bluuebunny/crossref_metadata_2025 huggingface dataset:
The Reaction‐Bonded Aluminum Oxide (RBAO) Process: II, The Solid‐State Oxidation of RBAO Compacts
Authors: Suvaci Ender , Simkovich George , Messing Gary L.
Published: July 2010

The oxidation kinetics and the fraction of aluminum that is oxidized via solid–gas reaction in reaction‐bonded aluminum oxide (RBAO) compacts are shown to be strongly dependent on the oxidation temperature and the characteristics (size and green density) of the RBAO compact. Based on the Biot number, the oxidation process of RBAO compacts is controlled by convective heat transfer. Low heat transfer from the surface of the compact results in too‐rapid oxidation, thermal gradients, and core–shell oxidation of the compacts. Uniform oxidation of RBAO compacts is possible by oxidizing at low temperatures (400°–470°C), where slow surface reaction of the aluminum particles controls the oxidation of the compact. A grain model is presented to cover both linear and nonlinear oxidation regimes during the oxidation of a RBAO compact, and this model predicts the experimental results when surface reaction of the aluminum particles is the rate‐controlling mechanism and oxidation of the compact occurs uniformly.
Thermal Conductivity of β‐Si3N4: II, Effect of Lattice Oxygen
Authors: Kitayama Mikito , Hirao Kiyoshi , Tsuge Akira , Watari Koji , Toriyama Motohiro , Kanzaki Shuzo
Published: July 2010

Dense β‐Si3N4 with various Y2O3/SiO2 additive ratios were fabricated by hot pressing and subsequent annealing. The thermal conductivity of the sintered bodies increased as the Y2O3/SiO2 ratio increased. The oxygen contents in the β‐Si3N4 crystal lattice of these samples were determined using hot‐gas extraction and electron spin resonance techniques. A good correlation between the lattice oxygen content and the thermal resistivity was observed. The relationship between the microstructure, grain‐boundary phase, lattice oxygen content, and thermal conductivity of β‐Si3N4 that was sintered at various Y2O3/SiO2 additive ratios has been clarified.
Stress‐Corrosion Cracking of Silicon Carbide Fiber/Silicon Carbide Composites
Authors: Jones Russell H. , Henager Charles H. , Lewinsohn Charles A. , Windisch Charles F.
Published: July 2010

Ceramic‐matrix composites are being developed to operate at elevated temperatures and in oxidizing environments. Considerable improvements have been made in the creep resistance of SiC fibers and, hence, in the high‐temperature properties of SiC fiber/SiC (SiCf/SiC) composites; however, more must be known about the stability of these materials in oxidizing environments before they are widely accepted. Experimental weight change and crack growth data support the conclusion that the oxygen‐enhanced crack growth of SiCf/SiC occurs by more than one mechanism, depending on the experimental conditions. These data suggest an oxidation embrittlement mechanism (OEM) at temperatures <1373 K and high oxygen pressures and an interphase removal mechanism (IRM) at temperatures of ≳700 K and low oxygen pressures. The OEM results from the reaction of oxygen with SiC to form a glass layer on the fiber or within the fiber–matrix interphase region. The fracture stress of the fiber is decreased if this layer is thicker than a critical value (d > dc) and the temperature below a critical value (T < Tg), such that a sharp crack can be sustained in the layer. The IRM results from the oxidation of the interfacial layer and the resulting decrease of stress that is carried by the bridging fibers. Interphase removal contributes to subcritical crack growth by decreasing the fiber‐bridging stresses and, hence, increasing the crack‐tip stress. The IRM occurs over a wide range of temperatures for d < dc and may occur at T > Tg for d > dc. This paper summarizes the evidence for the existence of these two mechanisms and attempts to define the conditions for their operation.
Experimental Investigation and Thermodynamic Description of the Constitution of the Ternary System Cr‐Si‐C
Authors: Du Yong , Schuster Julius C. , Perring Loic
Published: July 2010

The constitution of the ternary system Cr‐Si‐C is investigated using differential thermal analysis and X‐ray diffractometry techniques. Eight invariant reaction temperatures involving the liquid phase are determined. The crystal structures and phase partitions observed for the subsolidus temperature region confirm literature data. Thermodynamic modeling, using a minimum of adjustable ternary parameters and assuming the binary high‐temperature phase, β‐Cr5Si3, to be stabilized toward lower temperatures as ternary D88‐type Cr5Si3Cx, generally reproduces the measured temperatures within 10 K. The calculated liquidus surface contains four ternary eutectics, three transition reactions, three degenerated reactions, and five (pseudobinary eutectics) maxima. The reaction scheme shows that no solid‐phase reactions occur in the Cr‐Si‐C system at >1000°C, except for the binary transition β‐Cr5Si3↔α‐Cr5Si3. The activity diagram along the section Cr‐SiC at 1300°C, computed from the optimized data set, agrees with the observed diffusion path.
Atomistic Simulation of the Surface Energy of Spinel MgAl2O4
Authors: Fang Chang Ming , Parker Stephen C. , de With Gijsbertus
Published: July 2010

Atomistic simulations with atomic potentials including anion polarizibility have been performed for the low‐index surfaces of spinel MgAl2O4 with various terminations. The calculations show that for the most stable surface the surface energy is 2.27 J/m2 for the {100}, about 2.85 J/m2 for the {110}, and 3.07 J/m2 for the {111} orientation. The ratio between the experimental values to the calculated relaxed surface energies is about 1.5. Strong surface relaxation was found for the {110} and {111} orientation but only moderate surface relaxation for the {100} surface.
Thermochemical Reactions and Equilibria between Fluoromicas and Silicate Matrices: A Petromimetic Perspective on Structural Ceramic Composites
Authors: King Todd T. , Grayeski Walter , Cooper Reid F.
Published: July 2010

A petromimetic (geological–analog) approach is applied to the design of alumina‐fiber‐reinforced glass‐ceramic‐matrix composites that use a refractory, trioctahedral fluoromica fiber–matrix interphase and feldspar matrixes. Studies of the solid‐state reaction couples between these silicate phases are pursued to address the chemical tailorability of the interphase/matrix interface from an engineering perspective. The minimization of alumina and silica activities within polyphase, feldspar‐based matrixes via MgO buffering is shown to be an effective route toward a stable fluoromica interphase. An anorthite–2‐vol%‐alumina (CaAl2Si2O8+α‐Al2O3) substrate, chemically buffered with MgO, is shown to exhibit thermodynamic stability against fluorokinoshitalite (BaMg3[Al2Si2]O10F2), up to temperatures potentially as high as 1460°C. The key to the approach is the reduction of alumina activity via the formation of MgAl2O4 spinel. Similarly, the formation of forsterite (Mg2SiO4) stabilizes the mica in contact with matrix compositions otherwise containing excess silica. The cationic interdiffusion between solid‐solution feldspars and fluoromicas also is characterized. Coupled interdiffusion of K+ and Si4+ in exchange for Ba2+ and Al3+ was observed between K‐Ba solid‐solution celsian and the barium‐rich solid‐solution end‐member fluorokinoshitalite at 1300°C. A similar cationic exchange also is observed against the potassium‐rich end‐member fluorophlogopite (KMg3[AlSi3]O10F2), although in a reverse direction, at temperatures of <1280°C. The interfacial compositions identified via electron microprobe analysis specify one set of local equilibrium conditions from which global ceramic composite equilibrium can be achieved.
Microstructural Characterization of C60‐Doped Zirconia
Authors: Miyazawa Kun'ichi , Yano Junya , Honma Itaru , Akaishi Minoru
Published: July 2010

Zirconia powders doped with C60 molecules were prepared from an aqueous solution of zirconium oxynitrate dihydrate, C60 and C16TMA, and sintered at 600°C under 5.5 GPa for 2 h. C60 was found to be retained in the sintered specimens by HRTEM, and carbon was observed to be uniformly dispersed by the SEM‐EDX analysis. HRTEM observations of the sintered specimens exhibited the formation of ZrO2 crystal grains covered with thin graphitic or amorphous carbon films.
Ab Initio Calculations of the Atomic and Electronic Structure of β‐Silicon Nitride
Authors: Belkada Rachid , Shibayanagi Toshiya , Naka Masaaki , Kohyama Masanori
Published: July 2010

The atomic and electronic structure of the β‐silicon nitride (β‐Si3N4) crystal have been determined using the ab initio pseudopotential method based on the density functional theory. We have obtained the stable lattice parameters and the stable positions of 14 atoms in the unit cell for the structure P63/m for the first time. The electronic structure and the charge distribution indicate that the Si–N bond has both ionic and covalent characters. The band structure is in good agreement with the other first‐principles results and consistent with the experiments.
Coagulation Method of Aqueous Concentrated Alumina Suspensions by Thermal Decomposition of Hydroxyaluminum Diacetate
Authors: Laucournet Richard , Pagnoux Cécile , Chartier Thierry , Baumard Jean‐François
Published: July 2010

Consolidation of aqueous concentrated suspensions was used to shape alumina green bodies because it enabled us to obtain complex‐shape components with accurate sizes. A high state of alumina particle dispersion was achieved by using (HO)2C6H2(SO3Na)2 (Tiron), which allowed us to obtain stable alumina suspensions at pH 9 with a powder concentration higher than 60 vol%. The addition to the suspension of hydroxyaluminum diacetate, (CH3CO2)2AlOH, which decomposed as the temperature increased, permitted us to coagulate an alumina suspension dispersed with Tiron efficiently. Adsorption measurements, electrokinetic mobility, and the rheological behavior of the suspensions provided useful methods to characterize each processing stage. Dense green bodies with sufficient cohesion could be demolded and dried, demonstrating that the dispersant and the flocculant agent chosen permit one to optimize the direct coagulation casting processing of alumina components.
Chemical Compatibility between Silver Electrodes and Low‐Firing Binary‐Oxide Compounds: Conceptual Study
Authors: Valant Matjaz , Suvorov Danilo
Published: July 2010

The chemical compatibility of silver electrodes and low‐firing ceramics has been considered, in terms of the existence of a tie line between silver (and/or Ag2O) and the binary oxide compound in the corresponding ternary phase diagram. The probability of the existence of the tie line is related to the conditions in the subordinated silver‐based isothermal binary systems. Greater probabilities have been calculated for the systems with fewer silver‐based binary compounds. Based on the concepts that have been developed, several silver‐based isothermal binary systems have been investigated to identify the oxides suitable for the development of low‐temperature cofired ceramics. The developed concept has been tested by investigating the phase relations in the Bi2O3–Nb2O5 and Bi2O3–V2O5 ternary systems with silver. X‐ray and microstructural investigations of bismuth niobates and bismuth vanadates reveal that, as a result of the inertness of Bi2O3 and the reactivity of Nb2O5 and V2O5 toward silver, compounds that are rich in niobium or vanadium react with silver to form ternary Nb/V‐Bi‐Ag oxide compounds, whereas for compounds that are rich in bismuth, tie lines to silver and Ag2O do exist.
Determination of the In Situ Fiber Strength in Ceramic‐Matrix Composites from Crack‐Resistance Evaluation Using Single‐Edge Notched‐Beam Tests
Authors: Rausch Gerald , Kuntz Meinhard , Grathwohl Georg
Published: July 2010

A model for the single‐edge notched‐beam (SENB) test is presented which allows the load–displacement curve of fiber‐reinforced ceramic‐matrix composite (CMC) specimens to be calculated using the crack‐opening function. This function considers the effects of the main parameters of the composite components that govern the fracture of the composite (e.g., fiber strength and interfacial properties). Then, in an experimental study, the SENB curves of an oxide–oxide composite are measured in the as‐processed state and after exposure at elevated temperatures. Comparison between the measured and calculated SENB curves indicates that the characteristic loss of the maximum load in the SENB curves due to the annealing is caused by the corresponding reduction of the in situ fiber strength. This method then is proven to be an excellent tool to evaluate the actual fiber strength in CMCs, which is, otherwise, barely measurable. It will be shown that the SENB test is very much qualified for a proper investigation of the relationship between in situ fiber strength and composite strength (the maximum SENB load), because of the limited influence of other parameters (Weibull modulus of the fiber strength and the friction coefficient) on the maximum SENB load.
Synthesis and Structure of Three‐Dimensionally Ordered Graphitelike BC2N Ternary Crystals
Authors: Nicolich Jeffrey P. , Hofer Ferdinand , Brey Gerhard , Riedel Ralf
Published: July 2010

In this article we present the synthesis of three‐dimensionally ordered ternary BC2N crystals produced under high‐temperature and high‐pressure conditions. Unit cell dimensions are a= 2.4820 ± 0.0007 Å and c= 6.620 ± 0.003 Å. The composition BC2N is derived from Vegard's law and quantitative electron energy loss spectroscopy. The ordered arrangement of atoms in the unit cell is discussed.
Electrophoretic Deposition of Zirconia on Porous Anodic Substrates
Authors: Will Julia , Hruschka Martin K. M. , Gubler Lorenz , Gauckler Ludwig J.
Published: July 2010

The aim of the present work is the preparation of thin (<20 μm) zirconia layers on porous substrates with the electrophoretic deposition process. The preparation was completed with a cosintering step of substrate and layer. Through adjustment of shrinkage and the shrinkage rate of the deposited zirconia layer on the presintered porous substrate, thin, dense layers without cracks were prepared. A method for direct control of the layer thickness during the electrophoretic deposition process was developed. The solid oxide fuel cell application with porous anode substrates and thin zirconia electrolytes was chosen to demonstrate the potential of the electrophoretic deposition process.
Intrinsically Formed Trivalent Titanium Ions in Sol–Gel Titania
Authors: Muñoz Eduardo , Boldú José L. , Andrade Eduardo , Novaro Octavio , Bokhimi Xi. , López Tessy , Gómez Ricardo
Published: July 2010

Electron paramagnetic resonance (EPR) room‐temperature observation of trivalent titanium in sol–gel titania has been reported. This unprecedented detection of stable paramagnetic signals in this titania, which was treated at 200°–800°C, occurred regardless of whether the atmosphere was reducing, inert, or oxidating. The possibility that the paramagnetic signals may originate from an metal impurity other than titanium was completely excluded by analyzing the samples using three different ion‐beam‐analysis techniques: Rutherford backscattering spectroscopy, particle‐induced X‐ray emission resonance, and energy‐recoil detection analysis. The paramagnetic signals appeared at precisely the temperature range of the sample dehydroxylation, which suggests a mechanism for explaining these Ti3+ EPR spectra.
Apatite Formation on Calcium Phosphate Invert Glasses in Simulated Body Fluid
Authors: Kasuga Toshihiro , Hosoi Yoshimasa , Nogami Masayuki , Niinomi Mitsuo
Published: July 2010

Calcium phosphate invert glasses, which contain P2O72− and PO43− ions, have been prepared via the addition of a small amount of TiO2. The formation of bonelike calcium phosphate apatite on the surface of the phosphate invert glasses was examined in simulated body fluid (SBF) at a temperature of 37°C. Soaking for 20 d resulted in the deposition of leaflike apatite particles on 6CaO·3P2O5·TiO2 invert glass (based on molar ratio). The glass had much‐greater chemical durability against SBF, in comparison with a metaphosphate glass; P ions were not dissolved excessively from the 6CaO·3P2O5·TiO2 glass, so the apatite formation was not suppressed.
Effect of Acrylic‐Styrene Copolymer Chemical Structure on the Properties of Ceramic Tapes Obtained by Tape Casting
Authors: Szafran Mikolaj , Rokicki Gabriel
Published: July 2010

The results of studies on the effect of the chemical structure of acrylic‐styrene copolymers on the properties of ceramic tapes from Al2O3 obtained by the tape‐casting method are presented. The properties of the emulsions that serve as ceramic binders have been modified by insertion of selected amphiphilic macromonomers and acrylic acid into the polymer molecules. These amphiphilic macromonomers, because of the proper ratio of the hydrophilic to hydrophobic fragments, serve as internal plasticizers and modifiers of the adhesion of such binders to the ceramic powder particles. The presence of carboxylic groups as polymer pendant groups in an optimal amount has an additional effect on the ceramic tape density, uniformity, and mechanical properties, before and after the sintering process.
Effect of Substitution of Manganese for Iron on the Structure and Electrical Properties of Yttrium Ferrite
Authors: Cao Xueqiang , Kim Chan‐Soo , Yoo Han‐Ill
Published: July 2010

The solubility of manganese in YFe1−yMnyO3 reaches y= 0.4. Among these compositions, YFe0.6Mn0.4O3 has the highest conductivity with ς= 2.16 Ω−1·cm−1 (1000°C, air), which is >1 order of magnitude higher than that of yttrium ferrite, YFeO3 (ς= 0.10 Ω−1·cm−1, 1000°C, air). In the range 0.2 ≤y≤ 0.4, the conductivity is linearly proportional to the manganese content. The mechanism of direct transport of polarons among manganese sites (y≥ 0.2) is proposed to explain the electrical behavior of manganese‐doped samples. X‐ray diffractometry and infrared spectroscopy have been applied for the structural characterization of the samples.
Calculating the Thermal Conductivity of Heated Powder Compacts
Authors: Birnboim Amikam , Olorunyolemi Tayo , Carmel Yuval
Published: July 2010

This work is devoted to the theoretical understanding of the microstructure and thermal conductivity relationships of compacted ceramic powders in the initial, nondensifying stage of sintering. A model based on surface diffusion of vacancies for the growth of the neck between particles is combined with numerical fully three‐dimensional code calculations, which solve for the effective heat conductivity coefficient of lightly sintered particles. The predictions of the model are in agreement with experimental observations. The approach presented can be applied to solve a series of related problems, like dielectric properties, with arbitrary microstructure and multicomponent composite of the powders.
Electrokinetic Properties of Nanosized SiC Particles in Highly Concentrated Electrolyte Solutions
Authors: Wang Sheng‐Chang , Wei Wen‐Cheng J.
Published: July 2010

In this research, the electrokinetic behavior and stability of nanosized SiC particles suspended in various electroplating solutions were studied. Analyses were performed using electrophoretic mobility photometry and streaming current (SC) techniques. The electrolytes included NiCl2, Ni(SO3NH2)2, and Na3Co(NO2)6, which are currently used in composite plating solutions with concentrations as high as 0.5M. The results showed that the adsorption of dissolved Ni2+ ions onto the surface of the SiC in the pH range 4–8 changed the sign and magnitude of the surface potential. Moreover, trivalent complex species Co(NO2)63− replaced nickel species on the SiC surface and decreased the surface charge of SiC to between pH 3 and pH 5. Even in a highly concentrated electrolyte solution, the SiC particles still maintained a positive charge in a Ni(SO3NH2)2 suspension with nickel coplating on the cathode. The difference between the SC reading and the zeta potential, as well as the surface adsorption of various species onto the SiC, are discussed here.
Cadmium Sulfide Aggregates through Reverse Micelles
Authors: Chatterjee Minati , Patra Amitava
Published: July 2010

CdS aggregates, composed of primary nanocrystalline particles, were synthesized in cyclohexane‐sorbitan monooleate (Span 80) reverse micelles at 80°± 1°C under mechanical agitation and sonication with varying Cd2+ concentrations in the solutions. The critical micelle concentration (CMC) of the surfactant Span 80 in cyclohexane was determined from discernible breaks in the surface tension and optical (ultraviolet) absorbance versus surfactant concentration curves. The synthesized aggregates were characterized using transmission electron microscopy, X‐ray diffractometry, particle‐size analysis, Fourier‐transform infrared spectroscopy, and thermal analyses. The generated aggregates exhibited the characteristic green emission at 520 nm. A minor decrease in emission intensity with increased aggregate size at the same wavelength was observed.
Influence of the Compositional Profile of Functionally Graded Material on the Crack Path under Thermal Shock
Authors: Fujimoto Takao , Noda Naotake
Published: July 2010

Thermal cracking under a transient‐temperature field in a ceramic/metal functionally graded plate is discussed. When the functionally graded plate is cooled from high‐temperature, curved or straight crack paths often occur on the ceramic surface. It is shown that the crack paths are influenced by the compositional profile of the functionally graded plate. Transient‐thermal stresses are treated as a linear quasi‐static thermoelastic problem for a plane strain state. The crack paths are obtained using finite element method with Mode I and Mode II stress intensity factors.
Effect of Composition and Crystallization Temperature on Microstructure of Y‐ and Er‐SiAlON Iw‐Phase Glass‐Ceramics
Authors: MacLaren Ian , Falk Lena K. L. , Díaz Aranzazu , Hampshire Stuart
Published: July 2010

Glass‐ceramics were produced from Y‐ and Er‐SiAlON compositions containing the Iw phase as the only detectable crystalline phase. TEM analysis showed that these materials had fine‐scale glass‐ceramic microstructures with crystallite sizes <1 μm and glass contents between 20% and 40%, depending on the starting powder composition: Increased yttrium or erbium content of the starting material resulted in a reduced glass content. The Iw‐phase crystals in all specimens were found by EDX analysis in TEM to display a solid‐solution range with a clear anticorrelation between the yttrium or erbium content and the silicon content: A possible explanation for this effect was suggested. The average crystal composition approximated a cation ratio of Y:Si:Al = 3:2:1. The composition of the residual glass was also similar in all specimens. Some small silicon‐rich amorphous features were found in the microstructures, and it was believed that these had occurred because of phase separation in the glass during the crystallization process.
Differences between Zirconium Hydroxide (Zr(OH)4·nH2O) and Hydrous Zirconia (ZrO2·nH2O)
Authors: Huang Chuanyong , Tang Zilong , Zhang Zhongtai
Published: July 2010

Based on cause and effect rather than experiment data, zirconium hydroxide (Zr(OH)4·nH2O) is always considered as hydrous zirconia (ZrO2·nH2O). With the aid of nitric acid, XPS, XRD, XPF, and TGA, some differences between them have been confirmed. It is found that in contrast to zirconium hydroxide (the binding energy of zirconium = 183.6 eV), hydrous zirconia does not dissolve in nitric acid. The chemical properties of zirconium (181.8 eV) in hydrous zirconia are similar to those in zirconia (182.2 eV for the binding energy of zirconium), and the weight loss of hydrous zirconia is 21.5%, different from 32.19%, the weight loss of zirconium hydroxide. According to the experimental data, a structure for hydrous zirconia is proposed and then the different phenomena are explained.
Mullite–β‐Spodumene Composites from Aluminosilicates
Authors: Yamuna Ammini , Devanarayanan Sankaranarayanan , Lalithambika Malathi
Published: July 2010

Sintering studies were conducted using kaolin, metakaolin, zeolite 4A, and various synthetic mixtures of Al2O3 and SiO2 in the presence of Li2CO3 and LiCl as fluxing agents. Various compositions of the above were prepared, and conventional sintering studies were conducted at temperatures of 900°–1450°C with soaking periods of 1–3 h. Kaolin, metakaolin, and amorphized kaolin in the presence of Li2CO3 showed nucleation centers of β‐spodumene as pink specks, whereas synthetic mixtures of Al2O3 and SiO2 failed to behave in the same manner. To determine whether the pink specks formed were color centers or F centers, the samples were subjected to UV, IR, and X‐ray irradiation; however, the samples showed no tenebrescence properties. External addition of iron as an impurity in a nonlayered system also resulted in pink speck formation. This observation indicated that impurities present in the natural kaolin were the cause of this phenomenon. Moreover, the LiCl‐based samples did not result in pink specks, even though the kaolinitic samples contained iron as an impurity. Therefore, although β‐spodumene was formed in aluminosilicates in the presence of Li2CO3 and LiCl, the pink variety of β‐spodumene (kunzite) formation occurred only in the presence of lithium‐rich aluminosilicates and in the presence of iron as an impurity. The phase identification and microstructure were explained based on XRD, DTA, and SEM studies.
Control of Synthesis Conditions to Improve Zirconia Microspheres for Ultrafast Chromatography
Authors: Yan Bingwen , McNeff Clayton V. , Chen Fang , Carr Peter W. , McCormick Alon V.
Published: July 2010

Although it has been known for some time that monodisperse, micrometer‐scale spherical zirconia particles can be synthesized by the hydrolysis of zirconium alkoxides, the particles so made are frequently aggregated. Moreover, it is not clear whether they can be made nonporous without suffering aggregation; this would limit their usefulness in applications like ultrafast chromatography, which require that the particles be spherical and unaggregated (which we term “microspheres”) and that they be nonporous. Here we report that when using an alkoxide hydrolysis synthesis process, critical washing steps can eliminate irreversible aggregation and so enable reliable production of microspheres. We clarify important but practical precautions that easily ensure the reproducibility of the synthesis method. Finally, we demonstrate that the microspheres can be made nonporous with a suitable heating schedule.
Revisit to the Origin of Grain Growth Anomaly in Yttria‐Doped Barium Titanate
Authors: Lee Jung‐Kun , Hong Kug‐Sun , Chung Jang‐Hoon
Published: July 2010

In this study, the effect of atmosphere during calcining and sintering of Y2O3‐doped BaTiO3 was investigated to reveal the origin of the anomaly of grain growth and conductivity as a function of Y2O3 content. Samples with various atmospheric histories were prepared. Microstructural and electrical variations were observed using SEM, dielectric (ɛr vs T) and thermopower measuring techniques, and impedance spectroscopy. An abrupt decrease in grain growth and a transition from semiconducting to insulating behavior were observed when the Y2O3 concentration exceeded 0.3 mol% and the samples were heat‐treated in an O2 atmosphere. In contrast, the samples treated under N2+ 5% H2 atmosphere revealed neither grain growth anomaly nor conductivity anomaly even though the Y2O3 concentration was much greater than 3 mol%. The undoped BaTiO3 samples also demonstrated increased average grain size with increased oxygen partial pressure. Therefore, it was suggested that the grain growth anomaly did not result from the formation of cation vacancies V″Ba or V″″Ti. To investigate the origin of the grain growth anomaly, the Curie temperature, concentration of free electrons, and impedance spectra were measured. The grain growth anomaly and conductivity anomaly were associated with abrupt changes in all the measurements. These abrupt changes could be explained by the partial incorporation of donor dopants. Consequently, the origin of the anomaly in the microstructure and electrical properties are discussed in terms of grain‐boundary segregation of the donor dopant.
Oxidation Kinetics of an Amorphous Silicon Carbonitride Ceramic
Authors: Raj Rishi , An Linan , Shah Sandeep , Riedel Ralf , Fasel Claudia , Kleebe Hans‐Joachim
Published: July 2010

The oxidation kinetics of amorphous silicon carbonitride (SiCN) was measured at 1350°C in ambient air. Two types of specimens were studied: one in the form of thin disks, the other as a powder. Both specimens contained open nanoscale porosity. The disk specimens exhibited weight gain that saturated exponentially with time, analogous to the oxidation behavior of reaction‐bonded Si3N4. The saturation value of the weight gain increased linearly with specimen volume, suggesting the nanoscale pore surfaces oxidized uniformly throughout the specimen. This interpretation was confirmed by high‐resolution electron microscopy and secondary ion mass spectroscopy. Experiments with the powders (having a particle size much larger than the scale of the nanopores) were also consistent with measurements of the disks. However, the powder specimens, having a high surface‐to‐volume ratio, continued to show measurable weight gain due to oxidation of the exterior surface. The wide range of values for the surface‐to‐volume ratio, which included all specimens, permitted a separation of the rate of oxidation of the free surface and the oxidation of the internal surfaces of the nanopores. Surface oxidation data were used to obtain the rate constant for parabolic growth of the oxidation scale. The values for the rate constant obtained for SiCN lay at the lower end of the spectrum of oxidation rates reported in the literature for several Si3N4 and SiC materials. Convergence in the behavior of SiCN and CVD‐SiC is ascribed to the purity of both materials. Conversely, it is proposed that the high rates of oxidation of sintered polycrystalline silicon carbides and nitrides, as well as the high degree of variability of these rates, might be related to the impurities introduced by the sintering aids.
Flaw‐Insensitive Ion‐Exchanged Glass: I, Theoretical Aspects
Authors: Sglavo Vincenzo M. , Larentis Luca , Green David J.
Published: July 2010

Stable crack growth can be produced on residually stressed surfaces, provided that the residual‐stress profile is carefully chosen. Once identified, these conditions will lead to strengthening and an insensitivity of strength to the size of the initial surface flaw. Theoretical calculations are performed that involve choosing idealized forms of either the required toughness curve or the residual‐stress profile. The results of this analysis allow the range of stable growth and the strengthening to be determined. Thus, in the material‐design process, if the range of surface flaws in the stress‐free surface are known, then the required stress profile or toughness curve for optimum performance can be selected.
Origin of Gas Porosity in Gold‐Based Alloys Cast in Calcium Sulfate‐Bonded Investment and Influence of Metal Oxide Acid–Base Properties on Calcium Sulfate Thermal Stability
Authors: Ingo Gabriel Maria , Riccucci Cristina , Chiozzini Gianni
Published: July 2010

The origin of gas‐porosity in gold‐based alloys produced via lost‐wax casting in CaSO4‐bonded investment has been identified using a combination of microanalytical and thermal techniques. The occurrence of gas‐porosity is related to the thermal decomposition of CaSO4, which, with SiO2, constitutes the investment material and decomposes at a temperature very close to the casting temperature of some typical gold alloys used for jewelry production. The thermal reaction generates SO2, leading to gas‐porosity and, therefore, to defective products. Furthermore, the results show the detrimental effect of thermal decomposition caused by the presence of ZnO, Cu2O, CuO, NiO, and Ag2O formed on the surface of the gold‐based alloy during air melting or casting. Therefore, the solid‐state thermal decomposition of CaSO4 in the presence of other ceramic oxides has been investigated and found to be related to their surface acid–base properties, measured as isoelectric points on the solid surface.
Single‐Source Sol‐Gel Synthesis of Nanocrystalline ZnAl2O4: Structural and Optical Properties
Authors: Mathur Sanjay , Veith Michael , Haas Michel , Shen Hao , Lecerf Nicolas , Huch Volker , Hüfner Stefan , Haberkorn Robert , Beck Horst P. , Jilavi Mohammad
Published: July 2010

Nanometer‐sized zinc aluminate (ZnAl2O4) particles were synthesized from heterometal alkoxides, [ZnAl2(OR)8], possessing an ideal cation stoichiometry for the ZnAl2O4 spinel. ZnAl2O4 is formed at 400°C, which is the lowest temperature reported for the formation of monophasic ZnAl2O4. 27Al magic‐angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl2O4 possesses an inverse structure at <900°C, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in‐depth composition and Zn:Al stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence‐band spectra of ZnAl2O4 and ZnS suggested that the hybridization of O 2p and Zn 3d orbitals is responsible for lowering the bandgap in the latter. The average crystallite size showed an exponential relationship to the calcination temperature (X‐ray diffractometry and transmission electron microscopy data). The optical spectra of different spinel powders (average particle sizes, 20–250 nm) showed that the absorption edge exhibits a blue shift as particle size decreases.
High‐Temperature Proton Conductors for the Injection of Hydrogen into a Vacuum
Authors: Schober Tilman , Meuffels Paul
Published: July 2010

Tubes of ceramic high‐temperature proton conductors (CaZr0.9In0.1O3−x,BaZr0.9Y0.1O3−x, and Ba3Ca1.18Nb1.82O9−x)were used to introduce hydrogen into a vacuum system. A prerequisite was a leak rate below 10−8mbar/s for an assembly consisting of the active tube and a 8YSZ support tube. Mass spectrometry showed that the partial pressure of hydrogen in the vacuum system, pH2, increased linearly with the electric current flowing through the proton conductor. All other important partial pressures remained unaffected by this operation. Calculation of throughput during operation of the hydrogen source essentially revealed that the total current, J, is used to transport protons from the anode to the cathode (tH∼ 1). The number of moles of H2 transported per second is given by J/2F. In essence, the present proton conductor tube constitutes a precise, current‐controllable device, with a response time of a few seconds only, that delivers ultrahigh‐purity hydrogen to a vacuum system.
Gel‐to‐Ceramic Conversion during Hydroxyapatite Synthesis
Authors: Hsieh Ming‐Fa , Chin Tsung‐Shune , Perng Li‐Hsiang , Perng Huann‐Guang
Published: July 2010

Conversion of hydroxyapatite (HA, Ca10(PO4)6(OH)2) synthesis from gel to ceramic using solid‐state nuclear magnetic resonance (NMR) spectroscopy and X‐ray diffraction was studied. The sol was prepared by dissolving calcium nitrate tetrahydrate and triethyl phosphate in 2‐methoxyethanol. In solid‐state 31P magic‐angle spinning nuclear magnetic resonance (MAS NMR) spectra, gel calcined at 250°, 350°, and 600°C showed a conversion from gel to glassy phosphate, then HA ceramic. Spin‐lattice relaxation experiments of samples calcined at 600°C indicated that the existence of a CaO impurity might affect OH···O=PO3 linkages in a HA unit cell and result in longer 31P spin‐lattice time.
Ceramic Membrane Filter Using Ultrafine Powders
Authors: Kinemuchi Yoshiaki , Suzuki Tsuneo , Jiang Weihua , Yatsui Kiyoshi
Published: July 2010

A ceramic membrane filter was developed by depositing ultrafine powders onto a porous substrate. The powders, having an average particle size of tens of nanometers, were produced by a newly developed technology, pulsed wire discharge: The carrier gas was pumped through the porous substrate, which had been heated to 500°–700°C, so that the powders could be sintered on the substrate surface, resulting in a membrane filter. Gas separation experiments were conducted using this filter. The experimental results showed a ratio of gas permeation very close to that given by the Knudsen flow.
Crystallization Behavior of Amorphous Silicon Carbonitride Ceramics Derived from Organometallic Precursors
Authors: Iwamoto Yuji , Völger Wolfgang , Kroke Edwin , Riedel Ralf , Saitou Tomohiro , Matsunaga Katsuyuki
Published: July 2010

The crystallization behavior of organometallic‐precursor‐derived amorphous Si‐C‐N ceramics was investigated under N2 atmosphere using X‐ray diffractometry (XRD), transmission electron microscopy (TEM), and solid‐state 29Si nuclear magnetic resonance (NMR) spectroscopy. Amorphous Si‐C‐N ceramics with a C/Si atomic ratio in the range of 0.34–1.13 were prepared using polycarbosilane‐polysilazane blends, single‐source polysilazanes, and single‐source polysilylcarbodiimides. The XRD study indicated that the crystallization temperature of Si3N4 increased consistently with the C/Si atomic ratio and reached 1500°C at C/Si atomic ratios ranging from 0.53 to 1.13. This temperature was 300°C higher than that of the carbon‐free amorphous Si‐N material. In contrast, the SiC crystallization temperature showed no clear relation with the C/Si atomic ratio. The TEM and NMR analyses revealed that the crystallization of amorphous Si‐C‐N was governed by carbon content, chemical homogeneity, and molecular structure of the amorphous Si‐C‐N network.
Hydrothermal Treatment of a Silica Sand Complex with Lime
Authors: Watanabe Osamu , Kitamura Kazuko , Maenami Hiroki , Ishida Hideki
Published: July 2010

The feasibility of using relatively low‐purity silica as the starting material for hydrothermal solidification was investigated from the consideration of effective utilization of natural resources. Hydrothermal solidification was performed at 140°–180°C for periods ranging from 2 to 40 h using a mixture of 80% silica sand (containing 67% quartz, 22% clay minerals (mica, clinochlore, and kaolinite), and 7% feldspar) and 20% Ca(OH)2. Within the range of experimental conditions, the flexural strength of the solidified bodies increased with increased treatment temperature and treatment time, reaching values of up to 20 MPa. The flexural strength was proportional to the amount of, but independent of the type of, reaction products. Hydrogarnet and C‐S‐H were formed in the initial stages of the reaction; the amount of these phases tended to decrease, and 1.1 nm tobermorite formed as the reaction further progressed. These results indicated that hydrogarnet and C‐S‐H were precursors of 1.1 nm tobermorite.
Effect of Flaw State on the Strength of Brittle Coatings on Soft Substrates
Authors: Kim Hae‐Won , Deng Yan , Miranda Pedro , Pajares Antonia , Kim Do Kyung , Kim Hyoun‐Ee , Lawn Brian R.
Published: July 2010

A study is made of the role of flaw state on the strength properties of brittle ceramic coating layers bonded to soft polycarbonate substrates. We introduce Vickers radial cracks at prescribed loads into the coating undersurfaces prior to bonding to control the sizes and locations of the starting flaws. A spherical indenter is then loaded on the top bilayer surfaces, directly above the Vickers indentation sites, subjecting the radial cracks to flexural tensile stress. Radial crack responses are monitored in situ, using a camera located below the transparent substrate. Critical loads to cause radial crack instability, and ensuing growth of the arrested cracks, are recorded. Conventional biaxial flexure tests on corresponding monolith coating materials provide a baseline for data comparison. Relative to the monolith flexure specimens, the bilayers show higher strengths, the more so the larger the flaw, indicating enhanced flaw tolerance. A simple fracture mechanics analysis of the radial crack evolution in the concentrated‐load field, with due account for distribution of flexural tensile stresses at the coating undersurface, is unable to account completely for the enhanced bilayer strengths for the larger Vickers flaws. It is hypothesized that the epoxy used to bond the bilayer components enters the cracks, causing crack‐wall adherence and providing an increased resistance to radial crack instability. The fracture mechanics are nevertheless able to account for the arrest and subsequent stable extension of the radial cracks beyond the critical loads once this extraneous adherence has been overcome.
Damage Evolution and Stress Analysis in Zirconia Thermal Barrier Coatings during Cyclic and Isothermal Oxidation
Authors: Singh Jitendra P. , Nair Balakrishnan G. , Renusch Daniel P. , Sutaria Manish P. , Grimsditch Marcus H.
Published: July 2010

The failure mechanisms of air‐plasma‐sprayed ZrO2 thermal barrier coatings with various microstructures were studied by microscopic techniques after thermal cycling. The elastic modulus (E) and hardness (H) of the coatings were measured as functions of the number of thermal cycles. Initially, both E and H increased by ∼60% with thermal cycling because of sintering effects. However, after ∼80 cycles (0.5 h at 980°C), the accumulated damage in the coatings led to a significant decrease of ∼20% of the maximum value in both E and H. These results were correlated with stresses measured by a spectroscopic technique to understand specific damage mechanisms. Stress measurement and analysis revealed that the stress distribution in the scale was a complex function of local interface geometry and damage in the top coat. Localized variations in geometry could lead to variations in measured hydrostatic stresses from −0.25 to −2.0 GPa in the oxide scale. Protrusions of the top ZrO2 coat into the bond coat were localized areas of high stress concentration and acted as damage‐nucleation sites during thermal and mechanical cycling. The net compressive hydrostatic stress in the oxide scale increased significantly as the scale spalled during thermal cycling.
Hydride Vapor Phase Epitaxy of GaN Thick Films by the Consecutive Deposition Process Using GaCl3
Authors: Lee Soo Min , Kim Young Hoon , Chung Su Jin
Published: July 2010

GaN buffer and main layers were grown by the conventional hydride vapor phase epitaxy technique using GaCl3 consecutively. The deposited buffer layers were investigated by atomic force microscopy and X‐ray analysis. To examine the behavior of the buffer layers at main layer growth temperature, heat treatment was conducted at 900°C. Based on the results of the buffer layer study, GaN thick films were grown at 1050°C. Optimum deposition conditions of buffer layer from the buffer and main layer studies generally coincided. On the φ scanning pattern, the GaN films grown on (0001) Al2O3 were single‐crystalline. Band‐edge emission dominated photoluminescence was observed at room temperature.
Comparison between SEM and TEM Imaging Techniques to Determine Grain‐Boundary Wetting in Ceramic Polycrystals
Authors: Kleebe Hans‐Joachim
Published: July 2010

Two different non‐oxide ceramics, Si3N4 and SiC, were characterized with respect to their grain‐boundary structure employing both scanning and transmission electron microscopy. The latter method, which enables one to gain direct insight of the atomistic interface structure, was utilized to verify whether grain‐boundary wetting occurred. SEM imaging of plasma‐etched surfaces revealed a characteristic bright contrast along interfaces for both ceramics, Si3N4 as well as SiC, suggesting the presence of an intergranular glass film. High‐resolution TEM studies of the Si3N4 sample confirmed that these fine bright lines along grain boundaries represent intergranular glass films separating Si3N4 matrix grains. However, when high‐resolution TEM was employed on SiC samples, which showed a similar contrast variation across SiC grain boundaries in the SEM, the presence of residual glass films was not detected. The SiC materials showed clean grain boundaries with no indication of residual glass even at triple pockets. Chemical analysis monitored yttrium and aluminum segregation at interfaces, which creates a potential barrier (space charges) and therefore affects both the inner mean potential at the interface (Fresnel fringes) and the plasma‐etching response. Although SEM imaging showed a similar interface contrast for both Si3N4 and SiC ceramics, HRTEM studies clearly revealed grain‐boundary wetting in the former and clean interfaces in the latter material, respectively. Hence, SEM imaging and Fresnel fringe TEM imaging alone are not conclusive when characterizing interface wetting in ceramic polycrystals.
Bond Energetics at Intergranular Interfaces in Alumina‐Doped Silicon Nitride
Authors: Painter Gayle S. , Becher Paul F. , Sun Ellen Y.
Published: July 2010

In Si3N4 ceramics sintered with Al2O3, the interfacial strength between the intergranular glass and the reinforcing grains has been observed to increase with increases in the aluminum and oxygen content of the epitaxial β‐Si6‐zAlzOzN8–z layer that forms on the Si3N4 grains. This has been attributed to the formation of a network of strong bonds (cross bonds) that span the glass‐crystalline interface. This proposed mechanism is considered further in light of first‐principles atomic cluster calculations of the relative stabilities of bridge and threefold‐bonded atomic fragments chosen to represent compositional changes at the glass/Si3N4 grain interface. Calculated binding energies indicate Al‐N binding is favorable at the Si3N4 grain surface, where aluminum occupancy can promote the growth of SiAlON, further enhancing the cross‐bonding mechanism of interfacial strengthening.
First‐Principles Calculations of Anion Vacancies in Oxides and Nitrides
Authors: Tanaka Isao , Tatsumi Kazuyoshi , Nakano Masanobu , Adachi Hirohiko , Oba Fumiyasu
Published: July 2010

The formation energy, structural relaxation, and defect‐induced states of neutral anion vacancies of five oxides (i.e., MgO, Al2O3, ZnO, In2O3, and SnO2) and four nitrides (i.e., AlN, Si3N4, Ge3N4, and InN) are systematically discussed, based on first‐principles plane‐wave pseudopotential calculations. Two types of polymorphs for each compound are compared. The number of atoms included in the supercells ranged from 54 to 96. When a localized vacancy‐induced state appears within the band gap, as in a typical ionic crystal, the formation energy can be well scaled by the band gap of the perfect crystal. On the other hand, when an empty and localized vacancy‐induced state is located above the highest occupied band or no localized state is formed, the formation energy has a tendency to be smaller. In compounds such as ZnO and SnO2, the formation energy is dependent largely on the crystal structure. This result can be explained by the transition of the vacancy‐induced state from occupied to unoccupied, which is caused by the change in atomic arrangement, as represented by the cation coordination number.
Shock Synthesis of Cubic Silicon Nitride
Authors: Sekine Toshimori
Published: July 2010

The phase transitions of α‐Si3N4 and β‐Si3N4 have been investigated by shock compression through the recovery technique and Hugoniot measurements. α‐ and β‐Si3N4 are transformed into a cubic spinel structure (c‐Si3N4). The yield of c‐Si3N4 increases with increasing shock pressure and reaches 100% at 63 GPa. The shock‐synthesized c‐Si3N4 powders are nanocrystals and display a high‐temperature metastability up to about 1620 K. c‐Si3N4 is one of the hard materials based on the measured equation of state. c‐Si3N4 powders have been characterized by electron microscopy and 29Si magic angle spinning NMR spectroscopy. The purification and separation method has been developed to obtain pure c‐Si3N4 powders.