Showing papers on "Ceramic published in 1993"

Ceramic Fuel Cells

Abstract: A ceramic fuel cell in an all solid-state energy conversion device that produces electricity by electrochemically combining fuel and oxidant gases across an ionic conducting oxide. Current ceramic fuel cells use an oxygen-ion conductor or a proton conductor as the electrolyte and operate at high temperatures (>600°C). Ceramic fuel cells, commonly referred to as solid-oxide fuel cells (SOFCs), are presently under development for a variety of power generation applications. This paper reviews the science and technology of ceramic fuel cells and discusses the critical issues posed by the development of this type of fuel cell. The emphasis is given to the discussion of component materials (especially, ZrO2 electrolyte, nickel/ZrO2 cermet anode, LaMnO3 cathode, and LaCrO3 interconnect), gas reactions at the electrodes, stack designs, and processing techniques used in the fabrication of required ceramic structures.

Ceramic Powder Synthesis by Spray Pyrolysis

Abstract: A variety of spray pyrolysis (SP) techniques have been developed to directly produce ceramic powders from solutions. This paper reviews the current status of these processes in terms of the process parameters that enable the formation of particles with controlled morphology and composition. A model incorporating solute diffusion in the droplet and solvent evaporation from the droplet surface is presented to establish the critical parameters leading to solid particle formation. The model illustrates that solid particles can be obtained if solutes with high solubility and a large difference between the critical supersaturation and equilibrium concentration are used and if the process is designed to avoid solvent boiling. It is demonstrated that mixed metal oxide, non-oxide, and composite particles that are solid, hollow, porous, or fibrous can be produced by modifying the precursor characteristics, solution properties, and process parameters. The physical and chemical flexibility of SP processes offers numerous opportunities for the controlled synthesis of advanced ceramic powders and films. However, production rates are limited by the need to produce <5-[mu]m-diameter droplets and to avoid subsequent droplet coagulation. Developments in process controls, atomization, and system design are required for wider commercialization of SP-type processes.

Foundations of Materials Science and Engineering

Abstract: 1 Introduction to Materials Science and Engineering 2 Atomic Structure and Bonding 3 Crystal and Amorphous Structures in Materials 4 Solidification, Crystalline Imperfections 5 Thermally Activated Processes and Diffusion in Solids 6 Mechanical Properties of Metals I 7 Mechanical Properties of Metals II 8 Phase Diagrams 9 Engineering Alloys 10 Polymeric Materials 11 Ceramics 12 Composite Materials 13 Corrosion 14 Electrical Properties of Materials 15 Optical Properties and Superconductive Materials 16 Magnetic Properties 17 Biological and Biomaterials

Thermodynamic modeling of hydrothermal synthesis of ceramic powders

Abstract: A thermodynamic method is proposed for analyzing the hydrothermal synthesis of ceramic materials. The method utilizes standard-state thermodynamic data for solid and aqueous species and a compressive activity coefficients model to represent solution nonideality. The method is used to generate phase stability diagrams for the species that predominate in the system. The stability diagrams can be used to predict the optimum suspension synthesis conditions (i.e., feedstock composition, pH and temperature) for hydrothermal synthesis of ceramic materials. The synthesis of barium titanate and lead titanate are discussed as examples.

Injectable ceramic compositions and methods for their preparation and use

Abstract: Injectable implant compositions comprise a biocompatible ceramic matrix present in a fluid carrier, where the ceramic matrix comprises particles having a size distribution in the range from 50 μm to 250 μm. Optionally, the compositions may further comprise collagen, where the relative amounts of collagen and ceramic matrix at least partly determine the physical properties of implants formed by injecting the compositions. The fluid carrier is an aqueous buffered medium, typically including an organic polymer base material when there is no collagen present in the composition. The compositions are particularly suitable for repair and augmentation of soft and hard tissues by injection.

A new all-ceramic crown: A dense-sintered, high-purity alumina coping with porcelain

Abstract: A method of manufacturing an all-ceramic crown composed of a coping of dense-sintered, high-purity alumina with dental porcelain is described. This method takes the sintering shrinkage of alumina into consideration and makes it possible to produce individual dental copings in dense-sintered, high-purity alumina, which is a biocompatible implant material. The alumina used has density, grain size, and flexural strength within the limits of the values required in ISO 6474-1981, 'Implants for surgery, ceramic materials based on alumina'.

Bonding, structure, and properties of metal/ceramic interfaces: Part 1 Chemical bonding, chemical reaction, and interfacial structure

Abstract: Chemical bonding, chemical reaction, and the interfacial structure of metal/ceramic interfaces are reviewed with particular emphasis placed on Al and Ti alloys bonded to Al2O3 and SiC ceramics. It is shown that a number of important properties of metal/ceramic interfaces such as the wetting behaviour and work of adhesion can be predicted qualitatively from simple bonding models based on the elements in the metal and ceramic. A variety of chemical reactions can occur at metal/ceramic interfaces and equilibrium thermodynamics is useful in predicting possible reactions. The kinetics of reaction at metal/ceramic interfaces generally follow a parabolic rate law and reaction layers thicker than about 1 μm usually have a deleterious effect on interface strength. Interfacial energy minimisation dominates the atomic structure of metal/ceramic interfaces with the result that close packed planes and directions in the metal, ceramic, and reaction product when present, are often parallel across the interface. ...

Fundamental Research in Structural Ceramics for Service Near 2000°C

Abstract: Structural ceramics for high-temperature applications should embody the following properties: oxidation resistance, chemical stability, low volatility, resistance to creep deformation, resistance to creep cavitation at interfaces, sufficient toughness at ambient temperature, and thermalshock resistance. These criteria lead to five themes for fundamental research and design of high-temperature structural ceramics: chemical and environmental stability, grain-boundary sliding and cavitation, single-crystal microstructure design, room-temperature mechanical properties, and thermal shock. It is recommended that research that is confined to any one of these five areas takes into consideration the broader implications of research results. For example, microstructure designs that require weak interfaces for obtaining toughness at room temperature directly or indirectly conflict with creep and cavitation resistance needed for long-term service at high temperatures. New research should be directed at mechanisms that can simultaneously achieve good mechanical properties over a wide range of temperatures. This paper addresses the following recommendations: (i) although non-oxide systems can be viable for structural applications below 1500°C, oxidebased ceramics are necessary for service above 1500°C; (ii) microstructure designs based on acicular grain morphologies and/or single-crystal fiber reinforcements have the potential for meeting the mechanical property requirements from room temperature up to very high temperatures; (iii) for fundamental studies of mechanical properties at high temperatures, simple uniaxial tension experiments should be used in tandem with four-point bending and uniaxial compression experiments; (iv) the study of the reinforcement phase should center on very pure, highly stoichiometric materials in the case of non-oxides, and on mixed and alloyed single crystals of cubic symmetry, or crystals having isotropic properties, and large unit cells in the case of oxides; (v) the study of interfaces in non-oxides should focus on the chemistry of the intergranular glass phase, particularly the control of the oxygen content and the crystallization of this phase for improvement of high-temperature properties; (vi) the study of interfaces in oxides is best directed at the relationship between interface structure, defect chemistry, and interfacial mechanical properties over a wide range of temperature; (vii) the understanding of the micromechanisms of thermal-shock failure and the application of this understanding for designing graded interfaces that may be able to cope with thermal-expansion stresses without leading to microfracture and cavitation is important in all classes of ceramic materials, and is of critical importance in the development of oxides for very-high-temperature applications; and (viii) research in processing science should emphasize the study of basic mechanisms that lead to in-situ growth of acicular and fibrous microstructures.

Documentation of damping capacity of metallic, ceramic and metal-matrix composite materials

Abstract: High-damping materials allow undesirable mechanical vibration and wave propagation to be passively suppressed This proves valuable in the control of noise and the enhancement of vehicle and instrument stability Accordingly, metallurgists are continually working toward the development of high-damping metals (hidamets) and high-damping metal-matrix composites (MMCs) MMCs become particularly attractive in weight-critical applications when the matrix and reinforcement phases are combined to provide high-damping and low-density characteristics In selecting the constituents for an MMC, one would like to have damping capacity data for several prospective component materials Based upon data which have been published in the scientific literature, a concise documentation is given of the damping capacity of materials by three categories: (a) metals and alloys, (b) ceramic materials, and (c) MMCs

Application of magnetron sputtering for producing ceramic coatings on implant materials

Abstract: Radiofrequent magnetron sputtering was used to produce calcium phosphate coatings on metal and plastic substrates. Scanning electron microscopy showed that the deposited films had a uniform thickness and a dense columnar structure. Energy-dispersive X-ray analysis, X-ray crystal diffraction and atomic absorption spectrometry demonstrated that the sputtered layer was well-crystallized calcium phosphate ceramic with a Ca/P ratio varying between 1.9 and 2.5. The biocompatibility of the coatings was determined by in vitro and in vivo experiments. It was found that the coatings were biocompatible without any sign of adverse tissue reaction. It was concluded that magnetron sputtering is a promising method for forming a biocompatible ceramic coating onto an implant material. Nevertheless, several problems have to be solved before magnetron sputtering can be used on a routine basis for the production of Ca/P coatings.

Dielectric properties of tetragonal lanthanum modified lead zirconate titanate ceramics

Abstract: Tetragonal La‐modified lead zirconate titanate ceramics with a Zr/Ti ratio of 40/60 (x/40/60) with La contents (x) between 0 and 21 at. % were prepared by the mixed oxide method and were subsequently characterized using dielectric spectroscopy. Small La contents tended to diffuse the dielectric response in the temperature region around the phase transition, however, no distinct changes were observed in the paraelectric region. Compositions with intermediate La contents showed a thermally driven (spontaneous) transformation from a relaxor to a normal ferroelectric, i.e., micro‐macro domain switching. Materials with a high La content exhibited typical relaxor ferroelectric behavior. The relative thermodynamic stability of the ensemble of micropolar regions is then discussed in reference to that of the long‐range ferroelectric state.

Effective Elastic Moduli of Porous Ceramic Materials

Abstract: This paper compares the applicability of a few theoretical models for determining effective elastic moduli, using published experimental data on ceramic materials in a porosity range of 0–40% and on a cellular material with a porosity of about 90%. As the experimental data for the effective Poisson's ratio involve a large scatter, a set of numerical experiments using the finite element method was carried out to obtain the variation of the effective Poisson's ratio with porosity. These variations show that the effective Poisson's ratio approaches 0.25 with increasing porosity, irrespective of the material Poisson's ratio. The effect of pore shapes on the effective elastic moduli and the Poisson's ratio has also been analyzed using FEM.

Heater having a multiple-layer ceramic substrate and method of fabrication

Abstract: A heater having a multiple-layer ceramic substrate and a method for fabricating the heater are provided. The heater consists-of a plurality of ceramic layers which are laminated to form a single ceramic substrate. A plurality of resistive heating elements are deposited onto the multiple-layer ceramic substrate, which are connectable to a power source via conductive elements which extend through the substrate to the resistive heating elements. The heater may also include a terminal that allows for convenient electrical and mechanical interfacing to a smoking article.

Cordierite glass-ceramics for multilayer ceramic packaging

Abstract: Sinterable cordierite glass-ceramics developed for use in fabricating copper-glass-ceramic multilayer substrates have been studied to understand the important role of the composition in sintering, crystallization, and microstructural development

Drying of Granular Ceramic Films: I, Effect of Processing Variables on Cracking Behavior

Abstract: Drying of binder-free granular ceramic films was studied to identify processing variables which affect their cracking behavior. Films were prepared from electrostatically stabilized suspensions of α-alumina in water. A critical cracking thickness (CCT) was determined, above which films would spontaneously crack during drying. The effects of particle size, liquid surface tension, drying rate, dispersion stability, and sedimentation time were evaluated by a statistical design methodology. The CCT for films prepared on glass substrates was used as a measure of the effect of each variable on cracking. The statistically significant variables were particle size, dispersion stability, and sedimentation time. The effect of substrate constraint was also studied by producing films on a Teflon substrate and a pool of liquid Hg. The observations were consistent with a capillary formed tensile stress acting on the entire film rather than differential stress generated by a moisture gradient over the film thickness.

Ceramic Matrix Composites

Abstract: The present state of the knowledge of ceramic-matrix composites have been reviewed. The fracture toughness of present structural ceramics are not enough to permit design of high performance machines with ceramic parts. They also fail by catastrophic brittle fracture. It is generally believed that further improvement of fracture toughness is only possible by making composites of ceramics with ceramic fibre, particulate or platelets. Only ceramic-matrix composites capable of working above 1000 degree centigrade has been dealt with keeping reinforced plastics and metal-reinforced ceramics outside the purview. The author has discussed the basic mechanisms of toughening and fabrication of composites and the difficulties involved. Properties of available fibres and whiskers have been given. The best results obtained so far have been indicated. The limitations of improvement in properties of ceramic-matrix composites have been discussed.

Modeling 1-3 composite piezoelectrics: hydrostatic response

Abstract: A simple physical model of 1-3 composite piezoelectrics that was advanced for the material properties relevant to thickness-mode oscillations is extended to address the hydrostatic response. The model is valid when the lateral spatial scale of the composite is sufficiently fine that the composite can be treated as an effective homogeneous medium. Expressions are derived for the composite's material parameters in terms of the volume fraction of piezoelectric ceramic and the properties of the constituent piezoelectric ceramic and passive polymer. The results are similar to those derived by Haun and Newnham (1983, 1986) using a parallel-series connectivity model. The model is illustrated by analyzing composites made from conventional PZT5 and anisotropic modified lead titanate piezoelectric ceramics. For PZT5, the composite structure enhances its hydrostatic charge coefficient, hydrostatic voltage coefficient, hydrophone figure of merit, and hydrostatic coupling coefficient, while three of these quantities fall short of their pure ceramic values in the modified lead titanate composites. The shortfall is due to an enhanced composite that arises from lateral stress on the polymer being transferred to a longitudinal stress along the ceramic rods by the Poisson effect in the polymer, thus producing a charge through the ceramic's d/sub 33/. >

CVD of silicon-based ceramic materials on internal surface of a reactor

Abstract: In order to reduce the rate of coke formation during the industrial pyrolysis of hydrocarbons, the interior surface of a reactor is coated with a thin layer of a ceramic material, the layer being deposited by thermal decomposition of a non-oxygen containing silicon-nitrogen precursor in the vapor phase, in an inert or reducing gas atmosphere in order to minimize the formation of oxide ceramics.

Experimental and Numerical Study of Premixed Combustion Within Nonhomogeneous Porous Ceramics

Abstract: An experimental and numerical investigation of premixed methane combustion within a nonhomogeneous porous ceramic was performed. The burner consisted of two porous ceramic cylinders of equal length and diameter that were stacked together and insulated around the circumference. Four series of experiments were carried out to determine the lean limit using three different pore sizes in the downstream ceramic cylinder (SBR). The pore size in the upstream ceramic cylinder was constant in all four cases. A new definition of the lean limit was introduced to account for the effects of the porous ceramic. The burners were tested over the range of lean limit 0.41 < φ≤ 0.68 and the numerical simulations were performed over 0.43 < φ ≤ 1.0. The results demonstrated that porous ceramic burners provide a range of stable burning rates at a constant φ, The maximum flame speed inside the burners was much higher than the premixed, freely burning adiabatic laminar flame (free flame) speed. The lean limits in the por...

Principles of particle selection for dispersion-strengthened copper

Abstract: A new fundamental approach to the design of high strength, high thermal conductivity dispersion-strengthened copper alloys for applications in actively cooled structures is developed. This concept is based on a consideration of the basic principles of thermodynamics, kinetics and mechanical properties. The design requirements for these materials include a uniform distribution of fine particles for creep and fatigue resistance, a high thermal conductivity, thermodynamic and chemical stability at temperatures up to 1300 K, a small difference in the coefficients of thermal expansion between the particle and matrix, and low particle coarsening rates at the processing and service temperatures. The theory for creep of dispersion-strengthened metals developed by Rosler and Arzt is used to predict the optimum particle size for a given service temperature and to illustrate the need for a high interfacial energy. Resistance to coarsening leads to a requirement for low diffusivity and solubility of particle constituent elements in the matrix. Based on the needs for a low difference in the coefficients of thermal expansion to minimize thermal-mechanical fatigue damage and low diffusivity and solubility of the constituent elements, several candidate ceramic phases are compared using a weighted property index scheme. The results of this quantitative comparison suggest that CeO2, MgO, CaO and possibly Y2O3 may be good candidates for the dispersed phase in a copper matrix.

Flexible nonwoven mat

Abstract: The present invention relates to a flexible nonwoven mat comprising physically entangled shot-free ceramic oxide fibers. The flexible nonwoven mat is useful, for example, as filter material, mounting mat, thermal insulation, and sound insulation.

Advanced ceramics: Combustion synthesis and properties

Abstract: Fine-particle ceramic powders such as chromites, manganites, ferrites, cobaltites,aluminas ($\alpha-Al_30_3 , Cr^{3+}/A1_20_3$, zirconia-toughened alumina, mullite and cordierite), ceria,titania, zirconia (t, m, c and PSZ), dielectric oxides ($MTiO_3, PZT and PLZT) as well as high T, cuprates have been prepared by the combustion of redox compounds or mixtures.The combustion-derived oxide materials are of submicron size with a large surface area and are sinteractive.

Microwave-hydrothermal processing for synthesis of electroceramic powders

Abstract: Microwave-hydrothermal processing has so far been used only to dissolve inorganic solids for chemical analysis. We report herein the use of microwave-hydrothermal processing to synthesize various ceramic powders in binary and polynary systems. We describe the synthesis of some electroceramic powders such as BaTiO3, SrTiO3, Sr0.5Ba0.5TiO3, PbTiO3, BaZrO3, SrZrO3, Pb(Zr0.52Ti0.48)O3, and pyrochlore phases with the Pb(Mg1/3Nb2/3)O3 and Pb(Zn1/3Nb2/3)O3 compositions by this novel microwave-hydrothermal processing technique.

BaTi4O9/Ba2Ti9O20-based ceramics resurrected for modern microwave applications

Abstract: Ceramics based on BaTi 4 O 9 and Ba 2 Ti 9 O 2 0 have been largely ignored over the past decade by manufacturers of high Q, high dielectric constant, temperature compensated materials for commercial wireless communications at elevated frequencies. When processed correctly, these ceramics yield properties that rival if not exceed those of ZrTiO 4 -based products currently used. Materials incorporating ZnO/Me 2 O 5 (Me = Ta, Nb) and their properties are described.

Drying of Granular Ceramic Films: II, Drying Stress and Saturation Uniformity

Abstract: Films composed of ceramic particles were observed during drying. The films were prepared from 20 vol% aqueous dispersions of α-alumina and α-quartz and were free of any organic binder. Conditions for uniform film saturation during drying were established by consideration of a liquid transport model and by direct observation of the drying films. Drying stresses were measured in situ by a substrate deflection method based on an optical interference technique. Simultaneous stress and weight measurements were used to determine stress as a function of saturation. The maximum stress occurred near 100% saturation and was approximately 2 and 1.1 MPa for films produced from 0.35-and 0.68-μm particles, respectively. The maximum stress decreased from 2 to 0.9 MPa for films produced from the 0.35-μm particles when 0.005 wt% surfactant was added to the slurry. The surfactant decreased the liquid surface tension from 72 to 32 dyn/cm. These observations are direct evidence of the effects of capillary tension on the state of stress in a ceramic body. Mechanical properties of the green ceramic films were estimated by use of a linear elastic fracture model. Knowledge of the critical cracking thickness and maximum stress in the film was used to estimate the fracture resistance of the granular film. The fracture resistance values are approximately 0.02 and 0.007 MPa·m1/2 for films produced from alumina and silica, respectively. The difference in mechanical behavior of the silica and alumina films is similar to that expected by the difference in Hamaker constants between the two materials.

Indentation fatigue : a simple cyclic Hertzian test for measuring damage accumulation in polycrystalline ceramics

Abstract: A simple Hertzian contact procedure for investigating cyclic fatigue damage in brittle polycrystalline ceramics is described. Repeat loading of a spherical indenter on a coarse alumina ceramic produces cumulative mechanical damage. The mode of damage is one of deformation-induced intergranular microfracture, leading ultimately at large numbers of cycles and high contact pressures to severe grain dislodgement. In contrast to the classical Hertzian cone cracks that form in more homogeneous materials in the regions of tensile stress outside the contact circle, the damage in the coarse-grain alumina develops in a zone of high shear stress and hydrostatic compression beneath the contact circle