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Thermal Insulation
Pyrogenic silica is the most effective thermal insulation material, which is readily
available in large quantities [37]. Table 2 summarizes the thermal conductivity of
selected insulating materials.
Table 2:
Thermal conductivity of selected insulating materials at 293 K.
Material
λ
tot
[mWm
-1
K
-1
]
test specimen
pyrogenic silica
ca. 20
fiber inforced panel
silica aerogel
ca. 15
monolith
mineral wool
ca. 40
fiber mat
extruded polystyrene (XPS)
ca. 35
panel
expanded polystyrene (EPS)
ca. 40
panel
According to eq. 8, the total thermal conductivity
λ
tot
is a combination of gas-borne
thermal conductivity
λ
g
, solid body thermal conductivity
λ
s
, and radiation thermal
conductivity
λ
r
.
ߣ
௧௧
ൌ
ߣ
ߣ
௦
ߣ
(8)
For non-compacted pyrogenic silica powder at 300 K
λ
g
of ca. 20 mWm
-1
K
-1
,
λ
r
of ca.
2 mWm
-1
K
-1
, and
λ
s
of ca. 5 mWm
-1
K
-1
was obtained which results in total thermal
conductivity
λ
tot
of ca. 27 mWm
-1
K
-1
. The radiation thermal conductivity is a
consequence of IR radiation passing through the silica. This contribution is of minor
importance at ambient temperature and can be significantly reduced by means of IR
radiation opacifiers like carbon black or magnetite. Solid body thermal conductivity
results from thermal vibrations transmitted through the silica. This contribution is also
relatively weak due to the fractal structure of pyrogenic silica aggregates. Their fractal
nature gives rise to a distinct tortuosity of the silica chains within the aggregates and a
large number of dead chain ends. Even the gas-borne thermal conductivity, although
being the main contributor for pyrogenic silica, shows quite low numbers compared to
other insulation materials. The magnitude of gas-borne thermal conductivity is directly
related to the porosity and the mean pore size of the insulating material according to
eq.9 [38].
ߣ
ൌ
ஈήఒ
ǡబ
ሺ்ሻ
ଵାଶఉ
ವ
(9)
Π
is the porosity of the thermal insulator, which is for pyrogenic silica ca. 0.98,
λ
g0
is
the thermal conductivity of the gas,
β
is a gas-specific co-factor, I
m
is the mean free
path length of the gas, and D is the mean pore size of the insulator.
Pyrogenic silica is a mesoporous material (s. above) with pore sizes smaller than the
mean free path length of air at standard conditions (ca. 70 nm). The combination of all
factors discussed above is reason for the outstanding insulation properties of pyrogenic
silica. Reduction of
λ
g
is an efficient way to further improve the insulation properties
of pyrogenic silica. This concept is successfully applied in case of so called vacuum
insulation panels where a foil-wrapped pyrogenic silica panel is evacuated. A total
thermal conductivity of ca. 5 mWm
-1
K
-1
can be achieved.
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