The agglomerate structures of the low-density SiO2 are
determined by transmission electron microscopy (TEM) and
scanning electron microscopy (SEM). TEM is used to
measure the primary particle size and to observe the direct
aggregation of the primary particles. Powder samples are
dispersed in pure ethanol by ultrasonic treatment for 15 min
before examination by TEM. SEM is used to measure the
size, the shape and the structure of agglomerates. Powder
samples for SEM analysis are acquired directly from the
APF bed by adhesive tape.