Biomedical Engineering Reference
In-Depth Information
13.9
Densification behavior of nanocrystalline TiO
2
with three different
agglomerate sizes: note that the larger the agglomerate size, the higher
the sintering temperature (agglomerate size in bold, crystallite size
in light). For the non-agglomerated (N/A) powder, sintering time is
120 min;
69
for the 80 and 340 nm agglomerate powders, sintering
time is 30 min.
70,71
other hand, the finer the particle sizes, the lower the green density of powder
compacts, assuming the compaction pressure is the same. It should be
noted, however, that nanosized particles can be sintered from a green
density that is much lower than is possible for sintering coarse (micron or
submicron) particles.
It has been widely recognized that agglomeration of nano particles has a
critical impact on the sintering of nano particles. Due to the extremely fine
size and the strong interactive force between particles, nano particles tend to
form agglomerates. The size and strength of the agglomerated particles
affect the densification rate. The most direct investigation of the
agglomeration of densification was summarized by Mayo,
3
whose data
were based on numerous published experimental results as shown in
Fig. 13.9.
In essence, a powder compact can be viewed as consisting of a bi-level
hierarchical structure: the compact is made of agglomerates which consist of
nanosized particles. There is, therefore, a bi-model pore size distribution.
The pores existing within agglomerates are finer than the pores between
agglomerates. The densification of an individual agglomerate is relatively
easy, while the elimination of the inter-agglomerate pores is more difficult.
By tracking the evolution of pore size distributions, Petersson and A
˚
gren
72
studied the sintering of fine grain cemented tungsten carbide and cobalt
system (WC-Co). They showed that during the intermediate stage of
