High-tech Ball Mills Spur Nanotechnology Advances

Faster, more accurate and more consistent instrumentation is key to capitalizing on and furthering the advancements of the nanotechnology industry.

RETSCH’s EMax uses a unique combination of high friction and impact to produce extremely fine particles in a short amount of time.Nanotechnology is one of the most innovative developments of our time—it revolutionizes industries such as materials science, pharmaceutics, food, pigments and semi-conductor technology. Nanotechnology deals with particles in a range from 1 to 100 nm. These particles possess special properties because of their size, as their surface is greatly enlarged in relation to their volume (so-called “size-induced functionalities”). Ultrafine particles are, for example, harder and more break-resistant than larger particles. Nanotechnology brings effects that occur in nature to a commercial scale—for example, the lotus effect: nanocoated fabrics or paints that are water- and dirt-repellent, just like the lotus flower.

But how are nanoparticles produced? The “bottom-up” method synthesizes particles from atoms or molecules. The “top-down” method involves reducing the size of larger particles to nanoscale, for example with laboratory mills. Due to their significantly enlarged surface in relation to volume, small particles are drawn to each other by their electrostatic charges. Nanoparticles are produced by colloidal grinding, which involves dispersion of the particles in liquid to neutralize the surface charges. Both water and alcohol can be used as dispersion medium, depending on the sample material. In some cases, the neutralization of surface charges is only possible by adding a buffer, such as sodium phosphate or molecules with longer uncharged tails, like diaminopimelic acid (electrostatic or steric stabilization).

Factors such as energy input and size reduction principle make ball mills the best choice for the production of nanoparticles. The most important criteria for selecting a mill and appropriate accessories are: material of the grinding tools; grinding ball size; grinding balls/sample/dispersant ratio; grinding time; and energy input.

Top-down method

Nanoparticles are created with the top-down method by colloidal grinding using a suitable dispersant to keep the particles from agglomerating. To reduce small particles with mechanical force to even smaller sizes, a high energy input is required. The choice of suitable grinding tools and the correct grinding jar filling are further aspects to be considered.

Depending on the size of the initial sample material and desired final fineness, a preliminary size reduction step can be useful. A dry grinding process with grinding balls of >3 mm Ø is usually carried out by filling one third of the jar with grinding balls and one third with sample material. The obtained sample is then used for the actual colloidal process.

Grinding jars and balls made of an abrasion-resistant material, such as zirconium oxide, are best suited for colloidal grinding. Sixty percent of the grinding jar volume is filled with grinding balls of 0.1 to 3 mm Ø, providing a large number of frictional points. The actual sample fills about one third of the jar volume. By adding a suitable dispersant (e.g. water, isopropanol, buffer), the consistency of the sample should become pasty, thus providing ideal preconditions for colloidal grinding. If a very high final fineness is required, it is recommended to proceed with a second colloidal grinding with 0.1 or 0.5 mm Ø grinding balls, particularly if 2 to 3 mm balls were used in the first process (the balls need to be 3x bigger than the particle size of the initial material). To separate the sample from the grinding balls, both are put on a sieve (with aperture sizes 20 to 50 percent smaller than the balls) with a collecting pan. For the subsequent colloidal grinding, 60 percent of the jar is filled with small beads. The suspension from the previous grinding is carefully mixed with the grinding beads until a pasty consistency is obtained.

Some materials tend to become pastier during grinding, which prevents the grinding balls from moving around in the liquid, thus making further size reduction almost impossible. Therefore, it is recommended to check the consistency of unknown sample materials during the grinding process. If needed, the sample/ball mixture can be further diluted by adding more dispersant. If a sample is known to swell easily, the sample/dispersant ratio should be adapted accordingly. Another option is the addition of surfactant to stabilize the consistency.

Planetary ball mills

With a planetary ball mill, imagine that every grinding jar represents a planet. This planet is located on a circular platform, the so-called sun wheel. When the sun wheel turns, every grinding jar rotates around its own axis, but in the opposite direction. Thus, centrifugal and Coriolis forces are activated, leading to a rapid acceleration of the grinding balls. The result is very high pulverization energy, allowing for the production of very fine particles. The acceleration of the grinding balls from one wall of the jar to the other produces a strong impact effect on the sample material and leads to additional grinding effects through friction. For colloidal grinding and most other applications, the ratio between the speed of the sun wheel and the speed of the grinding jar is 1:-2. This means that during one rotation of the sun wheel, the grinding jars rotate twice in the opposite direction. 

Figure 2 shows the result of grinding of alumina (Al2O3) at 650 min-1 in RETSCH’s PM 100. After 1 hour of size reduction in water with 1 mm grinding balls, the mean value of the particle size distribution is 200 nm; after 4 hours it is 100 nm. In a further trial, the material was initially ground for 1 hour with 1 mm grinding balls and then for 3 hours with 0.1 mm grinding balls. In this case, an average value of 76 nm was achieved. The grinding results show that planetary ball mills can produce particle sizes in the nanometer range.

Faster ball mills

Manufacturers are developing even faster ball mills that can produce ultrafine particle sizes in the shortest amount of time as the next step in advanced ball mill technology. 

For example, RETSCH’s Emax can achieve a speed of 2,000 min-1 through a combination of impact, friction and circulating grinding jar movement. The combination is generated by the oval shape and the movement of the grinding jars that do not rotate around their own axis, as is the case in planetary ball mills. The interplay of jar geometry and movement causes strong friction between grinding balls, sample material and jar walls, as well as rapid acceleration that lets the balls impact with great force on the sample at the rounded ends of the jars. This improves the mixing of the particles, resulting in smaller grind sizes and a narrower particle size distribution. Thanks to a new liquid cooling system, excess thermal energy is quickly discharged, preventing the sample from overheating, even after long grinding times. This ensures that the Emax can handle continuous grinding without breaks.