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Inorganic Polymers 423

The good visible light transmission and good insulting power make aerogel materials of interest in
window manufacturing. The visible transmission spectra of light shows little absorption in the range
of about 300–2,700 nm giving aerogels a good visible light “window” making aerogels attractive
for day-lighting applications. Aerogels provide about 40 fold more insulation than does fi berglass
insulation. While such aerogels may eventually be used as the entire window component, for the
present time they may act as the material sandwiched between two pains of clear plastic or glass.
Thermoglass is generally simply glass sheets that are separated by a vacuum. The seals on such ther-
moglass often spring small leaks causing diminished insulation properties. Aerogel inner cores will
not suffer from this problem. Currently, about 40%–50% of a house’s heating bill literally goes out
the window because of lost heat or cold through windows. A single one-inch thick glass pane of aero-
gel offers the insulation equivalent to more than 30 windowpanes of R-20 insulation rated glass.
Another commercial area that is being considered is the use of aerogels as nanocomposite mate-
rials. Approaches are varied. In one approach, material is added to the silica sol before gelation. The
material can be inorganic, organic, polymeric, bulk fibers, woven cloths, and so forth. The additional

material must be able to withstand the subsequent processing steps, including carbon dioxide dry-
ing. The added material must be present in a somewhat homogeneous manner throughout the sys-
tem. Gentle agitation appears to be suffi cient to give a product with decent homogeneity. Aerogels
may be good materials for optical sensors. They have good visible transparency, high surface area,
good temperature and chemical stabilities, and facile transport of gases through their pores.

12.11 SILICON DIOXIDE (CRYSTALLINE FORMS)—QUARTZ FORMS
Just as silicon dioxide forms the basis of glass, so also does it form the basis of many of the rocks,
grains of sand, and dirt particles that compose the Earth’s crust. Most rocks are inorganic polymers,

but here we will deal with only a few of these containing significant amounts of silicon.
Silicon oxide crystallizes in mainly three forms—quarts, tridymite, and cristobalite. After the
feldspars, quart is the most abundant material in the Earth’s crust, being a major component of
igneous rocks and one of the most common sedimentary materials, in the form of sandstone and
sand. Quartz can occur as large (several pounds) single crystals, but normally is present as granular
materials. The structure of quarts (Figure 12.4) is a three-dimensional network of six-membered

R R

Si
R O O R
O O
Si Si Si Si
R O R
O O O O
Si Si
O O O O
O R
R Si Si Si Si
O O
O O
R R
Si
R R
(a) (b)

FIGURE 12.4 Structure of crystalline SiO 2 tetrahedra found in quartz (right structural formula and right
ball-and-stick model of one unit cell).

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