Biomedical Engineering Reference
In-Depth Information
pressure effects, Hahn fully sintered TiO
2
at 998−1098
o
C (~0.35 T
m
)
by applying 1 GPa while retaining the morphologically metastable
structure (i.e., with no grain growth) [17].
Table 4.1
Densities and grain sizes in selected nanopowders densiied by
conventional sintering
Sintering parameters:
Final properties:
Init. Gr.
size(nm)
Time
(hrs)
Gr. size
(nm)
Material:
Temp. (K)
Atm.
Density (%)
ZrO
2
6 to 9 1400
1.3 air full
80
ZrO
2
-3 mol.%
Y
2
O
3
<10
1373
1 NR 99.9
80
TiO
2
~6
873
NR NR 99
<60
4.2.2 Nanosintering
Thermodynamically, nanopowders are unstable due to large
surface area. Nanoparticles adopt different surface energies than
regular ones, for instance, by a different local atomic arrangement at
the surface. TEM studies showed that nanoparticles have a faceted
appearance with anisotropic surface energies (e.g., in γ-Al
2
0
3
[7]).
Kinetically, sintering of nanopowders is signiicantly enhanced.
Sintering of nanoparticles indicated depressed sintering onset
temperatures (0.2-0.3 T
m
) as compared to conventional powders
(0.5-0.8 T
m
).
Molecular dynamics (MD) simulations indicated
extremely fast sintering of nanoparticles [58]. Surface diffusion
cannot explain this behavior. Therefore, some other sintering
mechanisms have been suggested: dislocation motion, grain
rotation, viscous low, and grain boundary slip [14, 33]. The rapid
shrinkage was attributed to fast dislocation activity driven by the
contact Hertzian stresses that exceed the ideal shear strength
[58]. After the neck forms, the adjacent particles rotate to achieve
minimum grain boundary energy. Such rotation process was
conirmed by TEM studies of nanoparticles [37, 56].
Full densiication of nanopowders is completed at temperatures
lower than that for conventional powders. To rationalize this
decrease in the sintering temperature, different scaling laws have
been applied [1, 18]. Considering Herring's law, the sintering
temperature dependence on the particle size may be calculated.
