: June 2006

Direct electrical heating experiments were performed on irradiated fuel to study the fuel and cladding response as a function of burnup during a slow thermal transient. The results indicated that the nature and extent of the fuel and cladding behavior depended on the quantity of fission gas retained in the fuel. Fission‐gas‐driven fuel ejection occurred as the molten cladding flowed down the stack exposing bare, radially unrestrained fuel. The fuel dispersion occurred in the absence of molten fuel and the amount of fuel ejected increased with increasing burnup. In tests with medium‐ and high‐burnup fuel, the cladding ballooned prior to melting. In regions where the cladding remained intact, helium‐filled cavities were observed on the cladding. The swelling due to the cavities increased with increasing burnup.
Characterization of Grain‐Boundary Segregation in MgO
Authors: CHIANG Y. M. , HENRIKSEN A. F. , KINGERY W. D. , FINELLO D.
Published: June 2006

Using Auger electron spectroscopy, segregation of Ca and Si corresponding to the adsorption of a partial monolayer adjacent the boundary in polycrystalline MgO was observed. Segregation of Sc is different in nature; the distribution consists of a 3.0‐nm wide region corresponding to a positive space charge layer balancing a negative boundary charge. Various segregation models are applied to the observed behavior.
High/Low Modulus Si3N4‐BN Composite for Improved Electrical and Thermal Shock Behavior
Authors: MAZDIYASNI K. S. , RUH ROBERT
Published: June 2006

High‐density Si3N4+6% CeO2 composites with 5 to 50% BN were fabricated by hot‐pressing. BN remained as a discrete phase. Dielectric constants were 4 to 8 and loss tangents were 0.0008 to 0.06 for the room temperature to 1100°C range for compositions with 10 to 50% BN. Thermal‐expansion values perpendicular to the hot‐pressing direction were somewhat less than those of hot‐pressed Si3N4+6% CeO2. Flexure strengths at room temperature were considerably lower than those of hot‐pressed Si3N4+6% CeO2 but values at 1000°, 1250°, and 1400°C in air were only slightly lower. Young's modulus values were found to decrease with increasing BN content at all temperatures. Better thermal shock resistance was found than for commercial hot‐pressed Si3N4.
Equilibrium Oxygen Pressures of Mn‐Zn Ferrites
Authors: TANAKA T.
Published: June 2006

The equilibrium oxygen pressures of Mn‐Zn ferrites with various oxygen/metal ratios were obtained at 1000° to 1400°C. The method used is the dependence of the lattice constant on oxygen content obtained in Mn ferrites and the relation between oxygen content and the equilibrium oxygen pressure found in Mn and Ni ferrites. Previous results were compared with those of the present study.
Solubility of TiO2 in BaTiO3
Authors: SHARMA R. K. , CHAN N.‐H. , SMYTH D. M.
Published: June 2006

Scanning electron microscopy and electron probe micro‐analysis were used to investigate the microstructure of both slow‐cooled and quenched polycrystalline BaTiO3 specimens with a small excess of TiO2 (Ba/Ti=0.995 to 0.999) or of BaO (Ba/Ti=1.002 and 1.005). The electron micrographs of polished and etched TiO2‐excess BaTiOs samples, and of fracture surfaces of quenched samples, showed a second phase in the grain boundaries and triple‐point regions, whereas no second phase was observed in samples having Ba/Ti=1.000. Microprobe analysis of the second phase gave compositions near that of the reported adjacent phase of higher TiO2 content, Ba6Ti17O40. The results indicate that the solubility of TiO2 in BaTiO3 is <0.1 mol%.
Coalescence of Thin Films of Gold Condensed on Glass
Authors: FRECHETTE V. D. , PULVER J. C. , ROSSINGTON D. R.
Published: June 2006

Thin films of gold were deposited by evaporation onto glass surfaces generated by slow fracture in a vacuum. Such films characteristically show an agglomerated or coalesced structure. Deliberate contamination of the substrate and growing films with O2, CO2, H2O, and Ar by exposure to a pressure of 0.0013 Pa (1×10−5 torr) of the gas had no effect on the structure of films of an average thickness of 5 nm (50 Å). Similar treatment of the films with N2 resulted in a decrease in the extent of agglomeration of the gold. Treatment of the substrate with N2 prior to the deposition caused an even greater decrease in agglomeration. Films of an average thickness of 2 nm (20 Å) were not affected detectably by the N2 treatment. The available evidence indicated that the N2 acted directly on the glass and that the structure of the films was unaffected by minor variations in the age of the surface or by differences in the speed of the fracture which generated the surface. The film structure was not changed by extended exposure to the atmosphere.
Isostatic Hot‐Pressing of UO2
Authors: SOLOMON A. A. , COCHRAN K. M. , HABERMEYER J. A.
Published: June 2006

Final‐stage isostatic hot‐pressing of nearly stoichiometric UO2 was investigated. The rate of hot‐pressing is linearly dependent on the pressure driving force F=P+(2γ/r)—p, where P and p are the external and internal pressures, respectively, γ the average surface energy, and r average pore radius. The apparent activation energy for hot‐pressing agrees with that for U bulk diffusion. Grain growth during hot‐pressing follows d∝tn, where d is the grain diameter, t is time, and n=0.2. Grain size is also a function of temperature at constant density. The results indicate that Nabarro‐Herring creep is the controlling mechanism of hot‐pressing over the range of variables investigated, although the applicability of this model is questioned. The results also show that sintered UO2 can entrap gas that can lead to swelling.
Density and Ionic Conductivity of Sintered (CeO2)0.82(GdO1.5)o.18
Authors: RIESS I. , BRAUNSHTEIN D. , TANNHAUSER D. S.
Published: June 2006

A study is presented on sintering of the mixed oxide (CeO2)0.82(GdO1.5)0.18 where the density of the green samples, the density of the sintered samples, and the ionic conductivity of O2‐ ions in the sintered samples were measured. The dependence of these three parameters on the method of preparation of the powder and on the pressure in pressing of the green samples was determined. The pressures used varied from 25 to 1200 MPa. The different preparation methods used resulted in powders of different degree of mixing of the two oxides, which then determined the compositional homogeneity of the samples when sintered. An apparent relation exists between compositional homogeneity and ionic conductivity as well as density. However, when high pressures (∼1000 MPa) were applied, densities up to 98 % of theoretical value were obtained. A temperature dependence of the activation energy in seemingly homogeneous samples is attributed to a higher dopant concentration in the grain boundary than in the grain bulk.
Low‐Temperature Densification of Lead Zirconate‐Titanate with Vanadium Pentoxide Additive
Authors: WITTMER DALE E. , BUCHANAN RELVA C.
Published: June 2006

Additions of 0.1 to 6.0 wt% V2O, to lead zirconate titanate (PZT) ceramics promoted rapid densification below 975°C, thereby eliminating the need for PbO atmosphere control The base PZT, Pb(Zr0.53Ti0.47)O3, was prepared by coprecipitation from mixed oxides and butoxides. The V2O5was incorporated as a batch addition during the PZT coprecipitation process, as mill additions to the calcined precipitated powder, and to a commercial PZT powder. Densification rates were enhanced by the addition of V2O5(>98% of theoretical density was obtained in ∼15 min at 960°C by the addition of 0.1 to 1.0 wt% V2O5, compared to 4 h at 1280°C for the base PZT). Dielectric properties and piezoelectric coefficients varied slightly within the optimum range of 0.25 to 1.0 wt% V2O5addition but were at least comparable to the base PZT. Indications are that V2O5becomes incorporated into the surface layers of the oxide powders during mixing (or in the coprecipitation process) and that the accelerated densification is due to enhanced surface activation and liquid‐phase sintering.
Raman Study of Arsenic‐Containing Potassium Silicate Glasses
Authors: VERWEIJ H.
Published: June 2006

Raman spectra are given of potassium silicate glasses with batch composition xK2CO3‐(100—x)SiO2‐0.5As2O3 in which x varies from 10 to 50. To isolate the contribution of the As2O3 additions to the spectra from that of the silicate network, difference spectra were prepared from spectra of glasses with and without As2O3 added to the batch. To obtain a precision sufficient for this purpose, the spectral data were collected in multiple scans under full computer control using photon counting and a correction for laser‐source fluctuations. It is concluded from the polarized and depolarized difference spectra that arsenic in potassium silicate glasses with x>35 is mainly present in the pentavalent state as AsO43‐ ions. For glasses with x<35 it is concluded that As5+ is present in As2O74‐ or As3O105‐ ions and As3+ as AsmO2m+1‐m‐2 chains of AsO3 pyramids. In these chains, m increases with decreasing x. From the intensity ratio of Raman peaks, caused by arsenite (As3+) and arsenate (As5+) ions in the glasses, it is concluded that there is a significant increase of the As3+/As5+ ratio with x increasing from 50 to 10. The effect of melting temperature on the As3+/As5+ ratio in 30K2O‐70SiO2‐0.5As2O3 glasses at 1300° to 1600°C was also studied. No clear temperature effect could be concluded from the Raman spectra.
Effect of Air Exposure on the Resistivity of Sodium Beta and Beta Aluminas
Authors: DUNN BRUCE
Published: June 2006

The exposure of sodium beta and beta' alumina tubes to flowing air causes the room temperature resistivities to increase considerably. Nonohmic behavior is observed within 48 h. Measurements indicate that a hydrated surface region is primarily responsible. Heating the tubes to 600°C in dry air is able to reversibly restore the original resistivities. The evolution of CO2 at higher temperatures suggests that chemical reactions may be occurring during exposure.
Quantitative Phase Analysis of the Hydrothermally Treated System γ‐Ca2SiO4‐CaO
Authors: BEZJAK ALEKSANDAR , RUNJE VESNA
Published: June 2006

A multiphase product of hydrothermal reaction in a system γ‐Ca2SiO4‐CaO, at a Ca/Si molar ratio of 2.25 and 200°C, was subjected to quantitative phase analysis by XRD and selective dissolution in a methanolic solution of salicylic acid. The step‐wise procedure required by the general method of phase analysis included: (1) determination of the number of phases with overlapping XRD patterns, (2) determination of the individual XRO patterns from diffraction patterns of multiphase samples, and (3) determination of weight fractions for all coexisting crystalline constituents. The derived individual XRD patterns correspond to calciochondrodite, “γ‐dicalcium silicate hydrate,” and rankinite, a phase not hitherto reported to be formed at 200°C under saturated steam conditions. The method was checked on the model system xonotlitetobermorite‐calcite.
Temperature/Composition Phase Diagram of the System Bi2O3‐PbO
Authors: BIEFELD R. M. , WHITE S. S.
Published: June 2006

The Bi2O3‐PbO phase diagram was determined using differential thermal analysis and both room‐ and high‐temperature X‐ray powder diffraction. The phase diagram contains a single eutectic at 73 mol% PbO and 635°C. A body‐centered cubic solid solution exists above ∼600°C within a composition range of 30 to 65 mol% PbO. The compounds α‐Bi2O3, σ5‐Bi2O3, and γ‐PbO (litharge) have wide solubility ranges. Four compounds, 6Bi2O3·PbO, 3Bi2O3·2PbO, 4Bi2O3,5PbO, and Bi2O3·3PbO, are formed in this system and the previously unreported X‐ray diffraction patterns of the latter three compounds are reported. Diffraction patterns for some of these mixed oxides have been observed in ZnO‐based varistors grown using Bi2O3 and PbO as sintering aids.
Study of Titanium Carbide Precipitates in Titanium Diboride by Electron Energy Loss Spectroscopy
Authors: MOCHEL PEGGY , ALLISON CRAIG , WILLIAMS WENDELL S.
Published: June 2006

Precipitates occurring in single‐crystal titanium diboride grown by the arc‐Verneuil process are shown by electron diffraction to have an fee structure and by electron energy loss spectroscopy to consist of only titanium and carbon. The precipitates must thus be NaCI‐structure titanium carbide. Crystallographic considerations are involved to interpret the occurrence of these lamellar, coherent precipitates on prism planes of the hexagonal matrix.
Ion‐Exchange Method for Fabricating High‐Silica Glasses
Authors: SIMMONS CATHERINE J.
Published: June 2006

Porous glasses produced by phase separation and leaching are used as substrates for the exchange of metal cations from a basic solution for the protons of silicon‐bonded hydroxyl ions on the pore surfaces and on residual silica gel present in the pores. This method yields extremely homogeneous, fully dense high‐silica glasses with low to moderate concentrations of metal dopants after sintering to collapse the pores. The quantity and rate of ion exchange are studied as a function of time, solution pH, and solution ion concentration for various dopant salts. In addition, a double ion‐exchange method is described whereby, during the second exchange, ions which have a relatively low solubility in basic solutions can be exchanged from a neutral or acidic solution onto the glass surface.
Catalytic Activity of Nickel at the Glass/Metal Interface in the Porcelain Enameling System
Authors: SALAMAH M. A. , WHITE D.
Published: June 2006

It is suggested that the function of the nickel flash in the one‐coat enameling system is to atomize catalytic‐ally the hydrogen which forms at the interface and thus facilitate its diffusion into the steel. In the absence of nickel, the bubble which forms between the enamel and the metal has been shown to contain hydrogen. Of the metals tested, only platinum and palladium were effective replacements for the nickel. These two metals are known to catalyze the atomization of hydrogen.
Solubility of Lanthanum in Strontium Titanate
Authors: Balachandran U. , Eror N. G.
Published: June 2006

The compound LaxSr,1‐xTiO3 with x up to 40 at. % is observed to be homogeneous by microscopic examination. The observed X‐ray powder pattern corresponds to the cubic perovskite structure. The plot of lattice constant a0 vs x indicates a Vegard's law dependence within the experimental error. There were no extraneous lines in the X‐ray powder pattern.
Oxidation Kinetics of Hot‐Pressed and Sintered α‐SiC
Authors: COSTELLO J. A. , TRESSLER R. E.
Published: June 2006

The oxidation kinetics of sintered α‐ and hot‐pressed SiC were investigated over the temperature range 1200° to 1500°C. Oxide thicknesses were measured by ellipsometry, interferometry, and profilometry. The two materials behaved similarly, exhibiting parabolic behavior for short times (up to 100 min) followed by decreasing rates for longer times. The oxidation rates for the sintered material were lower than for the hot‐pressed material at all temperatures studied. Apparent activation energies varied with temperature, from 134 to 389 kJ/mol for the sintered alpha material and from 155 to 498 kJ/mol for the hot‐pressed variety. Platinum‐marker experiments indicated that new oxide is formed at the silicon carbide/ oxide interface. It is proposed that the diffusion of oxidant through the growing oxide film is the rate‐controlling process. The details of the oxidation kinetics were complicated by the partial crystallization of the oxide film at higher temperatures, leading to complex paths for transport of oxidant through the film so that the apparent activation energies are difficult to interpret.
Toughening of Glasses by Metallic Particles
Authors: KRSTIC VLADIMIR V. , NICHOLSON PATRICK S. , HOAGLAND RICHARD G.
Published: June 2006

The role of elastic, thermoelastic, and interfacial properties in the toughening of a brittle matrix by metallic second‐phase particles was studied. Two composites were studied: glass+partly oxidized Ni particles (thermal expansion coefficient of the glasses lower than, equal to, and higher than that of Ni) and glass+partly oxidized Al particles (thermal expansion and elastic moduli equal). Weak interfacial bonding between the nickel and its oxide and developed stress concentrations are the major toughness limitations found in the glass/Ni composites. When the thermal expansion coefficient and elastic modulus of the second phase are sufficiently greater than that of the glass matrix, a propagating crack will bypass the particles. When the thermal and elastic stresses are minimized and satisfactory bonding is achieved (glass/Al composites), a 60x toughness increase was realized.
Prismatic Slip of A12O3 Single Crystals Below 1000°C in Compression Under Hydrostatic Pressure
Authors: CASTAING J. , CADOZ J. , KIRBY S. H.
Published: June 2006

Alumina single crystals were compressed perpendicular to the [0001] axis at a constant strain rate between 20° and 950°C. At r>200°C, failure was suppressed by_hydrostatic pressures of 500 to 1500 MPa. Prismatic slip {1120}〈1100〉 was deduced from optical observations of the lateral surfaces and from stress‐optical features in thin sections cut from the specimens. The critical resolved shear stress (CRSS) decreased rapidly with increasing temperature, from a maximum of ∼3000 MPa at 200°C (strain rate 2±10‐−5 s−1). A simple linear law can be fitted with the logarithm of the CRSS as a function of temperature, up to 1800°C. The rate‐controlling mechanism for dislocation glide is likely to be either the Peierls barrier or barriers due to dissociation out of the glide plane.
Crack Branching in Transformation‐Toughened Zirconia
Authors: KIRCHNER H.P. , GRUVER R.M. , SWAIN M. V. , GARVIE R. C.
Published: June 2006

Crack propagation and branching were investigated in calcia partially stabilized zirconia aged at 1300°C for various times. The crack‐branching radii were measured and used to calculate the apparent stress intensity factors at crack branching (K8) which increase with increasing aging time. The KB/KIC ratios increased with aging time with a maximum of 5.3. This maximum ratio is much greater than the ratios previously observed for ceramics.