Consistent Milling on a Nano Scale

February 1, 2005
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Planetary ball mills are helping scientists and engineers advance nanotechnology by providing a consistent way to produce nanoscale powders.

Colloid and chemical scientists have been studying nanoparticles (extremely fine particles of less than 1 micron in diameter) for many years, but recently these tiny particles have been receiving increased attention in a variety of fields. As particles become smaller, their properties can change in mysterious and useful ways. This change in properties is often due to the increased reactivity of nanoparticles and their consequent desire to quickly bond with anything close to them. Unfortunately, this includes a tendency to bond with their own kind, which is one of the many challenges of making and working with these "little fellows." However, once these challenges are overcome, the potential benefits in fields ranging from cancer treatment and food to environmental remediation, ceramics and numerous others are virtually endless.

Before working with nanoparticles, it is first necessary to be able to produce them. One method is to generate particles of the correct size from scratch using crystallization, direct generation or other similar techniques, which is often referred to as "bottom up."

In the other method, referred to as "top down," larger particles are reduced through milling to achieve the desired nanoscale particles. Most mechanical mills use one fixed and one moving element to either compress/shatter or cut individual pieces into smaller particles. Due to mechanical limitations, these methods cannot produce very small particles. However, one technique that will consistently produce nanoscale particles is planetary ball milling.

Figure 1. The combined effects of centrifugal and Coriolis forces in planetary ball milling.

The Planetary Design

All ball milling is carried out in a vessel or jar, into which is added the material to be ground, along with balls or other suitably shaped grinding elements that are made of the same material as the jar. Grinding takes place as a result of the interaction between the balls, particles and grinding jar wall, and the technique works equally well on soft, medium-hard and even extremely hard, brittle and fibrous materials. Unlike knife and rotor mills, where the moving element is fixed to the mill, the grinding balls or elements used in ball mills are free to move around during the grinding process, which gives the technique the ability to produce much finer particles. Generally speaking, smaller balls and longer grinding times are required to produce smaller particles.

One relatively simple version of a ball mill, called a centrifugal ball mill, rotates a jar at controlled speeds around a central axis point. Planetary ball mills also use jars and grinding media, and operate on the same basic principle. One or more grinding jars can be used, with each occupying a station or "planet" that is mounted on a circular platform called a sun wheel. As the sun wheel turns, each of the "planets," or grinding jars, also rotates on its own axis but in the opposite direction (see Figures 1 and 2). This action rapidly accelerates the grinding balls through both centrifugal and Coriolis forces (named after a French engineer). As a result, the grinding energy is significantly increased, which allows even smaller particle sizes to be produced compared to other types of ball mills.

Figure 2. A single-station planetary ball mill. Different models can hold from one to eight grinding jars with volumes from 12 to 500 ml.
The rapid acceleration of the particles from one side of the jar to the other produces powerful impact forces between the balls and sample material while also providing additional grinding action through frictional forces. For colloidal grinding and most other applications, the standard ratio between the rotational speed of the "planets" and the sun wheel is 1:-2. For applications that require a higher level of impact forces, one of the major benefits of the planetary ball mill principle is that different models with different ratios can be provided. For example, in mechanical alloying, where two metals are physically forged together to form an alloy, a ratio of 1:-2.5 or 1:-3.0 would be optimal.

Non-colloidal planetary ball milling is performed dry, which allows for relatively easy recovery of the ground material. (In some cases, a few drops of methanol are added to prevent the ground material from sticking to the jars and balls.) However, to produce submicron or colloidal-sized particles, the grinding must be performed in a liquid medium. This is usually an alcohol (isopropanol or methanol), although other solvents and even water can be used in some applications. The presence of a liquid allows for better dispersion, thereby reducing agglomeration of the smaller particles, which can interfere with the grinding process.

In addition, a large number of small (typically 3 mm) grinding balls is used. Along with the liquid medium, the use of small balls virtually eliminates all impaction and employs primarily frictional forces to grind the material. As particles become smaller, they also become increasingly difficult to split; friction has proven to be one method that will reduce particle size in the colloidal size range.

Grinding jars and media for planetary ball milling can be obtained in a variety of different materials, including stainless steel, agate, tungsten carbide and various ceramic materials. For wet grinding, jars and media made from yttrium-stabilized zirconium oxide (ZrO2) will produce the best results with minimal sample contamination. (Sample contamination is always a concern with mechanical grinding.) This particular ceramic is as close to a perfect grinding media as can be found, since it is both hard and tough, wears very slowly, is non-corrosive and has excellent surface characteristics.

Although not considered brittle compared to other ceramic materials, ZrO2 will shatter if dropped or subjected to similar impact forces. Consequently, some high-quality yttrium-stabilized ZrO2 grinding jars can be obtained with a stainless steel protective jacket that does not come into contact with the material. This feature also allows for the inclusion of other benefits, such as airtight seals and gripping rims, which make the jars both easy and safe to use.

Ensuring Grinding Success

Success or failure in colloidal grinding depends heavily on the grinding parameters used. These include the size of the grinding balls, the ratio of material to the grinding balls and liquid, and the grinding time and speed. Table 1 shows a typical set of parameters for colloidal grinding.

While friction is a desired aspect of planetary ball mill grinding, it also generates a significant amount of heat, and therefore internal pressure, especially in processes such as colloidal grinding that require long grinding times. A tight seal between the jar and lid is therefore vital to prevent the sample and/or solvent from leaking out of the jar during grinding. As an added precaution and to prevent a dangerous sample/solvent loss when the jar is removed, the use of a safety clamping device is also recommended (see Figure 3). This device ensures the complete security of the sample and the safety of the user, since the jar can only be opened when the safety clamp is removed (typically in a glove box or other secure area).

Figure 3. A 250-ml zirconium oxide grinding jar with a safety clamp, ready for grinding. The stainless steel jacket is visible around the bottom of the jar and lid.
After the material has been ground, it must be separated from the grinding balls. The easiest way to accomplish this task is to pour the wet material and grinding media mixture through a 7- or 8-mesh sieve (smaller than the 3 mm grinding balls) into a sieve collection pan. The sample can also be allowed to dry, using a laboratory dryer if necessary. After the material is dry, the sieve and pan can be placed on a suitable three-dimensional sieve shaker, and the vibratory effect will remove the vast majority of the remaining material from the grinding balls. This material will fall through the sieve and into the pan below, leaving behind the relatively clean grinding balls on the sieve. The grinding balls can then go through a final cleaning step in an ultrasonic bath to remove any remaining material.

Figure 4. Results of grinding 100 g quartz at 500 rpm in a Retsch PM 100 planetary ball mill at various grinding times. (The particle size analyses were performed on a Horiba LA 920 laser diffraction analyzer.)

A Safe, Durable Technology

Tests with planetary ball mills have proven the success of this technology in nanomaterial applications. For example, in one test, 100 grams of quartz were ground at 500 rpm (see Figure 4). After one hour, the particle size (d50) was 0.99 µm; after five hours, the particle size had been reduced to 0.6 µm. The same material was also ground at different speeds (see Figure 5). Using 350 rpm, the size was just over 0.8 µm after five hours.

Figure 5. Comparison of grinding results at different speeds. At five hours using 350 rpm, the resultant size was just over 0.8 µm; at 500 rpm, the size was 0.6 µm.
The grinding results show that planetary ball mills are able to produce micron and submicron particles. The results also clearly illustrate that as the particles become smaller, an increased amount of energy is needed to further reduce their size. While the particle size was reduced from around 100 to 0.99 µm in just one hour, another four hours of grinding were required just to reduce the size by an additional 0.39 µm. Such long grinding times demand that the mill be extremely well engineered and able to withstand constant vibration, while at the same time providing users with the versatility to handle many different sample types. Additionally, the mill must be equipped with adequate safety features, since it will have to be left unattended for long periods and used with potentially dangerous solvents.

However, with the right design and the correct grinding parameters, planetary ball mills can be extremely useful and effective tools in the field of nanotechnology.

For more information about planetary ball mills, contact Retsch, Inc., 74 Walker Ln., Newtown, PA 18940; (866) 473-8724; fax (267) 757-0358; e-mail frank.bath@retsch-us.com ; or visit http://www.retsch-us.com .

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