Biomedical Engineering Reference
In-Depth Information
1.10.1 Polymer-derived ceramic nanocomposites
Intense efforts have been made to develop and use new materials for high-
temperature applications due to environmental and economical reasons
(Belmonte 2006). Higher efficiencies in energy production and decreasing
harmful emissions (e.g. CO
2
or NO
x
) can, for instance, be achieved by
increasing the operating temperature of engines or turbines as well as by
reducting the weight of their components. Within this context, ceramic
nanocomposites have been shown to be promising candidates for use under
extreme conditions as they exhibit good high-temperature properties such as
high strength, creep resistance, thermal shock resistance and stability in
oxidative and corrosive environments. Future ceramic nanocomposites will
need to have some crucial properties such as oxidation resistance, no phase
transformation from ambient to operating temperature, chemical stability,
low volatility, high resistance to creep deformation, sufficient toughness at
ambient temperature and thermal shock resistance. Polymer-derived SiOC/
HfO
2
ceramic nanocomposites have been prepared by Ionescu et al. (2010)
via chemical modification of a commercially available polysilsesquioxane by
hafnium tetra (n-butoxide). The modified polysilsesquioxane-based mater-
ials were cross-linked and subsequently pyrolyzed at 1100
8
C in an argon
atmosphere to obtain SiOC/HfO
2
ceramic nanocomposites. Annealing
experiments at temperatures between 1300 and 1600
C were performed and
the annealed materials were investigated with respect to chemical composi-
tion and microstructure. This ceramic nanocomposite will be able to exhibit
a remarkably improved thermal stability up to 1600
8
8
C in comparison with
hafnia-free silicon oxycarbide.
1.10.2 Sol-gel synthesized HfO
2
-TiO
2
nanostructured films
Hafnium titanate ceramics are low thermal expansion materials with a high
refractoryness (Carlson et al. 1977, Lynch and Morosin 1972, Ruh et al.
1976). Hafnium titanate films are generating increasing interest because of
their potential application as high-k dielectric materials for the semicon-
ductor industry. Kidchob et al. (2008) investigated sol-gel processing as an
alternative route to obtain hafnium titanate thin films. Hafnia-titania films
of different compositions were synthesized using HfCl
4
and TiCl
4
as
precursors. The HfO
2
-TiO
2
system with 50mol% HfO
2
allowed the
formation of a hafnium titanate film after annealing at 1000
C. The film
exhibited a homogeneous nanocrystalline structure and a monoclinic
hafnium titanate phase that has never been obtained before in a thin film.
The homogeneously distributed nanocrystals had an average size of 50 nm.
Different compositions, with higher or lower hafnia contents, have been
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