Intermediate stage sintering

The model of clusters or ensembles of sites and bonds (secondary supramolecular structure), whose size and structure are determined on the scale of a process under consideration. At this level, the local values of coordination numbers of the lattices of pores and particles, that is, number of bonds per one site, morphology of clusters, etc. are important. Examples of the problems at this level are capillary condensation or, in a general case, distribution of the condensed phase, entered into the porous space with limited filling of the pore volume, intermediate stages of sintering, drying, etc. 

Figure 2.31 Development of ceramic microstructure during sintering (a) Loose powder particles (b) initial stage (c) intermediate stage and (d) final stage. From W. E. Lee and W. M. Rainforth, Ceramic Microstructures, p. 37. Copyright 1994 by William E. Lee and W. Mark Rainforth, with kind permission of Kluwer Academic Publishers.

Initial powder Initial stage Intermediate stage Final stage Dense polycrystal-compact of sintering of sintering of sintering line compact... 

Hsueh, C.H., Evans, A.G. and Coble, R.L., Microstructure development during final/intermediate stage sintering -1. Pore/grain boundary separation , Acta. Metall., 1982 30 1269-1279. 

TABLE 16.6 Intermediate Stage Sintering Models Parameters fiar Simplified Models Represented by the Equation = K[(.y ClD)nR BTmf- t)... 

FIGURE 16S0 Sintering rate constant fw the (a) initial stage and (b) intermediate stage as a function of the log-normal size distribution width param r, lattice diffusion, g-b.d. giain-boundetiy difiiision, v. viscous flow). Taken from Chappell et al. [47]. 

Solid state sintering was discussed previously in this chapter. The sintering kinetics depend upon the rate determining step, which can be either viscous flow, grain boundary diffusion, or lattice diffusion. These sintering kinetics are summarized in Tables 16.4 and 16.6 for the initial and intermediate stage and Section 16.3.2.3 for the final stage. 

Problem 1 assumes that the BaTiOs powder is monosized. This is not actually the case, the powder used has a geometric mean size of 0.71 jum and a geometric standard deviation of 1.6. Determine the isothermal shrinkage at 1200°C of this BaTiOs sample during both initial and intermediate stage sintering. 

The pore properties of cast bulk porous material and coating layers from the same suspension become different above sinter temperatures where intermediate stage sintering in the bulk starts (see Section 6.2.5). At lower temperatures pore properties of free casts determined with Hg porosimetry can be used to compare the pore properties of consolidated dispersion coatings. 

The time and temperature required to form the desired degree of porosity in the dense solid depend mainly on the particle size of the alumina powder. The usual sintering sequence is imagined to be (1) neck formation between powder particles, (2) formation of open porosity with a continuous solid phase (intermediate stage), and (3) removal of closed pores imbedded in the dense solid. In the usual practical sintering of alumina, stage one is rapid, and the final density or porosity is determined mainly by stage three. 

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