Growth and Stabilization of Discrete Particles

Because of the nonuniform distribution of particle sizes, especially when particles are smaller than 10 nm, spontaneous particle growth occurs. The particle size that is attained strongly depends on the temperature. As shown in Table 3.5, at 50-100 C particles reach 4-8 nm in diameter, whereas at 350 C in an autoclave they may grow to 150 nm. This spontaneous growth is relatively independent of the silica concentration. The size of particles produced by autoclaving aqueous sols is limited at high temperature by the conversion of amorphous silica to crystalline quartz. Fyre and McKay have shown that the rate of conversion under autogenous pressure at 330 C is proportional to the square of the hydroxyl ion concentration (13). Oehler (14) reported that after 4 weeks at I50°C and 2000 bars microspheres of crystalline tridymite 50 microns in diameter were formed. 

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