High energy ball milling for nanoparticle synthesis

High energy ball milling of powder particles as a method for materials synthesis has been developed as an industrial process to successfully produce new alloys and phase mixtures in 1970’s. This powder metallurgical process allows the preparation of alloys and composites, which cannot be synthesized via conventional routes. In nanomaterials research, this top down technique is well used to fine-tune the grain sizes of the materials in nanoscales.
A variety of milling devices has been developed for different purposes including tumbler mills, attrition mills, shaker mills, vibratory mills, planetary mills etc. In this work, Fritsch planetary micro mill, ‘pulverisette 7’ was used for milling. (Here grinding bowls rotates on their own axis while simultaneously rotating through an arc around the central axis. The grinding balls and the material in the grinding bowl are thus acted upon by the centrifugal forces, which constantly change in direction and intensity resulting in efficient, fast grinding process. The grinding bowl and the supporting disc rotate in opposite directions, so that the centrifugal forces alternatively act in the same and opposite directions. This results in, as a frictional effect, the grinding balls running along the inner wall of the grinding bowl, and impact effect, the balls impacting against the opposite wall of the grinding bowl. The energy thus created by impact is many times higher than for traditional mills. This results in excellent grinding performance and considerably shorter grinding times. Atmospheric contamination can be minimized by sealing the vial with flexible ‘O’ ring after the powder has been loaded. If a milling medium-a fluid (usually an organic fluid) is used, contamination by the milling tools can be prevented and also it minimizes the wear.A few parameters exists in high energy ball milling which on changing, we can produce a wide range of fine particles with different sizes and consequently with different physical properties. These parameters are (1) Type of mill, (2) Milling atmosphere, (3) Milling media, (4) Intensity of milling, (5) Ball to powder weight ratio (BPR), (6) Milling time and (7) Milling temperature.
The reduction in grain size is accomplished by the kinetic energy transfer from balls to powder. Since the kinetic energy of the balls is a function of their mass and velocity, dense materials are preferred like steel or tungsten carbide. Other materials used as balls are agate, sintered corundum, zirconium dioxide, Teflon, chrome nickel, silicon nitride etc. In order to prevent excessive abrasion, the hardness of the grinding bowl used and of the grinding balls must be higher than that of the materials used. Normally grinding bowls and grinding balls of the same material should be chosen. In this work tungsten carbide vial and balls (with density~14.75 g/cm3) are used to mill ferrite system and steel vial and balls (with density~7.85 g/cm3) are used to mill aluminates systems.
In the initial stage of milling, a fast decrease of grain size occurs which slows down after extended milling. Once the minimum steady state grain size is reached, further refinement ceases. Initially the kinetic energy transfer leads to the production of an array of dislocations. This is accompanied by atomic level strains. At a certain strain level, these dislocations annihilate and recombine to form small angle grain boundaries separating the individual grains. Thus sub grains are formed with reduced grain size. During further milling, this process extends throughout the entire sample. To maintain this reduction in size, the material must experience very high stresses. But extended milling could not able to maintain the high stresses and hence reduction of grain size is limited in extended milling. The two other parameters which also causes this limit to grain size reduction are the local temperature developed due to ball collisions and the overall temperature in the vial. Temperature rise arises from balls to balls, balls to powder and balls to wall collisions.
The impact speed and the impulsive load of the grinding balls are the two key parameters, which determine the kinetic energy transfer. The impulsive load of grinding balls is given by,
F = mv/t ---------- (1)
Where ‘m’ is the ball mass, ‘v’ the ball velocity and‘t’ the ball-vial contact time.