bluuebunny/crossref_metadata_2025 huggingface dataset:
Slip Casting of Yttria‐Stabilized Tetragonal Zirconia Polycrystals
Authors: MORENO R. , REQUENA J. , MOYA J.S.
Published: March 2005

The rheological and casting parameters of 3‐mol%‐yttria‐stabilized tetragonal zirconia polycrystalline (Y‐TZP) powder obtained by a wet‐chemical coprecipitation route have been studied. Colloidal stability has been studied through zeta potential measurements. An organic surfactant has been used as deflocculant. Viscosity, casting rate, and green densities have been determined for suspensions with 28.2, 33.6, and 40.3 vol% solids content. Relative density, grain size, and t‐ZrO2 evolution versus temperature and soaking time are also reported.
Microstructure and Superconducting Properties of High‐Density Consolidated YBa2Cu3Ox
Authors: Shelton R.N. , Andreasen D. , Klavins P. , Chan H.W. , Oslin B.L. , Anderson R.L.
Published: March 2005

Sintered bulk samples of YBa2Cu3Ox of 95% to 99% theoretical density have been consolidated by a one‐step densification process. Superconducting transitions determined by dc magnetization experiments are above 90 K. The metallurgical state of the material, both before and after consolidation, is characterized by X‐ray diffraction and electron microscopy analysis as well as density and resistivity measurements. Improvement in critical current density (Jc) in the consolidated material is related to changes in the microstructure. Particular attention is focused on the granular nature of the compound and the influence of the consolidation process on the weak‐link structure.
Defect Structure and Electrical Properties of Single‐Crystal Ba0.03Sr0.97TiO3
Authors: CHOI GYEONG MAN , TULLER HARRY L.
Published: March 2005

The electrical conductivity and thermoelectric power of single‐crystalline Ba0.03Sr0.97TiO3 were measured over a wide temperature (800° to 1100°C) and oxygen partial pressure (105 to 10‐15 Pa) range. Our experimental data, like those of previous workers on nominally undoped BaTiO3 or SrTiO3, support a defect model based on doubly ionized oxygen vacancies, electrons, holes, and accidental acceptor impurities. The simultaneous measurement of electrical conductivity and thermoelectric power, together with precise experimental data obtained with an advanced thermoectric power measurement technique, enabled us to determine for the first time reliable values for the preexponential factors and the activation energies which characterize the defect equilibrium constants. These calculated values, together with the defect model, were found to give an excellent fit to the experimental data, and were used to generate the boundaries, in Po2‐1/T space, of the various defect regimes.
R‐Curve Behavior of a Polycrystalline Graphite: Microcracking and Grain Bridging in the Wake Region
Authors: SAKAI MOTOTSUGU , YOSHIMURA JUN‐ICHI , GOTO YASUO , INAGAKI MICHIO
Published: March 2005

The contributions of nonlinear fracture processes both in the microcracking frontal process zone and in the following wake region and of grain bridging to crack‐growth resistance parameters are discussed in terms of the R‐curve behavior of an isotropic polycrystalline graphite. The R‐curve behavior of the graphite is characterized by rapidly increasing values at the initial stage of crack extension (Δa≤1 to 2 mm) followed by a steady‐state plateaulike region and then a distinct decrease when the primary crack tip approaches the end surface of the test specimen. Scanning electron microscopy of fracture mechanics specimens revealed a dominant role of grain bridging in the following wake regions on the rising R‐curve behavior and confirmed the significant size effect of the large‐scale microcracking process zone on the falling R‐curve behavior. The stress‐derived fracture toughness (KR) and the energy fracture toughness (Rc) are discussed in relation to the micro‐cracking residual strain.
Formation Mechanisms of La3+‐Doped CaO·6Fe2O3 with the Magnetoplumbite Structue
Authors: Fang Tsang‐Tse , Lin Horng‐Bin , Lee Kuo‐Tong
Published: March 2005

Several experiments were conducted to investigate the formation mechanisms of the magnetoplumbite phase in La3+‐doped CaO 6Fe2O3. It is shown that the CaO·2Fe2O3 phase plays a crucial role in forming the magnetoplumbite phase. The formation mechanisms are proposed and verified.
Strength Improvement of Yttria‐Partially‐Stabilized Zirconia by Flaw Elimination
Authors: SUNG JASON , NICHOLSON PATRICK S.
Published: March 2005

A systematic improvement of the flexural strength of conventionally sintered 4.5‐wt%‐Y2O3‐partially‐stabilized zirconia was realized by identifying the fracture origins of four‐point‐bend bars and eliminating them by process modification. The average flexural strength increased from 880 to 1380 MPa, a value comparable with that of isostatically hot‐pressed or slip‐cast material. A part‐through crack extension model was used to correlate the fracture stresses and the fracture origin sizes and a “fracture origin severity parameter” was identified.
Effect of Grain Growth on Pore Coalescence During the Liquid‐Phase Sintering of MgO‐CaMgSiO4 Systems
Authors: OH UN‐CHOL , CHUNG YOUNG‐SOO , KIM DOH‐YEON , YOON DUK N.
Published: March 2005

During the liquid‐phase sintering of MgO‐CaMgSiO4 systems in N2 atmosphere, the total porosity and the average pore size increase while the number of pores decreases. The negligible permeability of entrapped nitrogen through the liquid matrix and the observed linear relationship between the number of pores and that of the MgO grains suggest that the pores coalesce as a consequence of grain growth during sintering. An analysis of the balance between the pressure of the entrapped N2 gas and the capillary pressure shows that pore coalescence in turn causes the observed porosity increase. When Fe2O3 or Cr2O3 is added to MgO‐CaMgSiO4, the pore size and the total porosity become larger or smaller, because the grain growth is accelerated or retarded, respectively.
Si‐C‐O‐N High‐Pressure Equilibria and ΔGfo for Si2ON2
Authors: EKELUND MAGNUS , FORSLUND BERTIL , ERIKSSON GUNNAR , JOHANSSON THOMAS
Published: March 2005

Phase relations in the Si‐C‐O‐N system were studied by subjecting mixtures of carbon and silica (C/SiO2= 2 to 3) to high temperatures (up to 2000 K) and elevated nitrogen pressures (0.5 to 2.0 MPa) in a gas autoclave. The condensed‐phase assemblages obtained at different temperatures, pressures, and N/Si ratios are reported and compared with thermodynamic calculations. Values of ΔGfo(Si2ON2) consistent with the experimental data are ‐397 kJ/mol at 1780 K and ‐369 kJ/mol at 1890 K. The influence on the synthesis of seeding the starting material is discussed.
Effect of the Matrix and Matrix Bonding on the Creep Behavior of a Unidirectional Carbon–Carbon Composite
Authors: Sines George , Yang Zheng , Vickers Brian D.
Published: March 2005

Creep tests were performed on single‐bundle carbon–carbon specimens at high temperatures (>2310°C) and at high stress levels (>770 MPa). It was found that the creep was very strongly dependent on the filament–matrix interfacial bond. When the bond was good, the typical creep was 3.6% after 5.9 h with the primary creep a high percentage of the total deformation. When the bond was absent (dry bundle), rupture with strain was approximately 140%, and it occurred after only 0.39 h. The marked improvement in creep resistance is attributed to the ability of the matrix to distribute loads evenly and to produce a plastic flow inhibiting triaxial stress state among the filaments.
Glass Formation in the System SiO2–PbO Containing Transition‐Metal Oxides
Authors: Nitta Atsumi , Miura Terumi , Komatsu Takayuki , Matusita Kazumasa
Published: March 2005

Glass‐forming regions, valence states, and viscosities in SiO2–PbO systems containing various transition‐metal oxides as a third component were investigated. The glasses were prepared by melting in an open atmosphere. The glass‐forming regions ranged as follows: MnO≡ZnO > FeO1.5>NiO. The ratios Fe2+/(Fe2++ Fe3+) and Mn3+/ (Mn3++ Mn2+) in the glasses were determined by chemical analysis. The Fe2+/ (Fe2++ Fe3+) ratio in SiO2–PbO–FeO1.5 glasses ranged from 0.016 to 0.050. The Mn3+/ (Mn3++ Mn2+) ratio in SiO2–PbO–MnO glasses ranged from 0.056 to 0.30. The fraction of manganese (III) ions in the glasses varies considerably with the glass composition. The effects of transitionmetal oxides on the viscosity are discussed.
Synthesis of Ultrafine Grain Ferrites
Authors: Clearfield Abraham , Gadalla Ahmed M. , Marlow William H. , Livingston Terry W.
Published: March 2005

Methoxides of nickel(II) and iron(III) were synthesized, mixed in the ratio to form NiFe2O4, aerosolized, hydrated, and then fired in a streamline tubular reactor. Submicrometersized spherical particles were obtained from pyrolysis at temperatures up to 570°. At 425°, the sudden loss of volatiles shattered the particles to much smaller spheres (with 30% less than 0.1 μm) giving a remarkable increase in surface area. Pyrolysis up to 570° caused a gradual increase in the powder density toward that of NiFe2O4. Heating the aerosol product up to just 600° with a heating rate of 5°/min yielded pure crystalline nickel ferrite.
New Aqueous Injection Molding Process for Ceramic Powders
Authors: Fanelli Anthony J. , Silvers Robert D. , Frei William S. , Burlew Joan V. , Marsh Gary B.
Published: March 2005

An aqueous injection molding process based on the gelling properties of certain naturally occurring polysaccharides, in particular agar, and its purified derivative, agarose, is described. The materials form nonviscous solutions at temperatures near 100°C which solidify to rigid gels upon cooling below the so‐called gel point temperature, 37°C. Gels formed by these materials are strong, nominally 1500 g/cm2 for agar and 2500 g/cm2 for agarose at 3 wt% concentration. Molded parts can be dried and fired without use of absorbent powders or special debinding operations of any kind.
Microwave Characteristics of A(B3+1/2B5+1/2)O3 Ceramics (A = Ba, Ca, Sr; B3+= La, Nd, Sm, Yb; B5+= Nb, Ta)
Authors: Takata Mitsuhiro , Kageyama Keisuke
Published: March 2005

Dielectric properties of A(B3+1/2B5+1/2)O3 (A = Ba, Ca, Sr; B3+= La, Nd, Sm, Yb; B5+= Nb, Ta) ceramics have been investigated at microwave frequencies. Sr(B3+1/2B5+1/2)O3 and Ca(B3+1/2B5+1/2)O3 ceramics have relative dielectric constants (εr) above 20 and negative temperature coefficients of resonant frequency (Tf). In the group of Ba(B3+1/2B5+1/2)O3 ceramics, Tf changes from + 118 ppm/° to nearly zero according to the kinds of B‐site ions. Among the ceramics investigated, Sr(Sm1/2Ta1/2)O3 ceramics have the highest Q values at microwave frequencies. For Sr(Sm1/2Ta1/2)O3 ceramics Q= 7000, εr= 27.7, and Tf=−62.5 ppm/° at 8.5 GHz. The microstructure of Sr(Sm1/2Ta1/2)O3 ceramics is composed of a matrix of the ternary compound (Sr‐Sm‐Ta‐O system) and secondary phase grains of the binary compound (Sm‐Ta‐O system).
Fractography of Thermal‐Shock‐Cracked Multilayer Capacitors
Authors: Koripella Chowdary R. , DeMatos Henry V.
Published: March 2005

Cracking of multilayer ceramic chips due to mechanical and thermal stresses during surface mounting is a major problem. Two types of tests were performed to study the susceptibility of chips for thermal shock cracking. In one test, chips were immersed in molten solder at different temperatures and, after cooling, chips were examined for visible cracks. In another test, chips were mounted on a printed circuit board and, then, wave soldered to simulate the actual usage conditions. After the soldering process, chips were exposed to load humidity conditions and, then, tested for insuiation resistance. Cracked chips from both tests were analyzed by fractographic methods to determine the source of failure. Causes of the defects leading to fracture and their relationship to the processing of multilayer capacitors are discussed. The relative advantages and disadvantages of the two thermal shock test methods in evaluating the integrity of the chips are presented. [Key words: capacitors, multilayer, thermal shock, cracks, fractography.]
Diffuse Phase Transition of Tantalum‐Bearing Strontium Barium Niobate
Authors: Tsurumi Takaaki , Hoshino Yoshio
Published: March 2005

The dielectric constants, ε′ and ε″, of tantalum‐bearing strontium barium niobate (SBNT) ceramics were measured as functions of temperature and frequency. The phase transition of SBNT became ambiguous by adding Ta and the apparent Curie temperature determined from the maximum of ε′ had a linear relation with the axial ratio c/a. A dielectric loss peak was observed in the phase transition region of SBNT, and its peak frequency increased with temperature. The loss peak was attributed to dielectric relaxation by the jump between two states with different polarizing directions as a preliminary phenomenon of the phase transition. A model based on the local variation from one unit cell to the next rather than the macroscopic compositional fluctuation in the crystal was proposed for the mechanism of diffuse phase transition.
Zino Oxide Varistors Made from Powders Prepared by Amine Processing
Authors: Hishita Shunichi , Yao Yao , Shirasaki Shin‐ichi
Published: March 2005

A powder preparation process suitable for varistor production was developed. The mixed acidic solution containing varistor components was added to the diethylamine solution to coprecipitate all components. The resulting mixtures were washed, dried, and calcined to produce active and uniform composition powders. Large nonlinearity coefficient (α∼ 50) were achieved in varistors fabricated from these powders by sintering in the entire range 1100° to 1250°C. Large breakdown fields were also achieved with decreasing sintering temperature. The breakdown voltage per grain boundary of these varistors was approximately 5.2 V.
Preparation of Highly Pure Fine Mullite Powder
Authors: Mizuno Mineo , Saito Hajime
Published: March 2005

The preparation of high‐purity mullite powders (3Al2O3· 2SiO2) was examined using aluminum salts or boehmite and fumed silica. The combination of aluminum sulfate and fumed silica was found to have the highest quality for mullite powder. A slurry consisting of an aqueous solution of aluminum sulfate and fumed silica was dried and heated at 1350°C and was attrition‐milled for 2 h. The mullite powder obtained was composed of submicrometer, uniform, and equiaxial particles with narrow size distribution. It was hot‐pressed at 1600°C for 1 h under 10 MPa or was sintered at 1650°C for 3 h. The bulk densities of the products made by both processes were 3.16 and 3.09 g/cm3.
Preparation of Proton‐Conducting Amorphous Films Containing Dodecamolybdophosphoric Acid by the Sol—Gel Method
Authors: Tatsumisago Masahiro , Minami Tsutomu
Published: March 2005

Silica gel films containing dodecamolybdophosphoric acid, H3PMo12O40·29H2O (MPA), were prepared by the sol—gel method. Amorphous films were obtained for the compositions with 0 to 42 wt% MPA. Infrared spectra revealed that the skeleton of heteropolyanions PMo12O3−40 remained in the silica gel structure. The gel films exhibited considerably high proton conductivities in a highly humid atmosphere, and the conductivity increased with increasing MPA content. The value of conductance (G) at 1 kHz for these films was very sensitive to the variation of relative humidity (rh). The relation between log G and rh was almost linear and was reproducible on repeated runs for the gel films with 42 wt% MPA heat‐treated at 200°C.
Weibull Crack Density Coefficient for Polydimensional Stress States
Authors: Gross Bernard , Gyekenyesi John P.
Published: March 2005

A structural ceramic analysis and reliability evaluation code has recently been developed encompassing volume and surface flaw induced fracture, modeled by the two‐parameter Weibull probability density function. A segment of the software involves computing the Weibull polydimensional stress state crack density coefficient from uniaxial stress experimental fracture data. The relationship of the polydimensional stress coefficient is derived herein for a shear‐insensitive material with a random surface flaw population. In addition, a critique is made on the derivation of this relationship by Ikeda and Igaki, which appeared in a recent article in the Journal of the American Ceramic Society.
Oxidation and Fracture Strength of High‐Purity Reaction‐Bonded Silicon Nitride
Authors: Haggerty John S. , Lightfoot A. , Ritter John E. , Gennari Paul A. , Nair S. V.
Published: March 2005

Reaction‐bonded Si3N4 (RBSN) made from high‐purity Si powder is unusually resistant to degradation caused by exposures to air for up to 50 h at temperatures up to 1400°C. The weight gain during oxidation of this SiH4‐originating RBSN is approximately 10 times less than conventional RBSN. Contrary to normally observed strength degradations, room‐temperature strengths of this high‐purity, oxidized RBSN (avg = 435 MPa, max. = 668 MPa) remained at their unusually high, as‐processed levels after 1000° and 1400°C oxidizing exposures. Fracture toughness values were unaffected by oxidation (KIC= 2.3 to 2.4 MPa · m1/2). This superior oxidation resistance results from the high purity and the small diameter pore channels (0.01 to 0.06 μm) achieved in this SiH4‐originating RBSN.
Mixed Intermediate Effect in Calcium Alumino/Galliosilicate Glasses
Authors: Balewick Laura Ann , Shelby James E.
Published: March 2005

Glasses in the calcium alumino/galliosilicate system were prepared. All glasses contained 30 mol% CaO. The density, refractive index, and infrared spectra were measured. These glasses exhibited additive behavior in all properties, which implies that Al and Ga play isostructural roles in the glass network. These results agree with those reported previously for lithium alumino/galliosilicate glasses.
Solubility of Ruthenium Dioxide in Lead Borosilicate Glasses
Authors: Palanisamy Ponnusamy , Sarma Dwadasi H. R. , Vest Robert W.
Published: March 2005

The solubility of RuO2 in a lead borosilicate glass was measured in the range of 700° to 1000°C. The effect of dissolved alumina in the glass was studied in the same range using 2%, 4%, 6%, and 10% dissolved alumina. The solubility showed an exponential dependence on temperature, and the dissolved alumina decreased the solubility at any given temperature; however, the solubility was essentially the same in all of the alumina‐containing glasses. The Kelvin equation was used to deduce the concentration of RuO2 in glass as a function of the curvature of the RuO2 particles, and the dependence was found to be quite small. The implications of the results in processing thick‐film resistors using RuO2 as the conductive phase and these glasses as insulating phases on an alumina substrate are discussed.
Indentation Creep of Na2O · 3SiO2 Glasses with Various Water Contents
Authors: Han Won‐Taek , Tomozawa Minoru
Published: March 2005

Time‐dependent deformation behavior induced by an indentation, called indentation creep, was investigated for Na2O · 3SiO2 glasses with various water contents by measuring the indentation depth as a function of time using a conical indenter. It was found that the indentation creep behavior of the glasses could be best explained as steady‐state inhomogeneous (non‐Newtonian viscous) flow. Water in the glasses appeared to reduce the stress (or yield stress) needed to cause this flow. Crack initiation of the glasses was promoted by water through this inhomogeneous flow.
Ceramic Dental Molds for the Precision Casting of Titanium and Titanium Alloys: I, Relationship between Particle‐Size Distribution of Aggregates and Mold Expansion
Authors: Nakamura Masahiko , Jyohzaki Katsuhito , Kimura Takatoshi , Togaya Toshihiro , Ida Kazuo
Published: March 2005

A new ceramic dental mold can cast excellent Ti restoratives by controlling expansion due to the oxidation of additive metal particles. The packing structure of the metal and aggregate particles in the mold can be considered a function of the size distribution of the aggregate grains. An investigation of the expansion mechanism reveals three different relationships of void fraction versus linear thermal expansion for the molds at the high temperature under which oxidation of the metal particles occurs. Room‐temperature casting, favorable for making high‐quality Ti restoratives, can also be achieved by precisely controlling both the amount of additive metal powder and the compositional distribution of aggregate grain sizes in the mold.
Effects of Carbon as a Sintering Aid in Silicon Carbide
Authors: van Rijswijk W. , Shanefield Daniel J.
Published: March 2005

Self‐diffusion data are collected from the literature in an attempt to better understand the strong effects of carbon as a sintering aid in SiC. These data indicate that the presence of excess carbon, in addition to reducing the native SiO2 layer of the SiC, probably enhances the rate‐controling bulk self‐diffusion rate of SiC by a factor of about 100.
Analysis of Hot Isotatic Pressing of Presintered Zirconia
Authors: Kim Jin‐Young , Uchida Nozomu , Saito Katsuichi , Uematsu Keizo
Published: March 2005

Hot isostatic pressing (HIP) of presintered Y‐TZP was studied at 1100° to 1400°C, 5 to 200 MPa, for 0.5 to 4 h. The effects of process variables of HIP and the characteristics of the presintered specimens on the densification behavior of HIP were examined. The microstructural development after HIP was also examined. Grain growth occurred during the densification in HIP. Empirically, a linear relation having a rather constant slope was found between the logarithms of porosities and grain sizes, for each starting condition. Provided this relationship was taken into account, the Ashby's model for HIP could express the densification process for this system satisfactorily.
Interfacial Structure in Heteroepitaxial Silicon on Sapphire
Authors: Aindow Mark
Published: March 2005

Several distinct structural models have been proposed for the interface in heteroepitaxial silicon on sapphire based on transmission electron microscopy observations. These models fall conveniently into three groups according to the mechanism whereby misfit is accommodated. In coherent models misfit is accommodated by twins, in semicoherent models by arrays of misfit dislocations, and in incoherent models by mixed‐mode aluminosilicate bonding at the interface. It is shown that particular specimen geometries or imaging modes are common to each group. Possible explanations for the controversy are proposed in terms of inherent ambiguities in the interpretation of electron microscope images of heterophase boundaries.
Superconducting Glass‐Ceramics in the Bi‐Sr‐Ca‐Cu‐O System
Authors: De Guire Mark R. , Bansal Narottam P. , Kim Cheol J.
Published: March 2005

Bi1.5SrCaCu2Oz was prepared in the glassy state by rapid quenching of the melt. The recrystallization of the glass during various heat treatments was studied by differential thermal analysis, X‐ray diffraction, scanning electron microscopy, and resistivity measurements. Activation energies and frequency factors for the crystallization events below 600°C were determined by nonisothermal differential scanning calorimetry. Heating at 450°C formed mainly the Bi2+xSr2−x‐CuOz solid solution (“R”). Between 765° and 845°C, R reacted slowly with the glass to form the 80 K superconductor Bi2(Sr,Ca)3Cu2Oz and CuO. Heating for 7 d at 845°C, followed by slow cooling, eliminated a low‐temperature resistive “tail” and raised the temperature of zero resistance to 77 K. Heating at 867°C caused partial melting, with segregation of a Bi‐ and Cu‐rich liquid, and loss of the superconducting phases.
Thermodynamic Study of the Cu‐Sr‐O System
Authors: Alcock Charles B. , Li Baozhen
Published: March 2005

The Gibbs energies of formation of the inter‐oxide compounds in the Cu‐Sr‐O system have been obtained from solid‐state electrochemical measurements in the temperature range 900 to 1300 K. Cells employing yttria‐stabilized zirconia or single‐crystal calcium fluoride as electrolyte were used in studies of SrCu2O2, Sr2CuO3, SrCuO2, and “Sr3Cu5O8+z.” The oxygen potential vs stoichiometry dependence was studied, particularly for this last compound, by thermogravimetry at a few selected temperatures in the oxygen partial pressure interval 1 × 101 to 1 × 105 Pa. Based on the results of emf measurement, thermogravimetric study, and X‐ray powder diffraction, phase relations in this system were calculated.
Mixed‐Mode Fracture Toughness of Ceramic Materials
Authors: Suresh S. , Shih C. F. , Morrone A. , O'Dowd N. P.
Published: March 2005

An experimental technique whereby pure mode I, mode II, and combined mode I‐mode II fracture toughness values of ceramic materials can be determined using four‐point bend specimens containing sharp, through‐thickness precracks is discussed. In this method, notched and fatigue‐precracked specimens of brittle solids are subjected to combined mode I‐mode II and pure mode II fracture under asymmetric four‐point bend loading and to pure mode I under symmetric bend loading. A detailed finite element analysis of the test specimen is performed to obtain stress intensity factor calibrations for a wide range of loading states. The effectiveness of this method to provide reproducible combined mode I‐mode II fracture toughness values is demonstrated with experimental results obtained for a polycrystalline Al2O3. Multiaxial fracture mechanics of the Al2O3 ceramic in combined modes I, II, and III are also described in conjunction with the recent experimental study of Suresh and Tschegg (1987). While the mode II fracture toughness of the alumina ceramic is comparable to the mode I fracture toughness KIc, the mode III fracture initiation toughness is 2.3 times higher than KIc. The predictions of fracture toughness and crack path based on various mixed‐mode fracture theories are critically examined in the context of experimental observations, and possible effects of fracture abrasion on the apparent mixed‐mode fracture resistance are highlighted. The significance and implications of the experimental methods used in this study are evaluated in the light of available techniques for multiaxial fracture testing of brittle solids.
Deep Levels Near the Grain Boundary in a Zinc Oxide Varistor: Energy Change Due to Electrical Degradation
Authors: Tanaka Junzo , Hishita Shun‐ichi , Okushi Hideyo
Published: March 2005

Localized deep levels in a ZnO varistor were investigated by isothermal capacitance transient spectroscopy, in relation to a degradation of nonlinear electrical characteristics caused by thermal treatment in a slightly reduced atmosphere. Two discrete deep levels were observed near the grain boundary. Before the degradation, they were located at 0.36 and 0.19 eV below a conduction band. After the degradation, they shifted toward the conduction band by 0.16 and 0.09 eV, respectively: the energy shift was not identical for the two levels. It was suggested that the energy shift of the localized levels reflected lattice disorders originally existing near the grain boundary or proper changes of chemical states induced by the reduction.
High‐Temperature Extrusion Behavior of a Superplastic Zirconia‐Based Ceramic
Authors: Kellett Bruce J. , Carry Claude , Mocellin Alain
Published: March 2005

Workability of 3‐mol%‐yttria‐stabilized tetragonal ZrO2 has been gauged through a series of extrusion experiments performed under vacuum with graphite dies at 1500°C and 35 MPa piston stress. It is shown that dense and smooth extrusions can be obtained from solid billets when graphite paper is used as a lubricant. Sigmoidal dies and conical dies with cone angles of 18.4°, 26.6°, and 45° and diameter ratios of 1.5, 2, and 3 were used to explore extrusion behavior. Observed piston velocities correspond to what may be predicted from the experimental uniaxial constitutive creep equation and a simple slab analysis. A precise analysis, however, is not attempted because of lack of steady‐state behavior of the material itself.
Thermal Expansion of Mullite
Authors: Schneider Hartmut , Eberhard Emil
Published: March 2005

The thermal expansion coefficients of transition‐metal‐free sinter mullite and fused mullite, and of chromium‐doped (11.5 wt% Cr2O3) and iron‐doped (10.3 wt% Fe2O3) sinter mullites are measured between 25° and 900°C by high‐temperature Guinier X‐ray diffraction techniques. Most mullites display low and nonlinear thermal expansions below, but larger and linear expansion above, ∼300°C. Although the temperature‐induced c‐axis expansion coefficients seem to be less dependent on the compositional state x and on transition‐metal incorporation of the Al4+2xSi2–2xO10–x mullite‐type phases (α(c) = 5.6 × 10−6/°C to 6.1 × 10−6/°C), thermal a‐ and b‐axis expansion coefficients change more significantly (α(a) = 3.1 × 10−6/°C to 4.1 × 10−6/°C and α(b) = 5.6 × 10−6/°C to 7.0 × 10−6/°C, where the values were calculated between 300° and 900°C). The larger temperature‐induced b than c and a expansions probably are caused by intense lengthening of the relatively long and elastic octahedral Al(1)–O(D) bonds in mullite, which form at an angle of about 30° with b, but of about 60° with a. With increasing x value of the transition‐metal‐free mullites, the volume thermal expansion decreases, while the anisotropy of thermal expansion is reduced simultaneously. We believe that the variation of the thermal expansion coefficients is controlled by the Al* occupancy and by the number of O(C) vacancies in the mullite structure, and also by the disordering distribution of both structural elements. Transition‐metal incorporation into mullite has no distinct influence on thermal expansion anisotropy, but does reduce thermal volume expansion. A prestressing of the crystal structure by substitution of Al3+ by the larger Fe3+ and Cr3+ ions may be the main reason for the latter effect.
Electronic Structure of High‐Temperature Superconductors: Comparison of Theory and Experiment
Authors: Kasowski Robert V.
Published: March 2005

The ab initio pseudofunction method has been used to compute electronic properties of superconductors in good agreement with experiment. La2CuO4 and CuO are computed to be antiferromagnetic and semiconducting in agreement with experiment. The 0.4‐eV peak in the optical conductivity spectrum has been shown to be an interband transition for YBa2Cu3O7. The bands for Bi2Sr2CaCu2O8 as well as YBa2Cu3O7 agree well with angle‐resolved photoemission near Ef. The shake‐up peaks near 9 and 12 eV observed in photoemissio can be understood in terms of two‐electron processes envolving the local densities of states of Cu and O. A mechanism for superconductivity based on these electronic properties will be advanced. All calculations use local density potentials with no parameters.
Fundamental Optical Absorption Edge of Sputter‐Deposited Zirconia and Yttria
Authors: Aita Carolyn Rubin , Kwok Chee‐Kin
Published: March 2005

Zirconia and yttria films were sputter deposited onto unheated fused silica substrates using a metal target and rare gas‐oxygen discharges. Double‐beam spectrophotometry was used to measure the transmission and reflection as a function of incident photon energy, E, from which the absorption coefficient, α(E), was calculated. An indirect interband transition at Ei= 4.70 eV and two direct interband transitions at Eg1= 5.17 eV and Eg2= 5.93 eV occur in monoclinic zirconia. Two direct interband transitions at Eg1= 5.07 eV and Eg2= 5.73 eV occur in cubic yttria. The absorption edge structure is modified when unusual phases, such as tetragonal zirconia, and zirconia and yttria with no longrange crystallographic order, are present.
Elastic Properties of Silica Xerogels
Authors: Abramoff Bennet , Klein Lisa C.
Published: March 2005

To calculate elastic constants, longitudinal and tranverse acoustic wave velocities were measured for silica xerogels as a function of relative humidity (rh). The silica xerogels studied are microporous with open porosity of 53 vol%. The longitudinal wave velocity exhibits a minimum at about 35% rh. The transverse wave velocity decreases to a constant value for 35% rh. Consequently, Young's modulus is a minimum at about 35% rh, whereas the shear modulus decreases to a constant value at 35% rh. The bulk modulus and Poisson's ratio exhibit minimum values at about 15% rh. Young's modulus decreases from 4.91 to 3.42 GPa at 35% rh and then increases to 3.60 GPa at 55% rh. Poisson's ratio decreases from 0.184 to 0.164 at 15% rh and then increases to 0.272 at 55% rh. Below 35% rh, silica xerogels adsorb a monolayer of hydroxyls, whereas above 35% rh silica xerogels show pore filling.
Processing and Characterization of Hexagonal Ferrite Thin Films from Organometallic Compounds
Authors: Ryder Daniel F. , Fenstermacher Janet H.
Published: March 2005

Thin films of BaFe12O19 were deposited on resistor‐grade alumina by a chemical solution processing method involving the hydrolysis of a barium alkoxide and iron carboxylate. Conversion of the deposited organometallic precursor to an amorphous oxide was accomplished by a thermal decomposition/oxidation at 370°C. Subsequent development of the M‐type hexagonal structure was accomplished with postannealing in air at temperatures in excess of 600°C. All films were characterized by scanning electron microscopy, X‐ray diffraction, and magnetic measurements.
Pyroxene‐Type (K,Na,Li)VO3 Solid Solutions
Authors: Chang Luke L. Y. , Huang Da‐Wei
Published: March 2005

Phase relations in the system were studied between 600° and 350°C. All three end members have the pyroxene‐type structure. Both LiVO3 and NaVO3 are monoclinic, whereas KVO3 takes the orthorhombic symmetry. At 360°C, the join LiVO3–NaVO3 is characterized by two series of pyroxene‐type solid solutions: NaVO3–(Na0.44Li0.56)VO3 and LiVO3‐(Na0.16Li0.84)‐VO3. (K0.5Na0.5)VO3 and (K0.5Li0.5)VO3 are two other stable phases at 360°C. The pyroxene‐type (K0.5Na0.5)VO3 has a range of solid solution from (K0.6Na0.4)VO3 to (K0.4Na0.6)VO3 along the join and extends into the ternary field with a maximum of 13 mol% LiVO3. (K0.5Li0.5)VO3 has no detectable range of solid solution, and its X‐ray powder diffraction data cannot be indexed based upon either the monoclinic or the orthorhombic unit cell.
Controlled‐Pore‐Size Composite Nickel Oxide Structures for Carbonate Fuel Cell Cathodes
Authors: Baumgartner Charles E.
Published: March 2005

The performance of a molten carbonate fuel cell (MCFC) is significantly eroded when fuel and oxidant gases are allowed to combine chemically as occurs with a loss of gas impermeability by the cell's electrolyte structure. This performance decline is eradicated when the cell's electrodes possess pore size distributions small enough to absorb sufficient electrolyte to act as a secondary gas barrier. Described here is a process for preparing composite MCFC NiO cathodes where the median pore size of the gas barrier region is varied from 3.2 to 0.4 μm by adjusting the starting powder's Ni/NiO ratio. When filled with liquid Li2CO3‐38 mol% K2CO3 at 923 K, these structures possess a gas pressure resistance of between 2.3 × 105 and 13.6 × 105 Pa, respectively. The incorporation of these composite cathodes in a MCFC can prevent the penetration of the oxidant gas into the fuel cell's interior.
Analysis of the Pyrolysis Products of Dimethyldichlorosilane in the Chemical Vapor Deposition of Silicon Carbide in Argon
Authors: Cagliostro Domenick E. , Riccitiello Salvatore R. , Carswell Marty G.
Published: March 2005

A study of the products and reactions occurring during the chemical vapor deposition of silicon carbide from dimethyl‐dichlorosilane in argon is presented. Reaction conditions were as follows: 700° to 1100°C, a contact time of ∽1 min, and a pressure of 1 atm (∽0.1 MPa). At these conditions, the gases that formed were mainly methane, hydrogen, silicon tetrachloride, trichlorosilane, and methyltrichlorosilane. Hydrogen chloride might also be present, but was not determined. The silicon carbide solid that formed showed the presence of hydrogen and chlorine as impurities, which might degrade the silicon carbide properties. These impurities were eliminated slowly, even at 1100°C, forming hydrogen, trichlorosilane, and silicon tetrachloride.
Solid Ceramic Fibers via Impregnation of Activated Carbon Fibers
Authors: Card Roger J. , O'Toole Michael P.
Published: March 2005

A process has been developed which allows the preparation of some solid ceramic fibers. In this process, activated carbon fibers which are porous throughout their entire diameter are impregnated with metal‐containing solutions such as silicon tetrachloride. Calcination and sintering result in the formation of solid SiO2 fibers. Use of woven mats or nonwoven felts of activated carbon fibers results in the formation of woven mats or nonwoven felts of silica fibers.
Topotactically Modified Stress in Coatings
Authors: Kalman Zwi H. , Johnson Lois A. , Wachtman John B.
Published: March 2005

The unexpected occurrence of compressive stresses in solgel‐derived alumina coatings following a densification transformation was found to be related to the topotactic transformation of boehmite (γ‐AlOOH) to γ‐alumina (γ‐Al2O3) on heating. The boehmite coating showed a pronounced fiber texture with the [001] crystallographic axis perpendicular to the substrate‐coating interface plane. At about 500°C, the orthorhombic boehmite transformed into tetragonal γ‐alumina with very little, if any, reorientation of the crystallites. Because of the topotactic nature of the transformation, the fiber texture persisted in the γ‐alumina phase. This resulted in an anisotropic change that generated compressive strains in the coating.
Electrical Properties of Polycrystalline Nickel Zinc Ferrites
Authors: Irvine John T. S. , Huanosta Alfonso , Valenzuela Raul , West Anthony R.
Published: March 2005

The electrical properties of three ferrites of composition (Ni0.36Zn0.64)1–xFe2+xO4, x=−0.03, 0, 0.13, were investigated by impedance spectroscopy. The observed impedance responses corresponded to that of a highly resistive grain‐boundary component in series with a less resistive bulk component. Bulk resistances gave linear Arrhenius conductivity plots (Ea≤0.25 eV) that were independent of thermal cycling. Grain‐boundary resistances changed greatly on cycling between room temperature and 200°C, probably because of changes in the extent of oxidation at the grain boundaries.
Low‐Temperature Metal‐Organic Chemical Vapor Deposition of Silicon Nitride
Authors: Du Honghua , Gallois Bernard , Gonsalves Kenneth E.
Published: March 2005

Amorphous silicon nitride films have been deposited on single‐crystal silicon from the gas mixture of methylsilazane and ammonia at 873 to 1073 K. The films have been characterized by ellipsometry, Fourier transform infrared spectroscopy, and Auger electron spectroscopy. The Si‐C, Si‐H, and C‐H bonds in methylsilazane can be effectively cleaved and the associated C and H species removed. The structure and composition of the films do not show any apparent dependence on the deposition temperature.
Crystal Orientation Dependence of Machning Damage–A Stress Model
Authors: Blackley Winston S. , Scattergood Ronald O.
Published: March 2005

The pitting damage occurring along preferred crystal orientations observed after single‐point diamond turning of brittle materials dictates a limiting feed rate at which the material can be machined. Using elasticity theory, the orientation dependence of the pitting can be qualitatively explained. Calculations based on a line‐force stress model predict the orientation dependece of the damage symmetry. The results are in good agreement with machining tests for a series of germanium single‐crystal wafers.
Crack‐Tip Transformation Zones in Toughened Zirconia
Authors: Marshall David B. , Shaw Michael C. , Dauskardt Reinhold H. , Ritchie Robert O. , Readey Michael J. , Heuer Arthur H.
Published: March 2005

Transformation zones surrounding cracks in several toughened magnesia‐partially‐stabilized zirconia ceramics are characterized by optical interference measurements of surface uplift and by Raman microprobe spectroscopy. The measurements demonstrate that the volume fraction of transformation is nonuniform within the zone and that the extent of the frontal zone is approximately the same as that of the wake. Results are used to evaluate the crack‐tip shielding stress intensity factor and to compare with the measured fracture toughness.
Transitional Phases at Ceramic–Metal Interfaces: Orthorhombic, Cubic, and Hexagonal Ti‐Si‐Cu‐N Compounds
Authors: Carim Altaf H.
Published: March 2005

Reaction products at the boundary between Si3N4 and Ag‐Cu‐Ti braze alloy have been characterized by transmission electron microscopy. Beyond the TiN adjoining the Si3N4 substrate is a continuous layer of a previously unreported, orthorhombic Ti‐Si‐Cu‐N compound. Evidence of additional cubic and hexagonal Ti‐Si‐Cu‐N phases is also found; these exist as isolated grains farther from the substrate. The cubic compound is probably an η‐type M6X phase of the sort observed at other ceramic‐metal interfaces. These “dilute ceramics,” in which metallic constituents dominate, are common transitional phases in ceramic‐metal systems.
Experimental Examination of the Push‐Down Technique for Measuring the Sliding Resistance of Silicon Carbide Fibers in a Ceramic Matrix
Authors: Weihs Timothy P. , Nix William D.
Published: March 2005

The use of a push‐down technique to measure the sliding resistance of NICALON fibers (SiC) in a lithium aluminosilicate (LAS) matrix has been examined experimentally. Tests have been conducted on more than 300 fibers in 2 different samples of the as‐processed SiC/LAS III composite. Results show that the push‐down measurements on an individual sample are reproducible, that the sliding resistance can vary significantly between samples, and that Poisson's expansion does not affect push‐down measurements of fibers in this system.
Microstructure of Solution‐Processed Lead Zirconate Titanate (PZT) Thin Films
Authors: Carim Altaf H. , Tuttle Bruce A. , Doughty Daniel H. , Martinez Sheri L.
Published: March 2005

Lead zirconate titanate (PZT) thin films with a composition near the morphotropic phase boundary (Zr/Ti = 53/47) were fabricated by spin deposition of an alkoxide‐derived solution and annealed at 650°C for 30 min. A complex microstructure is observed in which micrometer‐scale rosettes of the desired perovskite phase are surrounded by nanocrystalline (10 to 15 nm) grains of pyrochlore structure. Transmission electron microscopy (TEM) demonstrates that the perovskite rosettes—features of approximately circular cross section which grow rapidly within the confined conditions of the thin film—are single crystals despite being highly porous. Pockets of lead‐deficient pyrochlore extend throughout the thickness of the film. The only effects of Nb (2%) doping on the microstructure are to increase the fraction of the perovskite phase and the perovskite grain size. Despite the highly irregular shape of the perovskite particles and the presence of some pyrochlore, reasonable ferroelectric properties are measured (spontaneous polarization Ps∼ 0.2 C/m2).
Thermodynamic Study of Reduced Phases in the BaLa4Cu5O13.1−x System
Authors: Thiele Erik , Davies Peter K.
Published: March 2005

The thermodynamic stabilities of reduced, perovskiterelated cuprates with the BaLa4Cu5O13.1−x structure were investigated using differential scanning calorimetry techniques. For 0 < x 0.6 the partial molar enthalpy of oxygen in the structure is constant at ‐187.4 kJ/mol O2. This value is almost identical to those reported for other perovskite cuprates. These results suggest that −190 kJ is a reliable value for the oxidation of Cu from 2+ to 3+ and can be used with confidence in calculations of the stabilities of the p‐doped perovskite cuprates. Anomalously large enthalpies measured for samples prepared with higher values of x under forming gas probably result from trace contaminants of Cu metal.
Knoop Microhardness Anisotropy of Single‐Crystal Cassiterite (SnO2)
Authors: Li Hong , Bradt Richard C.
Published: March 2005

The Knoop microhardness anisotropy of single‐crystal cassiterite (SnO2) was measured on the (100), (110), (001), and (111). That anisotropy is depicted as the microhardness profiles for those planes. The results are addressed first in terms of the elastic anisotropy of SnO2 and then on the basis of the effective resolved shear stress (ERSS), the latter an approach initially advanced by Brookes and co‐workers. The load dependence of the Knoop microhardness is also evaluated in terms of the classical Meyer's law for which it is demonstrated that the Meyer's law coefficient and Meyer's law exponent are related.
Colloidal Processing and Mechanical Properties of Whisker‐Reinforced Mullite Matrix Composites
Authors: Hirata Yoshihiro , Matsushita Shinichi , Ishihara Yoshimi , Katsuki Hiroaki
Published: March 2005

Aqueous colloidal suspensions in the two systems of CVD‐processed ultrafine mullite powder (<0.1 μm), ‐Si3N4 whisker and ‐mullite whisker, were prepared near the isoelectric point of mullite (pH 7.0) to prevent cracking during drying of wet green compacts consolidated by filtration. The freeze‐dried porous green compacts were hot‐pressed with a carbon die at 1500°C for 1 h at a pressure of 39 MPa in N2 atmosphere. The relative densities of the mullite matrix composites with whiskers of 0 to 10 vol% were in the range of 95.2% to 99.8%. Increasing the fraction of Si3N4 whisker increased the density, flexural strength, and fracture toughness of the hot‐pressed composites. On the other hand, addition of the mullite whisker increased the fracture toughness but decreased the density and strength of the composites.
High‐Temperature Compressive Creep of Polycrystalline Mullite
Authors: Hynes Anne P. , Doremus Robert H.
Published: March 2005

Aluminosilicates of three compositions with mullite as the major phase were synthesized by a sol‐gel process and characterized with bulk and microchemical analyses and microstructural observation. An apparatus for measuring the compressive creep up to 1900 K with a sensitivity of ±1 μm was constructed and used to measure the creep of singlephase mullite, mullite with second‐phase glass, and mullite with second‐phase corundum. Measurements in air at stresses of 15 to 100 MPa and temperatures of 1471 to 1724 K determined that samples with second‐phase glass crept more rapidly than single‐phase mullite or mullite with secondphase corundum. The apparent creep activation energies determined at 100 MPa were 742 kJ/mol for the mullite containing glass, 819 kJ/mol for the single‐phase mullite, and 769 kJ/mol for the mullite with second‐phase corundum. The stress exponents determined at 1724 K were 1.6 for the mullite plus glass, 1.5 for the single‐phase mullite, and 1.2 for the mullite with α‐Al2O3. The creep behavior of the aluminosilicates containing glass were consistent with rate control by the viscous flow of the glass and the measured creep rates were in good agreement with creep rates calculated from a model by Dryden. The creep behavior of the completely crystalline aluminosilicates was consistent with rate control by diffusional creep.
Steady‐State Creep Behavior of Si–SiC C‐Rings
Authors: Chuang Tze‐jer , Liu Wei‐jun , Wiederhorn Sheldon M.
Published: March 2005

Because of the ease of experimental setup as well as economics in sample preparation, C‐ring specimens are sometimes chosen for the evaluation of mechanical behavior. In this paper, the long‐term creep of siliconized silicon carbide (Si–SiC) C‐rings is investigated. Creep tests on a number of Si–SiC C‐rings were carried out under constant compressive loads at 1300°C in air. Load‐point displacements were continually monitored as a function of time, thereby establishing the steady‐state regime as a function of load and ring geometry. Optical micrography on the postcrept specimens was performed to obtain damage zone sizes. A simple curved beam theory was employed to analyze the stress state developed throughout the body during steady‐state creep. Loadpoint displacement rates were numerically calculated using both geometric and energy methods. Observed damage zone sizes and shapes within the specimen agreed with those predicted theoretically. Results obtained on the stress solutions are useful as local loading parameters in the study of high‐temperture fracture behavior of a cracked C‐ring.