bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Purity of Raw Materials and Processing on the Microstructure and Properties of High‐Tc Ba2YCu3O7‐x
Authors: Dumas Philippe , Taylor Jenifer A. T.
Published: March 2005

Microstructure and properties of high‐Tc Ba2YCu3O7‐x ceramics were studied as a function of purity of raw materials and processing conditions. Technical‐grade (TG) and reagent‐grade (RG) raw materials were used to prepare the precursor superconducting powders, the characteristics of which were monitored by XRD, chemical analysis, and SEM. Bulk specimens were then prepared with the two master batches. XRD microphase analysis and SEM‐EDS investigation did not show phase differences. Jc and normal‐state resistivity measurements indicated that the TG specimens had more resistive grain boundaries than did RG specimens.
Effect of Heating Rate on the Sintering of Titanium Dioxide Thin Films: Competition between Densification and Crystallization
Authors: Keddie Joseph L. , Giannelis Emmanuel P.
Published: March 2005

Sintering of sol‐gel‐derived TiO2 thin films was studied as a function of heating rate by ion‐beam analysis, X‐ray diffraction, and ellipsometry. The degree of densification increases with increasing heating rate. Densest films containing the lowest amount of hydroxyls are obtained using rapid thermal annealing. The results are rationalized in terms of a competition between densification and crystallization.
Cracking and Damage in a Notched Unidirectional Fiber‐Reinforced Brittle Matrix Composite
Authors: Bordia Rajendra K. , Daigleish Brian J. , Charalambides Panos G. , Evans Anthony G.
Published: March 2005

Results of four‐point bend tests on notched beams of a laminated unidirectional fiber‐reinforced glass matrix composite are presented. The failure sequence has been established through in situ examination. The dominant damage mode is a mixed‐mode, split crack that runs parallel to the predominant fiber directions. The crack interacts with and crosses over imperfectly aligned fibers. The resulting bridging tractions are sufficient to cause the critical strain energy release rate to increase substantially as the crack extends. Several other damage modes are also observed. These include mode I (tensile) matrix cracks bridged by fibers, mode II (shear) cracks, and compressive damage at the loading points.
Polymeric Cyanoborane, (CNBH2)n: Single Source for Chemical Vapor Deposition of Boron Nitride Films
Authors: Maya Leon , Richards Hugh L.
Published: March 2005

Polymeric cyanoborane, (CNBH2)n, is a material readily prepared by passing hydrogen chloride through an ether suspension of sodium cyanotrihydroborate. This polymeric material was volatilized in a CVD reactor to produce, at 600°C, amorphous films containing boron, nitrogen, and carbon. Residual carbon present in the films was removed by ammonia treatment at 800°C, producing nearly stoichiometric boron nitride films that were adherent to a variety of substrates including silicon.
Densification and Shrinkage During Liquid‐Phase Sintering
Authors: Kang Suk‐Joong L. , Kim Kwan‐Hyeong , Yoon Duk N.
Published: March 2005

The process of densification and shrinkage during the final stage of liquid‐phase sintering is described. The densification occurs by the liquid filling of pores during grain growth. The pore filling results in an instantaneous drop of liquid pressure in the compact and causes gradual accommodation of grain shape. The grain shape accommodation by the growth causes the specimen shrinkage. At the same time, the grains tend to restore their spherical shape, resulting in microstructure homogenization around filled pores. The process of densification and shrinkage appears to be determined by the growth of grains during sintering.
Chemical Interactions in Diboride‐Reinforced Oxide‐Matrix Composites
Authors: Singh Mrityunjay , Wiedemeier Heribert
Published: March 2005

Thermochemical analyses of interfacial reactions in titanium, zirconium, and hafnium diboride reinforced oxidematrix composites have been carried out to evaluate the chemical compatibility. The chemical reactivity of these diborides with oxygen and the high volatility of B2O3(l) at reduced oxygen pressures are concerns during processing and operating conditions. The thermochemical stability and the vaporization behavior of B2O3(l) are discussed in terms of partial pressures of dominant gaseous species of the boron‐oxygen system at 1700 and 2300 K. The TiB2/ZrO2 and TiB2/HfO2 systems are thermodynamically stable in a limited oxygen pressure range. The TiB2/Al2O3 system is stable, but the reactions in this system may apparently be accompanied by formation of gaseous products (B2O3, AlO, Al2O, and lower boron oxides) in the presence of elemental oxygen. These thermochemical considerations are very useful in evaluating the effectiveness of oxides as diffusion barrier coatings on diboride reinforcements.
Impurity‐Enhanced Intergranular Cavity Formation in Silicon Nitride at High Temperatures
Authors: Tanaka Isao , Pezzotti Giuseppe , Matsushita Ken‐ichi , Miyamoto Yoshinari , Okamoto Taira
Published: March 2005

The effect of trace impurities on high‐temperature strength is examined in Si3N4 sintered without additives. Strength degradation above 1000°C occurs only in the low‐purity material, which also exhibits an intergranular slow crack growth (SCG) rate 2 orders of magnitude faster than that of the high‐purity material at 1400°C. A relaxation peak of internal friction is observed only in the low‐purity material, at ≅1200°C, and the origin of this peak is ascribed to the initial stage of SCG—that is, the cavity‐nucleation stage, enhanced by impurities. Based on the present results, a model for impurity‐enhanced SCG is proposed.
Fracture Resistance of a Transparent Magnesium Aluminate Spinel
Authors: Ghosh Asish , White Kenneth W. , Jenkins Michael G. , Kobayashi Albert S. , Brad Richard C.
Published: March 2005

The fracture resistance of a fully dense, transparent, polycrystalline magnesium aluminate spinel was measured from room temperature to 1400°C using the chevron‐notched beam and the straight‐notched beam macroflaw techniques, as well as the indentation‐induced, controlled‐microflaw test method, all in three‐point bending. Flexural strengths were also measured for the same range of temperatures to compare with the fracture toughness measurements. From the load vs load‐line displacement (P‐u) curves of the chevronnotched test specimens, the crack growth resistance curves (R‐curves) and the total work‐of‐fracture were determined. It was observed that polycrystalline MgAl2O4 exhibits rising R‐curve behavior which increases with increasing test temperature. The R‐curve increases are attributed to the geometric constraints due to grain bridging and grain wedging phenomena as well as secondary grain boundary microcracking processes, all of which occurred in the wake region behind the advancing crack. The work‐of‐fracture and the R‐curves increased rapidly above 800°C coincident with the onset of increased secondary grain boundary microcracking.
Chemical Characterization of Si‐Al‐C‐O Precursor and Its Pyrolysis
Authors: Babonneau Florence , Sorarú Gian D. , Thorne Kevin J. , Mackenzie John D.
Published: March 2005

Polycarbosilane was modified by reaction with an aluminum alkoxide to get a precursor for Si‐Al‐C‐O ceramics. The precursor was essentially characterized by magic‐angle spinning nuclear magnetic resonance (29Si, 27Al, and 13C) and appeared as a dispersion of Al(OH)3‐based particles in a polycarbosilane chain matrix. After the material was heat‐treated under argon at 1500°C, X‐ray diffraction showed that it crystallized mainly as SiC 2H. The presence of this rather unusual polytype for polycarbosilane‐derived ceramics seemed to be related to the presence of aluminum atoms.
Mechanistic Analysis of Time‐Dependent Failure of Oxynitride Glass‐Joined Silicon Nitride below 1000°C
Authors: O'Brien Michael H. , Coon Dennis N.
Published: March 2005

Time‐dependent failure at elevated temperatures currently governs the service life of oxynitride glass‐joined silicon nitride. Creep, devitrification, stress‐aided oxidation‐controlled slow crack growth, and viscous cavitation‐controlled failure are examined as possible controlling mechanisms. Creep deformation failure is observed above 1000°C. Fractographic evidence indicates cavity formation and growth below 1000°C. Auger electron spectroscopy verified that the oxidation rate. Time‐to‐failure is independent of oxygen concentration. Reasonable agreement is found between the observed time‐to‐failure data and those predicted using the Tsai and Raj, and Raj and Dang viscous cavitation models. It is concluded that viscous relaxation and isolated cavity growth control the rate of failure in oxynitride glassfilled silicon nitride joints below 1000°C. Several possible methods are also proposed for increasing the service lives of these joints.
Redox State of Iron and Its Related Effects in the CaO–P2O5–Fe2O3 Glasses
Authors: Kumar Binod , Lin Steve
Published: March 2005

Iron exists in Fe2+ and Fe3+ states in CaO–P2O5–Fe2O3 glasses and they impart characteristic optical absorption bands that allow analysis of relative proportion of the two species. This communication reports the redox states of glasses melted under air, argon, and oxygen atmospheres and relates them to the dissolution rates. The dissolution rate was found to be related to the redox state and it is lowered if the glass is melted under oxidizing atmospheres.
Evaluation of Slow Crack Growth Resistance in Ceramics for High‐Temperature Applications
Authors: Tanaka Isao , Pezzotti Giuseppe
Published: March 2005

The slow (subcritical) crack growth (SCG) resistance of Si3N4 and SiC ceramics has been evaluated by a stepwise loading test on bending bars precracked by Vickers indentation. Three highly refractory materials were selected for the evaluation: i.e., (1) high‐purity Si3N4 sintered by hot isostatic pressing (HIP) without additives and (2,3) α‐ and β‐ SiC pressureless sintered with B and C addition. Under the hypothesis of linear elastic behavior at high temperature, which was found satisfied in the present materials, the SCG resistance was expressed in terms of initial stress intensity factor critical for SCG failure within a predetermined lifetime. The present method was found useful in shortening the testing time and consistent with other traditional fatigue tests (e.g., static‐fatigue test): It is recommended as a screening test for materials under research and development. Among the materials tested in the present study, the highest SCG resistance up to 1440°C was found in the high‐purity Si3N4 without additives.
Direct Study of the Behavior of Flaw‐Forming Defect in Sintering
Authors: Uematsu Keizo , Miyashita Masayori , Kim Jin‐Young , Uchida Nozomu
Published: March 2005

The morphological change of natural pores with densification was examined in an alumina green body. The liquid‐immersion technique was used for observing pores inside the body before and after sintering. In this technique the specimen is made transparent by an adequate immersion liquid and observed with a transmission optical microscope. Growth of large pores with densification was observed and is discussed along with past thermodynamic consideration of pore growth. Supplemental investigation of the results was conducted by SEM and mercury porosimetry.
Liquid–Solid Equilibria in the System FeO–NiO–Fe2O3–SiO2
Authors: Grutzeck Michael W. , Muan Arnulf
Published: March 2005

Liquidus and solidus phase relations have been determined for the system NiO‐iron oxide–SiO2 in air and in CO2. In both cases the system is simple in that it contains only three primary crystalline phases: spinel (Fe3O4–NiFe2O3 solid solution), oxide (“FeO”–NiO solid solution) and SiO2, either tridymite or cristobalite. There is only one liquidus invariant point at which the above crystalline phases coexist with liquid (and the gas phase). In air, the invariant point (a eutectic) is at 1524°C, and a temperature maximum is present along the spinel–silica liquidus boundary curve at 1540°C. In CO2, the silica + spinel + oxide + liquid invariant temperature is considerably lower: 1433°C. Combination of these data with those obtained previously for the same system under strongly reducing conditions (CO2–CO gas mixtures of ratios only slightly above those of equilibrium coexistence of the oxide phases with Fe–Ni alloy) makes it possible to draw the main features of phase relations in the quaternary system FeO–NiO–Fe2O3–SiO2. This system also is relatively simple, in that only five crystalline phases are present in equilibrium with liquids, viz, silica (cristobalite or tridymite), olivine (Fe2SiO4‐Ni2SiO4 solid solution), spinel (Fe3O4‐NiFe2O4 solid solution), hematite (Fe2O3), and oxide (“FeO”–NiO solid solution). There is only one quaternary liquidus invariant point in the system, characterized by the equilibrium coexistence of silica, olivine, spinel, oxide, and liquid, at a temperature of ∼ 1410°C and an oxygen pressure of the gas phase of ∼10−5 atm.
Hydration in the System Ca2SiO4–Ca3(PO4)2 at 90°C
Authors: Barnes Mary W. , Klimkiewicz Maria , Brown Paul W.
Published: March 2005

Compositions along the Ca2SiO4–Ca3(PO4)2 join were hydrated at 90°C. Mixtures containing 15, 38, 50, 80, and 100 mol% Ca3(PO4)2 were fired at 1500°C, forming nagelschmidtite +a1‐CaSiO4, A‐phase and silicocarnotite and a‐Ca3(PO4)2, respectively. Hydration of these produces hydroxylapatite regardless of composition. Calcium silicate hydrate gel is produced when Ca2SiO4≠ 0 and portlandite when Ca2SiO4 is >50%. Relative hydration reactivities are a‐Ca3(PO4)2 > nagelschmidtite > α1‐Ca2SiO4 > A‐phase > silicocarnotite. Hydration in the presence of silica or lime influences the amount of portlandite produced. Hydration in NaOH solution produces 14‐A tobermorite rather than calcium silicate hydrate gel.
Barrier Layer Capacitor Using Barium Bismuth Plumbate and Barium Plumbate
Authors: Hiremath Basavaraj V. , Newnham Robert E. , Cross Leslie E.
Published: March 2005

A new type of barrier layer capacitor is described utilizing thin glass layers on highly conducting ceramics of barium plumbate and barium bismuth plumbate. The frequency dispersion of the apparent dielectric constant has been explained using a modified version of the Maxwell–Wagner model. These capacitors have a low temperature coefficient of capacitance and a high dispersion frequency in the megahertz range. Simple processing conditions together with low firing temperature make it possible to produce the barrier layer capacitors inexpensively.
Aerosol Synthesis of Aluminum Nitride Powder Using Metalorganic Reactants
Authors: Adjaottor Albert A. , Griffin Gregory L.
Published: March 2005

We describe a new laboratory‐scale aerosol process for making submicrometer AIN powder by reaction of gas phase Al(C2H5)3 and NH3. The process can produce spherical, near‐stoichiometric AIN powder with mean particle diameter ranging from 0.07 to 0.20 μm and polydispersity index between 0.2 and 0.5. The inlet TEA concentration is found to have the greatest effect on particle size, with the furnace temperature and carrier gas flow rate having lesser influence. Our results appear to favor a LaMer‐type mechanism (particle nucleation, followed by condensation growth) over fusion‐coalescence to explain particle growth.
Creep of 6H α‐Silicon Carbide Single Crystals
Authors: Corman Gregory S.
Published: March 2005

The compressive creep behavior of single‐crystal 6H α‐SiC was measured for orientations parallel to and at 45° to [0001]. Deformation of the 45° orientation was dominated by basal slip. Steady‐state creep rates above 10‐7/s were measured at temperatures as low as 800°C. An activation energy of 277 kJ/mol and a stress exponent of 3.32 were determined. Creep testing with applied stresses parallel to [0001] was performed at 1650°C to 1850°C, yielding a stress exponent and activation energy of 4.93 and 180 kJ/mol, respectively. The occurrence of basal slip in the [0001] specimens suggested that significant off‐axis stresses were present during testing.
Strength Evaluation of Reaction‐Bonded Silicon Carbide Radiant Tubes after Long‐Term Exposure to Combustion and Endothermic Gas Ambients
Authors: Darroudi Taghi , Hellmann John R. , Tressler Richard E. , Gorski Lynn
Published: March 2005

The strengths of reaction‐bonded silicon carbide radiant tubes were examined after a 14‐month campaign at 800°–950°C in an industrial heat‐treating furnace. The inner tube of the single‐ended radiant tube (SER) burner system was exposed to a combustion gas environment, and the outer tube was exposed to both a combustion gas (inner surface) and an endothermic gas (outer surface) environment. Diametral compression testing of C‐rings and O‐rings was employed to assess the effect of exposure on the strengths where failure initiated from the outer and inner surfaces of the tubes, respectively. Comparisons of strengths obtained on exposed versus as‐received tubes revealed that exposure to the combustion ambient yields an apparent moderate strengthening, with a concurrent increase in strength variability. No significant difference in strength distribution was observed for tubes exposed to combustion versus endothermic gas ambients. Failure most often originated at silicon‐rich regions remaining from silicon infiltration of macroporosity in the slip cast tubes during reaction sintering. Specimens sectioned from the outer tube exhibited unexpectedly high strengths (based on predictions from Weibull statistics) compared to those sectioned from the smaller inner tube after the same exposure. These results indicate that the larger tubes have flaws with characteristics different from those present in smaller tubes, and underscore the need to use data from the same material, with the same flaw populations, in making predictions of the effect of component size on strength distributions and reliability in service.
Strontium and Silicon Simultaneous Diffusion in Single‐Crystal MgO
Authors: Sakaguchi Isao , Yurimoto Hisayoshi , Sueno Shigeho
Published: March 2005

Diffusion profiles of Sr in single‐crystal MgO at 1473 to 1843 K have been determined by a depth‐profiling technique using secondary ion mass spectrometry. The diffusion experiments were performed using a focusing infrared furnace. The temperature dependences of lattice and dislocation diffusion are D1= 2.6 × 10−11 exp(–268.2/RT [kJ/mol])[m2.s‐1], Dd= 1.7 × 104 exp(–562.3/RT), respectively. The present lattice diffusion coefficients are three orders of magnitude smaller than those previously reported. Si diffused with Sr in MgO over 1583 K, and the diffusion coefficients of Si were also determined. The results of Si diffusion were equivalent to those of our previous study.
Apatite Formation Induced by Silica Gel in a Simulated Body Fluid
Authors: Li Panjian , Ohtsuki Chikara , Kokubo Tadashi , Nakanishi Kazuki , Soga Naohiro , Nakamura Takashi , Yamamuro Takao
Published: March 2005

It has been confirmed that the essential condition for glasses and glass‐ceramics to bond to living bone is the formation of an apatite layer on their surfaces in the body. It has been proposed that a hydrated silica formed on the surfaces of these materials in the body plays an important role in forming the surface apatite layer, but this has not yet been proved. In the present study, it is shown experimentally that a pure hydrated silica gel can induce apatite formation on its surface in a simulated body fluid when its starting pH is increased from 7.2 to 7.4.
Microstructure and Short‐Term Oxidation of Hot‐Pressed Si3N4/ZrO2(+Y2O3) Ceramics
Authors: Falk Lena K. L. , Rundgren Kent
Published: March 2005

Composite ceramic materials based on Si3N4 and ZrO2 stabilized by 3 mol% Y2O3 have been formed using aluminum isopropoxide as a precursor for the Al2O3 sintering aid. Densification was carred out by hot‐pressing at temperatures in the range 1650° to 1800°C, and the resulting micro‐structures were related to mechanical properties as well as to oxidation behavior at 1200°C. Densification at the higher temperatures resulted in a fibrous morphology of the Si3N4 matrix with consequent high room‐temperature toughness and strength. Decomposition of the ZrO2 grains below the oxidized surface during oxidation introduced radial stresses in the subscalar region, and from the oxidation experiments it is suggested that the ZrO2 incorporated some N during densification.
Formation of SiO2, Al2O3, and 3Al2O3. 2SiO2 Particles in a Counterflow Diffusion Flame
Authors: Chung Shyan‐Lung , Sheu Yu‐Cheng , Tsai Ming‐Shyong
Published: March 2005

SiO2, Al2O3, and 3Al2O3.2SiO2 powders were synthesized by combustion of SiCl4 or/and AlCl3 using a counterflow diffusion flame. The SiO2 and Al2O3 powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al2O3.2SiO2 powder produced with a low‐temperature flame was also amorphous and had a similar morphology. However, those produced with high‐temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame.
A Novel Method for the Uniform Incorporation of Grain‐Boundary Layer Modifiers in Positive Temperature Coefficient of Resistivity Barium Titanate
Authors: Ravi V. , Kutty T. R. N.
Published: March 2005

The presently developed two‐stage process involves diping the prefired porous disks of n‐BaTiO3 in nonaqueous solutions containing Al‐buty rate, Ti‐isopropoxide, and tetraethyl silicate and subsequent sintering. This leads to uniform distribution of the grain‐boundary layer (GBL) modifiers (Al2O3+ TiO2+ SiO2) and better control of the grain size as well as the positive temperature coefficient of resistivity characteristics. The technique is particularly suited for GBL modifiers in low concentrations (< 1%).
Synthesis of Submicrometer Mullite Powder via High‐Temperature Aerosol Decomposition
Authors: Moore Katherine A. , Cesarano Joseph , Smith Douglas M. , Kodas Toivo T.
Published: March 2005

High‐purity mullite powders (3Al2O3.2SiO2) have been prepared using a high‐temperature aerosol decomposition technique yielding submicrometer (0.6 μm average) particles of spherical morphology with no hard agglomerates using aluminum nitrate and fumed silica as precursors. Depending upon the reaction conditions used, the powders range from amorphous to crystalline with no evidence of secondary‐phase formation. This mullite synthesis approach has the advantages of not requiring postsynthesis milling, the ability to use a wide range of precursor systems, and enhanced control over chemical homogeneity and particle size/shape.
Deformation by the Stress‐Induced Transport of Helium Bubbles in Silicon Carbide
Authors: Mori Tsutomu , Suzuki Tetsuya , Iseki Takayoshi
Published: March 2005

Silicon carbide (SiC) containing He atoms expands by annealing above ∼ 1300°C, owing to the formation and growth of He bubbles. An external compressive stress applied during annealing retards expansion in the stress direction, while promoting it in the lateral directions. It is proposed that He atoms in the bubbles on grain boundaries perpendicular to the compression axis are transported to the bubbles on boundaries parallel to the stress axis. The transport of He accompanies changes in the volumes of the bubbles to maintain equal pressure in all the bubbles. The volume changes of the bubbles are caused by the flow of SiC‐constituting atoms from or to the boundaries where the volume‐changing bubbles exist. This process results in a jacking action which alters the thickness of boundary atom layers. It produces macroscopic strains. The plausibility of the proposed process is examined on the basis of energetics.
Subsolidus Phase Equilibria of the MgO─V2O5─SiO2 System
Authors: McGarry Michael J. , McCauley Ronald A.
Published: March 2005

The subsolidus phase equilibria of the MgO─V2O5─SiO2 system was studied by solid‐state reaction and powder X‐ray diffractometry. The resulting ternary is discussed with respect to corrosion of magnesia‐ and silica‐containing refractories by vanadium‐containing fuels.
Low‐Earth‐Orbit Effects on Strength of Glasses
Authors: Wiedlocher David E. , Tucker Dennis S. , Nichols Ron , Kinser Donald L.
Published: March 2005

The effects of a 5.8‐y exposure to low‐earth‐orbit environment upon the mechanical properties of five commercial glasses and a low‐expansion‐coefficient glass‐ceramic have been examined. The radiation components of the earth‐orbit environment did not degrade the mechanical strength of the samples examined within the limits of experimental error. Statistical problems arising from the low frequency of micrometeorite or space debris impacts upon the samples precluded statistically valid measurement of impacted sample strengths. Upper bounds for the magnitude of the impact event damage upon the strengths for impacted samples have been determined.
29Si MAS‐NMR Study of the Short‐Range Order in Lithium Borosilicate Glasses
Authors: Martin Steve W. , Bain Debra , Budhwani Karim , Feller Steven
Published: March 2005

29Si MAS‐NMR measurements have been made on a series of lithium borosilicate glasses of general composition RLi2O.B2O3·KSiO2. At low alkali contents (R < 1), the 29Si resonance envelope is broadened and indicates a distribution of Si sites. As R increases above 1, the FWHM of the 29Si resonance narrows considerably to that representative of a single chemical site. Simultaneously, the average chemical shift of the resonance shifts upfield in agreement with the trends found in the binary lithium silicate glass system. Using the chemical shifts for the individual Q species in the binary system it was found that very good agreement between the chemical shifts of the binary glasses and the ternary glasses examined here could be achieved if a model of proportional sharing of the added oxygen (from lithia) between silicate and borate units was used. In contrast to the 11B NMR studies of these same glasses, the 29Si NMR data are quantitatively best‐fit if it is assumed that the proportional sharing of the oxygen from the added lithia begins at R= 0. Models of sharing developed from the 11B NMR studies of these glasses, where proportional sharing above a certain fixed (independent of K) or variable (dependent on K) minimum R0, have been reexamined and were quantitatively shown through residual analysis to give consistently poorer fits to our data. At present the reasons for the discrepancy between the two sets of NMR data are unknown.
Microstructure and Densification of Pressureless‐Sintered Al2O3/Si3N4‐Whisker Composites
Authors: Tseng Wenjea J. , Funkenbusch Paul D.
Published: March 2005

Composite densification was studied by performing slip casting and sintering experiments on an Al2O3 matrix and Si3N4 whisker system. Even though all the slip‐cast powder compacts exhibited high green densities (up to 70% of the theoretical) and narrow pore‐size distribution (pore radius around 15 to 30 nm), significant differential densification on a microscopic scale was found due to the existence of local whisker agglomeration. The inhomogeneous whisker distribution resulted in a binary mixture of large and small pores in the sintered composites, in which whisker‐associated flaws remained stable even after prolonged sintering. The sintered microstructures showed that the spatial distribution as well as the volume fraction of the Si3N4 affect composite densification. Inhomogeneous whisker distribution dominated the complete densification of the composites.
Microstructural Studies in Alkoxide‐Derived Mullite/Zirconia/Silicon Carbide‐Whisker Composites
Authors: Zangvil Avigdor , Lin Chien‐Cheng , Ruh Robert
Published: March 2005

Mullite composites toughened with ZrO2 (with or without a MgO or Y2O3 stabilizer) and/or SiC whiskers (SiC(w)) were fabricated by hot‐pressing powders prepared from Al, Si, Zr, and Mg(Y) alkoxide precursors by a sol–gel process. Micro‐structures were studied by using XRD. SEM, and analytical STEM. Pure mullite samples contained prismatic, preferentially oriented mullite grains. However, the addition of ZrO2, as well as the hot‐pressing temperature, affected the morphology and grain size in the composites; a fine, uniform, equiaxed microstructure was obtained. The effect of SiC(W) was less pronounced than that of ZrO2. Glassy phases were present in mullite and mullite/SiC(W) composites, but were rarely observed in Al2O3‐rich or ZrO2‐containing samples. The formation of zircon due to the reaction between ZrO2 and SiO2 and the considerable solid solution of SiO2 in ZrO2 prevented the formation of the glassy phase, whereas the reaction between Al2O3 and MgO in MgO‐containing samples formed a spinel phase and also deprived the ZrO2 phase of the stabilizer. Intergranular ZrO2 particles were either monoclinic or tetragonal, depending on size and stabilizer content; small intragranular ZrO2 inclusions were usually tetragonal in structure.
Retrograde Densification of Calcined and Glass Precursors of Approximate (Bi,Pb)2Sr2Ca2Cu3O10 Stoichiometry
Authors: Argyriou Dimitri N. , Vance Eric R.
Published: March 2005

Retrograde densification of pelletized calcines and glasses having an approximate (Bi,Pb)2Sr2Ca2Cu3O10 starting stoichiometry and sintered at ∼850°C can be described by first‐order rate equations. Retrograde densification in the calcine precursors was largely due to the development of open pores, and was approximately proportional to the concentration of the (Bi,Pb)2Sr2CaCu3O10 phase. In the glasses, retrograde densification is mainly caused by porosity accompanying the growth of a needlelike Sr─Ca─Cu─O phase, together with (Bi,Pb)2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2CaCu2O8.
Estimation of the Nucleation Rate by Differential Scanning Calorimetry
Authors: Kelton Kenneth F.
Published: March 2005

A realistic computer model is presented for calculating the time‐dependent volume fraction transformed during the devitrification of glasses, assuming the classical theory of nucleation and continuous growth. Time‐ and cluster‐dependent nucleation rates are calculated by modeling directly the evolving cluster distribution. Statistical overlap in the volume fraction transformed is taken into account using the standard Johnson–Mehl–Avrami formalism. Devitrification behavior under isothermal and nonisothermal conditions is described. The model is used to demonstrate that the recent suggestion by Ray and Day,1 that nonisothermal DSC studies can be used to determine the temperature for the peak nucleation rate, is qualitatively correct for lithium disilicate, the glass investigated.
Densification of Large Pores: I, Expriments
Authors: Slamovich Elliott B. , Lange Frederick F.
Published: March 2005

Three different diameters (0.50, 1, and 2 μm) of monosized spherical pores were introduced, at different volume percents up to 15%, into two submicrometer commercial Zr(Y)O2 powders which exhibited either sluggish (ZrO2–3 mol% Y2O3) or rapid (ZrO2–8 mol% Y2O3) grain growth kinetics. Densification and grain growth behavior was studied as a function of heat‐treatment period at 1400°C and/or 1500°C. Pores in the finer‐grain Zr(3Y)O2 densified more rapidly. The grain growth kinetics of the Zr(3Y)O2 material were unaffected by porosity, because the pores were always larger than the grains. Consistent with the Zener concept applied to mobile pores on grain junctions, the spherical pores influenced the grain growth kinetics of Zr(8Y)O2 once the grain size was greater than the pore size. Measurements indicated that the number of pores per unit volume decreased dramatically during densification, whereas their average size remained approximately constant.
Raman Study of Glasses of Ba2TiSi2O8 and Ba2TiGe2O8
Authors: Markgraf Steven A. , Sharma Shiv K. , Bhalla Amar S.
Published: March 2005

Raman spectra are reported for fresnoite (Ba2Ti(Si,Ge)2O8 glasses, and comparison is made between the Raman spectra of the corresponding crystalline powders and glasses of Ba2TiSi2O8 and Ba2TiGe2O8. The Ba2TiGe2O8 glass spectra show correspondence with the Ba2TiGe2O8 crystalline Raman spectra; the vs(Ge–O–Ge) mode occurs at 518 cm−1 in the glass and at 521 cm−1 in the crystalline material. Five‐fold coordinated titanium is the majority species present in the Ba2TiGe2O8 glass as revealed by a strong band at 824 cm−1 in the I glass spectrum. The Ba2TiSi2O8 glass spectra are similar to the Ba2TiSi2O8 crystalline spectrum; the strongest band is found at 836 cm−1 in the I glass spectrum. Through comparison with the previous Raman data of other titania silicate glasses, we conclude that the Ba2TiSi2O8 glass has a structure similar to the crystalline phase.
Point Defect Hardening in MgO·3Al2O3
Authors: Suematsu Hisayuki , Iseki Takayoshi , Yano Toyohiko , Saito Yasutoshi , Suzuki Tetsuya , Mori Tsutomu
Published: March 2005

Vickers hardness in spinel increases by 23% after neutron irradiation to a fluence of 8.3 × 1022 n/m2 at 100°C. Annealing at high temperatures above 100°C gradually decreases the hardness. Above 500°C, the hardness is reduced to almost the same value as that of unirradiated material. The hardness of spinel, irradiated at 470°C to a fluence of 2.4 × 1024 n/m2, is unchanged both after irradiation and after annealing up to 1000°C. The length change during annealing was also measured and is similar to the hardness change. Frank dislocation loops with a density of 3 × 1014/cm3 are induced by neutron irradiation at 470°C but they apparently do not affect the hardening in spinel. Thus, point defects are concluded to act as obstacles against dislocation movement. The yield stress measured at 1400°C is also unchanged after irradiation. It is believed that not only the point defects but also the loops are annihilated by annealing at 1400°C.
Defect‐Chemical Aspects in the Synthesis of Metallic Conductors La1–xBaxCuO3 (x= 0.0 to 0.5)
Authors: Dwivedi Anurag , Rodriguez Mark A. , Cormack Alastair N.
Published: March 2005

Compositions of La1‐xBaxCuO3, where x ranges from 0.0 to 0.5, were fired in air, oxygen‐enriched air, and oxygen. Studies show that BaO cannot make a solid solution with LaCuO3 without changing the basic structure. The resulting phases, in all attempts, were the binary compounds La2CuO4, La1‐xBaxCuO3‐δ (x= 0.2 to 0.5), or their mixtures. All samples showed metallic conductivity. Extra oxygen in the reaction atmosphere appeared to encourage the formation of the LaCuO3‐based phases of La1‐xBaxCuO3‐δ (x= 0.2 or 0.5). We provide a defect‐chemical and thermodynamical explanation for this observation.
Relationship between Fracture Surface Roughness and Fracture Behavior of Cement Paste and Mortar
Authors: Lange David A. , Jennings Hamlin M. , Shah Surendra P.
Published: March 2005

Fracture surfaces of cement and mortar specimens were characterized using a confocal microscope to create three‐dimensional computer‐based topographic maps. The texture of the fracture surfaces was quantified using image analysis techniques to compute a roughness parameter and fractal dimension. Total porosity, pore‐size distribution, compressive strength, and fracture properties of the specimens were determined and compared with the roughness parameter. Fracture properties were determined by notched‐beam tests and the results were analyzed both by using conventional linear elastic fracture mechanics as well as a compliance‐based approach that incorporated R‐curve type of behavior. A positive correlation between fracture surface roughness and fracture toughness was demonstrated.
Metal Organic Resin Derived Barium Titanate: I, Formation of Barium Titanium Oxycarbonate Intermediate
Authors: Kumar Suresh , Messing Gary L. , White William B.
Published: March 2005

Crystallization of BaTiO3 from an X‐ray amorphous, metal organic precursor was investigated by comparing samples heated in O2, air, argon, and CO2. It is evident that an intermediate barium titanium oxycarbonate phase forms between 500° and 620°C and that BaTiO3 forms directly by the endothermic decomposition of this phase between 635° and 700°C. From thermodynamic calculations, thermal analysis, X‐ray diffraction, and Raman spectroscopy, it is concluded that the intermediate oxycarbonate is a highly disordered, metastable, and weakly crystalline phase with a stoichiometry close to Ba2Ti2O5CO3.
High‐Temperature X‐ray Study of the Cubic–Tetragonal Diffusionless Phase Transition in the ZrO2─ErO1.5System: II, Temperature Dependences of Oxygen Ion Displacement and Lattice Parameter of Compositionally Homogeneous 12 mol% ErO1.5─ZrO2
Authors: Yashima Masatomo , Ishizawa Nobuo , Yoshimura Masahiro
Published: March 2005

The tetragonal‐to‐cubic phase transition of rapidly quenched 12 mol% ErO1.5─ZrO2 was investigated in situ using high‐temperature X‐ray diffraction. Rapid quenching of the melt yielded a small oxygen ion displacement and a small tetragonality, axial ratio c/a. Although the tetragonality of the as‐quenched specimen increased with temperature up to about 1000°C, it became independent of temperature on cooling to room temperature from about 1000°C and reheating up to 1000°C. The tetragonality decreased continuously with temperature above 1000°C and became unity at about 1400°C. The atomic coordination z for the oxygen ion, which expresses the displacement from its ideal site in the fluorite‐type structure, increased with temperature and became 1/4 at about 1400°C. The tetragonality increased with annealing time through a thermal activation, which could explain the compositional dependence of the tetragonality in rapidly quenched ZrO2─RO1.5 samples (R = rare earths).
Rigidity, Connectivity, and Glass‐Forming Ability
Authors: Gupta Prabhat K.
Published: March 2005

Following the ideas of Zachariasen and of Cooper, it is argued that the glass‐forming ability of a system can be rationalized in terms of the degrees of freedom available to form an infinitely large topologically disordered network composed of rigid structural units connected at vertices, edges, or faces. Expressions are derived for the degrees of freedom for regular and irregular networks composed of regular or distorted polytopes sharing vertices, edges, or faces. The condition that degrees of freedom must be nonnegative for such a disordered network to exist is in agreement with the empirical results on glass‐forming abilities of a variety of covalent, ionic, and metallic systems. The present approach can be used to screen the proposed models of structure of a glass and to predict the boundaries of glass‐forming regions in composition spaces.
Thermal Stresses, Relaxation, and Hysteresis in Glass
Authors: Rekhson Simon M.
Published: March 2005

The theory of A. R. Cooper, Jr., and other viscoelastic theories of thermal stresses in glass are reviewed. The role of structural relaxation in the evolution of the free strain and stress relaxation times is examined. An explanation of the hysteresis loop observed on thermal contraction/expansion curves in the glass transition region is offered. Single‐exponential models are reviewed on the merit of the insight they provide into the physics of stress development. Their numerical performance is compared with that of the more comprehensive theories.
Study of Porosity in Corroded Refractories
Authors: Konsztowicz Krzysztof J. , Boutin Jacques
Published: March 2005

In this work, computer image analysis was used in order to characterize the complex porosity found in ceramic refractory materials. The porosities of SiC and Al2O3 refractory bricks before and after slagging tests have been examined by both mercury porosimetry and image analysis techniques; the results obtained by these two methods were compared and their differences discussed. Mercury porosimetry underestimated the volume fraction of pores larger than 100 μm, which contributed significantly to the total pore volume of analyzed materials. In contrast, image analysis did not reveal finer porosity, the contribution of which was, however, less important, Image analysis appeared to have the advantage of quantification of local heterogeneities of porosity, and the capabilities of this technique to quantify the variations in the volume of pores adjacent to reaction zones are documented and supported by microscopic observations.
High‐Performance Multilayer Capacitor Dielectrics from Chemically Prepared Powders
Authors: Bruno Salvatore A. , Swanson Donald K. , Burn Ian
Published: March 2005

Advantages of chemically prepared powders for electronic ceramics have been demonstrated for a number of multilayer capacitor (MLC) dielectrics. A cost‐efficient precipitation process was developed to produce undoped or doped crystalline barium titanate powder with a narrow particle size distribution close to 0.5 μm. More complex compositions, e.g., barium‐neodymium titanate, were amorphous as precipitated but could be crystallized by calcination below 1000°C. Additional compositional modifications, to adjust electrical properties or to lower sintering temperature, were accomplished by doping the surface of the powder particles using a solution coating process. Exceptional fired densities and electrical performance were obtained.
Hydrothermal Corrosion of Pure, Hot Isostatically Pressed Silicon Nitride
Authors: Oda Kohei , Yoshio Tetsuo , Miyamoto Yoshinari , Koizumi Mitsue
Published: March 2005

The aqueous corrosion behavior of hot isostatically pressed Si3N4 (HIPed‐Si3N4) without additives was studied under hydrothermal conditions at 300°C and 8.6 MPa (86 atm). The accelerated weight loss in the HIPed‐Si3N4 was attributed to uniform thinning of the specimen accompanied by dislodgement of Si3N4 grains from the substrate due to preferential attack at grain boundaries. Enhanced attack at grain boundaries was due to the presence of amorphous SiO2 from impurities in the starting powder.
Effect of Stoichiometry on the Dielectric Properties and Life Performance of (Pb0.875Ba0.125)[(Mg1/3Nb2/3)0.5(Zn1/3Nb2/3)0.3Ti0.2]O3 Relaxor Dielectric Ceramic: Part II, Life Performance
Authors: Kanai Hideyuki , Furukawa Osamu , Nakamura Shin‐ichi , Yamashita Yohachi
Published: March 2005

The accelerated life performance of a partially A‐site substituted relaxor dielectric ceramic, (Pb0.875Ba0.125)A‐[(Mg1/3Nb2/3) 0.5(Zn1/3Nb2/3)0.3Ti0.2] O3, was investigated for disk specimens with silver electrodes. The specimens were subjected to a humidity load life condition (85°C, 95% rh, 700 V), and the life performance was measured as a function of the stoichiometry of the relaxor. Stoichiometry was represented by the mole ratio A/B, which is the mole ratio of elements at A‐sites to those at B‐sites in the perovskite structure. The life performance was found to depend greatly on A/B, which was varied between 0.95 and 1.05. When A/B was equal to or less than 0.99, the failure rate at 500 h was 25% or below, while all the specimens failed within 350 h when A/B was equal to or greater than 1.00. All failed specimens had evidence of short‐circuit failures. The life performance had a strong correlation with the microstructure of the dielectrics. Transmission electron microscopy revealed a PbO‐rich secondary phase both at triple points and at grain boundaries in specimens with A/B greater than 1.00. Although a secondary phase was found at triple points in specimens with A/B = 1.00, it consisted of unreacted elements such as Pb, Ba, Nb, Zn, Mg, and Ti which did not form the perovskite structure. The results suggest that the lead component analyzed at triple points and grain boundaries is PbO, which has slight solubility in water. Thus, it appears that triple points and grain boundaries dissolve during the life test, and then migration of Agions through the dissolved grain boundaries leads to degradation.
Interaction of Silver/Palladium Electrodes with Lead‐and Bismuth‐Based Electroceramics
Authors: Wang Sea Fue , Huebner Wayne
Published: March 2005

In this study the phase relations between Pd or 70Ag/30Pd electrode systems with commercially important Bi‐or Pb‐based oxides were determined to establish the conditions under which detrimental interfacial chemical reactions may occur. For the reaction of Pd with Bi compounds, PdBi2O4 formation was observed if the reaction proceeded at temperatures less than 835°C. At higher temperatures a Pd (Bi) alloy formed containing the maximum solubility of the Bi, i.e., 16 at.%. For the reaction of Pd with Pb compounds, a Pd (Pb) alloy formed which in all instances exhibited the maximum solubility of the Pb, i.e., 14 at.%. These reactions had an adverse effect on the local stoichiometry and composition of the dielectric, causing depletion of Bi or Pb. Studies on commercial capacitor dielectrics from DuPont (X7R and Z5U formulations) and Pb (Mg1/3Nb2/3)O3 showed that the use of Pd electrodes decreased the dielectric constant substantially, which was due to the formation of a low permittivity phase in series connectivity with the unaltered dielectric.
Computer Simulation of Impedance Spectroscopy in Two Dimensions: Application to Cement Paste
Authors: Coverdale R. Tate , Garboczi Edward J. , Jennings Hamlin M. , Christensen Bruce J. , Mason Thomas O.
Published: March 2005

A random 2‐D digital‐image‐based microstructural model is used to simulate impedance spectroscopy of cement paste. Results of these simulations are used to interpret previous experimental results. The main features of the impedance spectra of cement paste are the result of the composite nature of cement paste; in other words, the electrical response varies as different phases, each with its own specific electrical properties, are consumed and generated. Percolation theory is invoked to help explore certain relationships between two‐ and three‐dimensional analysis.
Cation‐Exchange Characteristics of Sintered Hydroxyapatite in the Strongly Acidic Region
Authors: Suzuki Suguru , Fuzita Toshifumi , Maruyama Toru , Takahashi Minoru , Hikichi Yasuo
Published: March 2005

The ion‐exchange behavior of hydroxyapatite (Ca10(PO4)6‐OH)2) (HAp), fired at high temperatures, has been investigated in the strongly acidic region (pH 2 and 3). According to the present study, HAp fired above 1000°C can exchange ions from Ca2+ to Pb2+, even at pH 2, without dissolution. The molar ratios of Pb2+/Ca2+ after ion exchange are approximately unity in all cases. Ion exchange occurs in a thin layer near the surface of HAp particles fired at 1300°C. After ion exchange, Pb‐Cl‐apatite crystals are created in the strongly acidic region (pH 2).
Critical Evaluation of the Compression Creep Test
Authors: Debschütz Klaus D. , Caspers Bernhard , Schneider Gerold A. , Petzow Gunter
Published: March 2005

The compression creep test is analyzed with respect to the friction at the loaded ends of the specimen. An analytical and a finite‐element solution performed for large deformations reveals that friction leads to a strain‐independent reduction of the creep rate which only depends on the stress exponent of the creep law and the geometry of the specimen. The assumed material behavior is a time‐dependent, isochoric, nonlinear viscosity. Compression creep tests on 3‐mol%‐yttria‐stabilized tetragonal zirconia polycrystals (3Y‐TZP) have been analyzed. The determination of a stress exponent corresponding to the decreasing stress state in the specimen during large deformations results in a higher resolution of the stress–strain‐rate relationship derived from a certain number of creep tests. For 3Y‐TZP the compression creep tests reveal a decreasing stress exponent with increasing stress.