bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of Mullite from Sol‐Gel Mixtures and Kaolinite
Authors: Pask Joseph A. , Tomsia Antoni P.
Published: March 2005

Critical analyses of published reaction studies on kaolinite, and sol‐gel and powder mixtures equivalent to 3Al2O3·2SiO2mullite, were made. Supplementary experiments on reactions of kaolinite at different rates of heating were analyzed. Complete atomic homogeneity of precursors results in formation of tetragonal mullite. Colloidal precursors initially form spinel which reacts with the remaining silica by a diffusion‐nucleation mechanism to form orthorhombic mullite. Precursors with intermediate atomic homogeneity follow both reaction paths. Kaolinite with a layer lattice structure has two‐dimensional atomic homogeneity. Both reaction routes are followed and their degree of interaction is dependent on the crystallinity and impurities of the kaolinite.
Effect of Physical Structure on the Phase Development of Aluminosilicate Gels
Authors: Fahrenholtz William G. , Hietala Susan L. , Newcomer Paula , Dando Neal R. , Smith Douglas M. , Brinker C. Jeffrey
Published: March 2005

The surface area and density of aluminosilicate xerogels containing a one‐to‐one Al/Si molar ratio (47 wt% alumina) can be varied dramatically by changing the pore fluid prior to drying. The surface area of ethanol‐washed 47 wt% alumina gels was more than 500 m2/g, while gels dried from the mother liquor (approximately 75 vol% ethanol, 25 vol% water) had less than 1 m2/g surface area. Changes in the physical structure of the dried gels had dramatic effects on subsequent phase evolution and densification behavior during heat treatment. NMR, X‐ray diffraction, and DTA were used to follow the phase evolution of different gels. Differences in the amorphous gel structure were identified using 27Al and 29Si MAS NMR. Gels of identical composition prepared from the same precursor solutions crystallized to different phases, depending upon the surface area of the gel prior to heating. The high surface material (ethanol washed) formed mullite and amorphous silica, while the low surface area gel (unswashed) crystallized to mullite and cristobalite. These gels were prepared from alkoxide precursors. A low surface area gel with a different degree of chemical homogeneity was prepared by the nitrate method for comparison. Results indicate that the physical structure of aluminosilicate gels, i.e., pore structure and chemical homogeneity, has a dramatic influence on phase evolution.
Fabrication of Mullite and Mullite‐Matrix Composites by Transient Viscous Sintering of Composite Powders
Authors: Sacks Michael D. , Bozkurt Nazim , Scheiffele Gary W.
Published: March 2005

Mullite was fabricated by a process referred to as transient viscous sintering (TVS). Composite particles which consisted of inner cores of α‐alumina and outer coatings of amorphous silica were used. Powder compacts prepared with these particles were viscously sintered to almost full density at relatively low temperatures (∼1300°C). Compacts were subsequently converted to dense, fine‐grained mullite at higher temperatures (∼1500°C) by reaction between the alumina and silica. The TVS process was also used to fabricate mullite/zirconia/alumina, mullite/silicon carbide particle, and mullite/silicon carbide whisker composites. Densification was enhanced compared with other recent studies of sintering of mullite‐based composites. This was attributed to three factors: viscous flow of the amorphous silica coating on the particles, avoidance of mullite formation until higher temperatures, and increased threshold concentration for the development of percolation networks.
Phase Separation in Gelling Silica–Organic Polymer Solution: Systems Containing Poly(sodium styrenesulfonate)
Authors: Nakanishi Kazuki , Soga Naohiro
Published: March 2005

Phase separation which occurs in parallel to the hydrolysis and gelation of alkoxysilane solution containing poly(sodium styrenesulfonate) (NaPSS) has been investigated. Depending on the reaction conditions, gel morphologies such as isolated pores, particle aggregates, and interconnected continuous pores from 0.1 to 100 μm long have been observed. Time‐resolved light scattering of gelling solution suggested the occurrence of spinodal phase separation through the polymerization of silica and the subsequent freezing of the developing structure by sol‐gel transition. The effects of reaction parameters on the periodic size are explained mainly in terms of their influence on the “chemical cooling” rate which is determined by the polymerizatica rate of silica and the solubility of NaPSS in the reacting solution.
Water Entry into Silica Glass During Slow Crack Growth
Authors: Tomozawa Minoru , Han Won‐Taek , Lanford William A.
Published: March 2005

Hydrogen concentration depth profiles on surfaces of SiO2 glass fractured slowly in water and rapidly in oil were determined by nuclear reaction analysis. It was found that water enters SiO2 glass during slow crack growth in water.
Effect of Hot‐Pressing on the Bi─Pb─Sr─Ca─Cu─O System
Authors: Song Kang‐Ho , Sorrell Charles C. , Dou Shi‐Xue , Liu Hua‐Kun
Published: March 2005

Uniaxial hot‐pressing has been used to improve the relative density and preferred gain orientation of the nearly pure (Bi, Pb)2Sr2Ca2Cu3O10‐y (2223) phase in the Bi‐Pb‐Sr‐Ca‐Cu‐O system. Samples were prepared by freeze‐drying, calcining, and hot‐pressing the powders at 650° to 840°C at 50 and 200 MPa for 2 h. The hot‐pressed samples showed a high degree of grain alignment and increased density up to 99% of theoretical. At 200 MPa, the 2223 phase was partially destroyed during hot‐pressing, probably because of the weak bonding between the Bi─O2 layers. However, the 2223 phase and superconductivity were recovered with subsequent annealing. The critical current density (Jc) was improved 10‐fold over that of sintered samples through this technique, with the highest Jc being 2100 A°Cm−2 at 77.3 K and zero field. The hot‐pressed samples also showed significant improvements in the Jc as a function of applied magnetic field. The Jc of a hot‐pressed and postannealed sample at 0.2 T and 77.3 K was 2 orders of magnitude greater than that of a sintered sample.
Interpretation of Hydrated States of Sodium Silicate Glasses by Infrared and Raman Analysis
Authors: Uchino Takashi , Sakka Tetsuo , Iwasaki Matae
Published: March 2005

Infrared spectra of thin films (a few micrometers) of hydrated sodium silicate glasses after heating at temperatures from 100° to 400°C were measured in the spectral region from 400 to 4300 cm−1. Also, Raman spectra of hydrated glasses were measured in the region from 100 to 1400 cm−1. Water molecules in hydrated glasses were classified into three types by the bonding configuration: water molecules interacting with Si–OH groups, nonbridging oxygens, and bridging oxygens. On the basis of the fundamental bands observed here, we calculated the wave numbers of the combination‐overtone bands of water molecules and SiOH groups in hydrated glasses. The calculated wave numbers satisfactorily agreed with observed wave numbers in the nearinfrared region.
Modeling of Thermal Spalling During Electrical Discharge Machining of Titanium Diboride
Authors: Gadalla Ahmed M. , Bozkurt Bedri , Faulk Nancy M.
Published: March 2005

Erosion in electrical discharge machining has been described as occurring by melting and flushing the liquid formed. Recently, however, thermal spalling was reported as the mechanism for machining refractory materials with low thermal conductivity and high thermal expansion. The process is described, here, by a model based on a ceramic surface exposed to a constant circular heating source which supplies a constant flux over the pulse duration. The calculations were based on TiB2 mechanical properties along a and c directions. Theoretical predictions were verified by machining hexagonal TiB2. Large flakes of TiB2 with sizes close to grain size and maximum thickness close to the predicted values were collected, together with spherical particles of Cu and Zn eroded from cutting wire. The cutting surfaces consist of cleavage planes sometimes contaminated with Cu, Zn, and impurities from the dielectric fluid.
Controlled Gelation and Sintering of Monolithic Gels Prepared from γ‐Alumina Fume Powder
Authors: Tsai Dah‐Shyang , Hsieh Chien‐Chung
Published: March 2005

The gelation, phase transformation, and densification of a colloidal monolithic gel made from γ‐Al2O3 fume powder are investigated. Among the six gelation agents that we use, formamide and urea are quick in causing gelation and easy to burn off. The densification rate of this gel decreases rapidly after the γ‐to‐α phase transformation. TiO2 is an effective sintering aid to overcome this bottleneck of densification because (1) it enhances the phase transformation rate so that the sintering of α‐alumina occurs at a lower temperature, and (2) it promotes sintering rates at the initial and intermediate stages after phase transformation. On the other hand, MgO has an inappreciable effect on gel sintering. The effect of MgO at the final sintering stage is obstructed by this densification barrier after transformation. The titania‐doped gel monoliths can be sintered to high density and fine microstructure at 1400°C.
Superplastic Alumina Ceramics with Grain Growth Inhibitors
Authors: Xue Liang A. , Wu Xin , Chen I‐Wei
Published: March 2005

Superplastic deformation of alumina ceramics was studied at 1400° to 1450°C and at a strain rate of 4 × 10−5 to 5 × 10−4 s−1. MgO and ZrO2 were introduced to suppress dynamic grain growth. The latter was especially effective; grain growth was minimal in 10‐vol%‐ZrO2‐containing material. Both materials were superplastically stretched under biaxial tension to 100% engineering strain with good surface finishing, demonstrating the feasibility of superplastic forming for alumina ceramics.
Ferroelastic Toughening in Bismuth Vanadate
Authors: Baker Theresa L. , Faber K. T. , Readey D. W.
Published: March 2005

Bismuth vanadate has been used as a model system to examine toughening by stress‐induced domain motion in a ferroelastic materials. Fracture toughnesses in the ferroelastic and paraelastic states have been compared. Toughening and R‐curve behavior have been observed in the ferroelastic state. Single crystals of BiVO4 show domain nucleation and propagation under compressive loading and domain generation during fracture. The increase in toughening in the ferroelastic state can be explained by a combination of crack deflection and domain wall motion, the latter contributing to R‐curve behavior.
Cavitation Damage During Flexural Creep of SiAlON–YAG Ceramics
Authors: Chen Ching‐Fong , Wiederhorn Sheldon M. , Chuang Tze‐jer
Published: March 2005

Cavitation damage in flexure bars crept at 1170°C was studied by a density measurement technique. The cavity density within the flexure beams could be approximated by a linear function of position from the tensile surface. A threshold stress for cavitation damage is suggested from the results of this study. Below the threshold tensile stress, cavitation ceases, whereas above the threshold, cavitation damage is in the form of wedge‐shaped cracks at grain‐boundary triple junctions. Cavitation is not observed in compression for the conditions used in this study. From a stress analysis of the flexure bars, a cavitation threshold of 55 MPa is estimated for this material.
Temperature Dependence of the Elastic Moduli of Monoclinic Zirconia
Authors: Chan Sai‐Kit , Fang Yue , Grimsditch Marcos , Li Zhuang , Nevitt Michael V. , Robertson William M. , Zouboulis Elias S.
Published: March 2005

We have used Brillouin scattering to measure the temperature dependence of the sound velocities of the longitudinal and transverse vibrational modes in various directions in small single crystals of monoclinic ZrO2. Using these velocities and the Christoffel equation, we have calculated the 13 elastic stiffness moduli between room temperature and the monoclinic—tetragonal transformation temperature.
Thermodynamic Evaluation for the Y2O3–BaO–CuOx System
Authors: Lee Byeong‐Joo , Lee Dong Nyung
Published: March 2005

The thermodynamic data for the Y2O3–BaO–Cu2O–CuO quaternary system were optimized from measured thermodynamic data. A two‐sublattice model for ionic solution was used to express the Gibbs free energy of the liquid phase, and a two‐sublattice regular solution model was used for the nonstoichiometric YBa2Cu3O6+δ superconducting compound. The optimized thermodynamic data were used to calculate the phase diagrams of the Cu2O–CuO binary system and the CuOx–Y2Cu2O5 and CuOx–BaCuO2 quasi‐binary systems. The results were in good agreement with reported measured data. The liquidus projection and isothermal and vertical sections of the Y2O3–BaO‐CuOx quasi‐ternary system were calculated. The effect of oxygen pressure on some reaction temperatures was predicted by calculating them at various oxygen pressures, and the oxygen contents (6 +δ) in YBa2Cu3O6+δ were calculated at various temperatures and oxygen pressures. The results were compared with experimental data.
Indentation Fatigue of High‐Purity Alumina in Fluid Environments
Authors: Reece Michael , Guiu Frank
Published: March 2005

The influence of environment on the cyclic fatigue behavior of a high‐purity alumina bioceramic was investigated using the repeated indentation technique. Tests were conducted in the presence of water, a variety of alcohols, toluene, and simulated physiological fluid environments. The results show that these environments do not have any detectable effect on the damage produced by single indentations, but those containing water cause a significant degradation in cyclic fatigue resistance which cannot be quantified in terms of known subcritical crack growth behavior in static fatigue. It is concluded that the effects of fluid environments on the growth of cyclically driven cracks must be an integral part of the mechanism responsible for cyclic fatigue in ceramics.
Thermal Analysis of a SiO2─Al2O3─CaO─CaF2 Glass
Authors: Hill Robert G. , Goat Christopher , Wood David
Published: March 2005

A SiO2─Al2O3─CaO─CaF2 ionomer glass was investigated using thermal analysis, X‐ray diffraction, and scanning electron microscopy. The purpose of this investigation was to control the susceptibility of the glass to acid attack. The differential thermal analysis trace exhibited a sharp glass transition at about 645°C and two exotherms. The first exotherm corresponded to liquid–liquid phase separation followed by crystallization of fluorite. The second, much larger, exotherm was the result of crystallization of the remaining glass phase to form anorthite. Prolonged heat treatment below the glass‐transition temperature demonstrated that crystallization of fluorite can occur without prior liquid–liquid phase separation.
Control of Grain‐Boundary Pinning in Al2O3/ZrO2 Composites with Ce3+/Ce4+ Doping
Authors: Xue Liang A. , Meyer Karsten , Chen I‐Wei
Published: March 2005

The control of the microstructure of Ce‐doped Al2O3/ZrO2 componsites by the valence change of cerium ion has been demonstrated. Two distinctively different types of microstructure, large Al2O3 grains with intragranular ZrO2 particles and small Al2O3 grains with intergranular ZrO2 particles, can be obtained under identical presintering processing conditions. At doping levels greater than ∼ 3 mol% with respect to ZrO2, Ce3+ raises the alumina grain‐boundary to zirconia particle mobility ratio. This causes the breakaway of grain boundary from particles and the first type of microstructure. On the other hand, Ce4+ causes no breakaway and produces a normal intergranular ZrO2 distribution. The dramatic effect of Ce3+ on the relative mobility ratio is found to be associated with fluxing of the glassy boundary phase and is likewise observed for other large trivalent cation dopants. The ZrO2 second phase acts as a scavenger for these trivalent cations, provided their solubility limit in ZrO2 is not exceeded.
Experimental Observations of Frictional Heating in Fiber‐Reinforced Ceramics
Authors: Holmes John W. , Cho Chongdu
Published: March 2005

The influence of fatigue loading history and microstructural damage on the magnitude of frictional heating and interfacial shear stress in a unidirectional SiC fiber/calcium aluminosilicate matrix composite was investigated. The extent of frictional heating was found to depend upon loading frequency, stress range, and average matrix crack spacing. The temperature rise attained during fatigue can be significant. For example, the temperature rise exceeded 100 K during fatigue at 75 Hz between stress limits of 220 and 10 MPa. Analysis of the frictional heating data indicates that the interfacial shear stress undergoes an initially rapid decrease during the initial stages of fatigue loading: from an initial value over 20 MPa, to approximately 5 MPa after 25 000 cycles. Over the range of 5 to 25 Hz, the interfacial shear stress was not significantly influenced by loading frequency. The implications of frictional heating in fiber‐reinforced ceramics are also discussed.
Influence of Silica Fume on the Early Hydration of Portland Cements Using Impedance Spectroscopy
Authors: Christensen Bruce J. , Mason Thomas O. , Jennings Hamlin M.
Published: March 2005

Electrical properties of hydrating portland cements (PC) and portland cements containing silica fume were studied from 5 min to 90 days. Cement pastes with water to solids ratios (w/s) of 0.30, 0.35, and 0.40, as well as silica fume to portland cement ratios (s/c) of 0.05, 0.10, and 0.20, were made and impedance was measured within the frequencies of 13 MHz to 5 Hz. The impedance spectra exhibit electrode arcs at low frequencies and bulk material arcs at high frequencies. The bulk resistance of the paste increases with increasing silica fume content and/or decreasing water content. The conductivity of pore fluid from PC pastes increases rapidly with time during the early stages and then remains constant, while that of the silica fume pastes increases then decreases sharply.
Mullite—Alumina Composites by Extrusion
Authors: Blackburn Stuart , Lawson Toni A.
Published: March 2005

The rheological properties of a paste containing chopped alumina fiber and particulate silica suspended in a gelled boehmite liquid phase have been evaluated using a physically based extrusion model. When sintered, the paste formed a mullite‐alumina fiber composite. Extrudates with fiber volumes up to 30% in the sintered product were prepared. During extrusion, the pressure drop was largely independent of extrudate velocity, fiber length, and the fiber concentration. All pastes showed significant yield behavior leading to good postextrusion shape retention. For any given fiber length, it was shown that there exists a critical volume fraction above which fiber‐fiber interactions are so great that both yield and wall shear stresses increase. At these high concentrations of fiber, inhomogeneities also increase. Up to the critical volume fraction, dispersed wet fibers produced lower extrusion parameters than when dry fibers were used as the starting material. The observed behavior is explained in terms of low viscosity liquid formation above the yield point of the boehmite gel.
Plasma‐Sprayed‐Yttria Layers for Corrosion Resistance
Authors: Berkowski Marek , Bowen Paul , Liechti Thomas , Scheel Hans Jörg
Published: March 2005

Crucible corrosion in the growth of YBa2Cu3O7‐x (YBCO) superconducting single crystals is one of the major obstacles in obtaining high‐quality crystals. Both metallic and ceramic crucibles have shown significant corrosion that affects the size and properties of YBCO crystals. Plasma spraying has been successfully used to produce yttria coatings on alumina substrates. Yttria coatings with thicknesses between 30 and 100 μm have shown corrosion rates of 3 to 4 μm/h; these rates are 5 to 10 times lower than that of the alumina substrate.
Zeolite Formation in Class F Fly Ash Blended Cement Pastes
Authors: LaRosa Judith L. , Kwan Stephen , Grutzeck Michael W.
Published: March 2005

Zeolite formation in Class F fly ash blended cement pastes is under investigation. A Na–P type zeolite and Zeolite Y were synthesized from Class F fly ash and NaOH solution after 2 days of aging at room temperature and 6 days of curing at 80°C. However, no zeolites formed when KOH was used. In additional experiments, a Na–P type zeolite, Zeolite Y, and chabazite developed in cement pastes blended with Class F fly ash and NaOH solution which had been aged 2 days at room temperature and then cured 6 days at temperatures ranging from room temperature to 90°C. Seeding the pastes with natural zeolites was also investigated.
Phenomenological Theory of High Permittivity in Fine‐Grained Barium Titanate
Authors: BUESSEM W. R. , CROSS L. E. , GOSWAMI A. K.
Published: March 2005

The anomalously high permittivity in fine‐grained ceramic barium titanate is explained by the absence of 90° twinning within the grains, giving rise to internal stresses as the ceramic cools below the Curie temperature. Using the Devonshire phenomenological thermodynamic method and a simplified model, the stresses involved are calculated. At these stresses levels, the reduced strain calculated on this model agrees well with X‐ray data for the fine‐grained material. The limitations and possible developments of the proposed model are discussed.
Temperature Dependence of Ultra‐Low‐Frequency Dielectric Relaxation of Barium Titanate Ceramic
Authors: Chu Fan , Sun Hong‐Tao , Zhang Liang‐Ying , Yao Xi
Published: March 2005

Based on a linear voltage ramp method, ultraslow dielectric relaxation of undoped BaTiO3 ceramic has been carefully measured over the temperature range between − 180° and +120°C. Temperature dependence of the dielectric constant in the frequency range from 10 −5 to 10−1 Hz can be obtained using Kramers–Kronig equations. Close to the Curie temperature, the dielectric constant increases so rapidly that the Curie peak is overwhelmed. Two kinds of relaxation processes were observed in this experiment. Ferroelectric domain and mobile charge are proposed to be responsible for the relaxation over the temperature range. With temperature increasing, Maxwell–Wagner relaxation associated with mobile charge gradually dominates. From relaxation time constants at various temperatures, relaxation activation energies about 0.06 and 0.09 eV are identified.
Processing and Microstructural Development of In Situ TiN‐Reinforced Silicon Nitride/Silicon Oxynitride Composites
Authors: Lin Wai , Yang Jenn‐Ming , Ting Su‐Jen , Ezis Andre , Shih C. James
Published: March 2005

The development of a novel TiN‐reinforced silicon nitride/silicon oxynitride composite using an innovative in situ TiN‐forming technique was investigated. Silicon powder compacts containing various amounts of TiO2 (anatase structure) and sintering additives were nitrided under N2 atmosphere and then further densified by hot pressing. The microstructure of the resultant composite was analyzed by X‐ray diffraction, scanning electron microscopy, and solid‐state nuclear magnetic resonance spectroscopy. The results of this study show that TiO2 can be successfully converted to TiN during the nitridation of Si + TiO2 mixture. The TiO2 content affects not only the microstructure of the matrix but also the composition of the intergranular phase. The type and amount of sintering additives also show a significant effect on the microstructural development of the composite.
Solids and Liquids in the NaF–AlF3–Al2O3 System: A High‐Temperature NMR Study
Authors: Stebbins Jonathan F. , Farnan Ian , Dando Neal , Tzeng Shih‐Ying
Published: March 2005

Aluminofluoride and oxyfluoride melts are essential in the electrolytic smelting of aluminum, as the precursors of fluoride glasses, and as analogue systems for molten silicates. In these systems, the quantification of the abundances of structural species is crucial in the understanding of both thermodynamic and transport properties. 27Al NMR spectroscopy at high temperatures can provide important constraints on speciation and coordination number in aluminofluoride liquids, as well as structural and dynamical changes in the crystalline fluorides. We present results on a series of solids and liquids in the NaF–AIF3–Al2O3 system. These illustrate the dramatic effects of composition and temperature on liquid structure and suggest that most Al cations are four‐ or six‐coordinated, with a relatively low abundance of five‐coordinated sites.
Tribological Characteristics of α‐Alumina in High‐Temperature Water
Authors: Kitaoka Satoshi , Yamaguchi Yoshimi , Takahashi Yoshihide
Published: March 2005

The effects of temperature, sliding speed, and contact load on the tribological behavior of an α‐alumina ceramic sliding on the same material in water were investigated in the range from room temperature to 300°C under the corresponding saturated vapor pressures. The specific wear rate increased remarkably at elevated temperatures. The primary wear mechanisms in high‐temperature water are considered to be microfracture, promoted by the solution of grain boundary layers, and stress corrosion cracking. A film containing γ‐alumina particulates appeared on the worn surface with increasing sliding speed and increasing contact load in 300°C water.
Origin of Viscosity Increase of Phase‐Separated Borosilicate Glasses
Authors: Tomozawa Minoru , Sridharan Srinivasan , Takamori Takeshi
Published: March 2005

The viscosity of some borosilicate glasses increases with heat‐treatment time for phase separation. The viscosity increase in the initial stage of phase separation is attributed to the composition change of the separated phases. The origin of the viscosity increase in the later stage of phase separation is the subject of controversy. Namely, the viscosity increase has been attributed to two different mechanisms: (1) environmental relaxation and (2) the increase of silica content in the high‐viscosity silica‐rich continuous phase. Analyses are presented to show that the latter model is consistent with the experimental data.
Fabrication of Transparent Silicon Nitride from Nanosize Particles
Authors: Pechenik Alexander , Piermarini Gasper J. , Danforth Stephen C.
Published: March 2005

Compaction of ultrafine silicon nitride (Si3N4) powder at high pressures and various temperatures followed by pressureless sintering was investigated. The powder, consisting of nearly spherical particles (16 nm in diameter) of amorphous stoichiometric Si3N4, was pressed in a diamond anvil cell under pressures up to 5 GPa and temperatures ranging from liquid nitrogen to 500°C. Quality of compaction, evaluated by visual transparency and hardness of the produced compacts, depended on the amount of adsorbed gases on the surface of the particles and on the temperature of compaction. Visually transparent compacts were produced by pressing the starting powder without outgassing in liquid nitrogen under 5 GPa. The transparent compacts exhibited a hardness of 1200 kg/mm2 after pressing in the diamond anvil cell at 500°C for 3 h at 5 GPa. After subsequent pressureless sintering conducted for 1 h at 5 GPa. After subsequent pressureless sintering conducted for 1 h at 1400°C in a tube furnace under nitrogen, the hardness of these samples increased to over 2000 kg/mm2 and the visual transparency was maintained. The results demonstrated that transparency was maintained. The results demonstrated that transparent compacts of nanosize amorphous Si3N4 particles could be sintered to high hardness at relatively low temperatures without using sintering aids or applying pressure during sintering.
Comparative Measurement of Indentation Fracture Toughness with Berkovich and Vickers Indenters
Authors: Dukino Rodney D. , Swain Michael V.
Published: March 2005

Measurements of the load dependence of the radial crack size with Vickers and Berkovich indenters are compared for a range of materials. It is found that the extent of radial cracks was slightly larger for the Berkovich than for the Vickers indenter. The observations reveal that cracks from a Berkovich indenter are best described by an expression developed by Laugier combined with a modification proposed by Ouchterlony to account for the number of radial cracks. It was also found that the Berkovich indenter, which offers advantages for ultramicroindentation, gave more consistent toughness values at lower loads than a Vickers indenter.
Crystallization of Yttria–Alumina–Silica Glasses
Authors: Arita Irene H. , Wilkinson David S. , Purdy Gary R.
Published: March 2005

Bulk nucleation has been studied for glasses within the Y2O3–Al2O3–SiO2 system, using a two‐stage nucleation and growth heat treatment. The crystalline phases formed have been identified. Annealing in an inert atmosphere is required in order to prevent surface nucleation from dominating the results. Despite this, the phases observed are in general agreement with those observed in previous studies done in air. The nucleation kinetics of the y‐Y2Si2O7 phase have been measured and the optimum nucleation temperatures have been identified. The kinetic data have been analyzed using two existing models based on classical nucleation theory. The results of this study have been compared with crystallization results of a previous study of glass of similar composition within sintered silicon nitride samples. A two‐stage heat treatment is suggested as a process that may lead to improved devitrification of grain boundary glass in such materials.
Damage Mechanisms and the Mechanical Properties of a Laminated 0/90 Ceramic/Matrix Composite
Authors: Beyerle Douglas S. , Spearing S. Mark , Evans Anthony G.
Published: March 2005

The tensile properties of a 0/90 laminated CAS matrix composite reinforced with Nicalon fibers have been measured. Some effects of notches have also been explored. Changes in modulus and permanent strain caused by matrix cracking have been measured and compared with available models. For this comparison, independent measurements have been made of the constituent properties and the residual stress. The ultimate tensile strength has also been measured and compared with a global load‐sharing model. It is concluded that lower‐bound matrix cracking models provide good predictability of the stresses at which various matrix cracking mechanisms first operate. Also, the ultimate tensile strength is found to be consistent with a global load‐sharing model, based on the in situ strength properties of the fibers. Conversely, the evolution of matrix cracks at stresses above the lower bound has yet to be adequately modeled. In addition, a need is identified for improved models relating elastic properties and permanent strains to matrix crack spacing.
A Grüneisen Relation on Constant‐Pressure Lines
Authors: Burns S. J. , Dahake Girish , Ding Zhong , Kudallur Suresh , Kumar Sujeet , Li Kangjie , Srinivasan Ramakrishnan , Stack Ken , Venkataraman Natarajan , Wang Chuan , Yang Fuqian , Zhou Yiyang
Published: March 2005

Grüneisen's constant, γ, is ideally temperature‐independent on constant‐volume lines; a similar constant, λ, which is ideally temperature‐independent on isobaric lines, has been found. The comparison between γ and λ shows that both expressions contain the ratio of the volumetric thermal expansion coefficient to the specific heat capacity; γ has an additional temperature dependence due to the thermal part of the compressibility, whereas λ has only a slight thermal component due to the volume per unit mass. λ has been chosen so that λ=γ at the point where the temperature and pressure are both zero. For most materials it is expected that λ≥γ with a different temperature dependence. It is shown empirically for a selection of ceramics that λ is nearly temperature‐independent and has a value between 0.7 and 1.4.
Mixed‐Cation Oxide Powders via Resin Intermediates Derived from a Water‐Soluble Polymer
Authors: Tai Lone‐Wen , Anderson Harlan U. , Lessing Paul A.
Published: March 2005

Submicrometer powders of complex oxides were prepared via resin intermediates based on a starch type of organic precursor. A commercially available water‐soluble starch derivative was (for the first time) used as the organic base for solution synthesis of ceramic powders. Calcination of the charred, fluffy, amorphous resins at a temperature below 600°C for 4 h yielded perovskite powders of Sr‐doped LaMnO3 and Sr‐doped La(Fe,Co)O3. Sr‐doped LaCrO3 needed to be calcined above 750°C to ensure phase purity and to remove organic residue. Due to the low cost of starch derivatives, the process has the potential of being more economical than the commonly used Pechini's type process, which utilizes citric acid and ethylene glycol.
Factors Controlling the Sintering of Ceramic Particulate Composites: I, Conventional Processing
Authors: Fan Chen‐Lung , Rahaman Mohamed N.
Published: March 2005

The effect of processing and material parameters on the sintering and coarsening of model composites consisting of a fine‐grained ZnO matrix and coarse, rigid, inert particulate inclusions of ZrO2 was investigated. The green composites were formed by two methods: (i) mixing the matrix and inclusion powders in a ball‐mill followed by die‐pressing and (ii) slip casting. For both forming methods, the inclusions caused a significant reduction in the density and the grain size of the composite matrix but had almost no effect on the grain size vs density relationship. The effects of inclusion volume fraction and sintering temperature were somewhat independent of the forming method. However, for the slip‐cast composites, the effect of inclusion size was less severe and the packing of the matrix phase immediately surrounding the inclusion showed some improvement. The sintering kinetics and microstructural observations indicated that two main factors controlled the sintering of these composites: (i) interactions between the randomly distributed inclusion particles that created a constraint on the matrix and (ii) the packing of the matrix, especially in regions immediately surrounding the inclusion particles.
Sintering, Creep, and Electrical Conductivity of Model Glass‐Matrix Composites
Authors: Dutton Rollie E. , Rahaman Mohamed N.
Published: March 2005

The sintering, creep, and electrical conductivity of model glass‐matrix composites were investigated as functions of inclusion content and size. The composites consisted of spherical soda–lime glass particles and spherical nickel inclusions. At inclusion volume fractions below 10 vol% there was no inclusion size effect on the creep viscosity. Above 10 vol% there was an inclusion size effect, with the viscosity increasing significantly with decreasing inclusion size. The increase in electrical conductivity commenced at lower inclusion volume fractions as the inclusion size decreased. The data indicate that interactions between the inclusions might be responsible for the observed inclusion size effect. The sintering rates of the composites were compared with the predictions of Scherer's self‐consistent model. Good agreement was obtained when the measured creep viscosity was used in the model equations.
Lattice Constants of Boron Carbides
Authors: Aselage Terry L. , Tissot Ralph G.
Published: March 2005

The lattice constants of boron carbides have been determined by powder X‐ray diffraction for samples with compositions between about 7.7 and 20.5 at.% carbon. The boundaries of the single‐phase region are at about 9 at.% carbon and near, but likely somewhat less than, 20 at.% carbon. The composition dependence of the lattice constants thus established provides a method of assessing the carbon concentration of unknown materials. In particular, assignment of the approximate composition of single crystals used in previous studies allows for a systematic examination of changes in interatomic separation as a function of composition. These changes are discussed in terms of a structural model of the boron carbide solid solution.
Reaction Sintering and Properties of Silicon Oxynitride Densified by Hot Isostatic Pressing
Authors: Larker Richard
Published: March 2005

Silicon oxynitride ceramics were reaction sintered and fully densified by hot isostatic pressing in the temperature range 1700°C to 1950°C from an equimolar mixture of silicon nitride and silica powders without additives. Conversion to Si2N2O increases steeply from a level around 5% of the crystalline phases at 1700°C to 80% at 1800°C, and increases a few percent further at higher temperatures.α‐Si3N4is the major residual crystalline phase below 1900°C. The hardness level for materials containing 85% Si2N2O is approximately 19 GPa, comparable with the hardness of Si3N4hot isostatically pressed with 2.5 wt% Y2O3, while the fracture toughness level is around 3.1 MPa. m1/2, being approximately 0.8 MPa.m1/2lower. The three‐point bending strength increased with HIP temperature from approximately 300 to 500 MPa.
Fracture Toughness and Thermal Shock Behavior of Silicon Nitride–Boron Nitride Ceramics
Authors: Lutz Ekkehard H. , Swain Michael V.
Published: March 2005

Fracture toughness behavior, stress–strain behavior, and flaw resistance of pressureless‐sintered Si3N4‐BN ceramics are investigated. The results are discussed with respect to the reported thermal shock behavior of these composites. Although the materials behave linear‐elastic and exhibit no R‐curve behavior, their flaw resistance is different from that of other linear‐elastic materials. Whereas the critical thermal shock temperature difference (ΔTc) is enhanced by adding BN, the content of BN has no influence on the strength loss during severe thermal shocks.
Relationship between Doping and Ordering (1/3<111> and 1/2<111> Types) in A(B2+B5+)O3 and A(B3+B6+)O3 Systems
Authors: Wang Qiang , Rushan Han , Gan Z. Z.
Published: March 2005

The Madelung energies of A2+ (B1/32+ B2/35+) O32‐ and A2+ (B2/33+ B1/36+) O32‐ systems were calculated for different dopants on the A‐site. The theoretical results show that the higher‐valence dopant enhances the 1/2<111>‐type (1:1) ordered arrangement of B‐site cations in A2+ (B1/32+ B2/32+)O32‐, whereas the lower‐valence dopant promotes the 1:1 ordering in A2+ (B2/33+ B1/36+)O32‐, and no doping promotes the 1/3<111>‐type (1:2) ordering in the both systems. These calculation results are in agreement with the experimental observations of donor‐ and acceptor‐doped PMN systems.
Determination of Young's Modulus of Thin Films
Authors: Maddalena Amedeo
Published: March 2005

An improved vibrating‐reed method is described for determining Young's modulus of thin films deposited on both sides of a substrate. This technique consists of measuring the resonant frequency of a cantilever composite beam obtained by coating both sides of a substrate. The calculation procedure is presented to evaluate the film modulus from sample geometry, material density, and mechanical resonant frequency. For accurate determination of resonant frequency, the phase angle between the exciting and vibration signals is analyzed. Using the proposed technique, Young's modulus of ZrO2 thin films is calculated, obtaining a value in agreement with literature data.
2223 Phase Formation in Bi(Pb)─Sr─Ca─Cu─O: III, The Role of Atmosphere
Authors: Chen Yan Ling , Stevens Ronald
Published: March 2005

The formation of second phases during the preparation of the 2223 phase and their stability in the Bi system under various annealing temperatures and atmospheres have been studied. The 2201 precipitates developed at ∼830°C, and their conversion to the 2223 phase can be completed in the temperature range 810°C ≤T < 830°C. A Ca2PbO4‐like phase can precipitate from the liquid phase at ∼830°C during cooling. A (Sr,Ca)14Cu24O41 phase is usually found accompanying the synthesis of the 2223 phase. This secondary phase is stable in an oxidizing atmosphere but can be eliminated by annealing under a low oxygen atmosphere or by choosing a suitable starting composition and set of sintering conditions. The precipitation of Ca2PbO4‐like phase can be avoided by using a relatively fast cooling rate. Unlike the YBa2Cu3Ox superconductor, the 2223 phase can be stable under a wide range of atmospheres, such as argon, air, and oxygen.
Interaction of Ag/Pd Metallization with Lead and Bismuth Oxide‐Based Fluxes in Multilayer Ceramic Capacitors
Authors: Wang Sea Fue , Huebner Wayne
Published: March 2005

An ongoing challenge for future developments in high‐performance ceramic multilayer capacitors and integrated ceramics is to reduce the internal electrode cost and thickness without sacrificing yield or reliability. Key to these developments is a thorough understanding of the interactions which occur between flux‐sintered dielectrics and low‐cost, Ag/Pd electrodes. In this paper we present results on the pahse equilibria of Ag/Pd electrode systems with Bi2O3 and PbO fluxes. The conditions under which the bismuth reaction occurs and reactions in the Pd–PbO system are reported. Results show that the equilibrium phases which form depend strongly upon the Ag/Pd ratio and temperature. These phases include PdBi2O4, Pd(Bi), PdPbO2, Pd(Pb), and PbPd3. The PdBi2O4 and PdPbO2 phases decompose when PdO destabilizes, resulting in a series of reactions which result in oxygen evolution and partial melting of components. The exact phase relations of the Ag/Pd–Bi2O3–O2(air) system and Ag/Pd–PbO–O2(air) system have been established for the first time and are discussed in terms of their impact on multilayer, cofired structures.
Phase Transformations in Dicalcium Silicate: I, Fabrication and Phase Stability of Fine‐Grained β Phase
Authors: Nettleship Ian , Slavick Kurt G. , Kim Youn Joong , Kriven Waltraud M.
Published: March 2005

Fine‐grained β‐Ca2SiO4 containing small amounts of sodium was fabricated as an analogue to tetragonal zirconia polycrystals (TZP) in order to study the stress‐induced β→γ transformation. This avoided the problems associated with the fabrication and evaluation of composites containing β‐Ca2SiO4. The microstructure of dense β‐Ca2SiO4 exhibited severe intergranular strains and twin‐terminating microcracks as seen by TEM. The β‐phase twin widths were quantitatively correlated with grain sizes giving an average ratio of 0.04. Stress‐induced transformation was observed on ground surfaces but not on fracture surfaces. The stress–strain behavior and the mechanical properties were consistent with stress‐induced microcracking and microcrack coalescence. The elastic modulus of fully dense β phase was estimated to be 123 GPa.
Preparation of High‐Purity ZrSiO4 Powder Using Sol–Gel Processing and Mechanical Properties of the Sintered Body
Authors: Mori Toshiyuki , Yamamura Hiroshi , Kobayashi Hidehiko , Mitamura Takashi
Published: March 2005

Effects of the concentration of ZrOCl2, calcination temperature, heating rate, and the size of secondary particles after hydrolysis on the preparation of high‐purity ZrSiO4 fine powders from ZrOCl2.8H2O (0.2M to 1.7M) and equimolar colloidal SiO2 using sol–gel processing have been studied. Mechanical properties of the sintered ZrSiO4 from the high‐purity ZrSiO4 powders have been also investigated. Single‐phase ZrSiO4 fine powders were synthesized at 1300°C by forming ZrSiO4 precursors having a Zr–O–Si bond, which was found in all the hydrolysis solutions, and by controlling a secondary particle size after hydrolysis. The conversion rate of ZrSiO4 precursor gels to ZrSiO4 powders from concentrations other than 0.4M ZrOCl2.8H2O increased when the heating rate was high, whereupon the crystallization of unreacted ZrO2 and SiO2 was depressed and the propagation and increase of ZrSiO4 nuclei in the gels were accelerated. The density of the ZrSiO4 sintered bodies, manufactured by firing the ZrSiO4 compacts at 1600° to 1700°C, was more than 95% of the theoretical density, and the grain size ranged around 2 to 4 μm. The mechanical strength was 320 MPa (room temperature to 1400°C), and the thermal shock resistance was superior to that of mullite and alumina, with fairly high stability at higher temperatures.
Grinding Effects on the Synthesis and Sintering of Cordierite
Authors: Awano Masanobu , Takagi Hiroyoshi , Kuwahara Yoshitaka
Published: March 2005

Grinding effects on the synthesis and sintering of cordierite were investigated. The grinding of a precursor gel derived from colloidal processing promoted the homogeneous distribution of elements in the gel, resulting in improved reactivity for the formation of cordiertie. Grinding the calcined powder enhanced the homogeneity of the resulting powder and caused the accumulation of internal energy as crystal strain; consequently, the densities of the sintered bodies increased and the optimum temperature range for sintering widened.
Comparison of High‐Temperature Crack Growth in SiC‐Whisker‐Reinforced Mullite and Si3N4
Authors: Ritter John E. , Jakus Karl , Godin Micheal H. , Ruh Robert
Published: March 2005

Crack growth behavior under creep conditions was studied in SiC‐whisker‐reinforced mullite and silicon nitride. Tests of four‐point bend specimens with indentation cracks were periodically interrupted to observe the creep behavior. At each interruption the bulk creep strain of the specimen, the growth of the indentation cracks, and the nucleation and growth of creep‐induced cracks were measured. A strong linear correlation was observed in both materials between the crack growth rate and the creep strain rate. For a given strain rate, cracks in the silicon nitride composite propagated at velocities about an order of magnitude greater than those in the mullite composite. On the other hand, for similar nominal stresses, creep rates in the silicon nitride composites were about an order of magnitude less than with the mullite composite.
Phase Evolution during Autoclaving Process of Aerated Concrete
Authors: Mitsuda Takeshi , Sasaki Kaori , Ishida Hideki
Published: March 2005

The reactions were investigated that occur when lime, cement, and quartz sand are mixed together and molded, then treated at 180°C under saturated steam pressures to produce autoclaved aerated concrete. The hydrothermal treatment of mixtures gives Ca‐rich C‐S‐H with varying Ca/Si ratios as an initial product, which reacts further with silica dissolved from quartz to form 1.1‐nm tobermorite with increase of curing time. During autoclaving, the composition of C‐S‐H and tobermorite as a binder continues to change until after 8 h, when the Ca/(Al + Si) ratio becomes constant at 0.8. As the reaction proceeds, the number of micropores increases, and the strength also increases due to the binder effect of the tobermorite. However, the total pore volume does not change, remaining constant values.
Active Oxidation of SiC‐Based Ceramics in Ar–2% Cl2‐O2 Gas Mixtures at 1000°C
Authors: Ip Sik Y. , McNallan Michael J. , Park Dong S.
Published: March 2005

The corrosion in Ar–2% CI2 gas mixtures of four low‐cost SiC‐based materials suitable for use in high‐temperature heat exchangers has been invesigated. The oxygen potential was controlled by addition of O2 or H2 at 1000°C. Little attack was observed in the reducing environment composed of Ar–2% Cl2–1% H2 or the oxidizing environment composed of Ar–2% Cl2–20% O2 but all of the materials were subject to active corrosion at intermediate oxygen potentials. Selective attack of the free silicon phase was obsrved for the siliconized silicon carbide materials. The severity of the active oxidation and the oxygen potentital at which the corrosion changed from active to passive were affected by the nature of the sintering aids used in the materials