bluuebunny/crossref_metadata_2025 huggingface dataset:
Scratch Damage in Zirconia Ceramics
Authors: Kun Lee Seung , Tandon Rajan , Readey Michael J. , Lawn Brian R.
Published: July 2010

Scratch damage modes in zirconia‐based ceramics—Mg‐PSZ, Y‐TZP, and Ce‐TZP—are investigated. Precursor indentation tests with a tungsten carbide sphere foreshadow the nature of damage: in Mg‐PSZ, extensive (quasi‐)plastic deformation in the region outside and beneath the contact; in Y‐TZP, less plastic deformation beneath the contact but incipient cone cracking in the region of tension outside the contact; in Ce‐TZP, intermediate behavior. Scratch testing is conducted using a conical diamond indenter. In all materials the damage mode changes from smooth plastic deformation to limited cracking with increasing scratch load: in Mg‐PSZ, plastic deformation is predominant at lower loads, with microcracking at higher loads; in Y‐TZP, plastic deformation is predominant over the range of the test loads—macrocracks initiate only at relatively high loads, but penetrate to a relatively large depth; again, Ce‐TZP shows intermediate behavior, but with cracking patterns closer to that of Mg‐PSZ. Bending tests on specimens subjected to scratch damage indicate a relatively high damage tolerance in the Mg‐PSZ and Ce‐TZP; Y‐TZP shows the highest initial strength, but suffers relatively large strength loss above the critical load for macrocracking. Implications concerning relative merits of each zirconia type for wear properties, contact fatigue, and machining damage are briefly discussed.
Finite‐Element Analysis of Ceramic Multilayer Capacitors: Failure Probability Caused by Wave Soldering and Bending Loads
Authors: Franken Klaus , Maier Horst R. , Prume Klaus , Waser Rainer
Published: July 2010

A two‐dimensional numerical model that predicts the reliability of multilayer capacitors (MLCs) during soldering and bending is presented. The Weibull parameters used in the model are based on measurements of soldered MLC devices. The preheating and soldering temperatures have a dominant impact on the failure probability, in comparison to the thickness of the nickel layer, the soft solder geometry, and the number of inner electrodes. Comparison of calculated and measured reliability of three MLC sizes leads to the assumption that residual stresses due to the manufacturing process or size‐related microstructure are important.
Synthesis of Pure Monolithic Calcium, Strontium, and Barium Hexaluminates for Catalytic Applications
Authors: Di Filippo Lucia , Lucchini Elio , Sergo Valter , Maschio Stefano
Published: July 2010

Calcium, strontium, and barium hexaluminates have been prepared via a chemical route. In light of their interest as catalytic substrates the relation between attrition‐milling time and density was studied, with the scope of obtaining fired bodies with suitable pore structure. It was found that all hexaluminates grew in elongated structures; longer milling times resulted in higher aspect ratios for calcium hexaluminate, with less than 75% of the theoretical density. Strontium and barium hexaluminates were prepared with even lower densities, but the reason is the inherent poor densification, and not the growth of mutually interfering elongated structures, as in the case for calcium hexaluminate.
Deterioration in the Final‐Stage Sintering of Magnesium Aluminate Spinel
Authors: Ting Ching‐Jui , Lu Hong‐Yang
Published: July 2010

Sintering of magnesium aluminate spinel of the MgO‐excess, stoichiometric, and Al2O3‐excess compositions has been investigated under vacuum and in air for the effect of low oxygen partial pressures. Densification enhancement of the surface layer is due to MgO evaporation which generates oxygen vacancies in the host crystal lattice. Regions of different grain sizes are observed from samples sintered under both conditions. Microstructural features of pairwise breakup of particle chains representing differential sintering are characteristic of the less‐densified sample interior. The densification improved initially and yet was retarded in the intermediate sintering stage when the density exceeded 75% with vacuum‐sintering owing to MgO evaporation.
Alumina–Aluminide Alloys (3A) Technology: I, Model Development
Authors: Gaus Shaun P. , Harmer Martin P. , Chan Helen M. , Caram Hugo S. , Claussen Nils
Published: July 2010

The general reaction behavior of the 3A process under the thermal explosion mode of synthesis has been investigated via a continuum model. The continuum model uses mass and energy balances to predict temperature difference (Ts,avg−Tf) curves, as well as profile curves of the reactant conversions and sample temperature. In particular, the effect of the dimensionless parameters associated with the rate of local heat generation (β, the thermicity factor), the activation energy (γ, the Arrhenius number), the rate of heat redistribution (α, the modified thermal diffusivity), the rate of heat transfer by convection (Bi, the Biot number or convective heat transfer parameter), and the rate of heat transfer by radiation (Ω, the radiative heat transfer parameter) were investigated. Conditions to control the reaction process, which should produce high‐density final products, were determined. It was found that the overall maximum temperature may be reduced for high γ, low β, high α, and high Bi and Ω. In terms of processing conditions, this may be obtained by reducing the initial reactant concentrations, optimizing the particle size, using small sample sizes and high compaction pressure, and increasing the heat loss by using a high thermal conductivity inert gas.
Comparison of Microwave Hybrid and Conventional Heating of Preceramic Polymers to Form Silicon Carbide and Silicon Oxycarbide Ceramics
Authors: Danko Gene A. , Silberglitt Richard , Colombo Paolo , Pippel Eckhard , Woltersdorf Jörg
Published: July 2010

Six different preceramic polymers were pyrolyzed via conventional and microwave hybrid heating; these polymers provide a range of carbon content and local atomic coordination. The products were compared with each other using X‐ray diffractometry and transmission electron microscopy. Nanocrystalline β‐SiC was the principal crystal phase detected, and the amount and size of the nanocrystals increased as the processing temperature increased. Differences were observed in the amount and size of the β‐SiC nanocrystals and the graphitization of residual carbon between the microwave hybrid heating and the conventional oven heating of polycarbosilanes. Conventional heating of a high‐carbon polysiloxane in an oven (in flowing argon) produced a greater amount of β‐SiC from carbothermal reduction at high temperature. Microwave hybrid heating led to better β‐SiC nanocrystal development for polyureasilazane.
Morphological Changes in Process‐Related Large Pores of Granular Compacted and Sintered Alumina
Authors: Shinohara Nobuhiro , Okumiya Masataro , Hotta Tadashi , Nakahira Kenji , Naito Makio , Uematsu Keizo
Published: July 2010

Large pore defects clearly develop in Al2O3 ceramics during sintering. These large pores originate from voids caused by the incomplete deformation and adhesion of powder particles in collapsed dimples at the centers and boundaries of granules in the green compacts. The coalescence of pores, with limited shrinkage, during densification and grain growth in the late intermediate to final stages of sintering, is considered responsible for the development of the large pores. The mechanism of pore coalescence is explained by thermodynamic arguments, which demonstrate that the largest pores result in a stable system.
Green State Joining of Silicon Carbide Using Polycarbosilane
Authors: Zheng Jing , Ünal Özer , Akinc Mufit
Published: July 2010

Green state joining of SiC was investigated using a paste consisting of polycarbosilane polymer and SiC powder. The joining process and densification were described. Initial experiments resulted in the formation of symmetrical black bands and cracks on both sides of the joint. However, with modifications in processing conditions, the cracks were eliminated and the resulting joints were indistinguishable from the matrix. The flexural strength of joined samples was measured to be 234 MPa, which was comparable to that of the control sample with similar density. As the applied pressure during joining was increased from 34 to 138 MPa, the strength of the joined samples increased from 180 to 250 MPa.
Refinement of the Thermodynamic Properties of Ruthenium Dioxide and Osmium Dioxide
Authors: Jacob K. Thomas , Mishra Saurabh , Waseda Yoshio
Published: July 2010

The standard Gibbs energies of formation of RuO2and OsO2at high temperature have been determined with high precision, using a novel apparatus that incorporates a buffer electrode between the reference and working electrodes. The buffer electrode absorbs the electrochemical flux of oxygen through the solid electrolyte from the electrode with higher oxygen chemical potential to the electrode with lower oxygen potential. The buffer electrode prevents polarization of the measuring electrode and ensures accurate data. The standard Gibbs energies of formation (ΔfG°) of RuO2, in the temperature range of 900–1500 K, and OsO2, in the range of 900–1200 K, can be represented by the equationsimagewhere the temperatureTis given in Kelvin and the deviation of the measurement is ±80 J/mol. The high‐temperature heat capacities of RuO2and OsO2are measured using differential scanning calorimetry. The information for both the low‐ and high‐temperature heat capacity of RuO2is coupled with the ΔfG° data obtained in this study to evaluate the standard enthalpy of formation of RuO2at 298.15 K (ΔfH°298.15K). The low‐temperature heat capacity of OsO2has not been measured; therefore, the standard enthalpy and entropy of formation of OsO2at 298.15 K (ΔfH°298.15Kand S°298.15K, respectively) are derived simultaneously through an optimization procedure from the high‐temperature heat capacity and the Gibbs energy of formation. Both ΔfH°298.15KandS°298.15Kare treated as variables in the optimization routine. For RuO2, the standard enthalpy of formation at 298.15 K is ΔfH°298.15K(RuO2) =−313.52 ± 0.08 kJ/mol, and that for OsO2is ΔfH°298.15K(OsO2) =−295.96 ± 0.08 kJ/mol. The standard entropy of OsO2at 298.15 K that has been obtained from the optimization is given asS°298.15K(OsO2) = 49.8 ± 0.2 J·(mol·K)−1.
Crystal Nucleation and Growth in Lithium Diborate Glass by Thermal Analysis
Authors: Koga Nobuyoshi , Sestak Jaroslav
Published: July 2010

The kinetics of nucleation and crystal growth in a Li2B4O7 glass were investigated using differential thermal analysis (DTA) and differential scanning calorimetry (DSC). This melt forms a glass even when it is cooled at a very slow rate (2 K/min). The temperature range of nucleation was determined by comparing the heights of the exothermic DTA crystallization peaks for samples that were annealed at different temperatures for a fixed time (10 min). Nucleation for this glass occurred in the temperature region of glass transition, where the nucleation rate had a maximum value at ∼770 K. The real detectable enthalpy change that was due to crystallization occurred at temperatures well above the temperature range for nucleation; hence, the temperature regions for nucleation and crystal growth were assumed to be well separated. The glass sample with an almost‐saturated number of nuclei, which was prepared by annealing at a temperature of 770 K for 10 min while the melt was cooled, was subjected to DSC measurements at various heating rates (φh). The kinetics of crystal growth were analyzed via the conventional isoconversion method. For the sample without any annealing treatment, the number of nuclei for growth varied, depending on the thermal histories within the temperature region for nucleation, which was characterized by the linear cooling rate (φc) of the melt and the linear heating rate φh of the glass. A kinetic approach that considered the dependence of the number of nuclei on φc and φh was applied to the crystal growth in the nonannealed glass. The three‐dimensional growth of the preexisting nuclei with apparent activation energies (E) of 315.6 ± 4.6 kJ/mol for the annealed glasses and 300.6 ± 9.5 kJ/mol for the nonannealed glasses was determined. This agreement in the values of E indicates that the change in the number of nuclei, depending on φc and φh, can be treated successfully by the present kinetic approach for crystal growth.
Facile Synthesis of Aluminum‐Containing Mixed‐Metal Oxides Using Doped Carboxylate Alumoxane Nanoparticles
Authors: Callender Rhonda L. , Barron Andrew R.
Published: July 2010

A simple and rapid process has been developed that allows the formation of highly crystalline main‐group, lanthanide, and transition‐metal aluminates, including CaAl12O19 (hibonite), Y3Al5O12 (yttrium aluminum garnet, YAG), LaAl11O18, LaAlO3, Ce2Al3O8, NdAlO3, Er6Al10O24, and Al2TiO5. Carboxylate‐stabilized alumina nanoparticles (carboxylate alumoxanes) are reacted with the appropriate metal acetyl‐acetonate complexes to form metal‐doped carboxylate alumoxanes and aluminum acetylacetonate via a facile transmetalation reaction. Thermolysis of the metal‐doped carboxylate alumoxanes, up to a temperature of 1400°C, yields crystalline aluminates, as confirmed by X‐ray diffractometry, electron microprobe analysis, and scanning electron microscopy. The synthesis of different phases (i.e., LaAl11O18 versus LaAlO3) is controlled by the stoichiometry of the transmetalation reaction. A new, distorted, perovskite phase (with lattice parameters of a= 5.47 Å and c= 13.3 Å) of LaAlO3 is formed. The alumoxane methodology results in multicomponent aluminate materials that are obtained in shorter processing times and with greater phase purity than previously reported. This observation is undoubtedly due to the atomic‐scale mixing that is inherent in the alumoxane nanoparticle approach.
Fracture Energy of an Aligned Porous Silicon Nitride
Authors: Inagaki Yoshiaki , Ohji Tatsuki , Kanzaki Shuzo , Shigegaki Yasuhiro
Published: July 2010

The fracture energy of a porous silicon nitride with aligned fibrous grains was investigated, using a chevron‐notched‐beam technique. A crack was constrained to propagate normal to the grain alignment. The obtained fracture energy was ∼500 J/m2, which was ∼7 times larger than that of a dense silicon nitride with randomly oriented fibrous grains. The large fracture energy was attributable primarily to the sliding resistance associated with interlocking grains.
Effect of Additives on the Hydration Resistance of Materials Synthesized from the Magnesia–Calcia System
Authors: Chen Shujiang , Chen Guorong , Cheng Jijian , Tian Fengren
Published: July 2010

The effect of a variety of doping additives on the hydration resistance of calcined materials in the MgO–CaO system was investigated. Samples were prepared from lightly calcined flotation magnesite that was mixed with dolomite, as well as from analytically pure CaO and MgO; then, the samples were doped with additives that contained cations with various valences (monvalent to tetravalent). Both the hydration rate and the powdering rate were measured. The important role of the higher‐valence cations in improving the hydration resistance of the MgO–CaO materials was revealed. This behavior is believed to be due to the formation of vacancies in solid solutions of CaO or MgO with higher‐valence cations. The Ti4+ cation forms a solid solution with CaO, which reduces the Ca2+ concentration and leads to the improved hydration resistance of calcined materials from the MgO–CaO system.
Microstructure and Mechanical Properties of Fine‐Grained Magnesia‐Partially‐Stabilized Zirconia Containing Titanium Carbide Particles
Authors: Jang Joon Hyuk , Lee Jaehyung
Published: July 2010

Fine‐grained Mg‐PSZ has been fabricated by adding TiC particles. The average cubic grain size was smaller by more than an order of magnitude than without TiC when the TiC content was over 2.5 vol%. Pressurelessly sintered specimens contained numbers of relatively large pores while hot‐pressed ones were fully dense. For hot‐pressed specimens, addition of TiC particles did not affect the growth behavior of tetragonal precipitates during annealing. With increasing TiC content, the bend strength of hot‐pressed specimens increased while the fracture toughness decreased. The bend strength and the fracture toughness of fine‐grained Mg‐PSZ containing 5 vol% TiC were 980 MPa and 8.2 MPa·m1/2, respectively.
Structural Characterization of Aluminum Phosphate Binder
Authors: Vippola Minnamari , Keränen Jaakko , Zou Xiaodong , Hovmöller Sven , Lepistö Toivo , Mäntylä Tapio
Published: July 2010

The chemical structure of aluminum phosphate binder, which is used as a sealant for thermally sprayed coatings, was studied. Powder X‐ray diffraction (XRD) analysis showed that the binder was a mixture of several phases. None of these phases could be identified from the powder XRD data only. Electron diffraction revealed several distinct microcrystalline phases. The predominated phase was analyzed in more detail. The three‐dimensional reciprocal lattice was reconstructed from a tilt series of electron‐diffraction patterns that indicated that the main phase was the cyclohexaphosphate Al2P6O18. The similarity of the intensity distribution in the observed and simulated electron‐diffraction patterns of Al2P6O18 confirmed this observation. The following unit‐cell parameters were determined from the reconstructed reciprocal lattice: a= 8.8(7) Å, b= 15.5(6) Å, and c= 6.3(9) Å, β= 108°, and Z= 2. The reflection conditions indicated that the space group was P21/a.
Gelation, Consolidation, and Rheological Properties of Boehmite‐Coated Silicon Carbide Suspensions
Authors: Yang Chia‐Yi , Shih Wan Y. , Shih Wei‐Heng
Published: July 2010

We have studied the gelation, consolidation, and rheological properties of boehmite‐coated SiC suspensions. A boehmite‐coated SiC suspension consists of SiC particles covered with a boehmite layer of a few nanometers in thickness in the suspension. Similar to boehmite suspensions, the boehmite‐coated SiC suspension can gel over time. The gelation, as well as the rheological behavior of the boehmite‐coated SiC suspension with respect to pH, is shown to be similar to that of a boehmite suspension. However, because of the particle‐size difference, a boehmite‐coated SiC suspension gels more slowly than suspensions of smaller boehmite particles. The boehmite coating improves the consolidation density of SiC, increasing the sediment density from 39 to 52 vol% and the centrifuged density from 50 to 60 vol%. It, also, makes the consolidation behavior of a boehmite‐coated SiC suspension with respect to pH more consistent with the rheological behavior; i.e., lower suspension viscosity and storage modulus correlate with a higher consolidated density. In contrast, suspensions containing SiC particles partially covered with boehmite and individual boehmite particles in the suspensions show no improvement in the sediment density and no systematic correlation between the consolidation density and the rheological properties. This indicates that the complete coating of boehmite on the SiC particles is critical to the improvement in consolidation density.
Constrained Film Sintering of Nanocrystalline TiO2
Authors: Stech Martin , Reynders Peter , Rödel Jürgen
Published: July 2010

The sintering of thin, nanocrystalline TiO2 films either 140 or 65 nm thick is characterized and compared with the sintering of bulk material. Grain size, pore size distribution, and density data are obtained. Observation of the microstructural evolution during sintering suggests that grain growth, as well as pore growth, at low density can be attributed to differential sintering. A continuum mechanical model for the intermediate stage is useful until the grain size becomes one‐half the film thickness. From then on, the ratio of grain size to film thickness affects the degree by which both densification and grain growth decrease, as compared with the continuum computation results.
Polyelectrolyte Effects on the Rheological Properties of Concentrated Cement Suspensions
Authors: Lewis Jennifer A. , Matsuyama Hiro , Kirby Glen , Morissette Sherry , Young J. Francis
Published: July 2010

Polyelectrolyte species, known as superplasticizers, dramatically affect the rheological properties of dense cement suspensions. We have studied the influence of sulfonated naphthalene formaldehyde condensate (SNF) and carboxylated acrylic ester (CAE) grafted copolymers of varying molecular architecture on the surface (e.g., adsorption behavior and zeta potential) and rheological properties of concentrated cement suspensions of white portland cement and two model compounds, β‐Ca2SiO4 and γ‐Ca2SiO4. The adsorption of SNF species was strongly dependent on cement chemistry, whereas CAE species exhibited little sensitivity. The respective critical concentrations (Φ*) in suspension required to promote the transition from strongly shear thinning to Newtonian flow (flocculated → stable) behavior were determined from stress viscometry and yield stress measurements. Theoretical analysis of interparticle interactions suggested that only colloidal particles in the size range of ≤1 μm are fully stabilized by adsorbed polyelectrolyte species. Our observations provide guidelines for tailoring the molecular architecture and functionality of superplasticizers for optimal performance.
Preparation of Yttria‐Stabilized Zirconia Thin Films on Strontium‐Doped LaMnO3 Cathode Substrates via Electrophoretic Deposition for Solid Oxide Fuel Cells
Authors: Ishihara Tatsumi , Shimose Kuninobu , Kudo Takanari , Nishiguchi Hiroyasu , Akbay Taner , Takita Yusaku
Published: July 2010

A yttria‐stabilized zirconia (YSZ) thin film on an La0.8Sr0.2MnO3 porous cathode substrate was prepared, using electrophoretic deposition (EPD) to fabricate a solid oxide fuel cell (SOFC). The electrical conductivity of an La0.8Sr0.2MnO3 substrate is satisfactorily high at room temperature; therefore, YSZ powder could be deposited electrophoretically onto an La0.8Sr0.2MnO3 substrate without any extra surface treatment, such as a metal coating. Successive repetition of EPD and sintering was required to obtain a film without gas leakage, because of the thermal expansion coefficient mismatch between the YSZ and the La0.8Sr0.2MnO3 substrate. On the other hand, the electromotive force of the oxygen concentration in the cell that used YSZ film prepared via EPD increased and attained the theoretical value when the number of deposition and calcination cycles was increased. Six or more successive repetitions were required to obtain a YSZ film without gas leakage. A planar‐type SOFC was fabricated, using nickel as the anode and YSZ film (∼10 μm thick) that had been deposited onto the La0.8Sr0.2MnO3 substrate as the electrolyte and cathode. The cell exhibited an open circuit voltage of 1.0 V and a maximum power density of 1.5 W/cm2. Thus, the EPD method could be used as a colloidal process to prepare YSZ thin‐film electrolytes for SOFCs.
Y2O2S:Eu Red Phosphor Powders Coated with Silica
Authors: Jean Jau‐Ho , Yang Szu‐Ming
Published: July 2010

Y2O2S:Eu red phosphor powders were coated with silica (SiO2), using sol–gel and heterocoagulation techniques. Phosphor powders were dispersed in ethanol with tetraethyl orthosilicate and water. Hydrochloric acid was used to catalyze the sol–gel reaction, and an amorphous film 10–20 nm thick was observed via transmission electron microscopy (TEM). Colloidal SiO2 powders 10–70 nm in size were used, and the SiO2 powder coating was made by controlling pH values in the range of 4.5–8, in which a negatively charged surface of SiO2 powder and a positively charged surface of red phosphor powder were formed. Then, SiO2 powders were adsorbed electrically onto the phosphor powder surface, as evidenced by TEM, dissolution, and zeta potential measurements. Chemical bonding in the coating was studied using electron spectroscopy for chemical analysis and Fourier transform infrared spectroscopy.
Zirconia‐Based Metastable Solid Solutions through Self‐Propagating High‐Temperature Synthesis: Synthesis, Characterization, and Mechanistic Investigations
Authors: Maglia Filippo , Anselmi‐Tamburini Umberto , Spinolo Giorgio , Munir Zuhair A.
Published: July 2010

Cubic Zr1−xMexOy(Me = Fe, Co, Ni, Cu) metastable solid solutions with metal content significantly higher than equilibrium levels have been synthesized by the self‐propagating high‐temperature synthesis method based on a thermite reaction between metallic zirconium and the transition‐metal oxides CoO, Fe2O3, CuO, and NiO. Through in situ XRD analysis, it was determined that when heated to 1100°C, the cubic solid solution transformed to the tetragonal phase with the concomitant formation of iron oxide. When cooled to lower temperatures, the tetragonal phase transformed to the monoclinic phase at or below 500°C. Results of auxiliary experiments strongly suggest that the formation of the solid solution takes place behind the combustion front by a reaction between zirconia and the metal.
Pore Structure Evolution of Lanthana–Alumina Systems Prepared through Coprecipitation
Authors: Nair Jalajakumari , Nair Padmakumar , Mizukami Fujio , Van Ommen Jan G. , Doesburg Giel B. M. , Ross Julian R. H. , Burggraaf Anthonie J.
Published: July 2010

Pure Al2O3 and different compositions of La2O3–Al2O3 samples have been prepared through coprecipitation. Even after heating at 1300°C, the compositions La2O3·11Al2O3 and La2O3·13Al2O3 had higher surface area compared to the pure Al2O3 and the La2O3·Al2O3 composition. Ethanol washing is an effective way for improving the textural stability of pure Al2O3 and La2O3–Al2O3 samples. The effect of steam on the thermal stability of La2O3·11Al2O3 has also been studied. La2O3·11Al2O3 sample is found to be stable in steam.
Thermal Conductivity of Yttria–Zirconia Single Crystals, Determined with Spatially Resolved Infrared Thermography
Authors: Bisson Jean‐François , Fournier Danièle , Poulain Martine , Lavigne Odile , Mévrel Rémy
Published: July 2010

The thermal diffusivity of yttria–zirconia (Y2O3–ZrO2) single crystals, with YO1.5 concentrations in the range of 0–60 mol%, has been determined using a new method that is based on spatially resolved (20 μm) infrared mapping of a modulated thermal field. The decreasing trend of the thermal conductivity (K), as a function of the YO1.5 content (up to 20 mol%), can be described using a model based on a Debye approach that has been modified by introducing a cut‐off length for the phonon mean free path. At higher concentrations, K increases, as a result of a possible ordering of the point defects.
Toughening by Multiple Mechanisms in Ceramic‐Matrix Composites with Discontinuous Elongated Reinforcements
Authors: Zhai Hongxiang , Huang Yong , Wang Changan , Wu Xin
Published: July 2010

The dependence of toughening mechanisms on reinforcement orientation and the toughening effect governed by multiple toughening mechanisms were characterized for ceramic‐matrix composites (CMCs) with discontinuous elongated reinforcements. Two kinds of Si3N4‐based composites, with directionally oriented and randomly oriented SiC whiskers, respectively, were tested by the three‐point bending of chevron‐notched bars. Based on microscopic observations and micromechanical analyses, three mechanisms were confirmed to dominate the crack‐bridging behavior: (1) bridging and breaking of long reinforcements, (2) frictional pullout and breaking of short reinforcements, and (3) local matrix spalling. Both the occurrence of the multiple mechanisms and their toughening effects were proved dependent on the reinforcement orientation. The combined effect of the multiple mechanisms correlated with random orientation thus was characterized by a statistical approach to solve for the crack‐bridging stress function. The theoretical model was in good agreement with the experimental results.
Zirconates as New Materials for Thermal Barrier Coatings
Authors: Vassen Robert , Cao Xueqiang , Tietz Frank , Basu Debabrata , Stöver Detlev
Published: July 2010

Zirconates with high melting points were investigated for application as materials for thermal barrier coatings at operating temperatures >1300°C. SrZrO3, BaZrO3, and La2Zr2O7 powders were synthesized and sintered to compacts with various levels of porosity. The sintering results indicated a promising low‐sintering activity of the investigated materials. Thermal properties of these dense materials were determined. Thermal expansion coefficients were slightly lower than those of Y2O3‐stabilized ZrO2 (YSZ); thermal conductivities of SrZrO3 and BaZrO3 were comparable or slightly higher than those of YSZ. La2Zr2O7 had a lower thermal conductivity. SrZrO3 was not suitable for application as a thermal barrier coating because of a phase transition at temperatures between 700° and 800°C. Mechanical properties (hardness, fracture toughness, and Young's modulus) of dense BaZrO3 and La2Zr2O7 samples were determined by indentation techniques and showed lower hardness and Young's modulus compared to YSZ. BaZrO3 and La2Zr2O7 powders were optimized for application as powders for plasma spraying. Plasma‐sprayed coatings were produced and characterized. Thermal cycling with a gas burner at 1200°C showed a massive attack of the BaZrO3 coating, with loss of BaO. On the other hand, the La2Zr2O7 coating showed excellent thermal stability and thermal shock behavior.
Critical Factors in the Templated Grain Growth of Textured Reaction‐Bonded Alumina
Authors: Suvaci Ender , Messing Gary L.
Published: July 2010

The Templated Grain Growth (TGG) of Al2O3 occurs in three stages: densification, radial growth of individual template grains until template impingement, and template grain thickening. Significant template growth occurs only after densification to ∼90% density. Template loading determines the inter‐template spacing and, subsequently, the degree of growth. The spacing distance can be predicted from the initial template concentration and template dimensions. Thermodynamic conditions favorable for template growth occur when the size ratio between the template thickness and matrix grains is ≥1.5. The initial template concentration should be less than a critical template concentration, VT,C, to achieve textured ceramics with a narrow orientation distribution. For this system, which uses 20 ± 5 μm × 2 μm template particles and uniaxial pressing as a forming technique, VT,C is between 5 and 10 vol%.
Analysis of X‐ray Powder Diffraction Data Using the Maximum Likelihood Estimation Method
Authors: Tsur Yoed , Randall Clive A.
Published: July 2010

The details of substitutional chemistries can be deduced with the associated relaxation of the crystal structure. This paper demonstrates the use of the maximum likelihood estimation method (MLE) for X‐ray powder diffraction (XRD) analysis. Detailed calculations are performed for cubic and tetragonal systems. Analysis of yittrium‐doped BaTiO3 prepared under different conditions is shown as an example. The methodology outlined here gives rise to a correct evaluation of the standard deviations of the lattice parameters. In addition, MLE approaches asymptotically the Cramer–Rao lower bound (CRLB) and, therefore, has an advantage over other estimation methods. A link between the output of a commercial software and the standard deviation in the peak position is also suggested.
Effects of Grain‐Boundary Structure on the Strength, Toughness, and Cyclic‐Fatigue Properties of a Monolithic Silicon Carbide
Authors: Da Chen  , Zhang Xiao‐Feng , Ritchie Robert O.
Published: July 2010

An in situ‐toughened silicon carbide (ABC‐SiC) has been examined in the as‐processed condition, where the grain‐boundary films are predominantly amorphous, and following thermal exposure at a temperature of 1300°C, where the films become fully crystalline. Previous work has shown that, at elevated temperatures (up to 1300°C), after the grain‐boundary films crystallize in situ, only a marginal reduction in strength, fracture toughness, and cyclic‐fatigue crack‐growth properties is observed, in comparison with those of the as‐processed microstructure at 25°C. In the present study, the effect of such crystallization on the subsequent strength, toughness, and fatigue properties at 25°C is examined. Little or no degradation is observed in the room‐temperature properties with the crystallized grain‐boundary films/phase; in fact, although the strength shows a small reduction (∼3%), the fracture toughness and fatigue‐crack‐growth threshold both increase by ∼20%, compared with that of the as‐processed structure with amorphous grain‐boundary films.
Sintering of Partially Stabilized Zirconia by Microwave Heating Using ZnO–MnO2–Al2O3 Plates in a Domestic Microwave Oven
Authors: Fujitsu Satoru , Ikegami Makoto , Hayashi Takashi
Published: July 2010

Partially stabilized zirconia (PSZ) powders were fully densified by microwave heating using a domestic microwave oven. Pressed powder compacts of PSZ were sandwiched between two ZnO–MnO2–Al2O3 ceramic plates and put into the microwave oven. In the first step, PSZ green pellets were heated by self‐heating of ZnO–MnO2–Al2O3 ceramics (1000°C). In the second step, the heated PSZ pellets absorbed microwave energy and self‐heated up to a higher temperature (1250°C), leading to densification. The density of PSZ obtained by heating in the microwave oven for 16 min was 5.7 g/cm3, which was approximately equal to the density of bodies sintered at 1300°C for 4 h or 1400°C for 16 min by the conventional method. The average grain size of the sample obtained by this method was larger than the average grain size of samples sintered by the conventional method with a similar heating process.
Influence of Yttria–Alumina Content on Sintering Behavior and Microstructure of Silicon Nitride Ceramics
Authors: Yang Jian‐Feng , Ohji Tatsuki , Niihara Koichi
Published: July 2010

The present study investigates the influence of the content of Y2O3–Al2O3 sintering additive on the sintering behavior and microstructure of Si3N4 ceramics. The Y2O3:Al2O3 ratio was fixed at 5:2, and sintering was conducted at temperatures of 1300°–1900°C. Increased sintering‐additive content enhanced densification via particle rearrangement; however, phase transformation and grain growth were unaffected by additive content. After phase transformation was almost complete, a substantial decrease in density was identified, which resulted from the impingement of rodlike β‐Si3N4 grain growth. Phase transformation and grain growth were concluded to occur through a solution–reprecipitation mechanism that was controlled by the interfacial reaction.
Effect of Chondroitin Sulfate on the Crystal Growth of Hydroxyapatite
Authors: Rhee Sang‐Hoon , Tanaka Junzo
Published: July 2010

The effect of chondroitin sulfate (ChS) templates on the crystal growth of hydroxyapatite (HAp) was investigated. HAp particles and 60HAp/40ChS hybrid (in wt%) were synthesized by using a precipitation method with a calcium hydroxide suspension and phosphoric acid solutions without and with ChS, respectively. The sizes of HAp crystals formed without ChS templates were around 75 nm × 10 nm while those with ChS templates were around 12 nm × 6 nm. The inhibitory effect of ChS templates on the crystal growth of HAp is explained in terms of organic matrix regulated HAp formation following steric hindrance by adjacent crystals.
Testing Method for the Johnson–Mehl–Avrami Equation in Kinetic Analysis of Crystallization Processes
Authors: Málek Jiří , Mitsuhashi Takefumi
Published: July 2010

The kinetics of crystallization processes in solids are usually described by the Johnson–Mehl–Avrami equation. One of the most popular methods to test the applicability of this equation is based on a double logarithmic plot of fraction crystallized (α). However, such plot is not very sensitive to small changes in α, and its reliability is rather limited. A simple and convenient testing method based on rate data (dα/dt) is proposed. The method can be applied to virtually any type of kinetic data obtained under isothermal or nonisothermal conditions.
Development of Titanium Coatings on Particulate Diamond
Authors: Breval Else , Cheng Jiping , Agrawal Dinesh K.
Published: July 2010

The performance of many wear‐resistant materials can be improved further by incorporating diamond into the structure. Diamond that has been coated with titanium is known to adhere well to the matrix particles and also prevents reaction with H2, which often is used in the sintering atmosphere. Two coating methods of coating diamond with titanium have been explored in this study: a sputtering method and a procedure that involves heating the diamond in a mixture of salt and titanium. The latter, which uses a salt mix of NaCl, KCl, and CaCl2 and metallic titanium particles 3 μm in size, heated at a temperature of 750°C in an argon atmosphere, is determined to be the best method.
Subcritical Crack Growth in Soda–Lime Glass under Mixed‐Mode Loading
Authors: Ritter John E. , Huseinovic Armin , Chakravarthy Srinath S. , Lardner Thomas J.
Published: July 2010

Subcritical crack growth in glasses and ceramics has been studied extensively under Mode I loading. In this study, subcritical crack growth in soda–lime glass under mixed‐mode loading has been determined, using the double cleavage drilled compression (DCDC) specimens with a hole offset from the center line. With this test configuration, cracks are nucleated at the pole of the hole and propagated parallel to the centerline of the specimen under mixed‐mode loading. Under mixed‐mode loading, subcritical‐crack‐growth rates are significantly less than those under Mode I loading at the same energy‐release rates. Possible mechanisms for this increased resistance to subcritical crack growth under mixed‐mode loading are discussed.
Refinement of Nanoscale Grain Structure in Bulk Titania via a Transformation‐Assisted Consolidation (TAC) Method
Authors: Liao Shih‐Chieh , Colaizzi James , Chen Yijia , Kear Bernard H. , Mayo William E.
Published: July 2010

Bulk nanocrystalline TiO2 samples (100% rutile) with a relative density as high as 97% and a grain size of <20 nm have been produced via high‐pressure (up to 8 GPa)/low‐temperature (∼0.3Tm, where Tm is the melting temperature) sintering, using a toroidal‐type high‐pressure apparatus. Nanophase TiO2 powder with a metastable anatase structure and an initial grain size of ∼38 nm was used as the starting material. During sintering, the anatase phase transformed to either the rutile or srilankite phase, depending on the pressure–temperature (P–T) combination. The starting temperature of the anatase‐to‐rutile phase transformation decreased from ∼550°C at ambient pressure to ∼150°C at 2.5 GPa. Grain growth was limited by the low sintering temperature and the multiple nucleation events in the parent phase. The grain size of the transformed rutile decreased as the sintering pressure increased, which can be explained by the combined effect of increasing the nucleation rate and decreasing the growth rate with high pressure. We have demonstrated that it is possible to produce a dense sintered compact with a grain size even smaller than that of the starting powder. The high‐pressure srilankite phase was observed at P–T conditions as low as 4.75 GPa and 250°C, respectively; however, unlike the anatase‐to‐rutile phase transformation, the rutile‐to‐srilankite phase‐transformation temperature increased as the pressure increased. Also, in contrast to the irreversible anatase‐to‐rutile phase transformation, the srilankite will reversibly transform to rutile under the appropriate circumstances. This observation provides an opportunity to further refine the TiO2 grain structure by switching the sintering conditions (temperature and pressure) between the regions in which the rutile or srilankite phase are stable.
Simulating Microstructural Evolution and Electrical Transport in Ceramic Gas Sensors
Authors: Wang Yunzhi , Liu Yuhui , Ciobanu Cristian , Patton Bruce R.
Published: July 2010

An integrated computational approach to microstructural evolution and electrical transport in ceramic gas sensors has been proposed. First, the particle‐flow model and the continuum‐phase‐field method are used to describe the microstructural development during the sintering of a prototype two‐dimensional film. Then, the conductivity of the sintering samples is calculated concurrently as the microstructure evolves, using both resistor lattice models and effective medium theory for electrical transport in porous aggregates of lightly sintered particles. This approach, when combined with the modeling of resistivity at the grain–grain contacts as a function of neck geometry, ambient gas concentration and temperature, could facilitate the development and optimization of novel microstructures for advanced ceramic gas sensors.
Calcium‐ and Lanthanum‐Modified Lead Titanate (PCLT) Ceramic and PCLT/Vinylidene Fluoride‐Trifluoroethylene 0‐3 Nanocomposites
Authors: Zhang Q. Q. , Chan H. L. W. , Zhou Q. F. , Choy C. L.
Published: July 2010

Calcium‐ and lanthanum‐modified lead titanate (PCLT) powders with size in the nanometer range were prepared by a sol–gel process. The PCLT gel was annealed at 850°C to produce powder with an average particle diameter of 80 nm. A dense and fine‐grained PCLT ceramic, with grain size of ∼0.7 μm, was prepared by sintering the sol–gel‐derived powder at 1150°C. The piezoelectric and pyroelectric properties of the PCLT ceramic varied linearly with the degree of poling in the ceramic. PCLT/vinylidene fluoride‐trifluoroethylene (P(VDF‐TrFE)) 0‐3 nanocomposites with PCLT volume fractions of 0.1–0.5 were fabricated, using PCLT powders imbedded in a P(VDF‐TrFE) matrix. The ceramic data were used to model the piezoelectric and pyroelectric properties of the PCLT/P(VDF‐TrFE) composites, and good agreements were obtained.
Rapid Electroacoustic Method for Monitoring Dispersion: Zeta Potential Titration of Alumina with Ammonium Poly(methacrylate)
Authors: Beattie James K. , Djerdjev Alex
Published: July 2010

The zeta potential (ζ) of 2‐vol% and 10‐vol% suspensions of a commercial α‐alumina has been measured using the electroacoustic technique and found to be in good agreement with values reported previously from microelectrophoresis measurements. An increasingly negative ζ value was observed through semiautomatic addition of solutions of a ammonium salt of poly(methacrylic acid). The point of complete surface coverage was inferred from simultaneous conductivity measurements. The average particle size obtained from the electroacoustic dynamic mobility spectrum showed the effect of coagulation near the isoelectric point. The expected correlations among ζ, the viscosity, and the sedimentation volume were observed. The results illustrate the efficacy of the electroacoustic technique for examining colloidal dispersions.
Curing of Highly Loaded Ceramic Suspensions in Acrylates
Authors: Chu Tien‐Min Gabriel , Halloran John W.
Published: July 2010

The objective of this study is to use differential scanning calorimetry (DSC) to characterize the curing kinetics of Al2O3 and hydroxyapatite suspensions in acrylates. The heat evolution of the acrylate premix and ceramic suspensions in dynamic and isothermal curing was measured. The calculated activation energies for the acrylate premix and the two ceramic suspensions were 133, 74, and 78 kJ/mol, indicating a catalytic effect of the ceramic fillers. The curing behavior of the two ceramic suspensions modeled with the autocatalytic polymerization model with satisfactory results.
Sol–Gel Synthesis and Characterization of Ag and Au Nanoparticles in SiO2, TiO2, and ZrO2 Thin Films
Authors: Epifani Mauro , Giannini Cinzia , Tapfer Leander , Vasanelli Lorenzo
Published: July 2010

Silver and gold nanoparticles were synthesized by the sol–gel process in SiO2, TiO2, and ZrO2 thin films. A versatile method, based on the use of coordination chemistry, is presented for stabilizing Ag+ and Au3+ ions in sol–gel systems. Various ligands of the metal ions were tested, and for each system it was possible to find a suitable ligand capable of stabilizing the metal ions and preventing gold precipitation onto the film surface. Thin films were prepared by spin‐coating onto glass or fused silica substrates and then heat‐treated at various temperatures in air or H2 atmosphere for nucleating the metal nanoparticles. The Ag particle size was about 10 nm after heating the SiO2 film at 600°C and the TiO2 and ZrO2 films at 500°C. After heat treatment at 500°C, the Au particle size was 13 and 17 nm in the TiO2 and ZrO2 films, respectively. The films were characterized by UV–vis optical absorption spectroscopy and X‐ray diffraction, for studying the nucleation and the growth of the metal nanoparticles. The results are discussed with regard to the embedding matrix, the temperature, and the atmosphere of the heat treatment, and it is concluded that crystallization of TiO2 and ZrO2 films may hinder the growth of Ag and Au particles.
Dissolution and Deagglomeration of Silicon Nitride in Aqueous Medium
Authors: Laarz Eric , Zhmud Boris V. , Bergström Lennart
Published: July 2010

Silicon nitride undergoes hydrolysis and dissolution when subjected to an aqueous environment. Molecular dynamics simulations suggest that hydrolysis proceeds through nucleophilic attack of water with the formation of an intermediate molecular complex involving a pentacoordinated silicon. We found that the dissolution of an oxidized silicon nitride powder resembles that of silica; the dissolution rate could be described using a simple kinetic equation with a dissolution activation energy of 52 kJ·mol−1. The deagglomeration of a fine silicon nitride powder under mild agitation was evaluated; we show that the peptization kinetics at room temperature is dominated by the breakup of particle–particle bonds due to hydrodynamic friction and cluster attrition. For breakup of hard agglomerates of small particles the dissolution of interparticle necks will play an important role.
Solid Freeform Fabrication of Aqueous Alumina–Poly(vinyl alcohol) Gelcasting Suspensions
Authors: Morissette Sherry L. , Lewis Jennifer A. , Cesarano Joseph , Dimos Duane B. , Baer Tom
Published: July 2010

Solid freeform fabrication of aqueous alumina–poly(vinyl alcohol) (Al2O3–PVA) gel‐casting suspensions was conducted using a computer‐controlled extrusion apparatus fitted with a two‐nozzle delivery system. The impact of casting parameters on the shear rate profiles experienced during deposition was evaluated via conventional flow analysis and computer simulations. In addition, the influence of these parameters on line resolution/uniformity, printability, and as‐cast component properties was studied using laser profilometry, optical microscopy, and scanning electron microscopy. Continuous printablity was achieved for tip diameters ranging from 0.254 to 1.370 mm for all mixing rates and suspension compositions studied. Printed lines were uniform with good edge definition, and line dimensions were independent of mixing rate for these process conditions. The Al2O3 volume fraction (φAl2O3) in the as‐deposited layers depended on casting conditions and cross‐linking agent concentration, where (φAl2O3 increased with decreased tip diameter and increased cross‐linking agent concentration. The free‐formed Al2O3 components exhibited uniform particle packing, with minimal macrodefects (e.g., slumping or staircasing) and no discernable microdefects (e.g., bubbles or cracking).