bluuebunny/crossref_metadata_2025 huggingface dataset:
Low‐Temperature Sintering and Microwave Dielectric Properties of Zinc Metatitanate‐Rutile Mixtures Using Boron
Authors: Kim Hyo Tae , Kim Seong Ho , Nahm Sahn , Byun Jae Dong , Kim Yoonho
Published: July 2010

Mixtures of zinc metatitanate and rutile (ZnTiO3+xTiO2, where x= 0‐0.5) have been prepared via the conventional mixed‐oxide method. Centrifugal planetary milling with zirconia beads 1 mm in diameter produced very fine powders (mean particle size of 0.2 µm), which allowed the synthesis of ZnTiO3 and sintering at temperatures <945°C, which is the decomposition temperature of ZnTiO3. Sintering of the mixtures was enhanced further by the addition of B2O3. Densities of >94% of the theoretical density have been attained for the specimens that were sintered at 875°C for 4 h with B2O3 additions of <1 wt%. Microwave dielectric properties of the aforementioned compositions were as follows: dielectric constant of 29‐31, normalized quality factor of 56000‐69000 GHz, and a temperature coefficient of resonance frequency between ‐10 and +10 ppm/°C. Sintering was enhanced by the formation of a ZnO‐B2O3 liquid phase, which affected the microwave properties, because of variation in the phase composition.
Evolution of Mechanical Properties of Porous Alumina during Free Sintering and Hot Pressing
Authors: Ostrowski Thomas , Rödel Jürgen
Published: July 2010

This comparative study addresses the influence of microstructural evolution on mechanical properties of porous alumina sintered with and without the application of uniaxial pressure. A complete set of data on Young's modulus, long‐crack fracture toughness, and fracture strength for two alumina powders as a function of density was obtained. The evolution of fracture strength with increased density was modeled using a porosity‐dependent crack‐tip fracture toughness, linked to the contact area of grains and a porosity‐dependent size of the largest defect. The defect shape factor was found independent of porosity. A uniaxial pressure of 13 MPa during densification had negligible effect on the relation of strength to porosity.
Notched Fracture Behavior of an Oxide/Oxide Ceramic‐Matrix Composite
Authors: Kramb Victoria A. , John Reji , Zawada Larry P.
Published: July 2010

The fracture behavior of an oxide/oxide ceramic‐matrix composite, alumina/alumina‐silica (Nextel610/AS), was investigated at 23° and 950°C using a single edge notched specimen geometry with clamped ends. Crack growth and damage progression were monitored during the tests using optical microscopy, ultrasonic C‐scans, and crack mouth opening displacement. The net section strength of Nextel610/AS was less than the unnotched ultimate tensile strength. The failure mode was nonbrittle with considerable nonlinear deformation prior to and after the peak load at 23° and 950°C. The effect of temperature on the notched strength was significant. Net section failure stress decreased 50% when temperature was increased from 23° to 950°C. Observations of damage progression indicated that the reduction in notch strength with temperature was associated with self‐similar crack growth at 950°C. Ultrasonic C‐scans were found to be an effective method of monitoring damage progression. Ultrasonic attenuation ahead of the notch tip was correlated with surface matrix cracks and exposed fiber lengths on the fracture surface.
Thermal Conductivity of ß‐Si3N4: I, Effects of Various Microstructural Factors
Authors: Kitayama Mikito , Hirao Kiyoshi , Toriyama Motohiro , Kanzaki Shuzo
Published: July 2010

Calculations based on a simple modified Wiener's model for thermal conductivity of a composite material predict that the thermal conductivity of ß‐Si3N4 decreases quickly as the grain‐boundary film thickness increases within a range of a few tenths of a nanometer and also that it initially increases steeply with increased grain size, then reaches almost constant values. Because of the faceted nature of the ß‐Si3N4 crystal, the “average” grain‐boundary film thickness is much greater than that in equilibrium. The present study demonstrates both theoretically and experimentally that grain growth alone cannot improve the thermal conductivity of ß‐Si3N4.
Spherical and Vickers Indentation Damage in Yttria‐Stabilized Tetragonal Zirconia Polycrystals
Authors: Suganuma Motohiro
Published: July 2010

Damage induced in an yttria‐stabilized tetragonal zirconia polycrystal by spherical and Vickers indentations was investigated. Scanning acoustic microscopy revealed that, as indentation stress increased, the spherical indentation gradually developed subsurface damage, until it experienced a transition to a fully plastic state, characterized by a highly anisotropic variation in the leaky Rayleigh wave velocity, vR, and very similar to that for Vickers indentation. The transition was a result of the formation of a microcracked core beneath the contact. Indenter geometry had an appreciable effect only within the core; the distribution of microcracks differed depending on the indenter used, as confirmed by direct observations using a scanning electron microscope. In contrast, the residual stresses in the elastic‐plastic zone were insensitive to indenter geometry. The resulting plastic zone was not hemispherical but rather cylindrical, irrespective of indenter geometry.
Piezospectroscopic Analysis of the Residual Stresses in the Strontium Hexaluminate/Zirconia (SrAl12O19/ZrO2) System
Authors: Maschio Stefano , Lucchuni Elio , Sergo Valter
Published: July 2010

Samples of strontium hexaluminate (SrAl12O19), both pure and mixed with different amounts of ceria‐stabilized zirconia (CE‐TZP), have been prepared via a chemical route. The fluorescence spectrum of SrAl12O19is similar to that of ruby and yttrium aluminium garnet (YAG); consequently, this similarity has been attributed to the presence of a trace impurity of Cr3+ions. The most‐intense fluorescence peak of SrAl12O19exhibits a dependence on stress that is given by the average uniaxial piezospectroscopic coefficient ( = 1.5 cm‐1·GPa‐1). The piezospectroscopic coefficient has been used to convert to stress the fluorescence frequency shift of SrAl12O19mixed with zirconia (ZrO2). The stress, as a function of the volume fraction of ZrO2, resides between the Hashin‐Shtrikman bounds but is best reproduced by an effective medium approximation.
Crystallization Kinetics and Properties of Nonstoichiometric Cordierite‐Based Thick‐Film Dielectrics
Authors: Cho Yong S. , Schulze Walter A. , Amarakoon Vasantha R. W.
Published: July 2010

Dielectric thick films based on a nonstoichiometric cordierite (2.4MgO·2Al2O3·5SiO2, containing 3 wt% B2O3, 3 wt% P2O5, and 3 wt% PbO) were investigated, in regard to their microstructure, crystallization kinetics, and properties. A stable glass‐ceramic thick‐film microstructure that was formed on a 96% alumina substrate was observed after firing at a temperature of 915°C for 30 min in a nitrogen atmosphere. No µ‐cordierite was observed in the X‐ray diffraction (XRD) patterns of the thick film. The crystallization kinetics were studied via quantitative XRD analysis using the Avrami equation, and the rate constant increased as the temperature increased. The decreasing tendency of the Avrami parameter, relative to temperature, suggested a change in growth directionality during crystallization. The activation energy for crystallization of the thick film was determined to be ~83 kcal/mol (~350 kJ/mol). The coefficient of thermal expansion (CTE) and the dielectric constant of the glass phase were evaluated using the bulk‐sample data. For the case of a 3‐wt%‐PbO sample fired at 950°C for 30 min in a nitrogen atmosphere, the remaining glass was estimated, using the parallel mixing rule, to have a dielectric constant of 15.3 at 1 MHz. The dielectric constant of the remaining glass was dependent on the PbO content and the heat‐treatment temperature. The estimated CTE of the remaining glass for the 3‐wt%‐PbO sample was 19 × 10‐6/°C.
Crystallization of Barium Osumilite Glass
Authors: Jais Umi S. , Lee William E. , James Peter F.
Published: July 2010

The crystallization of stoichiometric barium osumilite (BaMg2Al6Si9O30) glass manufactured by melting or by sol‐gel processing using an all‐alkoxide route (AAR) or a partial‐alkoxide route (PAR) using mixed alkoxides and salts has been characterized by thermal methods, phase analysis, and microstructural analysis. In all glasses, crystallization with heat treatment at a finite heating rate (1 or 10 K/min) occurs via an initial phase separation, leading to regions devoid of and enriched in BaO. In PAR gel glass and melt glass, the BaO‐deficient regions crystallize as µ‐cordierite, but they appear to remain amorphous in AAR gel glass, in which hexacelsian is the first phase, to crystallize at the interface between the phase‐separated regions. With further heat treatment, barium osumilite becomes the dominant phase. Crystallization with isothermal heat treatment at 1250°C leads to direct crystallization of barium osumilite in bulk and powder samples of melt glass, but not in bulk and powder samples of sol‐gel glass, which again form precursor phases, such as µ‐cordierite, alpha‐cordierite, hexacelsian, and mullite. The crystallization mechanisms and morphologies are discussed here.
Interpretation of Si‐L2,3 Edge Electron Energy Loss Near Edge Structures (ELNES) from Intergranular Glassy Film of Si3N4 Ceramics
Authors: Yoshiya Masato , Adachi Hirohiko , Tanaka Isao
Published: July 2010

First‐principles molecular orbital calculations of model clusters for Si3N4, Si2N2O, SiO2 crystals are made by the discrete variational (DV)‐Xalpha method in order to interpret features that appear in the Si‐L2,3 edge ELNES spectra. They are understood in terms of interaction among molecular orbitals for their structural units, i.e., [SiX4]n‐ tetrahedra (X = N, O). Two parameters, i.e., the N/(N + O) ratio, MN, and the number of N atoms included in a unit tetrahedron, XN, are found to determine the spectral features. On the basis of this knowledge, spatially resolved ELNES from intergranular glassy films of high‐purity Si3N4‐SiO2 ceramics is interpreted. The shift in the first peak by 0.9 eV can be ascribed to MN= 0.43 ± 0.06 in the intergranular film. The N content satisfactorily agrees with that determined by line‐profiling of ELNES. Broadening of the first peak can also be explained consistently by considering this amount of N in the glassy film. An example model‐cluster of the glassy film, [Si7N13O7]25‐ is found sufficient to reproduce the ELNES from the glassy film without inclusion of further complexity or inhomogeneity. The presence of N in the intergranular film is proposed to be the result of the topological requirement imposed by the absence of a broken bond. A model structure that may be useful for further structural optimization is given.
Formation of TiO2(B) Nanocrystallites in Sol‐Gel‐Derived SiO2‐TiO2 Film
Authors: Kogure Toshihiro , Umezawa Taku , Kotani Yoshinori , Matsuda Atsunori , Tatsumisago Masahiro , Minami Tsutomu
Published: July 2010

TiO2(B), one of the polymorphs of TiO2, has been formed by annealing a sol‐gel‐derived SiO2‐TiO2 amorphous film on a silicon wafer at 900°C in air. Transmission electron microscopy (TEM) revealed that nanocrystallites with a size of 5‐10 nm were dispersed in the amorphous SiO2 matrix in the film. The X‐ray diffraction pattern and lattice fringe spacing in high‐resolution TEM images corresponded to those of TiO2(B). These TiO2(B) nanocrystallites are probably stable with the presence of surrounding SiO2 in the film at 900°C, because previous works reported that this phase should be converted to anatase at temperatures higher than 550‐700°C.
Oxygen Content in ß‐Si3N4 Crystal Lattice
Authors: Kitayama Mikito , Hirao Kiyoshi , Tsuge Akira , Toriyama Motohiro , Kanzaki Shuzo
Published: July 2010

ß‐Si3N4 crystals were obtained through the heat treatment of alpha‐Si3N4 powder with additives, Y2O3:SiO2= 1:2 and 2:1, and subsequent acid treatments that removed the secondary phases. The lattice oxygen contents of these crystals were determined by the hot‐gas extraction method to be 0.258 ± 0.006 and 0.158 ± 0.003 wt% for the additive compositions of Y2O3:SiO2= 1:2 and 2:1, respectively. The oxygen dissolved in the ß‐Si3N4 crystal lattice as much as in the alpha‐Si3N4 crystral lattice prepared by the chemical vapor deposition process and in the AlN crystal lattice that exhibited high thermal conductivity.
Structure of Spinel
Authors: Sickafus Kurt E. , Wills John M. , Grimes Norman W.
Published: July 2010

This paper reviews the crystal structure of compounds with the general formula AB2X4, which crystallize with the same atomic structure as the mineral spinel, MgAl2O4. Three degrees of freedom associated with the detailed atomic arrangements of spinels are considered here: (i) the lattice parameter, a; (ii) the anion parameter, u; and (iii) the cation inversion parameter, i. Oxide spinels are used as examples to explore the interrelationships between these parameters.
Cation Disorder and Vacancy Distribution in Nonstoichiometric Magnesium Aluminate Spinel, MgO·xAl2O3
Authors: Sheldon Robert I. , Hartmann Thomas , Sickafus Kurt E. , Ibarra Angel , Scott Brian L. , Argyriou Dimitri N. , Larson Allen C. , Von Dreele Robert B.
Published: July 2010

Neutron diffractometry and X‐ray diffractometry were used to characterize the cation disorder and vacancy distribution in nonstoichiometric spinel, MgO·xAl2O3, where x greaterthan equal to 1.0. Both synthetic and natural samples were examined. Least‐squares refinements of integrated intensities for single crystals and Rietveld profile refinements for powders yielded the average scattering “power” from tetrahedral and octahedral sites within the almost‐perfect cubic close‐packed oxygen sublattice. The cation disorder was calculated assuming models in which the vacancies resided on tetrahedral, octahedral, or both types of sites. No degree of cation disorder was consistent with the tetrahedral vacancy model, and vacancies most likely resided on octahedral sites.
Characterization of MgAl2O4 Precursor Powders Prepared by Aqueous Route
Authors: Montouillout Valérie , Massiot Dominique , Douy André , Coutures Jean Pierre
Published: July 2010

MgAl2O4 spinel precursors have been prepared by two different aqueous synthesis routes: spray drying and citrate complexing. The spinel precursors exhibit different morphology and crystallization behavior. 27Al high‐resolution magic‐angle spinning (MAS), multiple quantum MAS, and 27Al‐{1H} rational echo double resonance (REDOR) nuclear magnetic resonance experiments, show that the spinel precursors have different aluminum local structures. 27Al‐{1H} REDOR experiments detect AlIV‐ and AlV‐bearing proton‐free domains, possibly stabilized by residual carbonates, characteristic of the complexed precursor.
Superconductivity in the Spinel Compound LiTi2O4
Authors: Moshopoulou Evagelia G.
Published: July 2010

LiTi2O4 has been the subject of considerable experimental and theoretical interest since the discovery of its superconductivity by Johnston et al.1 This unique, low‐Tc (11.2 K) spinel oxide superconductor exhibits unusual characteristics, and interestingly, it presents properties that depend on various defects of the spinel structure. This article reviews experimental and theoretical studies of LiTi2O4 and discusses the basic, unresolved issues regarding the physics of this system. Special emphasis is given to methods for preparing the compound and to the structure‐properties relationship.
Comparison of Ion‐Beam Irradiation Effects in X2YO4 Compounds
Authors: Wang LuMin , Gong WeiLang , Wang ShiXin , Ewing Rodney C.
Published: July 2010

The effects of 1.5 MeV Kr‐ion irradiation on seven X2YO4 phases with the olivine (A2VIBIVO4), spinel (AIVB2VIO4), and phenakite structures have been investigated using in situ and high‐resolution transmission electron microscopy (HRTEM) over a wide temperature range (20‐873 K). At low temperatures (<200 K), the olivine and phenakite are susceptible to radiation‐induced amorphization with a critical amorphization dose of 0.2‐0.5 displacement per atom (dpa). The critical amorphization dose increases with increasing irradiation temperature at varying rates for the various phases, resulting in a distinct critical amorphization temperature for each phase. For the Mg‐Fe series of olivine, the susceptibility to amorphization at higher temperatures (room temperature or above) increases with increasing Fe content. Although the spinel phases are, in general, much more resistant to amorphization, a high‐pressure metastable spinel phase, gamma‐SiFe2O4, is easily amorphized at doses <0.2 dpa at temperatures below 723 K. This phase decomposes after irradiation at 873 K. At 20 K, complete amorphization of the FeCr2O4 spinel (chromite) is achieved at ~4 dpa, but no evidence of amorphization is observed in MgAl2O4 spinel after 5.4 dpa. The distinct differences in the relative susceptibility of these phases to amorphization are discussed in terms of the structural and chemical controls on the amorphization process.
Magnetic Properties of Ordered and Disordered Spinel‐Phase Ferrimagnets
Authors: Willard Matthew A. , Nakamura Yuichiro , Laughlin David E. , McHenry Michael E.
Published: July 2010

In spinel‐phase ferrimagnets, disorder in the cation distribution between octahedral and tetrahedral sites can influence such fundamental magnetic properties as saturation magnetization, exchange coupling, and ferrimagnetic ordering temperatures. Given the sensitivity of magnetic properties to structure, it has been suggested that magnetic properties may be used as a probe of disorder in ferrimagnetic spinels. In this article, we review the fundamentals of the magnetic properties of spinel ferrites, including the interesting manifestation of noncollinear spin structures in Yafet‐Kittel triangular spin configurations. This is followed by description of some experimental observations on the structure and magnetic properties of (MnxAl1‐x)3O4 spinels and hausmannites, which exhibit Yafet‐Kittel‐type ordering in their spin structures.
Effects of Neutron Irradiation on the Mechanical Properties of Magnesium Aluminate Spinel Single Crystals and Polycrystals
Authors: Yano Toyohiko
Published: July 2010

The effect of fast neutron irradiation on the mechanical properties of magnesium aluminate spinel single crystals and polycrystals, such as bending strength, fracture toughness, Young's modulus, and hardness, was summarized based on the reported data. Essentially, the changes in these properties are dependent on the nature and concentration of neutron‐irradiation‐induced defects. Furthermore, the efficiency of damage accumulation is known to be dependent on temperature. Spinel shows superior resistance to the formation of defect aggregates, and the recombination of point defects occurs efficiently under neutron irradiation; therefore, the change in mechanical properties is not critical, up to very high neutron fluences, and the spinel maintains its structural integrity.
Structure and Electronic States of Defects in Spinel of Different Compositions MgO·nAl2O3:Me
Authors: Gritsyna Vasily T. , Afanasyev‐Charkin Ivan V. , Kobyakov Vladimir A. , Sickafus Kurt E.
Published: July 2010

This paper reviews the nature of defects and optical centers in magnesium aluminate spinel. The identification of absorption and luminescence bands was used to investigate the influence of UV and electron irradiation on optical center formation. Crystals examined were nominally pure MgO‐nAl2O3 crystals with compositions in the series n= 1.0, 1.5, 2.0, and 2.5. MgO·Al2O3 crystals doped with transition‐metal ion dopants were also studied to determine the influence of impurities on the nature and concentration of lattice defects in spinel. Results strongly suggest that absorption bands at 3.78 and 4.15 eV, and luminescence bands near 5 eV, may be attributed to optical centers on antisite defects. It is also apparent that through the introduction of specific amounts of Mn and Cr ions, the concentration of optical centers (which are related to defects such as cation antisites in the spinel structure) can be considerably changed.
Electrokinetic Properties of Colloidal Zirconia Powders in Aqueous Suspension
Authors: Wei Wen‐Cheng J. , Wang Sheng‐Chang , Ho Fang‐Yuan
Published: July 2010

Pure zirconia, yttria, and three yttria‐doped zirconia powders of submicrometer size have been dispersed in various aqueous solutions. The zeta potential (zeta) of the zirconia powders is determined primarily via streaming‐current (SC) detection and is confirmed using electrophoretic spectroscopy techniques. The results reveal that the isoelectric point (IEP) of these zirconia powders is in the pH range of 5.6‐7.2 and zeta is controlled primarily by the yttrium content of the zirconia powders and the type of electrolyte. In addition, the yttria content strongly affects the potential and SC in zirconia suspensions only at high solids contents (>1 vol%). The electrokinetic data reveal that the surface of the yttria‐doped tetragonal zirconia powder (TZP) can be modified via the adsorption of ionic molecules or polymeric species in the suspension. The adsorption of an anionic polymer can stabilize zirconia particles in a solution that is almost neutral or weakly basic (in the IEP range of pure ZrO2). The interaction of the zirconia and yttria particles with the electrolytes in an aqueous suspension will be discussed to reveal the roles of hydrated oxide formation and zirconia surface interaction with polymeric dispersants.
Bimaterial Composites via Colloidal Rolling Techniques: I, Microstructure Evolution during Rolling
Authors: Menon Mohan , Chen I‐Wei
Published: July 2010

Laminate composites of Al2O3 and ZrO2 fabricated by repeated rolling and folding are demonstrated using aqueous slurries of ceramic powders without any plasticizers or binders. Crack‐free composites with a layer thickness of 4‐60 µm have been obtained. A novel interface instability during the rolling of bimaterial slurries, which leads to the entrapment of the rheologically harder phase in the softer phase, also has been discovered. This discovery has been exploited to obtain layered materials, by matching the yield stress of the slurries, or cellular materials with the desired included phase, by mismatching the yield stress of the slurries. Adjustment of the yield stress can be realized by varying only the NH4NO3 salt concentration while keeping the pH and the solids loading (64 vol%) constant.
Seasonal Variation of Microstructure and Sintered Strength of Dry‐Pressed Alumina
Authors: Shinohara Nobuhiro , Okumiya Masataro , Hotta Tadashi , Nakahira Kenji , Naito Makio , Uematsu Keizo
Published: July 2010

An origin was investigated for the variation of the density and the fracture strength of sintered alumina with the manufacturing season. Direct observation using immersion microscopy was utilized to examine the microstructures of granules, green bodies, and sintered samples for two specific cases: samples made in summer and others made in winter. This method revealed a seasonal difference in the pore structure of both green and sintered bodies. The variation of the density and the fracture strength with the manufacturing season was ascribed to the different concentrations of large pore defects in sintered bodies, which were developed from the green body structures. Formation of large pore defects resulted from void spaces at the center and at the boundary of granules in the green bodies. High temperature and humidity contributed to an increase in the deformability of granules, reducing defect sizes in summer and thus improving fracture strength.
Kinetic Study of Cobalt Silicide Infiltration into Silicon Carbide Preforms
Authors: Pan Yi , Yi Xiao S. , Baptista Joao L.
Published: July 2010

Infiltrations of titanium‐doped cobalt silicide into silicon carbide preforms were performed using an indirect infiltration method. The infiltration was found to proceed in two stages. In the early stage a melt spread over the pore surfaces and coated the solid particles, and in the final stage the melt filled the intervals between coated particles. The degree of infiltration is linearly dependent upon infiltration time in the early stage and slows down in the final stage. The Washburn equation and Darcy's law do not fit the infiltration process. A model involving melt spreading and coating during infiltration is proposed to explain the linear dependence between the degree of infiltration and the infiltration time.
1:1 Ordered Domain Growth in Pb(Mg1/3Ta2/3)O3‐La(Mg2/3Ta1/3)O3 Relaxor Ferroelectric Perovskites
Authors: Montgomery John K. , Akbas Mehmet A. , Davies Peter K.
Published: July 2010

Single‐phase perovskite solid solutions are formed across the entire (1 ‐ x)Pb(Mg1/3Ta2/3)O3‐xLa(Mg2/3Ta1/3)O3 (PLMT) pseudobinary system. Although as‐sintered (1300°C, 3 h) samples with x lessthan equal to 0.1 adopt a phase‐separated, “PMN‐type” microstructure comprising small (2‐3 nm) ordered domains dispersed in a disordered matrix, extensive domain growth and complete order can be induced by extended thermal annealing (1300°C, 24 h). These observations, and the alterations in the thermal stability and domain size across the system, can be interpreted using a charge‐balanced random site model for the cation order. No evidence is found to support the space‐charge models currently used to describe the cation ordering and properties of the corresponding niobate systems.
Finite‐Element Calculations on the Impedance of Electroceramics with Highly Resistive Grain Boundaries: I, Laterally Inhomogeneous Grain Boundaries
Authors: Fleig Jürgen , Maier Joachim
Published: July 2010

Lateral inhomogeneities of grain boundaries caused by partially wetting grain‐boundary phases or pores induced in the grain boundary give rise to current‐constriction resistances close to grain‐to‐grain contacts. The present work presents a quantitative three‐dimensional analysis of the resulting grain‐boundary impedance. Numerical finite‐element calculations of the impedance spectra of polycrystalline samples characterized by typical imperfect contact patterns are performed and analyzed. The validity of existing models is checked, and an interpolation formula for the grain‐boundary resistance is given, using the fraction of contacted area and the number of contact spots per grain as parameters. The magnitude of the effects that can be expected in realistic ceramics is emphasized, as well as the question of the extent to which further conclusions on the grain‐boundary mechanism can be drawn from the temperature, partial‐pressure, bias, and grain‐size dependence of the impedance spectra.
Oxidation of BN/Nicalon Fiber Interfaces in Ceramic‐Matrix Composites and Its Effect on Fiber Strength
Authors: Pérez‐Rigueiro José , Celemín José Antonio , LLorca Javier , Herrero Pilar
Published: July 2010

Microstructural changes at the interface were analyzed in two Nicalon‐fiber ceramic‐matrix composites with a dual BN/SiC coating on the fibers after thermal exposure at different temperatures (in the range 800°‐1400°C) and in different environments (air and argon). The outer SiC coating acted as a barrier to oxygen, which penetrated into the composite via pipeline diffusion along the BN/fiber interfaces. Oxygen penetration led to the formation of an SiO2 layer by oxidation of the fiber surfaces. The in situ fiber strength at different temperatures, as determined from the radius of the mirror region on the fiber fracture surface, indicated that this SiO2 layer severely degraded the fiber strength. Oxidation was highly dependent on the nature of the BN/fiber interface. The presence of a thin carbon‐rich interlayer, which burned out rapidly at high temperature, favored the entry of oxygen and accelerated oxidation of the fibers.
Young's Modulus and Compositional Parameters of Oxide Glasses
Authors: Inaba Seiji , Fujino Shigeru , Morinaga Kenji
Published: July 2010

The Young's moduli of various oxide glasses (silicate, borate, phosphate, and tellurite) were measured using an ultrasonic method. To predict the Young's moduli of the oxide glass systems, empirical compositional parameters Gi and Vi, based on the Makishima‐Mackenzie theory, were obtained, where Gi is the dissociation energy and Vi the packing density parameter of a single‐component oxide. The relationship between the calculated Young's modulus from the compositional parameters and the measured Young's modulus was investigated. Experimental results indicated that the Young's modulus of phosphate and tellurite glasses could not be predicted using these parameters. Thus, it was necessary to modify the Gi, by considering P2O5 and TeO2 as glass network formers. As for the phosphate glass, it exhibited a layered structure that consisted of P=O double bond and three chains of P‐O bond. In this paper, the modified Gi of P2O5 was calculated using the assumption that the P=O double bond is a nonbridging bond and does not contribute to Young's modulus. In the case of tellurite glass, the glass structure is mainly composed of TeO4 trigonal pyramids, and the addition of other oxides results in structural changes to the TeO3 trigonal pyramid. However, the mechanisms of such structural changes have not yet been clarified. Therefore, the modified Gi of TeO2 was calculated from the measured value using the density and Young's modulus of pure TeO2 glass. The results revealed that the calculated values using our proposed parameter were in good agreement with measured values all through the oxide glasses.
Probabilistic Thermal‐Shock Strength Testing Using Infrared Imaging
Authors: Wereszczak Andrew A. , Scheidt Rebecca A. , Ferber Mattison K. , Breder Kristin
Published: July 2010

A thermal‐shock strength‐testing technique has been developed that uses a high‐resolution, high‐temperature infrared camera to capture a specimen's surface temperature distribution at fracture. Aluminum nitride (AlN) substrates are thermally shocked to fracture to demonstrate the technique. The surface temperature distribution for each test and AlN's thermal expansion are used as input in a finite‐element model to determine the thermal‐shock strength for each specimen. An uncensored thermal‐shock strength Weibull distribution is then determined. The test and analysis algorithm show promise as a means to characterize thermal shock strength of ceramic materials.
Effects of Additives on Microstructure and Properties of Alumina‐Silicon Carbide Nanocomposites
Authors: Jeong Young‐Keun , Nakahira Atsushi , Niihara Koichi
Published: July 2010

Al2O3/5‐vol%‐SiC nanocomposites have been fabricated by using pressureless sintering with MgO and/or Y2O3 sintering aids and post‐hot isostatic pressing (HIPing), which circumvents the limitations of hot pressing. Al2O3/SiC nanocomposites that have been doped with 0.1 wt% MgO and 0.1 wt% MgO + 0.1 wt% Y2O3 show an increased sintering density and a homogeneous microstructure, as well as a high fracture strength (1 GPa) after HIPing. In contrast, using Y2O3 as a dopant has a negative impact on the microstructure and the fracture strength. The results suggest that MgO, as a sintering additive, has a key role in improving the densification and controlling the microstructure of Al2O3/SiC nanocomposites.
Effect of Al2O3 Additions on the Thermal Expansion Behavior of a Li2O‐ZnO‐SiO2 Glass‐Ceramic
Authors: Demirkesen Erdem , Erkmen Z. Engin , Yildiz Nil
Published: July 2010

The effect of Al2O3 substitution on the thermal expansion behavior of a Li2O‐ZnO‐SiO2 glass‐ceramic was investigated using differential thermal analysis, X‐ray diffractometry, and dilatometry. The coefficient of thermal expansion of the original Al2O3‐free glass‐ceramic measured between 20° and 900°C decreased from 12.4 × 10‐6°C‐1 to 6.3 × 10‐6°C‐1 with Al2O3 substitution for ZnO up to 11 wt%. The results were correlated to the changes in the phase assemblage with Al2O3 addition.
Silicon Nitride and Related Materials
Authors: Riley Frank L.
Published: July 2010

Silicon nitride has been researched intensively, largely in response to the challenge to develop internal combustion engines with hot‐zone components made entirely from ceramics. The ceramic engine programs have had only partial success, but this research effort has succeeded in generating a degree of understanding of silicon nitride and of its processing and properties, which in many respects is more advanced than of more widely used technical ceramics. This review examines from the historical standpoint the development of silicon nitride and of its processing into a range of high‐grade ceramic materials. The development of understanding of microstructure–property relationships in the silicon nitride materials is also surveyed. Because silicon nitride has close relationships with the SiAlON group of materials, it is impossible to discuss the one without some reference to the other, and a brief mention of the development of the SiAlONs is included for completeness.
The Reaction‐Bonded Aluminum Oxide Process: I, The Effect of Attrition Milling on the Solid‐State Oxidation of Aluminum Powder
Authors: Suvaci Ender , Simkovich George , Messing Gary L.
Published: July 2010

The effect of attrition milling on the solid‐state oxidation of aluminum powder is important for the reaction‐bonded aluminum oxide process. Attrition milling increased the surface area to 14.4 and 20.2 m2/g versus 1.2 m2/g for unmilled powder and smeared the Al particles, and the surface was hydrolyzed to form bayerite and boehmite. Upon heating the hydroxides decompose to form an 11–13 nm thick amorphous plus γ‐Al2O3layer which subsequently retards oxidation kinetics. The oxidation per unit area decreases for the higher surface area powders at temperatures below the critical temperature but the total oxidation of the milled powder is ∼70% versus ∼9% for the as‐received powder because of the higher surface area. The critical temperature depends on Al particle surface characteristics and is defined as the transition temperature above which the initial rate of oxidation is linear, not parabolic. Above the critical temperature the oxidation per unit area decreases significantly. In addition, linear oxidation occurs faster than parabolic oxidation and thus the initial fast oxidation kinetics (i.e., linear) can cause thermal runaway during oxidation. Therefore, oxidation below the critical temperature is essential to maximize solid‐state oxidation and to prevent thermal runaway. The critical temperatures for the as‐received (1.24 m2/g), the 6 h (14.4 m2/g), and 8 h (20.2 m2/g) attrition‐milled Al powders were 500°, 475°, and 500°C, respectively. A model for oxidation during the parabolic and linear oxidation stages is presented.
Strengthening and Prevention of Oxidation of Aluminum Nitride by Formation of a Silica Layer on the Surface
Authors: Koh Young‐Hag , Choi Jong‐Jin , Kim Hyoun‐Ee
Published: July 2010

A silica (SiO2) layer was deposited on the surface of an AlN ceramic in order to increase the strength and to prevent the high‐temperature oxidation of the material. The layer was formed on the surface by exposing coupons to the atmosphere downstream of a bed of SiC powder in a flowing H2–0.1% H2O atmosphere at 1450°C. A reaction between the SiC powder and H2O in the H2gas resulted in the generation of SiO2“smoke” in the product gas stream. Part of the SiO2smoke was subsequently deposited on the surface of the AlN specimen to form a dense and uniform SiO2layer. The strength of AlN was improved by about 20% apparently because of blunting of surface defects by SiO2. More importantly, the layer was very effective in protecting the AlN from the oxidation at elevated temperatures, through the inhibition of transport of oxidants to the sample surface.
Elevated‐Temperature Mechanical Properties of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics
Authors: Trice Rodney W. , Halloran John W.
Published: July 2010

A unique, all‐ceramic material capable of nonbrittle fracture via crack deflection and delamination has been mechanically characterized from 25° through 1400°C. This material, fibrous monoliths, was comprised of unidirectionally aligned 250 μm diameter silicon nitride cells surrounded by 10 to 20 μm thick boron nitride cell boundaries. The average flexure strengths of fibrous monoliths were 510 and 290 MPa for specimens tested at room temperature and 1300°C, respectively. Crack deflection in the BN cell boundaries was observed at all temperatures. Characteristic flexural responses were observed at temperatures between 25° and 1400°C. Changes in the flexural response at different temperatures were attributed to changes in the physical properties of either the silicon nitride cells or boron nitride cell boundary.
Elastic Modulus by Resonance of Rectangular Prisms: Corrections for Edge Treatments
Authors: Quinn George D. , Swab Jeffrey J.
Published: July 2010

The effects of edge chamfers on the resonant frequency of rectangular bar‐shaped ceramic specimens, as well as their effect on the dynamic elastic modulus, have been studied. A correction factor must be applied to obtain the true elastic moduli of specimen bars with chamfered or rounded edges.
Microstructural Characterization of Reactively Sintered Mullites
Authors: Kara Ferhat , Turan Servet , Little John A. , Knowles Kevin M.
Published: July 2010

Mullite ceramics have been produced by reaction sintering of powders prepared using pseudoboehmite–colloidal silica and aluminum sulfate–colloidal silica mixtures. The microstructural development of these mullites was studied by a number of transmission electron microscopy based techniques including diffuse dark field, Fresnel fringe defocus imaging, and high‐resolution transmission electron microscopy. This characterization procedure showed that mullite ceramics free from glassy phases at triple junctions and grain boundaries could be produced from both mixtures using suitable sintering temperatures and alumina/silica ratios. The wetting of grain boundaries by glass, occurring in the mullite ceramics from either incomplete reaction between alumina and silica components or release of silica from the mullite structure with increasing temperature, was found to depend on the prior thermal history of the ceramics.
Zirconium Oxide Structure Prepared by the Sol–Gel Route: I, The Role of the Alcoholic Solvent
Authors: Caracoche María C. , Rivas Patricia C. , Cervera Mario M. , Caruso Ricardo , Benavídez Edgardo , de Sanctis* Oscar , Escobar Marta E.
Published: July 2010

Sol–gel‐derived powder samples of zirconia (ZrO2) prepared via the dissolution of zirconium n‐propoxide in methanol, ethanol, and 2‐propanol have been characterized mainly using perturbed angular correlations spectroscopy, as a function of temperature. Results indicate that the nanostructures and subsequent thermal evolution are alcohol dependent: the shorter the alcohol chain, the more hydrolyzed the product. ZrO2 powder that has been obtained using ethanol as the solvent is the product that exhibits the better stabilization of the metastable tetragonal phase (t‐phase). It undergoes a clear and detailed t1‐form →t2‐form → monoclinic ZrO2 thermal transformation and shows the highest activation energy against the transformation to the monoclinic phase.
Phase Stability of Yttria‐Stabilized Zirconia with Dissolved Cerium and Neptunium Oxides Under Oxidizing and Reducing Atmospheres
Authors: Kinoshita Hajime , Kuramoto Ken‐ichi , Uno Masayoshi , Yanagi Tadashi , Yamanaka Shinsuke , Mitamura Hisayoshi , Banba Tsunetaka
Published: July 2010

Phase stability and effects of sintering atmosphere on the crystalline structure of Np‐doped yttria‐stabilized zirconia (YSZ) were evaluated in comparison with those same properties and conditions for Ce‐doped YSZ. Different sintering atmospheres for Ce‐doped YSZ led to differences in phase formation through reduction of the dopant from Ce4+ to Ce3+. On the other hand, YSZ specimens containing up to 40 mol% Np formed only a fluorite‐structure phase regardless of sintering atmosphere. Yttria‐stabilized zirconia thus seems to accommodate Np within a wide range of concentrations and to have excellent phase stability under both oxidizing and reducing atmospheres.
Contact‐Damage Behavior of Textured and Random Alumina Ceramics under Hertzian Indentation
Authors: An Linan
Published: July 2010

Two alumina ceramics with different microstructures were prepared by tailoring the processing conditions. In one of them, elongated alumina grains were aligned parallel to each other (“textured” material), whereas in the other, the elongated grains were uniformly distributed (“random” material). Hertzian indentation was performed on these two materials. The results showed that, although both materials exhibited a diffuse damage zone rather than typical cone cracks, the extent of damage in the textured material is significantly less than that in the random material. The difference in damage behavior between these two materials was interpreted with the aid of the contact‐stress field. A new microstructural design method for improving contact‐damage resistance is suggested, based on the present results.
Low‐Temperature Synthesis of Lead Tantalate Pyrochlore Solid Solutions Pb1.5+x(Ta2−yPby)O7−δ (0.0 < x < 0.5; 0.0 < y < 0.6)
Authors: Sreedhar Krishnan , Mitra Anjana
Published: July 2010

Synthesis of lead tantalate pyrochlores by the reaction of PbO and Ta2O5 in various molar ratios between 1.5 and 4.0 at low temperatures (500–650°C) has been carried out. It has been shown that when PbO is reacted with Ta2O5 in various molar ratios at low temperatures, metastable lead tantalate pyrochlore solid solutions having cubic symmetry are formed. X‐ray diffraction data give evidence for an increase in the a lattice parameter with increasing PbO content and partial occupancy of Pb in the Ta sites leading to the formula Pb1.5+x2+(Ta2−yPby4+)O7−δ (0.0 < x < 0.5; 0.0 < y < 0.6) for this phase. It has also been found that the intermediate members of the solid solutions are metastable and tend to segregate into Pb‐deficient and Pb‐excess compositions leading to significant compositional inhomogeneity for the intermediate members.
Effect of Poling Direction on R‐Curve Behavior in Lead Zirconate Titanate
Authors: dos Santos e Lucato Sergio L. , Lupascu Doru C. , Rödel Jürgen
Published: July 2010

R‐curves of lead zirconate titanate (PZT) have been measured with compact tension (CT) specimens for different poling conditions and grain sizes. Depending on poling direction the plateau value of the R‐curves ranged from 1.13 to 1.54 MPa·m1/2 for a grain size of 6.4 μm and from 1.14 to 1.30 MPa·m1/2 for a grain size of 5.2 μm. Poling in the thickness direction yielded the material with the highest fracture toughness while the direction parallel to the loading direction led to the lowest fracture toughness.
Low‐Temperature Synthesis of Nanocrystalline Titanium Nitride via a Benzene–Thermal Route
Authors: Hu Junqing , Lu Qingyi , Tang Kaibin , Yu Shuhong , Qian Yitai , Zhou Guien , Liu Xianming
Published: July 2010

A benzene–thermal reaction of TiCl4 and NaN3 at 350°–380°C was conducted for the preparation of nanocrystalline TiN. Powder X‐ray diffractometry patterns indicated that the powder was cubic‐phase TiN with a lattice constant a= 4.23656 Å. Transmission electron microscopy images showed the TiN powders consisted of uniform spherical particles with an average diameter of 50 nm. The binding energies of Ti 2p3/2 and N 1s core levels at the positions of 454.85 and 397.1 eV, respectively, and the Ti:N atomic ratio of 1.08:1.00 were detected by X‐ray photoelectron spectra. A possible formation mechanism of TiN was proposed.
Tensile Fracture Behavior and Strength of Surface‐Modified SiTiCO Fiber SiC‐Matrix Minicomposites Fabricated by the PIP Process
Authors: Mamiya Takayuki , Kagawa Yutaka , Shioji Yasuhiro , Sato Mitsuhiko , Yamamura Takemi
Published: July 2010

The effect of surface‐modified SiTiCO (Tyranno®) fiber on the tensile behavior of SiC‐matrix minicomposites has been examined. Standard SiTiCO fiber and two kinds of surface‐modified fibers were used and the composites were fabricated by a polymer infiltration pyrolysis process. The strengths of the surface‐modified‐fiber‐reinforced minicomposites were significantly improved compared to that of the standard fiber minicomposite. The scattering of tensile strength also decreased with the use of surface‐modified fibers. The results demonstrate the achievement of SiTiCO‐fiber‐reinforced SiC‐matrix composite by modifying the surface chemistry of the fiber.
Effect of Precursor Pyrolysis on the Dielectric Properties of Hydrothermally Derived Barium Titanate Thin Films
Authors: McCormick Mark A. , Slamovich Elliott B.
Published: July 2010

BaTiO3 thin films were processed hydrothermally on Ag‐coated quartz substrates at 90°C by reacting films of titanium dimethoxy dineodecanoate (TDD) in aqueous solutions of Ba(OH)2. Two reaction sequences were used: either the TDD was reacted directly in aqueous Ba(OH)2, or the TDD was first pyrolyzed in air at temperatures ranging from 200° to 500°C before hydrothermal reaction. Depending on the processing conditions, the dielectric constant of the thin films ranged from 5 to 170, the dielectric constant increasing with increasing pyrolysis temperature. Thin film porosimetry data suggest that the improvement in film dielectric performance is related to decreases in thin film residual porosity after hydrothermal reaction.
B6O–c‐BN Composites Prepared by High‐Pressure Sintering
Authors: Itoh Hideaki , Yamamoto Ryuuji , Iwahara Hiroyasu
Published: July 2010

High‐pressure sintering behavior in the B6O–c‐BN system was investigated using in‐laboratory‐synthesized B6O and commercially available c‐BN powders (with an average grain size of 0.5, 3, or 6 μm). No reaction occurred between the two components under the high‐pressure (4–6 GPa) and high‐temperature (1500°–1800°C) conditions that have been investigated. Well‐dispersed, sintered B6O–x(c‐BN) composites (wherex= 0–60 vol%) of almost‐full density were prepared by sintering at a pressure of 6 GPa and temperature of 1800°C for 20 min. The maximum Vickers microhardness (46 GPa) of these composites was attained by adding 40 vol%c‐BN with an average grain size of 0.5 μm. The fracture toughness of these composites increased as the c‐BN content increased; the maximum fracture toughness (1.5–1.8 MPa.m1/2) was observed for x= 40–60 vol%. Crack deflection along the B6O–c‐BN grain boundary contributed to increasing the fracture toughness.
Structure Formation in the Combustion Synthesis of Al2O3–TiC Composites
Authors: Xia Tian D. , Munir Zuhair A. , Tang Yan L. , Zhao Wen J. , Wang Tian M.
Published: July 2010

Structure formation in the combustion synthesis of Al2O3–TiC composites from TiO2, Al, and graphite powders was investigated using cylindrical samples and cone‐shaped “quenching samples.” It is shown that the phases Ti and Ti3Al exist as intermediates in the combustion synthesis process. Titanium carbide forms in a secondary step through reactions between graphite and liquid Ti or Ti3Al, then nucleates from a liquid mixture of the three phases Ti, Ti3Al, and alumina. The nucleated particles grow in the postcombustion stage. Liquid alumina, containing TiC as a dissolved phase, solidifies into corundum grains in the postcombustion stage. Moreover, it is shown that the temperature gradient in the postcombustion stage markedly affects the microstructures of the products. Higher‐temperature gradients, typical at the surface of the samples, give rise to the formation of corundum whiskers and TiC agglomerates. In contrast, lower gradients, typical in the center of the samples, lead to the formation of relatively large TiC particles and corundum grains.