bluuebunny/crossref_metadata_2025 huggingface dataset:
Packing of Ternary Mixtures of Nonspherical Particles
Authors: Yu Ai‐Bing , Zou Rui‐Ping , Standish Nicholas
Published: March 2005

An experimental investigation of the packing of some ternary mixtures of nonspherical particles has been conducted. The double extrapolation method is used to determine the packing density of a particle mixture in infinite space. The application of experimental design to particulate mixture problems is demonstrated. The results indicate that the similarity between the packing systems of spherical and nonspherical particles can be readily depicted in terms of specific volume variation.
Synthesis of Highly Oriented Lead Zirconate–Lead Titanate Film Using Metallo‐organics
Authors: Hirano Shin‐ichi , Yogo Toshinobu , Kikuta Koichi , Araki Yasushi , Saitoh Masahiro , Ogasahara Satoru
Published: March 2005

Crack‐free Pb(Zr,Ti)O3 (PZT) thin films with preferred orientation were prepared successfully on MgO (100), SrTiO3 (100), and Pt/Ti/SiO2/Si substrates from metal alkoxide solutions. Calcination of precursor films in a H2O─‐O2 gas mixture was found to be effective not only for low‐temperature crystallization of perovskite PZT, but also for obtaining the preferred orientation of PZT films. Single‐phase PZT films with high preferred orientation were synthesized on MgO (100) and Pt/Ti/SiO2/Si substrates at 550° and 600°C for 2 h, respectively. The PZT film on the Pt/Ti/SiO2/Si substrate showed a permittivity of 520, tan δ of 0.03, a remanent polarization of 24 μC/cm2, and a coercive field of 54 kV/cm.
2223 Phase Formation in Bi(Pb)─Sr─Ca─Cu─O: I, The Role of Chemical Composition
Authors: Chen Yan Ling , Stevens Ronald
Published: March 2005

The formation of superconducting phases in the Bi(Pb)─Sr─Ca─Cu─O system has been systematically investigated using DTA/TG, XRD, SEM/EDAX, TEM, EPMA, ICP‐AES, fourprobe dc resistance, and ac susceptibility. Starting compositions, firing temperature, and the duration of heat treatment, together with the atmosphere, were found to be critical in determining the preferred formation of the 2223 phase. This paper reports the effect of the initial chemical composition, emphasizing the importance of compositional control in the synthesis of the single 2223 phase. It has been shown that, with a correct starting composition and predetermined synthesis conditions, single 2223 phase can be obtained without intergrowth by the 2212 and other impurity crystalline phases. The optimum starting composition for the preferred growth of the 2223 phase was identified as being Bi1.7Pb0.3+ySr2Ca2Cu3Ox (y= 0.1), with excess Pb added in order to compensate for its loss at high temperatures. The effect of Pb doping and excess Cu on the phase formation in the Bi oxide based superconducting system is discussed.
Crystallization and Phase Transformation Process in Zirconia: An in situ High‐Temperature X‐ray Diffraction Study
Authors: Srinivasan Ram , Davis Burtron H. , Cavin O. Burl , Hubbard Camden R.
Published: March 2005

Amorphous zirconia precursors were made by the precipitation of a zirconium tetrachloride solution with either slow (8 h) or rapid additions of ammonium hydroxide at a pH of 10.5. Following calcination at 500°C for 4 h, the rapidly precipitated precursor exhibited predominantly monoclinic ZrO2 phase, while the slowly precipitated precursor produced the tetragonal ZrO2 phase. The crystallization and phase transformations were followed by in situ high‐temperature X‐ray diffraction (HTXRD) for both specimens in helium and in air. Each amorphous precursor first crystallizes as the tetragonal phase at about 450°C. A tetragonal‐to‐monoclinic phase transformation of the rapidly precipitated material was observed on cooling at about 275°C. Surface impregnation of sulfate ions following precipitation inhibited the tetragonal‐to‐monoclinic transformation for the rapidly precipitated ZrO2 sample. The crystallite size for the t‐ZrO2 of all samples, irrespective of whether they transform to monoclinic, was approximately 11 nm, indicating that the t→m transformation in these materials is not controlled by differences in crystallite size. It is therefore suggested that anionic vacancies control the tetragonal‐to‐monoclinic phase transformation on cooling, and that oxygen adsorption triggers this phase transformation.
Creep Mechanism of Polycrystalline Yttrium Aluminum Garnet
Authors: Parthasarathy Triplicane A. , Mah Tai‐Il , Keller Kristen
Published: March 2005

The high‐temperature deformation behavior of a fine‐grained polycrystalline yttrium aluminum garnet (YAG) was studied in the temperature range of 1400° to 1610°C using constant strain rate compression tests under strain rates ranging from 10−5/s to 10−3/s. The stress exponent of the creep rate, the activation energy in comparison with that for single‐crystal YAG, and the grain size dependence suggest that Nabarro–Herring creep rate limited by the bulk diffusion of one of the cations (Y or Al) is the operative mechanism.
Analysis of Residual Stress in 6H‐SiC Particles within Al2O3/SiC Composites through Raman Spectroscopy
Authors: DiGregorio John F. , Furtak Thomas E.
Published: March 2005

We measured the Raman spectrum associated with the E2‐TOquasi and LOquasi modes of 6H‐SiC particles as a function of hydrostatic pressure using a diamond anvil cell. The results of this calibration experiment were used to analyze the residual stress in 6H‐SiC particles within Al2O3/SiC composites with 12%, 20%, and 30% SiC by volume. The Raman spectra show that residual stress in the SiC near the surface of the composites is −2040 ± 120, −1841 ± 110, and −1615 ± 100 MPa for the 12%, 20%, and 30% SiC composites, respectively. The measured decrease in stress with increasing packing fraction is consistent with theoretical predictions based on micromechanics.
Improved Flaw Tolerance in Alumina Containing 1 vol% Anorthite via Crystallization of the Intergranular Glass
Authors: Padture Nitin P. , Chan Helen M.
Published: March 2005

The intergranular phase in an alumina containing 1 vol% anorthite glass was crystallized in order to enhance internal residual stresses within the microstructure. The influence of crystallization on the mechanical behavior was investigated by the indentation–strength method. Such crystallization was found to result in a marked improvement in the flaw tolerance of this alumina, indicative of strong R‐Or T‐curve behavior. These results are discussed in the light of a theoretical model which assumes grain‐localized crack bridging to be the predominant toughening mechanism. Particular reference is made to the influence of residual stresses and interfacial properties on grain pullout across the crack walls in the wake of the crack tip.
Eutectic Composition in the Pseudobinary of Y4Al2O9 and Y2O3
Authors: Mah Tai‐Il , Petry M. Dennis
Published: March 2005

The eutectic composition between Y4Al2O9 and Y2O3 was determined using electron probe microanalysis (EPMA) on directionally solidified specimens with hypo‐ and hypereutectic compositions. The microstructures of the specimens as a function of composition differ considerably with small deviation from the eutectic composition (70.5 mol% Y2O3 and 29.5 mol% Al2O3). Based on the current results and other published data, the pseudobinary system between Al2O3 and Y2O3 is revised.
Low‐Temperature Nucleation of Rutile Observed by Raman Spectroscopy during Crystallization of TiO2
Authors: Ocaña Manuel , Garcia‐Ramos Jose V. , Serna Carlos J.
Published: March 2005

The thermal evolution of amorphous TiO2 powders, consisting of spherical particles and prepared by hydrolysis of a titanium ethoxide aerosol, was studied by using Raman spectroscopy. On calcination at 350°C, the solid crystallized, giving anatase as a major phase. A small amount of rutile was also detected and attributed to small seeds localized at the particle outlayer. The nucleation of rutile at so low a temperature was ascribed to the presence of organic impurities in the powders. The transformation of anatase into rutile was clearly observed after heating at 660°C.
Ceramic Fuel Cells
Authors: Minh Nguyen Q.
Published: March 2005

A ceramic fuel cell in an all solid‐state energy conversion device that produces electricity by electrochemically combining fuel and oxidant gases across an ionic conducting oxide. Current ceramic fuel cells use an oxygen‐ion conductor or a proton conductor as the electrolyte and operate at high temperatures (>600°C). Ceramic fuel cells, commonly referred to as solid‐oxide fuel cells (SOFCs), are presently under development for a variety of power generation applications. This paper reviews the science and technology of ceramic fuel cells and discusses the critical issues posed by the development of this type of fuel cell. The emphasis is given to the discussion of component materials (especially, ZrO2electrolyte, nickel/ZrO2cermet anode, LaMnO3cathode, and LaCrO3interconnect), gas reactions at the electrodes, stack designs, and processing techniques used in the fabrication of required ceramic structures.
Superconducting Properties of Aerosol‐Generated YBa2Cu3O7–8 Powders
Authors: Zhou Derong , Biswas Pratim , Oostens Jean , Boolchand Punit
Published: March 2005

YBa2Cu3O7–8 superconducting powders are generated by an aerosol route and classified into different size intervals in a cascade impactor. Two length scales (penetration depth, λ, and coherence length, ζ) characterizing superconductivity in aerosol‐produced superconducting grains are established for the first time from magnetization measurements. Representing the aerosol size distribution by a lognormal function, the susceptibility (χ) versus particle diameter (dp) relation is analyzed to obtain the penetration depth (λ). The lower and upper critical field strengths, Hc1 and c2, are established for the samples. Intraparticle critical current densities are computed from the magnetization results for these aerosol‐processed powders.
Phase Equilibria in the System ZrO2─InO1.5
Authors: Sasaki Kazunari , Bohac Petr , Gauckler Ludwig J.
Published: March 2005

Phase equilibria in the system ZrO2─InO1.5 have been investigated in the temperature range from 800° to 1700°C Up to 4 mol%, InO1.5 is soluble in t‐ZrO2 at 1500°C. The martensitic transformation temperature m→t of ZrO2 containing InO1.5 is compared with that of ZrO2 solid solutions with various other trivalent ions with different ionic radii. The diffusionless c→t′ A phase transformation is discussed. Extended solid solubility from 12.4 ± 0.8 to 56.5 ± 3 mol% InO1.5 is found at 1700°C in the cubic ZrO2 phase. The eutectoid composition and temperature for the decomposition of c‐ZrO2 solid solution into t‐ZrO2+InO1.5 solid solutions were determined. A maximum of about 1 mol% ZrO2 is soluble in bcc InO1.5 phase. Metastable supersaturation of ZrO2 in bcc InO 1.5 and conditions for phase separation are discussed.
Processing and Characterization of Porous TiO2 Coatings
Authors: Kim Young‐Joo , Francis Lorraine Falter
Published: March 2005

Titania coatings were prepared by spin coating anhydrous titanium ethoxide solutions onto Si substrates. During deposition, Ti ethoxide in the solution layer reacted with atmospheric moisture to form precipitated particles. The resulting microstructures were composed of a network of particles and particle clusters. The induction time for precipitation, the particle diameter, and the size and packing of particle clusters were influenced by the Ti concentration in the sol and the spinning rate used for deposition. Individual particle sizes ranged from ∼150 to 250 nm. Smaller particles and more compact particle clusters were characteristic of coatings prepared from solutions with lower Ti concentrations and those prepared using faster spinning rates. Asdeposited coatings were amorphous and crystallized into the anatase phase at ∼400°C. Transformation to the rutile phase began at ∼850°C, and the transformation rate was influenced by the microstructure.
Thermal Diffusivity/Conductivity of Compacts of C60 Buckminsterfullerene and a C60–C70 Mixture
Authors: Withers James C. , Loutfy Raouf O. , Donaldson Kimberly Y. , Hasselman D. P. H.
Published: March 2005

The thermal diffusivity/conductivity at room temperature of a compact of C60 buckminsterfullerene with a density ∼94.5% of theoretical was found to be about one‐half of the literature value for a C60 single crystal. With increasing temperature to about 500°C, the thermal diffusivity was found to decrease with ∼T−1 dependence, indicative of phonon‐phonon scattering. A compact of a C60–C70 mixture with density comparable to that of the C60 compact exhibited a thermal diffusivity/conductivity about one‐half that of the C60 compact, this decrease possibly attributable to enhanced phonon scattering because of the mass differences of the C60 and the C70 molecules. The specific heat values of the C60 and the C60–C70 compacts were about the same as that of graphite, with a similar dependence on temperature. Although both the C60 and the C60–C70 compacts appeared to disintegrate spontaneously at elevated temperatures, this tendency appeared to be less pronounced for the C60 than for the C60–C70 compacts.
Effect of Interlayer Thickness on Residual Thermal Stresses in a Ceramic‐to‐Metal Cylindrical Joint
Authors: Kimura Osamu , Kawashima Toshio
Published: March 2005

It is well‐known that large thermal stresses are caused in a ceramic‐to‐metal joint by thermal expansion mismatch. Barton's analysis for stresses in cylindrical rods (M. V. Bartoh, J. Appl. Mech., 8, A97 (1941)) was reformulated in terms of stresses arising from thermal expansion mismatches in elastically homogeneous cylindrical joints, and qualitative agreement was shown between these results and those produced by finite element calculations. Here, we apply these results to homogeneous joints with interlayers having different thermal expansion coefficients, to examine the effect of interlayer thickness on diminishing thermal expansion mismatch stress.
Densification of Alkoxide‐Derived Fine Silica Powder Compact by Ultra‐High‐Pressure Cold Isostatic Pressing
Authors: Kamiya Hidehiro , Suzuki Hisao , Kato Daisuke , Jimbo Genji
Published: March 2005

Powder compacts of alkoxide‐derived fine silica powders were consolidated into a highly dense and uniform structure by ultra‐high‐pressure cold isostatic pressing of granules with controlled structure. The diameters of spherical and nearly monosized amorphous silica particles, prepared from metal alkoxide, were successfully controlled in the range of 9 to 760 nm by varying the concentration of ammonia. Close‐packed granules of these powders were produced by spray drying. These powders were isostatically pressed up to 1 GPa at room temperature. Although the average particle diameter was less than 100 nm, the maximum relative density of the compacts was more than 78% of theoretical density. The optimum particle size to obtain highly dense compacts was in the range of 30 to 300 nm at 1 GPa. Furthermore, the ratio of mode pore diameter in these compacts to particle diameter was less than 0.155, which corresponded to the minimum ratio of calculated three‐particle pore channel radii for hexagonal close packing. Viscous deformation of particles under ultra‐high isostatic pressure played an important role in the densification of the compacts.
Effect of pH on the Hydration of Tricalcium Silicate
Authors: Ramachandran A. R. , Grutzeck Michael W.
Published: March 2005

The suspension hydration of coarse‐grained (0.07 m2.g−1) tricalcium silicate (C3S) in relatively large amounts of water (1 g per 500 mL) was studied as a function of time (up to 48 h), temperature (10°, 25°, and 40°C), and pH (8, 10, 11, 11.5, 12). The pH of the solution was maintained with dilute HCl, and both solid and solution samples were withdrawn periodically for analysis. At earliest times, the microstructure of the hydration products suggests that a fine‐grained, highly conformable hydrate forms on the C3S grains as early as 2 min. Once formed, the growing hydrate layer imparts anywhere from 0.5 to 6 h of diffusion control to the dissolution reaction. The parabolic rate constants (kps) calculated for this period increase with increasing temperature and acidity of the solution. Similarly, activation energies calculated using the Arrhenius equation range from 2.0 kJ·mol−1 at pH 10 to 20.6 kJ·mol−1 at pH 12. Values less than 21 kJ·mol−1 are generally indicative of diffusion control in aqueous systems. Depending on the pOH of the solution, the initial calcium silicate hydrate (C─S─H) was observed either to persist throughout the 48‐h experiments (aOH <10−2.5) or to act as a nucleation site for a second, less soluble, highly reticulated C─S─H hydrate (aOH≥ 10−2.5). The chemical composition of the solution phase and the characteristics of the evolving microstructure over the course of the reaction lead to the conclusion that under high‐pH conditions (aOH≥ 10−2.5), the reticulated phase which is precipitating is identical to the Type I/II C─S─H observed in non‐pH‐controlled experiments. Experiments confirm and strengthen the hypothesis that the system CaO–SiO2–H2O contains at least two distinct hydrated phases and that the activity of the hydroxide ion plays a dominant role in determining the nature and microstructure of the hydration products which form.
Transformation, Microstructure Development, and Densification in α‐Fe2O3‐Seeded Boehmite‐Derived Alumina
Authors: McArdle James L. , Messing Gary L.
Published: March 2005

Addition of α‐Fe2O3 seed particles to alkoxide‐derived boehmite sols resulted in a 10‐fold increase in isothermal rate constants for the transformation of γ‐ to α‐Al2O3. Changes in porosity and surface area with sintering temperature showed no effect of seeding on coarsening of the transition alumina gels, but the 200‐fold decrease in surface area associated with transformation to α‐Al2O3 occurred ∼ 100°C lower in seeded gels compared with unseeded materials. As a result of high nucleation frequency and reduced microstructure coarsening, fully transformed seeded alumina retained specific surface areas >22 m2/g and exhibited narrow pore size distributions, permitting development of fully dense, submicrometer α‐Al2O3 at ∼ 1200°C.
High‐Temperature Hardness of Chemically Vapor‐Deposited Diamond
Authors: Mumm Daniel R. , Faber K T. , Drory Michael D. , Gardinier Clayton F.
Published: March 2005

The hardness of chemically vapor‐deposited diamond is examined at elevated temperatures. Diamond films of 400‐μm thickness are grown on silicon substrates by plasma‐enhanced chemical vapor deposition with subsequent removal of the substrate by chemical etching. Vickers hardness measurements were performed in the temperature range of 500°–950°C in an inert gas environment and under 7‐N load. A 30% reduction in hardness from the room‐temperature value is observed above 800°C.
Strength, Degradation, and Coating of Silica Lightguides
Authors: Kurkjian Charles R. , Simpkins Peter G. , Inniss Daryl
Published: March 2005

Silica and germania–silica glasses are most appropriate for use in making lightguide fibers because of their special optical and mechanical properties. They can be prepared easily in a highly pure state and thus exhibit very low optical absorption losses. In addition, because of their high bond strength and complete three‐dimensional bonding, they have a very high theoretical (and practical) mechanical strength. The two most serious drawbacks from a mechanical point of view are brittleness and reactivity with water. Their brittleness allows them to be easily damaged, while their reactivity with water gives rise to stress‐accelerated reaction or inhomogeneous corrosion in the presence (fatigue) and absence (aging) of stress. By the development of suitable polymeric and metallic coatings, both problems can be largely overcome.
Mass Transport in the Presence of Internal Defect Reactions—Concept of Conservative Ensembles: II, Evaluation of Electrochemical Transport Measurements
Authors: Maier Joachim
Published: March 2005

It is shown that in the general case of a defective solid, where the ionic point defects can change their valence states, the evaluation formulas of well‐known electrochemical techniques (such as concentration cell experiments, Wagner‐Hebb method, chemical and electrochemical polarization and relaxation) have to be modified. The generalized relationships are derived.
Mass Transport in the Presence of Internal Defect Reactions—Concept of Conservative Ensembles: III, Trapping Effect of Dopants on Chemical Diffusion
Authors: Maier Joachim
Published: March 2005

Trapping effects of dopant ions on the chemical diffusion due to valence changes of the impurity ions (association with electronic charge carriers) and/or due to association with ionic defects are considered. It is shown that the chemical or ambipolar diffusion coefficient may be substantially changed and may even be lower than both the diffusivity of the ionic and the electronic defects under consideration. Data reinterpretation is exemplified for SrTiO3 and ZrO2 (Y2O3).
Toughening by Metallic Lamina in Nickel/Alumina Composites
Authors: Chen Zheng , Mecholsky John J.
Published: March 2005

Tape casting and hot‐pressing were used to produce metal/ alumina laminate composites. The mechanical behavior of these laminates was characterized. The strength and toughness of the laminates were greatly improved in comparison to that of monolithic alumina. Indentation strength values of the laminates remained approximately constant for indent loads between 10 and 100 N. Thus, this type of laminate appears to be a good candidate for damage‐tolerant composite material design.
Low‐Temperature Synthesis of Hexagonal Anorthite via Hydrothermal Processing
Authors: Borglum Brian P. , Bukowski John M. , Young J. Francis , Buchanan Relva C.
Published: March 2005

Hexagonal anorthite (CaAl2Si2O8) has been prepared by hydrothermal processing of monocalcium aluminate and quartz at temperatures as low as 200°C. The successful development of this phase is dependent upon several processing parameters, including the hydration of the calcium aluminate precursor material to the hydrogarnet phase (Ca3Al2O6·6H2O) prior to hydrothermal treatment and the use of quartz as opposed to amorphous sources of SiO2. Quartz has partial solubility in the hydrogarnet lattice for additions up to 40 wt%. Increased SiO2 substitution has been shown to reduce the conversion of hydrogarnet to Ca4Al6O13·3H2O, thereby increasing its thermal stability and improving its strength characteristics at temperatures greater than 200°C. Quartz additions greater than 43 wt% lead to the formation of CaAl2Si2O8 as the sole reaction product. The moderate temperatures involved in forming this anhydrous material are an order of magnitude lower than those necessary to form this phase by melt crystallization, making it a true chemically bonded ceramic. The reaction can form a bonded matrix with strengths up to 40000 psi (280 MPa). Strengths are limited due to density changes during anorthite formation, but the matrix is thermally stable up to 1000°C.
Surface Engineering of Structural Ceramics
Authors: Reeber Robert R.
Published: March 2005

The role of surface engineering for controlling structural ceramic properties is reviewed. Initially, a perspective of the effects of an unmodified surface state on fracture, adhesion, tribological properties, and degradation is given. Ceramic surface modification techniques including laser, ion, ion‐beam‐assisted deposition, and plasma‐assisted deposition are then discussed with regard to how each non‐equilibrium technique affects bulk and surface properties. The importance of experiments, process and materials characterization, and modeling for understanding changes in localized bonding, chemistry, and structure is emphasized. Although not exhaustive, a broad guide to important literature is given.
Laser Specular Reflectance and the Optical rms Roughness of Alumina
Authors: Kusy Robert P. , Stuart James M. , Whitley John Q. , Saunders Carl R.
Published: March 2005

Using laser specular reflectance, the optical rms roughnesses (σ0'S) of 42 polycrystalline aluminas (PCA's) were determined. From these conventional experimental results, a nomogram was constructed so that single grazing angle (α) measurements could be used to predict σ0. Subsequently, the nomogram was reassessed using five commercially important PCA's and one single‐crystal sapphire (SC). The outcomes were twofold: that this nomogram was valid for PCA's over the range of 0.28 σ0 0.10 μm, but that the nomogram was invalid for SC sapphire. When the conventional experimental method was compared with the nomogram method at three different single‐angle values, a 1:1 correspondence was observed for α= 78°, 80°, and 82°. The present noncontact method is advocated for speed and cleanliness, particularly when a quality assurance method is desired.
Fracture Resistance Characteristics of a Metal‐Toughened Ceramic
Authors: Flinn Brian D. , Lo Calvin S. , Zok Frank W. , Evans Anthony G.
Published: March 2005

The fracture characteristics of an Al2O3/Al composite are examined. Measurements of resistance curves and work of rupture are compared with predictions of a micromechanical model, incorporating the effects of crack bridging by the Al reinforcements. The bridging traction law is assumed to follow linear softening behavior, characterized by a peak stress, σc, and a critical stretch‐to‐failure, uc. The values of σc and uc inferred from such comparisons are found to be broadly consistent with independent measurements of stretch‐to‐failure, along with the measured flow characteristics of the Al reinforcement. The importance of large‐scale bridging on the fracture resistance behavior of this class of composite is also demonstrated through both the experiments and the simulations.
Measurement of the Free Energy of Formation of Silicon Carbide Using Liquid Gold as a Silicon Potentiometer
Authors: Sambasivan Sankar , Capobianco Christopher , Petuskey William T.
Published: March 2005

The free energies of formation of 3C‐ and 6H‐silicon carbide were determined between 1623 and 1898 K by measuring the silicon solubility in liquid gold at equilibrium with silicon carbide and graphite. The solution behavior of the Au–Si system provides a wide compositional range for the small difference in the activity of silicon exhibited by the two polytypes, thus offering greater experimental precision over previous similar methods. For 6H‐SiC, ΔG (J/mol) = ‐116900(±7.2) + 38.2(±4.1)T (where T is in K) relative to liquid silicon and graphite standard states is in excellent agreement with other investigations, indicating that the method is capable of determining the free energy of formation with a precision of about 0.7 kJ/mol. Our measurements on 3C‐SiC exhibited greater experimental scatter, which may have been due to variations in the density of non‐equilibrium defects of the crystals used, although experimental problems encountered with the complex crystal shapes may have also contributed. Nevertheless, the precision of the technique is sufficient to address some issues of polytype stability and perhaps free energy contributions of nonequilibrium defects.
Sliding Wear of Silicon Carbide—Titanium Boride Ceramic‐Matrix Composite
Authors: Ajayi Oyelayo O. , Erdemir Ali , Lee Richard H. , Nichols Fred A.
Published: March 2005

The tribological behavior of a SiC–TiB2 ceramic‐matrix composite was evaluated using a pin‐on‐flat contact configuration in reciprocating sliding. A steady‐state friction coefficient of about 0.55 and an average specific wear rate of about 2 × 10‐6 mm3·N‐1·m‐1 were measured. The mechanisms of material removal involved failure of the particle–matrix interface, resulting in particle pullout and chipping of the matrix material. The generated wear debris was often recycled through the contact interface, resulting in the production of submicrometer fine particles that reattached to the surface and were later removed in flake form. Transitions were observed in the specific wear rate from higher to lower values as sliding progressed. These transitions were accompanied by transitions in the nominal contact pressure.
The U─Zr─C Ternary Phase Diagram above 2473 K
Authors: Butt Darryl P. , Wallace Terry C.
Published: March 2005

The U─Zr─C tertnary phase diagram is optimized by graphical and numerical curve fitting of self‐consistent thermodynamic and phase diagram data reported in the literature. The system is described from 2473 to 3693 K using isothermal ternary phase diagrams. The solidus and liquidus curves of the UxZr1−xCy solid solution were estimated from combined thermodynamic and phase diagram data. The solidus and liquidus curves in the ternary system were calculated using a modified form of a free‐energy minimization method developed by Rudy and Chang. The calculated curves agree with experimental data.
Effect of Composition and Size of Crystallite on Crystal Phase in Lead Barium Titanate
Authors: Saegusa Kunio , Rhine Wendell E. , Bowen H. Kent
Published: March 2005

Lead titanate, barium titanate, and lead barium titanate powders (>99.9% pure), the particle size of which varied from 0.03 to 0.15 μm depending on the calcination temperature and the composition, was prepared from barium lead titanyl oxalate, which was previously prepared by reacting high‐purity ammonium titanyl oxalate with barium and lead acetate. The critical crystallite size of BaTiO3 powder from the cubic to the tetragonal phase is around 1 μm. Pb0.3Ba0.7TiO3 powder with an average size of 0.057 μm showed the tetragonal phase.
Effect of Coherency Strains on Phase Separation in the AlN‐Al2OC Pseudobinary System
Authors: Chen Jong , Tian Qiang , Virkar Anil V.
Published: March 2005

AlN and Al2OC, both of 2H structure, form a homogeneous solid solution at elevated temperatures. Annealing at lower temperatures leads to the formation of two solid solutions, one AlN‐rich and the other Al2OC‐rich. Samples of near equimolar composition form disk‐shaped precipitates upon annealing at temperatures below ∼1900°C. In the present work, samples of several compositions were fabricated by hot‐pressing mixtures of AlN, Al4C3and Al2O3. Lattice parameters of the resulting solid solutions were measured by X‐ray diffraction. Samples of an equimolar composition were annealed at 1500°C for various periods of time up to a maximum of 512 h. Disk‐shaped precipitates with their axes along the [0001] direction were initially coherent. In the coherent stage, thealattice parameters of the precipitate and the matrix were identical because of coherent fitting of lattice planes. The shape of the precipitates and their orientation were in accord with predictions based on theoretical calculations of coherency strain energy. Using the experimentally measured values of the aspect ratios, the precipitate/matrix interfacial energy for coherent precipitates was estimated to be on the order of a few mJ/m2. Long‐term annealing led to the loss of coherency as manifested by the formation of interface dislocations.
Matrix Cracking in Ceramic‐Matrix Composites
Authors: Danchaivijit Sawai , Shetty Dinesh K.
Published: March 2005

Matrix cracking in ceramic‐matrix composites with unbonded frictional interface has been studied using fracture mechanics theory. The critical stress for extension of a fiber‐bridged crack has been analyzed using the stress‐intensity approach. The analysis uses a new shear‐lag formulation of the crack‐closure traction applied by the bridging fibers based on the assumption of a constant sliding friction stress over the sliding length of the fiber‐matrix interface. The new formulation satisfies two required limiting conditions: (a) when the stress in the bridging fiber approaches the far‐field applied stress, the crack‐opening displacement approaches a steady‐state upper limit that is in agreement with the previous formulations; and (b) in the limit of zero crack opening, the stress in the bridging fiber approaches the far‐field fiber stress. This lower limit of the bridging stress is distinctly different from the previous formulations. For all other conditions, the closure traction is a function of the far‐field applied stress in addition to the local crack‐opening displacement, the interfacial sliding friction stress, and the material properties. Numerical calculations using the stress‐intensity approach indicate that the critical stress for crack extension decreases with increasing crack length and approaches a constant steady‐state value for large cracks. The steady‐state matrix‐cracking stress agrees with a steady‐state energy balance analysis applied to the continuum model, but it is slightly less than the matrix‐cracking stress predicted by such theories of steady‐state cracking as that of Aveston, Cooper, and Kelly. The origin of this difference and a method for reconciliation of the two theoretical approaches are discussed.
A New Equation for Predicting the Dielectric Constant of a Mixture
Authors: Wakino Kikuo , Okada Tsutomu , Yoshida Norio , Tomono Kunisaburo
Published: March 2005

The dielectric constant of a compound consisting of two different materials was simulated using the Monte Carlo and finite element methods. In the high concentration range of the higher dielectric constant material, the simulated value of the dielectric constant showed a “parallel‐model”‐like tendency. On the other hand, in the low concentration range of the higher dielectric constant material, the compound showed a “serial‐model”‐like behavior. Around 65% content of the higher dielectric constant material, it exhibited results similar to the logarithmic mixing rule. After careful consideration of these results, a new equation to predict the dielectric constant of the compound was derived. The agreement between the dielectric constant calculated by the new equation and the measured values was better than in any other case reported before.
Mechanical Properties of Hot Isostatically Pressed Zirconia‐Toughened Alumina Ceramics Prepared from Coprecipitated Powders
Authors: Matsumoto Yosuke , Hirota Ken , Yamaguchi Osamu , Inamura Suguru , Miyamoto Hiroki , Shiokawa Nobuaki , Tsuji Kazuhiro
Published: March 2005

Amorphous Al2O3–ZrO2 composite powders with 5–30 mol% ZrO2 have been prepared by adding aqueous ammonia to the mixed solution of aqueous aluminum sulfate and zirconium alkoxide containing 2‐propanol. Simultaneous crystallization of γ‐Al2O3 and t‐ZrO2 occurs at 870°–980°C. The γ‐Al2O3 transforms to α‐Al2O3 at 1160°–1220°C. Hot isostatic pressing has been performed for 1 h at 1400°C under 196 MPa using α‐Al2O3–t‐ZrO2 composite powders. Dense ZrO2‐toughened Al2O3 (ZTA) ceramics with homogeneous‐dispersed ZrO2 particles show excellent mechanical properties. The toughening mechanism is discussed. The microstructures and t/m ratios of ZTA are examined, with emphasis on the relation between strength and fracture toughness.
Ceramic Powder Synthesis by Spray Pyrolysis
Authors: Messing Gary L. , Zhang Shi‐Chang , Jayanthi Gopal V.
Published: March 2005

A variety of spray pyrolysis (SP) techniques have been developed to directly produce ceramic powders from solutions. This paper reviews the current status of these processes in terms of the process parameters that enable the formation of particles with controlled morphology and composition. A model incorporating solute diffusion in the droplet and solvent evaporation from the droplet surface is presented to establish the critical parameters leading to solid particle formation. The model illustrates that solid particles can be obtained if solutes with high solubility and a large difference between the critical supersaturation and equilibrium concentration are used and if the process is designed to avoid solvent boiling. It is demonstrated that mixed metal oxide, non‐oxide, and composite particles that are solid, hollow, porous, or fibrous can be produced by modifying the precursor characteristics, solution properties, and process parameters. The physical and chemical flexibility of SP processes offers numerous opportunities for the controlled synthesis of advanced ceramic powders and films. However, production rates are limited by the need to produce < 5‐μm‐diameter droplets and to avoid subsequent droplet coagulation. Developments in process controls, atomization, and system design are required for wider commercialization of SP‐type processes.
Structural Changes of Sol–Gel‐Derived TiO2–SiO2Coatings in an Environment of High Temperature and High Humidity
Authors: Matsuda Atsunori , Kogure Toshihiro , Matsuno Yoshihiro , Katayama Shinya , Tsuno Toshio , Tohge Noboru , Minami Tsutomu
Published: March 2005

Structural changes in sol–gel‐derived TiO2–SiO2coatings were found to proceed in an environment of high temperature and high humidity as follows: (1) dissociation of Si–O–Ti bonds in the coating by the attack of water vapor, (2) formation of Ti–O–Ti bonds, and (3) nucleation and growth of anatase TiO2. The coating obtained with the addition of poly(ethylene glycol), PEG, reacts with water vapor more easily than the coating obtained without PEG, since the former is more porous than the latter due to the decomposition of PEG during heat treatment.
Fabrication of Transparent Tellurite Glasses Containing Potassium Niobate Crystals by an Incorporation Method
Authors: Komatsu Takayuki , Shioya Kazuhide , Matusita Kazumasa
Published: March 2005

To fabricate transparent oxide glasses containing ferroelectric KNbO3 crystals, a new method in which KNbO3 particles are directly incorporated into TeO2─K2O─Nb2O5 glasses has been developed. Transparent TeO2‐based glasses containing KNbO3 crystals with a diameter of ∼ 10 μm have been first successfully fabricated by adjusting temperature and time for incorporation. A small difference in the refractive indexes, n, between TeO2‐based matrix glasses (n= 2.0) and incorporated KNbO3 crystals (n= 2.21) is a significant reason for the transparency. This new method is applicable for the fabrication of new transparent glasses containing other functional materials with high refractive indexes.
Combined Buckling and Cracking of Films
Authors: Thouless M. D.
Published: March 2005

Films or laminates subjected to a compressive stress may fail by a mechanism that involves the propagation of a buckle. An additional driving force for delamination is provided when cracking of the film accompanies the buckling. This provides a mechanism by which cracks can tunnel through compressively stressed films. Although the mechanics of the process is very similar to previous analyses of simple buckling, the possibility of concurrent cracking and buckling allows delamination to occur under conditions excluded by these earlier analyses.
Fracture Toughness Anisotropy and Toughening Mechanisms of a Hot‐pressed Alumina Reinforced with Silicon Carbide Whiskers
Authors: Hansson Thomas , Warren Richard , Wasén Jonas
Published: March 2005

The fracture toughness of a hot‐pressed alumina and that of a hot‐pressed alumina/SiC‐whisker composite containing 33 vol% SiC whiskers were measured by four‐point bending on single‐edge precracked bend bars having sharp precracks created by “bridge indentation.” Two batches of the composite were examined, one exhibiting a greater degree of whisker clustering than the other. The fracture toughness of the alumina was around 4 MN·m‐3/2 whereas that of the composite varied between 5 and 8 MN·m‐3/2 depending on microstructural uniformity and crack‐propagation direction. Crack deflection in combination with a change in fracture from intergranular to transgranular fracture is proposed as an explanation of the superior fracture toughness of SiC‐whisker‐reinforced alumina as compared to unreinforced alumina. The composite exhibited a variation in fracture toughness with the crack‐propagation direction in identical crack planes. This effect could with good accuracy be described in terms of crack deflection for the composite with uniform whisker distribution. However, in the material with whisker clustering the variation of the fracture toughness with crack‐growth direction was greater and could not entirely be explained by crack deflection.
A Transmission Electron Microscopy Diffraction and Simulation Method for Early‐Stage Studies of the Evolution of Gel‐Derived Zirconia Precursors
Authors: Tong Jianzhu , Zuo Jian Min , Eyring LeRoy
Published: March 2005

A technique, utilizing transmission electron microscopy, is described that enables the modeling and diffraction simulation of structural development during the processing of gelderived products. This technique relies on the determination of the position and relative intensity of diffuse electron diffraction rings from small domains. X‐ray diffraction cannot provide a basis for this analysis. A qualitative microcrystal modeling of the structural changes accompanying the in situ transformation of a zirconia‐precursor gel to crystalline ZrO2 is described. A quantitative analysis is possible only after correction for the inelastic diffraction effect and a calibration of the nonlinear response of the photographic film.
Elimination of Large Artificial Pores During the Hot Isostatic Pressing of Presintered Alumina
Authors: Lee Sang‐Ho , Yang Jae‐Kyun , Kim Doh‐Yeon
Published: March 2005

Shrinkage of large pores during hot isostatic pressing (HIP) of presintered alumina has been studied. When alumina specimens containing artificially produced large spherical pores are HIP treated, pore shrinkage occurs predominantly by plastic deformation. When the temperature of the HIP treatment is 1850°C, the pores appear to collapse with particle fragmentation and sliding. The complete elimination of large pores is hard to achieve, because the pressure of N2 gas entrapped in the pores increases with their shrinkage. The practical implication of these results is that large voids or cracks inside ceramics can be removed by using the proper pressure/temperature regime.
Nonlinear Deformation of Ferroelectric Ceramics
Authors: Cao Hengchu , Evans Anthony G.
Published: March 2005

Ferroelectric ceramics used in advanced actuator systems may be subject to high stresses and large deformations that may lead to nonlinearity and performance degradation. A preliminary experimental investigation has been undertaken to study the relevant mechanical responses. For this purpose, polarization and strain are measured continuously on a compression cuboid. Severe nonlinear and hysteretic behavior is observed in several cases. An attempt has been made to establish constitutive laws, applicable to this nonlinear deformation, which may be used for design purposes.
Nature of Alumina in Phosphate Glass: II, Structure of Sodium Alurninophosphate Glass
Authors: Brow Richard K. , Kirkpatrick R. James , Turner Gary L.
Published: March 2005

We have used 27Al and 31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and X‐ray photoelectron spectroscopy (XPS) to describe quantitatively the relationships between the composition, structure, and properties of glasses in the Na2O.Al2O3.P2O5 (NAP) system. In general, the glass properties (evaluated in part I) are most sensitive to changes in Al coordination. 27 Al MAS NMR spectra reveal that octahedrally coordinated Al is most abundant in glasses with O/P ratios less than 3.5, the pyrophosphate structural limit. Tetrahedrally coordinated Al is most abundant in glasses with O/P greater than 3.5. Decreasing Al(OP)6/Al(OP)4 ratio generally correlates with decreasing glass transition temperature and refractive index. The compositional dependence of glass structure and properties can be qualitatively understood using a crystal chemical model based on oxygen charge balancing by the different Al and P moieties.
Deposition and Characterization of Li2O–SiO2–P2O5 Thin Films
Authors: Bates John B. , Dudney Nancy J. , Luck Chris F. , Sales Brian C. , Zuhr Raymond A. , Robertson J. David
Published: March 2005

Amorphous lithium electrolyte thin films, xLi2O·ySiO2·zP2O5, were deposited by rf magnetron sputtering of pure and mixed‐phase lithium silicate, lithium phosphate, SiO2, Li2O, and Li2CO3 targets, and their compositions were determined using proton‐induced y‐ray emission spectroscopy, energy‐dispersive X‐ray analysis, Rutherford backscattering spectrometry, and atomic‐emission spectroscopy. The deposition conditions were chosen to assure thermalization of the sputtered flux, which proved to be necessary in order to obtain a homogeneous distribution of Si and P in the films. Optical absorption and ac impedance measurements showed that glass‐in‐glass phase separation occurred in a large SiO2‐rich domain of the composition diagram. In contrast to bulk glasses, all of the Li2O–SiO2 films were phase‐separated, including those with lithia contents larger than lithium disilicate. High‐performance liquid chromatography measurements revealed that, analogous to bulk glasses, the addition of SiO2 to Li2O‐P2O5 compositions reduced the number of phosphate anion dimers, trimers, and higher anion polymers in the films through the formation of ‐Si‐O‐P‐bonds. However, in contrast to bulk glasses, the distribution of phosphate anion polymers followed closely the Flory distribution, with the fraction of anion polymers decreasing monotonically with increasing chain length.
Oxidation of Hafnium Carbide in the Temperature Range 1400° to 2060°C
Authors: Bargeron C. Brent , Benson Richard C. , Jette A. Norman , Phillips Terry E.
Published: March 2005

After hafnium carbide has been oxidized at temperatures in the range of 1400° to 2060°C, three distinct layers are present in the film cross section: (a) a residual carbide layer with dissolved oxygen in the lattice, (b) a dense‐appearing oxide interlayer containing carbon, and (c) a porous outer layer of hafnium oxide. Experimental measurements of layer thicknesses and oxygen concentrations are combined with an extended formulation of moving‐boundary diffusion theory to obtain the diffusion constants of oxygen in each of the three layers. The results indicate that the oxide interlayer is a better diffusion barrier for oxygen than either of the other layers. Based on X‐ray microanalysis, X‐ray diffraction, and resistance measurements, the interlayer is an oxygen‐deficient oxide of hafnium with a carbon impurity. The interlayer hardness equals that of the residual carbide layer.
Zirconia/Alumina Functionally Gradiented Composites by Electrophoretic Deposition Techniques
Authors: Sarkar Partho , Huang Xuening , Nicholson Patrick S.
Published: March 2005

An electrophoretic deposition and sintering route was used to prepare YSZ/Al2O3 composites with a compositional gradient. The YSZ content was continuously decreased from the YSZ‐rich surface to the Al2O3‐rich surface, Microstructural and Vickers hardness (16–24 GPa) evidence tracked the compositional development, and the indentation fracture toughness was found to vary across the section (10–3 MPa·m1/2).
Microstructure and Microchemistry of Polymer‐Derived Crystalline SiC Fibers
Authors: Xu Youren , Zangvil Avigdor , Lipowitz Jonathan , Rabe James A. , Zank Gregg A.
Published: March 2005

Stoichiometric and near‐stoichiometric silicon carbide fibers, derived from the polymer polycarbosilane, have been characterized by scanning Auger and transmission electron microscope techniques. Excess carbon usually appeared in the fibers, but it could be eliminated by controlling processing conditions. The fibers consisted mainly of submicrometer‐sized SiC microcrystals, with a considerably twinned and stacking‐faulted β‐SiC structure. Free carbon, which appeared in the form of graphite, was the only detectable intergranular phase in the nonstoichioinetric SiC fibers, while the stoichiometric SiC fibers contained no detectable second phase whatsoever.