Environmental Engineering Reference
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the resulting material properties (e.g., mechanical, thermal, catalytic properties).
In addition, more functionality can be imparted to the cellular ceramics by impreg-
nation processes.
2.3.1 Polymer-Derived Ceramic Foams
A speci
c approach of foam formation with preceramic polymers makes use of the
intrinsic hydroxy and alkoxy groups of silicone materials for direct foaming.
A common material of choice is a poly(methyl phenyl silsesquioxane) (commercial
name Silres H44, supplied by Wacker Silicones, Burghausen, Germany) having the
general formula [Ph
0.62
Me
0.31
R
0.07
SiO
1.5
]
n
with
n > 20, Ph = phenyl groups,
Me = methyl groups, and R =
OC
2
H
5
and
OH. The solid powder has a glass
-
−
C. The average molecular weight is 2,100 g mol
−
1
,
the density is 1.1 g cm
−
3
, and the amount of cross-linking-active hydroxy-
(
transition temperature of
43
°
*
OC
2
H
5
) groups is 7 mol%, related to a formula unit as
shown above. Heating of the H44 preceramic polymer results in condensation
reactions and siloxane bond formation at the atomic level (see reaction scheme in
Eqs. (
1a
) and (
1b
)):
−
Si
-
OH) and ethoxy- (
−
Si
-
Si
OH
þ
HO
Si
!
Si
O
Si
þ
H
2
O
"
ð
1a
Þ
Si
OC
2
H
5
þ
HO
Si
!
Si
O
Si
þ
C
2
H
5
OH
"
ð
1b
Þ
Under the right conditions, foam formation occurs on the macroscopic level. This
is because water and/or ethanol formed in Eqs. (
1a
) and (
1b
) act as blowing agents,
and the foam is prevented from collapsing due to the siloxane bond formation and
the resulting increase in size of the polymeric molecules and its viscosity. Reaction 1
also results in the formation of a cross-linked thermoset material, resulting in a
product that is neither meltable nor dissoluble in solvents. For the tailoring of the
material properties, the starting material can be loaded with particulate
fillers such as
silicon carbide (SiC), alumina (Al
2
O
3
), silica (SiO
2
), or silicon (Si). Subsequent
pyrolysis at temperatures between 800 and 1,500
C results in a ceramic composite
material. The properties of those ceramics are controllable by the temperature, the
pyrolysis atmosphere, and the pyrolysis time and by the composition and the
polymer/
°
ller ratio of the starting material (Gambaryan-Roisman et al.
2000
;
Zeschky et al.
2003
,
2005
).
The material properties of PDC foams manufactured as described above can be
modi
filler materials to the
starting material or by a post-manufacturing process. Examples for the direct
modi
ed either by the selection and amount of additional
cation are the addition of pore formers leading to a pronounced strut porosity
(Reschke et al.
2011
), or the addition of speci
uence the properties
of the foam, such as the addition of copper (I) oxide (Cu
2
O) which results in an
increase of the electric conductivity by several orders of magnitude even at low
Cu
2
O concentrations (Colombo et al.
2001
).
c agents to in
fl
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