Biomedical Engineering Reference
In-Depth Information
corresponds to the formation of junction zones between particles of the
compact. The fast formation of the junctions between particles, before the
shrinkage onset, involves the creation of a high concentration of vacancies
inside these junctions. The shrinkage of the compact results then from a
decrease of the distance between the centers of particles due to annihilation
of the trapped vacancies in the junction zone. Because the concentration of
trapped vacancies inside the junction zone largely exceeds the thermo-
dynamic equilibrium concentration, the diffusion can be considered as
independent of time and controlled only by the probability of jumping of
ions, as long as the concentration of vacancies exceeds the equilibrium
content. Any further sintering, after the initial non-equilibrated concentra-
tion of vacancies is exhausted, corresponds with the diffusion of equilibrated
vacancies.
Furthermore, based on the theory that excessive concentration of
vacancies exist (c
10
-4
), Trusov et al.
65
stipulated that there is a possibility
of liquid-like merging (coalescence) of particles into large ones. Liquid-like
coalescence, as well as slippage, causes the compact shrinkage of ultrafine
particles.
In another study focusing on size-dependent grain growth kinetics
observed in nanocrystalline Fe, Krill et al.
66
also established their model on
the basis of existence of excess volume at the grain boundaries. The 'excess'
volume is in the form of vacancies, which leads to a non-equilibrium
vacancy concentration. The issues of grain growth of nano particles during
sintering will be further discussed in later sections of this chapter.
Finally, the rapid densification mechanisms of nano particles are also
related to the preferential crystalline orientations. It has been observed that
in loose nanocrystalline powders, the first neck formation occurs not
randomly between particles, but by the orderly mating of parallel,
crystallographically aligned facets on the particle surfaces.
59,67
Some
nanocrystalline powder compacts also appear to reflect a kind of ordered
structure resulting from less than random type matings of particles during
the initial stage of sintering.
68
>
13.3.4 Effect of green density, agglomeration and pore
In the practice of sintering of nano particles, the densification behavior of
nano particles is affected not only by the intrinsic nature of the nanoscale
size of the particles, but also by the processing conditions and related
difficulties, such as green density and agglomeration.
First, similar to powder compacts of micron-sized powders, the
densification of a powder compact depends significantly on the green
density of the compact. Green density must be sufficiently high in order to
achieve adequate densification under similar sintering conditions. On the
