Page 117 - Book Hosokawa Nanoparticle Technology Handbook

2.4 COMPOSITE STRUCTURE FUNDAMENTALS
Amount of fine particles bonded to core particle surface (R) Amount of added fine particles particle Estimated temperature of particle surface − room temperature (K) 1500

Fine particle
Core

1000

500
0
Processing vessel temperature − room temperature (K)

Figure 2.4.24 50 100 150
The relationship between the vessel temperature and the
Specific surface area of processed fine particle (Sw) I II operation [4].
particle surface temperature during the particle composing

processing generates heat in the equipment, it is impor-
tant to control the processing temperature to prevent
melting or deformation of the particles. For example,
when resin is used as the feed material, the processing
temperature needs to be kept below its glass-transition
temperature. On the other hand, it is reported that the
higher the temperature is, the denser becomes the
Processing time t nanoparticle layer on the core particles when compos-
ing metal and ceramic particles [2].
Figure 2.4.23 To investigate the processing temperature effect on
The process of making core–shell type composite particles the properties of particle composites, it is necessary
by the mechanical process. to know the actual temperature at the particle surface
during the composing process. Figure 2.4.24 shows
an example of the relationship between the vessel
in case of composing titanium dioxide nanoparticles temperature and the particle surface temperature dur-
on the surface of glass beads, the nanoparticles can be ing the particle composing with the MechanoFusion
layered on the core particles under certain processing System as explained in Table 2.4.3 [4].
time with proper machine revolution. However, the The particle surface temperature was estimated
nanoparticle-coating layer can be ripped off from the thermodynamically from the reaction of oxide layer
surface of core particles as seen in Fig. 2.4.23, when on the surface of metal particle and the added carbon.
the machine revolution exceeds a critical value. The It is believed that the temperature at the particle sur-
critical revolution tends to reduce with the increasing face reaches about ten times higher than that of the
size of core particles [2]. This is attributed to the composing vessel. This temperature effect can initi-
increase in the mechanical energy per unit area due to ate highly specific phenomena at the particle surface
the reduction in specific surface area of the larger and enhance the bonding strength between the parti-
core particles. cles to a great extent during the particle-composing
Furthermore, there is an apparent difference in the process.
structure of particle composites depending on how the
nanoparticles are added, namely either charging all 2.4.4.3 Examples of particle composing
the feed mixture at once or feeding the nanoparticles Figure 2.4.25 shows an example of particle compos-
little by little repeatedly with a certain time interval. ing of nanoparticles on the surface of carbon
By feeding the nanoparticles to the core particles lit- nanofiber [5]. This technology enables the composing
tle by little repeatedly, smooth and uniform coating of nanoparticles not only on the surface of spherical
layer on the core particles can be obtained [3]. particles but also on plate-like and other shaped
The effect of processing temperature on the compos- particles. As seen in the figure, the surface of
ing process is the next to discuss. Since the mechanical nanofiber is coated densely with nanoparticles by

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