Biomedical Engineering Reference
In-Depth Information
13.3.3 Densification mechanisms during sintering of
nano particles
Using the various methods for figuring activation energies, activation
energies for sintering various nanosized powders were reported. For
example, a very low activation energy for densification is observed in initial
sintering - about 234 kJmol
-1
for nanocrystalline Al
2
O
3
and 96.2 kJmol
-1
for nanocrystalline TiO
2
,
53
268 kJmol
-1
for nanocrystalline ZnO,
54
66.2 kJmol
-1
for nanocrystalline nickel,
55
82 kJmol
-1
α
for nanocrystalline
titanium and 49 kJmol
-1
titanium.
56
It can be seen from the data that the majority of studies point toward
lower activation energies for early stages of sintering. This is reasonable for
the obvious reason of the huge surface areas and the expected high activity
of nano particles. Surface diffusion is one of the most cited mechanisms that
contribute to the sintering of nanosized particles. However, in conventional
sintering theories, surface diffusion is believed to induce initial neck
formation between particles, but not densification. This seemingly conflict-
ing theory of the effects of surface diffusion on sintering of nano particles
can be understood from a perspective of the indirect role of surface diffusion
to densification.
First of all, the rapid and active surface diffusion may lead to grain
boundary slip and rotation of particles that may result in the rearrangement
of particles, hence the increased density of the compact. The possibility of
grain boundary slip and rotation was mentioned in numerous sintering
studies.
2,3,57
Evidence of nano particle coalescence via surface diffusion was
presented in Shi's study on barium titanate
58
and Bonevich and Marks's
study of Al
2
O
3
59
using TEM. Figure 13.8 shows the formation of the neck
between two Al
2
O
3
particles after sintering at 1000
for nanocrystalline
β
C for just a fraction of a
second. This study shows that surface diffusion is the predominant
mechanism for sintering, as evidenced by the fact that the faceted interfaces
are similar to ledge growth, and the sintered particles retain their initial
adhesion structure with no reorientation occurring during sintering. The
driving force for sintering can be considered a chemical potential difference
between facet surfaces and the neck region.
The indirect role of surface diffusion on densification can also be
understood based on theories of the relationship between coarsening and
sintering of particles.
1,58,60,61
As discussed earlier, effects of pores on
sintering of nano particles, according to theories first proposed by Kingery
and Francois and further elaborated by Lange et al.,
62,63
a pore will shrink
during sintering only if the coordination number of the pore is smaller than
a critical value n
8
n
c
because only then the surface of the pore is concave.
Thermodynamic driving force dictates that mass will diffuse from convex
surfaces to concave surfaces. Initial sintering of a compact will develop an
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