Showing papers on "Sintering published in 1976"

Sintering of Covalent Solids

Abstract: The sintering behavior of primarily covalently bonded β-SiC, Si, and Si3N4 was studied using surface area and densification measurements as well as observations of microstructures developed during firing. The existence of highly dense, microscopic regions and large (≥100°) dihedral angles in fired compacts of β-SiC and Si which experience little macroscopic densification suggests that macroscopic densification is not intrinsically limited by the effects of surface energy. The mechanism proposed to explain the microstructure that develops in unsinterable covalent solids which do not undergo a phase change is based on the existence of a high ratio of surface and/or vapor-phase matter transport-to-volume and/or grain-boundary transport. The addition of boron to both β-SiC- and Si-containing carbon retards surface and/or vapor-phase transport and grain growth at lower temperatures, which results in enhanced densification at high temperatures. Macroscopic densification of β-SiC and α-Si3N4 can also be retarded by the formation of a continuous network of high-aspect-ratio grains of the polymorphic form that rigidities the sintering body. Finally, the sintering of pure Si depends sensitively on particle size in the submicron range. Nearly theoretical density is achieved in Si powder of ∼0.06-μm size. This result suggests that other pure covalently bonded solids can also be sintered to high density without applied pressure.

Dependence of Electrical Conductivity of ZnO on Degree of Sintering

Abstract: The electrical resistivity of ZnO doped with Al2O3 was measured in air and under reduced pressure (∼0.5 mm Hg) in the range from 30° to 680°C as a function of the degree of sintering. The data obtained were explained in terms of the effects of the microstructure of the sintered body and the chemisorbed oxygen. There are two mechanisms of electrical conduction involved, only one of which is affected by the microstructure.

Surface Area Reduction During Isothermal Sintering

Abstract: A kinetic model for the reduction in specific surface area during isothermal sintering is examined in detail. Experimental data for a variety of materials including A12O3, B, Ni, Cu, ZnO, TiO2, Fe2O3, and As-Se-S glasses are analyzed by the technique provided. The results of this technique agree well with those for previously identified sintering mechanisms for each material. The kinetic model is based on sintering driven by the curvature gradient in the interparticle neck region associated with initial-stage sintering. At the intermediate stage of sintering, a diminished curvature gradient shifts the sintering driving force to the excess surface free energy.

Pressure sintering of Si3N4

Abstract: The sintering of Si3N4 with 5% MgO was investigated at 1450 to 1900‡ C under a pressure of nitrogen. A maximum density of 95% of the theoretical value was obtained, which is greater than that obtained by pressureless sintering. The sintering process was inferred to be liquid-phase sintering and divided into two processes; rearrangement and solution-precipitation. The contribution of rearrangement to densification was about 10% in the present system, and the rest, up to 17% was due to solution-precipitation. Application of the present method of sintering Si3N4 with a high strength grain-boundary phase at high temperature is surveyed.

Sintered dense silicon carbide

Abstract: A method of making a dense silicon carbide ceramic is disclosed involving the steps of homogeneously dispersing silicon carbide with a sufficient amount of a boron containing additive and a carbonaceous additive, forming the powder mixture into a shaped green body and then sintering the body in a controlled atmosphere and in the absence of external pressure at a temperature of about 1900°-2100° C. such that density of the body is at least 85% of the theoretical density of silicon carbide. The complex shaped silicon carbide product formed thereby is also disclosed.

Tantalum metal powder

Abstract: Improved, high-purity agglomerated tantalum powders characterized by exceptionally high green strength when pressed without using a carbonaceous binder into low density anodes and high electrical capacitance along with low direct current leakage and dissipation factor in the anodes after sintering. Such powder is produced by milling hydrided high-purity tantalum metal ingots or powder to a critical particle size powder, subjecting the powder to a two-step heat treatment to degas and pre-agglomerate it, screening and milling the oversize portion of the degassed and pre-agglomerated powder to achieve intermediate mesh size powder, subjecting this powder to a higher temperature treatment to agglomerate it, milling the agglomerated powder to an intermediate mesh fraction, screening the powder to remove the fine particle size portion, subjecting this fine particle material to an additional heat treatment to re-agglomerate it, and blending the re-agglomerated fines with the coarser mesh size portions of the agglomerated powder.

The mechanism of simultaneous sintering and phase transformation in alumina

Abstract: The sintering behaviour of boehmitic alumina gels during the transformation to the stableα phase has been studied using dilatometry, transmission electron microscopy, X-ray analysis and differential thermal analysis. The specimens for transmission electron microscopy were prepared from gel specimens, sintered to various predetermined temperatures, using an ion-beam thinning technique. The transmission electron microscope study and X-ray analysis have revealed a characteristic sintering behaviour which is associated with theθ toα phase transition. The transformation to theα phase occurs by a nucleation and growth process. During the growth process considerable redistribution of the fine porosity existing within the transition alumina matrix occurs, in the form of large elongated interconnected pores trapped within the nucleatingα grains. These pores grow rapidly to a size approximately one hundred times that of theθ grains. This process results in a rapid fall-off in sintering rate at the end of the transformation. A study of theθ/α interphase interface by transmission electron microscopy has led to the development of a model that accounts satisfactorily for the redistribution of the porosity.

Enhanced low-temperature sintering of tungsten

Abstract: The effects of various transition metal additions on the sintering of a well-characterized, fine tungsten powder were analyzed using both isothermal and constant heating rate experiments in the temperature range 900 to 1400°C. Approximately four atomic mono-layers of palladium on the tungsten powder surface were found to be the optimal enhancer, followed by nickel, cobalt, platinum, and iron. The addition of Cu to the tungsten had no appreciable effect on the sintering kinetics. Sintering enhancement by these transition metals is related to their periodic chart position (i.e., electron structure). An overall non-Arrhenius shrinkage temperature dependence is attributed to grain growth in the activator-treated specimens. The activation energy for tungsten densification was determined to be 430 to 450 kJ/mol, which is in general agreement with a grain boundary diffusion process.

Method for sintering refractories and an apparatus therefor

Abstract: A method for sintering refractories comprising adding 0.05 to 10% by weight of an electro-conductive particles to a group of dielectric refractory particles to obtain a mixture thereof, and heating the mixture by means of microwave induction heating to sinter the refractions.

High Temperature Microwave Heating in Refractory Materials

Abstract: Microwave energy can be used to create high temperatures (> 1700° C for alumina) in refractory materials. Energy conversion takes place with a very good efficiency (90% in the conversion process of microwaves to thermal energy) if an appropriate applicator is utilized.Experiments at 2.45 GHz involving either the sintering of alumina or melting of silica for fiber fabrication are described which confirm the feasibility of this new process.

Thermal stabilization of an active alumina and effect of dopants on the surface area

Abstract: The α-AI2O3 transformation of a monolithic active alumina has been increased from 1200 to ∼1380‡ C through structural incorporation of silica. This shift is significant since α-Al2O3 transformation determines the limits of the usefulness of these materials as catalysts and catalyst carriers. The thermal stabilization effect is optimized at around 6% silica doping. At elevated temperatures, the material containing no silica rapidly loses surface area, primarily by α-Al2O3 transformation, whereas the material containing excess silica loses surface area by classical sintering.

Silicon carbide sintered body

Abstract: A silicon carbide sintered body characterized by its microstructural stability at high temperatures is produced by forming a mixture of β-silicon carbide powder, α-silicon carbide seeding powder, boron additive and a carbonaceous additive into a green body and sintering it to produce a sintered body with a density of at least 80% wherein at least 70% by weight of the silicon carbide is α-silicon carbide.

Densification Mechanisms in the Tungsten Carbide—Cobalt System

Abstract: An investigation has been made into the sintering of tungsten carbide in the presence of small amounts of various metal additives. It has been found that a very marked degree of densification occurs when a small amount (1 vol.-% or less) of iron-group metal is added to WC. This arises from a process of activated sintering due to enhanced surface and interfacial diffusion, which has been observed by other workers to take place when nickel or other Fe-group metals are added in small quantities to W. Tungsten carbide forms low-energy prismatic interfaces in the presence of cobalt and other Fe-group metals. Densification takes place by the close-packing of particles along irregular boundaries which comprise segments of these low-energy interfaces. This configuration is far removed from that required by the usual sintering criterion of minimum surface and interfacial area. The segments of low-energy interface set up activity/diffusion gradients which bring about rapid material transport to achieve this...

Lamps and discharge devices and materials therefor

Abstract: Conducting cermets with volume fractions of nickel down to 0.045 are produced by coating relatively coarse refractory oxide granules with fine metal powder particles and compacting and sintering the coated granules. Using oxide granules in the size range 400 to 800 microns it is possible to make conducting cermets having the same volume fraction of metal and thus the same thermal expansion coefficients as insulating cermets prepared from finer granules of the same oxide. The cermets can be used to fabricate tubes and components for electric lamps and may include integrally formed conducting and insulating regions.

Direct observation of liquid-phase sintering in the system tungsten carbide-cobalt

Abstract: The hot-stage of a scanning electron microscope has been used to observe liquid-phase sintering in the system tungsten carbide-cobalt. Densification behaviour and the mechanism for the first, fast stage of sintering have been determined; the influence of particle size and the amount of liquid phase has been investigated. In all samples the densification kinetics is that of a rearrangement process; direct observation confirmed this result.

Sintered material for tools of high hardness and its preparation

Abstract: PURPOSE: To produce the sintered material for cutting tools having excellent wear resistance, strength and heat resistance, by sintering BN of cubic system using carbides, nitrides, borides or silicides of the specified metals as the binding phase. COPYRIGHT: (C)1978,JPO&Japio

Fabrication of translucent magnesium aluminate spinel and its compatibility in sodium vapour

Abstract: The sintering behaviour of magnesium aluminate spinel on both sides of the stoichiometry has been investigated. It is found that the composition close to the stoichiometric composition, but slightly enriched in magnesia can be sintered to translucency in a free flowing hydrogen environment. The grain size decreases with magnesia content; the addition of yttria, however, results in a highly bimodal grain structure. Prolonged exposure of the spinel in a high temperature sodium vapour environment forms Na2O · 5Al2O3 and some traces of Na2O · 11Al2O3 predominantely at the grain boundaries and near the surface of the specimen. Some possible sintering mechanism and factors affecting the in-line transmission are discussed.

Polycrystalline silicon articles by sintering

Abstract: A polycrystalline silicon body is produced by shaping silicon powder having an average particle size less than 0.2 micron into a green body and sintering the body to a density of at least 60% of the theoretical density of silicon.

Particle Rearrangement in Solid State Sintering

Abstract: This paper shows that particle rearrangement occurs during solid state sintering of crystalline as well as amorphous particles. The reasons for this process which hitherto has not been adequately considered in sintering theory, are unsymmetric neck formation and induced stresses. Asymmetric necks may form due to asymmetric contact geometry, anisotropy of surface energy, and asymmetric packing of particles. The effects of asymmetric arrangement can best be studied experimentally as well as theoretically by means of the three particle model. This model shows that angle changes occur during sintering. Stresses are induced due to the formation of new contacts, differences in particle size, and other effects causing non-uniform center-to-center approach. Since systematic rearrangement has a strong influence on shrinkage, it should be considered in theoretical and practical sintering studies.

A Comparative Assessment of Explosive and Other Methods of Compaction in the Production of Tungsten—Copper Composites

Abstract: In the production of tungsten-copper composites by pressing and sintering mixed powders followed by hot working, or by liquid-phase infiltration techniques, difficulties arise in obtaining a fully dense material, particularly at tungsten contents > 70 wt-% W–Cu composites containing between 70 and 90 wt-% W have been made by various routes including liquid-phase sintering, hot forging, triaxial compaction, isostatic re-pressing, and explosive compaction, and a comparative assessment of their mechanical and electrical properties has been made In the case of composites containing 70 wt-% W, it has been shown that the maximum density achieved by liquid-phase sintering at up to 1753 K increases as the Cu particle size decreases A density of 95·5% theoretical was obtained using −350 mesh Cu powder Isostatic repressing at 1·4 GPa of liquid-phase sintered composites yielded a maximum density of 95·5% theoretical, irrespective of initial Cu particle size Triaxial compaction of the mixed powders wa

Cold compaction molding and sintering of polystyrene

Abstract: It is the objective of this paper to demonstrate the applicability of cold compaction molding followed by a sintering treatment to the processing of polystyrene powders. The influence of pressure, compaction speed, and peak pressure dwell time on the green (as compacted) density and the green tensile strength, as well as the effect of sintering on the tensile strength and dimensional change, were evaluated. The resulting data indicate that room temperature compaction alone is insufficient to provide adequate tensile strength for the compacts. Sintering the green compacts at temperatures of 150 to 173°C markedly improves the tensile strength while simultaneously causing a thickness change in the compacts. This thickness change results from gas evolution, pore shrinkage, and viscoelastic recovery of the residual stresses induced by pressure. For compacts of 0.225 in. thickness, an optimum sintering treatment of 173°C for 30 mins is recommended to provide a tensile strength of 4,000 psi and a thickness change of less than + 7 percent. Coining (repressing) the green compacts does not appreciably affect the sintered strength. However, a finer particle size improves the sintered properties. A review of the literature on the flow behavior of polystyrene suggests that a non-Newtonian viscous flow mechanism is followed by a Newtonian one as sintering progresses.

Silicon carbide-boron carbide sintered body

Abstract: A particulate mixture of β-silicon carbide, boron carbide and a carbonaceous additive is formed into a green body and sintered producing a sintered body with a density of at least about 85% containing boron carbide in an amount ranging from about 10% to about 30% by weight of the total amount of silicon carbide and boron carbide present.

Method of producing highly densified ceramic articles

Abstract: A method of producing ceramic articles having a density substantially equal to the theoretical density comprises heating a highly porous preliminary molding made of a ceramic powder to obtain a preliminarily sintered molding of a porosity below 30%, followed by sintering the preliminarily sintered molding under a uniform pressure by the use of a powder acting as a pressure transmitting medium. This method permits producing a ceramic article having a desired shape, and no deformation in it.

[Studies on the application of apatite to dental materials. (I) --Apatite ceramics-- (author's transl)].

Abstract: Apatite ceramics is composed of hydroxyapatite [Ca10(PO4)6(OH)2] sintered at high temperature. It is known that hydroxyapatite is the main component of bone and tooth minerals. There are two synthetic methods for the apatite powder. One is so called wet synthetic method: Synthesis by the reaction of Ca++ and PO4--- in the aqueous solution of approximately pH 7.0, the other is dry method: Synthesis by the solid state reaction at high temperature. The apatite powder stable below 1400 degrees C was prepared by the latter method in this work. After passing through a sieve, this powder was cold-pressed and then sintered at 1000 degrees C to 1300 degrees C in air. Biological apatite powders were also perpared as a reference. It was found that any apatite ceramics having porosity in the range of 5 to 50% could be obtained under the various sintering conditions. Compressive strength of these apatite ceramics increased with the reduction of the porosity, and those with porosity less than 20% were more than 100 kg/cm2. Vickers hardness was measured. This result showed the same tendency as that of compressibility. Hardness of the apatite ceramics with 90% relative density was almost the same or more as that of enamel. Solubility of the synthetic apatite powder in distilled water and aqueous solution of lactic acid (pH 4.0) was nearly the same as biological apatites. The dissolution rate decreased with the reduction of porosity of the ceramics. It was certified that hot pressing technique was extremely effective to obtain high density ceramics (more than 95% of density) and thus low parosity apatite ceramics. From the facts as described above, it is understood that sintered pure hydroxy-apatite is an excellent ceramics of high mechanical strength.