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The Big Deal About Very Small

Tue, 03/24/2009 - 9:44am
Nanoscale particle size-reduction equipment and processes are daily improving as the market expands for opportunity.

The idea of size reduction to the nanoscale is not new. In fact, long before nanotechnology was coined in 1974, artisans in the 15th century were using gold, copper and silver nanoparticles to affect the color and light reflection in stained glass and ceramics. Today, new discoveries in application and advantage continue. From inks, coatings and ceramics to drug delivery, nano-sized particles are fulfilling manufacturers' needs of efficacy and efficiency.

Nanotechnology is a collective term for a range of applications dealing with structures and processes with product dimensions of less than 100 nm (0.1 µm or 10-7 m). Specific applications include solid particles in suspensions, powders and dust, and liquid drops in emulsions, fog, sprays and foam. The nanotechnology business is booming with support from private and government research. According to a Lux Research study, $12.4 billion was spent on nanotech research and development worldwide in 2006. More than $50 billion worth of nano-enabled products were sold globally that same year. Why put so much money into something so small? Utility in an array of industries. Smaller than viruses or bacteria, nanoparticles can enter cells unhampered, making them beneficial to the pharmaceutical industry as transporting agents. They also have optimal optical properties, such as gloss, transparency, color strength and jetness—explaining why 15th-century artists instinctively used them. Furthermore, nanoparticles are extremely hard and scratchproof, which is valuable to the paint and coatings market, and provide new properties for low-sintering ceramics, amorphous metals, and materials with high-fracture strengths and toughness at low temperatures, or extreme plasticity at high temperatures.
Manufacturing Nano—Up Or Down

There are two places to start when manufacturing nanoparticles—from the bottom or top. The top-down approach requires breaking large pieces of material down to nano size, whereas bottom-up involves assembling single atoms and molecules into larger nanostructures. Bottom-up manufacture produces ultra-pure spherical particles, yet with narrow particle-size distributions of the primary particles. This method often results in agglomeration, thus limiting production capacities. Traditional plasma gas processes, for example, deliver superior particle uniformity, but cannot disperse particles in a solution at their primary size due to their large surface area and energy (which is what actually delivers the advantageous effects of nanomaterials). In contrast, it can also prevent easy particle dispersion in liquids. On the other hand, the top-down approach produces stable dispersions, facilitating scale-up when moving from testing to full production. Some top-down techniques include:

Revolutionary, contemporary top-down methods are in effect for applications crossing almost every industry, providing:

Popular due to their simplicity and scalability, fine-bead mills are used in nanogrinding to efficiently disperse output in primary particle sizes, assuming the proper stabilizing agents are used. The principle of agitator bead mills is based on grinding suspended solid particles by impact and shearing forces between grinding media. In bead milling, the agitator shaft in the grinding chamber transmits kinetic energy to the grinding media. The stress intensity and number of contact points between the beads and particles define the resulting particle fineness. Fine-particle distribution requires many contact points, which can be achieved via smaller media—somewhere between 50 and 200 µm. It's commonly known that particle sizes are about 1/1,000 of the size of grinding media.
Milling Through The Choices

Selecting the proper grinding media and mill is essential to developing a repeatable process and creating efficient, scalable work flow. Grinding beads are available in an assortment of materials, such as plastics, glass, ceramics, steel, tungsten carbide and even yttria zirconia, a high-strength ceramic that allows metal contact-free processing. The right material can prevent unwanted reactions and transfer contamination, avoiding wasted time, money and materials. Using conventional grinding media like alumina and zirconia isn't an option for manufacturers of silicon carbide (SiC) nanoparticles because of high contamination rates and bead wear, which increase cost. NETZSCH Fine Particle Technology, however, has developed a method using SiC grains as milling media, thereby reducing the continuous cost of replacing worn beads. These SiC grains additionally produce contamination-free output, since the process material and media are comprised of the same substance. Maintaining efficient nanogrinding requires constant grinding-media levels monitoring, and in some cases, constant media additions. Conventional methods for managing this process are costly, and can cause blocking and wear. To keep media at optimum levels, NETZSCH has created a media-addition system that incorporates a vibratory feeder, which runs off the mill's kilowatt draw. As media wears, the kilowatt draw drops. Using programmable logic control, the feeder turns on when the kilowatt draw reaches a low setpoint and off when target kilowatt draw is achieved. Current horizontal and vertical mills, moreover, offer ample throughput at low-energy motor speeds to prevent nanoparticle damage, while additionally providing practical methods for handling and removing grinding media at process end. Ideally, the mill would further include plug flow, which prevents product from bypassing the grinding process, to ensure that all material passes through the machine at the same velocity for a uniform grind and residence time distribution. One could consider the following features as well, depending on the equipment's intended application:

People are developing better ways of manufacturing nanoparticles every day. According to Plunkett Research, approximately 2,500 companies are involved in nanotechnology research, and that does not include those already putting nanotechnology to use. What is important now is that manufacturers in all industries embrace the benefits of nano-sized particles and choose the appropriate form of manufacture to produce a safe product.
John Hill is a process and applications advisor at NETZSCH Fine Particle Technology LLC. For more information, please visit www.netzschusa.com. "According to a Lux Research study, $12.4 billion was spent on nanotech research and development worldwide in 2006."
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