bluuebunny/crossref_metadata_2025 huggingface dataset:
Properties of Sm2O3─Al2O3─SiO2 Glasses for In Vivo Applications
Authors: Erbe Erik M. , Day Delbert E.
Published: March 2005

The compositional range for glass formation below 1600°C in the Sm2O3─Al2O3─SiO2 system is (9–25)Sm2O3─(10–35)Al2O3─(40–75)SiO2 (mol%). Selected properties of the Sm2O3─Al2O3─SiO2 (SmAS) glasses were evaluated as a function of composition. The density, refractive index, microhardness, and thermal expansion coefficient increased as the Sm2O3 content increased from 9 to 25 mol%, the values exceeding those for fused silica. The dissolution rate in 1N HCl and in deionized water increased with increasing Sm2O3 content and with increasing temperature to 70°C. The transformation temperature (Tg) and dilatometric softening temperature (Td) of the SmAS glasses exceeded 800° and 850°C, respectively.
Derivatization of Preformed Polymers to Produce Ceramic Precursors
Authors: Maya Leon
Published: March 2005

The feasibility of producing derivatized preformed polymers to be used as ceramic precursors was explored. A borane derivative of polyethyleneimine (PEI) was prepared through a transamination reaction involving Me3NBH3. Higher boron loadings were possible with Me3NB3H7. The borane derivative could be cast into films and produced boron nitride upon pyrolysis. Similarly, polyethyleneimine hydrochloride was converted into the cyanohydroborate derivative through a reaction with NaBH3CN. This derivative was also accessible through a transamination reaction. Both materials produced boron nitride upon pyrolysis. Polyallylamine hydrochloride could not be derivatized. Silyl derivatives of PEI were also prepared, but the products obtained afforded low ceramic yields upon pyrolysis.
Phase Distribution and Transparency in Glass‐Ceramics Based on a Study of the Sodium Niobate–Silica System
Authors: Herczog Andrew
Published: March 2005

High transparency in the NaNbO3 crystal–silica glass system at low silica content is attributed to maintenance of the glass‐to‐glass phase separation pattern in microstructure throughout the process of crystallization. Optical properties are explained by a characteristic of the phase‐separated glass, internal connectivity, applicable in this case to both crystal and glass phases. This type of microstructure allows light propagation in each phase independently, without interference between phases. Deviation from this ideal behavior and light‐scattering mechanisms which may apply to different phase distribution patterns here and in other glass‐ceramic systems are discussed.
Physical and Chemical Properties of Titania‐Silica Films Derived from Poly(ethylene glycol)‐Containing Gels
Authors: Matsuda Atsunori , Matsuno Yoshihiro , Katayama Shinya , Tsuno Toshio , Tohge Noboru , Minami Tsutomu
Published: March 2005

Physical and chemical properties of poly(ethylene glycol) (PEG)‐containing TiO2‐SiO2 gel films have been studied relative to their application to the fine‐patterning technique for optical memory disks. PEG added to TiO2‐SiO2 gel films decomposed completely at temperatures between 250° and 300°C. The combustion of PEG in the gel films led to decreases in the refractive index and in the hardness of the films, which should result from the increase in porosity of the films. The addition of PEG into the gel films, however, did not adversely influence the weathering resistance of the glass substrates coated with the sol‐gel‐derived films.
Aluminoborate Glass‐Ceramics with Low Thermal Expansivity
Authors: MacDowell John F.
Published: March 2005

The synthesis and properties of calcium, strontium, and barium aluminoborate glass‐ceramics are described in this paper. Glass powder compacts near the RO‐Al2O3‐B2O3 stoichiometry were sintered and crystallized below 1000°C to form dense glass‐ceramics containing RAI2B2O7 as the principal phase. Thermal expansion coefficients as low as 6 × 10−7/°C, 15 × 10−7/°C, and 23 × 10−7/°C were measured on SrAl2B2O7, CaAl2B2O7, and BaAl2B2O7 glass‐ceramics, respectively. Two forms of the ultra‐low‐expansion SrAl2B2O7 were not previously reported in the literature and were indexed as cubic and hexagonal crystals. The unique radial spherulitic interbladed microstructures and other important properties of these glass‐generated ceramics are also revealed.
Dynamic Fracture Responses of Alumina and Two Ceramic Composites
Authors: Yang Kwan‐Ho , Kobayashi Albert S.
Published: March 2005

A hybrid experimental‐numerical procedure was used to characterize the dynamic fracture response of alumina (Al2O3) and TiB2‐particulate/SiC‐matrix and SiC‐whisker/Al2O3‐matrix composites. Unlike metals and polymers, dynamic arrest stress intensity factors (SIFs) did not exist in the monolithic ceramics and the two ceramic composites considered. Thus a running crack in these materials cannot be arrested by lowering the driving force, i.e., the dynamic SIF. Fractography study of the alumina specimens showed that the area of transgranular failure varied from about 3% to about 16% for rapid crack extensions in statically and impact loaded specimens, respectively. The influence of kinematic constraints which enforces transgranular flat crack extension, despite the higher fracture energy of transgranular fracture, is discussed.
Heat Capacity of Monoclinic Zirconia between 2.75 and 350 K
Authors: Nevitt Michael V. , Fang Yué , Chan Sai‐Kit
Published: March 2005

We have measured the constant‐pressure heat capacity Cp of pure monoclinic ZrO2 between 2.75 and 350 K. At low temperatures we find heat capacity in excess of the thermal acoustic phonon contribution, which we attribute to the ingress of low‐lying optic mode vibrations. We have calculated the heat capacity at constant volume Cv at the high‐temperature end of our measurement range and used these values with the harmonic model to derive the moments of the phonon spectrum. Comparing the moments of the monoclinic phase with theoretically derived corresponding moments for the tetragonal phase, we find that a downward shift of optic mode frequencies accompanies the tetragonal‐to‐monoclinic transformation.
Stable and Metastable Phase Relationships in the System ZrO2–ErO1.5
Authors: Yashima Masatomo , Ishizawa Nobuo , Noma Tatsuo , Yoshimura Masahiro
Published: March 2005

Stable and metastable phase relationships in the system ZrO2–ErO1.5 were investigated using homogeneous samples prepared by rapid quenching of melts and by arc melting. The rapidly quenched samples were annealed in air for 48 h at 1690°C or for 8 months at 1315°C. Two tetragonal phases (t‐ and t′‐phases) were observed after quenching samples heated at 1690°C to a room temperature, whereas one t‐phase and cubic (c‐) phase were found in those treated at 1315°C. Since the t′‐phase is obtained through a diffusionless transformation during cooling from a high‐temperature c‐phase, t‐ and c‐phases can coexist at high temperature. The t‐ and c‐phases field spans from 4 to 10 mol% ErO1.5 at 1690°C and from 3 to 15 mol% ErO1.5 at 1315°C. The equilibrium temperature Tt‐m0 between the t‐ and monoclinic (m‐) phases estimated from As and Ms temperatures decreased with increasing ErO1.5 contents.
Electrical Properties of Ceria‐Doped Yttria
Authors: Marques Fernando M. B. , Wirtz Gerald P.
Published: March 2005

The electrical conductivity and ionic domain of several ceriadoped yttria compositions, with up to 10 cation% dopant, were studied as a function of temperature (800° to 1100°C) and oxygen partial pressure (10−15 to 105 Pa). The ionic conduction in ceria‐doped yttria involves the transport of interstitial oxygen ion defects. The increase of electronic conductivity under reducing conditions with increasing dopant concentration suggests hopping of small polarons between cerium ions with different valences as the dominant conduction mechanism.
Boron Carbide Reinforced Alumina Composites
Authors: Liu Jenq , Ownby P. Darrell
Published: March 2005

The mechanical properties of alumina have been successfully improved by adding isolated boron carbide particles of two different shapes. A KIc of 7.26 ± 0.20 MPa · m1/2 for alumina—boron carbide whiskerlike composites and of 5.27 ± 0.12 MPa · m1/2 alumina—boron carbide shardlike particle composites has been achieved. The fracture toughness of these composites is dependent on the volume fraction of the boron carbide particles as well as their size and shape. The flexural strength is also appreciably enhanced to a constant value with from 5 to 20 vol% boron carbide additions. The whiskerlike particles increase the flexural strength by 25% and the shardlike particles produce a 47% improvement.
Cyclic Fatigue from Frictional Degradation at Bridging Grains in Alumina
Authors: Lathabai Srinivasarao , Rödel Jürgen , Lawn Brian R.
Published: March 2005

Tension—tension cyclic loading tests have been conducted on a coarse‐grained alumina ceramic that exhibits toughnesscurve behavior by grain‐interlock bridging. Fatigue effects are observed in the regions of both short cracks, using indentation flaws, and long cracks, using compact‐tension specimens. A true mechanical fatigue effect is demonstrated by running the tests below the static fatigue limit. A custom‐made device for in situ observation of crack propagation in the scanning electron microscope enables us to identify bridge degradation as a cause of the fatigue process. “Wear” debris cumulates at the sliding intergranular frictional contact points, indicating a loss of traction at the junction. The basis of a fracture mechanics model describing the effect of this frictional degradation in reducing crack‐tip shielding is outlined and fitted to the data. It is suggested that the bridge degradation fatigue mechanism may be widespread in polycrystalline ceramics with pronounced toughness curves.
Thermodynamic Modeling of Equilibrium Subsolidus Phase Relations in the Ag–Pd–O2 System
Authors: Wang Sea Fue , Huebner Wayne
Published: March 2005

From literature values of the thermodynamic activity of Pd in Ag/Pd solid solutions and the Gibbs free energy of formation of PdO as a function of temperature, subsolidus equilibrium phase relations in the Pd–Ag–O2 system have been calculated and compared to results from high‐temperature XRD analyses. The developed model takes into account the nonideal activity of Pd in Ag/Pd solid solutions and confirms that increasing the Ag/Pd ratio or decreasing the oxygen activity decreases the temperature at which PdO reduces. It also accurately predicts that the temperature range over which the PdO reduces is broad for Ag‐rich compositions, and narrow for Pd‐rich compositions.
Selectivity of Silicon Carbide/Stainless Steel Solid‐State Reactions and Discontinuous Decomposition of Silicon Carbide
Authors: Chou Ting C. , Joshi Anne
Published: March 2005

Solid‐state reactions of SiC with a multicomponent system, stainless steel, have been studied at 1125°C. Four‐layered reaction products consisting of modulated carbon precipitation zone, random carbon precipitation zone, four‐phase mixture zone, and grain‐boundary precipitation zone were formed in the reaction zone. The carbon precipitates were embedded in a matrix of complex metal silicides. In addition, extensive interfacial melting was noted. Carbon atom was found to diffuse faster than Si and selectively reacted with Cr to form Cr carbide(s) along the grain boundaries of stainless steel. No Fe carbides or Ni carbides were ever detected. Among the consituents existing in stainless steel, Ni atoms have the highest affinity for Si. An uphill diffusion of Ni toward the SiC reaction front was observed. While the diffusion of Cr and Fe toward SiC followed a downhill concentration gradient, very small amounts of Cr reached the SiC interface. The selective reactions of Si and C with Ni, Fe, and Cr are discussed on the basis of Gibbs free energy of formation of various compounds. The diffusion kinetics of C and Si atoms in selected metal/SiC reactions are discussed on the basis of their chemical affinities for respective metals. The modulation of carbon precipitation is correlated with previous results from Ni/SiC, Ni3Al/SiC, Fe/SiC, Co/SiC, and Pt/SiC, reactions. A general model describing discontinuous decomposition of SiC is proposed to explain the origin of carbon modulation.
Kaolinite–Mullite Reaction Series: The Development and Significance of a Binary Aluminosilicate Phase
Authors: Chakravorty Akshoy Kumar , Ghosh Dilip Kumar
Published: March 2005

The semiquantitative estimations of 980°C exothermic reaction products of kaolinite by quantitative X‐ray diffraction (QXRD) and chemical leaching techniques show the formation of a significant amount of amorphous aluminosilicate phase (∼ 30 to 40 wt%). The theoretically expected AlO4/AlO6 ratio in the 980°C reaction is in close agreement with the value measured by the X‐ray fluorescence (XRF) technique and the experimental radial electron distribution (RED) profile agrees with the suggested 980°C formation of Si‐Al spinel with mullite‐like composition. Mullitization of kaolinite has been compared with a synthetic Al2O3—SiO2 mixture. In synthetic mixtures development of an intermediate amorphous aluminosilicate phase is an essential step prior to mullitization. Kaolinite forms mullite in two ways: (i) by polymorphic transformation of cubic mullite at 1150° to 1250°C and (ii) by nucleation of mullite in the amorphous aluminosilicate phase and its subsequent growth above 1250°C. Thus chemical continuity is maintained throughout the reaction series and the intermediate spinel phase is silicon bearing and its subsequent transformation to mullite confirms the topotactic concept in the kaolinite transformation.
Mechanical Properties of Silica Xerogels
Authors: Abramoff Bennett , Klein Lisa C.
Published: March 2005

Strengths have been measured for silica xerogels using four‐point bending. The silica xerogels studied are microporous with open porosity of 53 vol%. Strengths were measured on fresh samples, samples which had been outgassed at 250°C, samples which were exposed to ambient conditions for 2 d, and samples stored in ethanol. Fresh samples had strengths about 20 MPa which increased to 32 MPA with outgassing but returned to 20 MPa after exposure. The loss of strength could be arrested, but not reversed, by storing the samples in ethanol.
Thermal Conductivity of Li2O·Al2O3·nSiO2 Glass‐Ceramics between 5 and 100 K
Authors: Hanna Baher , Bohn Randy G.
Published: March 2005

The thermal conductivity of Li2O·Al2O3·nSiO2 glass‐ceramics is studied for n= 4, 6, 8, 10 between 5 and 100 K. A monotonic increase in conductivity is observed for all samples. This behavior is different from that of glassy counterparts which exhibit a plateau in thermal conductivity between 10 and 20 K. It is also observed that while the conductivity of glass‐ceramics is lower than that of glasses at low temperatures, the situation is reversed at higher temperatures. A crossover occurs around 40 K for all studied samples. The glass‐ceramic behavior is interpreted in the light of the acoustic mismatch theory of Little. At low temperatures, the thermal boundary resistance that exists at the crystalline‐amorphous mismatch is high and the thermal conductivity is low. At higher temperatures, the boundary resistance is very small and the high conductivity is mainly due to the crystalline region within the amorphous structure.
Corrosion Behavior of Alumina Ceramics in Caustic Alkaline Solutions at High Temperatures
Authors: Sato Tsugio , Sato Shigeki , Okuwaki Akitsugu , Tanaka Syun‐ichiro
Published: March 2005

The corrosion rate and changes in the microstructure and fracture strength of alumina ceramics (93.0% Al2O3 and 99.5% Al2O3) were studied in 0.1m to 25m NaOH solutions at 150°C to 200°C, where m= mol/(kg of H2O). The attack of the caustic alkaline solution started at the grain boundaries. Consequently, the corrosion resistance increased with decreasing SiO2 content in Al2O3 ceramics, and the corrosion resistance of 99.5% pure Al2O3 was similar to that of Si3N4 ceramics. Since large pits are formed by corrosion, the surface area increased first and the apparent corrosion rate increased with time in the initial stage of the corrosion. The corrosion rate of Al2O3 increased linearly with increasing NaOH concentration, and the activation energy was 102 kJ/mol. The fracture strength of corroded Al2O3 decreased monotonically as the degree of dissolution of alumina increased.
Effect of Lead Oxide on Niobium‐Doped Titania Varistors
Authors: Wu Jenn‐Ming , Lal Chih‐Huang
Published: March 2005

The effect of PbO on Nb‐doped TiO2 varistors was investigated. It was found that the PbO dopant had significant effect on the varistor properties of Nb‐doped TiO2 ceramics. There existed an optimal range of PbO starting from 0.25 to 1 mol% for the nonlinear I–V characteristics of the 0.25 at.% Nb‐doped TiO2 ceramics. Within this PbO range, an effective boundary energy barrier of about 0.70 eV was created which yielded nonlinear I–V characteristics with α= 7.6. In contrast, α values of the samples containing PbO dopant outside this optimal range are only about 2 to 3. The effect of PbO on TiO2 varistors was analyzed by impedance spectroscopy, C–V and dielectric measurements, and X‐ray diffraction, as well as scanning electron microscopy. The results are discussed in the text.
Relationship between Fractal Geometry and Fractography
Authors: Mecholsky John J. , Freiman Stephen W.
Published: March 2005

Fractal geometry has been used to describe irregular fracture surfaces in a quantitative way. The fractal dimensional increment has been related to the fracture toughness of the material through the elastic modulus and a characteristic structure parameter, a0. The study of fractography has shown the relationship between the flaw/mirror size ratio and the fracture toughness. An experimental observation has shown that the fracture toughness is related to the elastic modulus through another structure parameter, b0. Combining all of these relationships leads to the conclusion that the fractal dimensional increment, D*, is directly related to the flaw/mirror size ratio. Experimental measurements of the fractal dimension and the flaw/mirror size ratio on glasses, a glass‐ceramic, polycrystalline ceramics, and a single crystal all agree with the prediction. The implication of this finding is that there is a linear scaling law in operation at fracture between the energy of crack initiation and of microbranching and is reflected in the features on the fracture surface.
Kinetics of Hydroxyapatite Formation at Low Temperature
Authors: Brown Paul W. , Fulmer Mark
Published: March 2005

The influence of temperature on the kinetics of formation of calcium‐deficient hydroxyapatite by an acid‐base reaction has been determined. Calcium‐deficient hydroxyapatite is compositionally similar to bone apatite. Reactants used in this study were tetracalcium phosphate, a calcium phosphate more basic than hydroxyapatite, and dicalcium phosphate, which is more acidic than hydroxyapatite. The kinetics of hydroxyapatite formation are initially controlled by the surface areas of the reactants; however, eventually the rate of hydroxyapatite formation becomes diffusionally controlled. The origin of diffusion control appears to be associated with the epitaxial formation of hydroxyapatite on the surface of the dicalcium phosphate reactant. The microstructure of hydroxyapatite formed at 5°, 15°, and 25°C were generally similar and are characterized by the formation of a porous acicular product which covers the surfaces of the reactants. The microstructure of the hydroxyapatite formed at 38°C was distinct from this and exhibited a pseudomorphic relationship with the dicalcium phosphate reactant. Seeding with hydroxyapatite accelerated the initial reaction but did not appear to have major long‐term effects on the fractional degree of reaction or on microstructural development. Reaction was also accelerated in a neutral salt solution of high ionic strength.
X‐ray Absorption Studies of Ceria with Trivalent Dopants
Authors: Li Ping , Chen I‐Wei , Penner‐Hahn James E. , Tien Tseng‐Ying
Published: March 2005

The local structural environments of Sc3+ and Gd3+ in ceria were studied for the first time by using EXAFS (extended X‐ray absorption fine structure) and XANES (X‐ray absorption near edge structure) spectroscopy. Our results indicate that Sc3+ in 10‐mol%‐Sc2O3‐doped ceria has a 7‐fold‐coordinated Sc‐ordered structure with two different Sc─O distances (2.13 and 2.26 Å). An octahedrally coordinated Sc─O cluster is present in 5 mol% Sc2O3─5 mol% Gd2O3─CeO2, because of the strong scavenging effect of Sc3+ ions on oxygen vacancies created by Gd3+. The Gd3+ in this sample is largely unassociated. Sc2O3 (5 mol%) and Gd2O3 (5 or 10 mol%) form random solid solutions with CeO2. These results are consistent with the current concept of defect structures in this structural family.
Mechanical and Electrical Properties of Silicon Nitride–Silicon Carbide Nanocomposite Material
Authors: Sawaguchi Akihiro , Toda Kohji , Niihara Koichi
Published: March 2005

Mechanical and electrical properties of nanocomposite materials composed of a Si3N4 matrix and nanometer‐sized SiC particles are described. Composites containing less than 10 vol% SiC particles have the same order of resistivity and dielectricity as the non‐SiC material as well as highly improved mechanical properties. The composites are promising materials for use under harsh conditions.
Fabrication and Properties of Low‐Shrinkage Reaction‐Bonded Mullite
Authors: Wu Suxing , Claussen Nils
Published: March 2005

Mullite ceramics were fabricated according to the recently developed reaction‐bonded Al2O3 (RBAO) technology. Green compacts consisting of mechanically alloyed Al, SiC, and Al2O3 were heat‐treated in two steps. During the first hold at 1200°C, Al and SiC were oxidized to form Al2O3 and SiO2. On further heating, mullite was formed which then sintered during the second hold at 1550°C. All reactions involved in the process were associated with volume expansions that almost compensated for the shrinkage on sintering. Processing details and microstructure development are discussed. Reaction‐bonded mullite ceramics exhibit high fracture strength, e.g., 290 MPa at a density of 97% of theoretical density.
Solubility Limits of α'‐SiAION Solid Solutions in the System Si,Al,Y/N,O
Authors: Sun Wei‐Ying , Tien Tseng‐Ying , Yen Tung‐Sheng
Published: March 2005

The solubility limit of α'‐SiAION solid solutions on the Si3N4─YN:3AIN composition join in the system Si3N4─YN─AIN has been determined at 1800°C. The end members of these solid solutions are Y0.43Si10.7Al1.3N16 and Y0.8Si9.6Al2.4N16. Unit‐cell dimensions of the α'‐SiAION solid solutions in the system Si,Al,Y/N,O can be expressed as follows: ao(Å) = 7.752 + 0.045m+ 0.009n, co(Å) = 5.620 + 0.048m+ 0.009n, where the α'‐SiAION solid solution has the formula YxSi12‐(m+n)Alm+nN16‐nOn. The single‐phase boundary of the solid solution α'‐SiAION on the composition triangle Si3N4─YN:3AIN─AIN:Al2O3 is delineated. The present paper also reports the phase relationships involving α'‐SiAION.
Preceramic Polymers as “Reagents” in the Preparation of Ceramics
Authors: Seyferth Dietmar , Bryson Nathan , Workman David P. , Sobon Christine A.
Published: March 2005

Pyrolysis of preceramic polymer/metal powder composites resulted in formation of ceramic products containing the metal as well as element present in the polymer. Prepared in this manner were ceramics containing crystalline phases of metal silicides, metal carbides, metal nitrides, metal borides, and silicon carbide. The polymers used in this investigation included a polysilazane, a polycarbosilane, and decaborane (12)‐diamine polymers, and the metals used included aluminum and a number of early transition metals.
Low‐Stress Creep of Single‐Crystalline Calcium Oxide
Authors: Duong Hiep , Wolfenstine Jeff
Published: March 2005

The creep behavior of single‐crystalline CaO compressed along the 〈100〉 direction has been investigated over the temperature range 1350° to 1450°C at stresses between 5 to 15 MPa. Creep strains greater than 0.10 were required to achieve a steady‐state creep rate. Power‐law creep was exhibited over the entire stress range with a stress exponent equal to 4.4 and an apparent activation energy for creep of 345 kJ·mol−1.
Influence of Fiber and Interfacial Properties on Fracture Behavior of Fiber‐Reinforced Ceramic Composites
Authors: LLorca Javier , Singh Raj N.
Published: March 2005

The fracture and fracture resistance behaviors of zirconmatrix composites uniaxially reinforced with either uncoated or BN‐coated silicon carbide fibers are studied by performing experiments in three‐point flexure and by analyzing results analytically using a cohesive crack model that incorporates crack bridging and fiber pullout mechanisms. A comparison of experimental results with the model predictions demonstrates good agreement. This analytical approach is then used in a parametrical study to demonstrate the role of fiber and fiber‐matrix interfacial properties on the mechanical behavior of fiber‐reinforced ceramic‐matrix composites. Material parameters that enhance ultimate strength and ductility or toughness are elucidated.
Effect of Metal Particle Morphology on the Combustion of Refractory Metals in Nitrogen
Authors: Agrafiotis Christos C. , Puszynski Jan A. , Hlavacek Vladimir
Published: March 2005

The synthesis of niobium nitride by combustion of niobium metal powder under nitrogen atmosphere was investigated. Ignition and propagation characteristics were studied in the region of nitrogen pressure between 5 and 100 atmospheres and solid‐phase dilution up to 30 wt%. Within this region of operating parameters, the combustion reaction proceeds without any significant liquid‐phase formation, thus exhibiting many features common with the combustion of tantalum in nitrogen. However, the effect of metal particle size on the combustion characteristics was different for the two metal systems. This was attributed to the difference in morphology and microstructure between the niobium and tantalum reactant particles. The operating parameters for efficient synthesis of nearly stoichiometric cubic NbN were determined. Results from appropriately designed experiments under conditions where the filtration limitations were minimized were used to extract the apparent activation energy of the combustion nitridation process, which was found to be 40 809 cal/mol.
Substituted Tobermorites: 27Al and 29Si MASNMR, Cation Exchange, and Water Sorption Studies
Authors: Tsuji Masamichi , Komarneni Sridhar , Malla Prakash
Published: March 2005

27Al and 29Si magic angle spinning nuclear magnetic resonance spectroscopy revealed that the maximum amount of Al that can be substituted for Si in the tobermorite structure is 15 to 20 mol%. Powder X‐ray diffraction and cation exchange studies corroborate the above finding. Anomalous tobermorite structures resulted in all cases, and hydrogarnet appeared beyond the 15 to 20 mol% Al substitution limit for Si. The sorption of water molecules by synthetic [Al3++ Na+]‐substituted tobermorites and their cation‐exchanged forms heated at 200°C under vacuum (≥10−3 torr) was measured at 25°C in order to probe the nature of the ion‐exchange cavity. Samples with ≤30 mol% of Al substitution for Si showed isotherms of Brunauer, Demming, Demming, Teller (BDDT) type I and gave better linearity with the Langmuir plot than with the BET plot. Samples with ≥35 mol% of Al substitution for Si showed BDDT type II isotherms and gave better linearity with the BET than with the Langmuir equation. The Langmuir monolayer capacity was found to depend on the Al content and on the cationic form. The Li+‐ and Na+‐exchanged tobermorites with about 20 mol% of Al substitution for Si showed the highest Langmuir monolayer water sorption capacity. The Cs+‐exchanged tobermorites showed a smaller capacity than the Li+‐ or Na+‐exchanged samples, which can be ascribed to clogging of the ion‐exchange cavity by the large Cs+ ions.
Thermodynamic Assessment of the ZrO2─YO1.5 System
Authors: Du Yong , Jin Zhanpeng , Huang Peiyun
Published: March 2005

An optimal set of thermodynamic functions for the ZrO2─YO1.5 system are obtained using phase diagram and thermodynamic data. The liquid is described by a subregular solution model. Both cubic ZrO2 and YO1.5 solid solutions are regarded as one cubic solution, which is also treated as a subregular solution. The ordered Zr3Y4O12 phase is treated as a stoichiometric compound. A regular solution model is applied to the other solid solutions. Tentative equilibrium boundaries between monoclinic and tetragonal ZrO2 solid solutions are evaluated from information about the T0 line. The calculated phase diagram and thermodynamic functions agree well with experimental data.
Thermodynamics of Nitrogen in B2O3, B2O3─SiO2, and B2O3─CaO Systems
Authors: Wakasugi Takashi , Tsukihashi Fumitaka , Sano Nobuo
Published: March 2005

The BN solubilities for B2O3, B2O3─SiO2, and B2O3─CaO systems have been measured mainly at 1823 K using a graphite crucible. The capability of the systems for nitrogen dissolution is compared with that of silicate systems in terms of nitride capacity. The dependence of nitrogen solubility in molten CaO containing 15 mol% of B2O3 on oxygen and nitrogen partial pressures is also investigated. It has been found that there are two mechanisms for nitrogen dissolution, namely as chemically bonded nitrogen and as physically dissolved nitrogen gas.
Bioactive Bone Cement Based on CaO─SiO2─P2O5 Glass
Authors: Kokubo Tadashi , Yoshihara Satoru , Nishimura Naomi , Yamamuro Takao , Nakamura Takashi
Published: March 2005

A CaO─SiO2─P2O5─CaF2 glass powder hardened within 4 min when mixed with an ammonium phosphate solution to form CaNH4PO4·H2O. After it had soaked in a simulated body fluid for 3 d, forming hydroxyapatite, the cement showed a compressive strength of 80 MPa. Implanted into a rat tibia, the mixed paste formed a tight chemical bond to the living bone within 4 weeks. Such a bioactive cement could be useful not only for fixing various kinds of implants to the surrounding bones but also, by itself, as a bone filler.
Carbothermal Synthesis of Silicon Nitride: Effect of Reaction Conditions
Authors: Durham Simon J. P. , Shanker Kartik , Drew Robin A. L.
Published: March 2005

Conditions for carbothermal synthesis of α‐Si3N4 are presented with special emphasis on the reaction temperature, C:SiO2 ratio, and precursor mixing. With pure precursors, the conversion temperature is 1500° to 1550°C. An excess of C is necessary for complete conversion, and a simple sol–gelmixing technique provides excellent intermixing of the precursors. Copious flow of N2 gas throughout the reactor bed is essential if pure Si3N4 is to be produced; small concentrations of CO and O2 promote SiC and Si2N2O, respectively.
Enhanced Mechanical Properties of Alumina by Dispersed Titanium Diboride Particulate Inclusions
Authors: Liu Jenq , Ownby P. Darrell
Published: March 2005

The mechanical properties of composite ceramics composed of 0 to 20 vol% of titanium diboride particles dispersed in an α‐alumina matrix were investigated. The alumina–titanium diboride composite powder was hot‐pressed at 1470°C for 20 min to achieve over 98.8% of the theoretical composite density. The strength and fracture toughness of the twophase, hot‐pressed composite were both significantly improved compared to the single‐phase alumina. Results from different methods of measuring the stress intensity factor, (KIc) are compared and discussed.
Fracture of Alumina Tubes under Combined Tension/Torsion
Authors: Kim Kitae T. , Suh Jeong
Published: March 2005

A fracture criterion for Al2O3 tubes is investigated for various loading paths under combined tension and torsion. Experimental data are compared with the Weibull statistical fracture theory. As the stress state approaches a shear dominant state, a tensile principal stress at fracture from the Weibull theory underestimates strengthening effects. By including the effect of the minimum principal stress in the tensile principal stress criterion, an empirical fracture criterion is proposed. Agreement between the proposed criterion and experimental data for Al2O3 tubes under combined tension/torsion and balanced biaxial tension is very good.
Subcritical Crack Growth of Macrocracks in Alumina with R‐Curve Behavior
Authors: Fett Theo , Munz Dietrich
Published: March 2005

Subcritical crack growth of macroscopic cracks in two Al2O3 ceramics is investigated with single‐edge‐notched bending specimens under constant load. The resulting v‐KI‐curves are in complete contrast to the behavior of natural cracks. In spite of the monotonic increase of the externally applied stress intensity factor due to crack extension, the crack growth rates first decrease. This behavior is caused by crack shielding due to crack border interaction and can be described by a rising crack growth resistance. Two methods are applied to determine the R‐curve under subcritical crack growth conditions.
Crystallization in a Barium‐Containing Magnesium Aluminosilicate Glass‐Ceramic
Authors: Chaim Rachman , Heuer Arthur H.
Published: March 2005

Crystallization in a Ba‐containing magnesium aluminosilicate (Ba‐MAS) glass‐ceramic was studied using transmission electron microscopy. Ba‐MAS slabs were hot‐pressed at 1000°C to form dense glassy bodies, which were crystallized by further heating in air or argon between 850° and 1300°C. Heterogeneous nucleation of a metastable highquartz solid solution (μ‐cordierite) occurred at crystallization temperatures below 1100°C, followed by dendritic growth; the interdendrite phase was Ba‐rich silicate glass. The high‐quartz solid solution generally acted as a precursor for the nucleation of the stable β‐phase, which was the final crystallization product. Surprisingly, slower crystallization kinetics were observed in argon than in air. Further morphological changes at the higher temperatures and longer annealing, times resulted from a Rayleigh type instability, which led to breakup of the interdendrite silicate rods, and simultaneous polygonization of β‐cordierite.
Fracture Modes in MoSi2
Authors: Wade Robert K. , Petrovic John J.
Published: March 2005

The room‐temperature fracture behavior of polycrystalline MoSi2 was characterized using Vickers indentation fracture. Fracture analysis was aided by the optically active grain structure of MoSi2 revealed under polarized light. Radial crack propagation from indentations was found to be predominantly transgranular. The approximate indentation fracture toughness of MoSi2 was 3 MPa.m1/2, while the measured hardness was 8.7 GPa. Fracture behavior is believed to be controlled by anisotropy and cleavage energy of the tetragonal MoSi2 crystal structure.
Processing Effects in the Sol–Gel Preparation of PZT Dried Gels, Powders, and Ferroelectric Thin Layers
Authors: Lakeman Charles D. E. , Payne David A.
Published: March 2005

Infrared spectroscopy was used to monitor and characterize solutions used in the preparation of PZT 53/47 powders, dried gels, and thin layers deposited on platinized silicon substrates. It was found that careful control of the chemistry of the precursor solutions, in particular the control of the formation of esters and their elimination, resulted in reproducible solutions which could be used to prepare thin layers with superior properties. In addition, it was found that phase development and microstructure were related to the precursor chemistry. Single‐phase perovskite layers with uniform microstructures resulted from ester‐free solutions and were suitable for ferroelectric applications (Pr∼ 22 μC/cm2, Ec∼ 80 kV/cm, K∼ 800, tan δ∼ 0.01).
Reactive Laser Ablation Synthesis of Nanosize Alumina Powder
Authors: Johnston Gregory P. , Muenchausen Ross , Smith Douglas M. , Fahrenholtz William , Foltyn Steve
Published: March 2005

An aluminum (Al) target was laser ablated in an oxygen (O2) atmosphere, producing nanosize alumina (Al2O3) powder. The powder surface area decreased (and the particle size increased) with both increasing oxygen pressure and laser fluence. All powders produced had surface areas between 135 and 250 m2/g, corresponding to primary particle sizes ranging from 7 to 3 nm in radius. Phase evolution with temperature was studied via X‐ray diffraction. These powders showed a direct transformation from γ‐ to α‐alumina at approximately 1200°C, bypassing other transition alumina phases, while still maintaining small particle size ( 30 nm). Despite the nanosize particles, green densities equal to 54% of the skeletal density (i.e., true density of the solid phase) were obtained by uniaxial pressing at 40 MPa.
Correlation of Subsolidus Phase Relations in the Ag─Pd─O System to Oxidation Reduction Kinetics and Dilametric Behavior
Authors: Wang Sea Fue , Huebner Wayne , Huang Chi‐Yen
Published: March 2005

The relationship between the equilibrium subsolidus phase diagram of the Ag─Pd─O2 system, and the oxidation/reduction kinetics and microstructure development of 70Ag/30Pd coprecipitated and alloyed powders is presented. For the same Ag/Pd ratio and similar powder characteristics, the onset temperatures of Pd oxidation and PdO reduction for coprecipitated powders are lower than those of alloyed powders. For Ag/Pd alloyed powders, complete oxidation of Pd does not occur, because of kinetic limitations and the equilibrium phases present at high temperatures. Significant differences in microstructural evolution were observed and correlated to the oxidation processes and densification kinetics. The impact of these results on the use of the Ag/Pd system as conductors is discussed.
Long‐Term Tensile Creep Testing for Advanced Ceramics
Authors: Ohji Tatsuki , Yamauchi Yukihiko
Published: March 2005

A tensile creep test technique for long‐term (10 000 h or more) measurement is described. A specimen is gripped by ceramic grips with a simple design in the hot zone of a furnace. By optimizing the grip geometry, the ratio of the parasitic bending strain to the average tensile strain can be suppressed to less than 1%. The careful hand polishing to remove machining flaws has been found essential for reliable creep tests. The long‐term test is demonstrated for silicon carbide at 1400°C under a stress of 200 MPa, by routinely replacing the heat elements, thermocouples, etc., during the test. The stability of the creep curve, which has been obtained by an optical extensometry system, is normally within ± 2 μm.
Synthesis, Sintering, Microstructure, and Mechanical Properties of Ceramics Made by Exothermic Reactions
Authors: Cutler Raymond A. , Rigtrup Kevin M. , Virkar Anil V.
Published: March 2005

Al2O3‐TiC, Al2O3‐TiC0.5N0.5, Al2O3‐WC, Al2O3‐SiC, and Al2O3‐HfB2 powders were synthesized by the aluminothermic reduction of oxides in the presence of carbon or boron. The reacted powders were milled to reduce the size of agglomerates and subsequently densified without applied pressure to near‐theoretical density. Microstructures and mechanical properties of composites made from exothermically reacted powders were compared with similar ceramics made from commercially available powders. In situ sintering was possible in the Al2O3‐TiC system using a closed graphite crucible to contain reaction gases. The synthesis of β‐SiC at temperatures above 1400°C via the direct reaction of the elements (SHS) was compared with SiC made by the magnesiothermic reduction of SiO2 in the presence of C after removing the MgO by leaching.
Crystallization of Anorthite‐Seeded Albite Glass by Solid‐State Epitaxy
Authors: Liu Chunling , Komarneni Sridhar , Roy Rustum
Published: March 2005

Stoichiometric albite glass (NaAlSi3O8) was seeded with 5 wt% crystalline anorthite (CaAl2Si2O8) to make albite glass‐ceramics. The epitaxial crystallization of the albite glass to the glass‐ceramics was investigated by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). High albite was observed as the major crystallization product over the temperature range of 800–1200°C. No crystalline albite could be crystallized from pure albite glass without seeds. Small amounts of nepheline (NaAlSiO4), however, crystallized along with albite after heat treatments at temperatures lower than 1000°C. The platelike microstructure of albite crystals was revealed in the seeded glasses. The albite blades grew epitaxially from the anorthite seeds, and the Ca content decreased in the direction away from the seeds. The degree of crystallization and the grain size were dependent upon the heat treatment conditions. By increasing the particle size of the seed, the crystallization process was retarded and the resultant microstructure was degraded. The seeding efficiency was also lowered by adding nonisostructural hexagonal anorthite seeds which produced less albite but more nepheline crystals. Crystallization of albite glass by seeding with 5 wt% anorthite is much greater than with the surface nucleation which takes place in a homogeneous 95 wt% albite + 5 wt% anorthite glass.
Experimental Analysis of Fine–Particle Migration during Ceramic Filtration Processes
Authors: Hampton J. Holly D. , Savage Stuart B. , Drew Robin A. L.
Published: March 2005

Slip casting and filter pressing are ceramic forming processes which can be affected by the migration of fine particles during filtration. Fine particles can be carried along with the filtrate and deposited within the ceramic compact and /or filter medium, thereby clogging and reducing the permeabilities of the porous media. This in turn affects the growth rate as well as the porosity of the compact. Evidence of clogging of the ceramic compact and filter medium was obtained by (1) SEM analysis of the compact and filter media, (2) surface area measurements of cross sections of compacts, and (3) measurements of the thickness of compacts as a function of casting time.
Effect of Cr2O3 on Slag Resistance of Al2O3–SiO2 Refractories
Authors: Chan Chen‐Feng , Ko Yung‐Chao
Published: March 2005

Refractory bodies of 65 wt% Al2O3 were prepared from a mixture of calcined alumina and raw kaolin with the addition of Cr2O3 up to 15 wt%. The Cr2O3 addition effectively enhances slag resistance and reduces mullite formation. Petrographic analysis of the refractories after the slag test suggests that Cr2O3 increases the viscosity of both the glassy phase in the refractory as well as the slag, thereby retarding slag penetration and reaction at elevated temperature.
Cubic‐to‐Tetragonal (t') Transformation in Zirconia‐Containing Systems
Authors: Sheu Tzer‐Shin , Tien Tseng‐Ying , Chen I‐Wei
Published: March 2005

The coexistence of the cubic fluorite and tetragonal phases in rapidly quenched samples was studied in the ZrO2‐MO1.5 systems for M = Sc, In, Y, and rare earths (R). Spontaneous transformation from metastable cubic phase was triggered at room temperature by a mechanical force. Isolated tetragonal platelets in the cubic matrix were bounded by [101] habit planes and contained anti‐phase boundaries. The tetragonality decreased with stabilizer content and vanished at around 18 mol% for M = Y and R, 23 mol% for M = Sc, and 25 mol% for M = In, all at room temperature. With increasing temperature, the tetragonality initially increased because of anisotropic thermal expansion, then decreased rapidly, after reaching a maximum, as the temperature for the tetragonal‐to‐cubic transformation was approached. Being a first‐order martensitic transformation, the cubic‐to‐tetragonal transformation is accompanied by a discontinuous change of tetragonality and a hysteresis loop as the temperature or composition passes through the equilibrium value.
Protonic Conduction in Acceptor‐Doped Cubic Rare‐Earth Sesquioxides
Authors: Norby Truls , Dyrlie Oddvar , Kofstad Per
Published: March 2005

Using ac conductivity and the concentration cell emf method, conductivity contributions from protons, native ions, and electrons have been measured as a function of temperature (560° to 1156°C) in wet oxygen/air for the cubic systems Y2O3+ 1 mol% MgO, Sm2O3+ 1 mol% CaO, Gd2O3+ 1 mol% CaO, and YYbO3+ 5 mol% CaO. All exhibit significant proton conductivities as well as native‐ion and electronic conductivities at all temperatures. In wet atmospheres and reduced temperatures, the oxides dissolve protons to compensate for the acceptor doping, at the expense of native positive defects. This dissolution of protons seems to be relatively more favorable in oxides of smaller rare‐earth cations (e.g., Y3+ and Yb3+). On the other hand, larger cations (e.g., Sm3+ and Gd3+) give higher proton mobilities. As compared to oxides of the true lanthanides with similar cationic radii, yttrium‐containing oxides have lower proton mobilities.
Fatigue Deformation Mechanisms of Zirconia Ceramics
Authors: Liu Shih‐Yu , Chen I‐Wei
Published: March 2005

Two distinct fatigue deformation mechanisms, microcracking and transformation plasticity, have been identified in 3Y‐TZP and Mg‐PSZ. Microcracking is dominant in 3Y‐TZP, while transformation plasticity is more evident in Mg‐PSZ. However, the proportion of these two mechanisms is dependent on the frequency and stress amplitude, with transformation plasticity favored at low frequency and high stress. Generally, microcracks form in the tensile half‐cycle and partly close in the compressive half‐cycle, whereas transformation from the tetragonal to the monoclinic phase, as well as the reverse transformation, can occur in both tension and compression. Under stress‐controlled cycling, although the initial hysteresis loop is highly asymmetric with respect to the stress, cyclic softening and the development of an internal stress cause it to gradually evolve to yield an essentially symmetric cyclic stress–strain curve.
A Prediction on the Strength of Glass Following the Formation of Subsurface Flaws by Scribing
Authors: Cheng Weili , Finnie Iain
Published: March 2005

An analysis is presented to predict the strength of glass after subsurface cracking is produced by drawing a lightly loaded angular particle across the surface. It is shown that the presence of subsurface cracks combined with the residual stresses introduced by such scratches under very small loads greatly lowers the strength of glass. The predicted ranges of tensile strength for soda–lime glass in dry and moist environments are in agreement with typical values in the literature.
Liquid‐Phase‐Assisted Sintering of Calcium‐Doped Lanthanum Chromites
Authors: Sakai Natsuko , Kawada Tatsuya , Yokokawa Harumi , Dokiya Masayuki , Kojima Isao
Published: March 2005

Investigations have been made on the low‐temperature sinterability of calcium‐doped lanthanum chromites which are to be used as interconnectors in solid oxide fuel cells (SOFCs). Nominally chromium deficient lanthanum calcium chromites (La0.7Ca0.3Cr1‐yO3, y= 0.02) were found to be sinterable to 94% theoretical density at 1573 K in air, whereas no densification was observed for samples with y = 0. The two‐step shrinkage process suggests a liquid phase sintering mechanism with calcium oxychromates playing an important role as the liquid phase. After sintering at 1573 K, calcium‐rich substances remained at grain boundaries.
High‐Temperature X‐ray Study of the Cubic‐Tetragonal Diffusionless Phase Transition in the ZrO2─ErO1.5 System: I, Phase Change between Two Forms of a Tetragonal Phase, t′‐ZrO2 and t″‐ZrO2, in the Compositionally Homogeneous 14 mol% ErO1.5‐ZrO2
Authors: Yashima Masatomo , Ishizawa Nobuo , Yoshimura Masahiro
Published: March 2005

The cubic‐tetragonal phase transition of 14 mol% ErO1.5‐ZrO2 was investigated through high‐temperature powder X‐ray diffraction. A new tetragonal form without tetragonality, t″‐ZrO2, was taken into consideration, although only two forms of t‐ and t′‐ZrO2 have previously been distinguished. The metastable t″‐ZrO2, which could be obtained by rapid quenching of 14 mol% ErO1.5‐ZrO2 melts, partially changed into t′‐ZrO2 through a thermal activation process above 500°C. The t′‐ZrO2 continuously changed into t″‐ZrO2 or transformed into cubic phase at about 1200°C. The t′‐t″ change occurred reversibly below 1100°C.