Showing papers on "Sintering published in 1974"

Translucent Sintered MgAl2O4

Abstract: A translucent polycrystalline MgAl2O4 ceramic was prepared from finely divided coprecipitated spinel in which a small amount of CaO added as a sintering aid was uniformly distributed. The CaO promotes densification through the formation of a liquid phase at the sintering temperatures. Depending on the sintering treatment, the relative density of the sintered spinel was 99.7 to ∼100% of theoretical. The in-line optical transmission was > 10% from 0.3 to 6.5 μm. Total transmission in the visible region was between 67 and 78%.

A Study of Sintered Apatites

Abstract: Hydroxyapatite, fluorapatite, and chlorapatite materials were synthesized and their sintering characteristics were studied. The compressive strength of the apatites was determined and is given as a function of density. Scanning electron micrographs of the typical material structure are given. The degradation of strength of the hydroxyapatite after an extended acid soak also is reported.

Bearing members having coated wear surfaces

Abstract: The invention comprises bearing members having a coated wear surface applied by plasma-spraying an aggregate of particles thereon consisting of nickel-aluminum particles, nickel-molybdenum particles, tungsten carbide particles, and intermetallic alloy particles to form a high temperature, oxidation resistant, wear resistant, and scuff resistant coating. Alloying of the constituents in the final coating is desired and is achieved by encapsulating both the aluminum and molybdenum particles with nickel (alternatively the nickel-aluminum may be in the form of a bonded aggregate), by sintering the tungsten carbide in a matrix of cobalt, and by alloying nickel-cobalt, carbon, silicon, chromium, boron, and iron into an intermetallic alloy to form an aggregate of four types of particles for plasma-spraying onto the wear surface.

Sorption system for low-grade (solar) heat utilization

Abstract: A system for the effective utilization of low-grade heat sources such as solar energy, with a system including a molecular sieve material such as zeolite and a gaseous fluid adapted to be absorbed by the material which is in a closed container and circuit which includes a condenser and a gas expansion cooler member. When the container is heated, a gas is given off from the molecular sieve material, cooled in a condenser, and thereafter expanded for cooling purposes. In one embodiment, the cooled gas is received in a further container having absorbent material and subsequently, upon cooling of the first container, the gaseous fluid may be returned thereto via again a condenser and gas expansion cooler member to provide further cooling. In another embodiment, the molecular sieve material is formed by sintering same to form a pressure resistanr divider across the container. One side of the divider is heated to create a temperature gradient across the divider so that it functions as a heat energized pump for the gaseous fluid which is absorbed, a pressure as well as temperature differential developing within the container across the divider whereupon the heated pressurized gas, after giving up sone of its energy in a circuit which may include a condenser and gas expansion member, is returned to the container to be again pressurized and heated by the action of the divider composed of the molecular sieve material.

Nature of Intergranular Phase in Nonohmic ZnO Ceramics Containing 0.5 Mol% Bi2O3

Abstract: The intergranular phase obtained by sintering a binary mixture of ZnO + 0.5 mol% Bi2O3 was isolated by using a dilute solution of HCIO4, which etches ZnO preferentially. The combined results of selected-area electron diffraction and microscopy, microprobe analysis, and X-ray diffraction strongly indicate that the intergranular material is a polycrystalline phase of tetragonal β-Bi2O3 (P421c), rather than the amorphous ZnO-Bi2O3 phase reported earlier. It appears that the nonohmic behavior in this prototype metal-oxide varistor must be an interfacial property associated with the semiconducting ZnO grains separated by thin layers of high-resistivity Bi2O3.

Liquid sintered cobalt-rare earth intermetallic product

Abstract: A process for preparing novel sintered cobalt-rare earth intermetallic products which can be magnetized to form permanent magnets having stable improved magnetic properties. A particulate mixture is formed of a base CoR alloy and an additive CoR alloy, where R is a rare earth metal. The base CoR alloy is one which, at sintering temperature, exists as a solid Co5R intermetallic single phase. The additive CoR alloy is richer in rare earth metal than the base CoR alloy, and at sintering temperature is at least partly liquid. The base and additive alloys, in particulate form, are each used in an amount to form a mixture which has a cobalt and rare earth metal content substantially corresponding to that of the final desired sintered product. The mixture is pressed into compacts and sintered to the desired sintered product phase composition and density. At sintering temperature, the final sintered product has a phase composition lying outside the Co5R single phase on the rare earth richer side. Specifically, the final sintered product contains a major amount of the Co5R solid intermetallic phase and up to about 35 percent by weight of the product of a second solid CoR intermetallic phase which is richer in rare earth content than the Co5R phase.

Sintering behaviour of ultrafine NbC and TaC powders

Abstract: Compositional and microstructural changes upon firing ultrafine (300 to 400 A) stoichiometric NbC and TaC powder lots have been studied up to a temperature of 1600° C. Substantial amounts of oxygen impurities, mostly oxide particles or layers are eliminated by reductions with hydrogen, free carbon or the carbides themselves. TGA showed these reactions to take place at 700 to 1400° C with maxima around 1000 to 1100° C. Low temperature sintering is inhibited by this impurity and its removal is thus essential. Other impurities (Ni, Cr, Fe) were also found in the starting powders in total concentration 0.5 to 1%. They give rise to a liquid phase located at grain edges at temperatures as low as 1100° C which then controls microstructure development. It dissolves to some extent in the carbide matrix at high temperature, and has a tendency to rise to the free surface of the samples. Compositional and structural heterogeneities are thus produced between bulk and surface at high temperatures. Owing to these impurity effects, it was not possible to clearly evaluate the influence of powder granulometry.

Influence of Oxygen Activity on the Sintering of MgCr2O4

Abstract: The sintering behavior of MgCr2O4 powder compacts was investigated as a function of temperature, time, and oxygen activity. The results show that MgCr2O4 cannot be densified to >70% of theoretical density at temperatures up to 1700°C if the oxygen activity exceeds 10−6 atm. The oxygen activity must be decreased to <10−10 atm before densities exceeding 90% of theoretical can be achieved. Weight loss and X-ray data indicated that maximum density occurred at an oxygen activity just above that where MgCr2O4 becomes unstable.

The microstructures of silicon nitride/alumina ceramics

Abstract: The microstructures of materials formed by sintering or hot-pressing mixtures of silicon nitride and alumina have been studied by transmission electron microscopy. The probable mechanism of transformation of the reactants to form β′-silicon aluminium oxynitride (β′-sialon) via a liquid phase sintering process, which is analogous to a similar transformation in hot-pressed silicon nitride containing a magnesia additive, is proposed. The origin and crystal symmetry of an unknown second phase is discussed. The residual quantity of this phase, known as the X-phase, is controlled mainly by the silica impurity content of the initial silicon nitride powder.

Pressureless sintering silicon nitride powders

Abstract: A method of producing a silicon nitride article by powder techniques, wherein silicon nitride powder is used as a starting material. The silicon nitride powder, mixed with a densification aid, is heated rapidly to the sintering/densification temperature (1500° to 1750° C) in the absence of pressure, held there a short, closely controlled time (5 to 30 minutes) and thereafter rapidly cooled. This provides a strong product with controlled dimensional tolerances.

Process for producing a sintered article of a titanium alloy

Abstract: A sintered article of a titanium alloy is produced by powder metallurgy techniques, the alloy having uniform structure, and improved mechanical properties, machinability and weldability. The process comprises; partially sintering a powder mixture of (a) at least one selected from the group consisting of powdered titanium and titanium hydride having a particle size of minus 60 mesh and (b) at least one powdered additive selected from the group consisting of powdered Ni, Al, Cu, Sn, Pd, Co, Fe, Cr, Mn, and Si having a particle size of minus 60 mesh, to partially alloy the titanium with one or more additives employed; after furnace cooling the partially sintered mass to room temperature, pulverizing it to powder of minus 60 mesh in particle size to prepare mother alloy powder; mixing (a) the mother alloy powder having a particle size of minus 60 mesh, (b) at least one selected from the group consisting of powdered titanium and titanium hydride having a particle size of minus 60 mesh, and (c), if necessary, at least one additional element selected from the group consisting of powdered V, Mo, Zr and Al-V alloy, which are added for the purpose of avoiding excess formation of a liquid phase during the subsequent sintering; compacting the thus formed powder mixture into a compact having a predetermined shape; and sintering the compact at a temperature ranging from 1000°C to 1500°C in a non-oxidizing and non-nitriding atmosphere for from 30 minutes to 2 hours.

Shaped zirconium oxide bodies of high strength

Abstract: Shaped bodies of zirconium oxide of high mechanical strength can be prepared by sintering a pulverulent mixture of 30% to 90% monoclinic zirconium oxide, 7.8% to 69.5% zirconium oxide conventionally stabilized by bound oxides of metals having an ionic radius similar to that of zirconium, and 0.5% to 2.2% magnesium oxide and/or calcium oxide at above 1,600*C, the combined amount of the stabilizing oxide and of the magnesium and/or calcium oxide in the sintered body being 2.5% to 3.5%.

Microstructural changes in SmCo5 caused by oxygen, sinter-annealing and thermal aging

Abstract: The solubility of oxygen in SmCo 5 at 1125 °C (sintering temperature) in excess of the 800 °C solubility was determined to be 3500 ± 500 ppm. The source of oxygen during sintering is the samarium oxide subscale. Submicron oxide particles precipitate within SmCo 5 grains on cooling from the sintering temperature. Oxidation also causes depletion of samarium and precipitation of Sm 2 Co 17 particles within SmCo 5 grains. Both inclusions may be sources of domain wall nucleation. The oxide inclusions can be removed by a thermal aging treatment that collects the oxide into a few large grains outside the SmCo 5 grains by a solution/reprecipitation mechanism involving grain boundary transport of samarium. The Sm 2 Co 17 inclusions are not affected by thermal aging, but they can be prevented from occurring by including excess samarium in the sintered compact to replenish samarium lost by oxidation.

Method of manufacturing α-alumina

Abstract: A method of manufacturing α-alumina with a degree of purity of at least 98% which has a high sintering activity. The α-alumina contains at least 98% α-Al2 O3 and not more than 0.1% Na (calculated as Na2 O), and not more than 0.1% Ti (calculated as TiO2). The α-alumina also contains between 0.03% and 2% of Fe2 O3 and/or Cr2 O3. The α-alumina is formed by first fine grinding and then calcining, preferably at a temperature between about 1120° C and 1350° C, an aluminum hydroxide and/or hydrated aluminum oxide. The α-alumina is then cooled and finely ground to provide an α-alumina having excellent sintering activity.

Mechanism of the sintering of high-porosity materials in the presence of a volatile pore-forming agent

Abstract: 1. During the sintering of compacts with a filler, their volume shrinkage is a result of the densification of the regions with the fine natural pores, but their macroporosity remains unchanged (except in the case of compacts of very high starting porosity). 2. A matrix mixture of metal and voids predominates in the structure of porous solids produced with a coarse filler, while a statistical mixture is characteristic of the structure of solids produced with a fine filler. A formula is proposed for calculating the statistical weight of the matrix mixture in the structure. 3. The differences in the structures of porous materials associated with the particle size of the filler lead to differences in the structural weakening of these materials (the volume viscosity is less for porous solids with a mainly statistical distribution of the large pores), and this results in a decrease in macroporosity during the sintering of compacts with a fine filler. 4. The growth of interparticle contacts during the sintering of compacts with a filler is analogous to the growth of contacts in unpressed powders being sintered and is directly determined by the kinetics of the volume shrinkage of the porous solids.

Processing Parameters and Properties of Lithium Ferrite Spinel

Abstract: Polycrystalline compacts of lithium ferrite with varying stoichiometry were sintered using a packing-powder technique and high-O2 atmosphere to control the material loss from the system. These specimens were used to study the influence of sintering time and temperature and stoichiometry on the densification, microstructural characteristics, dc resistivity, and hysteresis-loop parameters of lithium ferrite. The influence of the packing-powder composition was also investigated.

Catalysts and methods of making

Abstract: A sintered catalyst article is prepared by: molding into a shaped article and aging (I) either A. a refractory powder of calcium aluminate substantially free of silicon dioxide, alone, or B. a homogenous mixture of 1. said refractory powder and 2. at least one member selected from the group consisting of aluminum oxide, calcium oxide, beryllium oxide, magnesium oxide, strontium oxide and a compound which is converted by heating to one of said oxides; (II) sintering said molded and aged article at a temperature above 1150°C; the thus sintered catalyst article containing 10 to 60 wt. percent of calcium oxide, 30 to 90 wt. percent of aluminum oxide, 0 to 30 wt. percent of at least one oxide selected from the group consisting of beryllium oxide, magnesium oxide and strontium oxide, and less than 0.2 wt. percent of silicon dioxide.