bluuebunny/crossref_metadata_2025 huggingface dataset:
Sliding Wear of Alumina/Silicon Carbide Nanocomposites
Authors: Rodríguez Jesús , Martín Antonia , Pastor Jose Ygnacio , LLorca Javier , Bartolomé José F. , Moya José S.
Published: July 2010

The wear resistance of four Al2O3/SiC nanocomposites that contained SiC particles of varying average size (40, 200, and 800 nm) was studied under dry sliding conditions and compared with the results obtained in unreinforced alumina. The wear rate of the alumina and the nanocomposites of equivalent grain size increased as the contact load increased; however, the nanocomposite wear resistance at high contact loads was better than that of the alumina by a factor of 3–5. The wear resistance of the nanocomposites of submicrometer grain size was fairly independent of the contact load, and their wear resistance at high contact loads was up to two orders of magnitude better than that of the alumina. The mechanisms responsible for these behaviors were discussed in terms of the microscopic wear mechanisms that were observed on the worn surfaces.
Microwave‐ versus Conventional‐Hydrothermal Synthesis of Hydroxyapatite Crystals from Gypsum
Authors: Katsuki Hiroaki , Furuta Sachiko , Komarneni Sridhar
Published: July 2010

Hydroxyapatite (HAp) crystals were synthesized from gypsum powder with diammonium hydrogen phosphate solution via microwave‐hydrothermal treatment at 100°C, and their properties were studied. Gypsum powder could be completely converted to fine HAp crystals 30‐300 nm long with 0.5M diammonium hydrogen phosphate at 100°C in 5 min. Compared to the formation of HAp crystals via conventional‐hydrothermal treatment, microwave‐hydrothermal treatment led to increased rate of formation (by two orders of magnitude).
High‐Temperature Flow Behavior of SiC‐Glass Composites in Compression and Tension
Authors: Banuprakash G. , Das Santanu , Murthy Varanasi S. R. , Murty Gollapudi S.
Published: July 2010

The high‐temperature flow behavior of different borosilicate composites was investigated in compression and tension. The flow behavior of composites changed from Newtonian to non‐Newtonian as the volume fraction and aspect ratio of reinforcements increased. Generally, platelet/particulate composites exhibited higher tensile elongations compared with short fiber/whisker composites. The tensile ductility increased with decreased volume fraction of reinforcements and temperature. Cavitation in these samples varied across the length, and maximum cavitation occurred near the fracture zone.
Sandwiched‐Beam Procedure for Precracking Brittle Materials
Authors: Sglavo Vincenzo M. , Bosetti Paolo , Trentini Elena , Ceschini Michele
Published: July 2010

A technique for precracking brittle materials is presented. This procedure, which is called the sandwiched‐beam (SB) technique, allows the production of sharp through‐thickness cracks with predetermined length in specimens with a rectangular section. A bar, in which an initial notch is produced by using a conventional saw, is inserted between two supporting beams and the sandwich assembly is loaded in three‐point bending. Conditions can be defined that allow the stable propagation of a sharp flaw from the notch as the applied load is increased. Then, the cracked bar can be used to determine the fracture toughness. The SB technique is applied to different brittle materials, including soda‐lime‐silica glass, alumina, Si3N4, a SiCw‐Si3N4 composite, graphite, a Ti‐Al intermetallic, and Carrara marble.
High‐Temperature Strength and Creep Behavior of Melt‐Infiltrated SiC‐Mo≤5Si3C≤1 Composites
Authors: Zhu Qingshan , Shobu Kazuhisa , Tani Eiji , Kishi Kazushi , Umebayashi Seiki
Published: July 2010

Molybdenum carbosilicide composites (SiC‐Mo≤5Si3C≤1) were fabricated via the melt‐infiltration process. The fracture behavior of the composites was studied from room temperature up to 1800°C in 1 atm (∼105 Pa) of argon. The bend strength of the composites slightly increased at ∼1200°C, because of the brittle‐ductile transition of the intermetallic phase. The composites retained ∼90% of their room‐temperature strength, even at 1700°C. Compressive creep tests were performed over a temperature range of 1760°‐1850°C and a stress range of 200–250 MPa. The creep rate of the SiC‐Mo≤5Si3C≤1 composites was approximately an order of magnitude higher than that of reaction‐bonded SiC.
Combustion Synthesis of Aluminum Nitride Particles and Whiskers
Authors: Bradshaw Steven M. , Spicer John L.
Published: July 2010

Aluminum nitride (AlN) was produced via the combustion synthesis of loosely packed aluminum powder (pore fraction of ∼0.8) in a graphite reactor that was lined with permeable carbon felt. Almost‐complete conversion was achieved with a forced flow of nitrogen for beds 50‐150 mm deep (mass of 200‐650 g). The product was in the form of a loosely aggregated bed, with regions of distinct morphology, and had a predominantly whisker morphology. Some control over the microstructure was possible by changing the processing parameters. The addition of 5% of ammonia to the nitrogen resulted in a uniform production of whiskers, whereas a 50% solid‐phase dilution with AlN favored the production of particles.
On‐Line Monitoring of Ceramic Paste Extrusion
Authors: Amarasinghe Shantha , Wilson Ian
Published: July 2010

Fluctuations in pressure signals that are measured at a die during the extrusion of α‐alumina‐based ceramic pastes have been shown to contain information about the homogeneity of the paste that is fed into the extruder and phenomena that occur during the extrusion process. Standard error and fractal‐analysis techniques give quantitative measures that have shown good correlation with agglomerate breakdown and variation in solid‐liquid mixing. Gross extrudate surface fracture could be detected via frequency analysis. The potential use of these techniques for the feedback control of extruders is discussed.
Nanocrystallization and Phase Transformation in Monodispersed Ultrafine Zirconia Particles from Various Homogeneous Precipitation Methods
Authors: Hu Michael Z.‐C. , Hunt Rodney D. , Payzant E. Andrew , Hubbard Camden R.
Published: July 2010

Monodispersed ultrafine (nano‐ to micrometer) zirconia precursor powders were synthesized by three different physicochemical methods: (I) forced hydrolysis, (II) homogeneous precipitation in inorganic salt solutions, and (III) hydrolysis/condensation of alkoxide. The forced hydrolysis method produced monoclinic nanocrystalline particles (cube shaped) of nanometer scale, which depended largely on the initial salt concentration. Methods II and III, both involving the use of alcohol as a solvent, exhibited a faster particle formation rate and generated amorphous ultrafine (submicrometer) monodispersed microspheres, indicating that the presence of alcohol may have stimulated particle nucleation due to its low dielectric property (and, thus, the low solubility of nucleus species in mixed water‐alcohol solutions). Nucleation and growth of the particles in solutions are discussed based on the measurements obtained by small‐angle X‐ray scattering (SAXS) and dynamic light scattering (DLS). High‐temperature X‐ray diffraction (HTXRD) and TGA/DTA studies elucidated the differences in phase transformation for different types of powders. The most interesting finding was the nonconventional monoclinic nanocrystal nucleation and growth that occurred prior to transformation to the tetragonal phase (at 1200°C) during the heat treatment of the nanocrystalline powders produced by the forced hydrolysis.
Synthesis of Si–C–O Bulk Ceramics with Various Chemical Compositions from Polycarbosilane
Authors: Kakimoto Ken‐ichi , Wakai Fumihiro , Bill Joachim , Aldinger Fritz
Published: July 2010

Si‐C‐O bulk ceramics with various atomic ratios were produced by pyrolysis and high‐temperature treatment (HTT) of a polycarbosilane (PCS) precursor with a given amount of oxygen. The oxygen introduction within the PCS molecular structure makes the compact infusible during the pyrolysis step, and supports sintering. The derived chemical compositions are mapped on a Si‐C‐O constitution diagram, with their transition behaviors due to HTT described. Upon heating between 1500° and 1700°C, the chemical compositions of the as‐pyrolyzed specimens were as follows: PCS‐6 (SiC1.36O0.26), PCS‐8 (SiC1.25O0.45), and PCS‐10 (SiC1.02O0.68), moving toward C/Si = 1.2, 1.0, and 0.8 with a reduction of the oxygen content, respectively. It is also shown that there are influences of the initial oxygen content on the resulting microstructure of both as‐pyrolyzed and heat‐treated specimens.
Development of Strongly Adherent Triboluminescent Zinc Sulfide Films on Glass Substrates by Ion Plating and Annealing
Authors: Xu Chao‐Nan , Watanabe Tadahiko , Akiyama Morio , Zheng Xu‐Guang
Published: July 2010

Crystalline ZnS films were prepared on glass substrates from a high‐crystallinity ZnS pellet using an ion‐plating method. The crystallinity and friction‐induced photon‐emitting capability (triboluminescence) of the films were greatly enhanced by postannealing under vacuum. The annealing treatment made the ZnS films strongly adherent to the glass substrate and enhanced their endurance to friction. This greatly enhanced ZnS as a stress/friction sensor for various applications.
Aggregation Effects on the Compressive Flow Properties and Drying Behavior of Colloidal Silica Suspensions
Authors: Guo J. Jiyou , Lewis Jennifer A.
Published: July 2010

The influence of aggregation phenomena on the compressive flow properties and drying behavior of nonaqueous and aqueous silica (SiO2) suspensions of varying electrolyte (NH4Cl) concentrations were studied. Compressive rheology measurements, including sedimentation and centrifugal consolidation, were first conducted to investigate consolidation behavior in the absence of solvent evaporation. The volume‐fraction‐dependent osmotic pressure and compressive yield stress were determined for dispersed and flocculated SiO2 suspensions, respectively. Consolidation behavior then was studied in situ by simultaneously measuring stress evolution and solvent loss as a function of drying time. The observed drying stress histories of the films were complex, consisting of several characteristic regions. First, there was an initial period of stress rise to a maximum drying stress. These measured stress values exhibited good agreement with the osmotic pressure and compressive yield stress at equivalent SiO2 volume fractions for the dispersed and flocculated systems, respectively. Beyond the maximum drying stress there was a subsequent region of stress decay, which coincided with the draining of liquid‐filled pores. No residual drying stress was detected for films prepared from salt‐free SiO2 suspensions, whereas salt‐containing films exhibited residual drying stresses likely due to salt‐bridging effects. Microstructural characterization of dried films prepared from aqueous SiO2 suspensions revealed nonuniformities in the spatial distribution of colloidal particles and precipitated salt, with the highest concentrations located at the outer edges of the films. Such features result from capillary‐induced transport of these species during drying, and they have important implications on colloidal processing of ceramic thick films and bulk forms.
Control of Microstructure and Orientation in Solution‐Deposited BaTiO3 and SrTiO3 Thin Films
Authors: Schwartz Robert W. , Clem Paul G. , Voigt James A. , Byhoff Elena R. , Van Stry Melanie , Headley Thomas J. , Missert Nancy A.
Published: July 2010

Columnar and highly oriented (100) BaTiO3 and SrTiO3 thin films were prepared by a chelate‐type chemical solution deposition (CSD) process by manipulation of film deposition conditions and seeded growth techniques. Randomly oriented columnar films were prepared on platinum‐coated Si substrates by a multilayering process in which nucleation of the perovskite phase was restricted to the substrate or underlying layers by control of layer thickness. The columnar films displayed improvements in dielectric constant and dielectric loss compared to the fine‐grain equiaxed films that typically result from CSD methods. Highly oriented BaTiO3 and SrTiO3 thin films were fabricated on LaAlO3 by a seeded growth process that appeared to follow a standard “two‐step” growth mechanism that has been previously reported. The film transformation process involved the bulk nucleation of BaTiO3 throughout the film, followed by the consumption of this matrix by an epitaxial overgrowth process originating at the seed layer. Both BaTiO3 and PbTiO3 seed layers were effective in promoting the growth of highly oriented (100) BaTiO3 films. Based on the various processing factors that can influence thin film microstructure, the decomposition pathway involving the formation of BaCO3 and TiO2 appeared to dictate thin film microstructural evolution.
Electronic Structure of Pristine and Solute‐Incorporated SrTiO3: III, Perfect‐Crystal, Grain‐Boundary Geometry, and Acceptor Doping
Authors: Rodrigues Richard P. , Ellis Donald E. , Dravid Vinayak P.
Published: July 2010

Electronic structure is investigated for donor‐impurity‐incorporated perfect‐crystal and 36.8° symmetric tilt Sigma5 (310) grain‐boundary geometries of SrTiO3. The relaxed model of the atomic structure of the grain boundary used in the present investigations is the same as that used in Part II, as derived by Ravikumar et al. using lattice‐statics simulations based on pair‐potential calculations. As in Part II, the methodology of one‐electron first‐principle cluster calculations, which is discussed in Part I, is extended to clusters with single‐donor impurity substitutions at the central titanium and strontium sites. The effects of niobium substitution at a titanium site and lanthanum substitution at a strontium site in the bulk and at the grain‐boundary core have been investigated by determining the aspects of the electronic structure discussed for the acceptors. The influence of grain‐boundary crystallography on donor impurity incorporation has been evaluated in terms of variations in densities of states, spatial charge densities, and charge populations at the grain boundary. As in Parts I and II, no additional local lattice relaxations around the impurity are considered for the impurity‐incorporated clusters. Donor compensation mechanisms reported in the literature are discussed in connection with the electrical activity of the material. The calculations reveal that, in perfect‐crystal geometry, although lanthanum exhibits almost the expected donor behavior at the strontium site, niobium does not exhibit very good donor behavior at the titanium site. Moreover, a decrease in the donor behavior of these impurities is observed in the grain‐boundary geometry. Such variations in the electronic behavior of donors are due to the crystallographic variations at the grain boundary and are likely to decrease the grain‐boundary‐charge and associated space‐charge effects in the presence of donors as compared with the increase of such effects in the presence of acceptors. Consequently, a decrease in space‐charge‐induced segregation of donors at grain boundaries and the effects thereof can be expected.
Factors Affecting the Electrical Conductivity of Donor‐Doped Bi4Ti3O12 Piezoelectric Ceramics
Authors: Villegas Marina , Caballero Amador C. , Moure Carlos , Durán Pedro , Fernández José F.
Published: July 2010

High‐temperature piezoelectric ceramics based on W6+‐doped Bi4Ti3O12 (W‐BIT) were prepared by both the conventional mixing oxides and the chemical coprecipitation methods. Sintering was carried out between 800° and 1150°C in air. A rapid densification, >99% of the theoretical density (rhoth) at 900°C/2 h, took place in the chemically prepared W6+‐doped Bi4Ti3O12 ceramics, whereas conventionally prepared BIT‐based materials achieved a lower maximum density, ∼94% of rhoth, at higher temperature (1050°C). The microstructure study revealed a platelike morphology in both materials. Platelike grains were larger in the conventionally prepared W‐BIT‐based materials. The sintering behavior could be related both to the agglomeration state of the calcined powders and to the enlargement of the platelets at high temperature. The W6+‐doped BIT materials showed an electrical conductivity value 2‐3 orders of magnitude lower than undoped samples. The electrical conductivity increased exponentially with the aspect ratio of the platelike grains. The addition of excess TiO2 produced a further decrease of the electrical conductivity.
Segregation in Yttrium Aluminum Garnet: II, Theoretical Calculations
Authors: Brown Kelly R. , Bonnell Dawn A.
Published: July 2010

Calculations based on ionic space‐charge models of doped yttrium aluminum garnet (YAG) have been compared to experimental measurements of surface segregation in crystals of various compositions. The comparison allows limits for vacancy‐formation energies to be set. The range for anion:cation formation‐energy ratios has been established to be 0.20‐0.23, based on the reasonable assumptions that the formation energy of the yttrium ion is 75% of that of the aluminum ion and the Schottky defect formation energy is 4.2 eV. The model explains the experimental observation of calcium at the surface regardless of net acceptor excess or net donor excess. The relationship between vacancy‐formation energy and dopant excess has been used to construct segregation maps for YAG, which are useful in materials design strategies.
Design of Ceramic Materials for Chemical Sensors: SmFeO3 Thick Films Sensitive to NO2
Authors: Traversa Enrico , Villanti Stefano , Gusmano Gualtiero , Aono Hiromichi , Sadaoka Yoshihiko
Published: July 2010

Ultrafine SmFeO3 powders were prepared by the thermal decomposition at 700°C of the corresponding hexacyanocomplex, Sm[Fe(CN)6]·4H2O. These powders were used for the preparation of pastes which were deposited as thick films on alumina substrates with comb‐type Au electrodes. The films were fired at different temperatures in the 800‐1000°C range. The content of α‐terpineol, a component of the organic vehicle, was varied in the range 0.0046‐4 wt%. The microstructure, the chemical composition at the surface, the electrical conductivity, and the NO2 sensing properties of the films were investigated. The content of α‐terpineol strongly influenced the electrical conductivity and its activation energy. A significant reduction in the NO2 response was observed for the films containing smaller amounts of α‐terpineol, together with an increase in conductivity. On the other hand, the largest NO2 response was observed for the films fired at 1000°C when 4 wt% of α‐terpineol was used. Such increase in conductivity is attributed to a different oxygen surface layer on the SmFeO3 surface, which is induced by the decomposition reaction of α‐terpineol during sintering. The materials processing parameters are thus of primary concern for the NO2 sensing properties of the SmFeO3 thick films. The correlations found between activation energy, NO2 sensitivity, and materials characteristics (influenced by the preparation parameters) are reported. These correlations can be used to design the gas‐sensing properties of SmFeO3 thick films for the optimization of their sensing characteristics.
Defect and Electronic Structures of Calcium‐Doped Lanthanum Cuprate
Authors: Okasinski John , Cohen Jerome B. , Hwang Jinha , Mason Thomas O. , Ding Zenong , Warschkow Oliver , Ellis Donald E.
Published: July 2010

Quenched calcium‐doped lanthanum cuprate (La2CuO4) samples were examined at room temperature using extended X‐ray absorption fine structure (EXAFS) analysis and X‐ray absorption near‐edge spectroscopy (XANES) near the CaK edge, and via atomic‐ and electronic‐structure modeling. Calculations and experiments show that the Ca atom, in a manner similar to that of Sr and Ba atoms, replaces the La atom and maintains its nine‐fold coordination above the center of a CuO4 square, which is the nominal position of the La atom. In contrast to the Sr and Ba atoms, the distances from Ca to its nearest neighbors are similar to the size of the La3+ cation. The distances that are obtained from the simulation are in reasonable agreement with the experimental results for calcium, as well as previous studies that involved strontium and barium. The binding of a calcium dopant to vacancies on the O(1) sites is stable, but only slightly; therefore, there are probably many different types of such clusters. This calculated binding also is supported by the EXAFS data. The overall electronic structure is similar to that observed in calculations for pure La2CuO4: the LaO planes remain ionic, even with the presence of the calcium dopant, and the Cu‐O(1) bond is covalent. The calcium is ionic in character, with a net charge of 1.6, and the narrow Ca 3d band lies ∼6 eV above the Fermi energy level.
Investigation of Toughening and Resistance‐Curve Behavior in Hybrid Molybdenum Disilicide
Authors: Ye Fan , Li Mingwei , Soboyejo Winston O.
Published: July 2010

This paper presents the results of a study of the toughening and resistance‐curve behavior in a hybrid molybdenum disilicide (MoSi2) composite that has been reinforced with 2 mol% yttria partially stabilized zirconia particles (TZ‐2Y) and niobium layers. Toughening and resistance‐curve behavior occur as a result of the combined effects of crack bridging and transformation toughening. Crack‐tip shielding that is due to large‐scale crack bridging and transformation toughening also is modeled using micromechanics concepts. The models show that the overall crack‐tip shielding from transformation toughening is very limited. However, large‐scale bridging by the ductile niobium layers promotes significant levels of toughening and resistance‐curve behavior.
Hi‐Nicalon/SiC Minicomposites with (Pyrocarbon/SiC)n Nanoscale Multilayered Interphases
Authors: Bertrand Sébastien , Forio Philippe , Pailler René , Lamon Jacques
Published: July 2010

SiC/SiC minicomposites that comprise different pyrocarbon/silicon carbide ((PyC/SiC)n) multilayered interphases and a tow of SiC fibers (Hi‐Nicalon) have been prepared via pressure‐pulsed chemical vapor infiltration. Pyrocarbon and SiC were deposited from propane and a CH3SiCl3/H2 mixture, respectively. The microstructure of the interphases has been investigated using transmission electron microscopy. The mechanical tensile behavior of the minicomposites at room temperature exhibits the classical features of tough composites, regardless of the characteristics of the (PyC/SiC) sequences. The interfacial shear stress has been determined from the width of hysteresis loops upon unloading/reloading and from the crack‐spacing distance at saturation. All the experimental data indicate that the strength of the fiber/interphase interfaces is rather weak (∼50 MPa).
Application of the Weight Function Method to Study theR‐Curve Behavior of Ceramics Using Chevron‐Notched Specimens
Authors: Sarrafi‐Nour G. Reza , Coyle Thomas W.
Published: July 2010

Chevron‐notched specimens are widely used to characterize fracture toughness and the fracture resistance behavior of ceramic materials. Quantitative analysis ofR‐curves resulting from crack wake bridging obtained by growing a crack in this notch geometry requires an accurate description of the contribution of the bridging tractions to the total stress intensity factor. Three approaches to describe the stress intensity factor due to crack wake tractions in chevron‐notched specimens for the case of the commonly employed bend bar geometry are compared to anR‐curve calculated using finite element methods for a zone of uniform bridging stress. The results obtained using a recently available fracture mechanics weight function for crack surface tractions in this geometry are shown to be the most accurate. An iterative procedure based on the weight function method which yields the distribution of bridging stresses as a function of crack opening displacement from measuredR‐curves is described. To illustrate the procedure, experimentalR‐curve results from an Al2O3‐20 vol% SiC platelet composite are analyzed to obtain the bridging tractions as a function of the crack opening displacement. For the specific curve analyzed here, a maximum bridging stress of 26 MPa was found, with a baseline toughness of 4.7 MPa·m1/2.
Mechanical‐ and Physical‐Property Changes of Neutron‐Irradiated Chemical‐Vapor‐Deposited Silicon Carbide
Authors: Osborne Matthew C. , Hay Jack C. , Snead Lance L. , Steiner Don
Published: July 2010

Indentation and density measurements have revealed important changes in the mechanical and physical properties of silicon carbide (SiC) due to neutron irradiation. Specifically, the changes in the elastic modulus, hardness, fracture toughness, and density with irradiation have provided an understanding of the expected performance of SiC and SiC composites in nuclear applications. After the accumulated damage has saturated, these mechanical properties were affected primarily by the irradiation temperature. Chemical‐vapor‐deposited (CVD) SiC was irradiated above the saturation fluence and yielded volumetric swelling of 2.6% and 1.3% for irradiation temperatures of 100°‐150°C and 500°‐550°C, respectively. At the same respective temperatures, the elastic modulus decreased from an unirradiated value of 503 GPa to ∼420 and 450 GPa. Conversely, the hardness increased from 36 GPa for the unirradiated CVD SiC to 38 and 40 GPa for the samples irradiated at 100°‐150°C and 500°‐550°C, respectively. Interestingly, these two independent properties approached almost‐constant levels after exposure to a fluence of 0.5 × 1025 n/m2, E > 0.1 MeV. Indentation fracture toughness measurements, which were within the range of values in the literature for conventional fracture toughness procedures for SiC, increased from ∼2.8 MPa·m1/2 for the unirradiated samples to 3.7 and 4.2 MPa·m1/2 for the samples that were irradiated at 100°‐150°C and 500°‐550°C, respectively.
High‐Pressure Synthesis of Heat‐Resistant Diamond Composite Using a Diamond‐TiC0.6 Powder Mixture
Authors: Hong Shi‐Ming , Akaishi Minoru , Yamaoka Shinobu
Published: July 2010

The sintering behavior of synthetic diamond with a grain size of 2‐4 µm was investigated at high pressure and high temperatures (6.5 GPa and 1600°‐1900°C, respectively) in the presence of TiC0.6 or TiC. No well‐sintered diamond composite was synthesized from a diamond‐TiC powder mixture under the examined conditions; however, a well‐sintered diamond composite with a homogeneous microstructure was synthesized under the conditions of 6.5 GPa and temperatures >1800°C using a diamond‐TiC0.6 powder mixture. The diamond composite was hard (Vickers hardness of 45 GPa) and was composed of diamond and TiC, in which TiC was formed via the reaction of TiC0.6 and carbon atoms of diamond. Neither graphitization nor cracking was observed in or on the composite after successive heat treatment at 900°‐1400°C for 30 min each under vacuum. The Vickers hardness of the composite that was treated at 1500°C decreased to 40 GPa, whereas the Vickers hardness values for composites treated at temperatures of <1400°C were not changed. The present diamond composite is believed to be one of the most heat‐resistant materials among the superhard composites and can be used as heat‐resistant tools for superhard materials.
Thermodynamic Approach to the Oxidation of Hi‐Nicalon Fiber
Authors: Vahlas Constantin , Laanani Fadela , Monthioux Marc
Published: July 2010

A thermodynamic investigation of the oxidation of Hi‐Nicalon fiber in air has been performed, based on the minimization of the Gibbs energy of the silicon‐carbon‐oxygen chemical system. The results illustrate the expected stable phases, as a function of temperature and the molar ratio of air over the fiber, which permits simulation of the diffusion‐controlled radial oxidation of the fiber. In agreement with literature information, the model has shown that (i) during oxidation, the external portion of the fiber is transformed to silica while the core remains unaffected, (ii) in certain conditions, a free carbon interphase is produced between the external portion and the core of the fiber, and (iii) low oxygen partial pressure may lead to consumption of the fiber, which illustrates an active oxidation.
Melting Properties of Bi2O3‐ZnO‐Nb2O5‐Based Dielectric Ceramics
Authors: Wei Jianzhong , Zhang Liangying , Yao Xi
Published: July 2010

Cubic‐pyrochlore‐structure α‐phase and low‐symmetry‐structure ß‐phase are the basic phases of dielectric ceramics that are based on Bi2O3‐ZnO‐Nb2O5 (BZN). In this paper, the melting behaviors of the α‐phase and the ß‐phase have been explored using differential thermal analysis and X‐ray diffractometry. The results show that BZN‐based α‐phase is a congruent compound; its congruent melting point is 1190°C. BZN‐based ß‐phase is an incongruent compound that dissociates to form α‐phase and a liquid phase, with an absorption of heat, at its incongruent melting point (peritectic temperature) of 1100°C. Upon cooling, the α‐phase reacts with the liquid phase to form the ß‐phase via peritectic reaction. Upon quenching, the BZN‐based cubic‐fluorite phase precipitates from the liquid phase.
Preparation and Characterization of Fine‐Grained 3Y‐TZP/BaFe12O19 In Situ Composites
Authors: Suzuki Yoshikazu , Awano Masanobu , Kondo Naoki , Ohji Tatsuki
Published: July 2010

Composites of BaFe12O19 particles dispersed throughout a 3‐mol%‐yttria‐doped zirconia (3Y‐TZP) matrix have been prepared using the pressureless reactive sintering of 3Y‐TZP, BaCO3, and γ‐Fe2O3 powders. The reaction behavior of the mixed powder was studied with an in situ, high‐temperature powder X‐ray diffraction technique. The composite that was sintered at 1300°C consisted of submicrometer‐sized 3Y‐TZP grains and BaFe12O19 particles; the size of the 3Y‐TZP grains was ∼100‐300 nm, and the size of the BaFe12O19 particles was ∼200‐500 nm. Based on the grain size, most of the BaFe12O19 grains presumably had a single‐magnetic‐domain structure. The 3Y‐TZP/20‐wt%‐BaFe12O19 composite showed high magnetization and coercivity values, despite the low concentration of ferromagnetic phase. Preliminary mechanical tests revealed that the composite possessed moderately good mechanical properties.
Processing and Properties of Nanograin Silicon Carbide
Authors: Vassen Robert , Stöver Detlev
Published: July 2010

The manufacture of fine‐grained SiC ceramics by using nanophase SiC powders with particle sizes of about 20 nm is described. Conventional sintering of these powders led to extreme grain growth and hence the nanophase microstructure was destroyed. Pressure‐assisted sintering was applied to reduce the sintering temperature and also grain growth. Hot isostatic pressing (HIPing) of samples with about 1 wt% carbon and 1 wt% boron addition at temperatures below 1700°C and with pressures up to 350 MPa resulted in densities of more than 95% of the theoretical density (TD) and grain sizes of 150 nm. A further reduction of grain size became possible by an optimized high‐temperature heat treatment of the samples prior to HIP. This procedure removed at least partially the oxygen layer on the surface of the nanophase SiC particles. Samples with densities of more than 97% TD and grain sizes below 80 nm were produced. Grain sizes were measured with scanning and transmission electron microscopes; both methods gave similar values. Grain sizes determined from the peak broadening of X‐ray diffraction peaks showed in some cases lower values. This was attributed to the relatively high amount of twinning boundaries and stacking faults produced during crystal growth. The influence of the grain size on different mechanical and thermal properties was investigated. Vickers hardness and indentation fracture toughness were measured for samples with different densities and grain sizes. The results revealed that besides grain size features like density and/or oxygen content can strongly influence hardness and fracture toughness. Similar results have been found for the wear resistance of fine‐grained materials. Results of pin‐on‐disk type experiments showed the importance of a high density of the samples for high wear resistance. Thermal diffusivity measurements were performed for samples with different grain sizes up to 1400°C. A large decrease with decreasing grain size was found at room temperature. At higher temperatures the difference in the thermal diffusivity of fine‐ and large‐grained materials was reduced.
Anisotropic Shrinkage in Tape‐Cast Alumina: Role of Processing Parameters and Particle Shape
Authors: Raj P. Markondeya , Cannon W. Roger
Published: July 2010

Aqueous suspensions of alumina and other powders containing no visible morphological anisotropy were tape cast under different conditions to study the role of processing parameters and powder characteristics in the origin of in‐plane sintering shrinkage anisotropy. High anisotropic shrinkage, with the typical shrinkage in the transverse direction exceeding that in the casting direction by more than 10%, was observed for tape‐cast alumina cast at high solids loading and high shear rates. Tape casting without any binder also resulted in similar anisotropy. Lower solids loading with increased agglomeration resulted in a drastic reduction of shrinkage anisotropy. Under similar casting conditions, BaTiO3 did not show any shrinkage anisotropy unlike alumina. Cubic zinc ferrite and titania powders also showed some consistent shrinkage anisotropy. A correlation was found between in‐plane particle orientation and anisotropic shrinkage proving that powder characteristics, particularly particle shape, control anisotropic shrinkage even in apparently equiaxed powders.
Mode I Fracture Toughness of a Small‐Grained Silicon Nitride: Orientation, Temperature, and Crack Length Effects
Authors: Trice Rodney W. , Halloran John W.
Published: July 2010

The Mode I fracture toughness (KIC) of a small‐grained Si3N4 was determined as a function of hot‐pressing orientation, temperature, testing atmosphere, and crack length using the single‐edge precracked beam method. The diameter of the Si3N4 grains was <0.4 µm, with aspect ratios of 2–8. KIC at 25°C was 6.6 ± 0.2 and 5.9 ± 0.1 MPa·m1/2 for the T–S and T–L orientations, respectively. This difference was attributed to the amount of elongated grains in the plane of crack growth. For both orientations, a continual decrease in KIC was observed through 1200°C, to ∼4.1 MPa·m1/2, before increasing rapidly to 7.5–8 MPa·m1/2 at 1300°C. The decrease in KIC through 1200°C was a result of grain‐boundary glassy phase softening. At 1300°C, reorientation of elongated grains in the direction of the applied load was suggested to explain the large increase in KIC. Crack healing was observed in specimens annealed in air. No R‐curve behavior was observed for crack lengths as short as 300 µm at either 25° or 1000°C.
Synthesis and Characterization of Suspension‐Derived, Porous Ion‐Conducting Ceramic Membranes
Authors: Kim Jinsoo , Lin Y. S.
Published: July 2010

Porous, oxygen‐ion‐conducting ceramic membranes can have applications as supports for fuel cells, sensors, and thin membrane films, or as filters for membrane filtration. This paper reports on the preparation of unsupported and supported yttria‐stabilized zirconia (YSZ) and yttria‐doped bismuth oxide (BY) membranes with submicrometer pore sizes. Fluorite‐structured BY powder that has been synthesized using the citrate method and commercial YSZ powder have been used to prepare stable aqueous suspensions. Unsupported and supported YSZ and BY membranes have been prepared from the stable suspensions of YSZ and BY. The supported BY membranes are crack free but contain small defects. Defect‐free YSZ membranes that are supported on porous alumina have been prepared under controlled conditions. The average pore size is 100 nm, with a porosity of 57%, for an unsupported YSZ membrane (measured by mercury porosimetry), and 114 nm for a supported membrane (as estimated via helium permeation). The ionic conductivity of the YSZ membranes is 0.00044–0.01 S/cm in the temperature range of 600°–900°C, which is lower than that of dense YSZ disks.
Surface Modification of Si3N4 Powders by Coprecipitation of Sintering Aids
Authors: Bertoni Fabrice , Galassi Carmen , Ardizzone Silvia , Bianchi Claudia Letizia
Published: July 2010

Two commercial Si3N4 powders were coated with sintering aids by coprecipitation. Lanthanum and yttrium nitrates were used as sintering aid precursors. Electrokinetic sonic amplitude measurements and X‐ray photoemission spectroscopy analysis were used to investigate electrokinetic behavior and surface properties, respectively. Coprecipitation produced different effects on the composition of the coating layer depending on the actual features of the starting Si3N4 powders. The electrokinetic behavior of aqueous suspensions with coated powders depended strongly on the additives, their solubility, and the rate of oxidation of the coated layer. The coprecipitation conditions had to be carefully controlled to obtain reproducible composition and morphology of the coating layers. Treatments of the starting powder, pH, and washing volumes were optimized to tailor the coating layer and improve the coprecipitation yield.
Carbothermic Synthesis of Monodispersed Spherical Si3N4/SiC Nanocomposite Powder
Authors: Choi Jae Young , Kim Chong Hee , Kim Do Kyung
Published: July 2010

The synthesis and structure of a monodispersed spherical Si3N4/SiC nanocomposite powder have been studied. The Si3N4/SiC nanocomposite powder was synthesized by heating under argon a spherical Si3N4/C powder. The spherical Si3N4/C powder was prepared by heating a spherical organosilica powder in a nitrogen atmosphere and was composed of a mixture of nanosized Si3N4 and free carbon particles. During the heat treatment at 1450°C, the Si3N4/C powder became a Si3N4/SiC composite powder and finally a SiC powder after 8 h, while retaining its spherical shape. The composition of the Si3N4/SiC composite powder changed with the duration of the heat treatment. The results of TEM, SEM, and selected area electron diffraction showed that the Si3N4/SiC composite powder was composed of homogeneously distributed nanosized Si3N4 and SiC particles.
Dielectric Properties of Fine‐Grained Barium Titanate Based X7R Materials
Authors: Wang Sea‐Fue , Dayton Gordon O.
Published: July 2010

The next generations of multilayer ceramic capacitors are expected to have active ceramic layer thicknesses of 3 μm or less. To retain a reliable ceramic device, it is advantageous to engineer a uniform and fine‐grained microstructure. In this study, a dense ceramic (>96% of theoretical) with fine‐grained size (<0.2 μm), and permittivity of 2800, meeting the EIA X7R temperature characteristic has been obtained using hydrothermally synthesized BaTiO3 powder with precipitation‐coated additives. The temperature coefficient of capacitance (TCC) and the dissipation factor (DF) are investigated as a function of active layer thickness and compared with that of a conventional coarse‐grained dielectric of similar composition and fired density. The fine‐grained ceramic maintained X7R characteristic in active layers as thin as 3 μm under 1 VAC and 1 kHz test conditions. For equivalent active thickness, the DF of the fine‐grained version was consistently lower than the coarse‐grained material. These results indicate that an improved voltage coefficient is possible for finer‐grained materials under otherwise equivalent conditions. The possible causes for this behavior are also discussed.
Fine‐Grained Silicon Carbide Ceramics with Oxynitride Glass
Authors: Kim Young‐Wook , Mitomo Mamoru
Published: July 2010

By using an oxynitride glass composition from the Y‐Mg‐Si‐Al‐O‐N system as a sintering additive, the effect of atmosphere on densification was investigated during the liquid‐phase sintering of SiC, and the resulting microstructure and mechanical properties of the sintered and subsequently annealed materials were investigated. SiC ceramics that were densified with 10 wt% oxynitride glass showed higher sinterability in a nitrogen atmosphere. Oxynitride glass enlarged the stability region of β‐SiC and suppressed β→ alpha phase transformation, which resulted in an equiaxed microstructure. Grain growth of fine‐grained SiC in some extent (up to ∼300 nm) was beneficial in improving both room‐temperature strength and toughness. The best results were obtained when the ceramics were hot‐pressed at 1800°C for 1 h in a nitrogen atmosphere and subsequently annealed at 1900°C for 3 h in an argon atmosphere. The room‐temperature flexural strength and fracture toughness of the material were 847 MPa and 3.5 MPa·m1/2, respectively.
Influence of Oxygen Partial Pressure and Oxygen Content on the Wettability in the Copper–Oxygen–Alumina System
Authors: Diemer Matthias , Neubrand Achim , Trumble Kevin P. , Rödel Jürgen
Published: July 2010

The influence of oxygen on the wetting behavior of copper on single‐crystal Al2O3 has been studied. By controlling the oxygen partial pressure (pO2) and oxygen content in the copper simultaneously, contact angle can be varied between 125° and 22°. Evaluation of the Gibbs adsorption equation for the liquid/solid interface at 1300°C suggests that adsorption of a Cu‐O complex at that interface plays a key role in promoting wetting. Formation of CuAlO2 and dissolution of Al2O3 in the melt also influence the contact angle, especially in the range of pO2 > 10−5 bar (1 Pa).
Processing and Mechanical Properties of Ti3SiC2: II, Effect of Grain Size and Deformation Temperature
Authors: El‐Raghy Tamer , Barsoum Michel W. , Zavaliangos Antonios , Kalidindi Surya R.
Published: July 2010

In this article, the second part of a two‐part study, we report on the mechanical behavior of Ti3SiC2. In particular, we have evaluated the mechanical response of fine‐grained (3–5 μm) Ti3SiC2 in simple compression and flexure tests, and we have compared the results with those of coarse‐grained (100–200 μm) Ti3SiC2. These tests have been conducted in the 25°–1300°C temperature range. At ambient temperature, the fine‐ and coarse‐grained microstructures exhibit excellent damage‐tolerant properties. In both cases, failure is brittle up to ∼1200°C. At 1300°C, both microstructures exhibit plastic deformation (>20%) in flexure and compression. The fine‐grained material exhibits higher strength compared with the coarse‐grained material at all temperatures. Although the coarse‐grained material is not susceptible to thermal shock (up to 1400°C), the fine‐grained material thermally shocks gradually between 750° and 1000°C. The results presented herein provide evidence for two important aspects of the mechanical behavior of Ti3SiC2: (i) inelastic deformation entails basal slip and damage formation in the form of voids, grain‐boundary cracks, kinking, and delamination of individual grains, and (ii) the initiation of damage does not result in catastrophic failure, because Ti3SiC2 can confine the spatial extent of the damage.
Scanning Transmission Electron Microscopy Analysis of Grain Boundaries in Creep‐Resistant Yttrium‐ and Lanthanum‐Doped Alumina Microstructures
Authors: Bruley John , Cho Junghyun , Chan Helen M. , Harmer Martin P. , Rickman Jeffrey M.
Published: July 2010

High‐spatial‐resolution analytical electron microscopy using energy‐dispersive X‐ray (EDX) and electron energy‐loss spectrometry (EELS) of yttrium‐ and lanthanum‐doped Al2O3 has been conducted to ascertain the level of segregation of these impurities to grain boundaries. Line profile analyses indicate that the segregation is confined to a layer thickness of <3 nm. Similar amounts of excess solute have been observed in both dopant systems: 4.4 ± 1.5 and 4.5 ± 0.9 at./nm2 for yttrium and lanthanum, respectively. Assuming all the segregant is uniformly distributed within ±0.5 nm of the boundary, this excess corresponds to 9 ± 3 at.% for yttrium‐doped Al2O3 and 10 ± 2 at.% for lanthanum‐doped Al2O3. For both dopant systems, examination of the spatially resolved electron energy‐loss near‐edge structures (ELNES) on the Al‐L2,3 edge suggests a loss in octahedral symmetry and a slight Al‐O bond‐length expansion. No significant change is noted in the O‐K edge.
Evidence of Tin Oxide Recrystallization in Opacified Lead Glazes
Authors: Molera Judit , Pradell Trinitat , Salvadó Nati , Vendrell‐Saz Marius
Published: July 2010

Tin oxide (SnO2) particles that are a few hundred nanometers in size and are embedded in a lead glaze act as a white opacifier. The insolubility of SnO2 particles in a lead glaze is responsible for this microstructure. Although it is assumed that SnO2 particles do not react during the glaze formation, we demonstrate that the development of metastable phases prior to the fusion of the PbO‐SiO2‐SnO2 mixture allows the SnO2 particles to react and then recrystallize subsequent to the fusion of the mixture. The recrystallization of SnO2 occurs during heating and/or cooling until the temperature is sufficiently low and/or the tin supply is exhausted.
Effect of Deposition Temperature on the Growth of Yttria‐Stabilized Zirconia Thin Films on Si(111) by Chemical Vapor Deposition
Authors: Hwang Sang‐Chul , Shin Hyung‐Shik
Published: July 2010

An experimental study was made on the effect of deposition temperature on the growth of yttria‐stabilized zirconia (YSZ) thin films in the chemical vapor deposition (CVD) process. The YSZ thin films were obtained in a temperature range of 650°–850°C, using β‐diketone chelates and a Si(111) substrate. Dense and mirrorlike YSZ films with uniform thickness were prepared; the deposition rate was 12–20 nm/min at those temperatures. An examination of the crystalline structure of the YSZ films was made, and the appropriate temperature for the growth of c‐axis‐oriented YSZ thin films using a Si(111) substrate was determined. The quality of the YSZ films was strongly dependent on the deposition temperature. As the temperature increased, the film growth mechanism changed from being controlled by surface reaction to being controlled by gas‐phase diffusion.
Synthesis of AlN Powder by Carbothermal Reduction‐Nitridation of Various Al2O3 Powders with CaF2
Authors: Ide Takayuki , Komeya Katsutoshi , Meguro Takeshi , Tatami Junichi
Published: July 2010

We investigated the effect of characteristics of raw Al2O3 powder on the synthesis of AlN by the carbothermal reduction‐nitridation method, in which CaF2 was added as a catalytic material. Four types of Al2O3 powders were selected. An Al2O3/C molar ratio of 0.29 was fixed, and the amount of CaF2 was varied from 3 to 30 wt%. The carbothermal reduction‐nitridation was conducted from 1350° to 1450°C in N2 flow. The nitridation rate tended to increase with decreasing particle size of raw Al2O3 and was found to depend on the Al2O3 synthesizing method. The particle sizes of the synthesized AlN increased somewhat with increasing reaction temperatures. However, even though different particle sizes of Al2O3 powders were used, AlN powders synthesized under the same conditions exhibited almost the same particle size, round shape, and narrow size distribution. From XRD analysis, CaO·6Al2O3 and CaO·2Al2O3 were identified as intermediate compounds during these reactions. The above phenomena suggest that the synthesis mechanism of AlN powder by carbothermal reduction‐nitridation of Al2O3 with CaF2 addition was the nitridation of the intermediate compounds through the liquid phase of the system CaF2‐CaO·6Al2O3‐CaO·2Al2O3.
Role of Ionic Depletion in Deposition during Electrophoretic Deposition
Authors: De Debnath , Nicholson Patrick S.
Published: July 2010

A model is developed for deposition during the electrophoretic deposition (EPD) process. It suggests that ions that move with the charged particles in suspension are depleted at the depositing electrode, locally changing the pH toward the isoelectric point (pHiep) to give coagulation. The variation of zeta (zeta) potential is modeled via chemical‐equilibrium and surface‐adsorption isotherms. The model successfully fits the experimental data for Al2O3 particles in ethanol when the Freundlich surface‐adsorption isotherm is assumed. Calculations predict the co‐ion concentration gradient as a function of location within the suspension, and the deposition time and its role in the coagulatoin process during EPD.