Percolation channels

Here, it was argued that the oxygen supply through the pores of the alumina preform was the rate-limiting step, and that growth became progressively easier as the alloy percolation channels approached the free surface. The oxidation rate was also found to decrease with increase in the silicon content of the melt. 

The crystallinity of alumina formed by DMO infiltration into SiC particulate preforms follows the expected behavior of heterogeneous nucleation on SiC particles and large monocrystalline regions with [0001] parallel to the growth direction within the larger voids [83,86]. Manor et al. [82] found that the infiltration rate increased as the SiC particle size decreased. However, no systematic trend was found in another work [83], most probably due to widely dilferent silicon concentrations in the percolation channels resulting from reduction of the silica layer during infiltration. 

The activation energy of dc relaxation (due to through current appearing when the percolation channels appear) E (caused by mobile protons and other small ions) corresponds to a linear... 

Figure 55 Modified random network as proposed by Greaves (1985) showing Si-rich zones connected by modifier-rich percolating channels marked in grey.

---