bluuebunny/crossref_metadata_2025 huggingface dataset:
Tin Oxide Thin‐Film Sensors for Aromatic Hydrocarbons Detection: Effect of Aging Time on Film Microstructure
Authors: Nicoletti Sergio , Dori Leonello , Corticelli Franco , Leoni Matteo , Scardi Paolo
Published: July 2010

SnO2thin films for gas‐sensor devices were deposited by pulsed laser ablation on  silicon wafers and on substrate heater elements with 200 nm thick Si3N4membranes as physical supports of the sensor stack. Microscopic analysis showed that the thin films were polycrystalline, with ∼40‐60 nm equiaxial grains, high porosity, and high specific surface area. The sensors were exposed to gas mixtures containing benzene, wet synthetic air, and CO, with 0.5 ppm of benzene added to 30% relative humidity synthetic air and 20 ppm CO. A 100% relative conductance variation, DeltaI/I0, of the sensing layer was observed. However, this DeltaI/I0value consistently decreased after prolonged testing (up to 2 weeks) in a gas‐sensor chamber. The sensing layer was studied using several analytical techniques to understand the aging time effects on sensor performance under working conditions. X‐ray diffractometry showed a marked decrease of the residual microstrain after prolonged heat treatments, suggesting a substantial reduction of lattice (point and line) defects. Furthermore, photothermal deflection spectroscopy highlighted a consistent decrease of the sensing layer absorption coefficient near 1 eV that could be related to a reduction of carrier concentration. Because structural defects in SnO2, such as oxygen vacancies or dislocations, are electrically active and variations in the carrier concentration are, at least in part, related with changes in the defect density, the annealing of these defects might have accounted for the observed worsening of sensor performance with the aging time.
Differential Thermal Analysis on the Crystallization Kinetics of K2O‐B2O3‐MgO‐Al2O3‐SiO2‐TiO2‐F Glass
Authors: Cheng Kangguo , Wan Julin , Liang Kaiming
Published: July 2010

The crystallization kinetics of a glass based on one type of mica, NaMg3AlSi3O10F2, with the addition of a nucleating agent, TiO2, has been studied using differential thermal analysis (DTA) under both isothermal and nonisothermal conditions. Two distinct crystallization exotherms in the DTA curve are observed and resolved that correspond to the initial formation of magnesium titanate (MgTi2O5) and the later formation of mica. The activation energy for precipitation of each crystalline phase has been evaluated, and the crystallization mechanism has been studied. The results indicate that the growth of mica is a two‐dimensional process, controlled by the crystal‐glass interface reaction. The average calculated values of activation energies are 256 ± 11 kJ/mol and 275 ± 6 kJ/mol for the precipitation of MgTi2O5 and mica from the glass matrix, respectively.
Notch‐Sensitivity of Fiber‐Reinforced Ceramic‐Matrix Composites: Effects of Inelastic Straining and Volume‐Dependent Strength
Authors: McNulty John C. , Zok Frank W. , Genin Guy M. , Evans Anthony G.
Published: July 2010

The effects of circular holes and sharp notches on the tensile strength of two Nicalon‐reinforced ceramic composites have been investigated. The influence of inelastic straining on the redistribution of stress has been elucidated through measurements of the local strains in the regions of high stress concentration, coupled with finite element simulations of the test geometries, using a nonlinear constitutive law appropriate to ceramic composites. The scale dependence of strength has been inferred from tests performed on specimens of varying size. The utility of two failure models that incorporate both the inelastic straining and the scale dependence has been assessed: one based on the point stress failure criterion and the other on weakest‐link fracture statistics. Both approaches provide a reasonably consistent description of the experimental measurements. Some of the implications and limitations associated with the failure models are discussed.
Experimental Evaluation of Toughening Mechanisms in Alumina‐Zirconia Composites
Authors: Shin Yu‐Seon , Rhee Young‐Woo , Kang Suk‐Joong L.
Published: July 2010

The contributions of transformation toughening and microcrack toughening in an 85Al2O315ZrO2 (vol%) composite were quantitatively evaluated. Three types of hot‐pressed samples with similar size (∼0.5 µm) and size distribution of ZrO2 grains, but with different contents (vol%) of monoclinic‐ (m‐) ZrO2 (0 (AZS), 45 (AZT), and 67 (AZM)) were prepared using Y2O3 and MoO2 dopants. Therefore, the possible effect of ZrO2 grain size on each toughening mechanism was eliminated. When the measured fracture toughnesses of m‐ZrO2 volume fractions before and after fracture were compared, the transformation‐toughening constant was estimated as 3.0 MPam1/2 and the microcrack‐toughening constant 0.2 MPam1/2 for a 100% monoclinic transformation of ZrO2 grains. This result indicates that transformation toughening was the dominant toughening mechanism in the studied composite.
In Situ Measurement of Bridging Stresses in Toughened Silicon Nitride Using Raman Microprobe Spectroscopy
Authors: Pezzotti Giuseppe , Muraki Naoki , Maeda Naoki , Satou Kouji , Nishida Toshihiko
Published: July 2010

Bridging stresses that result both from elastic tractions and frictional interlocking in the wake of an advancing crack have been evaluated quantitatively via in situ Raman microprobe spectroscopy in a toughened Si3N4 polycrystal. Crack opening displacement (COD) profiles of bridged cracks also have been measured quantitatively via scanning electron microscopy to substantiate the piezospectroscopic determination of microscopic stresses via Raman spectroscopy. The highest spatial resolution of the stress measurement in the Raman apparatus was 1 µm, as dictated by the optical lens that was used to focus the laser on the sample. Measurements of the bridging stresses were performed both at fixed sites (as a function of the applied load) and along the profile behind the crack tip (under a constant load). Rather high stress values (i.e., 0.4‐1.1 GPa) were measured that corresponded with unbroken ligaments that bridged the crack faces in elastic fashion, whereas frictional sites were typically under a lower tensile stress (0.1‐0.5 GPa). Mapping the near‐tip COD profile and the bridging stresses at the (normal) critical load for catastrophic fracture enabled us to calculate the crack‐tip toughness and to explain the rising R‐curve behavior of the material. From a comparison with conventional fracture‐mechanics data, a self‐consistent view of the mechanics that govern the toughening behavior of the Si3N4 polycrystal could be obtained. In particular, crack bridging is proven to be, by far, the most important mechanism that contributes to the toughening of polycrystalline Si3N4 materials.
Role of Cement Alkali Content in Acid Attack: A Pore‐Solution and Pore‐Size‐Distribution Study
Authors: Goñi Sara , Macías Angeles , Madrid Julio , Díez Jesús M.
Published: July 2010

The behavior of hydrated portland cement pastes in an acid‐buffered solution of acetic acid/sodium acetate (HAc/NaAc) (pH 4.5) during a period of 90 days at a temperature of 21°C has been studied. The changes that were caused by the acid ingress were monitored via analysis of the extracted pore solution. The evolution of the porosity and pore‐size‐distribution changes were evaluated by using intrusion mercury porosimetry, and those of crystalline compounds were evaluated by using X‐ray diffractometry. A similar study was conducted with an equivalent series of samples that were immersed in distilled water. The results showed the important role that cement alkali content (1.03% K2O, 0.2% Na2O) has in the mechanism of the acid attack. Interesting correlations were established between pore‐solution and porosity data.
Contact Fatigue in Silicon Nitride
Authors: Lee Seung Kun , Lawn Brian R.
Published: July 2010

A study of contact fatigue in silicon nitride is reported. The contacts are made using WC spheres, principally in cyclic but also in static loading, and mainly in air but also in nitrogen and water. Damage patterns are examined in three silicon nitride microstructures: (i) fine (F)‐almost exclusively fully‐developed cone cracks; (ii) medium (M)‐well developed but smaller cone cracks, plus modest subsurface quasi‐plastic damage; (iii) coarse (C)‐intense quasi‐plastic damage, with little or no cone cracking. The study focuses on the influence of these competing damage types on inert strength as a function of number of contacts. In the F and M microstructures strength degradation is attributable primarily to chemically assisted slow growth of cone cracks in the presence of moisture during contact, although the M material shows signs of enhanced failure from quasi‐plastic zones at large number of cycles. The C microstructure, although relatively tolerant of single‐cycle damage, shows strongly accelerated strength losses from mechanical degradation within the quasi‐plastic damage zones in cyclic loading conditions, especially in water. Implications concerning the design of silicon nitride microstructures for long‐lifetime applications, specifically in concentrated loading, are considered.
Mechanisms for Silanol Formation on Amorphous Silica Fracture Surfaces
Authors: D'Souza Andrew S. , Pantano Carlo G.
Published: July 2010

Amorphous SiO2fracture surfaces created under different partial pressures of water vapor (pH2O) were analyzed using temperature‐programmed static secondary ion mass spectroscopy. The results were used to develop an atomistic model for the formation of a fracture surface. It was found that substantial reconstruction of the SiO2fracture surface took place immediately after the fracture event. Formation of the fracture surface was modeled as three individual steps‐rupture of Si‐O‐Si bonds to form dangling Si* and Si‐O* bonds, reconstruction and relaxation of the surface to form both strained and unstrained siloxane bonds, and, lastly, reaction of H2O molecules with strained siloxane bonds to form surface silanol groups. The final concentration of surface silanol groups was found to have only a weak dependence on the pH2O in the ambient atmosphere during the fracture process. It was also found that the number of strained siloxane bonds on the SiO2fracture surface could be substantially reduced by heat treatment of the glass under vacuum.
Structural Instability during Cubic‐Tetragonal Transformation in Lead Titanate Single Crystals
Authors: Chou Chen‐Chia , Hou Chun‐Shou
Published: July 2010

In this paper, we report the structural instability and premartensitic transformation phenomena in PbTiO3single crystals during tetragonal‐to‐cubic phase transformation and vice versa, studied using hot‐stage X‐ray diffraction and polarized optical microscopy. The present results suggest that theoretical predictions of a habit plane based upon phenomenological crystallographic theory and experimental values may vary over a certain range if the lattice perturbation due to crystal defects and structural instability during transformation are taken into consideration. In the course of the tetragonal‐to‐cubic phase transformation, X‐ray diffraction shows a broad peak, associated with an intermediate phase, up to several tens of degrees above Tc. Using heating‐stage optical microscopy, patches of bright bands and moiré fringes were often seen to form in advance prior to the arrival of a phase front during cubic‐to‐tetragonal transformation, especially when the specimens exhibited severe internal stress, indicating a generalized lattice perturbation during phase transformation.
Silicon Substitution for Aluminum in Calcium Silicate Hydrates
Authors: Faucon Pascal , Petit J. Claude , Charpentier Thibault , Jacquinot J. François , Adenot Fredéric
Published: July 2010

27Al MAS and multiquantum (MQ) MASNMR (magic‐angle spinning nuclear magnetic resonance) spectroscopy were used to study the substitution of silicon by aluminum in calcium silicate hydrates (C‐S‐H), which are the main component of hydrated portland cement. Synthetic C‐S‐H samples were prepared, and their chemical stability was studied. Two‐dimensional 3Q‐MASNMR spectra revealed the chemical shift and quadrupolar parameters (deltaiso, nuQ) that labeled aluminum sites in the C‐S‐H. Tetrahedral aluminum was observed in the bridging and nonbridging sites of the silicate chains.
Phase Diagram of the BaO(BaCO3)‐CaO‐CuO System
Authors: Kusano Yoshihiro , Kikuchi Takeyuki , Takada Jun , Fukuhara Minoru , Ikeda Yasunori , Hiroi Zenji
Published: July 2010

The phase diagram of the BaO(BaCO3)‐CaO‐CuO system, especially in the barium‐rich region at 900°C in air, was studied. Two new different oxycarbonates were observed: Ba8Ca16/15Cu64/15O11.20(CO3)2.66and a solid‐solution series with a chemical composition of Ba2Cax+yCu1+(x/2)‐yO2+delta(CO3)1‐z(where 0 ≤ to x≤ to 2/39 and 0 ≤ to y≤ to 16x/5). The oxycarbonate solid solution was formed in a region of the compositional triangle Ba:Ca:Cu (in moles) = (2:0:1)‐(39:1:20)‐(65:7:28). The solid‐solution structure had P4/mmm symmetry, with lattice parameters a similar/congruent apand c similar/congruent 2ap, where aprepresents the perovskite cell. The Ba8Ca16/15Cu64/15O11.20(CO3)2.66compound, which had Pm3m symmetry with a lattice parameter a= 0.8116(2) nm, had no chemical‐solubility range.
Synthesis of Metastable Tetragonal (t′) ZrO2‐12 mol% YO1.5 by the Organic Polymerized Complex Method
Authors: Yokota Osamu , Yashima Masatomo , Kakihana Masato , Shimofuku Akira , Yoshimura Masahiro
Published: July 2010

Zirconia‐yttria (ZrO2‐12 mol% YO1.5) powders were prepared via an organic polymerized complex method that was based on the Pechini‐type reaction route. A mixed solution of citric acid, ethylene glycol, and zirconium and yttrium ions were polymerized to form a transparent resin, which then became a black powder after charring at ∼450°C. This product was used as a precursor for ZrO2‐12 mol% YO1.5. The formation of a metastable tetragonal (t´) phase with compositional homogeneity occurred when the precursor was heated at the rate of 10°C/min up to a temperature >560°C. X‐ray diffraction peaks became sharp with an increase in the heat‐treatment temperature. A t´‐form with cell parameters of a= 5.1265(6) Å and c= 5.1559(9) Å were obtained via heat treatment up to 1400°C. This study shows that the organic polymerized complex method is effective for obtaining the compositionally homogeneous metastable t´‐form in the ZrO2‐YO1.5system for short annealing times at relatively lower temperatures.
Deep Reactive Ion Etching of Lead Zirconate Titanate Using Sulfur Hexafluoride Gas
Authors: Wang Shinan , Li Xinghua , Wakabayashi Katsuhiro , Esashi Masayoshi
Published: July 2010

A deep reactive ion etching (RIE) technique that uses sulfur hexafluoride (SF6) gas has been developed for lead zirconate titanate (Pb(Zr,Ti)O3, PZT) three‐dimensional microfabrication from PZT ceramic blocks. The etching was performed by using an inductively coupled plasma that was generated in a narrow‐gap vacuum chamber. The etch depth was 70 µm with a maximum etch rate of 0.3 µm/min and a selectivity of PZT to the electroplated nickel mask of >35:1. The sidewalls of the PZT structures were tapered, with base angles of ∼75°. Both positive RIE lag and unexpected ultrafine‐slit etching phenomena were observed.
Two‐Dimensional‐Woven‐Carbon‐Fiber‐Reinforced Silicon Carbide/Carbon Matrix Composites Produced by Reaction Bonding
Authors: Tani Eiji , Shobu Kazuhisa , Kishi Kazushi , Umebayashi Seiki
Published: July 2010

Plane‐woven‐fabric carbon‐fiber‐reinforced SiC/C matrix composites were fabricated at 1450°C via reaction bonding and impregnation with phenolic resin. The relationship between the flexural strength and the open porosity of the composites is dependent on the heat‐treatment temperature before the impregnation. The flexural strength of composites heat‐treated at 1000°C (open porosity of ∼15%) was ∼300 MPa, whereas that of composites heat‐treated at 1450°C (open porosity of ∼12%) was only ∼240 MPa. The heat‐treatment temperatures before the impregnation step might control the interface properties between the fiber and the matrix.
Mechanochemically Synthesized Lead Magnesium Niobate
Authors: Wang John , Junmin Xue , Dongmei Wan , Weibeng Ng
Published: July 2010

Lead magnesium niobate (PMN) of high sintered density has been successfully prepared by a mechanochemical fabrication technique from mixed oxides of PbO, MgO, and Nb2O5. The novel technique skips the phase‐forming calcination steps at intermediate temperatures that are almost always required in the most widely employed columbite method. The solid‐state reactions among constituent oxides are activated by mechanical energy instead of calcination at elevated temperatures. Nanosized PMN particles of perovskite structure with a minimized degree of particle agglomeration were formed when the oxide mixture was mechanically activated for 20 h. The resulting PMN powder was sintered to a density of ∼99% theoretical density at 1050°C for 1 h. The sintered PMN exhibited a peak dielectric constant of 18 080 at a frequency of 100 Hz at ‐11°C.
Theoretical Structure Determination of a Complex Material: κ‐Al2O3
Authors: Yourdshahyan Yashar , Ruberto Carlo , Halvarsson Mats , Bengtsson Lennart , Langer Vratislav , Lundqvist Bengt I. , Ruppi Sakari , Rolander Ulf
Published: July 2010

The atomic and electronic structures of kappa‐Al2O3 are determined using theoretical first‐principles techniques based on density‐functional theory (DFT), plane waves, and pseudopotentials. The obtained structure is confirmed by analysis of powder X‐ray diffraction data. The structure is orthorhombic with oxygen ions in close‐packed ABAC stacking and aluminum ions occupying both tetrahedral (1/4) and octahedral (3/4) interstitial sites. A growth model for chemical vapor deposition of kappa‐Al2O3 is proposed based on the atomic structure. Calculated electronic structure and charge density yield a band gap of 5.3 eV and a high ionic character of the bonds. The study shows the applicability of DFT‐based methods to complex and metastable materials.
New Synthetic Method of Forming Aluminum Oxynitride by Plasma Arc Melting
Authors: Fukuyama Hiroyuki , Nakao Wataru , Susa Masahiro , Nagata Kazuhiro
Published: July 2010

A new synthetic method of forming the gamma‐phase of aluminum oxynitride (alon) has been proposed. alon has been successfully synthesized by the DC nitrogen plasma arc using alpha‐Al2O3and AlN as starting materials. Alon rapidly forms in a liquid state under thermal plasma. The obtained lattice parameter of alon has been determined as a function of the concentration of Al2O3. Evaporation takes place during arc melting. The condensed alon from the vapor consists of nanoscale‐sized spherical particles, and these particles are in single crystals. The evaporation mechanism of alon during arc melting is discussed. Thus, arc plasma processing is a promising method for synthesizing alon and producing the ultrafine powder.
Improved Low‐Temperature Environmental Degradation of Yttria‐Stabilized Tetragonal Zirconia Polycrystals by Surface Encapsulation
Authors: Koh Young‐Hag , Kong Young‐Min , Kim Sona , Kim Hyoun‐Ee
Published: July 2010

An encapsulating layer was deposited on the surface of tetragonal zirconia polycrystals doped with 3 mol% of yttria (3Y‐TZP), to prevent low‐temperature environmental degradation (aging) of the material. The layer, which was composed of silica and zircon, was formed on the surface by exposing the specimens next to a bed of silicon carbide powder in a flowing hydrogen atmosphere that contained ∼0.1% water vapor at 1450°C. The layer was ∼0.5 µm thick and is expected to be under strong residual compressive stress. This encapsulation process remarkably improved the low‐temperature degradation of the material. The strength of the specimens also was improved by this process.
Measuring the Influence of Grain‐Boundary Misorientation on Thermal Groove Geometry in Ceramic Polycrystals
Authors: Saylor David M. , Rohrer Gregory S.
Published: July 2010

We have used electron backscattered diffraction patterns to determine the misorientation of 201 adjacent pairs of grains in a magnesia polycrystal. The width and depth of the thermal grooves formed by these same grain boundaries were also measured by atomic force microscopy (AFM). By simulating the errors associated with the AFM observations and comparing our observations to existing data for magnesia and alumina, we show that, under appropriate experimental conditions, surface dihedral angles, relative grain‐boundary energies, and surface diffusivities determined from AFM measurements are consistent with data acquired by more laborious techniques. Correlation of the grain‐boundary misorientation and thermal groove geometry leads to the observation that grain boundaries with small misorientations, regardless of the rotation axis, have shallow thermal grooves and relatively low grain‐boundary energies. Furthermore, numerous boundaries with relatively large misorientations but shallow thermal grooves correspond to special boundaries near coincident‐site‐lattice (CSL) misorientations. Finally, the data set indicates that factors other than the boundary misorientation, such as anisotropy of the surface energy and the grain‐boundary tangent plane, play a role in determining the groove geometry.
Preparation of β‐Silicon Nitride Seeds for Self‐Reinforced Silicon Nitride Ceramics
Authors: Ramesh Peelamedu D. , Oberacker Rainer , Hoffmann Michael J.
Published: July 2010

This paper describes a method for the preparation of silicon nitride (Si3N4) seeds that have an average aspect ratio of ∼4. The seeds are prepared via heat treatment of a powder mixture that contains alpha‐phase‐rich Si3N4 and 0.5 wt% Y2O3 at a temperature of 1800°C and a nitrogen pressure of 35 kPa. A Y‐Si‐O‐N liquid forms during heat treatment; this liquid acts as a flux for seed precipitation. During cooling, the Y‐Si‐O‐N liquid transforms to a thin intergranular grain‐boundary phase and causes strong agglomeration of the seeds. The seeds can be isolated by dissolving the grain‐boundary phase in hot phosphoric acid, followed by an ultrasonic treatment (for 30 min). The method can be used to produce large quantities of seeds.
Dissolution of Yttrium Ions and Phase Transformation of 3Y‐TZP Powder in Aqueous Solution
Authors: Ho Fang‐Yuan , Wei Wen‐Cheng J.
Published: July 2010

In order to understand the reactions of yttria‐containing zirconia powder in aqueous solution, the dissolution behavior of yttrium and the formation of the monoclinic (m) phase of the Y‐TZP powder were investigated. The tests were conducted in aqueous solution at temperatures up to 95°C. The results showed that the reaction order of yttrium dissolution from the surface of TZP powder is 5‐9, and the activation energy is about 8.4 kJ/mol. However, the value of the reaction order is different from that (0.96‐1.20) of the formation of m‐phase. However, the yttrium dissolution of the slurries with the addition of 2 mol% dispersant (D‐134) was reduced greatly to an extremely low (<2 ppm) level up to 24 h at the testing temperatures. The results also showed that the amounts of m‐phase formation were nearly the same for the powders in the slurries with or without D‐134 addition. The decrease of yttrium content in the presence of D‐134 and the formation of the m‐phase of the TZP powder were not closely related. The degradation mechanism, or the formation of m‐phase, should not be directly related to the dissolution of yttrium ions from a zirconia surface.
Evidence for the Coexistence of Tetragonal and Cubic Phases in n‐BaTiO3 under Electric Field
Authors: Ravi V. , Aslam M.
Published: July 2010

Application of an appropriate electric field on donor‐doped polycrystalline BaTiO3 (tetragonal) leads to the formation of the cubic phase in the specimen because of Joule heating. Coexistence of the tetragonal and cubic phases depends on the magnitude of the applied field and on ambient and ceramic‐body temperature, and it is a prerequisite for the current‐limiting action of these ceramics. The contributions from the tetragonal and cubic phases for the charge transport is evidenced from a capacitance‐voltage measurement.
Mo5Si3‐Boron Composites Fabricated by Induction Plasma Deposition and Their High‐Temperature Oxidation Resistance
Authors: Fan Xiaobao , Ishigaki Takamasa
Published: July 2010

Induction plasma spraying was used to produce freestanding parts of Mo5Si3‐boron (Mo5Si3‐B) composite materials. Four different Mo5Si3‐B compositions were prepared and oxidized isothermally at 1210°C in air at atmospheric pressure. The high‐temperature oxidation performance of these materials was dependent strongly on the boron content in the specimens. The composite that contained 2.0 wt% boron exhibited excellent resistance to oxidation, as indicated by the almost‐zero change in specimen mass after oxidation for 24 h.
Synthesis and Characterization of Ti0.33Ta0.33Nb0.33C and Ti0.33Ta0.33Nb0.33CxN1‐x Whiskers
Authors: Carlsson Mats , Johnsson Mats , Nygren Mats
Published: July 2010

Ta0.33Ti0.33Nb0.33C and Ta0.33Ti0.33Nb0.33CxN1−x whiskers were synthesized via a carbothermal vapor‐liquid‐solid growth mechanism in the temperature range 900°‐1450°C in Ar or N2. The optimum temperature was 1250°C. Whiskers were obtained in a yield of 70‐90 vol%. The whiskers were 0.5–1 µm in diameter and 10–30 µm in length. The starting materials that produced the highest whisker yield were: TiO2, Ta2O5, Nb2O5, C, Ni, and NaCl. C was added to reduce the oxides, and Ni to catalyze whisker growth. NaCl was used as a source of Cl for vapor‐phase transportation of Ta and Nb oxochlorides and Ti chlorides to the catalyst. The catalyst metal was recycled several times during the synthesis and was transported as NiCl2(g) according to thermodynamic calculations. The rate of formation and the chemical composition of the whiskers depended on the synthesis temperature, the choice of catalyst, and the atmosphere. At low temperatures, the whiskers were enriched in Nb and Ta, whereas the Ti content increased with increased synthesis temperature.
Precipitation and Calcination Processes for Yttrium Aluminum Garnet Precursors Synthesized by the Urea Method
Authors: Matsushita Naoya , Tsuchiya Noriyoshi , Nakatsuka Katsuto , Yanagitani Takakimi
Published: July 2010

Yttrium aluminum garnet (YAG) precursors for transparent ceramics were synthesized by the urea method under various [urea]/[metal ions] ([U]/[M]) conditions. Monophase YAG was obtained from solutions with a high [U]/[M] ratio after calcination at a temperature of 1200°C. The condition of the precipitates seemed to indicate that the yttrium compounds had precipitated onto the aluminum compounds. The surface morphology and size of the particles were controlled by the [U]/[M] ratio. The different reaction sequences of YAG crystallization for low‐ and high‐ratio samples were dependent on morphology, size, and the quantity of chemical species that was precipitated as carbonate and/or sulfate compounds.
Effect of Particle Size on the Reaction Wave Propagation in the Combustion Synthesis of Al2O3‐ZrO2‐Nb Composites
Authors: Tomasi Roberto , Munir Zuhair A.
Published: July 2010

The effect of reactant particle size on the kinetics of wave propagation in the thermite system Nb2O5+ Al2Zr + nuAl2O3 (with alumina being a diluent) was investigated. Reactants in three size ranges were utilized: fine (1–3 µm), medium (<10 µm), and coarse (10–45 µm). Particle size had an effect on the mode and velocity of the self‐propagating combustion wave. For fine particles, the combustion wave propagated in a steady‐state mode for all values of dilutions investigated (0 ≤ nu ≤ 0.8). For medium particles with nu > 0.1 and for all coarse particles, wave propagation was in the spin mode. A significantly large difference between the calculated adiabatic combustion temperatures and the measured values was observed and is attributed to large heat losses arising from very wide reaction zones. For fine particles, the temperature dependence of the wave velocity showed a discontinuity at a temperature corresponding to the formation of the eutectic in the binary ZrO2‐Al2O3. The velocity can be more than a factor of 3 higher at the same temperature, depending on the amount of the eutectic liquid. The dependence of the wave velocity on particle size (r) was found to have the form v is proportional to 1/rm with 0.5 < m < 0.8. The upper limit approximates that of m= 1.0 for diffusion‐controlled reaction kinetics, according to reported modeling studies. In all cases, regardless of the mode of wave propagation the reactions resulted in the desired products, Nb + ZrO2+ Al2O3 with a microstructure exhibiting the eutectic of the binary ZrO2‐Al2O3.
Effect of Solids Loading on Slip‐Casting Performance of Silicon Carbide Slurries
Authors: Ferreira José M. F. , Diz Henrique M. M.
Published: July 2010

The influence of solids loading and particle shape on the green microstructure of slip‐cast bodies was investigated. Three commercial silicon carbide (SiC) powders (two coarse varieties with the same particle‐size distribution (PSD) but different particle shapes and a finer powder) were used to prepare bimodal PSDs designed to maximize the packing density. Various surface‐active agents (anionic, cationic, and non‐ionic) were tested. Anionic dispersants were the most effective in dispersing aqueous SiC slurries. The effectiveness of dispersants was evaluated by sedimentation tests using very dilute slurries, by rheology, and by the packing density of slip‐cast bodies prepared from suspensions loaded with 62.5 wt% solids, stabilized with a fixed amount of dispersant (0.25 wt%, relative to the solids). Then, the best dispersant was selected to study the effects of dispersant and solids concentrations on the degree of packing of bimodal suspensions that contained the sharper‐edged coarse particles. It could be observed that the green density was dependent on both parameters, initially showing an increase to a maximum, followed by a decreasing trend. A high value of 74.5% of the theoretical density (TD) was obtained from suspensions that contained 70 wt% solids and 0.1 wt% dispersant. The substitution of the angular coarse particles by similarly sized but more spherical particles resulted in an additional increase in green density to >76 wt% TD. The results can be interpreted in terms of freedom of particles upon deposition on the cast layer, which enables particle rearrangement, and segregation phenomena.
Electrophoretic Deposition of Silicon Carbide
Authors: Bouyer Frédéric , Foissy Alain
Published: July 2010

Electrophoretic deposition of silicon carbide (SiC) in ethyl alcohol was studied using the simplest bath formulation allowing the production of acceptable deposits. The object of the study was to explain the role of the different components in the bath, regarding both the dispersion of particles and the deposition mechanism. The elementary bath composition comprised SiC, ethyl alcohol, water, an aluminum salt, and a polymer chemically based on poly(vinyl butyral (PVB). AlCl3 partly dissociated in the solution, depending on the water content, and absorbed strongly onto the SiC. Adsorption of aluminum species promoted the positive charge and the dispersability of the particles, if some water was present. PVB did not adsorb onto SiC but decreased the sedimentation rate. Particle deposition was bound to the occurrence of an electrolytic reaction. That reaction combined the reduction of water and the formation of Al(OH)3, which reduced the surface charge and allowed close contact between particles in the deposit. Excess water, above ∼10 wt% of the solution, was harmful because it induced heavy hydrogen bubbling at the electrode and high bath conductivity. The diffusion of ionic species through the deposit controlled the electrolysis and deposition rates. Finally, a range of bath compositions for satisfactory deposition rates was established and explained.
Impregnation Molding of Particle‐Filled Preceramic Polymers: Process Modeling
Authors: Erdal Merve , Güçeri Selçuk I. , Danforth Stephen C.
Published: July 2010

A process model is presented for the analysis and simulation of impregnation molding of particle‐filled, continuous‐fiber ceramic composites. A multidimensional, transient particle filtration formulation is coupled with isothermal, anisotropic Darcy flow to predict filler‐concentration profiles during molding. The particle filtration is insensitive to the injection flow rate; however, in irregularly shaped molds, the geometry of the mold greatly influences the local velocities, which results in local variations in the rate of filtration. Although the preform volume fraction and inlet particle concentration influence the particle accumulation quantitatively, the filtration coefficient and the permeability have a dominant role in setting the filtration trend.
Novel Synthesis Method of Hydroxyapatite Whiskers by Hydrolysis of alpha‐Tricalcium Phosphate in Mixtures of Water and Organic Solvent
Authors: Nakahira Atsushi , Sakamoto Kiyoko , Yamaguchi Shunro , Kaneno Motoki , Takeda Shinichi , Okazaki Masayuki
Published: July 2010

A novel and simple nonhydrothermal method was developed for synthesizing hydroxyapatite whiskers under mild conditions. Hydroxyapatite whiskers were synthesized by the hydrolysis of alpha‐tricalcium phosphate at 80°C and atmospheric pressure. The effect of aliphatic alcohols on the transformation of alpha‐tricalcium phosphate to hydroxyapatite in water‐alcohol systems was examined. When the hydroxyapatite was characterized by XRD and TEM observation, its morphology and size were found to vary with the specific alcohol used during hydrolysis. The effect of reaction time on hydrolysis of alpha‐tricalcium phosphate to hydroxyapatite also was studied.
Preparation of YAG:Europium Red Phosphors by Spray Pyrolysis Using a Filter‐Expansion Aerosol Generator
Authors: Kang Yun Chan , Chung Yun Seup , Park Seung Bin
Published: July 2010

Europium‐doped yttrium aluminum garnet (YAG) phosphor particles were prepared from mixed nitrate solutions by the FEAG (filter expansion aerosol generator) process. The crystallinity, morphology, and luminescence of the YAG:Eu particles were investigated. The prepared particles had an amorphous phase, which turned into phase‐pure YAG particles after annealing above 1000°C. A cubic‐structure YAG phase was formed by the reaction of crystalline Y2O3 and the aluminum component. The prepared particles had spherical morphology. The mean size of the YAG:Eu particles increased from 0.45 to 1.0 µm when the overall solution concentrations were increased from 0.02 to 1.2 mol/L. The optimum doping concentration of europium for the maximum brightness of phosphor particles was 1.3 at.%. The cathodoluminescence (CL) intensity was strongly affected by the annealing temperatures. The maximum CL value of the particles was 55 cd/m2.
Topotaxy of Corundum‐Type Tetramagnesium Diniobate and Ditantalate Layers on Rock‐Salt‐Type Magnesium Oxide Substrates
Authors: Manabe Takaaki , Yamaguchi Iwao , Kondo Wakichi , Mizuta Susumu , Kumagai Toshiya , Nagahama Tsutomu , Tsuchiya Toshio
Published: July 2010

In‐plane alignments were found to occur in the product layers of corundum‐type Mg4Nb2O9 (MNO) and Mg4Ta2O9 (MTO) on rock‐salt‐type MgO(001) substrates as a result of a topotaxial interface reaction between a Nb2O5 (or Ta2O5) precursor and MgO at 800°C and higher. Such alignments were attributed to a topotaxial formation of (114)‐oriented MNO (or MTO) layers. This (114) orientation is hidden in the conventional XRD theta‐2theta scans due to an overlap of the film (114) peak with the substrate peak, MgO(002). Its presence was revealed by XRD pole‐figure measurements. The crystallographic relationship between MNO(114) [or MTO(114)] and MgO(001) is also discussed.
Sintering of Lanthanum Zirconate
Authors: Nair Jalajakumari , Nair Padmakumar , Doesburg Giel B. M. , Van Ommen Jan G. , Ross Julian R. H. , Burggraaf Anthonie J. , Mizukami Fujio
Published: July 2010

Lanthanum zirconate (La2Zr2O7) was prepared by coprecipitating lanthanum nitrate and zirconyl oxychloride at pH 10, followed by ethanol washing. The initial high surface area of ∼304 m2·g−1 decreased very rapidly with increased sintering temperature and decreased to an immeasurably small value after heating at 1200°C for 15 h. The major parameters studied were phase evolution, crystallite size, porosity, surface area reduction, and shrinkage during sintering. Three temperature regions were identified based on these studies: below the crystallization temperature, between the crystallization temperature and ∼1100°C, and above 1100°C. The main contribution of surface area reduction in the region 800°–1100°C was due to surface diffusion; the main contribution above 1100°C was due to grain‐boundary diffusion coupled with surface diffusion.
Controlled Cracking of Multilayer Ceramic Bodies
Authors: Grau Jason E. , Uhland Scott A. , Moon Jooho , Cima Michael J. , Sachs Emanuel M.
Published: July 2010

Cracking of granular ceramic films during drying is a well‐known phenomenon. Previous work has shown that films cast above a critical thickness on nonporous substrates crack spontaneously. Films cast on previously cast layers crack at thicknesses that depend on the solids content of the slurry. Cracking occurs in this instance when films are cast above a critical saturation thickness (CST). The CST is defined as the thickness of granular material that would be 100% saturated by the liquid contained in the last cast layer. The CST is remarkably similar in magnitude to the critical cracking thickness obtained with similarly prepared slurries on dense substrates. For example, 0.5 µm Al2O3 slurries in water have a CST of 65 µm when cast on previously deposited layers of Al2O3. Similarly prepared slurries crack spontaneously when cast on single‐crystal silicon at any thickness above ∼60 µm.
Effects of Sm3+ Substitution on Dielectric Properties of Ca1−xSm2x/3TiO3 Ceramics at Microwave Frequencies
Authors: Kim Woo Sup , Kim Eung Soo , Yoon Ki Hyun
Published: July 2010

Microwave dielectric properties of Ca1‐xSm2x/3TiO3 ceramics were investigated as a function of the amount of Sm3+ substitution (0.0 ≤x≤ 0.8). The structure was changed from orthorhombic perovskite at x= 0.0 to tetragonal at x= 0.6. As the calcium vacancy concentration increased with increased Sm3+ substitution, the unloaded Q value (similar/congruent 1/tan delta) increased up to the solid‐solution limit at x= 0.6 and then decreased because of formation of the secondary phase Sm2Ti2O7. The dielectric constant decreased with increased Sm3+ substitution. The effects of Sm3+ substitution on dielectric loss and dielectric constant of the specimens were analyzed by the infrared reflectivity spectra in the range 50–4000 cm−1, which were evaluated using the Kramers‐Kronig analysis and classical oscillator model. The correlations among dielectric constant, dielectric loss, and dispersion parameters were studied.
Low‐Temperature Aging of Y‐TZP Ceramics
Authors: Chevalier Jérôme , Cales Bernard , Drouin Jean Michel
Published: July 2010

The isothermal tetragonal‐to‐monoclinic transformation of a 3Y‐TZP ceramic is investigated from 70° to 130°C in water and in steam by X‐ray diffraction and optical interferometer techniques. Aging kinetics followed by X‐ray diffraction are fitted by the Mehl‐Avrami‐Johnson law, suggesting nucleation and growth to be the key mechanisms for transformation. Optical interferometer observations of highly polished samples effectively reveal a nucleation and growth micromechanism for tetragonal‐to‐monoclinic transformation. A model based on surface change analysis is developed that fits closely to the X‐ray diffraction results.
Electrochemical Determination of the Activity of Na2O in the Pyrochlore Phase of the Sb2O4‐NaSbO3 System
Authors: Kale Girish Madhav , Srikanth Srinivasan
Published: July 2010

Activity of Na2O in the pyrochlore phase of the system Sb2O4‐NaSbO3 has been measured as a function of temperature in the range 1073–1412 K by electromotive force (emf) measurements of the following solid‐state electrochemical cells:
image

The solid electrolyte used in the above electrochemical cells is Na‐ß‐Al2O3, which is an excellent sodium‐ion conductor in the temperature range of the measurements. From the measured reversible emf, the activity of Na2O in the Sb2O4‐NaSbO3 system has been calculated. The temperature dependence of the logarithm of the activity of Na2O in various two‐phase regions of the Sb2O4‐NaSbO3 system can be represented as
image

No thermodynamic data have been reported earlier in the literature for the system Sb2O4‐NaSbO3, and the present data constitute the first thermodynamic information.
Improvement in Reactivity and Grindability of Belite‐Rich Cement by Remelting Reaction
Authors: Fukuda Koichiro , Ito Suketoshi
Published: July 2010

A new modification of belite‐rich cement, a “remelted belite cement,” exhibits higher hydration activity and better grindability than conventional belite‐rich cement. The belite constituent in the new cement completely undergoes a remelting reaction and thus is distinctly different from the belite in conventional cement, which is rapidly quenched to depress the reaction. The remelted belite has a weaker resistance to fracture than does the quenched material and thus has better grindability. When the two cements, with nearly the same particle‐size distributions, are compared, the specific surface area of the remelted belite cement is higher, resulting in higher hydration activity.
High‐Temperature Anelastic and Viscoplastic Deformation of Fine‐Grained Magnesia‐ and Magnesia/Yttria‐Doped Alumina
Authors: Lakki Angeliki , Schaller Robert , Carry Claude , Benoit Willy
Published: July 2010

High‐temperature (1200–1550 K) mechanical‐loss and shear‐modulus measurements were performed on fine‐grained magnesia‐ and magnesia/ytrria‐doped alumina at low frequency (10−4−101 Hz), using a forced‐vibration torsion pendulum. Anelastic and viscoplastic relaxation phenomena were observed, which may be associated with grain‐boundary (GB) sliding and, consequently, may be dependent on the grain size and GB segregation of yttrium. The associated activation enthalpies were 800–1100 kJ/mol. GB precipitation of yttrium aluminum garnet led to enhanced mechanical loss and increased activation enthalpy. Viscoplastic deformation increased sharply as the amplitude of the applied stress increased above a “threshold” value of ∼3 MPa. The mechanical loss and creep rate of identical materials showed qualitatively similar dependence on the grain size and yttrium content. A possible micromechanism of mechanical loss could be the motion of GB dislocations, which is important for the accommodation process of GB sliding.
Microstructural Characterization of Small‐Particle Plasma Spray Coatings
Authors: de Arellano‐López Antonio R. , Faber K. T.
Published: July 2010

The microstructure of a small‐particle plasma spray (SPPS) aluminum oxide coating sprayed onto a prepared mild steel substrate has been characterized using a variety of microscopic techniques as part of a process optimization study. The coating was highly conforming to the substrate as evidenced in high‐resolution transmission electron microscopy, although some voids were present at the interface. The layered splat microstructure, characteristic of conventional plasma‐sprayed coatings but smaller in size, was discerned in partially thinned samples in a focused‐ion‐beam scanning electron microscope (FIB‐SEM). Microcracks and microporosity, generally less than 0.5 µm in size, was also seen between splats using transmission electron microscopy. Cubic alumina of the crystallographic form gamma‐Al2O3 was identified by electron diffraction.
Phase Stability and Microstructural Evolution in the System Bi2Sr2CaCu2O8+x
Authors: Jr. Medendorp Nicholas W. , Gaskell David R.
Published: July 2010

The dependence of the oxygen nonstoichiometry of Bi2Sr2CaCu2O8+x (2212) on temperature and oxygen partial pressure has been determined in the temperature range of 300° to 1000°C and the oxygen partial pressure range of 3 × 10−3 to 1 atm (1 atm = 105 Pa). The solidus temperature of 2212 was identified by differential thermal analysis over the same ranges of temperature and oxygen partial pressure. The nature of the decomposition of 2212 was determined by thermogravimetric analysis in the temperature range of 800° to 1000°C and oxygen partial pressure range of 3 × 10−3 to 1 atm. Microstructural evolution was examined by quenching materials, equilibrated with atmospheres of fixed partial pressures of oxygen, at various stages in the decomposition and identifying the phases present by electron microscopy and wavelength dispersive spectroscopy. With increasing temperature, at fixed oxygen partial pressure, the decomposition of 2212 in the presence of 1.9 mol% Sr0.4Ca0.6CuO2 follows the sequence 2212 + Sr0.4Ca0.6CuO2→ 2212 + Sr0.4Ca0.6CuO2+ Sr0.2Ca1.8CuO3→ Sr0.5Ca0.5CuO2+ Sr0.3Ca1.7CuO3+ Sr0.5Ca0.5Cu2O3+ L1→ Sr0.5Ca0.5CuO2+ Sr0.3Ca1.7CuO3+ L2→ L3.
Evidence for 6‐Coordinated Zirconium in Inactive Nuclear Waste Glasses
Authors: Galoisy Laurence , Pélegrin Emmanuèle , Arrio Marie‐Anne , Ildefonse Philippe , Calas Georges , Ghaleb Dominique , Fillet Catherine , Pacaud François
Published: July 2010

In inactive glasses used as models for French nuclear waste forms, Zr K‐ and L2,3‐edge X‐ray absorption spectroscopy spectra indicate that Zr is only 6‐coordinated, with d(Zr‐O) distances of 2.08 ± 0.01 Å and with a small radial disorder. Beyond, the first coordination shell, next‐nearest Si neighbors occur at about 3.4 Å. These distances exclude the presence of [7]Zr and/or [8]Zr and suggest [6]Zr‐O‐Si angles close to 130°, indicating corner‐shared ZrO6 and SiO4 polyhedra. This structural configuration is similar to those observed in some crystalline zirconosilicates and is in agreement with the stability of Zr in borosilicate melts and glasses.
Glass‐Ceramic Formation in the ZnO‐P2O5 System and the Effect of Silica as a Nucleating Agent
Authors: Jordery Sophie , Lee William E. , James Peter F.
Published: July 2010

Crystallization of a series of ZnO‐P2O5 based glasses was investigated. ZnO‐P2O5‐CaO glasses could be converted most readily to glass‐ceramics and crystallization of these led to formation of alpha‐Zn2P2O7, alpha‐CaZn2(PO4)2, and ß‐CaZn2(PO4)2 phases. A further phase has been tentatively identified as monoclinic (Zn,Ca)2P2O7. The most promising glass‐ceramic composition Z15 (59.4ZnO·33P2O5·6.6CaO·1SiO2) crystallized to alpha‐Zn2P2O7 and ß‐CaZn2(PO4)2, the latter phase being stabilized by the presence of SiO2 which also encouraged volume nucleation giving a fine‐scale (submicrometer) microstructure.
Formation of Nanocrystalline Silicon Carbide Powder from Chlorine‐Containing Polycarbosilane Precursors
Authors: Mitchell Brian S. , Zhang Haoyue , Maljkovic Nikica , Ade Martin , Kurtenbach Dirk , Müller Eberhard
Published: July 2010

Nanocrystalline ß‐SiC particulates with a grain‐size range of 5‐20 nm were prepared by heating a prepyrolyzed, chlorine‐containing polysilane/polycarbosilane (PS/PCS) to 1600°C. The transformation from the prepyrolyzed PS/PCS to nanocrystalline SiC was investigated by differential thermal analysis, thermogravimetric analysis, X‐ray diffractometry, mass spectrometry, and infrared spectroscopy. The results indicated that the nanocrystalline ß‐SiC was formed by the crystallization of the PS/PCS random network and the crosslinking of Si‐Si, Si‐Cl, and Si‐CH2‐Si bonds. Transmission electron microscopy observation showed that SiC particulates consisted of equiaxed, randomly oriented, ultrafine grains.