44 4 Gas-Phase Synthesis of Nanoparticles
distribution. Two points need special attention: There were no particles with sizes
below 3 nm detected and there is a long tail towards larger particle sizes. Both
features are characteristic for particles produced by a purely random process. As
an aid for visual perception, Figure 4.5 displays an electron micrograph of titania
powder with a broad particle-size distribution. In this product, the particle sizes
range from 5 to 50 nm. Lastly, the message provided by Figure 4.5 is the same as
that of Figure 4.4: there is a broad distribution of particle size containing a few
with large diameter.
Figure 4.4 Size distribution for a zirconia,
ZrO 2 , powder synthesized by the inert-gas
condensation technique. The asymmetric
particle-size distribution with a long tail
towards large particle sizes is typical for
products made by random processes. This
type of size distributions is, generally, fitted
with the log-normal distribution function [1].
0
5
10
15
20
25
30
35
40
particle diameter [nm]
0
0.04
0.08
0.12
0.16
class
frequency
Figure 4.5 Electron micrograph of titania powder with broad particle-size distribution as is
obtained by purely random synthesis processes. In this micrograph, the particle size ranges
from 5 nm to 50 nm. (Vollath, D., and Sickafus, K.E., Los Alamos Natoinal Laboratory, USA
(1992) unpublished results.)
40 nm