Showing papers on "Sintering published in 1970"

Alumina as a ceramic material

Abstract: INTRODUCTION. NOMENCLATURE. PREPARATION OF ALUMINA PHASES. Bauxite. Preparation of Bayer Alumina. Wet Alkaline Processes. Wet Acid Processes. Furnace Processes. Carbothermic Processes. Electrolytic Processes. Amorphous and Gel Aluminas. Preparation of the Alumina Trihydroxides. Gibbsite. Bayerite. Nordstrandite, Bayerite II, Randomite. Preparation of the Alumina Monohydroxides. Boehmite. Disapore. Transition Aluminas. Dehydration Mechanism. Sequence of Transition. Phases Formed on Aluminum. Rehydration. Alpha Alumina. Preparation. Factors Affecting Alumina Transitions. Special Ceramic Aluminas. Beta and Zeta Aluminas. Suboxides and Gaseous Phases. STRUCTURE AND MINERALOGICAL PROPERTIES. Structure of the Alumina Phases. Pseudomorphosis. Surface Area of Alumina. Porosity. Sorptive Capacity. MECHANICAL PROPERTIES OF ALUMINA. General Considerations. Bending, Compressive, Tensile, and Torsional Strength. Impact Strength. Moduli of Elasticity (E), and Rigidity (G). Poisson's Ration (i). Creep Characteristics. Thermal Shock. Internal Friction. Fatigue. Hardness and Abrasiveness of Alumina THERMAL PROPERTIES. Thermophysical and Thermochemical Constants. Specific Heat. Thermal Expansion. Thermal Conductivity. Thermal Diffusivity SONIC EFFECTS IN ALUMINA. Velocity of Sound in Alumina. Ultrasonic Absorption. ELECTRICAL PROPERTIES OF ALUMINA. Introduction. Electrical Conductivity of Alumina. Dielectric Constant and Loss Factor of Alumina. Dielectric Strength MAGNETIC PROPERTIES OF ALUMINA. Magnet Susceptibility. Magnetic Resonance of Alumina. OPTICAL PROPERTIES OF ALUMINA. Refractive Index of Alumina. Transmission, Emissivity, and Absorption of Alumina. Phosphorescence, Fluorescence, and Thermoluminescence. Optical Spectra of Alumina. Color in Alumina. Chromia-Alumina System, Laser Applications RADIATION AND ALUMINA. CHEMICAL PROPERTIES OF ALUMINA. Wet Chemical Reactions of Sintered Alumina. Reaction of the Chemical Elements with Alumina. Slagging Effects. Ash Slags. Slags Containing Sulfates. Steel Furnace Slags. Glass Furnace Reactions. Calcium Aluminate Slags. Aluminum Slag Reactions. Miscellaneous Reactions COLLOIDAL PROPERTIES OF ALUMINA. Plasticity. Surface Charge and Zeta Potential of Alumina. Flocculation and Deflocculation Effects. Additives GRINDING CERAMIC ALUMINA. FORMING ALUMINA CERAMICS. Cold Forming of Alumina. Hot-Pressing. Miscellaneous Forming Methods SINTERING. Introduction. Sintering Atmospheres. Sintering Additives ALUMINA IN REFRACTORIES. General. High-Alumina Refractories. Fused Cast Alumina Refractories. Clay-Bonded Alumina Refractories, Mullite Refractories. Spinel, Cordierite, Alumina-Chromite. Refractory Equipment. Refractories for Aluminum and Other Nonferrous Uses. Lightweight Alumina Refractories. Binders for Alumina Refractories ALUMINA AS AN ABRASIVE MATERIAL. Introduction. Loose Grain Abrasive. Grinding Wheels. Ceramic Tools ELECTRICAL APPLICATIONS. Spark Plug Insulators. Electron Tube Elements, High-Frequency Insulation. Alumina Porcelain Insulation. Resistors and Semiconductors CEMENT. Calcium Aluminate Cement. Barium Aluminates ALUMINA IN GLASS. Introduction. Bottle Glass. Devitrified Glasses Containing Alumina. Boron Glasses. Lithium Glasses, Phosphate Glasses. Optical Glasses ALUMINA IN COATINGS. Introduction. Anodic Coatings on Aluminum. Glazes and Enamels. Flame-Sprayed Coatings. Painted, Cast, or Troweled Coatings. Electrolytic Coatings. Evaporated Coatings. Dip Coatings, Cementation Coatings. Coatings on Alumina and Other Ceramic Bases. Alumina Coatings for Electrical Insulation. Alumina Coatings by Sputtering ALUMINA IN CERMETS AND POWDER METALLURGY. Introduction. Chromium-Alumina Cermets. (Iron, Nickel, Cobalt)-Alumina Cermets. Aluminum-Alumina Alloys. Miscellaneous Cermets ALUMINA IN AIRBORNE CERAMICS. Introduction. Gas-Turbine Accessories. Radomes and Rocket Equipment. SEALS, METALLIZING, WELDING. FIBERS, WHISKERS, FILAMENTS. Introduction. Alumina Fibers. Glass Fibers MISCELLANEOUS CERAMIC APPLICATIONS OF ALUMINA. References.

Initial Sintering with Constant Rates of Heating

Abstract: Initial sintering of several materials was studied by measuring powder compact densification at constant rates of heating (CRH). The CRH technique was extremely sensitive to the particle size distribution and other characteristics of the compacts. Although the CRH method circumvents several problems encountered in isothermal studies, it cannot be used to identify the mechanism of diffusion. Using the method on carefully prepared alumina powder compacts and assuming a grain-boundary diffusion mechanism, an activation energy of 115±10 kcal/mol was obtained. Zirconia (yttria-stabilized) and titania also exhibited a single densification mechanism with diffusion coefficients which correlate well with values obtained by isothermal measurements.

Effect of TiO2 on Initial Sintering of Al2O3

Abstract: Alpha alumina with additions of TiO2 sintered more rapidly than “pure” alumina. The rate of initial sintering increased approximately exponentially with titania concentration up to a percentage beyond which the rate of sintering remained approximately constant or decreased slightly with additional titania. The concentration which produces the maximum rate of sintering is thought to be the solubility limit of TiO2 in Al2O3. For alumina particles larger than about 2 μm, the kinetic process was mainly grain-boundary diffusion. With smaller particles, volume diffusion increased. The “solubility limit” increased with decreasing particle size, indicating an excess surface concentration of TiO2. The data may be interpreted in terms of a region of enhanced diffusion at the grain boundary that increases with TiO2 concentration. With small alumina particles, this region is large enough to become a significant portion of the volume of the particle, and the small particles appear to sinter by volume diffusion kinetics, but the diffusion coefficient corresponds to an enhanced diffusion coefficient.

Liquid-phase sintering.

Abstract: Industrial advances frequently depend on the development of new, special-purpose ma- terials possessing specific magnetic, electrical, optical, strength, friction, antifriction, and other properties. Metal alloys produced by the conventional technique of metallurgical reduc- tion often do not meet these new requirements. Powder metallurgy, therefore, is of consider- able importance in solving many problems of present-day materials science. Its production techniques-solid-phase and liquid-phase sintering, impregnation, hot pressing - make it pos,...sible to obtain materials from metallic components which are immiscible in the liquid state and also materials in which metals are combined with nonmetallic components such as refract- ory compounds- oxides, carbides, nitrides, borides, sUicides, sulfides, etc. The properties of sintered parts depend essentially on the processes occurring during their formation. One of the most promising methods of producing sintered materials of high density with the best combination of various properties is liquid-phase sintering. In recent years, many publications have appeared concerning processes of sintering specific combinations of com- ponents, the theoretical basis of liquid-phase sintering, and the laws governing this process. The present work examines liquid-phase sintering processes and the action of capillary forces in models of dispersed solid-liquid systems, and also gives data from theoretical and experi- mental studies of liquid-phase sintering in various metal and metal-ceramic systems. Some theoretical generalizations on the principles of sintering processes are presented, and the driv- ing forces of sintering and the effect of different conditions on liquid-phase sintering processes are considered.

Cobalt‐samarium permanent magnets prepared by liquid phase sintering

Abstract: The preparation and measurement of magnetic properties of cobalt‐samarium permanent magnets prepared by liquid‐phase sintering are discussed. Energy products in excess of 15×106 G Oe were observed. Long time exposure to air at 150°C did not degrade the samples.

Study of sintering of poly(methyl methacrylate)

Abstract: Sintering of poly(methyl metacrylate) spheres to the blocks of the same material was studied at temperatures ranging from 127° to 207°C. The analysis of the data based on the empirical Ostwald relation indicates that the predominant mechanism responsible for this process is non-Newtonian viscous flow. The type of the flow changes with temperature of sintering, being pseudo-plastic at lower temperatures and tending to dilatancy at higher ones.

Sintering steel-bonded carbide hard alloy

Abstract: A sintered alloy comprising a carbide of preferably titanium and a steel matrix of an alloy steel containing chromium, molybdenum, copper and vanadium as alloying elements provide high temperature hardness and wear resistance. Preferred alloys contain 0.8 to 1.9% by weight of manganese and up to 80% by weight of carbide.

Preparation and characterisation of high surface area semiconducting oxides

Abstract: The conventional method of preparing doped semiconducting oxides involves solid state sintering at elevated temperatures and the resultant products have a very low surface area (< 1 m2 g−1). This severely limits the activity of semiconducting oxide catalysts, such as lithiated NiO. A cryochemical method based on the freeze drying of mixed salt solutions, followed by low temperature calcining has yielded lithiated NiO of over 60 m2 g−1. DTA, conductivity measurements and chemical analyses confirmed that under these conditions, very uniform mixing between Li2O and NiO is achieved and that Li2O can diffuse into the NiO lattice at about 400° C, as compared to 950 to 1000° C for products prepared by conventional means.

Solid-State Sintering

Abstract: The theory of solid-state sintering is reviewed, with particular emphasis on the effects of several concurrent mechanisms on sintering behavior. Computer synthesis of both non-isothermal and isothermal sintering data has been used to predict initial sintering behavior under a wide variety of conditions. This approach has been successful in describing the sintering of compacts of spherical particles of Fe, Cu, Fe2 O3, Ag, and LiF. The relative importance of surface, grain boundary, and volume diffusion and vapor transport are discussed. Experimentally observed sintering behavior is discussed in terms of the theoretical model.

Kinetics of Initial Sintering with Grain Growth

Abstract: Kinetic equations for initial sintering were obtained by combining the conventional kinetic equation with an empirical expression for grain growth in the initial stage. The equations describe the isothermal shrinkage of ZnO in O2 at 80 torr and 800° to 950°C. The equation also successfully analyzed the sintering of powder compacts of Al2O3 studied by other workers.

Densification Mechanisms in Hot–Pressing of Magnesia with a Fugitive Liquid

Abstract: Densification of MgO with LiF additions depends on the formation of a liquid that is initially a lubricant for rearrangement of particles and later a material transport medium for pressure–enhanced liquid–phase sintering. The rate–controlling process in the latter stage is viscous flow of the liquid through grain–boundary channels. After the liquid is distributed as a uniformly thin film between the MgO grains, it can be removed by evaporation, and transparent polycrystalline specimens are produced.

Studies on sintering process of synthetic hydroxyapatite

Abstract: In this study the effect of sintering process in different temperatures on microstructure and morphological properties of sintered hydroxyapatite (HAp) was investigated. HAp powder was prepared by wet precipitation method from following reagents: Ca(OH)2 + H3PO4 in an alkaline conditions. Thermal analysis (TA), X-Ray diffraction method (XRD), FT-IR spectrometry (FT-IR) and scanning electron microscopy (SEM) were used to elaborate the phase composition and properties of sintered HAp samples and raw HAp powder as well. The total and apparent density and total porosity of sintered compacts, shrinkage and weight loss during the sintering were also measured. The results show that there is a difference in sintering behavior of synthetic hydroxyapatites depending on sintering temperature. The main differences refer to the loss of mass, shrinkage, changes in porosity and density of the investigated materials.

Microstructure Development in Yttrium Iron Garnet

Abstract: Sintering and grain-growth rates were determined for yttrium iron garnet as a function of the yttrium/iron ratio. Rates decreased with an increase in this ratio; this behavior is interpreted as a result of the oxygen content variation through the garnet field. For constant sintering time, density and grain size, as well as microstructure-dependent properties, varied through the garnet field. Remanent magnetization and coercive force in particular depended on composition and sintering temperature. The rf field for nonlinear spin-wave excitation, hcrit, measured for dense samples with grain sizes from ∼4 to 30 μm, varied by a factor of four.

Process for forming high density refractory shapes and the products resulting therefrom

Abstract: A PROCESS FOR FABRICATING LOW POROSITY, ESSENTIALLY DEFECT FREE, COMPOSITE REFRACTORY SHAPES OF NOVEL COMPOSITION. A POWDERED REFRACTORY MATERIAL IS MIXED WITH A CARBON CONTAINING SUBSTANCE AND THE MIXTURE IS COLD FORMED INTO A DESIRED SHAPE, THE PREFORMED SHAPE IS THEN HEAT TREATED TO CONVERT THECARBON CONTAINING SUBSTANCE TO CARBON. A MOLTEN METAL BATH IS PREPARED, COMPRISED OF AT LEAST TWO METALS, AT LEAST ONE OF WHICH IS CAPABLE OF REACTING WITH THE CARBON IN THE PREFORMED SHAPE, AND THE COMBINATION OF WHICH RESULTS IN A METALLIC MIXTURE GREATER THAN THAT OF THE REFRACTORY MATERIAL MAING UP THE MATRIX OF THE PREFORMED SHAPE. THEPERFORM IS THEN HEATED AND IMPREGNATED WITH THE MOLTEN METAL WHICH RESULTS IN SINTERING OF THE REFRACTORY MATERIAL MATRIX, AND THE REACTION OF AT LEAST ONE OF THE METALS WITH THE CARBON CONTAINED IN THE INTERSTICES FORMED BY THE PARTICULATE REFRACTORY MATERIAL, TO FORM METAL CARBIDE(S) THEREIN. THE COMPOSITE SHAPE IS THEN REMOVED FROM THE MOLTEN METAL BATH AND COOLED. BECAUSE THE COEFFICIENT OF THERMAL EXPANSION OF THE MIXTURE OF METALS IS CLOSE TO OR SLIGHTLY GREATER THAN THAT OF THE REFRACTORY MATRIX, THE COMPOSITE SHAPE COOLS TO ROOM TEMPERATURE ESSENTIALLY FREE OF CRACKS AND RESIDUAL STRESS.

Manganese zinc ferrite core with high initial permeability

Abstract: A FERROMAGNETIC STRUCTURAL ELEMENT HAVING INITIAL PERMEABILITY HIGHER THAN 4000 IN THE OPERATING TEMPERATURE RANGE OF -50*C. TO +120*C. COMPOSED OF 54 TO 58 MOL PERCENT OF FE2O3 10 TO 24 MOL PERCENT OF MNO AND 18 TO 36 MOL PERCENT OF ZNO AND CONTAINING LESS THAN 0.05% BY WEIGHT OF IMPURITIES AND A METHOD OF MANUFACTURING THE SAME. THE METHOD INCLUDES MIXING THE ABOVE COMPONENTS AND SINTERING THEM AT 1270*C. TO 1300*C. FOR 4 TO 20 HOURS, WITH AT LEAST THE LAST HALF OF THE SINTERING TAKING PLACE IN AN INERT ATMOSPHERE HAVING LESS THAN 0.2% BY VOLUME OF OXYGEN AND COOLING THE SINTERED FERRITE TO ABOUT 300*C. IN THE SAME INERT ATMOSPHERE.

A Study of Sintering Using Hot-Stage Electron Microscopy

Abstract: Hot-stage electron microscopy has been used to study the sintering of b.c.c. (Fe) and f.c.c. (Fe–Ni) powders in the size range 0·005–0·2 μm, where the particles are transparent to 100 kV electrons. The observations show that in the absence of an external load dislocations play no part in sintering, even in the earliest stages of neck growth. The sphere–sphere model is employed to calculate the total climbing forces (elastic plus chemical) acting on dislocations, and elasticity theory is applied to determine the principal and maximum shear stresses generated at the weld-neck of sintering particles. Only dislocations lying very close to the grain boundary can move to the neck or the grain boundary and so contribute to neck growth. However, the maximum shear stresses generated in sintering are too small to nucleate new dislocations and it is thus concluded that neck growth is controlled solely by diffusional processes.

Heat-aged sintered cobalt-rare earth intermetallic product and process

Abstract: A PROCESS FOR PREPARING HEAT-AGED NOVEL SINTERED COBALTRARE EARTH INTERMETALLIC PRODUCTS WHICH CAN BE MAGNETIZED TO FORM PERMANENT MAGNETS HAVING STABLE IMPROVED MAGNETIC PROPERTIES. A SINTERED COBALT-RARE EARTH METAL PRODUCT IS PROVIDED WHICH AT ITS SINTERING TEMPERATURE IS A COMPOSITION RANGING FROM A SINGLE SOLID CO5R INTERMETALLIC PHASE TO THAT COMPOSED OF CO5R SOLID INTERMETALLIC PHASE AND A SECOND PHASE OF SOLID COR WHICH IS RICHER IN RARE EARTH METAL CONTENT THAN THE CO5R PHASE AND WHICH IS PRESENT IN AN AMOUNT OF UP TO ABOUT 30 PERCENT BY WEIGHT OF THE PRODUCT. THE SINTERED PRODUCT IS HEATED AT A TEMPERATURE WITHIN 400*C. BELOW ITS SINTERING TEMPERATURE TO PRECIPITATE OUT OF THE CO5R PHASE OF A COR PHASE RICHER IN RARE EARTH METAL CONTENT THAN THE CO5R PHASE IN AN AMOUNT SUFFICIENT TO IMPROVE COERCIVE FORCE, EITHER INTRINSIC AND/OR NORMAL, OF THE PRODUCT BY AT LEAST 10 PERCENT.

The Sintering of Cavities by Grain-Boundary Diffusion

Abstract: A theoretical treatment is given of the sintering of cavities by grain-boundary diffusion, assuming vacancies to be annihilated at randomly distributed sinks. Satisfactory agreement is obtained with previously published experimental data.

Utilizing mixtures of yttria,magnesia,and lanthanum oxide in manufacture of transparent alumina

Abstract: A METHOD OF MANUFACTURING A POLYCRYSTALLINE TRANSLUCENT ALUMINA CONSISTING OF A HIGH PURITY ALUMINA AND HAVING EXCELLENT OPTICAL TRANSMISSION PROPERTY FOR THE LIGHT IN THE VISIBLE SPECTRUM, THERMAL SHOCK RESISTANCE AND MECHANICAL STRENGTH WHICH COMPRISES THE FOLLOWING STEPS CALCINING ALUMINA POWDERS HAVING A PURITY OF MORE THAN 99.00% BY WEIGHT AS A STARTING MATERIAL AT A TEMPERATURE OF 1,050-1,0250*C. IN AIR, ADDING TO SAID ALUMINA POWDERS 0.05-0.5% BY WEIGHT OF Y2O3, 0.05-0.5% BY WEIGHT OF LA2O3 AND 0.01-0.1% BY WEIGHT OF MGO, MIXING AND THEN MOLDING SAID ALUMINA POWDERS ADDED WITH SAID ADDI- TIVES, SINTERING AT FIRST SAID MOLDED MIXTURE AT A TEMPERATURE OF 1,200-1,450*C. AND SECONDLY SINTERING IN VACUUM OR A REDUCING ATMOSPHERE SUCH AS HYDROGEN OR DISSOCIATED AMMONIA GAS AT A TEMPERATURE OF 1,60001,800*C.

Electrical Properties of High‐Density Iron‐Deficient Nickel‐Zinc Ferrites

Abstract: The dc resistivity of nickel-zinc ferrite was studied as a function of nickel/zinc ratio, apparent density, temperature, and grain size. Resistivities of Ni0.40Zn0.51Fe1.90O4– and Ni0.35Zn0.65-Fe1.90O4– are similar. Evaluation of samples sintered between 1100° and 1220°C showed that densification proceeds rapidly for sintering temperatures 1170°C; for these specimens the room temperature resistivity increases to an equilibrium value with sintering time. Samples sintered to 99+% of theoretical density at lower temperatures densify slowly; the resistivity is invariant with sintering time. The Seebeck coefficient for the p-type ferrites is 550 μV/°C from 200° to 700°C; the dielectric constant varies from 17.3 at 0.5 MHz to 16.4 at 15 MHz.

Processes for the manufacture of tungsten-based alloys and in the corresponding materials

Abstract: The process comprises sintering very fine tungsten powder mixed in the cold with a metallic binder occurring in the liquid state at the sintering temperature. This metallic binder is constituted by a very fine powder, of a granulometry of the order of several microns, containing 65 to 90% of nickel, 5 to 20% of chromium and 5 to 15% of phosphorus, the phosphorus being introduced in the form of an alloy powder containing one at least of the two other binder constituents.

The Selection of Grain-growth Control Additives for the Sintering of Ceramics

Abstract: The close interrelation between sintering and grain growth is discussed critically, and the necessity for control, but not suppression, of grain growth is established. A distinction between discontinuous and exaggerated grain growth is drawn, and the superiority of grain boundary films as grain-growth control agents is demonstrated. Work on the establishment of tentative criteria for the selection of grain-growth control additives for alumina is reviewed and discussed, and the extension of these ideas to the selection of NaF as a successful aid for magnesia is described.

Liquid phase sintered molybdenum base alloys having additives and shaping members made therefrom

Abstract: Molybdenum alloys containing at least two metallic element components which form a molten alloy which dissolves appreciable Mo during liquid phase sintering may contain additives selected from Co, Mn, Cr, Ru, Zr, Ti, Hf, Re, refractory metals, and metalloids to obtain desired properties for certain applications. Many of these additives are particularly effective in shaping members.

Method of spark sintering electrically conductive particles onto a metallic substrate

Abstract: A method of spark sintering electrically conductive particles, e.g. copper, nickel, ferrochromium and tungsten carbide, onto a metallic substrate (e.g. of nickel, iron or steel) in which a mass of particles is positioned along the substrate in lightcontacting relationship therewith. An electric current pulse is applied across the mass of particles to effect a spark discharge among them and between the mass and the substrate to sinter the particles to one another and to the substrate. The method also increases the fatigue resistance of iron and steel when tungsten carbide is spark-sintered thereto.

Formation, Microstructure, and Properties of Barium Zirconium Sulfide Ceramics

Abstract: The preparation, microstructure, and properties of BaZrS3 ceramics are described. The compound was prepared by heating either an equimolar mixture of BaS and ZrS2 or BaZrO3 alone in CS2 gas. The reaction of the BaZrO3−CS2 system was extremely rapid, as would be expected in an autocatalytic reaction caused by very strong reduction by CS2. Well-sintered bodies were obtained only by the BaZrO3−CS2 reaction. The time dependence of grain growth was affected considerably when reaction and sintering occurred above 1250° C. Non-uniform grain growth which occurs when an excess of Ba is present can be interpreted in terms of secondary recrystallization resulting from segregation of BaS at the grain boundary during sintering. The BaZrS3 ceramic was stable in air up to 550°C and was oxidized above that temperature.