Page 120 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 2 STRUCTURAL CONTROL OF NANOPARTICLES

(a)

200 nm

(b) (c)

1μm 1μm

Figure 2.5.2
SEM images of porous structure particles. (a) ZnO, (b) ZnS, (c) CdS.

porous particles produced by the spray-pyrolysis other hand, doughnut-type SiO particles can be pro-
2
method [5]. Within this method, the sprayed droplets duced an under conditions of high evaporation rate [6].
are first heated at a low temperature, i.e. less than Figure 2.5.5 shows SEM images of particles pre-
o
400 C, to evaporate the solvent and to obtain particles pared from different sizes of silica nanoparticles [7].
that contain both PSL particles and reactant. The par- Figure 2.5.6 shows variation in effective refractive
ticles are then heated at a relatively high temperature index and porosity as a function of 8 and 100 nm pri-
to burn out the PSL particles, thus producing for mary particles content used as the precursor [8].
obtaining porous particles. Various types of oxide In the case of a suspension containing two differ-
porous particles can be successfully produced by the ent sizes of nanoparticles, the resulting particles may
spray-pyrolysis method [5]. have a composition in which larger particles are
encapsulated by smaller particles, since the smaller
2.5.1.3 Preparation of porous structure particles by
particles tend to move faster to surface of the droplet
spray-drying method during evaporation. Figure 2.5.7 shows variation in
Spray-drying involves spraying a suspension of the effective refractive index for spray-drying
droplets followed by a drying process, and results in microencapsulated particles derived from small Al O 3
2
the production of solid particles. or ZrO particles mixed with large SiO particles, as a
2
2
As shown in Fig. 2.5.4, spherical SiO particles can function of their content [8]. It is known that by using
2
be produced by spray-drying a sol of SiO nanoparti- a spray-drying process involving two different sizes
2
cles under conditions of small droplet size, high con- of particles and/or nanoparticles having two different
centration, and very large silica particle size. On the refractive indexes; the refractive index of the prepared
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