bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic Fatigue of a Lithia‐Alumina‐Silica Glass‐Ceramic
Authors: Tucker Dennis S.
Published: March 2005

A dynamic fatigue study was performed on a Li2O‐Al2O3‐SiO2 glass‐ceramic in order to assess its susceptibility to delayed failure. Fracture mechanics techniques were used to analyze the results for the purpose of making lifetime predictions for optical elements made from this material. The material has reasonably good resistance (N= 20) to stress corrosion in ambient conditions. Analysis also indicated the elements should survive applied stresses incurred during grinding and polishing operations.
Preparation of Monolithic Porous Titania‐Silica Ceramics
Authors: Hosono Hideo , Sakai Yasuhiro , Fasano Marcelo , Abe Yoshihiro
Published: March 2005

Monolithic porous ceramics composed of TiO2 (67 mol%) and SiO2 (33 mol%) were prepared via casting a melt in the Na2O‐CaO‐TiO2‐P2O5‐SiO2 system and subsequent acid leaching of the resultant ceramics which are constituted of NaCaPO4, TiO2, and amorphous SiO2. The median pore diameter and specific surface area of the resulting porous ceramics are approximately 1 μm and 40 m2, respectively. Amorphous silica surrounds the submicrometer‐sized particles of TiO2 acting as a binder and retaining monolithic forms. No significant shrinkage in the pore size occurred upon heating up to 1000°C.
Structural Investigations of Prehydrolyzed Precursors Used in the Sol‐Gel Processing of Lead Titanate
Authors: Ramamurthi Sangeeta D. , Payne David A.
Published: March 2005

A variety of titanate gels were prepared from lead acetate and titanium isopropoxide in methoxyethanol. In the present study, 1H and 13C Fourier transform‐nuclear magnetic resonance and mass spectroscopic techniques were used to identify the structure of the lead and titanium precursors formed from lead acetate and titanium isopropoxide, respectively, when refluxed in methoxyethanol. The lead and titanium precursors were determined to be Pb(OOCCH3) (OC2H4OCH3). xH2O, where x < 0.5, and Ti2(OC2H4OCH3)8, respectively. Similar spectroscopic procedures were applied to identify the lead titanium complex formed from the reaction between lead and titanium precursors.
Ultrasonic Dispersion of Ceramic Powders
Authors: Jorge Eric , Chartier Thierry , Boch Phillippe
Published: March 2005

Ultrasonication is very effective in dispersing alumina and barium titanate slurries. When optimum conditions are used, very short durations (2 to 3 min) are enough to achieve well‐dispersed, stable suspensions.
Direct Observation of Internal Structure in Spray‐Dried Alumina Granules
Authors: Uematsu Keizo , Kim Jin‐Young , Miyashita Masayori , Uchida Nozomu , Saito Katsuichi
Published: March 2005

The internal structure of spray‐dried alumina granules was characterized by optical microscopy by immersing them in a liquid having a refractive index close to that of alumina. This method provides a unique technique for the detailed analysis of the internal structure of spray‐dried granules.
Quick Analysis of Free Carbon in Metal Carbide by Plasma Oxidation
Authors: Ogawa Toru , Fukuda Kousaku
Published: March 2005

Plasma oxidation combined with emission monitoring was applied to the quantitative analysis of free carbon in ZrC powder. The emission was monitored with an optical color analyzer and was calibrated with standard samples of ZrO2+ C mixtures. Oxidation rates of the free and the combined carbons are so different that it is possible to estimate the amount of the former from the emission. No chemical treatment is necessary.
Effects of Y2O3 and ZrO2 on the Densification of Si3N4‐Zr(Y)O2 Composites
Authors: Kim Jae Ryong , Kim Do‐Hyeong , Kim Chong Hee
Published: March 2005

The effects of ZrO2 and Y2O3 on the densification of hotpressed Si3N4‐Zr(Y)O2 composites have been studied. High density could not be obtained by the addition of pure or 3‐mol%‐Y2O3‐doped ZrO2 in this composite; however, nearly full density (>97%) could be obtained in Si3N4 using 6‐ and 8‐mol%‐Y2O3‐doped ZrO2. It is concluded that Y2O3 diffusing out from the added Zr(Y)O2 promoted the densification and that ZrO2 also had some role in the formation of an oxynitride glass.
Microstructural and Chemical Studies of Stainless Steel/Refractory Interaction
Authors: Sillo Cyrill Eyewu
Published: March 2005

The experimental work investigates the behavior of commercial‐grade aluminosilicate and basic refractories in contact with stainless steels at 1630°C in an argon atmosphere. For the aluminosilicate refractories, Mn proves to be the most deleterious to these refractories. For the basic refractories, silicon is the most destructive element attacking the magnesioferrite phase to produce a complex silicate of magnesium, managanese, and calcium. The presence of metal droplets in the hot face of every steel/refractory system is explained on the basis of disproportionation and reaction of lower oxides, especially CrO.
New Phases in the CaO–M2O3–CuO (M = Nd, Gd, Y) Systems at 1000°C
Authors: Davies Peter K. , Caignol Elizabeth , King Todd
Published: March 2005

New layered cuprates with the general stoichiometry Ca2+xM2−xCu5O10 have been characterized for M = Nd, Gd, and Y. These insulating phases have structures closely related to that of NaCuO2 and contain one‐dimensional chains constructed from edge‐sharing [CuO4] square planes. The X‐ray patterns of these ternary cuprate phases are complicated by strong reflections from both incommensurate and commensurate supercells which result from the ordering of the Ca2+ and M3+ ions in the structure. The stoichiometry range, and resultant Cu valences in these cuprates depend upon the nature of the trivalent ion. Ca‐rich and trivalentrich compositions can be prepared for M = Nd and Gd, but only Ca‐rich compositions were formed in the Y system. No other ternary phases are observed under the conditions of this study, 1000°C in air, and the solid‐state equilibria are dominated by the single ternary phase identified above.
Direct Measurement of Transformation Zone Strains in Toughened Zirconia
Authors: Dadkhah Mahyar S. , Marshall David B. , Morris Winfred L. , Cox Brian N.
Published: March 2005

Residual strains responsible for crack tip shielding have been measured within transformation zones surrounding cracks in Mg‐PSZ. Two techniques were used for strain measurement: moiré interferometry and high‐resolution image matching. Both methods provide maps of differential in‐plane displacements within the specimen surface intersected by the crack, the latter method with the higher resolution. The results are compared with finite‐element analysis to assess surface relaxation effects, and the measured strains are used to evaluate the crack tip shielding stress intensity factor. Calculations based on the assumption that the unconstrained transformation strain is hydrostatic dilatation yielded results that were significantly smaller than the measured toughness increases.
Influence of Pyridine–Borane on the Sintering Behavior and Properties of α‐Silicon Carbide
Authors: Passing Gerd , Riedel Ralf , Petzow Günter
Published: March 2005

In the present study, α‐SiC powder is coated with pyridineborane (BH3·C5H5N), a liquid molecular compound, which forms a boron carbonitride (BC3.5N) layer by heat treatment at 1000°C under argon. The precipitation method leads to an improved chemical homogeneity in the compacted powder resulting in enhanced densification and significant reduction in grain growth during subsequent sintering at temperatures exceeding 2070°C. Thus, small average grain sizes of d50= 1.3 μm and a narrow grain size distribution (d10= 0.6 μm, d90= 2.2 μm) are detected in the liquid‐phase‐processed sample sintered at 2200°C for 0.5 h in argon. Final densities of at least 98% of theoretical could be obtained by pressureless sintering at 2100°C. These results as well as the microstructural distribution of the sintering aids in the densified samples are discussed.
Residual‐Stress‐Induced Grain Pullout in a 96% Alumina
Authors: Powell‐Doğan Cynthia A. , Heuer Arthur H. , Ready Michael J. , Merriam Kathy
Published: March 2005

A comparison of the surface and bulk microstructures of two 96% aluminas indicates that the residual stresses arising from thermal expansion mismatch created by crystallization of the amorphous boundary phase, when combined with stresses created during grinding, can lead to excessive pullout of the alumina grains. However, neither source of residual stress is, by itself, sufficient to cause such pullout. Residual stresses resulting from crystallization of the boundary glass are expected to play a significant role in determining the abrasive wear resistance of these materials.
Thermal Stress Failure of Ceramics under Repeated Rapid Heatings
Authors: Lamon Jacques , Pherson David
Published: March 2005

A plasma jet thermal shock testing method was developed for evaluating the mechanical response of ceramics to repeated thermal shock heat up. The extent of damage involving changes in the populations of critical microstructural heterogeneities was characterized through statistical distributions of residual strengths. This thermal shock testing method was applied to silicon‐based ceramics, including two grades of silicon nitride and a commercial silicon carbide. The thermal fatigue behavior was finally simulated using failure probability computations produced by the CERAM computer code for reliability analysis.
Fatigue of Yttria‐Stabilized Zirconia: I, Fatigue Damage, Fracture Origins, and Lifetime Prediction
Authors: Liu Shih‐Yu , Chen I‐Wei
Published: March 2005

Uniaxial tension–compression fatigue behavior of 3‐mol%‐yttria‐stabilized tetragonal zirconia polycrystals was investigated. Hysteresis in the stress–plastic strain curve featured cumulative plastic strain and weakened elastic stiffness. Fracture statistics in terms of cycle‐to‐failure depends strongly on the maximum stress and less on the stress amplitude. Preexisting processing flaws were identified as the fracture origins in all cases. We suggest that microcracking is the dominant mechanism of fatigue damage, that nucleation of fatigue crack is usually not necessary, and that fatigue lifetime is primarily controlled by crack propagation, which is most sensitive to the maximum stress.
Effect of Heating Rate on Sintering and Coarsening
Authors: Chu May‐Ying , Rahaman Mohamed N. , De Jonghe Lutgard C. , Brook Richard J.
Published: March 2005

The sintering of zinc oxide powder compacts has been investigated at constant rates of heating of 0.5° to 15°C/min. For samples with the same initial relative density (0.50), the temperature derivative of the densification strain versus density fits within a single, relatively narrow band. At low temperatures the densification rate as a function of temperature increases almost linearly with the heating rate. The data, covering a wide density range of 0.5 to 0.98, are consistent with an analysis that accounts for the coarsening (defined as an increase in the mean pore separation) in terms of two classes of microstructural coarsening processes: those associated with densifying and with nondensifying mechanisms.
Structure Evolution from Pb(Mg1/3Nb2/3)O3 to La(Mg2/3Nb1/3)O3
Authors: Lin Li‐Jiaun , Wu Tai‐Bor
Published: March 2005

The crystal structure of lanthanum‐modified lead magnesium niobates having composition (Pb1−xLax) (Mg(1+x)/3‐Nb(2−x)/3)O3 with X= 0 to 1 was investigated by X‐ray powder diffraction. It was found that the fundamental reflections from perovskite structure remain in the whole range of composition. The superlattice reflections from the A(B′1/2‐B″1/2)O3 ordered structure are also well preserved for La content greater than 50 at.%; however, a series of extra peaks of mixing indices appears, with intensities gradually enhanced with the increase of La content. For the complete substitution of Pb by La, a splitting of some reflections can be observed in the diffraction pattern. The results indicate that the crystal structure evolves continuously with the La content, from disordered cubic perovskite of space group Pm3m for X= 0, to ordered cubic perovskite of space group Fm3m for X= 0.5, distorted cubic perovskite of space group Pa3 for 0.5 < X < 0.9, and finally to a rhombohedral perovskite, possibly belonging to the space group R3, for X≥ 0.9. In the evolution of structure, a linear reduction of the lattice constant of the perovskite cell from 4.048 to 3.964 Å was observed.
Epitaxial Crystallization of Seeded Albite Glass
Authors: Selvaraj Ulagaraj , Liu Chun Ling , Komarneni Sridhar , Roy Rustum
Published: March 2005

Glasses that are extremely difficult to crystallize are generally avoided in making glass‐ceramics. It is now possible to crystallize such glasses epitaxially using isostructural seeds. The role of solid‐state epitaxy in the crystallization of such albite (NaAlSi3O8) glass to glass‐ceramic was investigated. The glass was seeded with extremely fine ZrO2 (nonisostructural) and albite (isostructural) seed crystals. X‐ray diffraction results indicated that the albite‐seeded glass, heat‐treated at 1000°C for 100 h, epitaxially crystallized to albite, while the ZrO2 and unseeded glasses did not crystallize in identical heat‐treatment conditions. In addition, the albite‐seeded glass, heat‐treated at 905°C for 10 d, crystallized mostly to albite, whereas the ZrO2 and unseeded glasses at the same conditions contained only a small amount (<5 wt%) of nepheline (NaAlSiO4). The microstructure of the epitaxially grown glass‐ceramics showed that extremely fine crystals (∼0.2 μm thickness) were formed around the seed.
Local Structure of Alkaline‐Earth Boroaluminate Crystals and Glasses: I, Crystal Chemical Concepts—Structural Predictions and Comparisons to Known Crystal Structures
Authors: Bunker Bruce C. , Kirkpatrick R. James , Brow Richard K.
Published: March 2005

This paper presents a model which predicts glass structures based on local charge balance considerations. The model is shown to be consistent with known structures of borate, aluminate, and boroaluminate crystals and predicts that boroaluminate glasses should contain 3‐ and 4‐coordinate boron, 4‐, 5‐, and 6‐coordinate aluminum, and oxygens coordinated to one, two, three, and perhaps even four network‐forming cations. The predicted glass structures are more complex than previous models for boroaluminate glasses, explaining the solid‐state NMR data for the boroaluminate glasses discussed in Part II.
Microstructure, Microchemistry, and Flexural Strength of Mullite Ceramics
Authors: Mizuno Mineo
Published: March 2005

The microstructure of mullite ceramics hot‐pressed and sintered at different temperatures was studied using transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) with EDS, and electron probe microanalysis (EPMA). The specimens, consisting of stoichiometric mullite grains without glassy phase, are obtained by hot‐pressing stoichiometric mullite powder at 1575°C for 1 h. Silica‐rich glassy phases are observed using TEM at three‐grain junctions of mullite grains in specimens heated at and above 1600°C. However, high‐resolution transmission electron micrographs show no glassy phase at two‐grain boundaries in all specimens. SEM with EDS analyses show that the average value of Al2O3 contents of mullite grains increases slightly with increasing temperature. These results are consistent with a published Al2O3–SiO2 phase diagram. The flexural strength of mullite specimens at room temperature depends on their microstructure, such as the grain size and grain size distribution of mullite grains. The strength is high at room temperature and up to 1200°C, and it decreases at and above 1350°C, irrespective of the presence of the glassy phase.
Crack Resistance and Fatigue of Transforming Ceramics: II, CeO2‐Stabilized Tetragonal ZrO2
Authors: Grathwohl Georg , Liu Tianshun
Published: March 2005

Crack resistance characteristics and fatigue properties have been investigated in Ce‐TZP ceramics with different grain sizes. The relatively low critical transformation stress allows the development of larger transformation zones (≦200 μm), leading to flaw‐tolerant behavior. However, autocatalytic transformation processes are found to be bound to grain sizes beyond a critical value; transformation is then very limited in finer microstructures. Fatigue as a specific cyclic effect is more pronounced in microstructures with larger grains. Thus, damaging processes in the course of extensive t–m transformation are suspected to be intensified during cyclic loading.
Phase Stability and Physical Properties of Cubic and Tetragonal ZrO2 in the System ZrO2–Y2O3–Ta2O5
Authors: Kim Dae‐Joon , Tien Tseng‐Ying
Published: March 2005

The cubic (c‐ZrO2) and tetragonal zirconia (t‐ZrO2) phase stability regions in the system ZrO2–Y2O3–Ta2O5 were delineated. The c‐ZrO2 solid solutions are formed with the fluorite structure. The t‐ZrO2 solid solutions having a c/a axial ratio (tetragonality) smaller than 1.0203 display high fracture toughness (5 to 14 MPa · m1/2), and their instability/transformability to monoclinic zirconia (m‐ZrO2) increases with increasing tetragonality. On the other hand, the t‐ZrO2 solid solutions stabilized at room temperature with tetragonality greater than 1.0203 have low toughness values (2 to 5 MPa · m1/2), and their transformability is not related to the tetragonality.
Mathematical Modeling of Flow and Heat‐Transfer Phenomena in Glass Furnace Channels and Forehearths
Authors: Choudhary Manoj K.
Published: March 2005

A three‐dimensional mathematical model was developed to calculate fluid‐flow and heat‐transfer phenomena in a channel/forehearth system used for delivering glass from the melting end to the forming end of a continuous‐glass‐fiber furnace. The model allowed for a varying cross section of the delivery system and for the temperature‐dependent physical properties of the molten glass. Also, a formulation was presented to calculate heat transfer by radiation at the combustion gas/glass interface. The model was used to investigate the effects of natural convection, throughput, and radiation in the glass on velocity and temperature distribution in a glass delivery system. The results showed that, although the natural convection velocities were much lower than the average velocity due to throughput, the presence of natural convection significantly affected the flow path lines in the system. The results also illustrated the distribution of surface and bottom glass streams entering a channel into different forehearths.
Preparation and Cu+‐Ion‐Conducting Properties of the Glasses in the System CuBr–Cu2MoO4–Cu3PO4
Authors: Shinkuma Yoshikane , Machida Nobuya , Minami Tsutomu
Published: March 2005

Cu+‐ion‐conducting glasses were prepared in the pseudoternary system CuBr–Cu2MoO4–Cu3PO4, and the ion‐conducting properties of the glasses obtained were compared to those of the glasses in the system CuI—Cu2MoO4—Cu3PO4, which contain CuI instead of CuBr. The CuBr—Cu2MoO4—Cu3PO4 glasses showed high ion conductivities in the range of 100 to 10−2 S · m−1 at room temperature. The change in ion‐conducting properties caused by the substitution of PO3‐4 anions for MoO2−4 anions was larger than the change caused by the substitution of Br− anions for I− anions in the glasses containing a constant amount of the cuprous halides.
Strength Development in Phosphate‐Bonded Calcium Aluminate Cements
Authors: Sugama Toshifumi , Carciello Neal R.
Published: March 2005

The chemical reaction at room temperature between calcium aluminate cement (CAC) and NH4H2PO4‐based fertilizer solution causes the formation of ammonium calcium pyrophosphate (AmCPP) as an amorphous cementitious phase. This phase was responsible for the development of strength in rapid‐setting phosphate‐bonded cement (PBC) specimens. Hydrothermal treatment of PBC at 200°C led to phase transformations of AmCPP and CAC into crystalline hydroxyapatite as the major phase and anorthite as the minor one. The moderate growth of these interlocked crystals in the interspaces of amorphous phases played an important role in creating a dense microstructure, thereby conferring a very high strength to the PBC. In contrast, an excessive crystal growth, which caused the formation of porous microstructure, resulted in a reduction in strength.
Rare‐Earth Aluminogermanate Glasses
Authors: Kohli Jeffrey T. , Shelby James E.
Published: March 2005

The present study is a comparison of the properties of rare‐earth aluminogermanate and rare‐earth aluminosilicate glasses. Several properties have been measured, including density, transformation temperature, softening point, thermal expansion, Vickers hardness, magnetic susceptibility, Verdet constant, and IR transmission. The property data are discussed in relationship to the structure of these glasses and the field strength of the rare‐earth ions. The Vickers hardness numbers of the glasses are discussed using a unique theory of hardness which involves the relationship between hardness and atom/ion packing in material structures.
Formation of the Perovskite Phase in the PbMg1/3Nb2/3O3─PbTiO3 System
Authors: Bouquin Olivier , Lejeune Martine , Boilot Jean‐Pierre
Published: March 2005

The reaction sequences during calcination of oxide mixtures were studied for the PbMg1/3Nb2/3O3─PbTiO3 (PMN‐PT) system. The effect of reactivity and composition of the starting mixtures was investigated. In the present study, a B‐site‐deficient, cubic pyrochlore phase in the PbO‐Nb2O5 system was formed at 500°C. The perovskite phase of PMN was formed at 7007deg;C through the diffusion of MgO into the pyrochlore phase. The lattice parameter of the pyrochlore phase decreased as this transformation to perovskite progressed.
Aqueous Processing of Mullite‐Containing Green Sheets
Authors: Ushifusa Nobuyuki , Cima Michael J.
Published: March 2005

An aqueous‐based slurry for tape‐casting mullite sheets was studied that is suitable for electronic packaging. A latex binder was used to attain the necessary organic content in the tape while maintaining a low slurry viscosity. The colloidal stability of these complex slurries was characterized by electrokinetic sonic amplitude (ESA) measurements and sedimentation experiments. Control of dispersion stability required the addition of a polyelectrolyte. Suitability of the aqueous‐based tapes for multilayer packaging was evaluated by measuring deformation during lamination and by optimizing conditions such that binder loading and total deformation were minimized.
Boron Nitride Interphase in Ceramic‐Matrix Composites
Authors: Naslain Roger , Dugne Olivier , Guette Alain , Sevely Jean , Brosse Christian Robin , Rocher Jean‐Philippe , Cotteret Jacques
Published: March 2005

A BN interphase has been deposited, by isothermal/isobaric chemical vapor infiltration (ICVI) from BF3─NH3, within a preform made from ex‐polycarbosilane (ex‐PCS) fibers, at about 1000°C. In a second step, the BN‐treated preform was densified with SiC deposited from CH3SiCl3─H2 at about the same temperature. From a thermodynamic standpoint, ex‐PCS fibers could be regarded as unreactive vs the BF3─NH3 gas phase assuming they are coated with a thin layer of carbon or/and silica. The as‐deposited interphase consists of turbostratic BN (N/B < 1) containing oxygen. The SiC infiltration acts as an annealing treatment: (i) the BN interphase becomes almost stoichiometric and free of oxygen; (ii) the fibers undergo a decomposition process yielding a SiO2/C layer at the BN/fiber interface. The weaker link in the interfacial sequence seems to be the BN/SiO2 interface. Deflection of microcracks arising from the failure of the matrix takes place at (or nearby) that particular interface.
Reaction Kinetics of Perovskite Phase Formation in Lead Zinc Magnesium Niobate Ceramics
Authors: Chen San‐Yuan , Wang Chien‐Min , Cheng Syh‐Yuh
Published: March 2005

The reaction kinetics of perovskite Pb[(Zn,Mg)1/3Nb2/3]O3─SrTiO3 (PZMN–ST) phase formation vary with the phase condition of the starting materials. Basically, the reaction is controlled by a diffusion process with either a fixed number of nuclei or a constant nucleation rate. When the Pb3Nb2O8 (P3N2) phase is used as the starting material, the reaction belongs to the former. The P3N2 phase acts as a nucleation center and fixes the number of nuclei for perovskite phase formation. If PbO and Nb2O5 instead of P3N2 are used as starting materials, the nucleation center will be the pyrochlore phase Pb2Nb2O7 or P3N2 and is continuously created by the reaction of residual PbO with the formed pyrochlore Pb3Nb4O13 phase. In the final stage of PZMN formation, the reaction is mainly through diffusion of ZnO into PMN or Zn‐containing PMN. The importance of the pyrochlore phase to perovskite PZMN phase formation is consequently asserted.
Cation Interdiffusion and Phase Stability in Polycrystalline Tetragonal Ceria–Zirconia–Hafnia Solid Solution
Authors: Sakka Yoshio , Oishi Yasumichi , Ando Ken , Morita Shoji
Published: March 2005

Zr–Hf interdiffusions were carried out at 1350° to 1520°C for polycrystalline tetragonal solid solutions of 14CeO2·86(Zr1‐xHfx)O2 with X= 0.02 and 0.10. Lattice and grain‐boundary interdiffusion parameters were calculated from the concentration distributions by using Oishi and Ichimura's equation. Lattice interdiffusion coefficients were described by D= 3.0 × 103 exp[‐623 (kJ/mol)/RT] cm2/s and grain‐boundary interdiffusion parameters by δD' = 0.29 exp[‐506 (kJ/mol)/RT] cm3/s. The cation diffusivity was lower than the anion diffusivity. The results were compared with diffusivities in the fluorite‐cubic solid solution. The critical grain radii for stabilization of the tetragonal phase in CeO2‐doped ZrO2 were 11 and 6 μm for the solutions with 2 and 10 mol% HfO2 substitution, respectively, both of which are much greater than in the Y2O3‐doped ZrO2 solid solution.
Nucleation and Crystallization Kinetics in Fragile Glass‐Forming Liquids
Authors: Senapati Hema , Angell C. Austen
Published: March 2005

Isothermal kinetics of crystallization in the “fragile” Ca(NO3)2─KNO3 melts and in AgI─Ag2SO4─Ag2WO4 melts of intermediate fragility have been investigated using singlestep and multistep calorimetric techniques. Time‐temperature‐transformation curves for crystal nucleation and growth have been delineated and the temperature for maximum nucleation rate (TNN) identified. The results are compared with the kinetics of nucleation observed in other fragile systems, such as fluoride glass melts, and with classical oxide melts (such as Li2Si2O5) which have “strong” liquid characteristics. Reduced‐temperature presentations of nucleation‐rate data show qualitative correlations between TNN/TL (TL is liquidus temperature) and liquid fragility. These correlations show that strong‐liquid glass formers survive much larger supercoolings without nucleation than do fragile liquids.
Isothermal Capacitance Transient Spectroscopy of Grain‐Boundary Interfacial States in Beryllium Oxide‐Doped Silicon Carbide Ceramics
Authors: Tanaka Shigeru , Takahashi Ken , Suzuki Yasutaka , Akita Chiyoshi , Ohashi Naoki , Haneda Hajime , Tanaka Junzo
Published: March 2005

Grain boundaries of SiC ceramics doped with 2 wt% BeO were investigated using isothermal capacitance transient spectroscopy (ICTS). ICTS signals were observed above 433 K and the resulting capacitance‐time curves (under bias and zero bias) were ascribed to interfacial states at the grain boundaries. The signals were sharp compared to those of the SrTiO3─Bi2O3 system, suggesting that the structural disorder of grain boundaries in the SiC─BeO system was relatively small. It was determined that the interfacial state was located 0.92 eV above the valence band.
Hardness and Fracture Toughness of SiC‐Particle‐Reinforced MoSi2 Composites
Authors: Bhattacharya Arun K. , Petrovic John J.
Published: March 2005

The indentation technique has been used to evaluate the hardness and fracture toughness of SiC‐reinforced MoSi2 composites made by hot‐pressing. It is seen that the toughness increases with increasing indentation crack length (under increasing load) and a probable mechanism responsible for this behavior is described. It is also observed that there is an optimum volume fraction of SiC particles for which the maximum fracture toughness of the composite can be achieved.
Carbon Additions to Molybdenum Disilicide: Improved High‐Temperature Mechanical Properties
Authors: Maloy Stuart , Heuer Arthur H. , Lewandowski John , Petrovic John
Published: March 2005

C addition (2 wt%) to MoSi2 acted as a deoxidant, removing the otherwise ubiquitous siliceous grain boundary phase in hot‐pressed samples, and causing formation of SiC and Mo5Si3C1 (a variable‐composition Nowotny phase). Both hardness and fracture toughness of the C‐containing alloy were higher than those of the C‐free (and oxygen‐rich) material; more significantly, the fracture toughness of the MoSi2+ 2% C alloy increased from 5.5 MPa·m1/2 at 800°C to ∼11.5 MPa·m1/2 at 1400°C.
Caged Alumina Particles in Reticular Foam
Authors: Pope Edward J. A. , Almazan Alex , Kratsch Kenneth
Published: March 2005

A novel approach has been employed to suspend highly porous alumina particles within the reticular structure of silicon carbide foam. The particles are slightly smaller than the reticular cells that they occupy, thereby permitting them to “rattle” freely when subjected to a gas stream. Potential applications include low‐pressure‐drop catalyst supports.
Effect of Alkalies and of a Reducing Atmosphere on the Corrosion of Refractories by Molten Aluminum
Authors: Allaire Claude , Desclaux Pau
Published: March 2005

The corrosion of aluminosilicate refractories by molten aluminum, such as in melting and holding furnaces, generally leads to the formation of an alumina deposit on the refractory. In the presence of alkalies and under a reducing atmosphere the extent of corrosion is increased. This could be related to the transformation of the alumina to sodium aluminate, whose formation kinetics are increased under a reducing atmosphere, promoting the formation of aluminum nitride. In melting and holding furnaces, the alkalies could originate from the presence of a cryothic bath in the metal charge and/or from the refractories. In the latter case, it was shown that aluminum nonwetting refractory castables containing aluminum fluoride as the nonwetting agent could lead to severe corrosion in the above applications if their alkali content is such that it promotes the formation of β‐alumina.
Grain‐Size Distribution in Al2O3─ZrO2 Generated by High‐Temperature Annealing
Authors: Okada Kazuki , Yoshizawa Yu‐ichi , Sakuma Taketo
Published: March 2005

Grain‐size distribution in various Al2O3─ZrO2 (2.5 mol% Y2O3) ceramics during high‐temperature annealing was examined. In alumina‐rich alloys, the grain size of major and minor phases was very different, while grain size was almost uniform in zirconia‐rich alloys. This difference in grainsize distribution was related to the difference in grain growth rate of the major phase and to the effectiveness of grain‐boundary pinning by minor‐phase grains.
Structure and Properties of Hot‐Pressed Clay Mineral/Acrifavine Complex Disk
Authors: Oya Asao , Saito Makoto , Otani Sugio , Hanaoka Hirokazu
Published: March 2005

Saponite Na(a‐b)+[(Mg6‐bAlb) (Si8‐aAla)O20(OH)4]−, a kind of clay mineral, was intercalated with acriflavine to form their complex (SAC). The resulting SAC powder was hot‐pressed into small disks (II mm in diameter, ∼10 mm long) under ∼20 MPa and temperatures up to 1300°C. Saponite powder (Sap) and saponite/carbon black mixture (SKM) reference samples were also hot‐pressed according to the same procedure. H2, CO, and CO2 gases evolved more abundantly, and at lower temperature, from the SAC than from the two reference samples. Fine carbon particles dispersed homogeneously throughout the SAC after the sample was heated. Enstatite and forsterite crystals formed in the SAC at higher temperatures and grew less favorably than in the reference samples. The electrical resistivity of the SAC disk sharply decreased with increased heat treatment temperature. At 1300°C, the SAC disk with only 3 wt% of carbon content exhibited a resistivity of 2 × 10−1Ω/cm, about 1/20 that of the SKM with a 5.5 wt% carbon after heating to 1300°C. The compressive strength of SAC after heating to 1300°C was near that of Sap and twice that of SKM.
La2Ti2O7 Ceramics
Authors: Fuierer Paul A. , Newnham Robert E.
Published: March 2005

La2Ti2O7 powders were prepared using three different techniques. Single‐phase material was obtained at 1150°C by calcination of mixed oxides, at 1000°C by molten salt synthesis, and at 850°C by evaporative decomposition of solutions. Particle sizes and morphologies of the powders differed substantially, as did the sintered microstructures and dielectric properties. Very dense (99%), translucent, grain‐oriented lanthanum titanate was fabricated by hot‐forging at 1300°C under a 200‐kg load. Anisotropy was demonstrated by X‐ray diffraction, scanning electron microscopy, thermal expansion, and dielectric measurements.
Precoarsening to Improve Microstructure and Sintering of Powder Compacts
Authors: Chu May‐Ying , De Jonghe Lutgard C. , Lin Mark K. F. , Lin Frank J. T.
Published: March 2005

MgO and Al2O3 were sintered by two types of processes: a conventional isothermal sintering and a two‐step sintering consisting of an initial low‐temperature precoarsening treatment before conventional isothermal sintering. The final microstructure from two‐step sintering can be more uniform and finer than that of compacts sintered conventionally. A narrow‐size‐distribution alumina powder was sintered under constant‐heating‐rate conditions, with and without a precoarsening treatment, and the results were compared. The differences between two‐step and conventional processing were clarified by experiments on precoarsened and as‐received ZnO powders. These compacts were precoarsened at 450°C for 90 h with virtually no increase in the overall density. The resulting grain size was 1.7 times the starting one, but the standard deviation of the precoarsened powder size distribution was smaller than that of the asreceived powder. Precoarsened compacts sintered to nearly full density showed improved homogeneity. The sintering stress of the precoarsened ZnO was approximately 0.8 that of the as‐received one. A computational model has been used with two components of coarsening to describe the differences in pore spacing evolution between the precoarsened and the as‐received system. The benefit of two‐step sintering is attributed to the increase in uniformity resulting from precoarsening. The increased uniformity decreases sintering damage and allows the system to stay in the open porosity state longer, delaying or inhibiting additional coarsening (grain growth) during the final stage of densification. Two‐step sintering is especially useful for nonuniform powder systems with a wide size distribution and is a simple and convenient method of making more uniform ceramic bodies without resorting to specialized powders or complicated heat schedules.
Formation of Refractory Metal Carbide Coatings on Carbon Fibers by a Reaction‐Bonding Process
Authors: MacInnes Andrew N. , Barron Andrew R. , Li Jiaxiang , Gilbert Thomas R.
Published: March 2005

Carbon fiber tows were impregnated by ethanolic solutions of refractory metal chlorides and fired at temperatures up to 2200°C to form metal carbide coatings incorporating carbon from the underlying substrate. Optimum firing sequences were determined for maximum metal carbide conversion. Fired tows were subsequently examined by X‐ray photoelectron and Auger spectroscopy, scanning electron microscopy, and thermogravometric analysis to characterize the coated material. Coatings produced were stable under flowing air at a temperature of 800°C. This process is illustrated for the production of niobium‐based coatings.
Composition and Microstructure of Chemically Vapor‐Deposited Boron Nitride, Aluminum Nitride, and Boron Nitride + Aluminum Nitride Composites
Authors: Hanigofsky John A. , More Karren L. , Lackey W. J. , Lee Woo Y. , Freeman Garth B.
Published: March 2005

The composition and microstructure of dispersed‐phase ceramic composites containing BN and AIN as well as BN and AIN single‐phase ceramics prepared by chemical vapor deposition have been characterized using X‐ray diffraction, scanning electron microscopy, electron microprobe, and transmission electron microscopy techniques. Under certain processing conditions, the codeposited coating microstructure consists of small single‐crystal AIN fibers (whiskers) surrounded by a turbostratic BN matrix. Other processing conditions resulted in single‐phase films of AIN with a fibrous structure. The compositions of the codeposits range from 2 to 50 mol% BN, 50 to 80 mol% AIN with 7% to 25% oxygen impurity as determined by electron microprobe analysis.
Crack Resistance and Fatigue of Transforming Ceramics: I, Materials in the ZrO2–Y2O3–Al2O3 System
Authors: Grathwohl Georg , Liu Tianshun
Published: March 2005

Crack resistance characteristics and fatigue properties have been studied in four types of Y2O3–TZP ceramics including one containing Al2O3. The largely linear‐elastic behavior connected with the very small transformation zone (<5 μm) explains the absence of any resistance‐curve behavior and the flaw‐controlled strength. The crack resistance shows high sensitivity to environment‐induced subcritical crack growth. This influence is also operative in both types of fatigue experiments, i.e., under static and cyclic stresses, leading to reduced fatigue thresholds compared with KIC. While for static conditions a benefit is observed from enhanced t‐m ZrO2 transformation, cyclic stresses provoke an additional fatigue effect. However, if the cyclic stresses are restricted to subthreshold values, cyclic stress‐induced effects in the process zone provide an improvement of the materials being visible as a strengthening effect.
Thermal Expansions and Chemical Durabilities of Yttria–Aluminosilicate Glasses Containing Na2O and ZrO2
Authors: Makishima Akio , Hara Tatsuo
Published: March 2005

Properties of glasses in the system Y2O3–Al2O3–SiO2 containing Na2O and ZrO2 were investigated. The difference between the thermal expansion coefficients (Δα) at temperatures above Tg and those below Tg, microhardness, density, and chemical durability were measured in relation to the Al2O3/Y2O3 molar ratio. These glasses were found to have a smaller value of Δα than that of a commercial coating glass.
Twinning in YNbO4
Authors: Quinn Candace , Wusirika Raja
Published: March 2005

The compound YNbO4 is a 3–5 analogue of ZrO2 with two polymorphs: low‐temperature monoclinic and high‐temperature tetragonal forms. YNbO4 powder with a surface area of 40 m2/g was prepared from citrate complexes. The powder sintered to theoretical density at 1550°C. Unlike pure ZrO2, YNbO4 can be cooled through the tetragonal‐to‐monoclinic phase transformation without cracking. The phase transformation is gradual and takes place by shear, resulting in twinning.
Humidity Sensors
Authors: Kulwicki Bernard M.
Published: March 2005

Humidity sensor technologies amenable to widespread application in electronic control systems are reviewed. The principal sensor groups of interest include resistive ceramics, dielectric polymers, and resistive polyelectrolytes. The physical mechanisms and application trade‐offs for each group are considered. Stability of sensor response is a primary concern. At the present time no single design option can be considered to be universally applicable.
Effect of Extrusion on the Critical Current Density of Bi1.4Pb0.6Sr2Ca2Cu3Ox Superconducting Wires
Authors: Suzuki Suguru , Asada Shinji , Takahashi Minoru , Hikichi Yasuo
Published: March 2005

The effect of extrusion on improving the critical current density (Jc) of Bi1.4Pb0.6Sr2Ca2Cu3Ox superconducting wires is investigated. Calcined powders (Bi1.4Pb0.6Sr2Ca2Cu3Ox) are first mixed with a forming aid—a thermoplastic polymer (polyethylene)—for workability, and then extruded, using a capillary rheometer, to form wires 2 mm in diameter. The Jc value, measured by the four‐probe method in liquid nitrogen at 77 K, is improved substantially by the following process: (1) the superconducting precursors are extruded at high viscosity with a forming aid, to align the platelike particles unidirectionally; (2) the forming aid alone is carefully burned out, without destroying the extruded configuration; and (3) the extruded wires are annealed at 850°C in air for more than 96 h.
Non‐Darcian Airflow through Ceramic Foams
Authors: Philipse Albert P. , Schram Henny L.
Published: March 2005

This paper presents the first results on fluid flow through (laboratory‐made) ceramic foams. Pressure drop (ΔP) vs airflow rate (U) measurements show that Darcy's law does not hold and that the non‐Darcian airflow through the foams satisfies an equation of the form ΔP =aU+bU2. A permeability quotient is defined, which appears to be a qualitative measure for the average pore size of a foam. A Reynolds number defined in terms of this quotient can be used to estimate the Darcian flow range for a certain fluid.
Reactions During the Processing of U3O8—Al Cermet Fuels
Authors: Marra John E. , Peacock Harold B.
Published: March 2005

The cermet fuel (U3O8 dispersed in Al) being considered for use in the Savannah River Site Reactors is thermodynamically unstable because of the potential for an exothermic metallothermic reduction reaction. This paper describes work performed to quantify the extent of reaction during powder metallurgy (P/M) processing of the U3O8—Al cermet fuel, and to determine the effect of partial reduction to U4O9 on the metallothermic reduction reaction. During the fabrication of the U3O8—Al cermet fuel by the P/M technique, a significant portion of the U3O8 is reduced to U4O9. The reaction between U4O9 and Al is also exothermic; however, the maximum heat released by the reaction is substantially less than that released for the U3O8—Al reaction, approximately 335 J (80 cal) per gram of oxide reacted compared to 940 J (225 cal). Metallothermic reduction reactions for U3O8/U4O9/Al mixtures do not occur at the normal reactor operating temperature, ∼ 370 K (∼ 100°C) or at temperatures below the melting point of aluminum, 930 K (660°C).
High‐Temperature Toughness and Tensile Strength of Whisker‐Reinforced Silicon Nitride
Authors: Ohji Tatsuki , Goto Yasuhiro , Tsuge Akihiko
Published: March 2005

The R‐curve behavior of hot‐pressed silicon nitride reinforced with silicon carbide whiskers is investigated from room temperature to 1300°C using the chevron‐notch bend test. The bridging stress, estimated from increment of fracture resistance in the rising R‐curve, is discussed in relation to tensile strength measured with various displacement rates at 1300°C. The reinforcing whiskers provide most of the tensile strength in the creep‐deformation range at 1300°C. The whiskers appear to bear a great deal of the applied tensile stress during slow crack growth.
Modeling of Chemical Vapor Infiltration for Ceramic Composites Reinforced with Layered, Woven Fabrics
Authors: Chung Gui‐Yung , McCoy Benjamin J.
Published: March 2005

A homogeneous model is developed for the chemical vapor infiltration by one‐dimensional diffusion into a system of layered plies consisting of woven tows containing bundles of filaments. The model predictions of the amount of deposition and the porosity of the sample as a function of time are compared with the predictions of a recent nonhomogeneous model with aligned holes formed by the weave. The nonhomogeneous model allows for diffusion through the aligned holes, into the spaces between plies, and into the gaps around filaments; i.e., three diffusion equations apply. Relative to the nonhomogeneous results, the homogeneous model underestimates the amount of deposition, since the absence of holes and spaces allows earlier occlusion of gaps around filaments and restricts the vapor infiltration.
Percolation Phenomenon for Dissolution of Sodium Borosilicate Glasses in Aqueous Solutions
Authors: Kinoshita Masahiro , Harada Makoto , Sato Yasuo , Hariguchi Yoichi
Published: March 2005

A percolation phenomenon has been discovered for the dissolution of sodium borosilicate glasses in aqueous solutions under low‐pH conditions (pH ∼2). The glasses fall into one of two categories which are markedly different in the dissolution behavior, in terms of whether the Si/B ratio in the glass network exceeds a threshold value or not. The transition from one of the categories to the other is very sharp when the Si/B ratio is changed as the key parameter. The qualitative aspects of the experimental observation can be explained very well by a simple model combined with the site percolation theory.
Indentation‐Induced Cracks and the Toughness Anisotropy of 9.4‐mol%‐Yttria‐Stabilized Cubic Zirconia Single Crystals
Authors: Pajares Antonia , Guiberteau Fernando , Dominguez‐Rodriguez Arturo , Heuer Arthur H.
Published: March 2005

The crack systems associated with room‐temperature Vickers indentations on {001} surfaces in 9.4‐mol%‐Y2O3‐stabilized cubic ZrO2 single crystals are examined. The indentation‐induced radial cracks are not always perpendicular to the free surface, as is usually assumed, and the surface traces are therefore not a reliable guide to the nature of the cracking. In fact, indentations with 〈100〉 diagonals do not form cracks on {010}, but form secondary radial cracks on inclined planes and noncoplanar median cracks on (110) or (110).
Role of Concurrent Cavitation in the Fracture of a Superplastic Zirconia—Alumina Composite
Authors: Chokshi Atul H. , Nieh Tai‐Gang , Wadsworth Jeffrey
Published: March 2005

Superplastic zirconia‐alumina composites exhibit very large elongations to failure of ≳500% under optimum conditions. A quantitative microstructural study reveals that the composite is susceptible to considerable internal cavitation, with the level of cavitation increasing with increasing strain rate. The observed inverse relationship between the level of cavitation and the elongation to failure suggests strongly that the tensile ductility is limited by the evolution of cavitation damage during superplastic deformation.
Effect of Phase Composition on the Mechanical Properties of Hot Isostatically Pressed SiAION Ceramics
Authors: Yabuta Kazuya , Nishio Hiroaki , Uematsu Keizo
Published: March 2005

Phase compositions were determined in SiAION ceramics, consisting of only Si, Al, O, and N, which were prepared by high‐pressure sintering followed by hot isostatic pressing. The o'‐phase contents were correlated with the high‐temperature fracture strengths and Vickers hardness. The high‐temperature bending strength obtained in this study was over 670 MPa when the o'‐phase content was under 18.0 vol% and decreased with increasing o'‐phase content. The hardness increase was approximately linear with increasing o'‐phase content.
In Situ Synthesis of Silicon Carbide Whiskers from Silicon Nitride Powders
Authors: Wang Hongyu , Fischman Gary S.
Published: March 2005

Silicon carbide whiskers were synthesized in situ by direct carbothermal reduction of silicon nitride with graphite in an argon atmosphere. Phase evolution study reveals that the formation of β‐SiC was initiated at 1400° to 1450°C; above 1650°C silicon was formed when carbon was deficient. Nevertheless, Si3N4 could be completely converted to SiC with molar ratio Si3N4:C = 1:3 at 1650°C. The morphology of the SiC whiskers is needlelike, with lengths and diameters changing with temperature. SiC fibers were produced on the surface of the sample fired at 1550°C with an average diameter of 0.3 μm. No catalyst was used in the syntheses, which minimizes the amount of impurities in the final products. A reaction mechanism involving the decomposition of silicon nitride has been proposed.