Ceramics for Sustainable Energy Technologies with a Focus on Polymer-Derived Ceramics - Novel Combustion Concepts for Sustainable Energy Development - page 497

structures from thermal stress and ensure operation at higher temperatures (and
hence higher ef ciency) that would be possible without the coatings. TBC in tur-

bine applications generally consist of a porous ceramic top layer, responsible for the
thermal insulation, and a metal alloy bond coat layer providing for a good interface
to the substrate (generally Ni-based superalloys). The current material of choice for
the TBC top-coat layer is yttria-stabilized zirconia in its metastable tetragonal
modi cation. The criterion to withstand high temperatures in oxidizing environ-

ments for long periods of time has led to a focus on refractory oxides in search of
novel and alternative TBC materials. Proposed candidates include

fluorite-struc-
tured oxides (HfO 2 , CeO 2 , ThO 2 ), zirconate pyrochlores such as codoped
La 2 Zr 2 O 7 , garnets (Y 3 Al x Fe 5 − x O 12 ), or monazite, LaPO 4 (Clarke and Levi 2003 ).
Closely related to TBC are environmental barrier coatings (EBC), which protect
an underlying substrate from aggressive combustion atmospheres. Furthermore, the
coating acts as a diffusion barrier, inhibiting the interaction of potentially critical
gas-phase components with the base material. Several authors of this paper have
been engaged in the development of environmental barrier coatings, and their
contributions are summarized in Sect. 2.2 .
In thermal processes, a further increase in process ef

fl

ciency can be achieved by
the recovery of thermal energy released during the process by using heat
exchangers. Heat exchange is also relevant for the utilization of thermal energy
sources such as solar thermal or geothermal energy. Ceramic materials have found
niches especially in high-temperature applications due to their resistance against
thermal loads and corrosive environments. While a number of ceramic materials
(Al 2 O 3 ,ZrO 2 ,Al 2 TiO 5 , AlN, Si 3 N 4 ) and ceramic matrix composites (C/C-SiC, SiC/
Al 2 O 3 ) have been used in heat exchange applications, SiC can generally be con-
sidered as one of the most promising ceramic materials for this application.
In summary, while the unique thermal and mechanical properties of ceramic
materials de le in energy and environmental applications,

their shortcomings also have to be considered during design and production of
components and systems. Owing to the inherent high temperature stability and
mechanical resistance, the forming of parts remains a major challenge in many of
the applications described above. This is particularly true for silicon-based ceramic
materials. As a result, in the recent past, there has been signi

ne their application pro

cant research in the
area of the precursor (or polymer)-derived ceramic route as an alternative to con-
ventional ceramic processing methods. This approach, as discussed below, has
speci

c advantages and is the focus of the rest of this paper.

1.2 Precursor or Polymer-Derived Ceramics

Preceramic polymers are organometallics that lose their organic components upon
pyrolysis to form an amorphous or nanostructured ceramic. There are many types of
preceramic polymers now available on the market; precursors to carbides, nitrides,
and oxides are used to form such ceramics as SiC, B 4 TiC, TiC, Si 3 N 4 , BN, AlN,