bluuebunny/crossref_metadata_2025 huggingface dataset:
Poly(methacrylate) Precursors to Forsterite
Authors: Martin M. Hogan E. , Ober Chris K. , Hubbard Camden R. , Porter Wally D. , Cavin O. Burl
Published: March 2005

A novel polymer precursor method for the preparation of silicates including enstatite‐free forsterite is being investigated. Precursors to crystalline materials were prepared via the copolymerization of the K and/or Mg salts of methacrylic acid with various combinations of a siloxane‐methacrylate and a methoxysilyl‐methacrylate Thermogravimetric/differential thermal, pyrolysis mass spectrometry, and high‐temperature X‐ray diffraction analyses were used to monitor the conversion of the precursors to inorganic oxide during air pyrolysis. Some precursors to crystalline oxides showed yields as high as 77% of theoretical.
Thermal Shock Damage Assessment in Ceramics Using Ultrasonic Waves
Authors: Hefetz Meir , Rokhlin Stanislav I.
Published: March 2005

Structural ceramics are susceptible to microcrack damage by thermal shock. There is a critical temperature for thermal shock damage initiation with damage severity increasing at greater shock temperatures. In this work the applicability of an ultrasonic method to determine the critical temperature and the accumulated damage is demonstrated in alumina. Information is obtained via velocity and attenuation measurements using surface and obliquely incident bulk ultrasonic waves. The elastic anisotropy effect due to preferred crack orientation has been estimated. The critical temperature for the alumina is about 200°C. The damage increases steeply from 200° to 400°C and grows significantly above 400°C. Changes of up to 17% from the original values in the effective shear moduli and up to 45% in the longitudinal effective modulus in the direction transverse to crack orientation are measured at high thermal shock temperatures.
Chemical Considerations in Carbon‐Fiber/Oxide‐Matrix Composites
Authors: Luthra Krishan L. , Park Hee Dong
Published: March 2005

Thermochemical calculations are performed to evaluate the chemical compatibility of oxides with carbon at temperatures of the order of 1650°C and to evaluate the implications of carbon oxidation. These calculations indicate that Al2O3, BeO, Y2O3, and Ce2O3 are stable with carbon provided there are no cracks or pores for CO gas to escape. In the presence of cracks or interconnected pores, both the direct reduction of oxides with carbon as well as the oxidation of carbon can proceed rapidly. The calculations also indicate that even in the absence of such cracks the oxidation of carbon can generate high enough CO partial pressures to degrade the composite properties by formation of CO gas bubbles or of cracks in the oxide. The results of these calculations are confirmed by conducting experiments on samples of pyrolytic and vitreous carbon disks hot‐pressed within Al2O3 and Y2O3disks and exposing them in argon and oxygen environments at 1650–1800°C. An oxidation model is then proposed that includes thermodynamic and kinetic considerations as well as creep behavior of oxides. The model quantitatively predicts the times required to initiate the formation of gas bubbles and qualitatively considers the factors influencing the growth rate of bubbles and formation of cracks.
Thermal Shock Behavior of Open‐Cell Ceramic Foams
Authors: Orenstein Robert M. , Green David J.
Published: March 2005

Specimens of heated, open‐cell ceramics were thermally shocked by immersion in water or oil. It was found the strength retained after thermal shock underwent a gradual decrease with increasing quench temperature, indicative of a cumulative damage mechanism which manifests itself with increasing thermal stress. This damage could also be monitored using measurements of the elastic constants before and after quenching. The thermal shock resistance of the open‐cell materials was found to be strongly dependent on cell size (increased with increasing cell size) and weakly dependent on density (increased with increasing density). Two possible sources of thermal stress were considered; one was associated with the temperature gradient across the microscopic struts and the other with the heating of the quenching medium as it infiltrates the cellular structure. Such heating was confirmed and it was concluded that this was the dominant source of thermal stress in this particular study, controlling the thermal stress in this particular study, controlling the thermal shock resistance of the open‐cell ceramics.
Distribution of Oxide in a Bed of Thermally Decomposed ZrF4· H2O and Its Effect on ZrF4 Sublimation
Authors: Ewing Kenneth J. , Wright Barbara M. , Jaganathan James , Aggarwal Ishwar D.
Published: March 2005

The oxide distribution in a bed of thermally decomposed zirconium fluoride monohydrate (ZrF4· H2O) is determined via inert gas carbon fusion analysis (LECO). It is shown that the oxide concentration in the bed of thermally decomposed ZrF4· H2O is a function of position in the crucible. The oxide concentration in the crucible decreases in the order, from greatest to least, surface ≫ bottom > sides > middle. XRD analysis and thermogravimetric analysis indicate that the predominant phase in the bottom, side, and middle regions of the bed is ZrF4, while in the surface region Zr7O9F10 is the predominant phase with traces of ZrO2 and ZrF4 present. It is proposed that the production of HF(g) from the thermal decomposition of the bottom and side regions of the bed of ZrF4· H2O causes the central region of the bed to contain a higher concentration of HF(g) than any other region in the bed. The higher concentration of HF(g) in the central region of the bed of decomposing ZrF4· H2O is expected to shift the equilibrium from formation of zirconium oxyfluoride and elimination of HF(g) toward the formation of ZrF4 and elimination of water. Thermal decomposition of different masses of ZrF4· H2O demonstrates that the oxide constration increases in the middle of the bed as the initial mass decreases. This result is consistent with the proposed mechanism for production of different phases in the bed of material. It is also shown that thermally decomposed ZrF4· H2O can be used as feedstock for preparation of low‐oxide zirconium fluoride via sublimation. The best results for sublimation from this feedstock material are obtained when the surface, which contains the highest oxide concentration, is removed before sublimation.
High‐Temperature Indentation Studies on the {110} Plane of Single‐crystal Magnesium Oxide
Authors: Khasgiwale Niranjan , Chan Helen M.
Published: March 2005

The indentation behavior (Vickers) of Single‐crystal MgO was studied as a function of temperature (20° to 1000°C). Indentations were made on the {110} plane, with the indents oriented such that one indent diagonal was parallel to the 〈001〉 direction. Using etchant techniques, the dislocation etch pit structures were examined both in the plane of the indentation and in cross section. All the observed slip traces were found to be consistent with primary slip ({110}〈110〉), with no evidence of secondary slip, even at 1000°C. Radial cracking was observed only at the pair of indent corners joined by the indent diagonal parallel to 〈001〉. The crack length increased with temperature (T) for indentations conducted at T < 800°C. For indents made at 800°C or higher, however, no cracking occurred. These results are discussed both with respect to an existing slip‐induced crack nucleation model, and the change in crack driving force and toughness with indentation temperature.
Resolution of Thermal Peaks of Kaolinite in Thermomechanical Analysis and Differential Thermal Analysis Studies
Authors: Chakraborty Akshoy K.
Published: March 2005

The recent findings for the kaolinite metakaolinite, cubic‐mullite, and orthorhombic‐mullite reaction series have been thoroughly examined by differential thermomechanical analysis (DTMA) and differential thermal analysis (DTA). Metakaolinite shows two differential contraction peaks in the vicinity of 980°C caused by final dehydroxylation at the endothermic dip just before 980°C in DTA with expulsion of 35–37 wt% SiO2, formation of a defect aluminosilicate phase and simultaneous contraction of the latter phase, and crystallization of cubic mullite at the 980°C exotherm in DTA. Mullitization takes place in two simultaneous reaction steps: (i) polymorphic transformation of cubic mullite to orthorhombic mullite during the ∼1250°C exotherm shown by DTA which coincides with the differential expansion peak in DTMA and (ii) nucleation followed by crystallization of orthorhombic mullite from the residual aluminosilicate compact phase during the ∼1330°C exotherm shown by DTA. The aluminosilicate formed during the large differential contraction at 1100°–1400°C as shown by DTMA. These results, obtained by the two physical techniques, corroborate earlier findings of the kaolinite transformation series.
Neutron Diffraction Study on the Structure of Na‐Si‐O‐N Oxynitride Glasses
Authors: Jin Jisun , Yoko Toshinobu , Miyaji Fumiaki , Sakka Sumio , Fukunaga Toshiharu , Misawa Masakatsu
Published: March 2005

The local structure of a Na2O─SiO2 oxide glass and a Na2O─iO2─Si3N4 oxynitride glass has been investigated by the pulsed neutron diffraction method. In the oxide glass, the first peak of radial distribution function (RDF) curves appeared at 0.1627 nm corresponding to Si─O boxh length. In the oxynitride glass, the first peak of the RDF curve was shifted to longer distances at 0.1650 nm due to the coexistence of Si─O and Si─N boxhs. The first peak in the RDF curve of the oxynitride glass was deconvoluted into two peaks whose positions are located at 0.1626 and 0.1709 nm. The one at 0.1626 nm is ascribed to Si─O boxhs and the other at 0.1709 nm to Si─N boxhs. The number of silicon atoms around a nitrogen was found to be 2.42, suggesting that not all of the nitrogen atoms are boxhed to three silicon atoms, but 58% twofold (─N─) and 42% threefold (>N─) nitrogens coexist in the present Na‐Si‐O‐N oxynitride glass. The formation of twofold nitrogens may arise from the reaction between ≡Si─O− and ≡Si3N groups. This model is compatible with the experimental observation that the density and Vickers hardness of oxynitride glasses increase with the nitrogen content.
Supersolidus Phase Investigation of the Bi‐Sr‐Ca‐Cu Oxide System in Silver Tape
Authors: Schartman Richard R. , Sakidja Ridwan , Hellstrom Eric E.
Published: March 2005

The phase assemblage in Ag tapes with molten or partially molten Bi‐Sr‐Ca‐Cu oxide cores of stoichiometries in the immediate vicinity of Bi:Sr:Ca:Cu = 2:2:1:2 was characterized. Four types of melt behavior were found. The results are discussed in terms of their relevance to the processing of superconducting Ag‐clad Bi‐Sr‐Ca‐Cu oxide wires.
Corrosion of Silicon‐Based Ceramics in Combustion Environments
Authors: Jacobson Nathan S.
Published: March 2005

Silicon‐based ceramics and composites are prime candidates for heat engine and heat exchanger structural components. In such applications these materials are exposed to combustion gases and deposit‐forming corrodents. In this paper combustion environments are defined for various applications. These environments lead to five main types of corrosive degradation: passive oxidation, deposit‐induced corrosion, active oxidation, scale/substrate interactions, and scale volatility. Each of these is discussed in detail. The key issues in oxidation mechanisms of high‐purity silicon carbide (SiC) and silicon nitride (Si3N4) in pure oxygen are discussed. The complicating factors due to the actual combustion environment and commercial materials are discussed. These discussions include secondary elements in the ceramics; additional oxidants, such as water and carbon dioxide (CO2); combustion environment impurities; long‐term oxidation effects; and thermal cycling. Active oxidation is expected in a limited number of combustion situations, and the active‐to‐passive transition is discussed. At high temperatures the limiting factors are scale melting, scale volatility, and scale/substrate interactions. Deposit‐induced corrosion is discussed, primarily for sodium sulfate (Na2SO4), but also for vanadate and oxide‐slag deposits as well. In applying ceramics in combustion environments it is essential to be aware of these corrosion routes and how they affect the performance of a component.
Deformation Behavior of an Al2O3─Y3Al5O12 Eutectic Composite in Comparison with Sapphire and YAG
Authors: Parthasarathy Triplicane A. , Mah Tai‐Il , Matson Lawrence E.
Published: March 2005

The relationship between a composite and its constituents with respect to high‐temperature deformation behavior was investigated using the Al2O3‐Y3Al5O12 (YAG) eutectic system. The eutectic is essentially a composite with sapphire and YAG as the two phases with a volume fraction of YAG of 0.45. The deformation behavior of the eutectic and those of single‐crystal sapphire and YAG were studied under identical conditions, using constant‐strain‐rate compression tests in air at 1530°C. The composite was also studied at 1650°C and compared with existing data on sapphire and YAG. The stronger YAG phase was found to reinforce the sapphire matrix at higher strain rates, but the composite crept faster than sapphire at lower strain rates. It is suggested using a simple semiempirical analysis that diffusional relaxation at the YAG‐sapphire interface boundaries may cause the inferior creep behavior at lower strain rates.
Viscous Glass Crack Bridging Forces in a Sintered Glassy Alumina at Elevated Temperatures
Authors: Jakus Karl , Ritter John E. , Schwillinski Roland H.
Published: March 2005

Time‐dependent crack bridging forces were measured at temperatures from 1150° to 1275°C in a vitreous bonded alumina using the rear‐notched, short double cantilever beam technique. Glassy ligaments were observed to bridge the crack even after the crack entered the rear notch. The crack bridging force exhibited both a temperature and displacement rate dependency. A model was developed that describes this temperature and time dependence of the bridging forces that result from the viscous glass bridges.
Nonaqueous Processing of Silicon for Reaction‐Bonded Silicon Nitride
Authors: DeLiso Evelyn M. , Bleier Alan
Published: March 2005

Ethanolic silicon suspensions, with and without a polyethoxylated amine of low molecular weight, were studied by rheological, adsorption, electrophoretic, and sedimentation methods. Pellets were pressure‐cast and nitrided to form reaction‐bonded silicon nitride. Density and binding strength in the green state relate well to rheological behavior and other collodial aspects of the suspensions used, particularly the additive's role and distribution. Density and degree of nitridation in the final state are not importantly affected by the additive's use. Its greatest benefit is to modify the binding strength in the green state. The mode by which this small molecule affects the processing of silicon consists of adsorption, combined with an increased electrostatic interparticle repulsion which increases the suspension viscosity and that of undried pellets. Although the improved binding strength is accompanied by decreased green and nitrided densities, high degrees of conversion to silicon nitride are still achieved.
Dynamic Stress‐Induced Transformation and Texture Formation in Uniaxial Compression of Zirconia Ceramics
Authors: Subhash Ghatuparthi , Nemat‐Nasser Sia
Published: March 2005

Transformation plasticity in Mg‐PSZ and Y‐TZP ceramics is investigated using a novel stress reversal Hopkinson bar technique recently developed at the University of California–San Diego (UCSD) for dynamic recovery experiments.1 The longitudinal and transverse strains are measured by strain gauges mounted on the specimen. The specimens are loaded until transformation reaches saturation. Reloading of the same specimens to higher stress levels does not reveal additional inelasticity. Cuboid specimens which have been loaded initially to attain transformation saturation are then reloaded in a direction perpendicular to the first loading. The second loading produces additional inelasticity and microcracking, indicating the formation of transformation texture in each loading under uniaxial compression. Microscopic observations, and the results of ultrasonic and X‐ray diffraction measurements, as well as micromechanical modeling of the damage evolution are discussed.
Evolution of Carbon during Burnout and Sintering of Tape‐Cast Aluminum Nitride
Authors: Yan Hongwen , Cannon W. Roger , Shanefield Daniel J.
Published: March 2005

The surface nature and the composition of AlN powder, as‐received and exposed to binder burnout, were studied using XPS and TEM. The surface of as‐received AlN powder was covered by a thin layer of aluminum oxynitride and oxide mixture. A small portion of residual carbon from binder burnout was bound to oxygen atoms on the AlN powder surface, and the majority of the carbon was amorphous graphitoid carbon which covered the AlN powder surface uniformly. AlN samples were made using tape casting and pressureless sintering. Surface‐carbon‐to‐oxygen ratio of AlN powder after binder burnout was evaluated using XPS. The surface C/O atomic ratios were observed to correlate with the sintering behavior, the composition of the second phase, the second phases distribution, and grain‐boundary composition, as well as thermal conductivity of AlN samples.
Effect of Compressive Stress on Fatigue of Alumina under Uniaxial Cyclic Loading
Authors: Kim Kitae T. , Suh Jeong
Published: March 2005

The effect of compressive stress on fatigue behavior of alumina was investigated under uniaxial cyclic loading. Experimental data for alumina tension specimens under uniaxial tension–unloading and tension–compression cyclic loadings were compared. This comparison suggests that compressive stress is effective in advancing the crack growth under tension–compression cycling.
Diffusion Bonds between a Stainless Steel and Zirconia
Authors: Qin Cai‐Dong , Derby Brian
Published: March 2005

Extremely strong diffusion bonds can be made directly between Y2O3‐stabilized TZP ZrO2 and Fe─18Cr─8Ni stainless steel above a critical joining temperature of about 1100°C. Fracture initiates at the metal/ceramic interface with plasticity occurring in the metal. Bonding is accompanied by a thin interfacial reaction layer which may possibly lead to local melting during the bonding cycle. This behavior is similar to that observed with Ni─Cr alloys bonded to TZP ZrO2, but a less vigorous interfacial reaction occurs with stainless steel, and stronger bonds can be made.
Numerical Models for Predicting Tracer Profiles in Silicon Double‐Oxidation Experiments
Authors: Cawley James D.
Published: March 2005

Standard numerical techniques are used to model the development of tracer profiles in “double‐oxidation” experiments involving silicon or silicon‐based ceramics. These models are more generally applicable than previously developed analytical models. In particular, the numerical model allows the relative contribution to oxidation by network and interstitial oxygen diffusion through the silica scale to be varied, and can accommodate a position‐dependent diffusion coefficient. The results of calculations are compared to previously published profiles.
Fiber Coating Concepts for Brittle‐Matrix Composites
Authors: Davis Janet B. , Löfvander Jan P. A. , Evans Anthony G. , Bischoff Ewald , Emiliani Mario L.
Published: March 2005

The current interest in tough, high‐temperature materials has motivated fiber coating development for brittle‐matrix composites with brittle reinforcements. Such coatings are needed for controlled interface debonding and frictional sliding. The system investigated in this study was sapphire fiber‐reinforced alumina. This system is thermochemically stable for severe use conditions, exhibits little thermal expansion mismatch, and utilizes the excellent strength and creep resistance of sapphire reinforcements. Porous oxide and refractory metal coatings which satisfy requirements for toughness improvement in these composites were identified by employing a variety of newly developed mechanical testing techniques for determining the interfacial fracture energies and sliding resistances.
Reactions between Synthetic Mica and Simple Oxide Compounds with Application to Oxidation‐Resistant Ceramic Composites
Authors: Cooper Reid F. , Hall Peter C.
Published: March 2005

Reaction‐couple experiments have been pursued in order to evaluate the potential of a phyllosiiicate to act as a chemically protective, fracture‐deflecting, oxidation‐resistant interphase for oxide fiber–oxide matrix composites. The synthetic mica fluorophiogopite (KMg3[AlSi3]O10F2) was reacted with single‐phase substrates of alumina (Al2O3), mullite (3Al2O3·2SiO2), forsterite (Mg2SiO4), or enstatite (MgSiO3). X‐ray spectroscopy, X‐ray diffraction, and scanning electron and transmitted polarized light microscopy were applied to the analysis of the reaction couples. Fluorophlogopite reacts strongly with alumina, mullite, and enstatite, resulting in substantial damage to the substrate as well as the breakdown of the mica. The chemical reactions between mica–alumina and mica–mullite are examined critically. In the case of alumina, the reaction results in the formation of a planar spinel (MgAl2O4) layer separating the substrate from the breakdown products of the mica. This unvarying result suggests, therefore, that a spinel diffusion barrier would prove effective in protecting alumina from fluorophlogopite. Experiments revealed such effectiveness: local equilibrium is established in the layer sequence alumina–spinel–fluorophlogopite; i.e., planar interfaces are established amongst these phases that are stable under conditions of high temperature and high oxygen fugacity. A similar chemical approach for protection of mullite is not obvious. Based on an understanding of its intrinsic fracture energy, the fluoromica interphase is expected to be effective in mechanically protecting adjacent oxides from propagating cracks, a behavior qualitatively demonstrated by indentation experiments on the kinetically persistent alumina–spinel–fluorophlogopite–spinel–alumina laminates.
Si3N4/SiC‐Platelet Composite without Sintering Aids: A Candidate for Gas Turbine Engines
Authors: Pezzotti Giuseppe
Published: March 2005

A dense and isotropic Si3N4 composite body containing 25 vol% of α‐SiC platelets, with average particle size of 24μm and aspect ratio of 8 to 10, was fabricated by hot isostatic pressing without any addition of sintering aids. In this composite, desirable properties for structural ceramics to be used in long‐term high‐temperature applications are conveniently combined: a fracture resistance comparable with that of Si3N4 sintered with conspicuous amounts of additives, as well as a superior time‐dependent strength and deformation behavior up to 1400°C, was found. The high‐temperature reliability in the present composite was improved further than that of the additive‐free Si3N4, mainly due to mechanisms operating in the wake of the crack. The key to the attainment of a valid synergism between nitride and carbide phase resides both in the presence of pure SiO2 glassy phase at the grain boundary and in the morphology of the reinforcement.
Slag–Portland Cement Based DSP Paste
Authors: Lu Ping , Young J. Francis
Published: March 2005

The partial replacement of Portland cement and silica fume by ground granulated blast furnace slag in DSP cement pastes has been investigated under different curing regimens. The influence of long‐term hydration at 25°C, as well as hydrothermal curing (at 80°C) and strong drying (at 200° and 400°C), on the composition and microstructures of slag–Portland cement DSP materials was investigated by nsing QXRD, TGA, MIP, and TMS‐GPC methods in this study. Slag is found to be an additive improving intrinsic properties of the DSP paste. CaO/SiO2 molar ratios of C‐S‐H phase in the DSP cement past have been calculated.
Low‐Temperature Structural Phase Transition in ZrO2–12 mol% CeO2 Studied by Infrared Spectroscopy and X‐ray Diffractometry
Authors: Hirata Toshiya , Zhu Haiyan , Furubayashi Takao , Nakatani Isao
Published: March 2005

The low‐temperature tetragonal (t) to monoclinic (m) phase transition in ZrO2–12 mol% CeO2 was studied by Fourier‐transform infrared spectroscopy and X‐ray diffractometry (XRD). When the ZrO2–12 mol% CeO2 was cooled, the t→m phase transition occurred at about 120 K; this caused significant changes in the infrared reflectance spectra and X‐ray diffractograms. As the phase transition proceeded, “difference spectra” obtained by subtracting the infrared spectra before and after the phase transition revealed two infrared modes at 575 and 740 cm−1 which were assigned to the m phase. The volume fraction of the m phase (determined by XRD) and the subtraction coefficient (defined such that the difference spectra reproduced the infrared spectrum of m‐ZrO2) were consistent with each other; this revealed a marked discontinuity at about 120 K. Full width at half maximum of the (111)m and (111)t reflections also indicated a similar discontinuity at about 120 K.
Powder Characteristics of Pb(Mg1/3Nb2/3)O3 Prepared by Molten Salt Synthesis
Authors: Yoon Ki Hyun , Cho Yong‐Soo , Lee Dong‐Heon , Kang Dong‐Heon
Published: March 2005

Lead magnesium niobate (Pb(Mg1/3Nb2/3)O3, PMN) was prepared by a molten salt synthesis method. Powder characteristics were greatly influenced by the processing parameters, such as amount of salt, species of salt, and the presence of excess PbO. Grown in sulfate salts, PMN crystallites showed faceted morphology with a bimodal particle‐size distribution in which the smaller particles were in the submicrometer range. The presence of excess PbO in the sulfate salts greatly influenced the faceted morphology and resulted in rounded PMN crystallites. The possible origin of particle morphology changes due to the processing parameters is discussed.
Zirconium–Hafnium Interdiffusion in Polycrystalline Fluorite‐Cubic CeO2─ZrO2─HfO2 Solid Solution
Authors: Sakka Yoshio , Oishi Yasumichi , Ando Ken , Ikeda Yoji
Published: March 2005

Zr–Hf interdiffusion was studied in the temperature range of 1650° to 1850°C in air for polycrystalline fluorite‐cubic systems of 90CeO2·10(Zr1‐xHfx)O2 and 60CeO2·40(Zr1‐xHfx)O2. Lattice and grain‐boundary diffusion parameters were calculated from the Zr–Hf concentration distributions by using the grain‐boundary diffusion equation of Oishi and Ichimura. The cation iattice diffusivity was close to that in the fluorite‐cubic Y2O2‐ZrO2 solid solution.
Fiber‐Reinforced Diamond Matrix Composites
Authors: Drory Michael D. , McClelland Robert J. , Zok Frank W. , Heredia Fernando E.
Published: March 2005

The feasibility of fiber‐reinforced diamond matrix composites was examined. For this purpose, monofilament composite specimens were fabricated using SiC fibers. The diamond was deposited onto the fibers using plasmae‐enhanced chemical vapor deposition. The mechanical properties of the composite were characterized using uniaxial tensile tests. The specimens exhibited multiple matrix cracking, followed by fiber fracture and fiber pullout. The measurements were used to obtain an estimate of the interface sliding stress and the in‐situ fiber strength.
Metal Atom‐Ceramic Binding Energies
Authors: Burns Richard P. , Gabriel Kenneth A. , Pierce Daniel E.
Published: March 2005

The binding of vapor‐deposited metal and metal oxide films to Al2O3(0001) and fused silica was investigated using thermal desorption spectroscopy combined with mass spectrometric detection of the desorbed species. The experimental value for Ed, the activation energy of desorption, is compared with HL, the calculated bond enthalpy value for an adsorbed metal‐surface oxygen bond. For Ga and Ga2O on Al2O3(0001) and fused silica, the calculated value 2HL (Ga‐O) = 200.6 kJ·mol‐1 agrees with experimental Ed values and indicates that two gallium‐oxygen bonds are broken in the desorption process (Ed= 2HL). For Pb and Sn on Al2O3(0001) and fused silica, the calculated values 3HL (Pb–O) and 3HL (Sn–O) agree with experimental Ed values (Ed= 3HL). For Cr adsorbed on Al2O3, HL (Cr–O) = 111.1 kJ·mol‐1 is calculated for one Cr‐surface oxygen bond. This value agrees, within experimental error, with Stubican's values for the activation energy of surface diffusion for Cr of 110 ± 12, 121 ± 12, and 119 ± 12 kJ·mol‐1 on Al2O3, MgO, and MgAl2O4, respectively. A calculated value of 3HL (Cr–O) = 333.3 kJ·mol‐1 agrees within experimental error with literature values for activation energy of volume diffusion of Cr in Al2O3, MgO, and MgAl2O4, Agreement between experiment and theory suggests that in these cases the surface bond breaking process involved in desorption and/or diffusion is described by an integral number of metal‐oxygen bonds similar to those in an appropriately chosen bulk oxide model. This agreement suggests that the binding model may be useful in predicting and/or correlating appropriate metal‐ceramic interactions.
Effect of Grain Boundary Impurities on the Mechanical and Tribological Properties of Zirconia Surfaces
Authors: Liang Hong , Fischer Traugott E. , Nauer Marco , Carry Claude
Published: March 2005

It is desirable to establish a relationship between the processing of ceramics, their mechanical properties that can be rapidly measured, for instance by indentation (hardness and toughness), their abrasion and scratch resistance, and their tribological performance. This paper examines such relationships for transformation‐toughened zirconia. Experiments were performed on two zirconium oxides of similar doping with yttria (3%), one of high purity and one containing grain boundary impurities. The results are compared with those of previous work, in which the composition of the zirconium oxide was changed, but the processing and grain boundary purity were constant. It is found that the relationship between mechanical properties and triboiogical performance is complex but understandable in terms of the scale of the material responses (plastic deformation and fracture) with respect to its microstructure. The yttria content of the zirconia has a large influence on the wear resistance of the material (which increases with the fourth power of toughness). At constant yttria content, impurities produce relatively small changes in mechanical properties and wear behavior. The impurities weaken the grain boundaries with the following consequences: zirconia with pure grain boundaries behaves much like a brittle continuum, the material is hard, and macroscopic cracks develop at the corners of indentations and underneath wear tracks. Grain boundary impurities lower the hardness and increase the apparent (macroscopic) toughness by crack diffusion. At low bearing load, wear occurs by microchipping, and grain boundary phases have no effect. In water, wear is increased by intergranular fracture and the presence of grain boundary phases increases the wear rate. At high loads (44.5N), macroscopic cracks underneath the wear track develop early in the pure material; these cracks are retarded by intergranular fissuration in the material with grin boundary impurities.
Chemistry and Structure of Beta Silicon Carbide Implanted with High‐Dose Aluminum
Authors: Du Honghua , Yang Zunde , Libera Matthew , Jacobson Dale C. , Wang Yu C. , Davis Robert F.
Published: March 2005

Single‐crystal β‐SiC was implanted with aluminum to 3.90 × 1017 ions/cm2 at 168 keV at 773 K. The resultant compositional and structural characteristics were studied by Rutherford backscattering spectrometry, Auger electron spectroscopy, X‐ray photoelectron spectroscopy, and cross‐sectional transmission electron microscopy. No aluminum redistribution was observed during implantation. The Si‐to‐C ratio exhibited a negative deviation from unity in the implanted region. The shift in the photoelectron binding energies indicated the formation of aluminum carbide. The studies by electron microscopy showed that the implanted region consists of slightly misoriented β‐SiC crystals and textured crystalline aluminum carbide precipitates
Coefficient of Friction Characterization of Surface‐Modified Polycrystalline Alumina
Authors: Kusy Robert P. , Keith Olga , Whitley John Q. , Saunders Carl R.
Published: March 2005

Applications in dental medicine sometimes require efficient and reproducible sliding, while maintaining biocompatibility and aesthetics. Several techniques of surface modification are explored for polycrystalline alumina materials: ion implantation, diamond‐like carbon coating, and plasma deposition. Using a light‐load, low‐velocity, single‐pass device, the frictional coefficients of orthodontic appliances are simulated. Materials that were investigated include couples of polycrystalline alumina (PCA) against stainless steel (SS) or β‐titanium (β‐Ti). Although all techniques of surface modification reduced the frictional coefficients below those values for the control couples, plasma deposition and diamond‐like carbon coating proved best for the PCA/SS and PCA/β‐Ti couples, respectively. In these cases, values decreased below those of conventional SS against SS and β‐Ti against SS, respectively.
Retention of Implanted Gases and Enhanced Thermal Expansion of Mica
Authors: Hishmeh Gary , Cartz Louis , Karioris Frank G. , Templier Claude , Chaumont Jacques , Clerc Catherine
Published: March 2005

Helium and xenon gases implanted into muscovite micas increase the reversible thermal expansion perpendicular to the silicate layers by one or two orders of magnitude. The gas‐mica composite behaves like a gas within an elastic membrane, and this can be developed into a thermal actuator or heat sensor. Helium gas is implanted into muscovite mica to simulate the anomalous thermal expansion behavior of water in phlogopite. The implanted Xe gas has been studied by Rutherford backscattering and by counting and measuring gas bubble radii. Helium gas appears to diffuse out of the structure, since the anomalous thermal expansion decreases slowly over a period of several months. All of the implanted Xe is found to remain within the mica structure.
Tribology and Hardness of Excimer‐Laser‐Processed Titanium Layers on Cubic Zirconia
Authors: Zaleski Mark A. , Jervis Thomas R. , Mayer James W.
Published: March 2005

We have examined the wear and friction and surface hardness of excimer‐laser‐processed Ti layers on cubic zirconia substrates. The film exhibits a complex array of cracking following processing that is related to the crystallographic orientation of the substrate. The friction between the laser‐processed layers and both steel and ruby pins is reduced by approximately one‐third relative to that of untreated zirconia. In the untreated case, wear is characterized by pin wear and debris, whereas the laser‐processed layer wears by film transfer to the pin. The surface hardness of the processed layer is lower than that of both the untreated zirconia and the deposited Ti film. Indentation tests indicate that the surface is brittle following processing.
Microstructural Evolution in Annealed and Crept Silicon Nitride
Authors: Chadwick Margaret M. , Wilkinson David S.
Published: March 2005

The microstructure of a commercial sintered silicon nitride has been examined in the as‐sintered condition, after annealing to a stable microstructure in either air or argon, and after creep deformation. Both as‐sintered material and material annealed to a stable microstructure were crept. Extensive analytical electron microscopy has been used to determine the composition of the intergranular material (amorphous and crystalline). The as‐sintered material contains an amorphous intergranular phase which partially devitrifies upon exposure to high temperatures. The residual intergranular glass is homogeneous throughout the samples (even though different crystalline products form near the surface and in the center of samples annealed in air) and stable. Creep deformation does not affect the microstructural evolution. It is, however, responsible for internal strain and some cavitation in the material.
Grain Size‐Microcracking Relation for NaZr2(PO4)3 Family Ceramics
Authors: Yamai Iwao , Ota Toshitaka
Published: March 2005

The grain size‐microcracking relation was examined for low thermal expansion NaZr2(PO4)3 family ceramics. By measurements of the strength, Young's modulus, thermal expansion, and grain size of polycrystalline ceramics sintered at appropriate conditions, the critical grain size for microcracking was determined. The critical grain size was proportional to the inverse square of the maximum thermal expansion difference.
Modification of SiC Ceramics Using Chemical Vapor Deposition Surface Treatment
Authors: Wang Ting‐ji , Liu Gui‐xing , Ding Pu , Jin Zong‐zhe
Published: March 2005

This paper describes experiments in depositing β‐SiC film onto SiC ceramics using a chemical vapor deposition surface treatment technique. The experimental results show that the best conditions for producing β‐SiC thin film are not at a specific „point” but rather within a „region” in the C/Si‐T graph; that X‐ray diffraction can be used to monitor the thickness of the thin film; and that, for SiC ceramics, surface treatment can improve fracture strength (by about 20%), oxidation resistance (by a factor of about 5), and resistance to heat damage. A simple model based on surface healing is presented, and two conditions for modifying ceramics are given.
Processing and Characterization of SiC‐Whisker‐Reinforced Alumina‐Matrix Composites
Authors: Smith Stanley M. , Singh Jitendra P. , Scattergood Ronald O.
Published: March 2005

Brittle monolithic alumina can be reinforced with highstrength single‐crystal SiC whiskers with the effect of increasing fracture toughness. In this study, well‐mixed and nearly fully dense SiC whisker/alumina composites were fabricated by wet‐blending the constituents and uniaxially hot‐pressing the resulting powder. The alumina‐matrix grain size depended on whisker volume fraction, whisker surface‐oxygen content, and hot‐pressing environment. Fracture toughness, measured by an indentation‐fracture method, increased from 3.0 MPa·m1/2 for the hot‐pressed unreinforced alumina to 10.7 MPa·m1/2 for the composite containing 25 vol% SiC whiskers. Fracture surfaces revealed evidence of toughening by the mechanisms of crack deflection, pullout, and crack bridging by the whiskers. The observed increase in fracture toughness of alumina due to the addition of SiC whiskers was correlated with existing models of toughening mechanisms.
Conductivity of Tazheranite Ceramics Synthesized by Controlling Composition
Authors: Kobayashi Hidehiko , Mitamura Takashi , Kuramochi Hideto , Mori Toshiyuki , Yamamura Hiroshi
Published: March 2005

Tazheranite ceramics with controlled compositions were synthesized by solid‐phase reaction, and their conductivity was measured using the complex impedance method. The CaO content in tazheranite compositions showed the highest conductivity at around 15 mol%. In particular, Ca0.81Ti0.04Oy sintered bodies revealed the highest conductivity, and this value agreed very closely with that of 15 mol% CaO‐stabilized ZrO2.
Microstructure of Mica‐Based Nanocomposite Glass‐Ceramics
Authors: Uno Tomoko , Kasuga Toshihiro , Nakayama Shin , Ikushima Akira J.
Published: March 2005

A high‐strength machinable glass‐ceramic was prepared by controlling the crystallization of ZrO2‐containing Ca0.43K0.14Mg3 (Si3AlO10)F2 glass. A unique microstructure of the glass‐ceramic was observed, using transmission electron microscopy (TEM): zirconia particles of nanometer size, 20–50 nm, were embedded in calcium mica crystal. After zirconia particles grew in nanometer size, the mica crystals were precipitated to embed the particles. The particles were identified as transformable tetragonal zirconia crystals. This texture leads to high strength (500 MPa in bending) and good machinability.
Homogeneous Fabrication and Densification of Cordierite–Zirconia Composites by a Mixed Colloidal Processing Route
Authors: Lim Byung C. , Jang Hyun M.
Published: March 2005

Based on the electrokinetic properties of aqueous silica, boehmite, and ZrO2 dispersions, cordierite‐ZrO2 composites were fabricated by a mixed colloidal processing route. The fabricated composite was characterized by a dense and homogeneous microstructure and by a uniform spatial distribution of submicrometer‐sized tetragonal ZrO2 particles throughout the matrix. Increasing ZrO2 content enhanced densification and resulted in a full density composite at 20 wt% ZrO2. Fracture toughness was also increased with increasing ZrO2 content. The enhanced toughening was partly attributed to the martensitic transformation of the dispersed tetragonal ZrO2 particles in a cordierite matrix. The formation of zircon was suppressed by suitably adjusting the heating schedule during sintering.
Effect of Humidity on Small‐Crack Growth in Silica Optical Fibers
Authors: Muraoka Mikio , Ebata Kiyoshi , Abé Hiroyuki
Published: March 2005

Subcritical small‐crack growth in silica optical fibers was measured directly in air at 25°C for several humidity conditions from 30% to 90% relative humidity. The crack velocity data were shown to be closely fitted by a power function of the stress intensity factor. It was found that the power n is independent of humidity and takes a value of 21.7. It was also revealed that crack velocity varies with relative humidity to a power of 2.5, which agrees with estimates from the published dynamic fatigue data concerning the humidity dependence of the strength of pristine silica fibers.
New Experimental Setup for Wettability Characterization under Monitored Oxygen Activity: I, Role of Oxidation State and Defect Concentration on Oxide Wettability by Gold
Authors: Chatain Dominique , Chabert Françoise , Ghetta Véronique , Fouletier Jacques
Published: March 2005

A sessile drop setup has been combined with an oxygen sensor, an electrochemical pump, and a renewable oxygen getter. The oxygen activity of a metal–oxide sample can be monitored in the 10−9–1 range, in a flowing gas, that enables the control of the defect concentration in the oxide and of the oxygen content in the metal. The stoichiometry of the substrate can also be modified by electrochemical reduction, allowing the oxygen activity of the sample to be monitored down to values as low as 10−20. The dependence on oxygen activity of the wetting and the thermodynamic adhesion of gold on sapphire, titanium oxides, and yttria‐stabilized zirconia is presented to illustrate the versatility of the equipment.
Effect of Introduction of Chloride on the Upconversion Emission Intensity in Er‐ and Yb‐Codoped ZBLAN Fluoride Glasses
Authors: Inoue Satoru , Soga Kohei , Makishima Akio
Published: March 2005

The effect of the introduction of chloride or bromide on the upconversion fluorescence intensity in Er‐ and Yb‐codopea ZBLAN fluoride glasses is investigated. The introduction of chloride enhanced the emission intensity by up to about 70% of the initial value at 550 nm for samples excited by a Nd:YAG laser. The addition of bromide increased the emission intensity at 550 nm by about 35%. The enhancement of the emission intensity seems to result from the reduction of the multiphonon relaxation rate.
Effect of Aliovalent Dopants on the Kinetics of Phase Transformation and Ordering in RE2O3‐Bi2O3 (RE = Yb, Er, Y, or Dy) Solid Solutions
Authors: Fung Kuan Zong , Chen Jong , Virkar Anil V.
Published: March 2005

Bi2O3‐based cubic solid solutions containing 20% Er2O3 undergo transformation to a rhombohedral phase when annealed at temperatures 635°C. This transformation is generally very sluggish and is accompanied by a decrease in conductivity. The kinetics of this transformation were enhanced by the addition of CaO and suppressed by the addition of ZrO2. The time constants for transformation kinetics at 600°C for CaO‐doped and undoped samples were ∼55 and ∼330 h, respectively, and the incubation periods ranged between ∼20 and ∼2000 h depending upon the dopant type, its concentration, and the temperature. This result is rationalized on the premise that cation interstitials are more mobile compared to cation vacancies. The same samples, originally cubic of CaF2 type, when annealed below about 600°C for a short period of time (a few hours) undergo a degradation in conductivity without a change in XRD patterns. The kinetics of this conductivity decay were observed to be significantly faster than the kinetics of cubic → rhombohedral phase transformation. A similar degradation of conductivity was also observed in samples containing other stabilizers such as Y2O3, Dy2O3, and Yb2O3. The kinetics of conductivity decay without the formation of the rhombohedral phase were also found to depend upon the presence of aliovalent dopants. Specifically, ZrO2 suppressed this decay. Electron diffraction showed the formation of a superstructure in samples annealed at temperatures <600°C. Analysis of the diffraction patterns suggests that the structure corresponds to a doubling of the unit cell with the ordering of cations responsible for the origin of the superstructure. The decay in conductivity in the ordered state is attributed to the expected differences in bonding between oxygen ions and the two different cations, Bi and RE.
Infrared Reflection of Ba(Mg1/3Ta2/3)O3 Ceramics
Authors: Sagala Djuniadi A. , Koyasu Shigeji
Published: March 2005

The dielectric function of the ordered Ba(Mg1/3Ta2/3)O3 ceramics was investigated by the infrared reflectance spectra taken over the 50–4000‐cm−1 range. The detailed crystal structure of the specimen was examined by the Rietveld method. The space group of trigonal D3d3 and the degree of long‐range order of 0.97 for Mg and Ta atomic arrangement were confirmed. The reflectance spectra were analyzed on the basis of the four‐parameter semiquantum model assuming 16 infrared active vibrational modes allowed for the related D3d3 structure. The lowest‐frequency optical mode was found at 60 cm−1, which can be assumed to involve the motions of the heavy TaO6 octahedra.
Energetics of Cobalt(II) Oxide with the Zinc‐Blende Structure
Authors: DiCarlo Joseph , Navrotsky Alexandra
Published: March 2005

Cobalt(II) oxide (CoO) has been prepared in the zincblende structure by the decomposition of cobalt(II) acetate under vacuum at 290°C. Calorimetric experiments have shown that the zinc‐blende structure is energetically 37 ± 10 kJ/mol less stable than the common rock‐salt form of CoO. This value is in good agreement with the value recently reported by Grimes and Lagerlof of 38 kJ/mol found by a lattice energy calculation
A Chemical Kinetics Model for Glass Fracture
Authors: Michalske Terry A. , Bunker Bruce C.
Published: March 2005

We utilize a chemical‐kinetics‐based model to describe the rate of crack extension in vitreous silica as a function of the applied stress and the presence of reactive species. Our approach builds upon previous fracture models that treat the atomic bond rupture process at the crack tip as a stressenhanced hydrolysis reaction. We derive the stress dependence for siloxane hydrolysis from measurements of hydrolysis rates for strained silicate ring structures. The stress dependence determined for siloxane hydrolysis yields an activation volume of 2.0 cm3/mol, which is in good agreement with the stress dependence determined for silicate glass fracture. This result supports previous fracture models that are based on absolute reaction rate theory and predicts an exponential dependence of crack extension rate on applied stress intensity.
Application of Sol‐Gel Processing Techniques for the Manufacture of Fiber‐Reinforced Ceramics
Authors: Russell‐Floyd R. S. , Harris B. , Cooke R. G. , Laurie J. , Hammett F. W. , Jones R. W. , Wang Tianhe
Published: March 2005

This paper discusses the application of freeze gelation of sil‐ica sols for the manufacture of low‐cost, net‐shape components in ceramic‐matrix composites (CMCs). A brief analysis is made of the central features which determine the stiffness, strength, and toughness of composite materials insofar as they may be applied to this particular class of CMCs. The critical processing parameters, relating mainly to the matrix phase, are reviewed and the results of the program so far obtained are interpreted in the context of this analysis. It is clear that interactions between processing parameters make optimization a complex process. In short‐fiber composites, matrix porosity dominates composite properties, while in continuous‐fiber systems, it is fiber degradation that controls the composite performance.