bluuebunny/crossref_metadata_2025 huggingface dataset:
High‐Strength Zirconia Fibers
Authors: Marshall David B. , Lange F. F. , Morgan Peter D.
Published: March 2005

Fine‐grained polycrystalline zirconia fibers have been formed from an acetate precursor. The fibers contained a Y2O3 additive, which inhibited grain growth (grain size ≤0.5 μm) and allowed the tetragonal phase to be retained at room temperature. Fibers with diameters in the range 2 to 5 μm had strengths in the range 1.5 to 2.6 GPa.
Static Fatigue of Optical Fibers in Bending
Authors: MATTHEWSON M. JOHN , KURKJIAN CHARLES R.
Published: March 2005

A two‐point bending technique for making static fatigue measurements on optical fibers is described which allows large quantities of data to be rapidly and conveniently gathered. Statistical analysis is used to compare the times to failure of the method with those of the commonly used mandrel and tensile methods. Direct experimental comparison of the two‐point bend and tensile methods shows that while the times to failure are generally longer for two‐point bend than tension, the essential fatigue behavior is identical for the two methods.
Analytical Electron Microscope Analysis of the Formation of BaO · 6Al2O3
Authors: MACHIDA MASATO , EGUCHI KOICHI , ARAI HIROMICHI
Published: March 2005

The formation process of barium hexaaluminate (BaO 6Al2O3) from BaCO3/γ‐Al2O3 powders or hydrolyzed alkoxides was studied by analytical electron microscopy. Barium hexaaluminate is produced by a two‐step solid‐state reaction from BaCO3 and Al2O3 via formation of BaO·Al2O3. Marked grain growth and inclusion of nonequilibrium phase were inevitable in this powder mixture process. However, in an alkoxide‐derived precursor, homogeneous mixing of components is attained and hence the formation of BaO·6Al2O3 proceeds readily. Powders obtained by this latter route consisted of fine planar particles with a uniform size and retained a large surface area (20.2 m2/g) even after heating at 1300°C. Electron diffraction results implied that suppression of crystal growth along the c axis is the reason for the large surface area of BaO·6Al2O3.
Micromechanical Analysis of Short Cracks in Fiber‐Reinforced Ceramics
Authors: Kennedy Timothy C. , Wang Min
Published: March 2005

The finite‐element method is used to perform a micromechanical analysis of a crack in a fiber‐reinforced ceramic, when the crack length is the same order of magnitude as the fiber spacing. The stress intensity factor is calculated under mode‐I loading for various crack lengths and fiber volume fractions. Cracks with and without fibers bridging the faces of the crack are analyzed.
Microwave Dielectric Properties of Antiferroelectric Lead Zirconate
Authors: LANAGAN MICHAEL T. , KIM J.H. , JANG SEI‐JOO , NEWNHAM ROBERT E.
Published: March 2005

The dielectric properties of ceramic lead zirconate and rutile are presented at microwave frequencies. Several techniques are required to span the frequency range from 100 Hz to 26 GHz, and rutile is utilized as a reference material to determine the consistency between measurement methods. A significant relaxation was seen for lead zirconate. At 10 GHz, the dielectric constant decreased 20% from the 1‐MHz value of 160, and the dielectric loss increased by more than 3 times the low‐frequency value of 0.007.
Zinc Iron Chromite Pigments
Authors: Murdock Stephen H. , Eppler Richard A.
Published: March 2005

The system ZnO–Fe2O3–Cr2O3 is the basis of most of the brown pigments used in the whitewares industry. The ability of iron to occur on both the tetrahedral and octahedral sites of the spinel structure leads to a versatile system yielding a variety of related shades.
Phase Equilibrium and Martensitic Transformation in Lanthana‐Doped Zirconia
Authors: BASTIDE B. , ODIER P. , COUTURES J.P.
Published: March 2005

The system ZrO2‐La2O3 has been studied in the 0 to 15 mol% La2O3 range using X‐ray diffraction and thermal analysis. Two kinds of diagrams were elaborated: First is a phase equilibrium diagram where the eutectoid decomposition of ZrO2,ss(T) → ZrO2,ss(M) + Pyss occurs at 1100°C and 0.75 mol% La2O3 (Py is the pyrochlore compound Zr2La2O7). The maximum solubility of La2O3 in ZrO2,ss(M) is 0.5 mol% at room temperature. Second, a nonequilibrium diagram is determined showing the correlations between grain size, La2O3 content, and the martensitic transformation temperature start (Ms).
Indirect Dissolution of Sapphire into Silicate Melts
Authors: SANDHAGE KENNETH H. , YUREK GREGORY J.
Published: March 2005

The forced‐convective dissolution of sapphire into CaO‐MgO‐Al2O3‐SiO2melts was investigated at 1450° and 1550°C. Spinel, MgAl2O4, formed on the sapphire under all conditions although the morphology of the spinel, which varied from individual particles to a complete layer, depended on the MgO content of the melt, the rate of rotation of the specimens in the melt, and the temperature. The thermodynamic and kinetic conditions required for formation of spinel are considered, and the factors that are important in the development of a coherent spinel layer on sapphire are discussed. Although spinel inhibits the dissolution of sapphire into silicate melts, it does not stop the dissolution. Because the dissolution process occurs in an indirect manner when a complete layer of spinel forms, this type of dissolution process is designated as indirect dissolution.
Fast Firing and Conventional Sintering of Lead Zirconate Titanate Ceramic
Authors: Baumgartner Charles E.
Published: March 2005

Conventional sintering and fast firing were examined as sintering techniques for PZT‐5 pressed compacts. Density maxima of 7.42 ± 0.05 and 7.66 ± 0.01 g/cm3 were obtained at 1350°C for conventionally sintered ceramics and at 1300°C for fast‐fired ceramics, respectively. Analysis of the ceramic obtained from these two sintering routes showed fast‐fired material to possess a three‐point fracture strength 33% greater and an average grain size almost 50% less than the conventionally sintered counterpart.
Temperature Dependence of Hardness of Alumina‐Based Ceramics
Authors: Alpert Charles P. , Chan Helen M. , Bennison Stephen J. , Lawn Brian
Published: March 2005

Hardness was measured as a function of temperature (20° to 1000°C) for several Al2O3 ceramics, including single‐crystal sapphire and polycrystalline aluminas containing different amounts of second phase. Hardness decreased steadily with increasing temperature for all materials tested, in accordance with a semiempirical relation of the form H=H0 (1 – T/T0). This behavior conformed with a thermally activated slip process, limited by Peierls stresses. At lower temperatures, the hardness values for debased aluminas were less (smaller H0) than for the pure materials, consistent with a reduction in shear modulus resulting from the “soft” phase. However, at higher temperatures the hardness values for all the aluminas converged (identical T0, i.e., material‐invariant activation energy). The latter behavior indicated that the temperature dependence of the indentation deformation was controlled predominantly by the Al2O3 component.
31P NMR Studies of Aqueous Colloidal Dispersions Stabilized by Polyphosphazene Adsorption
Authors: Exarhos Gregory J. , Ferris Kim F. , Friedrich Donald M. , Samuels William D.
Published: March 2005

The molecular interactions that act to stabilize a colloidal dispersion in solutions containing phosphazene trimers or polymers were investigated using 31P nuclear magnetic resonance spectroscopy.
Compressive Stress for Large‐Pore Removal in Sintering
Authors: Lin S.T. , German R.M.
Published: March 2005

The compressive stress for the shrinkage of a large pore during sintering has been analyzed by combining the effects of the pore size, the ratio of pore size to grain size, and the coordination number of the pore. Alumina has been selected to illustrate this model. The result shows that the compressive stress is a positive and increasing quantity during the sintering process with an increasing value depending on the rate of grain growth and pore shrinkage.
Sintering of Whisker‐Reinforced Ceramics and Glasses
Authors: Hsueh Chun‐Hway
Published: March 2005

The sintering rates of whisker‐reinforced ceramics and glasses have been calculated. The calculations reveal that retardation of sintering may occur because of the development of hydrostatic tensile stress in the matrix. The application of pressure to counteract the retardation and of an initial gap between the whisker and the matrix to delay material contact are addressed.
Determination of the Compositional Fluctuation in the Perovskite Ternary System PbZrO3–PbTiO3–Pb(Mg1/3Ta2/3)O3
Authors: Kakegawa Kazuyuki , Kawakami M. , Sasaki Y.
Published: March 2005

A method was developed for the determination of a region of compositional fluctuation in the perovskite ternary solid‐solution system PhZrO3‐PbTiO3‐Pb(Mg1/3Ta2/3)O3. The compositional fluctuation in the ternary system extends two‐dimensionally on the phase diagram. The method described in this paper utilized the fact that the compositional fluctuation region abuts on the curves of composition whose lattice spacing is the lower or higher fluctuation limit. The fluctuation limits of the lattice spacings were estimated from X‐ray diffraction analysis.
Neutron Powder Diffraction of Nb2BN1‐x
Authors: Rogl Peter , Klesnar Helmuth , Fischer Peter
Published: March 2005

The crystal structure of Nb2BN1‐x was determined from X‐ray powder photographs; precise atom parameters, atom occupancies, and atom order were derived from room‐temperature neutron powder diffraction data employing the Rietveld profile analysis method. The final reliability value of the refinement was R=0.031. Nb2BN1‐x is the first boron nitride representative of the Mo2BC‐type structure crystallizing with the space group Cmcm‐D172h No. 63. Boron atoms in triangular prismatic coordination [Nb6B] are at a distance of 0.184 nm and form zigzag chains parallel to the c axis; the angle in the boron zigzag chain is 115.4°. Nitrogen atoms occupythe octahedral voids and are coordinated by six Nb atoms. With no N‐N and no N‐B contacts, the crystal structure of Nb2BN1‐x represents the first typical transition‐metal boron nitride.
Tempering Glass with Modulated Cooling Schedules
Authors: GARDON ROBERT
Published: March 2005

Modulated quenching is a variant of the tempering process that gives one independent control over the strength and fracture characteristics of glass. Thus the strength of nondicing “heat‐strengthened” glass can be raised to that of “fully tempered” glass. Conversely, the break pattern of glass having the strength of fully tempered glass can be made to range from nondicing to fine dicing.
Theoretical Model of Phosphorus Incorporation in Silica in Modified Chemical Vapor Deposition
Authors: DiGIOVANNI D. J. , MORSE T. F. , CIPOLLA J. W.
Published: March 2005

Based on experiment, a theoretical model is developed of phosphorus incorporation in modified chemical vapor deposition. In the glass formation stage of this process, gas‐phase reaction of silicon tetrachloride and phosphorus oxychloride generates submicrometer particles of phosphorus‐doped silica. These particles deposit in a thin porous soot layer which is then viscously sintered. Equations for thermal and dopant transport through the soot voids and particles during sintering predict that gradients in each layer are due to inhibited diffusion of dopant through the gaseous voids of the soot, not to gas‐phase gradients. Thermal gradients create variations in gas and solid diffusion and dopant solubility which enhance concentration gradients. These results apply to viscous sintering of glasses produced by flame hydrolysis and sol‐gel processes.
Effects of Interfacial Films on Thermal Stresses in Whisker‐Reinforced Ceramics
Authors: HSUEH CHUN‐HWAY , BECHER PAUL F. , ANGELINI PETER
Published: March 2005

Toughening of whisker‐reinforced (or fiber‐reinforced) ceramics by whisker pullout requires debonding at the whisker/matrix interface. Compressive clamping stresses, which would inhibit interface debonding and/or pullout, are expected in composites where the matrix has a higher thermal expansion coefficient than the whisker. Because such mismatch in thermomechanical properties can result in brittle composites, it is important to explore approaches to modify the thermal stresses in composites. As a result, the effects of a film at the whisker/matrix interface on the stresses due to thermal contraction mismatch upon cooling are considered in this study. Analysis of various properties of the film are considered for the whisker/matrix systems, in particular for SiC/Al2O3, SiC/cordierite, and SiC/mullite composites. Reduction of thermomechanical stresses is shown to occur when the interfacial film has a low Young's modulus. Also, when the whisker has a lower thermal expansion coefficient than the matrix (e.g., SiC/Al2O3), the interfacial stresses generated during cooling decrease as the thermal expansion coefficient of the film increases.
Predicted Static Fatigue Behavior of Specially Coated Optical Glass Fibers
Authors: RITTER JOHN E. , SERVICE THOMAS H. , JAKUS KARL
Published: March 2005

Long‐term static fatigue behavior of optical glass fibers is controlled by their strength, fatigue resistance, and zerostress aging behavior. The effectiveness of four special coatings in preventing the long‐term static fatigue deterioration of optical glass fibers was evaluated by determining the dynamic fatigue behavior and the effects of zero‐stress aging on strength of the four specially coated optical glass fibers in water from 25° to 85°C. The results clearly show that the strength, fatigue resistance, and aging behavior varied significantly between these specially coated fibers. By analysis of these experimental results in terms of fracture mechanics principles, the predicted static fatigue behaviors of the four fibers were compared. Ideally the optimum fiber is one that exhibits a high strength, low strength variability, high fatigue resistance, and high aging resistance. Each of these specially coated fibers had a deficiency in at least one of these properties.
Destabilization of Zirconia Thermal Barriers in the Presence of V2O5
Authors: SUSNITZKY DAVID W. , HERTL W. , CARTER C. BARRY
Published: March 2005

Impurities, such as vanadium, degrade the operating performance of yttria‐stabilized zirconia thermal‐barrier coatings. V2O5 reacts preferentially with Y2O3, forms YVO4, and leads to the destabilization of zirconia thermal‐barrier material. A model experiment has been designed to monitor the destabilization of zirconia thermal barriers by directly exposing thin films of yttria‐stabilized zirconia to V2O5 vapor. The growth of YVO4 from yttria‐stabilized zirconia and the destabilization of cubic yttria‐stabilized zirconia into tetragonal and/or monoclinic zirconia polymorphs are monitored by selected‐area diffraction and energy‐dispersive X‐ray spectroscopy in the transmission electron microscope. A special crystallographic orientation relation between YVO4 and cubic zirconia is discussed.
Stereoscopic Fractography of Crack Propagation Phenomena in a TiB2—AIN Composite
Authors: Zdaniewski Wieslaw A.
Published: March 2005

Microstructural defects identified in a TiB2–AIN composite were poorly bonded agglomerates, foreign inclusions, and highly strained regions. There were also surface flaws. Fracture surfaces produced in air were highly irregular, suggesting that microcracking caused the composite to behave differently from classical brittle ceramics. Fracture at elevated temperature in Ar or in liquid Al produced the only crack branching observed. No fractographic evidence of slow crack growth in liquid Al was found. Fractographic observations support a proposed explanation of the R‐curve behavior which involves grain bridging across the crack.
Viscoelastic Analysis of Concentric Cylinders for Composite Materials and Seals
Authors: Jain Lalit K. , Wetherhold Robert C.
Published: March 2005

A viscoelastic analysis for concentric cylinders is performed for the case where an external axial load is applied. The results are applicable to glass‐to‐metal seals and as a representation for unidirectional composites. The analysis is for a general temperature and load history, and includes the important phenomenon of structural relaxation. A sample calculation is presented to illustrate the method.
Characterization of Material for Low‐Temperature Sintered Multilayer Ceramic Substrates
Authors: Hsu Jen‐Yan , Wu Nan‐Chung , Yu Shu‐Cheng
Published: March 2005

The sintering temperature of multilayer ceramic substrates must decrease to 1000° or below to avoid melting the conductors (Pd‐Ag, Au, or Cu) during sintering. In this study, SiO2, CaO, B2O3, and MgO were used as additives to Al2O3 to decrease the firing temperature by liquid‐phase sintering. Compositions with 18.0 and 22.5 wt% B2O3 were sintered at around 1000° in an air atmosphere to yield dense ceramics with good properties: relative dielectric contant between 6 to 7 (1 MHz), tan δ≤× 3 × 10−4 (1 MHz), insulating resistivity > 1014ω cm, coefficient of thermal expansion ∼ 7.0 × 10−6/°, and thermal conductivity ∼ 4.1 W/(m · K).
Strengthening of Zirconia–Alumina During Cyclic Fatigue Testing
Authors: Grathwohl Georg , Liu Tianshun
Published: March 2005

Recent fatigue results of advanced ceramics are reviewed. It is pointed out that current statements of fatigue properties often relate to specimens with long artificial cracks; in this study, a Y‐ZrO2+Al2O3 ceramic is tested under cyclic stresses using specimens with natural flaws. Cyclic fatigue is restricted to a narrow range of stresses near the ultimate strength. Survivor specimens reveal increased residual strength, possibly resulting from a higher ZrO2 phase transformation encouraged by long‐term cyclic stresses.
In Situ Polysilane‐Derived Silicon Carbide Particulates Dispersed in Silicon Nitride Composite
Authors: Riedel Ralf , Strecker Kurt , Petzow Günter
Published: March 2005

A dense (97% of theoretical density) Si3N4—SiC composite containing 10 wt%β‐SiC was prepared by introducing a SiC phase by the pyrolysis of a polymeric SiC precursor. The composite material was produced by mixing an alkyl/aryl‐substituted polysilane with Si3N4 powder and, by subsequently forming green compacts, pyrolyzing the polymeric species, and finally sintering the sample. Synthesis and characterization of the polymeric compound was described. Its transformation reactions to SiC and the characterization of the ceramic residue were also studied. High ceramic yields were obtained by curing the as‐synthesized polysilane at 500°C in an Ar atmosphere. The heat treatment had no effect on the good solubility of the polymeric precursor in organic solvents. This was important for processes such as infiltration, sealing, and coating and for the mixing of the polymer with powders for the preparation of homogeneous composite ceramics. The dense microstructure of the pyrolyzed and sintered Si3N4 powder–polysilane mixture exhibited reduced grain growth of the Si3N4 particles and a very homogeneous distribution of the in situ‐formed β‐SiC phase.
Viscoelastic Effect of Heat Treatment on the Fracture Toughness of Metal‐Particulate/Glass‐Matrix Composites
Authors: Jessen Todd L. , Mecholsky John J.
Published: March 2005

The fracture toughness (KIc) of a 90/10 glass‐matrix/metal‐particulate composite was measured as a function of treatment temperature and hold time. The toughness was 20% higher at a heat‐treatment temperature 15°C below Tg than at the other four treatment temperatures examined. KIc also varied as a function of time at temperature. The volume fraction of exposed particulate phase on the fracture surface was proportional to the increase in the fracture toughness.
Subsolidus Phase Relations in the Pseudoternary System SrO·6M2O3 (M = Fe, Cr, Al) in Air
Authors: Sandiumenge Felip , Galí Salvador
Published: March 2005

Crystalline powder samples belonging to the pseudoternary system SrO·6M2O3 (M = Fe, Al, Cr) were synthesized in air. The limit of the magnetoplumbite‐type (M‐type) phase solid solution is governed by the presence of Cr3+ in the structure and is accompanied by the formation of a chromate and a sesquioxide. The extension of the solid solution in the Cr‐containing pseudobinaries depends to a large extent on the crystallographic site preference of the host cation. The extension of the M‐type solid solution is also narrowed at a lower temperature.
Statistical Determination of Plating Process Variables for Multilayer Ceramic Chip Capacitors
Authors: Benm Doug A. , Feltz Carol J. , Haynes Richard , Pinault Steven C.
Published: March 2005

The effects of the following barrier layer plating process variables on reliability of multilayer ceramic chip capacitors are discussed: media type, nickel plating thickness, lead concentration, pH, tin plating current density, number of capacitors, and percent loading of the plating barrel. The dielectric was formulated from a lead magnesium niobate material. Evaluations were done with respect to capacitance, dissipation factor, conductance, dry life (temperature bias) test, solderability (with and without steam aging), and voltage surge test. Several of the in‐process variables studied were found to have a statistically significant effect on some of these response variables. The data indicate that the most significant inprocess parameters in the barrier layer process, of those included in this study, are media type and percentage loading of the barrel. Variables of marginal significance are number of capacitors, lead in the bath, and tin plating current density. The other variables had no discernible effect on the response variables studied. The presence of such a large experimental error in life test was a significant outcome of the study and indicates a need for further study on means of obtaining reproducible life‐test results after plating.
Electrode‐Based Causes of Delaminations in Multilayer Ceramic Capacitors
Authors: Pepin John G. , Borland William , O'Callaghan Patrick , Young Richard J. S.
Published: March 2005

Delaminations are a principal quality problem in the manufacture of multilayer ceramic capacitors (MLC's). They are defined as a separation of the electrode and dielectric layers and can result in electrical shorts and/or life failures. Delaminations originate from many sources in MLC manufacture, but we have identified four which are caused by the electrode. High levels of organic resin in the electrode paste lead to high resin content in dried electrode prints, requiring removal of large amounts of organic residues during burnout, causing delaminations. Catalysis of these organics by the precious metals also causes delaminations from rapid evolution of gas and heat release during burnout. Poor adhesion of the dielectric tape layers to dried electrode prints during MLC buildup can cause “green‐state” delaminations which remain through firing. Sintering shrinkage mismatch between the electrode and dielectric layers leads to internal stresses in MLC's, resulting in delaminations. We discuss these electrode‐based mechanisms of delamination and the design of electrode pastes which solve these problems.
Analysis of Heat Transfer in the Corroding Sidewall of a Furnace
Authors: Choudhary Manoj K.
Published: March 2005

A mathematical formulation is developed to calculate the rate of corrosion of a refractory sidewall of a furnace in the presence of free convection in the melt. The free‐convection phenomena considered are caused by the dependence of the melt density on its temperature and composition. The formulation involves the statement of equations for heat conduction in the refractory wall and movement of the solid‐melt interface. The governing equations and the boundary conditions are transformed into nondimensional forms, and the dimensionless parameters which characterize corrosion in the presence of free convection are identified. The equations are solved by using coordinate transformation and a finite‐difference method. Calculated results are presented on the shape of the solid‐melt interface, temperature distribution in the corroding wall, and the amount of cooling at the outside surface of the wall. The results show how free convection in the melt interacts with heat conduction in a refractory wall to determine the rate of corrosion of the wall.
Comparison of the Effects of Titania and Tantalum Oxide Nucleating Agents on the Crystallization of Li2O · Al2O3· 6SiO2 Glasses
Authors: Hsu Jen‐Yan , Speyer Robert F.
Published: March 2005

The crystallization mechanism in Li2O‐Al2O3‐SiO2 (1 : 1 : 6) glasses was investigated with TiO2 or Ta2O5 as nucleation agents, by differential thermal analysis, X‐ray diffractometry, and transmission electron microscopy. Phase separation occurred prior to crystallization in all of the glasses studied. Both TiO2 and Ta2O5 additives promoted nucleation and crystal growth in the bulk glasses by causing the precipitation of crystalline precursor phases dispersed within phaseseparated glass, which acted as heterogeneous nucleation sites for β‐quartz(ss) (solid solution) formation. Upon reheating glasses with Ta2O5as the nucleating agent, phase separation occurred, followed by LiTa3O8 and Ta2O5 nucleation and limited growth as the first‐formed crystalline phases, and then crystallization of β‐quartz structured solid solution, which nucleated and grew from the LiTa3O8/Ta2O5 crystallites. Tantalum was rejected from the growing β‐quartz(ss), further precipitating Ta2O5. Eventually, β‐quartz(ss) transformed to β‐spodumene(ss) with increasing temperature. Upon reheating glasses nucleated with TiO2 additive, they initially phase separated, followed by the formation of crystalline precursors, in turn followed by the near simultaneous crystallization of β‐quartz(ss) and β‐spodumene(ss), transforming to solely β‐spodumene(ss) with increasing temperature [Key words: glass, titania, tantalum oxide, nucleation, crystallization.]
Promotion of Densification by Grain Growth
Authors: Xue Liang An , Brook Richard J.
Published: March 2005

Comparison of the microstructural changes and of the densification kinetics during the sintering of doped and undoped BaTiO3 ceramics shows the existence of an increase in densification rate at the point of onset of abnormal grain growth. It is suggested that the increase is attributable to an enhancement of the driving force for densification brought about by the surface curvature changes when a matrix pore makes contact with a growing abnormal grain.
Strength Distribution of Reinforcing Fibers in a Nicalon Fiber/Chemically Vapor Infiltrated Silicon Carbide Matrix Composite
Authors: Eckel Andrew J. , Bradt Richard C.
Published: March 2005

The strength distribution of fibers within a two‐dimensional laminate ceramic/ceramic composite consisting of an eight harness satin weave of Nicalon continuous fibers within a chemically vapor infiltrated SiC matrix was determined from analysis of the fracture mirrors of the fibers. Comparison of the fiber strengths and the Weibull moduli with those for Nicalon fibers prior to incorporation into composites suggests that possible fiber damage may occur either during the weaving or during another stage of the composite manufacture. Observations also indicate that it is the higher‐strength fibers which experience the greatest extent of fiber pullout and thus make a larger contribution to the overall composite toughness than do the weaker fibers.
Solubility and Aging of Calcium Silicate Hydrates in Alkaline Solutions at 25°C
Authors: Macphee Donald E. , Luke Karen , Glasser Fred P. , Lachowski Eric E.
Published: March 2005

Calcium silicate hydrate (C‐S‐H) gels are the principal bonding material in portland cement. Their solubility properties have been described, enabling pH and solubilities to be predicted. However, the gels also interact with other components of cements, notably alkalis. C‐S‐H has been prepared from lime and silicic acid in solutions of sodium hydroxide or potassium hydroxide and by the hydration of tricalcium silicate (C3S) in sodium hydroxide solutions. Analyses of aqueous phases in equilibrium with 85 gels show that the aqueous calcium and silicon concentrations fit smooth curves over the range of increasing sodium concentrations. Where anomalous data occur, they correspond to solids with low lime contents: such gels are tentatively assumed to fall into a region where the presence of another gel phase influences the aqueous composition. Dimensional changes have been observed in the hydration products of C3S as a function of alkali content and these may be relevant to the alkali‐silica reaction. The significance of this and other data is discussed with reference to real cement systems.
Impurities in the Combustion Synthesis of Titanium Carbide
Authors: Kecskes L. J. , Niiler A.
Published: March 2005

The evolution of solid and gaseous impuritics from powders and their effects during the combustion synthesis (self‐propagating high‐temperature synthesis) of titanium carbide have been studied. First, the volatiles from the precursor powders, released during bakeout in a vacuum furnace, were measured by residual gas analysis. Second, both condensable and non‐condensable materials emitted during the reaction of the Ti + C system were analyzed. The results indicate that the major noncondensing species in both cases were water vapor, hydrogen, carbon monoxide, carbon dioxide, and hydrocarbons. Significant amounts of oxygen and TiO2 were also enitted during the reaction, but were not observed during bakeout. These impurities are shown to affect not only the reaction process, but also the microstructure of the reaction product.
Wear Perfomance of Ceramics in Ring/Cylinder Applications
Authors: Dufrane Keith F.
Published: March 2005

In support of efforts to apply ceramics to advanced heat engines, a study was performed of the wear mechanisms of ceramics at the ring/cylinder interface. A laboratory apparatus was constructed to reproduce most of the conditions of an actual engine. Ring and cylinder specimens having a simple geometry were used to facilitate their fabrication. Attempts to operate three zirconias, α‐SiC, Si3N4, and several plasma‐sprayed coatings without lubrication were unsuccessful because of high friction and wear rates. The ceramics responded well to liquid lubricants at a temperature of 260°C. Plasma‐sprayed coatings of Cr2O3 and hypersonic flame‐sprayed coatings of Co‐bonded WC performed particularly well as ring coatings. Similar performance was obtained with these coatings operating against SiC, Si3N4, SiC‐whisker‐reinforced Al2O3, and Cr2O3 coatings. The study demonstrated the critical need for lubrication.
Thermodynamics of Densification: II, Grain Growth in Porous Compacts and Relation to Densification
Authors: Lange F. F. , Kellett* Bruce J.
Published: March 2005

It is shown that grain‐boundary motion in a sintering array of particles can be limited by the geometry of the sintered particle network. Despite its curvature, the grain boundary can encounter an energy barrier when it attempts to move through smaller grains toward its center of curvature. This energy barrier decreases to zero when the radius ratio (R) between adjacent grains approaches a critical value, R→Rc=− [cos (Ψe)]−1, where Ψe is the dihedral angle relating surface and grain‐boundary energies. The change in radius ratio (R→Rc) can occur by interparticle mass transport, viz., coarsening. This result suggests grain growth kinetics within a sintering array can be controlled by coarsening kinetics, making it distinctly different from grain growth in the fully dense material which occurs by boundary motion. More significantly, it is shown that when a grain disappears from between larger grains by coarsening, the driving force for sintering between the newly joined grains is reinitiated. That is, grain growth via coarsening can continually drive sintering and array shrinkage. It is shown that coarsening by itself does not lead to significant array shrinkage, but can promote significant shrinkage when sintering is reinitiated. Densification is therefore expected to be predictably related to grain growth, and thus coarsening kinetics. It is shown that the relative density of a ring of spheres that undergoes cyclic coarsening and sintering is nearly linearly related to grain size. Conceptual relations between density and grain growth for real powder compacts are discussed and related to the function that describes the compact's distribution of pore coordination numbers. Finally, since coarsening can be the rate‐limiting step for densification, it is suggested that theory and related experiments associated with densification kinetics should be reevaluated.
The Role of Thermal Shock on Tool Life of Selected Ceramic Cutting Tool Materials
Authors: Wayne Steven F. , Buljan Sergej‐Tomislav
Published: March 2005

The introduction of Si3N4‐ and Al2O3‐based tools has greatly increased the speed of metal removal and the productivity of metal‐cutting operations. High‐speed machining elevates the temperature at the tool/workpiece interface and subjects the tool to considerably higher stress, resulting from increased mechanical forces and thermal gradients. Under these circumstances, the thermal shock resistance of tool materials would be expected to play a more important role in determining the tool life and should be more prominently considered in both material design and applications. It was of interest to devise a series of controlled thermal shock machining tests and to assess the modes of ceramic cutting‐tool wear. The resistance of ceramic cutting‐tool materials to wear or fracture under these conditions is discussed in terms of thermal shock parameters.
Evaluation of Strut Strength in Open‐Cell Ceramics
Authors: Brezny Rasto , Green David J. , Dam Chuong Quang
Published: March 2005

The strength of cellular materials is dependent on the strength of the solid making up the cellular structure, but this parameter is often difficult to quantify. In order to better evaluate these materials, a technique has been developed to measure the strength ofthe cell struts in open‐cell ceramics. The strut strength was measured in two commercially available, open‐cell ceramics and evaluated in terms of a two parameter Weibull distribution. The strut strength distribution was found to be very wide with the Weibull modulusin the range 1 to 3. In addition, it was found that the strut strength was invariant withdensity (at constant cell size) but could be dependent on cell size. This behavior was inqualitative agreement with the failure statistics of brittle materials, once the variations in the microstructural geometry are understood. Based on these data and the observations of flaws within the struts, it is clear that careful processing of these materials could significantly improve the strut strength distribution, in terms of reducing its width and increasing its magnitude. Such microscopic improvements would lead to similar benefits in the strength and toughness of the bulk cellular ceramics.
Advanced Porous Coating for Low‐Density Ceramic Insulation Materials
Authors: Leiser Daniel B. , Churchward Rex , Katvala Victor , Stewart David , Balter Aliza
Published: March 2005

The need for improved coatings on low‐density reusable surface insulation (RSI) materials used on the space shuttle has stimulated research into developing tougher coatings. The processing of a new porous composite “coating” for RSI called toughened unipiece fibrous insulation is diseassed. Characteristics including performance in a simulated high‐speed atmospheric entry, morphological structure before and after this exposure, resistance to impact, and thermal response to a typical heat pulse are described. It is shown that this coating has improved impact resistance while maintaining optical and thermal properties comparable to the previously available reaction‐cured glass coating.
Radiophase Development in Hot‐Pressed Alkoxide‐Derived Titanate Ceramics for Nuclear Waste Stabilization
Authors: Dickson Fiona J. , Mitamura Hisayoshi , White Timothy J.
Published: March 2005

Phase development as a function of hot‐pressing temperature was studied in alkoxide‐derived titanate‐based ceramics doped with a 10 wt% loading of a sodium‐rich (NAR) and a sodiumpoor (NAP) simulated high‐level waste. Pyrochlore was found to be the most abundant phase in both calcine powders. A pseudobrookite phase existed metastably at hot‐pressing temperatures between 890° and 920°C. After hot‐pressing at 1100°C, the final phase assemblage for the NAP material consisted of zirconolite, hollandite‐type, perovskite, alloy, and reduced rutile (Magnéli phases). In addition, NAR samples contained hibonite, freudenbergite, and loveringite. Phase development was driven to completion over a very narrow temperature range (50°C), beginning at 870° and 850°C for NAP and NAR, respectively, although full densification was not achieved below 1100°C. Both waste forms exhibited comparable microstructure and aqueous durability.
Porous Glass‐Ceramic in the CαO–TiO2–P2O5 System
Authors: Hosono Hideo , Zhang Zhibo , Abe Yoshihiro
Published: March 2005

A porous glass‐ceramic in the CaO–TiO2—P2O5 system has been prepared by crystallization and subsequent chemical leaching of the corresponding glass. By applying a two‐step heat treatment to 45CaO · 25TiO2· 30P2O5 glasses containing a few mol% of Na2O, volume crystallization results in the formation of dense glass‐ceramics composed of CaTi4(PO4)6 and β‐Ca3(PO4)2 phases. By leaching the resultant glass ceramics with HCI, β‐Ca3(PO4)2 is selectively dissolved out, leaving a crystalline CaTi4(PO4)6 skeleton. The surface area and mean pore radius of the porous glass‐ceramics were approximately 40 m2/g and 13 nm, respectively.
Continuous Processing of Monodispersed Titania Powders
Authors: Ogihara Takashi , Ikeda Masaki , Kato Masanori , Mizutani Nobuyasu
Published: March 2005

A new continuous precipitation process for monodispersed ceramic powders using a static mixer was developed. Monodispersed TiO2 powders were prepared continuously with high reproducibility and reliability by controlled hydrolysis of titanium ethoxide. Important steps were uniform and rapid mixing and aging using hydroxypropylcellulose as a dispersant. The concentrations of titanium ethoxide and water suitable to production of monodispersed TiO2 powders lay within a limited range. A minimum aging period of 10 min was required to obtain monodispersed powders.
High‐Temperature Microstructure of a Hot‐Pressed Silicon Nitride
Authors: Clarke David R.
Published: March 2005

The outstanding question as to the microstructure of silicon nitride at temperatures associated with potential high‐temperature applications of the material is addressed experimentally by quenching thin (transmission electron microscopy) samples from 1450°C and examining them in the microscope. The morphology of the microstructure is qualitatively unchanged compared to the materials slowly cooled, for example, after hot‐pressing, to room temperature. The most significant difference is that the thickness of the intergranular phase is larger, typically 2 to 10 nm, as compared to the ∼ 1 nm observed in the hot‐pressed material. In addition there is an apparent increase in the volume fraction of the intergranular phase at the three‐grain junctions. On the basis of a number of supporting experiments including both hot‐stage transmission electron microscopy (up to 1000°C) and Auger electron spectroscopy of material fractured and examined at 850°C, the change in microstructure is concluded to occur at temperatures above about 1000°C.
Mechanical Properties of Polycrystalline Lithium Orthosilicate
Authors: Chu Cha‐Yang , Bär Kai , Singh Jitendra P. , Goretta Kenneth C. , Billone Michael C. , Poeppel Roger B. , Routbort Jules L.
Published: March 2005

Room‐temperature mechanical properties and high‐temperature creep deformation of lithium orthosilicate (Li4SiO4) were studied. Elastic constants, flexural strength, and fracture toughness were determined for specimens with densities between 68% and 98% of theoretical. Critical quenching temperature and thermal‐shock resistance parameters for 90% dense specimens were also measured. High‐temperature creep deformation was investigated by a constant‐strain‐rate test in an argon atmosphere at temperatures between 750° and 1025°C and strain rates ranging from about 10−6 to 10−3 s−1. At 950°C and above, the stress exponent, n, was determined to be 3.6, with a creep activation energy of 715 kJ/mol. Selected results obtained for Li4SiO4 are compared with results obtained for other Li‐containing ceramics that are under consideration as candidates for fusion reactor breeder blankets.
Hot Isostatic Press Sintering and Properties of Silicon Nitride without Additives
Authors: Tanaka Isao , Pezzotti Giuseppe , Okamoto Taira , Miyamoto Yoshinari , Koizumi Mitue
Published: March 2005

Fully densified silicon nitride without additives was fabricated by means of hot isostatic pressing. The sintering process of highly pure powder was investigated with special interest in the evolution of α–β phase transformation, densification, and microstructure development. It was observed that the transformation occurred without a liquid phase below 1730°C, which corresponds to the melting point of SiO2. Above 1730°C, the densification and β‐grain elongation accelerated concurrently because of the appearance of liquid SiO2. However, full densification was attained at 1950°C together with marked grain growth. Flexural strength, microhardness, fracture toughness, and Young's modulus of sintered bodies were measured as a function of temperature. In the sintered body started from highly pure powder, excellent MOR behavior was found up to 1400°C. Impurity content of a few hundred ppm was found to be sufficient to make densification easy and to degrade high‐temperature strength.
Comparison of Mechanical and Dielectric Strength Distributions for Variously Surface‐Finished Titanium Dioxide Ceramics
Authors: Kishimoto Akira , Koumoto Kunihito , Yanagida Hiroaki
Published: March 2005

Mechanical and dielectric strength distributions were studied for variously surface‐finished TiO2 ceramics. Roughly finished specimens showed a small Weibull modulus for both kinds of failures, and clear analogies between mechanical and dielectric strength distributions were found in both rough‐and fine‐surface specimens. The validity of the dielectric strength measuring method as an alternative method to evaluate the structural reliability of brittle ceramics is discussed.
Effect of Volatilization on the Conduction Behavior of Acceptor‐Doped Chromium Oxide
Authors: Huang Rong‐Fong , Anderson Harlan U.
Published: March 2005

The effect of volatilization on the conduction behavior of Li‐ or Mg‐doped Cr2O3 was studied. The Po2 dependence on conduction changed from 1/4 to 3/8 when volatilization occurred. The high‐temperature conduction mechanism of Cr2O3 may be dominated by the formation of Cr vacancies due to the volatilization effect.
Sintering of Zinc Oxide Doped with Antimony Oxide and Bismuth Oxide
Authors: Kim Jinho , Kimura Toshio , Yamaguchi Takashi
Published: March 2005

The phase change, densification, and microstructure development of ZnO doped with both Bi2O3 and Sb2O3 are studied to better understand the sintering behavior of ZnO varistors. The densification behavior is related to the formation of pyrochlore and liquid phases; the densification is retarded by the former and promoted by the latter. The pyrochlore phase, whose composition is Bi3/2ZnSb3/2O7, appears below 700°C. The formation temperature of the liquid phase depends on the Sb/Bi ratio: about 750°C for Sb/Bi < 1 by the eutectic melting in the system ZnO—Bi2O3, and about 1000°C for Sb/Bi > 1 by the reaction of the pyrochlore phase with ZnO. Hence, the densification rate is determined virtually by the Sb/Bi ratio and not by the total amount of additives. The microstructure depends on the sintering temperature. Sintering at 1000°C forms intragrain pyrochlore particles in ZnO grains as well as intergranular layers, but the intragrain particles disappear at 1200°C by the increased amount of liquid phase, which enhances the mobility of the solid second phase.
Preparation of Void‐Free Calcium Phosphate Glass‐Ceramics
Authors: Watanabe Akira , Mitsudou Makoto , Kihara Seiji , Abe Yoshihiro
Published: March 2005

Voids frequently form on crystallization of calcium phosphate glasses because of the density difference between the original glass and the resulting glass‐ceramic. However, the calcium phosphate glass‐ceramic with a composition of 40CaO · 50P2O5· 10Al2O3 (mol%) has a density nearly equal to that of the original glass; this is caused by the formation of AlPO4, which has a lower density than Ca(PO3)2. The glass‐ceramic, being void‐free, shows improved mechanical properties.
Effects of Material Transfer by Liquid and Vapor on the Microstructure of Sintered Y‐Ba‐Cu‐O Superconductors
Authors: Cho Seog‐Hyun , Kim Doh‐Yeon , Khim Zheong‐G
Published: March 2005

Effects of a slight compositional change on the microstructure of sintered Y‐Ba‐Cu‐O superconducting ceramics were investigated. The compositional change was provided either by the liquid which has been formed at the hot end of the rectangular bar specimen sintered under gradient temperature or by the vapor from Y‐Ba‐Cu‐O melt. The results indicated that the Cu and Ba vapor pressure of the sintering atmosphere must be closely controlled to obtain an enhanced densification and a desired single‐phase microstructure.
Structural Investigation of a Rapidly Quenched 20Li4SiO4·80Li2WO4 Glass
Authors: Takahashi Masanari , Toyuki Hiroshi , Tatsumisago Masahiro , Minami Tsutomu , Umesaki Norimasa , Iwamoto Nobuya
Published: March 2005

A 20Li4SiO4·80Li2WO4 glass was prepared by the rapid quenching technique, and its structure was investigated by X‐ray diffraction (XRD) and Raman scattering. The coordination number of the oxygen atoms around a tungsten atom, NO/W, was determined to be 4.8 by XRD and 4.6 by Raman scattering. The deviation of NO/W from 4 resulted from the coexistence of condensed structural units, such as WO6, Si2O6−7 ions containing bridging oxygens, WO2−4 ions, and SiO4−4 ions.
Effect of Bismuth of Oxide Content on the Sintering of Zinc Oxide
Authors: Kim Jinho , Kimura Toshio , Yamaguchi Takashi
Published: March 2005

Densification and microstructure de velopment in Bi2O3‐doped ZnO have been studied with a special emphasis on the effect of the Bi2O3 content. A small amount of Bi2O3 in ZnO (0.1 mol%) retarded densification, but the addition of Bi2O3 to more than 0.5 mol% promoted densification by the formation of a liquid phase above the eutectic temperature (∼740°C). The liquid phase increased grain‐boundary mobility, which was responsible for the formation of intragrain pores and the decrease in the sintered density. The increase in the Bi2O3 content increased the probability of the formation of skeleton structure, which reduced the grain growth rate and the sintered density.
Preparation of 1‐2‐3 Superconductors from Hydroxide–Carbonate Coprecipitation
Authors: Morgan Douglas , Maric Milena , Luss Dan , Richardson James T.
Published: March 2005

A batch coprecipitation of 1‐2‐3 superconductor precursor powder was accomplished by adding an aqueous solution of mixed nitrates rapidly to a mixture of sodium or potassium hydroxide and carbonate. The blue hydrogel, an intimate mixture of Y(OH)3, BaCO3, and Cu(OH)2, was filtered and washed by decantation, using water sparged with CO2 to control the pH. After a second filtration, drying and calcination, the product was a homogeneous powder, with an average particle size of about 1 μm, free of alkali, and with excellent superconducting properties. The process is reproducible, inexpensive, and easy to scale to higher production rates. Important process parameters are the following: the time and temperature of precipitation; alkali ratios; volume, temperature, and pH of the wash water; and time of calcination.