Ceramics for Sustainable Energy Technologies with a Focus on Polymer-Derived Ceramics - Novel Combustion Concepts for Sustainable Energy Development - page 513

2010 ). To get a deeper understanding of the structure forming process and
parameters relevant for the control of the netlike structure, the solubility of the
preceramic polymers in different organic solvents was investigated (Woiton et al.
2011 ). It was found that a poly(methyl silsesquioxane), PMS, and a poly(methyl
phenyl silsesquioxane), PMPS, are immiscible, which results in a demixing process
during evaporation of the methanol in filler-free coating systems. In filler-con-

taining systems, the filler play an important role for the

structure forming process, and it was found that dewetting of the preceramic
polymer from the

filler size and the type of

filler particles is the major driving force for the structure for-
mation. More details might be found in (Woiton 2011).

2.3 Porous and Cellular Ceramics

Preceramic polymers have been used for the manufacturing of porous and cellular
ceramics, and some of these ceramics have been provided with an additional
functionality. Independent of the material, porous ceramics can be obtained by one
of three different general processing techniques:

Templating uses an organic template, e.g., a polymeric foam, which is
impregnated with a ceramic slurry. A thermal treatment decomposes the organic
template, and sintering of the ceramic particles in the shape of the template
occurs. One main disadvantage is hollow struts of the ceramic structure. This
process predominantly yields open-cell structures with a high pore connectivity.
Slurries of preceramic polymers with active or inert

￿

fillers are widely used for
the impregnation of polymer preforms to form ceramic foams (Adam et al.
2014 ; Krishnan et al. 2014 ; Ceron-Nicolat et al. 2010 ; de Sousa et al. 2008 ).
￿ Sacrificial particulate fillers such as polymeric spheres are added to a ceramic
slurry or to a powder bed. After shaping and forming, the sacri

cial templates
are thermally decomposed to form volatile gases; the initial pore shape less
some shrinkage remains in the final ceramic. This leads to predominantly

closed-cell structures, with cell windows mostly developing at the contact area
of two sacri

cial template particles.
￿ Direct foaming is an in situ foaming method which involves the generation of
gaseous products either by reaction, by the use of foaming agents in a ceramic
slurry, or by introduction of an external gas in a blowing process. The stabil-
ization of the foamed structures is the most critical part of the process. Due to
the foaming process (pore forming and release of gas), the formation of a
gradient structure is often the case. These gradient structures, if properly con-
trolled, are of interest for filtering applications or for thermal insulation (Wolff

et al. 2012 ; Ceron-Nicolat et al. 2012 ).
These three techniques are suitable for the generation of a wide variety of porous
ceramics for different applications and can be used and combined to design the
microstructure (viz., porosity, permeability, and interconnectivity) and, subsequently,