Page 91 - Electrical Properties of Materials

Bond types 73

abrasive materials include silicon carbide, a group IV compound with a similar
diamond type covalent bond, but less symmetry; and tungsten carbide, whose
bonds are partly ionic. The diamond structure also gives a high value of bulk
elastic modulus [eqn (5.7)].
Diamond has two other superlatives. It has a much greater thermal conduct-
ivity than metals at room temperature, conducting energy by lattice vibrations
rather than free electrons. It is thus a good heat sink for electronic devices
where heat can be removed without prejudicing the electrical behaviour. As
well as using ‘industrial’ rather than gem grade diamonds for this purpose, it
is possible to grow suitable plane layers of diamond by molecular beam epitaxy
and related techniques described in a little more detail in Section 8.11.
The second superlative that takes us out of science and into high finance
is the ‘sparkle’ of diamond in jewellery. But see Table 10.1; it is the high
refractive index which makes sparklers.
The sale of diamonds of jewel quality is controlled by the Central Selling
Organisation (CSO) of de Beers who in 1948 hired a New York advertising
agency to create a slogan to stimulate their industry. The young lady assigned
to this task thought long and hard, weeks passed with no idea. She thought
she would work on this forever, so said “diamonds are forever”. It was a great
success.
Carbon dating is an important impact of science on ancient history and
archaeology, removing some of the luxury of literary speculation from its prac-
titioners. Atmospheric CO 2 , which is the source of carbon in living organisms,
14
12
contains 1 atom of C 14 in 7.8 × 10 11 atoms of stable C .C , decays with a
half-life of 5700 years emitting an electron. These extreme numbers result in
appreciable radioactivity, giving 15 disintegrations per gram per minute. When
the organism dies the intake of atmospheric CO 2 stops and the C 14 within it
decays exponentially. Thus, a count of radioactivity in dead bones or wood etc.
will give a dating for the time of death.
In the two decades after 1950, scientific archaeology matured rapidly with
the advent of the nuclear physics-based methods of carbon and thermolumines-
cent dating. Samples of diamonds from ancient jewels and numerous samples
mined in the past two centuries were dated. They were all found to have been
9
formed within an order of magnitude of 10 years ago. On a human scale,
9
10 years is pretty close to ‘forever’. So the slogan is more accurate than the
average advertisement.
◦
Diamonds are not always forever. Their one vice is that at 700 Cinair
they burn to carbon dioxide. This rules out diamond for large-scale cutting of
steels and other hard metals. It has stimulated research for hard compounds
with better temperature stability and hardness perhaps even greater than 15 on
the Mohs scale. Compounds of C with N, B, and Si have shown promise, the
possible winner is C 3 N 4 ; but not yet.

5.3.4 The van der Waals bond
If the outer shell is not filled, atoms will exert themselves to gain some
extra electrons, and they become bonded in the process. But what happens
when the shell is already filled, and there are no electrostatic forces either,
as for example in argon? How will argon solidify? For an explanation some
quantum-mechanical arguments are needed again.