Most micron diamonds are produced by breaking larger diamond grains of poor quality. There are various ways to pulverize diamond, each with its range of size applicability (Fig. 5.1).
The breaking of diamond can be accomplished by stamping, compression, impact, shear, and attrition. Stamping and compression may be suitable for objects (e.g. stone) much larger (e.g. >1 cm) than diamond. Impact and shear may be used to pulverize diamond, but often the produced grains are irregular in shape (e.g. platy or slender). Attritors are effective in forming fine powders so they are capable to make micron diamond.
A more common technique is to mill diamond with balls (e.g. made of steel) in a rotating cylinder. The turning of the mill is rather fast so the balls can be raised high to smash diamond more effectively. In order to further increase the dropping height of the balls so to accelerate the production of diamond fines, the cylinder may be rotated around the center of cylinder instead of around the center of axis (Fig. 5.2).
Because the reduction ratio of diamond should be kept low to increase the efficiency of milling, so several ball mills may be used in stages to pulverize diamond to very fine sizes. In each stage, diamond is sized so the feedstock contains diamond of similar sizes. Typically, the load of ball mill is half full. The size of steel balls is about 1 cm. The total weight of balls can be four times higher than
Diamond Nanotechnology: Synthesis and Applications by James C Sung & Jianping Lin
Copyright © 2009 by Pan Stanford Publishing Pte Ltd
diamond. Water may be added to the charge to prevent powder from clinging on the wall.
Ball milling may be effective to reduce the size of diamond, but the broken diamonds are irregular in shape. The sharp corners may be chipped off by using a jet mill that force two streams of micron diamonds to collide each other (Fig. 5.3). Jet milling may not be suitable to reduce diamond in size, but it is effective to smooth the surface by removing weak protrusions.
After jet milling, the chipped diamond is mixed in with much debris and impurities derived from the interior of the milling machine. Most impurities can be dissolved in warm acids (e.g. HNO3 or HCl) or bases (e.g. KOH or NaNOs). The siliceous impurities can be etched away by HF or KF solution.
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