GROUP IV 165
thermal degradation of certain acrylic textile fibres. The basic
molecular orientation of the carbon atoms in the original fibre is
retained. Plastics reinforced with carbon fibres are light in weight
but have great strength, properties making them valuable to many
industries and to the aero industry in particular.
A process in which hydrocarbons are heated above 2300 K gives
a material called pyrographite. This has properties indicating con-
siderable ordering of the graphite crystals present. The thermal
conductivity along the planes of carbon atoms is almost 100 times
that at right angles to the planes, a property which makes the
material valuable in rocket nose cones where rapid conduction from
the hot zone is required and low conduction through to the interior.
Electric conductance along the planes is 1000 times that found at
right angles to the planes.Amorphous carbonIn addition to diamond and graphite, carbon appears to exist in a
number of other forms, collectively called amorphous carbon. Four
common examples are coke, animal charcoal, lampblack and sugar
carbon which can be prepared by heating coal, bones, oil and sugar
respectively in the virtual absence of air. X-ray diffraction studies
indicate that these and nearly all other forms of amorphous carbon
are in fact microcrystallme graphite. Truly amorphous carbon,
which gives random X-ray scattering, can be prepared by the low
temperature decomposition of hexaiodobenzene, C 6 I 6.
Charcoal and lampblack have enormous surface areas for a small
volume of sample, and are able to adsorb large amounts of gas or
liquid. The effectiveness of the carbon can be greatly increased by
heating the sample in a stream of steam to 1100-1300 K when
impurities adsorbed during the initial preparation are driven off.
This 'activated' charcoal has particularly good adsorption proper-
ties and is used as a catalyst. Lampblack is used in making printing
ink, pigments and as a filler for rubber to be used in tyres.SILICON
After oxygen, silicon is the most abundant element in the earth's
crust. It occurs extensively as the oxide, silica, in various forms, for
example, flint, quartz, sand, and as silicates in rocks and clays, but
not as the free element, silicon. Silicon is prepared by reduction of
silica, SiO 2 - Powdered "amorphous' silicon can be obtained by
heating dry powdered silica with either powdered magnesium or a