Showing papers on "Ceramic published in 1975"

The physics of metal oxide varistors

Abstract: This paper outlines our present understanding of the conduction mechanisms and physical processes relevant to the performance of ZnO−based ceramic varistors. Varistor behavior is determined by the gross ceramic microstructure of the device as well as by the localized conduction processes which occur between grains. We show that the qualitative features of the highly nonlinear conductivity are largely independent of the details of varistor composition or processing but rather appear to be a general effect engendered by a microstructure of conducting grains surrounded by thin insulating oxide barriers. Evidence is presented from a variety of sources that this intergranular layer is ∼100 A in thickness resulting in grain−to−grain fields of F∼106 V/cm. The conduction mechanism at breakdown is consistent with a Fowler−Nordheim tunneling process obeying a current−density−vs−field relation given by J∝exp(−γ/F), where γ is a constant. At somewhat lower fields (prebreakdown region) the conduction process follows a...

Engineering materials and their applications

Abstract: Partial table of contents: FUNDAMENTALS. The Structure of Engineering Materials. Effects of Stress on Structure. Effects of Temperature on Structure and Mechanical Properties. The Control of Nonequilibrium Structures. METALS AND ALLOYS. Processing Metals into Components. Plain-Carbon and Low-Alloy Steels. High-Alloy Steels, Superalloys, Cast Irons. CERAMIC MATERIALS. Conventional Ceramic and Glass Structures. Advanced Structural Ceramics. Processing of Ceramics. POLYMERIC MATERIALS. Special Polymer Products. Processing of Polymers. COMPOSITE MATERIALS. Synthetic Composite Materials. Traditional Composites: Concrete, Asphalt, and Wood. SPECIAL TOPICS: EFFECTS OF ENVIRONMENT ON MATERIALS. Analysis and Prevention of Failure. Electrical and Optical Properties of Materials. Magnetic Properties of Materials. References. Index.

Deformation of Ceramic Materials II

Abstract: In recent years there has been extensive progress in the research on plastic deformation of ceramic materials. This volume constitutes the proceedings of an international symposium held at Pennsylvania State University in July of 1983. It includes studies of semiconductors and minerals which are closely related to ceramic materials. This conference emphasized the deformation behavior of crystals and polycrystalline and polyphase ceramics with internationally recognized authorities as keynote lecturers on the major subtopics. The papers in this volume discuss dislocation dynamics and deformation of single crystals, including binary oxides, ternary oxides, silicates, nonstoichiometric oxides, covalent materials and halides; and the effects of point defects and twinning. Others address deformation of polycrystalline ceramics consider oxides, nonoxides, polyphase materials and superplastic deformation. There are also several papers dealing with cavity nucleation and creep crack growth, representing a major new research thrust in ceramics research.

Microstructure and phase transformation in a highly non−Ohmic metal oxide varistor ceramic

Abstract: The microstructure of a complex multicomponent varistor ceramic based on ZnO with small additions of antimony, bismuth, cobalt, manganese, and tin oxides has been elucidated using a variety of structural techniques. Three crystalline phases are found to coexist in the sintered material. The bulk phase consists of a polycrystalline matrix of ZnO doped with Co(II). The ZnO grains are separated from one another by a Bi2Zn4/3Sb2/3O6 pyrochlore phase which forms a three−dimensional threadlike network, indicative of a liquid phase at high temperatures. A Zn(Zn4/3Sb2/3)O4 spinel constitutes the third phase and forms well−faceted octahedral crystals located frequently at grain boundaries between the ZnO and occasionally within the grains. This phase acts as a grain growth moderator for ZnO by anchoring the boundaries during sintering, but plays no role in the nonlinear characteristic of the varistor. A quasiequilibrium between the pyrochlore and spinel has been established such that formation of the spinel is favored at high temperatures. The over−all varistor microstructure may be described as a three−dimensional series−parallel network of ZnO−pyrochlore junctions.

High resistivity BaTiO3 ceramics sintered in CO-CO2 atmospheres

Abstract: A barium titanate ceramic containing 13.5 mol % calcium zirconate was doped with up to 3 mol % of oxides of various metals that were considered likely on the basis of ionic size and valence to enter the small cation lattice with charge less than 4+. It was hoped in this way to compensate electrically for loss of oxygen during sintering in CO-CO2 mixtures, so as to obtain high resistivity dielectrics and allow the use of base metal electrodes in a monolithic capacitor. Doping with approximately 0.5 mol % Mn, Co, or Mg produced the highest resistivities, and dielectrics with nickel electrodes and relative permittivity up to 10 000 were obtained with resistivity in excess of 1012 Ω cm at room temperature. When the doped ceramic sintered in contact with nickel, the grain structure and permittivity-temperature characteristics depended on the oxygen partial pressure of the sintering atmosphere, apparently influenced by dissolution of Ni into the ceramic.

The Electrophoretic Forming of Beta‐Alumina Ceramic

Abstract: The forming or shaping of beta‐alumina ceramic is usually difficult with conventional ceramic processing. In this paper, electrophoretic deposition is shown to be an operationally simple, rapid, and reliable technique for forming a wide range of beta‐alumina shapes required in electrochemical devices. The following topics are discussed in detail: the basis for selection of a suitable vehicle for holding beta‐alumina particles in suspension, the control of electric charge on the particles, the milling of starting powder to provide a small particle size with minimum contamination by wear of the grinding media, the deposition procedure, and the removal of electrophoretic deposits from forming mandrels. Finally, some properties are given for sintered ware formed by electrophoretic deposition.

Strength-Size Relationships in Ceramic Materials: 1. Investigation of a Commercial Alumina. 2. Effect of Flaw Shape on Strength of Ceramics.

Abstract: : Bend strengths of a commercial sintered alumina, 3M Company's Alsimag 614, are reported. Fracture stresses in specimens differing in each linear dimension by a factor of five were measured at room temperature under environmental conditions which either minimized or enhanced subcritical crack growth prior to catastrophic fracture. Strength was found to be dependent on specimen size under both test conditions. Fractographic identification of strength-controlling flaws, coupled with the analysis of strength-size data, indicated a limited applicability of Weibull's statistical approach.

A comparison of iliac marrow and biodegradable ceramic in periodontal defects.

Abstract: Defects were created in the tooth-supporting bone of six beagle dogs. Biodegradable tricalcium phosphate was compared to cancellous marrow as a treatment method. The findings indicated that ankylosis and resorption of the tooth can occur when using marrow. The ceramic-filled defects healed slower; but the material itself was well tolerated by the tissues, initiated new bone formation, filled in the defect, and has good potential for the treatment of advanced periodontitis.

An Analysis of the Impedance of Polycrystalline Beta‐Alumina

Abstract: A procedure is described in this paper for distinguishing in the measured electrical properties of polycrystalline β‐alumina, the separate contributions of the grain boundaries and of the crystal, i.e., the grain interiors. This separation is brought about through the use of a model for these properties. Certain quantitative consequences of the model are developed and compared with experimental results. For most sintered β‐alumina ceramic, the electrical properties are determined more by the characteristics of the grain boundaries than by those of the interior of the grains.

A Model for Degradation of Ceramic Electrolytes in Na-S Batteries

Abstract: Ceramic electrolytes derived from β-alumina are cracked by electrolysis under certain conditions. Cracks form only when Na+ ions migrate through the ceramic and are converted to metallic Na, as in the charging of an Na-S battery. The origins of mechanical damage are postulated and examined. A plausible mechanism consists of ceramic dissolution at preexisting surface cracks; it depends on the interaction of capillarity, stress, and selective removal of electrolyte from crack tips by effluxing Na. Relative solute fluxes are calculated for each contributing process. The proposed model is shown to account for many of the experimental observations, most notably a threshold current density below which no degradation is apparent.

Oxygen sensors using CoO ceramics

Abstract: Ceramic CoO has been found to be a material suitable for oxygen sensing. The CoO sensor has good sensitivity and fast response time and appears particularly suited for applications involving high temperatures and corrosive or contaminating environments, e.g., combustion processes.

Method for producing ceramic cellular structure having high cell density

Abstract: A ceramic cellular structure having a cell density of up to 1600 cells per square inch and wall thicknesses down to 2 mils is produced by a process which comprises: (1) forming a slurry consisting essentially of finely divided sinterable solid particles of ceramic raw materials and a controlled amount of a plastic supporting matrix containing thermoplastic resin, a thermosetting resin, a plasticizer, an organic solvent and a small amount of a deflocculant; (2) ball milling the slurry; (3) casting the slurry in the form of a film; (4) removing the solvent to produce a self-supporting green ceramic tape; (6) molding a portion of the tape to form a corrugated first member; (7) providing another portion of the tape to form a substantially flat second member; (8) forming a first bilayer by bonding the second member to the nodes of the first member; (9) mutually bonding a predetermined number of bilayers substantially identical to the first bilayer to form a cellular green structure of a desired shape; and (10) firing the green structure at a temperature sufficient to form a sinter-welded polycrystalline ceramic structure. The fired structures, after being subjected to various finishing operations, are useful as rotary heat regenerators, stationary heat recuperators and catalyst supports.

Porous ceramic or metallic coatings and articles

Abstract: A method for preparing porous ceramic or metallic coatings on substrates and preparing perforated articles which are ceramic or metallic; the method involving utilization of a composition comprising ceramic powder or metallic powder, binder, and solvent whereby at least a portion of the solvent is rapidly volatilized to obtain the porosity in the ceramic or metallic material. The coated substrates and perforated articles are useful for example, as prosthetic devices, catalyst supports, and bone supplementation material.

Chemical bonding of metals to ceramic materials

Abstract: A refractory oxide ceramic is chemically bonded to a metal by disposing the surfaces to be joined in an abutting relationship and heating at least the abutting surfaces in air to a temperature below the melting point of the lowest melting component of the system, the temperature being sufficient to cause a chemical reaction between the ceramic oxide and the metal thereby forming a bond between the two abutting surfaces.

Method for producing a sintered silicon nitride base ceramic and said ceramic

Abstract: A silicon nitride based ceramic is formed of silicon nitride and at least two metal oxides of such a type that when the metal oxides are heated separately they form a spinel. Combination of said metal oxides and said silicon nitride as fine powders and sintering same at a specified temperature for a specified period of time results in a silicon nitride based ceramic having improved mechanical and chemical properties.

Ceramic mounting and heat sink device

Abstract: This relates to a multi-layer composite metallic sheet having a predetermined coefficient of thermal expansion for use as a heat sink and mounting arrangement when bonded to a ceramic substrate. Since copper has a coefficient of thermal expansion greater than that of ceramic and molybdenum has a coefficient of thermal expansion less than that of ceramic, layers of copper and molybdenum can be combined to provide a composite having a coefficient of thermal expansion equal to that of ceramic. To avoid a bimetallic effect, a core layer of a fused metal is sandwiched between two outer layers of the second metal.

Effect of the grain size on the conductivity of high‐purity pore‐free ceramics Y2O8–ZrO2

Abstract: Measurements of the electrolyte overall resistance as well as the grain volume and boundary resistances comprising it, reveal that the grain volume resistance remains constant while that of grain boundaries and accordingly of the electrolyte decreases with the increase of the average grain size and sintering temperature. The activation energy for conductivity of the grain boundaries is higher than that of the grain volume due to which at high temperatures the electrolyte conductivity is mainly determined by the grain conductivity, while at low temperatures it is the boundary conductivity that matters. Durch Messung der Impedanz des Elektrolyten und der diese zusammensetzenden Widerstande der Korngrenzen und des Kornvolumens wurde gezeigt, das der Widerstand des Kornvolumens konstant bleibt wahrend der Widerstand der Korngrenzen und damit der des Elektrolyten bei Erhohung der mittleren Korngrose und der Gluhtemperatur abnimmt. Die Aktivierungsenergie der Leitfahigkeit der Korngrenzen ist hoher als die des Kornvolumens. Infolgedessen wird die Elektrolytenleitfahigkeit bei erhohten Temperaturen im wesentlichen durch die Kornleitfahigkeit, bei den niedrigen durch Grenzenleitfahigkeit bestimmt.

Integrated ceramic-metal combustor

Abstract: Integration of separate ceramic parts and metal parts to form a unified combustor is described. The flame holding portion and any other portion where air is introduced into the combustor are made of metal while insulated, imperforate ceramic construction is used for the balance of the combustor liner. In this manner the regions of high local stress are accommodated by cooled metal and the regions of uniform thermal stress are accommodated by resiliently-mounted thermally-insulated ceramic.

Ceramic compositions and articles made therefrom

Abstract: This invention relates to slurry compositions which are adapted for the formation of ceramic articles. The compositions comprise ceramic particulate matter blended with an aqueous dispersion of an internally plasticized thermosetting resin. These compositions are formed into green (unfired) ceramic tapes which can be ultimately used for electronic purposes wherever fired rigid ceramic bodies are required. The use of these compositions overcomes many of the prior disadvantages relating to dimensional stability, flexibility, atmospheric stability and compatibility with a wide variety of conventional metallizing pastes.

Stoichiometric air/fuel ratio exhaust gas sensor

Abstract: A stoichiometric air/fuel ratio exhaust gas sensor construction particularly useful with variable resistance partial pressure of oxygen responsive sensor material is disclosed. A generally cylindrical mounting body, formed of corrosion resistant material, is arranged for receipt within the exhaust system of an internal combustion engine and is provided with a two-part ceramic insert member for support of a wafer of variable resistance sensor material. The ceramic insert member is comprised of a first portion having a slotted tip for receipt and support of the wafer of partial pressure of oxygen responsive ceramic sensor material. The ceramic insert member is arranged to position the wafer within a perforated shield member at a remote free end of the cylindrical housing body. The wafer includes a pair of extending high temperature resistant electrical conductors which are received within bores provided therefor in the ceramic insert member. The extending leads are electrically connected, proximate to the interface of the two-part ceramic insert member, to a pair of relatively flexible, electrically low resistance, high temperature resistant extending lead wires which are received within the second portion of the ceramic insert member.

Pyroelectric Response in Transparent Ferroelectric Glass‐Ceramics

Abstract: When the network system is limited, most ferroelectric glass-ceramics will produce a fully crystallized transparent body when cerammed at low temperatures. Higher-temperature heat treatment results in crystal growth and gradual loss of transparency. In the transparent state, the crystal size is limited to ∼2000 A. Piezoelectric and hysteresis loop measurements have failed to detect evidence of ferroelectricity, but the present pyroelectric measurements show that fully transparent glass-ceramics can be ferroelectric.

Measurement of thermal properties of nuclear materials by laser flash method

Abstract: The present status and prospects of the laser flash method for the measurement of thermal properties of nuclear materials are reviewed. Special emphasis is placed on the progress seen in experimental techniques for obtaining precise data on heat capacity and thermal diffusivity from 80 to 1,000 K. In this temperature range, the heat capacity and the thermal diffusivity of ceramic nuclear materials can be determined within a precision of ±2%. The data on heat capacity and thermal conductivity determined by the laser flash method are presented and discussed in respect of such nuclear materials as U02 U4O9 ThO2 BeO, ThO2-UO2 ThO2-Ce02 UO2-ZrO2 ThO2-Be0, UC, UN, US, UP, UC1-xNx and UP1-xSx.

Sintering in the Presence of Liquid Phase

Abstract: In recent years considerable progress in materials development has been achieved by applying the methods of liquid phase sintering well known from heavy alloy and hard metal production Sintering of cobalt rare earth magnets (1–4), hot pressing of silicon nitride structural ceramics in the presence of a liquid phase (5–8), supersolidus sintering of superalloys (9–12), accelerated diffusion alloying for powder forging (13) or the development of hardenable cemented carbides (14–16) may be listed here as the most prominent examples These technological achievements, however, were for a long time hardly matched by the scientific work which is necessary not only for a basic understanding of liquid phase sintering but also for an improvement of the materials made by this method

Ceramic foam and method of preparation

Abstract: Ceramic foams possessing controlled permeability and uniformity are prepared by the impregnation of an open-celled organic polymer foam material possessing a predetermined permeability and resilience with a aqueous slurry of a thixotropic ceramic composition while shearing said slurry an amount sufficient to maximize said impregnation, and expelling excess slurry from said material by conducting at least two passes of said material through preset rollers to effect a temporary compression ranging from about 50 to 90% for the first pass and 70 to 90% for the second pass. The resulting material is then dried and heated to remove the organic foam component. The foams prepared in accordance with the present invention are useful as filters for molten aluminum.

Nature of an intergranular thin−film phase in a highly non−Ohmic metal oxide varistor

Abstract: A detailed examination by transmission electron microscopy and selected−area diffraction revealed that the thin−film intergranular phase in a highly non−Ohmic multicomponent metal oxide varistor system based on ZnO with small additions of antimony, bismuth, cobalt, manganese, and tin oxides is amorphous and is chemically related to a Bi2O3−rich pyrochlore coexisting in the ceramic.

Continuous system for providing a catalytic coating on support members

Abstract: A processing system which can provide a continuous assembly-line procedure for placing a catalytic coating on ceramic honeycomb type members embodies the use of a plurality of pressure tight chambers such that each ceramic member can be vacuum treated and then rapidly coated, as well as pressure treated and air blown to effect rapid coating fluid removal and partial drying. After each coating operation, there is controlled drying and high temperature curing. Also, in a preferred controlled system, there is a weighing of each element before and after each coating step to determine an amount of coating retained for comparison with a predetermined standard.