Page 51 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 1 BASIC PROPERTIES AND MEASURING METHODS OF NANOPARTICLES
no difference on the crystal symmetry. Crystal struc-
cross section ture can be classified into the 7 crystal symmetries,
14 Bravais lattices and 32 crystal classes. These clas-
sifications are listed in Table 1.8.1. Typical crystal
symmetries are expressed by two types of close pack-
ing structures and their stuffed structures of atoms
and/or atom groups into the interstitial spaces of the
close packing structures [1].
analysis line
On the other hand, crystal structures of the nanopar-
ticles for advanced materials depend on the particle
size and the thermodynamic stability of the materials.
Namely, we have to take into account such parameters
for the crystal symmetries of the materials and these
are recognized as so-called quantum effect or quan-
tum size effect (hereafter, called size effect). The size
effect of nanoparticles is observed if the particle
diameter is small enough and then the surface energy
becomes considerably large compared to the lattice
energy of the nanocrystal. Therefore, the critical par-
ticle diameter, below which the size effect will be
expected, can be changed by the intrinsic and/or the
extrinsic conditions such as the kind of compound
and the atmosphere, the preparation method and so
Co element
on. Such variable factors make the interpretation of
1 μm
the size effect complicated. However, the develop-
ment of recent analytical methods shows the possibil-
Figure 1.7.7 ity to provide the means to determine the critical size
SEM image of the cross section of LiCoNiO coated NiO for advanced materials. In this section, all of these
2
grains [6]. cannot be shown but some case studies are listed,
which contain the crystal structure and the size effect
of zirconium oxide (zirconia) and the ferroelectric
References nanoparticles to consider the crystal symmetry of the
nanoparticle.
[1] K. Yasuda, T. Ioroi: Surface, 76, 563–567 (2000).
[2] K. Murata, T. Fukui, C.C. Huang, M. Naito, H. Abe and
K. Nogi: JCEJ, 37, 568–571 (2004). 1.8.1 Particle size dependence of crystalline phases
[3] M. Matsumoto, K. Kaneko, Y. Yasutomi, S. Ohara and of zirconia
T. Fukui: J. Ceram. Soc. Jpn., 110, 60–62 (2002).
[4] T. Fukui, T. Oobuchi, Y. Ikuhara, S. Ohara and K. Kodera: The phase transition of zirconia, which exhibits
high fracture toughness and strength to apply as
J. Am. Ceram. Soc., 80, 261–263 (1997).
structural ceramics and high ionic conductivity at
[5] T. Fukui, S. Ohara and K. Mukai: Electrochem. Solid-
relatively low temperature to apply as oxygen sen-
State Lett., 1, 120–122 (1998).
sor for automobile and solid electrolyte for fuel cell,
[6] T. Fukui, H. Okawa, T. Hotta, M. Naito and T. Yokoyama:
is the typical example to show the crystal structure
J. Am. Ceram. Soc., 84, 233–235 (2001). change by the thermodynamic reason. Zirconia
undergoes unique phase transition behavior, so-called
martensitic transformation. Crystal symmetry of
1.8 Crystal structure
zirconia changes from cubic to tetragonal at around
1000°C and tetragonal to monoclinic at lower tem-
The crystal structure of particles can be described by perature and these transition temperatures can be
the same crystal symmetry as a bulk material, if the controlled by dissolving the yttria (Y O ) and mag-
2
3
particle size is large enough. Therefore, we should nesia (MgO) to stabilize the tetragonal or cubic
take into account for the crystal chemistry of the bulk phase at room temperature (the so-called partially
materials because the crystal symmetry of the materi- stabilized and stabilized zirconia, respectively).
als is uniquely determined by the ambient atmos- The application of zirconia ceramics to the
phere, independent of the morphology. Namely, structural materials or the functional materials by uti-
crystal symmetry is determined only by the physical lizing unique phase transition mechanism and its
arrangement of the constituent elements, and the stabilizing technique are well-known and very
chemical bonding of the constituent elements makes useful [2, 3].
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