bluuebunny/crossref_metadata_2025 huggingface dataset:
Stress‐Enhanced Oxidation of Silicon Nitride Ceramics
Authors: Gogotsi Yury G. , Grathwohl Georg
Published: March 2005

Silicon nitride ceramics show an accelerated oxidation rate under load in air. This phenomenon was observed for porous and dense ceramics with and without additives in a wide temperature range (700°–1450°C) and can be interpreted as stress corrosion in oxygen‐containing environments. Stresses cause an alteration of the amount and composition of oxidation products, formation of pits and cracks on stressed parts of specimens, and changes of the surface coloration and oxide scale morphology. Both tensile and compressive stresses can affect the oxidation process. An exponential dependence of mass gain on stress was found. On the other hand, oxidation of silicon nitride‐based ceramics can affect the material response to mechanical stresses as, for example, deformation, cavitation and cracking. Stress‐assisted chemical reaction at lower temperatures and stress‐affected diffusion at higher temperatures seem to be the main reasons for the susceptibility of Si3N4 ceramics to stress corrosion. The effect of stress corrosion on mechanical properties is discussed.
Zirconia–Ceria Solid Solution Synthesis and the Temperature–Time–Transformation Diagram for the 1:1 Composition
Authors: Yashima Masatomo , Morimoto Kenji , Ishizawa Nobuo , Yoshimura Masahiro
Published: March 2005

Zirconia–ceria solid solutions of tetragonal symmetry (t’ZrO2) containing 30 to 65 mol% ceria were prepared by annealingc′‐ZrO2at 627°C samples sintered at 1660–1760°C, wherec′‐ZrO2is defined as a cubic or tetragonal phase whose axial ratioc/a(tetragonality) is equal to 1. The lattice parameters and the cube root of the unit cell volumes of the annealed samples increased linearly with the ceria content but, on the other hand, the axial ratioc/a(tetragonality) decreased to 1.000 at 70 mol%. During the annealing of each sample, the cell volume decreased because the residual Ce3+ions were oxidized to the smaller Ce4+. The temperature–time–transformation (TTT) diagram of the 1:1 solid solution was investigated for thec′‐t′diffusionless phase transition and thec′–(t + c)diffusional reaction. Thec′–t′transformation was found to behave as a thermally activated process with an activation energy estimated to be 113 kJ/mol. This value may indicate thatc′–t′transformation is controlled by oxygen diffusion accompanied by dimensional changes of the cerium ions.
Indentation Cyclic Fatigue of Single‐Crystal Magnesium Oxide
Authors: Guillou Marie‐Odile , Henshall John Leslie , Hooper Robert Maurice
Published: March 2005

Repeated point contact loading has been used to investigate the deformation and fracture of single‐crystal magnesium oxide subjected to cyclic loading by softer metallic cones at room temperature. The results of the investigation clearly indicate that MgO is susceptible to cyclic fatigue. An explanation for the mechanical cyclic fatigue effect is proposed.
Phase Separation and Crystallization of BiSrCaCu2Al0.5Ox Glass
Authors: Kasuga Toshihiro , Abe Yoshihiro
Published: March 2005

Al2O3 addition to the melt of a BiSrCaCu2Ox composition was found by TEM observation to cause the liquid‐liquid phase separation of the melt‐quenched glass, and to result in preferential precipitation of superconducting Bi2Sr2Ca1Cu2Ox crystals from the melt during the cooling process.
Polar‐Oriented Crystallization of Fresnoite (Ba2TiSi2O8) on Glass Surface Due to Ultrasonic Treatment with Suspensions
Authors: Ding Yong , Miura Yoshinari , Osaka Akiyoshi
Published: March 2005

Improved polar (c‐axis) oriented growth of fresnoite (Ba2TiSi2O8) was observed for surface crystallization of a glass 33.3BaO· 16.7TiO2· 50SiO2 (in mol%) due to ultrasonic surface treatment with an aqueous suspension of Ba2TiSi2O8, and an oriented film more than 50 μm in thickness was prepared. The effect of ultrasonic treatment on the polar orientation depends on the suspending particles; of these, foreign particles of a phase other than the crystallizing Ba2TiSi2O8 give poorer efficiency than the Ba2TiSi2O8 particles. It is assumed that fresnoite has a tendency toward preferred polar growth on the glass, and seeding of a large number of fine particles through ultrasonic bombardment realizes the tendency. The polar‐oriented growth kinetics of Ba2TiSi2O8 were also discussed.
Anomalous Temperature Dependence of Cyclic Fatigue‐Crack Growth in a Silicon Carbide Matrix Composite
Authors: Yao Daping , Shang Jian‐Ku
Published: March 2005

Fatigue‐crack growth behavior under cyclic tension‐tension loading has been examined in a 30‐vol%‐TiB2‐reinforced β‐SiC matrix composite, using compact tension specimens tested at temperatures from 750° to 850°C in a furnace air environment. Cyclic fatigue‐crack growth rate was found to depend strongly on testing temperature and was faster at a lower temperature. Such an anomalous dependence of crack growth rate on temperature is shown to result from the reduction in crack growth driving force, ΔK, by the wedging action of the oxide film formed behind the crack tip. Subtraction of the contribution from oxide films leads to a “normal” Arrhenius relation between cyclic crack growth rate and temperature, with the activation energy equal to that for the viscous flow of grain boundary phase.
Factors Determining the Carbon Monoxide Sensing Properties of Tin Oxide Thick Films Calcined at Different Temperatures
Authors: Song Kug‐Hyun , Park Soon Ja
Published: March 2005

Effects of calcinations temperature on the gas‐sensing properties of SnO2 thick films were explored by measuring the variations in resistance with CO concentration. As the calcining temperature increased from 500° to 1100°C, the absolute values of the slopes in the log (concentration)vs log (resistance) curves gradually decreased from −0.9 to −0.2. The data were analyzed with the equations proposed by Madou and Morrison. It was suggested that the reduction in the formation rate of oxygen adsorbates was responsible for the decrease in the absolute slope, and hence a relatively high sensitivity was attained in a low gas concentration with a largely crystallizes sample.
Phase Relations in the System Ce2O3–Ce2Si2O7 in the Temperature Range 1150° to 1970°C in Reducing and Inert Atmospheres
Authors: Tas A. Cuneyt , Akinc Mufit
Published: March 2005

The high‐temperature phase relations in the system Ce2O3‐Ce2Si2O7 have been studied in the temperature range 1150°–1970°C in inert and reducing atmospheres by conventional quenching and differential thermal analysis. Three eutectic reactions were found to occur at 1664°C and 27 mol% SiO2 between Ce2O3 and Ce2SiO5, at 1870°C and 54 mol% SiO2 between Ce2SiO5 and Ce4.67(SiO4)3O, and at 1762°C and 65 mol% SiO2 between Ce4.67(SiO4)3O and Ce2Si2O7. Microstructural characteristics of the eutectic reactions were observed. Cerium pyrosilicate exhibits a polymorphic transformation at 1274°C. The binary compounds of this system were all found to melt congruently. The enthalpy and entropy of melting of these binary compounds were calculated from the initial slopes of their experimentally determined liquidus lines.
Nanostructural Characterization of Silicon Oxycarbide Glasses and Glass‐Ceramics
Authors: Breval Else , Hammond Michael , Pantano Carlo G.
Published: March 2005

Silicon oxycarbide glasses were synthesized by the sol‐gel process using precursors such as methyl‐, propyl‐, and phenyltrimethoxysilanes. The final products contained 14–38 wt% carbon. A TEM study on the nanometer scale revealed that all of the materials were amorphous and monophasic, and that it was not possible to detect any crystalline or otherwise distinct carbon phases. Hot‐pressing the glasses led to the crystallization of graphite and silicon carbide within the amorphous matrix. X‐ray and electron diffraction showed increasing crystallinity at the higher hot‐pressing temperatures. Hot pressing at 1400°C resulted in the appearance of fine‐grained silicon carbide, whereas at the highest temperature (1650°C), graphite and both hexagonal and cubic silicon carbide were produced. Subsequent heat treatment of the hot‐pressed glasses under an argon atmosphere at 1400°C resulted in the formation of cristobalite. The glass‐ceramics produced at the highest hot‐pressing temperatures were more resistant to the crystallization of cristobalite during subsequent heat treatments.
Processing Conditions for Boron Nitride Coatings in Fiber Bundles via Chemical Vapor Deposition
Authors: Einset Erik O. , Patibandla Nag‐Bhushan , Luthra Krishan L.
Published: March 2005

A model for the transport and reaction of BCI3 and NH3 for the deposition of BN on individual filaments in a fiber bundle at low pressures has been developed using previously published kinetic data. An operating diagram is presented which summarizes the results of the model and facilitates the choice of deposition parameters for obtaining uniform coatings throughout the bundle. The uniformity of coatings deposited on individual fibers of Nicalon, Sumitomo‐A12O3 fiber bundles, and straight weave Nicalon cloth is consistent with the model predictions.
Densification and Grain Growth of Porous Alumina Compacts
Authors: Kwon Young S. , Son Geonseog , Suh Jeong , Kim Kitae T.
Published: March 2005

Densification and grain growth of porous alumina compacts during various high‐temperature processes were investigated. Experimental data were obtained for densification and grain growth of alumina powder during hot pressing. A set of constitutive equations was proposed based on the constitutive equations by Helle et al.1 for hydrostatic response and by Rahaman et al.2 for deviatoric response. Theoretical results from the proposed constitutive equations were compared with various experimental data for alumina powder compacts in the literature, including pressureless sintering, sinter forging, and hot pressing. The proposed model well predicts the densification and grain growth of alumina compacts.
Synthesis and Characterization of Zinc Sulfide/Gallium Phosphide Nanocomposite Powders
Authors: Han Yong , Voiles E. Todd , Chumbley L. Scott , Akinc Mufit , Naiini Ahmad A. , Verkade John G.
Published: March 2005

Nanocomposite powders between zinc sulfide and gallium phosphide were synthesized. Monodisperse, submicrometer, spherical, and porous clusters of ZnS nanocrystallites were used as a host material, into which a solution containing a phosphinogallane, [(t‐Bu)2GaPR2]xwherein R =i‐Pr,x= 2 or R =t‐Bu,x= 1, was impregnated. The pure ZnS powders and the composite powders obtained after flash pyrolysis at 600°C were subsequently heat‐treated at various temperatures ranging from 500° to 900°C and characterized using a variety of techniques. While pure ZnS powders undergo significant grain growth and morphology change at temperatures as low as 600°C, the composite Powders maintain their integrity up to 800°C.
Suppression of Acceptor Solubilities in BaTiO3 Densified in Highly Reducing Atmospheres
Authors: Bheemineni Veerabhadrarao , Chang Edward K. , Lal Misri , Harmer Martin P. , Smyth Donald M.
Published: March 2005

The solubilities of acceptor impurities are strongly suppressed when BaTiO3 is densified in highly reducing atmospheres. This is evidenced by a shift in the minimum in the equilibrium electrical conductivity to higher oxygen activities, a decrease in the ionic contribution to the conductivity, and a decrease in the leakage current and the rate of leakage current degradation under temperature–voltage stress. The normal solubility is restored by subsequent anneals in air that result in substantial grain growth, and the properties then revert to those of BaTiO3 sintered in air. The solubility suppression is attributed to a mass‐action interaction between the processes that generate oxygen vacancies, those that result from the compensation of acceptor centers, and those that result from reduction.
Elastic‐Plastic Indentation of Hard, Brittle Materials with Spherical Indenters
Authors: Adler Thomas A.
Published: March 2005

A method was developed at the U.S. Bureau of Mines to measure the plastic deformation of hard, brittle materials with low values of elastic modulus to hardness (E/H). The method involves indenting the hard materials with spherical indenters and measuring the resulting contact diameter and depth. Subsequent analysis separates the plastic deformation from the elastic deformation. The analysis is an extension of a theoretical study of the indentation of ductile materials by spherical indenters. The analysis is based on Meyer's law and the results include the distribution of the contact pressure and the profile of the deformed surface. Measurements were made on alumina and WC–Co.
Effect of Etching and Imaging Mode on the Measurement of Subsurface Damage in Microground Optical Glasses
Authors: Zhou Yiyang , Funkenbusch Paul D. , Quesnel David J. , Golini Donald , Lindquist Arne
Published: March 2005

Microgrinding of optical glass has been under intensive study recently. Characterization of the subsurface damage (SSD) generated by this process is very important because it is one of the criteria used to characterize the quality and efficiency of the microgrinding technique. SEM, as a new method, is applied for a detailed observation of the SSD. Etching with HF solutions of various concentrations is utilized to open the subsurface cracks. The etching condition has a significant influence on the measured SSD value. Using the SEM method, slightly larger SSD values are obtained compared to those obtained using conventional optical microscopy.
Electrical Properties of Manganese‐Doped Titanium Dioxide
Authors: Fujitsu Satoru , Hamada Tomosuke
Published: March 2005

The electrical conductivity and Seebeck coefficient for TiO2 doped with varying amounts of MnO2 have been measured over the temperature range of 900°–1100°C. The Hall coefficient was also measured over the temperature range of 300°–500°C. The electrical conductivity and carrier density were found to decrease initially, go through a minimum at approximately x = 0.0001–0.001, and then increase with increasing content of MnO. For dopant levels above 0.01 at.%, p‐type electrical conduction behavior was observed.
α‐Decay Damage Effects in Curium‐Doped Titanate Ceramic Containing Sodium‐Free High‐Level Nuclear Waste
Authors: Mitamura Hisayoshi , Matsumoto Seiichiro , Stewart Maartin W. A. , Tsuboi Takashi , Hashimoto Masaaki , Vance Eric R. , Hart Kaye P. , Togashi Yoshihiro , Kanazawa Hiroyuki , Ball Cliff J. , White Timothy J.
Published: March 2005

A polyphase titanate ceramic incorporating sodium‐free simulated high‐level nuclear waste was doped with 0.91 wt% of 244Cm to accelerate the effects of long‐term self‐irradiation arising from α decays. The ceramic included three main constituent minerals: hollandite, perovskite, and zirconolite, with some minor phases. Although hollandite showed the broadening of its X‐ray diffraction lines and small lattice parameter changes during damage ingrowth, the unit cell was substantially unaltered. Perovskite and zirconolite, which are the primary hosts of curium, showed 2.7% and 2.6% expansions, respectively, of their unit cell volumes after a dose of 12 × 1017α decays.g‐1 Volume swelling due to damage ingrowth caused an exponential (almost linear) decrease in density, which reached 1.7% after a dose of 12.4 × 1017α decays.g‐1. Leach tests on samples that had incurred doses of 2.0 × 1017 and 4.5 × 1017 a decays g‐1 showed that the rates of dissolution of cesium and barium were similar to analogous leach rates from the equivalent cold ceramic, while strontium and calcium leach rates were 2–15 times higher. Although the curium, molybdenum, strontium, and calcium leach fates in the present material were similar to those in the curium‐doped sodium‐bearing titanate ceramic reported previously, the cesium leach rate was 3–8 times lower.
Temperature–Time Texture Transition of Pb(Zr1−xTix)O3 Thin Films: I, Role of Pb‐rich Intermediate Phases
Authors: Chen San‐Yuan , Chen I‐Wei
Published: March 2005

Lead‐rich intermediate phases, in particular Pt5–7Pb and PbO, are found to form on Pt(111) atop Ti–buffered Si substrates prior to formation of Pb(Zr1−xTix)O3 (PZT) thin films. Pt5–7Pb is a [111] textured transient intermetallic phase that nucleates PZT[111] texture. PbO is a [001] textured layer compound that nucleates PZT[100] texture. The formation conditions and lattice matching of these intermediate phases have been examined. The presence of other possible intermediate phases, such as pyrochlore and Zr and Ti‐rich phases, has also been investigated but found unrelated to the texture selection of perovskite PZT.
Crystal Chemistry and Phase Equilibrium Studies of the BaO(BaCO3)–½R2O3–CuOx Systems in Air: VI, R = Neodymium
Authors: Wong‐Ng Winnie , Cook Lawrence P. , Paretzkin Boris , Hill Michael D. , Stalick Judith K.
Published: March 2005

Crystal chemistry and subsolidus phase equilibrium studies of the Ba‐Nd‐Cu‐O system near the CuO and Nd2O3 corners have been carried cut at 950°C in air. Two solid‐solution series have been identified in the Ba‐Nd‐Cu‐O system. The first series involves the high‐Tc superconductor phase, and has the formula Ba2–xNd1+xCu3O6+z, where × < ≅ 0.7. At the ideal compound stoichiometry of Ba2NdCu3O6+z, the transformation from the high‐Tc orthorhombic to tetragonal phase occurs at 550°–575°C in air. This temperature varies as a function of composition, and at x≅ 0.2 to 0.3 it occurs at 950°C. The second solid solution is the non‐superconducting “brown phase” represented by Ba2+2x‐Nd4–2xCu2–xO10–2z 0 ≤x≤ 0.1. Preliminary phase diagrams of the BaO–Nd2O3 and Nd2O3–CuOx systems are also presented. Standard X‐ray diffraction patterns of BaNd2–CuO5 and (Nd1.9Ca0.1)CuO4–z are provided.
Aspects in Grinding of Ceramics
Authors: Cho Seong‐Jai , Huh Yong‐Hak , Yoon Kyung‐jin , Ogawa Tomeyoshi
Published: March 2005

Aspects In the grinding of ceramics have been investigated. Si3N4, ZrO2, SiC, and Al2O3 were ground with resin‐bonded diamond wheels of grit numbers ranging from 80 to 800. Microstructural observations of ground surfaces show that grinding occurs predominantly by flow mode in Si3N4 and ZrO2, and by fracture and grain pullout in SiC and Al2O3. The measurement of grinding force shows that grinding resistances in Si3N4 and ZrO2 are significantly larger than those in SiC and AI2O3. As the grit number of the grinding wheel increases, maximum surface roughness significantly decreases in Si3N4, ZrO2, and SiC, but it apparently does not change in AI2O3. This unexpected result in AI2O3 is discussed in terms of the observed grinding mode and microstructure.
Fabrication and Characterization of PZT/Thermoplastic Polymer Composites for High‐Frequency Phased Linear Arrays
Authors: Stevenson Jeffry W. , Reidmeyer Mary R. , Huebner Wayne
Published: March 2005

A new method of fabricating PZT/potymer composites with 2‐2 connectivity is described. This fabrication technique offers significant advantages over conventional dice‐and‐fill fabrication methods, and the composites exhibit the high electromechanical coupling expected from conventional PZT/polymer composites. In this method, thin (≤20μm) sintered PZT plates and sheets of a thermoplastic polymer film (≤6 μm) are bonded together via thermal processing. A technique for sintering thin, flat PZT plates from tape cast materials was developed to provide the necessary PZT plates. The resulting composite blocks were cut to required dimensions, electroded, and poled. Electromechanical properties were measured to evaluate the composites.
Nonstoichiometry and the Long‐Range Cation Ordering in Crystals of (Na1/2Bi1/2) TiO3
Authors: Park Seung‐Eek , Chung Su‐Jin , Kim In‐Tae , Hong Kug Sun
Published: March 2005

Crystals of (Na1/2Bi1/2)TiO3 have grown by the flux technique and the Czochralski method. Nonstoichiometry, twin configurations, and long‐range cation ordering in the crystals have been investigated using X‐ray diffraction and an optical polarizing microscope. It has been found that nonstoichiometry was induced in the crystal grown by the Czochralski method owing to the volatilization of Bi‐rich phase during the crystal growth. This nonstoichiometry in the crystal resulted in less lattice distortion from cubic symmetry, a lower degree of cation ordering, and a larger domain width in twin configuration. Variations in twinning with temperature and isotropization have been investigated.
A New Route for the Synthesis of Tungsten Carbide–Cobalt Nanocomposites
Authors: Zhu Yuntian T. , Manthiram Arumugam
Published: March 2005

Synthesis of WC–Co nanocomposites generally involves gas‐phase carburization. A novel approach in which a polymer precursor such as polyacrylonitrile serves as an in situ carbon source has been developed. The WC–Co nanocomposites formed are characterized by X‐ray powder diffraction and electron microscopy. Nearly phase pure WC–Co nanocomposites with a particle size of 50–80 nm have been obtained. The phase purity of the products is strongly influenced by the synthesis and processing conditions such as the firing temperature, time, and atmosphere.
EELS Study of Oxygen Diffusion in Aluminum Nitride
Authors: Sterntzke Martin , Müller Gerd
Published: March 2005

The oxygen/nitrogen interdiffusion in AIN ceramics was investigated by electron energy‐loss spectroscopy (EELS) in combination with transmission electron microscopy (TEM). AL2O3/AIN diffusion couples were prepared by oxidation of an AIN ceramic. The samples were annealed for different times in the temperature range from 1500° to 1900°C. The diffusion couples were encapsulated in a tantalum ampoule to guarantee an inert atmosphere. Oxygen concentration profiles across the oxide/nitride interface were measured by EELS. The oxygen/nitrogen interdiffusion coefficient in AIN is given by D= 1.5–1.2+6.0× 10–8 exp[–240 ± 40 (kJ/mol)/(RT)] (cm2/s). The magnitude and temperature dependence of the N/O interdiffusion is comparable to diffusion coefficients reported in the literature for other non‐oxide ceramic materials. At standard AIN sintering conditions the O/N interdiffusion is so slow that it cannot provide an effective means for oxygen removal from the AIN grains.
Sintering and Electrical Properties of Titania‐ and Zirconia‐Containing ln2O3‐Sno2(ITO) Ceramics
Authors: Nadaud Nicolas , Nanot Monique , Boch Philippe
Published: March 2005

The deposition rate and film quality of In2O3‐SnO2 (ITO) transparent electrodes processed by sputtering are improved when using dense sputtering targets. Unfortunately, ITO ceramics do not sinter easily. It is shown that addition of TiO2 (<1 wt%) to ITO greatly increases densification without degrading electrical properties of sputtered films. The influence of ZrO2 and SiO2 was also investigated.
Microstructural Mechanics Model of Anisotropic‐Thermal‐Expansion‐Induced Microcracking
Authors: Sridhar Narayanaswamy , Yang Wuhua , Srolovitz David J. , Fuller Edwin R.
Published: March 2005

Thermal‐expansion‐induced microcracking in single‐phase ceramics has been simulated using a simple mechanics model based upon a regular lattice of brittle, elastic springs. Microcracks preferentially form at grain boundaries and propagate either into the bulk or along grain boundaries, depending on the toughness of the boundaries relative to the grain interiors. The present results show that anisotropic‐thermal‐expansion‐induced microcracking can be more severe for either large or small grain size samples depending on the damage measure employed. At very small misfit strains, the large grain microstructure develops microcracks before the small grain microstructure. However, over most of the misfit strain regime examined, the total length/area of all cracks in a sample is larger when the grain size is small. This is manifested in a larger decrement of the elastic modulus in small grain size samples as compared with large grain size samples at the same misfit (ΔT). However, large grain sizes are more detrimental with regard to fracture properties. This is because the fracture stress scales as inversely with the crack length and large grain samples exhibit larger microcracks than small grain samples. Unlike in the unconstrained samples, when a sample is constrained during a temperature excursion, the stress created by the overall thermal expansion can directly lead to fracture of the entire sample.
Creep and Stress Rupture Behavior of an Advanced Silicon Nitride: Part III, Stress Rupture and the Monkman–Grant Relationship
Authors: Menon Mamballykalathil N. , Fang Ho T. , Wu David C. , Jenkins Michael G. , Ferber Mattison K.
Published: March 2005

The applicability of the Monkman–Grant relationship to predict the stress rupture life of NT154 silicon nitride is examined. The data show that the Monkman–Grant lines relating rupture life to minimum creep rate are stratified with respect to temperature. A modification to the current expression for the Monkman–Grant relation is proposed to accommodate this temperature dependence. A phenomenological approach based on crack growth as the failure mechanism is presented to explain this temperature dependence. The results can be interpreted as suggesting that the stress dependence of the failure process is greater than that for creep.
Formation Processes of β‐C2S by the Decomposition of Hydrothermally Prepared C‐S‐H with Ca(OH)2
Authors: Okada Yoshihiko , Sasaki Kaori , Zhong Baiqian , Ishida Hideki , Mitsuda Takeshi
Published: March 2005

A mixture of CaO and silicic acid prepared with a Ca/Si ratio of 2.0 was hydrothermally synthesized at 80° to 200°C, and the thermal decomposition behavior of the products (C‐S‐H with Ca(OH)2) was analyzed using XRD, 29Si MAS NMR, and the trimethylsililation method (TMS). It was found that the main silicate anion structure of C‐S‐H was a mixture of a dimer and a single‐chain polymer (larger than Si5O16) and that polymerization advanced with an increase of the synthesizing temperature. On heating, the products decomposed to form β‐C2S. It was found that the decomposition was gradual and that the‐higher the temperature of hydrothermal synthesis, the lower was the temperature of the decomposition into β‐C2S. Although the decomposition proceeded to form a monomer (β‐C2S) from the polymer and dimer, this dimer was resistant to heat and did not decompose unless heated to above 400°C.
Grain Boundary Defect Chemistry of Acceptor‐Doped Titanates: Space Charge Layer Width
Authors: Vollman Markus , Waser Rainer
Published: March 2005

The grain boundary space charge depletion layers in acceptor‐doped SrTiO3and BaTiO3ceramics were investigated by impedance spectroscopy in the time and frequency domain. Based on the layer width and its dependence on the acceptor concentration, the temperature, and the oxygen partial pressure during annealing, a suggestion for a refined Schottky model is proposed. The local distribution of the donor‐type grain boundary states causing the depletion layer and the resulting band bending are discussed.
Defect Clustering and Color in Fe,Ti: α‐Al2O3
Authors: Moon Anthony R. , Phillips Matthew R.
Published: March 2005

A comprehensive theory is presented which successfully explains the polarization, isothermal, and isochronal behavior of the optical absorption bands responsible for the color in blue and blue‐green sapphire (Fe,Ti: α‐Al2O3). The experimental study on which this theory is based has conclusively shown that (Fe,Ti) and (Fe,Fe) vacancy‐containing defect clusters are responsible for the optical properties of Fe,Ti: α‐Al2O3 and H,Fe,Ti: α‐Al2O3. The experimental results also prove that V‴ is the charge‐compensating defect for Ti˙Al in Fe, Ti and Ti: α‐Al2O3 with an [Ti˙Al] = 3[V‴] electroneutrality condition and that V¨o provides the charge balance for Fe′Al in Fe: α‐Al2O3 with an [Fe′Al] = 2[V¨o]electroneutrality condition. Defect clusters were found to form via diffusion‐limited solid‐state reactions where the relative concentration of charged and neutral point defects depends on both the association energy and the diffusivity of the defects participating in the clustering reactions. In this paper, a model is presented which attempts to explain both the origin and the thermal behavior of the optical bands responsible for the color of Fe,Ti: α‐Al2O3.
First‐Principles Calculation of Electronic, Optical, and Structural Properties of α‐Al2O3
Authors: Ching W. Y. , Xu Yong‐Nian
Published: March 2005

The electronic structure, the linear optical properties, and the structural properties of α‐Al2O3 in the corundum structure are studied by means of first‐principles local density calculations. An indirect band gap of 6.29 eV is obtained, which is almost the same as the direct band gap of 6.31 eV at Γ. The calculated density of states are compared with X‐ray photoemission and photoabsorption measurements. Real space charge density analysis shows Al,2O3 to be highly ionic with an effective charge formula of Al2+2.75O3−1.83. The calculated dielectric function is in general agreement with the experimental vacuum ultraviolet data. It is shown that the component with c‐direction polarization is in better agreement with the experimental data because it is less affected by the exciton formation near the absorption edge. The intrinsic difference between the calculated local density approximation gap and the measured optical gap is pointed out. Various careful test calculations indicate that the remaining discrepancy in the optical spectra may be in the LDA approximation of the electronic structure theory. The total energy calculation for the ground‐state structural properties of α‐Al2O3 shows excellent agreement with experimental data: the calculated equilibrium volume, c/a ratio, bulk modulus, and internal parameters for Al and O atoms differ from measured values by 0.0, −4.3, −4.0,0.85, and 1.96 percent, respectively. The calculations for the electronic structure and the optical properties are repeated with crystal parameters obtained at 2000°C. The result shows only a slight reduction in the band gap (to 5.76 eV at Γ), with the optical spectra essentially unchanged.
Oxidation Mechanisms and Kinetics of 1D‐SiC/C/SiC Composite Materials: I, An Experimental Approach
Authors: Filipuzzi Ludovic , Camus Gerald , Naslain Roger , Thebault Jacques
Published: March 2005

The oxidation of unidirectional SiC/C/SiC model composites has been investigated through thermogravimetric analysis, optical/electron microscopy, and electrical measurements. The influence of temperature and carbon interphase thickness on the oxidation of the composites is discussed. The oxidation involves three phenomena: (i) reaction of oxygen with the carbon interphase resulting in pores around the fibers, (ii) diffusion of oxygen and carbon oxides along the pores, and (iii) reaction of oxygen with the pore walls leading to the growth of silica layers on both the fibers and matrix. In composites with a thin carbon interphase (e.g., 0.1 μm) treated at T > 1000°C the pores are rapidly scaled by silica. Under such conditions, the oxidation damages are limited to the vicinity of the external surface and the materials exhibit a self‐healing character. Conversely, long exposures (300 h) at 900°C give rise to the formation of microcracks in the matrix related to mechanical stresses arising from the in situ SiC/SiO2 conversion, fly, the self‐healing character is not observed in composites with a thick interphase (e.g., 1 μm) since carbon is totally consumed before silica can seal the pores.
Nucleation and Growth of β′‐SiAlON
Authors: Hwang Shyh‐Lung , Chen I‐Wei
Published: March 2005

Elongated β′‐SiAlON grains grown from several finegrained Ym/3Si12(m+n)Alm+nOnN16–r compositions with α‐Si3N4, AlN, Al2O3, and Y2O3 starting materials have been examined. These grains have large aspect ratios and are oriented along the [0001] axis. TEM structural and chemical analysis suggests that they are nucleated from various seed crystals, which can be α‐Si3N4, β‐Si3N4, or other β′‐SiAlON. The β′‐SiAlON seed and the initial precipitation on β‐Si3N4 show a higher content of Al and O, indicating that a large transient supersaturation of Al and O in the liquid is instrumental for β′‐SiAlON formation, whereas subsequent growth proceeds under a much lower driving force. The misfit between phases is accommodated by interfacial dislocations (c‐type and a‐type). Fully grown β′‐SiAlON grains usually contain several variants independently nucleated from the same seed. In particular, the two alternative α/β phase‐matching possibilities result in two [0001] growth habits separated by a twin boundary.
Short‐Crack Mechanical Properties and Failure Mechanisms of Si3N4‐Matrix/SiC‐Fiber Composites
Authors: Xu Hockin H. K. , Ostertag Claudia P. , Braun Linda M. , Lloyd Isabel K.
Published: March 2005

SiC‐fiber‐reinforced Si3N4 composites were fabricated by hot pressing. The indentation‐strength technique was applied to study the mechanical properties of these composites. This enabled the investigation of short‐crack behavior of continuous‐fiber ceramic composites (CFCCs). The flaw tolerance of composite ultimate strength, matrix‐cracking stress, and work‐of‐fracture were investigated. Scanning electron microscopy was used to examine crack–fiber interactions. The ultimate strength was found to be independent of indentation load at a fiber volume fraction f= 0.29, while at f= 0.14 it exhibited a transition from flawsensitive to flaw‐independent. The work‐of‐fracture was found to be independent of indentation load at both fiber volume fractions. The matrix‐cracking stress was found to correspond to the first load‐drop on the load–displacement curve. It decreased with increasing flaw size and therefore is the steady‐state matrix‐cracking stress. A failure mechanism transition from catastrophic failure to non‐catastrophic failure, coupled with the transition from flawsensitive to flaw‐tolerant behavior, was observed by varying the preexisting flaw size and the fiber volume fraction. These transitions were explained by analyzing the relations between ultimate strength, matrix‐cracking stress, fiber volume fraction, and preexisting flaw size of the composite materials. Experimental results were compared with predictions from available models.
High‐Temperature Mechanical Properties and Chemical Stability of Ba1+xZr4P6–2xSi2XO24 Low‐Thermal‐Expansion Ceramics
Authors: Harshé Girish , Agrawal Dinesh , Limaye Santosh
Published: March 2005

The NaZr2P3O12 (NZP) family of materials is attracting increasing attention due to its low‐thermal‐expansion behavior. The system Ba1+xZr4P6–2xSi2xO24 (0 ≤x≤ 1), belonging to the NZP family, shows ultralow thermal expansion over a wide temperature range. It also shows anisotropy in its lattice thermal expansion. This causes microcracking as the sintered specimens are cooled, which results in degradation of the mechanical properties. In this work, the chemical stability, strength, and Young's modulus of Ba1+xZr4P6–2xO24 (X= 0.25 and 0.5) ceramics at high temperatures have been determined. An attempt has been made to correlate the mechanical properties to the thermal expansion anisotropy.
High‐Temperature Hall Measurements on Cerium Dioxide Thin Films
Authors: Lohwasser Werner , Fleischer Maximilian , Gerblinger Josef , Meixner Hans
Published: March 2005

Simultaneous Hall and conductivity measurements have been performed on sputtered polycrystalline thin films and on bulk ceramic specimens of nearly stoichiometric CeO2 in the temperature range between 900° and 1040°C. The measurements have been performed in air using low‐frequency alternating current. In the case of the bulk ceramic specimens, an upper limit for the carrier mobility of ≤0.2 cm2/(V·s) has been obtained, which is in accordance with data from the literature for bulk samples. The conductivity of the thin films (l/1Ω·m) at 1000°C) is in accordance with data from the literature for bulk ceramics. The carrier density derived from the Hall measurements (3 × 1016/cm3 at 1000°C) increases with increasing temperature, whereas the Hall mobility (4 cm2(V·s) at 1000°C) decreases with increasing temperature. These values differ from literature data for bulk ceramic specimens. The difference may be duelo the small grain diameters (∼200 nm) in the 1‐μm‐thick thin films.
Unstable Neck Formation during Initial‐Stage Sintering
Authors: Rankin Janet , Boatner Lynn A.
Published: March 2005

By observing pairs of single‐crystal cubic‐faceted small particles of MgO in contact along faces, edges, and corners, it was possible to determine the effects of contact geometry on the stability of neck regions formed during the initial stages of sintering. Changes in the neck morphology were monitored during the sintering process by continuous TEM/ video observations of particles heated in situ between 1100° and 1260°C. Particles in face‐to‐face contact were found to coalesce in the classic manner to form a single particle, Particles in contact along either cube corners or edges, however, developed unstable necks that initially grew, then shrunk, and eventually broke so that such particle pairs effectively “de‐sintered.”
Role of Oxygen in Microstructure Development at Solid‐State Diffusion‐Bonded Cu/α‐Al2O3 Interfaces
Authors: Rogers Kirk A. , Trumble Kevin P. , Dalgleish Brian J. , Reimanis Ivar E.
Published: March 2005

Microstructure development at solid‐state diffusion‐bonded Cu/α‐Al2O3 interfaces has been studied using optical and electron microscopy. High‐purity Cu foil was bonded between basal‐oriented α‐Al2O3 single‐crystal plates at 1040°C for 24 h in a vacuum of ∼1.3 × 10−4 Pa (1 × 10−6 torr). Optical microscopy of as‐bonded specimens revealed a large Cu grain size, fine pores, and long needles of Cu2O at the interface. Bulk specimens were annealed at 1000°C for various times under controlled oxygen partial pressures in CO/CO2 mixtures. Consistent with a thermochemical analysis, CuAlO2 could be formed at the interfaces. The CuAlO2 was acicular and discontinuous, but occurred in a uniform distribution over the bulk specimen interfaces.
Effect of Network Modifier on Spontaneous Emission Probabilities of Er3+ in Oxide Glasses
Authors: Takebe Hiromichi , Nageno Yoshikazu , Morinaga Kenji
Published: March 2005

Compositional dependence of spontaneous emission probabilities of Er3+ was studied for silicate, borate, and phosphate glasses using the Judd–Ofelt theory. Through all of the glass systems, spontaneous emission probabilities of the 4I13/2→4I15/29→4I9/2→4IJ, and 4S3/2→4IJ (J= 9/2, 11/2,13/2,15/2) transitions increased with increased ionic packing ratio of the glass host, which varied markedly with the type of network modifier. The effect of Er–O covalency on the transition probabilities was discussed from the spectral profile of the 4I15/2→4I3/2transition in terms of nephelauxetic effect.
Preparation of Porous BaTiO3 PTC Thermistors by Adding Graphite Porosifiers
Authors: Su Shi‐Mei , Zhang Liang‐Ying , Sun Hong‐Tao , Yao Xi
Published: March 2005

Using an ordinary ceramic processing technique, a new method of preparing porous BaTiO3, PTC thermistors is introduced. Adding proper graphite powders into calcined BaTiO3, powder can increase porosity and enhance the PTC effect of the sintered sample. When the graphite addition is about 1.0 wt%, the resisivity of samples from low‐purity raw materials decreases to 20 Σ·cm at room temperature, while the PTC resistivity ratio is over 105 and 1–2 orders higher than that of samples without the porosifier. The porosifier has increased the reproducibility of resistivity from 60% up to 90%. In addition, the porous thermistor has a fast response to overcurrent and shows some improvement in heat resistance. With the aid of the Heywang model and the Kuwabara conclusion, the influence of graphite on grain surfaces is discussed. The experimental results show that this method is a useful technique to prepare good PTC thermistors from low‐purity raw materials.
Carbothermal Synthesis of Aluminum Nitride Using Sucrose
Authors: Baik Youngmin , Shanker Kartik , McDermid Joseph R. , Drew Robin A. L.
Published: March 2005

Several aluminum oxides (α‐Al2O3, θ‐Al2O3, and AIOOH) were examined to study the differences in reaction behavior and powder characteristics during carbothermal nitrida‐tion to AIN using sucrose and carbon black. The reaction conditions investigated were carbon‐to‐alumina ratio, reaction temperature, and time. Carburized sucrose resulted in Full conversion to AIN and produced a uniform powder morphology using a near‐istoichiometric ratio of C:Al2O3 while carbon black required higher C:Al2O3 ratios (i.e., >4:1) for full conversion and led to agglomeration of the AIN powder. The most favorable reaction temperature was 1600°C, with the reaction time to full conversion being dependent on the type of Al2O3. The particle and agglomerate size of the AIN powders did not change significantly with reaction time. However, the particle size and morphology were strongly dependent on that of the initial AI2O3 with sucrose, whereas agglomeration of the AIN occurs when using carbon black. A solid–solid reaction mechanism is proposed.
Quantitative Determination of the Cubic‐to‐Monoclinic Phase Transformation in Fully Stabilized Zirconias by Raman Spectroscopy
Authors: Kontoyannis Christos G. , Carountzos George
Published: March 2005

Vibrational Raman spectroscopy was used successfully for the quantitative determination of the cubic and monoclinic phases of zirconia. Pressed pellets with known composition were prepared using monoclinic and cubic powders from different manufacturers and were used as standard mixtures. An intensity ratio plot of the 617‐cm−1 band, which is attributed to the presence of both phases, to either the 177‐cm−1 or 476‐cm−1 monoclinic band against the inverse monoclinic molar fraction (χ) yielded a straight line which was described by ratio = 0.11χ−1+ 0.26 and ratio = 0.12 χ−1+ 0.29, respectively. The method was found to be independent of the origin of the powders.
Wicking Technique for Determination of Pore Size in Ceramic Materials
Authors: Li Zhaohui , Giese Ross F. , van Oss Carel J. , Kerch Helen M. , Burdette Harold E.
Published: March 2005

A technique is described for the measurement of mean pore size in ceramics by wicking. Pore size is derived from the measured rates of capillary rise of several apolar, low‐energy liquids of known surface tension through thin slices of porous materials, via the Washburn equation. The use of apolar, low‐energy liquids assures that the contact angle θ=0° (and cos θ= 1) so that the mean pore radius, R, is the only unknown in the Washburn equation. Fore‐size data are presented for two colloidal gel compositions with engineered pore‐size distributions. Pore diameters obtained from wicking are found to be smaller than those afforded by merciury porosimetry by a factor of ∼2.
Creep and Creep Rupture of an Advanced Silicon Nitride Ceramic
Authors: Ding Jow‐Lian , Liu Kenneth C. , More Karren L. , Brinkman C. R.
Published: March 2005

Creep and creep rupture behavior of an advanced silicon nitride ceramic were systematically characterized in the temperature range 1150° to 1300°C using uniaxial tensile creep tests. Absence of tertiary creep and the order‐of‐magnitude breaks in both creep rate and rupture lifetime at certain threshold combinations of stress and temperature were two characteristic features of the creep behavior observed. Thermal annealing was found to have enhanced both subsequent creep resistance and creep rupture life. The stress exponent (n) and the activation energy (Q) defined in the Norton relation were found to be 12.6 and 1645 kJ/mol for the material investigated. Both values appear to fall in the general range of those reported for other but similar types of Si3N4 ceramic materials. The stress exponent, m, equivalent to the slope of the Larson–Miller equation was found to be in the range 13 to 14.4, and that defined as p in the Monkman–Grant relation to be 0.91, based on the available experimental data. The values of m, n, and p obtained above approximately support the interrelationship of the three exponents given by p=m/n.
Electric and Dielectric Properties of Barium Titanate Schottky Barrier Diodes
Authors: Wang Da Yu
Published: March 2005

The electrode properties of yttrium‐doped barium titanate ceramics were studied. The current–oltage (I–V) and capacitance‐voltage (C–V) characteristics of fritted‐Pt‐paste electrodes, nonfritted‐Pt‐paste electrodes, vacuum‐deposited‐Pt electrodes, and vacuum‐deposited‐Au electrodes were measured. In general, the I–V data indicated that the diodes are metal–interface–semiconductor type junctions with the depletion–layer dielectric properties described by the Devonshire theory of barium titanate. The observed I–V and C–V data have unusual characteristics which could be related to the ferroelectric properties of barium titanate. The large diode ideal factors observed in the I–V data were attributed to the effect of the high dielectric constant of barium titanate. For fritted‐Pt‐paste electrodes, the forward and reverse bias had the same value for the diode factor near the zero‐bias region, and the interface layer thickness is estimated to be δ/ɛir= 1.60 Å. At the high–bias region, the fritted electrodes gave a positive‐temperature‐coefficient effect of the diode factor n, which was explained as an effect of the nonlinear dielectric properties of the barium titanate. The nonfritted and deposited types of electrode had different values of diode ideal factor n for forward bias and reverse bias. This difference of barrier‐height lowering effect between the forward and reverse bias was explained by the relative motion between the electrode and the semiconductor surface caused by the piezoelectric effect of the depletion layer on the loosely bonded elecrodes. The spontaneous polarizations of barium titanate were found to have two different effects on the electrode flat‐band barrier height, depending on the thickness of the interface layer. When a thick interface layer was involved in the case of nonfritted electrodes, the spontaneous polarizations would bend the band at the electrode surface, have the charges compensated for by the density states at the top of the valence band, and give a barrier height as high as the band gap. In the case of intimate‐contact electrodes, the charge compensation could be accomplished by the metal‐induced gap states (MIGS) of the electrode and yield a low barrier height, which, regardless of the electrode materials, could give an ohmic‐type electrode behavior due to the low electron mobility of barium titanate.
Characterization of Dip‐Coated Boron Nitride on Silicon Carbide Fibers
Authors: Shen Lu , Tan Beng Jit , Willis William S. , Galasso Francis S. , Suib Steven L.
Published: March 2005

Amorphous boron nitride thin coatings (∼0.2 μm) have been formed on Nicalon and C‐Nicaion (pre‐carbon‐coated Nicalon) yarns via dip coating in boric acid solution followed by heating and nitriding in NH3gas at 1000°C. X‐ray photoelectron spectroscopy (XPS) and Auger electron spec‐troscopy (AES) studies have shown the formation of boron nitride. The coating was boron rich and contains oxygen. The N/B and O/B ratios range from 0.6 to 0.8 and from 0.1 to 0.25, respectively. Tensile strength measurements revealed that the BN‐coated C‐Nicalon yarn maintained ∼85% of its original strength while BN‐coated Nicalon lost ∼85% of its original strength. Auger depth profiles showed that there was a consumption of carbon during the heating and nitridation process for both BN‐coated Nicalon and C‐Nicalon fibers. However, the depletion of carbon in BN‐coated Nicalon fibers was much more severe than that in BN‐coated C‐Nicalon fibers.