High performance thermal insulation materials for buildings - Nanotechnology in Eco-efficient Construction - page 193

how they achieve rarefaction of the gaseous material and its resulting high
Knudsen number, i.e., by a nanoporous solid structure with a low pore size
λ , such as fumed silica or aerogels (Baetens et al ., 2011; Dorchech & Abbasi,

2008; Hüsing & Schubert, 1998; Jelle et al ., 2010; Richter, 1995; Wang et al .,
2007), or by application of a partial vacuum such as for vacuum insulation
panels (Alam et al ., 2011; Baetens et al ., 2010a; Jelle, 2011; Jelle et al. , 2010)
with a low gas pressure p . The best results are achieved with a combination
of both.

9.3.1 Nanoporous thermal insulators

As most thermal insulators that apply a partial vacuum use a material with
nanoscale pores, nanoporous thermal insulators will be treated fi rst. Within
this context, the main nanoporous thermal insulators are aerogels (see Fig.
9.3) and microporous silica. Aerogels are used as a main thermal insulator
on its own, whereas microporous silica is generally only used as core mate-
rial for partial vacuum thermal insulators and will be treated later. Aerogels
are essentially the solid framework of a gel isolated from its liquid medium
and were discovered in the early 1930s by S.S. Kistler (Kistler, 1931). The
main type of aerogels as high performance thermal insulators are silica
aerogels, SiO 2 . Aerogels have an extremely high porosity and a pore size
distribution f ( ) below the mean free path for air molecules at standard

temperature and pressure, making them a high performance thermal
insulator.
The unique properties of aerogels are determined by its synthesis, which
can be divided into three general steps: gel preparation, aging of the gel,
and drying of the gel. A detailed comprehensive review on the synthesis of

Λ

￿ ￿ ￿ ￿ ￿ ￿

Multi-layer envelope
ilm

Core-bag

Pressed silica core
with opaciier

Welded seam

9.3 Typical VIP structure showing the main components and an
example of aerogel as a high performance thermal insulation material
for building applications (Baetens et al. , 2011; Jelle, 2011).