Page 267 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 4 CONTROL OF NANOSTRUCTURE OF MATERIALS
substantially more mass than the molecules used in [12] N. Asai, R. Matsuda, M. Watanabe, H. Takayama,
conventional methods such as sputter deposition. In S. Yamada, A. Mase, M. Shikida, K. Sato, M. Lebedev
addition, it is easy to form complicated oxide thin films and J. Akedo: Proc. MEMS 2003, Kyoto, Japan,
with the AD method, because the crystal structure of pp. 247–250 (2003).
the starting particles is preserved during deposition. [13] J. Akedo, M. Lebedev, H. Sato and J-H. Park: Jpn.
Therefore, the AD method is useful for developing J. Appl. Phys., 44(9B), 7072–7077 (2005).
composite or integrated materials with various ceram- [14] M. Lebedev, J. Akedo and Y. Akiyama: Jpn. J. Appl.
ics, metals, and polymers. However, because many
defects are introduced during deposition, a process Phys., 39, 5600–5603 (2000).
such as annealing is needed to achieve acceptable elec-
trical properties of the deposited films. One of the ori- 4.5.6 Suppression of particle growth in sintering
gins for these defects may be that the crystals in the nanoparticles
starting particles are crushed during collision with the
substrate. Or it may be that surface defects of the start- Increasing the density of bulk products with the sup-
ing particles are introduced into the deposited layer. pression of particle growth is important to fully utilize
Because integration of ceramics with low melting- the benefits of nanoparticle functions. In middle and
point materials is important for a broad range of later periods of the sintering process, usually, densifi-
applications, reduction of process temperature is a cation and particle growth proceed simultaneously;
requirement of future fabrication techniques. To accordingly, crystalline size of bulk products is of the
achieve these goals with the AD method, the order of micrometers, even if nanoparticles are used
deposition mechanism, formation of defects in the as the starting materials.
layer, and recovery of electrical and mechanical prop- Two techniques are available to suppress the parti-
erties by post-deposition annealing will be investi- cle growth: one increases the contribution of surface
gated in detail. Establishment of methods for and grain boundary diffusions in sintering by control-
controlling the particle size and degree of aggregation ling sintering conditions and the other accelerates
of the starting particles are also important. We expect plastic flow by applying external pressure. The former
that AD films will be useful for making MEMS method is called two-step sintering because of its spe-
devices, high-frequency components, optical integra- cific temperature profile, while the latter is called
tion devices, and components for fuel cells, which pressure sintering.
require ceramic thick films greater than 1
m thick.
(1) Two-step sintering
Two types of two-step sintering are available: one
References keeps nanoparticles at a relatively low temperature
where surface diffusion predominates and then
[1] J. Akedo, M. Lebedev: Materia, 41(7), 459–466 (2002) increases the temperature for densification [1, 2]
(in Japanese). (Pattern 1 in Fig. 4.5.33); and the other first brings
[2] J. Akedo, M. Lebedev: Jpn. J. Appl. Phys., 38(9B), particles to a high temperature and then, without
5397–5401 (1999). maintaining that high temperature, decreases the tem-
[3] J. Akedo: J. Am. Ceram. Soc., 89(6), 1834–1839 (2006). perature by about 100 C and finally maintains that
temperature for a long period of time [3] (Pattern 2 in
[4] M. Kiyohara, Y. Tsujimichi, K. Mori, H. Hatono, J.
Fig. 4.5.33). In both of them, the objective of the first
Migita, T. Kusunoki, N. Minami, M. Lebedev and J.
step is to induce pre-coarsening.
Akedo: Proc. 15th Ceram. Soc. Jpn. Autumn Symp.,
These are effective methods for homogeneous sin-
228 (2002).
tering of particles with a wide size distribution or with
[5] J. Akedo, M. Lebedev: Jpn. J. Appl. Phys., 40, aggregates often observed in nanoparticles. For exam-
5528–5532 (2001). ple, when using powder with a wide size distribution,
[6] J. Akedo, M. Lebedev: Jpn. J. Appl. Phys., 41, first sintering of small-sized particles starts; the pore
6980–6984 (2002). size distribution of the bulk products is wide reflect-
[7] M. Lebedev, J. Akedo: IEEJ Trans., 120-E(12), ing the inhomogeneous particle size distribution. That
600–601 (2000). is, both large and small pores coexist; the larger pores
being stable even in the last period of the sintering
[8] J. Akedo: Microsystem Technol., 6(11), 205–209 (2000).
process, where high sintering temperature is required
[9] T. Ide, T. Ito, H. Hatono, M. Kiyohara, M. Lebedev, J.
for densification. A similar situation occurs in the
Akedo: Proc. 15th Ceram. Soc. Jpn. Autumn Symp.,
presence of aggregates. That is, with the packing
229 (2002).
density of the aggregates different from that of the
[10] J. Akedo, M. Lebedev: Appl. Phys. Lett., 77, 1710–1712 other parts, the pore structure becomes inhomoge-
(2000). neous. Particularly, the sintering of nanoparticles that
[11] J. Akedo, M. Lebedev: J. Cryst. Growth, 235, 397–402 easily aggregate depends largely on the sintering
(2002). property of aggregates. As a result, a high final
242