Answer: nanomaterials are synthesised by top-down methods (ball milling, lithography, etching, laser ablation) or bottom-up methods (sol-gel, chemical vapour deposition, chemical reduction, physical vapour deposition, self-assembly). Ball milling is explained below.
Techniques
- Top-down (bulk material broken into nano-sized particles): ball milling, photolithography / electron-beam lithography, etching, laser ablation.
- Bottom-up (built atom by atom or molecule by molecule): sol-gel process, chemical vapour deposition (CVD), physical vapour deposition (evaporation–condensation), chemical reduction/precipitation, molecular beam epitaxy, self-assembly.
Ball milling (top-down method) in detail
Construction: a hollow cylindrical container (vial) made of hardened steel or tungsten carbide is partly filled with hard balls of the same material and the powdered bulk material. In a planetary ball mill, the vials rotate about their own axes while the supporting disc rotates in the opposite direction.
Working:
- When the container rotates, the balls are lifted and then fall or roll, striking the powder with high energy.
- The repeated impact and friction (shearing) between balls, powder and the wall break the particles into smaller and smaller pieces.
- Milling for many hours reduces the particle size to the nanometre range. Size depends on rotation speed, milling time, ball size and the ball-to-powder weight ratio. Milling can be done dry or wet, sometimes in an inert atmosphere to avoid oxidation.
Advantages: simple, inexpensive, works at room temperature, can produce large quantities, and can also make alloys (mechanical alloying).
Disadvantages: contamination from the balls and container, irregular particle shapes, wide size distribution, and defects or strain in the particles.