The Effect of Fractal Aggregates on Sintering Kinetics

Fine ceramic powders made by spray drying, by sol-gel powder synthesis, and sometimes by precipitation usually have very low packing densities. A possible reason for these low packing densities is that the basic packing units are porous agglomerates. In sol-gel s3mthesis. 

FIGURE 16.22 (a) Relative grain size versus relative density for a grain growth exponent g = 2 as a function of the log-normal size distribution width parameter, cr. Taken from Chappell et al. [47]. (b) Experimental grain size (GS) divided by initial particle size (PS() 0.3 pm) versus relative density for — monodisperse T1O2 [53] (cr = 0.1), — TiOj [54] ( r, = 0.3), and —- AljOg (o- = 0.5) [55]. 

FIGURE 16 3 Fractal a regate with three graerations. 

The processing of fractal ceramic powders is severely complicated by the evolving nature of the microstructure throu out the covuse of sintering. Changes in microstructure are influenced by a host of factors. 

Generalizing the sintering model discussed in the previous section to a fractal ceramic compact requires that equation 16.46 be used for the shrinkage of each generation of particles. Equation 16.55 can be rewritten as a product of the packing fractions of each generation, i, as a function of sintering time  

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