bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural Changes Induced on Mullite Precursors by Thermal Treatment: A 27Al MAS‐NMR Investigation
Authors: Sanz Jesus , Sobrados Isabel , Cavalieri Ana L. , Pena Pilar , de Aza Salvador , Moya Jose S.
Published: March 2005

NMR study of mullite precursors has shown that local arrangement of Al in samples synthesized by spray pyrolysis differs considerably from the one adopted by samples obtained by polymeric or colloidal routes. Aluminum is tetra‐ and pentahedrally coordinated in the first type of samples but is tetra‐ and octahedrally coordinated in the second ones. Segregation of SiO2 and Al2O3 is directly produced in colloidal preparation; however, this phenomenon occurs only in polymeric gels when they are heated between 980° and 1100°C. In polymeric samples, thermal treatment at ∼980°C produces the formation of γ‐Al2O3. A similar treatment in spray‐pyrolized powders gives directly 3:2 mullite. From these results, exothermic and expansive effects detected at ∼980°C were ascribed to changes in coordination of Al produced during the atomic rearrangement that accompanies formation of these two phases (γ‐Al2O3 or mullite). Above 1200°C, incorporation of Si in the Al‐rich phase induces the formation of 3:2 mullite in polymeric and colloidal samples.
Technique for Producing Mullite and Other Mixed‐Oxide Systems
Authors: Lewis Duane J.
Published: March 2005

Mixed‐oxide systems are of interest in a variety of technical ceramic applications. The key in many systems is to achieve a high degree of homogeneity, approaching the molecular level. A technique for producing mixed oxides via direct oxidation of mixtures of organometallic compounds is discussed as a potential route. Preliminary work performed on producing mullite is discussed.
Desiliconization of Mullite Felt
Authors: Zaykoski James , Talmy Inna , Norr Marriner , Wuttig Manfred
Published: March 2005

High‐temperature evaporation from 80% porous, rigid mullite (3Al2O3·2SiO2) whisker felt was studied under vacuum and at various helium pressures using gravimetry, X‐ray diffractometry, SEM, and EDS. Heat treatments at 1350° to 1550°C resulted in evaporation of SiO2 from mullite with a rate strongly dependent on temperature and pressure. A concentration gradient of SiO2 was observed through the cross section of samples after heating under vacuum, indicating that SiO2 preferentially evaporated from whiskers on the periphery of the samples. The SiO2 concentration gradient was accompanied by sharp microstructural changes across the specimens. The gradient was decreased by raising the ambient helium pressure during heat treatment. A mathematical model was developed to predict the SiO2 concentration profile. The model agrees well with experimental results and demonstrates that the diffusivity of SiO2 in the vapor phase controls the gradient of SiO2 through the cross section of mullite felt.
Tape Casting and Properties of Mullite and Zirconia–Mullite Ceramics
Authors: Boch Phillippe , Chartier Thierry
Published: March 2005

Mullite and ZrO2‐mullite ceramics have been prepared by tape casting mixtures of Al2O3, quartz, and ZrO2 powders and subsequent reaction sintering. Tape casting leads to homogeneous, high‐density green materials with good sinterability. The design of a thermal cycle which favors densification with respect to mullitization allows the preparation of nearly dense, nearly fully reacted materials at sintering temperatures below 1600°C. ZrO2 additions limit grain growth, but the ZrO2 content must not be too high when a high tetragonal:monoclinic ratio is required.
Perturbation Theory for the Transport Properties of Multicomponent Glass Melts
Authors: Shah Ritesh P. , Behrman E. C.
Published: March 2005

A first‐order theory is presented for the calculation of the effects of compositional variation on the transport properties of glass melts. The theory is based upon the modified Enskog theory applied to the high‐temperature expansion of the free energy. The theory is applied to a simple two‐component glass system and compared to experimental and previous theoretical results.
Anomalous Behavior of Ultrasonic Velocity in Rubidium Borate Glasses
Authors: Kodama Masao
Published: March 2005

By use of the ultrasonic pulse echo overlap method, ultrasonic velocity in rubidium borate glasses is measured as a function of composition at a temperature of 298 K, which is found to exhibit a maximum, a minimum, and another maximum in succession as the rubidium oxide content increases. This behavior is remarkable among the borate anomalies. The elastic property of these glasses is analyzed in terms of the three structural units defined as BØ3, Rb+BØ2O−, and Rb+BØ−4, where Ø represents a bridging oxygen and O− a nonbridging oxygen, from which the cause of this anomalous behavior is elucidated.
Simultaneous Chemical Vapor Deposition of Boron Nitride and Aluminum Nitride
Authors: Lee Woo Y. , Lackey W. J. , Agrawal P. K. , Freeman Garth B.
Published: March 2005

Two chemically different phases, hexagonal BN and AIN, were simultaneously produced by chemical vapor deposition (CVD) using an impinging jet reactor and the BCl3─AlCl3─NH3─Ar reagent system. The microstructure of the BN + AIN composite coatings was strongly dependent on temperature, pressure, and BCl3 and AlCl3 concentrations. The growth characteristics of BN and AIN in the codeposition system were similar to those expected from the single‐phase deposition processes (i.e., BN‐CVD and AIN‐CVD), except the growth of AIN whiskers was accentuated, and competition between BN and AIN deposition in the composites was suspected to be the cause of less‐crystalline deposits. In both BN + AIN‐CVD and AIN‐CVD, the growth of AIN whiskers became more apparent with increasing pressure or temperature. The codeposition behavior observed experimentally was compared with thermodynamic predictions.
Thortveitite‐Type Manganese (Mg, Zn, Ni) Vanadates
Authors: Liu Hui‐fang , Chang Luke L. Y.
Published: March 2005

By introducing manganese to (Mg,Zn,Ni) divanadates, a series of thortveitite‐type phases, Mn(MgxZnyNiz)V2O7 with x+y+z= 1, has been synthesized. The limits of x, y, and z can be represented by four phases: Mn(Mg0.2Zn0.8)V2O7, Mn(Mg0.5Zn0.5)V2O7, Mn(Mg0.1Zn0.8Ni0.1)V2O7, and Mn(Mg0.4Zn0.5Ni0.1)V2O7. Their cell dimensions are, respectively, a= 0.6734, 0.6737, 0.6749, and 0.6749 nm, b= 0.8806, 0.8819, 0.8816, and 0.8787 nm, c= 0.4748, 0.4750, 0.4752, and 0.4753 nm, and β= 102.16°, 102.15°, 102.23°, and 102.41°.
Effect of Oxidation on Single‐Fiber Pullout Interfacial Shear Stresses in a Silicon Carbide–Borosilicate Glass System
Authors: Barsoum Michel , Tung I‐Chung , Chou H.‐M.
Published: March 2005

The debonding, τd, and frictional, τf, interfacial shear strengths between 140‐μm diameter SiC (with a 1‐μm C‐rich coating in which SiC particles are embedded) monofilaments and borosilicate glass were measured using the single‐fiber pullout method as a function of postfabrication heat treatments [5 h at temperatures between 200° and 525°C] in 4% H2–96% Ar and air. The debonding shear stress remained virtually unchanged after extended H2 anneals. In air, however, τd, was found to initially increase and then subsequently decrease with increasing postfabrication heat treatment temperature. The initial increase in τd is due to enhanced bonding between the fiber and the borosilicate matrix as a result of the oxidation of the SiC particles embedded in the outermost carbon layer coating. At temperatures greater than about 450°C, oxidation of the C layer results in the severe degradation of the interfacial shear strengths.
Effect of Silicon Carbide Reinforcement on the Properties of Combustion‐Synthesized Titanium Silicide
Authors: Bhattacharya Arun K.
Published: March 2005

The formation of various phases in self‐propagating high‐temperature synthesis of titanium silicide with 15 wt% SiC was studied. It was observed that the addition of SiC as a reinforcing agent significantly influences the thermodynamics and kinetics of the combustion process when compared with the combustion synthesis of titanium silicide alone. Hardness and fracture toughness of this composite were directly obtained from the porous compact by using micro‐indentation technique and the fracture toughness of this composite was found to be significantly enhanced with the addition of SiC.
Failure Behavior of Cellular‐Core Ceramic Sandwich Composites
Authors: Van Voorhees Eric J. , Green David J.
Published: March 2005

The mechanical behavior of a novel ceramic sandwich composite system was investigated. This system was comprised of a low‐density, cellular alumina core bonded to dense alumina faceplates. The flexural strength and elastic properties of the core material alone were measured as a function of relative density. For the sandwich system, failure mechanisms and failure loads for a variety of core densities and face thicknesses were recorded. Initial failure mechanisms were observed to be exclusively core fracture but in a non‐catastrophic manner. In order to successfully predict these failure loads with existing theory, Weibull statistics were incorporated into the analysis. This allowed flexural strengths measured independently on the core material to be scaled to those expected from the stress distribution in the sandwich core.
Subsolidus Phase Relationships in Part of the System Si,Al,Y/N,O: The System Si3N4─AIN─YN─Al2O3─Y2O3
Authors: Sun Wei‐Ying , Tien Tseng‐Ying , Yen Tung‐Sheng
Published: March 2005

The subsolidus phase relationships in the system Si,Al,Y/N,O were determined. Thirty‐nine compatibility tetrahedra were established in the region Si3N4─AIN─Al2O3─Y2O3. The subsolidus phase relationships in the region Si3N4─AIN─YN─Y2O3 have also been studied. Only one compound, 2YN:Si3N4, was confirmed in the binary system Si3N4─YN. The solubility limits of the α′─SiAION on the Si3N4─YN:3AIN join were determined to range from m= 1.3 to m= 2.4 in the formula Ym/3Si12‐mAlmN16. No quinary compound was found. Seven compatibility tetrahedra were established in the region Si3N4─AIN─YN─Y2O3.
Tetragonal Tungsten Bronze Niobate, K0.2Sr0.4NbO3: A New Material for Capacitors with Flat Dielectric Curves
Authors: Boufrou Brahim , Desgardin Gilbert , Raveau Bernard
Published: March 2005

A new ceramic for capacitors, which exhibits flat dielectric curves with relative permittivity values (ɛ) as high as 3000 (Y7R or X7R class), has been synthesized. Use of a niobate, K0.2Sr0.4NbO3 (KSN), with a tetragonal tungsten bronze structure (TTB), as a major phase for the fabrication of such ceramics is shown for the first time. Nevertheless, the addition of perovskites as minor agents—PbMg1/3Nb2/3O3 (PMN)—is necessary to obtain stable temperature. Moreover, the use of lithium salts as sintering agents has been shown to be important for the fabrication of these ceramics.
Low‐Temperature Creep of Nanocrystalline Titanium(IV) Oxide
Authors: Hahn Horst , Averback Robert S.
Published: March 2005

Nanocrystalline TiO2 with densities higher than 99% of rutile has been deformed in compression without fracture at temperatures between 600° and 800°C. The total strains exceed 0.6 at strain rates as high as 10−3 s−1. The original average grain size of 40 nm increases during the creep deformation to final values in the range of 120 to 1000 nm depending on the temperature and total deformation. The stress exponent of the strain rate, n, is approximately 3 and the grain size dependence is d−q with q in the range of 1 to 1.5. It is concluded that the creep deformation occurs by an interface reaction controlled mechanism.
Electrical Properties of Sr0.7Ba0.3TiO3 Ceramics Doped with Nb2O5, 3Li2O · 2SiO2, and Bi2O3
Authors: Zhou Liqin , Jiang Zhihua , Zhang Shuren
Published: March 2005

The electrical properties of Sr0.5Ba0.3TiO3 in the presence of Nb2O5 as a donor, 3Li2O · 2SiO2 as a sintering agent, and Bi2O3 as a dopant have been studied. When the compositions of the ceramics were 1 mol Sr0.7Ba0.3TiO3+ 0.5 mol% Nb2O5+ 2 mol% 3Li2O · 2SiO2+ 0.2 mol% Bi2O3, the ceramics were sintered at 1100°C and exhibited the following characteristics: apparent dielectric constant ɛ, 25000; loss factor tan δ, 2%; insulating resistivity ρj, 1010Ω· cm; variation of dielectric constant with temperature Δɛ/ɛ (−25° to +85°C), +10%, −14%. ɛ and tan δ show only small changes with frequency. The study shows this ceramic can be used in multilayer technology.
Influence of Oxygen Vacancies on the Electronic Structure of Yttrium Oxide
Authors: Jollet François , Noguera Claudine , Gautier Martine , Thromat Nathalie , Duraud Jean‐Paul
Published: March 2005

The influence of oxygen vacancies on the electronic structure of yttrium oxide was investigated both experimentally and theoretically. By means of X‐ray absorption spectroscopy, at the yttrium K and L11 edges and at the oxygen K edge, information on the local densities of states of various orbital symmetries, around each type of atom, was obained. The total density of states in the valence band was studied using X‐ray photoelectron spectroscopy. Experimental results were compared with those obtained on stoichiometric yttria. The evolution of the densities of states is well reproduced by self‐consistent, semiempirical tight‐binding calculations applied to clusters of increasing size, in which oxygen vacancies are introduced. The charge transfer between oxygen and yttrium is modified, as experimentally observed from the study of the O KLL Auger line shape. The decrease of the local atomic charge on oxygen is theoretically confirmed by the tight‐binding calculations, showing a tendency toward a more covalent bond in nonstoichiometric yttria.
Etching of Silicon Carbide Materials at Elevated Temperatures in a Nitrogen‐Based Gas
Authors: Butt Darryl P. , Tressler Richard E. , Spear Karl E.
Published: March 2005

Two sintered SiC‐based materials were heat‐treated for 150 h at 1300°C in a nitrogen‐based gas (1.2% H2, 0.6% CO) at a total pressure of 130 Pa. Sintered SiC samples were also preoxidized and then exposed to this gas under the same conditions to evaluate the protective nature of an SiO2 scale. In this atmosphere, SiO gas and cyanogens are predicted to form, rather than SiO2. Experimental studies confirmed that etching of sintered SiC occurs. Preoxidation does not provide protection from etching, because of the rapid removal of SiO2 by H2 as H2O and SiO.
Three‐Dimensional Electron Paramagnetic Resonance Imaging Technique for Mapping Porosity in Ceramics
Authors: Kordas George , Kang Youn‐Ho
Published: March 2005

A three‐dimensional (3D) electron paramagnetic resonance imaging (EPRI) method was developed to probe the structure and size of pores in ceramic materials. The imaging device that was added to the EPR instrument consisted of a computer‐controlled current source and magnetic field gradient. This add‐on facility was tested using a well‐defined diphenylpicrylhydrazyl phantom sample. Pumice was then used to demonstrate the potential of the technique. This stone was immersed in a 0.5 mM15N‐substituted perdeutereted tempone water solution to fill the pores with spin labels. Images were reconstructed using a filtered backprojection technique. A two‐dimensional (2D) imaging plane was constructed by collecting 33 projection planes over 180°. A 3D image was derived from 22 planes each constructed by 22 projections. At present, the facility allows a resolution of 69 and 46μm for 2D and 3D imaging, respectively. Advancements of the imaging apparatus, software, and line width of the spin labels will be needed to enhance the resolution of this technique.
Cation Interdiffusion in Polycrystalline Calcium and Strontium Titanate
Authors: Butler Elizabeth Patricia , Jain Himanshu , Smyth Donald M.
Published: March 2005

A method has been developed to study bulk lattice interdiffusion between calcium and strontium titanate by fabrication of a diffusion couple using cosintering. The measured interdiffusion coefficients, D̃(C), indicate that strontium impurity diffusion in calcium titanate occurs at a faster rate than calcium impurity diffusion in strontium titanate. These interdiffusion coefficients are composition independent when the concentration of the calcium cation exceeds that of the strontium cation; otherwise D̃(C) is strongly composition dependent. Investigations into the effect of cation nonstoichiometry give results that are consistent with a defect incorporation reaction in which excess TiO2, within the solid solubility limit, produces A‐site cation vacancies as compensating defects. The interdiffusion coefficients increase with increasing concentrations of TiO2, so it is concluded that interdiffusion of these alkaline‐earth cations in their titanates occurs via a vacancy mechanism.
Heterogeneous Plantinum Nucleation and Crystallization of a Heavy‐Metal Fluoride Glass
Authors: Jewell John M. , Jaganathan James , Aggarwal Ishwar D.
Published: March 2005

The effect of controlled heterogeneous nucleation by platinum on the crystallization of a ZrF4‐BaF2‐LaF3‐AlF3‐NaF (ZBLAN) glass was studied. Various levels of platinum were incorporated into this glass by a combination of PtCl2‐doping and melting‐atmosphere variation. The effect of doping levels and melting conditions on the incorporation of platinum and the subsequent nucleation of crystals was studied by optical microscopy, scanning electron microscopy, differential scanning calorimetry (DSC), and X‐ray diffraction. Increased platinum in the glass resulted in an increased number of nucleation sites for the growth of β‐ZrF4‐BaF2 crystals. Analysis of isothermal and ramp‐rate DSC measurements indicated that the crystallization of this glass changed from surface controlled to bulk controlled with an increased number of nuclei. This was confirmed by optical microscopy. In addition, Avrami analysis of the isothermal crystallization data gave an accurate approximation of the number of nuclei in the glass.
A Monte Carlo Solution to the Problem of Diffusion in Grain Boundaries
Authors: Hodge James D.
Published: March 2005

A computer‐oriented Monte Carlo approach is used to generate solutions to the problem of simultaneous diffusion of particles along grain boundaries and in the bulk for polycrystalline materials. The calculation technique also utilizes a Monte Carlo technique to generate a random array of grains and grain boundaries through which particles diffuse. This technique has been used to calculate diffusional profiles for a range of grain sizes and relative mobilities. It is found that the exponential dependence of concentration on penetration depth, long characterized as being approximately equal to 1 for polycrystalline diffusion, is a function of the relative mobilities of material in boundaries versus the bulk.
Immobilization of Sodium in Perovskite
Authors: Vance Eric R. , Thorogood Gordon J.
Published: March 2005

Perovskite, a candidate phase for the immobilization of Nabearing nuclear waste, shows excellent Na leach resistance in deionized water, when hot‐pressed to 97% theoretical density at 1200°C. Surface reactions in two brines appeared to be minimal.
Static and Cyclic Fatigue of Alumina at High Temperatures
Authors: Lin Chih‐Kuang Jack , Socie Darrell F.
Published: March 2005

Fatigue behavior of alumina at 1200°C was investigated. Uniaxial tensile tests were conducted in both static and cyclic loading. A variety of loading wave forms were applied during the cyclic tests. Cyclic lifetime is found to be cycle shape dependent and controlled by the duration of the hold time at the maximum tensile stress in a cycle. Cyclic loading with a higher strain rate and a short duration of maximum stress during each cycle provides a beneficial effect on lifetime in comparison to static loading at the same maximum stress. The time to failure for cyclic loading with a longer hold time at maximum stress is very close to the static loading lifetime. Viscous boundary phase may be the primary contributor to the improved cyclic fatigue resistance for cyclic loading with a short duration of maximum stress.
Cell Models for Viscous Sintering
Authors: Scherer George W.
Published: March 2005

The kinetics of viscous sintering are determined for models consisting of cylinders intersecting so as to form cells with cubic, tetrahedral, and octahedral shapes. These models are shown to be quite resistant to Rayleigh instabilities, and to densify at similar rates. The constitutive parameters (free strain rate, uniaxial viscosity, and Poisson's ratio) are presented for all of the models.
The Effect of Green Density and the Role of Magnesium Oxide Additive on the Densification of Alumina Measured by Small‐Angle Neutron Scattering
Authors: Long Gabrielle G. , Krueger Susan , Page Richard A.
Published: March 2005

Small‐angle neutron scattering measurements were used to examine the effect of green density and the role of MgO additive on the evolution of the porous microstructure of alumina during intermediate‐ and final‐stage sintering. It was found that the initial connectivity in the green state plays a dominant role in establishing the channel diameters during the intermediate stage of sintering, and contributes also to determining the onset density at which the final stage of sintering begins. The role of MgO as a sintering aid lies, at least in part, in prolonging the stability of intermediate‐stage sintering such that the body achieves greater density before the transition to final‐stage sintering after which isolated pores are formed.
Role of Interfacial Debonding and Matrix Cracking in the Effective Thermal Diffusivity of Alumina‐Fiber‐Reinforced Chemical‐Vapor‐Infiltrated Silicon Carbide Matrix Composites
Authors: Hasselman D. P. H. , Venkateswaran A. , Tawil H.
Published: March 2005

The thermal diffusivity of a biaxial weave alumina‐fiberreinforced chemical‐vapor‐deposited (CVD) SiC composite heated to 1500°C, which is above the manufacturing temperature, was found to exhibit an increase for heat flow parallel to the fiber plane, whereas a decrease was observed perpendicular to the fiber plane. The increase parallel to the fiber plane was thought to be due to the annealing of the fibers and matrix. The decrease perpendicular to the fiber plane was found to be the result of interfacial debonding and matrix cracking within the plane of the fibers.
Carbon Interfacial Layers Formed by Oxidation of SiC in SiC/Ba‐Stuffed Cordierite Glass‐Ceramic Reaction Couples
Authors: Chaim Rachman , Heuer Arthur H.
Published: March 2005

Reaction couples between α‐SiC and cordierite (2MgO·2Al2O3·5SiO2═ Mg2Al4Si5O18) were prepared by sandwiching (and enclosing) SiC single crystals between plates of Ba‐stuffed magnesium aluminosilicate (Ba‐MAS) glass and hot‐pressing; the Ba‐MAS was subsequently crystallized at 1000° to 1200°C in argon or air. No reaction occurred at the SiC/Ba‐MAS interfaces during hot‐pressing, but crystallization heat treatments caused formation of amorphous carbon reaction layers at the SiC/cordierite interfaces, due to concurrent oxidation via the reaction SiC + O2→ SiO2+ C. The thickness of the carbon of the carbon layer was variable. These results suggest that formation of C layers at SiC/silicate interfaces in other composites (containing Nicalon fibers, for example) depends more on thermochemistry and less on the details of SiC nonstoichiometry than has heretofore been supposed.
Carbon–Nitrogen Pyrolyzates: Attempted Preparation of Carbon Nitride
Authors: Maya Leon , Cole David R. , Hagaman Edward W.
Published: March 2005

Precursors for the preparation of carbon nitride were selected from a number of relatively high nitrogen content organic materials. The precursors were pyrolyzed in a closed system at 700°C under 225 MPa. The resulting residues showed little nitrogen loss, but were amorphous with relatively low densities. Raman spectroscopy and 13C NMR indicate the presence of trigonal carbon.
Simple Method for Measuring the Friction Coefficient of Thin Fibers
Authors: Gupta Prabhat K.
Published: March 2005

A method is described for measuring the friction coefficient between a fiber, particularly thin fibers (with diameters down to a few micrometers), and a cylindrical substrate. The method allows for examination of the influences of the environment, temperature, and relative velocity between the fiber and the substrate.
Evaluation of the Crack Face Bridging Mechanism in a MgAl2O4 Spinel
Authors: White Kenneth W. , Kelkar Girish P.
Published: March 2005

The effectiveness of a grain bridging mechanism in the following wake zone of two MgAl2O4 spinel microstructures has been demonstrated through renotching experiments with double‐cantilevered‐beam (DCB) specimens. Measurements of dKR/dδa agree with previous crack growth resistance curves of similar materials, obtained from single‐edgenotch‐beam specimens. However, even the extended test ligaments available from the present DCB specimens did not permit the full development of the following wake zone of these coarse microstructures.
Theory of Filtration of Ceramics: II, Slip Casting on Radial Surfaces
Authors: Tiller Frank M. , Hsyung Niey B.
Published: March 2005

The theory of filtration of slip casting of incompressible beds of ceramic materials in planar molds is extended to deposition on internal and external cylindrical surfaces. Formulas are developed for (a) calculating the variation of the liquid pressure in both cake and mold as a function of the radius and (b) determining the time to produce a given thickness of a consolidated body. Assuming that the mold permeability Km and capillary suction Pcap are related by an inverse parabolic relation, KmPcap2=J, there is a mold with a specific permeability that produces a maximum rate of deposition from the slip. The theory presented in this paper strictly applies only to incompressible cakes and should be used cautiously with moderately compactible beds. For highly flocculated slurries yielding highly compressible bodies, the equations would not be expected to yield reliable results. Radial geometry leads to mathematical complexities not encountered in analyses of planar molds.
Cyclic Fatigue‐Crack Growth in a SiC‐Whisker‐Reinforced Alumina Ceramic Composite: Long‐ and Small‐Crack Behavior
Authors: Dauskardt Reinhold H. , James M. R. , Porter J. R. , Ritchie R. O.
Published: March 2005

The ambient‐temperature subcritical growth behavior of both long and microstructurally small cracks is investigated during cyclic‐fatigue loading in a SiC‐whisker‐reinforced alumina (Al2O3–SiCw) ceramic composite (fracture toughness, Kc∼ 4.5 MPa · m1/2). Based on long‐crack experiments using compact C(T) specimens, cyclic fatigue‐crack growth rates (over the range 10−11 to 10−5 m/cycle) are found to be sensitive to the applied stress‐intensity range and load ratio, and to show evidence of fatigue crack closure. Similar to other ceramic materials, under tension–tension loading the “long”(>3 mm) crack fatigue threshold, ΔKTH, was found to be on the order of 60% of Kc. Conversely, “small”(1 to 300 μm) cracks grown from micro‐indents on the surface of cantilever‐beam specimens were observed to grow at applied ΔK levels some 2 to 3 times smaller than ΔKTH. similar to behavior widely reported for metallic materials. The observed small‐crack behavior is rationalized in terms of the residual stress field associated with the indent, and the restricted role of crack‐tip shielding (from mechanisms such as crack bridging, closure and deflection) with cracks of limited wake, analogous to closure effects with small fatigue cracks in metals. Consistent with the lack of zone‐shielding mechanisms in Al2O3–SiCw, under variable‐amplitude cyclic loading crack‐growth rates do not exhibit the marked transient response following block overload sequences as do transformation‐toughened ceramics or ductile metallic materials. Possible mechanisms for cyclic crack advance in reinforced ceramic‐matrix composite materials are discussed.
Vapor Pressures of Bismuth, Lead, and Copper Components in Bi2Sr1.7CaCu2Oy and Bi1.7Pb0.3Sr1.7Ca2Cu3Oy Superconductor Ceramics
Authors: Sata Toshiyuki , Sakai Kazuo , Tashiro Suguru
Published: March 2005

The vapor pressures of the Bi system superconductor oxides Bi2Sr1.7CaCu2Oy (BSCCO) and Bi1.7Pb0.3Sr1.7Ca2Cu3Oy (BPSCCO) were determined by the transpiration method in atmospheres of O2, air, and N2, in the temperature range 730° to 82 0°C. The Bi/Cu atomic ratios of the vapor increased from 0.8 to 4.6 with increasing temperature for BSCCO, and the Bi/Cu/Pb ratio was 1.1/1/72 for BPSCCO at 800°C in O2. Vapor pressures for the vapor species Cu, CuO, Bi4O6, PbO, and Pb2O2 were calculated using the literature values. Vapor pressures for Cu were higher and those for both Bi and Pb lower than values estimated from the pure oxides. These values are discussed in the present paper in relation to crystal structure.
Fabrication and Properties of High‐Temperature Superconducting Wire by the Green‐Fiber Method
Authors: Halloran John W. , Hodge James D. , Chandler David , Klemptner Lori Jo , Neal Matthew , Parish Mark , Park Hyun , Pathare Viren , Bakis George , Eagles Dana
Published: March 2005

Continuous silver‐clad Y‐123 wire has been fabricated from cofired green fibers produced by melt‐spinning polymers loaded with Y‐123 powders. Metal claddings are provided by coating with silver‐alloy powder slurries. The sintered polycrystalline Y‐123 wires have 77 K self‐field critical current density (Jc) values up to 2900 A/cm2, with typical ranges in long lengths of about 1000 A/cm2, but are weak linked. High‐Jc wire is made by continuous melt texturing of the presintered wires. Values of Jc above 10 000 A/cm2 at 77 K have been achieved.
Metastable Phase Selection and Partitioning in ZrO2—MgO Processed from Liquid Precursors
Authors: Balmer Mari Lou , Lange Fred F. , Levi Carlos G.
Published: March 2005

Aqueous mixtures of either zirconium acetate or zirconium nitrate and magnesium nitrate were dried and subsequently pyrolyzed at fast heating rates (upquenching) to form metastable crystalline phases of ZrO2 with various degrees of MgO supersaturation. The crystallization temperature was determined to be 380°C for the zirconium acetate, and 270°C for the zirconium nitrate at a heating rate of 5°C/min. The crystalline structures were characterized as a function of MgO content and thermal history for specimens containing 0 to 30 mol% MgO. Upquenching to 900°C, where monoclinic (m) ZrO2 and MgO are the equilibrium phases, yielded single‐phase tetragonal (t) ZrO2 (<8 mol% MgO), single‐phase cubic (c) ZrO2 (9 to 17 mol% MgO), and two‐phase c‐ZrO2+ MgO structures (>17 mol% MgO). The composition for which T0(t/c) = 900°C was estimated as 9 ± 1 mol% MgO. Compositions crystallizing as metastable t‐ZrO2 (<8 mol% MgO) partitioned at higher temperatures and/or longer times into two‐phase mixtures, following the general sequence t→t+m→m+ MgO. Similarly, compositions forming metastable c‐ZrO2 (10 to 30 mol% MgO) partitioned in the following sequence: c→c+t+ MgO →t+ MgO →t+m+ Mgo →m+ Mgo. The initial phase selection and subsequent partitioning sequence are discussed in light of phase hierarchies predicted from thermodynamic concepts and kinetic constraints which are introduced by the solute partitioning required to achieve equilibrium.
Sol—Gel Synthesis of Strontium‐Doped Lanthanum Manganite
Authors: Bilger Serge , Syskakis Emmanuel , Naoumidis Aristides , Nickel Hubertus
Published: March 2005

Strontium‐doped lanthanum manganite powders were prepared using a peroxide acetate salt based solution. The stable sol was peptized by reacting ammonium hydroxide with the precursor solution. The amorphous dried gel powders exhibit a high energy level, due to their high cations coordination and small particles, to develop the perovskite phase. This crystalline phase development from powders containing monocarboxylate ligands was characterized by thermal analysis (TG, DTG, DTA), X‐ray diffraction, and IR spectroscopy. The transformation from amorphous powders into a crystallized homogeneous oxycarbonate phase in a first stage corresponds to an exothermal DTA peak at 270°C. X‐ray diffraction patterns and IR spectra showed similar behavior of the powders after complete organic removal, during the conversion into perovskite phase starting at approximately 630°C and achieved about 700°C and achieved about 700°C, as well as during the sintering process.
Deterioration of a Classical Final‐Stage Microstructure: A Study in Alumina
Authors: Thompson A. Mark , Harmer Martin P.
Published: March 2005

Classical final‐stage microstructures have been obtained in an ultra‐high‐purity alumina by a latex sphere impregnation and burnout technique. The isolated equilibrium‐shaped pores produced by the latex spheres deteriorated during long‐term, high‐temperature annealing under reducing atmospheres to produce a cracklike interconnected pore network. Several mechanisms of degradation are considered, including (i) the influence of entrapped gases, (ii) the effect of stresses produced during differential sintering, and (iii) the kinetics of competing mass transport mechanisms. Results suggest that the degradation is an inherent microstructural instability promoted by enhancing the ratio of coarsening rate/densification rate. Furthermore, it is shown that magnesia additions inhibit the process of microstructural deterioration.
Delayed‐Failure Resistance of High‐Purity Si3N4 at 1400°C
Authors: Tanaka Isao , Pezzotti Giuseppe
Published: March 2005

Delayed failure and creep behavior of high‐purity Si3N4 sintered without additives with a mean grain size of 1 μm has been measured at 1400°C. Lifetime under 300 MPa was >240 h, which showed good agreement with the value predicted in our previous report. Creep strain rate ranged from 1 × 10−5 to 3 × 10−5 h−1 between 200 and 360 MPa. These values demonstrate the excellent potential of high‐purity Si3N4 materials for structural application up to 1400°C.
Reactions Between Iron Oxides and Alumina‐Silica Refractories
Authors: MUAN ARNULF
Published: March 2005

Phase equilibrium data for the system FeO– Fe2O3–Al2O3–SiO2 are used to evaluate the attack of iron oxides on alumina‐silica refractories under various idealized conditions which approach those occurring in practice. The extent of attack and the nature of the reactions at various levels of O2 pressure are discussed.
Phase Relations in the System Iron Oxide–NiO–SiO2 under Strongly Reducing Conditions
Authors: Grutzeck Michael W. , Muan Arnulf
Published: March 2005

Liquidus and solidus phase relations have been determined for the system iron oxide–NiO–SiO2 under strongly reducing conditions obtained by using CO2–CO gas mixtures in controlled proportions. The phase relations were determined with the well‐known quenching method: oxide mixtures were equilibrated in vertical tube resistance furnaces, followed by quenching to room temperature and identification of phases with transmitted‐ and reflected‐light microscopy and X‐ray diffraction. Three crystalline phases are present on the liquidus surface: olivine (Fe2SiO4–Ni2SiO4 solid solutions), oxide (“FeO”–NiO solid solutions), and silica (tridymite or cristobalite, depending on temperature). The “ternary peritectic” point where these three phases coexist with liquid is at 1571°C, with a liquid composition of approximately 19 wt%“FeO”, 47 wt% NiO, 34 wt% SiO2.
Equilibrium Relations in the System NiO–TiO2 in the Temperature Range 1300° to 1750°C
Authors: Muan Arnulf
Published: March 2005

Phase relations in the system NiO–TiO2 have been determined by heating oxide mixtures in air at selected temperatures in the range 1300° to 1750°C for sufficient periods of time to attain equilibrium, followed by rapid quenching to room temperature. The phases have been characterized by optical microscopy, X‐ray diffraction, and electron microprobe analysis. The most striking feature is the presence, above 1430°C, of a spinel‐type phase that decomposes below this temperature to a mixture of remnant spinel, NiO of periclase‐type structure, and NiTiO3 of ilmenite‐type structure. There are two peritectic points in the system, one at 1730°C where spinel, NiO, and liquid coexist in equilibrium, and one at 1610°C where spinel, NiTiO3, and liquid are the coexisting phases. A eutectic is present at 1570°C, with NiTiO3, rutile, and liquid coexisting in equilibrium. Rapid transformation of the spinel phase, even during rapid quenching, imposes uncertainties on the interpretation of the experimental data obtained, but the equilibrium‐phase relations are deduced essentially as shown in the phase diagram presented. Results of a small number of calculated activities of NiO in oxide‐phase assemblages involving the spinel phase at high temperatures (∼1500°C) lend support to the interpretation of the phase relations as presented in this paper.
Activity–Composition Relations in Platinum–Chromium and Platinum–Vanadium Alloys at 1500°C
Authors: Pretorius Eugene B. , Muan Arnulf
Published: March 2005

Activity–composition relations in Pt–Cr and Pt–V alloys at 1500°C were determined from equilibration of these alloys with Cr2O3 and V2O3, respectively, in Co2–H2 atmospheres of known oxygen partial pressures, followed by quenching to room temperature and microprobe analyses of the quenched alloys. The activities of Cr and V were calculated from known standard free energies of formation of Cr2O3 and V2O3 from the respective elements, and the oxygen pressures were calculated from the C–H–O equilibrium in the gas phase. Both alloy systems show strong negative deviations from Raoultian behavior.
Activity–Composition Relations of Chromium Oxide in Silicate Melts at 1500°C under Strongly Reducing Conditions
Authors: Pretorius Eugene B. , Muan Arnulf
Published: March 2005

The solubility and activity–composition relations of chromium oxide in melts of the systems CaO–CrOx–SiO2 and CaO–Al2O3–CrOx–SiO2 have been determined at 1500°C by equilibrating melts with Pt–Cr alloys at known oxygen pressures. It is shown that the increase in the concentration of divalent chromium ions, as the oxygen pressure and the basicity of melt decrease, results in a dramatic increase in the solubility of chromium oxide in the liquid phase. An increase in the Al2O3 content of the melt leads to a decrease in the solubility of chromium oxide over the whole composition range studied. The activity coefficient of CrO has been found to increase with increasing melt basicity and decreasing oxygen pressure whereas the activity coefficient of CrO1.5 decreases sharply with increasing melt basicity for siliceous melts but levels off at a basicity ratio (wt% CaO/wt% SiO2) of about 0.7. An increase in the Al2O3 content of the melts results in an increase in the activity coefficient of CrO.
Spinel–Silicate Equilibria in the System MgO–FeO–Fe2O3–Al2O3–Cr2O3–SiO2
Authors: Schwessinger William T. , Muan Arnulf
Published: March 2005

We have investigated the distribution of Mg, Fe, Al, and Cr among olivine, orthopyroxene, spinel, and liquid in the system MgO–FeO–Fe2O3–Al2O3–Cr2O3–SiO2 for temperatures from 1309° to 1471°C under an atmosphere of constant CO2/H2= 10/1 at a total pressure of 1 atm. Crystal–liquid and crystal–crystal distribution coefficients defined (with Fe and Mg as examples) in the form Kc‐liqD= (Fe2+/Mg)crystal/(Fe2+/Mg)liquid, and Kc‐cD= (Fe2+/Mg)crystal 1/ (Fe2+/Mg)crystal 2, were calculated from the experimental data. The crystal‐liquid coefficients for the silicate minerals are relatively constant [Kol‐liqD (Fe2+/Mg) =0.33; Kopx‐liqD (Fe2+/Mg) = 0.29]. However, the crystal‐liquid coefficient for spinel is more variable [Ksp‐liqD (Fe2+/Mg) =1.2 to 1.7]. Chromium is strongly partitioned into spinel relative to coexisting liquid, olivine, and orthopyroxene (Dsp‐liqCr= 80 to 170, Dsp‐olCr= 100 to 230, and Dsp‐opxCr= 60 to 120), and Al is enriched in the liquid and the spinel relative to olivine and orthopyroxene. Values of (Al/Cr)opx/(Al/Cr)sp range from 4 to 8. The experimental results also show that the Al/Cr ratio in spinel increases with decreasing activity of SiO2 in the melts.
Activity–Composition Relations in FeCr2O4–FeAl2O4 and MnCr2O4–MnAl2O4 Solid Solutions at 1500° and 1600°C
Authors: Tsai Hwan‐Tang T. , Muan Arnulf
Published: March 2005

Activity–composition relations of FeCr2O4–FeAl2O4 and MnCr2O4–MnAl2O4 solid solutions were derived from activity–composition relations of Cr2O3–Al2O3 solid solutions and directions of conjugation lines between coexisting spinel and sesquioxide phases in the systems FeO–Cr2O3–Al2O3 and MnO–Cr2O3–Al2O3. Moderate positive deviations from ideality were observed.
Equilibrium Studies of the System′(Fe,Co)O′–(Fe,Co)3O4 by Solid‐State emf Measurements in the Temperature Range 970 to 1370 K
Authors: Lundberg Mats , Rosén Erik
Published: March 2005

The equilibrium reaction 3‘(Fe, Co)O’(ss) +1/2O2 (g) ⇄ (Fe, Co)3O4(ss) was studied in the temperature range 970 to 1370 K for seven different total compositions of molar ratios 0.5 < Fe/(Fe + Co) ≤ 1.0. The equilibrium pressures of oxygen were determined by using galvanic cells incorporating calcia stabilized zirconia as solid electrolyte and the Fe/Co ratios in the solid‐solution phases by wavelength dispersive spectrometry microprobe analyses. The activities of ‘FeO’ in the cobaltowüstite phase were then derived from the experimental results obtained.
Calculated Phase Relations in the Al–Li–O System at 1000 K with Implications for Refractories and Metal–Matrix Composites
Authors: Weirauch Douglas A. , Graddy G. E.
Published: March 2005

An isothermal section of the Al–Li–O system has been constructed at 1000 K using available thermodynamic data. Oxide stability is shown as a function of alloy composition and oxygen partial pressure pO2). Rigorous experimental verification of the calculated phase relations is precluded by the extremely low values of pO2) at equilibrium. The validity of the computed phase relations is considered from the results of laboratory studies of refractory compatibility and observations of interfacial phases in aluminum alloy matrix composites.
Activity of Nickel(II) Oxide in Silicate Melts
Authors: Pretorius Eugene B. , Muan Arnulf
Published: March 2005

Activity–composition relations of NiO have been determined at 1435°C in melts of the system CaO–NiO–SiO2 and at 1400°C in melts of the systems CaO–MgO–NiO–SiO2, CaO–MgO–NiO–Al2O3–SiO2, and CaO–MgO–NiO–K2O–SiO2. In the CaO–NiO–SiO2 and CaO–MgO–NiO–SiO2 systems the activity coefficient of NiO (γNiO) decreases as the polymerization of the melt increases (basicity decrease). γNiO stays contant up to several weight percent NiO for melts with similar basicity, an indication of Henry's law behavior. Minima in the NiO activity coefficient are observed in melts of the CaO–MgO–NiO–Al2O3–SiO2 and CaO–MgO–NiO–K2O–SiO2 systems at NBO/Si ratios between 1.0 and 1.5; i.e., γNiO decreases with decreasing basicity for NBO/Si ratios >1.5 and increases with decreasing basicity for melts with NBO/Si ratios <1.0 (NBO/Si; nonbridging oxygens per silicon). The addition of Al2O3 and K2O to the CaO–MgO–NiO–SiO2 system results in an increase in the activity coefficient of NiO.
Simple Geometric Model for Assessing Whisker Orientation in Axisymmetric SiC‐Whisker‐Reinforced Composites
Authors: Sandlin Michael S. , Lee Farnjeng , Bowman Keith J.
Published: March 2005

The nonuniform crystal structure of SiC whiskers complicates the interpretation of X‐ray texture data collected from whisker‐reinforced composites. A simple model for assessing whisker textures in alumina matrix composites which is able to account for the mixed polytype structure of SiC whiskers is discussed. Synchrotron X‐ray radiation was used to quantitatively measure SiC whisker texture in hot‐pressed and hot‐forged Al2O3–SiC whisker composites. Pole figures of the 2.51 Å reflection ({111} cubic, (001) hexagonal) were obtained and compared to pole figures calculated from hypothetical whisker axis disribution describing the whisker orientation distribution within the axisymmetric composites. The X‐ray data suggest that the degree of reinforcement phase texture in hot‐worked specimens is dependent upon final specimen strain. The data also demonstrate that for conventionally hot‐pressed composites the whiskers are angularly distributed about the compression plane, having an average orientation which does not approximate a two‐dimensionally random distribution.
Enhanced Densification by Seeding of Sol‐Gel‐Derived Aluminum Titanate
Authors: Prasadarao A. V. , Selvaraj Ulagaraj , Komarneni Sridhar , Bhalla Amar S. , Roy Rustum
Published: March 2005

Aluminum titanate was prepared by two different methods. A monophasic gel was obtained by cohydrolysis and polycondensation reactions of chelated aluminum sec‐butoxide and titanium isopropoxide. A diphasic gel was obtained from boehmite and anatase. Effects of homogeneous dispersion of magnesium oxide and iron oxide incorporated as alkoxides in the monophasic gel and also the effects of isostructural seed on the crystallization and densification of aluminum titanate obtained from monophasic and diphasic gels were investigated. Seed particles of MgTi2O5 facilitated lowering of crystallization temperature and enhanced densification of aluminum titanate. Microstructure studies indicated that almost‐microcrack‐free sintered bodies with smaller grains were formed from the seeded monophasic and diphasic gels.
Fabrication and Fracture Behavior of Novel SiC Ceramics Having Rodlike Grains
Authors: Kodama Hironori , Miyoshi Tadahiko
Published: March 2005

A new type of silicon carbide having rodlike grains has been fabricated by hot‐pressing or hot‐pressing followed by hot isostatic pressing using SiC whiskers as the raw powder. Preliminary relationships between sintering conditions and microstructure and fracture toughness of the processed ceramics were established. Ceramics with relative density greater than 98% were prepared at temperatures of 2050°C or greater. The fracture toughness of these ceramics was better than that of ordinary SiC, and its maximum value was 7.3 MPa · m1/2. Grain pullout, grain bridging, and crack deflection were considered to be the main operative mechanisms which led to improved fractrue toughness.