Page 87 - Visions of the Future Chemistry and Life Science

76 P. W. MAY

Table 5.1. Some of the outstanding properties of diamond

• Hardest known material giving extreme wear resistance
• Highest bulk modulus, i.e. stiffest material
• Least compressible
• Highest room temperature thermal conductivity
• Extremely low thermal expansion at room temperature
• Broad optical transparency from the deep ultraviolet to the far
infrared
• Highest speed of sound
• Very good electrical insulator
• Diamond can become a wide bad gap semiconductor
• Very resistant to chemical corrosion
• Biologically compatible
• Some surfaces exhibit very low or ‘negative’ electron affinity

range of physical properties. Indeed, a glance at any compendium of
material data properties will prove that diamond is almost always ‘the
biggest and best’. A selection of some of these is given in Table 5.1.
Amongst other properties, diamond is the hardest known material, has the
highest thermal conductivity at room temperature, is transparent over a
very wide wavelength range, is the stiffest material, the least compressible,
and is inert to most chemical reagents. With such a wide range of excep-
tional properties, it is not surprising that diamond has sometimes been
referred to as ‘the ultimate engineering material’.
Unfortunately, it has proved very difficult to exploit these properties,
due both to the cost and scarcity of large natural diamonds, and the fact
that diamond was only available in the form of stones or grit. It had been
known for 200 years that diamond is composed solely of carbon, and many
attempts were made to synthesise diamond artificially using as a starting
material another commonly occurring form of carbon, graphite. This
proved extremely difficult, mainly because at room temperature and pres-
sure, graphite is more stable than diamond. Although the difference in
stability between the two forms of carbon is actually quite small, their
structures are so different that it would require a large amount of energy to
convert between them. Ironically, this large energy barrier which makes
diamond so rare is also responsible for its existence, since diamond, once