bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Soaking Time on the Temperature Coefficient of Resistivity of Semiconducting Barium Titanate PTCR Ceramics
Authors: Lu Yuh‐Yih , Lai Chun‐Hung , Tseng Tseung‐Yuen
Published: March 2005

The influence of soaking time at 1200°C on the temperature coefficient of resistivity (TCR) of semiconducting barium titanate ceramics has been investigated. It is found that soaking duration has influence on the room‐temperature resistivity, the maximum resistivity, the temperature (Tmax) at which the maximum resistivity appears, and TCR of the positive temperature coefficient of resistivity (PTCR) ceramics. The increased room‐temperature resistivity with the increase of soaking time is contributed mainly by the increase of grain boundary resistivity examined by the complex‐plane impedance method. The obtained surface acceptor density (Ns) by capacitance–voltage measurement is found to increase with the soaking duration. The higher Ns contributes to higher built‐in potential and results in lower Tmax and higher maximum resistivity. Therefore, the increased surface acceptor density from increasing the soaking time at 1200°C increases the temperature coefficient of resistivity of PTCR ceramics.
Modeling of Point Defects in Corundum Crystals
Authors: Jacobs Patrick W. M. , Kotomin Eugene A.
Published: March 2005

Several different approaches including Hartree–Fock ab initio cluster calculations, semiempirical INDO calculations, and atom–atom potentials were used for modeling of the spatial and electronic structure as well as migration mechanisms of both intrinsic defects (self‐trapped and defect‐trapped holes, O and Al vacancies) and impurities (transition‐metal Ions like Co, Fe, Mg, Mn, Ti). The atomic structure of all hole centers is found to be similar to VK centers in alkali halides (two‐site model); their formation is energetically favorable. The energy required for 60° hole reorientations inside the basic oxygen triangles is found to be similar to both the energy for hops between such triangles and the experimental activation energy for self‐trapped hole migration (0.7 eV). A novel mechanism of hole polaron motion in ionic solids is presented on the basis of quantumchemical cluster calculations. The role of clustering in the solution of impurities is shown to be crucial. Lastly, five kinds of O vacancy hops are simulated. In several cases the activation energy is lowered considerably when the hopping ion is allowed to deviate from a straight path. Theory predicts the lowest activation energy to be 1.85 eV, in excellent agreement with the value observed experimentally below 1550°C. Theoretical predictions of the Arrhenius energy for diffusion at high temperatures are also in excellent agreement with Oishi and Ando's experimental values above 1590°C.
Fabrication of Oriented SiC‐Whisker‐Reinforced Mullite Matrix Composites by Tape Casting
Authors: Wu Mianxue , Messing Gary L.
Published: March 2005

Dense SiC‐whisker‐reinforced mullite composites with up to 50 vol% whiskers can be obtained by tape casting and hot pressing. The tape casting process results in high degrees of SiC whisker orientation as determined visually and by X‐ray diffraction. The ability to achieve dense composites with as much as 50 vol % whiskers is attributed to the higher percolation threshold of aligned whiskers. The factors affecting the degree of whisker orientation during tape casting are described using a fluid dynamics model derived from Jeffery's equations and show that the orientation of anisometric particles is enhanced primarily by the casting rate and particle aspect ratio.
Comparison of the Pyrolysis Products of Dichlorodimethylsilane in the Chemical Vapor Deposition of Silicon Carbide on Silica in Hydrogen or Argon
Authors: Cagliostro Domenick E. , Riccitiello Salvatore R , Ren Jian , Zaghi Farshad
Published: March 2005

A chemical analysis of the pyrolysis gases and solids formed during the deposition of silicon carbide from the decomposition of dichlorodimethylsilane in argon and hydrogen is reported. Depositions were performed at 1 atm pressure, temperatures from 700° to 1100°C, and a mean residence time of approximately 1 min. The chemical analysis shows that, under reactor conditions, the gases formed were mainly methane, hydrogen, silicon tetrachloride, trichloro‐silane, and trichloromethylsilane. The presence of hydrogen chloride was not examined. The use of hydrogen, as a carrier gas, decreased the trichloromethylsilane and solid aerosol (smoke) in the reaction products, compared to that present in the argon system, and increased methane, trichlorosilane, and silicon production. Primarily, silicon and silicon carbide were deposited when hydrogen was used as the carrier gas. When argon was used, a complex mixture of silicon carbide and organosilicon compounds was formed. It is hypothesized that, when hydrogen was used as the carrier gas, silicon carbide formed from chlorosilanes and methane, which were products from the decomposition of dichlorodimethylsilane. These products subsequently reacted to form silicon, which then reacted with methane to form silicon carbide. In argon, however, it is hypothesized that silicon carbide can be formed in two ways: (1) from the pyrolysis of solid organosilicon compounds which are products from the pyrolysis of dichlorodimethylsilane in argon and (2) as the reduction of dichlorodimethylsilane to chlorosilanes and methane, caused by the hydrogen produced from the pyrolysis of dichlorodimethylsilane in argon.
Effect of Particle Size on Binary Reactions Common to the Y‐Ba‐Cu‐O System
Authors: Sobolik Jeffrey L. , Wang Hong , Thomson William J.
Published: March 2005

A systematic study has been conducted to determine the effects of particle size on the reaction paths leading to binary phases in the Y‐Ba‐Cu‐0 system. Using dynamic X‐ray diffraction (DXRD), three binary reaction sets with stoichiometries equal to that present in the Y‐Ba‐Cu high‐Tc; superconductor and having two different particle sizes (nanometers and micrometers) were reacted in both air and helium environments. Results indicate that the absence of oxygen and carbon dioxide leads to lower reaction temperatures and to different products in the Y‐Cu and Ba‐Cu binaries. Particle size comparisons show not only a higher reactivity in the nanometer system, but also different products due to the diffusion limitations in the micrometer system. Comparisons between the binary and ternary reactivities point to the Ba‐Cu binary as the most critical reaction system for assuring high‐quality HTSC.
Sintering of Silicon Carbide/Molybdenum Disilicide Composites Using Boron Oxide as an Additive
Authors: Ting Jyh‐Ming
Published: March 2005

Sintering of silicon carbide reinforced molybdenum disilicide‐Composites using boron oxide as an additive was performed. The addition of boron oxide to the composites was found to be beneficial. It reduced the processing temperature, resulted in smaller grain sizes, and increased the flexural strength of the composites. Formation of glassy boundary phase was also found.
Flexural Properties of Brittle Multilayer Materials: I, Modeling
Authors: Folsom Craig A. , Zok Frank W. , Lange Fred F.
Published: March 2005

Multilayer systems comprising brittle materials can exhibit substantially different behaviors under flexural and tensile loadings. The present article addresses the origins of such differences, with emphasis on the modeling of the flexural stress–strain response. Systems with both a deterministic tensile strength and a distribution in strengths (characterized by Weibull statistics) are considered. The model predictions show that both the ultimate strength and strain‐to‐failure in flexure exceed the corresponding values in uniaxial tension. In addition, systems comprising alternating layers of two different materials are examined, and disparities in the flexural and tensile behaviors addressed.
Enhanced Surface Crystallization of β‐Barium Borate on Glass Due to Ultrasonic Treatment
Authors: Ding Yong , Osaka Akiyoshi , Miura Yoshinari
Published: March 2005

β‐Barium borate (BBO) surface crystallized glass composites were prepared for a glass of composition 40BaO·15Tio2–45B2O3 (in mol%). Enhanced nucleation was attained by ultrasonic surface modification with an aqueous suspension of BBO particles, resulting in dense BBO thin films on the glass surfaces. Second harmonic generation signals were observed for samples subjected to ultrasonic treatment.
Internal Friction in Lithium Aluminosilicate Glass‐Ceramics
Authors: Versteeg Vera A. , Kohlstedt David L.
Published: March 2005

This paper describes the results of an investigation of the anelastic behavior of two glass‐ceramic materials with similar compositions, one nucleated with TiO2 and the other with ZrO2. The influence of the grain size, residual glass fraction, and composition was examined. A low‐frequency torsion pendulum was constructed for this purpose. The as‐received TiO2‐nucleated. Corning Code 9608 ceramic exhibits an attenuation peak at about 750°C; when heat‐treated at 1200°C for 167 h, the attenuation peaks at approximately 850°C. The ZrO nucleated glass‐ceramic showed no such peak. Our results indicate that this anelastic behavior is due to grain boundary sliding, but the rates and magnitude of sliding are strongly dependent on boundary chemistry.
Elastic Properties of Two‐Phase Composites
Authors: Ravichandran Kakkaveri S.
Published: March 2005

Simple expressions were derived to predict the elastic properties of two‐phase systems containing discontinuous reinforcements, on the basis of approximation of composite microstructure to a unit cell incorporating isostrain and isostress type elements, arranged in two different ways. The bounds on elastic modulus obtained in this manner have been shown to accurately describe the variation of elastic modulus as a function of volume fraction of one of the phases, for a wide variety of two‐phase composites. The present expressions offer predictions of elastic modulus that are much closer to experimental data than the commonly used Hashin and Shtrikman bounds, particularly for composites with constituents having large differences in elastic moduli. Similarly, it has been shown that the shear moduli and the Poisson's ratios of composites as a function of second‐phase volume fraction can also be predicted. the present unit‐cell‐based method of calculation presents a promising approach for the prediction of properties of multiphase materials. especially for those consisting of more than two dissimilar phases.
Deformation and Fracture by Sharp Rolling Contacts
Authors: Cook Robert F.
Published: March 2005

The contact mechanics of sharp “glass‐cutting” rollers is analyzed. Assuming the contact to be plastic, expressions are developed for the width and length of a static contact and the width of a rolling track. The residual stress‐intensity factor and stable length for median cracks generated beneath a roller track are derived, assuming the track to have been created by an equivalent wedge loading and modeling the plastic deformation zone as an expanding cylindrical cavity. The strengths of components containing dominant roller flaws are then derived by superposing an applied stress field onto the residual contact field and solving for instability. The contact dimensions, median crack lengths, and strengths are all expressed in terms of the roller radius and included angle, contact load, and indented material hardness, modulus, and toughness. Tests on soda–lime glass and aluminum confirm the predicted dependences.
Coherency Strain Energy and the Direction of Chemically Induced Grain Boundary Migration in Al2O3–Cr2O3 and Al2O3–Fe2O3
Authors: Lee Ho Yong , Kang Suk‐Joong L. , Yoon Duk Yong
Published: March 2005

Using various pairs of Al2O3 crystals of known surface orientation and two solute species of Cr2O3 and Fe2O3, the direction of chemically induced grain boundary migration has been investigated on the basis of a calculation of coherency strain energy. For all the grain boundaries studied, the chemically induced migration always occurred in the direction of the grain with the surface orientation corresponding to higher coherency strain energy. The migration directions of some grain boundaries were different depending on the solute species because of different strain energy for the same orientation. The zigzag migration was also observed but only in grain boundaries between grains whose coherency strain energies were comparable to each other. These observations have confirmed the prediction of the migration initiation by the coherency strain theory.
Crack Propagation Behavior of Polycrystalline Alumina under Static and Cyclic Load
Authors: Kishimoto Hidehiro , Ueno Akira , Okawara Seiji , Kawamoto Hiroshi
Published: March 2005

Various causes for cyclic‐loading fatigue in ceramics have been proposed. Degradation in the grain‐bridging effect is the most important cause for cyclic‐loading fatigue in nontransforming ceramics. Cyclic‐ and static‐loading crack propagation behavior in terms of crack propagation rate, load—strain curve, and R‐curve in compact tension specimens of polycrysalline aluminas with two types of average grain size is reported. Significant bridging is observed in coarse‐grained alumina. The results are consistent with the proposal that grain bridging increases with grain size and that degradation in grain bridging is the most important cause for cyclic‐loading fatigue in alumina ceramics.
Fabrication of Continuous‐Fiber‐Reinforced Polycrystalline Oxide Composites via Molten Salt Infiltration
Authors: Chou Yeong‐Shyung , Ramberg C. Eric , Hellmann John R.
Published: March 2005

A novel molten nitrate salt infiltration technique was developed for the fabrication of continuous‐fiber‐reinforced polycrystalline‐alumina‐matrix composites containing a high volume fraction (47%) of small‐diameter fibers (Du Pont PRD 166 alumina/zirconia; 20‐μm diameter). A single infiltration resulted in sufficient matrix yield to permit densification of the resulting composites to >93% of theoretical density with excellent microstructural uniformity. Hot‐pressed composites fabricated in this manner exhibited Young's modulus of 270GPa, flexural strengths of 272 ± 20 MPa, and fracture toughness of 3.35 ± 0.37 MPa·ml/2. Primary fracture origins were localized regions of interfiber porosity, which were attributed to incomplete fiber tow infiltration. Fractographic analysis revealed lack of fiber pullout, and emphasized the need for interfacial debonding agents (coatings) to achieve further toughening. Results have demonstrated the utility of molten‐salt‐matrix precursors for the fabrication of polycrystalline‐matrix composites containing high volume fractions of continuous, small‐diameter ceramic fibers.
Porosimeter as a Means to Measure the Compactibility of Powders
Authors: Majling Jan , Znášik P. , Khandl V. , Komarneni Sridhar
Published: March 2005

Samples of silica fume ponder prepressed in the form of slabs under relatively small pressures and cut in spheres were coated with a thin flexible latex film by dipping them into the water emulsion of the natural latex, and their densification behavior was studied using the common mercury intrusion porosimeter. The samples were compacted at isostatic pressure conditions by increasing the mercury pressure, and the volume changes as a function of pressure were recorded. The recorded pressure/volume changes provided information about the compaction course of the original green powder compacts in a continuous fashion.
Observation of Hydrogenated Amorphous Carbon Films by Atomic Force Microscopy
Authors: Nakamura Yoshikazu , Watanabe Yoshihisa , Suefuji Yoshihito , Hirayama Shigekazu
Published: March 2005

Atomic force microscopy images were made of the surface of CVD hydrogenated amorphous carbon films. The films were deposited from toluene vapor with oxygen additions during deposition, producing very smooth surfaces. Average roughnesses of only 0.5 nm were measured for the films made with oxygen, whereas a surface roughness of 11 nm was obtained for the films deposited without oxygen. The results suggest that sp2‐ and sp3‐bonded carbons were removed by oxygen rosion for CVD hydrogenated amorphous carbon films deposited in the presence of oxygen.
Scattering‐Loss Mechanism in CaF2‐Film‐Assisted Diffusion Bonding of MgF2 Ceramics
Authors: Yen Tsung‐Fu , Chang Yen‐Hwei
Published: March 2005

The present study examined the bonding of MgF2 ceramics by hot‐pressing. A layer of CaF2 2 to ∼3 μm thick was used as an agent to decrease the bonding temperature and to avoid breaching. The principal factors influencing transmission loss were the cellular growth in the MgF2 layer, which caused about a 5% loss in transmittance, and grain growth in the films, as well as voids caused by the lattice mismatch, which, together with the formation of a recrystallization zone, caused about a 20% loss in transmittance. Grain growth in the CaF2 film was not the main cause of the scattering loss. The effect of the grain growth in the MgF2 matrix on transmittance was calculated as about 5%. Compositional fluctuations along the interfaces also had an important influence on scattering loss. For the sample that bonded at 650°C after 5 h, under a pressure of 430 kg/cm2 (42 MPa), the crystalline solubility limit of CaF2 in MgF2 was measured at ∼l7at.%.
Mechanical Properties and Microstructure of Ca2SiO4–CaZrO3 Composites
Authors: Hou Tien I , Kriven Waltraud M.
Published: March 2005

Three types of dicalcium silicate (Ca2SiO4–calcium zirconate (CaZrO3) composites were fabricated and their microstructures correlated with their mechanical properties. In the first type, Ca2SiO4 was added as a minor phase. The second type consisted of a 50 vol% Ca2SiO4‐50 vol% CaZrO3 mixture, while in the third type, CaZrO3 constituted the minor phase. Pure CaZrO3 was also studied as a control and found to have a toughness which depended on its grain size. In composites with Ca2SiO4 as the minor phase, a toughness increase was observed and found to be a function of matrix grain size. The composite with the second type of microstructure had the highest toughness of about 4.0 Mpa. m1/2, which was about double that of the monolithic CaZrO3. No evidence was found for transformation toughening by the orthorhombic (β) to monoclinic (γ) transformation in Ca2SiO4. The main toughening mechanisms identified were crack deflection and crack branching. Microstructural observations indicated the existence of weak grain boundaries in CaZrO3 agglomerates as well as weak interfaces between the two phases.
Low‐Temperature Processing of Ceramic Woven Fabric/Ceramic Matrix Composites
Authors: Shin Dong‐Woo , Tanaka Hidehiko
Published: March 2005

Two‐dimensional Al2O3 and SiC woven laminate composites, and oxide and nonoxide monolithic ceramics with 5 to 10 wt% of polycarbosilane binder, were consolidated up to 75% of TD (theoretical density) at 1150°C by the multiple impregnations of a polycarbosilane solution. The processing conditions were optimized without causing fiber damage. The near‐net‐shape composite fabricated by this process showed high reproducibility in terms of relative density and flexural strength. The mechanical properties were characterized by flexural testing with strain gauges. All of the woven laminate composites exhibited good composite‐type fracture behavior, e.g., load‐carrying capacity following maximum load. The room‐temperature flexural strength and first‐matrix cracking stress of SiC fabric/SiC composite with 73% TD were about 300 and 77 MPa, respectively.
Flat and Rising R‐Curves for Elliptical Surface Cracks from Indentation and Superposed Flexure
Authors: Krause Ralph F.
Published: March 2005

Flat and rising R‐curves, fracture resistance versus crack extension, were determined for a sintered 99%α‐silicon carbide and for a hot‐pressed composite of 25 wt% silicon carbide whiskers and alumina, respectively. The R‐curves were evaluated from a combination of measured crack lengths, which were produced over a range of Vickers indentation loading, and of measured strengths, which were correlated either with the indentation flaws or with the most severe natural flaws on flexure specimens. A published analysis of the stress‐intensity factor for a surface crack in flexure was interpreted to show that the crack front takes the form of a semiellipse where both the ratio of the minor to major radii and the configuration coefficient itself decrease with increasing crack extension. A power‐law function of the indentation load was fitted to the product of an effective configuration coefficient and the flexural strength to evaluate the R‐curves. When the configuration coefficient is assumed constant, a customary practice, the R‐curves appear to have steeper rises. The assumed constancy of the coefficient of the indentation driving force may also have an effect on R‐curves, but the effect would be much less.
Radiation Damage Effects in Alumina
Authors: Pells George Philip
Published: March 2005

The current understanding of radiation damage mechanisms in alumina produced by bombardment with energetic electrons, ions, or neutrons is reviewed with particular emphasis on the relative effects of displacement damage on the aluminum and oxygen sublattices. The displacement damage equations are presented and it is shown that a temperature dependence of damage efficiency should be introduced into displacement damage calculations. The evolution of the damage microstructure is discussed for three broad temperature ranges followed by the effects of point and aggregate defects on physical properties. These include dimensional stability, thermal conductivity, electrical conductivity, and dielectric properties. The review concludes with the effects of neutron irradiation on mechanical properties.
Infiltration of Reaction‐Bonded Silicon Nitride with Equilibrium Y‐Si‐O‐N Melt
Authors: Sheu Tzer‐Shin
Published: March 2005

An equilibrium Y‐Si‐O‐N melt was infiltrated to eliminate the open porosity of reaction‐bonded silicon nitride at 1600–1800°C. This oxynitride melt contained two equilibrium phases, a β‐Si3N4 solid phase and a liquid phase at high temperatures. Before infiltration, porous reaction‐bonded silicon nitride compacts were heat‐treated to completely transform to the β‐Si3N4 phase. After infiltration, the flexural strength of the reaction‐bonded silicon nitride material increased from 200 to 600 MPa at 25°C, from 200 to 300 MPa at 1400°C in air.
Deflocculation of Concentrated Aqueous Clay Suspensions with Sodium Polymethacrylates
Authors: Corradi Anna Bonamartini , Manfredini Tiziano , Pellacani Gian Carlo , Pozzi Paolo
Published: March 2005

The ability of sodium polymethacrylate salts with various molecular weights (1200 to 30000 g·mol−1) to deflocculate concentrated aqueous suspensions (58 wt%) of two clays, widely used for stoneware ceramic tile production, was investigated. Rheological measurements were made in distilled water or in water containing calcium ions (0 to 6 × 10−3 mol·dm−3) to evaluate the practical applications of these salts as deflocculants in the ceramic tile industry. All slips presented a non‐Newtonian behavior. Polymethacrylates with low molecular weights were the most efficient deflocculants for clay suspensions. The salts acted by an essentially electrosteric stabilization mechanism, in combination with a high sequestering ability toward flocculating cations. For their efficient deflocculating capabilities toward concentrated aqueous clay suspensions, sodium(I) polymethacrylates may be considered for practical industrial application, in particular for the preparation of liquid deflocculants.
Stress Corrosion Cracking in a Unidirectional Ceramic‐Matrix Composite
Authors: Spearing S. Mark , Zok Frank W. , Evans Anthony G.
Published: March 2005

A study of matrix cracking in a unidirectional ceramic‐matrix composite under static loading conditions has been conducted. The evolution of crack density with time has been measured using both flexure and uniaxial tension tests. Subcritical cracking has been observed at stresses below that required to develop matrix cracks in short‐duration, monotonic loading tests. Furthermore, a relatively high final crack density has been observed following extended periods (∼106 s) under static load. A fracture mechanics analysis applicable to subcritical crack growth has been developed and used successfully to model the evolution of matrix cracking with time and applied stress. The model incorporates the properties of the matrix, fibers, and interfaces, as well as the residual stress and the initial flaw distribution in the matrix.
Synthesis of Single‐Crystal Barium Titanium Isopropoxide Complex to Form Barium Titanate
Authors: Suyama Yoko , Nagasawa Mitsuru
Published: March 2005

Single‐crystal barium titanate double alkoxide complex with a composition presumed to be BaTi(OCH(CH3)2)6·xC6H6 was successfully synthesized in this work. The crystal was converted to BaTiO3 at 100°C. The preparation of the Oxides via single crystals of metal alkoxides has some advantages over the well‐known sol‐gel method in that it results in oxides with well‐defined and homogeneous compositions at the atomic level and in fine particle sizes, smaller than 50 nm.
Ambient‐Temperature Mechanical Response of Alumina–Fluoromica Laminates
Authors: King Todd T. , Cooper Reid F.
Published: March 2005

Flexural delamination experiments were used to evaluate the mechanical performance of thermochemically stable alumina–fluoromica laminates. Hot‐pressed, precracked laminate specimens, in which two MgAl2O4‐spinel‐coated alumina substrates were separated by a thin layer of fluorophlogopite (KMg3(AlSi3)O10F2), were tested in fourpoint flexure at room temperature. Two types of mechanical response were observed: steady‐state delamination and brittle failure. Microstructural analysis showed that the delamination response was associated with fine (≤5 μm) grains of the mica; the brittle response occurred when the mica interphase consisted of large (>30 μm) grains that bridged the interphase. The steady‐state strain‐energy release rate (Gss) measured on the graceful, delaminating beams was 9.1 ± 0.4 Jm–2 for randomly oriented ∼ 5–μm grains but only 2.8 ± 0.2 Jm–2 for ∼1–μm grains that were aligned with easy‐cleavage planes parallel to the laminate interfaces. The results suggested that debonding of the specimens occurred via cleavage of the mica grains. Observation of delamination cracks confirmed this point: propagation occurred within the fluoromica interphase rather than along the spinel/alumina or spinel/fluorophlogopite interfaces. The mechanical feasibility of laminate specimens without the protective spinel coating on the substrate containing the notch was also tested to address an issue related to the preparation of alumina fiber/mica interphase/alumina matrix composites. The delamination response again occurred for the case of a fine‐grained mica interphase.
Kinetics of the Reduction of Carbon/Alumina Powder Mixture in a Flowing Nitrogen Stream
Authors: Chen Hsi‐Kuei , Lin Chun‐I , Lee⋆ Chiapyng
Published: March 2005

The reduction of carbon/alumina powder mixture in a flowing nitrogen stream was investigated by weight loss measurement and X‐ray diffraction. Experimental results indicated that the reaction rate could be increased by increasing the nitrogen flow rate, weight ratio of C/Al2O3, or reaction temperature. The rate was also found to be increased by decreasing the sample size, the grain sizes of carbon or aluminum oxide, or initial bulk density. Empirical expressions of consumption rates of Al2O3 and C as well as production rate of AIN have been determined.
Metalorganic Chemical Vapor Deposition of Ferroelectric Pb(Zr,Ti)O3 Thin Films
Authors: Peng Chien H. , Desu Seshu B.
Published: March 2005

Ferroelectric Pb(ZrxT1–x)O3, films were successfully and reproducibly deposited by both hot–wall metalorganic chemical vapor deposition (MOCVD) and cold‐wall MOCVD. One of the important problems associated with the MOCVD techniques is the selection of ideal precursors. After an intensive investigation for the most suitable precursors for MOCVD PZT films, the safe and stable precursors, namely lead tetramethylheptadione [Pb(thd)2], zirconium tetramethylheptadione [Zr(thd)4], and titanium ethoxide [Ti(OEt)4], were chosen. The films were deposited at temperatures as low as 550°C and were single‐phase perovskite in the as‐deposited state. Also, the films were smooth, specular, crack‐free, and uniform, and adhered well to the substrates. The stoichiometry of the films can be easily controlled by varying the individual precursor temperature and/or the flow rate of the carrier gas. Auger electron spectroscopic (AES) depth profile showed good compositional uniformity through the thickness of the films. The AES spectra also showed no carbon contamination in the bulk of the films. As‐deposited films were dense and showed uniform and fine grains (≅0.1 μm).The optical properties of the films on the sapphire disks showed high refractive index (n= 2.413) and low extinction coeflicient (k= 0.0008) at a wavelength of 632.8 nm. The PZT (82/18) film annealed at 600°C showed a spontaneous polarization of 23.3 μC/cm2 and a coercive field of 64.5 kV/cm.
Structure and Properties of (Ba,Ca) TiO3 Ceramics Prepared Using (Ba,Ca)TiO3 Ceramics I, Crystallographic and Microstructural Studies
Authors: Tiwari Vidya S. , Singh Neelam , Pandey Dhananjai
Published: March 2005

Ba1–xCaxTiO3 powders have been prepared using a novel route involving solid‐state reaction in a mixture of Ba1–xCaxCO3 and TiO2. The Ba1–xCaxCO3 precursors used in this method were prepared by a chemical coprecipitation route to ensure a uniform supply of Ba and Ca ions during thermochemical reaction with TiO2. The compositional homogeneity of Ba1–xCaxTiO3 powder prepared by such a semiwet route is compared with those prepared by the conventional dry route, involving thermochemical reaction in a mixture of BaCO3, CaCO3, and TiO2, using principles of X‐ray line broadening. It is shown that the powders obtained by the semiwet route possess better compositional homogeneity, over a length scale of 1800 Å corresponding to the coherently scattering domain size, than those prepared by the conventional dry route. Microstructural studies have revealed grain sizes of the order of 1 μm and several micrometers, respectively, for ceramics prepared by the semiwet and conventional dry routes. The solid solubility limit of Ca in Ba1–xCaxTiO3 ceramics fired at 1300°C is 16 mol% for samples prepared by the semiwet route while conventionally processed ceramics had a Ca solid Solubility limit of 12 mol% at the same temperature.
Structure and Properties of (Ba,Ca)TiO3 Ceramics Prepared Using (Ba,Ca)CO3 Precursors: II, Diffuse Phase Transition Behavior
Authors: Tiwari Vidya S. , Pandey Dhananjai
Published: March 2005

Ferroelectric phase transition in (Ba,Ca)TiO3 ceramics has been investigated using dielectric measurements. It is shown that samples prepared using (Ba,Ca)CO3, precursors exhibit diffuse phase transition (DPT) while those prepared by the conventional route show BaTiO3‐type sharp transition. The diffuseness of the phase transition is found to increase with increasing calcium content. The critical exponent for the dielectric stiffness (1/ε′) departs significantly from the mean field value of unity and is shown to increase from 1.21 to 1.86 with increasing calcium content. Arguments are advanced to show that the DPT behavior in these ceramics is inexplicable in terms of either the internal stress model due to the absence of 90° domains in fine‐grain ceramics or the compositional inhomogeneity model. It is proposed that DPT behavidr is intrinsic to the system and is intimately linked with the appearance of polar phase in the critical regime whose width is shown to increase with increasing Ca content.
Microstructure Control of Silicon Nitride by Seeding with Rodlike β‐Silicon Nitride Particles
Authors: Hirao Kiyoshi , Nagaoka Takaaki , Brito Manuel E. , Kanzaki Shuzo
Published: March 2005

The effect of seeding on microstructure development and mechanical properties of silicon nitride was investigated by the use of morphologically regulated rodlike β‐Si3N4singlecrystal particles with a diameter of 1 μm and a length of 4 μm as seed crystals. Silicon nitride with a bimodal microstructure was fabricated under a relatively low nitrogen gas pressure of 0.9 MPa owing to the epitaxial growth of β‐silicon nitride from the seed particles. Grain growth from seeds followed the empirical equationDn–D0n=kt, with growth exponents of 3 and 5 for thec‐axis direction and thea‐axis direction, respectively, being analogous to the kinetics of matrix grain growth. By seeding morphologically regulated particles, fracture toughness of silicon nitride was improved from 6.3 to 8.4–8.7 MPa·m1/2, retaining high strength levels of about 1 GPa.
Low‐Temperature Phase Equilibria by the Flux Method and the Metastable–Stable Phase Diagram in the ZrO2–CeO2 System
Authors: Yashima Masatomo , Takashina Hiroyuki , Kakihana Masato , Yoshimura Masahiro
Published: March 2005

Low‐temperature phase equilibria ranging from 1000° to 1200°C in the ZrO2–CeO2 system were investigated by annealing compositionally homogeneous ZrO2–CeO2 solid solutions in a Na2B2O7.1 NaF flux. The 5 mol% CeO2 samples decomposed into monoclinic (m) and tetragonal (t) phases during annealing at 1100°2 and 1120°C, and the t‐phase transformed diffusionlessly into monoclinic (m′) symmetry during quenching. A eutectoid reaction, t→ (m + c), was confirmed to occur at 1055°± 10°C, where the equilibrium compositions of the t‐, m‐, and c‐phases were 11.2 ± 2.8, 0.9 ± 0.9, and 84 ± 1 mol% CeO2, respectively. The equilibrium phase boundaries were almost independent of the annealing time and/or the flux:sample ratio, which indicates that the flux accelerates the reaction rate withouts affecting the equilibration. The previous data are discussed using metastable–stable phase diagrams. The discrepancies of the low‐temperature phase diagram in the literature are attributable to either regarding the metastable phase boundaries as stable ones or ignoring the sluggish kinetics.
BaO–1/2Y2O3–CuOx Eutectic Melting in Air
Authors: Wong‐Ng Winnie , Cook Lawrence P.
Published: March 2005

Eutectic melting in the BaO–1/2Y2O3–CuOx system has been investigated, using three different compositions. Thermal events up to 1100°C were studied by means of differential thermal analysis/thermogravimetric analysis (DTA/TGA). Phase formation and compositions of melts were examined using powder X‐ray diffraction and scanning electron microscopy/energy‐dispersive X‐ray spectrometry (SEM/ EDS) of quenched experiments. The eutectic occurs at 923°± 10°C in CO2‐ and H2O‐scrubbed air, and the eutectic melt has a Ba:Y:Cu cation ratio of 23.2(±0.5): 0.5(±0.1):76.3(±1.0). The oxygen content of the quenched eutectic melt, as determined by hydrogen reduction, indicated that nearly all of the copper in the melt was presentas Cu+. A topological sequence, based on the X‐ray results, is presented which describes the events immediately following melting.
Effects of Fiber Volume Fraction on Mechanical Properties of SiC‐Fiber/Si3N4‐Matrix Composites
Authors: Xu Hockin H. K. , Ostertag Claudia P. , Braun Linda M. , Lloyd Isabel K.
Published: March 2005

The effects of fiber volume fraction on composite mechaniacl properties were examined in SiC‐fiber‐reinforced Si3N4 composites fabricated in our laboratories. Fiber volume fraction was found to have significant effects on important composite properties including failure mode, ultimate strength, matrix‐cracking stress, fiber–matrix interfacial shear stress, and work‐of‐fracture. The composite mechanical properties were improved with increasing fiber volume fraction. However, when the fiber volume fraction was sufficiently large, the composite ultimate strength was degraded. This was related to fiber strength loss as a result of fiber damage from contact with surrounding fibers and abrasive matrix particles during hot pressing.
Predictability of the Mechanical Properties of Inclusion‐Containing Ceramics
Authors: Lutz Ekkehard H.
Published: March 2005

The incorporation of inclusions into a brittle matrix can change its properties significantly. The quantitative change of strength, toughness, and retained strength after severe thermal shock and Vickers indentation damage in so‐called “duplex ceramics” can be reasonably well predicted by relating these properties to the empirically derived internal stress‐intensity factor K1. In the present paper the relationship between K1 and three other recently measured properties—flaw resistance behavior, KR‐curve behavior, and Stress–Strain behavior—is investigated. In addition, the strength‐degrading influence of pressurized inclusions observed in “duplex ceramics” is compared to the strength reduction and enhancement reported by other authors for composites containing a glass matrix and glass and alumina inclusions. An attempt is made to identify the important parameters which control the strength development in duplex structures.
AlN Matrix Composites Fabricated via an Infiltration and Reaction Approach
Authors: Aghajanian Michael K. , Biel John P. , Smith Russell G.
Published: March 2005

Novel AlN‐based ceramic matrix composites were produced using a two‐step infiltration and reaction process. Molten Al alloy was infiltrated without pressure into preforms containing Si3N4 particles (mixed α and β phases). The resultant composites were heat‐treated, causing hard phases (AlN and Si) to form by reaction of the soft Al with Si3N4. The ceramic content of the final composites was higher than that of the original preform due to volume changes that occurred during the reaction heat treatment. Initial characterization of mechanical properties demonstrated that the composites had hardnesses, Young's moduli, and compresive strengths that compared favorably with traditionally processed AlN. Flexural strength measurements above the melting point of Si indicated that the composites possessed an interconnected skeleton of ceramic.
Numerical Model of Polydispersed Silica Grain Dissolution in Glass Melting Furnaces
Authors: Ungan Aydin , Payli Resat U. , Balkanli Batu
Published: March 2005

The objective of this paper is to report on a numerical methodology addressing the defect problem in glass‐melting furnaces. Specifically, given a furnace, through use of the proposed models one can predict the individual grain dissolution behavior and global size distribution curves of the undissolved silica grains at every point in the glass melt and, of course, in the final glass. Through some illustrative results, the utility of the proposed models is illustrated.
Preparation of Uniform Submicrometer Particles of Calcium Titanate and Lead Niobate by Replacement Reactions
Authors: Kim Myung Jin , Matijević Egon
Published: March 2005

Uniform submicrometer particles of calcium titanium peroxo‐hydroxide were prepared by adding calcium salts to the solution of peroxotitaniun. species chelated with nitrilotriacetic acid (NTA). Similarly, corresponding leadniobium compodnd of [Pb]/[Nb] = 0.75 was obtained by introducing lead salts into the solution of chelated niobium complex with ethylenediaminetetraacetic acid (EDTA) at pH 3. Calcium ions in solutions containing a mixture of NTA‐chelated Pb and NTA‐chelated Nb complexes also caused the precipitation of lead niobium oxo‐hydroxide of [Pb]/[Nb] = 1.5 at pH 9. The so‐obtained precursor powders were converted on calcination to the corresponding composite oxides. The mechanisms of these reactions were suggested, based on the chemical, thermal, and X‐ray diffraction analyses.
Rheology and Particle Packing of Chem‐ and Phys‐Adsorbed, Alkylated Silicon Nitride Powders
Authors: Kramas Thomas , Lange Fred. F.
Published: March 2005

Three alcohols with extended carbon chain lengths between ∼1 and 2 nm were chem‐adsorbed on a Si3N4 powder by reacting with hydroxyl surface groups at temperatures 200°C. Slurry rheology, particle packing density, and body rheology were determined for toluene and dodecane slurries formed with these chem‐adsorbed powders. These same properties were determined for slurries where the alcohol was simply added, but not reacted with the powder (phys‐adsorbed powders). The viscosities of chem‐adsorbed slurries are shear‐thinning with longer chains producing lower viscosities at a given shear rate. The relative density of powder compacts produced by pressure filtration (10 MPa) was high (∼0.60) for octadecanol and dodecanol‐reacted powders, and lower (∼0.50) for the octanol‐reacted powder. When a sufficient amount > 10 times that required for chem‐adsorption) of the same alcohol was simply added to the unreacted Si3N4 slurry system, the phys adsorbed slurries exhibited similar rheological behavior as the chem‐dsorbed slurries, but, unlike chem‐adsorbed slurries, their packing densty was lower, and their slurries were destabilized by water vapor. Stress relaxation experiments showed that bodies formed with the octadecanol chem‐adsorbed powders were plastic after consolidation, whereas phys‐dsorbed bodies were brittle (fractured before flow). All evidence suggests that the short‐chained alkyl groups are steric “stabilizers” at small interparticle distances and thus prevent the particles from making surface‐surface contact in common organic liquids; i.e., they produce a short‐range interparticle, repulsive potential. Chem‐adsorbed molecules, but not phys‐adsorbed molecules, persist during particle packing and in moist environments.
Morphology and Crystallography of Aluminum Nitride Whiskers
Authors: Caceres Pablo G. , Schmid Herbert K.
Published: March 2005

Highly pure and dense AIN whiskers were produced by carbothermal reaction of an alumina and a carbon source at 1800°C under flowing nitrogen. The whiskers grew via a VLS mechanism where there is a strong interaction between the liquid catalyst and the substrate/support, resulting in a complete lack of droplets at the whisker tips. The whiskers can be described as long and straight single crystals, free of planar defects, having a diameter of 2–30 μm and a length of 0.5–10 cm. They exhibited two different morphologies, planar and serrated. SADPs indicate that their lateral surfaces fall in a great circle of a stereographiv projectiov, joining two pyramidal poles such as the (1101)–(1011). It is suggested that branching in these whiskers occurs by keeping active one of the pyramidal planes and activating a new one.
Superconductivity in the (Yba2Cu3)1−xNaxO7−δ System
Authors: Chen Christina H. , Kumar Binod
Published: March 2005

The (YBa2Cu3)1−xNaxO7–δ system in the range of x= 0–0.8 was investigated. Experimental data suggest that the sodium doping with x 0.26 does not affect the critical transition temperature Tc, and the crystal structure maintains the orthorhombic lattice with a slightly smaller unit cell. However, sodium doping increases the sintering and grain growth kinetics, resulting in a higher superconducting phase volume and an enhanced Meissner effect. It also lowers the processing temperaturel. The experimental data also suggest that the sodium atoms diffuse into the superconducting YBa2Cu3O7−δ crystallites, which stabilizes the orthorhombic phase. The transition temperature (ortho‐rhombic to tetragonal) in sodium‐doped materials increases with the increasing concentration of sodium.
Oxidation Behavior and Flexural Strength of Aluminum Nitride Exposed to Air at Elevated Temperatures
Authors: Kim Hyoun‐Ee , J. Moorhead Arthur
Published: March 2005

The oxidation behavior of a sintered aluminum nitride containing 3 wt% Y2O3 as a sintering aid was investigated. Samples were exposed to air at elevated temperatures for times up to 100 h. The weights of the samples were continuously monitored during exposure at various temperatures and humidity levels. The effects of oxidation on room‐temperature flexural strength were also determined, and correlated to the observed weight changes of the samples. At temperatures 1200°C, linear weight gains were observed. However, at temperatures above 1200°C, the weight gains became parabolic with respect to exposure time. The oxidation rates were significantly increased by water vapor in the air. The oxidation products were found by X‐ray analysis to be a mixture of Al2O3 and 5A12O3·3Y2O3. The oxide layer formed on the surface was severely cracked because of the thermal expansion mismatch between the oxide layer and the substrate. The cracks initiated in the oxide layer and propagated into the substrate, resulting in severe reduction in the room‐temperature flexural strength of the material. When exposed to ambient air for more than 50 h at temperatures greater than 1100°C, the strengths of the samples decreased to less than half that of the as‐received material.
Stoichiometry of Fluorotopaz and of Mullite Made from Fluorotopaz
Authors: Moyer John R. , Rudolf P. R.
Published: March 2005

Mixtures of alumina and silica containing 68–78 wt% alumina react above 600°C with 0.5 mol of SiF4 per mole of alumina to form fluorotopaz. High‐resolution X‐ray powder diffraction data were used to determine very accurate cell parameters of fluorotopaz as a function of 1 wt% compositional increments. The samples containing 69–76 wt% A12O3 were found to have a linear cell parameter relationship. Compositions outside that range show discontinuities in the cell parameters, indicating solid solution behavior between 69 and 76 wt% alumina. Within this range the composition of fluorotopaz may be written 2A12O3·xSiO2· SiF4 where 1.07 x 1.53. Pyrolysis of all compositions of fluorotopaz solid solution yields mullite whiskers containing 76.1 wt% alumina (65.2 mol%).
Microwave Joining of Yttrium‐Based Superconductor and Its Microstructure Features
Authors: Cai Jie , Xie Xiao‐ming , Zhang Liu‐wan , Li Xiang‐ting , Tian Yong‐lai , Chen Ting‐guo , Guo Jing‐kun
Published: March 2005

A new method for joining superconductors by microwave heating has been developed. Bars of microwave‐joined, Y‐based ceramic superconductors show good mechanical integrity. After they are postannealed, the samples show superconducting transition at around 89.7 K. Meanwhile, the backscattered electron images of the joined region after postannealing in air at 960°C for 15 h show a higher density in the joined region, which has larger pores that may be caused by an accumulation of small pores during post‐heat‐treatment.
Microstructure Characterization of Gas‐Pressure‐Sintered β‐Silicon Nitride Containing Large β‐Silicon Nitride Seeds
Authors: Hirosaki Naoto , Akimune Yoshio , Mitomo Mamoru
Published: March 2005

Fine β‐Si3N4 powders with or without the addition of 5 wt% of large β‐Si3N4 particles (seeds) were gas‐pressure sintered at 1900°C for 4 h using Y2O3 and Al2O3 as sintering aids. The microstructures were examined on polished and plasmaetched surfaces. These materials had a microstructure of in situ composites with similar small matrix grains and different elongated grains. The elongated grains in the materials with seeds had a larger diameter and a smaller aspect ratio than those in the materials without seeds. A core/rim structure was observed in the elongated grains; the core was pure β‐Si3N4 and the rim was β‐SiAION. These results show that the large β‐Si3N4 particles acted as seeds for abnormal grain growth and the rim was formed by precipitation from the liquid containing aluminum.