Page 271 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 4 CONTROL OF NANOSTRUCTURE OF MATERIALS
materials are pressured by their own weight. In this References
method, a sample is sintered under the acceleration of
about 10,000 times that of gravity. Since the pressure [1] M.Y. Chu, L.C. De Jonghe, M.F. Lin and F.J.T. Lin: J.
is caused by own weight of materials, they can be pres- Am. Ceram. Soc., 74, 2902–2911 (1991).
sured uniformly, in principle. [2] F.J.T. Lin, L.C. De Jonghe and M.N. Rahaman: J. Am.
The relation between density and particle size is Ceram. Soc., 80, 2269–2277 (1997).
shown in Fig. 4.5.34 for samples prepared by the [3] I.W. Chen, W.H. Wang: Nature, 404, 168–171 (2000).
same two-step sintering (Pattern 2) [sintering pattern, [4] G.C. Kuczynski: J. Metals. Trans. AIME, 185(2),
1st step temperature: 750 C, 2nd step holding tem- 169–178 (1949).
perature 650 C; centrifugal acceleration: 90,000 m/s 2 [5] Y.I. Lee, Y.W. Kim, M. Mitomo and D.Y. Kim: J. Am.
(kept constant during processing, pressure generated Ceram. Soc., 86, 1803–1805 (2003).
by the acceleration corresponds to several Mega
Pascal)]. The relations between density and particle [6] R.S. Averback, H.J. Höfler and R. Tao: Mater. Sci. Eng.,
size are similar in both the samples prepared by cen- A166, 169–177 (1993).
trifugal sintering and normal sintering. This is [7] Y. Kinemuchi, S. Uchimura and K. Watari: J. Euro.
because the process temperature is the same. Ceram. Soc., 24, 2061–2066 (2004).
Figure 4.5.38 shows increase of the density dur-
ing the holding time in the 2nd step. The density of 4.5.7 Fabrication of nanoceramics by colloidal
the sample is always higher than that of sample by processing
the normal sintering; while the shrinkage rate of the
centrifugal sintering is low. Judging from the fact Colloidal processing is a wet method for consolidat-
that the particle sizes are almost the same, improve- ing particles with a high density and homogeneous
ment in densification by the centrifugal process is microstructure by controlling the interparticle interac-
considered to be attributable to plastic flow caused tion in a suspension. Table 4.5.3 shows typical col-
by sliding of particles during the initial sintering loidal processing accompanied by a drain step, in
period. which the transport media and their driving forces are
To suppress particle growth in the sintering of indicated [1].
nanoparticles, it is important to suppress uneven sin- Slip casting is a common method in which a sus-
tering caused by aggregate structures and to increase pension is poured into a porous mold such as plaster
density. For this purpose, the two-step sintering is an and a consolidated layer is obtained. As is shown in
easy and effective method. In the pressure sintering, Fig. 4.5.39, the consolidation rate is controlled by the
rearrangement of particles and plastic flow can be drain rate in which the solvent is removed through the
expected which improve sintering property. Since the filter formed by the consolidated layer consisting of
sintering temperature is low, this method is effective packed particles. Therefore, during the drain process,
to suppress particle growth. the consolidation rate becomes slower when the parti-
cle size becomes smaller. Pressure or vacuum casting
is an enhanced filtration method in which the suspen-
sion is filtered from the outside of the mold by pres-
1
sure application or evacuation. Centrifugal casting is
Two staged sintering
the method for the sedimentation and consolidation of
Two staged sintering Centrifugal force
particles by centrifugal force in which the solvent is
removed at the top; therefore a porous mold is not
necessary.
0.9
Relative density in which the suspension is poured onto a moving
Tape casting is the method for forming a sheet film
sheet via a knife edge so-called doctor blade. This
process is widely used for making a laminated layer.
On the other hand, electrophoretic deposition (refer
0.8
to Section 4.2.3) is a colloidal processing technique, in
which charged particles are transported to the plate and
consolidated on it by an external DC application. The
consolidation rate is fast and independent of the parti-
0.7 cle size; therefore, this process is suitable for the con-
1 10 100 solidation of fine particles. Moreover, not only the
Retention time / h layer thickness but also the assembling of particles can
be controlled by adjusting the external electric field.
Figure 4.5.38 As another colloidal processing without accompa-
A relation between relative density and holding time in the nying dehydration, gel casting, and floc-casting have
two-step sintering (Pattern 2). received increased attention because of isotropic

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