Page 269 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 4 CONTROL OF NANOSTRUCTURE OF MATERIALS

1500
ZnO Particle size (20nm)
One staged sintering
Two staged sintering
Particle size / nm 500 Centrifugal force load
1000
Two staged sintering

0
0.7 0.8 0.9 1
Relative density

Figure 4.5.34
Particle growth in two-step sintering (Pattern 2).

solid-phase sintering of Y O ; later it was shown
3
2
that the method is also effective in liquid-phase sin-
tering [5].
Although this phenomenon is very interesting, no
reports have been published on details of this
mechanism. Chen et al. suggest that the activation
energy of the particle growth is higher than that of
sintering, because the densification proceeds with
suppression of the particle growth [3]. Particle
growth is influenced by relative size among nearby Figure 4.5.35
particles and presence of pore (pinning effect). Comparison of microstructures.
Therefore, it can be said that particle growth is dif-
ficult in the samples that have homogeneous particle
size and pore distributions; these microstructures are
probably formed in the 1st step of the two-step sin- molding at 30 MPa followed by CIP treatment at
tering process. 100 MPa. Accordingly, the above difference is not
Figure 4.5.35 shows the fractured surfaces of ZnO caused by the difference in the molding method. The
sintered by a normal temperature profile and the two- relative density is the same in both the samples, that
step profile of Pattern 2. The relative density of bulk is, the porosity is the same, suggesting that the pores
products is almost the same in these two examples. are homogeneously distributed in the sample
No significant difference is observed in the particle obtained by the two-step sintering process. From
distribution, although the two-step sintering gives these observations, it is considered that one of the
somewhat larger particle size. reasons why particle growth is suppressed in the
Figure 4.5.36 shows electron microscopic images two-step sintering is the homogeneous distribution
of polished surfaces of the same samples. Large of pores.
voids originated from aggregates are observed in
the sample prepared by the one-step sintering (a (2) Pressure sintering
normal sintering pattern). In the one-step sintering, The pressure sintering is another method to minimize
similar voids are observed very frequently, which particle size other than the method described in the
clearly shows the tendency of easy aggregation of former section, the heat profile controlling method.
nanoparticles. An advantage of pressure sintering is relatively low
On the other hand, the chance of finding these temperature with the suppression of particle growth.
voids decreases in samples prepared by two-step sin- Generally, the effect of externally applied pressure is
tering; the sintering is more homogeneous in the expressed by adding an external pressure term to the
two-step than in the one-step. Both the molds are sintering stress (driving force of sintering such as
prepared from nanoparticles by unidirectional surface tension) term. That is, the driving force for

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