bluuebunny/crossref_metadata_2025 huggingface dataset:
Rapid Rate Sintering of Nanocrystalline ZrO2−3 mol% Y2O3
Authors: Chen Da‐Jiang , Mayo Merrilea J.
Published: September 2005

Conventional ramp‐and‐hold sintering with a wide range of heating rates was conducted on submicrometer and nanocrystalline ZrO2–3 mol% Y2O3 powder compacts. Although rapid heating rates have been reported to produce high density/fine grain size products for many submicrometer and smaller starting powders, the application of this technique to ZrO2–3 mol% Y2O3 produced mixed results. In the case of submicrometer ZrO2–3 mol% Y2O3, neither densification nor grain growth was affected by the heating rate used. In the case of nanocrystalline ZrO2–3 mol% Y2O3, fast heating rates severely retarded densiflcation and had a minimal effect on grain growth. The large adverse effect of fast heating rates on the densification of the nanocrystalline powder was traced to a thermal gradient/differential densification effect. Microstructural evidence suggests that the rate of densification greatly exceeded the rate of heat transfer in this material; consequently, the sample interior was not able to densify before being geometrically constrained by a fully dense shell which formed at the sample exterior. This finding implies that rapid rate sintering will meet severe practical constraints in the manufacture of bulk nanocrystalline ZrO2–3 mol% Y2O3 specimens.
Electrophoretic Deposition of Y2O3‐Stabilized ZrO2 Electrolyte Films in Solid Oxide Fuel Cells
Authors: Ishihara Tatsumi , Sato Keiji , Takita Yusaku
Published: September 2005

An electrophoretic deposition (EPD) method was applied for the preparation of yttria‐stabilized zirconia (YSZ) films for solid oxide fuel cell (SOFC) applications. Dense YSZ films with uniform thickness can be readily prepared with the EPD method by using acetylacetone or acetone as a solvent. The open‐circuit voltages of SOFC, for which the YSZ films were prepared by the EPD method, increased with increasing repetitions of deposition and sintering. It was found that the open‐circuit voltage exceeded 1.0 V after five repetitions. When the planar SOFC was fabricated using La0.6Sr0.4MnO3 as a cathode, and electroless plating Pt as an anode, the open‐circuit voltage and the maximum power density attained were 1.03 V and 1.84 W·cm−2, respectively. Consequently, it became evident that the electrophoretic deposition was a suitable processing route for the formation of gas‐tight YSZ films with thickness less than 10 μm.
Surface Tension of Calcium Aluminosilicate Glass Using Computerized Drop Shape Analysis
Authors: Weirauch Douglas A. , Ziegler Donald P.
Published: September 2005

Numerical analysis is used in this study to derive surface tension from the shape of a liquid silicate sessile drop. The speed afforded by the drop shape analysis package facilitates analysis of experimentally derived drop profiles. Drop symmetry, contact angles, and local shape variations can be readily determined, thereby permitting the detection of experimental errors. The experimental and analytical technique was first validated by determining the surface tension of high‐purity gallium and aluminum under carefully controlled furnace atmospheres. Results for a calcium aluminosilicate melt are compared to pendant drop and maximum‐bubble pressure measurements reported in the literature. The use of an internal scaling factor and the effect of substrate interactions are discussed.
Oxidation Behavior of Boron‐Modified Mo5Si3 at 800°–1300°C
Authors: Meyer Mitchell K. , Akinc Mufit
Published: September 2005

Mo5Si3 shows promise as a high‐temperature creep‐resistant material. The high‐temperature oxidation resistance of Mo5Si3 has been found to be poor, however, limiting its use in oxidizing atmospheres. Undoped Mo5Si3 exhibits pest oxidation at 800°C. Mass loss occurs in the temperature range 900°–1200°C due to volatilization of molybdenum oxide, indicating that the silica scale that forms does not provide a passivating layer. The addition of boron results in protective scale formation and parabolic oxidation kinetics in the temperature range of 1050°–1300°C. The oxidation rate of Mo5Si3 was decreased by 5 orders of magnitude at 1200°C by doping with less than 2 wt% boron. Boron doping eliminates catastrophic pest oxidation at 800°C. The mechanism for improved oxidation resistance of borondoped Mo5Si3 is viscous sintering of the scale to close pores that form during the initial transient oxidation period, due to volatilization of molybdenum oxide.
Retardation of Fatigue Crack Growth in Ceramics by Glassy Ligaments: A Rationalization
Authors: Ramamurty Upadrasta
Published: September 2005

In high‐temperature fatigue crack growth (FCG) experiments on ceramic materials containing amorphous grain boundary phases, the crack growth rates under cyclic loads were observed to be lower than those predicted solely on the basis of crack growth velocities measured under static loads. In this paper, a rationalization was offered for such a behavior by means of a phenomenological glass‐bridging model which takes the relaxation behavior of glass into account. In ceramics which exhibit subcritical crack growth through cavitation ahead of the crack tip, the maximum stress intensity factor of the fatigue cycle required to initiate FCG was observed to be always greater than or equal to the threshold stress intensity factor for crack growth under sustained far‐field loads. This trend was also explained with the aid of the glass‐bridging model and invoking the equivalence between bridging and damage zones. The elevated temperature FCG behavior of nitride‐based ceramics which exhibit grain bridging in the wake during crack propagation was discussed and contrasted with oxide‐based ceramics which show glass bridging.
High‐Temperature Ultrasonic Characterization of the Mechanical and Microstructural Behavior of a Fibrous Composite with a Magnesium Lithium Aluminum Silicate Glass‐Ceramic Matrix
Authors: Cutard Thierry , Huger Marc , Fargeot Dominique , Gault Christian
Published: September 2005

The mechanical behavior and the microstructural modifications of a SiC‐fiber‐reinforced magnesium lithium aluminum silicate glass‐ceramic (SiC/MASL) have been characterized by ultrasonic measurement of uniaxial Young's modulus at high temperature. Under vacuum, long isothermal agings in the 750–1000°C temperature range have shown matrix modifications in terms of crystallization of residual glassy phases, and of phase transformations in the Li2O‐AI2O3‐SiO2 system. In air, long isothermal agings performed under the same conditions have led to the same matrix transformations but in competition with oxidation mechanisms of the carbon fiber‐matrix interphase. All of these matrix and/or interface transformations have been confirmed by X‐ray diffraction analysis, scanning electron microscopy, scanning acoustic microscopy, and microin‐dentation tests.
Structures and Phase Relations of Aluminum‐Substituted Calcium Silicate Hydrate
Authors: Kwan Stephen , LaRosa‐Thompson Judy , Grutzeck Michael W.
Published: September 2005

The structures of aluminum‐substituted calcium silicate hydrate (C‐S‐H) forming in a series of aqueous suspensions formulated with colloidal silica, reactive alumina, and lime and aged for 1 year have been studied using 29Si and 27Al magic angle spinning nuclear magnetic resonance spectroscopy (with and without cross polarization (CP)), solution pH, electron microscopy, and X–ray diffraction. As in earlier work dealing with the nature of C‐S‐H in the system CaO‐SiO2H2O2 two aluminum‐substituted C‐S‐H phases, having distinctly different anionic structures on the atomic level (Q2 versus Q1Q2), were found to extend into the system CaO‐Al2O3‐SiO2‐H2O2 X‐ray diffraction patterns of the two aluminum‐containing C‐S‐H phases are nearly identical, suggesting that their intermediate‐range order is very similar, but MASNMR spectra show that these two phases have markedly different silicate structures on the atomic‐level scale. Both C‐S‐H structures can accommodate approximately 5 mol% of Al2O3 in tetrahedral and possibly octahedral coordination as well.
Creep Cavitation in a Siliconized Silicon Carbide Tested in Tension and Flexure
Authors: Fields B. Anne , Wiederhorn Sheldon M.
Published: September 2005

Cavity formation was quantified in a grade of siliconized silicon carbide containing 33 vol% silicon. The type, size, and density of cavities were determined for smooth‐bar specimens tested in both tension and bending, and for indented specimens tested in tension. In both tension and bending, the volume fraction of cavities was found to be proportional to the tensile creep strain. Cavities nucleated at random locations throughout the test specimen, eventually coalescing into cracks that were the source of failure at high temperatures. In tension, the strain to failure was about 1%. In flexure, stress relaxation at the tensile surface of test specimens helped stabilize cracks that formed during creep. As a consequence, strains to failure were about twice as large in bending as in tension. In tensile specimens containing large, >300 μm, indentation cracks, cavitation was profuse near the crack tips. At a volume fraction of about 3%, cavities coalesced to form secondary cracks near the tip of the indentation crack. Cracks advanced by linkage of cavitation cracks with the indentation crack. Crack growth was intermittent, requiring the buildup of cavities in front of the crack tip before crack advance could occur. If the indentation crack length was less than about 200 μm, cavity formation at the tip of the crack was not sufficient for crack advance. In such case, failure would have to occur by cavity coalescence and crack formation at some other location in the test specimen.
Three‐Dimensional Dynamic Calculation of the Equilibrium Shape of a Coherent Tetragonal Precipitate in Mg‐Partially Stabilized Cubic ZrO2
Authors: Wang Yunzhl , Chen Long‐Qing , Khachaturyan A. G.
Published: September 2005

The equilibrium shape of a tetragonal precipitate coherently embedded in a cubic matrix is examined in three dimensions by a computer simulation. Independent experimental data of Mg‐partially stabilized ZrO2 are used as the input parameters. The equilibrium shape is obtained diffusional relaxation of an initially nonequilibrium spherical shape. The relaxation is described through a generalized field kinetic model which takes into account the transformation‐induced elastic strain arising from a cubic → tetragonal crystal lattice rearrangement. Without any a priori constraint on the geometry of the particle, the equilibrium shape is shown to be a rotation disk formed by two cones with the common base. It is far from the ellipsoidal shape assumed in the conventional analytical calculations based on Eshelby's model.
Crystallization and Properties of Fe2O3—CaO—SiO2 Glasses
Authors: Lee Yong‐Keun , Choi Se‐Young
Published: September 2005

Nucleation and crystal growth rates and properties were studied in a two‐stage heat treatment process for Fe2O3‐CaO‐SiO2 glasses. Glass transition (Tg) and crystallization temperatures (Tc) for the glasses lay between about 612.0° and 710.0°C, and 858.5° and 905.0°C, respectively, and magnetite was the main crystal phase. For a glass of 40Fe2O3. 20CaO·40SiO2 (in wt%) the maximum nucleation rate was (68.6 ± 7) × 106/mm3·s at 700°C, and the maximum crystal growth rate was 9.0 nm/min1/2 at 1000°C. The mean crystal size of the magnetite increased from 30 to 140 nm with variation of nucleation and crystal growth conditions. The glass showed the maxima in saturation magnetization and coercive force, 212.1 × Wb/m2 and 30.8 × 103 A/m, when heat‐treated for 4 h at 1000°C and 1050°C, respectively. The variation of the saturation magnetization could be quantitatively interpreted well in terms of the volume fraction of the magnetite, whereas that of the coercive forces could be explained only qualitatively in terms of the particle size of the magnetite. Hysteresis losses showed the maximum value of 1493 W/m3 when heat‐treated at 1000°C for 4 h prenucleated at 700°C for 60 min, and increased linearly with increasing heat treatment time under a magnetic field up to 800 × 103 A/m.
Single‐Grain Boundaries in PTC Resistors
Authors: Kuwabara Makoto , Morimo Ken‐ichiro , Matsunaga Tatsuya
Published: September 2005

Thin semiconducting barium titanate ceramic bars consisting of single grains joined together in series have been prepared, and the positive temperature coefficient of resistivity (PTCR) characteristics of strictly single‐grain boundaries in the materials were investigated. The resistivity (R)‐temperature (T) characteristics obtained for the present samples can be classified into typically three categories: (1) normal type PTCR characteristics, similar to those observed in usual ceramic samples, (2) saw‐tooth type PTCR characteristics, characterized by an abrupt increase in resistivity by more than three orders of magnitude at the Curie point, immediately follwoed by a monotonous decrease in it, and (3) flat type R–T characteristics, with substantially little or no resistivity jump. Of these R–T characteristics, normal type PTCR characteristics were the most frequently observed (about 60%; a total of 65 samples were examined). Flat type R–T characteristics were least frequently (about 10%) observed. Single boundaries with these three types of PTCR characteristics exhibited essentially the same ferroelectric capacitance–temperature characteristic; this demonstrates that the temperature dependence of the dielectric constant above the Curie point was not responsible for the PTCR anomalies. Single boundaries with normal and saw‐tooth type PTCR characteristics showed significantly nonlinear current‐voltage characteristics above the Curie point, which may be interpreted to be caused by a current strongly affected by traps (or surface acceptor states) present at the grain boundaries.
Hertzian Contact Response of Tailored Silicon Nitride Multilayers
Authors: Liu Haiyan , Lawn Brian R. , Hsu Stephen M.
Published: September 2005

The nature and degree of damage accumulation beneath Hertzian contacts in silicon nitride‐based laminates are studied. Specimens with alternating homogeneous and heterogeneous layers are fabricated by a tape‐casting route, with strong interlayer bonding. Homogeneous material consisting of relatively pure fine‐grain silicon nitride is used as the overlayers. Heterogeneous material containing 10 to 30 wt% boron nitride platelets in a silicon nitride matrix, with weak platelet/matrix interphase boundaries, forms the underlayers. Contact tests with spherical indenters are used to monitor the stress‐strain response of the laminates and to investigate the damage modes within the individual layers. The heterogeneous layer exhibits a distinctive “softening” in the stress‐strain curve, indicating a quasi‐plasticity in the silicon nitride associated with local microfailures at the platelet/matrix interfaces. In contrast to the welldefined cone cracks that develop within the tensile zone outside the contact area in bulk homogeneous silicon nitride, the damage in the laminates is widely distributed within the shear‐compression zone below the contact. Fractures form incompletely in the homogeneous layers, as downward‐propagating partial cone cracks and upwardpropagating stable cracks. Comparatively extensive, diffuse microscopic damage occurs in the heterogeneous layers, culminating in a macroscopic failure that traverses these layers at higher loads. A strong synergism between the interlayer damage modes is apparent. Implications concerning the design of composite laminates for improved damage tolerance, with retention of strength and wear resistance, are considered.
Exaggerated Growth of Hexagonal Barium Titanate under Reducing Sintering Conditions
Authors: Rečnik Aleksander , Kolar Drago
Published: September 2005

The influence of reducing sintering conditions on anisotropic grain growth in BaTiO3 was studied above the BaTiO3‐Ba6Ti17O40 eutectic temperature. The morphology and structure of exaggeratedly grown grains was examined by X‐ray powder diffractometry, scanning electron micros‐copy, and high‐resolution transmission electron microscopy. The results show that all anomalously grown anisotropic grains were hexagonal BaTiO3 in the form of platelike crystals with a/c ratios up to 10. The direction of preferential growth of hexagonal grains is crystallographically analogous with that of parallel (111) twins in a cubic phase. Ti3+ ions, induced by reducing atmosphere, play an important role in the formation of hexagonal stacking.
Controlled Epitaxial Nucleation of Nickel Oxide on Microfabricated Magnesium Oxide Substrates in a CVD Process
Authors: Hoghooghi Bahar , Raj Rishi
Published: September 2005

It is shown that the nucleation of nickel oxide crystals grown by CVD on MgO depends on the surface structure of the substrates. On freshly cleaved {100} surfaces of MgO, nucleation occurs readily on cleavage steps. The crystallographic orientation of the surfaces of these steps is {110}, and their height ranges from 1 to 8 μm. In contrast, nucleation is sparse on highly polished {100} surfaces having a step height of less than 0.02 μm. Nucleation on polished {111} surfaces is also poor. However, highly polished {110} surfaces show profuse nucleation under similar growing conditions. This preference for nucleation on {110} surfaces and on the step height was used to obtain periodically spaced nuclei of NiO crystals by etching and microlithography of the (100) MgO surface which exposed the {110} surfaces at periodic locations. In this way the spacing of the nuclei could be controlled from 10 μm up to 100 μm.
Influence of Calcination Temperature on the Microstructure and Mechanical Properties of a Gel‐Derived and Sintered 3 mol% Y‐TZP Material
Authors: Montanaro Laura , Ferroni Luca , Pagliolico Simonetta , Swain Michael V , Bell Trevor J.
Published: September 2005

A pure coprecipitated 3 mol% Y‐TZP powder was subjected to two calcination temperatures, 600° and 1000°C, prior to compaction and sintering. Significant differences in the initial sintering behavior were observed. The lower temperature calcined powder exhibited abnormal grain growth. The resultant mechanical properties mirrored the microstructure with the lower temperature calcined material having lower flexural strength. Hardness measurements of the two sintered bodies revealed significant differences in the two phases of the lower temperature calcined material. Differences in reactivity of the two powders after calcination are suggested as the basis for the difference in microstructure and resultant mechanical properties.
Determination of Interfacial Properties Using a Single‐Fiber Microcomposite Test
Authors: Morscher Gregory N. , Martinez‐Fernandez Julian , Purdy Mark J.
Published: September 2005

A single‐filament microcomposite consisting of 143‐μm‐diameter CVD SiC, a thin “interfacial” coating, and a thick (130–160 μm) CVD SiC matrix sheath was tested in tension to determine the interfacial shear properties of carbon and BN interfacial coatings. Load/displacement and acoustic emission were used to determine the mechanical behavior of the microcomposites and matrix cracking. Optical and electron microscopy were performed to confirm the extent and nature of matrix cracking and to analyze the micro‐composite microstructure. The interfacial properties were determined from matrix cracking and load/displacement hysteresis techniques. Direct measurement of the interfacial sliding stress from push‐out and pull‐out experiments were in good agreement with two of the three microcomposite systems studied.
Sol‐Gel Synthesis of a New Oxide‐Ion Conductor Sr‐ and Mg‐Doped LaGaO3 Perovskite
Authors: Huang Keqin , Feng Man , Goodenough John B.
Published: September 2005

Sr‐ and Mg‐doped LaGaO3 powders were prepared from a salt acetate solution. The stable solution was peptized by reacting ammonium hydroxide with the precursor solution. Thermal analysis (DTA/TGA) was used to characterize first the dehydration and then the thermal decomposition of the organic ligands of the dried gel. The transformation from amorphous powders into a crystallized, homogeneous oxide phase corresponds to two endothermic peaks in the DTA curve; the first one at 150°C is related to the removal of water and is followed by a shoulder at 250°C. The second peak at 300°C corresponds to a superposition of two decomposition reactions: acetate salt into its oxycarbonate and this oxycarbonate into its oxide. Two subsequent exothermic peaks correspond to oxidation of evolved gases such as methane, hydrogen, and carbon monoxide. TEM observations show an average 10 nm particle size of the LaGaO3powder after annealing at 600°C. X‐ray diffraction patterns indicate a pure primitive‐cubic phase is formed by 1300°C without formation of any SrLaGaO3 impurity. The impedance spectroscopy on a 93%‐dense sample exhibits no grain‐boundary contribution and an ac conductivity σ= 0.11 Ω−1·cm−1 at 800°C.
Microstructural Model on the Evolution of Domain Structure during Processing of PbTiO3 Thin Films on MgO(100) Substrates
Authors: KimM Sangsub , Kang Young Min , Baik Sunggi
Published: September 2005

A two‐dimensional microstructural model is proposed to explain the origin of the 90° domain structure of epitaxial PbTiO3 thin films grown on MgO(100) substrates by radio frequency sputter deposition. During film deposition at or above 600°C, the film grows epitaxially in a cubic phase without any domain structure. At or just below the Curie temperature, the film mainly consists of α‐domains with a small portion of c‐domains nucleated in a triangular shape. The c‐domains gradually expand with decresasing temperature to relax the strain energy caused by the difference in thermal contraction rate between the c‐axis of α‐domains and the substrate, forming consequently thin strips of α‐domains distributed in the matrix of c‐domains at room temperature.
Boron Carbide Particles Formed from an Amorphous Boron/Graphite Powder Mixture Using a Shock‐Wave Technique
Authors: Yamada Kenjiro
Published: September 2005

Boron carbide (B4C) particles with filamental, distorted ellipsoidal, platelike, and polyhedral shapes were formed from vapor generated from an amorphous boron/graphite powder mixture with 14% starting density using a cylindrical shock‐wave technique. The crystal phases of shocked compact and microstructures of the B4C particles were characterized by X‐ray diffractometry and electron microscopy, respectively.
Damage Tolerance of Silicon Carbide‐ and Alumina‐Matrix Surface Composites
Authors: Day P. S. , Skamser D. J. , Faber K. T. , Jennings H. M. , Johnson D. Lynn
Published: September 2005

A method for the fabrication of a ceramic‐matrix composite (CMC) layer on the surface of a monolithic substrate via chemical vapor infiltration (CVI) is described. Preforms consisted of tows of fibers wound onto the surface of monolithic cylindrical tubes. Nicalon fibers were wound onto mullite substrates and infiltrated with β‐SiC from CH3SiCl3/H2 gas mixtures in a cylindrical cold‐wall reactor. Similarly, Nextel fibers were wound onto A12O3 substrates and infiltrated with α‐Al2O3 from AlCl3/H2/CO2/N2 gas mixtures. Composites with densities as high as 88% of the theoretical value were fabricated in 8 h. The effective fracture strength of the SiC‐ and Al2O3‐matrix surface composites, as determined from diametral compression tests of C‐ring specimens, was found to be insensitive to damage caused to the outer diameter by a Vickers indentation. The tolerance of the SiC‐matrix surface composites to surface damage was retained in specimens subjected to oxidation at 1000°C for 6 h.
Growth Morphology and Mechanism of a Hollow ZnO Polycrystal
Authors: Hashimoto Shinobu , Yamaguchi Akira
Published: September 2005

When zinc oxide and graphite were placed into an alumina crucible, with a square hole (∼3 mm ×∼3 mm) near the top, and the crucible was covered by an alumina lid and heated at >1050°C, a hollow polycrystal composed of needlelike, ribbon(comb)‐like, dendritic, and bulk zinc oxide crystals grew from the hole toward the outside of the crucible. The diameter and length of the polycrystal grown at 1050°C after 1 h were ∼2.2 mm and ∼20 cm, respectively. The diameter increased, while the length decreased, with increasing temperature. Most of the needlelike zinc oxide crystals in the hollow polycrystal were elongated along the c‐axis.
Hydration Reactions in the System CaO‐P2O5‐SiO2‐(H2O)
Authors: Vanis Peter , Odler Ivan
Published: September 2005

Highly dispersed powders containing α‐tricalcium phosphate as the main constituent, produced by a sol‐gel process and subsequent heating to at least 1300°C, exhibit hydraulic properties if allowed to react with water. Both pastes and compacts made from such powders exhibit setting and hardening, especially in the presence of sodium phosphate, potassium phosphate, and ammonium phosphate, which act as catalysts. The product of the hydration process is hydroxy‐apatite in combination with a C‐S‐H and/or C‐S‐P‐H gel.
Transition of the Particle‐Growth Mechanism with Temperature Variation in the Molten‐Salt Method
Authors: Park Jun‐Hong , Lee Dae‐Hee , Shin Hyo‐Soon , Lee Byung‐Kyo
Published: September 2005

In this study, changes in the amount of potassium chloride liquid phase used for the molten‐salt synthesis of strontium ferrite were examined to understand the transition of the growth mechanism with temperature. The results of this investigation indicate that an increase in reaction temperature from 900° to 1200°C caused the particle shapes to become nearly spherical. In addition, the particle size decreased as the relative amount of liquid increased. The growth process of the strontium ferrite particles in the molten salt appears to have undergone a transition from an interfacial, reaction‐controlled mechanism to a diffusion‐controlled mechanism as temperature is increased.
Effects of Grain Size and Humidity on Fretting Wear in Fine‐Grained Alumina, Al2O3/TiC, and Zirconia
Authors: Krell Andreas , Klaffke Dieter
Published: September 2005

Friction and wear of sintered alumina with grain sizes between 0.4 and 3 μm were measured in comparison with Al2O3/TiC composites and with tetragonal ZrO2(3 mol% Y2O3). The dependence on the grain boundary toughness and residual microstresses is investigated, and a hierarchical order of influencing parameters is observed. In air, reduced alumina grain sizes improve the micromechanical stability of the grain boundaries and the hardness, and reduced wear is governed by microplastic deformation, with few pullout events. Humidity and water slightly reduce the friction of all of the investigated ceramics. In water, this effect reduces the wear of coarser alumina microstructures. The wear of aluminas and of the Al2O3/TiC composite is similar; it is lower than observed in zirconia, where extended surface cracking occurs at grain sizes as small as 0.3 μm.
Rheology and Consolidation of Colloidal Alumina‐Coated Silicon Nitride Suspensions
Authors: Shih Wei‐Heng , Kisailus David , Shih Wan Y. , Hu Yung‐Haw , Hughes Jerry
Published: September 2005

In an attempt to improve the colloidal processing of Si3N4 ceramics, we studied the rheology and consolidation of colloidal suspensions of Si3N4 particles (average particle size 0.7 μm) with small Al2O3 particles (average particle size 20 nm). It was found that at pH >7, the viscosity of the mixtures increased and then decreased with an increasing concentration of Al2O3. λpotential measurements, optical micrographs, and visible light absorptance measurement suggest that such viscosity behavior is due to clustering of Si3N4 particles bridged by the small Al2O3 particles. This is also supported by the Derjaquin‐Landau‐Verwey‐Overbeek (DLVO) potential calculations that show the barrier height in the DLVO potential between Al2O3 and Si3N4 is small. The small barrier height under current experimental conditions stems from the small size of the Al2O3 particles. The small barrier height allows the thermal motion of the two kinds of particles to overcome the barrier and attach to each other. The adsorption of small Al2O3 particles on Si3N4 can occur even when both Al2O3 and Si3N4 carry the same sign of charges. The adsorption of Al2O3 on Si3N4 also increases the density of consolidated compacts.
Computer Simulation Model for Coupled Grain Growth and Ostwald Ripening—Application to Al2O3‐ZrO2 Two‐Phase Systems
Authors: Chen Long‐Qing , Fan Danan
Published: September 2005

A kinetic model based on generalized continuum time‐dependent Ginzburg‐Landau (TDGL) equations is proposed for studying coupled grain growth and Ostwald ripening in multiphase systems. In this model, an arbitrary multiphase microstructure is described by many orientation field variables which represent crystallographic orientations of grains in each phase and by n— 1 composition field variables which distinguish the compositional differences among n phases. Microstructural development during simultaneous grain growth and Ostwald ripening is predicted by the temporal evolution of these field variables by numerically solving the TDGL equations. A particular example, Al2O3‐ZrO2 particulate composite, was considered. The effects of the volume fraction of ZrO2 on the microstructural features and their evolution were studied and compared to experimental observations and previous thermodynamic analysis.
Crack Initiation in Fiber‐Reinforced Brittle Laminates
Authors: Cox Brian N. , Marshall David B.
Published: September 2005

In fiber‐reinforced brittle laminates, crack growth under monotonic tension generally consists of crack tunneling along the weaker ply (usually the 90° ply) followed by plane strain crack growth through the adjacent, more resistant plies (the 0° plies). In this paper, the details of this transition in crack mode are examined. The tunneling crack configuration is generalized to allow the crack to penetrate the 0° ply during tunneling. The effects of crack bridging in the 0° plies on the energetics of tunneling are computed numerically for general cases and combined with analytical results for certain limits. The nature of the transition from tunneling to plane strain cracking is found to depend on the ratio of the toughnesses of the 90° and 0° plies. Implications for laminate design are discussed.
Correlation between Long and Short Crack R‐curves in Alumina Using the Crack Opening Displacement and Fracture Mechanical Weight Function Approach
Authors: Fett Theo , Munz Dietrich , Seidel Jurgen , Stech Martin , Rödel Jurgen
Published: September 2005

Both fine grain sized and coarse grain sized alumina has been used as a model material to correlate short crack R‐ curves by the surface‐flaw‐in‐flexure method with long crack R‐curves from a compact tension specimen. The analysis has been made using the crack opening displacement from long cracks combined with the appropriate weight function to compute the crack closure stresses by crack bridging as a function of crack opening displacement. Then the long crack R‐ curve, the short crack curve, as well as the crack opening displacement for short cracks are computed. The agreement with the experimental data is within an absolute value of about 0.3 MPa and attributed to uncertainties in determining the crack tip toughness and effects due to subcritical crack growth.
Synthesis of Silicon Carbide from Rice Husk in a dc Arc Plasma Reactor
Authors: Nayak Bijan Bihari , Mohanty Bishnu Charanarabinda , Singh Saroj Kumar
Published: September 2005

SiC particles have been synthesized in the α‐phase from a mixture of boiler‐burnt rice husk and graphite powder in a dc extended arc plasma reactor on a 200–250 g scale. A SiC yield as high as 72% was achieved at a 3.03:1 carbon‐to‐silica ratio. 21R polytypism, which is rare to grow, was observed in the SiC. About 90% of the SiC particles produced were found to lie under 163 μm, 50% under 90 μm, and 10% under 28 μm size. Very fine particles under 10 μm size occurred to the extent of 3%. XRD and SEM characterizations were carried out to study the phase and morphology of the particles. Heterogeneous gas‐phase reactions seem to be responsible for intermediate‐size SiC particle growth in the multitemperature zone dc extended arc plasma reactor.
Auger Electron Spectroscopy of Aluminum Nitride Powder Surfaces
Authors: Saito Naruki , Ishizaki Kozo
Published: September 2005

The chemical states of powder surfaces depend on the manufacturing processes of the powders. The surface chemistry of three different commercial AIN powders, which are processed by carbothermal nitridation of Al2O3, chemical vapor deposition (CVD), and direct nitridation of aluminum, were evaluated by using Auger electron spectroscopy (AES). In order to obtain reference AES spectra of aluminum compounds, α‐, γ‐, θ‐Al2O3, γ‐AIOOH, γ‐AION, and sintered AIN were also examined. Line shapes of aluminum LVV; Al(LVV), nitrogen KLL; N(KLL) and oxygen KLL; O(KLL) are discussed for the AIN powders and all the other aluminum compounds. The differential Auger electron spectra, i.e., E dn/AE were obtained directly, where n is the number of Auger electrons, and E is the kinetic energy of the electron. Their integrated spectra, i.e., n(E) are also employed for analysis. The results confirm the conclusions of our previous temperature‐programmed desorption work. The AES line shape analysis implies the presence of an oxide‐like θ‐Al2O3 containing AION phase on the carbothermal nitride AIN powder surfaces. The surfaces of CVD and direct‐nitrided AIN powders are covered by an oxide–like γ‐Al2O3 with an oxygen diffusion layer and does not have AION phase.
Textured Magnetoplumbite Fiber‐Matrix Interphase Derived from Sol‐Gel Fiber Coatings
Authors: Cinibulk Michael K. , Hay Randall S.
Published: September 2005

A calcium hexaluminate (CaAl12O19, magnetoplumbite structure) sol was used to coat alumina and yttrium‐aluminum‐garnet (YAG) single‐crystal fibers and single‐crystal alumina plates. When the coated substrates were either annealed or hot‐pressed in polycrystalline alumina and YAG matrices, the calcium hexaluminate basal cleavage planes were aligned parallel with the fiber‐matrix interface. A complex series of reactions and phase transformations contributed to texture formation on alumina substrates. The alumina fibers and plates seeded the phase transformation of sol‐derived transition aluminas to α‐Al2O3. CaAl12Ol9 and CaAl4O7 formed between the seeded α‐Al2O3, and CaAl4O7 later reacted with the seeded α‐Al2O3 to form CaAl12O19, resulting in a single‐phase coating. Several different mechanisms may be responsible for the texture. The microstructure, phase evolution, and possible mechanisms for texture formation of CaAl12O19 powders, sol‐derived thin films, and coated plates and fibers, with and without hot‐pressed matrices, were studied and are discussed. Deflection and propagation of cracks within the fiber‐matrix interphase in thin foils suggests that such an inter‐phase may protect fibers from matrix cracks.
Wear and Wear Transition in Silicon Carbide Ceramics during Sliding
Authors: Cho Seong‐Jai , Um Chang‐Do , Kim Seock‐Sam
Published: September 2005

Wear and wear transition in silicon carbide ceramics during sliding have been investigated. Three different microstructures, i.e., solid‐state‐sintered silicon carbide, liquidphase‐sintered silicon carbide, and a liquid‐phase‐sintered SiC‐TiB2 composite, were produced by hot pressing. Wear data and examinations of worn surfaces showed that the wear behavior of these silicon carbide ceramics was significantly different. In the solid‐state‐sintered silicon carbide, the wear occurred by a grooving process. In the liqudphase‐sintered silicon carbide and composite, on the other hand, an abrupt transition in the wear mechanism from an initial grooving process to a grain pullout process occurred during the test. The transition occurred significantly earlier in the composite than in the carbide. The different wear behavior in these silicon carbide ceramics is discussed in relation to the grain or interphase boundary strength.
Thermodynamic Analysis of the Metastable Regions for the AI2O3‐SiO2 System
Authors: Djuric Mirjana , Mihajlov Andjelka , Petrasinovic‐Stojkanovic Ljiljana , Zivanovic Branislav
Published: September 2005

The paper offers a short review of investigations conducted on the Al2O3‐SiO2 system, particularly from the point of view of an appearance of immiscibility gaps, below the liquidus temperatures. Based on theoretical criteria and using the data along the liquidus curves (for mullite and corundum), this investigation suggests immiscibility regions at both ends of the phase diagram. That is, it defines the Margules polynomials for calculation of the activity coefficients necessary for determining the Gibbs energy of mixing. Obtained results are in good agreement with the results reported by Risbud and Pask and other investigators who have found immiscibility gaps at rather high temperatures.
Pressure Consolidation of Fiber‐Reinforced Oxide Matrix Composites: Experiments and Simulations
Authors: Casagranda Albert , Xu Junde , Evans Anthony G. , McMeeking Robert M.
Published: September 2005

Uniaxial consolidation experiments conducted on porous A12O3 specimens with fine particle size have allowed the constitutive law to be determined. These experiments involved simultaneous measurements of load and strain, coupled with grain size determinations. The data over a wide range of stress and temperature are consistent with a power law, grain‐boundary‐climb mechanism of consolidation, at relative densities up to at least 0.9. Consolidation experiments on composite specimens have identified a spatial variation in matrix density near either a reinforcement or a hole. A finite element calculation performed using the matrix constitutive law compares well with a density map of the matrix made using image analysis.
Characterization of Bi0.5Na0.5TiO3‐PbTiO3M Dielectric Materials
Authors: Kuharuangrong Sutin , Schulze Walter
Published: September 2005

Bi0.5Na0.5TiO3 (BNT) modified with Pb showed an increased and broadened dielectric constant and limited grain growth. Pb also lowered the first transition temperature. The phases of BNT were confirmed to be ferroelectric at room temperature which transformed to antiferroelectric above 220°C. When BNT was doped with 10% Pb, the first transition decreased to 140°C and abruptly disappeared in the composition with 17% Pb. The crystal structure of 17%‐Pb‐doped BNT at room temperature is tetragonal, which differs from the rhombohedral structure at lower Pb contents. Thus, the phase boundary between rhombohedral and tetragonal ferroelectric phase was determined to be between 15% to 17% Pb in this study.
Evaluation of Ultrasonically Determined Elasticity‐Porosity Relations in Zinc Oxide
Authors: Martin Louis Peter , Dadon David , Rosen Moshe
Published: September 2005

Ultrasonically determined elastic moduli in ZnO samples sintered to various densities were evaluated using both an empirical model and the Mori‐Tanaka effective field theory. Both approaches have been successfully used to model the modulus‐porosity relations in several material systems. In the present investigation, application of the empirical model predicted elastic moduli which deviated significantly from the experimental results. The deviation was attributed to the porosity dependence of Poisson's ratio which was neglected in this model. When this dependence was accounted for empirically, the fit to the experimental data was improved dramatically. Analysis of the data in the context of the Mori‐Tanaka model indicates that the porosity shape changes significantly during the sintering process. This analysis may provide a convenient way to quantitatively compare such changes for ceramic materials prepared by different processing techniques, such as by conventional and microwave sintering.
Abrasive Wear Behavior of a Si3N4‐MoSi2 Composite
Authors: Hawk Jeffrey A. , Alman David E. , Petrovic JohnJ.
Published: September 2005

MoSi2 particles (20 vol%) have been added to Si3N4 to form ceramic matrix‐intermetallic composites. Benefits associated with the addition of the MoSi2 to Si3N4 include higher strength, higher fracture toughness, no loss in oxidation resistance, and lower electrical resistivity. However, because the hardness of MoSi2 is approximately half that of Si3N4, a decrease in the specific wear rate of the Si3N4‐20 vol% MoSi2 composite is expected to result from the incorporation of the MoSi2 into the Si3N4. In this U.S. Bureau of Mines and Los Alamos National Laboratory study, it is found, however, that the specific wear rate of the composite during two‐body abrasion by SiC particles is equivalent in magnitude to the specific wear rate of monolithic Si3N4. The specific wear rates of both the Si3N4‐20 vol% MoSi2composites and monolithic Si3N4 are four to five times less than that of monolithic MoSi2.
Microstructural Evolution of Copper Thick Films Observed by Environmental Scanning Electron Microscopy
Authors: Sample David R. , Brown Paul W. , Dougherty Joseph P.
Published: September 2005

A generic copper thick film ink system was formulated to be representative of the inks currently used in industry. The microstructural evolution of two of the copper‐based inks based on the generic composition was investigated using an environmental scanning electron microscope (ESEM) equipped with a hot stage. The first ink, consisting of copper and organic constituents (binder and solvent), oxidized heavily during firing while the second ink, consisting of copper, glass, and organic constituents, oxidized only slightly. The sequence of events observed in firing the ink comprised of copper, glass, and organics is as follows: The burnout process reached completion by about 320°C. Copper oxidation was observed to occur between 500° and 600°C in nitrogen. The glass was observed to soften over the same temperature range. By 700°C, the glass had softened and coated the copper particles. Prior to glass softening, oxidation occurred regardless of the presence of the glass. However, once the glass had softened, copper oxidation was retarded. Cu2O was found to be sufficiently soluble in the glass to allow it to be removed from the surfaces of the copper particles. Thus, the presence of glass facilitated the formation of conductive paths by two mechanisms. It inhibited oxidation and it dissolved the oxide that formed. Both processes allowed effective sintering.
Influence of Loading Rate and Processing Atmosphere on Pullout of SiC Fibers from Pyrex Glass
Authors: Tsuda Hiroshi , Enoki Manabu , Kishi Teruo
Published: September 2005

The interfacial shear strength of SiC/glass composites fabricated either under vacuum or in argon gas by hot‐pressing was estimated using a single‐fiber pullout test as a function of the loading rate. The interfacial shear strength of the composite fabricated under vacuum depended insignificantly on the loading rate. On the contrary, for the composite fabricated in argon gas, loading rates strongly influenced the interfacial shear strength as well as the interfacial debonding behavior. The influence of loading rates on the interfacial shear strength in the latter composite could result from the presence of the Si‐O bond, whose strength is affected by slow crack growth along the fiber‐matrix interface. These results indicate that the fracture behavior of fiber‐reinforced ceramics whose interfacial shear strength depends on the loading rate can differ widely under dynamic and static loading.
Processing of Highly Oriented LiNbO3 Films for Waveguides from Aqueous Solution
Authors: Ono Satomi , Takeo Takashi , Hirano Shin‐ichi
Published: September 2005

Stoichiometric and homogeneous LiNbO3 films were prepared by using a Li and Nb hydroxide aqueous coating solution from a double alkoxide. In this method, a double alkoxide precursor solution was hydrolyzed completely to form the gel, which was subjected to dissolution into water to form the aqueous coating solution. Processing with the aqueous coating solution was preferable in terms of much less carbonic residue in coated films than with the alkoxide ethanol solution. The coated films crystallized with high orientation by subsequent heat‐treatment at temperatures as low as 350°C. Ti‐doped (5 mol%) LiNbO3 homogeneous films (Ti:LiNbO3) were also prepared.
Thermodynamics of the Si‐O‐N System: II, Stability of Si2N2O(s) by High‐Temperature Mass Spectrometric Vaporization
Authors: Rocabois Philippe , Chatillon Christian , Bernard Claude
Published: September 2005

The Si2N2O(s) compound is vaporized in a multiple Knudsen‐cell connected to a mass spectrometer. Comparison of SiO(g) and N2(g) pressures between different samples in the Si‐N‐O system, as triphasic or diphasic mixtures from Si(s), Si3N4(s), and Si2N2O(s), allows the determination of the SiO(g) evaporation coefficient and consequently the calculation of the equilibrium pressure of SiO(g). Then the standard enthalpy of formation of Si2N2O(s) is determined: AfH°m(Si2N2O, s, 298.15 K) =−887.5 ± 10 kj.mol−1.
Gas Pressure and Temperature Dependences of Thermal Conductivity of Porous Ceramic Materials: Part 2, Refractories and Ceramics with Porosity Exceeding 30%
Authors: Litovsky Efim , Shapiro Michael , Shavit Arthur
Published: September 2005

Effective thermophysical properties of ceramic materials (mainly insulating materials) with porosity (II) >30% are reviewed. Nonmonotonic pressure and temperature dependences of the effective thermal conductivity (X) are analyzed, based on the ceramic microstructure (pores, cracks, and grain boundaries present in many industrial refractories) and several heat‐transfer mechanisms in composite multiphase materials. These mechanisms include heat conduction in solid and gas phases, thermal radiation, gas convection, and the mechanism originating from intrapore chemical conversion processes accompanied by gas emission. For high temperatures, λ of porous insulations is governed by thermal radiation. Contact‐heat‐barrier resistances play a less‐important role in highly porous ceramics than in their dense counterparts. This underlies a weaker pressure dependence at low temperatures (<500°C) of λ of the majority of industrial insulating materials than in dense materials possessing microcracks and small pores in the grain‐boundary region. For high gas pressure, λ of porous insulating materials is governed by free convective‐gas motion. For low gas pressures (normally <1 kPa), where heat transfer in pores occurs in the free‐molecular regime, X is controlled by the pressure‐dependent mean free path of gas molecules in pores. A classification of the porous material structure and thermophysical properties is proposed, based on the geometric model described in Part 1 of this series.
Computer Simulations of Binder Removal from 2‐D and 3‐D Model Particulate Bodies
Authors: Lewis Jennifer A. , Galler Michael A. , Bentz Dale P.
Published: September 2005

A series of computer simulations were developed to investigate the removal of multicomponent, thermoplastic binders from two‐ and three‐dimensional model particulate bodies. Monosized particles with varying diameters were randomly placed in such systems, and all unoccupied pixels were assigned to the binder phase at ratios of 1:9, 1:2, or 1:1 plasticizer (volatile) to polymeric (nonvolatile) species. Simulations were carried out under isothermal conditions to study the influence of liquid‐phase transport processes, i.e., plasticizer diffusion in the binder‐filled pore network and capillary‐driven redistribution of the binder phase, on plasticizer removal rates. Plasticizer diffusion was modeled by a random‐walk algorithm, and nonplanar pore development arising from capillary‐driven binder redistribution was modeled by an invasion percolation algorithm. For comparison, simulations were also carried out on systems in which binder redistribution was not permitted. In such cases, pore development was modeled as an advancing or nonadvancing planar front. Visualization of transport phenomena on a microscopic scale has provided the first quantitative assessment of plasticizer concentration profiles, C(t) and C(z), and binder‐vapor interfacial development during removal. Removal rates were significantly enhanced when capillarydriven binder redistribution was assumed, and they depended strongly on initial plasticizer content under those conditions.