bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic Structure of a Near Σ11 a‐axis Tilt Grain Boundary in α‐A12O3
Authors: Mo Shang‐Di , Ching Wai‐Yim , French Roger H.
Published: July 2010

The electronic structure and interatomic bonding of a near Σ11 α‐axis tilt grain boundary in α‐A12O3 are determined using first‐principles calculations based on a model structure constructed by Kenway. This Σ11 boundary, which has a low grain boundary energy, has been the focus of various previous studies, including high‐resolution transmission microscopy (HRTEM) and pair potential based structural studies. The relaxed structure model, which contains 72 Al atoms and 108 O atoms, is periodic in two dimensions and reproduces the grain boundary structure obtained by HRTEM. To accurately identify the changes arising in the boundary, parallel calculations on the bulk supercell models containing the same number of Al and O atoms with and without surfaces were also carried out. This grain boundary does not have deep levels in the band gap, and its defect states appear mainly near the O‐2p derived valence band maximum and the Al‐3s derived conduction band minimum. The calculated partial densities of states for Al in the conduction band region are in good agreement with recent measurements on the near‐edge energy loss spectrum and the valence electron energy loss spectrum. By means of Mulliken population analysis, the effective charges, the bond order, and the partial density of states of the grain boundary atoms are identified and elucidated. In the region of the grain boundary, there is increased charge transfer from Al to O, arising mainly from the lower coordination number of the grain boundary atoms and therefore the reduction in the covalent bond formation. In addition, it is found that as the Al‐O bond lengths decrease, the Al—O charge transfer increases. For this Σ11 boundary, the structural features most associated with grain boundary electronic structure changes are undercoordination and shortened bond lengths.
Microstructural Development on Crystallizing Hot‐Pressed Pellets of Cordierite Melt‐derived Glass Containing B2O3 and P2O5
Authors: Glendenning Malcolm D. , Lee William E.
Published: July 2010

Comparing the crystallization mechanism of stoichiometric and B2O3 and P2O5 containing glass reveals that the additives extend the gap between the glass transition and crystallization temperatures and suppress formation of μ, cordierite while promoting direct crystallization of α cordierite. Detailed TEM analysis of nucleation and growth of crystals in hot‐pressed pellets of B2O3/P2O5‐containing glass particles shows that nucleation occurs on unidentified heterogeneous nuclei at the sites of the previous particle surfaces. Growth of α cordierite with a cellular morphology or μ cordierite with a dendritic morphology is most likely controlled by the glass composition directly ahead of the growth front.
Microstructure Control and Wear of Al2O3‐SiC‐(Al, Si) Composites Made by Melt Oxidation
Authors: Jayaram Vikram , Manna Rampada , Kshetrapal Manjunath G. , Sarkar Jaydeep , Biswas Sanjay K.
Published: July 2010

Ceramic matrix composites of Al2O3‐SiC‐(Al, Si) have been fabricated by directed melt oxidation of aluminum alloys into SiC particulate preforms. The proportions of AI2O3, alloy, and porosity in the composite can be controlled by proper selection of SiC particle size and the processing temperature. The wear resistance of composites was evaluated in pin‐on‐disk experiments against a hard steel substrate. Minimum wear rate comparable to conventional ceramics such as ZTA is recorded for the composition containing the highest fraction of alloy, owing to the development of a thin and adherent tribofilm with a low coefficient of friction.
Compressive Creep of a Fine‐Grained Poly crystal line Yttria
Authors: Besson* Jean‐Louis , Murat Denis , Rouxel* Tanguy , Valin Frédéric , Boncoeur Marcel
Published: July 2010

Compressive creep of a high‐purity fine‐grained polycrys‐talline yttria ceramic processed by hot isostatic pressing has been investigated in the 1130–1210°C temperature range for stresses between 6 and 92 MPa. The main creep mechanism is grain boundary diffusion controlled by an interface reaction. The most striking feature is the strong dynamic grain growth whereas static grain growth is negligible in the studied temperature domain.
Stress Dependence of the Raman Spectrum of β‐Silicon Nitride
Authors: Sergo Valter , Pezzotti Giuseppe , Katagiri Gen , Muraki Naoki , Nishida Toshihiko
Published: July 2010

The stress dependence of the Raman bands of β‐silicon nitride,β‐Si3N4, has been investigated. In the stress range examined (from‐200 to +200 MPa), low‐frequency shift bands (namely the 183, 205, and 226 cm−1 lines) do not show any frequency change with the stress, whereas the high‐frequency shift bands (862, 925, and 936 cm−1) have been found to have a linear stress dependence. The pertinent piezo‐spectroscopic coefficients have been determined and are found to depend strongly on the additives used to promote densification presumably being taken into solid solution into the β‐Si3N4 phase.
Grain Boundary Films in Rare‐Earth‐Glass‐Based Silicon Nitride
Authors: Wang Chong‐Min , Pan Xiaoqing , Hoffmann Michael J. , Cannon Rowland M. , Riihle Manfred
Published: July 2010

The thickness of the intergranular films in Si3N4 densified with lanthanide oxides has been systematically investigated using high‐resolution transmission electron microscopy. Four lanthanide oxides—La2O3, Nd2O3, Gd2O3, and Yb2O3—as well as Y2O3 are chosen so that the results will reflect the overall trend in the effect of the lanthanide utilized. The film thicknesses increase with increasing ionic radius of the lanthanide. In addition, Si3N4 particles flocculated into isolated clusters in the lanthanide‐based glasses are also characteristically separated by an amorphous film whose thickness is similar to that in the comparable polycrystalline ceramics, demonstrating that the film thickness is dictated entirely by the composition and not by the amount of the glass phase present.
Photoinduced Formation of Small Gold Particles in Silica Gels
Authors: Tanahashi Ichiro , Tohda Takao
Published: July 2010

AuCl−4 ions in sol‐gel‐derived silica gels (Au/silica gels) were photoreduced by ultraviolet light irradiation at room temperature. In the presence of ethylene glycol in the silica gels, small Au particles were effectively formed by the irradiation. When the storage temperature of the gels was increased in the dark after the irradiation, the surface plas‐mon resonance absorption peak of the Au particles was shifted to longer wavelengths, indicating that the Au particles became large. At a storage temperature below 9°C, Au particles could not be observed in the silica gels. The apparent activation energy of Au particle formation was estimated to be 17. 3 kcal·tmol−1.
Microstructural Characterization of Alumina and Silicon Carbide Slip‐Cast Cakes
Authors: Smith Peter A. , Kerch Helen M. , Haerle Andrew G. , Keller Joseph
Published: July 2010

The effect of solids loading, particle‐size distribution, and suspension viscosity on the resultant microstructure of slipcast monolithic ceramics prepared from aqueous suspensions of alumina and silicon carbide was studied. Unimodal alumina suspensions (average particle size = 0.6 μm) were prepared at 35, 37, and 42 vol%. Silicon carbide suspensions (average particle size = 0.7 μm) were produced with different quantities of dispersant at 37 vol%. Similarly, aqueous alumina suspensions of 42 and 50 vol% were produced with a bimodal particle‐size distribution. The slip‐cast microstructures were characterized by mercury porosimetry and small‐angle neutron scattering, which provided pore size (distribution), pore fraction, and pore morphology. Essentially, the combination of these techniques deciphered packing differences obtained in the cake microstructures. For the alumina cakes produced from the 35,37, and 42 vol% suspensions, the individual characterization techniques, mercury intrusion, and the neutron scattering measurements showed that the cake microstructures were similar in pore size and quantity. However, comparison of the techniques and their assumptions showed differences in the pore shape. Mercury porosimetry and neutron scattering showed bimodal porosity for the cake produced from a mixture of 85% 6‐μm particles and 15% 0.6‐μm particles. Pore volume fraction and pore size increases were correlated with increased viscosity in the silicon carbide suspensions. In addition, the silicon carbide cake microstructures were measured, and homogeneity was evaluated as a function of position in the cast.
Temperature Profile of Anatase‐TiO2Powder Compact during Microwave Heating
Authors: Ikuma Yasuro , Shigemura Takashi , Hirose Takashi
Published: July 2010

Anatase‐TiO2 cylindrical pellets were microwave sintered and partially transformed to rutile‐TiO2.The rutile concentration profile was determined as a function of location within the pellet. The rutile distribution in the microwavesintered pellets was different from that in pellets sintered by conventional fast firing. Temperature profiles within the pellets were estimated using the irreversible nature of the anatase‐rutile transformation. The estimated temperature profile within each pellet was not uniform. In the axial direction, maximum temperature occurred in the middle of the pellet. In the radial direction, maximum temperature occurred at 1 mm below the surface of the cylinder wall. Analysis of the temperature profile indicated that, although heat flow occurred in the microwave‐sintered pellet, the temperature profile did not change during a microwave exposure of 5–90 min.
Heteroflocculation in Binary Colloidal Suspensions: Monte Carlo Simulations
Authors: Shih Wan Y. , Shih Wei‐Heng , Aksay Ilhan A.
Published: July 2010

We have examined heteroflocculation in binary colloidal suspensions by Monte Carlo simulations based on a diffusion‐limited‐cluster‐aggregation (DLCA) model modified with finite attraction energies. The simulations were performed in two dimensions. Under heteroflocculation conditions, i.e., attraction between unlike particles and repulsion between like particles, cluster size undergoes a maximum as the concentration of the second species of particles is increased, similar to the experimental results in both binary suspensions and suspensions with adsorbing polymers. The initial increase in cluster size at low concentrations of the second species of particles is due to the mutual attraction between unlike particles. The decrease in cluster size at higher concentrations of the second species of particles is due to the repulsion between the second species of particles. The distinction between heteroflocculation and particulate depletion flocculation is discussed.
Stress Analysis of Elastic‐Plastic Contact Damage in Ceramic Coatings on Metal Substrates
Authors: Fischer‐Cripps Anthony C. , Lawn Brian R. , Pajares Antonia , Wei Lanhua
Published: July 2010

An elastic‐plastic analysis of damage in ceramic coatings on metal substrates from contacts with spherical indenters is made using finite element modeling. Computations are carried out specifically for plasma‐sprayed alumina:titania on a soft steel. The algorithm assumes an elastic sphere with frictionless contact on a flat elastic‐plastic layered specimen, and incrementally evaluates the expanding contact field as a function of applied load. Two key aspects of the contact field are examined: (i) the indentation stress‐strain curve; (ii) the damage zone geometry. Composite coating/substrate indentation stress‐strain curves are computed for two coating thicknesses, using input material parameters from iterative fits to data from control tests on free‐standing coating and substrate materials. Contours of principal shear stresses, most notably those contours corresponding to yield zone boundaries in both the softer substrate and the harder coating, are mapped out in the fully plastic region. Corresponding distributions of tensile stresses are also mapped out, and are shown to correlate with the locations of transverse fractures in the coating. General implications concerning material and geometrical design of ceramic‐based layer structures are discussed.
Porous Silicon Oxycarbide Glasses
Authors: Singh Anant K. , Pantano Carlo G.
Published: July 2010

High‐surface‐area silicon oxycarbide gels and glasses were synthesized from mixtures of methyldimethoxysilane (MDMS) and tetraethoxysilane (TEOS) through acidic hydrolysis and condensation. A surface area of ∼275 m2/g and an average pore size of ∼30 Å was obtained for a 50% MDMS‐50% TEOS glass at 800°C under a flowing argon atmosphere. The average pore size was increased by aging the precursor gels in ammonium hydroxide. The increased average pore size and the higher strength of the mesoporous gel network enhanced the surface‐area stability of the glasses; in this case, surface areas >200 m2/g were retained at 1200°C under an argon atmosphere. 29Si MAS NMR spectra revealed that an oxycarbide structure was established in the mesoporous glasses obtained after pyrolysis of the aged gels. The role of carbon was demonstrated by comparing the surface‐area stability of the oxycarbide glasses with that of pure silica and that of oxycarbide glasses where all the carbon groups were removed through low‐temperature plasma‐oxidation treatments. In the absence of carbon, the thermal stability of the surface area decreased dramatically.
Formation of Monodisperse Spherical TiO2 Powders by Thermal Hydrolysis of Ti(SO4)2
Authors: Park Hong Kyu , Moon Young Tae , Kim Do Kyung , Kim Chong Hee
Published: July 2010

Titania powders have been prepared by the thermal hydrolysis of titanium sulfate at 80°C in a mixed solvent of n‐propyl alcohol (n‐PrOH) and water. The morphology of the powders was greatly influenced by the volume ratio in n‐PrOH to water (RH ratio) of the solvent. An RH ratio of 1.0 was essential for the formation of spherical TiO2 powders. When hydroxypropyl cellulose (HPC) was used as a steric dispersant, monodisperse spherical powders 0.7 (μm in diameter were produced. As the RH ratio was increased, the zeta potential of the powders was negatively increased and the dielectric constant of the solvent was decreased. However, powders obtained at an RH ratio of 1.0 were observed to have the highest energy barrier. The colloidal stability of the powders in a mixed solvent of n‐PrOH and water is discussed. As‐precipitated powders were amorphous hydrates of titania and were crystallized by calcination into anatase (>600°C) and rutile (>800°C).
A Simple System for the Preparation of Submicrometer Lead Titanate Powders by the Sol‐Gel Method
Authors: Kumar Viswanathan , Marimuthu Ramadoss , Patil Shashikumar G. , Ohya Yutaka , Takahashi Yasutaka
Published: July 2010

This paper reports a new system for the preparation of lead titanate powders by the sol‐gel method. In this system basic lead acetate, (CH3COO)2Pb.Pb(OH)2, is used as the lead precursor instead of the widely used Pb(OOCCH3)2.3H2O and titanium tetrabutoxide monomer. This new system simplifies the chemical processing of precursor solutions of lead titanate, increases their stability in air, and offers good control of Pb:Ti stoichiometry. The xerogel, obtained from the precursor solution by aging at room temperature, is found to have a higher inorganic content. Gel‐to‐ceramic conversion is achieved by calcining the xerogel at 600°C. The phase purity, particle size, morphology, and compositional homogeneity of the gel‐derived powders are examined by XRD, TEM, and ICP‐OES.
Ionic Conductivity and Sinterability of NASICON‐Type Ceramics: The Systems NaM2(PO4)3+yNa2o (M = Ge, Ti, Hf, and Zr)
Authors: Aono Hiromichi , Suginoto Eisuke
Published: July 2010

Sodium‐rich NASOCON‐type ceramics, the NaM2(PO4)3+yNa2O (M = Ge, Ti, Hf, Zr) system, were investigated in order to obtain a material having a high Na+ conductivity and high density. The ionic conductivity and the sinterability were greately improved by an increase in the valve of y for all of the system examined. Added Na2O was not souble in teh NASICON‐type skeletton, sice the lattice constants and teh X‐ray diffraction patterns were not changed by the Na2O addintion in all of the samples. Na2O acts as a flux for obtaining highly dense ceramics and highly conductive grain boundaries. Partial A2 site insertion by Na+ ions is effective for the enhancement of conductivity, because the conductivity for Na1.5M(III)0.5Zr1.5(PO4)3 (M = In or Y) is about 1 order of magnitude higher than the maximum conductivity of the NaZr2(PO4)3+yNa2O system.
Synthesis of Boron Nitride by Self‐Propagating Reactions at High Pressure
Authors: Solozhenko Vladimir L. , Turkevich Vladimir Z. , Will Georg
Published: July 2010

Peculiarities of the formation of BN in a self‐propagating reaction between LiBF4 and Li3N have been studied under a wide range of p,T conditions within the region of thermodynamic stability of cBN. Boron nitride crystallized mainly in the form of highly ordered hBN (1700–2100 K), of mesographitic BN (2200–3000 K), or of turbostratic BN (>3000 K). Cubic boron nitride was obtained with a low yield (up to 14 vol%) as a result of precipitation from the metastable eutectic liquid in the BN‐Li3N system. These results give further evidence for an alternative metastable behavior in the BN phase formation under high pressures.
Colloidal Processing of a Very Fine BaTiO3 Powder—Effect of Particle Interactions on the Suspension Properties, Consolidation, and Sintering Behavior
Authors: Bergstrm Lennart , Shinozaki Kazuo , Tomiyama Hiroyuki , Mizutani Nobuyasu
Published: July 2010

The relation between the suspension state and the rheological properties, the consolidation, and packing of a very fine (nanosized) BaTiO3 powder has been investigated. The BaTiO3 powder was suspended in a nonaqueous medium by adsorbing fatty acids and a polymeric dispersant, poly(12‐hydroxy stearic acid), (PHS), at the BaTiO3/decane interface. Calculated interparticle energies imply that the suspension with PHS adsorbed is colloidally stable, while the suspensions with oleic and octanoic acid can be characterized as weakly and strongly flocculated, respectively. Analysis of settling experiments and rheological measurements at high concentrations confirmed these characteristics. Pressure filtration resulted in nearly identical green body densities in spite of the differences in colloidal properties, but the preliminary sintering experiments and microstructural characterization showed that the strongly flocculated suspension displays a significantly retarded sinterability compared to the colloidally stable and the weakly flocculated suspensions. The absence of a correlation between green density and sintering behavior was explained by considering both the volume taken by the adsorbed fatty acids and the PHS polymer—which can be substantial for nanosized powders—and the state of the suspension. While a decrease in the thickness of adsorbed surfactant or polymer layer will enable a higher particle packing density, such a thin adsorbed layer results in a more strongly flocculated suspension which will resist dense packing. Hence, it is suggested that the green bodies of the colloidally stable and the weakly flocculated suspensions correspond to a relatively homogeneous, but loosely packed, green body microstructure. The strongly flocculated suspension results in a green body with a more inhomogeneous microstructure.
Effects of Palladium Dispersions on Gas‐Sensitive Conductivity of Semiconducting Ga2O3 Thin‐Film Ceramics
Authors: Bausewein Andreas , Hacker Birgitta , Fleischer Maximilian , Meixner Hans
Published: July 2010

The gas sensitivity of Ga2O3 thin‐film n‐type conductors was investigated at temperatures of 500–1000°C. Palladium dispersions whose particle sizes are dependent on the preceding annealing processes were deposited by a wet‐chemical technique onto Ga2O3 thin‐film ceramics. The palladium clusters and their temperature‐dependent growth were detected using scanning electron microscopy micrographs and X‐ray photoemission spectroscopy measurements. The effect of the palladium dispersions on the gas‐sensitive behavior of the Ga2O3 ceramics was investigated in various O2/H2 mixtures in the N2 carrier gas at 700°C. The conductivity of the ceramics treated in this way was dependent on the O2 partial pressure, as well as on the H2 partial pressure of the surrounding gas atmosphere. The ceramic conductivity can be described as a function of the O2:H2 ratio, in accordance with the relation σ(pO2/pH2/)−1/3.
Preparation and Structural Investigation of Bi16(Sr,Ca)14O38
Authors: Wong‐Ng Winnie , Roth Robert S. , Rawn Claudia J.
Published: July 2010

Single‐crystal X‐ray diffraction was used to investigate the structure of the ternary oxide of approximate formula Bi14(Sr,Ca)12O33. The crystal examined was found to have a composition Bi2Sr0.68Ca1.07O4.75 or Bi16Sr5.44Ca8.56O38 as a result of refinement. It crystallizes in the monoclinic space group (C2/m), with cell parameters a= 21.764 (4) A, b= 4.3850 (13)° A, c= 12.905 (3) A, β= 102.72 (2)°, and V 1201.2 (5) A3. Strontium and calcium ions were found to substitute for each other. The structure consists of a network of alkaline‐earth oxide polyhedra with broad channels parallel to the b axis, in which the bismuth ions reside. Two out of four crystallographically distinct Bi ions form infinite Bi—O zigzag ribbons of edge‐linked Bi—O units, while each of the other two form oxygen corner‐shared Bi—O chains along b. The lone pair electrons of the Bi ions point toward each other in the channels.
Mechanics of Constant‐Rate Filter Pressing of Highly Flocculated Slurries
Authors: Kellett Bruce J. , Lin Chiang‐Yao
Published: July 2010

The growth of a powder compact from a highly flocculated slurry has been simulated by a computer program that models non‐steady‐state Darcian flow. Computer simulations have been compared with filter‐pressing experiments. Constant‐rate filter‐pressing experiments are divided into two regimes of piston stress—displacement behavior: an initial, almost‐linear, but concave‐up, regime during cake growth, followed by a second region of rapidly increasing piston stress when the piston comes into contact with the cake. Linear piston stress—displacement behavior is expected, from theory based on a uniform cake model. Highly flocculated slurries show highly nonlinear behavior. Nonlinear behavior is shown to be consistent with nonuniform growth of the cake. The permeability and consolidation behavior of the cake has been determined by a consolidometer experiment. Computer simulations indicate that the particle‐packing density profiles during cake build‐up are surprisingly similar during cake growth. Conditions for uniform consolidation can be determined from a general equation for non‐steady‐state Darcian flow. Results are directly applicable to constant‐flow‐rate pressure casting or slip casting.
Microstructural Development during the Microwave Sintering of Yttria—Zirconia Ceramics
Authors: Nightingale Sharon A. , Worner H. K. , Dunne D. P.
Published: July 2010

The microstructural development of 3 and 8 mol% yttria—zirconia ceramics during microwave sintering was studied and compared to conventionally heated samples. Microwave heating enhanced the densification processes occurring during constant‐rate heating for both materials. No change was found in the grain size—density relationship for the 8 mol% yttria—zirconia. However, a small, but statistically significant shift favoring densification was found for the 3 mol% yttriazirconia. Differences in the responses of the two materials are not completely understood but may be due to the differences in the activation energy for grain growth and grain‐boundary mobility.
Atomic Structure of a 36.8 (210) Tilt Grain Boundary in TiO2
Authors: Wallis D. J. , Browning N. D. , Nellist P. D. , Pennycook S. J. , Majid I. , Liu Y. , Sande J. B. Vander
Published: July 2010

An understanding of the atomic structure at internal interfaces is of crucial importance for the electronic and structural properties of most advanced materials. Here, we present a detailed study of the atomic structure at a [001] tilt grain boundary of σ5(210) in Tio2 (rutile). Z‐contrast imaging is used to obtain a 2‐D atomic map of the cation positions at the interface. Details of the charge state of cations and atomic structure around anion sites are then provided using electron energy loss spectroscopy. In particular, the spectroscopic data for oxygen is interpreted using multiple scattering theory to give 3‐D structural information. These combined techniques allow a unique grain boundary structure to be defined.
High‐Temperature Bridging in a Vitreous Bonded Alumina
Authors: Webb James E. , Jakus Karl , Ritter John E.
Published: July 2010

The load supported by crack wake bridges was studied in glassy alumina at 1240°C using short double cantilever beam specimens with a rear notch wedge. Direct observation of fracture surfaces revealed the sizes and shapes of the high‐temperature bridges. The bridged area was stochastic in nature but there was a clear trend toward decreased area bridged with increasing crack opening displacement consistent with existing models. Fracture surface observations on specimens without a rear notch were compared to bridge distributions in wedged specimens. Because of the complex bridge morphology the load supported by the bridges cannot be modeled as viscous uniaxial columns.
Effect of Zwitterionic Surfactants on Interparticle Forces, Rheology, and Particle Packing of Silicon Nitride Slurries
Authors: Ducker William A. , Luther Erik P. , Clarke David R. , Lange Fred F.
Published: July 2010

Phosphocholine (PC) zwitterionic surfactants, with different hydrocarbon chain lengths (C6C6PC to C9C9PC), were absorbed on the surface of silicon nitride near the isoelectric point (pH 6). Adsorption of the surfactants changed the lateral and normal surface forces, the rheology, and the consolidation behavior of the particles. The normal force between two silicon nitride surfaces as a function of separation and the lateral (friction) forces were measured using an atomic force microscope (AFM). These measurements indicated that surfactant adsorption reduced the magnitude of the long‐range attractive van der Waals force and produced a repulsive short‐range force. Although the adsorbed layers provided a barrier to particle contact, they could be ejected with a critical force that increased with the hydrocarbon chain length. The effect of an adsorbed layer on the viscosity and consolidation of slurries was also measured. The viscosity of all slurries decreased with increasing shear rate, indicative of attractive particle networks. The highest viscosity was observed for slurries formulated at the isoelectric point without added surfactant. Much lower viscosities were observed when the surfactant concentration was greater than the critical micelle concentration (cmc). A relative density of 0.46 was obtained via pressure filtration at 4 MPa without a surfactant, and between 0.46 to 0.59 (C6C6PC to C9C9PC, respectively) for surfactant concentrations greater than the cmc. Comparing force measurements with rheology and packing density provides a basis for discussing the role of interparticle forces in ceramic powder processing via colloidal routes.
Thermal Stability of Hexaaluminate Film Coated on SiC Substrate for High‐Temperature Catalytic Application
Authors: Inoue Hiroshi , Sekizawa Koshi , Eguchi Koichi , Arai Hiromichi
Published: July 2010

A coating of barium hexaaluminate (Ba0.75Al11.0O17.25) on an α‐SiC substrate and the thermal stability of the formed film were investigated for a high‐temperature catalytic application. The film prepared by sol coating consisted of BaAl2Si2O8 and α‐Al2O3 phases and always contained many cracks or exfoliations after heating at 1200C. A hexaaluminate porous film was successfully formed by slurry coating without void formation at the interface between the film and the substrate and exfoliation due to the formation of the intermediate layer after heating at 1200°C. The microstructure of the film remained unchanged, even after heating at 1300°C.
Intermediate‐Temperature Environmental Effects on Boron Nitride‐Coated Silicon Carbide‐Fiber‐Reinforced Glass‐Ceramic Composites
Authors: Sun Ellen Y. , Lin Hua‐Tay , Brennan John J.
Published: July 2010

The environmental effects on the mechanical properties of fiber‐reinforced composites at intermediate temperatures were investigated by conducting flexural static‐fatigue experiments in air at 600° and 950°C. The material that was studied was a silicon carbide/boron nitride (SiC/BN) dual‐coated Nicalon‐fiber‐reinforced barium magnesium aluminosilicate glass‐ceramic. Comparable time‐dependent failure responses were found at 600° and 950°C when the maximum tensile stress applied in the bend bar was 60% of the room‐temperature ultimate flexural strength of as‐received materials. At both temperatures, the materials survived 500 h fatigue tests at lower stress levels. Among the samples that survived the 500 h fatigue tests, a 20% degradation in the room‐temperature flexural strength was measured in samples tested at 600°C, whereas no degradation was observed for the samples tested at 950°C. Microstructure and chemistry studies revealed interfacial oxidation in the samples that were fatigued at 600°C. The growth rate of the Si‐C‐O fiber oxidation product at 600°C was not sufficient to seal the stress‐induced cracks, so that the interior of the material was oxidized and resulted in a strength degradation and less fibrous fracture. In contrast, the interior of the material remained intact at 950°C because of crack sealing by rapid silicate formation, and strength/toughness of the composite was maintained. Also, at 600°C, BN oxidized via volatilization, because no borosilicate was formed.
Neutron Diffraction Observations of Ferroelastic Domain Switching and Tetragonal‐to‐Monoclinic Transformation in Ce‐TZP
Authors: Kisi Erich H. , Kennedy Shane J. , Howard Christopher J.
Published: July 2010

In‐situ neutron diffraction has been used to study the plastic deformation of a tetragonal zirconia polycrystal stabilized with 12 mol% ceria under compressive loads up to 1.6 GPa. The development of significant plastic strain in the ceramic has been found to be due to a combination of ferroelastic switching and the tetragonal‐to‐monoclinic phase transformation, both beginning at} 1.2 GPa. Evidence of a strong coupling between the two phenomena is present. Both transitions are partially reversed on removal of the load. The linear elastic response of the a and c crystal axes of the parent tetragonal phase suggests that the ferroelastic switching occurs directly by a shear mechanism rather than via a cubic intermediate state. Anisotropic distortion of the tetragonal unit cell, as the critical stress is approached, gives some insight into the shear transformation mechanisms.
Particle‐Precipitation‐Aided Chemical Vapor Deposition of Titanium Nitride
Authors: Dekker Jan Pieter , Van der Put Paul J. , Veringa Hubert J. , Schoonman Joop
Published: July 2010

Particle‐precipitation‐aided chemical vapor deposition (PP‐CVD) is a modification of the conventional CVD process, where an aerosol is formed in the gas phase and particles are deposited on a substrate. The driving force for particle deposition is thermophoresis. The synthesis of titanium nitride (TiN) has been studied. TiN is formed on the substrate, as well as in the gas phase. At low temperature differences, only dense microstructures with equiaxed grains are observed; porous coherent layers are found in experiments where larger temperature differences are applied. Additional increase in the temperature difference only leads to loose powder deposits. In principle, the PP‐CVD process is a suitable method for the synthesis of thin porous layers of ceramics.
Dielectric and Ordering Behavior in PbxCa1−xTiO3
Authors: Ganesh Ramaratnam , Goo Edward
Published: July 2010

The dielectric properties and microstructure of PbxCa1−xTiO3 are reported. At room temperature, PbxCa1−xTiO3 is orthorhombic for compositions where x is 0.1 and 0.2, pseudocubic for compositions where x is 0.3 and 0.4, and tetragonal for compositions where x $0.5. TEM studies on the calcium‐rich compositions reveal the presence of 1/2{100}‐, 1/2{110}‐, and 1/2{111}‐type superlattice reflections in three principal zone axes. An analysis of crystal structure, based on the atomic displacements and the ordering of lead and calcium atoms, is reported. No ordering behavior is observed for the compositions where x is 0.1 and 0.2; however, partial ordering of lead and calcium atoms occurs for compositions where 0.3 # x # 0.60. The dielectric measurements do not reveal any relaxor behavior; however, the addition of calcium titanate to lead titanate broadens the width of the transition region. Calculation of the diffuseness parameter, δ, from the power‐law model also is presented.
Effect of Composition on Mullitization Behavior of α‐Alumina/Silica Microcomposite Powders
Authors: Sacks Michael D. , Wang Keyun , Scheiffele Gary W. , Bozkurt Nazim
Published: July 2010

The effect of composition on mullite formation was investigated using submicrometer composite particles that consisted of α‐alumina cores and amorphous silica coatings. Powders with varying alumina/silica ratios were prepared by varying the thickness of the silica coating. The mullitization behavior was monitored using differential thermal analysis, X‐ray diffractometry, and scanning electron microscopy. The results were consistent with previous studies that indicated that mullite forms initially by nucleation and growth within the siliceous phase, but also that chemical interdiffusion within the mullite grains is required to complete the reaction. The reaction rate in both stages was affected by the alumina/silica ratio. Available evidence has indicated that the first stage of the reaction is controlled by the dissolution of alumina in the siliceous phase. Results for the second stage suggested that alumina diffuses more rapidly through mullite than silica.
Electron Paramagnetic Resonance Investigation of Acceptor Centers in Pb(Zr,Ti)O3 Ceramics
Authors: Warren William L. , Tuttle Bruce A. , Rong E. Christopher , Gerardi Gary J. , Poindexter Edward H.
Published: July 2010

The nature of extrinsic point defects in lead zirconate titanate (PZT) ceramics of various compositions prepared by solution chemistry has been explored. Using electron paramagnetic resonance (EPR), several impurity sites have been identified in the as‐received materials, which include Fe3+oxygen vacancy (VO‥) complex and isolated Cu2+ ions; both of these ions are incorporated into the lattice by replacing the Ti(Zr) ion. A Fe3+VO‥ complex serves as a sensitive probe of the local crystalline field environment of the ceramic; the symmetry of this defect is roughly correlated with its phase diagram as the composition is varied from PbTiO3 to PbZrO3. The Fe3+VO‥ complex experiences tetragonal, rhombic, or orthorhombic symmetry as the composition is varied from PbTiO3 to PbZrO3. As the composition of the Cu2+ ion is varied, it appears as though the addition of Zr, and not necessarily a change in phase, is largely responsible in determining the local environment of this acceptor impurity. Also, the Cu2+ resonance parameters weakly reflect the relative Ti–O(Zr–O) bond covalency in the perovskite lattice.
Titanium Nitride/Carbon Coatings on Graphite Fibers
Authors: Liu Yin , Treadwell David R. , Kannisto Mike R. , Mueller Brian L. , Laine Richard M.
Published: July 2010

Titanium nitride (TiN) coatings were applied to graphite‐fiber tows by dip coating desized (900°C in nitrogen) fibers in hexane solutions containing 0.5, 1, or 2 equivalent wt% TiN as the precursor, [Ti(NMe2)x(μ–NCHMe2)4−2x]n. The precursor was characterized by thermogravimetric analysis (TGA), chemical analysis, and nuclear magnetic resonance. Bulk samples of precursor were heated to selected temperatures in argon. Based on microstructural and chemical analyses, two pyrolysis temperatures— 900°C and 1200°C— were selected for more‐detailed studies predicated on fiber oxidation‐resistance behavior. At 900°C, the bulk material exhibited a powder X‐ray diffractometry (XRD) pattern consistent with microcrystalline TiN, although some free carbon remained. XRD analysis of the samples pyrolyzed to 1200°C indicated a higher degree of crystallinity and some carbon incorporation in the TiN lattice; e.g., a TiCN solid solution forms, as supported by chemical and TGA analyses. Fibers coated with precursor were heated at a rate of 10°C/min in argon to selected temperatures, followed by a hold for 1 h, and the resulting coatings were characterized by scanning electron microscopy, X‐ray photoemission spectroscopy, and oxidation resistance at 700°C for 2 h in air. Coated fibers pyrolyzed to 900° and 1200°C were studied in detail. Fibers heat treated at 900°C exhibited better oxidation resistance than fibers pretreated at 1200°C, as determined by mass loss. Uncoated fiber tows fully oxidized after 80 min. The oxidation‐resistance studies on fiber tows pretreated at 900°C and coated with 0.5, 1, and 2 equivalent wt% TiN indicated that the sample with a coating of 1 wt%, with thicknesses of 0.1–0.2 μm, was the most stable. The coatings of 0.5 equivalent wt% TiN were irregular, whereas the coatings of 2 equivalent wt% TiN exhibited process‐related cracking, leading to poor oxidation resistance.
Influence of Spray Angle on the Pore and Crack Microstructure of Plasma‐Sprayed Deposits
Authors: Ilavsky Jan , Allen Andrew J. , Long Gabrielle G. , Krueger Susan , Berndt Christopher C. , Herman Herbert
Published: July 2010

To understand the influence of processing parameters on the microstructure of plasma‐sprayed deposits, smallangle neutron scattering measurements were made of the processing‐parameter‐dependent specific surface area of the voids in gray alumina deposits. These studies indicate that the voids are in the form of pores between the splats and around inclusions or unmelted particles, and are also in the form of cracks within the splats which may develop during cooling. The porous volume increases as the angle between the spray gun and the substrate (the “spray angle”) decreases. This study also indicates that the cracks are preferentially oriented, and that the crack orientation also depends on the spray angle. The interlamellar pores, however, are preferentially oriented parallel to the substrate surface, and the orientation of the pore is independent of the spray angle.
Subcritical Crack‐Growth Behavior of Borosilicate Glass under Cyclic Loads: Evidence of a Mechanical Fatigue Effect
Authors: Dill Sara J. , Bennison Stephen J. , Dauskardt Reinhold H.
Published: July 2010

Amorphous glasses are generally considered immune to mechanical fatigue effects associated with cyclic loading. In this study surprising new evidence is presented for a mechanical fatigue effect in borosilicate glass, in both moist air and dry nitrogen environments. The fatigue effect occurs at near threshold subcritical crack‐growth rates (da/dt 3× 10‐8 m/s) as the crack extension per cycle approaches the dimensions of the borosilicate glass network. While subcritical crack growth under cyclic loads at higher load levels is entirely consistent with environmentally assisted crack growth, lower growth rates actually exceed those measured under monotonic loads. This suggests a mechanical fatigue effect which accelerates subcritical crack‐growth rates. Likely mechanisms for the mechanical fatigue effect are presented.
Probing 3‐D Structural Distortions and Coordination Changes at SrTiO3 Grain Boundaries Using Electron Energy Loss Spectroscopy
Authors: Wallis David J. , Browning Nigel D.
Published: July 2010

Critical to a fundamental understanding of the structureproperty relationships in ceramics is the ability to address the key issues of like‐ion repulsion, partial occupancy, and composition at defect cores. Here, multiple scattering theory is used to produce a symmetry‐projected, site‐specific, density of states for defect structures observed experimentally in a SrTiO3 high‐angle tilt grain boundary. The real space atomic clusters used in multiple scattering calculations do not rely heavily on symmetry, thus permitting the structural origins of particular features to be readily identified in low‐symmetry systems. Results are compared directly with experimental electron energy loss spectra and give strong evidence for partial occupied columns of atoms at specific interface cores and the persistence of bulklike atomic correlations at the grain boundary.
Effect of Matrix Grain Growth Kinetics on Composite Densification
Authors: Sudre Olivier , Lange Fred F.
Published: July 2010

The effect of matrix grain growth kinetics on the densification behavior of a particulate composite has been investigated. Large alumina inclusions were added to two types of zirconia matrices having different grain growth kinetics, i.e., Zr(8Y)O2 and Zr(3Y)O2. Despite the evolution of different grain sizes, both matrices reached an end‐point relative density of 0.92 and 0.94, respectively, even though they contained 0.25 volume fraction of inclusions (based on total solid volume). However, the morphology and spatial distribution of the residual pores were distinct. The Zr(8Y)O2 matrix, which developed very large grains, had a dual microstructure with porous regions surrounded by a connective, denser region, as previously reported for an alumina matrix containing zirconia inclusions. Conversely, the Zr(3Y)O2 matrix was fully dense and uniform, but the composite contained large cracklike voids at inclusion/matrix interfaces. The large opening displacement of these cracklike voids locally relaxed the constraint to matrix shrinkage imposed by the inclusion network.
Effects of Oxidation on the Flaw‐Limited Strengths of Brittle Materials
Authors: Munro Ronald G.
Published: July 2010

The weakest‐link statistical model is used to derive an approximate relation describing the influence of oxidation or corrosion on the fracture strengths of brittle materials. Differences in strength under comparable short‐term oxidizing and nonoxidizing exposures are found to be proportional to δm/m2 where m is the Weibull modulus.
Local Dendrite Development during Seeded Peritectic Solidification of Large YBa2Cu3O7−δ Grains
Authors: Lo Wai , Leung Hin‐Tak , Cardwell David A. , Chow James C. L.
Published: July 2010

The growth morphology of large YBa2Cu3O7−δ grains during peritectic solidification has been reported to be responsible for the generation of processing defects, such as platelets, and an inhomogeneous distribution of 211 particles, both of which influence significantly the superconducting properties of the fully processed material. The present study demonstrates both experimentally and theoretically the formation of local dendrites at macroscopically planar YBa2Cu3O7−δ growth fronts which propagate along different crystallographic directions and identifies these as key growth features of the peritectic solidification process.
Correlation between Surface Area Reduction and Ultrasonic Velocity in Sintered Zinc Oxide Powders
Authors: Martin Louis Peter , Rosen Moshe
Published: July 2010

Specific surface area and longitudinal wave ultrasonic velocity were measured in zinc oxide (ZnO) powder compacts partially sintered to different pore fractions (P). A linear correlation was observed between the reduction in surface area and the increase in ultrasonic velocity at all P up to the onset of pore closure. During the initial stage of sintering, the ultrasonic velocity increased by as much as 300%, with negligible reduction in P, whereas during the intermediate stage, the surface area reduction was accompanied by increasing ultrasonic velocity and decreasing P. During the final stage of sintering, the pores closed and became isolated. The linear relation between the reduction in surface area and the increase in longitudinal wave velocity encompassed the entire range of porosity from completely unsintered to the onset of pore closure, thus spanning the transition from initial‐ to intermediate‐stage sintering. Ultrasonically inferred surface areas were used to evaluate the kinetics of the initial stage of sintering, and the results compared favorably with previously published values.
High‐Temperature, High‐Pressure X‐ray Investigation of Dicalcium Silicate
Authors: Remy Corinne , Andrault Denis , Madon Michel
Published: July 2010

Energy‐dispersive X‐ray powder diffraction experiments have been investigated at high temperature and room pressure, and at high pressure and room temperature, starting from either γ‐ or β‐Ca2SiO4. High‐temperature studies were performed up to 1980 K, using a versatile heating cell. The high‐temperature phase transformations previously described were reexamined. Volume and linear thermal expansions were measured for each Ca2SiO4 polymorph, γ, β, α′;L,α′;H, and α. Volume thermal expansion increases with increasing temperature except for α′;H, whose thermal expansion tends to decrease at elevated temperature. High‐pressure investigations were performed in the 0–15 GPa pressure range, using a diamond anvil cell, with silicon oil as the pressure‐transmitting medium. The value of the room‐pressure bulk modulus K0, assuming a second‐order BirchMurnaghan equation of state with K'0= 4, is 140(8) GPa for γ‐Ca2SiO4. The γ olivine form exhibits anisotropic compression, with the c axis as the most compressible. From such in situ high‐pressure X‐ray investigations, the γ‐→Ca2SiO4 phase transformation induced by cold compression is clearly evidenced and extends from 2 to about 5 GPa.
Chemical Interactions between La‐Sr‐Mn‐Fe‐O‐Based Perovskites and Yttria‐Stabilized Zirconia
Authors: Kindermann Lutz , Das Dasarathi , Bahadur Dhirendra , Wei Reinhard , Nickel Hubertus , Hilpert Klaus
Published: July 2010

Orthoferrite‐based perovskites are of interest as materials for the cathode in solid oxide fuel cells (SOFCs). Therefore, the chemical compatibility between perovskites of the composition (La1−xSrx)zFe1−yMnyO3−δ (0 # x # 0.3; 0.2 # y # 1; z = 0.90, 0.95, 1.00) and the solid electrolyte zirconia (ZrO2) doped with 8 mol% yttria (Y2O3) (8YSZ) has been investigated. Powder mixtures of the two materials have been annealed at different temperatures. The formation of monoclinic ZrO2 at 1000°C, as well as of La2Zr2O7 and SrZrO3 at 1400°C, has been determined in some samples. The reactions that are observed are discussed, with respect to the thermodynamic activities, tolerance factor, and oxygen‐ion migration energies. Some perovskite compositions seem to be compatible with Y2O3‐stabilized ZrO2 (YSZ), thereby offering the possibility to use orthoferrite‐based perovskites in SOFCs with a solid electrolyte made of YSZ.
Influence of Acidity on the Electrostatic Stability of Alumina Suspensions in Ethanol
Authors: Wang Gonghou , Sarkar Partho , Nicholson Patrick S.
Published: July 2010

Alumina (Al2O3) powders have been dispersed in ethanol (EtOH) solutions of different acidity. In aqueous media, acidity is defined by pH. This definition can be extended to nonaqueous media using ion‐transfer functions. The electrical charge on the particle surfaces has been found to be acidity dependent. The electrostatic stability of Al2O3EtOH suspensions has been evaluated via electrophoresis and turbidity. An electrostatic stabilization mechanism is proposed and analyzed via Derjaguin–Landau–Verwey–Overbeek (DLVO) theory.