Showing papers on "Sintering published in 1969"

New Method of Obtaining Volume, Grain‐Boundary, and Surface Diffusion Coefficients from Sintering Data

Abstract: A sintering model is proposed by which all of the significant mechanisms of material transport may be identified, even though more than one mechanism may be operating simultaneously. For diffusion‐controlled sintering it is possible to calculate both the volume and the grain‐boundary diffusion coefficients from measurements of neck size, shrinkage, and shrinkage rate. Furthermore, the surface diffusion coefficient may be estimated through computer synthesis of the sintering curves. Preliminary results on iron and copper have yielded values of the diffusion coefficients which are in good agreement with those measured by other techniques.

Sintering of Glass Powders During Constant Rates of Heating

Abstract: Analytical expressions for the initial sintering of glass powders were developed to describe shrinkage of powder compacts at a constant rate of heating. Data describing sintering of soda-lime glass spheres as measured by shrinkage of powder compacts at rates of heating from 0.46 to 2.91°C/min are consistent with the model analytical expressions. Compacts of glass spheres decreased in size exponentially with increasing temperature as predicted by the model.

Initial Sintering Kinetics of MgAl2O4

Abstract: Results are presented for the initial-stage sintering of spinel in air at 1050° to 1300°C Spinel powders of high purity were prepared from magnesium-aluminum hydroxide coprecipitates by calcining at 1100°C for 4 h The sintering mechanism is volume diffusion, and the temperature dependence of the apparent diffusion coefficients can be represented by D = 186 exp [(-116,000 cal/mol)/RT] cm2/s

Sintering of aluminium nitride: Particle size dependence of sintering kinetics

Abstract: The effect of particle size (0.78 ∼ 4.4 μm) on the sintering kinetics of AIN powder was investigated in the temperature range from 1600 to 2000° C and the results were analysed on the basis of vacancy diffusion models. The mechanisms of sintering are discussed. Fractional shrinkage is proportional to the nth power of soaking time with n = 0.20 for 4.4 μm and 1.5 μm powders and 0.33 for 0.78 μm powder. For the 0.78 μm powder at 1900° C, however, n decreases gradually as grain growth proceeds. The experimental activation energy for sintering is between 92 kcal/mole for 4.4 μm and 129 kcal/mole for 0.78 μm powder. Unlike this activated energy, the rate of sintering and the diffusion constant calculated from it increase drastically with decrease of particle size; the derived diffusion constant for 1.5 μm powder is 101 to 102 times larger than that of 4.4 μm powder, and for 0.78 μm powder the diffusion constant is estimated to be still higher. The particle-size dependence of parameter n and the diffusion constant seems to be caused by a variation in predominant diffusion mechanisms; namely, bulk diffusion in coarse powder and surface or grain-boundary diffusion in fine powder.

Carbide grain growth during the liquid-phase sintering of the alloys NbCFe, NbCNi, and NbCCo

Abstract: An investigation has been made of the microstructural changes which occur during the liquid-phase sintering of the alloys NbC-22 wt.% Fe andNbC-22 wt.% Ni. It was found that the growth of the carbide grains, dispersed in either of the liquid metals, is consistent with the theoretical equation, derived by Wagner, for a solution/ precipitation growth process controlled by the diffusion of the solid constituent through the liquid. The same growth behaviour has been observed earlier in a similar alloy of NbC in cobalt. The effect of temperature on grain growth was measured and “activation energies” of the growth rates were determined.

Process for making aluminum modified boron carbide and products resulting therefrom

Abstract: A process for fabricating high density aluminum modified boron carbide compositions comprising the steps of blending specific quantities of powdered boron carbide and powdered aluminum together with a small quantity of temporary organic binder, preforming this mixture to the desired shape with high pressure at room temperature, followed by final densification of the finished product at a temperature below the sintering temperature of boron carbide and at a pressure as low as 500 psi. The resulting compositions are near theoretical density and possess a degree and type of strength which make them well suited as protective armor against penetration by ballistic projectiles.

Solid State Reactions in Lithium Ferrite

Abstract: The conventional sintering of lithium ferrite from the normal constituents lithium carbonate and hematite is facilitated by the decomposition of the lithium carbonate in the intimate mixture about 400°C below the decomposition point in the pure state. However, at temperatures in excess of 1000°C, there is an irreversible loss of lithium and oxygen from the ferrite with a marked change in the density and the magnetic moment due to solid solution or second phase formation. Attempts to reduce the loss of oxygen and, possibly, lithium, by the application of a substantial oxygen pressure during sintering are accompanied by a retardation of the formation and densification processes.

The preparation of iron powders consisting of submicroscopic elongated particles by pseudomorphic reduction of iron oxides

Abstract: The preparation of submicroscopic elongated iron particles by pseudomorphic reduction of α-FeOOH with hydrogen has been studied. At a temperature of 350°C rapid sintering occurs of the iron particles formed. At temperatures below 300°C sintering is largely absent but the reaction is too slow. It has been found that the reduction rate at low temperatures can be increased, and hence sintering diminished, by absorption of Ag+ or Co+ ions on the surface of the iron oxide particles, leading to iron powders with improved magnetic properties. When Sn2+is adsorbed the reaction rate decreases. Yet the coercivity of the iron powders obtained is considerably increased, probably also due to a diminished sintering.

Method of making a plastically shapeable cathode material

Abstract: A plastically shapeable cathode material is prepared by depositing on a nickel substrate a layer of a mixture of nickel or nickel oxide powder and barium, strontium and calcium carbonates; sintering in a neutral or reducing atmosphere the nickel substrate with deposited layer; compressing the sintered material; cold rolling the sintered and compressed material; annealing the resulting cold-rolled material; and repeating the latter two steps until the nickel substrate with deposited layer has a desired thickness.

Sintered tungsten carbide-base alloys

Abstract: SINTERED TUNGSTEN CARBODE-BASE ALLOYS CONTAINING FROM 40-80% BY WEIGHT OF AN IRON-BASE BINDER ALLOY WHICH IN TURN CONTAINS FROM ABOUT 5-25% BY WEIGHT NICKEL AND AN AMOUNT OF CARBON RANGING FORM ABOUT 0.5-2% BY WEIGHT OF THE BINDER. THE ALLOYS OF THE INVENTION ARE PREPARED BY SINTERING A PRESSED MIXTURE OF THE TUNGSTEN CARBIDE AND BINDER, WHICH MAY THEN BE MACHINED OR OTHERWISE SHAPED IN THE SOFT AS-SINTERED STATE AND SUBSEQUENTLY HARDENED BY HEAT TREATMENT TO FORM A FULLY DENSE, STRONG AND HARD CEMENTED CARBIDE COMPACT.

Microstructure and Magnetic Properties of Lead Ferrite

Abstract: Unoriented lead ferrite specimens were prepared by calcining and sintering techniques. Specimens having an initial composition of Pb0.5Fe2O3 had the best permanent magnet properties (for nondoped material). The effects of small amounts of added oxides on the microstructure and magnetic properties were also determined. Silica and boria were most effective in raising the (BaHa)max to over 1.4 × 106 G.Oe as compared to 0.9 to 1.1 × 105 G.Oe for commercial un oriented barium ferrite magnets. Optical and electron microscopy revealed that silica and boria form low melting phases as the specimens are sintered, which, acting as fluxes (probably by a solution-precipitation mechanism), enhance the densification of the magnets, thereby raising the Br, and (BaHa)max, values. On cooling, these nonmagnetic low-melting phases are retained at the ferrite grain boundaries where they inhibit domain wall motion which would otherwise lead to a reduction in coercive force. Although these magnets are termed “un oriented” to distinguish them from “oriented” ferrites which are purposely aligned before sintering, a slight orientation is imparted to the polycrystalline compact during the pressing operation before sintering. The silica and boria additions increase the degree of orientation by accelerating the rate of densification. The increase in orientation accounts for part of the increase in Br, and (Ba, Ha)max.

Activation of the sintering of tungsten by the iron-group metals

Abstract: 1. A study was made of the influence of iron-group metal additions on the sinterability of tungsten powder at temperatures of 1000–2000°C. It is shown that the activating effect of these additions increases in the sequence Fe → Co → Ni. 2. It is demonstrated that the method of introducing an addition has no effect on the course of sintering of tungsten powder. 3. An explanation, based on the model of configurational localization of the valence electrons of atoms, is proposed for the mechanism of activated sintering of tungsten.

Partially stabilized zirconia refractory

Abstract: Body consisting essentially of ZrO2 and CaO with CaO being 2-5 wt. percent of the body. Microstructure consists essentially of cubic grains collectively constituting volumetrically the major phase, smaller primary monoclinic zirconia grains randomly dispersed substantially at grain boundaries between adjacent cubic grains, and, as a principal new feature effecting increased strength and elasticity properties, extremely finely divided precipitate of monoclinic zirconia dispersed intragrangularly throughout the cubic grains and being substantially smaller than the primary monoclinic zirconia grains. Cubic grains composed essentially of a solid solution of ZrO2 in a compound CaZr4O9. Manufacturing process comprises preparing batch material of prescribed composition, heating the material at firing temperature of at least 1,800* C. that causes sintering or melting of the material, cooling fired material to an annealing temperature of 900*-1,700* C. and maintaining it at the annealing temperature for at least about one day and thereafter continuing cooling of the material. Product is either sintered or fusioncast body. Batch material can be either unfused or fused finely divided particles. Unfused batch can be either unreacted or prereacted raw materials. For optimum density from unreacted raw material batch, it is heated, prior to molding, to prefiring temperature of 900*-1,300* C. for at least about one day. CaO can be provided in batch by heat decomposable compound whose solid decomposition product is substantially all CaO, e.g. calcium acetate monohydrate.

Use of yttria-magnesia mixtures to produce highly transparent sintered alumina bodies

Abstract: By using a combination of magnesia and yttria, or their precursors, in small amounts as sintering aids for alumina, sintered bodies having very high in-line transmission properties are produced. In addition the sintering time and/or temperature required to achieve a given inline transmission is reduced as compared with using only magnesia as a sintering aid.

Sintering vapor deposited silica on a mandrel designed to reduce shrinkage

Abstract: A PROCESS FOR MANUFACTURING FUSED SILICA ARTICLES IN WHICH SILICON DIOXIDE IS DEPOSITED AND VITRIFIED ON THE SAME MANDREL. SILICON DIOXIDE FORMED BY VAPOR PAHSE HYDROLYSIS OF SILICON TETRACHLORIDE IS DEPOSITED ON A CYLINDRICAL GRAPHITE MANDREL HAVING REDUCED DEAMETER PORTIONS AT ONE OR AT EACH OF ITS ENDS WHILE THE MANDREL IS AT AN ELEVATED TEMPERATURE. THE "GREEN" SILICA ARTICLE THUS FORMED IS THEN DENSIFIED ON THE SAME MANDREL BY HEAT TREATMENT IS A CACUUM FRUNACE. INTERENGAGEMENT OF THE SILICA AND THE REDUCED DIAMETER PORTIONS OF THE MANDREL PREVENTS SHRINKAGE OF THE ARTICLE DURING HEAT TREATMENT.

Infiltrated cemented carbides

Abstract: CEMENTED CARBIDE COMPOSITIONS ARE PREPARED BY THE PRODUCTION OF A SINTERED CEMENTED CARBIDE COMPACT BY CONVENTIONAL LIQUID-PHASE SINTERING TECHNIQUES, FOLLOWED BY CHEMICAL REMOVAL OF THE METALLIC BINDER AND REPLACEMENT BY INFILTRATION WITH A SECOND METALLIC BINDER DIFFERENT THAN THE ORIGINAL BINDER. THE RESULTING COMPOSITIONS ARE FULLY DENSE, HAVE A FINE-GRAINED, HOMOGENEOUS STRUCTURE AND POSSESS THE HIGH HARDNESS NORMALLY ASSOCIATED WITH CEMENTED CARBIDE ALLOYS.

Apparatus for and method of de-waxing,presintering and sintering powdered metal compacts

Abstract: In this apparatus a de-waxing operation is performed in hydrogen, and the same vessel is used for vacuum sintering without moving of the part to be heat treated, and in one continuous operation.

Method of making fuel cell electrolyte matrix

Abstract: A heat resistant porous fuel cell matrix for holding liquid electrolyte in fuel cells which can be subjected to extreme temperature conditions without cracking and permanently deforming is made from a ceramic metal oxide such as magnesia, a binder such as an alkali metal phosphate, and a liquid phase sintering agent for the metal oxides such as lithium fluoride. To form the matrix, the matrix molding composition is formed containing a thoroughly mixed composition of a major portion of the ceramic metal oxide, and minor portions of the binder and liquid phase sintering agent. The composition is molded into suitable matrix form, and the binding agent is activated such as by heating to form a porous green matrix which has structural integrity at room temperature. The porous green matrix is then heated to a temperature above the melting point of the liquid phase sintering agent to cause substantial sintering between the metallic oxide particles and yield a thermal resistant porous fuel cell matrix.

Sintered chromium steel and process for the preparation thereof

Abstract: SINTERED CHROMIUM STEEL OF HIGH DENSITY, HIGH STRENGTH, AND HIGH HEAT RESISTANCE IS OBTAINED BY MIXING AN INTERMETALLIC COMPOUND FECR (IN THE SO-CALLED SIGMA PHASE), PULVERIZED TO LESS THAN 10 MICRONS, WITH IRON POWDER AND CARBON POWDER, AND IF DESIRED WITH OTHER ALLOY POWDERS, AND BY MOLDING AND SINTERING THE THUS OBAINED MIXTURE.

Glow Discharge Sintering of Alumina

Abstract: Experimental work is reported on the rate of sintering of alumina compacts sintered in a microwave-induced plasma. It is shown that the sintering rate is very much greater for plasma-sintered specimens than for specimens sintered by conventional means. Evidence indicates the generation of plasma inside the pores of the compact, and the increased rate of densification is related to an increased rate of diffusion.

Grain‐Size Effects on Switching Properties of Lithium Ferrite

Abstract: This study was to determine the effects of grain size on the switching properties of lithium ferrite and to establish the best grain size for a fast switching speed, with a minimum increase of coercive force. Lithium ferrite (Li0.5Fe2.5O4) powder prepared by four preparation techniques; flame spraying, commercial techniques, fluid‐bed reaction, and Li2CO3+Fe2O3 mixture, was densified by hot pressing or conventional sintering. There was a variation in the grain growth and densification characteristics of the powders, but the magnetic properties were similar for a given grain size. The magnetic properties which were evaluated as a function of grain size were squareness, switching coefficient, coercive force, and switching time. These properties were evaluated for a 0.7 to 15.0 μ grain‐size range in order to establish the effects of grain size on switching speed and coercive force. It is shown that samples with a reduced grain size have twice the switching speed of larger grain‐size samples with little or no...

Method of making powder composites

Abstract: DESCRIBED HEREIN IS A METHOD OF MAKING A COMPOSITE ARTICLE OF POWDERED METAL AND A REFACTORY HARD METAL COMPONENT WHICH INVOLVES FORMING A GREEN COMPACT OF POWDER METAL AND SINTERING THE COMPACT IN CONTACT WITH THE REFRACTORY HARD METAL COMPONENT TO METALLURGICALLY BOND THE TWO.

Prevention of crawling of metal oxide hollow articles along the support mandrel during sintering

Abstract: The method of sintering amorphous of amorphous metal oxide, and in a preferred embodiment crucibles and tubing of fused silicon dioxide, by depositing a porous layer of amorphous metal oxide on a deposition surface, and thereafter partially sintering the edges of the metal oxide article to less than 90 percent of its theoretical density and substantially sintering the remainder of the metal oxide article to greater than 90 percent of its theoretical density. By only partially sintering the edge portions sufficient frictional forces are produced with the mandrel to exceed the surface tension forces created in the remainder of the metal oxide article by the higher sintering temperature thus preventing longitudinal contraction and crawling encountered in the prior art processes. For a silicon dioxide article, the edges of the mandrel are sintered at a temperature in the range of 1,250* C. to 1,350* C.; the remainder of the article is sintered at a temperature in the range of 1,450* C. to 1,550* C. The partially sintered portion is then severed from the fully sintered portion to produce the desired article.

Thermal and particle size effects in magnesium oxide

Abstract: X-ray line widths, surface areas and heats of solution of MgO samples prepared by vacuum dehydration of Mg(OH)2 at different temperatures in the range 350–1 200°C have been studied. While the surface energy of particles shows a steady decrease with the temperature of preparation, the heats of solution shows an abrupt decrease over a narrow temperature range. The results have been interpreted in terms of pseudocrystallization of the highly disordered oxide followed by sintering; the pseudocrystallization of colloidal oxides appear to be similar to glass transitions.

The sintering of activated CaO

Abstract: The sintering of CaO has been investigated from oxides of different “activity” resulting from the decomposition of CaCO3 at 1000‡C or Ca(OH)2 between 600 and 1000‡C.

Glassy phase method for making pure alumina-to-metal hermetic seals

Abstract: A method is described for making reliable hermetic seals between essentially pure alumina and metal. The method comprises applying a metallizing paint to the alumina, sintering the metallizing paint, placing a molybdenum trioxide-manganese dioxide coating on the metallizing layer, sintering the coating, and then bonding the metal to the coated alumina. The metallizing paint comprises molybdenum trioxide and a glass frit in an alcohol suspension containing a nitrocellulose binder, the glass frit constituting from about 2 percent to about 40 percent of the paint by weight.

Influence of the sintering conditions on the densification of manganese-zinc ferrites

Abstract: In order to prepare reproducibly high permeability manganese-zinc ferrites, it is necessary to carefully control the sintering schedule for obtaining both a homogeneous chemical composition and the desired microstructure. The latter should consist of big pore-free crystals, the remaining porosity being a few number of big pores situated at the grain boundaries. In our theoretical analysis of the densification, a volume diffusion model is used taking into account the geometrical parameters of the average crystal in thermodynamical equilibrium in the polycrystalline material. In the intermediate stage of sintering, the grain growth and densification are correlated, the densification rate being approximately inversely proportional to the grain volume. Sintering experiments have been carried out under various conditions of temperature and duration, the atmosphere composition being adjusted for maintaining the same stoichiometry. These results permit one to define isoporosity curves. It is then possible to compute the activation energies Q and Q 1 responsible for grain growth and for densification. The values found are Q = 82 \pm 5 kcal/mole and Q_{1} = 85 \pm 5 kcal/mole, in agreement with the previous measurements of Paulus [5] and Ogawa [3]. The experimental results show that both processes are controlled by the diffusion energy of oxygen anions which are the slowest diffusing species. The variation of the porosity versus time follows a (t)^{-n} law, n being roughly equal to 0.5 in good agreement with our predicted model. The experimental results also confirm that the use of a more oxidizing atmosphere increases the grain growth rate but decreases the densification rate. A sintering schedule leading to very high permeability ferrites is determined taking into account these results. The microstructure is obtained during a first soak at high temperature under oxidizing atmosphere. This leads to big crystals and open porosity while preventing zinc losses. This first treatment is followed by a second soak under a more reducing atmosphere leading to a high density ferrite and an homogeneous chemical composition possessing a ferrous iron content so as to obtain a maximum permeability at room temperature.