Biomedical Engineering Reference
In-Depth Information
during sintering of densified ingredients (Chen and Chen 1994, 1996a,
Nowick et al. 1979). The effects of dopants Mg
2+
,Ca
2+
,Sr
2+
,Sc
3+
,Yb
3+
,
Y
3+
,Gd
3+
,La
3+
,Ti
4+
,Zr
4+
and Nb
5+
on the grain boundary mobility of
dense CeO
2
have been investigated from 1270
C. Parabolic grain
growth was observed in all instances. Together with atmospheric effects, the
results support the mechanism of cation interstitial transport being the rate-
limiting step. A strong solute drag effect has been demonstrated for
diffusion-enhancing dopants such as Mg
2+
and Ca
2+
, which, at high
concentrations, can nevertheless suppress grain boundary mobility. Severely
undersized dopants (Mg, Sc, Ti and Nb) have a tendency to markedly
enhance grain boundary mobility, probably due to the large distortion of the
surrounding lattice that apparently facilitates defect migration. Overall, the
most effective grain growth inhibitor at 1.0% doping is Y
3+
, while the most
potent grain growth promoter is either Mg
2+
(e.g. 0.1%) or Sc
3+
at high
concentration (greater than 1.0%) (Chen and Chen 1996b).
8
C to 1420
8
1.4
Thermal properties of particular ceramic
nanocomposites
1.4.1 Al
2
O
3
, MgO and mullite based nanocomposites
Conventional ball milling techniques have been used for homogeneous
mixing of nano-size oxide and non-oxide powder particles with highly pure
media for making nanocomposites. The dried mixture powders were hot-
pressed under N
2
or Ar atmosphere. Natural kaolin and Al
2
O
3
powders
were selected for the natural mullite/SiC system. Natural mullite/SiC
nanocomposites were prepared by reaction-sintering mixtures of kaolin,
Al
2
O
3
and SiC powders are processed at 1500
C. The fabrication
processes of Al
2
O
3
/Si
3
N
4,
Al
2
O
3
/SiC and MgO/SiC nanocomposites were
presented by Niihara (1991).
The variation of Vickers hardness with temperature for the Al
2
O
3
/
16 vol% Si
3
N
4
nanocomposite was noticed in the reported findings. The
temperature dependence of the fracture strength for Al
2
O
3
/5 vol% SiC and
MgO/30 vol% SiC nanocomposites was also reflected in the characteriza-
tion. Sudden slope changes in the reported figures correspond to the brittle-
to-ductile transition temperature (BDTT). The improvement of BDTT by
nano-size Si
3
N
4
dispersion reaches approximately 450
8
C to 1700
8
C. Tremendous
improvements in BDTT were also observed for the Al
2
O
3
/SiC and MgO/
SiC nanocomposites. This improvement in high-temperature hardness and
BDTT must be due to the pinning of dislocations by the nano-sized SiC and
Si
3
N
4
dispersions. The observed improvement in hardness from degradation
at high temperatures suggests that nano-size dispersions within matrix
8
