bluuebunny/crossref_metadata_2025 huggingface dataset:
Reaction Sequence and Microstructrual Development of CeO2‐Doped Reaction‐Bonded Mullite
Authors: Mechnich Peter , Schmücker Martin , Schneider Hartmut
Published: July 2010

A promising technique for the fabrication of mullite ceramics and mullite‐matrix composites with low dimensional changes (“near‐net‐shape processing”) is reaction bonding using Si metal and α‐Al2O3 as starting materials, because sintering‐induced shrinkage is compensated by Si‐oxidation‐induced volume expansion. A mullite reaction bonding (RBM) route which proceeds at much lower temperatures (lessthan equal to1350°C) than in conventional RBM systems (greaterthan equal to1500°C) is based on Ce doping which provides accelerated Si oxidation and mullite formation due to the formation of transient, low‐viscosity Ce‐Al‐Si‐O liquids. The present study shows that the required Ce‐Al‐Si‐O liquids form in a reducing environment with Ce occurring as Ce3+. In an oxidizing environment, Ce is present as Ce4+, giving rise to precipitation of crystalline CeO2. Ce3+ left and right arrow Ce4+ redox reactions in the temperature range under consideration appear to be controlled by the presence of nonoxidized Si in the samples. According to the present investigation the amount of CeO2 added to the starting powders must be tailored carefully: Exaggerated CeO2 content produces large amounts of low‐viscosity Ce‐Al‐Si‐O liquids which may have the disadvantage of excessive sealing of the open porosity. This slows the oxygen diffusion velocity into the specimen considerably, with the consequence that nonoxidized Si and a residual Ce‐Al‐Si‐O glass coexist in the ceramics after processing. A solution to this problem is to simultaneously enhance mullite crystal growth through seeding which works against excessive liquid‐phase‐induced shrinkage of the samples. This in turn enables complete oxidation and recrystallization of all liquid phases.
Microstructural Changes in Lanthanum‐Doped Barium Magnesium Niobate
Authors: Lee Hwack Joo , Park Hyun Min , Ryu Hyun , Paik Jong Hoo , Nahm Sahn , Byun Jae‐Dong
Published: July 2010

Observations of microstructural changes in (Ba0.95La0.05)‐(Mg0.35Nb0.65)O3 and (Ba0.925La0.075)(Mg0.36Nb0.64)O3 (BLMN) were carried out using high‐resolution transmission electron microscopy (HRTEM) and synchrotron powder X‐ray diffractometry (XRD). In both samples, not only 1:1 and 1:2 ordered domains coexisted in a single grain, but also the intermediate phase, whose structure had a superlattice modulation of 1.42 nm, which was equivalent to 6 times the unit cube of disordered perovskite found on the nanoscale. The ordered 1:2 domains gradually transformed to 1:1 ordered structure through the formation of an intermediate superlattice structure that comprised 6 × 6 × 6 cubic unit cells with different chemical orderings of B‐site ions in B‐site lattices. Also, the features of thin plates could be detected by XRD patterns and HRTEM. When the thicknesses were very thin, about several atomic distances, stacking faults occurred on (111) planes. However, when their thicknesses were >50 nm, the thin plates existed as a transition phase with their own structure. They were coherent with the matrix and continuously decomposed into the matrix phase by the lateral migration of the interfaces.
Synthesis of Silicon Nitride/Silicon Carbide Nanocomposite Powders through Partial Reduction of Silicon Nitride by Pyrolyzed Carbon
Authors: Tian Jiemo , Li Jinwang , Dong Limin
Published: July 2010

An alternative method to incorporate nanometer‐sized silicon carbide (SiC) particles into silicon nitride (Si3N4) powder was proposed and investigated experimentally. Novolac‐type phenolic resin was dissolved in ethanol and mixed with Si3N4 powder. After drying and curing, the resin was converted to reactive carbon via pyrolysis. Si3N4 powder was partially reduced carbothermally using the pyrolyzed carbon, and nanometer‐sized SiC particles were produced in situ at 1530°‐1610°C in atmospheric nitrogen. At temperatures <1550°C, the reduction rate was low and the SiC particles were very small; no SiC whiskers or barlike SiC was observed. At 1600°C, the reduction rate was high and the reaction was close to completion after only 10 min, with the appearance of SiC whiskers as well as curved, barlike, and equiaxial SiC, all of which were dozens of nanometers in diameter; this size is greater than that at observed temperatures <1550°C. A longer soaking time at 1600°C led to agglomerates. SiC particles were close to the surface of the Si3N4 particles. The SiC content could be adjusted by changing the carbon content before reduction and the reduction temperature. A reaction mechanism that involved the decomposition of Si3N4 has been proposed.
Effects of Cd‐Substitution Site on the Electrical Conductivities in Pb(Ni1/3Nb2/3)O3‐PbZrO3‐PbTiO3 Ceramics
Authors: Chung Hoon‐Taek , Cho Jeong‐Ho , Kim Ho‐Gi
Published: July 2010

The effects of Cd2+ substitution for Pb2+ and Ni2+ ions (on the A‐site and B‐site of the ABO3 perovskite structure, respectively) on the electrical conductivities in Pb(Ni1/3‐Nb2/3)O3‐PbZrO3‐PbTiO3 (PNN‐PZ‐PT) ceramics have been investigated. Generally, the compounds that contain PbO show p‐type conductivity, because of PbO evaporation. Thus, the conductivities are known to be proportional to the PbO evaporation. However, PNN‐PZ‐PT ceramics exhibit a reciprocal relationship between the conductivities and the evaporation of PbO. Generally, no vacancy change is observed with the substitution of the same‐charge valence ion. However, in the PNN‐PZ‐PT ceramic, lead vacancies could be created or annihilated by replacing Pb2+ and Ni2+ ions with Cd2+ ions, because of incomplete substitution. The electrical conductivities are influenced by this incomplete substitution.
Determination of Nickel(II) Oxide in Ceramic Pigments by In Situ X‐ray Diffraction Quantitative Analysis
Authors: Gualtieri Alessandro F. , Mazzucato Edoardo , Venturelli Paolo , Viani Alberto , Zannini Paolo , Petras Laszlo
Published: July 2010

Nickel(II) oxide (NiO) still is extensively used as a raw material for the synthesis of traditional ceramic pigments, despite the fact it is potentially carcinogenic. One of the reasons is that, unfortunately, no widely accepted methods exist for its determination in ceramics so far (the main reason being experimental difficulties). Quantitative phase analysis via powder X‐ray diffractometry, for example, may fail because of the presence of glass and because NiO diffraction peaks overlap those of spinel in spinel‐based pigments. This communication elucidates how to address these experimental difficulties and determine free NiO in the case of black ceramic pigments.
Solution‐Precipitation of Fine Powders of Barium Titanate and Strontium Titanate
Authors: Kumar Viswanathan
Published: July 2010

Hydrolytic reactions of metal alkoxides offer a broad range of possibilities for their use in the preparation of ceramic powders. This paper reports a unique and novel process to prepare fine powders of BaTiO3 and SrTiO3 from stable precursor solutions by carefully controlling the pH and temperature. This simple route offers good control of stoichiometry and the powders are agglomerate‐free with fine particles of size 0.06–0.1 µm and were well sintered at 1200° and 1350°C, respectively. The dielectric properties of the dense ceramics are also reported. The formation aspects of these perovskite phases are also discussed.
Depletion Flocculation Strategy for the Fabrication of Ceramic Powder Coatings on Ceramic Fibers
Authors: Koltay Judith A. , Feke Donald L.
Published: July 2010

A colloidal processing approach combining the phenomena of depletion flocculation and steric stabilization has been used in the preparation of high‐quality coatings (monolayer and thicker) of silicon particles on silicon carbide fibers. Grafting procedures for the steric additive and the use of the depletion flocculation method to achieve dispersions of relatively high solids concentration are described. The impact of processing parameters on coating quality is demonstrated through electron microscopic images. The relationship between optimal processing parameters and the size and shape characteristics of the solid particles used is also discussed.
Influence of Powder Agglomerates on the Structure and Rheological Behavior of Injection‐Molded Zirconia–Wax Suspensions
Authors: Liu Dean‐Mo , Tseng Wenjea J.
Published: July 2010

Surface‐modified zirconia powders with 20% to 100% of the particle surface area covered with stearic acid was blended with 50 vol% of an organic wax vehicle for rheological study. These fractions of surface coverage gave rise to different degrees of powder agglomeration. A further adsorption of the wax on partially modified particle surfaces was examined, which was likely to exhibit different degrees of solid surface‐wax affinity depending upon whether the particle surfaces were “bare” or “premodified.” The rheological behavior of the suspensions revealed that the shear viscosity as well as the yield stress increased appreciably with decreased fractions (or surface coverage) of the pre‐adsorption. The observed suspension rheology due to incomplete surface modification can be accounted for by the formation of agglomerates which suppress suspension flowability to a significant extent. The formation of the agglomerates alters the suspension structure by reduction of the maximum solid concentration (φmax) that is attainable for a given powder. This change in suspension structure (decrease in φmax) leads to a restriction of particle mobility, reflected as a linear function of the yield stress of the suspensions.
Development and Characterization of Y2O3‐Stabilized ZrO2(Y‐TZP) Composites with TiB2, TiN, TiC, and TiC0.5N0.5
Authors: Vleugels Jef , Van der Biest Omer
Published: July 2010

Up to 50 vol% of TiB2, TiC0.5N0.5, TiN, or TiC was added to Y2O3‐stabilized tetragonal ZrO2polycrystals (Y‐TZP) and hot pressed under vacuum. The influence of the type of secondary phase on the microstructure and mechanical properties was studied, as a function of the hot‐pressing temperature. The influence of the secondary‐phase content on the mechanical properties was studied by varying the TiB2content up to 50 vol%. Fully dense Y‐TZP‐based composites with very high toughness (up to 10 MPa·m1/2), excellent bending strength (up to 1237 MPa), and increased hardness, with respect to ZrO2(Vickers hardness up to 1450 kg/mm2), were obtained.
Comparison of Hydraulic‐Burst and Ball‐on‐Ring Tests for Measuring Biaxial Strength
Authors: Simpatico Anupa , Cannon W. Roger , Matthewson M. John
Published: July 2010

The statistics of failure of the hydraulic‐burst (HB) test were compared with those of the ball‐on‐ring (BOR) test. Polycrystalline Al2O3 tape‐cast specimens, both square and circular, in two different sizes, were tested. Both the mean strengths and the Weibull moduli from the BOR tests were approximately twice the values from the HB tests. The area (volume) under stress is much larger for the HB test than the BOR test; therefore, the HB data can be considered as a low‐probability‐of‐failure, low‐strength tail of the BOR curve that has a lower Weibull modulus than the high‐stress portion. Thus, BOR tests give a misleading picture of improvements in mechanical strength, because of changes in the fabrication and handling of substrates. However, previous observations that the incidence of edge and support failures was very high in the HB test were confirmed. Also, the apparent strength of the HB specimens was affected more strongly by size and shape than was that of the BOR specimens.
Oxidation Kinetics of Hexagonal‐Shaped Single‐Crystal Silicon Whiskers
Authors: Jiang Honggang , Zhu Yuntian T. , Butt Darryl P.
Published: July 2010

The oxidation kinetics of hexagonal‐shaped Si whiskers at high temperature were studied. Si whiskers were oxidized at 1000°C under a partial oxygen pressure of 0.14 atm. Oxide thickness was measured using scanning electron microscopy (SEM). A unique model was developed to describe the oxidation kinetics of hexagonal‐shaped fibers. The model takes into account the inward diffusion of oxygen, oxidation reaction at the oxide/Si interface, and the fiber diameter change caused by the molar volume difference between SiO2 and Si. Comparison of the model with experimental data shows good agreement.
Crystallization of Zircon in Stoneware Glazes
Authors: Castilone Robert J. , Sriram Dattaguru , Carty William M. , Snyder Robert L.
Published: July 2010

High levels of zircon are required to impart opacity in glazes. Zircon additions of <3 wt% dissolve into the glassy phase; additions of >12% cause all the zircon to crystallize from the glaze. Dynamic high‐temperature X‐ray diffraction has shown that, during heating, a portion of the zircon dissolves and subsequently recrystallizes, using undissolved zircon crystals as seeds. This recrystallization does not occur at lesser zircon levels, because of a lack of seed crystals. When zirconia is added to the glaze, zircon is formed as elongated crystals at the glaze surface. Lesser zircon levels have been observed in the glazes that have zirconia additions.
Crystallization Kinetics of Mullite in Alumina–Silica Glass Fibers
Authors: Takei Takahiro , Kameshima Yoshikazu , Yasumori Atsuo , Okada Kiyoshi
Published: July 2010

The mechanism of mullite crystallization in Al2O3‐SiO2 glass fibers with compositions of 49 and 69 mass% Al2O3 was investigated using isothermal methods. The activation energies (Ea) of the nucleation and nucleation‐growth of mullite were obtained from the temperature dependence of the incubation times and rate constants of mullite formation, respectively. The formation of mullite in both glasses occurred in two stages. The amount of mullite increased very steeply in the first stage within very short firing time, whereas the increase was more gradual in the second stage. The Ea values for nucleation and nucleation‐growth in the first stage were 864 and 1288 kJ/mol in the 49‐mass%‐Al2O3 glass fiber and 980 and 1138 kJ/mol in the 69‐mass%‐Al2O3 glass fiber, respectively. Arrhenius plots of the second‐stage data showed that the slope ratios were different at temperatures below and above 1200°C. The Ea values at <1200°C were 1195 and 1099 kJ/mol in the 49‐ and 69‐mass%‐Al2O3 glass fibers, whereas those at >1200°C were 696 and 645 kJ/mol, respectively. Observation of the microstructures and crystallite‐size data indicates that the crystallization of mullite proceeds via three different mechanisms: nucleation, nucleation–growth, and coalescence of mullite grains.
Formation of Ettringite from Monosubstituted Calcium Sulfoaluminate Hydrate and Gypsum
Authors: Clark Boyd A. , Brown Paul W.
Published: July 2010

The formation of ettringite (3CaO·Al2O3·3CaSO4·32H2O) from monosulfate (3CaO·Al2O3·CaSO4·12H2O) and gypsum (CaSO4·2H2O) was investigated by isothermal calorimetry and X‐ray diffraction (XRD) analyses. Hydration was carried out at constant temperatures from 30° to 80°C using deionized water and 0.2M, 0.5M, and 1.0M sodium hydroxide (NaOH) solutions. Ettringite was found to be the dominant crystalline phase over the entire temperature range and at all sodium hydroxide concentrations. A sodium‐substituted monosulfate phase was formed as a hydration product in the 1.0M sodium hydroxide solution regardless of temperature. XRD and calorimetry demonstrate that hydration in increasing sodium hydroxide concentrations decreases the amount of ettringite formed and retards the rate of reaction. The apparent activation energy for the conversion of the monosulfate/gypsum mixture to ettringite was observed to vary depending on the sodium hydroxide concentration. Ettringite formation was observed to depend upon the concentration of calcium in solution; thus the formation of calcium hydroxide and sodium‐substituted monosulfate phase competes with ettringite formation.
Microstructural Control for Strengthening of Silicon Carbide Ceramics
Authors: Zhan Guo‐Dong , Mitomo Mamoru , Kim Young‐Wook
Published: July 2010

Fine‐grained (<1 μm) silicon carbide ceramics with high strength were obtained by using ultrafine (∼90 nm) β‐SiC starting powders and a seeding technique for microstructural control. The microstructures of the as‐hot‐pressed and annealed ceramics without α‐SiC seeds consisted of fine, uniform, and equiaxed grains. In contrast, the annealed material with seeds had a uniform, anisotropic microstructure consisting of elongated grains, owing to the overgrowth of β‐phase on α‐seeds. The strength, the Weibull modulus, and the fracture toughness of fine‐grained SiC ceramics increased with increasing grain size up to ∼1 μm. Such results suggested that a small amount of grain growth in the fine grained region (<1 μm) was beneficial for mechanical properties. The flexural strength and the fracture toughness of the annealed seeded materials were 835 MPa and 4.3 MPa·m1/2, respectively.
Evaluation of Rule‐of‐Mixtures Predictions of Thermal Expansion in Powder‐Processed Ni–Al2O3 Composites
Authors: Bruck Hugh A. , Rabin Barry H.
Published: July 2010

Ni‐Al2O3 composites containing 5 to 80 vol% Ni were produced using powder processing methods and their thermal expansion behavior was studied using dilatometry. By varying composition and relative particle sizes, either interpenetrating‐phase or particle‐reinforced microstructures were obtained. Experimentally determined values of the mean thermal expansion coefficient were compared with predictions made using two different rule‐of‐mixtures (ROM) formulations. Experimental results for interpenetrating‐phase composites were in reasonable agreement with linear ROM estimates. Certain particle‐reinforced microstructures that contain processing‐induced damage did not correlate as well with ROM predictions. By accounting for this damage in ROM models, better agreement with experimental observations was obtained. However, linear ROM predictions were reasonable over the entire composition range.
Experimental Evidence for the Anisotropic Ostwald Ripening of β‐Silicon Nitride
Authors: Kitayama Mikito , Hirao Kiyoshi , Toriyama Motohiro , Kanzaki Shuzo
Published: July 2010

A powder mixture of α‐Si3N4, Y2O3, and SiO2 was heat‐treated in a loose powder state in the temperature range of 1750°–1900°C for 2 h; then, the mixture was acid‐rinsed to remove the glassy phase. The widths and lengths of the resulting β‐Si3N4 crystals were analyzed quantitatively. The width–aspect‐ratio distribution of the β‐Si3N4 crystals initially showed a strong negative correlation, and then the aspect ratio of crystals with small widths quickly decreased. After a stage in which aspect ratio was almost constant, regardless of the width, the width‐aspect‐ratio distribution evolved to show a positive correlation in the final stage. This pattern of morphology evolution of the β‐Si3N4 crystals was in good agreement with that predicted by the anisotropic Ostwald ripening model.
Effect of Sintering Aid Composition on the Processing of Si3N4/BN Fibrous Monolithic Ceramics
Authors: Trice Rodney W. , Halloran John W.
Published: July 2010

Si3N4/BN fibrous monoliths were prepared with 4 wt% Y2O3 added as a sintering aid to the Si3N4. Residual carbon, present in the billet before hot‐pressing, was shown to influence the final microstructure. The sintering aid glass, known to migrate into the BN cell boundaries during hot‐pressing, was not sufficient in quantity to prevent premature shear failure when samples were tested in flexure. Increasing the hot‐pressing temperature alleviated this problem. For flexure samples tested at 1400°C, fibrous monoliths fabricated with 4 wt% Y2O3 demonstrated linear‐elastic loading behavior at a greater stress than fibrous monoliths fabricated with 6‐wt%‐Y2O3/2‐wt%‐Al2O3 sintering aids.
Pore Growth during Initial‐Stage Sintering
Authors: Akash Akash , Mayo Merrilea J.
Published: July 2010

A quantitative model for pore growth during initial‐stage sintering is proposed. During initial‐stage sintering, neck formation leads to surface rounding of the pores, thereby causing a decrease in the surface area of the system. The decrease in surface area, without a concomitant decrease in pore volume, leads to a spurious increase in pore size, as calculated by the gas adsorption technique. Geometrical calculations predict a final pore size that is a function of the initial (green) density of the compact. For the 62% dense compacts of the present study, the model predicts a factor of 1.28 increase in pore size, compared to the factor of 1.27, which is experimentally observed. Interestingly, a common, constant factor of 1.27 can also be observed in pore growth data reported by a number of other researchers.
Growth of Pb(Mg1/3Nb2/3)O3‐35 mol% PbTiO3 Single Crystals from (111) Substrates by Seeded Polycrystal Conversion
Authors: Khan Ajmal , Meschke Frank A. , Li Tao , Scotch Adam M. , Chan Helen M. , Harmer Martin P.
Published: July 2010

Fully dense disks, each consisting of a single crystal (111) plate of the relaxor‐based ferroelectric Pb(Mg1/3Nb2/3)O3‐35 mol% PbTiO3 (PMN‐35PT) embedded in a 0.48 ± 0.05 µm grain size polycrystalline matrix of the same composition, were formed by hot‐pressing at 950°C for 30 min under 20 MPa. Specimens were subsequently annealed to promote migration of the single‐crystal boundary through the polycrystal (a process termed seeded polycrystal conversion). An anneal of 10 h at 1150°C using PMN‐35PT packing powder resulted in minimal single‐crystal boundary migration, and was accompanied by matrix grain coarsening to 1.86 ± 0.20 µm. In contrast, an anneal of 10 h at 1150°C using PbZrO3 (PZ) sacrificial powder resulted in significant migration of the single‐crystal boundary through the polycrystal, accompanied by matrix grain coarsening to 13.3 ± 0.3 µm. The shape of the grown crystal relative to the seed plate was consistent with the <111> direction being the fastest growth direction. Based on the grown crystal dimensions, a lower bound <111> growth velocity of 0.14 mm/h was calculated. The increased boundary mobility in the specimen that was annealed using PZ sacrificial powder is attributed to a boundary‐wetting liquid PbO‐based second phase that formed during the anneal. This phase is believed to have formed via PbO absorption from the surrounding vapor phase due to a higher equilibrium PbO vapor pressure above PZ than above PMN‐35PT. The grown single crystal exhibited a promising <100> strain of 0.5% at an applied electric field of 4 MV/m.
Densification and Mechanical Properties of Titanium Diboride with Silicon Nitride as a Sintering Aid
Authors: Park June‐Ho , Koh Young‐Hag , Kim Hyoun‐Ee , Hwang Cheol Seong , Kang Eul Son
Published: July 2010

Titanium diboride (TiB2) was hot‐pressed at a temperature of 1800°C, and silicon nitride (Si3N4) was added as a sintering aid. The amount of Si3N4 that was added had a significant influence on the sinterability and mechanical properties of the TiB2. When a small amount (2.5 wt%) of Si3N4 was added, the Si3N4 reacted with titania (TiO2) that was present on the surface of the TiB2 powder to form titanium nitride (TiN), boron nitride (BN), and amorphous silica (SiO2). The elimination of TiO2 suppressed the grain growth effectively, which led to an improvement in the densification of TiB2. The formation of SiO2 also was deemed beneficial for densification. The mechanical properties‐especially, the flexural strength‐were enhanced remarkably through these improvements in the sinterability and microstructure. On the other hand, when a large amount (greaterthan equal to5 wt%) of Si3N4 was added, the mechanical properties were not improved much, presumably because of the extensive formation of a glassy Si‐Ti‐O‐N phase at the grain boundaries.
Formation of Varistor Characteristics by the Grain‐Boundary Penetration of ZnO‐PrOx Liquid into ZnO Ceramics
Authors: Chun Sung‐Yong , Shinozaki Kazuo , Mizutani Nobuyasu
Published: July 2010

Penetration of a liquid (ZnO‐PrOx) into the grain boundaries of sintered, cobalt‐doped ZnO pellets resulted in varistors with breakdown voltages per grain boundary in the 1‐2 V range and nonlinearity coefficients of 22‐37. The varistors were fabricated by spreading a thin layer of Pr6O11 powder paste on the surface of ZnO pellets and heating to various temperatures (1200°‐1400°C) and times (0‐60 min). Comparing the varistor properties per grain boundary (e.g., threshold voltage, donor concentration, and barrier height) of liquid penetration to those of conventional method indicated the individual grain boundaries were electrically activated when the samples were heat‐treated above liquid‐phase formation temperature.
Effects of MgO Stoichiometry on the Dielectric and Mechanical Response of Pb(Mg1/3Nb2/3)O3
Authors: Koyuncu M. , Pilgrim S. M.
Published: July 2010

Careful control of the magnesia stoichiometry and hydration level overcomes the problem of pyrochlore formation in the synthesis of Pb(Mg1/3Nb2/3)O3 (PMN). Careful removal of volatiles from MgO showed that excess MgO is not required to form pyrochlore‐free PMN using the columbite method; this clarifies the previously reported need of excess MgO. The stoichiometric PMN shows a peak dielectric constant of ~15 000 for 1 kHz. In addition, the presence of volatiles in the MgO has been linked with a change in fracture mode. The relation between the MgO stoichiometry and the microstructural and dielectric properties is presented in an integrated study using X‐ray diffraction at various temperatures, dielectric response, and microscopy.
Subcritical Crack Growth in a Phosphate Laser Glass
Authors: Crichton Stephen N. , Tomozawa Minoru , Hayden Joseph S. , Suratwala Tayyab I. , Campbell John H.
Published: July 2010

The rate of subcritical crack growth in a metaphosphate Nd‐doped laser glass was measured using the double‐cleavage‐drilled compression (DCDC) method. The crack velocity is reported as a function of stress intensity at temperatures ranging from 296 to 573 K and in nitrogen with water vapor pressures ranging from 40 Pa (0.3 mmHg) to 4.7 × 104 Pa (355 mmHg). The measured crack velocities follow region I, II, and III behavior similar to that reported for silicate glasses. A chemical and mass‐transport‐limited reaction rate model explains the behavior of the data except at high temperatures and high water vapor pressures where crack tip blunting is observed. Blunting is characterized by an arrest in the crack growth followed by the inability to reinitiate slow crack growth at higher stresses. A dynamic crack tip blunting mechanism is proposed to explain the deviation from the reaction rate model.
Lattice‐Dynamic Treatment of Vibrational and Elastic Properties of Cotunnite‐Type ZrO2 and HfO2: Comparison with Ambient Pressure Polymorphs
Authors: Mirgorodsky Andrei P. , Quintard Pierre E.
Published: July 2010

The results of the joint lattice‐dynamical model treatment of the vibrational and elastic properties of the cotunnite‐type zirconia and hafnia are presented. The model is capable of reproducing the frequencies of the Raman‐active vibrations, and provides their symmetry assignment. Also it predicts the position of the infrared‐active bands. The model successfully describes the elastic properties of the low‐pressure polymorphs, but could not reproduce the bulk modulus of OII‐ZrO2 of 332 GPa obtained from a volume‐pressure dependence. The current results give an estimate of the bulk modulus of 254 GPa.
Proton Conductor Ba3Ca1.18Nb1.82O9‐delta (BCN18): Effect of Reducing Environments
Authors: Schober Tilman , Friedrich Jochen
Published: July 2010

The compound Ba3Ca1.18Nb1.82O9‐delta (BCN18) has been discussed recently as a promising high‐temperature proton conductor. Oxygen loss under very reducing conditions has been determined quantitatively using thermogravimetry. The thermodynamic parameters DeltaH°O= 2.58 ± 0.06 eV and DeltaS°O= 1.06 × 10‐3 eV/K provide a suitable thermodynamic description for the equation O×O right arrow ½O2+V··O+ 2e´. Enthalpy and entropy of the hydrogen absorption reaction have been determined indirectly for BCN18: DeltaH°H= ‐0.8 eV and DeltaS°H= ‐6.6 × 10‐4 eV/K. Hydrogen absorption is, therefore, an exothermic process. Valence changes where Nb5++e´ right arrow Nb4+ also have been taken into account under reducing conditions.
Subcritical Crack Propagation in 3Y‐TZP Ceramics: Static and Cyclic Fatigue
Authors: Chevalier Jerome , Olagnon Christian , Fantozzi Gilbert
Published: July 2010

A detailed analysis of crack propagation in tetragonal zirconia polycrystals doped with 3 mol% of Y2O3 (3Y‐TZP) ceramics is presented. Crack propagation tests have been conducted for crack velocities of 10‐12‐10‐3 m/s in several environments, including air, water (in the temperature range of 3°‐85°C), secondary vacuum (10‐5 mbar), and silicon oil. Analysis of the experimental results‐three propagating regimes that are dependent on the environment and a marked threshold below which no propagation occurs‐shows that stress corrosion by water molecules is the key mechanism for crack propagation. The effect of grain size on the crack velocity is quantified and analyzed in terms of transformation toughening. Experiments under cyclic loading have been conducted to quantify the effects of cyclic fatigue. Crack velocities are higher under cyclic loading than that predicted by stress corrosion alone, and the threshold is lower. Experiments that have been conducted at two different frequencies (0.1 and 1 Hz) and static‐fatigue/cyclic‐fatigue sequences show that both stress corrosion by water and pure cyclic‐fatigue effects are operative under alternative stresses.
Residual Stress Distributions in Ceramics
Authors: Zimmermann André , Fuller Edwin R. , Rödel Jürgen
Published: July 2010

Residual stress distributions in ceramics can be calculated for a coordinate system fixed by the specimen geometry as well as for the crystal system. Computations were performed using alumina as a model system with the residual stresses caused by thermal expansion anisotropy. Mean values and standard deviations of the stress distributions are calculated for a plane stress as well as a plane strain scenario in a laboratory coordinate system and compared to distributions of stresses occuring along thea‐axis andc‐axis of alumina. In addition, stress fluctuations in the whole assembly of grains are compared with stress fluctuations within and between individual grains.
Transmission Electron Microscope Investigation of the Interface between Titanium and Zirconia
Authors: Lin Kun‐Fung , Lin Chien‐Cheng
Published: July 2010

The interfaces between 3‐mol%‐yttria‐partially‐stabilized zirconia and commercially pure titanium after reaction at 1750°C were analyzed with a scanning electron microscope and an analytical transmission microscope. Zirconia was reduced to oxygen‐deficient zirconia (ZrO2‐x) with an O/Zr ratio as low as 1.53, causing the evolution of oxygen. Part of the oxygen could accumulate at grain boundaries, the remainder being dissolved in titanium as alpha‐Ti(O). An ordered titanium suboxide (Ti3O) could be formed from a solid solution of alpha‐Ti(O) during cooling. A fine crystalline ZrO2‐x phase (O/Zr similar/congruent 2) was also found along with alpha‐Zr near the interface on the zirconia side. The alpha‐Zr was twinned with one of the twin planes being indexed as {1012}. The yttria stabilizer was excluded from zirconia as the reaction was progressing, existing as oxygen‐deficient yttria. Extensive dissolution of zirconia in titanium gave rise to the formation of alpha‐Ti(Zr,O) solid solution. On cooling, lamellae of Ti2ZrO precipitated from alpha‐Ti(Zr,O) with an orientation relationship of {110}Ti2ZrO//{100}alpha‐Ti and <111>Ti2ZrO//<011>alpha‐Ti.
Determination of Texture from Individual Grain Orientation Measurements
Authors: Blendell John E. , Vaudin Mark D. , Fuller Edwin R.
Published: July 2010

We present a technique for determining the texture of a polycrystalline material based on the measurement of the orientation of a number of individual grains. We assumed that the sample has fiber (i.e., axisymmetric) texture and that the texture can be characterized by a function (the March‐Dollase function) with a single parameter. We simulated a large number, N, of orientation data sets, using the March‐Dollase function for a total of five different texture parameters, rinit. Using the maximum‐likelihood method, we solved for the texture parameter, r´, that best fits each simulated data set in order to determine the distribution of r´ and evaluate the precision and accuracy with which r´ can be determined. The 90% confidence limits of the ratio r´/rinit vary as N‐1/2 but were independent of rinit. Using the texture of slightly textured Al2O3 as determined by X‐ray diffraction, we calculated the 90% confidence limits for measurements of 131 grains. The orientations of 131 grains in textured Al2O3 were measured by electron backscatter diffraction, and the texture determined from those measurements lay within these 90% confidence limits.
Si NMR Study of the Hydration of Ca3SiO5 and ß‐Ca2SiO4 in the Presence of Silica Fume
Authors: Sun Guokuang , Brough Adrian R. , Young J. Francis
Published: July 2010

29Si magic‐angle spinning nuclear magnetic resonance (MASNMR) was used to study the room‐temperature hydration of C3S, ß‐C2S, and reactive ß‐C2S mixed with different amounts of silica fume (SF) that had been hydrated up to nine months and longer. The overall CaO:SiO2 molar ratios of the mixes were 0.12, 0.20, 0.35, 0.50, and 0.80. NMR spectroscopy was used to quantify the remaining starting materials and the resulting hydration products of different species. A broad peak assigned to Q3, appearing in both the fourier transform (FT) and the cross‐polarization (CP) modes, increased in intensity with increased SF content and with age. This Q3 species was attributed to two sources: (1) the surface hydroxylation of SF and (2) the cross‐linking of dreierketten (chains of silicate tetrahedra arranged in a repeating three‐unit conformation) in the calcium silicate hydrate (C‐S‐H) structure. A Q4 species also appeared in the CP spectra of samples with large SF additions after extended hydration and was attributed to cross‐polarization by adjacent hydroxylated Q3 species at the surface of amorphous SiO2.
Hydrothermal Synthesis of Pollucite (CsAlSi2O6) Powders
Authors: MacLaren Ian , Cirre Jesus , Ponton Clive B.
Published: July 2010

Pollucite (CsAlSi2O6) powders were produced by hydrothermal synthesis from relatively inexpensive precursors. The pollucite powders were comprised of polycrystalline particles with dimensions of hundreds of nanometers consisting of 25‐75 nm crystallites. The polycrystalline particles were near stoichiometric composition. The pollucite powders were mixed with a poly(ethylene glycol) binder, pressed into disks, and sintered at temperatures from 1450° to 1550°C for 10 h. The resulting sintered ceramics had densities >80% of theoretical density and, in the best case, >97% of theoretical density.
Structural Analysis of Heat‐Treated Chromium Oxycarbide Films
Authors: Kado Tetsuo
Published: July 2010

Chromium oxycarbide (Cr2CO) films were deposited on stainless steel and copper plates via plasma‐assisted chemical vapor deposition. The films were heated to 1073 K under high‐vacuum conditions, to examine the high‐temperature stability of the films. X‐ray diffractometry revealed that the films decomposed to a mixed structure of Cr2O3 and Cr3C2. Thermogravimetric analysis and differential thermal analysis revealed that the decomposition occurred in the temperature range of 858‐910 K.
Surface Modification of Sapphire by Magnesium‐Ion Implantation
Authors: Li Jihong , Nutt Steven R. , Kirby Kevin W.
Published: July 2010

Single crystals of Al2O3 were implanted at a temperature of 25°C with magnesium ions that were accelerated at 200 keV and then annealed in oxygen gas for 15 h at 1500°C. The microstructure and composition in the implanted region were examined using analytical electron microscopy techniques, with an emphasis on identification of the microstructural changes that were caused by implantation and annealing. Implantation created a damage zone 0.3 µm thick in the near‐surface region of sapphire, and implanted magnesium ions were distributed in this zone without detectable precipitation. Annealing in oxygen gas caused redistribution of the implanted magnesium ions and the formation of a discrete buried layer of spinel (MgAl2O4) that was epitactic with both the substrate and the cap layer.
Cation Vacancy Energetics in the Gadolinium Titanate/Zirconate System
Authors: Williford Ralph E. , Weber William J.
Published: July 2010

Atomistic simulations of coupled anion/cation disorder in the pyrochlore structure of Gd2(ZrxTi1‐x)2O7 show that homoatomic cation vacancy migration energies range from 3.8 to 7.9 eV, and increase with the addition of anion defects. For ordered materials, the energies for heteroatomic Ti or Zr vacancy migrations to Gd sites are 1.4 and 1.9 eV, respectively, but the addition of neighboring oxygen defects generally reduces these energies to less than 1 eV. The heteroatomic migration of Gd vacancies to Ti or Zr sites remains rather large (>2.7 eV) even with the presence of oxygen defects.
Magnetic Properties of Topochemically Sintered Barium Hexaferrite
Authors: Gumaste Jayaram Laxmanrao , Ray Hem Shanker , Godkhindi Mahadev Malhar , Mukunda Pudkotai Gopalayngar
Published: July 2010

This communication describes the method of topochemical reaction sintering for the production of barium hexaferrite. Topochemical reaction sintering overcomes crucial process steps that are used in the conventional wet magnetic compaction method, such as calcination, wet grinding, and wet magnetic compaction. Chemical reaction sintering of the above‐mentioned ferrite has been performed in air and under vacuum. Barium hexaferrite that has been topochemically sintered in air at a temperature of 1473 K for 60 min shows a maximum magnetic energy product ((BH)max) of 6.56 kJ/m3, whereas barium hexaferrite that has been topochemically sintered at a temperature of 1423 K for 30 min under vacuum (10‐2 mm Hg (∼0.133 Pa)) shows (BH)max= 7.6 kJ/m3. The variation of the magnetic properties of the topochemically sintered ferrites is interrelated to process variables such as temperature and sintering duration (time).
Dislocations and Mechanical Properties of MgO‐Al2O3 Spinel Single Crystals
Authors: Mitchell Terence E.
Published: July 2010

MgO·nAl2O3 spinel single crystals can be deformed plastically at high temperatures, displaying a range of interesting features. Stress‐strain curves often exhibit strong work hardening followed by prominent work softening due to glide and climb processes. The critical resolved shear stress (CRSS) at a given temperature decreases dramatically, by almost 2 orders of magnitude, with increasing deviation from stoichiometry, i.e., as n increases from 1 to 3.5. The CRSS is proportional to exp(‐T/T0) and to [Vc]‐2, where T is the temperature in kelvin, T0 a characteristic temperature, and [Vc] the concentration of charge‐compensating cation vacancies. The Burgers vector is 1/2<110>, and slip can occur on {111} and {110} planes. Slip on {111} planes is believed to occur between the Kagomé cation layer and the adjacent anion layer. Slip on {110} planes is slightly easier (and has a higher T0), because the planes are more widely separated. The temperature dependence of the CRSS can be explained in terms of the Peierls stress for partial dislocations, either in terms of a steep and high Peierls potential or in terms of temperature and stress‐dependent kink diffusion. The dependence of CRSS on [Vc]‐2 can be explained in terms of kink nucleation at cation vacancies.
Transparent Conducting Oxides Based on the Spinel Structure
Authors: Kawazoe Hiroshi , Ueda Kazushige
Published: July 2010

The structural characteristics of spinel (from both theoretical and experimental viewpoints) that are advantageous for producing transparent conducting oxide (TCO) materials are considered. A working hypothesis is presented for finding new TCO materials with spinel structure. Results of tight‐binding energy‐band calculations reveal the formation of extended conduction bands in selected TCOs. This is a necessary criterion for n‐type TCOs containing cations with d10s0 electronic configuration. Discovery of some transparent conducting spinels, namely MgIn2O4, CdGa2O4, and ZnGa2O4, provides experimental confirmation for the formation of extended conduction bands. Li+‐, He+‐, and H+‐ion implantations into MgIn2O4 films have been performed to explore a new technique to generate electron carriers. Carrier generation in implanted MgIn2O4 films has been verified from electrical conductivity and optical absorption measurements. Implanted Li+ ions have been observed to occupy vacant tetrahedral cation interstices in the spinel lattice. Each Li+ ion releases an electron to the conduction band upon occupying this tetrahedral vacancy. Observations suggest that the ion implantation technique is effective and feasible for carrier generation in TCO materials.
Crystallization Behavior of Potassium Niobium Silicate Glasses
Authors: Pernice Pasquale , Aronne Antonio , Sigaev Vladimir N. , Sarkisov Pavel D. , Molev Vladimir I. , Stefanovich Sergei Yu.
Published: July 2010

Crystallization behavior of potassium niobium silicate (KNS) glasses having compositions expressed by the general formula xK2O·xNb2O5·(1 ‐ 2x)SiO2, with x= 0.167, 0.182, 0.200, 0.220, and 0.250, has been studied by DTA, X‐ray diffraction, second harmonic optical generation (SHG), and electron microscopy. Bulk crystallization of potassium niobates in glasses with compositions near K2O·Nb2O5·2SiO2, as well as surface crystallization of KNbSi2O7 phase, has been established. Transparent glass‐ceramics, based on potassium niobates with remarkable SHG signal values, can be obtained from glasses with the lowest silica content, by heat treatment at temperatures just above Tg, while at higher temperatures from all of the glasses under investigation the main crystallizing phase is KNbSi2O7 ferroelectric. Applying a dc electric field, grain‐oriented crystallization is produced in KNS glasses with development of significantly anisotropic arrangements of KNbSi2O7 crystallites.
Role of the Powder Bed in the Densification of Silicon Carbide Sintered with Yttria and Alumina Additives
Authors: Winn E. Jennifer , Clegg William J.
Published: July 2010

The effect of a powder bed on the densification behavior of a compact made from a silicon carbide powder and containing yttria and alumina additives has been studied. It has been found that where the powder bed is made of just silicon carbide grit, relative densities of up to 0.92 of the theoretical density can be obtained. Additions of alumina to the powder pack increased the apparent relative density to 0.98. Further experiments have shown that the pack cannot act as a physical barrier to the diffusion of volatile components of the sintering aids. The alumina additions to the pack increase the relative density by allowing the diffusion of alumina species into the sample, rather than by affecting its shrinkage behavior. It is suggested that the predominant effect of the pack is to ensure that the partial pressure of silicon‐containing species, such as SiO and Si, within the sample is sufficient to allow reaction between them and the volatile sub‐oxides of yttrium and aluminum, forming a liquid phase.
Fired Porcelain Microstructures Revisited
Authors: Iqbal Yaseen , Lee William Edward
Published: July 2010

The microstructure of standard commercial porcelain consists of alpha‐quartz grains held in a complex matrix. Quartz grains are surrounded by amorphous silica‐rich solution rims while the matrix contains clay relicts of small (~200 nm × 40 nm × 40 nm) primary (2Al2O3:1SiO2) mullite crystals in aluminosilicate glass and feldspar relicts of acicular (>1 µm long) secondary (3Al2O3:2SiO2) mullite in potassium aluminosilicate glass. A continuous increase in mullite crystal size from the clay‐feldspar relict interface to the feldspar relict center and their compositions indicate a transformation from primary to secondary mullite. In aluminous porcelain, corundum grains are observed in addition to alpha‐quartz and regions of clay and feldspar relicts. Small (~50 nm × 10 nm × 10 nm) tertiary mullite crystals (formed by precipitation from Al2O3‐rich glass) were detected adjacent to these corundum grains.
Fracture Toughness of a Silicon Nitride/Silicon Carbide Nanocomposite at 1350°C
Authors: Dusza Jan , Sajgalik Pavol , Steen Marc
Published: July 2010

The fracture toughness of a hot‐pressed silicon nitride/silicon carbide (Si3N4/SiC) nanocomposite and reference monolithic Si3N4 has been investigated in four‐point bending at 1350°C in air, using different loading rates (0.01‐1 mm/min). Single‐edge V‐notched bend specimens that were prepared by polishing the notch tip to a radius of <10 µm, using 1 µm diamond paste, were used for the fracture toughness measurement. Slow crack growth (SCG) prior to catastrophic failure was detected at all applied loading rates at 1350°C. The fracture toughness at 1350°C, as calculated using the actual crack size measured on the fracture surface after the bend test, increased in both ceramics with decreasing loading rate and increasing area of the SCG region.
Effects of Ammonium Chloride on the Rheological Properties and Sedimentation Behavior of Aqueous Silica Suspensions
Authors: Guo J. Jiyou , Lewis Jennifer A.
Published: July 2010

The influence of ammonium chloride (NH4Cl) on the rheological properties and sedimentation behavior of aqueous silica (SiO2) suspensions of varying solids volume fraction (φs) was studied. SiO2 suspensions with low NH4Cl concentration (≤0.05M, pH 5.2) exhibited Newtonian behavior and a constant settling velocity (U). The volume fraction dependence was well described by the Richardson–Zaki form, U=U0(1 −φs)n, where n= 4.63 and U0= 1.0419 × 10−5 cm/s. At higher NH4Cl concentrations (0.07–2.0M, pH 5.2), suspensions exhibited shear thinning and more complicated sedimentation behavior due to their aggregated nature. For all suspensions studied, however, the apparent suspension viscosity, characteristic cluster size, and initial settling velocity were greatest at ∼0.5M NH4Cl and exhibited a similar dependence on salt concentration. Above 0.5M NH4Cl, considerable restabilization was observed. This behavior cannot be explained by traditional DLVO theory.
Evaluation of the Cyclic Fatigue Life of 3‐mol%‐Yttria‐Stabilized Zirconia Bioceramic Using Biaxial Flexion
Authors: Hulm Belinda J. , Evans William J.
Published: July 2010

The cyclic fatigue life of zirconia bioceramic was determined under biaxial flexion using disk‐shaped specimens. The fatigue life was not reduced by subjecting the material to a simulated physiological environment nor was it affected by testing at different loading frequencies. Tests performed at different stress ratios produced a large degree of scatter in the data but a statistical analysis proved that there was no significant effect on the fatigue life of the material. Failure of disk specimens was initiated either by flaws introduced during manufacturing or by the more typical inherent flaws commonly observed in ceramics. The former generally resulted in premature, low‐stress failures on initial loading whereas the latter acted as initiating defects for fatigue cracks that then propagated to failure. The distributions of the two categories of flaw were analyzed using two‐ and three‐parameter Weibull probability functions. It became clear that the Weibull modulus for the fatigue failures was consistent with previously reported work on other ceramics.
Growth of BaTiO3 Seed Grains by the Twin‐Plane Reentrant Edge Mechanism
Authors: Kang Myung‐Koo , Yoo Young‐Sung , Kim Doh‐Yeon , Hwang Nong M.
Published: July 2010

Two different types of BaTiO3 seed particles, normal and twinned seeds of ∼30 μm on the average, were prepared from crushed sintered specimens. Normal seeds were obtained from the usual BaTiO3 sintered compacts, while twinned seeds containing a double twin were obtained from BaTiO3 compacts sintered with 2 mol% of SiO2. The BaTiO3 powder compacts were again prepared with 5 wt% of seed grains and sintered under various conditions. The microstructural evolution was quite different in the two cases: the growth of normal seed grains was ultimately limited but that of the twinned seeds continued extensively. The observed difference is discussed in terms of the growth mechanism and the atomic structure of interfaces.