Page 247 - Book Hosokawa Nanoparticle Technology Handbook

4.5 STRUCTURE CONTROL OF NANOPARTICLE COLLECTIVES BY SINTERING AND BONDING FUNDAMENTALS
such as ceramic and powder metallurgy where parti-
cles of submicron to micron levels are now used.
Nanoparticles may drastically reduce the sintering
temperature as compared to the traditional powders.
They also reduce the grain sizes in microstructure, and Coarse grain
thus the properties of resultant products. However, a
very high barrier is present before reaching these goals.
Properly controlled microstructure is essential for
excellent materials. Starting powder of excellent char-
acteristics is only a single fulfillment among numerous pore
requirements for the production of excellent materials. Grain
Nanoparticles have a difficult aspect other than its high boundary
potential. Their handling and treatment are extremely nd
difficult. Improperly handled and treated, the proper- 2 phase
ties of resultant product could be inferior to those made
with conventional powders. In this section, fundamen-
tals of sintering and microstructure development will
be explained briefly [1, 2] and recent technology in sin-
tering of nanoparticles will be discussed.
grain
1. Microstructure
Two phenomena proceed simultaneously upon Figure 4.5.2
heating of powder compact as shown in Fig. 4.5.1. Schematic diagram of microstructure.
One is the formation of bonding between particles
(neck formation) and the other the growth or anni-
hilation of particles (Fig. 4.5.2). Particles in Desired features for microstructures depend widely
microstructure are largely single crystals. Grain on the intended application of material. Generally,
boundary corresponds to the bonding region high density, as well as fine and uniform microstruc-
between them and the atomic arrangement is irreg- tures are needed in the field of nanotechnology.
ular there. The properties of grain boundary may Namely, the pores must be small and few in number.
differ significantly from those of crystal, and thus Size and shape of grains must be fine and uniform.
the net properties of polycrystalline material may The distribution of secondary phases must be uni-
differ completely from those of single crystal. form. Abnormally large grains, pores, and secondary
Proper control of microstructure is often a key in phases are extremely undesirable, for they exert espe-
designing properties of material. cially bad effects on properties. To fulfill these
requirements in microstructure, precise control of sin-
tering process is absolutely needed, in addition to the
achievement of uniform green structure.
particle particle
2. Sintering and mechanism of microstructure
development
‹C E
pore The driving force for sintering is the change of inter-
Grain facial energy in the process. It is the difference
boundary between the total surface energy of particles in the
y
powder compact and the sum of total grain boundary
energy and surface energy of residual pores in the sin-
tered body. The similar driving force works on grain
particle
particle growth; the reduction of grain boundary area and thus
the total grain boundary energy. Tendency of grain
growth is inevitable in sintering. The extreme goal of
pore
‹C E
sintering is the single crystal without grain boundary
pore
‹C E or residual pore. The significance of the same driving
force involved in densification and grain growth is
that it is difficult to precede one of them preferen-
tially. A special technique is needed to achieve densi-
fication with minimal grain growth. Controlled grain
Figure 4.5.1 growth is used in the commercial production of ferrite
Formation of bonding between particles, grain growth, and single crystal. A seed crystal is used to convert poly-
microstructure development. crystalline feed ceramic to a large single crystal.
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