bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemical Vapor Infiltration of TiB2‐Matrix Composites
Authors: Besmann Theodore M. , Miller James H. , Cooley Kevin M. , Lowden Richard A. , Starr Thomas L.
Published: March 2005

The efficiency of the Hall–Heroult electrolytic reduction of aluminum can be substantially improved by the use of a TiB2 cathode. The use of TiB2 components, however, has been hampered by the brittle nature of the material and the grain boundary attack of sintering‐aid phases by molten aluminum. In the current work, TiB2 is toughened through the use of reinforcing fibers, with chemical vapor infiltration used to produce the TiB2 matrix. In early efforts it was observed that the formation of TiB2 from chloride precursors at fabrication temperatures below 900–1000°C may have allowed the retention of destructive levels of chlorine. At higher fabrication temperatures (>1000°C), using appropriate infiltration conditions as determined from the use of a process model, TiB2/THORNEL P‐25 fiber composites have been fabricated in 20 h. The improved composite material has been demonstrated to be stable in molten aluminum in short‐duration (24 h) tests.
Determination of Crack Velocity as a Function of Stress Intensity from Static Fatigue Data
Authors: Gupta Prabhat K. , Inniss Daryl , Kurkjian Charles R. , Brownlow Darryl L.
Published: March 2005

The estimation of lifetime of high‐strength (≥1 GPa) glass samples with small cracks at low stress levels (≤ 150 MPa) requires a knowledge of the crack velocity, V, as a function of the stress intensity factor, K, in the range of V values lower than the current limit of measurements (about 10 –11 m/s). It is shown that V(K) data in this range can be obtained directly from static fatigue measurements without assuming a priori any form for V(K). The method has been applied to the static fatigue data of Proctor, Whitney, and Johnson on silica fibers, and values of V in the range of 10–11 to 10–16 m/s (corresponding to K values of 0.35 to 0.20 Mpa·m0.5) are reported.
Osmotic Consolidation of Suspensions and Gels
Authors: Miller Kelly T. , Zukoski Charles F.
Published: March 2005

An osmotic method for the consolidation of suspensions of ceramic particles is demonstrated. Concentrated solutions of poly(ethylene oxide) are separated from a suspension of ceramic particles by a semipermeable membrane, creating a gradient in solvent chemical potential. Solvent passes from the suspension into the polymer solution, lowering its free energy and consolidating the suspension. Dispersions of stable 8‐nm hydrous zirconia particles were consolidated to over 47% by volume. Suspensions of α–alumina in three states of aggregation (dispersed, weakly flocculated, and strongly flocculated) were consolidated to densities greater than or equal to those produced in conventional pressure filtration. Moreover, the as–consolidated alumina bodies were partially drained of fluid during the osmotic consolidation process, producing cohesive partially dried bodies with improved handling characteristics.
Activity of NiO in (Ni,Mg)O Solid Solutions
Authors: Kreidler Eric R. , Park Hee Dong
Published: March 2005

Disagreement exists in the literature concerting the activity of NiO in (Ni,Mg)O solid solutions. Ideal behavior, as well as strong positive deviations and strong negative deviations from ideality, has been reported. In the present study, the activity of NiO was determined by electromotive force measurements using the solid‐state cell Pt/Ni‐NiO/0.92ZrO2‐0.08Y2O3/Ni‐(Ni,Mg)O/Pt. The compositions studied had NiO mole fractions of 0.1,0.3,0.5,0.7, and 0.9. In agreement with theoretical expectations, the solid solutions exhibited ideal behavior at 882° and 982°C. Possible reasons for the conflicting results in the literature arc presented.
Influence of Nd2O3 Doping on the Reaction Process and Sintering Behavior of BaCeO3 Ceramics
Authors: Lu Chung‐Hsin , De Jonghe Lutgard C.
Published: March 2005

The influence of Nd2O3 doping on the reaction process and sintering behavior of BaCeO3 is investigated. Formation of BaCeO3 is initiated at 800°C and completed at 1000°C. When Nd2O3 is added to the starting materials, the formation of BaCe1–xNdxO3–δ is delayed and the temperature for complete reaction is increased to 1100°C. Only a BaCe1‐xNdxO3–δ solid solution with an orthorhombic crystal structure is present in the specimens for x ≤ 0.1. A secondary phase rich in Ce and Nd is formed within grains and at grain boundaries, when the Nd2O3 content is greater than the solubility limit (x ≥ 0.2). Pure BaCeO3 is difficult to sinter, even at 1500°C, and only a porous microstructure could be obtained. However, doping BaCeO3 with Nd2O3 markedly enhances its sinterability. The enhancement of the sinterability of Nd2O3‐doped specimens at x ≤ 0.1 is attributed to the increase in the concentration of oxygen ion vacancies, which increases the diffusion rate. At x ≥ 0.2, the grain size is abnormally coarsened, which is caused by the formation of a liquid phase. While this liquid phase accelerates sintering, its beneficial effect on densification is counteracted by the segregation of the secondary grain‐boundary phase which inhibits sintering.
Thermal Degradation Behavior of WOP2O7/Phenanthroline Complex
Authors: Oya Asao , Saito Tsutomu , Otain Sagio
Published: March 2005

An intercalation compound of WOP2O7/phenanthroline and a mixture of WOP2O7/carbon black were prepared to have equal carbon contents. Both samples were heated below 1400°C under a nitrogen atmosphere and their thermal degradation behaviors compared, with the following results: (1) CO gas evolved through carbothermal reduction at a temperature 150°C lower in the intercalation compound than in the WOP2O7/carbon black mixture. (2) Both WP and W formed at temperatures approximately 100°C lower in the intercalation compound than in the mixture. (3) Thermal degradation occurred homogeneously throughout the intercalation compound particles, and the particle morphology remained favorable after thermal degradation.
Tensile Properties of Alumina Fibers Using Hot‐Grips
Authors: Ünal Özer , Lagerlöf K. Peter D.
Published: March 2005

Tensile testing of single‐crystal Al2O3 fibers was conducted using a hot‐grip method developing during this study. Fracture strength was measured between 25° and 1500°C and was found to decrease with increasing temperature. However, a strength minimum, observed by others between 400° and 900°C, appeared to be absent The experimental results are in good agreement with calculated fracture strength values based on the fracture toughness of cleavage planes up to 1200°C. Strain rate experiments carried out at 1300°C indicate that strength is strongly time dependent. Fracture originates from both surface flaws and internal pores in the fibers.
Raman Spectrometric Determination of the Oxygen Self‐Diffusion Coefficients in Oxides
Authors: Kim Byung‐Kook , Park Soon Ja , Hamaguchi Hiro‐o
Published: March 2005

New methods of determining the oxygen self‐diffusion coefficients (D*o) in oxides have been developed using Raman spectroscopy combined with the 16O–18O exchange technique. From the depth‐profiles of the 18O concentration in the 16O–18O exchanged oxides, which was measured by Raman microscope with a spatial resolution of 5 μm, D*o was determined for 2.8 mol% Y2O3‐containing tetragonal zirconia polycrystall (the depth‐profile method). Thus‐obtained results are expressed as D*O,D‐P= 1.82(+0.41−0.40) × 10−1·exp{−(139.3 ± 0.2) [kJ/mol]/RT} [cm2/s] in the temperature range of 700–950°C. We also determined D*o for the same sample from the Raman spectrometric monitoring of the ambient gas during the 16O–18O exchange reaction (the gas‐monitoring method). Thus‐obtained results are expressed as D*O,G‐M= 1.14(+0.05−0.04) × 10−2 exp{−(117.5 ± 0.4) [kJ/mol]/RT} [cm2/s] in the temperature range of 700–1165°C. The results obtained from the above two different methods virtually agree with each other, indicating that reliable D*o can be obtained by either of these two methods. We demonstrate that Raman spectroscopy is a useful tool for determining D*o in oxides.
Calculation of the Thermal Conductivity and Gas Permeability in a Uniaxial Bundle of Fibers
Authors: Skamser Daniel J. , Bentz Dale P. , Coverdale R. Tate , Spotz Mark S. , Martys Nick , Jennings Hamlin , Johnson D. Lynn
Published: March 2005

A model of the local microstructure of a bundle of fibers is simulated and used as the basis for calculations of transport properties. This, in turn, can be used in a macroscopic model of the chemical vapor infiltration process. An expanding/overlapping circle representation of the micro‐structure simulates the deposition of matrix in a uniaxial bundle of fibers. An iterative heat conduction algorithm is used to calculate the transverse thermal conductivity based on the thermal conductivities of the solid and gas phases. The permeability of gas through the microstructure is calculated for flow both parallel and transverse to overlapping cylinders using a Stokes equation and assuming a Darcy's law behavior. Both the simulations of the microstructure and associated calculations of the transport properties compare favorably with experimental data. Darcy's law for the behavior of gas in a bundle of fibers is shown to be valid for gas pressures of 5–13 kPa.
Comparison of the Creep and Creep Rupture Performance of Two HIPed Silicon Nitride Ceramics
Authors: Ferber Mattison K. , Jenkins Michael G. , Nolan Theodore A. , Yeckley Russell L.
Published: March 2005

Measurements of the tensile creep and creep rupture behavior were used to evaluate the long‐term mechanical reliability of a commercially available and a developmental hot isostatically pressed (HIPed) silicon nitride. Measurements were conducted at 1260° and 1370°C utilizing button–head tensile specimens. The stress and temperature sensitivities of the secondary creep rates were used to estimate the stress exponent and activation energy associated with the dominant creep mechanism. The stress and temperature dependencies of creep rupture life were determined by continuing individual creep tests to specimen failure. Creep deformation in both materials was associated with cavitation at multigrain junctions. Two‐grain cavitation was also observed in the commercial material. Failure in both materials resulted from the evolution of an extensive damage zone. The failure times were uniquely related to the creep rates, suggesting that the zone growth was constrained by the bulk creep response. The fact that the creep and creep rupture behaviors of the developmental silicon nitride were significantly improved compared to those of the commercial material was attributed to the absence of cavitation along two‐grain junctions in the developmental material.
Sol–Gel Preparation and Optical Nonlinearity of Lead(ll) Oxide–Titanium(IV) Oxide Amorphous Monoliths
Authors: Katagiri Yuji , Nasu Hiroyxiki , Matsuoka Jun , Kamiya Kanichi
Published: March 2005

PbO–TiO2 amorphous monoliths with TiO2 content from 60 to 80 mol% were prepared by fbc sol–gel method using Pb–Ti complex alkoxides synthesized from Pb(CH3,COO)2·3H2O and Ti(O·C3H7ia)4. The thermal evolution process of the gel was examined by means of DTA/TG, IR spectros–copy, and X–ray diffraction techniques. Dehydrated, transparent, amorphous monoliths were obtained in the temperature range from 350° to 550°C, depending on the composition. The refractive index of those amorphous monoliths was higher than 2.0, and the third–order optical nonlinear susceptibility, X(3), evaluated from third harmonic generation (THG) was on the order of 10‐12 esu.
Diffusional Crack Growth and Creep Rupture of Silicon Carbide Doped with Alumina
Authors: Ohji Tatsuki , Yamauchi Yukihiko
Published: March 2005

This study deals with tensile creep and crack growth behavior of silicon carbide doped with alumina at 1400°C. Excellent creep resistance was observed for stresses from 150 MPa to 200 MPa. From the creep exponent of 1.4 and the activation energy of 320 kj/mol, the principal creep mechanism was Coble creep. The creep failure was caused by slow crack growth from a preexisting flaw. The crack was found to grow subcritically along grain boundaries almost in isolation. The relation between the time–to–failure and the applied stress was well treated by a diffusive crack growth model, and the threshold stress of this material at 1400°C was estimated at 165 MPa.
Effect of Microstructural Scale on Thermal Shock Resistance of Aluminum‐Reinforced Alumina
Authors: Schön Stefan , Prielipp Helge , Janssen Rolf , Rödel Jürgen , Claussen Nils
Published: March 2005

Thermal shock data are presented for two types of Al/Al2O3 composites (35 vol% Al) with different microstructural scale and compared to a monolithic alumina. The fine‐scale Al/A12O3 material exhibits high room‐temperature strength (765 MPa) and a gradual decrease in retained strength with increasing quenching temperature difference, δT, and improved thermal shock resistance—as compared to monolithic alumina—even at δT= 600°C. The coarse‐scale Al/Al2O3 composite shows high long‐crack fracture toughness (10.5 MPa·m1/2) and almost no strength degradation up to δT= 500°C. A qualitative picture for the effect of microstructural scale on thermal shock resistance emerges by employing a simple fracture mechanics description contrasting the crack driving force for transient stress fields with R‐curve behavior.
Binder Distribution in Macro‐Defect‐Free Cements: Relation between Percolative Properties and Moisture Absorption Kinetics
Authors: Lewis Jennifer A. , Boyer Michelle , Bentz Dale P.
Published: March 2005

Macro‐defect‐free (MDF) cement is fabricated from a calcium aluminate cement and a poly(vinyl alcohol‐acetate) (PVA) copolymer. For the composites studied, it was determined that the interphase regions comprised 63 voi % of the total binder content, while the bulk PVA regions comprised 37 vol% of this phase. Mercury intrusion porosimetry showed that a bimodal pore size distribution developed as binder was removed in increasing amounts from heat‐treated samples. Larger pores with a characteristic diameter above 30 nm resulted from the removal of bulk PVA, whereas smaller pores approximately 5 nm in size resulted from the removal of water and PVA from the interphase regions. Simulation results obtained from a hard‐core/soft‐shell continuum percolation model of the MDF microstructure indicate that both the bulk PVA and interphase regions form percolative pathways through the system. Dramatic changes in both moisture absorption kinetics and flexural strength were observed only when a percolative network of larger pores was present in these composites. Hence, the bulk polymer regions are the dominant transport pathway for moisture in MDF cement. Based on this knowledge, processing guidelines have been developed to improve the moisture resistance of these materials.
Fatigue Mechanisms in Graphite/SiC Composites at Room and High Temperature
Authors: Morris Winfred L. , Cox Brian N. , Marshall David B. , Inman Richard V. , James Michael R.
Published: March 2005

Some deductions have been made from fractographic evidence about mechanisms of low‐cycle mechanical fatigue in plain woven graphite/SiC composites at room and high temperature in vacuum. At both room temperature and 830°C, fatigue appears to be confined to the crack wake, where attrition reduces the efficacy of bridging fibers. It is inferred that the crack tip advances at some critical value of the crack tip stress intensity factor, as in monotonic growth, rather than by any intrinsic fatigue mechanism in the matrix. However, the manifestations of attrition are very different at room and high temperatures. At high temperature, wear is greatly accelerated by the action of SiC debris within the crack. This distinction is rationalized in terms of the temperature dependence expected in the opening displacement of a bridged crack. This argument leads in turn to plausible explanations of trends in loadlife curves and the morphology of cracks as the temperature rises.
Thermal Shock Behavior of Silicon Nitride Flexure Beam Specimens with Indentation Cracks
Authors: Choi Sung R. , Salem Jonathan A.
Published: March 2005

The experimental results of thermal shock testing of silicon nitride flexure beam specimens containing indentation cracks are presented. The thermal stress induced by water quenching is much greater in the transverse direction than in the longitudinal direction, resulting in an insensitivity of residual bend strength to temperature differences up to 580°C. This result indicates that a flexure beam configuration is not an appropriate geometry for thermal shock testing when thermal shock behavior is to be evaluated from residual bend strength data.
Design of a Hot Tensile Stage for an Ultra‐High‐Voltage Electron Microscope and Its Application to in Situ Deformation of Sapphire at 1620 and 1720 K
Authors: Komatsu Masao , Mori Hirotaro , Iwasaki Katsuhiro
Published: March 2005

A hot tensile stage, which is capable of operation at temperatures of up to 2300 K, has been constructed for an ultra‐high‐voltage (3 MV) electron microscope. With this stage, in situ deformation experiments have been conducted on sapphire at 1620 and 1720 K. At 1620 K, almost all the dislocations activated are of the basal slip system. The motion of dislocations is not jerky but rather smooth and continuous. At 1720 K, dislocations frequently undergo motions not confined in the basal plane. Activation of the secondary slip system is also observed at this temperature.
Effects of Deposition Mechanisms in the Modeling of Forced‐Flow/Temperature‐Gradient Chemical Vapor Infiltration
Authors: Tai Nyan‐Hwa , Chou Tsu‐Wei , Ma Chen‐Chi M.
Published: March 2005

A model for the forced‐flow/temperature‐gradient chemical vapor infiltration (FCVI) process has been proposed in a previous study. The results demonstrate that the arrangement of the heating elements is important in the matrix deposition within a fibrous preform. In the present analysis, two cases (first‐order and zero‐order reaction mechanisms) of the CVI process are examined to evaluate the assumption of matrix deposition mechanisms. In the first case, the distribution of the deposited matrix depends on the competition between the temperature gradient within the preform and the rate of reactants consumed during infiltration. In the second case, the distribution of matrix deposition depends solely on the temperature gradient. The analysis concludes that the prediction of matrix deposition is influenced significantly by the selection of deposition kinetics in the model. A zero‐order reaction mechanism is appropriate only if the consumption of reactant vapor is insignificant. Furthermore, it is suggested that the use of the reaction mechanism and activation energy in the FCVI model should be based on the experimental deposition results from an FCVI system.
Modified Lead Calcium Titanate Ceramics with a Relatively Large Dielectric Constant for Hydrophone Applications
Authors: Troilo Louis M. , Damjanovic Dragan , Newnham Robert E.
Published: March 2005

Modified lead titanate of the base composition (Pb,Ca)Ti‐(Co,W)O3+ 0.01 mol% MnO was doped with 2 to 6 mol% strontium and barium to increase the dielectric constant, K, while preserving a high piezoelectric anisotropy (d33/d31). K was increased above 500 and a d33/d31 ratio of 34/1 was achieved when the composition Pb0.65Ca0.31Sr0.04Ti0.09‐(CO0.5W0.5)0.06O3+ 0.01 mol% MnO was fabricated. The large d33/d31 resulted in an impressive hydrostatic piezoelectric coefficient (dh= 81 × 10—12 m2/V). The relatively low transition temperature (Tc= 120°C) did not result in an increased aging rate when samples were tested at 40°C and 99% humidity for 120 h after poling. A small tetragonality ratio (1.008) enabled saturation of ceramic properties at poling fields as low as 30 kV/cm at 100°C.
Low‐Temperature Synthesis and Phase Equilibria in the Y‐Cu‐O Binary System
Authors: Brosha Eric L. , Garzon Fernando H. , Raistrick Ian D. , Davies Peter K.
Published: March 2005

The low‐temperature phase relations in the Y‐Cu‐O system were examined using ultrasonically prepared, freeze‐dried nitrate precursors. Under 1 atm of oxygen, Y2Cu2O5 was found to be stable above 955 ± 5 K; below this temperature, the component binary oxides CuO and Y2O3 are the only stable phases in the system. The entropy of formation of Y2Cu2O5, with respect to the component oxides, was calculated to be 5.3 ± 2.3 J/(mol.K) at 955 K. Bond valence analyses indicate that the entropy stabilization of this compound is most likely caused by the underbonded character of the copper ions.
Mechanical and Environmental Factors in the Cyclic and Static Fatigue of Silicon Nitride
Authors: Jacobs David S. , Chen I‐Wei
Published: March 2005

The effects of environment on cyclic and static fatigue behavior were investigated with hot‐pressed silicon nitride materials. Tests were conducted at ambient temperature on standard compact tension specimens, and a dc electric potential technique was used to monitor crack lengths in situ. The results indicate that the environmental sensitivity of our materials under both cyclic and static loading mirrors that of durable glasses in static fatigue. The materials were most sensitive to water in the environment, while changes in pH had no significant effect in the range tested. In addition, NH3 was much less reactive with our materials than with vitreous SiO2. In some cases, the intergranular glass appears to be the site of environmental interaction. Evidence was also found that cyclic fatigue is not simply a manifestation of static fatigue. Cyclic fatigue was seen to occur in the absence of measurable static fatigue, and the data indicate that the mechanism of cyclic fatigue involves damage to the crack wake shielding zone.
Fibrous Monolithic Ceramics: IV, Mechanical Properties and Oxidation Behavior of the Alumina/Nickel Systein
Authors: Baskaran Suresh , Nunn Stephen D. , Halloran John W.
Published: March 2005

Fibrous monolithic ceramics were fabricated in the alumina/nickel system. The microstructure consists of high‐aspect‐ratio polycrystalline cells of alumina separated by thin cell boundaries of nickel. The nickel content in the material is 3 to 8 vol%. The fibrous monolith with uniaxially aligned cells fails noncatastrophically in flexure. Bridging ligaments of nickel, crack deflection along cell boundaries, and crack branching in the axial direction are observed in flexure bars and notched beams. Strength values range from 246 to 375 MPa. Indentations cause controlled damage on the surface but do not introduce strength‐degrading flaws. The alumina/nickel fibrous monoliths also show potential for use at high temperatures in oxidizing environments. Noncatastrophic fracture behavior is observed at room temperature after 10 h at 1200°C in air. The Ni cell boundary network is oxidized to a depth of 50 to 100 μm by this heat treatment. The NiO oxidation product in the cell boundaries reacts partly with alumina from the cells to form NiAI2O4, which would provide better protection.
Oxide Matrix Composite by Directional Oxidation of a Commercial Aluminum‐Magnesium Alloy
Authors: DebRoy Tarasankar , Bandopadhyay Amitava , Roy Rustum
Published: March 2005

Oxidation rates of a commercial Al‐Mg alloy, Al‐5083, were investigated to understand the synthesis of an oxide‐based composite. In some experiments, the oxide layer was grown with several platinum wires embedded in the matrix to facilitate transport of electrons. The oxidation rate was not controlled by the rate of transport of the electrons and positive holes through the ceramic oxide layer. In the initial phase of the composite growth, an incubation period was observed due to formation of MgO on the alloy surface. At 1273 K or higher, the incubation period was followed by a period of sustained oxidation after MgO was converted to a spinel layer. It is demonstrated that, after the initial incubation period, the rate of oxidation is influenced by the rate of transport of ions through the oxidematrix composite.
Pressure Sensitivity for Particle Packing of Aqueous Al2O3 Slurries vs Interparticle Potential
Authors: Chang Jeanne C. , Lange Fred F. , Pearson Dale S. , Pollinger John P.
Published: March 2005

The relation between relative density and applied network pressure has been determined for aqueous Al2O3 slurries prepared with different interparticle potentials and consolidated by centrifugation. Attractive interparticle potentials were obtained by either changing the pH to the isoelectric point or adding an excess electrolyte. A range of centrifugal speeds produced consolidation pressures between 10–3 and 10 MPa. At lower speeds, the density gradient was determined with an X‐ray absorption technique. The maximum packing density (0.62 ± 0.02) was achieved for both dispersed and coagulated slurries at network pressures >0.5 MPa. At lower pressures, thepacking density of these slurries was pressure‐dependent, where the coagulated (salt added) slurries had a relative density between that of the dispersed and flocced slurries. Slurries flocculated by adjusting the pH to the isoelectric point never reached the highest packing density at the largest pressure.
Use of Boron Phosphate in the Solid‐State Preparation of Crystalline Iron(III) Phosphate and Manganese(II) Diphosphate
Authors: Vasović Dušanka D. , Stojaković Djordje R.
Published: March 2005

The possibility of using BPO4 for the solid‐state preparation of crystalline phosphates of trivalent iron, manganese, and chromium has been investigated. If BPO4 and Fe2O3 are heated at 900°C, a complete conversion of BPO4 to the quartz form of FePO4 results after 9 h. If FeCl3.6H2O is used instead of Fe2O3, a multiphase crystalline product is obtained in which the dominant phase is the quartz form of FePO4. The reaction of BPO4 with Mn2O3 or MnO2 at 830°C yields Mn2P2O7; the amount of conversion of BPO4, to Mn2P2O7 after 4 h is 80% (if Mn2O3 is used) and 75% (if MnO2 is used). However, the method is not successful in the preparation of phosphates of chromium: the reaction of BPO4 and some chromium compounds yields α‐Cr2O3 as the only chromium‐containing product.
Densification and Microstructure of Sodium‐Doped Colloidal Mullite
Authors: Fahrenholtz William G. , Smith Douglas M.
Published: March 2005

A series of mullite precursor gels to prepared by hetero‐coagulation of boehmite and colloidal silica. Samples were doped with up to 1.5 wt% Na2O. Na2O additions as low as 0.16 wt% led to the formation of α‐alumina and a stable glassy phase in addition to mullite. No evidence for the incorporation of sodium into the mullita structure was found by either XRD or 27AlMAS NMR. SEM showed mullite doped with 0.16 wt% and greater Na2O had an inter‐granular glassy phase and large crystalline grains. It is thought that the majority of the sodium was present in the glassy phase. Sodium addition did not decrease the mullite crystallization temperature and did not lead to enhanced sintering.
Thio Sol–Gel Process for the Synthesis of Titanium Disulfide
Authors: Sriram Mandyam A. , Kumta Prashant N.
Published: March 2005

A thio sol–gel process for the synthesis of titanium disulfide using titanium alkoxide as the metal source is demonstrated. The alkoxide reacts at room temperature with H2S to form a precipitate which is a precursor to the sulfide. Through infrared spectroscopy and chemical analysis of the powder and gas chromatographic analysis of the liquid byproducts of the reaction, it is shown that a partially sulfidized alkoxide precursor forms through the displacement of alkoxy groups from the alkoxide by sulfur‐bearing thiol groups. This thiolysis reaction is very similar to that which occurs in the case of sol–gel reactions to form oxides. The alkoxy–thiol species then undergo condensation–polymerization by the liberation of H2S. When it is heat‐treated in H2S, this precursor transforms to TiS2 at ∼700°C. Systematic heat treatments have been performed and the formation of TiS2 has been observed using X‐ray diffractometry. The change in the morphology has also been studied using scanning electron microscopy.
Thermodynamics and Phase Equilibria in the Ca─Cu─O System
Authors: Suzuki Ryosuke O. , Bohac Petr , Gauckler Ludwig J.
Published: March 2005

Phase equilibria in the CaO─CuO─Cu system were determined at 1173 K from the results of X‐ray diffraction measurements using specimens annealed in the oxygen partial pressure range from PO2= 1 to 10−8 atm. Electromotive force (emf) measurements using ZrO2 solid electrolyte cells were carried out in the ternary phase equilibria. Gibbs free energies for the chemical reactions were summarized by equations with linear temperature dependence, and the standard free energy of formation for Ca2CuO3 was derived. The stability conditions of the oxides are displayed in the p–T–x diagram, and the possible phase equilibria with the liquid are evaluated.
Carbon Dioxide Laser Cutting of a Carbon‐Fiber–Silicon Carbide‐Matrix Composite
Authors: Trubdja Mladen F. , Ramanathan Sriram , Modest Michael F. , Stubican Vladimir S.
Published: March 2005

CO2 laser scribing and cutting were studied on a carbon‐fiber‐silicon carbide‐matrix (C/SiO) composite nominally containing 45 vol% of carbon fibers. The scribing and cutting were performed in continuous‐wave (CW) mode using laser powers between 750 and 1500 W, and specimen translation velocities between 0.5 and 4 cm/s. The laser spot size was 300 μm in diameter. The groove width and depth were measured as functions of power and velocity. The results were compared to theoretically predicted values obtained by solving the quasi‐steady‐state heat conduction equation in three dimensions for a moving body. Reasonably good agreement between theory and experiment was found. The microstructures of the laser‐cut surfaces indicated the formation of redeposit by condensation from the vapor phase. X‐ray diffraction and Raman spectroscopy analyses of the redeposit showed the presence of β‐SiC and graphitic carbon. The four‐point bending strength of the laser‐cut composite was found to be approximately 20% lower than the corresponding strength of the diamond‐cut composite. The strength was fully recovered after removing 180 ± 10 μm of the material from the lased surface by grinding. The oxidation resistance of the laser‐cut and diamond‐cut composites was studied with a thermogravimetric balance at 1103°, 1304°, and 1402°C in air. The oxidation behavior at all investigated temperatures for both materials was dominated by a rapid initial mass loss due to the oxidation of carbon and a possible active oxidation of SiC, followed by a slow mass gain due to the passive oxidation of SiC. At 1304°C the rate of passive oxidation of SiC in the laser‐cut material was somewhat higher than in the diamond‐cut material. At 1402°C, the diamond‐cut surface oxidized more rapidly than the taser‐cut surface. The differences in oxidation rates were attributed to the differences in microstructure.
Fatigue Crack Growth of Silicon Nitride at 1400°C: A Novel Fatigue‐Induced Crack‐Tip Bridging Phenomenon
Authors: Liu Shih‐Yu , Chen I‐Wei , Tien Tseng‐Ying
Published: March 2005

high‐strength Si3N4with elongated β‐Si3N4 and equiaxed α‐sialon was tested in cyclic and static fatigue at 1400°C. At low stress intensity factors and high frequencies, the pullout process of the elongated grains was enhanced, which suppressed the crack growth. This provides a possible explanation for the increased lifetime under cyclic leading conditions reported for ceramics by several investigators. While crack‐healing by high‐temperature annealing was found to greatly reduce the subsequent static fatigue crack growth rate, it had only a modest effecf on cyclic fatigue and none at high frequencies.
Aluminum‐Containing Nilrogen Melilite Phases
Authors: Cheng Yi‐Bing , Thompson Derek P.
Published: March 2005

A significant solubility of Al in N‐melilite phases (M) has been observed, and this results in the formation of a melilite solid solution (M′ss) of general formula Ln2Si3 −xAlxO3 +xN4 −x (Ln = rare earth). Up to one Si can be replaced by Al without change of structure, and the M′solid solution terminates at Ln2Si2AlO4N3 in samarium SiAlON systems. M′ss may appear as an intermediate phase during the sintering of SiAlONs, and its melting temperature is critical to the densification of the materials. For example, samarium M′ss melts at a temperature lower than neodymium M′ss, and as a result, samarium oxide shows better densification behavior in the preparation of α‐SiAION ceramics than does neodymium oxide. Devitrification of M′ss from an amorphous grain boundary phase occurs above 1500°C during post heat‐treatment. The M′ss is refractory and may offer better oxidation resistance than N‐melilite because of the replacement of Al─O for Si─N in the structure. Therefore M′ss, is considered to be a most desirable grain boundary phase for α and α–β SiAlON ceramics.
Rutherford Backscattering and Channeling Studies of Cleaved, Polished, and Annealed MgO (100) Single Crystals
Authors: Kim Sangsub , Baik Sunggi
Published: March 2005

The structure and chemisitry of surfaces and near‐surface regions of cleaved, mechanically polished, and thermally annealed single‐crystalline MgO substrates were investigated by Rutherford backscattering and channeling. The cleaved crystals remained highly ordered, showing a nearly ideal bulk truncation. The mechanical polishing introduced extensive structural damage that was identified primarily as dislocations. Such damage was removed effectively by subsequent thermal annealing. Although the annealed surface showed the presence of impurities originating from internal or external sources, the annealed MgO (100) substrates seemed to be favorable for epitaxial thin him growth since structural damage had been relieved effectively by thermal annealing.
Alkoxide–Hydroxide Route for the Preparation of γ‐LiAlO2‐Based Ceramics
Authors: Renoult Oliver , Boilot Jean‐Pierre , Boncoeur Marcel
Published: March 2005

An alkoxide‐hydroxide route has been developed to prepare Li4 +xAl4 − 3xSi2xO8 (0 ≤x≤ 0.25) powders by taking into account fundamental aspects of the sol‐gel process. This technique allows one to prepare powders which exhibit the β‐LiAlO2 type of structure after drying at 150°C. The β→γ‐LiAlO2 topotactic transformation spreads over a large temperature range (746–839°C for x= 0.125) with no significant dilatometric and enthalpic change. Stoichiometric γ‐LiAlO2‐based ceramics with a large variety of uniform microstructures are fabricated by a direct sintering of β‐LiAlO2 powders in the temperature range of 900–1100°C.
Cluster Models of Bulk, Surface, and Impurity Structure.in α‐Alumina
Authors: Ellis Donald E. , Guo Jun , Lam Daniel J.
Published: March 2005

Self–consistent field embedded cluster models have been used to study the electronic structure of bulk, surface, and impurity regions of α‐alumina, Al2O3. First principles local density (LD) theory is used to make an expansion of one–electron wave functions in a linear combination of atomic orbitals which permits direct interpretation of bonding and effective atomic configurations. The clusters are embedded self–consistently in the host lattice, so that all long–range potential effects are properly included. The LD calculations give a reasonably good account of bulk cohesion and bond length; relative energies of several cleavage faces are predicted. The nature of surface states for different terminating surfaces is examined, and results of partial relaxation “experiments” are reported. Local densities of state of Ti, Cr, and Fe substitutional impurities are discussed.
Interaction between Barium Titanate and Binary Glasses
Authors: Kuromitsu Yoshirou , Wang Sea Fue , Yoshikawa Shoko , Newriham Robert E.
Published: March 2005

Interactions between BaTiO3, and three binary glasses were studied through the reaction of BaTiO3, powder with glass powder. For PbO–B2O3 and PbO–SiO2 glasses, the reaction led to stable compound formation, the substitution of Pb in the BaTiO3 structure, and noticeable grain growth of BaTiO3. The interaction phenomena for these two glass systems were very similar. The substitution of Pb into BaTiO3 is assisted by chemical reactions in which BaB2O4 or Ba2SiO4 is formed. The substitution into BaTiO3 also seems to be closely related to the grain growth of BaTiO3. On the other hand, only compound formation was observed during the processing of BaTiO3 with Bi2O3–B2O3 glass. Neither BaTiO3 grain growth nor Bi substitution took place with the Bi2O3–B2O3 glass system. Based on the observed reactions and the glass viscosity, several sintering aids for BaTiO3 ceramic products are suggested in this paper.
Statistical Analysis of the Fatigue Life Distribution of Monolithic Silicon Nitride
Authors: Ogasawara Toshio , Hirosaki Naoto , Akimune Yoshio
Published: March 2005

Cyclic fatigue testing of silicon nitride ceramics was performed at a frequency of f = 150 Hz and a stress ratio of R= 0.1 using a piezoelectric bimorph fatigue device. Based on the method proposed by Fett and Munz, a fatigue crack growth diagram (K1–V diagram) was estimated from fatigue lifetime distribution data. A change in the slope of the KI–V diagram was observed. A statistical analysis for predicting the lifetime of ceramics exhibiting a transition in a KI–V diagram was devised on the basis of this observation. Analytical results obtained with this method showed good agreement with experimental data.
Effect of Grain Size on Hertzian Contact Damage in Alumina
Authors: Guiberteau Fernando , Padture Nitin P. , Lawn Brian R.
Published: March 2005

The role of microstructural scale on deformation‐micro fracture damage induced by contact with spheres is investigated in monophase alumina ceramics over a range 3–48 μ in grain size. Measurement of a universal indentation stress–strain curve indicates a critical contact pressure ≅5 GPa, above which irreversible deformation occurs in alumina. A novel sectioning technique identifies the deformation elements as intragrain shear faults, predominantly crystallographic twins, within a confining subsurface zone of intense compression‐shear stress. The twins concentrate the shear stresses at the grain boundaries and, above a threshold grain size, initiate tensile intergranular microcracks. Below this threshold size, classical Hertzian cone fractures initiate outside the contact circle. Above the threshold, the density and scale of subsurface‐zone microcracks increase dramatically with increasing grain size, ultimately dominating the cone fractures. The damage process is stochastic, highlighting the microstructural discreteness of the initial deformation field; those grains which lie in the upper tail of the grain‐size distribution and which have favorable crystallographic orientation relative to local shear stresses in the contact field are preferentially activated. Initial flaw state is not an important factor, because the contact process creates its own flaw population. These and other generic features of the damage process will be discussed in relation to microstructural design of polycrystalline ceramics.
Tape Casting of Fine Alumina/Zirconia Powders for Composite Fabrication
Authors: Plucknett Kevin P. , Cáceres Carlos H. , Willinson David S.
Published: March 2005

Ceramic films, containing AI2O3, with up to 40 vol% ZrO2, have been fabricated using the tape casting process. Finer powders (average mean diameter of 250–300 nm) than have generally been reported for tape casting were used in this study. The optimum formulation for tape casting is affected substantially by decreasing particle size. For example, the amount of dispersant needed is increased. Moreover, the amount of plasticizer/binder must be increased so as to maintain the solids content in the dried tapes below a critical level (about 55 vol% in this case), which decreases with particle size. Rheological studies on the effectiveness of menhaden fish oil and phosphate ester as dispersants show that phosphate ester can be used in lower concentrations, for the preparation of higher solids loading slurries, and was therefore selected for further study. The amount of dispersant required to obtain minimum slurry viscosity was found to be primarily dependent upon the effective particle surface area, defined as that available to the dispersant molecules. In the case of particles composed of agglomerated crystallites (such as the ZrO2, powder used here), this may be considerably less than that measured by nitrogen absorption. Moreover, the porous internal structure of such powders is filled with solvent, which increases the effective solids loading of the slurry, and thus its viscosity. Particle morphology also influences the packing efficiency; i.e., the green density decreases as ZrO2, is added.
Formation of Alumina/Zirconia (3 mol% Yttria) Composite Powders Prepared by the Hydrazine Methods
Authors: Yamakata Köji , Hirota Ken , Yamaguchi Osamu , Kume Hideki , Inamura Suguru , Miyamoto Hiroki
Published: March 2005

Alumina/3 mol% yttria‐doped zirconia composite powders have been prepared by the hydrazine method. As‐prepared powders are AlO(OH) gel solid solutions and the mixtures of this and amorphous ZrO2 below and above 10 mol% ZrO2, respectively. The formation process leading to α–Al2O3–t‐ZrO2 composite powders is examined.
Residual α–Si3N4 in O′ Crystals in CeO2‐Doped O′+β′ SiAlON Ceramics
Authors: Xu Youren , Huang Chao M. , Kriven Waltraud M. , Zangvil Avigdor
Published: March 2005

The microstructure of a pressureless sintered (1605°C, 90 min) O′+β′ SiAlON ceramic with CeO2 doping has been investigated. It is duplex in nature, consisting of very large, slablike elongated O′ grains (20–30 μm long), and a continuous matrix of small rodlike β′ grains (< 1.0 μm in length). Many α‐Si3N4 inclusions (0.1–0.5 μm in size) were found in the large O′ grains. CeO2‐doping and its high doping level as well as the high Al2O3 concentration were thought to be the main reasons for accelerating the reaction between the α‐Si3N4 and the Si‐Al‐O‐N liquid to precipitate O′–SiAlON. This caused the supergrowth of O′ grains. The rapid growth of O′ crystals isolated the remnant α–Si3N4 from the reacting liquid, resulting in a delay in the α→β‐Si3N4 transformation. The large O′ grains and the α‐Si3N4 inclusions have a pronounced effect on the strength degradation of O′+β′ ceramics.
Processing, Microstructure, and Wear Behavior of Silicon Nitride Hot‐Pressed with Alumina and Yttria
Authors: Zutshi Ajoy , Haber Richard A. , Niesz Dale E. , Adams Jane W. , Wachtman John B. , Ferber Mattison K. , Hsu Stephen M.
Published: March 2005

Commercial silicon nitride powder with A12O3 and Y2O3 additives was hot‐pressed to complete density. The resulting microstructure contained elongated grains with no trace of remaining α‐Si3N4. The aspect ratio of the elongated grains increased with increasing soak time at a fixed hot‐pressing temperature. X‐ray diffraction analysis showed that the crystalline phase in the hot‐pressed samples was β‐sialon (Si6−zAlzOzN8−z) with z values that increased with soak time. The fracture strength and fracture toughness of the samples increased as the aspect ratio of the grains increased. The Vickers hardness decreased slightly as the soak time was increased, which was attributed to a grain size effect. Wear tests of silicon nitride against silicon nitride were conducted on a reciprocating pin‐on‐disk apparatus with paraffin oil as a lubricant. Correlation studies of wear with microstructure and mechanical properties were performed. The wear rate increased rapidly with increasing soak time in spite of the increased strength and toughness. This was attributed to increased third‐body wear caused by pullout of pieces from the wear surface. The pullout mechanism was not conclusively identified. However, TEM examination showed clear evidence of dislocation motion under the wear scar. Grain boundary microstresses caused by the anisotropic thermal expansion and elastic properties of the elongated grains may have contributed to the observed pullout.
Grain Growth Kinetics in Alumina–Zirconia (CeZTA) Composites
Authors: Alexander Kathleen B. , Becher Paul F. , Waters Shirley B. , Bleier Alan
Published: March 2005

Grain growth kinetics in alumina‐rich, zirconia–alumina composites have been examined as a function of volume fraction of zirconia. Both he microstructural scale and character alter during grain growth. Grain growth exponents between n= 3 and 4 were observed and have been rationalized in terms of current models for grain growth and particle coarsening. Grain growth constants wore measured and compared with available data. Coupled grain growth was observed in that the ratio of the alumina‐to‐zirconia grain size was constant, for a given zirconia content. This behavior is discussed in terms of the topological constraint present during growth and the observed microstructural evolution. The measured activation energies were consistent with boundary‐diffusion‐controlled grain growth.
Stable Crack Growth in Polycrystalline Magnesia under Monotonic and Cyclic Loads
Authors: Ogawa Takeshi , LLorca Javier
Published: March 2005

The quasi‐static, stable growth of cracks in polycrystalline magnesia under various loading conditions (cyclic compression, monotonic tension, and tension–compression fatigue) was studied. Crack length was monitored continuously through a conductive film attached to the specimen surface. The fracture surfaces and the crack path were examined by scanning electron microscopy. The different fracture mechanisms responsible for the observed stable crack growth in each loading condition are discussed.
Microstructures of Y2O3‐Stabilized ZrO2 Electron Beam‐Physical Vapor Deposition Coatings on Ni‐Base Superalloys
Authors: Unal Ozer , Mitchell Terrence E. , Heuer Arthur H.
Published: March 2005

The microstructures of ZrO2–20 wt% Y2O3 thermal barrier coatings formed by electron beam‐physical vapor deposition on a Nibase superalloy have been studied by transmission electron microscopy. The coating systems consist of several layers, including a superalloy substrate, a bond coat, an Al2O3 scale, and the PVD coating. The overall ceramic thermal barrier coatings were characterized, with special emphasis being given to the α‐Al2O3 scale which forms between the bond coat and the ZrO2−Y2O3 coating. The oxide scale exhibited various morphologies in different coating systems; the majority of the porosity formed in this region for all coatings.
Diffusion‐Controlled Growth or Dissolution of Spheres with Variable Temperature
Authors: Frade Jorge R. , Cable Michael
Published: March 2005

Theoretical calculation of growth or dissolution of spherical particles is a challenging problem even under isothermal conditions. This paper examines the diffusion‐controlled behavior of spherical particles for time‐dependent and spatially uniform temperature. Analytical solutions are possible for growth from zero size even when temperature changes, making diffusivity a variable. Numerical methods for cases not capable of analytical solution are also developed; these include the variation of interfacial concentration with temperature. Analytical solutions are restricted to growth from zero but it is shown that these solutions represent the asymptotic behavior for growth from finite initial size. Nearly all solutions for dissolving spheres must be computed numerically but quasi‐steady‐state analytical solutions applicable to both growth and dissolution, in suitable cases, are also developed.