bluuebunny/crossref_metadata_2025 huggingface dataset:
Ordering Structure and Dielectric Properties of Undoped and La/Na‐Doped Pb(Mg1/3Nb2/3)O3
Authors: Chen Jie , Chan Helen M. , Harmer Martin P.
Published: March 2005

Transmission electron microscopy was used to study the ordered domain structures in undoped and La/Na‐doped Pb(Mg1/3Nb2/3)O3 (PMN), where the compositions of the doped samples were specifically chosen so as to elucidate the ordering mechanism. The results showed that the Mg2+ ions and Nb5+ ions are short‐range ordered on the B‐site sublattice in undoped PMN, with a domain size of 2 to 5 nm. This short‐range ordering gives rise to B‐site composition fluctuations occurring on a nanometer scale, and it is this compositional inhomogeneity which is believed to be responsible for the diffuse phase transition behavior. Donor doping with La2O3 can compensate for the local charge imbalance resulting from the short‐range order and thus enhances the degree of ordering. Acceptor doping with Na2O, however, increases the charge effect, and hence ordering is suppressed. The effect of Na doping and La doping on the dielectric properties of PMN is also discussed.
Effect of Heat Treatments on the Wetting Behavior of Bismuth‐Rich Intergranular Phases in ZnO:Bi:Co Varistors
Authors: Gambino Jeffrey P. , Kingery W. David , Pike Gordon E. , Levinson Lionel M. , Philipp Herbert R.
Published: March 2005

The effect of annealing on the wetting behavior of Bi‐rich intergranular phases in ZnO:Bi:Co varistors was studied. The intergranular phase exhibits temperature‐dependent grain‐boundary wetting, with an average equilibrium dihedral angle of 0° at 1140°C and over 55° at 610°C. The temperature‐dependent wetting may be related to the temperature dependence of the ZnO concentration in the Bi2O3 liquid phase. The effect of the intergranular phase distribation on the electrical properties of ZnO varistors is discussed.
Preparation and Properties of Hot‐Pressed YBa2Cu3O7‐x
Authors: Loehman Ronald E. , Hammetter William F. , Venturini Eugene L , Moore Roger H. , Gerstle Frank P.
Published: March 2005

We have hot‐pressed dense, bulk specimens of the YBa2Cu3O7‐x composition to near‐theoretical density. Powders made by the mixed‐oxide method were pressed at 6.9 MPa (1000 psi) in Al2O3‐lined SiC dies at 950°C in air or oxygen for 30 min. Relative densities of 94% to 97% of theoretical were obtained for speciments as large as 200 g. In the aspressed condition the ceramic has an oxygen stoichiometry near YBa2Cu3O6.28, as determined by thermogravimetric reduction in hydrogen and by wet chemical analysis. Scanning electron microscoy shows a microstructure of elongated grains that are partially oriented by the hotpressing. Oxidation of bulk specimens of dense, as‐hot‐pressed material produced intergranular porosity, presumably from anisotropic contraction as individual grains are converted from the tetragonal to the orthorhombic structure. We studied the reoxidation kinetics of hot‐pressed specimens to the orthorhombic, superconducting state by thermogravimetry in flowing oxygen. Isothermal weight gains can be fitted to an Avrami model with an activation energy of 96 kJ / mol. Superconducting quantum interference detector (SQUID) measurements of magnetic susceptibility reveal a small anisotropy that is related to the preferred orientation seen in the microstructure. The average thermal expansion coefficient (CTE) of the ashot‐pressed, tetragonal material is 135 × 10−7°C−1 from room temperature to 800°C, which increases to 155 × 10−7°C−1 for reoxidized specimens. For reoxidized material the CTE is not linear with temperature, but shows a sharp break at approximately 450°C. Measurements of Young's modulus show there is a 36% decrease for reoxidized specimens as compared with ceramics in the as‐hot‐pressed condition. The decrease in modulus appears to be related to the development of microstructural porosity and cracking as a result of the reoxidation.
Preparation of Fine, Spherical Yttria‐Stabilized Zirconia by the Spray‐Pyrolysis Method
Authors: Dubois Bertrand , Ruffier Daniel , Odier Philippe
Published: March 2005

Fine yttria‐stabilized zirconia powders were prepared by the spray pyrolysis of aqueous solutions of ZrOCl2·8H2O and Y(NO3)3·5H2O (3 mol%). An appropriate thermal treatment resulted in slightly porous spherical particles with a narrow size distribution. The sintering ability of these powders was evaluated.
Spherical‐Impact Damage and Strength Degradation in Silicon Carbide Whisker/Silicon Nitride Composites
Authors: Akimune Yoshio , Katano Yasushi , Matoba Kazuo
Published: March 2005

The silicon carbide (SiC) whisker reinforcement of silicon nitride (Si3N4) improves fracture strength and toughness, hardness, and Young's modulus, resulting in higher resistance of the composites to sphere penetration and crack initiation at spherical impact. Sintered Si3N4 shows an elastic/plastic response and initiates median/radial cracks at 100 m/s impact velocity. SiC‐whisker/Si3N4 composites, on the other hand, demonstrate an elastic response, with Hertzian cone crack initiation, only when impact velocity exceeds 280 m/s. The SiC‐whisker/Si3N4 composites thus exhibit improved strength degradation versus critical impact velocity characteristics because of improved mechanical properties provided by the SiC whiskers.
Ionic Resistivity in Fused Silica: I, Conduction‐History Effects
Authors: Jain Vijay , Varshneya Arun K. , Bihuniak Peter P.
Published: March 2005

The steady‐state dc ionic resistivity measurements on GE 124 commercial fused silica glass were carried cut in nonblocking mode between 300° and 650°C using fused Li+, Na+, and K+ nitrate salts as electrodes. The alkali ions were electrolyzed through a sample disk in a U‐tube‐type dc conductivity cell until steady state was achieved. Sequential conduction experiments at varying temperatures using various ions in a given sample showed that the resistivity of an ion could vary by as much as 1 order of magnitude depending upon the conduction history of the specimen, i.e., which ions had been electrolyzed through the specimen during prior experiments.
Role of Ceria in Enhancing the Resistance of Aluminosilicate Refractories to Attack by Molten Aluminum Alloy
Authors: O'Brien Michael H. , Akinc Mufit
Published: March 2005

Rare‐earth doping of aluminosilicate refractory compositions has been shown to impart improved resistance to attack by molten aluminum alloys. The basis for this improved alloy attack resistance was investigated. The addition of rare‐earth oxide to bauxite was found to alter the microstructure to form large, uniformly shaped corundum crystals interlinked with cerium‐containing strands. It was concluded that this unique microstructure was responsible for the enhanced attack resistance of the refractories. Other possible causes for improved alloy attack resistance, such as enhanced sintering, excess oxygen provided by the CeO2 dopant, or higher thermodynamic stability of the phases formed in the doped refractory compositions, were not consistent with the experimental findings.
Phases and Microstructures in Zirconia‐Calcia‐Titania Multiphase Ceramics
Authors: Agrawal Dinesh C. , Gopalakrishnan R. , Chakravorty Dipankar
Published: March 2005

Two cubic phases of differing compositions and lattice parameters and a monoclinic phase are found in samples of zirconia with 8 mol% calcia and 4 to 10 mol% titania. Increasing amounts of titania reduce the amounts of the cubic phases. The monoclinic phase dissolves small amounts (∼1 mol%) of calcia and up to 4.7 mol% titania; these amounts for the cubic phases are much larger. The cubic lattices contract linearly with increasing amounts of titaia and a contraction in the monoclinic lattice is also evident.
Poling of Coprecipitated Lead Titanate‐Epoxy 0–3 Piezoelectric Composites
Authors: Lee Moon‐Ho , Halliyal Arvind , Newnham Robert E.
Published: March 2005

The 0–3 piezoelectric composites were prepared using a dielectric gel with 70 vol% of coprecipitated PbTiO3 powders with sizes of 12 to 1100 nm. Composites were cured at 80°C for 10 h and poled with E= 0 to 100 kV /cm for 0 to 15 min at 85°C. X‐ray diffraction patterns taken on the surface of the composites before and after poling indicated that saturation poling was nearly achieved at 80 kV /cm. d̄33 was strongly dependent upon the particle size. Composites of fine PbTiO3 powders, smaller than 200 nm, needed a much longer poling time to saturate than large particles probably because of the rotation of the single‐domain crystallites during poling.
A Machinable Calcia–Alumina–Yttria–Silica Glass‐Ceramic
Authors: Makishima Akio , Asami Mitsuaki , Ogura Yoshiji
Published: March 2005

It is found that a crystallized aluminosilicate glass having needlelike Ca4Y6O(SiO4)6 crystals is machinable. Densities, Vickers hardnesses, Young's moduli, thermal expansion coefficients, and fracture toughnesses of the crystallized glasses were measured.
Valid KIC Determination via In‐Test Subcritical Precracking of Chevron‐Notched Bend Bars
Authors: Sung Jason , Nicholson Patrick S.
Published: March 2005

The initiation of the crack in a chevronnotched bend bar was studied. Ideal testing conditions depend strongly on specimen preparation and material; valid KIC values do not always result. An in‐test precracking technique was developed for ceramics at room and elevated temperatures. A yttria‐partially‐stabilized zirconia ceramic was tested, and the results showed that ideal testing conditions were obtained, which guaranteed valid KIC determination.
Formation of Zirconia Titanate Solid Solution from Alkoxides
Authors: Yamaguchi Osamu , Mogi Hiroshi
Published: March 2005

In the system ZrO2–TiO2, ZrTiO4 solid solutions prepared by the simultaneous hydrolysis of zirconium and titanium alkoxides crystallize at low temperatures from amorphous materials between 30 and 70 mol% TiO2. As zirconium is substituted for titanium, the solid solutions can be indexed in an orthorhombic unit cell with a and c decreasing linearly from 0.4832 to 0.4778 nm and from 0.5063 to 0.5002 nm, respectively, and b increasing linearly from 0.5401 to 0.5478 nm. The volume of the unit cell decreases continuously with increasing TiO2 content. At higher temperatures the solid solutions decompose into ZrTiO4 and either ZrO2 (monoclinic) or TiO2 (rutile), depeanding on the starting composition.
Further Studies on Thermal Evolution of Glass‐Forming Gels
Authors: Montenero Angelo , Ugozzoli Franco , Schiroli Dino , Gnappi Guglielmina , Giori D. Camillo , Pye L. David
Published: March 2005

Glasses of composition Na2O·2SiO2 containing iron oxide are obtained by the gel technique. The results show that the iron atoms accelerate the formation of the glass, whereas when they are present in low percentage (∼2%), the temperature necessary to obtain the glass phase is higher. During heat treatment at 250°C, two silicates are formed: Na2SiO3 and Na4SiO4. When the iron content is low, they change at 650°C into Na2Si2O5, but when the iron content is higher (∼6%), this silicate does not form and the system becomes vitreous.
Decomposition Chemistry of Tetraethoxysilane
Authors: Desu Seshu B.
Published: March 2005

SiO2 films deposited by the decomposition of tetraethoxysilane (TEOS) at high temperatures have superior insulating properties and excellent step coverage, but are not compatible with aluminum metallization. Earlier work on the deposition of SiO2 from TEOS concentrated on the properties of the film and on the modeling of thickness uniformity, but no attempt was made to probe the decomposition chemistry. In the present work a three‐step model is presented to explain the TEOS deposition chemistry. A useful deposition temperature is determined by the first step where an intermediate is formed in the gas phase. This activated species adsorbs on the surface and later decomposes into SiO2. Equilibrium constants for the gas‐phase intermediate formation and surface adsorption respectively are 1.38 × 1010 exp[(– 299.24 kJ/mol)/RT] and 1.14 × 104 exp[(– 21.80 kJ/mol)/RT]. The rate constant for the surface decomposition of the intermediate is 1.29 × 10−6 exp[(– 12.26 kJ/mol)/RT]. This model successfully explains rate dependence on pressure and temperature. Approaches are suggested which would catalyze the formation of the activated species and thus lower the deposition temperature such that this process could be used instead of aluminum metallization.
Mechanical Properties and Microstructure of Whisker‐Reinforced Alumina–30 vol% Glass Matrix Composite
Authors: Chaim Rachman , Baum Laura , Brandon David G.
Published: March 2005

High‐density whisker‐reinforced composites of an alumina‐30 vol% glass matrix material were produced by hot‐pressing in the temperature range 1350° to 1400°C in air. Significant improvement was observed in the strength of composites containing 15 vol% SiC whiskers, up to ∼550 MPa, but with only a small effect on the fracture toughness. In composites containing Si3N4 whiskers, no reinforcement was achieved. Transmission electron microscopy showed the formation of a protective layer of amorphous silica on the SiC whiskers, while the Si3N4 whiskers were found to react with the matrix. The mechanical properties were related to the microstructure and the density of the samples.
Wetting Reaction between Lithium Fluoride and Barium Titanate
Authors: Lin Jiu‐Nun , Wu Tai‐Bor
Published: March 2005

The wetting reaction between LiF and BaTiO3 was studied by examining the reaction products on the LiF‐wetted surface of BaTiO3. A reduction reaction occurred between the melt of LiF and BaTiO3, causing the formation of LiTiO2 and volatilization of fluorine. The wetted surface consisted of spherical particles of LiTiO2 and eutectically decomposed phases of BaLiF3 and LiTiO2. The formation of LiTiO2 was repressed by the addition of excess BaO in the melt of LiF, and a well‐spread film was formed on the surface of BaTiO3. The wetting between BaLiF3 and BaTiO3 was also examined. The reaction of forming LiTiO2 did not occur, and a mosaic film was formed on the surface of BaTiO3.
Hybrid Gels for Homoepitactic Nucleation of Mullite
Authors: Huling Jeffrey C. , Messing Gary L.
Published: March 2005

Hybrid gels, defined as gels from mixtures of polymerically and colloidally derived sols, offer many opportunities for crystalline microstructure development upon heating. In this study, hybrid mullite gels are formed by mixing a colloidal boehmite—silica sol with a polymeric aluminum nitrate—tetraethoxysilane‐derived sol. The polymeric gel crystallizes in situ to form mullite that acts as seed crystals for homoepitactic nucleation during the subsequent transformation of the colloidal component of the hybrid gel. Compared with the entirely colloidal gel, the introduction of a 30 wt% polymeric gel fraction results in an increase in apparent nucleation frequency from ∼5x1011 to ∼1x1014 nuclei / cm3 at 1375°C, a reduction in high‐temperature grain size from 1.4 to 0.4 μm at 1550°C, and an increase in the degree of microstructural homogeneity, as evidenced by intragranular pore removal.
High‐Tc Superconductor Coating on Metal Substrates by an Electrophoretic Technique
Authors: Maiti H. S. , Datta S. , Basu R. N.
Published: March 2005

High‐Tc superconductor wire and sheet were fabricated by electrophoretic deposition of superconductor powders on silver‐metal substrates followed by an appropriate sintering schedule. The technique has some promise for technological applications.
Flow Microcalorimetric Estimation of Energy Changes During Surface Reactions of Glass Powders with Aqueous Media
Authors: Fletcher Joseph P. , Hudson Guy F. , Raghavan Srini , Risbud Subhash H.
Published: March 2005

Flow microcalorimetry was used for an investigation of the surface chemical nature of glass surfaces. A commercially available flow microcalorimeter allowed for the measurement of heat energy changes during surface reactions of two different glass powders with pyridine and water solutions. Pure fused silica glass and a fluorozirconate glass were the materials studied by flow microcalorimetry.
Dynamic and Static Fatigue Behavior of Sintered Silicon Nitrides: II, Microstructure and Failure Analysis
Authors: Rawlins Mary H. , Nolan Theodore A. , Allard Lawrence F. , Tennery Victor J.
Published: March 2005

The microstructures and failure mechanisms of a sintered silicon nitride, tested under dynamic and static fatigue conditions, were studied. For longer test times at 1000°C, all specimens showed a slow crack growth mechanism of failure, with failure generally initiating at a defect. Even more importantly, these specimens contained cracks in addition to the failure fracture which extended into the material surrounding the initiation site. The cracking always appeared to occur within an amorphous grain‐boundary phase.
Synthesis of High‐Purity Zirconium Fluoride for Fluoride Glass Fibers by Chemical Vapor Deposition
Authors: Fujiura Kazuo , Ohishi Yasutake , Sakaguchi Shigeki , Terunuma Yukio
Published: March 2005

A technique for synthesizing ultra‐high‐purity ZrF4 using chemical vapor deposition in a ZrBr4–HF system is developed and a purification mechanism is clarified. The Fe concentration in ZrF4 is evaluated at less than 10 ppb based on analysis of the transmission loss spectrum of a fiber prepared using synthesized ZrF4. Purification is achieved mainly in a sublimation process of ZrBr4, and purification efficiency is determined by sublimation temperature and activity of impurities in ZrBr4. The concentration of transition‐metal impurities in ZrF4, synthesized by chemical vapor deposition in the ZrBr4–HF system, is expected to be less than 1 ppb.
Preparation of Gadolinium Gallium Garnet [Gd3Ga5O12] by Solid‐State Reaction of the Oxides
Authors: Hellstrom E. E. , Ray R. D. , Zhang C.
Published: March 2005

We have prepared dense polycrystalline gadolinium gallium garnet (GGG) by solid‐state reaction of the oxides. The oxides were prereacted at 1350°C, ground, pressed, and sintered at 1650°C, yielding 97% dense samples. Ga2O3 evaporated from the sample surface leaving Gd4Ga2O9 that could spall off the sample. For the short times needed to sinter samples, the bulk composition of the material remained essentially constant. The microhardness of the GGG was 11.8 ± 1.2 GN · m−2.
Reaction Chemistry of Crystalline Allotropes of Germania
Authors: Seibles Lawrence , Mitchell James W. , Blitzer Lawrence D.
Published: March 2005

Chemical reactions potentially useful for converting intractable allotropes of germania (aged hexagonal and tetragonal forms) into reactive products are investigated. The effectiveness of reactions in aqueous solutions under basic, basic‐reductive, acidic‐oxidative, and acidic‐reductive conditions is reported in terms of germania solubilization. Solubility versus time of reaction at elevated temperatures is reported for the most effective medium, 2.5M NaOH. For six flux media, fusion reactions and respective temperatures are reported with the percentage of germania solubilized from the fused melt. Additionally, the crystalline germania materials are characterized by infrared and differential thermal analyses. Chemical procedures compatible with batch or continuous processing methods are discussed.
A Study of Conductivity in the Sr(Fe1−xTix)O3−δ System
Authors: Zhuang Yan , Lin Yanzheng , Zhu Dongyan , Zheng Yufang , Yu Zhengfang
Published: March 2005

The conductivity and conductivity‐temperature characteristics of the Sr(Fe1−xTixO3−δ system fired in air are studied by means of Mössbauer spectroscopy at room temperature and at 78 K. On the basis of quantitative analysis of the Fe4+ content in the solid solution, the concentrations of oxygen vacancies are calculated and the relationship between Fe4+ content and conductivity is found. It is also found that the turning points of conductivity and material constant B, as well as Fe4+ and the percentage of oxygen vacancies, occur at x=0.7 on the curve. When x=0.7, Fe3+ (II), which has never been reported before, disappears and the lattice parameters no longer vary with x. Besides Fe4+, Fe3+(II) is also a factor affecting the conductivity of this system.
Decoration of Dislocations by the Precipitation of Alumina in MgO · 2.9Al2O3 Spinel
Authors: Suematsu Hisayuki , Suzuki Tetsuya , Iseki Takayoshi , Mori Tsutomu
Published: March 2005

A thermal etching technique is developed to reveal dislocations in MgO · nAl2O3 (n=2.9) single crystals decorated by Al2O3 precipitation. Large plastic deformation leads to a larger density of thermal etch pits. Also, dense arrays of pits, parallel to the traces of slip planes with a surface, are observed around a Vickers identation. The cracks developed by an indentation run selectively along the 〈100〉 directions and accompany dislocations arrayed in the possible slip planes.
Electrical Properties of Porous Titania Ceramic Humidity Sensors
Authors: Yeh Yuan‐Chang , Tseng Tseung‐Yuen , Chang Deh‐An
Published: March 2005

Ceramic humidity sensors made with pure TiO2 powder as the starting material are investigated. The sensor can be reversibly operated without repeated high‐temperature thermal desorption, and the conductance versus relative humidity (rh) sensitivity of the sensor is more than 4 orders of magnitude in the range ∼15% to 95% rh at 400 Hz and 25°C. The sensor can be polarized similar to electrolytes in the charging process, and the degree of polarization is enhanced with increasing rh. The conduction carriers of the sensor in the humid atmosphere are ions and electrons, and ions are the dominant conduction carrier. The relative dielectric dispersion k′(ω) is almost proportional to ω−1.84 for rh ∼50% to 95% at low frequencies, where ω is the angular frequency. An inductive effect is also observed in a low‐humidity atmosphere, which implies desorption of water vapor with the regeneration of the adsorption sites.
Effect of Additives on the Pressureless Sintering of Aluminum Nitride between 1500° and 1800°C
Authors: Troczynski Thomas B. , Nicholson Patrick S.
Published: March 2005

Combined oxide additives (Y2O3, CaO, La2O3, CeO2, SiO2, TiO2, and Fe2O3) were investigated as AIN sintering aids. AIN can be fully sintered at 1600°C to substantial thermal conductivity (92 W/(m·K)) using a multiple sintering aid of Y2O3, CaO, SiO2, La2O3, and CeO2. This lowtemperature material has small grain size (1 to 3 μm).
Reversible Transformation and Elastic Anisotropy in Mg‐ZrO2
Authors: Marshall David B. , Swain Michael V.
Published: March 2005

Mechanical loading of polycrystalline ZrO2 ceramics causes reversible, out‐of‐plane distortions at free surfaces that are parallel to the direction of applied stress. These distortions have been measured using optical interference microscopy, and the separate contributions due to elastic anisotropy and reversible mantensitic tranformation have been identified.
Mullite Alumina Particulate Composites by Infiltration Processing: II, Infiltration and Characterization
Authors: Marple Basil R. , Green David J.
Published: March 2005

Mullite/alumina composites were fabricated by infiltrating porous alumina preforms with a hydrolyzed ethyl silicate sol. Evidence is presented which suggests that initial infiltration occurred without complete filling of the porosity by the sol. Multiple infiltrations were used to increase the amount of SiO2 introduced but led to blockage of the pores in the surface of the preform. Sintered bodies had concentration gradients, SiO2 decreasing from the surface inward. Although mullite limited grain growth in alumina, in partially modified bodies large grains (>1 mm) with a preferred orientation were observed at the interface between the two zones.
Formation of Zeolite During Caustic Dissolution of Fiberglass: Implications for Studies of the Kaolinite‐to‐Mullite Transformation
Authors: Jantzen Carol M.
Published: March 2005

Insoluble zeolitic reaction products are formed during the caustic dissolution of fiberglass filters. The zeolite that forms is Linde B1, the higher temperature form of the zeolite identified during caustic dissolution of free SiO2 in kaolinite‐to‐mullite transformations. The Linde B1 is a sodium aluminosilicate hydrate that preferentially incorporates Ca2+ and Mg2+. Formation of the Linde B1 zeolite from fiberglass dissolution in NaOH indicates that caustic dissolution of kaolinite does not preferentially dissolve free amorphous SiO2, but dissolves any multicomponent amorphous phase present.
Effects of Electric Fields on Slow Crack Growth in Glass
Authors: Caso Gary B. , Schwabel Mark G. , Frechette Van Derck
Published: March 2005

Electrical de fields applied through electrodes on either side of a slow‐running crack in soda‐lime‐silica glass resulted in a deviation of the crack plane and in delayed deceleration, arrest, and healing. In some cases crack closure occurred while still under load. Resumption of propagation resumed gradually upon removal of the field. It is suggested that the effects involve displacement of the dilated negatively charged zone at the crack tip, while healing is effected by reformation of bonds between the crack surfaces.
Densification of Lead Selenide and Lead Sulfide by Hot Isostatic Pressing
Authors: Hattori Takeo , Iwadate Yasuhiko , Sato Tatsunori
Published: March 2005

PbSe and PbS are both volatile and difficult to densify by normal sintering. We tried to sinter these compounds in a closed system, that is, by hot isostatic pressing in a glass capsule. Hot isostatic pressing was carried out for these compounds on the basis of their thermal behaviors in air as studied by TG‐DTA. The maximum bulk densities obtained were 8.10 and 7.50 g/cm3, respectively, corresponding to their theoretical densities. The microstructures of the isostatically hot‐pressed bodies were also observed by SEM. The present work shows that ceramic compounds which are normally volatile can be densified by encapsulation in glass followed by hot isostatic pressing.
High‐Tc Hydroxide‐ and Fluoride‐Containing Ceramic Superconductors
Authors: Kobayashi Keiji
Published: March 2005

Experimental data for maximum Tc, up to 125 K with temperature cycling, are reported with regard to Y‐Ba‐Cu‐O.OH systems. This increase in Tc is presumably associated with structural transition or phase equilibrium for [CuO6] local octahedra, occupied by hydroxyl or fluorine ions in the oxygen vacancy sites. Possible improvement in Tc using hydroxide ceramic compounds is described briefly.
Melt Infiltration and Reaction at the Fiber/Matrix Interface During the Brazing of a Fiber‐Reinforced Ceramic to Metal
Authors: Karunanithy Somasundaram
Published: March 2005

The presence of silica in the fiber/matrix interface of SiC‐fiber‐reinforced alumina facilitates the brazing of these composites with metals. In addition, molten braze infiltrates the interfacial pores to form reaction layers with the matrix and the fiber. The chemical compatibility, wetting, and thermodynamic favorability of the Ti‐based braze leads to a potentially useful method of joining a fiber‐reinforced ceramic to metal.
Cyclic Fatigue‐Crack Propagation in Magnesia‐Partially‐Stabilized Zirconia Ceramics
Authors: Dauskarat Reinhold H. , Marshall David B. , Ritchie Robert O.
Published: March 2005

The subcritical growth of fatigue cracks under (tension‐tension) cyclic loading is demonstrated for ceramic materials, based on experiments using compact C(T) specimens of a MgO‐partially‐stabilized zirconia (PSZ), heat‐treated to vary the fracture toughness Kc from ∼3 to 16 MPa·m1/2 and tested in inert and moist environments. Analogous to behavior in metals, cyclic fatigue‐crack rates (over the range 10−11 to 10−5 m/cycle) are found to be a function of the stress‐intensity range, environment, fracture toughness, and load ratio, and to show evidence of fatigue crack closure. Unlike toughness behavior, growth rates are not dependent on through0‐thickness constraint. Under variable‐amplitude cyclic loading, crack‐growth rates show transient accelerations following low‐high block overloads and transient retardations following high‐low block overloads or single tensile overloads, again analogous to behavior commonly observed in ductile metals. Cyclic crack‐growth rates are observed at stress intensities as low as 50% of Kc, and are typically some 7 orders of magnitude faster than corresponding stress‐corrosion crack‐growth rates under sustained‐loading conditions. Possible mechanisms for cyclic crack advance in ceramic materials are examined, and the practical implications of such “ceramic fatigue” are briefly discussed.
Thermally Stimulated Polarization and Depolarization in Halogen‐Containing Lead Silicate Glasses
Authors: Dutta Biprodas , Magruder Robert H. , Weeks Robert A. , Kinser Donald L.
Published: March 2005

Thermally stimulated polarization and depolarization current (TSPC/TSDC) measurements were made on lead halide silicate glasses having compositions (65 −x)PbO·xPbX2·35SiO2 where x= 0, 0.1, 2, 10 and X = F, CI, Br, and I. The addition of halogen ions to lead silicate glasses gives rise to a new high‐temperature TSDC peak in the vicinity of the peak previously observed in binary lead silicate glasses. The integrated area of the new peak is dependent on the amount and type of halogen ion present in the glass and does not saturate in the temperature range of our measurements. This new peak is attributed to space charge polarization of halogen ions.
Compressive Creep of SiC‐Whisker‐Reinforced Al2O3
Authors: de Arellano‐López Antonio R. , Cumbrera Francisco L. , Domínguez‐Rodríguez Arturo , Goretta Kenneth C. , Routbort Jules L.
Published: March 2005

Compressive creep of SiC‐whisker‐reinforced Al2O3 composites (0, 5, 15, and 25 wt% SiC) was measured in the temperature range of 1300° to 1500°C in air and argon. The creep resistance increased with increasing whisker concentration. The results indicated that the whiskers degraded in air, increasing strain rates compared to those in argon. Stress exponents between 1.0 and 2.0 and an activation energy of 620 ± 100 kJ/mol were measured. Transmission electron microscopy observations indicated that cavitation was minimal and that the deformed composites had the same dislocation structure as did the as‐received samples.
Microstructure and Electrical Properties of Chemically Prepared Nb2O5‐Doped SrTiO3 Ceramics
Authors: Raymond Mark V. , Amarakoon Vasantha R.W.
Published: March 2005

Chemically homogeneous SrTiO3 powders of submicrometer size were obtained by alcohol dehydration and subsequent calcination of citrate/format solutions. Nb2O5‐doped SrTiO3 was prepared with various Sr:Ti ratios resulting in an anomalous increase in the dielectric constant (K’up to ∼8000) for donor‐doped SrO‐excess SrTiO3. No semiconducting behavior was observed for donor‐doped TiO2‐excess SrTiO3 when fired in air. Therefore, a “brick‐wall” type of microstructure was formed as a result of the excess SrO, giving rise to anomalously high dielectric constants.
Active Corrosion of Sintered α‐Silicon Carbide in Oxygen–Chlorine Gases at Elevated Temperatures
Authors: Park Dong S. , McNallan Michael J. , Park Chan , Liang Winston W.
Published: March 2005

The active corrosion of sintered α‐silicon carbide from heat exchanger tubes in the temperature range 900° to 1100°C in gas mixtures containing 2% Cl2 by volume with additions of O2 or H2 has been investigated by thermogravimetric analysis and subsequent examination of the corrosion products. The presence of a small amount of oxygen accelerated rapid active corrosion in chlorine‐containing gas mixtures, but the corrosion was suppressed by an active‐to‐passive transition when the concentration of oxygen in the gas mixture was too high. Low rates of attack were observed in the environments containing H2 even when the chlorine potential was high. The concentration of oxygen necessary to produce the active‐to‐passive transition was found to vary from one material to another and may be related to the amount of excess carbon in the sintered silicon carbide.
High‐Pressure Self‐Combustion Sintering of Alumina‐Titanium Carbide Ceramic Composite
Authors: Adachi Seiji , Wada Takahiro , Mihara Toshihiro , Miyamoto Yoshinari , Koizumi Mitsue
Published: March 2005

Al2O3‐TiC ceramic composites were fabricated by highpressure self‐combustion sintering (HPCS). The samples had a unique microstructure which was quite different from that of commercial Al2O3‐TiC materials made by conventional processing. A possible mechanism for combustion reaction is suggested on the basis of the microstructure of the samples and the thermodynamical data. The HPCS sample had a higher KIC value than commercial Al2O3‐TiC materials.
Lateral Cracks and Microstructural Effects in the Indentation Fracture of Yttria
Authors: Cook Robert F. , Pascucci Marina R. , Rhodes William H.
Published: March 2005

The fracture properties of three polycrystalline Y2O3 materials: one fully cubic phase, one containing an Al2O3 grain‐boundary phase, and one containing hexagonal phase, were examined by indentation over a wide range of contact loads. The two former microstructures displayed tendencies at large indentation loads to radial crack lengths shorter than those extrapolated from the ideal response at low loads. The deviations correlated with the development of lateral cracks at the larger contacts, rather than with any observable change in the interaction between the cracks and the microstructure. After taking the lateral crack influence into account, the toughness of all three materials was estimated to be constant over the range of crack lengths studied, in contrast to the phenomena observed in similar grain size noncubic materials and inferred from earlier fractographic studies. The toughness of the partially hexagonal material was estimated to be 50% greater than the cubic materials. The general phenomenon of partitioning energy into lateral cracks at the expense of radial cracks at large indentation loads has been characterized by a lateral crack development parameter, LD, which varies from 0 to 1 as lateral cracks progressively develop and remove material.
Origin of Hysteresis Observed During Fatigue of Ceramic‐Matrix Composites
Authors: Kotil Temel , Holmes John W. , Comninou Maria
Published: March 2005

Possible mechanisms for the hysteresis in stress‐strain response observed during fatigue of fiber‐reinforced ceramics are examined analytically. In the model developed, the microstructure of a unidirectional composite is divided into adjacent cells, each containing a single fiber. The compliance of each cell is modeled by a series of springs, with frictional sliding of fibers represented by sliding blocks. Fatigue damage is modeled by allowing fibers to debond and fracture on a random cycle‐by‐cycle basis. The magnitude of the interfacial shear between the fibers and matrix is shown to play a significant role in determining the extent of hysteresis observed during fatigue loading of unidirectional composites. Practical considerations, such as the influence of fiber volume fraction on macroscopic fatigue behavior, are also discussed.
Pressureless Sintering of SiC‐Whisker‐Reinforced Al2O3 Composites: II, Effects of Sintering Additives and Green Body Infiltration
Authors: Lee Hae‐Weon , Sacks Michael D.
Published: March 2005

Pressureless sintering of SiC‐whisker‐reinforced Al2O3 composites was investigated. In Part II of the study, the effects of Y2O3/MgO sintering additives and green body infiltration on densification behavior and microstructure development are reported. Both sintering additives and green body infiltration resulted in enhanced densification. However, the infiltration approach was more effective for samples with high SiC whisker concentrations. Samples with 27 vol% whiskers could be pressureless sintered to ∼93% relative density and ∼3% open porosity. Fracture toughness values and microstructural features (e.g., grain size) for the infiltrated samples remained approximately the same as observed in the uninfiltrated samples.
Texture and Recrystallization on Ground Surfaces of Hafnia
Authors: Gasdaska Charles J. , Whalen Philip J. , Marti Jordi , Reidinger Franz
Published: March 2005

The texture in X‐ray diffraction patterns of polycrystalline monoclinic hafnia after surface grinding and annealing was examined and compared to that expected from ferroelastic twinning. The changes in relative peak intensity were consistent with textures expected for twinning. Examination by TEM verified the presence of twins just below the ground surface. At the ground surface, recrystallization was observed at temperatures as low as 450°C. The results support the proposal that similar changes in XRD patterns observed in surface‐ground zirconia materials are due partly to ferroelastic domain switching.
Migration of Intergranular Boundaries in Cubic Zirconia‐Yttria Induced by Magnesia Addition
Authors: Jeong Jun‐Whan , Yoon Duk N. , Kim Doh‐Yeon
Published: March 2005

When partially sintered cubic ZrO2–10 mol% Y2O3 specimens are heat‐treated at 1500°C with 5MgO·95(ZrO2–10 mol% Y2O3) powder mixture in the pores, the intergranular boundaries, which may be either grain boundaries or thin glass films, migrate, leaving behind them a new solid solution slightly enriched with MgO but depleted of Y2O3. The boundary curvatures generally increase during the migration and the average migration distance initially increases linearly with the heat treatment time. The observed boundary migration behavior is similar to that observed previously in a number of metallic systems, and its driving force is believed to arise from the coherency strain in the thin solute diffusion zone ahead of the moving boundaries.
Subcritical Crack Growth in Soda‐Lime Glass under Combined Modes I and III Loading
Authors: Yoda Mitsuo
Published: March 2005

Fracture and subcritical crack‐growth characteristics under combined Modes I and III loading were studied using the modified compact tension (CT) specimens of soda‐lime glass. The combined mode load was applied to the specimen in the direction β with respect to the initial crack. By superposition of Mode III, the advancing crack begins to rotate at an angle Ψ to the initial crack plane, which nearly maximizes the Mode I stress intensity factor KI(Ψ), and the crack continues to propagate in the same direction. In this case, unlike combined Modes I and II, the crack breaks into multiple partial fronts, and ligamentary bridging forms fracture lances when these segmented cracks are held together. The crack velocity da/dt was plotted versus the maximum Mode I stress intensity factor KI(Ψ) for combined Modes I and III loading. The da/dt values are initially high, and the crack growth tends to be discontinuous compared with the result for pure Mode I. The subcritical crack growth seems to occur when the KI value for the initial crack reaches a certain value. The da/dt‐KI(Ψ) curves for combined Modes I and III lie roughly on the same curve as that for pure Mode I as the crack growth increases.
Low‐Temperature Sol–Gel Synthesis of NaZr2P3O12
Authors: Agrawal Dinesh K. , Adair James H.
Published: March 2005

The NZP family of new low‐expansion materials has attracted wide interest for its potential in advanced technological applications. NaZr2P3O12, which is the parent composition of this family, has been synthesized by the solution sol‐gel method using special precursor solutions, which led to its formation (although poorly crystalline) at temperatures as low as 120°C. The lowest temperature of formation of a single phase of NaZr2P3O12 with a high degree of crystallinity was found to be 600°C.
Atomistic Simulation of High‐Tc Oxides
Authors: Allan Neil L. , Mackrodt William C.
Published: March 2005

Atomistic lattice simulations are reported of the normalstate crystal properties of high‐Tc oxides. Nonstoichiometry in La2CuO4 and Nd2CuO4 associated with oxygen interstitial defects is examined in some detail. The lattice structure and defect chemistry of Ca2CuO3, Sr2CuO3, La2SrCu2O6, and Sr2CuO4 are also considered. Finally, Jahn‐Teller effects and bipolaron formation are addressed in relation to high‐Tc behavior.
Some Observations on the Role of Dopants in Phase Transitions in Zirconia from Atomistic Simulations
Authors: Cormack Alastair N. , Parker Stephen C.
Published: March 2005

Attempts to calculate the free energy of the polymorphs of zirconia are reported. The results suggest a theoretical basis for the phase transitions in terms of the removal of lattice instabilities (evidenced by imaginary phonon frequencies). Dopant stabilization of the higher symmetry structures is seen to arise because of changes to the phonon frequencies due to the different mass and effective charge of the dopant.
Luminescence Studies of Oxygen‐Related Defects In Aluminum Nitride
Authors: Youngman Robert A. , Harris Jonathan H.
Published: March 2005

Aluminum nitride possesses a unique ability to accommodate oxygen via lattice dissolution to levels exceeding 4 at.%. The mechanism for this large accommodation of oxygen is of technological and scientific interest due to the established delecterious effects of oxygen on the thermal conductivity in this material. When doped with oxygen, AlN exhibits an intense, very broad (FWHM > 1 eV), luminescence peak in the near‐UV (∼ 375 nm). Though there is little doubt that this transition is associated with oxygen incorporation in the lattice, both the anomalous width of this feature and the specific complex from which it is derived have been a matter of debate. This paper reviews past studies of the luminescence of oxygen‐related defects in AlN and presents recent detailed photoluminescence experiments which delineate changes in the luminescence as a function of oxygen content. These data are utilized in conjunction with other measurements to elucidate the nature of the oxygen‐related defect and its evolution as a function of oxygen concentration. A defect‐cluster model is presented which accounts for a transition in the luminescent properties of AlN and is found to be in accord with measurements on thermal conduction and unit‐cell volume changes in AlN. This understanding of the oxygen‐related defect in AlN from the photoluminescence studies is then utilized in cathodoluminescence studies via cathodoluminescence imaging in a scanning electron microscope and a transmission electron microscope. Such techniques are extremely useful in elucidating the distribution and interaction of oxygen in the microstructure of sintered AlN ceramics, which has been heretofore an extremely difficult problem in microstructural and microchemical analysis of such sintered ceramics.
Interfacial Segregation in Perovskites: III, Microstructure and Electrical Properties
Authors: Desu Seshu B. , Payne David A.
Published: March 2005

A model is proposed to relate segregation of dopants with the development of fine ceramic microstructures and electrical properties in polycrystalline donor‐doped BaTiO3. As the average dopant concentration is increased, the dopant concentration at the grain boundary increases compared with the bulk. This has two important effects: (1) dopant incorporation at the grain boundary shifts from electronic to vacancy compensation and the formation of high‐resistive layers, and (2) grain‐boundary mobility is impeded and grain growth retarded with increased dopant additions. Thus, for high dopant concentrations, donor‐doped BaTiO3 becomes insulating. The model discusses in detail the nonequilibrium heterogeneous defect chemistry as a function of thermal‐processing and accounts for barrier layer phenomena.
Self‐Irradiation Damage of a Curium‐Doped Titanate Ceramic Containing Sodium‐Rich High‐Level Nuclear Waste
Authors: Mitamura Hisayoshi , Matsumoto Seiichiro , Miyazaki Takashi , White Timothy J. , Nukaga Kiyoshi , Togashi Yoshihiro , Sagawa Tamio , Tashiro Shingo , Levins Desmond M. , Kikuchi Akira
Published: March 2005

The polyphase titanate ceramic containing sodium‐rich simulated high‐level nuclear waste was doped with 0.69 wt% of 244Cm to accelerate long‐term self‐irradiation due to α decays. α autoradiography showed that α emissions were almost uniformaly distributed throughout the curium‐doped samples on a >20‐μm scale although micropore surfaces and titanium oxide agglomerates were free of α‐emitting nuclides. The phase assemblage of the curium‐doped titanate ceramic included freudenbergite and loveringite in addition to the more abundant oxide phases: hollandite, perovskite, and zirconolite. Accumulation of α decays was accompanied by a gradual decrease in density. The increment of density was – 1% after an equivalent age of 5000 yr. Leach tests showed a slight trend toward higher total release of curium with equivalent age. The release of soluble nonradioactive elements (e.g., Na, Cs, Sr, and Ca) in the oldest specimens (equivalent age, 2000 yr) varied from specimen to specimen but, on average, were higher than specimens that had suffered a lower radiation dose.
The System Na2O‐CaO‐SiO2‐H2O
Authors: Brown Paul Wencil
Published: March 2005

A portion of the quaternary phase diagram for Na2O‐CaO‐SiO2‐H2O has been constructed. Plotting concentrations as their 10th roots allows compounds having solubilities which differ by several orders of magnitude to be represented on a single diagram. The compositional relationships among sodium‐substituted calcium silicate hydrate, calcium‐substituted sodium silicate hydrate, calcium bydroxide, a quaternary compound of approximate composition 0.25Na2O · CaO · SiO2· 3H2O, sodium hydroxide monohydrate, and miscellaneous sodium silicate hydrates are presented. The quaternary diagram constructed shows the quaternary compound to exist in equilibrium with sodium‐substituted calcium silicate hydrate and calcium hydroxide. Conditions in concrete pore solutions which favor the formation of this quaternary compound may also favor the occurrence of the alkali‐silica reaction.
Mechanical Properties of β‐Si3N4‐Whisker/Si3N4‐Matrix Composites
Authors: Homeny Joseph , Neergaard Lynn J.
Published: March 2005

Composites containing 30 vol%β‐Si3N4 whiskers in a Si3N4 matrix were fabricated by hot‐pressing. The composites exhibited fracture toughness values between 7.6 and 8.6 MPa · m1/2, compared to 4.0 MPa · m1/2 for unreinforced polycrystalline Si3N4. The improvements in fracture toughness were attributed to crack wake effects, i.e., whisker bridging and pullout mechanisms.
Aluminum‐Substituted Gyrolite as Cation Exchanger
Authors: Miyake Michihiro , Iwaya Masaki , Suzuki Takashi , Kakehi Hiroyuki , Mitsuda Takeshi
Published: March 2005

A successful Al‐substituted gyrolite has been hydrothermally synthesized at 220°C under saturated steam pressure, and its cation exchange characteristics for K+ and Cs+ ions have been investigated. Al‐substituted gyrolite took up K+ and Cs+ ions of 14 and 20 mequiv/100 g, respectively, and these removal amounts are smaller than those by Al‐substituted tobermorites (ca. 80 mequiv/100 g). It was found that Al‐substituted gyrolite exhibited selectivity for univalent cations in the following order: Na+ < K+ < Cs+ in the low‐concentration regions of K+ and Cs+ ions. Al‐substituted gyrolite did not lose its crystallinity and morphology, and its basal spacing did not change in the cation exchange. These results, therefore, suggest that Al‐substituted gyrolite may be useful for separation and waste disposal.
Pyrolysis of Silicon‐Backbone Polymers to Silicon Carbide
Authors: Carlsson David J. , Cooney J. David , Gauthier Sylvie , Worsfold D. James
Published: March 2005

A systematic study of the role of the composition of polysilane homo‐, co‐, and terpolymers on their pyrolysis yield to silicon carbide (SiC) was performed. The ceramic yield was found to increase as the poly(dimethylsilane) structure was substituted with hexyl and, especially, phenyl groups. the best improvement in SiC yield came from the copolymerization of low levels of silane monomers containing unsaturated groups (vinyl or allyl). For a given composition, ceramic yield increased with increasing heating rate under argon. Infrared spectroscopy and thermogravimetry were used to identify the chemical changes accompanying the pyrolysis process up to 1200°C. These changes included the formation of carbosilane linkages and Si‐H groups.