Page 248 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 4 CONTROL OF NANOSTRUCTURE OF MATERIALS
Figure 4.5.3 shows various mechanisms of solid- success, the science of liquid-phase sintering has not
stage sintering and the route of mass transport. During been well-established. It involves too many compli-
sintering, substances move from the surfaces or grain cated phenomena for detailed analysis.
boundaries of particles as the starting points to the neck Nanoparticles contribute to a very high sintering
regions formed between particles as terminal points, rate, due both to their high driving force and the short
forming solid bonding. The starting point of substance transport distance for substance. The driving force gov-
governs shrinkage in sintering. If it is the grain bound- erned by the surface curvature of particle increases
ary, net shrinkage occurs in powder compacts, since the with decreasing particle size. The small particles in the
centers of adjacent particles get closer as the substance system also reduce the distance between the starting
is removed from the contact region between them. and terminal points. In general, the sintering rate varies
There is no shrinkage in powder compact, if the sub- with the minus second to third powder of particle size.
stance is removed from the surface of particles. The Unfortunately, the grain growth rate also increases
particles are thinned, but the centers of particles remain rapidly with decreasing particle size, i.e., with the
at the same position in this case. Explicit routes of mass minus second to third powder of particle size. Large
transport include bulk diffusion in solid, grain bound- grains are often noted after sintering of nanoparticles.
ary diffusion, surface diffusion on particle surface, and It is difficult to form nanostructure after sintering.
evaporation-diffusion in gas phase-condensation. Unlike conventional technique, additive for suppress-
Liquid-phase sintering is widely used industrially ing grain growth are ineffective in most cases of
and is the major sintering technique for commercial nanosystems. The driving force for grain growth is
production of materials. It uses additives which form too strong to suppress it by additives.
considerable amount of liquid phase between particles
and/or at particle surface at the sintering temperature 3. Sintering and control of microstructure in nanosystem
[3]. The liquid phase plays two roles. One is to pull Special technique is needed to obtain homogeneous
particles together by the surface tension and the other microstructure with fine grains in the sintering of
the rapid mass transport. In general, mass transport is nanoparticle systems. Conventional sintering does not
much faster in liquid than in solid. The bulk and/or lead to a desired microstructure. Rapid grain growth
film liquid phase dissolves substance, and allows rapid accompanying densification often results in microstruc-
transport through them. Compared to the industrial tures similar to those formed by conventional systems.
A novel two-step technique has been reported
recently by I.W. Chen to sinter nanoparticles into
ceramics with nanograins [4].
Figure 4.5.4 shows the relationship between the rel-
neck
Particle ative density and the grain size for the yttria nanopar-
Particle
ticle system that was sintered by the conventional
Volume diffusion technique with the time–temperature pattern shown in
Grain boundary Grain boundary the figure. The grain size increased with increasing
diffusion density, reaching about 100 nm, and cannot be cate-
gorized as nanoparticle at the relative density 70%.
The grain growth continues in the subsequent heating
and densification. This is the similar results often
noted in the conventional systems. Addition of nio-
(a)
surface diffusion Evaporation-condensation bium did not help the formation of nanoceramics,
although it suppressed the grain growth slightly.
Figure 4.5.5 shows the two-step sintering, in which
Particle Particle the powder compact is heated for a very short period
of time at a temperature normally selected for con-
Volume
diffusion neck ventional sintering (T ), and then sintered for a long
1
period of time at a lower temperature (T ) with the
2
temperature–time profile shown in the figure.
Densification was accompanied by the little grain
o
o
growth at 1,310 C and 1,250 C in the first step. In the
second step, only the densification proceeded with the
(b)
grain size kept virtually constant. Both high density
and fine grain with size under 100 nm have been
Figure 4.5.3 achieved with this technique.
Solid-stage sintering and the route of mass transport Figure 4.5.6 shows the grain size after the sintering
(a) Mass transport with contraction and (b) Mass transport of the first stage which is required for the achievement
without contraction. of densification in the sintering of the second stage.
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