Shock Syntheses of Nanodiamond

Besides the direct generation of nanodiamond in a detonation, the required pressure can also be achieved by the achon of an external shock wave. Usually, the latter is induced by an explosion too and compresses the carbon material that is enclosed in a kind of capsule. A catalyst Hke, for example, copper, iron, aluminum, nickel, or cobalt is frequently employed in this process. It has already been mentioned in the introduction that nanoscale diamond particles had been prepared quite early by the conversion of other carbon materials in a shock wave. Soon after this discovery, researchers of the DuPont Corp. developed a method also based on shock action that yields very small diamond particles. These are processed by subsequent sintering to give utterly durable cutting and poHshing tools. 

The resulting material consists of polycrystaUine diamond particles measuring 1-60 gm. They closely resemble the naturally occnuring carbonados (Section 1.3.2) in appearance and also in their properties. In particnilar, these diamonds do not feature preferred cleavage planes due to their polycrystaUine structure. As a consequence they show enhanced stabUity and mechanical durabUity and lead to a longer service life when employed in the manufacture of cutting or pohshing tools. 

More than 2 million karat ( 0.4t) of sintered nanodiamond per year is produced in this manner. 

Depending on the specific procedure, the duration of the shock action can range from 0.1 to 10 gs. After the fading of the shockwave, the problem arises again that the diamond initially formed is exposed to a temperature that decreases much less quickly. This means massive losses to the final yield as the reconversion into graphitic material is promoted. The solution for this problem consists in the 

The reaction itself is followed by the isolation of the product The metal powder and unconverted carbon are removed by the action of concentrated nitric acid. The individual diamond particles are generally covered with a graphitic shell, which is an indication of the processes taking place during particle growth. 

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