bluuebunny/crossref_metadata_2025 huggingface dataset:
Nucleation Energy Barriers for Volume‐Conserving Shape Changes of Crystals with Nonequilibrium Morphologies
Authors: Rohrer Gregory S. , Rohrer C. Lane , Mullins William W.
Published: July 2010

This paper quantifies the nucleation energy barrier that must be overcome if a faceted, defect‐free particle with a nonequilibrium morphology is to change shape by intraparticle transport. Two types of crystals are considered: those whose equilibrium form is a truncated sphere and those whose equilibrium form is a cube. Numerical estimates show that, for a particle near equilibrium, the barrier becomes insurmountable for a facet larger than a few tens of nanometers. For nonequilibrium shapes where material must be transferred from faceted surfaces to uniformly curved surfaces to reach the equilibrium shape, the facets enlarge without a nucleation barrier (at a rate limited by diffusion or surface attachment kinetics) until they reach a fraction of their equilibrium size that is typically between 0.5 and 0.75. At this point, a significant barrier is encountered that, in the absence of step producing defects, prevents the particle from continuing toward equilibrium. For nonequilibrium shapes where material must be transferred to faceted surfaces from other parts of the crystal for it to reach the equilibrium shape, significant energy barriers for the nucleation of new layers persist even when the shape is far from equilibrium. Predictions from our model are compared to experimental observations reported by other researchers.
Local Viscosity of Si‐O‐C‐N Nanoscale Amorphous Phases at Ceramic Grain Boundaries
Authors: Pezzotti Giuseppe
Published: July 2010

Internal friction characterization has been used to quantitatively assess the viscosity characteristics of Si‐O‐C‐N glasses segregated to nanometer‐sized grain boundaries of polycrystalline Si3N4 and SiC ceramics. A relaxation peak of internal friction, which arises with rising temperature from the viscous sliding of glassy grain boundaries, was systematically collected and analyzed with respect to its shift upon changing the oscillation frequency. As a result of such an analysis, both activation energy for viscous grain‐boundary flow and inherent viscosity of the intergranular glass film could be quantitatively evaluated. Two main features are shown: (i) the presence of N and/or C greatly affects the viscosity characteristics of SiO2 phases at Si3N4 and SiC grain boundaries; and (ii) the internal friction method has potential as a unique experimental tool for understanding the local properties of nanoscale amorphous phases in new ceramic materials.
Transverse Thermal Conductivity of Thin C/SiC Composites Fabricated by Slurry Infiltration and Pyrolysis
Authors: Berbon Min Z. , Dietrich Donald R. , Marshall David B. , Hasselman D. P. H.
Published: July 2010

Thin C/SiC composites were fabricated by infiltrating a woven carbon fiber fabric with a slurry of SiC powder and polymer precursor for SiC, followed by heat treatment for pyrolysis. The effects of heat treatment parameters on the crystallization of the polymer‐derived SiC, the composite microstructure, and the transverse thermal properties were assessed. Whereas composites heat‐treated at 1000°C were crack‐free and nearly fully dense, composites that were subjected to further multiple reinfiltration and heat treatment cycles at 1600°C developed porosity and cracking. However, the transverse thermal conductivity was increased significantly by the higher‐temperature heat treatment, to values higher than that of a composite with a chemical‐vapor‐infiltration SiC matrix and the same fiber reinforcement.
A Process for Cf/SiC Composites Using Liquid Polymer Infiltration
Authors: Rak Zbigniew S.
Published: July 2010

A continuous carbon fiber/silicon carbide matrix composite material has been produced by a low‐cost process. In this process the space in a two‐dimensional carbon fiber preform is filled with a SiC powder by a pressure infiltration method. High particle packing densities are achieved within the fiber preform in this way. The compact body is heat‐treated at 400°C to form a porous framework, which is then infiltrated with a liquid preceramic polymer, CerasetTM SN. Subsequently the infiltrated polymer is pyrolyzed in argon at 1300°C. The microstructure of the final composite is characterized, and mechanical properties of these composites are discussed.
Electrical Characterization of Polymethylsiloxane/MoSi2‐Derived Composite Ceramics
Authors: Cordelair Jens , Greil Peter
Published: July 2010

High‐temperature‐resistant ceramics in the system Mo‐Si‐O‐C were prepared from polymethylsiloxanes loaded with MoSi2 filler. Irreversible structural rearrangement reactions in the polymer‐derived Si‐O‐C matrix phase and at the filler‐matrix interface during heat treatment were detected by in situ electrical resistance measurements and impedance spectroscopy. At low temperatures a large difference in thermal expansion between the polymer and the filler resulted in an increase of resistance from 0.05 to 560 Ω·cm when the temperature was raised from 20° to 680°C. An increase of filler particle contact distances resulted in a high positive temperature coefficient (PTC200°C= 4.64 K−1). At high temperatures carburization reaction of the filler on the particle surface resulted in contact formation which gave rise to a resistance lower than 10−3Ω·cm above 1000°C.
Theoretical Investigation of Heat Transfer in Glass Forming
Authors: Viskanta Raymond , Lim Jong Mook
Published: July 2010

A theoretical study to investigate internal heat transfer in glass undergoing cooling between glass and mold, as well as plunger, during and after pressing, is described. A thermal model has been formulated to simulate the cooling. The heat‐transfer analysis accounts for the spectral nature of radiation in glass, the dependence of the thermophysical properties of glass on temperature, and the contact heat transfer between and after pressing, as well as subsequent cooling. Heat exchange between glass and mold by contact conduction across a very small gap and that by thermal radiation are considered separately. Numerical solutions have been obtained for typical conditions simulating symmetric and nonsymmetric cooling, and the results obtained are presented and discussed. During the dwell time, thermal‐contact conduction between glass and mold is the dominant mechanism for heat extraction from glass. Results show that radiation from the surface of the glass plays a relatively small role in the heat extraction from the glass, but that radiation from the interior of the glass is much more significant.
Low‐Frequency Measurements on the Grain‐Boundary Internal Friction Peak in Chlorine‐Doped Silicon Nitride
Authors: Pezzotti Giuseppe
Published: July 2010

A procedure is shown to quantitatively analyze the morphology of the internal friction peak resulting from grain‐boundary sliding. A Si3N4 polycrystal containing chlorine‐doped SiO2 at grain boundaries is selected as a model system for discussing chemical (e.g., anion) gradients at glassy grain boundaries. In this model material, grain boundaries lodging Cl− anions show nonuniform thickness characteristics, which suggests a non‐negligible dependence of the intergranular SiO2‐network structure upon grain misorientation. Both chemical and microstructural inhomogeneities existing in a polycrystalline ceramic body can result in peak broadening. The key for separating broadening contributions of chemical gradients from grain‐size/morphology distributions resides in analyzing the peak‐width change upon damping frequency. Groups of grain boundaries with different chemical characteristics may produce broadening because different peak components are generated that obey a spectrum of activation energies. On the other hand, microstructural inhomogeneities obey a single activation energy, but they generate a distribution of relaxation times. As a result, when a chemical gradient is present at grain boundaries, the peak may shift upon changing damping frequency with obeying a true activation energy, but its width increases with decreasing damping frequency. When peak broadening results only from microstructural inhomogeneities, the peak width is independent of damping frequency.
Preparation of Titanium Nitride: Microwave‐Induced Carbothermal Reaction of Titanium Dioxide
Authors: Peelamedu Ramesh D. , Fleming Mary , Agrawal Dinesh K. , Roy Rustum
Published: July 2010

Carbothermal reduction and nitridation of TiO2 was performed in a 2 kW, 2.45 GHz microwave furnace. Carbon, generally added for the removal of oxygen from TiO2 lattice also served as the low‐temperature susceptor in these experiments. At temperatures >1200°C, the mixtures started reacting vigorously (self‐burn), which was never observed with the respective pure compounds. In the self‐burning state, the minimum duration required for complete titanium nitride transformation was ∼20 min with stoichiometric amounts of carbon. With excess C, the transformation duration dropped to just 1 min. CO removal and subsequent Ni fixation occur more directly in microwave‐induced reactions than in conventional nitridation procedures. Intermediate formation of successive Magneli phases (Ti2nO2n−1) was not found in the microwave‐induced reactions. The combination of microwave processing and combustion makes this route of economical interest.
Synthesis, Characterization, and Processing of Cordierite‐Glass Particles Modified by Coating with an Alumina Precursor
Authors: Mei Sen , Yang Juan , Monteiro Regina , Martins Rodrigo , Ferreira José M. F.
Published: July 2010

The surfaces of cordierite and glass particles were modified by coating them with an alumina precursor using a precipitation process in the presence of urea. Scanning electron microscopy (SEM), high‐resolution transmission electron microscopy, X‐ray diffraction, electrophoresis, and rheological measurements were used to characterize the coated powders. SEM and transmission electron microscopy morphologies of the coated powders revealed that amorphous and homogeneous coatings have been formed around the particles. The morphology of the coated powders showed a coiled wormlike surface. The coating Al2O3 layer dominated the surface properties of the coated glass and cordierite powders. The influence of the coating layer on the processing ability of cordierite‐based glass‐ceramics substrates by tape casting was studied in aqueous media. It could be concluded that the coating of the powders facilitates the processing and yields green and sintered tapes with denser, more homogeneous microstructures compared with the uncoated powders.
Microstructure and Magnetic Properties of (Co‐Fe)‐Al‐O Thin Films
Authors: Ikeda Kenji , Kobayashi Kazuyoshi , Fujimoto Masayuki
Published: July 2010

(Co‐Fe)‐Al‐O soft magnetic thin films were fabricated by cosputtering Co‐Fe alloy and Al2O3 ceramic targets using an inductively coupled RF (radio frequency) sputtering system. The microstructure and magnetic properties of various (Co‐Fe)/(Al‐O) thin‐film compositions were systematically characterized. A specimen with the composition (Co72Fe17Al11)‐O exhibited excellent high‐frequency magnetic properties, with a value >300 for the real part of the permeability and >2 GHz for the self‐resonant frequency. High‐resolution transmission electron images revealed nanometer‐order Al2O3 amorphous phases separating the Co‐Fe alloy nanograins. The excellent high‐frequency magnetic properties were derived from the particular microstructure of these thin films.
Nucleation and Growth of Mullite Whiskers from Lanthanum‐Doped Aluminosilicate Melts
Authors: Regiani Inacio , Magalhães Washington L. E. , De Souza Dulcina P. Ferreira , Paiva‐Santos Carlos O. , De Souza Milton Ferreira
Published: July 2010

Nucleation and growth of mullite whiskers in the La2O3−Al2O3−SiO2 system were investigated in the 1500°–1700°C temperature range. A differentiaC Bhermal analysis (DTA) showed that the mullitization temperature decreases from 1350°C to 1240°C as a result of lanthania doping. In the temperature range of 1250°–1500°C, most of the mullite grains have an Al2O3/SiO2= 1.5 composition throughout the ceramic body; however, from 1400°C upward, the number of anisotropic grains with the Al2O3/SiO2= 1.3 composition begins to increase. The concentration of alumina in the composition of the grain‐boundary phase decreases as firing temperatures increase. At temperatures > 1500°C, alumina grains and whiskers grow on the internal and external surfaces of the ceramic body with the characteristic Al2O3/SiO2= 1.3 composition. Removal of the mullite whisker layer by acid attack revealed an alumina‐rich, rosace‐like patterned microstructure correlated with the process of whisker nucleation and growth. In the early stages, whisker growth rates were found to be near 60 μm/h. Experimental evidence pointed to nucleation inside the thin glass layer on the external surface.
Reactive Hot Pressing of Alumina‐Molybdenum Disilicide Composites
Authors: Fahrenholtz William G. , Loehman Ronald E. , Ewsuk Kevin G.
Published: July 2010

Al2O3‐MoSi2 composites were prepared by reactive hot pressing using molybdenum, aluminum, and mullite powders as precursors. The Gibbs free energy was highly negative for the composite‐forming reaction, which indicated that the products were stable relative to the reactants. After the reaction, the composites had high relative density, ∼96%. Based on the composite‐forming reaction, the composites should have contained 18 vol% MoSi2 in an Al2O3 matrix. Scanning electron microscopy revealed that the MoSi2 inclusions were elongated, with an average thickness of ∼5 μm and inclusion lengths that ranged from 5 to 50 μm. Average composite strength was 467 MPa, and toughness was 3.7 MPa·m1/2.
Thermodynamic and Structural Properties of Sodium Lithium Niobate Solid Solutions
Authors: Pozdnyakova Irina , Navrotsky Alexandra , Shilkina Lidia , Reznitchenko Larisa
Published: July 2010

Thermodynamics of the Na1−xLixNbO3 system is investigated by high‐temperature drop‐solution calorimetry in molten 3Na2O–4MoO3 solvent at 973 K. Standard molar enthalpies of formation are derived. The estimated heats of transition between hypothetical and stable structures, lithium niobate and perovskite for NaNbO3 and vice versa for LiNbO3 are −6 kJ/mol and −10 kJ/mol, respectively. X‐ray diffraction studies at room temperature showed for 0 ≤x≤ 0.14 there are three phases based on different ordering of the perovskite type lattice: orthorhombic with a quadrupled reduced perovskite cell at 0 ≤x≤ 0.02, orthorhombic with a doubled reduced perovskite cell at 0.015 ≤x≤ 0.14, and rhombohedral at 0.08 ≤x≤ 0.13. There are two two‐phase (morphotropic) regions with coexistence of the two orthorhombic phases at 0.015 ≤x≤ 0.02 and with the second orthorhombic phase coexisting with the rhombohedral phase at 0.08 ≤x≤ 0.13. A reproducible anomaly in specific heat at ∼600 K, not reported previously, has been observed in pure NaNbO3. Heat‐capacity measurements confirm a phase transition at 553 K for 0.07 ≤x≤ 0.09. With increasing lithium concentration, a gradual disappearance of high‐temperature phase transitions associated with tilting of oxygen octahedra has been observed.
Piezoresistivity of Hi‐Nicalon Type‐S Silicon Carbide‐Based Fiber
Authors: Kishimoto Akira , Toyoguchi Ginjiro , Ichikawa Hiroshi
Published: July 2010

Piezoresistivity effect of a silicon carbide‐based ceramic fiber (Nicalon family) was evaluated when a single fiber was buried at the tensile plane in a plastic rectangular bar subjected to bending to apply a stable tensile strain. Electrical resistivity change during the bending was monitored by a digital high‐resistance meter. From the fracture strain, Young's modulus, and the resistivity change, the piezoresistance coefficient is calculated to be 1.6 × 10−11 m2/N, which is in agreement with the reported value for silicon carbide single crystals and our own reported value for polycrystalline silicon carbide ceramics.
Biomimetic Hydroxyapatite Coating on Metal Implants
Authors: Habibovic Pamela , Barrère Florence , Van Blitterswijk Clemens A. , de Groot Klaas , Layrolle Pierre
Published: July 2010

The combination of the high mechanical strength of metals with the osteoconductive properties of calcium phosphates make hydroxyapatite coatings on titanium implants widely used in orthopedic surgery. However, the most popular coating method, plasma spraying, exhibits some important drawbacks: the inability to cover porous implants and to incorporate biologically active agents, delamination, and particle release. The aim of this study was to elaborate a dense, strong, and thick calcium‐phosphate coating on titanium and porous‐tantalum implants using a two‐step biomimetic procedure. In the first step, the implants were soaked in a solution that was 5 times more concentrated than regular simulated body fluid (SBF‐A solution). A thin but uniform amorphous calcium‐phosphate coating was deposited on the metal. Then, the implants were immersed in the SBF‐B solution, which had a similar composition as the SBF‐A solution, but with decreased contents of crystal growth inhibitors (i.e., Mg2+ and HCO3−). This resulted in the fast precipitation of a 30 μm thick crystalline calcium‐phosphate coating. The pH of the SBF‐B solution and the thickness of the crystalline coating layer were studied as a function of time. The Fourier transform infrared spectra and X‐ray diffraction patterns showed that this new coating closely resembles bone mineral. Our biomimetic coating should facilitate rapid bone formation around the implant, reducing therewith the patient's recovery time after surgery.
Formation Mechanism of Hydrous Zirconia Particles Produced by Hydrolysis of ZrOCl2Solutions: IV, Effects of ZrOCl2Concentration and Reaction Temperature
Authors: Matsui Koji , Ohgai Michiharu
Published: July 2010

The formation rate, crystal phase, and primary and secondary particle sizes of hydrous zirconia particles produced under the hydrolysis conditions of various ZrOCl2concentrations (0.1–0.4 mol/dm3) and reaction temperatures (343–373 K) were measured to investigate the effects of ZrOCl2concentration and reaction temperature on the formation process. Chemical analyses and X‐ray diffraction measurements revealed that hydrous zirconia particles synthesized under all of the hydrolysis conditions were similar to those observed in monoclinic crystalline zirconia, except that the crystal structure changed as the chlorine content increased under the synthesizing conditions of 0.4 mol/dm3 and ≤363 K. The primary particle size of monoclinic hydrous zirconia particles decreased as the ZrOCl2concentration increased and was independent of the reaction temperature. The secondary particle size of hydrous zirconia particles synthesized at ZrOCl2concentrations of 0.1, 0.2, and 0.4 mol/dm3 increased monotonously, first increased and then decreased, and decreased monotonously as the reaction temperature decreased, respectively. The rate constants (k) of the hydrous zirconia particles were determined experimentally by applying Avrami–Erofeev's equation. The determined k value decreased as the ZrOCl2concentration increased and the reaction temperature decreased. From Arrhenius plots of k, the activation energies for hydrous zirconia particles in the monoclinic phase and containing high chlorine contents were determined to be 387–396 and 249 kJ/mol, respectively. The nucleation and crystal‐growth mechanisms of primary particles and the formation mechanism of secondary particles formed by hard aggregations among primary particles were determined based on the present experimental results.
A‐ and B Site Cosubstituted Ba6−3xSm8+2xTi18O54Microwave Dielectric Ceramics
Authors: Chen Xiang Ming , Li Yi
Published: July 2010

Tin (Sn) substitution into the B‐site and Nd/Sn cosubstitution into the A‐ and B‐sites were investigated in a Ba 6−3xSm8+2xTi18O54solid solution (x= 2/3). A small amount of tin substitution for titanium improved the temperature coefficient of resonant frequency (τf) but led to a decrease of the relative dielectric constant (ɛ) and the quality factor (Qf). The Ba6−3xSm8+2x(Ti1−zSnz)18O54‐based tungsten‐bronze phase became unstable for compositions with a tin content of ≥10 mol%, where BaSm2O4and Sm2(Sn,Ti)2O7appeared, and finally, these phases became the major phases. On the other hand, Nd/Sn cosubstitution led to a good combination of high ɛ, high Qf, and near‐zero τf. Excellent microwave dielectric properties were achieved in Ba6−3x(Sm1−yNdy)8+2x(Ti1−zSnz)18O54ceramics with y= 0.8 and z= 0.05 sintered at 1360°C for 3 h: ɛ= 82, Qf= 10 000 GHz, and calculated τf=+17 ppm/°C. The tolerance factor and electronegativity difference exhibited important effects on the microwave dielectric properties, especially the Qf value. A large tolerance factor and high electronegativity difference generally led to a higher Qf value.
Effects of Coffee‐Bean‐Like Morphology and Graded Interlayer on Texture Evolution of Plasma‐Enhanced Chemical‐Vapor‐Deposited Ti‐C‐N Films
Authors: Shieh Jiann , Wang Hsiao Lei , Tsai Ming Shyong , Hon Min Hsiung
Published: July 2010

Titanium nitride (TiN), titanium carbide (TiC), and titanium carbonitride (TiCxN1−x) were prepared by plasma‐enhanced chemical vapor deposition. A graded interlayer was introduced to explore its effects on the texture of the TiC and TiN. The results showed that the topography of the TiCxN1−x, which resembled coffee beans, resulted from a twinning core structure that caused the dendritic grain shape and enhanced the 〈211〉 texture of TiC0.62·N0.38and TiC0.75·N0.25. As graded coating was used as a transition layer, the texture of TiC was changed from (220) to (200), and TiN was changed from (200) to (220) due to the pseudomorphic forces provided by the first layer plus graded layer. The residual stress contributions to this phenomenon are also discussed.
Effect of a Transverse Tensile Stress on the Electric‐Field‐Induced Domain Reorientation in Soft PZT: In Situ XRD Study
Authors: Li Xiaoping , Shih Wan Y. , Vartuli James S. , Milius David L. , Aksay Ilhan A. , Shih Wei‐Heng
Published: July 2010

The effect of a transverse tensile stress on the electric‐field‐induced 90°‐domain reorientation in tetragonal lead zirconate titanate (PZT) near the morphotropic phase boundary was investigated in situ using X‐ray diffraction (XRD). The XRD intensity ratio, I(002)/I(200), which represents the ratio of the volume of the c‐domains to that of the a‐domains on the PZT surface, was examined as a function of the electric field at various stress levels. It was found that a transverse tensile stress changes the electric‐field dependence of I(002)/I(200), especially at higher electric fields. Without a transverse tensile stress, I(002)/I(200) began to saturate at E≈ 800 kV/m. With a transverse tensile stress of 75 MPa, I(002)/I(200) increased with an upward curvature with the electric field, indicating that the transverse tensile stress enhanced the field‐induced 90°‐domain reorientation, and increased the effective piezoelectric coefficients at larger electric fields. At E= 900 kV/m, the estimated d31,domain changed from −200 × 10−12 V/m at zero stress, to −350 × 10−12 V/m at 75 MPa.
Compositional Effects on the Properties of Si‐Al‐RE‐Based Oxynitride Glasses (RE = La, Nd, Gd, Y, or Lu)
Authors: Becher Paul F. , Waters Shirley B. , Westmoreland C. Gary , Riester Laura
Published: July 2010

A series of silicon‐aluminum oxynitride‐glass compositions containing selected rare‐earth (RE) additions were prepared to examine the effects of specific RE, as well as Si:Al:RE and N:O ratio, on properties. The properties that were characterized included density, thermal expansion coefficient (α), glass‐transition temperature (Tg), hardness (H), and Young's modulus (E). The compositions (in equivalent percent) selected included: 55 Si‐20 RE‐25 Al oxide and 80 O‐20 N oxynitride, and 45 Si‐30 RE‐25 Al oxide and 70 O‐30 N glasses. The results show that the density increased significantly with an increase in the RE atomic mass and slightly with an increase in N:O ratio. For each of the fixed Si‐Al‐RE‐O‐N compositions, the E, H, and Tg values were each increased by substituting smaller RE ions, whereas the α value was decreased. For each specific cation composition and RE, increasing the N:O ratio systematically led to a decrease in the α values but an increase in the E, H, and Tg values. The observed response in the glass properties to changes in composition appears to reflect modifications in the bonding within the glass network.
Formation of Nanocrystalline Calcia by the Decomposition of Calcite
Authors: Singh Alok , Dash Sitaram , Kamruddin Mohammed , Ajikumar Puthupparampil K. , Tyagi Ashok K. , Raghunathan Vinjamoor S. , Raj Baldev
Published: July 2010

Formation of nanocrystalline calcia from calcite has been studied in situ via transmission electron microscopy. The crystallographic transformation occurred via two mechanisms: the first is by distortion of the cleaved rhombohedron of calcite, formed by {104} planes in hexagonal coordinates, into a cube. This produced a microstructure of oriented, elongated nanocrystals of calcia with planar boundaries. In the second mechanism, the micrometer‐sized parent calcite particles broke up into nano‐sized grains as the decomposition began, leading to irregularly shaped, randomly oriented nanocrystals of calcia.
New Process for the Preparation of Monodispersed, Spherical Silica Particles
Authors: Kim Ki Do , Kim Hee Taik
Published: July 2010

A new method is presented for preparing highly monodispersed silica particles using a two‐stage semibatch/batch hydrolysis reaction of Si(OC2H5)4. The slower rate of hydrolysis of the tetraethylorthosilicate (TEOS) that occurred during the semibatch process resulted in larger silica particles with a higher yield and narrower size distribution. This was in direct contrast to the batch process. In addition, the ability of four different mixed processes to produce silica particles with good packing density, narrower particle‐size distribution, and higher yield were evaluated. These were batch/batch (B‐B), batch/semibatch (B‐S), semibatch/batch (S‐B), and semibatch/semibatch (S‐S) processes. The S‐S system produced the largest particles with the highest yields. The size of the silica particles obtained by the S‐B method decreased with increasing reaction time, while the particles obtained by the B‐S process had the best particle‐size distribution and packing density. In conclusion, a mixed batch/semibatch system was the best way to produce an extremely narrow particle‐size distribution and a good packing density.
Lattice Model for Estimating the Opacity of White Coatings
Authors: Focke Walter W. , Manhique Arao , Carter Rachel
Published: July 2010

White pigments impart opacity to films and coatings by virtue of their ability to scatter incident light. This study considers a homogeneous coating that contains a low concentration of randomly dispersed, monodisperse pigment particles. The optical properties are determined using a lattice model with cell size defined by the diameter (d) of the particles. For pure scattering, the contrast ratio (CR) is an algebraic function of the scattering coefficient (κ), pigment volume fraction (φ), and coating layer thickness (L), as follows: [formula omitted] When tested against literature data for white films opacified with titanium dioxide, an empirical expression with a single adjustable constant provides a better interpolation formula than the above equation. This discrepancy is attributed to the contribution of a minor amount of absorption to the measured opacity.
Hydrothermal Synthesis and Low‐Temperature Sintering of Zinc Gallate Spinel Fine Particles
Authors: Hirano Masanori , Sakaida Naoshi
Published: July 2010

Nanosized powders of single‐phase zinc gallate (ZnGa2O4) spinel were hydrothermally synthesized from solutions in the presence of NaOH over the pH range of 1.9 to 7.0 and from solutions above pH 7.0, i.e., the very basic medium (pH of 13.85), by removing the residual ZnO phase by washing with aqueous H2SO4 from the precipitate mixtures of zinc gallate spinel particles and ZnO. A very wide compositional range (Zn/2Ga = 0.705–1.157) of zinc gallate spinel solid solutions could be hydrothermally synthesized in the form of nanosized particles from acid and very basic mediums (pH of 2.4–13.85) in the presence of NaOH. These hydrothermally synthesized spinel powders showed good sinterability and almost full densification at 1100°C for 1 h. Dense sintered bodies consisting of single‐phase zinc gallate spinel were fabricated at 1100°C using zinc gallate spinel powders having almost stoichiometric composition formed from the solution at pH 9.95 in the presence of aqueous ammonia.
Preparation and Luminescence Spectra of Calcium‐ and Rare‐Earth (R = Eu, Tb, and Pr)‐Codoped α‐SiAlON Ceramics
Authors: Xie Rong‐Jun , Mitomo Mamoru , Uheda Kyota , Xu Fang‐Fang , Akimune Yoshio
Published: July 2010

Rare‐earth‐doped oxynitride or nitride compounds have been reported to be luminescent and may then serve as new phosphors with good thermal and chemical stabilities. In this work, we report the photoluminescence (PL) spectra of europium‐, terbium‐, and praseodymium‐doped Ca‐α‐SiAlON ceramics. The highly dense ceramics were prepared by hot pressing at 1750°C for 1 h under 20 MPa in a nitrogen atmosphere. Europium‐doped Ca‐α‐SiAlON displayed a single broad emission band peaking at λ= 550–590 nm depending on the europium concentration. The emission bands in the spectra of europium‐doped Ca‐α‐SiAlONs were assigned to the allowed transition of Eu2+ from the lowest crystal field component of 4f65d to 8S7/2 (4f7) ground‐state level. The emission spectra of terbium‐ and praseodymium‐doped Ca‐α‐SiAlON ceramics both consisted of several sharp lines, which were attributed to the 5D4→7Fj (j= 3, 4, 5, 6) transitions of Tb3+ and 3P0→3Hj (j= 3, 4, 5) transitions of Pr3+, respectively. In particular, the terbium‐doped α‐SiAlON ceramics showed a strong green emission among these phosphors.
Characterization of the Free‐Carbon Phase in Si‐C‐N Ceramics: Part II, Comparison of Different Polysilazane Precursors
Authors: Trassl Stephan , Kleebe Hans‐Joachim , Störmer Heike , Motz Günter , Rössler Ernst , Ziegler Günter
Published: July 2010

The excess carbon of various polysilazane precursors with varying carbon contents was investigated using 13C MAS NMR, Raman and ESR spectroscopies, and microwave conductivity measurements. Microstructure characterization was investigated using TEM. The collected data were compared with those from a previous study on the same precursor. This study focused on the distribution and appearance of the free‐carbon phase. Although the spectroscopic techniques showed no differences in the structure of the free‐carbon phase, a clear distinction between the various precursors was found using microwave conductivity and high‐resolution TEM imaging.
Conversion of Polycrystalline Al2O3 into Single‐Crystal Sapphire by Abnormal Grain Growth
Authors: Scott Curtis , Kaliszewski Mary , Greskovich Charles , Levinson Lionel
Published: July 2010

In a given batch more than 30%–40% of polycrystalline, MgO‐doped Al2O3 tubes were converted into single crystals of sapphire by abnormal grain growth (AGG) in the solid state at 1880°C. Most crystals grew 4–10‐cm in length in tubes with wall thicknesses of 1/2 and 3/4 mm and outer diameters of 5 and 7 mm, respectively, and had their c‐axes oriented ∼ 90° and 45° to the tube axis. Initiation of AGG was associated with low values of bulk MgO concentration near 50 ppm. The unconverted tubes did not develop centimeter‐size crystals but instead exhibited millimeter‐size grains. The different grain structures in converted and unconverted tubes may be related to nonuniform concentration of MgO in the extruded tubes. The growth front of the migrating crystal boundary was typically nonuniformly shaped, and the interface between the single crystal and the polycrystalline matrix was composed of many “curved” boundary segments indicative of classical AGG in a single‐phase material. The average velocities of many migrating crystal boundaries were quite high and reached ∼1.5 cm/h. The average grain boundary mobility at 1880°C was calculated as 2 × 10−10 m3/(N·s), representing the highest value reported so far in Al2O3 and within a factor of 2.5 of the calculated intrinsic mobility. Under similar experimental conditions sapphire crystals did not grow when a codopant of CaO, La2O3, or ZrO2 was added in concentrations of several hundred ppm.
Preparation of a Bioactive Poly(methyl methacrylate)/Silica Nanocomposite
Authors: Rhee Sang‐Hoon , Choi Je‐Yong
Published: July 2010

The nanocomposite of a poly(methyl methacrylate)/silica containing calcium salt was synthesized through the sol–gel method. Methyl methacrylate was co‐polymerized with 3‐(trimethoxysilyl)propyl methacrylate and then co‐condensed with tetraethyl orthosilicate and calcium nitrate tetrahydrate. Low crystalline hydroxycarbonate apatite was successfully formed on the surface after soaking in simulated body fluid for 1 week at 36.5°C. This study demonstrates that this nanocomposite may be used as a bioactive bone substitute or filler for poly(methyl methacrylate) bone cement.
Model Analysis of Multicrack Mechanisms in Ceramic/Superplastic Laminates
Authors: Deng Zhen‐Yan , Zhang Guo‐Jun , Ando Motohide , Ohji Tatsuki
Published: July 2010

Previous experimental results showed that a ceramic/superplastic laminate exhibited multiple cracking in ceramic layers during a three‐point bending test. In this work, a model analysis has been developed based on bending theory. It reveals that there are two basic processes that occur after a ceramic layer fractures: one is a relaxation process of the residual stress in the ceramic layer, due to the confinement by the superplastic layers; the other is a shear process of superplastic flow, which originates from the difference in strain rate between the fractured and unbroken ceramic layers. The total stress in an as‐fractured ceramic layer is the sum of the residual stress and a shear‐accumulated stress, depending on time. When the total stress at a critical distance from the fractured surface exceeds the fracture strength of a ceramic layer, new cracking occurs. There is a critical roller speed below which no multiple cracking occurs, depending on specimen dimensions and material properties. The number of multicracks in one ceramic layer decreases with the progress of the fracture in the laminate, due to the decrease in shear‐accumulated stress. The theoretical predications are in good agreement with the experimental results. Moreover, the variations in fracture energy of the laminate due to the multiple cracking are discussed in detail.
Preparation of Aluminum Nitride–Silicon Carbide Nanocomposite Powder by the Nitridation of Aluminum Silicon Carbide
Authors: Itatani Kiyoshi , Tsukamoto Ryuji , Delsing Anne C. A. , Hintzen Hubertus T. , Okada Isao
Published: July 2010

Aluminum nitride (AlN)–silicon carbide (SiC) nanocomposite powders were prepared by the nitridation of aluminum‐silicon carbide (Al4SiC4) with the specific surface area of 15.5 m2·g−1. The powders nitrided at and above 1400°C for 3 h contained the 2H‐phases which consisted of AlN‐rich and SiC‐rich phases. The formation of homogeneous solid solution proceeded with increasing nitridation temperature from 1400° up to 1500°C. The specific surface area of the AlN–SiC powder nitrided at 1500°C for 3 h was 19.5 m2·g−1, whereas the primary particle size (assuming spherical particles) was estimated to be ∼100 nm.
Spark Plasma Sintering of Tungsten Bronze Sr2−xCaxNaNb5O15 (x= 0.1) Piezoelectric Ceramics: II, Electrical Properties
Authors: Xie Rong‐Jun , Akimune Yoshio , Wang Rui‐Ping , Hirosaki Naoto
Published: July 2010

Sr2−xCaxNaNb5O15 (SCNN, x= 0.1) piezoelectric ceramics with tetragonal tungsten bonze structure were successfully produced by spark plasma sintering. Part I of this study presented the sintering of these highly dense ceramics. The current work investigates their electrical (dielectric, ferroelectric, and piezoelectric) properties and relates them to the sintering parameters (temperature, heating rate, and soaking time) and crystallographic orientation. The results show that the electrical properties of SCNN are enhanced with increasing heating rate and soaking time. Dielectric study reveals a less pronounced diffuse phase transition of SCNN. Anisotropic electrical properties were observed as a result of the preferred crystallographic orientation in the sintered ceramics. The samples sintered at 1200°C for 30 min exhibited the highest electrical properties, and showed the following piezoelectric properties when poled across the direction perpendicular to the pressing direction: electromechanical planar and thickness coupling coefficients of kp= 9.5% and kt= 39.6%, and piezoelectric constant of d33= 121 pC/N (∼45% of the single‐crystal value).
Impedance Spectroscopy Study of Niobium‐Doped Strontium Titanate Ceramics
Authors: Abrantes Joao C. C. , Feighery Alan , Ferreira António A. L. , Labrincha Joao Antonio , Frade Jorge R.
Published: July 2010

Impedance spectroscopy was used to study the temperature dependence of the resistance and capacitance of niobium‐doped strontium titanate ceramics and also to identify the main contributions determining potential applications. Nyquist plots (Z″ versus Z′) show a main contribution in the intermediate‐frequency range, with a peak at ∼1 kHz at 500°C. The activation energy of this resistance contribution is close to 1.5 eV. Modulus representations (log M″ versus log f) show a second contribution in the high‐frequency range, with a peak at ∼100 kHz at 500°C. This resistance contribution is much smaller, and its activation energy is also lower (close to 0.8 eV). The capacitance of the intermediate‐frequency term increases from ∼0.1 μF/m at 700°C to ∼1 μF/m at 425°C. The capacitance values of the high‐frequency contribution are also >0.1 μF/m, much higher than expected for the bulk. These contributions might be related to differences between the intergrain interfaces and outer grain shells, as suggested by SEM microstructures with core–shell formations. Representations of 1/C against temperature suggest a Curie–Weiss law for the main contribution in the intermediate‐frequency range, with a TC of ∼400°C. However, the temperature dependence of the overall dielectric constant measurements (at constant frequency) fail to show a clear maximum, probably due to an additional low‐frequency contribution ascribed to the Pt/SrTi1−xNbxO3−δ interface.
Electromechanical Properties of Pb(Yb1/2Nb1/2)O3‐PbZrO3‐PbTiO3 Ceramics
Authors: Yoon Ki Hyun , Lee Hong Kyu , Lee Hong Ryul
Published: July 2010

The electromechanical and electric‐field‐induced strain properties of xPb(Yb1/2Nb1/2)O3·yPbZrO3·(1−x−y)PbTiO3 (x= 0.12, 0.25, 0.37; y= 0.10–0.40) ceramics have been studied systematically as a function of Pb(Yb1/2Nb1/2)O3 (PYN) content and PbZrO3/PbTiO3 (PZ/PT) ratio. In addition, the effect of MnO2 on the electromechanical properties of 0.12Pb(Yb1/2Nb1/2)O3·0.40PbZrO3·0.48PbTiO3 was also investigated. The maximum transverse strain values of 1.6 × 10−3 for x= 0.12, 1.45 × 10−3 for x= 0.25, and 1.36 × 10−3 for x= 0.37 were obtained at the compositions which were regarded as the morphotropic phase boundary (MPB). The transverse strain was maximized at the MPB composition. The value of the maximum electromechanical coupling coefficient was 0.69 for y= 0.40 and x= 0.12 composition. In the 0.12Pb(Yb1/2Nb1/2)O3·0.40PbZrO3·0.48PbTiO3 composition, the temperature of the maximum dielectric constant decreased and the grain size increased with an addition of MnO2. The electromechanical coupling coefficient decreased while the mechanical quality factor rapidly increased with an addition of MnO2. These resulted mainly from the acceptor effect of manganese ions that were produced by doping MnO2 into the perovskite structure.
Microwave Dielectric Properties of Low‐Fired Ba5Nb4O15
Authors: Kim Dong‐Wan , Kim Jeong‐Ryeol , Yoon Sung‐Hun , Hong Kug Sun , Kim Chang Kyung
Published: July 2010

The effect of B2O3 on the sintering temperature and microwave dielectric properties of Ba5Nb4O15 has been investigated using X‐ray powder diffraction, scanning electron microscopy, and a network analyzer. Interactions between Ba5Nb4O15 and B2O3 led to formation of second phases, BaNb2O6 and BaB2O4. The addition of B2O3 to Ba5Nb4O15 resulted in lowering the sintering temperature from 1400° to 925°C. Low‐fired Ba5Nb4O15 could be interpreted by measuring changes in the quality factor (Q×f), the relative dielectric constant (ɛr), and the temperature coefficient of resonant frequency (τf) as a function of B2O3 additions. More importantly, the formation of BaNb2O6 provided temperature compensation. The microwave dielectric properties of low‐fired Ba5Nb4O15 had good dielectric properties: Q×f= 18700 GHz, ɛr= 39, and τf= 0 ppm/°C.
Effect of Silicon Substitution on the Sintering and Microstructure of Hydroxyapatite
Authors: Gibson Iain R. , Best Serena M. , Bonfield William
Published: July 2010

The substitution of between 0 and 1.6 wt% silicon (Si‐HA) in hydroxyapatite (HA) inhibited densification at low temperatures (1000°–1150°C), with these effects being more significant as the level of silicon substitution was increased. For higher sintering temperatures (1200°–1300°C), the sintered densities of HA and Si‐HA compositions were comparable. Examination of the ceramic microstructures by scanning electron microscopy (SEM) showed that silicon substitution also inhibited grain growth at higher sintering temperatures (1200°–1300°C). The negative effect of silicon substitution on the sintering of HA at low temperatures (1000°–1150°C) was reflected in the hardness values of the ceramics. However, for higher sintering temperatures, e.g., 1300°C, where sintered densities were comparable, the hardness values of Si‐HA compositions were equal to or greater than that of HA, reflecting the smaller grain sizes observed for the former.
Fabrication of Arsenic Selenide Optical Fiber with Low Hydrogen Impurities
Authors: Nguyen Vinh Q. , Sanghera Jas S. , Pureza Pablo , Kung Frederic H. , Aggarwal Ishwar D.
Published: July 2010

Arsenic selenide glass optical fibers typically possess extrinsic absorption bands in the infrared wavelength region associated with residual hydrogen and oxygen related impurities, despite using purified precursors. We report a purification process based on the addition of 0.1 wt% tellurium tetrachloride (TeCl4) to the glass. During melting, the chlorine from TeCl4 reacts with the hydrogen impurities to produce volatile products (e.g., HCl) that can be removed by subsequent dynamic distillation. The processing conditions have been modified accordingly to give very low H–Se impurity content. Consequently, the H–Se absorption band centered at 4.57 μm has been reduced from tens of dB/m to 0.2 dB/m.
Chemical Heterogeneity in PMN–35PT Ceramics and Effects on Dielectric and Piezoelectric Properties
Authors: Leite Edson R. , Scotch Adam M. , Khan Ajmal , Li Tao , Chan Helen M. , Harmer Martin P. , Liu Shi‐Fang , Park Seung‐Eek
Published: July 2010

Pb[(Mg1/3Nb2/3)0.65Ti0.35]O3 (PMN–35PT) powder was prepared using the columbite precursor method. Fully dense compacts were formed by hot‐pressing the powder at 950°C, and then the compacts were annealed at 1150°C for 5 and 10 h, respectively. Dielectric and piezoelectric properties of the as‐hot‐pressed and annealed samples were measured and correlated with microstructure. The as‐hot‐pressed material exhibited relaxor–ferroelectric‐like behavior, with a relatively low dielectric constant maximum measured at 1 kHz (Km@1kHz) of 8160. Annealing resulted in a transition to weak normal‐ferroelectric behavior, a shift in the dielectric maximum temperature from 190°C to 169°C, and a dramatic increase of Km@1kHz to a maximum value of 41 720 for the longer anneal. The as‐hot‐pressed microstructure was chemically heterogeneous, characterized by submicrometer‐sized regions of varying magnesium, niobium, and titanium content that likely originated from chemical heterogeneities that were present in the as‐prepared PMN‐PT powder. The as‐hot‐pressed properties have been explained as being the integrated response of many discrete ferroelectric responses as dictated for each of these regions by the local chemistry. The transition on annealing has been explained in terms of chemical homogenization to a near‐morphotropic phase‐boundary composition that is intrinsically weak normal‐ferroelectric. Differences in polarization‐versus‐electric‐field and strain‐versus‐electric‐field behavior between the hot‐pressed and annealed materials have been discussed in terms of differences in domain mobility.
Intermediate Phases in Ti3SiC2 Synthesis from Ti/SiC/C Mixtures Studied by Time‐Resolved Neutron Diffraction
Authors: Wu Erdong , Kisi Erich H. , Riley Daniel P. , Smith Ronald I.
Published: July 2010

The reactive sintering of 3Ti/SiC/C to form the layered ternary carbide Ti3SiC2 was studied in situ by time‐resolved neutron powder diffraction. A number of intermediate processes occur during the synthesis beginning with the α‐β transition in Ti. Concurrent with the α‐β transition, two intermediate phases, TiCx and Ti5Si3Cx (x≤ 1), form. These phases account for almost the entire sample in the range 1500–1600°C beyond which they react with each other and a small amount of free C to form the product phase Ti3SiC2.
Synthesis of Nanosized Titanium‐Chromium Oxynitride Powders by Ammonolysis of Coprecipitated TiO2/Cr2O3 Precursors
Authors: Li Yaogang , Gao Lian
Published: July 2010

Interstitial titanium‐chromium oxynitrides in the solid solution series Ti1−zCrz(OxNy) (z= 0.2, 0.4, 0.5, 0.6, 0.8) have been obtained by ammonolysis of the TiO2/Cr2O3 precursors resulting from the coprecipitation method. The precursors and the resulting oxynitrides were characterized by auger electron spectroscopy, X‐ray diffraction analysis, electron probe microanalysis, transmission electron microscopy, and BET surface area techniques. Compounds in the Ti1−zCrz(OxNy) series are prepared as single phases by nitridation at 1073 K for 8 h. The as‐synthesized oxynitride powders contain only Ti1−zCrz(OxNy) with cubic structure and the particle size is in the nanometer scale.
Thin Yttrium‐Stabilized Zirconia Electrolyte Solid Oxide Fuel Cells by Centrifugal Casting
Authors: Liu Jiang , Barnett Scott A.
Published: July 2010

A centrifugal casting technique was developed for depositing thin 8‐mol%‐yttrium‐stabilized zirconia (YSZ) electrolyte layers on porous NiO‐YSZ anode substrates. After the bilayers were cosintered at 1400°C, dense pinhole‐free YSZ coatings with thicknesses of ∼25 μm were obtained, while the Ni‐YSZ retained porosity. After La0.6Sr0.4Co0.2Fe0.8O3 (LSCF)‐Ce0.9Gd0.1O1.95 (GDC) or La0.8Sr0.2MnO3 (LSM)‐YSZ cathodes were deposited, single SOFCs produced near‐theoretical open‐circuit voltages and power densities of ∼1 W/cm2 at 800°C. Impedance spectra measured during cell tests showed that polarization resistances accounted for ∼70%–80% of the total cell resistance.
Synthesis and Characterization of Zirconia Nanorods
Authors: Liu Yingkai , Zheng Chenglin , Wang Wengzhong , Zhan Yiongjie , Wang Guang Hou
Published: July 2010

ZrO2nanorods are prepared by annealing precursor powders produced in the novel inverse microemulsion system. The length and diameter of ZrO2nanorods are a few micrometers and 40–100 nm, respectively. The microstructure of the resultant nanorods are studied by XRD, TEM, selected area electron diffraction, and Raman spectroscopy. The ZrO2nanorods are single crystalline and have monoclinic structure. The formation of ZrO2nanorods is discussed.