Sintering initial stage

If we consider the microstructural changes to the particles, it is clear that the spherical particles will alter their shape with time to that of a prolate ellipsoid of revolution by tins transfer of material finm the spherical surface to that of the neck. In fact, the distance between the centers of the particles is not affected by vapor phase transport and only the shape of the pores is changed. Without a decrease in the center-to-center distance there is no densification or sintering. Therefore, this is one example of coarsening during the initial stage sintering. 

TABLE 16.3 Initial Stage Sintering Parameters for the Equation AL/Lq = Cj(t"/c ")... 

This is valid for all sintering mechanisms. Table 16.4 show the formula for initial stage sintering for all the possible mechanisms. These mechanisms use the previous formalism with different material sources and sinks and different transport pathways. These mechanisms all have the general formula ... 

The effect of the particle size distribution on shrinkage rates has been considered previously for the initial stage sintering [46]. The problem was treated for the linear shrinkage of a pair of different size spherical particles in contact. These results were extended to distributions of a few discrete sizes by a weighting of all possible contacts. For a continuous distribution in size, this pairwise interaction can be... 

Coble RL (1973) Effects of particle-size distribution in initial-stage sintering. J Am Ceram Soc 56 461 66... 

Figure 4.2. Two-particle model for initial stage sintering (a) without shrinkage and (b) with shrinkage.

Table 4.2. Summary of kinetic equations for various mechanisms of initial stage sintering...

USEFULNESS AND UMITATIONS OF THE INITIAL STAGE SINTERING THEORY... 

In the two-particle model of initial stage sintering, the neck growth is expressed as xldf = F T)d "t (Eq. (4.28)), where x is the neck radius and a the particle radius. The equation is, in general, acceptable for (x/u) < 0.2. With increasing values of (x/a) above 0.2, would the exponent n become smaller or larger Explain. 

Consider a powder compact in which the neck growth at the initial stage sintering occurs by lattice as well as surface diffusion. Discuss the relative importance of the two mechanisms according to particle size and sintering time. 

Johnson, D. L., Impurity effects in the initial stage sintering of oxides, in Sintering and Related Phenomena, G. C. Kucz5mski, N. A. Hooton and C. F. Gibbon (eds), Gordon and Breach, New York, 393-400, 1967. 

Figure 8.10 Geometrical parameters for the two-sphere model used in the derivation of the initial stage sintering equations for crystalhne particles. The geometries shown correspond to those for (a) the nondensifying mechanisms and (h) the densifying mechanisms.

Summary and Limitations of the Initial Stage Sintering Equations... 

In the study by Nordstrom and Yost, the sintering behavior of thick film Au conductors was monitored by electrical resistance measurements and scanning electron microscopy. Initial-stage sintering kinetic equations coupled with resistance/neck area correlations were used to develop a relationship between resistance R and sintering time T ... 

A variety of models [66-69] have been proposed for analysis of densifica-tion data obtained while heating a sample at a constant rate, q = dT/dt. The starting point is the general equation for isothermal initial-stage sintering [69] ... 

D. L. Johnson and I. B. Cutler, Diffusion Sintering I, Initial Stage Sintering Models and Their Application to Shrinkage of Powder Compacts, J. Am. Ceram. Soc. 46 541 (1963). 

A substantial dislocation motion contribution to the densification therefore appears to be present in the initial stage sintering. The isostatic pressing effect corroborates the significance of dislocations. ... 

Demirsk) D, Ragulya A, Agrawal D. Initial stage sintering of binderless tungsten carbide powder under microwave radiation. Ceramics International 2011 37 505-12. 

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