bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantitative Analysis of Brittle Fracture Surfaces Using Fractal Geometry
Authors: Mecholsky J. J. , Passoja D. E. , Feinberg‐Ringel K. S.
Published: March 2005

Fractal geometry is a non‐Euclidean geometry which has been developed to analyze irregular or fractional shapes. In this paper, fracture in ceramic materials is analyzed as a fractal process. This means that fracture is viewed as a self‐similar process. We have examined the fracture surfaces of six different alumina materials and five glass‐ceramics, with different microstructures, to test for fractal behavior. Slit island analysis and Fourier transform methods were used to determine the fractal dimension, D, of successively sectioned fracture surfaces. We found a correlation between increasing the fractional part of the fractal dimension and increasing toughness. In other words, as the toughness increasing the fracture surface increases in roughness. However, more than just a measure of roughness, the applicability of fractal geometry to fracture implies a mechanism for generation of the fracture surface. The results presented here imply that brittle fracture is a fractal process; this means that we should be able to determine processes on the atomic scale by observing the macroscopic scale by finding the generator shape and the scheme for generation inherent in the fractal process.
Experimental and Thermodynamic Study of Nonstoichiometry in 〈YBa2Cu3O7‐x〉
Authors: Lindemer Terrence B. , Hunley John F. , Gates Jacques E. , Sutton Alfred L. , Brynestad Jorulf , Hubbard Camden R. , Gallagher Patrick K.
Published: March 2005

The dependence of the nonstoichiometry of 〈YBa2Cu3O7−x〉 (solid) has been studied over 5 orders of magnitude in oxygen pressure and from 573 to 1173 K. Hydrogen‐reduction methods for determining the absolute oxygen to‐metal ratio were developed. The resulting data were used to derive a chemical thermodynamic representation of the experimental variables. The data were used to derive a chemical thermodynamic representaltion of the experimental variables. The data were also compared with several other investigations to indentify the selfconsistent sets of data. The present data and thermodynamic data from the literature were correlated on an Ellingham diagram.
Corrosion Behavior of Silicon Carbide in 290°C Water
Authors: Hirayama Hideo , Kawakubo Takashi , Goto Akira , Kaneko Tadashi
Published: March 2005

The fundamental corrosion behavior of silicon carbide (SiC) ceramics was investigated after immersion in 290°C water solutions with different pH and dissolved‐oxygen concentrations. The weight loss in the oxygenated solution was more than that in the deoxygenated solution and was accelerated by increasing pH. Preferential attack could be found at grain boundaries and around pores on the sample surface immersed in the oxygenated alkaline solution. The weight change, dW, followed the general rate law, (dW)m= kt. The exponent, m, was 1.11 in the alkaline solution and 0.45 in the acidic solution. Based on the above results, the SiC was considered to be directly hydrolyzed to a silica sol, with its dissolution kinetics dependent on the sol stability. This corrosion behavior is quite different from those in high‐temperature or vapor‐phase hydrothermal oxidation, where the oxidation rate is controlled by oxidant diffusion through the protective silica surface layer.
Fracture of Brittle Materials under a Simulated Wear Stress System
Authors: Rosenfield Alan R.
Published: March 2005

Currently no data are available for fracture toughnesses associated with the compression/shear (mixed mode) stress distribution of the sliding‐wear loading system. To provide such information, diametrically compressed disk specimens of electrical porcelain were tested and used to develop a fracture criterion for a body containing a crack which is subjected to mixed‐mode loading. The role of applied shear loads in creating local tensile stresses near the crack tip is discussed. While these stresses are embodied in the maximum‐hoop‐stress theory of mixed‐mode fracture, it was found that the toughness values had only a fair fit with theory. However, the direction of initial crack growth, which is out‐of‐plane, is consistent with theory. Since effects due to crack‐face rubbing were limited by using a finite‐thickness notch in the test specimen (instead of a crack), the data, although indicative, are not fully representative of debris formation during sliding wear. The paper concludes with the formulation of a simple model of the formation of a debris particle.
Elevated‐Temperature Fracture Resistance of a Sintered α‐Silicon Carbide
Authors: Ghosh Asish , Jenkins Michael G. , White Kenneth W. , Kobayashi Albert S. , Brad Richard C.
Published: March 2005

The fracture toughness of a dense, sintered commercial α‐silicon carbide was determined for temperatures from 20° to 1400°C using both straight‐ and chevron‐notched test specimens and also controlled‐surface‐microflaw specimens, all in three‐point bending. The flexural strengths were also measured for the same range of temperatures and the trend is compared with that of the toughness. Measurements from this study are discussed and also compared with other results in the literature. Analysis reveals the importance of contrasting sharp crack and blunt crack techniques and also the need for addressing the microhardness indentation method separately. It is concluded that the fracture toughness of this silicon carbide is about 3 MPa · m½ and that the crack growth resistance is characterized by a flat R‐curve behavior, both of which are independent of temperature from 20° to 1400°C.
New Inorganic Ultrafiltration Membranes: Titania and Zirconia Membranes
Authors: Larbot Andre , Fabre Jean‐Paul , Guizard Christian , Cot Louis , Gillot Jacques
Published: March 2005

Destabilization of colloidal solutions permits preparation of titania and zirconia ultrafiltration membranes. In this work, layer thickness, pore diameter as a function of sintering temperature, and water permeability are developed. These membranes have a very good chemical resistance and can be used in all pH ranges in ultrafiltration processes. A comparison between alumina, titania, and zirconia membranes is given.
Formation and Transformation of TiO2 (Anatase) Solid Solution in the System TiO2—Al2O3
Authors: Yamaguchi Osamu , Mukaida Yasumichi
Published: March 2005

In the system TiO2—Al2O3, TiO2 (anatase, tetragonal) solid solutions crystallize at low temperatures (with up to ∼ 22 mol% Al2O3) from amorphous materials prepared by the simultaneous hydrolysis of titanium and aluminum alkoxides. The lattice parameter a is relatively constant regardless of composition, whereas parameter c decreases linearly with increasing Al2O3. At higher temperatures, anatase solid solutions transform into TiO2 (rutile) with the formation of α‐Al2O3. Powder characterization is studied. Pure anatase crystallizes at 220° to 360°C, and the anatase‐to‐rutile phase transformation occurs at 770° to 850°C.
Titanate Ceramics for the Stabilization of Partially Reprocessed Nuclear Fuel Elements
Authors: Ball Clifford J. , Buykx William J. , Dickson Fiona J. , Hawkins Kate , Levins Desmond M. , Smart Roger St. C. , Smith Katherine L. , Stevens Geoffry T. , Watson Kenneth G. , Weedon David , White Timothy J.
Published: March 2005

A titanate ceramic designed to immobilize high‐level waste generated by Amine reprocessing of heavy‐water reactor fuel and fabricated by uniaxial hot‐pressing was characterized. X‐ray diffraction and selected‐area electron diffraction were used to identify a six‐phase assemblage consisting of betafite, a hollandite structure type, perovskite, uraninite, hibonite, and Magnéli phases. Secondary electron imaging of polished surfaces revealed many microvoids consistent with a measured density of 5.25 g · cm−2 (∼90% of theoretical density). The waste form was chemically heterogeneous at the hundreds of micrometers scale, as backscattered electron imaging and energy‐dispersive X‐ray spectroscopy indicated that regions rich in either uranium or titanium oxides were common. Grain sizes ranged from 0.1 to 0.5 μm. All crystals were faceted with the exception of anhedral uraninite grains. Dissolution experiments conducted at 90°C in distilled water gave uranium and titanium loss rates which were solubility limited (10−1 to 10−4 g · m−2· d−1), while cesium dissolution, which was not contrained by solubility limits, was more rapid (23.3 g · m−2· d−1). Hydrothermal testing at 150°C in distilled water resulted in precipitation of uranium(VI) titanate (UTiO5) and brookite (TiO2).
Chemical Stabilization of β‐Cristobalite
Authors: Perrotta Anthony J. , Grubbs Donald K. , Martin Edward S. , Dando Neal R. , McKinstry Herbert A. , Huarg Chi‐Yuen
Published: March 2005

Chemically stabilized β‐cristobalite has been synthesized in the Na2O‐Al2O3‐SiO2 and CaO‐Al2O3‐SiO2 systems. Highly reactive chemical precursors in the gel and zeolitic state are thermally transformed to stabilized β‐cristobalite crystalline solutions in each system. High‐temperature X‐ray diffraction, phase data, differential scanning calorimetry, thermochemical analysis, electron microscopy, and magic‐angle spinning NMR give information on the limits of stability of β‐cristobalite, the α—β phase transition, thermal expansion behavior, and the structural state of the crystalline solutions.
Elastic Properties of Alkali Phosphomolybdate Glasses
Authors: Damodaran K. V. , Rao K. J.
Published: March 2005

Ultrasonic velocities at 10 MHz have been measured in two series of lithium, sodium, and potassium phosphomolybdate glasses with two fixed P2O5 concentrations. Elastic moduli, Poisson's ratio, and Debye temperature have been calculated. The composition dependence of most of the properties of lithium glasses exhibits a trend opposite to that of potassium glasses. Properties of sodium glasses lie between the other two alkali systems. Alkali oxide modification is suggested to be accompanied by ring reformation in lithium and sodium glasses. Ring size effects have been shown to account for all of the composition dependence.
Effect of Powder Surface Chemistry on the Stability of Concentrated Aqueous Dispersions of Alumina
Authors: Kelso John F. , Ferrazzoli Thomas A.
Published: March 2005

The surface chemistry differences between two alumina powders is shown to have a significant effect on the properties of concentrated aqueous dispersions of the powders. The initial dispersion pH and magnitude and sign of the initial mobility of the particles (determined by acoustophoresis) are different for the two powders because of differences in surface impurity contents. As a result, the two powders require different amounts of dispersant to achieve stability in dispersions near their isoelectric points. The surface chemistry differences between these two aluminas illustrate the importance of examining ceramic dispersion properties at the concentrations used in processing the material.
Effect of Excess PbO on the Densification of PLZT Ceramics
Authors: Song Byung‐Moo , Kim Doh‐Yeon , Shirasaki Shin‐ichi , Yamamura Hiroshi
Published: March 2005

The densification behavior and electrical properties of PLZT ceramics as a function of excess PbO in the starting powder have been investigated. The hot‐pressed PLZT ceramics, prepared from powders containing 0% to 4% excess PbO, were transparent, and their electrical properties were similar in general. PLZT with PbO deficiency up to 1.4 mol% from stoichiometry can be regarded as a single phase‐PLZT.
Supercritical Fluid Technologies for Ceramic‐Processing Applications
Authors: Matson Dean W. , Smith Richard D.
Published: March 2005

The applications of supercritical fluid technologies for ceamic processing are reviewed. The novel physical and chemical properties of these densified gases are summarized and related to their use as solvents and processes benefit from the unique properties of supercritical fluids. The rapid expansion of supercritical fluid solutions provides a technique for producing fine uniform powders and thin films of widely varying materials. Supercritical drying technologies allow the formation of highly porous aerogel products with potentially wide application. Hydrothermal processes leading to the formation of large single crystals and microcrystalline powders can also be extended into the supercritical regime of water. Additional applications and potential applications are identified in the areas of extraction of binders and other additives from ceramic compacts, densification of porous ceramics, the formation of powders in supercritical microemulsions, and in preceramic polymer processing.
Transformation Zone Shape, Size, and Crack‐Growth‐Resistance [R‐Curve] Behavior of Ceria‐Partially‐Stabilized Zirconia Polycrystals
Authors: Yu Cheng‐Sheng , Shetty Dinesh K.
Published: March 2005

Transformation zone shape, size, and crack‐growth‐resistance (R‐curve) behavior were studied in precracked and annealed single‐edge notch bend specimens of commercial‐grade ceriapartially‐stabilized zirconia polycrystals as a function of applied load. Well‐defined transformation zones with a characteristic elongated shape in the plane of the crack were observed. It is shown that the observed zone shape is significantly different from the shape predicted by a combined shear/dilatation yield criterion and the stress field of the crack prior to the transformation. The length of the transformation zone directly ahead of the crack tip is in better agreement with the prediction of the Dugdale plastic strip zone model. The fracture toughness increment showed the characteristic square root dependence on the transformation zone width, but the magnitude of the toughness increment was not consistent with the predictions of the theoretical models of transformation toughening.
Microstructural Coarsening During Sintering of Boron Carbide
Authors: Dole Stephen L. , Prochazka Svante , Doremus Robert H.
Published: March 2005

The sintering behavior of boron carbide was investigated with particular attention given to microstructure development at various stages in the sintering process. Hot‐pressing and pressureless sintering techniques were employed and the effects of heating rate, firing atmosphere, and composition were used to characterize the sintering behavior. Pressureless sintering at temperatures up to 2300°C produces only limited densification. Microstructural coarsening is responsible for this since it leads to conditions where densification is slow. Hot‐pressing and carbon additions suppressed coarsening and permitted densification to >95% of theoretical density.
Effect of Ionic Strength on Boehmite Hydrogel Formation
Authors: Gieselmann Mary J. , Anderson Marc A.
Published: March 2005

Neutral salts were added to boehmite (γ‐AlOOH) sols, and the solution and solution and interfacial chemistries were investigated during hydrogel formation. In general, gel volume increases with higher salt concentrations. Gels form in the presence of salts with monovalent anions when a critical ionic strength is reached, the value of which increases with increasing size of the anion. At the same time, the effect of interparticle distance appears more important with larger anions. The action of divalent anions is more complicated and may involve specific adsorption and complexing with the dissolved aluminum species. The concept of a critical gelation concentration is introduced analogous to the critical coagulation concentration for the flocculation of colloidal particles. The results are interpreted in terms of available models for particle‐particle interactions.
Gas Diffusion During Containerless Hot Isostatic Pressing of Liquid‐Phase Sintered Ceramics
Authors: Kwon Oh‐Hun , Messing Gary L.
Published: March 2005

Diffusion and dissolution of gases are considerably higher in glasses than in most crystalline materials. Thus, materials with a glassy grain‐boundary phase are susceptible to gas permeation when they are containerless hot isostatic pressed. Annealing of sinter‐hot isostatic pressed alumina—magnesium aluminosilicate glass (3 to 10 vol%) composites and pure glass samples at 1200° to 1600°C results in dedensification by matrix bloating and swelling. The degree of dedensification increases with the hot isostatic pressing pressure, temperature, and time and increasing annealing temperature. A theoretical prediction of high‐pressure gas permeation is developed based on a diffusion model. The analysis allows a satisfactory explanation for the gas diffusion effect on hot isostatic press densification. The analysis is also useful for developing design criteria for the hot isostatic press schedule and encapsulation materials. Annealing of hot isostatic pressed samples at 1100°C prior to high‐temperature annealing results in no dedensification as a result of out‐diffusion of the internal gases.
Electron Microscopy Study of Ordinary Portland Cement and Ordinary Portland Cement–Pulverized Fuel Ash Blended Pastes
Authors: Rodger Sally A. , Groves Geoffrey W.
Published: March 2005

Many ordinary portland cement (OPC) OPC–pulverized fuel ash (pfa) blended pastes of various ages have been examined using transmission electron microscopy, energy dispersive X‐ray analysis, and scanning electron microscopy. In addition to the phases commonly identified in OPC pastes, such as CH, CSH, AFt, and AFm, we found areas of pooriy crystalline iron‐containing material intermixed with CSH and needles of hydrotalcite‐like composition. The iron‐containing material may be the precursor of hydrogarnet phases identified in older pastes by other authors. In the pfa‐blended cements, some of the pfa particles reacted to give a radially fibrillar gel, and others showed areas of well‐crystallized hydrogarnet within the clearly defined outer boundary of the particle. Unreactive pfa particles had a rim of fibrillar CSH gel around the particle. The general fine‐scale microstructure of the OPC‐pfa‐blended pastes was similar to that in OPC pastes alone, but the CSH gel material had a lower C:S ratio.
Observation of Poly{Vinyl Butyral}‐Dibutyl Phthalate Binder Capillary Migration
Authors: Cima Michael J. , Dudziak Mark , Lewis Jennifer A.
Published: March 2005

The movement of poly(vinyl butyral) (PVB) and dibutyl phthalate (DBP) plastic solution under the influence of capillary forces was observed directly by hot‐stage optical microscopy (T=160°C). Migration of a PVB‐DBP droplet within a tapered capillary was examined, allowing the surface tension of the binder system at elevated temperatures to be measured, while maintaining constant composition. Calculated surface tension values for PVB‐DBP plastic at T=160°C ranged from 8 × 10−3 to 20× 10−3 N/m (8 to 20 dyn/cm); the large error was attributable primarily to the difficulty in measuring the radii of curvature of the leading and trailing edges of the droplet.
Chemical Vapor Deposition for Silicon Cladding on Advanced Ceramics
Authors: Goela Jitendra S. , Taylor Raymond L.
Published: March 2005

Polycrystalline Si was used to clad several advanced ceramic materials such as SiC, Si3N4, sapphire, Al2O3, pyrolytic BN, and Si by a chemical vapor deposition (CVD) process. The thickness of Si cladding ranged from 0.025 to 3.0 mm. CVD Si adhered quite well to all the above materials except Al2O3, where the Si cladding was highly stressed and cracked or delaminated. A detailed material characterization of Si‐clad SiC samples showed that Si adherence to SiC does not depend much on the substrate surface preparation; that the thermal cycling and polishing of the samples do not cause delamination; and that, in four‐point bend tests, the Si–SiC bond remains intact, with the failure occurring in the Si.
Metastability of the Martensitic Transformation in a 12 mol% Ceria‐Zirconia Alloy: II, Grinding Studies
Authors: Swain Michael V. , Hannink Richard H. J.
Published: March 2005

Observations of the grinding‐induced transformation in singlephase Ce‐TZP materials, referred to in an earlier paper, are presented. Two techniques were used to grind the surface: by hand in a slurry of abrasive particles and with a high‐speed diamond‐impregnated wheel. Significant differences in X‐ray diffraction profiles between the two grinding methods was observed. Limited monoclinic ZrO2 was detected on the machineground surface, along with the reversal of the tetragonal ZrO2 (200) peak intensities. On the hand‐ground surface, considerable monoclinic phase was observed. The disappearance of the monoclinic phase with heating was followed by X‐ray diffraction, and the Af was found to exceed 700°C, while the reversal in tetragonal (200) peak symmetry and intensity remained unaltered up to at least 1000°C. Transmission electron microscope studies at various depths below the ground surface were undertaken to identify the differences between these surfaces and fractured surfaces. A simple explanation is proposed for the reversal of the tetragonal peak intensities. This reversal has previously led to the notion of a ferroelastic toughening mechanism in similar TZP materials.
Brittle‐to‐Ductile Transition in Silicon Carbide
Authors: Campbell Geoffrey H. , Dalgleish Brian J. , Evans Anthony G.
Published: March 2005

Observations of the microstructure and creep behavior of two commerical silicon carbides are presented. A combination of techniques has been used to characterize the microstructures. Sequential creep rupture testing has been carried out and scanning electron microscopy used to observe creep‐crack propagation and damage development. Basic theory for stress fields and creep rates around a crack tip is relatd to the observed brittle‐to‐ductile transition in these materials. Analogy with the brittle‐to‐ductile transition in steels is made and used to interpret the present observation.
Transparent Hydroxyapatite Prepared by Hot Isostatic Pressing of Filter Cake
Authors: Uematsu Keizo , Takagi Masaki , Honda Tsunehiro , Uchida Nozomu , Saito Katsuichi
Published: March 2005

Hydroxyapatite powder was synthesized and formed into a compact in an aqueous medium using a filter‐cake method. The compact was hot isostatically pressed at 700° to 1000°C and 100 MPa for 2 h. Fully dense, transparent materials were obtained above 800°C. Both forming and densification methods were found to be important in obtaining transparent materials.
Thermal Conversion of Preceramic Polyiminoalane Precursors to Aluminum Nitride: Characterization of Pyrolysis Products
Authors: Seibold Michael M. , Rüssel Christian
Published: March 2005

This study introduces a new method for the synthesis of aluminum nitride by pyrolysis of a polymeric precursor. Metallic aluminum is electrolytically dissolved in an electrolyte containing a primary amine. Subsequent drying and heating enables the formation of a polymeric, gellike intermediate polyiminoalane compound. Calcining in ammonia atmosphere at 850°C results in aluminum nitride powder with a mean crystallite size of about 30 nm.
Materials Development of Optical Fiber
Authors: MacChesney John B. , DiGiovanni David J.
Published: March 2005

Traditionally, new materials are developed in response to the requirements of an emerging device technology. In the optical fiber case, development of glass as a transmission medium came as the culmination of a century‐long quest for improved broadband telecommunications, a search that established the superiority of light signals transmitted through glass over electronic signals transmitted by wire.
Fabrication, Microstructural Characterization, and Mechanical Properties of Polycrystalline t'‐Zirconia
Authors: Jue Jan Fong , Virkar Anil V.
Published: March 2005

Large‐grained (100‐ to 200‐μm), yttria‐doped, polycrystalline t'‐zirconia ceramics were fabricated by heat‐treating presintered samples at temperatures 2100°C. Polarized light microscopy revealed the ferroelastic domain structure in the t' samples. XRD showed that no monoclinic phase was detected on as‐polished, ground and fracture surfaces, or on surfaces while under a tensile stress as high as 400 MPa. By contrast, relative changes occurred in the tetragonal peak intensities, which were attributed to ferroelasatic domain switching. The higher toughness of 3‐mol%‐Y2O3‐doped t' samples (7.7 MPa · m1/2) compared to that of 8 mol% Y2O3 cubic samples (2.4 MPa · m1/2) was explained in part by ferroelastic domain switching.
Discontinuous Coarsening of Tetragonal Precipitates in Partially Stabilized Zirconia Induced by Diffusional Coherency Strain under Applied Stress
Authors: Kim Jeong‐Joo , Park Chan , Kim Doh‐Yeon , Yoon Duk N. , Shapiro Alexander
Published: March 2005

When partially stabilized zirconia with 6 mol% MgO and 4 mol% CaO is aged at 1450°C, intragranular precipitation occurs and concurrently the boundaries between the grains migrate, forming a Ca‐enriched precipitate‐free cubic phase and large tetragonal precipitates behind them. At these compositions and temperature the boundary migration is rapid and shows the characteristics of a discontinuous coarsening. A uniaxial compressive stress applied to this specimen during the aging treatment increases or decreases the migration rate of the boundaries parallel or perpendicular to the stress axis, respectively, in agreement with the prediction that a compressive coherency strain due to the diffusion of Ca atoms is produced at the surface of the retreating grains and drives the migration. The diffusional coherency strain energy is thus expected to be the dominant driving force for the discontinuous coarsening in this solid.
Assessment of the CaO‐Al2O3 System
Authors: Hallstedl Bengt
Published: March 2005

A thermodynamic assessment of the quasi‐binary system CaO‐Al2O3 has been made using a computerized CALPHAD (calculation of phase diagrams) technique. The actual optimization was carried out with a computer program for the optimization of parameters in thermodynamic models called PARROT. The liquid phase is described by a simple two‐sublattice subregular solution model with Ca2+ and Al3+ as cationic species and O2‐ as anionic species. All solid phases are treated as stoichiometric compounds. A consistent set of parameters describing the system is presented, and numerous comparisons with experimental data are made. There is a lack of accurate data in the high‐alumina part of the system, and the importance of a better knowledge of the CaO·6Al2O3 phase is stressed.
Farbication of Optically Transparent Lead Magnesium Niobate Polycrystalline Ceramics Using Hot Isostatic Pressing
Authors: Kim Namchu , McHenry Dean A. , Jang Sei‐Joo , Shrout Thomas R.
Published: March 2005

The physical, dielectric, and optical properties of hot isostatically pressed lead magnesium niobate polycrystalline ceramics modified with 1/2 mol% La2O3, Pb1–3/2xLax□x/2‐(Mg1/3Nb2/3)O3, have been investigated. Methods used to characterize the ceramics included determination of the dielectric permittivity, optical transmittance, and refractive index dispersion. The materials exhibited relaxor ferroelectric type behavior with a peak dielectric constant K > 14000 and average Tc∼−35°C. Various sintering, hot isostatic pressing, and annealing conditions were examined to produce highly dense and optically transparent materials. Through the use of hot isostatic pressing, densities more than 99.5% theoretical and transmittance greater than 50% at 633‐nm wavelength were obtained. Hot isostatic pressing technique appears to be a good alternative to hot uniaxial pressing without the associated problem of PbO volatility, reactivity with the pressure vessel, and geometrical constraints.
Effect of Porosity on the Combustion Synthesis of Titanium Nitride
Authors: Eslamloo‐Grami Maryam , Munir Zuhair A.
Published: March 2005

The effect of porosity on the self‐propagating reaction between porous titanium and gaseous nitrogen was investigated. The relationship between total nitrogen uptake and porosity showed a maximum at about 44% porosity. High initial sample porosities lead to partial melting of the titanium and subsequently to a lower degree of conversion to the nitride. Low initial porosities limit the conversion through lower reaction interfacial areas and lower gas permeation. Wave velocity measurements, thermogravimetric determinations on the progression of the combustion front, and microstructural analyses demonstrated that the passage of the front is associated primarily with the formation of a surface layer of TiN1‐x with x being 0.10 and 0.06 for lowdensity (49%) and high‐density (59%) samples. The product of combustion contained titanium nitride and primary and β‐transformed α solid solutions. The relative abundance of the latter two phases was dependent on the initial relative density of the samples.
Toughening Behavior of Silicon Carbide with Additions of Yttria and Alumina
Authors: Kim Do‐Hyeong , Kim Chong Hee
Published: March 2005

The fracture‐toughness‐determining mechanism of silicon carbide with additions of yttria and alumina was studied. Observations of indentation crack profiles revealed that significant crack deflection had occurred. Median deflection angles increased with increased volume fractions of the second phases, which was accompanied by increased fracture toughness.
Chemical Preparation and Shock Wave Compression of Carbon Nitride Precursors
Authors: Wixom Michael R.
Published: March 2005

Two synthetic routes have been developed to produce high‐molecular‐weight organic precursors containing a high weight fraction of nitrogen. One of the precursors is a pyrolysis residue of melamine‐formaldehyde resin. The second precursor is the byproduct of an unusual low‐temperature combustion reaction of tetrazole and its sodium salt. These precursors have been shock compressed under typical conditions for diamond and wurtzite boron nitride synthesis in an attempt to recover a new ultrahard carbon nitride. The recovered material has been analyzed by X‐ray diffraction, FTIR, and Raman microprobe analysis. Diamond is present in the recovered material. This diamond is extraordinarily well ordered relative to diamond shock synthesized from carbonaceous starting materials.
Tensile Tests of Ceramic‐Matrix Composites: Theory and Experiment
Authors: Cao Hengchu , Thouless Michael D.
Published: March 2005

A model describing the salient features of tensile stress‐strain curves of ceramic–fiber composites has been developed. The model incorporates statistics of fiber failure. Furthermore, the compliance of the testing machine is included so that the onset of instability can be predicted. An experiment conducted on a composite consisting of a glass‐ceramic matrix reinforced with SiC fibers exhibits excellent agreement with the predicted behavior.
Subcritical Crack Growth in Sintered Silicon Nitride Exhibiting a Rising R‐Curve
Authors: Okada Akira , Hirosaki Naoto , Yoshimura Masahiro
Published: March 2005

Subscritical crack growth of sintered silicon nitride was analyzed in terms of the R‐curve. Provided that the stress intensity at the crack tip governs the subcritical crack‐growth velocity, the KI–V relationship of sintered silicon nitride exhibiting a rising R‐curve is shown to shift to the high‐KI region as the crack advances.
Effect of Sintering Atmosphere on the Densification and Electrical Properties of Alumina
Authors: Miranzo Pilar , Tabernero Leonor , Moya José S. , Jurado José R.
Published: March 2005

The effect of sintering atmosphere on the final density and electrical properties of alumina compacts has been investigated using two different oxygen pressures: air and CO/CO2. Measuring of electrical behavior has been considered a tool for determining the mechanism responsible for densification. Finally, the importance of a reducing atmoshphere on the electrical behavior of polycrystalline alumina is pointed out.
Borasilazane Polymeric Precursors for Borosilicon Nitride
Authors: Seyferth Dietmar , Plenio Herbert
Published: March 2005

The reaction of H3B·S(CH3)2with the (CH3SiHNH)ncyclic oligomers obtained by ammonolysis of methyldichlorosilane (CH3SiHCI2) results in evolution of hydrogen and formation of cross‐linked products that contain borazine rings as well as boron atoms that are linked to three nitrogen atoms. Pyrolysis of the products in a stream of argon gives a high yield of a black “borosilicon carbonitride,” whereas pyrolysis in a stream of ammonia gives white “borosilicon nitride” in high yield.
Electron Paramagnetic Resonance, Optical, and Electronic Structure Studies of Photorefractive Barium Titanate
Authors: Schwartz Robert N. , Wechsler Barry A. , Rytz Daniel
Published: March 2005

Barium titanate (BaTiO3) is an important electrooptic ferroelectric oxide material which exhibits the unusual optical nonlinearity of photorefractivity. There is evidence that the photorefractive effect, which refers to the change in the index of refraction of the crystal by the action of light, is related to the presence of defects/impurities such as transition metal ions and/or cation or anion vacancies. This paper will review our electron paramagnetic resonance, optical, and theoretical eletronic structure studies aimed at probing the nature of the defects/impurities in BaTiO3 and their role in the photorefractive effect.
Grain‐Boundary Chemistry of Barium Titanate and Strontium Titanate: II, Origin of Electrical Barriers in Positive‐Temperature‐Coefficient Thermistors
Authors: Chiang Yet‐Ming , Takagi Touichi
Published: March 2005

Scanning transmission electron microscopy (STEM) of positive‐temperature‐coefficient (PTC) BaTiO3 thermistors shows that the grain‐boundary oxygen content in as‐received (oxidatively cooled) materials is slightly enriched compared to quenched samples, and the acceptor‐rich space‐charge present at high temperatures is retained upon cooling. The defect density of the space charge is approximately equal to the acceptor state density at PTC boundaries determined by electrical measurements. Accordingly, it is proposed that the electrical barrier forms when acceptor defects already segregated in the ionic space charge at high temperature become active interface states when compensating donor defects in the grain‐boundary core are oxidized. These acceptor defects appear to be primarily barium vacancies, but need not form upon cooling in the manner proposed by Daniels and Wernicke. Acceptor solutes when present can also contribute to barrier formation through space‐charge segregation; the increase in interface state density upon addition of Mn is consistent with the magnitude of the expected segregation.
Liquid‐Phase Transport During Removal of Organic Binders in Injection‐Molded Ceramics
Authors: Barone M. R. , Ulicny J. C.
Published: March 2005

A method has been developed for calculating hydraulic pressures induced by thermal expansion of liquid binders early in the removal cycle, when evaporative losses are negligible and fully saturated conditions prevail. Specific results were obtained for flat compacts containing a common wax binder, mixed with varying amounts of low‐density polyethylne. In general, these results show how the risk of hydraulic fracture increases with heating rate and compact thickness. Although pressures are minimal when the binder consists entirely of wax, the continual addition of polyethylene eventually gives rise to unacceptable risk levels, even for relatively thin compacts. Binder removal at elevated temperatures is considered subsequently. In this case, vapor pressures eventually approach a critical level, thereby allowing mass removal by evaporation to overcome the effect of thermal expansion in maintaining full saturation. With the onset of void formation, the developing capillary pressure supersedes hydraulic pressure as the driving force in liquid transport. Besides representing capillary flow, the present formulation also accounts for thermal degradation of the binder during removal. The resulting system of equations was solved numerically for a variety of representative debinding conditions. Predictions for flat compact containing a balanced wax/polyethylene binder indicate that thermal degradation of the polyethylene can give rise to a marked improvement in debinding rates. It turns out, however, that this enhancement is far more effective in thinner compacts.
Spectroscopic Study of the Chemical State and Coloration of Chromium in Rutile
Authors: Ishida Shingo , Hayashi Masahiko , Fujimura Yoshikazu , Fujiyoshi Kaichi
Published: March 2005

The chemical states of Cr ions in rutile were studied using mainly electron spectroscopy for chemical analysis (ESCA), electron spin resonance (ESR), and diffuse reflectance spectroscopy. The study assumed that Cr3+ and Cr4+ which formed solid solutions with TiO2 possessed yellow and orange colors, respectively. A dark tinge accompanying the maple color of Cr‐doped TiO2 probably was produced by undissolved CrO3−x (x∼ 0.5) occurring on the TiO2 grains. Incorporation of Sb into the Cr‐doped TiO2 enhanced the solubility of Cr presumably because of the formation of Sb5+Cr3+ pairs and eventually decreased the dark tinge.
Deformation and Grain Growth of Low‐Temperature‐Sintered High‐Purity Alumina
Authors: Xue Liang A. , Chen I‐Wei
Published: March 2005

Through close control over green‐state powder processing, pure alumina ceramics of 0.5‐μm grain size were obtained by sintering at 1250°C. The static grain growth of this material was modest at temperatures below 1300°C. However, dynamic grain growth occurred rapidly during superplastic deformation. Therefore, although the ultrafine‐grained alumina exhibited rather low initial flow stress at relatively low deformation temperatures, dynamic grain‐growth‐induced strain hardening gave rise to high flow stress causing cavitation and cracking. As a result, superplastic deformation could not be achieved for the ultrafine‐grained pure alumina.
Critical Particle Size and Phase Transformation in Zirconia: Transmission Electron Microscopy and X‐ray Diffraction Studies
Authors: Srinivasan Ram , Rice Larry , Davis Burtron H.
Published: March 2005

A study was undertaken to examine the crystallite size effect on the low‐temperature transformation of tettragonal zirconia. Zirconia weas prepared by precipitation from a solution of zirconium tetrachloride by adding ammonium hydroxide to produce a pH of 2.95. Portions of the sample, after drying, were calcined at 500°C for various time intervals. Phase transformation was followed by X‐ray diffraction; the data show that the tetragonal phase was initially formed and it was transformed to the monoclinic phase at longer periods of calcination. It was observed by TEM particle size and XRD crystallite size that the transformation does not appear to be due to a critical particle size effect.
Preparation of Orthorhombic Ba2YCu3O7 Powder by Single‐Step Calcining
Authors: Spann Joseph R. , Lloyd Isabel K. , Kahn Manfred , Chase Mark T.
Published: March 2005

A single calcination step, solid‐state process that provides orthohombic Ba2YCu3O7 powder is described. BaCO3, Y2O3, and CuO are used as precursor materials. The only phase identifiable by X‐ray diffraction is the orthorhombic Ba2YCu3O7. The use of a vacuum during the inital stages of the calcining process promotes complete decomposition of the carbonate, and no residual carbonate is observed. An oxygen atmosphere during the later stages of calcining ensures proper oxidation to Ba2YCu3O7. The use of a similar combination vacuum‐oxygen calcining schedule should also be beneficial in the preparation of chemically derived powders.
Determining the Nucleation Rate Curve for Lithium Disilicate Glass by Differential Thermal Analysis
Authors: Ray Chandra S. , Day Delbert E.
Published: March 2005

The crystallization of lithium disilicate (Li2O·2SiO2) glass nucleated at various temperatures was studied by differential thermal analysis (DTA). A plot of the DTA crystallization peak height versus nucleation temperature closely resembles the classical nucleation rate curve for lithium disilicate glass whose maximum is at 453°C. The glass becomes saturated with internal nuclei when heated at 453°C for 10 h. The DTA technique is a rapid, alternative method for determining the temperature for maximum nucleation. The activation energy for crystallization, E, and the heat of crystallization, H, are independent of the concentration of nuclei and are 249±10 and 67±3 kJ/mol, respectively. The Avrami exponent, n, depends strongly on the concentration of nuclei in the glass.
Crack Growth and Acoustic Emission in Ceramics During Thermal Shock
Authors: Konsztowicz Krzysztof J.
Published: March 2005

It is recognized that microcracks may contribute to failure of ceramics undergoing thermal shock, although there is little direct experimental evidence. The combination of acoustic emission (AE) and SEM observations provides such evidence. In this work AE data were analyzed after rapid quenching of samples in silicone oil. A thermal shock resistant material, alumina, and a material resistant to thermal damage, advanced zirconia refractory, have been examined. Cracks were detected by analysis of AE amplitudes and durations and their growth was monitored by systematic SEM observations as thermal shocks of increasing severity were applied. Three‐point bending strengths were determined in air after quenching. For the first time SEM images are presented showing early stages of crack initiation for temperature differences less than ΔTcrit, i.e., where the fracture was not believed to occur. Further development of the cracks leads to abrupt strength reduction in alumina and controlled strength loss in zirconia, although AE data did not indicate any particular pattern of catastrophic crack propagation when substantial loss of strength occurred.
29Si Magic‐Angle‐Spinning Nuclear Magnetic Resonance Study of Hydrated Cement Paste and Mortar
Authors: Al‐Dulaijan Salan U. , Parry‐Jones Gwilym , Al‐Tayyib Abdul‐Hamid J. , Al‐Mana Abdulaziz I.
Published: March 2005

This paper presents 29Si magic‐angle‐spinning nuclear magnetic resonance measurements that trace the cement hydration process in cement paste and mortar specimens made from ordinary portland cement, type I. These specimens were moist‐cured for 3, 7, 14, and 28/31 d at temperatures ranging from 21° to 80°C. Compressive strength for all tested specimens was also determined. The results show that the degree of hydration (Q1+Q2) and the compressive strength increase with curing times and temperatures. However, at 80°C, the compressive strength decreases while the degree of hydration increases.
Steric Stabilization of Nonaqueous Silicon Slips: I, Control of Particle Agglomeration and Packing
Authors: Kerkar Awdhoot V. , Henderson Robert J. M. , Feke Donald L.
Published: March 2005

The behavior of silicon powder dispersed in two nonaqueous processing media (benzene and trichloroethylene) with various adsorbed polymeric stabilizers (poly(styrene), poly‐(methyl methacrylate), and their copolymer) has been studied. Measured adsorption isotherms indicate that poly‐(methyl methacrylate) is adsorbed onto silicon in significantly greater amounts than poly(styrene) from both benzene and trichloroethylene. The adsorption phenomena are explained on the basis of the polymer‐medium interactions, the chemical structure of the polymers, and the surface chemistry of the silicon powder. In the case of the copolymer, the poly(methyl methacrylate) segments act as anchors to the silicon surface. The poly(styrene) segments project out into the medium and impart the necessary stabilizing action against particle agglomeration. Adsorption of poly(methyl methacrylate) and copolymer effectively controls particle agglomeration and enables the preparation of nonaqueous slips of silicon having stability similar to that which can be achieved in water. Control of particle agglomeration with this steric stabilization strategy results in compacts with improved particle packing as observed through sediment volume analysis.
Micromechanics of Creep‐Crack Growth in a Glass‐Ceramic
Authors: Page Richard A. , Chan Kwai S. , Davidson David L. , Lankford James
Published: March 2005

The near‐tip strain and opening behavior of creep cracks in a glass‐ceramic were studied at temperatures for which grain‐boundary sliding is the dominant deformation mechanism. Using the stereoimaging displacement measurement technique, near‐tip creep strain and crack‐opening displacement (COD) increments were obtained as functions of distance from the crack tip and time of creep. Both the time and radial dependence of the strain and COD were observed to be consistent with the Riedel and Rice (RR) field when the time of creep and crack extension were small. Increasing creep time and crack extension led to the formation of a localized shear zone located directly ahead of the crack tip. Further crack extension tended to follow this shear zone within which creep damage accumulated. Neither the rate of COD nor the strain‐rate distribution within this shear zone could be described by the RR field or the Hui and Riedel (HR) field for growing cracks. Steady‐state, stable crack growth exhibiting both Mode I and II components was observed in the glass‐ceramic despite a creep exponent of ≅2. Varying the test temperature and applied stress intensity influenced both the development of the crack‐tip shear zone and the crack growth process. These observations are discussed on the basis of the interrelationships between the near‐tip field, creep damage accumulation, and grain‐boundary sliding.
Hydration Study of the System Ca3Al2O6–CaSO4· 2H2O–Ca(OH)2–H2O with and without Citric Acid
Authors: Singh Nakshatra B. , Singh Awadhesh K. , Singh Shashi Prabha
Published: March 2005

Hydration occurring in the system Ca3Al2O6–CaSO4· 2H2O–Ca(OH)2–H2O has been studied at different temperatures and it was found that the reactions are diffusion controlled. The kinetic data obeyed Jander's equation and the rate of reaction increased with increasing temperature. X‐ray diffraction studies and calorimetric measurements show that when gypsum is consumed, ettringite is converted into monosulfate. The rate of this conversion also increased with the increasing temperature and decreased in the presence of citric acid. Spectroscopic studies showed that there was some interaction between citric acid and the cement and that the product of hydration is of colloidal nature. Zeta potential measurements show that retardation of Ca3Al2O6 hydration in the presence of gypsum and Ca(OH)2 is not due to SO2−4 adsorption. Electrical conductivity and thermoelectric potential measurements of solid Ca3Al2O6 show that Ca3Al2O6 is an n‐type semiconductor and contains defects. The retardation of Ca3Al2O6 may be due to poisoning of reaction sites by gypsum and Ca(OH)2.
Study of Fracture Behavior of Very Fine‐Grained Silicon Carbide Ceramics
Authors: Kodama Hironori , Miyoshi Tadahiko
Published: March 2005

Dense, fine‐grained silicon carbide (SiC) ceramics were fabricated by a hot‐pressing technique using pyrolyzed polycarbosilane powders. Hot‐isostatic pressing treatments were also applied to some of these hot‐pressed samples. The grainsize range of the obtained sintered bodies was from 0.2 to 1.4 μm, which was much finer than that of ordinary sintered SiC ceramics. Relationships among sintering conditions, microstructures, and fracture toughness of the obtained ceramics were investigated. A clear grain‐size dependence of fracture toughness was observed in this very fine‐grain region (0.2 to 1.4 μm). Fracture toughness showed its maximum (5.1 MPa.m1/2) at the average grain size of ∼0.7 μm. Also, the fracture toughness of the samples having similar grain sizes increased with increasing relative density.
Influence of Magnesium‐Aluminum Spinel on the Directed Oxidation of Molten Aluminum Alloys
Authors: Sindel Manfred , Travitzky Nahum A. , Claussen Nils
Published: March 2005

The directed oxidation of some molten aluminum alloys was studied. Alloys containing Si, Zn, and Mg are readily converted into Al2O3/Al composites with the characteristic directed metal oxidation process (DIMOX) metal‐channel structure. No reaction took place when alloys without Mg were used. However, when Mg‐free melts containing Si and Zn were in contact with bulk spinel (MgAl2O4), the typical reaction product grew from the metal pool, although without formation of the spinel starter layer which was thought to be a prerequisite for DIMOX growth. Furthermore, the growth front was more planar than when Mg‐containing alloys were used.