bluuebunny/crossref_metadata_2025 huggingface dataset:
Reaction Synthesis of Titanium Silicides via Self‐Propagating Reaction Kinetics
Authors: Yen Bing K. , Aizawa Tatsuhiko , Kihara Junji
Published: July 2010

Self‐propagating exothermic reactions in the titanium‐silicon system induced by mechanical milling, shock loading, and thermal ignition of elemental powder mixtures have been investigated. After an induction period of 3 h, the 5Ti + 3Si powder mixture abruptly reacted during milling to form single‐phase Ti5Si3 via a mechanically induced self‐propagating reaction (MSR). Moreover, the formation of porous Ti5Si3 solid indicated that the melting of powder particles had occurred during the mechanical alloying process. The Ti + Si powder mixture also reacted via the MSR mode, but the end‐product was multiphase. Shock and combustion synthesis (thermal‐explosion mode) experiments essentially produced the same result.
Direct Observation of the Double Schottky Barrier in Niobium‐Doped Barium Titanate by the Charge‐Collection Current Method
Authors: Kataoka Noriyasu , Hayashi Katsuro , Yamamoto Takahisa , Sugawara Yoshihiro , Ikuhara Yuichi , Sakuma Taketo
Published: July 2010

Charge‐collection (CC) current was measured at a single grain boundary, which exhibited positive temperature coefficient of resistivity (PTCR) effects, in 0.1‐mol%‐Nb‐doped BaTiO3. The CC current systematically reversed across the grain boundary above the Curie point, which indicated the presence of a double Schottky barrier (DSB) at the grain boundary. In contrast, the CC current was constant across the grain boundary below the Curie temperature. The result obtained from the CC current measurement agrees with the classical DSB model in donor‐doped BaTiO3, indicating the PTCR effects.
Zircon Synthesis via Sintering of Milled SiO2 and ZrO2
Authors: Spearing Dane R. , Huang John Y.
Published: July 2010

The formation of zircon (ZrSiO4) via sintering of milled SiO2 and ZrO2 powders was studied, and the effects of slurry vs dry milling, sintering time, and particle size on zircon yield were examined. It was found that very high zircon yields could be obtained via slurry milling, cold pressing, and sintering of the oxide precursors. The controlling factor in determining zircon yield was found to be the particle size of the SiO2 and ZrO2 powders. Zircon yield as a function of sintering time was examined, and found to be similar to previous studies in which sol‐gel precursors seeded with zircon were used. SEM studies reveal a homogeneous product with particle sizes on the order of 1–5 µm. It was found that complete reaction to zircon can be achieved from a once‐through milling, pressing, and sintering process of SiO2‐ZrO2 powders.
Metastable Alumina Polymorphs: Crystal Structures and Transition Sequences
Authors: Levin Igor , Brandon David
Published: July 2010

The available literature on the crystal structure of the metastable alumina polymorphs and their associated transitions is critically reviewed and summarized. All the metastable alumina structures have been identified as ordered or partially ordered cation arrays on the interstitial sites of an approximately close‐packed oxygen sublattice (either face‐centered cubic or hexagonal close packed). The analysis of the symmetry relations between reported alumina polymorphs having an approximately face‐centered cubic packing of the oxygen anions allows for an exact interpretation of all the complex domain structures that have been observed experimentally. Possible mechanisms for the phase transitions between the different alumina polymorphs also are discussed.
Effect of Starting Powder on Damage Resistance of Silicon Nitrides
Authors: Lee Seung Kun , Lee Kee Sung , Lawn Brian R. , Kim Do Kyung
Published: July 2010

The role of starting powder in the resistance of silicon nitride (Si3 N4) ceramics to strength‐impairing contact damage is studied. Si2N4 materials are prepared from three starting powders, at selectively increasing hot‐pressing temperatures to coarsen the microstructures: (i) from relatively coarse α‐phase powder, essentially equiaxed α‐Si3N4 grains, with limited, slow transformation to β‐Si3N4 grains toward the latter half of the temperature range; (ii) from relatively fine α‐phase powder, a more rapid transfor‐mation to β‐Si3N4, with attendant grain elongation; (iii) from fine β‐phase powder, an essentially equiaxed β‐Si3N4 structure over the entire temperature range. The resulting microstructures thereby provide a spectrum of β/α phase ratios and grain sizes and shapes for investigation. Indentations with hard spheres (Hertzian test) are used to induce damage into the Si3N4 specimens. Examination of the indentation sites indicates a competition between brittle and quasi‐plastic damage modes: in structures with relatively equiaxed grains, the damage takes the form of classical cone cracking; in structures with large elongate grains, the damage is distributed beneath the contact as grain‐localized microfailures within a subsurface “yield” zone. Bend tests on specimens containing single‐cycle contact damage reveal those structures with well‐developed cone cracks to be highly susceptible to strength degradation. The microstructures with the greatest resistance to strength loss are those formed from fine α‐phase starting powder at intermediate firing temperatures (1700°C), with characteristic large, elongate β grains. Implications of the results in the context of other mechanical properties, e.g., toughness, wear, and fatigue resistance, are discussed.
Continuous Existence of Bismuth at Grain Boundaries of Zinc Oxide Varistor without Intergranular Phase
Authors: Kobayashi Kei‐Ichiro , Wada Osamu , Kobayashi Masahiro , Takada Yoshio
Published: July 2010

The state of Bi at the grain boundaries of ZnO varistors was analyzed using FE‐TEM with EDS point and mapping analysis with a spatial resolution of less than 1 nm. The continuous existence of Bi of less than 1/2 atomic layer has been confirmed at ZnO‐ZnO junctions without any secondary phase. The coexistence of a crystalline Bi2O3 and an amorphous phase has been found, and a new model has been proposed for the transition of the Bi‐O phase from crystalline Bi2O3 to atomic Bi via an amorphous phase with decreasing thickness of grain boundary phases.
Processing and Performance of an All‐Oxide Ceramic Composite
Authors: Levi Carlos G. , Yang James Y. , Dalgleish Brian J. , Zok Frank W. , Evans Anthony G.
Published: July 2010

Continuous fiber ceramic composites (CFCCs) based on oxides are of interest for high‐temperature applications owing to their inherent oxidative stability. An enabling element is a matrix with an optimum combination of toughness and strength, which may be achieved by incorporating a controlled amount of fine, well‐distributed porosity. Implementation of this concept by vacuum infiltration of aqueous mullite‐alumina slurries into two‐dimensional woven preforms of alumina fibers has been investigated. Evaluation of these materials shows stress‐strain characteristics similar to other CFCCs, especially carbon‐matrix composites. Moreover, promising notch and creep properties have been found. Microstructural and processing issues relevant to the attainment of these behaviors are discussed.
Effect of Firing Temperature on the Thermal Stability of Low‐Oxygen Silicon Carbide Fibers
Authors: Shimoo Toshio , Tsukada Ichiro , Seguchi Tadao , Okamura Kiyohito
Published: July 2010

Low‐oxygen SiC fibers which were fabricated by curing with electron‐beam irradiation and firing at 1273 and 1573 K (fiber‐A and fiber‐B) were exposed at 1673 to 1973 K. Significant differences in surface composition, crystallinity, specific resistivity, and tensile strength were found between fiber‐A and fiber‐B. On exposing the fibers to high temperatures, they both crystallized nearly completely into intermixed β‐SiC and free carbon, and consequently the resistivity decreased markedly. Deep voids were formed at the surface of fiber‐A as a result of rapid dehydrogenation at the earliest stage of high‐temperature exposure. On the other hand, fiber‐B, which decomposed mildly, had a dense pore‐free structure. The fiber strength was proportional to the reciprocal square root of SiC crystal size. At the same crystal size, the strength of fiber‐A was lower because of the surface flaws.
Construction of the Quasi‐ternary Phase Diagram in the NdO1.5‐BaO‐CuOx System in an Air Atmosphere: Part I, Equilibrium Tie Lines in the Nd1+xBa2−xCu3O6+δ Solid Solution and Liquid Region
Authors: Kambara Makoto , Umeda Takateru , Tagami Minoru , Yao Xin , Goodilin Eugene A. , Shiohara Yuh
Published: July 2010

Quasi‐ternary phase diagrams of the NdO1.5‐BaO‐CuOx system near the CuOx corner have been constructed near the peritectic temperature in air of the Nd1+xBa2−xCu3O6+delta (Nd123) solid‐solution phase. Liquidus curves were determined by measuring the temperature dependences of the neodymium, barium, and copper solubilities in Nd‐Ba‐Cu‐O solutions with different BaO:CuO ratios. Solidus line compositions and equilibrium tie lines were determined by analyzing the compositions of the Nd123 solid solution equilibrated with the melt by quenching samples held isothermally. Based on the tie‐line features in the Nd123‐and‐ liquid two‐phase field, the Nd123 solid solution with the smaller substitution content was observed to be equilibrated with the solution melt with a higher BaO:CuO ratio, even in an air atmosphere. Nd123 crystal with a substitution content of ∼0.02 could be formed from the solution with the BaO:CuO ratio of greaterthan equal to0.75, which resulted in higher critical superconducting transition temperatures.
Spectroscopic Ellipsometry of Nickel Oxide/Zinc Oxide Artificial Superlattices
Authors: Ma Yuefei , Tsurumi Takaaki , Nishizawa Shuichi , Ohashi Naoki , Fukunaga Osamu
Published: July 2010

Thin films of ZnO and NiO and their superlattices were made by an ion beam sputtering technique. The dielectric functions of these films were measured as a function of photon energy using a variable‐angle spectroscopic ellipsometer. Artificial superlattices were formed in [(NiO)12/(ZnO)6]10, [(NiO)6/(ZnO)4]16, and [(NiO)7/(ZnO)8]12 films, where ZnO had the rock salt structure rather than the wurtzite type of structure. The dielectric functions of these superlattices were markedly different from those of pure ZnO, NiO, and their mixtures. However, ZnO had the wurtzite type of structure in [(NiO)10/(ZnO)20]4. In this film, the epitaxial relation which enables the formation of an artificial superlattice was not observed between ZnO and NiO layers, and the dielectric function could be interpreted as a simple mixture of ZnO and NiO.
Oxidation and Strength Retention of Monolithic Si3N4 and Nanocomposite Si3N4‐SiC with Yb2O3 as a Sintering Aid
Authors: Park Hyoungjoon , Kim Hae‐Won , Kim Hyoun‐Ee
Published: July 2010

The oxidation behaviors of monolithic Si3N4 and nanocomposite Si3N4‐SiC with Yb2O3 as a sintering aid were investigated. The specimens were exposed to air at temperatures between 1200° and 1500°C for up to 200 h. Parabolic weight gains with respect to exposure time were observed for both specimens. The oxidation products formed on the surface also were similar, i.e., a mixture of crystalline Yb2Si2O7 and SiO2 (cristobalite). However, strength retention after oxidation was much higher for the nanocomposite Si3N4‐SiC compared to the monolithic Si3N4. The SiC particles of the nanocomposite at the grain boundary were effective in suppressing the migration of Yb3+ ions from the bulk grain‐boundary region to the surface during the oxidation process. As a result, depletion of yttribium ions, which led to the formation of a damaged zone beneath the oxide layer, was prevented.
Scattering in Polycrystalline Nd:YAG Lasers
Authors: Ikesue Akio , Yoshida Kunio
Published: July 2010

For the present study, 1.1‐at.%‐neodymium‐doped yttrium aluminum garnet (YAG) ceramics with a controlled pore volume (150‐930 vol. ppm) were fabricated via a solid‐state reaction method using high‐purity powders. The scattering coefficients of neodymium‐doped YAG (Nd:YAG) ceramics, obtained from Fresnel's equation, increased simply as the pore volume increased. The effective scattering coefficient and the laser‐oscillation performance of Nd:YAG ceramics with a pore volume of ∼150 vol. ppm were almost equivalent to those of a 0.9‐at.%‐Nd:YAG single crystal fabricated via the Czochralski method.
Structure and Dielectric Properties of the Ba(Mg1/3Nb2/3)O3‐La(Mg2/3Nb1/3)O3 System
Authors: Akbas Mehmet A. , Davies Peter K.
Published: July 2010

A complete range of perovskite solid solutions can be formed in the (1 −x)Ba(Mg1/3Nb2/3)O3‐xLa(Mg2/3Nb1/3)O3 (BMN‐LMN) pseudobinary system. While pure BMN adopts a 1:2 cation ordered structure, 1:1 ordered phases are stabilized for 0.05 ≤x≤ 1.0. Dark‐field TEM images indicate that the La‐doped solid solutions are comprised of large 1:1 ordered domains and no evidence was found for a phase‐separated structure. This observation coupled with the systematic variations in the intensities of the supercell reflections supports a charge‐balanced “random‐site” model for the 1:1 ordering. The substitution of La also induces a transformation from a negative to positive temperature coefficient of capacitance in the region 0.25 ≤x≤ 0.5.
Pyrolytic Conversion of Spherical Organo‐silica Powder to Silicon Nitride under Nitrogen
Authors: Choi Jae Young , Moon Young Tae , Kim Do Kyung , Kim Chong Hee , Seog In‐Sig
Published: July 2010

Monodisperse, spherical Si3N4 powder composed of fine particulates was synthesized by pyrolyzing spherical organo‐silica powder under nitrogen. The organo‐silica powder was prepared by hydrolyzing a mixture of phenyltrimethoxysilane (PTMS) and tetraethoxysilane (TEOS) in a methanol solution of water and ammonia. The organo‐silica powder consisted of 81.3 at.% silicon units derived from PTMS and 18.7 at.% silicon units derived from TEOS. During the pyrolysis under nitrogen, the organo‐silica powder decomposed to a mixture of free carbon and silica, with an increase of the surface area, at 500°‐600°C, followed by the formation of alpha‐Si3N4, with ß‐Si3N4 as a minor phase, at 1450° and 1500°C and ß‐SiC at 1550°C. The pyrolyzed powders, which retained the spherical shape and monodispersity of the organo‐silica powders, with a reduction in mean particle diameter, were composed of fine particulates that were ~40 nm in size.
Properties of Multilayered Interphases in SiC/SiC Chemical‐Vapor‐Infiltrated Composites with “Weak” and “Strong” Interfaces
Authors: Rebillat Francis , Lamon Jacques , Naslain Roger , Lara‐Curzio Edgar , Ferber Mattison K. , Besmann Theodore M.
Published: July 2010

The interfacial properties of SiC/SiC composites with interphases that consist of (C‐SiC) sequences deposited on the fibers have been determined by single‐fiber push‐out tests. The matrix has been reinforced with either as‐received or treated Nicalon fibers. The measured interfacial properties are correlated with the fiber‐coatingbond strength and the number of interlayers. For the composites reinforced with as‐received (weakly bonded) fibers, interfacial characteristics are extracted from the nonlinear portion of the stress‐displacement curve by fitting Hsueh's push‐out model. The interfacial characteristics are controlled by the carbon layer adjacent to the fiber. The resistance to interface crack growth and fiber sliding increases as the number of (C‐SiC) sequences increases. For the composites reinforced with treated (strongly bonded) fibers, the push‐out curves exhibit an uncommon upward curvature, which reflects different modes of interphase cracking and a contribution of fiber roughness.
Processing and Electrical Characterization of a Varistor‐Capacitor Cofired Multilayer Device
Authors: Toal Francis J. , Dougherty Joseph P. , Randall Clive A.
Published: July 2010

A comprehensive study of the materials selection, processing, cofiring, and electrical characterization of a multilayer varistor/capacitor (MLVC) device has been presented. The goal was to achieve component miniaturization and provide superior high‐frequency and high‐amplitude transient‐voltage protection. A ZnO‐based material and 0.75Pb‐(Mg1/3Nb2/3)O3‐0.25Pb(Zn1/3Nb2/3)O3 (PMN‐PZN) were chosen as the respective varistor and capacitor components. Using a controlled‐profile furnace, PMN‐PZN with excess PbO resulted in an optimum cofiring with a complex varistor composition. The sintering study made it clear that the ability to cofire is dependent not only on matching sintering temperatures and final densities but also sintering rates. Composite‐pellet studies assured the mechanical and electrical integrity of the device and indicated that interaction between the respective components was minimal. Prototype MLVC devices were fabricated using standard tape‐casting techniques. Microstructural analysis of the MLVCs revealed solid ceramic/ceramic and ceramic/electrode interfaces with little evidence of interaction. Current‐voltage and dielectric measurements both indicated good electrical properties that can be specifically tailored by changing the layer thickness.
Flow Method for Rapidly Producing Barium Hexaferrite Particles in Supercritical Water
Authors: Hakuta Yukiya , Adschiri Tadafumi , Suzuki Toshiyuki , Chida Toshihiro , Seino Kazuei , Arai Kunio
Published: July 2010

We propose a flow method to produce barium hexaferrite (BaO6Fe2O3) particles with hydrothermal crystallization in supercritical water. Aqueous iron(III) and barium nitrate solution at room temperature was pressurized to 30 MPa and then mixed with potassium hydroxide solution (OH:NO3 = 4) at the same conditions to generate metal hydroxides. This mixture was then rapidly heated to 400°C by mixing with supercritical water and then fed into a tubular reactor. Residence time was ~1 min. The reaction was terminated by cooling at the exit of the reactor. The Ba:Fe mole ratio was varied over a range of 0.1‐2. When the Ba:Fe ratio was ~1/12, which is the stoichiometric ratio for BaO6Fe2O3, the main products were alpha‐Fe2O3. However, for the case of Ba:Fe > 0.5, fine particles of single‐phase BaO6Fe2O3 were produced. Batch experiments (380°C, 30 MPa) at Ba:Fe = 0.5 in supercritical water at a reaction time of 10 min produced a mixture of alpha‐Fe2O3 and BaO6Fe2O3. This product transformed to the equilibrium phase, BaO2Fe2O3, in 4 h as the reaction time increased, which suggests that the BaO6Fe2O3 that formed in supercritical water with our proposed flow method under nonstoichiometric conditions was an intermediate but stable product. Furthermore, the nonstoichiometric and nonequilibrium (dynamic) conditions are important for producing single‐phase BaO6Fe2O3 particles. The single‐phase particles are highly stable and can be produced continuously in a reaction time of <1 min.
Room‐Temperature Fabrication of Lithium Cobalt Oxide Thin‐Film Electrodes by Lithium Hydroxide Solution Treatment
Authors: Han Kyoo‐Seung , Song Seung Wan , Yoshimura Masahiro
Published: July 2010

An economical, energy‐efficient, and environmentally friendly solution process was used to prepare lithiated thin‐film electrodes as a cathode for lithium rechargeable microbatteries. Well‐crystallized and electrochemically active lithium cobalt oxide thin‐film electrodes were spontaneously fabricated in a single synthetic step in a 4M lithium hydroxide solution at room temperature without any post‐synthesis annealing. The estimated film properties show that the obtained spinel lithium cobalt oxide films (space group Fd3m) has possible use as a desired cathode film.
Processing and Properties of Intermetallic/Ceramic Composites with Interpenetrating Microstructure
Authors: Klassen Thomas , Günther Robert , Dickau Birte , Gärtner Frank , Bartels Arno , Bormann Rüdiger , Mecking Heinrich
Published: July 2010

Intermetallic/ceramic composites represent an interesting class of materials for high‐temperature structural and functional applications. These materials can be prepared via high‐energy milling of pure metals with Al2O3 as well as of aluminum with metal oxides. During subsequent compaction via pressureless sintering, the components react to form dense composites that consist of interpenetrating networks of the ceramic and intermetallic phases. Microstructural investigations, mechanical properties, and resistivity and wear resistance measurements of selected composites are presented. Improved fracture toughness and bending strength, with respect to monolithic Al2O3, have been achieved.
Kinetics of Ceramic‐Metal Composite Formation by Reactive Metal Penetration
Authors: Fahrenholtz William G. , Ewsuk Kevin G. , Loehman Ronald E. , Lu Ping
Published: July 2010

The rate of composite formation via reactive metal penetration has been determined. The metal penetration depth (i.e., the reaction‐layer thickness) was measured from cross sections of partially reacted samples. Samples were fabricated by immersing dense mullite preforms in a bath of molten aluminum at temperatures of 900°–1300°C and reacting the combination for up to 250 min. In general, the reaction‐layer thickness increased linearly as the time increased. Penetration rates as high as 6.0 mm/h were measured; however, the aluminum penetration rate varied dramatically with time and temperature. The penetration rate increased when the reaction temperature was increased from 900°C to 1100°C, and the reaction‐layer thickness increased linearly as the time increased in this temperature range. At temperatures of 1150°C and above, reaction‐layer formation slowed or stopped after a relatively short period of rapid linear growth, because of an increase in silicon concentration near the reaction interface. The duration of the rapid linear growth period decreased from 25 min at 1150°C to <1 min at 1250°C. At temperatures of 1300°C and above, no reaction layer was detected by using optical microscopy. Kinetics data and transmission electron microscopy analysis suggest that the reaction was inhibited at higher reaction temperatures and longer times, because of silicon buildup and saturation at the reaction front. Calculations show that, as the reaction temperature increased, the silicon production increased faster than the silicon transport. The two rates were approximately equal at a temperature of 1100°C.
Superior Perovskite Oxide‐Ion Conductor; Strontium‐ and Magnesium‐Doped LaGaO3: I, Phase Relationships and Electrical Properties
Authors: Huang Keqin , Tichy Robin S. , Goodenough John B.
Published: July 2010

The single‐phase, cubic‐perovskite region of the LaO1.5‐SrO‐Gao1.5‐MgO phase diagram was determined from room‐temperature and high‐temperature X‐ray diffraction. Two impurity phases were identified, LaSrGaO4 and aSrGa3O7. The conductivity of the oxygen‐deficient perovskite phase was shown to be essentially a purely oxide‐ion conductivity sigmao over a wide range of oxygen partial pressures 10‐22 lessthan equal to PO2 lessthan equal to 1 atm. The highest values of sigmao = 0.17 and 0.08 S/cm were found for La0.8Sr0.2Ga0.83Mg0.17O0.2815 at 800° and 700°C, respectively; they remain stable over a week‐long test. The Arrhenius plot of sigmao is curved, dividing into a high‐temperature region T > T* similar/congruent 600°C and a low‐temperature region T < T*. Above T* all the oxygen vacancies appear to be mobile; below T* they progressively condense into clusters of ordered vacancies.
Superior Perovskite Oxide‐Ion Conductor; Strontium‐ and Magnesium‐Doped LaGaO3: II, ac Impedance Spectroscopy
Authors: Huang Keqin , Tichy Robin S. , Goodenough John B.
Published: July 2010

Alternating current impedance spectroscopy and electron microscopy were used to investigate the effect of grain boundaries in LSGM‐based materials. The impurity LaSrGa3O7 existing in the samples La0.8Sr0.2Ga1‐yMgy‐O3–0.5(0.2+y) (y= 0.05–0.10) gives a contribution to a perfect or depressed grain‐boundary semicircle in the impedance spectroscopy. The depressed semicircle represents a constant phase element (CPE) indicating LaSrGa3O7 is an oxide‐ion insulator. The grain‐boundary semicircle vanishes if a hydrogen‐containing atmosphere is applied. This fact may reflect proton conduction in LaSrGa3O7. In contrast, the impurity LaSrGaO4 existing, for example, in the samples x= 0.20, y= 0.25–0.30 gives no grain‐boundary contribution to the impedance.
Superior Perovskite Oxide‐Ion Conductor; Strontium‐ and Magnesium‐Doped LaGaO3: III, Performance Tests of Single Ceramic Fuel Cells
Authors: Huang Keqin , Tichy Robin , Goodenough John B. , Milliken Christopher
Published: July 2010

The performance of La0.8Sr0.2Ga0.83Mg0.17O2.815 (LSGM) as an optimized electrolyte of a solid oxide fuel cell was tested on single cells having a 500‐µm‐thick electrolyte membrane. The reactivity of NiO and LSGM suggested use of an interlayer to prevent formation of LaNiO3. The interlayer Sm‐CeO2 was selected and sandwiched between the electrolyte and anode. Comparison of Sm‐CeO2/Sm‐CeO2+ Ni and Sm‐CeO2+ Ni as anodes showed that Sm‐CeO2/Sm‐CeO2+ Ni gave an exchange current density 4 times higher than that of Sm‐CeO2+ Ni. The peak power density of the interlayered cell is 100 mW higher than that of the standard cell without the interlayer. This improvement is due to a significant reduction of the anode overpotential; the overpotential of the cathode La0.6Sr0.4CoO3‐delta (LSCo) remained unchanged. Comparison of the peak power density in this study and with that of a previous study, also with a 500‐µm‐thick electrolyte, indicates a factor of 2 improvement, i.e., from 270 mW/cm2 to 550 mW/cm2 at 800°C. The excellent cell performance showed that an LSGM‐based thick membrane SOFC operating at temperatures 600° < Top < 800°C is a realistic goal.
Stress Concentration Due to Fiber‐Matrix Fusion in Ceramic‐Matrix Composites
Authors: Glime William H. , Cawley James D.
Published: July 2010

Stress concentration in ceramic‐matrix composites due to fiber‐matrix fusion is investigated using finite‐element modeling (FEM) in conjunction with experiment. FEM results indicate substantial stress concentration can be expected when the fiber and matrix are locally fused. The focus of the investigation is sealing through the formation of a reaction product layer (e.g., SiO2 in a SiC‐SiC system). Direct sealing also is considered. Tensile experiments involving a novel SiC‐SiC microcomposite system confirm FEM predictions and unambiguously indicate the observed reduction in load‐bearing capacity is the result of a SiO2 reaction product that locally fuses the fiber and matrix.
Toughness‐Curve Behavior of an Alumina‐Mullite Composite
Authors: Khan Ajmal , Chan Helen M. , Harmer Martin P. , Cook Robert F.
Published: July 2010

Toughness‐curve (T‐ or R‐curve) behavior of a composite of 30 vol%, polycrystalline, coarse‐grained, spherical alumina agglomerates dispersed throughout a fine‐grained, 50/50 vol% alumina‐mullite matrix, and that of its microstructural end‐members (100% matrix and 100% alumina), were studied using the indentation‐strength‐in‐bending technique. T‐curves were deconvoluted from indentation‐strength data using an indentation fracture mechanics model. The monolithic matrix and alumina exhibited an invariant toughness and a moderate T‐curve, respectively. In comparison, the composite exhibited a pronounced T‐curve. The T‐curve of the composite is best explained as deriving from the interaction of a propagating crack with the alumina agglomerates: crack propagation experiments revealing two possible toughening mechanisms‐intra‐agglomerate frictional grain bridging and elastic bridging ligaments in the matrix that appeared to be associated with alumina agglomerates. Rule‐of‐mixtures toughness calculations indicated that intra‐agglomerate bridges could account for only a fraction of the toughening exhibited by the composite. It is suggested that the extra toughening arises from the elastic bridging ligaments.
Crystallization with Variable Temperature: Corrections for the Activation Energy
Authors: Frade Jorge Ribeiro
Published: July 2010

The Nakamura‐Avrami method has been revised to derive reliable solutions for describing the kinetics of crystallization on heating with variable temperature. The corresponding numerical solutions for first‐order phase‐boundary‐controlled growth were computed to evaluate the errors that should be expected when using simple approximate solutions. The relative differences increased as the average dimensionless temperature increased; thus, a method was proposed to obtain correct values for the activation energy and pre‐exponential factors. Our analysis of the effects of heating rate on the crystallization peak is consistent with the Kissinger equation, even for dimensionality values of >1. Numerical solutions were also used to demonstrate the validity of the Kissinger equation.
Synthesis of Controlled Spherical Zinc Sulfide Particles by Precipitation from Homogeneous Solutions
Authors: Vacassy Robert , Scholz Stefan M. , Dutta Joydeep , Plummer Christopher John George , Houriet Raymond , Hofmann Heinrich
Published: July 2010

Zinc sulfide (ZnS) powders have been obtained by precipitation from homogeneous solutions of various zinc salt compounds, with S2‐ as precipitating anion, formed by decomposition of thioacetamide. Spherical particles with a very narrow size distribution can be obtained by controlling the synthesis parameters. The particle sizes are influenced by the nature of the associated anion. For example, nanometer‐sized ZnS particles are formed using acetate or acetylacetonate anions under acidic pH conditions, controlled by the addition of acetic acid. Although the nucleation is accelerated by the use of acetic acid, limited particle growth occurs because of the formation of complexes with zinc cations that lowers the concentration of free cations in the solution. Also, the complexing‐attachment phenomena of the ZnS particles with acetate and acetylacetonate anions lead to the arrest of particle growth processes. The presence of complexed Zn2+ species in the acidic solution is demonstrated both theoretically, using a model based on the calculations of the solubility isotherms of the soluble species, and by Fourier‐transform infrared techniques. The nanostructured ZnS particles may provide a suitable semiconductor nanocluster material for optoelectronic applications as well as a phosphor suitable for application in flat‐panel display technology.
Thermal Depoling Effects on Anisotropy of Lead Zirconate Titanate Materials
Authors: Wan Shan , Bowman Keith J.
Published: July 2010

Changes in elastic anisotropy and fracture toughness anisotropy of a lead zirconate titanate (PZT) piezoelectric material as a function of depoling temperature were investigated using ultrasound and Vickers indentation. A gradual transition from anisotropic to isotropic properties occurs with increased depoling temperatures at and above the Curie temperature. Depoling is also evident through changes in domain character revealed by X‐ray diffraction. The relationship between the mechanical properties and domain structure variation with respect to different depoling conditions is discussed.
Low‐Temperature Synthesis of Cubic and Rhombohedral Y6WO12 by a Polymerized Complex Method
Authors: Yoshimura Masahiro , Ma Junfeng , Kakihana Masato
Published: July 2010

Cubic Y6WO12, which has a defect fluorite‐type structure with disordered cation and anion sublattices, was synthesized for the first time at the low temperature of 600°C by a polymerized complex method. The crystallization process of Y6WO12 from an amorphous precursor has been investigated in detail using TG‐DTA, XRD, and TEM. Above 400°C, the cubic phase began to crystallize upon decomposition of the organic materials in the precursor. The crystallite size of the cubic Y6WO12 increased from about 5 nm at 600°C up to about 35–40 nm at 1000°C. Above 1200°C, the cubic phase transformed to rhombohedral Y6WO12, which is a stable phase.
Platinum as a Weak Interphase for Fiber‐Reinforced Oxide‐Matrix Composites
Authors: Wendorff J. , Janssen R. , Claussen N.
Published: July 2010

Alumina/zirconia matrix composites with platinum‐coated sapphire fibers were fabricated using the technology of reaction bonding of aluminum oxide (RBAO) as the synthesis route. The interfacial behavior of both sintered and annealed specimens was investigated by crack path observations and fiber pushout tests. The results show that precious metals like platinum or related alloys can be used as high‐temperature interphases for oxide/oxide composites. Weak interfaces were obtained both between fiber and coating as well as between matrix and coating, which should be suitable to achieve a damage‐tolerant behavior of the composite.
Influence of Thermal Aging on Microstructural Development of Mullite Containing Alkalis
Authors: Baudín Carmen , Villar María Pilar
Published: July 2010

The influence of annealing treatments at temperatures of 900°C up to 1630°C on the microstructure of a 3Al2O32SiO2 mullite that contains a small amount of alkali (<3 wt%) has been studied. Annealing treatments of a base mullite material at the sintering temperature (1630°C) and at two temperatures lower (900°C) and higher (1200°C) than the lowest invariant points of the SiO2‐Al2O3‐Na2O system have been performed. Microstructures have been characterized by using scanning and transmission electron microscopy. Special attention has been given to grain‐boundary characteristics‐particularly the amount, composition, and distribution of the remaining glasses. Aging of this material at high temperature leads to a redistribution of the microstructure toward an equilibrium that involves the dissolution of the mullite grains, formation of a liquid phase, and liquid‐phase grain growth. As the aging temperature increases, liquid‐phase grain growth progressively overcomes the effect of the dissolution of mullite and a bimodal microstructure with an increasing number of large, tabular grains develops.
A Pervasive Mode of Oxidative Degradation in a SiC‐SiC Composite
Authors: Ogbuji Linus U. J. T.
Published: July 2010

The oxidative durability of a SiC‐SiC composite with Hi‐Nicalon fiber and a BN interphase was investigated at 800°C (where pesting is known to occur in SiC‐SiC composites) for exposure durations of up to 500 h and in a variety of oxidant mixes and flow rates, ranging from quasi‐stagnant room air, through slow‐flowing oxygen that contained 30%‐90% H2O, to the high‐velocity flame of a burner rig. Degradation of the composite was determined from residual strength and fracture stain in post‐exposure mechanical tests and correlated with microstructural evidence of damage to the fiber and interphase. The most‐severe degradation of composite behavior occurred in the burner rig and has been shown to be associated with the high oxidant velocity and substantial moisture content, as well as a thin sublayer of carbon that is indicated to form between the fiber and the interphase during composite processing.
Microstructural Design of Silicon Nitride with Improved Fracture Toughness: II, Effects of Yttria and Alumina Additives
Authors: Sun Ellen Y. , Becher Paul F. , Plucknett Kevin P. , Hsueh Chun‐Hway , Alexander Kathleen B. , Waters Shirley B. , Hirao Kiyoshi , Brito Manuel E.
Published: July 2010

Significant improvements in the fracture resistance of self‐reinforced silicon nitride ceramics have been obtained by tailoring the chemistry of the intergranular amorphous phase. First, the overall microstructure of the material was controlled by incorporation of a fixed amount of elongated ß‐Si3N4 seeds into the starting powder to regulate the size and fraction of the large reinforcing grains. With controlled microstructures, the interfacial debond strength between the reinforcement and the intergranular glass was optimized by varying the yttria‐to‐alumina ratio in the sintering additives. It was found that the steady‐state fracture toughness value of these silicon nitrides increased with the Y:Al ratio of the oxide additives. The increased toughness was accompanied by a steeply rising R‐curve and extensive interfacial debonding between the elongated ß‐Si3N4 grains and the intergranular glassy phase. Microstructural analyses indicate that the different fracture behavior is related to the Al (and O) content in the ß´‐SiAlON growth layer formed on the elongated ß‐Si3N4 grains during densification. The results imply that the interfacial bond strength is a function of the extent of Al and Si bonding with N and O in the adjoining phases with an abrupt structural/chemical interface achieved by reducing the Al concentration in both the intergranular phase and the ß´‐SiAlON growth layer. Analytical modeling revealed that the residual thermal expansion mismatch stress is not a dominant influence on the interfacial fracture behavior when a distinct ß´‐SiAlON growth layer forms. It is concluded that the fracture resistance of self‐reinforced silicon nitrides can be improved by optimizing the sintering additives employed.
Influence of the Addition of Bi2O3 on the Grain Growth and Magnetic Permeability of MnZn Ferrites
Authors: Drofenik Miha , Znidarsic Andrej , Makovec Darko
Published: July 2010

Additions of Bi2O3 were used to promote grain growth and to increase magnetic permeability during sintering of MnZn ferrites. The results showed that small additions of Bi2O3 of <0.05 wt% remarkably increase the permeability of MnZn ferrites. On the other hand, addition of 0.05 wt% Bi2O3 induced the formation of a microstructure composed of giant grains with trapped pores embedded in a normal microstructure. The permeability of these samples showed a pronounced secondary maximum in permeability. At still higher Bi2O3 concentrations, above 0.2 wt%, the grain growth was retarded and a normal microstructure appeared; however, the magnetic permeability was strongly reduced.
High‐Temperature Properties of Unburned MgO‐C Bricks Containing Al and Si Powders
Authors: Uchida Shigeki , Ichikawa Kenji , Niihara Koichi
Published: July 2010

Changes in the mechanical and thermal properties, as well as in the microstructure, of unburned MgO‐C bricks containing Al and Si powders were investigated at selected temperatures. Specimens with heat treatments at 500°C shrank and exhibited higher apparent porosity than untreated specimens. The bending strength and elastic modulus at 500°C were much lower than those of untreated specimens, and the apparent porosity increased and the mechanical properties at 500°C decreased with each repeating heat treatment. It was predicted that when the volatile matter was no longer generated, microstructure shrinkage would stop and the mechanical properties become constant. The bending strength and static elastic modulus at 800°, 1000°, and 1300°C were higher than those at 500°C because of the binding effect of the reaction products (i.e., Al4C3, SiC, and MgAl2O4), although the apparent porosity was higher than at 500°C. Repeated heat treatment from room temperature (RT) to the respective temperature, however, degraded the properties to nearly the same level as at 500°C because of the increased apparent porosity and the cracks generated in magnesia particles by the reaction products. Plastic deformation appeared to occur at 1300°C just before bricks were fractured. In addition, the thermal expansion ratio decreased through repeated heating and cooling from RT to 500°, 1000°, or 1300°C, and finally decreased to a constant value, as predicted.
Synthesis of BaAl2Si2O8from Solid Ba‐Al‐Al2O3‐SiO2Precursors: II, TEM Analyses of Phase Evolution
Authors: Zhang Xiao‐Dong , Sandhage Kenneth H. , Fraser Hamish L.
Published: July 2010

Conventional transmission electron microscopy (TEM), coupled with energy‐dispersive X‐ray (EDX) analysis, has been used to examine the intermediate phases produced during the transformation of Ba‐Al‐alpha‐Al2O3‐SiO2(cristobalite) precursors into celsian, BaAl2Si2O8. Analyses were conducted on samples that had been exposed to pure, flowing oxygen at peak temperatures of 300°C for 24 h, 650°C for 72 h, or 900°C for 24 h. Particles of Al2O3retained after the 300‐900°C heat treatments were coated with a layer of BaAl2O4. An amorphous, alumina‐poor Ba‐Al‐Si‐O (G1) phase was observed in contact with residual SiO2particles after the 650°C heat treatment. Although not a starting component of the precursor, elemental silicon was also detected after the 300‐900°C heat treatments. Silicon particles produced during the 300°C heat treatment were surrounded by an amorphous Ba‐Al‐Si‐O (G2) phase that contained more alumina than the G1 phase. Fine‐grained barium orthosilicate (Ba2SiO4) and sanbornite (BaSi2O5) were observed after the 650°C treatment, along with thin, plate‐shaped grains of BaAl2Si2O8. The latter two phases were surrounded by an amorphous Ba‐Al‐Si‐O (G3) phase that possessed less alumina than the G2 phase. Smaller platelets of BaAl2Si2O8were also detected after the 300°C treatment. After annealing at a peak temperature of 1650°C, BaAl2Si2O8was the only silicate compound detected by TEM. Novel reaction paths to BaAl2Si2O8, which are consistent with the present TEM observations and prior XRD and SEM/EDX analyses (Part I), are discussed.
Influence of Yttrium Doping on Grain Misorientation in Aluminum Oxide
Authors: Cho Junghyun , Chan Helen M. , Harmer Martin P. , Rickman J. M.
Published: July 2010

Oversized dopant ions such as yttrium and lanthanum segregate to grain boundaries and reduce the tensile creep rate of α‐Al2 O3 by 2‐3 orders of magnitude. One explanation for this behavior is that the oversized segregants give rise to a “site‐blocking” effect for grain boundary diffusion. It has also been speculated that the dopant ions modify the grain boundary structure in alumina and reduce the creep rate by promoting the formation of special (e.g., coincidence site lattice (CSL)) grain boundaries. In order to test the latter hypothesis, we have used electron backscattered Kikuchi diffraction to characterize the misorientation and special grain boundary distribution for undoped and 1000‐ppm‐yttrium‐doped alumina. The results show that the grain boundary structure in alumina (as characterized by the frequency of selected CSLs and misorientation distribution) was not significantly changed by the addition of yttrium, indicating that creep retardation results mainly from site‐blocking.
Abnormal Grain Growth of Niobium‐Doped Strontium Titanate Ceramics
Authors: Bae Choelhwyi , Park Jae‐Gwan , Kim Yoon‐Ho , Jeon Hyeongtag
Published: July 2010

We have investigated the influence of the Ti/Sr ratio and particle size on abnormal grain growth of Nb‐doped SrTiO3 ceramics. As the Ti/Sr ratio increases, the onset time and temperature of abnormal growth increase, for specimens sintered at temperatures above 1440°C. When the particle size is decreased by milling, the onset and completion temperatures of the abnormal growth and the final grain size are decreased. The abnormal grains growing during isothermal heating are nearly tetrakaidecahedral in shape, whereas those growing during rising temperature have the morphology of an edge‐cut cube.
Preparation and Properties of Alumina/Nickel‐Cobalt Alloy Nanocomposites
Authors: Oh Sung‐Tag , Sando Mutsuo , Niihara Koichi
Published: July 2010

High‐density nickel‐cobalt alloy‐dispersed Al2O3 (Al2O3/Ni‐Co alloy) composites were obtained via the hydrogen reduction and hot pressing of Al2O3, Ni(NO3)26H2O, and Co(NO3)26H2O powder mixtures. Microstructural investigations revealed that nanometer‐sized alloy particles were dispersed homogeneously at the matrix grain boundaries, forming the intergranular‐type nanocomposite. High strength (>1 GPa) was registered for the Al2O3/10 wt% Ni‐Co alloy composite. An inverse magnetostrictive response to applied stress was observed, because of the Ni‐Co alloy dispersions, which indicates promise for incorporating new functions such as stress and fracture sensing into the structural ceramics without any loss of mechanical properties.
Core‐Shell Structure Formation in Nb2O5‐Doped SrTiO3 by Oxygen Partial Pressure Change
Authors: Chung Sung‐Yoon , Lee Byoung‐Ki , Kang Suk‐Joong L.
Published: July 2010

A core‐shell structure was observed in SrTiO3 doped with 1.2 mol% of Nb2O5, after sintering in a reducing atmosphere (5H2‐95N2) and then in an oxidizing atmosphere (air). In undoped and Al2O3‐doped SrTiO3 specimens, no core‐shell structure formed after the same sintering treatments as those for SrTiO3 doped with 1.2 mol% of Nb2O5. The measured chemical compositions of the core and shell regions of 1.2‐mol%‐Nb2O5‐doped SrTiO3 grains showed that the Sr/(Ti + Nb) ratio of the shell regions grown in air was ~1% less than that of core regions grown in 5H2‐95N2, which was in good agreement with a value predicted by available defect equations. Therefore, the observed core‐shell structure is thought to result from the formation of strontium vacancies in an oxidizing atmosphere.
Microwave‐Hydrothermal Synthesis of Nanophase Ferrites
Authors: Komarneni Sridhar , D'Arrigo Maria Cristina , Leonelli Cristina , Pellacani Gian Carlo , Katsuki Hiroaki
Published: July 2010

This paper reports the synthesis of technologically important ferrites such as ZnFe2O4, NiFe2O4, MnFe2O4, and CoFe2O4 by using novel microwave‐hydrothermal processing. Nanophase ferrites with high surface areas, in the range of 72‐247m2/g, have been synthesized in a matter of a few minutes at temperatures as low as 164°C. The rapid synthesis of nanophase ferrites via an acceleration of reaction rates under microwave‐hydrothermal conditions is expected to lead to energy savings.
Nanophase Glass‐Ceramics
Authors: Beall George H. , Pinckney Linda R.
Published: July 2010

Future applications for glass‐ceramics are likely to capitalize on designed‐in, highly specialized properties for the transmission, display, and storage of information. Glass‐ceramics with microstructures comprised of uniformly dispersed crystals <100 nm in size offer promise for many potential new applications as well as provide unique attributes for many current products. This paper focuses on two types of nanocrystalline glass‐ceramics: transparent glass‐ceramics and tough, high‐modulus glass‐ceramics with precisely engineered surfaces. Transparent glass‐ceramics are formed from certain aluminosilicate glasses capable of efficient crystal nucleation and slow growth. The key crystalline phases include β‐quartz solid solutions, characterized by low‐thermal‐expansion behavior; spinel, with high hardness and elastic modulus; and mullite, which shows unique chromium‐luminescence behavior.
Plasticizing Aqueous Suspensions of Concentrated Alumina with Maltodextrin Sugar
Authors: Schilling Christopher H. , Bellman Robert A. , Smith Rachelle M. , Goel Honey , Giesche Herbert
Published: July 2010

Aqueous suspensions of submicrometer, 20 vol% Al2O3 powder exhibited a transition from strongly flocculated, thixotropic behavior to a low‐viscosity, Newtonian‐like state upon adding small amounts of maltodextrin (0.03 g of maltodextrin/(g of Al2O3)). These suspensions could be filter pressed to highly dense (57%) and extrudable pastes only when prepared with maltodextrin. We analyzed the interaction of maltodextrin with Al2O3 powder surfaces and quantitatively measured the resulting claylike consolidation, rheological, and extrusion behaviors. Benbow extrusion parameters were comparable to, but higher than, those of kaolin at approximately the same packing density of 57 vol%. In contrast, Al2O3 filter cakes without maltodextrin at 57 vol% density were too stiff to be extruded. Measurements of rheological properties, acoustophoresis, electrophoresis, sorption isotherms, and diffuse reflectance Fourier infrared spectroscopy supported the hypothesis that sorbate‐mediated steric hindrance, rather than electrostatic, interparticle repulsion, is important to enhancing the consolidation and fluidity of maltodextrin–Al2O3 suspensions. Viscosity measurements on aqueous maltodextrin solutions indicated that free maltodextrin in solution does not improve suspension fluidity by decreasing the viscosity of the interparticle solution.