bluuebunny/crossref_metadata_2025 huggingface dataset:
Improvement of the PTCR Effect in Ba1‐xSrxTiO3 Semiconducting Ceramics by Doping of Bi2O3 Vapor during Sintering
Authors: Qi Jianquan , Chen Wanping , Wu Yajing , Li Longtu
Published: July 2010

The influence on the PTCR effect in Ba1‐xSrxTiO3 semiconducting ceramics by doping of Bi2O3 vapor during its sintering is reported in this paper. Comparative experiments showed that the grains of Bi2O3‐vapor‐doped samples were much smaller than those of undoped ones; both the magnitude of the resistance jump and the positive temperature coefficient of resistance were considerably increased. Very dense samples with 8 orders of magnitude of resistance jump were obtained. The mechanism for the effect of Bi2O3 vapor doping is also discussed.
Hot Pressing of Annular Alumina–Zirconia Composites
Authors: Bhattacharya Sandeepan , Jakus Karl
Published: July 2010

An analysis is performed to predict the densification during and the state of residual stress after hot pressing of annular alumina/zirconia (3Y‐TZP) composites. The objective of the analysis was to study the residual stresses resulting from stress gradients during pressing and those from thermal expansion mismatch during the cooling of the compact from the pressing temperature to room temperature. It is predicted that the residual stresses are affected by the respective densification rates of the core and the annulus, their elastic modulus, and thermal expansion coefficient. For the system analyzed in this study, it is predicted that hot pressing reduces the residual stresses that result from the mismatch in thermal expansion coefficients. This is due in part to the high densification rate and in part to the high elastic modulus of the alumina annulus compared to the zirconia core. For surface compression strengthening, a system where the annulus would have similar elastic modulus but lower densification rate and lower thermal expansion coefficient than the core would be more beneficial.
Application of the Potts Model to Simulation of Ostwald Ripening
Authors: Tikare Veena , Cawley James D.
Published: July 2010

Many models have been developed to simulate microstructural evolution in recent years. Among these is the Monte Carlo Potts model used to simulate microstructural evolution under a wide range of conditions for a variety of microstructural evolution processes. An advantage of the Potts model is that the thermodynamic and kinetic characteristics are inherent to the technique. In this paper, grain growth by Ostwald ripening will be used as an example to demonstrate how a well‐designed Potts model need not explicitly specify the thermodynamics of the process; rather, it should rely on the statistical‐mechanical nature of the model to simulate the physics of the process. Simulation of Ostwald ripening requires two components forming two‐phase, solid grains in a liquid matrix with solubility of the components forming the solid phase in the liquid phase. Another requirement is Gibbs‐Thomson's behavior for the solid grains where solubility is inversely proportional to size. The incorporation of these characteristics into the Potts model using the statistical‐mechanical nature of the system will be demonstrated in this paper.
Atomistic Simulations of Materials Fracture and the Link between Atomic and Continuum Length Scales
Authors: Cleri Fabrizio , Phillpot Simon R. , Wolf Dieter , Yip Sidney
Published: July 2010

The macroscopic fracture response of real materials originates from the competition and interplay of several atomic‐scale mechanisms of decohesion and shear, such as inter‐planar cleavage and dislocation nucleation and motion. These phenomena involve processes over a wide range of length scales, from the atomic to the macroscopic. We briefly review the attempts to span these length scales in dislocation and fracture modeling by (1) fully atomistic large‐scale simulations of millions of atoms or more, approaching the continuum limit from the “bottom‐up”; (2) directly coupling atomic‐scale simulations and continuum mechanics, in a “top‐down” approach; and (3) by defining a set of variables common to atomistic simulations and continuum mechanics and feeding the results of atomistic simulations into continuum‐mechanics models in the form of constitutive relations. For this latter approach we discuss in detail the issues crucial to ensuring the consistency of the atomistic results and continuum mechanics. A case study of the constitutive‐relation approach is presented for the problem of dislocation nucleation from a crack tip in a crystal under stress; a comparison of the results of atom‐istic simulations to the Peierls–Nabarro continuum model is made.
Three‐Dimensional Finite‐Element Analysis of Viscous Sintering
Authors: Zhou Hua , Derby Jeffrey J.
Published: July 2010

A finite‐element model is developed and implemented on a massively parallel supercomputer to study the viscous sintering of a three‐dimensional (3‐D) configuration of three particles. Model development is described, implementation is discussed, and calculations are compared with benchmark results for a two‐dimensional (2‐D) axisymmetric case. For the first time, detailed simulation results are shown for the viscous sintering of a 3‐D, three‐particle system. The calculations predict center approach of particle pairs concurrent with asymmetric neck growth, effects which ultimately lead to an inward rotation of the outer particles. Anisotropic shrinkage arises due to this 3‐D rearrangement of the particles during sintering.
Atomic Force Microscopy Study of Ceramic Powder Compacts during Drying
Authors: Prica Mirjana , Kendall Kevin , Markland Svend A.
Published: July 2010

Atomic force microscopy (AFM) was used to image the surface of ceramic powder compacts that contained polymer and solvent during drying. Topographical information was continuously collected as a function of drying time. Surface changes were directly observed during imaging. Surface crystals appeared on damp zirconia sheets after drying, subsequently contributing to secondary flaws in the form of holes in fired ceramic products. In cases where few surface crystals were observed in the green stage, fine grain size and high strength were observed after firing. The fired material was viewed via AFM and shown to be directly related to the drying process that was observed earlier.
Quantitative Comparison of Transmission Electron Microscopy Techniques for the Study of Localized Ordering on a Nanoscale
Authors: Pan Xiaoqing , Kaplan Wayne D. , Rühle Manfred , Newnham Robert E.
Published: July 2010

Different transmission electron microscopy techniques have been compared for the detection and size measurements of ordered regions on a nanometer scale in a disordered matrix phase. As a model system, the perovskite lead magnesium niobate doped with varying amounts of La has been chosen for the present study. While selected area electron diffraction can reveal the existence of ordered structures, dark‐field contrast micrographs are usually used to reveal the spatial distribution and the size of ordered regions. However, for small ordered regions, a few nanometers in diameter, the measurements are no longer accurate because of superposition in projection. High‐resolution transmission electron microscopy (HRTEM) combined with image processing is shown to be a suitable technique to provide a quantitative measurement of such localized ordering. The detection limit of this method is discussed in detail, using the experimental results and HRTEM image simulations.
Free‐Energy‐Based Model of Chemical Equilibria in the CaO–SiO2‐H2O System
Authors: Thomas Jeffrey J. , Jennings Hamlin M.
Published: July 2010

The chemical equilibrium between the C‐S‐H gel phase and the aqueous phase in the CaO‐SiO2‐H2O system was modeled using the free‐energy‐based Thermo‐Calc commercial software system. Unlike programs that are based on solubility products, the minimization of free energy allows the composition and solubility of the C‐S‐H phase to vary, as in real cementitious systems. The C‐S‐H gel is modeled as a defect tobermorite structure, based on recent compositional data gathered using 29Si NMR. The composition is varied by removal of some of the bridging silica tetrahedra in the SiO layers and by incorporating different amounts of Ca in the interlayer spaces.
Fracture Resistance Measurement Method for in situ Observation of Crack Mechanisms
Authors: Sørensen Bent F. , Horsewell Andy , Jørgensen Ole , Kumar Amar N. , Engbæk Preben
Published: July 2010

A special test fixture has been developed for fracture mechanical testing of brittle materials inside an environmental scanning electron microscope. The fixture loads a double cantilever beam specimen with pure bending moments and provides stable crack growth. Crack growth is detected by in situ observation and acoustic emission. As an example, crack growth in a cubic‐phase yttria‐stabilized zirconia is detected easily by in situ observation of the crack‐tip region. Many fracture toughness measurements are obtained for each specimen, giving high confidence in the measured fracture toughness value. In situ observation is useful for the study of toughening mechanisms and subcritical crack‐growth behavior and to sort out erroneous measurements (e.g., due to crack branching).
Composition Dependence of the Electrical Conductivity of ZnO(n)–CuO(p) Ceramic Composite
Authors: Jun Seok‐Taek , Choi Gyeong Man
Published: July 2010

Electrical properties of semiconducting ZnO(n)‐CuO(p) ceramic composites with varying composition were investigated. The electrical conductivity increased with increasing CuO volume fraction between 1 to 95 mol%. Impedance response showed three semicircles, indicating three resistive elements contributing to the total resistance of the composite. A new model based on the equivalent circuits was developed to explain the contribution of grain boundaries to the resistance of the composite. The change of electrical conductivity was explained by the probability change of two equivalent circuits.
Phase Diagram for the 0.4Pb(Ni1/3,Nb1/3)O3–0.6Pb(Zr,Ti)O3Solid Solution in the Vicinity of a Morphotropic Phase Boundary
Authors: Robert Gilles , Demartin Marlyse , Damjanovic Dragan
Published: July 2010

A phase diagram based on dielectric‐permittivity‐versus‐temperature measurements and high‐temperature X‐ray diffractometry was proposed for 0.4Pb(Ni1/3,Nb1/3)O3‐xPbZrO3‐(0.6‐x)PbTiO3(0.2 lessthan equal toxlessthan equal to 0.32) relaxor‐ferroelectric solid solution, and a morphotropic phase boundary that sharply bends toward zirconium‐rich compositions was found. A spontaneous normal‐to‐relaxor ferroelectric transition was also observed when heating was performed for all the compositions tested near the morphotropic phase boundary. Additional considerations about previously published phase diagrams for Pb(Zn1/3,Nb2/3)O3‐PbTiO3and Pb(Mg1/3,Nb1/3)O3‐PbTiO3might lead to an extension of the presented diagram to these compositions.
Formation of Layered Magnesium Silicate during the Aging of Magnesium Hydroxide–Silica Mixtures
Authors: Temuujin Jadambaa , Okada Kiyoshi , MacKenzie Kenneth J. D.
Published: July 2010

The formation of a poorly crystalline layer‐lattice type of magnesium silicate has been demonstrated in mixtures of magnesium hydroxide with hydrated silicic acid that has been intimately combined via grinding and then aged at room temperature for six months. X‐ray diffractometry, Fourier transform infrared spectroscopy, and 29Si solid‐state magic‐angle spinning nuclear magnetic resonance show the product to be similar to that which has been obtained from the same mixture via hydrothermal treatment at a temperature of 80°C for 24 h. This reaction does not occur to any appreciable extent when the silicic acid is replaced by silica gel, which suggests that the hydration water has an essential role, probably in the dissolution of the silica in the alkaline environment of the magnesium hydroxide.
Mechanically Induced Phase Transformation and Surface Segregation in Bismuth‐Doped Tetragonal Zirconia
Authors: Gulino Antonino , Egdell Russel G. , Fragalà Ignazio
Published: July 2010

Compact pressure causes a martensitic transformation of partially stabilized tetragonal bismuth‐doped ZrO2 powders to the monoclinic phase. A similar effect is not observed when pelletization, followed by firing, of amorphous bismuth/ZrO2 is performed. The stress‐induced transition has been monitored via X‐ray diffractometry, Raman spectroscopy, and Fourier transform infrared reflectivity. Surface bismuth segregation, which involves few ionic planes, has been probed by angle‐resolved X‐ray photoelectron spectroscopy. The bismuth segregation enthalpy has been evaluated.
Kinetics and Mechanism of a Sintering Process for Macroporous Alumina Ceramics by Extrusion
Authors: Wang Huan‐ting , Liu Xing‐qin , Chen Fang‐lin , Meng Guang‐yao , Sørensen O. Toft
Published: July 2010

The paper describes an evaluation of kinetic parameters for sintering from the analysis of stepwise isothermal dilatometry data for the pore‐forming process of macroporous alpha‐Al2O3 ceramics fabricated by extrusion. The experimental results demonstrated that isothermal shrinkage data for the sintering process can fit the following rate equation: d Y/dt=nK(T)Y(1 ‐ Y)[(1 ‐ Y)/Y]1/n, where Y is the fraction of volume shrinkage. Kinetic parameters, i.e., the average exponent n and the activation energy E for the pore‐forming process, were calculated to be 0.232 and 415.5 kJ/mol, respectively, in the temperature range of 1200‐1400°C. The results also show the equivalence between isothermal sintering and stepwise sintering in alumina. In the latter case the temperature is increased in steps over a period of time. The equivalence is derived from understanding how the sintering rate changes with porosity. The sintering mechanism in different temperature ranges is discussed, with reference to microstructure development. The bend strengths of porous ceramics with different porosities are also presented.
Microstructural Development, Densification, and Hot Pressing of Celsian Ceramics from Ion‐Exchanged Zeolite Precursors
Authors: Hoghooghi Bahar , McKittrick Joanna , Helsel Eugene , Lopez Olivia A.
Published: July 2010

Dense monoclinic celsian ceramics (melting point of 1760°C) have been fabricated utilizing zeolite precursors. A sodium zeolite (Na86Al86Si106O384H2O) was ion‐exchanged in aqueous solutions to replace Na with Ba ions. The ion‐exchanged powders were then heat‐treated to effect the collapse of the zeolite structure and formation of an amorphous phase at 627°C, followed by crystallization of the celsian ceramic at 990°C. Inducing viscous flow from a thermal soak above the glass transition temperature was necessary to form a dense body from cold‐pressed powders. Hot pressing at a pressure >5 MPa and above the crystallization temperature resulted in densities >90% of theoretical and eliminated the necessity of adding seed particles to form monoclinic celsian. To fabricate shaped bodies, the amorphous phase was molded at a temperature just above the glass transition temperature and then crystallized to monoclinic celsian at 1050°C. This processing technique demonstrates the potential of using zeolites as precursors for the low‐temperature fabrication of shaped refractory parts.
High‐Temperature Stability of the Al2O3‐LaPO4 System
Authors: Marshall David B. , Morgan Peter E. D. , Housley Robert M. , Cheung Jeffrey T.
Published: July 2010

The compatibility of Al2O3 and LaPO4 at temperatures up to 1600°C is examined. Provided the ratio of La to P was close to 1:1, no reactions were observed after 200 h at 1600°C. Moreover, the Al2O3/LaPO4 interface remained sufficiently weakly bonded to cause deflection of cracks, as reported previously. In the presence of excess P or La, reactions occurred as expected, forming AlPO4 in the case of excess P, and LaAlO3 and LaAl11O18 in the case of excess La.
Strength‐Degrading Mechanisms for Chemically‐Vapor‐Deposited SCS‐6 Silicon Carbide Fibers in an Argon Environment
Authors: Bhatt Ramakrishna T. , Hull David R.
Published: July 2010

The room‐temperature tensile strengths of chemically‐vapor‐deposited SCS‐6 silicon carbide fibers were measured after 1 to 400 h heat treatments in 0.1 MPa of argon at temperatures up to 2100°C. The fibers heat‐treated for 1 h above 1400°C and those heat‐treated for 400 h above 1300°C showed strength degradation. Scanning and transmission electron microscopic examination of the degraded fibers showed formation of a recrystallization region within the outer zone of the SiC sheath and the growth of SiC particles in the carbon‐rich surface coating. The activation energy for the growth of the recrystallization region was ∼370 kJ/mol. The tensile strength of the fibers was found to vary as an inverse function of the recrystallized zone thickness.
Crack Deflection and Propagation in Layered Silicon Nitride/Boron Nitride Ceramics
Authors: Kovar Desiderio , Thouless M. D. , Halloran John W.
Published: July 2010

Crack deflection and the subsequent growth of delamination cracks can be a potent source of energy dissipation during the fracture of layered ceramics. In this study, multilayered ceramics that consist of silicon nitride (Si3N4) layers separated by boron nitride/silicon nitride (BN/Si3N4) interphases have been manufactured and tested. Flexural tests reveal that the crack path is dependent on the composition of the interphase between the Si3N4 layers. Experimental measurements of interfacial fracture resistance and frictional sliding resistance show that both quantities increase as the Si3N4 content in the interphase increases. However, contrary to existing theories, high energy‐absorption capacity has not been realized in materials that exhibit crack deflection but also have moderately high interfacial fracture resistance. Significant energy absorption has been measured only in materials with very low interfacial fracture resistance values. A method of predicting the critical value of the interfacial fracture resistance necessary to ensure a high energy‐absorption capacity is presented.
Densification and Microstructural Development of the Reaction Sintering of Strontium Barium Niobate
Authors: Lee Wen‐Jiung , Fang Tsang‐Tse
Published: July 2010

SrNb2O6 and BaNb2O6 were selected as the precursor powders for the reaction sintering of strontium barium niobate of the composition SrxBa1−xNb2O6 (x= 0.5 and 0.6) (SBN). In comparing the densification behavior of the reaction sintering and the normal sintering of SBN, it was observed that the densification rate of the former was slower during reaction but became faster after reaction. It was also found that the chemical reaction occurred prior to densification, but a high‐density and uniform microstructure could be obtained for the reaction‐sintered sample. The densification rate of the reaction‐sintered sample was pronouncedly enhanced when the reaction was almost completed. We also proposed the reason for the enhancement of the densification rate of the reaction‐sintered sample when the reaction was almost completed. A high‐density (>98% of the theoretical density), uniform, and fine‐grained (3‐4 µm) SBN microstructure could be achieved by using reaction sintering.
Low‐Temperature Sintering of Mullite/Yttria‐Doped Zirconia Composites in the Mullite‐Rich Region
Authors: Imose Masayuki , Ohta Akihiro , Takano Yoshihiko , Yoshinaka Masaru , Hirota Ken , Yamaguchi Osamu
Published: July 2010

Mullite/Y‐TZP composites in the mullite‐rich region, using powders of mullite prepared via the hydrazine method and a commercially available zirconia compound (ZrO2(2Y)), have been fabricated by sintering for 3 h at 1450°C in air. The grain sizes of mullite and ZrO2(2Y) are changed with increased ZrO2(2Y) content; the former decreases from 0.25 µm to 0.13 µm, and the latter increases from 0.21 µm to 0.35 µm. High strength (780 MPa) and fracture toughness (5.4 MPam1/2) are obtained in the 50/50 (vol%) mullite/ZrO2(2Y) composite with 99.6% of theoretical density.
Fabrication of Microconfigured Multicomponent Ceramics
Authors: Crumm Aaron T. , Halloran John W.
Published: July 2010

Microfabrication by coextrusion is a novel ceramic processing technique capable of creating complex ceramic‐metal structures. These structures are fabricated by multiple pass coextrusion of a feedrod comprised of several powder‐filled thermoplastic compounds. The compounds contain either ceramic, metal, or fugitive powders. To illustrate the capabilities of microfabrication, a demonstration part containing lead manganese niobate‐lead titanate ceramic and silver palladium was fabricated. The final part was microconfigured, with a fenestrated structure containing 3110 repeat units per square centimeter. The repeat unit feature sizes were 15 and 5 µm for the ceramic and electrode, respectively. Microfabrication by coextrusion is proposed as a fabrication technique for the production of smart structures and materials.
Nucleation and Growth of Silicon Oxynitride Grains in a Fine‐Grained Silicon Nitride Matrix
Authors: Wang Chongmin , Emoto Hideyuki , Mitomo Mamoru
Published: July 2010

Annealing induces significant microstructural changes in β‐Si3N4 ceramics densified with a silica containing additive, as signified by the precipitation of elongated Si2N2O grains in the fine‐grained β‐Si3N4 matrix, leading to a reinforced structure. The phases and microstructure evolution and the interface structural features in this composite system have been systematically studied by using X‐ray diffraction, and conventional and high‐resolution transmission electron microscopy. Traced from the indispensable features of the Si2N2O grains, typically the spatial distribution of the stacking faults and the size, morphology, phase, and the spatial distribution of the particles trapped within the Si2N2O, it has been generally established that Si2N2O grains nucleate heterogeneously on the β‐Si3N4 grain surface and grow rather quickly. The low nucleation rate and high growth speed of Si2N2O indicate a high nucleation barrier of Si2N2O and fast diffusion of (N,O) in the liquid. Experimentally, no epitaxial orientation relationship between Si2N2O grains and intragranularly trapped β‐Si3N4 grains which either serve as a nucleation site for Si2N2O or are simply encapsulated during Si2N2O grain growth can be observed; instead, the interface between the Si3N4 and the Si2N2O features an amorphous phase of uneven thickness. Sialon polytypoid phase particles similarly trapped in Si2N2O show an epitaxial relationship with Si2N2O grains. From the observed interface structure between the Si2N2O and β‐Si3N4, a possible partial ordering of a glass film in the form of Si2N2O is also discussed.
Toughening Behavior of a Two‐Dimensional SiC/SiC Woven Composite at Ambient Temperature: II, Stress‐Displacement Relationship in the Crack Process Zone
Authors: Wang Yu‐Lin , Nair Shanti V.
Published: July 2010

In this paper insight into the origin of the JR‐curve of a SiC/SiC woven composite was obtained by experimental characterization of the closure stress‐crack opening displacement, sigma(u), relationship in the process zone of the crack. This process zone included both a crack frontal zone and a crack wake damage zone so that quantitative estimates could be obtained of the magnitudes of toughening associated with these two separate zones. The research indicated that the closure stress‐crack opening displacement curve has a positive slope in the crack frontal zone and a negative slope in the wake zone with a maximum stress capability on the order of 350 MPa. The toughness contributions from the crack wake and from the crack front were consistent with the JR‐curve results obtained in the previous paper. The stresses supported locally in the crack frontal zone were almost twice as large as those supported by tensile specimens even though this zone was considerably damaged by matrix cracks. This appears to be the result of stabilization of matrix cracks by arrest at fiber bundles. Application of a previously derived theoretical function, sigmab(u), solely based on crack bridging by continuous unidirectional fibers, suggested that the efficacy of bridging in the woven composite may in part be related to the woven fiber architecture. Such an architecture apparently induces greater sliding resistance of the SiC bundles against the surrounding SiC matrix.
Liquid‐Phase Sintering of Alumina Coated with Magnesium Aluminosilicate Glass
Authors: Nakajima Akira , Messing Gary L.
Published: July 2010

Densification of alumina coated with a MgO‐Al2O3‐SiO2 (MAS) glass was investigated from 1400° to 1460°C. Spinel was observed to form at the liquid‐alumina interface, whereas mullite crystallized uniformly in the liquid. Spinel and mullite crystallization kinetics were accelerated by smaller alumina particle size. Mullite and spinel crystallization retarded densification by forming a percolating network. Boron doping suppressed spinel formation, and thus alumina sintered to higher densities at low temperature. The concept of glass basicity is proposed as a useful guide for selecting dopants for low‐temperature sintering.
High‐Frequency Fatigue Behavior of Woven‐Fiber‐Fabric‐Reinforced Polymer‐Derived Ceramic‐Matrix Composites
Authors: Chawla Nikhilesh , Tur Yahya K. , Holmes John W. , Barber James R. , Szweda Andy
Published: July 2010

The monotonic and high‐frequency (100 Hz) fatigue behavior of two Nicalon‐fabric‐reinforced SiCON matrix composites was investigated at room temperature. The matrix composition was varied by the addition of BN and SiC particulate fillers, to contain shrinkage from processing by polymer infiltration and pyrolysis (PIP). The composites had strong fiber/matrix bonding, which resulted in substantially less frictional heating than observed with weakly bonded composites. Both composites exhibited fatigue runout at 107 cycles at ∼80% of the monotonic strength. Comparison with existing fatigue data in the literature (for the same composites) at 1 Hz shows no change in fatigue life; i.e., no frequency effect was observed. Most of the stiffness reduction in the composites occurred in the first fatigue cycle, whereas subsequent decreases in moduli were attributed to limited fiber cracking. The major driving force for failure was the localized debonding of transverse and longitudinal plies at the crossover points in the fabric, which, when linked, resulted in interlaminar damage and failure in the composite.
Fabrication of Si3N4/SiC Composite by Reaction‐Bonding and Gas‐Pressure Sintering
Authors: Lee Soo Young
Published: July 2010

Si3N4/SiC composite materials have been fabricated by reaction‐sintering and postsintering steps. The green body containing Si metal and SiC particles was reaction‐sintered at 1370°C in a flowing N2/H2 gas mixture. The initial reaction product was dominated by alpha‐Si3N4. However, as the reaction processed there was a gradual increase in the proportion of β‐Si3N4. The reaction‐bonded composite consisting of alpha‐Si3N4, β‐Si3N4, and SiC was heat‐treated again at 2000°C for 150 min under 7‐MPa N2 gas pressure. The addition of SiC enhanced the reaction‐sintering process and resulted in a fine microstructure, which in turn improved fracture strength to as high as 1220 MPa. The high value in flexural strength is attributed to the formation of uniformly elongated β‐Si3N4 grains as well as small size of the grains (length = 2 μm, thickness = 0.5 μm). The reaction mechanism of the reaction sintering and the mechanical properties of the composite are discussed in terms of the development of microstructures.
Phase Relationships and Physical Properties of Homologous Compounds in the Zinc Oxide‐Indium Oxide System
Authors: Moriga Toshihiro , Edwards Doreen D. , Mason Thomas O. , Palmer George B. , Poeppelmeier Kenneth R. , Schindler Jon L. , Kannewurf Carl R. , Nakabayashi Ichiro
Published: July 2010

Equilibrium phase relationships in the ZnO‐In2O3 system were determined between 1100° and 1400°C using solid‐state reaction techniques and X‐ray diffractometry. In addition to ZnO and In2O3, nine homologous compounds, ZnkIn2Ok+3 (where k= 3, 4, 5, 6, 7, 9, 11, 13, and 15), were observed. Electrical conductivity and diffuse reflectance of the k= 3, 4, 5, 7 and 11 members were measured before and after annealing at 400°C for 1 h under forming gas (4%2‐96% N2). Room‐temperature conductivity increased as k decreased, because of increased carrier concentration as well as increased mobility. In general, transparency in the wavelength range of 450‐900 nm increased as k increased. Reduction in forming gas resulted in increased conductivity and reduced transparency for all compounds measured. The highest room‐temperature conductivity measured, 270 S/cm, was that of reduced Zn3In2O6.
Preparation of Magnesium‐Calcium Titanate Powders by Alkoxide Precursor Method
Authors: Piagai Robert , Kim In‐Tae , Park Jae‐Gwan , Kim Yoon‐Ho
Published: July 2010

Magnesium‐calcium titanate (Mg0.95Ca0.05)TiO3 powders have been prepared via two semialkoxide methods that use nitrate and carbonate precursors instead of magnesium and calcium alkoxides. The thermal decomposition and crystallization behavior of the prepared gels and the morphology of the heat‐treated powders have been investigated using thermal analysis, X‐ray diffractometry, Fourier transform infrared spectroscopy, and scanning electron microscopy. The ilmenite structure starts to crystallize at a temperature of ∼500°C without forming an intermediate, spinel‐structured phase. Powders obtained from carbonate precursors that have been dissolved in acetic acid reveal a more‐spherical morphology and a narrower particle‐size distribution than those from nitrate precursors.
High‐K Dielectric Ceramics from Donor/Acceptor‐Codoped (Ba1−xCax)(Ti1‐yZry)O3 (BCTZ)
Authors: Hansen Peter , Hennings Detlev , Schreinemacher Herbert
Published: July 2010

(Ba1−xCax)(Ti1−yZry)O3 ceramics doped with various donors and acceptors were sintered in a reducing atmosphere and then annealed at different oxygen partial pressures. The dielectric maxima at the Curie point increased up to ∼30000 after reoxidation, depending on the grain size, the type of acceptor, and the annealing conditions. The increase of the dielectric maxima seems to be correlated with oxidation of the grain boundaries. The dielectric properties show relaxation effects with increased frequency.
Size Control of α‐Alumina Particles Synthesized in 1,4‐Butanediol Solution by α‐Alumina and α‐Hematite Seeding
Authors: Bell Nelson S. , Cho Seung‐Beom , Adair James H.
Published: July 2010

The effects of seed particles and shear rate on the size and shape of α‐Al2O3 particles synthesized in glycothermal conditions are described. It is proposed that seed particles provide a low‐energy, epitaxial surface in solution to lower the overall surface energy contribution to the nucleation barrier, thus increasing nucleation frequency and subsequently reducing the particle size of hexagonal α‐Al2O3 platelets or polyhedra, depending on synthesis conditions, in 1,4‐butanediol solution. Seeds have a significant effect on the size of hexagonal α‐Al2O3 platelets in samples with high seed concentration. The particle size of α‐Al2O3 platelets decreases from 3 to 4 µm to 100 to 200 nm by increasing the number concentration of seeds. In the case of α‐Fe2O3 seeding, the effect of seeding on the size of α‐Al2O3 particles closely resembles the effects obtained with α‐Al2O3 seeding. Regardless of seed concentration, high stirring rate promotes the formation of hexagonal platelets with high aspect ratio, whereas medium and low stirring rates promote the formation of elongated platelets and polyhedra with 14 faces, respectively.
Synthesis of Ultrafine β″‐Alumina Powders via Flame Spray Pyrolysis of Polymeric Precursors
Authors: Sutorik Anthony C. , Neo Siew Siang , Treadwell David R. , Laine Richard M.
Published: July 2010

Flame spray pyrolysis of a polymeric precursor is used to prepare ultrafine powders that, when sintered, convert to essentially pure phase lithium‐doped sodium β″‐alumina. The precursor Na1.67 Al10.67 Li0.33 [N(CH2CH2O)3]10.67‐[OCH2CH2O]·x(HOCH2CH2OH) has been synthesized from stoichiometric amounts of metal hydroxides and tri‐ethanolamine (N(CH2CH2OH)3, TEA) in excess ethylene glycol. The precursor is dissolved in ethanol, and an atom‐ized spray of the solution is combusted in a specially con‐structed flame spray apparatus. Combustion occurs at ∼2000°C, followed by immediate quenching. This proce‐dure provides for a measure of kinetic control over the process. The resulting nanopowder particles are 50–150 nm in diameter and exhibit powder X‐ray diffractometry pat‐terns similar to β″‐alumina. Heating the nanopowder at 30°C/min to 1200°C with a 1 hisotherm converts it to pure β″‐alumina. In preliminary sintering studies, green powder compacts (∼65% theoretical density) sintered at 1600°C for 12 min densify to 3.0 ± 0.1 g/cm 3 (∼92% theoretical density) with minimal loss of Na2O. This procedure offers several processing and cost advantages over conventional β″‐alumina syntheses.
Model of Strength Degradation from Hertzian Contact Damage in Tough Ceramics
Authors: Lawn Brian R. , Lee Seung Kun , Peterson Irene M. , Wuttiphan Sataporn
Published: July 2010

A model of strength degradation for ceramics subject to damage from contact with hard spheres is developed. Primary attention is focused on tough ceramics with heterogeneous microstructures which deform in a quasi‐plastic mode. Brief consideration is also given to ideally brittle ceramics which form classical ring cracks, as a comparative baseline. Strength vs indentation load data from two microstructurally controlled ceramics, silicon nitride and a micaceous glass‐ceramic, illustrate distinctive strength degradation responses: in fine‐grain (F) form, ideally brittle failure from ring cracks, with abrupt strength loss at the critical load for crack initiation followed by a slow falloff at increasing load; in coarse‐grain (C) form, failure from within the quasi‐plastic zone, with continuous strength loss beyond a load well above that for the onset of yield, and with even slower falloff. Failure in the latter materials occurs from contact‐induced microdamage flaws with two essential elements: an inner closed shear crack with frictional sliding faces (“shear fault”), which forms within the confining compression–shear contact field; an outer annular, kinked crack that initiates at the fault edges (“wing crack”), and that extends in tensile local mode. The critical fault–crack is modeled as a virtual crack, with the residual field from the inner fault stabilizing the net driving force on the outer wing crack during ensuing tensile loading. Finite element modeling is used to evaluate the nonlinear elastic–plastic contact fields, and to provide a relationship between residual shear fault stress and contact load. The model accounts for the essential qualitative and quantitative features of the strength–load data, with provision for catastrophic degradation at high fault densities and extreme loads by microcrack coalescence. The model also contains the ingredients for analysis of contact fatigue, via attrition of the frictional tractions on the residual fault.
Chemical Reactions of Ultraphosphate Glasses with Water at Various Temperatures
Authors: Palavit Gérard , Mercier Cyrille , Montagne Lionel , Drache Michel , Abe Yoshihiro
Published: July 2010

The chemical reactions between P2O5‐ZnO‐H2O ultraphosphate glasses and water were characterized between room temperature and 500°C, using thermogravimetry, differential scanning calorimetry, X‐ray diffraction, and 31P nuclear magnetic resonance. Water adsorption and hydrolysis reactions of the glass leads to the formation of H3PO4 and crystalline ZnH2P2O7 below 200°C. The rate of water adsorption increases, owing to the hygroscopicity of the hydrolysis products of the glass. Devitrification occurs at 250°C via surface reactions. The microstructure of the devitrified glass consists of crystalline Zn2P4O12 and a liquid phase containing hydrolysis products of P2O5 like metaphosphoric acid (HPO3)n. Devitrification is finally followed by water desorption at higher temperatures.
Thermomechanical Analysis of Functionally Graded Thermal Barrier Coatings with Different Microstructural Scales
Authors: Pindera Marek‐Jerzy , Aboudi Jacob , Arnold Steven M.
Published: July 2010

The recently developed two‐dimensional version of the higher‐order theory for functionally graded materials (denoted as HOTFGM‐2D in previous communications) has been used to investigate the effects of microstructural architectures in graded thermal barrier coatings (TBCs) on stress distributions in the presence of a through‐thickness temperature gradient. In particular, the response of TBCs with different levels of functionally graded microstructural refinement and different arrangements has been investigated, and the results for the through‐thickness stress distributions are compared with those based on the standard micromechanical homogenization scheme. The examples presented illustrate the shortcomings of the standard micromechanics‐based approach that is applied to the analysis of functionally graded TBCs, particularly if the presence of creep effects is included in the analysis.
Sea‐Salt Corrosion and Strength of a Sintered α‐Silicon Carbide
Authors: Fox Dennis S. , Cuy Michael D. , Nguyen QuynhGiao N.
Published: July 2010

A commercially available, sintered silicon carbide was exposed to a temperature of 982°C for up to 50 h in a burner rig pressurized to 500 kPa. Synthetic sea salt added to the flame (5 ppm) resulted in the deposition of sodium sulfate and formation of a sodium magnesium silicate corrosion product. A 16% reduction in room‐temperature strength occurred after 5 h of exposure; this reduction was due to the formation of surface pits. Exposure for longer times resulted in continued strength reduction, up to 56% at 25 h. Samples exposed for 50 h were so degraded that mechanical tests could not be conducted. The strength after 25 h of exposure to a salt concentration of 2 ppm was similar to the as‐received strength, whereas exposures to 10 ppm of salt resulted in strengths similar to that observed with 5 ppm of salt.
Grain‐Boundary Sliding and Grain Interlocking in the Creep Deformation of Two‐Phase Ceramics
Authors: Muto Hiroyuki , Sakai Mototsugu
Published: July 2010

Grain‐boundary sliding and grain interlocking of two‐phase ceramics during creep are examined on the basis of the Dryden–Kucerovsky–Wilkinson–Watt theory. That theory is extended to the plane–strain creep deformation of model arrays of square and hexagonal grains embedded in a continuous grain‐boundary melt phase to develop consti‐tutive equations for their universal creep. Superposition is derived for the “time‐applied stress” and the “time–temperature” relations during creep. The “shift factor” for the time–temperature superposition characterizes the temperature dependence of the apparent viscosity of the ceramic. A two‐phase ceramic during creep acts as a non‐Newtonian fluid, the viscosity of which is dependent on the creep strain. The constitutive relation between the creep strain rate and the creep strain is then utilized to estimate the viscosity and the volume fraction of the intergranular melt phase. The equilibrium creep strain to achieve grain interlocking is considered through the rotational motion of grains. Creep tests for a β‐spodumene glass‐ceramic are conducted under simple shear to generate experimental results for scrutinizing the theoretical predictions. Satisfactory agreement is observed, giving important rheological information on the creep process.
Effect of Processing on the Structure, Oxygen Content, and Superconductivity of LaBa2Cu3Oy
Authors: Wu Ma‐Shine , Fang Tsang‐Tse
Published: July 2010

We have examined the structure, oxygen content, and superconductivity of a LaBa2Cu3Oy superconductor heat treated at different temperatures and in different atmospheres. It was found that the expansion of the c‐axis, which was caused by the removal of oxygen, might be the reason for the ordering of barium and lanthanum ions when the specimen is heated in a reducing atmosphere. The anomaly in the expansion of the c‐axis was also observed when oxygen was removed. Mechanisms of the orthorhombic‐tetragonal transition and the occurrence of the anomaly in the c‐axis have been proposed.
Elastic Constants of Tetragonal Zirconia Measured by a New Powder Diffraction Technique
Authors: Kisi Erich H. , Howard Christopher J.
Published: July 2010

Elastic constants for 12 mol% Ce‐doped tetragonal zirconia have been determined from peak shifts in neutron diffraction patterns recorded under applied uniaxial stress. When these diffraction data are combined with a measured value of Young's modulus, a complete set of elastic constants is obtained. The values arec11= 327,c12= 100,c13= 62,c33= 264,c44= 59, andc66= 64 (units: GPa). These are the first reported results using a new technique for the measurement of elastic constants for anisotropic materials via neutron diffraction measurements on polycrystalline samples.
Interfacial Structure of Ordered Domains in Barium Lanthanum Magnesium Niobate
Authors: Lee Hwack Joo , Ryu Hyun , Park Hyun Min , Paik Jong Hoo , Nahm Sahn , Byun Jae‐Dong
Published: July 2010

Microstructural studies on the interfacial boundaries of 1:1 and 1:2 ordered domains in (Ba0.9La0.1)(Mg0.37Nb0.63)O3 were conducted using high‐resolution transmission electron microscopy and X‐ray diffractometry. Both 1:2 and 1:1 ordered domains coexisted in a fully ordered single grain. Each ordered domain occupied its own region, and the interfaces were atomically sharp and coherent. The wavelength of the superlattice modulation was ∼0.47 nm in the 1:1 ordered domain and ∼0.71 nm in the 1:2 ordered domain. The transition from the 1:2 ordered region to the 1:1 ordered region was clearly shown at the interface. These observations well support the structural models that have been previously presented.
Synthesis of [NZP]‐Structure‐Type Materials by the Combustion Reaction Method
Authors: Breval Else , Agrawal Dinesh K.
Published: July 2010

Ca0.5Sr0.5Zr4P6O24 and Ba1.25Zr4P5.5Si0.5O24 compositions, which belong to the [NZP] family of low‐thermal‐expansion materials, were synthesized by the combustion reaction method using urea as a fuel and NH4NO3 as an oxidizer. The combustion method is a highly exothermic reaction often referred to as self‐propagating high‐temperature synthesis. The raw materials used were metal zirconates and fine ZrO2 powders. An earlier‐described combustion method using metal nitrates, zirconium oxynitrate, and ammonium dihydrophosphate to synthesize [NZP] compounds was modified in the present study by replacing these costly raw materials with less‐expensive oxides. The reactivity of different combinations of raw materials also is discussed.
Reinforcement of Hydroxyapatite Bioceramic by Addition of Ni3Al and Al2O3
Authors: Choi Je‐Won , Kong Young‐Min , Kim Hyoun‐Ee , Lee In‐Seop
Published: July 2010

The effects of Ni3Al and Al2O3 additions on the mechanical properties of hydroxyapatite (HAp) were investigated. The addition of Ni3Al particles increased the strength as well as the fracture toughness of HAp. However, the improvements in the properties were limited because of the formation of microcracks around the metal particles. The microcracks were formed because of the large difference in the coefficients of thermal expansion between HAp and Ni3Al, and because of the relatively large size of Ni3Al particles (∼20 µm). The addition of submicrometer Al2O3 powder was also effective in increasing the mechanical properties. The flexural strength and the fracture toughness were increased from about 100 MPa and 0.7 MPam1/2, respectively, to 200 MPa and 1.5 MPam1/2 by the addition of 20 vol% Al2O3. When Ni3Al and Al2O3 were added together, the fracture toughness was further increased to 2.3 MPam1/2. This increase in the fracture toughness was attributed to the synergistic effect of matrix strengthening and crack interactions with the metal particles.
Effect of Particle Additions on Drying Stresses and the Green Density of Sol‐Gel‐Processed Three‐Dimensional Ceramic‐Matrix Composites
Authors: Liu Hsien‐Kuang , Parvizi‐Majidi Azar
Published: July 2010

This paper investigates sol‐gel processing of three‐dimensional (3‐D) ceramic‐matrix composites, focusing on the effect of addition of particles on drying stresses and composite green density. A 3‐D woven carbon fiber/silica composite was used as the model material system for the investigation. Composites were fabricated with silica particle additions to the silica sol using a pressure infiltration technique, and the drying rate and matrix cracking were characterized. The particle size was varied in order to study the role of particle additions in the processing. Ammonium hydroxide (NH4OH) and a particle coating agent, 3‐(trimethoxysilyl)propyl methacrylate (TPM), were added in the mixture of particles and sol to improve particle packing in the preform. Sol‐gel processing with the addition of particles proves to be an effective route to fabricate 3‐D ceramic‐matrix composites with higher green density and less matrix cracking.
Crack Healing in a Silicon Nitride Ceramic
Authors: Zhang Yan Hui , Edwards Lyndon , Plumbridge William J.
Published: July 2010

A detailed study has been undertaken on crack healing at high temperatures in a silicon nitride containing 10 wt% additives in order to identify the dominant mechanism responsible for the phenomenon. Fracture toughness increased with annealing time and the crack growth rate decreased until arrest with increasing testing time. Differentiation between possible operating mechanisms was obtained using critical experiments involving detailed compliance measurements, crack wake removal, and crack reinitiation tests and a comprehensive TEM study of healed cracks. It was found that crack healing was not uniform in the crack wake. When the original crack path was either transgranular or intergranular, healing was associated with the appearance of a thin layer of silica glass due to the oxidation of Si3N4 grains. But when the crack went through multigrain junctions, the former crack path was completely obliterated and replaced by a new, crystalline phase formed by diffusion of the preexisting glass phase. It is concluded that the increased crack growth resistance and fracture toughness at high temperature is attributable to the partial recovery of the original strength from the crack segments at multigrain junctions due to vitreous phase flow and subsequent crystallization.
Accelerated Reaction Bonding of Mullite
Authors: Mechnich Peter , Schneider Hartmut , Schmücker Martin , Saruhan Bilge
Published: July 2010

A suitable way to process mullite ceramics and mullite‐matrix composites with low dimensional changes (“near‐net‐shape processing”) is reaction bonding, using silicon metal and corundum (α‐Al2O3) as starting materials, be‐cause sintering‐induced shrinkage is compensated by vol‐ume expansion caused by the silicon‐oxidation‐induced vol‐ume expansion. This work describes a new reaction‐bonded mullite (RBM) processing route that proceeds at much lower temperatures (≤1350°C) than in “normal” RBM systems (≥1550°C). Accelerated silicon oxidation and mullite formation are effected by adding low amounts of yttria (Y2O3) or ceria (CeO2) to the green powder mixtures, which causes the formation of transient, metastable, and low‐viscosity Y‐Al‐Si‐O and Ce‐Al‐Si‐O partial melts. After long‐term heat treatments at ≤1350°C, however, the intermediate liquids recrystallize and leave behind RBM ceramics that are homogeneous, substantially glass‐free, and relatively dense. These two RBM materials consist of mullite, α‐Al2O3, and Y2Si2O7 or CeO2, respectively.