Initial stage of sintering

The shrinkage that often accompanies densification is controlled by different mechanisms in the three stages of sintering. In the initial stage of sintering, the linear shrinkage, AL/Lq, is described by... 

W.D. Kingery and M. Berg. Study of the initial stages of sintering solids by viscouus flow, evaporation-condensation, and self-diffusion. J. Appl. Phys., 26(10) 1205-1212,... 

After preforming, there are still residual stresses and entrapped air in the preformed part, which invariably causes cracking mainly during the initial stage of sintering. The removal of entrapped air or degassing requires some time called resting time, which depends mainly on the wall thickness. 

The initial stage of sintering [20—23] is fiequently referred to as the neck formation stage, as is shown in Figure 16.1. The sintering driving force for the initial powder compact is due to the curvature difference between particle surface and that of the neck, see Figure 16.5. The six... 

Initial stage of sintering. Theoretical studies over the past 35 years, which have... 

The initial stage of sintering is that in which the connecting necks between the particles are formed and the contact area increases from zero to about 0.2 of the cross-sectional area of the particle (Coble, 1961). 

The initial stage of sintering is characterized by formation and growth of contact areas (necks) between the particles neither formation of closed pores nor crystal growth occur. The linear shrinkage due to sintering does not exceed about 5%. 

Although these initial stages of sintering ere important, it is perhaps the phase transformation of to y-Al O which Is of more interest in a catalyst... 

Figure 10.11 (a) Initial stage of sintering model represented by spheres in tangential... 

Calculate the effect of a 10 MPa applied pressure on the vacancy concentration in a powder compact when x/r = 0.2, that is, during the initial stages of sintering at 1500°C. State all assumptions. 

Equations for initial stage of sintering have been intensively studied in the open literature [20, 21, 29-32]. With the equations for neck growth and the densifying mechanisms, shrinkage can be expressed in the following general forms ... 

There have also been reports on numerical simulation of the initial stage of sintering, involving grain boundary, lattice, and surface diffusion at the same time [36]. However, this simulation also uses the circle approximation for the neck surface. Furthermore, the chemical potential gradients for the grain boundary diffusion and the lattice diffusion are assumed to be the same. The validity of numerical simulation has been compared with other methods [44, 45]. 

In the two-sphere model at the initial stage of sintering, is usually assumed to be equal to the area of the sphere projected onto the punch of the hot pressing die divided by the cross-sectional area of the neck, i.e., [Pg.351]

As in the case of flexural strength, variation of the Young s modulus with the degree of densification increases during the initial stages of sintering. 

Figure 8.9 Examples of real microstructures (planar sections) for (a) initial stage of sintering, (b) intermediate stage, and (c) final stage.

Table 8.5 Plausible Values for the Constants Appearing in Eqs. (8.34) and (8.35) for the Initial Stage of Sintering...

The other simplifying assumptions of the models must also be remembered. The extension of the two-sphere geometry to real powder compacts is valid only if the particles are spheres of the same size arranged in a uniform pattern. In practice, this system is, at best, approached only by the uniform consolidation of monodisperse powders by colloidal methods (see, for example, the work of Barringer and Bowen discussed in Chapter 1). Coble (17) considered the effect of a particle size distribution on the initial stage of sintering by considering a linear array of spheres. 

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