Mechanism of sintering

Fig. 19.2. The microscopic mechanism of sintering. Atoms leave the grain boundary in the neck between two particles and diffuse into the pore, filling it up.

Several of the early studies aimed at finding the governing mechanisms of sintering were done with metal powders. A famous study was by Kuczynski (1949) who also examined the sintering of copper or silver to single-crystal metal plates but... 

Note that the values of h depends upon the material and the mechanism of sintering. Also, these values depend upon r and x, the radii of the particles and the radius of joining. These equations apply only to spheroids and a shape factor must be considered as well. 

Arai and co-workers investigated the effects of cation composition in the mirror plane of Mn-substituted hexaaluminates.5,15 Investigation of a series of rare-earth-based hexaaluminates (AMnAluOi9, A=La, Pr, Nd, Sm) prepared via hydrolysis of alkoxides and calcined at 1200 °C for 5 h showed that surface area increases with the ionic radius of A. The La-substituted sample, the largest cation of the series, showed a surface area of 15 m2/g, which compared with 5 m2/g for the Sm-substituted one, i.e., the smallest cation of the series. These data are consistent with the mechanism of sintering resistance reported above. Indeed, it is expected that the larger the cation, the more difficult its diffusion along the c axis. 

Particle migration (up to 8nm particles migrated over 25 nm at 773 K) was also observed for Pt on alumina [43]. The two major mechanisms of sintering of supported Pt crystallites appeared to be (i) short-distance, direction-selective migration of particles followed by either collision and coalescence or by direct transfer of atoms between the two approaching particles, or (ii) localized direct ripening between a few immobile, adjacent particles. 

Effect of Oxygen on the Platinum Functions. On the basis of the mechanism of sintering in a non-hydrogen atmosphere (2) and in order to protect the platinum functions of the catalysts, we treated them at 482 494°C with different oxygen concentration from 0 to 21% in N2 for 4 8 hrs. The results are shown in Table III. 

It is clear from the above discussion that despite the importance of alumina as a catalyst support, the detailed mechanism of sintering is far from clear. There is good understanding of the effect of different factors on sintering, but the detailed ntechanlsra of phase transformations is, in many cases, open to considerable uncertainty. 

One of the principal objectives of model catalyst studies is to relate observed changes in the structure, con sition, and size of single crystallites or of small collections of crystallites to mechanisms of sintering and redispersion. It may be possible under favcaable circumstances to observe direcdy mechanistic processes such as crystallite migration and spreading. In this section, mechanistic evidence frtan model catalysts studies is Hcsented and discussed. 

This then leads to a rapid spreading of the crystallites, to torus formation and also fragmentation of the crystallites. New mechanisms of sintering based on wetting and spreading have been suggested to explain some experimental observations. 

NEW MECHANISMS OF SINTERING AND H1CD1SPERSI0N INDUCED BY SPREADING AND WETTING... 

Several mechanisms of sintering have been proposed (i) migration of... 

Figure 5.10 Mechanism of sintering Teflon . (Courtesy DuPont.)...

Mechanisms of Sintering 385 Sintering Technologies 389 Batch Sintering 390 Continuous Sintering 397... 

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