2. Top down appraoach: BACK
High-Energy ball milling: The milling of materials is of prime interest in the mineral, ceramic processing, and powder metallurgy industry. Typical objectives of the milling process include particle size reduction (comminution), solid-state alloying, mixing or blending, and particle shape changes. These industrial processes are mostly restricted to relatively hard, brittle materials which fracture, deform, and cold weld during the milling operation. While oxide-dispersion strengthened super alloys have been the primary application of mechanical attrition, the technique has been extended to produce a variety of nonequilibrium structures including nanocrystalline, amorphous and quasicrystalline materials. A variety of ball mills has been developed for different purposes including tumbler mills, attrition mills, shaker mills, vibratory mills, planetary mills, etc. The basic process of mechanical attrition is illustrated in fig below.
Fig: Schematic representation of the principle of mechanical milling.
Powders with typical particle diameters of about 50 µm are placed together with a number of hardened steel or tungsten carbide (WC) coated balls in a sealed container which is shaken or violently agitated. The most effective ratio for the ball to powder masses is five to 10.
High-energy milling forces can be obtained using high frequencies and small amplitudes of vibration. Shaker mills (e.g. SPEX model 8000) which are preferable for small batches of powder (approximately 10 cm3 is sufficient for research purposes) are highly energetic and reactions can take place one order of magnitude faster than with other types of mill. Since the kinetic energy of the balls is a function of their mass and velocity, dense materials (steel or tungsten carbide) are preferable to ceramic balls. During the continuous severe plastic deformation associated with mechanical attrition, a continuous refinement of the internal structure of the powder particles to nanometer scales occurs during high energy mechanical attrition. The temperature rise during this process is modest and is estimated to be less than or equal to 100 to 2000 C.
The difficulty with top-down approaches is ensuring all the particles are broken down to the required particle size. Furthermore, for all nanocrystalline materials prepared by a variety of different synthesis routes, surface and interface contamination is a major concern. In particular, during mechanical attrition, contamination by the milling tools (Fe) and atmosphere (trace elements of O2, N2, in rare gases) can be a problem. By minimizing the milling time and using the purest, most ductile metal powders available, a thin coating of the milling tools by the respective powder material can be obtained which reduces Fe contamination tremendously. Atmospheric contamination can be minimized or eliminated by sealing the vial with a flexible ‘O’ ring after the powder has been loaded in an inert gas glove box. Small experimental ball mills can also be enclosed completely in an inert gas glove box. As a consequence, the contamination with Fe based wear debris can generally be reduced to less than 1-2 % and oxygen and nitrogen contamination to less than 300 ppm. However, milling of refractory metals in a shaker or planetary mill for extended periods of time (>30 h) can result in levels of Fe contamination of more than 10% if high vibrational or rotational frequencies are employed. On the other hand, contamination through the milling atmosphere can have a positive impact on the milling conditions if one wants to prepare metal or ceramic nanocomposites with one of the metallic elements being chemically highly reactive with the gas (or fluid) environment. On the other side, main advantage of top-down approach is high production rates of nano powders.