bluuebunny/crossref_metadata_2025 huggingface dataset:
Piezoelectric Anisotropy and Polarization Sublattice Coupling in Perovskite Crystals
Authors: Uchino Kenji , Oh Ki‐Young
Published: March 2005

Anisotropic piezoelectricity has been intensively investigated in PbZrO3‐ or PbTiO3‐based perovskite ceramics, in comparison with the isotropic characteristic in the intermediate composition of the solid‐solution Pb(Zr, Ti)O3 system. In order to explain the piezoelectric anisotropy, a phenomenological treatment is proposed for a perovskite sublattice system, including electromechanical coupling terms. Electrostrictive couplings, especially between the two sublattices, provide significant contribution to the piezoelectric anisotropy.
Mullite for Structural, Electronic, and Optical Applications
Authors: Aksay Ilhan A. , Dabbs Daniel M. , Sarikaya Mehmet
Published: March 2005

Mullite (3Al2O3·2SiO2) is becoming increasingly important in electronic, optical, and high‐temperature structural applications. This paper reviews the current state of mullite‐related research at a fundamental level, within the framework of phase equilibria, crystal structure, synthesis, processing, and properties. Phase equilibria are discussed in terms of the problems associated with the nucleation kinetics of mullite and the large variations observed in the solid‐solution range. The incongruent melting behavior of mullite is now widely accepted. Large variations in the solid solubility from 58 to 76 mol% alumina are related to the ordering/disordering of oxygen vacancies and are strongly coupled with the method of synthesis used to form mullite. Similarly, reaction sequences which lead to the formation of mullite upon heating depend on the spatial scale at which the components are mixed. Mixing at the atomic level is useful for low‐temperature (<1000°C) synthesis of mullite but not for low‐temperature sintering. In contrast, precursors that are segregated are better suited for low‐temperature (1250° to 1500°C) densification through viscous deformation. Flexural strength and creep resistance at elevated temperatures are significantly affected by the presence of glassy boundary inclusions; in the absence of glassy inclusions, polycrystalline mullite retains >90% of its room‐temperature strength to 1500°C and displays very high creep resistance. Because of its low dielectric constant, mullite has now emerged as a substrate material in high‐performance packaging applications. Interest in optical applications mainly centers on its applicability as a window material within the mid‐infrared range.
Reactions and Microstructure Development in Mullite Fibers
Authors: Richards Elizabeth A. , Goodbrake Chris J. , Sowman Harold G.
Published: March 2005

Microstructural and compositional changes during heat treatment of sol–gel‐derived mullite fibers with additions of 2 wt% B2O3, 2 wt% P2O5, 2 wt% Cr2O3, and (1 wt% P2O5+ 1 wt% Cr2O3) were compared with those of undoped mullite fibers. For all compositions the sequence of phase development was the crystallization of a spinel phase (†‐Al2O3 or Al–Si spinel) from amorphous material, followed by the formation of mullite at higher temperatures. Differential thermal analysis showed that additions of B2O3 and P2O5 increased the temperature of spinel formation and that B2O3 significantly decreased the temperature of mullite formation. After 1 h at 1200°C, the size of mullite grains in fibers that contained B2O3 was less than 1000 Å the grains in fibers of other compositions were 6000 to 12000 Å. After 60 h at 1400°C, fibers modified with B2O3 had a grain size less than 2000 to 3000 Å the grains in fibers of other compositions were 6000 to 12000 Å. B2O3 was the most volatile additive.
Forces between Aluminum Oxide Grains in a Silicate Melt and Their Effect on Grain Boundary Wetting
Authors: Shaw Thomas M. , Duncombe Peter R.
Published: March 2005

Experiments in which the disperson and flocculation behavior of aluminum oxide grains in a Ca‐Al‐Si‐O silicate liquid are examined have been used to determine the nature of the forces between oxide grains in a silicate melt. From observations of the way in which the melt penetrates grain boundaries in a high‐purity polycrystalline aluminum oxide, it is concluded that at short ranges the interaction between most grains is strongly repulsive. The formation of aggregated groups of grains in settling experiments, however, indicates that at larger separations the forces between grains become attractive and hence that there is an energetic minimum in the interaction between grains. The results are discussed in relation to models for the stability of intergranular films in liquid‐phase‐sintered ceramics. Microstructural observations made on the melt‐penetrated aluminas are used to make a quantitative estimate of the fraction of grain boundaries that are wetted by the melt. The observations indicate that large clusters of grains connected by unwetted grain boundaries exist in liquid‐phase‐sintered aluminas.
Rising Crack‐Growth‐Resistance (R‐Curve) Behavior of Toughened Alumina and Silicon Nitride
Authors: Ramachandran Nageswaran , Shetty Dinesh K.
Published: March 2005

R‐curves for a sinter/HIPed SiC(whisker)‐reinforced alumina and a sintered silicon nitride were assessed by direct measurements of lengths of cracks associated with Vickers indentation flaws. The fracture toughness measurements based on (a) initial (as‐indented) crack lengths, (b) equilibrium growth of cracks during increasing far‐field loading, and (c) crack lengths corresponding to unstable fracture showed definitive trends of R‐curves for both materials. The fracture mechanics analyses employed an indenter‐material constant that was independently estimated using a physical model for the residual driving force and a free surface correction factor that accounted for the effects of size and shape of the cracks on stress intensity. It is shown that R‐curve estimations based on crack length measurements have the intrinsic advantage that crack length dependence of fracture toughness is not assumed a priori as is done in conventional analysis based on strength. The measured fracture toughness of SiC(whisker)‐reinforced alumina was in agreement with the prediction of a toughening model based on crack bridging by partially debonded whiskers.
Geometry‐Dependent R‐Curve for Quasi‐Brittle Materials
Authors: Ouyang Chengsheng , Shah Surendra P.
Published: March 2005

Fracture of quasi‐brittle materials such as ceramics and cement‐based materials can be described by an R‐curve, defined as fracture resistance. The R‐curve can be constructed as the envelope of the strain energy release rates (G). In this study, R‐curve is defined as an envelope of G‐curves obtained by varying the dimensions of identical specimens, while keeping the geometry and the initial flaw size constant. By approximating the G‐curve with a second‐order function of the crack length, a simple formulation of the R‐curve is derived by solving a differential equation. Three parameters are needed for the proposed R‐curve. These parameters can be obtained by testing a notched‐beam specimen and using linear elastic fracture mechanics. The proposed R‐curve simulates well the geometry dependency as well as other characteristics of the fracture response of quasi‐brittle materials.
Interrelation between Flaw Resistance,R‐Curve Behavior, and Thermal Shock Strength Degradation in Ceramics
Authors: Lutz Ekkehard H. , Swain Michael V.
Published: March 2005

Indentation‐induced flaw resistance, toughness behavior upon crack extension, and thermal shock behavior of a range of different ceramics are investigated. The results are analyzed and compared with previous theoretical considerations. An attempt is made to establish an interrelation among the above three properties of ceramics.
Progressive Changes in the Structure of Hardened C3S Cement Pastes due to Carbonation
Authors: Groves Geoffrey W. , Brough Adrian , Richardson Ian G. , Dobson Christopher M.
Published: March 2005

The structures of partially carbonated hardened C3S cement pastes have been investigated by a combination of 29Si magic angle spinning nuclear magnetic resonance spectroscopy and analytical transmission electron microscopy, supported by X‐ray diffraction and thermogravimetric analysis. Progressive changes in structure are reported for thin slices for a paste carbonated in pure CO2 for times from 1 to 16 h, and the results are compared with those for a paste carbonated for 2 months in air. C‐S‐H gel of reduced Ca:Si ratio and increased silicate polymerization was formed during the early stages of carbonation. The morphology of the original C‐S‐H was, in the main, retained. A cross‐linked silica‐rich gel formed at later times in paste carbonated in CO2 but not up to the time of 2 months in air. Calcium carbonate took the form of microcrystals of vaterite and calcite which formed dense masses between gel fibrils and around partially reacted CH crystals, possibly accounting for the observed slowing in the rate of reaction of CH with time.
Reaction Sintering of Polycarbosilane‐Impregnated Compact of Silicon Carbide Hollow Particles and the Resultant Thermoelectric Properties
Authors: Pai Chul‐Hoon , Sasaki Yasuhiro , Koumoto Kunihito , Yanagida Hiroaki
Published: March 2005

Porous SiC ceramics fabricated from hollow particles and polycarbosilane (PSC) are promising materials for high‐temperature thermoelectric energy conversion. Reaction sintering of PCS‐impregnated compacts of SiC hollow particles gave rise to porous microstructures with the hollow shape remaining. The repetition of the PCS‐impregnation and sintering process resulted in only a slight increase in density but in a great improvement in thermoelectric properties.
Si3N4/SiC‐Whisker Composites without Sintering Aids: III, High‐Temperature Behavior
Authors: Pezzotti Giuseppe , Tanaka Isao , Okamoto Taira
Published: March 2005

The fracture behavior at high temperature of a Si3N4‐based SiC‐whisker composite fabricated by hot isostatic pressing without sintering aids is compared with that of other highly refractory materials. Particular attention is directed toward evaluating the slow‐crack‐growth resistance of the composite up to 1440°C and relating this resistance to the microfracture behavior of Si3N4 grains, SiC whiskers, and the intergranular, glassy SiO2 phase. Only thick whiskers operate to bridge the wake of the crack; these whiskers may make a positive contribution to the slow‐crack‐growth resistance. Impurities detected by EDX microanalysis at the grain boundary, however, apparently degrade the high‐temperature properties, a finding supported by internal‐friction measurements. Nevertheless, the high potential of the system without sintering aids for high‐temperature structural applications has been demonstrated by the time to failure estimated from the measured slow‐crack‐growth resistance for a fixed flaw size.
Origin of Darkening in 8 wt% Yttria—Zirconia Plasma‐Sprayed Thermal Barrier Coatings
Authors: Ingo Gabriel Maria
Published: March 2005

The origins of darkening of 8 wt% Y2O3‐ZrO2 air plasmasprayed (APS) and low‐pressure plasma‐sprayed (LPPS) thermal barrier coatings (TBC) have been studied using X‐ray photoelectron spectroscopy. The change of valence states of zirconium, due to the reduction of ZrO2 to Zr2O3, was responsible for darkening of TBC. Quantification of Zr3+ oxide was related both to the black color of TBC and to the spraying technologies and parameters. Furthermore, impurity (Fe, Al, Si, and Na) segregation and exsolution phenomena were monitored as a function of the air thermal treatment (up to 1473 K) and it was demonstrated not to be the origin of darkening.
Effect of Barium Titanate on Microstructural Evolution and Properties of Lead Zinc Magnesium Niobate Ceramics
Authors: Chen San‐Yuan , Cheng Syh‐Yuh , Wang Chien‐Min
Published: March 2005

BaTiO3 (BT) addition vastly alters the dielectric behavior of lead zinc magnesium niobate (PZMN) relaxor ferroelectrics. Heat treatment, including the sintering process, influences the dielectric properties through changes in the distribution of heterogeneous compositions and in the extent of diffused phase transition. With more than 15 mol% BaTiO3 addition, two cubic phases composed of Ba‐rich and Ti‐rich PZMN substances exist. The destruction of the BaTiO3 phase and reconstruction of Ba‐rich and Ti‐rich regions are initiated during heat treatment, and the extent of destruction and reconstruction determines the dielectric properties of the PZMN ceramics. The BT‐rich PZMN phase, which occurs with higher BaTiO3 addition, plays a less important role in the present investigation.
Kinking of a Crack out of an Interface: Role of In‐Plane Stress
Authors: He Ming‐Yuan , Bartlett Andrew , Evans Anthony G. , Hutchinson John W.
Published: March 2005

A crack lying in the interface between two brittle elastic solids can advance either by continued growth in the interface or by kinking out of the interface into one of the adjoining materials. This competition can be assessed by comparing the ratio of the energy release rates for interface cracking and for kinking out of the interface to the ratio of interface toughness to substrate toughness. The stress parallel to the interface, σ0, influences the energy release rate of the kinked crack and can significantly alter the conditions for interface cracking over substrate cracking if sufficiently large. This paper provides the dependence of the energy release rate ratio on the in‐plane stress. The nondimensional stress parameter which emerges is, σ0(a/E* Ti)1/2, where a is the initial length of the kink into the substrate, E* is a modulus quantity, and Ti is the fracture energy of the interface. An experimental observation of the cracking of reaction product layers in bonds between Ti(Ta) and Al2O3 is rationalized by the theory.
Effects of Magnesium Oxide on Grain‐Boundary Segregation of Calcium During Sintering of Alumina
Authors: Baik Sunggi , Moon Jong Ha
Published: March 2005

The advantage of certain amounts of MgO addition in alumina sintering has been realized, and it is common practice. In an attempt to understand the role of MgO in the presence of CaO in commercial‐grade alumina, grain‐boundary segregation of Ca was investigated by scanning Auger electron microprobe (SAM) using an ultrapure alumina after controlled doping of CaO and/or MgO. The commercial‐grade alumina, which usually contains a small amount of CaO, is difficult to sinter to high density. The pure alumina composition (<99.999%) gives “clean” boundaries when it is sintered under “clean” conditions. As the powder was doped with 100 ppm of CaO and sintered at 1300° to 1500°C, all of the grain boundaries were enriched by Ca as observed by others. However, it was also discovered that some of the grain boundaries are enriched by an exceptionally high concentration of Ca. Such a large variation of Ca contents depending on the grain‐boundary facets disappeared when samples were codoped with small amounts of MgO. The results suggest that MgO plays a beneficial role in controlling the anisotropic segregation of Ca to various interfaces including grain boundaries and pore surfaces during sintering of alumina. MgO thus enhances chemical homogeneity of commercial‐grade alumina.
Synthesis of Solid Phosphate Compounds of Ti(III) and Ti(IV)
Authors: Bamberger Carlos E. , MacDougall C. Sue , Begun George M. , Hatmaker Terry L.
Published: March 2005

Several new methods for the synthesis of TiPO4 are described. Crystalline phosphate compounds containing boron and titanium in a single III or mixed (III, IV) oxidation states have been synthesized and identified by X‐ray diffraction. An additional family of phosphate compounds containing only Ti(III, IV) has also been synthesized and identified. All the compounds exhibit attractive colors, e.g., bright green, blue, purple, and black, and are stable at room temperature in air and in boiling water.
Preparation of Fiberlike Silicon Nitride from Diatomaceous Earth
Authors: Mizuhara Yukako , Noguchi Masao , Ishihara Tatsumi , Satoh Atsushi , Hiramatsu Katsuto , Takita Yusaku
Published: March 2005

Fiberlike Si3N4 was prepared by the carbothermal reduction of diatomaceous earth in a flow of nitrogen and ammonia. Diatomaceous earth, which is an inexpensive raw material, is composed of 82.5 wt% SiO2, 5.69 wt% Al2O3, and a very small amount of metal oxides (K2O, CaO, and Fe2O3). Two types of fiberlike Si3N4 were obtained, short needlelike fiber and woollike fiber with Fe droplets at 1350°C.
Microwave Heating of Grain Boundaries in Ceramics
Authors: Johnson D. Lynn
Published: March 2005

It has been suggested that enhanced sintering of ceramics by microwave heating may be the result of accelerated grainboundary diffusion caused by the grain boundaries being heated to temperatures significantly greater than the bulkspecimen temperature. Heat flow calculations show that the temperature difference between the grain boundary and grain interior will be a small fraction of a degree, even under extreme conditions.
Sol‐Gel Coating of Lithium Zinc Ferrite Powders
Authors: Brooks Keith G. , Amarakoon Vasantha R.W.
Published: March 2005

Lithium zinc ferrite powders of composition Li0.3Zn0.4Mn0.05− Fe2.25O4 were prepared by solid—state synthesis. Liquid‐phase borosilicate sintering additives were applied to the ferrite particle surfaces at room temperature via a sol—gel coating technique. Calcined and comminuted ferrite powder was dispersed in methanol with predetermined quantities of tetraethyl orthosilicate and triethyl borate. Hydrofluoric acid was used to catalyze the sol—gel reactions. Amorphous coatings of 10 to 20 nm thickness were observed on particle surfaces by TEM. Chemical bonding in the coatings was studied using diffuse reflectance FTIR spectroscopy.
Durability of Zirconium‐Containing Ceramic Waste Forms in Water
Authors: Kamizono Hiroshi , Hayakawa Issei , Muraoka Susumu
Published: March 2005

The durability of three ceramic waste forms containing Zr as a constituent is examined at 90° and 150°C in HCl or deionized water. The three ceramic waste forms examined are: (A) 10‐mol%‐Y2O3‐stabilized ZrO2, (B) La2Zr2O7 with a pyrochlore structure, and (C) CaZrO3 with a perovskite structure. Each form contains an appropriate amount of Ce, Nd, and Sr to simulate the leaching behavior of actinides, lanthanides, and alkaline‐earth fission products from highlevel nuclear waste. Two forms, (A) 10‐mol%‐Y2O3‐stabilized ZrO2 and (B) La2Zr2O7 with a pyrochlore structure, show excellent durability in HCl and deionized water. For example, leach rates of all constituents in the La2Zr2O7 waste form are less than about 10−4 g·m−2·d−1 at 150°C for a leach time of 32 d in deionized water.
Refractive Index of Liquid Aluminum Oxide at 0.6328 μm
Authors: Krishnan Shankar , Weber J. K. Richard , Schiffman Robert A. , Nordine Paul C. , Reed Robert A.
Published: March 2005

The real part, n, of the refractive index of pendant drops of laser‐heated liquid Al2O3 was measured by laser ellipsometry at 0.6328 μm. At temperatures from 2327 to 2600 K, n for the liquid was 1.744 ± 0.016. No significant differences were observed in the results obtained on ruby specimens and in oxygen or argon atmosphres.
Grain Growth in Microwave‐Annealed Alumina
Authors: Janney Mark A. , Kimrey Hal D. , Schmidt Monica A. , Kiggans James O.
Published: March 2005

Normal grain growth in dense, fine‐grained, aluminum oxide‐0.1 wt% MgO was studied under both conventional furnace and 28‐GHz microwave furnace annealing conditions. The microstructural changes that occurred were the same for both sets of samples; soap bubble microstructures were observed and the aspect ratios and shape factors did not change during the anneals. The kinetics of grain growth were greatly increased by the 28‐GHz microwave anneals; e.g., the grain growth rate at 1500°C in the microwave furnace was the same as the rate at 1700°C in the conventional furnace. Also, the activation energy for grain growth was reduced by the microwave anneal from 590 kJ/mol (conventional) to 480 kJ/mol (microwave). Finally, these results demonstrate that a “microwave effect” can exist in a dense ceramic body and that no free pore‐solid interfaces are necessary.
Molten Salt Synthesis of a Complex Perovskite, Pb(Fe0.5Nb0.5)O3
Authors: Chiu Chien C. , Li Chen C. , Desu Seshu B.
Published: March 2005

Pure, single‐phase stoichiometric Pb(Fe0.5Nb0.5)O3 (PFN) powders were successfully formed by molten salt synthesis using mixtures of NaCl and KCl salts. Lower temperatures and shorter times (1/2 h at 800°C) were needed for singlephase PFN formation from molten salts relative to those required for solid‐phase methods (4 h at 1000°C). A systematic study indicating the effects of process parameters, such as temperature, time, and amount of flux with respect to starting oxides, on the PFN formation mechanism and its resulting powder characteristics is reported. The particle size increased with increasing synthesis temperature; the rate of increase was greatest above 900°C, which is close to the melting point of PbO. PFN powders formed by molten salt synthesis are spheroidal, free from aggregates, and sintered to good densities at temperatures as low as 1000°C.
Crystallization Kinetics of a Lithia–Silica Glass: Effect of Sample Characteristics and Thermal Analysis Measurement Techniques
Authors: Ray Chandra S. , Huang Wenhai , Day Delbert E.
Published: March 2005

The crystallization kinetics of a 40Li2O · 60SiO2 (mol%) glass were studied using DTA and both isothermal and nonisothermal DSC techniques as a function of (1) particle size of the glass powder, (2) crucible material, e.g., alumina or platinum, (3) furnace atmosphere, e.g., nitrogen, oxygen, or argon, and (4) surface pretreatment of the glass powder with deionized water, HCl, or HF. The kinetic parameters for crystallization, namely, the activation energy, E, frequency factor, v, and Avrami exponent, n, for a particular sample particle size are in close agreement for all three techniques. E and v both decrease while n increases (from 0.9 to 3.0) as the particle size of the glass powder increases from 40 to ∼250 μm, whereupon these parameters become fairly constant for larger particles. Neither the furnace atmosphere nor crucible material had a significant effect on E, v, or n. Washing the glass with deionized water, HCl, or HF before the measurements decreased the crystallization temperature by 25° to 30°C. E and v remain unchanged by this treatment, but n decreased from 3 to 1.6 for a sample of particle size ∼250 μm.
Spherulitic Growth from a Phase‐Separated Vitreous Matrix in a Cordierite–Y‐Stabilized Zirconia Glass‐Ceramic
Authors: Sue Yuan‐Jang , Shen Pouyan , Chen San‐Yuan , Lu Hong‐Yang
Published: March 2005

Crystallization behavior, especially spherulitic growth of a cordierite–3 mol% Y2O3–ZrO2 glass composition annealed at 950° to 1370°C was studied by optical and electron microscopy. Microstructural analysis revealed that glass‐inglass phase separation occurred prior to the crystallization of β‐quartz solid solution (ss). ZrO2 was involved in promoting the nucleation of β‐quartz ss; the crystallization evolved from a cellular to a spherulitic morphology, and β‐quartz transformed to α‐cordierite as the temperature was raised. ZrO2 was eventually expelled intergranularly and intragranularly to form slightly misoriented domains upon annealing, and the dendritic or clustered tetragonal t‐ZrO2 crystals transformed to monoclinic (m) in symmetry. A possible dragging force exerted between α‐cordierite and a glassy droplet phase is proposed to explain the corrugated cordierite/glass interface.
Cohesive Crack Models for Cement Mortar Examined Using Finite‐Element Analysis and Laser Holographic Measurements
Authors: Miller Richard A. , Castro‐Montero Alberto , Shah Surendra P.
Published: March 2005

Unlike classically brittle materials, concrete and mortar toughen as cracks propagate. One of the methods to account for this quasi‐brittle response is to use cohesive crack models. Closing pressure, which is assumed to be a function of crack‐opening displacement, is applied on the crack faces in such models. In this paper, measurements of crack‐opening profiles by laser holography were used to examine the various closing‐pressure models. An iterative process based on linear elastic finite‐element analysis with singular elements was used to compute crack profiles when the closing pressure was applied to the crack faces. The predicted crack profiles were compared with the experimental values for evaluation. The validity of the assumption of eliminating crack tip singularity was studied. A crack‐length‐dependent cohesive model is proposed based on experimental measurements of the crack profile.
Oxygen Tracer Diffusion in Vitreous Silica
Authors: Kalen Joseph D. , Boyce Rethia S. , Cawley James D.
Published: March 2005

Oxygen diffusion in vitreous silica glass is studied using the gas exchange technique. The tracer concentration profiles are consistent with a model based on two mechanisms, one network and the other interstitial. These processes are coupled through limited network–interstitial exchange. Nuclear reaction analysis and secondary ion mass spectrometry techniques are performed and compared. These results are compared to experiments on transport in thin silica films grown on single‐crystal silicon.
Piezoelectricity in the Field‐Induced Ferroelectric Phase of Lead Zirconate‐Based Antiferroelectrics
Authors: Oh Ki‐Young , Saito Yutaka , Furuta Atsushi , Uchino Kenji
Published: March 2005

The piezoelectric characteristics were investigated in the field‐induced ferroelectric phase of the antiferroelectric Pb0.99 Nb0.02[(Zr0.6Sn0.4) 1 −yTiy]0.98O3 family. The electromechanical coupling factors kp and kt were measured, and the piezoelectric anisotropy was calculated. The piezoelectricity was controlled easily by the applied electric field pulse. The large piezoelectric anisotropy of these materials in comparison to that of conventional PZT piezoelectric materials makes them very applicable for ultrasonic probes and surface acoustic wave filters.
Low‐Temperature Chemical Vapor Deposition Tungsten Carbide Coatings for Wear/Erosion Resistance
Authors: Garg Diwakar , Dyer Paul N. , Dimos Duane B. , Sunder Swaminathan , Hintermann Hans E. , Maillat Michel
Published: March 2005

A new chemical vapor deposition (CVD) process has been developed to deposit hard coatings, containing tungsten carbide, at temperatures below 500°C. These coatings, which have been applied to both ferrous and nonferrous alloys, exhibit excellent resistance to wear and erosion. The coatings comprise a mixture of tungsten and the tungsten carbide, the latter being present as W2C, W2C + W3C, or W3C. The coatings' composition and properties can be controlled by varying the CVD process parameters. The unique lamellar, fine‐grained microstructures of these coatings contribute to their good tribological properties.
Preparation and Characterization of High‐Density PbO–Bi2O3–B2O3 Glasses
Authors: Van Kirk Susan E. , Martin Steve W.
Published: March 2005

Transparent glasses with densities greater than 8 g/mL, which are resistant to nuclear radiation damage and can serve as a host for rare‐earth scintillators, are being developed. Glasses in the PbO–Bi2O3 system are known to have some of the highest densities yet reported. In this study, a thorough reexamination of these glasses has been performed to examine their optical‐transmission windows and densities. The observed glass‐forming range found in this work agrees well with that reported in the literature. A minimum of ≅ 10 mol% B2O3 was required in some cases to vitrify the compositions. Glasses were examined in which the PbO:Bi2O3 ratios ranged from 10:90 to 90:10 with a maximum total B2O3 content of ≅50 mol%. Glasses with higher B2O3 contents were not examined because of the rapidly decreasing density with increasing B2O3 content. The densities of the glasses ranged from a low of 6.0 g/mL to a high of 8.1 g/mL. The highest densities were observed for glasses with the greatest PbO content. Ultraviolet/visible and infrared spectra were recorded for these glasses. The transmission window lies between ≅430 and 3850 nm. A systematic increase in the cutoff wavelength was found to occur with an increase in the density. Water‐absorption peaks were found in the infrared spectra at ≅3000 nm.
Sol–Gel Processed Y‐PSZ Ceramics with 5 wt% Al2O3
Authors: Tu Gan Feng , Sui Zhi Tong , Huang Qiang , Wang Chang Zhen
Published: March 2005

Y‐PSZ ceramics with 5 wt% Al2O3 were synthesized by a sol–gel route. Experimental results show that powders of metastable tetragonal zirconia with 2.7 mol% Y2O3 and 5 wt% Al2O3 can be fabricated by decomposing the dry gel powder at 500°C. Materials sintered in an air atmosphere at 1500°C for 3 have high density (5.685 g/cm3), high content of metastable tetragonal zirconia (>96%), and high fracture toughness (8.67 MPa.m1/2). Compared with the Y‐PSZ ceramics, significant toughening was achieved by adding 5 wt% Al2O3.
Thermodynamic Assessment of the System MgO–Al2O3
Authors: Hallstedt Bengt
Published: March 2005

The system MgO–Al2O3 was assessed with the CALPHAD technique using a computerized optimization procedure called PARROT. Two of the solid phases, MgO and spinel, showed extensive solid solubility at high temperature and were modeled with the compound energy model. The third solid phase, α‐Al2O3, was modeled as a stoichiometric phase. For the liquid phase, an ionic two‐sublattice model was used. In total, 13 adjustable parameters were optimized: 3 for the MgO, 8 for the spinel, and 2 for the liquid. This resulted in a consistent thermodynamic description for most of the available experimental data points on the phase diagram as well of the thermodynamic properties.
Application of an Ion‐Packing Model Based on Defect Clusters to Zirconia Solid Solutions: II, Applicability of Vegard's Law
Authors: Yashima Masatomo , Ishizawa Nobuo , Yoshimura Masahiro
Published: March 2005

Lattice parameter data of cubic phases and cube roots of unit cell volumes of tetragonal phases in homogeneous ZrO2‐containing solid solutions were compiled to examine the validity of Vegard's law. Except for ZrO2–CeO2 and ZrO2–UO2 systems, the data for cubic phases were expressed by the equation d=asX+b, where d, as, X, and b denote the lattice parameter, a constant depending on dopant species, the dopant content, and a constant independent of dopant species, respectively. For tetragonal phases, the cube roots of unit‐cell volumes could be fitted by a similar equation except for the data in the ZrO2–MO2 systems (M = Ge and U). The constant as was calculated using an ion‐packing model and was independent of the defect cluster models. The calculated as is close to the experimentally observed one, although the former is slightly smaller than the latter in the ZrO2–MOu systems (u= 1 and 1.5). This difference was ascribed to the lack of consideration of the ionic distortions from the ideal sites of the fluorite‐type structure.
In‐Situ Measurement of the Stress‐Induced Phase Transformations in Magnesia‐Partially‐Stabilized Zirconia Using Raman Spectroscopy
Authors: Lim Cheryl S. , Finlayson Trevor R. , Ninio Fred , Griffiths John R.
Published: March 2005

Raman spectroscopy has been used to examine the connection between room‐temperature creep and stress‐induced phase transformations in TS‐Grade Mg‐PSZ. The in‐situ phase measurements obtained show that the tetragonal phase transforms to both the orthorhombic and monoclinic structures under stress. The effect of precipitate orientation is indicated by the variation of creep strain with monoclinic volume fraction.
Reactions of Silicon Carbide and Silicon(IV) Oxide at Elevated Temperatures
Authors: Jacobson Nathan S. , Lee Kang N. , Fox Dennis S.
Published: March 2005

The reaction between SiC and SiO2 has been studied in the temperature range 1400–1600 K. A Knudsen cell in conjunction with a vacuum microbalance and a high‐temperature mass spectrometer was used for this study. Two systems were studied—1:1 SiC (2 wt% excess carbon) and SiO2; and 1:1:1 SiC, carbon, and SiO2. In both cases the excess carbon forms additional SiC within the Knudsen cell and adjusts to the direct reaction of stoichiometric SiC and SiO2 to form SiO(g) and CO(g) in approximately a 3:1 ratio. These results are interpreted in terms of the SiC‐O stability diagram.
Physical Stabilization of the β→γ Transformation in Dicalcium Silicate
Authors: Chan Chin Jong , Kriven Waltraud M. , Young J. Francis
Published: March 2005

It has been shown that the monoclinic β‐phase of dicalcium silicate (Ca2SiO4) can be stabilized against transformation to the orthorhombic γ‐phase by physical rather than chemical factors. Stabilization was studied in different types of microstructures fabricated under various processing conditions such as different powder or grain sizes, chemical additives, cooling kinetics, or high‐temperature annealing treatments. The observations can be explained in terms of a critical particle size effect controlling nucleation of the transformation. Rapid quenching through the high‐temperature hexagonal (α) to orthorhombic (a′H) transformation at 1425°C, which is accompanied by a −4.7% volume decrease, causes periodic fracture of β‐twins due to accumulated strains. Chemical doping with K2O or Al2O3 promotes the formation of amorphous phases which mold themselves around β‐Ca2SiO4 grains. Annealing treatments cause crystallization of the glass and subsequent transformation to the γ‐phase.
Carbon Monoxide and Hydrogen Detection by Anatase Modification of Titanium Dioxide
Authors: Birkefeld Lora D. , Azad Abdul M. , Akbar Sheikh A.
Published: March 2005

From the viewpoint of industrial and automobile exhaust pollution control, a sensor capable of detecting and measuring the concentration of harmful gases such as carbon monoxide and hydrogen in the ambient is desired. This paper describes the development and use of a titania (TiO2)‐based semiconducting oxide material as a reliable and rugged CO and H2 gas sensor in the temperature range of 773 to 1073 K. Significant change in the sensing characteristic of the anatase modification of TiO2 was observed when admixed with an insulating second phase, such as alumina or yttria. In the case of TiO2–10 wt% Al2O3, the sensor response was found to be exclusively dependent on the hydrogen concentration alone; the presence of CO or CO2 did not affect the sensitivity. On the other hand, the sensor based on TiO2–10 wt% Y2O3 showed increased sensitivity to CO and decreased sensitivity to H2, compared to that of the undoped TiO2. Addition of elemental iron, in small concentration, to the two‐phase mixture of titania and yttria seemed to further improve the sensitivity and selectivity of the latter to CO.
Mechanical Properties of Continuous‐Fiber‐Reinforced Carbon Matrix Composites and Relationships to Constituent Properties
Authors: Heredia Fernando E. , Spearing S. Mark , Evans Anthony G. , Mosher Paul , Curtin William A.
Published: March 2005

The tensile properties of three carbon matrix composites reinforced with SiC (Nicalon) fibers (materials A, B, C) have been measured with and without notches. One of the three materials (material B) had a relatively low strength and exhibited notch brittleness. This material had both a high interface sliding stress and a low fiber bundle strength, caused by particulates in the matrix. These characteristics have been shown to result in a change in failure mechanism that leads to the inferior properties exhibited by material B. The notch properties of the higher‐toughness materials were shown to involve splitting, which alleviates the notch stress concentration and diminishes the notch sensitivity.
Chemistry of Basal Plane Defects in Zinc Oxide–Antimony Oxide (0.1 mol%) Ceramics
Authors: Bruley John , Bremer Ulrich , Krasevec Viktor
Published: March 2005

Planar faults on the basal planes of zinc oxide are formed during sintering with 0.1 mol% antimony oxide at 1200°C. Spatially resolved energy‐dispersive X‐ray analysis and electron energy loss spectroscopy microanalysis reveal the presence of significant quantities of Sb. Quantification of the data suggests that up to a monolayer of Sb is distributed uniformly on the faulted zone.
Stabilization and Processing of Aqueous BaTiO3 Suspension with Polyacrylic Acid
Authors: Chen Zhien‐Chi , Ring Terry A. , Lemaître Jacques
Published: March 2005

Polyacrylic acid (PAA) has been chosen to stabilize BaTiO3 powder in aqueous solution. Zeta potential studies show that the Ba‐rich particle surface is positively charged in the pH range from 2 to 11 without PAA. Adsorption of PAA decreases the zeta potential in acidic solution and changes the surface to a negative charge in basic solution. Adsorption of PAA onto BaTiO3 surfaces is found to be related to the pH‐dependent PAA conformation in solution. The amount of PAA adsorbed at pH 1.5 is one order of magnitude more than at pH 10.5, due to a more compact polymer conformation. Colloid stability of aqueous BaTiO3 suspension is related to the dissociation of PAA as a function of pH. BaTiO3 suspensions can be stabilized at pH values higher than 7, where more than 99% of carboxylic groups are ionized. At pH 10.5, suspension stability is related to the PAA coverage on BaTiO3 particle surface. Stabilization can be achieved only when conditions of both PAA ionization and BaTiO3 surface coverage are satisfied, suggesting an electrosteric stabilization mechanism. Green density of 62% can be achieved by slip casting at a pH above 10, compared to the best density of 58.6% by isostatic pressing at 242 MPa.
The Effect of Inclusions on Densification; III, The Desintering Phenomenon
Authors: Sudre Olivier , Lange Fred F.
Published: March 2005

In the first two papers of this series, a model, cosistent with observations, was proposed for the densification of a ceramic powder matrix in the presence of dense inclusions. One aspect of the model concerned the breakup of grain bridges linking polycrystalline clusters. This breakup phenomenon results in the formation of large cracklike voids. The mechanism that produces this breakup phenomenon, which we called desintering, is the subject of this third paper, where we (1) present further experimental results concerning the breakup of polycrystalline bridges that undergo grain growth and (2) calculate the free energy of two evolving bridge configurations. The instability of grain bridges is shown to be a function of the grain size to bridge length ratio. That is, when the ratio exceeds a critical value, the bridge will break up, e.g., desinter, to reduce the free energy of the system. This instability condition can be independently achieved either by grain coarsening at constant bridge length or by extending the bridge length under an applied tensile stress at constant grain size.
Consolidation Behavior of Flocculated Alumina Suspensions
Authors: Bergström Lennart , Schilling Christopher H. , Aksay Ilhan A.
Published: March 2005

The consolidation behavior of flocculated alumina suspensions has been analyzed as a function of the interparticle energy. Consolidation was performed by a centrifugal force field or by gravity, and both the time‐dependent and equilibrium density profiles were measured by a gamma‐ray absorption technique. The interparicle energy at contact was controlled by adsorbing fatty acids of varying molecular weight at the alumina/decalin interface. We found that strongly attractive interactions result in a particle network which resists consolidation and shows compressible behavior over a large stress range. The most weakly flocculated suspension showed an essentially incompressible, homogeneous density profile after consolidation at different centrifugal speeds. We also found a significant variation in the maximum volume fraction, φm, obtained, with φm∼ 0.54 for the most strongly flocculated suspension to φm∼ 0.63 for the most weakly flocculated suspension. The compresive yield stresses show a behavior which can be fitted to a modified power law. In this paper, we discuss possible correlations between the fitting parameters and physical properties of the flocculated suspensions.
Dopants in Vapor‐Phase Synthesis of Titania Powders
Authors: Akhtar M. Kamal , Pratsinis Sotiris E. , Mastrangelo Sebastian V. R.
Published: March 2005

Gas‐phase synthesis of titania by TiCl4 oxidation in the presence of dopants (SiCl4, POCl3, and BCl3) was investigated in an aerosol reactor as a function of temperature (1300–1700 K) and dopant concentration (0–25 mol% of TiCl4). The addition of dopants, most notably silicon and phosphorus, drastically altered the morphology of titania particles from polyhedral to spheroidal, increased the extent of aggregation, increased the specific surface area, reduced the primary particle size, and decreased the rutile content. The observed morphology/crystallinity changes were explained in terms of ionic radii and valence of the dopant element.
Fracture Mechanisms of a Coarse‐Grained, Transparent MgAl2O4 at Elevated Temperatures
Authors: White Kenneth W. , Kelkar Girish P.
Published: March 2005

The fracture of a transparent, large‐grain‐sized MgAl2O4 spinel has been studied through temperatures of 1400°C. Fracture toughness values, falling between about 1.3 and 2.3 MPa.m12;1/2, behaved sigmoidally with temperature, with a lower shelf transition appearing near 800°C. Rising R‐curves, displaying a run‐arrest character, were found at both the lowest and the highest temperatures, while only minimal values of dKR/dΔa were produced at intermediate temperatures of 800°C. This fracture character is ascribed largely to finite concentrations of a residual LiF pressing aid identified on the fracture surface, while additional influences associated with the lower shelf region at the highest temperatures may include the increased fraction of intergranular crack path and the onset of plasticity. This cubic monolithic ceramic displayed strong nonlinear fracture behavior in both temperature regimes. In both cases the fracture character is linked directly to an active toughening mechanism in the wake region, which depends upon crack face bridging.
Determination of Chromium Oxidation States in Part of the System CaO─3CaO · Cr2O3 in Air
Authors: Mirtič Breda , Fajgelj Aleš , Lutar Karel , Schara Milan , Kaučič Venčeslav
Published: March 2005

The aim of the present work was to identify the structural characteristics of calcium chromium oxide formed at molar ratios CaO/Cr2O3≥ 3/1 and at elevated temperatures by the use of nondestructive techniques: instrumental neutron activation analysis (for determination of the total chromium content), electron paramagnetic resonance, magnetic susceptibility (for determination of chromium oxidation states), X‐ray diffraction, and scanning electron microscopy. Our results suggest formation of the 3CaO · Cr2O5 compound, while at larger molar ratios only a small portion of Cr is incorporated into the crystalline CaO lattice as paramagnetic Cr3+.
Phase Homogeneity and Segregation in PZT Powders Prepared by Thermal Decomposition of Metal–EDTA Complexes Derived from Nitrate and Chloride Solutions
Authors: Wang Hong‐Wen , Hall David A. , Sale Frank R.
Published: March 2005

Fine, homogeneous lead zirconate titanate (PZT) powders have been prepared by the thermal decompostion of metalorganic complexes derived from nitrate solutions using ethylenediaminetetraacetic acid (EDTA) as a complexing agent. It has been shown that nitrate ions accelerate the decomposition of the precursor and the crystallization of the PZT phase initiates at temperatures as low as 250°C. No intermediate phases, other than PbO, were found. The coexistence region of the rhombohedral and tetragonal phases in the sintered ceramic was found to be a little over 1 at.%. The chloride‐EDTA precursor did not form the desired PZT phase, even after calcination at 1000°C. The segregation and loss of lead was observed for this material, due to the formation and evaporation of PbCl2. DTA/TGA, SEM, and XRD were employed to characterize the powders. The use of TEM with nanoprobe energy dispersive X‐ray analysis (EDX), allowed the identification of the phase segregation in the chloride‐EDTA derived powder. A possible reaction mechanism for this phase segregation is suggested.
Densification and Microstructural Development of Silicon Nitride–Silica During Hot Isostatic Pressing
Authors: Zeng Jianren , Tanaka Isao , Miyamoto Yoshinari , Yamada Osamu , Niihara Koichi
Published: March 2005

The influence of SiO2 addition on the densification and microstructural development of high‐purity Si3N4 during hot isostatic pressing (HIP) was studied. During HIP, densification was promoted, but the phase transformation from α‐Si3N4 to β‐Si3N4 was impeded by SiO2. Analysis using a simple model shows that the enhanced densification was mainly due to the viscous flow of SiO2. The microstructure changed remarkably at between 10 and 20 wt% SiO2 additions. Analysis of the phase transformation kinetics suggests that the diffusion of Si3N4 through SiO2 glass is the ratecontrolling step for the transformation.
Infrared Absorption Coefficients of Silica Glasses
Authors: Sahba Nazanin , Rockett Thomas J.
Published: March 2005

The absorption coefficient values of several silicate glasses in the IR radiation range were determined. Four methods were employed in this study: (1) direct transmission by CO2 laser, (2) direct transmission through ultrathin sections using an IR spectrometer, (3) glass particle dispersions in KBr pressed pellets, and (4) IR reflection spectral analysis. Wide variations in values were observed for some of these techniques. The reasons for the variations were explored, and evidence is presented to support the conclusion that the reflection measurements and the low‐power transmission technique yield accurate values. The pellet technique was found to possess too many experimental variables which could not be controlled and gave erroneous data. The damage produced by direct transmission of high‐power radiation was severe and introduced effects which gave spurious results.
Oxidation and Degradation of Titanium Nitride Ultrafine Powders Exposed to Air
Authors: Sakka Yoshio , Ohno Satoru , Uda Masahiro
Published: March 2005

Titanium nitride ultrafine powders were synthesized by an active plasma‐metal reaction method. Gas desorption measurements were conducted to estimate the surface chemistry of the powders after exposure to air and storage at room temperature. H2O, H2, CO2, CO, and NH3 gases were mainly evolved. These gases were considered to be formed by the surface reaction of adsorbed gases on surface oxide of the powders and decomposition of hydroxide‐like or ammonialike compounds, which might be produced during a slow oxidation treatment and storage.
Sol–Gel Synthesis of Strontium Pyroniobate and Calcium Pyroniobate
Authors: Prasadarao Alamanda V. , Selvaraj Ulagaraj , Komarneni Sridhar , Bhalla Amar S.
Published: March 2005

Strontium and calcium pyroniobates were prepared by a sol–gel process, using strontium/calcium metal and niobium ethoxide as precursors. The formation of Sr2Nb2O7 occurred at 750°C via an intermediate perovskite phase of composition close to Sr0.82NbO3. The crystallization of Ca2Nb2O7 occurred at 600°C directly without any intermediate phases. Sintered Sr2Nb2O7 and Ca2Nb2O7 pellets showed a preferred grain orientation. Microstructural studies revealed an increase in grain growth and associated orientation with sintering temperature.
Synthesis of Nanocrystalline Alumina–Metal Composites by Room‐Temperature Ball‐Milling of Metal Oxides and Aluminum
Authors: Matteazzi Paolo , Caër Gerard Le
Published: March 2005

The synthesis of nanometer‐sized α‐Al2O3−M composites (nanocomposites) is described for M = Fe, V, Cr, Mn, Co, Ni, Cu, Zn, Nb, Mo, W, Si, and Fe alloys. The synthesis is performed by solid‐state reduction reaction which occurs during room‐temperature ball‐milling of mixtures of a metal oxide (MxOy) and aluminum (Al2y/3). A complete reduction is reached for milling times of ≅ 1.5 h in a planetary mill. The crystallite sizes of alumina and metal obatined, for powders processed in a vibratory mill, are in general about 10 nm.