Sintering Variables

The major variables which determine sinterability and the sintered microstructure of a powder compact may be divided into two categories material variables and process variables (Table 1.1). The variables related to raw materials (material variables) include chemical composition of powder compact, powder size, powder shape, powder size distribution, degree of powder agglomeration, etc. These variables influence the powder compressibility and sinterability (densification and grain growth). In particular, for compacts containing more than two kinds of powders, the homogeneity of the 

Variables related to sintering Temperature, time, pressure, atmosphere, heating and 

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Sintering aims, in general, to produce sintered parts with reproducible and, if possible, designed microstructure through control of sintering variables. Microstructural control means the control of grain size, sintered density, and... 

All of the sintering equations were derived under the assumption that a local equiUbrium of atoms with capillary pressure is maintained everywhere, in the atom source and in the atom sink. (This assumption is acceptable.) The dihedral angle of 180° is also an acceptable assumption because the dihedral angle affects only numerical constants in kinetic equations, and not the sintering variables (Eq. (4.7)). 

Klar, E. and Samal, P. K., "Effect of Density and Sintering Variables on the Corrosion Resistance of Austenitic Stainless Steels, Advances in Powder Metallurgy and Particulate Materials, MPIF, Vol. 3, 1996, pp. 11-3 to 11-17. 

The characteristics of a powder that determine its apparent density are rather complex, but some general statements with respect to powder variables and their effect on the density of the loose powder can be made. (/) The smaller the particles, the greater the specific surface area of the powder. This increases the friction between the particles and lowers the apparent density but enhances the rate of sintering. (2) Powders having very irregular-shaped particles are usually characterized by a lower apparent density than more regular or spherical ones. This is shown in Table 4 for three different types of copper powders having identical particle size distribution but different particle shape. These data illustrate the decisive influence of particle shape on apparent density. (J) In any mixture of coarse and fine powder particles, an optimum mixture results in maximum apparent density. This optimum mixture is reached when the fine particles fill the voids between the coarse particles. 

The hardness of boron carbide (carbon hexaboride) is not well defined because it is made as sintered compacts which have variable densities, compositions, and defect densities. It is very hard (up to 4400kg/mm2), and of relatively low density, so it has been used extensively as body-armor (McColm,... 

Supported metal catalysts are much easier to employ and have obvious attractions for industrial use from their ease of handling and economic considerations of obtaining maximum utilisation of the catalytically active metal, by using very small particles with a high surface-to-volume ratio, which are stable on the support and not susceptible to sintering. In spite of the inherent difficulties of variable activity, kinetics and activation energies [11] associated with their use, supported metals have been extensively used as hydrogenation catalysts. 

Parameters of sintered tungsten carbide with variable cobalt content and strength characteristics calculated from Palmquist cracks (specification based on tests of Peters, 1979)... 

Place a mixture of 275 ml of concentrated nitric acid with an equal volume of concentrated sulphuric acid in a 2-litre three-necked flask, fitted with a thermometer, a mechanical stirrer and a dropping funnel and assembled in the fume cupboard. Cool the mixture to 10 °C in an ice bath, and run in 100 g (98 ml, 0.85 mol) of benzyl cyanide (Expt 5.157) at such a rate (about 1 hour) that the temperature remains at about 10°C and does not rise above 20 °C. Remove the ice bath, stir the mixture for 1 hour and pour it on to 1200g of crushed ice. A pasty mass slowly separates more than half of this is p-nitrobenzyl cyanide, the other components being the ortho isomeride and a variable amount of an oil. Filter the mass on a sintered glass funnel, press well to remove as much oil as possible and then dissolve in 500 ml of boiling rectified spirit. The p-nitrobenzyl cyanide crystallises on cooling. Filter this off at the pump and recrystallise from 80 per cent ethanol. The yield of p-nitrobenzyl cyanide, m.p. 115-116 °C, is 75 g (54%). Another recrystallisation raises the m.p. to 116-117°C. 

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