Particle size sintering kinetics

Since improvements achievable with bulky electrodes are limited by the structure of the electrode itself, sintered, porous, Teflon bonded, or phosphate-bonded Ni electrodes have been proposed [386, 391, 399, 400]. A mere increase in surface area is observed without any change in Thfel slope. The same is the case with Ni wiskers in spite of their very large surface area and small particle size [401, 402], A decisive modification of the kinetic pattern is indeed obtained as Raney Ni is used [93, 403] (see Fig. 11). This form of Ni is well known also in the field of hydrogenation catalysis. As an electrocatalyst it was proposed by Justi et al. [404] long ago. Raney Ni is obtained by allowing Ni with a component (usually Al or Zn) which is then... 

Effect of Particle Size Distribution on Sintering Kinetics... 

Particle size may have a considerable influence on the respective type of the kinetic equation by changing the value of the exponent in the relation of the type x" kt (see eqn. (84)). The same applies to particle shape. For example, with two conical formations in contact, the exponent value falls to n = 3 compared with = 5 for spherical particles sintering by volume diffusion (Geguzin, 1967). This phenomenon has been verified experimentally by Kuczynski on AI2O3 of spherical shape. After a longer period of time, edges appeared on the sphere surfaces which were responsible for reducing the value of the exponent n. 

A formal derivation of steady state particle size distributions during catalyst sintering has not been made before. Their existence adds validity to the use of steady state dispersions during the kinetic analysis of sintering data. At the same time, this provides an interesting perspective for reanalyzing the present ideas about sintering mechanisms. 

Particle sintering involves atoms that escape and diffuse away from crystallites, eventually being captured by other particles (267), or crystallite diffusion on the surface and subsequent collision and coalescence with larger particles (262). The latter model is unlikely for particles over 50 A (267). Since practical supported catalysts have a broad particle size distribution, a combination of atom and small crystallite diffusion is probable. That more than one process is responsible for sintering is also apparent from the power law kinetics of the aging process (267-264) ... 

Sintering kinetics are dependent on the particle size and relative values of the transport coefficients, with smaller particles favoring grain boundary and surface diffusion and larger particles favoring bulk diffusion. 

The importance of a high green density with a uniform microstructure to improve the various properties of sintered technical ceramics has been documented in numerous studies. Thus, in recent years, extensive efforts have been made to increase the homogeneity of green compact microstructures [26]. Among these, a colloidal consolidation route using a kinetically stable slip with a narrow particle size distribution has received increasing attention [27-29]. 

Most attempts to characterize sintering via particle size distributions resort to classical Ostwald ripening analyses. As part of its long-time solution, a kinetic expression for is obtained, together with a predicted time-invariant PSD. It has been recurrently indicated in the literature that the distributions obtained from Ostwald ripening are not in agreement with... 

In analytical models, it is assumed that the particles in the initial powder compact are spherical with the same size and uniform packing, which is called the geometrical model [6]. With appropriate boundary conditions, the remainder of the powder system is considered as a continuum, having the same macroscopic properties, such as shrinkage and densification rate, as the isolated unit. The equations of the sintering kinetics can be derived from the established mass transport equations, which are solved under appropriate boundary conditions. 

In general, the sintering rate (densification rate) increases with decreased particle size and with increased sintering temperature and time, as shown schematically in Figure 4.5. A quantitative explanation of this general tendency can be drawn from the kinetic equations derived in Section 4.2 and also summarized in Table 4.2. 

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