bluuebunny/crossref_metadata_2025 huggingface dataset:
Aerosol Generation of Lanthanum Aluminate
Authors: Lux Byron C. , Clark Ronald D. , Salazar Abe , Sveum Larry K. , Krebs Michael A.
Published: March 2005

Spherical lanthanum aluminate powders approximately 1 μm in diameter were produced using spray pyrolysis techniques. These materials were characterized by X‐ray diffractometry and scanning electron microscopy. Two intermediate products were produced and identified as lanthanum oxide and aluminum oxide. These intermediates were present in varying amounts in all of the aerosol‐furnace‐prepared materials and reflect the short residence time of the aerosols in the furnace. Samples of each aerosol‐furnace preparation were further annealed in air for 1 h at 1100°C, and all of them converted completely to lanthanum aluminate.
Preparation of Coprecipitated Ferroelectric Ceramic Powders by Two‐Stage Calcination
Authors: Menegazzo Bruno Acácio , Eiras José Antonio
Published: March 2005

A coprecipitation method is described for preparing ferroelectric ceramic powders. The process is based on source solution preparation for coprecipitation after previous calcination of the constituent oxides or carbonates. Rhombohedral PZT 53/47 could be obtained by this method after calcination at 850°C. In addition, PLZT 9/65/35 was prepared by the same method.
Effective Elastic Moduli of Porous Ceramic Materials
Authors: Ramakrishnan N. , Arunachalam V. S.
Published: March 2005

This paper compares the applicability of a few theoretical models for determining effective elastic moduli, using published experimental data on ceramic materials in a porosity range of 0–40% and on a cellular material with a porosity of about 90%. As the experimental data for the effective Poisson's ratio involve a large scatter, a set of numerical experiments using the finite element method was carried out to obtain the variation of the effective Poisson's ratio with porosity. These variations show that the effective Poisson's ratio approaches 0.25 with increasing porosity, irrespective of the material Poisson's ratio. The effect of pore shapes on the effective elastic moduli and the Poisson's ratio has also been analyzed using FEM.
Infrared Spectroscopy of Wide Composition Range xNa2S + (1 –x)B2S3 Glasses
Authors: Bloyer Donald R. , Cho Jaephil , Martin Steve W.
Published: March 2005

Wide composition range studies of the IR spectra of xNa2S + (1 –x)B2S3 glasses are reported for the first time. Glasses can be prepared in two composition regions: 0 x 0.33 in a low‐alkali region and 0.55 x 0.80 in a highalkali region. The structures of glasses in the former region are dominated by the creation of tetrahedral boron units similar to those observed in the alkali borate glasses. For pure B2S3, a large fraction of six‐membered rings is found, as in glassy B2O3. As alkali is added, the vibrational frequency of the six‐membered ring mode decreases, but not the intensity, and a new band grows in which is used to propose, by analogy with the alkali borate glasses, that the fraction of tetrahedral borons increases with alkali sulfide content. These observations suggest that isolated six‐membered rings persist even in the presence of tetrahedral borons. For the high‐alkali glasses, both the six‐membered rings and tetrahedral boron structures are destroyed in preference to the formation of isolated orthothioborate groups, Na3BS3. The IR spectra of the high‐alkali glasses show a monotonic increase in symmetry and simplicity, indicating an increase in structural simplicity as the orthothioborate composition is approached. For the orthothioborate composition, both glass and polycrystal can be prepared. The IR spectra of the two phases are very similar, with the glass exhibiting a broadened absorption envelope.
Porosity Calculation of Binary Mixtures of Nonspherical Particles
Authors: Yu Ai‐Bing , Standish Nicholas , McLean Arnold
Published: March 2005

An investigation has been made of the packing of binary mixtures of either spherical or nonspherical particles. It is shown that this binary packing system can satisfactorily be described by the Westman equation. By use of the concept of “equivalent packing diameter,” nonspherical particle packing may be related to spherical particle packing. The porosity of binary mixtures of nonspherical particles can then be predicted by means of a model developed for spherical particles. This approach is verified by the good agreement between the calculated and experimental results.
Thermomechanical Fatigue Behavior of a Silicon Carbide Fiber‐Reinforced Calcium Aluminosilicate Composite
Authors: Butkus Lawrence M. , Holmes John W. , Nicholas Theodore
Published: March 2005

Isothermal fatigue and in‐phase thermomechanical fatigue (TMF) tests were performed on a unidirectional, continuous‐fiber, Nicalon®‐reinforced calcium aluminosilicate glass‐ceramic composite ([O]16, SiC/CAS‐II). Monotonic tensile tests were performed at 1100°C (2012°F) and 100 MPa/s (14.5 ksi/s) to determine the material's ultimate strength (σult) and proportional limit (σpl). Isothermal fatigue tests at 1100°C employed two loading profiles, a triangular waveform with ramp times of 60 s and a similar profile with a superimposed 60‐s hold time at σmax. All fatigue tests used a σmax of 100 MPa (40% of σpl), R= 0.1. TMF loading profiles were identical to the isothermal loading profiles, but the temperature was cycled between 500° and 1100°C (932° and 2012°F). All fatigued specimens reached run‐out (1000 cycles) and were tested in tension at 1100°C immediately following the fatigue tests. Residual modulus, residual strength, cyclic stress‐strain modulus, and strain accumulation were all examined as possible damage indicators. Strain accumulation allowed for the greatest distinction to be made among the types of tests performed. Fiber and matrix stress analyses and creep data for this material suggest that matrix creep is the primary source of damage for the fatigue loading histories investigated.
Calculation of the Electronic Structure of sp Elements in β‐Si3N4 with Correlation to Solubility and Solution Effects
Authors: Tanaka Isao , Niihara Koichi , Nasu Saburo , Adachi Hirohiko
Published: March 2005

Local electronic structures around the sp‐element solutes in β‐Si3N4 have been examined from first principles by the discrete‐variational (DV) Xα method for the first time. A solute atom, M, was put substitutionally into the Si sites of a (Si3N10)18‐ cluster, and Hartree–Fock–Slater equations were solved self‐consistently. The relationship between the calculated overlap population of electrons between the solute and nitrogen and the experimental solubility and the magnitude of the solution effects was examined, and a satisfactory correlation was found. Based on the present calculations, the Sp‐element solutes are classified into the following three categories: (1) Elements such as K, Ca, Na, and Mg are almost insoluble and localize at the intergranular glassy phase or precipitates. (2) Remarkable solution effects to lose the local covalency are expected for Li, Be, Ga, and Al. They may enhance the volume diffusivity and also decrease the elastic constants considerably. (3) For B, C, and Ge, no significant reduction in local covalency is expected when they dissolve. The correlations between the calculated overlap population and empirical ionic radii of solute atoms and Pauling's electronegativity were found to be weak, indicating that these empirical parameters are not sufficient to represent the electronic structure change due to the substitution and hence solubility and solution effects, although they are useful in describing general trends.
Chemical Vapor Infiltration of Boron Nitride Interphase in Ceramic Fiber Preforms: Discussion of Some Aspects of the Fundamentals of the Isothermal Chemical Vapor Infiltration Process
Authors: Cholet Vincent , Vandenbulcke Lionel
Published: March 2005

Boron nitride (BN) has been deposited in situ on carbon fibers of 3D fibrous preforms by isopressure/isothermal chemical vapor infiltration (ICVI) from BCl3–NH3–H2 mixtures, the aim being to produce a coating of uniform thickness in the whole preforms. The deposition temperature has been lowered to 773 K. Even at this temperature, the deposition rate is not totally controlled by the heterogeneous chemical kinetics; the optimization of the thickness uniformity necessitates an accurate control of both the convective plus diffusive mass transfer outside the preform and the diffusive mass transfer inside the porous medium coupled to an adjustment of the surface kinetics via the deposition temperature and the reactant composition. Under the best conditions, both the longitudinal and infiltration uniformities reach nearly 90% with a “relatively” high deposition rate (92 nm/h), and nearly similar deposition rates on bulk and porous substrates. On the other hand, still better infiltrations have been obtained by the use of protection screens (like honeycomb) to create a partial separation of the convective and diffusive mass transport influences. We propose that the basic insights deduced here be extended to the ICVI study of other chemical systems, and they should permit an easier extension to large‐scale reactors.
Effect of Nb2O5 Alloying on Thermal Expansion Anisotropy of 2 mol% Y2O3‐Stabilized Tetragonal ZrO2
Authors: Kim Dae‐Joon , Becher Paul F. , Hubbard Camden R.
Published: March 2005

Tetragonal ZrO2 (t‐ZrO2) solid solutions were prepared with addit ons of 2 mol% Y2O3 plus up to 0.45 mol% Nb2O5. The thermal expansion coefficients in both the a‐ and c‐axis lattice directions increased with Nb2O5 alloying and the thermal expansion in the c‐axis direction was greater than that in the a‐axis direction over the entire composition range. This anisotropic thermal expansion behavior was related to the 4‐fold coordination of Nb5+ with oxygen ions in t‐ZrO2 solid solutions in the system ZrO2–Y2O3–Nb2O5. The fracture toughness continuously increased with Nb2O5 alloying and suggested that the c/a axial ratio is a more significant factor than the internal stress that arises from the thermal expansion anisotropy, in the determination of the transformability of t‐ZrO2 in this system.
Cohesive Zone Approach to Interpreting the Fiber Push‐Out Test
Authors: Dollar Anna , Steif Paul S.
Published: March 2005

A means of inferring interface parameters from the fiber push‐out test is proposed. The significant feature of the approach is the use of a cohesive zone model to capture fiber‐matrix interfacial debonding. The approach is used to interpret push‐out data obtained from silicon carbide/silicon nitride composites. Comparisons are made with other analyses and with other interface models. It is shown that this approach may be particularly useful for composites in which there is substantial interfacial roughness, but little or no chemical cohesion. The advantage of the cohesive zone model in being able to predict the existence of qualitatively different load‐displacement curves associated with different specimen thicknesses is emphasized.
R‐Curve Behavior and Flaw Insensitivity of Ce‐TZP/Al2O3 Composite
Authors: Ramachandran Nageswaran , Chao Luen‐Yuan , Shetty Dinesh K.
Published: March 2005

A ceria‐partially‐stabilized zirconia‐aiumina (Ce‐TZP/Al2O3) composite optimized for transformation toughening was used to demonstrate its flaw insensitivity due to R‐curve behavior. Four‐point bend specimens fabricated with a controlled distribution of spherical pores showed nearly the same characteristic strength and strength variability (Weibull modulus) as specimens fabricated without the artificial pores. In situ observations confirmed stable growth of cracks initiated at pores and the crack lengths at fracture instability were much greater than the pore sizes, thus resulting in fracture strengths insensitive to the pores. The small variability in the fracture strength was found to be associated with variability in the R‐curve and the instability crack lengths. An analysis based on the fracture instability criterion for rising crack growth resistance accounted for the strength variability due to variability in the R‐curve. Comparable four‐point bend experiments were also conducted on a sintered yttria‐partially‐stabilized zirconia (2Y‐TZP) ceramic. This ceramic showed significant degradation of strength due to the presence of the pores. This flaw sensitivity is attributed to its steep rising R‐curve over short crack lengths.
Correlation of Machining‐Grain‐Size Effects on Tensile Strength with Tensile Strength–Grain‐Size Behavior
Authors: Rice Roy W.
Published: March 2005

Existing data on the effect of machining direction on the room‐temperature flexure strength (σ) of various ceramic materials are reviewed from the standpoint of their grain‐size (G) dependence and the correlation of this dependence with σ‐G‐1/2 behavior. The correlation shows that the reduction in strength from grinding specimens perpendicular versus parallel to their tensile axes is greatest at fine G, about zero at intermediate G, and then progressively increases again for larger G and for single‐crystal specimens (G=∞). The lower σ due to machining perpendicular to versus parallel to the tensile axis is due primarily to crack shape, not depth, i.e., elongated and approximately half‐penny cracks. Thus, the reduction in σ difference implies a reduction in flaw elongation at intermediate G. This is shown to correspond with the σ‐G‐1/2 behavior with larger G and finer G branches meeting when the flaw size is about equal to G. This result is supported by fractographic observations.
Structure and Properties at the Ferroelectric/Electrode Interface between Lead Zirconate Titanate and Copper
Authors: Cao Heng C. , De Graef Marc , Evans Anthony G.
Published: March 2005

In advanced actuator systems, ferroelectric ceramics are usually made into multilayered stacks to obtain optimum performance. This is done by interlacing noble metals (such as Pt/Pd) with ceramic green sheet and cofiring. The present study proposes an alternative, more economic electrode. Copper has been successfully diffusion bonded to several PZT‐based ferroelectric ceramics in a multilayer configuration. The microstructure and the relevant physical properties of the interfaces have been investigated. This has been achieved by using microscopy, as well as by mechanical and electrical characterization. These studies indicate that no reaction product forms at the interface and, furthermore, that the interface exhibits good cohesion.
Shape Evolution of a Coherent Tetragonal Precipitate in Partially Stabilized Cubic ZrO2: A Computer Simulation
Authors: Wang Yunzhi , Wang Hongying , Chen Long‐Qing , Khachaturyan A. G.
Published: March 2005

The kinetics of shape evolution of a tetragonal precipitate coherently embedded in a cubic matrix are examined. Specifically, the morphology of tetragonal ZrO2 particles in partially stabilized cubic ZrO2 is discussed. A computer simulation, carried out without any a priori constraint on possible kinetic paths and particle morphologies, shows that a lenslike shape appears during growth of a tetragonal particle. Upon further coarsening, the shape relaxes into a rhombus bounded by facets. Depending on the balance between interfacial and strain energies controlled by the particle size, the facets can be smoothly curved or straight. The predicted particle morphologies are in good agreement with the experimental observations. The kinetic model proposed is quite general for simulating microstructural developments during decomposition involving a crystal lattice symmetry change where elastic strain accommodation plays an important role.
Sol‐Gel Synthesis of High‐Quality Heavy‐Metal Fluoride Glasses
Authors: Dejneka Matthew , Riman Richard E. , Snitzer Elias
Published: March 2005

Fluoride glasses are excellent laser hosts and are very well suited for a broad spectrum of optical applications. However, current fluoride glass synthesis is expensive. The solgel method is an affordable alternative for producing highperformance, optical‐quality heavy‐metal fluoride glasses. The method involves forming a hydrous oxide gel of the constituent metal alkoxides and salts, polymerizing the solution to form a gel, fluorinating the gel with anhydrous HF, melting the amorphous material in an oxidizing atmosphere of SF6, and casting the melt into desired shapes. ZBLA (57ZrF4·36BaF2·4LaF3·3AlF3, in mol%) and Nd‐doped (0.3 mol%) ZBLA glass rods were prepared by this process and their properties were measured. The solgel‐based glasses had thermal and optical properties similar to those found in the literature for conventionally prepared fluorides.
In Situ Study on the Generation of Radiation Damage in Cubic‐Zirconia in the High‐Voltage Electron Microscope
Authors: Baufeld Bernd , Baither Dietmar , Messerschmidt Ulrich , Bartsch Martin , Merkel Ingolf
Published: March 2005

Deformed cubic‐zirconia single crystals have been irradiated in the high‐voltage electron microscope at room temperature. Prismatic dislocation loops of extrinsic character grow on {111} planes as secondary radiation damage, preferentially along dislocations. At about 200°C, the loops grow much faster and instantaneously transform into dislocations with a a/2〈110〉‐type Burgers vector.
Evidence of Grain‐Boundary‐Sliding‐Induced Cavitation in Ceramics under Compression
Authors: Blanchard Cheryl R. , Chan Kwai S.
Published: March 2005

Detailed microscopy of two crept aluminas, one with (AD99) and one without (Lucalox) a grain boundary glassy phase, has been performed to determine the pertinent damage mechanisms during creep. Evidence is presented for a nucleation‐controlled cavitation process where creep cavities nucleate primarily on two‐grain facets, followed by cavity growth and coalescence to form grain‐facet‐sized cavities and microcracks. A variety of creep cavity morphologies were observed in Lucalox, including spheroidal and irregularly shaped cavities. The latter finding implies a strong influence of crystallographic orientation and the corresponding surface energy of the cavitated planes on the cavity shape. In contrast, classical spheroidal cavities were observed in AD99 due to the presence of a viscous phase along grain boundaries. Direct evidence for grain boundary sliding as the process driving force for cavitation in Lucalox is presented together with evidence for the nucleation of creep cavities at grain boundary ledges. These findings are compared to the grain boundary sliding (GBS) and small‐angle neutron scattering (SANS) measurements performed previously on the same systems. Based on this study, the cavity nucleation process in the glassy‐phase‐ and non‐glassy‐phase‐containing aluminas is apparently similar as both involve the nucleation of rows of equally sized and equally spaced cavities.
Glass Transition and Infrared Spectra of Low‐Alkali, Anhydrous Lithium Phosphate Glasses
Authors: Hudgens James J. , Martin Steve W.
Published: March 2005

Anhydrous glasses in the series xLi2O + (1 ‐ x)P2O5 have been prepared and characterized in the range 0 ≤x≤ 0.5. FT‐IR spectroscopy and glass transition temperature measurements have been used to explore the structure and a key physical property of the low‐alkali phosphate glasses. The structure of v‐P2O5 is proposed to consist of a 3‐D network of trigonally connected tetrahedra decorated with a P=O unit. Contrary to what has long been proposed for these glasses, the addition of alkali degrades the 3‐D network through the generation of nonbridging oxygens rather than strengthen the network through the proposed alkali ion bridging. The Tg of v‐P2O5 is ∼653 K and decreases some 130 K with the addition of 10 mol% Li2O. Tg then reaches a minimum value at 20 mol% Li2O and increases with further Li2O additions. The increase in Tg, even though the fraction of nonbridging oxygens is still increasing, is interpreted in terms of an increasing entanglement of long‐chain PO2 groups in the glass. Such a structural transition from a 3‐D network of interconnected PO4 groups for P2O5 to a 1‐D chain structure for LiPO3 is one of the first examples of the importance of intermediate‐range order in governing the properties of glass.
Electromechanical Properties of Porous Piezoelectric Ceramics
Authors: Dunn Martin L. , Taya Minoru
Published: March 2005

A theoretical approach is forwarded to predict the electromechanical properties of porous piezoelectric ceramics. The analysis is able to account for the effects of porosity shape and concentration and is applicable to piezoelectric ceramics of arbitrary material symmetry. By coupling the exact solution for a single ellipsoidal pore embedded in an infinite piezoelectric matrix with an effective medium approximation, the theory considers, in an approximate manner, interaction effects at finite porosity concentrations. The theoretical estimates are developed using a matrix formulation which enables all elastic, dielectric, and piezoelectric moduli of the porous solid to be readily computed. Numerical results are presented to illustrate the effects of the shape and concentration of the porosity on the effective electroelastic moduli and transducer parameters of practical importance. Particular attention is devoted to assessing the sensitivity of the effective electromechanical properties to the accuracy of the input data. Finally, theoretical estimates are shown to be in good agreement with existing experimental results for porous piezoelectric ceramics with various microstructural geometries.
Effect of CaO on the Thermal Conductivity of Aluminum Nitride
Authors: de Baranda Pedro Sainz , Knudsen Arne K. , Ruh Edwin
Published: March 2005

The effect of CaO on the thermal conductivity of aluminum nitride pressureless sintered with 3 wt% Y2O3 as a sintering aid was investigated. Over the composition range of 0 to 2.0 wt% additions, CaO decreased the thermal conductivity of the sintered parts by 10%. CaO doping rendered the secondary oxide phases more wetting and thus with a greater tendency to penetrate along the grain boundaries. Furthermore, CaO segregation to the grain boundaries was observed even on those grain boundaries apparently free of secondary phases. These microstructural changes disrupted the connectivity of the high thermal conductivity AIN grains and were the main factors contributing to the decrease in the thermal conductivity of the ceramic parts. CaO additions to samples doped with SiO2 had the opposite effect, increasing the thermal conductivity. CaO removed SiO2 from the AIN grains and incorporated it into the oxide second phases, most likely through charge‐compensating substitutions Ca2++ Si4+ for Y3+ and/or Al3+. Thus, AIN samples containing both SiO2 and CaO had higher thermal conductivity than those containing comparable amounts of SiO2 alone.
Microstructure Evolution of SiC/Al2O3/Al‐Alloy Composites Produced by Melt Oxidation
Authors: Manor Eitan , Ni Hu , Levi Carlos G. , Mehrabian Robert
Published: March 2005

Alumina‐matrix composites with and without SiC particulates have been produced by directed melt oxidation of a multicomponent Al alloy. In the more general case, the microstructure consists of three interpenetrating phases: the SiC preform, a continuous α‐Al2O3 matrix, and a network of unoxidized metal. The volume fraction of metal within the oxidation product decreases with increasing processing temperature, and its distribution is less uniform when a preform is present. The preform does not show evidence of degradation by the molten alloy, but the growth front tends to climb up the particles, increasing the oxidation area and enhancing the rate of composite formation. The total porosity of the composite was found to increase with increasing Mg content, processing temperature, and/or SiC particle size. Porosity within the channels is associated primarily with insufficient metal flow to feed the solidification shrinkage. Larger pores are common in the SiC composites and seem to evolve by encroachment of interparticle spaces by the convoluted growth front. Coarse chemical segregation of the heavily alloyed residual metal is pervasive regardless of the presence or characteristics of the preform, but can be significantly refined by increasing the cooling rate of the composite after growth.
Color and Vanadium Valency in V‐Doped ZrO2
Authors: Ren Feng , Ishida Shingo , Takeuchi Nobuyuki
Published: March 2005

The distribution and chemical states of vanadium in V‐doped ZrO2 were studied to clarify the origin of the color of vanadium‐zirconium yellow pigment in comparison with vanadium‐tin yellow pigment. ESCA data and measurements of lattice constants of V‐doped ZrO2 revealed that vanadium was dissolved mainly as V4+ substituting for Zr in ZrO2 lattice, and its solubility limit was 0.5 wt% as V2O5. It was found that the yellow color of vanadium‐zirconium yellow was produced predominantly by the dissolved vanadium and that the contribution of vanadium oxide on ZrO2 grains to the yellow color was about 1/30 of that of the dissolved vanadium when compared on the basis of equimolar quantity of vanadium. Most of the undissolved vanadium oxide was in an amorphous or a poorly crystallized state.
Surface Roughness of Yttria‐Containing Aluminosilicate Glass‐Ceramics as Indicative of Their Machinability
Authors: Morita Kazuki , Umezawa Akira , Yamato Shinji , Makishima Akio
Published: March 2005

Several series of Y2O3‐containing aluminosilicate glasses were prepared by an ordinary melting method, and their physical properties, such as density and Vickers hardness, were measured. Heat treatment of these glasses at 1200°C precipitated tubular and hexagonal Ca4Y6O(SiO4)6 oxyapatite crystals which were characterized by scanning electron microscopy and X‐ray diffraction analysis. The homogeneously crystallized glass‐ceramics were lathed, and the machinabilities of these samples were estimated by measuring the lathed surface roughness. Then, the relationship between surface roughness and other properties was analyzed to optimize the glass composition. Substitution of BaO for CaO was conducted and the physical properties were examined.
Effect of Substitution of Ca on Thermal Expansion of Cordierite (Mg2Al4Si5O18)
Authors: Vepa Sai Sundar V. S. S. , Umarji Arun M.
Published: March 2005

The effect of substitution of calcium on the anisotropic axial thermal expansion of cordierite was investigated by using a high‐temperature X‐ray diffraction technique. The compositions were prepared by the sol–gel route. In the Mg2‐xCax‐Al4Si5O18 system, single‐phase cordierite can be prepared for x up to 0.5. Thermal expansion anisotropy (αa–αc) of cordierites reduces progressively by the substitution of increasing amounts of Ca for Mg.
Contributions of Transformation and Microcracking to the Plastic Deformation of Magnesia‐Partially‐Stabilized Zirconia
Authors: Batchelor Warren J. , Finlayson Trevor R. , Gross Anita K.
Published: March 2005

The measured plastic deformation of Mg‐PSZ under stress contains contributions from both transformation and microcracking. A method is described to separate the contributions from transformation and microcracking for samples deformed in uniaxial tension. Such a separation enables the ratio of the longitudinal to transverse strains arising from the transformation to be estimated. The results are compared to the predictions of the shear–dilatation model of Chen and Reyes‐Morel. Serious discrepancies are found between the experimental results and the predictions of the model and the possible reason for this is discussed.
Apatite Coating on Organic Polymers by a Biomimetic Process
Authors: Tanahashi Masami , Yao Takeshi , Kokubo Tadashi , Minoda Masahiko , Miyamoto Takeaki , Nakamura Takashi , Yamamuro Takao
Published: March 2005

Dense and uniform layers of a biologically active carbonate‐containing hydroxyapatite can be formed on various kinds of organic polymers by the following biomimetic method. First, a substrate is set in contact with particles of CaO–SiO2‐based glass soaked in a simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma for forming the apatite nuclei on the substrate. Second, the substrate is soaked in another solution highly supersaturated with respect to the apatite, e.g, with ion concentrations 1.5 times those of SBF (1.5SBF) for making the apatite nuclei grow on the substrate in situ. The induction period for the apatite nucleation, which is defined as the time of the first treatment required for forming enough of the apatite nuclei to make the continuous layer after the second treatment, was almost 24 h for most of the examined polymers. The adhesive strength of the formed apatite layer to the polymers was as high as 3 to 4 M Pa for poly(ethylene terephthalate), poly‐ether sulfone, and poly (vinyl alcohol) hydrogel. This type of apatite–organic polymer composite is expected to be useful for repairing not only living hard tissues but also soft ones.
Oxygen Transport in Aluminum Nitride Substrates
Authors: Solmon Héiène , Robinson David , Dieckmann Rüdiger
Published: March 2005

The diffusion of oxygen in commercially available aluminum nitride substrates has been studied by performing interdiffusion‐type experiments. The diffusion of oxygen both within AlN grains and along grain boundaries was investigated. By using as‐received AlN substrates and an electron microprobe as an analytical tool, it was found that the rate of oxygen diffusion along grain boundaries was strongly influenced by the presence of impurities and/or other phases at these boundaries. The diffusion of oxygen within AlN grains was studied between 1600° and 1900°C in a flowing nitrogen atmosphere by using secondary ion mass spectrometry (SIMS) for determining oxygen concentration profiles. The chemical diffusion coefficient of oxygen in the AlN lattice as a function of temperature is described by the equation
image

The oxygen concentration profiles determined by SIMS also show a contribution from diffusion along grain boundaries. Therefore, it was possible to determine values for the product δD′o (δ= width of grain boundaries, D′o= grain boundary diffusion coefficient of oxygen).
Creep of Duplex Microstructures
Authors: French Jonathan D. , Zhao Junhong , Harmer Martin P. , Chan Helen M. , Miller Gary A.
Published: March 2005

The tensile creep behavior of two ceramic composite systems exhibiting duplex microstructures was studied relative to their single‐phase constituents in the temperature and stress ranges of 1100–1350°C and 35–75 MPa. The equivolumetric compositions in the Al2O3:c–ZrO2 (8 mol% Y2O3) and Al2O3:Y3Al5O12 systems both exhibit lower creep rates than either of their single‐phase constituents. This effect is attributed to Y3+ (and possibly Zr4+) present in the A12O3 as a segregant which lowers the creep rate by ∼2 orders of magnitude. It is believed that the segregation of Y3+ to the A12O3 grain boundaries hinders the interface reaction believed to control the creep. If one of the single‐phase constituents is taken to be the Y3+‐doped Al2O3, the creep of the duplex microstructures can be modeled using standard composite theory applied to flow.
Origin of Density Gradients in Sintered β‐Silicon Carbide Parts
Authors: Ness Eric A. , Rafaniello William
Published: March 2005

Sintered densities of β‐silicon carbide compacts were observed to decrease as the thickness of the compact increased. The chemistry and density gradients of these compacts were analyzed, as well as the weight loss and the CO evolution during sintering. The sintering cycle was altered to include a vacuum hold between the temperatures of 1400° and 1700°C, which resulted in increased density. Depending on the temperature of the vacuum soak, densities of 98% could be achieved on tiles with a thickness greater than 2.5 cm. The density gradients within the compact were significantly reduced, but not totally eliminated. It is suggested that these gradients are due to the effect of microstructural coarsening which occurs simultaneously with CO evolution.
Metal Organic Resin Derived Barium Titanate; II, Kinetics of BaTiO3Formation
Authors: Kumar Suresh , Messing Gary L.
Published: March 2005

A physicochemical model has been developed for the kinetics of barium titanate formation from X‐ray‐amorphous, metal organic precursors by relating the changes in the physical structure of the precursor particles with the degree of transformation in isothermally heated powder samples. From electron microscopy and gas adsorption, it is evident that the precursor particles consist of 20‐to 60‐nm crystallites and < 10‐nm intraparticle pores. A Ba,Ti oxycarbonate phase forms on heating the Ba,Ti metal organic precursor, which subsequently decomposes to form BaTiO3 It is concluded that the formation of BaTiO3 follows the shrinking core model, and the overall transformation is rate‐controlled by the diffusion of CO2 through the nanometer‐size intraparticle pores.
High‐Temperature Crack Growth in Monolithic and SiCw‐Reinforced Silicon Nitride under Static and Cyclic Loads
Authors: Ramamurty U. , Hansson T. , Suresh S.
Published: March 2005

Experimental results are presented on subcritical crack growth under sustained and cyclic loads in a HIPed Si3N4 at 1450°C and a hot–pressed Si3N4–10 vol% SiCw composite in the temperature range 1300°–1400°C. Static and cyclic crack growth rates are obtained from the threshold for the onset of stable fracture with different cyclic frequencies and load ratios. Fatigue crack growth rates for both the monolithic and SiCw‐reinforced Si3N4 are generally higher than the crack growth velocities predicted using static crack growth data. However, the threshold stress intensity factor ranges for the onset of crack growth are always higher under cyclic loads than for sustained load fracture. Electron microscopy of crack wake contact and crack–tip damage illustrate the mechanisms of subcritical crack growth under static and cyclic loading. Critical experiments have been conducted systematically to measure the fracture initiation toughness at room temperature, after advancing the crack subcritically by a controlled amount under static or cyclic loads at elevated temperatures. Results of these experiments quantify the extent of degradation in crack–wake bridging due to cyclically varying loads. The effects of preexisting glass phase on elevated temperature fatigue and fracture are examined, and the creep crack growth behavior of Si3N4–based ceramics is compared with that of oxide‐based ceramics.
Effect of Al/Fe Ratio in Belite on the Microtexture Induced by the Remelting Reaction
Authors: Fukuda Koichiro , Maki Iwao , Ito Suketoshi , Yoshida Hideto , Kato Chika
Published: March 2005

During the remelting reaction, belite crystals with Al/Fe ratio <1 produced a liquid which formed droplets on the lamellar boundaries; the rate of remelting reaction was very low. Belite crystals with Al/Fe >1 produced a liquid which readily spread on the lamellae with a resulting high rate of reaction. At 1450°C the distribution coefficient of Al and Fe atoms between the belite and the interstitial liquid ((Al/Fe)Belite/(Al/Fe)Liquid) was about 0.8.
Generation and Mobility of Electronic Charge Carriers in Y1Ba2Cu3Ox
Authors: Yoo Han‐Ill , Lee Sang‐Min
Published: March 2005

The Hall effect and the conductivity of Y1Ba2Cu3Ox have been measured together for quenched specimens at room temperature as functions of oxygen content (x) in the range of 6 < × < 7. It has been reconfirmed that a hole is generated per every oxygen atom incorporated in the orthorhombic phase (x > 6.5) and per every two oxygen atoms in the tetragonal phase (x < 6.5). The exothermic enthalpy change for hole generation is 2 times or more larger in the tetragonal than in the orthorhombic phase. The mobility of the majority carrier holes has been found to vary discontinuously crossing the phase boundary (x ⋍ 6.5), reflecting a first‐order phase transition for these quenched specimens. It is independent of oxygen content in each structure: 0.34 ± 0.03 cm2/(V·s) in the tetragonal phase and 0.51 ± 0.06 cm2/(V·s) in the orthorhombic phase.
Effect of Interfacial Roughness Parameters on the Fiber Pushout Behavior of a Model Composite
Authors: Parthasarathy Triplicane A. , Barlage Douglas R. , Jero Paul D. , Kerans Ronald J.
Published: March 2005

The effect of interfacial roughness on the frictional sliding in composites has been studied using fiber pushout and pushback tests on a model composite of Plexiglas rods in an epoxy matrix. Different extents of roughness were introduced on the Plexiglas rods and the resulting roughness profiles measured. The roughness profiles were characterized using six different roughness parameters. An attempt was made to find a correlation between the sliding resistance and the selected roughness parameters. A parameter defined as the maximum coefficient in the Fourier transform of the roughness profile was found to yield the best correlation. If the roughness introduced is periodic, then the pushout traces exhibit periodic dips, but the magnitude of this periodic dip is significantly smaller than the seating drop obtained from pushback tests.
R‐Curve Behavior and Stable Crack Growth at Elevated Temperature (1500°–1650°C) in a Si3N4/SiC Nanocomposite
Authors: Rouxel Tanguy , Wakai Fumihiro , Sakaguchi Shuji
Published: March 2005

The crack growth resistance behavior and the stable crack growth regime of a Si3N4/SiC composite have been examined at high temperature (1500°–1650°C). SENB specimens were used and the load/unloading technique, with high deflection rates to ensure an elastic behavior, has been employed to estimate the crack lengths. Rising R‐curves have been obtained with a maximum crack growth resistance almost twice as high as the initial value. Above the Tg of the intergranular glassy phase, the behavior changes from brittle to visco‐plastic and, consequently, the fracture characteristics become strongly rate dependent. It is observed experimentally that in the enhanced ductile region the crack extension velocity during the stable crack propagation from a preexisting flaw decreases rapidly with time. This phenomenon has been tentatively attributed to dynamic crack‐tip stress relaxation resulting from the rapid flow of the glassy intergranular phase in the process zone. Thus, the rheological properties of the composite appear to be of major importance to gain insight into the mechanical behavior at such elevated temperatures.
Formation of N‐phase and Phase Relationships in MgO–Si2N2O–Al2O3 System
Authors: Huang Zhen‐Kun , Nunn Steve D. , Peterson Irene , Tien Tseng‐Ying
Published: March 2005

Formation of N‐phase in the system Mg,Si,Al/N,O was studied. Its composition was confirmed to be MgAl2Si4O6N4 (2Si2N2OMgAl2O4). Subsolidus phase relationships in the MgO–Si2N2O‐Al2O3 system were determined. The results are discussed by comparing with two similar systems, CaO‐and Y2O3–Si2N2O–Al2O3.
Use of Thin‐Film Substrates to Study Enhanced Solid‐State Phase Transformations
Authors: Kotula Paul G. , Erickson Dwight D. , Carter C. Barry
Published: March 2005

A thin‐film substrate geometry is described for the study of enhanced solid‐state phase transformations. A thin film of the seed material is grown on a suitable bulk substrate forming the thin‐film substrate. A sol of the material for which seeding is to be studied is then deposited and the sample heated. Cross sections are prepared to study both the extent of the transformation and its crystallography, using electron microscopy. The special feature of this approach is that the position of the original seed/coating interface is known precisely and the thickness of the transformed layer can be measured directly at different stages of the transformation. As an example of this approach, thin films of hematite have been used as substrates for the study of the seeded phase transformation of a boehmite‐derived transition‐alumina to α‐Al2O3
Recent Attempts to Detect Magnesium in a Heavily Doped Sapphire Bicrystal by Spatially Resolved Electron Energy‐Loss Spectroscopy
Authors: Bruley John , Hoche Thomas , Kleebe Hans‐Joachim , Rühle Manfred
Published: March 2005

Needle‐shaped spinel precipitates in a matrix of heavily MgO‐doped α‐AI2O3 bicrystal containing a near Σ= 11 tilt boundary were examined by spatially resolved electron energy‐loss spectroscopy. The Mg content was consistent with the precipitate being an alumina‐rich spinel of composition MgO Σ(Al2O3)2. The Al and Mg K and L2.3 absorption edge structures were recorded. No evidence could be found for preferential Mg segregation to the tilt boundary. Using the precipitates as a calibration standard, a limit of detection equivalent to about 0.01 monolayer is deduced. Comparison of the Al‐L2.3 edge recorded at the Σ= 11 boundary to that of the same boundary in undoped material suggests that the local structure is independent of Mg doping. The lack of Mg segregation to the Σ= 11 boundary is perhaps attributable to the low energy and close packing of this interface.
Development of Tantalum Pentoxide Coatings by Chemical Vapor Deposition
Authors: Graham David W. , Stinton David P.
Published: March 2005

Thermochemical considerations clearly indicate a need for the protection of SiC and Si3N4 materials from HC1‐ and Na2SO4‐induced corrosion. Ta2O5 has been identified as a material which may be suitable for such a protective coating. Thermodynamic calculations have indicated a range of temperatures, pressures, and reactant concentrations under which Ta2O5 coatings may be formed as a single phase by chemical vapor deposition. Ta2O5 coatings exhibiting a range of microstructures were obtained experimentally.
Shape Changes by {100} Lithium Fluoride Ridge‐Channel Arrays and of Lithium Fluoride Particles at Sintering Temperatures
Authors: Bullard Jeffrey W. , Glaeser Andreas M. , Searcy Alan W.
Published: March 2005

Parallel channels of rectangular cross section were etched into an LiF {100} single–crystal surface, by a photolithography technique, to produce ridge–channel arrays of controlled size and shape. The shape changes undergone by these arrays and by LiF isolated particles and powder beds at sintering temperatures were studied by SEM. The ridge–channel arrays and particles developed either faceted or rounded shapes, depending on temperature and atmosphere. Adsorption of an impurity, presumably H2O or O2, although it causes faceting,1 increases the rate of mass transport from the ridges to channels. The influence of array geometry on mass transport rates is compared to models to show that the rate‐limiting mass transfer step is probably a surface step when the surfaces remain faceted and may be a surface step when the surfaces are rounded.
Effect of Calcination on the Sintering of Gel‐Derived, Zirconia‐Toughened Alumina
Authors: den Exter Peter , Winnubst Louis , Leuwerink Theo H. P. , Burggraaf Anthonie J.
Published: March 2005

The densification behavior of ZrO2 (+ 3 mol% Y2O3)/85 wt% Al2O3 powder compacts, prepared by the hydrolysis of metal chlorides, can be characterized by a transition‐ and an α‐alumina densification stage. The sintering behavior is strongly determined by the densification of the transition alumina aggregates. Intra‐aggregate porosity, resulting from calcination at 800°C, partly persists during sintering and alumina phase transformation and negatively influences further macroscopic densification. Calcination at 1200°C, however, densifies the transition alumina aggregates prior to sintering and enables densification to almost full density (96%) within 2 h at 1450°C, thus obtaining a microstructure with an alumina and a zirconia grain size of 1 μm and 0.3–0.4 μm, respectively.
Effect of Yb3+‐Codoping on the Upconversion Emission Intensity in Er3+‐Doped ZBLAN Fluoride Glasses under 800‐nm Excitation
Authors: Inoue Satoru , Nukui Akihiko , Soga Kohei , Makishima Akio
Published: March 2005

The effect of Yb3+‐codoping on the upconversion emission intensity in Er3+‐doped ZBLAN fluoride glasses is investigated. The codoping of Yb enhanced the emission intensity for the samples excited by an 800‐nm laser diode beam. The enhancement in a constant YbF3 content (2 mol%) increased with increasing ErF3 content was about 70% of the initial value at 550 nm for the glasses containing 8 mol% of ErF3. The emission intensity at 550 nm in a constant ErF3 content (5 mol%) increased remarkably with the addition of YbF3 and was maximized around 7 mol% of YbF3 content, giving an increased ratio of about 200% of the initial value. The reason for the enhancement is discussed and it is derived mainly from two‐step excitation of Er3+ assisted by Yb3+ excited through the energy transfer from Er3+.
Porous Ceramic Filters Produced by Hot Isostatic Pressing
Authors: Nanko Makoto , Ishizaki Kozo , Fujikawa Takao
Published: March 2005

Porous TiO2 ceramics were produced by capsule‐free hot isostatic pressing (HIPing) and were evaluated by measuring gas permeability and pore size distribution. In comparison to conventionally sintered TiO2, the HIPed TiO2 has a higher permeability and narrower pore size distribution. The differences are caused by the effects of high‐pressure gas during sintering.
Microstructure of KTaxNb1−xO3 Thin Films on MgO (100) Single Crystals
Authors: Nazeri Azar , Kahn Manfred , Bender Barry , Allen Carlton
Published: March 2005

Thin KTaxNb1−xO3 (KTN) films were prepared by deposition of sol–gel precursor solutions on MgO (100) single crystals. Crystal structure and microstructure of the films as a function of processing parameters, such as rate, duration, and temperature of postdeposition heat treatment, were studied. Several techniques such as X‐ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were employed to analyze the films. It was observed that slow heating of KTN films promotes pyrochlore formation while fast‐firing of the films results in predominant formation of the perovskioe phase. In slow‐heated samples, TEM showed randomly oriented pyrochlore crystallites with a vermicular nanostructure of 10–30 nm with an interpenetrating porosity of the same range. In fast‐fired samples, large perovskite pockets with pyrochlore crystallites scattered among them were seen. The large perovskite grains were on the order of 0.1–0.5 μm, irregular in shape and porous. Transmission electron diffraction indicated these were single crystals, and ferroelectric domains were observed in them. Films of up to 1 μm thick were obtained by multiple deposition of the sol–gel KTN. Dense films were achieved when each layer was densified at 750°C for 2 h before the next layer was deposited.
X‐ray Diffraction Study of the t‐to‐m Phase Transformation in 12‐mol%‐Ceria‐Doped Zirconia at Low Temperatures
Authors: Zhu Hai‐Yan
Published: March 2005

The t‐to‐m phase transformation in a 12‐mol%‐ceria‐doped zirconia at subzero temperatures was studied in situ by XRD with a cryogenic sample stage. It was found that the t‐to‐m transformation continuously proceeded to 30 vol% as the temperature decreased to 125 K, when a burstlike transformation suddenly occurred with a transformation to 75 vol%. A preferred orientation of the (111)m plane parallel to the surface during the preburst transformation and XRD line broadening of both m and t phases after the burst were observed. The preburst transformation, which has not been revealed by previous dilatometry studies, is explained as surface martensitic transformation. This surface transformation occurs at higher temperatures than the burstlike transformation due to less matrix constraint and the higher probability of nucleation.