Probably more so than any other gemstone, diamonds feature more predominantly in the history and cultural heritage of the human race. They were prized for their scarcity for centuries, and still remain a symbol of wealth and prestige to this day. The word diamond comes from the Greek adamas, meaning indestructible. Diamonds were first mined in India over 4000 years ago, but the modern diamond era only began in 1866, when huge diamond deposits were discovered in Kimberley, South Africa, creating a huge rush of European prospectors. The wealth this created helped to underwrite the British Empire, and changed the fates of many African countries.
Apart from their appeal as gemstones, diamonds possess a remarkable
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 exceptional 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 pressure, 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 formed, cannot spontaneously convert to the more stable graphite phase. Diamonds are, indeed, forever!
To overcome these problems, researchers realised that in order to form diamond, they had to choose conditions where diamond, and not graphite, is the more stable phase. The knowledge of the conditions under which natural diamond is formed deep underground, suggested that diamond could be formed by heating carbon under extreme pressure. This process forms the basis of the so-called high-pressure high-temperature growth technique, first marketed by General Electric, and which has been used to produce 'industrial diamond' for several decades. In this process, graphite is compressed in a hydraulic press to tens of thousands of atmospheres, heated to over 2000 °C in the presence of a suitable metal catalyst, and left until diamond crystallises. The diamond crystals this produces are used for a wide range of industrial processes which utilise the hardness and wear resistance properties of diamond, such as cutting and machining mechanical components, and for polishing and grinding of optics. However, the drawback of this method is that it still produces diamond in the form of single crystals ranging in size from nanometres to millimetres, and this limits the range of applications for which it can be used. What is required is a method to produce diamond in a form which can allow many more of its superlative properties to be exploited - in other words, diamond in the form of a thin film.
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