Page 344 - Book Hosokawa Nanoparticle Technology Handbook

CHAPTER 6 EVALUATION METHODS FOR PROPERTIES OF NANOSTRUCTURED BODY

6.1 Functionality of nanostructures 6.1.1 What are nanostructures?
and their characteristic evaluation
Nanostructures play an important role in the actual
applications of nanoparticles. Typical images of
Nanoparticles are defined in “Nanoparticle nanostructures, such as composite nanoparticles,
Technology” (The Nikkan Kogyo Shimbun, Ltd., dense and porous materials, are illustrated in
2003) as “ultimate fine particles with sizes in the Fig. 6.1.1, where nanostructures composed of two
nanometer region, formed with their respective types of nanoparticles and micron-size host particles
chemical characteristics maintained, whose charac- are exemplified. Nanostructures can mainly be
teristics differ significantly from those in the form of divided into particle structure and bulk structure, all
bulk.” The vast increase in specific surface area by of which contain nanoscale particles.
scaling down fine particles to nanoscale elevates Composite nanoparticles can be roughly classified
their original physical and chemical characteristics into structures comprising nanoscale particles and
as materials or particles extensively. For instance, the nanostructures fabricated through the presence of
such nanosizing enhances the reactivity of each par- host particles. Composite nanoparticle structures,
ticle, enabling the low-temperature sintering of such as catalysts, have nanoparticles fixed onto the
ceramics and offering remarkable improvement in surface of host particles as a carrier, and their single
functions such as catalytic activity. This nanosizing nanoparticles generate high catalytic activity. They
can also generate the effects of unprecedented pecu- are also often attached with catalytic promoter parti-
liar electromagnetic and optical characteristics; for cles together with catalyst particles, enabling the cre-
instance, nanosizing iron particles to the size of a ation of multi-functional catalysts. In fields other
single magnetic domain, the smallest unit capable of than catalysts, there are some types of toners having
producing magnetism, allows them to change from a such a composite nanoparticle structure, where toner
soft magnetic material that is magnetized according resin particles about 10 m in particle size are coated
to a magnetic field to a hard magnetic material (per- with silica particles to improve fluidity and magnetic
manent magnet). Such characteristics and effects are particles for transfer printing. The above-mentioned
the reason that nanoparticles are called “the ultimate structures of partial coating, such as for catalysts,
fine particles”. However, nanosizing is prone to pro- aim to stabilize the functions of nanoparticles.
duce a physically and chemically unstable state due Meanwhile, the purpose of full coating structures is
to the tremendously increased surface energy. to regulate the high surface activity of nanoparticles
Nanoparticles are, therefore, difficult to exist inde- or control the surface characteristics of the micron-
pendently and undergo the agglomeration of parti- size particles, which is called “surface modification”.
cles, grain growth by sintering and other changes of This surface modification can be utilized to improve
state by oxidation and other reactions. Such poor sta- the dispersibility and handling ability of nanoparti-
bility, dispersibility and handling ability have ham- cles as well as enhance the powder characteristics of
pered the wide application of nanoparticles as micron-size particles such as in terms of fluidity.
ultimate fine particles. These structures are also utilized for the preparation
As a means to cope with such problems, the appli- of drugs to protect or compound active ingredients
cation of nanoscale structures is being developed. and to provide the slow-release of ingredients.
The functions inherent in nanoparticles can be main- Furthermore, it is possible to improve the applicabil-
tained while lowering their reactivity by fabricating ity or handling ability of nanoparticles while main-
nanoparticles into nanostructures. Thus, stable mate- taining their inherent functions by fabricating
rials, fully utilizing the functions of nanoparticles, nanoparticle composites. Forming a collective struc-
can be created by fabricating composite and bulk ture of nanoparticles is an effective way to lower their
materials from nanoparticles, depending on their surface activity, for which internal-dispersion and
intended use. surface-coating structures are employed. In addition
This chapter deals with such nanostructured to the powder form, the nanoparticle composite
materials and their various functions and the char- structure is often utilized for materials and members
acteristic evaluation by function. As an introduc- in the form of bulk, where the functions of nanopar-
tion, this section summarizes the nature of ticles are performed inside the bulk form. Four rep-
nanostructures, relevant examples of the relations resentative types of bulk-form structures are shown
between nanostructures and their characteristics and in Fig. 6.1.1.
the functionality and the evaluation of the charac- As dense structures, those fabricated from
teristics of nanostructures. nanoparticles and those in which nanoparticles are

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